Insights into the evolution of chemical structures in hydrochars from hydrothermal carbonization of PVC

Hydrothermal carbonization (HTC) treatment can induce the comprehensive evolution of PVC structure, which broadens the horizons for the development and optimization of hydrochars derived from PVC. However, there is a lack of understanding of the evolution of chemical structure in hydrochars during the HTC reactions. In this work, the dynamic evolution of the chemical structure of hydrochars during HTC at 220–260 °C within 60 min was comprehensively studied by ultimate analysis, XPS and FTIR. The results showed that hydrothermal temperature had a significant effect on the chemical structure of hydrochars. Raising the hydrothermal temperature facilitated the conversion of C–Cl and C–C/C–H of PVC to C–O, CO, OC–O and CC/C-(C, H), respectively. Under the combined action of hydrolysis, oxidation, aromatization, polymerization and esterification reactions, the hydrochar presented an alicyclic and aromatic structure with different degrees of condensation as its backbone structure. The main oxygen-containing functional groups of the hydrochar were aromatic ethers (C–O), hydroxyl groups (-OH), carboxylic acids (-COOH), and ester groups (-COOR). The maturity of hydrochar was close to that of lignite. Overall, this study provides helpful information for the preparation of diverse value-added products, and also for developing a molecular model of hydrochar for in-depth investigation of the HTC mechanism of PVC.