Effect of Ga and Zn modification on propylene aromatization over HZSM-5 catalysts

ZSM-5 catalysts were modified with Ga and Zn by using impregnation and hydrothermal methods to obtain zeolite catalysts with different acidities. XRD, SEM, NH₃-TPD and XPS were used to characterize the effects of the modification components (Ga and Zn) and methods (impregnation and hydrothermal methods) on the pore structure, skeletal structure and surface acidity of the resultant catalysts. The performances of the Ga and Zn modified catalysts for alkene aromatization were evaluated with propylene as a model compound. The results indicated that Ga and Zn modification has little influence on the morphology of catalysts particles, but it obviously changes the surface acidity and alkene aromatization reactivity of the catalysts. Zn modification reduces the acidity of ZSM-5 catalyst, while the effect of Ga modification on the catalyst acidity depends on the Ga addition methods. Ga introduced through impregnation slightly reduces the strong acid sites, while Ga introduced via hydrothermal method apparently increases the intensity of both weak and strong acid sites. Ga and Zn modifications are able to increase the aromatization activity and the aromatics selectivity, and to suppress the carbon deposition.