Metallic monatomic molecular sieve composite catalyst, preparation method and application thereof
By loading Fe-NC species onto ZSM-23 molecular sieves to create a composite catalyst, the problem of preparing C1 liquid products from methane oxidation at low temperatures was solved, achieving high efficiency and low cost catalytic effect, and generating products such as CH3OH, HCOOH and HOCH2OOH.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CHINA PETROLEUM & CHEMICAL CORP
- Filing Date
- 2024-12-13
- Publication Date
- 2026-06-16
AI Technical Summary
Existing technologies struggle to efficiently catalyze the direct oxidation of methane to C1 liquid products at low temperatures, especially because oxygen molecules are difficult to activate the CH bonds in methane under mild conditions, resulting in low methane conversion efficiency and a tendency for the target product to be over-converted.
A composite catalyst of metal single-atom molecular sieves, specifically Fe-NC species supported on ZSM-23 molecular sieves, was used to achieve the low-temperature oxidation of methane to prepare C1 liquid products by controlling the distribution of acidic sites and pore structure of the catalyst through the preparation method.
This catalyst achieves highly efficient catalytic activation of methane below 80°C, generating C1 liquid products such as CH3OH, HCOOH, and HOCH2OOH. It is inexpensive, highly active, selective, and stable, and uses low-concentration hydrogen peroxide as an oxidant, ensuring high safety.
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Figure CN122209451A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the technical field of direct methane conversion and utilization, specifically relating to a metal single-atom molecular sieve composite catalyst and its preparation method, as well as its application in the low-temperature direct oxidation of methane to produce C1 liquid products. Background Technology
[0002] Since the beginning of the 21st century, with China's sustained and rapid economic growth, accelerated urbanization and industrialization, and the increasing environmental protection requirements impacting energy consumption structures, China's natural gas consumption and production have grown rapidly, with growth rates far exceeding the world average. Simultaneously, the large-scale discovery of combustible ice in the ocean in recent years, along with advancements in shale gas, oilfield gas, and coalbed methane extraction technologies, has expanded the utilization of abundant and inexpensive natural gas beyond its primary use as fuel. Combustible ice, as a strategic high ground for future global energy development, possesses characteristics such as high calorific value, large reserves, and low pollution. International research on combustible ice has been consistently active. Both natural gas and combustible ice are primarily composed of methane, whose unique structure makes it highly stable compared to other organic molecules, requiring stringent reaction conditions to activate methane molecules. Therefore, researching how to selectively activate and directionally convert methane is one of the major scientific challenges currently facing the world, and an urgent need to effectively alleviate the energy crisis and achieve sustainable development.
[0003] Currently, among the various pathways for methane conversion and utilization, directly preparing C1 liquid products (methanol, formic acid, etc.) from methane in a single step not only has significant advantages in terms of technology and economy, but also, since the C1 main product is liquid at room temperature and pressure, storage and transportation are relatively convenient, making it considered an ideal method for methane conversion. However, the direct catalytic conversion of methane to high-value-added chemicals has always been a global challenge. This is mainly due to methane's low polarizability and high CH bond energy (439 kJ / mol), which typically requires harsh reaction conditions such as high temperatures (above 600°C), strong oxidants (e.g., fuming sulfuric acid), or external fields (e.g., plasma). However, these conditions can easily lead to over-conversion of the target product (e.g., the generation of CO2). The direct, directed conversion of methane using inexpensive and readily available oxygen at low temperatures has been a major research focus. For example, CN111195514A discloses a single-atom-dispersed rhodium-based catalyst for the low-temperature oxidation of methane, in which CH4, CO, and O2 can be converted into acetic acid, formic acid, and methanol with high activity and selectivity. CN112892588A discloses a method for preparing and applying an atomically monodisperse transition metal catalyst for the low-temperature catalytic oxidation of methane to acetic acid. However, oxygen molecules are extremely difficult to continuously form reactive oxygen species that can activate the CH bonds of methane under mild conditions, making the direct catalytic conversion of methane and oxygen at room temperature highly challenging. Summary of the Invention
[0004] To address the above-mentioned technical problems, the present invention aims to provide a metal single-atom molecular sieve composite catalyst and its preparation method. The metal single-atom molecular sieve composite catalyst provided by the present invention is applied to the low-temperature oxidation of methane to C1 liquid products. It can achieve the direct oxidation of methane to C1 liquid products at low temperatures below 80°C, and exhibits outstanding catalytic activity, selectivity, and good stability.
[0005] To achieve the above objectives, the first aspect of the present invention provides a metal single-atom molecular sieve composite catalyst, wherein the metal single-atom molecular sieve composite catalyst comprises a molecular sieve and an active component;
[0006] The molecular sieve is ZSM-23 molecular sieve; the active component is Fe-NC species.
[0007] Furthermore, the Fe metal in the Fe-NC species exists in single-atom form.
[0008] Furthermore, the Fe metal exists in single-atom form by being dispersed on the ZSM-23 molecular sieve as isolated single-atom sites.
[0009] Furthermore, in the catalyst, the iron content is 1wt%-1.5wt% based on the mass of the catalyst.
[0010] Furthermore, in the Fe-NC species, the molar ratio of Fe, N and C is 1:(3-5):(10-12).
[0011] Furthermore, the total Brønsted acid content of pyridine in the ZSM-23 molecular sieve is 0.10–0.37 mmol / g, preferably 0.13–0.35 mmol / g, more preferably 0.15–0.33 mmol / g; and the total Brønsted acid content of 2,6-dimethylpyridine in the infrared is 0.07–0.35 mmol / g, preferably 0.09–0.33 mmol / g, more preferably 0.12–0.30 mmol / g.
