A MXene-based catalyst for CO2 hydrogenation to methanol, preparation method and application

Abstract

This invention discloses a MXene-based catalyst for CO2 hydrogenation to methanol, its preparation method, and its application. The method first synthesizes a two-dimensional metal carbide layered material, such as niobium carbide, titanium carbide, and vanadium titanate. Then, a copper-zinc carbide catalyst, or MXene-based catalyst, is prepared by combining hydrothermal ligand synthesis with coprecipitation. The molar percentages of copper and zinc in the MXene-based catalyst are 10-30% and 5-15%, respectively. The catalyst prepared in this invention exhibits high carbon dioxide conversion, excellent methanol selectivity, and long-term catalyst stability in CO2 hydrogenation to methanol.

Description

Technical Field

[0001] The present invention belongs to the field of catalysts, and specifically relates to a MXene-based catalyst for CO2 hydrogenation to methanol, a preparation method and an application thereof. Background Art

[0002] With the rapid development of renewable energy such as wind and solar energy, the technical route of catalytically synthesizing green methanol from captured CO2 and green hydrogen produced from renewable energy has received widespread attention. It is expected to provide a transformative solution for the rapid realization of my country's "dual carbon" goals and the high-quality transformation of its energy structure.

[0003] Green methanol is a future development trend. Methanol is a basic organic chemical raw material with a wide range of applications. It can be used in chemical products such as synthetic fibers, formaldehyde, plastics, pharmaceuticals, pesticides, dyes, and protein synthesis. Methanol can also be used as a liquid fuel for direct methanol fuel cells (DMFCs) and improved diesel engines. The International Renewable Energy Agency (IRENA) predicts that global demand for green methanol will reach 385 million tons by 2050. The key to CO2 hydrogenation to methanol lies in the catalyst. Currently, the mainstream commercial catalyst is Cu / ZnO / Al2O3. However, its application is limited by low CO2 conversion, low methanol selectivity, and poor catalyst stability. Therefore, to achieve the sustainable development of efficient CO2 hydrogenation to methanol technology, the development of efficient CO2 hydrogenation to methanol catalysts is of great significance. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the present invention provides a MXene-based catalyst for CO2 hydrogenation to methanol, as well as its preparation method and application. The catalyst prepared by the present invention exhibits advantages such as high carbon dioxide conversion, excellent methanol selectivity, and long-term catalyst stability when used in CO2 hydrogenation to methanol.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for preparing a MXene-based catalyst for CO2 hydrogenation to methanol comprises the following steps:

[0007] (1) Synthesis of MXene carrier

[0008] The MAX powder is dissolved in hydrofluoric acid to obtain a suspension, stirred and etched under nitrogen protection, and then ultrasonicated, filtered, washed and dried to obtain a MXene support; the MAX powder is selected from one of Nb2AlC, Ti3AlC2, and V2AlC;

[0009] (2) Synthesis of MXene-based catalysts

[0010] The MXene carrier powder is dissolved in deionized water to obtain a suspension A, and copper salt and zinc salt are dissolved in deionized water to obtain a solution B; the suspension A and solution B are mixed by ultrasonication for 20-40 minutes, and then stirred and mixed at 65-75°C; the mixed suspension is placed in a hydrothermal reactor, first hydroheated at 75-85°C for 8-12 hours, and then further hydroheated at 160-180°C for 8-12 hours; then the pH of the suspension is adjusted, a precipitant is added, and co-precipitation aging is carried out at 55-65°C for 5-7 hours; finally, the MXene-based catalyst is obtained through washing, filtration, drying and calcination.

[0011] As a preferred embodiment of the present invention, in step (1), the ultrasonic treatment is performed under ultrasonic conditions of 100-150 W for 5-15 minutes.

[0012] As a preferred embodiment of the present invention, in step (1), the stirring speed is 500-1000 rpm, the etching control temperature is 25-80 °C, and the time is 18-48 h.

[0013] As a preferred embodiment of the present invention, in step (2), the copper salt and zinc salt refer to one or more of the nitrate, acetate, and chloride corresponding to copper and zinc, respectively.

