Process for the manufacture of carboxylic acids

Abstract

The present invention relates to a method of producing a carboxylic acid. A method of producing a carboxylic acid by reduction of carbon dioxide in the atmosphere includes a step of subjecting a hydrogen storage alloy negative electrode used in a nickel-hydrogen battery to a treatment in which the hydrogen storage alloy negative electrode is immersed in an alkaline electrolyte and then dried, and a step of applying a potential difference to a positive electrode used in the nickel-hydrogen battery and the hydrogen storage alloy negative electrode after the treatment, such that the potential of the positive electrode is higher than the potential of the hydrogen storage alloy negative electrode, in an electrolytic cell including an alkaline electrolyte configured to adsorb carbon dioxide, the positive electrode, the hydrogen storage alloy negative electrode after the treatment, and a power source connected to the positive electrode and the hydrogen storage alloy negative electrode, thereby reducing the carbon dioxide.

Description

Technical Field

[0001] This disclosure relates to a method for manufacturing carboxylic acids. Background Technology

[0002] Carbon dioxide (CO2) is one of the greenhouse gases that exists naturally in the atmosphere and is produced by the combustion of fossil fuels. Atmospheric CO2 levels have increased due to human activities and rising energy demands, which is believed to contribute to 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, have been extensively studied.

[0003] A method is known for reducing carbon dioxide using an electrochemical cell and extracting the resulting products in situ. In this method, a CO2-containing absorbent is introduced into the cathode, where the CO2 is electrochemically reduced to form products such as organic acids. These products are then extracted and collected in situ. Multiple membranes are arranged between the anode and the cathode to maintain an efficient reaction environment (Japanese Patent List No. 2024-517246 (JP 2024-517246 A)).

[0004] Photocatalysis is also known, in which specific catalytic materials are immobilized on a substrate, and visible light is used to convert carbon dioxide into acetic acid or formic acid. In this method, a mixed sol is prepared by co-encapsulating an electron acceptor slurry containing fine tungsten oxide particles, ruthenium (Ru)-based dyes, ethyl viologen dichloride, and artificial acetic acid bacteria enzyme using an organic solid solution binder, and the mixed sol is then coated onto a substrate. Carbon dioxide is thus converted into acetic acid or formic acid using a photocatalytic reaction (Japanese Patent Application Laid-Open No. 2022-76331 (JP 2022-76331 A)). Summary of the Invention

[0005] In conventional methods, electrochemical methods require a membrane in the construction of the electrolytic cell, while photocatalytic technology requires the use of a substrate with specific elements.

[0006] One embodiment of this disclosure aims to provide an efficient and environmentally friendly method for manufacturing carbon-neutral carboxylic acids.

[0007] The means to achieve the above objectives include the following aspects.

[0008] (1) A method for producing carboxylic acids by reducing carbon dioxide in the atmosphere, the method comprising: The step of treating the hydrogen storage alloy negative electrode used in a nickel-metal hydride battery, wherein the hydrogen storage alloy negative electrode is immersed in an alkaline electrolyte and then dried; and The step involves applying a potential difference between the positive electrode and the hydrogen storage alloy negative electrode in an electrolytic cell comprising an alkaline electrolyte configured to adsorb carbon dioxide, a positive electrode used in a nickel-metal hydride battery, the treated hydrogen storage alloy negative electrode, and a power source connected to the positive electrode and the hydrogen storage alloy negative electrode, such that the potential of the positive electrode is higher than that of the hydrogen storage alloy negative electrode, thereby reducing the carbon dioxide.

[0009] (2) According to the method of (1), wherein: the hydrogen storage alloy negative electrode contains MmNi5; and the positive electrode contains either or both of Ni(OH)2 and NiOOH.

[0010] (3) The method according to (1) or (2) wherein the hydrogen storage alloy anode has a diffraction peak in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements after the treatment.

[0011] (4) The method according to any one of (1) to (3), wherein the alkaline electrolyte contains: water, and at least one selected from KOH, K2CO3, Na2CO3 and KHCO3.

[0012] (5) The method according to any one of (1) to (4), wherein the carboxylic acid comprises at least one selected from formic acid, acetic acid and propionic acid.

[0013] One embodiment of this disclosure provides a highly efficient and environmentally friendly method for producing carbon-neutral carboxylic acids. Attached Figure Description

[0014] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention are described below with reference to the accompanying drawings, wherein like symbols denote like elements, and wherein: Figure 1 This is a graph showing the X-ray diffraction results of the MmNi5-based hydrogen storage alloy cathodes in the examples and comparative examples. Detailed Implementation

[0015] Embodiments of the present disclosure are described below with reference to the accompanying drawings.

