Highly synergistic effect of ionic liquids and Zn-based catalysts for synthesis of cyclic carbonates from urea and diols

Abstract The development of stable and efficient catalysts is an attractive topic for green chemistry reactions under mild reaction conditions. In order to improve solvent-free synthesis of cyclic carbonates from urea and diols, a binary catalyst systems of Zn-based and different ionic liquids (ILs) were developed and examined in this study. The yield of ethylene carbonate (EC) could reach to 92.2% in the presence of C16mimCl/ZnCl2 catalyst. Through exploring the structure-activity relationships of cation and anion, it was confirmed that a synergistic effect of cation and anion of catalyst had important influences on urea alcoholysis. Additionally, the controlling step of EC synthesis reaction involving the elimination of an ammonia molecule from intermediates had been revealed by in situ FT-IR. This could afford a guided insight for synthesizing cyclic carbonates with high yield. Furthermore, a possible mechanism for the catalytic process was proposed based on DFT and the experimental results via FT-IR, 1H-NMRand 13C NMR analysis, which revealed that not only a probable synergistic effects of cation-anion matters, but also C(2)-H of ILs and Zn2+ played a key role in accelerating the reaction of urea alcoholysis. This catalytic mechanism study is to provide a preliminary basis to develop novel catalysts for cyclic carbonates from urea and diols through a green synthetic pathway.