Synergistic Catalysis by Lewis Acid and Base Sites on ZrO2 for Meerwein-Ponndorf-Verley Reduction

摘要

Early transition metal oxides, TiO2, ZrO2, and Nb2O5, were studied as heterogeneous catalysts for the Meerwein-Ponndorf-Verley (MPV) reduction of cyclohexanone in 2-propanol. Despite a small amount of Lewis acid sites and weak Lewis acid strength, ZrO2 was clearly superior to TiO2 and Nb2O5 with respect to reaction rate. Fourier transform infrared spectroscopy (FT-IR) and temperature-programmed-desorption (TPD) measurements revealed that ZrO2 has large amounts of base sites that activate the methylene groups in 2-propanol bonded to Lewis acid sites. Various analyses, including experiments using isotopic 2-propanol, suggest that efficient MPV reduction over ZrO2 is due not only to Lewis acid strength and density, but also to a synergistic effect of base and Lewis acid sites.