Ionic halide impurity

All the halide exchange reactions mentioned above proceed more or less quantitatively, causing greater or lesser quantities of halide impurities in the final product. The choice of the best procedure to obtain complete exchange depends mainly on the nature of the ionic liquid that is being produced. Unfortunately, there is no general method to obtain a halide-free ionic liquid that can be used for all types of ionic liquid. This is explained in a little more detail for two defined examples the synthesis of [BMIM][(CF3S02)2N] and the synthesis of [EMIM][BF4]. 

Berthier, D., Varenne, A., Gareil, R, Digne, M., Lienemann, C.-P, Magnac, L., and Olivier-Bourbigouc, H., Capillary electrophoresis monitoring of halide impurities in ionic liquids. Analyst, 129,1257-1261, 2004. 

The above characterizations primarily concern the interactions between molecular solutes and ILs. However, ILs are also good solvents for ionic compounds, and have been studied extensively as media for transition metal catalysis [4, 38, 219] and for the extraction of heavy metals [23]. ILs are capable of solvating even simple salts, such as NaCl, to some degree [219], and in fact the removal of halide impurities resulting from synthesis can be a considerable challenge [68]. However, ionic complexes are generally far more soluble than simple salts [220], and we focus our attention on these systems as they have received greater study and are more relevant to the processes noted above. 

A further error in IL synthesis can originate from purification processes. In order to remove the often yellowish color of ionic liquids after synthesis they are commonly purified over silica or alumina powder (see above). Once we obtained a liquid where the supplier invested a lot of effort to deliver Endres-quality . [EMIMJTFSA was made with the best available educts in the add-base routine from diluted aqueous [EMIMJOH and H-TFSA. This approach exdudes metal and halide impurities. The supplier removed the slight yellowish color by purification over silica. For this purpose the supplier used quite a fresh silica, which had not been used in any purification process before. One has to bear in mind that the dominant impurities, even in hiqh quality silica, are aluminum species. Figure 11.26 shows the 1st, the 7th and the 15th cydes of this liquid on Au(l 11). Apparently... 

Metal ion and halide impurities are an issue in ionic liquids with discrete anions. As we have demonstrated in Chapter 11.5 Li+ (and K+) are common cationic impurities, especially in the bis(trifluoromethylsulfonyl)amides which typically contain 100 ppm of these ions from the metathesis reaction. Although Li and K are only electrodeposited in the bulk phase at electrode potentials close to the decomposition potential of the pyrrolidinium ions, there is evidence for the underpotential deposition of Li and K on gold and on other rather noble metals. For a technical process to deposit nickel or cobalt from ionic liquids the codeposition of Li and/or K, even in the underpotential deposition regime, has to be expected. 

Halide impurities can alter the complex chemistry in ionic liquids and can lead to unexpected oxidation reactions at the counter electrode. Furthermore even low amounts of e.g. chlorine can be formed, leading to some side reactions. 

Both the melting point and viscosity of an ionic liquid are highly dependent on their purity and values in the literature differ accordingly. In the 1-alkyl-3-methylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids, which are the ones most widely used in catalysis thus far, halide impurities are usually present to varying degrees, due to the metathesis route commonly used for their preparation (see Section 2.3). The effect of different chloride concentrations present on the viscosity of [C4Ciim][BF4] is shown in Figure 2.5.1161... 

One characteristic of Friedel-Crafts reactions in ionic liquids is their pronounced solvent dependence in that different anion-cation combinations can determine complete and fast conversion and total inactivity of a given catalyst. It is somewhat striking that the catalytic activity is often lowest in hydrophilic ionic liquids from which halide impurities are harder to remove. In any case, screening of a selection of cations and anions appears to be necessary in order to evaluate the suitability of any potential catalyst. 

Many investigators consider different variations of carbohalogenation as the most convenient way for removing oxygen-containing impurities from molten ionic halides [284-291]. 

Halide impurities are probably the most studied of the four general categories of impurities common to ionic liquids and, besides electrochemical analysis, two methods are currently being used to determine the level of residual halide impurities in ionic liquids [12]. The titration of the ionic hquid vnth AgN O3 is still widely used but suffers from a certain solubility of AgQ in the ionic hquid under investigation. This method can be enhanced by the Vollhard method for chlorine determination where the chloride is first precipitated with excess AgNO3 followed by back-titration of the mother liquor with aqueous potassium thiocyanate [13]. This method uses a visual endpoint through the formation of a complex between thiocyanate and an iron (III) nitrate indicator. 

