Epoxidation of styrene on Si/Ti/SiO2 catalysts prepared by chemical grafting Original Research Article

Titania–silica (Ti/SiO2) and silica–titania–silica (Si/Ti/SiO2) catalysts were prepared by chemical grafting using TiCl4 and tetraethyl orthosilicate (TEOS) as precursors and SiO2 as support. The prepared catalysts were characterized by UV Raman and visible Raman spectroscopies, XRD and the epoxidation of styrene. Ti/SiO2 catalyst grafted with only titanium species is not very active for epoxidation using H2O2 (30%), but is active and selective when one uses tert-butyl hydroperoxide (TBHP). The catalyst grafted at high temperatures shows better epoxide selectivity. Si/Ti/SiO2 catalyst, the titanium-silica grafted further with TEOS, is active and selective for the epoxidation of styrene using either dilute H2O2 or TBHP, possibly due to the fact that the grafting of Ti/SiO2 with TEOS modifies the coordination structure of titanium and makes the titanium sites of Si–O–Ti–O–Si species less hydrophilic. A characteristic band at 1085 cm−1 due to Ti–O–Si species is detected for the grafted catalysts by UV resonance Raman spectroscopy. Reaction between TiCl4 and SiO2 at high temperatures favors the formation of Ti–O–Si species. Better activity and selectivity to epoxide is found for the catalysts with more Ti–O–Si species. It is assumed that the active sites are the highly isolated Ti–O–Si species. For Si/Ti/SiO2 catalyst, the gas phase O2 can participate in the catalytic oxidation of styrene when H2O2 is present and it causes the formation of benzaldehyde.