The invention relates to a method for the preparation of perfluoroalkanes by reacting mixtures which contain perfluoroalkanes and hydrogen-containing polyfluoroalkanes with elemental fluorine under pressure at elevated temperature. 
Perfluorinated organic compounds are chemically very stable, non-combustible, non-toxic and odorless.  Due to their high capacity to dissolve oxygen, they are used, inter alia, as blood replacements.  They also are useful for other purposes, such as heat-transfer fluids or as solvents.  In this case, it is advantageous that the perfluorinated organic material be as free as possible of polyfluorinated compounds, since polyfluorinated compounds may be toxic and do not have the desired stability of perfluorinated organic compounds. 
Various methods are known for preparing perfluorinated compounds.  Examples include methods based on the exchange of hydrogen atoms for fluorine atoms, e.g. electrofluorination, fluorination with metal fluorides such as AgF.sub.2, or reaction with elemental fluorine.  In these methods, the starting material may appropriately comprise non-fluorinated or partially fluorinated compounds. 
There are various methods for purifying mixtures obtained during such processes which contain perfluorinated and polyfluorinated organic compounds.  Reacting the mixtures with strong bases involves loss of material, since the polyfluorinated compounds are destroyed and removed in this process.  Processes in which the polyfluorinated compounds are converted into perfluorinated compounds are preferable.  For example, U.S. Pat.  No.  4,220,606 discloses a method in which hydrocarbons are reacted with silver fluoride, cobalt fluoride or sulfur fluorides in three stages under increasingly severe conditions.  In this case, it is also possible in the third stage to start from partially fluorinated material which has been obtained in a different manner.  The method is very involved. 
U.S. Pat.  No.  2,496,115 discloses a method for stabilizing perfluorocarbons in which a mixture of perfluorinated and polyfluorinated compounds is treated with elemental fluorine in order to convert polyfluorinated compounds into perfluorinated compounds.  A similar procedure is disclosed in published European patent application No.  EP 271,272.  In EP 271,272, fluorine, which is optionally diluted with inert gas such as nitrogen, is passed through the mixture to be treated.  U.S. Pat.  No.  2,496,115 describes in Example 1 that complete perfluorination is only achieved when the reaction temperature is increased from an initial 150.degree.  C. to 300.degree.  C. Under these conditions, the yield is only 78%. 
Despite the efforts of the prior art, there remains a need for better methods of producing perfluoroalkanes free of polyfluoroalkanes.