Catalytic function of ionic liquids in polyethylene terephthalate glycolysis by molecular dynamics simulations

Abstract

Chemical recycling is of paramount importance to minimise the environmental impact of plastic waste. Polyethylene terephthalate (PET) is a polar thermoplastic widely used in fibres and packaging and is amenable to chemical depolymerisation. Recent efforts are devoted to its degradation via glycolysis. Even though it requires milder conditions than hydrolysis, catalysts are still necessary. In this case, ionic liquids (ILs) come into play to catalyse the reaction. In particular, we focus on choline-based liquids due to their low toxicity and cost compared to imidazolium-based ones. However, due to the complexity of the process, detailed information on the operating mechanism is scarce, which hinders the progress towards a rational design of new and more efficient systems. Herein, we present a computational study to address the role of IL catalysts during PET glycolysis under realistic catalytic conditions, i.e., considering time, concentration, and temperature. We perform classical molecular dynamics (MD) simulations on several systems, including a complex ternary mixture formed by ethylene glycol (EG), PET oligomers, and the [Ch]₃[PO₄] catalyst. By means of radial/spatial distribution functions, H-bond analysis, and domain count, we present a detailed solvation scenario of the catalytic system. Our findings suggest that the IL anion (and the IL cation to a lesser extent) does participate in the nucleophilic activation of EG, while the IL cation does not play a significant role in the electrophilic activation of PET.