JPH0794401B2 - Method for separating n-perfluorooctanoic acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to polytetrafluoroethylene (abbreviated as PTFE).
The method for purifying n-perfluorooctanoic acid, which is a raw material of ammonium n-perfluorooctanoate used as an emulsifier during emulsion polymerization during the production step of.

(Prior art and problems) Perfluorooctanoic acid (C ₇ F ₁₅ COOH) is obtained by electrolytically fluorinating octanoic acid chloride (C ₇ H ₁₅ COCl) into perfluorooctanoic acid fluoride (C ₇ F ₁₅ COF), which is then hydrolyzed. Decomposes to perfluorooctanoic acid. This product is an isomer (i-C ₇ F ₁₅ COOH) that is a by-product of electrolytic fluorination.
Is contained in about 10 to 20%, and to be used as an emulsifier during the production of PTFE, its allowable limit (0.2%
This isomer has to be removed to the following (preferably 0.1% or less).

To remove isomers, a recrystallization method using chloroform as a solvent has been conventionally performed, but the yield of perfluorooctanoic acid and the amount of isomers mixed after recrystallization vary greatly depending on the recrystallization conditions. There is a problem that the quality is not stable. In this case, if the amount of chloroform as the solvent is increased or the crystallization temperature is increased, the purity of n-C ₇ F ₁₅ COOH is improved, but the yield is decreased, and the amount of solvent is decreased or the crystallization temperature is decreased. It is known that the quality of ammonium perfluorooctanoate deteriorates due to a large amount of i-C ₇ F ₁₅ COOH mixed in. Therefore, there has been a demand for a more efficient method for purifying perfluorooctanoic acid, which improves the yield without lowering the purity.

(Issue findings leading to resolve) the present inventors, increase yield, and results to have intensively studied suppress contamination of isomers, that n-C ₇ F ₁₅ COOH is precipitated as monohydrate, also, Table As described in No. 1, as the water content in chloroform increases, the amount of perfluorooctanoic acid dissolved decreases, and n-C ₇ F ₁₅ C
OOH / i−C ₇ F ₁₅ COOH ratio is small, that is, n−
It was found that the solubility of the body in chloroform is relatively smaller. And, the n-body is efficiently and stably
In addition, it was found that the i-form content in the precipitated crystals can be reduced to 0.2% or less by adjusting the water content to be present in chloroform in order to cause the precipitation with a purity that can be used for the production of ammonium salts. The present invention has been completed.

(Structure of the Invention) That is, the present invention relates to n-perfluorooctanoic acid and i
-In the method of separating n-perfluorooctanoic acid from a mixture of perfluorooctanoic acid by recrystallization using chloroform as a solvent, the molar ratio of n-perfluorooctanoic acid is 0.9 times or more, and i-perfluorooctanoic acid in the precipitated crystals is N is characterized in that the amount of water that reduces the content to 0.2% or less is present in chloroform.
-Providing a method for separating perfluorooctanoic acid.

Perfluorooctanoic acid which is subjected to crystallization in the present invention include, for example, perfluorooctanoic acid fluoride obtained by electrochemical fluorination of octanoic acid chloride obtained by hydrolyzing a predominantly n-C ₇ F ₁₅ COOH i- C ₇ F ₁₅ COOH
A crude product of, usually n-form: 80-90%,
i-form: It has a composition of about 20 to 10%. As the recrystallization solvent chloroform, an industrial reagent is used, but since ethanol is added as a stabilizer in this chloroform, if it is used for recrystallization as it is, ethyl ester of perfluorooctanoic acid is produced. It is necessary to wash thoroughly with distilled water to remove ethanol. The chloroform after washing contains water only in the amount corresponding to the solubility, but it is not necessary to remove it, and the necessary water may be added.

