520 ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/cphc.200300663 CHEMPHYSCHEM 2003, 4, 520 Supercritical Carbon Dioxide- Induced Phase Changes in (Ionic Liquid, Water and Ethanol Mixture) Solutions: Application to Biphasic Catalysis. Vesna Najdanovic-Visak, [a] Ana Serbanovic, [a] Jose ¬ M. S. S. EsperanÁa, [b] Henrique J. R. Guedes, [b] LuÌs P. N. Rebelo, [a] and Manuel Nunes da Ponte* [a] KEYWORDS: biphasic catalysis ¥ carbon dioxide ¥ high-pressure chemistry ¥ ionic liquids ¥ supercritical fluids Ionic liquids (IL) are salts that are molten at room temperature. They have been viewed as a class of environmentally friendly, green solvents, with potential widespread applications. [1, 2] Ionic liquids have been extensively studied as reaction media for numerous types of reactions. In catalysis, they usually facilitate the recycling of the catalyst and/or the separation of the products, and they are often used in biphasic conditions, in contact with either an aqueous or an organic phase. [3±5] One of the most commonly investigated ionic liquids is 1-butyl-3-methyl-imidazolium hexafluorophosphate, [C 4 mim]- [PF 6 ]. At 40 8C, [C 4 mim][PF 6 ] is (partially) immiscible with either water or ethanol. In two independent investigations, both Swatloski et al. [6, 7] and Najdanovic-Visak et al. [8] discovered that the addition of ethanol (or water) to mixtures of [C 4 mim][PF 6 ] and water (or ethanol) increased mutual solubility, until a single phase was formed. The work from this laboratory [8] has shown that this surprisingly large cosolvent effect extends over a wide range of temperatures and compositions. Supercritical carbon dioxide has been proposed by Blanchard et al. [9, 10] as a convenient means to extract organic substances from an ionic liquid. The authors reported that the [C 4 mim][PF 6 ] and supercritical CO 2 (scCO 2 ) system shows specific phase behaviour where CO 2 can dissolve significantly into the [C 4 mim][PF 6 ] phase, but no ionic liquid dissolves in the scCO 2 . Here we present the effect of adding high-pressure CO 2 to mixtures of [C 4 mim][PF 6 ], ethanol and water. Figure 1 depicts the phase diagram of the ternary system, at 313.15 K. The shaded areas on the ethanol-rich and the water-rich sides are the immiscibility (two-phase) regions, the other areas of the diagram correspond to monophasic conditions. [8] In this work, we started with mixtures of [C 4 mim][PF 6 ], ethanol and water in the one- phase region (compositions shown as filled symbols in Figure 1), and gradually added carbon dioxide. This work was performed in a high-pressure stainless-steel view cell, with sapphire windows and a magnetic stirrer (see Supporting Information for details). As CO 2 was added, a third phase started to form between the liquid and gas phases. As pressure continued to increase, this middle phase increased significantly in volume. Finally, either the two upper phases merged into a single phase, after going through a critical point (entries 3 and 7 in Table 1), or the middle liquid phase completely filled the upper space available in the cell (all other mixtures). This succession of events is illustrated, for mixture 3 (Table 1), at 313.15 K, in the photographs shown in Figure 2. The first photograph (Figure 2A) shows the cell filled with the liquid mixture and CO 2 at a low pressure (3.5MPa). Slow addition [a] M. Nunes da Ponte, V. Najdanovic-Visak, A. Serbanovic, L. P. N. Rebelo Instituto de Tecnologia QuÌmica e Biolo ¬gica Universidade Nova de Lisboa, Apartado 127 2781±901 Oeiras (Portugal) Fax: ( 351)21±4411±277 E-mail: mnponte@itqb.unl.pt [b] J. M. S. S. EsperanÁa, H. J. R. Guedes REQUIMTE, Departamento de Quimica, Faculdade de Cie √ncias e Tecnologia Universidade Nova de Lisboa, 2829±516 Caparica (Portugal) Supporting information for this article is available on the WWW under http:// www.chemphyschem.org or from the author. Table 1. Three-phase pressures, p 3 , and critical pressures, p c , for ([C 4 mim][PF 6 ], water, ethanol and CO 2 ) mixtures. x i represents initial molar fractions, before adding carbon dioxide. Mixture no. x p 3 p c x IL x EtOH x H2O [MPa] [MPa] 1 0.347 0.653 0 5.8 ± 2 0.249 0.662 0.089 5.9 ± 3 0.111 0.740 0.149 6.5 9.4 4 0.107 0.752 0.141 5.7 ± 5 0.209 0.552 0.239 7.6 ± 6 0.398 0.302 0.300 10.0 ± 7 0.102 0.402 0.469 10.0 14.0 Figure 1. Phase diagram (mole fractions) of ternary ([C 4 mim][PF 6 ], water and ethanol) at 313.15 K. The shaded areas are immiscibility regions at 0.1MPa. Symbols indicate initial compositions of mixtures in this work. Dashed lines are visual aids connecting mixtures with similar three-phase pressures p 3 (the lowest that triggers the appearance of a third phase): !, 6 to 6.5MPa; ^, 7.6MPa; *, 10MPa.