Method for the mild aqueous phase synthesis of ionic covalent organic framework nanosheets and applications of the nanosheets

Abstract

This invention discloses a mild aqueous-phase synthesis method for ionic covalent organic framework nanosheets. Tp and Py are added to anhydrous ethanol at a molar ratio of 1:3. After reaction under stirring, a pale yellow solid is obtained by filtration, washing, and drying. This solid is used as an aldehyde monomer and reacts with an amino monomer containing ionic groups in a pure aqueous system under mild conditions catalyzed by Brønsted acid, thereby achieving the direct synthesis of ionic covalent organic framework nanosheets. By introducing Brønsted acid into the reaction system to regulate the hydrolysis rate of the aldehyde monomer and the formation rate of the covalent organic framework, the reaction can be completed in a pure aqueous system at room temperature and pressure, significantly simplifying the synthesis process and eliminating the need for toxic organic solvents, thus exhibiting good green sustainability. The obtained nanosheets have excellent processability and can be applied to the preparation of membrane materials such as nanofiltration membranes and tubular composite membranes.

Description

Technical Field

[0001] This invention relates to a method for synthesizing ionic covalent organic framework nanosheets, and more particularly to a method and application for directly synthesizing ionic covalent organic framework nanosheets in a pure water system under mild conditions. Background Technology

[0002] Covalent organic frameworks (COFs) are a new class of organic crystalline materials with rigid skeletons, ordered channels, and abundant organic functional groups, offering wide applications in separation, catalysis, and energy storage and conversion. Furthermore, the long-range ordered and tunable pore structure of COFs provides significant advantages in membrane separation. Traditional methods for preparing COFs involve solvothermal synthesis, which typically involves toxic organic solvents, high reaction temperatures (120–200℃), and complex operational steps such as freezing-vacuuming-freezing cycles. The final product is a COF powder, and these drawbacks significantly hinder the further development and practical application of COF materials. Therefore, it is essential to develop a method that is environmentally friendly, operates under mild conditions, is simple to operate, and synthesizes COFs with excellent processability.

[0003] Covalent organic framework nanosheets possess excellent processability and film-forming properties, making their assembly into membranes a key strategy for their application in membrane separation. During the growth of covalent organic framework nanosheets, in-plane expansion and out-of-plane stacking often occur simultaneously. However, due to significant π-π stacking interactions between layers, out-of-plane growth dominates, inhibiting the ordered stacking of nanosheets and ultimately leading to precipitation.

[0004] The synthesis methods for covalent organic framework nanosheets mainly include single-phase, two-phase, and three-phase methods. However, due to the poor solubility of aldehyde monomers in water, they usually need to be dissolved in organic solvents before reacting with amino monomers to construct the COF structure. Therefore, this synthesis process generally requires the use of organic solvents with certain toxicity, such as dimethyl sulfoxide (DMSO), mesitylene, tetrahydrofuran (THF), and dioxane. Summary of the Invention

[0005] In view of the above-mentioned prior art, the present invention provides a method for directly synthesizing ionic covalent organic framework nanosheets in a pure aqueous system under mild conditions. The method uses pre-synthesized TpPy as an aldehyde monomer and Brønsted acid as a catalyst to carry out a Schiff base reaction with an amino monomer containing ionic groups in a pure aqueous system under mild reaction conditions, thus successfully preparing ionic covalent organic framework nanosheets and avoiding the use of toxic organic solvents.

[0006] To address the aforementioned technical problems, this invention proposes a mild aqueous-phase synthesis method for ionic covalent organic framework nanosheets. First, trialdehyde phloroglucinol and pyrrolidine are added to anhydrous ethanol, wherein the molar ratio of trialdehyde phloroglucinol to pyrrolidine is 1:3. The reaction is carried out under stirring. After the reaction, a pale yellow solid is obtained by filtration, washing, and drying. Subsequently, this solid is used as an aldehyde monomer and reacted with an amino monomer containing an ionic group under Schiff base reaction conditions. Brønsted acid is introduced into the reaction system to regulate the hydrolysis rate of the aldehyde monomer and the formation rate of the covalent organic framework, thereby achieving in-situ synthesis of ionic covalent organic framework nanosheets in a pure aqueous phase system at room temperature and pressure. After the reaction, the product solution is diluted and dialyzed to finally obtain a homogeneous solution of ionic covalent organic framework nanosheets. The specific steps are as follows:

[0007] Step 1) Synthesis of aldehyde monomer: Dissolve an appropriate amount of trialdehyde phloroglucinol in anhydrous ethanol, then add an appropriate amount of pyrrolidine to form a suspension, wherein the molar ratio of trialdehyde phloroglucinol to pyrrolidine is 1:3; stir the reaction at room temperature for 2-4 hours. After the reaction is completed, filter, wash and dry to obtain a pale yellow solid, which is the aldehyde monomer, denoted as TpPy monomer.

