COF photocatalyst based on accurate regulation and control of benzene ring nitrogen sites as well as preparation method and application of COF photocatalyst

Abstract

The invention discloses a COF (Covalent Organic Framework) photocatalyst based on accurate regulation and control of benzene ring nitrogen sites as well as a preparation method and application thereof, and belongs to the technical field of COF materials. According to the preparation method, toxic reagents are not needed, high repeatability of a COF structure is achieved by accurately controlling monomer substitution positions, photocatalytic activity and stability in an acid environment are remarkably improved by designing ortho-N-substituted, meta-N-substituted and para-N-substituted COF materials, especially the photocatalytic hydrogen peroxide production rate of ortho-N-substituted oN-COF reaches 3015 [mu] mol.g <-1 >. H <-1 >, and the photocatalytic activity and stability in the acid environment are remarkably improved. The activity is obviously higher than that in the prior art (1.8 mmol.g <-1 >. H <-1 >, namely 1800 mu mol.g <-1 >. H <-1 > reported by Zhang and the like), and the activity is kept 92% after five times of circulation. The process is green and environment-friendly, and is suitable for large-scale production.

Description

Technical Field

[0001] The present invention belongs to the technical field of covalent organic framework (COF) materials, and particularly relates to a COF photocatalyst based on precise regulation of benzene ring nitrogen sites, a preparation method thereof, and an application thereof. Background Art

[0002] Photocatalysis technology has become an important way to solve energy crises and environmental pollution because it can convert solar energy into chemical energy. As the core of photocatalysis technology, the performance of photocatalysts directly affects the photocatalytic efficiency. Traditional inorganic semiconductor materials such as titanium dioxide have problems such as a narrow light response range, low solar energy utilization rate, and fast recombination of photo-generated carriers, which limit the further development of photocatalysis technology.

[0003] Since the report of covalent organic framework (COF) materials, they have become a research hotspot in the field of photocatalysis due to their designable pore structures and tunable electronic properties. COF materials have regular pore structures, high specific surface areas, tunable electronic properties, and abundant functionalization sites. These properties enable them to efficiently capture and utilize light energy during photocatalysis, promote the separation and migration of photo-generated carriers, and thus improve the photocatalytic efficiency. For example, by constructing an electron donor-acceptor (D-A) structure, introducing metal doping to form a heterojunction, and modifying functional groups and other strategies, the photocatalytic performance of COF materials can be effectively improved.

[0004] However, existing COF photocatalysts still face many challenges in acidic environments (such as industrial wastewater treatment, proton exchange membrane electrolysis water systems): 1. Lack of precision in the design of active sites: At present, researchers mostly improve the activity of COF photocatalysts by randomly introducing heteroatoms (such as N, S). However, using a random substitution strategy (such as a mixture of ortho, meta, and para substitutions), it is impossible to clearly understand the specific impact of the heteroatom position on the activity. For example, Zhang et al. ( J. Am. Chem. Soc. 2020, 142, 12367) reported that N-doped COF was used for photocatalytic hydrogen production, but the N substitution position was not distinguished, and the hydrogen production rate was only 1.8 mmol·g -1 ·h -1 . Different heteroatom introduction positions will cause significant changes in the electronic structure and chemical properties of the material, thereby affecting its photocatalytic performance. For example, introducing N atoms at different positions on the benzene ring may change the conjugation degree, electron cloud density distribution, and the interaction mode with reactants of the material. However, existing research lacks in-depth systematic research and precise design on this, resulting in unsatisfactory optimization effects of active sites.

[0005] 2. Insufficient stability: Under acidic conditions, N-unsubstituted COFs are prone to protonation, which can lead to framework collapse and loss of photocatalytic activity of the material. (For example, Adv. Mater. 2022, 34, 2107142). Although N-substituted COFs have improved stability to some extent, the specific mechanism of action remains unclear. For example, questions such as whether the introduction of N atoms will change the interaction mode between the COF material and the acidic medium, and how it affects the chemical bond stability and structural integrity of the material have not been fully answered, which severely restricts the practical application and long-term stability of COF photocatalysts in acidic environments.

