A method for preparing a nanofiltration membrane based on self-catalytic interfacial polymerization of molecularly encapsulated nanocapsules

Abstract

The application provides a self-catalytic interfacial polymerization method for controlling diffusion and polymerization reaction of monomers through a molecular encapsulation technology, and the reactive monomers are orderly decomposed and released under the catalysis of HCl through nanocapsules to participate in interfacial polymerization. The method is helpful to more uniform diffusion of amine monomers on a water / oil interface, and polyamide nanofilm with more uniform ion transmission channels is formed, and the nanofiltration membrane has highly controllable structure characteristics, high permeability and precise separation efficiency for mixed monovalent salt. The method can be used for preparation of a highly uniform polyamide membrane with precise molecular sieve.

Description

Technical Field

[0001] This invention relates to a method for preparing nanofiltration membranes based on the self-catalytic interfacial polymerization of molecularly encapsulated nanocapsules, belonging to the field of nanofiltration membrane preparation technology. Background Technology

[0002] Accurate and rapid ion-selective separation is essential for environmental, energy, and resource-related applications. In applications such as by-product recovery, seawater desalination, and hydrometallurgy, different mixed ion systems require suitable processes to effectively separate target ions from impurity ions. Nanofiltration membranes, due to their high cost-effectiveness, environmental friendliness, and high precision, have become one of the most effective ion separation technologies. Based on the unique separation mechanisms of size repulsion and the Donan effect, nanofiltration membranes exhibit extremely high selectivity for ions and organic matter with different radii and charges. Nevertheless, the precise separation of two similar ions remains technically challenging due to minute differences in hydration radius and net charge. Research indicates that the selectivity differences between different similar ions are based on dehydration phenomena. According to this theory, ions near the membrane pores temporarily shed and rearrange their surrounding water shell to more easily adapt to the pores. The degree of ion dehydration depends on the ion's hydration energy. The weaker the hydration energy, the easier it is for the ion to dehydrate. Generally, the difference in dehydration barriers mainly depends on the membrane pore size and the interaction between ions and membrane channels. To improve the selectivity of similar ions and achieve superselective ion separation, it is necessary to design membranes with highly uniform channels.

[0003] Polyamide-based nanofiltration membranes are prepared via interface polymerization (IP) on a porous support layer, a mature and energy-efficient fabrication technique. In a typical interfacial polymerization process, amine monomers diffuse from the aqueous solution into the organic phase and react violently with acyl chlorides at the water / organic interface. Generally, the interfacial polymerization process is controlled by the diffusion of amine monomers within the reaction zone. During film formation, the diffusion resistance of monomers within the reaction zone increases exponentially with the growth of the polyamide layer. Fluctuations in the diffusion rate and diffusion flux of amine monomers lead to spatially heterogeneous polymerization. Furthermore, the structural properties of polyamide nanomembranes are also influenced by the kinetics of the interfacial polymerization reaction. Nanomembrane formation typically occurs within seconds to minutes. While rapid reaction rates are beneficial for large-scale manufacturing, they also lead to uncontrolled crosslinking and pore size distribution. Therefore, uncontrolled diffusion and rapid polymerization induce the formation of multi-scale heterogeneous and non-uniform pore size nanomembranes, resulting in uncontrollable separation performance. Summary of the Invention

[0004] The technical problem this patent aims to solve is that in the process of preparing nanofiltration membranes by interfacial polymerization, the degree of crosslinking and pore size distribution are uncontrollable due to the fast reaction rate. This patent improves the controllability of the preparation process by encapsulating the target monomers and releasing them into the environment through a molecular sealing strategy.

[0005] A method for preparing nanofiltration membranes based on interfacial polymerization catalyzed by nanocapsules includes the following steps:

[0006] Step 1: Preparation of nanocapsules: A solution containing di-tert-butyl dicarbonate is slowly added to a solution containing polyethyleneimine and stirred to obtain a milky white dispersion; after centrifugation, washing and drying, nanocapsules are obtained.

[0007] Step 2: Preparation of molecularly encapsulated superselective nanofiltration membrane: Dissolve nanocapsules in a cosolvent, immerse the base membrane in the nanocapsule solution, and then remove excess solution and bubbles from the membrane surface; apply an aqueous solution containing piperazine monomers to the surface of the base membrane, and then wipe off the residual solution on the membrane surface; then apply an oil solution containing acyl chloride monomers to the base membrane to carry out an interfacial polymerization reaction to obtain the nanofiltration membrane.

[0008] The molecular weight range of polyethyleneimine is 1200-2500 Da; the molar ratio of polyethyleneimine to ditert-butyl dicarbonate is 1:0.01-0.04.

