Synthesis of Ionic Liquids

Posted by Tylor Keller on July 15th, 2019

The ionic liquid is in a liquid state at or near room temperature and is a salt composed entirely of anions and cations, also referred to as a low temperature molten salt. Most salts composed of anions and cations are generally ionic compounds with strong forces. If the force between the ions of the ionic compound is broken or weakened, the ionic compound can be liquid at room temperature or below, so that it is called an ionic liquid. Ionic liquids can be mainly divided into ammonium ionic liquids, imidazolium ionic liquids, morpholinium ionic liquids, phosphonium ionic liquids, piperidinium ionic liquids, pyridinium ionic liquids, pyrrolidinium ionic liquids and so on. Ionic liquids can be synthesized by several methods: one-step synthesis, two-step synthesis, and other methods including microwave-assisted synthesis and ultrasonic-assisted synthesis.

1 One-step synthesis

The one-step synthesis method refers to a method of synthesizing an ionic liquid in one step directly by an acid-base neutralization reaction or a quaternization reaction. The method is simple and convenient to operate has high atomic utilization. The product obtained by this method is easy to purify and has no by-products. For example, an ionic liquid of nitroethylamine is prepared by neutralization of ethylamine with nitric acid.

2 Two-step synthesis

The two-step synthesis involves the synthesis of a quaternary ammonium halide salt and the exchange of a halide anion. Taking an imidazolium salt ionic liquid as an example, a halogen salt containing a target cation ([cation] X-type ionic liquid) is first obtained by quaternization, and then the X-ion is replaced with a target anion Y- to obtain a desired ionic liquid.

3 Other synthetic methods

If the ionic liquid is synthesized by heating, an organic solvent is required, which will cause environmental pollution, and the reaction time is long and the yield is low, so this method is generally not used. Other synthetic methods include microwave enhancement, ultrasonic enhancement, and electrochemical method. Microwave-assisted and ultrasonic-assisted ionic liquid synthesis eliminates the need for organic solvents, reduces pollution during the synthesis process, speeds up the reaction and increases product yield and purity.

3.1 Microwave-assisted synthesis:

The synthesis of almost all common alkyl imidazoles and alkylpyridine ionic liquids can be carried out using a commercial microwave reaction system, and the reaction time can be greatly shortened, and the yield is not lower than the conventional synthesis method. However, the microwave assisted method is prone to side reactions while significantly shortening the reaction time. Moreover, microwave reactors are expensive, so this method can only be limited to small-scale synthesis in the laboratory, and is difficult to use for large-scale industrial production of ionic liquids.

3.2 Ultrasonic-assisted synthesis:

The results of research of ultrasound-assisted synthesis of ionic liquids show that this method can also significantly shorten the reaction time without reducing the yield. At present, the research on ultrasonic-assisted synthesis of ionic liquids is mainly the synthesis of common alkyl imidazoles and alkylpyridine ionic liquids, such as the synthesis of 1-butyl-3-methylimidazolium fluoroborate. Ultrasonic-assisted synthesis of ionic liquids has the advantages of simple synthesis conditions, short reaction time, low energy consumption and easy purification of products.