Molecular sieve photothermocatalyst for methane oxidation to methanol and method for preparing the same

Abstract

The application belongs to the technical field of photo-thermal catalysts, and particularly relates to a molecular sieve photo-thermal catalyst for methane oxidation to prepare methanol and a preparation method thereof, the catalyst has a structural formula of Cu / M-ZSM-5, and the preparation method comprises the following steps: S1: uniformly mixing a silicon source, an aluminum source, a metal source, a template agent and water to form a starting gel; S2: performing hydrothermal crystallization, centrifugal separation, washing until neutral, and drying to obtain a molecular sieve raw powder; S3: performing calcination to obtain M-ZSM-5 molecular sieve; and S4: under ultrasonic wave conditions, ion exchanging M-ZSM-5 with a copper salt solution, and performing rotary evaporation and drying to obtain the Cu / M-ZSM-5 photo-thermal catalyst. The catalyst is filled into a fixed bed reactor, helium is introduced, the temperature is raised to an activation temperature, the catalyst is activated, and a mixed gas of methane, oxygen and water is introduced; when the doped metal is Zn, the methanol yield of the Cu / Zn-ZSM-5 photo-thermal catalytic methane is 100.05 micromoles g cat ‑1 h ‑1 , which is increased by 91% compared to the methanol yield of Cu / ZSM-5. The catalyst exhibits excellent catalytic performance and stability in the reaction of preparing methanol by photo-thermal catalysis of methane.

Description

Technical Field

[0001] The present invention belongs to the technical field of photothermal catalysts, and in particular relates to a molecular sieve photothermal catalyst for producing methanol through oxidation of methane and a preparation method thereof. Background Art

[0002] Metal molecular sieve catalysts, such as Cu, Fe, Ni, and Co-based molecular sieve catalysts, are widely used due to their excellent catalytic effects. Doping heteroatom metals into the molecular sieve framework can impart photocatalytic properties. For example, TS-1 molecular sieve has been widely used in photocatalytic reactions. Compared to traditional semiconductor photocatalysts, molecular sieve photocatalysts have the advantages of larger specific surface area and facilitated separation of photogenerated charge carriers.

[0003] Compared with traditional catalytic processes, photothermal catalysis, as a new catalytic process, effectively integrates the advantages of both photocatalysis and thermal catalysis. Its performance is much higher than the simple linear combination of photocatalysis and thermal catalysis, and it can effectively inhibit catalyst deactivation and product overoxidation.

[0004] Chinese patent CN115069285B discloses a preparation method and application of a photothermal catalyst (Cu-MOR / g-C3N4) for direct conversion of methane to methanol. This method loads the photocatalytic center g-C3N4 onto the Cu-MOR pores. Excessive g-C3N4 content in the pores can block the Cu-MOR pores, preventing the reactants methane, oxygen, and water from reaching the catalyst's thermally active sites, ultimately leading to a decrease in catalytic performance. Chinese patent CN113967476B discloses a high-efficiency perovskite-supported cobalt catalyst (Co-STO), its preparation method, and a method for the partial oxidation of methane to methanol. Co-STO and water are used as the catalyst and oxidant, respectively, for a one-step photothermal methane-to-methanol synthesis process. However, this reaction system has three disadvantages: 1. When activated at low temperatures, the activity of the Co-based catalyst is lower than that of the Cu-based catalyst; 2. The oxidizing power of water is much lower than that of oxygen; and 3. The solubility of methanol in water is low. Therefore, achieving high methanol yields in this reaction system requires high pressure and batch reactions, which is not conducive to industrial development and places high demands on equipment. Summary of the Invention

[0005] The purpose of the present invention is to provide a molecular sieve photothermal catalyst for methane oxidation to methanol and a preparation method thereof, so as to solve the problems of low activity, easy pore blocking, inability to produce continuously and high equipment requirements of the current methanol production catalyst.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The molecular sieve photothermal catalyst used for methane oxidation to methanol has a structural formula of Cu / M-ZSM-5 and uses M-ZSM-5 molecular sieve as a carrier. The Cu in the molecular sieve pores is the active center, and the M on the molecular sieve framework is the photocatalytic active site.

