GB1587830A - Process for producing 1,1-difluoroethylene - Google Patents

Description

(54) A PROCESS FOR PRODUCING 1,1-DIFLUOROETHYLENE (71) We, DYNAMIT NOBEL AKTIENGESELLSCHAFT, a German Company of 521 Troisdorf, bez Köln, Postfach 1209, Germany do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for the production of 1 ,1-difluoroethylene and is an improvement in or modification of the process claimed in our application No. 23298/77, now accepted under serial No. 1566485.

It is known that 1,1-difluoroethylene may be produced in three stages. In a first stage, 1,1-difluoroethane is obtained by the hydrofluorination of acetylene and purified. The purified 1,1-difluoroethane is then chlorinated, with 1,1-difluoro-l-chloroethane being isolated from the chlorination products. In a third stage, hydrogen chloride is thermally liberated from the 1,1-difluoro-l-chloroethane to form 1,1-difluoroethylene.

It is also known that acetylene can be hydrofluorinated in the presence of a catalyst comprising a pelletised mixture of aluminium fluoride and bismuth fluoride. At most 97% of the acetylene can be reacted by means of this catalyst; the reaction products contain from 70 to 72% by weight of 1,1-difluoroethane and from 25 to 27 % by weight of vinyl fluoride.

A mixture such as this has to be worked up by distillation before the 1,1-difluoroethane which it contains can be used for further reactions.

Another catalyst which has already been proposed for the hydrofluorination of acetylene to form 1,1-difluoroethane is a catalyst containing aluminium fluoride which is produced by impregnating y- or aluminium oxide with a bismuth and manganese salt solution, predrying the impregnated aluminium oxide at temperatures of up to 1000C and then heating it at temperatures between 150 and 250"C, first in a nitrogen atmosphere and then in air with increasing concentrations of hydrogen fluoride up to a 100 % hydrogen fluoride atmosphere. In this regard, reference is made to our UK patent specification No. 1561535.

The production of a catalyst such as this is also described, for example, in German Offenlegungsschrift No. 2,000,200. The reaction product obtained by hydrofluorination with a catalyst such as this has been found to contain, in addition to difluoroethane, up to 2 to of acetylene and at most about 4.5% of vinyl fluoride.

Thus the secondary products produced by each of these catalyst systems have hitherto always necessarily been separated off before further processing of the difluoroethane.

The further processing of 1,1-difluoroethane in pure form into 1,1-difluoroethylene is disclosed and claimed in our copending accepted UK patent application No. 23298/77 (Serial No. 1566485). In that process pure 1,1-difluoroethane is photochlorinated to form a mixture comprising 1,1-difluoro-1-chloroethane and immediately afterwards that mixture is heated to a temperature of from 500 or 550 to 750"C without first isolating the 1, 1-difluoro-1-chloroethane.

Further investigation of the process has now shown that 1,1-difluoroethane in the form of an impure reaction product from the catalytic hydrofluorination of acetylene may be used as starting material.

Accordingly, the present invention provides a process for producing 1,1-difluproethylene comprising (a) in a first stage catalytically hydrofluorinating acetylene to form a first mixture comprising 1,1-difluoroethane, (b) in a second stage chlorinating the first mixture to form a second mixture comprising 1,1-difluoro-1-chloroethane, and (c) in a third stage heating the second mixture at a temperature of from 500 to 7500C to form a third mixture including the desired 1,1-difluoroethylene.

It is preferred that the first mixture, i.e. the hydrofluorination product of acetylene, contains at most 3%, more preferably up to 2% by volume of acetylene, and at most 8%, more preferably up to 7% by volume of vinyl fluoride.

Despite the presence of these impurities in the starting material of the second stage, overall yields of 1,1-difluoroethylene amounting to more than 90%, based on the acetylene used, have been obtained by the process of the invention. It would seem, then that the secondary products present, particularly the unreacted acetylene, have hardly any effect upon the reactions of the process as a whole; this is surprising insofar as it was not known hitherto that gas mixtures containing acetylene could be photochlorinated without difficulty.

Since in the process according to the invention the intermediate products are not isolated, it is possible for plant costs to be reduced. It is not essential in the process according to the invention to provide the distillation apparatus and storage and transport containers which, in conventional processes, have to be used in a large-scale industrial plant.

