The Principle of Multiphase Catalysis

The general principle of two-phase catalysis in polar solvents, for example, in water, is shown in the simplified diagram of Fig. 1. The metal complex catalyst, which can be solubilized by hydrophilic ligands, converts the reactants A + B into the product C. The product is more soluble in the second than in the first phase and can be separated from the catalyst medium by simple phase separation. Excellent mixing and contacting of the two phases are necessary for efficient catalytic reaction, and thus the reactor is normally well stirred. 

This is the simplest case of two-phase catalysis because the solubilities of the product C and of the catalyst are so different that a nearly perfect phase separation results. This kind of process would be ideal for industrial 

If the product C is partly dissolved in the catalyst phase S I, an extraction is necessary (Fig. 2a). In this case the extraction is performed with a phase S II (e.g., petroleum distillate) which is almost completely insoluble in the catalyst phase S I (e.g., water). 

It is also possible that the second phase may form during the reaction, because the product(s) C is (are) not soluble in the original catalyst phase S (Fig. 2b). A phase separation enables continuous return of the catalyst to the reactor. 

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