Structural transformation sequence occurring during the activation under n-butane–air of a cobalt-doped vanadium phosphate hemihydrate precursor for mild oxidation to maleic anhydride

The structural transformation of a cobalt-doped vanadium phosphate hemihydrate precursor, when heated in butane–oxygen, has been studied using transmission electron microscopy, XRD, 31P NMR by spin echo mapping and 31P MAS NMR. The cobalt-doped VOHPO4·0.5H2O precursor was prepared by refluxing V2O5 with isobutanol (containing cobalt acetylacetonate) in the presence of H3PO4. The precursor crystals have a characteristic rhomboidal plate-like morphology. A systematic series of ‘activated’ catalysts was then prepared in which the activation time was varied between 0.1 and 132 h. The activated samples retained the original rhomboidal plate-like morphology and were found to comprise mainly disordered vanadium phosphate throughout which the Co was uniformly distributed. Crystalline (VO)2P2O7 formed at the edge of the activated platelets; but, in contrast to the disordered interior, the (VO)2P2O7 crystallites did not contain Co. Selected area diffraction pattern analysis is used to show that, in addition to (VO)2P2O7, the platelets also contain some δ-VOPO4 and non-transformed VOHPO4·0.5H2O. By 31P NMR spin echo mapping and 31P MAS NMR, it is observed that the presence of the Co-dopant inhibits the transformation of the VOHPO4·0.5H2O precursor and stabilises an amorphous vanadium phosphate, for which Co acts as a promoter.