Ionic liquids extraction solvents, limitations

Catalysis in ionic liquids is not limited to biphasic reaction systems. When the reaction mixture is homogeneous, an extraction solvent that is immiscible with the ionic liquid can be used to remove the product. A number of organic solvents display little or only limited miscibility with these liquids. However, this advantage is of limited value in practice, because one major incentive for using ionic liquids is to avoid volatile organic compounds. 

When the solubility of reactants in ionic liquids is limited, mixed organic solvents and ionic liquids can be used advantageously to achieve homogeneous systems in which the reactants and catalysts have the best solubility. The product separation can be facilitated by the removal of the organic solvent from the mixed solvent containing the ionic liquid (e.g., by extraction or distillation), because the solubility of the product in the ionic liquid is reduced upon the removal of the organic solvent. 

In the previous sections the use of catalysts dissolved in ionic liquids has been documented with a variety of examples from the most recent literature. They were classified are catalytic systems based on the adoption of Strategies A, B and C, when solvent-less conditions were not adopted. In an ideal liquid-liquid biphasic system, the IL must dissolve the catalytic intermediates and, in part, the substrate to avoid that mass transfer limits reaction rates. Moreover, products should have a limited solubility in the IL to allow a facile product removal or extraction, and, possibly, the recycle of the ionic liquid-trapped catalyst. The separation of the catalyst from the products is made easier if solid support-immobilised ILs are used. The preference for a solid catalyst is dictated not only by the easier separation but also, as outlined by Mehnert in an excellent review article, " by (i) the possible use of fixed bed reactors, and (ii) the use of a limited amount of IL, a generally expensive chemical which can limit the economic viability of the process. In this section attention will be focused only on the most recent examples of solid-phase assisted catalysis using ionic liquids, following Strategy D. Examples prior to 2006 are covered in recent reviews and will not be discussed here. " ... 

While the monophasic reaction in organic solvents is known to suffer from product inhibition, the continuous reaction in the liquid-liquid biphasic system allowed to overcome this limitation by in situ product extraction from the ionic catalyst phase. To avoid metal leaching out of the ionic liquid phase, the catalyst... 

Since ionic liquids have extremely low vapor pressures, one of the usual methods of product isolation, evaporation of the solvent, is not an option. The product isolation techniques available are distillation/sublimation of the product firom the ionic liquid, precipitation of the product and extraction of the product into another solvent. In the latter case it is often imperative to have the product soluble in an extracting solvent but the catalyst still well immobilized in the ionic liquid, so that the catalyst remains in the ionic liquid phase. This prevents the need for subsequent purification steps to remove the catalyst from the product. In many situations, e.g. in cases where solid reactants have only limited solubility in the ionic liquid, it is preferable to have the extracting solvent already present during the reaction. 

The products are partially or totally miscible in the IL. This may be the case for polar products such as aldehydes or alcohols. Distillation can be envisioned but is limited to volatile products. In addition, it uses energy and may decompose the catalytic species. Products can be separated during or after the reaction by adding a solvent that is poorly miscible with the ionic liquid. In olefin hydro-formylation, the unconverted olefin can be recycled to optimize product extraction. This scheme presents the advantage of avoiding contamination of products by an additional organic solvent. 

While the products can be separated by several methods, the best when the products are not miscible with or have limited solubility in the ionic liquids. If the products dissolve partially or totally in ionic liquids, they can be extracted with a non-polar, non-miscible solvent. Of course, when the products are volatile and the system is thermally stable, they can be distilled from the reaction mixture. In some special cases the ionic liquids are separated by crystallization. An alternative separation approach is the immobilization to solid supports. ... 

The development of an efficient extraction system based on the ionic liquids (ILs) depends on the employment of an appropriate combination of the extractant and an ionic liquid. In the IL-based extraction system with neutral extractants such as CMPO and TOPO dissolved in the imidazolium ILs, the extraction efficiency and selectivity for rare-earth metals are greatly improved compared to that in an organic solvent system, and the stripping, however, is unfavorably difficult. The use of acidic extractants such as PC-88A is limited in the poor solubility in ILs, though the extraction is controllable by the acid concentration in the aqueous phase. 

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