[0012] Furthermore, the ratio of the total 2,6-dimethylpyridine infrared Brønsted acid content to the total pyridine infrared Brønsted acid content of the ZSM-23 molecular sieve is (65-99):100, preferably (70-97):100, and even more preferably (74-95):100.
[0013] Furthermore, in the 2,6-dimethylpyridine infrared acid of the ZSM-23 molecular sieve, the amount of weak Brønsted acid with a desorption temperature <250℃ is 0.09–0.31 mmol / g, preferably 0.11–0.29 mmol / g, and more preferably 0.13–0.28 mmol / g.
[0014] Furthermore, in the 2,6-dimethylpyridine infrared acid of the ZSM-23 molecular sieve, the ratio of the amount of weak Brønsted acid with a desorption temperature <250℃ to the total amount of Brønsted acid in the 2,6-dimethylpyridine infrared sieve is (64-95):100, preferably (67-93):100, and even more preferably (70-91):100.
[0015] Furthermore, the bulk SiO2 / Al2O3 (molar ratio) of the ZSM-23 molecular sieve is 40-400 higher than the outer surface SiO2 / Al2O3 (molar ratio), preferably 45-350 higher; the bulk SiO2 / Al2O3 (molar ratio) of the ZSM-23 molecular sieve is 80-500, and the outer surface SiO2 / Al2O3 (molar ratio) is 40-120; preferably, the bulk SiO2 / Al2O3 (molar ratio) is 90-400, and the outer surface SiO2 / Al2O3 (molar ratio) is 45-90.
[0016] The second aspect of the present invention provides a method for preparing the above-mentioned metal single-atom molecular sieve composite catalyst, the method comprising: mixing an iron source, a complex, and an organic solvent, then adding ZSM-23 molecular sieve for heating and reflux reaction, drying, and calcining to obtain the metal single-atom molecular sieve composite catalyst.
[0017] Furthermore, the preparation method of the ZSM-23 molecular sieve includes the following steps:
[0018] (1) Mix silicon source, template agent a and water, mix evenly and then carry out crystallization reaction. After the reaction, the material is separated into solid and liquid to obtain solid material.
[0019] (2) Mix the solid material obtained in step (1) with the fatty amine evenly;
[0020] (3) Add an alkaline source solution to the material obtained in step (2) and perform a first low-temperature treatment; then add ZSM-23 seed crystals and perform a second low-temperature treatment;
[0021] (4) Prepare an aqueous solution containing template agent b and aluminum source, and then mix it with the material obtained in step (3) to obtain a gel. After crystallization, filtration, washing, drying and calcination, ZSM-23 molecular sieve is obtained.
[0022] In step (1), the silicon source is one or more of silica, silica sol, water glass, fumed silica and tetraethyl orthosilicate, preferably silica and / or fumed silica.
[0023] In step (1), the template agent a is at least one of hexamethylenediamine, ethanol and n-hexamethylenediamine, preferably hexamethylenediamine and / or ethanol.
[0024] In step (1), the molar ratio of the template agent a to the silicon source (calculated as SiO2) is 0.1 to 1.0, preferably 0.15 to 0.8; the molar ratio of water to the silicon source (calculated as SiO2) is 20 to 80, preferably 30 to 70.
[0025] In step (1), the crystallization reaction is carried out in a reaction vessel with a polytetrafluoroethylene liner, and the operating conditions are as follows: the crystallization temperature is 120-220℃, the crystallization time is 8-48h, preferably the crystallization temperature is 140-200℃, and the crystallization time is 12-30h.
[0026] In step (1), the solid-liquid separation can be achieved by any of the existing technologies in the art, such as gravity sedimentation, filtration separation and centrifugal separation, preferably centrifugal separation.
[0027] In step (2), the fatty amine is one or more of oleylamine (9-octadeceneamine), octadecamine, and dodecylamine, preferably oleylamine.
[0028] In step (2), the liquid-to-solid ratio of the fatty amine to the solid material obtained in step (1) is 0.3 to 3 mL / g, preferably 0.5 to 2.0 mL / g.
[0029] In step (3), the alkaline source solution is any one of sodium hydroxide, potassium hydroxide and ammonia water, and the concentration of the alkaline source solution is 0.003 to 0.015 mol / L, preferably 0.005 to 0.010 mol / L; the liquid-solid ratio of the alkaline source solution to the solid material obtained in step (1) is 2 to 20 mL / g, preferably 5 to 15 mL / g.
[0030] In step (3), the temperature of the first low-temperature treatment is 20-40°C and the time is 3-15h; preferably, the temperature is 25-30°C and the time is 6-12h; the first low-temperature treatment is preferably carried out under stirring conditions and the stirring rate is 100-300rpm.
[0031] In step (3), the ZSM-23 seed crystals are added in the form of seed crystal solution. The specific operation process of the seed crystal solution is as follows: the ZSM-23 seed crystals are uniformly dispersed in water (preferably deionized water) to form seed crystal solution; wherein, the liquid-solid ratio of water to ZSM-23 seed crystals is 5-70 mL / g, preferably 10-60 mL / g.
[0032] In step (3), based on the weight of the solid material obtained in step (1), the amount of ZSM-23 seed crystals added is 0.1 to 8.0 wt%, preferably 0.5 to 5.0 wt%.
[0033] In step (3), the secondary low-temperature treatment is performed at a temperature of 60-120°C for 6-30 hours; preferably, the temperature is 80-100°C for 12-24 hours; the secondary low-temperature treatment is preferably performed under stirring conditions at a stirring rate of 100-300 rpm.
[0034] In step (4), the template agent b is one or more of pyrrolidine, isopropylamine and N,N-dimethylformamide, preferably pyrrolidine; the aluminum source is one or more of aluminum sulfate, aluminum isopropoxide, sodium aluminate and aluminum hydroxide, preferably aluminum sulfate.
