(597b) Liquid-Liquid-Vapor Phase Equilibrium of High Pressure Ternary Systems of CO2 + Toluene + Ionic Liquid

Ionic liquids (ILs) are versatile compounds applicable to a number of processes due to their large amount of possible anion-cation combinations. Among the advantages of using ILs are mainly their very low vapor pressure, a wide liquid range below 100 ºC and high thermal stability. According to these characteristics, ILs are suitable for replacing the commonly used volatile organic solvents and decreasing gas emissions to the environment.

ILs have been studied for applications in several separation processes, including liquid-liquid extractions, extractive distillation, aqueous extraction through aqueous biphasic system, etc. Scurto et al.(1), proposed another separation process: an organic compound (methanol) was separated in a second liquid phase from an IL ([bmim][PF₆]) by increasing the pressure and composition of CO₂ in the liquid mixture obtaining a liquid-liquid-vapor equilibrium, condition called “cloud point”.  The second liquid phase has a very low concentration of IL. If CO₂ pressure is increased even more, the second liquid phase disappears obtaining a liquid-vapor equilibrium. This condition is called “merging point”. There is no IL detectable in the gas phase after the merging point but the extracted organic is observed in this phase. This procedure has successfully separated water and other polar organic compounds from ILs(2–4), so it is important to explore the behavior of different ILs with non-polar compounds.

The aim of this work is to explore the phase equilibrium of the ternary system including CO₂ + toluene with different ILs. Since toluene is a non-polar aromatic compound, four hydrophobic ILs based in the bis(trifluoromethylsulfonyl) imide ([Tf₂N]^-) anion were selected. Cations used are 1-hexyl-3-methylimidazolium ([hmim]⁺), 1-hexyl-3-methylpyridinium ([hmpy]⁺), trihexyl tetradecyl phosphonium ([P₆₆₆₁₄]⁺) and trietyl octyl phosphonium ([P₂₂₂₈]⁺). The composition of CO₂ in the single liquid phase toluene + IL is measured in a stoichiometric phase equilibrium apparatus until the cloud point is observed. For measuring the compositions of the three phases in the liquid-liquid-vapor equilibrium, a sampling system coupled to a GC and a HPLC (or UV/VIS) is used. A full description of these experimental apparatuses is given elsewhere(3). Measurement conditions are 25 °C and 40 °C (also 60 °C for [hmim][Tf₂N]) with initial toluene amount in the IL of 30, 50 and 70 mol %. For comparison, solubility of binary system of CO₂ in toluene and the four ILs is measured.

High solubility of toluene is observed in the four ILs (>0.80 mole fraction at room conditions). Solubility of CO₂ in binary and ternary isotherms increases with increasing pressure and decreases at higher temperatures. In ternary systems, solubility of CO₂ is lower than the respective binary CO₂ + IL, but becomes higher over 5 MPa at 25 °C and 6 MPa at 40 °C. Solubility of CO₂ is lower at higher concentrations of toluene in the initial mixture toluene + IL. The higher CO₂ solubility is observed in the systems containing [P₆₆₆₁₄][Tf₂N] and is very similar for the other three ILs. Cloud point pressures decrease by decreasing the temperature and increasing the toluene concentration in the initial mixture. Again, systems with [P₆₆₆₁₄][Tf₂N] show the least favorable conditions for toluene separation, i.e. high cloud point pressure, and the best is [hmim][Tf₂N]. For the liquid-liquid-vapor equilibrium, a very small concentration of IL is observed in the organic-rich phase. The merging point slightly changes at constant temperature for all systems observing a small dependency on the initial concentration conditions. 

(1)      Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. F. CO₂ as a Separation Switch for Ionic Liquid/Organic Mixtures. J. Am. Chem. Soc. 2002, 124, 10276–10277.

(2)      Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. F. Carbon dioxide induced separation of ionic liquids and water. Chem. Commun. 2003, 572–573.

(3)      Aki, S. N. V. K.; Scurto, A. M.; Brennecke, J. F. Ternary Phase Behavior of Ionic Liquid (IL)−Organic−CO₂ Systems. Ind. Eng. Chem. Res. 2006, 45, 5574–5585.

(4)      Mellein, B. R.; Brennecke, J. F. Characterization of the Ability of CO₂ To Act as an Antisolvent for Ionic Liquid/Organic Mixtures. J. Phys. Chem. B 2007, 111, 4837–4843.