Method for preparing carboxylic acid through CO2 electrolysis

Abstract

The invention relates to a method for preparing carboxylic acid through CO2 electrolysis, which is characterized in that a cathode uses CO2 discharged by chemical engineering as a raw material, and carboxylic acid is prepared through one-step electroreduction; the anode uses aldehyde with the same carbon atom number and structure as carboxylic acid as a raw material, and carboxylic acid is prepared through one-step electrooxidation. The anion-exchange membrane is arranged between the cathode and the anode of the electrolytic cell, the two sides of the anion-exchange membrane are coated with the catalyst, the membrane electrode prepared through the anion-exchange membrane is used for carrying out cathode and anode co-electrolysis, a high-purity carboxylic acid aqueous solution is prepared at the anode, almost no separation problem exists, and pure carboxylic acid can be prepared with low power consumption.

Description

Technical Field

[0001] The present invention relates to the electrochemically assisted carbon neutralization of electrochemically reducing CO2, and the technology of co-electrolyzing carboxylic acids at the cathode and anode, and specifically relates to a method for electrochemically producing carboxylic acids from CO2 to assist carbon neutralization and co-electrolyzing aldehydes at the anode for electrooxidation to produce carboxylic acids. Technical Background

[0003] Currently, there are various known methods for converting CO2 into other carbon-containing compounds, such as chemical reforming, photocatalysis, biological methods, and electrocatalytic reduction methods. Among them, electrochemical CO2 conversion can be carried out at room temperature and ambient pressure. The electrochemical process has high controllability, can obtain higher conversion efficiency, and has high practicality and industrial application prospects.

[0004] There are many products in the electrochemical reduction of CO2. Currently, the electrochemical conversion of CO2 to produce carboxylic acids is a relatively popular research direction, and lower fatty acids are important chemical raw materials. For example, formic acid is widely used in industries such as pesticides, leather, dyes, pharmaceuticals, and rubber. Formic acid can be directly used in fabric processing, leather tanning, textile printing and dyeing, and storage of green fodder, and can also be used as a metal surface treatment agent, rubber additive, and industrial solvent. Pure acetic acid can be used to manufacture artificial fibers, plastics, fragrances, drugs, etc. Higher fatty acids are the basis of the oil industry. Therefore, developing new electrocatalytic processes to convert CO2 into carboxylic acids is of great significance.

[0005] However, in the current electrochemical reduction of CO2, most anodes are oxygen evolution reactions, and the generated oxygen has no commercial value. Only the cathode reaction is used to produce valuable products in the electrolysis process. If the cathode and anode are combined for co-electrolysis, and the anode produces valuable products, it will have high economic value and can reduce the comprehensive cost of CO2 electrolysis.

[0006] In the electrolysis of carbon dioxide, gas diffusion electrodes are often used to promote the mass transfer rate of the reaction and thus accelerate the reaction rate. Compared with traditional bulk electrodes, the reaction rate can be increased by more than two orders of magnitude. Existing gas diffusion electrodes mostly have a carbon substrate (such as commercial carbon fiber paper, carbon cloth, etc.) as the main structure, and a small amount of catalyst is further loaded on the carbon substrate. However, carbon-based gas diffusion electrodes have low strength and the amount of hydrophobic substances is insufficient to maintain long-term hydrophobicity, resulting in a low lifespan of the gas diffusion electrodes. If promoted to industrial use, it will lead to a significant increase in the cost of electrode materials.

[0007] Currently, the method for electrochemically reducing CO2 to formic acid is limited by the problems of low electrode lifespan or high power consumption, and the cost of electrochemically producing formic acid from CO2 remains high, making it difficult to promote it to industrialization.

[0008] The invention patent CN104641021B discloses a method for electrochemically reducing CO2 by alcohol co-oxidation. Both the anode and cathode of this electrolysis use methanol as the electrolyte to prepare products such as aldehydes, acids, and ketones. However, the cathode electrolyte is methanol or an organic solution, and the electrolysis efficiency is not high.

[0009] The invention patent CN103822487B discloses a method for co-electrolysis of electrochemically reducing carbon dioxide to methanol and decomposing organic wastewater by anodic electricity-producing microorganisms. This method realizes that the microbial nutrient source comes from the methanol produced by electrochemically reducing CO2, reducing the energy consumption problem of sewage biological treatment. However, the cathode product of this patent is methanol, and the anode uses electricity-producing microorganisms to achieve the electrochemical decomposition of organic matter. Essentially, there is no CO2 emission reduction.

[0010] The invention patent CN 114908363 A discloses a membrane electrode assembly reactor and its application. The cathode is for electrochemically reducing carbon dioxide to formic acid or CO, the anode is for electrolyzing water, and an anion exchange membrane is arranged between the cathode and anode. Between the cathode ion membrane and the gas diffusion layer is a catalyst, and the catalyst is coated on the gas diffusion layer rather than on the anion membrane. In this case, the overpotential of the anion membrane is high, and the gas diffusion layer is used for CO2 diffusion. Using this method greatly increases the resistance of the cathode; and the anode electrolyzes water to produce oxygen, and the anode is not fully utilized.

