A self-supporting metal-organic framework-carbon nanotube composite capacitive deionization membrane and a preparation method thereof

Abstract

This invention discloses a self-supporting metal-organic framework-carbon nanotube composite capacitor deionization membrane and its preparation method, belonging to the technical field of porous conductive materials and capacitor deionization membranes. Zirconium tetrachloride (ZrCl4) is ultrasonically dissolved in N,N-dimethylformamide (DMF); simultaneously, terephthalic acid (H2BDC) and 2-sulfonic acid monosodium terephthalate (H2BDC-SO3Na) are added to a mixed solution of DMF and a growth regulator, allowing for complete ultrasonic-assisted dissolution and uniform mixing of the two solutions; the self-supporting carbon nanotube (CNT) membrane is surface-treated in a plasma etching machine; then, the treated CNT membrane is placed in the above mixed solution, and the mixture is placed in a high-pressure reactor for hydrothermal reaction; the resulting composite membrane is washed, purified, and then thoroughly dried in a vacuum drying oven, finally obtaining the self-supporting metal-organic framework-carbon nanotube (UiO-66-SO3H-CNTs) composite capacitor deionization membrane material. It exhibits excellent capacitor desalination performance and has broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of porous conductive materials and capacitor deionization membrane technology, specifically relating to a self-supporting metal-organic framework-carbon nanotube composite capacitor deionization membrane and its preparation method. Background Technology

[0002] With the global freshwater supply and demand becoming increasingly strained, advanced desalination technologies have attracted widespread attention. Traditional desalination technologies such as reverse osmosis, electrodialysis, and membrane distillation suffer from drawbacks such as susceptibility to pollution and high energy consumption, necessitating the development of new desalination technologies. Compared to these technologies, capacitive deionization (CDI) is an environmentally friendly, energy-efficient desalination technology with high water utilization. CDI, also known as electroadsorption, is a novel water treatment technology based on the theory of electric double-layer capacitors (EDLC). It utilizes charged electrode materials to adsorb ions and other charged particles in water, thereby achieving water purification. The key and core of CDI technology is the electrode material, whose performance determines the ion removal rate, regeneration performance, and cycle life of the CDI process. CDI electrode materials should possess good conductivity, wettability, high specific capacitance, stable chemical and electrochemical properties, and also have a large specific surface area and a porous structure that facilitates ion migration. Due to their high specific surface area, good flexibility and low resistivity, carbon-based materials such as graphene, activated carbon (AC), carbon aerogels (CAs), and carbon nanotubes (CNTs) have been widely studied as CDI electrode materials in recent years. Patent CN110451615B discloses a method for preparing a metal-organic framework carbon nanotube hybrid desalination electrode. This method first mixes different cobalt salts with carboxylated CNT powder and an organic ligand solution, centrifuges, washes, and dries to obtain a hybrid electrode material. This material is then mixed uniformly with acetylene black and polyvinyl alcohol solution and coated onto graphite paper. Finally, it is dried to obtain the hybrid desalination electrode. Patent CN105513823B discloses a method for preparing a carbon nanotube self-supporting composite film. This method first adds nanomaterials to dispersant 1 to obtain dispersion system 1. Then, CNTs, terpineol, and ethyl cellulose are added to dispersant 2 to obtain dispersion system 2. Next, dispersion systems 1 and 2 are mixed at a certain volume ratio, stirred, and dispersed to obtain a precursor. Finally, the precursor is heated and evaporated to remove the dispersant to obtain a slurry. The slurry is then coated onto a substrate, and the substrate is annealed at 200℃-450℃ to obtain the composite film. The most common method for preparing CDI composite electrode materials is to combine modified CNTs with oxides, polymers or other porous materials, add a binder, and finally deposit them onto a current collector.

[0003] However, in the aforementioned composite electrode preparation process, CNTs are typically present in powder form and require activation or chemical modification to ensure good dispersibility in the final electrode material. These operations consume significant time and energy. The synthesis and modification processes of carbon-based materials are cumbersome, and the powders tend to agglomerate during use, necessitating the addition of binders. These processes further complicate electrode preparation. Furthermore, the addition of binders often leads to pore blockage in porous materials, resulting in a decrease in specific surface area and even failing to achieve ideal capacitive deionization performance. Therefore, there is an urgent need to develop advanced capacitive deionization membrane electrode materials with simple preparation processes and high energy efficiency.

