Multilayer crosslinked cation exchange membrane and method for producing same

Abstract

The present invention addresses the problem of providing a cation exchange membrane which has high selectivity with respect to specific cations, particularly ammonium ions, and which furthermore has exceptional strength and handling properties while undergoing little dimensional change. A multilayer crosslinked cation exchange membrane comprises a cation exchange layer that contains a metal cyano complex and a binder resin on at least one surface of a cation exchange membrane composed of a polystyrene crosslinked cation exchange resin and a porous base material.

Description

Multilayer cross-linked cation exchange membrane and method for manufacturing the same

[0001] The present invention relates to a multilayer cross-linked cation exchange membrane and a method for producing the same, characterized in that at least one side of the cation exchange membrane, which is made of a polystyrene-based cross-linked cation exchange resin and a porous substrate, has a cation exchange layer containing a metal cyano complex and a binder resin.

[0002] Currently, nitrogen compounds in exhaust gases and wastewater are treated to detoxify them using a great deal of energy. However, there are cases where nitrogen compounds are released without treatment, or where treatment is insufficient, resulting in a significant impact on the environment. Therefore, there is a need to develop technologies to convert wastewater nitrogen compounds into ammonia resources, thereby achieving detoxification and resource utilization (Clean Earth) of these harmful nitrogen compounds originating from human activities. To this end, technologies are being developed that focus on nitrogen compounds in wastewater and convert and concentrate them into ammonia (containing ammonium ions) to create a form that can be used as a valuable resource (resource ammonia). One important technology in this field is the energy-efficient separation and concentration of ammonium ions present in wastewater.

[0003] Technologies for concentrating and separating harmful or beneficial ions from wastewater include distillation, reverse osmosis (RO) membrane methods, forward osmosis (FO) membrane methods, brine concentration (BC) methods, ion exchange and adsorption methods, electrodialysis (ED) methods, and Donnan dialysis methods. Of these, distillation can handle a wide range of salinity and water quality in treated water and has excellent resistance to contamination, but it requires a large amount of energy and is unsuitable for treating particularly dilute solutions. RO membrane methods, FO membrane methods, and BC methods can achieve high levels of concentration, but in systems where polyvalent ions are generally present, scaling is likely to occur, and selective concentration of specific ions such as ammonium ions is difficult. On the other hand, various ion exchangers and adsorbents have been developed for ion exchange and adsorption methods, including adsorbents that selectively adsorb ammonium ions (Patent Document 1). However, ion exchangers and adsorbents require a regeneration process once saturated, requiring equipment and chemicals for regeneration, and are unsuitable for continuous treatment.

[0004] The ED method is an ion concentration and desalination technology that uses ion exchange membranes. In the concentration process, ions are concentrated by moving them, so if polyvalent ions that are prone to causing scale are present, scale will form on the concentration side, similar to RO membranes. Therefore, monovalent ion selective permeable membranes have been developed that block the permeation of such scale components and facilitate the permeation of monovalent ions, and these have been adopted in industries such as salt production (Patent Documents 3 and 4). However, when the target is a cation with the same valence as the target ion, such as ammonium ions, if monovalent ions such as sodium ions and potassium ions are present, even monovalent ion selective permeable membranes cannot separate them. As a result, power is consumed to concentrate ions other than the target ion, which reduces the energy efficiency of separating and concentrating the target ion. Therefore, there has been a need for the development of ion exchange membranes that selectively permeate specific target ions such as ammonium ions.

[0005] Patent Document 1 discloses an ammonium ion selective adsorbent, but it is not intended for use as a separation membrane. The membranes disclosed in Patent Documents 3 and 4 block the permeation of polyvalent ions, but they could not selectively allow ammonium ions, which are monovalent ions, to permeate more selectively than sodium ions.

[0006] International Publication WO2015 / 186819 Pamphlet Japanese Patent Publication No. 2012-250904 Japanese Patent Publication No. Hei 9-122501 Japanese Patent Publication No. 2001-206964

[0007] The object of the present invention is to provide a cation exchange membrane that exhibits high selectivity for specific cations, particularly ammonium ions, and also has excellent strength, handling properties, and minimal dimensional changes.

[0008] The inventors began investigating ion exchange membranes with high selectivity for specific ions, and proceeded to investigate ion exchange membranes with high selective permeability for ammonium ions as the target ion. In the course of their investigation, they unexpectedly discovered that a membrane made by mixing particles of metal cyano complexes (Patent Documents 1 and 2), which are known as adsorbents for ammonium and cesium ions, with a binder, became a cation exchange membrane with high selectivity for ammonium ions. Adsorbents are mainly used in batch processing because they retain the target ions. On the other hand, ion exchange membranes use driving forces such as electrical force or diffusive force to selectively and continuously move only the target ions through the membrane. Furthermore, adsorbents are used in particulate form, but ion exchange membranes can only fulfill their role when they are made into thin films. For these reasons, it had never been considered before to fabricate a cation exchange membrane using metal cyano complexes as adsorbents.

[0009] The inventors further investigated cation exchange membranes containing the above-mentioned metal cyano complex and binder resin from the viewpoint of industrial applications. As a result, they found that lower ion exchange capacity of the ion exchange membrane increases selectivity, but this increases electrical resistance. Lowering the resistance requires thinning the membrane, which makes it impractical in terms of strength. To achieve both, they found that by providing a cross-linked membrane as a base ion exchange membrane on at least one side of the cation exchange layer containing the above-mentioned metal cyano complex and binder resin, the strength can be improved, handling can be enhanced, and dimensional changes can be reduced, enabling a wider range of industrial applications. In particular, they found that using a cation exchange membrane made of a polystyrene-based cross-linked cation exchange resin and a porous substrate as the base ion exchange membrane provides excellent effects in improving strength and handling, as well as suppressing dimensional changes.

[0010] In other words, the present invention is defined by the following: (1) A multilayer cross-linked cation exchange membrane having a cation exchange layer containing a metal cyano complex and a binder resin on at least one side of a cation exchange membrane made of a polystyrene-based cross-linked cation exchange resin and a porous substrate. (2) The multilayer cross-linked cation exchange membrane of (1) above, wherein the polystyrene-based cross-linked cation exchange resin is a copolymer of a styrene monomer having cation exchange groups and a divinyl compound monomer. (3) The multilayer cross-linked cation exchange membrane of (1) or (2) above, wherein the porous substrate is a polyolefin-based substrate. (4) The multilayer cross-linked cation exchange membrane of (3) above, wherein the porous substrate is a polyolefin-based woven fabric with an aperture ratio of 35 to 50%, a thickness of 90 to 160 μm, and a single yarn diameter of 1 to 70 denier. (5) A multilayer crosslinked cation exchange membrane according to (1) or (2) above, wherein the metal cyano complex is a compound represented by the following general formula [1]. (In formula [1], M represents one or more metal atoms selected from the group consisting of vanadium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, lanthanum, europium, gadolinium, lutetium, barium, strontium, and calcium; M' represents one or more metal atoms selected from the group consisting of vanadium, chromium, molybdenum, tungsten, manganese, iron, ruthenium, cobalt, nickel, platinum, and copper; A represents a cation with a valence of 1 or more; x represents a value between 0 and 3, y represents a value between 0 and 1.5, and z represents a value between 0 and 6.) (6) A cation exchange layer comprising a metal cyano complex and a binder resin, wherein the binder resin comprises a styrene-based elastomer, as in (1) or (2) above. (7) The multilayer cross-linked cation exchange membrane according to (6), wherein the styrene elastomer is an acid-modified styrene elastomer. (8) The multilayer cross-linked cation exchange membrane according to (1) or (2), wherein the polystyrene cross-linked cation exchange resin contains a styrene elastomer. (9) A method for producing the multilayer cross-linked cation exchange membrane according to (1) or (2), comprising coating at least one side of a cation exchange membrane made of a polystyrene cross-linked cation exchange resin and a porous substrate with an organic solvent solution of a binder resin in which a metal cyano complex is dispersed, and drying the membrane. (10) A method for producing the multilayer cross-linked cation exchange membrane according to (9), wherein the binder resin in the organic solvent solution of the binder resin in which a metal cyano complex is dispersed is a binder resin containing a styrene elastomer.