[0008] Step 2) Synthesis of ionic covalent organic framework nanosheets: The TpPy monomer obtained in step 1) is dissolved in a solvent and sonicated to obtain an aldehyde monomer solution; the amino monomer with ionic groups is dissolved in a solvent and sonicated to obtain an amino monomer solution; the aldehyde monomer solution is transferred to a container, and the amino monomer solution is slowly added dropwise while stirring, wherein the molar ratio of aldehyde to amino is 1:1. After the addition is complete and sonicated, Brønsted acid is added to the above reaction system, wherein the molar ratio of Brønsted acid to amino monomer is 6-35:9, and the reaction is carried out at room temperature and pressure in the dark for 5-8 days.

[0009] Step 3) After the reaction is complete, the product solution is diluted and dialyzed to obtain a homogeneous solution of ionic covalent organic framework nanosheets.

[0010] Furthermore, in the synthesis method described in this invention, wherein:

[0011] In step 1), the process conditions for filtration, washing and drying are as follows: filter with anhydrous diethyl ether, wash with anhydrous diethyl ether until the washing liquid is colorless, and dry in a vacuum oven at 25°C for 24 hours.

[0012] In step 2), both the TpPy monomer and the amino monomer are dissolved in deionized water.

[0013] In step 2), the Brønsted acid is one of acetic acid, p-aminobenzenesulfonic acid, or sodium bicarbonate.

[0014] In step 2), depending on the selected amino monomer, the molar ratio of Brønsted acid to amino monomer in the reaction system is as follows: if the amino monomer is 2,5-diaminobenzenesulfonic acid, the molar ratio of Brønsted acid to amino monomer is 6–35:9; if the amino monomer is 2,5-diaminobenzoic acid, the molar ratio of Brønsted acid to amino monomer is 0.5–2:1; if the amino monomer is 1,3-phenylenediamine dihydrazide-5-sulfonic acid, the molar ratio of Brønsted acid to amino monomer is 0.5–2:1.

[0015] In step 3), the process conditions for diluting and dialysis of the product solution are as follows: the product solution is diluted with deionized water and then placed in a dialysis bag for dialysis for 3 days.

[0016] The ionic covalent organic framework nanosheets prepared by the synthesis method of this invention are used as membrane building units to construct nanofiltration membranes or tubular composite membranes for lithium-magnesium separation. The constructed nanofiltration membrane has a dense, uniform, smooth, and flat surface, and the pure water flux of the nanofiltration membrane is 30.1 L·m -2 ·h-1·bar -1 The lithium-magnesium selectivity was 22.3. The constructed tubular composite membrane exhibited a magnesium chloride rejection rate of 96.2% and a flux of 5.8 L·m⁻¹. -2 ·h-1·bar -1 .

[0017] Compared with the prior art, the advantages of the present invention are as follows:

[0018] In the synthesis method of this invention, TpPy monomer, which is synthesized by reacting trialdehyde phloroglucinol with pyrrolidine in advance, is proposed as the aldehyde monomer. Tp-Py has good water solubility, avoiding the use of highly toxic organic solvents in the process of synthesizing covalent organic framework nanosheets, which is green and environmentally friendly.

[0019] In the synthesis method of this invention, an amino monomer containing ionic groups is selected and reacted with TpPy via a Schiff base reaction to synthesize ionic covalent organic framework nanosheets. The electrostatic repulsion generated by the ionic groups inhibits the π-π stacking between nanosheet layers to a certain extent, promoting in-plane growth, thereby ultimately obtaining a covalent organic framework material existing in the form of nanosheets.

[0020] The covalent organic framework nanosheets synthesized in this invention exhibit good processability and can be further processed into nanofiltration membranes or tubular composite membranes. Attached Figure Description

[0021] Figure 1 This invention presents the chemical reaction formula for synthesizing TpPy monomers using trialdehyde phloroglucinol (Tp) and pyrrolidine (Py).

[0022] Figure 2 This is a simplified schematic diagram illustrating the synthesis of TpPa-SO3H in a pure aqueous phase system under mild conditions according to the present invention.

[0023] Figure 3 The images in a and b are optical photographs of the covalent organic frameworks prepared in Example 1 and Comparative Examples 1-3, respectively.

[0024] Figure 4 Fourier transform infrared spectra of TpPy, 2,5-diaminobenzenesulfonic acid, and TpPa-SO3H covalent organic framework.

[0025] Figure 5 The ae in the figures represent the physical morphology of the covalent organic frameworks prepared in Examples 1-3, 4-6, 7-9 and Comparative Examples 1-2 as observed under a scanning electron microscope (SEM).

[0026] Figure 6 The atomic force microscopy (AFM) characterization results are shown for the TpPa-SO3H covalent organic framework nanosheets prepared in Example 1.

[0027] Figure 7 The figures in a and b represent the powder X-ray diffraction results of the covalent organic frameworks prepared in Examples 1-3 and Comparative Example 3, respectively.

[0028] Figure 8 In Figures a and b, respectively, we can see the physical images of nanofiltration membranes and tubular composite membranes fabricated using the TpPa-SO3H covalent organic framework nanosheets prepared in Example 1.

