Highly active mesoporous Nb-W oxide solid-acid catalyst

Article abstract of DOI:10.1002/anie.200904791

Pore-ing acid: Mesoporous Nb-W oxides (Nb:W = 3:7) are found to be a recyclable, highly active solid acid. They surpass ionexchange resins (Naflon NR50 and Amberlyst-15) and zeolites (H-ZSM5 and H-Beta) in Friedel-Crafts alkylation and hydrolysis reactions (see picture). The high activity is due to strong acid sites and a mesoporous structure with a high surface area and easy reactant accessibility. (Figure Presented)

Full text of DOI:10.1002/anie.200904791

Communications
DOI: 10.1002/anie.200904791
Mesoporous Solid Acid
Highly Active Mesoporous Nb–W Oxide Solid-Acid Catalyst**
Caio Tagusagawa, Atsushi Takagaki, Ai Iguchi, Kazuhiro Takanabe, Junko N. Kondo,
Kohki Ebitani, Shigenobu Hayashi, Takashi Tatsumi, and Kazunari Domen*
The synthesis of mesoporous transition-metal oxides has been
extensively studied because of their wide range of potential
from the mesoporous structure and different acid properties
formed by specific Nb and W concentrations.
[
1]
applications. Examples of such compounds include meso-
Mesoporous Nb–W mixed oxides were prepared from
NbCl and WCl in the presence of a poly block copolymer
[
2,3]
[2,4]
[2,3b,5]
[2,6]
porous
TiO ,
ZrO2,
Nb O ,
Ta O ,
2 5
2
2
5
5
6
[
2,7]
[2,8]
[2]
(
Nb,Ta) O ,
SnO2,
and WO3, which are used as a
surfactant Pluronic P-123 as a structure-directing agent.
(Additional details are provided in the Supporting Informa-
tion) Peaks attributable to mesopores were observed from
2
5
variety of heterogeneous catalysts, such as solid-acid cata-
[
4b,e,f,h,5d,6f,g]
[3b,f–h,6b,h]
[5c]
lysts,
photocatalysts,
oxidation catalysts,
[
4d,g]
and catalyst supports.
Solid-acid catalysts, which are
Nb W
oxides with x values from 2 to 10 in the small-angle
x
(10Àx)
reusable and readily separable from reaction products, have
been widely investigated as direct replacements for liquid
acids to reduce the impact on the environment and to
decrease costs. The use of mesoporous transition-metal oxides
is an interesting approach to developing a solid-acid catalyst
with enhanced activity. The mesopores in the oxide allows the
reactants access additional active acid sites in the pores,
resulting in improved rates of acid catalysis. Sulfated meso-
porous niobium and tantalum oxides have been reported to
exhibit remarkable activity in acid-catalyzed Friedel–Crafts
powder X-ray diffraction (XRD) pattern (see Figure S1 in the
Supporting Information). Peaks attributed to (110) and (200)
of the two-dimensional hexagonal structure were observed
from an x = 10 sample (mesoporous Nb oxide), which was
consistent with previous studies. Wide-angle powder XRD
patterns revealed the presence of crystallized tungsten oxide
[
5]
(WO ) in W-rich samples (x = 0 to 2). The presence of
3
mesopores was also indicated by the N sorption isotherms
2
(Figure 1) for the same samples (x = 2 to 10). The surface
areas were estimated using the Brunauer–Emmett–Teller
(BET) method, and pore volumes were obtained by the
Barrett–Joyner–Halenda (BJH) method. Although the sur-
face area decreased gradually from 200 (mesoporous Nb
[
5d,6f,g]
alkylation and isomerization.
