Complementary Acid Site Mechanisms in Hydrogen-Free Polyethylene Upcycling: Elucidating the Distinct Roles of Brønsted and Lewis Sites in Ce-Modified Zeolites

Abstract

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