[0029] Figure 9 In Figures a and b, the separation performance of lithium and magnesium ions by nanofiltration membranes fabricated from TpPa-SO3H covalent organic framework nanosheets prepared according to Example 1 under mixed salt conditions is shown. Figure 9 The middle section (c) demonstrates the magnesium ion retention performance of the same material after it is processed into a tubular composite membrane. Detailed Implementation

[0030] The present invention proposes a method for the direct synthesis of ionic covalent organic framework nanosheets in a pure aqueous system under mild conditions. The method involves the reaction of trialdehyde phloroglucinol (Tp) with pyrrolidine (Py) to synthesize the water-soluble monomer TpPy. This monomer readily undergoes hydrolysis under the catalysis of Brønsted acid, regenerating trialdehyde phloroglucinol and pyrrolidine. Further, an amino monomer containing ionic groups is selected to undergo a Schiff base reaction with TpPy. During the reaction, the electrostatic repulsion between the ionic groups effectively inhibits the aggregation of covalent organic framework nanosheets (COF nanosheets) caused by π-π stacking during growth, thereby achieving the direct synthesis of ionic covalent organic framework nanosheets in a pure aqueous system under mild conditions. The ionic COF nanosheets prepared by this invention exhibit high crystallinity and excellent processability, making them a key component of high-performance nanofiltration membrane materials.

[0031] Specific embodiments of the present invention are given below. These specific embodiments are only used to further illustrate the present invention and do not limit the scope of protection of the claims of this application.

[0032] Example 1

[0033] The direct synthesis of ionic TpPa-SO3H covalent organic framework nanosheets in a pure aqueous system under mild conditions includes the following steps:

[0034] Step 1) Synthesis of TpPy monomer: At room temperature, 15 ml of anhydrous ethanol was added to a 100 ml beaker containing 2.0 g of trialdehyde phloroglucinol powder. Then, 2.553 ml of pyrrolidine solution (in this mixture, the molar ratio of trialdehyde phloroglucinol to pyrrolidine was 1:3) was added. The mixture was stirred continuously at room temperature and 700 rpm for 120 minutes to obtain a pale yellow suspension. 100 ml of anhydrous diethyl ether was added to the suspension. The suspension was filtered using filter paper and a glass funnel, and washed with anhydrous diethyl ether until the washings were colorless. The suspension, along with the filter paper, was then transferred to a vacuum drying oven and dried at 25°C under vacuum for 24 hours to obtain a pale yellow powder, denoted as TpPy monomer. The reaction equation is as follows: Figure 1 As shown.

[0035] Step 2) Synthesis of TpPa-SO3H covalent organic framework nanosheets: Add 5.0 ml of deionized water to a centrifuge tube containing 22 mg (0.06 mmol) of TpPy powder; add 10.0 ml of deionized water to a centrifuge tube containing 17 mg (0.09 mmol) of 2,5-diaminobenzenesulfonic acid powder, and sonicate for 15 minutes respectively until the powder is completely dissolved, obtaining TpPy solution and amino monomer solution respectively. Use a dropper to transfer 10.0 ml of amino monomer solution to a 20 ml black-capped vial, and slowly add 5.0 ml of TpPy solution dropwise under the conditions of room temperature and stirring at 700 rpm. Then sonicate the mixture for 1 minute; after sonication, add 2.0 ml of 6M acetic acid (in this example, the molar ratio of acetic acid to 2,5-diaminobenzenesulfonic acid is 4:3) to the mixture, and transfer to a constant temperature and humidity incubator at 25°C for 7 days in the dark. See the schematic diagram. Figure 2 .

[0036] Step 3) Post-treatment of covalent organic framework nanosheets: The nanosheet solution after 7 days of reaction, such as... Figure 3 As shown in Figure a, the solution was diluted 3 times with deionized water, filtered, and placed in a 50,000 molecular weight dialysis bag for dialyzing in deionized water. After dialysis for 3 days, an aqueous solution of TpPa-SO3H covalent organic framework nanosheets was obtained, denoted as TpPa-SO3H-1.

[0037] Infrared spectroscopy was performed on the TpPa-SO3H-1 covalent organic framework nanosheets synthesized in Example 1, and the results are as follows: Figure 4 As shown. Fourier transform infrared spectroscopy (FT-IR) was used to demonstrate the successful synthesis of the Schiff base covalent organic framework. The FT-IR spectrum based on the amino monomer 2,5-diaminobenzenesulfonic acid shows that the O=S=O stretching vibration peak is located at 1028 cm⁻¹. -1 At this location, the N–H stretching vibration peak is at 3300-3500 cm⁻¹. -1 Within the range. The Fourier transform infrared spectrum of TpPy shows that the CN stretching vibration peak is located at 1578 cm⁻¹. -1 At this location, the C=C stretching vibration peak is located at 1237 cm⁻¹. -1 The simultaneous presence of O=S=O, CN, and C=C stretching vibration peaks in the FTIR spectrum of TpPa-SO3H proves the successful synthesis of the TpPa-SO3H-1 covalent organic framework.

[0038] The physical morphology of the TpPa-SO3H-1 covalent organic framework nanosheets synthesized in Example 1 was observed under a scanning electron microscope (SEM). The results are as follows: Figure 5 As shown in Figure a, an interlaced, stacked, flexible sheet-like structure was observed, with relatively smooth sheet surfaces, proving that the synthesized TpPa-SO3H-1 covalent organic framework exists in the form of thin sheets.

