Catalytic conversion of alkylaromatics to aromatic nitriles
Stobbelaar, P.J.
DOI:
10.6100/IR538872
Published: 01/01/2000
Document Version
Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)
Please check the document version of this publication:
• A submitted manuscript is the author’s version of the article upon submission and before peer-review. There can be important differences
between the submitted version and the official published version of record. People interested in the research are advised to contact the
author for the final version of the publication, or visit the DOI to the publisher’s website.
• The final author version and the galley proof are versions of the publication after peer review.
• The final published version features the final layout of the paper including the volume, issue and page numbers.
Link to publication
Citation for published version (APA):
Stobbelaar, P. J. (2000). Catalytic conversion of alkylaromatics to aromatic nitriles Eindhoven: Technische
Universiteit Eindhoven DOI: 10.6100/IR538872
General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners
and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.
• You may not further distribute the material or use it for any profit-making activity or commercial gain
• You may freely distribute the URL identifying the publication in the public portal ?
Take down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Download date: 18. Jun. 2017
Catalytic Conversion of Alkylaromatics
to Aromatic Nitriles
Proefschrift
ter verkrijging van de graad van doctor aan de
Technische Universiteit Eindhoven, op gezag van de
Rector Magnificus, prof.dr. M. Rem, voor een
commissie aangewezen door het College voor
Promoties in het openbaar te verdedigen op
dinsdag 28 november 2000 om 16.00 uur
door
Pieter Johannes Stobbelaar
geboren te Driebergen-Rijsenburg
Dit proefschrift is goedgekeurd door de promotoren:
prof.dr. R.A. van Santen
en
prof.dr. B.K. Hodnett
CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN
Stobbelaar, Pieter J.
Catalytic conversion of alkylaromatics to aromatic nitriles / by
Pieter J. Stobbelaar. - Eindhoven : Technische Universiteit Eindhoven,
2000. - Proefschrift. - ISBN 90-386-2612-6
NUGI 813
Trefwoorden: katalytische oxidatie ; ammoxidatie / heterogene katalyse ;
zeolieten / overgangsmetaalverbindingen ; molybdeenverbindingen
Subject headings: catalytic oxidation ; ammoxidation / heterogeneous
catalysis ; zeolites / transition metal compounds ; molybdenum
compounds
The work described in this thesis has been carried out at the Schuit Institute
of Catalysis (part of NIOK: the Netherlands School for Catalysis
Research), Laboratory of Inorganic Chemistry and Catalysis, Eindhoven
University of Technology, The Netherlands. Financial support has been
supplied by the European Community under the Industrial & Materials
Technologies Programme (Brite-EuRam III).
Printed at Universiteitsdrukkerij, Eindhoven University of Technology.
Contents
Chapter 1: Nitrile formation and conversion reactions
Abstract
1.
Aromatic nitriles: Production and applications
1.1
Ammoxidation reactions
1.2
(Potential) Applications of nitriles
1.3
Aromatic nitriles as intermediates in selective oxidation reactions
2.
Scope of research
References
1
1
1
1
3
5
7
9
Chapter 2: Toluene ammoxidation mechanism
Abstract
1.
Main reaction steps during toluene ammoxidation
2.
Toluene activation
2.1
Hydrocarbon rupture
2.2
Effect of substituents on the aromatic ring
2.3
Effect of the catalyst basicity on the ammoxidation of
alkylaromatics
2.4
Nature of aromatic reaction intermediate
3.
Ammonia activation
4.
Catalyst reoxidation
5.
Toluene ammoxidation reaction schemes
5.1
The propylene ammoxidation mechanism
5.2
The ammoxidation of toluene
6.
Conclusions
References
11
11
11
12
12
13
15
Chapter 3: Screening of new toluene ammoxidation catalysts
Abstract
1.
Introduction
2.
Experimental methods
2.1
Catalyst preparation and characterization
2.2
Catalyst testing
3.
Results and discussion
3.1
Catalyst screening
3.2
Catalyst deactivation
3.2.1
Performance of ion-exchanged catalysts
41
41
41
43
43
45
46
46
50
52
18
23
27
29
29
32
35
36
Contents
3.2.2
3.2.3
3.2.4
3.3
3.4
3.5
4.
References
Performance of catalysts prepared by CVD of metal carbonyls
Performance of NaY based impregnated catalysts
Performance of γ-alumina supported catalysts
Benzonitrile selectivity
Temperature influence
Nitroxidation of toluene
Conclusions
52
53
55
57
60
61
64
64
Chapter 4: Faujasite encaged metal oxide toluene ammoxidation catalysts
prepared from metal carbonyl precursors
Abstract
1.
Introduction
2.
Materials and methods
2.1
Catalyst preparation
2.2
Catalyst characterization
2.2.1
Determination of the catalyst composition
2.2.2
X-Ray Photoelectron Spectroscopy
2.2.3
Transmission Electron Microscopy
2.2.4
Temperature Programmed Oxidative Decarbonylation
2.3
Catalytic tests
2.3.1
2-Methyl-3-butyn-2-ol decomposition
2.3.2
Toluene ammoxidation
3.
Results and discussion
3.1
Thermal activation of intra-zeolite Mo(CO)6
3.2
XPS analysis of Mo(CO)6/NaY and MoOx/NaY
3.3
Dispersion of molybdenum oxide clusters in NaY
3.4
Mo(CO)6 interaction with the faujasite lattice
3.5
Introduction of other transition metal carbonyls by CVD
3.5.1
Introduction of V(CO)6 into NaY
3.5.2
Introduction of Mn2(CO)10 into NaY
3.5.3
Introduction of Co(NO)(CO)3 into NaY
3.6
Catalytic activity in the ammoxidation of toluene
3.7
The effect of the Lewis acidity and basicity on the ammoxidation
of toluene over MoOx/Y
4.
Conclusions
References
67
67
67
71
71
72
72
73
73
73
74
74
74
75
75
80
84
87
92
92
94
96
97
99
100
101
Contents
Chapter 5: The effect of molybdenum oxide reducibility on the ammoxidation of
toluene
Abstract
1.
Introduction
1.1
Preparation methods of supported Mo catalysts
1.2
Notation of different Mo species
1.3
Molybdate surface species
1.4
Characterization of Mo surface species
1.5
Molybdate and Mo oxide reduction
2.
Materials and methods
2.1
Catalyst preparation
2.2
Catalyst characterization
2.2.1
Diffuse reflection UV-Vis spectroscopy
2.2.2
Temperature Programmed Reduction
2.2.3
Raman Spectroscopy
2.2.4
Transmission Electron Microscopy
2.2.5
X-Ray Diffraction
2.2.6
X-Ray Photoelectron Spectroscopy
2.2.7
Hydrogen–deuterium exchange reactions
2.3
Ammoxidation of toluene
3.
Results and discussion
3.1
Addition of a second metal to Mo/Al
3.2
Variation of the molybdenum oxide loading
3.3
DR-UVVis Spectroscopy
3.4
Reduction of Mo/Al catalysts
3.5
Hydrogen-deuterium exchange over Mo/Al catalysts
3.6
Transmission Electron Microscopy on Mo/Al samples
In situ treatment of Mo/Al
3.7
4.
Conclusions
References
105
Summary
143
Samenvatting
147
Dankwoord
151
Curriculum Vitae
153
105
105
106
107
108
110
112
114
114
114
114
115
115
115
116
116
116
117
117
117
120
121
124
126
131
131
137
138
Chapter 1
Nitrile formation and conversion reactions
Abstract
The background of the research project is described. Ammoxidation of
alkylaromatics is a simple gas-phase reaction that yields aromatic nitriles.
These nitriles have versatile applications, mainly as raw material in the
polymer industry. Additionally, alkylaromate ammoxidation can be applied
in the production of selective oxidation products since nitriles can be
converted by hydrolysis and hydrogenation reactions towards acids,
aldehydes, amines and amides. This two-step approach cleanly yields the
oxygenate without production of harmful side products. The project focuses
on the ammoxidation of toluene. For this reaction the development of new
faujasite-based catalysts was performed. Additionally, a comparison with
more conventional γ-alumina supported molybdenum oxide catalysts has
been made.
1.
Aromatic nitriles: Production and applications
1.1
Ammoxidation reactions
Aromatic nitriles can be formed by reacting an aromatic hydrocarbon with
ammonia and oxygen. The simplest example is benzonitrile production
from toluene, as shown in Equation 1.1.
CH3 + NH3 + 3/2 O2
CN + 3 H2O
(1.1)
The reaction of a reducible hydrocarbon with ammonia and oxygen are
referred to as ammoxidation reactions. Alkenes, alkanes and aromatics are
used most often in ammoxidation reactions. The catalysts that are active in
ammoxidation reactions consist mainly of mixed oxides containing
variable-valence transition metals.
For the ammoxidation of propylene bismuth-molybdate based systems are
applied industrially on a large scale [1]. The ammoxidation of propylene is
well developed and is commercially applied by Sohio since the early sixties
1
Chapter 1
[2]. The annual world production amounts to 4600 ktons [3]. Until recently
the production of acrylonitrile from propane could only be performed at
very high temperatures (750 – 1000 °C) [4]. In the late eighties propylene
ammoxidation to acrylonitrile has been patented frequently, for example by
BP America (previously SOHIO) [5]. Recently, a large variety of new
catalysts have been developed for the ammoxidation of propane. Mostly
vanadium antimony oxide [6] systems are reported, though also
molybdenum based multi-component catalysts are frequently patented [7].
Pilot-plant studies have been performed already [8] and commercial
production of acrylonitrile from propane was announced [9]. The feedstock
price of propane is significantly lower than that of propylene, but the
acrylonitrile yields are markedly lower because of the poor acrylonitrile
selectivity [10,11]. This lower acrylonitrile yield per mole of feedstock
delays commercial production of acrylonitrile from propane. To date
acrylonitrile production from propane has not started yet [12].
Aromatic ammoxidation reactions are performed mostly over vanadia
based catalysts [13]. Several companies practice commercial ammoxidation
of alkylated aromatics [14]. Showa Denko converts p-xylene and m-xylene
to the corresponding di-nitriles, terephthalonitrile and isophtalonitrile. Also
Mitsubishi Gas Chemical ammoxidizes m-xylene to isophtalonitrile on a
commercial scale. They operate two plants in the USA and in Japan. BASF
and Japan Catalytic Chem. Ind. produce phthalonitrile from o-xylene.
Phthalonitrile is applied as an important precursor in the manufacturing of
phthalocyanine dyes.
Typically, ammoxidation reactions are performed at temperatures between
400 and 500 °C. For propane ammoxidation the reaction temperature may
be somewhat higher, because the dehydrogenation of propane needs higher
temperatures to occur. During ammoxidation the catalysts is reduced by
ammonia and the hydrocarbon. It is generally accepted that lattice oxygen
reoxidizes the catalyst during ammoxidation. The oxygen insertion step
and catalysts reoxidation can be performed in separate reactors [15]. Under
these conditions the production of nitriles from hydrocarbons is referred to
as oxidative ammonolysis. In this way explosion hazards can be
eliminated, since the hydrocarbon mixture and oxygen are not mixed
together in one reactor.
2
Introduction and background
1.2
(Potential) Applications of nitriles
The most well-known ammoxidation reaction is the ammoxidation of
propylene to form acrylonitrile. Acrylonitrile is basically used for the
production of acrylic fibers, which can be used for manufacturing of
clothing and carpets [16]. Worldwide, about 65 % of the acrylonitrile
production is consumed for this purpose. Another important use of
acrylonitrile can be found in the resin production, of which acrylonitrilebutadiene-styrene (ABS) and styrene-acrylonitrile (SAN) are the main
applications. An extensive growth in the application of these resins has
occurred during the past decade. The largest increase among the end uses
of acrylonitrile, however, has come from adiponitrile, which is used as a
precursor for hexamethylenediamine (HMD) by Monsanto [17]. HMD is
used for the production of nylon-6,6. Recently, also the large-scale
production of caprolactam from adiponitrile was reported [18].
Caprolactam is used as precursor for nylon-6, which can be produced in the
same production site. Other applications of acrylonitrile are also found in
the polymer industry.
Catalytic hydrogenation of nitriles may result in several products. Among
these, amines, imines, aldehydes, amides and alcohols are the most
important products. The main product depends on the catalyst, substrate
and reaction conditions [19].
Aromatic nitriles find diverse applications, for example as dyes and in
pesticide and fungicide production but also in various nylons and
polyurethane foams. Benzonitrile is used as a precursor for resins and
coatings. Benzonitrile is also used as an additive in fuels and fibers. Table
1.1 lists the main application of some relatively simple substituted aromatic
nitriles. As already discussed nitriles derived from xylenes are primarily
used as precursors for the corresponding di-acids, for ultimate use in esters
and polyesters.
3
Chapter 1
Table 1.1: Applications of substituted benzonitriles.
Compound
Application
Remarks
2-chlorobenzonitrile
azo dyes
intermediate in the production of 2amino-5-nitrobenzonitrile
4-chlorobenzonitrile
2,6-dichlorobenzonitrile
2,6-difluorobenzonitrile
2-chloro-4-nitrobenzonitrile
4-chloro-2-nitrobenzonitrile
2-amino-5-nitrobenzonitrile
4-hydroxybenzonitrile
red pigment
for plastics
herbicide for
fruit and vine
cultivation
insecticides
intermediate in the production of 2,6difluorobenzonitrile and 2,6dichlorothiobenzamide
intermediate
azo dyes
intermediate
azo dyes
intermediate in the production of 2amino-4-chlorobenzonitrile
azo dyes
intermediate
herbicides
intermediate in the production of 3,5dibromo- and 3,5-diiodo-4-hydroxybenzonitrile
Data from [20].
The applications of more complex substituted aromatic and hetero
aromatic nitriles were described by Grasselli et al. [21]. For example, high
performance polymers are formed from atroponitrile. Related substituted
aromatic aldehydes such as atropaldehyde and cinemaldehyde, which can
be produced by direct gas-phase oxidation of the substrate, are used as
flavors or perfumes in different products. In Vitamin B complex
nicotinamide (niacinamide) and nicotinic acid (niacin) are used. These
products are formed from nicotinonitrile, which can be obtained readily by
ammoxidation of 3-methylpyridine over phosphorous molybdate-vanadate
catalysts. Additionally, fungicides can be prepared from heteroaromatic
nitriles such as 4-cyanothiazole. In Scheme 1.1 some examples are
summarized of aromatic nitriles and their applications. Also the most
commonly used catalysts are indicated for each example.
4
Introduction and background
Application as monomers for high performance polymers:
H2C
H2C
CH3
+ NH3 + 3/2 O2
CN
USb4.6Ox
+ 3 H2O
Application in resin and coating production:
CH3 + NH3 + 3/2 O2
V2O5
CN + 3 H2O
Application in Vitamin B complex:
CH3
+ NH3 + 3/2 O2
CN
PVMoOx
+ 3 H2O
N
N
Application as intermediate in fungicide production:
H3C
N
S
NC
+ NH3 + 3/2 O2
N
various catalysts
+ 3 H2O
S
Scheme 1.1: Applications of several ammoxidation reactions.
1.3 Aromatic nitriles as intermediates in selective oxidation
reactions
During the eighties ammoxidation reactions were frequently investigated,
especially the ammoxidation of alkylaromatics. The ammoxidation of
toluene to form benzonitrile was often used as a model reaction for other
alkylaromatics such as p-xylene. Showa Denko and Lummus ammoxidize
xylenes to mono- and di-nitriles. In the Lummus process aromatic nitriles
are prepared via an ammonolysis reaction. Xylene reacts with ammonia
and lattice oxygen to form the aromatic nitrile. Gas-phase oxygen is used
afterwards to regenerate the catalyst [15]. As described earlier by these
authors terephthalic acid can be produced via hydrolysis of the nitriles [22]
5
Chapter 1
produced in this oxidative ammonolysis reaction. The conversion of
alkylaromatics to oxidized products such as terephthalic acid is usually
performed by direct oxidation reactions. Since the performance of
alkylaromatic autoxidation reactions is relatively simple, because ring
oxidation does not occur, these reactions are performed basically in the
liquid phase [23]. The oxidizability of alkylaromatic hydrocarbons by
liquid-phase autoxidation reactions decreases significantly in the order
tertiary > secondary > primary benzylic C-H bonds [24]. Therefore, liquidphase autoxidations have somewhat limited applications. Especially for
primary alkylaromatics such as toluene, it is not possible to achieve high
selectivity to hydroperoxide at reasonable high reaction rates. Since the
oxidizability of toluene is about five orders of magnitude lower than the
oxidizability of benzaldehyde [25] production of benzaldehyde by
autoxidation is not possible. Nevertheless, terephthalic acid can be
produced in high yields by liquid-phase direct oxidation using a
Co/Mn/Br-acetic acid catalyst [26]. Though the yield of terephthalic acid
by the conventional liquid-phase process is high, future regulations may
restrict the application of this reaction, since the process conditions, which
require the highly corrosive bromine–acetic acid environment, are
reprehensible from environmental perspective. On the other hand due to
the low solubility of terephthalic acid in the solvent, most of it precipitates
as it forms. Separation of the product from the solvent is easy and the
production process will only be changed if future legislation so obliges.
Based on the atom utilization concept described by Sheldon and Dakka
[27], in general gas-phase oxidations are preferred over liquid-phase
oxidation processes. Moreover, the use of gas-phase oxygen as oxidant is
highly desirable since besides the oxidation product only water is produced.
The Environmental Quotient (EQ), which is defined by the amount of
waste per kilogram of product multiplied by an unfriendliness quotient (Q)
is as low as possible for oxidation reactions. In this respect aromatic nitriles
can be used as intermediates in selective oxidations. According to Equation
1.1 the aromatic nitrile is manufactured with high atom utilization; only
water (having a low Q value) is formed as side product. Conversion of the
aromatic nitrile in a second step cleanly yields the oxidation product.
Up to now the only industrially important manufacturing process for
benzaldehyde is the hydrolysis of benzal chloride or the partial oxidation of
6
Introduction and background
toluene [28]. The first route is highly productive and high benzonitrile
selectivity is obtained (> 95%). However, for each molecule of
benzaldehyde one hydrogen chloride molecule is produced as a side
product. Direct selective oxidation of toluene is a clean route. To date,
however, only moderate benzaldehyde yields are obtained. Very recently
the group of Centi developed a bulk-type Fe-Mo-Ce-oxide catalyst that
produces in high yield (50-55 mol%) 3-fluorobenzaldehyde from 3fluorotoluene [29]. Via classical organic chemistry aldehydes can be formed
from nitriles by performing a reduction with di-isobutylaluminumhydride
[30].
Chatterjee et al. [31] produced benzaldehyde from benzonitrile over
platinum and ruthenium loaded acidic zeolites with high selectivity by
vapour-phase reductive hydrolysis. This reaction can also be performed in
the liquid phase using Raney nickel [32] though a sulphuric acid or formic
acid medium has to be applied in this case. Also nickel and iron
precipitated on alumina catalysts have been described in literature for the
liquid-phase reductive nitrile hydrolysis [33]. By hydrogenation aromatic
nitriles can also be converted to aromatic amines [14]. The production of
benzamide can be performed selectively over hydrotalcite-like catalysts as
will be reported by Sychev et al. [34]. In our group theoretical work related
to aromatic nitrile conversion, was carried out by Barbosa et al. [35], who
studied the hydrolysis of acetonitrile over protonic zeolite catalysts.
2.
Scope of research
The research described in this thesis was aimed at the development of new,
selective and clean processes for alkylaromatic side chain oxidation. A gasphase process was chosen for the conversion of the alkylaromatic sidechain oxidation, based on the poor opportunities for liquid-phase processes.
Catalyst leaching complicates severely the possibilities of liquid-phase
heterogeneously catalyzed processes. Moreover, the higher cost of the
oxidant does not favor the economics of the process. Therefore, a two-step
vapour-phase process was studied, in which an alkylaromatic substrate is
converted by ammoxidation to an aromatic nitrile. In a second step this
aromatic nitrile is subjected to a hydrolysis reaction to form the oxygenate.
This reaction pathway cleanly yields oxygenated aromatics, as sketched in
Scheme 1.2.
7
Chapter 1
Amide formation:
CH3 +
NH3 + 3/2 O2
CN
+ 3 H2 O
O
CN + 2 H2O2
OH-
CNH2
+ O2 + H2O
Overall:
O
CH3 + NH3 + 2 H2O2 + 1/2 O2
CNH2 + 4 H2O
Aldehyde formation:
CH3 +
NH3 + 3/2 O2
CN
+ 3 H2O
O
CN + H2 + H2O
H
+ NH3
Overall:
O
CH3 + O2
H
+ H2O
Scheme 1.2: Ammoxidation and sequential hydrolysis to cleanly produce
oxygenated aromatic hydrocarbons.
For the ammoxidation reaction toluene was chosen as substrate, based on
the relative simplicity of the molecule. An elementary screening study was
performed in order to check the feasibility of faujasite-based catalysts for
the ammoxidation reaction. The performance of zeolite Y encaged
molybdenum oxide nanoclusters was compared to that of γ-alumina
supported molybdenum oxide. The properties of the latter catalysts were
studied in great detail by both in situ and ex situ characterization
techniques.
This thesis focuses on the ammoxidation of toluene. Theoretical work on
nitrile hydrolysis was performed by Barbosa [36]. The hydrolysis of
8
Introduction and background
benzonitrile was studied in the National Technical University of Ukraine,
Kiev by Prihod’ko [37].
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
J.F. Brazdil, Acrylonitrile, in: Kirk-Othmer Encyclopedia of Chemical
Technology, Vol 1. Wiley New York, 4th edition, pp. 352-369.
J.D. Idol, US Patent 2904580, J.D. Idol, 1959.
http://www.chemweek.com/productfocus/1996/acrylonitrile.html
P.W. Langvardt, Acrylonitrile, in: Ullmann’s Encyclopedia of Industrial
Chemistry, 6th (electronic) edition, 1999.
E.g. A.T. Gutmann, R.K. Grasselli, J.F. Brazdil, US Patent 4746641,
(1988).
G. Centi, F. Marchi, S. Perathoner, J. Chem. Soc. Faraday Trans., 92,
(1996), 5141-5149.
H. Midorikawa, N. Sugiyama, H. Hinago, US Patent 6973186, 1999.
H. Kazuyuki, S. Komada, US Patent 5907052, 1999.
K. Aoki, US Patent 5780664, 1998.
H. Midorikawa, K. Someya, K. Aoki, O. Nagano, US Patent 5658842,
1997.
H. Midorikawa, K. Someya, US Patent 5663113, 1997.
R. Canavesi, F. Ligorati, R. Ghezzi, US Patent 4609635, 1986.
P. Fairley, Chem. Week, 160(36), (1998), 45.
P. Layman, Chem. Eng. News, 73, (1995), 13-15.
B.K. Hodnett, Heterogeneous Catalytic Oxidation, John Wiley & Sons,
Chichester, 2000, pp. 240-263.
K. Weissermel, H-J. Arpe, Industrial Organic Chemistry, 3rd edition,
VCH, Weinheim, 1997, pp. 303-310.
R.K. Grasselli, Ammoxidation, in: Handbook of Heterogeneous
Catalysis, Eds. G. Ertl, H. Knözinger, J. Weitkamp, Vol. 5, VCH,
Weinheim, 1997, pp. 2302-2326.
R.G. Rizayev, E.A. Mamedov, V.P. Vislovskii, V.E. Sheinin, Appl.
Catal. A, 83, (1992), 103-140.
K. Weissermel, H-J. Arpe, Industrial Organic Chemistry, 3rd edition,
VCH, Weinheim, 1997, pp. 385-403.
M.C. Sze, A.P. Gelbein, Hydrocarbon. Proc., 1976, 103-106.
J.F. Brazdil, Acrylonitrile, In: Kirk-Othmer Encyclopedia of Chemical
Technology, 4th edition, Vol. 1, pp. 352-369.
M.M. Baizer, C.R. Campbell, R.H. Fariss, R. Johnson, US Patent
3193480, 1965.
Chemical Market Reporter, 254, (1998), 5.
G.V. Smith, F. Notheisz, Heterogeneous Catalysis in Organic
Chemistry, Academic Press, San Diego, 1999, p. 71-79.
P. Pollak, G. Romeder, F. Hagedorn, H-P. Gelbke, Nitriles, in:
Ullmann’s Encyclopedia of Industrial Chemistry, 6th (electronic) edition,
1999.
R.K. Grasselli, J.D. Burrington, R. Di Cosimo, M.S. Friedrich, D.D.
Suresh, in: Heterogeneous Catalysis and Fine Chemicals, Eds. M.
Guisnet, J. Barrault, C. Bouchoulle, D. Duprez, C. Montassier, G.
9
Chapter 1
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
10
Pérot, Stud. Surf. Sci. Catal., Vol. 41, Elsevier Science Publishers B.V.,
Amsterdam, 1988, pp. 317-326.
A.P. Gelbein, M.C. Sze, R.T. Whitehead, Hydrocarbon Processing,
(1973), 209-215.
R.A. Sheldon, J.K. Kochi, Metal-Catalyzed Oxidations of Organic
Compounds, Academic Press, 1981, pp. 315-339.
J.A. Howard, Adv. Free Radical Chem., 4, (1972), 49-173.
R.A. Sheldon, Liquid Phase Autoxidations, in: Catalytic Oxidation,
Principles and Applications, (Eds. R.A. Sheldon, R.A. van Santen),
World Scientific, London, 1995, pp. 150-174.
W. Partenheimer, Catal. Today, 23, (1995), 69-158.
R.A. Sheldon, J. Dakka, Catal. Today, 19, (1994), 215-246.
F. Brühne, E. Wright, Benzaldehyde, in: Ullmann’s Encyclopedia of
Industrial Chemistry, 6th (electronic) edition, 1999.
G. Centi, Private communications.
T.W.G. Solomons, Organic Chemistry, 5th edition, John Wiley & Sons
Inc., New York, 1992, pp. 686-687.
A. Chatterjee, R.A. Skaikh, A. Raj, A.P. Singh, Catal. Lett., 31, (1995),
301-305.
P. Tinapp, Chem. Ber., 102, (1969), 2770-2776.
B. Staskun, O.G. Backeberg, J. Chem. Soc., (1964), 5880-5881.
T. Es, B. Staskun, J. Chem. Soc., (1965), 5775-5777.
Z. Bodnar, T. Mallat, J. Petro, J. Mol. Catal., 70, (1991), 53-64.
R. Prihod’ko, I. Kolomitsyn, M. Sychev, P.J. Stobbelaar, R.A. van
Santen, Micr. Mesop. Mat., to be published.
L.A.M.M. Barbosa, R.A. van Santen, J. Catal., 191, (2000), 200-217.
L.A.M.M. Barbosa, Theoretical Study of Nitrile Hydrolysis by Solid
Acid Catalysts, PhD Thesis, Eindhoven University of Technology, 2000.
R. Prihod’ko, Synthesis and Characterization of some Heterogeneous
Catalysts for Fine Organic Chemistry, PhD Thesis, National Technical
University of Ukraine, Kiev, in preparation.
Chapter 2
Toluene ammoxidation mechanism
Abstract
The mechanism of the ammoxidation of toluene is reviewed.
Ammoxidation of toluene is mainly studied over vanadia-based catalysts.
Although the literature is not consistent with respect to the exact
mechanism some general trends can be observed. The rate-determining step
is the hydrocarbon activation. Most authors agree on the formation of an
oxygenated adsorbed organic intermediate. Toluene is adsorbed on the
catalyst surface as a benzyl fragment. This benzyl species is oxygenated to
form an adsorbed benzaldehyde surface structure. This structure is
sometimes also referred to as benzoate species. Additionally a reaction
pathway via sequential dehydrogenation of adsorbed benzyl species to an
adsorbed amine and imine is plausible. Oxygen is supplied as surface
oxygen, according to a Mars-Van Krevelen like mechanism. The exact
nature of the nitrogen insertion site is studied less extensively. The amount
of ammonia plays a decisive role in the catalyst oxidation state. Strong
ammonia adsorption leads to an inactive catalyst, whereas weak ammonia
adsorption leads to combustion reactions.
1.
Main reaction steps during toluene ammoxidation
Several groups have studied toluene ammoxidation in the past years.
Rizayev et al. [1], concentrating on Russian literature, have reviewed the
ammoxidation of simple alkylaromatics over vanadium oxide based
catalysts in 1992, but no literature overview exists that also discusses other
catalyst systems. In addition, toluene ammoxidation over VPO-based
catalysts as investigated intensively during the last five years by the group
of Lücke et al. [2,3] was not included in this review. Recently Centi et al.
[4] discussed in more detail ammonia activation with respect to the
ammoxidation of alkylaromatic compounds. This section discusses in a
more extensive manner the role of the several reaction steps in the
ammoxidation of toluene.
11
Chapter 2
In toluene ammoxidation reactions three important processes occur:
1. Toluene activation, during which the methyl group has to be
dehydrogenated.
2. Ammonia activation, leading to the formation of the nitrogen insertion
species.
3. Reoxidation of the catalysts by consumption of gas-phase oxygen.
Scheme 2.1 summarises these steps in toluene ammoxidation.
+
NH3
1/2 O2
+
(I)
(O)
+
C6H5CH3
(I)
()
C6H5CN
(1)
+
()
(O)
(2)
(3)
Scheme 2.1: Fundamental steps during toluene ammoxidation
It is generally agreed that activation of the methyl group is the ratedetermining step during toluene ammoxidation [E.g. 5,6]. The nature of
this activated species, however, is still under debate. According to Golodets
[7] partial oxidation reactions occur on oxide catalysts by a mechanism of
alternating reduction and oxidation of the catalyst surface. Total oxidation
reactions, on the other hand, proceed via both redox and associated
mechanisms. This is also true for ammoxidation reactions.
In this chapter the literature on the role of each of these three steps in the
ammoxidation of toluene is reviewed.
2.
Toluene activation
2.1
Hydrocarbon rupture
The pathway of hydrocarbon activation has been studied by several groups,
mostly by applying kinetic studies or IR Spectroscopy. If the nitrile
production occurs along the side chain three basic types of C-H activation
must be considered:
1. Heterolytic rupture, producing a carbocation and an H--ion. This
possibility is believed to occur over acidic catalysts. When this pathway
of C-H rupture occurs, the H--ion binds to the acid centre to give
hydrogen, which is oxidised to water in the presence of oxygen. This
pathway, however, was never proven experimentally.
12
Toluene ammoxidation mechanism
2. Heterolytic rupture, producing a H+ ion and a carbanion. This
mechanism is plausible over sufficiently strong basic sites. The
hydrocarbon acts as an acid when this C-H rupture mechanism applies
to the reaction.
3. Homolytic rupture. A hydrocarbon radical and a hydrogen radical are
formed. Contrary to the two heterolytic C-H rupture mechanisms the
presence of electron donating or electron withdrawing side groups on
the benzene ring should have little influence on the activity or selectivity
in the ammoxidation of toluene.
2.2
Effect of substituents on the aromatic ring
To examine in more detail these types of C-H rupture several groups have
studied the effect of electron donating and withdrawing side-groups on the
aromatic ring. The addition of an electron-withdrawing group (especially in
the para position) to the aromatic ring would increase the reactivity of the
methyl group if C-H rupture occurs according to heterolytic rupture via the
formation of a carbocation. In this case the formation of a carbanion would
be favoured if an electron-donating group is attached to the aromatic ring.
The effect of substituents on the aromatic ring can be divided into an
inductive effect, in which charges are stabilized by the aromatic ring and a
resonance effect, which applies to groups that contain a lone pair of
electrons. Generally, the resonance effect is much stronger than the
inductive effect. Moreover, the resonance effect is directed to the
substituent position. Electron donating groups such as -NH2 stabilize cation
formation only in the ortho- and para-position. Table 2.1 lists the most
important substrates used in ammoxidation reactions.
Table 2.1: Substituent effect of the most important alkylaromatic
ammoxidation substrates
Inductive effect
Electron withdrawing
Electron donating
Resonance effect
NO2
CN, CHO, COOH
OH
Halogens
CH3
C2H5
C6H5
NH2
OCH3
OH
Data from Morrison and Boyd [8].
13
Chapter 2
Over titania-supported vanadia catalysts the ammoxidation activity is
increased when a substituted toluene is applied as substrate, both with
electron withdrawing as with electron donating substituents as found by
Busca et al. [9]. The relative alkylaromatic ammoxidation rates are listed in
Table 2.2.
Table 2.2: Ammoxidation rates over substituted alkylaromatic substrates
Substrate
Toluene
m-Xylene
p-Methoxytoluene
p-Chlorotoluene
p-Xylene
Relative
Relative ammoxidation rate
1.00
1.12
1.27
1.42
1.45
Activities measured at T = 300 ° C over a V-Ti-O catalyst [9].
These data support the occurrence of homolytic C-H rupture over catalysts
that have well defined redox properties. Cavani et al. [6] reported the
ammoxidation activity of a series of para-substituted alkylaromatics over VTi-O. Compared to toluene they found a higher activity towards the nitrile
product for all substrates applied, irrespective the electron donating or
withdrawing properties of the substituents. This does support a homolytic
C-H rupture mechanism. The same group found similar ammoxidation
activity with respect to toluene ammoxidation when a methyl group was
situated in the meta-position [10]. The resonance effect predicts a strong
difference between the ortho- and para-position on the one hand and the
meta-position on the other hand. Differences that were found in selectivity
towards the nitrile products could be explained well by steric effects. Earlier
steric hindrance was found by Chmyr et al. [11] who found lower nitrile
yields when the aromatic ring was substituted in the 2 and 6 positions. The
methyl group was less accessible for reaction in this case as a result of the
presence of these chloro substituents in the 2 and 6 position. 3,5 Chloro
substitution protected ring oxidation without decreasing the ammoxidation
activity.
Cavani et al. [6], however, found significantly lower selectivities towards
alkylaromatic nitriles when a strong electron-donating group (methoxy)
was attached to the aromatic ring in the para-position. This significantly
lower nitrile production could not be explained by the homolytic rupture
mechanism proposed. The higher electron density in this case led to a more
14
Toluene ammoxidation mechanism
pronounced attack by electrophilic centres such as O2- or O-. This
electrophilic attack in general leads to degradation of the aromatic ring for
hydrocarbon oxidation reactions [12]. As a result the nitrile selectivity and
thus nitrile yield is decreased. When weaker electron donating groups were
applied the selectivity was similar to that in toluene ammoxidation.
N itrile y ield [% ]
Similar effects were
reported by Lücke
75
and Martin [13]
50
over
vanadium
25
phosphate (VPO)
0
catalysts.
The
2 -C l3 -C l4 -C ldifferences in nitrile
to luen e
to luen e
to luen e
yield,
however,
Figure 2.1: Substituent effect in ammoxidation
varied to a greater
over VPO catalysts. Data from Lücke et al. [13].
extend and seem to
indicate an ionic mechanism rather than homolytic C-H rupture. These
authors found also significant differences for the different substituent
positions in the aromatic ring. Figure 2.1 shows the effect of the different
ring positions of the chloro group. As would be expected from a heterolytic
C-H rupture mechanism the nitrile yield differs significantly. The lowest
yield was obtained over m-chlorotoluene as expected from theory assuming
heterolytic rupture with formation of a carbocation. This carbocation is
stabilized only in the ortho- and para-position according to the resonance
effect. The higher nitrile yield for the para-substituted toluene cannot be
explained by the resonance effect. Possibly steric effects account for the
higher nitrile yield.
100
2.3 Effect of the catalyst basicity on the ammoxidation of
alkylaromatics
It is found that the ammoxidation rate increases with decreasing C-H
dissociation energies, as shown by Suleimanov et al. [14] using V-Sb-O
catalysts. This observation is explained by heterolytic C-H rupture with the
formation of a proton and a carbanion. The rate of α-hydrogen exchange
correlates well with the alkylaromatic ammoxidation rate. This supports a
heterolytic C-H rupture mechanism, in which anion-like hydrocarbon
species are formed as shown in Figure 2.2.
15
Chapter 2
RC
δ−
H
A+ B -
δ+
RC
-
H
+
A+ B -
Figure 2.2: Heterolytic CH rupture [14].
The interaction of the catalyst surface and the hydrocarbon thus was seen
as an acid-base interaction. B- is the basic site, which can be formed by
surface oxygen ions (O2-) or nucleophilic forms of adsorbed nitrogen
species, such as NH2- or NH2-. The hydrocarbon interacts with the surface
as an acid [15]. The rate of isotope exchange of the hydrogen in the
hydrocarbon was taken as a first approximation to estimate the
hydrocarbon acidity. The data were measured in solution at low
temperature. Also, it must be noted that the rates of α-hydrogen exchange
do not correlate exactly with the ammoxidation rates reported in case the
methyl group is changed to ethyl or an isopropyl groups. Both substitutions
lead to a severe decrease of the α-hydrogen exchange rate, whereas the
ammoxidation rate is slightly higher for ethylbenzene and significantly
lower for i-propylbenzene. The order expected purely based on the rates of
α-hydrogen exchange is C6H5CH3 > C6H5CH2CH3 > C6H5CH(CH3)2. The
high reactivity of ethylbenzene, therefore, is surprising; bearing in mind the
fact that benzonitrile forms from ethylbenzene and from i-propylbenzene
via the formation of styrene, which is converted to benzonitrile in a
consecutive reaction step.
Assuming heterolytic C-H rupture, not only adjusting the substrate would
influence the ammoxidation rate, but also the acidity or basicity of the
catalyst. When a carbanion is formed as intermediate, stronger basic
centres of the catalyst would increase the ammoxidation rate. An
investigation of the influence of the ammonia partial pressure on the
ammoxidation of toluene showed that the increase in the ammonia
concentration led to an increase of the reaction rate over V-Sb-Bi-O
catalysts. It was shown that the number of basic sites was increased and the
number of acidic sites decreased at the same time [16]. Other work by the
same group showed that the introduction of small amounts of alkaline
metals or alkaline-earth metals to a V-Sb-Bi-O catalyst led to increase of the
toluene ammoxidation rate (referred to as rate of oxidative ammonolysis)
[17]. Doping of the catalyst by these compounds led to increase of the
basicity and decrease of the acidity as measured by adsorption of benzoic
16
Toluene ammoxidation mechanism
9
Basicity
[C6H5COOH/m2]
-1 5
2
R a t e [ ·1 0 -1 5 m o l t o l / (m 2 ·s )]
acid and butylamine. At low dopant amounts the increase of the reaction
rate is linear with the increase of the basicity as shown in Figure 2.3.
Stronger increase of the oxygen nucleophilicity would lead to lower
activity, since surface dehydroxylation then becomes too slow [18].
8
7
Acidity
[C4H9NH2/m2]
6
2
2.5
3
3.5
4
2
A c id ity / b a s ic ity [m o l a d s / m ]
Figure 2.3: Relation between acidity and basicity of the catalyst and the
toluene ammoxidation rate. Data from Guseinov et al. [16].
Addition of K2O or BaO did not lead to observable changes in the oxygen
bond energy with the catalyst neither to changes in the oxidation state of
the catalyst. Only the concentration of basic sites of the catalyst increased
[19]. Therefore, heterolytic C-H bond breaking is probable over alumina
supported V-Sb-Bi-O catalysts. A negatively charged benzyl intermediate is
proposed to form after this heterolytic α-hydrogen abstraction [20]. The
first step in the toluene ammoxidation thus writes as shown in Scheme 2.2.
..
HN2-
H-CH2
O2-Mn+ Mn+
H2N-
-CH
2
O2-Mn+ Mn+
H2N•
•CH2
O2-M(n-1)+ M(n-1)+
Scheme 2.2: Toluene ammoxidation over V-Sb-O catalysts according to the
group of Rizayev [20].
