Synthesis and characterization of mesoporous Ta-W oxides as strong solid acid catalysts

Article abstract of DOI:10.1021/cm903767n

Several mesoporous Ta_xW_10-x mixed oxides prepared from TaCl₅ and WCl₆ in the presence of poly block copolymer surfactant Pluronic P-123 were examined as potential solid acid catalysts. Amorphous wormhole-type mesopores were observed in samples with x values from 3 to 10, whereas W-rich samples (x = 0-2) formed nonmesoporous structures with crystallized tungsten oxide (WO₃). The acid strength increased with addition of W for mesoporous Ta_xW_10-x oxides, and mesoporous Ta₃W₇ oxide exhibited the highest acid catalytic activity for the Friedel-Crafts alkylation of anisole and the hydrolysis of disaccharides. The results are compared with those of nonporous Ta₂O₅-WO₃, HTaWO₆ nanosheets, which were a solid acid obtained by exfoliation and aggregation of layered HTaWO ₆ with strong Bronsted acid sites in the interlayer, and a range of conventional solid acid catalysts. Mesoporous Ta-W oxides exhibited a turnover rate higher than that of nonporous Ta₂O₅-WO ₃ and HTaWO₆ nanosheets, indicating that the mesoporous structure is an advantageous environment for the strong acid sites, because of the high surface area and easy reactant accessibility.

Full text of DOI:10.1021/cm903767n

3072 Chem. Mater. 2010, 22, 3072–3078
DOI:10.1021/cm903767n
Synthesis and Characterization of Mesoporous Ta-W Oxides as
Strong Solid Acid Catalysts
Caio Tagusagawa,† Atsushi Takagaki,‡ Ai Iguchi,§ Kazuhiro Takanabe,† Junko N. Kondo,§
Kohki Ebitani,‡ Takashi Tatsumi,§ and Kazunari Domen*,†
†Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1, Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan, ‡School of Materials Science, Japan Advanced Institute of Science and
Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan, and Chemical Resources Laboratory,
Tokyo Institute of Technology, 4259 Nagatsuta Midori-ku, Yokohama 226-8503, Japan
§
Received December 15, 2009. Revised Manuscript Received April 7, 2010
Several mesoporous TaxW10-x mixed oxides prepared from TaCl5 and WCl6 in the presence of
poly block copolymer surfactant Pluronic P-123 were examined as potential solid acid catalysts.
Amorphous wormhole-type mesopores were observed in samples with x values from 3 to 10, whereas
W-rich samples (x=0-2) formed nonmesoporous structures with crystallized tungsten oxide
(WO3). The acid strength increased with addition of W for mesoporous TaxW10-x oxides, and
mesoporous Ta3W7 oxide exhibited the highest acid catalytic activity for the Friedel-Crafts
alkylation of anisole and the hydrolysis of disaccharides. The results are compared with those of
nonporous Ta2O5-WO3, HTaWO6 nanosheets, which were a solid acid obtained by exfoliation and
aggregation of layered HTaWO6 with strong Brønsted acid sites in the interlayer, and a range of
conventional solid acid catalysts. Mesoporous Ta-W oxides exhibited a turnover rate higher than
that of nonporous Ta2O5-WO3 and HTaWO6 nanosheets, indicating that the mesoporous structure
is an advantageous environment for the strong acid sites, because of the high surface area and easy
reactant accessibility.
1. Introduction
solid acid catalyst with enhanced activity.4-6 Mesopores
in the oxide enable the reactants to access additional
active acid sites, resulting in improved rates of acid
catalysis. Sulfated mesoporous niobium5 and tantalum
oxides6 have been reported to exhibit remarkable activity
and selectivity in acid-catalyzed Friedel-Crafts alkyl-
ation and isomerization. However, the use of a recycled
catalyst remains difficult, because of leaching of sulfate
species, as reported for mesoporous silica and organosilicas
bearing sulfonic acid groups. Previous studies on meso-
porous Nb-W oxide solid acids7 have revealed that
wormhole-type mesoporous Nb-W oxides can be applied
as recyclable and highly active solid acid catalysts for
Friedel-Crafts alkylation, hydrolysis of disaccharides,
and esterification. The reaction rate and the acid strength
increased gradually with the addition of W, reaching the
highest reaction rate with mesoporous Nb3W7 oxide, which
exceeded the reaction rate of ion-exchange resins, zeolites,
and nonmesoporous metal oxides.
Solid acid catalysts, which are reusable and readily
separable from reaction products, have been widely inves-
tigated as direct replacements for liquid acids to reduce the
impact on the environment and to decrease costs.1 These
new concepts for designing industrial reactions, maximiz-
ing the amount of raw material that ends up in a product,
minimizing the environmental impact, and substituting
renewable sources for depleting sources called “green
chemistry” or sustainable chemistry2,3 have been widely
applied in organic chemistry, inorganic chemistry, bio-
chemistry, analytical chemistry, and physical chemistry.
