Organization of Catalysis

The catalyst is the same phase as the reactants and products. Examples are hydrolysis of esters by acids (liquid-liquid), oxidation of SOi by NO (vapor-vapor), and decomposition of potassium chlorate by MnOi (solid-solid). Usually, the liquid phase is most common, with both catalyst and reactants in solution. 

Catalysis occurs through comptexing and rearrangement between molecules and ligands of the catalyst. Reactions can be very specific, with high yields of desired products. Since the mechanisms involve readily identified species, these reactions are easily studied in the laboratory with the techniques of organometallic chemistry. They are, however, difficult to operate commercially. Liquid phase operation places restrictions on temperature and pressure, so equipment is complicated. The catalyst must be separated from the products, imposing additional difficulties. 

For these reasons, homogeneous catalysis is found only in limited industrial use, appearing usually in the manufacture of specialty chemicals, drugs, and food. Exceptions are acetic acid production, olefin alkylation, and hydroformylation. 

In these systems the reactants and catalyst exist in different phases. Most commonly, solid catalysts are used with gaseous or liquid reactants, sometimes both. Other permutations are possible but less often encountered. 

Nevertheless heterogeneous catalysis are convenient to use commercially. Easily prepared solid catalyst pellets, packed in tubes through which reactants flow, satisfy process requirements for simple construction and dependable operation. Control is good, product quality high. It is not surprising that the vast majority of industrial catalytic processes adopt this approach. 

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