Cellular Ceramics / 5

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5.4

Heterogeneously Catalyzed Processes with Porous Cellular Ceramic Monoliths

Franziska Scheffler, Peter Claus, Sabine Schimpf, Martin Lucas, and Michael Scheffler

5.4.1

Introduction

Catalytic processes are the driving force in industrial chemistry. More than 80 % of all base chemicals and products have encountered a catalyst (homogeneous or heterogeneous) during their production or conversion. Most of these processes are heterogeneously catalyzed, which means that the catalyst is in a different physical condition (solid) than the starting material/product (liquid and/or gas phase), and the majority of processes are operated in a small temperature window within the range 100–600 C. Only a few processes are carried out at higher temperatures.

In classical heterogeneous processes the catalyst typically consists of an active component, a binder, and several additives. Often starting from a paste, these mixtures are shaped, for instance, by extrusion or tablet pressing. The resulting products are spheres, cylinders, or tablets with lengths and diameters of several millimeters. These catalyst bodies are used in fixed bed reactors, often with dimensions of several or some tens of cubic meters, activated, and act for a certain time as a catalyst. Reviews on catalytic processes and how to select and make a catalyst can be found in [1–3]. Beside the well-established classical processes operated with shaped bodies novel processes were established in the automotive and chemical industries over the past 30 years making use of ceramic monolith catalysts. A well-known example is the three-way catalyst (TWC) used in automotive exhaust gas cleaning since 1974. This development was triggered by fouling of shaped catalysts in this process, caused by incomplete burning of fuel [4], and the power loss due to the resulting pressure drop over the fixed catalyst bed. Stacked corrugated sheets of cordierite, coated with alumina and impregnated with hexachloroplatinic acid, were used first in 1961, similar to the honeycombs which are used nowadays [5]. Figure 1 shows some characteristics of both types of ceramic monoliths (honeycombs and ceramic foams) compared to common fixed-bed catalysts.

From the viewpoint of catalysis, ceramic monoliths have a low geometrical surface area (about 1–5 m2 per liter of support volume). Therefore, to apply these structures in heterogeneously catalyzed processes, for instance, for the production of chemicals or as three-way catalytic converters for the transformation of pollutants into harmless gases before releasing them into the environment, they must be coated

Cellular Ceramics: Structure, Manufacturing, Properties and Applications.

Michael Scheffler, Paolo Colombo (Eds.)

Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-31320-6

5.4 Heterogeneously Catalyzed Processes with Porous Cellular Ceramic Monoliths 455

Fig. 1 Comparison of flow and heat-dynamic properties of shaped catalysts, honeycombs, and foams.

with a thin active layer, often of an inorganic oxide. This oxide layer (often applied as a so-called wash coat) contains the catalytically active material, for example, precious metals incorporated into the porous oxide structure, which is responsible for the catalytic transformation of the starting material. However, the high surface area of the wash coat components are not only needed for a high dispersion of the incorporated metals, but they also can take part in the catalytic reaction, for example, by strong metal–support interactions, by triggering the catalytic (electronic) properties of the metal component.

This chapter describes established and novel developments in heterogeneous catalysis using ceramic monoliths (honeycombs and foams). Exemplary methods of modifying the monolith surface, increasing the surface area, and loading with active components will be discussed, and an overview of the use of ceramic honeycombs and foams in industrial chemistry, automotive applications, and current research will be given. This chapter, however, can provide only a small insight into heterogeneously catalyzed processes. For detailed information, some references ranging from basic principles to common applications in heterogeneous catalysis are included to allow the reader a more profound study of this matter.

5.4.2

Making Catalysts from Ceramic Monoliths

Most of the available porous ceramic materials (for manufacture, see Part 2, especially Chapters 2.1, 2.2, and 2.6) do not themselves show sufficient catalytic performance for technically relevant processes. To exploit the fluid-dynamic and structural advantages of porous ceramics for heterogeneously catalyzed processes, generation of the desired type of catalytically active sites on the ceramic surface is necessary.

