A high-stability MOFs-based non-noble metal single-atom catalyst, a preparation method and application thereof

Abstract

The application belongs to the technical field of catalytic chemistry, and particularly relates to a high-stability MOFs-based non-noble metal monatomic catalyst as well as a preparation method and application thereof. The catalyst is composed of a zirconium-based MOFs carrier and a non-noble metal. The zirconium-based MOFs carrier is UiO-66-X, wherein X is one of -CH3, -H and -NO2. The non-noble metal is one of Cu, Ni, Mn and Zn, which is dispersed in the zirconium-oxo cluster nodes on the inner surface of UiO-66-X in the form of monatomic. The MOFs-based non-noble metal monatomic catalyst of the application has the advantages of mild reaction condition, high activity and high selectivity of oxygen-containing compounds when applied to the catalytic conversion of methane, and the monatomic anchored by the Zr-oxo nodes on the inner surface of MOFs has excellent circulation stability.

Description

Technical Field

[0001] This invention belongs to the field of catalytic chemistry technology, specifically relating to a highly stable MOFs-based non-noble metal single-atom catalyst, its preparation method, and its application in the low-temperature direct selective oxidation of methane. Background Technology

[0002] Methane, a major component of natural gas, associated petroleum gas, shale gas, and combustible ice, is of great significance for efficient conversion into high-value-added oxygenated compounds (such as methanol) [Chem. Rev. 2020, 120, 1438-1511; Sci. Sin. Chim. 2021, 51, 175-187; Chin. J. Chem. Eng. 2021, 38, 18-29; Chem. Soc. Rev. 2022, 51, 376-423]. Currently, industrial methane-to-methanol production mainly relies on energy-intensive (800-1000℃) indirect processes such as steam reforming and Fischer-Tropsch synthesis. Developing a direct route for the catalytic conversion of methane to methanol with lower energy consumption (<100℃) is increasingly important. This is because the CH bond energy of methane is as high as 104 kcal / mol. -1 [Appl. Catal. A-Gen. 1999, 186, 3-12; Science 2017, 356, 523-527; Catal. Today 2017, 285, 147-158], far exceeding the selectivity of the oxygen-containing compounds (such as methanol) produced. While activating methane, excessive oxidation of the products is easily caused, resulting in a "seesaw effect" in the entire catalytic process where the conversion of methane and the selectivity of oxygen-containing compounds are difficult to achieve simultaneously. Therefore, the activation of the CH bond of methane and its directional conversion to oxygen-containing compounds has always been considered the "holy grail" reaction in the field of catalysis [Chin. J. Catal. 2023, 52, 1-13; Chem 2019, 5, 2296-2325; Chin. J. Chem. Eng. 2021, 38, 18-29].

[0003] Single-atom catalysts are a class of metal-supported catalysts in which the active center is dispersed on a support in the form of a single atom. Due to their potential for 100% metal utilization and unique geometric-electronic structure, they typically exhibit excellent reactivity, target product selectivity, and catalyst stability, and are widely used in methane catalytic conversion research [J. Am. Chem. Soc. 2023, 145, 24, 13169-13180; Chem. Soc ... Soc. 2023, 145, 24, 13169-13180; 2018,4,1902-1910;Appl.Catal.B-Environ.2021,285,119827;Chem.Sci.2021,12,3152-3160;Nat.Commun.2020,11,954;Angew.Chem.Int.Ed.2019,131,18559-18 564; Angew.Chem.Int.Ed.2016,55,13441-13445; J.Am.Chem.Soc.2017,139,17694 -17699; Angew.Chem.Int.Ed.2020,59,1216-1219;ACSCatal.2018,8,6894-6907]. Compared to other noble metals commonly used in methane conversion (Ir, Rh, Pd, Pt, Au, etc.), non-noble metals such as Cu and Ni are not only inexpensive but also possess strong methane activation capabilities [ACS Nano 2022, 16, 8557-8618; Chem. Rev., 2023, 123, 6359-6411]. Therefore, preparing them into highly active single-atom catalysts has considerable application potential. However, single-atom catalysts supported on traditional inorganic materials (molecular sieves, oxides, activated carbon, etc.) often suffer from drawbacks such as easy loss of metal components and sintering, leading to decreased catalyst stability or even deactivation. UiO-66-X (X = -CH3, -H, and -NO2) is a class of MOF materials formed by the coordination self-assembly of zirconium-oxo cluster (Zr-oxo) nodes and terephthalic acid ligands. It possesses advantages such as high specific surface area, abundant anchoring groups (-OH / -OH2) on the inner surface Zr-oxo cluster (Zr-oxo) nodes, and excellent hydrothermal stability, making it an ideal single-atom catalyst support [Energy Environ. Sci., 2022, 15, 3722]. Therefore, utilizing the Zr-oxo nodes on the inner surface of UiO-66-X to stabilize non-noble metals and developing a low-cost, highly active, metal-resistant, and metal-sintering-resistant high-efficiency stable single-atom catalyst for the low-temperature direct selective oxidation of methane to oxygen-containing compounds is of great significance. Summary of the Invention

