Holmium-based covalent organic framework material as well as preparation method and application thereof

Abstract

The invention discloses a holmium-based covalent organic framework material and a preparation method and application thereof, and relates to the technical field of electrochemical catalytic materials, the holmium-based covalent organic framework material is a rare earth covalent organic framework successfully synthesized by 1, 3, 5-triformyl phloroglucinol, p-phenylenediamine and HO-NH2 through a three-in-one strategy, and the holmium-based covalent organic framework material is a holmium-based covalent organic framework material. The composite material has designable porosity, an adjustable active site structure and functional characteristics, can expose more active sites during electrocatalytic synthesis of urea, has good and appropriate adsorbability to NO3 <-> and CO2, and can promote proton transport. Experiments show that under the optimal pulse condition, the yield of synthesized urea reaches up to 1792 ug.h <-1 >. Mgcat <-1 > and is about four times of the yield of a HO-free metal node complex COF, the stability reaches up to 40 h, the cost is saved, waste is turned into wealth, operation is easy, and good commercial value and application prospects are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of electrocatalytic materials, and particularly to a holmium-based covalent organic framework material, a preparation method thereof, and an application thereof. Background Art

[0002] As is well known, more than 40% of the world's food production depends on chemical fertilizers. Urea (CO(NH 2 ) 2 )), as an important nitrogen source of fertilizers, has an annual global output of over 100 million tons. In addition to being used for agricultural fertilization, urea is also an important raw material for pharmaceutical and chemical production (J. Lim, C. A. Fernández, S. W. Lee, M. C. Hatzell, ACS Energy Lett. 2021, 6, 3676–3685. Y. Wang, C. Wang, M. Li, Y. Yu, B. Zhang, Chem. Soc. Rev. 2021, 50, 6720–6733). Among basic chemical raw materials, urea is not only the most important nitrogen fertilizer for crop growth but also the key chemical raw material for producing important chemical products such as urea-melamine-formaldehyde resin, urea formaldehyde, and barbiturates. Currently, industrially, urea is synthesized from liquid ammonia and carbon dioxide under high pressure and high temperature conditions. However, the current urea synthesis scheme has problems of high energy input and high ammonia added value, far from meeting the requirements of sustainable development (Chen, C. He, N. & Wang, S. Electrocatalytic C–N coupling for urea synthesis. Small Sci. 1, 2100070 (2021)).

[0004] Currently, reducing the impact of urea on the ecological environment and potential threats to human health during production and use has become an important issue in the research field. Scientific researchers are committed to developing more green and efficient production technologies and exploring more convenient usage methods to achieve comprehensive improvements in pollution reduction and safety. And the electrocatalytic coupling technology that realizes the electrocatalytic coupling of carbon sources (CO 3 ) and nitrogen sources (nitrogen, nitrate, nitrite) by skipping the NH 2 ) synthesis process can achieve sustainable urea synthesis under normal temperature and pressure conditions, which can simultaneously solve the above two major problems. It can be seen that the electrocatalytic coupling technology is a very promising green alternative to traditional urea synthesis technology. However, the main obstacle to realizing industrial urea synthesis by electrocatalytic coupling technology is the lack of catalysts with high selectivity and high activity. Therefore, in order to industrialize urea synthesis by electrocatalytic coupling technology, the research on related catalysts is extremely urgent. Summary of the Invention

[0005] The present invention provides a novel holmium-based covalent organic framework material, which has excellent electrocatalytic activity for NO 3 - and electrocatalytic CO 2 reduction, and can electrocatalytically couple NO 3 - (as the N source) and CO 2 (as the C source) in water to directly and selectively synthesize urea, thereby solving the technical problem of the lack of catalysts with high selectivity and high activity in the industrial urea synthesis process by existing electrocatalytic coupling technologies.

