Cathode electrode for acethylene-containing gas production and manufacturing apparatus

Abstract

To provide a new cathode electrode used in electrolysis using carbon dioxide as a raw material, and an acethylene-containing gas manufacturing apparatus using the cathode electrode.SOLUTION: The present invention provides a cathode electrode for an apparatus that produces acetylene gas-containing gas by reducing carbon dioxide through an electrochemical reduction reaction, and includes a metal catalyst and a polymer electrolyte, where the polymer electrolyte has a polystyrene backbone as the main chain structure and includes an ion exchange group in which an ammonium cation and a hydroxyl anion are paired, in which the ammonium cation is derived from an aromatic heterocycle having a ring proton bonded to the para position of the benzene ring of the polystyrene backbone via a methylene group, and an acetylene gas-containing gas manufacturing apparatus using the inventive cathode electrode as a cathode electrode.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a cathode electrode for an apparatus for producing a gas containing acetylene gas directly from carbon dioxide by an electrochemical reduction reaction, and to a production apparatus using the same. [Background technology]

[0002] Acetylene gas is one of the important basic chemicals that is indispensable to modern society because it has the highest combustion temperature characteristics among the various hydrocarbon gases used in metal processing and is used as a raw material for synthesizing a wide range of organic chemicals.

[0003] The most common method for producing acetylene gas is the carbide process, which uses relatively inexpensive calcium carbide (carbide) and water as raw materials. However, the crude acetylene generated by the carbide process contains hydrogen phosphide and hydrogen sulfide as impurities derived from the raw carbide, so there is a problem that it is necessary to install expensive and complicated refining equipment in order to use it in metal processing and as a raw material for chemical synthesis, which are the main applications of the process.

[0004] In addition, given the growing trend toward decarbonization in recent years, the carbide method, which uses calcium carbide (calcium carbide) as a raw material during the manufacturing process by adding energy of about 2000°C to quicklime and coke in an electric furnace, is not desirable from the perspective of life cycle assessment, and there is a demand for an acetylene production method with a lower environmental impact.

[0005] One possible means for overcoming the above problems is the use of electrochemical conversion devices (electrolysis devices) that can perform chemical conversion of the feed components under mild conditions with relatively little energy consumption.

[0006] Many electrochemical carbon dioxide fixation techniques have been proposed using carbon dioxide as a raw material. The products of electrolysis include the C1 compounds carbon monoxide (CO), methane (CH 4 ), methanol (CH 3OH), formic acid (HCOOH), and the C2 compound ethane (C 2 H 6 ), ethanol (C 2 H 5 OH), ethylene (C 2 H 4 ) are known.

[0007] Among them, ethylene, a C2 compound, is an important compound used as a starting material for various chemical products. When considering the conversion of ethylene, an olefin (alkene) compound with reactive double bonds, by electrolytic reduction from carbon dioxide, it is necessary to form a carbon-carbon double bond while realizing a multi-electron reduction reaction, and the selectivity is generally lower than that of various C1 compounds and C2 alkane products. For this reason, various studies have been reported (Patent Documents 1 and 2).

[0008] Patent Document 1 focuses on the cathode catalyst and reports an electrolysis device that uses carbon dioxide as a raw material and converts it to ethylene with a selectivity of about 70% by investigating a copper electrode coated with cuprous halide as a cathode catalyst for the electrolysis device.

[0009] Patent Document 2 also focuses on the cathode catalyst and reports that the use of crystalline copper phthalocyanine as a cathode catalyst is effective for the synthesis of ethylene by electrolytic reduction.

[0010] However, in neither case is acetylene produced, which is an alkyne that is more reactive than an alkene. Its electrochemical conversion to acetylene, which requires a multi-electron reduction to form a carbon-carbon triple bond, has not been reported. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] JP 2004-176129 A [Patent Document 2] JP 2018-141227 A Summary of the Invention [Problem to be solved by the invention]

[0012] An object of the present invention is to provide a novel cathode electrode for use in electrolysis using carbon dioxide as a raw material as a new method for producing an acetylene-containing gas, and to provide an apparatus for producing an acetylene-containing gas using the cathode electrode. [Means for solving the problem]

