Metal covalent organic framework material M-HATN-COFs, and preparation method and application thereof

Abstract

The application discloses a kind of metal covalent organic framework materials M-HATN-COFs and preparation method and application thereof, the structural formula of the material is M3·HATN, wherein M represents Mo or Ni, HATN represents six azacyclic substructure.The application constructs novel ultrathin conductive metal covalent organic framework electrocatalyst by using the characteristics of M3·HATN substructure high metal content.By simulating the chemical structure of nitric acid reductase (active enzyme in denitrifying bacteria), M-HATN-COFs electrochemical ammonia synthesis catalyst is constructed by using high unsaturation molybdenum atom or nickel atom.M-HATN-COFs can be prepared by simple coordination-condensation, and the electrocatalysis has ultrahigh metal content (12.5at%), ordered in-plane porous structure (1.2nm) and good conductivity of metallic property (1.5S·cm ‑2 ).In order to further enhance the ammonia synthesis catalytic activity, the application simultaneously applies salt template method to the synthesis of M-HATN-COFs to realize ultrathin nanosheet (≤1.5nm).

Description

Technical Field

[0001] This invention relates to electrochemical catalysts, specifically to a metal covalent organic framework material M-HATN-COFs, its preparation method, and its applications. Background Technology

[0002] Ammonia (NH3) is an essential component of all amino acids in living organisms and is fundamental to agriculture and aquaculture. Due to its high nitrogen content, it also plays a vital role in agriculture, textiles, pharmaceuticals, and the military industry. Recently, the pursuit of sustainable energy has driven new applications of NH3 in the renewable energy sector. With its high hydrogen content (17.7 wt%) and zero-carbon structure, NH3 is a promising hydrogen carrier with a yield of 4.3 kWh / kg⁻¹. 1 NH3 has a high energy density. Furthermore, unlike hydrogen, gaseous NH3 can be converted into a liquid state and obtained and stored under mild conditions (p≥10 bar or T≤-33℃). Therefore, NH3 is widely considered a next-generation sustainable energy carrier that can replace hydrogen.

[0003] To date, the Haber-Bosch process is used in NH3 production worldwide. This process demands stringent production conditions, requiring high pressure (≥200 atm) and high temperature (≥400°C), resulting in extreme energy consumption (≈38 GJ / t). NH3 ) and carbon emissions (≈2.9t) CO2 / t NH3 To meet the growing demand and the requirements for energy conservation and emission reduction, it is necessary to further expand the scale of ammonia production and develop new technologies. Electrochemical nitrogen reduction, utilizing nitrogen and water as fuel for the electrosynthesis of NH3 under mild conditions, is a novel ammonia production strategy. However, N≡N in nitrogen gas is extremely inert, and breaking its bond structure requires 941 kJ / mol. -1 The energy; and nitrogen has extremely low solubility in water (17.1 mg / kg). -1 water As a nonpolar molecule, nitrogen molecules also exhibit extremely low adsorption capacity on electrode surfaces. These three bottlenecks severely restrict the application of direct nitrogen electroreduction. As an alternative, nitrates (NO3) - It has unique advantages in promoting the electrochemical reduction of NH3: 1) Low bond energy (204 kJ / mol) -1 ); 2) High water solubility at room temperature (383g kg) -1 water 3) High chemical polarity is beneficial for the adsorption of catalytic active sites. Furthermore, due to the long-term accumulation from agricultural and industrial activities, NO3... - As a toxic environmental pollutant, excessive intake of NO3 can cause various diseases in human tissues. Therefore, NO3... -Reduced to HN3 (NRA, NO3) - +9H + +8e - The process of (NH3 + 3H2O, -0.12V vs. SHE) not only improves the production efficiency of NH3, but also greatly reduces environmental pollution.

[0004] As an emerging electrochemical technology, only a few electrocatalysts have been developed for NRA reactions, including noble metal nanoparticles, transition metal nanostructures, and oxides. These electrocatalysts mainly rely on inorganic metal-based nanomaterials, and their NRA active sites exist only on the surface and / or edges of the nanostructure, where unsaturated metal atoms are exposed. Apart from the surface, the internal metal atoms of inorganic catalysts are very inert, making it difficult to reduce NO3. - This inherent limitation results in very low metal atom utilization and catalytic efficiency in current NRA catalysts. Metal-organic frameworks (MOFs), due to their abundant exposed metal sites and ordered in-plane porous structure, offer an ideal approach for efficient NRA electrocatalysis, maximizing metal atom utilization (in principle, up to 100%) and producing electrocatalytic reactions with efficiencies similar to single-atom catalysts. However, the chemical structure of the metal linking sites in MOFs is very fragile, easily leading to framework collapse. Introducing conjugated covalent linkages into MOFs to form conductive metal-covalent organic frameworks (c-MCOFs) may help overcome this deficiency. Currently, only a few works have reported electrocatalysts constructed from c-MCOFs, whose structures are mainly based on porphyrinoid structures with extremely low metal content (metal content ≤2 at%), which cannot meet the production requirements of NH3. Summary of the Invention

