Alternative Reaction Media and Multiphasic Systems

If a catalyst is an insoluble solid, that is, a heterogeneous catalyst, it can easily be separated by centrifugation or filtration. In contrast, if it is a homogeneous catalyst, dissolved in the reaction medium, this presents more of a problem. This offsets the major advantages of homogeneous catalysts, such as high activities and selectivities compared to their heterogeneous counterparts (see Table 7.1). However, as Blaser and Studer have pointed out [6], another solution is to develop a catalyst that, at least for economic reasons, does not need to be recycled. 

This need for efficient separation of product and catalyst, while maintaining the advantages of a homogeneous catalyst, has led to the concept of liquid-liquid biphasic catalysis, whereby the catalyst is dissolved in one phase and the reactants and product(s) in the second liquid phase. The catalyst is recovered and recycled by simple phase separation. Preferably, the catalyst solution remains in the reactor and is reused with a fresh batch of reactants without further treatment or, ideally, it is adapted to continuous operation. Obviously, both solvents are subject to the same restrictions as discussed above for monophasic systems. The biphasic concept comes in many forms and they have been summarized by Keim in a recent review [7]  

Advantages - Mild reaction conditions - High activity selectivity - Efficient heat transfer - Facile separation of catalyst and products - Continuous processing 

Disadvantages - Cumbersome separation recycling of catalyst - Not readily adapted to a continuous process - Heat transfer problems - Low activity and/or selectivity 

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