Fast, High Monomer Yield from Post-consumer Polyethylene Terephthalate via Combined Microwave and Deep Eutectic Solvent Hydrolytic Depolymerization

Efficient low carbon foot print methods are critical to achieving circularity for the dominant post-consumer plastic polyethylene terephthalate (PET). In a strong sustainability advancement over previous technologies, depolymerization of waste PET bottles was performed using a dissolution/degradation approach optimized in accordance with polymer mechanical parameter inter-relationships. A dual functioning deep eutectic solvent (DES), comprising m-cresol and choline chloride, served as both the solubilizing and catalyzing agent for alkaline hydrolysis of PET using high energy efficiency microwave (MW) irradiation. The PET depolymerization process was optimized using Box–Behnken design while tailoring the DES volume, concentration of the depolymerizing agent (sodium hydroxide), and MW irradiation time as independent variables. The percentage PET weight loss as high as 84% was obtained using 15 mL of DES containing 10% (w/v) NaOH under 90 s MW irradiation. Simple, cost-effective purification steps were afforded by the DES’s advantageous physicochemical nature and were implemented to provide the terephthalic acid (TPA) monomer with acceptable yield. Validation of the PET depolymerization and identification of obtained monomers were carried out by a range of characterization techniques including FTIR, NMR, DSC, and HPLC. Post-consumer PET bottle depolymerization was evaluated, and a 91.55% TPA monomer yield ready for repolymerization as virgin PET demonstrates the high potential market application of this low energy, low carbon solvent virgin to virgin approach to PET circularity.