Effect of Na+ migration and the component proximity on direct conversion of aromatics into aromatics over Fe-based zeolite bifunctional catalysts

Abstract

Direct conversion of syngas into aromatics (STA) via Fischer-Tropsch (F-T) synthesis, using Fe-based zeolite bifunctional catalysts, has recently become a topic of great interest. The catalytic performance of the Fe-based zeolite bifunctional catalyst is influenced by the component proximity in the STA reaction, in addition to their physicochemical properties. However, the mechanism by which the component proximity affects the STA performance is still unclear. Herein, we investigate the effect of Na+ migration and the component proximity on the F-T reactivity, the aromatic distribution, and catalyst stability in a Fe-based ZSM-5 catalyst system. The correlation between the proximity of two components, the migration of alkali-metal promoters, the evolution of the Fe-based active phase, and the coke on the zeolite is analyzed using XRD, XPS, FT-IR, TG, Mössbauer spectroscopy techniques. The findings indicate that when the components are closer to each other, it encourages the formation of iron carbides and inhibits the coke deposition on the zeolite. However, the migration of alkali metals and the acidic environment of zeolite can decrease the catalyst’s stability and F-T reactivity. Moreover, by improving the hydrophobic nature of the zeolite, we can prevent the transfer of alkali-metal promoters from the Fe-based component to zeolite. This enhances the stability of Fe-based ZSM-5 catalysts in the STA reaction, suggesting a new approach to improve their STA performance.