Carbon dioxide absorption / reduction solution, carbon dioxide absorption / reduction device, and carbon dioxide absorption / reduction method

Abstract

To provide a carbon dioxide absorption and reduction solution capable of suppressing deposition of a metal complex as a catalyst for carbon dioxide absorption and reduction, a carbon dioxide absorption and reduction device, and a carbon dioxide absorption and reduction method.SOLUTION: A carbon dioxide absorption reduction solution contains 0.01 to 100 mM of a metal complex in a mixed solvent of water and a water-soluble solvent, and the metal complex contains a central metal that is either rhenium, manganese, or iron, and a ligand coordinating to the central metal, wherein the ligand comprises two or more carbonyl groups and two or more nitrogen atom-containing heterocycles, at least one of the two or more nitrogen atom-containing heterocycles has at least one substitution group containing a carboxyl group or hydroxy group. When the central metal of the metal complex is ruthenium, the nitrogen atom-containing heterocycle may not contain a hydroxyl group or hydroxy group.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a carbon dioxide absorption / reduction solution, a carbon dioxide absorption / reduction device, and a carbon dioxide absorption / reduction method. [Background technology]

[0002] Combustion of fossil fuels in power plants, chemical plants, and the like emits large amounts of carbon dioxide, which contributes to global warming. For this reason, attention has been focused on carbon cycle processes, such as recovering and effectively utilizing carbon dioxide or converting carbon dioxide into valuable materials. Methods for converting carbon dioxide into valuable materials include electrochemical reduction and photoelectrochemical reduction using light energy. Metal complexes can be used as catalysts for such electrochemical and photoelectrochemical reductions. A method for reducing carbon dioxide using such catalysts is described, for example, in Non-Patent Document 1. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] A.Nakada et al. “Selective Electrocatalysis of a Water-Soluble Rhenium(I) Complex for CO2 Reduction Using Water As an Electron Donor” ACS.Catal.2018,8,p354-363 Summary of the Invention [Problem to be solved by the invention]

[0004] However, gases containing carbon dioxide often contain moisture, and when a gas containing moisture is supplied to an electrolyte solution of a metal complex serving as a catalyst, if the metal complex is water-insoluble, the metal complex will not dissolve in water and will precipitate, which is a problem. In the method described in Non-Patent Document 1, the concentration of the metal complex in the electrolyte solution is kept low, such as 0.5 mM, in order to prevent such precipitation of the metal complex, but such a low concentration of the metal complex results in a problem of low efficiency of carbon dioxide reduction.

[0005] In view of the above circumstances, at least one embodiment of the present disclosure aims to provide a carbon dioxide absorption / reduction solution, a carbon dioxide absorption / reduction device, and a carbon dioxide absorption / reduction method that can suppress precipitation of a metal complex as a catalyst for carbon dioxide absorption / reduction. [Means for solving the problem]

[0006] In order to achieve the above object, the carbon dioxide absorption / reduction solution according to the present disclosure is a carbon dioxide absorption / reduction solution containing 0.01 to 100 mM of a metal complex in a mixed solvent of water and a water-soluble solvent, and the metal complex is having the following structure: , TIFF0007730500000001.tif54170 The concentration of the water-soluble solvent in the mixed solvent is 10% by mass or more and 50% by mass or less. . [Effects of the Invention]

[0008] According to the carbon dioxide absorption / reduction solution of the present disclosure, by dissolving a water-soluble metal complex as a catalyst for carbon dioxide absorption / reduction in a mixed solvent of water and a water-soluble solvent, precipitation of the metal complex can be suppressed. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram illustrating the configuration of a carbon dioxide absorption and reduction device according to an embodiment of the present disclosure. [Figure 2] FIG. 10 is a schematic diagram illustrating the configuration of a carbon dioxide absorption and reduction device according to a modified example of the present disclosure. [Figure 3] FIG. 10 is a schematic diagram illustrating the configuration of a carbon dioxide absorption and reduction device according to another modified example of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, a carbon dioxide absorption / reduction solution 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, and does not limit the present disclosure. The present disclosure can be arbitrarily modified within the scope of the technical concept of the present disclosure.

[0011] <Carbon dioxide absorption / reduction solution according to one embodiment of the present disclosure> A carbon dioxide absorption-reduction solution according to one embodiment of the present disclosure is used as an electrolyte for electrolysis, in which carbon dioxide supplied to the electrolyte is reduced by electrolysis into valuable substances such as carbon monoxide and formic acid. This carbon dioxide absorption-reduction solution contains a metal complex in a mixed solvent of water and a water-soluble solvent. The concentration of the metal complex in the mixed solvent is 0.01 to 100 mM, as disclosed in a document (Japanese Patent No. 6615175) based on a previous study by one of the inventors.

