Preparation method and application of charged covalent organic framework membrane with exogenous mesopores

Abstract

The invention discloses a preparation method and application of a charged covalent organic framework membrane with exogenous mesopores, and belongs to a membrane material preparation technology. A water-phase solution is a water solution containing a surfactant, an amine monomer and an acid catalyst, an oil-phase solution is a mixed organic solution containing an aldehyde monomer, the two-phase solution is mixed and then subjected to a polymerization reaction at an interface, and the charged covalent organic framework membrane is generated; and washing the obtained COF membrane, heating and soaking in an acid solution, and cleaning to obtain the charged covalent organic framework membrane containing exogenous mesopores. According to the invention, the surfactant and the amine monomer with opposite charges are selected and combined with the aldehyde monomer to introduce an exogenous mesoporous structure into a formed film, so that the template surfactant is prevented from being excluded from a COF growth domain due to strong crystallization driving force, the arrangement of the template in the COF film is ensured, and the performance of the COF film is improved. According to the method, the charged covalent organic framework membrane with high selectivity and high permeability can be simply, conveniently and efficiently prepared.

Description

Technical Field

[0001] This invention relates to membrane material preparation technology, specifically to a method for preparing and applying a charged covalent organic framework membrane with exogenous mesopores. Background Technology

[0002] Salinity gradient energy, as a renewable "blue energy" with huge reserves, is widely found in systems such as seawater-river interfaces and industrial wastewater. Its efficient utilization is of great significance for new energy development, environmental governance, pollution reduction and emission reduction, and freshwater regeneration.

[0003] Reverse electrodialysis, as a core technology for converting salinity gradient energy into electricity, hinges on the application of ion-selective separation membrane materials. When positive and negative ions selectively permeate through the separation membrane, a potential difference is created between the high-concentration solution side and the low-concentration solution side, generating current in the external circuit and achieving salinity gradient power generation. However, traditional separation membrane materials exhibit a significant trade-off between ion selectivity and permeability; that is, increasing ion selectivity often leads to a decrease in permeability, and vice versa. This contradiction makes it difficult to simultaneously improve energy conversion efficiency and output power density. Therefore, developing novel high-performance separation membrane materials is a prerequisite and key to improving salinity gradient power generation performance.

[0004] Covalent organic frameworks (COFs) are ideal substrates for preparing ion-selective separation membranes due to their regular nanoscale pore structure, tunable pore size, high specific surface area, and stable covalent bond framework. Separation membranes based on COFs exhibit ultra-high ion selectivity and are considered an emerging platform in the field of salinity gradient energy conversion, potentially propelling this technology from conceptual applications to large-scale industrial production. Although many related technologies are currently being researched regarding the preparation and application of COF membranes, existing COF membrane preparation technologies are generally limited by the micropore-scale ion channel size of COFs, making it difficult to achieve breakthroughs in ion flux and hindering further improvements in salinity gradient power generation performance. For example, CN118236858A discloses a surfactant-mediated method for preparing COF membranes. The method uses surfactants as structure-directing agents to reduce oil-water interfacial tension. The ordered aggregation of monomers is achieved through the electrostatic interaction between the hydrophilic groups of the surfactant and the amino monomers, resulting in a defect-free, dense COF membrane. This effectively improves the flux and rejection rate of the membrane in organic solvent nanofiltration. However, this technology is primarily aimed at the field of organic solvent nanofiltration, with the core objective being the preparation of a dense, defect-free COF membrane. The structure of the prepared COF membrane is still limited by the microporous channels of the COF itself, making it difficult to simultaneously improve ion selectivity and permeability. CN115814601A... A method for preparing an ion-modified COF membrane and its application in the conversion of salinity gradient energy and low-grade thermal energy is disclosed. The charged COF membrane is prepared by liquid-liquid interface polymerization, and the pore charge density can be controlled to improve ion selectivity. Energy conversion is achieved by building a reverse electrodialysis (RED) system. However, this technology also relies solely on the microporous ion channels of the COF itself to achieve ion transport. The ion permeation rate is limited, and it is difficult to break the trade-off effect between ion selectivity and permeability. As a result, there is a significant bottleneck in improving the energy efficiency and output power density of salinity gradient energy conversion.

