High-conjugation covalent organic framework as well as preparation method and application thereof

Abstract

The invention provides a high-conjugation covalent organic framework and a preparation method and application thereof.The high-conjugation covalent organic framework has the structure shown in the formula I. According to the high-conjugation covalent organic framework and the preparation method and application thereof, a compound Ni-HB-viCOF is subjected to a Scholl reaction under the catalytic action of ferric chloride, a hexaphenylbenzene unit in COF can be converted into a hexabenzene-hexabenzene unit, and the compound Ni-cHB-viCOF is obtained; and the conjugacy is obviously enhanced, so that the catalytic efficiency of the composite material serving as a catalyst in the process of photocatalytic reduction of CO2 is obviously improved. The material has the characteristics of rich active sites, good porosity and good chemical stability, can be recycled for multiple times, and is a good photocatalyst for CO2 reduction.

Description

Technical Field

[0001] The present invention belongs to the technical field of porous organic polymer materials, and particularly relates to a highly conjugated covalent organic framework and its preparation method and application. Background Art

[0002] Due to the rising sea level and the reduction of biodiversity caused by global warming, people's daily lives have been affected. In order to adhere to the principle of sustainable development, reducing greenhouse gas emissions and controlling climate warming to mitigate the greenhouse effect have become the consensus of the international community. Therefore, developing highly efficient and stable photocatalysts to convert carbon dioxide gas into utilizable resources has become the focus of people's attention.

[0003] Porous organic polymer materials have attracted extensive attention due to their high specific surface area and recyclability. In the development process of porous organic polymers, covalent organic frameworks (COFs) have attracted the attention of many researchers with their unique ordered structures. The regular combination and arrangement of molecules have laid a good foundation for the depth and breadth of scientific research. Currently, COF materials have been widely used in the fields of catalysis, adsorption, medicine, energy, etc. Among COF materials, sp 2 Carbon-conjugated COFs have received extensive attention due to their excellent semiconductor properties. Such materials often have a narrow bandgap width, which can increase the light absorption range. At the same time, as non-polar covalent bonds, carbon-carbon double bonds have good carrier transport ability and chemical stability, which are very compatible with the requirements of the photocatalysis field. Therefore, sp 2 Carbon-conjugated COFs are increasingly used in the field of photocatalysis.

[0004] Therefore, it is urgent to develop a highly conjugated covalent organic framework material for photocatalytic reduction of CO2. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a highly conjugated covalent organic framework and its preparation method and application, and the catalytic efficiency of the highly conjugated covalent organic framework in the process of photocatalytic reduction of CO2 as a catalyst is significantly improved. The highly conjugated covalent organic framework has the characteristics of rich active sites, good porosity and good chemical stability, and can be recycled multiple times.

[0006] To achieve the purpose of this invention, the following technical solutions are adopted:

[0007] In the first aspect, the present invention provides a highly conjugated covalent organic framework, and the highly conjugated covalent organic framework has a repeating unit with the structure shown in Formula I below:

[0008]

[0009] Among them, L includes Cl - 、CH3COO - or NO3 - 。

[0010] The highly conjugated covalent organic framework provided by the present invention has a large area of conjugated groups. The excellent conjugation provides a good electron transport platform and outstanding charge carrier transport efficiency for the framework material, which has a good synergistic effect with the catalytic activity of metal nickel ions. Nickel ions can achieve efficient adsorption and activation of carbon dioxide, thereby improving the conversion efficiency and selectivity of the photocatalytic reduction reaction. The highly conjugated covalent organic framework of the present invention has the characteristics of rich active sites, good porosity and good chemical stability, and can be recycled multiple times, and is a good photocatalyst for CO2 reduction.