[0012] The concentration of the water-soluble solvent in the mixed solvent is preferably from 1 to 60% by mass, more preferably from 10 to 50% by mass, and most preferably from 20 to 40% by mass.

[0013] The metal complexes usable herein are those represented by the following molecular structure (1) or (2).

[0014] [ka]

[0015] [ka]

[0016] In the molecular structure (1), M, the central metal of the metal complex, is either rhenium, manganese, or iron. In the molecular structures (1) and (2), the metal complex contains at least two carbonyl groups, at least two nitrogen-containing heterocycles A and B, and ligands X and Y as ligands for the central metals (M and ruthenium). Ligands X and Y are not limited to any particular groups, but may be any functional group, such as a linear or cyclic alkyl group or a functional group containing oxygen, nitrogen, sulfur, phosphorus, or a halogen atom; may be a carbonyl group or a nitrogen-containing heterocycle; or may be water or a hydroxy group.

[0017] The at least two nitrogen-containing heterocycles may have the same structure or different structures. Furthermore, at least one of the two nitrogen-containing heterocycles A and B of the metal complex represented by molecular structure (1) has a hydroxyl-containing substituent (-R-OH / -R'-OH) as a functional group. In molecular structure (1), both nitrogen-containing heterocycles A and B have a hydroxyl-containing substituent, but any one of the nitrogen-containing heterocycles may have a hydroxyl-containing substituent. When the metal complex has three or more nitrogen-containing heterocycles, at least one of the nitrogen-containing heterocycles may have a hydroxyl-containing substituent. The presence of a hydroxyl-containing substituent imparts water-solubility to the metal complex represented by molecular structure (1). However, since a large number of carbon atoms in a structure having a hydroxyl-containing substituent makes it difficult to impart water-solubility to the metal complex, a hydroxymethyl group, a hydroxyethyl group, or a carboxy group is preferred as the hydroxyl-containing substituent. The metal complex represented by molecular structure (2) having ruthenium as the central metal has water-solubility even when the nitrogen-containing heterocycles A and B do not contain a substituent containing a hydroxy group. However, even in the metal complex represented by molecular structure (2), at least one of the nitrogen-containing heterocycles A and B may have a substituent containing a hydroxy group.

[0018] The water-soluble solvent is a liquid containing an organic compound having a hydroxyl group or a carboxyl group. Examples of such organic compounds include alcohols, glycols, phenols, phenol derivatives, alkanolamines, and amino acids. Even organic compounds having a hydroxyl group or a carboxyl group have a lower water solubility as the number of carbon atoms increases, so substances with a lower number of carbon atoms are preferred. For example, among alcohols, methyl alcohol, ethyl alcohol, 1-propyl alcohol, or 2-propyl alcohol are preferred.

[0019] Alkanolamines have the property of absorbing carbon dioxide, and therefore, by using alkanolamines as a water-soluble solvent, carbon dioxide can be efficiently reduced. Note that alkanolamines can be classified by their structures as primary amine, secondary amine, or tertiary amine, and although there are differences in the ability to absorb carbon dioxide between amines of each structure, they have a higher property of absorbing carbon dioxide than other organic compounds, so alkanolamines of any structure may be used.

[0020] Examples of amino acids that can be used include, but are not limited to, glycine, alanine, etc. These are solids at room temperature, so they are used in a form dissolved in water or other liquid organic compounds.

[0021] The mixed solvent preferably contains an inorganic carbonate, an inorganic hydroxide, or an inorganic salt. Examples of inorganic carbonates that can be used include sodium carbonate, potassium carbonate, and calcium carbonate. Examples of inorganic hydroxides that can be used include sodium hydroxide, potassium hydroxide, and calcium hydroxide. Examples of inorganic salts that can be used include carbonates, nitrates, and sulfates. If such salts are not present in the mixed solvent, the ionic conductivity of the carbon dioxide absorbing / reducing solution may be low with only 0.01 to 100 mM of the metal complex, making it difficult for the electrolytic reaction to occur. In contrast, the presence of such salts in the mixed solvent can maintain the ionic conductivity necessary to promote the reduction reaction by electrolysis of carbon dioxide in the carbon dioxide absorbing / reducing solution.

