Effect of compositions of promoted VPO catalysts on the selective oxidation of n-butane to maleic anhydride

A series of vanadium phosphate (VPO) catalysts with different P/V ratios and with or without indium and TEOS additives have been characterized by controlled-environment XRD, ICP, ESCA, TEM, SEM, BET and chemical titration and used for n-butane oxidation to maleic anhydride. The best catalyst contains slightly excess P over V with indium and TEOS promoters. It was found that excess P increases the resistance of catalyst precursors toward oxidation and results in VPO catalysts with a large exposed platelet face of a layer morphology. The promoters reduce the thickness of the platelet and facilitate the oxidation of the precursor which contains disordered VOHPO<inf>4</inf> · 0.5H<inf>2</inf>O. The combination of the promoters and excess P results in a VPO catalyst with appropriate oxidizability and morphology and gives high yields of maleic anhydride.