Sub-nano channel membrane with bionic ion characteristics and preparation method and application thereof

Abstract

The invention discloses a sub-nano channel membrane with bionic ion characteristics as well as a preparation method and application of the sub-nano channel membrane. The sub-nano channel membrane comprises a PVDF (Polyvinylidene Fluoride) matrix and UiO-66-NH2 nanoparticles which are uniformly and continuously dispersed in the PVDF matrix, a bionic ion channel with an ion rectification characteristic and an ion specific transmission capability is formed between the UiO-66-NH2 nanoparticles which are uniformly and continuously dispersed and the PVDF matrix. According to the method disclosed by the invention, an MOGs precursor is encapsulated in a polymer through a polymer-mediated MOFs strategy, and the MOFs in the polymer are induced by utilizing solvent evaporation to realize in-situ growth. The high matching of the MOGs nucleation rate and the polymer film forming rate ensures the uniform and continuous distribution of MOFs in a polymer matrix, and a bionic channel with an ion rectification characteristic and a fluorine ion specific transmission capability is formed.

Description

Technical Field

[0001] The present invention relates to a sub-nanometer channel membrane with bionic ion characteristics, a preparation method thereof, and an application thereof, belonging to the technical field of the preparation of MOF mixed matrix membranes. Background Art

[0002] By polymer-mediated growth and dispersion of amino-functionalized ZIF, a polyamide selective layer with Turing structure was prepared, realizing the selective separation between large / small solute molecules. However, when the separation scale is further reduced to angstrom-scale solutes, due to the lack of specific recognition ability between the polyamide selective layer and solute molecules, its separation performance decreases significantly.

[0003] The uniform angstrom-scale pores and diverse functional groups of MOFs can achieve the selective recognition of angstrom-scale target ions, making them ideal materials for constructing bionic ion channels. MMMs based on MOFs combine the sub-nanometer channel properties of MOFs and the scalability of polymers, making them ideal platforms for constructing bionic ion channels. However, due to the mismatch between the nucleation rate of MOFs and the film-forming rate of polymers, and the lack of effective interaction between the interface of MOFs and polymers, it is difficult for MOFs to form continuous and uniform bionic ion channels in the polymer matrix. Summary of the Invention

[0004] The purpose of the present invention is to provide a sub-nanometer channel membrane with bionic ion characteristics, a preparation method thereof, and an application thereof. Through the "polymer-mediated MOFs" strategy, MOG precursors are encapsulated in polymers and in-situ growth of MOFs is achieved by solvent evaporation induction. The high matching between the nucleation rate of MOGs and the film-forming rate of polymers ensures the uniform and continuous distribution of MOFs in the polymer matrix, forming a bionic channel with ion rectification characteristics and fluoride ion specific transport ability.

[0005] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows: A sub-nanometer channel membrane with bionic ion characteristics, comprising a PVDF matrix and UiO-66-NH2 nanoparticles uniformly and continuously dispersed in the PVDF matrix; a bionic ion channel with ion rectification characteristics and ion specific transport ability is formed between the uniformly and continuously dispersed UiO-66-NH2 nanoparticles and the PVDF matrix.

[0006] Preferably, the size of the bionic ion channel is at the sub-nanometer level.

[0007] The preparation method of any of the above-mentioned sub-nanometer channel membranes with bionic ion characteristics is to disperse the UiO-66-NH2 gel precursor solution in the PVDF casting solution to obtain a mixed matrix membrane casting solution, and then perform membrane preparation; wherein, the mass of the UiO-66-NH2 gel precursor is 15-35 wt% of the mass of PVDF.

[0008] Preferably, the preparation method of the UiO-66-NH2 gel precursor solution is: dissolve ZrOCl2·8H2O and 2-aminoterephthalic acid in an equimolar amount in a solvent, and stir and ultrasonicate until a homogeneous solution is formed.

[0009] Preferably, the solvents used in the PVDF casting solution and the UiO-66-NH2 gel precursor solution are DMF.

[0010] Preferably, the specific method of membrane preparation is: scrape the mixed matrix membrane casting solution on a substrate, dry it at 100-150 °C, peel the membrane after cooling to room temperature, and soak it to remove residual solvents and then store it.

[0011] The application of any of the above-mentioned sub-nanometer channel membranes with bionic ion characteristics in ion separation.

[0012] A method for improving the tensile strength of a sub-nanometer channel membrane doped with UiO-66-NH2 nanoparticles is to add the UiO-66-NH2 gel precursor solution to the PVDF casting solution, and then perform membrane preparation after obtaining the mixed matrix membrane casting solution; wherein, The preparation method of the UiO-66-NH2 gel precursor solution is: dissolve ZrOCl2·8H2O instead of ZrCl4 and 2-aminoterephthalic acid in an equimolar amount in a solvent, and stir and ultrasonicate until a homogeneous solution is formed; and, the mass of the UiO-66-NH2 gel precursor is 15-35 wt% of the mass of PVDF.

[0013] The beneficial effects of the present invention are as follows: With the "polymer-mediated MOFs" strategy, the MOGs precursor is encapsulated in the polymer and the in-situ growth of MOFs is realized by solvent evaporation induction. During this process, the polymer regulates the in-situ growth of MOFs through a mediating effect and inhibits the formation of mesopores during the solvent evaporation stage. This strategy has the following advantages: (1) There is no need to pre-synthesize MOFs particles, and the size of MOFs can be regulated during the in-situ growth process, simplifying the preparation process of MMMs while improving the interfacial compatibility; (2) The high matching of the MOGs nucleation rate and the polymer film formation rate ensures the uniform and continuous distribution of MOFs in the polymer matrix. The uniformly dispersed MOFs form bionic channels with ion rectification characteristics and fluoride ion specific transport ability ( Figure 1) This strategy provides a new method for the construction of biomimetic ion channels in MMMs and a new perspective for the facile preparation of intelligent nanofluids. Description of the Drawings