[0011] The multilayer crosslinked cation exchange membrane of the present invention exhibits high selectivity for specific cations, particularly ammonium ions, as well as excellent strength, handling properties, and minimal dimensional changes.

[0012] Figure 1 is a schematic diagram showing one embodiment of the configuration of the multilayer cross-linked cation exchange membrane of the present invention. Figure 2 shows the membrane resistance measurement apparatus and measurement conditions used in the examples and comparative examples. Figure 3 shows the electrodialysis test apparatus used in the examples and comparative examples.

[0013] The multilayer cross-linked cation exchange membrane of the present invention is a cation exchange membrane (hereinafter sometimes referred to as the "base cation exchange membrane") made of a polystyrene-based cross-linked cation exchange resin and a porous substrate, having a cation exchange layer (hereinafter sometimes referred to as the "ion-selective layer") containing a metal cyano complex and a binder resin on at least one side. In the present invention, since different layers, a cation exchange membrane made of a polystyrene-based cross-linked cation exchange resin and a porous substrate and a cation exchange layer containing a metal cyano complex and a binder resin, are laminated, the cation exchange membrane of the present invention is called a multilayer cross-linked cation exchange membrane. The cation exchange layer containing a metal cyano complex and a binder resin functions as a selective layer for specific ions, such as ammonium ions. By using a polystyrene-based cross-linked cation exchange resin for the cation exchange resin of the base cation exchange membrane, the overall strength of the ion exchange membrane with a multilayered cation exchange layer containing a metal cyano complex and a binder resin can be improved, handling properties can be improved, and dimensional changes can be reduced. Furthermore, it is easy to manufacture, the cation exchange resin has high stability, and it can have a large ion exchange capacity and low membrane resistance. These features enable a wider range of industrial applications. Dimensional changes in ion exchange membranes occur due to swelling caused by water absorption in the ion exchange resin portion. If the base cation exchange membrane has a porous substrate, swelling is suppressed and dimensional changes are reduced. Also, dimensional changes are influenced by how easily the ion exchange resin portion swells due to water absorption; if the ion exchange resin portion has a cross-linked structure, it swells less easily, and the overall dimensional change of the ion exchange membrane is reduced. Conversely, if the base cation exchange membrane is not cross-linked, swelling due to water absorption in the ion exchange resin portion becomes large, resulting in a large dimensional change of the entire multilayer cation exchange membrane. Also, due to the difference in swelling between the base cation exchange membrane and the ion selective layer, the membrane curls significantly around the ion selective layer, impairing handling. In addition, among cross-linked cation exchange resins, using polystyrene-based cross-linked cation exchange resin can exhibit excellent effects in improving strength and handling, and suppressing dimensional changes.

[0014] A polystyrene-based crosslinked cation exchange resin is a cation exchange resin having a structure obtained by crosslinking and copolymerizing monomers having styrene units in their molecules (hereinafter also referred to as "styrene monomers"). This can be obtained by copolymerizing monomers without cation exchange groups and then introducing cation exchange groups, or by copolymerizing monomers with cation exchange groups. Crosslinking of polystyrene-based crosslinked resins is usually obtained by copolymerizing styrene monomers and crosslinkable monomers. Crosslinking can also be achieved by using chloromethylstyrene as the crosslinkable monomer and performing a Friedel-Crafts reaction of this monomer, but because the crosslinking reaction is easier, it is preferable that the polystyrene-based crosslinked resin in the present invention is obtained by copolymerizing styrene monomers and divinyl compound monomers.

[0015] Examples of styrene monomers include styrene, vinyltoluene, vinylxylene, α-methylstyrene, vinylnaphthalene, α-halogenated styrenes, styrenesulfonic acid and its salts. These may be used individually or in combination of two or more. Examples of divinyl compound monomers include divinylbenzene, divinylsulfone, butadiene, chloroprene, divinylbiphenyl, trivinylbenzenes, divinylnaphthalene, diallylamine, divinylpyridine, and other divinyl or trivinyl compounds, which may be used individually or in combination of two or more. In the production of the polystyrene-based crosslinked cation exchange resin in the present invention, as described later, it is preferable that the crosslinkable monomer, preferably a divinyl compound monomer, is produced in an amount of 0.1 to 40% by mass, and more preferably 1 to 30% by mass, of the total monomer components. In this specification, A (numerical value) to B (numerical value) means A or more and B or less.

[0016] The base cation exchange membrane in the present invention is composed of a polystyrene-based crosslinked cation exchange resin and a porous substrate. The porous substrate is a porous material that has the function of maintaining the strength of the separation membrane within the ion exchange membrane. Examples of its form include porous membrane-like materials such as mesh, woven fabric, nonwoven fabric, and porous film, but a woven fabric is preferable from the viewpoint of increasing the adhesion layer between the base cation exchange membrane and the ion selective layer and further improving strength. By using a cation exchange membrane having a porous substrate as the base cation exchange membrane, the strength of the cation exchange membrane is improved compared to, for example, a cation exchange membrane without a substrate obtained by the casting method, and it is less prone to tearing or defects during the manufacturing process, enabling stable industrial production. The material of the porous substrate is not particularly limited, but examples include polyolefin resins and polyvinyl chloride. These are commonly used as porous substrates, and polyolefin resins can be preferably used in terms of strength and heat resistance. As the porous substrate, a polyolefin resin substrate is preferred, and a woven fabric of polyolefin resin is more preferred.

[0017] Examples of polyolefin resins include homopolymers of α-olefins such as ethylene, propylene, 1-butene, and 4-methyl-1-pentene, as well as random or block copolymers thereof. Specifically, examples include low-density polyethylene, high-density polyethylene, polypropylene, poly-1-butene, and poly-4-methyl-1-pentene. Among these, low-density polyethylene, high-density polyethylene, or polypropylene are preferred, and polyethylene polymers such as low-density polyethylene and high-density polyethylene are most preferred in terms of availability and resistance to chemicals.

[0018] The aperture ratio of the porous substrate is preferably 35 to 50%. If the aperture ratio is less than 35%, the resistance of the cation exchange membrane may become too high, making it difficult to use, and if it is higher than 50%, the adhesion between the porous substrate and the polystyrene-based cross-linked cation exchange resin may be insufficient. When using woven fabric, the single filaments of the woven fabric can be either multifilament or monofilament, but monofilament is preferred because it can reduce the contact area with the polystyrene-based cross-linked cation exchange resin, thereby reducing the formation of gaps due to poor adhesion between the polystyrene-based cross-linked cation exchange resin and the porous substrate. Furthermore, while it is acceptable to select as appropriate depending on the application, in terms of balancing strength and membrane resistance, for example, when using a woven fabric such as a polyolefin-based woven fabric, the thickness of the woven fabric is preferably 90 to 160 μm, the porosity is preferably 50 to 80%, and the wire diameter of the single filament is preferably 1 to 70 denier.

[0019] The method for producing the base cation exchange membrane, which is a cation exchange membrane made of a polystyrene-based crosslinked cation exchange resin and a porous substrate in the present invention, is not particularly limited, but the following methods are preferred.

