Preparation method of an olefinic COF catalyst and application of the catalyst in photocatalytic production of hydrogen peroxide

Abstract

The application discloses an olefinic COF catalyst, a preparation method thereof and application of the catalyst in photocatalytic production of hydrogen peroxide. The catalyst is prepared from 2,2',6,6'-tetramethyl-4,4'-bipyridine and an aromatic aldehyde monomer containing two aldehyde groups through a Knoevenagel condensation reaction under a solvent-free condition, and has an olefinic covalent organic framework structure. The preparation method of the application does not need to add any solvent, is simple in operation, mild in reaction condition and easy to scale up, and has significant commercialization potential. The prepared catalyst has a highly conjugated two-dimensional layered porous structure, excellent light absorption capacity and light-generated carrier separation efficiency. When the catalyst is applied to photocatalytic decomposition of water to produce hydrogen peroxide, the catalyst exhibits excellent catalytic activity, and the hydrogen peroxide production rate can reach 1125 μM h ‑1 The application provides a new scheme of green, efficient and industrialized prospect for solving the problems of high energy consumption and high pollution of the traditional hydrogen peroxide production method.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor material photocatalysis and environmental protection technology, and relates to an olefinic COF catalyst, its preparation method and its application in photocatalytic hydrogen peroxide production. Background Technology

[0002] Hydrogen peroxide (H2O2), as a green and high-end oxidant, plays an indispensable role in bleaching, wastewater treatment, medical disinfection, and chemical synthesis. Its market value is increasingly prominent, especially with the growing demand for electronic products. However, the anthraquinone process commonly used in industry to produce H2O2 suffers from complex processes, high energy consumption, and the generation of large amounts of organic wastewater, which does not align with the development concept of green chemistry. To overcome the limitations of traditional thermocatalytic pathways, solar-driven photocatalytic synthesis of H2O2, which utilizes water and oxygen to directly generate the product under mild conditions, is considered a highly promising sustainable alternative.

[0003] Among numerous photocatalytic material systems, covalent organic frameworks (COFs) have shown great promise in the field of photocatalysis due to their high specific surface area, strong structural tunability, and excellent semiconductor properties. In particular, by constructing donor-acceptor structures, COFs can effectively broaden the visible light absorption range and promote the separation of photogenerated carriers, providing an ideal platform for improving the efficiency of photocatalytic H₂O₂ production. However, traditional imine-bonded COFs often face problems such as poor chemical stability and unfavorable exciton dissociation due to interlayer π-π stacking, limiting their long-term stability and quantum efficiency in photocatalytic water oxidation and oxygen reduction reactions. Compared to imine bonds, carbon-carbon double bonds not only enhance the planarity and electronic delocalization ability of materials but also optimize their band structure, promoting intramolecular charge transfer. However, current COF synthesis methods generally rely on solvothermal methods, which typically require large amounts of high-boiling-point organic solvents such as dioxane and mesitylene, along with complex vacuum operations and stringent reaction conditions, which to some extent restricts the large-scale production and practical application of COFs. Therefore, developing a novel method for preparing COF that is solvent-free, simple to operate, mild under mild conditions, and easy to scale up is of great research value for promoting the practical application of high-performance COF photocatalytic materials. Summary of the Invention

[0004] The purpose of this invention is to overcome the technical defects of existing imine bond COFs, such as poor chemical stability and low photogenerated carrier separation efficiency. At the same time, it solves the problems that existing COF preparation technologies generally require the use of large amounts of solvents, are complex to operate, have harsh conditions, and are not conducive to large-scale production. The invention provides a solvent-free, simple, mild, and commercially viable olefin bond COF catalyst, its preparation method, and its application in photocatalytic H2O2 production.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing an olefinic COF catalyst, the method being a solventless melt polycondensation method, comprising the following steps: mixing a monomer containing 2,2',6,6'-tetramethyl-4,4'-bipyridine and an aromatic aldehyde monomer containing two aldehyde groups in the presence of a catalyst, followed by degassing and then performing a melt polycondensation reaction under sealed conditions; the resulting solid product is then post-treated to obtain the olefinic COF photocatalyst.

