Effect of zinc content on the structure of Zn species and catalytic properties over Zn/ZSM-5

Studying the status and distribution of Zn species on Zn/ZSM-5 zeolite catalysts were of great significance for determining the active centers and establishing structure-activity relationships in the ethylene aromatization process. The effect of zinc contents of Zn/ZSM-5 zeolites prepared by incipient-wetness impregnation method on catalytic performances in ethylene aromatization were investigated. The structures and acidic properties of the catalyst were studied through X-ray powder diffraction (XRD), N₂ adsorption/desorption, and infrared spectra for pyridine adsorption (Py-FTIR). Besides, inductively coupled plasma-atomic emission spectrum (ICP), diffuse reflectance ultraviolet-visible spectrum (UV-vis DRS), extended X-ray absorption fine structure (EXAFS) and linear combination fitting (LCF) analysis on X-ray Absorption near edge spectra (XANES) had finely analyzed the structure and transition of Zn species and the losing rate of Zn species on HZSM-5 molecular sieve catalyst during ethylene aromatization process. The results showed that the introduction of Zn was advantage to improve the selectivity of aromatics hydrocarbon, and Zn contents of the catalyst had obvious influence on the structures, acidic properties, and the status of Zn species, as well as the catalytic performance of Zn/ZSM-5 catalysts. At low zinc loading, 1.5%-Zn(IM)/Z5 catalyst with more active 6-fold coordinated ZnOH⁺ species (55%) showed the highest selectivity to aromatics and catalyst stability. With the increase of zinc amount, the excessive Zn contents not only covered the acid sites and blocked the pore channel, but also changed the local coordination structure and state of Zn species. It was confirmed that the oxidizability of Zn species and the coordination number around Zn sites decreased, accompanied by a weakening of the interaction between Zn and zeolite, leading to the formation of large amounts of 4-fold coordinated ZnO clusters and ZnO crystallites. In the 4%-Zn(IM)/Z5 catalyst, Zn species composed of multi characteristics from ZnOH⁺, ZnO clusters inside the pores, and ZnO crystals on the external surface with relative contributions of 23.5%, 56.1%, and 20.4%, respectively. It meant that ZnO clusters and ZnO crystallites became the main component at the high Zn content. Furthermore, ZnO species located on the outer surface of Zn/ZSM-5 catalysts were easily reduced by H₂ and then transported as zinc vapor to the outer surface, which eventually lead to the loss of Zn species from the catalyst and the decline of the catalytic performance of Zn/ZSM-5 catalyst. The relative proportion of ZnOH⁺ decreased with that of ZnO clusters and ZnO crystallites correspondingly increased considerably with the increase of Zn loading on ZSM-5, accompanied with the elevated rate for Zn losing, and shortened catalyst life. Therefore, a positively correlated between the content of ZnOH⁺ obtained through the UV-vis DRS and LCF analysis on XANES and the rate of aromatics formation was established, further confirming the catalytic nature of ZnOH⁺ as the active center, which played an important role in the aromatization reaction that enhancing the formation of aromatic hydrocarbons. Meanwhile, ZnO on the outer surface of Zn/ZSM-5 catalysts was the main species that losing from catalyst, and influenced the catalytic properties on a certain degree.