Catalytic hydrothermal upcycling of end-of-life automotive plastic waste in near-supercritical water: process optimization and product characterization

Abstract

This study investigates the catalytic hydrothermal conversion of heterogeneous end-of-life automotive shredder residues (ASR) waste in near-supercritical water, focusing on process optimization, thermodynamic behavior, and multi-phase product characterization. Factorial screening identified temperature and residence time as critical parameters, with 350 °C and 90 minutes yielding maximum hydrochar through enhanced carbonization and repolymerization, accompanied by reduced liquid yields and CO₂-rich gas production. Thermodynamic analysis revealed significant increases in water ionization constant (K_w) during non-isothermal heating, peaking at 1.44 × 10⁻¹² mol² kg⁻² at 350 °C, while density decreased to 0.619 g cm⁻³ under autogenous pressure. Isothermal reactions at 350 °C exhibited highly exothermic behavior (ΔH ≈ −143 kJ mol⁻¹) and strong ordering effects, correlating with peak hydrochar and gas yields. Catalyst screening demonstrated Ni-based catalysts' superior selectivity for phenolics and aromatic amines in the oil-phase, increased aqueous-phase total organic carbon, and minimized tar formation. Ru/Al₂O₃ favored ketone production, while non-catalyzed runs produced a broad range of C₆–C₂₉ oil products and 341 g (CO₂)/kg(feed) as the only gas product. The optimized NiSiAl catalyst yielded a maximum hydrochar of 64.4 wt% with enhanced thermal stability (onset degradation ∼425 °C), higher calorific values at 5 wt% feedstock concentration and feedstock-to-catalyst ratio of 9. The highest H₂ production (5 g kg⁻¹ feed) occurred at a catalyst ratio of 5, while the best liquid yield (36%) was achieved at the lowest ratio of 1. The GC-MS analysis revealed feedstock concentration influenced reaction pathways, shifting from phenolics and amines at low solids loading to hydrocarbons and ketones at higher concentrations. These findings highlight catalytic hydrothermal conversion as a viable circular-economy route for ASR valorization, combining thermodynamic efficiency with targeted fuel and chemical production.