The use of mesoporous transition metal oxides as solid
acid catalysts is an interesting approach to developing a
*To whom correspondence should be addressed. Telephone: þ81-3-5841-
1148. Fax: þ81-3-5841-8838. E-mail: domen@chemsys.t.u-tokyo.ac.jp.
(1) (a) Okuhara, T. Chem. Rev. 2002, 102, 3641. (b) Tanabe, K.;
€
Holderich, W. F. Appl. Catal., A 1999, 181, 399.
(2) Anastas, P. T.; Williamson, T. C. In Green Chemistry; Anastas, P. T.,
Williamson, T. C., Eds.; American Chemical Society: Washington, DC,
1996; pp 1-20.
In this study, mesoporous Ta-W mixed oxides with
different Ta and W concentrations were examined as novel
solid acid catalysts. Our previous study demonstrated that
isomorphous replacement of Nb5þ with higher-valence
W6þ cations formed strong Brønsted acid sites in W-enriched
samples, resulting in a highly active mesoporous solid acid.7
(3) Clark, J. H. Green Chem. 1999, 1, 1.
(4) (a) Huang, Y.-Y.; McCarthy, T. J.; Sachtler, W. M. H. Appl. Catal.,
A 1996, 148, 135. (b) Chidambaram, M.; Curulla-Ferre, D.; Singh,
A. P.; Anderson, B. G. J. Catal. 2003, 220, 442. (c) Landge, S. M.;
Chidambaram, M.; Singh, A. P. J. Mol. Catal. A: Chem. 2004, 213,
257. (d) Hwang, C.-C.; Mou, C.-Y. J. Phys. Chem. C 2009, 113, 5212.
(5) Rao, Y.; Trudeau, M.; Antonelli, D. J. Am. Chem. Soc. 2006, 128,
13996.
(6) (a) Rao, Y.; Kang, J.; Antonelli, D. J. Am. Chem. Soc. 2008, 130,
394. (b) Kang, J.; Rao, Y.; Trudeau, M.; Antonelli, D. Angew. Chem.,
Int. Ed. 2008, 47, 4896.
(7) Tagusagawa, C.; Takagaki, A.; Iguchi, A.; Takanabe, K.; Kondo,
J. N.; Ebitani, K.; Hayashi, S.; Tatsumi, T.; Domen, K. Angew.
Chem., Int. Ed. 2010, 49, 1128.
r
2010 American Chemical Society
pubs.acs.org/cm
Published on Web 04/23/2010

Article
Chem. Mater., Vol. 22, No. 10, 2010 3073
Therefore, the mesoporous Ta-W mixed oxide was tested in
an attempt to obtain a solid acid more effective than non-
mesoporous Ta2O5-WO3 oxide and HTaWO6 nanosheet
aggregate, which is a two-dimensional crystalline oxide
obtained from exfoliation of cation-exchangeable layered
oxide.8 The acid properties of mesoporous Ta-W oxides
were evaluated by NH3 temperature-programmed desorp-
tion (NH3-TPD), coloration using Hammett indicators, and
Fourier transform infrared (FT-IR) spectroscopy. The acid
catalytic activities of mesoporous Ta-W oxides with differ-
ent Ta-W concentrations were examined during liquid-
phase Friedel-Crafts alkylation of anisole and hydrolysis
of disaccharides, and the results are compared with those of
conventional solid acids.
Figure 1. (A) Low and (B) wide-angle powder XRD patterns for meso-
porous (a) Ta, (b) Ta9W1, (c) Ta8W2, (d) Ta7W3, (e) Ta6W4, (f) Ta5W5, (g)
Ta4W6, (h) Ta3W7, and (i) Ta2W8 oxides and nonmesoporous (j) Ta1W9
and (k) W oxides.
2. Experimental Section
2.1. Preparation of Mesoporous Metal Oxides. Mesoporous
TaxW10-x and Ta oxides were prepared from tantalum penta-
chloride (TaCl5) (99.99%, Kojundo Chemical Laboratory Co.,
Ltd.) and tungsten hexachloride (WCl6) (99.99%, Kojundo
Chemical Laboratory Co., Ltd.). Poly block copolymer surfac-
tant Pluronic P-123 {[HO(CH2CH2O)20-[CH2CH(CH3)O]70-
(CH2CH2O)20H (Aldrich)} was used as a structure-directing
agent (SDA). We synthesized mesoporous TaxW10-x oxide by
dissolving 1 g of P-123 in 10 g of dehydrated 1-propanol and
adding TaCl5 and WCl6 (total of 6 mmol of metal chloride) for
mesoporous TaxW10-x and 7 mmol of TaCl5 for mesoporous Ta
oxide with vigorous stirring. Water (30 mmol) was added to the
solution with further stirring. The resulting sol was gelled in a
Petri dish at 313 K for 10-14 days. The aged gel samples were
then treated at 773 K for 5 h in static air to remove the SDA.
