PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and characterisation of acid mesoporous phosphate
heterostructure (PPH) materials
José Jiménez-Jiménez, Mónica Rubio-Alonso, Dolores Eliche Quesada, Enrique Rodrı́guez-Castellón and
Antonio Jiménez-López*
Received 28th April 2005, Accepted 16th June 2005
First published as an Advance Article on the web 22nd July 2005
DOI: 10.1039/b505973a
New stable acid mesoporous phosphate heterostructure materials, consisting of expanded
zirconium phosphate with silica galleries, have been prepared using as precursor MCM-50 type
lamellar zirconium phosphate, expanded with cetyltrimethylammonium as the surfactant,
hexadecylamine as the co-surfactant, and tetraethylorthosilicate (TEOS) as the silica source. The
amounts of co-surfactant and TEOS employed play a crucial role in the textural and acidic
properties of the mesostructures. The obtained solids present a strong Brønsted acidity as deduced
by the formation of iso-derivatives in the 1-butene isomerisation test.
Introduction
At the beginning of the last decade, Mobil Oil scientists
reported a new method of preparation of mesoporous materials
using surfactant molecules as a template for inorganic silica
arrays.1 By interaction of surfactant micelles with inorganic
species, three geometries can be obtained: hexagonal (MCM41), cubic (MCM-48) or lamellar (MCM-50) with subsequent
applications as catalysts and sorbents.2 Only for the first and
second types, when the surfactant molecules are removed,
generally by calcination, porous materials with high specific
surface areas and well defined porous diameters are obtained.
However, in the case of materials with a lamellar geometry,
the removal of the surfactant gives rise to the collapse of the
structure and a non-porous material results.
Some years later, Galarneau et al.3 reported a new process
for the synthesis of porous materials by using surfactant
molecules as a template for inorganic arrays in a clay interlayer
space, giving rise to a new family of materials called porous
clay heterostructures (PCH). In these materials, cationic
surfactant molecules are inserted in the interlayer space of a
clay host by means of a cationic exchange process. For this
purpose, a clay with a high cationic exchange capacity
(CEC) is necessary and host clays such as fluorohectorite
were used first.3–5 However, recently other clays such as
montmorillonite and saponite have also been used.6–9 In all
cases, a supplementary neutral surfactant is necessary as cosurfactant for the correct formation of silica galleries between
the layers by the hydrolysis and condensation of tetraethylorthosilicate (TEOS), which is used as the silica source. In this
way, the two main strategies for obtaining mesoporous
materials (pillared layer structures (PLS) and MCM-41) are
combined. On the other hand, the synthesis of a MCM-50 type
lamellar zirconium phosphate, expanded with cationic surfactant molecules, was reported by our group. This phosphate
Departamento de Quı́mica Inorgánica, Cristalografı́a y Mineralogı́a
(Unidad Asociada al Instituto de Catálisis del CSIC), Facultad de
Ciencias, Universidad de Málaga, 29071 Málaga, Spain.
E-mail: [email protected]; Fax: 34952132000; Tel: 34952131876
3466 | J. Mater. Chem., 2005, 15, 3466–3472
was used as a starting material for the insertion of gallium
oxide into its interlayer space by cationic exchange of the
surfactant guest with oligomeric gallium species, leading to a
porous structure, as is the case for PLS materials.10 Silica
pillared layered metal(IV) phosphates were firstly prepared11,12
on a- and c-zirconium phosphate by using two different
methods: i) by acid–base reaction between octa(aminopropylsiloxano) solutions formed by hydrolysis/polycondensation in
water–ethanol of aminopropyltriethoxysilane and zirconium
phosphate previously exfoliated with methylamine; ii) by
intercalation of TEOS previously hydrolysed in water–ethanol
and intercalated into expanded zirconium phosphate with
cationic surfactants. Here, we propose the use of a surfactant
expanded zirconium phosphate for the formation of silica
galleries into the interlayer space, analogous to the PCH
material, which shall be named a porous phosphate heterostructure (PPH). With this method, MCM-50 zirconium
phosphate materials can be used as precursor for the synthesis
of mesoporous solids and a new field for application of these
materials can be opened.
