Superior performance of Ga-MFI zeolite with a hierarchical structure and moderate acidity in the catalytic cracking of low-density polyethylene

Abstract

Addressing the global challenge of waste plastic management, catalytic cracking by utilizing zeolite catalysts offers a sustainable approach for converting plastics into valuable products. The acidity and diffusion of zeolites play a critical role in determining the efficiency of this process. However, conventional aluminium-containing zeolites exhibit excessive acidity, promoting excessive cracking and secondary bimolecular reactions, leading to coke formation. Additionally, microporous zeolites with limited diffusion of large molecules significantly impair their activity and overall utilization efficiency. In this study, we developed gallium-modified MFI zeolite catalysts (Ga-MFI) that possess a well-balanced micro- and mesoporosity along with controlled acidity. These Ga-MFI catalysts exhibited enhanced diffusion properties and moderate acidity, which contributed to an improved gasoline yield (64.84–77.69%), alongside a significant reduction in liquefied gas yield (from 23.21% to 13.07%) during the catalytic cracking of low-density polyethylene (LDPE) when compared to the ZSM-5 catalyst. Furthermore, the Ga-MFI catalysts demonstrated a significant enhancement in the selectivity for cycloalkenes within the liquid products, increasing from 35.62% to 51.53%. This improvement can be attributed to the promotion of bimolecular cracking reactions occurring on moderate acidic sites, which facilitate olefin formation, as well as the high concentration of Lewis acid sites that aid in the dehydrogenation of cycloalkanes to cyclo-olefins. These findings underscore the potential of Ga-MFI zeolite as a promising catalyst for the sustainable conversion of plastic wastes.