Effect of Ga doping on the catalytic performance of Zn-ZSM-5 for CO2-assisted oxidative dehydrogenation of C2H6

Abstract

Using a low silica-to-alumina ratio NaZSM-5 zeolite as the support, a bimetallic modified catalyst, Ga_xZnZ5, was prepared through ion-exchange method for CO₂-mediated oxidative dehydrogenation of C₂H₆ (CO₂-ODH). Based on systematic characterization and evaluation, it was found that Zn is the primary active component in the catalytic system for activating C₂H₆, but it is merely active towards CO₂, leading to rapid catalyst deactivation. Under the influence of Ga species, CO₂ can readily engage into the reaction network. During the initial stage of the reaction, the activated CO₂ primarily participants in dry reforming with C₂H_6-x species that rapidly formed on highly active Zn sites. However, the rapid cleavage of C-H bonds on these Zn sites leads to significant carbon deposition and subsequent deactivation. Following this rapid deactivation phase, CO₂ becomes involved in the reaction through the reverse water-gas shift (RWGS) reaction and carbon elimination reactions, thereby shifting the reaction equilibrium towards C₂H₄ formation while alleviating the accumulation of carbon deposition and thus enhancing stability. With the optimum catalyst (Ga_0.27ZnZ5), excellent and stable performance with 22.64 % C₂H₄ yield and nearly 100 % selectivity could be achieved after an induction period of ca. 100 minutes, these are among the highest values reported so far.