[0006] 3. Ambiguous structure-property relationship: The synthesis method of COF materials is difficult to precisely control the position of substituents, resulting in differences in the structure of COF materials prepared in different batches, and thus their properties are also different. (For example, Chem. Mater. 2019, 31, 4817). This problem of large differences between batches makes it difficult for researchers to accurately establish the corresponding relationship between structure and properties, increasing the difficulty of rational design and performance optimization of COF photocatalysts. For example, different positions of substituents may affect the pore size distribution, specific surface area, and electron transfer path of the material, thus having an important impact on the photocatalytic performance. However, due to the inability to precisely control the position of substituents, the relationship between these structural factors and properties becomes complex and ambiguous.

[0007] In view of the above key problems of existing COF photocatalysts in acidic environments, the present invention designs four new COF materials (Ph-COF, mN-COF, pN-COF, oN-COF) by precisely regulating the N-substitution position on the benzene ring, aiming to significantly improve the photocatalytic activity and stability in acidic environments and provide more efficient photocatalytic materials for solving energy crisis and environmental pollution problems. Summary of the Invention

[0008] Aiming at the technical problems of low yield, poor stability, and unclear structure-property relationship of existing COF photocatalysts under acidic conditions, the present invention provides a COF photocatalyst based on precise regulation of benzene ring nitrogen sites, its preparation method, and application.

[0009] The technical solution adopted by the present invention is as follows: A COF photocatalyst based on precise regulation of benzene ring nitrogen sites, with precisely regulated N-substitution positions on the COF benzene ring. The N-substitution positions are ortho-mononitrogen substitution, meta-mononitrogen substitution, para-mononitrogen substitution, or no N-substitution on the benzene ring. No N-substitution on the COF benzene ring corresponds to Ph-COF; meta-mononitrogen substitution on the benzene ring corresponds to mN-COF; para-mononitrogen substitution on the benzene ring corresponds to pN-COF; ortho-mononitrogen substitution on the benzene ring corresponds to oN-COF.

[0010] Furthermore, the COF photocatalyst is preferably oN-COF obtained by mono-N substitution at the ortho position of the benzene ring.

[0011] The above COF photocatalyst is prepared by a solvothermal method. Different substituted amine monomers and aldehyde monomers are dissolved in a mixed solvent composed of n-butanol, o-dichlorobenzene, and aqueous acetic acid solution, and then loaded into a Pyrex tube. The volume ratio of n-butanol, o-dichlorobenzene to aqueous acetic acid solution is 7-10:1:1. After ultrasonic treatment and freeze-pump-thaw cycles, the Pyrex tube is sealed under vacuum and placed in an oven at 100 - 140 °C for solvothermal reaction for 30 - 100 hours to form a yellow precipitate; and it is washed successively with tetrahydrofuran and acetone to obtain the products Ph-COF, mN-COF, pN-COF, or oN-COF.

[0012] Furthermore, the molar ratio of the amine monomer to the aldehyde monomer is 1.5-4:1, preferably 2-3:1.

[0013] Furthermore, the time for ultrasonic treatment is 1-5 minutes, preferably 2-4 minutes.

[0014] Furthermore, the freeze-pump-thaw cycle is achieved by liquid nitrogen at a temperature of 75-78 K.

[0015] Furthermore, the freeze-pump-thaw cycle is carried out 2-5 times, preferably 3-4 times.