[0005] To address the problem that current electrocatalysts constructed from c-MCOFs have extremely low metal content, which cannot meet the production requirements of NH3, this invention provides a metal covalent organic framework material, its preparation method, and its application. This metal covalent organic framework material contains a large number of ordered mesoporous channels, and its channels expose highly unsaturated variable-valence transition metal sites, which can serve as a highly efficient electrocatalyst for the electrochemical reduction of nitrate to ammonia. Furthermore, a simple, green, and mild preparation method is provided for it.

[0006] To achieve the above objectives, the present invention provides a metal covalent organic framework material M-HATN-COFs, wherein the structural formula of the material is M3·HATN, where M represents Mo or Ni and HATN represents a hexaazacyclic substructure.

[0007] When the structure of the metal covalent organic framework material is Mo-HATN-COFs, it belongs to the hexagonal crystal system, space group P6 / mmm, and its unit cell parameters are: α=90°, β=90°, γ=120°;

[0008] When the structure of the metal covalent organic framework material is Ni-HATN-COFs, it belongs to the hexagonal crystal system, space group P6 / mmm, and its cell parameters are: α=90°, β=90°, γ=120°.

[0009] A second aspect of the present invention provides a method for preparing the above-mentioned metal covalent organic framework material, the method comprising the following steps:

[0010] S1. Sodium chloride solution is slowly added dropwise to alcohols such as methanol or ethanol, and stirred to produce white flocculent sodium chloride microcrystals. After filtration and washing, sodium chloride template is obtained.

[0011] S2. Dissolve molybdenum pentachloride or nickel dichloride in N,N-dimethylformamide to form solution A;

[0012] S3. Dissolve 1,2,4,5-tetraaminobenzenetetrahydrochloride and cyclohexanehexanone octahydrate in N,N-dimethylformamide, and then add the sodium chloride microcrystalline template obtained in step S1 to form solution B.

[0013] S4. Under gas protection conditions, solutions A and B are mixed and heated to react. The resulting product is then washed and dialyzed to obtain metal covalent organic framework material M-HATN-COFs.

[0014] Preferably, in step S1, the concentration of the sodium chloride solution is 4-6 mol / L, and its volume ratio with methanol or ethanol is 1:(20-50).

[0015] Preferably, in step S2, the molybdenum pentachloride or nickel dichloride is dissolved in N,N-dimethylformamide by ultrasonication.

[0016] In step S3, the 1,2,4,5-tetraaminobenzenetetrahydrochloride and cyclohexanehexanone octahydrate are dissolved in N,N-dimethylformamide.

[0017] Preferably, in step S3, the molar ratio of 1,2,4,5-tetraaminobenzenetetrahydrochloride, molybdenum pentachloride or nickel dichloride, and cyclohexanehexanone octahydrate is 1:(0.5-2):(1-4).

[0018] Specifically, in step S4, the heating reaction conditions are 160–200°C for 12–24 hours.

[0019] A third aspect of the present invention provides a metal covalent organic framework material, which is prepared by the above-described preparation method.

[0020] A fourth aspect of this invention provides an application of the aforementioned metal covalent organic framework material in electrochemical ammonia production. Specifically, the metal covalent organic framework material serves as a catalyst for electrochemical ammonia production.

[0021] Through the above technical solution, the present invention achieves the following beneficial effects:

[0022] 1. This invention utilizes the high metal content of the M3·HATN substructure to construct a novel ultrathin conductive metal-covalent organic framework electrocatalyst. By simulating the chemical structure of nitrate reductase (an active enzyme in denitrifying bacteria), an M-HATN-COF electrochemical ammonia production catalyst was constructed using highly unsaturated molybdenum or nickel atoms. The M-HATN-COFs can be prepared via simple coordination-condensation. This electrocatalyst exhibits an ultra-high metal content (12.5 at%), an ordered in-plane porous structure (1.2 nm), and good metallic conductivity (1.5 S·cm). -2 To further enhance the catalytic activity for ammonia production, this invention also applies the salt template method to the synthesis of M-HATN COFs to achieve ultrathin nanosheets (≤1.5 nm).

