Hydrophobic titania-silica aerogels: epoxidation of cyclic compounds

Titania-silica mixed oxides with covalently bound methyl or phenyl groups w ere prepared from the corresponding alkyltrimethoxysilane and tetramethoxys ilane using a sol-gel process and ensuing low-temperature supercritical ext raction with CO2. Prehydrolysis of the organically modified silicon precurs or was applied to compensate for their lower sol-gel reactivity compared to the precursors of the other constituents. Trihexylamine was used as a gela tion catalyst. The dominantly mesoporous aerogels were characterized by the rmal analysis, N-2 adsorption, infrared spectroscopy, Si-29- and C-13-NMR s pectroscopy, and transmission electron microscopy. Thermal analysis and Si- 29 MAS NMR measurements indicated covalent incorporation of the modifying g roup. C-13 NMR spectra confirmed the integrity of the incorporated modifyin g groups. The materials were tested in the epoxidation of cyclohexene and c yclohexenol with tert-butylhydroperoxide. NH3 adsorption studied by DRIFT s pectroscopy indicated no significant acidity of the aerogel surfaces, which was traced to the use of trihexylamine as gelation catalyst. Addition of z eolite 4 Angstrom or NaHCO3 did not improve the performance of the unmodifi ed and phenyl-modified aerogel, corroborating the suppressed acidity of the surface. Interestingly, the phenyl-modified aerogel was more selective in the epoxidation of cyclohexenol, when using polar solvents (ethanol or acet one), compared to the reaction in toluene.