FIELD OF INVENTION:
The present invention relates to a process for preparation of tetrafluoroethane sulphonic acid. Tetrafluoroethane sulphonic acid more commonly known as TFESA is a compound with the chemical formula HCF2-CF2-S03H. It is a colorless, hygroscopic liquid at room temperature and is soluble in various polar organic solvents.
BACKGROUND OF THE INVENTION:
Tetrafluoroethane sulphonic acid is one of the super acid produced from Tetrafluoroethylene (TFE) and finds application as a catalyst in various chemical reactions, where strong acid such as sulfuric acid is used. Known process for the preparation of tetrafluoroethane sulphonic acid includes the following process:
According to US patent no: US-2006/0276671A1, TFESA is synthesized in a two step process. In the first step, aqueous solution of sodium or potassium metal sulfite is made to react with TFE, in presence of sodium or potassium metabisulphite , which acts as a buffer to suppress the formation of difluoroacetic acid , a side reaction due to hydration of TFE. In step -2 of the process, the tetrafluoroethane sulfonic acid sodium salt or potassium salt as the case may be is hydrolyzed using sulphuric acid and distilled to get monohydrate of tetrafluoroethane sulfonic acid which is then subsequently dehydrated and distilled. Alternately, there are processes which use 100 % Sulfuric acid and excess oleum to hydrolyze the alkali metal salt of tetrafluroethane sulfonic acid to get the acid in anhydrous form.
US Patent No. 7683209 and US Patent No. 7897807 disclose a process for manufacture of hydrofluoroalkanesulfonic acid comprising: contacting a fluoro olefin with sulfite in an aqueous solution adjusted to about pH 4 to pH 12; removing water from the solution to form a solid; directly treating the solid with oleum; and distilling the hydrofluoroalkanesulfonic acid therefrom. The process is useful for the manufacture of potassium hydrofluoroalkanesulfonate in high purity.
Journal of Organic Chemistry, 2008, 73, 711-714 discloses a process for the preparation of potassium 1,1,2,2-tetrafluoroethanesulfonate from tetrafluoroethylene in a buffered aqueous solution of potassium sulfite and bisulfite and then converted to 1,1,2,2-tetrafluoroethane sulfonic acid in 90% overall yield by distillation from oleum.
In addition, there are processes which describe the use of other buffering agents in place of alkali metal bisulfite, such as borax or disodium phosphate (Refer patent number GB-579897 & US 2403207) In either way the step -1 of the process results in the formation of sodium or potassium salt of tetrafluoroethane sulfonic acid. The processes suffer from the following draw back:
1. Boiling point of Tetrafluorethane sulfonic acid is 210- 211 degree C. It has a strong affinity to stay in the pool of sulfuric acid when subjected to hydrolysis using either sulfuric acid or oleum. Hence one has to subject to a high temperature and vacuum to recover the desired acid from hydrolysis mixture. However, it is not possible to extract the entire entrapped acid from the pool.
2. To carry out such process calls for exotic material of construction such as Hastalloy, inconel, Monel to name a few. This means high cost equipment for practicing such a process on an industrial scale.
Literature also cites a process where the alkali metal salt of Tetrafluoroethane sulfonic acid is hydrolyzed using dilute sulfuric acid ( a concentration of 30-35 % ), followed by removal of sodium sulfate by filtration. The resultant acid is extracted repeatedly in ether and the ether fraction is distilled under reduced pressure to get water & difluoroacetic acid as first fraction followed by tetrafluoroethane sulfonic acid monohydrate as a second main fraction. Monohydrate is then dehydrated to get pure acid. (JOC), (1949), 14, 747-53.
The process suffers from the drawback that dilute sulfuric acid at a concentration of 70 % or lower is much more corrosive compared to concentrated acid at 98 %. This means that the choice of equipment for performing such reaction is restricted to glass, which is not a preferred choice on an industrial scale. In addition, the process also has a problem of disposal of the spent sulphuric acid having hydrate of tetrafluoroethane sulfonic acid. Thus, there arise a need to develop a new economic process for the synthesis of tetrafluoroethane sulfonic acid using readily available commercial raw material which obviates the use of high boiling sulfuric acid.
OBJECTIVE OF THE PRESENT INVENTION:
An object of the present invention is to provide a synthetic route for tetrafluoroethane
sulfonic acid using readily available raw material.
Another object of the present invention is to provide a synthetic route for tetrafluoroethane sulfonic acid that obviates the use of high boiling acid such as sulfuric acid and poses problem for disposal of spent acid.
