Hydrogen-Free Catalytic Strategies for the Upcycling of Polyolefin Plastics

Polyolefins, such as polyethylene (PE) and polypropylene (PP), dominate modern plastic production due to their low cost and durability, driving widespread single-use consumption and contributing to a global waste crisis. Despite advances in mechanical recycling, most polyolefin waste—over 85%—is still landfilled or incinerated, and mechanical processes often degrade polymer properties, resulting in material downcycling. In contrast, catalytic upcycling offers a more sustainable route to convert polyolefin waste into fuels and chemical feedstocks. However, conventional catalytic processes, such as hydrogenolysis and hydrocracking, require high-pressure fossil-derived hydrogen, raising concerns over carbon emissions and process sustainability. This review highlights recent advances in hydrogen-free catalytic strategies for polyolefin upcycling, covering both thermocatalytic and electrified approaches. We discuss pyrolysis over solid acids at elevated temperatures; solvent-assisted systems where solvents donate hydrogen and influence degradation pathways; metal–acid catalysts that utilize hydrogen released from the polymer itself; and oxidative upcycling routes using O₂ and CO₂ as reactants for selective oxygenation and aromatization. The mechanistic roles of metal and acid sites, hydrogen transfer pathways, and confinement effects are analyzed to clarify how product selectivity and reaction efficiency are controlled. By comparing these diverse strategies, this review identifies key design principles for hydrogen-free polyolefin valorization and outlines future research directions toward circular, low-carbon plastic waste management.