Conversion of waste into wealth in chemical recycling of polymers: Hydrolytic depolymerization of polyethylene terephthalate into terephthalic acid and ethylene glycol using phase transfer catalysis

Abstract

Hydrolytic depolymerization of polyethylene terephthalate (PET) waste was studied using high pressure autoclave reactor at 240 °C (molten state) and autogenous pressure using excess of water in the presence of a phase transfer catalyst polyethylene glycol (PEG 400) and the product profile was traced at various time intervals. In comparison with zinc acetate (used before), PEG 400 was the best catalyst. Concentration profiles were developed for PET, oligomer and terephthalic acid (TPA) using HPLC. Effect of initial molar ratio of 22–110 mol of water/mol PET on depolymerization was studied by using both HPLC and end group analysis. Initial molar ratio of 55–110 mol of water/mol PET was found to give 99 percent conversion in 30 min at 240 °C. A complete conversion of ester linkages to acid group and the desired product TPA was studied. On the basis of ester linkage, 100% conversion was observed in 10–12 min reaction time. At 30 min additional time, it was found to give 95% yield of TPA. Molar concentration of PEG 400 of 2.0 × 10−5 mol/cm3 was sufficient to give the maximum conversion in breaking of ester linkages. The yield and purity of TPA was found to be 90 and 99.1%, respectively. A new mechanism of solid (polymer)-liquid (melt)-liquid (water) phase transfer catalysis (PTC) for hydrolysis was proposed and validated. A pseudo-first order rate equation was fitted for depolymerization with a rate constant of 1.4 min−1 at 240 °C and apparent activation energy of 34.4 kJ/mol. The rate of hydrolysis is very fast and complete depolymerization takes place within 30 min. This is an excellent example of circular economy and cleaner production from waste plastic.