Characterization of vanadia sites in V-silicalite, vanadia-silica cogel, and silica-supported vanadia catalysts: X-ray powder diffraction, Raman spectroscopy, solid-state V-51 NMR, temperature-programmed reduction, and methanol oxidation studies

摘要

The vanadia species in different silica environments (silicalite, cogel, and silica-supported) were characterized by XRD, Raman, solid state V-51 NMR, TPR, and methanol oxidation. Under dehydrated conditions, the dispersed vanadia species in all of the vanadia-silica systems possess an isolated and distorted VO4 coordination with minor differences. The VO4 species in the dehydrated 1% silica-supported vanadia catalyst contains a single terminal V=O bond and changes coordination from VO4 to VO5 or VO6 upon hydration. The VO4 species in the V-silicalite maintains its coordination upon hydration and essentially does not appear to possess a terminal V=O bond. A trace amount of crystalline V2O5 and two types of dispersed VO4 species are present in the 1% vanadia-silica cogel, One of the dispersed VO4 species is a surface vanadia species on silica and changes coordination upon hydration. All of the dispersed vanadia species exhibit similar reducibility and catalytic properties for methanol oxidation because they possess very similar V-O-Si bridging bonds that are the critical functionalities for methanol oxidation. (C) 1998 Academic Press. [References: 56]