Research progress on chemical upcycling of waste polyethylene terephthalate

<sec><p indent="0mm">The industrial production of plastics has greatly enhanced our daily life, but the rapid accumulation in waste plastics now places immense pressure on the ecological environment, making recycling and reuse a global priority. Polyethylene terephthalate (PET) is an excellent thermoplastic polyester, which is widely used in textiles, packaging, electronic devices, and other industries. Currently, chemical recycling of waste PET primarily involves hydrolysis and alcoholysis, converting it into monomers or oligomers that can be used to synthesize new PET or other high-value chemicals. This review summarizes and analyzes recent advancements in the pyrolysis, hydrolysis, alcoholysis, and hydrogenolysis of PET, aiming to elucidate the reaction mechanisms that govern product selectivity. PET pyrolysis involves the breaking down of PET into small molecular products, such as acids, esters, ketones, and aromatics under oxygen-free or oxygen-deficient conditions with high-temperature heating. The product composition is influenced by the choice of catalysts. PET hydrolysis can be classified into acidic hydrolysis, alkaline hydrolysis, and neutral hydrolysis based on the pH of the aqueous phase. OH⁻ or H₂O acts as a nucleophile to attack the ester bonds, facilitating PET depolymerization into terephthalic acid (TPA) and ethylene glycol (EG). The reaction process of PET hydrolysis is mainly controlled by increasing the concentration of OH⁻ or H⁺. PET alcoholysis is typically conducted in methanol and ethylene glycol, with acidic sites influencing the reaction pathway. Under high temperature and acid-base catalysis, PET undergoes ester exchange reactions, breaking the polymer’s ester bonds, and ultimately converting it into dimethyl terephthalate (DMT) or bis(2-hydroxyethyl) terephthalate (BHET). PET hydrogenolysis, on the other hand, involves breaking the ester bonds of PET using H₂ to produce short-chain saturated hydrocarbons (such as benzene, toluene, and xylene (BTX)). The catalytic active centers are designed and optimized to enhance the efficiency of PET hydrogenolysis. </sec><sec> Additionally, this review explores photocatalytic, electrocatalytic and microwave-assisted depolymerization of PET to improve the product selectivity. Photocatalysis is a clean, emerging catalytic method in which semiconductor catalysts generate photogenerated electrons and photogenerated holes under light excitation. The photogenerated electrons are excited from the valence band to the conduction band, enabling reduction reactions (such as reducing H₂O to H₂), while the holes in the valence band oxidize plastics into valuable small organic molecules. This approach enables mild plastics degradation. PET electrocatalytic upgrading includes processes like the PET hydrolysis and the oxidation of product ethylene glycol to high-value chemicals (such as terephthalic acid, formate, and glycolate). These processes include complex cascade reactions that produce various C₁/C₂ products depending on the catalyst used. Microwave-assisted depolymerization leverages the selective heating properties of microwaves, activating polar chemical bonds and reducing the activation energy of the reaction. Besides, this process enhances PET depolymerization, shortens reaction times and generates high-value products under mild conditions, supports the circular economy goals. </sec><sec> This review also summarizes the difficulties and challenges associated with various recycling methods and outlines the future directions for PET chemical upcycling. The development of efficient catalysts and in-depth exploration of reaction mechanisms to control product selectivity is recommended to advance the understanding of PET selective depolymerization process. Integrating new technologies, such as microwave-assisted processes and photo/electrocatalysis, could further enhance the reaction rates and efficiency at mild reaction conditions, which accelerates their application in real waste plastic scenarios. This review provides technical references for advancing the industrial recycling technologies and promotes the progress in plastic waste upcycling. </sec>