Method for producing anthraquinones

Abstract

To provide a method for efficiently producing anthraquinones having dissimilar substituents.SOLUTION: In a method for producing anthraquinones represented by the chemical formula below, at least one of R1-R8 is a hydroxyl group and at least one of them is an alkoxy group. The production method includes: a step for preparing a starting material represented by the formula below, wherein at least two of R1'-R8' are hydroxyl groups; and a step for reacting the starting material with an organic alkylating agent.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing anthraquinones. [Background technology]

[0002] Redox flow batteries are suitable for storing large amounts of power because the amount of power stored can be freely designed according to the capacity of the electrolyte tank, and are expected to be used to level out power supply and demand, including natural energy. Redox flow batteries consist of cells that charge and discharge, and an electrolyte tank that stores power, and are characterized by charging and discharging by circulating the electrolyte with a pump.

[0003] Currently, redox flow batteries that use vanadium as the active material in the electrolyte are mainstream. However, due to the recent rise in vanadium prices, development of redox flow batteries that use organic substances or metal complexes as the active material is underway. For example, Patent Document 1 describes a redox flow battery that uses anthraquinone or naphthoquinone as the negative electrode active material, and exemplifies numerous anthraquinones having sulfo groups. Patent Document 2 describes a redox flow battery that uses, as the active material, a composition containing a coordination compound in which a redox non-innocent ligand is coordinated to a metal center, rather than the active material itself, and exemplifies numerous anthraquinones in which various functional groups are bonded to positions 1 to 8 of the anthraquinone as the redox non-innocent ligand. Non-Patent Documents 1 and 2 also describe compounds in which various functional groups or elements are bonded to positions 1 to 8 of anthraquinone. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6574382 [Patent Document 2] Special Publication No. 2019-514170 [Non-patent literature]

[0005] [Non-Patent Document 1] K.Lin,Q.Chen,MRGerhardt,L.Tong,SBKim,L.Eisenach,AWValle,D.Hardee,R,G.Gordon,M,J.Aziz,MPMarshak,Science,349(2015) 1529-1532 [Non-patent document 2] DGKwabi,K.Lin,Y.Ji.F.Kerr,M.Goulet,DDPorcellinis,DPTabor,DAPollack,A.Aspuru-Guzik,RGGordon,MJAziz,Joule 2,19(2018) 1894-1906 Summary of the Invention [Problem to be solved by the invention]

[0006] Important parameters for the active material of a redox flow battery include redox potential, solubility, durability, and viscosity of the electrolyte, and in order to bring these parameters close to ideal values, it is important to appropriately select the substituents bonded to each of the 1st to 8th positions of the anthraquinone. Patent Document 1 and Non-Patent Documents 1 and 2 state that the substituents bonded to each of the 1st to 8th positions of the anthraquinone can be selected independently, but no method has been established for actually synthesizing anthraquinones into which specific combinations of substituents have been introduced, except for a very limited number of compounds.

[0007] In view of the above circumstances, at least one embodiment of the present disclosure has an object to provide a production method that can efficiently obtain anthraquinones having different types of substituents. [Means for solving the problem]

[0008] In order to achieve the above object, the present disclosure provides a method for producing anthraquinones to be used as active materials in redox flow batteries, the method comprising the steps of: The anthraquinones are represented by the following chemical formula: TIFF0007762380000001.tif37170 R 2 、R 3 、R 6 and R 7, at least one is a hydroxyl group and at least one is an alkoxy group; The remainder is either a hydroxyl group or an alkoxy group, and the R 1 、R 4 、R 5 and R 8 is hydrogen, The method for producing the compound includes using a starting material represented by the following chemical formula: TIFF0007762380000002.tif36170 Among the above R1' to R8' R 2 ’、R 3 ’、R 6 ' and R 7 ' is a hydroxyl group, and the rest are hydrogen atoms and reacting the starting material with an organic alkylating agent, wherein the amount of the organic alkylating agent reacted with the starting material is 0.05 mol or more and less than nmol per 1 mol of the starting material, where n is the number of hydroxyl groups contained in the starting material. [Effects of the Invention]

[0009] According to the method for producing anthraquinones of the present disclosure, when the number of hydroxyl groups contained in a starting material is represented by n, by reacting 0.05 mol or more and less than nmol of an organic alkylating agent per 1 mol of the starting material, some of the hydroxyl groups in the molecule of the starting material react with the organic alkylating agent, and therefore anthraquinones having different types of substituents can be efficiently obtained. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a flowchart illustrating a method for separating a mono-substituted product from a reaction solution obtained by chemical reaction formula (6). [Figure 2] 1 is a flowchart illustrating another method for separating a mono-substituted product from the reaction solution obtained by chemical reaction formula (6). [Figure 3]1 is a flowchart illustrating a method for separating a mono-substituted product from the product obtained by chemical reaction formula (7). DETAILED DESCRIPTION OF THE INVENTION

[0011]

[0023] Hereinafter, a method for producing anthraquinones according to an embodiment of the present disclosure will be described with reference to the drawings. The embodiment described below shows one aspect of the present disclosure, but does not limit the present disclosure and can be arbitrarily modified within the scope of the technical concept of the present disclosure.

