The one-pot synthesis, characterisation and catalytic behaviour of mesoporous silica-sulfated zirconia solids

Article abstract of DOI:10.1016/j.cattod.2011.08.003

A series of non-periodic mesoporous silicas impregnated with both framework and nanocrystalline sulfated zirconia was prepared by a sol-gel process using an ionic liquid as a template. The physico-chemical properties of the mesoporous sulfated zirconia ma

Full text of DOI:10.1016/j.cattod.2011.08.003

Catalysis Today 178 (2011) 187–196
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
The one-pot synthesis, characterisation and catalytic behaviour of mesoporous
silica-sulfated zirconia solids
Antony J. Ward, Ajit A. Pujari, Lorenzo Costanzo, Anthony F. Masters, Thomas Maschmeyer∗
Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, University of Sydney, NSW 2006, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2011
Received in revised form 4 August 2011
Accepted 8 August 2011
Available online 15 September 2011
A series of non-periodic mesoporous silicas impregnated with both framework and nanocrystalline sul-
fated zirconia was prepared by a sol–gel process using an ionic liquid as a template. The physico-chemical
properties of the mesoporous sulfated zirconia materials are described. Analysis of the new silicas indi-
cates isomorphous substitution of silicon with zirconium and reveals the presence of extremely small
(<10 nm) polydispersed zirconia nanoparticles in the materials with zirconium loadings of 28–41 wt.%.
The materials with higher zirconium loadings catalyse the Meerwin–Ponndorf–Verley reductions of alde-
hydes and ketones for substrates with hydrogen-bond basicities, pKHB > 1.10. In order to explain the
catalytic activity, analysis of the acid sites of the materials using pyridine as a probe molecule reveals
that in all cases the majority are Bronsted acid sites, with only a small number of Lewis acid sites present
in those materials with low zirconium loadings.
Dedicated to the memory of Professor D.L.
Trimm an esteemed colleague and valued
friend.
Keywords:
Mesoporous silica
Sulfated zirconia
One-pot
© 2011 Elsevier B.V. All rights reserved.
Catalysis
1. Introduction
Perhaps paradoxically, although there is a considerable indus-
trial need for green superacid catalysts, in particular to replace the
The interest in sulfated zirconia in particular, and the sulfated
metal oxides in general, stems in part from the observation that
relatively mild sulfating procedures, followed by calcination, dra-
matically increase the surface area, surface acidity and the catalytic
activity for reactions involving carbenium ion intermediates. ZrO2
is an amphoteric oxide with a monoclinic structure at room tem-
perature [1], which changes reversibly to tetragonal [2] and then
cubic (distorted fluorite) modifications at higher temperatures. In
contrast, the sulfation treatment cracks the sample into a high-
surface area and strongly acidic tetragonal sulfated ZrO2 material
[3].
chlorinated aluminium oxide and aluminium trichloride used in
isomerisations, alkylations and hydrations, there are relatively few
effective solid superacid catalysts [18,19]. Solid superacids, capable
of operating as proton exchange membranes at elevated temper-
atures, are also desirable materials for incorporation in fuel cells
[20].
However, in industrial practice, sulfated zirconia, although an
effective solid acid catalyst, is not cheap, is subject to deactiva-
tion by coke formation, and catalysts can be difficult to regenerate
[21,22]. The contrary drivers of promise and limitations, as well
as the recognition that the properties of sulfated zirconia are
highly dependent on the synthetic precursors and method [22],
have inspired a large number of synthetic strategies directed at
higher catalytic activities and greater chemical stability, frequently
by increasing the surface area and/or maximizing the fraction of
zirconium atoms available to reactants [23]. These attempts fall
predominantly into eight main categories: additives (e.g., Pt, WO3)
can be doped or added to the sulfated zirconia, sulfated zirconia can
be supported on high surface area oxides (mainly SiO2), high surface
area zirconia can be prepared and sulfated, the sulfated zirconia can
be prepared as a mesoporous solid [24], or as nanoparticles, either
unsupported or supported on high surface area oxides, or zirconium
can be substituted into high surface area oxides (e.g., silicalite [25],
APO-5 [26], MCM-41 [27–29], SBA-15 [30–35], MCM-48 and meso-
porous oxides [36,37]) and molecular zirconium precursors [38]
The structure of the sulfated surface remains the subject of con-
jecture. Bear and co-workers reported the structures of a series
of zirconium sulfates in the late 1960s [4–11]. Typically, these
2−
solids contain ZrO7 or ZrO8 polyhedra, linked by SO4 tetrahedra
[12–14]. Sulfated zirconia has been suggested to contain surface
layers of pyrosulfates or monosulfates, depending on the sulfur
loading, as well as (ZrO)3S O groups, and the involvement of
adjacent surface-bound water molecules with the sulfate species
[15–17].
∗
Corresponding author. Tel.: +61 293512581; fax: +61 293513329.
E-mail address: th.maschmeyer@chem.usyd.edu.au (T. Maschmeyer).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.08.003

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A.J. Ward et al. / Catalysis Today 178 (2011) 187–196
can be grafted onto suitable solids. Several microporous crystalline
zirconium silicate zeolitic materials have been reported (e.g., MFI
with 300 > Si:Zr > 50) [39].
surface area mesoporous supported sulfated zirconia either as sup-
ported nanocrystals or with silicon isomorphically substituted by
zirconium. We were also encouraged by the report of the incor-
poration of Zr into TUD-1 [55,56,62], although these materials had
not been sulfated. Recently, Natalie et al. have reported a novel
synthetic method, based on the copolymerisation of methacrylate-
functionalised silica monomers and zirconium tetramers, to form
mixed oxide materials of ZrO2 dispersed in an SiO2 matrix [63].
The acid/base properties of materials with high and low zirconium
contents (Zr/Si atomic ratios of 1:2.5 and 1:20, respectively) were
investigated by infrared spectroscopy. They concluded that under
appropriate heating conditions, the acidic properties of hydrogen-
bonded pairs of silanol groups were modified by the presence of
ZrO2, and that more acidic sites were present in samples with the
higher zirconium content. It is of interest, therefore, to examine
how these variations in acidity can be further modified by sulfating
the zirconia and how the combination of higher zirconium loading
and sulfating affects catalytic activity in a mesoporous material.
Our investigations into single-pot preparations of high surface
area mesoporous SiO2 support, impregnated with sulfated ZrO2,
and the characterisation and catalytic behaviour of the new mate-
rials in the Meerwin–Ponndorf–Verley reductions of aldehydes and
ketones, are described herein.
Porous ZrO2 aerogels can be prepared with surface areas in the
range 100–200 m2/g, but these materials readily sinter at tem-
peratures in excess of 400 ◦C, their surface areas decreasing as
they transform to the monoclinic and metastable tetragonal phases
[40]. The synthesis of mesoporous zirconia, which might then be
sulfated, was attempted soon after the reports of mesoporous sil-
ica [41–45]. However, these materials were often amorphous and
their pore structures were frequently unstable and collapsed in the
absence of the structure-directing template [46]. Mesoporous zir-
conia can be stabilised by the presence of sulfate groups, but this
material also frequently collapses when heated above 400 ◦C [47].
