| Title of Invention | METHOD FOR PREPARING TRIFLUOROMETHANESULPHINIC ACID |
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| Abstract | The present invention relates to a method for preparing a highly pure trifluorornethanesuiphinic acid. The method of the invention for preparing a highly pure trifluorornethanesuiphinic acid, starting from an aqueous mixture comprising a triflu-. oromethanesuiphinic acid salt, a trifluoroacetic acid salt and saline impurities resulting from the method for preparing same, is characterized in that said mixture is subjected to the following operations: acidification such that the trifluoroacetic acid salt and the triflinic acid salt are released in an acid form, - separation of the trifluoroacetic acid and trifluorornethanesuiphinic acid by distillation enabling the trifluomacetic acid to be recovered at the top of the distillation and the trifluommethanesuiphinic acid to be recovered at the bottom of the distillation, - separation by distillation of the trifluorornethanesuiphinic acid present in the distillation residue previously obtained. |
| Full Text | METHOD FOR PREPARING TRIFLUOROMETHANESULFINIC ACID A subject matter of the present invention is a process for the preparation of trifluoromethanesulfinic acid. The invention is targeted at the preparation of a trifluoromethanesulfinic acid of high purity. Trifluoromethanesulfinic acid, commonly known as "triflinic acid", or else its salified forms are products used in numerous fields (plant protection, pharmaceutical or other). One of the routes for the synthesis of said acid described in EP 0 735 023 consists in reacting trifluoroacetic acid, at least partially salified by an organic or inorganic cation, with sulfur dioxide in a polar organic solvent and heating the resulting mixture at a temperature of between 100°C and 200°C for a period of time of between 30 min and 20 hours. The relative amounts of the trifluoroacetic acid and of the sulfur dioxide are such that the ratio of the number of sulfur atoms per mole of trifluoroacetic acid is between 1 and 10, advantageously in the vicinity of two. At the end of the reaction, trifluoroacetic acid, trifluoromethanesulfinic acid in the saline form, preferably in the form of an alkali metal salt, preferably the sodium or potassium salt, and the organic solvent are obtained. Fluoride or sulfate salts, generally in the form of salt of an alkali metal, preferably the sodium or potassium salt, are also coproduced during the reaction. These salts formed are known as "saline impurities" in the continuation of the present text. Dilution is subsequently carried out by addition of water and then the polar organic solvent is extracted with an appropriate organic solvent, for example a chlorinated aliphatic hydrocarbon. The organic and aqueous phases are separated. An aqueous solution is generally obtained having a solids content of 10 to 40% by weight comprising: - from 5 to 35% by weight of a trifluoromethanesulfinic acid salt, preferably an alkali metal salt, - from 5 to 35% by weight of a trifluoroacetic acid salt, preferably an alkali metal salt, - from 0.5 to 2% by weight of saline impurities. Said aqueous solution preferably comprises from 15 to 20% by weight of a trifluoromethanesulfinic acid salt, from 10 to 15% by weight of a trifluoroacetic acid salt and from 0.5 to 2% by weight of saline impurities. The object of the present invention is to provide a trifluoromethanesulfinic acid of high purity starting from an aqueous phase comprising a trifluoromethanesulfinic acid salt in combination with a trifluoroacetic acid salt and saline impurities resulting from its process of preparation. The term "high purity" is understood to mean, in the present text, a trifluoromethanesulfinic acid having a purity of greater than or equal to 95%, preferably of greater than or equal to 98% and more preferably of greater than or equal to 99%. Thus, a subject matter of the present invention is a process for the preparation of a trifluoromethanesulfinic acid of high purity starting from an aqueous mixture comprising a trifluoromethanesulfinic acid salt, a trifluoroacetic acid salt and saline impurities resulting from its process of preparation, characterized in that said mixture is subjected to the following operations: - acidification in such a way that the salts of the trifluoroacetic acid and of the triflinic acid are released in an acid form, - separation of the trifluoroacetic acid and the trifluoromethanesulfinic