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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.

The chemical and processing conditions under which multicomponent vanadia−silica sol−gel derived glass is synthesized have been investigated to determine the factors leading to homogeneity in the final material. In conjunction, studies were also conducted to assess changes in the morphological properties (porosity) of the silica xerogel due to the presence of vanadium. It was found that both the water content of the initial sol and the humidity under which aging of the gel was carried out dramatically affected the homogeneity of the final material. High initial water content or high humidity aging conditions resulted in the formation of green gels containing partially reduced vanadia, which, upon drying at 500 °C, yielded opaque orange xerogels. Vanadia−silica gels made with low water and aged at low humidity remained transparent after drying. The amount of vanadium that could be incorporated while still maintaining homogeneity was increased significantly if low water and low humidity conditions were used in the process. The presence of the vanadium, even in low concentrations, dramatically affected the pore structure of the resultant xerogel. Materials with vanadium concentrations as low as 0.01 mol % were found to be significantly more microporous than pure silica control samples. Imaging by atomic force microscopy revealed that the increased microporosity was due to filling of the mesopore regions in the materials containing vanadium.

The transition metal alkoxides Ti(OPr i ) 4 and OV(OPr i ) 3 are shown to catalyze the transesterification of tetramethyl-and tetraethyl orthosilicates: Si(OR) 4 + R′OH T Si(OR) 3 -(OR′) + ROH. A detailed study of metal-catalyzed transesterification reactions and the factors that affect their efficiency and selectivity are presented. 29 Si NMR techniques were used to quantify the extent of transesterification of the two different silicon alkoxides under a variety of conditions while mechanistic aspects of the reaction were probed using 51 V NMR. It was found that tetramethyl and tetraethyl orthosilicates, when reacted with smaller, less sterically demanding alcohols, are substituted to a similar extent by both metals. However, transesterification of bulkier alcohols (2-propanol) revealed dramatic differences in their reactivity, with titanium being significantly more efficient than vanadium. The catalytic process and the differences in reactivity between the two metals are explained in terms of a mechanism involving ligand migration between the metal and the silicon. A comparison of the catalytic activity of these Lewis acids with a strong Brønsted acid (triflic acid) showed the latter to have far greater catalytic activity.