[0011] In a second aspect, the present invention provides a preparation method of the highly conjugated covalent organic framework as described in the first aspect, and the preparation method includes the following steps;

[0012] (1) Hexa(4-formylphenyl)benzene and 2,2'-([2,2'-bipyridine]-5,5'-diyl)dinitrile are subjected to a condensation reaction to prepare a compound HB-viCOF, and the reaction formula is as follows:

[0013]

[0014] (2) The compound HB-viCOF is subjected to an oxidative coupling reaction to prepare a compound cHB-viCOF, and the reaction formula is as follows:

[0015]

[0016] (3) The compound cHB-viCOF reacts with a nickel salt to prepare a compound Ni-cHB-viCOF, and the reaction formula is as follows:

[0017]

[0018] Among them, L includes Cl - 、CH3COO - or NO3 - 。

[0019] Preferably, the solvothermal method is adopted. The covalent organic framework material is prepared by the Knoevenagel condensation reaction of hexakis(4-formylphenyl)benzene and 2,2'-([2,2'-bipyridine]-5,5'-diyl)diacetonitrile under the catalysis of 1,8-diazabicyclo[5.4.0]undec-7-ene to obtain the compound HB-viCOF; the hexaphenylbenzene unit in the obtained compound HB-viCOF is converted into a hexabenzocoronene unit through the Scholl reaction to obtain the compound cHB-viCOF; and then nickel metal ions are loaded into the COFs framework to obtain the compound Ni-cHB-viCOF.

[0020] Preferably, in step (1), the molar ratio of hexakis(4-formylphenyl) to 2,2'-([2,2'-bipyridine]-5,5'-diyl)diacetonitrile is 1:3.

[0021] Preferably, in step (1), the condensation reaction is carried out in the presence of a catalyst.

[0022] Preferably, in step (1), the catalyst includes a 1,8-diazabicyclo[5.4.0]undec-7-ene solution.

[0023] Preferably, in step (1), the concentration of the 1,8-diazabicyclo[5.4.0]undec-7-ene solution is 2-4 mol / L. For example, it can be 2 mol / L, 3 mol / L, or 4 mol / L, etc., and further preferably 3 mol / L.

[0024] Preferably, in step (1), the condensation reaction is carried out in the presence of a solvent.

[0025] Preferably, in step (1), the solvent includes a mixed solution of o-dichlorobenzene and n-butanol.

[0026] Preferably, in step (1), the volume ratio of o-dichlorobenzene to n-butanol is 7:(2-4); for example, it can be 7:2, 7:3, or 7:4, etc., and further preferably 7:3.

[0027] Preferably, in step (1), the temperature of the reaction is 120-150 °C; for example, it can be 120 °C, 125 °C, 130 °C, 135 °C, 140 °C, 145 °C, or 150 °C, etc.

[0028] Preferably, in step (1), the reaction time is 3-5 days; for example, it can be 3 days, 3.5 days, 4 days, 4.5 days, or 5 days, etc.

[0029] Preferably, the preparation method of step (1) specifically includes: adding hexakis(4-formylphenyl)benzene, 2,2'-([2,2'-bipyridine]-5,5'-diyl)diacetonitrile, and 1,8-diazabicyclo[5.4.0]undec-7-ene into a solution mixed with o-dichlorobenzene and n-butanol; performing freeze-thaw cycling degassing 3 times and then sealing the tube under vacuum, heating the reaction system in a sealed vacuum state from room temperature to 120 °C, keeping the temperature for 3 days and then cooling to room temperature, and preparing the compound HB-viCOF through suction filtration and drying.

[0030] Preferably, in step (2), the oxidative coupling reaction is carried out in a solvent.

[0031] Preferably, in step (2), the solvent includes a combination of nitromethane and halogenated alkane solvents.

[0032] Preferably, in step (2), the volume ratio of nitromethane to halogenated alkane solvents is 1:(15 - 25), for example, it can be 1:15, 1:20, or 1:25, etc.

[0033] Preferably, in step (2), the halogenated alkane solvents include dichloromethane and / or chloroform.

