Hydrazone-linked fluorine-substituted covalent organic framework material and preparation method and application thereof

Abstract

This invention discloses a hydrazone-linked fluorine-substituted covalent organic framework material, its preparation method, and its applications, belonging to the field of semiconductor photocatalysis technology. The material is synthesized by hydrazone condensation of 1,3,5-trifluoro-2,4,6-tris(4-formylphenyl)benzene and terephthalohydrazide, denoted as F-TPDH-COF. A one-step solvothermal synthesis is employed, resulting in a mild, environmentally friendly, low-cost, and easily scalable process. The material obtained by this invention utilizes hydrazone bonds to achieve proton enrichment in acidic media, and fluorine atoms regulate the electronic structure to improve charge separation efficiency. Under pH=2 conditions, the photocatalytic hydrogen peroxide production rate reaches as high as 5825.2 μmolg. ‑1 h ‑1 Its activity and stability are significantly better than most existing COF photocatalytic materials, and it can be used for efficient photocatalytic preparation of hydrogen peroxide in acidic systems, showing good prospects for industrialization.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor photocatalytic materials technology, specifically relating to a hydrazone-linked fluorine-substituted covalent organic framework material, its preparation method, and its application. Background Technology

[0002] Hydrogen peroxide (H2O2) is an important chemical product that combines oxidizing properties with environmental friendliness. Its decomposition products are only water and oxygen, and it is recognized as a green and high-value oxidant. It is widely used in fine chemical synthesis, advanced wastewater treatment, soil remediation, semiconductor chip cleaning, medical device sterilization, paper bleaching, and fuel cell energy carriers, and is an indispensable basic chemical in modern industry and sustainable development systems.

[0003] Currently, the industrial production of hydrogen peroxide mainly employs the anthraquinone process. This process uses anthraquinone as a carrier and involves multiple cyclical steps including hydrogenation, oxidation, extraction, and purification. While technically mature and capable of high-yield production, it suffers from a series of inherent drawbacks: the process is lengthy and complex, involving high-pressure hydrogenation and high-temperature oxidation, resulting in extremely high energy consumption and equipment investment; the reaction process uses large amounts of organic solvents, which are prone to volatilization and leakage, generating organic byproducts and waste pollution; there are explosion and fire safety risks, limiting production safety; and it is difficult to prepare hydrogen peroxide in situ, leading to high transportation and storage costs and risks. Therefore, developing a mild, green, low-energy-consumption, safe, and in-situ oxygen-generating hydrogen peroxide synthesis technology has become a research hotspot and a significant industrial demand in the fields of catalysis and materials.

[0004] Photocatalytic synthesis of hydrogen peroxide uses solar energy as the sole energy input and water and oxygen as raw materials at room temperature and pressure. Hydrogen peroxide is directly generated through a two-electron oxygen reduction reaction (2e-ORR) or a two-electron water oxidation reaction (2e-WOR). It has outstanding advantages such as high atom economy, mild reaction conditions, green and pollution-free production, and the ability to be miniaturized and produced in situ. It is considered to be the most promising next-generation technology to replace the anthraquinone process.

[0005] In recent years, researchers have developed a variety of photocatalysts, including metal oxides, sulfides, nitrides, organic polymers, and covalent organic frameworks. However, existing systems still suffer from the following key bottlenecks: insufficient light absorption capacity, with most catalysts only responding to ultraviolet light, resulting in low solar energy utilization; severe recombination of photogenerated carriers, leading to low electron-hole separation and migration efficiency and poor quantum yield; slow surface reaction kinetics, insufficient number of active sites, limited proton transport, and poor selectivity; and poor adaptability to acidic environments. Hydrogen peroxide is much more stable in acidic media than in neutral or alkaline systems, but most photocatalysts experience structural collapse and rapid activity decay in acidic environments.

