Synthesis of highly active tungsten-containing MCM-41 mesoporous molecular
sieve catalyst
Zhaorong Zhang, Jishuan Suo, Xiaoming Zhang and Shuben Li*
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou, 730000, P.R. China
A tungsten-containing MCM-41 mesoporous molecular
sieve is synthesized by the hydrolysis of tetraethylorthosili-
cate and ammonium tungstate in the presence of cetylpyr-
idinium bromide as template in acidic medium and found to
be more active than the conventional WO3 catalyst in the
hydroxylation of cyclohexene using H2O2 as oxidant.
to be 7.1 mass% (SiO2 :WO3 ca. 50). The HK mean pore size
and BET surface area of the as-synthesized W-MCM-41
calculated on the basis of nitrogen adsorption–desorption
isotherms (Coulter, Omnisorp 360CX) were ca. 2.9 nm and
1059 m2 g21, respectively.
As mentioned above, different physicochemical characteri-
zations confirmed that in the W-MCM-41 sample prepared, the
W was highly dispersed in the silica-based framework structure,
and it was found to be active for carrying out catalytic
oxidations of unsaturated hydrocarbons using H2O2 as oxidant.
Here the H2O2 hydroxylation of cyclohexene was carried out
over W-MCM-41, Si-MCM-41 and conventional WO3 catalysts
suspended in acetic acid (HAc) media; where Si-MCM-41 was
a pure silica sample synthesized by the method described above
but leaving out the tungsten ion precursor. Since H2O2 in HAc
alone is a hydroxylating agent,2 a comparative experiment was
also made in the absence of any catalyst. The results
summarized in Table 1 clearly show that W-MCM-41 is a good
catalyst for the test reaction, on which the hydroxylation rate of
WO3-based catalysts are important not only in hydrodesulfuri-
zation and alkene metathesis1 but also in hydroxylation of
unsaturated compounds.2 Recently, the breakthrough discovery
of silica-based mesoporous molecular sieves M41S, including
the hexagonal MCM-41,3,4 offered new opportunities for
creating highly dispersed and more accessible catalytic sites by
incorporating transition-metal ions into their silica-based
frameworks.5–9 Many of the mesoporous molecular sieve
catalysts thus obtained therefore showed quite good catalytic
properties in different reactions.10,11 Here, we report the
synthesis, characterization and catalytic performance of the
tungsten-containing MCM-41 mesoporous molecular sieve
catalyst (W-MCM-41) in the hydroxylation of cyclohexene
with 30 mass% H2O2.
In a typical synthesis of the W-MCM-41, 5.67 g (20 mmol) of
ammonium tungstate [Aldrich, 99.99% (NH4)2WO4] was
dissolved in 100 ml of water to prepare solution A; 6.2 g (15
mmol) of cetylpyridinium bromide (Aldrich, 98%
C16H33NC5H5Br·H2O, CPBr) was combined with 60 ml of HCl
(5 mol dm23) to form solution B. Then 11.4 g (50 mmol) of
tetraethylorthosilicate [Aldrich, 98% Si(OEt)4, TEOS] and a
determined amount of solution A were simultaneously in-
troduced into solution B under vigorous stirring to give the
following composition: 1 TEOS:0.3 CPBr:0.02 W:6 HCl:60
H2O. After allowing the resulting gel to age at 323 K under
gentle stirring for 22 h, the solid product was centrifuged,
washed with distilled water and air-dried. The calcination of the
W-MCM-41 sample was carried out in air at ca. 533 K for 1.5
h, then at 873 K for 4 h.
The calcined W-MCM-41 sample was colorless, indicating
the absence of colored crystalline WO3 species outside the
framework. This result was verified by Raman and UV–VIS
spectroscopy.
An X-ray powder diffraction pattern (Rigaku, D/Max-2400,
with Cu-Ka radiation; l = 0.15418 nm) of the calcined sample
is depicted and indexed in Fig. 1 and corresponds to MCM-41
mesoporous silicas reported previously.4,5
12000
100
W-MCM-41
hkl d / nm
100 3.426
110 1.989
200 1.715
6000
110
200
0
2
4
6
8
10
2q / °
Fig. 1 PXRD pattern of the calcined W-MCM-41
804.1
Fig. 2 shows the Raman spectra (Nicolet, Raman 910) of
W-MCM-41 and crystalline WO3. Crystalline WO3 is a very
strong Raman scatterer, so the absence of intense peaks at ca.
804, 714, 327, 267 and 137 cm21 (in WO3) corresponding to
octahedral WO6 groups13 in the spectrum of W-MCM-41
indicated that the W was highly dispersed in the silica-based
framework structure. This result was also supported by the
diffuse reflectance (DR) spectra of W-MCM-41 and WO3
crystals in the UV–VIS region. In the DR UV–VIS spectrum of
the calcined W-MCM-41 sample, there is no absorption band
corresponding to crystalline WO3.
714.4
(a)
267.3
327.1
132.3
(b)
1000
800
600
400
–1
200
Raman shift / cm
The chemical analysis using ICP atomic emission spectro-
scopy (ARL 3520) showed the WO3 content in the W-MCM-41
Fig. 2 Raman spectra of (a) W-MCM-41 and (b) crystalline WO3
Chem. Commun., 1998
241
Zhang, Zhaorong