[0005] Therefore, how to develop a COF membrane that can maintain high ion selectivity while accelerating ion permeation rate through reasonable structural design, thereby improving the energy conversion efficiency and output power density of salinity gradient power generation, has become a key issue that urgently needs to be addressed in the field of salinity gradient energy utilization. Summary of the Invention

[0006] To address the aforementioned problems, the present invention aims to provide a charged covalent organic framework membrane with exogenous mesopores and its preparation method, thereby solving the problem that the ion selectivity and permeability of traditional separation membrane materials cannot be synergistically improved.

[0007] To achieve the above objectives, the present invention first provides a method for preparing a charged covalent organic framework membrane with exogenous mesopores, comprising the following steps: (1) Aqueous phase preparation: Mix the aqueous solution of surfactant and acid catalyst, and add amine monomer under ultrasonic conditions to obtain an aqueous solution; (2) Organic phase preparation: The aldehyde monomer is dissolved in an organic solvent and ultrasonically treated to obtain an organic phase solution; (3) Preparation of covalent organic framework membrane: The organic phase solution obtained in step (2) is slowly added to the aqueous phase solution in step (1), sealed and transferred to a constant temperature and humidity chamber at 25°C for static reaction to obtain the membrane material at the interface. (4) Charged covalent organic framework membrane with exogenous mesopores: Take out the membrane material at the interface obtained in step (3), clean it to remove the residual unreacted monomers and catalysts; after cleaning, add the membrane material to an acid solution and heat it to soak it, and finally soak and wash it to obtain a charged covalent organic framework membrane with exogenous mesopores. Both the surfactant and the amine monomer have charged groups, and the surfactant and the amine monomer carry opposite charges.

[0008] This invention selects surfactants and amine monomers with opposite charges. During aqueous phase preparation, the surfactants and amine monomers are connected through electrostatic attraction between ions. When an organic phase is subsequently added for reaction, the aldehyde monomers in the organic phase bind to the amine monomers around the structure formed by the surfactant and amine monomers, thereby introducing an exogenous mesoporous structure into the formed membrane. This prevents the template surfactant from being excluded from the COF growth domain due to its strong crystallization driving force. Because the binding force between the surfactant and amine monomers in this invention is strong, the surfactant easily blocks the mesopores in the membrane structure and is difficult to remove with conventional organic solvents (such as alcohols). This invention promotes the dissolution of the surfactant by immersing the covalent organic framework membrane in an acidic solution under heating conditions, facilitating better subsequent removal.

[0009] In one embodiment of the present invention, the surfactant is a cationic surfactant, selected from at least one of dodecyltrimethylammonium bromide, octadecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide, preferably dodecyltrimethylammonium bromide; the amine monomer is a negatively charged amine monomer selected from 2,5- Sodium diaminobenzenesulfonate, 2,5-diaminobenzenesulfonic acid, 4,4'-diaminostilbene-2,2'-disulfonic acid, 3,5-diaminobenzoic acid, and 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid, preferably 2,5-diaminobenzenesulfonic acid, are selected from the group consisting of sodium diaminobenzenesulfonate, 2,5-diaminobenzenesulfonic acid, 4,4'-diaminostilbene-2,2'-dicarboxylic acid. Diaminobenzenesulfonic acid.

[0010] In one embodiment of the present invention, the surfactant is an anionic surfactant selected from at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and sodium dioctyl sulfosuccinate, preferably sodium dodecyl sulfate, and the amine monomer is a positively charged amine monomer selected from at least one of dimethyl diallyl ammonium chloride, triaminoguanidine hydrochloride, and 1,3,5-tris(4-aminophenyl)benzene, preferably triaminoguanidine hydrochloride.

[0011] In one embodiment of the present invention, the molar ratio of the surfactant to the amine monomer is 1:8 to 5:1, preferably 1:1 to 2:1.

