ACIDITY AND CATALYTIC BEHAVIOR OF VANADIUM PHOSPHORUS OXYGEN CATALYSTS

The surface acidity of equilibrated and non-equilibrated vanadium-phos phorus oxide (VPO) catalysts was studied using basic probe molecules ( ammonia, pyridine and acetonitrile) via Fourier transform-IR spectrosc opy. The stronger bases detected the presence, in varying proportions, of both Lewis and Bronsted acid centers on all the solids assayed. Al l the catalysts used exhibited the X-ray diffraction pattern of vanady l pyrophosphate and much higher Lewis/Bronsted ratios than the beta-VO PO4 compound. This indicates that the Lewis acidity observed on the us ed catalysts is not due to the presence of small amounts of V(V) speci es. The Lewis/Bronsted ratio almost invariably decreased with temperat ure, suggesting that the Bronsted sites are stronger than the Lewis ce nters. Acetonitrile served to differentiate Lewis centers of varying s trength. The equilibrated catalysts which exhibited a better crystalli zed pyrophosphate phase showed an increase in very strong Lewis acidit y whereas the stacking fold disordered non-equilibrated catalyst showe d the presence of both medium strong and very strong Lewis centers. Th e oxidation of n-butane to maleic anhydride was studied using a plug f low reactor. Both the yield of maleic anhydride and the proportion of very strong Lewis sites increased with time-on-stream. These findings together with previous IR studies reported in the literature allow a b etter understanding of the relationship among structural features, aci dity and catalytic behavior of VPO formulations.