Absorption of carbon dioxide by ionic liquids with carboxylate anions

Abstract Experimental values of the absorption of carbon dioxide in two ionic liquids with carboxylate anions – the 1-butyl-3-methylimidazolium levulinate [C 1 C 4 Im][LEV] and the 1-butyl-1-methylpyrrolidinium acetate [C 1 C 4 Pyrro][OAc] – are reported as a function of temperature and at pressures close to atmospheric. Mole fraction absorption of carbon dioxide in [C 1 C 4 Im][LEV] and [C 1 C 4 Pyrro][OAc] is equal to 0.93 × 10 −2 and 1.10 × 10 −2 at 303.15 K and 89.2 kPa and 353.15 K and 67.1 kPa, respectively. The effect of the presence of controlled amounts of water on the absorption of carbon dioxide was measured for [C 1 C 4 Pyrro][OAc]. The presence of a 0.35 mole fraction of water in [C 1 C 4 Pyrro][OAc] decreases the viscosity of the ionic liquid phase and dramatically increases the amount of carbon dioxide absorbed, pointing toward a chemical reaction between the gas and the liquid absorbent. Increasing the amounts of water lowers the viscosity further but also the absorption capacity of the ionic liquid. Molecular dynamics simulations were used to interpret the molecular mechanism of solvation of carbon dioxide in [C 1 C 4 Pyrro][OAc]. Results show that carbon dioxide is solvated preferentially in the non-polar domain of the solvent, and that the CO 2 –anion interactions dominate over the CO 2 –cation interactions. Molecular simulations could reproduce the experimental solubility of CO 2 in [C 1 C 4 Im][TFA] (known to be a physical process), but not in [C 1 C 4 Pyrro][OAc] and reinforcing the conclusion that the higher solubility of carbon dioxide in the acetate based ionic liquid can be ascribed to a chemical reaction.