Studies on crystalline microporous vanadium silicates: II. FTIR, NMR, and ESR spectroscopy and catalytic oxidation of alkylaromatics over VS-2

Abstract

Three crystalline, microporous, vanadium silicates. VS-2 (Si/V = 79, 122, and 161, respectively) with MEL structure have been synthesized. The spectroscopic (IR, NMR, and ESR) features and catalytic properties (in the oxidation of toluene and xylenes by H2O2) have been elucidated. IR spectra of the adsorbed ammonia and pyridine reveal the presence of both weak Brønsted and Lewis acid sites. There is a direct correlation between the concentration of hydrogen-bonded hydroxyl groups and the amount of vanadium incorporated in the material. In carefully prepared samples of VS-2, the 51V MAS-NMR studies reveal the presence of only a single vanadium species with a chemical shift parameter (−573 ppm) and a signal linewidth (50 ppm) similar to those observed in monomeric orthovanadates having tetrahedrally coordinated vanadium ions. Clusters of vanadium as well as V2O5-like phases are absent. A linear increase in the intensity of the ESR peaks with both the vanadium content and unit cell volume of the MEL phase suggests that the V4+ ions are associated, in a manner yet undefined, with the MEL lattice. VS-2 oxidizes toluene to a mixture of cresols, benzyl alcohol, and benzaldehyde. The difference in the rates of oxidation of xylene isomers conhrms that most of the V ions are located inside the channel system of MEL