Depolymerization of polyethylene terephthalate with glycol under comparatively mild conditions

Abstract

The depolymerization of polyethylene terephthalate (PET) can proceed by using ethylene glycol (EG) as an alcoholysis solvent. However, high reaction temperature (≥180℃) of glycolysis process requires high energy consumption, and unfortunately results in yellowing of monomer product, i.e., bis(2-hydroxyethyl) terephthalate (BHET), limiting the production of high-quality and colorless recycled PET (rPET). To address this problem, a strategy involving acetonitrile as cosolvent was applied to decrease glycolysis temperature to 90℃ or even lower, and solvents as well as catalyst can be recycled and reused. The glycolysis of PET is hard to proceed at 90℃. However, the swelling effect of acetonitrile caused cracks and defects on the surface of PET, which promoted the depolymerization of PET and led to an S-shaped reaction curve. Besides, adding acetonitrile could significantly accelerate the degradation of oligomers into BHET. Under the optimal reaction conditions, the conversion of PET and yield of BHET could reach 96% and 90%, respectively.