Organo-bridged silsesquioxane titanates for heterogeneous catalytic epoxidation with aqueous hydrogen peroxide

Abstract

Organo-bridged silsesquioxane titanates for heterogeneous catalytic epoxidation with aqueous hydrogen peroxide were synthesized through the acid-catalyzed hydrolysis and co-condensation of organotrialkoxysilane monomers and α,ω-bis(trialkoxysilyl) alkane cross-linkers in ethanol–water solution, with subsequent titanium grafting. These hybrid catalysts show high activity and selectivity in the epoxidation of cyclooctene with both aqueous hydrogen peroxide and tert-butyl hydroperoxide (TBHP) as oxidants, although they are nonporous. The catalysts can be reused at least six times with no loss of activity with TBHP as the oxidant and at least four times (although with gradual activity loss) with aqueous H2O2 as the oxidant. Furthermore, the catalytic activities of these hybrid titanates relate to the nature and the composition of the starting materials used. The epoxidation activity of the catalysts using aqueous H2O2 as the oxidant seems to increase with increasing carbon length in the starting organosilane cross-linkers. UV–vis DRS and XPS measurements indicate that tetrahedrally coordinated Ti species are the active centers.