WO1991009000A1

WO1991009000A1 - Method for the production of bromodifluoromethane

Info

Publication number: WO1991009000A1
Application number: PCT/US1990/007105
Authority: WO
Inventors: Mark L. Robin; Victoria E. Lynch
Original assignee: Great Lakes Chemical Corporation
Priority date: 1989-12-15
Filing date: 1990-12-04
Publication date: 1991-06-27
Also published as: IL96629A0; CA2031887A1

Abstract

A method for the production of bromodifluoromethane includes reacting hydrogen and dibromodifluoromethane at between about 400 °C and about 600 °C and separating the bromodifluoromethane from the reaction product. Contact times, pressure and molar ratios are not critical, although contact times of about 0.1 to about 20.0 seconds, and molar ratios of hydrogen to dibromodifluoromethane of about 0.1 to about 2.0, are preferred. The reaction occurs without catalyst, and high conversion, yield and selectivity are achieved.

Description

METHOD FOR THE PRODUCTION OF BROMODIFLUOROMETHANE

BACKGROUND OF THE INVENTION

Field of the Invention:

The present invention relates to a method for the production of bromodifluoromethane, and particularly to a method having high conversion, yield and selectivity and using dibromodifluoromethane as a starting material.

Description of the Prior Art:

Numerous methods are disclosed in the prior art for the preparation of halogenated alkanes. These methods vary widely, due in part to the different halogens and alkanes involved. The prior art demonstrates that given methods do not readily translate into predictable applications for other compounds. The present invention provides a novel method for the preparation of monobromodifluoromethane by the reduction of dibromodifluoromethane. The reaction is characterized by its surprisingly high conversion, yield and selectivity.

Bromodifluoromethane is a known chemical, having been identified for many years to be useful as a refrigerant for air conditioning units and as a carrier or propellant (e.g., for insecticides, paints, etc.). These uses, are noted in United States Patent No. 2,639,301, issued to Ruh, et al. on May 19, 1953. For at least the forty years since the filing of that patent, there have been efforts to discover methods for the production of this compound.

Bromodifluoromethane has been prepared in one process of the past by the bromination of difluoromethane. This reaction is described in the previously identified Ruh patent, which calls for a vapor phase bromination with molecular bromine at 250°C to 600°C. This procedure has also been conducted for the purpose of preparing dibromodifluoromethane, a known flame re ardant, with the monobromodifluoromethane being formed as a byproduct. The Ruh patent also describes a prior art method for making bromodifluoromethane involving the fluorination of bromoform at 110°C to 120°C by the action of antimony trifluoride in the presence of bromine.

In United States Patent No. 3,210,430, issued to Knight on October 5, 1965, there is described a procedure for the preparation of tetrafluoroethylene which incidentally includes the preparation of bromodifluoromethane. In the Knight process, bromoform and hydrogen fluoride are combined at 100°C to 500°C in the presence of a catalyst, such as an activated, anhydrous chromium oxide, to yield bromodifluoromethane, fluoroform and hydrogen bromide. The bromodifluoromethane is thereafter pyrolyzed at 400°C to

1000°C to tetrafluoroethylene. In a surprising contrast, the present invention is directed to producing the bromodifluoromethane at these temperatures, whereas the Knight Patent indicates that tetrafluroethylene is produced. Other halogenation reactions have been described in the prior art, some of which have involved bromodifluoromethane. In addition to the bromination reaction above referenced, Ruh, et al. have described in United States Patent No. 2,639,300 the vapor phase chlorination of difluoromethane at 100°C to 400°C to yield the monochloro- and dichlorodifluoromethanes. The chlorination of bromodifluoromethane at elevated temperatures to produce bromochlorodifluoromethane is disclosed by Ruh, et al. in United States Patent No. 2,639,302. In accordance with British Patent No. 805,503, issued to Ruh, et al., a fluorination catalyst is prepared by impregnating dry, activated aluminum oxide with a solution of nickel chloride in hydrochloric acid, followed by treatment of the wet particles with anhydrous hydrogen fluoride. Passage of a vapor mixture of carbon tetrabromide and hydrogen fluoride at 300°C yields a mixture including 80% bromotrifluoromethane, dibromofluoromethane, tribromofluoromethane, dibromodifluoromethane and bromofluoromethane. Tetrafluorodichloroacetone subjected to heat in the presence of elemental bromine yields bromochlorodifluoromethane and dibromodifluoromethane, as described in United States Patent No. 2,885,450.