[0014] As a preferred embodiment of the present invention, in step (2), the mass ratio of the MXene support, copper salt, and zinc salt is 1-2:2-3:1; more preferably, the mass ratio of the MXene support, copper salt, and zinc salt is 2:2:1.

[0015] As a preferred embodiment of the present invention, in step (2), the pH is adjusted to a range of 6 to 10, more preferably 7.

[0016] As a preferred embodiment of the present invention, in step (2), the precipitant is selected from one of sodium carbonate, oxalic acid, and 3-aminopropyltriethoxysilane, more preferably sodium carbonate.

[0017] As a preferred embodiment of the present invention, in step (2), the calcination temperature is 400-500°C and the calcination time is 3-4 hours.

[0018] The present invention also provides a MXene-based catalyst prepared by the above preparation method.

[0019] The present invention also provides an application of the above-mentioned MXene-based catalyst in catalytic CO2 hydrogenation to methanol, comprising: loading the MXene-based catalyst into a high-pressure fixed-bed reactor, reducing it with a mixture of hydrogen and nitrogen in a volume ratio of 2:8 at 300°C, and then introducing a reaction gas under the conditions of a temperature of 220-300°C, a pressure of 2-3 MPa, and a space velocity of 20,000-32,000 mL / (gcat h) to achieve catalytic hydrogenation to produce methanol.

[0020] Principle of the present invention:

[0021] The present invention first synthesizes two-dimensional metal carbide layered materials, such as niobium carbide, titanium carbide, and vanadium titanate. A copper-zinc carbide catalyst, or MXene-based catalyst, is then prepared through a hydrothermal ligand synthesis combined with a coprecipitation method. The molar percentages of copper and zinc in the MXene-based catalyst are 10-30% and 5-15%, respectively. The present invention utilizes the metal carbides' combined properties of reducible oxides and strong diffusion properties of carbon supports to construct a heterogeneous catalyst with copper and zinc. The metal carbides possess a nanoscale layered structure that can accommodate metal active sites and reactive molecules, while also possessing exposed metal active sites on the surface. These features allow for better dispersion of the copper and zinc active sites on the support surface. This layered structure promotes the diffusion of small CO2 and H2 molecules within the catalyst, while the exposed metal active sites anchor the copper and zinc active sites and adsorb CO2, resulting in the catalyst exhibiting high carbon dioxide conversion, excellent methanol selectivity, and long-term catalyst stability.

[0022] The beneficial effects of the present invention are:

[0023] (1) The MXene-based catalyst of the present invention introduces a two-dimensional metal carbide layered material and uses the interlayer structure to regulate the diffusion rate of the reactants during the reaction, thereby changing the reaction equilibrium and enabling the catalyst to exhibit better carbon dioxide conversion and methanol selectivity.

[0024] (2) Traditional copper-zinc-aluminum methanol catalysts are prone to deactivation during the reaction process. This is mainly due to the agglomeration of metallic copper particles under the synergistic effect of high temperature and water vapor, which leads to reduced catalyst activity. The addition of the two-dimensional metal carbide structure of the present invention can effectively promote the diffusion of H2O molecules, using the exposed metal active sites to anchor the copper and zinc active sites, thereby extending the catalyst life. DETAILED DESCRIPTION

[0025] In order to make the technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0026] Example 1 Synthesis of copper zinc niobium carbide CZNC-1 catalyst:

[0027] Synthesis of niobium carbide support: 1 g of Nb2AlC MAX powder was dissolved in 30 mL of hydrofluoric acid to obtain a suspension. Nitrogen was continuously introduced into the suspension for 5 minutes, and the suspension was etched at 80°C with a stirring speed of 1000 rpm. Subsequently, the suspension was ultrasonicated at 100 W for 10 minutes, and then the suspension was filtered to form a filter cake. The filter cake was washed three times with deionized water, and the powder was dried at 80°C overnight.

