The preparation of TPA monomer through the pathway of waste PET steam pyrolysis using high H/C ratio gas as external hydrogen donors

A novel Polyethylene terephthalate (PET) steam pyrolysis method is developed by introducing high H/C ratio gas to produce terephthalic acid (TPA) monomer. CH 4 , C 2 H 6 , C 2 H 4 , C 3 H 8 , and C 3 H 6 were used as external hydrogen donors to upgrade the waste PET steam pyrolysis process, which were proven to be effective. Under optimized conditions (H 2 O flow rate of 0.1 mL/min, C 3 H 8 flow rate of 2 mL/min, heating rate of 0.5 °C/min, and final pyrolysis temperature of 450 °C), the H 2 O/C 3 H 8 /N 2 system achieved a product yield increase from 32.69% to 93.15% and a TPA purity improvement from 26.57% to 90.73%, with complete elimination of coke residues. Isotope tracing experiments using D 2 O confirmed the dual roles of H 2 O in hydroxyl substitution and hydrogen transfer, with C 3 H 8 compensating for hydrogen deficiency from H 2 O and enhancing the hydrogen transfer pathway. Through DFT calculations, the pathways and mechanisms of PET steam pyrolysis using C 3 H 8 as an external hydrogen donor were proposed, demonstrating that H 2 O and C 3 H 8 synergistically mediate the oriented pyrolysis of PET toward TPA by supplying ·OH and ·H radicals. This method offers a new idea for directional pyrolysis to prepare monomers, which is of great significance for achieving the recycling of waste plastics.