In the first stage of the process, acetylene is hydrofluorinated in the presence of a hydrofluorination catalyst. One catalyst which has proved to be particularly suitable is a catalyst containing from 0.1 to 20% by weight of bismuth, from 35 to 66% by weight of fluorine, and from 24 to 42% by weight of aluminium, the remainder consisting primarily of oxygen. This catalyst may be produced by impregnating y- or aluminium oxide with a solution of a bismuth salt or salts, predrying the impregnated oxide at a temperature of up to 1000C and heating to temperatures of from 150 to 250"C, first in a nitrogen atmosphere and then, after complete drying, with a mixture of air and increasing concentrations of hydrogen fluoride up to a 100 % hydrogen fluoride atmosphere until the exothermic reaction abates.

Another suitable catalyst may be prepared by the same method, but also impregnating the aluminium oxide with a manganese salt solution. In this case the final catalyst additionally contains from 0.1 to 10% of manganese, whilst its aluminium content decreases to from 20 to 38% by weight and its fluorine content to from 32 to 60% by weight.

It is preferred that the catalyst is produced as follows: y- or -aluminium oxide, which has preferably been heated in vacuo (below 1 Torr) for about 1 hour at 800C, is impregnated with an aqueous solution of a bismuth salt and optionally a manganese salt. Suitable bismuth salts are water-soluble or acid-soluble bismuth salts, the solution of which is optionally stabilised by a complex former. In the case of substantially insoluble bismuth and also manganese salts, these complex formers enable a homogeneous solution of these salts to be obtained. Suitable complex formers are, for example, hydroxyl-group-containing organic compounds from the group comprising sugar alcohols, such as for example mannitol, sorbitol or ribitol, or hydroxy acids, such as for example tartaric acid, lactic acid, or the sugar acids. Other suitable complex formers are amines and nitriles such as, for example, ethylene diamine, nitrilotriacetic acid or succinodinitrile.

A homogeneous solution can also be obtained by adjusting a suitable acid pH-value.

It is preferred to use the bismuth or bismuthoxy salts of nitric acid, sulphuric acid, hydrochloric acid or perchloric acid.

The concentration of the bismuth salt solution is not critical to the process and is selected according to the required bismuth content of the catalyst.

The manganese compound used is preferably the salt with the same anion as the bismuth salt used. However, it is also possible to use any other water-soluble or acid-soluble manganese salt, providing it does not form an insoluble deposit with the bismuth salt used under the conditions specified.

It is preferred to use the manganese (II) and bismuth or bismuthoxy salts of nitric acid, sulphuric acid, hydrochloric acid or perchloric acid.

The concentration of the manganese salt solution is also not critical to the process and is selected according to the required metal content of the catalyst.

The aluminium oxide impregnated with the bismuth or bismuth/manganese salt solution is then dried at temperatures of up to 1000C and subsequently heated in a nitrogen atmosphere to between 150 and 250"C. After complete drying of the catalyst, the nitrogen is replaced by air and increasing concentrations of hydrogen fluoride. The exothermic reaction which then begins is also carried out at temperatures of from 150 to 250"C.

Towards the end of the exothermic reaction, the reaction mixture is heated in a 100 % hydrogen fluoride atmosphere until the exothermic reaction has abated.

In a preferred embodiment, to carry out the hydrofluorination reaction, i.e. the first stage of the process according to the invention, acetylene is mixed with hydrogen fluoride and the resulting premixture is preheated to the necessary reaction temperature. The preheated premixture is passed over the catalyst which is preferably kept at a constant temperature of from 150 to 350"C. The catalyst may be arranged either in the form of a fixed bed or in the form of a fluidised bed.

After passage over the catalyst, the first gas mixture is washed in known manner partially dried and then directly subjected to photochlorination without separating off the impurities. However, it is also possible temporarily to store this first gas mixture in a gasometer and then directly to subject it to photochlorination.

When this procedure has been used, yields of from 96 to 98% of the required 1,1-difluoroethane have been obtained, the conversion of acetylene being almost complete.

This high conversion of acetylene is also obtained with a residence time of only from 5 to 30 seconds. The preferred residence time is from 5 to 45 seconds.

The preferred temperature range for this first stage is from 200 to 280"C. The higher the temperature selected, the shorter the residence time can be for substantially the same conversions and yields.

In contrast to some other processes, hydrofluorination of the acetylene does not have to be carried out with an excess of hydrofluoric acid over the stoichiometrically necessary quantity of 2 moles of HF per mole of acetylene. It takes place substantially quantitatively even with a stoichiometric ratio between the reactants. It is advisable to use an approximately 5% excess of hydrofluoric acid; in principle, it is also possible to use an excess of hydrofluoric acid of up to 50%.

The difluoroethane may also be produced by hydrofluorinating acetylene in the presence of other catalysts, resulting in the formation of first gas mixtures which preferably contain no more than 3% by volume and more preferably no more than 2% by volume of acetylene.