[0011] Therefore, it is necessary to develop a suitable method for electrochemically producing carboxylic acids from CO2, with electrodes having a high service life, achieving a low cost of electrode materials, high electrolysis efficiency, and low power consumption to prepare carboxylic acids to meet the requirements of actual industrialization. Summary of the Invention

[0012] The object of the present invention is to provide a method for directly electrochemically producing carboxylic acids from CO2 electrolysis. This electrochemical method for producing carboxylic acids has low material consumption, low power consumption, and high efficiency, meeting the industrial requirements for electrolytic production of carboxylic acids / salts.

[0013] To achieve the above-mentioned invention object, the technical solution adopted by the present invention is as follows:

[0014] A method for directly preparing carboxylic acid by CO2 electrolysis, comprising: using a plate-and-frame electrolytic cell, in which a membrane electrode prepared with an anion exchange membrane separates the cathode chamber and the anode chamber in the middle of the electrolytic cell. A mixed gas of water vapor and CO2 gas is introduced into the cathode chamber of the electrolytic cell, and CO2 electroreduction occurs to produce carboxylate ions, and the carboxylate ions enter the anode through the anion exchange membrane; an aqueous aldehyde solution with the same number of carbon atoms and structure as the carboxylic acid is introduced into the anode chamber, and an aldehyde electrooxidation reaction occurs to obtain carboxylic acid while protons are generated on the anode surface. The protons and the carboxylate ions entering from the cathode generate carboxylic acid, and finally a high-purity aqueous carboxylic acid solution is obtained at the anode. The carboxylic acid produced by cathode electrolysis of CO2 enters the anode through the anion membrane, and the anode electrolyzes aldehyde to produce carboxylic acid, and no salt needs to be introduced as an electrolyte during the electrolysis process, and the product carboxylic acid is produced from the anode. The carboxylic acid is preferably formic acid, acetic acid or propionic acid.

[0015] In the present invention, on the anode side of the anion membrane electrode, an electrode is prepared by combining an anion membrane and a catalyst. The electrode can promote the entry of carboxylate ions from the cathode into the anode, and protons are generated on the anode surface, which can efficiently promote the reaction between carboxylate ions and protons to prepare carboxylic acid, and has high activity, high efficiency and low energy consumption.

[0016] In the present invention, the cathode raw materials of the electrolytic cell are CO2 and water vapor, and the volume ratio is 1-2:1. The feeding temperature is 50-100 °C, and the feeding pressure is 0.1-1.3 MPaG.

[0017] In the present invention, the anode raw materials of the electrolytic cell are an aqueous aldehyde solution with 10-30% of the same number of carbon atoms and structure as the carboxylic acid, or an aqueous carboxylic acid solution with the same number of carbon atoms and structure as the corresponding carboxylic acid. The feeding temperature is 50-100 °C, and the feeding pressure is 0.1-1.3 MPaG.

[0018] In the present invention, the aldehyde with the same number of carbon atoms and structure as the carboxylic acid is formaldehyde, acetaldehyde or propionaldehyde.

[0019] In the present invention, the catalyst on the cathode side of the membrane electrode is a nano metal oxide containing Sn and Pb, and the molar ratio of Sn and Pb is 1:0.1-1. The particle size of the catalyst is 1-100 nm, preferably 5-50 nm.

[0020] And / or, the catalyst on the anode side of the membrane electrode is an electrode prepared by using a metal oxide catalyst of a Pb and Cd, Tl alloy as a raw material to prepare an anion membrane. Among the anode metal oxides, the molar ratio of Pb:Cd:Tl is 1:1-5:1-10.

[0021] In the present invention, the reaction temperature of the electrolytic cell is 50-100 °C, and the reaction pressure is 0.1-1.3 MPaG.

[0022] As a preferred embodiment, in the present invention, the cathode catalyst is prepared by using soluble salts of Pb and Sn, preferably SnCl2 and Pb(NO3)2, to prepare a solution. Add 2 - 3 times the molar amount of ammonia water to the solution, mix at a high temperature of 120 - 150 °C for 1 - 5 h, then cool to 0 - 20 °C and crystallize for centrifugal separation. The obtained solid is first calcined in a muffle furnace at 300 - 500 °C in an oxygen atmosphere for 1 - 4 h. The molar ratio of the raw materials SnCl2 and Pb(NO3)2 is 9:1 - 1:1.

[0023] As a preferred embodiment, in the present invention, the anode catalyst is prepared by using soluble salts of Pb, Cd, and Tl, preferably Pb(NO3)2, CdCl2, and Tl(NO3)3·3H2O, to prepare a solution. The molar ratio of Pb:Cd:Tl is 1:1 - 5:1 - 10. Add 2.5 - 3.5 times the molar amount of ammonia water to the solution, mix at a high temperature of 120 - 150 °C for 1 - 5 h, then cool to 0 - 20 °C and crystallize for centrifugal separation. The obtained solid is calcined in a muffle furnace at 300 - 500 °C for 1 - 4 h to obtain the anode catalyst.