[0012] <Method of producing anthraquinones> The anthraquinones obtained by the production method described below are represented by the following chemical formula (1), in which at least one of R1 to R8 bonded to positions 1 to 8 of the anthraquinone skeleton is a hydroxyl group and at least one is an alkoxy group. That is, anthraquinones having different types of substituents, hydroxyl and alkoxy groups, are produced.

[0013] [ka]

[0014] In this production method, a compound represented by the following chemical formula (2) is used as a starting material. In chemical formula (2), at least two of R1' to R8' bonded to the 1st to 8th positions of the anthraquinone skeleton are hydroxyl groups.

[0015] [ka]

[0016] In this production method, a starting material is reacted with an organic alkylating agent (RX) in the presence of a base, as shown in the following chemical reaction formula (3). In chemical reaction formula (3), R is an alkyl group, and X is any leaving group such as halogen, tosylate, mesylate, sulfonate, or phosphate. R has 1 to 6 carbon atoms, and when it has 4 to 6 carbon atoms, it has a linear or branched structure. The bond between the carbon atoms constituting R is not limited to a single bond and may include a double bond or a triple bond. R may also include an ether bond. Furthermore, at least one of the carbon atoms constituting R may be bonded with a halogen or any functional group, such as a sulfonic acid group, amino group, nitro group, carboxyl group, phosphoryl group, thiol group, or alkyl ester, instead of hydrogen. Furthermore, NaH, NaOH, KOH, K2CO3, or the like can be used as a base.

[0017] [ka]

[0018] In chemical reaction formula (3), the amount of organic alkylating agent reacted with the starting material is preferably 0.5 mol or more but less than nmol per mol of starting material, where n is the number of hydroxyl groups contained in the starting material. Under these conditions, some of the hydroxyl groups in the starting material molecule react with the organic alkylating agent, thereby efficiently producing anthraquinones having different substituents, i.e., hydroxyl groups and alkoxy groups. However, even if the amount of organic alkylating agent is less than 0.5 mol per mol of starting material, this effect can be obtained, although the amount of product recovery work increases. However, taking into account the increased amount of product recovery work, the amount of organic alkylating agent may be 0.05 mol or more but less than nmol per mol of starting material.

[0019] For example, when 2,6-dihydroxyanthraquinone (2,6-DHAQ) is used as the starting material in this production method, as shown in the following chemical reaction formula (4), by reacting, for example, 0.5 mol to 1.5 mol of an organic alkylating agent RX with 1 mol of 2,6-DHAQ, an anthraquinone having a hydroxyl group (OH) bonded to the 2-position and an alkoxy group (OR) bonded to the 6-position is produced, and an anthraquinone having an alkoxy group bonded to the 2-position and a hydroxyl group bonded to the 6-position is produced. In addition to these, unreacted starting material and anthraquinones having alkoxy groups bonded to the 2- and 6-positions may also be present, and a mixture of these four types of anthraquinones is obtained.

[0020] [ka]

[0021] Furthermore, for example, when 2,3,6,7-tetrahydroxyanthraquinone (2,3,6,7-THAQ) is used as the starting material in this production method, the amount of organic alkylating agent to be reacted with the starting material is 1 mol or more but less than 4 mol per mol of starting material because the starting material contains four hydroxyl groups. This production method produces a mixture of anthraquinones (compounds 1 to 5) with the combinations of substituents at the 2-, 3-, 6-, and 7-positions shown in Table 1 below, and anthraquinones with alkoxy groups bonded to all of the 2-, 3-, 6-, and 7-positions.

[0022] [Table 1]

[0023] <Variations of the manufacturing method of the present disclosure> In this production method, a halogenated alkyl carboxylic acid ester may be used as the organic alkylating agent. In this case, the halogenated alkyl carboxylic acid ester is a compound represented by the formula XR a -COOR b The R of the organic alkylating agent represented by RX can be expressed by the general formulaa -COOR b In this general formula, R a and R b are any alkyl groups. For example, when the starting material 2,6-DHAQ is reacted with such a halogenated alkyl carboxylic acid ester, the hydrogen atom of the hydroxyl group is converted to R, as shown in the first step of the reaction in the following chemical reaction formula (5). a -COOR b This produces an intermediate, an anthraquinone substituted with . In the second step, this intermediate is reacted in the presence of a base to cause ester hydrolysis, and treatment with any acid converts the alkoxy group to a carboxyl group at the end. This production method allows for efficient production of anthraquinones with alkoxy groups, including carboxyl groups.

[0024] [ka]

[0025] For example, in this production method, ethyl 4-bromobutanoate can be used as the halogenated alkyl carboxylic acid ester (R a =C3H6 and R b =C2H5(Et)). In this case, as shown in the following chemical reaction formula (6), when the starting material 2,6-DHAQ is reacted with ethyl 4-bromobutanoate, an intermediate anthraquinone is produced in which the hydrogen atom of one hydroxyl group is replaced with a 3-(ethoxycarbonyl)propyl group. Next, by reacting this intermediate in the presence of a base, ester hydrolysis occurs, and by treating with acetic acid (AcOH), an anthraquinone in which one hydroxyl group is bonded to one alkoxy group containing a carboxyl group (2-(3'-carboxypropyloxy)-6-hydroxy-9,10-anthraquinone (2,6-MHMBEAQ)) is obtained.

