SU925253A3 - Method for preparing homogeneous cation-exchange membrane - Google Patents

Description

for the side on which there is a polymer with a large EV. In order to achieve a high current utilization rate in combination with low energy consumption, it is necessary for a polymer with a large EV to increase the EV value and reduce the thickness. However, to obtain a composite cation-exchange membrane with current utilization factors of at least 90 when using a membrane containing only sulfonic acid groups, exclude-i

very difficult 2.

I

The closest to the proposed technical essence and the achieved effect is a method of obtaining a cation-exchange homogeneous membrane by chemical transformation of a film of perfluorinated copolymer containing sulfonyl groups as side substituents into a sulfonamide group, its salts or an N-monosubstituted sulfonamide group by treating an amine copolymer 3

However, such membranes, when used as diaphragms, do not have a sufficiently high current utilization rate during electrolysis.

The purpose of the invention is to increase (1) the current utilization ratio when using the membrane as the diaphragm in the electrolyzer.

The goal is achieved by the fact that in the method of producing a cation-exchange homogeneous membrane by chemically converting a film of perfluorinated acyclic copolymer containing sulfonyl groups as side substituents, these sulfonyl groups are reduced to carboxyl groups of the formula OCF2COOM, where M is selected from the group consisting of hydrogen, ammonium and metals.

As a copolymer, a perfluorinated copolymer containing sulfonic acid groups is used, which are side substituents of the formula - OSGsGy. The reduction of the sulfonyl groups of the perfluorinated copolymer is carried out before the formation of 20-100 mop.% Of carboxyl groups, which are side substitutes, in at least one layer at least 100 A thick on the surface or in the inner part of the membrane.

Only one surface of the copolymer film is subjected to chemical transformation.

The thickness of the membrane can vary widely depending on the purpose, but usually for many purposes a thickness of from 0.5 to 20 mils (from 1.27-10 to 50.8-10 mm) is suitable.

The specific fluorocarbon polymer forming the layer contains from 5 to 100 mol% of carboxylic acid substituents based on the total number of cation-exchange groups. Usually, as the distance from the surface increases, the relative content of carboxylic acid substituents decreases, and the relative content of sulfonic acid substituents increases. Only one surface represents co-ools, predominantly carboxylic acid substituents, and the number of sulfonic acid groups increases with increasing distance from this surface, so that the opposite surface is predominantly sulfonic acid groups. i It has been established that the presence of carb.oxyl groups on the surface of the cation-exchange membrane, especially on the surface facing the cathode, exists; 1 retarding backward migration

hydroxyl ions from the cathode space in the electrolysis of aqueous solutions of alkali metal halides, for example sodium chloride. This action is used when working with high current utilization rates, usually more than 90. In addition, membranes are particularly durable even in the presence of 20% or more sodium hydroxide aqueous solutions, and are also highly resistant to chlorine gas released at the anode.

As mentioned above, the surface density of the carboxylic acid groups can vary from 5 to 100 mol%. The preferred limit is 20-100 mol., And the optimum combination of economy and efficiency is usually achieved at a density of 40-100 mol. (based on the total number of all functional groups in the surface layer).

