A method for generating triethanolamine by electrocatalytic reduction of carbon dioxide through two-stage series reaction

Abstract

The present invention belongs to the technical field of electrochemical catalytic conversion of carbon-based small molecules, and is specifically a method for electrocatalytic reduction of carbon dioxide to generate triethanolamine. It first prepares a copper oxide nanosheet electrocatalyst for efficient electroreduction of carbon dioxide to ethylene. In the first-stage reaction, the electrocatalyst is used as a catalytic reaction cathode to achieve the conversion of carbon dioxide to ethylene products. In the second-stage reaction, an ethylene oxidation reaction occurs at the anode using a prepared metal phthalocyanine bifunctional catalyst and an electrolyte containing bromide ions, and 2-bromoethanol is selectively generated, while the generation of byproducts such as carbon dioxide and oxygen is suppressed; nitrite reduction occurs at the cathode to generate ammonia; the products generated by the two electrodes are mixed in situ to prepare triethanolamine containing multiple carbon atoms. The method of the present invention has a wide range of raw material sources, a simple preparation method, is environmentally friendly, and is low in price, which is conducive to popularization and application.

Description

Technical Field

[0001] The invention belongs to the technical field of electrocatalytic conversion of carbon-containing small molecules, and particularly relates to a method for generating triethanolamine by electrocatalytic reduction of carbon dioxide. Background Art

[0002] The electrocatalytic carbon dioxide reduction reaction is in line with the "dual carbon" goal. It is powered by renewable energy and converts carbon dioxide, a greenhouse gas, and water into high-value-added chemicals such as formic acid, ethylene and ethanol. So far, the selectivity of the reaction products has been limited due to the lack of optimal electrocatalysts that can effectively activate the carbon dioxide reactant and continuously grow the carbon chain, and the product contains up to four carbon atoms. It is of great significance to achieve the synthesis of carbon dioxide to multi-carbon products through electrochemical cascade reactions. Ethylene has an unsaturated carbon-carbon double bond, which makes it a basic unit for subsequent carbon chain extension. At the same time, various downstream products of ethylene are widely used in industry, ecology and agriculture. For a long time, people have been studying the electrocatalytic ethylene oxidation reaction to synthesize ethylene oxide and halogenated alcohols, but they usually have low Faraday efficiency and cannot achieve carbon addition. Summary of the invention

[0003] The object of the present invention is to provide an efficient and feasible method for producing triethanolamine by electrocatalytic reduction of carbon dioxide through two-stage series reaction.

[0004] The present invention relates to a catalyst with high activity and low price for two-stage series reaction electrocatalytic reduction of carbon dioxide to generate triethanolamine. The catalyst includes a catalyst for electrocatalytic reduction of carbon dioxide to generate ethylene, and a catalyst for electrocatalytic oxidation of ethylene to generate bromoethanol and electrocatalytic reduction of nitrite to generate ammonia.

[0005] The catalyst for electrocatalytic reduction of carbon dioxide to generate ethylene provided by the present invention is a copper oxide nanosheet, which is prepared by the following steps:

[0006] (1) Dissolve 3-6 mmol of copper salt and 0.1-0.2 mol of strong base in a certain volume (30-60 mL) of deionized water;

[0007] (2) transferring the mixed solution to an autoclave of suitable volume and reacting at 100-120°C for 10-16 h; after the reaction, centrifuging and separating the product, and washing with deionized water to obtain the catalyst.

[0008] In the present invention, the strong base is one or more of sodium hydroxide, potassium hydroxide, rubidium hydroxide and cesium hydroxide; the copper salt is one or more of copper chloride, copper acetate, copper sulfate and copper nitrate.

