Vanadia−Silica Low-Temperature Aerogels: Influence of Aging and Vanadia Loading on Structural and Chemical Properties

Abstract

Vanadia−silica mixed oxides were prepared via the sol−gel method involving acid catalysis together with prehydrolysis in order to achieve matching of the reactivities of vanadium(V) oxide triisopropoxide and tetraethoxysilicon(IV) precursors. Gelation was forced by the addition of basic solution. The as-received gels were supercritically dried by semicontinuous extraction with supercritical CO2 at 313 K (low-temperature aerogels). The effects of composition, aging, and calcination temperature on the chemical, structural, and textural properties of the solids were investigated. The oxides were characterized by N2 physisorption, XRD, vibrational spectroscopy, thermal analysis, UV−vis, and 51V NMR. The low-temperature vanadia−silica aerogels were mesoporous and highly disperse. The increasing V content from 5 to 20 wt % nominal V2O5 caused a gradual decline in V dispersion. For 30 wt % “V2O5” the continuous formation of V−O−V connectivity resulted in crystallization of V2O5. The effect of aging in basic medium confined to the textural properties, significantly increasing BET surface area and especially pore volume. The prepared aerogels revealed a marked lack of stability against both apolar solvents in the presence of peroxides and polar solvents. The marked thermal stability in air at ≤873 K, however, combined with mesoporosity and high V dispersion, render these solids promising catalysts for gas-phase reactions.