Preparation method and application of a three-step sulfonic acid functionalized COF material

Abstract

This invention belongs to the field of new materials technology, specifically relating to a three-step method for preparing and applying sulfonic acid-functionalized COF materials. Using 1,3,6,8-tetra-(p-aminophenyl)-pyrene and 2,5-dihydroxyterephthalaldehyde as monomers, this invention constructs a pyrene-based COF rich in phenolic hydroxyl groups and imine bonds via Schiff base condensation. To achieve high-density grafting of sulfonic acid groups within the pores, a three-step post-modification strategy is designed: 1,3-propanesulfonic acid lactone reacts with the phenolic hydroxyl groups in the framework to introduce the first batch of sulfonic acid groups; sodium cyanoborohydride is used to selectively reduce the imine bonds to secondary amine groups, providing new reaction sites without destroying the crystallinity of the COF; 1,3-propanesulfonic acid lactone is used again to undergo ring-opening grafting with the secondary amine groups to introduce the second batch of sulfonic acid groups. Through this step-by-step post-modification path, dual, high-density anchoring of sulfonic acid groups on the pore walls of COF was successfully achieved. Subsequent introduction of sodium ions further enhanced the synergistic effect of water absorption and proton density, providing a reliable guarantee for proton conduction under high temperature and humidity conditions. The proton conductivity reached 1.03 × 10⁻⁶ at 80℃–98%RH. ‑1 S cm ‑1 The preparation process of this invention is simple and low-cost, and it has good application prospects.

Description

Technical Field

[0001] This invention belongs to the field of new materials technology, specifically relating to a three-step sulfonic acid functionalized COF material preparation method and its application. Background Technology

[0002] Covalent organic frameworks (COFs), a class of crystalline porous materials formed by organic monomers linked by covalent bonds, possess high specific surface area, regular pore structure, and pre-designable framework functions, showing broad application prospects in gas separation, catalysis, sensing, and energy storage and conversion. Particularly in the field of proton exchange membrane fuel cells, COFs are considered highly promising proton conduction materials due to their ordered one-dimensional nanopores and functionalizable framework.

[0003] Currently, strategies for improving the proton conduction performance of COFs mainly fall into two categories: one is loading proton carriers (such as small molecules like phosphate, imidazole, and triazole) into the pores, and the other is introducing proton-conducting groups (such as sulfonic acid groups and phosphate groups) onto the framework through covalent modification. Compared to direct physical loading into the pores, covalent grafting of proton-conducting groups can achieve stable anchoring of the proton source on the framework, making it a reliable approach for constructing intrinsically proton-conducting COFs.

[0004] Among numerous proton-conducting groups, sulfonic acid groups (-SO3H) are widely used due to their strong acidity and good hydrophilicity. In existing technologies, sulfonic acid groups are typically introduced into the COF backbone in two ways: one is by directly using sulfonated monomers for polymerization, but sulfonic acid groups may interfere with the Schiff base condensation reaction under polymerization conditions, leading to reduced crystallinity or even failure to form a COF; the other is through post-modification strategies, utilizing active sites (such as hydroxyl and amino groups) on the backbone to react with reagents such as propanesulfonate lactone to graft sulfonic acid chains. For example, studies have reported the construction of hydroxyl-functionalized COFs using 2,5-dihydroxyterephthalaldehyde and 1,3,6,8-tetra-(p-aminophenyl)-pyrene as monomers, and the use of their phenolic hydroxyl groups to react with propanesulfonate lactone to achieve a single round of sulfonic acid grafting. However, this strategy only utilizes the phenolic hydroxyl group in the backbone as a single grafting site, resulting in a limited grafting density of sulfonic acid groups, which restricts further improvement in proton conduction performance. On the other hand, the imine bond (C=N) is the most common linking unit in Schiff base COFs, but it has poor chemical stability under acidic or basic conditions and does not possess proton conduction capabilities. Converting the imine bond to a secondary amine (-NH-) through a reduction reaction can not only significantly improve the skeletal stability of the COF, but also provide new nucleophilic sites for post-modification.

