Preparation method of alumina-supported copper-cobalt bimetallic catalyst and application thereof

Abstract

The application discloses a preparation method of an alumina-supported copper-cobalt bimetallic catalyst. The preparation method comprises the following steps: (1) a certain amount of cobalt salt, copper salt, aluminum salt, urea and ammonium fluoride are dissolved in an alcohol solvent to obtain a precursor through a solvothermal reaction; and (2) the precursor is reduced by hydrogen to obtain the alumina-supported copper-cobalt bimetallic catalyst. Compared with the prior art, the copper-cobalt bimetallic catalyst prepared by the method has excellent catalytic activity on the reaction of CO2 hydrogenation to ethanol, and can significantly improve the selectivity of ethanol product in the reaction.

Description

Technical Field

[0001] This invention belongs to the field of CO2 hydrogenation to ethanol, and specifically relates to a catalyst for CO2 hydrogenation to ethanol. Background Technology

[0002] Ethanol has wide applications in energy and chemical industries, with huge market demand. However, most ethanol is produced from biomass, which presents additional challenges for scaling up, increases pressure on land and water resources, and threatens food production. The overuse of fossil fuels has led to a rapid increase in carbon dioxide emissions, posing a serious and irreversible threat to global ecosystems. Directly using carbon dioxide as a feedstock for fuel and chemical production is one of the most promising methods for reducing carbon dioxide emissions. Thermocatalytic carbon dioxide hydrogenation is considered an effective technology for producing a variety of fuels and chemicals, including multi-carbon alkanes, gasoline, low-carbon olefins, long-chain olefins, aromatics, carboxylic acids, and alcohols. Therefore, directly producing ethanol through carbon dioxide hydrogenation can not only reduce dependence on fossil resources but also pave the way for carbon dioxide emission reduction and sustainable chemical and energy sources. Due to the inertness of carbon dioxide and the higher energy requirements of C-C bond coupling, CO2 conversion and ethanol selectivity are often low. In recent years, although a large number of catalysts have been designed and developed for CO2 hydrogenation to ethanol, most catalysts suffer from drawbacks such as low specific surface area and number of active sites, low ethanol selectivity, and easy deactivation. For example, patent document CN106975487A discloses a Co3O4 supported platinum catalyst with a specific morphology and its application in the CO2 hydrogenation synthesis of low-carbon alcohols. Its total alcohol selectivity is only about 20%, and ethanol accounts for less than 20% of the total alcohols. Moreover, the catalyst raw material cost is relatively high.

[0003] Alloy-phase CuCo catalysts are commonly used in the production of lower alcohols from syngas (a mixture of CO and H2), such as the CuCo catalyst for syngas-to-low alcohol production disclosed in patent document CN116474781A. This patent first prepares the CuCo-based hydrotalcite precursor Cu using a constant pH co-precipitation method or a nucleation crystallization isolation method. x Co y ZnA1-CO3 2- -LDHs were obtained by reducing the CuCo-based hydrotalcite precursor in a programmed temperature rise in an H2 atmosphere to yield the CuCo catalyst. However, compared with syngas, the direct production of ethanol from carbon dioxide hydrogenation is more difficult, requiring the integration of two catalytic active sites: CO2 activation and carbon-carbon bond coupling. Therefore, conventional CuCo alloy catalysts developed for the production of lower alcohols from syngas are insufficient to meet the requirements of CO2 hydrogenation to ethanol. Summary of the Invention

[0004] In view of the shortcomings of existing catalysts for CO2 hydrogenation to ethanol, such as high cost and low selectivity, the purpose of this invention is to provide a method for preparing an alumina-supported copper-cobalt bimetallic catalyst. The catalyst prepared by this method can significantly improve the selectivity of ethanol during CO2 hydrogenation.

[0005] A method for preparing an alumina-supported copper-cobalt bimetallic catalyst, characterized in that the preparation method includes the following steps:

[0006] (1) A certain amount of cobalt salt, copper salt, aluminum salt, urea and ammonium fluoride are dissolved in an alcohol solvent and a precursor is obtained by solvothermal reaction;

[0007] (2) The precursor is reduced by hydrogen to obtain the alumina-supported copper-cobalt bimetallic catalyst.

[0008] Preferably, the cobalt salt is cobalt sulfate, cobalt nitrate, or cobalt chloride; the copper salt is copper sulfate, copper nitrate, or copper chloride; and the aluminum salt is aluminum sulfate, aluminum nitrate, or aluminum chloride.

[0009] Preferably, the Co:Cu:Al molar ratio of the cobalt salt, copper salt and aluminum salt is 1-4:0.8-1.2:0.8-1.2.

[0010] Preferably, the mass ratio of urea to ammonium fluoride is 5-8:1.

[0011] Preferably, the amount of urea used is 1.4-2 times the total molar amount of Co, Cu and Al.

