Imidazolium-based ionic liquids with cyano groups for the selective absorption of ethane and ethylene

The density, viscosity and absorption of ethane and ethylene were determined experimentally, as a function of temperature and at atmospheric pressure, in different imidazolium-based ionic liquids: 1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)imide, [C₁C₈Im][NTf₂], 1-methyl-3-(propyn-3-yl)imidazolium bis(trifluoromethanesulfonyl)imide [C₁(C₂H₂CH)Im][NTf₂], 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C₁C₃CNIm][NTf₂], 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide [C₁C₃CNIm][DCA], 1-butyl-3-methylimidazolium dicyanamide, [C₁C₄Im][DCA] and 1-butyl-3-methylimidazolium methylphosphite [C₁C₄Im][C₁HPO₃]. The densities and viscosities of the ionic liquids studied are strongly dependent on the anion and, at 313K, follow the order: [C₁(C₂H₂CH)Im][NTf₂]>[C₁C₃CNIm][NTf₂]>[C₁C₃CNIm][DCA]>[C₁C₄Im][C₁HO₃]>[C₁C₄Im][DCA] and [C₁C₃CNIm][NTf₂]>[C₁C₃CNIm][DCA]>[C₁C₄Im][C₁HPO₃]>[C₁(C₂H₂CH)Im][NTf₂]>[C₁C₄Im][DCA], respectively. The differences in the molecular structures of the ionic liquids allowed the identification of the influence of increasing the alkyl side chain of the cation, of the presence of unsaturated CC and CN bonds on the alkyl side chain of the cation and finally of a phosphite based anion on the selective absorption of ethane and ethylene. The solubility of ethylene is higher than that of ethane in the ionic liquids studied and varies from 18.41×10^-3 in [C₁C₈Im][NTf₂] at 303.17K to 1.603×10^-3 in [C₁C₄Im][DCA] at 343.61K. The introduction of the different functional groups leads to a decrease on the gas absorption compared with that of [C₁C₈Im][NTf₂] but to an increase of the ideal separation selectivity of ethane and ethylene.