Imidazolium-based ionic liquids are a common type of ionic liquids. According to the number of substituents on the imidazole ring, it can be classified into monosubstituted imidazolium-based ionic liquids, disubstituted imidazolium-based ionic liquids and trisubstituted imidazolium-based ionic liquids. Among them, disubstituted imidazolium-based ionic liquids are the most widely used. Substituents on the imidazole ring can be introduced into alkyl groups, ether groups, unsaturated groups, etc., to form a wealth of disubstituted imidazolium-based ionic liquids. The diverse molecular structure gives the disubstituted imidazolium-based ionic liquids rich properties. Its application fields involve organic synthesis, electrochemistry, separation and extraction, biocatalysis, etc.
Figure 1. The general chemical structure of disubstituted imidazole cations
- Organic synthesis: The disubstituted imidazolium-based ionic liquid is non-volatile, non-flammable, and highly polar. It can dissolve many reaction substrates and many catalysts in organic synthesis, so that the heterogeneous reaction can be transformed into a homogeneous reaction. In addition, an organic solvent that is immiscible with the ionic liquid can be used to extract the product, while the catalyst remains in the ionic liquid to realize the separation and purification of the product and the catalyst. The catalyst and ionic liquid can be recycled to realize the greening of organic synthesis. This ionic liquids can be used in organic synthesis reactions such as Diels-Alder, Heck, Friedel-Crafts, esterification and nucleophilic substitution.
- Electrochemistry: Imidazolium-based ionic liquids can be used in electrolysis, electroplating, batteries and other fields. Disubstituted imidazolium-based ionic liquids can change the viscosity of ionic liquids by adjusting the types of substituents. Ionic liquids with moderate viscosity can greatly increase the conductivity of the electrolyte and increase the ionic conductivity. Coupled with the advantages of high boiling point, low vapor pressure, and wide potential window of this type of ionic liquid, it can also be used in the electrolyte of solar cells.
- Non-aqueous biocatalysis: As a biocatalyst, enzyme has the advantages of specificity, high efficiency, and mild reaction conditions. In order to expand the scope of use of enzyme catalysis, non-aqueous system enzyme biocatalysis has been developed. Ionic liquids can replace organic solvents as the reaction system to achieve green development. Disubstituted imidazolium-based ionic liquids can be used as reaction solvents to synthesize flavors and fragrances, glycolipids, and can also promote the hydrolysis of sugars.
- Separation and extraction: Disubstituted imidazolium-based ionic liquids can be used to extract heavy metal ions, organic and biomolecules, organic sulfur components in fuels, and gases. This type of ionic liquid has the advantages of polarity, solubility, and adjustability of selective gas absorption.
- Ryohei Kakuchi,Ryo Ito,Shuhei Nomura,Hadi Abroshan,Kazuaki Ninomiya,Tomoyuki Ikai,Katsuhiro Maeda,Hyung J. Kim,Kenji Takahashi. A mechanistic insight into the organocatalytic properties of imidazolium-based ionic liquids and a positive co-solvent effect on cellulose modification reactions in an ionic liquid [J]. RSC Adv., 2017,7, 9423-9430.