[0024] In the present invention, the preparation method of the anion exchange membrane electrode comprises the following steps:

[0025] (1) Mix the cathode catalyst with an adhesive, a dispersant, and an ionic polymer to make a slurry, and the viscosity of the slurry is 5 Pa·s - 50 Pa·s, preferably 10 Pa·s - 30 Pa·s;

[0026] (2) Place the anion exchange membrane on the surface of a heating device, and coat the slurry prepared in step (1) on the substrate through a coating process. The catalyst loading is 0.1 - 20 mg / cm 2 ;

[0027] (3) Press and fix the electrode obtained in step (2) through a hot pressing process, preferably press and fix it through a hot pressing process at 80 - 120 °C and 1 - 5 MPa;

[0028] (4) Mix the anode catalyst with an adhesive, a dispersant, and an ionic polymer to make a slurry, and the viscosity of the slurry is 5 Pa·s - 50 Pa·s, preferably 10 Pa·s - 30 Pa·s;

[0029] (5) Place the anion exchange membrane containing the cathode catalyst prepared in step (3) on the surface of a heating device, with the side without catalyst facing up, and coat the anode catalyst slurry obtained in step (4) on the substrate through a coating process at 40 - 80 °C. The catalyst loading is 0.1 - 20 mg / cm 2 ;

[0030] (6) Fix the electrode obtained in step (5) by hot pressing, preferably at 80 - 120 °C and 1 - 5 MPa, through the hot pressing process.

[0031] (7) Sinter the electrode compacted in step (6) at 100 - 200 °C for 20 - 100 min to obtain a membrane electrode.

[0032] In the preparation of the membrane electrode, the mass ratio of the catalyst:adhesive:dispersant:ionic polymer is 1:0.1 - 0.5:0.1 - 0.5:0.1 - 0.5, preferably 1:0.1 - 0.3:0.1 - 0.2:0.1 - 0.3. The adhesive can usually be selected as PTFE or PVDF; the dispersant can be selected as carboxymethyl cellulose, methyl cellulose, methyl hydroxyethyl cellulose, etc.; the ionic polymer can be selected as Sustainion XA - 9 powder.

[0033] The temperature of the coating process in step (2) is 40 - 80 °C.

[0034] The membrane electrode is placed in an electrolytic cell. Both sides of the membrane electrode are supported by PTFE nets to enhance CO2 dispersion; at the same time, as the support layer of the membrane, it improves the strength of the membrane. Conductive gold foils are added on both sides of the membrane electrode as current collectors, and the electrolytic cell is made of CPVC material; or both sides of the membrane electrode are supported by gold nets, the membrane electrode does not require conductive gold foils, and the electrolytic cell is made of titanium material.

[0035] The anion exchange membrane uses the constant current method to carry out an electrolysis reaction on the electrolyte solution, and the current density reaches 1000 A / m 2 -50000 A / m 2 . The voltage is strictly controlled below 5V. In the 100 - hour electrolysis experiment, the carboxylic acids generated by the reaction are enriched at the anode, and the average Faraday efficiency of carboxylic acids at the anode and cathode is greater than 92%.

[0036] Compared with the prior art, the positive effects of the present invention are:

[0037] The present invention provides a method for co - producing carboxylic acids at the anode and cathode using industrial - emitted CO2 as a raw material. The electrochemical synthesis method of carboxylic acids of the present invention realizes high efficiency, low power consumption, and low cost. Description of the Drawings:

[0038] Figure 1 It is a schematic diagram of the CO2 electrolytic cell structure of the membrane electrode. Detailed Embodiments

[0039] The following provides a specific and detailed description of the embodiments of the present invention. These embodiments are implemented on the premise of the technical solution of the present invention. Although it is impossible to list them all, representative detailed implementation methods and specific operation processes are given according to the experimental results. The protection scope of the present invention is not limited to the following embodiments.

[0040] Catalyst preparation raw materials:

[0041] SnCl2, purity 99.99%, Merck Chemicals, product number 452335 .

[0042] Pb(NO3)2, purity 99.999%, Merck Chemicals, product number 203580 .

[0043] CdCl2, purity 99.99%, Merck Chemicals, product number 202908 .

[0044] Tl(NO3)3·3H2O, purity 99.999%, Merck Chemicals, product number 163015 .

[0045] Ammonia water, ammonia solution 25%, Merck Chemicals.

[0046] Anion exchange membrane, Sustainion X37, Shanghai Xianren.

[0047] Anion polymer, Sustainion XA-9, Shanghai Xianren.

[0048] Proton exchange membrane, Nafion115, Shanghai Xianren.

[0049] Cationic polymer, 5wt% Nafion solution, Shanghai Xianren.

[0050] Reaction raw materials for CO2 electrolysis to formic acid: CO2, liquid CO2 cylinder gas, Mingju Gas.

[0051] Formaldehyde, Wanhua Chemical.

[0052] Formic acid, purity ≥95%, Merck Chemicals, product number F0507.

[0053] Analysis methods:

[0054] Viscosity analysis method, Brookfield viscometer, rotor 6#.

[0055] Analysis method for formic acid, Agilent 1260 liquid chromatography, external standard quantitative analysis.

[0056] Calculation method for average Faraday efficiency of cathode and anode:

[0057] The theoretical formic acid production of the cathode (anode) is equivalent, and the calculation formula is

[0058] m 理 = j*s*t*A / nF

[0059] Among them, j is the current density, unit A / m2;

[0060] S is the electrode area, with the unit of m2;

[0061] t is the reaction time, with the unit of h;

[0062] A is the molar mass of formic acid, in g / mol;

[0063] n is the number of electrons transferred in the reaction, 2;

[0064] F is the Faraday constant, 26.8 A·h / mol electron.

[0065] The total formic acid production of the cathode and anode is 2m 理 .