Reductions of fluorinated hydrocarbons are discussed in several prior art references. It has been reported that dibromofluoromethane may be reduced with zinc and ethanol to fluoromethane and bromofluoromethane, as described by F. Swarts, Bull. Sci. Acad. Roy. Belg., p. 113 (1910), referenced in "Aliphatic Fluorine Compounds", by Lovelace et al., Reinhold Publishing Corporation (1958). The reduction of dibromofluoromethane by treatment with an alkali metal amalgam, such as sodium amalgam, in an active hydrogen-containing reaction medium, such as an alcohol, is described in European Patent Application No. 88310323.6, filed February 11, 1988 by Imperial Chemical Industries PLC. This European Patent Application also discloses the electrolytic reduction of dibromofluoromethane in the presence of mercury.

In United States Patent No. 2,615,926, issued to Benrang,, et al. on October 28, 1952, there is described a process for preparing fluorine-containing unsaturated compounds. A saturated chlorofluorocarbon is reacted at 650°C to 800°C with hydrogen, without contact with a catalytic material, to yield tetrafluoroethylene. The Benrang patent states that prior art methods for making tetrafluoroethylene starting with chlorofluoroalkanes and bromofluoroalkanes are disfavored since these starting materials are difficult to obtain and expensive.

The preparation of fluoroform by the reduction reaction of chlorotrifluoromethane with hydrogen at elevated temperature in a palladium plated copper tube is described in United States Patent No. 3,439,052, issued to Bjornson on April 15, 1969. In United States Patent No. 3,042,727, issued to Olstowski, et al. on July 3, 1962, there is described the preparation of fluoroform by the vapor phase hydrogenation of trifluoromethylhalides. The reaction temperatures are in the range of 450°C to 900°C, with conversion and yields comparatively low below 600°C and a copper catalyst being preferred at the lower temperatures. Dichlorodifluoromethane will be reduced at high temperatures, e.g. 685°C, in the presence of hydrogen in a platinum tube, yielding a variety of products including difluoromethane, chlorodifluoromethane, tetrafluoroethylene and tetrafluoroethane. Dichlorodifluoromethane will also be reduced to chlorodifluoromethane, tetrafluoroethylene and chlorotrifluoromethane by reaction with methane at 720-760°C in the presence of platinum or copper, as described in "Methods for the Introduction of Hydrogen into Fluorinated Compounds", Fluorine Chemistry Reviews, Vol. 1(2), pp. 315-358 (1967).

Chlorofluoromethanes or -ethanes can be reduced with a lower alcohol, lower ketone or lower fatty acid at elevated temperature in a nickel tube. For example, dichlorodifluoromethane can be reduced with methanol and hydrogen at 550°C to yield chlorodifluoromethane and methyl chloride, as described in Japanese Patent No. 72 18,725, issued May 30, 1972. Electrochemical reduction of dibromodifluoromethane in electrolyte at platinum electrodes has been reported to result in the generation of difluorocarbene, as described in Fritz, et al., J.

Electroanal. Chem. Interfacial Electrochem. , No. 100, pp. 217-223 (1979) . Electroreduction of dichlorodifluoromethane yields chlorodifluoromethane and tetrafluoroethylene, as described in Stepanova,, et al., Electrokhimiya, Vol. 12(7), pp . H66-7 ( 1976 ) ; Dieta , et al . , J . Appl . Electrochem. , Vol . 3 ( 2 ) , pp . 143-151 ( 1973 ) . SUMMARY DF THE INVENTION

Briefly describing one aspect of the present invention, there is provided a method for the production of bromodifluoromethane which includes reacting hydrogen and dibromodifluoromethane at elevated temperatures, and thereafter recovering the resulting bromodifluoromethane from the reaction mixture.

It is an object of the present invention to provide a method for the production of bromodifluoromethane from readily obtainable starting materials.

A further object of the present invention is to provide a method which has high conversion, yield and selectivity for the desired bromodifluoromethane product.

