JPH0816172B2 - Method for producing improved anion exchange membrane - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing an anion exchange membrane having excellent durability and current efficiency.

[Problems to be Solved by Conventional Techniques and Inventions] In recent years, development of a process using a membrane has been active, and in particular, a process using an ion-exchange membrane has received a great deal of attention from the viewpoint of energy saving.

Among the ion exchange membranes, a cation exchange membrane has been developed which is a so-called perfluorocarbon type membrane represented by Nafion (trade name, manufactured by DuPont). The skeleton of this type of membrane is composed of a perfluorocarbon polymer, and its durability is extremely superior to that of a conventional cation exchange membrane.

On the other hand, conventional anion exchange membranes had many problems in terms of durability, but an anion exchange membrane having a novel structure in which the main chain is made of a perfluorocarbon polymer and having excellent durability was proposed ( (For example, JP-A-59-122520). This film has superior durability compared to conventional films,
Further, it has the same or better characteristics in terms of electrochemical properties, but it has a drawback that the current efficiency is low when it is used industrially, particularly when it is used in a system in which a proton exists.

The present invention has been made from the above viewpoint, and an object thereof is to provide a method for producing an anion exchange membrane having excellent durability and high current efficiency.

[Means for Solving Problems] The inventors of the present invention have conducted intensive studies based on such a background, and completed the present invention.

That is, the present invention provides the following general formula (1) (However, X is F or CF ₃ , Z is halogen, l is an integer of 0 to 5,
m is 0 or 1, n is an integer of 1 to 5, and r is 2 or 3. R ₁ ~
R ₃ is the same or different alkyl group having 1 to 5 carbon atoms, R ₄ is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, or R ₃ and R ₄ are integrated to form a polymethylene chain — (CH ₂ ) _b — ( b indicates that 2 or 3) is formed. Both p and q are positive numbers, and p / q is 2-16. The present invention relates to a method for producing an anion exchange membrane, which comprises treating an anion exchange membrane having a structure represented by (4) with an alkali metal hydroxide.

 The present invention will be described in detail below.

Examples of the alkyl group in the anion exchange membrane represented by the general formula (1) include a methyl group, an ethyl group, a propyl group, a butyl group, and a pentyl group.
Further, R ₃ and R ₄ may be integrated to form a polymethylene chain. Specifically, the following compounds can be mentioned as an example.

These anion exchange membranes can be produced, for example, by the method shown below.

A first tetrafluoroethylene, e.g. comonomer _{CF 2 = CFO-CF 2 -CF} 2 -SO 2 F CF 2 = CFO-CF 2 -CF (CF 3) as shown below O-CF ₂ -CF ₂ SO ₂ F CF ₂ = CF (OCF ₂ -CF (CF ₃ )) ₂ OCF ₂ -CF ₂ SO ₂ F CF ₂ = CFCF ₂ -CF ₂ -SO ₂ F by the following general formula (2) (However, X is F or CF ₃ , l is an integer of 0 to 5, m is 0 or 1, n
Is an integer of 1 to 5, r is 2 or 3, p and q are both positive numbers, and p / q is 2 to 16. ) To obtain a copolymer having a structure.

This copolymerization reaction can be obtained in the range of 0 to 200 ° C. and 1 to 200 atm in the presence of a free radical initiator.

These copolymers are formed into a film. For this molding, a general technique of melting and forming a thin film can be used. Then, the film having the structure of the above general formula (2) is formed into the following general formula (3) or (4) H ₂ NCH _{2 r} NR ₂ R ₃ (3) (However, b and r are 2 or 3, respectively, and R ₂ and R ₃ are the same or different alkyl groups having 1 to 5 carbon atoms.) A film having a sulfonamide group by reacting with an amine Convert to.

Examples of the amine represented by the general formula (3) or (4) include dimethylaminoethylamine, diethylaminoethylamine, dipropylaminoethylamine, dimethylaminopropylamine, diethylaminopropylamine, dibutylaminopropylamine and N-methylpiperazine. can do.

The reaction of these amines can be performed in amine or using a solvent. When a solvent is used, ethers such as diethyl ether, tetrahydrofuran and dioxane; hydrocarbons such as benzene, toluene and hexane can be used.

A reaction temperature of room temperature to 80 ° C and a reaction time of 60 hours are sufficient. Among the films having a sulfonamide group thus obtained, a film in which R ₄ is a hydrogen atom or a film in which R ₃ and R ₄ form a polymethylene chain using an amine represented by the general formula (4) In the case of, by alkylating by reacting this film with an alkylating agent, in the case of a film in which R ₄ is an alkyl group, an alkali metal hydroxide is allowed to act on this film to form a sulfonamide group. Anion exchange membrane having the structure of the general formula (1) can be produced by converting the alkali metal salt and then reacting with an alkylating agent for alkylation.