[0006] The technical solution adopted in the present invention is as follows:

[0007] 1. One of the objects of the present invention is to provide a holmium-based covalent organic framework material, abbreviated as HO-COF-30. HO-COF-30 is prepared by a solvothermal method from 1,3,5-triformylphloroglucinol, p-phenylenediamine and HO-NH 2 . Among them, the molar ratio of 1,3,5-triformylphloroglucinol to HO-NH 2 is 1:0.3. The structural formula of HO-NH 2 is shown in Formula 1, and the unit structure of HO-COF-30 is shown in Formula 2;

[0008]

[0009] Another object of the present invention is to provide a preparation method of the above-mentioned holmium-based covalent organic framework material, including the following steps:

[0010] (1) First, prepare Ho-NH 2 according to the reaction formula shown in Formula 3;

[0011]

[0012] (2) Then, prepare HO-COF-30 according to the reaction formula shown in Formula 4;

[0013]

[0014] Further, the specific operation of step (1) is as follows: HoCl with a molar ratio of 1:3 3 ·6H 2O and 4-aminophenylacetic acid are dissolved in a mixed solution of N,N-dimethylacetamide (DMAC) and water, stirred at room temperature for 4 to 10 hours. After the reaction is completed, NaOH solution is added to adjust the pH to neutral, and the solid precipitates from the reaction solution. Then the obtained solid is collected by filtration, washed several times with distilled water and N,N-dimethylformamide respectively, and finally the obtained product is dried under vacuum at 40 to 80 °C to obtain Ho-NH 2 。

[0015] Furthermore, the mixed solution of N,N-dimethylacetamide and water is composed of N,N-dimethylacetamide and water mixed in a volume ratio of 4:1.

[0016] Furthermore, the concentration of the NaOH solution is 0.1 M.

[0017] Further, the specific operation of step (2) is as follows: 1,3,5-triformylphloroglucinol, p-phenylenediamine and HO-NH with a molar ratio of 1:1.05:0.3 2 are placed in a Pyrex tube, and a mixed solution of mesitylene, N,N-dimethylacetamide and acetic acid is added. Then the mixture is ultrasonically treated to mix the mixture, quickly frozen at 77 K, and degassed in three freeze-pump cycles. Finally, the Pyrex tube is sealed and heated at 100 to 150 °C for 72 hours. After the reaction is completed, the mixed solution is allowed to cool naturally to room temperature, the precipitate is collected by centrifugation, and then washed several times with N,N-dimethylformamide and acetone. The collected precipitate is dried at 90 to 120 °C to obtain HO-COF-30.

[0018] Furthermore, in the mixed solution of mesitylene, N,N-dimethylacetamide and acetic acid, the volume ratio of mesitylene, N,N-dimethylacetamide and acetic acid is 3:3:1.

[0019] The third object of the present invention is to disclose the application of the holmium-based covalent organic framework material as a catalyst in the reaction process of electrocatalytic coupling of NO 3 - and CO 2 to synthesize urea.

[0020] Further, the reaction process of electrocatalytic coupling of NO 3 - and CO 2 to synthesize urea is as follows: HO-COF-30 is coated on carbon paper, and the prepared HO-COF-30 modified carbon paper electrode is used as the working electrode. The working electrode, reference electrode and counter electrode are separated by a Nafion 117 membrane and placed in a flow cell containing a mixed electrolytic solution of 0.1 M potassium bicarbonate and 0.1 M potassium nitrate. A voltage is applied through an electrochemical workstation, and high-purity CO2 Continuously introduced into the cathode chamber, electrocatalytic CO can be achieved under the condition of a pulse of 5 s - 10 s (react for 5 s and pause for 10 s). 2 and NO 3 - to co-reduce and synthesize urea products together.

[0021] Furthermore, the preparation process of the HO-COF-30 modified carbon paper electrode is as follows: Take 6 mg of HO-COF-30, 3 mg of 200 nm polytetrafluoroethylene, and add 12 μL of a 5 wt% Nafion solution, 500 uL of deionized water, and 500 μL of isopropanol. Ultrasonic for 1 h, take 25 μL of the mixed solution and drop it onto the surface of a carbon paper electrode with an area of 1 cm × 1 cm, and let it dry naturally to obtain the HO-COF-30 modified carbon paper electrode.