[0009] The catalyst provided by the present invention for electrocatalytic oxidation of ethylene to generate bromoethanol and electrocatalytic reduction of nitrite to generate ammonia is a carbon-supported metal phthalocyanine conductive catalyst, which is prepared by the following steps:

[0010] (1) Mix and grind metal chloride, urea, ammonium chloride, ammonium molybdate and phthalic anhydride in a certain molar ratio (7: (65-70): (15-20): (0.02-0.04): (14-18); react the mixture at 180-220°C for 2-5h, wash with water and acetone in turn and dry;

[0011] (2) Add 100-200 mL of concentrated nitric acid and concentrated sulfuric acid to the conductive carbon in a volume ratio of 1:0.8-1.2; reflux for 24-48 h; after removing most of the mixed acid, wash with ionized water and dry;

[0012] (3) Dispersing the carbon nanotubes described in step (2) and the metal phthalocyanine described in step (1) in acetone, ultrasonically dispersing for 24-72 hours, and centrifugally drying to obtain a carbon-supported metal phthalocyanine conductive catalyst.

[0013] In the present invention, the metal chloride is one or more of ferric chloride, cobalt chloride, cupric chloride and zinc chloride; the conductive carbon is one or more of conductive carbon black, conductive graphite, carbon nanotubes, graphene and carbon fiber.

[0014] The present invention provides a method for realizing a two-stage series reaction of electrocatalytic reduction of carbon dioxide to generate triethanolamine using the above catalyst, using an electrolyte containing bromide ions and nitrite or nitrate ions, using bromide ions to promote electrocatalytic oxidation of ethylene to generate brominated ethanol; using nitrite or nitrate ions to reduce and generate ammonia, which further participates in the reaction to generate triethanolamine from brominated ethanol (see Figure 1 ); The specific steps are as follows:

[0015] (1) For the first-order reaction, 0.5-2m / cm 2 A gas diffusion electrode containing CuO catalyst is loaded into a flow electrolytic cell; CuO is in situ electro-reduced by cyclic voltammetry; wherein the electrode is hydrophobic treated;

[0016] (2) Carbon dioxide gas with a purity greater than 99% is introduced to contact the side of the electrode that is not loaded with the CuO electrocatalyst. The carbon dioxide gas can pass through the hydrophobic electrode to contact the catalyst, while the electrolyte cannot diffuse to the other side of the electrode. The carbon dioxide gas flow rate is controlled to be 5-80 mL / min to achieve ethylene gas of different concentrations and yields.

[0017] (3) Apply negative voltage to the electrode and control the current size to 0.02-1A / cm2 , carbon dioxide is selectively reduced to ethylene;

[0018] (4) Further, 0.5-2 mg / cm 2 The carbon-supported metal phthalocyanine conductive catalyst is sprayed on the gas diffusion electrode; the ethylene prepared in step (3) is introduced into the anode side of the secondary reaction electrolytic cell, and the ethylene flow rate is controlled to be 1-40 mL / min, so that the electrolyte containing bromide ions contacts the electrode; and 0.5-1 mol / L nitrite is added to the cathode side of the reaction electrolytic cell;

[0019] (5) Apply voltage to the entire electrolytic cell device and control the total current density to 0.02-0.1A / cm 2 , ethylene is selectively oxidized to 2-bromoethanol, and nitrite is reduced to ammonia; the cathode and anode electrolytes are reacted and mixed to obtain triethanolamine.

[0020] In the present invention, the anolyte is one or more of NaBr and KBr, and the catholyte is NaNO 2 、KNO 2 The product generated by the electro-oxidation of ethylene is 2-bromoethanol; and the ethanolamine product obtained is triethanolamine.

[0021] In the present invention, an electrochemical-chemical coupling strategy for growing carbon chains is innovatively proposed. On the anode side, bromoethanol can be formed by bromine oxidation reaction in combination with ethylene gas. On the cathode side, nitrite is electrochemically reduced to ammonia, and ammonia is coupled with a certain stoichiometric amount of bromoethanol to form triethanolamine, and bromide ions are regenerated and circulated back to the anode to react again. In addition, by further combining carbon dioxide electroreduction, carbon dioxide is used as the only carbon source to generate ethylene, and the entire electrochemical reaction consumes only carbon dioxide, nitrite and water. In the absence of gas separation, a high ethylene single-pass conversion rate and a high selectivity for converting ethylene into triethanolamine are achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The schematic diagram of the method of the present invention is shown in Figure 1. a is the pathway for ethylene activation, where bromine species mildly oxidize ethylene and inhibit peroxidation at a lower potential; b illustrates the construction of triethanolamine by combining ethylene oxidation and nitrite reduction.

