Method for producing carboxylic acid

Abstract

A method for producing a carboxylic acid by reducing carbon dioxide in the atmosphere comprises: a process of immersing a hydrogen-absorbing alloy negative electrode used in a nickel-hydrogen battery in an alkaline electrolyte and then drying it; an electrolytic cell having an alkaline electrolyte capable of adsorbing carbon dioxide, a positive electrode used in a nickel-hydrogen battery, a hydrogen-absorbing alloy negative electrode after treatment, and a power source connected to the positive electrode and the negative electrode, and a process of reducing carbon dioxide by applying a potential difference to the positive electrode and the negative electrode such that the potential of the positive electrode becomes higher than the potential of the negative electrode.

Description

Technology Field

[0001] The present disclosure relates to a method for producing carboxylic acid. Background Technology

[0002] CO2 (i.e., carbon dioxide) is a greenhouse gas that naturally exists in the atmosphere and is produced by the combustion of fossil fuels. It is believed that atmospheric CO2 levels are increasing due to human activities and rising energy demands, causing global warming. The removal of atmospheric CO2 is attracting the attention of many researchers worldwide. Various CO2 conversion methods, including chemical, photocatalytic, and electrochemical methods, are being widely studied.

[0003] A method for reducing carbon dioxide using an electrochemical cell and extracting the product therefrom is known, wherein an absorbent containing CO2 is introduced to the cathode, CO2 is electrochemically reduced at the cathode to form a product such as an organic acid, and the product is extracted and collected therefrom, and there are multiple separators between the anode and the cathode to maintain an efficient reaction environment (Japanese Patent No. 2024-517246).

[0004] In addition, as a photocatalytic technology for converting carbon dioxide into acetic acid or formic acid using visible light by fixing a specific catalyst material to a substrate, a mixed sol comprising electron acceptor slurry containing tungsten oxide microparticles, Ru species dye, ethyl viologen dichloride, and artificial acetic acid bacteria enzymes is prepared by encapsulating the slurry in an organic solid solution binder, and the mixture is coated onto a substrate to convert carbon dioxide into acetic acid or formic acid using a photocatalytic reaction (Japanese Patent Publication No. 2022-76331).

[0005] In conventional methods, specifically in electrochemical methods, a separator was required in the configuration of the electrolytic cell. Additionally, in photocatalytic technology, a substrate utilizing specific elements was required.

[0006] One embodiment of the present disclosure aims to provide a method for manufacturing a carbon-neutral carboxylic acid efficiently and with a low environmental burden.

[0007] The means for solving the problem include the following modes.

[0008] <1> In a method for producing a carboxylic acid by reducing carbon dioxide in the atmosphere,

[0009] A method for producing a carboxylic acid, comprising: a process of immersing a hydrogen-absorbing alloy negative electrode used in a nickel-hydrogen battery in an alkaline electrolyte and then drying it; an alkaline electrolyte capable of adsorbing carbon dioxide; a positive electrode used in a nickel-hydrogen battery; a hydrogen-absorbing alloy negative electrode after treatment; and an electrolytic cell having a power source connected to the positive electrode and the negative electrode, and a process of reducing carbon dioxide by applying a potential difference to the positive electrode and the negative electrode such that the potential of the positive electrode becomes higher than the potential of the negative electrode.

[0010] <2> The hydrogen storage alloy negative electrode comprises MmNi5, and the positive electrode comprises at least one of Ni(OH)2 and NiOOH. <1> Method for preparing carboxylic acid as described in

[0011] <3> The hydrogen-absorbing alloy negative electrode after treatment has a diffraction peak in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements, <1> or <2> Method for preparing carboxylic acid as described in

[0012] <4> The alkaline electrolyte comprises water and at least one selected from KOH, K2CO3, Na2CO3, and KHCO3. <1> ~ <3> A method for preparing a carboxylic acid as described in any one of the following.

[0013] <5> The carboxylic acid comprises at least one selected from formic acid, acetic acid, and propionic acid. <1> ~ <4> A method for preparing a carboxylic acid as described in any one of the following.

[0014] According to one embodiment of the present disclosure, a method for manufacturing a carbon-neutral carboxylic acid efficiently and with a low environmental burden is provided. Brief explanation of the drawing

[0015] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, in which the same reference numerals indicate the same elements. Figure 1 is a graph of the X-ray diffraction results of each MmNi5-based hydrogen-absorbing alloy cathode in the examples and comparative examples. Specific details for implementing the invention

[0016] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0017] In the present disclosure, a numerical range indicated by "~" means a range that includes the values ​​listed before and after "~" as the minimum and maximum values, respectively.

