TITANIA-SILICA MIXED OXIDES .2. CATALYTIC BEHAVIOR IN OLEFIN EPOXIDATION

Various titania-silica aerogels prepared by an alkoxide-sol-gel route have been tested in the epoxidation of bulky olefins using cumene hydr operoxide as oxidant. The drying method, the titanium content between 2 and 20 wt%, and the calcination temperature between 473 and 1073 K w ere the most important preparation parameters, influencing the catalyt ic behaviour of the aerogels. The aerogels dried by semicontinuous ext raction with supercritical CO2 at low temperature (LT aerogel) were fo und to be much more efficient epoxidation catalysts than aerogels prep ared by high-temperature supercritical drying and conventionally dried xerogels. The reaction rate of cyclohexene epoxidation over LT aeroge ls increased monotonically with increasing Ti content. In the range of 333-363 K the catalysts containing 20 wt% Ti0(2)(20LT) showed high ac tivity and selectivity (79-93% to peroxide and 87-100% to epoxide) in the oxidation of various cyclic olefins, including cyclododecene, norb ornene, cyclohexene, and limonene. Catalytic experiments, FTIR, and UV -vis spectroscopy indicated that the LT aerogels consist of two differ ent types of active species: titanium well-dispersed in the silica mat rix and titania nanodomains. The key factors determining the activity and selectivity of sol-gel titania-silica catalysts are the morphology (surface area and pore size) and the relative proportions of Ti-O-Si and Ti-O-Ti structural parts. A comparative study of the epoxidation o f cyclohexene, cyclododecene, and norbornene over structurally differe nt titania-silica catalysts, indicates that 20LT shows better catalyti c behaviour in these reactions than Ti zeolites and silica-supported t itania. (C) 1995 Academic Press, Inc.