[0039] The TpPa-SO3H-1 covalent organic framework nanosheets synthesized in Example 1 were characterized by atomic force microscopy (TEM), and the results are as follows: Figure 6 As shown. Consistent with the scanning electron microscopy results, the TpPa-SO3H-1 covalent organic framework exists in the form of nanosheets. Figure 6 In addition, the nanosheets have a lateral dimension of about 10 μm and a very small and uniform thickness of only 3 nm. (See a) Figure 6 Medium b, with a diameter-to-thickness ratio exceeding 3000.

[0040] The TpPa-SO3H-1 covalent organic framework nanosheets synthesized in Example 1 were characterized by powder X-ray diffraction. The results are shown in the figure. Figure 7 As shown in Figure a, a distinct diffraction peak is observed at 4.7°, corresponding to the (100) crystal plane, demonstrating that the TpPa-SO3H-1 covalent organic framework nanosheets prepared in Example 1 are crystalline.

[0041] To verify the processability of the synthesized TpPa-SO3H covalent organic framework (COF) nanosheets, they were combined with polyethyleneimine using a vacuum-assisted self-assembly method, successfully preparing nanofiltration membranes, such as... Figure 8 As shown in Figure a. Experimental results show that the obtained nanofiltration membrane has a dense, uniform, and smooth surface, exhibiting good film formation quality. Relevant performance test results are shown in [Figure a]. Figure 9 ,like Figure 9 As shown in Figure a, under mixed salt conditions, the water flux of this membrane can reach a maximum of 29.8 (L·m). -2 ·h-1·bar -1 The lithium-magnesium selectivity reached a maximum of 22.3, demonstrating excellent separation performance. Furthermore, Figure 9 Figure b shows that the membrane exhibits good long-term stability. Furthermore, the TpPa-SO3H COF nanosheets were processed into a tubular composite membrane, such as... Figure 8 As shown in Figure b, its surface also exhibits dense, uniform, and smooth properties, while achieving a larger fabrication scale. Figure 9 As shown in Figure c, under cross-flow operation conditions, the tubular composite membrane achieves a maximum magnesium chloride rejection rate of 96.2%. After 7 days of continuous operation, its rejection performance did not show a significant decrease, demonstrating good long-term operational stability. These results indicate that the ionic covalent organic framework nanosheets synthesized by the method of this invention possess excellent processability, providing a new material option for the development of membrane separation technology and showing promising prospects for industrial application.

[0042] Example 2

[0043] Ionic TpPa-SO3H covalent organic framework nanosheets were directly synthesized in a pure aqueous system under mild conditions. The synthesis process in Example 2 was basically the same as in Example 1, except that in step 2), the amount of 2.0 ml 6M acetic acid added was changed to 2.0 ml 3M acetic acid (in this example, the molar ratio of acetic acid to 2,5-diaminobenzenesulfonic acid was 2:3). The resulting TpPa-SO3H covalent organic framework nanosheets were designated TpPa-SO3H-2.

[0044] The physical morphology of the TpPa-SO3H-2 covalent organic framework nanosheets was observed under a scanning electron microscope. Figure 5 As shown in Figure a, the prepared TpMbh-SO3H-2 covalent organic framework exists in the form of nanosheets.

[0045] Example 3

[0046] Ionic TpPa-SO3H covalent organic framework nanosheets were directly synthesized in a pure aqueous system under mild conditions. The synthesis process in Example 3 was basically the same as that in Example 1, except that in step 2), the amount of 2.0 ml 6M acetic acid added was changed to 2.0 ml 17.5M acetic acid (in this example, the molar ratio of acetic acid to 2,5-diaminobenzenesulfonic acid was 35:9). The resulting TpPa-SO3H covalent organic framework nanosheets were designated TpPa-SO3H-3.

[0047] The physical morphology of the TpPa-SO3H-3 covalent organic framework nanosheets was observed under a scanning electron microscope. Figure 5 As shown in Figure a, the prepared TpMbh-SO3H covalent organic framework exists in the form of nanosheets.

[0048] Example 4

[0049] Ionic TpMbh-SO3H covalent organic framework nanosheets were directly synthesized in a pure aqueous system under mild conditions. The synthesis process in Example 4 was basically the same as in Example 1, except that in step 2), the amount of TpPy was changed from 22 mg to 11 mg, and the amino monomer was changed from 17 mg of 2,5-diaminobenzenesulfonic acid to 12.3 mg of 1,3-phenylenediamine dihydrazide-5-sulfonic acid (Mbh-SO3H). In this example, TpPy and 1,3-phenylenediamine... The molar ratio of 1,3-phenylenediamine dihydrazide-5-sulfonic acid was 2:3; Brønsted acid was replaced with p-toluenesulfonic acid, and 15.5 mg of p-toluenesulfonic acid was dissolved in 2 ml of deionized water. This p-toluenesulfonic acid aqueous solution was added dropwise to the mixture of TpPy and Mbh-SO3H. The molar ratio of p-toluenesulfonic acid to the amino monomer 1,3-phenylenediamine dihydrazide-5-sulfonic acid was 2:1. Finally, TpMbh-SO3H covalent organic framework nanosheets were obtained, denoted as TpMbh-SO3H-1.

