Ammonium Ionic Liquids

The ammonium ionic liquid is a compound in which hydrogen atoms in the ammonium ion are substituted by alkyl groups, and its general formula is R₄NX, and the four alkyl groups may be the same or different groups. X may represent a halogen anion (F^-, Cl^-, Br^-, I^-), or an acid ion (RCOO^-, HSO₄^-).

Figure 1. The structure of ammonium ionic liquids

Applications

• Organic chemistry: Ammonium ionic liquids have proven to be valuable and useful in many different organic reactions, both as catalysts and as environmentally friendly solvents. For example, in the hydrolytic reaction of 1,1,1,3-tetrachloro-3-phenylpropane using ammonium ionic liquid, a volatile organic solvent and an additional catalyst are not required. In quaternary ammonium ionic liquids, the acrylic acid can be obtained with high conversion and yield. Good conversion and high yield of cinnamic acid can also be obtained by using ammonium ionic liquid [Et₃NH][HSO₄]. These ionic liquids provide a good alternative to the industrial synthesis of cinnamic acid.

Figure 2. Hydrolytic reaction in ionic liquids

• Polymer science: Ionic liquids are continued to be studied as important media for achieving various polymerizations. In polymer science, the ionic liquids can be used not only as solvents, but also as functional additives for polymer chains or hybrid materials. For example, poly(ionic liquid) made with ammonium ionic liquid can be used for CO₂ adsorption. Poly(p-vinylbenzyltrimethyl ammonium tetrafluoroborate) has a high CO₂ solubility, indicating that different cationic polymerized ionic liquids have different effects on CO₂ adsorption. Therefore, poly(ionic liquid) has been successfully used as a gas separation membrane for CO₂ separation by using this property.

• Electrochemistry: Ammonium ionic liquids with stable cations, such as tetraalkylammonium, have a broad electrochemical window (typically more than 5V). Lithium electrochemical intercalation of carbon materials in these ionic liquids has shown better prospects. The possibility of electrochemical formation of lithium graphite intercalation compounds in trimethyl-n-hexylammonium bis(trifluoromethanesulfone) imide (TMHATFSI) ionic liquids has also been reported.

References

1. Weng, J.; et al. (2006). “Novel quaternary ammonium ionic liquids and their use as dual solvent-catalysts in the hydrolytic reaction.” Green Chemistry 8(1), 96-99.
2. Blasig, A.; et al. (2007). “Carbon dioxide solubility in polymerized ionic liquids containing ammonium and imidazolium cations from magnetic suspension balance: P[VBTMA][BF₄] and P[VBMI][BF₄].” Ind Eng Chem Res 46, 5542–5547.
3. Katayama, Y.; et al. (2003). “Electrochemical intercalation of lithium into graphite in room-temperature molten salt containing ethylene carbonate.” Elelctrochem Solid-State Lett 5, 96–97.