JP3115434B2 - Method for producing perfluoro compound - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a perfluoro compound suitable for use as an electrically insulating liquid by an electrolytic fluorination method.

[0002]

BACKGROUND OF THE INVENTION Perfluoro (N, N-dimethylcyclohexylamine) and perfluoro (N, N-dimethylhexylamine) can be obtained as a mixture by electrolytic fluorination of N, N-dimethylaniline. Are known. Both have similar physical properties such as boiling points and are difficult to separate, and a considerable amount of loss cannot be avoided in order to obtain high purity. Therefore, for example, when used as an insulating liquid for a transformer, there is not much difference between the two in electrical properties and the like, and it is economically advantageous to use the mixture as it is without separating them.

[0003]

On the other hand, insulating liquids used particularly in large transformers are chemically stable and generate harmful decomposition products even when exposed to high temperatures due to some abnormalities. It is required to be difficult. However, perfluoro (N, N
-Dimethylcyclohexylamine) and perfluoro (N, N-dimethylhexylamine) were not sufficient in terms of thermal stability.

[0004]

DISCLOSURE OF THE INVENTION In view of the above-mentioned problems, the present inventors have developed a perfluoro (N, N-dimethylcyclohexylamine) having good thermal stability and less generation of harmful decomposition products. In order to obtain a mixture with perfluoro (N, N-dimethylhexylamine), an electrolytic fluorination method has been studied. As a result, the mixed composition of perfluoro (N, N-dimethylcyclohexylamine) and perfluoro (N, N-dimethylhexylamine) contained in the fluorinated product greatly changes depending on the conditions of electrolytic fluorination; Further, they have found that the thermal stability of the mixture is extremely excellent when this composition is within a specific range, and have led to the present invention.

That is, the present invention provides the following formula:

[0006]

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(Where A is a phenyl group or a cyclohexyl group which may be substituted by a fluorine atom;
¹ and R ² are the same or different alkyl groups. )
Is a method for producing a perfluoro compound, characterized by continuously performing electrolytic fluorination in an electrolytic cell while circulating an amine represented by the formula (1) and anhydrous hydrofluoric acid.

The raw material amine used in the present invention is represented by the above formula. In the above formula, A may be either a phenyl group or a cyclohexyl group, but a phenyl group can provide a more stable fluorinated product.
Further, the number of carbon atoms of the alkyl group represented by R ¹ and R ² is not particularly limited, but is usually preferably in the range of 1 to 3. Note that a part of the hydrogen atoms of the phenyl group and the clohexyl group represented by A may be previously substituted with a fluorine atom.

The hydrofluoric anhydride used in the present invention may be a commercially available hydrofluoric anhydride as it is, or a known method such as electrolysis of a small amount of water contained at a low current density in advance if necessary. Used after removal by

The present inventors have proposed the N, N-
In the electrolytic fluorination of dialkylaniline or N, N-dialkylcyclohexylamine, the conditions of electrolytic fluorination and the resulting perfluoro (N, N-dialkylcyclohexylamine) and perfluoro (N, N-dialkylhexylamine) The relationship with the thermal stability of the mixture was investigated in detail. As an index of thermal stability, the amount of perfluoroisobutylene (hereinafter abbreviated as PFIB) contained in a pyrolysis product generated by heating a mixture of both components was adopted. As a result, perfluoro (N,
The mixture of N-dialkylcyclohexylamine) and perfluoro (N, N-dialkylhexylamine) has a content of the former of 75 to 95% by weight, more preferably of 8 to 95% by weight.
When the content was 0 to 90% by weight, it was found that the mixture of both components had particularly excellent thermal stability at high temperatures.
The composition of both components can be determined from the area ratio of both components in the gas chromatogram by selecting measurement conditions for gas chromatography.

[0011] The mixture of the above specific composition is
It has been clarified that it can be obtained by continuous electrolytic fluorination in an electrolytic cell while circulating the amine represented by the above formula and hydrofluoric anhydride.

The circulation of the raw material amine and anhydrous hydrofluoric acid may be performed only in the electrolytic cell, or a circulating tank is provided separately from the electrolytic cell, and the electrolytic cell and the circulating tank are connected to each other by a tube. The connection may be made by forming a closed circuit between them and circulating through the closed circuit. As means for circulating, a normal pump may be used, and the amount of these circulations is such that the linear velocity of the electrolytic solution on the electrode surface in the electrolytic cell is 1.5 to 30 cm / cm.
It is preferable that the speed is set to be in seconds, preferably 2.5 to 20 cm / sec in order to obtain the effect of the present invention by performing electrolytic fluorination continuously for a long period of time.