[0013] As a result of extensive investigations aimed at solving the above problems, the present inventors have completed the invention described below. [1] A cathode electrode of an apparatus for producing a gas containing acetylene gas by reducing carbon dioxide through an electrochemical reduction reaction, the cathode electrode comprising a metal catalyst and a polymer electrolyte, the polymer electrolyte having a polystyrene skeleton as a main chain structure and an ion exchange group in which an ammonium cation and a hydroxyl anion are formed as an ion pair and the ion exchange group is an aromatic heterocycle having a ring proton bonded to the para position of the benzene ring of the polystyrene skeleton via a methylene group. [2] The cathode electrode according to [1], wherein the ammonium cation is at least one of an imidazolium cation, a pyridinium cation, and an isoquinolinium cation. [3] An apparatus for producing an acetylene-containing gas, comprising the cathode electrode according to [1] or [2] above as a cathode electrode. Effect of the Invention

[0014] According to the present invention, acetylene-containing gas can be directly obtained by an electrolytic reaction using carbon dioxide as a raw material, without using calcium carbide, which has a high environmental impact, and by introducing renewable energy as the electricity used, this will be an important technology that can greatly contribute to the realization of carbon neutrality. [Brief description of the drawings]

[0015] [Figure 1] Schematic diagram of an acetylene-containing gas production apparatus according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The cathode electrode of the present invention is a cathode electrode of an apparatus for producing a gas containing acetylene gas by reducing carbon dioxide through an electrochemical reduction reaction, and is characterized in that it contains, in addition to a metal catalyst, a polymer electrolyte having an ion exchange group in which an ammonium cation and a hydroxyl anion are formed as an ion pair, the ammonium cation being composed of an aromatic heterocycle having a ring proton bonded to the para position of the benzene ring of the polystyrene skeleton via a methylene group.

[0017] The ammonium cation having a polystyrene skeleton as a main chain structure and an aromatic heterocycle having a ring proton bonded to the para-position of the benzene ring of the polystyrene skeleton via a methylene group is a polymer electrolyte having a polystyrene skeleton as a main chain structure as shown in the following formula (1), and having an ion exchange group R at the benzylic carbon bonded to the para-position of the benzene ring of the styrene skeleton as indicated by the arrow in (formula 1).

[0018] [ka] In the formula (1), m is preferably 10 or more, and n is preferably about 0≦n≦3m.

[0019] The ion exchange group R is a functional group consisting of an ammonium cation and a hydroxyl anion, which are composed of an aromatic heterocycle having a proton on the ring, and the ion pair is an ammonium cation and a hydroxyl anion, and the aromatic heterocycle has at least one proton on the heteroaromatic ring and has little steric hindrance.

[0020] The ammonium cation is preferably at least one of an imidazolium cation, a pyridinium cation and an isoquinolinium cation, and the structural formula thereof is shown in formulae (2) to (4). [ka] At least one of R1, R2, and R3 in formula (2) is a proton, and if it is not a proton, it is a linear alkyl group having 3 or less carbon atoms with little steric hindrance. X is a linear alkyl group having 3 or less carbon atoms. At least one of R4 to R8 in formula (3) is a proton, and if it is not a proton, it is a linear alkyl group having 3 or less carbon atoms with little steric hindrance, or an amino group having no active proton and dialkylated with a linear alkyl group having 3 or less carbon atoms. At least one of R9 to R15 in formula (4) is a proton, and if it is not a proton, it is a linear alkyl group having 3 or less carbon atoms with little steric hindrance, or an amino group having no active proton and dialkylated with a linear alkyl group having 3 or less carbon atoms.

[0021] More preferred ammonium cations are those in which R1 to R3 are protons in the imidazolium cation of formula (2), R6 at the para position of the pyridinium ring where the cation is more delocalized in the pyridinium cation of formula (3) is a dimethylamino group and the others are protons, and R9 to R15 are protons in the isoquinolinium cation of formula (4), and further preferred are those in which R12 where the cation is more delocalized is a dimethylamino group and the others are protons.

[0022] The acetylene-gas-containing gas production apparatus of the present invention is an acetylene-gas-containing gas production apparatus having the cathode electrode according to the above [1] or [2] as a cathode electrode, and will be described below with reference to the drawings.