By performing pulse experiments Niwa et al. [21] also found evidence for
heterolytic C-H rupture. They observed nitrile production when toluene
was pulsed after ammonia treatment of the V2O5/Al2O3 catalyst, or when
they pulsed sequentially ammonia and toluene. When toluene was
admitted to the catalyst prior to the ammonia pulse no nitrile production
was observed. These authors, however, supported a mechanism in which
17
Chapter 2
benzoate ions were formed, rather than amine-like intermediates.
Ammonium ions were formed upon interaction of the hydrocarbon with
the catalyst.
2.4
Nature of aromatic reaction intermediate
The nature of the intermediate during toluene ammoxidation is under
debate since the first experiments were described in literature. Many studies
have been described to elucidate in more detail the nature of the
alkylaromatic intermediates formed, mostly based on infrared
spectroscopy. Due to the presence of many components that are infrared
active, spectral bands greatly overlap. The interpretation of the infrared
spectra under ammoxidation conditions, therefore, is very difficult. In
particular, the presence of water, one of the reaction products, complicates
data interpretation. The fact that species have been detected by IR does not
mean a priori that these species are involved in the mechanism.
Nevertheless, many attempts were made to describe the nature of the
intermediate based at least partly on infrared spectroscopy.
Table 2.3: IR experiments by Azimov et al. [20]
Species
Band
[cm-1]
Assignment
Intensity behaviour
upon thermal treatment.
Adsorbed benzyl
fragments
Benzylimine
1430
δ(CH2)
Increases at 20-100 ° C
1660
ν(C=N)
Coordinated
benzonitrile
H-bound benzonitrile
Benzamide ion
Benzamide ion
2280
ν(C≡N)
2240
1430
1565
ν(C≡N)
νs(CON)
νas(CON)
Increases at 150-175 ° C
Decreases at T>200 ° C
Increases at 150-250 ° C
Decreases at T>300 ° C
Only at T>250 ° C
Increases at 150-200 ° C
Benzoate ion
Benzoate ion
1420
1540
νs(COO)
νas(COO)
Shift towards νas(COO) at
T>250 ° C
Increase at T>250
Spectroscopic evidence for the heterolytic C-H rupture mechanism
supported by the group of Rizayev was found by Azimov et al. [22]. They
detected benzyl-species when V-Sb-Bi-O was treated with an ammoniaoxygen mixture after toluene adsorption. Treatment in either oxygen or
18
Toluene ammoxidation mechanism
ammonia alone did not lead to the presence of these benzyl species,
indicating that the degree of surface reduction determines the presence of
benzyl species. Table 2.3 summarises the results of the IR experiments
related to the ammoxidation of toluene over alumina supported V-Sb-Bi-O,
as reported by the group of Rizayev [20,22]. Toluene was adsorbed and
heated under NH3/O2 to 400 ° C. The increase of imine (150-175 ° C) and
amine (150 ° C and higher) structures at increasing temperature may
indicate an amine pathway. However, the intensity of the benzoate bands
also increased above 250 ° C. This was explained by the increase of total
combustion reactions at higher temperature.
The formation of benzoate species was also reported by Haber and
Wojciechowska [23]. They performed toluene ammoxidation over MgF2
supported vanadia catalysts. After adsorption of toluene at 400 ° C they
observed IR bands at 1410 cm-1 and 1550 cm-1, assigned to benzoate-like
species. These intermediates were only found over a freshly calcined
catalyst. When the catalyst was treated in NH3 no bands that could be
assigned to benzoate species were observed. These benzoate intermediates
were believed to react very fast with ammonia. No distinction was made
between a benzoate ion and a benzoate complex based on the experiments.
The benzoate species that were proposed have been sketched in Figure 2.4.
O
C
V
C
O
Benzoate ion
O
O
V O V
Benzoate complex
Figure 2.4: Benzoate intermediates as proposed in [23].
Niwa et al. found the formation of surface benzoate ions when they
adsorbed xylenes [21] or toluene [24] on vanadia by IR (bands at 1550 cm-1
and 1436 cm-1). After adsorbing p-xylene and p-tolualdehyde they found
very similar IR bands. The assignment of the bands was confirmed by
adsorption of deuturated p-xylene. A decrease in the wavenumbers by ca.
20 cm-1 was observed. When they contacted the catalyst with ammonia the
19
Chapter 2
intensity of these bands decreased and benzonitrile was formed. They
found similar results when adsorbing substituted toluene on this catalyst or
when using titania-supported vanadia as catalyst. The occurrence of an
oxygen containing intermediate is reported most frequently in the
literature. Besides benzoate ions the same group detected more stable
intermediates containing carbonyl groups [25]. The carbonaceous
intermediate was supposed to be stabilised on the support material as
benzoate (from toluene adsorption [26]) or methylbenzoate (from xylene
[21]) species. The greater stability of the carbonyl-like structures, however,
does not suggest transformation of these carbonyl groups to benzoate
intermediates.
The occurrence of aldehyde-like intermediates, on the other hand, is
consistent with the so-called aldehyde mechanism. This mechanism can be
generalised to the following reaction steps:
O2 + 2 *
2 Oads
NH3 + Oads
C6H5CH3 + *
NHads
+
H2O
C6H5CH3,ads
C6H5CH3,ads + 2 Oads
C6H5CHOads + H2O
C6H5CHOads + NHads
C6H5CN + H2O
Scheme 2.3: Aldehyde pathway for toluene ammoxidation.
The exact form of the nitrogen containing intermediate that is formed upon
ammonia adsorption will be discussed in Section 3 and the role of gaseous
oxygen in Section 4.
Evidence for benzoate and benzaldehyde-like reaction intermediates was
also reported by Busca et al. [9]. Using titania-supported vanadia catalysts
they found initial adsorption of toluene at room temperature as benzyl
species, as shown by IR. Ring vibrations were observed at 1604 cm-1 and
1494 cm-1. Upon heat treatment the aromatic ring was kept intact, but the
methyl group transformed into a carboxylate ion as evidenced by the
occurrence of strong absorption bands in the 1600-1500 cm-1 region and in
the 1450-1400 cm-1 region. A band present at 3070 cm-1 (νCH) and two
sharp bands near 1600 cm-1 were used to identify the adsorbed toluene
species as benzoate ion [27]. When ammonia was present the intensity of
20
Toluene ammoxidation mechanism
this band strongly decreased, whereas the band was strong during
toluene/oxygen adsorption. This indicates that the benzoate-species is
involved only in the oxidation of toluene. The spectra obtained were very
similar to those reported by Van Hengstum et al. [28] who studied toluene
oxidation over V-Ti-O. It was proposed that this benzoate ion was involved
in oxidation of the methyl group or even in complete oxidation of the
aromatic ring, since no nitrogen insertion species was present in this
experiment. When adsorbed toluene was heated in oxygen containing
conditions benzaldehyde was formed upon heating. This was evidenced by
adsorption of benzaldehyde itself, which gave a very similar IR spectrum.
Though the mechanism of toluene ammoxidation is often assumed to be
very similar to that of selective toluene oxidation these experiments did not
prove that benzaldehyde-like intermediates are really involved in the
ammoxidation reaction as intermediate. In fact, when coadsorption of
toluene and ammonia was performed no clear evidence could be found for
the formation of a benzaldehyde-like intermediate. The authors [9] assign
the obtained IR spectrum to adsorbed benzylamine, but the spectrum is
very complicated. On the other hand when benzaldehyde was adsorbed on
an ammonia covered surface the bands assigned to coordinated ammonia
(1610 cm-1 and 1230 cm-1) decrease with a corresponding increase of a band
at 2270 cm-1, which is assigned to coordinated benzonitrile, showing that
benzaldehyde can be converted by ammonia to benzonitrile.
Flow reactor studies by the same authors [5,10] show that using
benzaldehyde as feedstock under ammoxidation conditions leads to
formation of benzonitrile in high yields, as shown in Table 2.4. Formation
of benzonitrile from the reaction of benzaldehyde with ammonia and
oxygen was also found in an IR study by Murakami et al [26].
Table 2.4: Ammoxidation reaction of possible reaction intermediates
Substrate
Toluene
Benzaldehyde
Benzoic acid
Benzylamine
Substrate conversion
[mol%]
Benzonitrile selectivity
[mol%]
61
100
100
100
85
95
54
70
V-Ti-O catalyst; T= 310 ° C [10].
21
Chapter 2
The data reported in Table 2.4 suggest the feasibility of benzylamine and
benzaldehyde as possible reaction intermediates. The fact that benzoate
intermediates were detected by IR under reaction conditions probably
relates to proceeding of unselective total oxidation reactions. The selectivity
to benzonitrile from benzoic acid feedstock is significantly lower than the
selectivity to benzonitrile when feeding toluene. Benzaldehyde and
benzylamine were detected under toluene ammoxidation conditions at low
residence times and low partial pressures of ammonia (0.025 atm) and
oxygen (0.003 atm). Benzylamine intermediates were not found at low
residence times by Otimari et al. [29], who used similar catalysts. These
authors applied higher oxygen partial pressures in their experiments. They
found the presence of benzaldehyde in the reaction mixture under these
conditions. Benzaldehyde reaction intermediates are observed in toluene
ammoxidation reactions by several other authors, using different catalysts.
Over SAPO and VAPO catalysts Kulkarni et al. [30] detected
benzaldehyde as reaction product in low yields. They did not find the
presence of benzylamine or benzoic acid. Over Cu/ZSM-5 Kim et al. [31]
found the formation of benzonitrile from benzaldehyde in high yields. In
the absence of ammonia benzene was formed, in similar amounts as during
toluene oxidation. These authors propose a mechanism in which a
benzaldehyde-like cation acts as the selective intermediate towards
benzonitrile as well as the intermediate for benzene production. This
benzaldehyde-like cation is formed from a benzyl-cation, which is formed
upon toluene adsorption. The authors, however, did not take into account
the formation of combustion products, which were produced in significant
amounts. Moreover, the presence of small amounts of benzaldehyde in the
reaction mixture could possibly be caused by the occurrence of mild
toluene oxidation to benzaldehyde.
More extensively Martin et al. [32] studied the ammoxidation of toluene
over VPO catalysts by FT-IR and TAP experiments. By TAP they
measured the transient responses of benzaldehyde and benzonitrile. They
demonstrated that benzaldehyde evolved as first reaction product. At
longer contact times benzonitrile was found as the main product. It was
found that similar IR spectra were obtained when feeding toluene and
ammonia as feedstock as when feeding benzaldehyde and ammonia as
feedstock. Therefore, it can be concluded that benzaldehyde leads to the
formation of benzonitrile over VPO catalysts. It was found that the amount
22
Toluene ammoxidation mechanism
of activated ammonia controlled the formation of benzonitrile. If the
amount of activated ammonia was low high benzaldehyde selectivities
were found, whereas higher amounts of activated ammonia led to higher
yields of benzonitrile [33].
Besides benzylamine and benzaldehyde–like structures additionally also
benzylimine was reported as reaction intermediate in the ammoxidation of
toluene. The group of Rizayev found benzylimine-like species by
spectroscopic investigations [22,34] in the ammoxidation of toluene over VSb-Bi-O catalysts. Amine-like species were not detected, because of their
high reactivity. Benzylimine, however, is much more reactive under
ammoxidation conditions than benzyl amine is. Formation of imines could
occur, but is probably not involved as rate determining step during toluene
ammoxidation. Benzylimine intermediates were also reported by Den
Ridder [35] who reports the formation of benzylimine from benzaldehyde
intermediates. It was found that a homogeneous reaction of benzaldehyde
to produce benzylimine could occur at temperatures below 125 ° C. It
should be noted though, that the ammoxidation reaction temperature is
significantly higher.
3.
Ammonia activation
Ammonia plays multiple roles in the ammoxidation of toluene. It is the
source of nitrogen atoms, it also reduces the catalyst and/or it is adsorbed
on the catalyst blocking sites that otherwise could have weakly bound
oxygen, which could lead to total oxidation.
Guseinov et al. [36] found in the absence of ammonia mainly the
occurrence of total oxidation reactions over V-Sb-O catalysts. Admission of
ammonia to the toluene/oxygen mixture led to the production of
benzonitrile (selectivity of 95%). Additionally, the total conversion of
toluene was increased significantly. This was explained by the formation of
new basic sites upon ammonia adsorption [20,37]. They found by infrared
spectroscopy that Lewis acid sites were blocked by ammonia and partially
dehydrogenated ammonia species such as NH and NH2 were formed
[20,22 ,38]. Contrary, Busca et al. [9] and Niwa et al. [21,24] did not detect
partially dehydrogenated ammonia species on titania-supported or
alumina-supported vanadia catalysts respectively. Niwa et al. [21]
explained ammoxidation of toluene by the formation of NH4+ species
23
Chapter 2
interacting with toluene along the methyl group. They found indirect proof
for the formation of NH4+ by the decrease of the 3500 cm-1 OH band upon
ammonia adsorption.
Recently Centi and Perathoner [4] reviewed the role of ammonia adspecies
in several reactions, among which was the toluene ammoxidation reaction.
They described four main ways in which ammonia can be bound to the
metal oxide surface, based on IR experiments [39]:
1. Bonding via a hydrogen atom to a surface oxygen or the oxygen of a
surface OH-group
2. Bonding via the nitrogen atom to a hydrogen atom from a surface OH
group (Brønsted acid site), to form an ammonium ion
3. Coordination to an electron deficient metal atom (Lewis acid site)
4. Dissociative chemisorption of the ammonia atom to form a NH2 or NH
species and a (or two) OH group(s) with a surface oxygen atom
Centi et al. [40] studied the formation of NH4+-ions (“Type 2 species”) vs.
the NH3 coordinated to Lewis acid sites by IR, using a V-Sb-O catalyst.
They found a decrease of both species as function of temperature. At
temperatures from T= 200 ° C and higher no NH4+-ions were present at the
surface, whereas NH3 coordinated to the Lewis site was still present up to
temperatures of 400 ° C. This study was executed in vacuo, thus excluding
the influence of gas-phase ammonia, water and other compounds, which
are present under normal ammoxidation reaction conditions. They also
showed that the presence of other substances could influence the nature of
ammonia adspecies. The involvement of NH4+-ions in the ammoxidation
of toluene thus cannot be excluded from the reaction mechanism a priori.
For example at room temperature ammonia adsorbs mainly as NH4+-ions
on VPO, as shown by Busca et al. [41]. Upon heating they also observe
decrease of the relative amount “Type 2 species” and increase of the
relative amount of “Type 3 species”. Water presence, on the other hand,
leads to the transformation of NH3 coordinated to the Lewis acid sites to
NH4+.
The formation of metal-imido (e.g.. Mo=NH) and metal-amino (e.g. SbNH-Sb) species was discussed extensively for the (amm)oxidation of
propylene over Bi-Mo and Fe-Sb catalysts by the group of Grasselli [42,43].
These metal-imido and metal-amino species have been accepted quite
24
Toluene ammoxidation mechanism
generally as the only surface species responsible for ammonia insertion into
olefins. Also for the ammoxidation of toluene dehydrogenated ammonia
species were reported in the literature. For example Andersson et al.
[44,45] indicated the formation of V=NH and Cu=NH species, using
vanadium-oxide and barium-cuprate catalysts for the ammoxidation of
toluene. Spectroscopic evidence for the presence of these metal-imido and
metal-amino species to be the reactive ammonia species under reaction
conditions, however, is scarce. Therefore, attention should be given to the
nature of the ammonia adspecies under reaction conditions and the
participation of other forms of activated ammonia should not be excluded.
For example the presence of NH4+-ions influences the catalyst acid-base
properties. Since no clear evidence is available to answer the question
whether homolytic or heterolytic C-H rupture occurs this can play a main
role in the ammoxidation of toluene. The chemisorption of ammonia can
lead directly to modification of the Brønsted as well as the Lewis acidity of
the catalyst. Also the nucleophilic character of the oxygen surface sites is
increased by the NH3 adsorption properties. Indirectly, also other effects
can play a role. For example the reduction of acidity by ammonia
adsorption can lead to easier reduction of products or intermediates with a
basic character such as nitriles (or alkenes). Too strong Lewis acid sites,
additionally, can lead to total oxidation reactions. Toluene ammoxidation
reactions showed an optimum in ammonia concentration with respect to
the benzonitrile selectivity [5,45], indicating the effect of change of acidbase properties over vanadia catalysts. Similar relations were found for the
ammoxidation of propane to acrylonitrile. Centi and Perathoner [46]
describe a mechanism for this reaction over VPO pre-adsorbed with
ammonia in the presence of oxygen. At high NH3 coverage the propylene
yield was high and the acrylonitrile yield moderate. No combustion
products were produced. At low NH3 coverages hardly any other product
than COx was produced. NH3 coverages in between led to optimum
acrylonitrile yield. It was concluded that propane is converted via two
pathways. Propylene is formed by oxidative dehydrogenation and is
strongly co-ordinated to the Lewis acid sites. In the absence of oxygen a
propylene amine intermediate is formed. This intermediate can react with
amido-like (NH2-), which is formed by dissociatively adsorbed ammonia
and slowly forms acrylonitrile. In the presence of oxygen an acrylate
intermediate is formed as sketched in Figure 2.5. This intermediate reacts
25
Chapter 2
H
C
C
C
H
NH4+
H
O
V
O O
O
V
O
Figure 2.5: Active site
during propane
ammoxidation over VPO
P
faster with a nearby NH4+-ion to give
water and acrylonitrile as reaction
products. In the case ammonia occupies
all sites chemisorption of oxygen is
inhibited and the acrylate intermediate
cannot be formed. This is the situation at
very high ammonia coverages. Where no
nearby NH4+-sites are present, the
acrylate intermediate reacts to carbon
oxides [46].
Niwa et al. [24] find evidence for the presence of ammonium cations under
toluene ammoxidation conditions. They observe by IR the consumption of
OH bands after admission of ammonia. Similarly, they observe the
formation of benzonitrile. The OH bands were believed to originate from
the support. Ammonium cations were also reported by other groups [47]
when ammonia was adsorbed on titania supported vanadia monolayer
catalysts. When co-adsorption of toluene and ammonia was performed
Busca at el. [9] indeed detected the formation of ammonium cations by
infrared spectroscopy. Toluene adsorbed similarly as on clean vanadia
surfaces. The concentration of the ammonium cations was found to
decrease slightly when co-adsorption of toluene and ammonia was
performed at higher temperature (470 K). The intensity of the band
assigned to chemisorbed ammonia (1230 cm-1) was strongly reduced after
toluene adsorption at 570 K. New bands were observed at 3390 (a
shoulder), 3260 and 1642 cm-1. These bands were assigned to stretching and
deformation of NH2 groups. The authors conclude that ammonia
coordinated to Lewis acid sites (IR bands at 1230 and 1610 cm-1) is
involved in the formation of benzonitrile. As will be discussed in Section
5.2 the authors [9] support a mechanism in which adsorbed benzyl radicals
are stabilized as benzylamine intermediates, which react with ammonia
coordinated to Lewis acid sites (see Scheme 2.8).
Besides selective insertion of N atoms into the hydrocarbon to produce
benzonitrile also ammonia combustion occurs. Since the ammonia partial
pressure was found to have an important effect on the production of
benzonitrile it is important to avoid ammonia combustion as much as
26
Toluene ammoxidation mechanism
possible. Based on a kinetic evaluation Cavalli et al. [10] describe the role
of ammonia in terms of (1) increase of the benzonitrile selectivity at
increasing ammonia concentration and (2) decrease of the activity at
increasing ammonia concentration. The increase of selectivity was found to
correlate well with the amount of ammonia available in the reaction
mixture. Part of the ammonia was combusted to N2 and N2O. The decrease
of the activity was explained by a competitive adsorption effect: Ammonia
competes with toluene for the same active sites. Too high ammonia
concentrations, therefore, lead to depletion of the active sites. The active
site for toluene ammoxidation was found to be V(IV). The concentration of
V(IV) correlates linearly with the activity in toluene ammoxidation [48].
V(V) sites on the other hand catalyse the combustion of ammonia. A third
role of ammonia is to stabilise the intermediate of toluene activation. This
explains that V/TiO2 catalysts are unselective in toluene oxidation [49].
4.
Catalyst reoxidation
With respect to the oxygen inserting species in the alkylaromatic
(amm)oxidation mechanism a Mars–Van Krevelen mechanism [50] is
generally accepted. The hydrocarbon is oxygenated by lattice oxygen. In
the presence of ammonia an aromatic nitrile is produced from the
oxygenated intermediate as described above in Section 2.4. In a separate
reaction step the catalyst is reoxidized by gaseous oxygen. The mechanism
is drawn schematically in Figure 2.6.
CH3
O
1/
2
O2
[Intermediate]
NH3
-3 H2O
CN
Figure 2.6: Consumption of lattice oxygen during toluene ammoxidation
27
Chapter 2
Several groups have provided experimental evidence for the occurrence of a
Mars–Van Krevelen mechanism in alkylaromate ammoxidation. Pulse
experiments by Murakami et al. [51] indicate the consumption of surface
oxygen when producing benzonitrile over vanadia and over alumina
supported vanadia catalysts. They pulsed toluene–ammonia–toluene–
ammonia etc. and measured the benzonitrile production. Benzonitrile is
formed after pulsing ammonia, but the formation of benzonitrile strongly
decreases after a series of pulses. Admission of air and toluene to the
catalyst can reactivate the alumina supported vanadia catalyst. Toluene
thus forms an oxygen containing intermediate that reacts with gaseous
ammonia to form benzonitrile. The intermediate formed is stable on
alumina-supported vanadia, but reacts on unsupported vanadia to carbon
oxides. Niwa et al. [52] ascribe the more facile reoxidation to faster oxygen
diffusion through the thin vanadia layer on alumina, compared to bulk
vanadia. Similar mechanisms were suggested for the ammoxidation of
toluene over SiO2-Al2O3, SiO2-TiO2 and ZrO-SiO2. During the course of
reaction the initially very low activity of the catalysts is increased. This is
explained by the formation of a reactive carbonaceous layer on the catalyst
[25].
Similar pulse experiments were performed by the group of Rizayev [53].
They also found benzonitrile production after the ammonia pulse during
the pulse sequence: toluene–ammonia. The surface is reoxidized by
gaseous oxygen, but this surface reoxidation is not rate-determining.
Haber et al. [23] used pulse experiments over V/MgF catalysts. They found
that toluene/ammonia pulses led to the production of benzonitrile. After a
number of sequential toluene/ammonia pulses, however, the activity of the
catalyst dropped. Reoxidation by gaseous oxygen led to regain of the
activity. Also in flow experiments evidence for the occurrence of a Mars–
Van Krevelen mechanism was found. Benzonitrile was produced over V/Bi
catalysts when toluene and ammonia was fed to the catalyst. The
benzonitrile yield dropped as a function of time on stream, but could be
returned to its initial value after reoxidation of the catalyst in gaseous
oxygen [54].
The importance of oxygen in the feedstock, however, is not only limited to
reoxidation -and thus regeneration- of the active sites. Martin et al. [55]
28
Toluene ammoxidation mechanism
showed that for VPO catalysts the vanadium oxidation state has a very
delicate role. Using TAP they performed sequential pulse reactions of a
typical ammoxidation mixture. In between the ammoxidation feedstock
pulses the catalyst oxidation state was adjusted by either pulsing oxygen or
ammonia. CO2 was formed predominantly over oxidized sites. The total
activity of the catalyst was found to correlate with the oxidation state as
well. As a result optimum benzonitrile yield is obtained at intermediate
oxidation state. Indeed very small differences in the bulk oxidation state
were detected by titration. Therefore the concentration of oxygen in the
feedstock could have a significant influence on the ammoxidation reaction.
The optimum in nitrile production at intermediate V oxidation state is very
similar to the optimum in acrylonitrile yield from propane ammoxidation
found by Centi and Perathoner [46]. As already discussed in Section 3 the
oxidation state was also influenced by the surface ammonia concentration.
5.
Toluene ammoxidation reaction schemes
5.1
The propylene ammoxidation mechanism
Many reaction schemes for the ammoxidation of toluene proposed in
literature are based upon the ammoxidation of propylene. This process has
been patented already in the 1950s by SOHIO [56] and Distillers [57]. The
process is applied industrially on a large scale. Nowadays, over 5.000.000
tons of acrylonitrile is produced via the SOHIO Process [58]. Because of
its industrial importance [59] this reaction has been studied in great detail.
General agreement in literature exists about the mechanism. The early
stages of research were summarized in 1970 [60], but a more complete
review was published in 1981 [42]. The reactions of propylene oxidation to
acrolein and propylene ammoxidation to acrylonitrile occur simultaneously
over bismuth molybdate catalysts. Most experimental evidence for the
mechanism, however, is deduced from propylene oxidation reactions.
Oxygen insertion occurs via a Mars-Van Krevelen-type mechanism as
shown by Keulks [61] by 18O labeling experiments. The first step of the
reaction is the abstraction of a α-hydrogen in order to form an allylic
intermediate, as was shown by deuterium and 14C-labeling experiments
[62,63]. This intermediate then undergoes abstraction of a second hydrogen
atom and insertion of oxygen or nitrogen (while abstraction of a third
hydrogen takes place) to form acrolein resp. acrylonitrile. α-Hydrogen
29
Chapter 2
abstraction is the rate determining step of the reaction, as was shown by
measurement of the kinetic isotope effect after deuterium labeling [63,64].
MoO3
O
O
+ NH3
- H2O
HN
O
Mo
Mo
O
O
Oxidation
Ammoxidation
H
H
O
O
HN
O
Mo
Mo
O
O
O
N
+ O Mo OH
H
OH
Mo
OH
[O]
H
N
O
Mo
O OH
CN + MoO2 + H2O
Scheme 2.4: O-allyl and N-allyl surface species in the (amm)oxidation of
propylene (from [66]).
30
Toluene ammoxidation mechanism
Reoxidation of the catalyst lattice occurs at a higher rate [65]. By directly
reacting the allylic intermediates, which were formed by reacting
azopropene, Burrington and Grasselli [66] showed the formation of a πallyl surface complex, which is quickly converted to a σ-O-allyl species
(allyl molybdate ester) upon interaction with propylene. Davydov et al. [67]
found infrared evidence for the presence of σ-allyl and π-allyl complexes in
the oxidation of propylene over copper, chromium and molybdenum based
catalysts. Allyl radicals are not involved in the reaction mechanism as
selective intermediates, as shown by quantum chemical calculations [68].
Formation of a σ-O-allyl complex via a π-allyl complex seems to be more
probable in the oxidation of propylene. In a later publication, Burrington et
al. [69] described a more complete reaction scheme for the oxidation of
propylene. By substituting the Mo=O bond by a Mo=NH bond nitrile
formation was explained. As a result σ-N-allyl is formed instead of a σ-Oallyl intermediate. This reaction scheme is shown in Scheme 2.4.
O
O
M1 M2
NH3
Active site
H
O
H
2 [O]2[]
[]
M1 M2
2[]
NH
M1 M2
H2O
[O]2-
3M[]
Reoxidation site
[]
Reduced site
O
NH
M1 M2
Ammoxidation site
1½ O2
H2C
C
H
CN
+ 2 H2O
H2C C CH3
H
CH
H3C
CH3
HO
NH
M1 M2
Allylic surface complex
Scheme 2.5: Generalized alkene ammoxidation reaction scheme [58].
In a later review the propylene ammoxidation reaction scheme was
generalized. Bimetallic (or multi-component) catalysts show much higher
31
Chapter 2
acrylonitrile yields. Therefore, Scheme 2.4 is too simple. A generalized
propylene ammoxidation scheme for bimetallic metal oxides is described
by Grasselli [58] and is shown in Scheme 2.5. This reaction scheme can
also be applied to monometallic metal oxides as shown for antimonate
catalysts [43].
5.2
The ammoxidation of toluene
For the ammoxidation of toluene over molybdenum oxide catalysts
Scheme 2.4 rewrites to Scheme 2.6 [70].
CH2
CH2
O
O
MoO3
Mo
O
CH3
O
NH3
Mo
O
O
NH
O
O
+
O
H
H2
C
s-complex
HN•
C
O
N
O
O
Mo
O
H2O
O
••
Mo
O
OH
OH
H
C
O
N
[O]
Mo
O
O
OH
CN + MoO2 + H2O
Scheme 2.6: Toluene ammoxidation over supported MoO3 catalysts [70].
Sanati and co-workers [71] describe a reaction mechanism that was based
on DRIFTS experiments on V-Ti-O at temperatures not higher than 300
° C. They propose a reaction mechanism that involves the presence of all
hydrocarbon intermediates discussed before. Toluene adsorbs and reacts
with adsorbed or lattice oxygen to form a benzyl fragment and a surface
OH species. Further reaction of the benzyl fragments with an oxygen atom
32
Toluene ammoxidation mechanism
leads to formation of a C6H5-CH2O species, which in turn reacts with the
OH species to form water and a benzaldehyde like species.
This benzaldehyde-like species can desorb or can react with an oxygen
atom to form adsorbed benzoic acid that can also desorb. If the
benzaldehyde-like fragment reacts with two oxygen atoms an adsorbed
benzoate species and a hydroxyl group is formed. Ammonia reacts as an
adsorbed amine (NH2) species, which is formed by reaction with oxygen,
again producing a hydroxyl group. Reaction of the adsorbed benzaldehyde
with the adsorbed NH2 group forms a C6H5-CH(NH2)-O species, which is
converted to an adsorbed imine (again with the formation of a hydroxyl
group). This adsorbed imine reacts with oxygen to benzonitrile and water.
The benzylimine species, however, was not observed in the IR spectra. The
other pathway discussed is a reaction of the benzoate intermediate with the
NH2 group that was formed by ammonia adsorption to form benzonitrile as
sketched in Scheme 2.7.
O
O
δ+ C
[-NH2] +
(NH2)C
O
O
O
ads
HN
+ [-OH]
C
(NH2)C
ads
-O
O
ads
ads
HN
C
+ [-OH]
NC
ads
-O
ads
ads
Scheme 2.7: Benzonitrile formation from benzoate-like intermediate
according to Sanati et al. [71].
As already discussed in Section 2.4 benzaldehyde intermediates are not the
only reaction intermediates observed the in literature. Cavalli et al. [5]
propose another mechanism, which involves the presence of an amine like
intermediate. This so-called amine mechanism, or dehydrogenation
mechanism involves the presence of amine-like intermediates, which were
believed to be the species responsible for benzonitrile formation. This
33
Chapter 2
mechanism consists of stepwise dehydrogenation of the substrate forming
amine- and imine-like intermediates. A generalized form of the mechanism
was discussed, as show in Scheme 2.8.
CH3
.
CH2
4
.
CH
.
1
COOH
CHO
2
CH2NH2
CN
3
CH NH
Scheme 2.8: Reaction mechanism according to Cavalli et al. [10]
As discussed earlier these authors neglected the reaction pathway via
benzoic acid. Based on the high benzonitrile yields when the amine or
aldehyde were fed they proposed that both pathway 3 and pathway 4 are
possible over titania-supported vanadia. In both pathways the highly
reactive radical is attacked by adsorbed ammonia or oxygen. By infrared
spectroscopy it was shown that the intensity of the bands assigned to
ammonia coordinated to Lewis acid sites decreased and coordinated
benzonitrile was formed upon benzaldehyde adsorption on an ammoniacovered V-Ti-O catalyst. This was also observed upon toluene adsorption
on an ammonia-covered catalyst. Pathway 3, therefore, seems plausible.
On Reaction pathway 2 seems to be less probable, since carbene and imine
bi-radicals were not detected in any spectroscopic investigations. The
mechanism shows similarities with the reaction mechanism for propylene
ammoxidation over Bi-Mo-O catalysts, as was described by Grasselli et al.
[72]. Total combustion products can be formed from all intermediates
proposed, but not from benzonitrile [73]. Ammonia stabilizes the reaction
intermediate. Combustion of ammonia to nitrogen can occur over different
sites. This separate reaction leads to lower benzonitrile yields.
A mechanism similar to pathway 3 was proposed by the group of Martin
and Lücke [2] for the ammoxidation of toluene to benzonitrile over VPO
catalysts. They detected the presence of benzaldehyde as short-living
reaction intermediate and found also IR evidence for the presence of
benzoate surface species. Benzyl amine was not detected as intermediate.
This species was believed to react rapidly to benzoate and ammonium by
hydrolysis. The reaction mechanism is shown in Scheme 2.9. Toluene is
believed to adsorb as benzaldehyde-like intermediate on the catalyst. This
benzaldehyde-like species could be converted to a benzoate-like species by
the nearby oxygen atoms attached to the vanadium centres. This reaction,
34
Toluene ammoxidation mechanism
however, was not included in the mechanism according to Martin et al. [2].
The benzaldehyde intermediate reacts to an benzyl-imine intermediate with
desorption of water. Similar to the mechanism for propane ammoxidation
over the same catalyst the first step is oxygen addition to an organic
intermediate, which is attacked by adsorbed ammonium anions in a
subsequent reaction step.
NH4
O P
P
O
O
O
V
V
O
O
O
P
P
P O
P
O
O O
O
O
P
V
V
H2N
O
O
O
NH3
NH2
P
+O2
O
O
O
H4N
V
O
O
V
O
O
O
O
H4N
V
O
O
V
CH3
O
O
H2C
H
O
O
O
O
V
V
H4N
O
O
O
O
CN
+O2/NH3
-H2O
NH
O
HO
V
O
O
V
O
O
O
-H2O
H
H O
O
O
O
V
V
H4N
O
O
O
+O2
-H2O
Scheme 2.9: Reaction mechanism of toluene ammoxidation over VPO
catalysts according to Lücke et al. [2].
6.
Conclusions
Up to now, there is no general agreement in literature considering the exact
reaction mechanism for the ammoxidation of toluene. Many authors,
though, agree on the presence of an oxygenated adsorbed organic
intermediate as a first reaction step. Oxygen is supplied from the catalyst
35
Chapter 2
surface, implying a redox sequence according the Mars and Van Krevelen
mechanism. The benzaldehyde- or benzoate-like oxygenated species then
reacts with a nitrogen-insertion site to produce benzonitrile. The exact
nature of the nitrogen-insertion site is presently not known precisely.
Because of the lack of exact information about the reaction mechanism and
about the rate limiting steps, it is presently not known what defines the
optimal catalyst properties. The fact that most active catalysts consist of
two- or multi-compound systems suggests that hydrocarbon activation and
nitrogen insertion occur on different catalytic sites.
References
R.G. Rizayev, E.A. Mamedov, V.P. Vislovskii, V.E. Sheinin, Appl.
Catal. A, 83, (1992), 103-140.
2. A. Martin, H. Berndt, B. Lücke, M. Meisel, Topics in Catal., 3, (1996),
377-386.
3. A. Martin, B. Lücke, G.-U. Wolf, M. Meisel, Chem.-Ing.-Tech., 66
(1994), 948-949.
A. Martin, Y. Zhang, H.W. Zanthoff, M. Meisel, M. Baerns, Appl.
Catal. A., 139, (1996), L11-L16.
A. Brückner, A. Martin, N. Steinfeldt, G-U. Wolf, B. Lücke, J. Chem.
Soc., Faraday Trans., 92, (1996), 4257-4263.
H. Berndt, Y. Zhang, A. Martin, K. Büker, S. Rabe, M. Meisel, Catal.
Today, 32, (1996), 285-290.
Y. Zhang, A. Martin, H. Berndt, B. Lücke, M. Meisel J. Mol. Catal. A.,
118, (1997), 205-214.
A. Martin, Y. Zhang, M. Meisel, React. Kinet. Catal. Lett., 60, (1997),
3-8.
F.K. Hannour, A. Martin, B. Kubias, B. Lücke, E. Bordes, P. Courtine,
Catal. Today, 40, (1998), 263-272.
A. Martin, F.K. Hannour, A. Brückner, B. Lücke, React. Kinet. Catal.
Lett., 63, (1998), 245-251.
4. G. Centi, S. Perathoner, Catal. Rev. – Sci. Eng., 40, (1998), 175-208.
5. P. Cavalli, F. Cavani, I. Manenti, F. Trifirò, M. El-Sawi, Ind. Eng.
Chem. Res., 26, (1987), 804-810.
6. F. Cavani, F. Parrinello, F. Trifirò, J. Mol. Catal., 43, (1987), 117-125.
7. G.I. Golodets, Oxidation of Organic Substances by Heterogeneous
Catalysis, Naukova Dumka, Kiev, 1978.
8. R.T Morisson, R.N. Boyd, Organic Chemistry, 5th edition, Allyn and
Bacon Inc., Boston, 1987, pp. 499-526.
9. G. Busca, F. Cavani, F. Trifirò, J. Catal., 106, (1987), 471-482.
10. P. Cavalli, F. Cavani, I. Manenti, F. Trifirò, Catal. Today, 1, (1987),
245-255.
11. I.M. Chmyr, N.R. Bukeikhanov, B.V. Suverov, Izv. Akak. Nauk. Gruz.
SSR Ser. Khim., 1, (1983), 43. As referred to in [1].
12. J. Haber, in: Handbook of Heterogeneous Catalysis, Eds. G. Ertl, H.
Knözinger, J. Weitkamp, Vol. 5, VCH, Weinheim, 1997, pp. 2253-2274.
1.
36
Toluene ammoxidation mechanism
13. B. Lücke, A. Martin, in: Catalysis of Organic Reactions, Eds. M.G.
Scaros, M.L. Prunier, Marcel Dekker Inc., New York-Basel-Hong Kong,
1994, p. 479-482.
14. T.E. Suleimanov, V.P. Vislovskii, E.G. Guseinova, E.A. Mamedov,
R.G. Rizaev, Kinet. Catal., 30, (1989), 659-663.
15. O.A. Reutov, I.P. Beletskaya, K.P. Butin, in: CH-Acids, Ed. T.R.
Crompton, Pergamon, New York, 1978, 1-42.
16. A.B. Guseinov, E.A. Mamedov, R.G. Rizaev, React. Kinet. Catal. Lett.,
27, (1983), 371-374.
17. E.A. Mamedov, V.P. Vislovskii, R.G. Rizayev, Kinet. Catal., 27, (1986),
1201-1207.
A.B. Guseinov, Y.D. Pankratiev, E.A. Mamedov, R.G. Rizayev, Kinet.
Catal., 27, (1986), 785-788.
18. E.A. Mamedov, V.P. Vislovskii, R.M. Talyshinskii, R.G. Rizayev, in:
New Developments in Selective Oxidation by Heterogeneous Catalysis,
Eds. P. Ruiz, B. Delmon, Studies in Surface Science and Catalysis, Vol.
72, Elsevier Science Publishers B.V., Amsterdam, 1992, 379-386.
19. E.G. Guseinova, T.E. Suleimanov, A.V. Kalinkin, Y.D. Pankratiev,
V.P. Vislovskii, E.A. Mamedov, R.G. Rizaev, React. Kinet. Catal. Lett.,
43, (1991), 501-506.
20. A.B. Azimov, V.P. Vislovskii, E.A. Mademov, R.G. Rizayev, J. Catal.,
127, (1991), 354-365.
21. M. Niwa, H. Ando, Y. Murakami, J. Catal., 70, (1981), 1-13.
22. A.B. Azimov, A.A. Davydov, V.P. Vislovskii, E.A. Mamedov, R.G.
Rizayev, Kinet. Catal., 32, (1991), 96-104.
23. J. Haber, M. Wojciechowska, Catal. Lett., 10, (1991), 271-278
24. M. Niwa, H. Ando, Y. Murakami, J. Catal., 49, (1977), 92-96.
25. M. Niwa, M. Sago, H. Ando, Y. Murakami, J. Catal., 69, (1981), 69-76.
26. Y. Murakami, H. Ando, M. Niwa, J. Catal., 67, (1981), 472-474.
27. K. Machida, A. Kuwai, Y. Saito, T. Uno, Spectrochim. Acta A., 34,
(1978), 793-800.