2.2. Characterization. Samples were characterized by N2
desorption (BEL Japan, BELSORP-miniII), X-ray diffraction
(XRD) (RINT-UltimaIII, Rigaku), and scanning electron
microscopy (SEM) (S-4700, Hitachi). The Brunauer-Emmett-
Teller (BET) surface area was estimated over a relative pressure
(P/P0) range of 0.05-0.30. The pore size distribution was
obtained from analysis of the adsorption branches of the
isotherms using the Barrett-Joyner-Halenda (BJH) method
and the small-angle powder X-ray diffraction pattern.
100 mmol of anisole, and 10 mmol of benzyl alcohol in an oil
bath at 373 K for 1 h. The products (o-benzylanisole, p-
benzylanisole, and dibenzyl ether) were analyzed by flame
ionization gas chromatography (GC-2014, Shimadzu), using a
capillary column (J&W Scientific, DB-FFAP, length of 30 m,
inside diameter of 0.25 mm, and film of 0.25 μm).
For hydrolysis, the reaction was performed using 0.1 g of
catalyst, 0.5 g of sucrose (1.46 mmol) or cellobiose, and water
(10 or 5 mL, respectively) in an oil bath at 353 or 373 K, respectively,
for 3 h. The rates of glucose production (millimoles per gram per
hour) were obtained from the yields (percent) of glucose obtained
after 1 h reactions. The products (glucose and fructose) were
analyzed by high-performance liquid chromatography (HPLC)
(LC-2000 plus, Jasco) using a Shodex Asahipak NH2P-50 column.
A Bio-Rad Aminex HPX-87H column was also used for analysis of
5-hydroxymethylfurfural, levulinic acid, and formic acid.
The activities of niobic acid (Nb2O5 nH2O), Ta2O5-WO3,
3
ion-exchange resins (Amberlyst-15 and Nafion NR50), and
H-type zeolites [H-Beta, SiO2/Al2O3=25; JRC-Z-HB25 and
H-ZSM5, SiO2/Al2O3 = 90; JRC-Z-5-90H supplied by the
Catalysis Society of Japan (Japan Reference Catalyst)] were
used for comparison. Ta2O5-WO3 was prepared as follows.
The acid properties of the samples were determined by NH3-
TPD (acid strength and concentration of sites) using a TPD-1-
AT instrument (BEL Japan) equipped with a quadrupole mass
spectrometer, Hammett indicators (acid strength), and FT-IR
(acid type, Brønsted and Lewis acid) spectroscopy (Jasco, FT/
IR-6200). For TPD measurement, a 20 mg sample was heated at
423 K for 1 h under a helium flow, exposed to NH3 at 373 K for
adsorption, and finally heated at a rate of 2 K/min. For
coloration using Hammett indicators, a 0.1 g sample was heated
at 423 K for 1 h under a helium flow and then introduced in a
3 mL dehydrated benzene solution under stirring. Finally,
0.5 mL of a benzene solution with Hammett indicators (1%),
including dicinnnamalacetone, chalcone, anthraquinone, 4-ni-
trotoluene, and 2,4-dinitrotoluene, was added to observe the
coloration. For IR spectroscopic measurements, the samples
were evacuated in a conventional gas circulation system at 423 K
for 1 h, and then pyridine was introduced into the system to be
adsorbed on the acid sites for the study of the acid properties.
2.3. Acid-Catalyzed Reactions. For Friedel-Crafts alkyl-
ation, the reaction was performed using 0.2 g of catalyst,
Ta2O5 and (NH4)10W12O41 5H2O were dissolved in an oxalic
3
acid solution and distilled water, respectively, at 353 K. The
solutions were then mixed and stirred vigorously at 353 K. After
drying, the obtained materials were calcined at 773 K for 3 h to
yield Ta-W mixed oxides. The TOF (inverse hours) was
calculated from the rate of production (millimoles per gram
per hour) and acid site concentration (millimoles per gram)
obtained by NH3-TPD.
3. Results and Discussion
3.1. Structure of Mesoporous TaxW10-x Metal Oxides.
The presence of a mesoporous structure was evaluated
from XRD patterns (Figure 1), SEM images (Figure 2),
and N2 sorption isotherms (Figure 3). The small-angle
XRD patterns (Figure 1) contained peaks attributable to
mesopores for TaxW10-x oxides with x values from 3 to
10. The small-angle XRD peaks for W-enriched samples
(x = 3-7) were broader than those of Ta-concentrated
samples (x = 8-10), indicating the formation of more
randomly sized mesopores. Wide-angle powder XRD pat-
terns revealed the presence of an amorphous structure in
(8) Tagusagawa, C.; Takagaki, A.; Hayashi, S.; Domen, K. J. Phys.
Chem. C 2009, 113, 7831.

3074 Chem. Mater., Vol. 22, No. 10, 2010
Tagusagawa et al.
