1.2 O VERVIEW OF IL S
1.2.4 Properties and applications
Unlike molecular liquids, the ionic nature of these compounds (ionic liquids) results in a distinctive combination of properties. The most interesting properties of these compounds are listed below [23, 57-60]:
Extremely low vapour pressure; they are eco-friendly. This property makes them easy to use, contain and transfer in addition they can be used under high vacuum conditions which reduces the chronic exposure to solvent vapours.
A diverse series of organic, organometallic and inorganic compounds are soluble in ILs. They provide good solubility for gases such as CO2 which makes them attractive solvents for catalytic hydrogentaion, carbonylation, hydroformylation and aerobic oxidation.
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Low or reduced flammability hazards.
Tunable properties, such as the polarity and hydrophilicity. Hence the ILs can be tailored to be immiscible with some organic solvents and can be used in two-phase systems. In a similar manner, hydrophobic ILs are suitable for use in aqueous biphasic systems. In addition, some ILs can operate un triphasic system which is a very advantageous property for the extraction of the products.
Excellent solvation capacity for both polar and non polar compounds.
High thermal stability (up to 450 C) [51, 61] and wide liquid range which offers distinct advantages over the traditional solvent systems.
High electrical conductivity and wide electrochemical window (> 15 mScm-1) which make them attractive materials in numerous applications in the electrochemical applications [62].
Although ILs are generally known as safe and benign compounds, some of them are flammable and others are volatile [63, 64].
Fig 1-6 Selection of applications where ILs have been used [65]
The distinctive properties of these compounds enhanced their capabilities over some traditionally used volatile solvents and make them useful in many important areas of commercial applications such as: solvents for reactions, absorption media for gas separations, separating agent in extractive distillation, heat transfer fluids,
Solvents Bio-catalysis
Organic reactions & catalysis Nano-particle synthesis Polymerization
IONIC LIQUIDS
Electrolytes Fuel cells Sensors Batteries Metal finishing
Liquid crystals Displays
Electrostatic materials Artificial muscles Robotics
Lubricants & additives Lubricants
Fuel additives
Heat storage Thermal fluids Analytics
GC-head space solvents Protein crystalization
Separations Gas separations Extraction distillation Extraction
Membranes
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biomass processing, working fluid in a variety of electrochemical applications [66]
(batteries, capacitors, solar cells, etc.), lubricants [56, 67] and in biocatalysts with unique advantages [68]. Fig 1-6 shows a selection of applications where ILs have been used [65].
Both cationic and anionic components of ILs can be varied and modified for specific application with desirable properties. The versatility of these unique solvents also benefits other areas of chemical research. Among the latest successes, ILs were used in recovery of biofuels, in deep desulfurization of diesel fuel and also as versatile lubricants [27, 69].
Recently, the interest of the application of ILs in separations has increased, typically as replacements for the organic diluents employed in traditional liquid-liquid extraction or in membrane-based separations of organic solutes, metal ions, and gases.
Some of the studied applications are outlined below [70]:
Liquid extraction:
1. Extraction of organics from aqueous solution
RTILs having a miscibility gap with water have been shown to be effective solvents for a range of organic compounds [71]. Furthermore, the pH-dependent distribution of certain solutes can provide a route for reverse extraction [71, 72].
2. Metals extraction from aqueous solution
RTILs provide unique solvation environment for ionic species and have been shown to be highly effective as replacements for conventional organic solvents in the liquid-liquid extraction of metal ions. The partitioning of metal ions from aqueous solutions into ILs containing extractants far exceeds that obtainable with most conventional solvent [35, 72]. In addition, ―Task-specific‖
ILs (TSIL), incorporating a metal ion-ligand functional group into one of the ions of an RTIL functions as both the hydrophobic solvent and the extractants in liquid-liquid separations of metal ions [73, 74].
Hydrocarbon Processing
1. Sulfur removal from hydrocarbon fuels, including extraction of organosulfur compounds by selective solubilization, removal of mercaptans, and electrochemical oxidation [37, 75].
