Halogenation reactions, alumina surfaces

At first sight, the catalytic behaviour and the surface properties of pyrochlore Al(OH,F)3 does not fit this model since the pyrochlore structure (cf Fig. 9(c)) is a more open one than the HTB-A1F3 structure. However, since aluminium hydroxofluoride is susceptible to thermal decomposition, it is in fact no longer pyrochlore Al(OH,F)3 under the temperature conditions employed for the catalytic reactions. Thus, the behaviour of this phase in heterogeneous catalytic halogen exchange can be explained by the presence of amorphous alumina which determines the surface characteristics at the initial stage. Consequently, this phase acts in a manner similar to alumina and not until the surface becomes completely fluorinated does it reach its full catalytic activity. 

In neither case was it possible to propose definitive mechanisms due to the complexity of the systems in the 7-alumina study, it is suggested that adsorption-desorption processes are slow relative to rapid dismutation between two adsorbed species [105], while from the chromia study mono-molecular halogen exchange reactions with metal halide surface sites are indicated [38], The latter mechanism is reminiscent of the halogen exchange model proposed [95] for C2 CFCs on fluorinated chromia. 

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. 

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