Method for producing high-purity tetraalkylammonium hydroxide aqueous solution

Abstract

The present invention is a method for producing a tetraalkylammonium hydroxide aqueous solution by electrolyzing a tetraalkylammonium salt aqueous solution, wherein a tetraalkylammonium hydroxide aqueous solution having a reduced chloride ion concentration can be industrially produced efficiently by the content of tetraalkylammonium halide salts to the tetraalkylammonium salt in the tetraalkylammonium salt aqueous solution being 500 mass ppb or less.

Description

Method for producing a high-purity aqueous tetraalkylammonium hydroxide solution

[0001] The present invention relates to a method for producing a highly pure aqueous solution of tetraalkylammonium hydroxide.

[0002] Precision processing technologies for forming and microfabricating fine elements (devices) are becoming increasingly important in a wide range of fields, including the manufacture of semiconductor integrated circuits such as LSIs, display surfaces of FPDs, and circuit boards such as magnetic heads. Among these precision processing technologies, a processing method called photofabrication has become mainstream. Photofabrication involves applying a photosensitive resin composition called a photoresist to the surface of a workpiece to form a coating film (resist layer), exposing the coating film to light, patterning the exposed coating film with a developer (forming a resist pattern), and using the resist as a mask to perform electroforming (specifically, chemical etching, electrolytic etching, electroplating, or a combination thereof) to form various elements, wiring patterns, and the like. This photofabrication technology is used in the manufacture of various precision parts such as semiconductor packages.

[0003] Although alkaline aqueous solutions are used as developers in the photofabrication described above, if such alkaline aqueous solutions contain metal ions, they will adversely affect the electrical properties of the resulting semiconductor packages, etc. For this reason, alkaline aqueous solutions that do not contain metal ions, such as aqueous solutions of tetraalkylammonium hydroxide compounds such as tetramethylammonium hydroxide (TMAH) solutions, are widely used as developers.

[0004] Tetraalkylammonium hydroxide aqueous solution is produced by electrolyzing tetraalkylammonium salts in an electrolytic cell using a cation exchange membrane as a diaphragm. The tetraalkylammonium salts used in this production include chlorides, carboxylates, carbonates, and bicarbonates. When chlorides are used, they can be contaminated into the target tetraalkylammonium hydroxide aqueous solution on the cathode side. Because chloride ions can corrode wiring when used in semiconductor substrate processing, it is important to reduce chloride ions to concentrations on the ppb mass level during production. Possible causes of chloride contamination include trace amounts of chloride ions from the raw materials passing through the cation exchange membrane, or trace components contained as impurities in the raw materials reacting or decomposing in the electrolytic cell and contributing to chloride ions.

[0005] As a method for reducing chloride ions, a method has been proposed in which components called latent halides, which are precursors of chloride ions present in an aqueous solution of tetraalkylammonium hydroxide, are heated at high temperatures and then electrolyzed to improve purity (see Patent Document 1).

[0006] Patent No. 2690730

[0007] The method described in Patent Document 1 above can produce an aqueous tetraalkylammonium hydroxide solution in which the chloride ion content has been reduced to the level of several ppm by mass. However, no evaluation has been performed at the ppb level by mass, and it is unclear whether the method described in Patent Document 1 can produce an aqueous tetraalkylammonium hydroxide solution in which the chloride ion content has been reduced to the ppb level by mass. Furthermore, the inventors' studies have revealed that high-temperature treatment may not sufficiently reduce the chloride ion concentration.

[0008] Furthermore, the method described in Patent Document 1 involves heating an aqueous tetraalkylammonium hydroxide solution produced by electrolysis at a high temperature to decompose latent halides, and then electrolyzing the solution again to achieve high purity, which requires two electrolysis steps, and therefore there has been a demand for an industrially efficient method for producing an aqueous tetraalkylammonium hydroxide solution. That is, an object of the present invention is to provide an industrially efficient method for producing an aqueous tetraalkylammonium hydroxide solution having a reduced chloride ion concentration.

[0009] The present inventors conducted extensive research to solve the above-mentioned problems. They discovered that when a tetraalkylammonium salt contains, as an impurity, a compound in which at least one hydrogen atom of the alkyl group of the tetraalkylammonium salt is substituted with a chlorine atom (hereinafter referred to as a "halogenated tetraalkylammonium salt"), the impurity is stable even at a heat treatment of 90°C and cannot be sufficiently removed by heating. Furthermore, when an aqueous tetraalkylammonium hydroxide solution is produced by subsequent electrolysis, the chloride ion content cannot be reduced to the desired level. As a result of further research, they discovered that an aqueous tetraalkylammonium hydroxide solution with a reduced chloride ion concentration can be industrially and efficiently produced by electrolyzing a tetraalkylammonium salt containing a halide tetraalkylammonium salt within a predetermined range, thereby completing the present invention.

