Method for producing carboxylic acids

Abstract

To provide a method for efficiently producing carbon-neutral carboxylic acids with low environmental impact. [Solution] A method for producing a carboxylic acid by reducing carbon dioxide in the atmosphere, comprising the steps of: immersing a hydrogen storage alloy negative electrode used in a nickel-metal hydride battery in an alkaline electrolyte and then drying it; using an electrolytic cell equipped with an alkaline electrolyte capable of adsorbing carbon dioxide, a positive electrode used in a nickel-metal hydride battery, the treated hydrogen storage alloy negative electrode, and a power supply connected to the positive and negative electrodes, reducing carbon dioxide by applying a potential difference to the positive electrode so that the potential of the positive electrode is higher than the potential of the negative electrode.

Description

[Technical Field]

[0001] This disclosure relates to a method for producing carboxylic acids. [Background technology]

[0002] CO2 (i.e., carbon dioxide) is a greenhouse gas that exists naturally in the atmosphere and is produced by the combustion of fossil fuels. Increased human activity and energy demand are thought to be increasing atmospheric CO2 levels, causing global warming. Removing CO2 from the atmosphere has attracted the attention of many researchers worldwide. Various CO2 conversion methods, including chemical, photocatalytic, and electrochemical methods, are being widely studied.

[0003] A method is known for reducing carbon dioxide using an electrochemical cell and extracting the product in situ, wherein an absorbent containing CO2 is introduced to the cathode, the CO2 is electrochemically reduced at the cathode to form products such as organic acids, the products are extracted and collected in situ, and there are multiple separators between the anode and cathode to maintain an efficient reaction environment (Patent Document 1). Furthermore, as a photocatalytic technology in which a specific catalytic material is fixed to a substrate and carbon dioxide is converted to acetic acid or formic acid using visible light, a mixed sol is prepared by encapsulating an electron acceptor slurry containing tungsten oxide nanoparticles, Ru seed dye, ethyl viologen dichloride, and artificial acetic acid bacteria enzyme in an organic solid solution binder, and this is coated onto a substrate to convert carbon dioxide to acetic acid or formic acid using a photocatalytic reaction (Patent Document 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2024-517246 [Patent Document 2] Japanese Patent Publication No. 2022-76331 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Conventional methods, particularly electrochemical methods, required a separator in the electrolytic cell configuration. Furthermore, photocatalytic technology required a substrate containing specific elements.

[0006] One embodiment of this disclosure aims to provide a manufacturing method for efficiently producing carbon-neutral carboxylic acids with low environmental impact. [Means for solving the problem]

[0007] The means for solving the problem include the following: <1> A method for producing a carboxylic acid by reducing carbon dioxide in the atmosphere, comprising the steps of: immersing a hydrogen storage alloy negative electrode used in a nickel-metal hydride battery in an alkaline electrolyte and then drying it; and using an electrolytic cell equipped with an alkaline electrolyte capable of adsorbing carbon dioxide, a positive electrode used in a nickel-metal hydride battery, the treated hydrogen storage alloy negative electrode, and a power supply connected to the positive and negative electrodes, reducing carbon dioxide by applying a potential difference to the positive electrode such that the potential of the positive electrode is higher than the potential of the negative electrode. <2> The hydrogen storage alloy negative electrode contains MmNi5, and the positive electrode contains at least one of Ni(OH)2 and NiOOH. <1> A method for producing the carboxylic acid described above. <3> The treated hydrogen storage alloy anode exhibits diffraction peaks in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements. <1> or <2> A method for producing the carboxylic acid described above. <4> The alkaline electrolyte comprises water and at least one substance selected from KOH, K2CO3, Na2CO3, and KHCO3. <1> ~ <3> A method for producing a carboxylic acid as described in any one of the following. <5> The carboxylic acid comprises at least one selected from formic acid, acetic acid, and propionic acid. <1> ~ <4> A method for producing a carboxylic acid as described in any one of the following. [Effects of the Invention]

[0008] According to one embodiment of the present disclosure, a method for efficiently producing carbon-neutral carboxylic acids with low environmental impact is provided. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a graph showing the X-ray diffraction results of each MmNi5-based hydrogen storage alloy cathode in the examples and comparative examples. [Modes for carrying out the invention]

[0010] The embodiments of this disclosure will be described below with reference to the drawings. In this disclosure, a numerical range indicated using "~" means a range that includes the numbers written before and after "~" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. In the numerical ranges described in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the values ​​shown in the examples. In this disclosure, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes, as long as their intended purpose is achieved. In this disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In this disclosure, the "anode" in an electrolytic reaction is also referred to as the "positive electrode," and the "cathode" is also referred to as the "negative electrode."

