Ionic liquid

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A 1-butyl-3-methylimidazolium (BMIM) salt.
Ionic liquid model

An ionic liquid is a salt in the liquid state. In some contexts, the term has been restricted to salts that have a sufficiently low melting point, such as "below 100 °C", but this limit is only a convention, and is completely arbitrary. While ordinary liquids such as water and gasoline are predominantly made of eletrically neutral molecules, ionic liquids are largely made of ions and short-lived ion pairs. These substances are also called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses.

Ionic liquids have many applications, such as powerful solvents and electrically conducting fluids (electrolytes). Salts that are liquid at low temperature are important for electric battery applications, and have been used as sealants due to their very low vapor pressure.

Any salt that melts without decomposing or vaporizing will usually yield an ionic liquid. Sodium chloride (NaCl), for example, melts at 801 °C into a liquid that consists largely of sodium cations (Na+) and chloride anions (Cl−). Conversely, when an ionic liquid is cooled, it will often form an ionic solid — which may be either crystalline or glassy.

The bonding between ions is usually stronger than the Van der Waals bonds between the molecules of ordinary liquids. For that reason, the melting points of salts are can be higher than those of molecular substances, although one can encounter these salts as liquids at almost any temperature. Examples include 1-ethyl-3-methylimidazolium dicyanamide, (C2H5)(CH3)C3H3N+2·N(CN)2−, that melts at −21 °C;[1], pyridinium chloride, C5H6N+ · Cl− that melts at 144.5 °C;[2] and 1-butyl-3,5-dimethylpyridinium bromide which becomes a glass below −24 °C.[3]

Low-temperature ionic liquids can be compared to ionic solutions, which are liquids containing both ions and neutral molecules; in particular, to the so-called deep eutectic solvents, mixtures of ionic and non-ionic solid substances which have much lower melting points than the pure compounds.

The term "ionic liquid" in the general sense was used as early as 1943.[4]

Contents

[edit] History

The date of discovery of the "first" ionic liquid is disputed, along with the identity of the discoverer. Ethanolammonium nitrate (m.p. 52-55 °C) was reported in 1888 by S. Gabriel and J. Weiner.[5] One of the earliest truly room temperature ionic liquids was ethylammonium nitrate (C2H5)NH+3 ·NO−3 (m.p. 12 °C), synthesized in 1914 by Paul Walden.[6] In the 1970s and 1980s there was interest in ionic liquids based on alkyl-substituted imidazolium and pyridinium cations, with halide or trihalogenoaluminate anions, initially developed for use as electrolytes in battery applications.[7][8]

An important property of the imidazolium halogenoaluminate salts was that their physical properties — such as viscosity, melting point, and acidity — could be adjusted by changing the alkyl substituents and the imidazolium/pyridinium and halide/halogenoaluminate ratios.[9] Two major drawbacks for some applications were their moisture sensitivity and their acidity/basicity. In 1992, Wilkes and Zawarotko obtained ionic liquids with 'neutral' weakly coordinating anions such as hexafluorophosphate (PF6−) and tetrafluoroborate (BF4−), allowing a much wider range of applications for ionic liquids.[10] Recently a new class of air- and moisture stable, neutral ionic liquids became available.[clarification needed] Research has also been moving away from hexafluorophosphate and tetrafluoroborate towards less toxic alternatives 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 imidazolium.

[edit] Characteristics

Ionic liquids are often moderate to poor conductors of electricity, non-ionizing (e.g. non-polar), highly viscous and frequently exhibit a low vapor pressure. Their other properties are diverse: many have low combustibility, excellent thermal stability, wide liquidus regions, and favorable solvating properties for a range of polar and non-polar 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.[11] 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.

