Ionic liquid



  An ionic liquid is a liquid that contains essentially only Deep eutectic solvent, or DES, that have many similarities with ionic liquids.

 

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 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 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 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 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 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 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 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

References

  • F. Endres, S. Zein El Abedin (2006). "Air and water stable ionic liquids in physical chemistry". Phys. Chem. Chem. Phys. 8: 2101. doi:10.1039/b600519p.
  • S. Fujita, H. Kanamaru, H. Senboku and M. Arai (2006). "Preparation of Cyclic Urethanes from Amino Alcohols and Carbon Dioxide Using Ionic Liquid Catalysts with Alkali Metal Promoters" (open access). Int. J. Mol. Sci. 2006 (7): 438-450.
  1. ^ P. Walden, Bull. Acad. Sci. St. Petersburg 1914, 405-422
  2. ^ Electrochemical scrutiny of organometallic iron complexes and hexamethylbenzene in a room temperature molten salt H. L. Chum, V. R. Koch, L. L. Miller, R. A. Osteryoung Journal of the American Chemical Society 1975, 97, 3264.doi:10.1021/ja00844a081
  3. ^ Dialkylimidazolium chloroaluminate melts: a new class of room-temperature ionic liquids for electrochemistry, spectroscopy and synthesis J. S. Wilkes, J. A. Levisky, R. A. Wilson, C. L. Hussey Inorganic Chemistry 1982, 21, 1263-1264. doi:10.1021/ic00133a078
  4. ^ R. J. Gale, R. A. Osteryoung (1979). "Potentiometric investigation of dialuminum heptachloride formation in aluminum chloride-1-butylpyridinium chloride mixtures". Inorganic Chemistry 18: 1603. doi:10.1021/ic50196a044.
  5. ^ J. S. Wilkes, M. J. Zaworotko Chemical Communications 1992, 965-967
  6. ^ Adam J. Walker and Neil C. Bruce (2004). "Cofactor-dependent enzyme catalysis in functionalized ionic solvents". Chemical Communications 2004: 2570. doi:10.1039/b410467f.
  7. ^ Martyn J. Earle, José M.S.S. Esperança, Manuela A. Gilea, José N. Canongia Lopes, Luís P.N. Rebelo, Joseph W. Magee, Kenneth R. Seddon and Jason A. Widegren (2006 volume = 439). "The distillation and volatility of ionic liquids". Nature: 831. doi:10.1038/nature04451.
  8. ^ Applications of Functionalized Ionic Liquids, X. Li, D. Zhao, Z. Fei, L. Wang, Science in China: B, 2006, 35, 181
  9. ^ Zhao, D.; Fei, Z.; Geldbach, T. J.; Scopelliti, R.; Dyson, P. J. J. Am. Chem. Soc. 2004, 126, 15876 http://www.esi-topics.com/nhp/2006/july-06-ZhaofuFei.html
  10. ^ E. F. Borra, O. Seddiki, R. Angel, D. Eisenstein, P. Hickson, K. R. Seddon and S. P. Worden (2007). "Deposition of metal films on an ionic liquid as a basis for a lunar telescope". Nature 447 (7147): 979-981. doi:10.1038/nature05909.
  11. ^ Fort, D.A, Swatloski, R.P., Moyna, P., Rogers, R.D., Moyna, G. Chem. Commun. 2006, 714
  12. ^ (2006) "Acute toxicity of ionic liquids to the zebrafish (Danio rerio) C Pretti, C Chiappe, D Pieraccini, M Gregori, F Abramo, G Monni and L Intorre". Green Chem. 8: 238 - 240. doi:10.1039/b511554j.
  13. ^ D. Zhao, Y. Liao and Z. Zhang (2007). "Toxicity of Ionic Liquids". CLEAN - Soil, Air, Water 35 (1): 42-48. doi:10.1002/clen.200600015.
  14. ^ 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.. doi:10.1039/b618384k.
  15. ^ 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.
  16. ^ Richard P. Swatloski, Scott K. Spear, John D. Holbrey, and Robin D. Rogers (2002). "Dissolution of Cellose with Ionic Liquids". Journal of the American Chemical Society 124/18: 4974-4975. doi:10.1021/ja025790m.
  17. ^ Frank Hermanutz, Frank Gähr, Klemens Massonne, Eric Uerdingen, oral presentation at the 45th Chemiefasertagung, Dornbirn, Austria, September 20th – 22nd, 2006
 
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