Ionic Liquids and Water

Water is one of the typical impurities in ionic liquids, because these low 

A well-known effect of water impurities in ionic liquids is the viscosity decrease. However, also ionic liquids were reported, in which water impurities induces gelation. Spohr and Patey have shown with ionic liquid model systems that water tends to replace the counter ions from the ion solvation shell in ionic liquids with small ion size disparity, leading to a faster diffusion of the lighter ion-water clusters. However, water can increase viscosity of ionic liquids if the ion size disparity is too large, or if strong directional ion pairs are found. Spohr and Patey attributed this behavior to extended water-anion chains and strongly bound water-anion-cation clusters. A classical molecular dynamics study by Raju and Balasubramanian observed that the anion diffuse faster than the cation in water ionic liquid mixtures in contrast to neat ionic liquids. The larger 

Experimental studies on water imidazolium ionic liquid mixtures reported anions as preferred sites of interaction for water molecules.This is supported by classical molecular dynamics studies of water imidazolium ionic liquid mixtures which highlighted strong water-anion hydrogen bonds but did not find a specific interaction of water with the cations.In agreement with this, a recent study showed that the dipole moment of the imidazolium cation is nearly unaffected by the presence of water while the anion is significantly stronger polarized with increasing water content.  

In summary, water interacts stronger with the anions in nonprotic ionic liquids,while a similar important role of cations and anions is observed in protic ionic liquids.Interestingly, nonprotic as well as protic ionic liquids depolarize solvated water molecules. 

The application of ionic Hquids in catalysis has been reviewed by Hardacre and Parvulescu [127] hence, only selected examples are discussed herein. Molten salts have been used as an immobilization medium for homogeneous selox catalysis [128], for example, RuClj [129], Cu(C104)2 [130], or CuCl [131] (the latter in conjunction with molecular sieve 3A as a heterogeneous solid acid promoter). Van Doorslaer et al. [132] also reported the use of imidazolium ionic liquids for Pd(II)-acetate-catalyzed oxidation of alcohols to ketones, wherein, for 

Water is usually considered the most environmentally benign solvent however, the poor solubility of bulkier alcohols and molecular oxygen under ambient conditions has limited its application (in the absence of surfactants [25]). Pt nanoparticles supported on a water-soluble anion exchange resin exhibit excellent E-factors (12.8kilo waste per kilo product) in addition to very good activity and selectivity for alcohol oxidation [136]. Similarly, water has been used as solvent for, for example, benzyUc and primary alcohol oxidations over supported or stabilized Pd [137], Pt [138], and Au [139] clusters. 

Fundamental understanding of structure-function relationships is central for the design of improved selox catalysts, and has been greatly assisted by the development of new analytical tools with which to probe active sites at subnanometer spatial resolution [36] and subsecond time resolution. X-ray-based methods in particular can provide detailed insight into chemical composition and environment of active components and reacting adsorbates [140-142]. Quick and dispersive XAS have the capabihty to monitor dynamic changes in catalyst structure under reaction conditions (so-caUed operando spectroscopy) and have been applied to alcohol selox over Pd [96, 143-146], Pt [67, 147], and Ru [147] nanoparticles. 

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Anthony, J.L., Maginn, E.J., and Brennecke, J.R, Solution thermodynamics of imidazolium-based ionic liquids and water, /. Phys. Chem. B, 105,10942, 2001. 

Attempts have also been made to develop biphasic methodologies for the hydrogenation of aromatics. The hydrogenation of benzene derivatives was studied using various Ru complexes.468,469 A trinuclear cluster cationic species was isolated as the tetrafluoroborate salt and showed increased activity in hydrogenation.470 The air/ moisture-stable [bmim][BF4] ionic liquid and water with [Rh r -CgHg).,] [BF4] as the catalyst precursor is an effective system under usual conditions (90°C, 60 atm)471 A system composed of stabilized Rh(0) nanoparticles proved to be an efficient catalyst in the hydrogenation of alkylbenzenes 472... 

Anthony, J. L., Maginn, E. J., Brennecke, J. F. (2001), Solution Thermodynamics of Imidazolium-Based Ionic Liquids and Water, J. Phys. Chem. B 105, 10942-10949. 

The activity of microorganisms in organic solvents has been observed to have useful applications in the synthesis of organic materials. Using an ionic liquid solvent system of ionic liquid and water, Cull and coworkers have reported the a two-phase biotransformation with cells of Rhodococcus R312 [3]. Extensive research of Candida antarctica-cat lyzed reactions has also been carried out for catalyzing alcoholysis, ammoniolysis, and perhydrolysis reactions in ionic liquids ([bmim][BF4] or [bmimllPFe]), Scheme 10.1. 

Carbon dioxide is finding increasing use in combination with other green solvents, including ionic liquids and water. 

The miscibility of ionic liquids and water is temperature dependent and combinations are known where the ionic liquid and water form a single phase at higher temperatures and two separate phases at lower temperatures. Two examples that exploit this property in catalysis have been reported. One involves hydrodimerisation25 and the other hydrogenation26 and in both cases the reaction is conducted under homogeneous conditions and then the temperature of the system is reduced to bring about phase separation and subsequent product extraction. 

With regard to a possible water scavenging effect which would result in a shift of the equilibrium towards the products, studies were conducted in mixtures of ionic liquid and water. 