[0034] Preferably, in step (2), the solvent includes a combination of nitromethane and dichloromethane.

[0035] Preferably, in step (2), the oxidative coupling reaction is carried out in the presence of a catalyst.

[0036] Preferably, in step (2), the mass ratio of the catalyst to the compound HB-viCOF is 1:(0.5 - 1), for example, it can be 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, or 1:1, etc., and further preferably 1:0.7.

[0037] Preferably, in step (2), the catalyst includes iron chloride.

[0038] Preferably, in step (2), the reaction temperature is 30 - 60 °C; for example, it can be 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, or 60 °C, etc.

[0039] Preferably, in step (2), the reaction time is 1 - 4 h, for example, it can be 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, or 4 h, etc.

[0040] Preferably, the preparation method of step (2) specifically includes: adding compound HB-viCOF and iron chloride into a mixed solution of nitromethane and dichloromethane, reacting, filtering, washing with methanol and hydrochloric acid solution, and drying to obtain compound cHB-viCOF.

[0041] Preferably, in step (3), the reaction is carried out in a solvent.

[0042] Preferably, in step (3), the solvent includes methanol and water.

[0043] Preferably, in step (3), the volume ratio of methanol to water is 1:(0.5 - 2); for example, it can be 1:0.5, 1:1, 1:1.5, or 1:2.0, etc., and more preferably 1:1.

[0044] Preferably, in step (3), the mass ratio of the nickel salt to compound cHB-viCOF is 1:(0.5 - 1), for example, it can be 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, or 1:1, etc.

[0045] Preferably, in step (3), the nickel salt includes any one of nickel chloride hexahydrate, nickel acetate, or nickel nitrate.

[0046] Preferably, in step (3), the reaction temperature is 50 - 70 °C; for example, it can be 50 °C, 55 °C, 60 °C, 65 °C, or 70 °C, etc.

[0047] Preferably, in step (3), the reaction time is 20 - 30 h, for example, it can be 20 h, 22 h, 24 h, 26 h, 28 h, or 30 h, etc.

[0048] Preferably, the preparation method of step (3) specifically includes: adding compound cHB-viCOF and nickel chloride hexahydrate into a mixed solution of methanol and water, reacting, and then filtering the solid after the reaction, washing with methanol and acetone, and drying to obtain compound Ni-cHB-viCOF.

[0049] Preferably, in steps (1) - (3), after the reaction is completed, it also includes a post-treatment step respectively.

[0050] Preferably, the post-treatment includes filtering, washing, and drying.

[0051] Preferably, the drying is vacuum drying.

[0052] Preferably, the drying temperature is 50 - 70 °C; for example, it can be 50 °C, 55 °C, 60 °C, 65 °C, or 70 °C, etc.

[0053] Preferably, the drying time is 24 - 48 h. For example, it can be 24 h, 28 h, 32 h, 36 h, 40 h, 44 h, or 48 h, etc.

[0054] In a third aspect, the present invention provides an application of the highly conjugated covalent organic framework as described in the first aspect in photocatalytic reduction.

[0055] Preferably, the photocatalytic reduction includes photocatalytic reduction of carbon dioxide to carbon monoxide.

[0056] Preferably, the steps of the photocatalytic reduction are as follows: carbon dioxide undergoes a photocatalytic reduction reaction under the action of a catalyst and a light source to be converted into carbon monoxide.

[0057] Preferably, the catalyst includes the highly conjugated covalent organic framework as described in the first aspect.

[0058] Preferably, the light source includes a xenon lamp.

[0059] Preferably, the temperature of the photocatalytic reduction reaction is 5 - 25 °C. For example, it can be 5 °C, 10 °C, 15 °C, 20 °C, or 25 °C, etc.

[0060] Preferably, the time of the photocatalytic reduction reaction is 2 - 4 h. For example, it can be 2 h, 3 h, or 4 h, etc.

[0061] Preferably, the photocatalytic reduction reaction is carried out in the presence of a photosensitizer.

