Chemical recycling of polyethylene and polypropylene by catalysed dehydrogenation and metathesis

Abstract

Polyethylene (PE) and polypropylene (PP) constitute the largest fraction of global plastic waste and pose significant challenges for conventional chemical and mechanical recycling routes due to chemical inertness and mechanical downcycling. This study explores a novel chemical recycling pathway using tandem dehydrogenation and olefin cross-metathesis (TDOCM) to degrade virgin, recycled and consumer grade PE and PP under mild conditions (300 °C), producing exclusively linear alkane products in 3 hours. The process employs a fully heterogeneous catalyst system consisting of Pd/γ-Al₂O₃ for dehydrogenation and WOₓ/SiO₂ for metathesis, with n-decane as the light alkane co-reagent. It is also the first time that the degradation of PP using this reaction pathway has been reported. Initial validation experiments confirmed complete degradation of virgin HDPE, LDPE, and PP at 300 °C and moderate pressures, with both yielding exclusively alkane products and no detectable aromatics or olefins. For PE samples, including commercial-grade feedstocks with additives, the system exhibited consistent performance and high conversion (up to 80 wt.% liquid yields). PP, despite its branched and tertiary carbon structure, underwent rapid breakdown with >80 wt.% oil yields and showed enhanced iso-alkane selectivity compared to PE products, due to retained methyl side chains post-metathesis. Parametric studies revealed that product distributions are sensitive to polymer type, reaction time, catalyst loading, alkane-to-polymer ratios, and pressure. For PE, insufficient light alkane presence led to increased wax and gas yields due to redundant metathesis reactions, while for PP, lower alkane concentrations favoured β-scission over the TDOCM route, increasing gaseous byproducts. Elevated pressures improved alkane incorporation across both polymer types by enhancing hydrogen solubility and stabilising reactive intermediates. Lastly, petroleum ether was evaluated as a cost-effective substitute for n-decane. Despite its branched profile, it supported effective polymer breakdown with exclusively alkane products and offered advantages in ease of post-reaction separation. This work demonstrates the viability of TDOCM as a mild and selective method for PE and PP recycling, with the ability to tune product profiles and optimise reaction economics through thoughtful selection of conditions and reagents. It addresses the shortcomings of mechanical recycling and conventional chemical recycling alternatives by offering mild operating conditions and a narrow product spectrum suitable for downstream upgrading.