28. A.J. van Hengstum, J. Pranger, S.M. van Hengstum-Nijhuis, J.G. van
Ommen, P.J. Gellings, J. Catal., 101, (1986), 323-330.
29. J.C. Otimari, A. Andersson, Catal. Today, 3, (1988), 211-222.
30. S.J. Kulkarni, R. Ramachandra Rao, M. Subrahmanyam, A.V. Rama
Rao, A. Sarkany, L. Guczi, Appl. Catal. A., 139, (1996), 59-74.
31. S.H. Kim, H. Chon, Appl. Catal., 85, (1992), 47-60.
32. A. Martin, H. Berndt, B. Lücke, M. Meisel, Topics in Catal., 3, (1996),
377-386.
33. A. Martin, F.K. Hannour, A. Brückner, B. Lücke, React. Kinet. Catal.
Lett., 63, (1998), 245-251.
34. A.B. Azimov, A.A. Davydov, V.P. Vislovskii, E.A. Mamedov, R.G.
Rizaev, Kinet. Catal., 32, (1991), 104-110.
35. J.J.J. den Ridder, Ammoxidation of Toluene and Xylenes to
Nitriles, PhD Thesis, Delft University Press, 1981.
36. A.B. Guseinov, E.A. Mamedov, R.G. Rizayev, React. Kinet. Catal.
Lett., 27, (1985), 371-374.
37. R.G. Rizayev, E.A. Mamedov, A.B. Guseinov, F.M. Agayev, Kinet.
Catal. 27, (1986), 536-541
37
Chapter 2
38. A.B. Azimov, V.P. Vislovskii, E.A. Mamedov, R.G. Rizayev, Kinet.
Catal., 30, (1989), 983-988.
39. A.A. Davydov, Infrared Spectroscopy of Adsorbed Species on the
Surface of Transition Metal Surfaces, John Wiley & Sons, New York,
1990, pp. 27-37.
M.C. Kung, H.H. Kung, Catal. Rev. – Sci. Eng., 27, (1985), 425-460.
A.A. Tsyganenko, D.V. Pozdnyakov, V.N. Filimonov, J. Molec. Struct.,
29, (1975), 299-318.
40. G. Centi, F. Marchi, S. Perathoner, J. Chem. Soc., Faraday. Trans., 92,
(1996), 5151-5159.
41. G. Busca, G. Centi, F. Trifirò, V. Lorenzelli, J. Phys. Chem., 90, (1986),
1337-1344.
42. R.K. Grasselli, J.D. Burrington, Adv. Catal., 30, (1981), 133-163.
43. J.D. Burrington, C.T. Kartisek, R.K. Grasselli, J. Catal., 87, (1984), 363380.
44. J.C. Otimari, A. Andersson, Catal. Today, 3, (1988), 211-222.
J.C. Otimari, S.L.T. Andersson, A. Andersson, Appl. Catal., 65, (1990),
159-174.
45. M. Sanati, A. Andersson, Ind. Eng. Chem. Res., 30, (1991), 312-320.
46. G. Centi, S. Perathoner, J. Catal., 142, (1993), 84-96.
47. H. Miyata, Y. Nakagawa, T. Ono, Y. Kubokawa, Chem. Lett., (1983),
1141-1144.
48. F. Cavani, E. Foresti, F. Trifirò, G. Busca, J. Catal., 106, (1987), 251262.
49. F. Cavani, G. Centi, F. Trifirò, Chim Ind., 74, (1992), 182-193.
50. P. Mars, D.W. van Krevelen, Chem. Eng. Sci. Suppl., 3, (1954), 41-57.
51. Y. Murakami, M. Niwa, T. Hattori, S. Osawa, I. Igushi, H. Ando, J.
Catal., 49, (1977), 83-91.
52. M. Niwa, Y. Murakami, J. Catal., 76, (1982), 9-16.
53. A.B. Guseinov, E.A. Mamedov, Y.D. Pankratiev, R.G. Rizayev, Kinet.
Catal. 26, (1985), 126-130.
54. M-D. Lee, W-S. Chen, H-P. Chiang, Appl. Catal. A., 101, (1993), 269281.
55. A. Martin, Y. Zhang, M. Meisel, React. Kinet. Catal. Lett., 60, (1997),
3-8.
56. J.D. Idol, US Patent 2904580, 1959.
57. J.L. Barclay, J.B. Bream, D.J. Hadley, D.G. Stewart, Brit. Patent
876446, 1959.
J.L. Barclay, J.B. Bream, D.J. Hadley, D.G. Stewart, US Patent
3152170, 1959.
58. R.K. Grasselli, Ammoxidation, in: Handbook of Heterogeneous
Catalysis, Eds. G. Ertl, H. Knözinger, J. Weitkamp, Vol. 5, VCH,
Weinheim, 1997, pp. 2302-2326.
59. J. Haber, in: New Developments in Selective Oxidation by
Heterogeneous Catalysis, Eds. P. Ruiz, B. Delmon, Stud. Surf. Sci.
Catal., Vol. 72, Elsevier Science Publishers BV, Amsterdam, 1992, pp.
279-304.
60. J.L. Callahan, R.K. Grasselli, E.C. Milberger, H.A. Strecker, Ind. Eng.
Chem. Prod. Res. Dev., 9(2), (1970), 134-142.
61. G.W. Keulks, J. Catal., 19, (1970), 232-235.
38
Toluene ammoxidation mechanism
62. W.M.H. Sachtler, N.H. de Boer, in: Proc. 3rd Int. Congr. Catal., Eds.
W.M.H. Sachtler, G.C.A. Schuit, P. Zwietering, North Holland
Publishing Company, Amsterdam, 1965, 252-263.
63. C.R. Adams, T.J. Jennings, J. Catal., 2, (1963), 63-68.
64. C.R. Adams, T.J. Jennings, J. Catal., 3, (1964), 549-558.
65. J.F. Brazdil, D.D. Suresh, R.K. Grasselli, J. Catal., 66, (1980), 347-367.
66. J.D. Burrington, R.K. Grasselli, J. Catal., 59, (1979), 79-99.
67. A.A. Davydov, V.G. Mikhaltchenko, V.D. Sokolovskii, G.K. Boreskov,
J. Catal., 55, (1978), 299-313.
68. A.N. Orlov, S.G. Gagarin, Kinet. and Catal., 15, (1974), 1308-1312.
69. J.D. Burrington, C.T. Kartisek, R.K. Grasselli, J. Catal., 63, (1980), 235254.
70. J. Haber, M. Wojciechowska, J. Catal., 110, (1988), 23-36.
71. M. Sanati A. Andersson, J. Mol. Catal., 81, (1993), 51-62.
72. R.K. Grasselli, J.D. Burrington, J.F. Brazdil, Faraday Discuss. Chem.
Soc., 72, (1981), 203-223.
73. P. Cavalli, F. Cavani, I. Manenti, F. Trifirò, Ind. Eng. Chem. Res., 26,
(1987), 639-647.
39
40
Chapter 3
Screening of new toluene ammoxidation catalysts
Abstract
A broad range of new alkylaromatic ammoxidation catalysts was prepared.
Zeolite NaY or γ-alumina was used as matrix for different transition metal
oxides. Toluene ammoxidation was used as test reaction. Zeolite samples
were prepared from NaY by means of incipient wetness impregnation, ion
exchange or chemical vapour deposition of metal carbonyls. All γ-alumina
samples were prepared by means of incipient wetness impregnation. The
influence of dopants was studied for vanadia based NaY catalysts. The
selectivity towards benzonitrile increased; however, the catalyst activity was
less after dopant addition. γ-Alumina supported molybdenum oxide showed
high benzonitrile yields. High temperature ammonia treatment increases the
benzonitrile yield significantly, indicating the importance of nitrogen
containing species on the catalyst surface. The benzonitrile yield could be
improved by doping this catalyst with vanadia. The stability of the catalyst
samples under ammoxidation conditions was found to be an important
parameter. Copper exchanged NaY catalysts were found to have high
benzonitrile yields as well, but their activity decreases drastically during the
first hours on stream. By performing the conversion of toluene to
benzonitrile in the presence of NO and oxygen it was shown that nitrile
formation does not occur from ammonium nitrate or ammonium nitrite
intermediates, where presence of these intermediates might be proposed from
selective catalytic reduction of NO.
1.
Introduction
Aromatic nitriles are widely used as solvents in organic reactions. In
addition the nitrile group can be easily converted by means of
hydrogenation, hydration or hydrolysis reactions to amines, amides, imines
and other functional groups [1]. These functional groups are found in many
intermediates in polymer and pharmaceutical industry [2].
Current commercial catalysts for aromatic (di-)nitrile production are based
on supported V oxide, combined with other metal oxides [3]. For toluene
ammoxidation similar catalysts have been patented. V is combined with
elements such as Mo, Sb, Cr, Ti, Bi, P and other components [4,5]. Both
41
Chapter 3
supported and unsupported oxides are used. Usually alumina supported
catalysts show good performances. Other supports such as zeolites, TiO2,
SiO2, MgF2 have also been studied [6-10]. Numerous patents have been
published in which the ammoxidation activity of mixed transition metal
oxide is claimed; see for example [4,11]. A combination of V2O5 and Pt on
SiC has been patented [12] also. Benzonitrile yields up to 90 percent are
attainable.
V based catalysts are most widely reported to catalyse alkylaromatic
ammoxidation, due to their high activity. Several groups have studied
toluene ammoxidation over vanadia supported on titania [10,13], alumina
[14,15], or zirconia [16]. For Mo oxide also the use of MgF as support has
been reported [8]. Like for unsupported mixed oxide catalysts the
benzonitrile selectivity can be increased by addition of Sb to alumina
supported V catalysts [17]. Alkylaromatic ammoxidation over VPO
catalysts has recently been studied in detail by the group of Martin [18,19].
The ammoxidation of alkylaromatics over zeolite-based catalysts has been
studied less extensively, although copper loaded ZSM-5 [20-22] and other
zeolites [23] have been used by several authors for the ammoxidation of
different alkylaromatic substrates. Benzonitrile yields of over 80 % can be
reached via ammoxidation of toluene in the presence of a substantial
amount of water in the reactor feed. The application of other zeolite based
metal oxide catalysts is reported less frequently, though earlier NaX zeolites
containing Zn or Ag were reported to have moderate nitrile selectivity for
toluene ammonolysis, i.e. when nitrile formation occurs in an ammonia
flow and catalyst reoxidation occurs separately [6]. However, when
hydrocarbon and oxygen is passed over the catalyst simultaneously, these
zeolites as well as CrNaX, FeCrNaX, FeMnNaX give very poor yields to
aromatic nitriles [2]. V-zeolites [24,25] and -zeotypes [26] have also been
used for the ammoxidation of toluene and xylenes respectively. Zeolite
based transition metal oxide catalysts generally yield lower amounts of
selective oxidation products than alumina supported metal oxides do.
Recently, however, Fe-Mo oxides stabilized in pentasil-type zeolites have
shown high yields towards aldehyde reaction products in the oxidation of pxylene [27]. Benzaldehyde has been proposed as selective surface
intermediate for the production of benzonitrile from toluene by several
groups [7,13,20,26,28]. The performance of these catalysts in alkylaromatic
oxidations indicates the possibility for the use of zeolite-based catalysts to
42
Screening of new toluene ammoxidation catalysts
achieve high yields in selective oxidation reactions. In addition, Li and
Armor recently reported highly active and selective zeolite based catalysts
for the ammoxidation of ethane and other small hydrocarbons [29].
The research described in this chapter is aimed at the development of
improved catalysts for ammoxidation of alkylaromatics. The vapour phase
ammoxidation of toluene has been used as a test reaction. The use of
zeolite-based catalysts has not been reported extensively so far. Therefore, in
this research zeolite NaY was loaded with transition metals by means of ion
exchange, chemical vapour deposition (CVD) or incipient wetness
impregnation. For comparison, γ-alumina supported toluene ammoxidation
catalysts were prepared. Since NH3 activation plays a key role in
ammoxidation reactions [14,18] activation of the catalysts was performed
not only under oxidizing conditions, but also in NH3 atmospheres to
produce MoxN catalysts. The effect of the zeolite matrix has been examined
as well as dopant addition.
2.
Experimental methods
2.1
Catalyst preparation and characterization
Three different preparation methods were applied for the introduction of
transition metals into NaY and γ-alumina (Al): Ion-exchange (ie),
impregnation (im) and metalcarbonyl sublimation (s). Catalysts are
denotated according to the format Metal 1-Metal 2(preperation method,
wt% Metal 1,wt% Metal 2)/support. The catalyst loadings are expressed as
wt% of metal, unless otherwise mentioned.
Conventional ion exchange was used to prepare Co(ie)/NaY, Cu(ie)/NaY
and NH4(ie)/NaY. Nitrate salts were added to a NaY/water slurry and
stirred overnight at room temperature. The applied NaY batch (PA 73022)
was supplied by Akzo. The unit cell composition was Na55(AlO2)55(SiO2)137)
as confirmed by A.A.S. After centrifuging and thorough washing, the
samples were dried at 110 ° C. A 250-425 µm sieve fraction was used for the
catalytic tests.
All impregnated samples were prepared by incipient wetness impregnation.
V(im)/NaY and V(im)/Al were prepared by dissolving NH4VO3 in water.
The solution was heated until all NH4VO3 had dissolved. For the
43
Chapter 3
preparation of promoted V(im)/NaY catalysts sequential impregnation was
applied using solutions containing SbCl3, K2CO3, Bi(NO3)3∙ 5H2O and
(NH4)2HPO4 respectively. In between the impregnation steps the catalyst
precursors were dried at 110 ° C in order to remove the solvent. In the case
of the γ-alumina supported sample, NH4VO3 was added to a 2 M oxalic acid
solution. Prior to impregnation the samples were pelleted, crushed and
sieved into a 250-425 µm sieve fraction. The supports were heated to 400
° C in ambient air before reaction. V containing catalysts were prepared by
sequential impregnation, due to the low solubility of NH4VO3. In between
the two impregnations the catalyst precursors were heated to 110 ° C in
order to remove the solvent from the pores. Mn(im)/Al was prepared from
a manganese(II) nitrate solution, Mo/Al was prepared from (NH4)6Mo7O24
as described by Peeters et al. [30]. The catalyst was activated by heating at
500 ° C for one hour in flowing dry air (60 ml/min). Bi-metallic catalysts
were prepared by incipient wetness co-impregnation. γ-Alumina was
supplied by Akzo (surface area of 205 m2/, pore volume 0.55 ml/g).
Mo(im)/Al and commercial MoO3 were used as precursors to prepare bulk
and supported molybdenum nitride (MoxN and MoxN(im)/Al). The
precursors were heated in a fixed bed reactor to 360 ° C at a heating rate of
10 ° C per minute in an NH3(1 vol%)/He mixture. The heating rate was
then reduced to 1 ° C per minute and the sample was heated to 700 ° C.
After keeping the sample at this temperature for one hour the sample was
allowed to cool to room temperature. Finally the sample was passivated at
room temperature in an O2(1 vol%)/He flow (20 ml/min) for several hours.
Co(s)/NaY, Mo(s)/NaY, Mn(s)/NaY, V(s)/NaY and W(s)/NaY were
prepared by chemical vapour deposition by exposing dehydrated NaY (250425 µm) to Co(CO)3NO, Mo(CO)6, Mn2(CO)10, V(CO)6 or W(CO)6 vapour
respectively, in a N2 atmosphere. The samples were brought into glass
ampoules, which were evacuated (p = 1∙ 10-2 mbar). Ampoules were then
heated to 60 ° C or 120 ° C, depending on the volatility of the metal
carbonyl used. Previous experiments with Mo(CO)6 have shown that during
this treatment the metal carbonyl migrates into and is stabilised by the
zeolite supercages [31]. The catalysts were then transferred to a fixed bed
reactor, without exposure to ambient air and heated under controlled
conditions. In all cases the metal loading was two metal atoms per zeolite
44
Screening of new toluene ammoxidation catalysts
supercage; this corresponds to the saturation limit for Mo(CO)6 on zeolite
NaY [32].
Table 3.1: Toluene ammoxidation catalysts.
Catalyst
1
2
Loading1 [wt%]
Co(ie,4.4)/NaY
Cu(ie,4.3)/NaY
NH4(ie)/NaY
Mo(im,9.8)/Al
V(im,1.9)/NaY
V(im,1.7)/Al
V(im,4.4)/Al
Mn(im,3.0)/Al
Co(s,4.5)/NaY
4.4
4.3
9.8
1.9
1.72
4.42
3.02
4.5
Mo(s,11.4)/NaY
11.4
Mn(s,3.5)/NaY
3.52
V(s,4.2)/NaY
4.2
W(s,18)/NaY
18
2
Activation
None
None
None
Calcination at 500 ° C in He/O2
None
None
None
Calcination at 500 ° C in He/O2
Co(NO)(CO)3 introduction at 50 ° C.
Oxidation in He/O2. T= 400 ° C.
Mo(CO)6 introduction at 60 ° C.
Oxidation in He/O2. T= 400 ° C.
Mn2(CO)10 introduction at 120 ° C.
Oxidation in He/O2. T= 400 ° C.
V(CO)6 introduction at 60 ° C.
Oxidation in He/O2. T= 400 ° C.
W(CO)6 introduction at 60 ° C.
Oxidation in He/O2. T= 400 ° C.
determined by A.A.S. using a Perkin Elmer 3030 Atomic Absorption
Spectrophotometer.
loading was not determined.
2.2
Catalyst testing
Toluene ammoxidation was performed at atmospheric pressure between 300
° C and 460 °C in a quartz glass fixed bed reactor (4 mm internal diameter).
In all cases a plug flow regime was ensured. The amount of catalyst used
was between 0.2 and 0.3 g (250-425 µm particles). Thermal mass flow
controllers were used to control all gas flows. He was used as an inert gas.
Toluene vapour was introduced to the system by saturating a part of the He
carrier gas at 9.4 ° C and atmospheric pressure. Weight Hourly Space
Velocities (WHSV) between 0.7 and 1.0 gtoluene/(gcat∙ hr) were used. The
molar toluene: NH3: O2-ratio (T:N:O) was 1: 8: 13 and the total gasflow was
110 Nml/min, unless mentioned otherwise. Similarly, toluene nitroxidation
reactions were performed by using NO instead of NH3. All lines
45
Chapter 3
downstream of the reactor were thermostatted at 200 °C to prevent
condensation. The organic compounds were analysed using an HP 5980 gas
chromatograph, equipped with a 50 m HP-5 column and a flame ionisation
detector. CO, CO2, NH3 and H2O were detected by non-dispersive infrared
spectroscopy using a Fischer-Rosemound NGA-2000 MLT 4.2 analyser
platform. The analyser was equipped with a paramagnetic cell to estimate
the O2 concentration. Occasionally detection of inorganic products was
performed by GLC using a Carboplot P7 column (25 m). Prior to analysis
all organic products were removed by passing the gas stream through a trap
containing 1-hexanol. NH3 was removed using a trap containing 2 M
H2SO4. Qualitative analysis was also performed by quadruple mass
spectrometry. Occurrence of homogeneous gas-phase reactions was found
to be negligible under the experimental conditions applied.
Conversion (X), selectivity (S) and yield (Y) calculations are based on the
molar amount of toluene fed to the reactor. This amount was examined by
GLC prior to performance of the catalytic reactions. The production rate of
benzonitrile is defined as the molar amount of benzonitrile produced per
gram of catalyst per second.
3.
Results and discussion
3.1
Catalyst screening
Since the objective of the experiments described in this Chapter was to
screen new catalysts for the ammoxidation of toluene a V(im)/Al catalyst
was also prepared. Though vanadia catalysts suffer from low selectivity and
undesired ammonia decomposition vanadia based catalysts are traditionally
used for the toluene ammoxidation reaction [33]. Since the reaction
conditions reported in literature differ slightly the comparison based on
toluene conversion, benzonitrile selectivity and benzonitrile yield is
complicated. Therefore, the catalytic performance of this prepared
V(im,1.7)/Al was taken as a reference for the performance of the newly
prepared catalysts. The results are shown in Figure 3.1. From this figure it is
clear that the selectivity to benzonitrile over V(im,1.7)/Al is relatively stable
over the whole temperature range. The activity of this catalyst, however, is
low. Only at elevated temperature high activities were obtained. However,
the benzonitrile production rate, which amounts to 0.35 gbenzonitrile/(gcat∙ hr),
is comparable to that reported in literature. For example Azimov et al. [14]
46
Screening of new toluene ammoxidation catalysts
find toluene ammoxidation rates up to 37 µmol/(m2∙ hr) for toluene
ammoxidation over V-Sb-Bi-O/Al at 360 ° C. Assuming a surface area of 80
m2/g, on a gram basis benzonitrile productivities up to 0.27
gbenzonitrile/(gcat∙ hr) were achieved.
C o n v e rsio n , S e le c tiv it y [m o l% ]
100
Benzonitrile selectivity
80
60
40
Toluene conversion
20
0
300
350
400
450
T e m p e ra tu re [° C ]
Figure 3.1: The ammoxidation of toluene over V(im,1.7)/Al as a function of
temperature. WHSV = 0.8; T:N:O= 1: 8: 13.
It must be noted that the catalytic performance of supported vanadia
catalysts generally strongly depends on the loading. Cavalli et al. [34] found
higher benzonitrile yields upon increase of the V loading for V/Ti. When
the monolayer loading was reached no further increase of the benzonitrile
yield was observed. Contrary Sanati et al. [10] ascribe the activity of V/Ti
catalysts to the presence of V5+-species located on top of a vanadia
monolayer. V loadings even higher than the monolayer coverage therefore
are needed for the production of the most active toluene ammoxidation
catalyst.
Increase of the V loading led to increase of the activity as was expected from
literature [10,34]. The catalytic results at 360 and 380 ° C for two vanadia
loaded γ-alumina catalysts are shown in Table 3.2. Two drawbacks,
however, must be noticed. Though 55 % yield can be obtained over
47
Chapter 3
V(im,4.4)/Al at 360 ° C, temperature increase leads to severe benzonitrile
selectivity decrease, and thus to reduction of the benzonitrile yield.
Secondly, the preparation of vanadia catalysts is somewhat complex, since
the solubility of NH4VO3, the catalyst precursor, is low. Therefore,
sequential impregnation steps are necessary to reach V loadings higher than
2 wt%.
Table 3.2: The effect of V loading and reaction temperature on the toluene
ammoxidation over V(im)/Al catalysts.
Catalyst
V(im,1.7)/Al
V(im,1.7)/Al
V(im,4.4)/Al
V(im,4.4)/Al
T
[° C]
X(Toluene)
[mol%]
S(Benzonitrile)
[mol%]
Y(Benzonitrile)
[mol%]
360
380
360
380
14
22
65
95
74
74
85
35
10
16
55
33
WHSV = 0.8; T:N:O = 1: 8: 13.
Table 3.3 shows the effect of the support on the vanadia performance in the
ammoxidation of toluene at 400 ° C.
Table 3.3: Effect of the support for vanadia catalysts on the
performance in toluene ammoxidation at 400 ° C.
Catalyst
V(im,1.9)/NaY
V(im,1.5)/ZSM-5
V(im,1.7)/Al
X(Toluene)
[mol%]
S(Benzonitrile)
[mol%]
90
17
30
55
55
75
WHSV = 0.8; T:N:O = 1: 8: 13.
At low V loadings NaY is superior to the other supports that were applied.
High conversion levels could be obtained. The benzonitrile selectivity,
however, was lower than that over V(im,1.7)/Al. Nevertheless, since the
benzonitrile selectivity can be increased by promoter addition [4,17],
faujasite based catalysts show high potential for the ammoxidation of
alkylaromatics. High activities can be achieved at relatively low
temperatures. Moreover, due to the pore dimensions of the zeolite matrix,
size exclusion effects can be obtained. For example Beschmann et al. [20]
showed that only p-xylene was converted when a mixture of o-, m- and pxylene was fed to a Cu/ZSM-5 catalyst. Due to its larger pore size
48
Screening of new toluene ammoxidation catalysts
compared to ZSM-5, faujasite based catalysts can potentially be applied for
selective conversion of more complex alkylaromatic molecules.
In the remainder of this chapter the screening of a wide range of catalysts
for their ability to ammoxidize toluene will be described. The conversion
and selectivity were measured in order to determine the benzonitrile
production rate. The catalysts were evaluated at high conversion levels to
find the most productive samples. Catalyst deactivation was observed for
most catalysts. Therefore, initial data and data measured after 1000 minutes
on stream are shown. The activity of some catalysts increased slightly
during the first 100 minutes on stream. The maximum conversion level
obtained for those catalysts is shown between brackets. Generally the
selectivity to benzonitrile remained constant as a function of time on stream
for the catalysts investigated. The results of the toluene ammoxidation
screening tests are shown in Table 3.4
Table 3.4: Toluene ammoxidation results at 400 ° C.
Catalyst
X [mol%]
S [mol%]
Rp [µ
[µmole/g∙ s]
initial
final
initial final initial
final
NaY
22.5 (26.3) 22.5
62
62
0.34 (0.40) 0.34
1
1
Al
5.4
5.4
62
62
0.08
0.08
2
2
Co(ie,4.4)/NaY
98
43.7
35
57
1.06
0.77
Cu(ie,4.3)/NaY
100
872
0
742
0
1.79
NH4(ie)/NaY
98
37.52 37
572
1.01
0.59
1
1
Co(s,4.5)/NaY
39.0
36.3
76
76
0.61
0.57
Mo(s,11.8)/NaY
42.5
30
45
49
0.62
0.48
Mn(s,3.4)/NaY
100
100
0
1.7
0
0.05
V(s,4.2)/NaY
62 (66)
62
48
59
0.67 (0.71) 0.83
W(s,18)/NaY
41
25.2
45
45
0.57
0.35
1
1
Mn(im,4.2)/NaY 100
99
0
4
0
0.10
V(im,1.9)/NaY
97
95.8
54
54
1.41
1.39
Cu(im,3.2)/Al
67
67
1
1
0.02
0.02
Mo(im,9.8)/Al
94.6
92
80
80
1.67
1.62
1
1
Mn(im,3.0)/Al
76
75
23
23
0.37
0.36
V(im,4.4)/Al
73
66
86
86
1.50
1.35
WHSV = 0.8 – 1.0; T:N:O= 1: 8: 13
End time is after 1000 minutes on stream, except 1 400 min; 2 800 min
As can be seen the benzonitrile production rates obtained after 1000
minutes on stream were of the order of 1 µmole/g∙ s. This rate is
comparable to that reported earlier over VSb/Al (V/Sb = 0.2) [14]. The
main side reaction observed in our experiments was total combustion to
CO2. Additionally a small amount of CO was observed. In some cases some
49
Chapter 3
dark coloured higher molecular weight compounds were observed on the
catalyst bed or at the reactor outlet.
3.2
Catalyst deactivation
C o n v e rsio n , S e le ct iv ity
[m o le % ]
100
80
B en zon itrile selectivity [m ole % ]
60
40
Toluen e conversion [m ole % ]
20
0
0
200
400
600
T im e o n stre a m [m in ]
800
1000
Figure 3.2: Toluene ammoxidation over NaY. T= 400 ° C; WHSV = 0.78;
T:N:O= 1: 8: 13.
Figure 3.2 shows the change of conversion and selectivity with time on
stream for toluene ammoxidation over zeolite NaY. After a short period of
activation the catalyst deactivated continuously as a function of time on
stream. After reaction some dark brown products were found both on the
catalyst and on the wall of the condenser. Coking was also observed by
Niwa et al. [9]. These authors proposed that carbonaceous material was
responsible for the activity in the ammoxidation of toluene. The surface of
the SiO2-Al2O3 applied by Niwa et al. was covered with carbonaceous
material. This layer showed absorption bands belonging to carbonyl or
carboxyl groups in the IR spectrum. When ammonia was pulsed on this
layer benzonitrile was formed. Therefore toluene could be oxidized on
surface oxides, followed by stabilization as adsorbed benzoate ion to
produce benzonitrile. In performing pulse experiments they found
benzonitrile formation only when the pulse sequence toluene-ammonia was
applied [15], implying that ammonia reacts with the adsorbed alkylaromate
compound. Other experiments performed by this group showed that
50
Screening of new toluene ammoxidation catalysts
unsupported vanadia catalysts did not yield nitrile formation [35]. It was
concluded, based on IR experiments [15] that a methyl benzoate surface
species was formed initially from xylene. This species was stabilized after
migration to the alumina support and nitrile products are formed by
reaction of this methyl benzoate species, which was stabilized on the γalumina support. The formation of the methyl benzoate species was
assumed to occur on coke containing V sites.
Indeed the γ-alumina support does not show significant activity for the
formation of benzonitrile as shown in Figure 3.3. The benzonitrile
selectivity was in the order of 60 %. No activating effect was observed after
the start of the reaction.
10
400
T (righ t axis)
8
380
6
360
4
340
2
320
0
300
0
500
1000
1500
2000
T e m p e ra tu re [° C ]
C o n v e rsio n [m o l% ]
X (T oluen e)
2500
T im e o n stre a m [m in ]
Figure 3.3: Toluene ammoxidation over γ-alumina. WHSV = 0.75; T:N:O=
1: 8: 13.
The initial increase in activity for NaY in our experiments may be similarly
explained by the formation of carbonaceous deposits on the catalyst surface.
Eventually increase of the coke amount could have caused blocking of the
zeolite pores, resulting in decreased ammoxidation activity after a longer
time on stream. Similar deactivation behaviour was observed by
Ramachandra Rao et al. [36], who found a 50 % reduction in the
ammoxidation activity after 10 hours on stream during the ammoxidation
of 3-picoline over a VAPO catalyst. The pore dimensions of this catalyst are
similar to that of zeolite NaY. This suggests that modification of NaY
51
Chapter 3
should not only be directed at increasing the benzonitrile yield over freshly
prepared catalysts, but also at minimising coke forming sites.
3.2.1 Performance of ionion-exchanged catalysts
Toluene conversion can be increased significantly by introduction of metal
sites by means of ion exchange, as shown in Figure 3.4 and Table 3.4. The
selectivity towards benzonitrile is initially considerably lower than that of
NaY, but after a period of about two hours on stream the benzonitrile
selectivity equals the selectivity obtained when using NaY. Cu(ie,4.3)/NaY
shows an increase in benzonitrile selectivity up to 74 %. However, the
deactivation that occurs as a function of time on stream is even more
pronounced compared to NaY. After about 600 minutes on stream the
activity of the Co(ie,4.4)/NaY and NH4(ie)/NaY does become stable. Since
the brown products can still be observed on the ion-exchanged catalysts
after reaction, we can assume that coking again causes the deactivation.
This means that the catalyst activity can be improved by ion exchange with
transition metal ions, but this modification does not prevent the formation
of carbonaceous deposits completely.
C o n v e rsio n [m o le % ]
100
Cu(ie,4.3)/NaY
80
60
40
Co(ie,4.4)/NaY
NH4(ie)/NaY
20
NaY
0
0
200
400
600
800
T im e o n stre a m [m in ]
Figure 3.4: Performance of ion exchanged NaY. T= 400 ° C; WHSV= 0.8,
T:N:O= 1: 8: 13.
3.2.2 Performance of catalysts prepared by CVD of metal carbonyls
The performance of the catalysts that were prepared by means of CVD is
shown in Figure 3.5. These results show that the activity of the NaY zeolite
can be significantly improved by introducing Mn. However, this leads to a
52
Screening of new toluene ammoxidation catalysts
dramatic decrease in benzonitrile selectivity. For this catalyst only COx was
observed in the product stream. The introduction of V doubles the activity
where the selectivity remains at the same level compared to NaY.
Introduction of Mo and especially W leads to only a slight increase in
activity, whereas the benzonitrile selectivity is slightly lowered. After an
initial period of serious deactivation the stability of the catalysts that were
prepared by CVD of metal carbonyl molecules seems to be improved as
compared to the NaY catalyst. However, since the conversion level over
Mn(s,3.4)/NaY was 100% no judgement can be made about the
deactivation over this catalyst. Though the introduction of metal oxide
centres into the pores of faujasite catalysts can yield very specific catalytic
sites, which can be controlled accurately, the toluene ammoxidation activity
is generally low.
C o n v e rsio n [m o le % ]
100
Mn(s,3.4)/NaY
80
V(s,6.4)/NaY
60
40
Mo(s,11.8)/NaY
W(s,18)/NaY
NaY
20
0
0
200
400
600
T im e o n stre a m [m in ]
800
1000
Figure 3.5: Performance of catalysts prepared by CVD. T= 400 ° C;
WHSV= 0.8, T:N:O= 1: 8: 13.
3.2.3 Performance of NaY based impregnated catalysts
As shown in Figure 3.6 the ammoxidation activity is greatly improved by
impregnating NaY with a NH4VO3 solution. A low V loading can improve
the activity to a conversion level over 95 %. This result is contrary to Cavani
et al. [24] who found a low reaction rate in p-xylene ammoxidation over a
NH4VO3/NH4Y catalyst. Contrary to the V(im,1.9)/NaY catalyst described
here, their catalyst was prepared by wet impregnation of a NH4Y zeolite.
The activity of the V(im,1.9)/NaY is not significantly decreased upon time
on stream, although some higher molecular weight compounds condensed
at the reactor outlet.
53
Chapter 3
Though V(im,1.9)/NaY has the highest activity among all the catalysts
screened, the selectivity is not very high, it does not exceed 60% at 400 °C.
Therefore, the addition of dopants was examined to improve the
benzonitrile selectivity over this catalyst. Table 3.5 summarises the results
obtained for single doped V(im)/NaY catalysts.
C o nv e rsio n [m o le % ]
100
V(im,1.9)/NaY
80
60
40
NaY
20
0
0
200
400
600
800
1000
T im e o n stre am [m in]
Figure
Figure 3.6: Performance of V(im,1.9)/NaY. T= 400 ° C; WHSV= 0.9,
T:N:O= 1: 8: 13.
Table 3.5: Performance of two component catalysts.
Catalyst
V(im,1.9)/NaY
V-Sb(im,1.5-3.4)/NaY
V-K(im,1.1-1.4)/NaY
V-P(im,2.4-1.3)/NaY
V-Bi(im,1.5-1.6)/NaY
Me(2)/VMe(2)/V-ratio
[mol/mol]
X(toluene)
[mol%]
S(benzonitrile)
[mol%]
0.9
0.2
0.9
0.3
90
70
70
85
60
55
80
55
45
60
T=400°C; WHSV=0.9; T:N:O= 1: 8: 13.
All the catalysts listed in Table 3.5 were prepared by sequential incipient
wetness impregnation. Sb is the best dopant. The selectivity of V-Sb(im,1.53.4)/NaY increases to 80% at 400°C. When decreasing the temperature to
380°C, the selectivity can reach 90% with an overall conversion of 51%.
However, except for V-Bi(im,1.5-1.6)/NaY the stability of promoted
V(im)/NaY catalysts is decreased slightly compared to V(im,1.9)/NaY.
Figure 3.7 shows the activity as a function of time on stream of this series of
54
Screening of new toluene ammoxidation catalysts
doped V(im)/NaY catalyst. For comparison the non-promoted
V(im,1.9)/NaY catalyst has been plotted in Figure 3.7 as well.
V(im,1.9)/NaY
V-P(im,2.4-1.3)/NaY
V-K(im,1.1-1.4)/NaY
V-Sb(im,1.5-3.4)/NaY
V-Bi(im,1.5-1.6)/NaY
C o nv ersio n [m o l% ]
100
80
60
40
20
0
0
200
400
600
800
T im e o n stre am [m in]
Figure 3.7: Toluene ammoxidation over doped V(im)/NaY catalysts.
T= 400 ° C; WHSV = 0.9; T:N:O= 1: 8: 13.
3.2.4 Performance of γ-alumina supported catalysts
Figure 3.8 shows the activity of some γ-alumina supported catalysts as a
function of time on stream.
C o nv ersio n [m o le % ]
100
MoN(im,9.8)/Al
Mo(im,9.8)/Al
80
V(im,4.4)/Al
60
40
NaY
20
0
0
200
400
600
800
1000
T im e o n stream [m in ]
Figure
Figure 3.8: Performance of γ-alumina supported catalysts. WHSV = 0.8;
T:N:O= 1: 8: 13. T= 400 ° C, except V(im,4.4)/Al: T= 360 ° C.
Deactivation is generally less severe compared to the zeolite-based catalysts.
The performance of the catalysts was compared at 400 ° C, except for
V(im,4.4)/Al. At higher temperature the selectivity towards benzonitrile
drastically decreased to 30 mol%. Therefore, the performance was measured
55
Chapter 3
at 360 ° C for this catalyst. The activity of the γ-alumina supported metal
oxide catalysts generally equals or exceeds that measured on the most active
zeolite-based catalysts. The benzonitrile selectivity was also higher on γalumina supported catalysts. The highest yield of benzonitrile is obtained
over γ-alumina supported Mo oxide.
X (Toluene) [m ol% ]
When this catalyst was applied as precursor for the production of γ-alumina
supported Mo nitride the activity and the benzonitrile selectivity could be
increased slightly. The higher activity obtained for the toluene
ammoxidation reaction supports the importance of N containing
intermediates in the mechanism of alkylaromatic ammoxidation reactions.
Martin et al. [18] also stressed this in a study using isotopic NH3 labelling in
a TAP apparatus. These authors performed the ammoxidation of toluene
over an α-(NH4)2[(VO)3(P2O7)2] catalyst. The benzonitrile that was formed
initially contained only N atoms originating from the catalyst. In a later
stage of the reaction also N originating from NH3 was inserted into the
alkylaromatic compound.
B e n z o n itr ile s ele c tiv ity
[m o l% ]
100
90
100
80
60
40
20
0
360
380
400
420
440
Tem perature [°C ]
460
80
70
N o pre-treatm ent
C alcination at T=700 °C
60
0
20
40
60
80
T o lu e n e c o n v er s io n [m o l% ]
100
Figure 3.9: The effect of calcinations on the ammoxidation of toluene over
Mo(10.5)/Al; WHSV = 0.6; T:N:O= 1: 5: 8.
Since Mo nitrides are not stable in an oxygen-containing atmosphere it is
doubted that structural formation of surface MoxN has occurred. The
improvement of catalyst performance may have been caused by heat
56
Screening of new toluene ammoxidation catalysts
treatment. Indeed, high temperature treatment slightly increased the
selectivity towards benzonitrile, as shown in Figure 3.9. The activity was
not significantly changed upon heat treatment, as shown in the inset.
Mo based catalysts such as the well-known Bi-Mo oxide alkene
ammoxidation catalyst yield a large amount of hydrocarbon decomposition
products [37]. The selectivity for nitrile formation usually increases when V
is added to the catalyst, which was observed for V promoted Ce containing
Bi-Mo oxide catalysts [38]. As already discussed, Mo(im)/Al shows a fairly
toluene ammoxidation activity. The benzonitrile selectivity, which does not
exceed 70%, needs to be improved. As shown in Table 3.6 increasing the
temperature can improve slightly the benzonitrile selectivity for this type of
catalyst, unlike the performance of V based catalysts. This may indicate the
different mechanism of side reaction for these two catalysts.
Table 3.6: Effect of promoters Mo(im,11)/Al.