SUMMARY OF THE INVENTION:
The present invention provides a process for the preparation of Tetrafluoroethanesulfonic acid by reacting tertafluoroethylene with aqueous solution of sodium sulphite to obtain sodium salt of tetrafluoroethane sulfonic acid along with small amount of inorganic salts. The sodium salt of tetrafluoroethane sulfonic acid obtained is extracted with hot alcohol so as to separate sodium tetrafluorosulfonate from inorganic salt. The sodium salt of tetrafluoroethane sulfonic acid is dissolved in an aliphatic alcohol and hydrolyzing the salt with dry HCI. To the resultant reaction mixture, water was added to prevent the formation of ester. Alcohol & water is removed from the reaction mixture through distillation to obtain tetrafluoroethane sulfonic acid monohydrate. Further, thionyl chloride is added to the residual reaction mixture to dehydrate the acid followed by distillation to get the pure tetrafluoroethane sulfonic acid. The crude tetrafluroethane sulfonic acid obtained is then further purified by vacuum distillation to obtain pure tetrafluoroethane sulfonic acid having purity greater than 99.5 %.
STATEMENT OF THE INVENTION:
Accordingly, the present invention provides a process for preparation of tetrafluoroethane sulfonic acid comprising the steps of:
(a) reacting tertafluoroethylene with an aqueous solution of sodium sulphite , to obtain sodium salt of tetrafluoroethane sulfonic acid along with inorganic salt;
(b) separating the sodium salt of tetrafluoroethane sulfonic acid and inorganic salts obtained in step (a) by extracting with hot aliphatic alcohol;

(c) dissolving the sodium tetrafluoroethane sulfonate obtained in step (b) in an aliphatic alcohol and hydrolyzing the sodium tetrafluoroethane sulfonate with dry HCI;
(d) adding water to the resultant reaction mixture obtained in step (c);
(e) removing alcohol and water by distilling the reaction mixture obtained in step (d) to
obtain tetrafluoroethane sulfonic acid monohydrate;
(f) Adding thionyl chloride to the residual reaction mixture obtained in step (e) to
dehydrate the acid followed by distillation to obtain pure tetrafluoroethane sulfonic acid.
In an embodiment of the present invention, the aliphatic alcohol is selected from the group
consisting of methanol, ethanol, isopropanol and mixture thereof.
In another embodiment of the present invention, the amount of the alcohol ranges from 400-
1000 ml /mole of sodium salt of tetrafluoroethane sulfonic acid.
In yet another embodiment of the present invention, the mineral acid is selected from the
group consisting of halogen acids, preferably hydrogen chloride.
In yet another embodiment of the present invention, the amount of the mineral acid used
ranges from 1.1 moles to 3 moles / mole of the sodium salt of tetrafluoroethane sulfonic acid.
In yet another embodiment of the present invention, the amount of water used in the reaction
ranges from 15-30 moles / mole of tetrafluoroethane sulfonic acid.
In yet another embodiment of the present invention, the amount of the thionyl chloride ranges
from 1 to 1.5 moles / mole of the tetrafluoroethane sulfonic acid.
In yet another embodiment of the present invention, the temperature of reacting with
thionylchloride ranges from 40 - 80 degree C.
In yet another embodiment of the present invention, the hydrolysis is carried out at a
temperature of 70 degree C to 120 degree C.
DETAILED DESCRIPTION OF THE INVENTION:
The present invention provide a process for the preparation of tetrafluoroethanesulfonic acid, by reacting tertafluoroethylene with aqueous solution of sodium sulphite to obtain sodium salt of tetrafluoroethane sulfonic acid along with small amount of inorganic salts. The sodium salt of tetrafluoroethane sulfonic acid obtained is extracted with hot alcohol so as to separate sodium tetrafluorosulfonate from inorganic salt. The sodium salt of tetrafluoroethane sulfonic acid is dissolved in an aliphatic alcohol and hydrolyzing the salt with dry Hcl. To the resultant reaction mixture water is added so as to prevent the formation of ester. Removal of alcohol & water from the reaction mixture through distillation so as to obtain tetrafluoroethane sulfonic acid monohydrate. Further, thinoyl chloride is added to the residual reaction mixture to dehydrate the acid followed by distillation to get the pure tetrafluoroethane sulfonic acid. The
obtained crude tetrafluroethane sulfonic acid is then further purified by vacuum distillation to obtain pure tetrafluoroethane sulfonic acid having purity greater than 99.5 %.
The hydrolyzing step comprise of hydrolyzing sodium salt of tetrafluorethane sulphonic acid with methanolic halogen acid followed by addition of water ,to form tetrafluorethane sulfonic acid monohydrayte . Subsequently .dehydrating the mono hydrate with thionyl chloride at a temperature of 40 to 80 degree C to get crude anhydrous tetrafluorethane sulfonic acid followed by removing the impurities from the crude tetrafluorethane sulfonic acid by vacuum distillation to obtain the pure acid.
Typically the hydrolysis step is carried out at a temperature of 70 degree C to 120 degree C.
