Synthetic Control over the Production of Multicomponent Sol−Gel Materials: Fabrication of Homogeneous Vanadia−Silica Xerogels with High Vanadium Content

Abstract

The effect of Brønsted acid catalysts on the gelation times and final homogeneity of vanadia−silica sol−gel derived glass is reported. It was found that the addition of hydrochloric acid reduced the gelation times, especially at high acid concentrations. This acid also caused slow reduction of the V(V) to V(IV) during the process. Drying of the V(IV)-containing xerogels at 500 °C resulted in their reoxidation to V(V). Characterization of the reoxidized gels by Raman and 51V solid-state NMR spectroscopies verified that the structure of the vanadium(V) sites that were created from the reoxidation of the V(IV) sites were of pseudotetrahedral geometry with a short terminal oxo group and three Si−O−V linkages. This is the same site produced from direct incorporation of oxovanadium(V) into the gel without reduction occurring. A benefit is realized in this reduction−reoxidation process in that relatively high loadings (12.3 mol % or 17 wt % V2O5) of vanadium can be incorporated into the silica matrix while homogeneity is still maintained. Use of nonreducing acids such as HNO3 resulted in no particular gains in vanadium loading, and in fact, materials produced with this acid were similar to those made with no added acid. The transition between homogeneous and heterogeneous materials is also discussed.