[0028] Introduction of copper and zinc active sites. 300 mg of Nb2C powder was dissolved in deionized water to obtain suspension A. 568 mg of copper nitrate trihydrate and 275 mg of zinc nitrate pentahydrate were dissolved in deionized water to obtain solution B. Suspensions A and B were mixed and sonicated at 100 W for 30 minutes. The mixture was then stirred at 70°C for a predetermined period of time. The mixed suspension was placed in a polytetrafluoroethylene reactor and hydrothermally heated at 80°C for 10 hours, followed by a further hydrothermal treatment at 170°C for 10 hours. The pH of the suspension was adjusted to 7. 1 g of sodium carbonate was dissolved in the suspension and coprecipitated and aged at 60°C for 6 hours. The mixture was washed with deionized water, filtered, and dried. The brown powder was then calcined in a muffle furnace at 400°C for 3 hours at a heating rate of 5°C / min to obtain the copper-zinc composite niobium carbide heterogeneous catalyst, the MXene-based catalyst of the present invention, designated CZNC-1.

[0029] Example 2 Synthesis of copper zinc niobium carbide CZNC-2 catalyst:

[0030] The synthesis of the niobium carbide support was the same as in Example 1.

[0031] Introduction of copper and zinc active sites: 300 mg of Nb2C powder was dissolved in deionized water to obtain suspension A. 469 mg of copper acetate monohydrate and 168 mg of zinc acetate monohydrate were dissolved in deionized water to obtain solution B. Suspensions A and B were mixed and sonicated at 100 W for 30 minutes. The mixture was then stirred at 70°C for a predetermined period of time. The mixed suspension was placed in a polytetrafluoroethylene reactor and hydrothermally heated at 80°C for 10 hours, followed by a further hydrothermal treatment at 170°C for 10 hours. The pH of the suspension was adjusted to 7. 1 g of sodium carbonate was dissolved in the suspension and coprecipitated and aged at 60°C for 6 hours. The mixture was washed with deionized water, filtered, and dried. The brown powder was then calcined in a muffle furnace at 400°C for 3 hours at a heating rate of 5°C / min to obtain the copper-zinc composite niobium carbide heterogeneous catalyst, the MXene-based catalyst of the present invention, designated CZNC-2.

[0032] Example 3 Synthesis of copper zinc niobium carbide CZNC-3 catalyst:

[0033] The synthesis of the niobium carbide support was the same as in Example 1.

[0034] Introduction of copper and zinc active sites: 300 mg of Nb2C powder was dissolved in deionized water to obtain suspension A. 317 mg of copper chloride and 126 mg of zinc chloride were dissolved in deionized water to obtain solution B. Suspensions A and B were mixed and sonicated at 100 W for 30 minutes. The mixture was then stirred at 70°C for a predetermined period of time. The mixed suspension was placed in a polytetrafluoroethylene reactor and hydrothermally heated at 80°C for 10 hours, followed by a further hydrothermal treatment at 170°C for 10 hours. The pH of the suspension was adjusted to 7. 1 g of sodium carbonate was dissolved in the suspension and coprecipitated and aged at 60°C for 6 hours. The mixture was washed with deionized water, filtered, and dried. The brown powder was then calcined in a muffle furnace at 400°C for 3 hours at a heating rate of 5°C / min to obtain the copper-zinc composite niobium carbide heterogeneous catalyst, the MXene-based catalyst of the present invention, designated CZNC-3.

[0035] Example 4 Synthesis of copper zinc niobium carbide CZNC-4 catalyst:

[0036] The synthesis of the niobium carbide support was the same as in Example 1.

[0037] Introduction of copper and zinc active sites: 300 mg of Nb2C powder was dissolved in deionized water to obtain suspension A. 568 mg of copper nitrate trihydrate and 275 mg of zinc nitrate pentahydrate were dissolved in deionized water to obtain solution B. Suspensions A and B were mixed and sonicated at 100 W for 30 minutes. The mixture was then stirred at 70°C for a predetermined period of time. The mixed suspension was placed in a polytetrafluoroethylene reactor and hydrothermally heated at 80°C for 10 hours, followed by a further hydrothermal treatment at 170°C for 10 hours. The pH of the suspension was adjusted to 7. 1 g of oxalic acid was dissolved in the suspension and coprecipitated and aged at 60°C for 6 hours. The mixture was washed with deionized water, filtered, and dried. The brown powder was then calcined in a muffle furnace at 400°C for 3 hours at a heating rate of 5°C / min to obtain the copper-zinc composite niobium carbide heterogeneous catalyst, the MXene-based catalyst of the present invention, designated CZNC-4.

