Gallium-doped VPO catalysts for the oxidation of n-butane to maleic anhydride

Vanadium phosphate (VPO) catalyst materials have been doped with gallium and subsequently tested for the mild oxidation of n-butane to maleic anhydride. At low Ga concentrations, this impurity is shown to be beneficial for n-butane conversion. For low dopant levels (Ga/V ⩽ 1 at%), the crystallinity of the hemihydrate (VOHPO4·0.5H2O) precursor phase is improved and its specific area is increased relative to the undoped material as a result of decreased platelet thickness. Electron diffraction and energy dispersive X-ray analysis (XEDS) revealed that the Ga is uniformly distributed in a substitutional manner throughout the hemihydrate structure. The presence of Ga also significantly shortens the activation time required to convert the hemihydrate precursor into a well crystallized vanadyl pyrophosphate (VO)2P2O7 phase under an n-butane/air gas flow at 400 °C. The intimate presence of Ga distributed within the VOHPO4·0.5H2O unit cell has also been confirmed by XANES and EXAFS. Such studies also show that the Ga is partially redistributed within the (VO)2P2O7 structure after catalyst activation. A complementary electrical conductivity study on these materials revealed that Ga3+ substitutes for (VO)2+ species in (VO)2P2O7 giving rise to an n-type semiconductivity which partially compensates the natural p-type conductivity character of the (VO)2P2O7 phase. For higher Ga doping levels (Ga/V ≈ 5 at%), the excess of Ga concentrates as a GaPO4 impurity phase, which is shown to have a detrimental effect on the catalytic performance of the Ga-doped VPO catalyst.