[0016] Furthermore, the temperature of the solvothermal reaction is preferably 110-130 °C, more preferably 115-120 °C, and the time is preferably 60-100 hours, more preferably 70-80 hours. Furthermore, when the product is oN-COF, the amine monomer is prepared by the following method: 2,4-Dibromopyridine, 4-aminophenylboronic acid pinacol ester, tetrakis(triphenylphosphine)palladium, and potassium carbonate are dissolved in a mixed solvent of dioxane-water in a molar ratio of 1:1.5-2.5:0.01-0.03:2-4, and then refluxed for 40-80 hours under a nitrogen atmosphere and stirring conditions. After the reaction is completed, it is cooled to room temperature, the solvent is removed under reduced pressure, and the obtained residue is purified by column chromatography to obtain a brown powder, which is the amine monomer; When the product is Ph-COF, the amine monomer is prepared by the following method: 1,3-Dibromobenzene, 4-aminophenylboronic acid pinacol ester, tetrakis(triphenylphosphine)palladium, and potassium carbonate are dissolved in a mixed solvent of dioxane-water in a molar ratio of 1:1.5-2.5:0.01-0.03:2-4, and then refluxed for 40-80 hours under a nitrogen atmosphere and stirring conditions. After the reaction is completed, it is cooled to room temperature, the solvent is removed under reduced pressure, and the obtained residue is purified by column chromatography to obtain a brown powder, which is the amine monomer; When the product is mN-COF, the ammonia monomer is prepared by the following method: Dissolve 2,6-dibromopyridine, 4-aminophenylboronic acid pinacol ester, tetrakis(triphenylphosphine)palladium, and potassium carbonate in a mixed solvent of dioxane and water at a molar ratio of 1:1.5 - 2.5:0.01 - 0.03:2 - 4. Then, reflux the reaction mixture for 40 - 80 hours under a nitrogen atmosphere with stirring. After the reaction is completed, cool the mixture to room temperature and remove the solvent under reduced pressure. The resulting residue is purified by column chromatography to obtain a brown powder, which is the ammonia monomer. When the product is pN-COF, the ammonia monomer is prepared by the following method: Dissolve 3,5-dibromopyridine, 4-aminophenylboronic acid pinacol ester, tetrakis(triphenylphosphine)palladium, and potassium carbonate in a mixed solvent of dioxane and water at a molar ratio of 1:1.5 - 2.5:0.01 - 0.03:2 - 4. Then, reflux the reaction mixture for 40 - 80 hours under a nitrogen atmosphere with stirring. After the reaction is completed, cool the mixture to room temperature and remove the solvent under reduced pressure. The resulting residue is purified by column chromatography to obtain a brown powder, which is the ammonia monomer.

[0017] Furthermore, the aldehyde monomer is prepared by the following method: Add Pd(PPh3)4 to the degassed mixture of 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole, 5-bromoisophthalaldehyde, K2CO3, 1,4-dioxane, and water. The molar ratio of 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole, 5-bromoisophthalaldehyde, K2CO3, and Pd(PPh3)4 is 1:(2 - 4):(4 - 7):(1.5×10 -2 -4×10 -2 ) Heat the resulting mixture to 80 - 120 °C under stirring and a nitrogen atmosphere and maintain for 10 - 16 hours. Cool the reactant to room temperature and pour it into water. Wash the residue with excess water, methanol, toluene, and chloroform several times to obtain the aldehyde monomer.

[0018] The above COF photocatalyst is used for photocatalytic hydrogen production, pollutant degradation, or carbon dioxide reduction in an acidic environment with pH ≤ 2, and good results can be obtained.

[0019] The beneficial effects of the present invention are as follows: (1) The preparation method of the present invention does not require toxic reagents, and by precisely controlling the monomer substitution position, high repeatability of the COF structure is achieved.

[0020] (2)The COF photocatalyst obtained in the present invention has high activity. In particular, the photocatalytic hydrogen peroxide production rate of ortho-N-substituted oN-COF reaches 3015 μmol·g -1 ·h -1 , significantly higher than that of the prior art (such as 1.8 mmol·g -1 ·h -1 , that is, 1800 μmol·g -1 ·h -1 ) reported by Zhang et al.).