[0016] In this disclosure, the numerical range indicated by “to” represents a range that includes the minimum and maximum values ​​specified before and after “to”, respectively.

[0017] In the numerical ranges described in stages in this disclosure, the upper or lower limit of one numerical range can be replaced by the upper or lower limit of another numerical range. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit of the numerical range can be replaced by the values ​​disclosed in the embodiments.

[0018] In this disclosure, the term "step" includes not only independent steps, but also steps that cannot be clearly distinguished from other steps, as long as the intended purpose of the step is achieved.

[0019] In this disclosure, a combination of two or more preferred aspects is considered a more preferred aspect.

[0020] In this disclosure, the "anode" in the electrolysis reaction is also called the "positive electrode," and the "cathode" is also called the "negative electrode."

[0021] Methods for manufacturing carboxylic acids

[0022] The method for manufacturing carboxylic acids according to this disclosure is a method for producing carboxylic acids by reducing carbon dioxide in the atmosphere, and includes a processing step and a reduction step. In the processing step, a hydrogen storage alloy negative electrode used in nickel-metal hydride batteries is immersed in an alkaline electrolyte and then dried. In the reduction step, a potential difference is applied to the positive and negative electrodes in an electrolytic cell using a power source, such that the potential of the positive electrode is higher than that of the negative electrode, thereby reducing carbon dioxide. The electrolytic cell includes an alkaline electrolyte, a positive electrode, a negative electrode, and a power source. The alkaline electrolyte is capable of adsorbing carbon dioxide. The positive and negative electrodes are electrodes used in nickel-metal hydride batteries, and the negative electrode is pretreated by the processing step before being used for electrolysis. The power source is connected to the positive and negative electrodes. Hereinafter, the alkaline electrolyte is also referred to as "electrolyte".

[0023] Processing steps

[0024] In the aforementioned processing step, the hydrogen storage alloy negative electrode can be any negative electrode contained in hydrogen storage alloys used in nickel-metal hydride batteries (i.e., nickel-metal hydride secondary batteries (Ni-MH)). From an environmental impact perspective, the AB5 type hydrogen storage alloy, widely used in nickel-metal hydride batteries, is preferred; a hydrogen storage alloy based on mixed rare earth metals is more preferred; and a hydrogen storage alloy containing MmNi5 is even more preferred. The hydrogen storage alloy negative electrode can be an electrode before use in the battery or an electrode contained in a used nickel-metal hydride battery.

[0025] The alkaline electrolyte is preferably a solution used in nickel-metal hydride (NiMH) batteries. The alkaline electrolyte may be a solution not yet used in NiMH batteries, or a solution from previous use in NiMH batteries. The alkaline electrolyte may be a single type or a mixture of multiple types. The alkaline electrolyte may be the same type as the alkaline electrolyte used in the electrolytic cell.

[0026] 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).

[0027] From the viewpoint of carboxylic acid production, it is preferable that the alkaline electrolyte contains water and at least one selected from KOH, K₂CO₃, and Na₂CO₃. Since the KOH aqueous solution becomes a K₂CO₃ aqueous solution or a KHCO₃ aqueous solution by adsorbing CO₂ from the atmosphere, the KOH aqueous solution that has been in contact with air can be used as the alkaline electrolyte.

[0028] There are no restrictions on the method used to impregnate the hydrogen storage alloy negative electrode in an alkaline electrolyte. Any method can be used, as long as part or all of the hydrogen storage alloy negative electrode is impregnated in the alkaline electrolyte. When the hydrogen storage alloy negative electrode is a component of a used nickel-metal hydride battery, it has already been impregnated in the alkaline electrolyte. Therefore, the hydrogen storage alloy negative electrode removed from a used nickel-metal hydride battery can be used directly.

[0029] The hydrogen storage alloy negative electrode, which has been immersed in an alkaline electrolyte, is then dried. Drying does not necessarily mean, for example, complete drying to remove all moisture, but can be a process of concentrating the alkaline electrolyte adhering to the hydrogen storage alloy negative electrode.