[0006] Covalent organic frameworks (COFs) are crystalline porous organic polymers formed by covalently linking light elements (C, H, O, N, B, etc.). They possess unique advantages such as large specific surface area, ordered and designable structure, precise modification of functional groups, tunable photoelectric properties, and good stability, making them ideal supports for photocatalytic hydrogen peroxide production. However, existing COF-based photocatalysts still have significant technical shortcomings in acidic systems: the dissolved oxygen concentration in aqueous solutions is much higher than the proton concentration, resulting in insufficient local proton supply on the catalyst surface, severely limiting the 2e-ORR kinetics; traditional COF linkages (imine, borate esters, etc.) are easily hydrolyzed under acidic conditions, leading to poor structural stability; and the mismatch between the framework electronic structure and energy levels results in low charge separation and transport efficiency, making it difficult to achieve breakthroughs in catalytic activity.

[0007] Therefore, designing and fabricating covalent organic framework materials (COFs) capable of achieving efficient proton enrichment, structural stability, high charge transport efficiency, and excellent photocatalytic hydrogen peroxide production activity in acidic media has become a key problem urgently needing to be solved in this field. By rationally designing molecular structures and controlling bonding, overcoming the application bottlenecks of existing COF materials in acidic systems has significant theoretical and practical value for achieving efficient, stable, and highly selective photocatalytic hydrogen peroxide production under acidic conditions. Summary of the Invention

[0008] To address the technical problems of existing COF photocatalysts, such as structural instability and easy hydrolysis / collapse in acidic media, insufficient proton supply on the catalyst surface, and severe photogenerated electron-hole recombination, this invention provides a hydrazone-linked fluorine-substituted covalent organic framework material, its preparation method, and its applications. Using 1,3,5-trifluoro-2,4,6-tris(4-formylphenyl)benzene as a fluorine-substituted trialdehyde monomer and terephthalohydrazide (TPDH) as a dihydrazide monomer, a three-dimensional ordered crystalline porous F-TPDH-COF is constructed through hydrazone bond (-C=N-NH-) dehydration condensation. The hydrazone bonds provide Lewis basic sites, enabling H+ ionization under acidic conditions. + Reversible trapping and enrichment enhances surface proton concentration, while the strong electron-withdrawing effect of fluorine atoms modulates the electronic structure, optimizing band positions, promoting charge separation, and improving oxygen reduction selectivity. The resulting F-TPDH-COF is used for photocatalytic hydrogen peroxide production, exhibiting significantly superior activity and stability compared to similar COF materials.

[0009] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0010] A hydrazone-linked fluorinated covalent organic framework material, named F-TPDH-COF, is formed by the dehydration condensation of a fluorinated aldehyde monomer and an aromatic dihydrazide monomer via hydrazone bond, resulting in a highly crystalline, porous organic polymer. Its structural formula is shown in formula (I). The fluorinated aldehyde monomer is 1,3,5-trifluoro-2,4,6-tris(4-formylphenyl)benzene, whose electronic structure and band structure can be precisely controlled through the strong electron-withdrawing effect of fluorine atoms. The aromatic dihydrazide monomer is terephthalohydrazide, which provides dihydrazide active sites and efficiently forms hydrazone bonds with the aldehyde group, ensuring the structural stability and proton enrichment capacity of the framework under acidic conditions.

[0011] (I).

[0012] The above materials maintain a complete framework and stable structure in acidic media with pH values ​​ranging from 1.0 to 4.0, without exhibiting hydrolysis, collapse, or activity degradation. Under pH 2.0 conditions, the photocatalytic hydrogen peroxide production rate reaches as high as 5825.2 μmolg. -1 h -1 It is significantly superior to existing covalent organic framework photocatalysts and has outstanding potential for industrial application.

[0013] Furthermore, the above-mentioned material exhibits a fluffy nanoparticle cluster morphology with a large specific surface area and fully exposed active sites, which is conducive to substrate adsorption and charge transfer. The C, H, O, N and F elements in the framework are uniformly distributed without obvious segregation and agglomeration, and the structure is highly uniform. The X-ray diffraction pattern shows obvious and sharp characteristic diffraction peaks in the 2θ=2°~10° range, proving that the material has a typical covalent organic framework crystalline structure with high order and good crystallinity.