[0022] In summary, compared with the prior art, the present invention has the following advantages and beneficial effects:

[0023] 1. The rare earth covalent organic framework (MCOFs) coordination template successfully synthesized by the present invention through a three-in-one strategy has a composable porosity, adjustable active site structure and functional characteristics, enabling it to expose more active sites during the electrocatalytic synthesis of urea, and having good and appropriate adsorption properties for NO 3 - and CO 2 at the same time, and being more capable of promoting proton transport. High-efficiency catalysis of CO can be achieved at high Faraday efficiency under normal temperature and pressure 2 and NO 3 - to be reduced to form urea. Experiments show that under the optimal pulse conditions, the yield of synthesized urea is as high as 1792 μg h -1 mg cat. -1 , about four times that of the COF yield of the HO metal node complex without it, and the stability is as high as 40 h. This invention saves costs, turns waste into treasure, is simple to operate, and has good commercial value and application prospects;

[0024] 2. The HO-COF-30 provided by the present invention can adsorb more nitrate ions during the electrochemical reduction of CO 2 and NO 3 - to produce urea by using the pulse potential method, which is helpful for the forward reaction of the electrochemical reduction of CO 2 and NO 3 - to produce urea.

[0025] 3. The present invention prepared a covalent organic framework containing a rare earth node complex of Ho by a solvothermal method, and established a preparation method of Ho-COF-30. Description of the Drawings

[0026] Figure 1 is Ho-NH 2 High-resolution mass spectrometry diagram in positive ion mode, where a is the mass spectrometry diagram and b is the structural formula of Ho-NH 2 .

[0027] Figure 2 is the solid-state 13 C CP / MAS NMR spectrogram of Ho-COF-30, where a is the spectrogram and b is the schematic diagram of the topological structure formed by 1,3,5-triformylphloroglucinol in Ho-COF-30

[0028] Figure 3 is HoCl 3 ·6H 2 O, 4-aminophenylacetic acid (C 8 H 9 NO 2 ), Ho-NH 2 and the FT-IR spectrogram of Ho–COF-30

[0029] Figure 4 is the powder X-ray diffraction (XRD) pattern of each sample

[0030] Figure 5 is the SEM image of Ho–COF-30

[0031] Figure 6 is the TEM image of Ho–COF-30

[0032] Figure 7 is the nitrogen adsorption / desorption isotherm diagram

[0033] Figure 8 is the linear sweep voltammogram (LSV) of Ho-COF-30 in a mixed electrolyte of 0.1 M KHCO 2 and 0.1 M KNO 3 saturated with CO 3

[0034] Figure 9 is the self-made flow water tank diagram for testing

[0035] Figure 10 is the relationship diagram between the concentration of NH 3 standard solution and the light absorption value at a wavelength of 655 nm, where (a) is the ultraviolet-visible absorption spectrum of NH 3 standard solutions with different concentrations. (b) is the linear relationship diagram between the light absorption value at a wavelength of 655 nm and the concentration of NH 3 standard solution ​

[0036] Figure 11 are the impedance diagrams of Ho-COF-30, Ho-COF-0 and Ho-NH 2 .

[0037] Figure 12 are the active area diagrams of Ho-COF-30, Ho-COF-0 and Ho-NH 2 .

[0038] Figure 13 are the bar graphs of urea production rates of Ho-COF-30, Ho-NH 2 and Ho-COF-0 under different pulses.

[0039] Figure 14 are the line graphs of Faraday efficiencies of Ho-COF-30, Ho-NH 2 and Ho-COF-0 under different pulses. Detailed implementation manners

[0040] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in combination with various embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0041] The special term "embodiment" here, any embodiment described as "exemplary" does not have to be interpreted as superior to or better than other embodiments. For the performance index tests in the embodiments of the present method, unless otherwise specified, the conventional test methods in the art are adopted. The terms described in the present invention are only used to describe specific embodiments and are not used to limit the content disclosed in the present invention.