[0009] The stirring time is 6-10 h; the centrifugation rate is 8000-12000 rpm.

[0010] The base membrane is made of polyimide, polyethersulfone, polysulfone, polyketide-imide, polyurethane, polyelectrolyte, cellulose, polyamide, aromatic ester, ceramic membrane, metal membrane, polyvinylidene fluoride, polyamide, titanium dioxide, silicon dioxide, alumina, or zirconium oxide.

[0011] The piperazine monomers are selected from piperazine, piperazine-2-carboxylic acid, 1-(tert-butoxycarbonyl)piperazine, 3,5-bis(acryloylamino)piperazine or 3-mercaptopiperazine.

[0012] The acyl chloride monomers are selected from pyromellitic terephthaloyl chloride, terephthaloyl chloride, 2,2',4,4'-biphenyltetramethyl chloride or 5-(dichlorophosphoryl)isophthaloyl chloride.

[0013] The base film is a polyimide-based film prepared by a solvent-inducible phase inversion method.

[0014] In step two, the concentration of nanocapsules is 0.05-1 wt%.

[0015] The co-solvent is an 8-12 wt% aqueous solution of triethanolamine, the concentration of the aqueous solution of piperazine monomer is 0.015-0.02 wt%, and the contact time is 1-5 min.

[0016] The concentration of the oil phase solution containing acyl chloride monomers is 0.05-0.15 wt%, and the contact time is 10-20 s.

[0017] A nanofiltration membrane is prepared by the above-described preparation method.

[0018] The above-mentioned nanofiltration membranes are used in liquid filtration.

[0019] The liquid filtration includes the filtration of Mg 2+ and Li + Separation, or separation of Ac- and Cl-. Beneficial effects

[0020] This invention pre-presses nanocapsules onto a polyimide substrate, significantly mitigating the impact of substrate structural inhomogeneity on the selective layer and facilitating the fabrication of defect-free nanofilms. In this invention, the released amine reacts with acyl chloride to generate sufficient hydrochloric acid, further promoting nanocapsule decomposition and amine release. The amount of hydrochloric acid generated during interfacial polymerization controls nanocapsule disintegration and amine release, thereby increasing amine participation in the reaction. Orderly amine release enhances the uniformity of the diffusion process and constructs a uniform pore size distribution on the film surface, resulting in a membrane exhibiting tunable structural properties and precise separation efficiency of mixed monovalent salts. Attached Figure Description

[0021] Figure 1 Molecular encapsulation is used to prepare nanofiltration membranes with superselectivity and high uniformity, synergistically controlling diffusion rate and reaction process.

[0022] Figure 2 Synthetic route of nanocapsules.

[0023] Figure 3 FTIR spectra of PEI and PEI@Boc.

[0024] Figure 4 Characterization of the autocatalytic interfacial polymerization process. (a) Schematic diagram of the preparation of the original polyamide membrane; (b) FTIR spectra of PI, PI / PEI@Boc, TFC and i-TFC-PEI@Boc membranes; (c) Narrow N1s XPS spectrum of i-TFC-PEI@Boc membrane; (d) Water contact angle of the membrane; (e) Zeta potential of membranes with different pre-coated materials; (f, g) SEM morphology of TFC and i-TFC-PEI@Boc membranes.

[0025] Figure 5(a) Dimensional distribution of PEI; (b) Dimensional distribution of PEI@Boc.

[0026] Figure 6 A schematic diagram of the complete reaction process of the autocatalytic interfacial polymerization strategy.

[0027] Figure 7 O and N content of PEI@Boc films with different pre-coating amounts.

[0028] Figure 8 Zeta potential of different membranes.

[0029] Figure 9 SEM images of i-TFC and i-TFC-PEI membranes.

[0030] Figure 10 : Performance evaluation of the membrane. (a, b) MgSO4 performance of i-TFC-PEI@Boc membrane with different concentrations of PEI@Boc and PIP; (c) Comparison of membrane performance with different pre-coated materials; (d) Schematic diagram of autocatalytic interfacial polymerization process.

[0031] Figure 11 : Surface morphology characterization of the membrane. (ac) Different interfacial polymerization reaction times; (df) SEM images of the membrane at nanocapsule concentrations; (g,h) Schematic diagram of the precisely controlled polyamide membrane formed by the autocatalytic interfacial polymerization process.

[0032] Figure 12 SEM images of different PEI@Boc films.

[0033] Figure 13 (a) Schematic diagram of a laboratory-made gas measuring device; (b) Displacement volume caused by gases generated by traditional and novel autocatalytic interfacial polymerization processes.