[0030] The inventors have discovered that by using a material obtained by drying a hydrogen storage alloy negative electrode coated with alkaline electrolyte in an electrolytic cell, the resistance of the hydrogen storage alloy negative electrode in the alkaline electrolyte is significantly reduced. Although the reason is unclear, it is speculated that the change in surface state activates the surface of the hydrogen storage alloy negative electrode, thereby leading to the generation of carboxylic acids during electrolysis. One possible example of this change in surface state is that at least a portion of the alkaline electrolyte exists as crystals on the surface. It is speculated that as a result, a locally strongly alkaline environment is maintained, thereby forming carboxylate ions such as formate ions, acetate ions, and propionate ions in the CO2 reduction reaction.

[0031] From the viewpoint of manufacturing carboxylic acids, a relatively low drying temperature is preferred. As an example, drying in air at 80°C for several hours is possible. While drying conditions are not limited to these, it is presumed that drying at relatively low temperatures produces relatively uniformly adhered crystals.

[0032] The hydrogen storage alloy anode treated in the processing step preferably exhibits diffraction peaks in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements. These diffraction peaks in the 2θ = 32° to 33° range are not present in the XRD measurements of the hydrogen storage alloy anode before treatment and are newly observed in the treated anode. It is speculated that, as a result of the aforementioned processing step, KOH crystals, which are components of the alkaline electrolyte, are present on a portion of the surface of the treated hydrogen storage alloy anode, and the diffraction peaks originating from these KOH crystals are newly observed in the 2θ = 32° to 33° range. The treated hydrogen storage alloy anode also exhibits peaks originating from the hydrogen storage alloy itself before treatment.

[0033] Restoration steps

[0034] The reduction step uses an electrolytic cell. The alkaline electrolyte in this electrolytic cell is capable of adsorbing carbon dioxide (CO2) and can be the same as the alkaline electrolyte used in the treatment step. This alkaline electrolyte preferably contains KOH, KHCO3, and K2CO3. Since KOH aqueous solution becomes K2CO3 aqueous solution or KHCO3 aqueous solution by adsorbing CO2 from the atmosphere, KOH aqueous solution that has been in contact with air can be used as an alkaline electrolyte. The alkaline electrolyte in the electrolytic cell can be the same as the electrolyte in an alkaline secondary battery.

[0035] Potassium carbonate (K2CO3) reacts with atmospheric CO2 (400ppm, 40Pa) in the presence of water to produce potassium bicarbonate (KHCO3). This reaction is represented by the following equation (1).

[0036] K2CO3 + CO2 + H2O ←→ 2KHCO3 (1)

[0037] In the van't Hoff formula shown in equation (2) below, ΔH 0 The standard enthalpy change (-96.1 kJ / mol CO2) is represented by ΔS. 0 This represents the standard entropy change (-208 J / Kmol CO2). Substitute P0 for atmospheric pressure (0.1 MPa) and R for the gas constant (8.314 J / K). -1 mol -1 The temperature (T) was substituted into the equation (298 K). The results showed that the pressure P was 0.112 Pa, and the equilibrium concentration of CO2 absorption was 1.12 ppm. This indicates that potassium carbonate (K₂CO₃) can absorb atmospheric carbon dioxide (400 ppm), and potassium bicarbonate (KHCO₃) is electrochemically used as a CO₂ source. Subsequently, potassium bicarbonate (KHCO₃) was regenerated into potassium carbonate (K₂CO₃).

[0038]

[0039] Since KOH aqueous solution becomes K2CO3 aqueous solution or KHCO3 aqueous solution by adsorbing CO2 from the atmosphere, KOH aqueous solution that has been in contact with air can be used. KOH absorbs CO2 (400ppm) from the atmosphere to partially produce potassium carbonate (K2CO3) and water. This reaction is represented by the following equation (3).

[0040] 2KOH + CO2 ←→ K2CO3 + H2O (3)

[0041] In the van der Hoff formula shown in equation (2) above, ΔH 0 The standard enthalpy change (-194 kJ / mol CO2), ΔS 0 This represents the standard entropy change (-151 J / Kmol CO2). Substitute P0 for atmospheric pressure (0.1 MPa) and R for the gas constant (8.314 J / Kmol CO2). -1 mol -1 Substituting T into the temperature (298 K), the results show that the pressure P is 0.112 Pa and the equilibrium concentration of CO2 absorption is 7.92 × 10⁻⁶. - 21 ppm. This indicates that KOH can absorb carbon dioxide from the atmosphere. Since K2CO3 is thermodynamically stable, the above-mentioned K2CO3 aqueous solution is considered to make K2CO3 a more effective CO2 source for the production of carboxylic acids compared to KOH aqueous solution, and is therefore preferred as an alkaline electrolyte. When using KOH aqueous solution as an alkaline electrolyte in an electrolytic cell, electrolysis can be carried out, for example, by increasing the potential difference.