However, the use of the
recycled catalyst remains difficult, a result of the leaching of
sulfate species, as reported for mesoporous silica and organo-
silicas bearing sulfonic acid groups. Herein, mesoporous Nb–
W mixed oxides are examined as solid-acid catalysts, these
give very high catalytic performance in Friedel–Crafts
alkylation, hydrolysis, and esterification, which originates
2
À1
oxide) to 52 m g (non-mesoporous Woxide) with increasing
addition of W, up to x = 0, the pore volume decreased up to
x = 3. Then, the pore volumes increased in the non-mesopo-
rous W-rich oxides (x = 0 to 2) due to the formation of void
spaces between particles (Supporting Information, Fig-
ure S2). The pore diameter obtained by the BJH method
decreased from 7 (mesoporous Nb oxide) to 4.2 nm (meso-
[
*] C. Tagusagawa, Dr. K. Takanabe, Prof. K. Domen
Department of Chemical System Engineering, School of Engineer-
ing, The University of Tokyo
porous Nb W7 oxide) with increasing W content, and
3
mesopores were not observed in the Nb W oxide (Supporting
1
9
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Information, Figure S3). SEM and TEM images of the porous
Fax: (+81)3-5841-8838
E-mail: domen@chemsys.t.u-tokyo.ac.jp
Homepage: http://www.domen.t.u-tokyo.ac.jp
Dr. A. Takagaki, Prof. K. Ebitani
School of Materials Science, Japan Advanced Institute of Science
and Technology (JAIST)
1-1 Asahidai, Nomi, Ishikawa 923-1292 (Japan)
A. Iguchi, Prof. J. N. Kondo, Prof. T. Tatsumi
Chemical Resources Laboratory, Tokyo Institute of Technology
4259 Nagatsuta Midori-ku, Yokohama 226-8503 (Japan)
Dr. S. Hayashi
Research Institute of Instrumentation Frontier, National Institute of
Advanced Industrial Science and Technology (AIST)
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan)
[
**] This work was supported by the Development in a New Interdisci-
plinary Field Based on Nanotechnology 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.
Figure 1. N sorption isotherms of a) Nb, b) Nb W , c) Nb W ,
2
9
1
8
2
d) Nb W , e) Nb W , f) Nb W , g) Nb W , h) Nb W , i) Nb W oxides
7
3
6
4
5
5
4
6
3
7
2
8
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904791.
and non-mesoporous j) Nb W and k) W oxides. Traces are vertically
1 9
shifted for clarity.
1
128
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oxides are shown in Figure 2. The mesoporous Nb oxide had
hexagonally structured mesopores, as observed in the XRD
pattern. The mesoporous Nb W , Nb W , and Nb W oxides
7
3
5
5
3
7
Figure 3. a) Friedel–Crafts alkylation of anisole (~) and b) hydrolysis of
sucrose (*) with mesoporous Nb W
oxides. Reaction conditions:
x
(10Àx)
(a) anisole (100 mmol), benzyl alcohol (10 mmol), catalyst (0.2 g),
3
73 K, 1 h, and (b) sucrose (0.5 g, 1.46 mmol), H O (10 mL,
2
556 mmol), catalyst (0.1 g), 353 K, 1 h.
gradually with increasing W content, starting from a 0% yield
for mesoporous Nb oxide and reaching the highest yield
(
94%) for mesoporous Nb W oxide. The yield decreased
3 7
Figure 2. SEM images of mesoporous a) Nb, b) Nb W , c) Nb W , and
7
3
5
5
drastically for non-mesoporous oxide samples with x from 2 to
, reaching 0% for W oxide. The same pattern was found for
the hydrolysis of sucrose. The highest yield (65%) was
obtained for mesoporous Nb W oxide. These results indicate
d) Nb W oxides (scale bar: 50 nm). SEM images of non-mesoporous
3
7
0
e) Nb W and f) W oxides (scale bar: 50 nm). TEM images of
1
9
mesoporous g) Nb, h) Nb W , i) Nb W , and j) Nb W oxides (scale
7
3
5
5
3
7
bar: 10 nm).
3
7
the importance of the mesoporous structure to the reaction,
and demonstrate the drastic changes in the nature of the acid
sites.
had wormhole-type mesopores, and no mesoporous structure
was observed in Nb W or W oxides. Energy dispersive X-ray
The acid properties of mesoporous Nb W oxides were
1
9
x
(10Àx)
(
EDX) spectroscopy analysis of Nb and W were carried out to
evaluated by probing the vibrational frequencies of adsorbed
pyridine using Fourier transform infrared (FT-IR) spectros-
copy. FT-IR spectra for pyridine adsorbed by mesoporous
correlate the initial stoichiometry and the resulting compo-
sition of the products (Supporting Information, Table S1).