As the material of the electrode of the electrolytic cell used in the present invention, known materials can be used without any limitation. Nickel or a nickel alloy is usually used as the anode, and iron, stainless steel, copper, or the like is used as the cathode in addition to nickel or its alloy. In general, the distance between the negative and positive electrodes is preferably about 0.5 to 5 mm.
As the material of the electrolytic cell, the above-described material of the cathode can be used as it is, and a fluorine resin can also be used.

The circulating tank is used to effectively settle and separate the fluorinated product containing the target perfluoro compound produced in the electrolytic fluorination from the electrolytic bath solution, and to separate the raw material amine from the anhydrous hydrofluoric acid. It can be suitably used because the circulation can be performed efficiently. As the material of the circulation tank, the same material as in the case of the electrolytic tank can be used.

In the present invention, it is necessary to continuously perform electrolytic fluorination while continuously or intermittently supplying the raw material amine and hydrofluoric anhydride. When electrolytic fluorination is performed in a batch system without supplying anhydrous hydrofluoric acid and raw material amine during electrolytic fluorination, the composition of the fluorinated product changes over time,
Even if various conditions are changed, perfluoro (N, N-dialkylcyclohexylamine) and perfluoro (N, N-dialkyl) obtained in a single batch reaction from the start of electrolysis until the end of the electrolysis when the raw material amine disappears. (Hexylamine) is less than 75% by weight in the mixture, and only a mixture having a composition having a problem in thermal stability can be obtained.

The raw material amine and hydrofluoric anhydride are continuously added so that the total concentration of the raw material amine and its fluorinated intermediate in the electrolytic bath solution becomes substantially constant within the range of 5 to 40% by weight. Alternatively, intermittent supply is preferable because a fluorinated product having good thermal stability can be obtained and stable electrolytic fluorination can be performed.

The conditions for electrolytic fluorination are as follows: current density is 1 A /
dm ² or more, and more preferably 1.5 A / dm ² or more, and the voltage between the anode and the cathode can be usually used in the range of 4.5 to 6.5 V. The electrolysis temperature is preferably selected from the range of -15 to 20C.

When an electrolytic cell or a circulation tank is used for electrolytic fluorination, a mixture of perfluoro (N, N-dialkylcyclohexylamine) and perfluoro (N, N-dialkylhexylamine) obtained from the lower part of the circulation tank. Can be purified by separating and removing various fluorinated by-products and partially mixed hydrogen fluoride by a known method. The fluorinated product contains a considerable proportion of fluorinated products in which some hydrogen atoms remain. Therefore,
Further fluorination of a fluorinated product in which hydrogen atoms are partially left by performing direct fluorination using fluorine gas or the like includes perfluoro (N, N-dialkylcyclohexylamine) and perfluoro (N, N-dialkylhexylamine). This is an embodiment that is preferably employed in the present invention, because not only the yield of the mixture of (1) is improved, but also the thermal stability of the mixture is further improved. The mixture of perfluoro (N, N-dialkylcyclohexylamine) and perfluoro (N, N-dialkylhexylamine) newly produced by direct fluorination has a slightly different composition from that produced by electrolytic fluorination. It is considered that this also contributes to improvement in thermal stability. Needless to say, necessary purification treatment can be performed after fluorination such as direct fluorination.

[0019]

According to the present invention, the above-mentioned raw material amine is electrolytically fluorinated to obtain highly safe perfluoro (N, N-dialkylcyclohexylamine) and perfluoro ( (N, N-dialkylhexylamine). The resulting mixture does not substantially generate PFIB even when exposed to a high temperature of, for example, 250 ° C. or more, and thus can be suitably used for applications such as insulating liquids for large transformers.

Although the reason why the present invention can provide a great effect is not known in detail, perfluoro (N, N-dialkylcyclohexylamine) and perfluoro (N, N- (Dialkylhexylamine).

[0021]

The present invention will be described in more detail with reference to the following Examples and Comparative Examples, which by no means limit the scope of the present invention.

