Electrochemical reactor and artificial photosynthesis apparatus

Abstract

To provide an electrochemical reactor comprising an electrode for reduction reaction and an electrode for oxidation reaction, in which deterioration in characteristics can be suppressed, stable operation can be maintained, energy consumption in recovering generated organic acids can be suppressed, and an amount of recovered organic acids can be improved.SOLUTION: An electrochemical reactor 3 comprises: an electrochemical reaction cell 10 in which an electrode 16 for reduction reaction containing a reduction catalyst and an electrode 18 for oxidation reaction containing an oxidation catalyst are arranged in a container 20 and which generates organic acids by reducing a reaction substrate; a tank 12 into which an electrolytic solution obtained by dissolving a salt having a pka of 6 to 8, a neutral salt, or both thereof is stored; electrolytic solution supply means which supplies the electrolytic solution from the tank 12 to the container 20; and a liquid-liquid extraction device 14 which extracts an organic acid into an organic solvent containing at least one of a secondary amine and a tertiary amine from an electrolytic solution which is discharged from the container 20 and in which an organic acid produced by reduction of a reaction substrate is dissolved.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an electrochemical reactor including an electrode for a reduction reaction and an electrode for an oxidation reaction, and an artificial photosynthesis device including the electrochemical reactor. [Background technology]

[0002] Artificial photosynthesis, which uses solar energy to synthesize formic acid (HCOOH) and carbon monoxide (CO), is expected to be one of the technologies to reduce carbon dioxide emissions to prevent global warming, and research into it is being actively conducted. In the artificial photosynthetic reaction, water is oxidized at the oxidation reaction electrode to supply electrons and protons, and oxygen (O2) is generated. Carbon dioxide is reduced at the reduction reaction electrode using these electrons and protons.

[0003] For example, Non-Patent Documents 1 and 2 describe a large-scale artificial photosynthesis system that generates formic acid by pumping an electrolyte solution in which carbon dioxide is saturated and dissolved in a phosphate buffer solution in a tank to an artificial photosynthesis cell equipped with an anode (electrode for oxidation reaction) and a cathode (electrode for reduction reaction). In this large-scale artificial photosynthesis system, the electrolyte solution with formic acid dissolved in it is discharged from the artificial photosynthesis cell, returned to the tank, and then supplied to the artificial photosynthesis cell again, in a circulatory system.

[0004] Patent Document 1 describes a reduction product production system that has an oxidation reaction electrolytic cell equipped with an oxidation catalyst and a reduction reaction electrolytic cell equipped with a reduction catalyst, supplies an electrolyte solution in which carbon dioxide is dissolved to the reduction reaction electrolytic cell, and produces alcohol by reducing carbon dioxide. Patent Document 1 also describes a distillation column as a method for separating the produced alcohol (see FIG. 1 of Patent Document 1).

[0005] Patent Document 2 describes a method for recovering formic acid, which includes a recovery step of recovering formic acid from an aqueous solution containing formic acid by using an alkylamine represented by the following formula (1) and an organic solvent, and a distillation step of distilling the organic solvent from the organic solvent layer containing formic acid obtained in the recovery step under reduced pressure or normal pressure. R 1 R 2 R 3 N (1) (In formula (1), R 1 ~R 3 At least two of the groups are independently alkyl groups having 6 to 12 carbon atoms, and the remaining groups are hydrogen atoms.

[0006] Patent Document 3 describes a method for concentrating a formic acid solution as a method for recovering formic acid in an aqueous solution, the method including a first step of contacting a basic solution containing formate ions with a cation exchange resin to obtain an acidic solution containing formate ions, a second step of contacting this acidic solution with an anion exchange resin to adsorb the formate ions onto the anion exchange resin, and a third step of eluting the adsorbed formate ions from the anion exchange resin with an eluent containing an acid.

[0007] In the large-scale artificial photosynthesis system described in Non-Patent Documents 1 and 2, in order to effectively utilize the electrolyte and the carbon dioxide dissolved in the electrolyte, the electrolyte is discharged from the tank to the artificial photosynthesis cell and then returned to the tank in a circulating manner. Since the formic acid generated by the reaction in the artificial photosynthesis cell is discharged in a dissolved state in the electrolyte, the formic acid concentration in the electrolyte increases with continued operation. However, if the formic acid concentration increases, there is a problem that the characteristics of the artificial photosynthesis cell are reduced due to the decomposition of formic acid at the oxidation reaction electrode and the inhibition of formic acid production at the reduction reaction electrode.

[0008] In Patent Document 1, a distillation column is used as a method for recovering the product alcohol, but distillation requires a lot of energy for heating, and considering that the boiling point of formic acid is 101° C., it is difficult to separate formic acid from water (boiling point 100° C.). Furthermore, most of the formic acid dissolved in the phosphate buffer solution, which is the electrolyte, becomes formate (potassium formate), so the amount recovered by distillation is significantly reduced.

[0009] In Patent Document 2, the alkylamine extractant does not form a salt with potassium formate in the electrolyte, so formate cannot be extracted, resulting in a significant decrease in the extraction rate of formic acid.

[0010] In Patent Document 3, in order to extract formic acid from a basic solution containing formate ions, the solution is made acidic using a strongly acidic cation exchange resin, and then an anion exchange resin is used to adsorb the formate ions onto the anion exchange resin. However, this requires complicated steps, and furthermore, a regeneration process is required to reuse the electrolyte. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] JP 2017-057491 A [Patent Document 2] JP 2022-138278 A [Patent Document 3] JP 2020-015684 A [Non-patent literature]

[0012] [Non-Patent Document 1] “Solar Fuel Production from CO2 Using a 1 m-Square-Sized Reactor with a Solar-to-Formate Conversion Efficiency of 10.5%”, ACS Sustainable Chem. Eng., 2021, 9, pp.16031-16037 [Non-Patent Document 2] "A large-sized cell for solar-driven CO2 conversion with a solar-to-formate conversion efficiency of 7.2%", Joule 2021, 5, pp.687-705 Summary of the Invention [Problem to be solved by the invention]

[0013] The object of the present invention is to provide an electrochemical reactor equipped with an electrode for a reduction reaction and an electrode for an oxidation reaction, which is capable of suppressing deterioration of characteristics, maintaining stable operation, suppressing energy consumption when recovering generated organic acid, and improving the amount recovered, and an artificial photosynthesis device equipped with the electrochemical reactor. [Means for solving the problem]

[0014] The present invention provides an electrochemical reactor comprising: an electrochemical reaction cell in which a reduction reaction electrode containing a reduction catalyst and an oxidation reaction electrode containing an oxidation catalyst are disposed in a container, and which produces an organic acid by reducing a reaction substrate; a tank for holding an electrolyte solution in which a salt with a pka of 6 to 8, a neutral salt, or both are dissolved; electrolyte solution supply means for supplying the electrolyte solution from the tank to the container; and a liquid-liquid extraction device for extracting the organic acid from the electrolyte solution discharged from the container, into an organic solvent containing at least one of a secondary amine and a tertiary amine, the organic acid being dissolved in the electrolyte solution produced by the reduction of the reaction substrate.

[0015] In the electrochemical reactor, the organic acid is preferably at least one of formic acid, acetic acid, and propionic acid.

[0016] In the electrochemical reactor, the salt having a pka of 6 to 8 is preferably at least one of potassium dihydrogen phosphate, sodium dihydrogen phosphate, lithium dihydrogen phosphate, calcium dihydrogen phosphate, magnesium dihydrogen phosphate, cesium dihydrogen phosphate, rubidium dihydrogen phosphate, magnesium bicarbonate, magnesium chloride, cesium chloride, rubidium chloride, magnesium nitrate, calcium nitrate, cesium nitrate, rubidium nitrate, potassium thiocyanate, sodium thiocyanate, lithium thiocyanate, magnesium thiocyanate, cesium thiocyanate, and rubidium thiocyanate.

[0017] In the electrochemical reactor, the neutral salt is preferably at least one of potassium sulfate, sodium sulfate, lithium sulfate, magnesium sulfate, calcium sulfate, potassium chloride, sodium chloride, lithium chloride, potassium bromide, sodium bromide, lithium bromide, magnesium bromide, potassium iodide, sodium iodide, lithium iodide, potassium nitrate, sodium nitrate, lithium nitrate, potassium perchlorate, sodium perchlorate, and lithium perchlorate.