[0004] The purpose of this invention is to provide a MOFs-based non-noble metal single-atom catalyst, its preparation method, and its application. This catalyst uses UiO-66-X, rich in -OH / -OH2 groups, as a support. A stable dispersion of non-noble metal single atoms at Zr-oxo nodes on the inner surface of UiO-66-X is achieved through an impregnation reaction method. It is then used for the low-temperature direct selective oxidation of methane to oxygen-containing compounds.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention provides a MOFs-based non-noble metal single-atom catalyst, wherein the catalyst is composed of a zirconium-based MOFs support and a non-noble metal dual component; the zirconium-based MOFs support is UiO-66-X, wherein X is one of -CH3, -H and -NO2; the non-noble metal is one of Cu, Ni, Mn and Zn, and is dispersed in single-atom form in zirconium-oxygen cluster (Zr-oxo) nodes on the inner surface of UiO-66-X.

[0007] The UiO-66-X carrier is in powder form with a specific surface area of ​​500–3000 μm. 2 g -1 The pore size is 0.4–2 nm, and the pore volume is 0.2–1 mL g. -1 .

[0008] Furthermore, in the above technical solution, the non-precious metal content is 0.01 to 2 wt% of the total mass of the catalyst, preferably 0.1 to 1 wt%.

[0009] Another aspect of the present invention provides a method for preparing the above-mentioned MOFs-based non-noble metal single-atom catalyst, the method comprising the following steps:

[0010] (1) Dissolve zirconium chloride and terephthalic acid ligand in an equimolar ratio in a mixed solution of N,N-dimethylformamide (DMF) and a regulator, and obtain the UiO-66-X support after crystallization, filtration, washing and drying.

[0011] (2) A non-precious metal precursor solution is formed by mixing a non-precious metal precursor with an organic solvent. The UiO-66-X support is immersed in the non-precious metal precursor solution and impregnated at 50-100°C for 6-72 hours. After the reaction is completed, the product is filtered and washed. The crude product is dried to obtain the catalyst.

[0012] Furthermore, in the above technical solution, in step (1), the terephthalic acid ligand is one of 2-methylterephthalic acid, terephthalic acid, and 2-nitroterephthalic acid.

[0013] Furthermore, in the above technical solution, in step (1), the regulator is one of hydrochloric acid, formic acid, benzoic acid, acetic acid, and trifluoroacetic acid, preferably hydrochloric acid.

[0014] Furthermore, in the above technical solution, in step (2), the non-precious metal precursor is one or more of the following: chloride salt, nitrate salt, acetylacetone salt, and acetate salt of non-precious metals, preferably chloride salt.

[0015] Furthermore, in the above technical solution, in step (2), the organic solvent is one of acetylacetone, N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), and dichloromethane, preferably DMF.

[0016] Furthermore, in the above technical solution, in step (2), the filtration and washing process is as follows: filtration and washing with DMF 2 to 5 times, filtration and washing with ultrapure water at 50 to 90°C 2 to 5 times, and finally filtration and washing with a low-boiling-point organic solvent; the low-boiling-point organic solvent is one of acetone, tetrahydrofuran, dichloromethane, and n-hexane, preferably acetone.

