Ionic liquids characteristics

Fluorinated ionic liquids, characteristics, 1, 854 Fluorinated molecules, coordination, 1, 727 Fluorinated thiols, in nanoparticle preparation, 12, 80 Fluorination, and iridium carbonyl cluster complexes, 7, 299... 

It should be emphasized that ionic liquids are simply organic salts that happen to have the characteristic of a low melting point. Many ionic liquids have been widely investigated with regard to applications other than as liquid materials as electrolytes, phase-transfer reagents [12], surfactants [13], and fungicides and biocides [14, 15], for example. 

The following discussion concerns the thermal liquidus ranges available in different ionic liquids, as functions of cation and anion structure and composition. In particular, those structural features of cation and anion that promote these properties (while providing other desirable, and sometimes conflicting characteristics of the liquid, such as low viscosity, chemical stability, etc.) and variations in liquidus ranges and stabilities are the focus of this chapter. 

Ionic liquids are similar to dipolar, aprotic solvents and short-chain alcohols in their solvent characteristics. These vary with anion (from very ionic Cl to more covalent [BETI] ). IFs become more lipophilic with increasing alkyl substitution, resulting in increasing solubility of hydrocarbons and non-polar organics. 

Tunability of the solubility characteristics of the ionic liquids Solvent effect... 

Notwithstanding their very low vapor pressure, their good thermal stability (for thermal decomposition temperatures of several ionic liquids, see [11, 12]) and their wide operating range, the key property of ionic liquids is the potential to tune their physical and chemical properties by variation of the nature of the anions and cations. An illustration of their versatility is given by their exceptional solubility characteristics, which make them good candidates for multiphasic reactions (see Section 5.3.4). Their miscibility with water, for example, depends not only on the hydrophobicity of the cation, but also on the nature of the anion and on the temperature. 

Addition of co-solvents can also change the co-miscibility characteristics of ionic liquids. As an example, the hydrophobic [BMIM][PFg] salt can be completely dissolved in an aqueous ethanol mixture containing between 0.5 and 0.9 mole fraction of ethanol, whereas the ionic liquid itself is only partially miscible with pure water or pure ethanol [13]. The mixing of different salts can also result in systems with modified properties (e.g., conductivity, melting point). 

One of the key factors controlling the reaction rate in multiphasic processes (for reactions talcing place in the bulk catalyst phase) is the reactant solubility in the catalyst phase. Thanks to their tunable solubility characteristics, the use of ionic liquids as catalyst solvents can be a solution to the extension of aqueous two-phase catalysis to organic substrates presenting a lack of solubility in water, and also to moisture-sensitive reactants and catalysts. With the different examples presented below, we show how ionic liquids can have advantageous effects on reaction rate and on the selectivity of homogeneous catalyzed reactions. 

Flowever, information concerning the characteristics of these systems under the conditions of a continuous process is still very limited. From a practical point of view, the concept of ionic liquid multiphasic catalysis can be applicable only if the resultant catalytic lifetimes and the elution losses of catalytic components into the organic or extractant layer containing products are within commercially acceptable ranges. To illustrate these points, two examples of applications mn on continuous pilot operation are described (i) biphasic dimerization of olefins catalyzed by nickel complexes in chloroaluminates, and (ii) biphasic alkylation of aromatic hydrocarbons with olefins and light olefin alkylation with isobutane, catalyzed by acidic chloroaluminates. 

In comparison with classical processes involving thermal separation, biphasic techniques offer simplified process schemes and no thermal stress for the organometal-lic catalyst. The concept requires that the catalyst and the product phases separate rapidly, to achieve a practical approach to the recovery and recycling of the catalyst. Thanks to their tunable solubility characteristics, ionic liquids have proven to be good candidates for multiphasic techniques. They extend the applications of aqueous biphasic systems to a broader range of organic hydrophobic substrates and water-sensitive catalysts [48-50]. 

All these cations are bulky and asymmetric. In addition, their alkyl groups make it possible to modify them almost endlessly. The length of the alkyl chains can be varied, they can be straight or branched, and functional groups such as —OH can be substituted. This feature makes it possible to vary the characteristics of ionic liquids to suit a particular application. 

TABLE 7.6. Process characteristics for optimised nonanal production (using liquid-liquid biphasic catalysis with ionic liquids) and butanal production (using SILP catalysis) on a 100.000 tons/year scale... 

There is a wide range of possibilities for adjusting the solubility characteristics of ionic liquids, and this is one of their potential advantages for optimized performance in biphasic or multiphasic catalysis (/). Because of the generally weak coordinating ability of the anions, most catalysts can be isolated in the solvent in a stable state without loss of activity. The product selectivity can sometimes be improved as well by the phase isolation. Because the catalyst is concentrated in the ionic phase, the reaction volume can be much smaller than in classical... 

The solubilities of aromatic compounds in the ionic liquid are dramatically higher than those of saturated compounds. Benzene has a solubility of 4.9mol/mol of ionic liquid, and thiophene has a solubility of 6.7mol/mol of ionic liquid. A dramatic steric effect was observed on the solubility of aromatics the alkyl-substituted aromatics showed reduced solubility. Although the solubility of hexene in the ionic liquid is considerably lower than that of the aromatics, it is still measurably higher than that of hexane. Similar structure-solubility relationships characteristic of organic molecules were observed with the ionic liquids [BMIM]BF4, [BMIM]PFg, and [EMIM]BF4 (Fig. 10) (27). 

A. Characteristics of Ionic Liquid Catalyst Carriers A.l. Inert Catalyst Carriers... 

Recently, reports have appeared on a class of ILs known as task specific ionic liquids (TSIL). The term was introduced by J. H. Davis, Jr s group to refer to those ILs which have functional groups attached to them so as to give specific properties and functionalities. Thus, they not only perform specific functions like metal ion extraction,catalysisand capture of but also maintain the desired physical characteristics such as physical state, non-volatility, viscosity, etc. The implementation of TSILs further enhances the versatility of classical ILs where both reagent and medium are coupled. The union of reagent with medium has been... 

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