Organically Modified Titania–Silica Aerogels for the Epoxidation of Olefins and Allylic Alcohols

Abstract

Mesoporous titania–silica mixed oxides with varying amounts of covalently bound methyl groups were prepared from methyltrimethoxysilane and tetramethoxysilane using a sol-gel process and ensuing low temperature supercritical extraction with CO2. The aerogels containing 20 wt% TiO2were characterized by thermal analysis, N2and NH3adsorption, infrared spectroscopy, NMR, X-ray photoelectron spectroscopy, and transmission electron microscopy. The amorphous mesoporous structure of the aerogels did not depend markedly on the amount of modifier. The titania dispersion within the silica matrix was slightly lowered for methyl-modified materials.29Si CP-MAS NMR measurements indicated a more cross-linked network for the unmodified catalyst. Methyl groups located at the surface replaced part of the silanol groups present in the unmodified aerogel, resulting in a decreased ability to adsorb ammonia. The modified aerogels showed high thermal stability: a significant rate of Si–C bond breaking was detected only above 400°C. The materials were tested in the epoxidation of various olefins and allylic alcohols. The olefin epoxidation activity was generally lowered by the methyl modification, whereas in the epoxidation of allylic alcohol a maximum in activity was observed with increasing methyl content. In the epoxidation of cyclohexenol, 98% selectivity at 90% conversion was achieved in 10 min. Dependent on their basicity, inorganic salts and bases as additives exhibited a strong influence on the reaction over modified aerogels. This behavior is suggested to be due to: (i) different titanium complexes which form upon interaction with the reactant and basic additive and (ii) the resulting change of the stability/reactivity of the peroxo–titanium complex.