Traces of halide impurities (Cr, F, or even other anionic impurities) are often present in ionic liquids, notably when they are obtained by an anion exchange reaction. Most of the time, this presence of halides greatly influences the comse of the catalytic reaction. For example, a detrimental effect of chlorides was observed for hydrogenation or Michael addition [44,45], while a beneficial effect was reported for the Heck reaction [46]. It is therefore necessary to measure carefully the amount of halide impurity in ILs, and this amount can be quantified by high-performance ionic chromatography [47]. This precaution is especially important for colored commercial ionic liquids, which are likely to contain a significant amount of halide impurity as well as of other anions. 

As the predominance of Schottky defect situations have primarily been demonstrated in detail for ionic halides, e.g. alkali halides (NaCl, KBr, Lil a.o.), let us consider the effects of impurities/dopants on Schottky equilibria in a compound MX where the cations and anions have a valence of 1 and the cation and anion vacancies are singly charged, i.e. and. If divalent foreign cations, Mf2+, are dissolved substitutionally in the lattice and occupy the normal M-sites, the Mf2+ ions will have one effective positive charge, Mfl. (It has donated an electron, and Mf is called a donor dopant). If we disregard other native defects than the Schottky defects, the electroneutrality condition becomes... 

Apart from halide and protic impurities, ionic liquids can also be contaminated with other ionic impurities from the metathesis reaction. This is especially likely if the alkali salt used in the metathesis reaction shows significant solubility in the... 

For all research carried out with commercial ionic liquids we recommend a serious quality check of the product prior to work. As already mentioned, a good commercial ionic liquid may be colored and may contain some traces of water. However, it should be free of organic volatiles, halides (if not an halide ionic liquid), and all ionic impurities. 

It must be noted that impurities in the ionic liquids can have a profound impact on the potential limits and the corresponding electrochemical window. During the synthesis of many of the non-haloaluminate ionic liquids, residual halide and water may remain in the final product [13]. Halide ions (Cl , Br , I ) are more easily oxidized than the fluorine-containing anions used in most non-haloaluminate ionic liquids. Consequently, the observed anodic potential limit can be appreciably reduced if significant concentrations of halide ions are present. Contamination of an ionic liquid with significant amounts of water can affect both the anodic and the cathodic potential limits, as water can be both reduced and oxidized in the potential limits of many ionic liquids. Recent work by Schroder et al. demonstrated considerable reduction in both the anodic and cathodic limits of several ionic liquids upon the addition of 3 % water (by weight) [14]. For example, the electrochemical window of dry [BMIM][BF4] was found to be 4.10 V, while that for the ionic liquid with 3 % water by weight was reduced to 1.95 V. In addition to its electrochemistry, water can react with the ionic liquid components (especially anions) to produce products... 

It is noteworthy that the best results could be obtained only with very pure ionic liquids and by use of an optimized reactor set-up. The contents of halide ions and water in the ionic liquid were found to be crucial parameters, since both impurities poisoned the cationic catalyst. Furthermore, the catalytic results proved to be highly dependent on all modifications influencing mass transfer of ethylene into the ionic catalyst layer. A 150 ml autoclave stirred from the top with a special stirrer... 

As well as viscosity, other factors to be aware of include the purity of the ionic liquids. The presence of residual halide ions in neutral ionic liquids can poison transition metal catalysts, while different levels of proton impurities in chloroalumi-... 

In some ionic crystals (primarily in halides of the alkali metals), there are vacancies in both the cationic and anionic positions (called Schottky defects—see Fig. 2.16). During transport, the ions (mostly of one sort) are shifted from a stable position to a neighbouring hole. The Schottky mechanism characterizes transport in important solid electrolytes such as Nernst mass (Zr02 doped with Y203 or with CaO). Thus, in the presence of 10 mol.% CaO, 5 per cent of the oxygen atoms in the lattice are replaced by vacancies. The presence of impurities also leads to the formation of Schottky defects. Most substances contain Frenkel and Schottky defects simultaneously, both influencing ion transport. 

---