[0008] The M-ZSM-5 molecular sieve contains multi-level pores, including micropores and mesopores.

[0009] The preparation method of the molecular sieve photothermal catalyst comprises the following steps:

[0010] S1: Prepare the molecular sieve crystallization starting gel: uniformly mix aluminum isopropoxide, a template, and water, referred to as solution A; uniformly mix tetraethyl silicate and a metal source solution, referred to as solution B; slowly add solution B dropwise to solution A under stirring, fully hydrolyze for 1-8 hours, and then age the solution at 60°C to evaporate the ethanol produced during the hydrolysis process and replenish the water lost during the evaporation process;

[0011] S2: Hydrothermal crystallization: The molecular sieve crystallization starting gel obtained in step S1 is transferred to a stainless steel high-pressure hydrothermal reactor for hydrothermal crystallization. After completion, the product is centrifuged and washed with deionized water until neutral, and dried at 80-120°C for 12 hours to obtain molecular sieve raw powder;

[0012] S3: high temperature calcination: placing the molecular sieve powder obtained in step S2 in a muffle furnace and calcining it at a constant temperature in an air atmosphere to obtain M-ZSM-5;

[0013] S4: Ion exchange: Place the M-ZSM-5 molecular sieve obtained in step S3 in a copper salt solution, stir in a 30-50 kHz ultrasonic cleaner for 24 h, and adjust the pH to 5-6 with ammonia water. After completion, centrifuge and wash with deionized water three times. Dissolve the product and dry it at 55 °C by rotary evaporation for 1 h. Then place it in a muffle furnace and calcine it at a constant temperature in an air atmosphere to obtain Cu / M-ZSM-5.

[0014] Furthermore, in step S1, the molar ratio range of the reaction materials is Si:Al:M:TPAOH:H2O=1:(0.008-0.024):(0.008-0.024):0.18:18.

[0015] Furthermore, the metal source in step S1 is any one of anhydrous zinc acetate, tetrabutyl titanate, vanadyl sulfate and cobalt chloride.

[0016] Furthermore, in step S2, the hydrothermal crystallization temperature is 150-200° C., and the hydrothermal crystallization time is 48-120 h.

[0017] Furthermore, the copper salt solution in step S4 is any one of copper acetate, copper sulfate, copper nitrate, and copper chloride, the concentration of the copper salt solution is 0.001 M to 0.1 M, and the ratio of M-ZSM-5 to the copper salt solution is 1 g:60 mL.

[0018] Furthermore, in steps S3 and S4, the calcination temperature is 550-600° C., and the heating rate is 1-10° C. / min.

[0019] The application method of the above-mentioned molecular sieve photothermal catalyst is as follows: the catalyst is loaded into a fixed bed reactor, helium is introduced for programmed temperature increase, the temperature is raised to 500°C and the light source is turned on to activate the catalyst, then the reaction temperature is adjusted to 200-500°C, and finally the reaction gas is introduced, wherein the reaction gas is a mixture of methane, oxygen and water.

[0020] Furthermore, the gas flow ratio of methane, oxygen and water in the reaction gas is 19-27:1-6:6-12.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) The Cu / M-ZSM-5 molecular sieve photothermal catalyst of the present invention is applied for the first time to the direct photothermal oxidation of methane to methanol and shows excellent catalytic activity. The Cu in the pores is the active center, and the M on the molecular sieve framework is the photocatalytic active site. Under light conditions, M can generate active free radicals, increase the generation rate of active Cu species and reduce the reaction energy barrier, thereby improving the catalytic activity.