Experimental
Synthesis
The synthesis was carried out by using as precursor a
cetyltrimethylammonium (CTMA) expanded zirconium
phosphate which was prepared with a solution of CTMABr
(Aldrich) in n-propanol (Rectapur), H3PO4 (85%, BDH) and
zirconium(IV) propoxide (70% Aldrich) as described elsewhere.10 The solid obtained (CTMA-ZrP) was suspended in
water (10 g L21) and a solution of hexadecylamine (Aldrich) in
n-propanol (35 g L21) was added as co-surfactant. After one
day under stirring, a solution (50% v/v) of TEOS in n-propanol
was added as reactant to origin the silica galleries. This
suspension was stirred at room temperature for three days.
The solid obtained was then centrifuged, washed with ethanol
and dried at 333 K in air. This as-prepared material was
calcined at 823 K for 6 h (1.5 K min21 heating rate). Several
materials with different supplementary hexadecylamine/P and
This journal is ß The Royal Society of Chemistry 2005
TEOS/P molar ratios were prepared. The solids obtained
were denoted x-SiPPH(y), where x and y are the TEOS/P
and hexadecylamine/P molar ratios used in the synthesis,
respectively. A potassium exchanged material (denoted as
K-3Si-PPH(0.2)) was prepared by cationic exchange of 3SiPPH(0.2). For this purpose, 2 g of the pristine 3Si-PPH(0.2)
sample were firstly placed under NH3 vapours for 10 minutes.
For removing the adsorbed ammonia, the sample was left for
1 day in under an atmosphere controlled of H3PO4 (conc.).
Latter, ammonium cations were exchanged by K+, by placing
the solid in contact with an aqueous 0.1 M KCl solution,
which contained five times the total acidity of the 3SiPPH(0.2)
material. After one day under stirring, the solid was centrifuged and washed three times with distilled water and dried
in air at 333 K.
saturator–condenser at 303 K, which allowed a constant flow
of 25 mL min21 with 7.4% of isopropanol and a spatial space
velocity of 41 mmol g21 s21. Before the test, the samples
were pretreated at 493 K under static helium atmosphere. The
carrier gas was passed through a molecular sieve trap before
being saturated with isopropanol. The reaction products were
analysed by an on-line gas chromatograph provided with a
flame ionisation detector and a fused silica capillary column.
The catalytic 1-butene isomerisation is another widely
used test reaction to evaluate the acidity of a material. This
test was performed in a tubular glass flow microreactor.
Samples (133 mg) were pretreated for 2 h in He flow at 673 K.
Experiments were performed at t 5 0.60 gcat gbut21 h. The
1-butene was at 5% in He and the time on stream was 120 min.
Results and discussion
Characterisation
XRD powder patterns were recorded on a Siemens D501
diffractometer (Cu Ka radiation) provided with a graphite
monochromator. X-Ray photoelectron spectroscopy (XPS)
analyses were recorded using a Physical Electronics PHI
5700 spectrometer with non-monochromatic Mg Ka radiation
(300 W, 15 kV, 1253.6 eV) as the excitation source. N2
adsorption–desorption isotherms at 77 K were obtained
using a conventional volumetric apparatus, after outgassing
of samples at 473 K for 24 h, or using a Micromeritics
ASAP2020 apparatus. CHN elemental analysis was used
to determinate the surfactant content and was carried out
with a LECO instrument, while phosphorus was determined
colorimetrically.
Ammonia thermal programmed desorption (NH3-TPD) was
used to determine the total acidity of the samples. Before
the adsorption of ammonia at 373 K, the samples were heated
at 773 K in a He flow. NH3-TPD was performed between
373 and 773 K, at a heating rate of 10 K min21, and analysed
by a TC detector. Pyridine adsorption coupled to FT-IR
spectroscopy was employed to determine the nature of the
acid centres of the catalysts. For the adsorption of pyridine,
self-supported wafers, with a weight-to-surface ratio of about
8 6 1022 kg m22, were placed in a vacuum cell assembled
with Teflon stopcocks and CaF2 windows. Pretreatments
were carried out with an in-situ furnace. FT-IR spectra were
recorded on a Shimadzu 8300 spectrometer. The samples were
evacuated (623 K, 1022 Pa overnight), exposed to pyridine
vapour at room temperature, and then outgassed between
room temperature and 573 K. The concentrations of both
types of acid sites (Brønsted and Lewis) were estimated for
the integrated absorption at 1550 and 1450 cm21, using
the extinction coefficients obtained by Dakta et al.,13 EB 5
0.73 cm mmol21 and EL 5 1.11 cm mmol21, for Brønsted and
Lewis sites, respectively.
