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 51 V 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 % V 2 O 5 ) of vanadium can be incorporated into the silica matrix while homogeneity is still maintained. Use of nonreducing acids such as HNO 3 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.

Ionic liquids as dynamic templating agents for sol–gel silica systems: synergistic anion and cation effect on the silica structured growth

“…This entire process is highly dependent on several parameters like temperature, solvent, pH [11,12], presence of ions [11], air moisture [13], drying process and aging time [14]. The impact caused by changes in these parameters on the competing hydrolysis and condensation reactions, and as a consequence on the final products, is still not fully understood.…”

Vanadia?silica and vanadia?cesium?silica catalysts for oxidation of SO2

“…The use of sol-gel procedures received particular interest because of the characteristics reported for the materials obtained by this route. Due to large differences in the reaction rates between the metal alcoxides and the silicon alcoxides it is generally considered as being difficult to achieve homogeneity in mixed metal oxides using this technique [36]. However, studies made by Baiker and co-workers.…”

Platinum supported on doped alumina catalysts for propulsion applications. Xerogels versus aerogels

“…For the aerogel samples, the incorporation of silicon into the alumina network was demonstrated in a previous paper [10], leading to a more homogeneous material with better resistance to sintering and transformation into a-alumina. Homogeneous multicomponent systems are known, e.g., tetrahedrally coordinated silicon atoms are substituted by vanadium atoms (with reduction from V V to V IV ) in vanadia-silica sol-gel-derived materials prepared by co-condensation of the respective alkoxides, [18][19][20][21]. The similar coordination environments favor the formation of a homogeneous xerogel, which is not the case for the Si-doped boehmite sol-gel in which Si is in a tetrahedral environment and Al an octahedral one.…”