Collaborative role of seed solution and CTAB in the synthesis of hierarchical ZSM-5 zeolites and their templating mechanism

Abstract

Both seed solution and Cetyltrimethylammonium bromide(CTAB) serve as common template agents in the synthesis of hierarchical ZSM-5 zeolites. However, their specific roles in the synthesis process have not been clearly understood. In this study, hierarchical ZSM-5 zeolites were successfully synthesized using a 1.5% mass fraction (relative to the silicon source) seed solution in combination with varying amounts of CTAB. The physical and chemical properties of the hierarchical ZSM-5 zeolites were characterized using X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), low-temperature N₂ adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TG), ammonia temperature-programmed desorption (NH₃-TPD), pyridine-IR, and 2,4,6-collidine-IR. The investigation revealed that the CTAB content significantly influenced the synthesis of hierarchical ZSM-5 zeolites, with CTAB contributing to framework filling, charge balancing, and acting as a mesoporous template during the synthesis process, while the seed solution primarily served as a structure-directing agent. Moreover, utilizing the conversion of C8 aromatic hydrocarbons as a probe reaction, it was demonstrated that ZSM-5 zeolites synthesized with the co-assistance of seeds and CTAB exhibited enhanced conversion for ethylbenzene and m-xylene compared to samples synthesized exclusively with a seed solution.