The average elemental compositions were very close to the
initial stoichiometry, within 1% of differences for almost all
the samples. However, considerable standard deviation could
be observed for non-mesoporous W rich Nb W (4%) and
Nb W
oxides are shown in Figure 4. The tungsten-
x
(10Àx)
containing samples have both of Brønsted acid sites and
Lewis acid sites whereas Nb oxide sample has negligible
Brønsted acid sites. The FT-IR spectra indicate that the peak
2
8
À1
Nb W (5%) oxides. The lack of uniformity was observed for
intensity at 1532 cm , attributed to pyridinium ions formed
1
9
[
9,10]
samples with excess of W led to the formation of non-uniform
structure by the calcination of the material at 673 K to remove
the template, which induced the aggregation and crystalliza-
on strong Brønsted acid sites,
was enhanced by increasing
the W content. The Brønsted acid sites peak intensities have
doubled from mesoporous Nb W (2.5%) to Nb W (5.0%)
7
3
5
5
tion of pure WO . The aggregation and crystallization
3
resulted in the destruction of the original mesoporous
structure and the development of larger pores (between 5.4
and 21.5 nm) for W rich Nb W
oxides (x = 0 to 2) as
x
(10Àx)
interparticle voids. The addition of the transition metal Nb to
the Woxide should have improved the thermal stability of the
material in the amorphous phase by elevating the crystalli-
zation temperature beyond that required to completely
remove the mesoporous template (673 K). The same process
[
3i]
could be observed for mesoporous TiO oxides.
2
The acid-catalyzed reactions were first tested on the
mesoporous Nb W oxides using liquid-phase Friedel–
x
(10Àx)
Crafts alkylation of anisole with benzyl alcohol, and the
hydrolysis of sucrose (a disaccharide composed of glucose and
fructose) in water. A plot of the product yield of mesoporous
Figure 4. FT-IR spectra for pyridine adsorbed by mesoporous a) Nb,
b) Nb W , c) Nb W , and d) Nb W oxides. (B=Brønsted acid site;
7
3
5
5
3
7
Nb W
oxides with different Nb and W content in these
À1
x
(10Àx)
L=Lewis acid site) Assignments: 1590 cm (strong Lewis acid site),
reactions is shown in Figure 3. Variation of Nb and W content
resulted in remarkably different reaction rates of benzylani-
sole formation in the alkylation. The reaction rates increased
À1
À1
1
532 cm (strong Brønsted acid site), 1485 cm (very strong
À1
Brønsted acid site or strong Lewis acid site), 1440 cm (very strong
Lewis acid site).
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1129

Communications
À1
oxide and more than doubled from mesoporous Nb W5
0.39 mmolg
for Nb W . The increase in acid amount
7 3
5
(
5.0%) to Nb W oxide (13.0%). The trend of Brønsted
corresponded to the increase in surface area, indicating that
3
7
acid sites corresponded to the Friedel–Crafts alkylation rate,
the acid density of mesoporous Nb W
constant.
oxides was
x
(10Àx)
[11]
that is, the reaction was promoted by the Brønsted acid.
The acid properties of mesoporous Nb W oxides were
The NH3 temperature-programmed desorption (TPD)
results for all the mesoporous Nb W oxides were similar,
x
(10Àx)
3
1
also evaluated by P magic-angle spinning (MAS) NMR
x
(10Àx)
spectroscopy, using trimethylphosphine oxide (TMPO) as a
with a main broad peak at 420–480 K and a shoulder peak
above 515 K (Supporting Information, Figure S4). The
shoulder peak positions were 570 K for mesoporous Nb W ,
3
1
probe molecule. As the P chemical shifts of protonated
+
TMPO (that is, TMPOH ) tended to move downfield, higher
3
7
chemical shift values indicate higher protonic acid strength.
555 K for Nb W , 535 K for Nb W , reaffirming the acid
5 5 7 3
3
1
31
The P NMR spectra of mesoporous Nb W
oxides are
strength order obtained from P MAS NMR spectroscopy.
Heats of adsorption for ammonia on Nb W , Nb W , Nb W ,
x
(10Àx)
shown in Figure 5. A total of 0.8 mmol of TMPO was
3
7
5
5
7
3
and Nb were estimated to be ca. 145, 140, 135, and
À1
1
30 kJmol , respectively.
