A heteropoly acid supported bimetallic center catalyst, a preparation method and application thereof

Abstract

The application discloses a kind of heteropoly acid supported bimetallic center catalyst and its preparation method and application, its preparation includes the following steps: after dissolving heteropoly acid, anion solution is obtained by acidification;After dissolving ruthenium salt and cesium salt, cation solution is obtained by acidification;Anion solution and cation solution are mixed and reacted, and the catalyst of heteropoly acid supported metal single atom is obtained by centrifugation, washing, drying and grinding to the precipitate;After dispersion, it is mixed with Lewis acid solution and coupled reaction, and the catalyst is obtained after the precipitate is centrifuged, washed, dried and ground.The transition metal single atom is connected in series with Lewis acid in the application and is supported on polyoxometalate, and the obtained catalyst greatly improves the adsorption and activation of methane and carbon dioxide, and the synergistic effect in series connection catalyst makes the C-C coupling efficiency of methane and carbon dioxide increase, and the selectivity of target product acetic acid can be effectively improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of catalysis technology, in particular to a heteropoly acid supported bimetallic catalyst and its preparation method and application. BACKGROUND

[0002] Methane is the main component of natural gas, with abundant reserves and low price. However, direct combustion or emission of methane will cause energy waste and environmental problems. Converting methane into high-value-added acetic acid is an important way to achieve sustainable chemical industry. However, as reactants, methane and carbon dioxide are relatively stable in chemical properties, and the C-C coupling reaction between them is slow in kinetics, which leads to great challenges in the direct synthesis of acetic acid from methane and carbon dioxide.

[0003] CN117427627A discloses a kind of light catalyst for preparing ethanol by coupling methane and carbon dioxide and its preparation method and application, which is synthesized by in-situ one-step calcination method using urea, metal salt of cerium and zinc;The light catalyst is used for preparing ethanol by coupling methane and carbon dioxide, and by adjusting Ce\Zn ratio, water amount, reaction time, reaction temperature, gas ratio and total pressure, etc., methanol, peroxymethanol, acetic acid and the ethanol are obtained by reaction under light, while the generation of carbon monoxide and other side reactions is inhibited, and high selectivity of high-value multi-carbon liquid products is realized on the same catalyst interface by photochemistry.

[0004] CN115212898A discloses a catalyst for photocatalytic conversion of methane to acetic acid. The catalyst is a composite photocatalyst prepared by compounding two-dimensional layered transition metal sulfide confined metal monatomic material with nano-semiconductor. The two-dimensional layered transition metal sulfide prepared by this method has fewer layers and single crystal phase, and the type and concentration of confined metal atoms are easy to control, which is a universal method for preparing two-dimensional layered transition metal sulfide confined monatomic. In addition, the composite photocatalyst has a wider light absorption range and intensity, stronger oxidation-reduction, surface photocurrent and electron-hole separation capacity compared with single semiconductor, and can efficiently couple methane, oxygen and carbon monoxide to prepare acetic acid in one step under ultraviolet-visible light. The catalyst preparation method has the advantages of simple process, easy control and low cost.

[0005] The above-mentioned technologies aim to solve the problem of methane conversion, but still have some deficiencies. For example, although the tandem catalysis has made some progress in acetic acid synthesis, the reaction efficiency needs to be improved. In addition, the use of additional energy fields such as plasma, light and radiation to promote the reaction makes it difficult to achieve ideal acetic acid selectivity and scale-up. Developing high-efficiency catalysts is an important direction to solve the above problems, but the existing catalysts still have deficiencies in balancing the activity and selectivity of acetic acid. Therefore, it is of great significance to design a catalyst with stable activity and high selectivity for tandem catalysis of methane to prepare acetic acid. SUMMARY

[0006] The present application is directed to the problem that the reaction conditions are harsh and the product selectivity is low in the high-efficiency preparation of acetic acid by thermal catalytic reaction of methane and carbon dioxide, and provides a preparation method of a heteropoly acid loaded bimetallic center catalyst, which can realize low-temperature high-selectivity thermal catalytic C-C coupling and obtain high-value acetic acid chemicals.