[0020] For example, this method involves preparing a polymerizable composition for forming a polystyrene-based crosslinked cation exchange resin, which contains a styrene-based monomer into which a cation exchange group can be introduced, or a monomer component containing a styrene-based monomer having a cation exchange group and a crosslinkable monomer, a polymerization initiator, and, if necessary, a thickener such as polyvinyl chloride or polyethylene; immersing the composition in a porous substrate; copolymerizing the monomer component in the impregnated polymerizable composition after the immersion step; and introducing a cation exchange group as necessary. Examples of styrene-based monomers having functional groups into which a cation exchange group can be introduced include styrene, α-methylstyrene, and α-halogenated styrenes. Examples of styrene-based monomers having a cation exchange group include sulfonic acid monomers such as styrenesulfonic acid, their salts, and esters.

[0021] The crosslinkable monomer is not particularly limited, but examples include divinyl or trivinyl compounds such as divinylbenzene, divinyl sulfone, butadiene, chloroprene, divinylbiphenyl, trivinylbenzenes, divinylnaphthalene, diallylamine, and divinylpyridine, which can be used alone or in combination of two or more. The amount of the crosslinkable monomer is preferably 0.1 to 40% by mass, and more preferably 1 to 30% by mass, in the monomer component including the styrene monomer. By setting the amount of the crosslinkable monomer within this range, high ion selective permeability can be obtained without excessively increasing the electrical resistance of the cation exchange membrane.

[0022] Conventionally known polymerization initiators can be used without particular limitation. Specifically, organic peroxides such as octanoyl peroxide, lauroyl peroxide, t-butyl peroxy-2-ethylhexanoate, benzoyl peroxide, t-butyl peroxyisobutyrate, t-butyl peroxylaurate, t-hexyl peroxybenzoate, and di-t-butyl peroxycyclohexane can be used. Such polymerization initiators are preferably present in amounts of 0.1 to 20 parts by mass, and more preferably 0.5 to 10 parts by mass, per 100 parts by mass of the total monomer components.

[0023] In the polymerizable composition for forming the polystyrene-based crosslinked cation exchange resin described above, it is preferable to add a conventionally known thickener for the purpose of adjusting the viscosity so that the polymerizable composition can be uniformly adhered to a porous substrate such as a polyolefin-based woven fabric. Specifically, examples include polyvinyl chloride, polyethylene, acrylonitrile butadiene rubber and styrene butadiene rubber, as well as styrene-based elastomers such as polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymer, polystyrene-poly(ethylene-propylene)-polystyrene triblock copolymer, polystyrene-polyisoprene block copolymer and their hydrogenated products. These may be used individually or in combination of two or more. These thickeners are preferably contained in an amount of 5 to 50 parts by mass per 100 parts by mass of the total monomer components. In the present invention, it is preferable that the thickener contains a thickener that has high affinity for the solvent for forming the ion-selective layer described later. Specifically, it is preferable that it contains polyvinyl chloride, the styrene-based elastomer, etc. The inclusion of these thickeners improves the adhesion between the base cation exchange membrane and the ion-selective layer. The polymerizable composition for forming polystyrene-based crosslinked cation exchange resins in the present invention may also be used with various monomers used in conventionally known ion exchange membranes, as well as various additives such as plasticizers and hydrochloric acid scavengers, added in the required amounts.

[0024] In the production of the base cation exchange membrane of the present invention, the polymerizable composition described above is then impregnated into the porous substrate and filled into the voids of the porous substrate. There are no particular restrictions on the method of impregnating the voids of the porous substrate with the polymerizable composition. For example, this can be done by immersing the porous substrate in a tank filled with the polymerizable composition described above. Of course, instead of immersion, the polymerizable composition can also be impregnated by methods such as spray coating or coating using a doctor blade. In this way, the polymerizable composition impregnated into the porous substrate is heated in a polymerization apparatus such as a heating oven to copolymerize and harden. In this polymerization process, a method is generally employed in which the porous substrate filled with the polymerizable composition is sandwiched between films such as polyester and heated from room temperature under pressure. The pressure is generally about 0.1 to 1.0 MPa and is applied by an inert gas such as nitrogen or by applying pressure with a roll. This pressure pushes excess polymerizable composition present at the outer interface of the porous substrate into the voids of the porous substrate during polymerization, effectively preventing the occurrence of resin buildup. Other polymerization conditions depend on the type of monomer component, etc., and can be appropriately selected from known conditions. The polymerization temperature is usually set to a temperature lower than the melting point determined by the material of the porous substrate, and when a polyolefin substrate is used, it is usually in the range of 70 to 120°C. The polymerization time also varies depending on the polymerization temperature, etc., but is generally about 3 to 20 hours. When a styrene monomer having a cation exchange group is used in the polymerizable composition, a cation exchange film supported on the porous substrate is obtained upon completion of polymerization curing. When the polymerizable composition is a styrene monomer without a cation exchange group, a cation exchange group is introduced after polymerization curing of the polymerizable composition. The introduction of a cation exchange group is carried out by a known method, for example, by sulfonation, chlorosulfonation, phosphoniumization, hydrolysis, etc. Sulfonic acid groups, carboxylic acid groups, phosphonic acid groups, etc. are introduced as cation exchange groups by this treatment. Among these, sulfonic acid groups are preferred because they are strongly acidic and can lower the membrane resistance of the base cation exchange membrane.

[0025] In the base cation exchange membrane of the present invention obtained by the above method, the cation exchange capacity is usually set to about 0.5 to 3.0 meq / g-dry mass, and the moisture content is set to about 15 to 60%. The film thickness is preferably 95 to 250 μm in a wet state, and the Muhlene burst strength is preferably 0.5 to 2.0 MPa. Furthermore, even with such a high-strength cation exchange membrane, 0.5 N NH ４ The film resistance in an aqueous Cl solution was 4.0 Ω·cm. ２ Preferably, the following values ​​are 0.5 to 4.0 Ω·cm. ２ It can be made lower.

[0026] The metal cyano complex used in the cation exchange layer containing the metal cyano complex and binder resin in the present invention is not particularly limited, but examples include the metal cyano complex represented by the following general formula [1].

[0027]

[0028] Here, in formula [1], M represents one or more metal atoms selected from the group consisting of vanadium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, lanthanum, europium, gadolinium, lutetium, barium, strontium, and calcium; M' represents one or more metal atoms selected from the group consisting of vanadium, chromium, molybdenum, tungsten, manganese, iron, ruthenium, cobalt, nickel, platinum, and copper; A represents a cation with a charge of 1 or more charges; x represents a value between 0 and 3, y represents a value between 0 and 1.5, and z represents a value between 0 and 6. In formula [1], when M and M' are identified, it is called an M-M' cyano complex. For example, when M is copper and M' is iron, it is called a copper-iron cyano complex.

[0029] In formula [1], M is more preferably one or more metal atoms selected from the group consisting of vanadium, chromium, manganese, iron, ruthenium, cobalt, nickel, copper, and zinc, and even more preferably one or more metal atoms selected from the group consisting of manganese, iron, cobalt, nickel, copper, and zinc. M' is more preferably one or more metal atoms selected from the group consisting of manganese, iron, ruthenium, cobalt, and platinum, and even more preferably one or more metal atoms selected from the group consisting of iron and cobalt. A is preferably one or more cations selected from the group consisting of hydrogen, ammonium, alkali metals, and alkaline earth metals, and even more preferably one or more cations selected from the group consisting of hydrogen, lithium, sodium, potassium, rubidium, and cesium. x is more preferably 0 to 2.5, and even more preferably 0 to 2. y is more preferably 0.4 to 1.3, and even more preferably 0.5 to 1. z is more preferably 0.5 to 5.5, and even more preferably 1 to 5. Here, the most important composition is y. y represents the ratio of metal M to M'. The adsorption behavior of the metal cyano complex can be controlled by this ratio. For example, to effectively adsorb ammonium ions in water, y is preferably 0.6 or higher, and to adsorb ammonia molecules in gas, y is preferably 0.7 or lower. Therefore, if the present invention can be applied to any y, its applications are expected to be greatly expanded. The ratio of metal M to M' may be adjusted during the production of the metal cyano complex, but it can also be adjusted after production by contact with a surface treatment agent consisting of a metal salt such as potassium ferrocyanide trihydrate.