acid by distillation, making it possible to recover the trifluoroacetic acid at the distillation top and the trifluoromethanesulfinic acid at the distillation bottom, - separation, by distillation, of the trifluoromethanesulfinic acid present in the distillation concentrate obtained above. In the continuation of the account of the invention, trifluoromethanesulfinic acid is denoted "triflinic acid". The process of the invention is based on the fact that an acidification is carried out in such a way that the salts of the trifluoroacetic acid and of the triflinic acid are released in an acid form and then that said acids obtained are separated by distillation. According to a preferred alternative form of the process of the invention, the aqueous mixture collected is subjected to a concentration operation. Specifically, it is desirable to concentrate said aqueous mixture before the acidification operation. To this end, the aqueous mixture defined above can be concentrated so as to increase the concentration of the salts of the trifluoroacetic acid and the triflinic acid in such a way that between 2 and 60% by weight, preferably between 5 and 5 0% by weight, of water is present in the concentrated solution obtained. One embodiment for concentrating the reaction medium consists in carrying out the distillation of a portion of the water in order to achieve, in the reaction medium, the desired concentration of water. The distillation can be carried out at atmospheric pressure at a temperature of 100°C. The distillation can also be carried out under a pressure slightly lower than atmospheric pressure, for example of between 1 mbar and' 600 mbar, and at a temperature of less than 100°C. Generally, the pressure is chosen in order to have a distillation temperature lying between 40°C and 90°C. Another embodiment consists in carrying out an entrainment by injection of a fluid, for example steam or inert gas, in particular nitrogen. From a practical viewpoint, the concentration operations as well as the distillation operations described below can be carried out in an evaporator. Use may be made of those which are available commercially and mention may be made, inter alia, of wiped film evaporators or falling film evaporators of Luwa® type. The invention does not rule out the use of other concentration techniques, such as ultrafiltration or reverse osmosis. In accordance with the process of the invention, an acidification of the concentrated aqueous solution is subsequently carried out. The acidification is carried out using a strong acid having a pKa of less than or equal to 1. The pKa is defined as the ionic dissociation constant of the acid/base pair when water is used as solvent. The choice is made of a strong acid which advantageously does not exhibit an oxidizing nature. Thus, nitric acid is not preferred. More preferably, sulfuric acid, hydrochloric acid or phosphoric acid is resorted to. Sulfuric acid is preferably chosen. The amount of strong acid employed is such that the ratio of the number of moles of acid, expressed as H+ ions, to the number of moles of the salts of trifluoroacetic acid and of triflinic acid varies between 1 and 10, preferably between 1 and 4. Advantageously, a concentrated solution of strong acid is resorted to. Use is more particularly made of the commercial forms of acids. Mention may in particular be made of 95 or 98% by weight sulfuric acid solutions, the 37% by weight hydrochloric acid solution or 95-100% by weight phosphoric acid solutions. Use may also be made of hydrochloric acid in the gaseous form or of oleums, which correspond to sulfuric acid charged with sulfur trioxide SO3, the concentration of which can vary between 10 and 60% by weight. Oleums comprising 20, 40 or 60% by weight of S03 are commercially available. At the end of the acid treatment, an aqueous solution or suspension is collected which comprises the trifluoroacetic acid, the triflinic acid, the excess strong acid, the salts formed subsequent to the acidification and the acid forms corresponding to the saline impurities. Generally, the solution or suspension obtained comprises from 4 to 26% by weight of trifluoroacetic acid, from 4 to 27% by weight of triflinic acid and from 5 to 60% by weight of water. According to an alternative form of the invention, it is possible to add to the aqueous solution or suspension obtained, subsequent to the concentration but before the acidification, a compound capable of trapping hydrofluoric