Janus structure composite desalination membrane, preparation method and application thereof

Abstract

This invention discloses a Janus-structured composite desalination membrane, its preparation method, and its application. The composite desalination membrane comprises a porous supporting substrate and a Janus hierarchical separation layer with vertical heterogeneity constructed on its surface. The Janus hierarchical separation layer includes, from bottom to top, a hydrophobic nanochannel layer with directional low-friction water transport characteristics and a dense zwitterionic hydrophilic network layer covering its surface. This invention utilizes in-situ growth and interfacial polymerization technology to grow a low surface energy crystalline or semi-crystalline rigid porous framework material on the surface of the porous supporting substrate, constructing an ultra-low friction internal hydrophobic nano-slip microenvironment. Subsequently, using a surface tension modulation process, zwitterionic precursors are introduced to permeate and fill the intrigued boundary defects of the hydrophobic layer and undergo secondary interfacial polymerization. This invention achieves a balance between flux release and high sodium sulfate desalination rate without sacrificing the membrane surface antifouling performance.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, specifically relating to a Janus-structured composite desalination membrane, its preparation method and application, and particularly to the nanoscale microenvironment regulation of the functional separation layer of a thin-layer composite desalination membrane, the construction of a Janus hierarchical structure with an asymmetric hydrophilic and hydrophobic intrinsic structure, and the preparation process of the membrane. Background Technology

[0002] Thin-layer composite (TFC) membranes are core materials for modern water treatment, seawater desalination, brackish water desalination, and resource recovery. Traditional desalination membrane separation layer modification strategies often focus on enhancing the hydrophilicity of the membrane surface, aiming to repel pollutants such as oils, proteins, and microorganisms by constructing a macroscopic hydration layer on the surface. However, from the perspective of molecular dynamics and transport dynamics, hydrophilic groups (such as hydroxyl, carboxyl, and amino groups) form strong hydrogen bond networks and viscosity effects with water molecules, which greatly limits the migration rate of water molecules within the dense separation layer. This results in traditional membrane materials generally facing a "permeability-selectivity" trade-off, where "high flux and high desalination rate cannot be simultaneously achieved."

[0003] Within nanoscale hydrophobically confined channels, the hydrogen bond network of water molecules is restructured and tends towards one-dimensional order. Furthermore, the interaction between water molecules and the smooth hydrophobic walls is extremely weak, exhibiting an ultrafast "slip" transport characteristic similar to biological aquaporins. Successfully introducing this low-surface-energy hydrophobic nanoscale slip microenvironment into desalination membranes could potentially reduce water transport resistance and enhance the intrinsic flux of the membrane.

[0004] However, constructing such an efficient hydrophobic nanoscale microenvironment still faces challenges: Traditional thin-film nanocomposite (TFN) technology often uses physical blending or one-pot interfacial polymerization to introduce porous nanomaterials into the matrix. The compatibility between physically mixed nanomaterials and polymer matrix is ​​poor, and non-intrinsic boundary topological defects or boundary micro-gap are easily generated between layers, which become leakage channels for salt ions, resulting in a significant decrease in desalination rate. In actual operation, the completely hydrophobic membrane surface is very likely to strongly adsorb amphiphilic organic pollutants or microorganisms in the water through hydrophobic-hydrophobic interactions, causing irreversible membrane fouling.

[0005] Therefore, the key to high-performance desalination membranes lies in how to ingeniously construct a hierarchical asymmetric Janus structure nano-microenvironment that integrates an inner layer of directional low-friction sliding channels and an outer layer of high-efficiency anti-fouling barrier, and eliminate interface defects. Summary of the Invention