[0030] The crystal structure of metal cyano complexes is generally face-centered cubic, but is not necessarily limited to this. For example, K ０．６７ Zn[Fe(CN)] ６ ] ０．６７ ・zH ２O adopts a hexagonal crystal structure. While six cyano groups typically coordinate to M', some may be substituted with nitro groups, and any number between four and eight is acceptable. There are no particular restrictions on the synthesis method of the metal cyano complex, but a method that can uniformly achieve the desired composition is preferred. Furthermore, the complex surface may be modified with various materials for processing purposes. Specific methods include those described in Japanese Patent Publication No. 2006-256954 and Japanese Patent Publication No. 2013-173077, and nanoparticles with a large specific surface area are desirable. In practical terms, uniform particle size is desirable, so a method for producing uniform nanoparticles described in Japanese Patent Publication No. 2013-173077 is suitable. As described in Japanese Patent No. 5035767 and Re-publication 2008 / 081923, the dispersibility of the metal cyano complex in various solvents can be controlled by adding metal ions, ferrocyanide ions, or organic molecules to the particle surface.

[0031] The binder resin used in the cation exchange layer containing the metal cyano complex and the binder resin in the present invention is not limited as long as it can immobilize the metal cyano complex and form an ion-selective layer. In particular, a binder resin with good dispersibility with the metal cyano complex is preferred. Specifically, examples include polyolefins such as polyethylene and polypropylene, chlorine-containing resins such as polyvinyl chloride and polyvinylidene chloride, engineering plastics such as polyethersulfone and polyphenylene ether, rubbers such as acrylonitrile butadiene rubber and styrene butadiene rubber, styrene-based elastomers such as polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymer, polystyrene-poly(ethylene-propylene)-polystyrene triblock copolymer, polystyrene-polyisoprene block copolymer and its hydrogenated derivatives, polyacrylonitrile, cellulose triacetate, and polyvinyl alcohol. One of these may be used, or two or more may be mixed and used. If the binder resin does not completely dissolve in the ion-selective layer forming solvent described later, a powder form with an average particle size of 10 μm or less may also be used.

[0032] The binder resin described above preferably has an ion exchange capacity of less than 0.4 mmol / g, and more preferably less than 0.2 mmol / g. If the binder resin has an ion exchange capacity of 0.4 mmol / g or more, permeation of ions other than the target ion proceeds in the ion-selective layer without going through the metal cyano complex, and the selectivity of the target ion decreases. In the present invention, it is also preferable that the binder resin includes flexible resins such as rubbers and styrene elastomers among the binder resins described above, as this can enhance the selectivity of the ion-selective layer. When the ion-selective layer follows the shape of the ion-selective layer due to dimensional changes such as swelling of the base cation exchange membrane, a gap may occur between the metal cyano complex and the binder resin. However, it is presumed that using the flexible binder resin described above allows for high selectivity without creating a gap due to its high adhesion to the metal cyano complex.

[0033] Furthermore, the binder resin is preferably a flexible resin as described above, in that it can improve the adhesion between the base cation exchange membrane and the ion selective layer. In the present invention, since the base cation exchange membrane is composed of a polystyrene-based crosslinked cation exchange resin, the binder resin preferably contains a styrene-based elastomer. Among styrene-based elastomers, an acid-modified styrene-based elastomer, specifically an acid-modified hydrogenated polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymer, can be more preferably used, as it can improve dispersibility with metal cyano complexes, by modifying the terminal acid with maleic anhydride or the like. ３ It is ONa / g.

[0034] An ion-selective layer containing a metal cyano complex and a binder resin is formed on the base cation exchange membrane described above, resulting in the multilayer crosslinked cation exchange membrane of the present invention. The method for forming the ion-selective layer on the base cation exchange membrane is not particularly limited, but preferred methods include, for example, the following methods.

[0035] An ion-selective layer can be formed on a base cation exchange membrane by coating at least one side of the base cation exchange membrane with an organic solvent solution of a binder resin in which the above-mentioned metal cyano complex is dispersed (hereinafter also referred to as "dope") and drying it. The organic solvent of the dope can be used without restriction as long as it does not dissolve the metal cyano complex but dissolves or disperses the binder resin. An organic solvent with a moderate drying rate that does not take too long to volatilize is preferred. Examples include aprotic polar organic solvents such as dimethylacetamide (DMAC), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), but tetrahidofuran (THF) is even more preferred in terms of its high solubility of the binder resin and high volatilization rate. By controlling the volatilization rate by mixing THF with a solvent that is a good solvent but has a relatively slow volatilization rate, such as DMAC, rather than using THF alone, it is expected that the performance distribution in the film width direction will be made more homogeneous. The above-mentioned dope is obtained by mixing a predetermined amount of the metal cyano complex and the organic solvent, and then adding the binder resin. It is preferable to use metal cyano complexes that have been pre-dissolved in a mortar or ball mill to achieve good dispersion. The binder resin may be added as is, or it may be added after being dissolved in the same or different organic solvent. It is also preferable to further improve the dispersion by dissolving these in a ball mill for about 30 minutes to 30 hours after mixing.

[0036] When the mass ratio of the metal cyano complex to the binder resin is expressed as metal cyano complex / binder resin, the metal cyano complex / binder resin ratio is preferably 0.5 / 1 to 3 / 0.5, and more preferably 1 / 1 to 2 / 1. If the mass ratio of the metal cyano complex is less than 0.5 / 1, the film resistance may increase or the selective permeability of specific cations such as ammonium ions may decrease. If it is more than 3 / 0.5, the ion-selective layer may become brittle or the adhesion to the base cation exchange membrane may decrease. The total concentration of the metal cyano complex and binder resin in the dope is preferably 10 to 40% by mass, and more preferably 15 to 30% by mass. If the total concentration of the metal cyano complex and binder resin is too high, the resistance tends to increase, and if it is too low, homogeneous coating may not be possible when applied.

[0037] Next, the above-mentioned dope of binder resin in which the metal cyano complex is dispersed is applied to at least one side of the base cation exchange membrane. Further improvement in adhesion can be expected if the base cation exchange membrane is roughened in advance by sandblasting or the like on one or both sides to be coated. It is preferable that the base cation exchange membrane is moderately dry before coating. Typically, a membrane with a moisture content W of 15 to 60%, as defined by formula (6) described later, is used as the base cation exchange membrane, and is moderately dried. Since ion exchange membranes undergo dimensional changes due to drying, it is important to know how much the moisture content changes due to drying rather than the absolute value of the moisture content. Therefore, it is preferable to control the moisture content retention rate, that is, the ratio Wr (= ​​Wd / Wb) of the moisture content Wd after drying to the moisture content Wb of the original base cation exchange membrane when wet. It is preferable to use a membrane with a moisture content retention rate Wr of 10 to 90%, more preferably 20 to 70%, and especially 20 to 50%. If the moisture content falls below 10%, it becomes too dry, leading to increased elongation upon re-wetting, which can damage the coating layer or increase the likelihood of curling. Conversely, if there is too much moisture, for example, if water droplets are present on the coating surface, it can negatively affect the uniformity of the coating, such as causing uneven application. Therefore, it is preferable to maintain a moisture content of 90% or less.

[0038] There are no particular restrictions on the drying method to achieve the target moisture content retention rate; air drying, vacuum drying, and heat drying are possible, but leaving it in a room with controlled air conditioning is also acceptable. For coating, the base cation exchange membrane is fixed, the dope is dripped onto it, and it is coated using a coater with a set coating thickness. Roll-to-roll coating is also possible. In a laboratory setting, the base cation exchange membrane is fixed to a glass plate or the like, and the coating is applied using a film applicator with controlled coating thickness. Specifically, the membrane is fixed to a glass plate with vinyl tape or the like, the dope is dripped onto the base cation exchange membrane, and then stretched to a uniform thickness using a glass rod, bar coater, or non-wire bar coater. It is preferable to use a film applicator that allows for selection and adjustment of the dope coating thickness. The dope coating thickness varies depending on the composition and viscosity of the dope, but it is generally 100 to 600 μm. What is more important is the coating thickness after drying.

