Ionic Liquids IIIA - American Chemical Society

C h a p t e r 23

Multiphase Equilibrium in Mixtures of [C mim][PF ] with Supercritical Carbon Dioxide, Water, and Ethanol: Applications in Catalysis

Downloaded by PENNSYLVANIA STATE UNIV on June 2, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch023

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Vesna Najdanovic-Visak, A n a Serbanovic, José M. S. S. Esperança, Henrique J. R. Guedes, Luis P. N. Rebelo, and Manuel Nunes da Ponte Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Aptd. 127, 2781-901 Oeiras, Portugal REQUIMTE, Deparment of Chemistry, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal *Corresponding author: [email protected]

The ionic liquid [C4mim][PF ] and supercritical carbon dioxide produce multiphase systems when mixed with ethanol and water. Mixtures of these four solvents can be made to go, by small changes in composition, through a succession of phase changes, involving one, two and three-phase situations. Increasing carbon dioxide pressure induces first the appearance of an intermediate liquid phase and later the merging of this phase with the gas, leaving all the ionic liquid in a separate, denser liquid. This succession is suitable to carry out reaction cycles in ionic liquid-based solvents, with complete recovery of the reaction product by C O decompression. 6

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© 2005 American Chemical Society

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In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Supercritical carbon dioxide (ScC0 ) is a well-known clean solvent that found many applications, especially in separation and fractionation of natural products (/). More recently, it is being extensively studied as a green solvent for chemical reaction processes (2,3). The use of SeC0 + an ionic liquid as an ideal combination for many chemical processes was first suggested by Blanchard et al. (4). These authors reported that mixtures of ScC0 with die ionic liquid [C mim][PF ] show gasliquid equilibrium behaviour whereby carbon dioxide can dissolve significantly into the [C mim][PF6]-rich liquid phase, but no ionic liquid dissolves in the gas phase. Blanchard et al. (5) have also shown that organic compounds can be extracted from [C mim][PF ] using ScC0 . This work has led to the study of carbon dioxide as an extracting solvent for several reaction processes in ionic liquids, including its use in continuous flow mode, bringing in reactants and taking out products (6). In a more recent development, Scurto et al. (7) have presented phase behaviour results of [C mim][PF ] + methanol + C 0 . Their data show the formation of an additional liquid phase at relatively low pressures, leading to three phase, liquid-liquid-gas equilibrium. The same authors (8) have also found that the introduction of gaseous or liquid carbon dioxide into a mixture of water and an ionic liquid can cause the separation of both hydrophobic and hydrophilic ionic liquids from aqueous solution, with the formation of an intermediate third phase. Almost fifty years ago, Francis (9) published an extensive account of phase behaviour of binary and ternary mixtures containing liquid carbon dioxide. He lists 21 systems where completely miscible liquids are separated into two liquid phases by the introduction of C 0 , and many others where partial immiscibility of two liquids is enhanced. The behaviour discovered by Scurto el al. is therefore an expression in ionic liquid systems of the long-known capability of carbon dioxide as an inducer of immiscibility. The importance of this effect is that carbon dioxide may be used, at relatively low pressures, to control the number of phases in a reactive system where the reaction takes place in the ionic liquid. There is another interesting feature of the phase behaviour of [C mim][PF ] + methanol + C 0 . For the mixtures with lower content in ionic liquid, Scurto et al. (7) have shown that an increase in pressure leads to the disappearance of the intermediate liquid phase, by merging with the upper gas phase, through a critical point. The resulting fluid phase did not contain any ionic liquid, which was retained in the lower liquid phase. 2

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Phase behaviour of carbon dioxide + water + an alcohol + an ionic liquid Swatloski et al. (10,11) and Najdanovic-Visak et al. (12) discovered that the addition of water to binary mixtures of imidazolium-based ionic liquids with several alcohols, which show extensive immiscibility areas, increased mutual solubility, until a single phase was formed. Recent work from this laboratory (13) has shown that this surprisingly large co-solvent effect extends over wide ranges of temperatures and compositions. This work has also included the study of pressure and isotope-substitution effects on phase transitions. These water + alcohol + ionic liquid mixtures, due to the variety of their phase transitions, are especially appropriate solvents to carry out reactions where the rate may be controlled by switching, for instance, from two phases to one phase, by small composition changes. The separation of products from the reaction mixture would, however, become more complicated than in pure biphasic reactions. Najdanovic-Visak et al. (14) have recently shown that it is possible to separate [C mim][PF ] from water/ethanol mixtures using C 0 . As carbon dioxide is added to [C4mim][PF ]/ethanol/water, a third phase starts to form between the liquid and gas phases, in a similar fashion to what Scurto et al. reported on [C4mim][PF ] + methanol. At higher pressures, critical points involving the intermediate liquid phase and the vapour were observed, for different water/ethanol molar ratios. These findings indicate that the phase behaviour discovered by Scurto et al. is exhibited by many systems with ionic liquids, and that it is not confined to mixtures of low content in ionic liquid, as those authors suggested. A thermodynamic analysis can be based on a succession of phase diagrams for ternary diagrams, as shown in Figure 1. In the first (left) diagram of the Figure, at low pressures, simple liquid-vapour equilibrium is observed. The tie-line connecting the liquid and the vapour sides will depend on the initial composition of the liquid mixture A+B. The vapour side is essentially pure C 0 . In the second diagram, a third phase L has already been formed. An increase of C 0 pressure provoked further dissolution of carbon dioxide in the Li phase and induced the separation of L . The separation begins at the pressure where the overall composition of the mixture (phases liquid and vapour combined) hits the black triangle in the diagram, which corresponds to the three-phase area. This depends on the initial composition of the liquid mixture A+B, on the amount of C 0 added and on the available volume in the vessel containing the mixture (that is, the total volume less the volume occupied by the liquid phase). There is a limited range of compositions that lead to the formation of the third phase L , and its appearance (or not) is therefore dependent on the conditions of the experiment being carried out. 4

