First-Row Transition Metal-Containing Ionic Liquids as Highly Active Catalysts for the Glycolysis of Poly(ethylene terephthalate) (PET)

First-row transition metal-containing ionic liquids (ILs) were synthesized and used to catalyze the degradation of poly(ethylene terephthalate) (PET) in ethylene glycol (EG). One important feature of these IL catalysts is that they have good thermal stability, and most of them, especially [bmim]2[CoCl4] (bmim = 1-butyl-3-methyl-imidazolium) and [bmim]2[ZnCl4], exhibit higher catalytic activity, compared with traditional catalysts, conventional IL catalysts, and some functional ILs. For example, utilizing [bmim]2[CoCl4] as catalyst, the conversion of PET, selectivity of bis(hydroxyethyl) terephthalate (BHET), and mass fraction of BHET in products reach up to 100%, 81.1%, and 95.7%, respectively, under atmospheric pressure at 175 °C for only 1.5 h. Another important feature is that BHET can be easily separated from these IL catalysts and has high purity. Moreover, recycling results show that [bmim]2[CoCl4] worked efficiently after being used six times. These all show that [bmim]2[CoCl4] is an excellent IL catalyst for the glycolysis of PET. Finally, based on in situ IR spectra and experimental results, the possible mechanism of degradation with synthesized IL is proposed.