FIELD OF INVENTION
The present invention relates to an improved process for synthesis of potassium perfluoroalkane sulfonate represented by general formula CnF2n+1 SO3K where n is an integer from 1 to 3 .
BACKGROUND OF THE INVENTION
Perfluoroalkane sulfonates are useful starting materials for the synthesis of perfluoro alkanesulfonic acids which in turn are useful as catalysts in pharmaceutical applications.
US Patent No 2732398 describes the process for synthesis of perfluoroalkane sulfonic acid, in which the said acid is produced from alkyl sulfonyl halide having 1 to 3 carbon atom. The compound is subjected to electrochemical fluorination using hydrogen fluoride as a fluorinating agent to yield perfluoroalkanesulfonyl fluoride. This is further treated with potassium hydroxide to get potassium salt of perfluoroalkane sulfonic acid. Finally the purified potassium perfluoroalkane-sulfonate is decomposed in sulfuric acid to get the perfluoroalkane sulfonic acid.
The above process suffers from following drawbacks.
1. The perfluroalkane sulfonyl fluoride is condensed at very low temperature in liquid
air trap. This calls for huge energy to condense a product having a boiling point of -23
degree C.
2. Further the condensed product is treated with potassium hydroxide in a pressure vessel with 10 to 20 % excess of potassium hydroxide at a temperature of 95 degree C and 95 psi pressure for a period of 3 hours for completion of the reaction. The process calls for a pressure vessel for carrying out the reaction. Further the salt obtained is not pure enough to be carried for further processing.
3. The salt so obtained is then filtered from the reaction mixture and is crystallized from alcohol in order to get pure potassium trifluoromethane sulfonate. The process of this kind cannot be applied on a industrial scale for continuous production of product.
Japanese patent application No S64-61452, describes the process for the production of potassium trifluoromethane sulfonate. In this process the outlet of the electrochemical process cell which is perfluoromethane sulfonyl fluoride is absorbed in potassium hydroxide solution so as to convert the sulfonyl fluoride into potassium salt. The aqueous solution is concentrated, alkali is then added and finally subjected to filtration to isolate potassium perfluoromethane sulfonate. The filterate after isolation of salt is recycled back to the process. The process suffers from the following drawbacks.
1. The above process does not give complete recovery of potassium perfluoromethane sulfonate salt. A large amount of the salt remains in the filtrate, which needs to be recycled along with unreacted potassium hydroxide in the gas absorption step.
2. The potassium perfluoromethane sulfonate carries with it, potassium hydroxide and potassium fluoride. Thus process does not generate pure potassium trifluoromethane sulfonate. Purifying the potassium perfluormethane sulfonate is difficult on a commercial scale.
3. When above salt is subjected to acid hydrolysis, the potassium fluoride generate
hydrogen fluoride which causes corrosion of glass lined reactors. Similarly, the
potassium hydroxide present in potassium trifluormethane sulfonate tend to react with
sulfuric acid forming potassium sulfate and water. This water reacts with
trifluoromethane sulfonic acid forming hydrate. As hydrate has high boiling point, it is
hard to recover the acid from the distillation, resulting in loss of yield.
Japanese Patent number 3294323 describes the process wherein, methane sulfonyl fluoride is subjected to electrochemical fluorination with anhydrous hydrogen fluoride so as to generate trifluoromethane sulfonyl fluoride. The product so produced carries along with it some small amount of hydrogen fluoride which is washed with water to get trifluomethane sulfonyl fluoride free of HF. The gas is then purged through a slurry of lithium hydroxide to remove the byproduct of the reaction (lithium fluoride) as an
insoluble material leaving behind soluble lithium trifluoromethane sulfonate as an alkaline solution which is then concentrated to get the pure salt.
The process suffers from following drawbacks:-
1. Lithium fluoride is sparingly soluble in water. As a result, the formed lithium fluoride clogs the pipelines thereby causing problems in scrubbing the incoming gas (trifluoromethane sulfonyl fluoride).