Catalyst
Mo loading
Dopant loading
[mol%]
[mol%]
Mo(im,11)/Al
11.6
0
Mo-V(im,11-0.4)/Al
11.6
0.8
Mo-V(im,11-0.8)/Al
Mo-V(im,11-1.6)/Al
11.5
11.4
1.6
3.1
Mo-La(im,11-2.4)/Al
11.7
1.8
Mo-Ni(im,11-0.8)/Al
11.6
1.4
T
[°C]
380
400
380
400
380
380
400
380
400
380
X
[mol%]
S
[mol%]
50
91
65
100
95
88
100
51
93
51
70
76
75
77
82
83
76
80
85
75
WHSV = 0.8 gtoluene/(gcat∙ hr); T:N:O= 1: 8: 13.
A number of secondary components were tested on their promoter effect in
the Mo(im,11)/Al. The results of Table 3.6 show that La addition can
increase the selectivity from 76% to 85% while the activity remains
unchanged. V addition can greatly increase the activity of the catalyst but
has less effect on the selectivity.
3.3
Benzonitrile selectivity
Table 3.7 shows the selectivity towards benzonitrile at 20 % conversion.
Though in several cases the amount of combustion reactions increased
significantly all preparation methods applied in this research can produce
57
Chapter 3
more selective catalysts for the ammoxidation of toluene compared to NaY,
depending on the transition metal introduced. With respect to benzonitrile
selectivity all preparation methods applied can lead to high benzonitrile
selectivities. Ion exchange of Cu, impregnation of V and deposition of Co
into NaY lead to the most selective catalysts.
Table 3.7: Benzonitrile selectivity at toluene iso-conversion (20 mol%).
Catalyst
IonIon-exchanged
catalyst
Carbonyl based
catalysts
Impregnated
catalysts
Benzonitrile selectivity
[mol%]
NaY
NH4(ie)/NaY
Co(ie,4.4)/NaY
Cu(ie,4.3)/NaY
Co(s,4.5)/NaY
Mo(s,11.8)/NaY
Mn(s,3.4)/NaY
V(s,4.2)/NaY
W(s,18)/NaY
V(im,1.9)/NaY
Mo(im,10)/Al
V(im,1.7)/Al
63
Not measured
59
86
95
50
60
65
45
82
73
74
WHSV = 0.8 gtoluene/(gcat∙ hr); T:N:O= 1: 8: 13.
S e le c ti v i t y [m o l% ]
100
80
60
40
Cu(ie,4.3)/ N aY
20
V (im ,1.7)/ N aY
0
0
20
40
60
80
100
C o n v e r s i o n [m o l% ]
Figure 3.10: Conversion–selectivity plot for V(im,1.7)/NaY and
Cu(ie,4.3)/NaY. WHSV = 0.8; T:N:O= 1: 8: 13.
As discussed in earlier sections high benzonitrile selectivities could not be
obtained at high conversion levels for all catalysts. Generally the change in
58
Screening of new toluene ammoxidation catalysts
benzonitrile selectivity with toluene conversion is complex, as shown in
Figure 3.10. A volcano shape relation between conversion and selectivity is
found. At low conversion levels low selectivities are obtained. Increase of
the conversion level leads to increase of the benzonitrile selectivity over a
broad range. At higher conversion the benzonitrile selectivity drops again.
The complexity of the benzonitrile selectivity behaviour with the conversion
level can be understood well based on the reaction network of toluene
ammoxidation. Scheme 3.1 sketches the most important reactions during
toluene ammoxidation. Changes in the reactant concentrations influence
not only the toluene ammoxidation reaction itself, but also the combustion
of ammonia.
CH3
+ NH3, O2
- H2O
+ O2
- H2O
CN
+ O2
- H2O
(- HCN, N2, NOx)
CO, CO2
NH3
+ O2
- H2O
N2, NOx
Scheme 3.1: Most important processes during toluene ammoxidation.
A change in the benzonitrile selectivity was observed as a function of time
on stream for the ion exchanged catalysts as well as for the V(s,4.2)/NaY
catalyst. Initial benzonitrile values were low, as shown in Table 3.4. For
Co(ie,4.4)/NaY and NH4(ie)/NaY the increase in benzonitrile selectivity
during time on stream can possibly be related to the catalyst deactivation.
Total combustion probably occurs via a consecutive reaction over these
catalysts. A lower conversion level leads to higher benzonitrile selectivity
because consecutive combustion reactions are reduced.
For V(s,4.2)/NaY and Cu(ie,4.3)/NaY the increase in benzonitrile
selectivity could possibly be related to the presence of water. Water addition
to the reactant feed in toluene ammoxidation increases significantly the
benzonitrile selectivity [36,39]. During the ammoxidation or deep oxidation
59
Chapter 3
of toluene water is generated in a substantial amount. These water
molecules could possibly cause the increase of benzonitrile selectivity for
V(s,4.2)/NaY and Cu(ie,4.3)/NaY. This, however, does not explain why
selectivity changes during time on stream were not observed for all catalysts
used. Thus the beneficial effect of water relates strongly to the catalyst
composition. It has been proposed for V catalysts that adsorbing water
prevents oxygen from binding weakly to the active sites of the catalysts.
This weakly bound oxygen leads to total combustion [2 and references
therein, 39]. Water may also suppress ammonia decomposition, leading to a
decrease in total oxidation reactions.
3.4
Temperature influence
80
Mo(im,10)/Al
Y ie ld [m ol% ]
60
V(s,6.4)/NaY
V(im,1.9)/NaY
V(im,1.7)/A l
Co(s,4.4)NaY
40
Cu(ie,4.3)/NaY
Mo(s,11.8)/NaY
20
Mn(s,3.4)/NaY
0
300
340
380
420
460
500
Te m p e r atur e [° C ]
Figure 3.11: Toluene ammoxidation over NaY and Al based catalysts.
Benzonitrile yield as a function of T. WHSV = 0.8, T:N:O= 1: 8 :13.
Figure 3.11 shows the effect of the reaction temperature on the benzonitrile
yield during toluene ammoxidation over several catalysts. The reaction
temperature was varied between 300 ° C and 460 ° C. Increasing the
reaction temperature increases the catalyst activity, but in general the
selectivity is decreased due to the increase of total combustion. Though the
catalytic activity varied from catalyst to catalyst the catalytic activity at 400
° C could be taken as reference for comparison between the different
catalysts. For Cu(ie,4.3)/NaY and for Mo(im,10)/Al higher reaction
temperatures can be applied without selectivity loss. The temperature at
which the maximum yield of benzonitrile is achieved for Mn(s,3.4)/NaY is
significantly lower, at 400 ° C this catalyst leads to combustion reactions.
60
Screening of new toluene ammoxidation catalysts
3.5
Nitroxidation of toluene
As discussed extensively in Chapter 2 the exact nature of the nitrogen
insertion site for toluene ammoxidation is not known. Surface NH [40]
species, formed from dissociatively adsorbed ammonia, or NH4+-ions [41]
are proposed by most authors as reactive N insertion species. Another
possible way of ammonia activation is described by Centi and Perathoner
[42]. Upon interaction with Brønsted acid sites NH3 adsorbs on CuO as
NH4+. In the presence of NO, NH4NO2 and NH4NO3 surface intermediates
can be formed. Besides from other N producing intermediates, N2 and N2O
can be formed from these structures upon desorption as shown in Scheme
3.2.
NO
Cu O
NO
+
Cu
O
H
NH3
+
NO2 NH4
+
NO NH4
Cu
O
O
Cu
O
O2
+
NO3 NH4
Cu
N2 + H2O
O
N2O + H2O
Scheme 3.2: Surface NH4NO2 and NH4NO3 in the conversion of NO and
NH3 in the presence of O2 over Cu/Al. Adapted from Centi and Perathoner
[43].
The selective catalytic reduction of NO by hydrocarbons has been studied
extensively during the last ten years. Though most research is focused only
on the inorganic reaction products several groups have found nitriles in the
SCR product mixture [44] in the presence of hydrocarbons. A broad
overview of reaction data shows that every metal oxide that is active in
selective hydrocarbon oxidation catalysis can act as active component in the
SCR reaction [45].
Ammonia combustion occurs as side reaction during toluene
ammoxidation. Since at reaction temperatures around 400 ° C NO is
formed in this reaction [46] it seems plausible to consider also NO as
reaction intermediate during toluene ammoxidation. Indeed it has been
found by several groups that the reaction between toluene and NO yields
benzonitrile formation. This reaction is referred to as nitroxidation in the
61
Chapter 3
following. Alkylaromate nitroxidation is performed in the absence of
oxygen at reaction temperatures around 450 ° C.
CH3 + 3/2 NO
CN + 3/2 H2O + 1/4 N2
(3.1)
As shown in Equation 3.1 also N2 is formed in this reaction. For the
nitroxidation reaction mostly PbO catalysts are applied [47,48], but also
chromate [49] and NiO [48,50] catalysts have been reported in the literature.
The nitroxidation of toluene produces benzonitrile with high selectivity.
This reaction, however, leads to reduction of the catalyst [48]. Based on this
catalyst reduction it is not clear whether nitriles can be produced in steady
state or only during catalyst reduction. Moreover, reduction of the metal
oxides applied (NiO and PbO) is difficult. Therefore, a redox mechanism as
proposed by the authors seems not very probable.
C o n v e rs io n , S e le c tiv ity
[m o l% ]
Figure 3.12 shows the results of the nitroxidation of toluene in the presence
of gaseous oxygen over Cu(ie,4.3)/NaY. The catalytic performance was
compared to the ammoxidation of toluene using the same conditions. In
this reaction NH3 is present instead of NO. Though the activity towards
toluene conversion is comparable for both reactions significant amounts of
benzonitrile can only be achieved under ammoxidation conditions. In the
presence of oxygen toluene is combusted mainly to CO and CO2.
100
80
60
X (am m oxidation)
X (nitroxidation)
40
S(am m o xidatio n)
20
S(nitroxidation)
0
400
420
440
460
T e m p e ra tu re [°C ]
Figure 3.12: Toluene nitroxidation and ammoxidation over
Cu(ie,4.3)/NaY. WHSV = 0.84; T:N:O= 1: 5: 8.
Also other catalysts show very low benzonitrile selectivities as is shown in
Table 3.8. Though the reaction is not optimised towards temperature or
62
Screening of new toluene ammoxidation catalysts
reactant concentration the benzonitrile production from NO is significantly
lower than from NH3. Only Mo(s,11.8)/NaY shows benzonitrile yields that
are not negligible.
Table 3.8: Toluene nitroxidation in the presence of oxygen at 400 ° C.
Catalyst
Toluene conversion [mol%]
Benzonitrile Selectivity [mol%]
Cu(ie,4.3)/NaY
Mn(s,3.4)/NaY
Mo(s,11.8)/NaY
Mo(im,9.8)/Al
37
100
75
24
3
0
21
22
WHSV = 0.8 gtoluene/(gcat∙ hr); Toluene : NO : O2 = 1 : 5 : 8.
C o n v e rs i o n , S e l e c ti v i ty
[m o l% ]
If the oxygen concentration is optimised with respect to benzonitrile
selectivity, still only 30 % benzonitrile selectivity is obtained as shown in
Figure 3.13.
100
X (T o lu en e)
80
S (B en z o n itrile)
60
40
20
0
5
:1
.7
5
:3
.3
5
:5
5
:6
.7
5
:8
.3
5
:1
0
5
:1
1.
7
5
:1
3.
3
N O : O 2 m o l a r ra ti o ( T o l u e n e = 1 )
Figure 3.13: Toluene nitroxidation over Mo(s,11.8)/NaY as a function of
O2 concentration. T = 420 ° C; WHSV = 0.8; Toluene: NO= 1: 5.
From the nitroxidation results it is clear that benzonitrile formation from
NO can occur. Benzonitrile yields over catalysts that are active in toluene
ammoxidation are very low. Those catalysts that are very selective in
toluene ammoxidation do not show significant benzonitrile production
from NO. In the presence of oxygen mainly toluene combustion occurs.
Similarly Pajonk found that those catalysts active in toluene nitroxidation
did not show very high benzonitrile yields [48].
63
Chapter 3
4.
Conclusions
The toluene ammoxidation can be catalysed by zeolitic and by γ-alumina
supported metal oxide catalysts. Among the zeolite-based samples, both
incipient wetness and CVD methods can be applied. Although the
benzonitrile yield can be greatly improved by using ion exchange
techniques, the initial stability during time on stream is dramatically
lowered for Cu(ie,4.3)/NaY and Co(ie,4.4)/NaY. In spite of this
deactivation, still high benzonitrile yields can be obtained over
Cu(ie,4.3)/NaY after reaching steady state conditions. Mn(s,3.4)/NaY and
V(im,1.9)/NaY showed relatively high benzonitrile yields under optimum
conditions (T= 360 ° C). Better stability is obtained over γ-alumina
supported catalysts. Especially Mo(im,9.8)/Al shows good performance,
mainly because of the higher selectivity towards benzonitrile at high
conversion levels. The benzonitrile yield can be improved by high
temperature pre-treatment of Mo(im,9.8)/Al. The benzonitrile yield over
this catalyst remains high over a broad temperature range for this catalyst. V
addition can increase the activity without loss of benzonitrile selectivity. For
V(im)/NaY catalyst the selectivity can be increased by doping the catalyst
with a second metal, but the activity is decreased significantly upon dopant
addition. Surface NH4NO2 or NH4NO3 are not selective intermediates for
the ammoxidation of toluene, as shown by the low benzonitrile selectivities
in the reaction of toluene, NO and oxygen.
References
G.V. Smith, F. Notheisz, Heterogeneous Catalysis in Organic Chemistry,
Academic Press, 1999, San Diego, p. 71-79.
2. R.G. Rizayev, E.A. Mamedov, V.P. Vislovskii, V.E. Sheinin, Appl.
Catal. A., 83, (1992), 103-140.
3. T. Kudo, Chem. Econ. Eng. Rev., 2, (1970), 43-47.
4. D.J. Hadley , D.G. Steward, UK Patent 809 704, (1959).
5. Y. Ueda, M. Saito, N. Fujita, O. Mori, German Patent 1 809 795, (1970).
B.V. Surorov, A.D. Kagarlitskii, A.I. Loiko, UK Patent 1317064, (1973).
Th. Lussling, H. Schaefer, German Patent 2 039 497, (1973).
6. Socony Mobil Oil Co., Inc., UK Patent 956 892. (1964).
7. G. Busca, F. Cavani, F. Trifirò, J. Catal., 106, (1987), 471-482.
8. J. Haber, M. Wojciechowska, J. Catal., 110, (1988), 23-36.
9. M. Niwa, M. Sago, H. Ando Y. Murakami, J. Catal., 69, (1981), 69-76.
10. M. Sanati, A. Andersson, J. Mol. Catal., 59, (1990), 233-255.
11. C. Paparizos, W.G. Shaw, US Patent 5 183 793, (1993).
12. D.J. Hadley, D.G. Steward, UK Patent 902 880, (1962).
1.
64
Screening of new toluene ammoxidation catalysts
13. P. Cavalli, F. Cavani, I. Manenti, F. Trifirò, Ind. Eng. Chem. Res., 26,
(1987), 639-647.
14. A.B. Azimov, V.P. Vislovskii, E.A. Mamedov, R.G. Rizaiev, J. Catal.,
127, (1991), 354-365.
15. M. Niwa, H. Ando, Y. Murikami, J. Catal., 70, (1981), 1-13.
16. M. Sanati, A. Andersson, L.R. Wallenberg, B. Rebenstorf, Appl. Catal.,
106, (1993), 51-72.
17. A. Andersson, S.L.T. Anderson, G. Centi, R.K. Grasselli, M. Sanati, F.
Trifirò, Appl. Cat. A, 113, (1994), 43-57.
18. A. Martin, Y. Zhang, H.W. Zanthoff, M. Meisel, M. Baerns, Appl. Catal.
A, 139, (1996), L11-16.
19. A. Martin, B. Lücke, Catal. Today, 32, (1996), 279-283.
H. Berndt, K. Büker, A. Martin, S. Rabe, Y. Zhang, M. Meisel, Catal.
Today, 32 (1996) 285-290.
Y. Zhang, A. Martin, H. Berndt, B. Lücke, M. Meisel, J. Mol. Catal. A.,
118, (1997), 205-214.
A. Martin, Y. Zhang, M. Meisel, React. Kinet. Catal. Lett., 60, (1997), 38.
20. S.H. Kim, H. Chon, Appl. Catal., 85, (1992), 47-60.
21. K. Beschmann, L. Riekert, Chem. Eng. Tech., 65, (1993), 1231-1232.
22. J.C. Oudejans, Zeolite Catalysts in some Organic Reactions, PhD.
Thesis, Delft University of Technology, 1984, pp. 72-106.
D. Fraenkel, J. Mol. Catal., 51, (1989), L1-7.
23. J. Fu, I. Ferino, R. Monaci, E. Rombi, V. Solinas, L. Forni, Appl. Catal.
A., 154, (1997), 241-255.
24. F. Cavani, F. Trifirò, P. Jirů, K. Habersberger, Z. Tvaružková, Zeolites, 8
(1988), 12-18.
25. Z. Tvaružková, G. Centi, P. Jirů, F. Trifirò, Appl. Catal., 19, (1985), 307315.
26. S.J. Kulkarni, R. Ramachandra Rao, A.V. Rama Rao, L. Guczi, Appl.
Catal. A, 139, (1996), 59-74.
27. G. Centi, F. Fazzini, J.L.G. Fierro, M. Lopè z Granados, R. Sanz, D.
Serrano, in: Preparation of Catalysts VII, Eds. B. Delmon, P.A. Jacobs,
R. Maggi, J.A. Martens, P. Grange, G. Poncelet, Stud. Surf. Sci. Cat.,
Vol. 118, Elsevier, Amsterdam, 1998, pp. 577-591.
J.S. Yoo, J.A. Donohue, M.S. Kleefish, P.S. Lin, S.D. Elfine, Appl.
Catal. A., 105, (1993), 83-105.
J.S. Yoo, Catal. Today, 41, (1998), 409-432.
28. A. Martin, H. Berndt, M. Lücke, M. Meisel, Topics in Catal., 3, (1996),
377-386.
29. Y. Li, J.N. Armor, Chem. Commun., 20, (1997), 2013-2014.
Y. Li, J.N. Armor, J. Catal., 173, (1998), 511-518.
Y. Li, J.N. Armor, J. Catal., 176, (1998), 495-502.
Y. Li, J.N. Armor, Appl. Catal. A., 183, (1999), 107-120.
Y. Li, J.N. Armor, Appl. Catal. A., 188, (1999), 211-217.
J.N. Armor, Y. Li, Ammoxidation of Ethane to Acetonitrile: Substantial
Differences in Y vs. Dealuminated Y Zeolite, Proc. 16th meeting of the
North American Catalysis Society, 1999, Boston, p. 161.
65
Chapter 3
30. I. Peeters, A.W. Denier van der Gon, M.A. Reijme, P.J. Kooyman, A.M.
de Jong, J. van Grondelle, H.H. Brongersma, R.A. van Santen, J. Catal.,
173, (1998), 28-42.
31. H. Koller, A.R. Overweg, R.A. van Santen, J.W. de Haan, J. Phys.
Chem. B., 101, (1997), 1754-1761.
H. Koller, A.R. Overweg, L.J.M. van de Ven, J.W. de Haan, R.A. van
Santen, Microp. Mat., 11, (1997), 9-17.
32. Y. You-Sing, R.F. Howe, J. Chem. Soc., Faraday Trans. I, 82, (1986),
2887-2896.
S. Özkar, G.A. Ozin, K. Molbs, T. Bein, J. Am. Chem. Soc., 112, (1990),
9575-9586.
33. Ullmanns Encyclopedia of Industrial Chemistry, 6th electronic release
edition, Wiley, 1999.
34. P. Cavalli, F. Cavani, I. Manenti, F. Trifirò, Catal. Today, 1, (1987), 245255.
35. Y. Murakami, M. Niwa, T. Hattori, S. Osawa, I. Igushi, H. Ando, J.
Catal., 49, (1977), 83-91.
M. Niwa, H. Ando, Y. Murakami, J. Catal., 49, (1977), 92-96.
36. R. Ramachandra Rao, S.J. Kulkarni, M. Subahmanyam, A.V. Rama
Rao, Zeolites, 16, (1996), 254-257.
37. R.K. Grasselli, H. Garfield, US Patent 3 325 504, (1967).
38. (Montecatini Edison S.p.a.), French Patent 1 491 104, (1966).
39. J. Zhu, S.L.T. Andersson, Appl. Catal., 53, (1989), 251-262.
40. J. Haber, M. Wojciechowska, J. Catal., 110, (1988), 23-36
41. Y. Zhang, A. Martin, H. Berndt, B. Lücke, M. Meisel. J. Mol. Catal. A.,
118, (1997), 205-214.
42. G. Centi, S. Perathoner, Catal. Rev. – Sci. Eng., 40, (1998), 175-208.
43. G. Centi, S. Perathoner, Appl. Catal. A., 124, (1995), 317-337.
44. N.W. Hayes, W.Grunert, G.J. Hutchings, R.W. Joyner, E.S. Shpiro,
Appl. Catal. B., 6, (1995), 311.
C. Li, K.A. Bethke, H.H. Kung, M.C. Kung, J. Chem. Soc., Chem.
Commun., (1995), 813.
45. G. Busca, L. Lietti, G. Ramis, F. Berti, Appl. Catal. B., 18, (1998), 1-36
and references therein.
46. N.I. Ilchenko, Russ. Chem. Rev., 45, (1976), 1119-1134.
47. S. Abournadasse, G.M. Pajonk, S.J. Teichner, in: Heterogeneous
Catalysis and Fine Chemicals, Eds. M. Guisnet, J. Barrault, C.
Bouchoulle, D. Duprez, C. Montassier, G. Pé rot, Stud. Surf. Sc. Catal.,
Vol. 41, Elsevier Science Publishers, Amsterdam, 1988, pp. 371-378.
M. Belousov, S.B. Grinenko, in: New Developments in Selective
Oxidation, Eds. G. Centi, F. Trifirò, Stud. Surf. Sc. Catal., Vol. 55,
Elsevier Science Publishers, Amsterdam, 1990, pp. 3239-246.
48. G.M. Pajonk, in: New Developments in Selective Oxidation, Eds. G.
Centi F. Trifirò, Stud. Surf. Sc. Catal., Vol. 55, Elsevier Science
Publishers, Amsterdam, 1990, pp. 229-236.
49. S. Zine, A. Ghorbel, in: Heterogeneous Catalysis and Fine Chemicals II,
Eds. M. Guisnet, J. Barrault, C. Bouchoule Stud. Surf. Sci. Catal., Vol.
59, Elsevier Science Publishers, Amsterdam, 1991, pp. 455-462.
50. R. Prasad, A. Garg, S. Mathews, Can. J. Chem. Eng., 72, (1994), 164166.
66
Chapter 4
Faujasite encaged metal oxide toluene ammoxidation
catalysts prepared from metal carbonyl precursors
Abstract
Metal oxide loaded faujasite catalysts were prepared by deposition of metal
carbonyl compounds into the pores of a faujasite zeolite host. Mo(CO)6,
V(CO)6, Mn2(CO)10 and Co(CO)3NO catalyst precursors were used as metal
guests. Removal of the CO ligands under oxidative conditions led to the
oxidation of the transition metal. It was indicated by XPS that the resulting
Mo species remained in the faujasite supercage. Transmission Electron
Microscopy indicated that the Mo oxide clusters were well dispersed
throughout the NaY zeolite host. The cluster diameter was limited by the
size of the faujasite supercage, which is 13.4 Å. The introduction of
Mn2(CO)10 led to the deposition of manganese oxide clusters on the exterior
zeolite surface due to the low volatility of this compound. After contacting
the V(CO)6 guest into the NaY host immediate and uncontrolled oxidation
takes place as soon as the catalyst precursor is exposed to an oxygencontaining environment, as was evidenced by mass spectrometry. For Mo
loaded NaY it was shown by XPS that the electrons of the carbonyl group
interact with the Na+-ion. The effect of the faujasite extra-framework cation
on the Mo(CO)6 guest was examined by ion exchange of Na+ with a series
of alkali ions. It was shown by temperature programmed desorption that the
host–guest interaction between Mo(CO)6 and the alkali cation is strongest
for the least basic faujasite sample. For this purpose the basicity was
successfully determined using the decomposition reaction of 2-methyl-3butyn-2-ol. Toluene ammoxidation over alkali-exchanged catalysts showed
increased toluene conversion at higher acidity. The benzonitrile selectivity
on the other hand is enhanced at higher catalyst basicity, indicating
formation of the selective intermediate by heterolytic C-H rupture with
formation of a carbanion.
1.
Introduction
The introduction of high valent metal ions into zeolites via ion exchange is
impossible, due to insufficient stabilization of highly charged cations inside
the zeolite lattice. Application of conventional impregnation techniques
also is not preferable in most cases, because of the restricted size of the
zeolite pores. During impregnation oxy-anionic or neutral complexes are
67
Chapter 4
formed. These complexes cannot penetrate the zeolite pores in the presence
of water [1]. Additionally the presence of a significant amount of moisture
in the zeolite pores complicates well-defined incipient wetness
impregnation. Sublimation of volatile metal carbonyl compounds offers the
possibility to introduce these metals in a straightforward manner. The
process to produce heterogeneous catalysts by this technique requires two
basic steps: 1) volatilization and deposition of the organometallic
compound inside the zeolite cavities, and 2) decomposition of the
entrapped structure. The first step can be performed by applying a static
method in which the zeolite host is mixed with stoichiometric amounts of
the metal carbonyl. Upon heating the metal carbonyl compound sublimes
and disperses throughout the zeolite pores. Also a flow method can be
applied to introduce the metal carbonyl. By this flow method a metal
carbonyl containing vapor flow is admitted to the zeolite matrix [2].
Only the static method of metal carbonyl introduction was applied in the
experiments described in this chapter. This method can be controlled more
accurately with respect to catalysts loading. The zeolite matrix can be
loaded only with the saturation loading of the metal carbonyl compound
when the flow is applied. Additionally, the time of metal carbonyl vapor
exposure until saturation is reached is quite long, approximately 10 hours
[3]. Mo(CO)6 uptake in static experiments, on the other hand, occurs
within 20 minutes [4]. The equilibrium metal carbonyl loading equals two
metal hexacarbonyl molecules per supercage, as shown by gravimetric
analysis by several groups [3,5-9]. It is generally accepted that this
saturation loading applies to all metal hexacarbonyls. Özkar et al. [10]
indeed found the same saturation loading of 2 metal hexacarbonyl
complexes per supercage for Cr(CO)6, W(CO)6 and Mo(CO)6. The metal
loading can be increased by repeating metal carbonyl introduction after
decarbonylation. As was shown by Asakura et al. [3,11] two Mo(CO)6
molecules are introduced into NaY in each additional Mo(CO)6 deposition
sequence up to a loading of eight Mo atoms per faujasite supercage. On the
other hand Yong and Howe report Mo loadings up to 10 Mo atoms per
supercage [12].
Activation of this metal carbonyl loaded zeolite can occur in different ways,
in order to produce intra-zeolite, highly dispersed (zero-valent) metal sites,
metal oxides or metal sulfides, depending on the treatment. This activation
68
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
treatment can occur thermally or by means of UV irradiation. The latter
involves lower temperatures needed for catalyst activation. In contrast to
ion-exchange techniques with subsequent reduction by hydrogen, the
production of highly dispersed zero-valent metal sites is not accompanied
by the generation of protons, which would lead to the formation of a bifunctional catalyst. Recently, the absence of acidic protons has been shown
elegantly by Ugo et al., who found a 100 % selective methylcyclopentane
ring opening reaction using a real mono-functional Pd-Y catalyst prepared
by this technique [13].
In literature the formation of intra-zeolite metal or metal sulfide catalysts
has been studied most frequently. Zeolite encaged metal catalysts show
promising performance in hydrogenation reactions [14,15]. Zeolite encaged
molybdenum sulfide catalysts have been studied very extensively for
hydrodesulfurization reactions [9,16-18]. To date metal oxides occluded in
zeolite pores on the other hand have been studied mainly for the
production of well-defined semi-conductors [19]. Therefore, the research
described in this chapter focuses on the preparation of intra-zeolite metal
oxides. The properties of the zeolite host can be adjusted with respect to
pore structure and size. Moreover, the acid-base properties of the zeolite
host can be effectively tuned by means of exchange of the Na+-ions ions.
Therefore, these systems are very interesting from the fundamental point of
view.
Since the introduction of metal carbonyl vapor into the pores of faujasite
catalyst is well controlled these catalysts can be characterized quite well.
Uniform distribution of the metal carbonyl vapor is generally achieved and
the motion of these metal carbonyl compounds has been described well in
the literature. The introduction of metal carbonyl compounds can be
applied to several transition metals, which (potentially) act as catalysts in a
wide range of chemical reactions. The most frequently used metal carbonyl
compound is Mo(CO)6, due to its relative ease of handling and low price;
see for example [12,17,20,21]. Also Fe(CO)5, Cr(CO)6, Co2(CO)8,
Co(CO)3NO, Ni(CO)4 and W(CO)6 have been used formerly by several
groups [22-26]. Few articles have been published about the use of
Mn2(CO)10 precursors for the production of Mn loaded zeolite Y [13,27].
69
Chapter 4
Based on pore dimensions,
zeolite Y has been used as
metal carbonyl host most
frequently.
The
kinetic
diameters of metal hexaSI’
carbonyls are in the range of
SII
5.3-5.5 x 7.4-7.6 Å, just fitting
supercage
into the pores of zeolite Y
SI
[28]. The structure of faujasite
contains supercages, which
have an approximate diameter
of 13.4 Å. A part of the
faujasite structure, containing
hexagonal prism
one supercage is sketched in
sodalite cage
Figure 4.1. Three different
or β-cage
cation positions are indicated
with SI, SI’ and SII. The SII
Figure 4.1: Structure of Zeolite Y.
cations are located in the
supercage. Mo(CO)6 is stabilized in these supercages as was shown by
several authors using different characterization techniques. [4,18,29-31]
Although other zeolites contain large cages that might be able to stabilize
the metal carbonyl compound, the cage entrance is too small to enable
diffusion of the metal carbonyl complex. Therefore, the amount of
Mo(CO)6 introduced in these zeolites is significantly smaller; the Mo(CO)6
uptake is lower than 1 Mo atom per unit cell [32]. Nevertheless, Zeolite X,
ZSM-5, Zeolite L and Mordenite have been used also as metal carbonyl
hosts, but not as frequently as Zeolite Y. The first publications that deal
with metal carbonyl introduction onto a catalyst support describe
introduction of Mo(CO)6 onto alumina [33]. Metal hexacarbonyl
introduction to silica supports appeared to be only possible for very low
metal loadings [34], whereas other supports (MgO, TiO2) have been used
successfully [35]. Gallezot et al. [36] were the first to adsorb metal carbonyl
substrates onto a zeolitic support. They used a HY support for adsorption
and decomposition of Mo(CO)6, Re2(CO)10 and Ru3(CO)12. Adsorption
occurred by sublimation of the metal carbonyl at moderate temperatures
(60 to 120 ° C, depending on the metal carbonyl used). Decarbonylation
was performed by heating the samples in a closed system. All ligands were
removed gradually at temperatures below 300 ° C for Mo(CO)6.
70
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
The acidity and basicity of oxidation catalysts can influence severely the
selectivity in vapour phase mild oxidation reactions. Generally acidic sites
enhance the formation of deeper oxidation products such as acids and
anhydrides [37]. Ion exchange of Na+-ions for other alkali ions into
MoNaY offers the unique possibility to examine the influence of the
catalyst acidity and basicity on the toluene ammoxidation reaction.
The research described in this chapter focuses on the preparation and
characterization of different zeolite encaged metal oxides. The acidity of
the zeolite NaY host was varied by exchange of Na+ for a series of alkali
cations. Alcohol decomposition reactions were applied to determine the
acid-base properties of these catalysts.
2.
Materials and methods
2.1
Catalyst preparation
For all experiments zeolite NaY (Akzo, unit cell: Na55(AlO2)55(SiO2)137) was
used. In some cases the Na+-ions were exchanged with other alkali cations
by ion exchange. For this purpose alkali-metal nitrates were used. Batches
of 10 gram NaY were suspended in 600 ml 1 M alkali-metal nitrate
solution. Ion exchange was performed during 24 hours at 60 ° C while
continuously stirring the suspension. After separation by centrifuging at
4800 rpm ion exchange was repeated twice using fresh metal nitrate
solutions, according to the same procedure. After the third ion exchange
sequence the samples were washed three times and dried in ambient air at
60 ° C. For the exchange of Na+ by H+, NH4NO3 was applied. After
finishing ion exchange, the resulting NH4/Y was treated in N2 flow (300
ml/min) in order to remove NH3. The heating rate was 5 ° C per minute.
The temperature was kept at 110 ° C for one hour in order to remove
adsorbed water. After heating the catalyst at 510 ° C for 60 minutes the
temperature was reduced to 250 ° C and the flow was changed to N2/O2
(80%/20%, 300 ml/min). Then the temperature was increased with 5 ° C
per minute to 500 ° C and kept at this temperature for 20 minutes to
remove NH3 traces.
Co, Mn, Mo and V were introduced into the zeolite matrix by deposition of
metal carbonyl vapors. Co(NO)(CO)3, Mn2(CO)10, Mo(CO)6 and V(CO)6
71
Chapter 4
were applied as transition metal sources. Prior to deposition of the metal
carbonyl vapor the zeolite matrix was pelleted, crushed and sieved into a
sieve fraction of 250-425 µm. Batches of zeolite (1 gram) were carefully
dehydrated under continuous evacuation (p= 1∙ 10-2 mbar). The
temperature was raised with 1 ° C per minute to 400 ° C. The temperature
was kept at 400 ° C overnight, still under dynamic vacuum. The dehydrated
zeolite was then mixed with the metal carbonyl in nitrogen atmosphere,
without exposure to ambient air in between. The maximum amount of
metal carbonyl was kept equivalent to two transition metal atoms per
supercage, the saturation loading for NaY zeolites [12,38] in order to
exclude deposition at the outer zeolite surface. The metal carbonyl was
sublimated and diffused through the zeolite pores by heating the obtained
Mx(CO)y/NaY mixture in a closed ampoule (which was kept under static
vacuum; p= 1∙ 10-2 mbar) to 60 ° C. This ensured introduction of Mo(CO)6
into the zeolite supercages as described by Koller et al. [39]. The catalyst
was then cooled to room temperature and transferred to a tubular reactor,
still without exposing the sample to ambient air. CO ligands were
subsequently removed from the catalyst precursor by heat treatment as
described in Section 2.2.4.
The catalysts and catalyst precursors will be denoted according to the
following notation. Zeolite encaged metal carbonyls (Mex(CO)y) will be
indicated as Mex(CO)y/NaY, zeolite encaged decarbonylated zero-valent
metal as Me0/NaY and zeolite encaged oxidized metal as MeOx/NaY. The
catalysts will be indicated simply as Me/NaY when their oxidation state is
not discussed.
2.2
Catalyst characterization
2.2.1 Determination of the catalyst composition
The alkali-metal and transition-metal content of the catalysts was
determined by Atomic Absorption Spectroscopy, using a Perkin-Elmer
3030 Atomic Absorption Spectrophotometer. Destruction of the catalyst by
hydrochloric acid was used to dissolve the alkali metals. Prior to the
analysis the catalysts were destroyed and dissolved in sulfuric acid. The Mo
content was analyzed using the standard addition method.
72
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
2.2.2 X-Ray Photoelectron Spectroscopy
X-Ray Photoelectron Spectroscopy (XPS) was applied to determine the
catalyst surface composition. A VG ESCALAB 200 spectrometer equipped
with an Al Kα X-ray source and a hemispherical analyzer was applied for
the XPS experiments. Prior to recording of the spectra the samples were
ground and pressed on an indium film placed on an iron stub. The binding
energies were corrected for charging assuming a binding energy of 102.8 eV
for the Si 2p peak [22,40]. Charging was on the order of 6 eV. The data
were analyzed by a standard fit routine using a non-linear Shirley
background subtraction and a Gauss-Lorentzian curve-fit function.
Mo(CO)6/NaY catalysts precursors were transferred to the XPS chamber
without exposure to ambient conditions by mounting the sample on the
stub in a nitrogen filled glove box. The surface amounts of Na, Al and Si
were estimated using the atomic sensitivity factors determined by Wagner
et al. [41].
2.2.3 Transmission Electron Microscopy
Transmission Electron Microscopy (TEM) was performed using a Philips
CM 30 ST electron microscope, equipped with a LaB6 filament as electron
source, operated at 300 kV. The catalyst was mounted on a microgrid,
carbon polymer supported on a copper grid, by suspending the powdered
catalyst in ethanol and placing a few droplets of this suspension on the grid.
Drying was performed prior to the experiment under ambient conditions in
order to remove the ethanol. Several grains of the sample were analysed in
order to obtain a representative image of the sample, without focusing on
possible artefacts present.
EDX analysis was performed using a LINK EDX system by turning the
electron beam towards the detector. This enabled us to obtain information
about the chemical composition of a small part of the TEM image.
2.2.4 Temperature Programmed Oxidative Decarbonylation
Decarbonylation of the Mex(CO)y/NaY precursors was performed under
well-controlled conditions. 1 Gram batches of the catalyst precursor were
taken and heated while flowing a He or He/O2 gas flow (80%/20%, 60
Nml/min) through them. The heating rate was 5 ° C per minute. When
activation was performed in He, the resulting Me0/NaY was oxidized by
73
Chapter 4
subsequently flowing He/O2 (80%/20%, 60 Nml/min) through it at a
heating rate of 5 ° C/min. During the activation treatment the evolving
gases were analysed using a Balzers QMG-420 quadruple mass
spectrometer operated at an ionisation potential of 70 eV and an inlet
pressure of 1.0∙ 10-5 mbar. Prior to some of the experiments the CO signal
was calibrated using a He/CO feedstock with known CO concentration in
order to quantify the amount of evolved CO. To compensate for variations
of the inlet pressure a known large quantity of He was added to the reactor
effluent. In all cases the mass spectrometer signals were normalized to He.
2.3
Catalytic tests
2.3.1 2-MethylMethyl-3-butynbutyn-2-ol decomposition
To probe the catalyst acid-base features, decomposition of 2-methyl-3butyn-2-ol (MBOH) and 2-propanol was performed in a pulse micro reactor
at 150 ° C and 300 ° C. 0.5 µl Pulses were admitted to the catalyst and the
reaction products were analysed using a 3 meter Porapak Q GC column.
The catalysts were activated in situ at 350 ° C under an Ar flow (30
ml/min) prior to performance of the alcohol decomposition reactions. First
order rate constants were calculated as described elsewhere [42]. As will be
described in more detail in Section 3.1.4 the yield of 3-methyl-3-buten-1yne (mbyne) was measured to probe the catalyst acidity. The dehydration
function (DHD-value) is defined as the yield of mbyne/total conversion of
MBOH. On basic sites, the molecule decomposes to acetone and acetylene.
The dehydrogenation function (DHG-value) is defined similarly as the
yield of acetone and acetylene/total conversion of MBOH.
2.3.2 Toluene ammoxidation
The ammoxidation reaction of toluene was performed using a single-pass
tubular reactor (4 mm internal diameter) operated under plug flow
conditions. NH3, O2 and He (as an inert diluent) were controlled using
Brooks Thermal Mass-flow Controllers. Part of the He flow was saturated
with toluene (p.a.) using a three-step saturator that was kept at 9.4 ° C. No
purification of the gases was found necessary. All lines after the catalyst
bed were kept at a temperature of 200 ° C in order to prevent condensation
of products. Detection of the organic products was performed by on-line
gas chromatography, using a Hewlett Packard 5890 series II GLC,
74
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
equipped with a 50 m HP-5 column and a flame ionisation detector.