[0014] The above-mentioned method for preparing hydrazone-linked fluorine-substituted covalent organic framework materials is achieved through a mild route of stepwise monomer synthesis and one-pot solvothermal assembly. The reaction conditions are easy to control, reproducible, and suitable for scale-up production. The specific steps include:

[0015] S1. Dimethyl terephthalate and hydrazine hydrate were refluxed in a mixed solvent to prepare terephthalohydrazide monomer;

[0016] S2, 1,3,5-trifluoro-2,4,6-triiodobenzene and 4-formylphenylboronic acid undergo a Suzuki coupling reaction in the presence of potassium carbonate and the catalyst tetra(triphenylphosphine)palladium to prepare fluorinated aldehyde monomers. The palladium catalytic system enables efficient coupling of aryl iodine and arylboronic acid, while potassium carbonate activates boric acid and neutralizes byproduct acids, ensuring the high yield and high purity synthesis of the target fluorinated trialdehyde monomer.

[0017] S3. Fluorine-substituted aldehyde monomers and terephthalohydrazide monomers are reacted with an organic solvent and an acidic catalyst to obtain F-TPDH-COF via a solvothermal reaction. The acidic catalyst promotes the formation of hydrazone bonds, and the mixed solvent provides a suitable dissolution and assembly environment, allowing the two monomers to polymerize in an orderly manner to form a crystalline porous framework.

[0018] Further, in step S1, the mixed solvent is a mixture of methanol and toluene, and the hydrazine hydrate is a hydrazine hydrate solution with a mass fraction of 80-90%. The ratio of dimethyl terephthalate, methanol, toluene, and hydrazine hydrate is 6.0-9.0 mmol: 10-20 mL: 10-20 mL: 8-12 mL. The reaction atmosphere is under nitrogen protection, the reflux temperature is 65-75℃, and the reaction time is 20-28 h. The product is cooled, filtered, washed with methanol, and vacuum dried to obtain a white solid terephthalohydrazide, denoted as TPDH, with the following structural formula:

[0019] .

[0020] Further, in step S2, the molar ratio of 1,3,5-trifluoro-2,4,6-triiodobenzene, 4-formylphenylboronic acid, potassium carbonate, and tetrakis(triphenylphosphine)palladium is 1:5.0~7.0:5.0~7.0:0.04~0.06. Excess boric acid ensures complete conversion of the iodoaromatics, and the appropriate ratio of potassium carbonate to palladium catalyst maximizes coupling efficiency. The solvent is a mixture of 1,4-dioxane and water in a volume ratio of 4~6:0.8~1.2. Nitrogen degassing is performed for 10~20 min before the reaction, the reaction temperature is 90~110℃, and the reaction time is 60~84 h. The product is cooled, precipitated, and washed with water and ethyl acetate to obtain a fluorinated aldehyde monomer, denoted as F monomer, with the following structural formula:

[0021] .

[0022] Further, in step S3, the molar ratio of fluorinated aldehyde monomer to terephthalic acid hydrazine is 1:1.2~1.8. An appropriate excess of hydrazine monomer can ensure the complete reaction of the aldehyde group and improve the integrity and stability of the framework. The organic solvent is a mixture of n-butanol and o-dichlorobenzene in a volume ratio of 1:0.8~1.2. This mixed solvent has a high polarity matching degree, which is conducive to the growth of crystalline COF. The acidic catalyst is an aqueous solution of 2.5~3.5 mol / L acetic acid, which is added at 15%~25% of the total volume of the organic solvent to provide a mild acidic environment to promote the formation of hydrazone bonds, while avoiding framework destruction caused by strong acid. Before the reaction, the mixture is sonicated for 1~3 min, frozen in a liquid nitrogen bath at 77 K, pumped with nitrogen, and thawed in a cycle of degassing 2~5 times.

[0023] Further, in step S3, the solvothermal reaction temperature is 110~130℃, the reaction time is 2.5~3.5 days, after the reaction is completed, the product is collected by filtration, washed successively with tetrahydrofuran and acetone, and dried in air for 20~28 h to obtain a yellow final product, namely F-TPDH-COF.

