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

Shuo Yan; Dehong Xia; Nien-Chu Lai; Binfan Jiang; Xiangjun Liu

doi:10.1016/j.jhazmat.2023.131032

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

J Hazard Mater. 2023 May 5:449:131032. doi: 10.1016/j.jhazmat.2023.131032. Epub 2023 Feb 17.

Authors

Shuo Yan¹, Dehong Xia², Nien-Chu Lai¹, Binfan Jiang¹, Xiangjun Liu¹

Affiliations

¹ School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China.
² School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China; Beijing Key Laboratory for Energy Saving and Emission Reduction of Metallurgical Industry, Beijing 100083, China. Electronic address: xia@me.ustb.edu.cn.

PMID: 36821896
DOI: 10.1016/j.jhazmat.2023.131032

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.

Keywords: HTL; Plastic; ReaxFF; Supercritical water; Synergism; Tire.