Selective epoxidation of olefins by hydrogen peroxide in water using a polyoxometalate catalyst supported on chemically modified hydrophobic mesoporous silica gel

Article abstract of DOI:10.1016/S0040-4039(00)01787-1

A new heterogeneous catalyst prepared by immobilisation of polyoxometalates on chemically modified hydrophobic mesoporous silica gel has been successfully applied to the selective epoxidation of olefins with 15% aqueous H₂O₂ without the use of organic solvent. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01787-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 10009–10012
Selective epoxidation of olefins by hydrogen peroxide in
water using a polyoxometalate catalyst supported on
chemically modified hydrophobic mesoporous silica gel
Takaaki Sakamoto and Chyongjin Pac*
Kawamura Institute of Chemical Research, 631 Sakado, Sakura, Chiba 285-0078, Japan
Received 16 August 2000; revised 2 October 2000; accepted 6 October 2000
Abstract
A new heterogeneous catalyst prepared by immobilisation of polyoxometalates on chemically modified
hydrophobic mesoporous silica gel has been successfully applied to the selective epoxidation of olefins
with 15% aqueous H2O2 without the use of organic solvent. © 2000 Elsevier Science Ltd. All rights
reserved.
Keywords: catalysis; epoxidation; olefins; hydrogen peroxide; solid-supported reactions.
The catalytic epoxidation of olefins using environmentally benign oxidants, typically H2O2,
instead of stoichiometric reagents common in the field of synthetic organic chemistry has
received significant research attention in recent years, particularly from viewpoints of green
chemistry.1 Recently, Newmann and his coworkers reported interesting prototypical systems for
heterogeneous catalysis of olefin epoxidation in aqueous H2O2 without the use of organic
solvent.2,3 They prepared a polyoxometalate catalyst fixed on a polyether-modified silica surface,
which can catalyse the epoxidation of cyclooctene in 30% aqueous H2O2.2 However, this catalyst
has a crucial drawback in its synthetic application, since a relatively long reaction time is
required for the completion of the reaction. Moreover, no information has been given on the
applicability of this catalyst to various olefins other than cyclooctene. Another solid catalyst
reported by them is a silicate xerosol prepared by a sol–gel method, the reaction centre of which
is a quaternary ammonium/polyoxometalate ion pair covalently fixed on the silica surface.3 With
this catalyst, however, the olefin epoxidation in 30% aqueous H2O2 is not selective or efficient
except for a few particular olefins. While the Newmann’s catalysts have certainly opened up a
new route to heterogeneous catalysis of olefin epoxidation, there still remain crucial problems to
be explored for designing more efficient catalysts of synthetic significance. An essential factor is
* Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01787-1

10010
to control chemical features of the silica surfaces, since heterogeneous reaction rates should be
determined by such surface processes as the access of the reactants to the catalysis centre and
adsorption/desorption of the reactants and products, as well as by microscopic environments
surrounding the catalysis centre. From these viewpoints, we have been investigating surface
modifications of silica particles to explore the possible effects of hydrophobicity on the catalytic
efficiencies. In this paper, we wish to report a novel type of solid catalyst that has been prepared
by non-covalent fixing of the tungstophosphate anion on chemically modified hydrophobic
mesoporous silica gel. This catalyst has been found to catalyse the selective epoxidation of
various olefins by 15% aqueous H2O2 without the use of organic solvent (Scheme 1).
O
+
H2O
+
H2O2 (15%)
[R]3[PW12O40]/modified SiO2
in H2O
Scheme 1.
A typical example for the preparation of the catalyst is as follows. Mesoporous silica gel with
,
an average pore diameter of 100 A (Kieselgel 100, 2 g) was activated with hot concentrated
hydrochloric acid4 and then added to a dry toluene solution (10 cm3) containing either or both
ethoxytriphenylsilane (0.153 g, 0.5 mmol) and N,N-dimethylformamide dibenzylacetal5 (0.136 g,
0.5 mmol). After refluxing the mixture for 7 h under nitrogen, the resulting solid was collected
by filtration, washed thoroughly with toluene and acetone and then dried overnight in a vacuum
oven at room temperature. To a suspension of the solid (2.22 g) in dry acetone (10 cm3) was
added tris(cetylpyridinium)-12-tungstophosphate6 (TCPTP, [p-C5H5N+(CH2)15CH3]3(PW12O40))
(0.91 g, 0.24 mmol) in dry acetone (40 cm3). The mixture was refluxed for 5 h under nitrogen,
evaporated slowly at room temperature, carefully washed twice with water and acetone, and
then dried under vacuum at 80°C for 7 h to give the catalyst.
It was found that the catalytic epoxidation of various olefins in 15% aqueous H2O2 using the
solid catalyst does occur in various efficiencies and selectivities depending on different modifica-
tions of silica gel as well as on the organic counter cations of tungstophosphate. Table 1
summarises the catalytic epoxidation of 1-octene in 15% aqueous H2O2 using the various
catalysts prepared (Cat. 1–13). As a typical run, a mixture of 1-octene (3 mmol), Cat. 1 (0.5
mol%) and 15% H2O2 (6 mmol) was heated at 90°C for 10 h. GLC analysis showed the
quantitative formation of 1,2-epoxyoctane at the complete consumption of 1-octene. Prominent
features are as follows: (1) Modification of silica gel with both Ph3SiOEt and
Me3NCH(OCH2Ph)2 in a 1:1 ratio generally gave effective catalysts, i.e. Cat. 1, 10 and 11 among
which the first is the most effective. In contrast, the starting silica gel without surface
modification gave a poor catalyst (Cat. 4). (2) Catalytic activities were significantly or consider-
ably low when silica gel had been modified only with either Ph3SiOEt or Me2NCH(OCH2Ph)2
(Cat. 2 snd 3) or with a 1:3 or 3:1 mixture of them (Cat. 5 and 6). (3) The less the
hydrophobicity of the organic counter cation [R], the lower the catalytic activities become; the
reaction conversions decrease in the order Cat. 10ꢀCat. 11>Cat. 12>Cat. 13. (4) IR analysis of
Cat. 1 showed the complete disappearance of the silanol absorption of the starting silica gel at
3745 cm−1, whereas the silanol absorption still occurred significantly in the cases of Cat. 2 and
3. This implies that different types of silanol groups on the silica gel surface7 have different
reactivities with either Ph3SiOEt or Me2NCH(OCH2Ph)2. As a consequence, the high hydropho-