2. Lithium fluoride formed is not pure as it carries along with it, a small amount of lithium sulfate arising out of electrochemical decomposition of methaneslufonyl fluoride.
US patent no 2009/0143613 A1 describes the process for making potassium salt of perfluoro-alkanesulfonate in which alkane sulfonyl halide represented by CnH2n+1 SO2X where n is an integer of 1 to 3 and X is a halogen either C1 or F, is subjected to electrochemical fluorination to get perfluoroalkanesulfonyl fluoride. The reactor gas is then scrubbed in a 10% aqueous solution of potassium hydroxide, thereby to generate a gas absorbed solution containing potassium perfluoroalkanesulfonate represented by general formulae CnF2n+1 SO3K in which n is an integer of 1 to 3. In addition to potassium perfluoroalkanesulfonate, the gas absorbed solution contains impurities like potassium fluoride and potassium sulfate.
The above impurities are taken care by adding either alkali metal hydroxide or alkali earth metal hydroxide, suitably calcium hydroxide to the gas absorbed solution. This is done so as to convert the fluoride into insoluble calcium fluoride which gets precipitated. Similarly, the sulfate present in gas absorbed solution is converted to insoluble calcium sulfate. While converting the fluoride and sulfate as insoluble calcium salt, potassium hydroxide is generated in the aqueous phase which is then converted into potassium sulfate by the addition of sulfuric acid. All the insoluble impurities are filtered off (calcium fluoride, calcium sulfate & potassium sulfate) leaving behind aqueous solution of potassium perfluoroalkane sulfonate, which may be concentrated and dried to get the pure salt.
As an alternate process, the patent describes, the addition of alkali earth metal hydroxide to gas absorbed solution so as to remove the fluoride and sulfate as insoluble, followed by addition of perfluoroalkanesulfonic acid to convert potassium hydroxide in solution to Potassium perfluoroalkanesulfonate.
The patent also makes a mention in the purification step, that the potassium hydroxide generated after removal of insoluble fluoride and sulfate may be used to neutralize the sulfuric acid residue solution, generated in acid decomposition reaction of potassium salt of perfluormethane-sulfonate.
The process has following drawbacks:-
1. Although the process describes the purification of potassium perfluoroalkane
sulfonate by precipitating the impurities as mixture of insoluble calcium fluoride,
calcium sulfate & potassium sulfate, the fluoride, content of the byproduct is lost in the
process. Further these precipitates need to be disposed in an environmentally safe
manner, which is a concern.
2. The process comprises step of neutralizing the potassium hydroxide with
perfluoroalkane sulfonic acid. Perfluorosulfonic acids are considered expensive for
recovering the potassium content from the aqueous solution. Further, it will not be
possible to recover the entire amount of perfluoroalkane sulfonic acid added for
neutralization of potassium hydroxide from potassium perfluroalkane sulfonate.
3. The process does recognizes the importance of moisture free potassium
perfluoroalkane sulfonate for acid decomposition step. For achieving the same, the
process uses the method of concentrating the aqueous solution and drying to get the
product free of water. Such process is not energy efficient and cannot be applied on the
industrial scale.
The present invention has solved the above problems and provides a process for synthesis of potassium perfluoroalkanesulfonate which could be used as a starting material for commercial production of perfluoroalkanesulfonic acid. Further the present
invention provides a process which has reduced amount of impurities such as chloride, fluoride and sulfate ions in potassium perfluoroalkane sulfonate.
OBJECTIVES OF THE INVENTION
The principle objective of this invention is to provide an improved process for synthesis of potassium perfluoroalkane sulfonate represented by general formula CnF2n +1 SO3K where n is an integer from 1 to 3.
Another objective of this invention is to provide a process which result in less amount of impurities such as chloride, fluoride and sulfate ions in potassium perfluoroalkane sulfonate.
Still another objective of this invention is to provide an economical process which result in formation of potassium fluoride as a byproduct that can be recycled.
