Metal Ion Microwave-Assisted Depolymerization of Poly(Ethylene Terephthalate): A Zinc Salts-Based Deep Eutectic Solvent as Case Study

In this study, a new and very quick method to depolymerize PET plastics is reported. The depolymerization experiments were conducted using a type-IV deep eutectic solvent containing ZnCl2 and urea, and a microwave oven as reactor. Different combinations of power and reaction times were employed while keeping the total energy constant. Successful conversions were obtained carrying out the process at 180 W for 2 min and 360 W for 1 min, whereas at higher powers and shorter times, an inclusion likely occurs of some solvent into the structure of the recovered PET flakes, as suggested by the porosity of the flakes, imaged by SEM microscopy. The flakes increase their crystalline character during the treatment, as indicated by the appearance of narrow diffraction peaks in the XRD patterns, at variance with the broad signals observed in the case of the pristine amorphous polymer. The NMR analysis of the supernatant liquid above the partially solubilized PET shows the presence of terephthalic acid peaks. The infrared spectra of the solid powder achieved upon the acidic treatment of the extract reveal the presence of C=O stretching peaks and the absence of typical CH2 wagging absorptions that satisfactorily comply with the presence of terephthalic acid.