[0014] Figure 1 Schematic diagram of a biomimetic ion channel membrane constructed based on polymer molecule-encapsulated MOFs; Figure 2 XRD patterns (a), FTIR spectra (b), and SEM images (c) of PVDF, PVDF-U6N, and PVDF-U6N-gel; Figure 3 Zr 3d XPS spectra of (a) U6N powder, PVDF-U6N, and PVDF-U6N-gel, and stress-strain tensile curves of (b) PVDF, PVDF-U6N, and PVDF-U6N-gel; Figure 4 MSD curves between MOG / MOF molecules and PVDF; Figure 5 Intermolecular interactions between MOF / MOG precursors and polymer PVDF based on molecular dynamics simulations: (a) intermolecular forces, (b) numerical values of van der Waals interaction energy and intermolecular forces, (c) hydrogen bond length and number, (d) concentration-distance curves; Figure 6 Cartoon schematic diagrams of (a) the difference in crystallization rates between MOGs and MOF precursors, XRD patterns of (b) MOF precursors after different heating times to form UiO-66-NH2, XRD patterns of (c) MOG precursors after different heating times to form UiO-66-NH2, and kinetic curves in situ monitored by turbidimetry during the formation of UiO-66-NH2 from MOF and MOG precursors; Figure 7 Thermogravimetric curves of (a) U6N and U6N-gel nanoparticles, DTG curves of (b) U6N and U6N-gel nanoparticles, SEM images of (c) U6N-gel nanoparticles, full XPS spectra of (d) U6N-gel nanoparticles, Zr 3d spectra of (e) U6N-gel nanoparticles, and FTIR spectra of (f) U6N-gel nanoparticles; Figure 8 Surface and cross-sectional morphologies of MMMs formed with different MOG precursor addition amounts: (a) MOG addition amount of 20 wt%, (b) MOG addition amount of 30 wt%, (c) MOG addition amount of 40 wt%; Figure 9 TEM images of MMMs formed with 20% and 30% MOG precursors; Figure 10 SEM images of MMMs formed by adding 10% and 50% MOG precursors respectively Figure 11 XRD patterns of MMMs at (a) 20%, 30% and 40% MOG addition amounts, DSC curves of PVDF and PVDF-U6N-gel in (b), thermogravimetric curves of PVDF and PVDF-U6N-gel in nitrogen atmosphere in (c), and thermogravimetric curves of PVDF-U6N at different MOG addition amounts in air atmosphere in (d); Figure 12 EDX scan and (b) TEM images of PVDF-U6N-gel membrane Figure 13 SEM images of MMMs obtained with different MOF precursor addition amounts Figure 14 Schematic diagram of in-situ growth of MOF and MOG precursors in polymer membrane to form MMMs Figure 15 Schematic diagram of (a) I-V test device and I-V curves of PVDF-U6N in 0.01 M KCl and KF in (b); Figure 16 I-V curves of PVDF-U6N-gel membranes formed with different MOG doping amounts; (a) 0% MOG addition amount, (b) 10% MOG addition amount, (c) 20% MOG addition amount, (d) 30% MOG addition amount, (e) 40% MOG addition amount, (f) Trend chart of ion conductivity and selectivity changes of MMMs formed with different addition amounts Figure 17 Rectification ratios of MMMs prepared with different MOG addition amounts in 0.01 M KCl electrolyte solution in (a) and contact angles of MMMs prepared with different MOG addition amounts in (b); Figure 18 COMSOL simulation of ion transport in MMMs (PVDF-U6N membrane) with non-uniform MOFs distribution; (a)-(f) Concentration distribution diagrams of cations and anions in the membrane under different voltages and electrolyte conditions, (g)-(i) Two-dimensional concentration distribution diagrams Figure 19 COMSOL simulation of (a) potential distribution curve (KCl solution) and (b) current density under different electrolyte solutions during ion transport in PVDF-U6N membrane Figure 20COMSOL simulation of ion transport in MMMs (PVDF-U6N-gel membrane) with uniform MOF distribution: (a) Concentration distribution maps of cations and anions in the membrane under +2V voltage, (b) Concentration distribution maps of cations and anions under -2V voltage, (c) Potential-distance change curve, (d-f) Two-dimensional concentration maps of anions and cations in the membrane under different voltage conditions; Figure 21 Adsorption capacity of U6N-gel powder for 1 mM KF and KCl solutions; Figure 22 COMSOL simulation of the transport of KF solution in PVDF-U6N-gel membrane: (a) Two-dimensional concentration map of anions in the membrane under +2V voltage, (b) Two-dimensional concentration map of cations in the membrane under +2V voltage, (c) Concentration-distance curve in PVDF-U6N-gel membrane, (d) Current density in PVDF-U6N-gel nanochannels under different electrolytes and voltage conditions; Figure 23 Ion transport curves of PVDF-U6N-gel at different electrolyte concentrations; Figure 24 I-V curves under different pH conditions (KCl solution), (a) I-V curve at pH = 2.16, (b) I-V curve at pH = 5.8, (c) I-V curve at pH = 10.6; Figure 25 Schematic diagram for comparison of ion transport processes in PVDF-U6N and PVDF-U6N-gel membranes. Detailed implementation

[0015] In-situ preparation of MOF mixed matrix membrane

[0016] The preparation of MOF mixed matrix membrane is divided into the following 4 steps: (1) Preparation of pure PVDF casting solution Dissolve 2g of PVDF polymer in 10g of DMF, stir evenly and let stand for 12h to remove bubbles to form a casting solution.

[0017] (2) Preparation of metal-organic gel UiO-66-NH2 (MOG) precursor solution Dissolve different masses of ZrOCl2·8H2O (0.4, 0.8, 1.2, 1.6 and 2.0 mmol) and 2-aminoterephthalic acid (2-NH2-BDC) in 3g of DMF in equimolar amounts, stir and sonicate until a homogeneous solution is formed to form UiO-66-NH2 gel precursor solutions with different concentrations.

[0018] (3) Preparation of mixed matrix membrane casting solution Add the above-mentioned UiO-66-NH2 gel precursor solutions with different concentrations to the pure PVDF casting solution, stir ultrasonically to remove air bubbles, and then set aside. They correspond to UiO-66-NH2 gel casting solutions with concentrations of 10wt%, 20wt%, 30wt%, 40wt%, and 50wt% respectively (the percentage values are calculated by the ratio of the mass of the UiO-66-NH2 gel precursor to the mass of the polymer).

