Tunable and Recyclable Multi-Hydrogen-Bonded Protic Ionic Liquids for Efficient Methanolysis of PET under Mild Conditions

The recycling of waste PET is of great importance to alleviate resource depletion and environmental pollution. Methanolysis with basic catalysts is considered to be one of the most promising industrial application routes. However, it still requires a high temperature and pressure. Moreover, uncontrolled hydrolysis side reactions lower the selectivity and damage catalysts. Herein, multihydrogen-bonded azole-based protic ionic liquids (PILs) with different basicity were designed and applied in PET methanolysis. The results showed that [TBDH][Triz] with moderate basicity could achieve high selectivity and a yield of 94.2% at 70 °C, representing one of the best performances reported to date. Furthermore, the "volatility" of PILs arising from proton transfer mechanisms enabled their selective recovery from hydrolysis byproducts through vacuum distillation, thereby preventing catalyst deactivation and leading to excellent recycling of PILs. Finally, DFT investigations indicated that the high yield under mild conditions originated from the dual activation mechanism induced by multiple hydrogen bonds, where the reaction energy barrier was significantly reduced.