The pressure of the reaction could be any where between atmospheric pressure to 6 Kg/cm2.
Pressure above 6kg/cm2 does not give any distinct advantage to the process of current
invention.
Typically aliphatic alcohol used in the process of current invention is selected from group
consisting of methanol, ethanol, isopropanol and mixture thereof to form a hydrolysable
mass. The quantity of alcohol used in the reaction is about 400 to 1000 ml per mole of
sodium salt of tetrafluoroethane sulfonic acid.
The mineral acid used for the purpose of hydrolysis is selected from group of halogen acid of
which hydrogen chloride is preferred due to ease of handing the reaction.
Although, theoretically one mole of mineral acid is needed to hydrolyze one mole of sodium
salt of tetrafluoroethane sulfonic acid, in actual practice, it is seen that slight excess of the
acid is needed to ensure completion of reaction. It is seen that the mineral acid needed for
hydrolysis is between 1.1 moles to 3 moles per every mole of the salt. The completion of the
reaction is monitored by the formation of sodium halide which is analyzed by the ion
chromatography. The inorganic halide so formed is separated from the alcoholic phase by
filtration.
It is well known that any acid would tend to react with an alcohol forming ester and water. The
recovery of the methanol from the reaction mixture is difficult and a higher temperature leads
to the formation of ester and water. This means a loss of yield and separation of such
unwanted impurity from the desired product. To care of the above, water is intentionally
added to the reaction mixture. The addition of water converts the tetrafluoroethane sulfonic
acid to its mono hydrate thereby the formation of ester is inhabited. Although the quantity of water needed for formation of mono hydrate of tetrafluoroethane sulfonic acid is mole per mole, in practice it is seen that higher quantity of water is needed to prevent the formation of ester. The quantity of water added is 15-30 mole per mole of crude tetrafluoroethane sulfonic acid. The addition of water also facilitates easy removal of the methanol from the reaction mixture. The methanol so obtained from the reaction can be recycled for the subsequent batches.
After the removal of the methanol, the excess water present in the reaction mixture is also removed by the distillation. It is observed that some small amount of water tend to stay in the reaction mixture beyond tetrafluoroethane sulfonic acid monohydrate. To take care of the water and to dehydrate the tetrafluoroethane sulfonic acid thionyl chloride is added without having to use any solvent. Typically, tetrafluoroethane sulfonic acid mono hydrate is treated with thionyl chloride at a temperature of about 40 degree C to about 80 degree C, more preferably between 50 - 60 degree C to dehydrate the acid and excess water completely. Theoretically the amount of thionyl chloride needed for dehydrating the mono hydrate is one mole per mole, in actual the quantity of thionyl chloride is about 1 to 1.5 moles per mole of the acid mono hydrate. The addition of thionyl chloride to water leads to the formation of hydrogen chloride and sulphur dioxide. As both sulphur dioxide and hydrogen chloride being gas, they get out of the reaction system without affecting the purity of the product. The obtained crude tetrafluoroethane sulfonic acid is then further purified by vacuum distillation to get pure tetrafluoroethane sulfonic acid having purity greater than 99.5 %.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.
Example -1
151g of Sodium sulphite was dissolved in 500ml water and charged in 1000 ml hastelloy reactor, 100g of tetrafluoroethylene was transferred to the reactor at -130°C. Then the temperature was increased to 120°C and maintained for 12 hrs. The reactor was cooled to room temperature and vented off the unreacted tetrafluoroethylene. Water was removed from the reaction mass by azeotropic distillation. The resulting solid was extracted with hot ethanol using soxhlet apparatus and ethanol solution was concentrated to get pure sodium
tetrafluoroethane sulphonate. 50 g of Sodium tetrafluoroethane sulphonate obtained was charged in the RBF, followed by 96 gm of methanol. After charging the raw materials, the mass temperature was cooled to 15°C. To the reaction mass, 73 g (35%) of dry HCI was passed at this temperature over 45 minutes. After completion of the addition, the stirring was continued for half an hour. Then the reaction mass was filtered through sintered funnel. The Sodium chloride was washed with 15 gm of Methanol. 4 gm from the filtrate was given for analysis. The wet sodium chloride cake was kept in the oven for drying. The remaining filtrate was added to 125 gm of water and taken for distillation using one foot column. 182 gm of distillate was obtained at 60°C/10mm Hg.
The crude material obtained was 56 gm which contains 30.52% moisture content, 69.3% Tetrafluoroethanesulphonic acid. 116Gms (1.2 moles w.r.t moisture content) thionyl chloride was added drop wise to the crude material at 25°C over three hours. After completion of the addition, the mass temperature was slowly raised to 80°C and refluxed for half an hour. After the reflux, the moisture content of the reaction mass was analyzed (mc :600 ppm) and the mass temperature was cooled to 40°C. The crude material 38 gms was taken for vacuum distillation. The product distills at 95°C/1-2 mm Hg (Yield: 80%).
