Functionalized ionic liquids are mainly obtained through three methods: cationic functionalization, anionic functionalization, and anion-cation dual functionalization. Among them, the functionalization of cations mainly includes amino functionalization, hydroxyl functionalization, ether functionalization, carboxyl functionalization, ester functionalization, etc.; and the most common functionalization of anions is the introduction of hydroxyl and carboxyl groups. Anion and cation dual functionalization is composed of a combination of cationic and anionic functional groups. Functionalized ionic liquids have the generality of ordinary ionic liquids, and through the "designability" of ionic liquids, functionalized ionic liquids synthesized for specific application purposes or actual needs have higher selectivity and higher practicality than traditional ionic liquids. Functionalized ionic liquids can qualitatively adjust a number of physical and chemical properties to achieve applications in catalysts, electrochemistry, organic synthesis, materials, separation, etc.
Figure 1. Part of the cation of functionalized ionic liquids
- Catalyst: The functionalized ionic liquid obtained by introducing a catalytically active group into the cation or anion of the ionic liquid is a new type of catalyst. In addition to having excellent catalytic performance, this type of catalyst also has special physical and chemical properties, and it is easy to separate the product from the catalyst. According to the functional classification of catalysts, functionalized ionic liquids can be divided into acid functionalized ionic liquids, alkali functionalized ionic liquids, carbonyl metal anion functionalized ionic liquids, chiral ligand functionalized ionic liquids and polyoxometalate functionalized Ionic liquid. The acid-functionalized ionic liquid can catalyze the esterification reaction. Alkali-functionalized ionic liquids can catalyze Henry reaction, Michael addition, Heck coupling reaction, carbonylation reaction, etc.
- Food industry: On the one hand, functionalized ionic liquids have excellent solubility properties, can be used as solvents and catalyst carriers, and can be used to synthesize food industry solvents, spices, and food packaging materials. On the other hand, the functionalized ionic liquid has the advantages of no foul smell, pollution-free, non-flammable, easy to separate from the product, volatile, can be used repeatedly, convenient to use, etc., so that it can effectively solve the serious environment caused by traditional solvents, health and safety issues.
- Electrochemical materials: Functionalized ionic liquids have good application prospects as green electrochemical materials, which involve electrolysis, electroplating, electrocatalysis, batteries, electrodeposition, electroanalytical chemistry and capacitance. In terms of electrolyte, ionic liquids can be functionally modified according to the requirements of different electrochemical systems to prepare electrolytes with high ionic conductivity, wide potential window, and thermal stability.
- Solid surface modifier: Adjusting the hydrophilic/hydrophobic part of the ionic liquid can adjust the wettability of the solid surface. The thiol-functionalized ionic liquid can selectively modify the gold surface, and endow the solid surface with adjustable wettability.
- Mechanical lubricant: Functionalized ionic liquids have excellent friction reduction, anti-fatigue and high loading effects, so that they can meet the special requirements of being used as lubricating oil in extreme environments. In order to improve the lubricating performance of the ionic liquid, the phosphonate group is introduced into the imidazole cation skeleton to obtain excellent lubricating oil. The principle is that the phosphine group on the functionalized ionic liquid undergoes partial hydrolysis under ambient conditions and reacts with the fresh surface of aluminum or iron to form a stable substance dissolved in the ionic liquid. This kind of functionalized ionic liquid lubricating oil has been widely researched and applied.
- Dayuan Zheng, Dan Hua, Yiping Hong, Abdul-Rauf Ibrahim, Ayan Yao, Junyang Pan, Guowu Zhan. Functions of Ionic Liquids in Preparing Membranes for Liquid Separations: A Review[J]. Membranes 2020, 10(12), 395.