Distillation of Ionic Liquids

Distillation of ionic liquid Volatile ionic liquid, non-volatile product, thermal stability of product Macrocyclic compounds and monoarylidene ketones from DIMCARB [72, 73] Ready recycling of ionic liquid High energy use, possible incomplete removal of ionic liquid... 

WO 2005/068404, to BASF AG, Distillation of Ionic Liquids, (Inventor Maase, M ... 

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. 

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. 

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. 

Yet, there are still many hurdles that must be overcome before these substances can be put to widespread use. Because ionic liquids boil at high temperatures, conventional methods of separating reaction products from solution cannot be used. Pharmaceutical manufacturing processes, for instance, often involve the process of distillation. The higher temperatures required to distill using ionic liquids (as opposed to organic liquids) would decompose many of the desired pharmaceutical compounds. 

For catalytic application where a transition metal catalyst is dissolved in the ionic liquid or the ionic liquid itself acts as the catalyst two additional aspects are of interest. Firstly, the special solubility properties of the ionic liquid enables a biphasic reaction mode in many cases. Exploitation of the miscibility gap between the ionic catalyst phase and the products allows, in this case, the catalyst to be isolated effectively from the product and reused many times. Secondly, the non-volatile nature of ionic liquids enables a more effective product isolation by distillation. Again, the possibility arises to reuse the isolated ionic catalyst phase. In both cases, the total reactivity of the applied catalysts is increased and catalyst consumption relative to the generated product is reduced. For example, all these advantages have been convincingly demonstrated for the transition metal catalysed hydroformylation [17]. 

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... 

Fig. 11.16 Recycling of ionic liquid (1 ChCl 2 ethylene glycol) used to electropolish stainless steel (a) used liquid containing Fe Cr and Ni salts, (b) as (a) with 1 equiv. v/v added water, (c) as (b), after gravity filtration and subsequent removal of residual water by distillation.

However, it can be assumed for most electrochemical applications of ionic liquids, especially for electroplating, that suitable regeneration procedures can be found. This is first, because transfer of several regeneration options that have been established for aqueous solutions should be possible, allowing regeneration and reuse of ionic liquid based electrolytes. Secondly, for purification of fiesh ionic liquids on the laboratory scale a number of methods, such as distillation, recrystallization, extraction, membrane filtration, batch adsorption and semi-continuous adsorption in a chromatography column, have already been tested. The recovery of ionic liquids from rinse or washing water, e.g. by nanofiltration, can also be an important issue. 

This study focuses firstly on the transfer of regeneration principles as they have been developed in the field of water-based electroplating and of purification options for ionic liquids as they are experienced in other fields of ionic liquid application. A number of purification procedures for fresh ionic liquids have already been tested on the laboratory scale with respect to their finishing in downstream processing. These include distillation, recrystallization, extraction, membrane filtration, batch adsorption and semi-continuous chromatography. But little is known yet about efficiency on the technical scale. Another important aspect discussed is the recovery of ionic liquids from rinse or washing water. 

Solvent free methods have also impacted on the preparation of other alternative reaction media. Namely, a range of ionic liquids (ILs) was prepared (including imidazolium, pyridinium and phosphonium salts) through halidetrapping anion metathesis reactions (Figure 2.17). The alkyl halide by-product was easily removed by vacuum or distillation and the products were obtained quantitatively in high purity. In addition to being solvent free, this route is more atom economic than the usual route to room temperature ionic liquids (RTILs) as it does not use silver(i), alkali metal or ammonium salts which are normally used in an anion metathesis reaction. 

In 1972, Parshall used an ionic liquid for the first time for the immobilization of a transition metal catalyst in a biphasic reaction set-up [7]. He described the hydrogenation of CC-double bonds with PtCl2 dissolved in tetraethylammonium chloride associated with tin dichloride ([Et4N][SnCl3], m.p. 78 °C) at temperatures between 60 and 100 °C. A substantial advantage of the molten salt medium [over conventional organic solvents]. .. is that the product may be separated by decantation or simple distillation . The use of ionic liquids as novel media for transition metal catalysis started to receive increasing attention when in 1992 Wilkes reported on the synthesis of... 

The property of ionic liquids which has led to them being classed as green solvents is that most of them have very low vapour pressure at ambient temperatures and hence do not emit volatile organic compounds (VOCs) [83-85], Their nonvolatility also allows the recovery of products by distillation. 

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