Advances in chemical recycling of polyolefins by hydrothermal liquefaction in supercritical water: A comprehensive review

Plastic waste, particularly polyolefins such as polyethylene (PE) and polypropylene (PP), poses a persistent environmental challenge due to their chemical inertness and extensive use in packaging and consumer products. Conventional recycling methods, including mechanical and chemical approaches, face substantial limitations, especially for mixed or contaminated waste streams. Hydrothermal liquefaction (HTL), and specifically supercritical water liquefaction (SCWL), has emerged as a promising green alternative for converting polyolefins into valuable oils and chemicals. SCWL operates above the critical point of water (≥ 374 °C, ≥ 22 MPa), where water exhibits unique transport and solvent properties that facilitate radical-mediated depolymerisation. Optimised conditions (425 – 450) °C, (15 – 60) min, ≥ 22 MPa, can yield up to 95 wt% oil. Critical challenges remain in understanding reaction kinetics, refining process parameters, and managing additives and impurities in post-consumer plastics. This review identifies these knowledge gaps and outlines prospective research directions to advance SCWL as a sustainable component of a circular plastic economy. • SCWL yields up to 95 wt% oil at 425 °C to 450 °C, 15 min to 60 min, ≥ 22 MPa. • Temperature and time influence selectivity; longer times boost aromatics. • Water/polymer ratios of 2:1–4:1 improve productivity without yield loss. • PE favours paraffins; PP favours aromatics due to polymer structure differences. • SCWL tolerates mixed waste, but additives and heterogeneity pose challenges.