PHYSICOCHEMICAL CHARACTERIZATION AND CATALYTIC BEHAVIOR OF VPO FORMULATIONS

The effect of the preparation method of VPO catalysts, with varying phosphorus/vanadium ratios, on the solid composition and structure was studied using electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Their catalytic performance was evaluated using a flow reactor system. In the organic preparations varying degrees of structural disorder in the stacking fold of the precursor were observed. This disorder persisted in non-equilibrated solids but disappeared after longer time on stream. The water based preparations did not show this type of disorder. All the formulations showed surface enrichment of phosphorus. The butane conversion decreased with increasing (P/V)s ratios while the selectivity to maleic anhydride did not following such a simple pattern. These results are discussed in terms of the current literature. It is suggested that the production of anion vacancies during preparation are related to the origin of the active sites while the stacking fold defects are only secondary, visible signs of more profound changes in the solid.