Shaping of Solid Catalysts

Bettina Kraushaar-Czametzki and Steffen Peter Muller 

Synthesis of Solid Catalysts. Edited by K.P. de Jong 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-32040-0 

The requirements for moving-bed catalysts are basically the same as those for fixed-bed systems. However, there is no choice of the shape because moving-bed particles must be spherical. In addition, crush and attrition strengths need to be higher. 

Catalysts for fluidized-bed reactors have to be spherical as well. The appropriate particle size fraction for gas-solid systems can be estimated after Geldart [1] from the density difference between soKd and gas. Most widely used catalysts for fluidized beds and risers are Geldart-type B powders with particle diameters ranging from 40 to 500 pm or solid densities between 1.4 X 10 and 4 x 10 kg/m, respectively. When fluidization is provided by a Kquid as in ebullated-bed reactors, the particle sizes may be substantially larger because of the higher buoyancy in these systems. However, all types of fluidized-bed catalysts must exhibit high mechanical stability because they are exposed to abrasion on reactor walls and internals, collisions between particles and shear forces exerted by the surrounding fluid. 

In summary, the following aspects have to be taken into account upon formulation and shaping of solid catalysts through appropriate choice of materials, catalyst size, and shape, and by applying a suitable manufacturing technique  

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Solid catalysts can be subdivided further according to the reactor chosen. Dependent on the type of reactor the optimal dimensions and shapes of the catalyst particles differ. Catalysts applied in fixed beds are relatively large particles (typically several mm in diameter) in order to avoid excessive pressure drops. Extrudates, tablets, and rings are the common shapes. Figure 3.9 shows some commonly encountered particle shapes. 

Even though the synthesis of many medium pore SAPO molecular sieves are well documented, only SAPO-11 has been studied in detail with respect to its shape selectivity and catalytic activity in acid catalyzed reactions. The reaction of m-xylene on zeolites, besides its industrial importance, is abundantly described in literature not only because it provides information on the geometry of the zeolite channels, but also because it is considered as an appropriate reaction to give information on the acidic properties of solid catalysts. Both isomerization and disproportionation are catalyzed by Bronsted acid sites , the disproportionation reactions requiring stronger acid sites than isomerization reactions. Hence SAPO molecular sieves with medium acidity should give better selectivity for m-xylene isomerization than zeolites. 

The use of solid catalysts in halogenation processes will avoid corrosion and disposal problems. Work-up procedures to isolate and recover the desired product will also be easier leading to simpler and cleaner process routes. In addition, the use of zeolites as the solid catalysts in nuclear aromatic halogenations might lead to enhanced yields of the para isomer, because of the shape selectivity of the zeolite. 

In contra.st to their interaction with applied electric, magnetic or electromagnetic fields, porphyrins and metalloporphyrins can also interact with other chemical species. One might view such interactions as chemo-responsive rather than field-responsive. The development of chemo-responsive materials based on porphyrins, however, is somewhat less advanced. One example of such applications is that porphyrin solids, being highly porous, are involved in the current development of molecularly based molecular sieves or shape-selective solid catalysts. Porphyrins and metalloporphyrins have also been examined for a variety of sensor applications, further proving their importance as a class of chemo-responsive materials. 

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