SUMMARY OF THE INVENTION
The present invention relates to an improved process for synthesis of potassium perfluoroalkane sulfonate of general formula CnF2n+1 SO3K where n is an integer from 1 to 3, the process comprising the steps of:
i. subjecting alkanesulfonyl chloride to halogen exchange reaction with aqueous
KF to obtain alkanesulfonyl fluoride;
ii. reacting alkanesulfonyl fluoride obtained in step (i) with anhydrous HF in an electrochemical cell to obtain product gas containing perfluoro alkane sulfonyl fluoride;
iii. absorbing the product gas obtained in step (ii) in aqueous solution of potassium hydroxide (KOH) till most of the potassium hydroxide gets exhausted to obtain scrubbed mixture;
iv. distilling the scrubbed mixture obtained from step (iii) azeotropically using toluene as a solvent to remove water from the mixture till about 80% of the toluene is removed to obtain a slurry;
v. mixing acetone to the slurry obtained in step (iv);
vi. filtering the mixture obtained in step (v) to obtain solid KF and filtrate;
vii. washing KF obtained from step (vi) with acetone followed by drying to obtain dried KF;
viii. optionally recycling dried KF obtained from step (vii) for use in step (i);
ix. distilling the filtrate obtained from step (vi) and step (vii) to separate acetone, toluene and Potassium perfluoroalkane sulfonate;
x. optionally recycling toluene and acetone obtained from step (ix) to step (iv) and step (v) respectively.
In an embodiment, the molar ratio of alkanesulfonyl chloride to aqueous KF in step (i) is in the range of 1:1.2 to 1:1.5.
In an embodiment, the molar ratio of alkanesulfonyl fluoride to anhydrous HF in step (ii) is in the range of 1:1.2 to 1:1.4.
In another embodiment, the electrochemical fluorination is carried out at a temperature in the range of 5 to 10°C, at a voltage of 4-6 V with current density in the range of 0.12 to 0.21 A/dm2.
In yet another embodiment, the initial concentration of aqueous KOH in step (iii) is in the range of 10 to 30%.
In yet another embodiment, the product gas is absorbed till the concentration of aqueous KOH reaches less than 0.1%.
In still another embodiment, the azeotropic distillation in step (iv) is carried out in the presence of toluene.
In still another embodiment, me azeotropic distillation is carried out at a temperature in the range of about 80-100°C.
In another embodiment, the ratio of acetone: slurry in step(v) is in the range of 1:1.2 to 1:1.4 (wt/wt).
In another embodiment, the mixing of acetone to the slurry in step (v) is carried out at a temperature in the range of 25 to 35°C for a period in the range of 1 to 2 hrs.
In another embodiment, potassium fluoride obtained has fluoride content in the range 31 to 34%.
In yet another embodiment, potassium perfluoroalkane sulfonate has fluoride content is less than 300 ppm.
DESCRIPTION OF THE INVENTION
In accordance with the present invention, there is provided an improved process for producing potassium perfluoroalkane sulfonate of general formula CnF2n+1 SO3K where n is an integer from 1 to 3; the process comprising the following steps:
(i) subjecting alkanesulfonyl chloride to halogen exchange reaction with aqueous KF to obtain alkanesulfonyl fluoride;
(ii) reacting alkanesulfonyl fluoride obtained in step (i) with anhydrous HF in an electrochemical cell to obtain product gas containing perfluoro alkane sulfonyl fluoride;
(iii) absorbing the product gas obtained in step (ii) in aqueous solution of potassium hydroxide (KOH) till most of the potassium hydroxide gets exhausted to obtain scrubbed mixture;
(iv) distilling the scrubbed mixture obtained from step (iii) azeotropically using toluene as a solvent to remove water from the mixture till about 80% of the toluene is removed to obtain a slurry;
(v) mixing acetone to the slurry obtained in step (iv);
(vi) filtering the mixture obtained in step (v) to obtain solid KF and filtrate;
(vii) washing KF obtained from step (vi) with acetone followed by drying to obtain dried KF;
(viii) optionally recycling dried KF obtained from step (vii) for use in step (i);
(ix) distilling the filtrate obtained from step (vi) and step (vii) to separate acetone, toluene and potassium perfluoroalkane sulfonate;
(x) optionally recycling toluene and acetone obtained from step (ix) to step (iv) and step (v) respectively.