A starting fluorocarbon polymer containing sulfoxyl groups as side chains is obtained by copolymerizing fluorinated ethyl5, for example tetrafluoroethylene, and vinyl fluorocarbon monomer containing a sulfonyl fluoride group of the following general formula FOzSCFj CPaO (I) I already I can use it. the number of carbon atoms is from 1 to 5i; and P is an integer having a value from 0 to 3, if necessary in combination with perfluorovinyl ether or hexafluoropropylene. Of the vinyl fluorocarbon monomers containing a sulfonyl fluoride group, perfluoro- (3,6-dioxa-methyl-7-octenesulfonyl fluoride) is most preferred. A typical example of a fluorovinyl ether is perfluoromethylvinyl ether. The proposed method is applicable to all known sulfonyl-substituted polymers (II). It is advisable to first make a membrane from a sulfonyl-substituted polymer, and then convert it to the membrane proposed by means of reactions, which are described in more detail below. The composition of the copolymers used as starting materials is such that the content of fluoride-ethylene monomer is 30%. The tO wt.% and the content of perfluorovinyl monomer containing a sulfonyl fluoride group is 70-10 wt. D, preferably 6025 wt.%. These substances are obtained by homopolymerization or copolymerization of fluorinated ethylene. It is desirable that the copolymer has an equivalent weight (E6) in the range of 1000-2000, and the membrane has a low EW, since its proportionally low electrical resistance. However, the copolymer membrane with a very low EV has insufficient mechanical strength. From a copolymer with a very high E, it is difficult to form a membrane well. Thus, the most preferred limit of eV is 10001500. To obtain the proposed products, the side sulfonyl groups are in the form represented by the formulas. -QCfy Cf i 3 and / or li -OCFjCF SOa. -OCFjCFjSO / in which X is halogen, for example, fluorine or chlorine, hydroxyl, alkyl of the composition C | -C, aryl or OZ, where Z is a metal atom, for example, an alkali metal, alkyl of the composition C | —Ci or aryl, is converted to —OCP2 COOM by treatment with a reducing agent. Sulfonyl fluoride groups can be directly treated with a reducing agent to convert to acidic carboxyl groups, or they can be first converted to any of the other sulfonic acid derivatives indicated in formulas A and B using known reactions and subsequent conversion to acidic carboxyl groups. Sulfonyl chloride groups are particularly preferred due to their high reactivity. Therefore, it is better to convert the sulfonyl fluoride to any of the other sulfonic acid derivatives listed above with the designation X. These reactions can be easily carried out using known techniques. The preparation of the carboxyl group is carried out in various ways. It can be obtained by reduction to sulfinic acid using a relatively weak reducing agent, followed by heating OCPj СРа S02X SfOCP2 СООМ. Conversion to carboxylic acid groups is easier if M is hydrogen in the formulas given. Alternatively, the treatment can be carried out step by step, whereby sulfinic acid is first produced, which is then converted to a carboxyl group with a strong reducing agent. Possible process flow: -OCFj CFj BOGHwe G-OSG SG SOaWOCFjCOOM. With the help of some reducing agents, it is possible to obtain the carboxyl group directly from the sulfo group, —OSPr CPj SOjM - OCPj СООМ. It is desirable that the concentration of the sulfinic acid group in the final product is relatively low. In line with this, it is recommended to oxidize sulfonic acid groups into sulfonic acid groups according to the following scheme -OCFzCFjSO M OCFjCFj SOjM This is carried out by known methods using aqueous mixtures of sodium hydroxide and hypochlorite. Reducing agents that may be used according to the invention are listed below. Metal hydrides, for example, lithium aluminum hydride, lithium borohydride, sodium hydride and calcium hydride, are reducing agents of the first group. The second group of reducing agents include inorganic acids with reducing properties, for example, hydroiodic acid, hypophosphorous acid hydrobromic acid, hydrogen sulfide and mouse acid. The third group of reducing agents are mixtures of metals and acids. Examples of such mixtures are tin, iron, zinc and zinc amalgam; acids include hydrochloric, sulfuric, and acetic acid. The fourth group of reducing agents include low valence metal compounds. Examples of such compounds are iron chloride (II) tin chloride sulfate and titanium trichloride. They can be used together with acids, for example hydrochloric or sulphurous. The reducing agents of the fifth group include organometallic compounds. Examples of such reducing agents are butyl lithium, Grignard recipe, triethyl aluminum and tri-isobutyl aluminum. The reducing agents of the sixth group include salts of inorganic acids with reducing properties, and similar compounds. Examples of such reducing agents are iodides, sulfides, alkali metal or ammonium sulfites, etc. The seventh group of reducing agents are mixtures of metals with water, pa. rum, alcohols and alkali. Examples of metals that can be used in these mixtures are sodium, lithium, aluminum, magnesium, zinc, iron, and their amalgams. Examples of alkalis are alkali metal hydroxides and alkoxides. The eighth group of reducing agents are organic compounds. possessing reducing properties, for example, triethanolamine and acetaldehyde. Among these groups, the second, third, fourth, and sixth are preferred. The optimal treatment conditions for the reducing agent should be chosen depending on the chosen reducing agent and on the type of substitute X in the -SOjX group. Typically, the reaction temperature is in the range of from -50 to 250 ° C, preferably from 0 to, and the reducing agent is used in the gaseous, liquid state or as a solution. Water, polar organic solvents such as methanol, tetrahydrofuran, diglyme, acetonitrile, propionitrile or benzonitrile or non-polar organic solvents such as n-hexane, benzene or cyclohexane, or mixtures of these solvents can be used as a solvent for the reduction reaction. . The amount of the reducing agent must not be less than the EV of the sulfonyl group contained on the surface. The pH value of the reaction system is chosen depending on the specific reducing agent used. The reaction can be carried out under reduced, normal or elevated pressure. 