[0007] To achieve the above object, the technical scheme adopted by the present application is:

[0008] A preparation method of a heteropoly acid loaded bimetallic center catalyst, comprising the steps of:

[0009] Step 1: dissolving and acidifying the heteropoly acid to obtain an anion solution; dissolving and acidifying the ruthenium salt and cesium salt to obtain a cation solution;

[0010] Step 2: mixing and reacting the anion solution and the cation solution, centrifuging, washing, drying and grinding the precipitate to obtain a heteropoly acid loaded metal monatomic catalyst;

[0011] Step 3: dispersing the catalyst obtained in step 2 and mixing and coupling with a Lewis acid solution, and centrifuging, washing, drying and grinding the precipitate to obtain the catalyst.

[0012] In the present application, the transition metal monatomic atom is coupled with the Lewis acid and loaded on the heteropoly acid, which greatly improves the adsorption and activation of methane and carbon dioxide, and the synergistic effect in the tandem catalyst leads to the improvement of the C-C coupling efficiency of methane and carbon dioxide, which can effectively improve the selectivity of the target product acetic acid.

[0013] The heteropoly acid includes one or more than two combinations of silicotungstic acid, phosphotungstic acid, silicomolybdic acid and phosphomolybdic acid; preferably, the heteropoly acid is phosphotungstic acid, which has the best effect.

[0014] The ruthenium salt includes one or more than two combinations of halide, nitrate or other complex salt of ruthenium;

[0015] The cesium salt includes one or more than two combinations of halide, nitrate or other complex salt of cesium.

[0016] Preferably, the solvent for dissolving the heteropoly acid, ruthenium salt, cesium salt and Lewis acid includes water or alcohol, and the alcohol includes methanol, ethanol, ethylene glycol, n-butanol, benzyl alcohol, etc.

[0017] The Lewis acid includes one or more than two combinations of aluminum chloride, zinc dichloride, iron trichloride, copper dichloride and boron trifluoride ether complex. Preferably, the Lewis acid is aluminum chloride, which has the best effect.

[0018] The mass of the ruthenium salt is 0.1-2% of the mass of the heteropoly acid, the mass of the cesium salt is 0.13-2.6% of the mass of the heteropoly acid, and the total mass of the ruthenium salt and the cesium salt is 0.1-4% of the mass of the heteropoly acid.

[0019] The molar ratio of the metal in the Lewis acid to the metal in the ruthenium salt is 1:0.1-5; preferably, the molar ratio of the metal in the Lewis acid to the metal in the ruthenium salt is 1:0.5-2, and further preferably 1:1.

[0020] The acidification is adding an inorganic acid dropwise to the solution until the pH of the solution is 1-4.

[0021] The temperature of the mixed reaction in step 2 is -20-0°C, and the reaction time is 1-10h.

[0022] The temperature of the coupling reaction in step 3 is -5-20°C, and the reaction time is 1-5h.

[0023] All the product washing in the steps is collectively washed with the solvent used, and the drying is performed at -50-0°C for 1-6 hours.

[0024] The application also provides a heteropoly acid supported bimetallic center catalyst prepared according to the preparation method.

[0025] The application also provides the use of the catalyst in the preparation of acetic acid from methane in a cascade reaction. The catalyst prepared by the application can realize the high selective catalysis of methane to acetic acid under mild conditions. Preferably, the reaction process for preparing acetic acid comprises: a reaction temperature of 150-400°C, a reaction time of 0.5-2h, a methane pressure of 0.50-3.00MPa, a carbon dioxide pressure of 0.50-3.00MPa, a total gas pressure of 1.00-5.00MPa, and a catalyst mass of 5-50mg.

[0026] Further preferably, the total gas pressure is below 3 MPa, and the reaction temperature is 180-300°C. The catalyst of the application can realize high catalytic activity under lower temperature and lower gas pressure, and the selectivity of acetic acid reaches more than 40%, and the best effect can reach more than 95%.