The decomposition of isopropanol is widely used as a test
reaction to determine the effective acidity and/or redox centres
on a given surface.14 The catalytic activity of the samples in the
decomposition of isopropanol was tested at 463 K in a fixed
bed tubular glass microreactor at atmospheric pressure using
about 30 mg of sample without dilution. The isopropanol was
fed into the reactor by bubbling a flow of helium through a
This journal is ß The Royal Society of Chemistry 2005
The starting material, CTMA-ZrP, has an expanded MCM-50
structure where the cationic molecules of cetyltrimethylammonium are inserted into the interlayer space of zirconium
phosphate. The XRD pattern shows a single peak at low angle
corresponding to the d001 diffraction at 33 Å. From this basal
spacing of 33 Å, it is possible to deduce that surfactant
molecules are forming a bilayer in the interlayer space with the
alkyl chains slanted at 55u with respect to the layers.15 The
expected basal spacing would be 45.5 Å (45.5 Å 5 2 6 23.9
sin 55u + 6.3, where 23.9 Å is the length of the CTMA molecule
and 6.3 Å the thickness of a-zirconium phosphate layer). As
the observed value is 33.0 Å, this supposes an imbrication of
45.5 2 33.0 5 12.5 Å that corresponds to 10 -CH2- units. On
the other hand, because the cross-section of the polar head
of CTMA is 55 Å2, higher than that corresponding to
an exchange site (24 Å2), and since the alkyl chains are
imbricated, the minimum distance between two polar heads
neutralising two acid sites of the same side of one layer, must
correspond to two chain diameters, that is 2 6 5.04 Å 5 10.1 Å,
as a consequence two non-adjacent acid sites separated by
9.2 Å cannot be neutralised. This means that only two of every
three acid sites will be neutralised. When hexadecylamine
is added, this basic molecule, with a cross-section of 20 Å,
can enter between the chains of CTMA and neutralise the
remaining acid sites on the a-zirconium phosphate layer.
These expanded materials (with and without the neutral cosurfactant) are used for the formation of silica galleries in the
interlayer space using these organic molecules themselves as
a template (Scheme 1). Several materials are synthesised by
varying the TEOS/P and the co-surfactant/P molar ratios.
Almost all the prepared solids exhibit a diffraction peak at
low angle, which is preserved after calcination (Table 1). No
diffraction lines were observed in the high angle region,
indicating that coprecipitated silica is not present.
The preservation of the d001 diffraction peak at low angle
after calcination indicates that the structure does not collapse
after the removal of the surfactant, thus the formation of the
silica galleries in the interlayer space of the phosphate holds
the zirconium phosphate layers apart. In contrast to the
method previously used in the synthesis of PCH,3 these PPH
materials can also be obtained without the addition of
the neutral co-surfactant. This is possible due to the high
J. Mater. Chem., 2005, 15, 3466–3472 | 3467
Scheme 1
Table 1
Textural properties of x-SiPPH(y) heterostructure materials
a
a
SBETb/
2 21
c
c
Material
d001/
Å
Si/P
added
Si/P
XPS
m g
Vp /
cm3 g21
dp /
Å
1-SiPPH(0)
3-SiPPH(0)
5-SiPPH(0)
10-SiPPH(0)
1-SiPPH(0.2)
3-SiPPH(0.2)
5-SiPPH(0.2)
10-SiPPH(0.2)
1-SiPPH(0.4)
3-SiPPH(0.4)
5-SiPPH(0.4)
10-SiPPH(0.4)
1-SiPPH(0.6)
3-SiPPH(0.6)
5-SiPPH(0.6)
10-SiPPH(0.6)
42
38
37
35
37
40
38
42
46
37
39
43
—
39
48
—
1
3
5
10
1
3
5
10
1
3
5
10
1
3
5
10
0.65
1.13
1.70
2.11
1.15
4.71
6.01
7.29
1.3
7.12
7.76
27.69
1.98
7.69
10.12
15.04
340
445
438
378
463
619
568
520
376
427
494
422
377
502
546
507
0.32
0.32
0.79
0.33
0.62
0.55
0.58
0.45
0.37
0.58
0.54
0.67
0.42
0.62
0.67
0.60
34.8
28.9
73.6
38.2
43.7
30.9
36.7
28.3
43.7
43.3
40.7
57.6
41.9
47.4
47.8
52.4
a
Molar ratio. b Using the BET method. c dp 5 average pore
diameter. Using the Cranston and Inkley method.