[0062] Preferably, the photosensitizer includes ruthenium(III) tris(2,2'-bipyridyl) hexahydrate.

[0063] Preferably, the photocatalytic reduction reaction is carried out in a solvent, and the solvent includes a mixed solution of deionized water, acetonitrile, and triethanolamine.

[0064] Preferably, the specific steps of the photocatalytic reduction reaction are as follows:

[0065] Put the conjugated covalent organic framework and ruthenium(III) tris(2,2'-bipyridyl) hexahydrate into a reactor, add a mixed solvent according to the volume ratio of deionized water, acetonitrile, and triethanolamine of 1:3:1, introduce carbon dioxide, use a 300 - watt xenon lamp as the light source, use a 420 - nm cut - off filter, pass 5 °C condensed water through the double - wall quartz reactor using a constant - temperature water bath, the reaction time is 4 h, and a quantitative sampling valve extracts the gas in the system every 30 min and transfers it to a gas chromatograph for quantitative analysis.

[0066] Compared with the prior art, the present invention has the following beneficial effects:

[0067] The highly conjugated covalent organic framework provided by the present invention has a large area of conjugated groups. The excellent conjugation provides a good electron transport platform and outstanding charge carrier transport efficiency for the framework material, which has a good synergistic effect with the catalytic activity of metal nickel ions. Nickel ions can achieve efficient adsorption and activation of carbon dioxide, thereby improving the conversion efficiency and selectivity of the photocatalytic reduction reaction. The highly conjugated covalent organic framework of the present invention has the characteristics of rich active sites, good porosity and good chemical stability, and can be recycled multiple times, which is a good photocatalyst for CO2 reduction. In addition, the method used in the present invention to enhance conjugation has the advantages of simplicity, feasibility and mild conditions, and is expected to be applied to most organic photocatalysts to help effectively improve the photocatalytic performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 Schematic diagram of the synthesis of the covalent organic framework Ni-cHB-viCOF described in Example 1;

[0069] Figure 2 PXRD patterns of the covalent organic frameworks HB-viCOF, cHB-viCOF, Ni-HB-viCOF and Ni-cHB-viCOF described in Example 1;

[0070] Figure 3 Nitrogen adsorption and desorption isotherm curves of the covalent organic framework Ni-cHB-viCOF described in Example 1;

[0071] Figure 4 N1s XPS spectrum of Ni-cHB-viCOF described in Example 1;

[0072] Figure 5 Photocurrent response diagrams of HB-viCOF, cHB-viCOF, Ni-HB-viCOF and Ni-cHB-viCOF described in Example 1;

[0073] Figure 6 Comparison diagram of CO2 photoreduction rates of HB-viCOF, cHB-viCOF, Ni-HB-viCOF and Ni-cHB-viCOF described in Example 1;

[0074] Figure 7 Schematic diagram of the selectivity of the photocatalytic process of Ni-cHB-viCOF described in Example 1;

[0075] Figure 8 Infrared comparison diagram of the materials before and after photocatalysis of Ni-cHB-viCOF described in Example 1;

[0076] Figure 9Photocatalytic cycle performance diagram of Ni-cHB-viCOF described in Example 1. Detailed implementation mode

[0077] The technical solution of the present invention will be further described below through specific implementation modes. Those skilled in the art should understand that the described embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0078] Example 1

[0079] This example provides a highly conjugated covalent organic framework Ni-cHB-viCOF. The preparation method of the highly conjugated covalent organic framework includes the following steps:

