Effect of Water on the Carbon Dioxide Absorption by 1‑Alkyl-3-methylimidazolium Acetate Ionic Liquids

The absorption of carbon dioxide by the pure ionic liquids 1-ethyl-3-methylimidazolium acetate ([C₁C₂Im]­[OAc]) and 1-butyl-3-methylimidazolium acetate ([C₁C₄Im]­[OAc]) was studied experimentally from 303 to 343 K. As expected, the mole fraction of absorbed carbon dioxide is high (0.16 at 303 K and 5.5 kPa and 0.19 at 303 and 9.6 KPa for [C₁C₂Im]­[OAc] and [C₁C₄Im]­[OAc], respectively), does not obey Henry’s law, and is compatible with the chemisorption of the gas by the liquid. Evidence of a chemical reaction between the gas and the liquid was found both by NMR and by molecular simulation. In the presence of water, the properties of the liquid absorber significantly change, especially the viscosity that decreases by as much as 25% (to 78 mPa s) and 30% (to 262 mPa s) in the presence of 0.2 mol fraction of water for [C₁C₂Im]­[OAc] and [C₁C₂Im]­[OAc] at 303 K, respectively. The absorption of carbon dioxide decreases when the water concentration increases: a decrease of 83% in CO₂ absorption is found for [C₁C₄Im]­[OAc] with 0.6 mol fraction of water at 303 K. It is proved in this work, by combining experimental data with molecular simulation, that the presence of water not only renders the chemical reaction between the gas and the ionic liquid less favorable but also lowers the (physical) solubility of the gas as it competes by the same solvation sites of the ionic liquid. The lowering of the viscosity of the liquid absorbent largely compensates these apparent drawbacks and the mixtures of [C₁C₂Im]­[OAc] and [C₁C₂Im]­[OAc] with water seem promising to be used for carbon dioxide capture