Structure–activity relationships for Co- and Fe-promoted vanadium phosphorus oxide catalysts

Abstract

The effect of Co and Fe doping on vanadium phosphate catalysts, prepared by the reaction of V₂O₅ and H₃PO₄ with isobutanol, for the oxidation of n-butane to maleic anhydride is described and discussed. At low levels, both Co and Fe dopants significantly enhance the selectivity and the intrinsic activity to maleic anhydride. A combination of powder X-ray diffraction, ³¹P NMR spin-echo mapping spectroscopy and transmission electron microscopy, together with catalyst test data, is utilised to analyse the origin of the effects of Co doping. Co appears to be essentially insoluble in crystalline (VO)₂P₂O₇ and is preferentially distributed in and stabilises an amorphous VPO material. It is suggested that the origin of the promotional effect of Co is associated with its interaction with the disordered VPO phase. The same techniques have been used to analyse the Fe-doped catalyst, but at present it is not possible to be definitive concerning the specific location of the Fe-dopant within the phases present. Previous studies have indicated that Fe can form a solid solution within (VO)₂P₂O₇ and therefore it is probable that the Fe may act as an electronic promoter for this phase. The role of Co, however, emphasises the importance of amorphous vanadium phosphate phases in the catalyst system.