insoluble polyoxotungstates,7 immobilized peroxotungstates,8
triphasic phosphotungstate,9 or pseudo-heterogeneous sys-
tems10 have been recently reported.
percentage of the immobilization could be controlled at the
desired weight percentage. X-ray photoelectron spectroscopy
(XPS, see the Supporting Information) indicates that the
binding energy of the supported tungstens’s 4f2/7 is 35.8 eV,
which is in a range of typical values of WO3.19
While bulk powder WO3 exhibited a low activity in the
epoxidation of cis-cyclooctene using 50% aqueous H2O2
(Table 1, entry 1), downsizing the particles (30-100 nm)
Recent advances in the preparation of size-controlled
nanoparticles have led us to search for applications in
catalysis with a prediction that large surface areas of such
particles may offer novel catalytic activities.11 Much attention
has been also paid to the use of supported nanocatalyst
systems.12 Along this line, preparative methods for the
W-containing mesoporous sieves were disclosed,13 and their
catalytic activities were also preliminarily presented. Among
the reactions examined, representative examples are hy-
droxylation of cyclohexene,14 oxidative cleavage of cyclo-
pentene to glutaraldehyde,15 and dehydrogenation of 2-pro-
panol.16 Herein, we describe our studies on the preparation
of supported WO3 nanoparticles and their catalytic activities
in the oxidations of olefins, sulfides, and cyclic ketones.17
Table 1. Effects of Supporting Materials on the Epoxidationa
entry
supporting materials
convb (%)
selecc (%)
1d
2
30
91
33
91
e
nano-WO3
We prepared tungsten(VI) oxide nanoparticles supported
on a range of templates.18 With the use of ordered mesopo-
rous supports, the resulting WO3 nanoparticles are uniformly
dispersed on the surface of the channels. Inductively coupled
plasma (ICP) analysis showed that tungstate was quantita-
tively supported on the supporting materials, and the weight
3
alumina
45
71
4
5
6
7
TiO2
activated carbon
CeO2
57
79
62
10
>98
94
>98
<5
MgO
8
9
10
Montmorillonite K 10
MCM-48f
MCM-48g
28
>99
78
<5
>98
>98
(4) (a) Sato, K.; Aoki, M.; Takagi, J.; Noyori, R. J. Am. Chem. Soc.
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a Average ca. 5 wt % of WO3 except entry 10. b Conversion was
determined by GC using an internal standard (dodecane). c Selectivity for
the formation of cyclooctene oxide. d Bulk powder WO3 was used.
e Nanoparticles with an average size of 30-100 nm. f Average size of 2-3
nm for WO3 (5.0 wt %). g Average size of 3-10 nm for WO3 (20 wt %).
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resulted in a dramatic increase in the activity and selectivity
(entry 2). When we examined the effects of supporting
materials, it was observed that catalytic actiVity of the
supported tungstate was closely related to the nature of the
supporting materials and size of the nanoparticles. Whereas
the use of certain supporting materials such as alumina, TiO2,
active carbon, or CeO2 gave moderate activities, employment
of MgO and Montmorillonite K 10 resulted in a sharp
decrease in the epoxidation efficiency (entries 7-8). It is of
special interest to see that when MCM-48, ordered silica-
based 3D mesoporous materials,20 was applied as a support,
the catalytic activity of the resulting WO3/MCM-48 was
dramatically increased (entry 9). More interestingly, a diol
that normally forms by an acid-catalyzed hydrolysis of
epoxide was not observed. Additionally, it was seen that the
catalytic activity was changed inversely to the metal content
and size of the nanoparticles (entry 10).21
The scope of the prepared WO3/MCM-48 catalyst system
was in turn tested in the epoxidation of various olefins (Table
2).22 It displays an excellent activity and selectivity especially
(18) Preparation of WO3/MCM-48. WCl6 (120 mg, 0.3 mmol), MCM-
48 (1.0 g), and benzene (20 mL) were placed in a 50 mL two-neck flask
with a magnetic stirring bar. The color of the reaction mixture became blue
from gray after being stirring for 12 h at room temperature under an argon
atmosphere. The solution was filtered and washed with benzene, and the
obtained solid was dried in vacuo. The light blue solid was calcined for 2
h at 300 °C flowing with H2 gas. After being cooled to room temperature,
the solid was treated with O2 flow for 1 h at the same temperature to give
a gray powder (1.0 g).
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188, 90. (b) Alvarez-Merino, M. A.; Carrasco-Mar´ın, F.; Fierro, J. L. G.;
Moreno-Castilla, C. J. Catal. 2000, 192, 363.
(20) Stucky et al. reported the preparation of grafted tungsten on MCM-
48 via W-O-Si bonds through a metal alkoxide route: Morey, M. S.;
Bryan, J. D.; Schwarz, S.; Stucky, G. D. Chem. Mater. 2000, 12, 3435.
(21) (a) Iwamoto, M.; Tanaka, Y.; Sawamura, N.; Namba, S. J. Am.
Chem. Soc. 2003, 125, 13032. (b) Kumar, D.; Varma, S.; Gupta, N. M.
Catal. Today 2004, 93-95, 541.
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