Figure 2. SEM images of mesoporous (a) Ta, (b) Ta7W3, (c) Ta5W5, and (d) Ta3W7 oxides and (e) nonmesoporous W oxides.
Figure 3. N2 sorption isotherms of (a) Ta, (b) Ta9W1, (c) Ta8W2, (d) Ta7W3, (e) Ta6W4, (f) Ta5W5, (g) Ta4W6, (h) Ta3W7, and (i) Ta2W8 oxides and
nonmesoporous (j) Ta1W9 and (k) W oxides.
samples for which x = 3-10, and crystallized tungsten
oxide (WO3) in W-rich samples (x = 0-2). SEM images of
the prepared porous oxides are shown in Figure 2. Meso-
porous Ta, Ta7W3,Ta5W5,andTa3W7 oxides had wormhole-
type mesopores, but no mesoporous structure was observed
in the W oxides, supporting the XRD results. N2 sorption
isotherms (Figure 3) indicated the presence of mesoporous
structures in samples for which x = 3-10. These exhibited a
typical type IV pattern with an H1-type hysteresis loop at
high relative pressures for mesopores with high Ta (x =
8-10) concentrations. In W-enriched samples (x = 3-7),
hysteresis loops similar to the H2 type were observed,
confirming the XRD and SEM results.
Characterization of the mesopores was conducted by
both Brunauer-Emmett-Teller (BET) (surface area
estimation) and Barrett-Joyner-Halenda (BJH) (pore
size distributions) methods. Figure 4 shows the variation
in surface area and pore volume versus W concentration.
The BET surface area decreased gradually from 196
(mesoporous Ta oxide) to 52 m2/g (nonmesoporous W
oxide) with increasing W content, up to x = 0, due to the
formation of crystallized W oxide in the W-rich samples.
Aggregation and crystallization resulted in the destruc-
tion of the original mesoporous structure and the devel-
opment of larger pores (between 4.8 and 21.5 nm) for
W-rich TaxW10-x oxides (x = 0-2) due to interparticle
voids. The addition of the transition metal Ta to the W
oxide should have improved the thermal stability of the
material in the amorphous phase by elevating the crystal-
lization temperature beyond that required for complete
removal of the mesoporous template.9 The pore volume
obtained by BJH decreased up to x = 3, and then the pore
volumes increased in the nonmesoporous W-rich oxides
(x=0-2) due to the formation of void spaces between
particles, which can be observed in the SEM image of
nonporous W oxide (Figure 3). The BJH pore size
distributions of samples with different concentrations
are presented in Figure 5, and the corresponding struc-
tural properties obtained from N2 sorption isotherms and
XRD patterns are summarized in Table 1. The BJH pore
size distributions indicate the formation of mesopores in
(9) Kondo, J. N.; Yamashita, T.; Nakajima, K.; Lu, D.; Hara, M.;
Domen, K. J. Mater. Chem. 2005, 15, 2035.

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Chem. Mater., Vol. 22, No. 10, 2010 3075
Figure 4. BET surface areas (square meters per gram) and mesopore
volumes (microliters per gram) of mesoporous TaxW10-x oxides.
Figure 6. NH3-TPD (m/e 16, 2 K/min) curves for mesoporous (a) Ta,
(b) Ta7W3, (c) Ta5W5, and (d) Ta3W7 oxides.
and Lewis acid sites together, obtained from NH3-TPD
were 0.20 (mesoporous Ta3W7 oxide), 0.21 (mesoporous
Ta5W5 oxide), 0.23 (mesoporous Ta7W3 oxide), and
0.12 mmol/g (mesoporous Ta oxide). The acid strength
was also evaluated by Hammett indicators. The colora-
tion obtained with each indicator reagent is summarized
in Table 2. For mesoporous Ta oxide, a yellow color
developed in chalcone (pKa =-5.6) could be observed,
indicating that the acidity of the sample corresponds to
g71% sulfuric acid. The addition of W to the mesoporous
Ta oxide increased the pKa value, yielding acidic colora-
tion of anthraquinone for mesoporous Ta7W3 oxide
(pKa = -8.2) and of 4-nitrotoluene for mesoporous
Ta5W5 oxide. The coloration could not be clearly ob-
served for mesoporous Ta3W7 oxide because of the dark
green coloration of the catalyst. The ranking of catalyst
acidity increased with addition of W, reaffirming the acid
strength order obtained from NH3-TPD.
We evaluated the acid properties of mesoporous
TaxW10-x oxides (x = 3, 5, 7, and 10) also by probing the
vibrational spectra of adsorbed pyridine using IR spectro-
scopic measurements, as shown in Figure 7. For compar-
ison, nonporous Ta2O5-WO3 (1:1 Ta:W ratio) was also
included. Both mesoporous TaxW10-x oxide and nonpor-
ous Ta5W5 oxide were confirmed to possess the Lewis
(LAS) acid site at 1435 cm-1 and the Brønsted (BAS) acid
site at 1532 (BAS) cm-1. The W-containing samples had
both Brønsted acid sites and Lewis acid sites, whereas the Ta
oxide sample had a negligible number of Brønsted acid sites.