CEC of zirconium phosphate (6.63 meq g21) in comparison
with clay minerals which exhibit much lower CEC values
(0.7–1.12 meq g21).
Several differences as a function of the amount of cosurfactant and TEOS added are observed. The most significant
difference is that of the definition and intensity of the d001
diffraction peak. As showed in Fig. 1A, the most intense peaks
are observed for the solid with a molar ratio C16NH2/P of 0.2.
This behaviour can be explained taking into account that a
critical amount of surfactant is necessary for templating the
Fig. 1 A: XRD patterns of a) 3Si-PPH(0.0), b) 3Si-PPH(0.2), c) 3SiPPH(0.4) and d) 3Si-PPH(0.6); B: XRD patterns of a) 1Si-PPH(0.2), b)
3Si-PPH(0.2), c) 5Si-PPH(0.2) and d) 10Si-PPH(0.2).
3468 | J. Mater. Chem., 2005, 15, 3466–3472
silica galleries. In the case of samples prepared without cosurfactant, there are not enough surfactant molecules for the
adequate formation of cylindrical micelles throughout the
available interlayer space. However, the opposite occurs for
the samples synthesised with hexadecylamine/P molar ratios of
0.4 and 0.6. In these solids, it is first necessary to remove the
excess of the co-surfactant molecules for the formation of the
galleries, preferentially as neutral hexadecylamine, because
the removal of CTMA requires a cationic exchange with other
cationic species which are not present in the medium. The
chemical analysis of samples before and after the addition of
TEOS support this hypothesis, indicating that the total
surfactant/P molar ratios for expanded precursors are 0.78
and 0.86 (for added co-surfactant/P molar ratios of 0.4 and
0.6, respectively) and they decrease to values of 0.60 and 0.64
respectively after the formation of silica galleries. These values
remain practically constant for materials with different added
amounts of TEOS. In the case of the solid with a co-surfactant/
P molar ratio of 0.2, similar surfactant/P ratios are present in
expanded precursors and after the formation of silica galleries
(0.46 and 0.42 respectively). In this case, a partial loss of the
surfactant involves a more difficult synthesis process and thus
leads to a less regular structure, and therefore the observed d001
diffraction peak is poorly defined.
The influence of the added amount of TEOS on the
definition of the d001 diffraction peak for x-SiPPH(0.2) is
shown in Fig. 1B. In this case, only the 1-SiPPH(0.2) material
exhibits a low intensity d001 diffraction peak due to the small
amount of added TEOS which is insufficient for the adequate
formation of the galleries. However, for the other materials of
this series, relatively high intensity d001 diffraction peaks are
obtained, this being indicative of the existence of quite well
ordered structures.
This partial removal of co-surfactant molecules during the
formation of the silica galleries could also be responsible
for the superficial composition observed of the different
materials studied, as was determined by XPS analysis. For
the x-SiPPH(0) series, the surface Si/P molar ratio is smaller
than that of the corresponding added amounts of reagents.
This indicates that only a part of the added TEOS is
hydrolysed, which is present in the interlayer space of the
zirconium phosphate. For the x-SiPPH(0.2) series the surface
and added Si/P molar ratios are similar and therefore, there is
no co-precipitated silica outside the interlayer space. However,
for the x-SiPPH(0.4) and x-SiPPH(0.6) series, the surface Si/P
molar ratio exceeded that added for all the materials studied.