[0018] In the numerical ranges described stepwise in this disclosure, an upper or lower limit value described in any numerical range may be substituted with an upper or lower limit value of a numerical range described in another stepwise manner. In the numerical ranges described in this disclosure, an upper or lower limit value described in any numerical range may be substituted with a value shown in the examples.

[0019] In the present disclosure, the term “process” includes not only independent processes but also cases where the process cannot be clearly distinguished from other processes, provided that the intended purpose of the process is achieved.

[0020] In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment.

[0021] In the present disclosure, the “anode” in an electrolytic reaction is also referred to as the “positive electrode,” and the “negative electrode” is also referred to as the “negative electrode.”

[0022] Method for preparing carboxylic acid

[0023] A method for producing a carboxylic acid related to the present disclosure comprises a treatment process and a reduction process in a method for producing a carboxylic acid by reducing carbon dioxide in the atmosphere. In the treatment process, a hydrogen-absorbing alloy negative electrode used in a nickel-hydrogen battery is immersed in an alkaline electrolyte and then dried. In the reduction process, carbon dioxide is reduced by using an electrolytic cell and applying a potential difference to the positive electrode and the negative electrode by means of a power source such that the potential of the positive electrode becomes higher than the potential of the negative electrode. The electrolytic cell comprises an alkaline electrolyte, a positive electrode, a negative electrode, and a power source. The alkaline electrolyte is capable of adsorbing carbon dioxide. The positive electrode and the negative electrode are each used in a nickel-hydrogen battery, and the negative electrode is pretreated by a treatment process before being used for electrolysis. The power source is connected to the positive electrode and the negative electrode. Furthermore, hereinafter, the alkaline electrolyte is also referred to as the electrolyte.

[0024] Processing process

[0025] In the processing step, the hydrogen storage alloy negative electrode may be a negative electrode containing a hydrogen storage alloy used in nickel-hydrogen batteries (i.e., nickel-hydrogen secondary batteries (Ni-MH)). As for the hydrogen storage alloy, from the perspective of environmental burden, an AB5 type hydrogen storage alloy widely used in nickel-hydrogen batteries is preferred, a hydrogen storage alloy made from mischmetal is more preferred, and one containing MmNi5 is more preferred. The hydrogen storage alloy negative electrode may be one that is not yet used as a battery, or it may be a hydrogen storage alloy negative electrode included in a nickel-hydrogen battery after use.

[0026] As for the alkaline electrolyte, an alkaline electrolyte used in a nickel-hydrogen battery is preferred. The alkaline electrolyte may be one that has been used before being used in a nickel-hydrogen battery, or one that has been used after being used in a nickel-hydrogen battery. The alkaline electrolyte may be of a single type or a mixture of multiple types. The alkaline electrolyte may be of the same type as the alkaline electrolyte used in the electrolytic cell.

[0027] Examples of alkaline electrolytes include aqueous solutions of potassium carbonate (K2CO3), potassium hydroxide (KOH), potassium bicarbonate (KHCO3), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), and lithium hydroxide (LiOH).

[0028] From the perspective of carboxylic acid production, it is preferable that the alkaline electrolyte comprises water and at least one selected from KOH, K2CO3, and Na2CO3. Additionally, 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.

[0029] The method of immersing the hydrogen storage alloy negative electrode in the alkaline electrolyte is not limited. It is sufficient if part or all of the hydrogen storage alloy negative electrode is immersed in the alkaline electrolyte. If the hydrogen storage alloy negative electrode is one contained in a used nickel-hydrogen battery, it is already immersed in the alkaline electrolyte, so the hydrogen storage alloy negative electrode extracted from the used nickel-hydrogen battery may be used as is.

[0030] The hydrogen-absorbing alloy anode immersed in the alkaline electrolyte is then dried. The drying does not have to be complete, such as the removal of all moisture, but is sufficient if the alkaline electrolyte attached to the hydrogen-absorbing alloy anode is concentrated.