[0050] The physical morphology of the TpMbh-SO3H-1 covalent organic framework nanosheets was observed under a scanning electron microscope. Figure 5 As shown in Figure b, the prepared TpMbh-SO3H covalent organic framework exists in the form of nanosheets. Powder X-ray diffraction characterization was performed on it. Figure 7 As shown in Figure b, the prepared TpMbh-SO3H covalent organic framework nanosheets are crystalline.

[0051] Example 5

[0052] Ionic TpMbh-SO3H covalent organic framework nanosheets were directly synthesized in a pure aqueous system under mild conditions. The synthesis process in Example 5 was basically the same as that in Example 4, except that in step 2), the amount of p-toluenesulfonic acid added was changed from 15.5 mg to 3.9 mg (the molar ratio of p-toluenesulfonic acid to 1,3-phenylenediamine dihydrazide-5-sulfonic acid was 1:2). The resulting TpMbh-SO3H covalent organic framework nanosheets were designated TpMbh-SO3H-2.

[0053] The physical morphology of the TpMbh-SO3H-2 covalent organic framework nanosheets was observed under a scanning electron microscope. Figure 5 As shown in Figure b, the prepared TpMbh-SO3H-2 covalent organic framework exists in the form of nanosheets.

[0054] Example 6

[0055] Ionic TpMbh-SO3H covalent organic framework nanosheets were directly synthesized in a pure aqueous system under mild conditions. The synthesis process in Example 6 was basically the same as that in Example 4, except that in step 2), the amount of p-toluenesulfonic acid added was changed from 15.5 mg to 7.8 mg (the molar ratio of p-toluenesulfonic acid to 1,3-phenylenediamine dihydrazide-5-sulfonic acid was 1:1). The resulting TpMbh-SO3H covalent organic framework nanosheets were designated TpMbh-SO3H-3.

[0056] The physical morphology of the TpMbh-SO3H-3 covalent organic framework nanosheets was observed under a scanning electron microscope. Figure 5 As shown in Figure b, the prepared TpMbh-SO3H-3 covalent organic framework exists in the form of nanosheets.

[0057] Example 7

[0058] Ionic TpPa-COOH covalent organic framework nanosheets were directly synthesized in a pure aqueous system under mild conditions. The preparation process of Example 3 was basically the same as that of Example 1, except that in step 2), the amount of TpPy was changed from 22 mg to 44.3 mg, and the amino monomer was changed from 17 mg of 2,5-diaminobenzenesulfonic acid to 27.4 mg of 2,5-diaminobenzoic acid. In this example, the molar ratio of TpPy to 2,5-diaminobenzoic acid was 2:3. Brønsted acid was changed from acetic acid to bicarbonate ions. 30.24 mg of sodium bicarbonate was dissolved in 2 ml of deionized water, and this sodium bicarbonate aqueous solution was added dropwise to the mixture of TpPy and 2,5-diaminobenzoic acid. The molar ratio of sodium bicarbonate to 2,5-diaminobenzoic acid was 2:1. Finally, TpPa-COOH covalent organic framework nanosheets were obtained, denoted as TpPa-COOH-1.

[0059] The physical morphology of the TpPa-COOH-1 covalent organic framework nanosheets was observed under a scanning electron microscope. Figure 5 As shown in Figure c, the prepared TpPa-COOH-1 covalent organic framework exists in the form of nanosheets. Powder X-ray diffraction characterization was performed on it. Figure 7 As shown in Figure c, the prepared TpPa-COOH-1 covalent organic framework nanosheets are crystalline.

[0060] Example 8

[0061] Ionic TpPa-COOH covalent organic framework nanosheets were directly synthesized in a pure aqueous system under mild conditions. The synthesis process in Example 8 was basically the same as in Example 7, except that in step 2), the amount of sodium bicarbonate added was changed from 30.24 mg to 7.56 mg (the molar ratio of sodium bicarbonate to 2,5-diaminobenzoic acid was 1:2). The resulting TpPa-COOH covalent organic framework nanosheets were designated TpPa-COOH-2.

[0062] The physical morphology of the TpPa-COOH-2 covalent organic framework nanosheets was observed under a scanning electron microscope. Figure 5 As shown in Figure c, the prepared TpPa-COOH-2 covalent organic framework exists in the form of nanosheets.

[0063] Example 9

[0064] Ionic TpPa-COOH covalent organic framework nanosheets were directly synthesized in a pure aqueous system under mild conditions. The synthesis process in Example 9 was basically the same as that in Example 7, except that in step 2), the amount of sodium bicarbonate added was changed from 30.24 mg to 15.12 mg (the molar ratio of sodium bicarbonate to 2,5-diaminobenzoic acid was 1:1). The resulting TpPa-COOH covalent organic framework nanosheets were designated TpPa-COOH-3.

