Direct Synthesis of para-Xylene from CO2 Hydrogenation with a Record-High Space-Time Yield.

Abstract

The direct synthesis of para-xylene (p-X) from CO₂ hydrogenation with high space-time yield (STY) remains a significant challenge due to two primary limitations: the Anderson-Schulz-Flory distribution, which restricts the C₈ selectivity to ∼6.8 C%, and the thermodynamic equilibrium, which confines the p-X content among xylene isomers to 15-25%. Herein, we report a composite catalyst, K-FeMn/Hollow ZSM-5, that enables the efficient hydrogenation of CO₂ to p-X by integrating two synergistic catalytic functions. The K-FeMn component facilitates the reverse water-gas shift reaction and Fischer-Tropsch synthesis to olefin processes, generating light olefin intermediates. These intermediates are subsequently transformed to p-X within the hollow ZSM-5 zeolite through oligomerization, cyclization, and aromatization. The hollow ZSM-5 features a suitable pore size to facilitate p-X diffusion only, while its passivated external acid sites effectively suppress isomerization and alkylation of p-X outside the zeolite. As a result, the K-FeMn/Hollow ZSM-5 catalyst achieves a p-X STY of 41.7 g kg_cat^-1 h^-1 at a CO₂ conversion of 46.1%, surpassing all previously reported values. This work demonstrates a novel approach to overcome the local thermodynamic equilibria by specific catalyst design and the spatial separation of processes toward CO₂ hydrogenation into p-X.