The Catalytic Function of Phosphorus Enriched on the Surface of Vanadium-based Catalysts in Selective Oxidations

Vanadium phosphates are established as the benchmark system for the selective oxidation of n-butane towards maleic anhydride. By varying the phosphorus content on the surface of three V-based catalysts with diverse performance, this study experimentally elaborates on the catalytic function of phosphorus. Contact time variation and cofeed studies revealed, that surface phosphates, deposited in sub-monolayers via atomic layer deposition, significantly contribute to an increased product selectivity. Furthermore, our results suggest that the phosphorus particularly suppresses the consecutive combustion of the (re-)adsorbed product. The recently introduced solid solution catalyst V1-xNbxOPO4 with medium maleic anhydride selectivity could be tuned by the surface enrichment with phosphorus towards product selectivities of up to S-MAN= 60%, under optimized alkane-rich feed conditions. Therefore, POx-V0.3Nb0.7OPO4 is introduced as promising catalyst, which is not based on vanadyl(IV) pyrophosphate, to access significantly higher MAN formation rates at increased alkane partial pressures of c(n-butane)> 10%vol in n-butane oxidation.