10011
bic modification of silica gel surface can be achieved by the treatment with both of the agents.
All these features strongly suggest that the hydrophobic environment on the catalyst surface
should play essential roles in the efficient and selective epoxidation. Presumably, high hydropho-
bicity of both the silica gel surface and the counter cation [R] might favour strong binding of
the catalysis centre [PW12O40]3− on the silica gel surface and also easy access of the hydrophobic
reactant 1-octene to the catalysis centre.
Table 1
a
Epoxidation of 1-octene with 15% aqueous H2O2
Cat.
Catalystb
Conversion of Selectivity of
1-octene (%)c
epoxide (%)c
Polyoxometalate
[R]3(PW12O40)3−
Silane coupling Alkylating agent (AA)
agent (SA)
(CH3)2NCH(OX)2
1
2
3
R=p-C5H5N+(CH2)15CH3 Ph3SiOC2H5
R=p-C5H5N+(CH2)15CH3 Ph3SiOC2H5
R=p-C5H5N+(CH2)15CH3 None
X=benzyl
None
X=benzyl
None
X=benzyl
X=benzyl
X=benzyl
X=n-butyl
X=cyclohexyl
X=benzyl
X=benzyl
X=benzyl
X=benzyl
100
59
45
18
72
47
79
81
78
90
88
41
21
\98
97
97
4
R=p-C5H5N+(CH2)15CH3 None
96
5d
6e
7
R=p-C5H5N+(CH2)15CH3 Ph3SiOC2H5
R=p-C5H5N+(CH2)15CH3 Ph3SiOC2H5
R=p-C5H5N+(CH2)15CH3 Ph2Si(OC2H5)2
R=p-C5H5N+(CH2)15CH3 Ph3SiOC2H5
R=p-C5H5N+(CH2)15CH3 Ph3SiOC2H5
\98
\98
\98
97
97
97
97
97
96
8
9
10
11
12
13
R=C12H25(CH3)3N+
R=CH3(C8H17)3N+
R=(C4H9)4N+
R=H
Ph3SiOC2H5
Ph3SiOC2H5
Ph3SiOC2H5
Ph3SiOC2H5
a For reactions of 3 mmol 1-octene, 0.015 mmol of catalyst (0.5 mol%) and 6 mmol 15% H2O2 at 90°C for 10 h.
b Unless otherwise stated, SA:AA=1:1.
c By GC analysis using Bu2O as internal standard.
d SA:AA=3:1.
e SA:AA=1:3.
It was confirmed that Cat. 1 can be successfully applied to the epoxidation of different types
of olefins, i.e. terminal, cyclic, highly substituted and aromatic olefins. The corresponding
epoxides were produced in >97% (Table 2). In the cases of cyclohexene, cycloheptene,
cyclooctene, 1-methyl-1-cyclohexene, 2,3-dimethyl-2-butene, a-pinene, trans-stilbene and cis-stil-
bene, the epoxidation was completed in 56 h at 70°C and the reactions of 1-octene, 1-decene,
allyl phenyl ether and styrene occurred quantitatively in 8–10 h at 90°C. Large-scale reactions
of olefins can be easily achieved. As a typical run, a mixture of cyclohexene (24.7 g, 0.3 mol),
Cat. 1 (13.6 g, 1.5 mmol) and 15% aqueous H2O2 (136 g, 0.6 mol) was heated at 70°C for 6 h
under N2 to give pure cyclohexene oxide (27.1 g, 92% yield) after conventional workup followed
by simple distillation of the crude product.

10012
Olefin
Table 2
a
High yield (\97%) epoxidation of various olefins with 15% aqueous H2O2
T (°C)
t (h)b
Olefin
T (°C)
t (h)b
Cyclohexene
Cycloheptene
Cyclooctene
1-Methyl-1-cyclohexene
1-Octene
70
70
70
70
90
90
3
3
2
4
10
10
2,3-Dimethyl-2-butene
Allyl phenyl ether
a-Pinene
70
90
70
90
70
70
3
10
2
Styrene
8
trans-Stilbene
cis-Stilbene
6c
6
1-Decene
a For reactions of 3 mmol olefin, 0.015 mmol of catalyst (0.5 mol%) and 6 mmol 15% H2O2.
b Approximate time required for the disappearance of the starting olefin analysed by GC.
c Toluene (0.5 g) was added.
The present investigation has provided an effective methodology for designing efficient solid
catalysts for olefin epoxidation in water using a low concentration of H2O2. Further improve-
ments of the solid catalyst could be achieved by fine tuning both the silica-surface morphology
(e.g. porosity) and the surface-modification agents.
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