Subcritical CO2–H2O hydrolysis of polyethylene terephthalate as a sustainable chemical recycling platform†

The development of an efficient and environmentally sustainable chemical hydrolysis process for recycling waste plastics, based on green chemistry principles, is a key challenge. In this work, we investigated the role of subcritical CO₂ on the hydrolysis of polyethylene terephthalate (PET) into terephthalic acid (TPA) at 180–200 °C for 10–100 min. The addition of CO₂ into the reaction mixture led to the in situ formation of carbonic acid that helps to catalyze PET hydrolysis relative to hot compressed H₂O (i.e. N₂–H₂O). The highest TPA yield of 85.0 ± 1.3% was obtained at 200 °C, PET loading of 2.5 g PET in 20 mL H₂O for 100 min, and 208 psi of initial CO₂ pressure. In addition, the subcritical CO₂–H₂O system demonstrated high selectivity toward hydrolyzing PET in a mixture with polyethylene (PE) at 200 °C for 100 min, thus providing “molecular sorting” capabilities to the recycling process. The robustness of the process was also demonstrated by the ability to hydrolyze both colored Canada Dry and transparent Pure Life® waste PET bottles into high yields of TPA (>86%) at 200 °C. In addition, subcritical CO₂–H₂O hydrolysis of colored PET bottles resulted in a white TPA product similar to that generated from transparent PET bottles. Overall, this work shows that, under optimized reaction conditions, subcritical CO₂ can provide acid tunability to the reaction medium to favor waste PET hydrolysis for subsequent recycling.