It is another object of the present invention to provide a method as described which does not produce significant amounts of undesirable by-products.

Further objects and advantages of the present invention will be apparent from the description of the preferred embodiment which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the preferred embodiment and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the invention, and such further applications of the principles of the invention being contemplated as would normally occur to one skilled in the art to which the invention relates.

Bromodifluoromethane has been identified for more than forty years as a compound useful for a variety of applications, including use as a refrigerant or propellant.

The present invention is based on the discovery that dibromodifluoromethane may be reduced with hydrogen at elevated temperatures to yield bromodifluoromethane in high yield. The conversions and selectivities are also unexpectedly high for this process.

The basic method of the present invention involves the hydrogenation of dibromodifluoromethane according to the following reaction (I):

I. ^CF ₂ ^Br2 ^{+ H}2 ^{→ CF}2^{HBr + HBr} The reaction (I) is carried out by contacting the dibromodifluoromethane and the molecular hydrogen at elevated temperature. Although preferred ranges for contact times and molar ratios of reactants are stated hereafter, these ranges are not critical. Also, no significant effect on the reaction (I) has been observed based on pressure, and it therefore may be economically and conveniently performed at about atmospheric pressure. An additional aspect of the present invention is that the described reaction will occur without the presence of a catalyst.

The temperature of the reaction is generally one which is high enough to provide a desired amount and rate of conversion of the dibromodifluoromethane, and low enough to avoid deleterious effects such as the production of decomposition products. The reaction is therefore preferably carried out at a temperature between about 400°C and about 600°C. A more preferred range for the reaction is between about 450°C and about 550°C. It will be appreciated that the selected temperature for the reaction will depend in part on the molar ratios of the reactants and even more so on the time provided for the reaction to occur. For example, in general the desired temperature for the reaction will be inversely related to the time for the reaction.

The method of the present invention involves contacting the dibromodifluoromethane and molecular hydrogen at the elevated temperature for a given period of time. The time of the reaction, or contact time, will vary depending on the extent of conversion desired, the temperature and other factors. As used herein, the term "contact time" refers to the time during which the reactants are held at within about 100°C of the reaction temperature. The appropriate contact time generally will be inversely related to the temperature of the reaction and directly related to the -extent of conversion of the dibromodifluoromethane.

The reaction will typically be conducted as a continuous flow of reactants through a heated reaction vessel in which heating of the reactants may be very rapidly effected. Under these circumstances, the residence time of the reactants within the vessel is desirably between about 0.1 seconds and about 20.0 seconds, and is preferably about 10.0 seconds. An advantage of the reaction is that extremely short contact times are involved, which minimizes the equipment sizing and cost associated with producing the bromodifluoromethane. The reactants may be preheated before combining or may be mixed and heated together as they pass through the vessel. Alternatively, the reaction can be carried out in a batch process with contact time varying accordingly, although this is less preferred.

The molar ratio of the reactants may vary widely and is not critical to the inventive method. Limitations on this ratio are more determined by practical considerations, since excessive amounts of either reactant will not significantly affect the result. For example, a molar ratio of the hydrogen which is extremely low will simply require greater recycle due to the low conversion, whereas a ratio that is very high will be wasteful of the hydrogen. A preferred range for the molar ratio of hydrogen to dibromodifluoromethane is between about 0.1 and about 2.0, with a ratio of about 1.0 being most preferred.

It is a particular aspect and advantage of the present invention that the indicated reaction occurs without the benefit of catalyst. As indicated in the prior art discussion, past reduction reactions for halogenated alkanes have been characterized by the presence of catalysts. The present reaction does not require the presence of a catalyst, and is desirably effected in an inert reactor, such as one of 316 stainless steel. As used herein, the term "inert" is used to indicate that the material of the reaction vessel does not react with any of the reactants or the reaction products, and does not act as a catalyst to the reaction. Aside from the 316 stainless steel material identified, various other inert materials are well known in the art and are therefore not specifically listed herein. The non-catalytic process would also involve the absence of catalyst within the inert reaction vessel.