[0006] The purpose of this invention is to overcome the defects in existing desalination membrane modification, such as disordered distribution of hydrophobic channels, weak interlayer bonding, numerous interface defects leading to decreased desalination rate, and easy macroscopic surface fouling. This invention provides a Janus-structured composite desalination membrane, its preparation method, and its application, to at least solve the above-mentioned technical problems in the prior art. Through in-situ growth and interfacial chemical heterogeneous construction, this invention integrates a continuous low surface energy nano-slip microenvironment inside the separation layer, while constructing a dense zwitterionic hydrophilic network layer on the outermost layer, thus achieving the essential unity of low-resistance directional ultrafast transport of water molecules, high desalination rate, and long-term anti-fouling performance.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] In a first aspect, the present invention provides a Janus-structured composite desalination membrane, the composite desalination membrane comprising a porous supporting substrate and a Janus hierarchical separation layer having vertical heterogeneous characteristics constructed on the surface of the porous supporting substrate; the Janus hierarchical separation layer comprising, from bottom to top: a hydrophobic nanochannel layer having directional low-friction water transport characteristics, and a dense zwitterionic hydrophilic network layer covering the surface of the hydrophobic nanochannel layer; the hydrophobic nanochannel layer is formed by in-situ oriented growth of a crystalline or semi-crystalline rigid porous framework material, and the inner walls of the pores of the porous framework material are modified with low surface energy functional groups to construct a low-friction internal localized hydrophobic nano-microenvironment; the dense zwitterionic hydrophilic network layer covers the surface of the hydrophobic nanochannel layer, and its three-dimensional cross-linked network permeates and fills the gaps of the non-intrinsic microscopic boundary defects of the hydrophobic nanochannel layer, forming a heterogeneous interpenetrating structure with the hydrophobic nanochannel layer.

[0009] Furthermore, the porous support substrate membrane includes a polysulfone, polyethersulfone, polyacrylonitrile, polyimide, or polyvinylidene fluoride ultrafiltration membrane.

[0010] Furthermore, the crystalline or semi-crystalline rigid porous framework material includes covalent organic framework materials, metal-organic framework materials, hydrogen-bonded organic framework materials, porous polymer networks, or supramolecular organic frameworks.

[0011] Furthermore, the low surface energy modified functional groups include fluoroalkyl, perfluoroaryl, trifluoromethyl, highly branched alkyl, or organosiloxane groups.

[0012] Furthermore, the dense zwitterionic hydrophilic network layer is generated by interfacial polycondensation reaction of zwitterionic modified polyamine or zwitterionic modified polyol with polyacrylamide monomer, and the dense zwitterionic hydrophilic network layer contains any one or more of the following structures: sulfonic acid inner salt, carboxylic acid inner salt, phosphate inner salt or trimethylamine N oxide.

[0013] Secondly, the present invention provides a method for preparing the Janus structure composite desalination membrane, comprising the following steps:

[0014] S1: Contact the porous support substrate with a first solution containing a first framework precursor and a second framework precursor for 5-10 minutes, so that the framework precursors are directionally adsorbed onto the surface and surface micropores of the porous support substrate; the mass fraction of the first framework precursor in the first solution is 0.6-0.9 wt%, and the mass fraction of the second framework precursor is 0.5-0.7 wt%.

[0015] S2: Subsequently, it is introduced into a second solution containing a catalyst and a surfactant, and in-situ crystallized and grown at 20~80℃ for 5~60 min. A directional, ordered network assembly is achieved through covalent bonds, coordination bonds, or supramolecular forces, generating a hydrophobic nanochannel layer in situ on the surface of the porous support substrate. The substrate is then washed and dried. The mass fraction of the catalyst in the second solution is 0.02~0.4wt%, and the mass fraction of the surfactant is 0.2~0.35wt%.

[0016] S3: Recoat with an aqueous solution for 1-5 min; the aqueous solution contains zwitterionic modified polyamine or zwitterionic modified polyol and a surface tension modifier, the surface tension modifier guiding the zwitterionic precursor small molecules to directionally penetrate into the intrigued boundary defects and grain boundary pinholes of the underlying hydrophobic nanochannel layer; the mass fraction of the zwitterionic modified polyamine or zwitterionic modified polyol in the aqueous solution is 0.5-2.0 wt%, and the mass fraction of the surface tension modifier is 0.05-0.5 wt%.

[0017] S4: Remove excess aqueous solution, introduce organic phase solution containing polyacrylamide chloride monomer into the membrane surface for secondary interfacial polymerization reaction for 20~60 s, after the reaction is completed, heat cure and dry at 60~100℃ for 5~10 min, that is, construct a dense zwitterionic hydrophilic network layer in situ above the hydrophobic nanochannel layer.

