Catalysis multiphase 431 - immobilized homogeneous

There are numerous types of multiphasic chemical processes. The most common are biphasic although triphasic, tetraphasic and even higher number of phases can also be used to conduct chemical synthesis. All the multiphasic methods aim to overcome the major problem of homogeneous catalysis, which is catalyst recovery and product separation. The simplest systems are biphasic ones that involve immobilizing a catalyst in one solvent, which is immiscible with a second solvent in which the substrates/products are dissolved. If a gas is required as a substrate then the system could be regarded as triphasic (i.e. liquid-liquid-gas), although for the purposes of this book (and as is most commonly defined elsewhere) such as system will be referred to as biphasic. In other words, only the number of different liquid solvent phases will be used to define the phasicity of a system. 

Fluorous biphasic catalysis is another active area in multiphasic homogeneous catalysis. The term fluorous was introduced [90] as the analogue to the term aqueous, to emphasize the fact that one of the phases of a biphase system is richer in fluorocarbons than the other. Fluorous biphase systems can be used in catalytic chemical transformations by immobilizing catalysts in the fluorous phase. A fluorous catalyst system consists of a fluorous phase containing a preferentially fluorous-soluble catalyst and a second product phase, which may be any organic or inorganic solvent with limited solubility in the fluorous phase (Figure 2.8a). 

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