[0019] (4) Cast the casting solution into a film Use a 200-μm-thick scraper to cast the above casting solution on the surface of a clean glass plate, and transfer it to an oven for heating at 120 °C for different times. After the oven cools to room temperature, put the film and the glass plate into deionized water to peel the film from the glass plate, and then soak the film alternately in ethanol and water multiple times to remove the excess solvent. Finally, store it in deionized water for later use. This MMMs is named PVDF-U6N-gel.

[0020] Replacing ZrOCl2·8H2O in the above gel UiO-66-NH2 precursor solution with an equimolar mass of ZrCl4 can obtain MMMs of non-metal-organic gels (conventional MOF), named PVDF-U6N.

[0021] Preparation of MOG and MOF nanoparticles

[0022] Preparation of UiO-66-NH2 gel (MOG) nanoparticles Dissolve 0.2632 g of ZrOCl2·8H2O (0.8 mmol) and 0.147 g of 2-aminoterephthalic acid (0.8 mmol) in 13 g of DMF. After stirring and ultrasonically mixing until uniform, transfer the solution to a reaction kettle and then put it into an oven for heating. After the heating is completed, wash it 3 times with DMF and methanol respectively, and then put it into a vacuum drying oven for drying, named U6N-gel. Similarly, replacing ZrOCl2·8H2O with an equimolar mass of ZrCl4 can obtain conventional MOF nanoparticles, named U6N.

[0023] In-situ preparation of MMMs based on polymers and MOF / MOG precursors

[0024] A common preparation method for MMMs is to add synthesized UiO-66-NH2 particles into a polymer solution to form a casting solution. However, due to interfacial compatibility issues, the dispersibility of MOF particles in the polymer solution is often poor. Preliminary experiments showed that when the synthesized UiO-66-NH2 powder was added to the PVDF casting solution, it could be observed that the UiO-66-NH2 powder would precipitate at the bottom of the casting solution. By irradiating the casting solution with a laser, obvious scattering phenomena could be observed. This further proved that the post-synthesized MOF crystal powder had poor dispersibility in the polymer solution.

[0025] Subsequently, an in-situ synthesis method was adopted, and two UiO-66-NH2 precursors (MOF and MOG) were added to the PVDF casting solution. The difference between these two precursor solutions lies in the zirconium salts used. The MOF precursor used ZrCl4, while the MOG precursor used ZrOCl2·8H2O. This slight change disrupted the coordination equilibrium of MOF, resulting in the formation of MOG. Different from the post-synthesis addition, the PVDF casting solutions containing two UiO-66-NH2 precursors (MOF precursor and MOG precursor) both exhibited obvious Tyndall effects. This indicated that both MOF precursors could be uniformly dispersed in the casting solution to achieve molecular-level mixing. Subsequently, membranes were formed by casting and solvent evaporation, and the obtained membranes were named PVDF-U6N and PVDF-U6N-gel respectively. By directly heating the casting solution (120 °C, 30 minutes), the binding situation between the MOG / MOF precursor and PVDF was observed. There was no obvious change in the fluidity of the PVDF-U6N casting solution before and after heating. However, the solution containing the MOG precursor turned into a non-flowing gel state after heating. Subsequently, the casting solution was further inverted to verify the stability of its gel structure. It should be noted that the polymer mass in the casting solution was five times that of the precursor. However, the presence of the polymer did not affect the formation of the gel network because there was a strong supramolecular interaction and good compatibility between MOG and PVDF.

[0026] The prepared MMMs were further characterized structurally using XRD and FTIR. The results showed that obvious diffraction peaks appeared at approximately 7.8° for both the PVDF-U6N and PVDF-U6N-gel membranes, which were related to the (110) and (002) crystal planes of UiO-66-NH2 ( Figure 2 in region a). A characteristic peak belonging to the N-H bending vibration appeared at 1571 cm -1 in the infrared spectrum, indicating that UiO-66 was successfully aminated ( Figure 2region b). The above results indicate that both MOF precursors and MOG precursors can achieve in-situ growth within the PVDF matrix, but there are significant differences in their distribution patterns. Taking the MMMs prepared with MOF precursors (represented by an addition amount of 20%) as an example, obvious defects and particle aggregation phenomena are shown in their cross-sectional images ( Figure 2 region c). In contrast, the surface and cross-sectional SEM images of the MMMs containing MOG precursors show a uniformly distributed nanoparticle morphology. The defects in the PVDF-U6N membrane are caused by the poor interfacial compatibility between the MOF precursor and PVDF and the hygroscopic properties of ZrCl4.

[0027] The interfacial compatibility differences between PVDF-U6N and PVDF-U6N-gel can also be verified by stress-strain curves and XPS spectra ( Figure 3 ). Due to the poor interfacial compatibility, the tensile strength of the PVDF-U6N membrane is much lower than that of PVDF-U6N-gel. In the narrow spectrum of Zr 3d, a large shift in the binding energy occurs for PVDF-U6N-gel (referenced to the binding energy of UiO-66-NH2 crystals). This indicates that due to the presence of the polymer, the coordination environment of Zr has changed significantly, and there is a stronger interaction between PVDF and MOG. The above results show that although both MOF and MOG can crystallize and grow in PVDF, there are significant differences in their interfacial compatibility and interfacial distribution with PVDF.

[0028] Molecular dynamics analysis of the interfacial interaction mechanism between polymers - MOF / MOG

[0029] Molecular dynamics simulations were used to further explore the differences in the interactions between MOG precursors and MOF precursors and polymers. The MSD of MOF and MOG precursors in different systems was calculated to describe the molecular forces and motion of MOF and MOG precursors in the whole system. As Figure 4 shown, the MOG precursor diffuses relatively fast in the initial stage of the reaction; however, as time extends, the diffusion shows a trend of sub-diffusion (restricted diffusion). In contrast, the MSD curve of the MOF precursor always maintains a relatively high growth rate, showing a state of free diffusion. This is because in the initial stage of the reaction, the MOG precursor is less restricted by the polymer, enabling it to be quickly and evenly distributed throughout the system. As time goes on, the MOG precursor is more strongly restricted by the polymer, the degree of diffusion freedom decreases, and the growth of MSD tends to level off, thus presenting a state of restricted diffusion. The free diffusion behavior of the MOF precursor in the system is due to the weak interaction between the MOF precursor and the polymer.