[0024] The aforementioned hydrazone-linked fluorine-substituted covalent organic framework material is used as a photocatalyst to photocatalytically prepare hydrogen peroxide using water and oxygen as raw materials in an acidic medium. Leveraging its proton-enriching ability through hydrazone bonds, its electronic modulation capability of fluorine atoms, and its stable crystalline framework, the material can efficiently achieve photogenerated charge separation and two-electron oxygen reduction reactions, significantly improving the rate and selectivity of hydrogen peroxide generation.

[0025] Furthermore, the acidic medium has a pH value of 1.0 to 4.0, preferably pH=2.0, and the reaction light source is visible light or full spectrum, which can make full use of solar energy and does not need to rely on high-energy-consuming ultraviolet light sources; the reaction system temperature is 20 to 60°C, and oxygen is continuously introduced or exposed to the air atmosphere during the reaction process.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] (1) The F-TPDH-COF obtained in this invention produces H2O2 at a rate as high as 5825.2 μmolg in a strongly acidic medium with pH=2. -1 h -1 It is 2.8 times that of F-TADH-COF and 5.4 times that of F-HZ-COF, and is superior to most COF photocatalysts reported in the current literature.

[0028] (2) The material obtained by the present invention has strong acid resistance of hydrazone bonds and fluorine atoms enhance the hydrophobicity and chemical stability of the skeleton. The structure does not collapse and the activity does not decrease in a strong acid system.

[0029] (3) The material obtained by the present invention can efficiently capture, store and transport protons through the basic sites of hydrazone bonds, effectively solving the industry problem of local proton shortage in acidic systems.

[0030] (4) The present invention is prepared by a one-step solvothermal method, which is mild, and the raw materials are readily available, low in cost and simple in post-processing, making it suitable for large-scale industrial production. Attached Figure Description

[0031] Figure 1 XRD patterns of F-TPDH-COF, F-TADH-COF, and F-HZ-COF.

[0032] Figure 2 This is a scanning electron microscope (SEM) image of F-TPDH-COF.

[0033] Figure 3Mapping diagram of C, O, F, and N elements for F-TPDH-COF.

[0034] Figure 4 This is a comparison chart of the photocatalytic H2O2 production rates of the three materials under acidic or neutral conditions.

[0035] Figure 5 The photocurrent-time response curves for the three materials are shown. Detailed Implementation

[0036] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0037] Example 1

[0038] (1) Synthesis of terephthalic hydrazide (TPDH) monomer: 7.7 mmol of dimethyl terephthalate, 15 mL of methanol, 15 mL of toluene and 10 mL of 85% hydrazine hydrate were mixed and refluxed under nitrogen protection for 24 h. After the reaction solution was cooled, it was filtered, and the filter cake was washed with methanol and dried under vacuum to obtain white solid terephthalic hydrazide (TPDH).

[0039] (2) Synthesis of fluorinated aldehyde monomer (F monomer): 0.200 g (0.392 mmol) of 1,3,5-trifluoro-2,4,6-triiodobenzene, 0.354 g (2.35 mmol) of 4-formylphenylboronic acid, 0.325 g (2.35 mmol) of potassium carbonate, and 0.0227 g (0.0196 mmol) of tetra(triphenylphosphine)palladium were dissolved in a mixed solvent of 5 mL of 1,4-dioxane and 1 mL of distilled water. After mixing thoroughly, nitrogen gas was purged for 15 min to degas the mixture. The reaction tube was sealed and stirred at 100 °C for 72 h. After the reaction solution was cooled to room temperature, the precipitate was collected and washed thoroughly with water and ethyl acetate to obtain the fluorinated aldehyde monomer (F monomer).