[0022] <Carbon dioxide absorption and reduction device according to one embodiment of the present disclosure> As shown in Fig. 1, a carbon dioxide absorption / reduction device 1 according to an embodiment of the present disclosure includes an electrolyzer 3 that contains the above-described carbon dioxide absorbing / reducing solution 2. The interior of the electrolyzer 3 is divided into a first chamber 3a and a second chamber 3b by a diaphragm 4. A cathode 5 is provided so as to be immersed in the carbon dioxide absorbing / reducing solution 2 in the first chamber 3a, and an anode 6 is provided so as to be immersed in the carbon dioxide absorbing / reducing solution 2 in the second chamber 3b. The cathode 5 and the anode 6 are each electrically connected to a DC power source 7.

[0023] The electrolyzer 3 is not limited to the configuration shown in FIG. 1 , i.e., a configuration in which the cathode 5 and the anode 6 are each immersed in the carbon dioxide absorption / reduction solution 2 in the electrolytic cell. Other configurations include an electrolytic cell, a power source electrically connected to the electrolytic cell, piping for supplying liquid or gaseous reactants to the electrolytic cell, and piping for discharging the product. Another example is an electrolytic cell including an anode section, a cathode section, an electrolyte section, and an electrolytic cell container equipped with an anode terminal and a cathode terminal, with the anode section electrically connected to the anode terminal and the cathode section electrically connected to the cathode terminal disposed opposite each other within the electrolytic cell via the electrolyte section. Another example is a system in which the anode section and the cathode section are formed from a single member, and a catalytic electrode layer for promoting the electrolysis reaction is provided on a power supply substrate. Another example is a system in which a diaphragm for separating the product from the anode section from the product from the cathode section is disposed in the electrolyte section between the anode section and the cathode section. The diaphragm may be a porous membrane permeated with an electrolytic solution or an ion-permeable non-porous membrane. As another example, the anode and cathode parts may be spaced apart from the diaphragm or may be in contact with the diaphragm and joined together. As yet another example, the battery may have an electrode assembly membrane in which electrodes (cathode 5 and anode 6) are joined to an ion exchange membrane.

[0024] The operation of the carbon dioxide absorption / reduction device 1 will be described later; carbon dioxide dissolved in the carbon dioxide absorption / reduction solution 2 is reduced by electrolysis to produce at least one of carbon monoxide and formic acid in the first chamber 3a, and oxygen in the second chamber 3b. Connected to the carbon dioxide absorption / reduction device 1 are first outlet lines 8a and 8b through which carbon monoxide and formic acid, respectively, flow out of the first chamber 3a, and a second outlet line 9 through which oxygen flows out of the second chamber 3b. The first outlet line 8a is provided to communicate with the gas phase portion of the first chamber 3a so that the carbon monoxide produced in the first chamber 3a can flow out of the first chamber 3a. A first outflow line 8b is provided to communicate with the liquid phase portion of the first chamber 3a so that the carbon dioxide absorbing / reducing solution 2 containing formic acid produced in the first chamber 3a can flow out from the first chamber 3a, and the first outflow line 8b may be provided with a separation device 10, such as a distillation device or a membrane separation device, that separates formic acid from the carbon dioxide absorbing / reducing solution 2 that has flowed out from the first chamber 3a, and a return line 11 may be provided to return the carbon dioxide absorbing / reducing solution 2 from which formic acid has been separated in the separation device 10 to the first chamber 3a. Note that if only carbon monoxide and not formic acid is produced in the first chamber 3a, there is no need to provide the separation device 10.

[0025] When reducing carbon dioxide in a batchwise manner, such as when the carbon dioxide absorbing / reducing solution 2 having dissolved carbon dioxide is supplied to the electrolyzer 3 for electrolysis, or when a gas containing carbon dioxide gas is supplied to the carbon dioxide absorbing / reducing solution 2 supplied to the electrolyzer 3 to dissolve the carbon dioxide and then decompose it, the carbon dioxide absorbing / reducing device 1 has the above-described configuration. However, in order to continuously supply carbon dioxide to the electrolyzer 3 and perform continuous reduction of carbon dioxide, it is necessary to provide a supply line 12 for supplying the carbon dioxide absorbing / reducing solution 2 having dissolved carbon dioxide to the first chamber 3a and an outlet line 13 for allowing the carbon dioxide absorbing / reducing solution 2 to flow out from the first chamber 3a, both of which communicate with the first chamber 3a.

[0026] As an apparatus for dissolving carbon dioxide in the carbon dioxide absorbing / reducing solution 2, for example, an absorption tower 14 configured to bring a gas containing carbon dioxide into gas-liquid contact with the carbon dioxide absorbing / reducing solution 2 as an absorbing liquid can be provided, although it is not limited thereto. A gas supply line 15 for supplying a gas containing carbon dioxide, such as a combustion gas, to the absorption tower 14, and a gas outlet line 16 for allowing the gas from which carbon dioxide has been removed to flow out from the absorption tower 14 are connected to the absorption tower 14. The supply line 12 is connected to the bottom of the absorption tower 14, and the outlet line 13 is connected to the absorption tower 14 above the position where the gas supply line 15 is connected to the absorption tower 14. The supply line 12 and the outlet line 13 are provided with pumps 17 and 18, respectively.

