A MIL-101(Fe) derived iron-based catalyst for ammonia synthesis and a method of making the same

Abstract

The application discloses a MIL-101(Fe) derived iron-based catalyst for ammonia synthesis and a preparation method thereof, and relates to the technical field of preparation of ammonia synthesis catalyst materials. The catalyst comprises an iron-based active component, and the iron-based active component is coated with a carbon layer; the carbon layer is a doped carbon layer or an undoped carbon layer; and the doped carbon layer is doped with nitrogen, nickel or silicon dioxide. The application uses terephthalic acid or amino terephthalic acid as an organic ligand, uses ferric chloride as an iron source, obtains a MIL-101(Fe) precursor through a hydrothermal reaction, and obtains the catalyst through calcination. The application realizes the synergistic optimization of the composition and structure of the catalyst through the design of the structure of the precursor, the introduction of a nitrogen-containing component, the modification of a nickel component and the regulation and control of silicon dioxide polymerization inhibition, so that the application performance of the catalyst in ammonia synthesis is improved. The application solves the problem of insufficient catalytic performance of the existing iron-based catalyst.

Description

Technical Field

[0001] This invention relates to the field of ammonia synthesis catalytic material preparation technology, specifically to a MIL-101(Fe)-derived iron-based catalyst for ammonia synthesis and its preparation method. Background Technology

[0002] Ammonia is an important basic chemical raw material with wide applications in fertilizers, energy storage carriers, hydrogen transportation, and fine chemicals. Currently, industrial ammonia synthesis mainly relies on the Haber-Bosch process, which typically requires high temperatures and pressures, placing high demands on catalyst activity, stability, and structural tunability. Iron-based catalysts have long been an important catalyst system in ammonia synthesis due to their low cost, abundant sources, and suitability for industrial applications. However, existing iron-based catalysts still suffer from insufficient dispersion of active sites, limited electronic structure control, and easy particle agglomeration under high-temperature conditions, thus affecting their catalytic activity and long-term stability. Metal-organic frameworks (MOFs) possess characteristics such as large specific surface area, tunable pore structure, and uniform metal site distribution, providing new ideas for constructing high-performance catalysts. Among them, MIL-101(Fe), as a typical iron-containing MOF, can serve as an excellent precursor for iron-based catalysts. By designing its organic ligands, auxiliary ligands, and modifying components, and further calcining it at high temperatures, it is expected to obtain iron-based catalysts with carbon coating, nitrogen doping, bimetallic synergy, and anti-agglomeration structures. Therefore, there is an urgent need for a structurally tunable iron-based catalyst suitable for ammonia synthesis and its preparation method, in order to improve the overall performance of iron-based catalysts. Summary of the Invention

[0003] To address the aforementioned technical problems, the present invention aims to provide a MIL-101(Fe)-derived iron-based catalyst for ammonia synthesis and its preparation method, thereby solving the problem of insufficient catalytic performance of existing iron-based catalysts.

[0004] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: a MIL-101(Fe)-derived iron-based catalyst for ammonia synthesis is provided. The MIL-101(Fe)-derived iron-based catalyst for ammonia synthesis includes an iron-based active component, the iron-based active component is coated with a carbon layer, the carbon layer is a doped carbon layer or an undoped carbon layer, and the doped carbon layer is doped with nitrogen, nickel or silicon dioxide.

[0005] Based on the above technical solution, the present invention can be further improved as follows: This invention also provides a method for preparing the above-mentioned MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis, characterized by comprising the following steps: (1) Mix the organic ligand and the iron source and carry out a hydrothermal reaction to obtain an iron-containing metal-organic framework material precursor; (2) The iron-containing metal-organic framework material precursor obtained in step (1) is calcined under an inert atmosphere to obtain the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis.

[0006] Furthermore, in step (1), the organic ligand is terephthalic acid or aminoterephthalic acid, and the iron source is ferric chloride.