Also as is well known in the art, the reaction vessel may assume a variety of forms suitable to continuous flow or batch reactions. A preferred embodiment of the invention is to pass the hydrogen and dibromodifluoromethane through an inert tube heated to the desired temperature, with the tube size and flow rates being selected to provide the appropriate contact time within the tube. The inventive process has several advantageous aspects in addition to those already listed. The reaction involves readily obtainable reactants, namely the dibromodifluoromethane and molecular hydrogen. The reaction products are the desired bromodifluoromethane (CF_HBr) and hydrogen bromide (HBr), the latter being a useful byproduct for other applications. Therefore, the reaction is efficient in its use of the reactants without producing miscellaneous undesirable byproducts. The yield of bromodifluoromethane based on dibromodifluoromethane consumed is as high as about 80%. Also, the selectivity of the reaction is as much as 95-100%. These results show the reaction to be a highly efficient and advantageous method for the production of bromodifluoromethane.

Another benefit of the present invention is that it is combinable with an overall production scheme by which dichloromethane is converted to bromodifluoromethane. In the prior art it has been known to convert dichloromethane to dibromodifluoromethane and bromodifluoromethane in a two step process. In the first step, as described in British Patent No. 805,503, dichloromethane is reacted with hydrogen fluoride to produce difluoromethane in accordance with the reaction (II) :

II. ^H ₂ ^C12 + HF → ^CH2^F2 ^{+ 2 HC1} The difluoromethane is then converted to the mono- and dibromodifluoromethanes by reaction with molecular bromine by the following reaction (III) described in U.S. Patent No. 2,639,301:

III. CH₂F₂ + Br₂ → CF₂HBr + CF₂Br₂ This reaction (III) is one of the methods previously identified as being used in the prior art to produce bromodifluoromethane, although it has frequently been carried out for the primary purpose of forming the dibromo- compound. In accordance with the present invention, the dibromodifluoromethane is converted to the monobromo- compound ( I) , with hydrogen bromide being produced as a byproduct. The hydrogen bromide may then be recovered by conventional techniques and converted to molecular bromine for use in the second step (III) above shown. The combination of the above second step (III) and the process of the present invention (I) therefore yields an overall reaction scheme by which dichloromethane, molecular bromine, molecular hydrogen and HF are used to produce bromodifluoromethane in high yields based upon the amount of reactants consumed.

The unexpected and significant efficacy of the present invention is illustrated by the following specific examples.

EXAMPLE 1 15.5 g (0.074 mole) of dibromodifluoromethane and 0.086 mole of hydrogen were simultaneously fed through a one-half by twelve inch 316 ss reactor heated to 450°C in a tube furnace. The contact time for the reactants was 11.0 seconds. The reaction products were passed through a scrubber containing water to facilitate removal of HBr and then through a dry ice cooled trap. A total- of 12.1 grams of product was collected in the dry ice trap and was found to contain 3.0 grams of bromodifluoromethane and 9.0 grams of unreacted dibromodifluoromethane. This corresponds to a yield of CF_HBr of 73% based on consumed CF Br_. The selectivity of CF₂HBr was 96%, and the conversion of CF₂Br₂ was 42%.

EXAMPLE 2 The procedure of Example 1 was followed, except various parameters were manipulated in accordance with Table 1. As these results demonstrate, the present invention is operable in a wide range of temperatures, contact times and molar ratios. Runs 1-3 and 13-14 used identical contact times (10 seconds) and molar ratios (1.0) for the reactants, and resulted in yields ranging from 12% to 41%, and selectivities of 86-98%. Runs 3-9 were conducted at 450°C and a 1.0 molar -11-

ratio, but with varying contact times of 4-14 seconds. The yields were 5-40% and the selectivities were 93-100%. Runs 3 and 10-12 were conducted with the same temperature (450°C) and contact time (10 seconds), but at varying molar ratios ranging from 0.9 to 1.7 (H₂:CF₂Br₂). The resulting yields were 22-35% and selectivities were 92-96%.

TABLE 1

% Yield¹ % Selectivity CF₂HBr CF₂HBr

1 %yield CF₂HBr = % of theoretical moles CF2HBr produced _x I_QO moles CF2Br2 fed

While the invention has been described in detail in the foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been described and that all changes and modifications that come •within the spirit of the invention are desired to be protected.

