Ionic liquids are generally composed of specific organic cations with relatively large volume and asymmetric structure and inorganic anions with relatively small volume. Ionic liquids have a low melting point and they are liquid at or near room temperature. Compared with other solid or liquid materials, ionic liquids often exhibit special physical and chemical properties and special functions. Ionic liquids have good chemical and thermal stability and wide electrochemical stable potential window, which is a kind of new functional materials worthy of research and development. According to the type of cations, ionic liquids can be classified as quaternary ammonium salts-based ionic liquids, imidazole-based ionic liquids, pyridinium-based ionic liquids and others. Among them, pyridinium-based ionic liquids have been more and more widely studied and applied because of their excellent properties. In the structure of pyridinium-based ionic liquids, pyridinyl groups can be replaced at multiple sites. The pyridinium-based ionic liquids with only one active site replaced by other functional groups is called monosubstituted pyridinium-based ionic liquids. The general structural formula of the monosubstituted pyridinium-based ionic liquid is shown in Figure 1. R is usually an alkyl chain. Y- can be PF6-, Br-, BF4- and others.
Figure 1 The general structural formula of monosubstituted pyridinium-based ionic liquids.
Commonly used monosubstituted pyridinium-based ionic liquids include N-octyl pyridinium bromide, N-hexyl pyridinium bis(trifluoromethyl sulfonyl)imide, N-hexyl pyridinium hexafluorophosphate, N-hexyl pyridinium tetrafluoroborate and others. The chemical formulas of these monosubstituted pyridinium-based ionic liquids are shown in Figure 2.
Figure 2. The chemical formulas of the typical monosubstituted pyridinium-based ionic liquids.
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