[0034] Figure 14 Nanostructure characterization of the membranes. (ac) Cross-sectional views of i-TFC, i-TFC-pei, and i-TFC-PEI@Boc; (d) UV-Vis spectra of PIP in the organic phase after diffusion on PI (solution only), PI / PEI, and PI / PEI@Boc for 120 s; (e) Average exothermic rate (negative exothermic rate represents exothermic behavior) of adding TMC / hexane solution to TEOA / PIP, TEOA-PEI / PIP, and TEOA-PEI@Boc / PIP solutions as measured by ITC analysis; (f) Pore size distribution of the membranes under different pre-coating materials.

[0035] Figure 15 Add TMC / hexane solution to TEOA / PIP, TEOA-PEI / PIP and TEOA-PEI@Boc / PIP.

[0036] Figure 16 Performance of i-TFC-PEI@Boc membranes for single salt solutions.

[0037] Figure 17 : Membrane separation performance. (a) Separation performance of i-TFC-PEI@Boc membrane for Ac- and Cl- at different pH (1000:1000 pm, 6 bar); (b) Separation performance of i-TFC-PEI@Boc membrane for mixed salts with different feed concentrations; (c) Long-term stability of membrane for mixed salt separation; (d) Performance comparison with similar reported ion separation membranes; (e) Schematic diagram of the mechanism for membrane separation of Ac- and Cl-; (f) Relationship between the retention cut-off of different solutes and Stokes radius in i-TFC-PEI and (g) i-TFC-PEI@Boc; (h) Separation performance of MgCl2 and LiCl at different feed ratios. Detailed Implementation

[0038] The following materials are used in the following embodiments:

[0039] Polyamide PI (P84) ® Polyethylene glycol (PEG, MW=400 Da) and N-methylpyrrolidone were used as polymers, non-solvent additives, and solvents, respectively, in the coating solution to prepare the membrane substrate. Di-tert-butyl dicarbonate ((Boc)₂O) was used as an encapsulant to encapsulate polyethyleneimine (PEI, MW=1800 Da), which served as an active agent. Dichloromethane (DCM, AR) was used as the solution in the nanocapsule synthesis. Triethanolamine (TEOA) was used as a co-solvent to improve the solubility of the nanocapsules in water. A polyamide layer was prepared using nanocapsules, piperazine (PIP), trimesoyl chloride (TMC), and n-hexane to improve the membrane selectivity. Example

[0040] Synthetic nanocapsules (PEI@Boc)

[0041] PEI 1800 (5 g, 2.78 mmol) and (Boc)₂O (10.4 g, 0.048 mol) were dissolved separately in DCM and stirred at a constant speed. After the solutions were homogenized, the (Boc)₂O solution was slowly added to PEI. 1800 The solution was stirred at 25°C for 8 hours, and the milky white dispersion was collected. The dispersion was centrifuged at 10,000 rpm and washed three times to remove residual impurities. The resulting white powder after drying was identified as nanocapsules, denoted as PEI@Boc.

[0042] Preparation of molecularly encapsulated superselective nanofiltration membranes

[0043] PI-based films were prepared using a solvent-inducible phase inversion method. PI was dried in a vacuum oven at 60°C for 10 h to remove moisture. Then, the PI particles were dissolved in a mixed solution of NMP and PEG 400 (concentration percentages of 20%, 64%, and 16%, respectively). The coating solution was stirred for 12 h. After homogenization, the solution was allowed to stand for 10 h to remove air bubbles. The solution was then cast onto a nonwoven fabric using a 100 μm thick blade. The prepared substrate was washed with deionized (DI) water to remove residual solvent and stored in DI water before use.

[0044] A molecularly encapsulated superselective nanofiltration membrane was prepared. In the autocatalytic interfacial polymerization process, all nanofiltration membranes were pre-coated with nanocapsules, and then a trace amount of amine monomer was used as an initiator to generate HCl to release the active monomer, achieving diffusion and reaction synergistic control of interfacial polymerization. The specific preparation process is as follows: 0.1 wt% PEI@Boc powder was dissolved in 10 wt% TEOA solution and stirred for 2 h; further, in a comparative experiment, the concentration was adjusted so that the PEI@Boc concentration was changed from 0.02 wt% to 0.5 wt%. After the solution was mixed evenly, the PI-based membrane was immersed in PEI@Boc, and then excess solution and air bubbles on the membrane surface were removed using a soft rubber roller. Next, PIP was added to the surface of the base membrane in a very small amount as an initiator. The step was to pour 0.0175 wt% PIP solution onto the newly prepared membrane for 2 minutes, and then wipe off the residual solution on the membrane surface. The membrane was then immersed in a 0.1 wt% TMC / hexane solution for 20 seconds and immediately washed with hexane to quench the interfacial polymerization reaction. The prepared membrane, designated i-TFC-PEI@Boc, was stored in deionized water for later use. Unlike traditional interfacial polymerization, the i-TFC-PEI@Boc membrane fabrication process utilizes HCl, a byproduct of the trigger reaction, to maximize the release of amine monomers into the interfacial polymerization reaction. This HCl then continues to be generated to further promote the interfacial polymerization reaction, ultimately leading to the construction of the membrane structure.

