SELECTIVE OXIDATION OF TOLUENE OVER V2O5 TIO2 CATALYSTS - EFFECT OF VANADIUM LOADING AND OF MOLYBDENUM ADDITION ON THE CATALYTIC PROPERTIES/

The influence of vanadium loading and molybdenum presence on the catal ytic performance of vanadia-titania (anatase) catalysts for the select ive oxidation of toluene was investigated, Two series of V2O5/TiO2 and V2O5-MoO3/TiO2 catalysts were prepared. In the first series the loadi ng of vanadium varied from 0 to 8 mole %, whereas in the second the at omic ratio V/(V+Mo) varied from 0 to 1 while the total loading of acti ve elements (V+Mo) was kept constant and equal to 8 mole %. The sample s were characterized by XPS, TPR, XRD and BET measurements. It was fou nd that the activity for the oxidation of toluene and the selectivity for side chain partial oxidation products (benzaldehyde and benzoic ac id) exhibited by V2O5/TiO2 catalysts increased with the vanadium loadi ng up to monolayer coverage. This increase was attributed to the paral lel increase of the surface concentration of easily reducible isolated vanadium species interacting with the anatase surface. When the vanad ium content increased above monolayer coverage both activity and selec tivity decreased. V2O5-MoO3/TiO2 catalysts were found to be less activ e and selective than the corresponding V2O5/TiO2 ones. Molybdenum spec ies supported on anatase were less active for this reaction than vanad ium and, in addition, the presence of molybdenum inhibited the interac tion between vanadium and anatase leading to a poor vanadium dispersio n.