Supported Monolithic Structures

Catalysts can be metals, oxides, sulfides, carbides, nitrides, acids, salts, virtually any type of material. Solid catalysts also come in a multitude of forms and can be loose particles, or small particles on a support. The support can be a porous powder, such as aluminium oxide particles, or a large monolithic structure, such as the ceramics used in the exhaust systems of cars. Clays and zeolites can also be solid catalysts. 

The optimization of the catalyst formulation is relevant not only to the active species but also to the structure of the support. Indeed, structured catalysts in the form of monolith or foam offer great advantages over pellet catalysts, the most important one being the low pressure drop associated with the high flow rates that are common in environmental applications. 

A wide range of polymeric materials can be prepared from HIPEs. Polymerisation of the continuous phase yields highly porous cellular polymers with a monolithic structure. These are known as PolyHIPE polymers, and possess a number of unique properties including, in most cases, an interconnected cellular structure and a very low dry-bulk density. Their very high porosity favours their use as supports for catalytic species, precursors for porous carbons and inert matrices for the immobilisation of enzymes and micro-organisms. 

Monolith reactor This type of reactor is used extensively for the abatement of automobiles exhaust emissions. The gas flows continuously through the reactor, whereas the catalyst is a continuous phase consisting of a ceramic support and the active phase, which is dispersed onto the support. The support is structured in many channels and shapes that achieve large catalytic surface at small volume. A typical application of monolith reactors is the exhaust gas cleaning. 

Figure 2 shows the shape and size of the Monolith alumina supports. These are in the form of cylindrical segments of about 2.54 cm in length and about 1.0 cm in diameter. These have longitudinal and parallel channels along their length. The size, shape and thickness of the walls of the channels are also shown in Figure 2. The Monolith structure has about 60 to 80 percent of its cross-sectional area open. Therefore, a bed of regularly stacked Monoliths would offer significantly less pressure drop than that encountered in conventional packed beds. This has been observed by Satterfield and Ozel (1) for a water-air system.

FIGURE 9 Monolithic structure composed of catalyst particles or support. The catalyst loading can be high. 

A bare monolithic structure can be coated with a catalyst support layer in several ways. Figure 21 shows a SEM image of a typical commercial cordierite monolith structure. Washcoating can be done by (partly) filling the pores of the macroporous walls with the washcoat material or by depositing a washcoat as a layer on top of the walls. These methods are shown schematically in Figure 22. 

It is clearly evident that the texture of the support is different from that of bare cordierite (Figure 23A), and the typical layered structure of bare cordierite is no longer visible (Figure 23B), either in the channels or in the pores inside the channel walls, indicating a complete coverage of the monolithic structure. If the thickness of the coating is calculated from the specific surface area of the cordierite (0.7 m /g) and the washcoat characteristics (loading of 10 wt% with a density of 1600 kg/m ), a... 

The porous mass of the monolithic structure has a low pressure drop. If further reduction of the pressure drop in the bulk support is desired, there arc a few possibilities [Atomic Energy Commission (France), 1971]. Additional grooves can be machined or swaged with dowels which will produce channels, or a network of additional channels perpendicular to the honeycomb feed channels is provided by using materials such as carbon that can be burned off. 

Ceramic and metallic monolith structures have a geometrical surface area in the range 2.0-4.0m T support volume. This is much too low to adequately perform the catalytic conversion of the exhaust gas components. Therefore, these structures are coated with a thin layer of a mixture of inorganic oxides, some of which have a very high internal surface area. This mixture is called the washcoat (Fig. 36). 

The introduction of automobile exhaust catalysts in the United States and elsewhere has produced a major market for platinum-type oxidation and reduction systems. An innovative consequence of this industry has been the development of ceramic honeycombed monoliths as catalyst supports. These structures contain long, parallel channels of less than 0.1 mm in diameter, with about SO channels per square centimeter. The monolith is composed of cordierite (2MgO - 2AI2O) SSiOj) and is manufactured by extrusion. A wash coat of stabilized alumina is administered prior to deposition of the active metal, either by adsorption or impregnation methods. 

The catalysts were prepared as monolithic structures of parallel channels of square section with a density of 8 cells/cm and wall thickness of 0.90 mm. All the monolithic supports were subsequently heat treated at 500°C, if not otherwise stated, for 4 hours in an air atmosphere. 

The selection of the carrier is relatively simple. It may be imposed by the type of reaction to be promoted. For instance, if the latter requires a bifunctional catalyst (metal + acid functions), acidic supports such as silica-aluminas, zeolites, or chlorinated aluminas, will be used. On the other hand, if the reaction occurs only on the metal, a more inert support such as silica will be used. In certain cases, other requirements (shock resistance, thermal conductivity, crush resistance, and flow characteristics) may dominate and structural supports (monoliths) have to be used. For the purpose of obtaining small metal particles, the use of zeolites has turned out to be an effective means to control their size. However, the problem of accessibility and acidity appearing on reduction may mask the evidence of the effect of metal particle size on the catalytic properties. 

Optimize the choice of metals and metal loadings for the CGO-based catalysts supported on structured forms, such as monoliths or foams. 

Recently, there has been a growing interest in the use of monohthic structures for (bio)chemical conversion and adsorption processes. A very versatile type of monolith is based on carbon. The combined properties of carbon and monolithic structures create a support with great potential. In this chapter we describe recent developments in the field of carbon-based monolithic structures with respect to preparation, support properties, and application in catalytic processes. Furthermore, two examples are used to demonstrate the approach and possible pitfalls when using carbon (coated) monoliths in catalysis. 

Carbon-based monoliths can be of the integral or coated type [5,6]. For practical applications, several requirements are set to monolithic structures. For a coated type of monolithic support, the mechanical properties are adopted from the ceramic or metallic monoliths. It is, however, important that the coating adhere well to the monolith to prevent flaking and subsequent loss of active... 

Two different types of monolithic structures are to be distinguidied coated monoliths and integral monoliths. The former consist of a monolithic backbone and a support layer that is coated onto or in the structure. The monolithic structure, which is either a ceramic or metallic monolith, supplies the mechanical and geometrical properties, whereas the support layer provides the adsorptive and/or catalytic properties. The second type, the integral structure, is... 

The second concept for catalytic column internals is the use of catalytically active structures instead of those filled with catalyst. Such structures are either carrier-supported catalysts or solid catalytic structures. Carrier supports can be coated with any kind of catalyst (e.g. GPP rings and some specific structured packing [39], KATAPAK-M [40]). Moreover, it is possible to develop solid catalytic structures without any carrier. The so-called BP-rings, for example, are produced by polymerization in an annular gap [39], whereas the monolithic structures are made by extrusion of catalytic material [41]. 

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