Chapter IV
~10 atom% Cu containing ZnAl-LDH showed higher activity
compared to MgAl, NiAl or CoAl containing LDHs
XPS analysis confirmed the formation of active Cu+ for CuZnAlLDH
STEM analysis confirms the presence of highly dispersed Cu2+ in
the CuZnAl-LDH
Addition of sulfolane as co-solvent increased the phenol
selectivity
Benzene hydroxylation over Cu-containing LDHs:
Influence of co-bivalent ions and solvent
Appl. Clay. Sci., doi:10.1016/j.clay.2011.01.024 (2011)
Benzene hydroxylation over Cu-containing
LDHs: Influence of co-bivalent ions and
solvent
4.1 Introduction
4.1.1 Sulfonation process
4.1.2 Chlorination process
4.1.3 Toluene-benzoic acid process
4.1.4 Cumene process
4.1.5 Direct hydroxylation of benzene
4.1.5.1 Molecular oxygen
4.1.5.2 Nitrous oxide
4.1.5.3 Hydrogen peroxide
4.2 Experimental
4.2.1 Sample preparation
4.2.2 Benzene oxidation
4.3 Results and discussion
4.4 Conclusions
4.5 References
Benzene hydroxylation
Chapter-IV
4.1 Introduction
Phenol is common name of hydroxy benzene (C6H5OH) and belongs to the
class of compounds, containing one or more hydroxyl groups attached to an aromatic
ring. Phenol has also been called carbolic acid, phenolic acid, phenylic acid or
oxybenzene. The history of phenol goes back to 1834, when it was first isolated from
coal tar and named carbolic acid. Until the advent of synthetic phenol production, just
before World War-1, coal tar remained as the only source of phenol. In the early years
of the 20th century, Belgian chemist entrepreneur Leo Baekeland created the world's
first completely synthetic plastic. Baekeland mixed phenol and formaldehyde,
subjected them to heat, pressure and produced the sticky, amber-colored resin named
as Bakelite [1]. Due to this finding requirement of phenol increased and forced the
synthetic production of phenol from other petroleum fraction. Several industrial
applications have been developed during this century based on phenol and their
derivatives. Demand for phenol and their derivatives has been the driving force to
discover new technologies. Worldwide production of phenol was more than 5.5
megatons in 2000 and 7.2 megatons in 2006 and it’s continuous to grow.
4.1.1 Sulfonation process
The first synthetic phenol was produced by sulfonation of benzene and
hydrolysis of sulpfonate (Scheme 4.1, BASF- 1899). More than 99% of phenol was
produced worldwide in the 1890s from synthetic processes [2].
Scheme 4.1 Sulfonation process
In spite of aggressive reagents and large amounts of waste sodium sulphite, this
process was used for about 80 years.
71
Benzene hydroxylation
Chapter-IV
4.1.2 Chlorination process
In 1924, a new chlorination process was put in operation by Dow Chemicals in the
USA:
Scheme 4.2 Chlorination process
Independently, I. G. Farben industrie Co., developed a similar technology in Germany
(Scheme 4.2).
4.1.3 Toluene-benzoic acid process
Dow-Canada Ltd., first introduced the toluene benzoic acid process in 1961.
It accounts for 5% of the total synthetic phenol capacity in the world today (Scheme
4.3). The three chemical reactions in the toluene- benzoic acid process are oxidation
of toluene to form benzoic acid, oxidation of benzoic acid to form phenyl benzoate
and hydrolysis of phenyl benzoate to form phenol. A typical process consists of two
steps, in the first step, the oxidation of toluene to benzoic acid is achieved with air and
cobalt salt catalyst at a temperature 121-177 oC, the reactor operated at 206 kPa and
the catalyst concentration is between 0.1 and 0.3%, the reactor effluent is distilled and
the purified benzoic acid is collected. The overall yield of this process is believed to
be about 68 mol% of toluene.
Scheme 4.3 Toluene- benzoic acid process
The second processing step, in which benzoic acid is oxidized and hydrolyzed
to phenol, is carried out in two reactors in series. In the first reactor, the benzoic acid
is oxidized to phenyl benzoate in the presence of air and a catalyst mixture of copper
72
Benzene hydroxylation
Chapter-IV
and magnesium salts. The reactor is operated at 240 oC and 147 kPa. The phenyl
benzoate is then hydrolyzed with steam in the second reactor to yield phenol and CO 2.
The overall yield of phenol from benzoic acid is around 88 mol%.
4.1.4 Cumene process
The cumene process was developed and first realized in the former Soviet
Union (Scheme 4.4), 95% of phenol is produced now using this method. The route
was discovered in 1942 by a group of excellent chemists P. G. Sergeev, R. Yu. Udris
and B. D. Kruzhalov. In 1949, the first industrial plant was put in operation in
Dzerzhinsk city. There are several licensed processes to produce phenol, which are
based on cumene. Benzene on reaction with propene to give cumene, which is
oxidized with air to form cumene hydroperoxide and cumene hydroperoxide is
cleaved to yield phenol and acetone. In this process, approximately 0.46 kg of acetone
and 0.75 kg of phenol are produced per kg of cumene feedstock.
Scheme 4.4 Cumene process
Oxidation of cumene to cumene hydroperoxide is usually achieved in three to
four oxidizers in series, where the fractional conversion is about the same for each
reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled
in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are
operated at low to moderate pressure. Due to the exothermic nature of the oxidation
reaction, heat is generated and must be removed by external cooling. A portion of
73
Benzene hydroxylation
Chapter-IV
cumene reacts to form dimethyl benzyl alcohol and acetophenone. Methanol is
formed in the acetophenone reaction and is further oxidized to formaldehyde and
formic acid. A small amount of water is also formed by various reactions. The
selectivity of the oxidation reaction is a function of oxidation conditions, temperature,
conversion level, residence time and oxygen partial pressure. Typical commercial
yield of cumene hydroperoxide is about 95 mole percent in the oxidizers. The reaction
effluent is striped off un-reacted cumene, which is then recycled as feedstock. Spent
air from the oxidizers is treated to recover 99.99% of the cumene and other volatile
organic compounds. Due to environmental consideration, many phenol plants are
equipped with a special water treatment facility where acetone and phenol are
recovered from the wastewater stream.
4.1.5 Direct hydroxylation of benzene
The market growth is quite different for phenol and acetone produced from
cumene process, so much effort is currently being devoted to decouple their
productions and the day-to-day increasing demand of phenol lead researches to find
new economical and environmental friendly processes. The practical development of
the one-step production of phenol will have advantages in cost reduction and energy
saving. To achieve direct hydroxylation of aromatic compounds, a neutral oxygen
.
.
species or radical oxygen species, such as HO or HOO , might work as the active
species. The efficient production of active oxygen species on a catalyst remains
difficult which can be obtained through three different methods
 Molecular oxygen (O2) as oxidizing agent in presence of different reducing
agents like H2, NH3, CO, dithioalcohols and ascorbic acid etc.
 Nitrous oxide (N2O) as oxidizing agent.
 Hydrogen peroxide (H2O2) as oxidizing agent.
4.1.5.1 Molecular oxygen
Benzene hydroxylation using oxygen requires the activation of C–H bonds in
the aromatic ring and the subsequent insertion of oxygen. Most heterogeneous
catalysts that contain transition metals, however, supply an oxygen species that has a
negative charge, such as O -, O2- and O2-, all of which cause oxidation of benzene via
the electrophilic reaction mechanism [3]. More than 100 years ago, Friedel and Crafts
74
Benzene hydroxylation
Chapter-IV
examined the conversion of benzene to phenol using oxygen, these experiments were
conducted in the gas phase in the presence of aluminium chloride [4]. Later studies
include molybdenum, tungsten, copper, uranium, and vanadium oxides as catalysts [57]. The selectivity of phenol obtained was very low and hence they were of no
importance with regard to practical applications. Investigations on liquid phase
oxidation in the presence of CuCl [8], iron or palladium in heteropoly acids [9, 10] or
palladium acetate [11] also gave only low selectivities. Table 4.1 shows literature
results of benzene hydroxylation using O2 as oxidant. Tsuruya et al. worked diversely
on this reaction using O2 as oxidizing agent and ascorbic acid as reducing agent, at
>0.5 MPa and different temperatures. They used both vanadium and copper supported
materials like V/SiO2, CuO–Al2O3, CuHZAM-5, V/Al2O3, V-HPAs and Cu/Ti/HZSM-5 as catalysts among them V containing materials showed higher phenol yield
as compared to Cu containing materials [12]. Both V and Cu substituted materials
showed the leaching of active materials in different quantities. Several other materials
like titanium silicate, microporous SBA-15 containing Mo or V heteropolyacid and
organo-transition metal incorporated on solid matrix also found to be active for
phenol production [13-15].
