Enhancement of Epoxidation Efficiencies for Ta-SBA15 Catalysts. The Influence of Modification with 鈥揈Me3 (E = Si, Ge, Sn) Groups
文摘
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鈥揙H and Ta鈥揙H sites of TaSBA15-O2were modified with a series of trimethyl group 14 species, Me3E鈥? by treatment with Me3E鈥揘Me2 (E = Si, Ge, Sn) reagents. The resulting surface-modified catalysts (Me3E)capTaSBA15 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-O2. The rate of nonproductive H2O2 decomposition, as monitored via 1H 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鈥搗isible absorbance and diffuse-reflectance UV鈥搗isible spectroscopy. A Ta(V)(畏2-O2) intermediate species is proposed for the TaSBA15-O2, (Me3Si)capTaSBA15, and (Me3Ge)capTaSBA15 catalysts, while intermediate species for the (Me3Sn)capTaSBA15 catalysts could not be characterized.