Monolithic reactors for environmental applications

The main aspects of preparation technologies for the manufacture of environmental monolithic catalysts have been reviewed on the basis of the patent literature, the authors' own experience in their laboratories, and the current open literature. The main topics covered in this review are the preparation of massive monoliths by extrusion, the coating of ceramic monoliths, and metallic monoliths. Extrusion, one of the most extensively used processes for the production of ceramic monoliths, should fulfill a number of conditions as adequate plasticity of the pastes to allow their extrusion and their immediate conformation into rigid structures under monolithic shape. This is no trivial matter, since in developing monoliths with new materials a great effort should be devoted to finding appropriate conditions. With respect to coated monoliths, the convenient characteristics of a good monolithic catalyst should be met by managing preparation procedures. Thus, preparation technologies are composed of both materials science and engineering aspects. One emerging technology is the growth of zeolites on monolithic walls. Zeolites are among the most studied materials for environmental applications. Among metallic monoliths, those based on ferritic steel containing alumina are the most important, but those made of aluminum coated by alumina produced by anodisation are excellent materials to prepare metallic monoliths. The melting point of aluminum (660 °C) limits its use to medium-low temperature processes, but the especial characteristics of the anodisation alumina offer exciting prospects.