Hydrophobic titania-silica aerogels: epoxidation of cyclic compounds

Titania-silica mixed oxides with covalently bound methyl or phenyl groups were prepared from the corresponding alkyltrimethoxysilane and tetramethoxysilane using a sol-gel process and ensuing low-temperature supercritical extraction with CO2. Prehydrolysis of the organically modified silicon precursor was applied to compensate for their lower sol-gel reactivity compared to the precursors of the other constituents. Trihexylamine was used as a gelation catalyst. The dominantly mesoporous aerogels were characterized by thermal analysis, N-2 adsorption, infrared spectroscopy, Si-29- and C-13-NMR spectroscopy, and transmission electron microscopy. Thermal analysis and Si-29 MAS NMR measurements indicated covalent incorporation of the modifying group. C-13 NMR spectra confirmed the integrity of the incorporated modifying groups. The materials were tested in the epoxidation of cyclohexene and cyclohexenol with tert-butylhydroperoxide. NH3 adsorption studied by DRIFT spectroscopy indicated no significant acidity of the aerogel surfaces, which was traced to the use of trihexylamine as gelation catalyst. Addition of zeolite 4 Angstrom or NaHCO3 did not improve the performance of the unmodified 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 acetone), compared to the reaction in toluene.