Influence of fine structural characteristics of VPO catalysts on the formation of maleic and phthalic anhydrides in the oxidation of n-pentane

We report on the influence of the stages of preparation of a vanadium phosphate on the selectivity to phthalic anhydride (PA) and maleic anhydride (MA) in n-pentane oxidation. The attention was mainly focused on the extent of structural defects observed in precursors and catalysts. Vanadium phosphate catalysts were obtained from precursors prepared by a two-step synthesis. In the first step VOPO4-mixed isobutanol-water intercalates, with the amount of isobutanol per VOPO4 molecule varying from 1.6 to 0.05, were prepared by precipitation from a solution containing vanadyl isobutoxide and H3PO4 and a carefully adjusted water content. In the second step the precursors were formed by reflux using two different procedures: (i) in an inert medium (n-octane) or (ii) in a reductive medium (isobutanol). Catalysts were obtained by treating the precursors under the reaction conditions for about 40 h. By such procedures VPO precursors and catalysts with bulk P/V atomic ratio equal to 1.05 and displaying widely different structural defects were obtained. Precursors and catalysts were characterised by elementary chemical analysis, carbon analysis, oxidation state of vanadium, BET, XRD, FTIR, and XPS. Long range and short range orders were considered. Results show that the parallel routes of the n-pentane oxidation into MA and PA require different structural features of the catalyst. The formation of phthalic anhydride demands an ordered structure while maleic anhydride could be formed on a highly defective VPO catalyst. It is suggested that this high structural order for PA formation is necessary to create the complex active structure to provide the concerted process of PA formation. (C) 1999 Academic Press.