[0027] 2 and 3 show several forms of the carbon dioxide absorption / reduction apparatus 1 configured to reduce carbon dioxide in a batchwise manner as described above. The carbon dioxide absorption / reduction apparatus 1 shown in FIG. 2 includes, as a carbon dioxide capture apparatus 20 that captures carbon dioxide from a gas containing carbon dioxide, an absorption tower 14 configured to bring the gas containing carbon dioxide into gas-liquid contact with an absorbing liquid, and a regeneration tower 21 that releases carbon dioxide from the absorbing liquid that has absorbed carbon dioxide in the absorption tower 14. Unlike the carbon dioxide absorption / reduction apparatus 1 shown in FIG. 1, the absorbing liquid used in the absorption tower 14 is a different absorbing liquid from the carbon dioxide absorbing / reducing solution 2. The absorption tower 14 and the regeneration tower 21 are connected by a supply line 12.

[0028] The regeneration tower 21 is provided with a heat exchanger (reboiler) 23 including a heat medium flow passage 22 through which a heat medium (e.g., steam) flows and an absorption liquid circulation passage 24 through which the absorption liquid in the regeneration tower 21 circulates so that it flows out of the regeneration tower 21 and returns to the regeneration tower 21, and is configured so that heat exchange occurs between the heat medium flowing through the heat medium flow passage 22 and the absorption liquid flowing through the absorption liquid circulation passage 24. An extraction line 25 for extracting the absorption liquid is connected to the bottom of the regeneration tower 21. One end of a gas supply line 26, the other end of which is connected to the bottom of the first chamber 3a of the electrolyzer 3, is connected to the top of the regeneration tower 21, and a compressor 27 is provided on the gas supply line 26.

[0029] The carbon dioxide capture device 20 is not limited to the above-described configuration including the absorption tower 14 and the regeneration tower 21. Any configuration may be used as long as it can capture carbon dioxide from a gas containing carbon dioxide and supply the captured carbon dioxide to the carbon dioxide absorption / reduction solution 2 in the first chamber 3a of the electrolysis device 3. For example, the carbon dioxide absorption / reduction device 1 shown in FIG. 3 includes, as the carbon dioxide capture device 20, a membrane separation device 30 capable of separating carbon dioxide. A gas outflow line 16 is connected to the membrane separation device 30, through which the gas from which carbon dioxide has been separated flows out of the membrane separation device 30. A gas supply line 26 connects the membrane separation device 30 to the bottom of the first chamber 3a so that the separated carbon dioxide can be supplied into the first chamber 3a. A compressor 27 is provided in the gas supply line 26.

[0030] <Operation of the carbon dioxide absorption and reduction device according to one embodiment of the present disclosure> Next, the operation of the carbon dioxide absorbing / reducing solution 2 (carbon dioxide absorption / reduction method) will be described with reference to FIG. 1 . A gas containing carbon dioxide, for example, combustion gas discharged from a combustion facility (not shown), is supplied to the absorption tower 14 via a gas supply line 15. The combustion gas supplied to the absorption tower 14 rises within the absorption tower 14. The carbon dioxide absorbing / reducing solution 2 is supplied as an absorbing liquid to the absorption tower 14 via an outlet line 13. The absorbing liquid supplied to the absorption tower 14 falls within the absorption tower 14. Within the absorption tower 14, the rising combustion gas comes into gas-liquid contact with the falling absorbing liquid, whereby the carbon dioxide contained in the combustion gas is absorbed by the absorbing liquid, and the combustion gas from which the carbon dioxide has been removed flows out of the absorption tower 14 via a gas outlet line 16. The absorbing liquid that has absorbed the carbon dioxide remains in the lower part of the absorption tower 14, but is extracted from the absorption tower 14 by a pump 17 and supplied to the first chamber 3 a of the electrolyzer 3 via a supply line 12.

