Stable co-production of olefins and aromatics from ethane over Co2+-exchanged HZSM-5 zeolite

Abstract

Catalytic conversion of ethane into olefins and aromatics is an effective approach for the utilization of low-cost light paraffins. In this study, with the ion-exchange method, Co-exchanged HZSM-5 catalysts were prepared in which Co²⁺ ions were stably anchored at exchangeable sites, i.e., the Brønsted acid sites (–Al_F–O(H)–Si_F–), to form isolated extra-framework Co(II) species (–Al_F–O–Co(II)–O–Al_F–) in the zeolite. In comparison to the CoO_x clusters mainly formed in an impregnated catalyst, the isolated Co(II) species can withstand the reduction by H₂ even at a high temperature, and are only active for C–H bond dissociation but almost inactive for the C–C bond scission, ensuring a high selectivity to the primary ethylene product without serious coke and CH₄ formation. Further with appropriate acidity and right pore size of the employed HZSM-5 zeolite, the formed olefins can be partly converted into aromatics with a total selectivity to olefins and aromatics up to 83% under reaction conditions of 600 °C, 0.1 MPa, 3000 mL g⁻¹ h⁻¹ and 90%C₂H₆/10%N₂. Fortunately, at a moderate temperature, the Co(II) species did not suffer a significant de-anchoring from the zeolite framework to form sintered Co⁰ clusters, ensuring good catalyst stability.