[0045] Comparative experiment

[0046] To conduct comparative analysis, three types of nanofiltration membranes were prepared and compared with the i-TFC-PEI@Boc membrane. The same operating procedures were used.

[0047] The first method involves preparing a membrane labeled TFC using only trace amounts of PIP and TMC without pre-coating with PEI@Boc solution. The specific parameters are the same as in Example 1.

[0048] The second method involves pre-coating a 10% TEOA aqueous solution (without adding PEI@Boc) and preparing a membrane labeled i-TFC.

[0049] The third method involves pre-coating a 10% TEOA solution containing dissolved PEI (PEI concentration 0.1%), and the resulting membrane is labeled i-TFC-PEI.

[0050] Characterization methods

[0051] The diffusion rate of the monomer was determined using ultraviolet spectrophotometry. First, 10 ml of PEI or PEI@Boc solution dissolved in 10 wt% TEOA solution was pre-loaded onto the PI substrate for 3 minutes, and any residual solution was wiped off. Then, 10 ml of 0.02 wt% PIP / water solution was poured onto the treated PI substrate membrane for 2 minutes, and the solution was removed. Finally, 10 ml of hexane was poured onto the membrane for 20 seconds, and the hexane solution was transferred to a test tube to monitor the PIP content. For comparison, a 10 wt% TEOA solution was also pre-coated onto the PI substrate membrane to eliminate the influence of the solution on PIP diffusion.

[0052] The heat release rate was evaluated using isothermal titration calorimetry. 100 μl of 0.0175 wt% PIP / water solution was slowly injected into 100 μl of TEOA solution, 0.1 wt% TEOA-PEI, and 0.1 wt% TEOA-PEI@Boc, respectively. 40 μl of 0.1 wt% TMC / hexane solution was added to the syringe. Titration was performed at 25 °C, with 30 injections of 2 μl TMC / hexane solution each time. The initial injection was 0.4 μl to minimize volume errors caused by residual liquid in the syringe. During the measurement, the syringe was raised by 1 cm to ensure the needle was above the surface of the PIP / water solution. Each injection was performed at a fixed time interval of 90 seconds. A control ITC experiment was conducted beforehand using pure hexane and water to eliminate the influence of the solvent. No stirring was performed during the experiment to ensure interfacial reaction.

[0053] The concentration of positively charged ions in the mixed salt was measured by ICP. Ion chromatography was used to analyze the concentration of negatively charged ions in the mixed solution. In addition, the concentration of single ions was measured by a conductivity meter, and the concentration of acetate ions was measured by a total organic carbon analyzer.

[0054] To further confirm the autocatalytic interfacial polymerization process, the nanobubbles generated in the new process and in traditional interfacial polymerization reactions were compared. Figure 13The amount of CO2 gas generated is increased by increasing the volume of the solvent. First, 15 ml of 0.1 wt% PEI@Boc solution and 15 ml of 0.0175 wt% PIP solution are added to the flask, followed by injecting 2 ml of 0.1 wt% TMC solution into the sealed system using a syringe. For conventional interfacial polymerization processes, only 0.0175 wt% PIP solution is added to the flask before TMC injection. The amounts of PIP, PEI@Boc, and TMC are consistent with those used in the membrane fabrication process.

[0055] The pore size distribution of the membrane was analyzed using four neutral solutes with different molecular weights (MW): raffinose, sucrose, glucose, and diethylene glycol. The relationship between molecular weight and the Stokes radius of the solute (MW) was discussed. r s The relationship between the two can be calculated using the following equation:

[0056]

[0057] The pore size distribution of the membrane is calculated by the following equation:

[0058]

[0059] in r p This is the geometric mean radius corresponding to a 50% rejection rate, representing the effective aperture. μ p It is the average effective aperture. s p It is the geometric standard deviation that determines the sharpness of the aperture distribution.