[0010] That is, the first invention is a method for producing an aqueous tetraalkylammonium hydroxide solution by electrolyzing an aqueous tetraalkylammonium salt solution, characterized in that the content of tetraalkylammonium halide salt relative to the tetraalkylammonium salt in the aqueous tetraalkylammonium salt solution is 500 ppb by mass or less. In the first invention, the content of tetraalkylammonium halide salt relative to the tetraalkylammonium salt in the aqueous tetraalkylammonium salt solution is preferably 100 to 500 ppb by mass, and the tetraalkylammonium salt is preferably tetraalkylammonium chloride. Furthermore, it is preferable to contact the obtained aqueous solution containing tetraalkylammonium hydroxide with a cation exchange resin or a chelating resin.

[0011] The second aspect of the present invention is an aqueous tetraalkylammonium hydroxide solution having a tetraalkylammonium halide ion concentration of 100 to 400 ppb by mass relative to the tetraalkylammonium ion. In the second aspect of the present invention, the chloride ion concentration relative to the tetraalkylammonium ion is preferably 20 to 60 ppb by mass. Furthermore, the content of each of the metal impurities Li, Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, Ag, Cd, Sn, Ba, and Pb is preferably 5 ppt by mass or less.

[0012] According to the manufacturing method of the present invention, the chloride ion concentration of a tetraalkylammonium hydroxide aqueous solution produced by using a tetraalkylammonium salt containing a tetraalkylammonium halide salt within a predetermined range as a raw material can be reduced. Specifically, it is possible to reduce the chloride ion concentration to the order of ppb by mass relative to the tetraalkylammonium ion. While the details of why the method of the present invention can reduce the chloride ion concentration in tetraalkylammonium hydroxide are unclear, the inventors speculate as follows. Specifically, they speculate that tetraalkylammonium halide salts contained as impurities in the tetraalkylammonium salt undergo electrolytic reduction near the cathode during the electrolysis process, resulting in a decomposition reaction that generates chloride ions. This is thought to result in an increase in the chloride ion concentration in the tetraalkylammonium hydroxide aqueous solution. Furthermore, because tetraalkylammonium halide salts are stable even under high-temperature conditions, it is speculated that they are difficult to fully remove from the tetraalkylammonium salt by high-temperature treatment. Therefore, it is speculated that the chloride ion concentration in a tetraalkylammonium hydroxide aqueous solution can be significantly reduced by using a tetraalkylammonium salt containing a tetraalkylammonium halide salt within a predetermined range as a raw material.

[0013] FIG. 1 is a simplified diagram showing the configuration of an electrolytic cell used in an example of the present invention.

[0014] The production method of the present invention is characterized by using as a raw material a tetraalkylammonium salt having a tetraalkylammonium halide salt content within a predetermined range. The method of the present invention will be described in detail below. Unless otherwise specified, in this specification, the notation "A to B" for numerical values ​​A and B means "A or more and B or less." In such notation, when a unit is assigned only to numerical value B, the unit also applies to numerical value A.

[0015] (Tetraalkylammonium hydroxide aqueous solution) Examples of tetraalkylammonium hydroxide (hereinafter also referred to as TAAH) include tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and trimethylethylammonium hydroxide. Among the above TAAHs, tetramethylammonium hydroxide is preferably used because it is widely used as a developer in semiconductor manufacturing processes.

[0016] The halide ion concentration in the aqueous tetraalkylammonium hydroxide solution can be analyzed by ion chromatography.

[0017] (Tetraalkylammonium Salt Aqueous Solution) The TAAH aqueous solution containing the above-mentioned TAAH can be produced by electrolyzing an aqueous solution of a tetraalkylammonium salt having a desired tetraalkylammonium group, as described below. Specific examples of the tetraalkylammonium salt include halide salts, carbonate salts, carboxylate salts, bicarbonates, etc. Among these, halide salts and carbonate salts are preferred, and halide salts are particularly preferred, since they allow for easy production of a TAAH aqueous solution with a low content of metal impurities. The halide may be appropriately selected from chlorides, bromides, and iodides.

[0018] (Halogenated Tetraalkylammonium Salt) In the production method of the present invention, the content of the tetraalkylammonium halide salt relative to the tetraalkylammonium salt in the aqueous tetraalkylammonium salt solution must be 500 ppb by mass or less. The halogenated tetraalkylammonium salt in the production method of the present invention refers to a compound in which at least one hydrogen atom of four alkyl groups bonded to a nitrogen atom is halogenated. The halogen in the halogenated alkyl group is chlorine, bromine, or iodine. While the number of halogenated alkyl groups is not critical, considering that the general production method for tetraalkylammonium salts is the quaternization reaction of trialkylamines with alkyl halides and ease of purification, it is preferable that the number of halogenated alkyl groups is one, i.e., a monohalogenated alkyl (trialkyl) ammonium salt. The number of carbon atoms in the halogenated alkyl group is also not particularly critical, but examples include halogenated alkyl groups having 1 to 10 carbon atoms, such as halogenated methyl groups, halogenated ethyl groups, and halogenated propyl groups.

[0019] The content of the tetraalkylammonium halide salt relative to the tetraalkylammonium salt in the tetraalkylammonium salt aqueous solution may be appropriately determined depending on the TAAH aqueous solution obtained after electrolysis, so long as it is 500 ppb by mass or less. However, from the viewpoint of obtaining a TAAH aqueous solution with a low chloride ion content, it is preferably 100 to 500 ppb by mass.