[0018] In the electrochemical reactor, at least one of the secondary amine and the tertiary amine is preferably an alkylamine represented by the following formula (1). R 1 R 2 R 3 N (1) (In formula (1), R 1 ,R 2 ,R 3 At least two of the groups are independently an alkyl group having 6 to 12 carbon atoms, and the remaining groups are hydrogen atoms.

[0019] In the electrochemical reactor, the liquid-liquid extractor is preferably at least one of a mixer-settler type extractor, a column type extractor, and a centrifugal type extractor.

[0020] In the electrochemical reactor, it is preferable that the reaction substrate is carbon dioxide, and the organic acid produced by reduction of the reaction substrate is formic acid.

[0021] In the electrochemical reactor, the reduction catalyst is preferably a ruthenium complex polymer, and the oxidation catalyst is preferably iridium oxide.

[0022] It is preferable that the electrochemical reactor further comprises a circulation means for circulating the electrolytic solution to the tank after the electrolytic solution is discharged from the container of the electrochemical reaction cell.

[0023] The present invention is an artificial photosynthesis device comprising the electrochemical reactor and a solar cell that generates electric power to be supplied to the oxidation reaction electrode and the reduction reaction electrode. Effect of the Invention

[0024] The present invention makes it possible to provide an electrochemical reactor equipped with an electrode for a reduction reaction and an electrode for an oxidation reaction, which can suppress deterioration of characteristics, maintain stable operation, reduce energy consumption when recovering the generated organic acid, and improve the amount recovered, as well as an artificial photosynthesis device equipped with the electrochemical reactor. [Brief description of the drawings]

[0025] [Figure 1] FIG. 1 is a schematic configuration diagram showing an example of an artificial photosynthesis device including an electrochemical reactor according to an embodiment of the present invention. [Diagram 2] 1(a) to 1(e) are schematic configuration diagrams showing examples of electrochemical reaction cells in an electrochemical reactor according to an embodiment of the present invention. [Diagram 3] FIG. 11 is a schematic configuration diagram showing another example of an artificial photosynthesis device including an electrochemical reactor according to an embodiment of the present invention. [Figure 4] FIG. 1 shows the 1H-NMR spectrum of trioctylamine in Experiment 1. [Diagram 5] FIG. 1 shows 1H-NMR spectra when formic acid was extracted using Aqueous Solution 1 or Aqueous Solution 2 in Experiment 1. [Figure 6] FIG. 1 shows 1H-NMR spectra in the case where an extraction process was carried out using aqueous solutions 3 to 5 in Experiment 1. [Figure 7] Photographs showing the appearance of the triple mixer-settler device used in Experiment 2. (a) is a photograph showing each pump and each part of the mixer-settler bodies 1 to 3, and (b) is a photograph showing the tubes of the mixer parts 1 to 3. [Figure 8] Photographs showing the formation of a slug flow in Experiment 2. [Figure 9]Photographs showing continuous separation of the aqueous solution and the extractant due to two-phase separation in each settler section in Experiment 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes an embodiment of the present invention. The embodiment is merely an example of implementing the present invention, and the present invention is not limited to the embodiment.

[0027] The electrochemical reactor according to the present embodiment is an electrochemical reactor comprising: an electrochemical reaction cell in which a reduction reaction electrode containing a reduction catalyst and an oxidation reaction electrode containing an oxidation catalyst are arranged in a container, and which reduces a reaction substrate to produce an organic acid; a tank for holding an electrolyte in which a salt with a pka of 6 to 8, a neutral salt, or both are dissolved; an electrolyte supply means for supplying the electrolyte from the tank to the container; and a liquid-liquid extraction device for extracting the organic acid from the electrolyte in which the organic acid produced by the reduction of the reaction substrate is dissolved and discharged from the container, into an organic solvent containing at least one of a secondary amine and a tertiary amine. The electrochemical reactor can be, for example, a carbon dioxide reduction device, or can be an artificial photosynthesis device in which an electrochemical reactor and a solar cell are combined.

[0028] An electrochemical reactor according to an embodiment of the present invention and an example of an artificial-photosynthesis device including the electrochemical reactor are shown in FIG. 1, and the configuration thereof will be described.

[0029] The electrochemical reactor 3 includes, for example, an electrochemical reaction cell 10 in which a reduction reaction electrode 16 containing a reduction catalyst that causes a reduction reaction of a reaction substrate (e.g., carbon dioxide) and an oxidation reaction electrode 18 that is electrically connected to the reduction reaction electrode 16 and causes an oxidation reaction and contains an oxidation catalyst are disposed at positions facing each other at a distance from each other in a container 20, and an electrolytic solution containing a reaction substrate flows between the reduction reaction electrode 16 and the oxidation reaction electrode 18. The electrochemical reactor 3 includes a tank 12 for holding an electrolytic solution in which a salt with a pka of 6 to 8, a neutral salt, or both are dissolved; a pump 22 and an electrolytic solution supply pipe 24 as an electrolytic solution supply means for supplying the electrolytic solution from the tank 12 to the container 20 of the electrochemical reaction cell 10; and a liquid-liquid extraction device 14 for extracting an organic acid from the electrolytic solution discharged from the container 20 and containing an organic acid generated by the reduction of the reaction substrate, into an organic solvent containing at least one of a secondary amine and a tertiary amine as an extractant. The electrochemical reactor 3 may include a reactant supply source 30 that supplies reactant to the tank 12 or the like.

[0030] The artificial photosynthesis device 1 includes an electrochemical reactor 3 and a solar cell 34 that generates power supplied to the reduction reaction electrode 16 and the oxidation reaction electrode 18 of the electrochemical reactor 3.

[0031] In the electrochemical reactor 3, the electrolyte outlet of the tank 12 and the electrolyte inlet of the electrochemical reaction cell 10 are connected by an electrolyte supply pipe 24 via a pump 22. The electrolyte outlet of the electrochemical reaction cell 10 and the electrolyte inlet of the liquid-liquid extraction device 14 are connected by an electrolyte discharge pipe 26. The electrolyte outlet of the liquid-liquid extraction device 14 and the electrolyte inlet of the tank 12 are connected by a circulation pipe 28. The reaction substrate outlet of the reaction substrate supply source 30 and the reaction substrate inlet of the tank 12 are connected by a reaction substrate supply pipe 32. In the artificial photosynthesis device 1, the reduction reaction electrode 16 and the oxidation reaction electrode 18 of the electrochemical reactor 3 are electrically connected to the solar cell 34.

[0032] In the electrochemical reactor 3, the tank 12 contains an electrolyte in which a salt with a pka of 6 to 8, a neutral salt, or both of them are dissolved in a solvent such as water. A reactant (e.g., carbon dioxide) is supplied to the tank 12 from a reactant supply source 30 through a reactant supply pipe 32, and the reactant is dissolved in the electrolyte in the tank 12. The electrolyte in which the reactant and a salt with a pka of 6 to 8, a neutral salt, or both of them are dissolved and the reactant are supplied from the electrolyte outlet of the tank 12 through the electrolyte supply pipe 24 by a pump 22 to the container 20 from the electrolyte inlet of the electrochemical reaction cell 10, and is discharged from the electrolyte outlet of the container 20 through a flow path between the reduction reaction electrode 16 and the oxidation reaction electrode 18.

[0033] When an appropriate bias voltage is applied between the reduction reaction electrode 16 and the oxidation reaction electrode 18, an oxidation reaction occurs at the oxidation reaction electrode 18, for example, water (H2O) is oxidized to obtain oxygen (1 / 2O2) and electrons are generated. At the reduction reaction electrode 16, by receiving the electrons generated by the oxidation reaction, a reaction substrate such as carbon dioxide (CO2) is reduced to generate an organic acid such as formic acid (HCOOH) (electrochemical reaction process).

[0034] The electrolyte discharged from the electrolyte outlet of the container 20 of the electrochemical reaction cell 10, into which the organic acid generated by the reduction of the reaction substrate is dissolved, is sent to the liquid-liquid extraction device 14 from its electrolyte inlet through the electrolyte discharge pipe 26. In the liquid-liquid extraction device 14, the organic acid is extracted from the electrolyte into an organic solvent (extractant) containing at least one of a secondary amine and a tertiary amine (liquid-liquid extraction step). The electrolyte discharged from the electrolyte outlet of the liquid-liquid extraction device 14, from which the organic acid has been extracted and in which the amount of organic acid has been reduced, may be circulated to the tank 12 through the circulation pipe 28 (circulation step). For example, the pump 22, the electrolyte discharge pipe 26, the circulation pipe 28, etc. function as the circulation means.