[0027] Compared with the prior art, the application has the following beneficial effects:

[0028] (1) The application can effectively solve the problems of methane and carbon dioxide activation and C-C coupling by simply dispersing and loading the transition metal cascade Lewis acid on the polyoxometalate carrier through a simple precipitation-impregnation method, and realizes the efficient conversion of methane to acetic acid in a cascade reaction.

[0029] (2) The preparation process of the catalyst is relatively simple, easy to operate, and the raw materials are simple and easy to obtain, which has industrial application potential, is conducive to reducing production cost, and promotes the development of related industries.

[0030] (3) Compared with the existing catalyst, the catalyst of the application has excellent selectivity and activity in acetic acid, and can realize high selectivity and high activity under relatively mild conditions, providing a new effective way for the resource utilization of methane and carbon dioxide. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The X-ray diffraction spectrum of the catalyst prepared for Example 2, Comparative Example 1 and Comparative Example 2 is shown in the following figure.

[0032] Figure 2 The pyridine infrared spectrum of the catalyst prepared for Example 2, Comparative Example 1 and Comparative Example 2 is shown in the following figure.

[0033] Figure 3 The TEM image of the tandem catalyst prepared for Example 2 and the EDX mapping image of each element in the catalyst are shown in the following figure.

[0034] Figure 4 The performance stability test figure of the tandem catalyst prepared for Example 2 in the preparation of acetic acid by co-conversion of methane and carbon dioxide is shown in the following figure. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical scheme and advantages of the application more clear, the application will be further described in detail below in combination with examples. It should be understood that the specific examples described herein are only used to explain the application and not to limit the application. Those skilled in the art can modify or replace the equivalent without departing from the spirit and scope of the application, which should be covered by the protection scope of the application.

[0036] The raw materials used in the following specific embodiments are purchased from Aladdin or National Pharmaceutical Reagent and are used directly without treatment.

[0037] Example 1

[0038] Step 1: Dissolve 2.0 g of phosphotungstic acid in 30 mL of deionized water, add 4 drops of concentrated hydrochloric acid (65-68%) to acidify, and stir uniformly to obtain an anion solution.

[0039] Step 2: Dissolve 9.93 mg of ruthenium chloride and 450 mg of cesium nitrate in 30 mL of deionized water, add 4 drops of concentrated hydrochloric acid (65-68%) to acidify, and stir uniformly to obtain a cation solution.

[0040] Step 3, under the condition of stirring at 500 r / min, the cationic solution of step 2 was slowly added to the anionic solution of step 1 at a rate of 1 mL / min, and the reaction was carried out at 0℃, during which a pink precipitate gradually formed. After the addition was completed, the stirring was continued at 0℃ for 2 h, and then the solid product was collected by centrifugation, washed with ethanol three times, freeze-dried and ground to obtain Ru / POMs.

[0041] Step 4, 60.0 mg of aluminum chloride hydrate was dissolved in 10 mL of deionized water, and stirred uniformly to obtain a Lewis acid solution.

[0042] Step 5, 2 g of Ru / POMs obtained in step 3 was dispersed in 10 mL of deionized water to form a suspension. Under the condition of stirring at 500 r / min, the Lewis acid solution of step 4 was slowly added to the Ru / POMs suspension, and the impregnation reaction was carried out at 0℃ for 2 h. After the reaction was completed, the solid was separated by centrifugation, washed with ethanol three times, freeze-dried and ground to obtain Ru-Al / POMs catalyst. The molar ratio of the metal content in the Lewis acid to the metal ruthenium was 2:1.

[0043] Example 2-4

[0044] According to the preparation process of example 1, the loading amount of the Lewis acid in step 4 was changed, and the molar ratio of the metal content in the Lewis acid to the metal ruthenium was controlled to be 1:1, 1:2 and 1:3, respectively.

[0045] Example 5

[0046] According to the preparation process of example 1, the aluminum chloride hydrate in step 4 was replaced by an equal amount of boron trifluoride complex to prepare Ru-B / POMs.

[0047] Comparative Example 1

[0048] According to the preparation process of example 1, steps 4 and 5 were not performed to prepare Ru / POMs.