[0035] In step (4), the molar ratio of template agent b to the solid material (calculated as SiO2) obtained in step (1) is 0.01 to 0.1, preferably 0.02 to 0.08; the molar ratio of aluminum source (calculated as Al2O3) to the solid material (calculated as SiO2) obtained in step (1) is 0.002 to 0.015, preferably 0.005 to 0.01; and the molar ratio of water to aluminum source (calculated as Al2O3) in the aqueous solution containing template agent b and aluminum source is 300 to 3000, preferably 400 to 2000.
[0036] In step (4), the crystallization temperature is 180-220℃ and the crystallization time is 24-72h; the drying temperature is 80-120℃ and the drying time is 6-12h; the calcination temperature is 540-560℃ and the calcination time is 3-8h.
[0037] In the preparation method of the composite catalyst, the iron source is selected from at least one of ferric acetate and ferric acetylacetone.
[0038] In the preparation method of the composite catalyst, the complex is selected from 1,10-phenanthroline.
[0039] In the preparation method of the composite catalyst, the organic solvent is selected from at least one of ethanol and acetone.
[0040] In the preparation method of the composite catalyst, the mass ratio of iron source, complex, organic solvent and ZSM-23 molecular sieve is 1:(3-4):(350-450):(25-30).
[0041] In the preparation method of the composite catalyst, after adding ZSM-23 molecular sieve, it is preferable to first subject it to ultrasonic treatment, and then carry out a heating reflux reaction. The ultrasonic frequency is 20-30 kHz, and the treatment time is 10-50 min.
[0042] In the preparation method of the composite catalyst, the temperature of the heating and reflux reaction is 40-90℃, the reflux time is 10-12h, and the rotation speed during the reflux process is 400-800rpm.
[0043] In the preparation method of the composite catalyst, the drying is preferably carried out by rotary evaporation at a temperature of 50-60°C for 1-2 hours.
[0044] In the preparation method of the composite catalyst, the calcination is carried out under an inert atmosphere, which includes at least one of argon and helium. The calcination temperature is 400-800℃ and the calcination time is 2-4h.
[0045] The third aspect of this invention provides the application of the above-mentioned metal single-atom molecular sieve composite catalyst in the low-temperature oxidation of methane to C1 liquid products.
[0046] Furthermore, the low-temperature direct oxidation reaction of methane uses pure methane as the reaction gas and low-concentration hydrogen peroxide as the oxidant, wherein the concentration of hydrogen peroxide in the hydrogen peroxide is 0.5–1 mol / L.
[0047] Furthermore, the volume ratio of methane to hydrogen peroxide in the reaction is 1:(1-1.5).
[0048] Furthermore, the reaction is carried out using a high-pressure autoclave reactor, with a reaction temperature of 30–80°C, preferably 50–80°C, a reaction time of 15–60 min, a reaction pressure of 1–3 MPa, and a catalyst dosage of 2.5–12.5 g per liter of hydrogen peroxide.
[0049] Furthermore, the C1 liquid products include CH3OH, HCOOH, CH3OOH, and HOCH2OOH.
[0050] The present invention has the following advantages:
[0051] (1) The composite catalyst provided by the present invention can directly obtain C1 liquid products (including CH3OH, HCOOH, CH3OOH and HOCH2OOH) by catalyzing the methane oxidation reaction under low temperature conditions. The reaction temperature can be as low as below 80℃, which is different from the reaction temperature of more than 150℃ required in other methods, and can greatly reduce energy consumption.
[0052] (2) The composite catalyst provided by the present invention uses non-precious metal iron as the active component, and the catalyst cost is low.
[0053] (3) The ZSM-23 molecular sieve provided by the present invention has a surface and pores rich in Brønsted acid sites, with weak Brønsted acid sites being the main component. It is particularly suitable as a catalyst support component for the low-temperature oxidation of methane to C1 liquid products. It exhibits excellent activity, selectivity and stability in the low-temperature oxidation of methane to C1 liquid products.
[0054] (4) In the preparation method of ZSM-23 molecular sieve provided by the present invention, firstly, an inexpensive template agent a is selected to assist the aging of silicon source to generate hydroxyl-rich molecular sieve primary structural unit silicon-oxygen tetrahedron and secondary structural unit. Then, hydrophobic long-chain aliphatic amine molecules are combined with the hydroxyl groups on the surface of the structural unit through hydrogen bonds and arranged in an orderly and uniformly dispersed manner. Afterwards, under the action of alkaline solution, they are dissociated into uniform structural fragments with specific channels. Then, with the assistance of ZSM-23 seed crystals, they are rapidly assembled into metastable ZSM-23 nanocrystals of pure silicon. Then, Al species and template agent b are combined to generate chelates, which are combined with hydroxyl groups at specific sites on the surface of metastable ZSM-23 nanocrystals. During the high-temperature crystallization process, they are embedded in the framework structure and grown into ZSM-23 molecular sieve with stable structure and surface rich in weak Brønsted acid sites. The template agent used in this method is inexpensive and requires only a small amount, enabling precise control of the Al sites of ZSM-23 molecular sieves. This facilitates the enrichment of weak B acid sites at the pore openings and on the surface, significantly improving the reactivity of ZSM-23 molecular sieves, which are primarily based on pore opening adsorption.
[0055] (5) When the composite catalyst provided by the present invention is used to catalyze the methane oxidation reaction, the reaction system is simple (including methane, hydrogen peroxide solution and catalyst), and a low concentration of hydrogen peroxide solution is used as the oxidant, which is safer than using oxygen or air directly.
