A sustainable approach for polyethylene terephthalate waste valorization: optimization and mechanistic study of glycolysis using Zn(OAc)2·2H2O/urea deep eutectic solvent

The chemical recycling of polyethylene terephthalate (PET) via glycolysis is a promising pathway for the circular economy; however, it often requires harsh conditions or expensive catalysts. This study investigates the depolymerization of waste PET using ethylene glycol (EG) mediated by a deep eutectic solvent (DES) synthesized from zinc acetate dihydrate (Zn(OAc)2·2H2O) and urea. The formation of the DES via hydrogen bonding was confirmed by Fourier transform infrared analysis. Experimental results demonstrated a remarkable enhancement in catalytic activity owing to the DES, increasing the depolymerization efficiency from negligible levels (using only EG) to complete conversion. Under optimal conditions, a Zn(OAc)2·2H2O : urea molar ratio of 1 : 1.5, reaction temperature of 180°C, and reaction time of 90 min, the PET conversion reached 100% with a bis(2-hydroxyethyl) terephthalate (BHET) yield of 63.8%. The structure of the recovered BHET monomer was verified by 1Hnuclear magnetic resonance spectroscopy. Furthermore, a synergistic mechanism involving the Lewis acidity of Zn2+ and the hydrogen-bonding network of urea is proposed to explain the superior catalytic performance. This work presents a cost-effective, green and highly efficient protocol for PET waste valorization.