[0023] FIG. 1 is a schematic diagram of an apparatus for producing an acetylene-containing gas according to the present invention. The electrolysis cell 10 of the electrolysis device is composed of a membrane electrode assembly 9, a cathode separator 2, and an anode separator 6, and current collectors 18a, 18b are provided on the outer sides of the cathode separator 2 and the anode separator 6. The membrane electrode assembly 9 has a cathode electrode 5 and an anode electrode 8 laminated with an electrolyte membrane 1 interposed therebetween, and the cathode electrode 5 has a cathode catalyst layer 3 on the electrolyte membrane 1 side and a gas diffusion layer 4 on the cathode separator 2 side, and the anode electrode 8 has an anode catalyst layer 3 on the electrolyte membrane 1 side and a gas diffusion layer 4 on the anode separator 6 side.

[0024] To carry out the electrolytic reaction, power is supplied to the electrolytic cell 10 by connecting the cathode of the DC power supply 16 and the current collector 18a with a cathode side wiring 17a, and by connecting the anode of the DC power supply 16 and the current collector 18b with an anode side wiring 17b.

[0025] The cathode separator 2 and the anode separator 6 can be made of titanium with a platinum-plated surface. The cathode separator 2 and the anode separator 6 are both of the same shape, and at the point where they contact the cathode electrode 5 or the anode electrode 8, they form straight flow paths with a width of 1 mm, a depth of 1 mm, and a pitch width of 1 mm.

[0026] Carbon dioxide gas is supplied to the cathode separator 2 from a cathode supply gas cylinder 11 via a gas supply line 12a, and an acetylene-containing gas generated by an electrochemical reduction reaction is taken out from a cathode produced gas discharge line 12b. The anode separator 6 forms a liquid flow path, and an anode circulating liquid is circulated and supplied to the anode separator 6 by a circulation pump 14 connected to an anode circulating liquid storage tank 13. The circulating liquid containing the gas generated by the anode reaction is returned to the anode circulating liquid storage tank 13 and separated into gas and liquid in the anode circulating liquid storage tank 13. The anode produced gas is discharged from an anode discharge line 15 of the anode circulating liquid storage tank 13.

[0027] The electrolyte membrane 1 that can be used in the present invention is not particularly limited as long as it is an anion-exchange polymer electrolyte membrane. Commercially available membranes include PiperION (registered trademark, manufactured by Versogen), Sustanion (registered trademark, manufactured by Dioxide Materials), and Fumasep (registered trademark, manufactured by Fumatech).

[0028] The ionic group in the electrolyte membrane 1 that can be used in the present invention is an atomic group having a positive charge, and the type of functional group that constitutes the atomic group is not particularly limited.

[0029] When the ionic group is an atomic group having a positive charge, the ionic group of the electrolyte membrane is a salt having a counter anion and has anion exchange ability. The atomic group having a positive charge is generally a quaternary ammonium salt or an imidazolium salt.

[0030] The cathode electrode 5 used in the present invention is a gas diffusion electrode including a cathode catalyst layer 3 formed on a gas diffusion layer 4a. The cathode catalyst layer 3 is composed of a metal catalyst and a polymer electrolyte.

[0031] The polymer electrolyte used in the cathode catalyst layer 3 of the present invention is a polymer electrolyte having the polystyrene skeleton described in [1] above as a main chain structure, and having an ion exchange group in which an ammonium cation and a hydroxyl anion form an ion pair, the ammonium cation being composed of an aromatic heterocycle having a ring proton bonded via a methylene group to the benzylic carbon at the para-position of the benzene ring of the polystyrene skeleton.

[0032] The metal catalyst contained in the cathode catalyst layer 3 in the present invention is copper, and among them, a Cu catalyst is preferable, and a particulate Cu catalyst having a particle size of about 20 to 30 nm is more preferable.

[0033] The gas diffusion layer 4 in the present invention is a porous material having pores, and carbon paper, carbon nonwoven fabric, carbon cloth, etc. can be used, without any particular limitation.

