GB2030981A - Process for the preparation of tetrafluorethane - Google Patents

Abstract

1,2,2,2-Tetrafluoroethane is prepared by reacting 1-chloro-2,2,2- trifluoroethane with excess of hydrogen fluoride in the presence of an inorganic chromium (iii) compound as a catalyst. Addition of oxygen to the reaction system prevents the deterioration of the activity of the catalyst and assures a high yield of 1,2,2,2-tetrafluoroethane even after a long time run.

Description

SPECIFICATION Process for the preparation of tetrafluoroethane The present invention relates to a process for the preparation of tetrafluoroethane. More particularly, it relates to a process for preparing 1 ,2,2,2-tetrafluoroethane by reacting 1 reacting -chloro-2,2,2- trifluoroethane with hydrogen fluoride.

1 ,2,2,2=Tetrafluoroethane (hereinafter referred to as "tetrafluoroethane") is useful as a refrigerant. A well known process for the preparation of tetrafluoroethane comprises reacting trifluoroethylene with hydrogen fluoride. However, this process is not suitable for the industrial production of tetrafluoroethane, since trifluoroethylene is not readily available.

Fluorohydrocarbons are generally prepared by reacting chlorohydrocarbons or chlorofluorohydrocarbons with hydrogen fluoride in the presence of a catalyst such as chromium (III) fluoride, chromium (III) oxide, aluminium fluoride or iron (III) fluoride. However, the application of this general procedure to the preparation of tetrafluoroethane from 1 -chloro-2,2,2-trifluoroethane (hereinafter referred to as "chlorotrifluoroethane") has not previously been reported.

The reaction of chlorotrifluoroethane with hydrogen fluoride in equimolar proportions in the presence of an inorganic chromium (III) compound as a catalyst does not afford tetrafluoroethane with a high efficiency, since at a low temperature of about 2000C, the'conversion of chlorotrifluoroethane is less than 1% and at a high temperature of about 4000 C, 1 -chloro-2 2-difluoroethylene produced overwhelmingly according to the following reaction formula: CF3CH2CI < CH2=CHCI + HF Furthermore, 1-chloro-2,2-difluoroethylene has a boiling point close to that of tetrafluoroethane, so that its elimination from tetrafluoroethane by distillation is quite difficult.

We have now discovered that a high conversion of chlorotrifluoroethane selectively to tetrafluoroethane is achieved by reacting chlorotrifluoroethane with a large excess of hydrogen fluoride at a relatively high temperature in the presence of an inorganic chromium (III) compound. However, this process suffers from the disadvantage that the activity of the catalyst deteriorates significantly after considerable time run.

We have furthermore found that to overcome this disadvantage the existence of oxygen in a certain proportion to chlorotrifluoroethane is effective in preventing of the deterioration of catalytic activity.

Accordingly, the present invention provides a process for preparing 1 ,2,2,2-tetrafluoroethane which comprises reacting 1-chloro-2,2,2-trifluoroethane with hydrogen fluoride in the presence of an inorganic chromium (III) compound with the introduction of from 0.002 to 0.05 mole of oxygen per 1 mole of 1 -chloro-2,2,2-trifluoroethane into the reaction system.

Preferred inorganic chromium (III) compounds to be used as the catalyst are chromium (III), oxide, hydroxide, halogenide (e.g. chloride, bromide, iodide, fluoride) and inorganic acid salts such as chromium (Ill) sulfate, nitrate, carbonate and phosphate, and their hydrates. Chromium oxyfluoride which is disclosed in Japanese Patent Publication No. 10601/1968 and U.S. Patent No. 2,745,886 may also be used as the catalyst.

The catalytic activity of the inorganic chromium (Ill) compound is preferably stabilized by keeping it in an atmosphere of hydrogen fluoride under the conditions of temperature and pressure which are to be adopted for the reaction for a considerable period of time (e.g. 1 to 5 hours).

As stated above in carrying out the present invention 0.002 to 0.05 mole of oxygen to 1 mole of chlorotrifluoroethane is introduced into the reaction system. When the oxygen content is below the lower limit, satisfactory prevention of the deterioration of the catalytic activity is not achieved. When the oxygen content is more than the upper limit, the deterioration of the catalytic activity is prevented more effectively but the selective conversion to tetrafluoroethane decreases.

The molar ratio of chlorotrifluoroethane to hydrogen fluoride is generally from 1:3 to 1:20, more particularly from 1:5 to 1:12. When the proportion is equimolar, 1 -chloro-2,2-difluoroethylene is produced overwhelmingly with an extremely low conversion to tetrafluoroethane. The greater the proportion of hydrogen fluoride the higher the selectivity to conversion to tetrafluoroethane. For example, the selective conversion to tetrafluoroethane is more than 90% molar when the molar ratio of chlorotrifluoroethane to hydrogen fluoride is 1:3 or above, and is above 95% molar when the molar ratio is 1:5 or above.However, when the hydrogen fluoride is used in an amount not less than twenty times that of the chlorotrifluoroethane, the selective conversion to tetrafluoroethane and the conversion of chlorotrifluoroethane both become almost constant The preferred reaction temperature is in the range of from 300 to 4500C, more preferably from 350 to 4200 C. At a temperature of about 200 or, chlorotrifluoroethane hardly undergoes any conversion. At a temperature above about 3000 C, the conversion of chlorotrifluoroethane is above 5% molar and at a temperature above about 3500C, it is more than 20% molar. The conversion of chlorotrifluoroethane increases and the selectivity of conversion to tetrafluoroethane decreases with the elevation of the reaction temperature.For example, the selectivity of conversion to tetrafluoroethane is more than 95% molar at a temperature lower than about 4200C and more than 90% molar at a temperature lower than about 45 00 C, when the molar ratio of chiorotrifluoroethane to hydrogen fluoride is 1:8.