[0021] (3)The COF photocatalyst obtained in the present invention has high stability. In particular, the activity of ortho-N-substituted oN-COF remains 92% after 5 cycles.

[0022] (4)The present invention reveals the regulation law of the N-substitution position on the photocatalytic activity in an acidic environment and clarifies the mechanism of the structure-performance relationship. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 XRD patterns of four COFs, showing sharp absorption peaks, proving good crystallinity.

[0024] Figure 2 SEM image of Ph-COF, showing that COF is a one-dimensional bar-like structure.

[0025] Figure 3 SEM image of mN-COF, showing that COF is a one-dimensional bar-like structure.

[0026] Figure 4 SEM image of pN-COF, showing that COF is a one-dimensional bar-like structure.

[0027] Figure 5 SEM image of oN-COF, showing that COF is a one-dimensional bar-like structure.

[0028] Figure 6 Comparison chart of photocatalytic hydrogen peroxide production rates, showing that oN-COF has the highest activity.

[0029] Figure 7 Fourier infrared spectra of four COFs, confirming the formation of imine bonds (1627 cm -1 ). DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the present invention is not limited thereto.

[0031] Example 1 Preparation of oN-COF (ortho-N-substituted) Step 1, synthesis of ammonia monomer: 2,4-Dibromopyridine (1.19 g, 5.0 mmol), 4-aminophenylboronic acid pinacol ester (2.30 g, 10.5 mmol), tetrakis(triphenylphosphine)palladium (284 mg, 0.25 mmol), and potassium carbonate (2.07 g, 15 mmol) were refluxed and stirred in dioxane / water (60 mL / 15 mL) under nitrogen for 72 h. After cooling to room temperature, the solvent was removed under reduced pressure. The resulting residue was purified by column chromatography on alumina gel (200 mesh) with ethyl acetate / tetrahydrofuran (volume ratio 1 / 1) as the eluent, and the resulting brown powder was the ammonia monomer.

[0032] Step 2, synthesis of aldehyde monomer: Pd(PPh3)4 (30.0 mg) was added to a degassed mixture of 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole (0.20 g, 0.52 mmol), 5-bromoisophthalaldehyde (0.28 g, 1.30 mmol), K2CO3 (0.43 g, 3.12 mmol), 1,4-dioxane (10.0 mL), and H2O (10.0 mL). The resulting mixture was heated to 100 °C with stirring under a nitrogen atmosphere and maintained for 14 h. The reaction mixture was cooled to room temperature and poured into 100.0 mL of water. The residue was washed several times with excess water, methanol, toluene, and chloroform to obtain the aldehyde monomer.

[0033] Step 3, synthesis of oN-COF: The above-synthesized ammonia monomer (19 mg, 0.08 mmol), aldehyde monomer (15 mg, 0.04 mmol), n-butanol (1.8 mL), o-dichlorobenzene (0.2 mL), and acetic acid aqueous solution (6 M, 0.2 mL) were charged into a Pyrex tube. The Pyrex tube was sonicated for 2 min, then rapidly frozen and degassed by three freeze-pump-thaw cycles at 77 K using liquid nitrogen. The Pyrex tube was sealed under vacuum and then placed in an oven at 120 °C for a solvothermal reaction for 72 h to form a yellow precipitate, which was separated by filtration and washed with tetrahydrofuran, acetone, and ethanol, respectively. The sample was then dried in an oven at 60 °C for 12 h to obtain a yellow powder (yield 71%).

[0034] Example 2 Preparation of Ph-COF (without N substitution) Step 1, synthesis of ammonia monomer: 1,3 - dibromobenzene (1.18 g, 5.0 mmol), 4 - aminophenylboronic acid pinacol ester (2.30 g, 10.5 mmol), tetrakis(triphenylphosphine)palladium (284 mg, 0.25 mmol) and potassium carbonate (2.07 g, 15 mmol) were refluxed and stirred in dioxane / water (60 mL / 15 mL) under nitrogen for three days. After cooling to room temperature, the solvent was removed under reduced pressure. The resulting residue was purified by column chromatography on alumina gel (200 mesh) using ethyl acetate / tetrahydrofuran (volume ratio 1 / 1) as the eluent, and the resulting brown powder was the ammonia monomer.