[0034] The anode electrode 8 used in the present invention is a gas diffusion electrode including an anode catalyst layer 7 formed on a gas diffusion layer 4. The anode catalyst layer 7 is composed of a metal catalyst and a polymer electrolyte. The metal catalyst is not particularly limited as long as it is a metal that has water oxidation activity. For example, iridium oxide, platinum, etc. can be used. The polymer electrolyte is not particularly limited as long as it is an anion exchange type polymer electrolyte.

[0035] Next, a method for producing the gas diffusion electrodes (cathode electrode 5 and anode electrode 8) used in the present invention will be exemplified. A catalyst ink is prepared by the ink preparation method described below using a predetermined metal catalyst powder. The obtained catalyst ink is applied to the gas diffusion layer 4 to obtain the cathode electrode 5 and the anode electrode 8. The application method to the gas diffusion layer 4 can be either a method using a spray application device or a method using a bar coater. The catalyst layer sides of the prepared cathode electrode 5 and anode electrode 8 are laminated so as to face each other with the electrolyte membrane 1 interposed therebetween to produce a membrane electrode assembly 9.

[0036] The method for preparing the catalyst ink in the present invention will be exemplified. The catalyst ink is prepared by dispersing an arbitrary catalyst that is responsible for activity and an anionic polymer electrolyte that is responsible for ion conductivity and the function as a binder in an arbitrary solvent. As the dispersion solvent, various organic solvents such as lower alcohols such as methanol and ethanol, ketones such as acetone, methyl ethyl ketone, and cyclohexanone, ethers such as tetrahydrofuran, dioxane, and dibutyl ether, and polar solvents such as dimethylformamide, dimethylacetamide, and ethylene glycol can be used. These can be used alone or as a mixed solvent of two or more kinds. In addition, water can be used as a mixed solvent to reduce the risk of ignition of the catalyst ink. In addition, any method such as an ultrasonic bath, an ultrasonic homogenizer, a centrifugal stirrer, and a ball mill can be used for dispersing the catalyst.

[0037] Next, the method for producing an acetylene-containing gas in the present invention will be illustrated with reference to Fig. 1. The acetylene production in the present invention is achieved by applying a voltage from a DC power source 16 to a cathode electrode 5 and an anode electrode 8 separated by an electrolyte membrane 1, and directly converting carbon dioxide gas supplied from a cathode separator 2 into acetylene gas through electrochemical reduction.

[0038] A voltage is applied to the cathode electrode 5 and the anode electrode 8 so that the potential of the cathode electrode is negative relative to the potential of the anode electrode. At this time, a part of the applied voltage is consumed not only for the carbon dioxide reduction reaction at the cathode electrode but also for the water oxidation reaction at the anode electrode. The applied voltage is preferably about -2.0 to -4.0 V, but from the viewpoints of energy consumption and product selectivity, about -2.5 to 3.0 V is more suitable.

[0039] Examples of cathode supply gases that can be used in the present invention are given below. The cathode supply gas of the present invention is preferably 100% carbon dioxide gas from the viewpoint of reaction rate, but carbon dioxide diluted gas can also be used as long as it contains 10% or more carbon dioxide. The base gas of the dilution gas is not particularly limited as long as it is a stable gas such as nitrogen or argon. The cathode supply gas is supplied to the cathode separator as a humidified gas by passing through a humidification bubbler 19 set at the same temperature as the electrolysis reaction temperature. The linear velocity of the supply gas in the cell is not particularly limited as long as it is about 100 to 8000 cm / min, but is preferably about 600 to 800 cm / min from the viewpoint of achieving both saving of the supply gas and suppression of retention of the generated gas.

[0040] The anode circulation fluid that can be used in the present invention will be exemplified. The anode circulation fluid of the present invention can be an alkaline aqueous solution with a pH of about 12 to 14. The anode circulation fluid is stored in an anode circulation fluid storage tank 13, and is supplied to the anode separator by a circulation pump 14 during electrolysis. The linear velocity of the anode circulation fluid in the cell is not particularly limited as long as the anode generated gas does not remain, but is preferably about 50 to 400 cm / min in terms of pressure loss. As the solute of the anode circulation fluid, hydroxides of alkali metals such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide can be used. Potassium hydroxide is preferred in terms of solubility and versatility.