acid, for example boric acid or silica, for example at a content of 1% by weight. Before carrying out the distillation of the trifluoroacetic acid, it is possible but not essential to carry out a separation of the solids present in the aqueous suspension (salts, silica, and the like) according to conventional solid/liquid separation techniques, preferably by filtration. In a following stage, a distillation operation is carried out which makes it possible to bring about the separation of the trif luoroacetic acid and the triflinic acid. The abovementioned aqueous phase comprising the acids to be separated is introduced into a distillation column and the trifluoroacetic acid and optionally water are removed at the distillation top and the triflinic acid, accompanied by the excess strong acid, the salts of the strong acids, in particular those originating from the acidification, and water, is recovered at the distillation bottom. The distillation is carried out at a temperature in the reboiler of between 60°C and 90°C under a pressure ranging from 7 00 mbar to 5 0 mbar. It is carried out in a conventional distillation apparatus. A person skilled in the art is fully in a position to choose the means to be employed according to the compounds to be separated. The following will simply be restated. The size (in particular the diameter) of the distillation columns depends on the circulating stream and on the internal pressure. They will thus be sized mainly according to the flow rate of mixture to be treated. The internal parameter which is the number of theoretical stages is determined in particular by the purity of the starting compound and the purity of the product which has to be obtained at the distillation top. It will be specified that the columns can be packed without distinction with plates or with stacked packing, as is fully known to a person skilled in the art. The plant being established, a person skilled in the art adjusts the operating parameters of the column. Thus, the distillation column can advantageously but not limitingly be a column having the following specifications: - number of theoretical stages: from 1 to 10, preferably from 1 to 5, - reflux ratio R of between 1 and 20, preferably between 5 and 10. A distillation concentrate, comprising the triflinic acid, is recovered at the column bottom and a gas phase, composed of the trifluoroacetic acid optionally accompanied by water, is recovered at the column top. The gas phase is cooled and converted to the liquid form by cooling to a temperature, for example, of between -2 0°C and 2 0°C, preferably of between -10°C and 10°C. This operation is carried out by passing through a condenser which is a conventional device, for example a tube heat exchanger fed with water or with a fluid maintained at a temperature in the vicinity of the cooling temperature chosen. The choice is advantageously made, in carrying out the distillation operation, of equipment capable of withstanding the corrosion brought about by the compounds to be separated. To this end, the choice is made of materials which are advantageously enameled steels. The distillation concentrate comprising the triflinic acid which it is desired to obtain is subsequently treated. To this end, the triflinic acid is distilled at a temperature which, for safety reasons, is maintained below 100°C, preferably below 90°C, under a reduced pressure preferably chosen below 50 mbar and more preferably between 0.1 mbar and 30 mbar. The choice is advantageously made of a pressure between 5 mbar and 30 mbar. The invention does not rule out the addition of a third solvent in order to facilitate the distillation. Mention may in particular be made of aliphatic hydrocarbons, such as, for example, decane, decalin or petroleum fractions sold under the Isopar name; aromatic hydrocarbons, such as in particular toluene, xylenes or mesitylene; halogenated aromatic hydrocarbons and very particularly monochlorobenzene, dichlorobenzene or their mixtures. The amount of third solvent employed, expressed with respect to the weight present in the reboiler, can vary fairly widely. It can be equal, for example, to 10 to 10 0% by weight and preferably to 2 0 to 5 0% by weight. The triflinic acid, optionally accompanied by the third solvent, is recovered at the distillation top and is liquefied by passing through a condenser maintained at a temperature of between -20°C and 20°C. A concentrate comprising residual triflinic acid, the salts of