MXPA00004788A - An improved process for purifying alpha-keto acids - Google Patents

Abstract

The present invention relates to an improved process for purifying alpha-keto acids. In particular, the present invention pertains to the isolation of pivalic acid and other organic impurities from alpha-keto acids. The process of the invention generally includes (i) the partial acidification of an aqueous solution of a sodium salt of the alpha-keto acid (a"keto salt"), (ii) a first solvent extraction of the keto salt solution to remove organic impurities, (iii) further acidification of the keto salt solution, (iv) a second solvent extraction to remove the alpha-keto acid, and (v) isolation of the alpha-keto acid from the solvent. In a preferred embodiment, the present invention relates to the preparation of trimethylpyruvic acid (TMPA) of enhanced purity.

Description

AN IMPROVED PROCEDURE TO PURIFY ALPHA-CETOACY FIELD OF THE INVENTION The present invention relates to an improved process for purifying α-keto acids. Specifically, the present invention relates to the isolation of pivalic acid and other organic impurities of α-keto acids. In a preferred embodiment, the present invention is related to the preparation of trimethylpyrubic acid (ATMP) of higher purity. BACKGROUND OF THE INVENTION It is known that a-keto acids and their salts are useful as starting materials in the production of herbicides. Specifically, an aqueous solution of the sodium salt of the ATMP is used in the synthesis of the Sencor herbicide. In the prior art, various methods have been proposed for producing α-keto acids and salts thereof. These procedures include (1) the reaction of sodium cyanide and acetyl chloride to form acetyl cyanide and hydrolysis after cyanide; (2) dry distillation of tartaric acid in the presence of potassium hydrogen sulfate; (3) the fermentation of lactic acid; (4) the oxidation of lactic esters with potassium permanganate, and (5) the oxidation of propylene glycol. In U.S. Pat. No. 4,175,188, a process for producing ATMP from trimethylpyrubic acid N-t-butylamide is described. The process includes the acid hydrolysis of the N-t-butylamide of trimethylpyruvic acid by refluxing in hydrochloric acid and then extracting with dilute sodium hydroxide, hydrochloric acid and ethyl acetate. The yield of the desired product is only 75% of the theoretical. In U.S. Pat. No. 4,614,822, a method for preparing ATMP is described by oxidation of the corresponding α-hydroxy acid. The processes identified above have several drawbacks, among which the following are included: (i) the yield of the desired product is low and (ii) the formation of considerable amounts of by-products makes it difficult to separate and purify the desired product. SUMMARY OF THE INVENTION It is an object of the present invention to provide a process for preparing higher purity a-keto acids. The object of the invention is achieved by (i) adding to an aqueous solution of a sodium salt of the α-keto acid (a "ce-tosal") a mineral acid in an amount such that the pH of the acidified solution is about 3, 0 to about 4.0; (ii) adding a solvent to the acidified solution and allowing the resulting mixture to separate into a first organic phase and a first aqueous phase; (III) eliminating the first organic phase of the first aqueous phase, wherein the first organic phase includes the solvent and organic impurities and where the first aqueous phase includes water and the sodium salt of the α-keto acid ("ketosal"); (iv) adding a mineral acid to the first aqueous phase in an amount such that the pH of the first acidified aqueous phase is less than 1; (v) adding a solvent to the first acidified aqueous phase and allowing the resulting mixture to separate into a second organic phase and a second aqueous phase; (vi) separating the second organic phase from the second aqueous phase, where the second organic phase includes the solvent and a-ketoacid and the second aqueous phase includes water, and (vi) recovering the a-ketoacid of higher purity from the second phase organic DETAILED DESCRIPTION OF THE INVENTION The process of the present invention relates to the preparation of a-keto acids of a higher purity. The procedure includes, in general, the partial acidification of an aqueous solution of a sodium salt of the α-ketoacid (a "ketosal"), a first extraction with solvent of the ketosal solution to remove organic impurities, subsequent acidification of the solution of cetosal, a second extraction with solvents to separate the a-ketoacid and isolation of the α-ketoacid with respect to the solvent. In the present invention, a mineral acid is added to an aqueous solution of a sodium salt of the α-keto acid (a "ketosal"). Suitable mineral acids include hydrochloric acid, sulfuric acid and phosphoric acid. In a preferred embodiment, the mineral acid is hydrochloric acid. The mineral acid is added in an amount such that the pH of the acidified solution is from about 3.0 to about 4.0 and, preferably, from about 3.3 to about 3.7. The mineral acid used in the reaction mixture is preferably an aqueous solution containing from about 30% by weight to about 100% by weight of mineral acid and, preferably, from about 37% to about 85% by weight of mineral acid The precise amount of mineral acid used in the reaction depends on the particular mineral acid selected. In addition, in a preferred embodiment, the ketosal solution is an aqueous solution of trimethylpyruvic acid salt (ATMP) and the purified a-keto acid is ATMP. A solvent is then added to the acidified ketosal solution. Preferably, the solvent is an organic solvent. Such suitable organic solvents are known in the art and include any ketone, ester, ether, alcohol, hydrocarbon and chlorine hydrocarbon insoluble in water. In a preferred embodiment, the solvent is ethyl acetate or methyl isobutyl ketone (MIBC). The ratio of the solution of ketosal to solvent in volume is from about 1: 1 to about 10: 1 and, preferably, from about 3: 1 to about 5: 1. The addition of the solvent to the ketosal solution results in a mixture that is separated into a first organic phase and a first aqueous phase. The first organic phase contains the solvent and organic impurities. The first aqueous phase includes water and ketosal. The first organic phase, which contains the impurities, is separated from the first