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

Abstract

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.