Ionic liquids are salts composed entirely of anions and cations that appear as liquid at or near room temperature. Generally, ionic liquids are functionalized by changing the type of cations, which gives them designability. There are a variety of cations for ionic liquids, among which ammonium is an important cation. The general structure of ammonium-based ionic liquid is shown in Figure 1. The cations usually can be 1-aminoethyl-3-methyl imidazole, 1-aminopropyl-3-methyl imidazole and others. Anions commonly can be chlorine, bromine, tetrafluoroboric acid, hexafluorophosphoric acid, nitric acid and others. Ammonium-based ionic liquids have become a research hotspot in recent years because of their advantages such as low vapor pressure, non-flammability, large heat capacity, good electrical conductivity and others. Moreover, compared with the traditional solvents, the ammonium-based ionic liquids have the advantage of non-volatile, and they can be completely recycled. Therefore, ammonium-based ionic liquids are also known as a new generation of green solvents.
Figure 1. The general structure of ammonium-based ionic liquids.
- Carbon dioxide adsorption field: Carbon dioxide adsorption by organic amines is the most commonly used conventional method of carbon dioxide removal. However, the traditional organic amines used in this method have a certain vapor pressure, which inevitably produces volatile organic compounds in the process of decarbonization. Therefore, it will lead to certain environmental problems and seriously affects the economy of the process. Ammonium-based ionic liquids are organic liquid substances composed entirely of ions. It is characterized by almost no vapor pressure and good thermal stability, so it can be used as an environmentally friendly reaction and separation medium. A series of studies have shown that carbon dioxide is much more soluble in ammonium-based ionic liquids than other gases. In addition, the ammonium-based ionic liquids can be prepared into a spongy form, which can achieve efficient absorption of carbon dioxide under mild conditions.
Figure 2. Novel sponge-like ammonium-based ionic liquids with rich exposed anion, excellent textural properties and carbon dioxide adsorption were prepared for efficient carbon dioxide cycloaddition.
- Electrochemical sensor field: Ammonium-based ionic liquids can be used as modifiers to modify electrodes, which has been widely used in the field of electrochemical sensors. For example, using 1-aminopropyl-3-methylimidazolium tetrafluoroborate to modify the surface of glassy carbon electrode can increase the current response of the electrode and improve the sensitivity of detection. The chemical sensor prepared by the modified electrode can detect phenolic pollutants, which has the advantages of low cost, fast and convenient, high sensitivity and accuracy, wide detection concentration range and others.
- Others: In addition to the above applications, it has been reported that lignin degradation can be achieved under mild conditions by using ammonium-based ionic liquids.
Common ammonium-based ionic liquids include 1-aminopropyl-3-methylimidazolium nitrate, 1-aminopropylimidazolium bis(trifluoromethylsulfonyl)imine, 1-aminopropylimidazolium hexafluorophosphate, 1-aminopropyl-3-methylimidazolium tetrafluoroborate, 1-aminoethyl-3- methylimidazolium bromide, 1-aminoethyl-3-methylimidazolium tetrafluoroborate and others.
- Xie Y, Sun Q, Fu Y, et al. Sponge-like quaternary ammonium-based poly(ionic liquid)s for high CO2 capture and efficient cycloaddition under mild conditions[J]. Journal of Materials Chemistry A, 2017:10.1039.C7TA09656A.