Biphasic water/scCO2 system

In addition to this, related areas such as liquid CO2 and C02-expanded solvents should not be overlooked. Many additives and complex modifiers are being used to facilitate reactions in SCCO2 and perhaps the use of a small amount of organic solvent (perhaps from a bio-feedstock) could be justified in order to reduce the cost of a process and therefore lead to its uptake by industry. In addition to this, continued research into biphasic systems C02-water, C02-ionic liquids, CO2-PEG/surfactants and CC -solids (including heterogeneous catalysts) is needed to deliver pure products and reduced cost to future end-users of this technology. 

Water is particularly suitable for use in biphasic catalysis. It readily separates from organic solvents because of its polarity, density and because of the hydrophobic effect. Water will form biphasic systems with fluorous solvents, some ionic liquids, many volatile organic solvents, and also with SCCO2 [18]. 

Except for transitions from heterogeneous to homogeneous catalysis, there is also common groimd for the various methodologies described here the application of SCCO2 is described in the presence of ionic liquids [32a] or of fluorous solvents [32b,c] as well in aqueous operation [33a-d] and aerobic epoxidations have been attained in fluorous biphasic systems using ionic liquids [33e]. On the other hand, ionic liquids [34] or fluorous solvents [35a,c] have been used together with aqueous operations and water-soluble polymers are the focal point of the application of... 

Transesterification has also been used to demonstrate that enzyme catalysis can be carried out in SCF-IL biphasic systems. ILs can stabilize enzymes, thereby enabling their use at higher temperatures [61, 62]. SCCO2, on the other hand, may cause reductions in activity for enzymes either because of changes in pH, as a result of the acidic CO2 dissolving in water, or as a result of conformational changes... 

Extraction processes with SCCO2 from aqueous phases are of interest to natural product isolation and water pttrification (Section 6.2). The knowledge of the chemical and physio-chemical behavior of the binary mixture emerging from these studies provides an excellent basis for the concept of using the combination of compressed carbon dioxide and aqueous phases for biphasic catalytic systems. 

As outlined in general in Section 6.3, two distinct engineering concepts can be envisaged for transition metal catalysis in scC02-based biphasic systems. In one case the compressed CO2 phase is used as the compartment for substrates and/or products, whereas the catalyst is contained in the liquid phase, in the present case water. We will refer to this scenario as H2O/SCCO2 biphasic catalysis and examples are discussed in this section. In the other case, carbon dioxide is used as the catalyst-containing phase, whereas substrates and products are retained in the polar aqueous phase. We refer to this system as inverted biphasic catalysis , denoted as SCCO2/ H2O, and it will be subject of the discussion in Section 6.4.4.4. 

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