[0031] The inventors have discovered that by using a dried hydrogen-absorbing alloy negative electrode to which an alkaline electrolyte is attached in an electrolytic cell, the resistance of the hydrogen-absorbing alloy negative electrode in the alkaline electrolyte is significantly reduced. Although the reason is not clear, it is presumed that the surface of the hydrogen-absorbing alloy negative electrode is activated by a change in surface state, thereby producing carboxylic acids during electrolysis. As one of the changes in surface state, it is presumed that at least some of the components of the alkaline electrolyte exist on the surface as crystals. It is presumed that, due to the maintenance of a localized strong alkaline environment, carboxylate ions such as formate ions, acetate ions, and propionate ions are formed during the CO2 reduction reaction.

[0032] As for the drying temperature, a relatively low temperature is desirable from the perspective of carboxylic acid production, and as an example, drying is performed in the atmosphere at 80°C for several hours. Although drying is not limited to these conditions, it is presumed that drying at such a relatively low temperature produces crystals that adhere relatively uniformly.

[0033] Due to the treatment process, it is desirable for the hydrogen-absorbing alloy negative electrode after treatment to have a diffraction peak in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements. The diffraction peak in the range of 2θ = 32° to 33° is a diffraction peak that is newly included in the hydrogen-absorbing alloy negative electrode after treatment and does not exist in the X-ray diffraction (XRD) measurement results for the hydrogen-absorbing alloy negative electrode before treatment. It is presumed that due to the treatment process described above, crystals of KOH, a component of the alkaline electrolyte, exist on a part of the surface of the hydrogen-absorbing alloy negative electrode after treatment, and a new diffraction peak in the range of 2θ = 32° to 33° appears as a diffraction peak originating from these KOH crystals. Furthermore, the hydrogen-absorbing alloy negative electrode after treatment also has a peak caused by the hydrogen-absorbing alloy itself before treatment.

[0034] Reduction process

[0035] In the reduction process, an electrolytic cell is used. The alkaline electrolyte provided by the electrolytic cell is capable of adsorbing carbon dioxide (CO2) and may be the same as the alkaline electrolyte in the treatment process. It is preferable that the alkaline electrolyte contains KOH, KHCO3, and K2CO3. Additionally, 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 in an alkaline secondary battery.

[0036] 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).

[0037] K2CO3+CO2+H2O←→2KHCO3 (1)

[0038] In the van't Hoff equation shown in the following equation (2), ΔH 0 ε is the standard enthalpy change (-96.1 kJ / molCO2), ΔS 0 ε is the standard entropy change (-208 J / KmolCO2). P0 is atmospheric pressure (0.1 MPa), R is the gas constant, 8.314 JK -1 mol -1 , T is set to the temperature (298K). As a result, the pressure P is 0.112 Pa, and the equilibrium concentration of CO2 absorption is 1.12 ppm. Therefore, potassium bicarbonate (KHCO3) can absorb carbon dioxide (400 ppm) from the atmosphere, and potassium bicarbonate (KHCO3) is electrochemically used as a CO2 source. Afterwards, potassium bicarbonate (KHCO3) is regenerated into potassium carbonate (K2CO3).

[0039] (Formula 1)

[0040]

[0041] Since the aqueous KOH solution becomes an aqueous K2CO3 solution or a potassium bicarbonate (KHCO3) solution by adsorbing CO2 from the atmosphere, the aqueous KOH solution may be used in contact with the atmosphere. KOH absorbs CO2 (400 ppm) from the atmosphere and produces some potassium carbonate (K2CO3) and water. The reaction equation is shown in Equation (3).

[0042] 2KOH+CO2←→K2CO3+H2O (3)

[0043] In the van't Hoff equation shown in the above equation (2), ΔH 0 ε is the standard enthalpy change (-194 kJ / molCO2), ΔS 0 ε is the standard entropy change (-151 J / KmolCO2). P0 is atmospheric pressure (0.1 MPa), R is the gas constant, 8.314 JK -1 mol -1 , substitute the temperature (298K) for T. As a result, the pressure P is 0.112 Pa, and the equilibrium concentration of CO2 absorption is 7.92 × 10⁻⁶ -21 KOH can absorb carbon dioxide from the atmosphere at ppm. Since this K2CO3 is thermodynamically stable, the aforementioned aqueous K2CO3 solution is considered to be more effective than the aqueous KOH solution in terms of carboxylic acid production, as it allows for the effective utilization of K2CO3 as a CO2 source, and is therefore preferred as an alkaline electrolyte. Furthermore, when using an aqueous KOH solution as the alkaline electrolyte in an electrolytic cell, electrolysis may be performed by increasing the potential difference, etc.