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Figure 1 - Correspondence between the succession ofstates observed when carbon dioxide is added isothermally to a mixture of ionic liquid (A) and an alcohol or alcohol + water (B). (See page 3 of color insert.)


305 The third diagram on the left corresponds to the situation depicted above it, where phase L has grown significantly in volume by further addition of carbon dioxide. The black triangle has moved, indicating that phase Li is now much richer in component A (the ionic liquid) and L has increased its content of component Β and carbon dioxide. In some cases of very precise matching of compositions, a four-phase equilibrium situation might appear at a fixed pressure. A four-phase line in the (p,T) projection of a phase equilibrium diagram of a ternary mixture is limited between two fixed (p,T) points, the lower and the upper critical end points (LCEP and UCEP), as is very clearly explained in the paper of Wendland et al. (75). When carbon dioxide continues to be added to the system, two types of event may be observed: (a) either the pressure vessel fills up to the top with liquid L , or (b) a critical point (L =V) is formed, and the two phases become indistinguishable. This last situation is the one depicted in Figure 1. It can only be observed, of course, when binary mixtures of components Β and C 0 exhibit vapour-liquid critical points. The important peculiarity of ionic liquidcontaining mixtures is, as pointed out by Scurto et al., that the ionic liquid is totally confined to the L phase when the critical point L =V is reached. This last observation allows further thermodynamic analysis of the phase diagram, in terms that are now specific of ionic liquid solutions, at pressures close to the critical pressure In the case of the methanol-based mixtures studied by Scurto et al. (7), the L + V part of the pressure vessel (Figure 1) can be viewed as a binary C 0 + methanol. As the authors point out, the critical pressures for all compositions where a critical point is observed are within experimental error of the critical pressure determined for the binary. Their case where no formation of L was found, corresponding to a high concentration of IL (49.3% molar in the initial liquid mixture) can be interpreted as a situation where the small free volume above the liquid did not allow enough space for the required quantity of methanol to be drawn into the vapour phase, so that it would cross into the vapour-liquid equilibrium area. This is depicted in Figure 2, where two different paths are drawn for the vapour phase. The path on the left produces a second L liquid phase when it meets the two-phase area envelope, and leaves this area at higher pressures through the critical point. The path on the right, which does not cross this envelope, should correspond to the situation where no third phase is formed. As this is dependent on the available volume in the cell, the conclusion of Scurto et al. that, "when the concentration in IL is high, apparently it is not possible to induce liquid-liquid phase split by the addition of C 0 " should be revisited. 2


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Figure 2 - Binary mixture isothermal phase diagram, where two different paths for the pressure increase on carbon dioxide addition are shown.

In the case presented by Najdanovic-Visak et al. (14), component Β is a mixture of water and ethanol. The diagrams of Figure 1 do not apply rigorously to this situation, but the quaternary IL + C 0 + water + ethanol may be treated as a pseudo-ternary. Especially in the vicinity of the critical points (L2 = V) observed by these authors, the L + V part of the overall mixture can be interpreted in terms of the ternary phase diagram for C 0 + water + ethanol. As in this case there is one more degree of freedom than in the methanol + IL mixtures, there are more experiments where no critical point is observed, because the compositions are not the right ones, and the high pressure cell is filled with liquid L (with disappearance of V) at pressures lower than the critical. When critical points were indeed observed, those authors concluded, by comparison with the critical pointe for the pure ternary mixture (without IL) that water is taken out preferentially from ethanol-rich IL mixtures, while for water-rich initial mixtures, ethanol is preferentially withdrawn from the ionic liquid. This effect might be related to the more recent finding by Najdanovic-Visak et al. (13) that pressure increases mutual solubility of [C mim][PF ] and ethanol, but decreases it in the case of the same ionic liquid and water. 2

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Applications in catalysis


The cascade of phase changes in ionic liquid aqueous induced by ethanol and carbon dioxide can be used to allow reaction cycles to proceed as shown in Figure 3.