aqueous phase. A mineral acid is then added to the first aqueous phase. The mineral acid for this second acidification step can be selected from the same group as previously indicated for the first acidification step of the invention. In a preferred embodiment, the mineral acid is hydrochloric acid. The mineral acid is added in an amount such that the pH of the acidified solution is less than 1 and, preferably, from about 0.4 to about 0.8. The mineral acid has a concentration of from about 30% to about 100% by weight and, preferably, from about 37% to about 85% by weight. The precise amount of mineral acid used in the reaction will depend on the selected concrete mineral acid. A solvent is then added to the first acidified aqueous phase. The solvent can be selected from the same group as that indicated in the previous step of solvent extraction. In a preferred embodiment, the solven-te is MIBC. The ratio of solution of ketosal to solvent in volume is from about 1: 1 to about 10: 1 and, preferably, from about 3: 1 to about 5: 1. The addition of the solvent results in a mixture that is separated into a second organic phase and a second aqueous phase. The second organic phase contains the solvent and a-keto acid. The second aqueous phase includes water and inorganic salts. The second organic phase is separated from the second aqueous phase. The a-keto acid of higher purity is then isolated from the solvent in the second organic phase. In a preferred embodiment, the isolation of the desired product is achieved by distillation of the solvent from the second organic phase. The process of the present invention can be carried out by a batch process or a continuous process. In addition, the solvent can be recovered from the first and second organic phases and recycled for later use in the process of the present invention. The following examples illustrate preferred embodiments of the present invention and are not limiting of the description or the claims in any way. EXAMPLES Example 1 - Isolation of ATMP in a batch process Approximately 3057 grams of an aqueous solution of ATMP salt was loaded into a container. The ATMP salt solution was stirred and then approximately 68 were added., 4 grams of concentrated hydrochloric acid (HCl). The resulting acidified solution had a pH of about 3.3. About 500 ml of MIBC was added to the acidified solution. This mixture was then stirred and allowed to settle. After about one minute, an organic layer and an aqueous layer were formed. The organic layer was separated from the aqueous layer. The aqueous layer was then divided into two portions. a.) The first portion of the aqueous layer contained approximately 1,552.5 grams of an acidified ATMP salt solution having a pH of 3.3. Approximately 67.7 grams of concentrated HCl was added to the ATMP salt solution, resulting in an acidified mixture having a pH of about 0.7. 310 grams of MIBC were then added to this acidified mixture. The mixture was stirred and allowed to settle. After approximately two minutes, an organic layer and an aqueous layer were formed. The organic layer was separated from the aqueous layer. The ATMP was isolated from the organic layer by distillation of the MIBC at about 45 ° C. The net yield of the ATMP recovered in the organic layer was from about 90% to about 93%. ATMP recovery increased to approximately 98% when a second MIBC extraction of the aqueous layer was performed. b. ) The second portion of the aqueous layer contained approximately 1,552.5 grams of an acidified ATMP salt solution having a pH of 3.3. Approximately 68.9 grams of concentrated HCl was added to the ATMP salt solution, resulting in an acidified mixture having a pH of about 0.7. The mixture was stirred and allowed to settle. After approximately eight minutes, an organic layer and an aqueous layer were formed. The organic layer was separated from the aqueous layer. The net yield of the ATMP recovered in the organic layer was approximately 61%. The ATMP in the aqueous layer can be recovered by extraction with MIBC. Example 2 - Isolation of ATMP in a continuous process An aqueous solution of ATMP salt and concentrated hydrochloric acid (HCl) was continuously pumped into a first container Bl. The ATMP salt solution had a pH of about 7 to about 11 and a purity of about 7% to about 8%. The molar ratio of HCl to ATMP salt solution was such that the resulting mixture had a constant pH of about 3 to about 4. This acidified ATMP salt solution was stirred and allowed to overflow to a second vessel B2, in the which was mixed continuously with methyl isobutyl ketone (MIBC). The acidified salt solution ratio of ATMP to MIBC was maintained at about 5: 1. The mixture of B2 then overflowed to a first separator Sl, in which the organic layer containing MIBC and impurities was separated from the aqueous layer, which contained water and salt of ATMP. The aqueous layer flowed to a third vessel B3. In B3, HCl was added continuously to the mixture in an amount such that a pH of about 0.7 was achieved. At this point, the ATMP salt became completely ATMP. The acidified mixture of B3 then flowed into a fourth reactor B4, in which a second stage of solvent addition took place. In B4, MIBC was added continuously in an amount sufficient to maintain the acidified solution ratio of ATMP to solvent of 5: 1. The mixture of B4 then overflowed to a second separator S2, in which the organic layer containing MIBC and ATMP was separated from the aqueous layer, which contained water. The desired ATMP product contained in the organic layer was isolated from the MIBC by distillation of the MIBC. The distillation was carried out at a reduced pressure, so that the temperature did not exceed about 80 ° C (the ATMP can decompose at higher temperatures). A solvent extraction of the aqueous layer of separator S2 resulted in the complete recovery of ATMP. The MIBC was recovered from the first and second organic layers by (i) distillation or (ii) separation of the acids by extraction with dilute sodium hydroxide. Although the invention has been described in detail in the foregoing for purposes of illustration, it should be understood that said detail has only that purpose and that those skilled in the art can make variations therein without deviating from the spirit and scope of the invention, except as may be limited by the claims. ^? ÜM A¡-Í-