[0026] [ka]

[0027] If the carbon chain of the organic alkylating agent is short, part of the organic alkylating agent will decompose during the E2 reaction and will no longer contribute to the reaction. In contrast, if ethyl 4-bromobutanoate is used as the organic alkylating agent, the carbon chain length constituting the butanoic acid skeleton of ethyl 4-bromobutanoate is appropriate, making ethyl 4-bromobutanoate less likely to decompose. Therefore, most of the added ethyl 4-bromobutanoate can contribute to the reaction, thereby increasing the yield of the desired product.

[0028] Chemical reaction formula (6) is described as producing an anthraquinone having one hydroxyl group and one alkoxy group bonded thereto (hereinafter referred to as "mono-substituted anthraquinone"), but in reality, a mixture containing unreacted starting material and an anthraquinone having two alkoxy groups bonded thereto (hereinafter referred to as "di-substituted anthraquinone") is obtained. Next, two methods for separating the mono-substituted anthraquinone from the mixture obtained by the above-mentioned production method will be described.

[0029] The flowchart for the first method is shown in Figure 1. Water is added to the reaction solution of chemical reaction formula (6). When this reaction solution is filtered, the filtrate contains mainly the mono-substituted isomer and unreacted starting material. Meanwhile, the residue on the filter contains mainly the di-substituted isomer and the mono-substituted isomer. Because the filtrate from this filtration is alkaline, acetic acid is added to the filtrate to neutralize it, and a second filtration is performed. Chloroform is added to the residue from the second filtration, and a third filtration is performed. The filtrate from the third filtration is concentrated and dried, allowing the mono-substituted isomer to be separated.

[0030] The flowchart for the second method is shown in Figure 2. Because the reaction solution of chemical reaction formula (6) is alkaline, hydrochloric acid is added to neutralize it. The neutralized reaction solution is filtered. Tetrahydrofuran (THF) is added to the filtered residue and stirred at a temperature range of 40–60°C for 4 hours, then allowed to cool. This is then filtered, and the filtrate is concentrated and dried. Ethyl acetate is added to the solid obtained by concentration and drying, and the mixture is stirred at a temperature range of 40–70°C for 1 hour. This is then filtered at a temperature range of room temperature to 50°C. The filtrate is concentrated and dried. Chloroform is added to the solid obtained by concentration and drying, and the mixture is stirred at a temperature range of 40–60°C for 1 hour. This is then filtered, and the filtrate is concentrated and dried. Methanol is added to the solid obtained by concentration and drying, and the mixture is heated to 45–55°C. This is then allowed to cool and stand. This is then filtered, and the filtrate is concentrated and dried to isolate the monosubstituted isomer. In the above description, the stirring time is not an exact time but an approximate guideline, and can be changed as long as it is not significantly shortened or extended compared to the above time.

[0031] Experiments conducted by the inventors of the present disclosure confirmed that the reaction represented by chemical reaction formula (6) yielded a reaction solution containing approximately 40% by mass of the mono-substituted product, approximately 30% by mass of the di-substituted product, and approximately 30% by mass of unreacted starting material. When this reaction solution was subjected to the two methods described above to separate the mono-substituted product, the yield of the mono-substituted product was 12-18% when the first method was used, whereas the yield of the mono-substituted product was approximately 40% when the second method was used. With the first method, the mono-substituted product was contained in the filter cake from the first filtration, resulting in an increase in the amount of mono-substituted product that could not be recovered. While both methods can separate the mono-substituted product, the second method offers a higher yield and is therefore preferred.

[0032] The hydrolysis reaction of chemical reaction formula (6) is carried out in an organic solvent such as isopropyl alcohol or 1,2-dimethoxyethane in the presence of a base. However, hydrolysis can also be carried out without such an organic solvent using an aqueous potassium hydroxide or sodium hydroxide solution (an alkaline aqueous solution). In this case, the subsequent acid treatment can be omitted. Furthermore, even when hydrolysis is carried out using an organic solvent, the organic solvent can be removed by vacuum concentration without acid treatment after the hydrolysis reaction. In this case, an alkaline aqueous solution is obtained. When hydrolysis is carried out using this method, the hydroxyl group in the final product of chemical reaction formula (6) becomes -OK or -ONa, and the carboxyl group becomes -COOK or -COONa. When a compound containing hydroxyl or carboxyl groups, such as 2,6-MHMBEAQ, is dissolved in an alkaline electrolyte and used as an active material, alkali is consumed to neutralize the hydroxyl or carboxyl groups. To prepare an alkaline electrolyte for use in a redox flow battery, alkali is required to neutralize the hydroxyl or carboxyl groups. However, the solution in which the final product obtained by hydrolysis with potassium hydroxide or sodium hydroxide is dissolved can be used as it is in a redox flow battery as an alkaline electrolyte.

[0033] Although the production method of the present disclosure has been specifically described using ethyl 4-bromobutanoate as an example of a halogenated alkyl carboxylate, alkyl 4-bromobutanoates in which any alkyl group, such as a methyl group, propyl group, or butyl group, is bonded to the carboxyl group instead of the ethyl group constituting the ester group, may also be used. However, it is preferable to use alkyl 4-bromobutanoates in which the ester group is an alkyl group having a carbon chain in which four or more carbon atoms are bonded in a branched manner. By using such halogenated alkyl carboxylates, the difference in solubility between anthraquinones having different substituents and by-produced anthraquinones having the same substituents becomes greater, facilitating separation in the washing and extraction steps.

