Ionic liquids potential applications

Holbrey JD (2007) Heat capacities of common ionic liquids - potential applications as thermal fluids Chim Oggi - Chem Today 25 24-26. 

We expect that ionic liquids will find, in addition to organometallic synthesis, catalysis, and electrochemistry, a fourth area of application—the synthesis of nanostructured solids, either to make nanoobjects (e.g. particles and fiber) or for the design of nanopores and nanochannels in solids. The unique combination of adaptability towards other molecules and phases plus the strong H-bond-driven solvent structure makes ionic liquids potential key tools in the preparation of a new generation of chemical nanostructures. 

An alternative avenue for the exploration of the polarity of a solvent is by investigation of its effect on a chemical reaction. Since the purpose of this book is to review the potential application of ionic liquids in synthesis, this could be the most productive way of discussing ionic liquid polarity. Again, the field is in its infancy, but some interesting results are beginning to appear. 

From all this, it becomes understandable why the use of traditional solvents (such as water or butanediol) for biphasic catalysis has only been able to fulfil this potential in a few specific examples [23], whereas this type of highly specialized liquid-liquid biphasic operation is an ideal field for the application of ionic liquids, mainly due to their exactly tunable physicochemical properties (see Chapter 3 for more details). 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

This is only one of some very promising potential non-synthetic applications of ionic liquids that have emerged recently. Many others - some more, some less fully documented in patent or scientific literature - have been published. Table 9-1 gives a few examples, showing that most of the non-synthetic applications of ionic liquids can be grouped into three areas. Electrochemical applications benefit from the wide electrochemical window of ionic liquids and/or from the distinct variation of con-... 

Actually, it is quite likely that the first area of broader technical ionic liquid use will indeed be a non-synthetic application. Why Certainly not because non-synthetic applications have shown more potential, more performance, or more possibilities, but because many of these are relatively simple, with clearly defined technical targets. The improvement over existing technology is often based on just one or a very few specific properties of the ionic liquid material, whereas for most synthetic appli-... 

Several of the examples in Table 9-1 are looking quite promising for technical realization on a short to medium timescale. Other ideas are still in their infancy, and there is still a lot of potential for the development of other new non-synthetic applications of ionic liquids in the years to come. 

Of course, these concentration effects will be highly dependent on the nature of the substrate dissolved in the ionic liquid, as well as on the nature of the ionic liquid s cation and anion. Given the enormous opportunity to vary these last two, it becomes clear that a detailed understanding of the role of the ionic liquid in reaction mixtures is far from complete. Clearly, this limited understanding is currently restricting our opportunities to benefit from the full potential of an ionic liquid solvent in a given synthetic application. 

Recently, room temperature ionic liquids (RT-ILs) have attracted much attention for their excellent properties, e.g., wide temperature range of liquid phase, ultra-low vapor pressure, chemical stability, potential as green solvents, and high heat capacities [64,65]. These properties make them good candidates for the use in many fields, such as thermal storage [66], electrochemical applications, homogeneous catalysis [67], dye sensitized solar cells [68], and lubricants [69,70]. 

Before leaving ionic liquids it is worth mentioning their potential value in separation processes. Organic solvents are frequently used in multiphase extraction processes and pose the same problems in terms of VOC containment and recovery as they do in syntheses, hence ionic liquids could offer a more benign alternative. Interesting applications along this line which have been studied include separation of spent nuclear fuel from other nuclear waste and extraction of the antibiotic erythromycin-A. 

The strategy of using two phases, one of which is an aqueous phase, has now been extended to fluorous . systems where perfluorinated solvents are used which are immiscible with many organic reactants nonaqueous ionic liquids have also been considered. Thus, toluene and fluorosolvents form two phases at room temperature but are soluble at 64 °C, and therefore,. solvent separation becomes easy (Klement et ai, 1997). For hydrogenation and oxo reactions, however, these systems are unlikely to compete with two-phase systems involving an aqueous pha.se. Recent work of Richier et al. (2000) refers to high rates of hydrogenation of alkenes with fluoro versions of Wilkinson s catalyst. De Wolf et al. (1999) have discussed the application and potential of fluorous phase separation techniques for soluble catalysts. 

Recently, there has been considerable interest in developing molten salts that are less air and moisture sensitive. Melts such as l-methyl-3-butylimidazolium hexa-fluorophosphate [211], l-ethyl-3-methylimidazolium trifluoromethanesulfonate [212], and l-ethyl-3-methylimidazolium tetrafluoroborate [213] are reported to be hydro-phobic and stable under environmental conditions. In some cases, metal deposition from these electrolytes has been explored [214]. They possess a wide potential window and sufficient ionic conductivity to be considered for many electrochemical applications. Of course if one wishes to take advantage of their potential air stability, one loses the opportunity to work with the alkali and reactive metals. Further, since these ionic liquids are neutral and lack the adjustable Lewis acidity common to the chloroaluminates, the solubility of transition metal salts into these electrolytes may be limited. On a positive note, these electrolytes are significantly different from the chloroaluminates in that the supporting electrolyte is not intended to be electroactive. 

The possibility of adjusting solubility properties is of particular importance for liquid-liquid biphasic catalysis. Liquid-liquid catalysis can be realised when the ionic liquid is able to dissolve the catalyst, especially if it displays partial solubility of the substrates and poor solubility of the reaction products. Under these conditions, the product phase, which also contains the unconverted reactants, is removed by simple phase decantation. The ionic liquid containing the catalyst can then be recycled. In such a scenario the ionic catalyst solution may be seen as part of the capital investment for a potential technical process (in an ideal case) or at least as a working solution (only a small amount has to be replaced after a certain time of application). A crucial aspect of this concept is the immobilisation of the transition metal catalyst in the ionic liquid. While most transition metal catalysts easily dissolve in an ionic liquid without any special ligand design, ionic ligand systems have been applied with great success to... 

Among the potential impurities in ionic liquids water, halide ions and organic starting material are of great importance for transition metal chemistry while the colour of an ionic liquid is not a critical parameter in most applications. 

The discovery of ionic liquids is proving a fertile field for researchers. Ionic solvents are revealing new mechanisms of reaction and enhancing our understanding of the molecular world. As well, their potential applications in both industry and the home have encouraged several companies, world-wide, to further explore and develop these unusual substances. 

In SILP carbonylation we have introduced a new methanol carbonylation SILP Monsanto catalyst, which is different from present catalytic alcohol carbonylation technologies, by using an ionic liquid as reaction medium and by offering an efficient use of the dispersed ionic liquid-based rhodium-iodide complex catalyst phase. In perspective the introduced fixed-bed SILP carbonylation process design requires a smaller reactor size than existing technology in order to obtain the same productivity, which makes the SILP carbonylation concept potentially interesting for technical applications. 

Ionic liquids are salts that are often defined as those with melting points < 100°C (1) many are liquids even at room temperature. These salts present a wide range of properties for applieations as new solvents and eatalysts. A large number of new ionic liquids have been reported reeently, and the seope of their potential applications has been expanding rapidly. For eatalytie applieations, ionie liquids can be viewed as salts that are stable in the liquid state under the proeess conditions. 

An area of broad interest in catalysis is the search for viable replacements for the widely used Bronsted liquid acids such as HF and H2SO4 and solid Lewis acids such as AICI3 and MgCl2. The liquid acids are corrosive and also costly, because of the need to work up the products by neutralization and repeated washing. In many cases, the contamination of the products by these acids induces degradation over time and limits the application of the products. The cost of multi-step washing can be quite high. Acidic ionic liquids therefore offer potential alternatives for such reactions (//). 

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