TITANIA-SILICA AEROGELS WITH SUPERIOR CATALYTIC PERFORMANCE IN OLEFINEPOXIDATION COMPARED TO LARGE-PORE TI-MOLECULAR SIEVES

The paper provides a short overview on the various Ti- and Si-containi ng epoxidation catalysts, with a focus on the development and characte risation of titania-silica mixed oxides prepared by the sol-gel-aeroge l method. Titania-silica aerogels, dried by semi-continuous extraction of the solvent with supercritical CO2 at low temperature, possess hig h surface area, an amorphous mesoporous structure, and a rather even d istribution of Ti in the silica matrix. The catalytic activities of mi xed oxides, silica-supported titania and Ti-substituted molecular siev es (TS-I, Ti-beta and Ti-MCM-41) are compared in the epoxidation of l- hexene, 1-octene, cyclohexene, cyclododecene and norbornene, based on literature data. The activities (i.e. the amount of converted olefin p er unit time and catalyst weight) vary in a broad range of several ord ers of magnitude. It is shown that the low temperature aerogel contain ing 20 wt% TiO2 is superior to any other Ti- and Si-containing catalys t for the epoxidation of cyclic olefins. In the epoxidation of linear alkenes TS-1 has outstanding activity. Ti-substituted large and ultra- large pore molecular sieves require further development concerning the ir catalytic performance.