Conversions, selectivities and benzonitrile production rates were calculated,
based on the toluene inlet signal, which was measured before starting the
reaction. Toluene conversion levels and selectivities towards organic
products could be analysed accurately by using this method, irrespective of
a lack of carbon balance.
3.
Results and discussion
3.1
Thermal activation of intra-zeolite Mo(CO)6
The decarbonylation of the Mo(CO)6/NaY catalyst precursor in He flow is
plotted in Figure 4.2. The CO ligands gradually desorb from the catalyst
precursor during heating. The main desorption peak is observed at 237 ° C.
Mo(CO)6 melts between 147 and 149 ° C. The boiling point is 155 ° C [43].
The processes observed during heating are usually observed as sublimation,
based on the small temperature window in which melting and boiling
occur. The high CO desorption temperature that is observed in our
experiments clearly shows that Mo(CO)6 is stabilized by the zeolite host.
During preparation of the NaY-Mo(CO)6 mixture a slow color change from
clear white to pale yellow was observed. This indicates that some
decarbonylation occurred during preparation of the catalyst precursor.
During decarbonylation the sample colour changes from pale yellow to
black, indicating the removal of all CO ligands. Small amounts of H2 are
observed between 200 ° C and 250 ° C, indicating the consumption of traces
of OH-groups present in the faujasite structure. The amount of H2,
however, is very small. Earlier, the reaction of adsorbed Mo(CO)6 with
surface hydroxyl groups was reported for alumina supports by Brenner and
Burwell Jr. [44]. These authors found that hydroxylated alumina was able
to accommodate more Mo(CO)6 than dehydroxylated alumina. Nakamura
et al. [45] proved that metallic Mo could be obtained by loading completely
dehydroxylated alumina. More recently the interaction of σ-OH with
adsorbed Mo(CO)6 was described by Jaenicke and Loh, who report lower
stability of adsorbed Mo(CO)6 in the presence of surface hydroxyls [46].
For faujasite based catalysts it was reported by several authors that surface
hydroxyl groups of HY are able to oxidize Mo. The average Mo oxidation
number increases with increasing concentration of hydroxyl groups [15] in
the host structure. XPS experiments verified the increase of the Mo
75
Chapter 4
N o r m a li sed M S i n ten si ty [ a. u . ]
oxidation number [7]. The slightly lower temperature of the main H2 peak
compared to the main CO desorption peak agrees with this. Thermal
desorption occurs during TPD, but also trace OH-groups oxidize the zerovalent Mo atom.
m/e = 28: CO
m/e = 44: CO2 (·100)
m/e = 2: H2 (·100)
0
100
200
300
400
500
T em perature [°C]
Figure 4.2: Temperature programmed decarbonylation of Mo(CO)6/NaY.
Temperature ramp 5 ° C/min; He flow (40 Nml/min).
Since the faujasite host structure is in the sodium form, the H2 amount in
our experiments was very low, more than 100 times smaller than the
amount of desorbed CO. Therefore, the reaction of hydroxyl groups with
adsorbed Mo(CO)6 can be neglected in our experiments. CO2 production
was observed in two discrete desorption steps at 80 ° C and 237 ° C.
Formation of CO2 in the absence of O2 can be explained by occurrence of
the CO disproportionation reaction (Equation 4.1). For Pt/SiO2 catalysts
this reaction occurs on small Pt particles [47].
2 CO
C + CO2
(4.1)
Since the dispersion of the metal carbonyl guests in the zeolite matrix is
very high, possibly this reaction also occurs on Mo(CO)6/NaY during
decarbonylation. Also for other supported metal carbonyl complexes the
occurrence of CO disproportionation was observed [48]. As was verified by
MS analysis O2 was not present in the gas mixture applied during
decarbonylation. Therefore, this CO2 production cannot be related to
oxidation of the CO ligands. Similar to the amount of H2 observed in our
experiments the CO2 evolution can be neglected in further discussion, since
the amount was less than 1 percent compared to CO.
76
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
The desorption profile in Figure 4.2 differs significantly from that reported
by Okamoto et al. [20,49]. These authors find two CO desorption peaks at
100 ° C and 266 ° C respectively upon temperature programmed
decarbonylation in vacuo. The evolution of other molecules was not
reported. Based on the desorption profile measured and on infrared data
these authors propose formation of a Mo(CO)3 subcarbonyl species as first
step during decarbonylation. However, they report a ratio of 1.3 to 1.4 for
the peak areas of the low temperature and the high temperature desorption
peak. Assuming complete removal of all six CO ligands this result would
indicate “Mo(CO)2.5” formation after the first decarbonylation step. Our
result indicates a rather slow, but continuous removal of small amounts of
CO up to 200 ° C. Consecutively, the majority of the CO ligands desorbs at
237 ° C. The formation of a stable Mo(CO)3 subcarbonyl species, therefore,
is not very likely. During decarbonylation a gradual color change from pale
yellow to black was
pale yellow
black
observed over the catalyst
bed in the direction of the
Helium
Helium, CO
gas flow. Figure 4.3
sketches the gradual
Figure 4.3: Colour change over the catalyst
colour change over the
bed during decarbonylation in He.
catalyst bed.
The absence of a second CO desorption peak may be explained by the
experimental conditions. In comparison with experiments using catalysts in
the form of thin wafers we have used a relatively large amount of catalyst.
During temperature programmed desorption the He flow becomes partly
saturated with the desorbing CO. As the proposed subcarbonyl Mo species
can be re-saturated with six carbonyl ligands upon CO addition [10,39] this
CO presence possibly leads to inhibition of formation of the highly reactive
subcarbonyl species. At higher temperature all resulting CO ligands are
irreversibly removed, leading to a quite broad desorption peak of CO. Quite
similar to our experiments Asakura et al. [50] report a single CO desorption
peak at around 200 ° C. The amount of desorbed CO was 5.5 moles per
mole Mo, indicating incomplete decarbonylation. Further heating of the
Mo(CO)6/NaYprecursor, however, did not lead to further evolution of CO.
When the sample was exposed to O2 at 400 ° C after decarbonylation, the
remaining CO ligand was desorbed as CO2. Temperature Programmed
Oxidation (TPO) of decarbonylated Mo0/NaY shows that in addition to
77
Chapter 4
the consumption of O2, the remaining CO ligands are removed as CO and
CO2 at temperatures between 25 and 300 ° C. TPO results are plotted in
Figure 4.4.
N o r m a lise d M S I n ten si ty
O2
CO (•10)
0
100
CO 2 (•10)
200
300
T em p er a tu r e [ ° C ]
400
500
Figure 4.4: TPO of Mo0/NaY. He/O2 flow (80%/20%; 60 Nml/min).
The CO and CO2 production observed during oxidation can also result
from carbon fragments adsorbed on the catalyst surface. As was explained
above, C was probably formed during decarbonylation according to the
Boudart disproportionation reaction. CO2 production indeed was observed
during decarbonylation in He. The C formed was oxidized by O2 as shown
in Figure 4.5. The C oxidation probably was catalyst by Mo, since the
reaction occurred at relatively low temperature. The Boudart reaction has
been proposed also by Brenner and Hucul to account for CO2 production
during decarbonylation of W(CO)6/γ-alumina catalysts [51]. These authors
examined a broad range of W(CO)6/γ-alumina catalysts and found similar
broad CO desorption patterns as we did for Mo(CO)6/NaY.
Figure 4.5 shows the oxidative decarbonylation of Mo(CO)6/NaY. During
heating of the catalyst precursor only the evolution of CO was observed.
The signal at m/e= 44 was assigned to the formation of CO2 at the mass
spectrometer filament, as was confirmed by calibration experiments using
CO. Simultaneously with the evolution of CO from the catalyst
consumption of O2 was observed. Mo is oxidized during decarbonylation.
Mo oxidation was confirmed by the colour change of the sample. A white
colour was assigned to oxidized Mo. Asakura et al. [3] observe CO
desorption at 473 K when applying heat treatment in vacuo. The sample
78
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
colour turned from pale yellow to black, as was expected since Mo is in
valence state zero in Mo(CO)6. Oxidation of the Mo thus facilitates the
decarbonylation of Mo(CO)6.
0.6
0.1
0.4
0.0
m / e = 28
0.2
-0.1
m / e = 44
0.0
I n t e n sit y [a .u .]
I n t e n sit y [a .u .]
m / e = 32
-0.2
0
100
200
300
400
500
T im e [m in ]
Figure 4.5: Temperature Programmed Oxidative Decarbonylation of
Mo(CO)6/NaY. He/O2 flow (80%/20%; 60 Nml/min).
The Temperature Programmed Decarbonylation (TPOD) experiments that
were described above can be summarized in Scheme 4.1.
Mo(CO)6 + NaY
Mo(CO)6/NaY
MoNaY
mixing
He, 100 °C
(6-x) CO
O2, 300 °C
Mo(CO)6/NaY
(1)
Mo(CO)6/NaY
Mo(CO)x/NaY
He, 240 °C
Mo0/NaY + (6 [-x]) CO
(2)
(3)
MoOx/NaY
O2, 100 °C
MoOx/NaY + 6 CO
(4)
Scheme 4.1: Decarbonylation and oxidation of NaY encaged Mo(CO)6.
When decarbonylation occurs in an inert atmosphere, NaY encaged
molybdenum oxide is prepared according to steps 1-3.
1) Mo(CO)6 is mixed together with the NaY host and diffuses
throughout the pores.
2) Upon heating in He, the CO ligands are removed and a zero-valent
Mo0/NaY precursor is formed.
3) By heating this Mo0/NaY precursor in O2 to at least 300 ° C the
formation of MoOx/NaY completes.
79
Chapter 4
When O2 is present during decarbonylation, zeolite encaged molybdenum
oxide is formed according to reaction steps 1 and 4. The Mo(CO)6/NaY
precursor that is formed in step 1 is converted to zeolite encaged
molybdenum oxide in a single step. Decarbonylation and oxidation of
Mo(CO)6/NaY can be achieved at temperatures below 100 ° C in this case.
The amount of CO that evolved from the Mo(CO)6/Y catalyst precursor
was quantified for some alkali-exchanged catalysts during TPOD. Table 4.1
shows the results.
Table 4.1: TPOD of Mo(CO)6/Y catalyst precursors. Amount of CO
produced.
Catalyst
Mo(CO)6/CsY
Mo(CO)6/RbY
Mo(CO)6/NH4Y
Amount of CO [mmol]
Expected from Mo(CO)6 loading
Measured
4.6
5.2
7.0
5.0
5.4
6.4
Within the experimental error the amount of CO measured indicates
complete decarbonylation. The experimental error is estimated at around
10 %, since electrostatic charging in the glove box severely complicated
accurate sample weighing. For the NH4/Y host loss of NH3 ligands was
observed at temperatures between 360 ° C and 490 ° C. Small amounts of
NH3 were removed as N2 at 100 ° C. The CO signal was corrected for this
N2 production.
3.2
XPS analysis of Mo(CO)6/NaY and MoOx/NaY
The Mo(CO)6/NaY catalyst precursor and the decarbonylated and
oxidized MoOx/NaY sample were analyzed by XPS. Some evaporation
and premature decarbonylation of the Mo(CO)6/NaY catalyst precursor
was observed during introduction of the sample into the vacuum chamber,
as indicated by the strong increase of the pressure in the vacuum chamber.
Decomposition of bulk Mo(CO)6 is known to occur when exposed to
vacuum [52]. Mo(CO)6 sublimates readily in the range of 50-90 ° C at 1
Torr [43]. The high vacuum applied in the XPS experiments may have led
to further decrease of the sublimation temperature. Mass spectrometry
analysis showed the evolution of CO during the introduction of the sample
80
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
into the vacuum chamber. After sample introduction the sample was
allowed to stabilize and the XP spectrum was measured.
P eakfit
X P S In te n s ity
E b = 230.2 eV
E b = 233.4 eV
240
236
232
B in d in g E n e rg y [e V ]
228
224
Figure 4.6: XP Spectrum of 12 wt% loaded Mo(CO)6/NaY.
Figure 4.6 shows the Mo(3d) peak of the Mo(CO)6/NaY catalyst precursor.
The spectrum was fitted with two Mo(3d) peaks. A correct fit could not be
obtained when only one Mo(3d) peak was applied for fitting. This indicates
that Mo exists in at least two different states, although also the low peak
intensity in combination with the broad lines complicates determination of
the exact binding energy. The binding energies found for the two Mo(3d)
doublets were 230.2 and 233.4 eV. As can be seen in Table 4.2 the intensity
of the low binding energy peak was around 14 times larger than the high
binding energy peak. This small high-energy peak can be attributed to the
formation of oxidized Mo during the evaporation and premature
decarbonylation, which was observed during introduction into the vacuum.
The highly reactive decarbonylated Mo0/NaY species is probably oxidized
by trace OH-groups of the NaY metal carbonyl host. Evidence for the
presence of two Mo states can also be found in the literature, though the
authors do not always ascribe the XP spectrum to two Mo states [49]. The
poor quality of the Mo XP spectra further complicates peak assignment.
Komatsu et al. qualitatively conclude to a the shift of the Mo(3d) peak to
higher binding energies when the catalyst is briefly (4 minutes) exposed to
air [7].
81
Chapter 4
Table 4.2: XPS Binding energies of Mo/NaY catalysts and precursors.
Sample
Mo(3d)
Peak 1
Peak 2
Mo(CO)6/NaY 12 wt%
MoO
MoOx/NaY 5 wt%
MoOx/NaY 12 wt%
230.2 (93%)
232.2 (83%)
231.9 (41%)
Al(2p)
233.4 (7 %)
233.1 (17%)
233.3 (59%)
Na(1s)
74.6
74.5
74.6
1071.3
1072.2
1072.3
Binding energies were corrected for charging by normalization to the Si(2p) peak
at 102.8 eV.
Table 4.3: Comparison of Binding energies (BE) of metals and metal
carbonyls..
Metal
BEa [eV]
Metal carbonyl
BEa [eV]
Shift [eV]
Co
Cr
Fe
Mn
Ni
Mo
778.3
574.4
707.0
639.0
852.7
228.0
Co(CO)3NO
Cr(CO)6
Fe(CO)5
Mn2(CO)10
Ni(CO)4
Mo(CO)6/NaY
780.7
576.3
709.6
641.6
854.8
230.2b
2.4
1.9
2.6
2.6
2.1
2.2
a
b
data from [54] and references therein except b
this work
The binding energy of the main peak, which amounted at 230.2 eV is
assigned to Mo(CO)6. Schwartz and Hercules report a binding energy of
226.6 eV for the Mo(3d) peak [53] in bulk Mo(CO)6. These authors report a
value of 226.1 eV for metallic Mo. Except for these quite old literature
values, to our knowledge no other XP spectra of bulk Mo(CO)6 have been
reported in the literature, probably due to premature decarbonylation of the
sample in the sample XPS chamber. If we compare the binding energy
found for the main peak it is around 2 eV higher than that of zero-valent
Mo [54]. This increase of the binding energy is explained by the strong
electron withdrawing effect of the carbonyl groups present in the molecule.
For other, more stable, carbonyl compounds similar shifts in binding
energies were reported, as shown in Table 4.3. On the other hand,
Andersson and Howe [22] report a binding energy of 227.8 eV for
Mo(CO)6/NaY recorded at –100 ° C. Upon decarbonylation in vacuo at
400 ° C they find a shift in binding energy to 229.2, which was explained by
accidental oxidation during transport to the XPS chamber. The Mo(3d)
peak fit, however, was not completely correct. An interfering signal at
82
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
lower binding energy was observed as well. This peak was not included in
defining the reported binding energy.
Peakfit
X P S In te n sity
Eb= 233.3 eV
Eb= 231.9 eV
240
238
236
234
232
230
228
C o rre cte d B in din g E n e rg y [e V ]
Figure 4.7: XP Spectrum of 12 wt% loaded MoOx/NaY.
The Mo(3d) XP Spectrum of the decarbonylated MoOx/NaY sample is
shown in Figure 4.7. Again at least two Mo states were observed, since the
spectrum could not be fitted when only one Mo doublet was applied. The
binding energies were found at significantly higher values compared to the
catalyst precursor. The Mo(3d) doublets were found at 231.9 and 233.3 eV
respectively. These values show that Mo is in a high oxidation state. The
binding energy for bulk MoO3 is 232.6 eV [54 and references therein]. The
high binding energy value observed in our experiments is consistent with
the binding energy reported by other authors [7,20,22]. This shift to higher
binding energy is explained by the electron deficiency in the zeolite host
[55,56].
Table 4.4 shows the Mo:Si atomic ratio and the Mo:Na atomic ratio as
determined by XPS. These ratios are very close to the atomic ratios as
calculated from bulk composition for MoOx/NaY. No indication was
observed for possible high concentrations of Mo at the external surface of
the faujasite lattice. For the Mo(CO)6/NaY the Mo concentration
measured by XPS was much smaller. This is explained well by the
evaporation of Mo(CO)6 in the vacuum chamber as discussed above. The
actual amount of Mo in the sample, therefore, is much lower than that
83
Chapter 4
expected from the amount of Mo(CO)6 initially mixed together with the
zeolite host. Since chemical analysis of the bulk could not be performed
after performing the XPS experiment the bulk Mo concentration is strongly
overestimated.
Table 4.4: Relative XPS intensities energies of Mo/NaY catalysts.
Sample
Mo(CO)6/NaY 12 wt%
MoOx/NaY 5 wt%
MoOx/NaY 12 wt%
Mo:Si
Bulk
XPS
Bulk
XPS
0.12
0.05
0.13
0.29
0.12
0.32
0.03
0.09
0.31
0.03
0.05
0.18
Mo:Na
XPS surface Mo:Si and Mo:Na atomic ratios determined according to [41].
3.3
Dispersion of molybdenum oxide clusters in NaY
As was shown in Sections 3.1.1 and 3.1.2 Mo(CO)6/NaY catalyst
precursors can be decarbonylated at low temperature. No indication of
MoOx clustering by sintering was given. XPS experiments indicated high
Mo dispersion, since the Mo:Si ratio was close to the ratio expected from
the bulk composition of the catalyst.
Figure 4.8: TEM image of 12 wt% loaded MoOx/NaY.
Transmission Electron Microscopy (TEM) experiments were performed in
order to verify the Mo dispersion. Figure 4.8 shows the d-spacings of the
84
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
zeolite crystal, showing that the catalyst structure is not damaged during
the electron microscopy experiment.
Figure 4.9: TEM image of 6 wt% loaded Mo(CO)6/NaY.
Figure 4.10: TEM image of 6 wt% loaded MoOx/NaY.
Figure 4.9 shows the TEM image of Mo(CO)6/NaY. No Mo oxide clusters
were observed. This means that no Mo oxide clusters with a size larger
85
Chapter 4
than approximately 5 Å are present, since the detection limit of the
microscope is estimated at 5 Å. After oxidative decarbonylation metal oxide
clusters have formed as is indicated in Figure 4.10. The Mo clusters are
well dispersed throughout the sample as was verified by EDX analysis. The
Mo concentration was the same throughout the sample. Segments of the
TEM images that had an apparent higher concentration of TEM-visible
MoOx clusters did not show higher Mo concentrations than the bulk. The
size of the MoOx clusters was estimated by measuring the diameter of the
MoOx clusters visible in the microscope.
20
Frequency [-]
MoOx/NaY
15
MoOx/NaY after
ammonia treatment
10
5
0
5
7
9
12
14
16
18
21
23
25
28
Particle diam eter [Ångstrom ]
Figure 4.11: The MoOx cluster size of MoOx/NaY estimated from TEM.
Figure 4.11 shows the distribution of the MoOx cluster diameter. Since only
MoOx clusters having a diameter larger than approximately 5 Å could be
visualized by TEM, the method overestimates the cluster size. Therefore,
Figure 4.11 can only be taken as a rough estimate for the upper limit of the
MoOx cluster size. For this reason no attempts were made to give a
statistical evaluation of the MoOx cluster size measured by TEM. However,
from the XP Spectra in combination with the TEM experiments and the
low decarbonylation temperature it is clear that the MoOx clusters formed
are well dispersed. No indication was found for the presence of high
concentrations of MoOx clusters at the exterior zeolite surface. The fact that
two different Mo oxidation states were observed by XPS and the presence
of MoOx clusters larger than 14 Å for the calcined sample, however,
suggests that Mo is possibly deposited partly on the external zeolite surface.
NH3 treatment after calcination does not lead to growth of the average
86
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
MoOx cluster size. TEM does not show any indication for the presence of
significant amounts of Mo containing clusters on the external zeolite
surface after NH3 treatment either. The diameter of the MoOx clusters does
not exceed 14 Å after NH3 treatment and the Mo concentration at the edges
of crystals does not differ from the bulk Mo concentration as was verified
by EDX.
3.4
Mo(CO)6 interaction with the faujasite lattice
X P S In te n s i ty [a . u . ]
The binding energy of the Na(1s) peak at 1071.3 eV is about 1.1eV lower
than expected from the literature [40]. This shift can be explained well by
the interaction of the Na+-ion with the Mo(CO)6 guest. Donation of
electrons from the carbonyl ligands to the Na+-ion occurs and results in
lower Na(1s) binding
1071.3 eV
energies. The interaction
of adsorbed Mo(CO)6
NaY
with Na+ was discussed
earlier by Andersson and
Howe. They compared
NaX and NaY encaged
Mo(CO)6. The Mo(3d)
a
binding energy of the
NaX encaged Mo(CO)6
1072.3 eV
sample was slightly (0.4
b
eV) higher than that of
NaY encaged Mo(CO)6.
1080
1075
1070
1065
This can be explained by
the higher density of
B i n d i n g e n e r g y [e V ]
Na+-ions and thus lower
Figure 4.12: XP Spectrum of the Na(1s)
electron density around
region. a. Mo(CO)6/NaY, b. MoOx/NaY.
the
Mo
containing
23
clusters in NaX than in NaY. More convincingly, Na-NMR experiments
showed changes in the NMR shift attributed to the SII Na+-ions. W(CO)6
guest molecules interact with the Na+-ions of NaY as was shown by Ozin
and co-workers [57]. When performing 23Na DOR-NMR experiments these
authors found an increase of the intensity of the shoulder at –16 ppm upon
increase of the W loading of the zeolite host. This increase can be explained
by an increased contribution of SII Na+-ions interacting with the W(CO)6
guest. The interaction of Mo(CO)6 guests with the NaY host is quite
87
Chapter 4
similar, as shown by the same group. In our group the molecular motion of
Mo(CO)6 in NaY hosts was studied by Koller et al. [39]. They showed by
13
C-NMR that Mo(CO)6 interacts with Na+.
After oxidative treatment the Mo(3d) binding energies are the same within
the experimental error (0.2 eV). Our experiments clearly show that the
Na(1s) binding energy is affected by the Mo(CO)6 guest. After
decarbonylation the Na(1s) binding energy is almost equal to that reported
for NaY that does not contain any transition metal guests.
Figure 4.13 shows the temperature programmed desorption profiles of CO
from Mo(CO)6 loaded faujasite based catalyst precursors. The NaY mother
batch was ion exchanged with different alkali-metal nitrates. Based on the
periodic trend the electronegativity of the alkali ions applied increases in
the order HY<LiY<NaY<KY<RbY<CsY. Therefore, the strongest
interaction of the Mo(CO)6 guest with the alkali cation -and thus the
highest decarbonylation temperature- is expected for HY and for LiY.
M S I n te n s ity [ a . u . ]
M o/C sY
M o/R b Y
M o/K Y
M o/N aY
M o/L iY
M o/N H 4 Y
0
1 00
2 00
3 00
4 00
5 00
T e m p e ra tu re [ ° C ]
Figure 4.13: CO evolution during TPOD of Mo(CO)6/Y precursors.
Heating rate 5 ° C/min; He/O2 flow (80%/20%; 60 Nml/min).
To explain the deviation from this trend the base strength of the Mo loaded
samples was estimated. For this purpose alcohol decomposition reactions
were performed. It was already described in literature that the
decomposition reaction of 2-methyl-3-butyn-2-ol (MBOH) can be applied
to estimate the basic and the acidic function of the catalyst [58]. With good
results the basic strength of a wide range of catalysts was evaluated using
88
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
this reaction [59]. Meyer and Hölderich [60] recently evaluated the
decomposition of MBOH upon its ability to probe the basic strength of
alkali-containing zeolites. They found good correlation between CO2 TPD
experiments and MBOH decomposition results in probing the basicity of
NaX based catalysts. Catalysed by Lewis acids, 3-methyl-3-butene-1-yn
(mbyne) and 3-methyl-2-butenaldehyde (prenal) are formed, where basic
sites decompose MBOH into acetone and acetylene (see Scheme 4.2).
Additionally amphoteric sites (i.e. a combination of acidic and basic sites)
can catalyse the production of 3-hydroxy-2-methyl-2-butanone and 3methyl-3-buten-2-one [61]. When the selectivity towards acetylene and
acetone is high compared to all other reaction products the yield of these
reaction products can be taken as a measure for the catalyst basicity. Also
strong Lewis centres can be estimated, since Lewis acid sites catalyse the
dehydration of MBOH to 3-methyl-3-buten-1-yne [62]. The production of
3-methyl-3-buten-2-on occurs over Brønsted acid sites.
Quite similarly 2–propanol decomposition yields propylene over acidic sites
and acetone over basic sites [63]. In earlier experiments Lercher et al. [64]
however, observed only propene as desorption product. Acetone was
formed, but it was trapped in the pores of alkali-exchanged ZSM-5 as was
evidenced by infrared spectroscopy. Therefore, the decomposition of
MBOH is preferred over 2-propanol decomposition to estimate the basicity
of alkali-exchanged zeolites. When performing MBOH decomposition the
yields towards the aforementioned products can be taken as a measure for
the basic and acidic functions of the catalyst studied. Scheme 4.2 shows the
proposed reaction pathways.
Performing the decomposition of MBOH, Fouad et al. [65] found infrared
evidence for the presence of alcoholate-like species adsorbed on Cs+promoted MgO. Accordingly, desorption of acetone and acetylene was
observed at 70 ° C. For Ba2+-promoted MgO, MBOH adsorbed more
strongly to the surface via the acetylene bond. This stronger interaction
with the surface leads to surface reactions such as the polymerisation of
acetone to di-acetone or mesityl oxide, showing that not only the reaction
products mentioned in Scheme 4.2 are important, but also these
polymerisation products that may cause a strong deactivating effect in the
MBOH decomposition reaction.
89
Chapter 4
O
H3C
Lewis basic
pathway
CH3
+
acetone
HC CH
acetylene
CH3
H3C
H
OH
2-methyl-3-butyn-2-ol
(MBOH)
Lewis acid
pathway
H2C
CH
H3C
Amphoteric
pathway
Bronsted
acid pathway
CH3 O
H3C
OH
CH3
3-hydroxy-3-methyl2-butanone
H3C
+
H3C
3-methyl-3-buten-1-yn
(mbyne)
H
C C
H
O
3-methyl-2-butenaldehyde (prenal)
H3C
O
H2C
CH3
3-methyl-3-buten-2-one
Scheme 4.2: Determination of acidic and basic functions by MBOH
decomposition.
Figure 4.14 shows the acidic and basic function of alkali-exchanged NaY.
The values were corrected for the ion-exchange level, which was
determined by Atomic Absorption Spectroscopy, according to Equation
4.2. The exchange level varied between 69 and 91 percent. This is slightly
higher than those reported in related experiments [4,18]. The exchange
levels are shown in Table 4.4. The DHG-values were obtained similarly.
1 
1

 • ( DHD exp− (1 − q ) DHDNaY )
DHD =   • 
 q   DHDNaY 
where: q = fraction of exchanged Na+
DHD = Corrected dehydrydation function
DHDexp = measured DHD-value
DHDNaY = DHD-value of NaY
90
(4.2)
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
As discussed earlier these trends are accounted for by the periodic changes
of the electronegativity of the alkali cations applied. An increase of the
basicity is expected from HY to CsY. The acidic function is in the opposite
order. Deactivation was observed only for the HY based catalysts. This led
to the slight deviation from the observed trend for these catalysts.
1.4
HY
2.4
1.6
NaY
LiY
KY
0.8
0
RbY
basic function
acid ic fun ctio n
3.2
RbY
1.2
LiY
1
0.8
KY
NaY
HY
0.6
Figure 4.14: Acidic and basic function of alkali-exchanged NaY. The basic
and acidic functions are corrected for the alkali ion exchange level
according to Equation 4.2 and normalised to NaY = 1.
After introduction of Mo into the supercages of alkali-exchanged faujasite
the basicity does not correspond to the periodic trend. For Mo/NaY the
basicity is strongly increased compared to NaY, but for the other catalysts
generally a lower DHG value was observed. The values in Table 4.5
suggest that the basicity is not solely determined by the alkali ion present in
the zeolite lattice, but also by the Mo(CO)6 guest molecule. Table 4.5
shows the results of the MBOH decomposition reaction. The DHG values
in Table 4.5 were corrected according to Equation 4.3, which is very similar
to Equation 4.2.
1 
1

DHG =   • 
 • (DHG exp − (1 − q) DHGMoNaY )
q
DHG
NaY

  
(4.3)
Table 4.5: Basic function of Mo encaged faujasite catalysts.
Catalyst
NaY
Mo/HY
Mo/LiY
Mo/NaY
Mo/KY
Mo/RbY
Exchange level1 [%]
DHGexp [%]
DHG2 [[- ]
76
69
91
72
33.1
23.4
18.3
59.2
38.9
34.2
n.d
1
0.37
0.00
1.79
1.11
0.74
n.d
Mo/CsY
72
1
Exchange levels determined by A.A.S.
2
DHG value corrected for exchange level and normalised to DHGNaY=1
91
Chapter 4
T d e so r p t io n [° C ]
150
Mo/LiY
130
Mo/RbY
Mo/KY
110
90
Mo/NaY
70
50
10
20
30
40
50
60
70
B a sic f u n c t io n - D H G v a lu e [% ]
Figure 4.15: The effect of the basicity on the CO desorption temperature.
To determine the effect of the Mo(CO)6 host on the catalyst basicity Figure
4.15 shows the effect of the DHG value on the CO desorption temperature
during oxidative Mo(CO)6/Y decomposition. A good correlation between
the basic function and the desorption temperature was observed. The more
basic the catalyst is, the lower the CO desorption temperature. This effect
can be explained well by the higher electron density of the more Lewis
basic catalysts. The electrons of the carbonyl group are stabilized better for
the least basic catalysts. This means that the interaction of the Mo(CO)6
guest with the zeolite host takes place basically with the alkali cation,
confirming characterization studies as described above.
3.5
Introduction of other transition metal carbonyls by CVD
3.5.1 Introduction of V(CO)6 into NaY
Figure 4.16 shows the evolution of products during exposure of
V(CO)6/NaY to artificial air. The carbonyl groups are immediately
removed upon contact with the O2 flow. The decarbonylation is
accompanied with uncontrolled oxidation of the vanadium. Heat
generation was observed during oxidation, indicating heat transfer
limitations. Therefore, the preparation of V/NaY by V(CO)6 deposition
cannot be controlled, even under well-defined conditions as were applied in
our experiments.
92
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
1.5
2.5
1.0
m / e = 32
2.0
0.5
1.5
0.0
m / e = 28
1.0
-0.5
m / e = 44
0.5
-1.0
0.0
-1.5
0
2
4
6
Tim e [m in]
8
10
N orm alised Intensity [-]
N orm alised Intensity [-]
3.0
12
Figure 4.16: Room temperature oxidative decarbonylation of V(CO)6/NaY
Figure 4.17: TEM image of VOx/NaY.
TEM confirmed the expected low dispersion of vanadia clusters over the
sample. EDX analysis showed that some segments of the sample did
contain only vanadium whereas other parts of the sample did not contain
any vanadium at all. The TEM image shown in Figure 4.17 illustrates the
low vanadia dispersion over the catalysts. The vanadia clusters are
significantly larger than 13.4 Å, the size of the supercage of NaY. Some of
the vanadia clusters are indicated by arrows.
93
Chapter 4
3.5.2 Introduction of Mn2(CO)10 into NaY
The decarbonylation of Mn2(CO)10 occurs less effectively than
decarbonylation of Mo(CO)6/NaY catalyst precursors. Since the volatility
of Mn2(CO)10 is lower than the volatility of Mo(CO)6 the effect of the
preparation temperature on the decarbonylation was examined. Figure 4.18
shows the temperature programmed decarbonylation of a series of 6 wt%.
loaded Mn2(CO)10/NaY catalyst precursors. Two main differences can be
observed. Firstly the amount of CO evolution differs greatly as function of
evaporation temperature. The amounts of CO were not quantified, since no
extra He was added to the product mixture after decarbonylation, contrary
to the decarbonylation experiments of Mo(CO)6/NaY. The sample that
was evaporated at 120 ° C shows the highest amount of CO produced,
whereas hardly any CO evolution was observed for the catalyst precursor
that was prepared at 160 ° C. Secondly the onset of Mn2(CO)10
decomposition (83 ° C) of Mn2(CO)10/NaY prepared at 60 ° C was
significantly higher than that of the other catalyst precursors (43 ° C).
125
Tem perature
(right axis)
4
100
3
75
T(ev ap)= 120 °C
2
50
T(ev ap)= 60 °C
1
25
T(ev ap)= 160 °C
0
0
20
40
60
T im e [ m in ]
80
T e m p e r a tu r e [° C ]
M S In te n s ity [ a .u .]
5
0
100
Figure 4.18: Decarbonylation of Mn2(CO)10/NaY (6 wt%). Heating by 5
° C/min to 100 ° C.
The low amount of CO evolving from the catalyst that was prepared at 160
° C can be explained by the low thermal stability of Mn2(CO)10. When the
Mn2(CO)10/NaY catalyst precursor was heated to 220 ° C the pressure of
the capsule containing the powder mixture was so high that the sample was
lost during opening of the ampoule. Premature decarbonylation of
Mn2(CO)10 caused the high pressure in the capsule. For Mn/NaY prepared
94
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
at 160 ° C also premature decarbonylation has taken place, though the total
pressure in the capsule was not raised so high that the sample was
completely lost. The very low amount of CO produced during
decarbonylation evidences the loss of the greater part of the CO ligands
from Mn2(CO)10 prior to activation of the catalyst precursor. The high
onset temperature of CO production during decarbonylation of the
Mn/NaY catalyst that was prepared at 60 ° C can be explained by low
volatility of Mn2(CO)10. Heating to 60 ° C was insufficient to transport the
Mn2(CO)10 through the pores of the NaY host. Therefore, Mn2(CO)10
probably was deposited mainly on the exterior zeolite surface or as bulk
Mn2(CO)10. The onset temperature of CO evolution of bulk Mn2(CO)10 was
found to be 79 ° C, which is similar to that of Mn2(CO)10/NaY prepared at
60 ° C.
Figure 4.19: TEM image of 1.7 wt% loaded MnOx/NaY.
For all Mn/NaY catalysts TEM reveals the presence of large manganese
oxide clusters. Moreover, the distribution of Mn over the catalyst surface
was inhomogeneous, as was examined by EDX analysis. A representative
TEM image showing the presence of some large Mn containing clusters is
shown in Figure 4.19. The electron microscopy studies are consistent with
the information obtained from the temperature programmed desorption
95
Chapter 4
experiments. Low volatility of Mn2(CO)10 leads to deposition of Mn on the
exterior of the zeolite structure. Increase of the preparation temperature in
principle leads to enhanced diffusivity of the Mn2(CO)10. However, the low
temperature stability of the molecule results in deposition of Mn on the
external surface of the zeolite surface as well.
3.5.3 Introduction of Co(NO)(CO)3 into NaY
The introduction of Co via deposition of carbonyl complexes be done by
using Co2(CO)8 and via Co(NO)(CO)3. Since it was shown that the use of
di-nuclear complexes could lead to uncontrolled catalyst preparation due to
lower volatility of the carbonyl compound, Co2(CO)8 was not applied in the
research described here. Alternatively Co(NO)(CO)3 was introduced into
the supercages of NaY.
Normalised MS Signal [-]
1.2
1.0
m/e= 28
0.8
0.6
0.4
m/e= 44
0.2
m/e= 30
0.0
0
100
200
300
Temperature [°C]
400
500
Figure 4.20: TPOD of Co(NO)(CO)3/NaY.
As shown in Figure 4.20 decomposition of the molecule occurs stepwise in
a rather complex way. At 85 ° C the main desorption peak of CO is
observed. At slightly higher temperature some of the CO ligands are
removed as CO2. The removal of NO ligands occurs in three steps. NO
production is observed at 185 ° C and 310 ° C. The main peak of m/e= 28
at 85 ° C consists not only of CO, but also a part of the NO ligands is
desorbed as N2, as was verified by deconvolution of the spectrum using N
and C fragments.
96
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
Figure 4.21: TEM image of CoOx/NaY
Though the low temperature desorption peak during TPOD suggests wellcontrolled Co oxidation, larger CoOx clusters were detected by TEM, as
shown in Figure 4.21. Arrows indicate some of the CoOx clusters.
3.6
Catalytic activity in the ammoxidation of toluene
100
Benzonitrile
selectivity
[m ol% ]
80
Toluene
conversion
60
40
Benzonitrile yield
20
0
360
380
400
420
440
460
T e m p e ra tu re [ ° C ]
Figure 4.22: Toluene ammoxidation over Co/NaY as function of
temperature. WHSV = 0.7; T: N: O= 1: 5: 8.
The benzonitrile yields over zeolite encaged metal oxides prepared from
metal carbonyl precursors are generally low, as discussed in Chapter 3.
Based on the high selectivity to benzonitrile that can be achieved, however,
these catalysts can be of interest at low reaction temperature. In this respect
the performance of Co/NaY is most interesting. Figure 4.22 shows the
97
Chapter 4
activity in toluene ammoxidation. At increasing temperature the
benzonitrile selectivity decreases.
An even stronger decrease in the benzonitrile yield was observed for
Mn/NaY as was explained in Chapter 3. Moreover, a strong deactivation
of the conversions of toluene, ammonia and oxygen occurs as function of
time on stream (see Figure 4.23).
C o n v e r s io n [ m o l % ]
100
Toluene
80
Oxygen
60
40
Ammonia
20
0
0
50
100
150
200
T im e o n s t r e a m [ m in ]
250
300
Figure 4.23:
4.23: Deactivation of Mn/NaY during toluene ammoxidation. T=
360 ° C; WHSV = 0.9; T: N: O= 1: 8: 13.
Not only the toluene conversion decreases strongly with time, but also the
conversion of oxygen and ammonia. Though ammonia depletion did not
occur at any of the temperatures applied the benzonitrile selectivity may
have been limited by the ammonia combustion reaction. This reaction leads
to the consumption of oxygen. Since all oxygen was consumed at high
temperatures and thus high conversion levels, benzonitrile could not be
reached. The selectivity towards benzonitrile is proportional to the
concentration of oxygen in the reactor as shown in Figure 4.24. Clearly, the
strong deactivation observed over Mn/NaY makes it almost impossible to
compare the catalytic performance on a more quantitive basis. The initial
reaction rate should be compared, but the cause of deactivation should be
studied in detail in order to be able to define the initial concentrations of
reactants and (intermediate) reaction products.
98
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
S e le c t iv it y [m o l% ]
50
40
30
20
10
0
0
5000
10000
15000
20000
25000
30000
35000
40000
C (O 2 ) [p p m ]
Figure
Figure 4.24: Relation between O2 concentration and benzonitrile selectivity
during toluene ammoxidation at 400 ° C.