[0023] Figure 2 The morphology and characterization of the metal phthalocyanine on the carbon nanotube in Examples 1 to 3. Wherein, a is an electron microscope photo of the metal phthalocyanine loaded on the carbon nanotube; b is the distribution of the corresponding metal, carbon, oxygen, and nitrogen elements.

[0024] Figure 3The performance test of ethylene oxidation with metal phthalocyanine as electrocatalyst under different electrolyte conditions in Example 1. The bar graph corresponds to the left axis, which is the Faraday efficiency of different products. The line graph corresponds to the right axis, which is the total current density.

[0025] Figure 4 This is a stability test of two-stage series electrolytic cells under the conditions of two-stage series reaction in Example 2. The horizontal axis represents the test time, the line graph corresponds to the left axis, representing the change of voltage or current over time; the dot graph corresponds to the right axis, representing the Faraday efficiency of generating ethylene, 2-bromoethanol and triethanolamine, respectively.

[0026] Figure 5 The performance of bromoethanol corresponding to different flow rates of carbon dioxide under the conditions of the two-stage series reaction in Example 4, with 3 mol / L KBr as the electrolyte and metal phthalocyanine as the electrocatalyst. The horizontal axis represents the flow rate of carbon dioxide gas, the bar graph corresponds to the right axis and represents the yield of bromoethanol; the dotted line graph corresponds to the left axis, representing the single-pass conversion rate of carbon dioxide to ethylene and ethylene to bromoethanol, respectively. DETAILED DESCRIPTION

[0027] Below, the present invention is further described by examples, which will help to understand the present invention, but do not limit the content of the present invention.

[0028] Example 1, by designing a secondary electrocatalytic reaction, nickel phthalocyanine is used to promote the electrocatalytic oxidation of ethylene by bromide ions in the electrolyte to produce 2-bromoethanol, and then combines with ammonia water to form a triethanolamine product. The specific steps are as follows:

[0029] (1) Synthesis of CuO nanosheets as primary reaction catalyst:

[0030] Accurately weigh 0.75 g of copper acetate and dissolve it in 20 mL of deionized water; weigh 7.2 g of sodium hydroxide and dissolve it in 40 mL of deionized water; under stirring, add 20 mL of copper acetate solution dropwise into 40 mL of sodium hydroxide solution to form a clear blue liquid; transfer the mixed solution to a 100 mL autoclave and react at 100 °C for 12 h; after the autoclave is cooled to room temperature, centrifuge and separate the product, wash it with deionized water, and then disperse it; repeat the above steps until the solution becomes neutral, and finally dry the CuO at 80 °C overnight;

[0031] (2) Synthesize nickel phthalocyanine catalyst as a secondary reaction catalyst:

[0032] A mixture of 1.60 g nickel chloride, 4.10 g urea, 1.00 g ammonium chloride, 0.025 g ammonium molybdate and 2.10 g phthalic anhydride was ground; the mixture was reacted at 210°C for 3 h; after slowly cooling to room temperature, it was washed with water and acetone in sequence, and finally dried at 70°C overnight;

[0033] Add 200 mL of 1:1 (volume ratio) concentrated nitric acid and concentrated sulfuric acid to 200 mg of carbon nanotubes, reflux for 48 h for carboxylation; slowly cool to room temperature and remove most of the mixed acid; slowly add 500 mL of deionized water to dilute the mixture, and wash the product with deionized water 3 to 5 times; accurately weigh 20 mg of carboxylated carbon nanotubes and 1 mg of metal phthalocyanine, disperse the carbon nanotubes and metal phthalocyanine in 50 mL of acetone, and ultrasonicate for 72 h; finally, centrifuge and dry at 70°C overnight;