[0049] Comparative Example 2

[0050] According to the preparation process of example 1, the addition of ruthenium chloride in step 2 was cancelled to prepare Al / POMs.

[0051] Comparative Examples 3-5

[0052] According to the preparation process of example 1, the aluminum chloride hydrate in step 4 was replaced by an equal amount of zinc chloride, iron chloride and copper chloride, respectively, to prepare Ru-Zn / POMs, Ru-Fe / POMs and Ru-Cu / POMs, respectively.

[0053] Product characterization:

[0054] Figure 1 XRD spectra of catalysts prepared in Example 2, Comparative Example 1 and Comparative Example 2 were shown. It can be seen that the tandem catalyst was successfully synthesized, and there were no diffraction peaks of transition metal ruthenium and Lewis acid nanoparticles, indicating that the particle size of transition metal ruthenium and Lewis acid nanoparticles was small and the dispersion was high.

[0055] Figure 2 Pyridine infrared spectra of catalysts prepared in Example 2, Comparative Example 1 and Comparative Example 2 were shown. There were no Lewis acid sites in Comparative Example 1, and it can be seen that the Lewis acid was successfully doped in Example 2 and Comparative Example 2.

[0056] Figure 3 TEM images of the tandem catalyst prepared in Example 2 and EDX mapping images of each element in the catalyst were shown. It can be seen that the metal ruthenium and the Lewis acid aluminum chloride were effectively loaded on the phosphotungstic heteropolyacid carrier.

[0057] Catalytic performance evaluation:

[0058] The catalysts prepared in the examples and comparative examples were respectively evaluated for the performance of co-conversion of methane and carbon dioxide to acetic acid, and the reaction was carried out in a high-pressure reactor.

[0059] Application Example 1

[0060] Step 1, 10 mg of catalysts prepared in the examples and comparative examples were respectively dispersed in 20 mL of deionized water, and the suspension was transferred to a high-pressure reactor and sealed.

[0061] Step 2, the reactor was purged with ultra-pure argon (volume fraction 99.999%) for 10 minutes to completely remove air. Then, methane and carbon dioxide (both with a volume fraction of 99.999%) were injected, so that the molar ratio of methane to carbon dioxide in the reaction system was 3:2, and the total pressure reached 3.0 MPa.

[0062] Step 3, the reactor was heated to 250°C and kept at this temperature for 2 hours. After the reaction was completed, the reactor was placed in an ice bath to cool.

[0063] Step 4, after the reactor was cooled to room temperature, the post-reaction liquid was centrifuged, and the supernatant was analyzed by liquid nuclear magnetic resonance. The product yield and acetic acid selectivity were calculated by external standard method, and the results are shown in Table 1.

[0064] Table 1 Performance evaluation of co-conversion of methane and carbon dioxide to acetic acid in examples and comparative examples

[0065]

[0066] From Table 1, it can be seen that the comparative example 2 without transition metal ruthenium cannot synthesize acetic acid, and when there is no cesium metal involved, due to the lack of precipitant, the heteropoly acid, ruthenium metal salt and Lewis acid cannot form a precipitate to obtain a catalyst product.

[0067] Compared with comparative example 1, the doping of Lewis acid significantly improves the selectivity and conversion frequency of acetic acid product. The catalytic performance is optimal when the molar ratio of metal to ruthenium metal in the Lewis acid is 1:1. The reason for the performance improvement of example 2 compared with example 1 is that by optimizing the synthesis conditions of the catalyst, the molar ratio of metal ruthenium to Lewis acid is formed, and metal ruthenium and Lewis acid promote the adsorption and activation of methane and Lewis acid, respectively. The synergistic effect between metal ruthenium and Lewis acid promotes C-C coupling to improve the selectivity of acetic acid product. The performance of example 3 and example 4 is general because the high content of Lewis acid reduces the activation of methane by ruthenium, and too much Lewis acid weakens the synergistic effect between ruthenium and Lewis acid, making C-C coupling difficult and thus reducing the selectivity of acetic acid.