3.7 The effect of the Lewis acidity and basicity on the
ammoxidation of toluene over MoOx/Y
10
X (T o l u e n e )
9
Mo/NaY
8
7
Mo/KY
6
Mo/RbY
5
4
Mo/LiY
3
0
0.4
0.8
1.2
1.6
2
B a s ic f u n c t io n
Figure 4.25: Toluene ammoxidation activity over alkali-exchanged Mo/Y
as a function of the basicity. WHSV= 0.8; T= 400 ° C; T: N: O= 1: 5: 8.
Figure 4.25 shows the effect of the basicity of 12 wt% loaded Mo/Y
catalysts on the toluene ammoxidation activity. Higher basicity of the
catalysts leads to higher activity. The benzonitrile selectivity, on the other
hand, seems to increase with the Lewis acidity of the catalyst as is indicated
by Figure 4.26. This effect might be influenced by the size of the alkali
cation. The catalysts that contain larger cations show higher benzonitrile
99
Chapter 4
S (B en z o n it r ile) [m o l% ]
selectivities. The size of the alkali cation, and thus the size of the zeolite
pores, does not seem to influence the rate of toluene activation.
Interpretation of the results is rather difficult and preliminary, since few
data have been measured and the selectivity towards benzonitrile is rather
low over faujasite encaged MoOx catalysts. Nevertheless, the higher activity
over the more basic catalysts could be explained by the formation of a
carbanion as a first step of toluene activation. Stabilization of the carbanion
occurs on the basic centers of the catalyst. Also Guseinov et al. found
higher toluene activation rates over V-Sb-Bi-O catalysts during toluene
ammoxidation [66]. Since no further characterization on the nature of
reaction intermediates was performed conclusive explanations can be made
based only on the reaction data given here.
50
Mo/RbY
40
30
Mo/NaY
20
10
Mo/LiY
0
0
1
2
3
4
5
A c id it y
Figure 4.26: Benzonitrile selectivity over alkali-exchanged Mo/Y as a
function of acidity. WHSV= 0.8; T= 400 ° C; T: N: O= 1: 5: 8.
4.
Conclusions
Intra-zeolite molybdenum oxide nanoclusters can be produced by
entrapment of Mo(CO)6 in faujasite zeolites. Thermal desorption of the
encaged Mo(CO)6 structure in an oxygen containing gas mixture leads to
low temperature decarbonylation of Mo(CO)6. During this decarbonylation
Mo is oxidized to a 6+-oxidation state. Decarbonylation and oxidation
occurs more easily compared to decomposition in helium, followed by a
separate oxidation treatment. In this process subcarbonyl species could
have been formed, though we did not find direct experimental evidence for
this. The subcarbonyl species appear to be very reactive, both towards
100
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
oxidation by oxygen and by recarbonylation using gas phase CO. The
Mo(CO)6 structure interacts with the extra-framework cations of the zeolite
host. Higher electron density of the cation leads to weaker interaction with
the Mo(CO)6 guest.
Introduction of Mn, or V by means of metal carbonyl deposition cannot be
performed in a reproducible manner. The low volatility of Mn2(CO)10
complicates diffusion through the zeolite pores. This leads to deposition of
large manganese oxide clusters on the external zeolite surface. The
transport of Mn2(CO)10 can be slightly improved by increase of the
preparation temperature. This, however, leads to premature
decarbonylation of the Mn2(CO)10 guest. V(CO)6/NaY mixtures are
extremely sensitive to oxygen, leading to instantaneous and uncontrolled
decarbonylation and oxidation of the V(CO)6 guest.
Over Mn/NaY catalysts the ammoxidation of toluene was accompanied
with strong deactivation. Lack of available oxygen seemed to govern the
low benzonitrile selectivity. Toluene ammoxidation over Mo/NaY was
influenced by the acid/base properties of the catalyst. Higher Lewis basicity
leads to higher toluene conversion, indicating heterolytic toluene rupture.
References
R. Cid, F.J.G. Llambias, J.L.G. Fierro, A.L. Agudo, J. Villasenor, J.
Catal., 89, (1984), 478-488.
J.L.G. Fierro, J.C. Conesa, A.L. Agudo, J. Catal., 108, (1987), 334-345.
2. M.J. Vissenberg, Preparative Aspects of Supported Metal Sulfide
Hydrotreating Catalysts, PhD Thesis, 1999, Eindhoven University of
Technology, pp. 61-82.
3. K. Asakura, Y. Noguchi, Y. Iwasawa, J. Phys. Chem. B., 103, (1999),
1051-1058.
4. B.R. Müller, G. Calzaferri, J. Chem. Soc., Faraday Trans., 92, (1996),
1633-1637.
5. S. Abdo, R.F. Howe, J. Phys. Chem., 87, (1983), 1713-1722.
6. K. Asakura, Y. Noguchi, Y. Iwasawa, J. Phys. Chem. B., 103, (1999),
1051-1058.
7. T. Komatsu, S. Namba, T. Yashima, K. Domen, T. Ohnishi, J. Mol.
Catal., 33, (1985), 345-356.
8. S. Özkar, G.A. Ozin, R.A. Prokopowicz, Chem. Mater., 4, (1992), 13801388.
9. M. Laniecki, W. Zmierczak, Zeolites, 11, (1991), 18-26.
10. S. Özkar, G.A. Ozin, K. Moller, T. Bein, J. Am. Chem. Soc., 112,
(1990), 9575-9586.
1.
101
Chapter 4
11. T. Shido, K. Asakura, Y. Noguchi, Y. Iwasawa, Appl. Catal. A., 194195, (2000), 365-374.
12. Y-S. Yong, R.F. Howe, J. Chem. Soc., Faraday. Trans. I, 82, (1986),
2887-2896.
13. R. Ugo, C. Dossi, R. Psaro, J. Mol. Catal. A:, 107, (1996), 13-22.
14. R.F. Howe, Jiang Ming, Wong She-Tin, Zhu Jian-Hua, Catal. Today, 6,
(1989), 113-122.
Y. Okamoto, A. Maezawa, H. Kane, T. Imanaka, in: Proc. 9th Int.
Congr. on Catal. Eds. M.J. Phillips, M. Ternan, The Chemical Institute
of Canada, Ottawa, 1988, pp. 11-18.
15. T. Yashima, T. Komatsu, S. Namba, Chem. Exp., 1, (1986), 701-704.
16. W.J.J. Welters, V.H.J. de Beer, R.A. van Santen, Appl. Catal. A., 119,
(1994), 253-269.
G. Vorbeck, W.J.J. Welters, L.J.M. van de Ven, H.W. Zandbergen,
J.W. de Haan, V.H.J. de Beer, R.A. van Santen, in: Zeolites and related
microporous materials: State of the art, 1994, Eds. J. Weitkamp, H.G.
Karge, H. Pfeifer, W. Hölderich, Elsevier, Amsterdam, 1994, pp. 16171624.
M.L. Vrinat, C.G. Gachet, L. de Mourges, in: Catalysis by zeolites, Eds.
B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier, H.
Praliaud, Elsevier, Amsterdam, 1980, pp. 219-225.
J.A. Anderson, B. Pawelec, J.L.G. Fierro, P.L. Arias, F. Duque, J.F.
Cambra, Appl. Catal. A., 99, (1993), 55-70.
17. Y. Okamoto, H. Katsuyama, Ind. Eng. Chem. Res., 35, (1996), 18341844.
18. Y. Okamoto, A. Maezawa, H. Kane, T. Imanaka, J. Molec. Catal., 52,
(1989), 337-348.
19. G.D. Stucky, J. Mc Dougall, Science , 247, (1990), 669-678.
20. Y. Okamoto, Y. Kobayashi, T. Imanaka, Catal. Lett., 20, (1993), 49-57.
21. Y.S. Yong, R.F. Howe, In: New developments in zeolite science and
technology. Proceedings of the 7th international zeolite conference, Eds.
Y. Murakami, A. Ijima, J.W. Ward, Stud. Surf. Sc. Catal., Vol 28,
Elsevier, Amsterdam, 1986, pp. 883-889.
22. S.L.T. Anderson, R.F. Howe, J. Phys. Chem., 93, (1989), 4913-4920.
23. G-C. Shen, T. Shido, M. Ichikawa, J. Phys. Chem., 100, (1996), 1694716956.
N.K. Indu, H. Hobert, I. Weber, J. Datka, Zeolites, 15, (1995), 714-718.
24. G.A Ozin, S. Kirkby, M. Meszaros, S. Özkar, A. Stein, D. Stucky, in:
Chemical perspectives, Eds. S.R. Mardner, J.E. Sohn, G.D. Stucky,
ACS Symposium Series 455, American Chemical Society, Washington
D.C., 1991, pp. 554-581.
25. S. Özkar, G.A. Ozin, K. Moller, T. Bein, J. Am. Chem. Soc., 112,
(1990), 9575-9586.
26. R.K. Unger, M.C. Baird, J. Chem. Soc., Chem. Commun., (1986), 643645.
27. C. Dossi, A. Fusi, S. Recchia, R. Psaro, Mater. Eng., 5, (1994), 249-259.
28. W.M. Meier, D.H. Olson, Atlas of Zeolite Structure Types, 2nd edition,
Buttersworths, London, 1987.
29. H. Koller, A.R. Overweg, R.A. van Santen, J.W. de Haan, J. Phys.
Chem. B., 101, (1997), 1754-1761.
102
Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors
for the ammoxidation of toluene
30. G.A. Ozin, R.A. Prokopowicz, S. Özkar, J. Am. Chem. Soc., 114,
(1992), 8953-8963.
G.A. Ozin, S. Özkar, R.A. Prokopowicz, Acc. Chem Res., 25, (1992),
553-560.
31. C. Bré mard, G. Ginestet, J. Laureyns, M. Le Maire, J. Am. Chem. Soc.,
117, (1995), 9274-9284.
C. Bré mard, G. Ginestet, M. Le Maire, J. Am. Chem. Soc., 118, (1996),
12724-12734.
32. Y-S. Yong, R.F. Howe, in: New developments in zeolite science and
technology, Proceedings of the 7th international zeolite conference, Eds.
Y. Murakami, A. Iijima, J.W. Ward, Elsevier, Amsterdam, 1986, pp.
883-889.
33. A. Brenner, R.L. Burwell Jr., J. Catal., 52, (1978), 353-363.
A. Brenner, R.L. Burwell Jr., J. Catal., 52, (1978), 364-374.
34. A. Brenner, D.A. Hucul, S.J. Hardwick, Inorg. Chem., 18(6), (1979),
1478-1484.
35. J. Evans, B.E. Hayden, G. Lu, J. Chem. Soc., Faraday. Trans., 92(23),
(1996), 4733-4737.
E. Guglielminotti, A. Zecchina, J. Chim. Phys., 78, (1981), 891-895.
36. P. Gallezot, G. Coudurier, M. Primet, B. Imelik, Adsorption and
Decomposition of Metal Carbonyls Loaded in Y-Type Zeolite, in:
Molecular Sieves II, Ed. J.R. Katzer, ACS Symposium Series, Vol. 40,
Washington D.C., 1977, pp. 144-155.
37. S. Albonetti, F. Cavani, F. Trifirò, Catal. Rev. - Sci. Eng., 38, (1996),
413-438.
38. C.L. Tway, T.M. Apple, Inorg. Chem., 31, (1992), 2885-2888.
39. H. Koller, A.R. Overweg, L.J.M. van de Ven, J.W. de Haan, R.A. van
Santen, Microp. Mat., 11, (1997), 9-17.
40. C.D. Wagner, D.E. Passoja, H.F. Hillery, T.G. Kinisky, H.A. Six, W.T.
Jansen, J.A. Taylor, J. Vac. Sci. Technol., 21, (1982), 933-944.
41. C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.H. Raymond,
L.H. Gale, Surf. Interface Anal., 3, (1981), 211-225.
42. D. Bassett and H.W. Habgood, J. Phys. Chem., 64, (1960), 769-773.
43. E.M. Fedneva, J.V.K. Kryukova, Russ. J. Inorg. Chem., 11, (1966), 141143.
44. A. Brenner, R.L. Burwell Jr., J. Catal., 52, (1978), 353-363.
45. R. Nakamura, R.G. Bowman, R.L. Burwell Jr., J. Am. Chem. Soc., 103,
(1981), 673-674.
46. S. Jaenicke, W.L. Loh, Catal. Today, 49, (1999), 123-130.
47. S. Ichikawa, H. Poppa, M. Boudart, J. Catal., 91, (1985), 1-10.
48. K. Tanaka, K.L. Watters, R.F. Howe, J. Catal., 75, (1982), 23-38.
K. Tanaka, K.L. Watters, R.F. Howe, S.L.T. Andersson, J. Catal., 79,
(1983), 251-258.
49. T. Okamoto, A. Maezawa, H. Kane, I. Mitsushima, T. Imanaka, J.
Chem. Soc. Faraday Trans. I, 84, (1988), 851-863.
50. K. Asakura, Y. Noguchi, Y. Iwasawa, J. Phys. Chem. B., 103, (1999),
1051-1058.
51. A. Brenner, D.A. Hucul, J. Catal., 61, (1980), 216-222.
103
Chapter 4
52. F.A. Cotton, G.F.R.S. Wilkinson, Advanced Inorganic Chemistry, a
comprehensive text, 3rd edition, Interscience Publishers, New York,
1972, pp. 682-719.
53. W.E. Swartz Jr., D.A. Hercules, Anal. Chem., 43, (1971), 1774-1779.
54. J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of Xray Photoelectron Spectroscopy, Ed. J. Chastain, Perkin-Elmer
Corporation, Minnesota, 1992.
55. P. Gallezot, in: Metal clusters, Ed. M. Moskovits, Wiley, New York,
1986, pp. 219-247.
56. M.G. Mason, Phys. Rev. B, 27, (1983), 748-762.
57. R. Jelinek, S. Özkar, G.A. Ozin, J. Phys. Chem., 96, (1992), 5949-5953.
58. P.E. Hathaway, M.E. Davis, J. Catal., 116, (1989), 263-278.
M. Lasperas, H. Cambon, D. Brunel, I. Ridriguez, P. Geneste, Micr.
Mat., 1, (1993), 343-351.
59. M. Huang, S. Kaliaguine Catal. Lett., 18, (1993), 373-389.
C. Lahausse, J. Bachelier, H. Lauron-Pernot, A.M. Le Govic, J.C.
Lavalley, J. Mol. Catal., 87, (1994), 329-332.
V.R.L. Constantino, T.J. Pinnavaia, Catal. Lett. 23, (1993), 361-367.
60. U. Meyer, W.F. Hölderich, Appl. Catal. A., 142, (1999), 213-222.
61. H. Lauron-Pernot, F. Luck, J.M. Popa, Appl. Catal., 78, (1991), 213225.
62. A. Corma, H. Garcí a, Catal. Today, 38, (1997), 257-308.
63. M. Gasoir, J. Haber, T. Machej, Appl. Catal. 33, (1987), 1-14.
G. Bond, S. Flamers, Appl. Catal., 33, (1987), 219-230.
M. Ponzi, C. Duscatzky, A. Carrascull, E. Ponzi, Appl. Catal. A., 169,
(1998), 373-379.
64. J.A. Lercher, G. Warecka, M. Derewinski, in: Proceedings 9th Int.
Congr. On Catalysis, Calgary 1988, Eds. M.J. Phillips, M. Ternan, 1988,
pp. 364-371.
65. N.E. Fouad, P. Thomasson, H. Knözinger, Appl. Catal. A., 194-195,
(2000), 213-225.
66. A.B. Guseinov, E.A. Mamedov, R.G. Rizaev, React. Kinet. Catal. Lett.,
27, (1983), 371-374.
104
Chapter 5
The effect of molybdenum oxide reducibility on the
ammoxidation of toluene
Abstract
The effect of molybdenum oxide reduction on the ammoxidation of toluene was
studied using γ-alumina supported molybdenum oxide (Mo/Al) catalysts, which
were prepared by pore volume impregnation. The reducibility was studied by
hydrogen TPR as well as by temperature programmed ammonia decomposition. In
situ Raman spectroscopy proved that ammonia reduces molybdenum oxide. Under
toluene ammoxidation reaction conditions, however, XPS showed only the
presence of Mo6+ at the catalyst surface. It was shown that the benzonitrile
production rate is increased at increasing Mo loading. At low loadings only
tetrahedrally coordinated isolated surface molybdate is present at the catalyst, as
was shown by Raman Spectroscopy and diffuse reflectance UV spectrometry. These
isolated molybdates show low toluene ammoxidation activity. Hydrogen–
deuterium exchange was used to probe the Mo dispersion. When the Mo loading is
increased the activity for the hydrogen exchange reaction strongly decreases. At Mo
loadings higher than 8.5 wt% no H-D exchange activity was observed, indicating
that only tetrahedrally coordinated surface molybdates are active for the H2-D2
exchange reaction.
The effect of vanadia dopants on the toluene ammoxidation activity of Mo/Al was
studied also. As was shown by Raman spectroscopy MoO3-crystallites are formed
when Mo/Al is doped with vanadia. XRD showed that the MoO3 clusters have a
very small size, since no long range ordering was observed. Transmission Electron
Microscopy confirmed the small MoO3 cluster size. MoO3 clusters were not formed
when the catalyst contained only Mo, since the loading of the catalyst did not
exceed the monolayer capacity of the support. Both benzonitrile selectivity and
catalyst activity increased when Mo/Al was doped with vanadia.
1. Introduction
Supported molybdenum oxide catalysts have been described in the literature very
frequently. Since Mo based catalyst formulations are applied in many types of
catalytic reactions, such as selective (amm)oxidation, metathesis and
hydrotreating, both the preparation and the surface properties of these catalysts
have been intensively studied. Recently, industrial interest for Mo-based
ammoxidation catalysts increased, because of the high nitrile yields that can be
obtained in alkane ammoxidation [1]. Although ammoxidation reactions usually
105
Chapter 5
involve multi-component catalysts there is still a lack of knowledge about the exact
role of Mo in these reactions. Therefore, this study systematically examines the
influence of molybdenum oxide reduction and reoxidation during the
ammoxidation of toluene. γ-Alumina was chosen as a support. By varying the Mo
loading the properties of the Mo and O containing surface species were varied.
1.1
Preparation methods of supported Mo catalysts
The preparation of γ-alumina supported molybdenum oxides (Mo/Al) can be
achieved by several methods. In principle, the simplest technique is just making a
physical mixture of MoO3 and the support. Upon heating in an O2 containing flow
MoO3 spreads over the surface and forms a layer on the alumina surface [2,3],
when applying the appropriate Mo:Al2O3-ratio. Recently, the preparation of
Mo/Al catalysts by mechanically mixing MoO3 with the support was investigated
in more detail [4,5]. It was shown that besides thermal activation, also mechanical
activation by ball milling can spread the MoO3 phase over the γ-alumina support
[6]. Though in principle the method is very simple, control of the preparation by
this method is not very easy. For example, moisture can play an inhibiting role [5].
Organometallic precursors can be applied to introduce Mo onto a variety of
supports, including γ-alumina. For example allyl-precursors (Mo(η3-C3H5)4 have
been described by several authors [7-10]. The preparation using this method is
quite complex [11]. Though authors sometimes assign special properties and Mo
sites to this preparation method, others compared these catalysts with differently
prepared supported molybdenum oxide without finding differences [10-12].
Similarly MoCl5 can be used for the preparation of supported molybdenum oxide
catalysts. MoCl5 reacts with the hydroxyl groups of the support, producing socalled grafted Mo and HCl. A washing step is required to remove weakly bound
Mo [13]. Other precursors have been used, most notably Mo(CO)6 [14,15].
Though these methods are quite versatile and the Mo oxidation state can be tuned
by heat treatment, the maximum Mo loading that can be achieved is rather low.
Therefore, in general these preparation methods are not preferred for the
introduction of Mo onto γ-alumina supports.
Using ammonium heptamolybdate (AHM) as Mo source the equilibrium
adsorption method can be applied [16-19]. This method consists of adsorption of
molybdates onto the support at a fixed pH value of the aqueous solution applied.
AHM adsorption onto the support takes an extended period of time, usually of the
order of days [20]. Therefore, this method is applied basically for studying the
106
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
effect of the surface charge on the Mo surface chemistry. By varying the pH the
Mo loading of the catalyst can be controlled accurately [21].
Though catalysts prepared by equilibrium adsorption are very homogeneous [22],
impregnation of AHM solutions into the γ-alumina support has been used most
often. Molybdates are attached to the surface OH groups and decompose upon
high temperature. Three different OH-groups can be distinguished in γ-alumina, as
determined by infrared spectroscopy [23]. The different OH groups are drawn in
Figure 5.1.
H
O
Al
H
O
Al
H
O
Al
Al
Al
Al
1: basic
2: neutral
3: acidic
Figure 5.1: Surface OH groups present in γ-alumina.
The basic OH groups are involved when AHM is adsorbed onto the γ-alumina
surface, as shown by the decrease of only the 3775 cm-1 line upon MoO2(acac)2
adsorption. Upon protonation of the basic sites molybdate adsorption can also
occur reversibly via electrostatic interaction [21]. Additionally, γ-alumina contains
coordinatively unsaturated Al3+ sites. These sites are only involved with
physisorbed adsorbates [16,18].
1.2
Notation of different Mo species
Species containing Mo surrounded by O ligands exist in many different forms,
both at the catalyst surface and in the impregnation solution. For the sake of
convenience, the way these species will be indicated in this chapter is outlined
here.
In solution Mo species basically exist as molybdates. They will be referred to as
monomolybdate or MoO42-, heptamolybdate or Mo7O246-, octamolybdate or
Mo8O264-, or simply as polymolybdate in the case where their degree of
polymerisation is unknown.
At the catalyst surface Mo species can exist as molybdates, surrounded by
counterions provided by the support surface and also as Mo oxides.
107
Chapter 5
Similarly, the surface molybdates will be notated as surface monomolybdate or
surface MoO42-, surface heptamolybdate or surface Mo7O246-, surface
octamolybdate or surface Mo8O264-, or simply as surface polymolybdate in the case
where the degree of polymerisation is unknown. Surface Mo oxides are notated as
MoO3, MonO(3n-1) and MoO2, dependent on their degree of reduction. When the
degree of oxidation is unknown the surface molybdenum oxide will be notated
simply as surface Mo oxide or MoOx.
Unless otherwise mentioned the Mo loading of the catalyst is always expressed as
wt% Mo metal, irrespective of the chemical nature of the Mo species.
Based on the difference in structure tetrahedrally coordinated and octahedrally
coordinated Mo can be distinguished. Monomolybdates generally have tetrahedral
coordination and polymolybdates generally have octahedral coordination.
1.3
Molybdate surface species
Many groups have studied the effect of the pH of the aqueous AHM solution. The
pH value determines the structure of the molybdate species that are present in the
impregnation solution. Equation 5.1 shows the equilibrium between
heptamolybdate (Mo7O246-) and monomolybdate (MoO42-) species.
7 MoO42- + 8 H+
Mo7O246- + 4 H2O
(5.1)
According to this equation the MoO42- ions are stable at pH higher than 6.5 [24],
whereas pH values lower than 4.5 lead to complete formation of Mo7O246- [25] in
the impregnation solution. Further decrease of the pH to 1.5 leads to formation of
octamolybdate (Mo8O264-) [26], or to polymolybdates with an even higher degree
of polymerisation. At low Mo loading the impregnation of AHM leads to the
formation of monomolybdate surface species only, as was shown by Jeziorowski
and Knözinger [27]. These authors impregnated alumina with AHM solutions at
pH= 6 and pH= 11. The metal loading was 3 wt%. Only surface monomolybdate
was observed by Raman spectroscopy. The presence of only surface
monomolybdates at low Mo loading can be explained by the occurrence of an
8 Al-OH + Mo7O246-
4 (Al)2MoO4 + 3 MoO42- + 4 H2O
(5.2)
irreversible reaction of heptamolybdate anions with the basic hydroxyl groups of
alumina according to Equation 5.2 [18,28].
During catalyst drying, ammonia desorbs at a lower temperature than water,
leading to a decrease in the pH of the solution. Therefore, even at high initial pH
values surface monomolybdates and surface polymolybdates can be present as
well.
108
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
-2 0
[•1 0 -2 0 m o l/ g ca t ]
A d so rb e d m o ly b d e n u m
The influence of the
solutions pH-value can
be explained well by a
6
change in the surface pH
at the point of zero
charge (PZC), or iso4
electric point (IEP). The
charge of the surface can
2
be either positive or
negative, depending on
0
the
pH
of
the
0
2
4
6
8
10
impregnation solution.
When the pH value of
F in a l p H v a lu e
the solution is above the
Figure 5.2: Molybdate adsorption on Al2O3 [19].
IEP, the surface becomes
negatively charged and
no adsorption of any of the negatively charged molybdate anions occurs. On the
other hand, when the pH of the impregnation solution is lower than the IEP,
molybdate anions can adsorb on the positively charged alumina surface [29]. By
performing equilibrium adsorption on alumina Wang and Hall showed that the
equilibrium Mo loading is a function of the pH of the solution [19]. At lower pH
the amount of Mo that was adsorbed onto the alumina support was much higher
than at higher pH values, which is consistent with this model. Figure 5.2 shows
the adsorption curve. The break in the curve is at pH= 6-8, as was expected from
the PZC, which is reported to be 6-8 [30] or 9.1 [31], depending on the crystal
plane.
8
According to Van Veen et al. [16] molybdates adsorb primarily on the basic AlOH sites. In a second stage adsorption on the coordinatively unsaturated (cus)
Al3+ occurs. They performed equilibrium adsorption experiments at different pH
values. As expected from the PZC model a higher number of molybdates adsorbed
at the surface at lower pH values. Irrespective of the pH applied it was found that
after consumption of all basic Al-OH groups still molybdate adsorption from the
liquid phase occurred. It was indicated that the cus Al3+ sites were consumed
during the adsorption of additional molybdate. It was shown also that precipitated
molybdates could be formed on γ-alumina [18]. As was discussed above
protonated OH groups are involved via electrostatic interaction [21].
Applying a series of 13 wt% Mo loaded alumina catalysts Okamoto et al. [32] did
not find the presence of a larger amount of surface polymolybdates at lower pH
109
Chapter 5
values. Contrary, they observed a slight decrease of Mo dispersion when the pH of
the impregnation solution was increased from 2 to 8. They also explained this
slightly lower dispersion by the IEP of alumina, which was assumed to be 8.8 [33].
During impregnation most of the molybdate anions are in solution, without
adsorption to the alumina surface, resulting in agglomerated Mo species during
drying. The fraction of adsorbed molybdates during impregnation is higher when
the difference between the pH of the solution and the PZC of the support is larger.
To conclude: the exact nature of the Mo species on γ-alumina is still not
completely clear. Though in equilibrium adsorption experiments the PZC model
explains well the presence of surface monomolybdates (high pH), surface
polymolybdates (low pH) and precipitated molybdates on the surface no
equilibrium can be reached during incipient wetness impregnation. Since the pores
of alumina are exactly filled with the impregnation solution when this method is
applied deposition of additional amounts of AHM occurs. Upon calcination Mo
originating from deposited AHM could be bound to the alumina support, since
usually no significant Mo loss takes place upon catalyst calcination. This means
that the Mo loading of the catalyst can be higher than expected from equilibrium
adsorption according to the PZC-theory only.
1.4
Characterization of Mo surface species
Raman spectroscopy has been applied very frequently to study the Mo surface
species, which are formed after calcinations of the impregnated catalysts. As
explained above, AHM impregnation followed by calcination can lead to different
surface molybdates and molybdenum oxide species. Depending on the Mo
loading, generally three different Mo species can be distinguished. Furthermore
aluminium molybdate can be formed [34]. At low Mo loadings tetrahedral surface
monomolybdates are formed. If the Mo loading is increased, octahedral
coordinated molybdates are formed, having Raman shifts in the 940 to 975 cm-1
range [35]. Wang and Hall claim that the intensity ratio of the Raman bands at
950 cm-1 and 970 cm-1 can be taken as a measure for the ratio between the
tetrahedral and the octahedral molybdates [36]. However, other work does not
suggest any shift in the Mo=O stretching band position between tetrahedral and
octahedral coordinated molybdates [37]. Subsurface Al2(MoO4)3 was found to be
formed at Mo loadings near 20 wt% (γ-Al2O3 surface area 240 m2/g) [38]. If the
Mo loading is increased even further, MoO3 phases are formed. The loading at
which MoO3 formation occurs is generally denoted as monolayer loading. This
monolayer loading is calculated by assuming complete coverage of the catalyst
110
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
surface estimating a size of 0.15 nm2 per surface interacting MoO3 group [39]. No
evidence is described in the literature that only MoO3 is formed when the Mo
loading exceeds the so-called monolayer coverage. In fact Brown et al. [40] find
evidence for the presence of both Al2(MoO4)3 and MoO3 in a 18 wt% loaded
Mo/Al catalyst. Nevertheless, it is generally accepted that at Mo loadings lower
than the so-called monolayer loading MoO3 is not present.
Based on X-ray diffraction data as described by Stencel [41] three distinct
molybdenum oxide structures can be distinguished in MoO3.
1) One terminal O atom is bound to one Mo atom
2) Two bridged O atoms are bound to two Mo atoms
3) Three bridged O atoms are bound to three Mo atoms
The three different Mo-O bonds are sketched in Figure 5.3. For the sake of
convenience the coordination of only one O atom is sketched in this figure.
I: OMo II: OMo2 III: OMo3
O atom
Mo atom
Figure 5.3: Different Mo-O bonds in MoO3
The resulting Raman and infrared bands are shown in Table 5.1.
Table 5.1: Position of Raman and infrared bands for MoO3
Assignment (stretch)
MoMo-O bond length [Å]
I:O-Mo B3u
1.67
I: O-Mo A1g, B1g
II: O-Mo2 B3u
II: O-Mo2 B1g
III: O-Mo3 B3g
III: O-Mo3 B1u
1.67
1.73, 2.25
1.73, 2.25
1.95, 1.95, 2.33
1.95, 1.95, 2.33
IR [cm-1]
Raman [cm-1]
998 (1004)
885
993 (997)
810 (840)
817 (820)
664 (668)
566 (545)
Experimental data: Nazari et al. [42] and Beattie and Gilson [43] (in brackets).
The interpretation of complex Raman spectra is often based on the strongest
Raman lines of the MoO3 spectrum. E.g. if only one strong raman band is
observed at 993 cm-1 bridging O atoms are not present in the surface molybdate.
Such a Raman spectrum suggests the presence of isolated surface
monomolybdates.
111
Chapter 5
When Al2(MoO4)3 has formed, Raman bands are present at 877, 845, 791 and 321
cm-1 as measured by Brown et al. [40], who recorded the spectra of several
molybdate reference salts. The difference between the Raman spectra of
tetrahedral and octahedral surface molybdates is less well defined. Both species
show bands in the 940 to 990 cm-1 range. Raman spectra of solutions containing
polymolybdate anions show a strong line at 943 for Mo7O246- and at 965 cm-1 for
Mo8O264- indicating frequency increase of the bands in this region when the degree
of polymerisation increases [26]. Though distinguishing between the exact nature
of the surface molybdate species by Raman spectroscopy is difficult, all surface
polymolybdate species contain not only a band in the 940 to 990 cm-1 range, but
also a less intense band in the 840 to 890 cm-1 range. Additionally in the low
frequency range of the spectrum the Mo-O-Mo bending mode can be observed at
220 cm-1. This band dominates the low-frequency portion of the spectrum [10].
The absence of Raman bands in these regions indicates the absence of
polymolybdate surface species. The Mo=O bending mode of tetrahedrally
coordinated monomolybdate is found around 320 cm-1, as was measured for
MoO42--solutions [44]. For octahedrally coordinated Mo several bands are found
for the Mo=O bending. The 320 cm-1 band is shifted to higher wavenumber,
approximately 365 cm-1 [45]. Though the intensity of the low wavenumber bands
is rather low, these bands can be used to distinguish between tetrahedrally and
octahedrally coordinated Mo in supported Mo catalysts.
1.5
Molybdate and Mo oxide reduction
Reduction of surface molybdates can be described by the so-called shear plane
model [46]. This model,
which
was
originally
developed for the reduction
of bulk MoO3 is also
Reduction
Reduction
applicable
to
Bi-Mo
catalysts as described by
Brazdil et al. [47]. Oxygen
vacancies
arise
during
Surface
reduction of tetrahedral and
reconstruction
octahedral molybdate, as
sketched in Figure 5.4 and
5.5. After reduction ordered
Figure 5.4: Mo oxide tetrahedra reduction and
arrays of oxygen vacancies
reconstruction into shears [46].
are formed. The layers,
112
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
which were initially linked via the corners, are rearranged into edge-linked
octahedra. After surface reconstruction these vacancies are filled and the catalyst is
reoxidized. The surface reconstruction requires a minimum number of oxygen
vacancies, depending on the bulk structure of the metal oxide applied [48]. If this
reduction model is applied to supported Mo oxide surface reconstruction cannot
occur when the Mo surface species are isolated. This is the case when the Mo
loading of Mo/Al is low, as was indicated by UV-VIS and by Raman
spectroscopy. Surface polymolybdates that are formed at higher Mo loadings,
therefore, are easier to reduce.
Reduction
Reoxidation
Surface
reconstruction
Figure 5.5: Mo oxide octahedra reduction and reconstruction into shears [46].
A recent publication clearly showed that well-defined molybdenum suboxides
(MonO(3n-1), e.g. Mo4O11) are not formed during MoO3 reduction [49]. MoO3 is
converted to MoO2 in one step at temperatures below 420 ° C. During reduction
the growth of the crystallite size was observed for all phases present. This explains
accelerated MoO3 reduction at higher temperature assuming a nucleation-growth
mechanism. At higher temperature the solid-state reaction of MoO3 and MoO2 can
result in the formation of Mo4O11 when the hydrogen concentration is high (over
40%). These severe reduction conditions are not applied in selective oxidation
reactions. Therefore the formation of long-range ordered structures (e.g. Mo8O23)
can be excluded during MoO3 reduction. Shear planes (having short-to-mediumrange disorder possibly form from oxygen vacancies as explained above.
113
Chapter 5
2. Materials and methods
2.1
Catalyst preparation
Mo/Al catalysts were prepared by incipient wetness impregnation. The pores of γalumina (Ketjen, surface area 205 m2/g, pore volume 0.55 ml/g) were exactly
filled with aqueous AHM solutions, containing different amounts of AHM in
order to vary the Mo loading of the catalyst. In most cases the impregnation
solution was kept at pH= 10 by adding NH3, but also some catalysts were
prepared using neutral impregnation solutions. After impregnation, the catalyst
precursors were heated overnight in ambient air to 110 ° C. In order to gently
remove the water the catalyst precursors were kept for two hours at 60 ° C and 80 °
prior to heating to 110 ° C. Calcination was then performed by heating the catalyst
with 50 ° C increments from 200 ° C to 400 ° C in static air. The sequence described
above was repeated for those catalysts that contained a Mo loading higher than 14
wt%, due to limited solubility of AHM in water.
MoV/Al catalysts were prepared by co-impregnation. The impregnate contained
not only AHM, but also NH4VO3 in this case. The pH of these impregnates was
neutral. Due to the low solubility of NH4VO3, impregnation was performed twice
in order to obtain the desired Mo and V loadings. The catalysts are denoted using
the format Mo(wt%)V(wt%)/Al. Unless otherwise mentioned catalyst loadings are
given as weight percentage of metal. In the case loadings are calculated on a per
mole basis, the loadings are given as mole of metal per mole of Al2O3.
The metal loading of all catalysts was analysed by Atomic Absorption
Spectrometry, using a Perkin-Elmer 3030 Atomic Absorption Spectrophotometer.
The standard addition method was applied for the analysis of Mo.
2.2
Catalyst characterization
2.2.1 Diffuse reflection UVUV-Vis spectroscopy
Diffuse reflectance UV-Vis (DR-UVVis) spectra were recorded on a Shimadzu
UV-2401 spectrometer, equipped with a Shimadzu ISR-240A integrating sphere.
Cuvettes having a path length of 5.0 mm constructed from Suprasil quartz were
used. γ-Alumina shows some absorption at low wavelengths (lower than 500 nm).
Since all Mo oxide bands show intensities in this region γ-alumina was used as
reference for all experiments. All samples were ground before the experiment.
114
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
2.2.2 Temperature
Temperature Programmed Reduction
Temperature Programmed Reduction (TPR) was performed by heating 25 mg of
catalyst in an 8 ml/min N2/H2 flow (96%/4%). An equal degree of oxidation was
assured for all catalysts by heating the samples in situ to 500 ° C in He/O2 (8
ml/min, 96%/4%) flow prior to TPR experiment. The samples were flushed in He
and N2 subsequently to remove all oxygen present at the catalyst after this
treatment. H2 and O2 consumption was monitored using thermistor detectors
(Gow Mac Instrument). Water formed during reduction was removed using a
zeolite 4A molecular sieve before detection of the hydrogen signal. The
temperature resolution was 1 ° C.
2.2.3 Raman Spectroscopy
Raman Spectroscopy was performed using a single monochromator Renishaw
1000 spectrometer equipped with a cooled CCD detector (200 K) and a
holographic Notch filter. The samples were excited with the 514 nm Ar line in an
in situ cell (Linkam, TS-1500). The spectral resolution was better than 2 cm-1. The
samples were heated in N2 to 500 ° C by 15 ° C per minute prior to recording the
Raman signal, in order to remove fluorescence. The Raman shift was recorded
after cooling the sample to room temperature. Some in situ analyses were
performed while continuously flowing He, O2 and/or NH3. The Raman signal was
recorded under He flow at room temperature after treatment at different
temperatures and under different flow conditions.
UV Raman spectra were recorded using a Jobin-Yvon/Spex T64000 Raman
spectrometer. Excitation of the catalysts took place using a Lexel Laser Inc. model
95 ion laser, using the 244 nm line. The spectral resolution was 2.2 cm-1 in this
case. The catalyst samples were not pre-treated prior to the experiments, since the
use of the UV laser prevented the occurrence of fluorescence in the spectra.
2.2.4 Transmission Electron Microscopy
Transmission Electron Microscopy (TEM) was performed using a Philips CM 30
ST electron microscope, equipped with a LaB6 filament as electron source,
operated at 300 kV. The catalyst was mounted on a carbon polymer microgrid
supported on a copper grid, by placing a few droplets of a suspension containing
the powdered catalyst in ethanol and evaporating the ethanol under ambient
conditions. Several grains of the sample were analysed in order to obtain a
representative image, without focusing on possible local artefacts.
115
Chapter 5
EDX analysis was performed using a LINK EDX system by turning the electron
beam towards the detector. This enabled us to obtain information about the
chemical composition of a small part of the TEM image.
2.2.5 X-Ray Diffraction
X-Ray powder diffraction (XRD) patterns were recorded on a Rigaku Geigerflex
X-ray powder diffractometer using Cu-Kα radiation. Prior to the experiment the
catalysts were grinded and pressed into a sample holder containing vaseline. The
applied scanning speed was 1° per minute. Background subtraction was not
applied.
2.2.6 X-Ray Photoelectron Spectroscopy
X-ray Photoelectron Spectroscopy (XPS) experiments were performed using a VG
ESCALAB 200 spectrometer equipped with an Al Kα X-ray source. A
hemispherical analyzer was used for detection at a pass-energy of 20 eV. The
catalyst samples were ground and pressed in an indium film placed on an iron
stub. γ-Alumina based samples were corrected for charging assuming a binding
energy of 74.4 eV for the Al 2p peak. Charging was usually on the order of 9 eV.