[0030] Furthermore, the interactions between the precursors and the polymer in the two systems were quantitatively calculated. The interaction energy between the MOG precursor and PVDF is less than that between the MOF precursor and PVDF. This indicates that the MOG precursor has a better binding ability with PVDF ( Figure 5 in region a of Figure 5 ). The main difference in the binding force between the two is due to the different van der Waals interactions between different precursors and the polymer. The van der Waals force between the MOF precursor and PVDF is positive, indicating that the two repel each other. While the van der Waals force between the MOG precursor and PVDF is negative, indicating that the two attract each other ( Figure 5 in region b of Figure 5 ). At the same time, the hydrogen bond interaction between MOG and PVDF is also stronger (

[0031] Comparison of Nucleation and Crystallization Behaviors of MOF and MOG Precursors

[0032] To further explore the reasons for the different growth conditions of the two in the membrane, the crystal formation processes of MOF and MOG were investigated separately in the absence of the polymer PVDF. As Figure 6 shown in region a of Figure 6 , in the absence of a regulator, the crystal growth of UiO-66-NH2 with ZrCl4 as the metal salt was slow, and no MOF crystals were detected within the first 30 minutes of heating. And even after heating for 12 hours, only weak intensity UiO-66-NH2 XRD diffraction peaks could be formed. This is because in the absence of a regulator, the deprotonation degree of the MOF ligand is slowed down, thus delaying the crystallization of UiO-66. While the MOG precursor showed a "rapid crystallization" behavior, and the characteristic peaks of UiO-66 could be observed within 30 minutes. The characteristic peaks showed obvious broadened Bragg diffraction peaks. And with the extension of the heating time, the peak shape did not change significantly.in the b-c region). This indicates that the MOG precursor can rapidly generate stable UiO-66-NH2 crystals. Further, the difference in the crystallization rate during the growth process of the MOF and MOG precursor was quantitatively monitored by a turbidimeter ( Figure 6 in the d region). MOG began to rapidly nucleate and crystallize from the 10th minute, and the crystallization rate began to slow down after 60 minutes. The crystallization rate of MOG was 1.8×10 -3 min -1 . In contrast, the MOF began to rapidly crystallize from 2 hours, and its crystallization rate was 1.53×10 -4 min -1 (in the first 60 minutes), and the crystallization rates of the two differed by about 11 times.

[0033] The MOG precursor can rapidly synthesize UiO-66-NH2 nanoparticles within 30 minutes. To verify the structural stability of the rapidly synthesized material, its physical and chemical properties were characterized. To verify the thermal stability of U6N-gel, thermogravimetric analysis was performed on it. The weight loss of U6N-gel in the range of 20 - 150 o °C can be attributed to the removal of water molecules and solvent molecules adsorbed by the material itself. And its weight loss in the range of 250 - 550 o °C can be attributed to the pyrolysis of the ligands in the MOF crystal, making its structure gradually show thermal instability. The thermal stability of U6N-gel is similar to that of UiO-66-NH2( Figure 7 in the a region).

[0034] By comparing the DTG images of U6N and U6N-gel, it can be found that the maximum peak of the DTG of U6N is located at 464 °C, while the maximum peak of the DTG of U6N-gel is located at 547 °C, and the DTG peak shape of U6N is wider than that of U6N-gel( Figure 7 in the b region). This characteristic of a lower maximum peak and a wider peak shape further indicates that the thermochemical stability of U6N is significantly lower than that of U6N-gel. Further, the morphology of the synthesized U6N-gel was characterized, and the size of its nanoparticles was 5 - 25 nm, and the nanoparticles aggregated with each other( Figure 7 in the c region). XPS scanning was performed on U6N-gel, and Zr3d 3 / 2 and Zr 3d 5 / 2 orbits appeared in the narrow spectrum of Zr3d, indicating that Zr exists in the framework material in the +4 valence state. This is consistent with the valence state of Zr in normal UiO-66-NH2. At the same time, a bending vibration peak of −NH2 appeared at 1660 cm -1 and a stretching vibration peak of −NH2 appeared at around 3400 cm -1 , which confirmed the presence of -NH2 groups in U6N-gel(Figure 7 (in the d-f region). At the same time, the micropore size distribution of U6N-gel was analyzed, and it was found that its micropore size distribution was at 0.7 nm and 1.24 nm. This is consistent with the result of synthesizing UiO-66-NH2 by the solvothermal method for a long time. The above results indicate that the UiO-66-NH2 generated from the MOG precursor in a short time is consistent with the UiO-66-NH2 that needs to be generated for a long time in a conventional solvent in terms of crystal structure, thermal stability, and pore size distribution.

[0035] Controllable growth of MOF in MMMs regulated by MOG

[0036] In the traditional preparation process of MMMs, pre-synthesized MOF particles are usually added as fillers to the polymer matrix. In this case, the particle size of the nanoparticles in MMMs depends on the fillers themselves, making it difficult to precisely control the particle size of the nanoparticles. The "polymer-mediated MOFs" strategy proposed in this application can achieve in-situ controllable growth of MOF in the polymer, and thus achieve linear regulation of the MOF particle size in MMMs. During the crystal growth process, the precursor concentration often determines the nucleation and growth rate of the crystal. An increase in the precursor concentration often leads to a decrease in the crystallization rate, and thus an increase in the size of the generated nanoparticles. This phenomenon is also fully demonstrated in the MMMs membrane with MOG as the precursor. When the addition amount of the MOG precursor increases from 20% to 40%, the particle size of MOF in the MMMs membrane increases from 300 nm to about 1600 nm ( Figure 8 ).

[0037] This particle size change can be more clearly observed in the TEM image ( Figure 9 ). When the addition amount of the MOG precursor is 10%, no obvious UiO-66-NH2 morphology is shown in the cross-sectional SEM image of the PVDF-U6N-gel membrane. When the doping amount reaches 50%, the UiO-66-NH2 nanoparticles in the membrane show agglomeration and uneven distribution ( Figure 10 ). This is because when the content of the MOG precursor is low, it is difficult to form a continuous MOFs distribution in the polymer matrix, so the MOFs nanoparticles are difficult to be observed due to their too small size or too high dispersion. When the addition amount of the MOG precursor is too high, the polymer chains cannot confine the excessive MOG precursor, and at this time, the excess MOG precursor is prone to nano-agglomeration, thus forming MOFs particles with a wide particle size distribution in the polymer.