The vinyl fluoride content of the mixture may amount to 8% by volume, although it preferably amounts to 7% by volume. In these cases, too, the first gas mixture formed during the hydrofluorination reaction may be directly subjected to the second stage photochlorination reaction without separating off the acetylene or other reaction products.

In order to increase the conversions and yields of the first stage it is preferred to carry out the first stage in two process steps. Thus two reactors filled with the same catalyst are arranged in tandem, the second reactor preferably having a temperature from 20 to 50 degrees C lower than the first reactor. In this way the vinyl fluoride which is the predominant secondary product in the almost completely converted gas mixture issuing from the first reactor is, after passing through the second reactor, itself also largely converted into difluoroethane by reaction with the excess hydrogen fluoride present.

In addition to acetylene and vinyl fluoride, other secondary products which are formed in the catalytic hydrofluorination of acetylene may also be present in small quantities in the first gas mixture to be chlorinated in the second stage.

The chlorination of the first mixture of 1,1-difluoroethane, acetylene and vinyl fluoride is carried out in known manner. It is preferably carried out in the presence of light. The wavelength of the light rays may lie both in the visible region and also in the UV region.

The preferred region is from 500 to 600 nm.

The molar ratio between the hydrofluorination product, i.e. the first mixture, and chlorine during the chlorination reaction is preferably 1:1. A slight excess of chlorine is possible, however, but the molar ratio preferably does not exceed 1:1.2.

The chlorination reaction is preferably carried out at a temperature of from 0 to 150C the particularly preferred temperature range is from 20 to 700C. The residence time in the chlorination reactor, which preferably consists of glass, is preferably from 20 to 100 seconds, based on a temperature of 0 C and an empty reactor.

Immediately after chlorination, the chlorination products i.e. the second mixture, are subjected to dehydrochlorination by pyrolysis in a third reaction stage. The second mixture consists predominantly of 1,1-difluoro-1-chloroethane and hydrogen chloride, together with small quantities of more highly chlorinated fluorochloroethanes and unreacted difluoroethane. The proportion of higher boiling secondary products generally amounts to between 2 and 4% by volume.

Dehydrochlorination takes place at temperatures of from 500 to 750"C. The effect of the reaction products formed during chlorination of the 1,1-difluoroethane (primarily the hydrogen chloride liberated) is that pyrolysis of the 1,1-difluoro-1-chloroethane generally gives the required 1,1-difluoroethylene in a yield of almost 100 %. These generally high yields are obtained in particular when the dehydrochlorination reactor is filled with a material of high thermal conductivity such as, for example, metal chips which are not affected under the reaction conditions, for example nickel chips. The preferred pyrolysis temperature is from 550 to 750"C, more preferably from 650 to 720"C.

The residence time in the pyrolysis reactor is preferably from 1 to 150 seconds, based on an empty reactor and a temperature of 0 C. The particularly preferred range is from 2.5 to 90 seconds.

The reactor must consist of a material which is not affected under the reaction conditions.

Nickel reactors in the form of tube reactors are particlarly suitable.

In general, the dehydrochlorination (third) stage is carried out under atmospheric pressure or under the pressure which is spontaneously adjusted during the reaction. This pressure is generally no higher than about 1.1 atmosphere. In principle, however, it is also possible to work under higher pressures.

The process according to the invention is preferably carried out continuously. To this end, the hydrofluorination reactor, the chlorination reactor and the dehydrochlorination reactors are arranged immediately one behind the other in that order. A washing stage using water is best provided between the hydrofluorination reaction and the chlorination reactor in order to remove excess hydrogen fluoride. After leaving the last reactor, the reaction products are washed with water and/or dilute alkali hydroxide solution and subsequently dried. The third gas mixture obtained then mainly contains as impurities more highly chlorinated products which can readily be separated off from the 1,1difluoroethylene in known manner. The yields of 1, 1-difluoroethylene have generally been found to amount to more than 90%, based on the acetylene used.

The following Examples illustrate the invention.

Example 1 To produce the catalyst, 700 g of y-aluminium oxide in the form of 3 mm diameter pellets were evacuated for 1 hour at 80"C to < 1 Torr in a glass tube provided with a heating jacket.

After cooling in vacuo to room temperature, a solution of 160 g of Bi(NO3)3.5H2O, 65 g of Mn(NO3)2.4H2O and 80 ml of 14 N nitric acid in 900 ml of water was run into the tube.

After the impregnation batch had been aired, it was left standing for 1 hour at 800C; the aqueous phase was then run off and the catalyst mass was predried under the suction of a water jet pump.

