Mesoporous SiO2-TiO2 epoxidation catalysts: Tuning surface polarity to improve performance in the presence of water

Abstract

Herein, we present the preparation by the non-hydrolytic sol-gel (NHSG) route of a mesoporous SiO2-TiO2 epoxidation catalyst with high specific surface area and large pores, and we discuss its modification by methylation. The pristine NHSG catalyst is tested for the catalytic conversion of cyclohexene with H2O2 in acetonitrile, showing performance comparable to a reference TS-1 catalyst. To expand the scope of the catalysts, we study their performance in the presence of water – present in aqueous solutions of H2O2, formed as a by-product of the epoxidation reaction, or used as a co-solvent. While TS-1 maintains high epoxidation activity, the pristine amorphous and mesoporous SiO2-TiO2 catalysts strongly deactivates when water is used as co-solvent. In order to circumvent this deactivation in the presence of water, we investigate the effect of surface hydrophobization with methyl groups on the catalytic activity and selectivity. Two functionalization methods are investigated: one-pot synthesis with methyltrichlorosilane and post-synthesis silanization with methyltrimethoxysilane. On the one hand, the one-pot methylation does not increase water resistance, in spite of an increased surface hydrophobicity. We interpret this result in the light of a poorer Ti dispersion and lower Ti4+ surface content in these catalysts. On the other hand, post-synthesis grafting was successful, allowing to maintain good epoxidation activity even in the presence of water. This result is explained by a combination of a hydrophobic surface with a high active site content in this catalyst.