Ultra-purification of ionic liquids by melt crystallization

Ionic liquids (ILs) have received a great deal of attention in the field of engineering during the last decade due to their unique properties. ILs are a very important new class of non-volatile solvents (T_m < 100 °C) in (bio)catalysis applicable to many ionic, polar and non-polar structure groups and as efficient electrolytes [Wasserscheid, P., and Welton, T., 2003, Ionic Liquids in Synthesis (Wiley–VCH, Weinheim, Germany)]. The applications range from electrochemistry, sensors, analysis, and separation techniques to catalysis and reaction engineering. Given their growing importance, it is vital to develop low cost production methods for ionic liquids including efficient techniques for purification and ultra-purification.This paper will present results of purification and ultra-purification of EMIM-chloride and EMIM-bromide (EMIM, 1-ethyl-3-methyl-imidazolium) by melt crystallization. Different techniques for purification are discussed including zone melting, layer crystallization and dry sweating in lab scale [König, A. and Wasserscheid, P., Ultra Purification of Ionic Liquids by Melt Crystallization, Proceedings of the 13th International Workshop on Industrial Crystallization BIWIC 2006, September 13–15, 2006, Delft, The Netherlands, pp. 79–84] and layer crystallization for static and dynamic crystallization conditions in pilot scale. In the case of EMIM-chloride, segregation coefficients are in the range of 0.05 < k_seg < 0.6 depending on crystallization rate, yield, feed impurity concentration and techniques used. The crystallization behavior of purified ionic liquids is discussed in detail relative to those of organic substances with similar melting points. Purification potential of EMIM-chloride is discussed with respect to different crystallization techniques and different scales used for crystallization.The excellent purification results of EMIM-chloride suggest melt crystallization techniques offer purification potential for other ionic liquids, creating a new innovative class of solvents and reactants. Melt crystallization can be used as a very efficient method to purify ionic liquids at different scales from 0.5 g up to 1000 kg with purity of w_IL>99.99%.