[0011] <Method for producing carboxylic acids> The method for producing a carboxylic acid according to the present disclosure is a method for producing a carboxylic acid by reducing carbon dioxide in the atmosphere, and includes a treatment step and a reduction step. In the treatment step, a hydrogen storage alloy negative electrode used in a nickel-metal hydride battery is immersed in an alkaline electrolyte and then dried. In the reduction step, an electrolytic cell is used, and a power supply applies a potential difference between a positive electrode and a negative electrode such that the potential of the positive electrode is higher than that of the negative electrode, thereby reducing carbon dioxide. The electrolytic cell comprises an alkaline electrolyte, a positive electrode, a negative electrode, and a power supply. The alkaline electrolyte is capable of adsorbing carbon dioxide. The positive electrode and the negative electrode are each used for a nickel-metal hydride battery, and the negative electrode is pretreated by the treatment step before being used for electrolysis. The power supply is connected to the positive electrode and the negative electrode. In the following description, the alkaline electrolyte is also referred to as an electrolyte.

[0012] (Treatment Step) In the treatment step, the hydrogen storage alloy negative electrode only needs to be a negative electrode comprising a hydrogen storage alloy used for a nickel-metal hydride battery (that is, a nickel-metal hydride secondary battery (Ni-MH)). As the hydrogen storage alloy, from the viewpoint of environmental load, AB5-type hydrogen storage alloys widely used in nickel-metal hydride batteries are preferred, hydrogen storage alloys using misch metal as a raw material are more preferred, and the hydrogen storage alloy more preferably contains MmNi5. The hydrogen storage alloy negative electrode may be one that has not been used as a battery, or may be a hydrogen storage alloy negative electrode contained in a used nickel-metal hydride battery.

[0013] As the alkaline electrolyte, alkaline electrolytes used in nickel-metal hydride batteries are preferred. The alkaline electrolyte may be one before being used in a nickel-metal hydride battery, or may be an alkaline electrolyte after being used in a nickel-metal hydride battery. The alkaline electrolyte may be of a single type, or a mixture of a plurality of types. The alkaline electrolyte may be the same type of electrolyte as the alkaline electrolyte used in the electrolytic cell.

[0014] Examples of alkaline electrolytes include aqueous potassium carbonate (K₂CO₃) solution, aqueous potassium hydroxide (KOH) solution, aqueous potassium bicarbonate (KHCO₃) solution, aqueous sodium hydroxide (NaOH) solution, aqueous sodium carbonate (Na₂CO₃) solution, and aqueous lithium hydroxide (LiOH) solution.

[0015] From the viewpoint of carboxylic acid production, the alkaline electrolyte preferably contains water and at least one selected from the group consisting of KOH, K₂CO₃ and Na₂CO₃. Note that a KOH aqueous solution becomes a K₂CO₃ aqueous solution or a potassium bicarbonate (KHCO₃) aqueous solution by adsorbing CO₂ in the atmosphere, so a KOH aqueous solution brought into contact with the atmosphere may be used as the alkaline electrolyte.

[0016] There are no particular limitations on the method for immersing the hydrogen storage alloy negative electrode in the alkaline electrolyte. It is sufficient that part or all of the hydrogen storage alloy negative electrode is immersed in the alkaline electrolyte. When the hydrogen storage alloy negative electrode is a hydrogen storage alloy negative electrode contained in a used nickel-metal hydride battery, it has already been immersed in an alkaline electrolyte, so the hydrogen storage alloy negative electrode taken out from the used nickel-metal hydride battery may be used as-is.

[0017] The hydrogen storage alloy negative electrode immersed in the alkaline electrolyte is subsequently dried. The drying does not need to be complete drying such as removal of all moisture, and may be to a degree that the alkaline electrolyte adhering to the hydrogen storage alloy negative electrode is concentrated.

[0018] The inventors have discovered that by using a hydrogen storage alloy anode with an alkaline electrolyte attached and then dried, the resistance of the hydrogen storage alloy anode in the alkaline electrolyte is significantly reduced. Although the reason for this is not clear, it is presumed that a change in the surface state activates the surface of the hydrogen storage alloy anode, leading to the production of carboxylic acids during electrolysis. One possible change in the surface state is that at least some of the components of the alkaline electrolyte are present on the surface as crystals. This maintains a localized environment of strong alkalinity, which is presumed to lead to the formation of carboxylic acid ions such as formate ions, acetate ions, and propionate ions during the CO2 reduction reaction.

