In recent years, the application of ionic liquids (ILs) as (co)solvents and/or reagents in enzymatic catalysis for the production of biofuels is an emerging research area (Fig. 11.1).

ILs are composed entirely of ions and are liquids at ambient or far below ambient temperature, and have been extensively used as a potential alternative to toxic, hazardous, flammable and highly volatile organic solvents. ILs, particularly, have been shown to be exceptionally interesting non-aqueous reaction media for enzy­matic transformations [8, 9]. Typical ILs are based on organic cations, e. g. 1,3- dialkylimidazolium, tetraalkylammonium, etc., paired with a variety of anions that have a strongly delocalized negative charge (e. g. [BF4], [PF6], etc.), which results in colourless and easily handled materials with very interesting properties as solvents [10]. Their interest as green solvents resides in their negligible vapour pressure, excellent thermal stability, high ability to dissolve a wide range of organic and inorganic compounds, and their non-flammable nature, which avoids the problem of the emission of volatile organic solvents to the atmosphere. Moreover, their solvent properties, such as miscibility or immiscibility with water or some

organic solvents (e. g. hexane), can be tuned by selecting the appropriate cation and anion, which increases their usefulness for recovering products from the reaction mixture [8]. So, an important aspect for an IL is that it able to partially or completely solubilize the reactants, as well as low solubility for the reaction products. Solubility with reactants improves the reaction by allowing reactants to come into contact with each other, while the product can be separated by simple decantation as it is insoluble with ionic liquids, which can therefore be recycled.

The use of ILs as a reaction medium for enzymes, the application of such green compounds as cosolvents and/or reagents for biotransformation is well recognized. Studies on enzymatic reactions in ILs over the last 8-9 years have revealed not only that ILs are environmentally friendly alternatives to volatile organic solvents, but also that they have excellent selectivity, including substrate, regio — and enantios — electivity. Besides, many enzymes maintain very high thermal and operational stability in ILs. In this strategy, enzymes are simply suspended in the ILs, and then the resulting mixture can be used for biocatalytic reactions. It has been reported that lipase from Candida antarctica immobilized with IL is active at very high temperatures (95 °C) in hexane — and solvent-free conditions [11].

The most interesting biotransformations in ILs were observed at low water contents or in nearly anhydrous conditions because of the ability of hydrolases to carry out synthetic reactions. Furthermore, it is possible to design two-phase reaction systems that easily permit product recovery [12—14]. In anhydrous condi­tions, most of the ILs miscible with water clearly act as enzyme deactivating agents (e. g. [BMIM][Cl] or [BMIM][NO3]), with a few exceptions (e. g. [BMIM][BF4]) [8, 15, 16]. However, all the water-immiscible ILs assayed (e. g. [BMIM][PF6] or [BMIM][NTf2]) were shown to be suitable reaction media for biocatalytic reactions at low water content (<2 %, v/v) or in anhydrous conditions. In this context, lipases
are enzymes that have been most widely studied because of the high level of activity and stereoselectivity displayed in synthesizing many different compounds, e. g. aliphatic and aminoacid esters [17], chiral esters by kinetic resolution of sec-alcohols [12—14, 18, 19], flavonoid derivatives [20], polymers [15, 21], etc. Numerous other enzymatic reactions have also been reported in ILs [22]. Further­more, water-immiscible ILs also have an important stabilizing effect on hydrolases (lipases, esterases, proteases, etc.) in nearly anhydrous conditions [23—26].

We will focus on the use of enzymes in ILs for biofuels production.