Simple But Tricky: Investigations of Terephthalic Acid Purity Obtained from Mixed PET Waste

• Published in: Industrial & engineering chemistry research 2021-09, Vol.60 (35), p.12792-12797
• Main Authors: Cosimbescu, Lelia, Merkel, Daniel R, Darsell, Jens, Petrossian, Gayaneh
• Format: Article
• Language: English
• Subjects: Applied Chemistry
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/acs.iecr.1c02604

In this study, we report, for the first time, the basic depolymerization of mixed waste polyethylene terephthalate (PET) by hydrolysis and subsequent terephthalic acid monomer recovery with high purity using benign reaction conditions. Several conditions were tested for depolymerization such as PET chip size, concentration of aqueous sodium hydroxide (20 or 30%), organic co-solvent (ethylene glycol or ethanol), temperature at which the reaction was run, and duration of the heating. More importantly, several purity grades of PET were utilized as starting materials including commercial PET, chopped PET obtained from clean bottles whose caps and labels were removed, and chopped PET containing caps, rings, and labels. The purity of the product from each condition was evaluated via nuclear magnetic resonance (1H NMR and 13C NMR), differential scanning calorimetry (DSC), and powder X-ray diffraction. Unsurprisingly, the conversion of PET is dependent on the particle size, varying from 100% conversion for fine powder to 73% conversion (for 300 μm, mesh 6–20, or mesh 14–20). Ethanol appears to be more efficient as a co-solvent than ethylene glycol, with higher PET depolymerization conversions (94% vs 75–80%), shorter reaction times (2 h vs 6 h), and lower temperatures (80 °C vs 110 °C). The terephthalic acid (TPA) recovered appeared to have only subtle differences among the batches, most notably a pink color when the reaction was run in ethanol/aqueous base. The DSC curve of the compounds produced in ethylene glycol–water appears to display a melting point (280–288 °C), while the samples prepared in ethanol and a commercial sample did not. Overall, the purity of the various TPA batches is comparable and similar to that of commercial TPA, demonstrating the utility of the method to depolymerize realistic waste streams. The method is simple, demonstrated on a multigram scale (15–30 g), and allows for the complete removal of waste other than PET while being unaffected by alkaline conditions. Eliminating the need to remove caps, labels, and rings and the copious amounts of water required for cleaning makes this process green and environmentally friendly.