Glycolysis of poly(ethylene terephthalate) waste catalyzed by mixed Lewis acidic ionic liquids

Widely used poly(ethylene terephthalate) (PET) material induces environmental concern on its wastes. In this paper, different 1-hexyl-3-methylimidazolium (Hmim) halometallates, including [Hmim]ZnCl3, [Hmim]CoCl3, [Hmim]FeCl4 and [Hmim]CuCl3, are synthesized as Lewis acidic ionic liquids (LAIL) catalysts for PET degradation in excess ethylene glycol (EG). By using individual or mixed LAIL catalysts, product bis(hydroxyethyl) terephthalate (BHET) is characterized by FT-IR, 1H-NMR and DSC et al. From the PET conversion and yield of BHET product, a synergistic effect is found in mixed [Hmim]ZnCl3 and [Hmim]CoCl3 complexes. 87.1% BHET from original PET wastes catalyzed by equimolar [Hmim]ZnCl3 and [Hmim]CoCl3 mixture is higher than any individual IL halometallate. The filter residues after removing the BHET products with different reacting time using [Hmim]ZnCl3 and [Hmim]CoCl3 as catalyst, respectively, are characterized by 1H-NMR. The area ratio of the methylene protons of COO–CH2 (δ = 4.30 ppm) and the aromatic protons of the benzene ring (δ = 8.12 ppm) of filter residues suggests that more by-products will be produced by [Hmim]ZnCl3 because of its relatively higher catalytic activity in the chain scission stage. The glycolysis synergy comes from the balance between high reactivity of [Hmim]ZnCl3 and high selectivity of [Hmim]CoCl3.