Commonly used cations for ionic liquids

Despite their extremely low vapor pressures, some ionic liquids can be distilled under vacuum conditions at temperatures near 300 °C.[12] In the original work by Martyn Earle, et al., the authors wrongly assumed the vapor to be made up of individual, separated ions,[13] but it was later proven that the vapors formed consisted of ion-pairs.[14] 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. Thermal stability of various RTILs are available. The thermal stability of a task-specific ionic liquid, protonated betaine bis(trifluoromethanesulfonyl)imide is of about 534 K and N-Butyl-N-Methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide was thermally stable up to 640 K.[15]

The solubility of different species in imidazolium ionic liquids depends mainly on polarity and hydrogen bonding ability. Saturated 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 may vary relatively little between the more commonly used ionic liquids. Different analytical techniques have yielded somewhat different absolute solubility values.

[edit] 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 are employed, from simple halides, which generally inflect high melting points, to inorganic anions such as tetrafluoroborate and hexafluorophosphate and to large organic anions like bistriflimide, 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 ([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 catalysts, 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 coordinating groups,[16] 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.[17]

[edit] Low temperature ionic liquids

Low temperature ionic liquids (below 130 K) have been proposed as the fluid base for an extremely large diameter spinning liquid mirror telescope to be based on the Earth's moon.[18] 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.

[edit] Food science

Ionic liquids have been used in 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.[19]

[edit] Applications

Nowadays ionic liquids find a number of industrial applications which vary greatly in character. A few of their industrial applications are briefly described below; more detailed information can be found in a recent review article.[20]

[edit] BASIL

The first major industrial application of ILs was the BASIL (Biphasic Acid Scavenging utilizing Ionic Liquids) process by BASF, in which a 1-alkylimidazole was used to scavenge the acid from an existing process. This then results in the formation of an IL which can easily be removed from the reaction mixture.[21] But the easier removal of an unwanted side-product (as an IL rather than as a solid salt) is not the only advantage of the IL based process. By using an IL it was possible to increase the space/time yield of the reaction by a factor of 80,000. It should, however, be kept in mind that improvements of such scale are rare.[citation needed]

[edit] 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 bio-renewable 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, real solutions of cellulose can now be produced for the first time at technically useful concentrations.[22] This technology therefore opens up great potential for cellulose processing. Although it has been presented as a new idea, the use of ionic liquids in cellulose processing originally dates back to 1934 in a patent by Graenacher where mixtures of 1-ethylpyridinium chloride with free nitrogen containing bases were used.[23]

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.[24] The dissolution of cellulose based materials like tissue paper waste, generated at chemical industries and at research laboratories, in room temperature ionic liquid, 1-butyl-3-methylimidazolium chloride, bmimCl, was studied and the recovery of valuable compounds by electrodeposition was studied from this cellulose matrix.[25]

[edit] Eastman chemical’s DHF plant

Eastman operated an ionic liquid-based plant for the synthesis of 2,5-dihydrofuran from 1996 to 2004. However, the plant is now defunct because demand for the product has ceased.[citation needed]

[edit] Dimersol - Difasol

The dimersol process is a traditional way to dimerize short chain alkenes into branched alkenes of higher molecular weight. Nobel laureate Yves Chauvin and Hélène Olivier-Bourbigou at IFP (France) have developed an ionic liquid-based add-on to this process called the Difasol process. However, while it may be licensed it has as yet not been put into commercial practice.

[edit] Petrochina

Petrochina have announced the implementation of an ionic liquid-based process called Ionikylation. This process, the alkylation of C4 olefins with iso-butane, is retrofitted into a 65,000 tonne per year alkylation plant, making it the biggest industrial application of ILs to date.

[edit] Evonik paint additives

Ionic liquids can enhance the finish, appearance and drying properties of paints. Evonik is marketing such ILs under the name of TEGO Dispers. These products are also added to the Pliolite paint range.

[edit] Air products - ILs as a transport medium for reactive gases

Air products make use of ILs as a medium to transport reactive gases in. Reactive gases such as trifluoroborane, phosphine or arsine, BF3, PH3 or AsH3, respectively, are stored in suitable ILs at sub-ambient pressure. This is a significant improvement over pressurised cylinders. The gases are easily withdrawn from the containers by applying a vacuum.