[0018] Further, in step S1, one of the first skeletal precursor and the second skeletal precursor is a polyamine monomer containing low surface energy groups, and the other is a polyaldehyde monomer; in step S2, the catalyst is an acid catalyst.

[0019] Further, the polyamine monomer containing low surface energy groups includes 2,2-bis(4-aminophenyl)hexafluoropropane, 2,5-diaminotrifluorotoluene, or 1,3,5-tris(2,3,5,6-tetrafluoro)benzene; the polyaldehyde monomer includes pyromellitic methyl methacrylate, 2,4,6-triformylphloroglucinol, terephthalaldehyde, or 2,3,5,6-tetrafluoro-terephthalaldehyde; the acid catalyst includes acetic acid, hydrochloric acid, or trifluoromethanesulfonic acid; and the surfactant includes sodium dodecyl sulfonate.

[0020] Further, in step S3, the surface tension modifier includes sodium dodecyl sulfate; the zwitterionic modified polyamine includes polyethyleneimine modified with sulfonic acid inner salt, polyethyleneimine modified with carboxylic acid inner salt, or polyethyleneimine modified with trimethylamine N-oxide; the zwitterionic modified polyol includes N-methyldiethanolamine N-oxide or N-benzyldiethanolamine N-oxide; in step S4, the polyacrylamide chloride monomer includes pyromellitic acid trimesoyl chloride, isophthaloyl chloride, or 1,3,5-benzenetrisulfonyl chloride, and the mass fraction of the polyacrylamide chloride monomer in the organic phase solution is 0.01wt%~0.2wt%.

[0021] Thirdly, the present invention also provides an application of the Janus structure composite desalination membrane described above in seawater desalination, brackish water desalination, and saline-alkali water desalination.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] 1. The present invention adopts a hierarchical construction method to precisely control the internal structure of the desalination membrane. First, a confined low surface energy microenvironment framework is constructed inside the membrane. The low surface energy groups densely anchored on the inner wall of the channel can significantly improve the water molecule slip coefficient, making the water migration and diffusion coefficient several orders of magnitude higher than that of traditional hydrophilic channels.

[0024] 2. At the same time, the present invention constructs an asymmetric gradient Janus structure, which locks a high hydration energy zwitterionic network on the membrane surface to form a dual-effect hydration barrier layer with both space and charge. The membrane surface has both strong hydrophilicity and near-neutral properties, which can effectively block the pollution and invasion of organic matter, protein and microorganisms in the raw water.

[0025] 3. In addition, with the help of the surface tension modifier, the surface zwitterionic cross-linking network can penetrate and seal the grain boundary pinholes of the underlying oriented skeleton in situ, eliminate membrane defects, and form a highly efficient ion screening structure. Combined with the underlying low-friction ultrafast water transport channel, it can maximize the release of water flux while maintaining a stable high salt rejection rate. It takes into account the comprehensive excellent performance of the membrane material, which has ultra-high desalination accuracy, high water permeability efficiency and long-term anti-fouling and stable operation. Attached Figure Description

[0026] The above and other objects, features, and advantages of exemplary embodiments of this application will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings. Several embodiments of this application are illustrated in the drawings by way of example and not limitation, in which:

[0027] In the accompanying drawings, the same or corresponding reference numerals indicate the same or corresponding parts.

[0028] Figure 1 A schematic diagram of the Janus structure composite desalination membrane described in this invention is shown.

[0029] Figure 2 A water flux comparison chart of embodiments and comparative examples of the present invention is shown.

[0030] Figure 3 The accompanying diagram shows a comparison of sodium sulfate retention in embodiments and comparative examples of the present invention. Detailed Implementation

[0031] To make the objectives, features, and advantages of this application more apparent and understandable, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0032] It should be noted that the covalent organic framework materials, metal-organic framework materials, hydrogen-bonded organic framework materials, porous polymer networks, and supramolecular organic frameworks in this invention all possess the characteristics of crystalline or semi-crystalline rigid porous structures, regular pore sizes, and strong framework stability. They can all be used to construct low-resistance hydrophobic nanochannels through precursor directional assembly, which is suitable for the construction requirements of the Janus heterogeneous hierarchical structure of this invention. Therefore, they can all achieve the technical effects of this invention. This invention takes covalent organic framework materials as an example.

