Understanding the correlation between zinc speciation and coupling conversion of CO2 and n-butane on zinc/ZSM-5 catalysts

Abstract

The coupling reaction of alkanes and CO₂ into high value-added bulk chemical products is a promising way for CO₂ utilization. The Zn-introduced ZSM-5 catalyst plays an essential role in this process; however, the correlation between the catalytic performance and Zn species of the catalyst has yet to be established. Herein, the structural properties, the acid sites, and the existence status of the Zn species in the Zn-introduced catalysts were systematically characterized by several techniques. And the influence of the state of Zn species in the coupling reaction was discussed. The results indicate that the Zn species exist in the form of ZnO cluster, Zn-OH⁺, and (Zn-O-Zn)²⁺ species, thereinto (Zn-O-Zn)²⁺ species are produced by the Zn-OH⁺ group with the increasing Zn loading. The decrease of Brønsted acid sites, the formation of newly active sites caused by Zn species, and the accumulation of coarse ZnO species, are responsible for the change of n-butane conversion. The Zn-OH⁺ group serves as the primary catalytic center for the conversion of CO₂. Both the Zn-OH⁺ group and the (Zn-O-Zn)²⁺ species enhance the dehydrogenation performance of the Zn-introduced catalysts, thereby promoting the generation of aromatics. The Zn5%-ZSM-5 sample showed the most excellent catalytic performance; the n-butane conversion was 94.71%, the CO₂ conversion was 30.43%, and the aromatics selectivity was 53.71%. Simultaneously, we propose a more specific mechanism for the coupling reaction.