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

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-but ane to maleic anhydride is described and discussed. At low levels, both Co and Fe dopants significantly enhance the selectivity and the intrinsic acti vity to maleic anhydride. A combination of powder X-ray diffraction, P-31 N MR spin-echo mapping spectroscopy and transmission electron microscopy, tog ether with catalyst test data, is utilised to analyse the origin of the eff ects of Co doping. Co appears to be essentially insoluble in crystalline (V O)(2)P2O7 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 sa me techniques have been used to analyse the Fe-doped catalyst, but at prese nt it is not possible to be definitive concerning the specific location of the Fe-dopant within the phases present. Previous studies have indicated th at Fe can form a solid solution within (VO)(2)P2O7 and therefore it is prob able 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.