Common industrial catalysts often consist of several catalytically active components (e.g. metals, metal cations, simple binary or complex multicomponent metal

5.4 Heterogeneously Catalyzed Processes with Porous Cellular Ceramic Monoliths 457

and polymerization coating of carbon) are discussed for coating monoliths with alumina, silica, and carbon. An example of surface area enlargement of an a-alumina foam with a pore size of 30 ppi with alumina is given in Ref. [8]: A slurry consisting of 36 wt % boehmite, Al(NO3)3 9 H2O as binder, calculated amounts of La(NO3)3 9 H2O to give La2O3, which prevents the c-Al2O3 to a-Al2O3 transition during calcination, and small amounts of starch and glycerol as viscosity modifier was used. The predried ceramic a-Al2O3 foam disks were immersed in the slurry, and excess slurry was removed by draining after immersion. The coated disks were dried and calcined at 800 C. Immersion and calcination were repeated several times until a desired surface area was achieved. After repeated processing a wash-coat loading of up to 13.5 wt % was achieved, and the BET surface area was increased from 2 m2 g–1 (uncoated foam) to 12 m2 g–1 at this degree of loading. This same method can be applied to honeycombs and foams.

5.4.2.2

Loading with Catalytically Active Components and Activation

Loading with catalytically active component can be performed with a number of methods and chemical compounds bearing the desired component. The most common in the field of supported catalyst preparation are wash or slurry coating, impregnation, adsorption, ion exchange and deposition or precipitation. After loading with active component thermal treatment for solvent removal, calcination, and consolidation are necessary. In the case of transition metals or precious metals as the active component, chemical reduction to form the dispersed metal from loaded metal oxides or metal ions, often carried out in situ prior to the start of the catalytic process, is a further step. Among the numerous publications dealing with specific methods of catalyst preparation some economical aspects and aspects of catalyst development are described in Ref. [9], and some general aspects of catalyst preparation and a classification of methods are given in Ref. [10]. The principles described for honeycomb monoliths therein are also applicable for ceramic foam monoliths. A scheme illustrating the different steps and routes from monolithic ceramics (extrusion and foam formation as well) to the final monolithic catalyst is given in Fig. 3.

A large group of heterogeneously catalyzed reactions proceeds on metal cations or well-dispersed metal nanoclusters or on a surface that provides an interaction between a metal and a second surface-active component such as a metal oxide or a zeolite structure. The most common way to distribute the desired metal ions across the support surface is impregnation with an aqueous solution of a suitable soluble salt. Nitrates or simple salts of organic acids (e.g., acetate, oxalate) are preferred, since these anions are completely removable by thermal treatment. The impregnation conditions (concentration, solid/liquid ratio, temperature, and pH value) can vary over a broad range and depend on the metal type and the desired amount.

A specific example of the coating of a ceramic foam with active component and its activation is given in [11]: Pt was deposited by impregnation of an a-Al2O3 ceramic foam with a saturated solution of hexachloroplatinic acid (H2PtCl6) in water. After

458 Part 5 Applications

Monolith: extruded honeycomb or ceramic foam

Application of active phase

Activation procedure, e.g. ion exchange, reduction, calcination

Monolith catalyst: ceramic honeycomb catalyst, ceramic foam catalyst

Fig. 3 Path leading to a monolithic catalyst. Adopted from

Ref. [7] and modified.

impregnation the monolith was dried in nitrogen, calcined in air, and reduced in hydrogen. The Pt amount was 3–6 wt %. To fabricate a Pt–Au bimetallic catalyst the Pt/a-Al2O3 catalyst was impregnated thrice with a warm, saturated solution of gold trichloride (AuCl3) in water, and the calcination and reduction steps were repeated as described above.

5.4.2.3

Zeolite Coating: A Combination of High Surface Area and Catalytic Activity

Zeolites are a large group of crystalline, porous materials with a broad diversity of porosity (type of pore system and pore size created by the specific order of the crystallographic arrangement) and chemical composition [12, 13]. Some special zeolites are used in a great number of catalytic and sorption processes. Due to their welldefined pore sizes zeolites can restrict accessibility to the educts (educt selectivity), products (product selectivity), or transition states (transition state selectivity) during a reaction. In combination with their adjustable acidity and their capability as metal (cluster) hosts, the catalytic properties of those catalysts can be tailored over a wide range [14–17].

5.4 Heterogeneously Catalyzed Processes with Porous Cellular Ceramic Monoliths 459

Since zeolites are powders with a particle size in the micrometer range after hydrothermal synthesis, they must be shaped before use in technical applications. Instead of conversion to shaped bodies they can also be attached to the surface of ceramic honeycombs or of ceramic foams.