[0033] A schematic diagram of the Janus-structured composite desalination membrane provided by this invention is shown below. Figure 1 As shown.

[0034] Example 1

[0035] This embodiment prepares a Janus-structured composite desalination membrane, as detailed below:

[0036] (1) Provide a porous polyimide (PI) ultrafiltration base membrane as a porous support substrate membrane, and immerse the porous polyimide ultrafiltration base membrane in dichloromethane containing 0.8 wt% 2,2-bis(4-aminophenyl)hexafluoropropane (6F-PDA) and 0.6 wt% trimethylolpropane (TMA) for 5 min to allow the monomers to be adsorbed on the surface of the porous polyimide ultrafiltration base membrane and inside the surface pores.

[0037] (2) Take out the membrane, gently wipe off the surface liquid with a dust-free filter paper, and then immerse it in an aqueous solution containing 0.02wt% acetic acid catalyst and 0.2wt% sodium dodecyl sulfonate. Crystallize and grow in situ at 60℃ for 30 min, and generate COF-based hydrophobic nanochannel layers through covalent bond directional assembly. After the reaction, wash the membrane with anhydrous ethanol and deionized water in sequence, and dry it at room temperature for later use.

[0038] (3) Prepare an aqueous solution containing 1.0 wt% N-methyldiethanolamine N-oxide and 0.2 wt% sodium dodecyl sulfate surface tension modifier; coat the dried membrane surface evenly with the aqueous solution and immerse for 2 min.

[0039] (4) Pour off excess aqueous solution from the membrane surface, purge the surface liquid membrane with nitrogen, and then coat with an organic phase solution of n-hexane containing 0.1 wt% trimesoyl chloride (TMC) and carry out interfacial polymerization reaction for 60 s. After the reaction is completed, place the membrane in an 80 ℃ oven for heat curing for 5 min. Finally, a dense N-oxide-based zwitterionic hydrophilic network layer is constructed on the surface of the hydrophobic nanochannel layer to form a composite desalination membrane with a Janus structure.

[0040] Example 2

[0041] This embodiment prepares a Janus-structured composite desalination membrane, as detailed below:

[0042] (1) Provide a porous polyethersulfone (PES) ultrafiltration base membrane as a porous support substrate membrane, and immerse the porous polyethersulfone (PES) ultrafiltration base membrane in dichloromethane containing 0.7wt% 2,5-diaminotrifluorotoluene and 0.7wt% 2,4,6-tricarboxymethyl phloroglucinol (BTB) for 6 min to allow the monomers to be adsorbed on the surface of the PES membrane and inside the surface pores.

[0043] (2) Take out the membrane, gently wipe off the surface liquid with a dust-free filter paper, and then immerse it in an aqueous solution containing 0.2wt% hydrochloric acid catalyst and 0.25wt% sodium dodecyl sulfonate. Crystallize and grow in situ at 50°C for 25 min, and generate COF-based hydrophobic nanochannel layers through covalent bond directional assembly. After the reaction, wash the membrane with anhydrous ethanol and deionized water in sequence, and dry it at room temperature for later use.

[0044] (3) Prepare an aqueous solution containing 1.2 wt% sulfonic acid-modified polyethyleneimine and 0.15 wt% sodium dodecyl sulfate surface tension modifier; coat the dried membrane surface evenly with the aqueous solution and immerse for 2 min.

[0045] (4) Pour off excess aqueous solution from the membrane surface, purge the surface liquid membrane with nitrogen, and then coat with an organic phase solution of n-hexane containing 0.08 wt% trimesoyl chloride (TMC) and carry out interfacial polymerization reaction for 45 s. After the reaction is completed, place the membrane in a 75 ℃ oven for heat curing for 6 min. Finally, a dense sulfonic acid-based zwitterionic hydrophilic network layer is constructed on the surface of the hydrophobic nanochannel layer to form a composite desalination membrane with a Janus structure.