[0012] In one embodiment of the present invention, the acid catalyst is selected from at least one of acetic acid, p-toluenesulfonic acid, trifluoroacetic acid, and trifluoromethanesulfonic acid, and the molar concentration of the acid catalyst in the aqueous solution of the acid catalyst is 1 mol / L to 10 mol / L, preferably 9 mol / L.

[0013] In one embodiment of the present invention, the total ultrasonic time in step (1) is 10~60 min. The aqueous solution of surfactant and acid catalyst is mixed and ultrasonicated for a period of time. After micelles are formed, amine monomers are added and ultrasonic dispersion is continued.

[0014] In one embodiment of the present invention, in step (2), the aldehyde monomer is selected from trialdehyde phloroglucinol, 2,5-dimethylformaldehyde ... Dihydroxyterephthalaldehyde, 1,3,5 The aldehyde monomer is selected from at least one of pyromellitic pyrogallol and 2-hydroxy-1,3,5-benzyltricarboxaldehyde, preferably trialdehyde-resorcinol; the molar ratio of the aldehyde monomer to the amine monomer is 3:1 to 1:3, preferably 1:2 to 2:1, and the ultrasonic dispersion time is 5 to 60 min, preferably 10 min.

[0015] In one embodiment of the present invention, in step (2), the organic solvent is preferably a mixed organic solvent, which is selected from toluene / n-hexane, ethyl acetate / n-hexane, and chloroform / n-hexane, preferably toluene / n-hexane. The volume ratio of the two solvents in the mixed organic solvent is 1:5 to 2:1, preferably 2:5 to 2:8. The volume ratio of the two solvents refers to the volume ratio of the first solvent to the second solvent. For example, for toluene / n-hexane, the volume ratio of toluene to n-hexane is 1:5 to 2:1.

[0016] In one embodiment of the present invention, in step (3), the organic phase solution is added dropwise to the aqueous phase, and the settling time is 0.5 to 2 days, preferably 1 day.

[0017] In one embodiment of the present invention, in step (4), the cleaning refers to soaking and cleaning with DMF, methanol, ethanol and deionized water in sequence. The acid solution is one of acetic acid, hydrochloric acid and sulfuric acid, preferably hydrochloric acid, with a concentration of 0.3~3 mol / L, preferably 0.5 mol / L, a soaking time of 0.5~2 days, preferably 1 day, and a heating temperature of 30~90 ℃, preferably 60 ℃. In one embodiment of the present invention, in step (4), the final soaking and cleaning is to soak and clean the membrane with methanol and deionized water in sequence.

[0018] The present invention also provides a charged covalent organic framework membrane with exogenous mesopores prepared according to the above preparation method.

[0019] This invention also provides the application of the above-mentioned charged covalent organic framework membrane with exogenous mesopores in the field of salt gradient energy conversion.

[0020] The beneficial effects of this invention are as follows: (1) This invention uses surfactants and amine monomers with opposite charges. The surfactant and catalyst are first mixed, and then the amine monomer is added as the aqueous phase. An organic solution of the aldehyde monomer is used as the organic phase. A polymerization reaction occurs at the interface to generate a charged covalent organic framework membrane. During the preparation of the covalent organic framework membrane, due to the strong interionic interaction between the surfactant and the amine monomer, when the aldehyde monomer is added, it binds to the amine monomer around the structure formed by the surfactant and amine monomer, thereby introducing an exogenous mesoporous structure into the formed membrane. This prevents the template agent surfactant from being excluded from the COF growth domain due to its strong crystallization driving force, ensuring the arrangement of the template agent within the COF membrane. The synergistic effect of the mesoporous structure and ionic functional sites introduced by the method of this invention gives the membrane material higher ionic conductivity, which helps reduce energy loss and improve the overall system efficiency.

[0021] (2) Due to the strong binding force between the surfactant and the amine monomer in this invention, the surfactant easily blocks the mesopores in the membrane structure and is difficult to remove with conventional organic solvents (such as alcohols). In this invention, after cleaning the COF formed by the polymerization of the aqueous and organic phases, the covalent organic framework membrane is immersed in an acidic solution under heating conditions to promote the dissolution of the surfactant, while ensuring that the crystallinity of the COF membrane and the structure of the mesopores are not affected.

