“Redox switches” of Fe species on zeolite catalysts: Modulating the acidity and the para-xylene yield from methanol

Abstract

Molecular switches are widely studied in optical devices, computer science, DNA sensor systems, and chiral synthesis; however, their use in heterogeneous catalytic processes is rarely reported. Herein, we report a Fe-based redox switch for tuning the acidity of a ZSM-5-based catalyst in the methanol-to-aromatics reaction. In this reaction, the yield of the target product, para-xylene (PX), is low because various types of acids on the catalyst activate side reactions. Fe oxides and zeolite generate medium-strength Lewis acids, which activate the aromatization of methanol but suppress the dealkylation of xylene. Gradual reduction of Fe oxides during the reaction simultaneously decreases the conversion of methanol, the yield of aromatics, and the yield of PX. The oxidation state of the Fe species and the associated catalytic performance can be regenerated in the air at 550 °C. The redox switches caused regular fluctuation in the catalytic performance and remained stable throughout 16 regeneration cycles (up to 80 h). The employed strategy enabled a PX yield of up to 60% (carbon base) using a SiO₂-coated Zn/P/Fe/ZSM-5 catalyst, which is 3–6 times higher than previously reported values. The result showed a new mode of acidity modulation of the catalyst.