Enhancement of Epoxidation Efficiencies for Ta-SBA15 Catalysts. The Influence of Modification with -EMe3 (E = Si, Ge, Sn) Groups

Site-isolated Ta(V) centers were introduced onto the surface of a mesoporous SBA-15 support via the thermolytic molecular precursor method. After thermal treatment under oxygen, the resulting Si OH and Ta OH sites of TaSBA15-O-2 were modified with a series of trimethyl group 14 species, Me3E-, by treatment with Me3E-NMe2 (E = Si, Ge, Sn) reagents. The resulting surface-modified catalysts (Me3E)(cap)TaSBA15 exhibit a significantly increased rate of cyclohexene epoxidation with H2O2 as an oxidant, and provided a decreased amount of allylic oxidation products with respect to the unmodified material, TaSBA15-O-2. The rate of nonproductive H2O2 decomposition, as monitored via H-1-NMR spectroscopy, significantly decreased after the surface modification. The structure of the TaSBA15 catalysts and potential Ta(V) epoxidation intermediates (formed upon treatment of Ta(V) materials with H2O2) were probed using UV-visible absorbance and diffuse-reflectance UV visible spectroscopy. A Ta(V)(eta(2)-O-2) intermediate species is proposed for the TaSBA15-O-2, (Me3Si)(cap)TaSBA15, and (Me3Ge)(cap)TaSBA15 catalysts, while intermediate species for the (Me3Sn)(cap)TaSBA15 catalysts could not be characterized.