Degradation of poly(ethylene terephthalate) catalyzed by metal-free choline-based ionic liquids

Abstract

Glycolysis of poly(ethylene terephthalate) (PET) is a prospective way for degradation of PET to its monomer bis(hydroxyethyl) terephthalate (BHET) which can be polymerized again to form new qualified PET materials, and hence provides possibilities for a permanent loop recycling. However, most of the reported glycolysis catalysts are metal-based, leading to high cost and negative environmental impact. In this study, we developed a series of choline-based ionic liquids (ILs) without metals and applied them in the glycolysis of PET as catalysts. Choline acetate ([Ch][OAc]), which is cheaper, more biologically compatible and environmentally friendly in comparison with conventional imidazolium metal-based ILs, can achieve a comparable or even better performance than them. Under optimum conditions (PET (5.0 g), ethylene glycol (EG) (20.0 g), [Ch][OAc] (5 wt%), 180 °C, 4 h, atmospheric pressure), the yield of BHET reached up to 85.2%. Additionally, the reaction kinetics was studied and proved to be the shrinking-core model. The apparent activation energy is 131.31 kJ mol⁻¹, and the pre-exponential factor is 1.21 × 10¹³ min⁻¹. Finally, based on the experimental results and density functional theory (DFT) calculations, a possible mechanism was proposed. The promotion of the glycolysis reaction is attributed to the activation of EG by the formation of hydrogen bonds between EG and the IL.