[0017] Furthermore, in the above technical solution, in step (2), the crude product is dried in a vacuum drying oven at 25 to 250°C for 4 to 24 hours or placed in a freeze dryer at -50 to -30°C for 8 to 48 hours.

[0018] This invention also provides an application of the above-mentioned catalyst in a low-temperature direct selective oxidation reaction of methane. The reaction is carried out in a high-pressure reactor, and the reactants are methane and an aqueous solution of hydrogen peroxide. The amount of catalyst used is 1 × 10⁻⁶ of the aqueous hydrogen peroxide solution. -4 ~2.5×10 -3 wt%; reaction temperature 30–70℃; reaction methane pressure 0.5–3 MPa; reaction time 0.25–2 h.

[0019] Furthermore, in the above technical solution, the high-pressure reactor has an effective volume of 38 mL, the temperature is 30–70 °C, the reaction methane pressure is 0.5–3 MPa, and the catalyst dosage is 1 × 10⁻⁶ aqueous hydrogen peroxide solution. -4 ~2.5×10 -3 The solvent volume was 10–20 mL of water (wt%), the concentration of the hydrogen peroxide aqueous solution was 0.25–1.0 M, the stirring speed was 800–1200 rpm, and the reaction time was 0.25–2 h. Under these conditions, methanol exhibited the highest selectivity and yield, and showed very good cycle stability. The main products of the reaction were methanol (CH3OH), methyl hydrogen peroxide (CH3OOH), hydroxymethyl hydrogen peroxide (HOCH2OOH), formic acid (HCOOH), and other methane oxidation products.

[0020] Compared with the prior art, the essential features of this invention are:

[0021] (1) This invention utilizes the chemical reaction between the non-noble metal precursor and the -OH / -OH2 groups at the Zr-oxo nodes on the inner surface of UiO-66-X as the driving force to achieve atomic-level stable dispersion of various non-noble metals such as Cu, Ni, Mn, and Zn, which has a certain degree of universality. The impregnation reaction method used in catalyst preparation is simpler, more efficient, and lower in cost than other methods for preparing MOFs-based metal single atoms, such as atomic layer deposition, ligand anchoring, and in-situ synthesis.

[0022] (2) The metal single atoms on the catalyst of the present invention have a “dual-site synergistic” catalytic effect with the Zr-oxo nodes on the inner surface of UiO-66-H, which makes the activity and selectivity of methane on the catalyst to oxygen-containing compounds at low temperature direct selective oxidation far higher than that of metal single atoms or nanoparticle catalysts supported on other supports.

[0023] (3) The Zr-oxo nodes -OH / -OH2 groups on the inner surface of the carrier UiO-66-X of this invention have a stronger interaction with non-noble metal single atoms. Compared with the single atoms adsorbed on the outer surface, the node anchoring bonding is stronger, which can effectively prevent the loss and sintering of single-atom metal and maintain the stability of this type of metal single-atom catalyst. Attached Figure Description

[0024] Figure 1 XRD patterns of different M / U6-H catalysts prepared in Examples 1-4 and Comparative Examples 1-2;

[0025] Figure 2 Transmission electron microscopy (TEM) images of different M1 / U6-H catalysts prepared in Examples 1-4 and Comparative Example 1, where a is Example 1, b is Example 2, c is Example 3, d is Example 4, and e is Comparative Example 1.

[0026] Figure 3 The X-ray absorption fine structure R-space spectrum of the catalysts prepared in Example 1 and Comparative Example 1 is shown below;

[0027] Figure 4 X-ray photoelectron spectra of the catalysts prepared in Example 1 and Comparative Example 1;

[0028] Figure 5 Cu for Comparative Example 2 NPs Transmission electron microscopy image of U6-H catalyst;

[0029] Figure 6 The graph shows the low-temperature direct selective oxidation performance of methane by the M1 / U6-X-in catalysts prepared in Examples 1-4.

[0030] Figure 7The graph shows the low-temperature direct selective oxidation performance of methane by Cu catalysts supported on different supports prepared in Examples 1, 5, 6 and Comparative Examples 1-2.

