Design and synthesis of novel ionic liquids with unique structures and excellent catalytic activity for transesterification

Abstract

A series of novel ionic liquids (ILs) with near-neutrality, moderate nucleophilicity, and zwitterionic structure were synthesized using a one-step method. Notably, these ILs could be successfully synthesized by activating 1-methylimidazole with carbonate, carboxylic, and oxalate esters. The structures and properties of the synthesized ILs were qualitatively and quantitatively analyzed using Fourier-transform infrared spectroscopy, ¹H/¹³C, nuclear magnetic resonance spectroscopy, thermogravimetry–mass spectrometry, and Hammett indicator. A mechanism was proposed for activating 1-methylimidazole, and the negative charge densities of the ILs were calculated. Considering MI-EC as an example, the best conditions for the synthesis of ILs were optimized reaction at 85 °C for 18 h, and the synthesis pathway was determined through density functional theory calculations. Herein, MI-EC exhibited excellent catalytic activity for transesterification reactions, and the corresponding ethylene carbonate (EC) conversion, dimethyl carbonate (DMC) yield, and turnover frequency (TOF) reached 50.4 %, 30.5 %, and 127.8 h⁻¹, respectively, with a catalytic reaction of only 30 min. Furthermore, the mechanism underlying the transesterification reaction catalyzed by MI-EC was investigated. The catalytic activity and structure of MI-EC remained unchanged after six reuses, demonstrating its better stability. In addition, MI-EC displayed a wide range of substrate universality, such as carbonates, oxalates, and acetic esters. Thus, this study not only provides a theoretical and practical support foundation for designing and synthesizing ILs, but also provides a new perspective for preparing alkaline catalysts.