Insight into pyrolysis mechanism of plastic waste with C-O/C-N bonds in the backbone

Pyrolysis is an important method for efficiently recovering plastic monomers, fuels and chemicals from plastic waste. The depolymerization of the backbone structure of plastic waste is a key step of the pyrolysis process. Currently, researches on the pyrolysis mechanism of plastics with C-O/C-N bonds in the backbone are still not sufficiently in-depth and also lack systematic and comprehensive investigation. Therefore, this study for the first time comprehensively investigated both macroscopic and microscopic pyrolysis processes of plastics with C-O/C-N bonds in the backbone, and evaluated the difficulty of breaking different backbone linkages via bond dissociation energy (BDE) obtained by density functional theory (DFT) calculations to deeply reveal the pyrolysis mechanism. The results indicated that polyethylene terephthalate (PET) had a higher initial pyrolysis temperature and its thermal stability was slightly stronger than nylon 6. The backbone of PET was mainly decomposed via the cleavage of Cα-O on the alkyl side, while the degradation of nylon 6 backbone began with NH₂ groups at the end of the backbone. The pyrolysis products of PET were mainly derived from the small molecular fragments, which were generated by the degradation of the backbone through the cleavage of CO bonds or CC bonds, while the pyrolysis products of nylon 6 were always dominated by caprolactam. In addition, based on the results of DFT calculations, it could be inferred that the cleavage of CC bond in PET backbone and the cleavage of its adjacent Cα-O were most likely to occur, which followed a competitive reaction mechanism. However, in pyrolysis of nylon 6, the conversion to caprolactam was mainly via the concerted reaction of amide CN bonds. Compared with the concerted cleavage of amide CN bond, the cleavage of CC bond in the backbone of nylon 6 was not predominant.