[0046] Example 3

[0047] (1) Provide a porous polyvinylidene fluoride (PVDF) ultrafiltration base membrane as a porous support substrate membrane, and immerse the porous polyvinylidene fluoride (PVDF) ultrafiltration base membrane in dichloromethane containing 0.9wt% 2,2-bis(4-aminophenyl)hexafluoropropane (6F-PDA) and 0.6wt% 2,3,5,6-tetrafluorop-dibenzaldehyde for 5 min to allow the monomers to be adsorbed on the surface of the PVDF membrane and inside the surface pores.

[0048] (2) Take out the membrane, gently wipe off the surface liquid with a dust-free filter paper, and then immerse it in an aqueous solution containing 0.4wt% acetic acid catalyst and 0.15wt% sodium dodecyl sulfonate. Crystallize and grow in situ at 70℃ for 15 min. HOF-based hydrophobic nanochannel layers are generated through supramolecular forces. After the reaction, wash the membrane with anhydrous ethanol and deionized water in sequence, and dry it at room temperature for later use.

[0049] (3) Prepare an aqueous solution containing 0.8 wt% N-methyldiethanolamine N-oxide and 0.05 wt% sodium dodecyl sulfate surface tension modifier; coat the dried membrane surface evenly with the aqueous solution and immerse for 1 min.

[0050] (4) Pour off excess aqueous solution from the membrane surface, purge the surface liquid membrane with nitrogen, and then coat with an organic phase solution of n-hexane containing 0.05 wt% trimesoyl chloride (TMC) and carry out interfacial polymerization reaction for 25 s. After the reaction is completed, place the membrane in a 65 ℃ oven for heat curing for 10 min. Finally, a dense polyol-type zwitterionic hydrophilic network layer is constructed on the surface of the hydrophobic nanochannel layer to form a composite desalination membrane with a Janus structure.

[0051] Example 4

[0052] This embodiment prepares a Janus-structured composite desalination membrane, as detailed below:

[0053] (1) Provide a porous PI ultrafiltration base membrane as a porous support substrate membrane, and immerse the porous PI ultrafiltration base membrane in dichloromethane containing 0.75wt% 2,5-diaminotrifluorotoluene and 0.65wt% p-phenylenedialdehyde for 9 min to allow the monomers to be adsorbed on the surface of the PI membrane and inside the surface pores.

[0054] (2) Take out the membrane, gently wipe off the surface liquid with a dust-free filter paper, and then immerse it in an aqueous solution containing 0.25wt% hydrochloric acid catalyst and 0.2wt% sodium dodecyl sulfonate. Crystallize and grow in situ at 30°C for 55 min, and generate PPN-based hydrophobic nanochannel layers through covalent bond directional assembly. After the reaction, wash the membrane with anhydrous ethanol and deionized water in sequence, and dry it at room temperature for later use.

[0055] (3) Prepare an aqueous solution containing 2.0 wt% sulfonic acid-modified polyethyleneimine and 0.5 wt% sodium dodecyl sulfate surface tension modifier; coat the dried membrane surface evenly with the aqueous solution and soak for 5 min.

[0056] (4) Pour off excess aqueous solution from the membrane surface, purge the surface liquid membrane with nitrogen, and then coat with an organic phase solution of n-hexane containing 0.2 wt% 1,3,5-benzenetrisulfonyl chloride for interfacial polymerization for 60 s. After the reaction is completed, place the membrane in a 90 ℃ oven for heat curing for 8 min. Finally, a dense sulfonic acid zwitterionic hydrophilic network layer is constructed on the surface of the hydrophobic nanochannel layer to form a composite desalination membrane with a Janus structure.

[0057] Example 5

[0058] This embodiment prepares a Janus-structured composite desalination membrane, as detailed below:

[0059] (1) Provide a porous polyethersulfone (PES) ultrafiltration base membrane as a porous support substrate membrane, and immerse the porous polyethersulfone (PES) ultrafiltration base membrane in dichloromethane containing 0.65wt% 1,3,5-tris(2,3,5,6-tetrafluoro)benzene and 0.7wt% trimethylolpropane (TMA) for 5 min to allow the monomers to be adsorbed on the surface of the PES membrane and inside the surface pores.