Claims

What is claimed is:

1. A method for the production of bromodifluoromethane which comprises contacting hydrogen and dibromodifluoromethane in a reactor at a temperature between about 400°C and about 600°C and recovering the bromodifluoromethane from the resulting reaction product.

2. The method of claim 1 in which said contacting in the reactor is for a period of time between about 0.1 and about 20.0 seconds.

3. The method of claim 2 in which said contacting in the reactor is for a period of about 10.0 seconds.

4. The method of claim 1 in which said contacting is of a molar ratio of hydrogen and dibromodifluoromethane between about 0.1 and about 2.0.

5. The method of claim 4 in which said contacting in the reactor is for a period of time between about 0.1 and about 20.0 seconds.

6. The method of claim 5 in which said contacting in the reactor is for a period of about 10.0 seconds.

7. The method of claim 4 in which said contacting is of a molar ratio of hydrogen and dibromodifluoromethane of about 1.0.

8. The method of claim 7 in which said contacting in the reactor is for a period of time between about 0.1 and about 20.0 seconds.

9. The method of claim 8 in which said contacting in the reactor is for a period of about 10.0 seconds.

10. The method of claim 1 in which said contacting is in a reactor at a temperature between about 400°C and about 600°C.

ll. The method of claim 10 in which said contacting in the reactor is for a period of time between about 0.1 and about 20.0 seconds.

12. The method of claim 11 in which said contacting in the reactor is for a period of about 10.0 seconds.

13. The method of claim 10 in which said contacting is of a molar ratio of hydrogen and dibromodifluoromethane between about 0.1 and about 2.0.

14. The method of claim 13 in which said contacting is of a molar ratio of hydrogen and dibromodifluoromethane of about 1.0.

15. The method of claim 1 in which said contacting is in an inert reactor.

16. The method of claim 15 in which said contacting is in a reactor at a temperature between about 450°C and about 550°C.

17. The method of claim 15 in which said contacting in the reactor is for a period of time between about 0.1 and about 20.0 seconds.

18. The method of claim 15 in which said contacting is of a molar ratio of hydrogen and dibromodifluoromethane between about 0.1 and about 2.0.

19. A method for the production of bromodifluoromethane which comprises passing hydrogen and dibromodifluoromethane through an inert tube heated to a temperature between about 400°C and 600°C while avoiding contact with a catalyst, and recovering the bromodifluoromethane from the resulting reaction product.

20. The method of claim 19 in which said passing through the inert tube is to provide a residence time of the hydrogen and the dibromodifluoromethane in the inert tube of between about 0.1 and about 20.0 seconds.

21. The method of claim 20 in which said passing is to provide a residence time of about 10.0 seconds.

22. The method of claim 19 in which said passing is of a molar ratio of hydrogen and dibromodifluoromethane between about 0.1 and about 2.0.

23. The method of claim 22 in which said passing is of a molar ratio of hydrogen and dibromodifluoromethane of about 1.0.

24. The method of claim 22 in which said passing through the inert tube is to provide a residence time of the hydrogen and the dibromodifluoromethane in the inert tube of between about 0.1 and about 20.0 seconds.

25. The method of claim 19 in which said passing is through an inert tube heated to a temperature between about 450°C and about 550°C.

26. The method of claim 25 in which said passing through the inert tube is to provide a residence time of the hydrogen and the dibromodifluoromethane in the inert tube of between about 0.1 and about 20.0 seconds.

27. The method of claim 25 in which said passing is of a molar ratio of hydrogen to dibromodifluoromethane between about 0.1 and about 2.0.

28. The method of claim 27 in which said passing through the inert tube is to provide a residence time of the hydrogen and dibromodifluoromethane in the inert tube of between about 0.1 and about 20.0 seconds.

29. A method for the production of bromodifluoromethane which comprises reacting a mixture of hydrogen and dibromodifluoromethane at a temperature between about 400°C and about 600°C and recovering the bromodifluoromethane from the resulting reaction product.

30. The method of claim 29 and which comprises reacting a mixture having a hydrogen to dibromodifluoromethane ratio between about 0.1 and about 2.0.

31. The method of claim 29 in which said reacting is at a temperature between about 450°C and about 550°C.

32. The method of claim 29 in which said reacting is in the absence of a catalyst.