Ionic-Liquid-Catalyzed Depolymerization of PET: BHET Recovery and Re-Polymerization

Abstract

The accumulation of polyethylene terephthalate (PET) waste presents significant environmental challenges due to its extensive usage and low global recycling rates. Among the available recycling routes, chemical depolymerization by glycolysis offers a promising way to recover high-value monomers. In this study, six ionic liquids (ILs) were tested as homogeneous catalysts for the glycolysis of post-consumer PET bottles in ethylene glycol (EG) at 200 °C for 3 h, alongside an uncatalyzed control. 1-Butyl-3-methylimidazolium acetate ([BMIM][OAc]) and trihexyltetradecylphosphonium decanoate ([P6,6,6,14][Dec]) gave complete PET conversion with isolated bis(2-hydroxyethyl) terephthalate (BHET) yields of 33.6% and 72.4%, respectively. Moreover, trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate ([P6,6,6,14][TMPP]) reached 56% conversion with no separable product; however, the other three chloride-based ILs and the control reaction demonstrated negligible conversion at the given process parameters. The recovered BHET was characterized by FT-IR, 1H and 13C NMR and GC-MS, and further, was re-polymerized by melt polycondensation at 270 °C. The FT-IR fingerprint, thermogravimetric decomposition profile and solid-state 13C NMR spectrum indicated that the properties of the recycled PET compete with those of the bottlegrade PET. Their melting peak lay 7.7 °C below the reference on first heating (240.6 vs. 248.3 °C) but rose to 248.5 °C on second heating, 3.3 °C above the reference, suggesting that the initial difference arises mainly from crystal perfection and thermal history rather than from an altered chain structure.