[0060] Comparison and Analysis of Nanocapsule Polyamide Membranes

[0061] Nanocapsules (PEI@Boc) were synthesized using PEI (Mw=1800) and di-tert-butyl dicarbonate (Boc)₂O as raw materials. Figure 2 1594 cm -1 The NH peak disappears at 1684 cm. -1 A new secondary amide peak appeared at the point, which strongly proves the successful introduction of (Boc)₂O. Figure 3 The particle size distribution increased from 2.24 nm to 82.91 nm, which also demonstrates the synthesis of nanocapsules. Figure 5 In preparing the polyamide film, 10% TEOA was used as the dissolving solution for PEI@Boc, which was pre-loaded onto the polyimide film surface. TEOA improved the solubility of the nanocapsules by reducing the surface tension of the solution. Then, a trace amount of PIP was poured onto the film as an initiator to react with triformyl chloride (TMC). This method facilitates the orderly progress of the interfacial polymerization reaction. Figure 2 (a) and Figure 6 ). Elemental and functional group analysis can confirm the concept of autocatalysis. Figure 7 and Figure 4 (b) and (c). After pre-coating with PEI@Boc, the contents of O and N increased with increasing PEI@Boc concentration, which is due to the presence of ester groups in (Boc)₂O and amine groups in PEI. Figure 7 In addition, such as Figure 4 As shown in (b), the membrane spectrum is at 3250 cm⁻¹ -1 A relatively broad absorption band is observed at 1065 cm⁻¹, which is attributed to the OH group of TEOA. Furthermore, the PI / PEI@Boc film exhibits a broad absorption band at 1065 cm⁻¹ due to the carbonyl peak of (Boc)₂O. -1 A new peak appeared at [a certain value]. After interfacial polymerization treatment, all prepared polyamide films showed peak values ​​at 1613, 1540, and 1451 cm⁻¹. -1 Distinct bands appeared at all locations, due to the formation of the polyamide network. Within the concentration range of 0–0.1%, the peak intensity increased due to the influence of the degree of interfacial polymerization. Notably, when the concentration of PEI@Boc increased to 0.1%, the peak intensity increased at 2360 cm⁻¹. -1 A new wavelength band appeared, which may be due to the release of CO2 during the disintegration of the nanocapsules, with some CO2 molecules trapped in the dense polyamide network. The XPS peak at 398.5 eV corresponds to the N1s nuclear level spectrum of the primary amine, confirming that the nanocapsules disintegrate and release reactive amines to participate in the interfacial polymerization process. Figure 4 (c)).

[0062] Membranes prepared by autocatalytic interfacial polymerization exhibit significantly different properties. Compared to PI-based and TFC membranes, the presence of -OH groups in the TEOA solution enhances the hydrophilicity of i-TFC-PEI@Boc. After interfacial polymerization, the i-TFC-PEI@Boc membrane shows stronger hydrophilicity than the i-TFC membrane. This can be attributed to the release of amines from PEI by the HCl generated during interfacial polymerization, which contributes to enhanced hydrophilicity. Figure 4 (d)). Increased hydrophilicity is beneficial to membrane permeability. The i-TFC-PEI@Boc membrane has less negative charge than other membranes ( Figure 4(e)). For TFC and i-TFC membranes, the lower the PIP concentration, the fewer amino groups participate in the reaction, and the more acyl chlorides remain on the membrane surface. The acyl chlorides will hydrolyze, resulting in the membrane carrying more negative charges. In i-TFC-PEI@Boc and i-TFC-PEI membranes, -NH reacts more with TMC. However, the nanocapsules absorb HCl generated during interfacial polymerization, and the autocatalytic process promotes a more complete reaction. Therefore, the i-TFC-PEI@Boc membrane has a high degree of reaction and fewer unreacted acyl chlorides remaining on the membrane surface. Thus, the i-TFC-PEI@Boc membrane has the least negative charge. Similarly, the positive charge of the membrane also increases with the increase of nanocapsule content. Figure 8 Meanwhile, due to the trace concentration of PIP, the surface of the TFC membrane is indistinguishable from that of the PI-based membrane. Figure 4 (f)). After pre-coating with TEOA, the i-TFC membrane surface exhibits only limited nodular structural features. When PEI is pre-coated onto the membrane surface, these structures gradually increase due to the increased degree of reaction. Figure 9 The Turing structure can be observed on the i-TFC-PEI@Boc membrane. Figure 4 (g) In the autocatalytic interfacial polymerization process, the active monomer is provided by the disintegration of nanocapsules. The on-demand release of monomers enhances the uniformity and controllability of amine diffusion, but also restricts the free diffusion of monomers, thus leading to changes in the membrane structure.

