CE process

Figure 5.2. Separation scheme to recycle homogeneous catalysts using the CESS process.

Leitner reported that lower densities of SCCO2 can be used to extract product from a reaction mixture without coextraction of a homogeneous catalyst. This process, which he called CESS (catalysis and extraction using supercritical solution), was demonstrated with styrene hydroformylation [Eq. (23), ligand L ] and imine hydrogenation [Eq. (22)]. The catalyst was used for three cycles before drops in the conversion and selectivity was observed (101,111,133). Sellin and Cole-Hamilton used catalysts specifically chosen for their insolubility in SCCO2 to facilitate the catalyst/product separation in a CESS process (134). 

Detailed investigations of the course of reaction with catalyst 7d revealed that a much lower catalyst loading was required for efficient catalysis in SCCO2 than in CH2CI2 under otherwise identical conditions. It was shown that this effect was due to a different kinetic behavior in the two solvent systems, rather than to an increase in the overall reaction rate by eliminating mass-transfer limitations. Furthermore, it was demonstrated that the reaction product could be effectively separated from the catalyst by SFE, using the same solvent for the reaction and the purification step (CESS process). A single catalyst load was used in seven subsequent catalytic runs with almost constant enantioselectivity and a decrease of activity was noticeable only after the fourth run. 

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