Example -2
201 gm of Sodium sulphite was dissolved in 600ml water and charged in 1000ml hastelloy reactor, 150g of tetrafluoroethylene was transferred to the reactor at -130°C. Then the temperature was increased to 120°C and maintained for 12hrs. The reactor was cooled to room temperature and vented off the unreacted tetrafluoroethylene. Water was removed from the reaction mass by azeotropic distillation. The resulting solid was extracted with hot ethanol using soxhlet apparatus and ethanol solution was concentrated to get pure sodium tetrafluoroethanesulphonate. 100 gm of Sodium tetrafluoroethanesulphonate obtained was charged in the RBF, followed by 140 gm of methanol. After charging the raw materials, the mass temperature was cooled to 15°C. To the reaction mass 100 gm (35%) of dry HCI was passed at this temperature for 45 minutes. After completion of the addition, the stirring was continued for half an hour. Then the reaction mass was filtered through sintered funnel. The Sodium chloride was washed with 15 gms of Methanol. 4 gm from the filtrate was given for analysis. The wet sodium chloride cake was kept in the oven for drying. The remaining filtrate 244 gm was added to 200 gm of water and taken for distillation using one foot column. 319
gm of distillate was obtained at 60°C/10mm Hg.
The crude material obtained was about 96 gm which contains 23.3% moisture content, 72.3% Tetrafluoroethanesulphonic acid.160 gm (1.2 moles w.r.t moisture content) of thionyl chloride was added drop wise to the crude material at 25°C for three hours. After completion of the addition, the mass temperature was slowly raised to 80°C and refluxed for half an hour. After the reflux, the moisture content of the reaction mass was analyzed (mc:600 ppm) and the mass temperature was cooled to 40°C. The crude material 69 gms was taken for vacuum distillation. The product distills at 95°C/1-2 mm Hg (Yield:80.5%).
ADVANTAGES:
1. The process provides a synthetic route for the preparation of tetrafluoroethane sulfonic acid using readily available raw material.
2. The process provides improved yield.
3. The process provides highly pure tetrafluoroethane sulfonic acid.
4. The process is cost effective.
5. The process provides a synthetic route for tetrafluoroethane sulfonic acid that obviates the use of high boiling acid such as sulfuric acid and poses problem for disposal of spent acid.

We Claim:
1. A process for synthesis of tetrafluoroethane sulphonic acid comprising the steps of:
(a) reacting tertafluoroethylene with an aqueous solution of sodium sulfite, to obtain
sodium salt of tetrafluoroethane sulfonic acid along with inorganic salt;
(b) separating the sodium salt of tetrafluoroethane sulfonic acid and inorganic salt obtained in step (a) by extracting with an aliphatic alcohol to obtain sodium tetrafluoroethane sulfonate;
(c) dissolving the sodium tetrafluoroethane sulfonate obtained in step (b) in an aliphatic alcohol and hydrolyzing the sodium tetrafluoroethane sulfonate with a mineral acid to obtain a reaction mixture;
(d) adding water to the resultant reaction mixture obtained in step (c);
(e) distilling the reaction mixture obtained in step (d) to remove alcohol and water from the reaction mixture to obtain tetrafluoroethane sulfonic acid monohydrate;
(f) adding thionyl chloride to the residual reaction mixture obtained in step (e) followed by distillation to obtain pure tetrafluoroethane sulfonic acid.

2. The process as claimed in claim 1, wherein the aliphatic alcohol is selected from the group consisting of methanol, ethanol, isopropanol and mixture thereof.
3. The process as claimed in claim 1, wherein amount of the alcohol used in step (b) ranges from 400-1000 ml /mole of sodium salt of tetrafluoroethane sulfonic acid.
4. The process as claimed in claim 1, wherein the mineral acid is selected from the group consisting of halogen acids, preferably hydrogen chloride.
5. The process as claimed in claim 1, wherein amount of the mineral acid used ranges from 1.1 moles to 3 moles / mole of the sodium salt of tetrafluoroethane sulfonic acid.
6. The process as claimed in claim 1, wherein the amount of water used in the reaction ranges from 15 - 30 moles / mole of tetrafluoroethane sulfonic acid.
7. The process as claimed in claim 1, wherein amount of the thionyl chloride ranges from 1 to 1.5 moles / mole of the tetrafluoroethane sulfonic acid.
8. The process as claimed in claim 1, wherein the temperature of reacting with thionyl
chloride ranges from 40 - 80 degree C. 9. The process as claimed in claim 1, wherein the hydrolysis is carried out at a temperature of 70 degree C to 120 degree C.