[0056] (6) The composite catalyst provided by this invention exhibits a high reaction rate in its catalytic process, wherein the C1 generation turnover frequency (TOF) can be greater than 16 mmol. C1 / g cat / h -1 Furthermore, the catalyst exhibits good stability, and the C1 yield remains essentially unchanged after multiple cycles of use. Attached Figure Description
[0057] Figure 1 The XRD pattern of the ZSM-23 molecular sieve prepared in Example 1;
[0058] Figure 2 STEM image of the composite catalyst prepared in Example 6;
[0059] Figure 3 Stability testing of the composite catalyst prepared in Example 6;
[0060] Figure 4 STEM image of the composite catalyst prepared in Comparative Example 1. Detailed Implementation
[0061] The catalyst of the present invention, its preparation method, and its effects are further illustrated below through examples. These examples are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures; however, the scope of protection of the present invention is not limited to the following examples.
[0062] Unless otherwise specified, the experimental methods used in the following examples are conventional methods in the art. Unless otherwise specified, the experimental materials used in the following examples were purchased from conventional biochemical reagent stores.
[0063] In this invention, the pyridine infrared acid content was determined by pyridine adsorption infrared spectroscopy; the 2,6-dimethylpyridine infrared acid content was determined by 2,6-dimethylpyridine adsorption infrared spectroscopy. The acid content was calculated according to Lambert-Beer's law, using a 1540 cm⁻¹ spectroscopy method. -1 The acidity of Brønsted acid is calculated from the area of the absorption peak.
[0064] In this invention, the acid content of different intensities at the pore openings and outer surface of molecular sieves is determined by 2,6-dimethylpyridine adsorption infrared spectroscopy. The specific process is as follows: the molecular sieve sample is prepared into a self-supporting wafer (5-6 mg / cm²). 2 The sample was placed in an in-situ cell and treated under vacuum at 400℃ for 4 hours, then cooled to 50℃, and the spectrum was collected. After adsorbing 2,6-dimethylpyridine for 10 minutes, the sample was heated to 150℃ for desorption for 1 hour, cooled to room temperature, and the spectrum was collected. At this time, the total amount of 2,6-dimethylpyridine in the infrared spectrum could be calculated. After desorption at 250℃ for 1 hour, the sample was cooled to room temperature, and the spectrum was collected. The amount of weak acid with a desorption temperature of 2,6-dimethylpyridine <250℃ was calculated.
[0065] In this invention, the bulk SiO2 / Al2O3 (molar ratio) was obtained by X-ray fluorescence spectroscopy (XRF) analysis using a ZSX100e X-ray fluorescence spectrometer with Kα spectral line, LiF1 crystal, Rh target material, SC scintillation detector, timing of 20s, and vacuum atmosphere.
[0066] In this invention, the SiO2 / Al2O3 molar ratio on the outer surface was measured by X-ray photoelectron spectroscopy (XPS). The elemental composition and state of the catalyst surface were determined using a Thermofisher Multilab2000 electron spectrometer, with Mg Kα as the excitation source and a cathode voltage and current of 13 kV and 20 mA, respectively. The electron binding energy was calibrated using C1s (284.6 eV).
[0067] In this invention, the analysis of metallic iron species was performed using an aberration-corrected scanning transmission electron microscope (AC-STEM) with a JEOL JEM ARM200F electron microscope. A trace amount of powder sample was ultrasonically dispersed in anhydrous ethanol, and a few drops of suspension were added to the microgrid supporting the carbon film. After the sample dried, its microstructure was observed under a transmission electron microscope.
[0068] Example 1: Preparation of ZSM-23 molecular sieve
[0069] 4.68 g of hexamethylenediamine was dissolved in 54 g of deionized water, and then 6 g of fumed silica was added. After stirring at room temperature (25°C) for 1 h, the mixture was transferred to a 100 mL reactor lined with polytetrafluoroethylene (PTFE). The mixture was crystallized at 150°C for 24 h, then quenched and centrifuged. 10 mL of oleylamine was added to the resulting solid, and the mixture was thoroughly mixed in a shaker. The mixture was stirred at room temperature (25°C) for 0.5 h at a stirring rate of 200 rpm. 42 mL of 0.008 mol / L NaOH solution was added to the mixture, and the mixture was stirred at 25°C for 6 h at a stirring rate of 200 rpm to obtain mixture A. 0.18 g of ZSM-23 seed crystals were added to deionized water, and 10 mL of ZSM-23 seed solution was added to mixture A. The mixture was stirred at 80°C for 16 h at a stirring rate of 300 rpm to obtain mixture B.
[0070] Add 0.67g Al2(SO4)3·18H2O and 0.40g pyrrolidine to 20mL of deionized water to obtain a clear solution C;
[0071] Solution C was added to mixture B and mixed thoroughly. The final mixture was then transferred to a 150 mL reactor lined with polytetrafluoroethylene and crystallized at 180 °C for 48 h. After crystallization, the mixture was filtered, washed, dried at 100 °C for 12 h, and then calcined at 550 °C for 4 h to obtain the final product, ZSM-23 molecular sieve. Analysis using pyridine adsorption infrared spectroscopy and 2,6-dimethylpyridine adsorption infrared spectroscopy revealed that the total Brønsted acid content of pyridine in ZSM-23 molecular sieve was 0.184 mmol / g, and the total Brønsted acid content of 2,6-dimethylpyridine was 0.164 mmol / g. The ratio of the total Brønsted acid content of 2,6-dimethylpyridine to the total Brønsted acid content of pyridine was 89.1:100. Among the Brønsted acid content of 2,6-dimethylpyridine in ZSM-23 molecular sieve, the amount of weak Brønsted acid below 250℃ was 0.149 mmol / g, and the ratio of the amount of weak Brønsted acid below 250℃ to the total Brønsted acid content of 2,6-dimethylpyridine was 90.8:100. The bulk SiO2 / Al2O3 molar ratio was 97, and the outer surface SiO2 / Al2O3 molar ratio was 46.
