Density, Viscosity, CO₂ and CH₄ Solubility, and CO₂/CH₄ Selectivity of Protic and Aprotic Imidazolium and Ammonium Bis(trifluoromethanesulfonyl)amide and Tetrafluoroborate Ionic Liquids

We have investigated the density, viscosity, and CO₂ and CH₄ solubility of protic and aprotic imidazolium- and ammonium-based ionic liquids (ILs). Protic ILs are 1-butylimidazolium bis(trifluoromethanesulfonyl)amide ([bimH][TFSA]), 1-butylimidazolium tetrafluoroborate ([bimH][BF₄]), and N-butyldimethylammonium bis(trifluoromethanesulfonyl)amide ([N_411H][TFSA]). Aprotic ILs are 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([bmim][TFSA]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]), and N-butyltrimethylammonium bis(trifluoromethanesulfonyl)amide ([N₄₁₁₁][TFSA]). Their density and viscosity were measured at 273.15 and 363.15 K under atmospheric pressure. Protic ILs exhibited higher density and viscosity than aprotic ILs. The measured densities were fitted to quadratic functions of temperature, and the measured viscosities were analyzed using the Vogel–Fulcher–Tammann equation. The CO₂ and CH₄ solubilities in the protic and aprotic ILs were measured at 313.15 and 333.15 K using a magnetic suspension balance and exhibited a typical pressure dependence of physical absorption. The CO₂ and CH₄ solubilities of imidazolium-based protic ILs were lower than those of aprotic ILs; however, ammonium-based protic ILs were higher than aprotic ILs. Henry’s law constants for CO₂ and CH₄ were calculated from the experimental solubility data at mole fractions <0.1, and their ratios were used to calculate the CO₂/CH₄ selectivity. The CO₂/CH₄ selectivities of the protic and aprotic ILs did not differ significantly.