Claims (13)

Claims

1. A process for preparing an a-keto acid of higher purity, consisting of the following steps: a. Add a mineral acid to an aqueous solution of a sodium salt of the a-keto acid, in an amount such that the pH of the acidified solution is from about 3.0 to about 4.0; b. adding a solvent to the acidified solution and allowing the resulting mixture to separate into a first organic phase and a first aqueous phase; c. separating the first organic phase from the first aqueous phase, wherein the first organic phase includes the solvent and organic impurities and the first aqueous phase includes water and the sodium salt of the a-keto acid; d. adding a mineral acid to the first aqueous phase in an amount such that the pH of the first acidified aqueous phase is less than 1; and. adding a solvent to the first acidified aqueous phase and allowing the resulting mixture to separate into a second organic phase and a second aqueous phase; F. separating the second organic phase from the second aqueous phase, wherein the second organic phase includes the solvent and the α-keto acid and the second aqueous phase includes water and inorganic salts, and g. recover the a-keto acid of higher purity of the second organic phase.

2. The process of Claim 1, wherein the mineral acid is selected from the group consisting of hydrochloric acid, phosphoric acid and sulfuric acid.

3. The process of Claim 2, wherein the mineral acid is hydrochloric acid.

4. The method of Claim 1, wherein the mineral acid has a concentration of about 30% to about 100% by weight.

5. The method of Claim 1, step a), wherein the pH of the acidified solution is from about 3.3 to about 3.5.

6. The method of Claim 1, wherein the solvent is an organic solvent.

7. The process of Claim 1, wherein the solvent is selected from the group consisting of ketones, esters, ethers, alcohols, hydrocarbons and chlorinated hydrocarbons insoluble in water.

8. The process of Claim 7, wherein the solvent is selected from the group consisting of ethyl acetate and methyl isobutyl ketone.

9. The method of Claim 1, wherein the solvent is methyl isobutyl ketone.

10. The process of Claim 1, step d), wherein the pH of the first acidified aqueous phase is from about 0.4 to about 0.8.

11. The process of Claim 1, step g), wherein the α-keto acid is isolated by distillation of the solvent.

12. The method of Claim 1, wherein the process is carried out in a continuous system.

13. The method of Claim 1, wherein the solvent is recovered from the first and second organic phases and recycled for use in steps b) and e).