Cation exchange membrane and method for producing potassium hydroxide aqueous solution

1. A cation exchange membrane comprising at least one polymer having cation exchange groups, wherein in cations (100 mol %) contained in the cation exchange membrane, the total of potassium ions and sodium ions is at least 99 mol %, and in the total (100 mol %) of potassium ions and sodium ions contained in the cation exchange membrane, potassium ions are from 80 to 98 mol %, and sodium ions are from 20 to 2 mol %.

2. The cation exchange membrane according to claim 1 , wherein the at least one polymer having cation exchange groups is either one or both of a perfluorocarbon polymer having alkali metal salt form carboxylic acid groups and a perfluorocarbon polymer having alkali metal salt form sulfonic acid groups.

3. The cation exchange membrane according to claim 2 , which comprises a layer comprising the perfluorocarbon polymer having alkali metal salt form carboxylic acid groups, and a layer comprising the perfluorocarbon polymer having alkali metal salt form sulfonic acid groups.

4. The cation exchange membrane according to claim 3 , wherein the ion exchange capacity of the perfluorocarbon polymer having alkali metal salt form carboxylic acid groups is from 0.7 to 1.1 meq/g dry resin, and the ion-exchange capacity of the perfluorocarbon polymer having alkali metal salt form sulfonic acid groups is from 0.9 to 1.15 meq/g dry resin.

5. The cation exchange membrane according to claim 4 , wherein the layer comprising the perfluorocarbon polymer having alkali metal salt form carboxylic acid groups and the layer comprising the perfluorocarbon polymer having alkali metal salt form sulfonic acid groups, are in contact with each other, and the difference in ion exchange capacity between the layer comprising the perfluorocarbon polymer having alkali metal salt form carboxylic acid groups and the layer comprising the perfluorocarbon polymer having alkali metal salt form sulfonic acid group, adjacent to each other, is from 0.1 to 0.45 meq/g dry resin.

6. The cation exchange membrane according to claim 3 , wherein the thickness of the layer comprising the perfluorocarbon polymer having alkali metal salt form carboxylic acid groups is from 5 to 50 μm, and the thickness of the layer comprising the perfluorocarbon polymer having alkali metal salt form sulfonic acid groups is from 55 to 200 μm.

7. The cation exchange membrane according to claim 3 , wherein a reinforcing material is embedded in the layer comprising the perfluorocarbon polymer having alkali metal salt form sulfonic acid groups, and the ion exchange capacity of the layer comprising the perfluorocarbon polymer having alkali metal salt form sulfonic acid groups on an anode side of the reinforcing material, is higher than the ion exchange capacity of the layer comprising the perfluorocarbon polymer having alkali metal salt form sulfonic acid groups on a cathode side of the reinforcing material.

8. The cation exchange membrane according to claim 7 , wherein the thickness of the layer comprising the perfluorocarbon polymer having alkali metal salt form sulfonic acid groups on the anode side of the reinforcing material is from 10 to 60 μm.

9. The cation exchange membrane according to claim 1 , which comprises a gas release layer on at least either one of an anode side surface and a cathode side surface.

10. The cation exchange membrane according to claim 2 , wherein the perfluorocarbon polymer having alkali metal salt form carboxylic acid groups is one obtained by hydrolysis treatment of a copolymer having structural units derived from a monomer represented by formula (1), and structural units derived from a monomer represented by formula (2), and the perfluorocarbon polymer having alkali metal salt form sulfonic acid groups is one obtained by hydrolysis treatment of a copolymer having structural units derived from a monomer represented by formula (1), and structural units derived from a monomer represented by formula (3):

CF 2 ═CX 1 X 2   (1)

CF 2 ═CF(OCF 2 CFX 3 ) n O(CF 2 ) m Y  (2)

CF 2 ═CF(OCF 2 CFX 4 ) s O(CF 2 ) t W  (3)

wherein X 1 and X 2 are each independently a fluorine atom, a chlorine atom or a trifluoromethyl group,

X 3 is a fluorine atom or a trifluoromethyl group,

m is an integer of from 1 to 5,

n is 0 or 1,

Y is a precursor group which can be converted to a carboxylic acid group by hydrolysis,

X 4 is a fluorine atom or a trifluoromethyl group,

s is an integer of from 1 to 3,

t is an integer of from 0 to 2, and

W is a precursor group which can be converted to a sulfonic acid group by hydrolysis.

11. The cation exchange membrane according to claim 7 , wherein the material of the reinforcing material is polytetrafluoroethylene.

12. The cation exchange membrane according to claim 9 , wherein the gas release layer is a layer comprising inorganic particles and a binder.

13. The cation exchange membrane according to claim 12 , wherein the inorganic particles are SiO 2 , SiC, ZrO 2 or ZrC, and the binder is methyl cellulose, or a fluoropolymer having carboxylic acid groups or sulfonic acid groups.

14. A method for producing a potassium hydroxide aqueous solution, the method comprising contacting a potassium chloride aqueous solution with the cation exchange membrane as defined in claim 1 to electrolyze the potassium chloride aqueous solution by an ion-exchange membrane method.

15. The method according to claim 14 , wherein the concentration of potassium chloride in the potassium chloride aqueous solution is from 150 to 200 g/L, and the concentration of potassium hydroxide in the potassium hydroxide aqueous solution is from 20 to 40 mass %.