[0042] The alkaline electrolyte can be the electrolyte contained in a nickel-metal hydride battery (i.e., a nickel-metal hydride secondary battery (Ni-MH)). The electrolyte can be the electrolyte contained in a nickel-metal hydride battery before use or after use. The alkaline electrolyte may contain trace elements or compounds other than those mentioned above.

[0043] Regarding the electrodes set in the electrolytic cell, the positive electrode used in nickel-metal hydride batteries serves as the positive electrode. Preferably, the positive electrode contains either or both of Ni(OH)₂ and NiOOH. Elements other than Ni(OH)₂ or NiOOH, such as Co or Fe, can be added to the positive electrode. The positive electrode can be a positive electrode contained in a nickel-metal hydride battery before use or a positive electrode contained in a nickel-metal hydride battery after use.

[0044] From the viewpoint of carboxylic acid manufacturing, it is preferable that the positive electrode undergoes the same processing steps as the negative electrode. The processing of the positive electrode is the same as that of the negative electrode.

[0045] The power source is not limited, as long as it can perform electrolysis using an electrolytic cell. Carbon dioxide is reduced by applying a potential difference between the positive and negative electrodes, making the potential of the positive electrode higher than that of the negative electrode. A power source that does not use fossil fuels is preferred. This is because carboxylic acids, used as fuel feedstock, can be produced from atmospheric carbon dioxide with a relatively low environmental impact.

[0046] Electrolytic cells are used to perform electrolysis in an alkaline electrolyte environment where it can be exposed to the atmosphere. As a result, CO2 adsorbed by the alkaline electrolyte can be electrochemically reduced. In the method for producing carboxylic acids according to this disclosure, carboxylate ions are generated by reducing CO2, and these are considered to largely exist as potassium carboxylate in the alkaline electrolyte. The alkaline electrolyte containing potassium carboxylate is recovered, and the carboxylic acid can be recovered by neutralizing the electrolyte.

[0047] In the method for producing carboxylic acids according to this disclosure, carboxylate ions may be generated. The carboxylate ions may include at least one selected from formate ions, acetate ions, and propionate ions. In the method for producing carboxylic acids according to this disclosure, alcohols such as methanol may also be generated. Oxygen is generated from the positive electrode by electrolysis. The generated oxygen may be released into the atmosphere or may be recovered and used.

[0048] The method for manufacturing carboxylic acids according to this disclosure enables the electrolysis of carbon dioxide from the atmosphere to produce carboxylic acids, including acetic acid, which is used as a feedstock for, for example, vinegar, polymers, or ethanol as fuel. The electrolytic cell used in the electrolysis can utilize the electrodes and electrolyte of a used nickel-metal hydride battery. The method for manufacturing carboxylic acids according to this disclosure can be performed, for example, by opening a used nickel-metal hydride battery, removing the negative electrode, subjecting the negative electrode to a treatment step, returning it to the battery, and then applying a potential difference to the electrodes. Even when using electrolytes, positive electrodes, or negative electrodes not used in nickel-metal hydride batteries, existing components can be used directly. Therefore, the method for manufacturing carboxylic acids according to this disclosure enables the efficient production of carbon-neutralized carboxylic acids with low environmental impact.

[0049] The present disclosure will now be described in further detail with reference to embodiments. The embodiments described herein are exemplary and not intended to be limiting. Those skilled in the art to which this disclosure pertains will understand that materials, compositions, manufacturing methods, and applications can be suitably modified, altered, or substituted without departing from the spirit and scope of the invention. The term "Ni(OH)2 / NiOOH cathode" refers to a cathode comprising Ni(OH)2 or NiOOH, or both.

[0050] Measurement methods

[0051] In the following examples and comparative examples, after electrolysis was performed using an electrolytic cell under predetermined conditions, ion chromatography analysis was performed to measure the amounts of formate, acetate, and propionate ions produced. The amounts produced were measured based on mass. The measurement results are shown in Table 1.

[0052] Example 1

[0053] Disassemble the nickel-metal hydride battery and remove the Ni(OH)₂ / NiOOH positive electrode and the MmNi₅ type hydrogen storage alloy negative electrode. The positive electrode used was obtained by immersion in water and subsequent drying. The hydrogen storage alloy negative electrode used in the processing steps was an electrode that had been dried after being removed from the nickel-metal hydride battery. Drying was carried out in air at 80°C for 3 hours. The positive electrode was used as the anode, the negative electrode as the cathode, and a 2.5 M (mol / L) KHCO₃ aqueous solution was used as the electrolyte. Electrolysis experiments were conducted in air. The electrolysis conditions were as follows: the electrode area of ​​both the anode and cathode was 6 cm². 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℃, and the electrolyte volume was 200 mL. Subsequently, the electrolyte was analyzed by ion chromatography. The amounts of formate, acetate, and propionate ions produced by electrolysis, as well as the total amount of these ions produced as carboxylate ions, are shown in Table 1.