[0038] Investigation of the mechanism of controllable growth of MOF regulated by MOG

[0039] The physical and chemical properties of MMMs prepared with different MOG addition amounts were analyzed to clarify their in situ growth mechanism. As the amount of MOG precursor added increased, the XRD diffraction peak broadened ( Figure 11 This indicates that the increase in MOG precursor concentration leads to an increase in the size of the UiO-66-NH2 filler and inhibits the lattice order due to the accelerated nucleation rate, thereby reducing the crystallinity. The interfacial interaction between UiO-66-NH2 and the polymer substrate was tested by DSC ( Figure 11 Compared with pure PVDF-based membrane, the melting temperature of PVDF-U6N-gel (20% MOG addition) is ( T m ) decreased slightly. This phenomenon is attributed to the dispersion of UiO-66-NH2 nanoparticles between PVDF segments, whose steric hindrance effect weakens the interaction between polymer chains, resulting in a decrease in crystallinity. Thanks to the inherent high thermal stability of UiO-66, the in situ grown UiO-66-NH2 significantly improves the overall thermal stability of MMMs ( Figure 11 middle c area).

[0040] In addition, the actual content of UiO-66-NH2 in the PVDF-U6N-gel membrane was determined by thermogravimetric analysis (air atmosphere). Figure 11 As shown in the middle d region, the residual mass is almost 0% based on pure PVDF. As the MOG precursor loading increases from 10% to 40%, the residual mass of the MMMs gradually increases from 1.98% to 9.29%. Based on these data, the actual UiO-66-NH2 loading in the MMMs can be calculated to range from 4.40% to 20.62%. Even at an actual loading of 20%, the MOF can be uniformly dispersed, demonstrating the superiority of this method for preparing uniformly dispersed MMMs.

[0041] The structural stability of the PVDF-U6N-gel membrane was evaluated by immersing it in water for 30 days. The XRD characteristic peaks of the PVDF-U6N-gel membrane treated with water did not change significantly, indicating that UiO-66-NH2 can maintain the integrity of its crystal structure in a long-term aqueous environment. This stability is due to: (1) the high chemical stability of UiO-66-NH2 itself; (2) the PVDF matrix effectively inhibits the corrosion of water molecules on MOFs through the physical coating effect. The above results prove that the PVDF matrix does not significantly affect the physical and chemical properties of UiO-66-NH2 during the in situ preparation of MMMs.

[0042] To further analyze the in-situ growth mechanism of MMMs, EDX energy spectrum analysis was carried out. Among them, the F element signal corresponds to the presence of PVDF in MMMs, while the Zr element represents the distribution of U6N-gel nanoparticles therein. There is no negative correlation between the signal intensities of MOFs and PVDF ( Figure 12 in region a of Figure 12 ). This is due to the difference between the confinement molecular encapsulation strategy adopted in this application and the traditional physical blending method. In this application, after the MOG precursor is uniformly dispersed in the polymer matrix, it undergoes heat-induced crystallization growth. This hypothesis was verified by TEM images ( Figure 12 in region b of Figure 12 ): Nanoparticles of about 200 nm can be observed to be uniformly dispersed in the polymer in the TEM image. After magnifying the area around the nanoparticles, particles of 2-5 nm can be seen to be uniformly dispersed. These particles are metal clusters that have not grown into MOF crystals.

[0043] In contrast, the growth of MOF precursors in PVDF shows a disordered distribution. As the concentration of the precursor increases, some nanoparticles can be observed in the cross-section of the polymer matrix, showing a random distribution and obvious agglomeration phenomenon ( Figure 13 ).

[0044] Based on the above data, the in-situ growth process of MOG in the polymer matrix can be inferred: (1) The high electronegativity of F in the PVDF molecule makes it easy to combine with Zr in the MOF precursor. At the same time, the spatial confinement effect of PVDF ensures the uniform dispersion of metal clusters. This is confirmed by the uniformly dispersed metal clusters in the TEM image; (2) In the growth stage, continuous heating not only drives the casting liquid to turn into a film, but also provides the necessary energy for the transformation of MOFs precursors in the polymer matrix into MOF crystal particles. It should be noted that the MOG precursor can grow orderly in the polymer matrix, while the growth of the MOF precursor is significantly restricted. This difference stems from the effective coordination between the fast nucleation rate of MOG and the film-forming rate of polymer solvent evaporation. Specifically, the nucleation rate of MOG is significantly faster than that of MOF ( 4+ in region d of Figure 6 ): Its nucleation process mainly occurs within the first 30 minutes of heating, while the MOF precursor starts to nucleate only after 2 hours. By monitoring the mass change of the casting liquid during the solvent evaporation process, it is found that the solvent evaporation and film-forming process mainly occurs within the first 30 minutes of heating. This indicates that a complete polymer film has been formed after 30 minutes of heating. This matching of time scales enables the fast crystallization process of MOG to be well synchronized with the film-forming process of the polymer, providing an abundant crystal nucleus basis for the subsequent growth of MOG in the polymer matrix. Figure 6 in region d of Figure 6 ): Its nucleation process mainly occurs within the first 30 minutes of heating, while the MOF precursor starts to nucleate only after 2 hours. By monitoring the mass change of the casting liquid during the solvent evaporation process, it is found that the solvent evaporation and film-forming process mainly occurs within the first 30 minutes of heating. This indicates that a complete polymer film has been formed after 30 minutes of heating. This matching of time scales enables the fast crystallization process of MOG to be well synchronized with the film-forming process of the polymer, providing an abundant crystal nucleus basis for the subsequent growth of MOG in the polymer matrix.