[0039] Thereafter, the dopant organic solvent is volatilized and dried. The temperature is about 10°C to 40°C, but it is preferable for stable quality that the temperature can be controlled constantly. Furthermore, it is preferable to use a draft that sucks the volatilized organic solvent. The drying time is about 5 minutes to 1 hour, depending on the type of organic solvent, coating thickness, and temperature. After drying, it is wetted again by putting it in water. The thickness of the coating layer can be directly measured by observing with SEM, but as a production control method, it is measured by measuring the thickness of the base cation exchange membrane to be coated and subtracting it from the thickness of the coating membrane. The thickness of the coating layer in the wet state is preferably 1 to 100 μm, more preferably 5 to 80 μm.

[0040] The membrane resistance of the multilayer crosslinked cation exchange membrane of the present invention is 0.5 N ４ in an aqueous Cl solution, 5.0 Ω·cm ２ or less is preferable, and 4.0 Ω·cm ２ or less is more preferable. The NH ４ selectivity (to Na) is preferably 3 or more, more preferably 5 or more, and even more preferably 10 or more. The NH ４ selectivity (to Ca) is preferably 10 or more, more preferably 20 or more. The bursting strength is desirably 0.2 MPa or more, more preferably 0.3 MPa or more, and even more preferably 0.4 MPa or more, and can be measured by the Mullen method.

[0041] Hereinafter, the present invention will be specifically described with reference to examples of the present invention, but the technical scope of the present invention is not limited to these examples.

[0042] First, the method for evaluating the physical properties of the obtained membrane will be described.

[0043] (Measurement of film thickness) The film thickness of the obtained film was measured using a film thickness gauge (ABS digital blade thickness, Mitutoyo Corporation). The layer thickness of the ion selective layer was obtained by subtracting the thickness of the base cation exchange membrane to be coated from the thickness of the obtained film.

[0044] (Measurement of membrane resistance) Fig. 2 shows the measurement apparatus and measurement conditions for membrane resistance. First, an acrylic cell with a current-carrying area of 0.949 cm having a platinum electrode ２ was used as the measurement solution, and NH４ Cl aqueous solution (NH ４ Add Cl concentration (0.5 N) and measure the solution resistance (R) at 25°C using an LCR meter at a frequency of 10 kHz. ０ The film resistance (R) was measured. Then, the sample film was placed between two cells and the resistance was measured in the same way. ｍ The film resistivity K is calculated from the following equation (1), and the obtained film resistance is divided by the film thickness to obtain equation (2). ｍ The value [Ωcm] was calculated. In equation (1), R [Ωcm] ２ ] is the measured resistance measured with a sample film sandwiched between two cells, R ０ [Ωcm] ２ ] is the solution resistance measured without the sample film in between, and R ｍ [Ωcm] ２ ] is the film resistance. In equation (2), K ｍ [Ωcm] is the film resistivity, and d [cm] is the film thickness. The measurement solution is 0.5 N NH ４ Cl was used.

[0045]

[0046]

[0047] [NH ４ [Selectivity Evaluation] (Electrodialysis) The effective membrane area is 4.0 cm² as shown in Figure 3. ２ The measurement film was placed in a (2.0 cm x 2.0 cm) apparatus with the ion-selective layer facing the anode, and measured in a 25°C atmosphere using 0.1 M NH4. ４ Cl, 0.1 M NaCl, and 0.2 M CaCl ２ A mixed salt solution is placed in two cells, and a constant current of 90 mA (current density of 2.25 A / dm²) is applied between the Ag and AgCl electrodes using a DC stabilized power supply (PMC35-2A, Kikusui Electronics Co., Ltd.). ２ Electrodialysis was performed for approximately 120 minutes, and the solution in the cell was sampled at predetermined intervals. Subsequently, NH in the sampled solution was analyzed using an ion chromatograph. ４ ＋ Na ＋ and Ca ２＋By quantifying the concentrations of these ions, we measured the time evolution of their concentrations. Using these values, we calculated the flux of each ion using the following equation. In equation (3), J ｉ [mol・m －２ ・s －１ ] represents the permeation flux, V[m] ３ ] represents the volume of the concentrated solution, and S [m ２ ] represents the effective film area, t[s] represents the time the current was applied, and ΔC ｉ / Δt[mol・m] －３ ・s －１ ] represents the initial concentration gradient. Also, i is NH ４ ＋ Na ＋ or Ca ２＋ This represents P. In equations (4) and (5), P ＮＨ４ Ｎａ and P ＮＨ４ Ｃａ This represents the ion selective permeability coefficient [-].

[0048]

[0049]

[0050]

[0051] [Evaluation of K selectivity and Cs selectivity] (Electrodialysis) Effective membrane area of ​​4.0 cm² as shown in Figure 3 ２ A measurement film was placed in a (2.0 cm × 2.0 cm) apparatus with the ion-selective layer facing the anode. For K selectivity evaluation, a mixed salt solution of 0.1 M KCl and 0.1 M NaCl was placed in two cells, and for Cs selectivity evaluation, a mixed salt solution of 0.1 M CsCl and 0.1 M NaCl was placed in two cells. A source measuring unit GS-610 (Yokogawa Electric Corporation) was used between the Ag and AgCl electrodes, and a constant current of 90 mA (current density of 2.25 A / dm²) was applied at 25°C. ２ Electrodialysis was performed for approximately 120 minutes, and the solution in the cell was sampled every 60 minutes. Subsequently, the potassium content of the sampled solution was analyzed using an ion chromatograph. ＋ Ions, Na ＋ Ions and Cs ＋By quantifying the concentrations of these ions, we measured the time evolution of their concentrations. Using these values, we calculated the flux of each ion using the following equation. In equation (6), J ｉ [mol・m －２ ・s －１ ] represents the permeation flux, V[m] ３ ] represents the volume of the concentrated solution, and S [m ２ ] represents the effective film area, t[s] represents the time the current was applied, and ΔC ｉ / Δt[mol・m] －３ ・s －１ ] represents the initial concentration gradient. Also, i is K ＋ Na ＋ or Cs ＋ This represents P in equation (7). Ｋ Ｎａ and P in equation (8) Ｃｓ Ｎａ This represents the ion selective permeability coefficient [-]. NH ４ While AC resistance was measured in the first method, for K and Cs, the potential difference during electrodialysis was measured as an indicator equivalent to AC resistance. A smaller potential difference indicates lower membrane resistance. A salt bridge was set up near the front and back surfaces of the test membrane and connected to a potentiometer via a glass electrode to read the membrane potential difference at the end of the operation. Current density was adjusted by changing the current setting input value of the source measure unit GS-610.

[0052]

[0053]

[0054]

[0055] (Mullen burst strength) The pressure at the time of bursting was read using a Mullen burst strength measuring instrument manufactured by Toyo Seiki Seisakusho, and one data point was obtained for each sample (unit: [MPa]). The burst strength was measured as follows: The membrane sample was fixed by sandwiching it between upper and lower clamping plates with an opening of approximately 3 cm in diameter. Stress was applied from the lower side of the opening by the expansion of rubber under internal pressure, and the pressure at the moment of bursting was read with a red indicator needle.

[0056] (Moisture content) The base cation exchange membrane was immersed in a 0.5 mol / L NaCl aqueous solution for 8 hours or more, washed with deionized water, and the surface moisture was wiped off with tissue paper. Wet weight W ２ The following was measured. Furthermore, the same film was dried under reduced pressure at 60°C for 3 hours, and its dry weight was measured. １ The dry weight W was measured. １ and wet weight W ２ The moisture content W was calculated using the following formula (9).