[0020] The tetraalkylammonium halide salt in the aqueous tetraalkylammonium salt solution can be analyzed by liquid chromatography / mass spectrometry (LC / MS) or ion chromatography.

[0021] Tetraalkylammonium halide salts are relatively stable to heat and do not undergo thermal decomposition even when heated at 90° C., making removal by heating difficult. Methods for removing tetraalkylammonium halide salts from tetraalkylammonium salts include treatment using column chromatography filled with ion exchange resins and treatment of electrolyzing the tetraalkylammonium halide by passing an electric current through an electrolysis tank.

[0022] It is believed that the reason why tetraalkylammonium halide salts exist in tetraalkylammonium salts is that the alkyl halides used as the raw material for tetraalkylammonium salts contain impurities with two or more halogen atoms, and that these impurities react with trialkylamines to quaternize, thereby synthesizing tetraalkylammonium halide salts. For example, methyl chloride, the raw material for tetramethylammonium salts, contains dichloromethane as an impurity at levels of several hundred ppm by volume. If the dichloromethane content in methyl chloride can be reduced to almost zero, then the (chloromethyl)trimethylammonium content in tetramethylammonium salts will also be reduced to almost zero, and it is predicted that the chloride ion concentration relative to the tetraalkylammonium ions can be reduced to 20 ppb by mass or less.

[0023] (Electrolysis Method) In the production method of the present invention, the above-mentioned aqueous tetraalkylammonium salt solution is electrolyzed to obtain an aqueous TAAH solution. Any known method can be used as the electrolysis method, without any particular limitations. A common method involves supplying an aqueous tetraalkylammonium salt solution to an electrolytic cell having an intermediate chamber formed by arranging only a cation exchange membrane or a combination of a cation exchange membrane and an anion exchange membrane between a cathode and an anode, supplying an acid to an anode chamber formed in one of the compartments, and obtaining an aqueous tetraalkylammonium hydroxide solution from the other cathode chamber.

[0024] The reaction is carried out by supplying an aqueous acid solution such as hydrochloric acid to the anode chamber, an aqueous tetraalkylammonium salt solution to the intermediate chamber formed by the cation exchange membrane and the anion exchange membrane, and highly pure water, particularly ultrapure water or a highly pure aqueous tetraalkylammonium hydroxide solution to the cathode chamber.

[0025] The cation exchange membrane and anion exchange membrane used in the electrolysis can be any known membrane conventionally used in the production of aqueous tetraalkylammonium hydroxide solutions by electrolysis, without any particular limitations. For example, cation exchange membranes include membranes having sulfonic acid groups, carboxylic acid groups, phosphate groups, etc., while anion exchange membranes include membranes having one or more strongly basic ion exchange groups such as quaternary ammonium bases, sulfonium bases, and phosphonium bases, or primary, secondary, or tertiary amines. The substrate of the ion exchange membrane can be, for example, a hydrocarbon-based, fluorocarbon-based, or perfluorocarbon-based resin. In particular, the ion exchange membrane constituting the cathode chamber is preferably one based on a perfluorocarbon resin, which is stable and durable under a basic atmosphere. Furthermore, since oxidizing gases such as halogen gas and oxygen gas are generated in the anode chamber, it is preferable to use an ion exchange membrane based on an oxidation-resistant perfluorocarbon resin.

[0026] The anode is preferably an insoluble electrode, such as carbon, platinum-coated titanium, or titanium coated with ruthenium, iridium, or the like. The cathode is preferably one that is stable in a strongly basic atmosphere and has a low overvoltage. Suitable materials include SUS316, platinum, and Raney nickel.

[0027] In the above electrodialysis, the current density is 1 to 50 A / dm ２ is suitable, and the temperature is preferably controlled not to exceed 90°C, preferably between 30 and 50°C.

[0028] The production method of the present invention described above can provide an aqueous tetraalkylammonium hydroxide solution having a low chloride ion concentration. Specifically, the aqueous tetraalkylammonium hydroxide solution can provide a tetraalkylammonium halide ion concentration relative to the tetraalkylammonium ion of 100 to 400 ppb by mass, preferably 100 to 300 ppb by mass, and particularly preferably 100 to 200 ppb by mass.

[0029] (Removal of Metal Impurities) When metal impurities in the aqueous tetraalkylammonium hydroxide solution obtained by the production method of the present invention pose a problem, the metal impurities can be removed from the aqueous tetraalkylammonium hydroxide solution to obtain an aqueous tetraalkylammonium hydroxide solution with a reduced content of metal impurities. Specific methods for removing metal impurities include contacting the aqueous tetraalkylammonium hydroxide solution with a cation exchange resin or a chelating resin to adsorb the metal impurities onto the cation exchange resin or the chelating resin.

[0030] In the present invention, the metal impurities specifically refer to elements consisting of Li, Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, Ag, Cd, Sn, Ba, and Pb, and the content thereof is evaluated as the total content of these elements. Note that in the present invention, the content of the metal impurities in the aqueous tetraalkylammonium hydroxide solution refers to the value analyzed by inductively coupled plasma mass spectrometry (ICP-MS).

