Upcycling Waste PET: I. Ammonolysis Kinetics of Model Dimethyl Terephthalate and the Catalytic Effects of Ethylene Glycol

Chemical upcycling of waste plastics can play an important role in developing greater circularity in the material flows associated with the plastics industries. In this study, a fundamental understanding of upcycling poly(ethylene terephthalate) (PET) using ammonolysis is established. First, rate constants are determined for model studies of the ammonolysis of dimethyl terephthalate (DMT) in methanol. Ammonolysis proceeds sequentially, and a first ester group of DMT reacts with ammonia to produce methanol and the monoamide methyl 4-carbamoylbenzoate (MCB). Next, MCB reacts with ammonia to yield methanol and terephthalamide (TPD). At 100 °C, the pseudo first order rate constants are k ₁ ^' = 0.25 ± 0.02 h^-1 and k ₂ ^' = 0.11 ± 0.02 h^-1. Experiments conducted at 50, 75, 100, and 125 °C yield activation energies for the first and second reactions of E _a1 = 27.9 ± 2.2 kJ/mol and E _a2 = 37.3 ± 3.3 kJ/mol. Ammonolysis is demonstrated to be catalyzed by ethylene glycol (EG) with a first order concentration dependence. At 100 °C with EG present in a 3:1 excess, the pseudo first order rate constants are k ₃ ^' = 6.3 ± 0.7 h^-1 and k ₄ ^' = 1.7 ± 0.3 h^-1, representing a 22-fold increase. Demonstration experiments with reclaimed mixed postconsumer thermoform containers reveal that the ammonolysis of PET is self-catalyzed by the generated EG; the upcycling reaction on polymer substrates is autocatalytic. This new detailed understanding of the self-catalyzed chemical kinetics of ammonolysis suggests EG as the natural choice for the solvent, a topic pursued in part II of this work.