[0005] The above methods are highly dependent on the control between COF pore size and sulfonic acid density. There are no reports in the existing technology of combining hydroxyl sulfonation with secondary sulfonation after imine reduction, nor are there any designs that synergistically enhance water absorption and proton density by introducing sodium ions. Summary of the Invention

[0006] In order to solve the technical problems in the background art, the present invention provides a novel three-step sulfonic acid functionalized COF material.

[0007] The present invention designs a high-density sulfonic acid-grafted covalent organic framework material, which is a sulfonic acid-functionalized reduced imine pyrene covalent organic framework. The covalent organic framework is prepared by the Schiff base condensation reaction of monomers 1,3,6,8-tetra-(p-aminophenyl)-pyrene and 2,5-dihydroxyterephthalaldehyde, and then prepared by a post-modification strategy.

[0008] The beneficial effects of this invention are: by employing a dual grafting strategy of hydroxyl and secondary amine, propanesulfonic acid lactone is introduced twice, achieving high-density anchoring of sulfonic acid groups within the COF channels, significantly increasing the proton-conducting functional group loading of the material. The resulting material achieves a proton conductivity of 10 under high temperature and high humidity conditions. -1 S·cm -1 Magnitude.

[0009] This invention also provides a three-step method for preparing sulfonic acid-functionalized COF materials, wherein the materials are obtained sequentially through the following post-modification steps: The first step involves introducing the first batch of sulfonic acid groups (COF-S) by using 1,3-propanesulfonic acid lactone to undergo a ring-opening reaction with the phenolic hydroxyl groups in the backbone; The second step involves using sodium cyanoborohydride to selectively reduce imine bonds to secondary amine groups, providing new reaction sites without compromising the crystallinity of the framework. The third step involves using 1,3-propanesulfonic acid lactone to undergo a ring-opening grafting reaction with secondary amino groups again, introducing a second batch of sulfonic acid groups, thereby achieving high-density anchoring of COF-2S with sulfonic acid groups within the pores; Optionally, further treatment with sodium hydroxide solution converts some of the sulfonic acid groups into sodium sulfonate form to enhance the water absorption properties of the material COF-2S-Na. + .

[0010] The beneficial effects of this invention are: 1. This invention selectively reduces imine bonds to secondary amines using sodium cyanoborohydride, which enhances the chemical stability of the COF skeleton and provides highly reactive nucleophilic sites for the second sulfonic acid grafting. At the same time, all sulfonic acid groups are covalently linked, effectively avoiding the leakage problem of traditional supported proton carriers and ensuring the long-term stability of the material.

[0011] 2. This invention further introduces sodium ions to convert sulfonic acid groups into sodium sulfonate, which significantly enhances the water absorption performance of the material and forms a more continuous hydrated hydrogen bond network in the pores. This network works synergistically with the high-density sulfonic acid groups to provide a reliable guarantee for efficient proton conduction under high temperature and high humidity conditions. Attached Figure Description

[0012] Figure 1 The present invention relates to a three-step sulfonic acid functionalized COF material, COF-2S-Na. + A schematic diagram of the synthesis; Figure 2 The PXRD diagrams for each stage of the three-step sulfonic acid functionalized COF material of this invention are shown below. The vertical axis represents angle and the vertical axis represents intensity. Figure 3 The image shows the infrared spectra of each stage of the three-step sulfonic acid functionalized COF material of this invention. The horizontal axis represents the wavenumber, and the unit is usually cm. -1 The vertical axis represents transmittance; Figure 4 This is a comparison chart of sulfur content in the three-step sulfonic acid functionalized COF materials of this invention. The horizontal axis represents the material name, and the vertical axis represents the sulfur content. Figure 5 This is a comparison chart of the theoretical and actual sodium content of the three-step sulfonic acid functionalized COF material of this invention. The vertical axis represents the sodium content. Figure 6 This is a comparison chart of the contact angles of the three-step sulfonic acid functionalized COF materials of this invention. The horizontal axis represents the material name, and the vertical axis represents the contact angle. Figure 7 The present invention relates to a three-step sulfonic acid functionalized COF material, COF-2S-Na. + The humidity-dependent proton conductivity at 80 °C is represented by Z' on the x-axis and Z on the y-axis.