[0007] Furthermore, in step (1), the molar ratio of the organic ligand to the iron source is 1:(1-3). Further, in step (1), after mixing the organic ligand and the iron source, a DMF solution of 2-methylimidazole or tetraethyl orthosilicate is added, or a DMF solution of 2-methylimidazole and nickel nitrate are added, and then a hydrothermal reaction is carried out.

[0008] Furthermore, the organic ligand is terephthalic acid, and the iron source is ferric chloride.

[0009] Further, in step (1), after mixing the organic ligand and the iron source, a DMF solution of 2-methylimidazole is added, and then a hydrothermal reaction is carried out.

[0010] Furthermore, the molar ratio of the organic ligand to 2-methylimidazole is 1:6.

[0011] Furthermore, the concentration of the 2-methylimidazole DMF solution was 30.8 mg / mL.

[0012] Further, in step (1), after mixing the organic ligand and the iron source, a DMF solution of 2-methylimidazole and nickel nitrate are added, and then a hydrothermal reaction is carried out.

[0013] Furthermore, the molar ratio of the organic ligand, 2-methylimidazole, and nickel nitrate is 1:0.6:8.

[0014] Furthermore, in step (1), after mixing the organic ligand and the iron source, tetraethyl orthosilicate is added, and then a hydrothermal reaction is carried out.

[0015] Furthermore, the molar ratio of the organic ligand to tetraethyl orthosilicate is 1:0.25.

[0016] The beneficial effects of adopting the above-mentioned further technical solution are as follows: SiO2 formed by the pyrolysis of tetraethyl orthosilicate is used to suppress the agglomeration of iron particles during calcination.

[0017] Furthermore, in step (2), the hydrothermal reaction is carried out at 80-150℃ for 20 hours. Furthermore, in step (3), the sample is calcined at 500-800℃ for 3 hours.

[0018] Furthermore, in step (3), the inert atmosphere is argon.

[0019] The present invention also provides the application of the above-mentioned MIL-101(Fe)-derived iron-based catalyst for ammonia synthesis in ammonia synthesis.

[0020] The present invention has the following beneficial effects: 1. This invention uses terephthalic acid or aminoterephthalic acid as organic ligands and ferric chloride as the iron source to prepare the MIL-101 (Fe) precursor via hydrothermal reaction, followed by calcination to obtain the catalyst. Furthermore, by introducing 2-methylimidazole and nickel nitrate or tetraethyl orthosilicate, nitrogen-doped carbon-coated, Ni-modified, and SiO2-inhibited iron-based catalysts can be obtained, namely Fe@C, NH2-Fe@C, Fe@CN, Ni-Fe@CN, and iron-based catalysts containing SiO2 inhibition structures. These catalysts exhibit good structural tunability; carbon coating improves stability; nitrogen doping and nickel modification help regulate the electronic structure of the iron active center; and the SiO2 component helps inhibit Fe aggregation during calcination. Therefore, they have promising applications in ammonia synthesis reactions.

[0021] 2. This invention achieves synergistic optimization of catalyst composition and structure through precursor structure design, introduction of nitrogen-containing components, modification of nickel components, and regulation of silica polymerization inhibition, thereby improving its application performance in ammonia synthesis. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the synthesis of the catalyst in Example 3 of the present invention; Figure 2 The XRD patterns of the catalysts prepared in Examples 1-2 are shown below. Figure 3 This is a schematic diagram of the catalyst performance evaluation process for Examples 1-5. Detailed Implementation

[0023] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0024] Example 1: A MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis is prepared by the following steps: (1) The organic ligand (terephthalic acid) and the iron source (ferric chloride) were mixed at a molar ratio of 1:2 and hydrothermally reacted at 110°C for 20 h to obtain an iron-containing metal-organic framework material precursor. (2) The iron-containing metal-organic framework precursor obtained in step (1) is calcined at 800°C for 3 hours under an argon atmosphere to obtain the MIL-101 (Fe)-derived iron-based catalyst (denoted as Fe@C) for ammonia synthesis.

