Imidazolium-Based Ionic Liquids
Ionic liquid refers to a salt composed of ions that is liquid at room temperature or low temperature. The number of cations and anions in the ionic liquid is equal, and it is macroscopically neutral. There are many types of ionic liquids, and the current researches are mainly imidazoles, pyridines, quaternary phosphorus and quaternary ammonium cations. Imidazolium-based ionic liquids are widely used as green solvents, not only because of some basic physical and chemical properties of the ionic liquid, but also because of its designability. Imidazole contains two meta-nitrogen atoms, and different groups can be introduced into the nitrogen atoms to prepare monosubstituted imidazolium-based ionic liquids, disubstituted imidazolium-based ionic liquids and trisubstituted imidazolium-based ionic liquids. Due to the properties and advantages of imidazolium-based ionic liquids, they are widely used in chemical reactions, electrochemistry, and separation processes.
Figure 1. Imidazole cations with different numbers of substituents
- Dissolution of cellulose: The application of ionic liquids in the field of cellulose dissolution and processing has unique advantages. Among them, imidazolium-based ionic liquids have a significant effect on the dissolution of cellulose. Different types of imidazolium-based ionic liquids have different solubility to cellulose. The solubility of disubstituted imidazolium-based ionic liquids is the best. Among them, N-methylimidazole ionic liquid can dissolve wood pulp cellulose, wheat straw fiber, and emperor bamboo grass cellulose. The N-vinylimidazole ionic liquid has stronger cellulose dissolving ability.
- Catalysis of chemical reactions: In the process of chemical reactions, ionic liquids can provide a brand-new environment, which can improve the activity and reaction speed of reaction catalysts, and increase the reaction conversion rate. In the separation, the ionic liquid has no measurable vapor pressure and a wide liquid phase temperature range. Simple operation methods such as distillation and extraction can be used to separate the products. Imidazolium-based ionic liquids can significantly catalyze the reaction of dimethyl carbonate, a green methylating reagent, reduce the pressure required for the reaction, and improve the selectivity and yield of the reaction.
- Phase change energy storage materials: As a new type of functional material, imidazolium-based ionic liquid has the advantages of low vapor pressure, stable chemical properties, wide liquid range, non-toxic, no pollution to the environment, and low corrosivity to metals such as steel. More importantly, it has the advantages of phase change. Other points with high enthalpy and high energy storage density can be used to prepare energy storage and heat transfer materials. During the melting phase change process, the material absorbs heat from the environment; during the solidification phase change process, the material releases heat to the environment. Imidazolium-based ionic liquids have the advantages of high phase change temperature, high latent heat of phase change, high stability, small phase change volume change, and good thermal conductivity, and have obvious advantages in phase change energy storage performance.
- Preparation of phase-change energy storage materials: Compared with traditional organic solvents, ionic liquids can simplify sample pretreatment, increase extraction rate, and reduce environmental pollution during the process of extracting natural products. Imidazolium-based ionic liquids can produce hydrogen bonding, electrostatic, dispersion and other intermolecular forces with the extract to achieve extraction and improve the efficiency of separation and purification. Imidazolium-based ionic liquids can produce hydrogen bonding, electrostatic, dispersion and other intermolecular forces with the extract to achieve extraction and improve the efficiency of separation and purification.
- Yaqin Zhang,Hongyan He,Kun Dong,Maohong Fan,Suojiang Zhang. A DFT study on lignin dissolution in imidazolium-based ionic liquids [J]. RSC Adv., 2017,7, 12670-12681.
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