Application of Ionic Liquids in Biocatalysis

Abstract

Biotransformations have been of enormous economic and social importance throughout the history of mankind (Liese et al., 2000). Biocatalysis may be the most efficient way of producing fine chemicals. Today, several chemicals like pharmaceuticals, amino acids, saccharides and polysaccharides, esters and vitamins are produced by enzymatic biotransformations on industrial scale (Liese et al., 2000). The production of fine chemicals results in output of considerable volume of waste. Most of wastes are solvents such as water, volatile organic compounds (VOCs) etc. Solvents comprise 2/3 of all industrial emissions and 1/3 of all VOC emissions nationwide. These emissions have been linked to a host of negative effects (global climate change, pollution of air, human illness etc.) (Brennecke & Maginn, 2001). In recent years, green chemistry is become a growing area of research. Therefore the search for new environmental friendly and benign solvents and catalysts which can be easily recycled or reused is of significant interest. The ideal solvent should be chemically and physically stabile, recyclable, and reusable, should have a very low volatility, should allow selective and rapid transformations and should be easy to handle. For the biocatalysis, there are five main “green” solvent systems: supercritical fluids (SCFs), fluorinated solvents, ionic liquids (ILs), water, and solvent free reactions (Hobbs & Thomas, 2007). Enzymatic reactions could be performed under preferred conditions with minimized yield of the undesired by-products. Meanwhile, low yields, selectivity, and poor solubility of substrates in aqueous medium may require the enzymatic reactions to be carried out in non-aqueous medium (Sureshkumar & Lee, 2009). SCF is any substance at a temperature and pressure above its critical point. Close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a SCF to be “fine-tuned” (Jessop & Leitner, 1999). There are several advantages using the SCFs as solvents in chemical synthesis, where all are based on unique thermo-physical properties of SCFs for their mixtures with reactants. The main advantage of biocatalysis in SCFs is the tunability of the properties of the solvent by changes in the pressure and/or the temperature. The application of SCFs enables also design of integrated reaction and separation processes. In mass transfer limited processes the reaction rate can be increased if SCFs are applied due to higher diffusivity and to reduce viscosity of reaction system. SCFs display unique substrate specificity at relative mild reaction conditions.