Organic compounds -- part of the class 532-570 series – Organic compounds – Halogen containing
Reexamination Certificate
1993-11-01
2001-08-14
Siegel, Alan (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Halogen containing
Reexamination Certificate
active
06274781
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the production of halocarbons and their azeotropes and more particularly to the production of difluoromethane (i.e., CH
2
F
2
or HFC-32) and azeotropes of chlorine-containing precursors thereof (i.e., CH
2
ClF or HCFC-31, and CH
2
Cl
2
) with HF.
BACKGROUND
Chlorofluorocarbons (i.e., CFCs) are compounds containing only carbon, fluorine and chlorine. Various CFCs have been used for many years as refrigerants, heat transfer media, foam expansion agents, aerosol propellants, solvents and power cycle working fluids. However, there has been recent concern that CFCs may be detrimental to the Earth's ozone layer. Consequently, there is a worldwide effort to find alternative compounds which contain fewer or preferably no chlorine substituents. The hydrofluorocarbon difluoromethane has been proposed as a replacement for some CFCs, particularly, in refrigeration, air-conditioning and other applications (see e.g., European Patent Application Publication No. 508,660 A1). Accordingly, there is interest in developing efficient processes for the production of HFC-32.
Japanese Patent Publication 59-225131 discloses preparation of HFC-32 by reacting dichloromethane with HF in the vapor phase at 200° to 500° C in the presence of a catalyst which consists of chromium fluoride or which has been obtained by molding a mixture of chromium fluoride and a carrier or by supporting chromium fluoride on a carrier. Chromium fluoride may be obtained by fluorinating a chromium-containing compound. Immersion of activated carbon in a solution of chromium chloride, followed by treatment with HF is disclosed (see e.g., Example 2). It is well known that activated carbon typically has an ash content comprising metal-containing compounds (e.g., potassium and sodium oxides) and other impurities (see e.g., M. Smisek et al. “Active Carbon”,pages 61-70 (1970) and J. W. Hassler “Activated Carbon”,pages 344-345 (1963)).
1,1-Dichlorotetrafluoroethane (i.e., CCl
2
FCF
3
or CFC-114a) is of interest as an intermediate for producing 1,1,1,2-tetrafluoroethane (i.e., CF
3
CH
2
F or HFC-134a). CF
3
CH
2
F can be obtained by the catalytic hydrogenolysis of CCl
2
FCF
3
using a supported metal hydrogenation catalyst (see e.g., C. Gervasutti et al., J. Fluorine Chem., 1981/82, 19, pgs. 1-20). The HFC-134a is an environmentally acceptable potential replacement for CFC refrigerants, blowing agents, aerosol propellants and sterilants that are implicated in the destruction of stratospheric ozone.
SUMMARY OF THE INVENTION
The present invention provides a process for producing difluoromethane. The process comprises the step of contacting a gaseous mixture containing CH
2
Cl
2
and hydrogen fluoride with a catalyst containing a catalytically effective amount of trivalent chromium supported on a carbon having an ash content of less than 0.5 percent by weight, at a temperature of from about 180° C. to about 375° C. The catalyst and temperature conditions of this process allow the concurrent reaction of 1,1,1-trichlorotrifluoroethane (i.e., CCl
3
CF
3
or CFC-113a) with HF to form CFC-114a. CH
2
ClF and unreacted CH
2
Cl
2
, each of which may be recovered as an azeotrope with HF, may be recycled.
The present invention also provides compositions which consist essentially of hydrogen fluoride in combination with an effective amount of a compound selected from the group consisting of CH
2
Cl
2
and CH
2
ClF to form an azeotrope or azeotrope-like composition with hydrogen fluoride, said composition containing from about 5 to 29 mole% CH
2
Cl
2
or from about 58 to 65 mole% CH
2
ClF.
