Titania–Silica Epoxidation Catalysts Modified by Polar Organic Functional Groups

Abstract

Mesoporous titania–silica mixed oxides with covalently bound 3-chloropropyl, 3-acetoxypropyl, and N,N-dimethyl-3-aminopropyl groups were prepared from the corresponding alkyltrimethoxysilane and tetramethoxysilane using a sol–gel process and ensuing low temperature supercritical extraction with CO2. The aerogels were characterized by thermal analysis, N2 and NH3 adsorption, infrared spectroscopy, and 29Si and 13C NMR spectroscopy. The modified aerogels showed reduced thermal stability compared to the unmodified titania–silica aerogel. Si–C bond breaking was detected already around 150°C. 29Si MAS NMR measurements indicated covalent incorporation of the modifying group. DRIFT and NMR measurements confirmed the incorporation of 3-acetoxypropyl and N,N-dimethyl-3-aminopropyl groups and indicated the partial hydrolysis of 3-chloropropyl group to the corresponding alcohol. The activity in the epoxidation of cyclohexene and cyclohexenol with tert-butylhydroperoxide was significantly higher for 3-acetoxypropyl- and N,N-dimethyl-3-aminopropyl-modified catalysts than for the unmodified aerogel. Cyclohexenol oxide selectivity with the N,N-dimethyl-3-aminopropyl-modified catalyst was 94% at 91% peroxide conversion. The new aerogels are compared to other known Ti- and Si-containing epoxidation catalysts, including titania–silica mixed oxides modified by apolar functional groups.