JPS6116276B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for purifying hexafluoropropylene oxide. More specifically, the present invention relates to a method for purifying crude hexafluoropropylene oxide obtained by oxidizing hexafluoropropylene.

Crude hexafluoropropylene oxide obtained by oxidizing hexafluoropropylene with oxygen or a peroxide contains byproducts such as carbonyl fluoride, trifluoroacetic fluoride, and hexafluoroacetone. These by-products are
Since both of them have the effect of lowering the degree of polymerization when hexafluoropropylene oxide is oligomerized, it is necessary to completely remove them. Among these by-products, carbonyl fluoride and trifluoroacetic acid fluoride can be easily separated by distillation using the difference in their boiling points, but hexafluoroacetone, which has very similar boiling points, can be separated by distillation. It is difficult to remove by.

Therefore, the present inventors investigated a method for purifying crude hexafluoropropylene oxide by utilizing the following difference in reactivity with water between hexafluoropropylene oxide and hexafluoroacetone.

Hexafluoropropylene oxide is quite reactive with water near room temperature, so the contact time must be extremely short. In addition, the vapor pressure of water and hydrofluoric acid is quite high near room temperature, so dehydration using molecular sieves, calcium chloride, etc. and dehydrofluoric acid treatment using sodium fluoride are required as post-treatments, but this post-treatment is often necessary. Because hexafluoroacetone and pentafluoropropionic acid fluoride are produced in the process,
To avoid this, it is necessary to perform purification at low temperatures. Therefore, a water-soluble organic solvent is used to maintain the purification system at a low temperature and treat hexafluoropropylene oxide, which is a gas at room temperature, in the form of a solution.

As the water-soluble organic solvent, methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, tetrahydrofuran, etc. are used. These organic solvents can be mixed with water in any ratio as long as they do not solidify at low temperatures, for example -60°C, but generally they are used in a ratio of about 50 to 300 parts by weight per 100 parts by weight of water. used in

By the way, when crude hexafluoropropylene oxide (HFPO) is purified using a mixture of water and a water-soluble organic solvent, hexafluoroacetone (HFA) is produced under highly acidic processing conditions in the presence of hydrofluoric acid produced in the purification system. It was found that pentafluoropropionic acid fluoride (PFP) was produced as a by-product, and on the other hand, when excessive alkali was added to the mixed solution, the amount of pentafluoropropionic acid fluoride (PFP) produced increased. From these facts, it is inferred that the following reactions occur in the purification system.

(1) Purification reaction of crude hexafluoropropylene oxide containing hexafluoroacetone: (2) Side reactions under highly acidic treatment conditions: (3) Side reactions when adding excess alkali: According to this reasoning, the cause of the decrease in recovery rate in the purification system is that hexafluoroacetone or pentafluoropropionic acid fluoride is produced as a by-product in reaction (2) or (3) above, and the reaction of these by-products with water This is thought to be because the speed is higher than that of hexafluoropropylene oxide.

That is, in the case of hexafluoroacetone, (HFA.H ₂ O) is formed as described above, and in the case of pentafluoropropionic acid fluoride, it is reacted with water as follows.

And the reaction rate k ₁ for each water
(HFPO), k ₂ (HFA) and k ₃ (PFP) are k ₁
The relationship is ≪k ₃ <k ₂ , and in the low-temperature region, k ₁ is so small that it can be almost ignored. Furthermore,
This is thought to be because hexafluoropropylene oxide is an electron acceptor, so its reaction with an alkali is faster than its reaction with an acid.

From these inferences, it is predicted that by keeping the PH of the purification system somewhat more acidic than neutral, it is possible to purify hexafluoropropylene oxide without producing hexafluoroacetone or pentafluoropropionic acid fluoride as by-products, and in fact, such The correctness of the prediction was proven by the facts.

Therefore, the present invention relates to a method for purifying hexafluoropropylene oxide, and the purification of hexafluoropropylene oxide is carried out by mixing a water-soluble organic solvent and an alkaline aqueous solution while keeping the crude hexafluoropropylene oxide at pH 3 to 7, preferably 4 to 6. Low temperature conditions (-20 to -70℃) that can form liquids and solutions
This is done by making contact with the

The pH of the purification system can be adjusted by adding alkali. As the alkali, all commonly used alkaline compounds can be used, such as sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia, and organic amines.

Sodium fluoride can also be added to the mixed solution, and in this case, the recovered hexafluoropropylene oxide gas will not contain hydrofluoric acid, so the presence of hydrofluoric acid in the post-treatment process will cause Side reactions can be prevented in advance.