[0072] The XRD patterns of the prepared molecular sieves are shown in the figure. Figure 1 It is ZSM-23 molecular sieve.
[0073] Example 2: Preparation of ZSM-23 molecular sieve
[0074] 17.43 g of hexamethylenediamine was dissolved in 216 g of deionized water, and then 18 g of fumed silica was added. After stirring at room temperature (25°C) for 1 h, the mixture was transferred to a 300 mL reactor lined with polytetrafluoroethylene. The reactor was heated at 200°C for 12 h and then quenched. After centrifugation, 35 mL of oleylamine was added to the resulting solid. The mixture was thoroughly mixed in a shaker and stirred at room temperature (25°C) for 0.5 h at a stirring rate of 150 rpm. Then, 20 mL of 0.01 mol / L NaOH solution was added to the mixture, and the mixture was stirred at 25°C for 12 h at a stirring rate of 150 rpm to obtain mixture A. 0.9 g of ZSM-23 seed crystals were added to deionized water to prepare 30 mL of ZSM-23 seed crystal solution, which was then added to mixture A. The mixture was stirred at 100°C for 20 h at a stirring rate of 300 rpm to obtain mixture B.
[0075] Add 0.67g Al2(SO4)3·18H2O and 0.85g pyrrolidine to 20mL of deionized water to obtain a clear solution C;
[0076] Solution C was added to mixture B and mixed thoroughly. The final mixture was then transferred to a 150 mL reactor lined with polytetrafluoroethylene and crystallized at 200 °C for 36 h. After crystallization, the mixture was filtered, washed, dried at 100 °C for 12 h, and then calcined at 550 °C for 4 h to obtain the final product, ZSM-23 molecular sieve. Analysis using pyridine adsorption infrared spectroscopy and 2,6-dimethylpyridine adsorption infrared spectroscopy revealed that the total Brønsted acid content of pyridine in ZSM-23 molecular sieve was 0.176 mmol / g, and the total Brønsted acid content of 2,6-dimethylpyridine was 0.160 mmol / g. The ratio of the total Brønsted acid content of 2,6-dimethylpyridine to the total Brønsted acid content of pyridine was 91:100. Among the Brønsted acid content of 2,6-dimethylpyridine in ZSM-23 molecular sieve, the amount of weak Brønsted acid below 250℃ was 0.130 mmol / g, and the ratio of the amount of weak Brønsted acid below 250℃ to the total Brønsted acid content of 2,6-dimethylpyridine was 81.0:100. The bulk SiO2 / Al2O3 molar ratio was 246, and the outer surface SiO2 / Al2O3 molar ratio was 57.
[0077] The XRD patterns of the prepared molecular sieves are similar to those of... Figure 1 Similar to ZSM-23 molecular sieve.
[0078] Example 3: Preparation of ZSM-23 molecular sieve
[0079] 4.68 g of hexamethylenediamine was dissolved in 54 g of deionized water, and then 6 g of fumed silica was added. After stirring at room temperature (25°C) for 1 h, the mixture was transferred to a 100 mL reactor lined with polytetrafluoroethylene. The reactor was heated at 170°C for 24 h and then quenched. After centrifugation, 6 mL of oleylamine was added to the resulting solid. The mixture was thoroughly mixed in a shaker and stirred at room temperature (25°C) for 0.5 h at a stirring rate of 200 rpm. Then, 24 mL of 0.005 mol / L NaOH solution was added to the mixture, and the mixture was stirred at 25°C for 6 h at a stirring rate of 200 rpm to obtain mixture A. 0.24 g of ZSM-23 was added to deionized water, and 10 mL of ZSM-23 seed solution was added to mixture A. The mixture was stirred at 80°C for 16 h at a stirring rate of 200 rpm to obtain mixture B.
[0080] Add 0.17g Al2(SO4)3·18H2O and 0.40g pyrrolidine to 20mL of deionized water to obtain a clear solution C;
[0081] Solution C was added to mixture B and mixed thoroughly. The final mixture was then transferred to a 100 mL reactor lined with polytetrafluoroethylene and crystallized at 200 °C for 48 h. After crystallization, the mixture was filtered, washed, dried at 100 °C for 12 h, and then calcined at 550 °C for 4 h to obtain the final product, ZSM-23 molecular sieve. Analysis using pyridine adsorption infrared spectroscopy and 2,6-dimethylpyridine adsorption infrared spectroscopy revealed that the total Brønsted acid content of pyridine in ZSM-23 molecular sieve was 0.176 mmol / g, and the total Brønsted acid content of 2,6-dimethylpyridine was 0.160 mmol / g. The ratio of the total Brønsted acid content of 2,6-dimethylpyridine to the total Brønsted acid content of pyridine was 76:100. Among the Brønsted acid content of 2,6-dimethylpyridine in ZSM-23 molecular sieve, the amount of weak Brønsted acid below 250℃ was 0.126 mmol / g, and the ratio of the amount of weak Brønsted acid below 250℃ to the total Brønsted acid content of 2,6-dimethylpyridine was 79.0:100. The bulk SiO2 / Al2O3 molar ratio was 387, and the outer surface SiO2 / Al2O3 molar ratio was 71.
[0082] The XRD patterns of the prepared molecular sieves are similar to those of... Figure 1 Similar to ZSM-23 molecular sieve.