[0031] Since the tetraalkylammonium hydroxide aqueous solution exhibits a neutral to basic pH, the cation exchange resin can be suitably used whether it is a strongly acidic cation exchange resin in which the cation exchange group is a sulfonic acid group, or a weakly acidic cation exchange resin in which the cation exchange group is a carboxyl group, a phenolic hydroxyl group, or the like.

[0032] However, since the aqueous solution of tetraalkylammonium hydroxide may be acidic due to the inclusion of trace amounts of acid during the production process, it is preferable to use a strongly acidic cation exchange resin that can exchange ions even in the strongly acidic range. Alternatively, the acid may be neutralized with a hydroxide having the same quaternary ammonium ion as the aqueous solution of tetraalkylammonium hydroxide before contacting it with a weakly acidic cation exchange resin.

[0033] The strongly acidic cation exchange resin generally has a structure in which a sulfonic acid group is introduced into a crosslinkable resin matrix composed of a copolymer of a monovinyl monomer, which is a precursor for introducing a cation exchange group, and a crosslinkable monomer. Examples of the monovinyl monomer, which is a precursor for introducing a cation exchange group, include alkyl-substituted styrenes such as styrene, methylstyrene, and ethylstyrene, and halogen-substituted styrenes such as bromostyrene. These may be used alone or in combination of two or more. Among these, styrene or a monomer mainly composed of styrene is particularly preferred as the monovinyl monomer.

[0034] Examples of the crosslinkable monomer include crosslinkable monomers having multiple vinyl groups, such as divinylbenzene, trivinylbenzene, divinyltoluene, divinylnaphthalene, divinylxylene, divinylbiphenyl, bis(vinylphenyl)methane, bis(vinylphenyl)ethane, bis(vinylphenyl)propane, and bis(vinylphenyl)butane, and these may be used alone or in combination of two or more. Of these, divinylbenzene is particularly preferred as the crosslinkable monomer.

[0035] On the other hand, examples of weakly acidic cation exchange resins include copolymers of a monovinyl monomer having the weakly acidic cation exchange group, such as acrylic acid or methacrylic acid, with a crosslinkable monomer. Particularly preferred are acrylic acid-divinylbenzene copolymers and methacrylic acid-divinylbenzene copolymers.

[0036] Such cation exchange resins may be in any form, such as a gel type or a macroporous type, but the larger the contact area between the ion exchange resin and the liquid, the easier it is for metal ions to diffuse into the ion exchange resin, so the macroporous type is more preferably used in the present invention.

[0037] Additionally, the cation exchange resin used in the present invention preferably has a cation exchange capacity in the range of 1.0 to 5.0 equivalents / L, more preferably 1.5 to 3.0 equivalents / L. The shape of the cation exchange resin is not particularly limited and may be a membrane or the like, but is usually preferably in the form of particles.

[0038] The chelating resins mentioned above refer to resins that have been introduced with functional groups that form complexes with metal ions, and because they have the ability to selectively adsorb specific metal ions, they are used in purifying brine, purifying plating solutions, and removing toxic metals. They are also suitable for recovering precious metals such as platinum and palladium.

[0039] The functional groups of chelating resins generally have structures such as N-O, S-N, N-N, O-O, and P-N, which contain two or more electron-donating elements such as N, S, O, and P.

[0040] Examples of functional groups of such chelating resins include iminodiacetic acid, polyamine, primary amine, methylglucamine, amidoxime, isothiouronium, thiol, sulfonic acid, phosphonic acid, aminophosphate, bispicolylamine, semithiocarbalinic acid, etc. Among these, iminodiacetic acid and aminophosphate are more preferred because they are typical functional groups of chelating resins and exhibit a wide range of selectivity for metals.

[0041] The shape of such a chelating resin may be any shape, such as a gel type or a macroporous type, but the larger the contact area between the chelating resin and the liquid, the easier it is for metal ions to diffuse into the chelating resin, so the macroporous type is more preferably used in the present invention.

[0042] Here, the counter ions of the cation exchange resin or chelating resin (hereinafter collectively referred to as "cation exchange resin, etc.") are either metal ion type or non-metal ion type, and either can be used. Specific examples of non-metal ion type counter ions include hydrogen ions and ammonium ions (NH ４ ＋ ) or primary to quaternary ammonium ions, etc. are used.

[0043] In the present invention, the counter ions of the cation exchange resins and the like are preferably non-metal ions, and more preferably hydrogen ions. ４ ＋) or primary to quaternary ammonium ion types, unless the cation is the same quaternary ammonium ion as in the aqueous tetraalkylammonium hydroxide solution, the elution of the other cation may cause contamination of the resulting aqueous tetraalkylammonium hydroxide solution with other ammonium compounds, making separation difficult.

