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An ionic liquid refers to a liquid composed entirely of ions. All ionic liquids have some properties in common. The most important of these is the low vapor pressure exhibited by most ionic liquids at room temperature. This special feature has stimulated a great deal of researches on the green application of ionic liquids as a substitute for traditional organic solvents. Other desirable properties commonly found in ionic liquids include high thermal stability, low flammability, electrical conductivity, and the ability to solvate various organic and inorganic compounds. Ionic liquids can generally be easily separated from the reaction product. Most importantly, ionic liquids can be recycled. A number of applications of ionic liquids as environmentally friendly media have been described. Ionic liquids have opened up a field of research for the future of chemistry, electrochemistry, biology, physics, materials science and medicine.

Category

• • Ammonium ionic liquids: The ammonium ionic liquid is a compound, in which a hydrogen atom in the ammonium ion is substituted by an alkyl group, and it has a formula of R₄NX, and the four alkyl groups may be the same or different groups. X can represent a halogen anion or an acidic ion.
  • Functionalized ionic liquids: Functionalized ionic liquids are ionic liquids containing special functional groups, including hydroxyl, amine, sulfonate, nitrile, ether, benzyl, carboxyl, ester, decyl, alkenyl, phosphate group and so on.
  • Imidazolium ionic liquids: The imidazole ring is a versatile ionic liquid scaffold. The adjustable structure, thermal stability, relatively high ionic conductivity, wide electrochemical window and the amphoteric behavior of the imidazolium ionic liquid in solution have also attracted widespread attention.
  • Morpholinium ionic liquids: Morpholinium ionic liquids are a class of compounds having a morpholine ring. Typically, the morpholine in the compound contains two R groups on the N atom, and variations in the group can impart different functions and properties to the ionic liquid.
  • Phosphonium ionic liquids: The phosphonium ionic liquid is an ionic liquid having the general formula [PR₄]⁺ and there are four substituents on the phosphonium cation. The change in substituents and available anions represents a large number of possible salts.
  • Piperidinium ionic liquids: The piperidinium ionic liquid is a compound containing piperidine ions. Typical anions in piperidinium ionic liquids are chloride, bromide, acetate, diethyl phosphate, triflate, dicyandiamide, tetrafluoroborate, and so on.
  • Pyridinium ionic liquids: The pyridinium-based ionic liquid is an ionic liquid containing a pyridinium cation. Its stability, reactivity and catalysis in organic synthesis make it widely available in a variety of organic reactions.
  • Pyrrolidinium ionic liquids: The pyrrolidinium ionic liquid is an ionic liquid containing a pyrrolidinium cation. X^- represents different kinds of anions such as NO₃^- , HSO₄^- , CH₃COO^- and other acid ions.

Applications

• Chemical synthesis: Due to their unique properties, ionic liquids are widely used in various fields of chemical research. Ionic liquids have been used as solvents for many types of reactions. The advantage of using an ionic liquid instead of a common solvent in the reaction is that the reaction rate is several times faster than that of a conventional organic solvent, and the mixture of the ionic liquid and the catalyst used can be reused. Studies have shown that ionic liquids act as both solvents and catalysts during the reaction process.
• Separation and purification: Due to their unique physicochemical properties, ionic liquids are well suited as solvents for separation and purification. A large number of reports have been made, for example, extraction of butanol from fermentation broth by using ionic liquids.
• Electrochemical: Since the ionic liquid is electrically conductive, involatile, non-combustible, and its electrochemically stable potential window is much larger than other aqueous electrolyte solutions, the research of ionic liquid in electrochemical is undergoing. In molten salts, ionic liquids have been used as electrolytes in the manufacture of new high-performance batteries, solar cells, and capacitors.