[0083] Example 4: Preparation of ZSM-23 molecular sieve
[0084] Measure 4.68g of hexamethylenediamine and dissolve it in 54g of deionized water. Then add 6g of fumed silica. Stir at room temperature (25℃) for 1h and transfer to a 100mL reactor with a polytetrafluoroethylene liner. Heat at 150℃ for 24h and then quench. After centrifugation, add 3mL of oleylamine to the resulting solid and mix thoroughly in a shaker. Stir at room temperature (25℃) for 0.5h at a stirring rate of 250rpm. Add 12mL of 0.08mol / L NaOH solution to the mixture and stir at 25℃ for 8h at a stirring rate of 250rpm to obtain mixture A. Add 0.18g of ZSM-23 seed crystals to deionized water to prepare 5mL of ZSM-23 seed crystal solution and add it to A. Stir at 60℃ for 30h at a stirring rate of 200rpm to obtain mixture B.
[0085] Add 0.44 g Al2(SO4)3·18H2O and 0.15 g pyrrolidine to 15 mL of deionized water to obtain a clear solution C;
[0086] Solution C was added to mixture B and mixed thoroughly. The final mixture was then transferred to a 100 mL reactor lined with polytetrafluoroethylene and crystallized at 180 °C for 24 h. After crystallization, the mixture was filtered, washed, dried at 100 °C for 12 h, and then calcined at 550 °C for 4 h to obtain the final product, ZSM-23 molecular sieve. Analysis using pyridine adsorption infrared spectroscopy and 2,6-dimethylpyridine adsorption infrared spectroscopy revealed that the total Brønsted acid content of pyridine in ZSM-23 molecular sieve was 0.123 mmol / g, and the total Brønsted acid content of 2,6-dimethylpyridine was 0.104 mmol / g. The ratio of the total Brønsted acid content of 2,6-dimethylpyridine to the total Brønsted acid content of pyridine was 85:100. Among the Brønsted acid content of 2,6-dimethylpyridine in ZSM-23 molecular sieve, the amount of weak Brønsted acid below 250℃ was 0.091 mmol / g, and the ratio of the amount of weak Brønsted acid below 250℃ to the total Brønsted acid content of 2,6-dimethylpyridine was 87.5:100. The bulk SiO2 / Al2O3 molar ratio was 137, and the outer surface SiO2 / Al2O3 molar ratio was 64.
[0087] The XRD patterns of the prepared molecular sieves are similar to those of... Figure 1 Similar to ZSM-23 molecular sieve.
[0088] Example 5: Preparation method of Fe-NC-600@ZSM-23 composite catalyst
[0089] 106 mg of ferric acetate and 330 mg of 1,10-o-phenanthroline were weighed and poured into a 150 mL round-bottom flask. 50 mL of ethanol was added, and the mixture was stirred at room temperature for 30 min. Then, 3 g of ZSM-23 molecular sieve powder prepared in Example 1 was slowly added, and the mixture was sonicated at room temperature for 30 min. Afterward, the mixture was placed in a water bath and heated to 60 °C for reflux with stirring at 600 rpm for 12 h. After reflux, the sample was transferred to a rotary evaporator for rotary evaporation to obtain the precursor. The precursor was then transferred to a tube furnace and calcined under an argon atmosphere at 600 °C for 2 h. The resulting sample was labeled Fe-NC-600@ZSM-23.
[0090] The mass content of iron and the molar ratio of Fe, N, and C are shown in Table 1. Additionally, STEM and... Figure 2 Similarly, the catalyst was observed by aberration-corrected scanning transmission electron microscopy, which revealed that Fe in the active component Fe-NC species of this embodiment mainly exists in a highly dispersed single-atom form.
[0091] Example 6: Preparation method of Fe-NC-800@ZSM-23 composite catalyst
[0092] 106 mg of ferric acetate and 330 mg of 1,10-o-phenanthroline were weighed and poured into a 150 mL round-bottom flask. 50 mL of ethanol was added, and the mixture was stirred at room temperature for 30 min. Then, 3 g of ZSM-23 molecular sieve powder prepared in Example 1 was slowly added, and the mixture was sonicated at room temperature for 30 min. Afterward, the mixture was placed in a water bath and heated to 60 °C for reflux with stirring at 600 rpm for 12 h. After reflux, the sample was transferred to a rotary evaporator for rotary evaporation to obtain the precursor. The precursor was then transferred to a tube furnace and calcined under an argon atmosphere at 800 °C for 2 h. The resulting sample was labeled Fe-NC-800@ZSM-23.
[0093] The mass content of iron and the molar ratio of Fe, N, and C are shown in Table 1. Additionally, STEM data... Figure 2 As can be seen, observation of the catalyst by aberration-corrected scanning transmission electron microscopy revealed that Fe in the active component Fe-NC species of this embodiment mainly exists in a highly dispersed single-atom form.
[0094] Example 7: Preparation method of Fe-NC-800@ZSM-23 composite catalyst
[0095] The preparation process is the same as in Example 6, except that ferric acetate is replaced with ferric acetylacetone, and the amount used is 196 mg; the amounts of 1,10-o-phenanthroline, ethanol and ZSM-23 molecular sieve from Example 2 are 780 mg, 100 ml and 5 g, respectively.
[0096] The mass content of iron and the molar ratio of Fe, N, and C are shown in Table 1. Additionally, STEM and... Figure 2 Similarly, the catalyst was observed by aberration-corrected scanning transmission electron microscopy, which revealed that Fe in the active component Fe-NC species of this embodiment mainly exists in a highly dispersed single-atom form.
[0097] Comparative Example 1: Preparation Method of Fe-NC-600@ZSM-5 Composite Catalyst
[0098] Weigh 106 mg of ferric acetate and 330 mg of 1,10-o-phenanthroline into a 150 mL round-bottom flask, add 50 mL of ethanol, and stir at room temperature for 30 min. Then, slowly add 3 g of ZSM-5 molecular sieve powder, sonicate at room temperature for 30 min, and then place in a water bath and heat to 60 °C for reflux with stirring for 12 h at 600 rpm. After reflux, transfer the sample to a rotary evaporator for rotary evaporation to obtain the precursor. Then, transfer the precursor to a tube furnace and calcine under an argon atmosphere at 600 °C for 2 h. The obtained sample is labeled Fe-NC-600@ZSM-5.