[0019] From the viewpoint of carboxylic acid production, a relatively low drying temperature is preferred. For example, drying is performed in air at 80°C for several hours. While drying is not limited to these conditions, it is presumed that drying at such a relatively low temperature will produce crystals that adhere relatively uniformly.

[0020] The processing step preferably results in the treated hydrogen storage alloy anode exhibiting diffraction peaks in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements. These diffraction peaks in the range of 2θ = 32° to 33° are not present in the X-ray diffraction (XRD) measurements of the hydrogen storage alloy anode before processing, and are newly present in the treated hydrogen storage alloy anode. It is presumed that, due to the processing step described above, crystals of KOH, a component of the alkaline electrolyte, are present on a portion of the surface of the treated hydrogen storage alloy anode, and that these KOH crystals are the origin of the new diffraction peaks in the range of 2θ = 32° to 33°. The treated hydrogen storage alloy anode also exhibits peaks from the hydrogen storage alloy itself before processing.

[0021] (Reduction process) In the reduction process, an electrolytic cell is used. The alkaline electrolyte contained in the electrolytic cell is capable of adsorbing carbon dioxide (CO2) and may be the same as the alkaline electrolyte used in the processing process. Preferably, the alkaline electrolyte contains KOH, KHCO3, and K2CO3. Note that since an aqueous KOH solution becomes an aqueous K2CO3 solution or an aqueous potassium bicarbonate (KHCO3) solution by adsorbing CO2 from the atmosphere, an aqueous KOH solution brought into contact with the atmosphere may be used as the alkaline electrolyte. The alkaline electrolyte contained in the electrolytic cell may be the same as the electrolyte used in an alkaline secondary battery.

[0022] Potassium carbonate (K2CO3) reacts with atmospheric CO2 (400 ppm, 40 Pa) in the presence of water to produce potassium bicarbonate (KHCO3). The reaction equation is shown in equation (1).

[0023] K2CO3 + CO2 + H2O ←→ 2KHCO3 (1)

[0024] In the van't Hoff equation shown in equation (2) below, ΔH 0 This represents the standard enthalpy change (-96.1 kJ / molCO2), ΔS 0 This represents the standard entropy change (-208 J / KmolCO2). P0 is atmospheric pressure (0.1 MPa), R is the gas constant, and 8.314 JK. -1 mol -1 、 Substitute temperature (298K) for T. As a result, the pressure P becomes 0.112Pa and the equilibrium concentration for CO2 absorption becomes 1.12ppm. Therefore, 400ppm of carbon dioxide from the atmosphere can be absorbed, and potassium bicarbonate (KHCO3) is used electrochemically as a CO2 source. Subsequently, potassium bicarbonate (KHCO3) is regenerated into potassium carbonate (K2CO3).

[0025]

number

[0026] A KOH aqueous solution adsorbs CO₂ from the atmosphere to become a K₂CO₃ aqueous solution or a potassium hydrogen carbonate (KHCO₃) aqueous solution, so the KOH aqueous solution may be used while being in contact with the atmosphere. KOH absorbs CO₂ (400 ppm) in the atmosphere to partially generate potassium carbonate (K₂CO₃) and water. The reaction formula is shown in formula (3).

[0027] 2KOH+CO₂←→K₂CO₃+H₂O (3)

[0028] In the van't Hoff equation shown in the above formula (2), ΔH 0 is the standard enthalpy change (-194kJ / mol CO₂), ΔS 0 is the standard entropy change (-151J / K·mol CO₂). Substitute atmospheric pressure (0.1MPa) for P0, 8.314 J·K -1 ⁻¹mol -1 、 for the gas constant R, and temperature (298K) for T. As a result, the pressure P is 0.112 Pa, and the equilibrium concentration of CO₂ absorption is 7.92×10 -21 ppm, which indicates KOH can absorb carbon dioxide from the atmosphere. Since this K₂CO₃ is thermodynamically stable, it is considered that the above-mentioned K₂CO₃ aqueous solution can more effectively utilize K₂CO₃ as a CO₂ source from the perspective of carboxylic acid production than a KOH aqueous solution, and is therefore preferable as an alkaline electrolyte. When a KOH aqueous solution is used as the alkaline electrolyte in an electrolytic cell, electrolysis may be performed by, for example, increasing the potential difference.

[0029] The alkaline electrolyte may be an electrolyte contained in a nickel-hydrogen battery (that is, a nickel-hydrogen secondary battery (Ni-MH)). The electrolyte may be an electrolyte contained in an unused nickel-hydrogen battery, or may be an electrolyte contained in a used nickel-hydrogen battery. The alkaline electrolyte may contain trace amounts of elements or compounds other than those described above.

