alumina chromium derivatives in particular a chromium iii oxide or mixed catalysts based on chromiumand another constituent such as for example nickel magnesium or zinc in making use of the process according to the invention more particular preference is given to bulk cr 2 o 3 catalysts bulk ni cr catalysts as described in patent application fr 9115228 the content of which is incorporated here by reference and mixed catalysts based on nickel and chromium oxides halides and or oxyhalides deposited on a support consisting of aluminium fluoride or of a mixture of aluminium fluorides and alumina this last type of catalyst is described in patent application fr 2 669 022 the content of which is incorporated here by reference in the case of a given catalyst the operating conditions of the gas phase fluorination of the f125 f115 mixture to be purified depend on the desired target in respect of residual f115 and are generally the following a the reaction temperature is generally between 300 and 600 u00b0 c the higher it is the higher the conversion of f115 however since excessively high temperatures may give rise to a considerable decomposition of f125 and may also be prejudicial to catalyst lifetime the preferred temperature range is between 350 and 550 u00b0 c and more particularly between 380 and.