[0031] Figure 8 The graphs show the cycle stability test results of the Cu1 / U6-H catalysts prepared in Example 1 and Comparative Example 1.

[0032] Figure 9 This is a transmission electron microscope (TEM) image of the Cu1 / U6-H-in catalyst prepared in Example 1 after reaction. Detailed Implementation

[0033] The present invention will be further described below through specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0034] Unless otherwise specified, the materials used in the embodiments of the present invention can be obtained commercially or prepared according to conventional methods known to those skilled in the art.

[0035] Example 1

[0036] (1) Preparation of UiO-66-H support:

[0037] 1.64 g of anhydrous zirconium chloride (ZrCl4) and 1.16 g of terephthalic acid (H2BDC) were dissolved in a mixed solution of 80 mL DMF and 0.7 mL hydrochloric acid. The mixture was stirred for 30 min until the precursor was completely dissolved. The mixture was then transferred to a 200 mL stainless steel hydrothermal reactor lined with para-polystyrene and placed in an oven at 120 °C for 24 h for crystallization. After the reaction was completed and cooled to room temperature, the resulting white solid was washed with DMF and 80 °C ultrapure water by vacuum filtration. The water and other impurity molecules in the pores were then replaced with acetone. Finally, the crude product was transferred to a vacuum drying oven and dried at 150 °C for 12 h to obtain a UiO-66-H support rich in -OH / -OH2 groups, denoted as U6-H.

[0038] (2) Preparation of Cu1 / U6-H-in catalyst:

[0039] 55 mg of copper chloride hydrate (CuCl2·2H2O) was dissolved in a scintillation flask containing 9 mL of DMF. After sonication to dissolve the hydrate, 0.6 g of UiO-66-H powder was added, and the mixture was impregnated at 85 °C for 24 h. The mixture was then centrifuged and washed three times with DMF solution to remove unreacted metal precursors. The solid was transferred to a Buchner funnel and thoroughly washed with ultrapure water at 80 °C to remove solvent and residual Cl ions. The water in the pores was replaced with acetone, and the resulting solid powder was transferred to a vacuum drying oven and dried at 60 °C for 12 h to remove physically adsorbed water and impurity molecules from the pores. After cooling, a catalyst in which Cu is dispersed in single-atom form on the Zr-oxo nodes of the inner surface of UiO-66-H was obtained, denoted as Cu1 / U6-H-in. The loading of metallic Cu was determined by ICP to be 0.67 wt%, with the remainder being UiO-66-H support, denoted as 0.67Cu1 / U6-H-in.

[0040] Example 2

[0041] Preparation of Ni1 / U6-H-in catalyst:

[0042] The catalyst preparation method in this embodiment is the same as in Example 1, except that the metal precursor used is 24 mg of nickel chloride (NiCl2·6H2O). The loading of metallic Ni was determined by ICP to be 0.48 wt%, with the remainder being UiO-66-H support, denoted as 0.48Ni1 / U6-H-in.

[0043] Example 3

[0044] Preparation of Mn1 / U6-H-in catalyst

[0045] The catalyst preparation method in this embodiment is the same as in Example 1, except that the metal precursor used is 32 mg of manganese chloride (MnCl2·4H2O). The loading of metal Mn was determined by ICP to be 0.6 wt%, with the remainder being UiO-66-H support, denoted as 0.6Mn1 / U6-H-in.

[0046] Example 4

[0047] Preparation of Zn1 / U6-H-in catalyst

[0048] The catalyst preparation method in this embodiment is the same as in Example 1, except that the metal precursor used is 22 mg of zinc chloride (ZnCl2). The loading of metallic Zn was determined by ICP to be 0.53 wt%, with the remainder being UiO-66-H support, denoted as 0.53Zn1 / U6-H-in.