The IR spectroscopic measurements indicated that the peak
intensity of the bond at 1532 cm-1, attributed to pyridi-
nium ions formed on Brønsted acid sites,10,11 was enhanced
with an increase in W content. The IR spectroscopic
Figure 5. Pore size distributions of mesoporous (a) Ta, (b) Ta7W3, (c)
Ta5W5, and (d) Ta3W7 oxides and (e) nonmesoporous Ta1W9 oxide.
Table 1. Structural Properties of Mesoporous TaxW10-x Oxides
mesoporous
TaxW10-x oxides
SBET
(m2/g)
pore size
(nm)
pore volume
(mL/g)
D(100)
(nm)
Ta0W10
Ta1W9
Ta2W8
Ta3W7
Ta4W6
Ta5W5
Ta6W4
Ta7W3
Ta8W2
Ta9W1
Ta10W0
52
68
89
21.5
8
0.20
0.16
0.14
0.12
0.12
0.12
0.13
0.15
0.19
0.19
0.20
-
-
4.8
4.2
3.7
3.7
3.7
3.7
3.7
3.7
3.7
-
110
120
128
142
152
168
179
196
7.2
7.4
7.1
6.5
6.4
6.3
6.0
5.7
TaxW10-x oxides with x values from 3 to 10 and exhibited
decreased levels of mesopore formation in W-enriched
samples. The BJH pore size distribution peaks also
showed a small increase in the W-enriched samples.
3.2. Characterization of Acid Sites. The acid properties
of mesoporous TaxW10-x oxides (x = 3, 5, 7, and 10) were
evaluated by NH3 temperature-programmed desorption
(NH3-TPD) and Fourier transform infrared (FT-IR)
spectroscopic measurements of adsorbed pyridine. The
NH3-TPD profiles for mesoporous TaxW10-x (x =
3-10) exhibited a broad main peak at 495-520 K and a
shoulder peak above 560 K (Figure 6). The order of acid
strength of the mesoporous TaxW10-x oxides (x = 3-10)
increased with an increasing W oxide concentration. The
shoulder peak was observed at 520 K for mesoporous Ta
oxide, at 560 K for mesoporous Ta7W3 oxide, at 565 K for
mesoporous Ta5W5 oxide, and at 575 K for mesoporous
Ta3W7 oxide. The acid concentrations, both Brønsted
(10) (a) Parry, E. P. J. Catal. 1963, 2, 371. (b) Hughes, T. R.; White, H. M.
J. Phys. Chem. 1967, 71, 2192.
(11) (a) Hino, M.; Kurashige, M.; Arata, K. Catal. Commun. 2004, 5,
107. (b) Hino, M.; Kurashige, M.; Matsuhashi, H.; Arata, K. Appl.
Catal., A 2006, 310, 190. (c) Yamashita, K.; Hirano, M.; Okumura, K.;
Niwa, M. Catal. Today 2006, 118, 385.

3076 Chem. Mater., Vol. 22, No. 10, 2010
Tagusagawa et al.
Table 2. Coloration by Color Indicator Reagents for Mesoporous
TaxW10-x Oxides
coloration
indicator reagent
pKa
Ta
Ta7W3
Ta5W5
dicinnamalacetone
chalcone
anthraquinone
4-nitrotoluene
2,4-dinitrotoluene
-3.0
-5.6
þ
þ
-
-
-
þ
þ
þ
-
-
þ
þ
þ
þ
-
-8.2
-11.4
-12.8
measurement peak observed in nonporous Ta2O5-WO3
oxide was very similar to that of mesoporous Ta5W5 oxide,
indicating that the acid sites formed in both mesoporous
and nonporous Ta5W5 are similar, with differences only in
the formation of mesoporous structure. The increase in the
number of Brønsted acid sites corresponded to the increas-
ing number of desorption peaks observed in W-enriched
mesoporous TaxW10-x oxides by NH3-TPD.
Figure 7. FT-IR spectra for pyridine-adsorbed mesoporous (a) Ta, (b)
Ta7W3, (c) Ta5W5, and (d) Ta3W7 oxides and (e) nonmesoporous Ta5W5
oxide (B for the Brønsted acid site; L for the Lewis acid site). Assignments:
1532 cm-1 (Brønsted acid site) and 1435 cm-1 (Lewis acid site).