Table 4.1 Catalytic hydroxylation of benzene using O2
S.
No
Catalyst
Phenol
yield %
(Convers
ion %)a
3.8
0.9
7.6
Reducing
agent
Phenol
selectivi
ty %
Conditions
Ref.
1
2
3
V (2wt%) / SiO2
CuO–Al2O3
Pt / Pd on Amberlyst
or Nafion / silica
membrane
V / Al2O3
Ascorbic acid
Ascorbic acid
H2
56
333 K / 24 h
303 K / 5 h
303 K / 0.7–
1.4 MPa
[16]
[17]
[18]
8.2
Ascorbic acidd
-
[19]
1.2b
13.3
0.3
1.3c
19-23
H2
Ascorbic acid
H2
85.3
65
77
2.2
-
0.02
H2, Acetic acid
99
12
13
Cu–Ca phosphates
Cu/Al2O3
1.3
2.1
NH3
Acetic acid
96
-
28 kV / 100
Hz
338 K / 2 h /
0.69 MPa
303 K / 24 h /
[25]
11
CuH-ZSM-5
Pd / porous alumina
VCl3
CuH-ZSM-5
4 µm-thick palladium
film on porous
alumina tube
Pulse
DC corona discharge
V / 5% Pd / Al2O3
303 K / 0.4
MPa
673 K
523 K
323 K / 17 h
673 K
473 K
4
5
6
7
8
9
10
[20]
[21]
[22]
[23]
[24]
[26]
[27]
[28]
75
Benzene hydroxylation
Chapter-IV
[ReO4] / H-ZSM-5
Copperhydroxyapatite
Cu-ZSM-5
3
NH3
40
97
0.1 MPa
0.1 MPa
723 K / 5 h
[29]
[30]
1.7
Air
26
673 K
[31]
-(0.3 )
H2
98
473 K
[32]
-(9.2)
Ascorbic acid
91.8
-(59.2)
Acetic acid
13.9
20
21
22
Pd membrane reactor
Cu / H-ZSM-5
V-HPAs
-(15)
3.3
6
H2
H2O
Ascorbic acid
95
30.1
-
23
24
-(4.7)
-(12)
Ascorbic acid
LiOAc
74.8
0.9
Acetic acid
[40]
-
NH3
90.6
353 K / 24 h /
0.4 MPa
553 K
27
V / Al2O3
Pd(Oac)2 /
Heteropolyacid
(HPA)
V-Substituted
Heteropoly Acid
Re10 cluster on HZSM-5
Cu-ZSM-5
323 K / 12 h /
1 MPa
423 K / 3.5
MPa
423 K
673 K
303 K / 24 h /
0.4 MPa
423 K / 1 MPa
393 K
[33]
19
1% Pt–20% PMo12 /
SiO2
[(C4H9)4N]5[PW11Cu
O39(H2O)]
Pt / V / ZrO2
10
673 K
[42]
28
VOx / CuSBA-15
27
Without and
with H2
Ascorbic acid
[43]
29
MEMS-based Pd
membrane
Pd–silicalite-1
composite membrane
Py3PMo10V2O40
20 (56)
H2
2.9 (4.8)
H2
Low
level
100
353 K / 5 h /
0.7 MPa
473 K
473 K
[45]
11.5
Ascorbic acid
100
[46]
VCl3 ( reactionextractionregeneration system)
Schiff base
manganese complex molybdovanadophos
phoricheteropolyacid
Cu / Ti / H-ZSM-5
Laurylamine
modified
H5PMo10V2O40
Vanadium oxide
nano plate
-
-
-
373 K / 10 h /
2.0 MPa
-
12.2
Ascorbic acid
-
373 K / 10 h /
2 MPa
[48]
4
11.6
Ascorbic acid
673 K
373 K / 10 h /
2 MPa
[49]
[50]
3.7
Acetic acid
423 K / 10 h /
1 MPa
[51]
14
15
16
17
18
25
26
30
31
32
33
34
35
36
a
100
100
[34]
[35]
[36]
[37]
[38]
[39]
[41]
[44]
[47]
Benzene conversion%, b 3% CO2 and 1.8% CO , c 2.1% CO2, 1.2% CO, d 37.6% V leaching observed.
Kubacka et al. achieved phenol yield of 10% using Cu-ZSM-5 catalyst and
both O2 and O2/H2 mixtures as oxidant. Followed by them, Gu et al. showed highest
phenol yield of 27% using VOx/CuSBA-15 as catalyst. However to overcome the
76
Benzene hydroxylation
Chapter-IV
leaching of active metal ion from the material [52], which was commonly observed in
benzene hydroxylation, Laufer et al. used the novel Pt/Pd supported on acid resins
and showed best phenol yield of 7.6% in a semi continuous flow reactor. Rhenium
supported metal oxide was also reported for this reaction using ammonia as reducing
agent [53] and vanadium complex inclusion zeolite as catalyst and saccharides as
reducing agents was also used for the synthesis of phenol in acetic acid water solution
as solvent [54].
Pioneering work on hydroxylation using palladium membrane was done in
2002, Mizukami et al. [21] from Japan used Pd/porous alumina membranes for the
continuous production of phenol from benzene using H2/O2 mixture. Since H2 and O2
were not mixed simultaneously it has the advantage of low probability of causing
detonating gas reaction and phenol yield of 1.5 Kg per Kg of catalyst per hour at
150 oC was achieved. Followed by them Itoh et al. [24] used 4 µm thick palladium
film formed on a porous alumina tube as membrane and showed that appropriate
H2/O2 ratio giving a larger benzene yield (Fig 4.1).
Fig. 4.1 Palladium membrane reactor (reproduced from ref. 24).
This novel idea improved the phenol yield up to 20% at 473 K. Several other
works are also reported on palladium membranes for benzene hydroxylation; however
the stability of the Pd membrane is still a challenge for longer operations [35]. Guo et
al. prepared Pd membranes of excellent stability and flux with multi-legged anchors
by electroless plating of Pd on a porous support modified by a zeolite barrier layer
containing Pd seeds [45].
Ye et al. improved the stability of the Pd membrane by using a Pd membrane
made by chemical vapour decomposition (CVD) or a Pd-Ag membrane instead of the
Pd membrane made by sputtering in the Pd membrane micro reactor [44]. Recently
77
Benzene hydroxylation
Chapter-IV
Shu et al. observed that, the surface area of the dense palladium membrane is low, it
cannot be sufficiently utilized because of the hydrogen combustion and concluded
that the membrane concept is not effective for the one step hydroxylation of benzene
to phenol with hydrogen and oxygen [55].
Bortolotto et al. used novel microstructured membrane reactor with distributed
dosing of hydrogen and oxygen with palladium-coated Pd/Cu foil membrane. The
highest phenol selectivity obtained at 423 K was 9.6% with carbon dioxide being the
dominant product [56].