[0013] Figure 8 The present invention relates to a three-step sulfonic acid functionalized COF material, COF-2S-Na. + Temperature-dependent proton conductivity at 98% humidity, x-axis is Z', y-axis is Z. Detailed Implementation

[0014] The principles and features of the present invention will be described in detail below with reference to implementation. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention. Example

[0015] A three-step method for preparing sulfonic acid-functionalized COF materials, the preparation process including the following steps: A was prepared using 1,3,6,8-tetra-(p-aminophenyl)-pyrene (0.05 mmol) and 2,5-dihydroxyterephthalaldehyde (0.05 mmol) as monomers, added in an equimolar ratio to a mixed solvent of o-dichlorobenzene / n-butanol (1:1, v / v), with 0.2 mL of 6 M aqueous acetic acid solution as a catalyst. After degassing via a freeze-vacuum-thaw cycle, the mixture was sealed and reacted at 120 °C for 72 hours. The resulting precipitate was purified by Soxhlet extraction (tetrahydrofuran), washed, and vacuum dried to obtain the target COF.

[0016] B. Weigh 20 mg of the dried COF and place it in a round-bottom flask. Measure 3.6 mL of 1,3-propanesulfonyl lactone solution and add 10 mL of anhydrous toluene as the reaction solvent. Under a nitrogen atmosphere, heat the reaction system to 120 °C and stir for 24 hours to allow propanesulfonyl lactone to fully undergo a ring-opening grafting reaction with the phenolic hydroxyl groups in the COF skeleton. After the reaction is complete, allow it to cool naturally to room temperature, centrifuge to collect the solid product, and wash it three times each with acetone and deionized water to remove unreacted 1,3-propanesulfonyl lactone and byproducts. Finally, dry it under vacuum at 60 °C for 12 hours to obtain the first batch of sulfonated products, denoted as COF-S.

[0017] C. 50 mg of COF-S was added to 10 mL of anhydrous tetrahydrofuran and ultrasonically dispersed for 15 min. Glacial acetic acid (100 μL) was added dropwise to adjust the system to weakly acidic. 100 mg of sodium cyanoborohydride (NaBH3CN) was added, and the mixture was stirred at room temperature for 12 hours under nitrogen protection to selectively reduce the imine bond to a secondary amine. After the reaction was complete, the solid was collected by centrifugation and washed three times each with methanol, 0.1 M hydrochloric acid, deionized water, and acetone. The solid was then dried under vacuum at 60 °C for 12 hours to obtain the reduced COF. 40 mg of the reduced COF was added to 10 mL of anhydrous toluene and stirred to disperse. 7.2 mL of propanesulfonic acid lactone was added, and the mixture was heated to 120 °C for 12 hours under nitrogen protection to achieve the second sulfonic acid grafting of the secondary amine site. After the reaction was complete, the product was centrifuged and washed successively with DMF, anhydrous ethanol, deionized water, and acetone. The product was then dried under vacuum at 60 °C for 12 hours to obtain a high-density sulfonic acid-grafted COF, denoted as COF-2S.

[0018] D. The above COF-2S material was immersed in a 0.01 M sodium hydroxide aqueous solution and magnetically stirred at room temperature for 3 hours to partially convert the sulfonic acid groups into sodium sulfonate, thereby enhancing the water absorption of the material. After treatment, the solid was collected by centrifugation, repeatedly washed with deionized water, and finally vacuum dried at 60°C for 12 hours to obtain the final sodium ion functionalized high-density sulfonated COF material, denoted as COF-2S-Na⁺.