[0035] In the electrochemical reactor according to the present embodiment, an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode uses an electrolyte solution containing a salt with a pka of 6 to 8 or a neutral salt that is unlikely to form an organic acid salt such as a formate, and is equipped with a liquid-liquid extraction device for recovering an organic acid such as formic acid from the electrolyte solution, thereby suppressing deterioration of characteristics, maintaining stable operation, suppressing energy consumption when recovering the generated organic acid, and improving the recovery amount. It becomes possible to easily extract an organic acid such as formic acid in the electrolyte solution and to reuse the electrolyte solution, and a stable operation can be maintained by suppressing an increase in the concentration of an organic acid such as formic acid in the electrolyte solution.

[0036] In the electrochemical reactor, the reaction substrate may be, for example, a carbon compound, such as carbon dioxide (CO2) or carbon monoxide (CO). The reaction substrate is preferably carbon dioxide in view of its solubility in the electrolyte.

[0037] In the electrochemical reactor, examples of organic acids generated by reduction of a reaction substrate include formic acid, acetic acid, propionic acid, etc. From the viewpoint of the selectivity of the reduction reaction, the organic acid is preferably at least one of formic acid, acetic acid, and propionic acid, and more preferably formic acid.

[0038] In the electrochemical reactor, the salt having a pka of 6 to 8 contained in the electrolyte is not particularly limited, and examples thereof include potassium dihydrogen phosphate, sodium dihydrogen phosphate, lithium dihydrogen phosphate, calcium dihydrogen phosphate, magnesium dihydrogen phosphate, cesium dihydrogen phosphate, rubidium dihydrogen phosphate, magnesium bicarbonate, magnesium chloride, cesium chloride, rubidium chloride, magnesium nitrate, calcium nitrate, cesium nitrate, rubidium nitrate, potassium thiocyanate, sodium thiocyanate, lithium thiocyanate, magnesium thiocyanate, cesium thiocyanate, and rubidium thiocyanate. The salt having a pka of 6 to 8 is preferably potassium dihydrogen phosphate in terms of solubility and the like. The salt having a pka of 6 to 8 may be used alone or in combination of two or more of these. The pka of the salt can be determined by a neutralization titration method.

[0039] In the electrochemical reactor, the neutral salt contained in the electrolyte may be an inorganic salt that exhibits a pH of 6.0 to 8.0 when dissolved in water, and is not particularly limited. Examples of the neutral salt include potassium sulfate, sodium sulfate, lithium sulfate, magnesium sulfate, calcium sulfate, potassium chloride, sodium chloride, lithium chloride, potassium bromide, sodium bromide, lithium bromide, magnesium bromide, potassium iodide, sodium iodide, lithium iodide, potassium nitrate, sodium nitrate, lithium nitrate, potassium perchlorate, sodium perchlorate, and lithium perchlorate. The neutral salt is preferably potassium sulfate in terms of solubility. The neutral salt may be used alone or in combination of two or more of these.

[0040] A salt having a pka of 6 to 8 may be used alone, a neutral salt may be used alone, or a salt having a pka of 6 to 8 and a neutral salt may be used in combination.

[0041] The solvent of the electrolyte is, for example, water such as pure water.

[0042] The concentration of the salt having a pka of 6 to 8, the neutral salt, or both in the electrolyte may be, for example, in the range of 0.01 M to 5.0 M, and is preferably in the range of 0.1 M to 1.0 M. If the concentration of the salt having a pka of 6 to 8, the neutral salt, or both in the electrolyte is less than 0.01 M, the conductivity may be insufficient, and if it exceeds 5.0 M, precipitation of the salt may occur.

[0043] At least one of the secondary amines and tertiary amines used in the liquid-liquid extraction device 14 is not particularly limited, and examples thereof include secondary amines such as didodecylamine and diundecylamine, tertiary amines such as trioctylamine and triheptylamine, and alkylamines represented by the following formula (1). One of these secondary amines and tertiary amines may be used alone, or two or more of them may be used in combination. R 1 R 2 R 3 N (1) (In formula (1), R 1 ,R 2 ,R 3 At least two of the groups are independently an alkyl group having 6 to 12 carbon atoms, and the remaining groups are hydrogen atoms.

[0044] Among these, the alkylamine represented by the above formula (1) is preferred. The alkylamine represented by the above formula (1) is a Bronsted base and is water-insoluble. Therefore, by contacting an organic solvent containing the alkylamine represented by the above formula (1) with an electrolyte containing an organic acid such as formic acid, the alkylamine accepts protons from the organic acid such as formic acid, which is a Bronsted acid, to form an organic acid salt such as a formate. Furthermore, the formed organic acid salt such as a formate can be easily separated from water by leaving it to stand. Therefore, the generated organic acid can be easily recovered without consuming a large amount of energy such as heating.

[0045] In the alkylamine represented by the above formula (1), R 1 ~R 3At least two of the alkyl groups are independently alkyl groups having 6 to 12 carbon atoms. The alkyl groups having 6 to 12 carbon atoms may be linear, branched, or cyclic. The alkyl groups are preferably alkyl groups having 6 or more carbon atoms, more preferably alkyl groups having 8 or more carbon atoms, and even more preferably alkyl groups having 8 carbon atoms, so that the amine can be easily separated from water. As the carbon number of the alkyl group increases, the volume of the solution after recovery increases, and the concentration of the organic acid such as formic acid decreases relatively, so that the alkyl groups are preferably alkyl groups having 10 or less carbon atoms. In addition, the alkyl groups are preferably linear alkyl groups because they preferably have a space in which the amine can interact with the organic acid such as formic acid to form a salt.

[0046] The alkylamine represented by the above formula (1) is R 1 ~R 3 two of R are independently an alkyl group having 6 to 12 carbon atoms, and the remaining one is a secondary amine having a hydrogen atom; 1 ~R 3 In order to facilitate separation of the amine from water, R 1 ~R 3 It is more preferable that all of are independently an alkyl group having 6 to 12 carbon atoms.

[0047] R 1 ~R 3 may all be different alkyl groups, or R 1 ~R 3 Two of the alkyl groups may be the same, or all of the alkyl groups may be the same. From the viewpoint of the cost of the amine, etc., 1 ~R 3 are preferably all the same alkyl group.

[0048] R 1 ~R 3 The total number of carbon atoms in R is preferably 12 or more, and more preferably 16 or more. 1 ~R3 If the total number of carbon atoms in R is 11 or less, the amount of amine dissolved in water increases, and the amount of organic acid such as formic acid extracted may decrease. In addition, from the viewpoint of operability, it is preferable that the compound is a liquid at room temperature and normal pressure. 1 ~R 3 The total number of carbon atoms is preferably 36 or less.

[0049] Among the alkylamines satisfying the above conditions, one kind may be used alone, or a plurality of kinds may be used in combination.

[0050] The equivalent of at least one of the secondary amines and tertiary amines used is preferably in the range of 0.5 equivalents or more and 10 equivalents or less with respect to the amount of organic acid in the aqueous solution containing formic acid to be recovered. From the viewpoint of the recovery rate of organic acid, etc., the equivalent of at least one of the secondary amines and tertiary amines used is more preferably 1 equivalent or more with respect to the amount of organic acid. Since using more than necessary does not contribute to the recovery of organic acid, the equivalent of at least one of the secondary amines and tertiary amines used is more preferably 5 equivalents or less with respect to the amount of organic acid, and even more preferably 2 equivalents or less.

[0051] The organic solvent used must be phase-separated from the electrolytic solution. Therefore, a solvent with low solubility in water is preferable. In addition, a solvent with a low boiling point is preferable so that it can be distilled off without consuming a large amount of energy. When the organic acid is formic acid, the boiling point of the organic solvent is preferably 80°C or less, more preferably 60°C or less, considering the fractional distillation with formic acid, which has a boiling point of 101°C. The lower limit of the boiling point of the organic solvent is preferably 25°C or more, more preferably 35°C or more, since it is desirable for the organic solvent to be liquid at room temperature. In addition, it is desirable for the organic solvent to dissolve the amine used together and also dissolve the amine containing the organic acid.