Alkanesulfonyl chloride is subjected to halogen exchange reaction with aqueous KF so as to get alkanesulfonyl fluoride. The halogen exchange reaction is carried out by known method as described in US3920738A. Alkane sulfonyl fluoride is prepared by reacting alkane sulfonyl chloride with an aqueous solution of potassium fluoride at temperature of about 50°C. The molar ratio of alkanesulfonyl chloride to aqueous KOH is taken in the range of about 1:1.2 to 1:1.5. Following completion of the reaction, the crude alkane sulfonyl fluoride layer is separated from the brine layer. Pure alkane sulfonyl fluoride at quantitative yield is recovered by distillation procedure.
Alkane sulfonyl fluoride obtained in step-1 is reacted with anhydrous HF in an electrochemical cell thereby generating product gas containing mainly perfluoroalkanesulfonyl fluoride along with small amount of decomposition product such as SO2F2 and CF4. HF acts as electrolyte and raw material in this step. The molar ratio of alkanesulfonyl fluoride to anhydrous HF may be taken in the range of 1: 1.2 to 1:1.4.
The outlet of the electrochemical reactor i.e. the product gas is then absorbed in aqueous potassium hydroxide solution. The absorption of the gases is done using a scrubber. The product gas from the reactor is absorbed in aqueous solution of preferably 10-30% potassium hydroxide at a temperature of about 50 to 85°C till the time most of the potassium hydroxide gets exhausted, resulting in scrubbed aqueous mixture of potassium perfluoroalkane sulfonate, potassium fluoride and small amount of potassium sulfate. The molar ratio of aqueous KOH: product gas may be about 2.2:1 to 2.5:1.
Water present in the scrubbed mixture of potassium perfluoroalkanesulfonate, potassium fluoride and potassium sulfate is then removed by heating the mixture preferably to about 80-100°C along with toluene where water is removed azeotropically with toluene.
When about 80% of toluene is removed from the process stream, acetone is added to the slurry to separate potassium perfluoroalkane sulfonate from potassium fluoride. In the present process, the ratio of acetone: slurry may be about 1:1.2 to 1:1.4 (wt/wt).
Potassium perfluoroalkane containing toluene and acetone is distilled thereby recovering the solvent for recycle and reuse. The potassium salt so obtained has fluoride content less than 300 ppm, preferably in the region of 100-150 ppm and can be safely used for the industrial production of perfluoroalkane sulfonic acid.
The potassium fluoride obtained in the process is used for step-1 to convert alkanesulfonyl chloride to alkanesulfonyl fluoride.
According to a non-limiting embodiment of the process, methane sulfonyl fluoride was obtained by reacting methane sulfonyl chloride with an aqueous solution of potassium fluoride at temperature of about 50°C. 1.2 to 1.5 moles of aqueous KF was used per mole of alkanesulfonyl chloride. Methane sulfonyl fluoride obtained was continuously fluorinated in an electrochemical cell using anhydrous hydrogen fluoride. 1.75 to 1.85 Kg of hydrogen fluoride was taken in the electrochemical cell. The electrochemical cell was operated at a temperature of 5-10° and at voltage of 4 to 6V with a current density of 0.12 to 0.21 A/dm2 and at a substrate concentration of 1-10%, thereby generating product gas containing mainly perfluoromethanesulfonyl fluoride along with 2-5% small amount of decomposition product such as SO2F2 and CF4. The outlet of the electrochemical reaction was absorbed in 2 to 2.5 kg of aqueous potassium hydroxide solution whose initial concentration was seen in the range of 10-30%. The absorption of the gases was done using a scrubber. The depletion of potassium hydroxide solution was monitored and when the concentration of potassium hydroxide was seen lower than 0.1%, the scrubber was taken out of the process stream and fresh scrubber was connected to the system. About 600-1000 grams of toluene was then added to the above aqueous solution and the water was removed through azeotropic distillation. When about 80% of toluene was removed from the process stream, the mass left in the reactor was in the form of slurry containing potassium perfluoromethane sulfonate, potassium fluoride and small amount of potassium sulfate. To mis slurry, about 800-1000 grams of acetone was added and stirred for a period of 1 hr at room temperature and was filtered.
