Influence of Metal Salts on the Hydrolysis of Postconsumer Poly(ethylene Terephthalate)

This study reveals the effects of 23 metal salts on the hydrolysis of postconsumer poly(ethylene terephthalate) (PET). Isothermal reactions were conducted at 200 °C for 2 h and with a 1/10 (w/w) loading of PET and water. Terephthalic acid (TPA) yields of at least 80% were obtained with indium and ytterbium triflates, zinc iodide, potassium and sodium carbonates, potassium phosphate, and sodium bicarbonate. Without additives, the TPA yield was less than 10%, suggesting that these salts are potential candidates to improve the chemical recycling of PET. We expect complete catalytic depolymerization of PET to be available at longer times or higher temperatures. Reuse of zinc acetate solution for sequential hydrolysis experiments consistently gave high TPA yields for six cycles. We used experimental dataset and machine learning to determine that TPA yields and PET conversion were primarily influenced by the mass ratio of metal salt to PET, Lewis acidity, solution ionic strength, and pH. The catalytic effect of some metal triflates first increased with increasing Lewis acidity but peaked and declined as the Lewis acidity increased further. This suggests that higher Lewis acidity not only increases the intrinsic rate of hydrolysis but also increases the strength of the solvation shell around the catalyst, thereby hindering its access to the ester bonds in PET.