The presence of nanoparticles of silica outside the interlayer
space could be favoured by the removal of hexadecylamine
from the vicinity of each layer package. This basic environment enhances the hydrolysis and condensation of TEOS
molecules outside the interlayer space resulting in an increase
This journal is ß The Royal Society of Chemistry 2005
in the surface Si/P molar ratio, as was observed by XPS
analysis. This coprecipitated silica could also act as cementing
agent of different grains and thus it would have a clear
influence on the textural properties (Table 1).
In Fig. 2 the N2 adsorption–desorption isotherm at 77 K
for 3Si-PPH(0.2) is shown as a representative example. Type
IV isotherms corresponding to mesoporous materials are
observed for all the prepared materials, but in spite of the
N2 adsorption observed at low relative pressures, these solids
do not exhibit micropores. The BET surface values observed
(340–619 m2 g21) are very high in comparison to those
observed for CTMA-ZrP after calcination at 823 K (22 m2 g21).
These values indicate the presence of a silica structure between
the interlayer space of the phosphate, resulting in a porous
system accessible to the N2 molecules. The evolution of the
BET surface as a function of the co-surfactant/P and TEOS/P
molar ratios used is shown in Fig. 3. The highest values
are obtained with a co-surfactant/P molar ratio of 0.2, and
a maximum value of 619 m2 g21 is found for sample
3-SiPPH(0.2). The lamellar nature of this material can clearly
be observed in the TEM image (Fig. 4). Materials synthesised
without co-surfactant show the lowest surface area values.
This is probably due to the existence of an insufficient amount
of surfactant for adequate formation of the galleries. For the
Fig. 3 BET surface evolution as a function of the added cosurfactant/P and TEOS/P molar ratios.
Fig. 4 TEM image for sample 3-SiPPH(0.2).
x-SiPPH(0.4) and x-SiPPH(0.6) series, intermediate values are
obtained. In these cases, as indicated above, the formation of
the silica galleries is not direct and requires a certain amount of
the co-surfactant to be removed from the interlayer space,
provoking a decrease in the specific surface area. Also, the
possible formation of nanoparticles of silica outside the interlayer space could block the entrance of N2 into the porous
galleries, thus reducing the surface area. However, this process
could gives rise to pores of greater diameter (Fig. 2B) due to
face–edge or edge–edge interactions among different particles,
as the pore size distribution and the average pore diameter
suggest.16
Evaluation of the acidity
Fig. 2 Nitrogen adsorption (-$-)–desorption (-#-) isotherms at 77 K
of A) 3Si-PPH(0.2) and B) 3Si-PPH(0.6). The pore size distributions
are shown in the inserts.
This journal is ß The Royal Society of Chemistry 2005
Ammonia TPD is widely used to determine the total acidity of
solids, although this method lacks selectivity because ammonia
can titrate acid sites of any strength and type. The amount of
ammonia desorbed at some characteristic temperatures is
taken as a measure of the number of acid centres while the
temperature range in which ammonia is desorbed is an
indicator of the strength of such acid sites. The specific surface
area values obtained for Si-PPH materials are similar to or
slightly lower than those observed in the case of the analogous
PCH materials. However, the total acidity values, determined
by the thermal programmed desorption of NH3, are higher
for PPH (1.07–1.67 mmol NH3 g21) than for PCH (about
0.70 mmol NH3 g21).4,6,8 Fig. 5 shows the total acidity values
J. Mater. Chem., 2005, 15, 3466–3472 | 3469
Fig. 5 Amount of ammonia desorbed at different temperature ranges,
total acidity and density of acid sites from thermal programmed
desorption of NH3.
