Purification of Ionic Liquids

The lack of significant vapor pressure prevents the purification of ionic liquids by distillation. The counterpoint to this is that any volatile impurity can, in principle, be separated from an ionic liquid by distillation. In general, however, it is better to remove as many impurities as possible from the starting materials, and where possible to use synthetic methods that either generate as few side products as possible, or allow their easy separation from the final ionic liquid product. This section first describes the methods employed to purify starting materials, and then moves on to methods used to remove specific impurities from the different classes of ionic liquids. 

In practical terms, it is suggested that, in any application where the presence of halide ions may cause problems, the concentration of these be monitored to ensure the purity of the liquids. This may be achieved either by the use of an ion-sensitive electrode, or alternatively by use of a chemical method such as the Vollhard procedure for chloride ions [52]. Seddon et al. have reported that effectively identical results were obtained with either method [37]. 

If the liquids remain discolored even after these precautions, it is often possible to purify them further by first stirring with activated charcoal, followed by passing the liquid down a short column of neutral or acidic alumina as discussed in Section 2.1.3.2 [33]. 

It is hoped that this section will give the reader a better appreciation of the range of ionic liquids that have already been prepared, as well as a summary of the main techniques involved and the potential pitfalls. While the basic chemistry involved is relatively straightforward, the preparation of ionic liquids of known purity may be less easily achieved, and it is hoped that the ideas given here may be of assistance to the reader. It should also be noted that many of the more widely used ionic liquids are now commercially available from a range of suppliers, including some specializing in the synthesis of ionic liquids [53]. 

Advances in Molten Salt Chemistry 6, Eds. G. Mamantov, C.B. Mamantov, and J. Braunstein, Elsevier New York 1987, pp 211-346. 

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When ionic liquids are used as replacements for organic solvents in processes with nonvolatile products, downstream processing may become complicated. This may apply to many biotransformations in which the better selectivity of the biocatalyst is used to transform more complex molecules. In such cases, product isolation can be achieved by, for example, extraction with supercritical CO2 [50]. Recently, membrane processes such as pervaporation and nanofiltration have been used. The use of pervaporation for less volatile compounds such as phenylethanol has been reported by Crespo and co-workers [51]. We have developed a separation process based on nanofiltration [52, 53] which is especially well suited for isolation of nonvolatile compounds such as carbohydrates or charged compounds. It may also be used for easy recovery and/or purification of ionic liquids. 

Purification of ionic liquids following single or multiple use. Side- or decomposition products of chemical reactions or catalysts will accumulate in the solvent. Cheap and reliable methods have to be developed for purification. Very probably there will not be a universal method. More likely for each type of process a specific method will be set up. 

It is commonly accepted in the ionic liquids community that the purification of ionic liquids can be relatively complex. Currently, they cannot be distilled at reasonable rates, crystallized or sublimed. Thus, the only reasonable solution is to synthesize them from high quality starting materials. Apart from organic impurities... 

Another thermal separation unit often used for the laboratory scale purification of ionic liquids is recrystallization [125]. It is an attractive option for those ionic liquids that can form solids with a high degree of crystallinity. Crystals of ionic liquids are expected to be pure because each molecule or ion must fit perfectly into the lattice as it leaves the solution. Impurities preferentially remain in solution as they do not fit as well in the lattice. The level of purity of the crystal product finally depends on the extent to which the impurities are incorporated into the lattice or how much solvent is entrapped within the crystal formed. 

Optically Pure Ionic Liquids - Synthesis and Purification of Ionic Liquids. 286... 

Some detailed overviews that describe synthesis and purification of ionic liquids have appeared [5],... 

Gordon ChM, Muldoon MJ (2008) Synthesis and purification of ionic liquids. In Wasserscheid P, Welton (eds) Ionic liquids in synthesis T, 2nd edn. Wiley, Weinheim... 

Purification of ionic liquids formed by anion metathesis can throw up a different set of problems, as already noted in Section 2.1.3.2. In this case the most common impurities are halide anions, or unwanted cations inefficiently separated from the final product. The presence of such impurities can be extremely detrimental to the... 

For less volatile compounds such as phenylethanol the use of pervaporation has been reported by Crespo and coworkers [89-91]. A separation process based on nanofiltration [92] has been developed that is especially well suited for isolation of non-volatile compounds such as carbohydrates or charged compounds. It may also be used for an easy recovery and/or purification of ionic liquids. 

In industrial applications of ILs, the cost of the IL is a very important point. To be applied in an industrial process, ILs must meet a certain number of requirements. In general, ionic liquids are expensive in comparison to common organic solvents such as toluene, acetone, and ethanol, even if they are produced at an industrial scale. Ionic liquids are not readily accessible from cheap industrial process streams in a simple isolation or conversion step and also they are not easy to isolate and purify, due to their nonvolatile character and low melting points. The impurities of ionic liquids such as halide, water, and amine content will dramatically affect the price. Likewise, purification of ionic liquids will be a big challenge for any industrial applications (who uses ionic liquids) and manufacturer (who produce ionic liquids). 

Keywords Synthesis and purification of ionic liquids Properties of ionic liquids Rare-earth elements Solvent extraction based on ionic liquids TALSPEAK-like extraction for REEs... 

Baiker A, Andtnson JM, Jutz F (2010) Purification of ionic liquids by supercritical CO2 monitored by inlxared spectroscopy. J Supeiciit Fluids 55 395-400... 

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