Homogenous Catalytic Reactions in Biphasic Systems

A drawback of homogenous catalytic processes is often the complex and costly separation and recycling of the catalyst. Therefore, considerable efforts are made to combine the easy separation of heterogenous catalysts with the high potential activity and selectivity of homogenous molecular catalysts. 

Different methods are proposed to facilitate the recovery of the catalyst. A very successful way is to use biphasic systems of two immiscible liquids. The catalyst should be soluble only in one phase, in which the transformation takes place, while the products and sometimes the reactants should be preferentially soluble in the second. The catalyst is thus immobilized within a liquid support. The immiscible liquids can be separated after the reaction and the catalyst is recycled. This can be done without any thermal or chemical treatment. As the reaction is carried out in the presence of dissolved catalyst, the advantages of homogenous catalysis are fully preserved. 

The liquid support may be water, supercritical fluids, ionic liquids, organic liquids or fluorous liquids [12]. The Shell higher olefin process (SHOP) and the Oxo synthesis (hydrofomylation) are examples of important industrial processes based on biphasic catalytic systems. 

As the reaction takes place in the catalyst containing phase, the reactants must, first of all, be transferred from the second and eventually gas phase to the reaction phase. Therefore, special attention has to be paid to the mixing and dispersion of one phase within the other and mass transfer efficiency between phases. The mass transfer rate between the different phases depends on the area of the interface and the mass transfer coefficient. Whether the reaction will take place in the bulk of the reaction phase or near the interface depends on the ratio between the characteristic reaction time t ) and the characteristic time for mass transfer (t ). This ratio is known as the Hatta number Ha). 

The discussion can be facilitated on the basis ofthe film model and by supposing a first order irreversible reaction in the reaction phase and neglecting the mass transfer resistance in the non-reactive phase I [13-15]. 

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Homogenous Catalytic Reactions in Biphasic Systems 47 The bulk concentration c jj is a rather complex function of the intrinsic reaction... 

General aspects of fluid-fluid reactions are discussed in detail in Section 2.4 within the context of homogeneous catalytic reactions in biphasic systems. Mostly, the reaction takes place only in one phase and the reactant must be transferred from the nom-eactant phase, for example, the gas phase to the reaction phase. In consequence, the mass transfer between the different phases plays an important role on the overall kinetics and may strongly influence the... 

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