Advantages
 Low cost
 Environmentally friendly
Disadvantages
 Usage of reducing agents like CO, NH3 and ascorbic acid are industrially not
attractive
 Oxygen-hydrogen mixture in a large-scale synthesis is not preferred
 Low selectivity - over oxidation to maleic acid and CO2
4.1.5.2 Nitrous oxide
N2O is one of the best mono oxygen donor used in the literature, the O- ion
can be formed by surface decomposition of N2O in the presence of available electrons
[57, 58], so that catalytic oxidation reactions of hydrocarbons using N2O as an oxidant
are of interest in connection with usual oxidations by O2. Pioneering work on benzene
hydroxylation using N2O was done by Iwamoto et al. in 1983 [59], where they
showed O2 oxidation leads to the cleavage of C-C bond to maleic acid or maleic
anhydride where as N2O oxidation selectively forms the phenol (Scheme 4.5). They
used V2O5/SiO2 as catalyst at 823 K and showed 11% conversion of benzene with
45% phenol selectivity.
Scheme 4.5 Hydroxylation of benzene using N2O
78
Benzene hydroxylation
Chapter-IV
In 1988, three groups of researchers: Suzuki et al. [60] from Japan (Tokyo
Institute of Technology), Gubelmann et al. [61, 62] from France (Rhone Poulenc
Co.), and G. I. Panov et al. from Boreskov institute of catalysis (BIC) [63-66]
independently discovered that ZSM-5 zeolites are the best catalysts for this reaction.
In ZSM-5 zeolites, reaction proceeded at much lower temperature and more
importantly, with selectivity approaching 100% [67]. Fe-ZSM-5 with 0.055 wt% of
Fe showed 27% benzene conversion with 98% phenol selectivity at 623 K [68]. In
subsequent years several patents published in nitrous oxide hydroxylation of benzene
[69-75] using several catalysts.
In Fe-ZSM-5, ‘α-sites’ found to be the active sites for this reaction and
minimum concentration of Fe atom were needed for the generation of ‘α-sites’. The
mechanism of formation of active oxygen species, their interaction with the catalyst
and mechanism of phenol hydroxylation were well reviewed recently by G. I. Panov
[67]. Solutia Inc., is a USA chemical company was one of the major world producers
of adipic acid, an intermediate for Nylon 6,6, and BIC jointly made a process called
AlphOx (after the name of α-oxygen). They used waste nitrous oxide from adipic acid
plant for the oxidation of benzene to phenol, incorporating this reaction as a key stage
in a new modified adipic acid production (scheme 4.6).
Scheme 4.6 AlphOx process
The reaction was performed in a simple adiabatic reactor, an essential feature
of this process economy. The process has also very good environmental features.
Instead of spending efforts and money on N2O neutralization, the new phenol process
used N2O as a valuable raw material, thus mimicking the waste less strategy of nature.
Followed by these findings several other materials like Fe/Mo/DBH (partially
deboronated borosilicate) synthesized through chemical vapor deposition method
[76], unsupported and supported FePO4 [77], Fe β-zeolites (where coke formation was
higher), iron-exchanged ferrierite [78], copper–calcium–phosphates [79], tetrahedrally
79
Benzene hydroxylation
Chapter-IV
coordinated iron in framework substituted microporous AlPO-5 (gave less selectivity)
[80] and iron-functionalized Al-SBA-15 [81] were studied however they failed to give
either good conversion or selectivity compared to Fe-ZSM-5.
To further improve this process, ZSM-5 coated on stainless steel grids [82],
Fe-ZSM-5 coated stainless-steel supports in micro reaction engineering studies [83],
(NH4)2SiF6-modified iron exchanged ZSM-5 [84], Fe/ZSM-5 prepared by chemical
vapor deposition [85], Fe-ZSM-5 sintered metal fibers composites as catalytic beds
[86], hierarchical Fe/ZSM-5 with a strongly improved lifetime [87], wet milling of HZSM-5 zeolite [88] and microwave radiation during the reaction [89] were used and
obtained improved conversions and selectivity. Meloni et al. showed that the addition
of steam to the feed increased to the benzene conversion over Fe-ZSM-5 catalyst with
improved selectivity and durability [90].
Several works were done to find out mechanism and nature of active
sites for benzene hydroxylation using N2O. Nature of the active sites in HZSM-5 with Fe concentration was studied [91] and showed that there was no
direct correlation between the concentrations of Bronsted and Lewis acid sites. The Fe
ions with complex oxo structure were found to be the active sites. Hensen et al.
showed extraframework Fe–Al–O species occluded in MFI zeolite as the active
species for benzene hydroxylation [92], they observed only negligible conversion in
the absence of aluminium. Yuranov et al. observed three types of Fe(II) active sites in
the zeolites upon activation and which could be assigned to Fe(II) sites in
mononuclear species, oligonuclear species with at least two oxygen-bridged Fe(II)
sites, and Fe(II) sites within Fe2O3 nanoparticles and amount of different sites were
measured by the transient response of CO2 during CO oxidation [93]. Only
mononuclear and oligonuclear Fe(II) sites were active and Fe2O3 nanoparticles were
inactive. Fe(II) ions present in Fe2O3 nanoparticles (inactive in hydroxylation) were
probably irreversibly reoxidized by N2O to Fe(III), which were known to be
responsible for the total oxidation of benzene. Sun et al. studied the mechanism of this
reaction using Fe-ZSM-5 catalyst obtained through solid state ion exchange process
and proved by spectroscopic method that oligonuclear and binuclear, iron sites
appeared most favorable for nitrous oxide decomposition, whereas the mononuclear
iron sites were active for benzene hydroxylation to phenol [94]. Recently, Pirutko et
80
Benzene hydroxylation
Chapter-IV
al. studied the kinetics and mechanism of nitrous oxide decomposition over Fe-ZSM5 and showed that both N2O decomposition and benzene hydroxylation were shown to
have kindred mechanisms, which can be described by two main steps [95]. The first
step was a common one and includes oxidation of the α-site by the deposition of
oxygen from N2O:
The α-oxygen can be safely quantified. The second step was the reduction of the site.
It proceeded due to the removal of α-oxygen by the interaction of the later with either
N2O decomposition or benzene hydroxylation.
Fellah et al. have done density functional theory calculations for the oxidation
of benzene to phenol by N2O on a model (FeO)1+-ZSM-5 cluster and
[(SiH3)4AlO4(FeO)] cluster. They suggested that the experimentally observed
preference of Fe2+ sites over (FeO)1+ on ZSM-5 for benzene oxidation to phenol by
N2O was due to the reduced formation of adsorbed phenolate, which was possibly an
intermediate for deactivation [96]. Rana et al. synthesized hierarchically mesoporous
Fe-ZSM-5 zeolite with catalytically active surface Fe species by introduction of Fe3+
in the synthesis gel of mesoporous ZSM-5 and the material synthesized with the
optimum Fe content and porosity. Samples with very high surface area, strong acidity,
low zeolitic wall crystallinity and 0.93 wt% Fe content showed 1.3 times higher
phenol formation rate than a steamed H-ZSM-5 sample [97].
Advantages
 Low capital expenses
 No by-product
 No highly reactive intermediates
Disadvantages
 Applicable only where N2O is available cheaply such as adipic acid plants
81
Benzene hydroxylation
Chapter-IV
4.1.5.3 Hydrogen peroxide
Next to air, H2O2 is the most environmentally friendly oxidizing agent and
water alone formed as by product. Phenol is more reactive towards oxidation than
benzene, and hence substantial formation of over-oxygenated by-products (catechol,
hydroquinone, benzoquinones, and tars) usually occurs [98, 99] and hydrogen
peroxide selectivity is always low for these reactions. Since catalytic decomposition
of H2O2 is also a competitive reaction during hydroxylation and hydrogen peroxide is
more expensive, this method will be economically attractive only when H 2O2
selectivity is high.
Hydroxylation of benzene to phenol was studied initially in the literature in the
view of understanding the biological processes which showed that formation of
biphenyl through cyclohexadienyl radical and in the understanding of Fenton’s
mechanism [100, 101]. It was also reported that presence of copper in the Fenton’s
system reduces the formation of biphenyl [102, 103]. After the synthesis of titanium
silicates [104] Perego et al. first reported the synthesis of phenol using titanium
silicate [105] and followed by them Thangaraj et al. reported the titanium silicate
catalyzed phenol synthesis from benzene using H 2O2 as oxidant [106,107]. Balducci
et al. patented the synthesis of phenol using spherical silica/zeolite composite
materials with submicronic dispersion of titanium silicate and other titanium silicates
[108, 109]. Amorphous microporous mixed oxides containing transition metals and
copper containing molecular sieves were also found to be active for this reaction with
15 to 35% yield of phenol with 100% selectivity [110, 111]. Table 4.2 shows
literature results of benzene hydroxylation using H2O2 as oxidant. H2O2 selectivity
was calculated from the reported conversion values wherever it is needed, however
reported conversion and phenol selectivity critically depends on the experience of the
analyst and analytical instrument sensitivity.
