New insight into the synergistic reactions involved in the hydrothermal co-liquefaction of synthetic polymer wastes by molecular dynamics and DFT methods

Abstract

Coliquefying synthetic aliphatic and aromatic polymer wastes using supercritical water has drawn considerable research attention. However, the mechanisms of chemical reactions between different types of polymers are ambiguous. Herein, depolymerization mechanisms for individual polymers and reaction mechanisms for binary polymer mixtures were investigated using molecular dynamics and density functional theory (DFT). The innovative approach showed that the production of oil from individual polymers during HTL was hindered by (1) volatile C1–C4 molecules emitted from aliphatic polymers and (2) polycyclic aromatic hydrocarbons (PAHs) produced from aromatic polymers. Interestingly, synergistic reactions among these byproducts from different polymers could promote oil production during coliquefaction. Specifically, the synergistic radical-related reactions included (1) the ring-opening of PAHs caused by C2H2 molecules emitted from aliphatic polymers and (2) the recombination of PHA branches and short-chain aliphatics. A considerable synergy between aromatic polymers with higher benzene ring contents and aliphatic polymers with lower H/C atomic ratios was observed near the critical temperature of 649 K. This work provides new insights into the synergistic reactions involved in the coliquefaction of synthetic polymers and gives useful guidance for realizing efficient oil production from mixed organic wastes.