[0007] The aromatic aldehyde monomer containing two aldehyde groups is at least one of 2,2'-bipyridine-5,5'-dicarboxaldehyde, 6-(4-aldehydephenyl)pyridine-3-carboxaldehyde, or 4,4'-biphenyldicarboxaldehyde; the molar ratio of the 2,2',6,6'-tetramethyl-4,4'-bipyridine monomer to the aromatic aldehyde monomer containing two aldehyde groups is 1:1 to 1:3.

[0008] The catalyst is a mixture of organic acid and organic acid anhydride; preferably, the organic acid is at least one of benzoic acid, acetic acid or p-toluenesulfonic acid; and the organic acid anhydride is at least one of benzoic anhydride, acetic anhydride or trifluoromethanesulfonic anhydride.

[0009] The catalyst is benzoic acid and benzoic anhydride, with molar ratios of benzoic acid to 2,2',6,6'-tetramethyl-4,4'-bipyridine of 0.1-0.3:1 and 1.5-2.5:1, respectively.

[0010] The degassing process involves a freezing-pumping-thawing cycle of degassing 2-5 times.

[0011] The temperature of the melt polycondensation reaction is 120-250℃, and the reaction time is 1-10 days.

[0012] The post-processing includes the steps of sequentially soaking the solid obtained from the reaction in an alkaline solution, crushing, extracting with an organic solvent, and drying.

[0013] In some preferred embodiments, the method is a solvent-free melt polycondensation method, comprising the following steps: (1) 2,2',6,6'-tetramethyl-4,4'-bipyridine, 2,2'-bipyridine-5,5'-dicarboxaldehyde, benzoic acid and benzoic anhydride were added to the reaction vessel in sequence.

[0014] (2) After the above mixture is subjected to freezing-thawing cycle degassing treatment, it is gradually heated to room temperature, and then the reaction vessel tube is placed in an oven.

[0015] (3) After the reaction is complete, the resulting blocky solid is soaked in a mixed solution of sodium hydroxide aqueous solution and methanol.

[0016] (4) The soaked solid was crushed into powder with a hammer and ground with a mortar. The resulting powder was then extracted by reflux in a Soxhlet extractor with a mixed solvent of acetone / methanol. The extracted product was dried under vacuum and collected to obtain a yellow powder, which is the olefinic COF catalyst.

[0017] The molar ratio of 2,2',6,6'-tetramethyl-4,4'-bipyridine, 2,2'-bipyridine-5,5'-dicarboxaldehyde, benzoic acid and benzoic anhydride in step (1) is 1:2:0.2:2.

[0018] The oven reaction time and temperature in step (2) are 72 h and 180 ℃, respectively.

[0019] In step (3), the concentration of the sodium hydroxide aqueous solution in the mixed solution is 1 M, and the volume ratio of the sodium hydroxide aqueous solution to methanol is 1:1.

[0020] In step (4), the volume ratio of the acetone / methanol mixed solvent is 1:1, and the Soxhlet extraction time is 12 h.

[0021] The olefinic COF catalyst can be used for photocatalytic production of H2O2. The application method is to add the olefinic COF catalyst as a photocatalyst to a water system to carry out a photocatalytic reaction, thereby generating H2O2.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention is the first to synthesize the target olefinic COF using a solvent-free melt polycondensation method. The preparation process of this invention does not require the addition of any organic solvent, which fundamentally avoids the problems of solvent use and recycling, greatly reduces costs and environmental burden, and at the same time the operation process is simple and the reaction conditions are mild, which has significant commercial application potential.

[0023] (2) Addressing the problems of poor chemical stability and easy recombination of photogenerated carriers in traditional imine-bonded COFs. This invention provides an alkene-bonded COF catalyst with high stability and high charge separation efficiency. The COF used in this invention is formed by alkene bonds, creating a highly conjugated two-dimensional layered porous structure and 1.5 nm-2.5 nm nanopores. This structure not only provides a large specific surface area to expose more active sites and adsorb more reaction substrates, but also enhances the chemical stability and electron delocalization ability of the framework by alkene bond connection, constructing a highly efficient charge transport channel, which significantly promotes the separation and migration of photogenerated electron-hole pairs, providing an effective solution to the problems of poor stability and low quantum efficiency of traditional COF materials.