[0060] (2) Take out the membrane, gently wipe off the surface liquid with a dust-free filter paper, and then immerse it in an aqueous solution containing 0.1 wt% trifluoromethanesulfonic acid catalyst and 0.35 wt% sodium dodecyl sulfonate. Crystallize and grow in situ at 80°C for 5 min. SOF-based hydrophobic nanochannel layer is generated by directional assembly through supramolecular forces. After the reaction, wash the membrane with anhydrous ethanol and deionized water in sequence, and dry it at room temperature for later use.

[0061] (3) Prepare an aqueous solution containing 0.5 wt% N-benzyldiethanolamine N-oxide and 0.25 wt% sodium dodecyl sulfate surface tension modifier; coat the dried membrane surface evenly with the aqueous solution and immerse for 3 min.

[0062] (4) Pour off excess aqueous solution from the membrane surface, purge the surface liquid membrane with nitrogen, and then coat with an organic phase solution of n-hexane containing 0.1 wt% isophthaloyl chloride (IPC) and carry out interfacial polymerization reaction for 30 s. After the reaction is completed, place the membrane in a 100 ℃ oven for heat curing for 5 min. Finally, a dense polyol-type zwitterionic hydrophilic network layer is constructed on the surface of the hydrophobic nanochannel layer to form a composite desalination membrane with a Janus structure.

[0063] Example 6

[0064] This embodiment prepares a Janus-structured composite desalination membrane, as detailed below:

[0065] (1) Provide a porous polyacrylonitrile (PAN) ultrafiltration base membrane as a porous support substrate membrane, and immerse the porous polyacrylonitrile (PAN) ultrafiltration base membrane in dichloromethane containing 0.85wt% 2,2-bis(4-aminophenyl)hexafluoropropane (6F-PDA) and 0.6wt% 2,4,6-tricarboxymethyl phloroglucinol (BTB) for 7 min to allow the monomers to be adsorbed on the surface of the PAN membrane and inside the surface pores.

[0066] (2) Take out the membrane, gently wipe off the surface liquid with a dust-free filter paper, and then immerse it in an aqueous solution containing 0.35wt% acetic acid catalyst and 0.25wt% sodium dodecyl sulfonate. It is then crystallized and grown in situ at 20℃ for 60 min. A crystalline COF-based hydrophobic nanochannel layer is generated through covalent bond directional assembly. After the reaction, the membrane is washed with anhydrous ethanol and deionized water in sequence and dried at room temperature for later use.

[0067] (3) Prepare an aqueous solution containing 1.0 wt% carboxylic acid-modified polyethyleneimine and 0.4 wt% sodium dodecyl sulfate surface tension modifier; coat the dried membrane surface evenly with the aqueous solution and immerse for 4 min.

[0068] (4) Pour off excess aqueous solution from the membrane surface, purge the surface liquid membrane with nitrogen, and then coat with an organic phase solution of n-hexane containing 0.15 wt% trimesoyl chloride (TMC) and carry out interfacial polymerization reaction for 55 s. After the reaction is completed, place the membrane in an 80 ℃ oven for heat curing for 9 min. Finally, a dense carboxylic acid zwitterionic hydrophilic network layer is constructed on the surface of the hydrophobic nanochannel layer to form a composite desalination membrane with Janus structure.

[0069] Comparative Example 1

[0070] In this comparative example, a composite desalination membrane without a Janus heterostructure was prepared, as detailed below:

[0071] (1) Provide a porous polyimide (PI) ultrafiltration base membrane as a porous support membrane, select a clean and unmodified pure PI membrane as the substrate, and do not perform the process of adsorbing the framework precursor and constructing the hydrophobic nanochannel layer.

[0072] (2) Prepare an aqueous solution containing 1.0 wt% N-methyldiethanolamine N-oxide and 0.2 wt% sodium dodecyl sulfate surface tension modifier; uniformly coat the surface of the porous polyimide (PI) ultrafiltration membrane with the aqueous solution and soak for 3 min.

[0073] (3) Pour off excess aqueous solution from the membrane surface, purge the surface liquid membrane with nitrogen, and then coat with an organic phase solution of n-hexane containing 0.1 wt% trimesoyl chloride (TMC) and carry out interfacial polymerization for 60 s. After the reaction is completed, place the membrane in an 80 ℃ oven for heat curing for 5 min. Finally, a single dense N-oxide-based zwitterionic hydrophilic network layer is directly constructed on the surface of a pure PI substrate to obtain a composite membrane without hydrophobic nanochannel layer and without Janus vertical heterostructure interpenetrating structure.