[edit] Linde's IL 'piston'

Main article: Ionic liquid piston compressor

Whereas Air Product’s Gasguard system relies on the solubility of some gases in ILs, Linde are exploiting other gases’ insolubility in ILs. As mentioned above, the solubility of hydrogen in ILs is very low. Linde now make use of this insolubility by using a body of ionic liquid to compress hydrogen up to 450 bar[26] in filling stations; and in so doing they reduced the number of moving parts from about 500 in a conventional piston pump engine down to 8.

[edit] IOLITEC's dispersions of nano-materials

It is well known that ILs can be used for the synthesis of nano-scaled materials. IOLITEC has developed a couple of methods for the dispersion of different combinations of solvents and nano-materials, using the unusual physical properties of ionic liquids. The dispersion is useful, since it enables a safer use of nano-scaled materials in typical applications.

[edit] Nuclear industry

RTILs are extensively explored for various innovative applications in nuclear industry. It includes application of ionic liquid as extractant/diluent in solvent extraction systems, as alternate electrolyte media for the high temperature pyrochemical processing, etc. Fundamental studies on the extraction methods for electrodeposition of fission products like uranium, palladium etc., from spent nuclear fuel using RTILs as extractants are reported. Reports on employing using Ionic liquids as non-aqueous electrolyte media for the recovery of uranium,[27] lanthanides,[28] and useful fission products like palladium[29], rhodium[30] and even ruthenium [31] from spent nuclear fuel are also available. Ionic liquids have been explored as diluents for the liquid-liquid extraction of actinides. Recently a novel process using ionic liquids, namely, Extraction-Electrodeposition was developed and demonstrated for the recovery of palladium [32]. Studies on the electrochemical behavior of uranium(VI) in ionic liquid, 1-butyl-3-methylimidazolium chloride and also the recovery of valuable fission products from tissue paper waste was studied in room temperature ionic liquids.[33] The dissolution properties of uranium oxides, UO3, UO2, and U3O8 and their individual separation was studied using a task-specific ionic liquid, namely protonated betaine bis(trifluoromethanesulfonyl) imide, [Hbet][NTf2].[34]

[edit] Solar energy applications

Ionic liquids show great potential for use as a heat transfer and storage medium in solar thermal energy systems. Concentrating solar thermal facilities such as parabolic troughs and solar power towers utilize the energy of the sun by focusing it onto a receiver which can generate temperatures of around 600 °C. This heat can then be used to generate electricity in a steam or other cycle. For buffering during cloudy periods or to enable generation overnight, some of this energy can be stored by heating an intermediate fluid. Although nitrate salts have been the medium of choice since the early 1980s, they freeze at 220 °C and thus require heat tracing overnight to prevent solidification. Ionic liquids such as [C4mim][BF4] have been identified with more favorable liquid-phase temperature ranges (-75C to 459 °C) and could therefore be excellent liquid thermal storage media and heat transfer fluids in solar thermal power plants.[35]

[edit] Hydrogen storage

Ionic liquids have several properties that make them viable options for hydrogen storage systems. For instance, the vapor pressure of ionic liquids is very low and is negligible in most situations. These liquids are also stable at high temperatures. In addition, ionic liquids are able to act as solvents for a wide variety of compounds and gases, they also have weakly coordinating anions and cations which are able to stabilize polar transition states. Finally, the liquids are able to be reused with minimal loss of activity. In their research Karkamkar et al. used 1-butyl-3-methylimidazolium chloride (bmimCl) in the dehydrogenation of ammonia borane. Immediately upon heating the sample, hydrogen evolution took place with a final amount of hydrogen evolved as high as 5.4 wt% H2.[36]

[edit] Natural product extraction

Ionic liquids are proving superior to conventional solvents in the extraction of specific natural compounds from plant biomass for pharmaceutical, nutraceutical and cosmetic applications. For example, a series of protic ionic liquids have been evaluated as solvents for the isolation of the important antimalarial drug artemisinin from the plant Artemisia annua. Lapkin et al. conducting a benchmarking study taking in a consideration of operational parameters, in which the ionic liquid equalled or outperformed the alternatives.[37] Following subsequent process development studies, Bioniqs is currently commerciallizing a protic ionic liquid for artemisinin extraction.