Figure 3 - Water + [C mim][PFJ mixtures change the number ofphases with successive addition of ethanol (first transition) and carbon dioxide. (See page 3 of color insert.) 4

A reaction usually carried out in biphasic water + IL conditions, can benefit from monophasic conditions, for increased rates, by addition of ethanol, without losing the advantages of biphasic systems for catalyst recycling and product separation, because carbon dioxide can then be used to extract the reaction products and regenerate the ionic liquid phase. The reaction chosen (14) to carry out a proof-of -principle experiment was the (usually slow) epoxidation of isophorone by hydrogen peroxide, catalysed by sodium hydroxide:

Ο Ο

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isophorone

isophorone oxide

Bortolini et al. (16) carried out this epoxidation, and the epoxidation of several other electrophilic alkenes, dissolved in [C mim][PF ], by contact with 4

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an aqueous solutions of hydrogen peroxide. These reactions were performed in biphasic conditions, due to the above-mentioned immiscibility of water and the ionic liquid. Ethyl acetate was used to extract the products from the reaction mixture. The difference in the rate of this reaction when carried out either in biphasic conditions, similar to those of Bortolini et al. (16) or in one phase, by addition of ethanol to the water + ionic liquid immiscible system, (14) is shown in Figure 4.

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Figure 4 - Yield of the epoxidation reaction of isophorone carried out in a onephase [C mim][PF ] + water + ethanol solvent or in a biphasic mixture of [C mim][PF ] and water (ref 14). 4

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In the one-phase conditions, extraction with carbon dioxide, at 12 MPa and 313 K, completely removed the reaction product from the reaction mixture. In a more recent work, Bortolini et al. (17) contact their two-phase reaction mixture (for a different substrate, 2-cyclohexen-1 -one), with ScC0 , also at 313 K, but at the somewhat more drastic pressure of 20 MPa. Their results are essentially solubilities of the epoxide in the carbon dioxide-rich phase. They conclude that extraction of the reaction product with carbon dioxide in those conditions is viable. 2


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Acknowledgements This work was financially supported by Fundaçâo para a Ciência e a Tecnologia (FC&T, Portugal), through contracts POCTI/EQU/35437/99 and POCTI/QUI/38269/2001. VNV, AS and JMSSE thank FC&T for doctoral fellowships.


References 1. Brunner, G. Gas Extraction; Springer: New York, 1994 2. Noyori R. (ed), "Supercritical Fluids: Introduction", Chem. Rev., 1999, 99 (2), 353-354. 3. Jessop, P.G.; Leitner, W.; (eds). Chemical Synthesis in Supercritical Fluids, Wiley-VCH, Weinheim, 1999. 4. Blanchard, L.A.; Hancu, D.; Beckman, E.J.; Brennecke, J.F. Nature, 1999, 399, 28 5. Blanchard, L.A.; Brennecke, J.F. Ind. Eng. Chem. Res.; 2001, 40, 287292 6. Sellin,M.F.; Webb; P. B.; Cole-Hamilton, D .J. Chem. Commun., 2001, 781 7. Scurto, A . M . ; Aki, S.N.V.K.; Brennecke, J.F. J. Am. Chem. Soc., 2002, 124, 10276 8. Scurto, A . M . ; Aki, S.N.V.K.; Brennecke, J.F. Chem. Commun., 2003, 572 9. Francis, A.W. J. Phys. Chem. 1954, 58, 1099 10. Swatloski, R.P.; Visser, A.E.; Reichert, W.M.; Broker, G.A.; Farina, L.M.; Holbrey, J.D.; Rogers, R.D. Chem. Commun., 2001, 20, 20702071. 11. Swatloski, R.P.; Visser, A.E.; Reichert, W.M.; Broker, G.A.; Farina, L . M . ; Holbrey, J.D.; Rogers, R.D. Green Chemistry, 2002, 4, 81-87 12. Najdanovic-Visak, V.; Esperança, J.M.S.S.; Rebelo, L.P.N.; Nunes da Ponte, M . ; Guedes, H.J.R.; Seddon, K.R.; Szydlowski, J. Phys. Chem. Chem. Phys., 2002, 4, 1701-1703 13. Najdanovic-Visak, V.; Esperança, J.M.S.S.; Rebelo, L.P.N.; da Ponte, M.N.; Guedes, H.J.R.; Seddon, K.R.; Sousa, H.C.; Szydlowski, J. J. Phys. Chem. Β 2003, 107, 12797-12807 14. Najdanovic-Visak, V.; Serbanovic, Α.; Esperança, J.M.S.S.; Guedes, H.J.R.; Rebelo, L.P.N.; Nunes da Ponte; M . ChemPhysChem, 2003, 4, 520


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15. Wendland, M . ; Hasse, H.; Maurer, G. J. Supercritical Fluids, 1993, 6, 211-222 16. Bortolini, O.; Conte, V.; Chiappe, C.; Fantin, G.; Fogagnolo, M.; Maietti, S. Green Chemistry, 2002, 4, 94-96 17. Bortolini, O.; Campestrini, S.; Conte, V.; Fantin, G.; Fogagnolo, M . ; Maietti, S. Eur. J. Org. Chem., 2003, 24, 4804-4809


Ionic Liquids IIIA - American Chemical Society

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