8 reactions involving a gas exchange reducing agent, the increase in pressure contributes to an increase in the reaction rate. The reaction time ranges from one minute to 100 hours. The course of the reaction can be traced using infrared spectroscopy of the membrane. The fabrication of a cation-exchange membrane from a fluorocarbon polymer substituted by a carboxyl group was extremely difficult, due to the complexity of the synthesis of the necessary substituted fluorocarbon monomers. In addition, the copolymers obtained by copolymerization of such monomers with perfluorovinyl monomers are thermally unstable, and from them it is impossible to form membranes in the usual extrusion process at high temperature. The invention overcomes these difficulties by converting the sulfonic acid group of the fluorocarbon polymer to a carboxylic acid group. Example 1. Tetrafluoroethylene and perfluoro- (3,6-dioxa-methyl-7-octenesulfonyl fluoride) are copolymerized in 1,1,2-trichloro-1,2,2-trifluoroethane in the presence of perfluoropropio peroxide as an initiator nila The copolymerization process maintains a temperature and pressure of 5 atm. The exchange capacity of the obtained polymer, measured after saponification, is 0.95 mEq / g of dry resin. When the copolymer is formed, a film with a thickness of 0.3 mm is formed by heating, and washed in a mixture of 2.5N. caustic soda and go methanol at 60 ° C for 16 h, converted into the H-form in 1 n. hydrochloric acid and heated under reflux at 120 ° C for 20 hours in a mixture of equal amounts of phosphorus pentachloride and phosphoric acid chloride for conversion to sulfonyl chloride. At the end of the reaction, the copolymer membrane is washed with carbon tetrachloride. The measured spectrum of impaired total internal reflection (denoted below by the ATR) shows a strong one: cm band, characteristic of sulfonyl chloride. In a solution of crystal violet (nitrosulfuric acid), the membrane does not stain. Two sheets of such memoria are pulled onto acrylic resin frames with polytetrafluoroethylene gaskets. The frames are immersed in a 57% aqueous solution of hydroiodic acid so that one surface of each membrane is reacted for 2A hour. Then the ATR of the membrane is measured. In the spectrum, the absorption band with characteristic for the sulfonyl chloride group disappears, and the absorption band of 1780 cm2, characteristic of the carboxyl group, appears instead. In a growing crystal violet layer, a layer about 15 microns thick is painted on one side of the membrane surface. By measuring the ATR, it has been found that the cation-exchange groups on the surface are carboxylic acid groups (100). When saponification of the membrane in a mixture of 2.5 n. an aqueous solution of caustic soda and $ 0% methanol at bOS for 16 hours, a homogeneous membrane with a high cation exchange (capacity. In a 0.1N aqueous solution of caustic soda, this membrane shows an electrical conductivity of 10.0 x 10 mOm / cm. The Na-shaped electrolysis diaphragm is kept in a 2.5N aqueous sodium hydroxide solution for 16 hours and then placed in the electrolyzer so that the treated surface is on the cathode side and is used as a membrane in the electrolysis of sodium chloride. Utilization rate current is 95. Example 1a (comparative). The membrane in the sulfonyl chloride form obtained in Example 1 is washed with a mixture of 2.5N sodium hydroxide / 50% methanol. The conductivity of the resulting ion-exchange membrane in the form of sulfonic acid is measured and current utilization factor under the same conditions as used in example 1. The values of 13.0 x 10 mΩ / cm and 55%, respectively, are obtained. Example 2 The procedure described in example 1 is carried out by terpolymerization of monomers used in Example 1, plus perfluoropropyl vinyl ether. Example 3 - Terpolymerization was carried out using the monomers used in Example 1, plus perfluoro-3,6-diox-5-methylnonei-1. After processing the obtained membrane as described in example 1, the same results are obtained. An example. According to a method similar to that described in Example 1, a membrane in a sulfonyl fluoride form with an exchange capacity of 0.65 mEq / g of dry resin is obtained. The membrane is placed on 6 flasks and tetrahydrofuranone is poured. Lithium borohydride is added in large excess and the resulting reaction mixture is heated under reflux for 50 hours. At the end of the reaction, ATR is determined at the membrane. In the spectrum, the absorption band of cm mostly disappears, characteristic of sulfonyl fluoride, and instead there is a strong absorption band) 690 cm, characteristic of COOLi, weak absorption bands and 1010 cm, characteristic of the salt of sulfinic acid. This membrane is left for 3 min in an aqueous 0.1% solution of crystalline