[0063] The improved performance reflects the increased reactivity of the polyamide network, indicating that the HCl generated during interfacial polymerization will decompose the nanocapsules, releasing amines to participate in further interfacial polymerization. Even at an initiator concentration as low as 0.0175%, the membrane still exhibits a high MgSO4 rejection rate. Figure 10 (b) Specifically, the reaction with TMC requires only a limited amount of initiator to generate enough HCl to catalyze the decomposition of the nanocapsules and release reactive groups. The released amine participates in the interfacial polymerization process and releases HCl, further catalyzing the release of nanocapsules and the interfacial polymerization reaction. Finally, through a self-limiting effect, the polymerization rate slows down significantly with increasing polyamide thickness; through self-regulation, a uniform, defect-free nanofilm can be obtained. Figure 10 (d)

[0064] Comparative analysis with three contrast membrane materials ( Figure 10(c) When no deposition occurs, the trace amount of PIP cannot fully react to form a defect-free polyamide nanofilm, therefore the TFC membrane does not retain MgSO4. Since TEOA contains -OH groups, which can react with TMC, the i-TFC membrane with only TEOA pre-coating has a retention rate of 68.4%. Furthermore, comparing PEI deposition on the membrane surface with no deposition, although i-TFC-PEI has a MgSO4 retention rate of 90%, it is still lower than that of i-TFC-PEI@Boc. This is because HCl is consumed during nanocapsule release, increasing the degree of autocatalytic interfacial polymerization, resulting in a denser membrane structure of i-TFC-PEI@Boc.

[0065] Morphology, structure and performance analysis of membranes

[0066] Polyamide nanofilms prepared by autocatalytic interfacial polymerization exhibit a significantly different morphology from the nodular structure of traditional polyamide films. The disintegration of nanocapsules and the release of active monomers are closely related to the interfacial polymerization time. Therefore, the content of PEI@Boc was controlled at 0.1%, and the effect of interfacial polymerization time on the visualization of the nanochain structure was investigated. Figure 11 As shown in (a)-(c), when the interfacial polymerization reaction lasts for 10 s, a small number of nodular structures appear on the membrane surface. However, as the polymerization time increases, Turing structures gradually form until the entire membrane surface is covered. The reasons for the morphological changes are as follows: (1) When the polymerization time is short, the initiator PIP reacts mainly with TMC, and the surface exhibits a typical nodular structure. However, due to the low concentration of PIP, only a limited nodular structure is shown on the surface. (2) The formation of the polyamide layer is a self-catalytic interfacial polymerization process. The HCl generated in the polymerization reaction will continue to catalyze the decomposition of the nanocapsules and enter the next reaction cycle. The interfacial polymerization process mainly depends on the reaction between the amines released after the nanocapsules disintegrate and TMC. Only after enough HCl is generated in the first cycle reaction can the nanocapsules decompose and release amine groups, enter the next cycle, and finally achieve the polymerization reaction. This process inhibits the diffusion of amine monomers, and the difference in diffusion characteristics between the activator and the inhibitor leads to the appearance of Turing structures. (3) As the reaction time increases, monomers are continuously released, the restricted diffusion of monomers becomes more and more obvious, and the nanochain structure gradually grows until it covers the membrane surface. Figure 11 ).

[0067] Furthermore, with increasing PEI@Boc concentration, the density of the nanochain structure significantly increased after 30 s of reaction. When the PEI@Boc content reached 0.5 wt%, irregularly structured nanobubbles appeared on the membrane surface. Figure 11 (d) and (f) and Figure 12Specifically, the evolution of the nanofilm surface structure involves three aspects. First, during autocatalytic interfacial polymerization, the sequence of monomer release significantly improves the diffusion process of active monomers transferring to the organic phase, leading to the formation of nanochain structures. The disintegration of nanocapsules causes the consumption of HCl and the absorption of heat, resulting in changes in the concentration and thermal gradient at the water / organic interface, thus leading to convective instability. Fluctuating interfaces may also contribute to the formation of nanochain structures. Furthermore, during the disintegration of nanocapsules, in addition to the active amine, small amounts of CO2 and isobutylene are also generated. With increasing nanocapsule concentration, the reaction intensity can be enhanced, and gases cannot pass through the dense polyamide network, leading to the formation of nanobubble structures. Figure 11 (h)). To verify the accuracy of the above results, the solvent volume change caused by the autocatalytic interfacial polymerization reaction was measured in a sealed flask. Compared with the traditional interfacial polymerization process, the solvent volume increased significantly after the introduction of nanocapsules (h). Figure 13 This can be explained by the disintegration of the nanocapsules generating additional CO2 gas.