[0044] On the other hand, in the case of the hydrogen ion type, hydrogen ions are released by exchange with metal ions, but if the tetraalkylammonium hydroxide aqueous solution is a hydroxide, it will be neutralized to water and will not cause contamination. Also, even if the tetraalkylammonium hydroxide aqueous solution is something other than a hydroxide, if the acid derived from the released hydrogen ions is neutralized by adding a hydroxide having the same quaternary ammonium ion as the tetraalkylammonium hydroxide aqueous solution, the acid will become a tetraalkylammonium hydroxide aqueous solution and water, and contamination will be easily removed. The hydrogen ion type is also preferred even if it is only a part of the counter ions, and in this case, it is preferable that at least 1 mol % is in the hydrogen ion type, more preferably at least 3 mol %, particularly preferably at least 10 mol %, and most preferably at least 20 mol %.

[0045] In the present invention, the counter ions of the cation exchange resin or the like are brought into contact with an acid to elute metal ions and non-metal ions excluding hydrogen ions, and each component can be quantified by ion chromatography. Hydrogen ions can also be quantified by bringing the resin into contact with a salt to elute the hydrogen ions, followed by neutralization titration.

[0046] In the case where only metal ion counterions are available, or where a certain amount of metal ions are contained and the metal ion concentration needs to be reduced, the cation exchange resin or the like having a non-metal ion counterion can be obtained by contacting the resin with an aqueous solution containing cations other than metal ions to remove the metal ions and obtain a non-metal ion counterion. However, as mentioned above, in the case of a highly crosslinked cation exchange resin or the like, large cations such as tetraalkylammonium ions do not diffuse easily into the resin, so it is preferable to contact the resin with an aqueous solution containing small cations that can easily penetrate the resin and quickly exchange with metal ions. A preferred form is an aqueous solution in which the cations are hydrogen ions, i.e., contact with an acid is preferred.

[0047] The acid aqueous solution used for the contact is an inorganic acid aqueous solution such as sulfuric acid, hydrochloric acid, or nitric acid, and from the viewpoint of preventing deterioration of the resin material, it is preferable to use an aqueous hydrochloric acid solution or an aqueous sulfuric acid solution. Furthermore, the acid concentration can be used without any restrictions, but contact with a high-concentration acid may result in counter ions being replaced in a short period of time, causing a rapid change in volume, which may lead to cracking or destruction. Therefore, the acid concentration is preferably 1 mol / L or less, more preferably 0.5 mol / L or less, and even more preferably 0.3 mol / L or less. Treatment with an acid equivalent to 1.2 to 5 times the total amount of exchange groups on the cation exchange resin or the like used is preferred, and treatment with an acid equivalent to 1.5 to 3 times the total amount is more preferred.

[0048] The contact treatment between the aqueous acid solution and the cation exchange resin or the like may be performed by either a batch method or a flow-through method. In the case of a flow-through method using a resin tower, either an ascending flow or a descending flow may be used, and it is preferable to pass an acid containing an amount equivalent to at least three times the amount of exchange groups on the cation exchange resin or the like to be treated. The flow rate is not particularly specified, but from the viewpoint of treatment time and efficiency, it is preferable to perform the treatment in a range of SV = 1 to 20 (1 / hr). In addition, in the case of a batch method, it is preferable to exchange the liquid three or more times with an acid equivalent to 3 to 15 times the total amount of exchange groups on the cation exchange resin or the like to be treated.

[0049] Furthermore, if the content of metal ions contained in the acid aqueous solution is high, the metal ions will remain in the cation exchange resin or the like depending on the concentration, which may reduce the exchange amount of the metal ions during purification or, in some cases, cause contamination by being released. For this reason, it is preferable that the total content of metal impurities consisting of Li, Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, Ag, Cd, Sn, Ba, and Pb contained in the acid aqueous solution is 100 ppt or less, more preferably 10 ppt or less, and even more preferably 1 ppt or less.

[0050] After contact with the acid aqueous solution, the cation exchange resin or the like is preferably washed with ultrapure water. This removes excess acid that has adhered to the surface of the resin and not incorporated as a counter ion, thereby preventing the acid from being mixed into subsequent processes. Because the tetraalkylammonium hydroxide aqueous solution is a hydroxide, if acid remains in the cation exchange resin or the like, a neutralization reaction will generate sudden heat, which may cause the resin to swell rapidly and destroy the resin. Therefore, washing with ultrapure water is preferred.

[0051] In addition, when a cation exchange resin or the like having a hydrogen ion counter ion is contacted with an aqueous tetraalkylammonium hydroxide solution, the concentration, contact method, and metal ion content are preferably the same as those described for the aqueous acid solution. Furthermore, it is also preferable to wash the cation exchange resin or the like with ultrapure water after contact with the aqueous tetraalkylammonium hydroxide solution.

[0052] Next, in the present invention, the method of contacting the cation exchange resin or the like with the aqueous tetraalkylammonium hydroxide solution may be any type, such as a batch type or a flow-through type, but industrially, the flow-through type is preferred. In the flow-through type, purification is carried out by passing the liquid through a resin tower. The liquid may be passed in either an ascending or descending direction, with a descending direction being preferred from the viewpoint of increasing purification efficiency. The liquid passing conditions are preferably a space velocity SV of 1 to 20 (1 / hr), more preferably 5 to 20 (1 / hr).

