The Influence of Surface Modification on the Epoxidation Selectivity and Mechanism of TiSBA15 and TaSBA15 Catalysts with Aqueous Hydrogen Peroxide

Abstract

The thermolytic molecular precursor method was used to introduce site-isolated Ti(IV) and Ta(V) centers onto the surface of a mesoporous SBA15 support. The resulting surface SiOH, TiOH, and TaOH sites were modified with a series of (N,N-dimethylamino)trialkylsilanes, Me2NSiMe2R (where R = Me, n Bu, or n Oc). Compared to the unmodified catalysts, the surface-modified catalysts are more active in the epoxidation of cyclohexene with H2O2 and exhibit a markedly greater selectivity for cyclohexene oxide (up to 58% for Ti(IV) and greater than 95% for Ta(V)). In situ spectroscopies were used to probe this phenomenon, and it was determined that a siloxy-capped Ti(IV) or Ta(V) site is essential to achieve the high epoxide selectivity.