[0022] (3) The membrane material prepared by this invention has an asymmetric structure, which can reduce mass transfer resistance, effectively suppress ion concentration polarization, and improve ion permeation performance. In addition, the membrane material prepared by interfacial polymerization of this invention has controllable thickness, making it easy to prepare ultrathin separation layers. By adjusting parameters such as monomer concentration, reaction time, and interfacial tension, the membrane thickness can be precisely controlled to achieve nanoscale ultrathin separation layers.

[0023] (4) Based on the above design, the charged covalent organic framework membrane with exogenous mesopores quickly constructed exhibits excellent ion selectivity and permeation performance, with the highest permeation power reaching 24.56 W / m² and the ion selectivity reaching 0.85. It is beneficial to alleviate the energy shortage problem and has broad application prospects in the field of salinity gradient power generation. Attached Figure Description

[0024] Figure 1 The image shows the XRD pattern of the charged covalent organic framework membrane with exogenous mesopores prepared in Example 1 of this invention.

[0025] Figure 2 This is a scanning electron microscope cross-sectional image of the charged covalent organic framework membrane with exogenous mesopores prepared in Example 1 of the present invention. Detailed Implementation

[0026] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0027] The charged covalent organic framework membrane with exogenous mesopores prepared in this invention is used for permeation energy conversion. Ion selectivity and permeability are two important parameters for evaluating the permeation energy conversion of charged covalent organic framework membranes with exogenous mesopores.

[0028] Test methods for salinity gradient power generation performance: The transmembrane ion current was characterized using a Keithley 6487 picoammeter to analyze the ion transport properties of the membrane. The membrane sample was fixed between two equal-volume compartments. Depending on the research objective, solutions of the same or different concentrations were placed in both compartments. During measurement, Ag / AgCl salt bridge electrodes were placed in each compartment to apply a voltage to the membrane. The change in transmembrane ion current as a function of voltage was recorded, thus obtaining the current-voltage (IV) curve. An external adjustable resistance box was connected to the test setup to measure the ion current (I) at different resistances. For a given resistance (R), the output power density (P), i.e., the permeation power, can be calculated using the following formula: = 2 × (1) Energy conversion efficiency ( The energy conversion efficiency (ECE) is defined as the ratio of the output electrical energy to the Gibbs free energy of the mixture, with a maximum value of 50%. For systems with cation-selective transport, the ECE can be calculated using the following formula: = (2 + 1) 2 (2) Ion transport number formula: + = ( (3) Where γ and c are the ion activity coefficient and ion concentration, respectively; E diff denoted as diffusion potential; T as Kelvin temperature; z as ion valence state; F as Faraday constant; R as ideal gas constant.

[0029] Example 1 A method for preparing a charged covalent organic framework membrane with exogenous mesopores includes the following steps: (1) Add 49.33 mg of dodecyltrimethylammonium bromide (DTAB) to 10 mL of 9 M acetic acid solution, sonicate for 10 min to form micelles, and then add 18.82 mg of 2,5-dimethylammonium bromide. Diaminobenzenesulfonic acid (DABA) was further sonicated for 5 min to achieve uniform dispersion, forming the aqueous phase. 10 mg of trialdehyde phloroglucinol was added to 10 mL of a 2:5 (v / v) toluene / n-hexane organic solution and sonicated for 10 min to achieve uniform dispersion, forming the organic phase. The aqueous phase was added to a 20 mL glass bottle, and the organic phase was slowly added dropwise along the bottle wall. The bottle was sealed and placed in a 25 °C incubator for 1 day.

[0030] (2) Take out the membrane after the reaction in step (1) and wash it with DMF, methanol, ethanol and deionized water in sequence to remove the residual unreacted monomers and catalysts.

[0031] (3) The membrane after cleaning in step (2) is placed in a prepared 50 mL 0.5 M 60 ℃ hydrochloric acid solution. The surfactant dodecyl dimethyl ammonium bromide is removed by ion exchange. After reacting for 1 day, the membrane is taken out and washed with methanol and deionized water in sequence to remove residual hydrochloric acid. Finally, a charged covalent organic framework membrane with exogenous mesopores is obtained.