[0066] The actual formic acid production m 实 : The total mass of formic acid (root) in the reaction solution after the cathode and anode reactions - the mass of formic acid in the initial reaction solution.

[0067] The average Faraday efficiency of the cathode and anode = m 实 / 2m 理 *100%.

[0068] Example 1

[0069] Preparation of the cathode catalyst

[0070] Weigh 18.96 g of SnCl2 and 33.12 g of Pb(NO3)2 and dissolve them in deionized water to prepare a mixed solution. Add about 54.4 g of 25% ammonia water to the solution. After mixing at 120 °C for 1 h, cool it to 20 °C and then perform centrifugal separation. The obtained solid is fired in a muffle furnace at 300 °C for 4 h to obtain the cathode catalyst. The molar ratio of the raw materials SnCl2 and Pb(NO3)2 is 1:1.

[0071] Weigh 33.12 g of Pb(NO3)2, 18.33 g of CdCl2, and 44.44 g of Tl(NO3)3·3H2O and dissolve them in water to prepare a solution. The molar ratio of Pb:Cd:Tl is 1:1:1. Add about 102 g of 25% ammonia water to the solution. After mixing at 130 °C for 4 h, cool it to 20 °C and then perform centrifugal separation. The obtained solid is fired in a muffle furnace at 300 °C for 4 h to obtain the anode catalyst.

[0072] Preparation of the membrane electrode:

[0073] (1) Weigh 0.1 g of the cathode catalyst, weigh the adhesive PTFE, the dispersant methyl cellulose, and the ionic polymer Sustainion XA-9 and mix them to make a pulp. The mass ratio of the catalyst to the adhesive, dispersant, and ionic polymer is 1:0.1:0.1:0.1, and the viscosity of the pulp is 5 pa·s;

[0074] (2) Place a 100 cm 2 anion exchange membrane on the surface of a heating device, and coat the slurry on the substrate at 40 °C through a coating process, with a catalyst loading of 1 mg / cm 2 ;

[0075] (3) Press and fix the electrode in (2) at 100 °C and 1 MPa through a hot pressing process;

[0076] (4) Weigh 0.1 g of anode catalyst, weigh and mix the adhesive PTFE, dispersant methyl cellulose, and ionic polymer Sustainion XA-9 to make a slurry. The mass ratio of the catalyst to the adhesive, dispersant, and ionic polymer is 1:0.1:0.1:0.1, and the viscosity of the slurry is 5 pa·s;

[0077] (5) Place the anion exchange membrane containing the cathode catalyst prepared in (3) on the surface of a heating device, with the side without catalyst facing up, and coat the anode catalyst slurry in (4) on the substrate at 40 °C through a coating process, with a catalyst loading of 1 mg / cm 2 ;

[0078] (6) Press and fix the electrode in (5) at 100 °C and 1 MPa through a hot pressing process;

[0079] (7) Sinter the pressed electrode in (6) at 150 °C for 30 min.

[0080] Place a 10 cm 2 membrane electrode assembly in a plate-and-frame electrolytic cell. The material of the electrolytic cell is titanium. Gold meshes are connected to both sides of the membrane electrode. A mixture of CO2 and water (volume ratio 1:1) at 50 °C and 0.16 MPa is introduced into the cathode chamber of the electrolytic cell at a flow rate of 30 mL / min. A 10% formaldehyde aqueous solution is introduced into the anode chamber, and the temperature and pressure are equivalent to the temperature of the cathode feed. The current density of the electrolytic cell is 1000 A / m 2 , the voltage of the electrolytic cell is 3.7 V. After reacting for 100 h, the average formic acid Faraday efficiency of the cathode and anode is measured to be 96%.

[0081] Example 2

[0082] Preparation of cathode catalyst

[0083] Weigh 189.62 g of SnCl2 and 33.12 g of Pb(NO3)2 and dissolve them in deionized water to prepare a mixed solution. Add about 374 g of 25% ammonia water to the solution. After mixing at 150 °C for 2 h, cool it to 10 °C and then perform centrifugal separation. The obtained solid is fired in a muffle furnace at 320 °C for 3 h to obtain the cathode catalyst. The molar ratio of the raw materials SnCl2 and Pb(NO3)2 is 1:0.1.

[0084] Weigh 33.12 g of Pb(NO3)2, 18.33 g of CdCl2, and 444.44 g of Tl(NO3)3·3H2O, dissolve them in water to prepare a solution. The molar ratio of Pb:Cd:Tl is 1:1:10. Add approximately 571.2 g of 25% ammonia water to the solution, mix at 150 °C for 1 h, then cool to 10 °C and perform centrifugal separation. The obtained solid is fired in a muffle furnace at 350 °C for 3 h to obtain the anode catalyst.

[0085] Prepare the electrode:

[0086] (1) Weigh 1 g of the cathode catalyst, weigh the adhesive PVDF, the dispersant carboxymethyl cellulose, and the ionomer Sustainion XA-9 and mix them to make a slurry. The mass ratio of the catalyst to the adhesive, the dispersant, and the ionomer is 1:0.3:0.2:0.3, and the viscosity of the slurry is 20 Pa·s.

[0087] (2) Place a 100 cm 2 anion exchange membrane on the surface of the heating device, and coat the slurry on the substrate at 50 °C through a coating process. The catalyst loading is 10 mg / cm 2 ;

[0088] (3) Fix the electrode in (2) firmly by hot pressing at 120 °C and 3 MPa.

