Influence of the oxidation state of vanadium on the reactivity of V/P/O, catalyst for the oxidation of n-pentane to maleic and phthalic anhydrides

In this paper the role of V5+ and V3+ ions in V/P/O, catalyst for the oxidation of n-pentane to maleic and phthalic anhydrides, has been investigated. These species have transient lifetime, since the stable, fully equilibrated catalyst only contains vanadyl pyrophosphate, (VO)2P2O7, and V4+ ions. Therefore, first the formation of V5+ and V3+ species has been induced in catalysts, either by means of a suitable oxidative treatment on a equilibrated V/P/O catalyst, to generate controlled amounts of V5+, or by adopting a preparation procedure which leads to the formation of discrete amounts of V3+ in fresh catalysts. Thereafter, the evolution of catalytic performance was followed along with time-on-stream. A relationship was found between the distribution of products, and the oxidation state of V in catalysts. Higher concentrations of V5+ led to the preferred formation of maleic anhydride, while samples having average V valence state close to 4.00+ and well-crystallized vanadyl pyrophosphate were the most selective to phthalic anhydride. The latter samples were also characterized by the higher Lewis-type acid strength. The reduced catalyst, containing a large amount of V3+ ions, was instead non selective towards either of phthalic and maleic anhydrides; the progressive catalyst oxidation in the reaction environment led to the increase in the yield to both products.

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