[0025] Example 2: A MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis is prepared by the following steps: In step (1), terephthalic acid was replaced with aminoterephthalic acid, and the rest was the same as in Example 1, to prepare the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis (denoted as NH2-Fe@C).

[0026] Example 3: A MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis is prepared by the following steps (process flow is shown in [link]). Figure 1 ): In step (1), after mixing the organic ligand (terephthalic acid) and the iron source (ferric chloride), a DMF solution of 2-methylimidazole (concentration of 30.8 mg / mL) was added. The molar ratio of the organic ligand to 2-methylimidazole was 1:6. Then, a hydrothermal reaction was carried out. The rest was the same as in Example 1 to prepare the MIL-101 (Fe)-derived iron-based catalyst (denoted as Fe@CN) for ammonia synthesis.

[0027] Example 4: A MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis is prepared by the following steps: In step (1), after mixing the organic ligand (terephthalic acid) and the iron source (ferric chloride), a DMF solution of 2-methylimidazole (concentration of 30.8 mg / mL) and nickel nitrate were added. The molar ratio of the organic ligand, 2-methylimidazole and nickel nitrate was 1:0.6:8. Then, a hydrothermal reaction was carried out, and the rest was the same as in Example 1, to obtain the MIL-101 (Fe)-derived iron-based catalyst (denoted as Ni-Fe@CN) for ammonia synthesis.

[0028] Example 5: A MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis is prepared by the following steps: In step (1), after mixing the organic ligand (terephthalic acid) and the iron source (ferric chloride), tetraethyl orthosilicate is added. The molar ratio of the organic ligand to tetraethyl orthosilicate is 1:0.25. Then, a hydrothermal reaction is carried out. The rest is the same as in Example 1 to obtain the MIL-101 (Fe)-derived iron-based catalyst (denoted as SiO2-Fe@C) for ammonia synthesis.

[0029] Test case I. XRD Detection The catalysts prepared in Examples 1-2 were subjected to XRD analysis. Specifically, the phase composition and grain size of the samples were characterized using a Rigaku Ultima IV X-ray powder diffractometer (XRD). The instrument's radiation source was Cu Kα1, wavelength λ was 1.5405 Å, operating current was 40 mA, operating voltage was 40 kV, wide-angle diffraction scanning rate was 5° / min, and scanning range was 10–80°. The results are shown below. Figure 2 ( Figure 2 In the image, (a) shows the XRD pattern of the MIL-101(Fe) precursor and the simulated MIL-101; (b) shows the XRD pattern of the Fe@C catalyst and the metallic iron standard card; (c) shows the XRD pattern of the NH2-MIL-101(Fe) precursor and the simulated MIL-101; and (d) shows the XRD pattern of the NH2-Fe@C catalyst and the Fe3C standard card. Depend on Figure 2 It can be seen that, from Figure 2 It can be seen that both the MIL-101(Fe) precursor and the amino-modified MIL-101(Fe) precursor exhibit characteristic diffraction peaks corresponding to the simulated MIL-101 in the low-angle region, indicating that the prepared precursors have a MIL-101-type crystal structure. Compared with the simulated spectrum, the diffraction peaks of the measured samples show certain broadening and intensity differences, which may be related to the sample grain size, crystallinity, and ligand modification. After high-temperature calcination, the Fe@C sample shows obvious diffraction peaks around 44° and 65°, which match the standard card for metallic iron, indicating that an active phase of metallic iron was formed after calcination. The diffraction peaks of the NH2-Fe@C sample correspond to the Fe3C standard card, indicating that the amino ligands promoted the formation of the iron carbide phase after pyrolysis.

