Ionic liquids, also known as cryogenic molten salts, are liquid at or near room temperature. As ionic compounds, the main reason for the low melting point of ionic liquids is that the asymmetry of some substituents in their structures prevents the ions from accumulating into crystals regularly. Ionic liquids generally consist of organic cations and inorganic or organic anions. When the organic cation that constitutes an ionic liquid is a pyridine group, it is called a pyridinium-based ionic liquid. The general structure formula of the pyridinium-based ionic liquid is shown in Figure 1. R1, R2, R3, R4 and R5 in the structural formula can be the same or different, usually hydrogen, acyl, alkyl, benzyl, ester and others. The substituent group R is usually an alkyl chain. Moreover, the anion Y- can be trifluoroacetate, trichloroacetate, tetrafluoroborate, perchlorate, acetate and others. Furthermore, according to the number of pyridinyl sites replaced by other functional groups, it can be divided into monosubstituted pyridinium-based ionic liquid, disubstituted pyridinium-based ionic liquid and others.
Figure 1 The general structural formula of pyridinium-based ionic liquid.
- Direct synthesis method: The direct synthesis method can synthesize pyridinium-based ionic liquid through acid-base neutralization reaction or quaternary amination reaction in one step. This method has the advantages of simple and economical operation, no by-products and easy purification. Some halogenated pyridinium-based ionic liquid are obtained by direct synthesis method.
- Two-step synthesis method: For complex pyridinium-based ionic liquid which are difficult to obtain by direct synthesis, a two-step synthesis method is usually adopted. The first step is to prepare brine salt containing target cation through quaternary amination reaction. The second step is to get the target ionic liquid by replacing the halogen ion with the target anion or adding Lewis acid. Compared with the direct synthesis method, the two-step synthesis method can obtain the pyridinium-based ionic liquids with more diverse structures.
- Fuel oil desulfurization technology: Sulfide emissions in fuel tail gas can not only cause acid rain phenomenon, but also the main component of haze. Moreover, sulfide emissions can cause certain harm to our body. Therefore, fuel oil desulfurization technology has become one of the focuses of our researches. Traditional catalytic hydrodesulfurization requires reaction conditions of high temperature and pressure and has poor desulfurization effect. As a result, the research on non-catalytic hydrodesulfurization technology is on the rise. Extraction desulfurization is one of the non-catalytic hydrodesulfurization technologies, in which pyridinium-based ionic liquid plays an important role. The deep desulfurization of fuel tail gas can be realized by using pyridinium-based ionic liquid as extractant.
- Carbon dioxide absorption: Carbon dioxide is the main greenhouse gas in the atmosphere. The high concentration of carbon dioxide in the earth will cause the greenhouse effect and lead to global warming, which will lead to a series of environmental problems such as global climate anomalies. From the perspective of resource chemistry, carbon dioxide is a kind of abundant, safe and non-toxic carbon resource. Therefore, the efficient absorption and recycling of carbon dioxide has become a hot topic in recent years. Pyridinium-based ionic liquid has a good absorption effect on carbon dioxide and can be used in carbon dioxide enrichment technology.
- Others: In addition to the applications mentioned above, pyridinium-based ionic liquids have a variety of other applications, including hydrogenation, asymmetric catalysis, electrochemical studies, and others.