Efficient glycolysis of Poly(ethylene terephthalate) to bis(hydroxyethyl) terephthalate catalyzed by non-metallic deep eutectic solvent

Polyethylene terephthalate (PET) can be depolymerized using glycolysis process into bis(hydroxyethyl) terephthalate (BHET) monomer under relatively mild conditions to realize the closed-loop recycling of PET waste plastics. However, commonly used metal-based catalysts not only bring serious negative impacts on the environment, but also the residue of metals can affect the further application of products. In this work, a green and highly active 1,5-diazabicyclo[4.3.0]non-5-ene deep eutectic solvent (DBN-DESs) was developed for efficient glycolysis of PET. The optimal reaction conditions of PET glycolysis were obtained with 100 % conversion rate of PET and 78.7 % yield of BHET, respectively. The analysis of molecular weight distribution of depolymerized products shows that the glycolysis of PET varies from heterogeneous process to homogeneous process, following the shrinking-core model. First-order reaction kinetic of PET glycolysis with DBN/urea was further built with the activation energy of 155.65 kJ/mol. In particular, the catalytic mechanism of PET glycolysis by DBN/urea was analyzed in details, which reveals that DBN/urea can promote the swelling behavior of PET, and the hydrogen-bonding interaction between DBN/urea, EG and PET can also enhance the attack of hydroxyl oxygen in EG on carbonyl carbon in PET to easily breaking the ester group of PET, thus accelerating PET glycolysis.