Characterization of Vanadia Sites in V-Silicalite, Vanadia-Silica Cogel, and Silica-Supported Vanadia Catalysts: X-Ray Powder Diffraction, Raman Spectroscopy, Solid-State51V NMR, Temperature-Programmed Reduction, and Methanol Oxidation Studies

Abstract

The vanadia species in different silica environments (silicalite, cogel, and silica-supported) were characterized by XRD, Raman, solid state51V NMR, TPR, and methanol oxidation. Under dehydrated conditions, the dispersed vanadia species in all of the vanadia-silica systems possess an isolated and distorted VO4coordination with minor differences. The VO4species in the dehydrated 1% silica-supported vanadia catalyst contains a single terminal V=O bond and changes coordination from VO4to VO5or VO6upon hydration. The VO4species 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 V2O5and two types of dispersed VO4species are present in the 1% vanadia-silica cogel. One of the dispersed VO4species 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.