ZrO2 can also be deposited on the walls of mesoporous SBA-15 [48].
An alternative approach to mesoporous zirconia has been the
replicate method, in which ZrO2 is formed within the pores of the
“hard template”, SBA-15, which is then removed by leaching with
aqueous sodium hydroxide, leaving a crystalline, porous tetragonal
ZrO2, in which the existence of Si–O–Zr crosslinks are suggested to
stabilise the mesoporous structure of the crystalline material [49].
Ye et al. have recently reported the preparation of a mesoporous
ZrO2 with improved thermal stability (to 700 ◦C) by heating the
freshly prepared material in its mother liquor at 110–150 ◦C before
calcination. Nevertheless, the BET surface areas of these meso-
porous materials were only moderate (85–269 m2/g) and their
thermal stabilities, although an improvement, did not match those
of silicon-based MCM-41, nor were they treated with sulfate [50].
Furthermore, nanocrystalline ZrO2 with crystalline walls consisting
mainly of tetragonal ZrO2 and containing macro- and mesopores
has been reported recently [51].
Alternatively, a triblock copolymer can be used to synthesise
crystalline cubic ZrO2 nanoparticles of about 7 nm diameter which
self-assemble into a mesoporous structure [52]. This material can
be sulfated and the Lewis acidity of the sulfated material demon-
strated by its ability to catalyse the benzylation of mesitylene at
75 ◦C. Other recent examples of nanostructured sulfated ZrO2 are
prepared by precipitation in the presence of structure directing
agents such as hexadecyltrimethylammonium bromide, Pluronic
123 and Brij-56 [53]. These materials have been used as catalysts
for the isomerisation of n-butane [54].
Recently, zirconium has been incorporated into the worm-like
mesoporous material, TUD-1, giving stable catalytic materials with
enhanced acidity [55,56]. Similarly, Zr can be incorporated into
SBA-15 as both isolated Zr centres within the framework and (at
higher Zr loadings) as small aggregates on the pore walls [57].
This material catalysed the pinacol rearrangement. Other examples
have been reported in which Zr is incorporated in SBA-15 [35,58].
Similarly, ZrOCl2·8H2O and TEOS can be hydrolysed in the pres-
ence of NaCl and the triblock copolymer P123 to produce, after
aging, calcination, impregnation with H2SO4 and recalcination, an
ordered mesoporous silica, similar to SBA-15, with nominal Si/Zr
ratios between 2 and 5 [59].
2. Experimental
2.1. Materials
Zirconyl chloride octahydrate, tetraethoxysilane (TEOS),
tetraethylammonium hydroxide, hydrofluoric acid (48 wt.% in
water), (all Sigma Aldrich), boric acid, hydrochloric acid (35%)
(all Merck), and nitric acid (70%) (Univar) were all used without
further purification. Silicon (1000 ppm) and zirconium (1000 ppm)
standards for ICP analysis were obtained from Choice Analytical.
1-Hexadecyl-3-methyl-imdazolium bromide was prepared using
a reported procedure [64].
2.2. Instrumentation
The nitrogen sorption isotherms at 77 K were measured using a
Micromeritics ASAP2020 Surface Area and Porosity Analyzer. The
data were obtained by liquid nitrogen adsorption and desorption
at various nitrogen partial pressures and were analysed by the BJH
(Barrett–Joyner–Halenda) and the BET (Brunauer–Emmett–Teller)
methods. The pore size distribution curve was derived from the
analysis of the adsorption branch of the isotherm.
Powder X-ray diffraction patterns were collected on a PAN-
alytical X’Pert PRO X-ray diffractometer using Cu K␣ radiation
and a PIXcel solid-state detector. Low angle diffraction patterns
were recorded in a continuous mode over the range of 1–10◦ 2Â
using a step size of 0.040◦ 2Â with a count time/step of 26 s. Wide
angle diffraction patterns were recorded in the continuous scan-
ning mode over the range 10–70◦ 2Â using a step size of 0.0131◦ 2Â
with a count time/step of 300 s.
Metal and silicon concentrations were determined by ICP anal-
yses, which were performed on a Varian Vista AX ICP-AES. The
samples were prepared in the following manner. To a silica sample
in a polyethylene bottle (20–30 mg, dried for 12 h at 100 ◦C) were
added concentrated HNO3 (1 mL) and HF (2.5 mL, 48 wt.%) and this
mixture was stirred at room temperature for 8 h. Then a saturated
boric acid solution (20 mL) was added. The resulting solution was
then diluted to 50 mL with deionised water.
Diffuse reflectance UV–Vis spectra were recorded using a Cary
5E UV–Vis–NIR spectrophotometer in diffuse reflectance mode
using an integration sphere. Reflectance data were converted to
absorbance using the Kubelka–Munk method (arbitrary units).
Mesoporous materials are often effective catalysts or catalyst
supports. However, few catalytic reactions require that the pores
be regularly ordered. Since, in the case of ZrO2-derived materials,
regular pore ordering of mesoporous materials frequently leads to
thermally unstable materials, and given the high costs of the tem-
plates used in these syntheses, a more general strategy in catalyst
syntheses is to concentrate on the mesoporosity, rather than the
pore ordering, of the catalytic materials.
On the basis of our recent demonstration that ionic liquids could
be used to prepare high surface area silicas with a sponge-like, 3-
dimensional topology [60], as well as to stabilise metal chalcoginide
nanoparticles [61], we reasoned that ionic liquids might be suit-
able media for the single-pot preparation of thermally stable, high

A.J. Ward et al. / Catalysis Today 178 (2011) 187–196
189
Visible Raman spectroscopy was performed using a Renishaw
inVia Raman microscope fitted with a 25 mW 488 nm excitation
source, 1800 l/mm grating, and measured using a 50× objective.
Raman spectra were recorded using a Bruker MultiRAM Raman
spectrometer with a Nd YAG laser at 1064 nm. Spectra were
acquired using a laser power of 500–1000 mW and 500–5000 scans
and were sample dependent. The spectra were measured using a
was stirred at 40 ◦C for 4 h, and then the required amount of
ZrOCl2·8H2O was added to the stirred mixture. The resulting mix-
ture was then stirred at 40 ◦C for 20 h. The mixture was then
transferred into an autoclave and heated at 100 ◦C for 2 days. The
pH of the synthesis gel was adjusted to 7.5 by drop-wise addition
of aqueous ammonia at room temperature. The resulting mixture
was hydrothermally treated at 100 ◦C for another 2 days. The final
solid was collected by filtration, washed with water until no chlo-
ride was detected in the washings, and dried at room temperature.
