Tetrafluoroethane isomerization

Abstract

A method for the preparation of 1,1,1,2-tetrafluoroethane whereby 1,1,2,2-tetrafluoroethane is contacted with the fluorination catalyst at elevated temperature to effect isomerization.

Description

This invention relates to a chemical process and more particularly to a process for the manufacture of 1,1,1,2-tetrafluoroethane.

Several methods have been proposed for the manufacture of 1,1,1,2-tetrafluoroethane which is a useful refrigerant, aerosol propellant, blowing agent and solvent. Thus, our United Kingdom Pat. No. 1578933 describes a process for making tetrafluoroethanes by hydrogenating dichlorotetrafluoroethanes at elevated temperatures, for example temperatures in the range 200.degree.-450.degree. C. Also, our United Kingdom Pat. No. 2004539 describes the manufacture of 1,1,1,2-tetrafluoroethane by reacting trifluoroethylene in the vapour phase with hydrogen fluoride in the presence of chromium oxide, suitable reaction temperatures being in the range 200.degree.-500.degree. C. Both of these processes are expensive to operate.

It has now been found that 1,1,1,2-tetrafluoroethane can be prepared in high yield and high selectivity by the isomerisation of 1,1,2,2-tetrafluoroethane under the conditions hereinafter described.

Thus, according to the invention, there is provided a method for the preparation of 1,1,1,2-tetrafluoroethane which comprises contacting 1,1,2,2-tetrafluoroethane with a fluorination catalyst at an elevated temperature whereby to effect isomerisation.

Fluorination catalysts useful in the method of the invention have been fully described in the prior art and include aluminium fluoride, sodium fluoride, gammaalumina and, especially, chromia. Unmodified chromia may be used but it is advantageous to use a pre-fluorinated chromia that has been used in other fluorination reactions. After use, the catalyst may be regenerated by treatment with air at an elevated temperature, for example 400.degree.-420.degree. C.

The isomerisation temperature should be sufficiently high to effect conversion of the 1,1,2,2,-tetrafluoroethane but not so high that excessive amounts of by-products, such as pentafluoroethane, are obtained. In general, suitable temperatures are found in the range from about 300.degree. to about 900.degree. C. Lower temperatures can generally be employed when using chromia as the fluorination catalyst, for example temperatures from about 300.degree. to about 550.degree. C., more especially from 350.degree. to 500.degree. C. and preferably from 400.degree. to 450.degree. C. At these temperatures, the 1,1,2,2-tetrafluoroethane, optionally in conjunction with an inert diluent such as nitrogen, is suitably contacted with the catalyst for times of between about 1 and about 50 seconds.

The 1,1,2,2-tetrafluoroethane use as starting material in the method of the invention may be obtained by known methods, for example by the method described in United Kingdom Pat. No. 1578933 or by the hydrogenation of tetrafluoroethylene. The latter reaction may be conveniently effected at normal or elevated temperatures, for example up to 250.degree. C., in the presence of a hydrogenation catalyst, for example palladium on alumina.

The invention is illustrated but not limited by the following Example.

EXAMPLE

Hydrogenation of Tetrafluoroethylene

Hydrogen at 1000 ml/min and tetrafluoroethylene at 100 ml/min were passed over 120 g 5% palladium/alumina at ambient temperature in a glass reactor, the off gases being collected in two traps cooled in trichloroethylene and Drikold. There was an immediate exotherm and the temperature rose to 250.degree. C. After 210 min, the hydrogen and tetrafluoroethylene flows were turned off and the reactor and lines purged with nitrogen. The material condensed in the traps was weighed and analysed. The product was 84.4 g (94% yield) of 1,1,2,2-tetrafluoroethane (purity 98.5%).

Isomerisation of 1,1,2,2,-Tetrafluoroethane

1,1,2,2-Tetrafluoroethane and diluent nitrogen were passed through a Hastelloy reactor tube packed with 80 ml chromia, the reactor being heated in a furnace. After the reactor, the gases were passed through an empty tube to effect cooling and then through a scrubber containing 25% potassium hydroxide solution to remove hydrogen fluoride. The products were finally condensed in a trap cooled in trichloroethylene and Drikold. Gas samples were taken after the scrubber for analysis by gas chromatography.

Details of a number of runs using pre-fluorinated chromia, which had been dried at 400.degree. C. for 4 hours and then "activated" by passage of air at 200-300 ml/min at 400.degree. C. overnight, are given below in Table 1. In the Table:

A134=1,1,2,2-tetrafluoroethane A134a=1,1,1,2-tetrafluoroethane 3FE=trifluoroethylene A125=pentafluoroethane A flow rate of 50 ml/min A134 and 100 ml/min nitrogen corresponds to a catalyst contact time of approximately 12 sec.

Table 2 gives details of runs using the same catalyst after regeneration with 400 ml/min air at 400.degree. C. for 16 hours.

TABLE 1______________________________________ Tem-Catalyst pera- A134 Nitrogen Concentration in Off GasVolume ture Flowrate Flowrate mole %ml .degree.C. ml/min ml/min A134 A134a 3FE A125______________________________________80 288 50 100 93.5 1.0 0.1 0.280 325 50 100 91.4 3.0 0.1 0.480 420 50 100 69.9 21.5 0.2 0.780 460 50 100 59.4 34.1 0.3 1.880 497 50 100 52.6 34.6 0.2 8.580 550 50 100 48.1 28.1 0.3 27.980 510 50 100 56.0 26.0 0.6 12.780 465 50 100 73.4 21.0 1.0 2.780 410 50 100 88.5 9.1 0.6 0.480 470 50 100 74.9 19.4 0.2 3.8______________________________________ TABLE 2______________________________________80 420 50 100 29.1 4.2 -- 60.280 343 50 100 66.9 25.7 0.4 3.780 328 50 100 75.5 21.1 -- 1.180 375 50 100 43.6 48.8 0.1 5.480 388 50 200 52.3 43.0 0.2 2.880 400 50 300 46.3 48.1 0.1 3.980 412 50 400 49.8 44.3 0.1 4.480 410 50 200 40.3 50.2 0.9 7.180 430 50 300 47.1 43.6 1.6 6.280 440 50 400 53.1 36.8 2.2 6.580 445 50 100 28.4 52.9 0.5 16.680 450 50 150 41.4 45.1 0.7 11.180 455 50 100 40.6 42.5 0.6 14.780 455 50 100 47.9 42.2 0.8 7.9______________________________________

Claims

1. A method for the preparation of 1,1,1,2-tetrafluoroethane which comprises contacting 1,1,2,2-tetrafluoroethane with a fluorination catalyst at an elevated temperature whereby to effect isomerisation.

2. A method according to claim 1 wherein the catalyst comprises chromia.

3. A method according to claim 2 wherein the catalyst comprises pre-fluorinated chromia.

4. A method according to any preceding claim wherein the temperature is in the range from about 300.degree. to about 900.degree. C.

5. A method according to claim 4 wherein the temperature is in the range from about 300.degree. to about 550.degree. C.

6. A method according to claim 5 wherein the temperature is in the range from 350.degree. to 500.degree. C.

7. A method according to claim 6 wherein the temperature is in the range from 400.degree. to 450.degree. C.