[0040] (3) Preparation of F-TPDH-COF: 0.2 mmol of F monomer, 0.3 mmol of TPDH, 1 mL of n-butanol, 1 mL of o-dichlorobenzene, and 0.2 mL of 3 mol / L acetic acid aqueous solution were loaded into a Pyrex tube. The reaction tube was sonicated for 2 minutes, and then rapidly frozen and degassed using a liquid nitrogen bath at 77 K in three cycles of freezing-nitrogen pumping-thawing. After vacuum sealing, the tube was placed in an oven at 120 °C for 3 days. A yellow precipitate was formed, which was separated by filtration and washed with tetrahydrofuran and acetone, respectively. The sample was dried in air for 24 hours to obtain F-TPDH-COF.

[0041] Experimental results show that the photocatalytic H2O2 production rate of F-TPDH-COF obtained in Example 1 under an acidic system with pH=2 is 5825.2 μmolg. -1 h -1 .

[0042] Comparative Example 1

[0043] 0.2 mmol of monomer F, 0.3 mmol of dihydrazine tartrate (TADH), 1 mL of n-butanol, 1 mL of o-dichlorobenzene, and 0.2 mL of 6M aqueous acetic acid solution were added to a Pyrex tube. All other operations, reaction conditions, and post-treatment steps were exactly the same as in Example 1, yielding F-TADH-COF, whose structural formula is as follows:

[0044] .

[0045] Experimental results show that the photocatalytic H2O2 production rate of F-TADH-COF obtained in Comparative Example 1 under an acidic system with pH=2 is 2038.9 μmolg. -1 h -1 Aliphatic dihydrazides cannot provide a stable aromatic conjugated framework, have weak proton enrichment capacity, and significantly reduced charge transport efficiency, thus resulting in a substantial decrease in catalytic activity.

[0046] Comparative Example 2

[0047] 0.2 mmol of monomer F, 0.3 mmol of hydrazine (HZ), 0.5 mL of n-butanol, 0.5 mL of o-dichlorobenzene, and 0.2 mL of 6M aqueous acetic acid solution were added to a Pyrex tube. All other operations, reaction conditions, and post-treatment steps were exactly the same as in Example 1, yielding F-HZ-COF, whose structural formula is as follows:

[0048] .

[0049] Experimental results show that the photocatalytic H2O2 production rate of F-HZ-COF obtained in Comparative Example 2 under an acidic system with pH=2 is 1078.1 μmolg. -1 h -1 The single hydrazine unit tends to result in low framework cross-linking and structural disorder, making it impossible to form effective proton enrichment sites. This leads to severe recombination of photogenerated carriers, resulting in a significant decrease in catalytic activity.

[0050] F-TPDH-COF is illustrated using the product obtained in Example 1 as a typical example.

[0051] Depend on Figure 1 The XRD characterization diagrams show that F-TPDH-COF, F-TADH-COF, and F-HZ-COF all exhibit sharp characteristic diffraction peaks in the range of 2θ = 2° to 10°, proving the successful synthesis of crystalline covalent organic framework structures.

[0052] Depend on Figure 2 SEM morphology shows that F-TPDH-COF exhibits a fluffy nanoparticle cluster morphology with a large specific surface area and fully exposed active sites.

[0053] Depend on Figure 3 As can be seen from the element mapping diagram, the C, O, F and N elements in F-TPDH-COF are evenly distributed, with no obvious aggregation, and the structure is highly homogeneous.

[0054] Depend on Figure 4 It can be seen that under acidic conditions of pH=2, the hydrogen peroxide production rate of F-TPDH-COF is as high as 5825.2 μmolg. - 1 h -1 It is 2.8 times that of F-TADH-COF and 5.4 times that of F-HZ-COF, and exceeds most currently reported COF photocatalysts.

[0055] from Figure 5 It can be seen that F-TPDH-COF has a higher response value than other materials, indicating its excellent photogenerated carrier separation efficiency.