[0065] The physical morphology of the TpPa-COOH-3 covalent organic framework nanosheets was observed under a scanning electron microscope. Figure 5 As shown in Figure c, the prepared TpPa-COOH-3 covalent organic framework exists in the form of nanosheets.

[0066] Comparative Example 1

[0067] Ionic TpPa-SO3H covalent organic frameworks were directly synthesized in a pure aqueous system under mild conditions. The preparation process differed from that in Example 1 in that steps 1) and 3) were omitted; that is, TpPy monomers were not synthesized as aldehyde monomers beforehand, and the aldehyde monomer trialdehyde phloroglucinol was directly reacted with the amino monomer 2,5-diaminobenzenesulfonic acid in a pure aqueous system to synthesize the ionic covalent organic framework. The steps are as follows:

[0068] Add 5.0 ml of deionized water to a 50 ml centrifuge tube containing 12.6 mg of trialdehyde phloroglucinol powder; add 10.0 ml of deionized water to a 50 ml centrifuge tube containing 17.0 mg of 2,5-diaminobenzenesulfonic acid. The molar ratio of trialdehyde phloroglucinol to 2,5-diaminobenzenesulfonic acid is 2:3. Sonicate for 15 minutes to obtain aldehyde monomer and amino monomer solutions. Transfer 10.0 ml of the amino monomer solution to a 20 ml black-capped vial using a dropper. Slowly add 5.0 ml of the aldehyde monomer solution at room temperature and with stirring at 700 rpm. Sonicate the mixture for 1 minute. After sonication, add 2.0 ml of 6M acetic acid to the mixture. The molar ratio of acetic acid to 2,5-diaminobenzenesulfonic acid is 4:3. Transfer to a 25°C constant temperature and humidity incubator and react in the dark for 7 days. After the reaction is complete, a TpPa-SO3H covalent organic framework suspension is obtained. Figure 3 As shown in b.

[0069] The physical morphology of the prepared TpPa-SO3H covalent organic framework was observed under a scanning electron microscope. Figure 5 As shown in Figure d, it exhibits an aggregated blocky structure with an extremely rough surface, forming a porous structure similar to a sponge, proving that the TpPa-SO3H covalent organic framework exists in a blocky form.

[0070] Comparative Example 2

[0071] Nonionic TpPa covalent organic frameworks were directly synthesized in a pure aqueous system under mild conditions. The preparation process of Comparative Example 2 was basically the same as that of Example 1, except that in step two, p-phenylenediamine, which does not contain ionic groups, was selected as the amino monomer, and the amount used was 9.7 mg. The molar ratio of TpPy to p-phenylenediamine was 2:3. The finally obtained TpPa covalent organic framework precipitated in the form of a precipitate, as shown in [reference needed]. Figure 3 As shown in c.

[0072] The physical morphology of the prepared TpPa covalent organic framework was observed under a scanning electron microscope. Figure 5 As shown in Figure e, it exhibits blocky aggregation with a rough and porous surface. The block surface is rough and covered with fine pores, proving that the TpPa covalent organic framework exists in a blocky form.

[0073] Comparative Example 3

[0074] TpPa-SO3H covalent organic framework (COF) nanosheets were directly synthesized in a pure aqueous system under mild conditions. The preparation method of Comparative Example 3 was basically the same as that of Example 1, except that in step 2), after ultrasonic treatment, Brønsted acid was not added as a catalyst, but 2 mL of deionized water was added to continue the subsequent reaction process. The reaction system was carried out under mild conditions, and an aqueous solution of TpPa-SO3H covalent organic framework (COF) nanosheets was successfully obtained. However, the solution color was lighter than that of Example 1, as shown in Figure 3d, indicating that the reaction proceeded slowly.

[0075] The obtained product was analyzed by powder X-ray diffraction (PXRD), and the results are as follows: Figure 7 As shown in figure d, the TpPa-SO3H covalent organic framework nanosheets synthesized in this comparative example do not exhibit obvious diffraction peaks, indicating that they do not possess crystallinity.

[0076] The types of monomers used in Examples 1-9 and Comparative Examples 1-3, and the types and forms of covalent organic frameworks obtained are shown in Table 1.

[0077] Table 1

[0078]

[0079] This invention provides a method for the direct synthesis of ionic covalent organic framework nanosheets in a pure aqueous system under mild conditions. This method can synthesize covalent organic frameworks at 25°C and 1 bar, offering advantages such as simple process and convenient operation. Furthermore, this method eliminates the need for toxic organic solvents, demonstrating good environmental friendliness.

[0080] Covalent organic frameworks (COFs) are mainly composed of light elements such as C, H, O, and N linked by strong covalent bonds, exhibiting characteristics such as low density, good thermal stability, and high porosity. They have wide applications in research fields such as adsorption, catalysis, and energy storage. Furthermore, the long-range ordered and tunable pore structure of COFs offers significant advantages in membrane separation. However, the covalent framework structure of COFs makes them difficult to dissolve and melt, hindering their fabrication into membranes and impeding further practical applications. The aspect ratio is a crucial parameter for evaluating the quality of COF nanosheets, typically defined as the ratio of the nanosheet's lateral dimension to its thickness. Currently, COF nanosheets generally have a thickness below 10 nm, a lateral dimension reaching 10 μm, and an aspect ratio typically exceeding 1000, demonstrating excellent processability and film-forming properties. However, due to the poor solubility of aldehyde monomers in water, they are difficult to dissolve directly in the aqueous phase; therefore, existing direct synthesis methods generally rely on toxic organic solvents. Furthermore, during the growth of nanosheets, the π-π stacking effect between layers can easily induce out-of-plane growth along the Z-axis, which is not conducive to the formation of high-quality nanosheet structures.

