Production of difluoromethane

Details Production of difluoromethane Production of difluoromethane

1995-09-06

1997-09-30

Siegel, Alan

Organic compounds -- part of the class 532-570 series

Organic compounds

Halogen containing

570166, 570167, 570168, 570169, C07C 1708

Patent

active

056727860

DESCRIPTION:

BRIEF SUMMARY
This application is a 377 of PCT/GB94/00497 filed Mar. 14, 1994, now abandoned.
This invention relates to a process for the production of difluoromethane.
In recent years there has been increasing international concern that chlorofluorocarbons, which are used on a large scale around the world, may be damaging the earth's protective ozone layer and there is now in place international legislation to ensure that their manufacture and use is completely phased out. Chlorofluorocarbons are used, for example, as refrigerants, as foam blowing agents, as cleaning solvents and as propellants for aerosol sprays in which the variety of applications is virtually unlimited. Consequently, much effort is being devoted to finding suitable replacements for chlorofluorocarbons which will perform satisfactorily in the many applications in which chlorofluorocarbons are used but which will not have the aforementioned damaging effect on the ozone layer. One approach in the search for suitable replacements has centred on fluorocarbons which do not contain chlorine but which contain hydrogen. The hydrofluorocarbon difluoromethane, also known as HFA 32, is of interest as one such replacement, in particular in a blend thereof with other hydrofluoroalkanes, for example HFA 134a and HFA 125, as a replacement for R-22 and R-502 in refrigeration, air-conditioning and other applications.
Processes have been proposed for the production of difluoromethane. Thus, in U.S. Pat. No. 2,744,148, there is described a process for the production of difluoromethane comprising contacting dichloromethane with hydrogen fluoride in the presence of a fluorination catalyst which comprises nickel, chromium, cobalt, copper or palladium carried on aluminium fluoride. Many other catalysts have been proposed for use in the hydrofluorination of dichloromethane, for example, chromium fluoride on alumina in U.S. Pat. No. 4,147,733; aluminium fluoride, chromium fluoride, mixtures thereof, aluminium fluoride on active carbon or ferric chloride on active carbon in EP 128510; chromium oxyfluoride in U.S. Pat. No. 2,745,886 and chromia in GB 1,307,224.
However, a serious problem with the production of difluoromethane by the hydrofluorination of dichloromethane is that a substantial amount of a highly toxic by-product, monochloromonofluoromethane, HCFC 31, is produced as an intermediate. HCFC 31 has an estimated Occupational Exposure Limit of 10 parts per billion, and may be produced in substantial quantities, indeed as much as 20% or more of the product from the hydrofluorination of dichloromethane.
We have now found that, rather than following the obvious course in finding a solution to this problem, that is to search for conditions under which the production of HCFC 31 is reduced, the problem may be solved by suppressing the toxicity problems associated with the HCFC 31 which is produced.
According to the present invention there is provided a process for the production of difluoromethane comprising: (a) contacting dichloromethane with hydrogen fluoride in the presence of a fluorination catalyst to produce a product stream comprising difluoromethane, monochloromonofluoromethane and unreacted starting material and (b) separating difluoromethane from the product stream from step (a), wherein sufficient hydrogen fluoride is employed in the process such that during step (b) the molar ratio of hydrogen fluoride to monochloromonofluoromethane is at least about 100:1.
Preferably during step (b) the molar ratio of hydrogen fluoride to monochloromonofluoromethane is at least about 150:1, more preferably at least about 200:1 and especially at least about 300:1.
The Occupational Exposure Limit (O.E.L.) for HCFC 31 is estimated at 10 parts per billion, whilst that for hydrogen fluoride is 3 parts per million. Whilst hydrogen fluoride is therefore toxic, it is estimated to be about 300 times less toxic than HCFC 31. Furthermore, the toxicity problems associated with hydrogen fluoride usually exist in reactions in which it is employed as the reagent, and in particular where it is

REFERENCES:
patent: 2744148 (1956-05-01), Ruh et al.
patent: 2745886 (1956-05-01), Ruh et al.
patent: 5208395 (1993-05-01), Elsheikh
patent: 5495057 (1996-02-01), Nam et al.
patent: 5569795 (1996-10-01), Tsuji et al.

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