A Versatile KF/EG Deep Eutectic Solvent that Lunges PET Depolymerization Forward through Reusability, Tunable Selectivity and Energy Efficiency

This study investigates the depolymerization of PET in a potassium fluoride/ethylene glycol deep eutectic solvent (KF:EG, 1:6 molar ratio) a metal-free, halide-based DES employed here as both reaction medium and catalyst for PET chemical recycling using conventional heat-induced and microwave-assisted reactions, examined under anhydrous and hydrated conditions in the 0–43 mol% water range. The system combines hydration-controlled product selectivity, microwave acceleration enabled by the exceptional dielectric properties of the DES, and mechanistic insight into terminal-chain scission rationalized through molecular dynamics simulation. Depolymerization efficiency was evaluated by measuring PET and fluoride consumption yields and monomer product ratios via quantitative NMR, FTIR, and XRD analyses. Mechanistic pathways were explored through molecular dynamics simulations. Under heat-induced anhydrous conditions, both PET consumption and monomer recovery reach a stable 100% over five cycles at 180°C with 15 min heating confirming the purely catalytic role of fluoride under these exact conditions and demonstrating the full recyclability of the DES system. Under hydrated conditions, progressive fluoride losses of up to 85%, attributed to HF formation via proton abstraction from water and in situ-generated terephthalic acid followed by volatilization at the reaction temperature, indicate a transition of fluoride from a catalytic to a reagent-like role. The safety implications of HF generation, the associated engineering requirements for larger-scale implementation, and the impact on the green metrics of the hydrated pathways are discussed explicitly in the manuscript. The product distribution is rich in mono(2-hydroxyethyl) terephthalate (MHET), with bis(2-hydroxyethyl) terephthalate (BHET) and terephthalic acid (TA) formed as co-products. MD simulations indicate that the high MHET abundance arises from a coiled PET morphology in the DES, exposing terminal ester groups to solvent attack and supporting a mechanistic hypothesis of favoring chain-end scission over random glycolysis. The addition of water enhances PET consumption under moderate heating conditions and shifts product selectivity toward TA, reaching up to 97%. Microwave-assisted depolymerization achieved quantitative PET conversion within 3 min at 90 W under anhydrous conditions, yielding MHET as the primary product, whereas water addition reduced efficiency due to low microwave absorption relative to EG. Finally, the sustainability of the anhydrous processes was quantified using the energy economy coefficient (ε), providing a value of 0.037 and an estimate of 0.169 for the heat-induced and microwave-assisted processes, placing the proposed system among the most energy-efficient approaches reported so far.