3.3. Acid-Catalyzed Reactions. The acid-catalyzed reac-
tions were first tested on mesoporous TaxW10-x oxides
using liquid-phase Friedel-Crafts alkylation of anisole
with benzyl alcohol, and hydrolysis of sucrose (a disacc-
haride composed of glucose and fructose) in water. A plot
of the product yield for different Ta and W contents in the
Friedel-Crafts alkylation of anisole and the hydrolysis of
sucrose, reacted for 1 h at 373 and 353 K, respectively, is
shown in Figure 8. Variation of the Ta and W contents
resulted in remarkably different reaction rates of benzy-
lanisole formation in alkylation and hydrolysis. The
reaction rates increased gradually with an increase in
W content, starting from 0% yield for mesoporous Ta
oxide and reaching the highest alkylation yield (59% of
benzylanisole) for mesoporous Ta3W7 oxide. The yield
decreased drastically for nonmesoporous oxide samples
with x values from 2 to 0, reaching 0% for W oxide. A
similar curve was plotted for the hydrolysis of sucrose,
where the highest yield (24%) was obtained for mesopor-
ous Ta3W7 oxide. These results demonstrate the impor-
tance of the mesoporous structure to the reaction and
indicate drastic changes in the nature of the acid sites. The
reaction yield increased for W-enriched mesoporous
TaxW10-x oxide samples, consistent with the acid
strength determined by NH3-TPD, with Hammett indi-
cators, and with the increase in the number of Brønsted
acid sites, which promotes the reaction,12 observed by IR
spectroscopic measurements.
Figure 8. (a) Friedel-Crafts alkylation of anisole and (b) hydrolysis of
sucrose over mesoporous TaxW10-x oxides. Reaction conditions: anisole
(100 mmol), benzyl alcohol (10 mmol), and catalyst (0.2 g) at 373 K for 1 h
or sucrose (0.5g, 1.46 mmol), H2O (10 mL, 556 mmol), and catalyst (0.1 g)
at 353 K for 1 h.
Mesoporous Ta5W5 oxide also exhibited a turnover
frequency (53 h-1) higher than those of bulk Ta2O5-
WO3 oxide (47 h-1) and HTaWO6 nanosheets (28 h-1),
indicating that the mesoporous structure enhanced the
reaction rate of the accessible acid sites. After the reac-
tion, o-benzylanisole, p-benzylanisole, and dibenzyl
ether were formed. The selectivity of o-benzylanisole via
p-benzylanisole observed for mesoporous NbxW10-x oxi-
des for Friedel-Crafts alkylation gradually increased
with an increase in W content (36.9, 39.8, and 42.2%
for mesoporous Ta7W3, Ta5W5, and Ta3W7 oxides, res-
pectively). The same selectivity of o-benzylanisole was
observed for bulk Ta2O5-WO3 (Ta5W5) oxide (39.3%).
The variation of selectivity was probably not caused by
the mesoporous structure, but by variations of the acid
properties (Brønsted acid and Lewis acid sites) due to
changing Ta and W concentrations.7 The selectivity for
dibenzyl ether, a byproduct of benzyl alcohol, was 20, 17,
and 15% for mesoporous Ta7W3, Ta5W5, and Ta3W7
oxides, respectively. These results are consistent with
the IR spectroscopic measurements, which increase as
the Brønsted acid site at 1532 cm-1 promotes a decrease in
the selectivity of dibenzyl ether, a byproduct formed by
The acid catalytic activities of mesoporous Ta-W
oxides for Friedel-Crafts alkylation were also compared
to those of conventional solid acids. The acid concentra-
tion and surface area of the tested solid acids and the
results of the Friedel-Crafts alkylation of anisole are
summarized in Table 3. Nonmesoporous Ta2O5-WO3
and HTaWO6 nanosheet aggregates, obtained by exfolia-
tion of layered HTaWO6,8 were also used for comparison.
Mesoporous Ta3W7 oxide also exhibited the best perfor-
mance in this Friedel-Crafts alkylation reaction, produ-
cing the highest yield and turnover frequency (147 h-1).
(12) Shishido, T.; Kitano, T.; Teramura, K.; Tanaka, T. Catal. Lett.
2009, 129, 383.

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Chem. Mater., Vol. 22, No. 10, 2010 3077
Table 3. Friedel-Crafts Alkylation of Anisole over Several Solid Acid Catalysts
alkylation of anisolea
TOF (h-1 b selectivity of o-benzylanisole (%)c
catalyst
SBET (m2/g)
acid site concentration (mmol/g)
yield (%)
)
mesoporous Ta3W7
mesoporous Ta5W5
mesoporous Ta7W3
mesoporous Ta
bulk Ta5W5
HTaWO6 nanosheets
H-Beta d
Amberlyst-15
110
128
152
196
38
47
420
50
0.20e
0.21e
0.23e
0.12e
0.11e
0.48e
1.0 f
58.6
22.1
9.3
168
70
21
not detected
42.2
39.8
36.9
not detected
10.4
27.0
not detected
39.3
33.1
62
29
19
9
30.6
42.1
42.3
42.1
45.0
49.2
4.8f
Nafion NR50
0.02
0.9 f
27
a Reaction conditions: anisole (100 mmol), benzyl alcohol (10 mmol), and catalyst (0.2 g) at 373 K for 1 h. b Calculated from the rate of produced
benzylanisole (30 min) and acid site concentration. c Selectivity (%) = (o-benzylanisole)/(o-benzylanisole þ p-benzylanisole) ꢀ 100. d SiO2/Al2O3 = 25,
JRC-Z-HB25. e Determined by NH3-TPD. f From ref 17.