[0023] 2. Electrochemical test results show that the ammonia yield of the electrocatalyst M-HATN-COFs of this invention is as high as 8.52 mg·h⁻¹ under a low voltage of -0.5 V. -1 ·cm -2 Its Faraday efficiency (FE) can reach up to 91.3%. More importantly, unlike inorganic catalytic structures, M-HATN-COFs containing single metal sites can serve as the preferred model for studying the electrocatalytic mechanism of metal atoms.

[0024] 3. This invention is of great significance for revealing the intrinsic mechanism of the NRA reaction and designing a new generation of electrocatalysts. Attached Figure Description

[0025] Figure 1 This is the crystal structure of the Mo-HATN-COFs electrocatalyst prepared in Example 1 of this invention;

[0026] Figure 2 This is the X-ray diffraction pattern of the Mo-HATN-COFs electrocatalyst prepared in Example 1 of this invention;

[0027] Figure 3 This is a scanning electron microscope image of the Mo-HATN-COFs electrocatalyst prepared in Example 1 of this invention;

[0028] Figure 4 This is a transmission electron microscope image of the Mo-HATN-COFs electrocatalyst prepared in Example 1 of this invention;

[0029] Figure 5 This is a simulation diagram of the density of states of the Mo-HATN-COF electrocatalyst prepared in Example 1 of this invention;

[0030] Figure 6 This is a scanning electron microscope image of the Ni-HATN-COFs electrocatalyst prepared in Example 2 of this invention;

[0031] Figure 7 This is a scanning electron microscope image of the HATN-COFs electrocatalyst prepared in the comparative proportion;

[0032] Figure 8 These are polarization curves of the c-COFs electrocatalysts prepared in Example 1 and the comparative example of this invention;

[0033] Figure 9 This is a polarization curve of the Ni-HATN-COFs electrocatalyst prepared in Example 2 of this invention;

[0034] Figure 10 This is a graph showing the ammonia yield of the c-COFs electrocatalysts prepared in the embodiments and comparative examples of this invention;

[0035] Figure 11 This is a chronoamperometry diagram of the Mo-HATN-COFs electrocatalyst prepared in Example 1 of this invention;

[0036] Figure 12 This is a cyclic stability diagram of the Mo-HATN-COFs electrocatalyst prepared in Example 1 of this invention;

[0037] Figure 13 This is a scanning electron microscope image of the Mo-HATN-COFs electrocatalyst prepared in Example 1 of this invention after electrochemical testing. Detailed Implementation

[0038] The specific embodiments of the present invention will be described in detail below with reference to examples. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0039] Example 1

[0040] 1. Preparation of sodium chloride microcrystalline template

[0041] 10 mL of 4 M sodium chloride solution was slowly added dropwise to 500 mL of ethanol while stirring vigorously, resulting in white flocculent sodium chloride microcrystals. After filtration and washing with ethanol, a sodium chloride template for synthesizing ultrathin conductive covalent organic framework materials was obtained.

[0042] 2. Preparation of ultrathin conductive covalent organic frameworks (Mo-HATN-COFs) with high molybdenum content sites

[0043] 0.25 g of molybdenum pentachloride was dissolved in 1 mL of N,N-dimethylformamide and sonicated to form solution A. 0.12 g of 1,2,4,5-tetraaminobenzenetetrahydrochloride and 0.09 g of cyclohexanehexanone octahydrate were dissolved in 5 mL of N,N-dimethylformamide and sonicated. Then, 20 g of synthesized sodium chloride microcrystalline template was added to form solution B. Under nitrogen protection, solution A was added dropwise to solution B, and the mixture was heated to 160 °C and reacted for 24 hours. After cooling to room temperature, the mixture was washed with water and dialyzed to obtain a black conductive metal-covalent organic framework Mo-HATN-COFs electrocatalyst. It belongs to the hexagonal crystal system, space group P6 / mmm, and its cell parameters are: α=90°, β=90°, γ=120° (see Figure 1 The planar framework contains an ordered porous structure with a pore size of 1.2 nm.

