The application of deep eutectic solvents (DESs) in the catalytic degradation of polyethylene terephthalate (PET) has attracted increasing attention due to the high efficiency, biodegradability, and low toxicity of DESs. Herein, we report an efficient 1,5,7-triazabicyclo[4.4.0]decium chloride/zinc chloride (HTBD-Cl/ZnCl2) DES catalyst for recycling waste PET based on a combination of experimental and theoretical studies. First, the HTBD-Cl/ZnCl2 DES structure, in which the HTBD-Cl moiety and ZnCl2 were connected through a hydrogen bonding interaction, was clearly determined through theoretical calculations and experimental verification. Then, the HTBD-Cl/ZnCl2 was found to be effective for waste PET glycolysis, where the PET conversion reached 100%, and the bis(hydroxyalkyl) terephthalate (BHET) yield reached 84% at 185 °C within 60 min in atmosphere. Density functional theory (DFT) calculations demonstrated that the PET glycolysis reaction mediated by the HTBD-Cl/ZnCl2 DES included two stages, and the 1,5,7-triazabicyclo[4.4.0]decium cation (HTBD) played a crucial role. In the first stage, the active species [Zn]-OCH2CH2OH anion was generated through proton transfer from HTBD to Cl-, which was followed by HOCH2CH2O-H bond activation through the cooperation of HTBD and Zn. In the second stage, catalytic depolymerization of PET occurred through the cooperative functions of the Zn center and HOCH2CH2O- anion ligand.

Molecular mechanism of waste polyethylene terephthalate recycling by the 1,5,7-triazabicyclo[4.4.0]decium chloride/zinc chloride deep eutectic solvent

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