The Mechanism and Pathway of Selective Partial Oxidation of n-Butane to Maleic Anhydride Studied on Titanium Phosphate Catalysts

Abstract

The partial selective oxidation of n-butane to maleic anhydride with molecular oxygen is commercially well-established and strongly associated with the vanadium phosphorus oxide (VPO) catalyst. We report that also titanium phosphate (TiPO) exhibits the rare feature of accomplishing the most demanding complex selective oxidation reaction industrially applied. A facile molten salt method was used to prepare TiPO catalysts from mixtures of (NH4)2HPO4 and TiO2 (P25). In a continuous flow process under industrially relevant conditions with TiPO, conversions above 50% resulted in 20% overall selectivity for maleic anhydride with 90% oxygenate selectivity. Due to a high tendency to total oxidation (>60%), the performance of TiPO catalysts cannot yet compete with the industrial VPO catalyst. However, herein we want to highlight our studies on the reaction pathway and mechanism for the complex multistep conversion of n-butane to maleic anhydride, which is still under debate for the VPO catalyst after more than four decades of research. A complete chain of reaction intermediates was identified via online mass spectroscopy, under industrially relevant conditions, and in pulse experiments, including the consecutive formation of 2-butene, 1,3-butadiene, furan, and 2-furanone as C4 intermediates. Cyclic pulse experiments complemented with EPR measurements revealed a combined mechanism involving carbocation chemistry via Brønsted acid sites and a redox mechanism according to Mars van Krevelen.