Closed-loop recycling of polyesters through glycolysis using catalytic deep eutectic solvents and the influence of glycol types on reaction kinetics

Poly(butylene terephthalate) (PBT) is a widely used polymer in the plastics industry. However, its waste is resistant to natural degradation, posing environmental concerns. Conventional physical recycling can extend service life but often compromises material properties, making it difficult to achieve a sustainable solution. In contrast, glycolysis offers a high-value and environmentally friendly route for recycling waste polyesters. Among various catalysts, deep eutectic solvents (DESs) have gained increasing attention in glycolysis research due to their high efficiency, sustainability, and ease of preparation. Most reported studies focus on DESs composed of metal catalysts and third-party hydrogen bond donors (e.g., choline salts, amines, or urea derivatives) for polyester depolymerization. To date, no literature has reported the direct application of DESs from metal catalysts and reactant glycols in the glycolysis depolymerization. In this study, a DES was directly prepared by combining zinc chloride with reactant glycols (e.g., ethylene glycol and butane diol), serving simultaneously as solvent and catalyst without any additional components, and applied in the glycolysis of PBT. Both systems effectively promoted PBT glycolysis, with the ZnCl₂/ethylene glycol system achieving the highest performance at 200 °C and a molar ratio beyond 1:8, yielding 88–100% conversion and up to 57% bis(2-hydroxyethyl) terephthalate. Kinetic analysis revealed that PBT depolymerization followed a first-order reaction model with an activation energy of 213 kJ mol^–1. These findings confirm the feasibility of directly forming catalytic DESs and demonstrate their dual functionality in polyester depolymerization. Furthermore, the recovered bis(2-hydroxyethyl) terephthalate from ethylene glycol and bis(2-hydroxybutyl) terephthalate from butane diol were successfully repolymerized into polyesters, reducing dependence on virgin raw materials and enabling closed-loop recycling. From an economic perspective, this process offers sustainable viability by lowering both raw material procurement and waste management costs.