[0074] Comparative Example 2

[0075] In this comparative example, a hydrophobically modified membrane without Janus heterostructure was prepared, as detailed below:

[0076] (1) Provide a porous polyimide (PI) ultrafiltration base membrane as a porous support substrate membrane, and immerse the porous support substrate membrane in dichloromethane containing 0.8 wt% 2,2-bis(4-aminophenyl)hexafluoropropane (6F-PDA) and 0.6 wt% trimethylolpropane (TMA) for 5 min to allow the monomers to be adsorbed on the surface of the porous polyimide (PI) ultrafiltration base membrane and inside the surface pores.

[0077] (2) Take out the membrane, gently wipe off the surface liquid with a dust-free filter paper, and then immerse it in an aqueous solution containing 0.3wt% acetic acid catalyst and 0.2wt% sodium dodecyl sulfonate. Crystallize and grow in situ at 60℃ for 30 min, and generate COF-based hydrophobic nanochannel layers through covalent bond directional assembly. After the reaction, wash the membrane with anhydrous ethanol and deionized water in sequence, and dry it at room temperature for later use.

[0078] (3) The zwitterionic aqueous phase coating, interfacial polymerization and thermal curing processes are omitted. Only the PI substrate and surface hydrophobic nanochannel layer are retained. A dense zwitterionic hydrophilic network layer is not constructed, and a heterogeneous interpenetrating Janus hierarchical structure cannot be formed, resulting in a single hydrophobic modified film.

[0079] Comparative Example 3

[0080] In this comparative example, a hydrophobically modified membrane with a Janus heterostructure was prepared, as detailed below:

[0081] (1) Provide a porous polyimide (PI) ultrafiltration base membrane as a porous support substrate membrane, and immerse the porous support substrate membrane in an aqueous solution of sodium hydroxide (pH=13) containing 0.5 wt.% piperazine and 0.5 wt.% 3,6-dihydroxy-9-trifluoromethyl-9-phenyloxanthracene for 5 min to allow the monomers to be adsorbed on the surface and inside the surface pores of the porous polyimide (PI) ultrafiltration base membrane.

[0082] (2) Take out the membrane, gently wipe off the surface liquid with a dust-free filter paper, and then immerse it in a hexane solution containing 0.1 wt.% 1,3,5-benzenetricarboxyl chloride for 30 seconds to remove the residual liquid and obtain the primary membrane.

[0083] (3) The nascent membrane was heat-treated in an 80°C drying oven for 5 minutes to obtain a fluorinated polyester amide composite desalination membrane. The hydrophobic layer is formed by the polymerization of ordinary aromatic amines and fluorinated monomers to form an amorphous polyester amide hydrophobic layer with no directional low-resistance water transport channels.

[0084] (4) Dissolve N-methyldiethanolamine N-oxide monomer and 0.5 molar amount of 4-dimethylaminopyridine catalyst of N-methyldiethanolamine N-oxide monomer in deionized water to prepare a modified solution with a zwitterionic monomer concentration of 1.0 wt.%. Contact the zwitterionic modified fluorinated polyesteramide composite desalination membrane for 5 min and rinse to obtain a zwitterionic modified fluorinated polyesteramide composite desalination membrane.

[0085] Test case

[0086] To verify the desalination performance of the Janus-structured composite desalination membrane of the present invention, cross-flow nanofiltration performance tests were conducted on the desalination membranes prepared in Examples 1-6 and Comparative Examples 1-3. Sodium sulfate (Na2SO4) was used to simulate saline wastewater, and the pure water flux and sodium sulfate retention performance of each desalination membrane prepared in Examples 1-6 and Comparative Examples 1-3 were systematically evaluated.