The solids obtained were impregnated with aqueous H2SO4 (0.5 M
25 mL) at room temperature for 30 min. The sulfated samples were
then filtered, dried at 100 ◦C and calcined at 550 ◦C in air for 10 h.
liquid N2 cooled Ge detector with a resolution of 4 cm−1
.
Infrared spectroscopy was performed using a Bruker IFS 66v
infrared spectrometer equipped with a KBr beam splitter. Data
were collected over a spectral range of 1000–375 cm−1 using a
Pike Miracle Single Bounce ATR accessory. The internal reflection
element was a composite diamond KRS5 crystal. Spectra were
recorded with co-addition of 32 scans at a spectral resolution of
2.4. General procedure for the Meerwin–Ponndorf–Verley
reductions
4 cm−1
.
Transmission electron micrographs were recorded digitally
with a Gtan slow-scan charge-coupled device (CCD) using a Phillips
CM120 Biotwin electron microscope operating at 120 kV. The sam-
ples were prepared by dispersing the powder products as an
ethanol slurry, which was then deposited and dried on a holey
carbon film on a Cu grid.
High-resolution transmission electron microscopy (HRTEM)
was performed using a JEOL 3000F microscope operating at 300 kV.
Specimens for HRTEM studies were dispersed in ethanol, accumu-
lated on a copper holey carbon grid and dried at room temperature.
The specimens were additionally evaporated with carbon in order
to prevent excessive charging and decomposition of the sample
under the electron beam. During the microscope session the objec-
tive lens voltage was adjusted to sufficiently high underfocus values
(∼1000 nm).
All reductions were performed under a nitrogen atmosphere
at 80 ◦C. SZ–SiO2 (8, 25 mg) was added to a 2-neck flask prior to
the addition of 2-propanol (4 mL) and mesitylene (0.1 mL, internal
standard) and the reaction mixture was heated to 80 ◦C. When the
desired temperature was reached, the ketone or aldehyde (2 mmol)
was added. The reaction was monitored by taking small samples
(∼0.1 mL). The samples were centrifuged to remove the catalyst
and diluted with hexane prior to analysis by GC.
3. Results and discussion
The mesoporous silicas impregnated with sulfated zirconia
(2–9) were prepared using a modification of the method described
by Du and co-workers [59]. The method (schematically repre-
sented in Fig. 1) involved the hydrolysis of the silica precursor
(TEOS) in dilute HCl in the presence of the template 1-hexadecyl-
3-methylimdazolium bromide at 40 ◦C for 4 h after which time the
required amount of ZrOCl2·8H2O was added to achieve the desired
Si:Zr ratio. In addition, a purely siliceous material (1) was prepared
using an identical process. This synthesis differs from that of Zr-
TUD-1 in that it does not proceed through silatrane intermediates
[65].
X-ray photoelectron spectroscopy (XPS) measurements were
made using a Specs spectrometer (Specs, Germany), equipped with
Al X-ray source with monochromator operating at 200 W, a hemi-
spherical analyser and a line delay detector with 9 channels. Survey
spectra were acquired for binding energies in the range 50–1200 eV,
using a pass energy of 30 eV. O 1s, Zr 3p3/2 and Si 2s region spectra
were acquired at a pass energy of 23 eV with 10 scans to obtain
higher spectral resolution (0.1 eV step) and lower the noise level.
Spectra were analysed using CASA software.
Catalytic reactions were monitored with a Shimadzu GC-17A
Gas Chromatograph (GC) fitted with an SGE capillary column
(BPX5, 30 m × 0.25 mm × 0.25 ␮m) after including calibration with
mesitylene as internal standard.
Zirconium oxychloride, used in these syntheses, is present as the
8+
tetranuclear species [Zr4(OH)8(H2O)16
]
in the crystal [66] and in
aqueous solution [67] at low pH. It has been shown that the more
acidic the solution, the less polymerisation to higher molecular
weight species occurs [68]. Hence, at pH ∼1 used herein, the zir-
conium species can be expected to be predominately the tetramer.
From the mechanistic point of view, it is likely that the synthesis of
mesoporous sulfated zirconia using [C16MIM]Br as the templating
agent is mediated by the presence of Br−/Cl− ions in the synthe-
sis mixture as was observed by Wong et al. in the analogous CTAB
templated system [32]. In such a case the synthesis was shown
to proceed by a S+X−I+ pathway [32], in which S+ stands for the
[C16MIM] cation, X− for Br− anion and I+ for cationic zirconium
species [69].
The nature of the acid sites was determined through the adsorp-
tion of pyridine onto the materials (dried at 110 ◦C for 24 h). This
was achieved by placing the silicas in a desiccator (using P2O5 as
the desicant) with a vial of pyridine for 72 h at room temperature.
After this time the vial of pyridine was removed and the desic-
cator was evacuated and filled with N2. This process was repeated
until no pyridine persisted. The samples were then analysed using a
Linkam FTIR600 environmental cell purged with N2 using a Bruker
MultiRAM Raman spectrometer with a Nd YAG laser at 1064 nm
(800 mW and 2000 scans).
Nitrogen adsorption–desorption isotherms recorded at 77 K
reveal classic Type IV curves which indicate the materials are
mesoporous in nature (Fig. 2). These materials have pore sizes
in the range of 2–50 nm, and the Type IV isotherms exhibiting
hysteresis indicate that capillary condensation is occurring during
the adsorption process. The surface area, pore volume and pore
size data of mesoporous sulfated silica–zirconia samples are pre-
sented in Table 1. The BET surface area and pore volumes show
the expected decreases with increasing zirconia content: the sur-
face areas decrease from 482 to 343 m2/g and the pore volumes
decrease from 1.34 to 0.43 cm3/g. The dramatic decrease in surface
area with increasing Zr loading indicates that pore blocking may be
occurring as ZrO2 nanoparticles are deposited into the pores of the
2.3. Syntheses
Zirconium oxide was prepared by the addition of water to an
ethanolic solution of zirconium butoxide such that the Zr:H2O was
1:2. Immediately a white gel was formed. The sample was dried
and calcined at 450 ◦C for 2 h. The surface area of this material was
found to be 90.7 m2/g.
A typical synthesis of the mesoporous sulfated zirconia mate-
rials with various Si/Zr molar ratios was performed as follows:
1-hexadecyl-3-methylimidazolium bromide (0.8 g, 2.23 mmol)
was dissolved in aqueous dilute HCl (25 mL, 2 M, 0.05 mol) and
to this solution was added TEOS (1.7 g, 8.16 mmol). The mixture

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A.J. Ward et al. / Catalysis Today 178 (2011) 187–196
Fig. 2. Nitrogen adsorption (–)/desorption (· · ·) isotherms for the materials 1–9 at
77 K.
have incorporation levels of between 61 and 69%. As a consequence,
the zirconium loadings range from 1.0 to 41.4 wt.%.
