Sulfated mesoporous Au/TiO2 spheres as a highly active and stable solid acid catalyst

Article abstract of DOI:10.1039/c2jm16921e

In this work, we reported several kinds of highly stable, effective mesoporous Au/TiO₂ sphere-based solid acid catalysts with large specific surface areas and good monodispersity in the reaction medium. In this process, monodisperse titanium glycolate spheres were first formed by an antisolvent precipitation route, and were then converted to mesoporous TiO ₂ by in situ hydrolysis under hydrothermal conditions. The resultant uniform TiO₂ spheres were subsequently functionalized with sulfate anions at different temperatures or incorporated with tungstophosphoric acid (TPA); the solid acid catalysts were thus fabricated. In particular, our monodisperse spherical catalysts with large specific surface area have shown remarkable performance in a wide range of acid-catalyzed reactions such as Friedel-Crafts alkylation, esterification and hydrolysis of acetate. The TiO₂-based catalysts could be separated and reused showing no deactivation.

Full text of DOI:10.1039/c2jm16921e

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PAPER
Sulfated mesoporous Au/TiO2 spheres as a highly active and stable solid acid
catalyst†
a
Cheng Chao Li, Yan Ping Zheng and Tai Hong Wang
b
a
*
Received 30th December 2011, Accepted 20th April 2012
DOI: 10.1039/c2jm16921e
In this work, we reported several kinds of highly stable, effective mesoporous Au/TiO2 sphere-based
solid acid catalysts with large specific surface areas and good monodispersity in the reaction medium. In
this process, monodisperse titanium glycolate spheres were first formed by an antisolvent precipitation
route, and were then converted to mesoporous TiO2 by in situ hydrolysis under hydrothermal
conditions. The resultant uniform TiO2 spheres were subsequently functionalized with sulfate anions at
different temperatures or incorporated with tungstophosphoric acid (TPA); the solid acid catalysts
were thus fabricated. In particular, our monodisperse spherical catalysts with large specific surface area
have shown remarkable performance in a wide range of acid-catalyzed reactions such as Friedel–Crafts
alkylation, esterification and hydrolysis of acetate. The TiO2-based catalysts could be separated and
reused showing no deactivation.
because polymerization of the neat reactants may have occurred
Introduction
over the course of the reaction, resulting in coverage of active
sites. In addition, the use of toxic organic ligands and solvents in
the synthesis of these solid acid catalysts is still a crucial issue.
Recently, molybdenum oxides have been demonstrated as
a highly effective solid acid catalyst in benzylation of a broad
range of arenes with various benzylic alcohols as alkylating
agents.31–33 However, despite their good catalytic performance,
the practical use of MoO3 as a solid acid catalyst is still largely
hindered by the high cost and large scale synthesis. Despite recent
advances, there is still a demand for other low cost and envi-
ronmentally friendly solid acid alternatives. Titanium oxide and
titanium-containing catalysts have recently attracted great
interest due to their high stability and strong acid properties
which can be preserved in polar liquids.34–39 Furthermore, tita-
nium is the second most abundant transition metal in the earth’s
crust. In view of catalytic reactions, it is known that the catalytic
activity of a solid acid catalyst is greatly dependent on the
chemical active sites and diffusion rate of reaction substrate. For
this reason, nanostructured or/and porous titanium oxide cata-
lysts with large surface area hold much promise for the deve-
lopment of solid acid catalysts with enhanced activity. The short
diffusion channels and large surface area in theory allow more
facile diffusion of the reaction substrate compared to bulk
materials, and thus a higher reaction rate is achieved. In pursuing
this research, there have been a number of synthetic methods
proposed in recent years, such as solvent–thermal/hydrothermal
routes, template routes, electrospinning, sol–gel, and so on.40–44
In addition to these methods, in the present study, we develop
a novel in situ hydrolysis approach to synthesize monodisperse
mesoporous Au/TiO2 spheres and their solid acid catalyst
derivatives. In the synthesis, acetone acts as an antisolvent. When
Heterogeneous acid catalysts (e.g. AlCl3, BF3, HCl, and H2SO4),
a very important class of catalysts, are commonly used in many
industrially important reactions such Friedel–Crafts alkylation,
esterification, and hydrolysis.1–8 However, major drawbacks of
these catalysts include separation and recovery from the reaction
solution, corrosion, high toxicity, and disposal of spent catalyst,
etc.8–17 Besides, some of the catalysts are very moisture sensitive,
which increases the difficulty of reaction operation and requires
high-level reaction equipment. Thus, development of reusable
solid catalysts with high activity and low cost is of great practical
importance. In comparison with the liquid acid catalysts, solid
acid catalysts are environmentally friendly with regard to
corrosiveness and safety, and readily separable from the reaction
system. Intrigued by this, increasing research interest has focused
on the substitution of hazardous Lewis and Brønsted acid cata-
lysts by heterogeneous analogues. To date, silica-supported solid
acid catalysts, styrene-based sulfonic acid resins, sulfonated
mesoporous carbon, zeolites and heteropolyacids, and metal
oxides have been developed and investigated as solid acid cata-
lysts.18–30 However, their heterogeneous nature does not guar-
antee the stability of catalytic performance after their recovery
aKey Laboratory for Micro-Nano Optoelectronic Devices of Ministry of
Education, State Key Laboratory of Chemo/Biosensing and
Chemometrics, Hunan University, Changsha, 410082, P. R. China.