[0012] Preferably, the alcohol solvent is methanol or ethanol.

[0013] Preferably, the total ion concentration of Co, Cu and Al is 0.2-0.3 mol / L, more preferably 0.24-0.26 mol / L.

[0014] Preferably, the temperature of the solvothermal reaction is 120-200℃ and the time is 8-24 h; more preferably, the temperature of the solvothermal reaction is 150-180℃ and the time is 8-12 h.

[0015] Preferably, the temperature for hydrogen reduction is 300-500°C.

[0016] More preferably, the precursor is first calcined in an air atmosphere and then reduced in a hydrogen-containing atmosphere.

[0017] Preferably, the roasting temperature is 300-500℃ and the time is 4-8h.

[0018] The alumina-supported copper-cobalt bimetallic catalyst prepared by the above method has a flower-like structure.

[0019] The above-mentioned alumina-supported copper-cobalt bimetallic catalyst is used in the hydrogenation of carbon dioxide to ethanol.

[0020] Preferably, the reaction conditions for the hydrogenation of carbon dioxide are: H2 / CO2 molar ratio of 2.5-3.5:1, reaction pressure of 1-3.5 MPa, reaction temperature of 200-350℃, and space velocity of 6000-12000 ml·g. cat -1 ·h -1 .

[0021] Beneficial effects:

[0022] Compared with existing catalysts for the hydrogenation of CO2 to ethanol, the copper-cobalt bimetallic catalyst prepared by the method of this invention exhibits excellent catalytic activity for the hydrogenation of CO2 to ethanol and can significantly improve the selectivity of ethanol products in the reaction. Attached Figure Description

[0023] Figure 1 These are SEM images of the LDHs precursor (a), after calcination (b), and after reduction (c) of the present invention.

[0024] Figure 2 The XRD patterns of the LDH precursor of this invention (a), after calcination (b), and after reduction (c) are shown. Detailed Implementation

[0025] The technical solution of the present invention will be further described in detail below with reference to the embodiments.

[0026] Example 1

[0027] Weigh 3.10 g (0.0107 mol) Co(NO3)2·6H2O, 0.64 g (0.00265 mol) Cu(NO3)2·3H2O, and 1.00 g (0.00267 mol) Al(NO3)3·9H2O. Simultaneously weigh 1.57 g (0.0262 mol) urea and 0.22 g (0.00595 mol) NH4F. Place the above raw materials into a 100 mL clean beaker, add 65 mL of methanol, and stir magnetically for 1 h to ensure complete dissolution. Transfer the clear solution to a 100 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene (PTFE). React at 150 °C for 10 h. Cool to room temperature, filter, dry the solid in a drying oven, and then calcine it in a muffle furnace at 450 °C for 4 h. The calcined sample was then placed in a tube furnace and reduced at 300°C for 2 h in a 10% H2 / Ar mixed atmosphere with a flow rate of 25 mL / min to obtain flower-shaped Cu-Co / alumina.

[0028] Figure 1 The morphology of the prepared LDHs precursor (a), after calcination (b), and after reduction (c) is shown. Both the precursor and the prepared catalyst maintained a good flower-shaped structure.

[0029] 0.3 g of flower-shaped Cu-Co / alumina was loaded into a fixed-bed reactor for CO2 hydrogenation to ethanol synthesis. The reaction conditions were: reaction pressure 1.5 MPa, reaction temperature 240℃, H2 / CO2 molar ratio 3:1, and space velocity 12000 ml·g. cat -1 ·h -1 (The timing begins after the reaction conditions are met. The initial stage of the reaction is 0-120 min. Data after the reaction stabilizes for 2 hours are used to calculate the CO2 conversion rate and product selectivity, the same below). The test results are as follows: CO2 conversion rate is 12.6%, and the total alcohol selectivity reaches 26.3%, of which ethanol accounts for 51.6% of the total alcohol.

[0030] Example 2

[0031] Weigh 2.79 g (0.0096 mol) Co(NO3)2·6H2O, 0.65 g (0.0027 mol) Cu(NO3)2·3H2O, and 1.00 g (0.00267 mol) Al(NO3)3·9H2O. Simultaneously weigh 1.57 g (0.0262 mol) urea and 0.22 g (0.00595 mol) NH4F. Place these raw materials into a 100 mL clean beaker, add 60 mL of methanol, and stir magnetically for 1.5 h. Transfer the clear solution to a 100 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene (PTFE). React at 160 °C for 9 h. Cool to room temperature, filter, dry the solid in a drying oven, and then calcine it in a muffle furnace at 420 °C for 5 h. The calcined sample was then placed in a tube furnace and reduced at 400°C for 3 h in a 15% H2 / N2 mixed atmosphere with a flow rate of 20 mL / min to obtain flower-shaped Cu-Co / alumina.