[0045] On the other hand, the nucleation rate of the MOF precursor is relatively slow. By the time it starts to gradually nucleate and grow, the key growth solvent DMF has almost completely evaporated. Since DMF decomposes into dimethylamine during heating, and dimethylamine plays a crucial role in the binding of metal clusters and ligands, the absence of DMF further delays the crystallization process of MOF. Moreover, during the heating process, the MOG precursor forms a unique gel network structure, which consists of a large number of interconnected MOF particles rather than isolated particles. This gel network structure significantly improves the degree of ordered arrangement of MOF in the polymer matrix ( Figure 14 ), thus optimizing the overall performance of the material.

[0046] Bionic ion channel transport characteristics and ion rectification effect

[0047] Linear sweep voltammetry was used to test the current-voltage curve to investigate the ion transport in MMMs ( Figure 15 in region a). The PVDF-U6N membrane was tested for ion transport using KCl and KF solutions as electrolytes, and it was found that the PVDF-U6N membrane had neither separation performance nor bionic ion channel characteristics for KCl and KF ( Figure 15 in region b).

[0048] Subsequently, ion transport experiments were carried out on MMMs with different MOG doping amounts. In the pure PVDF membrane (0% MOG doping), the ionic conductances of KF and KCl were similar, indicating that PVDF has almost no separation ability for F - and Cl - ( Figure 16 in region a). When the MOG doping amount reached 10%, the separation ratio of F - to Cl - increased to 4.25. This is because the Zr metal clusters and amino groups in UiO-66-NH2 can selectively bind F - , enabling F - to be transported rapidly, thus increasing the separation ratio ( Figure 16 in region b). When the doping amount increased to 20%, the separation ratio of F - to Cl - rapidly rose to 30.0, accompanied by an ion rectification phenomenon ( Figure 16 in region c). Two key conditions are usually required for the occurrence of the ion rectification phenomenon: (1) the size of the nanochannel is at the sub-nanometer level; (2) there is asymmetry in the nanochannel (charge asymmetry or structural asymmetry). When the MOG addition amount increased to 30%, the rectification phenomenon of MMMs weakened, and the separation of F - from Cl -The separation ratio decreased to 2.68. When the MOG addition was further increased to 40%, although the ionic conductivity of the MMMs increased rapidly, their separation performance for F - and Cl - was almost non-existent ( Figure 16 in the d-e region of

[0049] ). This is because when the MOG addition was 10%, the MOF particles failed to penetrate the MMMs matrix. This hindered the continuous transport of ions in the MOF pores, so the ion rectification phenomenon did not occur, and the separation performance was thus limited. When the MOF content was too high, the presence of excessive large-particle MOFs would destroy the formation of the ion biomimetic channels, making the generated MMMs have certain defects. This was verified by the rapidly increasing ionic conductivity and decreasing contact angle of the PVDF-U6N-gel membrane (40% MOG addition) ( Figure 16 in the f region of o ). The addition of MOG significantly affected the surface wettability of the MMMs. As the MOG content increased, the hydrophilicity of the MMMs surface also increased. The initial contact angle of the PVDF-based membrane was 109.3 o , showing certain hydrophobic characteristics. As the MOG addition increased, the contact angle decreased continuously ( Figure 17 in the a region of Figure 17 ). This is because the porous structure and amino groups of the UiO-66-NH2 crystals could improve the membrane hydrophilicity. The F - / Cl - selectivity first increased and then decreased with the increase of the MOG content, and the rectification ratio value measuring the ion rectification performance also showed the same change trend as the selectivity ( Figure 17 in the b region of Figure 17 ).

[0050] Compared with the PVDF-U6N membrane, the PVDF-U6N-gel membrane showed more excellent ion separation and rectification performance. This was due to the uniform distribution of MOF particles in PVDF-U6N-gel, while there were MOF agglomerations and defects in PVDF-U6N. To further clarify the structure-performance relationship between the two, low-field NMR was used to analyze their approximate pore size distribution range. In the low-field NMR spectrum, the T2 relaxation time was positively correlated with the sample pore size, and a larger T2 value corresponded to a larger pore size. A T2 value in the range of 10 -2 to 10 0 indicated the presence of sub-nanometer channels in the membrane, while a T2 value between 10 0 and 10 3 indicated the presence of nano-scale channels in the membrane. Compared with PVDF-U6N-gel, PVDF-U6N had a T2 value in the range of 10 0 to 10 3An additional characteristic peak appears within the range. This indicates that in addition to sub-nanometer scale pores, there are also additional nano-scale channels in the PVDF-U6N membrane. This stems from the interfacial defects between the MOFs particles and the polymer matrix. Although the area occupied by these defects is small, their impact on the ion separation performance at the sub-nanometer scale cannot be ignored. Further micropore distribution tests on the PVDF-U6N-gel membrane found that there are 0.7 nm and 1.1 nm pores belonging to UiO-66-NH2 in the PVDF-U6N-gel membrane.

[0051] COMSOL Simulation of Ion Transport Mechanism in Bionic Channels

[0052] Furthermore, the COMSOL software was used to simulate the distribution of anions and cations in the nanochannels of the PVDF-U6N and PVDF-U6N-gel membranes in order to explain the ion rectification phenomenon and F - selective transport situation. When the arrangement of UiO-66-NH2 nanoparticles in the polymer becomes disordered (PVDF-U6N membrane), although the surface of UiO-66-NH2 is positively charged, due to the disordered distribution of the nanochannels, the disordered nanochannels lack a continuous path and cannot constrain ion transport. From the concentration distribution diagrams of the PVDF-U6N membrane with different electrolytes at different voltages, it can be found that the concentrations of anions and cations have not changed significantly compared to the initial concentrations, both being 10 mM ( Figure 18 in regions a-f). Through the two-dimensional concentration distribution cloud diagram, it can be clearly observed that there is no ion concentration gradient difference generated within the PVDF-U6N membrane ( Figure 18 in regions g-i), that is, the ion concentration in the nanochannel is equal to the total electrolyte ion concentration.

[0053] From the potential distribution curve, the potential distribution of this nanochannel shows a linear decreasing trend. That is, the surface charge of the disordered MOFs has little effect on the potential distribution within the ion channel ( Figure 19 in region a). Further, by calculating the current density within this nanochannel, it can be obtained that the absolute values of the current densities obtained by this nanochannel at ±2V voltages are equal, that is, no ion rectification phenomenon occurs. When the electrolyte solution becomes KF solution, although the diffusion rate of F - is faster than that of Cl - , due to the lack of effective interaction between the nanochannel and F - , the separation ratio between KF and KCl is only 2 ( Figure 19 in region b).