The data were analysed by a standard fit routine using a non-linear Shirley
background subtraction and a Gauss-Lorentzian curve-fit function. Samples that
were transferred to the XPS chamber without exposure to ambient conditions by
mounting the sample in the indium film in a N2 filled glove box. For this purpose
reaction and pre-treatment of the catalysts was performed in a reactor that could
be closed and transported to the glove box preventing exposure of the catalyst to
ambient air. The XPS cell could also be closed from ambient air.
2.2.7 Hydrogen–
Hydrogen–deuterium exchange reactions
Hydrogen-deuterium exchange reactions were performed in a recirculation reactor
setup equipped with a membrane pump. Oxygen traces were removed from the
hydrogen-deuterium exchange gas mixture using a BTS catalyst. After purging the
system sampling of 10 ml H2 and 10 ml D2 was performed. The carrier gas applied
was Ar. During recirculation a small amount of the reaction mixture was leaked
into a quadruple mass spectrometer (Balzers QMG 200M system) equipped with a
secondary electron multiplier operated at 1200 V. The total decrease in pressure
during a reaction was not larger than 1 %. Typically mass spectra were recorded
every 30 seconds. The initial reaction rate was calculated by extrapolation of the
116
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
signals to t= 0 s. Two hundred milligrams of catalyst were used for the reaction.
The reaction temperature was 150 ° C. The circulation flow rate was 60 Nml/min.
2.3
Ammoxidation of toluene
Toluene ammoxidation reactions were carried out in a single-pass tubular reactor
(4 mm internal diameter) under plug flow conditions. The flows of NH3, O2 and
He (as an inert diluent) were controlled using Brooks Thermal Mass-flow
Controllers. Part of the He flow was saturated with toluene (p.a.) using a threestep saturator that was kept at a temperature of 9.4 ° C. No purification of the gases
was found to be necessary. All lines after the catalyst bed were kept at a
temperature of 200 ° C in order to prevent condensation of products. The organic
products were analysed by on-line gas chromatography, using a Hewlett Packard
5890 series II GLC, equipped with a 50 m HP-5 column and a flame ionisation
detector.
Conversions, selectivities and benzonitrile production rates were calculated, based
on the toluene inlet signal, which was measured before starting the reaction.
Toluene conversion levels and selectivities towards organic products could be
analysed accurately by using this method, irrespective of a lack of carbon balance.
The molar ratio of toluene: NH3: O2 is represented as T: N: O.
3. Results and discussion
3.1
Addition of a second metal to Mo/Al
As is shown before [50] Mo/Al produces benzonitrile in high yields in the
ammoxidation of toluene and addition of vanadia dopants leads to increase of the
benzonitrile yield. In this section the characterization of MoV/Al is described in
more detail. To facilitate interpretation catalysts loadings are expressed as mole
percentage of metal in this section. The influence of V addition to Mo/Al catalysts
on the toluene ammoxidation is shown in Figure 5.6. Rate constants were
calculated per mol of metal assuming first order kinetics in toluene. The toluene
ammoxidation kinetics are not well described in the literature. Only empirical rate
equations are available. Therefore this assumption is rather speculative. As a
rough estimate, however, first order hydrocarbon dependence can be applied [51].
The reaction is assumed to be independent in NH3 and O2 above a minimum
concentration of both reactants. Consecutive and side reactions were neglected for
the calculation of the rate constants. The Mo(11.6)/Al and V(3.3)/Al catalysts,
117
Chapter 5
also prepared using neutral impregnation solutions, are included for comparison.
These catalysts were. Addition of V to Mo(11.6)/Al leads to a strong increase in
toluene ammoxidation activity, with respect to both undoped Mo/Al and
undoped V/Al catalysts. As shown before [50], the benzonitrile selectivity is
hardly influenced upon V doping.
16
Mo/Al
(11.6)
MoV/Al
(11.6 0.8)
MoV/Al
(11.5 1.6)
MoV/Al
(11.4 3.1)
V/Al
(3.3)
12
8
3
k [m 3 / (m m o l M e t a l ·s)]
20
4
0
C a ta ly st
Figure 5.6: Effect of V addition to Mo/Al on the toluene ammoxidation rate.
T: N: O= 1: 8: 13; WHSV = 0.7. Numbers in parentheses are mole percentages of
Mo resp. V.
Raman spectroscopy studies were performed on the MoV/Al samples, as shown
in Figure 5.7. The spectrum of Mo(11.6)/Al contains a somewhat broadened band
at 996 cm-1 only. This band was observed also by Vuurman et al. [52]. The band
can be assigned to the stretching vibration of Mo=O groups [27]. Since no other
Raman bands are present this Mo=O stretching must originate from an isolated
surface monomolybdate [53]. Studying Mo/SiO2 De Boer et al. [54] and Bañares
et al. [55] also observed the formation of isolated surface molybdates. They
observed a slightly lower frequency (986 cm-1) for the Raman band of dehydrated
Mo/SiO2. It is well known that the Mo=O stretching frequency is higher when the
sample is dehydrated [56]. Due to interaction with surrounding H2O molecules the
Mo=O bond is weakened when the sample is exposed to ambient air. The effect of
dehydration is generally more pronounced for Mo/γ-Al2O3 than for Mo/SiO2 [11].
The broadening of the Mo=O band in our Mo(11.6)Al sample (see Figure 5.7)
may indicate the small cluster size of the surface Mo species when no V is present
[57]. The absence of Raman bands at around 820 cm-1 (antisymmetric O-Mo-O
stretching [27]) and 220 cm-1 (Mo-O-Mo deformation) indicates that surface
118
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
R a m a n sig n a l [a .u .]
polymolybdates were not formed in significant amounts when only Mo was
present [10,54].
M o(11.4)V(3.1)/A l
M o(11.5)V(1.6)/A l
M o(11.6)/A l
M oO 3
200
500
800
1100
-1
W a v e le n g th [c m ]
Figure 5.7: Raman Spectroscopy for Mo/Al and MoV/Al samples. Numbers in
parentheses are mole percentages.
Conversely, for the MoV/Al samples at V loadings higher than 0.8 mol% (higher
than 0.4 wt%) the presence of MoO3 was clearly observed. The Mo(11.6)V(0.8)Al
spectrum is not plotted since it was highly distorted due to the presence of a strong
fluorescence background. No indication for the presence of MoO3 was found for
this catalyst, though. For reference also the spectrum of bulk MoO3 was recorded,
showing the same bands as the MoV/Al samples. The formation of MoO3 is not
expected based on the Mo loading. If we calculate the theoretical monolayer a Mo
loading of around 23 mol% is found. Apparently V addition reduces monolayer
coverage. As a result MoO3 is formed at lower loadings. Competition between Mo
and V for reaction with the alumina hydroxyl groups was also observed by
Vuurman et al. [58]. These authors found an increase in polymerised vanadium
oxide species when molybdena or tungsten oxide was present.
The presence of MoO3 could explain the higher toluene ammoxidation activity of
MoV/Al. The dispersion of the MoO3 clusters over the γ-alumina support is quite
good, since XRD did not reveal any peaks that correlate with MoO3 or
Al2(MoO4)3 as can be seen from Figure 5.8. The detection limit is determined by
the crystal size, which is estimated at around 4 nm [59]. Only γ-alumina spacings
could be clearly identified. Some evidence was found for the formation of
Al2V10O28∙ 22H2O (lattice spacings of 10.6 and 7.0 Å). The presence of the strong
119
Chapter 5
line at a diffraction angle of 47 ° (lattice spacing of 3.1 Å) could not be explained.
This line is present in the pure γ-alumina batch applied. The diffractogram of the γalumina support was reported earlier by Peeters et al. [60]. The noisy background
level of the spectrum shows the presence of an amorphous fraction in the catalyst.
Intensity [a.u.]
• • **
5
*
*
*
40
20
80
60
100
2 Theta [°]
Figure 5.8: Röntgen diffractogram of Mo(11.4)V(3.1)/Al. * typical γ-Al2O3 lines; •
typical Al2V10O28∙ 22H2O lines.
3.2
Variation of the molybdenum oxide loading
Figure 5.9 shows the effect of increasing the Mo loading of Mo/Al on the
ammoxidation of toluene.
[m m o l B N / m o l M o ·s]
B e n z o n itrile p ro d u c tio n
0.18
0.16
0.14
0.12
0.10
0.08
4
6
8
10
12
14
M o lo a d in g [w t% ]
Figure 5.9: Toluene ammoxidation over Mo/Al as a function of Mo loading.
T= 380 ° C; T: N: O= 1: 5: 8; WHSV = 0.8.
120
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
The activity per mole Mo, which was measured under differential conditions, is
plotted as a function of Mo loading. When all Mo sites would have similar toluene
ammoxidation activities, a constant activity is expected since the activity is
expressed per mole Mo. However, after a slight decrease until a Mo loading of
around 8 wt% the activity increases with the catalyst loading. This increase was
mainly caused by the higher benzonitrile selectivity observed at higher Mo
loading. At Mo loadings higher than 8.5 wt%, Mo sites are formed that are
significantly more active for the ammoxidation of toluene. With increasing above
8.5 wt% Mo the number of these active clusters increased, leading to higher
benzonitrile production rates. As will be explained in great detail in the following
sections the Mo surface chemistry is strongly influenced by the Mo loading. At
low Mo loading surface monomolybdates are formed, whereas at higher loading
surface polymolybdates are present.
Recently, Han et al. [61] discussed the relationship between metal oxide cluster
size and propylene oxidation activity over silica supported bismuth molybdate
catalysts. They found poor acrolein yields over those catalysts that contained
highly dispersed metal oxide clusters. Assuming a Mars and Van Krevelen
mechanism a rather bulky layer that can play the role of an active oxygen reservoir
is required. At low Mo loadings only highly dispersed surface monomolybdates
are present, which cannot supply oxygen, as explained in Section 1.5. This could
explain the low (amm)oxidation activity of this catalyst. Similarly Iwasawa [7]
reports much higher turnover frequencies for propylene oxidation over silica
supported surface polymolybdates compared to surface monomolybdates. Also the
selectivity to acrylaldehyde is much higher over surface polymolybdates than over
surface monomolybdates. For Mo/Al Peeters et al. [60] found the presence of
larger MoOx clusters. They performed the oxidative ammonolysis of ethylene to
acetonitrile. By hydrogen pre-treatment or by exposure for a longer time to
reaction conditions the catalyst was reduced to a MoO2-like structure and loss of
dispersion occurred. Compared to fresh catalysts they found a significantly higher
activity for those reduced catalysts. This is in accordance with the low toluene
ammoxidation activity found in our experiments when the Mo loading is low.
3.3
DR-UVVis Spectroscopy
Figure 5.10 shows the DR-UVVis spectra of Mo/Al as function of the Mo loading.
All bands can be ascribed to the ligand to metal charge transfer processes O2- →
Mo6+, since Mo has the d0 electronic configuration. The spectra were recorded
121
Chapter 5
against a γ-alumina blank sample since γ-alumina itself shows some absorbance at
lower wavelength as shown in the inset.
A b so rb a n c e
0.4
1.5
Absorbance
0.9
0.2
0.1
0
200
Mo(11)Al
1.2
0.3
300
400
W av elen gth [n m ]
500
Mo(3)Al
0.6
0.3
Mo(18)Al
Mo(6)Al
0
200
250
300
350
400
450
500
Wavelength [nm]
Figure 5.10: DR-UVVis spectra of Mo/Al catalysts. The inset shows the γ-alumina
spectrum (against a BaSO4 reference).
Those samples having a Mo loading up to 6 wt% show only bands located at
around 230 nm and at around 290 nm. These bands are usually ascribed to
tetrahedral surface molybdates. Wang and Hall for example found similar bands
using molybdate reference salts [19]. Small differences were found since Wang and
Halls experiments were performed in solution; the values found in our
experiments are slightly higher. Ashley and Mitchell [62] also found a shift to
higher wavelength in reflection compared to absorption experiments in solution.
For Mo(18)/Al a shoulder appears in the 320-370 nm region of the spectrum. This
band was frequently assigned to octahedral surface polymolybdates [2,8,62,63].
Usually, the assignment of the UV-bands to either tetrahedral or octahedral
molybdenum oxide is based on comparison with the UV-Vis data of model
compounds such as Na2MoO4 (tetrahedral Mo coordination) or (NH4)6Mo7O24 or
MoO3 (octahedral Mo coordination), as described formerly by several authors
[64]. The spectra of these compounds, however, vary greatly. Jeziorowski and
Knözinger [27] were the first to reinterpret the assignment of the UV bands. They
ascribed the 270-295 band to ligand to metal charge transfer in the Mo-O-Mo
bridge of polymolybdate rather than to tetrahedral monomolybdate. Since the
literature reports differences in both the position of the bands as well as their
122
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
widths, it is important to analyse the UV-Vis data carefully. Fournier et al. [65]
interpreted DR-UVVis spectra of various polymolybdate salts differing in size,
thus in Mo-Mo interaction, and in symmetry. This approach excluded differences
in the spectra that could be caused by the comparison of solid samples and
solutions, influence of the degree of hydration and the influence of absorption by
the support. Especially the high-energy bands, which appear at low wavenumbers,
can be influenced strongly by the support. These authors found that the local
symmetry (i.e. the symmetry of the inner coordination sphere of the Mo centers)
did not have any clear influence on the LMCT bands of the polymolybdate
anions. The overall symmetry (i.e. the symmetry of the polymolybdates
themselves) did only show a marginal influence on the width of the low energy
band at around 350 nm. The main difference of the spectra was ascribed to the
effect of the polymolybdate cluster size. Increase of the size led to broadening and
a shift of the low energy band to higher wavelengths. This concept can be applied
successfully to supported catalysts. When Mo is well dispersed, the interaction
between the Mo atoms is low, leading to a small band at low energy. On the other
hand, when the dispersion is decreased the surface molybdate clusters grow and
have more Mo-Mo interactions. This results in a shift to higher wavenumber as
well as to broadening of the low energy band. Broadening of this band can be
clearly observed in Figure 5.10 upon increase of the Mo loading. For the γalumina samples described in this chapter also a shift to higher wavenumbers was
observed when the Mo loading increases.
Additionally, UV Raman spectroscopy clearly shows the development of a
shoulder at 820 cm-1 when the Mo loading is increased from 6 to 10 wt%, as
shown in Figure 5.11. This shoulder is assigned to formation of surface
polymolybdates. The band at 960 cm-1, assigned to (isolated) monomolybdates
remains unaltered. The presence of both the 820 cm-1 shoulder and the 960 cm-1
bands proves that both surface polymolybdates and surface monomolybdates are
present in the Mo(10)/Al catalyst. In the low wavenumber part of the Raman
spectrum a poorly developed band can be observed in the 320-370 cm-1 region.
Though the intensity of this band is quite low and the resolution is poor a shift
from lower (328 cm-1) to higher (360 cm-1) wavenumber can be seen. This shift
does support the presence of polymolybdates in the Mo(10)/Al sample. However,
also for Mo(6)/Al this band is broad and besides the peak maximum at 328 cm-1
also a very small peak at 354 cm-1 can be observed. This indicates the presence of
some octahedrally coordinated Mo surface sites for this sample as well. This is
consistent with the data measured by Li [66], who found the presence of both
123
Chapter 5
R am a n in t e n sity [a.u .]
tetrahedrally coordinated Mo and octahedrally coordinated Mo at Mo loadings as
low as 0.1 wt%.
M o(10)/A l
M o(6)/A l
100
300
500
700
900
1100
W a v e n u m b e r [c m -1 ]
Figure 5.11: UV Raman spectra of Mo(6)/Al and Mo(10)/Al.
To summarize, the UV-VIS data of Mo/Al support a decrease in dispersion for
those catalysts that have a Mo loading of 10 weight percent and higher, though the
technique cannot discriminate between the exact nature of the Mo surface species.
3.4
Reduction of Mo/Al catalysts
Hydrogen TPR experiments were conducted in order to explain the behaviour of
the benzonitrile production rate as a function of Mo loading. In Figure 5.12 the
TPR results are shown as a function of Mo loading for Mo/Al catalysts in the 250650 ° C temperature range. As described in Section 2.2.2, all catalysts were treated
in He/O2 flow prior to performance of the TPR experiment. During this treatment
O2 consumption was not observed for any of the catalysts, as expected since all
catalysts were in a calcined form.
The peak maximum of the reduction peak clearly shifts to lower temperature at
increasing Mo loading. The decrease of Tmax was strong for Mo loadings lower
than approximately 8 wt% and levelled off at higher Mo loading. Park et al. [67]
and Van Veen et al. [16] observed a similar behaviour for Mo/Al catalysts that
were prepared under acidic and neutral conditions. The shift of Tmax can be
explained by the presence of Mo7O246- sites at Mo loadings higher than 8.5 weight
percent. At low Mo loading only tetrahedral coordinated surface monomolybdate
is present. De Beer and Schuit found that these clusters are difficult to reduce [68].
124
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
T P R sig n a l [a .u .]
Also TPR and XPS experiments using differently loaded Mo/α-Al2O3 catalysts
showed that reduction occurred easier at higher Mo loadings [69].
T re d [K ]
900
850
Increasing Mo loading
300
500
700
900
T e m p e ra tu re [K ]
1100
800
750
0
5
10
15
20
M o ly b d e n u m lo a d in g [w t % ]
Figure 5.12: Peak maximum temperature of the main reduction peak during
hydrogen TPR of Mo/Al catalysts. Inset shows the temperature profiles.
When NH3 was used as reducing agent the NH3 dissociation reaction took place,
producing H2 and N2.
M S In ten si ty [a . u . ]
0.03
NH3
H2
0.02
N2
0.01
0.00
775
825
875
925
975
T em p er a tu r e [K ]
Figure 5.13: Ammonia dissociation over Mo(2.6)/Al.
As an example Figure 5.13 shows the NH3 dissociation over Mo(2.6)/Al. NH3
was consumed for Mo reduction, as was observed by the formation of water
125
Chapter 5
during the experiment and the brown colour of the catalyst at the end of the
experiment. The water formation could not be quantified, since the amount of
water produced was very low and mass interference of water and ammonia
occurred. Therefore no attempt was made to quantify the degree of reduction,
based on the amount of water produced.
N itr o ge n o n se t [ K ]
900
850
800
750
700
0
5
10
15
20
M o lo ad in g [ w t.% ]
Figure 5.14: NH3 dissociation over Mo/Al catalysts as function of Mo loading.
Similar to the relation between the maximum of the reduction peak in the H2-TPR
experiments the relation was plotted between the onset of N2 production and the
Mo loading. The onset of N2 production, i.e. the temperature at which the NH3
dissociation reaction started to occur (determined by the temperature at which the
m/e=2 signal increased by at least 10 %) was lowered when the Mo loading was
increased, as shown in Figure 5.14. The curve had a similar behaviour as was
found for the H2-TPR experiments. This means that N activation occurs more
easily when the Mo loading is increased. At Mo loadings of 8.5 wt% and higher
no significant further decrease in the temperature required for NH3 dissociation
into H2 and N2 was observed.
3.5
Hydrogen-deuterium exchange over Mo/Al catalysts
To verify the influence of the Mo loading on the dispersion H2-D2 exchange
experiments were performed. Known amounts of H2 and D2 were sampled to the
catalyst. Subsequently Ar was flowed over the catalyst bed. While continuously
measuring the concentration of all H containing molecules present, the reaction
126
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
mixture was recirculated through the catalyst bed. H2 and D2 adsorb dissociatively
on the catalyst surface. Therefore, consecutive desorption leads to scrambling of
the H and D atoms. When no other processes occur than dissociative adsorption
and desorption theoretically a mixture consisting of 50 % HD, 25 % D2 and 25 %
H2 would be obtained after reaching chemical equilibrium. As shown in Figure
5.15, dissociation of H2 and D2 occurred and a mixture of H2, D2 and HD was
formed.
M o le fr a c tio n
0.6
HD
H2
D2
0.4
0.2
0.0
0
200
400
600
800
1000
T i m e [ m in ]
Figure 5.15: H2–D2 exchange over Mo(1.2)/Al at T= 150 ° C.
As was demonstrated for platinum catalysts by Hanson and Boudart, H2
dissociation is structure sensitive [70]. The reaction rate increases with the
dispersion. In the experiments reported here the rate of HD formation has been
taken as a measure for the dispersion. The initial HD formation rates were
measured at 150 ° C as a function of Mo loading for a series of Mo/Al catalysts.
The rates were expressed per mole of Mo, to enable easy interpretation of the
influence of Mo loading on the Mo dispersion. The initial HD formation and the
initial H2 and D2 consumption rates were calculated by interpolation to time is
zero, using a fit routine to describe the rates. Figure 5.16 shows the HD formation
rate as function of Mo loading. The HD formation rate is high at low Mo loading
and quickly decreases when the Mo loading is increased. This can be explained
well by a decrease of dispersion upon increasing Mo loading. At low Mo loading
well-dispersed tetrahedrally coordinated surface monomolybdates are formed.
These sites are highly active for H2 and D2 dissociation and lead to high HD
formation rates. When the Mo loading is increased surface polymolybdates are
formed. As a consequence, the dispersion of Mo decreases and the HD formation
rate decreases. When the Mo loading is higher than 8.5 wt% the HD formation
rate becomes close to zero. At this loading the amount of tetrahedrally
127
Chapter 5
coordinated surface monomolybdates has strongly decreased as was indicated by
the UV Raman spectra discussed in Section 3.3 (Figure 4.11).
R at e [m o l/ (m o l M o •h r)]
35
30
25
20
15
10
5
0
0
4
8
12
16
20
M o lo a d in g [w t% ]
Figure 5.16: The HD formation rate over Mo/Al as a function of Mo loading.
To estimate the Mo dispersion at higher loadings, the H2-D2 exchange reaction
was also performed at higher temperature. Figure 5.17 shows the temperature
programmed H2-D2 exchange reaction over Mo(14)/Al. The HD fraction is
plotted as a function of temperature. Also the H2 consumption during TPR is
plotted in this figure. Obviously H-D exchange occurs only at temperatures higher
than 250 ° C. When further reduction occurs, the HD production increases. At
temperatures lower than 250 ° C Mo is in 6+-oxidation state. This shows that for
Mo(14.2)/Al only (partly) reduced Mo is able to catalyse the H-D exchange
reaction. As shown in Section 3.4 the reduction temperature strongly increases at
lower Mo loading. Higher reaction temperatures for the H2-D2 exchange reaction,
therefore, could not be applied to compare the dispersion of Mo, since the
oxidation state is not the same over the whole range of Mo/Al catalysts.
The decrease of the H-D exchange rate with the Mo loading was similar to that
observed by Hensen [71] over sulfided Mo/Al catalysts, who showed that high
HD formation rates at Mo loadings lower than 5 wt% can be explained by the
presence of basic hydroxyls of the γ-alumina support. As explained in Section 1.1,
the basic hydroxyls primarily react with the impregnation solution. At Mo
loadings higher than 5 wt% all basic hydroxyls have been consumed. The higher
HD formation rates at lower Mo sulfide loading therefore could be explained by
the presence of spillover hydrogen, originating from basic hydroxyls. In the case of
128
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
γ-alumina supported Mo oxide the H-D exchange rates are much higher at low
Mo loading compared to γ-alumina supported Mo sulfide.
0.3
H 2 - D 2 exchange
0.2
0.1
TP R curv e
[a .u .]
H 2 con su m p tion
H D fra ctio n [-]
H D pro d uctio n during
0.0
200
300
400
500
600
700
T e m p e ra tu re [°C ]
Figure 5.17: TPR and Temperature Programmed H2-D2 exchange over
Mo(14.2)/Al.
Table 5.2 compares these rates. Note that the same catalyst precursors were
applied for both the calcined and sulfided catalysts. Spillover hydrogen alone,
therefore, does not explain the decrease of the HD formation rate with increasing
Mo loading for the Mo oxide catalysts. Tetrahedrally coordinated surface
monomolybdate catalyses H-D exchange. When the Mo loading is increased
surface polymolybdates are formed and the HD formation rate decreases.
Table 5.2: H2-D2 exchange rates over γ-alumina supported Mo oxide and sulfide
Mo loading [wt%]
1.2
2.6
4.9
6.7
8.5
10.3
14.2
16.8
1
HD exchange rate [molHD/(molcat∙ hr)]
Mo oxide
Mo sulfide1
38.7
12.4
6.8
1.6
0.9
0.3
0.1
0.0
6.9
5.8
3.7
3.2
3.0
2.7
2.3
Not measured
Data from Hensen [71].
129
Chapter 5
E q u i li b r i u m fr a cti o n [ m o l/ m o l]
0.6
0.5
HD
H2
0.4
0.3
D2
0.2
0.1
0
0
2
4
6
8
10
12
14
16
18
M o ly b d en u m lo a d i n g [ w t. % ]
Figure 5.18: Equilibrium fractions of H2, D2 and HD as function of Mo loading of
Mo/Al catalysts.
H 0 [m m ol/ g c at ]
Spillover H atoms could explain the deviation from the equilibrium composition.
Figure 5.18 shows the equilibrium fractions of HD, H2 and D2, again plotted
against the Mo loading. The equilibrium fractions were obtained after performing
the reaction for such a long time that no change was observed in the
concentrations of H2, D2 and HD. Usually the time of reaction was in the order of
16 hours before equilibrium was obtained. At Mo loadings up to 7 wt% the H2
fraction is slightly higher than the
0.5
D2 fraction. This can be explained
0.4
qualitatively by the participation
0.3
of spillover H. However, the
0.2
molar amount of spillover H
0.1
0.0
decreases to zero if the Mo
0
1
2
3
4
5
loading is increased from 1.2 wt%
M olybden u m lo a din g [w t.% ]
to 4.9 wt%, as shown in Figure
Figure 5.19: Amount of H0 as function
5.19. This result is very similar to
of Mo loading of Mo/Al.
the result described by Hensen
[71].The amount of spillover H
follows from the mass balance, according to Equation 5.3. Gas-phase molecules
are notated with the suffix g and the surface hydrogens that are initially present on
the alumina surface are indicated as H0.
 H   2 H2 , g + H0 
 =

 D   2 D2 , g 
initial
130
 2 H2 , g + HDg 
=

 2 D2 , g + HDg equilibrium
(5.3)
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
Though the decrease of H0 correlates well with the decrease of basic OH-groups
upon increase of the Mo loading, the equilibrium fractions over Mo(6.7)/Al and
Mo(8.5)/Al do not equal the values expected from dissociative adsorption,
recombination and desorption only. The H0 amount increased to 0.48 and 0.30
mmol/gcat. This can be explained by assuming that increase of the Mo loading
leads to clustering of isolated surface monomolybdates to surface polymolybdates,
which are less active in hydrogen activation. When surface polymolybdates have
formed, those sites that contain spillover H become available for reaction again.
3.6
Transmission Electron Microscopy on Mo/Al samples
Transmission Electron Microscopy (TEM) experiments were performed on fresh
Mo/Al samples. MoOx clusters were not observed by TEM, even not at a Mo
loading of 17 wt%. Since MoOx clusters can only be detected when their size is
larger than ca. 1 nm this means that in all cases the size of the MoOx clusters was
smaller than ca. 1 nm. EDX analysis showed the presence of Mo throughout the
whole sample. No inhomogeneities were observed for any of the catalysts.
On the other hand some small clusters were detected on the MoV/Al samples.
Additionally, the sample with the highest toluene ammoxidation activity,
Mo(11)V(0.8)/Al, displayed many Mo containing structures. The clusters could
not be convincingly demonstrated to be MoO3 clusters, however, since they were
to small to perform electron diffraction. The MoO3 clusters as were observed by
Raman Spectroscopy (see Figure 5.7), therefore, must be highly dispersed over the
surface, as was also concluded also from the XRD results described in Section 3.2.
3.7
In situ treatment of Mo/Al
The lower reducibility of Mo/Al samples that have a Mo loading below 10 wt%
may explain the lower toluene ammoxidation. In situ Raman Spectroscopy was
applied to examine the effect of thermal treatment using various gas flows on the
chemistry of surface molybdates. Figure 5.20 shows the Raman spectra of
Mo(11)/Al, which are all recorded at room temperature after treating the catalyst
in situ applying the indicated conditions. Subsequently the catalyst was calcined at
700 ° C, treated in He/NH3 at 360 ° C and 700 ° C respectively, passivated in
artificial air and treated in an NH3 and O2 containing flow at 450 ° C. Finally
recalcination of the catalyst was performed at 700 ° C.
131
Chapter 5
Raman Intensity [A.U.]
He/O2 (700 °C)
He/NH3/O2 (450 °C)
He/O2 (25 °C)
He/NH3 (700 °C)
He/NH3 (360 °C)
He/O2 (700 °C)
200
400
600
800
1000
-1
Wavenumber [cm ]
Figure 5.20: In situ Raman Spectroscopy of Mo(11)/Al.
Freshly calcined Mo(11)/Al shows two Raman bands at 860 cm-1 and at 990 cm-1.
The 860 cm-1 band can be assigned to the asymmetric Mo-O-Mo bending and the
990 cm-1 band to the Mo=O stretching mode. Isolated surface monomolybdates as
well as surface polymolybdates therefore are present at the surface of Mo(11)/Al
as discussed in Section 3.2. After cooling of the sample in He and heating in
He/NH3 by 10 ° C per minute to 360 ° C both bands are still present, although their
intensities were decreased and their frequency was decreased by 15 cm-1. This
decrease upon exposure to NH3 was also found by Stencel et al. [56] and is similar
to the decrease in frequency upon hydration caused by the decrease of the Mo=O
bond strength due to adsorption of H2O. Further heating of the sample in He/NH3
by 1 ° C per minute to 700 ° C did remove the Mo-O bands completely. No other
bands appeared during treatment in He/NH3 flow or after subsequent cooling to
room temperature, in the presence of a He/O2 flow. Heating the sample in a flow
that contained both NH3 and O2 by 5 ° C per minute to 450 ° C led to the
reappearance of the Raman bands that were present initially. Further oxidation in
He/O2 to 700 ° C did not change the Raman spectrum significantly. These
observations indicate that a redox mechanism is operative on Mo/Al catalysts.
Oxidation occurs by O2 and reduction of Mo occurs by NH3.
132
R a m an int e n sit y [a .u .]
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
Re-calcined at 300 °C
NH3 treated at 400 °C
Calcined at 700 °C
200
400
600
800
-1
W a v e n u m b e r [c m ]
1000
1200
Figure 5.21: Raman spectra of Mo(2.6)/Al during NH3-O2 cycles. Oxidation in
O2/Ar (20/80, 100 ml/min); NH3 treatment in NH3/Ar (5/95, 100 ml/min).
R a m an in te n sity [a .u .]
3000
2500
Recalcined at room temperature
2000
1500
1000
NH3 treated at 300 °C
500
Calcined at 700 °C
0
200
400
600
800
-1
W a v e n u m b e r [c m ]
1000
1200
Figure 5.22: Raman spectra of Mo(8.5)/Al during NH3-O2 cycles. Oxidation in
O2/Ar (20/80, 100 ml/min); NH3 treatment in NH3/Ar (5/95, 100 ml/min).
133
R a m a n in te n s ity [a .u .]
Chapter 5
Recalcined at 700 °C
Recalcined at 500 °C
Recalcined at 300 °C
NH3 treated at 700 °C
Calcined at 700 °C
200
400
600
800
-1
W a v e n u m be r [c m ]
1000
1200
Figure 5.23: Raman spectra of Mo(16.8)/Al during NH3/O2 cycles. Oxidation in
O2/Ar (20/80, 100 ml/min); NH3 treatment in NH3/Ar (5/95, 100 ml/min).
Irrespective of the Mo loading redox cycles were observed for Mo/Al. Figures
5.21-5.23 show the effect of reduction by NH3 and sequential reoxidation in
artificial air on the Raman spectra of Mo(2.6)/Al, Mo(8.5)/Al and Mo(16.8)/Al.
For Mo(2.6)/Al and Mo(8.5)/Al similar decreases in frequency of the highwavenumber band were observed, indicating weakening of the Mo=O band upon
ammonia adsorption. Ammonia adsorption, however, does not lead to complete
removal of the Raman bands. Therefore Mo still is in oxidised form. For
Mo(16.8)/Al such a frequency decrease was not observed. For this catalyst a
strong decrease of the intensity of all bands was observed. This could have been
caused by the more easy reduction of the catalyst, as was observed in the H2-TPR
and NH3 dissociation experiments. It should be mentioned though that Mo
reduction also leads to blackening of the catalyst. Though the black colour
strongly supports Mo reduction it also leads to a decrease of the Raman
intensities.
For Mo(16.8)/Al recalcination at 300 ° C is sufficient to re-oxidize the catalyst to
its original state, as is shown in Figure 5.23. After (re)calcinations, the Raman
spectrum basically shows the presence of aluminium molybdate and MoO3. For
Mo(2.6)/Al and Mo(8.5)/Al this temperature does not seem sufficient to
completely reoxidise the catalyst. Strong fluorescence, however, prevented us from
determining by Raman spectroscopy the temperature necessary for complete Mo
reoxidation.
134
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
Toluene ammoxidation
Toluene oxidation
Calcination
240
237
234
231
Binding Energy [eV]
228
Figure 5.24: XP Spectra of Mo(16.8)/Al after calcination, toluene oxidation and
toluene ammoxidation.
To check whether Mo/Al was in a reduced state upon NH3 treatment quasi in situ
XPS experiments were performed using a Mo(16.8)/Al sample. Various
treatments were applied at 400 ° C. After two hours the catalyst was cooled to
room temperature in a He flow. Without exposure to ambient air the catalyst was
transferred to the XPS equipment. Figure 5.24 shows the XP Spectra after
calcination in artificial air, toluene oxidation and toluene ammoxidation
conditions. All spectra could be fitted correctly using only one Mo 3d doublet,
indicating the presence of only one Mo oxidation state. The binding energy was
232.8 eV (± 0.1 eV). This value, which is slightly higher than that of bulk MoO3
[59,72], corresponds to Mo(VI) [73,74], as was expected since the spectra did not
change compared to the calcined sample. The increase of the binding energy
compared to bulk MoO3 is caused by the interaction of the molybdate with the
support [75].
Though Figure 5.24 suggests that the catalyst surface is unaltered upon toluene
(amm)oxidation, reduction of the molybdenum oxide is expected during reaction
as explained earlier [76]. Toluene is adsorbed in an oxygenated form, mostly
referred to as the aldehyde-like intermediate. Oxidation of toluene occurs by lattice
oxygen according to a Mars–Van Krevelen mechanism. The reduced catalyst is
reoxidized by gaseous oxygen.
135
Chapter 5
X P S I n te n s i ty [ a .u . ]
X P sp ectrum
Peak 1: 232.8 eV
Peak 2: 231.5 eV
244
240
236
232
228
224
B in d i n g E n e rg y [ e V ]
Figure 5.25: Mo 3d peaks of calcined and subsequently NH3 treated Mo(16.8)/Al.
Figure 5.25 shows that the catalyst indeed can be partly reduced when the catalyst
is contacted with NH3 at 400 ° C. Compared to completely oxidised Mo(16.8)/Al a
shift of 1.3 eV to lower binding energy was observed. According to Haber et al.
[46], who found a shift of 1.4 eV in Mo 3d binding energy upon reduction of
MoO3 this can be explained with a reduction of Mo6+ to Mo4+. Haber et al. found
two doublets for fresh MoO2. A similar spectrum was measured by Peeters et al.
[59]. The highest value for the Mo(IV) binding energy accounts for “isolated” Mo,
whereas a lower value was found for Mo4+ ions paired in clusters of edge sharing
octahedra. These nuclei can be considered to have an apparent oxidation state of
2+. This apparent oxidation state correlates linearly with the 3d binding energy
values, found by these authors. Grünert et al. [77] confirmed this correlation based
on thermal decomposition of MoO3 in the XPS chamber. The Mo 3d binding
energies decrease proportionally with the Mo valency by a factor of 0.8 eV.
However, at this relatively high Mo loading at least two different Mo species are
present (isolated tetrahedral surface monomolybdate and octahedral surface
polymolybdates), as was shown by Raman spectroscopy (see Figure 5.23). XPS
cannot distinguish between these two types of Mo species, although the chemical
environment is not the same. This means that a linear relationship between the
Mo oxidation state and the binding energy measured by XPS could be affected by
changes in the structure of the (6+) molybdate species. Other authors prefer to
assign the value for the binding energy to Mo(V) [20,72,78]. Poulston et al. [79]
also used XPS to study the reduciblity of ammoxidation catalysts. These authors
found that Bi-Mo oxide was reduced by NH3 at a temperature of 347 ° C. Besides
Mo6+ they found the presence of Mo 3d peaks at Eb = 229.0 eV after reduction by
NH3. This peak, which was ascribed to Mo4+ was stable, even after annealing the
136
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
catalyst at 547 ° C. The reduction of Bi-Mo oxide and Fe-Sb oxide was found to be
easier than reduction of VSbO4, USb3O10 and Bi4V2O11. This order correlates well
with the activity generally found in ammoxidation reactions.
To summarize, Mo/Al is in fully oxidized state after toluene ammoxidation and
toluene oxidation reactions. Our Raman and XPS results show that NH3 is
capable to reduce Mo from a hexavalent to a lower oxidation state. Reoxidation of
the Mo occurs by O2 present in the ammoxidation feedstock.
4. Conclusions
A redox mechanism applies to the ammoxidation of toluene over γ-alumina
supported molybdenum oxide. Reduction of Mo is facilitated when the Mo
loading is increased. The minimum reduction temperature is obtained when the
Mo loading is higher than 8 weight percent. Surface polymolybdates have formed
at this loading. The ammoxidation activity increases when the Mo loading is
increased to values higher than 8 wt%. Surface polymolybdates therefore are more
reactive towards the ammoxidation of toluene. It was shown by TPR and NH3
dissociation that reduction of surface polymolybdates occurs more easily than the
reduction of isolated monomolybdate, which is tetrahedrally coordinated. This
indicates that the rate of toluene ammoxidation is determined by the Mo
reducibility. This is consistent with the fact that toluene activation as oxidised
species (adsorbed benzaldehyde or adsorbed benzoate) is rate determining in the
toluene ammoxidation reaction. Hydrogen–deuterium exchange reactions confirm
this; high exchange rates are obtained over surface monomolybdates. At
increasing Mo loading the H-D exchange rate decreases and becomes close to zero
when the Mo loading is higher than 8.5 wt%. At this Mo loading the amount of
surface monomolybdates is close to zero.
For vanadia containing Mo/Al catalysts the benzonitrile yield is significantly
higher than for the corresponding vanadia-free catalysts. It was shown that
addition of small amounts of V dopants leads to the formation of crystalline MoO3
clusters. These clusters are believed to lead to high benzonitrile yields, since their
reducibility is high.
137
Chapter 5
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
138
H. Midorikawa, N. Sugiyama, H. Hinago, US Patent 6973186, 1999.
H. Kazuyuki, S. Komada, US Patent 5907052, 1999.
K. Aoki, US Patent 5780664, 1998.
H. Midorikawa, K. Someya, US Patent 5663113, 1997.
H. Midorikawa, K. Someya, K. Aoki, O. Nagano, US Patent 5658842, 1997.
R. Canavesi, F. Ligorati, R. Ghezzi, US Patent 4609635, 1986.
N. Giordano, A. Bart, A. Vaghi, A. Castellan, G. Martinotti, J. Catal., 36,
(1975), 81-92.
J.M.J.G. Lipsch, G.C.A. Schuit, J. Catal., 15, (1969), 174-178.
F.E. Massoth, J. Catal., 30, (1973), 204-217.