[0042] Unless otherwise specified, the technical and scientific terms used herein have the same meanings as those commonly understood by those of ordinary skill in the technical field to which the present invention belongs; the other raw materials, reagents, test methods and technical means not specifically noted in the present invention refer to the raw materials and reagents commonly used by those of ordinary skill in the art, as well as the experimental methods and technical means commonly adopted.

[0043] Embodiment 1

[0044] This embodiment discloses a holmium-based covalent organic framework material and its preparation process. The preparation process is as follows:

[0045] (1) As shown in Formula 5, prepare Ho-NH 2 : Add HoCl 3 ·6H 2O (379.38 mg, 1.000 mmol) and 4-aminophenylacetic acid (453.51 mg, 3.000 mmol) were dissolved in 5 mL of a solution prepared by mixing N,N-dimethylacetamide (DMAC) and water in a volume ratio of 4:1, and stirred at room temperature for 5 hours. After the reaction was completed, 0.1 M NaOH solution was added to adjust the pH to neutral, causing the solid to precipitate from the reaction solution. The resulting solid was collected by filtration and washed three times with distilled water and dimethylformamide (DMF) respectively. Finally, the product was dried in vacuo at 50 °C for 12 hours to obtain Ho-NH 2 ;

[0046]

[0047] (2) As shown in Formula 6, the HO-COF-30 material was prepared: 0.06 mmol of 1,3,5-triformylphloroglucinol, 0.063 mmol of p-phenylenediamine and 0.018 mmol of HO-NH 2 were combined and then 1.2 mL of m-xylene, 1.2 mL of N,N-dimethylacetamide, and 0.4 mL of acetic acid were added. First, they were ultrasonically mixed, then quickly frozen at 77 K and degassed through three freeze-pump cycles, and finally heated at 120 °C for 72 hours. After the reaction, it was naturally cooled to room temperature, and the solid product was collected by filtration, washed three times with N,N-dimethylformamide and acetone, and then dried in vacuo at 100 °C to obtain the HO-COF-30 material;

[0048]

[0049] Combined as Figure 1 described high-resolution mass spectrum and as Figure 3 shown Fourier transform infrared (FT-IR) spectrum, it can be found that the calculated value (m / z) of its [M+H] + for Ho-NH 2 was 496.40132; the measured value was 496.14245, proving the successful synthesis of the Ho-NH 2 monomer.

[0050] The crystal structure of HO-COF-30 was characterized by X-ray diffraction method (XRD). From the XRD ( Figure 4 ), it was shown that HO-COF-30 is an MCOF material; further, the synthesis of Ho-COF-30 was proven by the solid-state Figure 2 C CP / MAS NMR spectrum. The morphology of HO-COF-30 was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the results were as 13 shown in Figure 5 and Figure 6As shown, it can be seen from the SEM image and TEM image that the morphology of HO-COF-30 is a mixture of needle shape and particle shape; through Figure 7 it can be seen that its BET is about 340 m 2 / g. This indicates that HO-COF-30 with ultra-high electrocatalytic activity was successfully prepared in this example.

[0051] Example 2

[0052] In this example, the HO-COF-30 prepared in Example 1 was used to conduct linear voltammetry tests on the electrocatalytic coupling reduction of CO 2 and KNO 3 for the synthesis of urea. The specific steps are as follows:

[0053] (1) A carbon paper electrode loaded with HO-COF-30 with a loading of 0.12 mg·cm -2 was used as the working electrode. When preparing the working electrode, 6 mg of HO-COF-30 catalyst and 3 mg of 200-nm polytetrafluoroethylene nanoparticles were added to the mixed solution (500 μL of distilled water, 500 μL of isopropanol, and 12 μL of 5 wt% Nafion), and ultrasonic treatment was carried out for 1 hour. 25 μL of the mixed solution was taken and dropped onto the surface of a 1 cm × 1 cm carbon paper electrode, and it was left to dry naturally to prepare a HO-COF-30 modified carbon paper working electrode for standby.

