Migration and transformation mechanism of Cl during polyvinyl chloride pyrolysis: the role of structural defects

A profound understanding of the migration and transformation of Cl is crucial for controlling the formation of hazardous substances and efficiently recycling Cl-containing waste plastics. In the present work, the conversion of Cl into HCl and chlorinated hydrocarbons was studied based on a polyvinyl chloride model with defect structure (d-PVC) using density functional theory (DFT) calculations and wavefunction analysis. Particularly, the fate of the Cl atoms connected with primary, secondary, and tertiary C atoms was explored. The concerted reactions exhibit low energy barriers and large rate coefficients at low temperatures. Whereas homolytic reactions are favored at higher temperatures, and the rate coefficient of C–C homolysis will surpass other reactions when the temperature exceeds 830K. The energy barriers for concerted reactions to generate HCl are the lowest at low temperatures, followed by the C–Cl homolysis induced channel to produce HCl. Light chlorinated hydrocarbons chiefly result from dechlorinated unsaturated intermediates at lower temperatures. At higher temperatures, light chlorinated hydrocarbons tend to be formed by dechlorinated radical intermediates. The debranched radical intermediate is more favorable than dechlorinated intermediates for the formation of chlorinated aromatics. In total, the defective structure primarily affects the conversion of Cl into HCl and chlorinated hydrocarbons by influencing the initial pyrolysis of d-PVC.