In this way, in the method for purifying crude hexafluoropropylene oxide according to the present invention, undesirable side reactions during purification are effectively suppressed;
Purified hexafluoropropylene oxide with good purity can be efficiently recovered.

Next, the present invention will be explained with reference to examples.

Example: Methanol in a separable flask with a capacity of 3.
900ml, add 300g of sodium fluoride and 5 drops of the indicator Methylred (color change range PH4.4-6.2), -60
After cooling to 50°C, 1005g of crude hexafluoropropylene oxide (synthesized by oxidation of hexafluoropropylene and containing several percent hexafluoroacetone as an impurity) was
Prepare in minutes. During the charging of hexafluoropropylene oxide, if the methyl red changes color from yellow to red, 37.5% potassium hydroxide aqueous solution is added dropwise each time to maintain the methyl red in yellow.
The total amount of potassium hydroxide aqueous solution dropped at the end of the preparation was 140 ml. After stirring for about 30 minutes, the temperature was raised to -20℃, and the generated gas was
Pass through sieves and sodium fluoride sequentially, cool with dry ice-methanol, and finally
The gas generated by raising the temperature to ℃ is collected. 938g
of purified hexafluoropropylene oxide was recovered, with a recovery rate of 93.3%.

NMR (CH ₂ Cl ₂ solvent) δ ^EXT _CF3COOH 0.5ppm
None 9.0ppm None Infrared absorption spectrum 1890cm ^-1 None Comparative Example 1 The same operation as in Example 1 was performed. However, the potassium hydroxide aqueous solution was not added dropwise at all, so the pH at the end of the reaction was 1 or less. For 1070g of crude hexafluoropropylene oxide,
559 g of purified hexafluoropropylene oxide was recovered, with a recovery rate of 52.2%.

NMR (CH ₂ Cl ₂ solvent) δ ^EXT _CF3COOH 0.5ppm
Yes 9.0ppm No Infrared absorption spectrum 1890cm ^-1 No By comparing these Examples and Comparative Example 1, the following can be said. That is, when purification is performed in the absence of alkali, a decrease in PH is observed,
In addition, since the presence of hexafluoroacetone was observed in the recovered gas by ¹⁹ F-NMR, it can be considered that this is produced during purification and that this reduces the recovery rate of hexafluoropropylene oxide. On the other hand, in purification in the presence of an alkali, by controlling the pH, the production of hexafluoroacetone and other by-products can be effectively suppressed, and the recovery rate of hexafluoropropylene oxide can be increased.

Comparative example 2 900 methanol in a stainless steel container with a capacity of 30 methanol
After cooling to -50°C, 9.77 kg of crude hexafluoropropylene oxide was charged over about 3 hours. Stirring was continued for 1 hour, and then the temperature was raised to -30°C. The gas generated was passed through a molecular sieve and sodium fluoride in sequence, cooled with dry ice and methanol, and finally heated to 0°C. Collect the gas generated. PH at the end of the reaction
is less than or equal to 1. 7.30Kg of purified hexafluoropropylene oxide was recovered and the recovery rate was
It is 74.7%.

Comparative Example 3 In Comparative Example 2, 300g of potassium hydroxide,
Also, crude hexafluoropropylene oxide
10.02Kg was used. In this case as well, the pH at the end of the reaction is 1 or less. 5.98Kg of purified hexafluoropropylene oxide was recovered with a recovery rate of 59.7%
It is.

NMR (CH ₂ Cl ₂ solvent) δ ^EXT _CF3COOH 0.5ppm
None 9.0ppm Present Infrared absorption spectrum 1890cm ^-1 None From these Comparative Examples 2 and 3, the following can be said. That is, in Comparative Example 2, the pH of the purification system was always 3.
Since the purification was not performed while the temperature was maintained within the range of ~7, the recovery rate decreased. In addition, in Comparative Example 3, compared to Comparative Example 2, approximately 2.5 times as much alkali compound was added per the same amount of crude hexafluoropropylene oxide, resulting in the presence of pentafluoropropionic acid fluoride in the recovered gas. Since this was observed, it is thought that this is generated during purification and that this further reduces the recovery of hexafluoropropylene oxide.

Claims (1)

[Claims] 1 PH of crude hexafluoropropylene oxide
A method for purifying hexafluoropropylene oxide, which comprises bringing it into contact with a water-soluble organic solvent-alkaline aqueous solution mixture under low temperature conditions capable of forming a solution while maintaining a temperature of 3 to 7.