The cake obtained after filtration contained mainly potassium fluoride which was washed with acetone to take care of any entrapped potassium salt of perfluoroalkane sulfonate and was dried to get potassium fluoride. Upon distillation of the filtrate, the potassium salt of perfluoromethane sulfonate was obtained having fluoride concentration in the range less than 300 ppm which can be safely employed in the industrial production of perfluroalkane sulfonic acid. The recovered solvents, acetone and toulene obtained in the distillation were recycled.
The process generates potassium salt of perfluoroalkane sulfonate without generating any waste (Green chemistry). The potassium fluoride generated as by product is recycled to step-1 for its use in converting methane sulfonyl chloride to methane sulfonyl fluoride.
The process does not generate solid waste like calcium fluoride and calcium sulphate which otherwise tend to clog the process stream. Hence current process facilitates ease of the reaction at all the stages of the process.
Solvent (Both toluene and acetone) are recovered and reused. Hence the cost of processing is much lower than the prior art.
The following examples are given by way of illustration of the present invention and therefore should not be construed to limit the scope of the present invention.
EXAMPLE -1
Methane sulfonyl fluoride was continuously fluorinated in an electrochemical cell using anhydrous hydrogen fluoride. The outlet of the electrochemical reactor was absorbed in 2072 grams of aqueous potassium hydroxide solution whose initial concentration was seen at 19.54 %. The absorption of the gases was done using a scrubber. The depletion of potassium hydroxide solution was monitored and when the concentration of potassium hydroxide was seen lower than 0.1 %, the scrubber was taken out of the process stream and fresh scrubber was connected to the system.
The composition of the scrubbed liquid was seen as under.
1. Moisture content 70.40 %
2. potassium hydroxide concentration of 0.03 %
3. potassium fluoride content of 12.09 %.

4. pH of the solution between 7-8
5. potassium trifluoromethanesulfonate (potassium triflate) 10.04%
6. potassium sulfate 0.7%
Now 1000 grams of toluene was added to the above aqueous solution and the water was removed through Azeotropic distillation. The amount of water collected was weighed and it amounts to 1530 grams indicating that all the water present in the aqueous solution has been removed. After removal of the water, toluene to the tune of 781 grams was removed from the system which is roughly 80% of initial amount of toluene added to the system. As a result of removal of toluene, the mass left in the reactor was in the form of slurry containing potassium perfluoroalkane sulfonate, potassium fluoride and small amount of potassium sulfate. To this slurry, 900 grams of acetone was added and was stirred for a period of 1 hr at room temperature and was filtered. The cake containing mainly potassium fluoride was washed with 200 ml of acetone twice to take care of any entrapped potassium salt of perfluoroalkane sulfonate and was dried to get 294 grams of potassium fluoride. The potassium fluoride so obtained was analyzed for its fluoride content and was seen at 33.08 % and the moisture content was seen at 1.34 %.
The filtrate was distilled to drive out the solvents (toluene and acetone) leaving behind 233 grams of potassium perfluoromethane sulfonate. The product so obtained was analyzed and had following composition:
Fluoride concentration by ionometer 260 ppm, toluene by HPLC was seen below detection limit, Moisture content is 0.32 % and sulfate content of 490 ppm. The melting point of the material was seen at 236 degree C.
All these parameters confirm that the product is of high purity and can be taken for acid decomposition step to generate perfluoralkane sulfonic acid matching the commercial quality.