[0032] 0.4 g of flower-shaped Cu-Co / alumina was loaded into a fixed-bed reactor for CO2 hydrogenation to ethanol synthesis. The reaction conditions were: reaction pressure 1.5 MPa, reaction temperature 240℃, and space velocity 10000 ml·g. cat -1 ·h -1 When the H2 / CO2 molar ratio is 3.2:1, the CO2 conversion rate is 15.9% and the total alcohol selectivity reaches 41.5%, of which ethanol accounts for 60.7% of the total alcohol.

[0033] Example 3

[0034] Weigh 3.17 g (0.0109 mol) Co(NO3)2·6H2O, 0.97 g (0.004 mol) Cu(NO3)2·3H2O, and 1.50 g (0.004 mol) Al(NO3)3·9H2O. Simultaneously weigh 1.90 g (0.0317 mol) urea and 0.35 g (0.00946 mol) NH4F. Place these raw materials into a 100 mL clean beaker, add 75 mL of methanol, and stir magnetically for 1.5 h. Transfer the clear solution to a 100 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene (PTFE). React at 160 °C for 12 h. Cool to room temperature, filter, dry the solid in a drying oven, and then calcine it in a muffle furnace at 500 °C for 4 h. The calcined sample was then placed in a tube furnace and reduced at 420°C for 3 h in a 10% H2 / Ar mixed atmosphere with a flow rate of 20 mL / min to obtain flower-shaped Cu-Co / alumina.

[0035] 0.3 g of flower-shaped Cu-Co / alumina was loaded into a fixed-bed reactor for CO2 hydrogenation to ethanol synthesis. The reaction conditions were: reaction pressure 1.0 MPa, reaction temperature 210℃, and space velocity 12000 ml·g. cat -1 ·h -1 When the H2 / CO2 molar ratio is 3.1:1, the CO2 conversion rate is 11.2% and the total alcohol selectivity reaches 43.6%, of which ethanol accounts for 44.5% of the total alcohol.

[0036] Example 4

[0037] Weigh 1.77 g (0.00608 mol) Co(NO3)2·6H2O, 1.47 g (0.0061 mol) Cu(NO3)2·3H2O, and 2.00 g (0.00534 mol) Al(NO3)3·9H2O. Simultaneously weigh 1.57 g (0.0262 mol) urea and 0.22 g (0.00595 mol) NH4F. Place these raw materials into a 100 mL clean beaker, add 70 mL of methanol, and stir magnetically for 1.5 h. Transfer the clear solution to a 100 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene (PTFE). React at 180 °C for 8 h. Cool to room temperature, filter, dry the solid in a drying oven, and then calcine it in a muffle furnace at 450 °C for 4 h. The calcined sample was then placed in a tube furnace and reduced at 430°C for 3 h in a 15% H2 / N2 mixed atmosphere with a flow rate of 20 mL / min to obtain flower-shaped Cu-Co / alumina.

[0038] 0.5 g of flower-shaped Cu-Co / alumina was loaded into a fixed-bed reactor for CO2 hydrogenation to ethanol synthesis. The reaction conditions were: reaction pressure 1.5 MPa, reaction temperature 240℃, and space velocity 10000 ml·g. cat -1 ·h -1 When the H2 / CO2 molar ratio is 3.5:1, the CO2 conversion rate is 10.7% and the total alcohol selectivity reaches 40.5%, of which ethanol accounts for 51.3% of the total alcohol.

[0039] Comparative Example 1

[0040] With the same molar ratio of Cu, Co, and Al as in Example 4, this invention further employs a co-precipitation method to prepare Cu-Co / alumina. The specific preparation process is as follows:

[0041] Weigh 1.77 g (0.00608 mol) Co(NO3)2·6H2O, 1.47 g (0.0061 mol) Cu(NO3)2·3H2O, and 2.00 g (0.00534 mol) Al(NO3)3·9H2O, and dissolve them completely in 65 mL of deionized water. Separately, dissolve 1.6 g NaOH and 1.06 g Na2CO3 in 100 mL of deionized water. Add both solutions dropwise to a flask under vigorous stirring. Crystallize the resulting suspension at 60 °C for 6 h, filter, wash the precipitate with deionized water until pH=7.0, dry the solid in a drying oven, and then calcine it in a muffle furnace at 450 °C for 4 h. After calcination, place the sample in a tube furnace and reduce it at 300 °C for 2 h in a 10% H2 / Ar mixed atmosphere at a flow rate of 15 mL / min to obtain Cu-Co / alumina.

[0042] The microstructure of Cu-Co / alumina prepared by the above method changes, transforming from a co-spherical shape to a blocky shape.

