Ionic liquids supercritical fluids biphasic

EXAMPLES OF REACTIONS IN IONIC LIQUIDS - SUPERCRITICAL FLUIDS BIPHASIC SYSTEMS... 

When biocatalysis is conducted in ionic liquid/supercritical fluids biphasic systems, the major function of ILs is to protect the enzyme, whereas the supercritical fluid dissolves the substrate and extracts the product Both enzyme stability and... 

In this chapter, latest advancements in solvent engineering in bioreductions and greener needs for bioreaction media have been discussed in depth with recent examples. Solvents for bioreductions may be categorized as (i) aqueous (ii) water/water-miscible (monophasic aqueous-organic system) (iii) water/ water-immiscible (biphasic aqueous-organic system) (iv) nonaqueous (mono-phasic organic system, including solvent-free system) and (v) nonconventional media (e.g., ionic liquids, supercritical fluids, gas-phase media, and reverse micelles). 

These alternative processes can be divided into two main categories, those that involve insoluble (Chapter 3) or soluble (Chapter 4) supports coupled with continuous flow operation or filtration on the macro - nano scale, and those in which the catalyst is immobilised in a separate phase from the product. These chapters are introduced by a discussion of aqueous biphasic systems (Chapter 5), which have already been commercialised. Other chapters then discuss newer approaches involving fluorous solvents (Chapter 6), ionic liquids (Chapter 7) and supercritical fluids (Chapter 8). 

Supercritical fluids (e.g. supercritical carbon dioxide, scCCb) are regarded as benign alternatives to organic solvents and there are many examples of their use in chemical synthesis, but usually under homogeneous conditions without the need for other solvents. However, SCCO2 has been combined with ionic liquids for the hydroformylation of 1-octene [16]. Since ionic liquids have no vapour pressure and are essentially insoluble in SCCO2, the product can be extracted from the reaction using CO2 virtually uncontaminated by the rhodium catalyst. This process is not a true biphasic process, as the reaction is carried out in the ionic liquid and the supercritical phase is only added once reaction is complete. 

Webb, P.B.and Sellin, M.F. and Kimene, T.E. and Williamson, S. and Slawin, A.M.Z. and Cole-Hamilton, D.J. (2003). Continuous Flow Hydroformylation of Alkenes in Supercritical Fluid-Ionic Liquid Biphasic System. J. Am. Chem. Soc., 125, 15577-15588. 

Sellin MF, Webb PB, Cole-Hamilton DJ (2001) Continuous flow homogeneous catalysis hydroformulation of alkenes in supercritical fluid-ionic liquid biphasic mixtures. Chem Commun 8 781-782... 

Recent research in the application of supercritical (sc) fluids and ionic liquids (IL) as solvents in homogeneous catalysis (see Sections 7.3 and 7.4), opened the way to the development of biphasic water/scCOz [171, 172] and water/IL [173] systems for the hydrogenation of various substrates, e.g., alkenes, aldehydes, etc. with water-soluble catalysts. The catalytically highly active, versatile and robust transition metal - N-heterocyclic carbene complexes [174] have also been applied for hydrogenation reactions [175], Given that water-soluble complexes with N-heterocy-clic carbene ligands are known [176], catalytic applications in aqueous systems are also foreseen. 

This chapter explores the application of biocatalytic polymerization in exotic solvents. These solvents are often termed unconventional , in that they would not generally be considered as a polymerization media. However, their use over the previous decade has dramatically increased due to the international push for cleaner, greener reaction pathways in an effort to reduce volatile organic compounds (VOCs). The first solvents to be discussed (and by far the most fully investigated in the literature) are supercritical fluids. Within this field, supercritical C02 has been the most highly reported solvent. The second solvent class is ionic liquids. These have become increasingly popular over the last five years. Biphasic solvents will then be described and their application to biocatalytic polymerization. This section will be limited to biphasic solvents that are more unusual and, apart from a brief mention, will not encompass the broad field of emulsion polymerization in water. Finally, the use of fluorous solvents will be described. In all cases, the physical properties of the solvent imparts interesting,... 

Lipases are able to work in very different media. They work in biphasic systems and in monophasic (in the presence of hydrophilic or hydrophobic solvents) systems where the water content can vary significantly between aqueous and anhydrous media. They have been tested also in ionic liquid media (Lau et al. 2000 Wasserscheid and Keim 2000 Kamal and Chouhan 2004 Ha et al. 2007), in supercritical fluids (Laudani et al. 2007) and in gaseous media (Cameron et al. 2002). The different media for enzymatic catalysis has been outlined before (see section 1.6) and it will not be further discussed here. However, some examples of modulation of activity and selectivity of lipases by medium engineering will be described in this section. 

Looking back, it must be stated that Manassen and Beck/Joo s ideas were developed independently of each other. Remarkably, the fundamental papers of Jo6 and Kuntz created little interest and only found a wider echo in academic research once Shell and Rtihrchemie had managed to achieve industrial scale-up of their biphase catalyses in organic/organic or in aqueous systems. In a drastic departure from the normal pattern, here basic research lagged considerably behind industrial research and application [25]. This has changed with the introduction of other liquid phases such as ionic liquids (as defined today), supercritical liquids, polymeric fluids, and fluorous liquids. 

Recent research concerning the application of supercritical (sc) fluids and ionic liquids (IL) as solvents in homogeneous catalysis, [5, 6] opened the way to the development of biphasic water/scC02 [7] and water/IL [8] systems for the hydrogenation of various substrates with water-soluble catalysts (see also Chapters 5 and 6). 

Ionic liquids, fluorous biphasic systems, and supercritical fluids have all been studied as alternatives to conventional organic solvents. However, because of their nature, some of these novel systems require additional hardware for utilization. For example, some supphers have designed advanced mixing systems to enable polyphasic systems to be intimately mixed at the laboratory scale. There has also been considerable rethinking of the green credentials of some of these alternative solvents in recent years and many ionic liquids are no longer considered suitable because of their complex syntheses, toxicity, or other unacceptable properties, or difficulty in separation and puriflcation. Fluorous solvents (which are based on heavily fluorinated usually aliphatic compounds) are not considered to be environmentally compatible (as they persist in the environment). 

The liquid support may be water, supercritical fluids, ionic liquids, organic liquids or fluorous liquids [12]. The Shell higher olefin process (SHOP) and the Oxo synthesis (hydrofomylation) are examples of important industrial processes based on biphasic catalytic systems. 

---