[0057]

[0058] Next, we will describe the method for producing the base cation exchange membrane and the metal cyano complex.

[0059] (Production Example 1: Production of Base Cation Exchange Membrane-1) (i) A monomer mixture of the following formulation was prepared: ・Styrene (St) 55.4% by mass ・Chloromethylstyrene (CMS: m-isomer / p-isomer = 50 / 50) 9.6% by mass ・Divinylbenzene (DVB: purity 57%, remainder ethyl vinylbenzene) 9.1% by mass ・Acrylonitrile (AN) 22.9% by mass ・α-Methylstyrene (α-MeSt) 3.0% by mass (ii) The following was added to 100 parts by mass of the above monomer mixture and stirred for 10 hours to obtain a homogeneous composition. - 0.8 parts by mass of ethylene glycol diglycidyl ether - 17.4 parts by mass of tributyl acetyl citrate (ATBC) - 64.2 parts by mass of vinyl chloride powder (ZEST P22 manufactured by Shin-Daiichi Vinyl Chloride Co., Ltd.) - 1.0 part by mass of acrylonitrile butadiene rubber (N230SH manufactured by JSR Corporation) (iii) The following was added to the above composition and mixed for 2 hours to obtain a polymerizable composition: - 2.0 parts by mass of lauroyl peroxide (Perloyl L manufactured by Nippon Oil & Fats Co., Ltd.) (iv) Next, a high-density polyethylene monofilament woven fabric (PE33D 120 / 120) was prepared. The prepared high-density polyethylene monofilament woven fabric had a mesh count of 120, a linearity of 76 μm (33 denier), a thickness of 132 μm, an open area ratio of 41%, and a bursting strength of 1.0 MPa. (v) The polymerizable composition obtained in (iii) was applied to the high-density polyethylene monofilament fabric (PE33D 120 / 120) described above, and a polyester film was coated on both sides as a release agent. After preheating at 45°C for 1 hour, polymerization was carried out at 70°C for 3 hours. The resulting film-like polymer was sulfonated with a mixture of chlorosulfonic acid and sulfuric acid in a mass ratio of 1:1 at 40°C for 45 minutes, and then hydrolyzed with an aqueous sodium hydroxide solution to obtain a cation exchange membrane. The properties of the obtained cation exchange membrane were as follows: Wet film thickness: 228 μm Ion exchange capacity: 2.0 meq. / g-dry weight Water content: 49% Electrical resistance: 1.9 Ω・cm ２ ・Burst strength: 1.0MPa

[0060] (Manufacturing Example 2: Manufacturing of Base Cation Exchange Membrane-2) (i) A polymerizable composition was prepared by mixing each component containing p-chloromethylstyrene as a crosslinkable monomer according to the following formulation: ・Styrene 73.5 parts by mass ・p-chloromethylstyrene 16.5 parts by mass ・Acrylonitrile 10.0 parts by mass ・Tributyl acetylcitrate 2.5 parts by mass ・Styrene oxide 2.9 parts by mass ・Triganox B (di-t-butyl peroxide, manufactured by Kayaku Nurion Co., Ltd.) 1.9 parts by mass (ii) 500 g of the prepared polymerizable composition was placed in a 1000 mL glass container, and a porous film made of high molecular weight polyethylene with a thickness of 130 μm and a porosity of 50% was immersed therein as a base film, and the polymer composition was filled into the voids of the film. (iii) The porous substrate film filled with the above polymer composition was removed, and both sides of the porous substrate film were covered with a polyester film with a thickness of 188 μm as a release agent. Then, under nitrogen pressure of 0.4 MPa, the temperature was raised from 20°C to 50°C over 20 minutes, then from 50°C to 110°C over 60 minutes, then from 110°C to 130°C over 80 minutes, and held at 130°C for 180 minutes to carry out thermal polymerization. (iv) The obtained film was immersed in a 1:1 (mass ratio) mixture of 98% concentrated sulfuric acid and chlorosulfonic acid with a purity of 90% or more at 40°C for 60 minutes. After that, the film was sequentially immersed in 90% sulfuric acid, 60% sulfuric acid, and deionized water, and further immersed in a 4 mol / L-NaOH aqueous solution for 12 hours, and washed with water to obtain a sulfonic acid type cation exchange membrane. The properties of the obtained ion exchange membrane were as follows. • Wet film thickness: 1528 μm • Ion exchange capacity: 2.5 meq. / g-dry weight • Moisture content: 43% • Electrical resistance: 1.9 Ω·cm ２ ・Burst strength: 0.5MPa

[0061] (Production Example 3: Production of Metal Cyano Complexes: Preparation of KZnHCF) A KZnHCF slurry containing 100 g of KZnHCF (assuming tetrahydrate) was obtained by mixing raw material solution C, which was prepared by dissolving 0.38 mol of zinc salt in 537 mL of water, with raw material solution D, which was prepared by dissolving 0.26 mol of potassium ferrocyanide trihydrate in 551 mL of water. Next, potassium sulfate, a by-product, was removed by coagulation precipitation to obtain a washed KZnHCF slurry (KZnHCF-1). Furthermore, 41 g of potassium ferrocyanide trihydrate was added and shaken for 3 days to obtain a surface-treated KZnHCF slurry (KZnHCF-2). Subsequently, each slurry was placed in a petri dish and dried on a hot plate at 80°C until the weight change was negligible. Furthermore, the composition of KZnHCF-1 is estimated to be x = 0.66, y = 0.66 in equation [1], and the composition of KZnHCF-2 is estimated to be x = 0.98, y = 0.95 in equation [1].

[0062] (Production Example 4: Production of Metal Cyano Complexes: Preparation of KCuHCF) Copper-iron cyano complex (K ２ Cu ３ [Fe(CN)] ６ ] ２ The substance was synthesized as follows: Copper sulfate pentahydrate was dissolved in pure water to prepare an aqueous solution with a copper ion concentration of 0.52 mol / L. Potassium ferrocyanide trihydrate was dissolved in pure water to prepare an aqueous solution with a ferrocyanide ion concentration of 0.35 mol / L. When these solutions were mixed, a brown precipitate was formed. After centrifugation and washing twice with MilliQ water, the precipitate was evaporated to dryness, pulverized, and classified by sieving to obtain only the powder with a particle size of 90-300 μm, thereby obtaining a brown powder. When the obtained powder was evaluated using an X-ray diffractometer, it was found to be similar to the K in the K in the database of diffraction patterns of known substances. ２ Cu ３ [Fe(CN)] ６ ] ２ This coincided with the peak position.

[0063] [Example 1] 30 g of the pre-crushed KZnHCF-1 produced in Production Example 3 was weighed and placed in a 500 ml Teflon container. 300 ml equivalent amounts of silicon nitride balls with a diameter of 1 cm were added, the container was sealed, and then ball-milled for 48 hours on a 40 rpm roller. 6 g of the crushed KZnHCF-1 and 39 g of tetrahydrofuran (THF) were placed in a 100 ml Teflon container. 60 ml equivalent amounts of silicon nitride balls with a diameter of 1 cm were added, the container was sealed, and then ball-milled for 48 hours on a 40 rpm roller. Subsequently, a maleic anhydride modified product of the hydrogenated polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymer (ToughTec M1913, manufactured by Asahi Kasei Corporation, acid value 10 mgCH) was added. ３ Four g of ONa (Oxygen-based Na / g) was added, the mixture was sealed, and then ball milling was performed at 40 rpm for 18 hours to obtain the desired dope (organic solvent solution for ion-selective layer formation). As a result, the mass ratio of the metal cyano complex to the binder resin, which is metal cyano complex KZnHCF-1 / binder resin, was 1.5 / 1, and the total concentration of metal cyano complex KZnHCF-1 and binder resin in the overall solution was 20.4% by mass.