[0032] Characterization of exogenous mesoporous charged covalent organic framework membranes The exogenous mesoporous charged covalent organic framework membrane prepared in Example 1 was characterized, and the results are shown in the figure. Figure 1~ 2. The XRD pattern clearly shows that, in addition to the intrinsic crystallization peak at 4.8° COF, an ordered mesoporous peak of the template agent appears at 2.2°. Simultaneously, the cross-sectional structure of the film... Figure 2 It is evident that the membrane has an asymmetric structure with a dense surface layer and a loose underlying layer.

[0033] Example 2 The difference between Example 2 and Example 1 is that the amount of dodecyltrimethylammonium bromide (DTAB) added in step (1) is changed, so that DTAB and 2,5 The molar ratio of diaminobenzenesulfonic acid (DABA) is 0.4:1.0.

[0034] Example 3 The difference between Example 3 and Example 1 is that the amount of dodecyltrimethylammonium bromide (DTAB) added in step (1) is changed, so that DTAB and 2,5 The molar ratio of diaminobenzenesulfonic acid (DABA) is 0.8:1.0.

[0035] Example 4 The difference between Example 4 and Example 1 is that the amount of dodecyltrimethylammonium bromide (DTAB) added in step (1) is changed, so that DTAB and 2,5 The molar ratio of diaminobenzenesulfonic acid (DABA) is 1.2:1.0.

[0036] Example 5 The difference between Example 5 and Example 1 is that the amount of dodecyltrimethylammonium bromide (DTAB) added in step (1) is changed, so that DTAB and 2,5 The molar ratio of diaminobenzenesulfonic acid (DABA) is 2.0:1.0.

[0037] The exogenous mesoporous charged covalent organic framework membranes prepared in Examples 1-5 were tested. The results are shown in Table 1.

[0038] Table 1. Permeation power and ion selectivity of the interfacial evaporators prepared in Examples 1-5

[0039] As shown in Table 1, with the increase of molar ratio, the micelle content increases, and the permeation power and ion selectivity both show a trend of first increasing and then decreasing. This is because there is a certain balance between the self-assembly of the surfactant and the crystallization driving force of COF. The arrangement of the template agent in the COF growth domain will have nodes from disorder to order and then back to disorder.

[0040] Example 6 The difference between Example 6 and Example 1 is that the concentration of trialdehyde phloroglucinol in step (1) is different. In Example 6, the concentration of trialdehyde phloroglucinol in step (1) is 0.5 mg / mL.

[0041] Example 7 The difference between Example 7 and Example 1 is that the concentration of trialdehyde phloroglucinol in step (1) is different. In Example 7, the concentration of trialdehyde phloroglucinol in step (1) is 1.5 mg / mL.

[0042] The exogenous mesoporous charged covalent organic framework membranes prepared in Examples 6 and 7 were tested. The results are shown in Table 2.

[0043] Table 2. Permeation power and ion selectivity of the exogenous mesoporous charged covalent organic framework membranes prepared in Examples 6-7

[0044] As shown in Table 2, with the increase of the concentration of trialdehyde phloroglucinol, the diffusion rate increases. Both excessively slow and excessively fast diffusion rates will lead to discontinuity of the interfacial polymerized film and a decrease in crystallinity. Therefore, the permeation power and ion selectivity first increase and then decrease.

[0045] Example 8 The difference between Example 8 and Example 1 is that 2,5 The amount of diaminobenzenesulfonic acid (DABA) added was such that its amount was 0.05 mmol.

[0046] Example 9 The difference between Example 9 and Example 1 is that 2,5 The amount of diaminobenzenesulfonic acid (DABA) added was 0.2 mmol.

[0047] The exogenous mesoporous charged covalent organic framework membranes prepared in Examples 8-9 were tested. The results are shown in Table 3.

