The chemical and physical properties of ionic liquids depend on the combination of cations and anions, and the length of the alkyl chains and functional groups also have a significant effect on their properties. The preparation of functionalized ionic liquids is usually accomplished by introducing functional groups into the cations, including hydroxyl groups, amine groups, sulfonic acid groups, nitrile groups, ether groups, benzyl groups, carboxyl groups, ester groups, sulfhydryl groups, alkenyl groups, phosphate groups and the like.
Figure 1. One of the common functionalized ionic liquids
Reaction media: Special reactions can be carried out using the specific properties of the functionalized ionic liquid. The functionalized ionic liquid used as the reaction medium can be servd as an immobilization and coordination ligand in a homogeneously catalyzed process. Such a system can be used to avoid leaching of the catalyst from the ionic layer, which is especially important for expensive transition metals and expensive ligands. The ionic liquid system based on nitrile-functionalized pyridine is a highly efficient catalytic system. The nitrile-functionalized pyridines are highly active and can be reused multiple times. In addition, nitrile-functionalized ionic liquids can also stabilize the reaction intermediates in the glycosidation (C-O coupling) reaction.
Figure 2. Catalyst immobilization by CN functionalized ionic liquids
Porous material: Porous materials such as zeo-type backbones are typically prepared in a sealed autoclave in an aqueous solution at elevated temperature and pressure. Since the vapor pressure is low, the synthesis of the porous material can be carried out in an ionic liquid. In principle, ionic liquids can also be recycled for further use compared to other solvents, reflecting the green advantage of ionic liquids as solvents.
Figure 3. Solvothermal methods using ionic liquid to form porous materials
Surface science: Ionic liquids can interact with solid surfaces such as silica gel and molecular sieves, and the adjustability of the hydrophilicity and hydrophobicity of ionic liquids makes them useful as surface modifiers. The fact that ionic liquids are composed of cations and anions makes it simple to control surface wettability by anion exchange, which is not possible when using organic solvents.
Mechanical lubrication: Many commonly used lubricants have caused significant concern, especially for lubricants used under extreme conditions. The low vapor pressure of the ionic liquid allows, for example, alkylimidazolium tetrafluoroborate to be a promising general-purpose lubricant for the contact of steel/metal, steel/SiO2, Si3N4/SiO2, steel/ceramic systems. When used as a lubricant, ionic liquids have many advantages such as excellent friction reduction, wear resistance and high load carrying capacity. In order to improve the lubricating ability of ionic liquids, a phosphate group can be introduced into the imidazolium cation. The resulting functionalized ionic liquid typically has better friction reducing ability than conventional imidazolium hexafluorophosphate. This is particularly prominent at relatively high loads, suggesting that phosphoester functionalized ionic liquids may be promising candidates for new high temperature liquid lubricants.
Figure 4. Phosphorus ester ionic liquids for mechanical lubrication
Nanotechnology: The combination of ionic liquids and nanotechnology is also widely used. Palladium nanoparticles can also be separated in the Stille reaction process by using CN functionalized pyridine ionic liquid, and they differ from those isolated from non-functionalized pyridine-based ionic liquids. The CN group in the cation can be weakly coordinated with zero covalent palladium to prevent aggregation.
Clean technology: As an alternative to green processes, ionic liquids are now used not only to replace traditional solvents, but also as materials for other cleaning technologies, such as fuel desulfurization and flow gas desulfurization. Sulfur-containing fuels are the main cause of air pollution, so the amount of sulfur is strictly controlled around the world. Sulfhydryl functionalized ionic liquid can be used to adsorb SO2 from the atmosphere. The adsorption of SO2 by sulfhydryl functionalized ionic liquid can reach 0.305 g SO2/g ionic liquid. The adsorbed SO2 can be desorbed under vacuum at 40°C. The advantages of using functionalized ionic liquids to treat harmful substances in the atmosphere is that there are many types of ionic liquids that can be used, and the moderate interaction of ionic liquids between harmful substances enables controlled adsorption/desorption, thus opening up a new approach to control the atmospheric pollution.
Figure 5. The speculated desulfurization mechanism of guanidinum ionic liquids
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2. Zhao, D.; et al. (2004). “Nitrile-functionalized pyridinium ionic liquids: Synthesis, characterization, and their application in carbon-carbon coupling reactions.” J Am Chem Soc 126, 5876-15882.
3. Wu, W.; et al. (2004). “Desulfurization of flue gas: SO2 absorption by an ionic liquid.” Angew Chem Int Ed 43(18), 2415-2417.