AMMOXIDATION OF TOLUENE OVER TIO2(B)-SUPPORTED VANADIUM-OXIDE CATALYSTS

TiO2(B) was used as a support for vanadium oxide with loadings in the range 1 4 to 10 theoretical layers. The vanadium could be separated into two parts, which were soluble and insoluble in NH3(aq), respectively. Insoluble vanadium, in an amount corresponding to a complete monolayer at high loadings, was present as V4+ in freshly prepared catalysts. Diffuse reflectance FTIR spectroscopy was used to characterize the catalysts. The bands assigned to stretching vibrations of hydroxyl groups on TiO2(B) were found to decrease in intensity with increasing vanadium loading. Spectra in the VO and V-O-V vibration region revealed a rather complex growth pattern. For a catalyst covered with a complete monolayer, vanadium strongly interacting with the support is present as interconnected tetrahedral V4+ units. When the loading is increased, several additional layers of V5+ in tetrahedral coordination can exist on top of the monolayer. At higher loadings, there is an abrupt change in structure, resulting in the formation of an amorphous phase built up from distorted VO6 octahedra. Also, crystalline V2O5 is formed. The catalysts were used in the ammoxidation of toluene to benzonitrile. Multilayer catalysts showed better catalytic performance in comparison with crystalline V2O5 and catalysts exposing monolayer species. © 1990.