[0031] When DC power supply 7 applies a voltage between cathode 5 and anode 6 in electrolyzer 3, at least one of carbon monoxide and formic acid is produced in first chamber 3a by the catalytic action of the metal complex dissolved in carbon dioxide absorbing and reducing solution 2, as shown in the following half-reactions (A) and (B). Meanwhile, oxygen is produced in second chamber 3b as shown in the following half-reaction (C). CO2+H2O+2e - →CO+2OH - (A) CO2+2H2O+2e - →HCOOH+2OH - (B) 2OH - →(1 / 2)O2+H2O+2e - (C)

[0032] It is rare for only one of half-reactions (A) or (B) to occur; usually, both reactions occur simultaneously, and the ratio of the two reactions varies depending on the type of metal complex used. That is, the ratio of carbon monoxide and formic acid produced varies depending on the type of metal complex used.

[0033] The carbon monoxide produced in the first chamber 3a flows out of the first chamber 3a via a first outlet line 8a and is sent to a facility that uses the carbon monoxide or a carbon monoxide storage facility, etc. The formic acid produced in the first chamber 3a flows out of the first chamber 3a together with the carbon dioxide absorbing and reducing solution 2 via a first outlet line 8b, and the formic acid is separated from the carbon dioxide absorbing and reducing solution 2 in a separator 10, and the formic acid is sent to a facility that uses the formic acid or a formic acid storage facility, etc. The carbon dioxide absorbing and reducing solution 2 from which the formic acid has been separated in the separator 10 can be returned to the first chamber 3a via a return line 11. The oxygen produced in the second chamber 3b flows out of the second chamber 3b via a second outlet line 9 and is sent to a facility that uses the oxygen or an oxygen storage facility, etc.

[0034] Pump 18 causes a portion of the carbon dioxide absorbing / reducing solution 2 in first chamber 3a to flow out of first chamber 3a via outflow line 13. The carbon dioxide absorbing / reducing solution 2 flowing through outflow line 13 is supplied to absorption tower 14, falls within absorption tower 14, and comes into gas-liquid contact with the combustion gas rising within absorption tower 14 as an absorption liquid.

[0035] In this way, by dissolving a water-soluble metal complex as a catalyst for carbon dioxide absorption and reduction in a mixed solvent of water and a water-soluble solvent, it is possible to suppress precipitation of the metal complex, thereby improving the efficiency of carbon dioxide reduction. The effect of suppressing such precipitation of the metal complex will be explained based on the following examples.

[0036] In the carbon dioxide absorption / reduction apparatus 1 shown in FIG. 2, a gas containing carbon dioxide comes into gas-liquid contact with an absorbing liquid in an absorption tower 14, causing the absorbing liquid to absorb carbon dioxide. The absorbing liquid that has absorbed carbon dioxide is supplied to a regeneration tower 21 via a supply line 12. In the regeneration tower 21, the absorbing liquid is heated in a heat exchanger 23, causing the carbon dioxide to be released. The released carbon dioxide is supplied into the first chamber 3a via a gas supply line 26 by a compressor 27. At least a portion of the carbon dioxide supplied into the first chamber 3a is dissolved in the carbon dioxide absorbing / reducing solution 2. After a certain amount of carbon dioxide has dissolved in the carbon dioxide absorbing / reducing solution 2, a DC power supply 7 applies a voltage between the cathode 5 and the anode 6, and the carbon dioxide dissolved in the carbon dioxide absorbing / reducing solution 2 is reduced according to the principle described above.

[0037] In the carbon dioxide absorption / reduction device 1 shown in Fig. 3, carbon dioxide is separated from a gas containing carbon dioxide in a membrane separation device 30. The separated carbon dioxide is supplied into the first chamber 3a via a gas supply line 26 by a compressor 27. At least a portion of the carbon dioxide supplied into the first chamber 3a is dissolved in the carbon dioxide absorbing / reducing solution 2. After a certain amount of carbon dioxide has been dissolved in the carbon dioxide absorbing / reducing solution 2, a DC power supply 7 applies a voltage between the cathode 5 and the anode 6, and the carbon dioxide dissolved in the carbon dioxide absorbing / reducing solution 2 is reduced according to the above-mentioned principle. [Example]

[0038] An experiment was conducted to check the presence or absence of precipitation for the following metal complexes (3) and (4). Note that metal complex (3) corresponds to molecular structure (1), but metal complex (4) does not correspond to molecular structure (1) because it does not contain a hydroxyalkyl group in the nitrogen atom-containing heterocycle.

[0039] [ka]

[0040] [ka]

[0041] As shown in Table 1 below, Examples 1 to 5, in which metal complex (3) was dissolved in various liquid organic compounds, Comparative Examples 1 and 2, in which metal complex (4) was dissolved, and Comparative Example 3, in which metal complex (3) was dissolved, were prepared as mixed solutions of predetermined concentrations, i.e., mixed solvents of water and a water-soluble solvent. Each metal complex was added to the target concentration for each Example or Comparative Example, and the solution was stirred with a stirrer to confirm whether the metal complex could be dissolved. Note that Examples 1 to 5 and Comparative Example 1 contain a trace amount of water. After confirming that the metal complex had completely dissolved in each solution, carbon dioxide-containing gas (carbon dioxide concentration: 10 vol%) was blown into each solution to confirm whether the complex precipitated.