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2. Hydrocarbon separations based on interaction of metal salts dissolved in ILs [76].
3. Extractive distillation is an apparent processing application for ILs, given the possibility for selective solubility and no volatility of RTILs for separation of azeotropic mixtures (azeotrope is a special class of liquid mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation). This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture) and in multiple hydrocarbon separation processes (separation of hydrocarbons with close boiling points, such as C4 mixtures) [76].
Membrane separations
1. Supported liquid membrane system employing ILs as carriers utilize the characteristics of selective solubility and low volatility, while minimizing volume of potentially costly solvent [77].
2. Method for separating substances from solutions containing ILs by means of a membrane. The main function of this method is to perform fine separation of undesirable constituents from the catalytic system after phase decantation has already performed the coarse separation of the catalyst from the products [37].
Metals separation by electrorefining
IL processes developed for production, refining, and recycling of metals, including aluminum processing and spent nuclear fuel treatment. The metal ion extraction in the ionic liquid/aqueous two phase system indicated high efficiency and selectivity that expelled most of the organic solvents. The metal ion partitioning always rely on the species of the ionic liquid, metal ion and ligand. In addition, it has been demonstrated that room-temperature ionic liquids can be used for solvent extraction of metal species from aqueous media; this is an area of great significance to the nuclear industry which currently uses solvent extraction in the process for reprocessing spent nuclear fuel. [78].
Analytical/small-scale separations
1. Stationary phases for gas chromatography: ILs coated onto fused silica capillaries exhibit a dual behaviour, acting as low-polarity phases with
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nonpolar compounds and in the opposite manner for compounds bearing strong proton-donor groups. The chromatographic properties of these materials can be readily tuned by minor changes in the cationic or anionic constituent of the IL.
ILs appear to act as a low-polarity stationary phase to nonpolar compounds.
However, molecules with strong proton donor groups, in particular, are tenaciously retained. The nature of the anion can have a significant effect on both the solubilizing ability and the selectivity of ionic liquid stationary phases.
[79].
2. Electrolytes in capillary electrophoresis: RTILs have been shown to be suitable as running electrolytes in capillary electrophoresis for compounds such as basic proteins and polyphenols [80].
Gas Separation
Selective solubility of specific gases in ILs has been measured, leading to the possibility for gas separations. Carbon dioxide exhibits a relatively high solubility in imidazolium-based ILs; oxygen, nitrogen, hydrogen, carbon monoxide, argon have low solubility (the solubility of CO2 in [bmim]PF6 is greater than 0.2 mol fraction while that of O2 and Ar in the same IL is less that 0.02 mol fraction) [81].
The solubility of CO2 in the conventional ionic liquids, such as imidazolium, phosphonium and sulfonate based ILs is relatively higher than the solubility of CO2 in some conventional organic solvents such as heptane, ethanol, benzene, cyclohexane.
The equilibrium solubility of CO2 in conventional ILs ([bmim]PF6 and [bmim]BF4) is about 0.10-0.15 wt% at room temperature and atmospheric pressure, which is obviously too low for industrial application for CO2 capture [61].
Task-specific ILs have been demonstrated for selective gas solubility and the incorporation of functional groups to ILs was adopted to increase the solubility of CO2 in ILs [82]. Increasing the free volume (unoccupied part of the molar volume of a substance [83]) also can enhance the CO2 solubility capacity. The structure of an imidazolium-based cation was modified by appending an amine substituent, yielding an IL with elevated carbon dioxide capture from gas mixtures [14]. In addition, 1- alkyl-3-fluoroalkylimidazolium-based ILs such as 1-methyl-3-(nonafluorohexyl) imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C6H4F9mim][NTf2]) and 1-methyl-
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3-(tridecafluorooctyl)imidazolium bis[(trifluoromethyl)sulfonyl]imide [C8H4F13mim]
[NTf2] have a high solubility capacity for CO2 compared to the conventional ILs [82].