[0044] The prepared conductive c-COF electrocatalyst with high metal content was characterized using a Bruker D8 Advance X-ray diffractometer, a HITACHI SU8020 field emission scanning electron microscope, and a Tecnai G2F20 field emission transmission electron microscope. The results are shown in the figure. Figures 2-5 . Figure 2 The results show that Mo-HATN-COFs exhibit a prominent peak at 26.5°, belonging to their (002) plane, which matches the simulated XRD results. This result demonstrates that the method of this invention can successfully prepare Mo-HATN-COFs. Figure 3 It can be seen that the microstructure of the prepared Mo-HATN-COFs electrocatalyst is a two-dimensional layered structure, and this layered structure has obvious inter-layer interconnection. Combined with its in-planar channel structure, this catalyst has a highly efficient hierarchical channel structure, which can effectively improve the mass transport efficiency. Figure 4 It can be determined that the conductive metal c-COFs electrocatalyst has a thin-layer structure with a thickness of approximately 1.5 nm. Figure 5 Density of states (PDOS) simulations of Mo-HATN-COFs show that their electronic structure has a zero band gap, exhibiting a metallic structure and excellent electron transport capabilities. In contrast, the metal-free HATN-COF electrocatalysts exhibit a band gap of 1.73 eV, indicating a semiconductor electronic structure. This suggests that the inclusion of a metal complex leads to π-d conjugation between the HATN-COF structure and the metal ions, further enhancing their conductivity.

[0045] Example 2

[0046] 1. Preparation of sodium chloride microcrystalline template

[0047] 10 mL of 6 M sodium chloride solution was slowly added dropwise to 200 mL of ethanol while stirring vigorously, resulting in white flocculent sodium chloride microcrystals. After filtration and washing with ethanol, a sodium chloride template for synthesizing ultrathin conductive covalent organic framework materials was obtained.

[0048] 2. Preparation of ultrathin conductive covalent organic frameworks (Ni-HATN-COFs) with high molybdenum content sites

[0049] 0.03 g of nickel dichloride was dissolved in 1 mL of N,N-dimethylformamide and dissolved by sonication to form solution A. 0.142 g of 1,2,4,5-tetraaminobenzenetetrahydrochloride and 0.336 g of cyclohexanehexanone octahydrate were dissolved in 5 mL of N,N-dimethylformamide and dissolved by sonication. Then, 20 g of synthesized sodium chloride microcrystalline template was added to form solution B. Under nitrogen protection, solution A was added dropwise to solution B, and the mixture was heated to 200 °C and reacted for 12 hours. After cooling to room temperature, the mixture was washed with water and dialyzed to obtain a black conductive metal-covalent organic framework Ni-HATN-COFs electrocatalyst. It belongs to the hexagonal crystal system, space group P6 / mmm, and its cell parameters are: α = 90°, β = 90°, γ = 120°. Its morphology is similar to that of the Mo-HATN-COFs electrocatalyst in Example 1 (see...). Figure 6 ).

[0050] Comparative Example

[0051] Other conditions are the same as in Example 1, except that in step 2, no metal ions are introduced to prepare a conductive organic covalent framework catalyst without metal sites. Its structure is HATN-COFs, belonging to the hexagonal crystal system, space group P6 / mmm, and its cell parameters are: α = 90°, β = 90°, γ = 120°, and its morphology is similar to that of the Mo-HATN-COFs electrocatalyst in Example 1 (see Figure 7 ).

[0052] The metal-conductive organic-covalent framework electrocatalysts prepared in Examples 1, 2, and the comparative example were ultrasonically dispersed in a mixture of ethanol and water to prepare a suspension with a concentration of 2 mg / mL. This suspension was then drop-coated onto a pre-treated carbon cloth surface to obtain the modified carbon cloth as the working electrode. The metal-conductive organic-covalent framework electrocatalyst was tested using a three-electrode system and an H-type electrolytic cell on an electrochemical workstation. A saturated calomel electrode served as the reference electrode, a carbon rod as the counter electrode, and 0.1 M Na₂SO₄ and 0.1 M NaNO₃ were used as the electrolytes. Under argon protection, linear sweep voltammetry (LSV) curves showed that the current density of the metal-conductive organic-covalent framework electrocatalyst in an electrolyte containing nitrate was significantly higher than that in an electrolyte without nitrate, indicating that an NRA reaction occurred on the surface of the metal-conductive organic-covalent framework electrocatalyst (see...). Figure 8 and Figure 9 The Mo-HATN-COF electrocatalyst exhibits an onset potential of -0.31 V (vs. RHE) for NRA; at -0.5 V, its NRA catalytic current density reaches 60.7 mA·cm⁻¹. -2 It can be deduced that the optimal catalytic potential of the Mo-HATN-COFs electrocatalyst for NRA is -0.5 V. In contrast, the bulk Mo-HATN-COFs material only shows 23.2 mA·cm⁻¹. -2 The lower NRA catalytic current indicates that the ultrathin strategy can improve the electrochemical activity of MCOF catalysts. The HATN-COF electrode exhibits some NRA catalytic current, but it is much lower than that of the Mo-HTN-MCOF electrode, because NRA lacks highly active metal sites. However, on the Ni-HATN-COF electrode, although the catalytic current density is higher than that of HATN-COFs, its catalytic current is mainly provided by the hydrogen evolution reaction.

