Characterization of caprolactam based eutectic ionic liquids and their application in SO2 absorption

Five caprolactam (CPL) based eutectic ionic liquids (EILs) were prepared and their melting point, density, conductivity, viscosity, and decomposition temperature were detected. The EILs were investigated as potential absorbents for SO2 capture at 30 °C to 70 °C and 1 atmospheric pressure of pure gas. These properties of these EILs are similar to traditional ionic liquids and quite different from molecular solvents. In the tested temperature range, the density increases linearly with the increase of temperature, and the temperature dependence conductivity or viscosity is described by a VTF equation. CPL-imidazole (1:1) has the highest conductivity of 32.3 μs/cm at 30 °C and lowest viscosity of 48.6 mPa·s at 30 °C. The SO2 absorption capacities of CPL-organic amines based EILs are higher than CPL-organic acids based EILs. The solubility of SO2 in CPL-acetamide (1:1) is higher than BMImBF4 and lower than DMF. The solubility of SO2 in CPL-acetamide (1:1) is 0.497 g/g of mass fraction at 30 °C and the absorption is practically reversible. The absorption in CPL-acetamide (1:1) may probably follow a physical process by 1H NMR analysis.