Supported VPO catalysts for selective oxidation of butane. II. Characterization of VPO/SiO₂ catalysts

SiO₂-supported VPO catalysts with different P/V ratios were prepared, used in selective oxidation of butane, and characterized by X-ray diffraction (XRD), laser Raman spectroscopy (LRS), near-edge X-ray absorption spectroscopy (XANES), and ³¹P magic angle spinning NMR. Similar to the SiO₂-supported catalysts of lower metal loadings studied previously, the maleic anhydride selectivity increased with increasing P/V ratio, to greater than 50 mol % for P/V = 2. XRD and LRS detected predominantly α₁-VOPO₄ or a phase that is structurally very similar for both P/V = 1 and P/V = 2 samples. However, within the detection limit of the XANES experiment, the P/V = 1 catalyst contained mostly V⁵⁺ under reaction conditions, whereas the P/V = 2 catalyst contained 100% V⁴⁺.