Other Methods for Catalyst Recovery

Although very successful for propylene hydroformylation, the Rh-TPPTS-based process cannot be used for the hydroformylation of long-chain alkenes. This is because alkenes with six or more carbon atoms have negligible solubility in water, which results in unacceptably low rates of hydroformylation. A general solution to this problem is to carry out the reaction in a single phase and to transfer the catalyst into the second phase once the reaction is complete. Special ligands such as 5.20-5.22 mentioned earlier have been used for this purpose, but the mechanism of triggering the phase separation is different in each case. 

The complex RhH(CO)Lj, where L = 5.20, is soluble in a water-immiscible organic solvent such as toluene. Hydroformylation of an alkene such as 1 -octene could therefore be carried out in toluene without any difficulty. However, at the end of the reaction when water is added and the biphasic system is exposed to CO, the catalyst becomes soluble in water. 

The reason for this is reaction 5.2.4.1 the ligand acquires ionic character due to protonation by carbonic acid (aqueous solution of CO ). By bubbling through the biphasic system, the dissolved CO can be driven off. This makes the catalyst soluble in organic solvent again, and the organic layer can then be recycled. 

The complex RhH(CO)L3 where L is 5.21 is soluble in a fluori-nated solvent such as CgFjjCFj (perfluorinated methylcyclohexane) but not in an organic solvent like toluene. At 100°C, the fluorinated solvent 

At the end of the reaction, the single phase is separated into a nonpolar and a polar phase by the addition of water or methanol or by a change in temperature. The catalyst remains in the polar phase, while the product goes into the nonpolar phase. Finally, it is important to note that the techno-commercial viability of 5.20-5.22 remain to be established on an industrial scale in a manufacturing plant. 

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