[edit] Waste recycling

Ionic liquids can be developed for the recycling of synthetic goods, plastics and metals. They offer the specificity required to separate similar compounds from each other, such as in the separation of polymers from plastic waste streams. This has achieved this using lower temperature extraction processes than current approaches[1]and could be the answer to avoiding tonnes of plastics being incinerated or consigned to landfill each year.

[edit] Gas treatment

As profiled in the July 13, 2009 issue of C&E News, ION Engineering is commercializing technology using ionic liquids and amines for CO2 capture and natural gas sweetening.[38]

[edit] Batteries

Researchers have identified IL's that can replace water as the electrolyte in metal-air batteries. IL's have great appeal because evaporate at much lower rates than water, increasing battery life, drying out more slowly. Further, IL's have an electrochemical window of up to six volts [39](versus 1.23 for water) supporting more energy-dense metals. Energy densities from 900-1600 watt-hours per kilogram appear possible.[40]

Metal-air battery draw oxygen through a porous ambient "air" electrode (-cathode) and produce hydroxyl ions on contact with the electrolyte. These ions reach the anode and oxidize the metal, releasing electrons and producing current.

[edit] 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.[41] Review papers on this aspect have been published in 2007.[42][43] 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.[44]

Despite their low vapor pressure many ionic liquids have also found to be combustible and therefore require careful handling.[45] 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.

[edit] See also

[edit] References

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  26. ^ Zemships - Ionic liquid compression
  27. ^ Electrochemical behavior of uranium(VI) in 1-butyl-3-methylimidazolium chloride and thermal characterization of uranium oxide deposit, Electrochimica Acta, Volume 52, Issue 9, 15 February 2007, Pages 3006-3012, P. Giridhar, K.A. Venkatesan, T.G. Srinivasan and P.R. Vasudeva Rao
  28. ^ ELECTROCHEMICAL BEHAVIOR OF EUROPIUM (III) IN N-BUTYL-NMETHYLPYRROLIDINIUM BIS(TRIFLUOROMETHYLSULFONYL)IMIDE, "Electrochimica Acta", Accepted article, Ch Jagadeeswara Rao; K.A. Venkatesan; K Nagarajan; T G Srinivasan; P R Vasudeva Rao
  29. ^ Electrochemical behavior of fission palladium in 1-butyl-3-methylimidazolium chloride Electrochimica Acta, Volume 52, Issue 24, 1 August 2007, Pages 7121-7127, M. Jayakumar, K.A. Venkatesan and T.G. Srinivasan, http://dx.doi.org/10.1016/j.electacta.2007.05.049
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  31. ^ Electrochemical Behavior Of Ruthenium (III), Rhodium (III) And Palladium (II) In 1-Butyl-3-Methylimidazolium Chloride Ionic Liquid , M. Jayakumar, K.A. Venkatesan, T.G. Srinivasan, P.R. Vasudeva Rao, http://dx.doi.org/10.1016/j.electacta.2009.06.043
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  38. ^ C&E News
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  44. ^ Xuehui Li, Jinggan Zhao, Qianhe Li, Lefu Wang and Shik Chi Tsang (2007). "Ultrasonic chemical oxidative degradations of 1,3-dialkylimidazolium ionic liquids and their mechanistic elucidations". Dalton Trans.: 1875. doi:10.1039/b618384k. 
  45. ^ Marcin Smiglak, W. Mathew Reichert, John D. Holbrey, John S. Wilkes, Luyi Sun, Joseph S. Thrasher, Kostyantyn Kirichenko, Shailendra Singh, Alan R. Katritzky and Robin D. Rogers (2006). "Combustible ionic liquids by design: is laboratory safety another ionic liquid myth?". Chemical Communications 2006: 2554–2556. doi:10.1039/b602086k. 
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[edit] Further reading

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