the strong acids, the excess strong acid used in the acidification, water and optionally the third solvent added is obtained at the distillation bottom. The invention does not exclude the case where an additional distillation is carried out after the acidification operation which makes it possible to collect a mixture of the trif luoroacetic acid and the trifluoromethanesulfinic acid and then, starting from the mixture obtained of said acids, the separation of the trifluoroacetic acid and the trifluoromethanesulfinic acid is carried out, which separation is carried out as described above, namely a first distillation, which makes it possible to recover the trif luoroacetic acid at the distillation top and the trifluoromethanesulfinic acid at the distillation bottom, and then subsequently a second distillation of the distillation concentrate obtained above, which makes it possible to recover the expected trifluoromethanesulfinic acid at the distillation top. In this alternative form, the distillation conditions which make it possible to recover the mixture of the trifluoroacetic acid and the trifluoromethanesulfinic acid are those defined for the recovery of the 10 trifluoromethanesulfinic acid. The process of the invention is highly advantageous as it results in triflinic acid with a high purity preferably of greater than or equal to 95%, more 15 preferably of greater than or equal to 98% and more particularly of between 99 and 99.5%. Implementational examples of the invention are given below by way of indication and without a limiting 0 nature. Example 1 An aqueous solution (774 g) comprising 16% by weight of 5 potassium trifluoromethanesulfinate and 13.5% by weight of potassium trifluoroacetate is charged to a 1 liter jacketed glass reactor equipped with a central stirrer and maintained under a nitrogen atmosphere. The combination is placed under a pressure varying from 14 mbar to 2 mbar while maintaining a temperature of 50°C. 225 g of a suspension are obtained, which suspension remains fluid at 50°C. The concentrated solution is run onto concentrated 92% sulfuric acid (780 g) in a reactor surmounted by a distillation column and by a condenser cooled with water (15°C). The temperature of the medium is kept below 3 0°C 5 the operation in which the concentrated agueous solution is run in. The pressure is lowered to 280 mbar and the temperature of the reboiler is maintained at 68°C. During the distillation, the pressure is gradually lowered to 86 mbar in order to maintain a satisfactory flow rate of distillate. 15 The first distillation fraction is collected (56 9) when the temperature of the vapors at the column top lS between 26°C and 36°C. This fraction is composed of 85% by weight 20 trifluoroacetic acid and 15% of triflinic acid. The pressure is subsequently lowered to 3 mbar. A second fraction (44 g) is collected when the5 temperature of the vapors at the column top is between 29°C and 41°C. During this operation, the temperature of the reboiler varies from 70°C to 79°C. This fraction comprises 99% pure trifluoromethanesulfinic acid. Example 2 An aqueous solution (3000 g) comprising 16% by weights of potassium trifluoromethanesulfinate and 13.5% by weight of potassium trifluoroacetate is concentrated on a wiped film evaporator of Luwa type with an exchange surface area of 314 cm2 maintained at a temperature of 80°C and a pressure of 50 mbar. The feed flow rate is adjusted so as to obtain 980 g of concentrated solution at the outlet of the evaporator. The concentrated solution is run onto 2940 g of concentrated 96% sulfuric acid while maintaining the temperature below 2 0°C. The pressure of the reactor is adjusted to 280 mbar and the temperature of the reaction mass is brought to 78°C. A first fraction of 274 g is collected when the temperature at the column top is between 41°C and 45°C. The pressure is subsequently maintained at 1 mbar and a second fraction of 245 g is collected. This fraction comprises 99% pure trifluoromethanesulfinic acid. Example 3 A mixture composed of potassium trifluoroacetate (84.0 g, 0.55 mol), potassium trifluoromethanesulfinate (93.4 g, 0.54 mol) and water (44 g, 2.4 mol) is added to phosphoric acid (600 g, 6.1 mol) brought to 60°C and left stirring for one hour. The medium is subsequently distilled under reduced pressure using a Vigreux column. The temperature of the medium is brought to 7 0°C under a reduced pressure of 150 mbar. A fraction of 62.9 g having a boiling point of 24°C at this