[0080] (1) Put hexakis(4-formylphenyl)benzene (0.0160 g, 0.0228 mmol), 2,2'-([2,2'-bipyridine]-5,5'-diyl)diacetonitrile (0.0160 g, 0.0683 mmol), o-DCB / n-BuOH (2 mL, 7 / 3 v / v) into a Pyrex tube (10 mL) and ultrasonicate for 3 minutes. Then, add 0.2 mL of 1,8-diazabicyclo[5.4.0]undec-7-ene (3M) to the above system, quickly freeze the Pyrex tube under liquid nitrogen (77K), and degas through three freeze-thaw cycles. Flame seal under vacuum conditions and heat at 120 °C for 5 days. After the reaction is completed, slowly cool the Pyrex tube, collect the precipitate by centrifugation, and wash it 3 times with THF and acetone respectively. The product is dried at 60 °C under vacuum for 24 h to obtain a yellow-green powder, which is the compound HB-viCOF.

[0081] (2) Add the compound HB-viCOF (0.1 g) and anhydrous ferric chloride (0.14 g, 20.0 equiv) to a 250 mL flask, add 20 mL of dichloromethane and 1 mL of nitromethane, evacuate and then introduce nitrogen. This process is repeated three times, and stir at 40 °C for 1 h. After the reaction is completed, repeatedly wash the obtained powder product with methanol and 0.5 M hydrochloric acid. The product is dried at 60 °C under vacuum for 24 h to obtain an orange-yellow powder, which is the compound cHB-viCOF.

[0082] (3) In a 10 mL reaction tube, add 30 mg of compound HB-viCOF, 2 mL of MeOH, and 2 mL of H2O. Ultrasonicate the suspension in an ultrasonic bath for 10 minutes. Then, add 51.8 mg (0.4 mmol) of nickel(II) chloride hexahydrate to the suspension and ultrasonicate the mixture in the ultrasonic bath for another 10 minutes. After sonication, immerse it in a preheated oil bath at 60 °C and stir for 24 h. After heating, remove the tube from the oil bath and cool it to room temperature. Wash the residue with a mixture of MeOH (5 mL) and H2O (5 mL). The product is dried at 60 °C under vacuum for 24 h to obtain a light green powder, which is compound Ni-HB-viCOF.

[0083]

[0084] (4) In a 10 mL reaction tube, add 30 mg of compound cHB-viCOF, 2 mL of MeOH, and 2 mL of H2O. Ultrasonicate the suspension in an ultrasonic bath for 10 minutes. Then, add 51.8 mg (0.4 mmol) of nickel(II) chloride hexahydrate to the suspension and ultrasonicate the mixture in the ultrasonic bath for another 10 minutes. After sonication, immerse it in a preheated oil bath at 60 °C and stir for 24 h. After heating, remove the tube from the oil bath and cool it to room temperature. Wash the residue with a mixture of MeOH (5 mL) and H2O (5 mL). The product is dried at 60 °C under vacuum for 24 h to obtain a light green powder, which is compound Ni-cHB-viCOF.

[0085] Material Characterization and Property Testing:

[0086] (1) PXRD Characterization: The test results are as Figure 2 shown. The test was carried out using a SAXS / WAXS X-ray diffractometer produced by Xenocs, France. In the test results of HB-viCOF, the sharp diffraction peak at 3.99° corresponds to the (100) plane, the peak at 8.22° corresponds to the (200) plane, and the broad diffraction peak around 17.03° corresponds to the (400) plane of HB-viCO. The Scholl reaction and the metal coordination reaction did not cause serious damage to the crystallinity of the COFs.

[0087] (2) Nitrogen Adsorption-Desorption Isotherm and Specific Surface Area Testing: The test was carried out using a 3-flex three-station full-function multi-purpose gas adsorption instrument produced by Micromeritics, USA. The test results are as Figure 3As shown, the nitrogen adsorption-desorption isotherm was measured at 77 K to calculate the specific surface area (SBET) value of the sample. The specific calculation results were based on the Brunauer–Emmett–Teller theory, and the specific surface area of Ni-cHB-viCOF was calculated to be 113 m 2 g -1 .