Kakiuchi T (2008) Mutual solubility of hydrophobic ionic liquids and water in Uquid-hquid two-phase systems for analytical chemistry. Anal Sci 24 1221-1230... 

By using (PhH)4Ru4Bp4 as catalyst, the hydrogenation of arenes in reaction media containing ionic liquid and water has been described. ... 

In many cases, the formation of azeotropes does not allow the separation of two or more compounds by a simple distillation. Very well known azeotropes with an industrial relevance are, for example, water/ethanol or water/THF. Sometimes the azeotrope can be broken by addition of another compound. These compounds are called entrainers. It was found that ionic liquids work as entrainers for a whole range of azeotropic systems [10]. Very high separation factors can be achieved, especially if water is part of the azeotropic mixture. Ionic liquids are usually hygroscopic materials with a strong affinity to water. Obviously, the interactions between ionic liquid and water are much stronger than those between water and the other component of the azeotrope. The ionic liquids literally grabs the water and releases the second compound, which can be distilled off as a pure material. Figure 9.5 displays the... 

Gutowski, K. E., Broker, G. A, Willauer, H. D., Huddleston, J. G., Swatloski, R. R, Holbrey, j. D., and Rogers, R D. (2003]. Controlling the aqueous miscibility of ionic liquids Aqueous biphasic systems of water-miscible ionic liquids and water-structuring salts for recycle, metathesis, and separations, /. Am. Chem. Soc, 125, pp. 6632-6633. 

Ficke, L.E., Rodriguez, H.C., and Brermecke, J.F. (2008) Heat capacities and excess enthalpies of l-ethyl-3-methylimidazolium-based ionic liquids and water. J. Chem. Eng. Data, 53 (9), 2112-2119. 

Jones A, Kronenwetter H, Manchanayakage R (2012) Electrochemical reductive coupling of 2-cyclohexen-l-one in a mixture of ionic liquid and water. Electrochem Commun 25 8-10. doi 10.1016/j.elecom.2012.09.020... 

All the components are well-defined pure substances which are organie or inorganic nonelectrolytes, including low-melting ionic liquids and water. Only data obtained by direct experimental measurements are considered. 

Most ionic liquids are immiscible with water at room temperature. However, at higher temperatures the mutual solubilities between ionic liquid and water increase, eventually ending in upper critical solution temperature (UCST) behaviour as shown in Figure 11.4. Examples of ionic liquids that show UCST behaviour with water are imidazolium-based ionic liquids with tetra-fluoroborate anions, hexafluorophosphate anions " and bis(trifluoro-methylsulfonyl)amide anions, ° and other ionic liquids. ... 

It can be noticed from Figure 11.4 that the mutual solubilities of ionic liquids and water are not symmetric. Generally, a much higher mole fraction of water is present in the ionic liquid phase than ionic liquid present in the water phase at the same temperature. The UCST is thus found at very low fractions of the ionic liquid. This asymmetry is also observed in (polymer + water) systems. Therefore, there are analogies between the phase diagrams of ionic liquid solutions and polymeric ones, although this may be due to different mechanisms. 

The mutual solubilities of ionic liquids and water are primarily defined by the type of anion and the cation alkyl side chain length of the ionic liquid. An increase in the alkyl chain length of the cation decreases the mutual solubility. Ionic liquids with bis(trifluoromethylsulfonyl)amide anions and hexafluorophosphate anions are more hydrophobic than ionic liquids with tetra-fluoroborate anions. However, even the most hydrophobic ionic liquids still... 

Ionic allenylidene-ruthenium in catalysis into unusual media ionic liquids and water... 

Pandey, S., Fletcher, K. A., Baker, S. N., and Baker, G. A. 2004. Correlation between the fluorescent response of microfluidity probes and the water content and viscosity of ionic liquid and water mixtures. AwaZy f 129, 569-573. 

Chakrabarty, D., Chakraborty, A., Hazra, P., Setb, D., and Sarkar, N. 2004. Dynamics of pbotoisomerisation and rotational relaxation of 3,3 -dietbyloxadicarbocyanine iodide in room temperature ionic liquid and binary mixture of ionic liquid and water. Chem. Phys. Lett. 397, 216-221. 

Disubstituted 1,2,3-triazoles are also formed from terminal alkynes and in situ-generated azides. For example, reactions of sodium azide, alkynes and aryl iodides in the presence of Cu(i) in a mixture of ionic liquids and water afford 1,4-disubstituted 1,2,3-triazoles Likewise, reaction of terminal alkynes with sodium azide and formaldehyde affords 2-hydroxymethyl-2/f-1,2,3-triazoles 20 (major isomer) in high yields... 

Table 4.2.1 Mutual solubility of ionic liquid and water for moderately hydrophobic ionic liquids suitable to ionic liquid salt bridge at 25 °C...

Chen, Y., Wang, Y., Cheng, Q., Liuand, X. Zhang, S. (2009). Carbohydrates-tailored phase tunable systems composed of ionic liquids and water, J.Chem. Thermodyn. Vol.41 (No.9) 1056-1059. 

B S Mark, A Yokozeki. Absorption cycle using ionic liquids and water as working... 

Fig. 5. Behavior of refractve index as a function of temperature for some ionic liquids and water.

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