Monoliths in Heterogeneous Catalysis

The use of structured catalysts in the chemical industry has been considered for years. Conventional fixed-bed reactors have some obvious disadvantages such as maldistributions of various kinds (including a nonuniform access of reactants to the catalytic surface), high pressure drop in the bed, etc. Structured catalysts are promising as far as elimination of these setbacks is concerned. Two basic kinds of structured catalysts can be distinguished:

1. Structural packings covered with catalytically active material, similar in design to those used in distillation and absorption columns and/or static mixers. Good examples of catalysts of this kind are those offered by Sulzer, clearly developed by Sulzer column packings and static mixers. As in packed beds, there is an intensive radial convective mass transport over the entire cross-section of these packings. Structural packing catalysts and the reactors containing them are, however, not within the scope of this review.
2. Monolithic catalysts are continuous unitary structures which contain many small, mostly parallel passages. A ceramic or metallic support is coated with a layer of material in which active ingredients are dispersed. An interaction between these passages can occur if walls are permeable. The catalytically active material is present on or inside the walls of these passages. Radial mass transport can occur only by diffusion through the pores of the permeable walls.