[0034] For example, as shown in the following chemical reaction formula (7), the starting material 2,6-DHAQ is reacted with 4-bromobutanoic acid (2'-ethyl)hexyl as a halogenated alkyl carboxylate. This produces a monosubstituted product in which one of the two hydroxyl groups is substituted with an alkoxy group, and a disubstituted product in which each of the two hydroxyl groups is substituted with an alkoxy group. The reaction solution obtained by chemical reaction formula (7) contains unreacted starting material in addition to the monosubstituted and disubstituted products.

[0035] [ka]

[0036] Next, the method for separating the monosubstituted isomer from the reaction solution obtained by chemical reaction formula (7) is explained based on the flowchart in Figure 3. Water is added to the reaction solution, and the mixture is suction filtered and washed with water. Hexane is added to the resulting filter cake, which is then centrifuged. Because the solubilities of the monosubstituted and disubstituted isomers in hexane are significantly different, the liquid phase obtained after centrifugation contains primarily the disubstituted isomer, while the precipitate obtained by centrifugation contains primarily the monosubstituted isomer. The monosubstituted isomer can be isolated by collecting this precipitate and drying it in vacuum. Hydrolysis of this monosubstituted isomer yields 2,6-MHMBEAQ in high yield, in which the 2-ethylhexyl group constituting the ester group has been replaced with a hydrogen atom.

[0037] Since 2,6-DHAQ is mass-produced industrially, it is readily available as a starting material, thereby reducing the production costs of the target anthraquinones. On the other hand, 2,6-DHAQ can be easily synthesized from 2,6-diaminoanthraquinone (2,6-DAAQ) by the well-known Sandmeyer reaction. Because 2,6-DAAQ is less expensive than 2,6-DHAQ (approximately one-tenth or less), the production costs of the target anthraquinones can be further reduced by using 2,6-DHAQ synthesized from 2,6-DAAQ by the Sandmeyer reaction as a starting material. [Example]

[0038] Example 1 2,6-MHMBEAQ was synthesized from 2,6-DHAQ using the procedure shown in chemical reaction formula (8) below. The synthesis process is outlined as follows: From 2,6-DHAQ as the starting material, an intermediate is synthesized that has an alkoxy group in which the hydrogen of one hydroxyl group is replaced with an ethyl butanoate group, and from this intermediate the target substance, 2,6-MHMBEAQ, is synthesized.

[0039] [ka]

[0040] A 1-L recovery flask was charged with 40.0 g (167 mmol) of 2,6-DHAQ (Tokyo Chemical Industry Co., Ltd.) and 500 mL of N,N-dimethylformamide (DMF). While stirring, 23.1 g (167 mmol) of potassium carbonate was added, followed by 23.9 mL (167 mmol) of ethyl 4-bromobutanoate. The mixture was then heated and stirred at 100°C for 17 hours. After cooling, 600 mL of distilled water was added, and the precipitate was suction filtered. The filter cake was washed with distilled water. While stirring, 6 M hydrochloric acid was added to the filtrate (pH > 9). Hydrochloric acid was added until the pH of the filtrate became less than 3 and no carbon dioxide was generated upon addition of the hydrochloric acid. The mixture was then stirred at room temperature for 1 hour. The precipitate was transferred to a 200 mL centrifuge tube and centrifuged to separate the precipitate. The precipitate was filtered under suction, washed with distilled water, and then dried under vacuum at 80°C for 6 hours, yielding 11.4 g of a mixture of the raw material and intermediate. The resulting solid was pulverized into powder and suspended in 200 mL of chloroform. Insoluble matter was removed by suction filtration, and the mixture was washed with 200 mL of chloroform until all soluble matter was dissolved. This procedure yielded 11.1 g of unreacted raw material. The filtrate was again filtered under suction to completely remove insoluble matter, and the filtrate was concentrated under reduced pressure. The residue was suspended in distilled water, filtered under suction, washed, and dried under vacuum at 80°C for 4 hours, yielding 6.96 g of the intermediate as a reddish-brown solid (yield: 12%).

[0041] Next, 6.96 g (19.6 mmol) of the intermediate was placed in a 1 L recovery flask, along with 190 mL of isopropyl alcohol and 380 mL of distilled water. 4.48 g (79.9 mmol) of potassium hydroxide was added, and the mixture was heated and stirred at 60 °C for 20 hours. After cooling, 550 mL of distilled water was added, and the mixture was transferred to a 2 L Erlenmeyer flask. While stirring, 2 M hydrochloric acid was added until the pH was less than 3. After stirring for 2 hours, the precipitate was separated by centrifugation. The supernatant and precipitate were each filtered under suction, and the residue was washed with distilled water. The residue was vacuum dried at 80 °C for 4 hours to obtain 6.25 g of the target substance (98% yield from the intermediate). In Example 1, 1,2-dimethoxyethane can also be used instead of isopropyl alcohol.

