Amine-modified titania-silica hybrid gels as epoxidation catalysts

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 N-2 adsorption, and Si-29 and C-13 NMR spectroscopy. The surface areas in the range 306-383 m(2) g(-1) were mostly unaffected by the structure of the amine modifier. The ratios of Q(4)/Q(3) species in the Si-29 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. (C) 2000 Elsevier Science B.V. All rights reserved.