[0024] (3) In existing photocatalytic H2O2 production technologies, most systems require the addition of hole sacrificial agents to improve reaction efficiency. This not only increases raw material costs but may also introduce impurities due to sacrificial agent residue, leading to increased difficulty in H2O2 purification and even secondary pollution. The production method of this invention uses ultrapure water as the sole reaction medium. It can achieve efficient H2O2 production under visible light irradiation and normal temperature and pressure conditions without the addition of any hole sacrificial agents, and has significant advantages such as low energy consumption, low cost, and no secondary pollution. The catalyst achieves a H2O2 yield of up to 1125 μM h under visible light. -1 It has excellent performance, which reduces production costs and pollution risks, and simplifies the operation process. Attached Figure Description

[0025] Figure 1 The diagram shows the synthesis process of Examples 1, 2 and 3.

[0026] Figure 2 Comparison of X-ray diffraction (XRD) images of the COFs photocatalysts synthesized in Examples 1, 2 and 3.

[0027] Figure 3 Comparison of Fourier transform infrared (FT-IR) spectra of the COFs photocatalysts synthesized in Examples 1, 2 and 3.

[0028] Figure 4 The images show a comparison of scanning electron microscope (SEM) images of the COFs photocatalysts synthesized in Examples 1, 2 and 3.

[0029] Figure 5 Comparison of N2 adsorption-desorption isotherms of the COFs photocatalysts synthesized in Examples 1, 2 and 3.

[0030] Figure 6 Comparison of solid-state carbon NMR spectra of the COFs photocatalysts synthesized in Examples 1, 2 and 3.

[0031] Figure 7 The image shows a comparison of the photocurrent response of the COFs photocatalysts synthesized in Examples 1, 2, and 3.

[0032] Figure 8 Comparison of electrochemical impedance spectroscopy (EIS) spectra of the COFs photocatalysts synthesized in Examples 1, 2 and 3.

[0033] Figure 9 This is a comparison chart showing the yield of H2O2 produced by photocatalysis in an aqueous system for the COFs photocatalysts synthesized in Examples 1, 2, and 3. Detailed Implementation

[0034] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0035] Example 1 Preparation of olefin-bonded COF photocatalysts: (1) 2,2',6,6'-tetramethyl-4,4'-bipyridine (84.92 mg, 0.4 mmol), 2,2'-bipyridine-5,5'-dicarboxaldehyde (169.8 mg, 0.8 mmol), benzoic acid (9.8 mg, 0.08 mmol) and benzoic anhydride (181.0 mg, 0.8 mmol) were added sequentially to the reaction vessel; (2) After the above mixture is degassed by three cycles of freezing-pumping-thawing, it is gradually heated to room temperature and then the reaction tube is sealed. (3) Place the sealed reaction tube in an oven and heat it to 180 °C for 3 days; (4) After the reaction is complete, the resulting blocky solid is soaked in a mixed solution of 1M NaOH aqueous solution and methanol (volume ratio 1:1) for 24 h; (5) The soaked solid was crushed into powder with a hammer and ground with a mortar. The resulting powder was then extracted by reflux in a Soxhlet extractor with a mixed solvent of acetone / methanol (volume ratio 1:1) for 12 h. The extracted product was dynamically vacuum dried overnight at room temperature and the yellow powder was collected, which is the olefinic COF catalyst TMBpy-Bpy.