violet (containing 10% ethanol). A microscopic examination of the cross section shows that there is a highly colored layer on the both sides of the membrane surface, about 10 microns thick.

The content of carboxylic acid groups in the surfaces of the layers, as determined by the ATR, is about 60%. 1 Example 5. The membrane in the sulfonyl chloride form obtained in Example 1 is reduced in an aqueous solution of hypophosphorous acid for 10 hours and then its ATR is determined. The band characteristic of the sulfonyl chloride group has disappeared, but the 1780 cm band characteristic of the carboxylic acid group is not very strong. When the membrane was washed with water and treated with 47% hydrobromic acid for 20 hours, the intensity of the 1780 cm band characteristic of the carboxylic acid group increased. The content of cation-exchange groups on the surface, determined by the ATR, is 90%.

After saponification of the membrane in water 2.5 and. sodium hydroxide solution and 50% methanol followed by an ATR measurement, the 1780 cm band characteristic of the carboxylic acid group shifted to 1690, the carboxylic acid salt showed a weak band and the sulfonic acid salt groups showed 1010 cm.

Example 6. The membrane in the sulfonyl chloride form obtained in Example 5 is heated under reflux in an aqueous solution and hydrochloric acid for 50 hours, then measured by ATR. In the spectrum, the absorption band characteristic of the sulfonyl chloride group disappears and the 1780 absorption band appears, which is characteristic of the carboxylic acid group. By measuring the ATR, it was found that there are carboxylic acid groups on the surface (O 00). This membrane was saponified in aqueous 2.5N. caustic soda solution / 50% methanol.

Example 7. The experiment according to Example 6 is repeated, except that instead of an aqueous solution of tin and hydrochloric acid, an aqueous solution of tin chloride and hydrochloric acid are used. Get similar results.

Example 8. The membrane in the sulfonyl chloride form obtained in Example 1 is immersed in a 15% aqueous solution of sodium sulfide with a continuous flow of gaseous nitrogen for 1 hour. At the end of the reaction, the ATR of the membrane is measured. the sulfonyl chloride group completely disappears and strong absorption bands and 1010 characteristic of the sulfonic acid salt appear. The membrane is converted to the H form by immersion in 1N. hydrochloric acid solution at 60 ° C for 10 h, and then heated under nitrogen at 150 ° C for 120 min. It is then converted to Na form by immersion in 2.5N. caustic soda solution and measured IPVO. The spectrum completely absorbs the absorption bands of 9tO and 1010 cm, characteristic of the sulfonic acid salt, and a sharp polral of 1690 cm appears, which is characteristic of the carboxylic acid group. Found using an ATR, the HTO cation-exchange groups on the membrane surface are carboxyl acid groups (100%). The membrane is immersed in water 2.5 n. sodium hydroxide / 50% methanol solution for saponification of sulfonyl chloride groups still remaining in the inner parts of the membrane.

Example 9. The membrane in the sulfonyl chloride form obtained in Example 1 was scrubbed, converted to the H form with hydrochloric acid, dried thoroughly and reacted with lithium aluminum hydride dissolved in tetrahydrofuran for 20 hours. In the ATR spectrum there is a weak absorption band of 1780, characteristic of the carboxylic acid group.

Claims (3)

Example 10 The treated membrane prepared in Example 1 was subjected to separate durability tests by immersion in 5% sodium hydroxide solution or 5% sodium hypochlorite solution for 300 hours. Thereafter, the ATR of the treated membrane surface was measured. The spectrum shows a visible absorption band of 1690, characteristic of the carboxylic acid group. After transformation of the membrane into the H-form using 1 n. hydrochloric acid absorption band shifts to 1780 cm. Thus, in the course of testing for a long time, the membrane remains largely unchanged. Claim 1. A method of producing a cation-exchange homogeneous membrane by chemical transformation of a perfluorinated acyclic film of a copolymer containing sulfonyl groups that are side substitutes, characterized in that, in order to increase the current utilization rate when using fieM6ra (as a diaphragm in the electrolyzer, the sulfonyl groups are reduced to the carboxyl groups of the formula, OC1 2COOM, where M is selected from the group consisting of hydrogen, ammonium and metal atoms. 2. The method according to claim 1, exc. This is based on the use of a per-31-fluorinated copolymer containing sulfonic acid groups, which are side substituents, of the formula OCTjCFjSO M. The method according to Clause 7 and that the reduction of the sulfonyl groups of the perfluorinated copolymer lead to the formation of 20 -100 mol of carboxyl groups are side substituents in at least one layer at least 100 A thick on the surface or in the inner part of the membrane. k. The method according to paragraphs. 1 and 3, wherein only one surface of the copolymer film is subjected to chemical transformation. Sources of information accepted in the examination at examination 1. US patent number 3282875, cl.2bO29, published. 1366. 2. US patent W 3909378, cl. publish 1975. 3. US patent IP, cl. 117621, published. 1974 (prototype).