Amorphous phosphorus oxynitride as a robust catalyst for steam-free direct dehydrogenation of ethylbenzene to styrene: effect of calcination temperature

Abstract

A metal-free catalyst of phosphorus oxynitride (simplified as PNO) for direct dehydrogenation (DDH) of ethylbenzene was synthesized via calcinating hexachlorocyclotriphosphazene (HCCP) under an ammonia–oxygen atmosphere. The XRD, SEM and TEM characterizations results showed that calcination temperature had a profound effect on the crystal type, morphology and composition of PNO. And the PNO catalyst calcined at 800 °C displayed an amorphous P₃N₅ structure with a tubular shape decorated with abundant holes and delivered both the highest ethylbenzene conversion of 52.86% and the highest styrene selectivity of 98.24%. The XPS, FT-IR, ³¹P MAS NMR results and the control experiments revealed that both the PN₄ units in the PNO framework and the surface N₂PO/N₃P–OH groups acted as the active sites for DDH of ethylbenzene. Moreover, the outstanding performance of PNO-800 can be ascribed to the synergistic effect of its amorphous P₃N₅ structure, the suitable oxygen content, and the appropriate ratio of N₂PO and N₃P–OH groups. Additionally, when PNO-800 was in situ grown on the surface of commercial alumina, it showed a more satisfying performance with ethylbenzene conversion of 94.07% and styrene selectivity of 97.92%.