Vanadium Phosphorus Oxide Catalyst Modified by Niobium Doping for Mild Oxidation of n-Butane to Maleic Anhydride

The interest of doping (VO)2P2O7 with Nb5+ ions has been studied for n-butane oxidation to maleic anhydride. Nb was solubilized as niobium (V) ethoxide into isobutanol and used as a reducing reagent to prepare the VOHPO4·0.5H2O precursor. Under n-butane fuel-lean oxidation conditions, the VNbPO precursor was activated at a temperature 20°C lower than for undoped VPO because of a modification of its morphology. Electron microscopy showed that the activated VNbPO catalyst is more disorganized than the activated VPO. It contains more defects and concentrates Nb at the surface. Nb acts as an n-type dopant for the p-type (VO)2P2O7 semiconductor, as observed in electrical conductivity measurements. 31P NMR by spin-echo mapping and XPS spectroscopy provide evidence that the VNbPO catalyst is more oxidized, particularly at the surface. Doping with Nb creates defects, responsible for C–H n-butane activation, which have been observed to be associated with Lewis acid sites of low acidity. This is the reason for the enhancement of the n-butane conversion. Selectivity to maleic anhydride is not modified by Nb-doping while CO2/CO formation is increased due to the higher surface V5+/V4+ ratio.