Although an aqueous solution of lithium hydroxide, sodium hydroxide, potassium hydroxide or the like can be used as the alkali metal hydroxide used in the step (2), the use of a solvent such as methanol is excellent in terms of reaction efficiency. A reaction temperature in the range of room temperature to 90 ° C is sufficient.

Further, as the alkylating agent, for example, methyl iodide, ethyl iodide, butyl iodide or the like can be used.

At this time, methanol, ethanol, dimethylformamide or the like can be used as a solvent.

When it is necessary to exchange the counterion of the membrane having an anion exchange group obtained here, it can be exchanged by treating with an alkali metal salt by a conventional method.

The thickness of the film used in the present invention is usually 50 μm to 500 μm, and an appropriate thickness can be selected in consideration of the strength, specific electric conductivity and current efficiency of the film. Further, such a film may be reinforced with a Teflon fiber cloth or the like in order to improve the strength of the film.

The shape of the film is not particularly limited and may be smooth or tubular.

It is a feature of the present invention that one or both surfaces of the anion exchange membrane represented by the general formula (1) thus obtained is further treated with an alkali metal hydroxide. As a method of treating with an alkali metal hydroxide, an anion exchange membrane is directly immersed in a solution of an alkali metal hydroxide to treat both sides, or two anion exchange membranes are superposed and heat-sealed on four sides. Then, the outer surface is treated by immersing it in a solution of an alkali metal hydroxide, and then the heat-sealed portion is cut to treat one surface.

At the time of the above treatment, the desired anion exchange membrane can be obtained without stirring the alkali metal hydroxide solution, but it is preferable to stir to prevent unevenness due to the main treatment.

Examples of the alkali metal hydroxide used for the treatment include lithium hydroxide, sodium hydroxide, potassium hydroxide and the like. These may be usually used as an aqueous solution, but in order to increase the processing speed, methanol,
It is preferable to add a solvent such as ethanol. At this time, the alkali metal hydroxide having a concentration of 0.1 to 70 wt% can be used, but a concentration of 1 to 30 wt% is suitable for controlling the reaction.

Further, the treatment temperature and the treatment time are preferably 10 to 80 ° C. and 1 to 200 hours, respectively, because the reaction can be easily controlled.

The treatment amount of the film can be controlled by appropriately changing the alkali metal hydroxide concentration, the treatment temperature and the treatment time, but these treatment conditions depend on the membrane resistance after treatment being 10 Ω · cm ² or less. It is preferable to select This is because when the film resistance exceeds 10 Ω · cm ² , the power consumption becomes too large compared with the improvement of the current efficiency, which is industrially undesirable. In addition, the above-mentioned membrane resistance means a resistance value measured by using an alternating current of 25 ° C. and 1000 Hz 1 V after sufficiently equilibrating the membrane in a 0.5 N sodium chloride aqueous solution.

Although the reason why the membrane obtained by the present invention exhibits high current efficiency is not clear, when the anion exchange membrane represented by the general formula (1) is treated with an alkali metal hydroxide, the anion exchange group of the membrane surface layer is partially removed. It is considered that the fixed ion concentration becomes high because the water content of the surface layer is reduced and the current efficiency becomes high as a result. Particularly under acidic conditions, the expansion of the film is larger than that under other conditions, so that the effect is considered to be remarkable. However, these estimations do not limit the present invention in any way.

[Effect of the Invention] According to the method of the present invention, it is possible to obtain a film that is stable against repeated drying-wetting, has excellent acid resistance, heat resistance, solvent resistance, and exhibits high current efficiency.

By using this membrane as a membrane for various electrolysis that requires durability and current efficiency, such as recovery of acid from inorganic salt and recovery of valuable metal in the presence of acid, it can be used for a long time and production efficiency can be improved. Production cost can be reduced because it can be increased, and its industrial utility value is extremely large.

EXAMPLES Next, the present invention will be described in more detail with reference to Examples.
The present invention is not limited to these.

In the following examples, the resistance of the film is 0.5 N
After fully equilibrating in an aqueous solution of sodium chloride, exchange 1
Measured at V, 1000 Hz and temperature of 25 ° C.

Example 1 CF ₂ = CF ₂ and _{CF 2 = CFOCF 2 CF (CF} 3) O-CF 2 -CF 2 -SO 2 F
The copolymer obtained by copolymerization with
170 to 160 μ, p / q = 6.5).

Then, the film was immersed in dimethylaminoethylamine and reacted at 40 ° C. for 50 hours. After the reaction, wash with water,
(Conversion rate in this case was 97%, based on elemental analysis) It was immersed in a 10 wt% sodium hydroxide aqueous solution / methanol (volume ratio 1: 1), treated at 50 ° C. for 30 hours, washed with water, and air-dried.
Then methyl iodide / dimethylformamide (volume ratio 1:
In 4), it was treated at 60 ° C for 7 days. The obtained film was washed with methanol and then treated in a methanol solution of lithium chloride (10 wt%) at 50 ° C. for 24 hours to obtain an anion exchange membrane represented by the following structural formula.