Table 4.2 Catalytic hydroxylation of benzene using H2O2
S.
No
Catalyst
1
2
3
[Zr]-ZSM-5
Ti / MCM-41
TS-1
4
V-MCM-41
Benzene
conversion
%a
2
68
28.8
(Triphase)
18
H2O2
selectivity,
%
16
23.4
90.5
Phenol
selectivity,
%
91.8
>98
95
Conditions
Ref.
333 K / 4 h
335 K / 2 h
333 K / 2 h
[112]
[113]
[114]
6
50
333 K / 2 h
[115]
82
Benzene hydroxylation
5
Fe2+ and 5-Carboxy-2methylpyrazine-N-oxide
1% CuAPO-5
25-Ti-MCM-41
Ti grafted / MCM-41
FeSO4 pyrazine-3carboxylic
acid N-oxide
Ts-1B
Bi-ZSM-5 (5wt%
bismuth oxide)
VO2+ (APTS)
functionalized / MCM41
1.1% V / MCM-41
Nb-MCM-41
Ferric tri
(dodecanesulfonate)
Fe(DS)3 in OMImBF4
Ferrocene-SBA-15
7.54
73.2
85
28
75
92
8.1
16.2
12.4
30.7
81
100
NM
95
97
8.6
14
80.5
14
58.6
Chapter-IV
308 K / 4 h /
Acetic acid
343 K / 3 h
333 K / 3 h
338 K / 1.5 h
310 K / 4 h /
Acetic acid
[116]
94
NM
373 K / 2 h
343 K / 1.5 h
[121]
[122]
58.6
18.5
333 K / 1 h
[123]
10
19.4
54
7.5
6.5
54
67
84.5
100
343 K / 20 h
343 K / 48 h
H2SO4
[124]
[125]
[126]
-
-
84
[127]
Fe3+ / Al2O3
LaMn-MCM-41
Co (V, Nb, La)-MCM41
V / Activated carbon
Iron / Activated carbon
Fe (DDS)3
Microemulsion (H2O /
C6H6 / CH3COOH /
NaDDS)
CuM(II)Al-HT
H4PMo11VO40.3H2O
12
65
60
2.5
24.8
20
100
NM
NM
RT / 3 h /
H2SO4
333 K / 6 h
323 K / 2 h
343 K / 24 h
13.2
50
21.9
4.4
16.7
93.1
49.4
40
92.9
338 K / 5 h
336 K / 5 h
323 K / 5 h
[131]
[132]
[133]
4.8
26
14.4
15.5
100
91
[134]
[135]
14
10
94
13.5
1.2
(Biphasic
system)
0.47
(Biphasic
system)
10.1
7.9
96.8
94
99.9
298 K / 10 h
308 K / 4 h /
Acetic acid
[137]
[138]
37.99
99.9
308 K / 6 h /
Acetic acid
[139]
29
V / Clay (composed of
chlorite, illite,
attapulgite, etc.,)
NaVO3
FeSO4 / hydrophobic
PTFE porous support
(pore size 0.2 mm)
FeSO4 /
polydimethylsiloxane
membrane
H4PMo11VO40
338 K / 24 h
RT / 1.67 h /
Acetic acid
313 K / 4 h /
Acetic acid
4.7
93
[140]
30
Na3VO4 12H2O
12
13.1
NM
31
32
Fe-TpyP–Al-MCM- 41
Macrocomplex KU-2-8 /
Fe3+
Pd / c-Al2O3–NH4VO3
<1
32
<1
4.6
100
NM
0.57
26.6
100
343 K / 1.67
h / Acetic
acid
323 K / 1.5 h
/ Acetic acid
343 K / 20 h
323 K / 0.25
h
313 K / 2 h
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
33
[117]
[118]
[119]
[120]
[128]
[129]
[130]
[136]
[141]
[142]
[143]
[144]
83
Benzene hydroxylation
Chapter-IV
34
H5PMo10V2O40
34.5
(Insitu)
11.5
100
35
36
19.6
8.6
4.5
78.4
89.3
91
37
Fe / Activated carbon
Modified Titanium
Silicalite (TS-1B)
Ferrocene-SBA-15
15.1
15.1
88.3
38
39
40
VB-ZSM-5
Fe (III) / TiO2
Cu / MCM-41
19.1
14.8
21
57.5
7.4
15.2
98
18
94
41
42
[(CH3)4N]4PMo11VO40
Pt-Fe / TiO2
12.4
6.5
0.6
3.25
85.7
91
43
44
45
Rh6(CO)16
Fe / V / Cu / TiO2
Py1-PMo10V2O40
17
14.8
20.5
17
7.4
6.8
95
49
98
46
47
48
3.8
48.2
22.4
19.4
16.1
22.4
92.4
81.2
100
343 K / 24 h
337 K / 8 h
[157]
[158]
[159]
24.4
0.4
4.9
59
100
NM
333 K / 2 h
293 K / 2 h
[160]
[161]
51
52
53
54
TS-PQTM
Co / V / MCM-41
Vanadyl
tetraphenoxyphthalocya
nine (VOPc)
H5PV2Mo10O40
Tetrakis(2pyridylmethyl)
ethylenediamine–
iron(II)
complex((TPEN)FeII
complex)
5% Cu / Al-PILCs
CuAPO-11
V2O5–Al2O3
V-HMS
54.9
10.5
13.8
11.1
20.9
21
1.8
6.4
80.7
100
100
NM
[162]
[163]
[164]
[165]
55
56
TS-PQTM
Fe2O3 / MCM-41
3.9
11
19.4
3.26
92
NM
57
58
59
60
61
Fe3+ / MgO
MWCNTs
Fe2O3 / MWCNTs
VOx / SBA-16
Ferrocene / Mesoporous
organosilica
Nanocrystalline Fe2O3
36
1.6
16
14
6.8
6
1.3
13
4.1
20.5
100
98.5
90
98
65.2
17.4
6.7
NM
333 K / 3 h
343 K / 6 h
333 K / 6 h
333 K / 3 h /
Acetic acid
343 K / 3 h
348 K / 6 h /
Acetic acid
333 K / 6 h
333 K / 2.5 h
333 K / 3.5 h
333 K / 4 h
RT / 5 h /
H2SO4
343 K / 3 h /
Acetic acid
49
50
62
a
338 K / 6 h /
Acetic acid
303 K / 7 h
333 K / 2 h
[145]
293 K / 5 h /
H2SO4
353 K / 8 h
303 K / 4 h
RT / 1.67 h /
Acetic acid
333 K / 2 h
303 K / 4 h /
Ascorbic
acid
343 K / 7 h
303 K / 4 h
353 K / 5 h
Acetic acid
[148]
[146]
[147]
[149]
[150]
[151]
[152]
[153]
[154]
[155]
[156]
[166]
[167]
[168]
[169]
[170]
[171]
[172]
[173]
As per the initial mol of benzene, NM - Not Mentioned
Bhaumik et al. used triphasic catalysis using TS-1 as catalyst and showed
28.8% conversion with 95% phenol selectivity and 90% H 2O2 selectivity [114].
