Pyrolysis kinetics, volatile evolution, and backbone degradation mechanisms of typical nitrogen-containing plastics based on TG-FTIR-GC/MS and density functional theory

Abstract

Pyrolysis mechanisms of nitrogen (N)-containing plastics hold significant importance for recovering high-value fuels and chemicals from real plastic waste, whereas current insights remain unclear. This work systematically investigated the pyrolysis behaviors, volatile release characteristics, and backbone decomposition mechanisms of polyamide 6 (PA6), thermoplastic polyurethane (TPU), and polyacrylonitrile (PAN) through integrating TG-FTIR-GC/MS and density functional theory (DFT). Kinetic analysis based on Coats-Redfern and Achar methods suggested a higher average activation energy of PA6 (228 kJ/mol) compared to TPU (94 kJ/mol) and PAN (189 kJ/mol). PA6 pyrolysis predominantly produced c aprolactam (92.01 %) at 481 °C through the nucleophilic attack of terminal amino group on adjacent amide structure, forming a four-membered transition state with a free energy barrier of 217.0 kJ/mol. Owing to the lower bond dissociation energies (BDEs) of the acyloxy bonds in urethane groups compared to the alkoxy bonds in polyester structures, the hard and soft segments of TPU occurred sequential cracking at 358 and 448 °C, releasing 4,4’-diphenylmethane diisocyanate (85.12 %) and cyclopentanone (69.12 %), respectively. PAN backbone preferentially cleaved at mid-chain C-C bonds with lower BDEs, forming acrylonitrile dimer and monomer fragments, which coupled with each other and combined with hydrogen radicals to generate abundant NH₃ and fatty nitriles like 2-methylglutaronitrile, 1,3,6-hexanetricarbonitrile, and acrylonitrile. These findings elucidated the intrinsic connection between pyrolysis volatile evolution and backbone degradation mechanisms of N-containing plastics at the molecular level, thereby offering theoretical guidance for optimizing the pyrolysis process of real plastic waste to prepare high-value products.