[0054] When the MOFs are arranged into ordered channels in the polymer matrix (PVDF-U6N-gel membrane), after applying a voltage of +2 V, Cl -The concentration changes from the initial 10 mM to 23 mM (the highest point), while the concentration of K + changes from the initial 10 mM to 4 mM (the lowest point), and obvious changes occur in the concentrations of anions and cations. This is because the positive charges on the surface of UiO-66-NH2 cause the asymmetric transport of anions and cations, leading to the rearrangement of the concentration distribution of anions and cations in the ion channels. This asymmetry in ion distribution can be visually observed through two-dimensional concentration profiles and non-linear potential profiles ( Figure 20 in the a-c regions). During this process, when a positive voltage is applied, K + moves downward in the region, while Cl - moves upward. Cl - is attracted to the positively charged nanochannels and can thus pass through the nanopores smoothly. However, K + has difficulty passing through the ion channels due to the electrostatic repulsion with the nanochannels. This phenomenon is manifested as the enrichment of K + and the dissipation of Cl - in the upper region of the two-dimensional concentration profile. When a negative voltage is applied, the moving directions of K + and Cl - are opposite to those when a positive voltage is applied. Similarly, due to the positive charge of the nanochannels, anion-selective transport is exhibited ( Figure 20 in the d-f regions). In summary, the ion rectification phenomenon occurs due to the differences in ion concentrations within the ion channels under different voltages.

[0055] When the electrolyte is changed from KCl to KF solution, due to the specific binding between F - and UiO-66-NH2, the diffusion rate of F - is faster than that of Cl - . The difference in the transport rates of F - and Cl - is related to their binding abilities to MOFs. To investigate the difference in the binding abilities of UiO-66-NH2 to F - and Cl - , adsorption performance tests were carried out using U6N-gel nanoparticles ( Figure 21 ). The results show that the adsorption amounts of U6N-gel for K + and Cl - are low, but the adsorption amount for F - is high. This is due to the strong interaction between the μ3-OH groups of the Zr6 clusters and the Zr6 metal sites and F - .

[0056] The peak concentration of F - in the PVDF-U6N-gel nanopores is 133 mM, which is that of Cl -Six times the peak concentration. This indicates that the interaction between UiO-66-NH2 and F - results in a higher F concentration within the nanochannels - . The two-dimensional concentration distribution map further shows that the enrichment region of F - is wider compared to that of Cl - . The rapid diffusion process of F - makes the distribution of anions and cations more uneven. The ion concentration within the nanochannels is the main factor affecting the current density (region a-c in Figure 22 ). The rectification effect and ion transport rate are reflected by calculating the current density of the nanochannels in different voltages and different electrolyte solutions. The current density of the PVDF-U6N-gel nanochannels under positive voltage is significantly greater than that under negative voltage, showing an obvious ion rectification characteristic. The rectification ratios in KCl and KF solutions are 2.95 and 3.10 respectively. Due to the relatively high concentration of F - within the nanochannels, the current density of KF is 32.38 times that of KCl (region d in Figure 22 ).

[0057] Effect of electrolyte concentration and pH on ion transport behavior

[0058] Concentration dependence is one of the significant characteristics of biomimetic ion channels. To explore this property, linear voltammetry tests were performed on the PVDF-U6N-gel membrane at different electrolyte concentrations (0.1 mM - 1 M). The results show that when the electrolyte concentration increases from 0.1 mM to 1 mM, the I-V curve begins to exhibit ion rectification phenomenon ( Figure 23 ). With the further increase of the electrolyte concentration, the rectification ratio gradually decreases, and when the concentration reaches 1 M, the ion rectification phenomenon almost disappears. It is worth noting that when the solution concentration reaches 0.01 M, the rectification ratio reaches the maximum value of 3.29.

[0059] This is because the generation of the ion rectification phenomenon is mainly affected by the size of the nanopores within the membrane and the thickness of the electric double layer in the bulk solution. When the electrolyte concentration is higher than 0.01 M, the thickness of the electric double layer generated by the bulk solution decreases significantly, resulting in the incomplete overlap of the electric double layers within the nanopores, thus causing a decrease in the rectification ratio. On the contrary, when the electrolyte concentration is lower than 1 mM, although the thickness of the electric double layer increases at this time, its stability decreases, and the ion rectification phenomenon also weakens accordingly. When the size of the nanochannels is close to the thickness of the electric double layer, the ion transport process is more easily regulated by the surface charges within the nanopores, thus showing a higher rectification ratio. In a 0.01 M solution (the thickness of the electric double layer is approximately 3.1 nm), due to its proximity to the pore size of PVDF-U6N-gel (approximately 1.1 nm), the system exhibits the maximum rectification ratio. However, the offset phenomenon of the maximum rectification ratio is related to the presence of surface functional groups on the MOF.

[0060] In order to further understand the effect of MOFs surface groups on ion transport, KCl was selected as the electrolyte (K + With Cl - The IV tests were conducted under different pH conditions (with the same migration rate and size). When the pH value was 2.16, the PVDF-U6N-gel membrane showed ion current saturation (ICS) within the voltage range. That is, as the voltage was further increased, the current change amplitude was limited, which showed a certain gating property. This is also one of the typical properties of biomimetic ion channels ( Figure 24 (Area a in the middle). The ICS phenomenon is related to concentration polarization. Specifically, if the subnanopores are ion-selective, the ions transported through them will be enriched by counterions and repelled by the double layer. This causes the ion concentration to increase on one side of the membrane while decreasing on the other, creating a concentration gradient and, in turn, the ICS phenomenon.

