Can the tricyanomethanide anion improve CO2 absorption by acetate-based ionic liquids?

Abstract

Carbon dioxide absorption by mixtures of two ionic liquids with a common cation—1-butyl-3-methylimidazolium acetate, [C₄C₁Im][OAc], and 1-butyl-3-methylimidazolium tricyanomethanide, [C₄C₁Im][C(CN)₃]—was determined experimentally at pressures below atmospheric pressure as a function of temperature between 303 K and 343 K, and at 303 K as a function of pressure up to 10 bar. It is observed that the absorption of carbon dioxide decreases with increasing tricyanomethanide anion concentration and with increasing temperature, showing a maximum of 0.4 mole fraction of carbon dioxide in pure [C₄C₁Im][OAc] at 303 K. At this temperature, the CO₂ absorption in the mixtures [C₄C₁Im][OAc]_(1−x)[C(CN)₃]_x is approximately the mole-fraction average of that in the pure ionic liquids. By applying an appropriate thermodynamic treatment, after identification of the species in solution, it was possible to calculate both the equilibrium constant, K_eq, and Henry's law constant, K_H, in the different mixtures studied thus obtaining an insight into the relative contribution of chemical and physical absorption of the gas. It is shown that chemical sorption proceeds through a 1 : 2 stoichiometry between CO₂ and acetate-based ionic liquid. The presence of the C(CN)₃⁻ anion does not significantly affect the chemical reaction of the gas with the solvent (K_eq = 75 ± 2 at 303 K) but leads to lower Henry's law constants (from K_H = 77.8 ± 0.6 bar to K_H = 49.5 ± 0.5 bar at 303 K), thus pointing towards larger physical absorption of the gas. The tricyanomethanide anion considerably improves the mass transfer by increasing the fluidity of the absorbent as proven by the larger diffusivities of all the ions when the concentration of the C(CN)₃⁻ anion increases in the mixtures.