Hydrothermal Liquefaction of Polycarbonate (PC) Plastics in Sub-/Supercritical Water and Reaction Pathway Exploration

Plastic pollution is one of the most important environmental issues being faced today. In this work, the hydrothermal liquefaction performance of polycarbonate (PC) in sub-/supercritical water was investigated using a quartz tube reactor. Response surface methodology (RSM) was employed to demonstrate the correlation between reaction conditions and liquefaction efficiency of PC. The conversion selectivity and product recovery efficiency of PC were also analyzed. Moreover, according to the experimental results, the liquefaction reaction pathways of PC were speculated. In addition, based on the established reaction pathways, the lumped parameter method was also used to calculate the liquefaction kinetics of PC. The results showed that phenol was the largest liquefied product of PC, followed by 4-isopropylphenol (IPrP) and 4-isopropenylphenol (IPP), and the recovery efficiency of these three components determined the level of liquefaction efficiency. The PC liquefaction favored a mild reaction temperature, long residence time, and low feedstock concentration. Finally, for identified products, a 57.70 wt % carbon liquefaction efficiency was obtained with a 5 wt % feedstock concentration at 400 °C for 60 min. The liquefaction pathways showed that in the initial stage of depolymerization, 4-tert-butylphenol was first formed as a structural regulator, and phenol and IPP were the primary liquefied products. The IPrP, 4-ethylphenol, and p-cresol were the secondary products. The kinetic results indicated that the liquefied intermediates were easily converted to form phenol and IPP, and the IPP and IPrP were not directly converted to phenol, but there was a clear conversion relationship between them.