Effect of anion type of imidazolium based polymer supported ionic liquids on the solvent free synthesis of cycloaddition of CO₂ into epoxide

Abstract

Catalytic CO2 fixation into value added products have attracted ample devotion due to the rapid increasing greenhouse gas effect. Especially, by using heterogeneous catalysts, particularly, the designer nature of organic functionality immobilized on polymer supports has driven significant attention. In this work, various tailor made imidazolium based ionic liquids (ILs) covalently immobilized on polymeric support were synthesized successfully (denoted as PSIL) and applied in the cycloaddition of CO2 into epoxides. Imidazolium (C3H5N2+) and various counter anions such as Cl-, Br-, BF4-, PF6-, and NTf2-, were selected to construct less coordinated cations with anions in PSILs. In the fixation of CO2 into epoxide, all the prepared PSILs showed efficient catalytic activity, among them 0.1 equiv. of PSIL-NTf2- composed of imidazolium cations and NTf2- anions was the most efficient, showing 100% conversion and 91% yield of the respective cyclic carbonate under 100 degrees C reaction temperature using 8 bar pressure in 8 h reaction time. This might be due to the least co-ordinated NTf2- anions with imidazolium cations played a vital role in the ring -opening of epoxide, enhancing the reaction rate and favourable for the worthy yield. In addition, the effects of various reaction parameters including amount of catalyst, temperature, and pressure were studied and numbers of substituted cyclic carbonates were prepared. It was found that PSILs were easily separated from the reaction mixture and reused several times without any significant loss of catalytic activity. The present solvent free catalytic system was found mild, kinetically fast, and naturally benign for the coupling of CO2 into epoxide.