[0004] Compared with composite membrane electrodes doped with CNT powder, self-supporting CNT fabrics with a randomly interlaced fiber structure are a novel and very promising carbon nanoporous materials. These materials have excellent mechanical and electrical properties, as well as good thermal and chemical stability, and have broad application prospects in electrodes, microwave absorption, filtration and composite materials. However, their strong hydrophobicity greatly hinders their application in the CDI field. Therefore, there is an urgent need for a technology that can impart hydrophilicity and capacitive deionization properties to self-supporting CNT fabrics in a simple and controllable manner. Metal-organic frameworks (MOFs) are a class of novel porous crystalline materials composed of organic linkers and inorganic metal ion nodes. They possess extremely high specific surface area and porosity, showing great application potential in adsorption, catalysis, chemical sensing, and membrane separation. Meanwhile, the UiO series, as typical Zr-based MOFs, has attracted widespread attention and research due to its excellent chemical stability, thermal stability, and large specific surface area. However, research on the grafting modification of this type of MOF material and its further application as a self-supporting conductive porous material in the field of capacitive deionization is still lacking. Summary of the Invention

[0005] Based on the advantages and disadvantages of the aforementioned materials, the present invention aims to develop a self-supporting metal-organic framework-carbon nanotube composite capacitor deionization membrane electrode and disclose its preparation method. The goal is to prepare a composite capacitor deionization membrane material with uniform and controllable pores, high specific surface area, and good conductivity, flexibility, high temperature resistance, and resistance to acids, alkalis, and organic solvents under simple operation, mild conditions, and simple equipment and processes.

[0006] The technical solution of this invention is as follows: A self-supporting metal-organic framework-carbon nanotube composite capacitor deionization membrane and its preparation method are disclosed below: (1) Under ultrasonic assistance, ZrCl4 was completely dissolved in DMF solution to obtain solution I; (2) Under ultrasound assistance, H2BDC and H2BDC-SO3Na were completely dissolved in a mixed solution of DMF and growth regulator to obtain solution II; (3) Add solution I to solution II and mix them thoroughly to obtain solution III; (4) The self-supporting CNT film is processed in a plasma etching machine; (5) The treated CNTs membrane is placed in solution III, the mixture is placed in a high-pressure reactor with a polytetrafluoroethylene liner, and then placed in an oven for hydrothermal reaction at a certain temperature. (6) Take out the reaction vessel after step (5) and let it cool naturally to room temperature. The resulting composite membrane is washed with DMF and ethanol and then immersed in ethanol. (7) The composite membrane obtained in step (6) is placed in a vacuum drying oven and dried thoroughly to obtain a self-supporting metal-organic framework-carbon nanotube composite capacitor deionization membrane material.

[0007] More preferably, the molar ratio of ZrCl4, H2BDC and H2BDC-SO3Na in steps (1) and (2) is 1:(0.90-0.95):(0.22-0.25).

[0008] More preferably, the growth regulator used in step (2) is one or more combinations of water, acetic acid, formic acid and hydrochloric acid.

[0009] More preferably, the plasma etching process in step (4) is carried out in an oxygen atmosphere for 30-60 seconds. More preferably, the hydrothermal reaction temperature in step (5) is 85-120°C and the reaction time is 16-24 hours.

[0010] More preferably, in step (6), the total immersion time of the composite membrane in ethanol is 48-72 h and the solution is replaced every 12 h.

[0011] More preferably, in step (7), the drying temperature of the composite film in the vacuum drying oven is 100-130℃ and the drying time is 18-24h.

[0012] The beneficial effects obtained by this invention are as follows: (1) The preparation of the UiO-66-SO3H-CNTs composite capacitor deion membrane of the present invention is completed by a one-step hydrothermal method on a plasma-activated fabric. There is no need to consider the agglomeration of raw materials during dispersion and the problem of adding conductive agents. The preparation process is simple and easy to control, the conditions are mild, and it is suitable for large-scale production.