Sulfur loadings were determined by elemental analysis of the
materials prepared. As expected, the sulfur content of the materials
increases with zirconium loading (Table 1). In the case of 5, the
sulfur loading is lower than expected and this may indicate the
presence of larger ZrO2 domains, compared to that of 4, resulting
in a lower available surface area for sulfate coordination. However,
the amounts of sulfur are low for all materials, which indicates that
the concentration of zirconium at the surface is low. Consistent
with that interpretation, the surface sulfur densities range from 1
S atom/46 nm2 (2) to 1 S atom/1 nm2 (9), which can be compared
with the estimated 4 sulfate groups for each nm2 for a monolayer
of sulfate groups on tetragonal sulfated ZrO2 [70]. In the present
work the highest sulfur loading is 1.69 wt.% for material 9 which
has a zirconium loading of 61.5 wt.%. The sulfur loadings for the
sulfated zirconia–silica materials prepared by Du and co-workers
range from 2.0 to 3.9 wt.% for zirconium contents ranging from 20.1
to 33.8 wt.% [59].
Fig. 1. Schematic representation of the syntheses of the SZ–SiO2 materials.
materials. In addition to this, the very high Zr loadings in materi-
als 6–9 may have resulted in a collapse of the pore structure thus
giving rise to the observed decrease in the surface areas. The sil-
ica prepared without any zirconium added had a surface area of
709 m2/g and a pore volume of 0.82 cm3/g.
The ratio of zirconia to silica was measured using ICP-OES anal-
ysis. As can be seen from Table 1 there is a significant loss of the
added zirconium as a result of the washing process. In the case
of the SZ–SiO2 materials with the lowest Zr loadings (i.e. 2–5),
only 31–49% of the added zirconium was incorporated into the
final product. At the higher loadings, however, there is a signifi-
cant increase in the incorporation of the zirconium: the silicas 6–9
The calcination temperature may explain the low sulfur loadings
observed in these materials. It has been shown that for calcination
temperatures greater than 480 ◦C there is a significant decrease in
the sulfate concentration on pure zirconia samples [71,72] indi-
cating that the coordinated sulfate is highly labile at elevated
Table 1
Physical properties of the prepared mesoporous sulfated zirconia–silica materials.
Zr loadinga (wt.%)
Si/Zr (mol/mol)
Sulfur loadingb (wt.%)
Zr/S
Surface area (m2/g)
Pore volume (cm3/g)
Pore Size (A)
˚
Sample
Added Zr (wt.%)
1
2
3
4
5
6
7
8
9
–
2.2
3.6
7.9
10.9
25.0
39.5
45.0
61.5
–
1.0
1.3
2.4
∞
148
111
60
–
–
709
435
416
482
470
424
378
380
343
0.82
1.34
1.29
1.17
1.08
1.17
0.76
0.56
0.43
46
124
124
97
91
110
81
0.1
0.1
0.3
0.3
0.7
1.1
1.2
1.7
7.1
4.7
2.6
6.6
7.4
8.8
8.0
8.6
5.3
27
15.4
26.9
27.8
41.4
7.8
3.6
3.4
1.6
59
50
a
Determined by ICP-OES analysis.
Determined by elemental analysis.
b

A.J. Ward et al. / Catalysis Today 178 (2011) 187–196
191
Table 2
Relative concentrations of O, Zr and Si (%) at the surface of the SZ–SiO2 materials as
determined by ZPS.
Sample/absorption
O 1s
Zr 3p3/2
Si 2s
Si/Zr
2
3
4
5
6
7
8
9
68.3
69.6
75.9
72.1
78.3
69.5
84.0
74.6
0.2
0.2
0.2
1.2
1.0
1.3
5.7
9.0
31.5
30.1
31.5
26.7
20.7
29.2
10.3
16.4
158
151
158
22
21
22
2
2
temperatures. In addition, the concentration of the H2SO4 solution
that has been used to treat the zirconia has been found to have a
marked effect on the final sulfate concentration in the final mate-
rial: an increase in H2SO4 concentration from 0.02 M to 1.5 M sees
a 7.2 times increase in sulfate concentration in the final materials
[71]. Furthermore, recent work has shown that significant sulfate
is lost when the calcinations are performed in a shallow bed, as the
samples being investigated here prepared, compared to a calcina-
tion performed using a bed depth of a few centimetres [73]. As a
consequence, it can be concluded that the low sulfate concentra-
tions seen in the materials 2–9 are the result of two factors: the loss
of coordinated sulfate during the calcination procedure and the low
concentration of zirconia at the surface of the materials.
Fig. 3. Infrared spectra in the skeletal region of bulk sulfated zirconia (SZ–ZrO2),
silica (1) and the prepared SZ–SiO2 samples 2–9.
absorptions assigned as silica framework stretches become con-
siderably broader and less defined which is indicative of the
incorporation of Zr into the silica framework through isomorphous
substitution. Silica 1, in particular, shows a clear absorption at
960 cm−1 that is attributed to the Si–O stretch of silanol groups,
and this absorption becomes broader and less well resolved with
an increase in zirconium loading, which is indicative of the presence
of Zr coordinated to the hydroxyls on the surface of these materi-
als. An infrared absorption in zirconium-impregnated silicalite-1
at 965 cm−1 has been attributed to a Si–O–Zr asymmetric stretch
[39]. The silicas with the highest Zr loadings, 8 and 9, show the
appearance of a shoulder at ∼920 cm−1, which has previously been
attributed to a Si–O–Zr stretch [76].
In addition, the spectra of 8 and 9 show a weak absorption at
600 cm−1 that corresponds to the presence of ZrO2 in the materi-
als (cf. the spectra of bulk ZrO2 in Fig. 3). The absence of this weak
absorption in silicas 2–7 indicates that the Zr has been incorporated
into the silica framework by isomorphously replacing Si atoms.
This observation is supported by TEM images of these materials
(vide infra). The decreasing amount of silica with increasing zirconia
loading is clearly visible in the intensity decrease of the absorption
at 1000–1100 cm−1. A similar trend is observed for the absorp-
tion at 460 cm−1. The second derivative of the spectra (not shown)
resolves a number of peaks under the absorption envelope for the
peak at 1000–1100 cm−1 and clearly shows the influence of the
zirconium in the silica framework: 1 has a maximum at 1036 cm−1
whereas materials 2–7 have maxima between 1053 and 1061 cm−1
while 8 and 9 have maxima at 1046 and 1034 cm−1, respectively.
A similar trend is observed for the absorption at 1200 cm−1, which
shifts to 1215 cm−1 as the Zr loading is increased. An absorption at
877 cm−1 observed in samples with high zirconium loadings, and
assigned as a Zr O stretch may also contribute to this envelope
[39,77].