E-mail: thwang@aphy.iphy.ac.cn; Fax: +86-0731-88822137; Tel: +86-
0731-88823407
bState Key Laboratory of Physical Chemistry of Solid Surfaces, College of
Chemistry and Chemical Engineering, Xiamen University, Xiamen,
361005, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm16921e
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the solvent containing titanium precursors is poured into
acetone, titanium glycolate segregates from the mixed solvents to
form the spherical product due to large supersaturation with the
assistance of water.45–48 The obtained mesoporous Au/TiO2 in
our work can be easily functionalized with sulfate anions at
different temperatures or incorporated with tungstophosphoric
acid (TPA) to obtain a series of recyclable solid acid catalysts.
Thanks to their large surface area and short diffusion channels,
the prepared catalysts exhibit superior catalytic activities for
Friedel–Crafts alkylation, esterification, and hydrolysis of
acetates. In particular, the catalysts still maintained a similar
catalytic performance to the fresh one after using eight times.
Characterization
Crystallographic phases of the prepared products were investi-
gated by X-ray power diffraction method (XRD) using Shi-
madzu XRD-6000 with Cu Ka radiation. The morphologies of
the as-prepared samples were characterized by field-emission
scanning electron microscopy (FESEM; JSM-6700F), trans-
mission electron microscopy (TEM; JEM-2010, 200 kV), selected
area electron diffraction (SAED), and high-resolution trans-
mission electron microscopy (HRTEM; JEM-3010, 300 kV). The
Fourier transform infrared (FTIR) spectrum was recorded for
a KBr dilute sample using a Bio-Rad FTS-135 FTIR spectro-
meter. Thermogravimetric analysis (TGA; Shimadzu TGA-50)
was also performed to determine the composition of the sample.
The TGA measurements were carried out at a heating rate of
10 ꢀC minÀ1 from 50 ꢀC to 900 ꢀC with an air flow rate of 100 mL
minÀ1. Specific surface area measurement and porosity analysis
for the titanium oxide spheres samples were performed using N2
adsorption–desorption isotherms (Quantachrome NOVA-3000
system at 77 K). NH3-TPD was carried out using a Micromeritics
Auto Chem II 2920 instrument equipped with a Pfeiffer Vacuum
ThermoStar GSD 301 T2 mass spectrometer. After the sample
was heated at 373 K for 2 h under He flow, a certain amount of
the sample was exposed to NH3 at 353 K for adsorption and then
heated at 10 K minÀ1. Slightly compacted ground sulfated TiO2
was previously heated up to 100 ꢀC for 30 min in a catalytic cell.
After this period of time, pyridine vapor was introduced into the
cell, the inlet valve was left open for 3 min, after which a vacuum
was produced in order to remove ꢀexcess pyridine. The infrared
spectra were then acquired at 200 C.
Experimental section
The sulfated mesoporous Au/TiO2 spheres were prepared by
a three-step route: (i) synthesis of titanium glycolate precursor
spheres; (ii) hydrolysis of the titanium glycolate spheres to
fabricate mesoporous Au/TiO2 spheres; and (iii) treatment of Au/
TiO2 spheres by sulfuric acid and TPA.
