STUDIES ON THE CATALYTIC-OXIDATION OF ALKANES AND ALKENES BY TITANIUMSILICATES

Titanium containing, aluminum-free ZSM-5 (TS-1) and amorphous TiO2-SiO 2 coprecipitate are investigated as catalysts for the selective oxidat ion of alkanes and alkenes using a variety of oxidants at temperatures below 100 degrees C. Comparisons between the activities of TS-1 and t he TiO2-SiO2 coprecipitate for alkane oxidation and alkene epoxidation using nonaqueous H2O2 indicate that the absence of water is crucial f or the catalytic activity of silica-supported titanium. Due to the hyd rophobicity of TS-1, the concentration of water surrounding the titani um is maintained at a low value, and thus TS-1 can be used as an oxida tion catalyst with aqueous H2O2 as oxidant. Alkyl hydroperoxides are a ctive as oxidants for alkene epoxidation on the TiO2-SiO2 coprecipitat e but not for alkane oxidation reactions on both TS-1 and the TiO2-SiO 2 coprecipitate. A plausible explanation for the above results is prov ided. The presence of stereoscrambling without any ''radical clock'' r earrangement during alkane oxidation on TS-1 indicates that the radica ls formed may have a very short life-time, or their movements are rest ricted such that no rearrangement can occur. A proposal for the mechan ism of alkane oxidation on TS-1 is given and compared to a mechanism s uggested for alkene epoxidation on TS-1 and the TiO2-SiO2 coprecipitat e. (C) 1994 Academic Press, Inc.