Characterization of Vanadia/Silica Mixed Gel Catalysts by 51V and 29Si CP/MAS Solid State NMR Spectroscopy

Abstract

AbstractThe structure of V2O5/SiO2 mixed oxides prepared by sol‐gel techniques is investigated by solid state NMR experiments. For low V2O5 contents ( < 10 mol%), 51V wideline spectra reveal a predominance of tetrahedrally coordinated vanadia, as found in oligomeric vanadate species. With higher vanadia concentrations (15–25 mol%), an increasing fraction of vanadium centers in an octahedral environment is observed. This transition indicates an increasing degree of cross‐linking of the vanadia, to form two‐ and three‐dimensional aggregates. The influence of the vanadia component on the compactness of the network is reflected in the 29Si MAS spectra, where the fraction of completely cross‐linked Q4 sites is observed to increase with the vanadia contents. The 1H → 29Si cross polarization dynamics is characterized by a bi‐exponential time dependence. Evidence for the existence of Si—O—V bridges at vanadia contents of 1 wt.% is derived from an analysis of the relevant cross polarization time constants.