pressure is thus obtained. This fraction comprises 94% by weight of trifluoroacetic acid and 6% by weight of water. The trifluoroacetic acid yield is 84%. The temperature is subsequently raised to 90°C and the pressure is lowered to 0.3 mbar. 71.1 g of a second distillation fraction having a boiling point of 28°C are obtained. This fraction comprises 79% by weight of trifluoromethanesulfinic acid and 21% by weight of trifluoroacetic acid. The trifluoromethanesulfinic acid yield is 77%. This fraction is redistilled under a pressure of 2 5 mbar. A first fraction (16 g) comprising 94% by weight of water and a second fraction (47.8 g) comprising 98% by weight of trifluoromethanesulfinic acid are then obtained. Example 4 Concentrated 95% by weight sulfuric acid (162 g, 1.6 mol) is added to an aqueous solution (3 64 g) comprising potassium trifluoroacetate (95 g, 0.63 mol) and potassium trifluoromethanesulfinate (108 g, 0.63 mol) while maintaining the temperature of the medium below 10°C. A solid is formed in the medium. Stirring of the medium is maintained for 30 minutes and then the solid (205 g) is removed by filtration through a sintered glass funnel with a porosity of 3. The filtrate obtained (301 g) is distilled under reduced pressure. The medium is brought to a bulk temperature of 5 5°C under a pressure of 18 mbar. A first fraction (201 g) having a boiling point of approximately 2 5°C is obtained. This fraction comprises 27% by weight of trifluoroacetic acid and 73% by weight of water. The trifluoroacetic acid yield is 77%. The medium is subsequently brought to a temperature of 95°C under a pressure of 0.2 mbar. A second distillation fraction (57 g) comprising 72% by weight of trifluoromethanesulfinic acid and 2 8% by weight of water is obtained. The trifluoromethanesulfinic acid yield is 64%. This fraction is redistilled under a pressure of 2 5 mbar. A first fraction (17 g) comprising 94% by weight of water and a second fraction (32 g) comprising 98% by weight of trifluoromethanesulfinic acid are then obtained. WHAT IS CLAIMED IS: 1. A process for the preparation of a trifluoromethanesulfinic acid of high purity starting from an aqueous mixture comprising a trifluoromethanesulfinic acid salt, a trifluoroacetic acid salt and saline impurities resulting from its process of preparation, characterized in that said mixture is subjected to the following operations: - acidification in such a way that the salts of the trifluoroacetic acid and of the triflinic acid are released in an acid form, - separation of the trifluoroacetic acid and the trifluoromethanesulfinic acid by distillation, making it possible to recover the trifluoroacetic acid at the distillation top and the trifluoromethanesulfinic acid at the distillation bottom, - separation, by distillation, of the trifluoromethanesulfinic acid present in the distillation concentrate obtained above. 2. The process as claimed in claim 1, characterized in that the aqueous mixture is an aqueous solution having a solids content of 10 to 40% by weight comprising: - from 5 to 35% by weight of a trifluoromethanesulfinic acid salt, preferably an alkali metal salt, - from 5 to 35% by weight of a trifluoroacetic acid salt, preferably an alkali metal salt, - from 0.5 to 2% by weight of saline impurities. 3. The process as claimed in claim 2, characterized in that said aqueous solution comprises from 15 to 2 0% by weight of a trifluoromethanesulfinic acid salt, from 10 to 15% by weight of a trifluoroacetic acid salt and from 0.5% to 2% by weight of saline impurities. 4. The process as claimed in one of claims 1 to 3, characterized in that the reaction medium is concentrated so as to increase the concentration of the salts of the trifluoroacetic acid and the triflinic acid in such a way that between 2 and 60% by weight, preferably between 5 and 50% by weight, of water is present in the concentrated solution obtained. 5. The process as claimed in one of claims 1 to 4, characterized in that water is removed by distillation at atmospheric pressure or under a pressure lower than atmospheric pressure or by injection of a steam or inert gas fluid. 6. The process as claimed in claim 1, characterized in that the acidification is carried out using sulfuric, hydrochloric or phosphoric acid, an oleum or gaseous hydrochloric acid. 7. The process as claimed in claim 6, characterized in that the amount of strong acid employed is such that the ratio of the number of moles of acid, expressed as H+ ions, to the number of moles of the salts of trifluoroacetic acid and of triflinic acid varies between 1 and 10, preferably between 1 and 4. 