[0052] Considering the above, examples of the organic solvent to be used include cyclohexane (boiling point 81°C), benzene (81°C), carbon tetrachloride (77°C), diethyl ether (35°C), chloroform (62°C), ethyl acetate (77°C), dichloromethane (40°C), petroleum ether (30 to 60°C), hexane (69°C), pentane (69°C), methyl ethyl ketone (80°C), carbon disulfide (47°C), etc. Among these, dichloromethane and diethyl ether, which easily separate from water and have low boiling points, are preferred.

[0053] The organic solvent to be used may be one of the above alone or a combination of two or more of the above. It is preferable to use one of the above since it is easy to set the conditions for distilling off the organic solvent in the distillation step.

[0054] The amount of organic solvent used is not particularly limited as long as the amine used is dissolved and homogeneous. It is not necessary to use an unnecessarily large amount of organic solvent, and it is preferable to use as little as possible in consideration of the energy required for distillation in the distillation step. The amount of organic solvent used is preferably adjusted so that the concentration of the amine is in the range of 1 to 100 times the concentration of the organic acid in the electrolytic solution.

[0055] The liquid-liquid extractor 14 is not particularly limited as long as it can extract a liquid from a liquid, and examples thereof include a mixer-settler type extractor, a tower type extractor, and a centrifugal type extractor. Examples of tower type extractors include a plate tower, a packed tower, a pulsation tower, and a rotating disk type extractor. The liquid-liquid extractor is preferably a mixer-settler type extractor because it can reduce the ratio of the volume of the extractant to the volume of the electrolyte in which the organic acid is dissolved. The mixer-settler type extractor is a device that includes a mixer section that mixes two liquids (e.g., an aqueous phase and an oil phase) and a settler section that separates the two liquids by gravity, and performs specific gravity separation into two liquids while continuously performing solvent extraction. In the mixer section, it is preferable to generate a slug flow by a microchannel because it can shorten the extraction time.

[0056] 2(a), the electrochemical reaction cell 10 may be of a type in which a reduction reaction electrode 16 containing a reduction catalyst and an oxidation reaction electrode 18 containing an oxidation catalyst are disposed in a container at positions facing each other with a space therebetween, and a flow path 42 is provided between the reduction reaction electrode 16 and the oxidation reaction electrode 18 for flowing an electrolyte solution containing a reaction substrate such as carbon dioxide. Alternatively, the electrochemical reaction cell 10 may be of a type in which a reduction reaction electrode 16 containing a reduction catalyst and an oxidation reaction electrode 18 containing an oxidation catalyst are disposed in a container with an electrolyte membrane 44 sandwiched therebetween, and a flow path 42a is provided on the side opposite to the surface of the reduction reaction electrode 16 in contact with the electrolyte membrane 44, through which an electrolyte solution containing a reaction substrate such as carbon dioxide flows, and a flow path 42b is provided on the side opposite to the surface of the oxidation reaction electrode 18 in contact with the electrolyte membrane 44. Alternatively, the electrochemical reaction cell 10 may be of a type in which, for example, as shown in Fig. 2(c), a reduction reaction electrode 16 containing a reduction catalyst and an oxidation reaction electrode 18 containing an oxidation catalyst are disposed in a container at positions facing each other with a distance therebetween, a flow path 42 is provided through which an electrolyte flows between the reduction reaction electrode 16 and the oxidation reaction electrode 18, and a surface of the reduction reaction electrode 16 opposite to the surface in contact with the electrolyte is exposed to a gas phase to supply a reaction substrate such as carbon dioxide from the gas phase. In this case, for example, as shown in Fig. 2(d), an electrolyte membrane 44 may be disposed in contact with the electrolyte side surface of the reduction reaction electrode 16, or in contact with the electrolyte side surface of the oxidation reaction electrode 18, or between the reduction reaction electrode 16 and the oxidation reaction electrode 18. Alternatively, for example, as shown in FIG. 2( e), the electrochemical reaction cell 10 may be of a type in which a reduction reaction electrode 16 containing a reduction catalyst and an oxidation reaction electrode 18 containing an oxidation catalyst are arranged in a container with an electrolyte membrane 44 sandwiched therebetween, the surface of the reduction reaction electrode 16 opposite to the surface in contact with the electrolyte membrane 44 is exposed to a gas phase to supply a reaction substrate such as carbon dioxide from the gas phase, and a flow path 42 through which an electrolyte flows is provided on the surface of the oxidation reaction electrode 18 opposite to the surface in contact with the electrolyte membrane 44.

[0057] When supplying an electrolyte solution containing a reaction substrate such as carbon dioxide, the electrochemical reactor 3 may be configured such that the container 20 and the tank 12 of the electrochemical reaction cell 10 are separate as shown in FIG. 1, or the container 20 and the tank 12 may be configured as a single container.

[0058] In the electrochemical reactor 3, the liquid-liquid extractor 14 may be installed in-line between the electrolyte outlet of the container 20 of the electrochemical reaction cell 10 and the electrolyte inlet of the tank 12 as shown in FIG. 1, or may be installed in-line connected to the tank 12 as shown in FIG. 3.

[0059] In the electrochemical reactor 4 in the artificial photosynthesis apparatus 2 shown in FIG. 3, the first electrolyte outlet of the tank 12 and the electrolyte inlet of the electrochemical reaction cell 10 are connected by an electrolyte supply pipe 24 via a pump 22. The electrolyte outlet of the electrochemical reaction cell 10 and the first electrolyte inlet of the tank 12 are connected by a circulation pipe 36. The second electrolyte outlet of the tank 12 and the electrolyte inlet of the liquid-liquid extraction device 14 are connected by a pipe 38, and the electrolyte outlet of the liquid-liquid extraction device 14 and the second electrolyte inlet of the tank 12 are connected by a pipe 40. The reaction substrate outlet of the reaction substrate supply source 30 and the reaction substrate inlet of the tank 12 are connected by a reaction substrate supply pipe 32. In the artificial photosynthesis apparatus 2, the reduction reaction electrode 16 and the oxidation reaction electrode 18 of the electrochemical reactor 4 are electrically connected to the solar cell 34.

[0060] In the electrochemical reactor 4, the tank 12 contains an electrolyte in which a salt with a pka of 6 to 8, a neutral salt, or both of them are dissolved in a solvent such as water. A reactant (e.g., carbon dioxide) is supplied to the tank 12 from a reactant supply source 30 through a reactant supply pipe 32, and the reactant is dissolved in the electrolyte in the tank 12. The electrolyte in which the reactant and a salt with a pka of 6 to 8, a neutral salt, or both of them are dissolved and the reactant are supplied from the electrolyte outlet of the tank 12 through the electrolyte supply pipe 24 by a pump 22 to the container 20 from the electrolyte inlet of the electrochemical reaction cell 10, and is discharged from the electrolyte outlet of the container 20 through a flow path between the reduction reaction electrode 16 and the oxidation reaction electrode 18.

[0061] When an appropriate bias voltage is applied between the reduction reaction electrode 16 and the oxidation reaction electrode 18, an oxidation reaction occurs at the oxidation reaction electrode 18, for example, water (H2O) is oxidized to obtain oxygen (1 / 2O2) and electrons are generated. At the reduction reaction electrode 16, by receiving the electrons generated by the oxidation reaction, a reaction substrate such as carbon dioxide (CO2) is reduced to generate an organic acid such as formic acid (HCOOH) (electrochemical reaction process).

[0062] The electrolyte discharged from the electrolyte outlet of the container 20 of the electrochemical reaction cell 10 and dissolving the organic acid produced by the reduction of the reaction substrate may be circulated to the tank 12 through the circulation pipe 36 (circulation step). For example, the pump 22, the circulation pipe 36, etc. function as the circulation means. The electrolyte dissolving the organic acid is sent from the second electrolyte outlet of the tank 12 through the pipe 38 to the liquid-liquid extraction device 14 through the electrolyte inlet of the liquid-liquid extraction device 14. In the liquid-liquid extraction device 14, the organic acid is extracted from the electrolyte into an organic solvent (extractant) containing at least one of a secondary amine and a tertiary amine (liquid-liquid extraction step). The electrolyte discharged from the electrolyte outlet of the liquid-liquid extraction device 14, from which the organic acid has been extracted and in which the amount of organic acid has been reduced, is sent to the tank 12 through the pipe 40.