[0081] In the preparation method employed in this invention, the TpPy monomer is synthesized from trialdehyde phloroglucinol (Tp) and pyrrolidine (Py), exhibiting good water solubility. Under the catalysis of Brønsted acid, TpPy can be efficiently hydrolyzed into trialdehyde phloroglucinol and pyrrolidine, thereby achieving the synthesis of covalent organic frameworks in a pure aqueous system without the introduction of toxic organic solvents. Furthermore, by selecting amino monomers containing ionic groups, the electrostatic repulsion between them can effectively suppress the interlayer π-π stacking of nanosheets during growth, guiding them to preferentially grow along the in-plane direction, thus obtaining covalent organic framework nanosheets with high aspect ratios.

[0082] In this invention, based on the results of Examples 1-3, three different types of ionic covalent organic framework nanosheets, namely TpPa-SO3H, TpMbh-SO3H, and TpPa-COOH, were synthesized. It can be concluded that this method has a certain degree of universality and is suitable for the synthesis of various ionic covalent organic framework nanosheets.

[0083] In this invention, a comparison between Example 1 and Comparative Example 1 shows that using the TpPy monomer synthesized from trialdehyde phloroglucinol and pyrrolidine as the aldehyde monomer is advantageous due to its good water solubility and hydrolysis under the catalysis of Brønsted acid, yielding trialdehyde phloroglucinol and pyrrolidine. Under these conditions, trialdehyde phloroglucinol and 2,5-diaminobenzenesulfonic acid achieve homogeneous nucleation in the aqueous phase, resulting in a homogeneous solution of TpPa-SO3H covalent organic framework nanosheets. A significant Tyndall effect can be observed when the solution is irradiated with a laser pointer. In contrast, trialdehyde phloroglucinol itself has poor water solubility and exists as particles in the aqueous phase. This leads to a heterogeneous nucleation reaction between it and 2,5-diaminobenzenesulfonic acid in the aqueous phase, ultimately forming a suspension of TpPa-SO3H covalent organic framework, in which the covalent organic framework exists in block form. Therefore, it can be seen that replacing Tp with Tp as the aldehyde monomer with TpPy can realize the direct synthesis of covalent organic framework nanosheets in a pure aqueous system, which significantly improves the processability and structural controllability of the material.

[0084] In this invention, a comparison between Example 1 and Comparative Example 2 shows that 2,5-diaminobenzenesulfonic acid, containing sulfonic acid groups, was chosen as the amino monomer. The sulfonic acid groups carry a negative charge, and the electrostatic repulsion they generate effectively counteracts the π-π stacking effect between layers during the growth of the covalent organic framework, thus promoting in-plane growth and successfully preparing a homogeneous solution of TpPa-SO3H covalent organic framework nanosheets. In contrast, when p-phenylenediamine, which does not contain ionic groups, was used as the amino monomer, the π-π stacking effect between layers dominated, causing the covalent organic framework to grow mainly in the out-of-plane direction and undergo self-stacking, ultimately precipitating the TpPa covalent organic framework. Therefore, using amino monomers with ionic groups facilitates the direct synthesis of covalent organic framework nanosheets in a pure aqueous system.

[0085] In this invention, by comparing the experimental results of Example 1 and Comparative Example 3, it can be seen that when Brønsted acid is added as a catalyst during the synthesis process, the obtained covalent organic framework nanosheets exhibit obvious diffraction peaks corresponding to the (100) crystal plane in the powder X-ray diffraction (PXRD) pattern; while without the addition of Brønsted acid, the obtained product does not show the diffraction signal of this crystal plane. Therefore, the introduction of Brønsted acid helps to improve the crystallinity of covalent organic frameworks (COFs) and promotes their ordered crystal growth. The synthesis of Schiff base COFs involved in this invention is generally based on the condensation reaction between an aldehyde group and a primary amine to generate an imine bond (C=N). This reaction is essentially a reversible nucleophilic addition-dehydration process. Introducing a Brønsted acid catalyst into the reaction system can enhance the electrophilicity of the aldehyde group by protonating it, thus making it easier to undergo nucleophilic addition with an amino group to form a Schiff base intermediate. Subsequently, this intermediate is further dehydrated, ultimately generating a stable C=N bond structure. The addition of acid catalysts significantly improved the reaction rate and kinetic efficiency, enabling the formation of more Schiff base linkages in a shorter time, thus providing sufficient "building blocks" for the self-assembly of COFs. Furthermore, Schiff base bonds readily undergo hydrolysis under alkaline or neutral conditions, leading to the breakage of C=N bonds and reverting to the initial states of aldehydes and amines. The presence of acids effectively lowers the pH of the system, inhibiting the hydrolysis of Schiff bases and thereby improving the stability of Schiff base bonds. Therefore, by promoting the forward reaction and inhibiting the reverse reaction, Brønsted acids play a crucial role in the synthesis of Schiff base COFs, significantly enhancing the crystallinity and structural order of the product.