The amount of water present in chloroform is 0.9 times or more in molar ratio with respect to the dissolved perfluorooctanoic acid. As shown in FIG. 1 below, the range of the amount of water present in chloroform depends on the amount of recrystallization solvent used to dissolve perfluorooctanoic acid. That is, when the weight ratio of the recrystallization solvent is 30, the molar ratio of water to perfluorooctanoic acid is about 0.9 to 1.2. Similarly, the weight ratio
In the case of 60, the molar ratio of water and perfluorooctanoic acid is
It is about 0.9 to 2.0. If the amount of water is too large, the yield will increase, but the amount of isomers mixed will increase, and if the molar ratio is less than 0.9, the yield will decrease and the amount of isomers will also increase.

The amount of the recrystallization solvent is usually 10 to 60 times, preferably 10 to 40 times, and more preferably 25 to 35 times the weight of the perfluorooctanoic acid to be dissolved. If the amount of solvent is small, it will take time to dissolve. Further, when the amount of solvent is large, the amount of isomer (i-form) mixed is extremely small, but a large-capacity crystallization tank is required, which is not economical.

The recrystallization operation procedure is as follows: Weigh a certain amount of perfluorooctanoic acid (n-form / i-form mixture) that has been previously dissolved and made uniform,
Charge washed chloroform and calculated amount of water to this, adjust the molar ratio of H ₂ O / n-C ₇ F ₁₅ COOH, and heat to about 50 ° C with stirring to completely remove perfluorooctanoic acid. Dissolve. After complete dissolution, the solution is recrystallized by cooling with appropriate stirring. The final crystallization temperature is preferably 0 to 5 ° C. After crystallization, the mixture is rapidly filtered under reduced pressure by a previously cooled filtration device, and dried at room temperature until the odor of chloroform disappears.

(Specific Disclosure of the Invention) Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.

Perfluorooctanoic acid used in Example 1 recrystallization, GC purity (area percentage) in _{n-C 7 F 15 COOH:} 85.06% i-C 7 F 15 COOH: is of 13.70%.

As a recrystallization solvent, industrial chloroform was used, and it was thoroughly washed with distilled water at room temperature to remove ethanol. Therefore, the washed chloroform contains water in an amount corresponding to the solubility.

1 Perfluorooctanoic acid (n- / i-form mixture) 4 previously dissolved in a glass beaker to homogenize the composition
Weigh 0.00g, 0.8g water, 1200g washed chloroform
Was added to adjust the molar ratio of H ₂ O / n-C ₇ F ₁₅ COOH to 1.08.
While stirring on an oil bath, this was heated to about 50 ° C. to completely dissolve perfluorooctanoic acid and make the system uniform. After the dissolution, the beaker was put in ice, externally cooled with stirring, and recrystallized. When the temperature reached 3 ° C., the beaker was taken out, and suction filtration was performed quickly with a Buchner filter which had been cooled to about the crystallization temperature in advance to obtain white crystals. It was dried at room temperature until the odor of chloroform disappeared, and the yield and GC purity were measured.

The resulting white crystals, 28.52g, GC purity _{n-C 7 F 15 COOH:} 99.88%, i-C 7 F 15 COOH: 0.12%.

Comparative Example 1 40.01 g of perfluorooctanoic acid (n- / i-form mixture) was weighed in the same manner as in Example 1, and 1200 g of water-washed chloroform was added without adding water to carry out the same recrystallization operation as in Example 1. I did. _{H 2 O / n-C 7} F 15 COOH molar ratio of = 0.54.

The resulting white crystals, 26.49g, GC purity _{n-C 7 F 15 COOH:} 98.45%, i-C 7 F 15 COOH: often isomer content 1.55%.

Comparative Example 2 By the same operation as in Example 1, the added water content was increased and H ₂ O / n was added.
A recrystallization operation was performed by adjusting the molar ratio of —C ₇ F ₁₅ COOH to 1.4.

The resulting white crystals, 30.35G, GC purity _{n-C 7 F 15 COOH:} 99.32%, i-C 7 F 15 COOH: 0.68%, and the yield becomes greater as compared with Example 1, The amount of isomers increased.