[0042] <Example 2> 2,6-DHAQ was synthesized from 2,6-DAAQ by the following reaction scheme (9): 95.2 g (400 mmol) of 2,6-DAAQ (Tokyo Chemical Industry Co., Ltd.) and 1.6 L of 20% diluted sulfuric acid were placed in a 3 L reaction vessel. While stirring the mixture in an ice bath, 250 mL of an aqueous solution containing 71.8 g (1.04 mol) of sodium nitrite was added over 1 hour. The mixture was stirred between -10°C and -17°C for 14 hours to obtain a suspension of bisdiazonium salt. A separate 5 L reaction vessel was charged with 1.6 L of warm water, and the suspension of bisdiazonium salt was added to the warm water over approximately 2 hours while maintaining the temperature at 85-90°C. After the addition of the suspension, stirring was continued for 2 hours while maintaining the same temperature. After cooling to room temperature, the precipitate was filtered under suction, washed with distilled water, and dried by heating for 15 hours to obtain 92.0 g of 2,6-DHAQ (yield: 96%). 2,6-MHMBEAQ can be synthesized from the 2,6-DHAQ thus obtained by the method of Example 1.

[0043] [ka]

[0044] Furthermore, instead of the method of Example 1, 2,6-MHMBEAQ can also be synthesized by the following two-step method according to chemical reaction formula (10).

[0045] [ka]

[0046] A 3-L three-neck flask was charged with 192 g (800 mmol) of 2,6-DHAQ and 2.2 L of DMF. The temperature was raised to 85-95°C, and 84.0 g (608 mmol) of potassium carbonate was added with stirring. 67.4 g (345 mmol) of ethyl 4-bromobutanoate was then added. Stirring was continued for 1 hour at this temperature. After cooling, 3 L of cold water (0-10°C), 220 mL of 6 M hydrochloric acid, and 2 L of water (15-30°C) were added. The precipitate was suction filtered, washed with water, and then dried by heating to obtain 217.7 g of solid. 2.1 L of THF was added to 150 g of this solid, and insoluble material was removed by filtration. The filtrate was concentrated to obtain 103 g of solid. 1.5 L of ethyl acetate was added to this solid, and insoluble material was removed by filtration. The filtrate was concentrated under reduced pressure to obtain 72 g of solid. 1.1 L of chloroform was added to this solid, insoluble matter was removed by filtration, and the filtrate was concentrated under reduced pressure to give 52 g of a solid. 800 mL of methanol was added to this solid, insoluble matter was removed by filtration, and the filtrate was concentrated under reduced pressure to give 36.6 g of a solid (yield of the first step reaction was 21%).

[0047] 25.0 g (71 mmol) of the solid obtained in the first-stage reaction was placed in a 1 L recovery flask, and 275 mL of ethylene glycol dimethyl ether was added to form a solution. 178 mL (178 mmol) of 1 M aqueous sodium hydroxide was added, and the temperature was raised. The mixture was stirred at 60°C for 30 minutes. After cooling, the reaction solution was acidified by adding 33 mL of 6 M hydrochloric acid, and the mixture was concentrated under reduced pressure to remove the organic solvent. The precipitate was filtered by suction, and the residue was washed with distilled water. The washed residue was air-dried at 90°C for 16 hours to obtain 22.2 g of 2,6-MHMBEAQ (yield of the second-stage reaction was 96%).

[0048] Example 3 A compound (target substance) in which hydroxyl groups are bonded to two of the 2-, 3-, 6-, and 7-positions of the anthraquinone skeleton and alkoxy groups (-OC3H6COOH) having carboxyl groups are bonded to the remaining two positions was synthesized using the reactions represented by the following chemical reaction formulas (11) to (15) in the following procedure: First, 2,3,6,7-THAQ was synthesized from 1,2-dimethoxybenzene using the reactions represented by the following chemical reaction formulas (11) to (13) in the following procedure.

[0049] [ka]

[0050] A 500 mL beaker was charged with 42 g of ice and 100 mL of concentrated sulfuric acid. While carefully preventing the temperature of the reaction solution from exceeding 5°C, a mixture of 25.1 g (182 mmol) of 1,2-dimethoxybenzene (available from Tokyo Chemical Industry Co., Ltd.) and 7.3 mL (308 mmol) of acetaldehyde was added dropwise to the stirred reaction solution over 2.5 hours, followed by stirring at room temperature for 22 hours. The reaction solution was poured into a 1000 mL Erlenmeyer flask containing 350 mL of ethanol and washed down with 60 mL of methanol. The precipitate was filtered under suction, washed with 160 mL of ethanol and 320 mL of distilled water, and then vacuum-dried at 60°C for 5 hours to obtain 22.3 g of a white solid (75% yield for chemical reaction equation (11)).

[0051] 15.1 g (46.1 mmol) of the above white solid was placed in a 1 L recovery flask and suspended in 750 mL of acetic acid, followed by the addition of 85.4 g (287 mmol) of sodium dichromate dihydrate. The reaction solution was heated under reflux in an oil bath for 5 hours. After the reaction, the mixture was allowed to cool and stand, and the resulting precipitate was filtered under suction. The residue was washed with distilled water and vacuum-dried at 70°C for 4 hours to obtain 12.4 g of a yellow solid (82% yield for chemical reaction formula (12)).