8. The process as claimed in either of claims 6 and 7, characterized in that a concentrated solution of strong acid is resorted to. 9. The process as claimed in one of claims 1 to 8, characterized in that, at the end of the acid treatment, an aqueous solution or suspension is collected which comprises the trifluoroacetic acid, the triflinic acid, the excess strong acid, the salts formed subsequent to the acidification and the acid forms corresponding to the saline impurities. 10. The process as claimed in claim 9, characterized in that the solution or suspension obtained comprises from 4 to 2 6% by weight of trifluoroacetic acid, from 4 to 27% by weight of triflinic acid and from 5 to 60% by weight of water. 11. The process as claimed in claim 1, characterized in that a compound capable of trapping hydrofluoric acid is added to the aqueous solution or suspension obtained, subsequent to the concentration but before the acidification. 12. The process as claimed in claim 1, characterized in that the aqueous phase obtained subsequent to the acidification is subjected to a distillation operation which makes it possible to recover, at the distillation top, the trifluoroacetic acid and optionally water and, at the distillation bottom, the triflinic acid, accompanied by the excess strong acid, the salts of the strong acids, in particular those originating from the acidification, and water. 13. The process as claimed in claim 12, characterized in that the distillation is carried out at a temperature in the reboiler of between 60°C and 90°C under a pressure ranging from 700 mbar to 50 mbar. 14. The process as claimed in claim 12, characterized in that the gas phase comprising the trifluoroacetic acid and optionally water is cooled and converted to the liquid form by cooling to a temperature of between -2 0°C and 2 0°C. 15. The process as claimed in claim 1, characterized in that the distillation concentrate comprising the triflinic acid is subjected to a distillation operation which makes it possible to recover, at the distillation top, the triflinic acid and, at the distillation bottom, a concentrate comprising residual triflinic acid, the salts of the strong acids, the excess strong acid used in the acidification and water. 16. The process as claimed in claim 15, characterized in that the triflinic acid is distilled at a temperature below 100°C, preferably below 90°C, and under a reduced pressure preferably chosen below 5 0 mbar and more particularly between 0.1 mbar and 30 mbar and more preferably still between 5 mbar and 30 mbar. 17. The process as claimed in claim 16, characterized in that a third solvent, chosen from aliphatic hydrocarbons, preferably decane, decalin or petroleum fractions sold under the Isopar® name; aromatic hydrocarbons, preferably toluene, xylenes or mesitylene; halogenated aromatic hydrocarbons, preferably monochlorobenzene, dichlorobenzene or their mixtures, is added to the distillation concentrate comprising the triflinic acid. 18. The process as claimed in claim 16, characterized in that the triflinic acid, optionally accompanied by a third solvent, is recovered at the distillation top and is liquefied by cooling to a temperature of between -20°C and 20°C. 19. The process as claimed in one of claims 1 to 18, characterized in that the triflinic acid obtained has a purity of greater than or equal to 95%, preferably of greater than or equal to 98% and more preferably of between 99 and 99.5%. |
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| Patent Number | 279621 | |||||||||||||||
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| Indian Patent Application Number | 3112/CHENP/2010 | |||||||||||||||
| PG Journal Number | 05/2017 | |||||||||||||||
| Publication Date | 03-Feb-2017 | |||||||||||||||
| Grant Date | 27-Jan-2017 | |||||||||||||||
| Date of Filing | 26-May-2010 | |||||||||||||||
| Name of Patentee | RHODIA OPERATIONS | |||||||||||||||
| Applicant Address | 40, RUE DE LA HAIE-COQ, F-93306 AUBERVILLIERS. | |||||||||||||||
Inventors:
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| PCT International Classification Number | C07C313/04 | |||||||||||||||
| PCT International Application Number | PCT/EP08/066161 | |||||||||||||||
| PCT International Filing date | 2008-11-25 | |||||||||||||||
PCT Conventions:
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