[0054] Example 2

[0055] Except that the electrolyte in the electrolytic cell was a 3.5 M aqueous solution of K2CO3, the electrolysis experiment was conducted in the same manner as in Example 1. The amounts of formate, acetate, and propionate ions produced by electrolysis, as well as the total amount of these ions produced as carboxylate ions, are shown in Table 1.

[0056] Example 3

[0057] Except that the electrolyte in the electrolytic cell was an 8M KOH aqueous solution, the electrolysis experiment was conducted in the same manner as in Example 1. The amounts of formate, acetate, and propionate ions produced by electrolysis, as well as the total amount of these ions produced as carboxylate ions, are shown in Table 1.

[0058] Comparative Example 1

[0059] Electrolysis was conducted in air using platinum (Pt) electrodes as both anode and cathode, and a 2.5 M aqueous solution of KHCO3 as the electrolyte. The current was kept constant at 300 mA, and electrolysis was carried out for 24 hours. The electrolyte was then analyzed by ion chromatography. The amounts of formate, acetate, and propionate ions produced by electrolysis, as well as the total amount of these ions produced as carboxylate ions, are shown in Table 1.

[0060] Comparative Example 2

[0061] Except that the electrolyte in the electrolytic cell was a 3.5 M aqueous solution of K₂CO₃, the electrolysis experiment was conducted in the same manner as in Comparative Example 1. The amounts of formate, acetate, and propionate ions produced by electrolysis, as well as the total amount of these ions produced as carboxylate ions, are shown in Table 1.

[0062] Comparative Example 3

[0063] Except that the electrolyte in the electrolytic cell was an 8M KOH aqueous solution, the electrolysis experiment was conducted in the same manner as in Comparative Example 1. The amounts of formate, acetate, and propionate ions produced by electrolysis, as well as the total amount of these ions produced as carboxylate ions, are shown in Table 1.

[0064] Table 1

[0065] Example 4

[0066] The MmNi5-based hydrogen storage alloy cathode, removed from the 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 below. Figure 1 middle.

[0067] Comparative Example 4

[0068] exist Figure 1 The image shows the X-ray diffraction intensity curves of a MmNi5-based hydrogen storage alloy cathode that has been removed from a nickel-metal hydride battery, cleaned, and has not undergone any treatment steps.

[0069] As a result of the heat treatment of the MmNi5 alloy in Example 4, a new peak not present in Comparative Example 4 was observed at 2θ = 32° to 33°. This peak is believed to be caused by the crystallization of KOH, which is used as the electrolyte in the nickel-metal hydride battery. Other new peaks were observed at 12.9° and 25.7°. These peaks are also believed to be caused by the crystallization of KOH, which is used as the electrolyte in the nickel-metal hydride battery.

Claims

1. A method for producing carboxylic acids by reducing carbon dioxide in the atmosphere, the method comprising: A step of treating a hydrogen storage alloy negative electrode used in a nickel-metal hydride battery, wherein the hydrogen storage alloy negative electrode is immersed in an alkaline electrolyte and then dried; and The step of reducing carbon dioxide is performed by applying a potential difference between the positive electrode and the hydrogen storage alloy negative electrode in an electrolytic cell comprising an alkaline electrolyte configured to adsorb carbon dioxide, a positive electrode used in a nickel-metal hydride battery, the treated hydrogen storage alloy negative electrode, and a power source connected to the positive electrode and the hydrogen storage alloy negative electrode, such that the potential of the positive electrode is higher than that of the hydrogen storage alloy negative electrode.

2. The method according to claim 1, wherein: The hydrogen storage alloy negative electrode contains MmNi5; and The positive electrode contains either or both of Ni(OH)2 and NiOOH.

3. The method according to claim 1, wherein the hydrogen storage alloy anode has a diffraction peak in the range of 2θ = 32° to 33° in X-ray diffraction measurements after the treatment.

4. The method according to claim 1, wherein the alkaline electrolyte contains: Water, and It is selected from at least one of KOH, K2CO3, Na2CO3 and KHCO3.

5. The method 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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