Insights into the reaction mechanism of polybutylene terephthalate by hydrogen halide depolymerization

Efficient management of waste plastics through thermal recycling and disposal plays a crucial role in environmental protection. Addressing the issue of hydrogen halide emissions is essential in combating environmental pollution and improving energy efficiency. However, current research lacks the in-depth understanding of the evolutionary pathways and decomposition mechanisms of waste plastics during thermal processing. This study investigated the influence of hydrogen halides (HF, HCl, and HBr) on the thermal decomposition mechanism of waste plastics at the B3P86/6-311++G(d,p) level, combined with the verification through Fourier-transform infrared spectroscopy. Computational analysis revealed that the depolymerization of waste polybutylene terephthalate (PBT) dimer in the presence of hydrogen halides proceeds with low reaction energy barriers, indicating that hydrogen halides markedly promote the thermal depolymerization of waste PBT dimer, which makes the reaction process easier, with a promotion sequence of HF (153 kJ/mol) < HCl (130 kJ/mol) < HBr (120 kJ/mol). Furthermore, the study investigated how temperature affects the thermal decomposition process of PBT dimer with hydrogen halides and evaluated the environmental impact of the resulting products. These findings will improve the understanding of waste transformation and the assessment of aquatic environmental risk. Consequently, these findings provide a theoretical basis for alleviating pollutant generation during thermal treatment and recycling PBT-containing waste in the presence of hydrogen halides.