[0049] Example 5

[0050] (1) Preparation of UiO-66-CH3 support:

[0051] 1.64 g of anhydrous zirconium chloride (ZrCl4) and 1.27 g of 2-methylterephthalic acid (CH3-BDC) were dissolved in a mixed solution of 80 mL DMF and 0.7 mL hydrochloric acid. The mixture was stirred for 30 min until the precursor was completely dissolved. The mixture was then transferred to a 200 mL stainless steel hydrothermal reactor lined with para-polystyrene and placed in an oven at 120 °C for 24 h for crystallization. After the reaction was completed and cooled to room temperature, the resulting white solid was washed by filtration with DMF and 80 °C ultrapure water. The water and other impurity molecules in the pores were then replaced with acetone. Finally, the crude product was transferred to a vacuum drying oven and dried at 150 °C for 12 h to obtain a UiO-66-CH3 support rich in -OH / -OH2 groups, denoted as U6-CH3.

[0052] (2) Preparation of Cu1 / U6-CH3-in catalyst:

[0053] 55 mg of copper chloride hydrate (CuCl2·2H2O) was dissolved in a scintillation flask containing 9 mL of DMF. After sonication to dissolve the hydrate, 0.6 g of UiO-66-CH3 powder was added, and the mixture was impregnated at 85 °C for 24 h. The mixture was then centrifuged and washed three times with DMF solution to remove unreacted metal precursors. The centrifuged solid was transferred to a Buchner funnel and thoroughly washed with ultrapure water at 80 °C to remove solvent and residual Cl ions. The water in the pores was replaced with acetone, and the resulting solid powder was transferred to a vacuum drying oven and dried at 60 °C for 12 h to remove physically adsorbed water and impurity molecules from the pores. After cooling, the Cu1 / U-6-CH3-in catalyst was obtained. ICP analysis showed that the loading of metallic Cu was 0.35 wt%, with the remainder being UiO-66-CH3 support, denoted as 0.35Cu1 / U6-CH3-in.

[0054] Example 6

[0055] (1) Preparation of UiO-66-NO2 support:

[0056] 1.64 g of anhydrous zirconium chloride (ZrCl4) and 1.48 g of 2-nitroterephthalic acid (NO2-BDC) were dissolved in a mixed solution of 80 mL DMF and 0.7 mL hydrochloric acid. The mixture was stirred for 30 min until the precursor was completely dissolved. The mixture was then transferred to a 200 mL stainless steel hydrothermal reactor lined with para-polystyrene and placed in an oven at 120 °C for 24 h for crystallization. After the reaction was completed and cooled to room temperature, the resulting white solid was washed by filtration with DMF and 80 °C ultrapure water. The water and other impurity molecules in the pores were then replaced with acetone. Finally, the crude product was transferred to a vacuum drying oven and dried at 150 °C for 12 h to obtain a UiO-66-NO2 support rich in -OH / -OH2 groups, denoted as U6-NO2.

[0057] (2) Preparation of Cu1 / U6-NO2-in catalyst:

[0058] 55 mg of copper chloride hydrate (CuCl2·2H2O) was dissolved in a scintillation flask containing 9 mL of DMF. After sonication to dissolve the hydrate, 0.6 g of UiO-66-NO2 powder was added, and the mixture was impregnated at 85 °C for 24 h. The mixture was then centrifuged and washed three times with DMF solution to remove unreacted metal precursors. The solid was transferred to a Buchner funnel and thoroughly washed with ultrapure water at 80 °C to remove solvent and residual Cl ions. The water in the pores was replaced with acetone, and the resulting solid powder was transferred to a vacuum drying oven and dried at 60 °C for 12 h to remove physically adsorbed water and impurity molecules from the pores. After cooling, the Cu1 / U-6-NO2-in catalyst was obtained. ICP analysis showed that the loading of metallic Cu was 0.56 wt%, with the remainder being UiO-66-NO2 support, denoted as 0.56Cu1 / U6-NO2-in.

