Silver-impregnated solids

Silver-impregnated solids may also serve as catalysts in [4+2] cycloadditions. Thus, Mayoral reported that silver ion-exchanged montmorillite K10 functioned as a catalyst for the reaction of methyl 2-cyanocinnamate and cyclopentadiene 185 (Scheme 2.49). The use of such heterogeneous catalysts may enhance catalyst recovery, an important consideration for industrial applications. 

All present industrial catalyst systems are based on silver deposited on a slightly porous solid. The most widely used support is x-alumina, but silica-alumina and carborundum can also be employed. The specific surface area of the support, its porosity, and the pore size exert a considerable influence on the metal distribution at the surface, and consequently on catalytic activity. Several techniques are also available for fixing the silver, either by impregnation from a solution, or by deposition from a suspension. An initiator, usually consisting of alkaline earth or alkaline metals, can be added to the catalyst, but other metallic additions have also been recommended. Certain halogenated organic derivatives, such as dichloropropane, may increase selectivity in trace amounts (10 ppm in the feed), by reducing combustion side reactions. 

The efficiency of the adsorption of many gases on solid adsorbents may be increased by using specific impregnation substances. These are applied onto carriers or directly onto the charcoal. In this way it is possible to increase strongly, for instance, the adsorption efficiency for ammonia (with the help of bromine), formaldehyde (with the help of sodium sulphide) and hydrogen sulphide (with the help of silver cyanide or lead acetate). 

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