Mechanistic proposal for the conversion of syngas to light alkanes in Zn-ZSM-5 zeolite linking theoretical calculations to experimental characterizations

Abstract

The direct syngas conversion to olefins, aromatics and other hydrocarbons has attracted increasing attention while unraveling the complex reaction mechanism is of great challenge. It was recently demonstrated that zinc-exchanged zeolites could selectively convert syngas to alkanes, particular ethane, and [Zn–O–Zn] motif is likely the intrinsic active site. The underlying reaction network however remains ambiguous. Herein, we proposed a reaction mechanism for the direct conversion of syngas to light alkanes including methane and ethane using density functional theory calculations in periodic [Zn–O–Zn]-ZSM-5 zeolite model. The evolution of syngas to ethane follows the sequence of CO → CH2O → CH2CO → CH3CHO → C2H6. The activation of CO to formaldehyde initiates the reaction. The [Zn–O–Zn] site can readily be reduced by CO to [Zn–Zn] site, which either inserts aldehydes to form [Zn–O-CnH2n-Zn] motif for chain propagation to higher aldehydes, or converts aldehydes to alkanes for chain termination. Ketene is the first intermediate after the C–C bond coupling between [Zn–O–CH2–Zn] and CO. Both the structures and the evolution sequence of the involved intermediates in the proposal (formyl, methylene, acetyl, ethyl) coincide quite well with experimentally quasi in-situ characterized results. The proposed reaction network consisting of initiation, propagation and termination sub-cycles unifies the formation pathway of methane, ethane, and higher alkanes, and bears some resemblance to the Fischer-Tropsch synthesis for syngas conversion. This theoretical work thus further vindicates the critical role of [Zn–O–Zn] site and may proffer some implications to tailor alkane selectivity in metal-exchanged zeolites for syngas conversion.