Many enzymatic reactions in ionic liquids have been reported over the last decades. The performance of biocatalyst in ionic liquids reveals that ionic liquids are not only environmentally friendly alternatives to organic solvents but also good solvents for many enzymes and whole cell catalysts. The first example of biotransformation ionic liquids was reported by Lye et al. in 2000 [17]. It involved a whole cell biotransformation of 1,3-dicyanobenzene to 3-cyanobenzamide, with a Rhodococcus sp. in a biphasic [Bmim][PF6]/H2O medium. In this example, ionic liquids essentially act as a reservoir for the substrate and product, thereby decreasing the substrate and product inhibition observed in water. In principle, an organic solvent could be used for the same purpose but it was found that ionic liquids caused less damage to the microbial cell than organic solvents, for example, toluene [18].

The first use of isolated enzymes in ionic liquids was commonly recognized by the report of Erbeldinger et al. in 2000 [19] although Magnuson et al. earlier demonstrated the activity and stability of alkaline phosphatase in aqueous mixtures of [EtNH3][NO3] in 1984. However, the finding of Magnuson et al. did not attract much attention due to the lack of knowledge of ionic liquids at that time. In the work of Erbeldinger et al., thermolysin-catalyzed synthesis of Z-aspartame in [Bmim][PF6]/H2O (95/5, v/v) showed comparable reaction rate to those observed

in ethylacetate/H2O. In addition, the enzyme exhibited a higher stability in the ionic liquids/water medium although the small amount (3.2 mg • mL-1) of enzyme that dissolved in ionic liquids was catalytically inactive. At the same time, Sheldon et al. [20] showed that Candida antarctica lipase B(CALB), either free enzyme (SP525) or an immobilized form (Novozym 435) is able to catalyze a variety of biotransformation in [Bmim][BF4] or [Bmim][PF6]. Since then, a wide number of enzymes have been examined in ionic liquids such as lipase, protease, oxidoreduc — tase, peroxidase, cellulase, whole cells, and so forth (Table 10.1). Among them, lipase is the major enzyme reported so far in ionic liquids since it is considered as “work horses” of biocatalyst in various potential applications from fine chemical, chiral compounds, biopharmaceuticals to bulk chemicals such as surfactants, and biodiesel.

The uses of ionic liquids in biocatalysis can be classified into anhydrous ionic liquid system, aqueous/ionic liquids system using ionic liquids as co-solvent or additives, aqueous/ionic liquid two phase system, organic solvent/ionic liquids system, and supercritical CO2/ionic liquid systems. A large number of enzymes (e. g. lipases, proteases, peroxidases, dehydrogenases, glycosidases) and reactions (e. g. esterification, kinetic resolution, reductions, oxidations hydrolysis, etc.) have been tested in monophasic system based on ionic liquid [14, 15, 33]. While most water-miscible ionic liquids have been shown to act as enzyme — deactivating agents at low water content, water-immiscible ionic liquids (e. g. [Bmim][Tf2N], [Bmim] [PF6], etc.) act as suitable reaction media for enzymatic catalysis in the same conditions. The hygroscopic character of water-immiscible ionic liquids could be regarded as an additional advantage of these solvents, because enzymes require a certain degree of hydration to become active [34]. In addition, biphasic systems based on ionic liquid-water or ionic liquid-organic solvent have been assayed for biocatalytic processes [35, 36]. For example, the lipase catalyzed kinetic resolution of rac-ibuprofen in a water — ionic liquids biphasic medium [37], lipase catalyzed production of isoamyl acetate in an ionic liquid-alcohol biphasic system [38], and lipase catalyzed kinetic resolution of rac-1-phenylethanol in both ionic liquid/ hexane [39]. In these biphasic reaction systems, the products can be easily recov­ered by liquid-liquid extraction. However, in the case of hydrophobic compounds, liquid-liquid extraction with organic solvents might breakdown the greenness of the process. However, product recovery from ionic liquid-enzyme reaction media by another non-aqueous green solvent, such as supercritical CO2, is nowadays considered the most interesting strategy for developing integral clean chemical processes [34].

Many works and excellent reviews have been published during the last decade regarding the use of enzymes in ionic liquids [14—16, 40—47]. The results showed that enzymes in ionic liquids have many advantages compared to those in organic solvents such as high activity, high selectivity including substrate, region- and enantioselectivity, better enzyme stability as well as better recoverability and recyclability. In addition, ionic liquids can be used for carrying out biotransforma­tion with polar or hydrophilic substrates (e. g. amino acid, carbohydrates) which are not soluble or sparingly soluble in most organic solvents.

Advantages of using enzymes in ionic liquids are summarized as follows:

— Ionic liquids can be designed for particular bioprocesses

— Enzymes show excellent operational and thermal stability in ionic liquids. Therefore, bioprocesses can be conducted at high temperature due to high thermal stability of enzymes in ionic liquids

— Enzymes can be easily recovered and recycled simply by filtration or centrifugation

— Products and substrates can be recovered by evaporation, extraction with organic solvents or with supercritical CO2