Selective oxidation of toluene over V2O5/TiO2 catalysts. Effect of vanadium loading and of molybdenum addition on the catalytic properties Original Research Article

The influence of vanadium loading and molybdenum presence on the catalytic performance of vanadia-titania (anatase) catalysts for the selective oxidation of toluene was investigated. Two series of V2O5/TiO2 and V2O5-MoO3/TiO2 catalysts were prepared. In the first series the loading of vanadium varied from 0 to 8 mole %, whereas in the second the atomic ratio V/(V + Mo) varied from 0 to 1 while the total loading of active elements (V + Mo) was kept constant and equal to 8 mole %. The samples were characterized by XPS, TPR, XRD and BET measurements. It was found that the activity for the oxidation of toluene and the selectivity for side chain partial oxidation products (benzaldehyde and benzoic acid) exhibited by V2O5/TiO2 catalysts increased with the vanadium loading up to monolayer coverage. This increase was attributed to the parallel increase of the surface concentration of easily reducible isolated vanadium species interacting with the anatase surface. When the vanadium content increased above monolayer coverage both activity and selectivity decreased. V2O5-MoO3/TiO2 catalysts were found to be less active and selective than the corresponding V2O5/TiO2 ones. Molybdenum species supported on anatase were less active for this reaction than vanadium and, in addition, the presence of molybdenum inhibited the interaction between vanadium and anatase leading to a poor vanadium dispersion.