[0053] The treatment amount of the tetraalkylammonium hydroxide aqueous solution to be contacted is generally 100 (L / L-resin) or more for efficiency. As described above, the excellent metal ion removal effect in the method of the present invention is significantly exhibited when a large amount of crude tetraalkylammonium hydroxide aqueous solution is treated. Therefore, the treatment amount is more effective when the ratio of cation exchange resin or the like to the crude tetraalkylammonium hydroxide aqueous solution is 2000 (L / L-resin) or more, more preferably 10,000 (L / L-resin) or more. However, if the treatment amount is still too large, the metal ion removal effect decreases, so the treatment amount is generally 10 ８ (L / L-resin) or less, and particularly 50,000 (L / L-resin) or less is preferred.

[0054] For cation exchange resins or the like whose metal ion removal effect has been reduced in this way, it is efficient to subject the resin to the acid treatment as described above as a resin regeneration treatment, and then resume the flow of liquid and repeat the purification. In addition, the liquid may be passed through two or more resin towers in succession.

[0055] On the other hand, in the batch purification, it is preferable to immerse the resin in an aqueous tetraalkylammonium hydroxide solution at a ratio of 200 (L / L-resin) or more in order to significantly demonstrate the excellent effect of removing the metal ions. Furthermore, it is preferable to repeat the batch treatment, since this can further reduce the metal ions. The number of repetitions is preferably 2 to 5.

[0056] In the method of the present invention, if the aqueous tetraalkylammonium hydroxide solution contains particulate metal impurities, it is preferable to remove them by filtering before or after contact with a cation exchange resin or the like. If the crude aqueous tetraalkylammonium hydroxide solution contains particulate metal impurities or resin particles, they may interfere with the metal ion removal action by a cation exchange resin or the like in the present invention, or may cause clogging in the case of a flow-through system. Therefore, filtering is preferable as a pre-treatment. Furthermore, filtering is also preferably performed as a post-treatment because there is a risk of particulate metal impurities remaining after contact with a cation exchange resin or, in some cases, broken pieces of the cation exchange resin or the like being mixed in.

[0057] The average pore size of the filter used is preferably 1 μm or less, and more preferably 0.02 to 1 μm. If the average pore size of the filter is unknown, the value measured by the bubble point method is used as the average pore size.

[0058] The inner walls of the equipment, such as containers, tanks, resin towers, and piping, used to carry out the production method of the present invention are preferably made of a material that is chemical-resistant and minimizes metal contamination. Therefore, they are preferably made of or lined with polyethylene, polypropylene, or fluororesin materials. Fluororesin is particularly suitable from the viewpoint of preventing metal contamination. It is preferable to clean the surfaces of these components before use, for example, by cleaning with an aqueous tetraalkylammonium hydroxide solution or acid.

[0059] (Tetraalkylammonium hydroxide aqueous solution) The above method effectively produces a high-purity tetraalkylammonium hydroxide aqueous solution with reduced metal ions. Specifically, it is possible to obtain a tetraalkylammonium hydroxide aqueous solution in which the total content of metal impurities consisting of Li, Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, Ag, Cd, Sn, Ba, and Pb is 100 ppt or less, more preferably 50 ppt or less, and even more preferably 20 ppt or less. The tetraalkylammonium hydroxide aqueous solution can also have the content of each of these metal elements reduced to 30 ppt or less, more preferably 10 ppt or less, and even more preferably 5 ppt or less.

[0060] Furthermore, since the tetraalkylammonium hydroxide aqueous solution is strongly alkaline, some metal elements other than the above-mentioned metal elements may take an anionic or non-ionic form, and the removal effect may be insufficient.

[0061] The concentration of tetraalkylammonium hydroxide in the aqueous tetraalkylammonium hydroxide solution may be appropriately set depending on the desired purpose, specifically within the range of 0.5 to 40% by mass.

[0062] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following examples 1 to 3 and comparative examples 1 to 3, a three-compartment structure in which an anion exchange membrane and a cation exchange membrane are arranged as shown in FIG. 1 and having an effective area of ​​1 dm ２ An electrolysis apparatus was prepared as follows: The anode was a platinum-plated titanium plate, and the cathode was SUS316.

[0063] In the anode chamber of the electrolysis device, 0.5N sulfuric acid was added, and a 2.5N aqueous solution of tetramethylammonium chloride (test solution) was added between the anion exchange membrane and the cation exchange membrane on the cathode side. A quaternary ammonium hydroxide aqueous solution was circulated in the cathode chamber, and a current density of 30 A / dm ２Electrolysis was carried out continuously while maintaining the temperature at 40°C. During continuous operation, the concentration of tetramethylammonium hydroxide in the cathode chamber was kept at 2.0N. Similarly, to maintain a constant concentration of the solution circulating in each chamber, pure water was added when the concentration became high, and the corresponding component was added when the concentration became low.

[0064] After continuous operation for one month, the (chloromethyl)trimethylammonium ion concentration and chloride ion concentration in the cathode chamber were confirmed by the following methods.