[0035] Step 2, synthesis of aldehyde monomer: Pd(PPh3)4 (30.0 mg) was added to a degassed mixture of 4,7 - bis(4,4,5,5 - tetramethyl - 1,3,2 - dioxaborolan - 2 - yl) - 2,1,3 - benzothiadiazole (0.20 g, 0.52 mmol), 5 - bromoisophthalaldehyde (0.28 g, 1.30 mmol), K2CO3 (0.43 g, 3.12 mmol), 1,4 - dioxane (10.0 mL) and H2O (10.0 mL). The resulting mixture was heated to 100 °C under a nitrogen atmosphere with stirring and maintained for 14 hours. The reaction mixture was cooled to room temperature and poured into 100.0 mL of water. The residue was washed several times with excess water, methanol, toluene and chloroform to obtain the aldehyde monomer.

[0036] Step 3, synthesis of Ph - COF: The synthesized ammonia monomer (19 mg, 0.08 mmol), aldehyde monomer (15 mg, 0.04 mmol), n - butanol (1.8 mL), o - dichlorobenzene (0.2 mL) and acetic acid aqueous solution (6 M, 0.2 mL) were loaded into a Pyrex tube. The Pyrex tube was sonicated for 2 minutes, then rapidly frozen and degassed using three freeze - pump - thaw cycles at 77 K using liquid nitrogen. The Pyrex tube was sealed under vacuum and then placed in an oven at 120 °C for a solvothermal reaction for 72 hours to form a brown precipitate, which was separated by filtration and washed with tetrahydrofuran, acetone and ethanol respectively. The sample was then dried in an oven at 60 °C for 12 hours to obtain a brown powder (yield 83%).

[0037] Example 3 Preparation of mN - COF (meta - N - substituted) Step 1, synthesis of ammonia monomer: 2,6-Dibromopyridine (1.19 g, 5.0 mmol), 4-aminophenylboronic acid pinacol ester (2.30 g, 10.5 mmol), tetrakis(triphenylphosphine)palladium (284 mg, 0.25 mmol), and potassium carbonate (2.07 g, 15 mmol) were refluxed and stirred in dioxane / water (60 mL / 15 mL) under nitrogen for 72 h. After cooling to room temperature, the solvent was removed under reduced pressure. The resulting residue was purified by column chromatography on alumina gel (100 - 200 mesh) with ethyl acetate / petroleum ether (1 / 1) as the eluent, and the resulting brown powder was the ammonia monomer.

[0038] Step 2, Synthesis of aldehyde monomer: Pd(PPh3)4 (30.0 mg) was added to a degassed mixture of 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole (0.20 g, 0.52 mmol), 5-bromoisophthalaldehyde (0.28 g, 1.30 mmol), K2CO3 (0.43 g, 3.12 mmol), 1,4-dioxane (10.0 mL), and H2O (10.0 mL). The resulting mixture was heated to 100 °C with stirring under a nitrogen atmosphere and maintained for 14 h. The reaction mixture was cooled to room temperature and poured into 100.0 mL of water. The residue was washed several times with excess water, methanol, toluene, and chloroform to obtain the aldehyde monomer.

[0039] Step 3, Synthesis of mN-COF: The above-synthesized ammonia monomer (19 mg, 0.08 mmol), aldehyde monomer (15 mg, 0.04 mmol), n-butanol (1.8 mL), o-dichlorobenzene (0.2 mL), and acetic acid aqueous solution (6 M, 0.2 mL) were loaded into a Pyrex tube. The Pyrex tube was sonicated for 2 min, then rapidly frozen and degassed by three freeze-pump-thaw cycles at 77 K using liquid nitrogen. The Pyrex tube was sealed under vacuum and then placed in an oven at 120 °C for a solvothermal reaction for 72 h to form a yellow precipitate, which was separated by filtration and washed with tetrahydrofuran, acetone, and ethanol, respectively. The sample was then dried in an oven at 60 °C for 12 h to obtain a yellow powder (yield 65%).