For hydrofluorination, the catalyst was introduced into a double-jacketed Ni-tube 150 cm long and 5 cm in diameter, the temperature of which was regulatable by means of an oil circuit. The catalyst was thoroughly dried with nitrogen at 2000C and subsequently activated with a mixture of air and increasing concentrations of hydrogen fluoride. The temperature was kept below 250"C by varying the concentration of HF. After a 100% HF stream had been attained, the activation treatment was continued for 1 hour, followed by drying with air for 1 hour. The bismuth content of the catalyst then amounted to approximately 5%, the manganese content to approximately 3% and the fluoride content to approximately 50%.

A gaseous mixture of 12.6 moles/hour of hydrogen fluoride and 6 moles/hour of acetylene (molar ratio 2.1:1) was passed over the catalyst which had been activated as described above, in a nickel reactor with a heatable volume when empty of 1.8 litres at a temperature of 230 to 2400C.

After passing through a water wash, the reaction mixture was mixed with 6.6. moles/hour of chlorine and subjected to gas phase reaction at around 30"C in a 3.6 litre glass reactor under the effect of light from a 250 watt halogen metal vapour lamp (OSRAM-Power-Star HQJ type; OSRAM is a registered trade mark).

The mixture issuing from the chlorination reactor was directly introduced into a nickel reactor heated to around 690"C (heated volume 0.75 litre); the reaction product leaving this hot reactor was washed with water and dilute sodium hydroxide solution, and dried.

Analysis showed that a mixture of organic compounds having the following composition, as determined by gas chromatography, was obtained: CF2=CH2 91.9 % CHF2-CH3 0.5 % CF2Cl-CH3 0.2 % CF2=CHCl 1.4 % higher boiling fractions 6.0 %.

Accordingly, for a 100 % conversion of the acetylene used, the three stage reaction produced a total yield of 91.9 %.

The structure of the test installation was such as to enable samples, freed from hydrogen halides and chlorine by washing with water, to be taken after each reactor in order to be able to follow the individual reaction stages. In this way, it was possible to determine the following stage results by gas-chromatographic analyses: a) Hydrofluorination (Stage 1): CHF2-CH3 97.4 % CHF=CH2 2.5 % CHCH 0.1 % corresponding to a 99.9 % conversion; yield 97.5 % b) Chlorination (Stage 2): CF2Cl-CH3 94.1 % CHF2-CH3 2.3 % higher boiling fractions 3.6 % which, taking into account the composition indicated under a) above for the starting mixture which was subjected to chlorination stage 2, corresponded to a yield of 98.9 % for a 97.6 % conversion.

c) Thermal cleavage (Stage 3): The composition of the product of this stage is identical with the overall product given above.

It corresponds to a yield of 97.9 % for a 99.8 % conversion, again taking into account the composition of the starting mixture defined in b) above.

Example 2 The test on which this Example is based was carried out in the apparatus described in Example 1. In order to obtain as complete as possible a conversion of acetylene into difluoroethane, the hydrofluorination reactor was followed by a second reactor filled with the same catalyst but at a temperature of about 200"C, i.e. approximately 30 to 40 degrees C lower than in the first reactor.

Under otherwise the same conditions, the following overall result was obtained.

CF2=CH2 93.7 % CHF2-CH3 1.8 % CF2Cl-CH3 0.2 % CF,=CHC1 1.1 % higher boiling fractions 3.3 % The overal reaction produced a 93.7 % yield of 1,1-difluoroethylene for a 100 % conversion of the acetylene used.

A breakdown according to Example 1 produced the following stage results: a) Hydrnfluorinafion: CHF2-CH3 99.0 % CHF=CH2 0.5 % CHCH 0.5 % 99.5 % conversion; 99.5% yield.

b) Chlorination: CF2Cl-CH3 95.8 % CHF2-CH3 2.0 % higher boiling fractions 2.2 % 98.0% conversion; 98.8 % yield c) Cleavage: The composition, as determined by gas chromatography, was, of course, the same as that of the overall product; it corresponded to a 99.8 % conversion; 98.00/0 yield.

Example 3 In order to demonstrate the effectiveness of the overall reaction, a test was selected in which the hydrofluorination catalyst showed distinctly reduced activity through prolonged use and was ready for regeneration. The test was carried out with the same apparatus and in the same way as in Example 1, i.e. using only one hydrofluorination reactor in order deliberately to have a relatively high proportion of CHF=CH2 in the crude product of reaction stage 1.