[0030] The electrodes in the electrolytic cell use the same positive electrode as those used in nickel-metal hydride batteries. The positive electrode preferably contains at least one of Ni(OH)2 and NiOOH. Other elements besides Ni(OH)2 or NiOOH, such as Co and Fe, may be added to the positive electrode. The positive electrode may be the positive electrode contained in a nickel-metal hydride battery before use, or the positive electrode contained in a nickel-metal hydride battery after use.

[0031] From the viewpoint of carboxylic acid production, it is preferable to perform the same processing steps on the positive electrode as on the negative electrode. The processing of the positive electrode is the same as that on the negative electrode.

[0032] The power source can be any power source capable of performing electrolysis using an electrolytic cell, and is not limited to that. By applying a potential difference to the positive and negative electrodes using the power source, such that the potential of the positive electrode is higher than the potential of the negative electrode, carbon dioxide is reduced. It is preferable to use a power source that does not use fossil fuels, etc., because it is possible to produce carboxylic acids, which are raw materials for fuel, from carbon dioxide in the atmosphere with less environmental impact.

[0033] The electrolytic cell performs electrolysis in a state where the alkaline electrolyte can come into contact with the atmosphere. This allows for the electrochemical reduction of CO2 adsorbed by the alkaline electrolyte. In the carboxylic acid production method of this disclosure, carboxylic acid ions are generated by the reduction of CO2, and it is thought that much of these exist as potassium carboxylate in the alkaline electrolyte. The carboxylic acid can be recovered by recovering the alkaline electrolyte containing potassium carboxylate and neutralizing this electrolyte.

[0034] In the method for producing carboxylic acids of this disclosure, carboxylic acid ions may be generated. The carboxylic acid ions may include at least one selected from formate ions, acetate ions, and propionic acid. In the method for producing carboxylic acids of this disclosure, alcohols such as methanol may be generated. Oxygen is generated from the positive electrode by electrolysis. The generated oxygen may be released into the atmosphere or recovered and used.

[0035] According to the method for producing carboxylic acids described in this disclosure, carboxylic acids containing acetic acid, which can be used as a raw material for vinegar, polymers, and fuels such as ethanol, can be produced from carbon dioxide in the atmosphere by electrolysis. The electrolytic cell used for electrolysis may utilize the electrodes and electrolyte of a used nickel-metal hydride battery. The method for producing carboxylic acids described in this disclosure can be carried out, for example, by opening a used nickel-metal hydride battery, removing the negative electrode and subjecting it to a processing step, returning it to the battery, and applying a potential difference to both electrodes. Furthermore, even when using electrolyte, positive electrode, or negative electrode that have not been used in a nickel-metal hydride battery, existing components can be used as they are. Therefore, according to the method for producing carboxylic acids described in this disclosure, carbon-neutral carboxylic acids can be produced efficiently and with low environmental impact. [Examples]

[0036] The present disclosure will be further described below with reference to examples. The examples herein are illustrative and not intended to limit the scope. A person with ordinary skill in the art relating to this disclosure may modify, transform, or substitute materials, compositions, manufacturing methods, and applications as appropriate without departing from the spirit and scope of the invention. Note that "Ni(OH)2 / NiOOH cathode" means that the cathode contains at least one of Ni(OH)2 and NiOOH, and may contain both.

[0037] (Measurement method) In the following examples and comparative examples, electrolysis was performed using an electrolytic cell under predetermined conditions, followed by ion chromatography analysis to measure the amounts of formate ions, acetate ions, and propionate ions produced. The amounts were measured by mass. The measurement results are shown in Table 1.

[0038] (Example 1) A nickel-metal hydride battery was disassembled to extract the Ni(OH)2 / NiOOH positive electrode and the MmNi5-based hydrogen storage alloy negative electrode. The positive electrode was used after being immersed in water and then dried. For the processing step, the hydrogen storage alloy negative electrode was used after being removed from the nickel-metal hydride battery and dried. Drying was carried out in air at 80°C for 3 hours. Using the positive electrode as the anode and the negative electrode as the cathode, an electrolysis experiment was conducted in air using a 2.5 M (mol / L; the same applies hereafter) KHCO3 aqueous solution as the electrolyte. The electrolysis conditions were as follows: the electrode area was 6 cm² at both the anode and negative electrodes. 2 The current density is 50 mA / cm². 2 The current was kept constant at 300 mA, the electrolysis time was 24 hours, the temperature was 30°C, and the liquid volume was 200 mL. Subsequently, ion chromatography analysis was performed on the electrolyte. The amounts of formate ions, acetate ions, and propion ions generated by electrolysis, and the total amount of these ions, are listed in Table 1 as the amount of carboxylate ions generated.