[0059] Example 7

[0060] Activity evaluation test of methane low-temperature direct oxidation catalyst:

[0061] The reaction was carried out in a high-pressure reactor lined with para-polystyrene. First, 5 mg of catalyst was weighed and placed in a reactor with a 38 mL para-polystyrene liner, and 20 mL of 1 M H₂O₂ dilute solution was added. Then, residual air in the reactor was removed by purging with methane five times, followed by purging with 3 MPa methane. The reaction was carried out at 60 °C for 0.25 h with stirring at 1200 rpm. After the reaction was completed, stirring was stopped, and the temperature was lowered to 10 °C under ice-water bath conditions. The gaseous products were collected using a gas bag, and the liquid products were filtered out. The liquid products were quantitatively analyzed using 700 MHz NMR, and the gaseous products were detected using gas chromatography (equipped with an FID detector for a Ni conversion furnace). The selectivity for oxygen-containing compounds was n. (含氧化合物 ) / n (总产物)×100%, Yield of oxygen-containing compounds = n (含氧化合物 ) / m (催化剂) ×Reaction time, Transition frequency (TOF) per unit active site of a single-atom catalyst = n (含氧化合物) / n (单原子) × Reaction time. Test results are shown in Table 1. Figure 5-6 As shown.

[0062] Example 8

[0063] Stability test of Cu1 / U6-H catalyst:

[0064] In this embodiment, the catalyst activity evaluation test conditions were the same as in Example 8. Each test used the catalyst from the previous reaction, and multiple parallel tests were performed for each test to ensure consistent catalyst quality for subsequent tests. The test results are shown in Table 2. Figure 7 As shown.

[0065] Comparative Example 1

[0066] Cu single-atom catalyst with external surface node anchorage: Cu1 / U6-H-out was prepared by electrostatic adsorption.

[0067] In 10 mL of copper nitrate (CuNO3·4H2O) aqueous solution (0.42 mg / mL) -1 Ammonia (calculated as Cu) was added to adjust the pH to 10. 0.6 g of the UiO-66-H support prepared in Example 1 was added, and the mixture was stirred at room temperature for 0.5 h. The solid product was obtained by filtration. Unreacted precursors were thoroughly washed with ultrapure water. The obtained solid was then ground and freeze-dried for 12 h to obtain the sample. Before use, the sample was reduced at 100 °C for 0.5 h under a 10% H2 / He atmosphere to obtain the Cu1 / U6-H-out catalyst. The loading of metallic Cu, as determined by ICP, was 0.7 wt%, with the remainder being the UiO-66-H support, denoted as 0.7Cu1 / U6-H-out.

[0068] Comparative Example 2

[0069] Cu NPs Preparation of U6-H catalyst:

[0070] Under room temperature bath stirring conditions, 0.6 g of the UiO-66-H support prepared in Example 1 and 9 mL of copper nitrate (CuNO3·4H2O) aqueous solution (0.42 mg / mL) were added. -1The solution was added to a 100 mL beaker containing Cu, and then stirred at room temperature for 3 h. The temperature was then raised to 70 °C and stirred until the reaction solution was evaporated to dryness. The resulting solid was ground, dried overnight in an oven at 110 °C, and then calcined in a muffle furnace at 250 °C for 2 h. Before evaluation or characterization, the dried sample was subjected to a 10% H₂ / Ar solution (30 mL min) in a tube furnace. -1 Reduction at 250℃ for 0.5 h yields Cu. NPs / U6-H catalyst. ICP analysis showed that the loading of metallic Cu was 0.65 wt%, with the remainder being UiO-66-H support, denoted as 0.65Cu. NPs / U6-H.

[0071] Results analysis:

[0072] like Figure 1 The XRD patterns of the catalysts prepared in Examples 1-4 and Comparative Examples 1-2 of this invention are shown. The results show that the crystal form of the support UiO-66 is not changed regardless of whether metal single atoms or nanoparticles are loaded.

[0073] like Figure 2 The transmission electron microscope (TEM) images of the catalysts prepared in Examples 1-4 and Comparative Example 1 of this invention show that the metal in the M1 / U6-H catalyst is dispersed as single atoms.

[0074] like Figure 3 As shown, the X-ray absorption fine structure R-space spectrum of the catalysts prepared in Example 1 and Comparative Example 1 of this invention shows that the Cu in both Cu1 / U6-in and Cu1 / U6-out catalysts is a single-atom dispersion.