[0086] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many improvements and changes under the guidance of the present invention without departing from the spirit of the present invention, and these improvements and changes are all within the protection scope of the present invention.

Claims

1. A method for mild aqueous phase synthesis of ionic covalent organic framework nanosheets, characterized in that, First, trialdehyde phloroglucinol and pyrrolidine are added to anhydrous ethanol, wherein the molar ratio of trialdehyde phloroglucinol to pyrrolidine is 1:

3. The reaction is carried out under stirring. After the reaction is completed, a pale yellow solid is obtained by filtration, washing and drying. Subsequently, using this solid as an aldehyde monomer, it reacts with an amino monomer containing ionic groups under Schiff base reaction conditions. Brønsted acid is introduced into the reaction system to regulate the hydrolysis rate of the aldehyde monomer and the formation rate of the covalent organic framework, thereby achieving in-situ synthesis of ionic covalent organic framework nanosheets in a pure aqueous system at room temperature and pressure. Specifically: the amino monomer is 2,5-diaminobenzenesulfonic acid; the molar ratio of Brønsted acid to amino monomer in the reaction system is 6~35:9; or the amino monomer is 2,5-diaminobenzoic acid; the molar ratio of Brønsted acid to amino monomer in the reaction system is 0.5~2:1; or the amino monomer is 1,3-phthalic acid dihydrazide-5-sulfonic acid; the molar ratio of Brønsted acid to amino monomer in the reaction system is 0.5~2:

1. After the reaction was completed, the product solution was diluted and dialyzed to obtain a homogeneous solution of ionic covalent organic framework nanosheets.

2. The method for mild aqueous phase synthesis of ionic covalent organic framework nanosheets according to claim 1, characterized in that, The specific steps are as follows: Step 1) Synthesis of aldehyde monomer: Dissolve an appropriate amount of trialdehyde phloroglucinol in anhydrous ethanol, then add an appropriate amount of pyrrolidine to form a suspension, wherein the molar ratio of trialdehyde phloroglucinol to pyrrolidine is 1:3; stir the reaction at room temperature for 2-4 hours. After the reaction is completed, filter, wash and dry to obtain a pale yellow solid, which is the aldehyde monomer, denoted as TpPy monomer. Step 2) Synthesis of ionic covalent organic framework nanosheets: The TpPy monomer obtained in step 1) is dissolved in a solvent and sonicated to obtain an aldehyde monomer solution; the amino monomer with ionic groups is dissolved in a solvent and sonicated to obtain an amino monomer solution; the aldehyde monomer solution is transferred to a container, and the amino monomer solution is slowly added dropwise while stirring, wherein the molar ratio of aldehyde to amino is 1:

1. After the addition is complete and sonication is performed, Brønsted acid is added to the above reaction system, and the reaction is carried out at room temperature and pressure in the dark for 5-8 days. Step 3) After the reaction is complete, the product solution is diluted and dialyzed to obtain a homogeneous solution of ionic covalent organic framework nanosheets.

3. The method for mild aqueous phase synthesis of ionic covalent organic framework nanosheets according to claim 2, characterized in that: In step 1), the process conditions for filtration, washing, and drying are as follows: filter with anhydrous diethyl ether, wash with anhydrous diethyl ether until the washing liquid is colorless, and dry in a vacuum oven at 25°C for 24 hours; in step 2), the solvents in which the TpPy monomer and the amino monomer are dissolved are both deionized water; in step 2), the Brønsted acid is one of acetic acid, p-aminobenzenesulfonic acid, and sodium bicarbonate; in step 3), the process conditions for diluting and dialysis of the product solution are as follows: dilute the product solution with deionized water, put it into a dialysis bag, and dialyze for 3 days.

4. An application of an ionic covalent organic framework nanosheet, characterized in that, The ionic covalent organic framework nanosheets prepared by the method of mild aqueous phase synthesis of ionic covalent organic framework nanosheets according to any one of claims 1-3 are used as membrane building units to construct nanofiltration membranes or tubular composite membranes for lithium-magnesium separation.

5. The application according to claim 4, characterized in that, The constructed nanofiltration membrane has a dense, uniform, smooth, and flat surface. The pure water flux of the nanofiltration membrane under cross-flow test conditions is 30.1 L·m⁻¹. -2 ·h -1 ·bar -1 The lithium-magnesium selectivity is 22.3%.

6. The application according to claim 4, characterized in that, The constructed tubular composite membrane has a uniform and defect-free surface. Under cross-flow operation conditions, this tubular composite membrane achieves a magnesium chloride retention rate of 96.2%, while also exhibiting a viscosity of 5.8 L·m⁻¹. -2 ·h -1 ·bar -1 The flux.

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