[0065] The (chloromethyl)trimethylammonium ion was measured under the following LC / MS conditions, and the concentration was calculated using the peak area.

[0066] (LC / MS conditions) Apparatus system: Shimadzu Corporation liquid chromatograph mass spectrometer, model LCMS-2020 Column: Atlantis HILIC silica (manufactured by Nippon Waters) - particle diameter 3 μm, inner diameter 2.1 mm x length 150 mm Column temperature: 40°C Mobile phase: acetonitrile / 50 mM ammonium formate aqueous solution containing 0.02% formic acid = 30 / 70 (V / V) Flow rate: 0.2 mL / min Ionization method: ESI positive Interface voltage: +4.5 kV Measurement mode: SIM mode m / z 118 Nebulizer gas flow rate: 1.5 L / min DL temperature: 250°C Heat block temperature: 400°C Drying gas flow rate: 15 L / min Chloride ions were measured under the following ion chromatography conditions, and the concentration was calculated using the peak area.

[0067] (Ion Chromatography Conditions) Device system: Thermo Fisher Scientific, model Integration Separation column: AS-18 IC column (Thermo Fisher Scientific) - particle size 7.5 μm, inner diameter 2 mm × length 250 mm Column temperature: 35°C Mobile phase: 20 mM potassium hydroxide aqueous solution Detection method: electrical conductivity Sample loop: 100 μL Sample conditions: 25% TMAH aqueous solution + ultrapure water (diluted 1:1)

[0068] Example 1 A tetramethylammonium chloride aqueous solution containing 221 ppb of (chloromethyl)trimethylammonium chloride by mass relative to tetramethylammonium chloride was used as a raw material and subjected to electrolysis to produce a tetramethylammonium hydroxide aqueous solution. The tetramethylammonium hydroxide aqueous solution produced had a (chloromethyl)trimethylammonium ion concentration relative to tetramethylammonium ions of 246 ppb by mass and a chloride ion concentration relative to tetraalkylammonium ions of 25 ppb by mass.

[0069] Example 2 A tetramethylammonium chloride aqueous solution containing 442 ppb by mass of (chloromethyl)trimethylammonium chloride relative to tetramethylammonium chloride was used as a raw material and subjected to electrolysis to produce a tetramethylammonium hydroxide aqueous solution. The tetramethylammonium hydroxide aqueous solution produced had a (chloromethyl)trimethylammonium ion concentration relative to tetramethylammonium ions of 295 ppb by mass and a chloride ion concentration relative to tetraalkylammonium ions of 59 ppb by mass.

[0070] Example 3 An aqueous solution of tetramethylammonium chloride containing 660 ppb of (chloromethyl)trimethylammonium chloride by mass relative to tetramethylammonium chloride was purified by electrolysis. A single-chamber electrolysis apparatus (effective area 1 dm) was used, in which the anion exchange membrane and the cation exchange membrane were removed from the electrolysis apparatus shown in FIG. ２ The same electrodes were also used. A tetramethylammonium chloride aqueous solution was added to the electrolysis device, and a current density of 30 A / dm ２A current was passed continuously for 10 hours while maintaining the temperature at 40°C. As a result, the (chloromethyl)trimethylammonium chloride content relative to tetramethylammonium chloride was reduced to 332 ppb by mass. This tetramethylammonium chloride aqueous solution was used as a raw material and subjected to electrolysis to produce a tetramethylammonium hydroxide aqueous solution. The produced tetramethylammonium hydroxide aqueous solution had a (chloromethyl)trimethylammonium ion concentration relative to tetramethylammonium ions of 345 ppb by mass, and a chloride ion concentration relative to tetraalkylammonium ions of 39 ppb by mass.

[0071] Comparative Example 1: A tetramethylammonium chloride aqueous solution containing 663 ppb of (chloromethyl)trimethylammonium chloride by mass relative to tetramethylammonium chloride was used as a raw material and subjected to electrolysis to produce a tetramethylammonium hydroxide aqueous solution. The tetramethylammonium hydroxide aqueous solution produced had a (chloromethyl)trimethylammonium ion concentration relative to tetramethylammonium ions of 443 ppb by mass and a chloride ion concentration relative to tetraalkylammonium ions of 98 ppb by mass.

[0072] Comparative Example 2: A tetramethylammonium chloride aqueous solution containing 1,769 ppb of (chloromethyl)trimethylammonium chloride by mass relative to tetramethylammonium chloride was used as a raw material and subjected to electrolysis to produce a tetramethylammonium hydroxide aqueous solution. The tetramethylammonium hydroxide aqueous solution produced had a (chloromethyl)trimethylammonium ion concentration relative to tetramethylammonium ions of 1,477 ppb by mass and a chloride ion concentration relative to tetraalkylammonium ions of 246 ppb by mass.

