Cooperative Dual-Site Hydrogen-Bonding Organocatalysis for Efficient Metal-Free PET Glycolysis

The development of efficient catalysts for polyethylene terephthalate (PET) recycling is essential for advancing plastics circularity. Deep eutectic solvents (DESs) have garnered growing attention as catalytic media due to their low volatility, compositional tunability, and organized hydrogen-bonding networks. Herein, we report TBD@TP, an efficient DES catalyst prepared through the equimolar complexation of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and tropine (TP) for PET glycolysis. Under mild reaction conditions (170 °C, 50 min), complete PET conversion and a high bis(2-hydroxyethyl) terephthalate (BHET) yield of 85.1% were attained. Spectroscopic characterization combined with density functional theory calculations supports a proposed dual-site cooperative activation pathway, in which the nitrogen site of TBD in the TBD@TP catalyst enhances the nucleophilicity of ethylene glycol through hydrogen-bonding interactions, while the hydroxyl group of the TP unit contributes to the activation of the ester carbonyl groups of the PET, thereby facilitating the cleavage of ester bonds. Remarkably, the TBD@TP catalyst exhibits excellent thermal stability and maintains stable catalytic performance over at least five consecutive reaction cycles without significant deactivation. Furthermore, the isolated BHET was repolymerized into R-PET with molecular-weight characteristics and intrinsic viscosity comparable to those of the original PET, although differences in thermal behavior were observed, demonstrating the feasibility of a closed-loop recycling pathway.