Ionic liquid media, biocatalysis

The aspects of medium engineering summarized so far were a hot topic in biocatalysis research during the 1980s and 1990s [5]. Nowadays, all of them constitute a well-established methodology that is successfully employed by chemists in synthetic applications, both in academia and industry. In turn, the main research interests of medium engineering have moved toward the use of ionic liquids as reaction media and the employment of additives. 

The use of ionic liquids (ILs) to replace organic or aqueous solvents in biocatalysis processes has recently gained much attention and great progress has been accomplished in this area lipase-catalyzed reactions in an IL solvent system have now been established and several examples of biotransformation in this novel reaction medium have also been reported. Recent developments in the application of ILs as solvents in enzymatic reactions are reviewed. 

The choice of solvents for enzymatic reaction has been widened from organic solvents to various types of solvents such as supercritical fluids, ionic liquids, etc. The enzymatic reaction in organic solvent has been reported already in 1970s, the first biocatalysis in ionic liquids [4] was in 2000, and the first biocatalysis in supercritical fluids [5] was in 1985. Currently four kinds of liquid or fluid solvents, aqueous, organic solvents, ionic liquids, and supercritical fluids, are available for biocatalysis as shown in Figure 3.2. Moreover, biphasic or triphasic solvent systems consisting of two or more kinds of the solvents are also often employed for biocatalysis. Solid phase of immobilized enzymes and/or hydrophobic polymer to adsorb substrate and product may also exist. The performance of a biocatalyst depends significantly on the solvent system. The best medium should provide optimum reaction rates and simplify work-up procedure to make the process both economical and environment friendly. 

The interest in ILs has been generated due to their unique properties and potential uses in areas as diverse as synthesis, biocatalysis, electrochemistry, etc. Thus, this class of molecules is increasingly employed in organic chemistry, material sciences and physical chemistry [3,4], ILs are salts - substances composed exclusively of cations and anions. This fact differentiates them from simple ionic solutions, in which ions are dissolved in a molecular medium. They are also different from inorganic molten salts because their melting points are lower than 100°C (most of them exist in liquid form at or near room temperature). 

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