The deactivation mechanism of Zn/HZSM-5 zeolites in ethylene aromatization reaction

Zn/HZSM-5 zeolites, with zinc contents of 1%, 2%, and 3%, were prepared by impregnation method and characterized by XRD, N₂ adsorption, NH₃-TPD, Py-FTIR, XPS, and TG-DTA techniques to investigate the deactivation mechanism in ethylene aromatization reaction. It shows that coking is the main reason for catalysts deactivation, which is considerably depressed with the presence of Zn in the HZSM-5 catalysts. The deactivation is slow for the catalysts with low Zn loading. However, high content of Zn in the catalysts brings in problems such as decrease of surface area and microporous volume, and hence accelerates the deactivation. In the reaction, zinc species lose from the catalysts, accompanied with the migration and redistribution of Zn from bulk to surface of zeolite. The losing rate was constant with the time on stream, but influenced by the zinc content of the catalyst. ZnO on the external surface of the catalyst are the main species leaching from zeolite. Zn leaching is accelerated with the increase of zinc content, and has correlation with coking rate to some extent.