(However, p / q = 6.5) The obtained anion exchange membrane was colored yellow-orange with cresol red in the staining test, and was not stained with crystal violet at all. The membrane resistance was 4.0 Ω · cm ² in 0.5 M sodium chloride solution.

The obtained anion exchange membrane was immersed in a 4 wt% sodium hydroxide aqueous solution at 23 ° C. for 12 hours with stirring, and treated from both sides. The resistance of the obtained film was 5.8 Ω · cm ² .

Next, a cathode, a cation exchange membrane (Nafion 324 manufactured by DuPont), an anion exchange membrane (the membrane obtained in this example),
A three-chamber type electrolytic cell was assembled in the order of the anode.

Then, feed water so that a 20 wt% sodium hydroxide aqueous solution can be obtained between the cathode and the cation exchange membrane, and 7M sodium nitrate is placed in the intermediate chamber between the cation exchange membrane and the anion exchange membrane. An aqueous solution was fed, water was fed between the anion exchange membrane and the anode so that a 25 wt% nitric acid aqueous solution could be obtained, and electrolysis was carried out at 70 ° C. at 30 A / dm ² for 6 months. The current efficiency of nitric acid acquisition was stable in the range of 60%.

 The electrolysis voltage was stable at 5.7V.

Example 2 Instead of the 4 wt% aqueous sodium hydroxide solution used in Example 1, 4 wt% lithium hydroxide water was used for immersion for 15 hours, and treatment was performed from both sides. Others were the same as in Example 1. The resistance of the obtained film was 5.8 Ω · cm ² .

Further, an electrolytic cell similar to that of Example 1 was assembled and operated using the obtained membrane. In this case, the current efficiency of nitric acid acquisition is 64.
%Met. The electrolysis voltage was 6.2V.

Example 3 The same procedure as in Example 1 was carried out except that a 4 wt% aqueous solution of potassium hydroxide was used instead of the 4 wt% aqueous solution of sodium hydroxide used in Example 1 for immersion for 12 hours, and both sides were treated. . The resistance of the obtained film was 6.0 Ω · cm ² .

Further, an electrolytic cell similar to that of Example 1 was assembled and operated using the obtained membrane. In this case, the current efficiency of nitric acid acquisition is 62.
%Met. The electrolysis voltage was 6.0V.

Comparative Example 1 The untreated film obtained in Example 1 (membrane resistance: 4.0Ω.
cm ² ), an electrolytic cell was assembled and operated in the same manner as in Example 1. In this case, the current efficiency was 39%. The electrolysis voltage was 5.0V.

Example 4 CF ₂ = CF ₂ and _{CF 2 = CFO-CF 2 CF} (CF 3) O-CF 2 -CF 2 -SO
_The copolymer obtained by copolymerization with ₂ F was formed into a film (film thickness 125 μm, p / q = 7.6).

Example 1 except that the obtained copolymer film was immersed in dimethylaminopropylamine and reacted at 40 ° C. for 50 hours
The same treatment as above was performed to obtain an anion exchange membrane represented by the following structural formula.

The obtained anion exchange membrane was colored yellow-orange with cresol red in the staining test and was not stained with crystal violet at all. The membrane resistance was 4.5 Ω · cm ² .

The obtained anion exchange membrane was immersed in a 4 wt% sodium hydroxide aqueous solution at 23 ° C. for 10.5 hours and treated from both sides. The resistance of the obtained film was 5.7 Ω · cm ² .

Further, using the obtained film, an electrolytic cell was assembled and operated in the same manner as in Example 1. In this case, the current efficiency of nitric acid acquisition was 59%. The electrolysis voltage was 5.5V.

Comparative Example 2 Untreated film obtained in Example 4 (membrane resistance: 4.5Ω
.Cm ² ), an electrolytic cell was assembled and operated in the same manner as in Example 1. In this case, the current efficiency was 44%. The electrolysis voltage was 5.4V.

Claims (3)

[Claims] 1. The following general formula (However, X is F or CF ₃ , Z is halogen, l is an integer of 0 to 5,
m is 0 or 1, n is an integer of 1 to 5, and r is 2 or 3. R ₁ ~
R ₃ is the same or different alkyl group having 1 to 5 carbon atoms, R ₄ is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, or R ₃ and R ₄ are integrated to form a polymethylene chain — (CH ₂ ) _b — ( b indicates that 2 or 3) is formed. Both p and q are positive numbers, and p / q is 2-16. ) A method for producing an anion exchange membrane, which comprises treating an anion exchange membrane having a structure represented by (4) with an alkali metal hydroxide. 2. The production method according to claim 1, wherein the anion exchange membrane has a flat membrane shape. 3. The method according to claim 1, wherein the anion exchange membrane is tubular.