[0044] The alkaline electrolyte may be an electrolyte contained in a nickel-hydrogen battery (i.e., a nickel-hydrogen secondary battery (Ni-MH)). The electrolyte may be an electrolyte contained in a nickel-hydrogen battery before use or an electrolyte contained in a nickel-hydrogen battery after use. The alkaline electrolyte may contain trace amounts of elements or compounds other than those mentioned above.

[0045] As for the electrodes provided in the electrolytic cell, a positive electrode used in a nickel-hydrogen battery is utilized. It is preferable that the positive electrode comprises at least one of Ni(OH)2 and NiOOH. Elements other than 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 included in the nickel-hydrogen battery before use or the positive electrode included in the nickel-hydrogen battery after use.

[0046] From the perspective of carboxylic acid production, it is desirable to perform the same treatment process on the positive electrode as on the negative electrode. The treatment for the positive electrode is the same as the treatment for the negative electrode.

[0047] As for the power source, it is sufficient if it is capable of performing electrolysis using an electrolytic cell, but it is not limited to that. Carbon dioxide is reduced by applying a potential difference to the positive and negative electrodes using the power source, such that the potential of the positive electrode becomes higher than the potential of the negative electrode. It is preferable to use a power source that does not utilize fossil fuels, etc. This is because carboxylic acids, which serve as fuel raw materials, can be produced from carbon dioxide in the atmosphere with less environmental burden.

[0048] The electrolytic cell performs electrolysis in a state where the alkaline electrolyte can come into contact with the atmosphere. By doing so, CO2 adsorbed by the alkaline electrolyte can be electrochemically reduced. In the method for producing carboxylic acid disclosed in this invention, carboxylate ions are generated by the reduction of CO2, and it is believed that a large number of these ions also exist as potassium carboxylate in the alkaline electrolyte. By recovering the alkaline electrolyte containing potassium carboxylate and neutralizing this electrolyte, carboxylic acid can be recovered.

[0049] In the method for producing a carboxylic acid of the present disclosure, carboxylate ions may be generated. The carboxylate ions may include at least one selected from formate ions, acetate ions, and propionic acid. In the method for producing a carboxylic acid of the present disclosure, an alcohol such as methanol may be generated. Additionally, oxygen is generated from the positive electrode by electrolysis. The generated oxygen may be released into the atmosphere or recovered and utilized.

[0050] According to the method for producing carboxylic acid related to the present disclosure, carboxylic acid containing acetic acid, which serves as a raw material for, for example, vinegar, polymers, and fuel 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-hydrogen battery. The method for producing carboxylic acid related to the present disclosure can be implemented, for example, by opening a used nickel-hydrogen battery, subjecting the negative electrode to a removal processing process, returning it, and applying a potential difference to both electrodes. Furthermore, even when using an electrolyte, positive electrode, or negative electrode that is not currently in use as a nickel-hydrogen battery, existing components can be utilized as they are. Accordingly, according to the method for producing carboxylic acid related to the present disclosure, carbon-neutral carboxylic acid can be produced efficiently and with a low environmental impact.

[0051] The present disclosure is further explained in detail below by way of examples. The examples in this specification are illustrative and are not intended to be limiting. A person skilled in the art related to the present disclosure can appropriately modify, alter, or substitute materials, compositions, manufacturing methods, and objects of application without departing from the spirit and scope of the present invention. Furthermore, "Ni(OH)2 / NiOOH positive electrode" means that the positive electrode comprises at least one of Ni(OH)2 and NiOOH, and may comprise both.

[0052] measurement method

[0053] In the following examples and comparative examples, electrolysis was performed using an electrolytic cell under predetermined conditions, and then ion chromatography was performed to measure the respective amounts of formate ions, acetate ions, and propionate ions produced. In addition, the amounts produced were measured by mass. The measurement results are shown in Table 1.

[0054] Example 1

[0055] A nickel-hydrogen battery was disassembled, and a Ni(OH)2 / NiOOH positive electrode and an MmNi5-based hydrogen storage alloy negative electrode were extracted. The positive electrode used was dried after being immersed in water. As a processing step, the hydrogen storage alloy negative electrode used was dried after being extracted from the nickel-hydrogen battery. Drying was performed 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 under atmospheric conditions using a 2.5 M (mol / L; hereinafter the same) aqueous KHCO3 solution as the electrolyte. The electrolysis conditions were as follows: the electrode area for both the anode and cathode was 6 cm², the current density was 50 mA / cm², 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 propionate ions produced by electrolysis, and the total amount of these ions, are listed in Table 1 as the amount of carboxylate ions produced.

