Ionic liquid

  An ionic liquid is a liquid that contains essentially only ions. Some ionic liquids, such as ethylammonium nitrate, are in a dynamic equilibrium where at any time more than 99.99% of the liquid is made up of ionic rather than molecular species. In the broad sense, the term includes all molten salts, for instance, sodium chloride at temperatures higher than 800 °C. Today, however, the term "ionic liquid" is commonly used for salts whose melting point is relatively low (below 100 °C). In particular, the salts that are liquid at room temperature are called room-temperature ionic liquids, or RTILs. There also exist mixtures of substances which have low melting points, called Deep eutectic solvent, or DES, that have many similarities with ionic liquids.

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History

The date of discovery, as well as discoverer, of the "first" ionic liquid is disputed. However, one of the earlier known ionic liquids was [EtNH₃]⁺ [NO₃]^- (m.p. 12 °C), the synthesis of which was published in 1914.^[1] Much later, series of ionic liquids based on mixtures of 1,3-dialkylimidazolium or 1-alkylpyridinium halides and trihalogenoaluminates, initially developed for use as electrolytes, were to follow.^[2],^[3] An important property of the imidazolium halogenoaluminate salts was that they were tuneable – viscosity, melting point and the acidity of the melt could be adjusted by changing the alkyl substituents and the ratio of imidazolium or pyridinium halide to halogenoaluminate.^[4]

A major drawback was their moisture sensitivity and, though to a somewhat lesser extent, their acidity/basicity, the latter which can sometimes be used to an advantage. In 1992, Wilkes and Zawarotko reported the preparation of ionic liquids with alternative, 'neutral', weakly coordinating anions such as hexafluorophosphate ([PF₆]^-) and tetrafluoroborate ([BF₄])^-, allowing a much wider range of applications for ionic liquids.^[5] It was not until recently that a class of new, air- and moisture stable, neutral ionic liquids, was available that the field attracted significant interest from the wider scientific community.

More recently, people have been moving away from [PF₆]^- and [BF₄]^- since they are highly toxic, and towards new anions such as bistriflimide [(CF₃SO₂)₂N]^- or even away from halogenated compounds completely. Moves towards less toxic cations have also been growing, with compounds like ammonium salts (such as choline) being just as flexible a scaffold as imidazole.

Characteristics

Ionic liquids are electrically conductive and have extremely low vapor pressure. (Their noticeable odours are likely due to impurities.) Their other properties are diverse. Many have low combustibility, excellent thermal stability, a wide liquid range, and favorable solvating properties for diverse compounds. Many classes of chemical reactions, such as Diels-Alder reactions and Friedel-Crafts reactions, can be performed using ionic liquids as solvents. Recent work has shown that ionic liquids can serve as solvents for biocatalysis ^[6]. The miscibility of ionic liquids with water or organic solvents varies with sidechain lengths on the cation and with choice of anion. They can be functionalized to act as acids, bases or ligands, and have been used as precursor salts in the preparation of stable carbenes. Because of their distinctive properties, ionic liquids are attracting increasing attention in many fields, including organic chemistry, electrochemistry, catalysis, physical chemistry, and engineering; see for instance magnetic ionic liquid.

Despite their extremely low vapor pressures, some ionic liquids can be distilled under vacuum conditions at temperatures near 300 °C.^[7] Some ionic liquids (such as 1-butyl-3-methylimidazolium nitrate) generate flammable gases on thermal decomposition. Thermal stability and melting point depend on the components of the liquid.

The solubility of different species in imidazolium ionic liquids depends mainly on polarity and hydrogen bonding ability. Simple aliphatic compounds are generally only sparingly soluble in ionic liquids, whereas olefins show somewhat greater solubility, and aldehydes can be completely miscible. This can be exploited in biphasic catalysis, such as hydrogenation and hydrocarbonylation processes, allowing for relatively easy separation of products and/or unreacted substrate(s). Gas solubility follows the same trend, with carbon dioxide gas showing exceptional solubility in many ionic liquids, carbon monoxide being less soluble in ionic liquids than in many popular organic solvents, and hydrogen being only slightly soluble (similar to the solubility in water) and probably varying relatively little between the more popular ionic liquids. (Different analytical techniques have yielded somewhat different absolute solubility values.)