Synthesis of titanium glycolate precursor spheres
All chemicals were available commercially and used without
further purification. The titanium glycolate spheres were
synthesized according to the route reported previously with
minor modification.44 In a typical synthesis, tetrabutoxytitanium
(TBT, Aldrich) was added to ethylene glycol (EG, 50 mL, Merck)
in a 100 mL flask under vigorous stirring; then nitrogen was
bubbled through the system for about 30 min to remove water
and oxygen. After that, the flask was sealed and stirred for 12 h at
room temperature. The obtained mixture was poured into
200 mL acetone in a beaker, followed by the addition of 1.9 mL
water and 0.8 mL 30 mM HAuCl4. The beaker was then sealed
with Parafilm and stirred for 1 h. The white precipitate was
harvested by centrifugation, washed with ethanol three times and
Catalytic tests
The catalytic reaction of mesoporous titanium oxide spheres was
carried out in a 100 mL 3-neck-round bottom flask with a reflux
condenser. 50 mL of toluene, 5 mL of benzyl alcohol and 0.5 mL
internal standard hexadecane were added to the reactor with
0.5 g catalyst in each run. Nitrogen was introduced into the flask
through one of the gas inlets. The second inlet was equipped with
a septum for sample removal. The reaction mixture was heated to
130 ꢀC for a certain time using an oil bath. Samples of the
reaction mixture were periodically withdrawn from the flask and
analyzed by gas chromatography (GC). The temperature was
programmed from 70 ꢀC to 280 ꢀC. The results from GC identify
the o- and p-benzylation products and benzyl ether as the main
products of these reactions. Activities were calculated on the
basis of percentage conversion of benzyl alcohol in the starting
mixture.
ꢀ
dried at 50 C in an oven for further usage.
Synthesis of mesoporous Au/TiO2 spheres
0.5 g titanium glycolate precursor spheres were dispersed into
40 mL water under the assistance of ultrasonication. The mixture
was then transferred to a Teflon-lined autoclave and kept inside
ꢀ
an electric oven at 180 C for 4 h. The product was collected,
washed several times with water and ethanol, and then dried at
60 ꢀC in an oven.
Catalysts preparation
Sulfated mesoporous titanium oxide. The obtained mesoporous
titanium oxide spheres were dispersed in 20 mL methanol solu-
tion containing 1 M sulfuric acid under vigorous stirring for 6 h.
The obtained product was collected by centrifugation and
Catalyst recycling and reuse
ꢀ
ꢀ
calcined at 100 C or 400 C for 4 h to obtain the sulfated tita-
nium oxide.
The catalyst was separated from the reaction mixture by centri-
fugation, and then rinsed with toluene three times. The collected
catalyst was directly dispersed into the fresh reaction mixture
(5 mL benzyl alcohol and 50 mL toluene) to conduct the next
recycling test. In all reuse tests, the sampling time was 40 min for
convenience.
Sulfated titanium oxide particles. The titania particles
purchased from Merck were sulfated using the same route as our
mesoporous titanium oxide spheres.
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Results and discussion
Fig. 1 depicts the synthetic steps for the sulfated mesoporous Au/
TiO2 spheres solid acid catalysts prepared in this work. First,
tetrabutoxytitanium was dissolved in ethylene glycol and reacted
with ethylene glycol to form glycolate derivatives according to
eqn (1) and (2).44,49–51
Ti(OBu)4 + HOCH2CH2OH / Ti(OCH2CH2O)
(OBu)2 + 2HOBu
(1)
(2)
Ti(OBu)4 + HOCH2CH2OH / Ti(OCH2CH2O)2
+ 4HOBu
Due to their low solubility in acetone, glycolate derivatives
À
segregate when poured into acetone. At the same time, AuCl4
adsorbs on the surface of these spheres and makes them more
stable against aggregation by increasing their surface potential.
The morphology of the obtained titanium glycolate spheres was
investigated by transmission electron microscopy (TEM). As
depicted in Fig. 2a, it reveals the products are uniform and
spherical with sizes around 500 nm. By hydrothermal treatment
at 180 ꢀC, titanium glycolate spheres were converted to meso-
porous titanium oxide spheres by in situ hydrolysis. From the
SEM images (Fig. 2b), the spherical morphology was completely
preserved and the surface of the spherical particles became very
rough. The TEM images further reveal the porous nature of the
materials, as shown in Fig. 2c and d. The pores in mesoporous
spheres (several nanometers) are clearly observed. The meso-
porous spheres are composed of a lot of interconnected nano-
particles with sizes around 10 nm, which is compatible with
results calculated by Scherrer’s equation. During the hydro-
Fig. 2 (a) The representative TEM images of titanium glycolate spheres,
(b) SEM images and (c–e) TEM of the as-prepared mesoporous titanium
oxide spheres (MTOS), (f) DLS size-distribution diagram of MTOS.
