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

The vanadia species in different silica environments (silicalite, coge l, and silica-supported) were characterized by XRD, Raman, solid state V-51 NMR, TPR, and methanol oxidation. Under dehydrated conditions, t he dispersed vanadia species in all of the vanadia-silica systems poss ess an isolated and distorted VO4 coordination with minor differences. The VO4 species in the dehydrated 1% silica-supported vanadia catalys t contains a single terminal V=O bond and changes coordination from VO 4 to VO5 or VO6 upon hydration. The VO4 species in the V-silicalite ma intains its coordination upon hydration and essentially does not appea r to possess a terminal V=O bond. A trace amount of crystalline V2O5 a nd two types of dispersed VO4 species are present in the 1% vanadia-si lica cogel, One of the dispersed VO4 species is a surface vanadia spec ies on silica and changes coordination upon hydration. All of the disp ersed vanadia species exhibit similar reducibility and catalytic prope rties for methanol oxidation because they possess very similar V-O-Si bridging bonds that are the critical functionalities for methanol oxid ation. (C) 1998 Academic Press.