Room temperature ionic liquids

Room temperature ionic liquids consist of bulky and asymmetric organic cations such as 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, N-methyl-N-alkylpyrrolidinium and ammonium ions. A wide range of anions is employed, from simple halides, which generally inflect high melting points, to inorganic anions such as tetrafluoroborate and hexafluorophosphate and to large organic anions like bis-trifluorsulfonimide, triflate or tosylate. There are also many interesting examples of uses of ionic liquids with simple non-halogenated organic anions such as formate, alkylsulfate, alkylphosphate or glycolate. As an example, the melting point of 1-butyl-3-methylimidazolium tetrafluoroborate or [bmim][BF₄] with an imidazole skeleton is about -80 °C, and it is a colorless liquid with high viscosity at room temperature.

It has been pointed out that in many synthetic processes using transition metal catalyst, metal nanoparticles play an important role as the actual catalyst or as a catalyst reservoir. It also been shown that ionic liquids (ILs) are an appealing medium for the formation and stabilization of catalytically active transition metal nanoparticles. More importantly, ILs can be made that incorporate co-ordinating groups,^[8], for example, with nitrile groups on either the cation or anion (CN-IL). In various C-C coupling reactions catalyzed by palladium catalyst, it has been found the palladium nanoparticles are better stabilized in CN-IL compared to non-functionalized ionic liquids; thus enhanced catalytic activity and recyclability are realized. ^[9]

Low temperature ionic liquids

Low temperature ionic liquids (below 130 kelvins) have been proposed as the fluid base for an extremely large diameter spinning liquid mirror telescope to be based on the earth's moon.^[10] Low temperature is advantageous in imaging long wave infrared light which is the form of light (extremely red-shifted) that arrives from the most distant parts of the visible universe. Such a liquid base would be covered by a thin metallic film that forms the reflective surface. A low volatility is important for use in the vacuum conditions present on the moon.

Food science

The application range of ionic liquid also extends to food science. [bmim]Cl for instance is able to completely dissolve freeze dried banana pulp and the solution with an additional 15% DMSO lends itself to Carbon-13 NMR analysis. In this way the entire banana compositional makeup of starch, sucrose, glucose, and fructose can be monitored as a function of banana ripening.^[11]

Safety

Due to their non-volatility, effectively eliminating a major pathway for environmental release and contamination, ionic liquids have been considered as having a low impact on the environment and human health, and thus recognized as solvents for green chemistry. However, this is distinct from toxicity, and it remains to be seen how 'environmentally-friendly' ILs will be regarded once widely used by industry. Research into IL aquatic toxicity has shown them to be as toxic or more so than many current solvents already in use ^[12]. A review paper on this aspect has been published in 2007.^[13] Available research also shows that mortality isn't necessarily the most important metric for measuring their impacts in aquatic environments, as sub-lethal concentrations have been shown to change organisms' life histories in meaningful ways. According to these researchers balancing between zero VOC emissions, and avoiding spills into waterways (via waste ponds/streams, etc.) should become a top priority. However, with the enormous diversity of substituents available to make useful ILs, it should be possible to design them with useful physical properties and less toxic chemical properties.

With regard to the safe disposal of ionic liquids, a 2007 paper has reported the use of ultrasound to degrade solutions of imidazolium-based ionic liquids with hydrogen peroxide and acetic acid to relatively innocuous compounds.^[14]

Despite their low vapor pressure many ionic liquids have also found to be combustible and therefore require careful handling ^[15]. Brief exposure (5 to 7 seconds) to a flame torch will ignite these IL's and some of them are even completely consumed by combustion.

Cellulose Processing

Occurring at a volume of some 700 billion tons, cellulose is the earth’s most widespread natural organic chemical and, thus, highly important as a bio-renewable resource. But even out of the 40 billion tons nature renews every year, only approx. 0.2 billion tons are used as feedstock for further processing. A more intensive exploitation of cellulose as a biorenewable feedstock has to date been prevented by the lack of a suitable solvent that can be used in chemical processes. Robin Rogers and co-workers at the University of Alabama have found that by means of ionic liquids, however, real solutions of cellulose can now be produced for the first time at technically useful concentrations ^[16]. This new technology therefore opens up great potential for cellulose processing.

For example, making cellulosic fibers from so-called dissolving pulp currently involves the use, and subsequent disposal, of great volumes of various chemical auxiliaries, esp. carbon disulfide (CS₂). Major volumes of waste water are also produced for process reasons and need to be disposed of. These processes can be greatly simplified by the use of ionic liquids, which serve as solvents and are nearly entirely recycled. The “Institut für Textilchemie und Chemiefasern” (ITCF) in Denkendorf and BASF are jointly investigating the properties of fibers spun from an ionic liquid solution of cellulose in a pilot plant setup. ^[17]

See also

• 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF₆) for an often encountered ionic liquid.
• Aza-Baylis-Hillman reaction for the use of a chiral ionic liquid in asymmetric synthesis.
• Deep eutectic solvent

References