CO2-promoted ethylbenzene dehydrogenation catalyzed by zeolite-encaged single chromium sites

Abstract

The selective activation of C-H bonds is pivotal in catalysis for converting hydrocarbons into value-added chemicals. Ethylbenzene dehydrogenation to styrene is crucial process to produce polystyrene and its derivatives used in synthetic materials. Herein, K-Cr@Y with zeolite-encaged isolated O=Cr(VI)=O species modified by extraframework potassium ions is constructed, showing remarkable performance in CO₂-promoted ethylbenzene dehydrogenation with initial ethylbenzene conversion of 66% and styrene selectivity of 96%, outperforming other M-Cr@Y catalysts (M = Li, Na, Rb, Cs). Extraframework potassium ions can modulate the electron density of zeolite-encaged Cr(VI) species and therefore facilitate C–H bond activation in ethylbenzene molecules. The gradual reduction of zeolite-encaged O=Cr(VI)=O to less active Cr(IV)=O species by dihydrogen during ethylbenzene dehydrogenation is evidenced by comprehensive characterization results, and Cr(IV)=O can be re-oxidized to O=Cr(VI)=O species upon simple calcination regeneration. The results from in situ DRIFT spectroscopy elucidate the critical promotion role of CO₂ in ethylbenzene dehydrogenation over K-Cr@Y by retarding the over-reduction of zeolite-encaged Cr species to inactive Cr(III) species and suppressing coke deposition. This study advances the rational design of non-noble metal catalysts for CO₂-promoted ethylbenzene dehydrogenation with zeolite-encaged high valence transition metal ions modulated by extraframework cations.