Separation of organics from aqueous solutions, as is frequently encountered in recovery of products from fermentation, usually requires energy intensive distillation. Here we explore the potential use of ionic liquids to remove alcohols from aqueous solutions using ionic liquids. Ionic liquids are low melting salts that exhibit negligible volatility at ambient conditions. Therefore, the recovered alcohol could be removed from the ionic liquid extraction solvent in a single evaporation stage. To evaluate various ionic liquids for this application, this study details the phase behavior of binary and ternary systems of ionic liquids with water and alcohols. The mutual solubilities of various ionic liquids with water were determined in order to study the effect of the anion, and the alkyl chain length on the cation and substitution on the cation. As expected, an increase in the alkyl chain length or in the substitution decreases the mutual solubility of the ionic liquid with water. Of the ionic liquids tested, the ones with the tetrafluoroborate anion have the largest mutual solubility with water, followed by ionic liquids with tetracyanoborate anion, hexafluorophosphate anion, bis-(trifluoromethylsulfonyl)imide anion, methide anion, and tris(pentafluoroethyl)trifluoro-phosphate anions.[1]

The second component of our work is to optimize ionic liquids for various liquid-liquid separations. The general feasibility of separating water and alcohols using ionic liquids has been shown previously.[2-4] Ternary diagrams are necessary in order to design liquid-liquid separations. With the binary data as starting points, ternary diagrams of systems with water and alcohols for various ionic liquids were developed, measuring all three components of each phase. We also test the ability of excess Gibbs energy models to predict these ternary data over the entire composition range, using only parameters determined from binary and pure component data. Further the Hunter-Nash method was used to determine the number of stages that would be required in order to separate water and alcohols using a liquid-liquid extractor.[5]

Replacing distillation with liquid-liquid extraction could reduce energy consumption and make the production of alcohols cheaper and more environmentally sound. To that end, this study also measured enthalpies of mixing for mixtures of ionic liquids and alcohols. With this data we use a thermodynamic cycle to calculate the energy required to evaporate the alcohol from the ionic liquid and compare this to energy requirements for conventional distillation.

References

[1] A. Chapeaux, L. D. Simoni, M. A. Stadtherr and J. F. Brennecke, J. Chem. Eng. Data, 2007, 52, 2462-2467.

[2] M. M. M. Fadeev A. G., Chem. Comm., 2001, 295-296.

[3] X. S. Hu, J. Yu and H. Z. Liu, J. Chem. Eng. Data, 2006, 51, 691-695.

[4] V. Najdanovic-Visak, L. P. N. Rebelo and M. N. da Ponte, Green Chem., 2005, 7, 443-450.

[5] A. Chapeaux, L. D. Simoni, M. A. Stadtherr and J. F. Brennecke, Green Chem., 2008, Submitted.