(25c) High-Pressure Vapor-Liquid Equilibria of 1-Alkyl-1-Methylpyrrolidinium bis(trifluoromethylsulfonyl)imide Ionic Liquids and CO2

Room temperature ionic liquids (RTILs) are salts that have a melting point below 373.15 K and composed of an organic cation and an organic or inorganic anion. RTILs have tunable physical and chemical properties as well as extremely low vapor pressures (i.e. < 1.0 x 10^-3 Pa). Carbon dioxide (CO₂) is the most studied gas in ionic liquids, and pyrrolidinium-based ionic liquids have gained interest in recent years due to their high thermal stability and lower toxicity compared to pyridinium, phosphonium, imidazolium and ammonium ILs even though the viscosities are slightly higher than imidazolium ILs. In this study, vapor-liquid equilibrium for the binary systems of CO₂ and 1-alkyl-1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids ([C_nC₁pyr][NTf₂] (n = 3,4,6)) were measured at 298.15, 318.15 and 338.15 K and at pressures up to 20 MPa using gravimetric (IGA and XEMIS microbalances) and volumetric (high-pressure view cell) methods. The solubility of CO₂ in pyrrolidinium ionic liquids increases with decreasing temperature and increasing pressure for all three ionic liquids. The CO₂ solubility also slightly increases with increase in alkyl chain length on the pyrrolidinium cation which may be due to entropic effects. The mixtures exhibit physical absorption with no indication of a chemical reaction between the components. The molar volume of the mixtures decreased with an increase in pressure; however,. the density displayed relatively little change due to the increased mass of the liquid phase with CO₂ absorption. The Fickian diffusion of CO₂ in pyrrolidinium based ionic liquids (~10^-10 m²·s) was calculated at pressures up to 2 MPa and found to be slightly lower than the diffusivity of CO₂ in an imidazolium based ionic liquid with the [NTf₂] anion.