[0099] The mass content of iron and the molar ratio of Fe, N, and C are shown in Table 1. Additionally, according to... Figure 4 As can be seen, observation of the catalyst by aberration-corrected scanning transmission electron microscopy revealed that a small amount of Fe aggregates appeared in the Fe-NC species of the active component in this comparative example.
[0100] Comparative Example 2Fe-ZSM-23 Preparation
[0101] 106 mg of ferric acetate was weighed and poured into a 150 mL round-bottom flask. 50 mL of ethanol was added, and the mixture was stirred at room temperature for 30 min. Then, 3 g of ZSM-23 molecular sieve powder prepared in Example 1 was slowly added, and the mixture was sonicated at room temperature for 30 min. The flask was then placed in a water bath and heated to 60 °C for reflux stirring at 600 rpm for 12 h. After reflux, the sample was transferred to a rotary evaporator for rotary evaporation to obtain the precursor. The precursor was then transferred to a tube furnace and calcined under an argon atmosphere at 800 °C for 2 h. The resulting sample was labeled Fe-ZSM-23.
[0102] Test 1: Performance Evaluation
[0103] The low-temperature direct oxidation reaction of methane is carried out in a high-pressure reactor equipped with a heating mantle.
[0104] First, at room temperature, 0.3 g of the catalysts prepared in Examples 1, 5-7, and Comparative Examples 1-2, and 80 ml of hydrogen peroxide solution (0.5 mol / L) were added to a 160 ml reactor. Nitrogen gas was introduced twice for purging, followed by methane gas purging three times. The reactor was then purged with methane gas to 3 MPa and stirred at 200 rpm while simultaneously heating to 50°C. The stirring speed was then adjusted to 1500 rpm, and the reaction was allowed to proceed for 30 min. After the reaction, stirring was stopped, and the temperature was lowered to below 20°C. The liquid was then collected and filtered. Nuclear magnetic resonance (NMR) analysis was performed to quantitatively analyze the C1 products (including CH3OH, HCOOH, CH3OOH, and HOCH2OOH) in the filtrate. The conversion frequency and yield of C1 liquid products generated on the composite catalyst at 50°C are shown in Table 1. The conversion frequency of C1 liquid product generation was calculated as (molar amount of C1 liquid product) / (catalyst amount × reaction time).
[0105] Table 1. Composition and catalytic performance of the examples and comparative examples.
[0106]
[0107]
[0108] Test 2: Cyclic Activity Test
[0109] The Fe-NC-800 / ZSM-23 composite catalyst prepared in Example 6 was subjected to cyclic activity testing. The catalyst preparation and activity evaluation conditions were the same as in Example 1, except that the catalyst from the previous reaction was used in each test, and multiple parallel tests were performed for each test to ensure consistent catalyst quality for subsequent tests. The results are shown below. Figure 3 As can be seen, the yield of C1 liquid products remained basically unchanged after multiple cycles of catalyst use, indicating that the catalyst is very stable.
[0110] The specific embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including combining the various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.
Claims
1. A metal single-atom molecular sieve composite catalyst, comprising a molecular sieve and an active component; wherein, The molecular sieve is ZSM-23 molecular sieve; the active component is Fe-NC species.
2. The catalyst according to claim 1, characterized in that, The Fe metal in the Fe-NC species exists in single-atom form.
3. The catalyst according to claim 1, characterized in that, In the catalyst, the iron content is 1wt%-1.5wt% based on the mass of the catalyst. And / or, in the Fe-NC species, the molar ratio of Fe, N and C is 1:(3-5):(10-12).
4. The catalyst according to claim 1, characterized in that, The total Brønsted acid content of pyridine in infrared spectroscopy for the ZSM-23 molecular sieve is 0.10–0.37 mmol / g; the total Brønsted acid content of 2,6-dimethylpyridine in infrared spectroscopy is 0.07–0.35 mmol / g. And / or, the ratio of the total Brønsted acid content of 2,6-dimethylpyridine in infrared to the total Brønsted acid content of pyridine in infrared is (65-99):100; And / or, in the 2,6-dimethylpyridine infrared acid of the ZSM-23 molecular sieve, the amount of weak Brønsted acid with a desorption temperature <250℃ is 0.09 to 0.31 mmol / g.
5. The catalyst according to claim 4, characterized in that, In the ZSM-23 molecular sieve, the ratio of the amount of weak Brønsted acid with a desorption temperature <250℃ to the total amount of Brønsted acid in the 2,6-dimethylpyridine infrared sieve is (64~95):
100.
6. The catalyst according to claim 1, characterized in that, The bulk SiO2 / Al2O3 molar ratio of the ZSM-23 molecular sieve is 40 to 400 higher than that of the outer surface SiO2 / Al2O3 molar ratio.
7. The catalyst according to claim 6, characterized in that, The bulk SiO2 / Al2O3 molar ratio of the ZSM-23 molecular sieve is 80–500, and the outer surface SiO2 / Al2O3 molar ratio is 40–120.
8. A method for preparing the catalyst according to any one of claims 1-7, the method comprising: Iron source, complex, and organic solvent are mixed, and then ZSM-23 molecular sieve is added for heating and reflux reaction. After drying and calcination, a metal single-atom molecular sieve composite catalyst is obtained.