[0068] Analysis of the influence of reactive monomers on film thickness and pore size

[0069] Changes in the diffusion coefficient of the reactive monomers lead to a smaller reaction zone, which is beneficial for thinning the polyamide layer and improving mass transfer efficiency. Therefore, changes in film thickness were also investigated. Figure 14 As shown in (a)-(c), the polyamide layer thickness of the i-TFC-PEI@Boc film is significantly reduced compared to i-TFC and i-TFC-PEI. The reason for the thickness change can be divided into two stages: the initial film formation stage and the film formation stage with diffusion-limited growth behavior. In the first stage, PIP diffuses into the oil phase as an initiator and reacts with TMC. When more PIP diffuses into the organic phase, it accumulates less in the support layer. The reduced PIP penetration of the PI layer reduces the intrusion of polyamide into the substrate pores. The concentration of PIP in the organic solution was determined by UV-Vis spectrophotometry at a fixed time of 30 s, and PIP showed a maximum absorption peak at approximately 220 nm. When pre-coated with PEI or PEI@Boc, the diffusion rate of PIP is greatly increased due to the competitive diffusion of different monomers. Figure 14(d) Therefore, compared with i-TFC, i-TFC-PEI and i-TFC-PEI@Boc exhibit thinner selective layers, which is due to the synergistic effect of reduced PIP penetration through the support layer and increased diffusion rate into the organic phase. Furthermore, the concentration of amine groups in i-TFC-PEI and i-TFC-PEI@Boc is much higher than that in i-TFC films, thus facilitating the formation of a dense polyamide layer in the early stages of the reaction, thereby inhibiting subsequent monomer diffusion into the organic phase. This is another reason why i-TFC-PEI and i-TFC-PEI@Boc exhibit thinner polyamide layers. Once the continuous film is formed, the reaction enters the second stage. Monomers diffuse through the initial polyamide nanofilm at a significantly reduced rate, slowly thickening the film. The exothermic rate of the interfacial polymerization process plays a crucial role in the formation of the polyamide nanofilm. Strong heat release causes local temperature increases, leading to "interfacial boiling" and accelerating the interfacial polymerization process. Figure 14 (e) and Figure 15 As shown, the introduction of nanocapsules significantly increases the average exothermic rate, thus exhibiting higher resistance to heat dissipation from the aqueous phase during interfacial polymerization. Furthermore, after the introduction of nanocapsules, the byproduct HCl generated during polymerization is absorbed during nanocapsule disintegration, allowing more amines to react with TMC, thereby releasing more heat. This enhanced exothermic performance promotes a highly active interfacial polymerization process. The simultaneous decrease in diffusion rate and the acceleration of local reaction rate lead to the shrinkage of the reaction zone, ultimately resulting in a thinner i-TFC-PEI@Boc.

[0070] Nanocapsule-mediated release of reactive monomers can effectively regulate the diffusion and transport of amines at the water / organic interface, enhancing the kinetics of interfacial polymerization, which is a necessary condition for the formation of nanofilms with uniform pore size distribution. Figure 14 As shown in (f), the geometric standard deviation of i-TFC-PEI@Boc is 1.28, exhibiting narrower pores and more uniform pore size distribution compared to i-TFC and i-TFC-PEI membranes. The pore size decreases with increasing nanocapsule content due to the enhanced reactivity of the nanofilm. This is because the disintegration process of the nanocapsules acts as a promoter of interfacial reactions, altering the transport at the water / organic interface. The uniformity of diffusion flux and spatial polymerization contributes to a more uniform pore size distribution in the polyamide layer.

[0071] Separation mechanism and performance analysis of ions

[0072] The selectivity of ions, especially those with similar hydration radii and net charges, is of great significance for the application of nanofiltration in wastewater resource recovery and seawater desalination. The membrane rejection rate was tested using five single salts. Figure 16 As shown, the i-TFC-PEI@Boc membrane affects Mg 2+and SO4 2- The removal rate is high, but due to the difference in ion size, it has a limited effect on Na+. + The removal rate of Cl- was low. The separation performance of the membrane was evaluated using a cross-flow nanofiltration device with a binary solute mixture of NaAc and NaCl generated during the post-treatment of Pigment Yellow 12. Different pH conditions of the feed solution had a significant impact on this separation system. Figure 17 As shown in (a), the membrane's rejection rate for Cl- is less than 20% in the pH range of 2–10. Conversely, the membrane's inhibition of Ac- ions increases as the pH of the feed solution increases from 2 to 10, indicating that the membrane effectively separates the two ions.

[0073] Different concentrations of feed solution were measured at the initial neutral pH. For example... Figure 17 As shown in (b), in both cases, the membrane exhibits high selectivity for both Ac- and Cl-, but not for Cl-. - The rejection rate for α- is approximately 10%, while the rejection rate for Ac- is greater than 95%. This can be attributed to the fact that the polyamide nanomembrane enhances partial mass transfer, thereby improving solute back-diffusion into the bulk, reducing concentration polarization, and thus improving the membrane's separation performance. Furthermore, long-term filtration tests of this membrane show that it maintains excellent separation performance even after 120 h. Figure 17 (c)).

