CATALYTIC SYNTHESIS OF 1,1,1,2-TETRAFLUOROETHANE FROM 1,1,1,2-TETRACHLOROETHANE - A MECHANISTIC CONSIDERATION

1,1,1,2-Tetrachloroethane and its fluorinated derivatives as well as t richloroethene are fluorinated by hydrogen fluoride in the presence of a pre-conditioned chromia catalyst. The reaction pathways are derived under different conditions. Fluorinated haloalkanes are formed both b y dehydrochlorination/hydrofluorination mechanism as well as chlorine/ fluorine exchange mechanism. Thus, beside fluorinated alkanes consider able amounts of haloolefins occur in the product mixture. A survey is given on the reaction pathway showing dependence on the reaction condi tions. It is discussed with respect to calculated thermodynamic data. Kinetic and mechanistic investigations of the isomerisation reactions of 1.1.2.2-tetrafluoroethane on a CFC-conditioned chromia catalyst are also presented. The desired 1.1.1.2-tetrafluoroethane can be obtained from its symmetric isomer in the presence of a chromia catalyst condi tioned exclusively with chlorine-free fluorocarbons. Mechanistic infor mation is obtained by employing DCl which behaves similar to HF during the consecutive isomerisation reaction of 1.1.2.2-tetrafluoroethane. Thus, it is most probable that dehydrohalogenation/hydrohalogenation p rocesses (elimination/addition mechanism) are mainly responsible for t he formation of the haloolefins and halocarbons observed on chromia.