[0063] In the electrochemical reactor 3, as shown in Fig. 1, the electrolytic solution may be discharged from the container 20 of the electrochemical reaction cell 10 and circulated to the tank 12 after a liquid-liquid extraction process by the liquid-liquid extraction device 14, or may be discharged outside the system after a liquid-liquid extraction process by the liquid-liquid extraction device 14. In the electrochemical reactor 4, as shown in Fig. 3, the electrolytic solution may be discharged from the container 20 of the electrochemical reaction cell 10 and circulated to the tank 12 and further circulated to the tank 12 after a liquid-liquid extraction process by the liquid-liquid extraction device 14, or may be discharged outside the system after a liquid-liquid extraction process by the liquid-liquid extraction device 14. By circulating the electrolytic solution, it becomes easy to reuse the electrolytic solution.

[0064] (Electrode for reduction reaction) The reduction reaction electrode 16, which is a negative electrode (cathode electrode), is an electrode used to reduce a substance by a reduction reaction. The reduction reaction electrode 16 includes, for example, a conductive layer and a conductor layer formed in this order on a substrate.

[0065] The substrate is a member that structurally supports the reduction reaction electrode. The substrate is not particularly limited in material, but examples thereof include a glass substrate. The substrate may include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but examples thereof include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pb), and Ti (titanium). The semiconductor used as the substrate is not particularly limited, but examples thereof include titanium oxide (TiO2), tin oxide (SnO2), silicon (Si), strontium titanate (SrTiO3), zinc oxide (ZnO), and tantalum oxide (Ta2O5). When the substrate includes a metal or a semiconductor, an insulating layer may be formed between the conductive layer and the substrate. The insulating layer is not particularly limited, but examples thereof include an oxide, a nitride, and a resin of a semiconductor. The metal substrate may be electrically connected directly to the conductive layer and the conductor layer, or, if a metal substrate is used, the conductive layer may not be provided.

[0066] The substrate may contain carbon fiber. The substrate may be, for example, a composite substrate of carbon fiber and carbon heat-treated at high temperature, such as carbon paper and carbon cloth. Carbon paper is, for example, a composite substrate made of organic fibers such as polyacrylonitrile (PAN) fibers impregnated with a dispersion of polyvinyl alcohol and an aqueous medium, carbonized at about 2000°C, and bonded into a sheet. Carbon paper may contain about 25% by mass of Teflon (registered trademark)-based material. Carbon cloth is a woven material made of carbon fibers obtained by baking and carbonizing organic fibers. Carbon paper and carbon cloth are porous bodies and have countless pores of several tens of μm (about 10 μm to 100 μm). The thickness of carbon paper and carbon cloth is, for example, in the range of 0.1 mm to 0.4 mm per sheet. As this composite substrate, a multi-layered substrate (for example, 5 to 10 layers) having a thickness of about 10 times that of the original substrate, in the range of 1 mm to 4 mm, may be used. A multilayered composite substrate can be formed, for example, by laminating multiple pieces of carbon paper, carbon cloth, etc., using a carbon-based adhesive containing a conductive carbon material, such as a polymer containing graphite or graphene (e.g., an acrylic polymer binder) as the adhesive.

[0067] The conductive layer is provided to effectively collect current at the reduction reaction electrode. The conductive layer is not particularly limited, but examples thereof include transparent conductive layers such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, in consideration of thermal and chemical stability, it is preferable to use fluorine-doped tin oxide (FTO).

[0068] The conductor layer is composed of a conductor containing a material having a reduction catalyst function as a reduction catalyst. The conductor can be composed of a material containing a carbon material (C). The size of the single unit of the carbon material structure is preferably 1 nm or more and 1 μm or less. Examples of the carbon material include at least one of carbon nanotubes such as multi-wall carbon nanotubes (MWCNTs), graphene, and graphite. In the case of graphene and graphite, the size is preferably 1 nm or more and 1 μm or less. In the case of carbon nanotubes, the diameter is preferably 1 nm or more and 40 nm or less. The conductor can be formed, for example, by applying a carbon material mixed with a liquid such as ethanol by spraying and heating. Instead of spraying, it may be applied by spin coating. Also, it is possible to apply the solution by directly dripping it and drying it without using spin coating.

[0069] A complex catalyst or the like can be used as a material having a reduction catalyst function. For example, a ruthenium complex can be used as the complex catalyst. For example, the complex catalyst can be [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(MeCN)Cl2], [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2Cl2], [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2] nand [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(CH3CN)Cl2]. Furthermore, the ruthenium complex may be a Ru complex polymer in which a Ru complex containing a Ru complex monomer and a polymerization initiator (e.g., pyrrole and an oxidizing agent (e.g., an Fe-based oxidizing agent such as iron chloride)) is polymerized (polymerized). The reduction reaction electrode is, for example, an electrode containing a carbon-based material (carbon nanotubes such as multi-wall carbon nanotubes (MWCNTs) and graphite) and containing a Ru complex polymer as a reduction catalyst and an Fe-based oxidizing agent (FeCl3).

[0070] The support of the reduction catalyst can be prepared, for example, by dissolving a metal complex (catalyst) in an acetonitrile (MeCN) solution, coating the solution on a carbon-based material such as carbon paper or carbon cloth, and drying it. For example, the support of a Ru complex polymer can be prepared by dissolving a Ru complex monomer, a polymerization initiator (e.g., pyrrole and an oxidizing agent (e.g., an Fe-based oxidizing agent such as ferric chloride) in a solvent such as acetonitrile (MeCN), coating the solution (Ru complex polymer solution) on a carbon-based material such as carbon paper or carbon cloth, and drying it. The support of the reduction catalyst can also be performed by electrolytic polymerization. For example, a carbon-based material electrode can be used as the working electrode, a glass substrate covered with fluorine-containing tin oxide (FTO) as the counter electrode, and an Ag / Ag reference electrode can be used. + Using an electrode, Ag / Ag was placed in an electrolyte containing a reduction catalyst. + After applying a cathodic current to the electrode to create a negative voltage, the Ag / Ag + By passing an anode current so that the electrode has a positive potential, the carbon-based material can be supported as a reduction catalyst. For example, acetonitrile (MeCN) can be used as the electrolyte solution, and tetrabutylammonium perchlorate (TBAP) can be used as the electrolyte.

[0071] (Oxidation reaction electrode) The oxidation reaction electrode 18, which is a positive electrode (anode electrode), is an electrode used to oxidize a substance by an oxidation reaction. The oxidation reaction electrode 18 includes, for example, a base material having a conductive layer formed on a substrate and an oxidation catalyst layer formed thereon.

[0072] The substrate is a member that structurally supports the oxidation reaction electrode. The substrate is not particularly limited in material, but examples thereof include a glass substrate. The substrate may include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but examples thereof include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pb), titanium (Ti), etc. The semiconductor used as the substrate is not particularly limited, but examples thereof include titanium oxide (TiO2), tin oxide (SnO2), silicon (Si), strontium titanate (SrTiO3), zinc oxide (ZnO), tantalum oxide (Ta2O5), etc. When the substrate includes a metal or a semiconductor, an insulating layer may be formed between the conductive layer and the substrate. The insulating layer is not particularly limited, but examples thereof include an oxide, a nitride, or a resin of a semiconductor. The metal substrate and the conductive layer may be directly electrically connected. In order to make the oxidation reaction electrode 18 translucent, it is preferable that the substrate is, for example, a glass substrate or plastic. When a metal substrate is used, the conductive layer may not be included. A composite substrate of carbon fiber and carbon may be used as the substrate. This has the advantages of ensuring light weight and high conductivity, and eliminating the need for a separate conductive layer.

[0073] The conductive layer is provided to effectively collect electricity at the oxidation reaction electrode 18. The conductive layer is not particularly limited, but examples thereof include transparent conductive layers such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, in consideration of thermal and chemical stability, it is preferable to use fluorine-doped tin oxide (FTO).

[0074] The oxidation catalyst layer is configured to include a material having an oxidation catalyst function as an oxidation catalyst. Examples of the material having an oxidation catalyst function include a material containing iridium oxide (IrOx). Iridium oxide can be supported on the surface of the conductive layer as a nanocolloid solution (see T. Arai et.al, Energy Environ. Sci 8, 1998 (2015)).