[0074] By pre-coating nanocapsules, the solute separation behavior of polyamide nanofilms was significantly improved. For example... Figure 17 As shown in (f), based on the synergistic effect of Donnan and steric sieve, the i-TFC-PEI membrane exhibits solute repulsion over a wide range of Stokes radii. Simultaneously, the solute adsorption of the i-TFC-PEI@Boc membrane largely depends on the Stokes radius of the ions, and the adsorption curve shows... r s A sharp transition occurs at ~2.5 Å ( Figure 17 (g). The reduction in the transition range makes precise separation of ions with sub-1 Å selectivity possible. Figure 17 (h) shows the Mg in the feed liquid 2+ / Li + The effects of concentration ratio and salt concentration on the separation performance of the i-TFC-PEI@Boc membrane. With the increase of Mg... 2+ / Li + As the ratio increased from 10 to 50, the feed solute ion concentration increased from 5.5 g / L to 25.5 g / L. The membrane's effect on Mg... 2+ The retention rate is over 90%, for Li + The retention rate is around 10%.

Claims

1. A method for preparing nanofiltration membranes based on interfacial polymerization of molecularly encapsulated nanocapsules via autocatalysis, characterized in that, Includes the following steps: Step 1: Preparation of nanocapsules: A solution containing di-tert-butyl dicarbonate is slowly added to a solution containing polyethyleneimine and stirred to obtain a milky white dispersion; after centrifugation, washing and drying, nanocapsules are obtained. Step 2: Preparation of molecularly encapsulated ultraselective nanofiltration membrane: Dissolve nanocapsules in a co-solvent, immerse the base membrane in the nanocapsule solution, and then remove excess solution and bubbles from the membrane surface; Next, an aqueous solution containing piperazine monomers is applied to the surface of the base membrane, and then the residual solution on the membrane surface is wiped off. An oil-phase solution containing acyl chloride monomers is then applied to the base membrane to carry out an interfacial polymerization reaction, resulting in a nanofiltration membrane.

2. The method for preparing nanofiltration membranes based on self-catalytic interfacial polymerization of molecularly encapsulated nanocapsules according to claim 1, characterized in that, The molecular weight range of polyethyleneimine is 1200-2500; the molar ratio of polyethyleneimine to ditert-butyl dicarbonate is 1:0.01-0.04; the stirring time is 6-10 h; and the centrifugation rate is 8000-12000 rpm.

3. The method for preparing nanofiltration membranes based on molecularly encapsulated nanocapsules through autocatalytic interfacial polymerization according to claim 1, characterized in that, The base membrane is made of polyimide, polyethersulfone, polysulfone, polyketide-imide, polyurethane, cellulose, polyamide, aromatic ester, ceramic membrane, metal membrane, polyvinylidene fluoride, or polyamide. The piperazine monomers are selected from piperazine, piperazine-2-carboxylic acid, 1-(tert-butoxycarbonyl)piperazine, 3,5-bis(acryloylamino)piperazine or 3-mercaptopiperazine; The acyl chloride monomers are selected from pyromellitic terephthaloyl chloride, terephthaloyl chloride, 2,2',4,4'-biphenyltetramethyl chloride or 5-(dichlorophosphoryl)isophthaloyl chloride.

4. The method for preparing nanofiltration membranes based on self-catalytic interfacial polymerization of molecularly encapsulated nanocapsules according to claim 1, characterized in that, The base film is a polyimide-based film prepared by a solvent-inducible phase inversion method.

5. The method for preparing nanofiltration membranes based on molecularly encapsulated nanocapsules through autocatalytic interfacial polymerization according to claim 1, characterized in that, In step two, the concentration of the nanocapsule solution is 0.05-1 wt%.

6. The method for preparing nanofiltration membranes based on self-catalytic interfacial polymerization of molecularly encapsulated nanocapsules according to claim 1, characterized in that, The co-solvent is an 8-12 wt% aqueous solution of triethanolamine, the concentration of the aqueous solution of piperazine monomer is 0.015-0.02 wt%, and the contact time is 1-5 min.

7. The method for preparing nanofiltration membranes based on self-catalytic interfacial polymerization of molecularly encapsulated nanocapsules according to claim 1, characterized in that, The concentration of the oil phase solution containing acyl chloride monomers is 0.05-0.15 wt%, and the contact time is 10-20 s.

8. A nanofiltration membrane, characterized in that, It is prepared by the preparation method according to any one of claims 1-7.

9. The application of the nanofiltration membrane according to claim 8 in liquid filtration.

Images