[0034] (3) Using bromide ions in the electrolyte to promote electrocatalytic oxidation of ethylene to produce 2-bromoethanol, the specific steps are as follows:

[0035] Accurately weigh 1 mg of catalyst and disperse it in 3 mL of solvent. Spray the catalyst dispersion on a 15 cm 2 The electrolytic cell was placed on a commercial gas diffusion electrode; ethylene was introduced into the anode side of the electrolytic cell so that the electrolyte containing 0.1 mol / L KBr was in contact with the electrode; a voltage was applied to the entire electrolytic cell device, and the total current density was controlled to be 0.02~0.1 A / cm 2 , ethylene is selectively oxidized to 2-bromoethanol and a small amount of ethylene glycol; the product current density and total current are as follows Figure 3 For example, at a potential of 2.1 V, the partial current density of 2-bromoethanol can reach 40 mA / cm 2 ;

[0036] (4) Combining the first-stage reaction of converting carbon dioxide into ethylene, ethylene is converted into bromoethanol in series, and finally into triethanolamine. The specific steps are as follows:

[0037] First, a carbon dioxide flow electrolysis cell was used to produce ethylene as the reaction source for ethylene oxidation. Copper oxide nanosheets were used as carbon dioxide reduction electrocatalysts, which can reduce carbon dioxide to ethylene with high selectivity. In 1 mol / L KOH, the partial current density of ethylene production reached a peak of 300 mA / cm at -2.0 V. 2 ; The mixed gas after the reaction was then introduced into a secondary reaction tank for ethylene oxidation; the Faraday efficiency of generating 2-bromoethanol in 0.5 mol / L NaBr reached 98%, and the yield of generating triethanolamine after mixing with 0.1 mol / L ammonia water was 77%.

[0038] Example 2, by designing a secondary electrocatalytic reaction, phthalocyanine nickel is used to oxidize the electrolyte to generate 2-bromoethanol, and the ammonia generated by nitrite reduction is combined with cathode reduction to form triethanolamine product, and the specific steps are as follows:

[0039] (1) Synthesis of CuO nanosheets as primary reaction catalyst;

[0040] Accurately weigh 0.75 g of copper chloride and dissolve it in 30 mL of deionized water; weigh 8.0 g of potassium hydroxide and dissolve it in 40 mL of deionized water; under stirring, add 30 mL of copper chloride solution dropwise into 40 mL of potassium hydroxide solution to form a clear blue liquid; transfer the mixed solution to a 100 mL autoclave and react at 110°C for 10 h; after the autoclave is cooled to room temperature, centrifuge and separate the product, wash it with deionized water, and then disperse it; repeat the above steps until the solution becomes neutral, and finally dry the CuO at 90°C overnight;

[0041] (2) Synthesize nickel phthalocyanine catalyst as a secondary reaction catalyst;

[0042] (3) Using bromide ions in the electrolyte to promote electrocatalytic oxidation of ethylene to produce 2-bromoethanol, using nitrite in the electrolyte to reduce ethylene to produce ammonia, and finally to produce triethanolamine;

[0043] Accurately weigh 1 mg of catalyst and disperse it in 3 mL of solvent. Spray the catalyst dispersion on a 15 cm 2 The electrolytic cell was placed on a commercial gas diffusion electrode; ethylene was introduced into the anode side of the electrolytic cell, and 0.5 mol / L NaBr was brought into contact with the electrode; 0.1 mol / L nitrite ions were used as the cathode electrolyte; a voltage was applied to the entire electrolytic cell device, and the total current density was controlled to be 0.02~0.1 A / cm 2 , ethylene is selectively oxidized to 2-bromoethanol and a small amount of ethylene glycol, while nitrite is reduced to ammonia; for example, at a full cell voltage of 3.5 V, the separate current densities of 2-bromoethanol and ammonia can both reach 40 mA / cm 2 ; The electrolytes after 24 h of reaction were mixed to obtain a triethanolamine yield of 62%.

