Acidity and Catalytic Behavior of Vanadium Phosphorus Oxygen Catalysts

LAURA CORNAGLIA, EDUARDO LOMBARDO; JAMES ANDERSON, JOSÉ LUIS GARCÍA FIERRO

APPLIED CATALYSIS A-GENERAL

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1993 vol. 100 p. 37 - 50

0926-860X

The surface acidity of equilibrated and non-equilibrated vanadium-phosphorus oxide (VPO) catalysts was studied using basic probe molecules (ammonia, pyridine and acetonitrile) via Fourier transform- IR spectroscopy. The stronger bases detected the presence, in varying proportions, of both Lewis and Brensted acid centers on all the solids assayed. All the catalysts used exhibited the X-ray diffraction pattern of vanadyl pyrophosphate and much higher Lewis/Brensted ratios than the @VOPO, compound. This indicates that the Lewis acidity observed on the used catalysts is not due to the presence of small amounts of Vv species. The Lewis/Brensted ratio almost invariably decreased with temperature, suggesting that the Brensted sites are stronger than the Lewis centers. Acetonitrile served to  differentiate Lewis centers of varying strength. The equilibrated catalysts which exhibited a better crystallized pyrophosphate phase showed an increase in very strong Lewis acidity whereas the stacking fold disordered non-equilibrated catalyst showed the presence of both medium strong and very strong Lewis centers. The oxidation of n-butane to maleic anhydride was studied using a plug flow 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 better understanding of the relationship among structural features, acidity and catalytic behavior of VP0 formulations.