[0023] (2) The preparation method of the present invention introduces metal M into the molecular sieve framework through an in-situ synthesis method, effectively avoiding pore blockage; ultrasonic ion exchange improves the Cu ion exchange rate and increases the catalyst yield; compared with oven drying, rotary evaporation drying makes the obtained sample more delicate and uniform. This invention provides a new idea for molecular sieve catalysts for methane oxidation to methanol;

[0024] (3) The molecular sieve photothermal catalyst of the present invention is used for the production of methanol under simple reaction conditions, can be completed at low temperature, has low requirements on equipment, and can increase the methanol yield, which is conducive to industrial development. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The XRD patterns of the catalysts of Example 1, Example 2, Example 3, Example 4 and Comparative Example 1 are shown;

[0026] Figure 2 The infrared spectra of the catalysts of Example 1, Example 2, Example 3, Example 4 and Comparative Example 1 are shown;

[0027] Figure 32 are N2 adsorption / desorption isotherms of the catalysts of Example 1, Example 2, Example 3, Example 4 and Comparative Example 1;

[0028] Figure 4 Graph showing the methanol yields from the photothermal catalysis of methane at 300°C for the catalysts of Example 1, Example 2, Example 3, Example 4 and Comparative Example 1. DETAILED DESCRIPTION

[0029] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments. Example 1

[0030] The preparation process of Cu / Zn-ZSM-5 catalyst is as follows:

[0031] S1: Preparation of molecular sieve crystallization starting gel: Aluminum isopropoxide, template, and water were mixed and stirred for 30 min, recorded as solution A; tetraethyl silicate and zinc acetate solution were mixed and stirred for 30 min, recorded as solution B; solution B was slowly added dropwise to solution A under stirring and hydrolyzed for 120 min; the solution was aged at 60°C for 3 h, and the ethanol produced during the hydrolysis process was evaporated to replenish the water lost during the evaporation process; the molar ratio of the reaction materials was Si:Al:Zn:TPAOH:H2O=1:0.016:0.016:0.18:18;

[0032] S2: Hydrothermal crystallization: The molecular sieve crystallization starting gel obtained in step S1 was transferred to a stainless steel high-pressure hydrothermal reactor and crystallized at 170°C for 48 hours. After crystallization, the gel was centrifuged and washed with deionized water until neutral, and dried at 110°C for 12 hours to obtain molecular sieve raw powder.

[0033] S3: High-temperature calcination: The molecular sieve powder obtained in step S2 is placed in a muffle furnace and calcined at 550°C for 5 h at a heating rate of 2°C / min in an air atmosphere to obtain Zn-ZSM-5;

[0034] S4: Ion exchange: The Zn-ZSM-5 molecular sieve obtained in step S3 was placed in a 0.01 M copper acetate solution, stirred in a 40 kHz ultrasonic cleaner for 24 h, adjusted to pH 5.5 with ammonia water, then centrifuged and washed three times with deionized water. The product was dissolved and dried by rotary evaporation at 55 °C for 1 h, then placed in a muffle furnace and calcined at 550 °C for 5 h in an air atmosphere at a heating rate of 2 °C / min to obtain Cu / Zn-ZSM-5. Example 2

[0035] The preparation process of Cu / Ti-ZSM-5 catalyst is as follows:

[0036] Except that the metal source solution in step S1 is tetrabutyl titanate, the rest is the same as in Example 1. Example 3

[0037] The preparation process of Cu / V-ZSM-5 catalyst is as follows:

[0038] Except that the metal source solution in step S1 is vanadyl sulfate, the rest is the same as in Example 1. Example 4

[0039] The preparation process of Cu / Co-ZSM-5 catalyst is as follows:

[0040] Except that the metal source solution in step S1 is cobalt chloride, the rest is the same as in Example 1.

[0041] Comparative Example 1

[0042] The preparation process of Cu / ZSM-5 catalyst is as follows:

[0043] S1: Preparation of molecular sieve crystallization starting gel: Aluminum isopropoxide, template, and water were mixed and stirred for 30 min. Tetraethyl silicate was slowly added dropwise to the mixed solution and hydrolyzed for 120 min. The solution was aged at 60°C for 3 h to evaporate the ethanol produced during the hydrolysis process and replenish the water lost during the evaporation process. The molar ratio of the reaction materials was Si:Al:TPAOH:H2O = 1:0.016:0.18:18.

[0044] S2: Hydrothermal crystallization: The molecular sieve crystallization starting gel obtained in step S1 was transferred to a stainless steel high-pressure hydrothermal reactor and crystallized at 170°C for 48 hours. After crystallization, the gel was centrifuged and washed with deionized water until neutral, and dried at 110°C for 12 hours to obtain molecular sieve raw powder.

