Research progress on chemical depolymerization and upcycling of PET waste plastics

Plastics are ubiquitous in daily life due to their outstanding performance and cost-effectiveness. However, plastics are not without their drawbacks. Plastics are synthesized from fossil resources, and their widespread use accelerates the depletion of non-renewable resources. Additionally, their inherent stability makes them resistant to degradation under natural conditions, and they continuously release microplastics that pose significant harm to living organisms. As a result, plastic consumption and pollution are considered to be a critical environmental crisis that threatens sustainable human development. Against the backdrop of China’s heightened environmental protection efforts and its active promotion of “dual carbon” goals, the resource utilization of plastic waste has become a central societal issue. Polyethylene terephthalate (PET), one of the most widely produced polyester plastics, is extensively used in the manufacture of various everyday products and single-use packaging materials. PET synthesis began in the 1940s and has been used on a large scale for disposable packaging since the 1970s. PET is synthesized through the polycondensation of bis(2-hydroxyethyl) terephthalate (BHET), which can be produced by esterification of terephthalic acid (TPA) with ethylene glycol or from dimethyl terephthalate (DMT) and ethylene glycol. Despite existing initiatives aimed at recycling PET, the current recycling rate remains low. PET recycling is generally classified into primary, secondary, tertiary, and quaternary recycling. Primary and secondary recycling involve physical methods that recover PET waste without altering its chemical structure, for instance, PET waste plastic bottles are melt-spun to make recycled polyester fibers. Tertiary recycling restores the molecular value (chemical recycling), while quaternary recycling creates energy from the waste. With the rise of the circular economy, there has been a marked increase in research on PET recycling technologies, particularly in chemical recycling, which aims to establish a closed-loop system in which discarded PET is reintegrated into production. Chemical depolymerization pathways have been developed, allowing PET waste to be broken down into aromatic monomers and ethylene glycol. Through further purification and polycondensation, recycled PET can be produced from these monomers, thereby enabling closed-loop recycling. The depolymerization processes are typically hydrolysis, methanolysis, and glycolysis. Among them, hydrolysis usually requires the participation of water and the reaction with acidic or alkaline substances. Methanolysis requires an excess of methanol to react with PET. Glycolysis likewise requires the addition of large amounts of additional glycol for the reaction. These chemical depolymerization routes have a common by-product, ethylene glycol. However, due to the high boiling point of ethylene glycol, the chemical depolymerization of PET presents significant challenges in terms of separation and purification. In recent years, new recycling strategies have emerged that combine organic synthesis, electrochemical, and photochemical techniques to enhance the utilization of ethylene glycol including selective oxidation reactions to make carboxylic compounds. These innovative strategies not only preserve the valuable aromatic monomers of PET but also facilitate the recovery of new high-value chemicals. This paper reviews the production and recycling technologies for PET plastics, with a particular focus on the chemical depolymerization pathways involved in PET chemical recycling. It systematically examines the current status of traditional PET chemical depolymerization techniques and new strategies for their further development. Additionally, the paper explores the key challenges, research priorities, and prospects for producing high-value monomers from waste PET. This review offers new perspectives and insights for advancing sustainable development and the circular economy while promoting the importance of the waste plastic recycling industry.