[0075] like Figure 4 As shown, the X-ray photoelectron spectra of the catalysts prepared in Example 1 and Comparative Example 1 of this invention show that, under the condition of similar metal loading, the Cu 2p signal intensity of Cu1 / U6-out is higher, indicating that the Cu in Cu1 / U6-in is dispersed on the inner surface nodes of UiO-66-H.

[0076] like Figure 5 The image shows a transmission electron microscope (TEM) image of the catalyst prepared in Comparative Example 2 of this invention. The results indicate that Cu... NPs The Cu in the / U6-H catalyst is mainly in the form of nanoparticles.

[0077] like Figure 6 As shown in the figure, the activity evaluation test diagram of the catalysts in Examples 1-4 of the present invention for the low-temperature direct oxidation of methane shows that the selectivity of oxygen-containing compounds on the M1 / U6-H-in catalyst is >95%, and the yield on Cu1 / U6-H-in is relatively the highest.

[0078] like Figure 7As shown in the diagrams, the catalysts in Examples 1, 5, and 6 of this invention and Comparative Examples 1-2 exhibit activity evaluation test results for the low-temperature direct oxidation of methane. The results indicate that a small amount of oxygen-containing compounds are generated on the UiO-66-X support, while the yield significantly increases after the introduction of Cu single atoms, and the selectivity for oxygen-containing compounds is >93%. These results demonstrate that, compared to Cu single atoms and nanoparticles dispersed on the outer surface, Cu single atoms anchored at nodes on the inner surface of UiO-66-X exhibit superior low-temperature direct oxidation performance and oxygen-containing compound selectivity for methane.

[0079] like Figure 8 As shown in the figure, the low-temperature direct oxidation of methane by the catalysts in Example 1 and Comparative Example 1 of this invention is subjected to a cycle test diagram. The results show that the stability of Cu single atoms anchored at the inner surface nodes of UiO-66-H is better than that of Cu single atoms dispersed on the outer surface.

[0080] like Figure 9 As shown in the transmission electron microscope image of the catalyst in Example 1 of this invention after the low-temperature direct oxidation of methane under cyclic testing, it can be seen that Cu in Cu1 / U6-H-in still maintains single-atom dispersion after the cyclic testing, indicating that the Cu single atoms anchored by the Zr-oxo nodes on the inner surface of UiO-66-H have excellent stability.

[0081] Table 1 Catalytic performance of different catalysts in the low-temperature direct selective oxidation of methane

[0082]

[0083] Note: The reaction conditions were: 3 MPa methane, 20 mL 1 M H₂O₂ aqueous solution, 60 °C for 0.25 h. Oxygen-containing compounds included: methanol (CH₃OH), methyl hydrogen peroxide (CH₃OOH), hydroxymethyl hydrogen peroxide (HOCH₂OOH), and formic acid (HCOOH).

[0084] Table 1 shows the activity evaluation results of the catalysts prepared in Examples 1-6 and Comparative Examples 1-2 for the low-temperature direct oxidation of methane. Sequences 1-3 show that the oxygen-containing compound selectivity on UiO-66-H is 99.9%, with the highest yield, and the highest content of Zr-oxo nodes for stabilizing the metal. Therefore, it is preferred as the main support for loading metal M. Sequences 4-7 show that among the Cu, Ni, Mn, and Zn single atoms supported on the UiO-66-H support, the oxygen-containing compound selectivity on Cu1 / U6-H-in is >97%, and the yield and TOF are relatively the highest, indicating that the combination of the UiO-66-H support and single-atom Cu can effectively promote methane oxidation. Based on this, a representative Cu single-atom catalyst was selected to study the influence mechanism of metal distribution position and electronic properties on the performance of the methane oxidation reaction. Sequences 7-9 show that Cu single atoms anchored at the inner surface nodes of the UiO-66-H support exhibit superior low-temperature direct oxidation performance and selectivity for oxygen-containing compounds compared to Cu single atoms and nanoparticles anchored at the outer surface. Sequences 7, 10-11 show that Cu single atoms anchored at Zr-oxo nodes with moderate electron density on the inner surface are beneficial for the low-temperature direct oxidation of methane. These results indicate that, compared to the Cu1 / U6-H-in catalyst, the lack of effective synergy between the outer surface nodes and Cu single atoms / nanoparticles leads to poorer performance in the low-temperature direct selective oxidation of methane to oxygen-containing compounds.

