Deuterium isotope studies of the hydrofluorination of chloroethenes over chromia catalysts

Article abstract of DOI:10.1016/S0022-1139(00)80072-X

The mechanism of the catalytic fluorination of chloroalkenes over a chromia catalyst has been investigated by examining the effects of substituting DF for HF as the fluorine source for reaction with tetrachloroethene and trichloroethene.At a temperature of 250 degC and a HF (DF)/alkene molar ratio of 4.2:1, the rate of conversion of tetrachloroethene is increased by using DF and there is a concomitant increase in the selectivity to some chlorofluoroalkanes and a decrease in the selectivity to chlorofluoroalkenes.The opposite behaviour observed for the halogenated alkenes and alkanes indicates that the alkenes are not important intermediates in the production of the alkanes by a series of hydrofluorination and dehydrochlorination reactions.For tetrachloroethene, the main reaction pathway to the alkanes is a direct chlorine/fluorine exchange over a heavily fluorinated chromia surface with minimal C-H/C-D cleavage of intermediates.Substitution of DF for HF causes no change in the product selectivities from reaction with trichloroethene over chromia catalysts.However, the presence of dideutero - and monodeutero - products shows that both chlorine/fluorine exchange and HF addition / HCl elimination pathways are occurring for the less-substituted alkene.