Unlocking the Structure-Activity Relationship of Hierarchical MFI Zeolites Towards the Hydrocracking of HDPE

The use of metal-loaded zeolites in hydrocracking waste plastics is gaining attention due to their improved catalytic activity, enabling sustainable upcycling. However, the microporous nature of zeolites limits the diffusion of bulkier polymers to their active sites. Additionally, the specific role of their textural and acidic properties during the hydrocracking process remains unclear. In this study, we enhanced the textural properties of an MFI-based zeolite by varying the degree of demetallation through dealumination and desilication routes. The distinct role of textural and acidic properties of the parent and hierarchical zeolites, with and without the addition of Ni, were studied for the hydrocracking of high-density polyethylene at 300 ◦C for 1 h under 30 bar cold H2 pressure. Compared to parent zeolite, both hierarchical zeolites with enhanced textural properties showed higher activity and selectivity of lighter oils (~46 %). The activity and selectivity of low-boiling point hydrocarbons (C5-C18) were further improved by Ni addition. The Ni-loaded desilicated zeolite exhibited the maximum activity of 95.3
% (C5-C18 = 48.5 %), closely followed by the Ni-loaded dealuminated zeolite with 89.4 % (C5-C18 = 45.6 %). To further understand the structure–activity relationship of different catalysts, various activity driven factors, including number of silanol groups, Lewis/Brønsted acidity, acidic strength and textural properties were combined to develop an empirical relation, which perfectly correlates with the obtained conversions over the
different catalysts. Notably, both Ni-loaded hierarchical zeolites demonstrated reusability for consecutive runs and ability to be regenerated. Finally, a life cycle assessment showed that the synthesized hierarchical zeolites in this study exhibited better results and exhibited lower environmental impact as compared to the best catalysts found in the literature. In conclusion, this study suggests that the use of easily modified hierarchical HZSM-5 zeolites present a viable route for the development of efficient bi-functional catalysts for the hydrocracking of waste plastics.