THE EFFECT OF THE PHASE-COMPOSITION OF MODEL VPO CATALYSTS FOR PARTIAL OXIDATION OF N-BUTANE

X-ray diffraction, Raman spectroscopy, P-31 MAS-NMR and spin-echo NMR indicated that model vanadium phosphorus oxide (VPO) precursors and ca talysts contained various minor phases depending on both the synthetic approach and P/V ratios used. Raman spectroscopy revealed the presenc e of a number of micro-crystalline and amorphous V(IV) and V(V) phases not evident by XRD. The presence of VOPO4 phases was detrimental to t he performance of the VPO catalysts for n-butane oxidation. The best m odel organic VPO catalyst contained only vanadyl pyrophosphate with th e highest degree of stacking order and virtually no VOPO, phase impuri ty. Raman spectroscopy detected vanadyl metaphosphate, VO(PO3)(2), in the catalysts derived from aqueous precursors possessing P/V ratios gr eater than 1. Pure vanadyl metaphosphate catalyst was inactive in n-bu tane oxidation. P-31 NMR demonstrated the absence of vanadyl metaphosp hate and other impurity phases in the best catalyst derived from organ ic precursors at P/V = 1.18. The experimental data strongly indicate t hat the best VPO catalysts for n-butane oxidation contain only vanadyl pyrophosphate with well-ordered stacking of the (200) planes.