[0061] When the solution pH increases to near neutral, the ICS phenomenon turns into an ion rectification phenomenon. This is because as the pH increases, the protonation degree of MOFs decreases, but UiO-66-NH2 still has anion selectivity ( Figure 24 This asymmetric surface charge characteristic enhances the ICR effect caused by morphological asymmetry, and the ion transport presents an asymmetric structure. When the pH is further increased to 10.6, the ion rectification phenomenon disappears, and the IV curve only shows a slight asymmetry ( Figure 24 This is because the ion selectivity of UiO-66-NH2 changes from anion selectivity to cation selectivity under alkaline conditions, but the cation selectivity is weak and is not sufficient to trigger asymmetric ion rectification. The effect of pH on the ion transport properties of PVDF-U6N-gel can be attributed to the effect of pH on UiO-66-NH2 in MMMs. There are multiple pH-responsive units in the UiO-66-NH2 structure. The -OH at the metal node of UiO-66-NH2 will be protonated under low pH conditions and converted into a positively charged -OH2 + The -NH2 group in the ligand will combine with the proton under low pH and neutral pH conditions and transform into -NH3 with positive charge. + At the same time, the uncoordinated -COOH in the UiO-66-NH2 crystal structure is neutral at low pH, but is converted to -COO at high pH. -. When the pH is less than or equal to 5.8, UiO-66-NH2 exhibits obvious positive charge. That is, in addition to being affected by voltage, the ion transport is also affected by the surface charge of MOFs, showing anion selectivity. In summary, the uniform distribution of MOFs in the polymer matrix is the primary condition for the formation of ion rectification and ion separation. The appearance of the ion rectification phenomenon means that the anion selective transport characteristics are generated in the channel, which makes it easier to separate anions, thus making F - and Cl - have excellent separation performance ( Figure 25 ).

[0062] In addition, this method is prepared by a one-step method without subsequent addition of MOFs, and the operation is simple. At the same time, MMMs itself has good scalability, so this method shows the potential for large-scale preparation of biomimetic ion channels. Through the comparison of separation performance (Table 1), it can be seen that although some materials perform better in a single separation accuracy index, considering the preparation simplicity and separation accuracy comprehensively, the MMMs prepared in this application have the best comprehensive performance.

[0063] Table 1 Comparison of monovalent ion separation performance

[0064] In summary, based on the "polymer-mediated MOFs" strategy, this application constructs MMMs with biomimetic ion channels by regulating the dynamic transformation of MOFs-MOGs and the polymer film-forming synergistic mechanism, realizing the efficient separation of F - / Cl - . This strategy uses the polymer PVDF as the confinement medium and regulates the rapid nucleation and self-assembly of MOF precursors to form MOGs. Molecular dynamics simulations show that compared with MOF precursors, there are strong van der Waals forces and hydrogen bond interactions between MOG precursors and PVDF, and this interaction drives the MOG precursors and the subsequent generated MOF nanoparticles to achieve uniform and continuous dispersion in the polymer matrix. The nucleation rate of MOGs (1.8*10 -3 min -1 ) is 11 times higher than that of conventional MOFs (1.53*10 -4 min -1 ). Due to its high matching with the polymer film-forming rate, it realizes the uniform and continuous in-situ growth of MOFs in the polymer matrix. The PVDF-U6N-gel membrane generated from MOG precursors exhibits significant ion rectification effect (rectification ratio 3.10) and F - specific recognition ability (F - / Cl -Separation ratio reaches 30.0). COMSOL simulation reveals that uniformly and continuously distributed MOFs are the key to constructing biomimetic ion channels for ion rectification and specific ion separation. In addition, electrolyte concentration and pH have a significant impact on ion transport behavior. Under specific concentration and pH conditions, the PVDF-U6N-gel membrane exhibits typical characteristics of biomimetic ion channels such as ion rectification and ion gating. This application provides a new method for the construction of biomimetic ion channels in MMMs and also offers a new perspective for the facile preparation of intelligent nanofluids.

Claims

1. A sub-nanometer channel membrane with bionic ion characteristics, characterized in that, It includes a PVDF matrix and UiO-66-NH2 nanoparticles uniformly and continuously dispersed in the PVDF matrix; a bio-inspired ion channel with ion rectification characteristics and ion-specific transport ability is formed between the uniformly and continuously dispersed UiO-66-NH2 nanoparticles and the PVDF matrix.

2. The sub-nanochannel membrane with bionic ionic properties according to claim 1, characterized in that, The size of the bio-inspired ion channel is at the sub-nanometer level.

3. The preparation method of the sub-nanochannel membrane with bionic ion characteristics according to any one of claims 1-2, characterized in that, It is to disperse the UiO-66-NH2 gel precursor solution in the PVDF casting solution to obtain a mixed matrix membrane casting solution, and then make the membrane; among them, the mass of the UiO-66-NH2 gel precursor is 15-35 wt% of the mass of PVDF.

4. The method for preparing a sub-nanochannel membrane with bionic ion characteristics according to claim 3, wherein, The preparation method of the UiO-66-NH2 gel precursor solution is: dissolve ZrOCl2·8H2O and 2-aminoterephthalic acid in an equimolar amount in a solvent, and stir and ultrasonicate until a uniform solution is formed.

5. The preparation method of the sub-nanochannel membrane with bionic ion characteristics according to claim 3, characterized in that, The solvents used in the PVDF casting solution and the UiO-66-NH2 gel precursor solution are DMF.

6. The preparation method of the sub-nanochannel membrane with bionic ion characteristics according to claim 3, characterized in that, The specific method of making the membrane is: scrape the mixed matrix membrane casting solution on a substrate, dry it at 100-150 °C, peel the membrane after cooling to room temperature, and soak it to remove the residual solvent and then store it.

7. Application of the sub-nanometer channel membrane with bio-inspired ion characteristics according to any one of claims 1-2 in ion separation.

8. A method for improving the tensile strength of a sub-nanopore membrane doped with UiO-66-NH2 nanoparticles, characterized in that, It is to add the UiO-66-NH2 gel precursor solution to the PVDF casting solution, and then make the membrane after obtaining the mixed matrix membrane casting solution; among them, The preparation method of the UiO-66-NH2 gel precursor solution is: dissolve ZrOCl2·8H2O instead of ZrCl4 and 2-aminoterephthalic acid in an equimolar amount in a solvent, and stir and ultrasonicate until a uniform solution is formed; moreover, the mass of the UiO-66-NH2 gel precursor is 15-35 wt% of the mass of PVDF.

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