[0045] S3: High-temperature calcination: The molecular sieve powder obtained in step S2 is placed in a muffle furnace and calcined at 550°C for 5 h at a heating rate of 2°C / min in an air atmosphere to obtain ZSM-5;

[0046] S4: Ion exchange: The ZSM-5 molecular sieve obtained in step S3 was placed in a 0.01 M copper acetate solution, stirred in a 40 kHz ultrasonic cleaner for 24 h, and adjusted to pH 5.5 with ammonia water. After completion, the solution was centrifuged and washed three times with deionized water. The product was dissolved and dried at 55 °C by rotary evaporation for 1 h. The solution was then placed in a muffle furnace and calcined at 550 °C for 5 h at a heating rate of 2 °C / min in an air atmosphere to obtain Cu / ZSM-5.

[0047] The catalyst prepared by the present invention was structurally characterized:

[0048] Figure 1 The XRD patterns of the molecular sieve photothermal catalysts prepared in Example 1, Example 2, Example 3, Example 4 and Comparative Example 1 are shown. Figure 1 It can be seen that the catalysts prepared in the four examples and one comparative example all exhibit characteristic diffraction peaks of the MFI topological structure, indicating that metal doping and Cu loading do not destroy the molecular sieve structure.

[0049] Figure 2 The infrared spectra of the molecular sieve photothermal catalysts prepared in Example 1, Example 2, Example 3, Example 4 and Comparative Example 1 are shown. Figure 2 It can be seen that the catalysts prepared in the four examples have a higher -1 A characteristic absorption peak appears, which is due to the stretching vibration of the Si-OM bond of the molecular sieve or the vibration of the Si-O bond caused by the framework M, indicating that the metal is doped into the molecular sieve framework.

[0050] Figure 3 The N2 physical adsorption and desorption isotherms of the molecular sieve photothermal catalysts prepared in Example 1, Example 2, Example 3, Example 4 and Comparative Example 1 are shown. Figure 3 It can be seen that the catalyst prepared in the comparative example exhibits a typical Type I curve, indicating that it is a microporous molecular sieve; the catalysts prepared in the four examples exhibit the characteristics of both Type I and Type IV curves, indicating that they have a microporous-mesoporous composite structure and are multi-level pore molecular sieves.

[0051] The performance of the catalyst prepared by the present invention in photothermal catalytic oxidation of methane to methanol was tested.

[0052] The molecular sieve photothermal catalysts prepared in Examples 1-4 and Comparative Example 1, 100 mg in 40-60 mesh, were respectively taken and placed in a catalyst activity evaluation device. The activity evaluation was carried out in a modified fixed-bed reactor.

[0053] Using He as the protective atmosphere, the reactor was heated from room temperature to the activation temperature of 500°C by programmed temperature increase, and O2 was introduced and light was continuously irradiated to activate the photothermal catalyst for 2 h. The reaction temperature was adjusted to 300°C, and the reaction gases methane, oxygen, and water were introduced at normal pressure with a CH4:O2:H2O gas flow ratio of 24:3:8. The liquid and gas products were collected every 1 h and analyzed using a gas chromatograph (Agilent 7890B) equipped with a methane reformer, a hydrogen flame ionization detector (FID), and a thermal conductivity detector (TCD).

[0054] Figure 4The methanol yields of the catalysts prepared in Example 1, Example 2, Example 3, Example 4, and Comparative Example 1 for the methane oxidation to methanol reaction are shown in Table 1. As can be seen from the figure, at a reaction temperature of 300°C, the yields of methane oxidation to methanol over Cu / Zn-ZSM-5, Cu / Ti-ZSM-5, Cu / V-ZSM-5, and Cu / Co-ZSM-5 were 100.05, 67.41, 65.38, and 60.35 μmol g, respectively. cat -1 h -1 The yield of methanol produced by oxidation of methane over Cu / ZSM-5 catalyst was 52.44 μmol g cat -1 h -1 , the catalytic performance is significantly improved compared with Cu / M-ZSM-5; therefore, the doping of elements Zn, Ti, V and Co can make the catalyst have photothermal catalytic ability and improve the activity of Cu / M-ZSM-5 catalyst.