[0019] Morphological and structural characterization: like Figure 1As shown, through a stepwise post-modification process, the first covalent introduction of sulfonic acid groups is achieved by reacting propanesulfonic acid lactone with the phenolic hydroxyl groups in the COF framework. Subsequently, propanesulfonic acid lactone is grafted again onto the secondary amine sites after reduction treatment with sodium cyanoborohydride, achieving high-density anchoring of sulfonic acid groups within the pores. Further immersion in sodium hydroxide solution converts some of the sulfonic acid groups into sodium sulfonate form, enhancing the material's water absorption. After the reaction, the product is thoroughly washed with deionized water and organic solvents to completely remove unreacted reagents and byproducts remaining on the surface, ultimately yielding a high-density sulfonic acid-grafted covalent organic framework material.

[0020] like Figure 2 The figure shows the PXRD patterns of the three-step sulfonic acid functionalized COF material at each stage. It can be seen from the figure that after several steps of modification, the intensity of the characteristic diffraction peaks decreased and the crystallinity decreased to a certain extent. This phenomenon is mainly attributed to the interference of the connection of sulfonic acid branches in the pores with the original lattice regularity. This indicates that the post-modification does not significantly damage the crystallinity of the structure and still makes the structure intact.

[0021] like Figure 3 As shown, each stage of the three-step sulfonic acid functionalized COF material was detected by Fourier transform infrared spectroscopy (FT-IR). The infrared measurement was performed using the KBr pellet method, and samples were collected from 4000 to 450 cm⁻¹ on the spectrometer. -1 Infrared spectrum within the region. From Figure 3 It can be seen from the inside that at 1035 cm -1 An infrared characteristic peak with S=O appeared at 1160 cm⁻¹. -1 The infrared characteristic peak of successfully grafted COC with hydroxyl groups was observed at 1073 cm⁻¹. -1 The infrared characteristic peaks of the reduced CN were observed, and the C=N peak was weakened. The appearance of these three characteristic peaks directly confirms that the hydroxyl group and the reduced secondary amine bond were successfully grafted with sulfonic acid in this invention.

[0022] like Figure 4 As shown, the sulfur content of the three-step sulfonic acid functionalized COF material was determined by inductively coupled plasma mass spectrometry (ICP-MS). By comparing the sulfur content in the first and second steps, it can be seen that the density of the grafted sulfonic acid gradually increases, confirming that the present invention can effectively increase the content of grafted sulfonic acid.

[0023] like Figure 5 As shown, the sodium content of the three-step sulfonic acid functionalized COF material was determined by inductively coupled plasma mass spectrometry (ICP-MS). The theoretical sodium content was 10.69%, and the experimental sodium content was 5.72%, proving that sodium was successfully replaced in the sulfonic acid-modified COF.

[0024] like Figure 6As shown, the three-step sulfonic acid-functionalized COF material was measured using an optical contact angle meter. Images of droplets on a solid surface were captured using a high-resolution camera, and the droplet profile was analyzed using specialized software to automatically calculate the angle. Figure 6 It can be concluded that the hydrophilicity of the material is improved with the increase of sulfonic acid content. After the addition of sodium ions, the hydrophilicity of the material changes abruptly, confirming that the sodium ions in this invention are successfully incorporated.

[0025] like Figure 7 As shown, the proton conduction performance of the three-step sulfonic acid functionalized COF material was tested using an AC impedance meter. Figure 7 This is a graph showing the proton conductivity of the material as a function of temperature. It can be seen that at 40 ℃ and 98% RH, COF-2S-Na... + The proton conductivity reached 2.4 × 10⁻⁶. -2 S cm -1 .

[0026] like Figure 8 As shown, the proton conduction performance of the three-step sulfonic acid functionalized COF material was tested using an AC impedance meter. Figure 8 This is a graph showing the proton conductivity of a material as a function of humidity, compared to the temperature change ( Figure 7 Humidity has a significant impact on the material; at 80 ℃ and 60% RH, the proton conductivity is 8.04 × 10⁻⁶. -3 S cm -1 When the temperature is increased to 80 °C and 98% RH, the proton conductivity increases to 1.03 × 10⁻⁶. -1 S cm -1 .