[0089] (4) Weigh 1 g of the anode catalyst, weigh the adhesive PVDF, the dispersant carboxymethyl cellulose, and the ionomer Sustainion XA-9 and mix them to make a slurry. The mass ratio of the catalyst to the adhesive, the dispersant, and the ionomer is 1:0.3:0.2:0.3, and the viscosity of the slurry is 20 Pa·s.

[0090] (5) Place the anion exchange membrane containing the cathode catalyst prepared in (3) on the surface of the heating device with the side without the catalyst facing up, and coat the anode catalyst slurry in (4) on the substrate at 50 °C through a coating process. The catalyst loading is 10 mg / cm 2 ;

[0091] (6) Fix the electrode in (5) firmly by hot pressing at 120 °C and 3 MPa.

[0092] (7) Sinter the electrode in (6) at 100 °C for 100 min.

[0093] Take 10 cm 2The membrane electrode assembly is placed in a plate-and-frame electrolytic cell made of titanium. Gold meshes are connected to both sides of the membrane electrode. A mixed gas of CO2 and water at 100 °C and 1.3 MPa with a flow rate of 30 mL / min is introduced into the cathode chamber of the electrolytic cell, and a 20% aqueous formaldehyde solution is introduced into the anode chamber at a temperature and pressure equivalent to that of the cathode feed. The current density of the electrolytic cell is 5000 A / m 2 , and the electrolytic cell voltage is 4.9 V. After reacting for 100 h and measuring, the average Faraday efficiency of formic acid at the cathode and anode is 92.8%.

[0094] Example 3

[0095] Preparation of cathode catalyst

[0096] Weigh 94.8 g of SnCl2 and 33.12 g of Pb(NO3)2 and dissolve them in deionized water to prepare a mixed solution. Add about 163.2 g of 25% ammonia water to the solution. After mixing at 120 °C for 3 h, cool it to 1 °C and then perform centrifugal separation. The obtained solid is calcined in a muffle furnace at 350 °C for 2 h to obtain the cathode catalyst. The molar ratio of the raw materials SnCl2 and Pb(NO3)2 is 1:0.2.

[0097] Weigh 33.12 g of Pb(NO3)2, 91.65 g of CdCl2, and 222.22 g of Tl(NO3)3·3H2O and dissolve them in water. The molar ratio of Pb:Cd:Tl is 1:5:5. Add about 448.8 g of 25% ammonia water to the solution. After mixing at 120 °C for 3 h, cool it to 1 °C and then perform centrifugal separation. The obtained solid is calcined in a muffle furnace at 400 °C for 2 h to obtain the anode catalyst.

[0098] Preparation of electrode:

[0099] (1) Weigh 0.01 g of the cathode catalyst, and weigh the adhesive PVDF, the dispersant methylhydroxyethyl cellulose, and the ionic polymer Sustainion XA-9 to make a slurry. The mass ratio of the catalyst to the adhesive, the dispersant, and the ionic polymer is 1:0.2:0.2:0.2, and the viscosity of the slurry is 10 pa·s;

[0100] (2) Place a 100 cm 2 anion exchange membrane on the surface of the heating device, and coat the slurry on the substrate by a coating process at 60 °C. The catalyst loading is 0.1 mg / cm 2 ;

[0101] (3) Compact and fix the electrode in (2) by a hot pressing process at 100 °C and 5 MPa;

[0102] (4) Weigh 0.01 g of the anode catalyst, weigh the adhesive PVDF, the dispersant methyl hydroxyethyl cellulose, and the ionic polymer Sustainion XA-9 and mix them to make a slurry. The mass ratio of the catalyst to the adhesive, the dispersant, and the ionic polymer is 1:0.2:0.2:0.2, and the viscosity of the slurry is 10 Pa·s;

[0103] (5) Place the anion exchange membrane containing the cathode catalyst prepared in (3) on the surface of the heating device, with the side without the catalyst facing up. At 60 °C, coat the anode catalyst slurry in (4) on the substrate through a coating process, and the catalyst loading is 0.1 mg / cm 2 ;

[0104] (6) Press and fix the electrode in (5) at 80 °C and 1 MPa through a hot pressing process;

[0105] (7) Sinter the pressed electrode in (6) at 200 °C for 20 min.

[0106] Assemble a 10 cm 2 membrane electrode into a plate-and-frame electrolytic cell. The material of the electrolytic cell is CPVC. Gold foils are connected to both sides of the membrane electrode for conducting electricity. PTFE meshes are used as supports on both the anode and cathode sides. A mixed gas of CO2 and water at 60 °C and 0.5 MPa is introduced into the cathode chamber of the electrolytic cell at a flow rate of 30 mL / min, and a 30% aqueous formaldehyde solution is introduced into the anode chamber. The temperature and pressure are equivalent to the temperature of the cathode feed. The current density of the electrolytic cell is 2000 A / m 2 , the voltage of the electrolytic cell is 4 V. After reacting for 100 h, the average Faraday efficiency of formic acid for the cathode and anode is 93.5%.