Overall result: CF2=CH2 90.0% CHF2-CH3 1.1% CF2Cl-CH3 0.1% CF2=CHCl 2.1% higher boiling fractions 6.8% overall conversion 100%, overall yield 90.0%.

The breakdown into component reaction steps produced the following results: a) Hydrofluorination: CHF2-CH3 94.9% CHF=CH2 4.2 % CHCH 1.0 % conversion 99.0%; yield 95.8% b) Chlorination: CF2Cl-CH3 92.2% CHF2-CH3 2.4% higher boiling fractions 5.4 % conversion 97.5%; yield 99.6%.

c) Cleavage: The composition after this stage was as for the overall product, corresponding to a conversion of 99.9% and a yield of 97.7 %.

Claims (30)

WHAT WE CLAIM IS:

1. A process for producing 1,1-difluoroethylene comprising (a) in a first stage catalytically hydrofluorinating acetylene to form a first mixture comprising 1,1difluoroethane, (b) in a second stage photochlorinating the first mixture to form a second mixture comprising 1 ,1-difluoro-1-chloroethane, and (c) in a third stage heating the second mixture at a temperature of from 500 to 7500C to form a third mixture including the desired 1, 1-difluoroethylene.

2. A process according to claim 1 wherein the first mixture includes up to 3% by volume of acetylene.

3. A process according to claim 2 wherein the first mixture includes up to 2% by volume of acetylene.

4. A process according to claim 1, 2 or 3 wherein the first mixture includes up to 8% by volume of vinyl fluoride.

5. A process according to claim 4 wherein the first mixture includes up to 7% by volume of vinyl fluoride.

6. A process according to any one of the preceding claims wherein the catalyst employed in the first stage consists of from 0.1 to 20% by weight of bismuth, from 35 to 66% by weight of fluorine, and from 24 to 42% by weight of aluminium, with the remainder being oxygen.

7. A progress according to any one of claims 1 to 5 wherein the catalyst employed for the first stage consists of from 0.1 to 20% by weight of bismuth, from 0.1 to 10% by weight of manganese, from 32 to 60% by weight of fluorine, and from 20 to 38% by weight of aluminium, with the remainder being oxygen.

8. A process according to any one of the preceding claims wherein the first stage is carried out at a temperature of from 150 to 350"C.

9. A process according to claim 8 wherein the first stage is carried out at a temperature of from 200 to 280"C.

10. A process according to any one of the preceding claims wherein the first stage is carried out over a residence time of from 5 to 45 seconds.

11. A process according to any one of the preceding claims wherein in the first stage up to 50% excess hydrogen fluoride is used.

12. A process according to claim 11 wherein in the first stage up to 5% excess hydrogen fluoride is used.

13. A process according to any one of the preceding claims wherein the first stage is .carried out in two process steps, the reaction temperature of the second step being from 20 to 50 degrees C lower than the reaction temperature of the first step.

14. A process according to any one of the preceding claims wherein the second stage is carried out by irradiating the reactants with radiation having a wavelength of from 500 to 600 nm.

15. A process according to any one of the preceding claims wherein the second stage is carried out at a temperature of from 0 to 1500C.

16. A process according to claim 15 wherein the second stage is carried out at a temperature of from 20 to 700C.

17. A process according to any one of the preceding caims wherein the second stage is carried out over a residence time of from 20 to 100 seconds.

18. A process according to any one of the preceding claims wherein in the second stage the molar ratio of the first mixture to chlorine is from 1:1 to 1:1.2.

19. A process according to any one of the preceding claims wherein the second stage is carried out in a glass reactor vessel.

20. A process according to any one of the preceding claims wherein the third stage is carried out at a temperature of from 550 to 7500C.

21. A process according to claim 20 wherein the third stage is carried out at a temperature of from 650 to 7200C.

22. A process according to any one of the preceding claims wherein the third stage is carried out over a residence time of from 1 to 150 seconds.

23. A process according to claim 22 wherein the third stage is carried out over a residence time of from 2.5 to 90 seconds.

24. A process according to any one of the preceding claims wherein the third stage is carried out in the presence of metal chips which are inert to reactants and products.

25. A process according to claim 24 wherein the metal chips are nickel chips.

26. A process according to any one of the preceding claims wherein the third stage is carried out in a nickel reactor.

27. A process according to any one of the preceding claims when carried out continuously.

28. A process according to any one of the preceding claims and including the further step of removing the 1,1-difluoroethylene from the third mixture.

29. A process according to claim 1 substantially as hereinbefore described in any one of the Examples.

30. 1,1-Difluoroethylene whenever produced by the process according to any one of the preceding claims.