Demands on Solid Catalysts

In the preparation of solid catalysts to be employed in the fine-chemical industry the structure and chemical composition of the catalysts must meet conflicting demands which must be carefully considered. Three characteristics of solid catalysts are decisive, viz.  

In the previous section we dealt with the surface area per unit weight or unit volume required to achieve technically acceptable conversions. The conclusion was that porous catalyst particles must usually meet the demands both of pressure drop (fixed bed catalysts) or of viability of separation (suspended catalyst particles) and of the extent of the catalytically active surface area. When the catalyst must be thermally pretreated, the active surface area should not severely drop as a result of sintering of the active particles. Although solid catalysts in fine-chemistry operations are usually employed in liquid phases, i. e. not at highly elevated temperatures, sintering of the active component(s) should not occur during the reaction. 

Some catalytic reactions proceed only on surfaces with a specific structure. In fine chemistry, the size of metal particles can significantly affect catalytic activity and selectivity. With small metal particles, penetration of foreign atoms into the metal surface can proceed easily, because the number of neighboring metal atoms in the surface is small. As might be expected, the catalytic reactions of foreign atoms penetrating into the metal surface is often different from that of foreign atoms adsorbed on the surface. The most well known example is the activity of 

As discussed above, the transport properties of porous catalyst particles of ca 3 to 100 pm are extremely important for the selectivity of catalytic reactions in which the desired initial products are liable to further reaction to undesired material. The ratio of the rate of catalytic reaction to that of transport within the pore system of catalyst particles is represented by Thiele s modulus [1], which is proportional to the pore length and to the square root of the diameter of the pores. Accordingly reducing the size of the catalyst particles is more elfective than increasing the diameter of the pores. 

Small catalyst particles of diameter larger than ca 3 pm, but not larger than ca 10 pm, are attractive. The number of catalyst particles per unit weight of catalyst is inversely proportional to the third power of the diameter. The surface area per catalyst particle is proportional to the second power of particle size. Consequently the surface area exposed per unit weight of catalyst is inversely proportional to the first power of the size of the catalyst particles. Both transport from the bulk of the liquid to the external surface of the catalyst particles and transport within the pores of the catalyst particles are therefore favorably affected by a drop in size. Because the mass transfer coefficient from the bulk of the liquid to the external surface of suspended catalyst particles is inversely proportional to the diameter, transport to the external surface is inversely proportional to the square of the diameter of the particles. Special procedures are involved in the technical production of 3-10-pm particles this will be covered later. 

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