[0042] [Table 1]

[0043] In all of Examples 1 to 5, no precipitation of the metal complex was observed. In contrast, in Comparative Example 1, in which metal complex (4) not having water-soluble properties was used, no precipitation of the metal complex was observed because water was not mixed into the organic compound. However, in Comparative Example 2, in which conditions allowed water to be mixed into the organic compound, precipitation of the metal complex was observed. Moreover, in Comparative Example 2, precipitation of the metal complex was observed even though the concentration of the metal complex was reduced to make it difficult for the metal complex to precipitate. In Example 3, in which metal complex (3) was used, precipitation of the metal complex was observed. These results demonstrate that the use of the carbon dioxide absorption / reduction solution of the present disclosure can suppress precipitation of the metal complex.

[0044] As in Example 1, under conditions in which water and ethanol coexist, the complexes with Re as the central metal change into a total of six types of complexes due to the equilibrium reactions of each complex formed, as shown below. This makes it possible to suppress or prevent precipitation of the metal complexes. Based on this mechanism, by using a water-soluble metal complex under conditions in which water and a water-soluble solvent coexist, carbon dioxide can be added to the metal complex without precipitation.

[0045] [ka]

[0046] The contents described in each of the above embodiments can be understood, for example, as follows.

[0047] [1] A carbon dioxide absorption / reduction solution according to one embodiment is A carbon dioxide absorption / reduction solution (2) containing 0.01 to 100 mM of a metal complex in a mixed solvent of water and a water-soluble solvent, The metal complex is a central metal that is either rhenium, manganese, or iron; a ligand coordinated to the central metal; Including, The ligand comprises two or more carbonyl groups and two or more nitrogen-containing heterocycles, at least one of which has at least one substituent comprising a carboxy group or a hydroxy group.

[0048] According to the carbon dioxide absorption / reduction solution of the present disclosure, by dissolving a water-soluble metal complex as a catalyst for carbon dioxide absorption / reduction in a mixed solvent of water and a water-soluble solvent, precipitation of the metal complex can be suppressed.

[0049] [2] A carbon dioxide absorbing / reducing solution according to another embodiment is the carbon dioxide absorbing / reducing solution according to [1], The ligand comprises two or more carbonyl groups and two or more nitrogen atom-containing heterocycles, at least one of which is at least one hydroxymethyl group, hydroxyethyl group, or carboxy group.

[0050] The presence of a hydroxy group at the end of the side chain of the nitrogen-containing heterocycle of the ligand coordinated to the central metal makes the metal complex water-soluble, but as the number of carbon atoms in the carbon chain increases, the metal complex loses its water-solubility. In contrast, if the hydroxyalkyl group is a hydroxymethyl group, a hydroxyethyl group, or a carboxy group, as in the structure [2] above, the metal complex becomes water-soluble and can suppress precipitation of the metal complex.

[0051] [3] The carbon dioxide absorption / reduction solution according to one embodiment is A carbon dioxide absorption / reduction solution containing 0.01 to 100 mM of a metal complex in a mixed solvent of water and a water-soluble solvent, The metal complex is Ruthenium and A ligand that coordinates to ruthenium Including, The ligand contains two or more carbonyl groups and two or more nitrogen atom-containing heterocycles.

[0052] According to the carbon dioxide absorption / reduction solution of the present disclosure, by dissolving a water-soluble metal complex as a catalyst for carbon dioxide absorption / reduction in a mixed solvent of water and a water-soluble solvent, precipitation of the metal complex can be suppressed.

[0053] [4] A carbon dioxide absorbing / reducing solution according to another embodiment is any one of the carbon dioxide absorbing / reducing solutions according to [1] to [3], The water-soluble solvent is a liquid containing an organic compound having a hydroxyl group.

[0054] According to this configuration, the water-soluble metal complex dissolves in the mixed solvent, so that precipitation of the metal complex can be suppressed.

[0055] [5] A carbon dioxide absorbing / reducing solution according to yet another embodiment is the carbon dioxide absorbing / reducing solution according to [4], The organic compound is methyl alcohol, ethyl alcohol, 1-propyl alcohol, or 2-propyl alcohol.