[0048] Table 3. Permeation power and ion selectivity of the exogenous mesoporous charged covalent organic framework membranes prepared in Examples 8-9

[0049] As shown in Table 3, the diffusion rate of trialdehyde phloroglucinol slows down as the amount of DABA increases. Too slow or too fast diffusion rates can lead to discontinuity in the interfacial polymerized film and a decrease in crystallinity. Therefore, the permeation power and ion selectivity first increase and then decrease.

[0050] Example 10 The difference between Example 10 and Example 1 is that the surfactant in step (1) is different. In Example 10, the surfactant is an equimolar amount of the anionic surfactant sodium dodecyl sulfate (SDS).

[0051] Example 11 The difference between Example 11 and Example 1 is that the surfactant in step (1) is different. In Example 11, the surfactant is an equimolar amount of the nonionic surfactant Pluronic F-127.

[0052] Example 12 The difference between Example 12 and Example 1 is that the surfactant in step (1) is different. In Example 12, the surfactant is an equimolar amount of the cationic surfactant cetyltrimethylammonium bromide (CTAB).

[0053] The exogenous mesoporous charged covalent organic framework membranes prepared in Examples 10-11 were tested. The results are shown in Table 4.

[0054] Table 4. Permeation power and ion selectivity of the exogenous mesoporous charged covalent organic framework membranes prepared in Examples 10-11

[0055] As shown in Table 4, different types of surfactants result in variations in permeation power and ion selectivity. Specifically, since the amine monomers used in the examples are anionic amine monomers, if anionic surfactant SDS and nonionic surfactant Pluronic F-127 are used, the strong repulsion and weak interaction between the surfactants and anionic amine monomers cause the template agent to be excluded from the COF growth domain. The space occupied by the template agent forms a large number of defects, resulting in poor film formation and a significant decrease in permeation power. Compared to Example 1, the cationic surfactant CTAB in Example 12 has a longer chain length, leading to larger mesopores and a relatively increased permeation power, but a relatively decreased ion selectivity.

[0056] Example 13 The difference between Example 13 and Example 1 is that the surfactant and amine monomer in step (1) have different charges. In Example 13, the surfactant is an equimolar amount of the anionic surfactant sodium dodecyl sulfate, and the amine monomer is an equimolar amount of the cationic amine monomer triaminoguanidine hydrochloride. The membrane obtained by interfacial polymerization also has a permeation power of 23.58 W / m² and an ion selectivity of 0.87, which shows the universality of the strategy.

[0057] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the step of adding surfactant in step (1) is omitted.

[0058] When a surfactant is not used as a template agent, the membrane lacks self-supporting ability, exhibits poor mechanical properties, and suffers from decreased permeability (5.87 W / m). 2 .

[0059] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the cationic surfactant (DTAB) in step (1) was replaced with the polymer polyvinylpyrrolidone. The polymerized membrane does not have an asymmetric structure, has poor mechanical properties, and has a permeation power of 3.45 W / m. 2 .

[0060] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the concentration of hydrochloric acid solution in step (3) is 0.1 M.

[0061] Testing revealed that the charged covalent organic framework membrane prepared in Comparative Example 3 had a permeability of 6.04 W / m². This was attributed to insufficient hydrochloric acid concentration, leading to incomplete surfactant removal and reduced porosity. In fact, the immersion process in acidic solutions significantly impacts surfactant removal. Immersion at room temperature, low acid concentrations, or no acid immersion all result in incomplete surfactant removal, thus affecting the permeability and selectivity of the prepared membrane.

[0062] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the acetic acid solution in step (1) is replaced with an equal amount of aqueous solution.

[0063] When acetic acid is not added as a catalyst, the formed film does not have a fibrous structure and its film-forming properties deteriorate due to the slowed reaction rate, and it does not have self-supporting ability.

[0064] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that in step (1), toluene / n-hexane is replaced with an equal volume of o-dichlorobenzene.

[0065] When o-dichlorobenzene is used instead, the reaction rate is very slow, and a complete film cannot be formed in 1 day. Even if the film is formed by heating the reaction, it is very thin and does not have self-supporting ability.

[0066] The embodiments provided above are not intended to limit the scope of the invention, nor are the described steps intended to limit the order of execution. Any obvious modifications made to the invention by those skilled in the art based on existing common knowledge also fall within the scope of protection defined by the claims.