[0013] (2) The composite capacitor deionization membrane structure framework prepared by the present invention is a uniform combination of self-supporting CNTs and UiO-66-SO3H, which has good properties such as high specific surface area, conductivity, flexibility, high temperature resistance and acid and alkali resistance.

[0014] (3) The composite capacitor deionization membrane prepared by the present invention has low energy consumption, is environmentally friendly and pollution-free during use, and has high ion removal efficiency in water. It has good application prospects in capacitor desalination, adsorption, membrane separation and catalysis. Attached Figure Description

[0015] Figure 1 This is a scanning electron microscope image of the composite capacitor deionization membrane prepared in Example 1.

[0016] Figure 2 This is a comparison of the Raman spectra of the composite capacitor deionization membrane and the CNTs fabric prepared in Example 2.

[0017] Figure 3 This is a graph showing the change in electroadsorption capacity of the composite capacitor deionization membrane prepared in Example 3 at different cycles. Detailed Implementation

[0018] The following description of the embodiments will provide a more detailed explanation of the specific implementation of the present invention, in order to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention.

[0019] Example 1 At room temperature, 80.8 mg of zirconium tetrachloride (ZrCl4) was weighed and placed in 10 mL of N,N-dimethylformamide (DMF), and sonicated for 8 min to completely dissolve it to obtain solution I. 53.5 mg of terephthalic acid (H2BDC) and 21.3 mg of monosodium 2-sulfonate terephthalate (H2BDC-SO3Na) were weighed and placed in 15 mL of DMF, and sonicated for 10 min to completely dissolve the solids to obtain solution II. Then, 25 µL of H2O, 1.35 mL of HCl, and 1 mL of HAc were added to solution II as growth regulators. Next, solution I was quickly added to solution II and mixed thoroughly to obtain solution III. The self-supporting CNTs fabric was placed in a plasma etching machine and activated in an oxygen atmosphere for 45 seconds. The treated CNTs fabric was then placed in solution III and placed in a high-pressure reactor with a polytetrafluoroethylene liner. The reactor was then placed in an oven and reacted at 85°C for 24 hours. The reactor was then removed and allowed to cool naturally to room temperature. The composite membrane was washed sequentially with DMF and ethanol and then immersed in ethanol for 48 hours, with the solution being changed every 12 hours. Finally, the composite membrane was placed in a vacuum drying oven and dried at 120°C for 24 hours.

[0020] Figure 1The image shows a scanning electron microscope (SEM) image of the composite capacitor deionization membrane prepared in this embodiment. As can be seen from the image, the prepared metal-organic framework material has good crystal form and uniform particle size. It grows along the surface of CNTs and forms a three-dimensional interconnected spatial network structure, which improves conductivity, enriches the pore structure, and increases the specific surface area of ​​the composite membrane.

[0021] Example 2 At room temperature, 76.5 mg of zirconium tetrachloride (ZrCl4) was weighed and placed in 8 mL of N,N-dimethylformamide (DMF), and sonicated for 6 min to completely dissolve it to obtain solution I. 51.8 mg of terephthalic acid (H2BDC) and 19.7 mg of monosodium 2-sulfonate terephthalate (H2BDC-SO3Na) were weighed and placed in 12 mL of DMF, and sonicated for 8 min to completely dissolve the solids to obtain solution II. Then, 20 µL of H2O and 1 mL of HAc were added to solution II as growth regulators. Next, solution I was quickly added to solution II and mixed thoroughly to obtain solution III. The self-supporting CNTs fabric was placed in a plasma etching machine and activated in an oxygen atmosphere for 30 seconds. The treated CNTs fabric was then placed in solution III and placed in a high-pressure reactor with a polytetrafluoroethylene liner. The reactor was then placed in an oven and reacted at 100°C for 24 hours. The reactor was then removed and allowed to cool naturally to room temperature. The composite membrane was washed sequentially with DMF and ethanol and then immersed in ethanol for 48 hours, with the solution being changed every 12 hours. Finally, the composite membrane was placed in a vacuum drying oven and dried at 110°C for 24 hours.