[0055] The above embodiments are only preferred embodiments of the present invention. All equivalent changes and modifications made within the scope of the patent application of the present invention should fall within the protection scope of the present invention.

Claims

1. Application of a molecular sieve photothermal catalyst in the production of methanol by oxidation of methane, characterized in that: The molecular sieve photothermal catalyst has a structural formula of Cu / M-ZSM-5, with M-ZSM-5 molecular sieve as a carrier. The Cu in the molecular sieve pores is the active center, and the M on the molecular sieve framework is the photocatalytic active site. M is any one of zinc, titanium, vanadium and cobalt. The molecular sieve photothermal catalyst is loaded into a fixed bed reactor, and helium is introduced for programmed temperature increase. The temperature is raised to 500°C and the light source is turned on to activate the catalyst. The reaction temperature is then adjusted to 200-500°C, and finally a mixed gas of methane, oxygen and water is introduced.

2. The use of a molecular sieve photothermal catalyst in methane oxidation to methanol according to claim 1, characterized in that: The M-ZSM-5 molecular sieve contains multi-level pores, including micropores and mesopores.

3. The use of a molecular sieve photothermal catalyst in methane oxidation to methanol according to claim 1, characterized in that: The preparation method of the molecular sieve photothermal catalyst comprises the following steps: S1: Prepare the molecular sieve crystallization starting gel: uniformly mix aluminum isopropoxide, a template, and water, denoted as solution A, where the template is TPAOH; uniformly mix tetraethyl silicate with a metal source solution, denoted as solution B, where the metal source is any one of anhydrous zinc acetate, tetrabutyl titanate, vanadyl sulfate, and cobalt chloride; slowly add solution B dropwise to solution A under stirring, fully hydrolyze for 1-8 h, and then age the solution at 60°C to evaporate the ethanol produced during the hydrolysis process and replenish the water lost during the evaporation process; S2: Hydrothermal crystallization: The molecular sieve crystallization starting gel obtained in step S1 is transferred to a stainless steel high-pressure hydrothermal reactor for hydrothermal crystallization. After completion, the product is centrifuged and washed with deionized water until neutral, and dried at 80-120°C for 12 h to obtain molecular sieve raw powder; S3: high temperature calcination: placing the molecular sieve powder obtained in step S2 in a muffle furnace and calcining it at a constant temperature in an air atmosphere to obtain M-ZSM-5; S4: Ion exchange: The M-ZSM-5 molecular sieve obtained in step S3 is placed in a copper salt solution, stirred in a 30-50 kHz ultrasonic cleaner for 24 h, and the pH is adjusted to 5-6 with ammonia water. After completion, the solution is centrifuged and washed with deionized water three times. The product is dissolved and dried at 55 °C by rotary evaporation for 1 h, then placed in a muffle furnace and calcined at a constant temperature in an air atmosphere to obtain Cu / M-ZSM-5.

4. The use of a molecular sieve photothermal catalyst in methane oxidation to methanol according to claim 3, characterized in that: The molar ratio of the reaction materials in step S1 is Si:Al:M:TPAOH:H2O=1:(0.008-0.024):(0.008-0.024):0.18:

18.

5. The use of a molecular sieve photothermal catalyst in methane oxidation to methanol according to claim 3, characterized in that: In step S2, the hydrothermal crystallization temperature is 150-200° C., and the hydrothermal crystallization time is 48-120 h.

6. The use of a molecular sieve photothermal catalyst in methane oxidation to methanol according to claim 3, characterized in that: The copper salt solution in step S4 is any one of copper acetate, copper sulfate, copper nitrate, and copper chloride, the concentration of the copper salt solution is 0.001 M to 0.1 M, and the ratio of M-ZSM-5 to the copper salt solution is 1 g:60 mL.

7. The use of a molecular sieve photothermal catalyst in methane oxidation to methanol according to claim 3, characterized in that: In the steps S3 and S4, the calcination temperature is 550-600° C., and the heating rate is 1-10° C. / min.

8. The use of a molecular sieve photothermal catalyst in methane oxidation to methanol according to claim 1, characterized in that: The gas flow ratio of methane, oxygen and water is 19-27:1-6:6-12.

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