for the x-SiPPH(0.2) series. The highest value is obtained with
the 3-SiPPH(0.2) material, and, interestingly, this solid also
exhibits the highest specific surface area. This acidity comes
from the presence of free P–O–H groups located on the layers
of zirconium phosphate, as well as from the presence of silanol
groups on the surface of the silica galleries. Moreover silanol
groups will display enhanced acidity due to the inductive effect
from the layer. When the total acidity is normalised by taking
into account the specific surface area of the acid solids, the
superficial density of acid sites of the studied materials is
obtained (Fig. 5). According to this parameter, the density of
acid sites decreases with the increase in the TEOS/P molar
ratio used in the synthetic process. This indicates that when
TEOS molecules are hydrolysed into the interlayer space of
zirconium phosphate, the P–O–H groups are either neutralised
or covered by the silica framework and in this case the acidity
generally arises from free Si–O–H groups only. This produces
a decrease in the number of the available P–O–H groups and
thus a concomitant reduction of the acidity. The 3-SiPPH(0.2)
material was chosen as a representative material taking into
account its high surface area, acidity and crystallinity. Table 2
compiles the total acidity per gram together with that corresponding to different temperature intervals for 3-SiPPH(0.2)
and K-3-SiPPH(0.2) materials. The 3-SiPPH(0.2) material
exhibits 1.70 mmol NH3 g21 desorbed between 373–823 K,
revealing it to be a very acidic solid, especially having strong
acid centres, since 58% of the total ammonia desorption
takes place in the temperature range of 573–823 K. When
this solid is fully exchanged with K+ ions, the total acidity
falls to 1.00 mmol NH3 g21, but in this case only 38% of
ammonia molecules are desorbed in the temperature interval
Fig. 6 FT-IR spectra of adsorbed pyridine on A) 3Si-PPH(0.2) and
B) K-3Si-PPH(0.2) after outgassing at different temperatures.
of 573–823 K. This fraction of the total acidity could
correspond to the amount of strong Lewis acid sites because
it is assumed that the strongest Brønsted sites have been
exchanged by K+ ions. In fact the IR spectrum of this solid
shows a weak peak at 3747 cm21 corresponding to PO–H
groups, which was present as a strong peak in the IR spectrum
of the pristine material.
Pyridine is a molecule with a weaker basic character
(pKb 5 9) than ammonia, thus it can only titrate stronger
acid sites than ammonia, but is able to neutralise both
Brønsted and Lewis sites. The FT-IR spectra of adsorbed
pyridine in the 1400–1700 cm21 region have characteristic
bands of neutralisation of both types of acid centres.17,18 The
band at 1550 cm21 is assigned to pyridinium ion formed on
Brønsted sites whilst the band at 1450 cm21 corresponds to
pyridine coordinated to Lewis acid sites.
Fig. 6 shows the IR spectra of 3-SiPPH(0.2) and K-3SiPPH(0.2) samples and Table 3 compiled the concentration of
both Brønsted (CB) and Lewis (CL) sites calculated from the
integrated absorbances of bands at 1550 and 1450 cm21,
respectively. At 298 K, The 3-SiPPH(0.2) material exhibits
higher concentration of both types of acid sites than K-3SiPPH(0.2), as expected. When the temperature is raised up to
Table 3 Concentration of Brønsted (CB) and Lewis (CL) acid sites
from pyridine adsorption on PPH materials
Sample
Table 2 Total acidity and acidity for different temperature interval
data of PPH samples determined by NH3-TPD
3-SiPPH(0.2)
Acidity/mmol NH3 g21
Sample
373–473 K 473–573 K 573–673 K 673–823 K Total
3-SiPPH(0.2) 0.261
K-3-SiPPH(0.2) 0.233
0.490
0.391
0.310
0.143
3470 | J. Mater. Chem., 2005, 15, 3466–3472
0.640
0.235
1.701
1.003
K-3-SiPPH(0.2)
Outgassing
temperature/K
CL/
mmol g21
CB/
mmol g21
CL/CB
RT
273
493
623
RT
273
493
623
614
105
—
—
515
90
—
—
274
159
110
19
217
129
26
—
2.2
0.7
—
—
2.4
0.7
—
—
This journal is ß The Royal Society of Chemistry 2005
Table 4 Product distribution for 1-butene conversion on the PPH materials
Product gas composition (yield (%))
Sample
C3
isobutane
n-butane
isobutene
trans-2-butene
cis-2-butene
1,3-butadiene
Conv. (%)
3-SiPPH(0.2)
K-3-SiPPH(0.2)
0.77
0.0
0.69
0.65
0.60
0.0
8.26
1.44
36.31
32.95
26.56
29.03
0.55
0.61
73.74
64.69
373 K, the concentration of acid sites is dramatically decreased
in both materials (Table 3). Only at 493 K are Brønsted acid
sites retained the pyridine molecules in both materials, but at
this temperature the 3-SiPPH(0.2) sample has a CB four times
higher than that of K-3-SiPPH(0.2), revealing that this solid
only retains a small fraction of strong Brønsted acid sites after
being exchanged with K+ ions.