[0038] Example 5 Synthesis of copper zinc niobium carbide CZNC-5 catalyst:

[0039] The synthesis of the niobium carbide support was the same as in Example 1.

[0040] Introduction of Cu / Zn active sites: 300 mg of Nb2C powder was dissolved in deionized water to obtain suspension A. 568 mg of copper nitrate trihydrate and 275 mg of zinc nitrate pentahydrate were dissolved in deionized water to obtain solution B. Suspensions A and B were mixed and sonicated at 100 W for 30 minutes. The mixture was then stirred at 70°C for a specified period of time. The resulting suspension was placed in a polytetrafluoroethylene reactor and hydrothermally heated at 80°C for 10 hours, followed by a further hydrothermal treatment at 170°C for 10 hours. The pH of the suspension was adjusted to 7, and 1 g of 3-aminopropyltriethoxysilane was dissolved in the suspension. The mixture was coprecipitated and aged at 60°C for 6 hours. The product was washed with deionized water, filtered, and dried. The brown powder was then calcined in a muffle furnace at 400°C for 3 h with a heating rate of 5°C / min to obtain a copper-zinc composite niobium carbide heterogeneous catalyst, namely the MXene-based catalyst of the present invention, which is recorded as CZNC-5.

[0041] Example 6 Testing the catalytic reaction performance of carbon dioxide hydrogenation reaction in a fixed bed reactor:

[0042] The test steps are as follows:

[0043] 0.15 g of different types of copper-zinc niobium carbide catalysts (40-60 mesh) were poured into a stainless steel reactor for loading, and both ends were plugged with quartz wool to maintain the bed height to assemble a fixed-bed reactor.

[0044] The catalyst was reduced with 20% hydrogen / 80% nitrogen (volume percentage) at a space velocity of 40,000 mL / (gcat·h) at 300°C for 2 h.

[0045] A reaction gas having a molar ratio of hydrogen / carbon dioxide / nitrogen (H2 / CO2 / N2) of 72.0 / 24.0 / 4.0 was introduced into a high-pressure fixed-bed reactor. The catalyst bed temperature was controlled at 240°C, the reaction pressure was 3 MPa, and the volumetric space velocity of the feed gas was in the range of 18,000 mL / (gcat·h).

[0046] The experimental results are shown in Table 1 below:

[0047] Table 1 Catalytic results of the copper-zinc-niobium carbide catalysts prepared in Examples 1-5 in the CO2 hydrogenation to methanol reaction

[0048]

[0049] The experimental results above show that CZNC-1 achieved a CO conversion rate of 4.2% and a methanol selectivity of 75.3%. Furthermore, the catalyst achieved a methanol yield of 3.2%, demonstrating the best overall performance among the five samples. Therefore, CZNC-1 was selected as the catalyst for subsequent copper-zinc-niobium carbide catalyst studies.

[0050] Example 7 Catalytic Reaction Performance Test of Carbon Dioxide Hydrogenation Reaction at Different Reaction Temperatures:

[0051] (1) 0.15 g of CZNC-1 copper zinc niobium carbide catalyst (40-60 mesh) of different types was poured into a stainless steel reactor for filling, and quartz wool was used to fill both ends to maintain the bed height to assemble a fixed bed reactor;

[0052] (2) The catalyst was reduced at 300 °C for 2 h using 20% ​​hydrogen / 80% nitrogen at a space velocity of 40,000 mL / (gcat·h).

