HFC-125 (pentafluoroethane) can be synthesized by reaction in gas phase of HCFC-124 (1, 1, 1, 2-tetrafluoro-2-chloroethane) with HF over chromium oxides supported on Al_F_3. The AlF_3 support shows catalytic activity in the halogen exchange reaction but also in the production of CFC-1113 (1, 1, 2-trifluoro-2-chloroethene) by 1, 2 HF elimination and in the production of oligomers adsorbed on the catalyst. The introduction on the support of amorphous or microcrystalline phases of chromium oxide determines a progressive increase of the catalytic activity and a reduction of the yield in the olefmic by-product CFC-1113. To limit the production of oligomers, the composition of the catalyst is modified by introduction of elements like magnesium and calcium both by coimpregnation and by sequence impregnation.
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