The conversion of isopropanol is used as a reaction to
test the effective acid–base and redox properties of catalysts.
Strong acid sites are not required, and the reaction products
can be propene, formed by a dehydration reaction; diisopropyl
ether, formed by an intermolecular coupling reaction;
and acetone, originating from the presence of basic/redox
sites via an oxidative dehydrogenation reaction.19 At 493 K,
3-SiPPH(0.2) material shows 100% isopropanol conversion;
for this reason the catalytic test was run at 463 K. Working
with these conditions, 3-SiPPH(0.2) and K-3-SiPPH(0.2)
samples had 100% selectivity to propene. The 3-SiPPH(0.2)
material shows a conversion of isopropanol of 58.3%, whereas
the K+ exchanged material exhibits only a conversion of 5.0%
(propene being the only product). The decrease in the
decomposition of isopropanol is greater than that observed
in the total acidity of both materials, as determined by
NH3-TPD. It could mean that many acid centres of the
K-3-SiPPH(0.2) sample are not active for the isopropanol
dehydration reaction at this temperature.
To further investigate the acidity of these materials the
catalytic isomerisation of 1-butene was chosen as a test
reaction.20,21 The conversion and yield of different products
of this reaction strongly depend on the concentration, type and
strength of the acid sites. Dimerisation and oligomerisation
can occur when porous acid materials are used as catalysts.
Thus the most frequent by-products of isomerisation of
1-butene are the C3 and C5 alkenes derived from cracking of
C8 dimeric species. Moreover, carbonaceous compounds
originating from cracking and coke deposition of oligomerised
compounds can also be present. However, the main products
of this reaction are: isobutene formed by skeletal isomerisation
on strong Brønsted acid sites (Hammet function HR , 26.63);
cis/trans 2-butene originated by double bond migration on
Brønsted acid sites of moderate strength (0.82 . HR . 24.04).
The reaction is thought to occur via carbenium ion intermediates by a three-step mechanism. The hydrogenated C4
compounds, butane and isobutane, were also found among
the other products, which are usually generated by direct
hydrogenation of the butene isomers through hydride transfer
on the corresponding monomeric carbenium ions.
After 120 min of time-on-stream, 1-butene conversions at
673 K are very high, 73.7 and 64.7% for 3-SiPPH(0.2) and K-3SiPPH(0.2) materials, respectively; revealing these materials as
very acidic. Table 4 compiles the yields of different products.
This journal is ß The Royal Society of Chemistry 2005
Note the high yields of skeletal isomerisation (isototal) for
both solids, meaning the presence of an important number
of strong acid sites, especially in the pristine 3-SiPPH(0.2)
(yield of 8.95%). However, both samples exhibit similar yields
of 2-butenes (close to 62%) indicating that in the K-3SiPPH(0.2) sample the Brønsted acid sites of medium
strength were not fully exchanged by K+ ions, as was discussed
before. The stability of both catalysts is very good because
they hardly show any loss of activity after 2 h on stream.
Thus, the 3-SiPPH(0.2) catalyst was found to contain only
0.66 wt% of carbon as carbonaceous compounds after
the catalytic reaction, whereas K-3-SiPPH(0.2) contains only
0.34 wt%.
The preparation of mixed oxide galleries is also possible
when using zirconium or titanium alkoxides, in addition to
TEOS in the synthetic pathway, giving rise to mesoporous
materials with modified acidity.
In summary, new stable mesoporous phosphate mesostructure materials with adjustable surface area and high
acidity can be obtained using this methodology, permitting the
synthesis of several new mesoporous solids with potential
applications in adsorption and catalysis.
Acknowledgements
Financial support from the E.U. GR5D-2001-00537 contract
and MCYT MAT03-2986 and CYTED V.8 is acknowledged.
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