Catalytic synthesis of 1,1,1,2-tetrafluoroethane from 1,1,1,2-tetrachloroethane - A mechanistic consideration

Abstract

1,1,1,2-Tetrachloroethane and its fluorinated derivatives as well as trichloroethene 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 by dehydrochlorinationlhydrofluorination mechanism as well as chlorine/ fluorine exchange mechanism. Thus, beside fluorinated alkanes considerable amounts of haloolefins occur in the product mixture.A survey is given on the reaction pathway showing dependence on the reaction conditions. It is discussed with respect to calculated thermodynamic data. Kinetic and mechanistic investigations of the isomerisation reactions of 1.1.2.2-tetrafhioroethane 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 conditioned exclusively with chlorine-free fluorocarbons. Mechanistic information is obtained by employing DCI which behaves similar to HF during the consecutive isomerisation reaction of 1.1.2.2-tetrafluoroethane. Thus, it is most probable that dehydrohalogenationlhydrohalogenation processes (elimination/addition mechanism) are mainly responsible for the formation of the haloolefins and halocarbons observed on chromia.