Formation of Bis(hydroxyethyl) terephthalate from waste plastic using ionic liquid as catalyst

•1,1,3,3-tetramethylguanidine and different metal salt as raw material to synthesize ionic liquid catalyst, that is, [TMG]Cl/ZnCl2, [TMG]Cl/CoCl2, [TMG]Cl/FeCl3 and [CnTMG]Cl/ZnCl2 (n=2,4,6,8).•The influence of structure of cation and experimental factors on PET glycolysis were investigated. The catalytic effect of catalyst was mainly affected by the electron donating and steric hindrance.•DFT calculation was performed to analysis of influence of structure of cations on glycolysis of PET.•[C6TMG]Cl/2ZnCl2 as catalyst obtain the optimal reaction conditions: m (PET): m(ethylene glycol (EG)): m(catalyst) = 1: 5.0: 0.1, and atmospheric pressure at 195°C for 70 min, the conversion of PET is 100%, the BHET yield is 92.7%. To our best knowledge, this is first report on ionic liquid catalyst of PET degradation with superior BHET yield.•The degradation of commercial beverage bottles was studied. The purity of main product was around 90wt% and BHET yield was 84%.•6. [TMG]Cl/ZnCl2, [TMG]Cl/CoCl2 and [TMG]Cl/FeCl3 were characterized by ESI-MS, FT-IR, and TGA.•7. Main product was characterized by UPLC, ESI-MS, NMR and FT-IR. The accumulation of waste PET leads to serious environmental problem. PET degradation has been hot topic and is studied by lots of researchers. At present, numerous catalysts have been developed, among which ionic liquid catalyst is attractive due to its excellent advantages like convenient recycling and highly catalytic effect. In this work, a series of ionic liquids based on 1,1,3,3-tetramethylguanidine (TMG) were synthesized and used as catalyst of PET degradation. The influence of different structure of cation and different metal salt on PET degradation were studied. DFT calculation showed that the interaction between the ionic liquid and ethylene glycol can be changed by adjustment of the structure of the cation. At optimal condition, using [C6TMG]Cl/2ZnCl2 as catalyst, PET conversion was 100% and BHET yield was 92.7%. To our best knowledge, this is first report about ionic liquid catalyst of PET degradation with superior BHET yield. Catalytic effect of [C6TMG]Cl/2ZnCl2 did not obviously decrease after it was reused for six times. Moreover, the degradation of commercial beverage bottles was investigated using [C6TMG]Cl/2ZnCl2 as the catalyst, the BHET yield of 84% was obtained, and the quality purity of the product was 90%. In addition, possible mechanism was explored.