Unraveling the Mechanisms of Ketene Generation and Transformation in Syngas-to-Olefins Conversion over ZnCrOx|SAPO-34 Catalysts

Abstract

Ketene was identified as an intermediate in syngas-to-olefins (STO) conversion catalyzed by metal oxide-zeolite composites, which sparked the hot debate on its formation mechanism and catalytic roles. Here we employed large-scale atomic simulations using global neural network potentials to explore the STO reaction pathways, and microkinetic simulations to couple the reaction kinetics in ZnCrO_x|SAPO-34 composite sites. Our results demonstrate that a majority of ketene (86.1%) originates from the methanol carbonylation-to-ketene route (CH₃OH* + H* -> CH₃* + H₂O -> CH₃* + CO* -> CH₂CO* + H*) nearby zeolite acidic sites, where methanol is produced through conventional syngas-to-methanol on Zn₃Cr₃O₈ (0001) surface, while the minority of ketene (13.9%) arises from a direct CHO*-CO* coupling (CHO* + CO* + H* -> CHOCO* + H* -> CH₂CO + O*) on Zn₃Cr₃O₈. The presence of the ketene pathway significantly alters the catalytic performance in zeolite, as methanol carbonylation to ketene is kinetically more efficient in competing with conventional methanol-to-olefins (MTO) and thus predominantly drives the product to ethene. Based on our microkinetic simulation, it is the methanol carbonylation activity in zeolite that dictates the performance of STO catalysts.