Table 4. Hydrolysis of Cellobiose over Several Ssolid Acid Catalysts
hydrolysis of sucrosea
hydrolysis of cellobioseb
acid site concentration
(mmol/g)
rate of glucose production
(mmol g-1 h-1
rate of glucose
production (mmol g-1 h-1
f
f
catalyst
)
TOF (h-1
)
)
TOF (h-1
)
mesoporous Ta3W7
mesoporous Ta5W5
mesoporous Ta7W3
bulk Ta5W5
0.20d
0.21d
0.23d
0.11d
0.48d
0.4e
7.0
5.6
2.7
1.2
5.2
0.3
0.1
6.1
1.2
2.0
45.6
35.0
26.7
11.7
10.9
10.8
0.6
0.7
1.4
8.5
2.4
0.15
0.11
0.04
0.01
0.09
0
0.75
0.52
0.17
0.09
0.19
0
HTaWO6 nanosheets
Nb2O5 nH2O
3
H-ZSM5c
0.2e
0
0
Amberlyst-15
Nafion SAC 13
Nafion NR50
H2SO4
4.8e
0.51
0.03
0.44
6.73
0.11
0.34
0.48
0.33
0.1e
0.9e
20.4
2.2
a Reaction conditions: sucrose (0.5 g, 1.46 mmol), H2O (10 mL, 556 mmol), and catalyst (0.1 g) at 353 K. b Reaction conditions: cellobiose (0.5 g, 1.46
mmol), water (5 mL, 278 mmol), and catalyst (0.1 g) at 373 K. c SiO2/Al2O3 = 90, JRC-Z-5-90H. d Determined by NH3-TPD. e From ref 17. f Calculated
from the rate of glucose production and acid site concentration.
the Lewis acids. However, dibenzyl ether is also a good
alkylating agent, and its concentration decreased as it was
consumed, along with the benzyl alcohol, at the end of the
alkylation reaction.
acid with water-tolerant acid sites.15,16 The activity of
mesoporous Ta3W7 oxide, however, substantially exc-
eeded the performance of all other materials tested,
achieving a glucose formation rate of 7.0 mmol g-1 h-1
for sucrose hydrolysis. This reaction rate was significantly
higher than that of niobic acid or H-ZSM5, and 3 times
that of Nafion NR50. Moreover, the turnover frequency
of mesoporous Ta3W7 oxide was more than 15 times that
of Nafion NR50 and Amberlyst-15. Mesoporous Ta5W5
oxide also exhibited a reaction rate and turnover fre-
quency more than 3 times greater than that of bulk
Ta2O5-WO3 oxide; this considerable difference between
mesopore and bulk Ta5W5 cannot be explained in the
view of acid properties, once they were very similar at IR
spectroscopic measurements. This increase in the TOF
indicates that the mesoporous structure enhanced the
reaction rate at the accessible acid sites of mesoporous
Ta5W5 oxide. The high surface area should have helped in
the usage of acid sites by optimization of the contacts
between acid sites and reactants. The low acid site density
helps in the effective usage of all acid sites and would
Mesoporous TaxW10-x oxides were also tested for
hydrolysis of disaccharides (sucrose and cellobiose).
The hydrolysis of saccharides requires sufficient acid
strength and water-tolerant acid sites and is an important
class of reaction, used to convert biomass into sugars,
important raw materials for obtaining bioethanol and
other useful chemicals with minimal environmental
impact.13,14 The rate of glucose production and turnover
frequency (TOF) for hydrolysis of sucrose and cellobiose
over several solid acid catalysts are listed in Table 4.
Ion-exchange resins like Amberlyst-1513e are powerful
catalysts for the hydrolysis of saccharides because of the
strong sulfonic acid sites, and niobic acid is a unique solid
(13) (a) Sasaki, M.; Fang, Z.; Fukushima, Y.; Adshiri, T.; Arai, K. Ind.
Eng. Chem. Res. 2000, 39, 2883. (b) Takagaki, A.; Tagusagawa, C.;
Domen, K. Chem. Commun. 2008, 5363. (c) Suganuma, S.; Nakajima,
K.; Kitano, M.; Yamaguchi, D.; Kato, H.; Hayashi, S.; Hara, M. J. Am.
Chem. Soc. 2008, 130, 12787. (d) Onda, A.; Ochi, T.; Yanagisawa, K.
Green Chem. 2008, 10, 1033. (e) Rinaldi, R.; Palkovits, R.; Sch€uth, F.
Angew. Chem., Int. Ed. 2008, 47, 8047.
(14) (a) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Angew. Chem., Int.