EXAMPLE -2
Following the process described in example-1, 2070 grams of potassium hydroxide of 19.52% was taken for scrubbing the electrochemical reactor outlet gas. The composition of the aqueous stream after completion of the reaction was:-
1. KOH content: less than detectable limit.
2. Moisture content: 81.81%
3. potassium fluoride content: 13.46 %
4. potassium trifluoromethanesulfonate 9.2%
5. potassium sulfate 0.6%
600 grams of toluene was used for azeotropic distillation to remove 1890 grams of water present in the aqueous solution. To the above 900 grams of acetone was added to solubilize the potassium salt of perfluoromethane sulfonate. The slurry was filtered so as to remove the potassium fluoride from the soluble fraction. The weight of dry potassium fluoride was seen at 324 grams and the fluoride analysis (32.2% of fluoride) confirmed to pure potassium fluoride salt.
From the filterate, 184 grams of potassium perfluoromethane sulfonate was recovered, the analysis of which showed:
Fuoride content below detection limit and moisture content of 0.25%. The melting point of the material was seen at 236 degree C.
ADVANTAGES OF THE INVENTION
1. The process generates potassium salt of perfluoroalkane sulfonate without generating any waste (Green chemistry). The potassium fluoride generated as by product is recycled to step-1 for its use in converting alkane sulfonyl chloride to alkane sulfonyl fluoride.
2. The process does not generate solid waste like calcium fluoride and calcium sulphate which otherwise tend to clog the process stream. Hence current process facilitates ease of the reaction at all the stages of the process.
3. Solvent (Both toluene and acetone) are recovered and reused. Hence the cost of processing is much lower than the prior art.

We Claim:
1. An improved process for synthesis of potassium perfluoroalkane sulfonate of
general formula CnF2n +1 SO3K where n is an integer from 1 to 3, the process comprising
the steps of:
i. subjecting alkanesulfonyl chloride to halogen exchange reaction with aqueous
KF to obtain alkanesulfonyl fluoride;
ii. reacting alkanesulfonyl fluoride as obtained in step (i) with anhydrous HF in an electrochemical cell to obtain product gas containing perfluoro alkanesulfonyl fluoride;
iii. absorbing the product gas obtained in step (ii) in aqueous solution of potassium hydroxide till most of the potassium hydroxide gets exhausted to obtain scrubbed mixture;
iv. distilling the scrubbed mixture obtained from step (iii) azeotropically using toluene as a solvent to remove water from the mixture till about 80% of the toluene is removed to obtain a slurry;
v. mixing acetone to the slurry obtained in step (iv);
vi. filtering the mixture obtained in step (v) to obtain solid KF and filtrate;
vii. washing KF obtained from step (vi) with acetone followed by drying to obtain dried KF;
viii. optionally recycling dried KF obtained from step (vii) for use in step (i);
ix. distilling the filtrate obtained from step (vi) and step (vii) to separate acetone, toluene and potassium perfluoroalkane sulfonate;
x. optionally recycling toluene and acetone obtained from step (ix) to step (iv) and
step (v) respectively.
2. The process as claimed in step (i) of claim 1, wherein the molar ratio of alkanesulfonyl chloride to aqueous KF is in the range of 1:1.2 to 1:1.5.
3. The process as claimed in step (ii) of claim 1, wherein the molar ratio of alkanesulfonyl fluoride to anhydrous HF is in the range of 1:1.2 to 1:1.4.
4. The process as claimed in step (ii) of claim 1, wherein the electrochemical fluorination is carried out at a temperature in the range of 5° to 10°C, at a voltage of 4-6 V with a current density in the range of 0.17 to 0.21 A/dm2.
5. The process as claimed in step (iii) of claim 1, wherein the initial concentration of aqueous KOH is in the range of about 10 to 30%.
6. The process as claimed in any of the preceding claims, wherein the product gas is absorbed till the concentration of aqueous KOH reaches less than 0.1%.
7. The process as claimed in step (iv) of claim 1, wherein the azeotropic distillation is carried out in the presence of toluene.
8. The process as claimed in any of the preceding claims, wherein the azeotropic distillation is carried out at a temperature in the range of about 80-100°C.
9. The process as claimed in any of the preceding claims, wherein the mixing of acetone to the slurry is carried out at a temperature in the range of 25 to 35°C for a period in the range of 1 to 2 hrs.