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

Area of Science:

• Green Chemistry
• Catalysis
• Polymer Science

Background:

• Poly(ethylene terephthalate) (PET) recycling is crucial for sustainability.
• Existing depolymerization methods often require harsh conditions or less eco-friendly catalysts.
• Developing efficient and green catalytic systems for PET chemical recycling is a significant challenge.

Purpose of the Study:

• To develop a greener and efficient iron(II) catalyst system for PET depolymerization.
• To investigate the catalytic mechanism and active species involved in the transesterification of PET.
• To explore the potential for chemical recycling of PET using renewable ethanol.

Main Methods:

• Depolymerization of PET using FeCl2 and 1,2-bis(diphenylphosphino)ethane (dppe) in ethanol.
• Kinetic profiling to understand reaction progression and active species formation.
• Solid- and solution-phase X-ray absorption fine structure (XAFS) analysis.
• Density functional theory (DFT) calculations and FEFF fitting for structural elucidation.

Main Results:

• Efficient depolymerization of PET into diethyl terephthalate (DET) and ethylene glycol (EG) within 2 hours at 180°C.
• Identified an induction period followed by sigmoidal product formation, suggesting in situ catalyst activation.
• Demonstrated that pulverizing PET significantly accelerates the depolymerization process.
• Characterized the transformation of the iron catalyst from FeCl2(μ2-dppe) to mononuclear [FeCl2(dppe)] and finally to Fe-alkoxide species like [Fe(OEt)2(dppe)].

Conclusions:

• The developed Fe(II)/dppe system offers an efficient and greener route for PET chemical recycling via transesterification.
• In situ formation of iron alkoxide species is critical for the high catalytic activity in PET alcoholysis.
• The findings highlight the dynamic nature of iron catalysts and provide insights into optimizing PET depolymerization processes.