Deep eutectic solvents as highly active catalysts for the fast and mild glycolysis of poly(ethylene terephthalate)(PET)

Deep eutectic solvents (DESs) have attracted broad attention due to their low cost, easy preparation, low toxicity, good biological compatibility and similar characteristics to those of ionic liquids (ILs). In this study, we found that not only the glycolysis time is sharply shortened under mild reaction conditions, but also the high selectivity of monomer bis(hydroxyalkyl) terephthalate (BHET) is obtained when DESs were used as catalysts. Then, the influences of technological parameters on PET degradation were investigated and the optimization conditions were obtained. Under the optimization conditions of ethylene glycol (EG) (20 g), catalyst (n(urea)/n(ZnCl2) 4/1, 0.25 g), PET (5 g), and atmospheric pressure at 170 degrees C for 30 min, the conversion of PET and selectivity of BHET were 100% and 83%, respectively. This time is equal to that taken by a supercritical method under 15.3 MPa at 450 degrees C. In addition, the degradation mechanism of PET wastes catalyzed by DESs is proposed through the experiments and DFT calculations. The high catalytic activity is attributed to the synergetic catalysis of H-bonds and coordination bonds formed between the DES catalyst and EG.