Claims

1. A hydrazone-linked fluorine-substituted covalent organic framework material, characterized in that, The hydrazone-linked fluorinated covalent organic framework material is named F-TPDH-COF, and its structural formula is shown in formula (I). It is a crystalline porous polymer formed by the hydrazone condensation of a fluorinated aldehyde monomer and an aromatic dihydrazide monomer. The fluorinated aldehyde monomer is 1,3,5-trifluoro-2,4,6-tris(4-formylphenyl)benzene, and the aromatic dihydrazide monomer is terephthalohydrazide. （I）。 2. The hydrazone-linked fluorine-substituted covalent organic framework material as described in claim 1, characterized in that, The material has a fluffy nanoparticle cluster morphology, with C, H, O, N and F elements uniformly distributed in the framework. The X-ray diffraction pattern shows obvious characteristic diffraction peaks in the range of 2θ=2°~10°, exhibiting a typical covalent organic framework crystalline structure.

3. The method for preparing hydrazone-linked fluorine-substituted covalent organic framework materials as described in claim 1 or 2, characterized in that, Includes the following steps: S1. Dimethyl terephthalate and hydrazine hydrate were refluxed in a mixed solvent to prepare terephthalohydrazide monomer; S2, 1,3,5-trifluoro-2,4,6-triiodobenzene and 4-formylphenylboronic acid undergo a Suzuki coupling reaction in the presence of potassium carbonate and the catalyst tetra(triphenylphosphine)palladium to prepare fluorinated aldehyde monomers. S3. F-TPDH-COF is obtained by reacting fluorinated aldehyde monomers with terephthalohydrazide monomers in an organic solvent and an acidic catalyst via a solvothermal reaction.

4. The preparation method according to claim 3, characterized in that, In step S1, the mixed solvent is a mixture of methanol and toluene, the hydrazine hydrate is a hydrazine hydrate solution with a mass fraction of 80-90%, and the ratio of dimethyl terephthalate, methanol, toluene and hydrazine hydrate is 6.0-9.0 mmol: 10-20 mL: 10-20 mL: 8-12 mL.

5. The preparation method according to claim 3, characterized in that, In step S1, the reaction atmosphere is nitrogen protection, the reflux temperature is 65~75℃, and the reaction time is 20~28h.

6. The preparation method according to claim 4, characterized in that, In step S2, the molar ratio of 1,3,5-trifluoro-2,4,6-triiodobenzene, 4-formylphenylboronic acid, potassium carbonate, and tetra(triphenylphosphine)palladium is 1:5.0~7.0:5.0~7.0:0.04~0.

06. The solvent is a mixture of 1,4-dioxane and water in a volume ratio of 4~6:0.8~1.

2. Nitrogen is used for degassing before the reaction for 10~20 min, the reaction temperature is 90~110℃, and the reaction time is 60~84 h. The product is cooled, precipitated, and washed with water and ethyl acetate to obtain a fluorinated aldehyde monomer, denoted as F monomer.

7. The preparation method according to claim 4, characterized in that, In step S3, the molar ratio of fluorinated aldehyde monomer to terephthalohydrazide is 1:1.2~1.8, the organic solvent is a mixture of n-butanol and o-dichlorobenzene in a volume ratio of 1:0.8~1.2, the acid catalyst is an aqueous solution of 2.5~3.5 mol / L acetic acid, and its addition amount is 15%~25% of the total volume of organic solvent. Before the reaction, the mixture is subjected to ultrasonication for 1~3 min, freezing in a liquid nitrogen bath at 77 K, nitrogen pumping, and thawing cycle degassing 2~5 times.

8. The preparation method according to claim 4, characterized in that, In step S3, the solvothermal reaction temperature is 110~130℃, the reaction time is 2.5~3.5 days, after the reaction is completed, the product is collected by filtration, washed with tetrahydrofuran and acetone in sequence, and dried in air for 20~28h to obtain the yellow final product, namely F-TPDH-COF.

9. The application of the hydrazone-linked fluorine-substituted covalent organic framework material according to claim 1 or 2, characterized in that, The hydrazone-linked fluorine-substituted covalent organic framework material is used as a photocatalyst to photocatalytically prepare hydrogen peroxide using water and oxygen as raw materials in an acidic medium.

10. The application according to claim 9, characterized in that, The acidic medium has a pH value of 1.0 to 4.0, the reaction light source is visible light or full spectrum, the reaction system temperature is 20 to 30°C, and oxygen is continuously introduced or the system is exposed to air during the reaction process.

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