X-ray photoelectron spectroscopic (XPS) experiments were
used to probe the surface composition of the SZ–SiO2 materials. As
can be seen in Table 2, the Zr surface concentrations for materials
2–7 are relatively low, indicating the bulk of the zirconia is located
within the silica matrix. If nanoparticles were present on the surface
of these materials Si/Zr ratios higher than those observed for the
ICP would be expected: in the case of materials 2–7 the observed
Si/Zr ratios are lower or correspond with the ICP data which indi-
cates that there is no significant deposition of Zr at the surface;
for materials 8–9 the Si/Zr ratio is significantly greater than that
determined by ICP analysis indicating the presence of Zr at the sur-
face of the material. The relative concentrations of zirconia increase
considerably for silicas 8 and 9, suggesting the presence of ZrO2
nanocrystallites on the surface of the silica (vide infra). Assuming
that the silicon present within the samples is present as silica and
setting the Si 2p binding energy at 103.6 eV (the value for SiO2)
[74], then the binding energy for Zr 3d5/2 has a corrected value of
between 182.8 and 183.7 eV. These values are higher than for bulk
ZrO2 (182.2 eV) and closer to that of ZrSiO4 and of Zr in Zr-TUD-1
(183.3 and 183.2 eV, respectively) [56,75]. Similarly, the O 1s values
have a range of 531.7–532.3 eV, which are significantly higher than
the binding energy seen in ZrO2 but approximate those in Zr-TUD-
1 and in SiO2 (530.2, 532.4 and 532.9 eV, respectively) [56,75]. The
values in Zr-TUD-1 were interpreted in terms of a larger number of
Si–O–Si than Si–O–Zr bridges and most of the zirconium being in
the framework, rather than present as domains of ZrO2, consistent
with a similar situation being present here [56]. Further confirma-
tion of the incorporation of the Zr into the framework was obtained
by IR and Raman studies of the materials (vide infra). The use of XPS
to probe the S 2p binding energy was not possible in our study, due
to the low sulfur loadings (as determined by elemental analysis),
which resulted in the observed peaks having poor signal-to-noise.
The infrared spectra for the skeletal region of materials 1–9
are shown in Fig. 3. The spectra are similar to those of Zr-TUD-1,
which were interpreted as consistent with most of the zirconium
being present in the framework [56]. The spectra are dominated
by absorptions as−s1ignable to the silica support: 460 cm−1 (O–Si–O
bending), 800 cm (Si–O–Si deformation), 1000–1100 cm−1 (Si–O
stretch), ∼1200 cm−1 (asymmetric Si–O–Si stretch), and 1625 cm−1
(molecular water bending). It should also be noted that the
Attempts to record the visible Raman spectra of the prepared
SZ–SiO2 materials were unsuccessful due to the fluorescence of the
samples at 488 nm. The Raman spectra were recorded using an exci-
tation line of 1064 nm and are shown in Fig. 4. Broad unresolved
features characterise the Raman spectra. The strongest feature
occurs at 906 cm−1, and based on the asymmetric peak shape, this
absorption is clearly composed of a number of overlapping vibra-
tions. For in the bulk sulfated ZrO2 notable absorptions are observed

192
A.J. Ward et al. / Catalysis Today 178 (2011) 187–196
Fig. 4. Raman spectra (excitation: 1064 nm) of bulk sulfated ZrO2 (SZ–ZrO2) and the
prepared SZ–SiO2 samples 2–7.
Fig. 5. High angle X-ray diffraction patterns of the prepared sulfated zirconia–silica
samples 2–9.
at 640 and 478 cm−1. The absorption at 640 cm−1 (in silicas 2–7)
shifts to lower wavenumber as the zirconium loading decreases.
The peak at 478 cm−1 is obscuredin the silica samples by the intense
absorption that results from O–Si–O bending. The absorptions at
275 and 335 cm−1 in the SZ–ZrO2 sample can be seen at slightly
higher wavenumber in the silica samples with decreasing intensity
with lower zirconium loading. In the region 1200–800 cm−1, the
silicas display a series of overlapping absorptions at 1079, 1003,
906 and 801 cm−1 which are assigned to symmetric Si–O stretch-
ing modes. The positions are characteristic of tetrahedral units with
three (Q3), two (Q2), one (Q1) and zero (Q0) bridging oxygen atoms,
respectively [78]. Unfortunately, Raman spectra of 8 and 9 could not
be obtained due to the large elastic scattering signal obtained from
these samples. In such cases, several percent of the incident laser
light can be scattered elastically into the spectrometer giving rise to
a signal that is many orders of magnitude greater than that of the
Raman scattering [79]. When the samples are weak Raman scat-
terers, as is the case with these samples, obtaining spectra is near
impossible. The infrared and Raman spectra of ZrO2/SiO2 glasses
contain an absorption at 975 cm−1, (954 cm−1, Raman) which is
attributed to Zr–O–Si vibrations [80].
The diffuse reflectance UV–Vis spectra (not shown) of the
SZ–SiO2 materials 2–9 are effectively featureless and have no
resemblance to pure zirconia (which exhibits a maximum at
∼235 nm), indicating the absence of a bulk phase in the samples
prepared. Bulk ZrO2 has an absorption at ∼235 nm [37] (monoclinic
ZrO2 absorbs at 240 and 310 nm [39]), whereas all SZ–SiO2 materi-
als have maxima that are below 200 nm, other than sample 9, which
exhibits a weak maximum at ∼205 nm. This single absorption is
attributed to the O → Zr ligand-to-metal charge transfer transition
[37]. An electronic spectral absorption at 197 nm is attributed to
an oxygen to metal charge transfer in isolated tetrahedral Zr(IV)
ions in MCM-41 [29], and in Zr-TUD-1 [56], which is differentiated
from the broad absorption at 227 nm (or 212 nm [39]) in ZrO2 [48],
assigned as an oxygen to Zr(IV) charge transfer.
that observed for materials 2–6 and this broadness may be due to
the significant amounts of zirconium that have been incorporated
into the silica framework. In all cases, there is no diffraction associ-
ated with a crystalline phase of ZrO2 indicating no bulk crystalline
zirconia is present.
The TEM images of the SZ–SiO2 samples 3, 4 and 8 are shown
in Fig. 6. As can be seen from these images, the silica does not pos-
sess long range order, which is consistent with our observations
when using similar templates to produce high surface area meso-
porous silica [60]. In addition, no large clusters of nanoparticulate
zirconia are visible at the magnifications used, which indicates that
the zirconia is present as either very small nanoparticles and/or is
incorporated into the silica framework.
The dark field and HR-TEM images of several of the sulfated
zirconia–silica samples are shown in Fig. 7. Dark field images show
strong atomic number (Z) contrast, thus allowing heavier elements
to be visualized. Hence, the dark field image of 7 (Fig. 7a) shows
the dispersion of the Zr atoms within the silicon matrix and is rep-
resentative of similar images obtained for materials 8 and 9 (not
shown). In all the TEM images small crystalline particles associated
with the walls of the silica surface are observable. In the case of
silica 7 it was very difficult to observe the very small crystalline
ZrO2 particles in the HR-TEM but inspection of the electron diffrac-
tion pattern clearly indicates that polydispersed ZrO2 particles are
present. From the diffraction pattern it is evident that the material
is poly-crystalline with some crystal orientation evident. The HR-
TEM images of 8 and 9 clearly show the crystalline phase of ZrO2
on the surface of the silica, with the particles being in the range
5–10 nm in size.
