Liquid-SCF Systems

In liquid-supercritical systems, the liquid phase most commonly acts as a support to retain the catalyst rather than using the selective solubility properties of SCCO2 as for the CESS approach described below. Thus, there is a permanent phase separation between the mobile phase which is (usually, but not exclusively) CO2 and the stationary liquid phase. The fact that the catalyst is molecularly dispersed 

It is also important to note that in most cases the stationary phase cannot be regarded as an irmocent component of the reaction mixture. The interactions of substrates, catalyst, or CO2 with the stahonary phase can all influence the course of the reaction. In ophmal cases this can be a useful factor, for example in the hydrovinylation system discussed below, where the ionic hquid (I L) acts as a catalyst activator which must otherwise be added separately. In water—CO2 systems, however, the formation of carbonic acid when CO2 is dissolved in water means that the pH of the aqueous phase can be as low as 3. Thus all reagents and catalysts must be stable under acidic conditions when using this approach. 

Except for water as the stationary phase, no significant cross-contamination of the liquid phase into CO2 can be tolerated, as this would result in its depletion during continuous-flow operation, along with contamination of the product. The mixture critical points of most organic solvents are in the vicinity of those of the pure solvents. 

F = flow meter M = mixer MF = metal filter P = FIPLC pump  

The low solubility of most organometaUic catalysts in COj means that in ideal cases no modification catalyst is required, provided it is suffidenfly soluble in the IL. This was foimd to be the case for the hydrovinylation system which employed a nickel-based chiral catalyst first reported by Wilke and co-workers [23], In the case of the hydroformylation system, however, the activity was foimd to be highest when using [Rh(acac)(CO)2] combined with a sulfonated triphenylphosphine analogue that had an imidazoHum cation as its counterion. In practice it is likely that neutral catalysts may require charged ligands, while those which are themselves charged can be used unmodified. 

---

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. 

---