[0044] Example 3, by designing a secondary electrocatalytic reaction, using different phthalocyanine metal catalysts, the chloride ions in the electrolyte promote the electrocatalytic oxidation of ethylene to produce 2-bromoethanol, and combined with nitrate reduction to form triethanolamine product, the specific steps are as follows:

[0045] (1) Synthesis of CuO nanosheets as primary reaction catalyst;

[0046] (2) Synthesize different metal phthalocyanine catalysts as secondary reaction catalysts;

[0047] A mixture of 8 mmol nickel chloride (ferric chloride, cobalt chloride), 4.10 g urea, 1.00 g ammonium chloride, 0.025 g ammonium molybdate and 2.10 g phthalic anhydride was ground and reacted at 220°C for 3 h; after slowly cooling to room temperature, the mixture was washed with water and acetone in turn, and finally dried at 70°C overnight; finally, nickel phthalocyanine, iron phthalocyanine and cobalt phthalocyanine catalysts were obtained respectively;

[0048] Accurately weigh 20 mg of conductive carbon black and 1 mg of metal phthalocyanine, disperse the conductive carbon black and metal phthalocyanine in 50 mL of acetone, and ultrasonicate for 48 h; finally, centrifuge and dry at 80 °C overnight;

[0049] (3) The first-stage reaction of converting carbon dioxide into ethylene is as follows:

[0050] First, a carbon dioxide flow electrolysis cell was used to produce ethylene as the reaction source for ethylene oxidation. Copper oxide nanosheets were used as carbon dioxide reduction electrocatalysts, which can reduce carbon dioxide to ethylene with high selectivity. In 2 mol / L NaOH, the partial current density of ethylene production reached a peak of 380 mA / cm at -2.1 V. 2 ;

[0051] (4) The ethylene product is passed into the secondary reaction tank, and the bromide ions in the electrolyte are used to promote the electrocatalytic oxidation of ethylene to produce 2-bromoethanol. The specific steps are as follows:

[0052] Accurately weigh 1 mg of catalyst and disperse it in 3 mL of solvent. Spray the catalyst dispersion on a 15 cm 2 The electrolytic cell was placed on a commercial gas diffusion electrode; ethylene was introduced into the anode side of the electrolytic cell, and an electrolyte containing 1 mol / L KBr was brought into contact with the electrode; a voltage was applied to the entire electrolytic cell device to obtain 2-bromoethanol; the phthalocyanine iron catalyst had the best performance, reaching 122 mA / cm at a potential of 2.0 V. 2 ; Phthalocyanine cobalt catalyst is second, with 100mA / cm 2 ; Phthalocyanine iron catalyst has 25mA / cm 2 ;

[0053] (5) The generated 2-bromoethanol is mixed with ammonia water obtained by reduction of nitrite at the cathode in equal proportions to finally prepare triethanolamine; wherein a nickel phthalocyanine catalyst is used as the anode and an iron phthalocyanine catalyst is used as the cathode, and the Faradaic efficiency of the triethanolamine obtained reaches 43%.

[0054] Example 4, by designing a secondary electrocatalytic reaction, by regulating CO 2 Gas flow rates achieve different yields of triethanolamine:

[0055] By reducing the initial carbon dioxide gas input flow rate, the ethylene single-pass conversion rate and the bromoethanol formation rate can be increased. For example, at 30 mL / min, the bromoethanol yield was 162 μmol / h and the ethylene single-pass conversion rate was 13%; when it was reduced to 10 mL / min, the bromoethanol yield was 265 μmol / h and the ethylene single-pass conversion rate was 92.6%. At a flow rate of 10 mL / min, after continuous electrolysis for 9 h, the Faraday efficiency of the secondary reaction cell to generate triethanolamine was 37.6% when the anode and cathode electrolytes were mixed.