[0054] (2) A Pt sheet was used as the counter electrode, and a saturated Ag / AgCl electrode was used as the reference electrode. In an H-type electrolytic cell, the cathode chamber and the anode chamber were separated by a Nafion 117 membrane. In this example, the linear voltammetry curves of HO-COF-30 in 0.1 M KHCO 2 and 0.1 M KNO 3 mixed electrolyte saturated with argon atmosphere and CO 3 atmosphere, as well as in 0.1 M KHCO 3 saturated with argon atmosphere, were first studied. Electrochemical tests were carried out in a three-electrode H-type electrolytic cell. Before the test, the electrolyte in the cathode part was pre-saturated with the corresponding gas. Polarization curves were obtained by linear sweep voltammetry, and the scanning rate was 5 mV s -1 , as Figure 8 shown, in the case of containing NO 3 - , regardless of whether the gas environment is Ar or CO 2 , the current density of the system was significantly increased relative to the reversible hydrogen electrode (RHE), indicating that an obvious electrocatalytic reduction reaction occurred.

[0055] Example 3

[0056] This example further explored the performance of HO-COF-30 prepared in Example 1 in the electrocatalytic coupling reduction of CO 2 and KNO 3 during the synthesis of urea. To better demonstrate the excellent performance of HO-COF-30, we used Ho-NH 2 and Ho-COF-0 (without Ho element, that is, in the process of preparing HO-COF-30 in Example 1, Ho-NH 2 was not added, and directly used 0.06 mmol of 1,3,5-triformylphloroglucinol and 0.063 mmol of p-phenylenediamine, prepared under the same preparation steps and reaction conditions) as a comparison. The specific operation is as follows:

[0057] (1) Add 20 mg of HO-COF-30 catalyst, 10 mg of 200 nm polytetrafluoroethylene nanoparticles, and add 60 μL of Nafion solution (5 wt%), 2.5 mL of deionized water, and 2.5 mL of isopropanol. Ultrasonic for 1 h, take 25 μL of the above mixture and drop it onto the surface of a 1 cm×1 cm carbon paper electrode, and dry it naturally to make a HO-COF-30 modified carbon paper electrode. Using the HO-COF-30 modified carbon paper as the working electrode, the Hg / HgO electrode as the reference electrode, and the platinum mesh as the counter electrode, place the working electrode and the reference electrode in a self-made flow electrolytic cell ( Figure 9 ) cathode chamber containing a mixed electrolyte solution of 0.1 M KHCO 3 solution and 0.1 M KNO 3 (as the N source), place the counter electrode in the anode chamber of the flow cell containing a mixed electrolyte solution of 0.1 M KHCO 3 solution and 0.1 M KNO 3 . Separate the cathode chamber and the anode chamber of the flow cell electrolytic cell with a Nafion 117 membrane. Apply a corresponding pulsed potential of 5 s - 10 s (react for 5 s, pause for 10 s) on the HO-COF-30 modified carbon paper electrode through an electrochemical workstation. Continuously introduce high-purity CO 2 (as the C source) into the cathode chamber, and set the peristaltic pump to 60% of the total power. The flow rate of CO 2 is 60 mL·min -1 . The reaction time is 20 minutes. Collect the electrolyzed products. For the quantification of urea in the electrolyzed products, the urease decomposition method is used (the urease decomposition method is a reliable urea quantification method and is not interfered by coexisting by-products. Therefore, this example uses the urease decomposition method, which is suitable for research in this field. Briefly, add 0.2 mL of urease solution with a concentration of 0.5 mg·mL -1 to 1.8 mL of urea electrolyte solution, and then react in a 37 °C constant temperature water bath for 90 minutes. Urea ((NH 2 ) 2CO is decomposed by urease into CO 2 and two molecules of NH 3 . After decomposition, the concentration of NH 3 in the urea electrolyte containing urease is detected by the indophenol blue method. At the same time, the concentration of NH 3 in the urea electrolyte without urease is also quantified by the indophenol blue method. The total number of moles of ammonia (m urease ) in the electrolyte is measured by a spectrophotometer and expressed as 2n urea +n ammonia , where 2n urea represents the number of moles of ammonia from urea decomposition. Therefore, the number of moles of urea generated (n urea ) can be calculated by the formula (m urease -n ammonia ) / 2, Figure 10 shows the ultraviolet-visible absorption spectra of NH 3 standard solutions with different concentrations and the linear relationship between the light absorption value at a wavelength of 655 nm and the concentration of the NH 3 standard solution. It can be seen that there is an obvious linear relationship between the light absorption value at a wavelength of 655 nm and the concentration of the NH 3 standard solution, indicating that the quantitative result of urea in the electrolysis product by the urease decomposition method in this example is reliable).

