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, Keq, and Henry's law constant, KH, 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 (Keq = 75 ± 2 at 303 K) but leads to lower Henry's law constants (from KH = 77.8 ± 0.6 bar to KH = 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.
ABSTRACT
The equilibrium and transport properties of mixtures of two ionic liquids - [C4C1Im][OAc] and [C4C1Im][C(CN)3] - were determined and interpreted at the molecular level using vibration spectroscopy, NMR and molecular dynamics simulation. The non-ideality of the mixtures [C4C1Im][OAc](1-x)[C(CN)3]x was characterized by V(E) = +0.28 cm(3) mol(-1) (293 K, x = 0.65) and H(E) = -2.2 kJ mol(-1) for x = 0.5. These values could be explained by a rearrangement of the hydrogen-bond network of the mixture that favours the interaction of the acetate anion with the imidazolium cation at position C2. The dynamic properties of the mixture are also dramatically influenced by the composition with a decrease of the viscosity and an increase of self-diffusion coefficients of the ions when the amount of tricyanomethanide anion increases in the mixture.