structure of the as-synthesized samples was characterized using
power X-ray diffraction (XRD). Fig. 3 shows a representative
diffraction pattern. All these peaks can be indexed to the pure
anatase structure of TiO2. The lattice parameter a is found to be
0.378 nm by analysis of the XRD pattern, which is in agreement
with the reported values (JCPDS card no. 21-1272). Compared
with those of the bulk counterpart, the peaks are relatively
broadened, which demonstrates the small size of the nano-
particles. No characteristic peaks of other impurities are
À
thermal process, the AuCl4 adsorbed on the surface of the
spheres was also reduced into gold nanoparticles (Fig. 2d). The
dynamic light scattering (DLS) results (Fig. 2f) show that the
average hydrodynamic diameters of the spheres are about 500 Æ
50 nm, which is consistent with the SEM results. The phase
Fig. 1 Typical preparation process of sulfated mesoporous Au/TiO2 spheres solid acid catalysts: (a) mixing EG (solvent) containing TBT and acetone
(antisolvent), (b) hydrothermal treatment of titanium glycolate spheres, (c) functionalization with sulfate groups or TPA, (d) calcination at different
temperatures.
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Fig. 5 FTIR spectra of the sulfated MTOS catalysts after various heat
treatments: (a) at 100 ꢀC, (b) at 200 ꢀC, (c) at 300 ꢀC, (d) at 400 ꢀC for 4 h
and (e) non-sulfated MTOS for reference.
Fig. 3 XRD pattern of as-prepared mesoporous Au/TiO2 spheres.
observed, indicating the purity of the product. In order to esti-
mate the surface areas, the mean diameters and volumes of the
pores, nitrogen adsorption–desorption measurements at 77 K
have been carried out. Fig. 4 presents the isotherm of the Au/
TiO2 spheres and its corresponding pore size distribution curve.
The isotherm can be classified as type IV isotherm, characteristic
of mesoporous materials. The corresponding pore size distribu-
tion calculated from the isotherm by the Barrett–Joyner–
Halenda (BJH) method shows a narrow pore size distribution
ranging from 2 nm to 8 nm. The Brunauer–Emmett–Teller (BET)
atoms.52–54 These bands are related to the amount of sulfate
linked to the metal oxide in the chelate form. The bands at 1234
and 1140 cmÀ1 can be attributed to the asymmetric and
symmetric stretching vibrations of S]O groups, respectively,
while the peaks centered at 1048 cmÀ1 are ascribed to asymmetric
stretching vibrations of S–O groups. For mesoporous Au/TiO2
calcined at 400 ꢀC, the band intensities of the sulfate ion decrease
slightly because of the partial decomposition of sulfate ions.
The surface element composition and chemical state of the
sample were further confirmed by XPS results, and the corre-
sponding experimental results are shown in Fig. 6. The Ti 2p XPS
spectrum shows two major peaks with binding energies at
surface area and pore volume are 153.3 m2 gÀ1 and 0.3 cm3 gÀ1
,
respectively.
The as-prepared mesoporous Au/TiO2 spheres were treated by
sulfuric acid or tungstophosphoric acid (TPA) at different
temperatures to obtain the solid acid catalysts. There are three
states—simple coordination, chelation coordination and bridge
coordination—for the sulfate moiety existing on the surface of
TiO2. The bonding situations of sulfated species are revealed by
the FTIR spectrum (Fig. 5). The band at 1625 cmÀ1 is assigned to
the deformation vibration mode of the adsorbed water. In
comparison with TiO2, new absorption bands between 1250 and
950 cmÀ1 are observed in all sulfated MTOS catalysts, and are
assigned to a bidentate sulfate ion coordinated to the Ti
464.9 eV and 459.1 eV, corresponding to Ti 2p3/2 and Ti 2p1/2
,
respectively, characteristic of a TiO2 phase. Fig. 6b is an XPS
spectrum of S, which confirms the existence of SO42À. It can be
fitted into two peaks (S 2p3/2 and a S 2p1/2 doublet) by nearly
Gaussian curves. The spectrum of O 1s with an additional
shoulder at higher binding energy is observed and can be fitted
into three peaks. The lower energy peak located at 530.4 eV
corresponds to the coordination of oxygen in Ti–O–Ti, while the
higher energy peak centered at 531.9 eV can be ascribed to the O–
S binding. Another small peak located at 533.1 eV should be
assigned to OH species in H2O molecules. The XPS peaks of 86.4
and 81.2 eV with a constant separation of spin orbit coupling of
5.2 eV are clearly observed, which correspond to Au 4f7/2 and
4f5/2, respectively.