[0058] (2) To demonstrate the optimal performance of HO-COF-30, we optimized the pulse time and the CO 2 ventilation rate. The efficient transport of NO 3 - is crucial for improving the efficiency of the entire catalytic process. The charge transfer resistance of each catalyst was evaluated using electrochemical impedance spectroscopy (EIS). It can be seen from the impedance diagram ( Figure 11 ) that the interfacial charge transfer resistance of Ho-COF-30 is significantly smaller than that of Ho-COF-0 and Ho-NH 2 . This indicates that Ho-COF-30 can significantly accelerate the charge transfer process during the electro-synthesis of urea. Figure 12 shows that the Cdl of HO-COF-30 reaches 0.0342 mF cm -2 , which is better than that of Ho-COF-0 (0.0173 mF cm -2 ) and Ho-NH 2 (0.021 mF cm -2 ), indicating that Ho-COF-30 has the largest ECSA and provides more active sites for the reaction. Through the systematic optimization of the electrocatalytic urea synthesis process, the optimal experimental conditions were finally determined, including the CO 2 concentration (45 mL / min), the peristaltic pump flow rate (60%), the pulse interval time tocp = 10 s, pulse potential time t E = 5 s, and nitrate concentration (0.1 mol / L), Figure 13 is the urea yield corresponding to Ho-COF-30 at a potential of -1.0 (V vs. RHE) under different voltages. To highlight the advantages of this catalyst, through Ho-COF-0 (without Ho active sites) and Ho-NH 2 Comparing the catalysts shows that Ho-COF-30 has the highest yield. And it is four times that of Ho-COF-0. Figure 14 is the corresponding Faraday efficiency diagram. Obviously, Ho-COF-30 has the highest Faraday efficiency.

[0059] In summary, the HO-COF-30 with ultra-high electrocatalytic activity prepared by the method of the present invention can be used as a good electrocatalyst to achieve high-yield electrocatalytic reduction of CO 2 and NO 3 - to synthesize urea. The present invention shows that under the optimal conditions, HO-COF-30 has a urea synthesis yield as high as 1792 μg·h -1 .mg cat -1 The results show that the ultra-high electrocatalytic activity HO-COF-30 has good application prospects in electrocatalytic urea synthesis.

[0060] The above embodiments only represent the specific implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation to the protection scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the technical solution of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application.

