Two-step Sequential Reactions of Supercritical CO2 and Photocatalysis for PET Degradation and Hydrogen Production

Abstract

With the growing global demand for polyethylene terephthalate (PET), the associated environmental burden has become increasingly serious, making waste PET management a critical global issue. This work proposed a two-step strategy involving supercritical CO2 (ScCO2) and photocatalysis for efficient PET degradation, as well as synergistic H2 production. The addition of a small amount of H2O in ScCO2 as a solvent effectively swelled the PET matrix, weakening intermolecular interactions within the polymer. Meanwhile, the excellent fluidity of ScCO2 facilitated the penetration of H2O into the PET molecular chains, enhancing the hydrolysis reaction. Under the optimized ScCO2 conditions, PET was completely hydrolyzed to terephthalic acid (TPA), ethylene glycol (EG) and a minor amount of ethanol within 6 h. Notably, CO2 could be easily removed as a gas at ambient conditions, thus avoiding the contamination of depolymerization products. Subsequently, commercial P25 was used as a catalyst, enabling the oxidation of EG and ethanol into glycolic acid and acetaldehyde during photocatalytic water splitting, accompanied by H2 production with a rate of 1416.7 μmol h-1 g-1. This method not only achieved efficient and green recycling of waste PET, but also demonstrated the potential of ScCO2 as a sustainable reaction medium for polymer depolymerization. Finally, the reaction mechanism was investigated by analyzing intermediate and final products, offering guidance for ScCO2 and photocatalytic degradation of plastic.