[0043] Figure 2 The images show the XRD patterns of the LDH precursors prepared in Examples 1-4 and by the co-precipitation method described above: (a) after calcination (b) and after reduction (c). Figure 2(a) It can be seen that all samples exhibit characteristic diffraction peaks of LDHs, indicating that LDH molecular sieves were successfully prepared by both the solvothermal and coprecipitation methods. Cu-Co-LDH precursors with different morphologies and molar ratios all exhibit characteristic diffraction peaks resembling hydrotalcite (PDF#37-0630 / 51-0045), proving the successful preparation of layered double hydroxide precursors. After calcination, no characteristic diffraction peaks of CuO and alumina were observed in the obtained Cu-Co-MMO calcined samples, which showed a series of mixed phases of Co3O4 spinel (PDF#78-1970) and Cu-Co alloy oxides (PDF#36-1189 / 78-2173). This indicates that Cu successfully incorporated into the Co lattice to form Co-Cu alloy oxides, while Al may exist in amorphous AlO2. x It exists in the form of Co3O4. After reduction with H2, the diffraction peaks related to Co3O4 disappear, and characteristic diffraction peaks of CoO (PDF#48-1719) and metallic Co (PDF#15-0806) appear. As the Cu / Co molar ratio decreases, the diffraction peaks gradually shift towards metallic Cu, indicating that some Co atoms are incorporated into the Cu lattice, forming a CuCo alloy.

[0044] 0.5 g of Cu-Co / alumina prepared by the above co-precipitation method was loaded into a fixed-bed reactor for CO2 hydrogenation to ethanol synthesis experiment. The reaction conditions were: reaction pressure 1.5 MPa, reaction temperature 240℃, and space velocity 10000 ml·gcat. -1 ·h -1 When the H2 / CO2 molar ratio is 3.5:1, the CO2 conversion rate is 8.9%, and the total alcohol selectivity reaches 30.2%, of which ethanol accounts for 18.7% of the total alcohols.

[0045] This application also optimized the amounts of Cu, Co, and Al used in the above coprecipitation method, and found that the Cu-Co / alumina prepared by the above coprecipitation method under the conditions of 2.79 g Co(NO3)2·6H2O, 0.77 g Cu(NO3)2·6H2O, and 1.200 g Al(NO3)3·9H2O had the best catalytic activity, with a CO2 conversion rate of 9.2% and a total alcohol selectivity of 31.7%, of which ethanol accounted for 19.4% of the total alcohol pair.

[0046] The comparative experiments above show that, compared with the existing coprecipitation method, the copper-cobalt bimetallic catalyst prepared by the method of the present invention can significantly improve the selectivity of ethanol products in the CO2 hydrogenation reaction.

[0047] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing an alumina-supported copper-cobalt bimetallic catalyst, characterized in that, The preparation method includes the following steps: (1) A certain amount of cobalt salt, copper salt, aluminum salt, urea and ammonium fluoride are dissolved in an alcohol solvent and a precursor is obtained by solvothermal reaction; (2) The precursor is reduced by hydrogen to obtain a flower-shaped alumina-supported copper-cobalt bimetallic catalyst; The Co:Cu:Al molar ratio of the cobalt salt, copper salt, and aluminum salt is 1-4:0.8-1.2:0.8-1.2; The mass ratio of urea to ammonium fluoride is 5-8:1, and the amount of urea used is 1.4-2 times the total molar amount of Co, Cu and Al. The solvothermal reaction is carried out at a temperature of 120-200℃ for a duration of 8-24 h. The temperature for hydrogen reduction is 300-500℃; The precursor is first calcined in an air atmosphere, and then reduced in a hydrogen-containing atmosphere. The calcination temperature is 300-500℃ and the time is 4-8h.

2. The preparation method according to claim 1, characterized in that: The cobalt salt is cobalt sulfate, cobalt nitrate, or cobalt chloride; the copper salt is copper sulfate, copper nitrate, or copper chloride; and the aluminum salt is aluminum sulfate, aluminum nitrate, or aluminum chloride.

3. The preparation method according to claim 1, characterized in that: The alcohol solvent is methanol or ethanol.

4. The preparation method according to claim 3, characterized in that: The total concentration of Co, Cu and Al ions is 0.2-0.3 mol / L.

5. The preparation method according to claim 4, characterized in that: The total ion concentration of Co, Cu and Al is 0.24-0.26 mol / L.

6. The flower-shaped alumina-supported copper-cobalt bimetallic catalyst prepared by the preparation method according to any one of claims 1-5.

7. The application of the flower-shaped alumina-supported copper-cobalt bimetallic catalyst of claim 6 in the hydrogenation of carbon dioxide to ethanol.

8. The application according to claim 7, characterized in that: The reaction conditions for the hydrogenation of carbon dioxide are: H2 / CO2 molar ratio of 2.5-3.5:1, reaction pressure of 1-3.5 MPa, reaction temperature of 200-350℃, and space velocity of 6000-12000 ml·g. cat -1 ·h -1 .

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