The catalytic activity of these materials was probed using
the Meerwin–Ponndorf–Verley reductions [83] of aldehydes and
ketones as a test reaction and SZ–SiO2 (27.77 wt.% Zr (8) as a
representative material, containing both nanoparticulate SZ–SiO2,
and framework zirconium. This reaction, and its reverse, the
Oppenheimer oxidation, has been catalysed by calcined hydrous
zirconium oxide [84–87], The results using SZ–SiO2 (8) are collected
in Table 3.
The lack of a significant XRD reflection at low angle suggests
little long range mesostructure in the material [81,82]. The diminu-
tion in peak intensity compared to the purely siliceous material 1
is attributable to the decrease in the pore size and the filling of the
mesopores with the zirconium oxide nanoparticles at the highest
loadings. The absence of additional reflections at low angle indi-
cates that there is no ordering within the silica. The high angle
(2Â > 10◦) X-ray diffraction patterns of the mesoporous SZ–SiO2
materials 2–9 (Fig. 5) reveal a broad reflection at 20–35◦ 2Â which
is due to the amorphous nature of the silica support. In the case
of 8 and 9, this reflection is significantly broadened compared to
Sample SZ–SiO2 (8) catalyses the Meerwin–Ponndorf–Verley
reductions of a representative aldehyde (benzaldehyde) and a
ketone (acetophenone). The conversions are comparable to those
using Zr-TUD-1 as catalyst. The activity of the catalyst appears to
correlate with the pKHB of the substrate [88], inasmuch as sub-
strates with pKHB > 1.10 are not converted. This effect and the
substituent effects in arene substrates can be observed in the

A.J. Ward et al. / Catalysis Today 178 (2011) 187–196
193
Table 3
Meerwin–Pondorff–Verley catalysed by SZ–SiO2 (27.77 wt.% Zr, (8)).a
Conversion (%)c
Reaction time (h)
b
Substrate
Products
pKHB
0.78
48
96
(37% (A); 21% (B))d
1.10
1.13
9
0
96
20
N/A
N/A
6
96
96
0e
1.11
1.24
12
48
96
0
0.93
0.69
8
96
96
26

194
A.J. Ward et al. / Catalysis Today 178 (2011) 187–196
Table 3 (Continued)
b
Substrate
Products
pKHB
Conversion (%)c
0
Reaction time (h)
48
1.18
a
Reaction conditions: 25 mg catalyst, 4 mL 2-propanol, 2 mmol substrate, 100 ␮L mesitylene (internal standard), 80 ◦C, performed under N2 atmosphere.
Values of pKHB obtained from Ref. [88].
The products were identified using GC–MS.
b
c
d
The formation of B results from the reaction of the generated alcohol and subsequent reaction with one equivalent of the starting aldehyde to form a hemiacetal and the
resulting hemiacetal reacting with the solvent to form the observed acetal.
e
No products from the reduction of the aldehyde were seen, however, products resulting from the reaction of the aldehyde with the solvent (i.e. hemiacetal and acetal)
were observed.
Fig. 6. Representative TEM images of the prepared SZ–SiO2 samples: (a) silica 3 (1.34 wt.% Zr); (b) silica 4 (2.43 wt.% Zr); and, (c) silica 8 (27.77 wt.% Zr).
Fig. 7. HR-TEM images of the prepared SZ–SiO2 samples 7–9: (a) dark-field image of silica 7 (26.89 wt.% Zr); (b) silica 8 (27.77 wt.% Zr); (c) silica 9 (41.40 wt.% Zr).
results for the substituted phenylmethylketones, in which the p-Me
derivative (pKHB 1.24) is not converted, and the greatest conver-
sion is observed for the m-NO2 derivative. Although we observe no
detectable reaction of cinnamaldehyde with 2-propanol over 20 h,
this reaction is slow, even with ZrO2 catalysts of “moderate activity”
[89].
Given the low catalytic activities observed for the
Meerwin–Ponndorf–Verley reductions, the nature of the acid
sites on these materials was determined with FT-Raman spec-
troscopy using pyridine as a probe molecule. The most sensitive
Raman vibration of pyridine is the symmetric ring breathing
(vCCN) at 991 cm−1. The interaction of pyridine with the acid sites
of a support results in a shift of this Raman band to higher values,
thus the position of the skeletal vibration band can be used to
determine the nature of these acid sites. It has been determined
that the interaction of pyridine with weak protonic sites through
hydrogen bonding results in bands between 996 and 1008 cm−1
,
or chemisorption at strong Bronsted sites results in bands between
1007 and 1015 cm−1 or Lewis acid sites give rise to bands between
1018 and 1028 cm−1 on solid surfaces [89–91]. As can be seen in
Fig. 8, all the materials show a significant peak at 1007 cm−1, which
Fig. 8. Raman spectra of pyridine chemisorbed onto the SZ–SiO2 materials 2–9 in
the pyridine symmetric ring breathing vibration region.

A.J. Ward et al. / Catalysis Today 178 (2011) 187–196
195
increases in intensity with increasing zirconium content, that is
attributable to strong Bronsted sites. In addition, the peak centred
at ∼998 cm−1 reveals the presence of weak hydrogen bonded sites.
Only materials 2–5 have Lewis acid sites that are indicated by the
peak at ∼1024 cm−1. All of the materials do show some evidence
of hydrogen bonding to the pyridine, as evidenced by the peak
[15] A. Hofmann, J. Sauer, J. Phys. Chem. B 108 (2004) 14652–14662.
[16] M. Bensitel, O. Saur, J.-C. Lavalley, B.A. Morrow, Mater. Chem. Phys. 19 (1988)
147–156.
[17] K. Arata, Green Chem. 11 (2009) 1719–1728.
[18] V.V. Brei, Teoreticheskaya i Eksperimental’naya Khimiya 41 (2005) 165–175.
[19] A. Corma, H. García, Chem. Rev. 103 (2003) 4307–4366.
[20] W.H.J. Hogarth, J.C. Diniz da Costa, G.Q. Lu, J. Power Sources 142 (2005) 223–237.
[21] G.D. Yadav, J.J. Nair, Micropor. Mesopor. Mater. 33 (1999) 1–48.
[22] A. Corma, Chem. Rev. 95 (1995) 559–614.
between 997 and 1002 cm−1
.
[23] S. Jaenicke, G.K. Chuah, V. Raju, Y.T. Nie, Catal. Surv. Asia 12 (2008) 153–169.
[24] D.J. McIntosh, R.A. Kydd, Micropor. Mesopor. Mater. 37 (2000) 281–289.
[25] M.K. Dongare, P. Singh, P.P. Moghe, P. Ratnasamy, Zeolites 11 (1991) 690–693.