Ed. 2007, 46, 7164. (b) Corma, A.; Iborra, S.; Velty, A. Chem. Rev.
2007, 107, 2411.
(16) (a) Hanaoka, T.; Takeuchi, K.; Matsuzaki, T.; Sugi, Y. Catal.
Today 1990, 8, 123. (b) Hanaoka, T.; Takeuchi, K.; Matsuzaki, T.
Catal. Lett. 1990, 5, 13.
(17) (a) Tanabe, K. Mater. Chem. Phys. 1987, 17, 217. (b) Tanabe, K.;
Okazaki, S. Appl. Catal. A: Gen. 1995, 133, 191. (c) Harmer, M. A.;
Sun, Q. Appl. Catal., A 2001, 221, 45. (d) Matsuhashi, H.; Arata, K.
Phys. Chem. Chem. Phys. 2004, 6, 2529.
(15) (a) Iizuka, T.; Ogasawara, K.; Tanabe, K. Bull. Chem. Soc. Jpn.
1983, 2927. (b) Tanabe, K.; Okazaki, S. Appl. Catal., A 1995, 133, 191.
(c) Nowak, I.; Ziolek, M. Chem. Rev. 1999, 99, 3603.

3078 Chem. Mater., Vol. 22, No. 10, 2010
Tagusagawa et al.
prevent the formation of polymerization of reactants.
Cellobiose is the subunit of cellulose composed of β-1,4-
glycosidic bonds, which are much more stable than the
R-1,2-glycosidic bonds comprising sucrose and are thus
more resistant to hydrolysis.13 Accordingly, the rate of
production of glucose from the hydrolysis of cellobiose
was much lower than that of sucrose over all of the acid
catalysts tested, because of the β-1,4-glycosidic bonds.
Nevertheless, mesoporous Ta3W7 oxide exhibited the high-
est TOF (0.75 h-1) of glucose production among the solid
acids tested, including ion-exchange resins, niobic acid, and
zeolites, and achieved twice the turnover frequency of
sulfuric acid. The total production of glucose for mesopor-
ous Ta3W7 oxide for the 3 h reaction was estimated to be 0.4
mmol, corresponding to a turnover number of 2.0. We have
confirmed that no product was observed by decomposition
ofglucoseor fructose, including 5-(hydroxymethyl)furfural
(HMF), levulinic acid, and formic acid for all samples after
the 3 h hydrolysis reaction of sucrose and cellobiose, which
is determined by using a Bio-Rad Aminex HPX-87H
column for HPLC.
The very high level of catalytic performance of meso-
porous Ta3W7 oxide was due to a high-surface area
mesoporous structure and the formation of strong
Brønsted acid sites by the isomorphous replacement of
Ta5þ by higher-valence W6þ cations in these W-enriched
samples like that observed at WO3/ZrO2.18 It is reported
that replacement of ZrO2 with WO3 forms acids sites
similar to that of SO4/ZrO218 with strong acid sites. The
highest activity of WO3/ZrO2 has been obtained over a
range of surface tungsten densities, which maximizes the
quantity of amorphous surface polytungstate species
relative to isolated surface WOx and crystallized WO3.19
Similar to WO3/ZrO2, formation of strong Brønsted acid
sites could be observed in IR spectroscopic measurement
results of mesoporous TaxW10-x oxides. The Brønsted
acid sites could have formed which led to the high surface
tungsten densities in the tantalum matrix with no crystal-
lized WO3. Higher concentrations of W oxide deformed
the mesoporous structure, forming the crystallized WO3
and decreasing the reaction rate.
4. Conclusion
Wormhole-type mesoporous TaxW10-x oxides were
found to function as highly active mixed metal oxide solid
acid catalysts for Friedel-Crafts alkylation and hydrolysis,
exceeding the reaction rate of nonporous Ta2O5-WO3
oxide and HTaWO6 nanosheets. The formation of crystal-
lized WO3 was observed in W-rich samples (x = 0-2),
which formed a nonmesoporous structure with a low
reaction activity. The reaction rate and acid strength
increased gradually with the addition of W, reaching the
highest reaction rate with mesoporous Ta3W7 oxide, which
exceeded the TOF of ion-exchange resins, zeolites, and
nonmesoporous metal oxides. The very high level of cata-
lytic performance of mesoporous Ta3W7 oxide was due to a
high-surface area mesoporous structure and the formation
of strong Brønsted acid sites.
Acknowledgment. This work was supported by the Devel-
opment in a New Interdisciplinary Field Based on Nano-
technology and Materials Science program of the Ministry of
Education, Culture, Sports, Science and Technology
(MEXT) of Japan and the Global Center of Excellence
Program for Chemistry.
(18) Arata, K. Catal. Today 2003, 81, 17.
(19) Ross-Medgaarden, E. I.; Knowles, W. V.; Kim, T.; Wong, M. S.;
Zhou, W.; Kiely, C. J.; Wachs, I. E. J. Catal. 2008, 256, 108.