[0036] Example 2: Preparation of olefin-bonded COF photocatalyst TMBpy-Bah: (1) 2,2',6,6'-tetramethyl-4,4'-bipyridine (84.92 mg, 0.4 mmol), 6-(4-aldehydephenyl)pyridine-3-carboxaldehyde (170.0 mg, 0.8 mmol), benzoic acid (9.8 mg, 0.08 mmol) and benzoic anhydride (181.0 mg, 0.8 mmol) were added sequentially to the reaction vessel; (2) After the above mixture is degassed by three cycles of freezing-pumping-thawing, it is gradually heated to room temperature and then the reaction tube is sealed. (3) Place the sealed reaction tube in an oven and heat it to 180 °C for 3 days; (4) After the reaction is complete, the resulting blocky solid is soaked in a mixed solution of 1M NaOH aqueous solution and methanol (volume ratio 1:1) for 24 h; (5) The soaked solid was crushed into powder with a hammer and ground with a mortar. The resulting powder was then extracted by reflux in a Soxhlet extractor with a mixed solvent of acetone / methanol (volume ratio 1:1) for 12 h. The extracted product was dynamically vacuum dried overnight at room temperature and the yellow powder was collected, which is the olefinic COF catalyst TMBpy-Bah.

[0037] Example 3 Preparation of the olefinic COF photocatalyst TMBpy-Bph: (1) 2,2',6,6'-tetramethyl-4,4'-bipyridine (84.92 mg, 0.4 mmol), 4,4'-biphenyldicarboxaldehyde (168.2 mg, 0.8 mmol), benzoic acid (9.8 mg, 0.08 mmol) and benzoic anhydride (181.0 mg, 0.8 mmol) were added sequentially to the reaction vessel; (2) After the above mixture is degassed by three cycles of freezing-pumping-thawing, it is gradually heated to room temperature and then the reaction tube is sealed. (3) Place the sealed reaction tube in an oven and heat it to 180 °C for 3 days; (4) After the reaction is complete, the resulting blocky solid is soaked in a mixed solution of 1M NaOH aqueous solution and methanol (volume ratio 1:1) for 24 h; (5) The soaked solid was crushed into powder with a hammer and ground with a mortar. The resulting powder was then extracted by reflux in a Soxhlet extractor with a mixed solvent of acetone / methanol (volume ratio 1:1) for 12 h. The extracted product was dynamically vacuum dried overnight at room temperature and the yellow powder was collected, which is the olefinic COF catalyst TMBpy-Bph.

[0038] Figure 2 The image shows a comparison of XRD patterns of the prepared COF photocatalysts. The peak appearing near 4.50° in the image is a characteristic diffraction peak of COF, proving the successful synthesis of COFs.

[0039] Figure 3 The image shows a Fourier transform infrared (FTIR) comparison of the prepared COF photocatalysts. The image is approximately 1620 cm⁻¹. 1 The stretching vibration corresponding to the carbon-carbon double bond clearly shows that carbon-carbon double bonds are generated in the synthesized COFs, further proving the successful preparation of COFs photocatalysts.

[0040] Figure 4The images show a comparison of scanning electron microscope (SEM) images of the prepared COFs photocatalysts. As can be seen from the images, the material exhibits a sheet-like amorphous morphology.

[0041] Figure 5 Comparison of nitrogen adsorption-desorption isotherms for the prepared COF photocatalysts. In the figure, all four COFs show typical type IV adsorption isotherms, indicating the presence of mesoporous structures. The Brunauer-Emmett-Teller (BET) specific surface areas of TMBpy-Bpy, TMBpy-Bah, and TMBpy-Bph are 829.6, 603.7, and 1346.2 m², respectively. 2 / g.

[0042] Figure 6 Comparison of solid-state carbon NMR spectra of the prepared COF photocatalysts. Characteristic peaks belonging to alkene bonds appeared around 130 ppm in the three COFs, confirming the formation of alkene bonds.

[0043] Figure 7 The image shows a comparison of the photocurrent response of the prepared COFs photocatalysts. Under visible light irradiation, all samples can generate photocurrent, among which TMBpy-Bpy from Example 1 has the strongest response, indicating that it has the best photogenerated charge separation effect.

[0044] Figure 8 The image shows a comparison of electrochemical impedance spectroscopy (EIS) spectra of the prepared COFs photocatalysts. In the figure, the TMBpy-Bpy prepared in Example 1 exhibits the smallest radius of curvature, indicating that it has the lowest charge transfer resistance, which is beneficial for the effective separation of photogenerated electron-hole pairs and the rapid migration of interfacial charges, thereby enhancing photocatalytic activity.

