Pore-Structure Engineering of Hierarchical β Zeolites for the Enhanced Hydrocracking of Waste Plastics to Liquid Fuels

Abstract

Hydrocracking of plastics over bifunctional hierarchical zeolites is promising for the upcycling of plastics into value-added products. However, the exact role of their acidic and textural properties toward the catalytic activity remains unclear. Herein, we modified the structure of a β zeolite via dealumination and desilication routes, resulting in hierarchical zeolites. The parent and hierarchical modified β zeolite samples were loaded with Ni and studied for the hydrocracking of virgin HDPE. In comparison to the parent and dealuminated β zeolite, desilicated β zeolite showed a higher conversion of 87.8% with 66.7% of the products in the gasoline range, owing to its significantly high textural properties. The conversion and selectivity of gasoline-range hydrocarbons over the desilicated zeolite were further improved to 95.9 and 69.2%, respectively, by Ni addition. To unlock the structure–activity correlation of the various zeolite samples, the role of different activity-driven factors was studied, resulting in an empirical relationship that aligns with the observed conversions over different zeolite samples. Moreover, it was observed that it is possible to achieve high selectivity of iso-paraffins in gasoline-range hydrocarbons via the optimization of the balance between metal-acid sites on bifunctional hierarchical zeolites. Furthermore, both Ni-loaded hierarchical β zeolites showed good stability and the ability to be regenerated under cyclic runs. The best-performing Ni-loaded desilicated β zeolite was also maintained over various postconsumer waste plastics (conversion = 85–95%) and when using a mixture of postconsumer waste plastics (88.4%). A life cycle assessment and a comparison with the recent literature also demonstrated the advantages of the proposed hierarchical modification routes in achieving high gasoline productivity (6.6–7.6 g_gasoline/g_cat·h) and less environmental impact. Overall, these findings highlight the role of improved textural properties of noble-metal-free, easily modifiable, and environmentally friendly bifunctional hierarchical β zeolites for the enhanced conversion of waste plastics into liquid fuels.