SCF IL Biphasic Systems

More work on batch alkene hydrogenation in an IL with scCOj extraction was carried out by Liu et al. [34] and Sohnas et al. [31]. The latter also showed how hydrogen solubihty in IL increased with the addition of CO2, another example of CO2 enhancing the solubihty of gases in ILs. 

Further studies by the same group [38] with optimized reaction parameters showed activities comparable with those required for commercial systems, but with a lower linear selectivity than desired. Nevertheless, the linear selectivity was increased later with specially designed ligands [39]. 

Bosmann et al. [40] reported the continuous-flow hydrovinylation of styrene with a system closer to a fixed-bed reactor. The ionic catalyst solution was confined in 

Continuous-flow biphasic IL-SCCO2 has also been applied in biocatalysis. Thus enzymatic reactions including transesteriflcation [41], acylation [42], and kinetic resolution of enantiomers [43] have been reported. 

More examples of this system can be found for Friedel-Crafts [44] and metathesis [45] reactions, which have also been reviewed [46]. 

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Following the elegant work of Brennecke, Beckman, and co-workers on phase behavior and separation [4, 15], a number of groups have investigated SCF-IL biphasic systems for other reviews c [32-36]. 

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... 

Figure 18.6 Schematic of continuous-flow homogeneous catalysis using an SCF-IL biphasic system. (Source Webb et al. [38]. Reproduced with permission of Royal Society of Chemistry.)...

ILs/SCFs Biphasic Systems as Promising Media for Biocatalysis... 

The SCF/water, SCF/IL, and SCF/Uquid polymer biphasic systems, which have been studied extensively, have great potential for applications in catalytic reactions because they can combine the advantages of different greener solvents. 

As with classical multiphase catalysis, the organometallic catalyst is retained here in a liquid phase that is immiscible with the second phase containing substrates and/or products. For hydrogenation, the liquid/SCF system is always biphasic, whereas conventional systems are usually triphasic (liquid-1 /liquid-2/ H2). The liquid phase must provide a stable environment for the organometallic catalyst and should be insoluble in the SCF phase. Water, ILs and PEG have been used successfully for this purpose, together with scC02 as the mobile phase. Again, the products must not be too polar in order to be effectively extracted if C02 is used as the SCF. 

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