[0045] In the examples, COFs photocatalytically produce hydrogen peroxide: 5 mg of photocatalyst was weighed and placed in a beaker containing 10 mL of ultrapure water. The suspension was continuously stirred and bubbled with air for 20 min in the dark to reach adsorption-desorption equilibrium. Subsequently, the air-saturated solution was illuminated with a 300 W xenon lamp equipped with a 420 nm cutoff filter at a light intensity of 100 mW / cm². 2 Xenon lamp irradiation was applied, and quantitative samples of the reaction solution were collected at regular intervals. After filtration to remove the catalyst, the H2O2 concentration was analyzed using the DPD / POD method, and the absorbance at 552 nm was measured using a UV-2600 UV-Vis spectrophotometer (Shanghai Tianmei Scientific Instruments Co., Ltd.). The concentration of hydrogen peroxide in the reaction solution was then calculated based on the standard curve. The results are shown in Table 1. Figure 9 : Table 1 Reaction conditions and photocatalytic performance of the examples

[0046] Experimental results show that the olefinic COF catalyst TMBpy-Bpy prepared in this invention achieves a hydrogen peroxide yield of up to 1125 μM h⁻¹ in a pure water system under visible light irradiation. -1 It is far superior to the comparative material TMBpy-Bah (277 μM h) -1 ) and TMBpy-Bph (60 μM h -1 ).

Claims

1. A method for preparing an olefinic COF catalyst, characterized in that, The method is a solvent-free melt polycondensation method, which includes the following steps: mixing a monomer containing 2,2',6,6'-tetramethyl-4,4'-bipyridine and an aromatic aldehyde monomer containing two aldehyde groups in the presence of a catalyst, degassing the mixture, and then carrying out a melt polycondensation reaction under sealed conditions. The resulting solid product is then post-processed to obtain the olefinic COF photocatalyst.

2. The preparation method according to claim 1, characterized in that, The aromatic aldehyde monomer containing two aldehyde groups is at least one of 2,2'-bipyridine-5,5'-dicarboxaldehyde, 6-(4-aldehydephenyl)pyridine-3-carboxaldehyde, or 4,4'-biphenyldicarboxaldehyde; the molar ratio of the 2,2',6,6'-tetramethyl-4,4'-bipyridine monomer to the aromatic aldehyde monomer containing two aldehyde groups is 1:1 to 1:

3.

3. The preparation method according to claim 1, characterized in that, The catalyst is a mixture of organic acid and organic acid anhydride; preferably, the organic acid is at least one of benzoic acid, acetic acid or p-toluenesulfonic acid; and the organic acid anhydride is at least one of benzoic anhydride, acetic anhydride or trifluoromethanesulfonic anhydride.

4. The preparation method according to claim 3, characterized in that, The catalyst is benzoic acid and benzoic anhydride, with molar ratios of benzoic acid to 2,2',6,6'-tetramethyl-4,4'-bipyridine of 0.1-0.3:1 and 1.5-2.5:1, respectively.

5. The preparation method according to claim 1, characterized in that, The degassing process involves a freezing-pumping-thawing cycle of degassing 2-5 times.

6. The preparation method according to claim 1, characterized in that, The temperature of the melt polycondensation reaction is 120-250℃, and the reaction time is 1-10 days.

7. The preparation method according to claim 1, characterized in that, The post-processing includes the steps of sequentially soaking the solid obtained from the reaction in an alkaline solution, crushing, extracting with an organic solvent, and drying.

8. A photocatalyst of olefinic covalent organic framework (COF), characterized in that, The catalyst, prepared by the method according to any one of claims 1-7, has a two-dimensional layered porous structure connected by alkene bonds and a BET specific surface area of ​​500-1500 m². 2 / g, with a pore size of 1-3 nm.

9. The application of the olefinic COF photocatalyst of claim 8 in the photocatalytic preparation of hydrogen peroxide.

10. The application according to claim 9, characterized in that, The olefinic COF photocatalyst was dispersed in water and subjected to a photocatalytic reaction to generate hydrogen peroxide under an oxygen-containing atmosphere and visible light irradiation.

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