84
Benzene hydroxylation
Chapter-IV
Several other reports on Ti or V containing MCM-41 were reported however showed
poor H2O2 selectivity [113, 115, 118, 119]. Bianchi et al. used homogeneous catalysis
using Fe2+ co-ordinated with different bidentate ligands with (N,N) (N,O) and (O,O)
co-ordination out of which (N,N) ligand was almost inactive (O,O) ligand showed
good activity but poor selectivity and (N,O) gave good activity as well as good
selectivity. 5-Carboxy-2-methylpyrazine-N-Oxide gave highest H2O2 conversion of
94%, 78% H2O2 selectivity and 85% benzene conversion whereas Fe SO4 pyrazine-3carboxylic acid N-oxide gave 8.1% benzene conversion with 80% H2O2 selectivity
and 95% phenol selectivity (116, 120). Balducci et al. showed for the first time
heterogeneously catalyzed reaction with 8.6% conversion with 80.5% H 2O2
selectivity and 94% phenol selectivity using TS-1 as catalyst (121,174). They used
sulfolane as solvent for the first time for this reaction and the complex formation of
sulfolane with phenol increased the phenol selectivity. Several other attempts were
made to improve the benzene conversion using VO2+ functionalized (APTS)/MCM41, LaMn-MCM-41, Co (V, Nb, La)-MCM-41 and Fe/activated carbon as catalyst and
achieved conversion of benzene from 50 to 65% whereas H2O2 selectivity of 17 to
50% [123, 129, 130, 132]. Recent patents claims the catalytic application of different
materials like vanadyl pyrophosphate, vanadium phthalocyanine complex, vanadium
loaded SBA-16 and vanadium containing aluminium phosphate molecular sieves over
these reactions [175-178].
Peng et al. used biphasic hydroxylation using 1-n-octyl-3-methylimidazoliumtetrafluoroborate (OMImBF4) ionic liquid as solvent and ferric tri(dodecanesulfonate)
Fe(DS)3 catalyst. They screened different ionic liquids and found that length of alkyl
chains on the 1-alkyl-3-methylimidazolium cations and the anions had some effect on
the conversion and selectivity however detailed mechanism was not mentioned clearly
[126]. After the reaction, oxidation products and un-reacted substrate were extracted
from the aqueous and ionic liquid phase with ether at the end of each reaction and
aqueous–ionic liquid catalyst system was reused, however this method has the
disadvantage of using 50 mM H2SO4.
Liu et al. recently reported that novel micro-emulsion system, which was
made of water, benzene, acetic acid, sodium dodecylbenzene sulfonate as a surfactant
and ferric dodecane sulfonate as a catalyst [133]. They achieved a maximum
85
Benzene hydroxylation
Chapter-IV
conversion of 21.9% with 93.1% H2O2 selectivity, 92.9% phenol selectivity and this
can be reused for several times however it has the disadvantage of using acetic acid as
co-solvent.
Bianchi et al. used modified titanium silicate (TS-1B) obtained by the postsynthesis treatment of TS-1 with NH4HF2 and H2O2, as catalyst and obtained 8.6%
benzene conversion with 78.4% H2O2 selectivity and 91% phenol selectivity [147].
Several other reports were also done using both vanadium and copper containing
materials however they failed to give good H2O2 selectivity or benzene conversion;
further research is needed in order to make this process commercially attractive.
Recently, Barbera et al. studied the effect of solvent and crystallite size of
TS-1 over the benzene hydroxylation reaction in both biphasic and triphasic
conditions and also in presence of co-solvent sulfolane. Small-sized TS-1 crystallites
produced the highest primary selectivity to phenol and different titanium
environments and defective hydroxyls population was evaluated by means of diffuse
reflectance UV–vis and IR spectroscopy [179]. Mesoporous aluminium oxide
containing transition metals were also found to be active for this reaction under mild
reaction conditions [180].
Advantages
 Environmentally friendly oxidant
 Only water formed as by-product
Disadvantages
 Highly expensive with respect to oxygen
 H2O2 selectivity is always low for benzene oxidation
 Unproductive decomposition of H2O2
Molinari and Poerio, recently reviewed the direct production of phenol in
conventional and membrane reactors. Attention was devoted to phenol production
processes using various configurations of membrane reactors (MRs) and a
photocatalytic membrane reactor (PMR). They classified the oxidation according to
oxidant type such as N2O, O2, and H2O2 and each of them shows that direct oxidation
of benzene to phenol was a difficult task. Concluded that further efforts are needed to
86
Benzene hydroxylation
Chapter-IV
search and replace the three step traditional process of converting benzene into phenol
with a process of direct oxidation [181].
With this knowledge we have attempted to study this challenging reaction
using H2O2 as oxidant. It was already reported that copper-containing LDHs with high
Cu content (Cu/M(II)=75/25) catalyzed the benzene hydroxylation [134] at a substrate
to oxidant mol ratio of 3:1 using pyridine as the solvent. One of the difficulties
encountered in that work was that the catalysts were difficult to reuse. The present
study discusses the utility of both Cu-containing LDHs with low copper content and
their calcined oxides as catalysts for this reaction using environmentally acceptable
water as the solvent under triphasic conditions. Since peroxide oxidation is vigorous
in water and the formation of over-oxidation products are possible; we have used ~10
atom% copper containing LDHs (Cu/M(II)=10/90) diluted with different co-bivalent
metal ions (M(II) = Co, Ni, Mg or Zn) and at low substrate to oxidant mol ratio of
10:1.
4.2 Experimental
4.2.1 Sample preparation
Samples were prepared by co-precipitation under conditions of low
supersaturation as reported earlier [182]. In brief, a solution (A) containing the desired
amount of 1M solution of metal nitrates (M(II), Cu, Al, where the (Cu+M(II)/Al)
atomic ratio was 3.0 and the Cu:M(II) ratio was 10:90; M(II) = Co, Ni, Mg or Zn)
were mixed thoroughly to make a homogeneous solution. Slowly (1ml min -1) and
simultaneously, solution (A) was mixed with a solution (B) containing precipitating
agents (i.e., NaOH and Na2CO3; 2 M NaOH and 0.2 M Na2CO3 in 80 ml) in a Schott
electronic titrator, at a constant pH 9.5 0.1. During addition the solution was
vigorously stirred at room temperature. The addition took approximately 90 min and
the final pH was adjusted to 10 by adding few drops of precipitant solution. The
samples were aged in the mother liquor at 65 °C for 18 h in an oil bath, filtered,
washed (until nitrates and sodium were totally absent in the washing liquids), and
dried in an air oven at 100-110 °C for 12 h. These samples are referred here as
CuM(II)Al-LDH. All materials were calcined at 700 oC for 12 h in air and the
calcined samples are referred to as CuM(II)Al-CLDH-700.
87
Benzene hydroxylation
Chapter-IV
4.2.2 Benzene oxidation
Benzene oxidation was done in a two-neck 50 ml glass reactor using 5 g (64.1
mmol) of benzene, the calculated amount of catalyst and 15 ml of solvent. The reactor
was placed in a pre-heated oil bath at 65 oC. As soon as the solution reached 65 oC,
0.7 ml (6.2 mmol) of H2O2 (30% W/V) was added through a septum at once, and the
reaction mixture was kept for 6 h with stirring at 800 rpm. Immediately after the
reaction, 5 ml of ethylacetoacetate and 5 ml of water were added, mixed well, and the
aqueous and organic layers were collected separately. The yields of phenol and other
hydroxylated aromatics (given here as total hydroxylated products) were determined
in both the organic and aqueous layers by gas chromatography (Varian 450-GC with a
Factor Four VF-1ms capillary column) by injecting 0.25 μl of sample.
Yield and selectivity were calculated as follows:
Yield of hydroxylated products = [(Moles of hydroxylated products in organic layer +
Moles of hydroxylated products in aqueous
layer) / Initial moles of benzene]*100
Selectivity of phenol = [Total moles of phenol formed (organic + aqueous) / Total
moles of hydroxylated products]*100
Our experimental error in the yield of phenol was ± 0.1 as determined from the GC
calculations. Further, each measurement was done in triplicate and the average yield
and selectivity are given.
4.3 Results and discussion
Table 4.3 summarizes the elemental compositions and the calculated formulae
of the as-synthesized LDH samples. The carbonate content was computed from the
M(II)/Al atomic ratio and the water content was calculated from the TG analysis.
