Distillable ionic liquids

Zhang J, Bhatt AI, Bond AM, Wedd AG, Scott JL, Strauss CR (2005) Voltammetric studies of polyoxometalate microparticles in contact with the reactive distillable ionic liquid DIMCARB. Electrochem Commun 7(12) 1283-1290. doi 10.1016/j.elecom.2005.09.008... 

The next major subset of ILs we will discuss is the distillable ionic liquids or dlLs. These IL systems have a significant vapour pressure associated with them and at relatively low temperatures can be dissociated into a gaseous or liquid state and can be reformed at lower temperatures [13,14], hence the distillable nature of them. Clearly this subset of ILs has unique properties that are a cross between conventional molecular solvent systems and also posses the ionic character associated with ILs. Previous work has shown that again, as for the IL and PIL systems, dlLs can have a range of melting points. Thus, as before the temperature distinction shown in Fig. 7.1 can be applied to this special case. 

Table 7.10 Formal potentials of Cc ICc and Fc IFc° in distillable ionic liquids against an AglAg quasi-referoice electrode...

Distillable ionic liquid Ei ofCcICc of DmFclDmFc Referaices... 

If a drybox is not available, the preparation can also be carried out by use of a dry, unreactive solvent (typically an alkane) as a blanket against hydrolysis. This has been suggested in the patent literature as a method for the large-scale industrial preparation of Eewis acid-based ionic liquids, as the solvent also acts as a heat-sink for the exothermic complexation reaction [28]. At the end of the reaction, the ionic liquid forms an immiscible layer beneath the protecting solvent. The ionic liquid may then either be removed by syringe, or else the solvent may be removed by distillation before use. In the former case it is likely that the ionic liquid will be contaminated with traces of the organic solvent, however. 

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. 

From Section 2.1 it has become very clear that the synthesis of an ionic liquid is in general quite simple organic chemistry, while the preparation of an ionic liquid of a certain quality requires some know-how and experience. Since neither distillation nor crystallization can be used to purify ionic liquids after their synthesis (due to their nonvolatility and low melting points), maximum care has to be taken before and during the ionic liquid synthesis to obtain the desired quality. 

With ionic liquids now commercially available, it should not be forgotten that an ionic liquid is still a quite different product from traditional organic solvents, simply because it cannot be purified by distillation, due to its nonvolatile character. This, combined with the fact that small amounts of impurities can influence the... 

Section 2.1 excellently describes methods used to produce colorless ionic liquids. From this it has become obvious that freshly distilled starting materials and low-temperature processing during the synthesis and drying steps are key aspects for avoidance of coloration of the ionic liquid. 

Ionic liquids have been described as designer solvents [11]. Properties such as solubility, density, refractive index, and viscosity can be adjusted to suit requirements simply by making changes to the structure of either the anion, or the cation, or both [12, 13]. This degree of control can be of substantial benefit when carrying out solvent extractions or product separations, as the relative solubilities of the ionic and extraction phases can be adjusted to assist with the separation [14]. Also, separation of the products can be achieved by other means such as, distillation (usually under vacuum), steam distillation, and supercritical fluid extraction (CO2). 

A quantitative study of the nucleophilic displacement reaction of benzoyl chloride with cyanide ion in [BMIM][PFg] was investigated by Eckert and co-workers [52]. The separation of the product, 1-phenylacetonitrile, from the ionic liquid was achieved by distillation or by extraction with supercritical CO2. The 1-phenylacetonitrile was then treated with KOH in [BMIM][PF6] to generate an anion, which reacted with 1,4-dibromobutane to give 1-cyano-l-phenylcyclopentane (Scheme 5.1-23). This was in turn extracted from the ionic liquid with supercritical CO2. These... 

Other methods of nitration that Laali investigated were with isoamyl nitrate in combination with a Bronsted or Lewis acid in several ionic liquids, with [EMIM][OTf] giving the best yields (69 %, 1.0 1.0 o p ratio). In the ionic liquid [HNEt( Pr)2] [CE3CO2] (m.p. = 92-93 °C), toluene was nitrated with a mixture of [NH4][N03] and trifluoroacetic acid (TEAH) (Scheme 5.1-37). This gave ammonium trifluoroacetate [NH4][TEA] as a by-product, which could be removed from the reaction vessel by distillation (sublimation). 

