Construction of Hierarchical High Acid-Capacity ZSM-5 for Enhancing Mass Transfer and Aromatization of Lignite Pyrolysis Volatiles

ZSM-5 is an effective catalyst for the catalytic reforming of volatiles from lignite. An increased density of acidic sites significantly enhances catalytic activity. However, its inherent microporous structure and abundant external surface acids lead to issues including catalyst deactivation. In this work, a nondiscriminatory etching method using NH4F was employed to simultaneously remove Si and Al species from highly acidic zeolites. This process introduces mesopores while reducing external surface acidity, maintains high crystallinity, and preserves the total acid content essentially unchanged. The mesoporous surface area of HZ5-6.5 reaches 129 m2/g. This significantly enhances the mass transfer efficiency of volatile component molecules within the zeolite, effectively improving the accessibility of active sites. Furthermore, compared to the parent zeolite, the modified HZ5-6.5 exhibits a 50% reduction in acidic sites on its crystal surface while maintaining essentially unchanged total acidity. Concurrently, light aromatic yields increase, while coke deposition decreases by 40%.