EXAMPLE A pair of nickel negative anodes having an area of 5.6 dm ² (width 8.0 cm, height 70 cm) and a thickness of 2 mm were arranged at an interval of 2 mm, and the electrolytic solution always passed between the negative and positive anodes. A stainless steel electrolytic cell with the periphery of the anode closed as described above,
Similarly, a stainless steel circulating tank (capacity: 3 L) was connected with a tube to form a closed circuit using an electrolytic device.
Electrolytic fluorination of dimethylaniline was performed. Hydrofluoric anhydride and N, N-dimethylaniline are continuously supplied to the circulation tank so that the total amine concentration in the electrolyte in the steady state maintains 10% by weight, and the electrolyte from the circulation tank is supplied. The liquid flowed between the electrodes at a flow rate of 4 cm / sec from the lower part of the electrolytic cell, and returned to the circulation tank again by overflowing from the upper part of the electrode. The current value is 16.8 A (current density 3 A / dm ² )
The voltage between the negative and positive electrodes was about 5.75V. The electrolysis temperature was maintained at about 12 ° C. by externally cooling the electrolysis tank and the circulation tank. The generated hydrogen gas was provided at the top of the electrolytic cell.
Discharged through a reflux condenser at 40 ° C.

The fluorinated product obtained from the lower part of the circulation tank was further fluorinated by flowing an excessive amount of fluorine gas for 15 hours. After removing hydrogen fluoride, distillation was performed to obtain a mixture of perfluoro (N, N-dimethylcyclohexylamine) and perfluoro (N, N-dimethylhexylamine).

The content of perfluoro (N, N-dimethylcyclohexylamine) determined from the result of gas chromatography analysis of the obtained mixture was 83% by weight. 10 mL of this mixture was sealed in a 100 mL stainless steel container and heated at 270 ° C. for 15 minutes. Thereafter, the gas in the container was collected by a sampler and analyzed by gas chromatography. As a result, no PFIB was produced (detection limit: 0.1 ppb).

COMPARATIVE EXAMPLE The same pair of negative anodes as those shown in the examples (the back surfaces of the opposing electrode surfaces were coated with a fluorine resin) were placed in a stainless steel electrolytic cell having a capacity of 3 L without blocking the periphery of the negative anode. It was arranged, and the electrolytic fluorination of N, N-dimethylaniline was performed in a batch system without circulating the electrolytic bath solution. The concentration of N, N-dimethylaniline at the start of electrolysis was 10% by weight, and electrolysis was started at a current value of 16.8 A (voltage between negative and positive electrodes: 5.87 V). The electrolysis was stopped when the raw material disappeared and the voltage rose sharply.

The fluorinated products accumulated at the bottom of the electrolytic cell are:
First, the mixture was treated under reflux for 120 hours in an equal volume mixture of a 40% by weight aqueous sodium hydroxide solution and diisobutylamine, and then washed with water. This is for removing a fluorinated product in which a hydrogen atom contained in the fluorinated product partially remains. Then, distillation was performed after removing hydrogen fluoride in the same manner as in the example to obtain a mixture of perfluoro (N, N-dimethylcyclohexylamine) and perfluoro (N, N-dimethylhexylamine). Using this, the composition was analyzed in the same manner as in the example, and the decomposability at high temperature was examined. As a result, the content of perfluoro (N, N-dimethylcyclohexylamine) in the mixture was 43% by weight. 2.0 ppb of PFIB was detected in the gas phase after heating at 270 ° C.

Next, the fluorinated product obtained by carrying out electrolytic fluorination exactly in the same manner as described above is fluorinated by flowing fluorine gas in the same manner as in the example,
Then, distillation was carried out after removing hydrogen fluoride to obtain a mixture of perfluoro (N, N-dimethylcyclohexylamine) and perfluoro (N, N-dimethylhexylamine). The content of perfluoro (N, N-dimethylcyclohexylamine) in this mixture was 52% by weight. Also, 1.1 ppb was added to the gas phase after heating at 270 ° C.
Of PFIB were detected.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-25671 (JP, A) JP-A-2-30785 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C25B 1/00-15/08

Claims (1)

(57) [Claims] (1) The following formula: (However, A is a phenyl group or a cyclohexyl group which may be substituted by a fluorine atom, and R ¹ and R
² is the same or different alkyl group. ), Wherein the amine and hydrofluoric anhydride are circulated, and the electrolytic fluorination is continuously carried out in the electrolytic cell.