[0064] The base cation exchange membrane of Production Example 1 was dried at room temperature to achieve a water content retention rate Wr of 30%. A 175 μm thick vinyl tape was applied around the base cation exchange membrane to fix the substrate membrane and set the dope coating thickness to 175 μm. The prepared dope was dripped onto the substrate membrane, and a stainless steel scraper was placed on two rows of vinyl tape and pulled towards the user to coat the dope. The vinyl tape thickness was 175 μm. After drying at room temperature for one hour, the entire glass plate was submerged in pure water. The multilayer cross-linked cation exchange membrane of the present invention was obtained in about one hour. The obtained membrane had a 0.5 N NH ４ It was immersed in an aqueous solution of Cl and stored.

[0065] [Examples 2 and 3] In Example 2, the vinyl tape was doubled and the coating thickness was 350 μm, and in Example 3, it was tripled and the thickness was 525 μm, except that the conditions were the same as in Example 1.

[0066] [Examples 4 and 5] Example 4 was applied under the same conditions as Example 2, except that the amount of binder resin was half that of Example 1. Example 5 was applied under the same conditions as Example 2, except that the amount of binder resin was 1.5 times that of Example 1.

[0067] [Example 6] Example 6 was conducted under the same conditions as Example 2, except that the base cation exchange membrane was a commercially available membrane, NeoSepta CMX (manufactured by Astrom Co., Ltd.), which has a styrenedivinylbenzene type crosslinked structure with a PVC woven fabric as the base material.

[0068] [Examples 7 and 8] Example 7 was conducted under the same conditions as Example 2, except that the binder resin was acrylonitrile butadiene rubber (NBR rubber, N230SH, manufactured by JSR Corporation). Example 8 was conducted under the same conditions as Example 2, except that the binder resin was polyethersulfone and the solvent was dimethylacetamide.

[0069] [Examples 9 and 10] Example 9 was carried out under the same conditions as Example 2, except that a base cation exchange membrane made of a porous substrate film obtained in Production Example 2 was used. Example 10 was carried out under the same conditions as Example 2, except that a base cation exchange membrane made of a porous substrate film obtained in Production Example 2 was used, polyethersulfone, an engineering plastic, was used as the binder resin, and dimethylacetamide was used as the solvent.

[0070] [Comparative Example 1] Comparative Example 1 consists only of a base cation exchange membrane without doping.

[0071] [Comparative Example 2] Comparative Example 2 is under the same conditions as Example 2, except that the dope was applied to a glass plate with a dope coating thickness of 350 μm instead of being coated on a substrate film, and the film was obtained by peeling it off the glass plate.

[0072] [Comparative Example 3] Polyphenylene oxide was dissolved in chloroform, reacted with chlorosulfonic acid, neutralized with sodium hydroxide, and then the solvent was removed to obtain sulfonated polyphenylene oxide with a cation exchange capacity of 1.6 mmol / g. Next, the obtained sulfonated polyphenylene oxide was dissolved in N,N-dimethylformamide to prepare a sulfonated polyphenylene oxide solution having a viscosity of 11 dPa·s. As a substrate, a polyethylene terephthalate nonwoven fabric sheet (fiber diameter 13 μm, basis weight 50 g / m ２ ) was prepared, and the sulfonated polyphenylene oxide solution was cast in this sheet form, and the solvent was heated and dried at 60 °C to obtain a base cation exchange membrane made of non-crosslinked sulfonated polyphenylene oxide. An ion selective layer was formed in the same manner as in Example 2 except that the obtained membrane was used as the base cation exchange membrane.

[0073] The physical properties of the membranes obtained in Examples 1 to 10 and Comparative Examples 1 to 3 are shown in Table 1. In Table 1, the dope coating thickness is the thickness when the dope is applied, and the ion selective layer thickness is the thickness of the ion selective layer obtained by drying the applied dope.

[0074]

[0075] In Example 1, a membrane with a membrane resistance of 1.66 Ω·cm ２ , NH ４ selectivity (against Na) of 10.8, and good membrane with respect to NH ４ selectivity (against Ca) was obtained. In Examples 2 and 3, as the dope coating thickness increased, the coating layer thickness also increased, and the membrane resistance increased compared to Example 1, but the membrane resistance was within a preferable range, and the NH ４ selectivity improved for both against Na and against Ca compared to Example 1. In Example 4, the amount of the binder resin was half that of Example 1, but the balance between the membrane resistance and the selectivity was good, and changing the amount of the binder resin did not significantly affect the physical properties. In Example 5, since the amount of the binder resin was 1.5 times that of Example 1, the membrane resistance increased, but a very high NH ４Selectivity (to Ca) was obtained. In Example 6, due to the change of the base cation exchange membrane, the Mullen burst strength decreased compared to Example 1, but it was still high enough, and the membrane resistance and NH ４ The balance of selectivity was good. In Examples 7 and 8, due to the change of the binder resin, the NH ４ Selectivity became lower than that of other examples, but still had a higher selectivity compared to the cation exchange membrane without an ion selective layer as in Comparative Example 1, and a membrane with low membrane resistance and high strength was obtained. The membranes obtained in Examples 9 and 10 had good membrane resistance and NH ４ Selectivity, and a membrane with a sufficient membrane strength with a Mullen burst strength of 0.5 MPa was obtained. The membrane of Comparative Example 1 did not have an ion selective layer, so the NH ４ Selectivity (to Na) was 1.92, and the NH ４ Selectivity (to Ca) was very low at 0.87. The membrane of Comparative Example 2 consisted only of an ion selective layer, so the Mullen burst strength was as low as 0.1 MPa, and a practical strength could not be obtained. The membrane of Comparative Example 3 had a non-crosslinked base cation exchange membrane, so the membrane had a low Mullen burst strength of 0.2 MPa.

[0076] [Example 11] In Example 1, a membrane was prepared as in Example 1, except that the means for defining the coating thickness was not a vinyl tape but a bar coater with a gap of 100 μm for coating.

[0077] [Example 12] Preparation and membrane formation were carried out under the same conditions as in Example 11, except that KZnHCF-2 was used as the metal cyanide complex in Production Example 3.

[0078] The physical properties of the membranes obtained in Examples 11 and 12 are shown in Table 2. In Table 2, the dope coating thickness is the thickness when the dope is coated, and the ion selective layer thickness is the thickness of the ion selective layer obtained by drying the coated dope. In Example 11, a membrane excellent in membrane resistance, NH ４ Selectivity (to Na) and NH ４ Selectivity (to Ca) was obtained. In Example 12, a membrane with a thin ion selective layer thickness was obtained, but the membrane resistance was low, and a sufficient burst strength similar to that of Example 1 was obtained, and NH ４The selectivity was also higher compared to Comparative Example 1.

[0079]

[0080] [Example 13] 30 g of the KZnHCF-1 produced in Production Example 3, which had been pre-crushed, was weighed and placed in a 500 ml Teflon container. 300 ml equivalent amounts of silicon nitride balls with a diameter of 1 cm were added, the container was sealed, and then ball-milled for 48 hours on a 40 rpm roller. 6 g of the crushed KZnHCF-1 and 40 g of tetrahydrofuran (THF) were placed in a 100 ml Teflon container. 60 ml equivalent amounts of silicon nitride balls with a diameter of 1 cm were added, the container was sealed, and then ball-milled for 48 hours on a 40 rpm roller. Subsequently, a maleic anhydride modified product of the hydrogenated polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymer (ToughTec M1913, manufactured by Asahi Kasei Corporation, acid value 10 mgCH) was added. ３ Three g of ONa (Oxygen-based Na / g) was added, the mixture was sealed, and then ball milling was performed at 40 rpm for 18 hours to obtain the desired dope (organic solvent solution for ion-selective layer formation). As a result, the mass ratio of the metal cyano complex to the binder resin, which is metal cyano complex KZnHCF-1 / binder resin, was 2 / 1, and the total concentration of metal cyano complex KZnHCF-1 and binder resin in the overall solution was 18.4% by mass.