The wash coating procedures (dip, wash, or slurry coating [18]) make use of binders such as alumina, silica, or layered silicates. Other additives are organic agents such as cellulose or starch to improve adhesion between the zeolite-loaded slurry and the support surface during the coating procedure. Depending on the coating procedure and the texture of the support the solid/liquid ratio of such slurries can vary between 1/1 and 1/100. Considering the viscosity of the slurries a minimum cell size of the ceramic foams or a minimum channel size of the ceramic honeycomb is required, and pore filling can be accelerated by vacuum treatment. To achieve a homogeneous layer either the amount and composition of the slurry is adjusted such that it completely soaks the support, or the excess slurry can be drained. The remaining solvent can be removed, for example, by microwave drying

[19]or thermal treatment at slightly elevated temperatures, and this is often followed by a calcination/activation step.

Whereas the slurry and dip-coating methods either require the addition of binder

[20]or are very limited in terms of layer thickness [21], in situ crystallization of zeolites on ceramic surfaces can be used to prepare binder-free zeolite layers with good adherence to the ceramic surface and a broad variability of zeolite structure, chemical composition, particle size, and controlled layer thickness. The first work to synthesis zeolite on porous ceramics can be traced back to attempts to synthesize dense layers of zeolite A or silicalite on a-Al2O3 for membrane applications [22]. Further work was carried out on commercially available ceramic honeycombs and foams made of silicon carbide, a- and c-alumina, mullite, cordierite, and zirconia, or alternatively with polymer-derived microcellular SiOC foams. These ceramics have a high chemical resistance with respect to zeolite crystallization conditions, such as a high alkalinity up to pH 14, temperatures up to 200 C, and autogenous water pressure at this temperature. The advantage of the use of “inert” materials is that the synthesis procedures can easily be covered by recipes from conventional zeolite powder syntheses, for instance, as described in Refs. [23–26].

Whereas the above-mentioned materials were considered to be inert supports

under common conditions of zeolite synthesis, other porous ceramics act not only as supports but also as a source of framework builders (SiIV and AlIII) of the zeolite.

Examples are ceramic foams fabricated by a self-foaming procedure of specific silicone resins (see Chapter 2.1.) with a tailored amount of SiO2, SiC, Si metal, or c-Al2O3. Such tailored materials allow partial transformation of the porous ceramic into the desired zeolite type by hydrothermal treatment [27, 28]. The ceramic foam acts as substrate and as silicon source under synthesis conditions (175 C, tetrapropylammonium hydroxide in sealed, Teflon-lined stainless steel autoclaves under autogenous pressure). Figure 4 shows an SEM image of a zeolite silicalite-1 layer on the surface of a polymer-derived ceramic foam (pyrolyzed in argon at 1000 C) after 72 h of zeolite synthesis time.

5.4 Heterogeneously Catalyzed Processes with Porous Cellular Ceramic Monoliths 461

5.4.3

Some Catalytic Processes with Honeycomb Catalysts

Monolith catalysts were long confined to applications in gas–solid processes, mainly in the area of exhaust gas purification. The largest application is in the automotive industry for cleaning exhaust gases. A short and general overview of the applications of monoliths for gas-phase catalytic reactions covering ceramic honeycombs, ceramic foams, and metallic honeycombs is given in Ref. [30]. These applications are:

In recent years, monolithic catalysts were also used for multiphase reactions, in which gas and liquid phase pass the reactor simultaneously.

The following sections deal with catalytic applications of honeycomb catalysts, starting with the three-way catalysts for automobile exhaust cleaning, diesel engines, and NOx removal from industrial flue gas, followed by other applications in catalysis, in both gas-phase and multiphase reactions, mainly research applications.

5.4.3.1

Automotive Catalysts

Based on the number of the monoliths the cleaning of automotive exhaust gas is the most common use of honeycomb catalysts. The exhaust gases emitted by combustion engines contain, besides carbon dioxide (CO2) and water, nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (HC), sulfur dioxide (SO2), and oxygen in variable amounts depending on the manner of driving and type of engine. The main pollutants which should be removed are carbon monoxide, hydrocarbons, and nitrogen oxides. Catalytic purification of exhaust gases has been known since the 1930s [31]. A patent on the production of structured ceramic articles by Corning [32], submitted in September 1958, refers to the use of honeycomb monolithic structures as catalyst supports in emission control. Starting from 1970, due to air pollution control acts, first total oxidation catalysts [33, 34], the predecessors of the gasoline three-way catalysts (TWC), were introduced as exhaust gas cleaning catalysts [35, 36]. As catalyst support, activated alumina was used in the form of beads and honeycomb monolith. Active metal was in both cases platinum and palladium [37]. These oxidation catalysts merely removed CO and hydrocarbons from the exhaust gas stream, and NOx passed unhindered. The breakthrough for the solution of this problem was achieved in 1976 by Volvo with the introduction of the TWC with lambda probe.