[0088] (3) X-ray photoelectron spectroscopy (XPS) analysis: The test was carried out using a Xeuss SAXS / WAXS X-ray diffractometer produced by Xenocs Company in France. The test results are as Figure 4 shown. In the N1s XPS spectrum of Ni-cHB-viCOF, three characteristic peaks can be observed. The characteristic peak at 399.1 eV corresponds to pyridine nitrogen, the characteristic peak at 399.7 eV corresponds to cyano nitrogen, and the characteristic peak at 400.1 eV corresponds to the coordination bond between pyridine nitrogen and nickel ions.

[0089] (4) Photocurrent test: The test results are as Figure 5 shown. A three-electrode system was adopted. Indium tin oxide (ITO) coated with COFs powder was used as the working electrode, a platinum sheet was used as the counter electrode, and Ag / AgCl was used as the reference electrode. The electrolyte used was a Na2SO4 solution (0.2 M). The photocurrent response intensity of Ni-cHB-viCOF was much higher than that of the other three comparative materials.

[0090] (5) Photocatalytic carbon dioxide reduction rate test: The test results are as Figure 6 shown. A 300-watt xenon lamp was used as the light source in the photocatalytic process, and a 420 nm cut-off filter was used. During the photocatalytic reaction process, the double-walled quartz reactor was passed through with 5°C condensed water by using a constant temperature water bath. The rate of photocatalytic reduction of CO2 to CO by Ni-cHB-viCOF was much higher than that of the other three comparative materials.

[0091] (6) Photocatalytic carbon dioxide reduction selectivity test: The test results are as Figure 7 shown. The by-product of Ni-cHB-viCOF during the photocatalytic reduction of CO2 is H2, and the selectivity of CO compared to H2 is as high as 90%.

[0092] (7) Infrared spectrum characterization: The test was carried out using a Perkin-Elmer Spectrum One type infrared spectrometer produced by Perkin-Elmer Instruments Co., Ltd. in the United States. The test results are as Figure 8 shown. Fourier transform infrared (FT-IR) spectroscopy tests (KBr pellet sample preparation) were carried out on Ni-cHB-viCOF before and after catalysis. The infrared spectrum data remained good, indicating that the material has good chemical stability.

[0093] (8) Cyclic stability test: The test results are as Figure 8 shown. The compound Ni-cHB-viCOF that has completed the photocatalytic process is washed with acetone and dried, and then put into the next photocatalytic experiment to verify its cyclic stability. After four cyclic experiments, the rate of photocatalytic CO generation remains basically unchanged, indicating that the material has good cyclic stability.

[0094] The applicant declares that the present invention uses the above embodiments to illustrate the highly conjugated covalent organic framework and its preparation method and application of the present invention, but the present invention is not limited to the above embodiments, that is, it does not mean that the present invention must rely on the above embodiments to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent replacement of the raw materials selected for the present invention, the addition of auxiliary components, and the selection of specific methods, etc., all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A highly conjugated covalent organic framework, characterized in that, The high-conjugated covalent organic framework has a repeating unit with the structure shown in Formula I below: Among them, L includes Cl - , CH3COO - or NO3 - .

2. A method for preparing the highly conjugated covalent organic framework according to claim 1, characterized in that, The preparation method includes the following steps; (1) Hexa(4-formylphenyl)benzene and 2,2'-([2,2'-bipyridine]-5,5'-diyl)diacetonitrile are subjected to a condensation reaction to prepare a compound HB-viCOF, and the reaction formula is shown below: (2) The compound HB-viCOF is subjected to an oxidative coupling reaction to prepare a compound cHB-viCOF, and the reaction formula is shown below: (3) The compound cHB-viCOF reacts with a nickel salt to prepare a compound Ni-cHB-viCOF, and the reaction formula is shown below: Among them, L includes Cl - , CH3COO - or NO3 - .