[0075] For example, a nanocolloid of iridium oxide (IrOx) is synthesized. Next, a yellow solution adjusted to pH 13 by adding 10 wt% sodium hydroxide (NaOH) aqueous solution to 50 mL of 2 mM potassium chloroiridate (IV) (K2IrCl6) aqueous solution is heated at 90 °C for 20 minutes using a hot stirrer. The blue solution obtained is cooled in ice water for 1 hour. Then, 3 M nitric acid (HNO3) is dropped into the cooled solution (20 mL) to adjust the pH to 1, and the solution is stirred for 80 minutes to obtain a nanocolloidal solution of iridium oxide (IrOx). Furthermore, a 1.5 wt% NaOH aqueous solution (1-2 mL) is dropped into this solution to adjust the pH to 12. The nanocolloidal solution of iridium oxide (IrOx) obtained in this way is applied to a conductive layer at pH 12 and dried by keeping it in a drying oven at 60 °C for 40 minutes. After drying, the precipitated salt is washed with ultrapure water to form the oxidation reaction electrode 18. Note that the application and drying of the nanocolloidal aqueous solution of iridium oxide (IrOx) may be repeated multiple times.

[0076] In addition, when a composite substrate of carbon fiber and carbon is used as the substrate in the oxidation reaction electrode 18, for example, a nanocolloidal aqueous solution of iridium oxide (IrOx) may be used as an oxidation catalyst, and the oxidation catalyst may be supported on a composite substrate of carbon fiber and carbon, such as carbon paper, which may contain carbon materials such as carbon nanotubes including multi-walled carbon nanotubes (MWCNTs), and the sheet may be bonded to the composite substrate of carbon fiber and carbon, such as carbon paper, with a carbon-based adhesive.

[0077] The current collecting wiring may be provided to enhance the effect of collecting electricity in the oxidation reaction electrode 18. That is, when the area of ​​the oxidation reaction electrode 18 is increased, the conductive layer alone cannot ensure the conductivity for promoting the reaction sufficiently over the entire surface of the oxidation reaction electrode 18, so the current collecting wiring is provided to enhance the conductivity of the oxidation reaction electrode 18. The current collecting wiring may be configured, for example, by combining linear finger electrodes arranged in a comb shape at intervals with a bus electrode for further collecting electricity from the finger electrodes. The current collecting wiring is preferably configured from a conductive part, a first seal part, and a second seal part. The conductive part is preferably configured from a material having high conductivity and a material containing a metal. For example, it is preferably configured from a material containing silver (Ag), copper (Cu), or the like. The first seal part and the second seal part are provided so as to cover at least a part of the conductive part in order to chemically and mechanically protect the conductive part. The first seal part may be a glass coating material having a low melting point. In addition, the second sealing portion can be made of resin such as silicone rubber (oxime-free type, low molecular weight siloxane reduced material, oil-resistant and solvent-resistant fluorosilicone, etc.), polyisobutylene, polypropylene, methacrylate (acrylic), polycarbonate, fluororesin (Teflon (registered trademark)), epoxy resin, etc.

[0078] The reduction reaction electrode 16 and the oxidation reaction electrode 18 may each have a single-layer configuration, or the reduction reaction electrode 16 and the oxidation reaction electrode 18 may each have a configuration in which a plurality of electrodes are laminated (stacked). Only the reduction reaction electrode 16 may be laminated (stacked), or only the oxidation reaction electrode 18 may be laminated (stacked). When used as an artificial photosynthesis device, it is preferable that the reduction reaction electrode 16 and the oxidation reaction electrode 18 each have a configuration in which a plurality of electrodes are laminated (stacked).

[0079] The container 20 is a member that supports the reduction reaction electrode 16 and the oxidation reaction electrode 18, and also constitutes a flow path through which the electrolyte flows. The container 20 is made of a material that has the mechanical strength required to constitute the electrochemical reaction cell 10. For example, the container 20 can be made of metal, plastic, etc.

[0080] A separator (for example, an electrolyte membrane 44 as shown in FIGS. 2(b), (d), and (e)) that separates liquid and gas and allows protons to move may be provided between the reduction reaction electrode 16 and the oxidation reaction electrode 18. The separator may be made of any material that separates liquid and gas and allows protons to move, and is not particularly limited; for example, Nafion (registered trademark), which is a solid polymer electrolyte membrane, may be used.

[0081] The tank 12 is a tank that contains an electrolyte, and can be made of, for example, metal, plastic, or the like.

[0082] The reaction substrate supply source 30 is not particularly limited as long as it can supply a reaction substrate, and is, for example, a reaction substrate cylinder such as a carbon dioxide cylinder.

[0083] The reduction reaction electrode 16 and the oxidation reaction electrode 18 are electrically connected to each other, and an appropriate bias voltage is applied therebetween. The means for applying the bias voltage is not particularly limited, and examples thereof include a chemical battery (including a primary battery, a secondary battery, etc.), a constant voltage source, a solar cell, etc. At this time, the positive electrode is connected to the oxidation reaction electrode 18, and the negative electrode is connected to the reduction reaction electrode 16.

[0084] As shown in Figs. 1 and 3, by using a solar cell 34 as a means for applying a bias voltage, an artificial photosynthesis device 1 or 2 can be provided that includes an electrochemical reactor 3 or 4 such as a carbon dioxide reduction device, and a solar cell 34 that generates power to be supplied to the reduction reaction electrode 16 and the oxidation reaction electrode 18. When the solar cell 34 is used as a means for applying a bias voltage, the solar cell 34 can be disposed adjacent to the reduction reaction electrode 16 or the oxidation reaction electrode 18, for example. For example, the solar cell 34 may be disposed on the back of the reduction reaction electrode 16, and the positive electrode of the solar cell 34 may be connected to the oxidation reaction electrode 18, and the negative electrode may be connected to the reduction reaction electrode 16.

[0085] When synthesizing formic acid (HCOOH) from carbon dioxide (CO2), water (H2O) is oxidized to supply electrons and protons to carbon dioxide (CO2). At around pH 7, the oxidation potential of water (H2O) is 0.82 V and the reduction potential is -0.41 V (both at the standard hydrogen electrode (NHE)). The reduction potentials of carbon dioxide (CO2) to carbon monoxide (CO), formic acid (HCOOH), and methyl alcohol (CH3OH) are -0.53 V, -0.61 V, and -0.38 V, respectively. Therefore, the potential difference between the oxidation potential and the reduction potential is 1.20 to 1.43 V. When reducing carbon dioxide (CO2), which is a carbon compound, it is preferable to use a solar cell with a configuration in which, for example, 4 to 6 crystalline silicon solar cells are directly connected or an amorphous silicon triple-junction solar cell.

[0086] It is preferable to provide a window material on the light-receiving surface side of the solar cell. The window material is a member that protects the solar cell. The window material is a member that transmits light of a wavelength that contributes to power generation in the solar cell, and can be, for example, glass, plastic, etc.

[0087] This specification includes the following embodiments. [1] An electrochemical reaction cell in which a reduction reaction electrode containing a reduction catalyst and an oxidation reaction electrode containing an oxidation catalyst are disposed in a container, and which reduces a reaction substrate to produce an organic acid; A tank for holding an electrolyte containing a salt with a pka of 6 to 8, a neutral salt, or both dissolved therein; an electrolyte supply means for supplying the electrolyte from the tank to the container; a liquid-liquid extraction device that extracts the organic acid from the electrolytic solution discharged from the container and dissolving the organic acid produced by the reduction of the reaction substrate into an organic solvent containing at least one of a secondary amine and a tertiary amine; An electrochemical reactor comprising:

[0088] [2] The electrochemical reactor according to [1], The organic acid is at least one of formic acid, acetic acid, and propionic acid.

[0089] [3] The electrochemical reactor according to [1] or [2], The salt having a pka of 6 to 8 is at least one of potassium dihydrogen phosphate, sodium dihydrogen phosphate, lithium dihydrogen phosphate, calcium dihydrogen phosphate, magnesium dihydrogen phosphate, cesium dihydrogen phosphate, rubidium dihydrogen phosphate, magnesium bicarbonate, magnesium chloride, cesium chloride, rubidium chloride, magnesium nitrate, calcium nitrate, cesium nitrate, rubidium nitrate, potassium thiocyanate, sodium thiocyanate, lithium thiocyanate, magnesium thiocyanate, cesium thiocyanate, and rubidium thiocyanate.