[0056] According to this configuration, the water-soluble metal complex dissolves in the mixed solvent, so that precipitation of the metal complex can be suppressed.

[0057] [6] A carbon dioxide absorbing / reducing solution according to yet another embodiment is the carbon dioxide absorbing / reducing solution according to [4], The organic compound is an alkanolamine.

[0058] According to this configuration, carbon dioxide can be efficiently reduced by using an alkanolamine capable of absorbing carbon dioxide as the water-soluble solvent.

[0059] [7] A carbon dioxide absorbing / reducing solution according to yet another embodiment is any one of the carbon dioxide absorbing / reducing solutions [1] to [6], The mixed solvent includes an inorganic carbonate, an inorganic hydroxide, or an inorganic salt.

[0060] According to this configuration, by adding an inorganic carbonate, an inorganic hydroxide, or an inorganic salt to the mixed solvent, it is possible to maintain the ionic conductivity necessary for progressing the reduction reaction of carbon dioxide in the carbon dioxide absorption-reduction solution by electrolysis.

[0061] [8] A carbon dioxide absorbing / reducing solution according to yet another embodiment is any one of the carbon dioxide absorbing / reducing solutions according to [1] to [7], The water-soluble solvent comprises an amino acid.

[0062] According to this configuration, the water-soluble metal complex dissolves in the mixed solvent, so that precipitation of the metal complex can be suppressed.

[0063] [9] A carbon dioxide absorption / reduction device according to one aspect includes: The apparatus includes an electrolyzer (3) containing a carbon dioxide absorption / reduction solution (2) according to any one of [1] to [8].

[0064] According to the carbon dioxide absorption / reduction device of the present disclosure, by dissolving a water-soluble metal complex as a catalyst for carbon dioxide absorption / reduction in a mixed solvent of water and a water-soluble solvent, it is possible to suppress precipitation of the metal complex, thereby improving the efficiency of carbon dioxide reduction.

[0065]

[10] A carbon dioxide absorption / reduction device according to another embodiment is the carbon dioxide absorption / reduction device according to [9], a supply line (12) for supplying the carbon dioxide absorbing / reducing solution having absorbed carbon dioxide into the electrolyzer (3); an outflow line (13) through which the carbon dioxide absorbing and reducing solution (2) flows out from the electrolysis device (3); Equipped with.

[0066] With this configuration, carbon dioxide absorption and reduction can be carried out continuously.

[0067]

[11] A carbon dioxide absorption / reduction device according to yet another embodiment is the carbon dioxide absorption / reduction device according to

[10] , an absorption tower (14) for bringing a gas containing carbon dioxide into contact with the carbon dioxide absorbing / reducing solution to absorb carbon dioxide; The carbon dioxide absorbing / reducing solution that has absorbed carbon dioxide in the absorption tower (14) is supplied into the electrolyzer (3) through the supply line (12), and the carbon dioxide absorbing / reducing solution (2) that has flowed out of the electrolyzer (3) through the outflow line (13) is supplied to the absorption tower (14) and comes into contact with the gas.

[0068] According to this configuration, it is possible to continuously reduce the carbon dioxide recovered from the gas containing carbon dioxide.

[0069]

[12] A carbon dioxide absorption / reduction device according to yet another embodiment is the carbon dioxide absorption / reduction device according to [9], a carbon dioxide recovery device (20) that recovers carbon dioxide from a gas containing carbon dioxide; a gas supply line (26) for supplying the carbon dioxide recovered in the carbon dioxide recovery device (20) to the carbon dioxide absorbing and reducing solution (2) contained in the electrolyzer (3); Equipped with.

[0070] According to this configuration, the carbon dioxide recovered from the gas containing carbon dioxide can be reduced in a batchwise manner.

[0071]

[13] A carbon dioxide absorption / reduction device according to yet another embodiment is the carbon dioxide absorption / reduction device according to

[12] , The carbon dioxide recovery device (20) an absorption tower (14) that brings the gas containing carbon dioxide into contact with an absorption liquid to absorb the carbon dioxide into the absorption liquid; a regeneration tower (21) for releasing carbon dioxide from the absorbing solution that has absorbed carbon dioxide; Equipped with The carbon dioxide released in the regeneration tower (21) is supplied to the carbon dioxide absorbing and reducing solution (2) contained in the electrolyzer (3) via the gas supply line (26).

[0072] According to this configuration, the carbon dioxide recovered from the gas containing carbon dioxide can be reduced in a batchwise manner.

[0073]

[14] A carbon dioxide absorption / reduction method according to one embodiment includes: A step of supplying carbon dioxide to the carbon dioxide absorption / reduction solution (2) of any one of [1] to [8]; Electrolyzing the carbon dioxide absorbing and reducing solution (2) to which carbon dioxide has been supplied; Includes.