[0040] Example 4 Preparation of pN-COF (para N-substituted) Step 1, Synthesis of ammonia monomer: 3,5-Dibromopyridine (1.19 g, 5.0 mmol), 4-aminophenylboronic acid pinacol ester (2.30 g, 10.5 mmol), tetrakis(triphenylphosphine)palladium (284 mg, 0.25 mmol) and potassium carbonate (2.07 g, 15 mmol) were refluxed and stirred in dioxane / water (60 mL / 15 mL) under nitrogen for 72 h. After cooling to room temperature, the solvent was removed under reduced pressure. The resulting residue was purified by column chromatography on alumina gel (100 - 200 mesh) with ethyl acetate / petroleum ether (2 / 1) as the eluent, and the resulting brown powder was the ammonia monomer.

[0041] Step 2, Synthesis of aldehyde monomer: Pd(PPh3)4 (30.0 mg) was added to a degassed mixture of 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole (0.20 g, 0.52 mmol), 5-bromoisophthalaldehyde (0.28 g, 1.30 mmol), K2CO3 (0.43 g, 3.12 mmol), 1,4-dioxane (10.0 mL) and H2O (10.0 mL). The resulting mixture was heated to 100 °C with stirring under a nitrogen atmosphere and maintained for 14 h. The reaction mixture was cooled to room temperature and poured into 100.0 mL of water. The residue was washed several times with excess water, methanol, toluene and chloroform to obtain the aldehyde monomer.

[0042] Step 3, Synthesis of pN-COF: The synthesized ammonia (19 mg, 0.08 mmol), aldehyde (15 mg, 0.04 mmol), n-butanol (1.8 mL), o-dichlorobenzene (0.2 mL) and acetic acid aqueous solution (6 M, 0.2 mL) were loaded into a Pyrex tube. The Pyrex tube was sonicated for 2 min, then rapidly frozen and degassed by three freeze-pump-thaw cycles at 77 K using liquid nitrogen. The Pyrex tube was sealed under vacuum and then placed in an oven at 120 °C for a solvothermal reaction for 72 h to form a yellow precipitate, which was separated by filtration and washed with tetrahydrofuran, acetone and ethanol respectively. The sample was then dried in an oven at 60 °C for 12 h to obtain a yellow powder (yield 78%).

[0043] The XRD patterns of the four COFs are as Figure 1 shown. It can be seen from Figure 1 that there are very sharp absorption peaks between 2 - 10°, proving the successful preparation of COF.

[0044] The scanning electron microscopy (SEM) images of the four COFs are as Figures 2 to 5 shown. It can be seen from the figure that the prepared COFs are all one-dimensional strip-shaped columns.

[0045] Under acidic test conditions (pH = 2), the photocatalytic hydrogen peroxide production rates of the four COFs under visible light irradiation are shown in Table 1 and Figure 6 as follows. It can be seen from Figure 6 that oN-COF reached an astonishing 3015 μmol·g -1 ·h -1 , exceeding most of the currently reported COF photocatalysts and showing excellent photocatalytic activity. This is because oN-COF has large and strong interfacial contacts, excellent visible light absorption ability, porous structure, spatially separated redox sites, and the rapid separation of photoinduced charge carriers. The adjacent N forms local charge centers, which can significantly promote the adsorption of H + .

[0046] The Fourier transform infrared spectra of the four COFs are as shown in Figure 7 . It can be seen from Figure 7 that the absorption peak of the aldehyde group (1700 cm -1 ) disappeared, and the formation of the imine bond (1627 cm -1 ) occurred.

