Acid sites in hydrogen-involved polyolefin upcycling: mechanistic insights and design principles

The escalating global crisis of plastic pollution, dominated by polyolefin (PO) wastes like polyethylene and polypropylene, necessitates a shift from traditional disposal methods to advanced chemical recycling strategies. Catalytic upcycling has emerged as a highly promising route to convert these otherwise persistent wastes into valuable fuels and chemical feedstocks, fostering a circular economy. This review systematically examines the critical function of acid sites in POs valorization, with particular emphasis on how acid identity, strength, and population govern product selectivity. We begin by contrasting metal-site-dominated hydrogenolysis with bifunctional hydrocracking, highlighting how the integration of acid sites with metal functionalities enables superior performance under milder conditions. A central focus is placed on elucidating the distinct yet complementary roles of Brønsted acid sites (BAS) and Lewis acid sites (LAS) in dictating reaction mechanisms, including C–C bond cleavage, isomerization, and aromatization. Furthermore, we explore the critical concept of the metal-acid balance (MAB) and survey state-of-the-art strategies for modulating catalyst acidity through elemental doping, structural modifications, and post-synthesis treatments. Finally, this review summarizes the current challenges—including catalyst deactivation, diffusion limitations, and the complexity of mixed plastic feeds—and provides an outlook on future research directions required to translate these catalytic technologies from the laboratory to industrial-scale applications.