[0073] Comparative Example 3: A tetramethylammonium chloride aqueous solution having a (chloromethyl)trimethylammonium chloride content of 660 ppb by mass relative to tetramethylammonium chloride was subjected to heat treatment at 90°C for 5 hours. As a result, the (chloromethyl)trimethylammonium chloride content relative to tetramethylammonium chloride was 652 ppb by mass, showing almost no change. This tetramethylammonium chloride aqueous solution was used as a raw material and subjected to electrolysis to produce a tetramethylammonium hydroxide aqueous solution. The produced tetramethylammonium hydroxide aqueous solution had a (chloromethyl)trimethylammonium ion concentration relative to tetramethylammonium ions of 492 ppb by mass, and a chloride ion concentration relative to tetraalkylammonium ions of 103 ppb by mass.

[0074] The results of Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1.

[0075]

[0076] Example 4 The aqueous tetramethylammonium hydroxide solution having a chloride ion concentration of 25 ppb by mass obtained in Example 1 was used as a crude aqueous tetramethylammonium hydroxide solution. The contents of metal impurities in this crude aqueous tetramethylammonium hydroxide solution were measured, and the contents of each metal impurity consisting of Li, Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, Ag, Cd, Sn, Ba, and Pb are shown in Table 2.

[0077] (Method for measuring the content of metal impurities in aqueous tetraalkylammonium hydroxide solution) The concentrations of Li, Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, Ag, Cd, Sn, Ba, and Pb were measured using an inductively coupled plasma mass meter (ICP-MS). The lower limit of measurement was 1 ppt, and concentrations less than 1 ppt were considered to be <1 ppt.

[0078] Next, 100 ml of macroporous styrene-based iminodiacetic acid functional group chelating resin D5341 (cation exchange capacity 2.5 equivalents / L, manufactured by Purolite Co., Ltd.) was packed into a 22 mm diameter x 750 mm column. The following pretreatments were passed through this packed column in this order: 1) ultrapure water washing, 2) 0.3 mol / L hydrochloric acid treatment, 3) ultrapure water treatment, 4) 0.45 mol / L tetramethylammonium hydroxide aqueous solution treatment, and 5) ultrapure water treatment. The volume of each solution passed was 3 L, and the space velocity was SV = 5 (1 / hr).

[0079] The 0.3 mol / L hydrochloric acid and 0.45 mol / L tetramethylammonium hydroxide aqueous solutions each had a total content of metal impurities consisting of Li, Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, Ag, Cd, Sn, Ba, and Pb of 100 ppt or less, and the ultrapure water used had a total content of metal impurities consisting of Li, Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, Ag, Cd, Sn, Ba, and Pb of 1 ppt or less.

[0080] A crude aqueous solution of tetramethylammonium hydroxide was passed through the column at a flow rate of 2,000 L and an SV of 20 (1 / hr). After the flow rates reached 200 L (2,000 L / L resin), 500 L (5,000 L / L resin), 1,000 L (10,000 L / L resin), and 2,000 L (20,000 L / L resin), 1 L of each solution was sampled and the content of metal impurities was measured. The results are shown in Table 2.

[0081]

[0082] In Example 4, in which an aqueous tetramethylammonium hydroxide solution containing 500 ppt by mass or less of the metal impurities Li, Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, Ag, Cd, Sn, Ba, and Pb was treated, an excellent effect of reducing the metal impurity concentration was exhibited at the beginning of treatment when the flow rate was 200 L (2,000 L / L-resin) due to contact with the chelating resin whose counter ions were non-metal ions, and this effect was maintained up to at least 2,000 L (20,000 L / L-resin).

[0083] 1: Anode 2: Cathode 3: Power source 4: Anode chamber 5: Raw material chamber 6: Cathode chamber 9: Proton permeation suppression layer 10: Base layer A: Anion exchange membrane C1: Cation exchange membrane

Claims

1. A method for producing an aqueous solution of tetraalkylammonium hydroxide by electrolyzing an aqueous solution of a tetraalkylammonium salt, wherein the content of tetraalkylammonium halide salt relative to the tetraalkylammonium salt in the aqueous solution of tetraalkylammonium salt is 500 ppb by mass or less.

2. The method for producing an aqueous tetraalkylammonium hydroxide solution according to claim 1, wherein the content of the tetraalkylammonium halide salt relative to the tetraalkylammonium salt in the aqueous tetraalkylammonium salt solution is 100 to 500 ppb by mass.

3. The method for producing an aqueous tetraalkylammonium hydroxide solution according to claim 1 or 2, wherein the tetraalkylammonium salt is a tetraalkylammonium chloride.

4. A method for producing an aqueous solution of tetraalkylammonium hydroxide, which comprises contacting an aqueous solution containing tetraalkylammonium hydroxide obtained by the method of claim 1 with a cation exchange resin or a chelating resin.

5. A tetraalkylammonium hydroxide aqueous solution having a tetraalkylammonium halide ion concentration of 100 to 400 ppb by mass relative to the tetraalkylammonium ion.

6. The aqueous tetraalkylammonium hydroxide solution according to claim 5, wherein the chloride ion concentration relative to the tetraalkylammonium ion is 20 to 60 ppb by mass.

7. The aqueous tetraalkylammonium hydroxide solution according to claim 5 or 6, wherein the content of each of the metal impurities consisting of Li, Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, Ag, Cd, Sn, Ba, and Pb is 5 ppt by mass or less.

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