n-butane oxidation to maleic anhydride: effect of Co and Fe addition by the method of incipient wetness on vanadium phosphate catalysts prepared by the aqueous HC1 method

The effect of the addition of Co²⁺ and Fe³⁺ on the catalytic performance and structure of vanadium phosphate catalysts is described and discussed. The catalyst precursor VOHPO₄.0.5H₂O was prepared using the aqueous HC1 route and Co²⁺ and Fe³⁺ were incorporated using the incipient wetness method. Addition of Co²⁺ improves the selectivity to maleic anhydride, but is found to decrease the specific activity for the formation of maleic anhydride, whereas the addition of Fe³⁺ increases the specific activity up to a maximum level at ca. 2 mol% Fe. The effects are not due to changes in the specific surface area of the final catalyst, which remains 6.5 ±0.5 m² g^–1 for all the catalysts studied. Part of the effect is due to the formation of VOPO₄ phases in the catalyst precursor due to the method of addition of the Co²⁺ or Fe³⁺ and this is both related to the effect of the oxidation potential of the additive cation and to the pH of the impregnating solution. It is apparent that the use of the incipient wetness method for the addition of promoter compounds should be used with care for the VPO system at variance with the direct incorporation of the promoter during the preparation of the VOHPO₄.0.5H₂O precursor.