Quantitative solid-state NMR investigation of V5+ species in VPO catalysts upon sequential selective oxidation of n-butane

Bulk VPO catalysts (VPO/bulk) and VPO catalysts supported on mesoporous SBA-15 material (VPO/SBA-15) were treated sequentially in a flow of nitrogen loaded with n-butane and of synthetic air loaded with n-butane in order to reach a complete reduction and reoxidation, respectively, of the catalyst surface. The V5+ species on these VPO catalysts were quantitatively investigated by solid-state NMR spectroscopy and correlated with the catalytic activities of the corresponding materials in the selective oxidation of n-butane to maleic anhydride. By 31P{1H} cross-polarization MAS NMR experiments on the n-butane-loaded VPO/SBA-15 catalyst was evidenced that n-butane is preferentially adsorbed at surface sites of δ-VOPO4-like phases. Based on the number of P/V5+ species in δ-VOPO4-like phases as determined by MAS NMR spectroscopy, the turnover frequency of the surface sites on the VPO/SBA-15 catalyst was estimated to be about 0.2 s−1.