Organic compounds -- part of the class 532-570 series – Organic compounds – Halogen containing
Reexamination Certificate
2002-07-12
2004-04-20
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Halogen containing
C570S165000, C570S166000, C570S167000, C570S168000
Reexamination Certificate
active
06723887
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for the preparation of difluoromethane (HFC-32) by vapor phase fluorination of methylene chloride with anhydrous hydrogen fluoride (AHF) using a coprecipitated chromia-alumina impregnated with a zinc salt as catalyst and purification of difluoromethane thus obtained, after removal of HCl and heavier components for enhancing the forward reaction of HCFC-31 to HFC-32, is passed over the fluorination catalyst in a second reactor to reduce the concentration of chlorofluoromethane (HCFC-31) by converting to pure HFC-32.
BACKGROUND OF THE INVENTION
Hydrochlorofluorocarbons and Chlorofluorocarbons are used, extensively, as foam blowing agents, refrigerants, cleaning solvents and propellants for aerosol sprays. However, in the recent years there has been increasing international concern that chlorofluorocarbons may be damaging the earth's protective ozone layer, which helps in containing the UV rays coming to earth. To this effect an international legislation has been put in place to ensure that their manufacture/use is phased out, completely. Extensive research work is being done to take care of the said concern and efforts are on to find suitable replacement(s) for chlorofluorocarbons. One such replacement could be fluorocarbons which do not contain chlorine, but which contain hydrogen.
The hydrofluorocarbon difluoromethane (HFC-32) is of interest as a substitute by itself or in the form of a blend with other hydrofluoroalkanes like 1,1,1,2-tetrafluoroethane (HFC-134a) and pentafluoraethane (HFC-125) for substituting ozone depleting HCFC's like chlorodifluoromethane (R-22) and R-502 in refrigeration, air-conditioning and other applications.
PRIOR ART
It is known in the art that the catalytic vapor phase fluorination of haloalkanes containing at least one halogen atom selected from chlorine or bromine in the molecule with hydrogen fluoride results in the formation of fluorine rich haloalkanes. In the early stages aluminium fluoride and chromium fluoride were found suitable as catalysts for vapour phase fluorination of haloalkanes.
One of the earliest patent, U.S. Pat. No. 2,744,148, disclosed a metal halide of elements selected from nickel, chromium, cobalt, copper or palladium carried on aluminium fluoride as catalyst for fluorination of methylene chloride. However, the yield of HFC-32 was only 15%. The use of an oxygenated chromium trifluoride disclosed in U.S. Pat. No. 2,745,886 gave an yield of 35.7%.
The fluorination of methylene chloride to give HFC-32 involves two reaction steps. The first step is the exchange of one chlorine in dichloromethane by fluorine to give chrofluoromethane (HCFC-31). In the second step HCFC-31 is further fluorinated to give HFC-32. Both the steps referred above are reversible. All subsequent efforts were directed to the development of the fluorination catalyst that gave high conversion of methylene chloride with high selectivity of HFC-32.
UK patent GB 1307224 teaches the preparation of a chromium oxide catalyst for use in fluorination reaction. The examples carried out with methylene chloride and HF at 320° C. gave a conversion of 51.7% and selectivity of 63.8% for HFC-32. The U.S. Pat. Nos. 6,337,299 and 6,300,531 reports use of chromium oxide as a fluorination catalyst for methylene chloride conversion to HFC-32. A conversion of 62% and selectivity of 80% for HFC-32 was reported. The U.S. Pat. No. 5,569,795 utilising chromia based, U.S. Pat. No. 5,900,514 utilising bulk chromia, U.S. Pat. No. 6,242,659 utilising chromium oxide with nickel and U.S. Pat. No. 5,763,704 utilising co-precipitated chromium oxide and Zinc as fluorination catalyst for the conversion of methylene chloride to HFC-32 is known. The highest conversion and selectivity of 99% and 93%, respectively were reported in U.S. Pat. No. 5,763,704 using Zn/Cr2O3 catalyst. However this was achieved only when the mole ratio of HF to methylene chloride was as high as 27.
