Theory and applications of ceramic foam catalysts

Reticulated ceramic foams can now be prepared from a range of materials, and they have characteristics that make them desirable as substrates for structured heterogeneous catalysts. They exhibit extremely high porosities, with a significant degree of interconnectivity that results in low pressure-drop. High convection in the tortuous megapores gives enhanced mass and heat transfer. Furthermore, ceramic foam catalysts, unlike their honeycomb monolithic counterparts, have a considerable degree of radial mixing, which is an advantage in processes limited by heat transfer and can even out flow distribution. The low surface areas of ceramic foams means they have to be coated with a higher surface area material, and this increases the pressure drop and radial heat transfer conductivity in a predictable way. Reactions that require short contact times to control product selectivity and processes that are limited by heat transfer to or from the catalyst bed can benefit from the use of foam-based catalysts. This is particularly true if the effectiveness factor for a specific reaction with conventional catalyst particles is low. Examples of reactions of this type include many industrially important processes, such as the partial oxidation of hydrocarbons, water gas shift, methanol synthesis, methanation and Fischer-Tropsch synthesis. In this paper, the structure of ceramic foam catalysts, and correlations for estimating their transport properties are reviewed, and their use to potential applications are considered.