Xie Youchang, Gui Linlin, Liu Yingjun, Zhang Yufen, Zhao Biying, Yang
Naifang, Guo Qinlin, Duan Lianyun, Huang Huizhong, Cai Xiaohai, Tang
Youchi, in: Adsorption and catalysis on oxide surfaces, Eds. M. Che, G.C.
Bond, Elsevier, Amsterdam, 1985, pp. 139-149.
J. Leyrer, M.I. Zaki, H. Knözinger, J. Phys. Chem., 90, (1986), 4775-4780.
G. Mestl, N.F.D. Verbruggen, F.C. Lange, B. Tesche, H. Knözinger, Langmuir,
12, (1996), 1817-1829.
J. Leyrer, R. Margraf, E. Taglauer, H. Knözinger, Surf. Sci., 201, (1988), 603623.
M. del Arco, S.R.G. Carrazá n, V. Rives, J.V. Garcí a Ramos, Mater. Chem. and
Phys., 31, (1992), 205-211.
S.R. Stampfl, Y. Chen, J.A. Dumesic, C. Niu, C.G. Hill, J. Catal., 105, (1985),
445-454.
R. Margraf, J. Leyrer, H. Knözinger, E. Taglauer, Surf. Sci., 189/190, (1987),
842-850.
G. Mestl, H. Knözinger, Langmuir, 14, (1998), 3964-3966.
Y. Iwasawa, Adv. Catal., 35, (1987), 189-264.
Y. Iwasawa, S. Ogasawara, J. Chem. Soc., Faraday Trans. I, 75, (1979), 14651476.
J.M. Stencel, J.R. Diehl, J.R. d’Este, L.E. Makovsky, L. Rodrigo, K.
Marcinkowska, A. Adnot, P.C. Roberge, S. Kaliaguine, J. Phys. Chem., 90,
(1986), 4739-4743.
Y. Iwasawa, Y. Sato, H. Kuroda, J. Catal., 82, (1983), 289-298.
J.L. Brito, B. Griffe, Catal. Lett., 50, (1998), 169-172.
C.C. Williams, J.G. Ekerdt, J-M. Jehng, F.D. Hardcastle, I.E. Wachs, J. Phys.
Chem., 95, (1991), 8791-8797.
E.g. C.C. Williams, J.G. Ekerdt, J-M., Jehng, F.D. Hardcastle, A.M. Turek, I.
E. Wachs, J. Phys. Chem., 95, (1991), 8781-8791.
M.A. Bañares, H. Hu, I.E. Wachs, J. Catal., 150, (1994), 407-420.
C. Louis, M. Che, J. Catal., 135, (192), 156-172.
A. Brenner, R.L. Burwell Jr., J. Catal., 52, (1978), 353-363.
A. Brenner, R.L. Burwell Jr., J. Catal., 52, (1978), 364-374.
A. Brenner, D.A. Hucul, S.J. Hardwick, Inorg. Chem., 18 (6), (1979), 14781484.
S. Jaenicke, W.L. Loh, Catal. Today, 49, (1999), 123-130.
J.A.R. van Veen, P.A.J.M. Hendriks, E.J.G.M. Romers, R.R. André a, J. Phys.
Chem., 94, (1990), 5275-5282.
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
17. M.A. Aulmann, G.J. Siri, M.N. Blanco, C.V. Caceres, H.J. Thomas, Appl.
Catal., 7, (1983), 139-149.
G.J. Siri, M.N. Morales, M.I. Blanco, H.J. Thomas, Appl. Catal., 19, (1985),
49-63.
C.V. Cá cares, J.L.G. Fierro, A. López Agudo, M.N. Blanco, H.J. Thomas, J.
Catal., 95, (1985), 501-511.
18. J.A.R. van Veen, H. de Wit, C.A. Emeis, P.A.J.M Hendriks, J. Catal., 107,
(1987), 579-582.
19. L. Wang, W.K. Hall, J. Catal., 77, (1982), 232-241.
20. J. Sarrí n, O. Noguera, H. Royo, M.J. Pé rez Zurita, C. Scott, M.R. Goldwasser,
J. Goldwasser, M. Houalla, J. Mol. Catal. A., 144, (1999), 441-450.
21. M.J. Vissenberg, Preperative Aspects of Supported Metal Sulfide Hydrotreating
Catalyst, PhD Thesis, Eindhoven University of Technology, 1999, pp. 83-95.
22. Y. Okamoto, Y. Arima, K. Nagai, S. Umeno, N. Katada, H. Yoshida, T.
Tanaka, M. Yamada, Y. Akai, K. Segawa, A. Nishijima, H. Matsumoto, M.
Niwa, T. Uchijima, Appl. Catal. A., 170, (1998), 315-328.
23. J.A.R. van Veen, J. Coll. Interf. Sci., 121, (1988), 214-219.
24. I. Lindqvist, Nova Acta Regiae Soc. Sco. Ups. IV, 15(1), (1950), 3-22.
25. I. Lindqvist, Acta Chem. Scand., 5, (1951), 568-577.
Y. Sasaki, I. Lindqvist, L.G. Sillé n, J. Inorg. Nucl. Chem., 9, (1959), 93-94.
O. Glemser, W. Holznagel, S.I. Ali, Z. Naturforsch., 20 B, (1965), 192-199.
26. J. Aveston, E.W. Anacker, J. S. Johnson, Inorg. Chem., 3, (1964), 735-746.
27. H. Jeziorowski, H. Knözinger, J. Phys. Chem., 83, (1979), 1166-1173.
28. J.A.R. van Veen, P.A.J.M. Hendriks, Polyhedron, 5, (1986), 75-78.
29. N. Spanos, L. Vordonis, C. Kordulis, A. Lycourghiotis, J. Catal., 124, (1990),
301-314.
N. Spanos, L. Vordonis, C. Kordulis, P.G. Koutsoukos, A. Lycourghiotis, J.
Catal., 124, (1990), 315-323.
N. Spanos, A. Lycourghiotis, J. Catal., 147, 1994), 57-71.
30. G.A. Parks, Chem. Rev., 65, (1965), 177-198.
31. R.J. Hunter, in: Zetapotential in Colloid Science, Principle and Application,
Eds. R.H. Ottewill, R.L. Rowell, Academic Press, London, 1988, 233.
32. Y. Okamoto, S. Umeno, Y. Shiraki, Y. Arima, K. Nakai, O. Chiyoda, H.
Yoshida, K. Inamura, Y. Akai, S. Hasegawa, T. Shishido, H. Hattori, N.
Katada, K. Segawa, N. Koizumi, M. Yamada, I. Mocida, A. Ishihara, T. Kabe,
A. Nishijima, H. Matsumoto, M. Niwa, T. Uchijima, Appl. Catal. A., 170,
(1998), 359-379.
33. J.P. Brunelle, Pure Appl. Chem., 50, (1978), 1211-1229.
34. D.S. Zingg, L.E. Makovsky, R.E. Tischer, F.E. Brown, D.M. Hercules, J. Phys.
Chem., 84, (1980), 2898-2906.
35. R. Thomas, M.C. Mittelmeyer-Hazeleger, F.P.J.M. Kerkhof, J.A. Moulijn, J.
Medema, V.H.J. de Beer, in: Proc. 3rd International Conference on Chemistry
uses of molybdenum, Eds. H.F. Barry, P.C. Mitchell, 1999, pp. 85-91.
36. L. Wang, W.K. Hall, J. Catal., 66, (1980), 251-255.
37. C.P. Cheng, G.L. Schrader, J. Catal. 60, (1979), 276-294.
38. J. Medema, C. van Stam, V.H.J. de Beer, A.J.A. Konings, D.C. Koningsberger,
J. Catal., 53, (1978), 386-400.
39. J.T. Richardson, Ind. Eng. Chem. Fundam., 3, (1964), 154-158.
J.M.J.G Lipsch, G.C.A. Schuit, J. Catal., 15, (1969), 174-178.
J. Sonnemans, P. Mars, J. Catal., 31, (1973), 209-219.
139
Chapter 5
40. F.R. Brown, L.E. Makovsky, K.H. Rhee, J. Catal., 50, (1977), 162-171.
41. J.M. Stencel, Raman spectroscopy for Catalysis, Van Nostrand Reinhold, New
York, 1990, p. 53-79.
42. G-A. Nazri, C. Julien, Solid State Ionics, 53-56, (1992), 376-382.
43. I.R. Beattie, T.R. Gilson, J. Chem Soc. (A), (1969), 2322-2327.
44. W.P. Griffith, J. Chem. Soc. A., (1970), 286-291.
N.F.D. Verbruggen, G. Mestl, L.M.J. von Hippel, B. Lengeler, H. Knözinger,
Langmuir, 10, (1994), 3063-3072.
45. G. Mestl, T.K.K. Srinivasan, Catal. Rev.-Sc. Eng., 40, (1998), 451-570.
46. J. Haber, W. Marczewski, J. Stoch, L. Ungier, Ber. Bunsenges., 79, (1975), 970974.
47. J.F. Brazdil, D.D. Suresh, R.K. Grasselli, J. Catal., 66, (1980), 347-367.
48. I. Matsuura, G.C.A. Schuit, J. Catal., 25, (1972), 314-325.
L. Kihlborg, Arkiv. Kemi., 21, (1963), 443-460.
49. T. Ressler, R.E. Jentoft, J. Wienold, M.M. Günter, O. Timpe, J. Phys. Chem.
B., 104, (2000), 6360-6370.
50. This thesis, Chapter 3.
51. R.G. Rizayev, E.A. Mamedov, V.P. Vislovskii, V.E. Sheinin, Appl. Catal., 83,
(1992), 103-140.
52. M.A. Vuurman, I.E. Wachs, J. Phys. Chem., 96, (1992), 5008-5016.
53. F.D. Hardcastle, I.E. Wachs, J. Raman Spectr., 21, (1990), 683-691.
54. M. de Boer, A.J. van Dillen, D.C. Koningsberger, J.W. Geus, M.A. Vuurman,
I.E. Wachs, Catal. Lett., 11, (1990), 227-240.
55. M.A. Bañares, N.D. Spencer, M.D. Jones, I.E. Wachs, J. Catal. 146, (1994),
204-210.
56. J.M. Stencel, L.E. Makovsky, T.A. Sarkus, J. de Vries, R. Thomas, J.A.
Moulijn, J. Catal. 90, (1984), 314-322.
57. G. Mestl, T.K.K. Srinivasan, H. Knözinger, Langmuir, 11, (1995) 3795-3804.
G. Mestl, N.F.D. Verbruggen, F.C. Lange, B. Tesche, H. Knözinger, Langmuir,
12, (1996), 1917-1829.
58. M.A. Vuurman, D.J. Stufkens, A. Oskam, G. Deo, I.E. Wachs, J. Chem. Soc.,
Faraday Trans., 92, (1996) 3259-3265.
59. S. Rajagopal, H.J. Marini, J.A. Marzari, R. Miranda, J. Catal. 147, (1994), 417428.
60. I. Peeters, A.W. Denier van der Gon, M.A. Reijme, P.J. Kooyman, A.M. de
Jong, J. van Grondelle, H.H. Brongersma, R.A. van Santen, J. Catal., 173,
(1998), 28-42.
61. Y-H. Han, W. Ueda, Y. Moro-Oka, Appl. Catal. A., 176, (1999), 11-16.
62. J.H. Ashley, P.C.H. Mitchell, J. Chem. Soc. (A), (1968), 2821-2827.
J.H. Ashley, P.C.H. Mitchell, J. Chem. Soc. (A), (1969), 2730-2735.
63. P.C.H. Mitchell, F. Trifiró, J. Chem. Soc. (A), (1970), 3183-3188.
G.N. Asmolov, O.V. Krylov, Kinet. Catal., 11, (1970), 847-852.
M. Che, F. Figueras, M. Forissier, J.C. McAteer, in: Proc. 6th Int. Conf. Catal.
London 1976, Eds. G.C. Bond, P.B. Wells, F.C. Tompkins, Vol. 1, The
Chemical Society, London, 1977, pp. 261-269.
H. Pralieud, J. Less-Common Met., 54, (1977), 387-399.
P. Gajardo, P. Grange, B. Delmon, J. Phys. Chem., 83, (1979), 1771-1779.
P. Gajardo, D. Pirotte, P. Grange, B. Delmon, J. Phys. Chem., 83, (1979), 17801786.
64. A. Bartecki, D. Dembicka, J. Inorg. Nucl. Chem., 29, (1967), 2907-2916.
140
The effect of molybdenum oxide reducibility on the ammoxidation of toluene
65.
66.
67.
68.
69.
70.
71.
72.
73.
74.
75.
76.
77.
78.
79.
Y.J. Israeli, Bull. Soc. Chim. Fr., (1964), 2692.
A. Iannibello, S. Marengo, P. Tittarelli, G. Morelli, A. Zecchina, J. Chem. Soc.,
Faraday Trans. I, 80, (1984), 2209-2223.
M. Fournier, C. Louis, M. Che, P. Chaquin, D. Masure, J. Catal., 119, (1989),
400-414.
G. Xiong, C. Li, Z. Feng, P. Ying, Q. Xin, J. Liu, J. Catal., 186, (1999), 234237.
J-N. Park, J-H. Kim and H-I. Lee, Bull. Korean Chem. Soc., 19, (1998), 13631368.
V.H.J. de Beer, G.C.A. Schuit, in: Preparation of Catalysts, Eds. B. Delmon,
J.A. Jacobs, G. Poncelet, Elsevier, Amsterdam, 1976, pp. 343-363.
F. Barath, M. Turki, V. Keller, G. Maire, J. Catal., 185, (1999), 1-11.
F.V. Hanson, M. Boudart, J. Catal., 53, (1978), 56.
M. Boudart, G. Djé ga-Mariadassou, Kinetics of heterogeneous catalytic
reactions, Princeton University Press, New Jersey, 1984, pp. 155-193.
E.J.M. Hensen, Hydrodesulfurization catalysis and mechanism of supported
transition metal sulfides, PhD Thesis, Eindhoven University of Technology,
2000, pp.157-192.
S.O. Grim, L.J. Matienzo, Inorg. Chem. 14, (1975), 1014-1018.
E.g. M.E. Harlin, L.B. Backman, A.O.I. Krause, O.J.T. Jylhä , J. Catal., 183,
(1999), 300-313.
Z. Paá l, P. Té té nyi, M. Muhler, U. Wild, J-M. Manoli, C. Potvin, J. Chem.
Soc., Faraday Trans., 94, (1998), 459-466.
Y.V. Plyuto, I.V. Babich, I.V. Plyuto, A.D. van Langeveld, J.A. Moulijn, Appl.
Surf. Sci., 119, (1997), 11-18.
This thesis, Chapter 2.
W. Grünert, A.Y. Stakheev, R. Feldhaus, K. Anders, E.S. Shpiro, K.M.
Minachev, J. Phys. Chem., 95, (1991), 1323-1328.
D.S. Zingg, L.E. Makovsky, R.E. Tischer, F.R. Brown, D.M. Hercules, J. Phys.
Chem., 84, (1980), 2898-2906.
S. Poulston, N.J. Price, C. Weeks, M.D. Allen, P. Parlett, M. Steinberg, M.
Bowker, J. Catal., 178, (1998), 658-667.
141
142
Summary
Alkylaromatics oxidation is performed nowadays as a liquid phase reaction,
catalysed by metal oxide salts dissolved in acidic solution. Although this process
yields high amounts of the oxygenate, the reaction environment is highly
demanding on both the equipment and the natural environment. The high demand
on the natural environment is expected to lead to more stringent legislation.
Therefore, development of new, clean and selective alkylaromatic oxidation
processes is of great importance. In this respect it is important to minimize the
amount of side- and by-products and to exclude the use of harmful solvents. The
application of heterogeneously catalysed vapour phase direct alkylaromatics
oxidation in principle satisfies all these demands. However, it is difficult to
exclude the formation of by-products due to the higher reactivity of the oxygenate
compared to the substrate. This generally leads to over-oxidation and the
production of CO2. Alkylaromatic ammoxidation -the reaction of alkylaromatic
compounds with ammonia and oxygen to form alkylaromatic nitriles- can be
applied as first step of a highly selective route towards alkylaromatic oxygenates.
The aromatic nitrile can be converted in a second reaction by hydrolysis.
Dependent on the conditions of this second reaction step aromatic aldehydes,
amines, amides or acids can be produced. In this second reaction step ammonia is
regained so this reaction route cleanly yields the alkylaromatic oxygenate.
This thesis focuses on the alkylaromatic ammoxidation. Toluene has been chosen
as substrate, since its relative simplicity ensures a comprehensive catalytic study.
The reaction is performed in the vapour phase at temperatures between 300 and
460 ° C. Generally high selectivity towards benzonitrile can be achieved, especially
compared to aldehydes and alcohols, which are less stable towards combustion.
A broad range of zeolite Y based catalysts was developed for the ammoxidation of
toluene. Due to the well-defined pore structure of the zeolite, NaY based catalysts
offer the unique possibility to introduce transition metals by several methods.
Moreover, the acid-base properties of the zeolite matrix can be adapted easily.
Therefore, zeolite based catalysts potentially offer an advantage catalysts
compared to mixed oxide catalysts, which are currently applied for alkylaromatic
ammoxidation reactions.
143
Summary
Metal oxides were introduced into the matrix of zeolite Y by means of ion
exchange, chemical vapour deposition and incipient wetness impregnation. Also
the catalytic performance of γ-alumina supported metal oxide catalysts was
explored. Since the stability of the catalysts was found to be an important
parameter, catalytic studies were executed over a relative long period, usually
about 15 hours. The catalytic performance of γ-alumina supported molybdenum
oxide was superior over all other catalysts examined. At lower temperature,
however, equal benzonitrile yields were obtained over Cu/NaY catalysts that were
prepared by ion exchange.
High temperature ammonia treatment of γ-alumina supported molybdenum oxide
increases significantly the benzonitrile selectivity at complete toluene conversion,
indicating the importance of the nitrogen containing surface species. The role of
the nitrogen insertion site was examined in more detail using NO instead of NH3
as nitrogen source. Mainly combustion of toluene occurs when the nitroxidation
reaction is performed. Though benzonitrile formation can occur by this so-called
nitroxidation reaction, NH4NO3 or NH4NO2 surface species cannot be considered
as the selective nitrogen insertion sites.
Metal oxide encaged NaY catalysts prepared by metalcarbonyl deposition were
used as model catalysts based on their well-defined structural properties. Mo(CO)6
can be introduced into the supercages of the zeolite Y, unlike to V(CO)6,
Mn2(CO)10 and Co(NO)(CO)3. Transmission Electron Microscopy (TEM) and XRay Photoelectron Spectroscopy (XPS) analyses showed that Mo(CO)6 is
dispersed homogeneously throughout the zeolite pores. Detailed temperature
programmed decarbonylation studies showed complete decarbonylation of the
Mo(CO)6 catalyst precursor.
In the presence of oxygen, thermal activation ensures low temperature Mo
oxidation. All molybdenum oxide clusters were in the 6+-oxidation state as was
indicated by XPS. The dispersion of the resulting molybdenum oxide clusters is
high as was confirmed by TEM analysis. The approximate molybdenum oxide
cluster size did not exceed 14 Å, thus suggesting the presence of intra-zeolite
molybdenum oxide clusters.
XPS experiments were applied to examine the interaction between the Mo(CO)6
guest and the zeolite host. As was shown by the low Na (1s) binding energy of
Mo(CO)6/NaY, Mo(CO)6 interacts with the Na+-cation. The Na (1s) binding
144
Summary
energy increases by 1 eV when oxidative decarbonylation is performed. To study
in greater detail the interaction of the zeolite cation with the Mo(CO)6 guest the
Na+-ions were exchanged with a series of other alkali ions. Based on the
electronegativity difference of the cations the basicity of the zeolite host is varied.
By performing the decomposition of 2-methyl-3-butyn-2-ol the acid-base properties
of the resulting catalysts were probed. It was found by performing control
experiments on the alkali exchanged zeolite host materials that this decomposition
reaction probes well the amount of Lewis acidity and basicity. Further proof of the
interaction of the encaged Mo species with the zeolite cations was found by the
effect of the zeolite basicity on the decomposition of the encaged Mo(CO)6. Lower
decarbonylation temperatures were found when the Lewis basicity of the zeolite
host is higher. Compared to the less basic cations the electron-rich Mo(CO)6 guest
is less stable, which leads to lower decarbonylation temperatures.
Preliminary catalytic tests on the effect of the Lewis acid/base properties showed
an increase of the catalyst activity with increasing basicity, confirming formation
of a carbanion as the first step in toluene activation, as was proposed in the
literature.
γ-Alumina supported molybdenum oxide catalysts were studied in great detail.
The nature of the molybdenum surface species was varied by varying the Mo
loading. A combination of Temperature Programmed Reduction, in situ and ex
situ Raman Spectroscopy, X-Ray Photoelectron Spectroscopy, X-Ray diffraction,
Transmission Electron Microscopy, diffuse reflectance ultraviolet spectroscopy
and hydrogen-deuterium exchange test reactions was applied to study the
morphology and the reduction behaviour of γ-alumina supported molybdenum
oxide. At low Mo loading tetrahedrally coordinated Mo is present on the γalumina support as surface monomolybdate. The reducibility of Mo is low for
these catalysts. At the increase of the Mo loading polymolybdates start to form.
These polymolybdate clusters have octahedral Mo coordination. The reduction
temperature of these catalysts is lowered by approximately 140 ° C compared to
surface monomolybdates.
Only tetrahedrally coordinated Mo seems to catalyse the hydrogen–deuterium
exchange reaction. At Mo loadings higher than 8.5 wt% no H-D exchange activity
was observed. A small amount of hydrogen is present at the support. This
hydrogen participates in the H-D exchange reaction by spillover.
145
Summary
In situ Raman spectroscopy indicated that Mo is reduced by NH3 at the
ammoxidation reaction temperature for γ-alumina supported molybdenum oxide,
when the loading is higher than 10 wt%. At this Mo loading no tetrahedrally
coordinated monomolybdate is present at the catalyst surface. This result was
confirmed by quasi in situ XPS. The Mo oxidation state is 4+ after NH3 treatment
at 400 ° C. When the loading is lower than 10 wt% the Raman active Mo-O bands
are shifted to lower wavenumber, which is very similar to the Mo-O shift to lower
wavenumber observed for hydrated supported molybdenum oxide catalysts. This
shows that the Mo-O bond is weakened by the presence of ammonia. Reoxidation
in oxygen recovers the initial oxidation state. Since the ammoxidation of toluene
occurs via a redox mechanism the catalyst reducibility strongly influences the
benzonitrile production rate. At molybdenum oxide loadings below 8.5 wt% low
benzonitrile production rates are observed. This can be understood from the fact
that reduction by ammonia does not occur for these catalysts. When the
molybdenum loading is increased over 10 wt% the toluene ammoxidation activity
strongly increases, as expected from the higher molybdenum reducibility at higher
Mo loading. Similarly, the low reducibility of intra-zeolite molybdenum oxide
could explain the low activity of NaY encaged molybdenum oxide. This, however,
was not extensively studied in this thesis.
Addition of small amounts of vanadia dopants leads to increase of the
ammoxidation activity. Raman Spectroscopy showed that MoO3 clusters are
formed upon V doping. The presence of MoO3 is unexpected, since the monolayer coverage of molybdenum oxide (plus vanadium oxide) is significantly higher
than was applied. The MoO3 clusters were highly dispersed over the γ-alumina
surface, since no MoO3 was observed by XRD. Transmission Electron Microscopy
confirmed the high dispersion, since no TEM-detectable MoO3-clusters were
observed.
146
Samenvatting
De oxidatie van alkylaromaten wordt heden ten dage uitgevoerd als een
vloeistoffase proces, gekatalyseerd door metaalzouten in een zure oplossing.
Alhoewel in het algemeen hoge opbrengsten van het gewenste oxidatie product
worden behaald, stellen deze processen hoge eisen aan zowel de apparatuur als
aan het natuurlijk milieu. Naar verwachting zal in de toekomst strengere
wetgeving hogere eisen gaan stellen aan deze processen, wat het belang van de
ontwikkeling van nieuwe, schone en selectieve alkylaromaat oxidatie processen
onderstreept. Hiertoe is het van groot belang om vorming van bijproducten en van
schadelijke oplosmiddelen te voorkomen. In principe voldoen heterogeen
gekatalyseerde gasfase alkylaromaat oxidatie processen aan de bovenstaande
eisen. Omdat het oxidatieprodukt doorgaans reactiever is dan de reactant, het
alkylaromaat, is het echter niet eenvoudig om de vorming van bijproducten te
minimaliseren. Met name over-oxidatie van het substraat tot CO2 treedt doorgaans
in hoge mate op. Een selectief twee-staps proces voor de vorming van
alkylaromaat oxidatieproducten is alkylaromaat ammoxidatie – de reactie van een
alkylaromaat met ammoniak en zuurstof tot een alkylaromatisch nitril – gevolgd
door hydrolyse van het gevormde nitril. Afhankelijk van de exacte condities van
de hydrolyse reactie kunnen op deze wijze aromatische aldehydes, amines, amides
of zuren worden geproduceerd. In deze tweede processtap wordt ammoniak
gevormd, zodat het oxidatieprodukt op zeer schone wijze wordt gevormd. Daar de
ammoxidatie reactie eenvoudiger verloopt dan directe zijketen oxidatie speelt de
vorming van CO2 een minder grote rol in dit proces.
Dit proefschrift richt zich op de ammoxidatie van alkylaromaten. Tolueen is
gekozen als substraat, aangezien de relatieve eenvoud van dit substraat uitvoering
van een katalytische studie mogelijk maakt. De reactie wordt uitgevoerd in de
gasfase bij temperaturen tussen 300 en 460 ° C. In het algemeen kan een hoge
selectiviteit naar benzonitril worden verkregen, met name in vergelijking met de
directe oxidatie van tolueen tot benzaldehyde en benzylalcohol.
Een groot aantal katalysatoren gebaseerd op zeoliet Y zijn ontwikkeld voor de
ammoxidatie van tolueen. Met zijn goed gedefinieerde poriestructuur vormt
zeoliet NaY een uniek dragermateriaal voor de introductie van katalytisch actieve
147
Samenvatting
overgangsmetalen. Deze overgangsmetalen kunnen middels verschillende
methoden in de porië n van de zeoliet matrix worden gebracht. Ook kunnen de
zuur-base eigenschappen op eenvoudige wijze worden gevarieerd. Dit biedt een
voordeel ten opzichte van gemengde oxiden, welke thans voor de ammoxidatie
van tolueen worden gebruikt.
Metaaloxides werden in de matrix van zeoliet NaY ingebracht middels ion
wisseling, metaalcarbonyl depositie en porievolume impregnatie. Ook werden γalumina gedragen metaaloxide katalysatoren bereid middels porievolume
impregnatie. Aangezien in het algemeen de katalysator stabiliteit een belangrijke
rol speelt tijden de ammoxidatie van tolueen over de gebruikte katalysatoren, zijn
de katalytische tests uitgevoerd gedurende een relatief lange tijd; doorgaans in de
orde van 15 uur. De opbrengst aan benzonitril was het hoogst over γ-alumina
gedragen molybdeenoxide. Bij lagere temperatuur kunnen over koper
uitgewisselde NaY katalysatoren gelijke opbrengsten worden verkregen.
Wanneer γ-alumina gedragen molybdeenoxide bij hoge temperatuur
voorbehandeld wordt met ammoniak wordt een significant hogere benzonitril
selectiviteit verkregen bij volledige tolueen omzetting. Dit duidt op het belang van
stikstof houdende oppervlakte structuren in de ammoxidatie van tolueen. De rol
van het stikstof insertiecentrum is onderzocht door het gebruik van NO als stikstof
bron in plaats van ammoniak. Tijdens deze zogenaamde nitroxidatie reactie trad
voornamelijk tolueen verbranding op. Ondanks het feit dat een kleine hoeveelheid
benzonitril gevormd werd kan hieruit geconcludeerd worden dat ammoniumnitraat of –nitriet oppervlakte centra uitgesloten kunnen worden als selectieve
stikstof insertie centra.
Op basis van hun goed gedefinieerde structuur zijn metaaloxide houdende NaY
katalysatoren bereid door middel van depositie van metaalcarbonylen. Mo(CO)6
kan succesvol in de superkooien middels deze techniek. Dit in tegenstelling tot de
introductie van V(CO)6, Mn2(CO)10 en Co(NO)(CO)3. Transmissie Electron
Microscopie (TEM) en Röntgen Foto-elektron Spectroscopie (XPS) analyses tonen
aan dat Mo(CO)6 homogeen gedispergeerd is in de zeolite structuur.
Gedetailleerde temperatuur geprogrammeerde experimenten toonden tevens aan
dat het Mo(CO)6 volledig ontdaan kan worden van alle CO liganden.
Met XPS werd aangetoond dat, in de aanwezigheid van zuurstof, het in de
superkooi aanwezige molybdeen volledig wordt geoxideerd tot een 6+-oxidatie-
148
Samenvatting
toestand. Ook van deze molybdeenoxide clusters is de dispersie hoog, zoals met
TEM werd onderzocht. De molybdeenoxide clustergrootte was niet hoger dan 14
Å. Dit suggereert dat de clustergrootte wordt gelimiteerd door de grootte van de
superkooien van de zeoliet.
Met XPS werd tevens de gast-gastheer interactie van de Mo(CO)6 met NaY
onderzocht. De lage Na(1s) bindingsenergie na Mo(CO)6-introductie duidt op een
directe interactie van de Mo(CO)6-gast met de NaY-gastheer. Na oxidatieve
Mo(CO)6 decarbonylering stijgt de Na(1s) weer met 1 eV naar de in de literatuur
gerapporteerde waarde. Om de interactie van de kationen van de zeoliet en de
Mo(CO)6-gast op meer gedetailleerde wijze te onderzoeken zijn de Na+-ionen
uitgewisseld met andere alkali ionen. Op basis van de elektronegativiteit wordt op
deze manier de basiciteit van de zeoliet gevarieerd. De zuur-base eigenschappen
van de katalysatoren werden onderzocht door uitvoering van de ontleding van 2methyl-3-butyn-2-ol. Controle experimenten toonden aan dat middels deze reactie
de Lewis aciditeit en basiciteit correct kan worden gemeten. Deze
ontledingsreactie bewees de interactie van de Mo(CO)6-gast met de kationen van
de zeolite. Hoe hoger de basiciteit van de zeoliet is, hoe zwakker de interactie met
de het Mo(CO)6-gastmolecuul. Ten gevolge van deze zwakkere interactie vindt
Mo(CO)6 decarbonylering bij lagere temperatuur plaats.
Katalytische tests duiden op een hogere katalysator activiteit bij hogere basiciteit.
Dit is in overeenstemming met de vorming van een carbanion als eerste tolueen
activeringsstap, zoals ook in de literatuur wordt voorgesteld.
Zeer gedetailleerde studies werden uitgevoerd naar γ-alumina gedragen
molybdeenoxide katalysatoren. Door de molybdeen belading te varië ren werden
de molybdeen en zuurstof bevattende oppervlakte structuren gevarieerd. Een
combinatie van temperatuur geprogrammeerde reductie, in situ en ex situ Raman
spectroscopie, XPS, Röntgen diffractie (XRD), Transmissie Electron Microscopy
(TEM), diffuse reflectie ultraviolet spectroscopie en waterstof–deuterium
uitwisselingsreacties werd gebruikt om het reductiegedrag en de morfologie van γalumina gedragen molybdeenoxide te onderzoeken. Bij lage molybdeen
beladingen is tetraedisch omringd molybdeen aanwezig als monomolybdaat
oppervlaktestructuur. Deze structuur reduceert bij hoge temperatuur. Wanneer de
molybdeen belading wordt verhoogd worden poly-molybdaat oppervlakte
structuren gevormd. Deze structuren hebben octaedrische molybdeen coördinatie.
149
Samenvatting
Ten opzichte van oppervlakte monomolybdaten is de reductie temperatuur met
ongeveer 140 ° C verlaagd.
De waterstof–deuterium uitwisselingsreactie lijkt alleen te worden gekatalyseerd
door tetraedisch gecoördineerd molybdeen; bij molybdeen beladingen hoger dan
8.5 gewichtsprocent was er geen waterstof–deuterium uitwisselingsactiviteit. Een
kleine hoeveelheid waterstofatomen is aanwezig op het oppervlak van de alumina
drager. Deze waterstofatomen nemen deel aan het H-D uitwisselingsproces via
een spillover proces.
In situ Raman spectroscopie experimenten toonden aan dat molybdeen onder
reactiecondities kan worden gereduceerd door ammoniak, wanneer de molybdeen
belading hoger dan 10 gewichtsprocent is. Quasi in situ XPS bevestigde deze
resultaten. Na ammoniak behandeling is het molybdeen in de 4+-oxidatie toestand.
Wanneer de belading lager is dan 10 gewichtsprocent zijn de Mo-O Raman
banden verschoven naar lager golfgetal. Deze verschuiving wordt ook
waargenomen voor gehydrateerd alumina gedragen molybdeenoxide. Dit
suggereert dat de Mo-O binding wordt verzwakt door ammoniak adsorptie.
Reoxidatie door middel van zuurstof herstelt de oorspronkelijke oxidatie toestand.
Daar de ammoxidatie van tolueen een redox proces is, wordt de benzonitril
productie sterk beï nvloed door de reduceerbaarheid van de katalysator. Bij
molybdeen beladingen onder de 8.5 gewichtsprocent wordt een lage benzonitril
productiesnelheid waargenomen. Deze katalysatoren kunnen bij de heersende
reactietemperatuur niet door ammoniak worden gereduceerd. Wanneer de
molybdeen belading hoger is dan 10 gewichtsprocent wordt, in overeenstemming
met de eenvoudigere reductie een hogere benzonitril productiesnelheid gemeten.
De lage reduceerbaarheid van molybdeen oxide in NaY zou de gemeten lage
ammoxidatie activiteit kunnen verklaren. Hier is echter geen gedetailleerd
onderzoek naar gedaan.
Wanneer kleine hoeveelheden vanadium worden toegevoegd aan γ-alumina
gedragen molybdeen oxide, wordt een hogere ammoxidatie activiteit
waargenomen. Raman spectroscopie toonde voor deze katalysatoren de vorming
van MoO3 aan. De aanwezigheid van dit MoO3 kan niet worden verwacht op
basis van de belading, omdat de monolaag bezetting bij een hogere belading wordt
bereikt. De MoO3 clusters werden niet waargenomen met behulp van XRD, of
TEM. Dit duidt op een hoge dispersie van de MoO3 clusters.
150
Dankwoord
Aan bijna alle dingen komt een eind, zo ook aan het schrijven van een
proefschrift. Dit zou echter niet het geval zijn geweest zonder de steun en hulp van
een groot aantal mensen. Rutger, bedankt voor het vertrouwen dat je me altijd
hebt gegeven in de uitvoering van mijn onderzoek. Ik heb de vrijheid die je me gaf
in het onderzoek en zeker ook in de samenwerking met de verschillende
onderzoekspartners zeer prettig gevonden. Joop was altijd in staat om mij binnen
de vrijheid van het onderzoek het hoofdpad weer te laten vinden, ook wanneer ik
een doodlopend zijpad ingeslagen was. Kieran, I have appreciated very much your
help, both in helping me understanding better the oxidation catalysis and in
accurately correcting the manuscript during writing of my thesis. During your stay
in Eindhoven you have motivated me to a great extent in enjoying the PhD
research project. Rob, dank je voor de zeer grondige wijze waarop je mijn
manuscript hebt gelezen en de nuttige suggesties die je hierbij hebt gegeven. De
prettige wijze waarop jij correcties en verbeteringen voorstelt heb ik zeer fijn
gevonden. San, van harte bedankt voor de buitengewoon secure manier waarop jij
mijn manuscript hebt gelezen. Jouw correcties waren voor mij erg waardevol. Ook
Hans wil ik graag van harte bedanken voor alle correcties en suggesties.
Mijn collega’s wil ik graag bedanken voor alle tips bij zowel praktische als
inhoudelijke problemen en natuurlijk voor alle gezelligheid. Met name wil ik
Alina, Annemieke, Bruce, Darek, Frank, Imre, Marco, Mayela en Noud hierbij
noemen. Lu Gang greatly helped me in the early stage of my research. Darek,
Niels, Jeroen en Maarten dank ik voor hun enthousiasme tijdens het uitvoeren van
hun researchstage. Betlem and Gemma, thanks for the enthusiasm you put in the
work during your stay in Eindhoven. This was of great value for me.
Ook buiten de metaalgroep heb ik op de TUE veelvuldig bij een groot aantal
mensen kunnen aankloppen voor hun hulp. Emiel, bedankt voor al je hulp, niet in
de laatste plaats bij de H-D uitwisselingsreacties. Roelant, jouw mening over mijn
proef-proefschrift was zeer stimulerend. Arian en Marcel dank ik voor de hulp met
het fijne werk aan de carbonyl katalysatoren in de glove-box. Ook Tiny en Leon
kwamen hier graag een kijkje nemen, maar hen dank ik, samen met Peter, meer
voor alle waardevolle hulp bij de XPS experimenten. Wout, jou bedank ik voor
151
Dankwoord
alle hulp en meedenken wanneer ik weer eens met een groot of klein verzoek bij je
kwam. Natuurlijk wil ik binnen SKA ook het secretariaat bedanken. Charlotte,
Edith, Ine, Ingrid en Joyce, het was altijd leuk om even binnen te lopen voor een
vraag, verzoek of zomaar voor een praatje.
I would like to thank all partners of the EC-Brite project. I have really enjoyed our
half-yearly meetings. Manolo, thank you very much for you contribution to my
thesis, especially on the Raman and IR experiments. A great deal of the thesis
could not have been written without your help. I really appreciated our
collaboration and your hospitality during my stays in Madrid. Misha, many
thanks for our very pleasant collaboration. Together with Roma you really have
done everything one can imagine to help me in my research. Unfortunately I could
not visit Kiev yet to see your laboratory during my PhD research. Patricia
Kooyman (National Centre for High Resolution Electron Microscopy), jouw
TEM metingen zijn zeer waardevol geweest voor mijn onderzoek. Ik heb onze
samenwerking altijd zeer prettig gevonden.
Mijn ouders wil ik graag bedanken voor de stimulans die zij mij hebben gegeven
om deze promotie te volbrengen. Janneke, ten tijde van het schrijven van het
proefschrift was je mijn vriendin, maar bij het verschijnen van het proefschrift ben
jij mijn vrouw! Dank je wel voor al de liefde en steun die jij me gaf en die jij me
geeft. Jij geeft mij de zekerheid dat toch niet aan alle dingen een eind komt.
Eindhoven, 7 september 2000.
152
Curriculum Vitae
Pieter Stobbelaar werd op 4 februari 1971 geboren te Driebergen-Rijsenburg. Hij
behaalde het Atheneum B diploma in 1989 aan het Eindhovens Protestants
Lyceum. Aansluitend startte hij de studie Scheikundige Technologie aan de
Technische Universiteit Eindhoven. Na een bedrijfsstage bij Raychem in KesselLo (België ) studeerde hij in september 1995 af bij prof.dr. R.A. van Santen op het
onderzoek getiteld “Hydroisomerisation of n-hexane”. Hij startte zijn
promotieonderzoek bij de capaciteitsgroep Anorganische Chemie en Katalyse,
geleid door prof.dr. R.A. van Santen, in oktober 1995. Het promotieonderzoek
werd uitgevoerd in samenwerking met zeven onderzoeksgroepen in het kader van
een EG consortium. De resultaten van het onderzoek zijn beschreven in dit
proefschrift. Na voltooiing van het promotieonderzoek vervolgde hij in november
2000 zijn loopbaan als development engineer bij Central Development Lamps,
Philips Lighting.
153