Oxygen-Induced Enhancement in Low-Temperature Dechlorination of PVC: An Experimental and DFT Study on the Oxidative Pyrolysis Process
Linzheng Wang, Ruizhi Zhang, Ruiqu Deng, Yonghao Luo
Abstract
Thermochemical utilization of chlorine-containing organic wastes requires pre-dechlorination to eliminate the corrosive and toxic chlorides. A dechlorination procedure through low-temperature oxidative pyrolysis is proposed and investigated using polyvinyl chloride (PVC). Experimental assessments were performed through simultaneous thermogravimetry-differential scanning calorimetry coupled with mass spectrometry under inert, air, and oxygen atmospheres. Oxygen significantly promoted the HCl release in a low-temperature range (200–350 °C) and caused exothermicity, and over 97.26% of chlorine could be eliminated at 275 °C using air as oxidant. After the dechlorination process, the presence of oxygen caused less than 1% of extra weight loss, indicating that the valuable carbonaceous residues could be largely preserved. The PVC chars produced under different conditions were characterized by Fourier transform infrared spectroscopy, which revealed that chlorine was preferentially removed at lower temperature in the presence of oxygen, accompanied by formation of oxygen-containing functional groups. Density functional theory calculations indicated that oxygen chemisorption on the alkene moieties derived from initial dehydrochlorination could form dioxetane intermediates, followed by chain cleavage, which contributed to the exothermicity. Oxygen-induced additional dechlorination channels, including bimolecular substitution and intramolecular substitution, have similar activation energy values with the conventional dehydrochlorination reaction and, therefore, enhanced the overall dechlorination efficiency.