The effect of the phase composition of model VPO catalysts for partial oxidation of n-butane

X-ray diffraction, Raman spectroscopy, ³¹P MAS-NMR and spin-echo NMR indicated that model vanadium phosphorus oxide (VPO) precursors and catalysts contained various minor phases depending on both the synthetic approach and P/V ratios used. Raman spectroscopy revealed the presence 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 the performance of the VPO catalysts for n-butane oxidation. The best model organic VPO catalyst contained only vanadyl pyrophosphate with the highest degree of stacking order and virtually no VOPO₄ phase impurity. Raman spectroscopy detected vanadyl metaphosphate, VO(PO₃)₂, in the catalysts derived from aqueous precursors possessing P/V ratios greater than 1. Pure vanadyl metaphosphate catalyst was inactive in n-butane oxidation. ³¹P NMR demonstrated the absence of vanadyl metaphosphate and other impurity phases in the best catalyst derived from organic precursors at P/V = 1.18. The experimental data strongly indicate that the best VPO catalysts for n-butane oxidation contain only vanadyl pyrophosphate with well-ordered stacking of the (200) planes.