[0041] The production temperature of the acetylene gas-containing gas in the present invention is exemplified below. The reaction temperature is not particularly limited as long as it is about room temperature to 60° C. at which the electrolytic cell in the present invention can be used, but it is preferable to carry out the reaction at room temperature from the viewpoint of energy consumption and product selectivity.

[0042] In the electrolytic reaction of the present invention, the acetylene-containing gas produced by the reduction reaction of carbon dioxide is mainly ethylene and acetylene as C2 compounds, and methane and carbon monoxide as C1 compounds. In addition, hydrogen gas is produced as a by-product. Other liquid products that may be produced include formic acid, methanol, and ethanol. EXAMPLES

[0043] The present invention will be further described below with reference to examples and comparative examples, but the present invention is not limited to these.

[0044] <Example 1> (Synthesis of Polyelectrolyte 1) Styrene, 4-vinylbenzyl chloride, and chlorobenzene were added to a heated and dried Schlenk flask under a stable gas flow. 2,2'-azodiisobutyronitrile was then added, and the mixture was heated to 65°C and stirred overnight. After cooling, the polymer was precipitated with a mixed solvent of tetrahydrofuran / methanol. The precipitate was washed with ethanol and reprecipitated with pure water to isolate the polymer as a white solid. The polymer was dissolved in dimethylformamide in a Schlenk flask, 1-ethylimidazole was added, and the mixture was stirred at 115°C for 2 days. After cooling, the reaction solution was dropped into ethyl acetate, and the precipitated white solid was collected. The white solid was dissolved again and treated with an aqueous potassium hydroxide solution to ion-exchange the counter ion, chlorine ion, with hydroxide ion, to obtain the ion exchange group R in the formula (1) as the formula (2), in which R1, R2, and R3 in the formula (2) are H and X is C. 2 H 5 Thus, polymer electrolyte 1 was obtained.

[0045] (Synthesis of polymer electrolyte 2) The synthesis was carried out in the same manner as for polymer electrolyte 1, except that N,N-dimethyl-4-aminopyridine was used instead of 1-ethylimidazole as the ion exchange group. The ion exchange group R in formula (1) was represented by formula (3), in which R4, R5, R7, and R8 in formula (3) were H, and R6 was N(CH 3 ) 2 The polyelectrolyte electrolyte 2 was obtained.

[0046] (Preparation of Membrane Electrode Assembly 9) Iridium oxide (manufactured by Tokuriki Honten) was used as the anode catalyst, and copper nanoparticles (manufactured by Aldrich) were used as the cathode catalyst. The catalyst was weighed into a glass container, and pure water and ethanol were added and stirred with a centrifugal stirrer. Then, commercially available PiperION was added as the anode binder, and the polymer electrolyte 1 prepared above was added as the cathode binder, and each was subjected to a dispersion treatment with an ultrasonic homogenizer. The obtained anode ink and cathode ink were applied to the respective substrates (SIGRACET, 28BB) with an ultrasonic spray to form catalyst layers. The basis weight of the cathode catalyst and anode catalyst was 1.0 mg / cm for both. 2The resulting anode catalyst layer and cathode catalyst layer were laminated with the anion-exchange polymer membrane PiperION interposed between them so that the catalyst-coated surfaces faced each other to produce a membrane electrode assembly 9 of the present invention. The produced membrane electrode assembly 9 was sandwiched between a cathode separator 2 and an anode separator 6 obtained by molding titanium with a platinum-plated surface, and current collectors 18a and 18b were placed on the outside and assembled to complete the electrolysis cell of the present invention.

[0047] (Electrolytic Test) The assembled electrolysis cell was connected to a potentiostat, and a 1M potassium hydroxide aqueous solution was introduced to the anode side at a linear velocity of 70 to 250 cm / min, and 100% CO2 gas was introduced to the cathode side at a linear velocity of 700 to 800 cm / min. The electric potential of the cathode electrode was set to a negative value with respect to the anode electrode at a linear velocity of 0.2 A / cm 2 Electrolysis was performed for 30 minutes at a constant current density of 1000 kcal / kg, and the generated gas was collected in a gas collection bag connected to the cathode generated gas exhaust line between 20 and 30 minutes. The electrolysis product was analyzed by a gas chromatograph (Shimadzu Corporation).

