Mechanistic Insights on Fe(II)-Catalyzed Depolymerization of Poly(Ethylene Terephthalate) by Transesterification

Depolymerization of poly(ethylene terephthalate) (PET: pellets) by transesterification in renewable EtOH has been achieved using a much greener Fe(II) catalyst system. FeCl₂ in combination with 1,2-bis(diphenylphosphino)ethane (dppe) efficiently converts PET into diethyl terephthalate (DET) and ethylene glycol (EG) at 180°C within 2 h under neutral conditions. Kinetic profiling reveals a distinct induction period followed by sigmoidal product formation, indicating in situ generation of the catalytically active species. Pulverization of PET markedly accelerates depolymerization, consistent with a surface-dominated process. To elucidate the catalyst speciation during the reaction, solid- and solution-phase X-ray absorption fine structure (XAFS) analysis was conducted in conjunction with FEFF fitting and density functional theory (DFT) calculations. It was determined that the semi-infinite solid-state structure of FeCl₂(μ²-dppe) transforms into a mononuclear tetrahedral [FeCl₂(dppe)] complex upon dissolution in EtOH. Under depolymerization conditions, the iron center undergoes ligand exchange to generate Fe-alkoxide-containing species, for which [Fe(OEt)₂(dppe)] provides the best structural model based on EXAFS fitting. These results highlight the dynamic nature of Fe(II) catalysts in alcoholysis of PET and support the key role of the in situ iron alkoxide formation in promoting efficient transesterification-based chemical recycling.