[0052] 18.8 g (57.1 mmol) of the above yellow solid was placed in a 1 L recovery flask, suspended in 250 mL of 47% hydrobromic acid, and heated to reflux in an oil bath at 150 °C for 6 days. During the 6 days, 90 mL of 47% hydrobromic acid was added. The reaction solution was allowed to cool, then transferred to a settling tube and centrifuged to remove the supernatant. 400 mL of distilled water was added to the residue, dispersed, and the mixture was centrifuged again to remove the supernatant. The insoluble matter was filtered by suction and washed with distilled water. The residue was vacuum-dried at 70-80 °C for 13 hours to obtain 15.1 g of 2,3,6,7-THAQ (yield of 98% for chemical reaction equation (13)).

[0053] The 2,3,6,7-THAQ obtained in this manner was synthesized using the following reaction represented by chemical reaction formula (14): The outline of this synthesis is as follows: From 2,3,6,7-THAQ, an intermediate mixture is obtained having alkoxy groups in which the hydrogen atoms of two hydroxyl groups have been replaced with ethyl butanoate, and from this intermediate mixture, the target substance is obtained.

[0054] [ka]

[0055] A 1-L recovery flask was charged with 19.8 g (72.2 mmol) of 2,3,6,7-THAQ and 280 mL of DMF. To this was added 19.9 g (144 mmol) of potassium carbonate and 20.7 mL (144 mmol) of ethyl 4-bromobutanoate. The mixture was then heated and stirred at 100°C for 23 hours. After cooling, 150 mL of distilled water was added, and the precipitate was collected by suction filtration. 6 M hydrochloric acid was added to the filtrate with stirring until the pH reached approximately 3-4, and the precipitate was collected by centrifugation and suction filtration. This precipitate was subjected to Soxhlet extraction using chloroform. The extract was concentrated under reduced pressure to obtain 5.04 g of an intermediate mixture (yield: 14%).

[0056] 5.15 g of the intermediate mixture was placed in a 500 mL recovery flask, to which 90 mL of isopropyl alcohol and 180 mL of distilled water were added. After adding 4.62 g (82.3 mmol) of potassium hydroxide, the temperature was raised and the mixture was heated and stirred at 60 °C for 20 hours. After cooling, the mixture was poured into a 1 L beaker containing 300 mL of distilled water, and 2 M hydrochloric acid was added with stirring until the pH was below 3. After stirring for 1 hour, the precipitate was separated by centrifugation. This precipitate was collected by suction filtration while washing with distilled water, and the residue was vacuum dried at 70 °C for 2.5 hours to obtain 3.71 g of the target product (a 91% yield from the intermediate mixture).

[0057] Example 4 2,6-MHMBEAQ was synthesized by the reactions represented by the following chemical reaction formulas (15) to (17). First, 4-bromobutanoic acid (2'-ethyl)hexyl was synthesized by the reaction represented by the following chemical reaction formula (15) in the following procedure.

[0058] [ka]

[0059] A 200 mL recovery flask was charged with 8.56 g (51.3 mmol) of 4-bromobutanoic acid and 70 mL of cyclohexane. 7.40 mL (47.2 mmol) of 2-ethyl-1-hexanol and 0.98 g (5.14 mmol) of p-toluenesulfonic acid monohydrate were added, and the mixture was heated to reflux for 21 hours using a Dean-Stark apparatus. After cooling, the reaction solution was transferred to a separatory funnel, and 30 mL of cyclohexane and 50 mL of saturated aqueous sodium bicarbonate were added. The organic phase was washed twice with 50 mL of saturated aqueous sodium bicarbonate and dried over anhydrous sodium sulfate. After removing the desiccant by filtration, the mixture was concentrated under reduced pressure and dried in vacuo to yield 11.8 g of a pale yellow oil (89% yield for chemical reaction (15)).

[0060] Next, 2,6-DHAQ (Tokyo Chemical Industry Co., Ltd.) and the above pale yellow oil (2'-ethylhexyl 4-bromobutanoate) were reacted according to the following reaction scheme (16): 1.20 g (4.99 mmol) of 2,6-DHAQ and 19 mL of N-methylpyrrolidone were placed in a 50 mL recovery flask. 0.52 g (3.78 mmol) of potassium carbonate was added and the mixture was heated to 95°C. 0.70 g of the above pale yellow oil was added and stirred for 21 hours while maintaining the temperature at 95°C. After cooling on ice, 25 mL of distilled water was added to the reaction solution, and the separated viscous solid (0.61 g) was collected by suction filtration and washing with water. This solid was dispersed in 25 mL of hexane, and the precipitate was collected by centrifugation. The same procedure was repeated with the addition of another 25 mL of hexane, yielding 0.49 g of a pale yellow solid (yield of chemical reaction (16) was 22%).