Claims

1. A holmium-based covalent organic framework material, characterized in that: The holmium-based covalent organic framework is referred to as HO-COF-30, and HO-COF-30 is prepared from 1,3,5-triformylphloroglucinol, p-phenylenediamine and HO-NH2 by a solvothermal method, wherein the molar ratio of 1,3,5-triformylphloroglucinol to HO-NH2 is 1:0.3, the structural formula of HO-NH2 is shown in Formula 1, and the unit structure of HO-COF-30 is shown in Formula 2; 2. The method for preparing a holmium-based covalent organic framework material according to claim 1, characterized in that: The steps include: (1) First, prepare Ho-NH2 according to the reaction formula described in Formula 3; (2) HO-COF-30 is obtained according to the reaction formula described in formula 4; 3. The method for preparing a holmium-based covalent organic framework material according to claim 2, characterized in that: The specific operation of step (1) is as follows: dissolving HoCl3·6H2O and 4-aminophenylacetic acid in a molar ratio of 1:3 in a mixture of N,N-dimethylacetamide and water, stirring at room temperature for 4 to 10 hours, and after the reaction is completed, adding NaOH solution to adjust the pH to neutral, so that a solid is precipitated from the reaction solution; collecting the obtained solid by filtration, and washing it with distilled water and N,N-dimethylformamide for multiple times, respectively, and finally drying the obtained product in vacuum at 40 to 80°C to obtain Ho-NH2.

4. The method for preparing a holmium-based covalent organic framework material according to claim 3, characterized in that: The mixed liquid of N,N-dimethylacetamide and water is prepared by mixing N,N-dimethylacetamide and water in a volume ratio of 4:

1.

5. The method for preparing a holmium-based covalent organic framework material according to claim 3, characterized in that: The concentration of the NaOH solution is 0.1M.

6. The method for preparing a holmium-based covalent organic framework material according to claim 2, characterized in that: The specific operation of the step (2) is as follows: 1,3,5-triformylphloroglucinol, p-phenylenediamine and HO-NH2 in a molar ratio of 1:1.05:0.3 are placed in a Pyrex tube, and a mixed solution of mesitylene, N,N-dimethylacetamide and acetic acid is added; then the mixture is mixed by ultrasonic treatment, and then rapidly frozen at 77K, and degassed in three freezing-evacuation cycles; finally, the Pyrex tube is sealed, and heated at 100-150°C for reaction for 72 hours. After the reaction is completed, the mixed solution is naturally cooled to room temperature, and the precipitate is collected by centrifugation, and then washed with N,N-dimethylformamide and acetone for multiple times, and the collected precipitate is dried at 90-120°C to obtain HO-COF-30.

7. The method for preparing a holmium-based covalent organic framework material according to claim 6, characterized in that: In the mixed solution of mesitylene, N,N-dimethylacetamide and acetic acid, the volume ratio of mesitylene, N,N-dimethylacetamide and acetic acid is 3:3:

1.

8. The holmium-based covalent organic framework material according to claim 1 is used for electrocatalytic coupling of NO3 - It is used as a catalyst in the process of synthesizing urea from CO2.

9. The use according to claim 8, characterized in that The electrocatalytic coupling of NO3 - The reaction process of synthesizing urea with CO2 is as follows: HO-COF-30 is coated on carbon paper, and the prepared HO-COF-30 modified carbon paper electrode is used as the working electrode. The working electrode, reference electrode and counter electrode are separated by Nafion 117 membrane and placed in a flow cell containing a mixed electrolytic solution of 0.1M potassium bicarbonate and 0.1M potassium nitrate. Voltage is applied through an electrochemical workstation, and high-purity CO2 is continuously introduced into the cathode chamber. Electrocatalysis of CO2 and NO3 can be achieved under pulse conditions of 5s-10s. - Co-reduction to synthesize urea product.

10. The use according to claim 9, characterized in that The preparation process of the HO-COF-30 modified carbon paper electrode is as follows: 6 mg of HO-COF-30 and 3 mg of 200 nm polytetrafluoroethylene are taken, and 12 μL of 5 wt% Nafion solution, 500 uL of deionized water and 500 μL of isopropanol are added, ultrasonicated for 1 h, 25 μL of the mixed solution is added dropwise to the surface of a carbon paper electrode with an area of ​​1 cm×1 cm, and dried naturally to prepare a HO-COF-30 modified carbon paper electrode.

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