Mechanistic aspects of the isomerization reactions of 1,1,2,2-tetrafluoroethane on a CFC-conditioned chromia catalyst

Article abstract of DOI:10.1016/S0022-1139(96)03419-7

The mechanism of the isomerization reactions of 1,1,2,2-tetrafluoroethane on a CFC-conditioned chromia catalyst was investigated. The desired 1,1,1,2-tetrafluoroethane can be obtained from its symmetric isomer in the presence of a chromia catalyst conditioned exclusively with chlorine-free fluorocarbons. In the presence of chlorofluorocarbons, hydrogen chloride is formed as a result of the decomposition reactions of the haloalkane used and is strongly adsorbed on to the surface. This HCl causes a very complex system of side-reactions. In addition, a second kind of chlorine, inactive chloride bonded to chromium, is formed which can only be removed as the result of a very slow solid-gas reaction. Mechanistic information was obtained by examining the effects of substituting DCl for HCl as the chlorine source for the reaction with consecutive isomerization products of 1,1,2,2-tetrafluoroethane. Thus, it is most probable that dehydrohalogenation/hydrohalogenation processes (elimination/addition mechanism) are mainly responsible for the formation of the halo-olefins and halocarbons observed on chromia. As the HCl/DCl adsorbed on the surface is very quickly removed by the reactions with the halocarbons and halo-olefins, it is advantageous to employ pulse techniques to evaluate this reaction system.