Claims

1. A method for producing triethanolamine by electrocatalytic reduction of carbon dioxide through a two-stage series reaction, characterized in that: An electrolyte containing bromide ions and nitrite ions is used, and bromide ions are used to promote electrocatalytic oxidation of ethylene to generate brominated ethanol; nitrite ions are used to reduce and generate ammonia, which further participates in the reaction to generate triethanolamine from brominated ethanol. The specific steps are as follows: (1) For the first-order reaction, 0.5-2m / cm 2 A gas diffusion electrode containing CuO catalyst is loaded into a flow electrolytic cell; CuO is in situ electro-reduced by cyclic voltammetry; wherein the electrode is hydrophobic treated; (2) Carbon dioxide gas with a purity greater than 99% is introduced to contact the side of the electrode that is not loaded with the CuO electrocatalyst. The carbon dioxide gas can pass through the hydrophobic electrode to contact the catalyst, while the electrolyte cannot diffuse to the other side of the electrode. The carbon dioxide gas flow rate is controlled to be 5-80 mL / min to achieve ethylene gas of different concentrations and yields. (3) Apply negative voltage to the electrode and control the current size to 0.02-1A / cm 2 , carbon dioxide is selectively reduced to ethylene; (4) 0.5-2mg / cm 2 The carbon-supported metal phthalocyanine conductive catalyst is sprayed on the gas diffusion electrode; the ethylene prepared in step (3) is introduced into the anode side of the secondary reaction electrolytic cell, and the ethylene flow rate is controlled to be 1-40 mL / min, so that the electrolyte containing bromide ions contacts the electrode; and 0.5-1 mol / L of nitrite is added to the cathode side of the reaction electrolytic cell; (5) Apply voltage to the entire electrolytic cell device and control the total current density to 0.02-0.1A / cm 2 , ethylene is selectively oxidized to 2-bromoethanol, and nitrite is reduced to ammonia; the cathode and anode electrolytes are reacted and mixed to obtain triethanolamine.

2. The method according to claim 1, characterized in that The anolyte is one or more of NaBr and KBr, and the catholyte is one or more of NaNO2 and KNO2.

3. The method according to claim 1, characterized in that The CuO catalyst is a copper oxide nanosheet, which is prepared by the following steps: (1) Dissolve 3-6 mmol of copper salt and 0.1-0.2 mol of strong base in 30-60 mL of deionized water; (2) The mixed solution is transferred to an autoclave of suitable volume and reacted at 100-120° C. for 10-16 h. After the reaction is completed, the product is separated by centrifugation and washed with deionized water to obtain the CuO catalyst.

4. The method according to claim 3, characterized in that The strong base is one or more of sodium hydroxide, potassium hydroxide, rubidium hydroxide and cesium hydroxide; the copper salt is one or more of copper chloride, copper acetate, copper sulfate and copper nitrate.

5. The method according to claim 1, characterized in that The carbon-supported metal phthalocyanine conductive catalyst is prepared by the following steps: (1) mixing and grinding metal chloride, urea, ammonium chloride, ammonium molybdate and phthalic anhydride in a molar ratio of 7:(65-70):(15-20):(0.02-0.04):(14-18), reacting the obtained mixture at 180-220°C for 2-5h, washing with water and acetone in turn and drying to obtain the final metal phthalocyanine; (2) Add 100-200 mL of concentrated nitric acid and concentrated sulfuric acid to the conductive carbon in a volume ratio of 1:0.8-1.2; reflux for 24-48 h; after removing most of the mixed acid, wash with ionized water, and dry to obtain the final carbon nanotubes; (3) Dispersing the carbon nanotubes described in step (2) and the metal phthalocyanine described in step (1) in acetone, ultrasonically dispersing for 24-72 hours, and centrifugally drying to obtain a carbon-supported metal phthalocyanine conductive catalyst.

6. The method according to claim 5, characterized in that The metal chloride is one or more of ferric chloride, cobalt chloride, cupric chloride and zinc chloride; the conductive carbon is one or more of conductive carbon black, conductive graphite, carbon nanotubes, graphene and carbon fiber.

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