[0090] [4] The electrochemical reactor according to any one of [1] to [3], the neutral salt is at least one of potassium sulfate, sodium sulfate, lithium sulfate, magnesium sulfate, calcium sulfate, potassium chloride, sodium chloride, lithium chloride, potassium bromide, sodium bromide, lithium bromide, magnesium bromide, potassium iodide, sodium iodide, lithium iodide, potassium nitrate, sodium nitrate, lithium nitrate, potassium perchlorate, sodium perchlorate, and lithium perchlorate.

[0091] [5] An electrochemical reactor according to any one of [1] to [4], At least one of the secondary amine and the tertiary amine is an alkylamine represented by the following formula (1): R 1 R 2 R 3 N (1) (In formula (1), R 1 ,R 2 ,R 3 At least two of the groups are independently an alkyl group having 6 to 12 carbon atoms, and the remaining groups are hydrogen atoms.

[0092] [6] An electrochemical reactor according to any one of [1] to [5], The electrochemical reactor, wherein the liquid-liquid extractor is at least one of a mixer-settler type extractor, a column type extractor, and a centrifugal type extractor.

[0093] [7] An electrochemical reactor according to any one of [1] to [6], The electrochemical reactor, wherein the reaction substrate is carbon dioxide, and the organic acid produced by reduction of the reaction substrate is formic acid.

[0094] [8] An electrochemical reactor according to any one of [1] to [7], The electrochemical reactor, wherein the reduction catalyst is a ruthenium complex polymer and the oxidation catalyst is iridium oxide.

[0095] [9] An electrochemical reactor according to any one of [1] to [8], The electrochemical reactor further comprises a circulation means for circulating the electrolytic solution to the tank after the electrolytic solution is discharged from the container of the electrochemical reaction cell.

[0096]

[10] An electrochemical reactor according to any one of [1] to [9]; a solar cell that generates power to be supplied to the oxidation reaction electrode and the reduction reaction electrode; An artificial photosynthesis device comprising: EXAMPLES

[0097] EXAMPLES The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0098] 1. Experiment 1 1.1. Preparation of various aqueous solutions Aqueous solutions 1 to 5 were prepared as follows.

[0099] (Reference Example) Aqueous solution 1: Formic acid (0.76 mL, 20 mmol) was dissolved in pure water (200 mL). (Comparative Example) Aqueous solution 2: Potassium dihydrogen phosphate (pka=7.2, 5.44 g, 40 mmol) and dipotassium hydrogen phosphate (pka=12.35, 6.97 g, 40 mmol) were added to water (200 mL) and formic acid (0.76 mL, 20 mmol) was dissolved in the solution to prepare aqueous solution 2. (Reference Example) Aqueous solution 3: Potassium formate (1.68 g, 20 mmol) was added to water (200 mL) to obtain a 0.10 mol / L solution. (Reference Example) Aqueous solution 4: Potassium dihydrogen phosphate (10.89 g, 80 mmol) was dissolved in water (200 mL). (Reference Example) Aqueous solution 5: Aqueous solution 5 was prepared by adding dipotassium hydrogen phosphate (13.93 g, 80 mmol) to water (200 mL).

[0100] The types and concentrations of the various aqueous solutions are shown in Table 1.

[0101] [Table 1]

[0102] 1.2. Preparation of NMR samples using various aqueous solutions NMR samples were prepared using Aqueous Solutions 1 to 5 as follows.

[0103] (1) Dichloromethane (DCM, 10 mL) and trioctylamine (1.75 mL, 4.0 mmol) were added to Aqueous solution 1 (20 mL) and stirred in a separatory funnel. (2) After standing, the organic layer was separated and dichloromethane was removed using an evaporator (200 torr, 40°C). (3) 0.25 mL of (2) was added to 1 mL of a solution of deuterated chloroform (CDCl3, 5 mL) and 1,2-dichloroethane (DCE, 10 μL), and then CDCl3 was added to make the total volume 2.00 mL. (4) Take a portion of (3) in an NMR sample tube. 1 H-NMR was measured.

[0104] The same procedures (1) to (4) were carried out using aqueous solutions 2 to 5 instead of aqueous solution 1. In order to compare with the state of trioctylamine not forming a salt, the same extraction procedure was also carried out using pure water only (20 mL).

[0105] 1.3. 1 H-NMR measurement 1 H-NMR measurements were performed using a JNM-ECX-400P (JEOL Ltd.). Chemical shifts are shown in δ values ​​(ppm), with the value of 0.03% (v / v) tetramethylsilane (TMS) contained as an internal standard in the CDCl3 reagent set at 0.00 ppm. The integral value of DCE was set at 1.00.

[0106] 1.4 Results 1 Figure 4 shows the structure of trioctylamine. 1 H-NMR spectrum is shown in Fig. 5, and formic acid is extracted using aqueous solution 1 or aqueous solution 2. 1 The H-NMR spectrum is shown. Table 2 shows the formic acid extraction rate (%) when Aqueous Solution 1 or Aqueous Solution 2 is used. Figure 6 shows the results when the extraction process was performed using Aqueous Solutions 3 to 5. 1The H-NMR spectrum is shown. The formic acid extraction rate (%) was calculated by dividing the amount of formic acid in the DCM layer residue by the amount of formic acid contained in the aqueous solution before extraction. The amount of formic acid in the DCM layer residue was calculated from the volume of the DCM layer residue by determining the concentration of formic acid in the DCM layer residue from the relative integral value of formic acid to DCE.

[0107] [Table 2]

[0108] Compared to Aqueous solution 1 (formic acid dissolved in pure water), the extraction rate of formic acid dissolved in Aqueous solution 2 (standard electrolyte) was significantly lower (see Figures 4, 5, and Table 2). Because trioctylamine hardly forms a complex salt with potassium formate or potassium phosphate (potassium dihydrogen phosphate or dipotassium hydrogen phosphate) (see Figure 6), most of the formic acid dissolved in Aqueous solution 2 becomes potassium formate due to the buffering effect, which is thought to be the reason for the significant decrease in the formic acid extraction rate.

[0109] 2. Experiment 2 2.1. Preparation of various aqueous solutions Aqueous solutions 6 to 8 were prepared as follows.

[0110] (Example) Aqueous solution 6: This was prepared by adding potassium dihydrogen phosphate (10.88 g, 80 mmol) as a salt with a pka value of 6 to 8 to water (200 mL) and dissolving formic acid (0.15 mL, 4 mmol). (Example) Aqueous solution 7: Potassium sulfate (pH=6.6, 13.94 g, 80 mmol) as a neutral salt was added to water (200 mL) and formic acid (0.15 mL, 4 mmol) was dissolved therein to prepare aqueous solution 7. (Comparative Example) Aqueous solution 8: This was prepared by adding potassium dihydrogen phosphate (5.44 g, 40 mmol) and dipotassium hydrogen phosphate (6.97 g, 40 mmol) to water (200 mL) and dissolving formic acid (0.15 mL, 4 mmol).

[0111] The types and concentrations of the various aqueous solutions are shown in Table 3.

[0112] Extractants 1–3: Trioctylamine (1.75 mL, 4.0 mmol) as a tertiary amine was dissolved in dichloromethane (DCM, 30 mL) as an organic solvent.

[0113] [Table 3]

[0114] 2.2. Triple mixer-settler configuration and formic acid extraction process The experimental configuration of the triple mixer settler is shown in Figure 7. The four pumps for the aqueous solution and the extractants 1 to 3 were Smoothflow pumps (Q-100-TT-PS) manufactured by TACMINA. Hydrophobic PFA (copolymer of tetrafluoroethylene and perfluoroalkoxyethylene) tubes (inner diameter: 1 mm) for piping in high performance liquid chromatography (HPLC) were used for the connections between each pump and each part of the mixer settler bodies 1 to 3 (see Figure 7(a)), and for the tubes (length: 2 m) of the mixer parts 1 to 3 (see Figure 7(b)). In the process in which the aqueous solution containing dissolved formic acid is sent by the aqueous solution sending pump from the aqueous solution container to the mixer settler body 1 to the mixer settler body 2 to the mixer settler body 3 to the aqueous solution recovery container, the pumps were connected to each part of the mixer settler body so that three extractions using the extractants 1 to 3 could be performed in a continuous process. In the process of pumping aqueous solutions 6-8 (60mL) from mixer settler body 1 to mixer settler body 2 to mixer settler body 3 in this order, slug flows were formed with the extractants 1-3 (30mL each), and then the liquids were separated into two phases in each settler section. The aqueous solutions were collected in an aqueous solution collection container, and the extractants were returned to the original extractant container and circulated.