[0061] [ka]

[0062] Next, 2,6-MHMBEAQ was synthesized by the following reaction represented by chemical reaction formula (17): 0.294 g of the above pale yellow solid was placed in a 50 mL recovery flask, along with 10 mL of isopropyl alcohol and 20 mL of distilled water. 0.211 g (2.68 mmol) of potassium hydroxide was added, and the mixture was heated and stirred at 60°C for 20 hours. After cooling, 20 mL of distilled water was added, and 2 M hydrochloric acid was added with stirring until the pH became less than 3. At this time, a yellow precipitate was dispersed throughout the solution. After stirring for 2 hours, the precipitate was separated by centrifugation. The supernatant and precipitate were each filtered under suction, and the residue was washed with distilled water. The solid was dried in vacuo at 80°C for 2 hours to obtain 0.206 g of 2,6-MHMBEAQ (a 94% yield for chemical reaction (17)).

[0063] [ka]

[0064] The contents described in each of the above embodiments can be understood, for example, as follows.

[0065] [1] A method for producing anthraquinones according to one embodiment includes the steps of: A method for producing anthraquinones represented by the following chemical formula: [ka] At least one of R1 to R8 is a hydroxyl group and at least one is an alkoxy group, The manufacturing method includes: A starting material represented by the following chemical formula: [ka] preparing a starting material in which at least two of R1' to R8' are hydroxyl groups; reacting said starting material with an organic alkylating agent; Including, The amount of the organic alkylating agent reacted with the starting material is 0.05 mol or more and less than nmol per 1 mol of the starting material, where n is the number of hydroxyl groups contained in the starting material.

[0066] According to the method for producing anthraquinones of the present disclosure, when the number of hydroxyl groups contained in a starting material is represented by n, by reacting 0.05 mol or more and less than nmol of an organic alkylating agent per 1 mol of the starting material, some of the hydroxyl groups in the molecule of the starting material react with the organic alkylating agent, and therefore anthraquinones having different types of substituents can be efficiently obtained.

[0067] [2] Another embodiment of the method for producing anthraquinones is the method for producing anthraquinones according to [1], Of the R1' to R8', the R2' and R6' are hydroxyl groups, and the rest are hydrogen atoms.

[0068] According to this configuration, since 2,6-dihydroxyanthraquinone, which is mass-produced industrially, is used as a starting material, the production cost of anthraquinones can be reduced.

[0069] [3] A method for producing anthraquinones according to yet another embodiment is the method for producing anthraquinones according to [1], Of R1' to R8', R2', R3', R6' and R7' are hydroxyl groups, and the rest are hydrogen atoms.

[0070] According to this method, 2,3,6,7-tetrahydroxyanthraquinone is used as a starting material, which can be synthesized in high yield and is readily available as a raw material, thereby reducing the production cost of anthraquinones.

[0071] [4] A method for producing anthraquinones according to yet another embodiment is a method for producing anthraquinones according to any one of [1] to [3], the organic alkylating agent is a halogenated alkyl carboxylic acid ester; The method includes the steps of reacting the starting material with the halogenated alkyl carboxylic acid ester, followed by alkaline hydrolysis and acid treatment.

[0072] According to this method, anthraquinones having alkoxy groups containing carboxyl groups bonded thereto can be efficiently obtained.

[0073] [5] A method for producing anthraquinones according to yet another embodiment is a method for producing anthraquinones according to any one of [1] to [3], the organic alkylating agent is a halogenated alkyl carboxylic acid ester; The method includes reacting the starting material with the halogenated alkyl carboxylic acid ester, followed by hydrolysis in an alkaline aqueous solution.

[0074] According to this configuration, an alkaline aqueous solution containing anthraquinones is obtained, and can be used as it is as an alkaline electrolyte in a redox flow battery.

[0075] [6] A method for producing anthraquinones according to yet another embodiment is a method for producing anthraquinones according to any one of [1] to [3], the organic alkylating agent is a halogenated alkyl carboxylic acid ester; The method includes the steps of reacting the starting material with the halogenated alkyl carboxylic acid ester, followed by hydrolysis with alkali and concentration under reduced pressure.

[0076] According to this configuration, an alkaline aqueous solution containing anthraquinones is obtained by vacuum concentration after alkaline hydrolysis, and can be used as is as an alkaline electrolyte in a redox flow battery.

[0077] [7] A method for producing anthraquinones according to yet another embodiment is the method for producing anthraquinones according to any one of [4] to [6], The halogenated alkyl carboxylic acid ester is alkyl 4-bromobutanoate.

[0078] If the carbon chain of the halogenated alkyl carboxylate is short, a portion of the halogenated alkyl carboxylate will decompose during the E2 reaction and will no longer contribute to the reaction. In contrast, with this configuration, the carbon chain length constituting the butanoic acid skeleton of the alkyl 4-bromobutanoate is appropriate, making the alkyl 4-bromobutanoate less susceptible to decomposition, and therefore most of the added alkyl 4-bromobutanoate can contribute to the reaction, thereby increasing the yield of the target product.

[0079] [8] A method for producing anthraquinones according to yet another embodiment is the method for producing anthraquinones according to [7], Water is added to the reaction mixture after hydrolysis and the mixture is filtered. An aqueous solution of acetic acid is added to the filtrate and the mixture is filtered. Chloroform is added to the residue on the filter and the mixture is filtered. The filtrate is concentrated and dried.

[0080] According to this method, anthraquinones having different kinds of substituents can be obtained in high yield.