The acid catalytic activity of mesoporous Nb–W oxides
was compared to that of conventional solid acids. The rate of
glucose production and turnover frequency (TOF) for
hydrolysis of sucrose over several solid-acid catalysts are
shown in Figure 6. The hydrolysis of saccharides requires
3
1
Figure 5. Left: P MAS NMR spectra for TMPO adsorbed by mesopo-
rous a) Nb, b) Nb W , c) Nb W , and d) Nb W oxides, measured at
7
3
5
5
3
7
room temperature. Right: enlargement of the acid-strength region.
À1
TMPO/catalyst: 0.8 mmolg ; the spinning rate of the sample was
1
0 kHz.
adsorbed per gram of mesoporous Nb W
oxide. Meso-
(10Àx)
x
porous Nb, Nb W , and Nb W oxides had two principal
7
3
5
5
peaks: a broad peak at d = 65–70 ppm and a sharp peak at d =
[12]
3
9 ppm. The latter is ascribed to physisorbed TMPO.
Mesoporous Nb W had three peaks indicating acid strength:
3
7
Figure 6. Hydrolysis of sucrose over several solid-acid catalysts. Reac-
a main peak at d = 75 ppm indicating acid strength compara-
tion conditions: sucrose (0.5 g, 1.46 mmol), H O (10 mL, 556 mmol),
catalyst (0.1 g), 353 K 1 h.
2
[
13]
ble to that of H-Beta zeolite (d = 78 ppm), another peak at
d = 63 ppm indicating comparable strength to HY zeolite,
[
12]
(
d = 65 ppm), and a distinct small sharp peak at d = 86 ppm
indicating acid strength greater that than that of ion-exchange
resin (d = 81 ppm for Amberlyst-15) and comparable in
strength to those of strongly acidic zeolites (d = 86 ppm for
sufficient acid strength, and is an important class of reaction,
used to convert biomass into bioethanol and other useful
[
17,18]
chemicals with minimal environmental impact.
Ion-
have strong
sulfonic acid sites and as a result are powerful catalysts for
[
14]
[13]
[15]
31
[18e]
HZSM-5 and HMOR ) and sulfated zirconia. The
MAS NMR results show an enhancement of acid strength in
mesoporous Nb W oxides, with shifts of the main peaks
P
exchange resins, such as Amberlyst-15,
the hydrolysis of saccharides. Niobic acid (Nb O ·nH O) is a
x
(10Àx)
2
5
2
[
19,20]
from d = 67 ppm (x = 7) to d = 70 ppm (x = 5) or d = 75 ppm
unique solid acid resistant in water solution.
The activity
(
x = 3). The acid strength of HY zeolite (d = 65 ppm) was be
of mesoporous Nb W oxide, however, substantially exceeded
3 7
the maximum performance of any of the other materials
[6g]
evaluated to H = À6.6. Based on theoretical calculations,
0
3
1
Zheng et al. proposed that a P chemical shift of adsorbed
tested, achieving
a
glucose production rate of
À1 À1
TMPO above d = 86 ppm can be attributed to superacidity of
11.9 mmolg h . This reaction rate was significantly higher
than that of niobic acid or H-ZSM5, and six times that of
Nafion NR50 and two times that of Amberlyst-15. Moreover,
the turnover frequency of mesoporous Nb W was over 15
[
16]
the solid acid (H < À12). Therefore, it could be considered
0
that mesoporous Nb W
oxides have a range of acid
x
(10Àx)
strength between À12 ꢀ H < À6.6. The total acid amounts
0
3
7
were also estimated from the NMR peaks assigned to
adsorbed TMPO. The acid amounts obtained were
times that of Nafion NR50 and Amberlyst-15. The mesopo-
rous Nb W was recoverable by filtration and washing with
3
7
À1
À1
0
.30 mmolg
for Nb W , 0.36 mmolg
for Nb W , and
water to remove residue, and the material was confirmed to
3
7
5
5
1
130
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Angewandte
Chemie
be reusable with no change in activity after three reuse cycles.