[0107] Example 4

[0108] Preparation of cathode catalyst

[0109] Weigh 18.96 g of SnCl2 and 33.12 g of Pb(NO3)2 and dissolve them in deionized water to prepare a mixed solution. Add about 62.56 g of 25% ammonia water to the solution. After mixing at 135 °C for 4 h, cool it to 5 °C and then perform centrifugal separation. The obtained solid is fired in a muffle furnace at 400 °C for 1 h to obtain the cathode catalyst. The molar ratio of the raw materials SnCl2 and Pb(NO3)2 is 1:1.

[0110] Weigh 33.12 g of Pb(NO3)2, 91.65 g of CdCl2, and 444.44 g of Tl(NO3)3·3H2O and dissolve them in water to prepare a solution. The molar ratio of Pb:Cd:Tl is 1:5:10. Add about 696.32 g of 25% ammonia water to the solution. After mixing at 135 °C at high temperature for 2 h, cool it to 5 °C and then perform centrifugal separation. The obtained solid is fired in a nitrogen atmosphere at 450 °C for 1 h to obtain the anode catalyst.

[0111] Preparation of electrode:

[0112] (1) Weigh 0.5 g of the cathode catalyst, weigh and mix the adhesive PTFE, the dispersant carboxymethyl cellulose, and the ionomer Sustainion XA-9 to make a slurry. The mass ratio of the catalyst to the adhesive, the dispersant, and the ionomer is 1:0.4:0.4:0.4, and the viscosity of the slurry is 30 Pa·s;

[0113] (2) Place a 100 cm 2 anion exchange membrane on the surface of the heating device, and coat the slurry on the substrate at 70 °C through a coating process. The catalyst loading is 5 mg / cm 2 ;

[0114] (3) Press and fix the electrode in (2) at 110 °C and 4 MPa through a hot pressing process;

[0115] (4) Weigh 0.5 g of the anode catalyst, weigh and mix the adhesive PTFE, the dispersant carboxymethyl cellulose, and the ionomer Sustainion XA-9 to make a slurry. The mass ratio of the catalyst to the adhesive, the dispersant, and the ionomer is 1:0.4:0.4:0.4, and the viscosity of the slurry is 30 Pa·s;

[0116] (5) Place the anion exchange membrane containing the cathode catalyst prepared in (3) on the surface of the heating device, with the side without the catalyst facing up, and coat the anode catalyst slurry in (4) on the substrate at 70 °C through a coating process. The catalyst loading is 5 mg / cm 2 ;

[0117] (6) Press and fix the electrode in (5) at 110 °C and 4 MPa through a hot pressing process;

[0118] (7) Sinter the electrode pressed in (6) at 120 °C for 80 min.

[0119] Assemble a 10 cm 2 membrane electrode assembly in a plate-and-frame electrolytic cell. The material of the electrolytic cell is titanium. Gold meshes are connected to both sides of the membrane electrode. A mixed gas of CO2 and water at 80 °C and 0.9 MPa is introduced into the cathode chamber of the electrolytic cell at a flow rate of 30 mL / min. An aqueous solution of 10% formaldehyde and 5% formic acid is introduced into the anode chamber, and the temperature and pressure are equivalent to the temperature of the cathode feed. The current density of the electrolytic cell is 3000 A / m 2 , the electrolytic cell voltage is 4.3 V. After reacting for 100 h, the average Faraday efficiency of formic acid at the cathode and anode is measured to be 95%.

[0120] Example 5

[0121] Preparation of cathode catalyst

[0122] Weigh 56.88 g of SnCl2 and 33.12 g of Pb(NO3)2 and dissolve them in deionized water to prepare a mixed solution. Add approximately 152.32 g of 25% ammonia water to the solution. After mixing at 130 °C for 5 h, cool it to 20 °C and then perform centrifugal separation. The obtained solid is calcined in a muffle furnace at 330 °C for 3 h to obtain the cathode catalyst. The molar ratio of the raw materials SnCl2 and Pb(NO3)2 is 1:0.33.

[0123] Weigh 33.12 g of Pb(NO3)2, 18.33 g of CdCl2, and 44.44 g of Tl(NO3)3·3H2O and dissolve them in water to prepare a solution. The molar ratio of Pb:Cd:Tl is 1:1:1. Add approximately 114.24 g of 25% ammonia water to the solution. After mixing at 130 °C for 2 h, cool it to 20 °C and then perform centrifugal separation. The obtained solid is calcined in a nitrogen atmosphere at 500 °C for 1 h to obtain the anode catalyst.

[0124] Prepare the electrode:

[0125] (1) Weigh 1.5 g of the cathode catalyst. Weigh the adhesive PTFE, the dispersant carboxymethyl cellulose, and the ionic polymer Sustainion XA-9 and mix them to make a slurry. The mass ratio of the catalyst to the adhesive, the dispersant, and the ionic polymer is 1:0.5:0.5:0.5, and the viscosity of the slurry is 50 Pa·s.

[0126] (2) Place a 100 cm 2 anion exchange membrane on the surface of the heating device. At 80 °C, coat the slurry on the substrate through a coating process, and the catalyst loading is 15 mg / cm 2 ;

[0127] (3) Clamp and fix the electrode in (2) through a hot pressing process at 90 °C and 5 MPa.

[0128] (4) Weigh 1.5 g of the anode catalyst. Weigh the adhesive PTFE, the dispersant carboxymethyl cellulose, and the ionic polymer Sustainion XA-9 and mix them to make a slurry. The mass ratio of the catalyst to the adhesive, the dispersant, and the ionic polymer is 1:0.5:0.5:0.5, and the viscosity of the slurry is 50 Pa·s.