[0047] Table 1 Performance comparison of four groups of catalysts under the same acidic test conditions (pH = 2)

[0048] Comparative Example 1 (random N-substituted COF)

[0049] Synthesis method: Mix the ammonia monomers obtained in Example 1, Example 3, and Example 4 in a molar ratio of 1:1:1, and then mix them with the aldehyde monomer in a molar ratio of 2:1. The solvothermal reaction conditions are the same as those in Example 1 (120 °C / 72 hours). The obtained product is a mixed N-substituted COF (denoted as mixN-COF).

[0050] Performance test: Photocatalytic hydrogen peroxide production rate: 1420 μmol·g -1 ·h -1 (pH = 2).

[0051] Cyclic stability: The activity decreased to 65% after 5 cycles.

[0052] The experimental results show that the mixed N-substitution leads to the disorder of the COF structure, low separation efficiency of photogenerated carriers, and significantly lower activity and stability than the precisely regulated oN-COF.

[0053] Comparative Example 2 (stability of N-free substituted COF under acidic conditions)

[0054] Test method: Place Ph-COF (without N substitution) in an acidic solution with pH = 1 and carry out a continuous light reaction for 12 hours to observe the structural changes and activity decay.

[0055] Test results: Initial activity: The hydrogen peroxide production rate is 580 μmol·g -1 ·h -1 .

[0056] After 12 hours: The material framework collapses and the activity drops to 305 μmol·g -1 ·h -1 , and it cannot be restored.

[0057] Meanwhile, in an acidic condition with pH = 1, the stability of other N-substituted (especially ortho-position) COFs is hardly affected.

[0058] The experimental results show that COF without N substitution is more prone to protonation with the further increase of acidity, resulting in poor framework stability, while N substitution (especially ortho-position) effectively inhibits protonation through local charge regulation and improves the stability.

Claims

1. A COF photocatalyst based on precise regulation of benzene ring nitrogen sites, characterized in that, The COF benzene ring has a precisely regulated N-substitution position, which is ortho-mononitro substitution, meta-mononitro substitution, para-mononitro substitution or no N-substitution on the benzene ring. The COF benzene ring without N-substitution corresponds to Ph-COF; the meta-mononitro substitution on the benzene ring corresponds to mN-COF; the para-mononitro substitution on the benzene ring corresponds to pN-COF; the ortho-mononitro substitution on the benzene ring corresponds to oN-COF.

2. The COF photocatalyst based on precise regulation of benzene ring nitrogen sites according to claim 1, characterized in that, The COF photocatalyst is oN-COF obtained by ortho-mononitro substitution of the benzene ring.

3. The preparation method of the COF photocatalyst based on precise regulation of benzene ring nitrogen sites according to claim 1 or 2, characterized in that, Prepared by the solvothermal method. Different substituted ammonia monomers and aldehyde monomers are dissolved in a mixed solvent composed of n-butanol, o-dichlorobenzene and acetic acid aqueous solution, and then loaded into a Pyrex tube. The volume ratio of n-butanol, o-dichlorobenzene to acetic acid aqueous solution is 7-10:1:

1. After ultrasonic treatment and freeze-pump-thaw cycle, the Pyrex tube is sealed under vacuum and then placed in an oven at 100 - 140 °C for solvothermal reaction for 30-100 hours to form a yellow precipitate; and washed successively with tetrahydrofuran and acetone to obtain the products Ph-COF, mN-COF, pN-COF or oN-COF.

4. The preparation method according to claim 3, characterized in that, The molar ratio of ammonia monomer to aldehyde monomer is 1.5-4:

1.

5. The preparation method according to claim 3, characterized in that, The time of ultrasonic treatment is 1-5 minutes.

6. The preparation method according to claim 3, characterized in that, The freeze-pump-thaw cycle is achieved by liquid nitrogen at a temperature of 75-78 K, and the freeze-pump-thaw cycle is carried out 2-5 times.

