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-