Here we have assumed that carbonate was the only interlayer anion because excess
Na2CO3 was used in the synthesis. This assumption was substantiated by PXRD and
FT-IR measurements, the results of which are shown below. BET specific surface
area (SA) measurements showed the highest surface area for CuNiAl-LDH and the
lowest surface area for CuZnAl-LDH.
88
Benzene hydroxylation
Chapter-IV
Table 4.3 Elemental analysis, textural and catalytic activity of the as-synthesized
samplesa
Name
Molecular formula
CuCoAlLDH
CuNiAlLDH
CuMgAlLDH
CuZnAlLDH
ZnAlLDH
MgAlLDH
[Cu0.07Co0.71Al0.22(OH)2](CO3)0.11.0.66H2O
[Cu0.07Ni0.70Al0.23(OH)2](CO3)0.11.0.64H2O
[Cu0.08Mg0.69Al0.23(OH)2](CO3)0.12.0.65H2O
[Cu0.07Zn0.70Al0.22(OH)2](CO3)0.11.0.70H2O
[Zn0.77Al0.23(OH)2](CO3)0.11.0.50H2O
[Mg0.74Al0.26(OH)2](CO3)0.13.0.62H2O
Yield
(%)b
Phenol
Selectivity
(%)
157
0.52
100
0.51
120
0.52
100
117
0.48
165
0.93
100
49
0.14
113
0.98
100
28
0.05
71
0
0
100
0.45
181
0
0
SA
(m2/g)
PV
(cm3/g)
APD
(Å)
125
0.49
168
a
Substrate: Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: 10 mg;
Temperature/Time: 50 oC/6 h, bTotal hydoxylated products formed
PXRD patterns of the as-synthesized materials are shown in Fig. 4.2; for
comparison, diffraction patterns of as-synthesized binary MgAl-LDH and ZnAl-LDH
110
113
018
015
012
1000
006
003
compounds are also included.
Intensity, C/s
a
b
c
d
e
f
10
20
30
40
50
60
70
2 Theta, deg
Fig. 4.2 PXRD of as-synthesized LDHs (a) CuCoAl-LDH, (b) CuNiAl-LDH, (c)
CuMgAl-LDH, (d) CuZnAl-LDH, (e) ZnAl-LDH, (f) MgAl-LDH
Irrespective of the co-bivalent ion, all samples showed patterns similar to the
HT-like structure with no observable peaks from crystalline impurity phases. The
samples showed sharp and symmetric reflections at low diffraction angles (peaks
close to 2
= 11 and 24o; ascribed to diffraction by basal planes (003) and (006)
respectively) and broad asymmetric reflections at higher angles (peaks close to 2
=
38, 46, and 60o; ascribed to diffraction by (012), (015) and (110) planes). These
diffraction features are characteristic of hydrotalcite [183]. Among the co-bivalent
89
Benzene hydroxylation
Chapter-IV
metal ions studied, zinc resulted in the maximum crystallinity followed by Mg, Ni,
and Co respectively. This is probably a consequence of the similarity of the octahedral
ionic radii of Zn2+ and Cu2+ (Zn2+ = 0.74 Å; Cu2+ = 0.73 Å; Mg2+ = 0.72 Å; Ni2+ =
0.69 Å; Co2+ = 0.65 Å) [184]. Further, no significant variation in the crystallinity
upon substitution of 10 atom% of Cu2+ as evidenced by PXRD profiles of binary and
ternary LDHs (see Fig. 4.2 c and f or d and e). The d-spacing values calculated for
(003) diffraction peaks for all materials were between 7.6 to 7.8 Å, which was
consistent with interlayer gallery height of 2.8 – 3.0 Å, allowing 4.8 Å for the
thickness of the brucite-like sheets.
This gallery height was consistent with the
presence of carbonate ion [185]. FT-IR spectra of the as-synthesized materials, shown
Transmittance, %
in Fig. 4.3, support the presence of carbonate ions.
a
b
c
d
4000
3000
2000
-1
1000
Wavenumber, cm
Fig. 4.3 FT-IR spectra of (a) CuCoAl-LDH, (b) CuNiAl-LDH, (c) CuMgAl-LDH, (d)
CuZnAl-LDH
The sharp and intense band around 1350-1380 cm-1 was attributed to the ν3
asymmetric stretching vibration of carbonate, probably present in D3h symmetry.
CuZnAl-LDH alone exhibited an additional shoulder band around 1490-1510 cm-1,
which might be due to symmetry lowering of free carbonate (to C2v or C3v symmetry)
and/or to partial co-ordination of the carbonate ion with Cu2+ in the cationic layer,
owing to Jahn-Teller distortion of the Cu2+ coordination environment. Such a
symmetry change for the carbonate anion was known for LDHs on mild heating
[186]. The TG/DTG curves of the samples recorded in a flow of nitrogen are shown in
Fig. 4.4. They showed two transitions; the first one, which was reversible,
endothermic, and occurs at low temperature, corresponds to the loss of interlayer
water.
90
Benzene hydroxylation
Chapter-IV
a
dw/dt
b
c
d
100
200
300
400
500
600
700
800
o
Temperature, C
Fig. 4.4 DTGA traces of (a) CuCoAl-LDH, (b) CuNiAl-LDH, (c) CuMgAl-LDH, (d)
CuZnAl-LDH
The second one, which was endothermic, at higher temperature and was due to
the loss of hydroxyl groups from the brucite-like layers as well as decomposition of
the carbonate anions. The higher second transition temperatures for Mg- and Nicontaining samples (around 350 oC) confirm the presence of strongly held hydroxyl
groups. However, for CuZnAl-LDH, high temperature transitions around 560 oC and
765 oC were also observed. These transitions were probably due to the release of
strongly bound carbonate anions, as substantiated by FT-IR spectroscopy.
Catalytic activities of the as-synthesized LDHs for benzene hydroxylation are
given in Table 4.3 (P. No. 88). Among the materials studied, CuZnAl-LDH showed
the highest activity whereas CuCoAl-LDH showed the lowest yield of phenol. Binary
LDHs under similar conditions did not show any conversion substantiating the fact
that copper was indispensible for this reaction. Because all four samples have nearly
the same concentration of copper, the variations in the activity suggested the influence
of the co-bivalent metal ion. Further, it was worth mentioning that although CuZnAlLDH has the lowest surface area it showed the maximum yield. This suggested that
the surface area was not the factor that determines the activity. To probe whether the
kinetics of benzene oxidation or the catalytic competitive decomposition of H 2O2 was
controlling the yield, we carried out independent experiments in which the amount of
H2O2 remaining in the reaction mixture was quantified under similar conditions after
30 min. These experiments were done in the absence of benzene, because benzene
oxidation produces dark brown to brownish black colored products that complicate
the titrimetric analysis of H2O2. These experiments showed that almost all of the H2O2
remained in the solution with CuZnAl-LDH (> 85%), but no H2O2 was detected for
91
Benzene hydroxylation
Chapter-IV
CuCoAl-LDH after 30 min. The CuNiAl-LDH and CuMgAl-LDH samples gave
intermediate values of 43 and 57%, respectively. These results suggest that
competitive decomposition of H2O2 is the predominant factor in limiting the yield of
phenol. However, one must also consider that the redox chemistry of Cu2+, which was
influenced by the presence of the co-bivalent metal ion, was also critical. To quantify
this effect, X-ray photoelectron spectroscopy (XPS) measurements were done. Fig.
4.5 shows the Cu 2p3/2 core level XPS spectra of CuZnAl-LDH and CuCoAl-LDH.
+
2+
Intensity, a.u.
Cu Cu
a
b
930
935
940
945
Binding Energy, eV
Fig. 4.5 Cu 2p core level X-ray photoelectron spectra of (a) CuZnAl-LDH, (b)
CuCoAl-LDH
The main line at 933.4 eV in both CuZnAl-LDH and CuCoAl-LDH
corresponded to Cu2+. An additional peak could be discerned at 932.1 eV for CuZnAlLDH (missing for CuCoAl-LDH) arising from Cu+ species [187]. Since the zinc
content of the material was very large (in comparison with copper), it masked the CuLMM Auger signal. It has been reported that Cu+ was the active intermediate for the
hydroxylation of phenol using H2O2 [188], a reaction that follows Fenton’s
mechanism. From these observations, one can conclude that the formation Cu+, which
is an active intermediate in benzene hydroxylation, was regulated by the nature of the
co-bivalent metal ion. Among the metal ions studied, Zn2+ most evidently facilitated
the formation of Cu+ species. The previously reported presence of Cu+ species in
thermally treated CuZnAl-LDHs by XPS measurements corroborates our findings
[189, 190].
