Structure Evolution of Zn Species on Fresh, Deactivated, and Regenerated Zn/ZSM-5 Catalysts in Ethylene Aromatization

Zn-modified ZSM-5 catalysts were widely used in ethylene aromatization; however, the deactivation and regeneration behavior of these catalysts have not been fully understood. In this work, the structure evolution of Zn species on various x-Zn(y)/Z5 catalysts prepared by ion exchange, incipient wetness impregnation, and physical mixture in ethylene aromatization were systemically investigated. The results of X-ray diffraction, X-ray photoelectron spectroscopy, DR UV–vis, 27Al/29Si MAS NMR, EXAFS, and X-ray absorption near edge spectra with LCF analysis indicated that the generated coke species on spent Zn/ZSM-5 can not only cover the acid sites and block the pore channel but also lead to the transformation of active 6-fold coordinated ZnOH+ species into unactive 4-fold coordinated ZnO. After regeneration to eliminate these carbon deposits, the crystallinity, specific surface area, pore volume, and total acid content of regenerated Zn/ZSM-5 were fully recovered. More importantly, most of the ZnO clusters/crystals were converted into ZnOH+ species again. As a result, the regenerated Zn/ZSM-5 catalysts showed even higher aromatic selectivity than the fresh one in ethylene aromatization.