Example 2 Using perfluorooctanoic acid containing 12% (GC purity) of i-C ₇ F ₁₅ COOH, the charged amount, the solvent amount, and the added water amount were the same as in Example 1, and the crystallization temperature was 5 ° C. Recrystallization was performed. The molar ratio of H ₂ O / n-C ₇ F ₁₅ COOH is 1.06.

Obtained crystals, 27.76G, GC purity _{n-C 7 F 15 COOH 99.92} %, a _{i-C 7 F 15 COOH 0.08} %, purity but is satisfactory, yield compared to Example 1 Fell below.

Comparative Example 3 Recrystallization was performed under the same conditions as in Example 2 without adding water. _{H 2 O / n-C 7} F 15 COOH molar ratio of = 0.53.

The resulting white crystals, 25.48 g, GC purity _{n-C 7 F 15 COOH 99.52} %, i-C 7 is F ₁₅ COOH 0.48%, contamination of i-C ₇ F ₁₅ COOH compared to Example 2 There is a large amount.

Comparative Example 4 Recrystallization was performed under the same conditions as in Example 2 without adding water. _{H 2 O / n-C 7} F 15 COOH molar ratio of = 1.62.

The resulting white crystals, 25.48 g, GC purity _{n-C 7 F 15 COOH 99.28} %, i-C 7 is F ₁₅ COOH 0.72%, contamination of i-C ₇ F ₁₅ COOH compared to Example 2 There is a large amount.

Example 3 Perfluorooctanoic acid used in Example 1 (n-form / i-
20.01g of the body mixture) and washed with water
Recrystallization was carried out under the same conditions as in Example 1 except that 00 g was added and water was not added. _{H 2 O / n-C 7} F 15 COOH molar ratio = 1.08
Is.

The resulting white crystals, 13.52 g, GC purity _{n-C 7 F 15 COOH 100.00} %, an i-C ₇ F ₁₅ COOH traces of very high purity n-C ₇ F ₁₅ COOH was obtained.

Example 4 Perfluorooctanoic acid used in Example 1 (n-form / i-
Body mixture) was weighed at 20.05 g, 0.8 g of water and 1200 g of water-washed chloroform were added thereto to adjust the molar ratio of H ₂ O / n-C ₇ F ₁₅ COOH to 1.48, and the same conditions as in Example 3 were applied. Was recrystallized.

The resulting white crystals, 13.83 g, GC purity _{n-C 7 F 15 COOH 100.00} %, an i-C ₇ F ₁₅ COOH traces of very high purity n-C ₇ F ₁₅ COOH was obtained.

Comparative Example 5 Perfluorooctanoic acid (n-form / i-used in Example 1)
Body mixture) was weighed 20.04 g, water 6.0 g and water-washed chloroform 1200 g were added to adjust the H ₂ O / n-C ₇ F ₁₅ COOH molar ratio = 2.7, and the same conditions as in Example 3 were applied. Was recrystallized.

The resulting white crystals, 14.23 g, GC purity _{n-C 7 F 15 COOH 98.34} %, i-C 7 is F ₁₅ COOH 1.66%, a large amount of isomers i-C ₇ F ₁₅ COOH exceeds 1.0% Mixed in.

The above results are summarized in Table 2 and FIG.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of water and the amount of impurities based on the results of Examples and Comparative Examples.

Claims (1)

[Claims] 1. A method for separating n-perfluorooctanoic acid from a mixture of n-perfluorooctanoic acid and i-perfluorooctanoic acid by recrystallization using chloroform as a solvent, wherein the molar ratio is 0.9 times that of n-perfluorooctanoic acid. The above is the method for separating n-perfluorooctanoic acid, characterized in that the amount of water that reduces the i-perfluorooctanoic acid content in the precipitated crystals to 0.2% or less is present in chloroform.