DETAILED DESCRIPTION
The present invention provides a process for the vapor-phase catalytic fluorination of CH
2
Cl
2
to CH
2
F
2
. The process may be used for the vapor-phase catalytic fluorination of mixtures of CH
2
Cl
2
and CCl
3
CF
3
to produce both CH
2
F
2
and CCl
2
FCF
3
. The process of this invention employs catalyst compositions comprising trivalent chromium. In addition to a catalytically effective amount of trivalent chromium such fluorination catalysts can include other components to increase catalyst activity and/or life such as one or more divalent metal cations (e.g., zinc, magnesium and/or cobalt). While unsupported trivalent chromium catalysts (e.g., Cr
2
O
3
) or trivalent chromium catalysts supported on non-carbon supports (e.g., on alumina, aluminum fluoride, or magnesium fluoride) may also catalyze the reaction of CH
2
Cl
2
to CH
2
F
2
, this invention advantageously involves trivalent chromium (e.g., CrCl
3
and/or CrF
3
) supported on a carbon which has an ash content of less than 0.5 weight percent. A CrF
3
on carbon is disclosed in U.S. Pat. No. 3,632,834, the contents of which are incorporated herein by reference. Catalysts suitable for use in the process of this invention can be prepared in a manner similar to that disclosed in U.S. Pat. No. 3,632,834, provided the carbon used has an ash content of less than 0.5 weight percent. Preferred catalysts include a low ash content carbon support (as described herein) containing chromium chloride (CrCl
3
), fluorided chromium chloride (e.g., CrCl
3
treated with HF to produce chromium chlorofluoride(s)), or mixtures of chromium chloride and chromium fluoride (CrF
3
). While such low-ash carbon supports may be obtained using a variety of methods, a preferred carbon support is acid-washing activated carbon prior to impregnating it with trivalent chromium. Preferably the chromium content (expressed as CrCl
3
) is from about 5 to 60 weight percent of the carbon supported catalyst.
The initial acid treatment typically uses an acid other than hydrofluoric acid. Preferred acids used for the acid treatment contain neither phosphorus nor sulfur. Examples of acids which may be used in the initial acid wash during the catalyst preparation process include organic acids such as acetic acid and inorganic acids such as hydrochloric acid or nitric acid. Preferably, hydrochloric acid or nitric acid is used. The second acid treatment, when employed, advantageously uses hydrofluoric acid. Preferably, the carbon is treated with acid such that after such treatment the carbon contains less than about 0.2% by weight ash.
Commercially available carbons which may be treated with acids to provide suitable supports include those sold under the following trademarks: Darco™, Nuchar™, Columbia SBV™, Columbia MBV™, Columbia MBQ™, Columbia JXC™, Columbia CXC™,Calgon PCB™, Norit™ and Barnaby Cheny NB™.The carbon support can be in the form of powder, granules, or pellets, etc.
The acid treatment may be accomplished in several ways. A suitable procedure is as follows. A carbon support is soaked overnight with gentle stirring in a 1 molar solution of the acid prepared in deionized water. The carbon support is then separated and washed with deionized water until the pH of the washings is about 3. Preferably, the carbon support is then soaked again with gentle stirring in a 1 molar solution of the acid prepared in deionized water for 12 to 24 hours. The carbon support is then finally washed with deionized water until the washings are substantially free of the anion of the acid (e.g., Cl
-13
or NO
3
-13
), when tested by standard procedures. The carbon support is then separated and dried at about 120° C. The washed carbon is then soaked, if necessary, in 1 molar HF prepared in deionized water for about 48 hours at room temperature with occasional stirring. The carbon support is separated and washed repeatedly with deionized water until the pH of the washings is greater than 4. The carbon support is then dried followed by calcination at 300° C. for about 3 hours in air prior to its use as a support. Reference is made to U.S. Pat. No. 5,136,113 for further details relating to producing acid-washed carbon catalysts.
Typically, where the starting material to be fluorinated consists essentially of CH
2
Cl
2
, the molar ratio of HF added to CH
2
Cl
2
starting material added (whether or not CH
2
ClF, unreacted CH
2
Cl
2
and/or HF are also recycled) ranges from about 0.5:1 t
Furmanek Paul S.
Glasscock David A.
Keane, Jr. Michael
Mahler Barry A.
Rao V. N. Mallikarjuna
E. I. Du Pont de Nemours and Company
Siegel Alan
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