Electrical properties of doped vanadium phosphate phases and VPO catalysts used in the partial oxidation of n-butane to maleic anhydride

Abstract

Vanadium phosphate phases and VPO catalysts, obtained by activation of the precursor VOHPO₄· 0.5 H₂O in the reaction mixture used for the selective oxidation of n-butane to maleic anhydride, have been prepared with various contents of two promoters (Fe and Co).

An electrical conductivity technique was used to determine if these promoters could be considered as dopants, i.e. if they are dissolved in the lattice of the matrix. Vanadium phosphate catalysts that are active for the mild oxidation of n-butane to maleic anhydride exist as a (VO)₂P₂O₇ matrix supporting a δ-VOPO₄ phase which decreases with the time of activation in the reaction mixture. The effect of the promoters on these two phases was investigated. It was found that both Fe³⁺ and Co³⁺ are dissolved in (VO)₂P₂O₇ by substitution for vanadyl ions (VO)²⁺, whereas they were scarcely dissolved in δ-VOPO₄. An optimum atomic ratio Fe/V and Co/V was found to be 2% for the solid solution [(VO)_1–xM_x]₂P₂O₇.