Copper-supported catalysts for sustainable PET depolymerization: a cost-effective approach towards dimethyl terephthalate (DMT) production†

Chemical upgrading as a recycling technology for poly(ethylene terephthalate) (PET) waste plastic is an effective way to help solve the current challenges of plastic pollution. Catalytic methanolysis technology enables the depolymerization of PET into high-value dimethyl terephthalate (DMT) monomers under relatively mild conditions, thereby achieving closed-loop recycling of PET waste plastic. In this study, we compared the catalytic efficiency of Cu/SiO₂ catalysts prepared by three different methods for the depolymerization of PET to DMT. Cu/SiO₂-IM, prepared through the impregnation method, exhibited the highest activity, with PET conversion and DMT selectivity reaching 92.35% and 99.0%, respectively, at 200 °C without any initial gas pressure. The exceptional performance of Cu/SiO₂-IM can be attributed to its small metal particles, substantial specific surface area, a high ratio of Brønsted acid sites (BAS) to Lewis acid sites (LAS) and the balance of Cu⁺ and Cu⁰ active sites on the catalytic surface. Furthermore, possible reaction mechanisms of Cu-based catalysts in the catalytic methanolysis of PET were investigated: Cu⁺ initiates the attack on the carbon groups along the long PET chain and enhances the nucleophilic attack during the depolymerization reaction, while Cu⁰ readily interacts with the hydroxyl group of methanol, leading to the formation of a new ester group between the oxygen atom in the hydroxyl group of methanol and the carbon atom in the CO group of PET polyester. This low-cost and high-efficiency catalytic methanolysis study provides a practical approach to PET waste plastic recycling.