Amine-modified titania–silica hybrid gels as epoxidation catalysts Original Research Article

Mesoporous titania–silica mixed oxides with covalently bound 3-aminopropyl, 3-(methylamino)propyl, 3-(diethylamino)propyl, 3-(phenylamino)propyl, and 3-(dimethylamino)propyl groups were prepared from the corresponding aminoalkyltrimethoxysilane and tetramethoxysilane precursors using a sol–gel process and ensuing low temperature supercritical extraction with CO2. The aerogels were characterized by N2 adsorption, and 29Si and 13C NMR spectroscopy. The surface areas in the range 306–383 m2 g−1 were mostly unaffected by the structure of the amine modifier. The ratios of Q4/Q3 species in the 29Si NMR spectra as a measure of cross-linkage were similar for the investigated materials. The activity in the epoxidation of cyclohexene and cyclohexenol with tert-butylhydroperoxide (TBHP) was significantly higher for all modified aerogels (by up to a factor of 4), as compared to the unmodified aerogel. Peroxide selectivity in the epoxidation of cyclohexene was 100% for a 2% 3-dimethylaminopropyl-modified catalyst at 70% TBHP conversion. This catalyst afforded also the highest initial rate in the epoxidation of cyclohexene and cyclohexenol (both 7.2 mmol g−1 min−1). Cyclohexenol oxide selectivity with the 10% 3-dimethylaminopropyl-modified catalyst was 96% at 70% peroxide conversion. The catalytic behaviour depended not only on the type but also on the concentration of modifier in the aerogel.