Monolithic Catalysts for the Chemical Industry

and Charles M. Sorensen
Process Technologies, Corning Incorporated, MP-HQ-W2-21, Corning, New York 14831
Honeycomb-shaped monolithic catalysts are the standard catalyst shape in most environmental applications. In the processes of the chemical industry, however, their current use is very limited. In this paper, the current status of the monolith technology for applications in the chemical industry is reviewed. Application areas in which monolithic catalysts have superior performance characteristics are discussed. Especially pre- and postreactors as well as replacement concepts for slurry reactors are expected to have a very good near-term potential for application in commercial operations. In addition, it is expected that several adiabatic fixed-bed processes, especially those constrained by pressure drop or mass transfer, can benefit from the use of monolithic catalysts because of their excellent pressure drop to mass transfer ratio. A new and promising area for monoliths with a high thermal conductivity is their use in various gas-phase processes where multitubular reactors are employed.
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waste stream. Another benefit, especially important in SCR reactors, is that the open-channel structure is capable of processing streams having high loads of particulates and dust. For automotive catalytic converters, mechanical integrity and durability are key attributes because of the thermal and mechanical stresses that occur during the many rapid changes in operating conditions. Additional important attributes for the success of monoliths in mobile applications are the low weight and low thermal mass, which is essential for a fast lightoff of the catalyst. Finally, another key factor for the relatively fast introduction of monolithic catalysts into these applications is that they were always driven by regulations. As catalysts and catalyst supports for the conversion and synthesis processes of the chemical industry, monoliths have found very limited commercial use so far. Public information indicates that monoliths are used in the production of hydrogen peroxide2 and in a postreactor for the synthesis of pththalic anhydride.3 The first example represents a selective multiphase hydrogenation reaction whereas the second one is a selective gas-phase oxidation. The limited use of monoliths in the chemical industry can be attributed to several reasons, and on the basis of our experience, the following items are some of the most important ones: (a) Today a large number of different catalysts, often with tailored properties, are employed in the many processes of the chemical industry. Often the annual volume of each catalyst is small, and it is often difficult to justify dedicated research and development efforts as well as capital investment to achieve a monolithic product with the same intrinsic catalyst properties. During the selection and evaluation of candidate monolithic catalysts, one needs to consider how much catalyst development work will be required. In the case of transferring previously developed pellet technology to monoliths, it will in many cases be required either that the benefits that come from the monolithic catalyst shape are significant enough to allow for some short-
Introduction Since their introduction in the mid-1970s, monolithic catalysts have become the standard catalyst shape in most environmental applications. The most prominent examples are the automotive catalytic converter and the DeNOx catalyst used in selective catalytic reduction (SCR) units used, for example, at power plants and incinerators. Annually more than 100 000 m3 of monolithic catalysts and catalyst supports are produced worldwide to meet the demand of these and other related applications.48 The term monolithic catalyst is used for a number of different structures, but we will limit the scope of this contribution to those commonly used in the abovementioned applications. Figure 1 shows a photo of a monolith composed of a large number of straight and parallel channels that extend throughout the body. These structures can be made from a range of materials.1 They are characterized by the size and geometry of the channel openings (dh), the void fraction ( ) or open frontal area (OFA), and the cell density usually characterized by cells per square inch (cpsi) or per square centimeter. Table 1 gives some examples of structures and materials available commercially or that were made within our R&D laboratories. One of the key reasons for the success of monoliths in environmental applications is clearly their excellent ratio of pressure drop to geometric surface area. This is shown in Figure 2. Compared to any other catalyst structure such as pellets or foams, a significantly lower pressure drop is observed at a given geometric surface area. This is especially important in environmental applications because large volumetric flow rates often have to be handled. Energy consumption is an important part of the overall operating costs because there is no commercial value created and the feedstock is a