Comparison of Homogeneous and Heterogeneous Catalysis

Due to their high degree of dispersion, homogeneous catalysts exhibit a higher activity per unit mass of metal than heterogeneous catalysts. The high mobility of the 

The most prominent feature of homogeneous transition metal catalysts are the high selectivities that can be achieved. Homogeneously catalyzed reactions are controlled mainly by kinetics and less by material transport, because diffusion of the reactants to the catalyst can occur more readily. Due to the well-defined reaction site, the mechanism of homogeneous catalysis is relatively well understood. Mechanistic investigations can readily be carried out under reaction conditions by means of spectroscopic methods (Fig. 1-3). In contrast, processes occurring in heterogeneous catalysis are often obscure. 

Owing to the thermal stability of organometallic complexes in the Uquid phase, industrially realizable homogeneous eatalysis is limited to temperatures below 200 °C. In this temperature range, homogeneous catalysts can readily be stabilized or modified by addition of hgands eonsiderable solvent effects also occur. 

In industrial use, both types of catalyst are subject to deactivation as a result of chemical or physical processes. Table 1-3 summarizes the advantages and disadvantages of the two classes of catalyst. 

The major disadvantage of homogeneous transition metal catalysts is the difficulty of separating the catalyst from the product. Heterogeneous catalysts are either automatically removed in the process (e.g., gas-phase reactions in fixed-bed reactors), or they can be separated by simple methods such as filtration or centrifugation. In the case of homogeneous catalysts, more complicated processes such as distillation, liquid-liquid extraction, and ion exchange must often be used [3]. 

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