Pyrrolidinium-Based Ionic Liquids

Ionic liquids are liquid compounds composed of cations and anions, and organic salts that are liquid at room temperature. Ionic liquids are called "green solvents" because of their low volatility, strong conductivity, high viscosity, low vapor pressure, stable properties, strong designability, good solubility for many substances, and no pollution. There are many types of ionic liquids, and different combinations of cations and anions can be changed to design different ionic liquids. Generally, cations are organic components and can be classified according to different cations. The cation of the pyrrolidine-based ionic liquid is usually N-alkyl-N-methylpyrrolidine (where the alkyl group includes ethyl, propyl, butyl, hexyl, octyl, etc.). The anion is usually tetrafluoroboric acid, hexafluorophosphoric acid, and bistrifluoromethanesulfonimide. The characteristics of ionic liquids can be adjusted by its composition, making it widely used.

Figure 1. Common composition of pyrrolidine-based ionic liquids

Applications:

• Separation and purification: Separation, purification and recovery of products has always been a difficult problem in synthetic chemistry. Water extraction and separation is only suitable for hydrophilic products, and distillation technology is not suitable for products with poor volatility. The use of organic solvents will cause cross-contamination. Ionic liquids have unique physical and chemical properties and are very suitable as solvents for separation and purification. Especially in liquid-liquid extraction and separation, ionic liquids can dissolve certain organic compounds, inorganic compounds and organometallic compounds, but are immiscible with a large number of organic solvents, and are very suitable as a new liquid-liquid extraction medium. For example, benzene derivatives such as toluene, aniline, benzoic acid, chlorobenzene, etc. have a high partition coefficient in the aqueous phase of pyrrolidine-based ionic liquids, and are suitable for use as separation solvents.
• Electrolyte: Due to the inherent ionic conductivity, non-volatility, non-combustibility and other advantages of ionic liquids, the electrochemical window is much larger than that of aqueous electrolyte solutions, which can reduce self-discharge. It can be used as a battery electrolyte without the high temperature like molten salt, and can be used to manufacture new high-performance batteries. N-alkyl-N-methylpyrrolidine can be used as the positive electrode of the electrolyte in the battery, and the negative ions are usually BF₄-, PF₆-, CF₃SO₃-.
• Ionic liquid liquid crystal: Ionic liquid liquid crystals combine the solvent properties of ionic liquids and the self-assembly behavior of liquid crystals on instruments, and have important theoretical significance and broad application prospects in the fields of nanomaterial synthesis, biotechnology, electrochemical processes, and stereoselective material synthesis. Tetrahydropyrrolidine ionic liquid has the characteristics of wide electrochemical window, high thermal stability, full saturation of positive ions, wide liquid crystal range, etc., and can form lamellar liquid crystal above the melting point.

Reference

• Gebrekidan Gebresilassie Eshetu, Sangsik Jeong,Pascal Pandard, Amandine Lecocq, Guy Marlair, Stefano Passerini. Comprehensive Insights into the Thermal Stability, Biodegradability, and Combustion Chemistry of Pyrrolidinium-Based Ionic Liquids[J]. ChemSusChem. 2019, 15, 10, 3146–3159.