[0081] [9] A method for producing anthraquinones according to yet another embodiment is the method for producing anthraquinones according to [7], Hydrochloric acid is added to the reaction liquid after hydrolysis and the mixture is filtered. Tetrahydrofuran is added to the residue on the filter and the mixture is filtered. The filtrate is concentrated and dried to obtain a solid, to which ethyl acetate is added and the mixture is filtered. The filtrate is concentrated and dried to obtain a solid, to which chloroform is added and the mixture is filtered. The filtrate is concentrated and dried to obtain a solid, to which methanol is added and the mixture is filtered. The filtrate is concentrated and dried.

[0082] According to this method, anthraquinones having different kinds of substituents can be obtained in high yield.

[0083]

[10] A method for producing anthraquinones according to yet another embodiment is the method for producing anthraquinones according to any one of [4] to [6], The alkyl group of the ester moiety of the halogenated alkyl carboxylic acid ester has a carbon chain in which four or more carbon atoms are bonded to form a branch.

[0084] According to such a configuration, the difference in solubility between anthraquinones having different types of substituents and by-produced anthraquinones having the same type of substituents becomes large, which facilitates separation in the washing and extraction steps.

[0085]

[11] A method for producing anthraquinones according to yet another embodiment is the method for producing anthraquinones according to any one of [1] to

[10] , the starting material is 2,6-dihydroxyanthraquinone, The method further includes a step of synthesizing the 2,6-dihydroxyanthraquinone using 2,6-diaminoanthraquinone as a raw material.

[0086] According to this configuration, the starting material, 2,6-diaminoanthraquinone, is produced from 2,6-dihydroxyanthraquinone, which is cheaper than 2,6-dihydroxyanthraquinone, thereby reducing the production cost of anthraquinones.

Claims

1. A method for producing anthraquinones used as active materials in redox flow batteries, comprising: The anthraquinones are represented by the following chemical formula: The R ２ , R ３ , R ６ and R ７ At least one of the R groups is a hydroxyl group, at least one of the R groups is an alkoxy group, and the remaining R groups are either a hydroxyl group or an alkoxy group. １ , R ４ , R ５ and R ８ is hydrogen, The manufacturing method includes: A starting material represented by the following chemical formula: The R １ '~R ８ ', the R ２ ', R ３ ', R ６ ' and R ７ providing a starting material in which ' are hydroxyl groups and the remainder are hydrogen atoms; reacting said starting material with an organic alkylating agent; Including, a method for producing anthraquinones, wherein the amount of the organic alkylating agent to be reacted with the starting material is 0.05 mol or more but less than nmol per 1 mol of the starting material, where n is the number of hydroxyl groups contained in the starting material.

2. A method for producing anthraquinones used as active materials in redox flow batteries, comprising: The anthraquinones are represented by the following chemical formula: The R １ ~R ８ Among them, the R ６ is a hydroxyl group, and the R ２ is an alkoxy group and the remainder are hydrogen; The manufacturing method includes: synthesizing 2,6-dihydroxyanthraquinone as a starting material using 2,6-diaminoanthraquinone as a raw material; reacting said starting material with an organic alkylating agent; Including, a method for producing anthraquinones, wherein the amount of the organic alkylating agent to be reacted with the starting material is 0.05 mol or more but less than nmol per 1 mol of the starting material, where n is the number of hydroxyl groups contained in the starting material.

3. the organic alkylating agent is a halogenated alkyl carboxylic acid ester; 3. The method for producing anthraquinones according to claim 1, further comprising the steps of reacting the starting material with the halogenated alkyl carboxylic acid ester, followed by hydrolysis with an alkali and treatment with an acid.

4. the organic alkylating agent is a halogenated alkyl carboxylic acid ester; 3. The method for producing anthraquinones according to claim 1, further comprising the step of reacting the starting material with the halogenated alkyl carboxylic acid ester and then hydrolyzing the reacted material in an alkaline aqueous solution.

5. the organic alkylating agent is a halogenated alkyl carboxylic acid ester; 3. The method for producing anthraquinones according to claim 1, further comprising the steps of reacting the starting material with the halogenated alkyl carboxylic acid ester, hydrolyzing the reacted material with an alkali, and concentrating the reacted material under reduced pressure.

6. The method for producing anthraquinones according to any one of claims 3 to 5, wherein the halogenated alkyl carboxylic acid ester is alkyl 4-bromobutanoate.

7. The method for producing anthraquinones according to claim 6, wherein water is added to the reaction solution after hydrolysis and the mixture is filtered, an aqueous acetic acid solution is added to the filtrate and the mixture is filtered, chloroform is added to the residue on the filter and the mixture is filtered, and the filtrate is concentrated and dried.

8. The method for producing anthraquinones according to claim 6, wherein hydrochloric acid is added to the reaction solution after the hydrolysis and the mixture is filtered, tetrahydrofuran is added to the residue obtained by filtration, the filtrate is concentrated and dried to obtain a solid, ethyl acetate is added to the solid, the mixture is filtered, chloroform is added to the solid obtained by condensing and drying the filtrate, and the solid is filtered, methanol is added to the solid obtained by condensing and drying the filtrate, and the filtrate is concentrated and dried.

9. The method for producing anthraquinones according to any one of claims 3 to 5, wherein the alkyl group of the ester moiety of the halogenated alkyl carboxylic acid ester has a carbon chain in which four or more carbon atoms are bonded so as to have a branch.

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