[0022] Figure 2 This is a comparison of the Raman spectra of the composite capacitor deionization membrane and the CNTs fabric prepared in this embodiment. As can be seen from the figure, both the composite capacitor deionization membrane and the CNTs fabric exhibit strong characteristic peaks at 1335 cm⁻¹ and 1581 cm⁻¹, respectively. These peaks can be attributed to the D peak (vibrational mode caused by the presence of microcrystalline plane edges or defects, representing defects and disorder in carbon materials) and the G peak (caused by the six-ring plane stretching symmetry vibration, representing the order of carbon nanotubes). The intensity of the G peak is much greater than that of the D peak, indicating that the CNTs fabric has a high degree of graphitization, high order of carbon nanotubes, and relatively low defects and disorder. The composite capacitor deionization membrane prepared in this embodiment also shows new characteristic peaks at 892 cm⁻¹ and 1457 cm⁻¹, respectively. The comparison shows that the characteristic peaks of the CNTs fabric itself have not changed significantly, indicating that the combination of metal-organic framework materials and CNTs has not damaged or affected the inherent structural properties of the CNTs fabric.

[0023] Example 3 At room temperature, 90.6 mg of zirconium tetrachloride (ZrCl4) was weighed and placed in 12 mL of N,N-dimethylformamide (DMF), and sonicated for 10 min to completely dissolve it to obtain solution I. 58.2 mg of terephthalic acid (H2BDC) and 23.3 mg of monosodium 2-sulfonate terephthalate (H2BDC-SO3Na) were weighed and placed in 18 mL of DMF, and sonicated for 10 min to completely dissolve the solids to obtain solution II. Then, 1.2 mL of HAc was added to solution II as a growth regulator. Next, solution I was quickly added to solution II and mixed thoroughly to obtain solution III. The self-supporting CNTs fabric was placed in a plasma etching machine and activated in an oxygen atmosphere for 40 seconds. The treated CNTs fabric was then placed in solution III and placed in a high-pressure reactor with a polytetrafluoroethylene liner. The reactor was then placed in an oven and reacted at 90°C for 18 hours. The reactor was then removed and allowed to cool naturally to room temperature. The composite membrane was washed sequentially with DMF and ethanol and then immersed in ethanol for 48 hours, with the solution being changed every 12 hours. Finally, the composite membrane was placed in a vacuum drying oven and dried at 130°C for 24 hours.

[0024] Figure 3 This figure shows the change in electroadsorption capacity of the composite capacitor deionization membrane prepared in this embodiment after different cycles. As can be seen from the figure, the electroadsorption capacity of the membrane prepared in this embodiment did not decrease significantly after 5 cycles, with an average electroadsorption capacity of 25.66 mg / g. This indicates that the composite capacitor deionization membrane has high electroadsorption capacity and excellent recycling performance. Electroadsorption capacity (EC, mg / g) is defined as the amount of ions adsorbed per gram of membrane, and its calculation formula is as follows:

[0025] Where Co and Ce represent the initial and equilibrium concentrations (mg / L) of the NaCl solution, respectively, V (L) is the total volume of the NaCl aqueous solution, and m (g) is the total mass of the membrane.

[0026] Example 4 At room temperature, 83.1 mg of zirconium tetrachloride (ZrCl4) was weighed and placed in 10 mL of N,N-dimethylformamide (DMF), and sonicated for 10 min to completely dissolve it to obtain solution I. 55.2 mg of terephthalic acid (H2BDC) and 20.7 mg of monosodium 2-sulfonate terephthalate (H2BDC-SO3Na) were weighed and placed in 10 mL of DMF, and sonicated for 10 min to completely dissolve the solids to obtain solution II. Then, 20 µL of H2O and 1.2 mL of HCl were added to solution II as growth regulators. Next, solution I was quickly added to solution II and mixed thoroughly to obtain solution III. The self-supporting CNTs fabric was placed in a plasma etching machine and activated in an oxygen atmosphere for 60 seconds. The treated CNTs fabric was then placed in solution III and placed in a high-pressure reactor with a polytetrafluoroethylene liner. The reactor was then placed in an oven and reacted at 110°C for 20 hours. The reactor was then removed and allowed to cool naturally to room temperature. The composite membrane was washed sequentially with DMF and ethanol and then immersed in ethanol for 60 hours, with the solution being changed every 12 hours. Finally, the composite membrane was placed in a vacuum drying oven and dried at 130°C for 18 hours.