One of the limitations encountered with the as-synthesized materials was that
they could not be reused. This was mainly because of the deposition of significant
amounts of coke (the recovered catalyst was brown in color). Attempts to remove the
coke at low temperature (as one would like to retain the layered structure) by Soxhlet
92
Benzene hydroxylation
Chapter-IV
extraction in various organic solvents were unsuccessful. The PXRD profile of the
recovered sample showed an amorphous pattern probably due to the presence of large
amount of high molecular weight organic residue (referred to here as coke) in the
interlayer galleries. However, when the recovered sample was calcined, the PXRD
showed the oxide phase (white in color) confirmed the presence of the catalyst (Fig.
4.6).
500
Intensity, C/s
a
b
c
10
20
30
40
50
60
70
2 Theta, deg
Fig. 4.6 PXRD patterns of the CuZnAl-LDH (a) as-synthesized (b) after the reaction
referred here as used catalyst (c) ‘b’ calcined at 700 oC for 12 h
Likewise, when CuO was tested as an oxidation catalyst for this reaction,
although it had high activity (the copper content in CuO was five times that of
CuM(II)Al-CLDH), it could not be recovered for reuse after the reaction. In an effort
to prepare catalysts that could be reused, the as-synthesized materials were calcined at
700 oC in air for 5 h, and their PXRD profiles are given in Fig. 4.7.
500
*
*
*
@
Intensity, C/s
@
*
* *
*
d
@
#
c
#
#
b
+
+
+
+
+
+
10
20
30
40
++ +
a
50
60
70
2 Theta, deg
Fig. 4.7 PXRD of (a) CuCoAl-CLDH-700, (b) CuNiAl-CLDH-700, (c) CuMgAlCLDH-700, (d) CuZnAl-CLDH-700, * Co3O4 (JCPDS-43-1003), @ NiO (JCPDS-22-1189),
# MgO (JCPDS-01-1235), + ZnO (JCPDS-36-1415)
93
Benzene hydroxylation
Chapter-IV
All materials showed reflections of the oxide phase typical of the co-bivalent metal
ion namely, ZnO, MgO, NiO and Co 3O4. No discrete reflections of CuO were seen for
these samples, suggested that CuO was well dispersed in the host oxide. Catalytic
activity of calcined LDHs and their surface properties are also given in Table 4.4.
Table 4.4 Benzene hydroxylation over calcined materialsa
Name
CuCoAl-CLDH-700
CuNiAl-CLDH-700
CuMgAl-CLDH-700
CuZnAl-CLDH-700
ZnAl-CLDH-700
MgAl-CLDH-700
CuOc
Without catalyst
SA
(m2/g)
41
106
53
38
35
23
-
PV
(cm3/g)
0.16
0.41
0.11
0.17
0.08
0.39
-
Yield
(%)b
0.65
0.76
0.85
0.88
0.2
0
1.42
Nil
Phenol
Selectivity (%)
89
90
100
100
100
0
92
a
Substrate: Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: 10 mg;
Temperature/Time: 50 oC/6 h, bTotal hydoxylated products formed, c5 times excess Cu
The surface areas and pore volumes of the calcined materials followed the
same trend as those of the as-synthesized materials. Here again, CuZnAl-CLDH
showed the highest activity while CuCoAl-CLDH showed the lowest, similar to the
trend of as-synthesized LDHs.
Fig. 4.8 SEM analysis of calcined materials
94
Benzene hydroxylation
Chapter-IV
In order to gain insight into the distribution of CuO on the oxide matrix and to
correlate the variation in the activity with sample morphology, SEM measurements
were done, and the results are summarized in Fig. 4.8. None of the samples showed
discrete CuO oxide domains, again supporting the idea that the CuO phase was well
dispersed and intermixed with the host oxide phase. Magnesium-containing CLDH
showed agglomerated platelets with a sponge-like morphology whereas zinccontaining CLDH showed a platelet-like morphology with less agglomeration.
Nickel- and cobalt-containing CLDHs showed severe agglomeration of platelets. We
presume that the well-defined platelet morphology exhibited by CuZnAl-CLDH
offered appropriate surface for the adsorption of reactant molecules compared to the
severely agglomerated spongy structure. The influence of the calcinations temperature
on the activity of CuZnAl-CLDH was also studied.
*
A
Intensity, C/s
2000
*
*
*
*
*
#
#
#
#
@
@
@
#
#
@
@
*
*
@
c
b
a
10
20
30
40
50
60
70
2 Theta, deg
1.4
B
1.2
Yield, %
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
900
o
Calcination Temperature, C
Fig. 4.9 (A) PXRD of (a) CuZnAl-CLDH-450, (b) CuZnAl-CLDH-700, (c) CuZnAlCLDH-900, * ZnO (JCPDS-36-1415), # ZnAl2O4 (JCPDS-01-1146), @ CuO (JCPDS-05-0661);
(B) Catalytic activity of CuZnAl-LDH calcined at different temperatures, (Substrate:
Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: 20 mg; Temperature/Time:
65 oC/6 h)
95
Benzene hydroxylation
Chapter-IV
Fig. 4.9 shows the PXRD of materials calcined at different temperatures (450,
700 and 900 oC as deduced from the TG profiles; three different regimes are evident,
namely the structural decomposition of the LDH lattice below 450
o
C, high
temperature transitions around 700 oC and no further decomposition above 800 oC)
along with their catalytic activity. CuZnAl-LDH calcined at 450 oC and 700 oC
showed mixed oxide phases without any discrete reflections of CuO. However, when
the temperature was increased to 900 oC, the calcined material showed the reflections
of a discrete CuO phase along with ZnO and ZnAl2O4. Activity studies revealed
nearly similar activity for the samples calcined at 700 and 900 oC. Since it was known
from our earlier experiments that the discrete CuO phase may facilitate the formation
of further oxidation products and pose difficulty for reuse, further studies were done
with the sample calcined at 700 oC (CuZnAl-CLDH-700).
In anticipation of increasing the yield of phenol, substrate:catalyst mass ratio
(Fig. 4.10) variation studies were done with CuZnAl-CLDH-700.
Yield, %
1.2
Selectivity
Yield
100
1.0
80
0.8
60
0.6
40
0.4
20
0.2
0.0
Phenol Selectivity, %
1.4
0
1000:1
500:1
250:1
125:1
62.5:1
Substrate:Catalyst
Fig. 4.10 Influence of substrate:catalyst mass ratio over CuZnAl-CLDH-700, Substrate:
Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: 10 mg; Temperature/Time:
50 oC/6 h)
Although a 500:1 substrate:catalyst ratio gave high selectivity of phenol, the
net yield of hydroxylated products was lower compared to 250:1 (500:1, 0.9% vs
250:1, 1.2%). Further, when the reaction was carried out at lower than 250:1
substrate:catalyst mass ratio, coke was formed. Hence the ratio of 250:1 was chosen
for further studies, which will also be reasoned later in the manuscript. Temperature
variation studies were done over CuZnAl-CLDH-700 and the results are given in Fig.
4.11. As the temperature of the reaction increased, the yield of phenol increased;
however, the formation of coke also increased at higher temperature.