[0027] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing and applying a three-step sulfonic acid-functionalized COF material, characterized in that, COF was synthesized by amine-aldehyde condensation of 1,3,6,8-tetra-(p-aminophenyl)-pyrene and 2,5-dihydroxyterephthalaldehyde. The hydroxyl groups and the two active sites of the reduced secondary amine were sulfonated. Then, some of the sulfonic acid groups were converted into sodium sulfonate groups by acid-base neutralization, thereby introducing hydrophilic sodium ions.

2. The preparation method of the three-step sulfonic acid functionalized COF material according to claim 1, characterized in that, Includes the following steps: A. Using 1,3,6,8-tetra-(p-aminophenyl)-pyrene (0.05 mmol) and 2,5-dihydroxyterephthalaldehyde (0.05 mmol) as monomers, they were added in an equimolar ratio to a mixed solvent of o-dichlorobenzene / n-butanol (1:1, v / v). A 6 M aqueous acetic acid solution (0.2 mL) was used as a catalyst. After degassing via a freeze-vacuum-thaw cycle, the mixture was sealed and reacted at 120 °C for 72 hours. The resulting precipitate was purified by Soxhlet extraction (tetrahydrofuran), washed, and vacuum dried to obtain the target COF. B. Weigh 20 mg of COF, measure 3.6 mL of 1,3-propanesulfonyl lactone solution, add 10 mL of toluene solution, stir at 120 °C for 24 hours under nitrogen atmosphere, and finally wash the product with acetone and water to remove unreacted 1,3-propanesulfonyl lactone to obtain COF-S. C. Add 50 mg of sulfonated COF to 10 mL of anhydrous tetrahydrofuran and sonicate for 15 min. Add 100 μL of glacial acetic acid. Add 100 mg of NaBH3CN and stir at room temperature for 36 h under nitrogen protection. Centrifuge and wash successively with methanol, 0.1 M HCl, water, and acetone. Dry under vacuum at 60 °C for 12 h to obtain reduced COF. Add 40 mg of reduced COF to 10 mL of anhydrous toluene and stir. Add 7.2 mL of propanesulfonate lactone and react at 120 °C for 12 h under nitrogen protection. Centrifuge and wash successively with DMF, ethanol, water, and acetone. Dry under vacuum at 60 °C for 12 h to obtain high-density sulfonated COF-2S. D. The above-mentioned high-density sulfonic acid product was soaked in 0.01 M NaOH aqueous solution for 3 h, centrifuged, and dried at 60℃ to obtain the COF material COF-2S-Na. + .

3. The preparation method of the three-step sulfonic acid functionalized COF material according to claim 2, characterized in that, The synthesized COF was obtained by a solvothermal reaction.

4. The method for preparing the three-step sulfonic acid functionalized COF material according to claim 2, characterized in that, The hydroxyl groups in the structure are modified to activate the sulfonic acid, which is then grafted into the COF structure to increase the sulfonic acid density as a first step.

5. The method for preparing the three-step sulfonic acid functionalized COF material according to claim 2, characterized in that, The imine bond is reduced, and the resulting secondary amine is grafted with sulfonic acid to increase the sulfonic acid density in the structure.

6. The method for preparing the three-step sulfonic acid functionalized COF material according to claim 2, characterized in that, The sodium hydroxide solution was partially neutralized with the post-modified high-density sulfonic acid to make the structure contain both hydrophilic groups and proton sources.

7. The application of the high-density sulfonic acid-grafted covalent organic framework material according to claim 1 in the field of proton conduction, characterized in that, The high-density sulfonic acid-grafted covalent organic framework material was used as a proton conduction material, and its proton transport performance under high temperature and high humidity conditions was tested using an AC impedance meter.

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