[0027] Example 5 At room temperature, 78.7 mg of zirconium tetrachloride (ZrCl4) was weighed and placed in 8 mL of N,N-dimethylformamide (DMF), and sonicated for 8 min to completely dissolve it to obtain solution I. 50.6 mg of terephthalic acid (H2BDC) and 22.3 mg of monosodium 2-sulfonate terephthalate (H2BDC-SO3Na) were weighed and placed in 12 mL of DMF, and sonicated for 8 min to completely dissolve the solids to obtain solution II. Then, 1 mL of HCl and 1 mL of HAc were added to solution II as growth regulators. Next, solution I was quickly added to solution II and mixed thoroughly to obtain solution III. The self-supporting CNTs fabric was placed in a plasma etching machine and activated in an oxygen atmosphere for 45 seconds. The treated CNTs fabric was then placed in solution III and placed in a high-pressure reactor with a polytetrafluoroethylene liner. The reactor was then placed in an oven and reacted at 120°C for 16 hours. The reactor was then removed and allowed to cool naturally to room temperature. The composite membrane was washed sequentially with DMF and ethanol and then immersed in ethanol for 48 hours, with the solution being changed every 12 hours. Finally, the composite membrane was placed in a vacuum drying oven and dried at 120°C for 24 hours.

[0028] Table 1 below shows the molar ratios of ZrCl4, H2BDC, and H2BDC-SO3Na in Examples 1-5. As can be seen from Table 1, the molar ratio of ZrCl4, H2BDC, and H2BDC-SO3Na is 1:(0.90-0.95):(0.22-0.25).

[0029] Table 1

[0030] The above embodiments are merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical concept proposed in this invention shall fall within the scope of protection of this invention. Technologies not covered in this invention can be implemented using existing technologies.

Claims

1. A method for preparing a self-supporting metal-organic framework-carbon nanotube composite capacitor deionization membrane, characterized in that, The steps include the following: (1) Zirconium tetrachloride (ZrCl4) was completely dissolved in N,N-dimethylformamide (DMF) solution using an ultrasonic-assisted method to obtain solution I; (2) Terephthalic acid (H2BDC) and 2-sulfonic acid monosodium terephthalate (H2BDC-SO3Na) were completely dissolved in a mixed solution of DMF and growth regulator using an ultrasonic-assisted method to obtain solution II; (3) Add solution I to solution II and mix them thoroughly to obtain solution III; (4) The self-supporting carbon nanotube (CNT) film was placed in a plasma etching machine for processing in an oxygen atmosphere for 30-60 seconds. (5) The treated CNTs membrane is placed in solution III, and the mixture is further placed in a high-pressure reactor with a polytetrafluoroethylene liner and placed in an oven for hydrothermal reaction. The hydrothermal reaction temperature is 85-120℃ and the reaction time is 16-24h. (6) Take out the reaction vessel after step (5) and let it cool naturally to room temperature. The resulting composite membrane is washed with DMF and ethanol respectively and then immersed in ethanol. (7) The composite membrane obtained in step (6) is placed in a vacuum drying oven and dried thoroughly to obtain a self-supporting metal-organic framework-carbon nanotube composite capacitor deionization membrane material.

2. The method according to claim 1, characterized in that, In steps (1) and (2), the molar ratio of ZrCl4, H2BDC and H2BDC-SO3Na is 1:(0.90-0.95):(0.22-0.25).

3. The method according to claim 1, characterized in that, The growth regulator used in step (2) is one or more of water, acetic acid, formic acid and hydrochloric acid.

4. The method according to claim 1, characterized in that, In step (6), the total immersion time of the composite membrane in ethanol is 48-72 hours, and the solution is replaced every 12 hours.

5. The method according to claim 1, characterized in that, In step (7), the composite membrane is dried at a temperature of 100-130℃ in a vacuum drying oven for 18-24 hours.

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