The U.S. Pat. No. 5,710,353 reports the use of alumina or compounds of the element selected from Ti, V, Zr, Ge, Sn, Mo and Pb on alumina for the preparation of fluorination catalyst. This patent reports a very low conversion of 16.4% and a selectivity of 46.3% for HFC-32 using alumina alone. But the co-precipitated TiO
2
/Al
2
O
3
gave a conversion of 75.7% and selectivity of 82% at a high mole ratio of HF to methylene chloride of about 17.
EP Patent 0128510 reports the use of aluminium chloride or alumina or mixed halide of chromia and alumina or mixed oxides of chromia and alumina or ferric chloride on carbon to generate catalysts for the fluorination of methylene chloride to HFC-32. The highest conversion of methylene chloride reported was 93% with 82% selectivity for HFC-32.
U.S. Pat. Nos. 5,155,082 and 5,763,708 report the use of a mixed oxide catalyst comprising CrO
3
/Al
2
O
3
for the fluorination of methylene chloride to give HFC-32. The conversion and selectivity obtained was 82% and 89% in U.S. Pat. No. 5,763,708. EP 0805136 discloses a fluorination catalyst prepared by chromium and nickel compounds on aluminium fluoride for the fluorination of methylene chloride to HFC-32 to obtain a 61% conversion of methylene chloride and selectivity of 96% for HFC-32.
A serious problem with the production of difluoromethane by the fluorination of dichloromethane is that a substantial amount of a highly toxic intermediate, chlorofluoromethane (HCFC 31), is produced as an impurity.
To address this impurity, the conventional distillation process is of little help despite the difference in relative volatility between HCFC-31 (b.p. −9.1 degree. C.) and HCFC-32 (b.p.: −51.7 degree C.) to bring down HCFC-31 below permissible limits.
European patent application EP 0508 630 describes a process for the contact of HFC-32 with an activated carbon to lower the HCFC-31 impurity in HFC-32. However, in the process, the selectivity is not very high and difluoromethane is adsorbed in the same proportions as HCFC-31, which in turn takes away the useful product as well as exhausts the capacity of the adsorbent at a faster rate than desired.
The use of molecular sieves for the purification of fluorohydrocarbons is known in the art. The purification treatments are usually performed at around ambient temperature. A process has been described in U.S. Pat. No. 5,608,129, to remove the traces of HCFC-31 present in HFC-32 by passing over a 13×molecular sieve at a temperature of at least 60 degree C.
An article by Rao, J. M. et al [See J. M. Rao et al Journal of Fluorine Chemistry 95 (1999) 177-180] teaches a process for preparing a catalyst based on co-precipitated chromia-alumina doped with zinc and/or magnesium involving the steps of co-precipitating chromia and alumina, washing and drying, shaping and impregnation of zinc chloride. The article also explains the occurrence of dismutation reaction in catalytical vapor phase halogen exchange reaction, which was attributed to the presence of strong acid centres in the catalyst. PCT application no. WO 01/74483 also mentions coprecipitated catalyst promoted with zinc.
SUMMARY OF THE INVENTION
The main object of the present invention is to provide a process for the preparation of difluoromethane and minimize the quantity of HCFC-31 in the product stream.
The coprecipitated chromia-alumina with zinc salt catalyst, when employed for the fluorination of methylene chloride, exhibited higher conversion and higher selectivity for HFC-32 under specific process conditions and also minimising formation of side products like methyl chloride and chloroform. The invention also embodies the reduction in the relative percentage of strong acid sites in the catalyst in order to achieve high selectivity.
The present invention relates to a process for the preparation of difluoromethane (HFC-32) by vapor phase fluorination of methylene chloride with anhydrous hydrogen fluoride (AHF) in the presence of a coprecipitated chromia-alumina impregnated with zinc salt as catalyst at a temperature ranging from 225-375° C., preferably 250
Anand Rajdeep
Jain Anurag
Ramanathan Rajasekaran
Rao Jampani Madhusudana
Price Elvis O.
Richter Johann
Schweitzer Cornman Gross & Bondell LLP
SFR Limited
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