9. The preparation method according to claim 8, characterized in that, The preparation method of the ZSM-23 molecular sieve includes the following steps: (1) Mix silicon source, template agent a and water, mix evenly and then carry out crystallization reaction. After the reaction, the material is separated into solid and liquid to obtain solid material. (2) Mix the solid material obtained in step (1) with the fatty amine evenly; (3) Add an alkaline source solution to the material obtained in step (2) and perform a first low-temperature treatment; then add ZSM-23 seed crystals and perform a second low-temperature treatment; (4) Prepare an aqueous solution containing template agent b and aluminum source, and then mix it with the material obtained in step (3) to obtain a gel. After crystallization, filtration, washing, drying and calcination, ZSM-23 molecular sieve is obtained.
10. The method according to claim 9, characterized in that, In step (1), the silicon source is one or more of silica, silica sol, water glass, fumed silica and tetraethyl orthosilicate; And / or, in step (1), the template agent a is at least one of hexamethylenediamine, ethanol and n-hexamethylenediamine.
11. The method according to claim 9, characterized in that, In step (1), the molar ratio of the template agent a to the silicon source (calculated as SiO2) is 0.1 to 1.0; the molar ratio of water to the silicon source (calculated as SiO2) is 20 to 80.
12. The method according to claim 9, characterized in that, In step (1), the operating conditions for the crystallization reaction are as follows: the crystallization temperature is 120-220℃, and the crystallization time is 8-48h; And / or, in step (1), the solid-liquid separation is performed by any one of gravity sedimentation, filtration separation and centrifugal separation.
13. The method according to claim 9, characterized in that, In step (2), the fatty amine is one or more of oleylamine, octadecylamine, and dodecylamine.
14. The method according to claim 9, characterized in that, In step (2), the liquid-to-solid ratio of the fatty amine to the solid material obtained in step (1) is 0.3 to 3 mL / g.
15. The method according to claim 9, characterized in that, In step (3), the alkaline source solution is any one of sodium hydroxide, potassium hydroxide and ammonia water, and the concentration of the alkaline source solution is 0.003 to 0.015 mol / L, preferably 0.005 to 0.010 mol / L; the liquid-solid ratio of the alkaline source solution to the solid material obtained in step (1) is 2 to 20 mL / g, preferably 5 to 15 mL / g.
16. The method according to claim 9, characterized in that, In step (3), the temperature of the first low-temperature treatment is 20-40°C and the time is 3-15 hours; the first low-temperature treatment is preferably carried out under stirring conditions, and the stirring rate is 100-300 rpm.
17. The method according to claim 9, characterized in that, In step (3), the ZSM-23 seed crystals are added in the form of seed crystal solution. The specific operation process of the seed crystal solution is as follows: the ZSM-23 seed crystals are uniformly dispersed in water to form seed crystal solution; wherein, the liquid-solid ratio of water to ZSM-23 seed crystals is 5 to 70 mL / g.
18. The method according to claim 9, characterized in that, In step (3), based on the weight of the solid material obtained in step (1), the amount of ZSM-23 seed crystals added is 0.1 to 8.0 wt%.
19. The method according to claim 9, characterized in that, In step (3), the secondary low-temperature treatment is carried out at a temperature of 60 to 120°C for a time of 6 to 30 hours; the secondary low-temperature treatment is preferably carried out under stirring conditions, with a stirring rate of 100 to 300 rpm.
20. The method according to claim 9, characterized in that, In step (4), the template agent b is one or more of pyrrolidine, isopropylamine and N,N-dimethylformamide; the aluminum source is one or more of aluminum sulfate, aluminum isopropoxide, sodium aluminate and aluminum hydroxide.
21. The method according to claim 9, characterized in that, In step (4), the molar ratio of template agent b to the solid material obtained in step (1) based on SiO2 is 0.01 to 0.1; the molar ratio of aluminum source based on Al2O3 to the solid material obtained in step (1) based on SiO2 is 0.002 to 0.015; and the molar ratio of water to aluminum source based on Al2O3 in the aqueous solution containing template agent b and aluminum source is 300 to 3000.
22. The method according to claim 9, characterized in that, In step (4), the crystallization temperature is 180-220℃ and the crystallization time is 24-72h; the drying temperature is 80-120℃ and the drying time is 6-12h; the calcination temperature is 540-560℃ and the calcination time is 3-8h.
23. The method according to claim 8, characterized in that, The iron source is selected from at least one of ferric acetate and ferric acetylacetone. And / or, the complex is selected from 1,10-o-phenanthroline; And / or, the organic solvent is selected from at least one of ethanol and acetone.
24. The method according to claim 8, characterized in that, After adding ZSM-23 molecular sieve, it is preferable to first treat it with ultrasound and then perform a heating and reflux reaction; the ultrasound frequency is 20-30 kHz and the treatment time is 10-50 min. And / or, the temperature of the heating reflux reaction is 40-90°C, the reflux time is 10-12 h, and the rotation speed during the reflux process is 400-800 rpm; And / or, the calcination is carried out under an inert atmosphere, the inert atmosphere including at least one of argon and helium, the calcination temperature is 400-800°C, and the calcination time is 2-4 hours.
25. The application of the metal single-atom molecular sieve composite catalyst according to any one of claims 1-7 in the low-temperature oxidation of methane to C1 liquid products.
26. The application according to claim 25, characterized in that, The methane low-temperature direct oxidation reaction uses pure methane as the reaction gas and low-concentration hydrogen peroxide as the oxidant, wherein the concentration of hydrogen peroxide in the hydrogen peroxide is 0.5-1 mol / L. And / or, the volume ratio of methane to hydrogen peroxide in the reaction is 1:(1-1.5); And / or, the reaction conditions are as follows: reaction temperature of 30-80℃, reaction time of 15-60 min, reaction pressure of 1-3 MPa, and catalyst dosage of 2.5-12.5 g per liter of hydrogen peroxide.