Synthesis of hydrophobic TiO2-SiO2 mixed oxides for the epoxidation of cyclohexene

A series of Ti-silica mixed oxides were prepared using sol-gel polycondensation of Ti and Si alkoxydes, The polarity of the surface was changed by substituting partly tetraethoxysilane by phenyl or methyl triethoxysilane. This type of solid adsorbs much less water but more n-hexane than the mixed oxide obtained by the normal procedure; thus it shows hydrophobic character, From UV-vis and TR spectra, titanium is incorporated in the lattice of silica, Drying with supercritical carbon dioxide at low temperature produces a mesoporous solid which was used as catalyst for the reaction of cyclohexene with hydrogen peroxide or tert-butyl hydroperoxide (TBHP). The selectivity of epoxidation with TBHP is very high and reaches >95% in acetonitrile as solvent, In acetonitrile as solvent, the rate of epoxidation of cyclohexene with TBHP decreases when the polarity of the surface decreases, but the reverse effect is observed when tert-butanol is the solvent, The reaction with hydrogen peroxide (30% in water) produces both cyclohex-2-en-1-ol and epoxide by two parallel paths, In presence of an aprotic solvent the oxygen balance overpasses 100%, which means that radical reactions involving atmospheric oxygen are involved, The selectivity of the allylic attack leading to cyclohex-2-en-1-ol is sensitive to the nature of the solvent and is minimized in tert-butanol. The epoxide is partly hydrolyzed to glycol, whatever the solvent and polarity of the surface, These catalysts epoxidize norbornene with high selectivity using hydrogen peroxide as oxidant. (C) 1997 Academic Press.