[0068] From example 2, example 5 and comparative examples 3-5, it can be seen that different Lewis acids have different effects on the selectivity and conversion frequency of acetic acid product. The reason for the performance improvement of example 2 compared with example 5 is that the type and synthesis of Lewis acid are optimized. Among them, the doping of zinc chloride, iron chloride and copper chloride in comparative examples 3-5 has strong carrier-Lewis acid interaction, which reduces the adsorption and activation effect of metal in Lewis acid on CO2. The difference in acetic acid activity between example 2 and example 5 is that aluminum chloride has stronger electron-donating ability than boron trifluoride, and its activation degree of CO2 is higher, thus showing higher acetic acid generation activity and selectivity.

[0069] Figure 4 The performance stability test diagram of the tandem catalyst prepared for example 2 for catalyzing the co-conversion of methane and carbon dioxide to prepare acetic acid. From Figure 4 It can be seen that in 5 catalytic cycles, the selectivity of acetic acid product can be maintained at more than 93%, and the conversion frequency remains at a high level of 18.0 h -1 , indicating that the stability of the tandem catalyst is excellent.

Claims

1. A process for the preparation of a heteropoly acid supported bimetallic center catalyst, characterized by, The preparation method comprises the following steps: Step 1: dissolving and acidifying the heteropoly acid to obtain an anion solution; dissolving and acidifying the ruthenium salt and the cesium salt to obtain a cation solution; Step 2: mixing and reacting the anion solution and the cation solution, centrifuging, washing, drying and grinding the precipitate to obtain a heteropoly acid loaded single-atom metal catalyst; Step 3: dispersing the catalyst obtained in step 2, mixing and coupling the solution of the Lewis acid, and centrifuging, washing, drying and grinding the precipitate to obtain the heteropoly acid loaded bimetallic center catalyst; The Lewis acid comprises one or a combination of two of aluminum chloride, boron trifluoride ether complex.

2. The method for preparing the heteropolyacid-supported bimetallic center catalyst according to claim 1, characterized in that, The heteropoly acid comprises one or a combination of two or more of silicotungstic acid, phosphotungstic acid, silicomolybdic acid and phosphomolybdic acid; And / or, the ruthenium salt comprises one or a combination of two or more of a halide salt, a nitrate salt or other complex salt of ruthenium; And / or, the cesium salt comprises one or a combination of two or more of a halide salt, a nitrate salt or other complex salt of cesium.

3. The method for preparing the heteropolyacid-supported bimetallic center catalyst according to claim 1, characterized in that, The mass of the ruthenium salt is 0.1-2% of the mass of the heteropoly acid, the mass of the cesium salt is 0.13-2.6% of the mass of the heteropoly acid, and the total mass of the ruthenium salt and the cesium salt is 0.1-4% of the mass of the heteropoly acid.

4. The method for preparing the heteropolyacid-supported bimetallic center catalyst according to claim 1, characterized in that, The molar ratio of the Lewis acid to the metal in the ruthenium salt is 1:0.1-5.

5. The method for preparing the heteropolyacid-supported bimetallic center catalyst according to claim 1, characterized in that, The acidification is adding inorganic acid dropwise to the solution until the pH of the solution is 1-4; And / or, the temperature of the mixing reaction in step 2 is -20-0℃, and the reaction time is 1-10h.

6. The method for preparing the heteropolyacid-supported bimetallic center catalyst according to claim 1, characterized in that, The temperature of the coupling reaction in step 3 is -5-20℃, and the reaction time is 1-5h.

7. The heteropoly acid loaded bimetallic center catalyst prepared by the preparation method according to any one of claims 1-6.

8. The use of the catalyst according to claim 7 in the preparation of acetic acid from methane in a cascade reaction.

9. Use according to claim 8, characterized in that, The reaction process for preparing acetic acid comprises the following conditions: the reaction temperature is 150-400℃, the reaction time is 0.5-2h, the methane pressure is 0.50-3.00MPa, the carbon dioxide pressure is 0.50-3.00MPa, the total gas pressure is 1.00-5.00MPa, and the mass of the catalyst is 5-50mg.

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