[0053] (3) A reaction gas with a molar ratio of hydrogen / carbon dioxide / nitrogen (H2 / CO2 / N2) = 72.0 / 24.0 / 4.0 was introduced into a high-pressure fixed-bed reactor. The catalyst bed temperature was controlled at 220-300°C, the reaction pressure was 3 MPa, and the volumetric space velocity of the feed gas was 18,000 mL / (gcat·h).

[0054] The experimental results are shown in Table 2 below:

[0055] Table 2 Catalytic results of the copper-zinc niobium carbide catalyst prepared in Example 1 in the CO2 hydrogenation to methanol reaction at different temperatures

[0056]

[0057] From the above test results, it can be seen that increasing the reaction temperature can increase the CO2 conversion rate of the CZNC-1 catalyst, but it will also cause a decrease in methanol selectivity. For example, when the reaction temperature is 220°C, the CO2 conversion rate is only 2.1%, but the methanol selectivity is as high as 100%; when the reaction temperature is raised to 300°C, although the CO2 conversion rate is also increased to 15.9%, the methanol selectivity drops to 26.8%. Overall, when the reaction temperature is 260°C, the catalyst shows the best performance, with the CO2 conversion rate and methanol selectivity reaching 7.2% and 62.6%. At this time, the methanol yield is 4.50%, and the space-time yield of methanol reaches 384 mg / g cat ·h.

[0058] Example 8 Catalytic Reaction Performance Test of Carbon Dioxide Hydrogenation Reaction at Different Reaction Flow Rates:

[0059] (1) 0.15 g of CZNC-1 copper-zinc-niobium carbide catalyst (40-60 mesh) was poured into a stainless steel reactor for filling, and both ends were filled with quartz wool to maintain the bed height, and a fixed bed reactor was assembled;

[0060] (2) The catalyst was reduced at 300 °C for 2 h using 20% ​​hydrogen / 80% nitrogen at a space velocity of 40,000 mL / (gcat·h).

[0061] (3) A reaction gas with a molar ratio of hydrogen / carbon dioxide / nitrogen (H2 / CO2 / N2) = 72.0 / 24.0 / 4.0 was introduced into a high-pressure fixed-bed reactor. The catalyst bed temperature was controlled at 260°C and the reaction pressure was 3 MPa. The feed gas volumetric space velocity was 16,000-32,000 mL / (gcat·h).

[0062] The experimental results are shown in Table 3 below:

[0063] Table 3 Catalytic results of the copper-zinc niobium carbide catalyst prepared in Example 1 in the CO2 hydrogenation to methanol reaction at different space velocities

[0064]

[0065] The experimental results above show that increasing the reaction space velocity (SSV) improves methanol selectivity and methanol space-time yield (SHY) over the CZNC-1 catalyst, but also reduces CO2 conversion. For example, at a SSV of 16,000 mL / (gcat·h), CO2 conversion reached 11.3%, but methanol selectivity was only 47.6%. At a SSV of 32,000 mL / (gcat·h), CO2 conversion dropped to 5.6%, but methanol selectivity increased to 61.5%. Considering the actual methanol production, a SHS of 20,000-24,000 mL / (gcat·h) is the optimal reaction space velocity, achieving a methanol SHY of 322-384 mg / gcat·h.

[0066] Example 9 Catalytic Reaction Performance Test of Carbon Dioxide Hydrogenation Reaction at Different Reaction Pressures:

[0067] (1) 0.10 g of CZNC-1 copper zinc niobium carbide catalyst (40-60 mesh) of different types was poured into a stainless steel reactor for filling, and quartz wool was used to fill both ends to maintain the bed height to assemble a fixed bed reactor;

[0068] (2) The catalyst was reduced at 300 °C for 2 h using 20% ​​hydrogen / 80% nitrogen at a space velocity of 40,000 mL / (gcat·h).

[0069] (3) A reaction gas with a molar ratio of hydrogen / carbon dioxide / nitrogen (H2 / CO2 / N2) = 72.0 / 24.0 / 4.0 was introduced into a high-pressure fixed-bed reactor. The catalyst bed temperature was controlled at 240°C and the reaction space velocity was 18,000 mL / (gcat·h). The reaction pressure was 1.5-3.5 MPa.