[26] M.K. Dongare, D.P. Sabde, R.A. Shaikh, K.R. Kamble, S.G. Hegde, Catal. Today 49
(1999) 267–276.
[27] X.X. Wang, F. Lefebvre, J. Patarin, J.M. Basset, Micropor. Mesopor. Mater. 42
(2001) 269–276.
[28] M.L. Occelli, S. Biz, A. Auroux, Appl. Catal. A 183 (1999) 231–239.
[29] L.F. Chen, J.A. Wang, L.E. Norena, J. Aguilar, J. Navarrete, P. Salas, J.A. Montoya,
P. Del Angel, J. Solid State Chem. 180 (2007) 2958–2972.
[30] W. Hua, Y. Yue, Z. Gao, J. Mol. Catal. 170 (2001) 195–202.
[31] F. Li, F. Yu, Y. Li, R. Li, K. Xie, Micropor. Mesopor. Mater. 101 (2007) 250–255.
[32] M.S. Wong, H.C. Huang, J.Y. Ying, Chem. Mater. 14 (2002) 1961–1973.
[33] J. El Haskouri, S. Cabrera, C. Guillem, J. Latorre, A. Beltran, D. Beltran, M.D. Maros,
P. Amoros, Chem. Mater. 14 (2002) 5015–5022.
It is well established that the Meerwin–Ponndorf–Verley reduc-
tion is promoted by Lewis acid sites [83], however, more recent
work has shown that Bronsted sites can also be active for the catal-
ysis of this reaction [89]. Thus, the lack of Lewis acidic sites can
explain the observed low catalytic activity. The lack of activity of
the Bronsted sites is attributable to two factors: the first is that a
high density of Bronsted sites has been observed to give lower cat-
alytic activity for the Meerwin–Ponndorf–Verley reduction [89];
the second is that when zirconia materials are calcined at temper-
atures above 400 ◦C a significant decrease in catalytic activity for
the Meerwin–Ponndorf–Verley reduction results [87].
[34] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. Engl. 38 (1999)
56–77.
4. Conclusions
[35] B.L. Newalkar, J. Olanrewaju, S. Komarneni, J. Phys. Chem.
8356–8360.
B 105 (2001)
This study has shown that 1-hexadecyl-3-methylimidazolium
bromide can be successfully used as the structure directing agent
in a one pot synthesis of mesoporous sulfated zirconia. The meso-
porosity is confirmed by the nitrogen absorption, XRD and electron
microscopy measurements. The materials, prepared with varying
ratios of zirconium to silica (up to ∼40 wt.%), displayed surface
areas in the range 343–482 m2/g and mesopore volumes ranging
from 0.43 to 1.34 cm3/g. This method of preparation of the SZ–SiO2
materials for loadings up to 41 wt.% Zr results in the formation of
nanocrystalline zirconia in the silica host, with the zirconium also
present as polydispersed nanoparticles of ZrO2 on the surface at the
higher loadings. These conclusions are based on the lack of spec-
troscopic and X-ray diffraction evidence for large domains of ZrO2
or sulfated ZrO2, even at relatively high zirconium loadings. XPS
revealed only small amounts of zirconium on the surface, consistent
with the majority of the zirconium being incorporated in the SiO2
framework. The materials display solid acid behaviour, active in the
catalysis of the Meerwin–Ponndorf–Verley reductions of aldehy-
des and ketones with pKHB values less than 1.1. Investigation of the
nature of the acidic sites of the SZ–SiO2 materials using pyridine as a
probe molecule revealed predominantly Bronsted acid sites, which,
when the materials are calcined above 400 ◦C results in a con-
siderable decrease in activity for the Meerwin–Ponndorf–Verley
reduction.
[36] D.J. Jones, J. Jiménez-Jiménez, A. Jiménez-López, P. Maireles-Torres, P. Olivera-
Pastor, E. Rodriguez-Castellón, J. Rozière, J. Chem. Soc. Chem. Commun. (1997)
431–432.
[37] M.S. Morey, G.D. Stucky, S. Schwarz, M. Froba, J. Phys. Chem. B 103 (1999)
2037–2041.
[38] X.-X. Wang, L. Veyre, F. Lefebvre, J. Patarin, J.-M. Basset, Micropor. Mesopor.
Mater. 66 (2003) 169–179.
[39] B. Rakshe, V. Ramaswamy, S.G. Hegde, R. Vetrivel, A.V. Ramaswamy, Catal. Lett.
45 (1997) 41–50.
[40] D.A. Ward, E.I. Ko, Chem. Mater. 5 (1993) 956–969.
[41] J.A. Knowles, M.J. Hudson, J. Chem. Soc. Chem. Commun. (1995) 2083–2084.
[42] M.J. Hudson, J.A. Knowles, J. Mater. Chem. 6 (1996) 89–95.
[43] U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, F. Schüth, Angew. Chem. Int. Ed.
Engl. 35 (1996) 541–543.
[44] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Nature (London) 396
(1998) 152–155.
[45] J. Fan, S.W. Boettcher, G.D. Stucky, Chem. Mater. 18 (2006) 6391–6396.
[46] M.S. Wong, J.Y. Ying, Chem. Mater. 10 (1998) 2067–2077.
[47] A. Mondal, A. Zachariah, P. Nayak, B.B. Nayak, J. Am. Ceram. Soc. 93 (2009)
387–392.
[48] C.K. Krishnan, T. Hayashi, M. Ogura, Adv. Mater. 20 (2008) 2131–2136.
[49] B. Liu, R.T. Baker, J. Mater. Chem. 18 (2008) 5200–5207.
[50] F. Ye, Z. Dong, H. Zhang, Mater. Lett. 64 (2010) 1441–1444.
[51] H. Chen, J. Gu, J. Shi, Z. Liu, J. Gao, M. Ruan, D. Yan, Adv. Mater. 17 (2005)
2010–2014.
[52] S.K. Das, M.K. Bhunia, A.K. Sinha, A. Bhaumik, J. Phys. Chem. C 113 (2009)
8918–8923.
[53] M. Lutecki, O. Solcova, S. Werner, C. Breitkopf, J. Sol-Gel Sci. Technol. 53 (2010)
13–20.
[54] M. Lutecki, C. Breitkopf, Appl. Catal. A: Gen. 352 (2009) 171–178.
[55] A. Ramanathan, D. Klomp, J.A. Petersa, U. Hanefeld, J. Mol. Catal. A 260 (2006)
62–69.
[56] A. Ramanathan, M.C.C. Villalobos, C. Kwakernaak, S. Telalovic, U. Hanefeld,
Chem. Eur. J. 14 (2008) 961–972.
Acknowledgements
[57] S.-Y. Chen, J.-F. Lee, S. Cheng, J. Catal. 270 (2010) 196–205.
[58] S.-Y. Chen, L.-Y. Jang, S. Cheng, Chem. Mater. 16 (2004) 4174–4180.
