The upcycling of polyethylene terephthalate using protic ionic liquids as catalyst

The recycling of waste PET is of great significance to alleviate environmental pollution and resource depletion. Ionic liquid, as environmentally friendly “green” solvent, has been applied in PET glycolysis. However, most of ionic liquids have some defects such as the cost and relatively complex preparation process, which is not conducive to its practical application. Herein, a series of protic ionic liquids were synthesized by simply mixing Bronsted acid and Bronsted base to catalyze PET glycolysis. The influence of different anion on PET glycolysis was investigated and results suggested that anion containing electron donating group (CH3-) has ideal activity and the electron donating group at different position of benzene ring also affects catalytic performance of protic ionic liquids. 1H NMR spectrum of PIL-EG system and DFT calculation showed that there is interaction between ethylene glycol and protic ionic liquid and presence of hydrogen transfer process in PET glycolysis catalyzed by protic ionic liquid. Possible reaction pathway was proposed. At optimal reaction condition, PET conversion is 100% and BHET yield reached 87.3% using PIL3 as catalyst. Moreover, protic ionic liquid still has excellent catalytic performance after being reused for eight times. The kinetic of PET glycolysis catalyzed by PIL3 was explored and apparent activation energy is 162.82 kJ/mol.