[0048] The mass of each gas component was determined from the concentration of the produced gas, and the Faraday efficiency was calculated from the ratio of the amount of charge required to produce those gas components through the reduction reaction of carbon dioxide to the amount of charge actually input during the electrolysis reaction. Faraday efficiency (%) = charge required to generate gas / charge used x 100 As a result of the experiment, it was confirmed that acetylene was produced with a faradaic efficiency of 10% and ethylene with a faradaic efficiency of 56%, resulting in a total faradaic efficiency of 66% for the C2 compounds (see Table 1).

[0049] <Example 2> An experiment was carried out in the same manner as in Example 1, except that a polymer electrolyte in which the counter ion before ion exchange of the polymer electrolyte 1 was a chloride ion was used as the cathode binder, and CO was introduced after circulating a 1M potassium hydroxide aqueous solution on the cathode electrode side for 30 minutes after assembling the cell. As a result of the experiment, it was confirmed that acetylene was produced with a faradaic efficiency of 1% and ethylene with a faradaic efficiency of 28%, resulting in a total faradaic efficiency of 29% for the C2 compounds (see Table 1).

[0050] <Example 3> An experiment was carried out in the same manner as in Example 1, except that polymer electrolyte 2 was used as the cathode binder. As a result of the experiment, it was confirmed that acetylene was produced with a faradaic efficiency of 6.6% and ethylene with a faradaic efficiency of 34%, giving a total faradaic efficiency of 41% for the C2 compounds (see Table 1).

[0051] <Comparative Example 1> The experiment was carried out in the same manner as in Example 1, except that commercially available Sustainion XA-9 was used as the cathode binder, and after the cell was assembled, a 1M potassium hydroxide aqueous solution was circulated on the cathode electrode side for 30 minutes before introducing CO2. As a result of the experiment, it was confirmed that no acetylene was produced, and ethylene was produced with a faradaic efficiency of 35% (see Table 1).

[0052] <Comparative Example 2> An experiment was carried out in the same manner as in Example 1, except that a commercially available PiperION was used as the cathode binder. As a result of the experiment, it was confirmed that no acetylene was produced, and ethylene was produced with a faradaic efficiency of 23% (see Table 1).

[0053] <Comparative Example 3> The experiment was carried out in the same manner as in Example 1, except that commercially available Nafion was used as the cathode binder. As a result of the experiment, it was confirmed that no acetylene was produced, and ethylene was produced with a faradaic efficiency of 20% (see Table 1).

[0054] [Table 1] [Industrial Applicability]

[0055] According to the present invention, acetylene-containing gas can be directly obtained by an electrolytic reaction using carbon dioxide as a raw material, and by introducing renewable energy as the electricity used, this will become an important technology that can greatly contribute to the realization of carbon neutrality. [Explanation of symbols]

[0056] REFERENCE SIGNS LIST 1...electrolyte membrane, 2...cathode separator, 3...cathode catalyst layer, 4...gas diffusion layer, 5...cathode electrode, 6...anode separator, 7...anode catalyst layer, 8...anode electrode, 9...membrane electrode assembly, 10...electrolysis cell, 11...cathode supply gas cylinder, 12...cathode generated gas exhaust line, 13...anode circulating liquid storage tank, 14...circulation pump, 15...anode generated gas exhaust line, 16...DC power supply unit, 17...wiring, 18...current collector, 19...humidifying bubbler

Claims

1. A cathode electrode of an apparatus for producing a gas containing acetylene gas by reducing carbon dioxide through an electrochemical reduction reaction, the cathode electrode comprising a metal catalyst and a polymer electrolyte, the polymer electrolyte having a polystyrene skeleton as a main chain structure and an ion exchange group in which an ammonium cation and a hydroxyl anion are formed as an ion pair and the cation is composed of an aromatic heterocycle having a ring proton bonded to the para position of a benzene ring of the polystyrene skeleton via a methylene group.

2. 2. The cathode electrode according to claim 1, wherein the ammonium cation is at least one of an imidazolium cation, a pyridinium cation, and an isoquinolinium cation.

3. 3. An apparatus for producing an acetylene-containing gas, comprising the cathode electrode according to claim 1 or 2 as a cathode electrode.

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