The Design and Preparation of Supported Catalysts

BY G. J. K. ACRES, A. J. BIRD, J. W. JENKINS AND F. KING

1 Introduction

In this Report of catalyst-preparation technology we have placed particular emphasis on catalyst design as opposed to preparation. A properly designed catalyst should have the essential attributes of activity, stability, selectivity, and regenerability. These can be related to the physical and chemical properties of the catalyst, which in turn can be related to the variable parameters inherent in the method used for the preparation of the catalyst. In the past much of the literature on supported catalysts has not included this information. In part this was due to the lack of techniques for physically and chemically characterizing supported catalysts. Many advances have been made in recent years in this area, as described in Chapter 2, so that the design of supported catalysts has become a feasible activity.

In addition to a wide range of techniques for the preparation of supported catalysts a substantial number of supports are available for such systems. In this Chapter we highlight the technology of catalyst preparation and the role of the support in its application. In Table 1 are listed the total U.S. sales of catalyst support materials for 1977.

The predominence of alumina and zeolites is reflected in the literature on the preparation of supported catalysts and hence in the contents of this Chapter.

2 General Methods of Preparation for Supported Catalyst Systems

The principal catalyst-preparation technique involves two stages. First, rendering a metal-salt component into a finely divided form on a support and secondly; conversion of the supported metal salt to a metallic or oxide state.

The first stage is known as dispersion and is achieved by impregnation, adsorption from solution, co-precipation, or deposition, while the second stage is variously called calcination or reduction. It is brought about by a thermal treatment in either an inert atmosphere or an active atmosphere of either oxygen or hydrogen. When the active atmosphere is hydrogen the process is known as reduction. Although calcination/reduction can cause major problems in catalyst preparation on a large scale, it is a generalization to say that once the metal species has been bound to the support surface its degree of dispersion and location will be retained during subsequent treatments. This Chapter therefore concentrates on the dispersion stage of catalyst preparation rather than the thermal treatment stage, although where this is known to cause difficulty it is discussed.

The primary aim of applying a catalytically active component to a support is to obtain the catalyst in a highly dispersed form and hence in a highly active form when expressed as a function of the weight of the active component. This feature of supported catalysts is especially important with regard to precious-metal catalysts, because it allows more effective utilization of the metal than can be achieved in bulk-metal systems. However, in the case of base-metal catalysts the use of the support is often primarily aimed at improving the catalyst stability. This can be achieved by suitable interaction between the active material and the support. For example unsupported copper oxide is a very active oxidation catalyst but suffers from thermal instability at high temperatures. However, when copper oxide is supported on a high-surface-area alumina, its thermal stability is improved.

A wide range of techniques has been employed for the incorporation of a catalytically active species onto a support material. A summary of the most widely used techniques is given below as an introduction to later Sections in this Chapter, which describe the more important chemical and physical factors involved in the dispersion of metal salts onto supports and their influence on the activity, selectivity, and durability of the catalyst system.

Impregnation. – Impregnation as a means of supported catalyst preparation is achieved by filling the pores of a support with a solution of the metal salt from which the solvent is subsequently evaporated. The catalyst is prepared either by spraying the support with a solution of the metal compound or by adding the support material to a solution of a suitable metal salt, such that the required weight of the active component is incorporated into the support without the use of excess of solution. This is then followed by drying and subsequent decomposition of the salt at an elevated temperature, either by thermal decomposition or reduction. With this method of preparation it is essential to have an understanding of both chemical and physical properties of the support and the chemistry of the impregnating solution in order to control the physical properties of the finished catalyst. Comment on these factors is reserved for discussion in a later Section of this Chapter. When used for the preparation of mixed metal catalysts, care has to be taken to confirm that a component in an impregnating solution of metal salts is not selectively adsorbed, resulting in an unexpectedly different and undesirable concentration of metals in a mixed-metal catalyst. This technique has been widely used for the preparation of small amounts of catalyst for basic studies,

Adsorption from Solution. – Adsorption is defined as the selective removal of metal salts or metal ion species from their solution by a process of either physisorption or chemical bonding with active sites on the support. Depending upon the strength of adsorption of the adsorbing species, the concentration of the active material through the catalyst particle may be varied and controlled. This technique is widely used in the preparation of industrial catalysts as it permits a greater degree of control over the dispersion and distribution of the active species on the support. In some systems, however, the weight of the active component that can be incorporated into the support is limited. Although multiple adsorption is often possible it is not recommended when close control of physical parameters is required.

Co-precipitation. – The preparation of supported catalysts by the co-precipitation of metal ions with the support ions usually produces an intimate mixing of catalysts and support. An example of this technique is the co-precipitation of metal ions with aluminium ions to produce a precipitated alumina gel...