[0129] (5) Place the anion exchange membrane containing the cathode catalyst prepared in (3) on the surface of the heating device with the side without the catalyst facing up. At 80 °C, coat the anode catalyst slurry in (4) on the substrate through a coating process, and the catalyst loading is 15 mg / cm 2 ;

[0130] (6) Clamp and fix the electrode in (5) through a hot pressing process at 90 °C and 5 MPa.

[0131] (7) Sinter the pressed electrode at 140 °C for 60 min.

[0132] Place a 10 cm 2 membrane electrode assembly in a plate-and-frame electrolytic cell made of CPVC. Gold foils are connected to both sides of the membrane electrode for conduction. PTFE meshes are used as supports on both the anode and cathode sides. A mixture of CO2 and water at 90 °C and 1 MPa with a flow rate of 30 mL / min is introduced into the cathode chamber of the electrolytic cell, and an aqueous solution of 10% formaldehyde and 10% formic acid is introduced into the anode chamber at a temperature and pressure equivalent to that of the cathode feed. The current density of the electrolytic cell is 1000 A / m 2 , the electrolytic cell voltage is 3.7 V. After reacting for 100 h, the average Faraday efficiency of formic acid at the cathode and anode is 95.2%.

[0133] Example 6

[0134] Preparation of cathode catalyst

[0135] Weigh 132.72 g of SnCl2 and 33.12 g of Pb(NO3)2 and dissolve them in deionized water to prepare a mixed solution. Add about 217.6 g of 25% ammonia water to the solution. After mixing at 140 °C for 1 h, cool it to 20 °C and then perform centrifugal separation. The obtained solid is fired in a muffle furnace at 380 °C for 2 h to obtain the cathode catalyst. The molar ratio of the raw materials SnCl2 and Pb(NO3)2 is 1:0.14.

[0136] Weigh 33.12 g of Pb(NO3)2, 18.33 g of CdCl2, and 44.44 g of Tl(NO3)3·3H2O and dissolve them in water to prepare a solution with a molar ratio of Pb:Cd:Tl of 1:1:1. Add about 122.4 g of 25% ammonia water to the solution. After mixing at 140 °C for 1 h, cool it to 20 °C and then perform centrifugal separation. The obtained solid is fired in a muffle furnace at 380 °C for 2 h to obtain the anode catalyst.

[0137] Preparation of electrode:

[0138] (1) Weigh 2 g of the cathode catalyst, and weigh the adhesive PTFE, dispersant carboxymethyl cellulose, and ionic polymer Sustainion XA-9 and mix them to make a slurry. The mass ratio of the catalyst to the adhesive, dispersant, and ionic polymer is 1:0.1:0.3:0.3, and the viscosity of the slurry is 20 Pa·s;

[0139] (2) Place a 100 cm 2 anion exchange membrane on the surface of a heating device, and coat the slurry on the substrate through a coating process at 50 °C. The catalyst loading is 20 mg / cm 2 ;

[0140] (3) Press and fix the (2) electrodes tightly through a hot pressing process at 100 °C and 5 MPa;

[0141] (4) Weigh 2 g of the anode catalyst, weigh and mix the adhesive PTFE, dispersant carboxymethyl cellulose, and ionic polymer Sustainion XA-9 to make a slurry. The mass ratio of the catalyst to the adhesive, dispersant, and ionic polymer is 1:0.1:0.3:0.3, and the viscosity of the slurry is 20 Pa·s;

[0142] (5) Place the anion exchange membrane containing the cathode catalyst prepared in (3) on the surface of the heating device, with the side without the catalyst facing up, and coat the anode catalyst slurry of (4) on the substrate through a coating process at 50 °C. The catalyst loading is 20 mg / cm 2 ;

[0143] (6) Press and fix the (5) electrodes tightly through a hot pressing process at 100 °C and 5 MPa;

[0144] (7) Sinter the electrodes pressed in (6) at 180 °C for 20 min.

[0145] Assemble a 10 cm 2 membrane electrode assembly in a plate-and-frame electrolytic cell. The material of the electrolytic cell is titanium. Gold meshes are connected to both sides of the membrane electrode. A mixture of CO2 and water at 60 °C and 0.1 MPa is introduced into the cathode chamber of the electrolytic cell at a flow rate of 30 mL / min, and a 10% aqueous formaldehyde solution is introduced into the anode chamber. The temperature and pressure are equivalent to the cathode feed temperature. The current density of the electrolytic cell is 4000 A / m 2 , the electrolytic cell voltage is 4.6 V. After reacting for 100 h, the average formic acid Faraday efficiency of the cathode and anode is 93%.

[0146] Comparative Example 1

[0147] This comparative example provides a method for co-electrolyzing CO2 to formic acid / salt and electro-oxidizing formaldehyde at the anode to formic acid. The difference compared with Example 1 is only that the ion exchange membrane selected in the membrane electrode preparation process is the proton exchange membrane Nafion115, and the ionic polymer used in the catalyst pulping is a 5 wt% Nafion solution of a cationic polymer, making the membrane electrode different from that in Example 1.