96
Benzene hydroxylation
1.6
120
Sel-Phenol
Sel-CAT
Sel-HQ
Yield
100
Yield, %
80
1.4
60
1.2
40
1.0
Selectiviy, %
1.8
Chapter-IV
20
0.8
0
40
50
60
70
80
90
o
Temperature, C
Fig. 4.11 Influence of reaction temperature on benzene hydroxylation over CuZnAlCLDH-700, (Substrate: Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: 20
mg; Time: 6 h)
In other words, reactions at high temperatures favored further oxidation products,
such as dihydroxybenzenes. This was also evidenced by the darker color of the
reaction mixture at higher temperatures, and under these conditions it was difficult to
recover the catalyst from the reaction mixture. Hence, further studies were carried out
at 65 oC. Attempts were then made to increase the yield of phenol by using mixed
solvents.
Table 4.5 Solvent variation studies over CuZnAl-CLDH-700a
Solvent (15 ml)
Sulfolane % (V/V)
Yield (%)b
Nil
Acetonitrile
Sulfolane
Acetone
Pyridine
Water
Water
Water
Water
Water
Water
Water
Water
Water
0.6
1.2
1.8
2.4
3
4.1
6.7c
33.3d
0.17
0.13
0.14
0.04
0.57
1.47
1.22
1.36
1.26
1.25
1.27
1.23
0.44
0.41
Phenol
Selectivity (%)
83
100
100
100
100
78
100
100
100
100
100
100
100
100
a
Substrate: Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: CuZnAl-CLDH700 (20 mg); Temperature/Time: 65 oC/6 h, bTotal hydoxylated products formed, cSolvent: Water (14
ml), dSolvent: Water (10 ml)
Solvent variations studies were done and the results are given in Table 4.5.
Interestingly, among the solvents screened, water showed the highest yield, but with
lower selectivity for phenol. The high activity using water as a solvent under triphasic
conditions was advantageous from an environmental perspective. To improve the
97
Benzene hydroxylation
Chapter-IV
selectivity for phenol, the addition of co-solvents was attempted and this was another
reason for using the 250:1 substrate:catalyst mass ratio (which showed lower
selectivity for phenol) in order to discern the effect of co-solvents on the selectivity of
the reaction. Sulfolane is one of the interesting solvents reported in literature for the
hydroxylation of benzene because it controls the further oxidation of easily oxidizable
(relative to benzene) phenol to dihydroxybenzenes [191, 147]. The reaction using
sulfolane alone as the solvent over CuZnAl-CLDH-700 failed to give high product
yields under our conditions. Hence, attempts were made to add sulfolane as a cosolvent along with water, and the results are also given in Table 4.5. The addition of
larger amounts (33.3 vol.% and 6.7 vol.%) of sulfolane decreased the conversion of
benzene whereas the addition of small amounts (0.6 vol.% to 4.1 vol.%) gave nearly
equivalent conversion of benzene with 100% phenol selectivity. Under equivalent
conditions, water alone gave slightly higher conversion but with only about 80%
selectivity. To surmise, addition of suflolane enhance the selectivity for phenol as
reported in literature; however the quantity also has an influence on the activity,
probably due to change in the polarity of the medium. Due to its high dielectric
constant (43.26), sulfolane has the peculiar property of forming complexes with
phenolic compounds [191], and this sterically hindered complex does not allow
further hydroxylation of phenol, thus improving the selectivity. A time variation study
was done under optimized conditions in the hope of improving the yield of the
reaction, and the results are given in Fig. 4.12.
120
Selectivity
Yield
2.0
1.8
Yield, %
1.4
80
1.2
60
1.0
0.8
40
0.6
0.4
20
Phenol Selectivity, %
100
1.6
0.2
0.0
0
5
10
15
20
0
25
Time, h
Fig. 4.12 Time variation studies under optimized conditions
A maximum yield of 1.95% of hydroxylated products was obtained with 95%
selectivity for phenol. This was the highest yield obtained in this study.
98
Benzene hydroxylation
Chapter-IV
The reusability of the CuZnAl-CLDH-700 was assessed as follows.
Immediately after the reaction, the catalyst was separated using centrifuge, washed
with water and calcined at 700 oC to remove the adsorbed organics. Fig. 4.13 shows
the activity of the recycled catalysts (referred to here as cycle 2 and cycle 3) along
with PXRD profiles of the corresponding used catalysts. Activity studies indicated
that the catalyst could be reused for up to three cycles without significant change in
the yield of phenol.
A
Intensity, C/s
1000
a
b
c
10
20
30
40
50
60
70
2 Theta, deg
1.4
B
1.2
Yield, %
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
No. of cycles
Fig. 4.13 (A) Powder X-ray diffraction patterns of used catalysts (a) CuZnAl-CLDH700, (b) CuZnAl-CLDH-700-cycle 2, (c) CuZnAl-CLDH-700-cycle 3 ; (B) Recycle
studies for CuZnAl-CLDH-700, (Substrate: Benzene (5 g); Solvent: Water (15 ml) and Sulfolane
(180 μl); Oxidant: H2O2 (0.7 ml); Catalyst: 20 mg; Temperature/Time: 65 oC/6 h)
However, PXRD showed an interesting trend. Immediately after the first use,
specific leaching of the zinc oxide occurred. This was evident from the absence of
characteristic reflections at 2θ = 34.6o, 36.4o, 47.7o 56.7o and 63.0o for zinc oxide
(JCPDF: 36-1415). However, on further recycling, no phase change was observed,
that showed characteristic reflections of ZnAl2O4 (JCPDF: 1-1146). Similar activity
99
Benzene hydroxylation
Chapter-IV
even after leaching of ZnO suggested that the nature of active copper species and its
dispersion were not affected.
Cu
Cu
Zn
Zn
Al
Al
Fig. 4.14 STEM-EDAX analysis of (A) CuZnAl-CLDH-700 (B) CuZnAl-CLDH-700cycle 3
100
Benzene hydroxylation
Chapter-IV
Further, no discrete CuO domains could be seen even after repetitive
calcinations. To confirm the absence of leaching of copper during the reaction and
also to find the morphological changes that were anticipated from leaching of ZnO,
TEM-STEM-EDAX analysis was done and the results are shown in Fig. 4.14.
Significant changes in the morphology of the catalyst were found after the reaction.
Large irregular platelets (~ 200 - 300 nm) with nano-sized punctures (due to
decarbonation and dehydroxylation of the layered lattice) for the calcined catalyst
before the reaction, whereas oblong condensed particles of smaller size (~ 20 - 50 nm)
were seen after the reaction. This contraction of the particles could be due to
intermittent calcinations of the catalyst between cycles. However, the elemental
mapping analysis revealed a uniform distribution of copper both before and after the
reaction (on both large platelets and oblong particles respectively) with no Curich/poor domains. This suggested that although the variation in the morphology
occurs after repetitive calcinations, the dispersion/distribution of copper was not
influenced significantly. This retention of dispersion could be responsible for
recyclability of the catalyst.
4.4 Conclusions
Hydroxylation of benzene using H2O2 as the oxidant was studied using ~10
atom% copper containing LDHs with different co-bivalent ions (Mg, Zn, Ni or Co)
both in their as-synthesized and calcined forms, using water as the solvent. Zinccontaining samples are the most active in both the as-synthesized and calcined forms.
The high activity of the zinc-containing LDH was attributed to the formation of active
Cu+ species as identified by XPS measurements. Solvent variation studies reveal that
addition of small amounts of sulfolane significantly increases the selectivity for
phenol. High activity (1.36% yield of phenol) with 100% phenol selectivity was
achieved using a substrate:oxidant molar ratio of 10:1 at 65 oC for CuZnAl-CLDH700. This catalyst was reused for up to three cycles without any significant change in
the activity. Although significant changes in the phase and morphology exist between
the calcined catalyst and the used catalyst, STEM-EDAX measurements showed that
the dispersion of copper was not significantly affected. The retention of good copper
dispersion was the likely reason for the recyclability of the catalyst. Although the
catalysts reported in the study show lower yields of phenol, (compared to the cumene
process), it has the advantage of avoiding environmentally unfriendly catalysts and
101
Benzene hydroxylation
Chapter-IV
dangerous intermediates. Water can be used as a benign solvent and 100% selectivity
of phenol can be obtained using an environmentally friendly LDH-derived catalyst.
Because of the low product yield and low H2O2 selectivity, further research is needed
in order to make this process commercially attractive.
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