[0074] According to the carbon dioxide absorption / reduction method of the present disclosure, by dissolving a water-soluble metal complex as a catalyst for carbon dioxide absorption / reduction in a mixed solvent of water and a water-soluble solvent, it is possible to suppress precipitation of the metal complex, thereby improving the efficiency of carbon dioxide reduction.

[0075]

[15] A carbon dioxide absorption / reduction method according to another embodiment is the carbon dioxide absorption / reduction method according to

[14] , At least one of carbon monoxide and formic acid is produced by electrolyzing the carbon dioxide absorbing and reducing solution (2).

[0076] According to this method, carbon dioxide is reduced to produce at least one of carbon monoxide and formic acid, thereby converting the carbon dioxide into a valuable resource for use. [Explanation of symbols]

[0077] 1. Carbon dioxide absorption and reduction device 2. Carbon dioxide absorption and reduction solution 3. Electrolyzer 12 Supply Line 13 Outflow Line 14 Absorption tower 20 Carbon dioxide capture equipment 21 Regeneration Tower 26 Gas supply line

Claims

1. A carbon dioxide absorption / reduction solution containing 0.01 to 100 mM of a metal complex in a mixed solvent of water and a water-soluble solvent, The metal complex has the following structure: A carbon dioxide absorption / reduction solution, wherein the concentration of the water-soluble solvent in the mixed solvent is 10% by mass or more and 50% by mass or less.

2. The carbon dioxide absorption / reduction solution according to claim 1 , wherein the water-soluble solvent is a liquid containing an organic compound having a hydroxyl group.

3. The carbon dioxide absorption / reduction solution according to claim 2, wherein the organic compound is methyl alcohol, ethyl alcohol, 1-propyl alcohol, or 2-propyl alcohol.

4. The carbon dioxide absorption / reduction solution according to claim 2 , wherein the organic compound is an alkanolamine.

5. The carbon dioxide absorption / reduction solution according to any one of claims 1 to 4, wherein the mixed solvent contains an inorganic carbonate, an inorganic hydroxide, or an inorganic salt.

6. The carbon dioxide absorption / reduction solution according to any one of claims 1 to 5, wherein the water-soluble solvent contains an amino acid.

7. A carbon dioxide absorption / reduction device comprising an electrolysis device containing the carbon dioxide absorption / reduction solution according to any one of claims 1 to 6.

8. a supply line for supplying the carbon dioxide absorbing / reducing solution having absorbed carbon dioxide into an electrolysis device; an outflow line through which the carbon dioxide absorption / reduction solution flows out from the electrolysis device; The carbon dioxide absorption / reduction device according to claim 7, comprising:

9. An absorption tower is further provided in which a gas containing carbon dioxide is brought into contact with the carbon dioxide absorbing / reducing solution to absorb carbon dioxide into the carbon dioxide absorbing / reducing solution, 9. The carbon dioxide absorption / reduction device according to claim 8, wherein the carbon dioxide absorbing / reducing solution that has absorbed carbon dioxide in the absorption tower is supplied into the electrolyzer via the supply line, and the carbon dioxide absorbing / reducing solution that has flowed out from the electrolyzer via the outflow line is supplied to the absorption tower and comes into contact with the gas.

10. a carbon dioxide capture device that captures carbon dioxide from a gas containing carbon dioxide; a gas supply line that supplies the carbon dioxide recovered in the carbon dioxide recovery device to the carbon dioxide absorbing and reducing solution contained in the electrolyzer; The carbon dioxide absorption / reduction device according to claim 7, comprising:

11. The carbon dioxide capture device an absorption tower in which the gas containing carbon dioxide is brought into contact with an absorption liquid to absorb the carbon dioxide into the absorption liquid; a regeneration tower that releases carbon dioxide from the absorption liquid that has absorbed carbon dioxide; Equipped with The carbon dioxide absorption / reduction device according to claim 10, wherein the carbon dioxide released in the regeneration tower is supplied to the carbon dioxide absorption / reduction solution contained in the electrolysis device via the gas supply line.

12. A step of supplying carbon dioxide to the carbon dioxide absorbing / reducing solution according to any one of claims 1 to 6; Electrolyzing the carbon dioxide absorbing and reducing solution supplied with carbon dioxide; A method for reducing carbon dioxide, comprising:

13. 13. The carbon dioxide reduction method according to claim 12, wherein at least one of carbon monoxide and formic acid is produced by electrolyzing the carbon dioxide absorbing and reducing solution.

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