The Role of Gel Processing in the Preparation of Catalyst Supports
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THE ROLE OF GEL PROCESSING IN THE PREPARATION OF CATALYST SUPPORTS
J.A. CAIRNS, D.L. SEGAL AND J.L. WOODHEAD Chemistry Division, AERE Harwell, Didcot, Oxfordshire,
OXIl ORA, England.
ABSTRACT Catalyst supports are required to meet a wide range of objectives, including chemical purity and composition, optimised surface area/porosity and thermal stability. Gel processing offers means by which many of these requirements can be achieved. In addition, the use of sols of controlled rheology allows the support material to be prepared in a form which facilitates its application to substrates (including metals) as a coating, thereby greatly improving its versatility for producing novel catalysts. The preparation of typical support materials will be discussed, together with examples of their use in catalysis.
INTRODUCTION A catalyst is a substance which accelerates the rate of a chemical reaction. It does so by adsorbing one or more of the reactants on to its surface, thereby converting them to a more active form. Hence, the specific activity of a catalyst (i.e. its activity/mass) can be increased by converting it to a high surface area form. One of the most convenient ways of doing this is to disperse one component of the catalyst (usually a metal or metal oxide) over a high surface area material known as the support. The support plays many roles, apart from acting as a dispersing medium. Thus it minimizes the tendency of the dispersed catalyst phase to aggregate into larger clusters, thereby losing activity; it can shield the dispersed phase from poisons; and it can, by virtue of its structure (surface area and pore size distribution), confer specificity on the catalytic reactions by imposing a restriction on the dimensions of reactants or products. Alumina and silica are two of the most widely used supports. The purpose of this work is to describe the special attributes of sol-gel technology in producing these materials (and others) in a form suitable for use as versatile catalyst supports.
SYNTHESIS OF HETEROGENEOUS CATALYST SUPPORTS The most widely used supports in heterogeneous catalysis are alumina and silica. Therefore we begin with a brief description of how these may be prepared conventionally. The simplest route to the former involves precipitation of hydroxide or hydrous oxide from an aqueous salt solution [1]. The nature of the precipitate, i.e. whether amorphous or crystalline, is determined by the precise precipitation conditions such as rate, concentration, temperature and pH. Furthermore, the degree of crystallinity can be increased by ageing in the aqueous environment, at either ambient or elevated temperatures. For example, boehmite, a crystalline a-alumina monohydrate is prepared by the addition of NH40H to an Al(III) salt solution under controlled conditions of pH. Similarly silica may be prepared by mixing sodium silicate solution with sulphuric acid at both alkaline and acid pHs to produce precipitated silicas and silica gels respectively.
Mat. Res. Soc. Symp. Proc. Vol. 32 (1984)§Elsevier Scienc
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