Ionic liquids are salts that consist only of ions and they are liquid at room or low temperatures. When the cation that makes up the ionic liquid is pyridine group, it is called a pyridinium-based ionic liquid. In the structural formula, when two active sites on the pyridine group are replaced by other functions, it is called a disubstituted pyridinium-based ionic liquid. The general structural formula is shown in Figure 1. In general, R is an alkyl chain. Another more common site to be replaced by other functional groups is R2 or R3. Y- can be CF3SO3-, BF4-, PF6-, CF3COO-, SbF6- and others. Compared with traditional organic solvents and electrolytes, disubstituted pyridinium-based ionic liquids have a series of outstanding advantages. First, disubstituted pyridinium-based ionic liquids have almost no vapor pressure, which makes them nonvolatile, colorless and tasteless. Secondly, disubstituted pyridinium-based ionic liquids have wide stable temperature range, good chemical stability and wide window of electrochemical stable potential. Thirdly, through the design of cationic and anionic structures, the solubility degree of disubstituted pyridinium-based ionic liquids to inorganic matters and organic matters can be adjusted. Therefore, in recent years, disubstituted pyridinium-based ionic liquids, as environmentally friendly solvents, have been developed rapidly in the world.
Figure 1 The general structural formula of disubstituted pyridinium-based ionic liquid.
Applications:
- Solvents for chemical reactions: Disubstituted pyridinium-based ionic liquids have some remarkable advantages in chemical reactions, which are mainly used as solvents in reaction systems. First of all, disubstituted pyridinium-based ionic liquid can provide an environment different from that of traditional molecular solvents for chemical reactions. It can change the reaction mechanism to make the catalyst more active, more stable and improve the reaction yield. Secondly, the catalyst dissolved in disubstituted pyridinium-based ionic liquid can be recycled together with ionic liquid, making the catalyst easy to separate. Finally, because disubstituted pyridinium-based ionic liquids have no vapor pressure and wide temperature range in the liquid phase, the resulting products are easy to separate. Therefore, disubstituted pyridinium-based ionic liquids have a wide range of applications in cycloaddition, selective hydrogenation, oxidation and other reactions.
- Separation and purification: The traditional solvent purification technology has the disadvantages of narrow application range and serious environmental pollution. With the improvement of people's sense of environmental protection, the voice of green chemistry is getting higher and higher. However, disubstituted pyridinium-based ionic liquids have unique physical and chemical properties and are suitable for separation and purification process, especially for liquid-liquid purification. Moreover, disubstituted pyridinium-based ionic liquids are pollution-free and recyclable, so it has received more and more attention, and the research on them has become more and more in-depth.
- Electrochemistry: Disubstituted pyridinium-based ionic liquids are liquid electrolytes composed entirely of ions. Disubstituted pyridinium-based ionic liquids are widely used in batteries, capacitors, transistors and electrodeposition due to their electrochemical characteristics such as wide electrochemical point window and good ionic conductivity.
Classification:
Commonly used disubstituted pyridinium-based ionic liquids include N-butyl-3-methylpyridine chloride, N-butyl-3-methylpyridine bromide, N-butyl-3-methylpyridine tetrafluoroborate, N-butyl-3-methylpyridine hexafluorophosphate and others.