[0148] Under the same operating conditions as in Example 1, the electrolytic cell voltage is 3.5 V, but after 100 h, hydrogen evolution reaction occurs on the surface of the membrane electrode at the cathode, and the average formic acid Faraday efficiency of the cathode and anode is 35%.

[0149] Comparative Example 2

[0150] This comparative example provides a method for co-electrolyzing CO2 to formic acid / salt and electro-oxidizing formaldehyde at the anode to formic acid. The difference compared with Example 1 is only that the anode composition is pure water.

[0151] Under the same operating conditions as in Example 1, the electrolytic cell voltage was 3.6 V. After 100 h, hydrogen evolution reaction occurred on the surface of the membrane electrode at the cathode, and water electrolysis reaction occurred at the anode. The average Faraday efficiency of formic acid at the cathode and anode was 30%.

Claims

1. A method for preparing carboxylic acids by electrolyzing CO2, characterized in that: It includes: A plate-and-frame electrolytic cell is adopted. The membrane electrode prepared with an anion exchange membrane in the middle of the electrolytic cell separates the cathode chamber and the anode chamber. A mixed gas of water vapor and CO2 gas is introduced into the cathode chamber of the electrolytic cell, and CO2 electroreduction is carried out to prepare carboxylic acids, preferably formic acid, acetic acid or propionic acid. The carboxylate ions enter the anode through the anion exchange membrane; an aqueous aldehyde solution with the same number of carbon atoms and structure as the carboxylic acid is introduced into the anode chamber, and an aldehyde electrooxidation reaction is carried out to prepare carboxylic acids; at the same time, protons are generated on the anode surface, and the protons and the carboxylate ions entering from the cathode generate carboxylic acids, and finally an aqueous carboxylic acid solution is obtained at the anode; wherein, catalysts are coated on both sides of the anion exchange membrane for electroreduction and electrooxidation.

2. The method according to claim 1, characterized in that The cathode feed temperature of the electrolytic cell is 50-100 °C, and the feed pressure is 0.1-1.3 MPaG.

3. The method according to claim 1 or 2, characterized in that The anode feed temperature of the electrolytic cell is 50-100 °C, and the feed pressure is 0.1--1.3 MPaG.

4. The method according to any one of claims 1 to 3, characterized in that, The catalyst on the cathode side of the membrane electrode is a metal oxide containing Sn and Pb, and the metal molar ratio of the two is 1:0.1-1, and / or the particle size of the catalyst is 1-100 nm, preferably 5-50 nm.

5. The method according to any one of claims 1-4, characterized in that, The catalyst on the anode side of the membrane electrode is a metal oxide containing Pb, Cd and Tl, and the molar ratio of Pb:Cd:Tl is 1:1-5:1-10.

6. The method according to any one of claims 1-5, characterized in that, The catalyst loading on the cathode side of the membrane electrode is 0.1 - 20 mg / cm 2 , preferably 1 - 10 mg / cm 2 , and / or the catalyst loading on the anode side of the membrane electrode is 0.1 - 20 mg / cm 2 , preferably 1 - 10 mg / cm 2 .

7. The method according to any one of claims 1-6, characterized in that The method for preparing the membrane electrode includes the following steps: (1) Mix the cathode catalyst with an adhesive, a dispersant and an ionic polymer to make a slurry, and the viscosity of the slurry is 5 pa·s - 50 pa·s, preferably 10 pa·s - 30 pa·s; (2) Place the anion exchange membrane on the surface of the heating device, and coat the slurry obtained in step (1) on the substrate through a coating process. Preferably, the catalyst loading is 0.1 - 20 mg / cm 2 ; (3) Press and fix the electrode obtained in step (2) by a hot pressing process, preferably press and fix it by a hot pressing process at 80-120 °C and 1-5 MPa; (4) Mix the anode catalyst with an adhesive, a dispersant and an ionic polymer to make a slurry, and the viscosity of the slurry is 5 pa·s - 50 pa·s, preferably 10 pa·s - 30 pa·s; (5) Place the anion exchange membrane containing the cathode catalyst prepared in (3) on the surface of a heating device, with the side without the catalyst facing up, and coat the anode catalyst slurry in step (4) on the substrate through a coating process. Preferably, the catalyst loading is 0.1 - 20 mg / cm 2 ; (6) Press and fix the electrode obtained in step (5) by a hot pressing process, preferably press and fix it by a hot pressing process at 80-120 °C and 1-5 MPa; (7) Sinter the electrode pressed in step (6), preferably sinter for 20-100 min to obtain the membrane electrode.

8. The method according to claim 7, characterized in that, In steps (1) and (4), the mass ratio of the catalyst: adhesive: dispersant: ionic polymer is 1:0.1-0.5:0.1-0.5:0.1-0.5, preferably 1:0.1-0.3:0.1-0.2:0.1-0.

3.

9. The method according to any one of claims 1-8, characterized in that, The membrane electrode is placed in the electrolytic cell. Both sides of the membrane electrode are supported by PTFE nets, and conductive gold foils are added on both sides of the membrane electrode as current collectors. The electrolytic cell is made of CPVC material; or both sides of the membrane electrode are supported by gold nets, and the membrane electrode does not require conductive gold foils, and the electrolytic cell is made of titanium material.

10. The method according to any one of claims 1-9, characterized in that, In the electrolysis reaction, the current density reaches 1000 A / m 2 -50000 A / m 2 , and the voltage is strictly controlled below 5V.

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