7. The preparation method according to claim 3, characterized in that, The temperature of the solvothermal reaction is 110~130 °C, and the time is 60-100 hours.

8. The preparation method according to claim 3, characterized in that, When the product is oN-COF, the ammonia monomer is prepared by the following method: 2,4-Dibromopyridine, 4-aminophenylboronic acid pinacol ester, tetrakis(triphenylphosphine)palladium and potassium carbonate are dissolved in a mixed solvent of dioxane-water according to a molar ratio of 1:1.5-2.5:0.01-0.03:2-4, and then refluxed and reacted for 40-80 hours under a nitrogen atmosphere and stirring conditions. After the reaction is completed, it is cooled to room temperature, the solvent is removed under reduced pressure, and the obtained residue is purified by column chromatography to obtain a brown powder, which is the ammonia monomer; When the product is Ph-COF, the ammonia monomer is prepared by the following method: 1,3-Dibromobenzene, 4-aminophenylboronic acid pinacol ester, tetrakis(triphenylphosphine)palladium and potassium carbonate are dissolved in a mixed solvent of dioxane-water according to a molar ratio of 1:1.5-2.5:0.01-0.03:2-4, and then refluxed and reacted for 40-80 hours under a nitrogen atmosphere and stirring conditions. After the reaction is completed, it is cooled to room temperature, the solvent is removed under reduced pressure, and the obtained residue is purified by column chromatography to obtain a brown powder, which is the ammonia monomer; When the product is mN-COF, the ammonia monomer is prepared by the following method: 2,6-Dibromopyridine, 4-aminophenylboronic acid pinacol ester, tetrakis(triphenylphosphine)palladium and potassium carbonate are dissolved in a mixed solvent of dioxane-water according to a molar ratio of 1:1.5-2.5:0.01-0.03:2-4, and then refluxed and reacted for 40-80 hours under a nitrogen atmosphere and stirring conditions. After the reaction is completed, it is cooled to room temperature, the solvent is removed under reduced pressure, and the obtained residue is purified by column chromatography to obtain a brown powder, which is the ammonia monomer; When the product is pN-COF, the ammonia monomer is prepared by the following method: 3,5-dibromopyridine, 4-aminophenylboronic acid pinacol ester, tetrakis(triphenylphosphine)palladium, and potassium carbonate are dissolved in a mixed solvent of dioxane-water in a molar ratio of 1:1.5 - 2.5:0.01 - 0.03:2 - 4, and then refluxed for 40 - 80 hours under a nitrogen atmosphere and stirring conditions. After the reaction is completed, it is cooled to room temperature, the solvent is removed under reduced pressure, and the resulting residue is purified by column chromatography to obtain a brown powder, which is the ammonia monomer.

9. The preparation method according to claim 3, characterized in that The aldehyde monomer is prepared by the following method: Pd(PPh3)4 was added to the degassed mixture of 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole, 5-bromoisophthalaldehyde, K2CO3, 1,4-dioxane and water. The molar ratio of 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole, 5-bromoisophthalaldehyde, K2CO3 and Pd(PPh3)4 was 1: (2 - 4): (4 - 7): (1.5×10 -2 -4×10 -2 ). The resulting mixture was heated to 80 - 120 °C under stirring and a nitrogen atmosphere and maintained for 10 - 16 hours. The reaction mixture was cooled to room temperature and poured into water. The residue was washed several times with excess water, methanol, toluene and chloroform to obtain the aldehyde monomer.

10. Use of the COF photocatalyst with precise regulation of benzene ring nitrogen sites according to claim 1 or 2, or the COF photocatalyst with precise regulation of benzene ring nitrogen sites obtained by the preparation method according to any one of claims 3 to 9, in photocatalytic hydrogen production, pollutant degradation, or carbon dioxide reduction in an acidic environment with pH ≤ 2.

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