There is no particular limitation to the reaction pressure. From the viewpoint of production efficiency, a pressure of from atmospheric pressure to 10 Kg/cm2G. is preferred. The contact time varies with the reaction temperature and is generally 1 to 30 seconds.

The following Examples illustrate the invention.

REFERENCE EXAMPLE Commercially available CrF.3H20 was shaped into pellets 6 mm in diameter and 6 mm in length.

50 ml of the pellets were charged into a Hastelloy C (heat resistant nickel alloy manufactured by Haynes Stellite Co.) pipe 1 inch in diameter, and the pipe was heated to 5000C over a period of 30 minutes while introducing therein air at a rate of 0.5 to 11/mien. The pipe was kept at the above temperature for 2 hours, during which time the introduction of air was continued. Then, the temperature was lowered to 45000, and at this temperature, hydrogen fluoride was passed through the pipe at a rate of 200 ml/min.

for 2 hours.

EXAMPLE 1 50 ml of the catalyst prepared as in the Reference Example were charged into a tubular Hastelloy C reactor 3/4 inch in diameter and 1 m long and heated to 4000C by an electric furnace.

Chlorotrifluoroethane and hydrogen fluoride in a molar ratio of 1:8.3 were passed over the catalyst at 4000C under atmospheric pressure at a space velocity of 670 he~'. The exit gas was washed successively with water and an alkaline solution dried over calcium chloride and trapped in a dry iceacetone cold trap. The dried exit gas was analyzed by gas-chromatography. After 5 hours from the initiation of the reaction, the conversion of chlorotrifluoroethane was 31 % molar and the selectivity of conversion to tetrafluoroethane was 98% molar.

The reaction was continued for additional 80 hours by feeding chlorotrifluoroethane admixed with oxygen in an amount of 1.7% molar, during which the conversion of chlorotrifluoroethane was 28% molar, the selectivity of conversion to tetrafluoroethane was 96% molar and the yield (=conversion x selectivity) of tetrafluoroethane was 27%.

COMPARATIVE EXAMPLE 1 300 ml of the catalyst prepared in Reference Example were charged into a tubular Hastelloy C reactor 1-1/2 inch in diameter and 2 m long and heated by an electric furnace. Chlorotrifluoroethane and hydrogen fluoride in a molar ratio of 1:7.7 were passed over the catalyst at 4000C under atmospheric pressure at a space velocity of 746 hut1, the chlorotrifluoroethane not being admixed with oxygen. Other reaction conditions were the same as those in Example 1.

The yield of tetrafluoroethane is given in Table 1.

Table 1

Reaction time (hrs) Yield (mole %) 5 26 10 26 29 24 44 22 The yield was 26% molar after 5 hours and 22% molar after 44 hours. Thus, the activity of the catalyst shows a 1 5.4% deterioration in about 40 hours.

EXAMPLES 2 TO 6 The reaction was carried out following the procedure of Example 1 but adding oxygen from the very beginning. The results are given in Table 2.

Table 2

Amount of oxygen Example added Reaction time Sel ecti vity Yield No. (mole o/o) (hrs) (mole /0) (mole K) 2 1.5 5 97 27 40 97 27 85 97 27 3 5 40 93 26 85 93 26 4 10 40 86 24 85 86 24 5 17 40 87 22 85 87 22 6 0.2 5 97 29 40 97 28

Claims (8)

1. A process for preparing 1 ,2,2,2-tetrafluoroethane which comprises reacting 1 -chloro-2,2,2trifluoroethane with hydrogen fluoride in the presence of an inorganic chromium (III) compound with the introduction of from 0.002 to 0.05 mole of oxygen per 1 mole of 1 -chloro-2,2,2-trifluoroethane into the reaction system.

2. A process as claimed in claim 1 wherein the molar ratio of 1 -chloro-2,2,2-trifluoroethane to hydrogen fluoride is from 1:3 to 1:20.

3. A process as claimed in claim 2 wherein the molar ratio of 1 -chloro-2,2,2-trifluoroethane to hydrogen fluoride is from 1:5 to 1:12.

4. A process as claimed in any one of the preceding claims wherein the reaction temperature is from 300 to 45000.

5. A process as claimed in claim 4 wherein the reaction temperature is from 350 to 42000.

6. A process as claimed in any one of the preceding claims wherein the inorganic chromium (III) compound is chromium (III) oxyfluoride, oxide, hydroxide or halogenide, or inorganic acid salts of chromium (III), or hydrates thereof.

7. A process as claimed in claim 1 substantially as hereinbefore described with reference to any one of the Examples.

8. 1 ,2,2,2-Tetrafluoroethane whenever produced by a process as claimed in any one of the preceding claims.