Claims

1. A method for preparing a charged covalent organic framework membrane with exogenous mesopores, characterized in that, Includes the following steps: (1) Aqueous phase preparation: Mix the aqueous solution of surfactant and acid catalyst, and add amine monomer under ultrasonic conditions to obtain an aqueous solution; (2) Organic phase preparation: The aldehyde monomer is dissolved in an organic solvent and ultrasonically treated to obtain an organic phase solution; (3) Preparation of covalent organic framework membrane: The organic phase solution obtained in step (2) is slowly added to the aqueous phase solution in step (1), sealed and transferred to a constant temperature and humidity chamber at 25°C and left to stand for 0.5 to 1 day to obtain the membrane material at the interface; (4) Charged covalent organic framework membrane with exogenous mesopores: Take out the membrane material at the interface obtained in step (3), clean it to remove the residual unreacted monomers and catalysts; after cleaning, add the membrane material to an acid solution and heat it to soak it, and finally soak and wash it to obtain a charged covalent organic framework membrane with exogenous mesopores. Both the surfactant and the amine monomer have charged groups, and the surfactant and the amine monomer carry opposite charges.

2. The preparation method according to claim 1, characterized in that, When the surfactant is a cationic surfactant, the amine monomer is a negatively charged amine monomer, and the surfactant is selected from at least one of dodecyltrimethylammonium bromide, octadecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide; the amine monomer is selected from 2,5- Sodium diaminobenzenesulfonate, 2,5-diaminobenzenesulfonic acid, 4,4'-diaminostilbene-2,2'-disulfonic acid, 3,5-diaminobenzoic acid, and 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid are at least one of the following:

3. The preparation method according to claim 1, characterized in that... When the surfactant is an anionic surfactant, the amine monomer is a positively charged amine monomer, and the surfactant is selected from at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and sodium dioctyl sulfosuccinate, and the amine monomer is selected from at least one of dimethyl diallyl ammonium chloride, triaminoguanidine hydrochloride, and 1,3,5-tris(4-aminophenyl)benzene.

4. The preparation method according to claim 1, characterized in that, The molar ratio of the surfactant to the amine monomer is 1:8 to 5:

1.

5. The preparation method according to claim 1, characterized in that, The acid catalyst is selected from at least one of acetic acid, p-toluenesulfonic acid, trifluoroacetic acid, and trifluoromethanesulfonic acid, and the molar concentration of the acid catalyst in the aqueous solution is 1 mol / L to 10 mol / L.

6. The preparation method according to claim 1, characterized in that, In step (2), the aldehyde monomer is selected from trialdehyde phloroglucinol, 2,5-dimethyl phloroglucinol, etc. Dihydroxyterephthalaldehyde, 1,3,5 At least one of pyromellitic benzoate and 2-hydroxy-1,3,5-benzyltricarboxaldehyde; the molar ratio of the aldehyde monomer to the amine monomer is 3:1 to 1:3, preferably 1:2 to 2:1, and the ultrasonic dispersion time is 5 to 60 min.

7. The preparation method according to claim 1, characterized in that, In step (2), the organic solvent is a mixed organic solvent, which is selected from toluene / n-hexane, ethyl acetate / n-hexane, and chloroform / n-hexane. The volume ratio of the two solvents in the mixed organic solvent is 1:5 to 2:

1.

8. The preparation method according to claim 1, characterized in that, In step (4), the cleaning is to soak and clean the membrane in DMF, methanol, ethanol and deionized water in sequence. The acid solution is one of acetic acid, hydrochloric acid and sulfuric acid, with a concentration of 0.3~3 mol / L, a soaking time of 0.5~2 days and a heating temperature of 30~90 ℃. Finally, the membrane is soaked and cleaned in methanol and deionized water in sequence.

9. The charged covalent organic framework membrane with exogenous mesopores prepared by the preparation method according to claim 1.

10. The application of the charged covalent organic framework membrane with exogenous mesopores as described in claim 9 in the field of salt gradient energy conversion.

Images