[0039] (Example 2) An electrolysis experiment was conducted in the same manner as in Example 1, except that the electrolyte in the electrolytic cell was a 3.5 M K2CO3 aqueous solution. The amounts of formate ions, acetate ions, and propion ions produced by electrolysis, and the total amount of these ions, are listed in Table 1 as the amount of carboxylate ions produced.

[0040] (Example 3) An electrolysis experiment was conducted in the same manner as in Example 1, except that the electrolyte in the electrolytic cell was an 8M KOH aqueous solution. The amounts of formate ions, acetate ions, and propion ions produced by electrolysis, and the total amount of these ions, are listed in Table 1 as the amount of carboxylate ions produced.

[0041] (Comparative Example 1) Electrolysis experiments were conducted under atmospheric pressure using Pt electrodes for the anode and cathode, and a 2.5 M KHCO3 aqueous solution as the electrolyte. The current was kept constant at 300 mA and the experiment was carried out for 24 hours. Subsequently, ion chromatography analysis was performed on the electrolyte. The amounts of formate ions, acetate ions, and propion ions produced by electrolysis, and the total amount of these ions, are listed in Table 1 as the amount of carboxylate ions produced.

[0042] (Comparative Example 2) An electrolysis experiment was conducted in the same manner as in Comparative Example 1, except that the electrolyte in the electrolytic cell was a 3.5 M K2CO3 aqueous solution. The amounts of formate ions, acetate ions, and propion ions produced by electrolysis, and the total amount of these ions, are listed in Table 1 as the amount of carboxylate ions produced.

[0043] (Comparative Example 3) An electrolysis experiment was conducted in the same manner as in Comparative Example 1, except that the electrolyte in the electrolytic cell was an 8M KOH aqueous solution. The amounts of formate ions, acetate ions, and propion ions produced by electrolysis, and the total amount of these ions, are listed in Table 1 as the amount of carboxylate ions produced.

[0044] [Table 1]

[0045] (Example 4) A MmNi5-based hydrogen storage alloy cathode extracted from a nickel-metal hydride battery was heat-treated in air at 80°C for 3 hours. The X-ray diffraction intensity curve of the heat-treated MmNi5-based hydrogen storage alloy cathode is shown in Figure 1.

[0046] (Comparative Example 4) Figure 1 shows the X-ray diffraction intensity curve of an MmNi5-based hydrogen storage alloy cathode that was removed from a nickel-metal hydride battery, washed, and then left untreated.

[0047] In Example 4, heat treatment of the MmNi5 alloy produced a novel peak at 2θ = 32°~33° that was not present in Comparative Example 4. This peak is thought to be due to the crystallization of KOH, the electrolyte used in nickel-metal hydride batteries. Novel peaks were also present at 12.9° and 25.7°, and these are also thought to be due to the crystallization of KOH, the electrolyte used in nickel-metal hydride batteries. [Explanation of Symbols]

[0048] X-ray diffraction results of a 10 mm mNi5-based hydrogen storage alloy cathode. 11. X-ray diffraction intensity curve of heat-treated MmNi5-based hydrogen storage alloy cathode. 12. X-ray diffraction intensity curve of MmNi5-based hydrogen storage alloy cathode without processing steps.

Claims

1. A method for producing carboxylic acids by reducing carbon dioxide in the atmosphere, A process of immersing a hydrogen storage alloy negative electrode used in nickel-metal hydride batteries in an alkaline electrolyte and then drying it. The method includes a step of reducing carbon dioxide by using an electrolytic cell comprising an alkaline electrolyte capable of adsorbing carbon dioxide, a positive electrode used in a nickel-metal hydride battery, a hydrogen storage alloy negative electrode after the treatment, and a power supply connected to the positive and negative electrodes, and applying a potential difference to the positive and negative electrodes such that the potential of the positive electrode is higher than the potential of the negative electrode. A method for producing carboxylic acids.

2. The hydrogen storage alloy anode is MmNi ５ Includes, The aforementioned positive electrode is Ni(OH) ２ A method for producing a carboxylic acid according to claim 1, comprising at least one of and NiOOH.

3. The method for producing a carboxylic acid according to claim 1, wherein the hydrogen storage alloy anode after processing has a diffraction peak in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements.

4. The aforementioned alkaline electrolyte consists of water, KOH, and K ２ CO ３ Na ２ CO ３ , and KHCO ３ A method for producing a carboxylic acid according to claim 1, comprising at least one selected from the following.

5. The method for producing a carboxylic acid according to claim 1, wherein the carboxylic acid comprises at least one selected from formic acid, acetic acid, and propionic acid.

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