F125 is an HFC (hydrofluorocarbon) which can be employed as substitute for F115 (a CFC or chlorofluorocarbon) in the field of low-temperature refrigeration.
Known processes for obtaining F125 include, inter alia, the fluorination of perchloroethylene or its fluorinated derivatives like 1,1-difluoro-l,2,2-trichloroethane (F122), 1,1-dichloro-2,2,2-trifluoroethane (F123) and 1-chloro-1,2,2,2-tetrafluoroethane (F124), the fluorination of chlorotrifluorethylene (F1113) or the chemical reduction of F115, especially the hydrogenolysis of the latter. In most cases these synthesis routes to F125 produce a crude F125 which is contaminated by significant quantities (several hundred ppm to several tens per cent) of F115, either through formation of byproducts due to the high temperatures needed to obtain high yields of F125 (in the case of fluorination reactions), or because of nonquantitative conversions of the starting material (in the case of the hydrogenolysis of F115).
As indicated in U.S. Pat. No. 5,087,329, the separation of the compounds F125 and F115 by distillation is very difficult, or even impossible if it is desired to obtain a very low F115 content.
Nevertheless, there is at present a trend towards a commercial F125 of high purity in which the F115 content might be of the order of a hundred ppm or so. Because of the difficulties encountered in separating the compounds F125 and F115, obtaining such low F115 contents does not appear to be feasible with a distillation line of realistic size. Processes for purifying F125 which make it possible to gain access to these very low F115 contents are therefore being investigated at present.
The process for separating F125 and F115 which is described in the abovementioned U.S. Pat. No. 5,087,329 relies on extractive distillation consisting in adding to the F125-F115 mixture before distillation an extractant consisting of an optionally hydrogenated and/or chlorinated C.sub.1 -C.sub.4 fluorinated hydrocarbon which has a boiling point of between -39.degree. C. and +50.degree. C. As examples of such extractants the patent mentions 1,2-dichlorotetrafluoroethane (F114), 1,1-dichlorotetrafluoroethane (F114a), 1,1,2-trichlorotrifluoroethane (F113), 1,1,1-trichlorotrifluoroethane (F113a), 2-chloro-1,1,1,2-tetrafluoroethane (F124), 2,2-dichloro-1,1,1-trifluoroethane (F123), trichlorofluoromethane (F11) and octafluorocyclobutane (F-C318).
In Patent Application EP 508,631, which describes the production of HFC compounds by liquid-phase chemical reduction of chlorinated, brominated or iodinated compounds with a metal hydride or a complex of such a hydride, it is indicated that this process may be of interest for purifying certain HFCs like F125. The purification of F125 is not exemplified. On the other hand, example 6 describes the reduction of F115 with LiAlH.sub.4 in diglyme; the best result indicates a 60% conversion of F115 to F125.