[0085] Table 2. ICP-OES results before and after the reaction with Cu1 / U6-H catalyst.

[0086]

[0087] As shown in Table 2, the Cu single-atom catalyst supported on the outer surface of U6-H suffered significant loss during the application test, while the one anchored on the inner surface showed no significant change. This indicates that the bonding between the Zr-oxo nodes on the inner surface and the Cu single atoms is stronger, resulting in higher cycle stability.

[0088] In summary, the catalyst prepared in this invention exhibits excellent performance and mild reaction conditions. The 0.67Cu1 / U6-H-in catalyst, when applied to the low-temperature direct selective oxidation of methane to methanol, demonstrates a high yield of 3253.0 μmol g under reaction conditions of 60°C, 3 MPa CH4, 1200 rpm, and 1 M H2O2. cat -1 h -1 The methanol yield, especially in the cyclic reuse experiment, remained in a good and stable state.

[0089] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a MOFs-based non-noble metal single-atom catalyst, characterized in that: The catalyst is composed of a zirconium-based MOF support and non-noble metals; The zirconium-based MOFs support is UiO-66-X, wherein X is one of -CH3, -H and -NO2; The non-noble metal is one of Cu, Ni, Mn, and Zn, and is dispersed in the form of single atoms in the zirconium-oxygen cluster nodes on the inner surface of UiO-66-X; The method for preparing the catalyst includes the following steps: (1) Dissolve zirconium chloride and terephthalic acid ligand in an equimolar ratio in a mixed solution of N,N-dimethylformamide and a regulator, and obtain the UiO-66-X support after crystallization, filtration, washing and drying. (2) Mix the non-precious metal precursor with an organic solvent to form a non-precious metal precursor solution. Immerse the UiO-66-X support in the non-precious metal precursor solution and impregnate it at 50~100 ℃ for 6~72 h. After the reaction is completed, filter and wash the product. Dry the crude product to obtain the MOFs-based non-precious metal single-atom catalyst. In step (2), the organic solvent is one of acetylacetone, N,N-dimethylformamide, N,N-diethylformamide, and dichloromethane; In step (2), the filtration and washing process is as follows: filtration and washing with N,N-dimethylformamide 2 to 5 times, filtration and washing with ultrapure water at 50 to 90 °C 2 to 5 times, and finally filtration and washing with a low-boiling-point organic solvent.

2. The preparation method according to claim 1, characterized in that: The non-precious metal content is 0.01~2 wt% of the total catalyst mass.

3. The preparation method according to claim 1, characterized in that: In step (1), the terephthalic acid ligand is one of 2-methylterephthalic acid, terephthalic acid, and 2-nitroterephthalic acid.

4. The preparation method according to claim 1, characterized in that: In step (1), the regulator is one of hydrochloric acid, formic acid, benzoic acid, acetic acid, and trifluoroacetic acid.

5. The preparation method according to claim 1, characterized in that: In step (2), the non-precious metal precursor is one or more of the following: chloride salt, nitrate salt, acetylacetone salt, and acetate salt of non-precious metals.

6. The preparation method according to claim 1, characterized in that: In step (2), the low-boiling-point organic solvent is one of acetone, tetrahydrofuran, dichloromethane, and n-hexane.

7. The preparation method according to claim 1, characterized in that: In step (2), the crude product is dried in a vacuum drying oven at 25~250 ℃ for 4~24 h or placed in a freeze dryer at -50~-30 ℃ for 8~48 h.

8. The application of a catalyst prepared by the method according to any one of claims 1-7 in the low-temperature direct selective oxidation reaction of methane, characterized in that: The reaction was carried out in a high-pressure reactor, with methane and an aqueous hydrogen peroxide solution as the reactants; the amount of catalyst used was 1 × 10⁻⁶ of the aqueous hydrogen peroxide solution. -4 ~2.5×10 -3 wt%; reaction temperature 30~70 ℃; reaction methane pressure 0.5~3 MPa; reaction time 0.25~2 h.

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