[59] Y. Du, Y. Sun, Y. Di, L. Zhao, S.H. Liu, F.-S. Xiao, J. Porous Mater. 13 (2006)
163–171.
[60] A.A. Pujari, J.J. Chadbourne, A.J. Ward, L. Costanzo, A.F. Masters, T. Maschmeyer,
New J. Chem. 33 (2009) 1997–2000.
Support from the Australian Research Council is gratefully
acknowledged. L.C. gratefully acknowledges the receipt of a Uni-
versity of Sydney International Scholarship.
[61] T. Maschmeyer, M. Che, Angew. Chem. Int. Ed. 49 (2010) 1536–1539.
References
[62] S. Telalovic´, J.F. Ng, R. Maheswari, A. Ramanathan, G.K. Chuah, U. Hanefeld,
Chem. Commun. (2008) 4631–4633.
[63] M.M. Natile, A. Galenda, A. Glisenti, S. Mascotto, S. Gross, J. Non-Cryst. Solids
355 (2009) 481–487.
[64] C.J. Adams, A.E. Bradley, K.R. Seddon, Aust. J. Chem. 54 (2001) 679–681.
[65] S. Telalovic´, A. Ramanathan, G. Mul, U. Hanefeld, J. Mater. Chem. 20 (2010)
642–658.
[66] A. Clearfield, P.A. Vaughan, Acta Crystallogr. 9 (1956) 555–558.
[67] C. Walther, J. Rothe, M. Fuss, S. Büchner, S. Koltsov, T. Bergmann, Anal. Bioanal.
Chem. 388 (2007) 409–431.
[68] A. Clearfield, G.P.D. Serrette, A.H. Khazi-Syed, Catal. Today 20 (1994) 295–312.
[69] Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A.
Firouzi, B.F. Chmelka, Chem. Mater. 6 (1994) 1176–1191.
[1] J.D. McCullough, K.N. Trueblood, Acta Crystallogr. 12 (1959) 507–511.
[2] G. Teufer, Acta Crystallogr. 15 (1962) 1187.
[3] M. Hino, K. Arata, J. Chem. Soc. Chem. Commun. (1980) 851–852.
[4] I.J. Bear, Aust. J. Chem. 19 (1966) 357–361.
[5] I.J. Bear, Aust. J. Chem. 20 (1967) 415–428.
[6] I.J. Bear, Aust. J. Chem. 22 (1969) 875–889.
[7] I.J. Bear, G.M. Lukaszewski, Aust. J. Chem. 19 (1966) 1973–1975.
[8] I.J. Bear, W.G. Mumme, J. Chem. Soc. Chem. Commun. (1968) 609–611.
[9] I.J. Bear, W.G. Mumme, J. Chem. Soc. Chem. Commun. (1969) 230–232.
[10] I.J. Bear, W.G. Mumme, Spectrochim. Acta A 26 (1970) 755–760.
[11] I.J. Bear, W.G. Mumme, Rev. Pure Appl. Chem. 21 (1971) 189–211.
[12] Y.-L. Fu, Z.-W. Xu, J.-L. Ren, S.W. Ng, Acta Crystallogr. 61E (2005) m1520–m1522.
[13] Z.-W. Xu, Y.-L. Fu, S.-Q. Teng, S.W. Ng, Acta Crystallogr. 62E (2006)
m3420–m3422.
[70] C. Breitkopf, ChemCatChem 1 (2009) 259–269.
[71] F.R. Chen, G. Coudurier, J.-F. Joly, J.C. Vedrine, J. Catal. 143 (1993) 616–626.
[72] R.A. Comelli, C.R. Vera, J.M. Parera, J. Catal. 151 (1995) 96–101.
[73] X. Li, K. Nagaoka, L.J. Simon, R. Olindo, J.A. Lercher, Catal. Lett. 113 (2007) 34–40.
[14] Z.-W. Xu, Y.-L. Fu, S.-Q. Teng, S.W. Ng, Acta Crystallogr. 63E (2006) m140–m142.

196
A.J. Ward et al. / Catalysis Today 178 (2011) 187–196
[74] L. Armelao, H. Bertagnolli, D. Bleiner, M. Groenewolt, S. Gross, V. Krishnan,
C. Sada, U. Schubert, E. Tondello, A. Zattin, Adv. Funct. Mater. 17 (2007)
1671–1681.
[83] M. Boronat, A. Corma, M. Renz, J. Phys. Chem. B 110 (2006) 21168–21174.
[84] M. Shibagaki, H. Kuno, K. Takahashi, H. Matsushita, Bull. Chem. Soc. Jpn. 61
(1988) 4153–4154.
[75] A. Infantes-Molina, J. Mérida-Robles, P. Maireles-Torres, E. Finocchio, G. Busca,
E. Rodríguez-Castillón, J.L.G. Fierro, A. Jiménez-López, Micropor. Mesopor.
Mater. 75 (2004) 23–32.
[76] Z. Dang, B.G. Anderson, Y. Amenomiya, B.A. Morrow, J. Phys. Chem. 99 (1995)
14437–14443.
[77] K. Dehnicke, J. Weidlein, Angew. Chem. Int. Ed. Engl. 5 (1966) 1041.
[78] M. Bouchard, D.C. Smith, C. Carabotos-Nédelec, Spectrochim. Acta A 68 (2007)
1101–1113.
[79] R.L. McCreery, Raman Spectroscopy for Chemical Analysis, John Wiley and Sons,
New York, 2000, p. 86.
[85] M. Shibagaki, K. Takahashi, H. Kuno, H. Kawakami, H. Matsushita, Chem. Lett.
17 (1988) 1633–1636.
[86] H. Kuno, K. Takahashi, M. Shibagaki, K. Shimazaki, H. Matsushita, Bull. Chem.
Soc. Jpn. 63 (1990) 1943–1946.
[87] S.H. Liu, S. Jaenicke, G.K. Chuah, J. Catal. 206 (2002) 321–330.
[88] F. Besseau, M. Lucon, C. Laurence, M. Berthelot, J. Chem. Soc. Perkin Trans. 2
(1998) 101–107.
[89] F.J. Urbano, R. Romero, M.A. Aramendía, A. Marinas, J.M. Marinas, J. Catal. 268
(2009) 79–88.
[90] A.A.M. Ali, M.I. Zaki, Colloid Surf. A 139 (1998) 81–89.
[91] M.A. Aramendia, V. Borau, C. Jimenez, J.M. Marinas, J.R. Ruiz, F.J. Urbano, J.
Colloid Interface Sci. 217 (1999) 186–193.
[80] S.W. Lee, R.A.S. Condrate, J. Mater. Sci. 23 (1988) 2951–2959.
[81] Y.-Y. Huang, T.J. McCarthy, W.M.H. Sachtler, Appl. Catal. A 148 (1996) 135–154.
[82] J.C. Jansen, Z. Shan, L. Marchese, W. Zhou, N.v.d. Puil, T. Maschmeyer, Chem.
Commun. (2001) 713–714.