Complementary acid site mechanisms in hydrogen-free polyethylene upcycling: elucidating the distinct roles of Brønsted and Lewis sites in Ce-modified zeolites

The environmental burden of petrochemical-derived plastics, particularly polyolefins such as polyethylene and polypropylene, has spurred the search for sustainable upcycling strategies. Conventional hydrogenolysis and hydrocracking processes rely on external H₂, over 98% of which is produced via steam methane reforming or coal gasification; both methods yield significant CO₂ emissions. In this study, we demonstrate a hydrogen-free approach to PE upcycling using zeolite Y ion-exchanged with various cations (Na⁺, Li⁺, K⁺, H⁺, La³⁺, and Ce³⁺). Among these, the Ce-exchanged mesoporous zeolite (Ce_meso_Y) achieved complete PE conversion with an 88.7% yield of naphtha-range hydrocarbons (C₅-C₁₂). NH₃-TPD and pyridine-DRIFTS analyses revealed that Brønsted acid sites (BASs) drive C-C bond cleavage, while strong Lewis acid sites (LASs) promote intramolecular hydrogen transfer from the polymer backbone, thereby eliminating the need for external H₂. Extending this approach to post-consumer polyolefin waste (including HDPE bottles, LDPE film, and PP cases) delivered 70.5-82.6% conversion and 77.8-84.8% naphtha selectivity. Our findings establish a sustainable, hydrogen-free route for plastic upcycling by harnessing intrinsic polymer hydrogen and fine-tuning acid site functionality.