[0053] After electrocatalysis, the NH3 yield of various c-COFs electrocatalysts was analyzed by electrolyte colorimetric method. Figure 10 Compared with the absorbance calibration curve, the Mo-HATN-COFs electrocatalyst exhibited a higher NH3 yield of 8.52 mg·h⁻¹. -1 ·cm -2 The ammonia production efficiency was significantly better than that of blocky Mo-HTN-MCOFs (3.37 mg·h⁻¹). -1 ·cm -2 Meanwhile, the ammonia production efficiency of Ni-HATN-COF was 1.92 mg·h⁻¹. -1 ·cm -2 HATN-COFs also showed a 0.49 mg h -1 cm -2The NRA electrocatalytic behavior was observed. This indicates that the NRA electrocatalytic activity at the Mo site is the highest, with the activity order being Mo site > Ni site > N site. Stability determines the practicality of the electrocatalyst; after 10 hours of NRA electrocatalysis, the NRA catalytic current density of the Mo-HATN-COFs electrocatalyst showed only a slight loss of 13.4%. Figure 11 Furthermore, in five consecutive electrochemical tests, the NH3 yield of FEs remained at 8.31 mg·h⁻¹. -1 ·cm -2 high level ( Figure 12 The effects of electrochemical processes on its structure were investigated using SEM. SEM images revealed a thin-layer morphology with a porous structure, similar to an untested Mo-HATN-COF electrocatalyst. Figure 13 ).

[0054] The preferred embodiments of the present invention have been described in detail above with reference to the examples. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0055] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0056] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing a metal covalent organic framework material M-HATN-COFs, characterized in that, Includes the following steps: S1. Sodium chloride solution is slowly added dropwise to alcohol, and stirring produces white flocculent sodium chloride microcrystals. After filtration and washing, sodium chloride template is obtained. S2. Dissolve molybdenum pentachloride or nickel dichloride in N,N-dimethylformamide to form solution A; S3. Dissolve 1,2,4,5-tetraaminobenzenetetrahydrochloride and cyclohexanehexanone octahydrate in N,N-dimethylformamide, and then add the sodium chloride microcrystalline template obtained in step S1 to form solution B. S4. Under gas protection conditions, solution A and solution B are mixed and heated to react. The resulting product is washed and dialyzed to obtain metal covalent organic framework material M-HATN-COFs. The material has the structural formula M3∙HATN, where M represents Mo or Ni, and HATN represents a hexaazacyclic substructure.

2. The preparation method according to claim 1, characterized in that, When the structure of the metal covalent organic framework material is Mo-HATN-COFs, it belongs to the hexagonal crystal system, space group P6 / mmm, and the cell parameters are: a=16.54Å, b=16.54Å, c=3.47Å, α=90°, β=90°, γ=120°. When the structure of the metal covalent organic framework material is Ni-HATN-COFs, it belongs to the hexagonal crystal system, space group P6 / mmm, and the cell parameters are: a=16.48Å, b=16.48Å, c=3.48Å, α=90°, β=90°, γ=120°.

3. The preparation method according to claim 1, characterized in that, In step S1, the concentration of the sodium chloride solution is 4~6 mol / L, and its volume ratio with alcohol is 1:(20~50).

4. The preparation method according to claim 1, characterized in that, In step S1, the alcohol is methanol or ethanol.

5. The preparation method according to claim 1, characterized in that, In step S2, the molybdenum pentachloride or nickel dichloride is dissolved in N,N-dimethylformamide by ultrasonication; In step S3, the 1,2,4,5-tetraaminobenzenetetrahydrochloride and cyclohexanehexanone octahydrate are dissolved in N,N-dimethylformamide by ultrasonication.

6. The preparation method according to claim 1, characterized in that, The molar ratio of 1,2,4,5-tetraaminobenzenetetrahydrochloride, molybdenum pentachloride or nickel dichloride, and cyclohexanehexanone octahydrate is 1:(0.5~2):(1~4).

7. The preparation method according to any one of claims 1 to 6, characterized in that, In step S4, the heating reaction conditions are 160~200 °C for 12~24 h.

8. A metal covalent organic framework material, characterized in that, It is prepared by any one of claims 1 to 7.

9. The application of the metal covalent organic framework material according to claim 8 in electrochemical ammonia production.

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