Industrial Production of Ionic Liquids

The nature of the cations of ionic liquids (ILs) is quite diverse, with the possibility of having a positive charge on a nitrogen, phosphorus, or sulfur atom. Numerous anions can be associated with these cations. A good idea of the extent of the field is given by the fact that today several hundreds of ILs are commercially available. Some new ILs have been presented in the preceding sections. 

In spite of this diversity, most of ILs are synthesized using similar procedures. The nature of the reagents, as well as the conditions, are changed from one product to the other but the principle remains the same. As a consequence, this contribution will be limited to the synthesis of imidazolium ILs, because they are the most widely used ILs. Phosphonium-based ILs are reviewed in Section 5.3.3.2. 

Purity of the salts is essential for many solvent applicahons and for the characterization of their physical and chemical properties [9]. It is important to note that ILs are completely nonvolatile and thus cannot be purified by dishUation. Chromatographic purification is also of very limited utility due to the tendency of the salts to absorb on a common stationary phase. Consequently the synthesis method chosen must necessarily be highly selective. 

Common contaminants from these reactions are unreacted starting material halide anions, alkaline cations, organic bases, and possibly solvent of reachon. Moreover, without a special drying procedure and handling in completely inert surroundings, water is omnipresent in ILs. Indeed, even hydrophobic ILs are to some degree hygroscopic. Some methods have consequently been developed to detect and quantify these impurities. 

Due to their coordinating properties, halide impurities can have disastrous effects on metal-catalyzed reactions, so special attention has to be given to the control of halide in ILs when they are obtained in a two-step synthesis. Halide impurities can be effectively removed from hydrophobic ILs by simply washing them several times with water. The preparahon of water miscible ILs is a more demanding process the IL can be dissolved in dichloromethane and washed with successive small portions of deionized water. However, this method results in a lowering of the yield and cannot be applied for the most hydrophilic ILs. In the latter case, the direct alkylation of N-alkyl imidazole should be preferred. 

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