Upcycling of PET: A Kinetic Investigation into the Ammonolysis of Bhet and the Impact of Particle Size on Reaction Rate

Poly(ethylene terephthalate) (PET) is ideal for chemical recycling because the ester linkages in the polymer backbone are susceptible to chemical attack. Chemical processes that can be used to break down PET include hydrolysis, methanolysis, glycolysis, and ammonolysis. Ammonolysis is of great interest because it has the potential to enhance product value by enabling a nitrogen-based functionality. High-purity bis-hydroxyethyl terephthalate (BHET) can be obtained by depolymerization of waste PET, and was used as the model system for this work. The ammonolysis of BHET produces terephthalamide, which can be reduced to para-xylylenediamine, and subsequently used to generate polyamides. The effects of mass transfer on depolymerization reactions are not always included in kinetic studies. However, accounting for diffusion limitations could lead to a more realistic model for the chemical breakdown of PET, and was therefore of interest in this work. After the kinetic data for BHET conversion were obtained, ammonolysis experiments were conducted using various particle sizes of PET, and the apparent rate constants were determined. Comparison of the rate constant for the BHET reaction to the apparent rate constants obtained as a function of PET particle size enables the assessment of mass transfer effects.