OXIDATION OF TOLUENE OVER V2O5/NB2O5 CATALYSTS

Nb2O5 supported vanadium oxide catalysts of theoretical monolayer coverages from 0.05 up to 5 were prepared by impregnation with vanadyl acetylacetonate in ethanol. Nb2O5 of the high temperature form [niobia(2)] and a less well-defined Nb2O5 [niobia(1)] obtained by calcining the hydrated form, were used. Phase and surface composition were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies. XPS spectra showed that vanadium was present at V5+ on all fresh samples. At very low loading, V4+ was also detected. The V/Nb atom.ratio showed considerable agglomeration of vanadia to occur at loadings above approximately one monolayer. The dispersion of the vanadia phase appears to be higher over niobia(2) than over niobia(1). Crystalline V2O5 could be detected by XRD at the highest loading over niobia (1 ). The catalytic activity for oxidation of toluene could, with niobia(2) as support, be correlated to the V/Nb atom ratio measured by XPS. Thus, the activity was roughly proportional to the surface area of exposed vanadium oxide species. Benzaldehyde selectivity was almost constant for these catalysts. Niobia (1) as support behaved differently with a large increase in activity and a decrease in benzaldehyde selectivity at loadings above approximately one monolayer, without a corresponding increase in the V/Nb atom ratio. A lower activity of niobia bonded vanadium species than vanadia bonded ones was suggested.