Hydrothermal Liquefaction of Post-Consumer Recycled Linear Low-Density Polyethylene with Different Molar Masses in Supercritical Water

Abstract

Hydrothermal liquefaction (HTL), which uses subcritical and supercritical water as reaction media, was used to chemically deconstruct post-consumer recycled linear low-density polyethylene (PCR-LLDPE) with different molar masses (from 7.20 × 10³ to 1.29 × 10⁵ g/mol) into oil at 425 °C for 2 h. When the molar mass was within the range of 7.20 × 10³–1.36 × 10⁴ (g/mol), a comparable oil yield (86–90%) was obtained. However, the oil yield was reduced to 66% when the molar mass increased to 1.20 × 10⁵ (g/mol). The chemical compositions of the oil converted from PCR-LLDPE with varying molar masses were mainly composed of paraffins, olefins, cyclics, and aromatics. Additionally, it was found that PCR-LLDPE and the post-HTL solid residues contained metallic impurities that may participate in HTL of LLDPE. The density functional theory was also used to elucidate the role of supercritical water under HTL in aiding deconstruction of LLDPE. The calculated electronic and thermodynamic parameters suggested that alkanes with smaller molar mass require lower energy to be deconstructed in HTL.