3. The preparation method according to claim 2, wherein, In step (1), the molar ratio of hexa(4-formylphenyl)benzene to 2,2'-([2,2'-bipyridine]-5,5'-diyl)diacetonitrile is 1:3; Preferably, in step (1), the condensation reaction is carried out in the presence of a catalyst; Preferably, in step (1), the catalyst includes a 1,8-diazabicyclo[5.4.0]undec-7-ene solution; Preferably, in step (1), the concentration of the 1,8-diazabicyclo[5.4.0]undec-7-ene solution is 2-4 mol / L, and more preferably 3 mol / L; Preferably, in step (1), the condensation reaction is carried out in the presence of a solvent; Preferably, in step (1), the solvent includes a mixed solution of o-dichlorobenzene and n-butanol; Preferably, in step (1), the volume ratio of o-dichlorobenzene to n-butanol is 7:(2-4), and more preferably 7:3; Preferably, in step (1), the temperature of the reaction is 120-150 °C; Preferably, in step (1), the reaction time is 3-5 days.

4. The preparation method according to claim 2 or 3, characterized in that, In step (2), the oxidative coupling reaction is carried out in a solvent; Preferably, in step (2), the solvent includes a combination of nitromethane and a halogenated alkane solvent; Preferably, in step (2), the volume ratio of nitromethane to the halogenated alkane solvent is 1:(15-25); Preferably, in step (2), the halogenated alkane solvent includes dichloromethane and / or chloroform; Preferably, in step (2), the solvent includes a combination of nitromethane and dichloromethane; Preferably, in step (2), the oxidative coupling reaction is carried out in the presence of a catalyst; Preferably, in step (2), the mass ratio of the catalyst to the compound HB-viCOF is 1:(0.5-1), and more preferably 1:0.7; Preferably, in step (2), the catalyst includes ferric chloride; Preferably, in step (2), the temperature of the reaction is 30-60 °C; Preferably, in step (2), the reaction time is 1-4 h.

5. The preparation method according to any one of claims 2-4, characterized in that, In step (3), the reaction is carried out in a solvent; Preferably, in step (3), the solvent includes methanol and water; Preferably, in step (3), the volume ratio of methanol to water is 1:(0.5-2); more preferably 1:1; Preferably, in step (3), the mass ratio of the nickel salt to the compound cHB-viCOF is 1:(0.5-1); Preferably, in step (3), the nickel salt includes any one of nickel chloride hexahydrate, nickel acetate or nickel nitrate; Preferably, in step (3), the temperature of the reaction is 50-70 °C; Preferably, in step (3), the reaction time is 20-30 h.

6. The preparation method according to any one of claims 2-5, characterized in that, In steps (1)-(3), after the reaction is completed, a post-treatment step is further included respectively; Preferably, the post-treatment includes filtration, washing and drying; Preferably, the drying is vacuum drying; Preferably, the drying temperature is 50-70 °C; Preferably, the drying time is 24-48 h.

7. An application of the highly conjugated covalent organic framework according to claim 1 in photocatalytic reduction; Preferably, the photocatalytic reduction includes photocatalytic reduction of carbon dioxide to carbon monoxide.

8. The application according to claim 7, wherein The steps of the photocatalytic reduction are as follows: Carbon dioxide undergoes a photocatalytic reduction reaction under the action of a catalyst and a light source to be converted into carbon monoxide; The catalyst includes the highly conjugated covalent organic framework according to claim 1.

9. The application according to claim 7 or 8, characterized in that The light source includes a xenon lamp; Preferably, the temperature of the photocatalytic reduction reaction is 5-25 °C; Preferably, the reaction time of the photocatalytic reduction is 2-4 h.

10. The application according to any one of claims 7-9, characterized in that, The photocatalytic reduction reaction is carried out in the presence of a photosensitizer; Preferably, the photosensitizer includes ruthenium terpyridine hexahydrate; Preferably, the photocatalytic reduction reaction is carried out in a solvent, and the solvent includes a mixed solution of deionized water, acetonitrile and triethanolamine.

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