Further, NH3 temperature-programmed desorption (NH3-
TPD) experiments have been carried out to investigate the acidity
of these solid acid catalysts based on MTOS (Fig. SI-1†). As we
know, sulfate species themselves are Lewis acids and they can
generate Lewis and Brønsted acidity centers by adsorption on the
metal oxide surfaces. Also, Lewis and Brønsted sites on the metal
oxide surfaces are easily changeable by adsorption or desorption
of water molecules. The chemical state of the sulfate groups
sometimes determines the acidity of the oxide surfaces. Gener-
ally, the ionic character between a metal center and sulfate anions
is changed to covalent character after calcination at high
temperature, which is responsible for generating strongly acidic
active centers on the oxide surfaces. It is proposed that sulfate
species are combined with Ti elements in the bridging bidentate
Fig. 4 N2 adsorption–desorption BET isotherm and pore size distribu-
tion curve.
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Fig. 6 XPS survey spectra of mesoporous Au/TiO2 spheres: (a) Ti 2p, (b) S 2p, (c) O 1s, (d) Au 4f.
state.55 The double-bond nature of the complex (Ti–OOSOO–Ti)
is much stronger compared to that of a simple metal sulfate
(Fig. SI-2†). Due to the electron-withdrawing effect of SO42À, the
Lewis acid strength of Ti4+ would be remarkably stronger. For
TPD analysis, the peak temperature is a measure of the strength
of acid sites while the area under the peak represents the total
amount of acid sites present on the catalyst. The NH3-TPD
profile of sulfated MTOS at 100–400 ꢀC shows a wide NH3
desorption band ranging from 100 to 400 ꢀC, which can be
considered an indication of the presence of a high concentration
of acid sites with moderate strength. In addition to this broad
peak, a shoulder peak above 420 ꢀC was also observed for
agreement with TPD results, the catalysts treated at 200 and
300 ꢀC contain the maximum number of acid sites. However, for
the catalyst treated at 400 ꢀC, the intensities of absorption bands
assigned to Brønsted acidity (1640 and 1538 cmÀ1) decrease,
indicating that Brønsted acidity will be largely destroyed at
higher calcination temperature. As we know, Brønsted acidity is
due to free surface hydroxyl groups located near active sulfate
species such that the electron-withdrawing effect of the active
sulfate species makes proton donation easier. Therefore, the
content of active sulfate species on the catalyst surface plays
a very important role in the generation of Brønsted acidity. A
certain minimum threshold of active sulfate species is required to
generate strong Brønsted acidity.
ꢀ
sulfated MTOS at 300 and 400 C. The formation of superacid
sites could be responsible for the presence of the high-tempera-
ture desorption peaks, which is in agreement with literature
reports. It should be noted that the peak intensity for sulfated
MTOS at 400 ꢀC is much weaker than that of sulfated MTOS at
The catalytic activities of these solid acid catalysts based on
Au/TiO2 spheres were evaluated in the benzylation of toluene
with benzyl alcohol in liquid phase at a temperature of 130 ꢀC. In
order to illustrate the excellent diffusion advantages inherited
from the mesoporous sphere structure, nonporous sulfated tita-
nium oxide particles were also tested for comparison. As shown
in Fig. 7, the titanium oxide particles show very low catalytic
activity in this Friedel–Crafts alkylation reaction: the conversion
of alcohol is only 3.12% in 8 h. The pure MTOS shows moderate
performance as a catalyst. The TPA/MTOS has a higher catalytic
activity than pure MTOS; it shows 100% conversion in less than
8 h. However, the catalytic activity decreases with time. Out of
the four catalysts, the best catalytic activity was achieved by the
sulfated MTOS treated at 300 ꢀC. The conversion of benzyl
alcohol in 30 min is up to 100%. Compared to the solid acid
catalyst reported previously, our sulfated MTOS treated at
ꢀ
300 C. This can be related to the partial decomposition of the
sulfate ion, which is consistent with FTIR results. To detect the
presence of Lewis and Brønsted acid sites on the catalysts, as well
as their acid strength, pyridine adsorption analysis was carried
out in this study. The pyridine adsorption experiments were
performed by acquisition of infrared spectra at 200 ꢀC. The
corresponding FT-IR spectra of pyridine adsorption are shown
in Fig. SI-3.† The spectra show bands of absorption at 1640,
1607, 1538, 1487 and 1445 cmÀ1, typical of adsorbed pyridine.