[0115] 2.3 Result 2 Figure 8 shows a photograph of the formation of the slug flow, and Figure 9 shows a photograph of the continuous separation of the aqueous solution and the extractant due to two-phase separation in each settler section. A slug flow of the aqueous solution and the extractant was formed in mixer settlers 1 to 3. Furthermore, due to two-phase separation in each settler section, the aqueous solution and the extractant could be separated in a flow-through manner (see Figure 9). This confirmed three consecutive extraction operations. Table 4 also shows the formic acid extraction rate (%) when aqueous solutions 6 to 8 were used.

[0116] [Table 4]

[0117] The extraction rate of formic acid using the triple mixer settler was 11% for aqueous solution 8, whereas the extraction rate was significantly improved to 96% for aqueous solution 6 (see Table 4). This is believed to be because aqueous solution 6, which uses a salt with a pka of 6 to 8, hardly forms potassium formate. Even aqueous solution 7, which uses a neutral salt, achieved a formic acid extraction rate of 40%, exceeding the value of aqueous solution 8.

[0118] 3. Experiment 3 3.1. Preparation of a small reduction reaction electrode (cathode electrode) A porous carbon sheet (CS) composed of porous carbon fibers was dip-coated with an ink containing 5% by mass of multi-walled carbon nanotubes (MWCNTs) dispersed in an ethanol solvent. The ink was then dried at 25°C and heated at 350°C in an argon atmosphere to produce an MWCNTs-supported MWCNTs / CS sheet.

[0119] The components of the Ru complex polymer solution (for one application) were the Ru complex monomer [Ru{4,4'-di(1-H-1-pyrrolipropyl carbonate)-2,2'-bipyridine}(CO)2Cl2] (5.02 × 10 -7 mol / cm 2 ), and pyrrole (1.99 × 10 -9 mol / cm 2 ), and iron chloride (FeCl3) (2.79 × 10-6 mol / cm 2 ), and ethanol as a solvent (14 mL / cm 2 ), acetonitrile (53 mL / cm 2 ), which was applied 10 times and dried in a vacuum to support Ru complex polymer (RuCP) on the MWCNTs / CS sheet. The surface of a 20mm x 15mm Ti plate (JIS Ti type 1, thickness 0.5mm) was polished, and a 10mm x 10mm piece of reduction catalyst RuCP / MWCNTs / CS (thickness 320μm, hereafter abbreviated as RuCP / MWCNTs / CS) was cut and bonded to the Ti substrate with graphite adhesive. The end of the titanium substrate and a conductor with a crimp terminal were joined using titanium bolts, nuts, and washers. The terminal joint was then covered with silicone rubber.

[0120] 3.2. Cathode characterization The cathode electrode was used as the working electrode, a platinum wire as the counter electrode, and Hg / Hg2SO4 as the reference electrode. A small cathode electrode of Ti / adhesive / RuCP / MWCNTs / CS and carbon dioxide (CO2) gas as a reaction substrate were bubbled into aqueous solution 6 (Example) or aqueous solution 8 (Comparative Example) at 1 mA / cm using a potentio-galvanostat (VMP3, Bio-Logic Sciences Instruments). 2 Constant current measurements were carried out. In aqueous solution 6, the generated formic acid was extracted to suppress the increase in the formic acid concentration in the aqueous solution of the cell, and then the measurement was performed. In aqueous solution 8, the generated formic acid concentration was increased as it was, and then the measurement was performed. The amount of generated formic acid was quantified using an ion chromatograph (Integrion RFIC EG equipped with IonPac EGC-500-KOH, CR-ATC600 columns and a conductometric detector, Dionex Corporation), and the Faraday efficiency (FE) of formic acid was calculated.

[0121] 3.3 Result 3 Table 5 shows the Faraday efficiency (FE) retention rate of formic acid when Aqueous Solution 6 and Aqueous Solution 8 were used.

[0122] [Table 5]

[0123] By suppressing the increase in the formic acid concentration in the aqueous solution of the cell, the formic acid FE retention rate was significantly improved and stable operation was obtained (see Table 5).

[0124] In this way, according to the embodiment, in an electrochemical reactor equipped with a reduction reaction electrode and an oxidation reaction electrode, it is possible to suppress deterioration of characteristics and maintain stable operation, and it is possible to reduce energy consumption when recovering the generated organic acid and to increase the amount of recovery. [Explanation of symbols]

[0125] 1,2 artificial photosynthesis device, 3,4 electrochemical reactor, 10 electrochemical reaction cell, 12 tank, 14 liquid-liquid extraction device, 16 reduction reaction electrode, 18 oxidation reaction electrode, 20 container, 22 pump, 24 electrolyte supply pipe, 26 electrolyte discharge pipe, 28,36 circulation pipe, 30 reaction substrate supply source, 32 reaction substrate supply pipe, 34 solar cell, 38,40 pipe, 42,42a,42b flow path, 44 electrolyte membrane.

Claims

1. an electrochemical reaction cell in which a reduction reaction electrode containing a reduction catalyst and an oxidation reaction electrode containing an oxidation catalyst are disposed in a container, and which reduces a reaction substrate to produce an organic acid; a tank for holding an electrolyte solution containing a salt having a pka value of 6 to 8, a neutral salt, or both of them dissolved therein; an electrolyte supply means for supplying the electrolyte from the tank to the container; a liquid-liquid extraction device that extracts the organic acid from the electrolytic solution discharged from the container and having the organic acid produced by the reduction of the reaction substrate dissolved therein into an organic solvent containing at least one of a secondary amine and a tertiary amine; An electrochemical reactor comprising:

2. 2. The electrochemical reactor of claim 1, The organic acid is at least one of formic acid, acetic acid, and propionic acid.

3. 2. The electrochemical reactor of claim 1, The salt having a pKa of 6 to 8 is at least one of potassium dihydrogen phosphate, sodium dihydrogen phosphate, lithium dihydrogen phosphate, calcium dihydrogen phosphate, magnesium dihydrogen phosphate, cesium dihydrogen phosphate, rubidium dihydrogen phosphate, magnesium bicarbonate, magnesium chloride, cesium chloride, rubidium chloride, magnesium nitrate, calcium nitrate, cesium nitrate, rubidium nitrate, potassium thiocyanate, sodium thiocyanate, lithium thiocyanate, magnesium thiocyanate, cesium thiocyanate, and rubidium thiocyanate.

4. 2. The electrochemical reactor of claim 1, The neutral salt is at least one of potassium sulfate, sodium sulfate, lithium sulfate, magnesium sulfate, calcium sulfate, potassium chloride, sodium chloride, lithium chloride, potassium bromide, sodium bromide, lithium bromide, magnesium bromide, potassium iodide, sodium iodide, lithium iodide, potassium nitrate, sodium nitrate, lithium nitrate, potassium perchlorate, sodium perchlorate, and lithium perchlorate.

5. 2. The electrochemical reactor of claim 1, At least one of the secondary amine and the tertiary amine is an alkylamine represented by the following formula (1): R １ R ２ R ３ N (1) (In formula (1), R １ , R ２ , R ３ At least two of the groups are independently an alkyl group having 6 to 12 carbon atoms, and the remaining groups are hydrogen atoms.

6. 2. The electrochemical reactor of claim 1, The electrochemical reactor, wherein the liquid-liquid extractor is at least one of a mixer-settler type extractor, a column type extractor, and a centrifugal type extractor.

7. 2. The electrochemical reactor of claim 1, 1. An electrochemical reactor, comprising: a reaction substrate comprising carbon dioxide; and an organic acid produced by reduction of the reaction substrate comprising formic acid.

8. 2. The electrochemical reactor of claim 1, 1. An electrochemical reactor, comprising: said reduction catalyst being a ruthenium complex polymer; and said oxidation catalyst being iridium oxide.

9. 2. The electrochemical reactor of claim 1, The electrochemical reactor further comprises a circulation means for circulating the electrolytic solution to the tank after the electrolytic solution is discharged from the container of the electrochemical reaction cell.

10. The electrochemical reactor of claim 1 ; a solar cell that generates power to be supplied to the oxidation reaction electrode and the reduction reaction electrode; An artificial photosynthesis device comprising:

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