[0056] Example 2

[0057] Electrolysis experiments were conducted in the same manner as in Example 1, except that the electrolyte provided in the electrolytic cell was a 3.5 M aqueous K2CO3 solution. The amounts of formate ions, acetate ions, and propionate ions produced by electrolysis, and the total amount of these ions, were listed in Table 1 as the amount of carboxylate ions produced.

[0058] Example 3

[0059] Electrolysis experiments were conducted in the same manner as in Example 1, except that the electrolyte provided in the electrolytic cell was an 8M aqueous KOH solution. The amounts of formate ions, acetate ions, and propionate ions produced by electrolysis, and the total amount of these ions, were listed in Table 1 as the amount of carboxylate ions produced.

[0060] Comparative Example 1

[0061] Electrodes made of Pt were used as the anode and cathode, and a 2.5 M aqueous KHCO3 solution was used as the electrolyte to conduct an electrolysis experiment under atmospheric conditions. The current was kept constant at 300 mA and the experiment was conducted for 24 hours. Afterward, ion chromatography analysis was performed on the electrolyte. The amounts of formate ions, acetate ions, and propionate ions produced by electrolysis, and the total amount of these ions, were listed in Table 1 as the amount of carboxylate ions produced.

[0062] Comparative Example 2

[0063] Electrolysis experiments were conducted in the same manner as Comparative Example 1, except that the electrolyte provided in the electrolytic cell was a 3.5 M aqueous K2CO3 solution. The amounts of formate ions, acetate ions, and propionate ions produced by electrolysis, and the total amount of these ions, were listed in Table 1 as the amount of carboxylate ions produced.

[0064] Comparative Example 3

[0065] Electrolysis experiments were conducted in the same manner as Comparative Example 1, except that the electrolyte provided in the electrolytic cell was an 8M aqueous KOH solution. The amounts of formate ions, acetate ions, and propionate ions produced by electrolysis, and the total amount of these ions, were listed in Table 1 as the amount of carboxylate ions produced.

[0066]

[0067] Example 4

[0068] An MmNi5-based hydrogen storage alloy cathode extracted from a nickel-hydrogen 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 Fig. 1.

[0069] Comparative Example 4

[0070] Figure 1 shows the X-ray diffraction intensity curve of an MmNi5-based hydrogen-absorbing alloy cathode that is removed from a nickel-hydrogen battery, cleaned, and does not undergo a processing process.

[0071] In Example 4, by heat-treating the MmNi5-based alloy, a novel peak was formed at 2θ=32°~33°, which was not present in Comparative Example 4. This peak is thought to be the crystallization of KOH in the electrolyte used in nickel-hydrogen batteries. Additionally, novel peaks also exist at 12.9° and 25.7°, and these are also thought to be the crystallization of KOH in the electrolyte used in nickel-hydrogen batteries.

Claims

Claim 1 A method for producing a carboxylic acid by reducing carbon dioxide in the atmosphere, comprising: a process of immersing a hydrogen-absorbing alloy negative electrode used in a nickel-hydrogen battery in an alkaline electrolyte and then drying it; using an electrolytic cell having an alkaline electrolyte capable of adsorbing carbon dioxide, a positive electrode used in a nickel-hydrogen battery, a hydrogen-absorbing alloy negative electrode after the above treatment, and a power source connected to the positive electrode and the negative electrode, and a process of reducing the carbon dioxide by applying a potential difference to the positive electrode and the negative electrode such that the potential of the positive electrode becomes higher than the potential of the negative electrode. Claim 2 A method for producing a carboxylic acid according to claim 1, wherein the hydrogen absorption alloy negative electrode comprises MmNi5, and the positive electrode comprises at least one of Ni(OH)2 and NiOOH. Claim 3 A method for producing a carboxylic acid, wherein the hydrogen-absorbing alloy negative electrode after treatment has a diffraction peak in the range of 2θ=32°~33° in X-ray diffraction (XRD) measurement. Claim 4 A method for producing a carboxylic acid according to claim 1, wherein the alkaline electrolyte comprises water and at least one selected from KOH, K2CO3, Na2CO3, and KHCO3. Claim 5 A 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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