The catalyst used in the first and third runs had glucose yields
at 2 h of 85.9 and 84.1%, respectively. The mesoporous
Nb W oxide also exhibited a reaction rate and turnover
and acid strength increased gradually with the addition of W,
reaching the highest reaction rate with mesoporous Nb W
3
7
oxide, which exceeded the reaction rate of ion-exchange
resins, zeolites, and non-mesoporous metal oxides. The very
high catalytic performance of mesoporous Nb W oxide was
3
7
frequency twice that of bulk Nb W oxide, indicating that the
3
7
3
7
mesoporous structure enhanced the reaction rate of the
accessible acid sites. Mesoporous Nb W oxides were also
attributed to a high surface area mesoporous structure, strong
acid sites which are comparable in strength to those of
strongly acidic zeolites (HZSM-5 and HMOR), and the
formation of strong Brønsted acid sites by the isomorphous
x
(10Àx)
tested for hydrolysis of cellobiose (Supporting Information,
Table S2). The subunit of cellulose consists of b-1,4-glycosidic
bonds, which are much more stable than the a-1,2-glycosidic
5
+
6+
replacement of Nb ions by higher-valence W ions in
[18]
bonds of sucrose, and are thus more resistant to hydrolysis.
tungsten-enriched samples, as observed in WO /ZrO (H =
3
2
0
[
22]
Accordingly, the rate of glucose production by hydrolysis of
cellobiose was much lower than that of sucrose over all of the
acid catalysts tested. Nevertheless, mesoporous Nb W oxide
À14.6). In that case it is reported that replacement of ZrO2
by WO forms strong acid sites similar to that of SO /ZrO
3
4
2
[
22]
(H = À16.1).
The highest activity of WO /ZrO was
3 2
obtained at surface tungsten densities, which maximize the
quantity of amorphous surface polytungstate species relative
3
7
0
exhibited the highest rate of glucose production of the solid
acids tested, including ion-exchange resins, niobic acid, and
zeolites, and had a turnover frequency four-times that of
sulfuric acid.
[
23]
to the isolated surface WO and crystallized WO . Similar
x
3
to WO /ZrO , strong acid sites in the mesoporous Nb W
7
3
2
3
The acid amount and surface area of the tested solid acids,
and the results of the Friedel–Crafts alkylation of anisole are
summarized in Table S3 of the Supporting Information. Non-
oxide could have formed leading to the high surface tungsten
densities in the niobium matrix with no crystallized WO3.
Higher concentrations of W oxide deformed the mesoporous
structure, decreasing the reaction rate. Mesoporous Nb W
7
oxide enabled both a high reaction rate and reusability, two
essential characteristics of solid acids for industrial applica-
tions.
[10]
porous Nb O –WO
obtained by exfoliation of layered HNbWO6,
and HNbWO nanosheet aggregates,
6
2
5
3
3
[
21]
were also
used for comparison. The mesoporous Nb W7 oxide also
3
exhibited the highest performance in this Friedel–Crafts
alkylation reaction, giving the highest yield and turnover
frequency. After the reaction, ortho-benzylanisole, para-
benzylanisole, and dibenzylether were formed. The selectivity
of ortho-benzylanisole over para-benzylanisole observed for
Received: August 27, 2009
Revised: October 18, 2009
Published online: December 28, 2009
mesoporous Nb W
oxides (Supporting Information,
x
(10Àx)
Keywords: Friedel–Crafts alkylation · heterogeneous catalysis ·
mesoporous materials · metal oxides · solid acid
Table S3) on Friedel–Crafts alkylation gradually increased
with increasing the W content (36.3%, 40.3%, and 42.4% for
mesoporous Nb W , Nb W , and Nb W oxides, respectively).
.
7
3
5
5
3
7
The same selectivity behavior towards o-benzylanisole could
be observed for nonporous Nb W oxides (Nb W , Nb W ,
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(10Àx)
7
3
5
5
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3
7
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the acid properties (Brønsted acid and Lewis acid sites) of Nb
and W concentrations. The selectivity of dibenzylether, a by-
product of benzyl alcohol, was 18%, 13%, and 11% for
mesoporous Nb W , Nb W , and Nb W oxides, respectively.
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3
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5
3
7
These results are consistent with the results obtained by FT-
IR spectroscopy, which show an increase in Brønsted acid
2813 – 2826.
À1
sites (1532 cm ) correlates with a decrease in dibenzyl ether
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acetic acid and lactic acid with ethanol, exceeding the
turnover frequencies of ion-exchange resins and zeolites
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(
Supporting Information, Table S4).
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Angew. Chem. Int. Ed. 2010, 49, 1128 –1132
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