[0081] The base cation exchange membrane of Production Example 1 was dried at room temperature to achieve a water content retention rate Wr of 30%. A 350 μm thick vinyl tape was applied around the base cation exchange membrane to fix the substrate membrane and set the dope coating thickness to 350 μm. The prepared dope was dripped onto the substrate membrane, and a stainless steel scraper was placed on two rows of vinyl tape and pulled towards the user to coat the dope. The vinyl tape thickness was 350 μm. After drying at room temperature for one hour, the membrane, along with the glass plate, was submerged in pure water. The multilayer cross-linked cation exchange membrane of the present invention was obtained in about one hour. The obtained membrane was immersed in a 0.5 N KCl aqueous solution and stored.

[0082] [Examples 14-17] Example 14 was conducted under the same conditions as Example 13, except that the dope coating thickness was changed to 175 μm. Example 15 was conducted under the same conditions as Example 13, except that the amount of binder resin was changed. In Example 16, a film was obtained under the same conditions as Example 15, but a portion with a thicker ion-selective layer was cut out for measurement (3.0 cm × 3.0 cm) to be used as the film of Example 16. In Example 17, the KCuHCF produced in Production Example 4, which had been pre-disintegrated, was replaced with a metal cyano complex, and the dope composition was changed as shown in Table 3, with the dope coating thickness set to 700 μm, except that the conditions were the same as Example 13.

[0083] The physical properties of the films obtained in Examples 13 to 17 are shown in Table 3. In Table 3, the ion-selective layer thickness is the thickness of the ion-selective layer obtained by drying the coated dope.

[0084]

[0085] [Example 18] The Cs selectivity of the film prepared in the same manner as in Example 13 was evaluated. The results are shown in Table 4. In Table 4, the ion-selective layer thickness is the thickness of the ion-selective layer obtained by drying the coated dope.

[0086] [Comparative Example 4] NeoSepta CXP-S (a monovalent selective membrane manufactured by Astrom Co., Ltd.), a commercially available monovalent selective membrane, was used as the membrane for Comparative Example 4, and the Cs selectivity was evaluated in the same manner as in Example 18. The results are shown in Table 4. P in Comparative Example 4 Ｃｓ Ｎａ The ratio was 1.3, and the membrane potential difference was 0.8. In the monovalent selective membrane of Comparative Example 4, Cs could not be separated almost completely. On the other hand, the membrane of Example 18 was P Ｃｓ Ｎａ The result was 17.4, demonstrating very high selectivity for Cs.

[0087]

[0088] [Examples 19-21] K selectivity was evaluated using films prepared in the same manner as in Example 13. For the evaluation, in Examples 19 and 21, a mixed salt solution of 0.01 M KCl and 0.1 M NaCl was used as the mixed salt solution, and in Example 20, a mixed salt solution of 1 M KCl and 1 M NaCl was used as the mixed salt solution. The current densities used for the evaluation are shown in Table 5.

[0089] [Comparative Examples 5-7] NeoSepta CXP-S (a monovalent selective membrane manufactured by Astrom Co., Ltd.), a commercially available monovalent selective membrane said to have excellent selectivity between polyvalent and monovalent ions, was used as the membrane for Comparative Examples 5-7, and its K selectivity was evaluated. In Comparative Example 5, a mixed salt solution of 0.1 M KCl and 0.1 M NaCl was used as the mixed salt solution. In Comparative Example 6, a mixed salt solution of 0.01 M KCl and 0.1 M NaCl was used as the mixed salt solution. In Comparative Example 7, a mixed salt solution of 1 M KCl and 1 M NaCl was used as the mixed salt solution. The results are shown in Table 5. The current density during evaluation was as shown in Table 5.

[0090]

[0091] As shown in the comparison between Examples 13-17 and 19-21 and Comparative Examples 5-7, the multilayer crosslinked cation exchange membrane of the present invention having an ion-selective layer is K ＋ It also exhibited excellent selectivity. Furthermore, the thicker the ion selective layer, the higher the K ＋ Selectivity improved, and the membrane potential difference also increased. Furthermore, as can be seen from the results of Examples 13 and 19-21 and Comparative Examples 5-7, K ＋ Even when the concentration is changed, the multilayer cross-linked cation exchange membrane of the present invention, which has an ion-selective layer, has a higher K than Comparative Examples 5-7. ＋ It demonstrated selectivity. Furthermore, the multilayer crosslinked cation exchange membrane of the present invention maintains high K even when the current density is changed. ＋ Show selectivity, K ＋ Even at low concentrations, by lowering the current density, K ＋ We were able to improve selectivity.

[0092] The multilayer cross-linked cation exchange membrane of the present invention exhibits excellent selective permeability of ammonium ions, superior strength and handling properties, and minimal dimensional changes. Therefore, it can be used for various applications involving the concentration of ammonium ions, such as in the treatment of wastewater.

[0093] 1. A cation exchange membrane (base cation exchange membrane) consisting of a polystyrene-based crosslinked cation exchange resin and a porous substrate. 2. A cation exchange layer (ion-selective layer) containing a metal cyano complex and a binder resin.

Claims

1. A multilayer cross-linked cation exchange membrane having a cation exchange layer containing a metal cyano complex and a binder resin on at least one side of a cation exchange membrane made of a polystyrene-based cross-linked cation exchange resin and a porous substrate.

2. The multilayer cross-linked cation exchange membrane according to claim 1, wherein the polystyrene-based cross-linked cation exchange resin is a copolymer of a styrene-based monomer having cation exchange groups and a divinyl compound-based monomer.

3. The multilayer crosslinked cation exchange membrane according to claim 1 or 2, wherein the porous substrate is a polyolefin-based substrate.

4. The multilayer crosslinked cation exchange membrane according to claim 3, wherein the porous substrate is a polyolefin woven fabric having an aperture ratio of 35 to 50%, a thickness of 90 to 160 μm, and a single filament diameter of 1 to 70 denier.

5. The multilayer crosslinked cation exchange membrane according to claim 1 or 2, wherein the metal cyanide complex is a compound represented by the following general formula [1]. (In formula [1], M represents one or more metal atoms selected from the group consisting of vanadium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, lanthanum, europium, gadolinium, lutetium, barium, strontium, and calcium; M' represents one or more metal atoms selected from the group consisting of vanadium, chromium, molybdenum, tungsten, manganese, iron, ruthenium, cobalt, nickel, platinum, and copper; A represents a monovalent or higher cation; x represents a numerical value of 0 to 3, y represents a numerical value of 0.1 to 1.5, and z represents a numerical value of 0 to 6, respectively.) 6. A multilayer cross-linked cation exchange membrane according to claim 1 or 2, comprising a metal cyano complex and a binder resin, wherein the binder resin comprises a styrene-based elastomer.

7. The multilayer crosslinked cation exchange membrane according to claim 6, wherein the styrene elastomer is an acid-modified styrene elastomer.

8. The multilayer cross-linked cation exchange membrane according to claim 1 or 2, wherein the polystyrene-based cross-linked cation exchange resin contains a styrene-based elastomer.

9. A method for producing a multilayer crosslinked cation exchange membrane according to claim 1 or 2, comprising coating at least one side of a cation exchange membrane made of a polystyrene-based crosslinked cation exchange resin and a porous substrate with an organic solvent solution of a binder resin in which a metal cyano complex is dispersed, and drying, and then drying the membrane.

10. A method for producing a multilayer crosslinked cation exchange membrane according to claim 9, wherein the binder resin used in the organic solvent solution of the binder resin in which a metal cyano complex is dispersed is a binder resin containing a styrene-based elastomer.

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