[0070] The experimental results are shown in Table 4 below:

[0071] Table 4 Catalytic results of the copper-zinc niobium carbide catalyst prepared in Example 1 in the CO2 hydrogenation to methanol reaction at different pressures

[0072]

[0073] The experimental results above demonstrate that increasing the reaction pressure improves the CO2 conversion, methanol selectivity, and methanol space-time yield of the CZNC-1 catalyst. For example, at a pressure of 1.5 MPa, the CO2 conversion reached only 1.2%, and the methanol selectivity was only 61.3%. However, at a space velocity of 3.5 MPa, the CO2 conversion increased to 3.8%, while the methanol selectivity rose to 74.2%, resulting in a methanol yield of 360 mg / gcat·h. However, excessively high pressures increase the system's energy consumption. Considering the actual methanol production, a pressure of 3 MPa is the optimal space velocity, achieving a methanol space-time yield of 318 mg / gcat·h.

[0074] The embodiment described above is only a preferred solution of the present invention and does not limit the present invention in any form. Other variations and modifications are possible without exceeding the technical solution described in the claims.

Claims

1. A method for preparing a MXene-based catalyst for CO2 hydrogenation to methanol, characterized in that: The following steps are involved: (1) Synthesis of MXene supports The MAX powder is dissolved in hydrofluoric acid to obtain a suspension, stirred and etched under nitrogen protection, and then ultrasonicated, filtered, washed and dried to obtain a MXene support; the MAX powder is selected from one of Nb2AlC, Ti3AlC2, and V2AlC; (2) Synthesis of MXene-based catalysts The MXene carrier powder is dissolved in deionized water to obtain a suspension A, and copper salt and zinc salt are dissolved in deionized water to obtain a solution B; the suspension A and solution B are mixed by ultrasonication for 20-40 minutes, and then stirred and mixed at 65-75°C; the mixed suspension is placed in a hydrothermal reactor, first hydroheated at 75-85°C for 8-12 hours, and then further hydroheated at 160-180°C for 8-12 hours; then the pH of the suspension is adjusted, a precipitant is added, and co-precipitation aging is carried out at 55-65°C for 5-7 hours; finally, the MXene-based catalyst is obtained through washing, filtration, drying and calcination.

2. The preparation method according to claim 1, characterized in that In the step (1), the ultrasonic treatment is carried out under an ultrasonic condition of 100-150W for 5-15 minutes.

3. The preparation method according to claim 1, characterized in that In the step (1), the stirring speed is 500-1000 rpm, the etching temperature is controlled at 25-80° C., and the etching time is 18-48 hours.

4. The preparation method according to claim 1, characterized in that In step (2), the copper salt and zinc salt refer to one or more of the nitrate, acetate and chloride corresponding to copper and zinc, respectively.

5. The preparation method according to claim 1, characterized in that In the step (2), the mass ratio of the MXene support, the copper salt, and the zinc salt is 1-2:2-3:

1.

6. The preparation method according to claim 1, characterized in that In the step (2), the pH is adjusted to a range of 6 to 10.

7. The preparation method according to claim 1, characterized in that In the step (2), the precipitant is selected from one of sodium carbonate, oxalic acid, and 3-aminopropyltriethoxysilane.

8. The preparation method according to claim 1, characterized in that In the step (2), the calcination temperature is 400-500° C. and the calcination time is 3-4 hours.

9. A MXene-based catalyst prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the MXene-based catalyst according to any one of claims 1 to 9 in catalytic CO2 hydrogenation to methanol, characterized in that: include: The MXene-based catalyst was loaded into a high-pressure fixed-bed reactor and reduced at 300°C with a mixture of hydrogen and nitrogen in a volume ratio of 2:

8. The reaction gas was then introduced at a temperature of 220-300°C, a pressure of 2-3 MPa, and a space velocity of 20,000-32,000 mL / (gcat h) to achieve catalytic hydrogenation to methanol.

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