The bands around 1640 cmÀ1 and 1538 cmÀ1 were attributed to
the pyridinium ion (Brønsted acid sites), and bands at about 1607
cmÀ1, 1487 cmÀ1 and 1445 cmÀ1 were ascribed to coordinated
pyridine (Lewis acid sites). The ratio between the Brønsted and
Lewis acid sites can be determined from the relative intensity
ꢀ
300 C also shows much better catalytic performance. The high
catalytic performance demonstrates that nanosized porous
particles are very effective for heterogeneous catalytic reactions.
In our work, the ultrahigh catalytic activity of sulfated MTOS
(band heights) of the bands located at 1538 cmÀ1 and 1445 cmÀ1
,
related to Brønsted and Lewis acid sites respectively. In
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a high activity for the esterification of acetic acid. The sulfated
MTOS treated at 200–300 ꢀC exhibits the highest reaction rate of
all the catalysts, achieving a reaction rate of 1.35 mmol gÀ1 minÀ1
.
The catalytic activity substantially exceeds the performance of
most conventional solid acid catalysts, such as Amberlyst-15, H-
Beta, Nafion NR50, and other catalysts reported previously.32,56
In the hydrolysis of ethyl acetate, interestingly, all the sulfated
MTOS show comparable catalytic activity. The reaction rates of
sulfated MTOS treated at 100, 200, 300, and 400 ꢀC are 0.28,
0.32, 0.32, and 0.30 mmol gÀ1 minÀ1, respectively.
Conclusion
In summary, we developed a series of highly stable, effective
mesoporous Au/TiO2 sphere-based solid acid catalysts with large
specific surface area and good monodispersity by a three-step
route. First, titanium glycolate spheres were prepared and then
converted into mesoporous Au/TiO2 spheres by hydrothermal
reaction. Subsequently, Au/TiO2 spheres were functionalized
with sulfate anions at different temperatures or incorporated
with tungstophosphoric acid (TPA) to fabricate solid acid cata-
lysts. Due to the large surface area and short diffusion channels,
the prepared catalyst exhibited superior catalytic activity for
Friedel–Crafts alkylation, esterification and hydrolysis of
acetates. In particular, the catalyst still maintained a similar
catalytic performance to the fresh one after using eight times.
Fig. 7 Percentage conversion of benzyl alcohol in the benzylation of
toluene catalyzed by various catalysts.
can be attributed to two advantages: (i) the monodisperse and
mesoporous spherical structure; the short diffusion path and
large surface area facilitate the diffusion of reactants and pro-
ducts, greatly increasing the reaction rate; (ii) high percentage of
sulfate groups covalently immobilized on titanium oxide,
increasing the acidity level of MTOS. The heat treatment
temperature of sulfated MTOS is a key parameter to catalytic
activity, which greatly affects the amount of sulfate groups
covalently immobilized on MTOS. Thꢀ e suitable heat treatment
temperature ranges from 200 to 300 C (Fig. SI-4†). When the
temperature is above 300 ꢀC, some of the sulfate groups will
decompose. However, if the temperature is too low (<200 ꢀC) the
percentage of covalently immobilized sulfate groups decreases.
After a catalytic reaction, the sulfated MTOS catalyst was easily
separated by centrifugation and washed with toluene, and could
be used several times. Eight test recycles were performed to
evaluate the reusability of sulfated MTOS, as shown in Fig. 8.
The reused sulfated MTOS still retained high catalytic perfor-
mance similar to that of the fresh catalyst; no decrease in cata-
lytic activity was observed after eight uses.
Acknowledgements
This work was partly supported by the Scholarship Award for
Excellent Doctoral Student granted by Ministry of Education
(2010), ‘‘973’’ National Key Basic Research Program of China
(Grant no. 2007CB310500), National Natural Science Founda-
tion of China (Grant no. 21003041, 21103046), and Hunan
Provincial Natural Science Foundation of China (Grant
no.10JJ1011).
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The mesoporous TiO2 spheres-based solid acid catalysts were
also applied for the esterification of acetic acid with ethanol and
the hydrolysis of ethyl acetate. The sulfated MTOS also show
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