Obviously, the use of a nonvolatile ionic liquid simplifies the distillative workup of volatile products, especially in comparison with the use of low-boiling solvents, where it may save the distillation of the solvent during product isolation. Moreover, common problems related to the formation of azeotropic mixtures of the volatile solvents and the product/by-products formed are avoided by use of a nonvolatile ionic liquid. In the Rh-catalyzed hydroformylation of 3-pentenoic acid methyl ester it was even found that the addition of ionic liquid was able to stabilize the homogeneous catalyst during the thermal stress of product distillation (Figure 5.2-1) [21]. This option may be especially attractive technically, due to the fact that the stabilizing effects could already be observed even with quite small amounts of added ionic liquid. 

The authors describe a stabilizing effect of the ionic liquid on the palladium catalyst. In almost all reactions no precipitation of elemental palladium was observed, even at complete conversion of the aromatic halide. The reaction products were isolated by distillation from the nonvolatile ionic liquid. 

A co-solvent that is poorly miscible with ionic liquids but highly miscible with the products can be added in the separation step (after the reaction) to facilitate the product separation. The Pd-mediated FFeck coupling of aryl halides or benzoic anhydride with alkenes, for example, can be performed in [BMIM][PFg], the products being extracted with cyclohexane. In this case, water can also be used as an extraction solvent, to remove the salt by-products formed in the reaction [18]. From a practical point of view, the addition of a co-solvent can result in cross-contamination, and it has to be separated from the products in a supplementary step (distillation). More interestingly, unreacted organic reactants themselves (if they have nonpolar character) can be recycled to the separation step and can be used as the extractant co-solvent. 

Jacobsen subsequently reported a practical and efficient method for promoting the highly enantioselective addition of TMSN3 to meso-epoxides (Scheme 7.3) [4]. The chiral (salen)Cl-Cl catalyst 2 is available commercially and is bench-stable. Other practical advantages of the system include the mild reaction conditions, tolerance of some Lewis basic functional groups, catalyst recyclability (up to 10 times at 1 mol% with no loss in activity or enantioselectivity), and amenability to use under solvent-free conditions. Song later demonstrated that the reaction could be performed in room temperature ionic liquids, such as l-butyl-3-methylimidazo-lium salts. Extraction of the product mixture with hexane allowed catalyst recycling and product isolation without recourse to distillation (Scheme 7.4) [5]. 

From reviewing much of the literature it is easy to conclude that ionic liquids are excellent solvents for catalysts and reagents but not for products, which is obviously not the case. Whilst some products can be decanted from the liquid and others can be recovered by distillation, there are many useful reactions in which removal of the product (or residual reactants) from the ionic liquid is challenging. Extraction with an organic solvent, or even water, would reduce the overall eco-efficiency. Initial... 

Upon opening the autoelave the eause of the deactivation was clear. The volume in the reaetor had deereased until it now oeeupied too little volume to reaeh the stirrer in the reaetor. After evaporating a portion of the effluent and examining the residue by NMR, we found that the residue eontained a ca. 3 2 mixture of MePy pyH indieating that we had lost a portion of ionic liquid to dealkylation. No further effort to analyze this effluent was attempted and at this stage it was unelear whether the volume loss was to aspiration or distillation. 

A wide variety of new approaches to the problem of product separation in homogeneous catalysis has been discussed in the preceding chapters. Few of the new approaches has so far been commercialised, with the exceptions of a the use of aqueous biphasic systems for propene hydroformylation (Chapter 5) and the use of a phosphonium based ionic liquid for the Lewis acid catalysed isomerisation of butadiene monoxide to dihydrofuran (see Equation 9.1). This process has been operated by Eastman for the last 8 years without any loss or replenishment of ionic liquid [1], It has the advantage that the product is sufficiently volatile to be distilled from the reactor at the reaction temperature so the process can be run continuously with built in product catalyst separation. Production of lower volatility products by such a process would be more problematic. A side reaction leads to the conversion of butadiene oxide to high molecular weight oligomers. The ionic liquid has been designed to facilitate their separation from the catalyst (see Section 9.7)... 

The first example of biphasic catalysis was actually described for an ionic liquid system. In 1972, one year before Manassen proposed aqueous-organic biphasic catalysis [1], Par shall reported that the hydrogenation and alkoxycarbonylation of alkenes could be catalysed by PtCh when dissolved in tetraalkylammonium chloride/tin dichloride at temperatures of less than 100 °C [2], It was even noted that the product could be separated by decantation or distillation. Since this nascent study, synthetic chemistry in ionic liquids has developed at an incredible rate. In this chapter, we explore the different types of ionic liquids available and assess the factors that give rise to their low melting points. This is followed by an evaluation of synthetic methods used to prepare ionic liquids and the problems associated with these methods. The physical properties of ionic liquids are then described and a summary of the properties of ionic liquids that are attractive to clean synthesis is then given. The techniques that have been developed to improve catalyst solubility in ionic liquids to prevent leaching into the organic phase are also covered. 

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