[0030] II. Catalytic performance The catalysts prepared in Examples 1-5 were subjected to ammonia synthesis catalytic performance testing. The specific testing method was as follows: evaluation was conducted using a pressurized fixed-bed reactor. First, 0.2 g of catalyst was diluted with quartz powder at a volume ratio of 1:10 and then loaded into a Hastelloy tube with an inner diameter of 10 mm. The catalyst was pretreated for 2 h at 450 °C in an atmosphere of 25 wt% N2-75 wt% H2. Subsequently, it was subjected to a pressure of 1 MPa and 30000 mL·g -1 ·h -1 Under the given space velocity conditions, the system temperature was lowered to the predetermined reaction temperature, while the feed gas composition remained 25 vt% N2 - 75 vt% H2. Ammonia in the reaction outlet gas was quantitatively detected using ion chromatography. Finally, the ammonia synthesis rate of the catalyst was calculated based on the outlet ammonia concentration. (See attached diagram for apparatus details.) Figure 3 The results are shown in Table 1.

[0031] Table 1. Performance evaluation results of the catalysts prepared in Examples 1-5

[0032] As shown in Table 1, the MIL-101(Fe)-derived iron-based catalysts prepared in Examples 1-5 all exhibited certain ammonia synthesis catalytic activity. Among them, Example 5 showed the highest ammonia synthesis rate, at 4.78 mmol·g⁻¹. cat -1 h -1 It was significantly higher than the 2.12 mmol·g in Example 1. cat -1 h -1 This indicates that the introduction of a silica-based polymerization inhibitor structure can effectively suppress the agglomeration of iron species during calcination and improve the utilization rate of the catalyst's active sites. The ammonia synthesis rates in Examples 3 and 4 were 3.42 mmol·g⁻¹, respectively. cat -1 h -1 and 3.96 mmol·g cat -1 h -1 The ammonia synthesis rate in Example 2 was higher than that in Example 1, indicating that nitrogen doping and nickel modification are beneficial to improving the catalytic performance of the iron-based catalyst. The lower ammonia synthesis rate in Example 2 suggests that when only aminoterephthalic acid is used to regulate the precursor structure, the formation of the effective active phase or the exposure of active sites in the resulting catalyst may be limited. Among the iron-based catalysts obtained in this invention, Example 5 has shown better catalytic activity, demonstrating that structural regulation methods such as silica polymerization inhibition, nitrogen doping, and nickel modification can significantly improve the ammonia synthesis performance of the MIL-101(Fe)-derived iron-based catalyst.

[0033] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. 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 MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis, characterized in that, The MIL-101(Fe)-derived iron-based catalyst for ammonia synthesis comprises an iron-based active component coated with a carbon layer, which may be a doped carbon layer or an undoped carbon layer, wherein the doped carbon layer is doped with nitrogen, nickel or silicon dioxide.

2. The method for preparing the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis according to claim 1, characterized in that, Includes the following steps: (1) Mix the organic ligand and the iron source and carry out a hydrothermal reaction to obtain an iron-containing metal-organic framework material precursor; (2) The iron-containing metal-organic framework material precursor obtained in step (1) is calcined under an inert atmosphere to obtain the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis.

3. The method for preparing the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis according to claim 2, characterized in that, In step (1), the organic ligand is terephthalic acid or aminoterephthalic acid, and the iron source is ferric chloride.

4. The method for preparing the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis according to claim 2, characterized in that, In step (1), the molar ratio of the organic ligand to the iron source is 1:(1-3).

5. The method for preparing the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis according to claim 2, characterized in that, In step (1), after mixing the organic ligand and the iron source, a DMF solution of 2-methylimidazole or tetraethyl orthosilicate is added, or a DMF solution of 2-methylimidazole and nickel nitrate are added, and then a hydrothermal reaction is carried out.

6. The method for preparing the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis according to claim 2, characterized in that, In step (2), the hydrothermal reaction is carried out at 80-150℃ for 20 hours.

7. The method for preparing the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis according to claim 2, characterized in that, In step (3), calcination is carried out at 500-800℃ for 3 hours.

8. The method for preparing the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis according to claim 2, characterized in that, In step (3), the inert atmosphere is argon.

9. The application of the MIL-101 (Fe)-derived iron-based catalyst for ammonia synthesis according to claim 1 in ammonia synthesis.

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