[0087] Water flux and sodium sulfate rejection rate tests: Water flux and sodium sulfate rejection rate are two important parameters for evaluating the performance of desalination membranes. This experimental example uses a cross-flow filtration device to evaluate the performance of the desalination membranes prepared in Examples 1-6 and Comparative Examples 1-3. Water flux (J) is defined as the volume of water permeating through a unit area per unit time under certain operating conditions, expressed in L / m². -2 h -1Rejection (R) is defined as the difference between the feed solution salt concentration and the permeate salt concentration under certain operating conditions, divided by the feed solution salt concentration.

[0088] The separation performance of the desalination membranes prepared in Examples 1-6 and Comparative Examples 1-3 was tested under the following operating conditions: feed solution was 1000 ppm sodium sulfate aqueous solution, operating pressure was 0.6 MPa, operating temperature was 25℃, and pressure stabilization time was 60 min. The test results are shown in Table 1.

[0089] Table 1

[0090] Example 1 118.6 98.9 Example 2 112.3 97.1 Example 3 125.4 95.8 Example 4 108.2 98.0 Example 5 122.7 95.3 Example 6 102.5 97.8 Comparative Example 1 (Single zwitterionic layer) 76.3 96.5 Comparative Example 2 (Single hydrophobic nanochannel layer) 83.8 72.1 Comparative Example 3 (Amorphous Polymer Hydrophobic Layer) 89.1 98.6

[0091] Table 1 Figure 2 and Figure 3 The water flux and sodium sulfate rejection rate of the desalination membranes in Examples 1-6 and Comparative Examples 1-3 are listed. The results show that the Janus-structured composite desalination membrane prepared in this invention exhibits excellent desalination performance, with a pure water flux stably maintained between 102.5 and 125.4 L / m³. -2 h -1 The sodium sulfate retention rate is higher than 95%, and the desalination performance is excellent and the overall performance is balanced.

[0092] Comparative Example 1 is a single hydrophilic membrane without a hydrophobic nanochannel layer. It only constructs a zwitterionic hydrophilic sieving network on the PI substrate surface, lacking a lower-layer low-friction hydrophobic directional water transport channel. The complete and dense hydrophilic cross-linked layer can effectively retain sodium sulfate through electrostatic repulsion and pore size sieving, resulting in excellent membrane retention performance. The sodium sulfate retention rate can reach 96.5%, similar to the high desalination levels of the embodiments of this invention. However, due to the lack of hydrophobic nanopermeable channels, the substrate's native pore resistance is high, mass transfer efficiency is low, and there is no rapid water transport path, resulting in a pure water flux of only 76.3 L / m³. -2 h -1 Its water permeability is far lower than that of the Janus double-layer membrane, and it has a significant shortcoming in flux.

[0093] Comparative Example 2 is a single hydrophobic channel membrane without a hydrophilic separation layer. It only constructs a COF-based hydrophobic nanochannel layer and lacks an upper dense zwitterionic hydrophilic sieving network. The COF membrane polymerized at the polymer membrane interface has certain defects: it cannot effectively intercept divalent sulfate ions, with a sodium sulfate rejection rate of only 72.1%; furthermore, the purely hydrophobic membrane surface has poor wettability and high water wetting resistance. Lacking the water absorption and conduction assistance of the hydrophilic layer, it cannot achieve efficient water transport, with a pure water flux of only 83.8 Lm. -2 h -1 Its water permeability is significantly inferior to that of the Janus composite membrane of this invention, which combines hydrophobic channels and a hydrophilic surface, and it has no practical desalination application value overall.

[0094] Comparative Example 3 consists of an amorphous hydrophobic polymer layer and a dense zwitterionic hydrophilic sieving network. It lacks regular and controllable nano-hydrophobic channels and grain boundary defect gap structures. The channels are disordered and chaotic, and the mass transfer resistance is relatively large. Ultimately, the pure water flux is 89.1 L·m·h and the sodium sulfate rejection rate is 98.6%. The overall performance is significantly inferior to that of the embodiments of the present invention.

[0095] In summary, this invention, by constructing a heterogeneous interpenetrating Janus hierarchical structure consisting of a lower layer of hydrophobic and low-friction nanochannels and an upper layer of dense zwitterionic hydrophilic network, synergistically achieves a two-way improvement in both high flux and high rejection rate. It solves the technical bottleneck of the mutual constraint between flux and rejection rate in traditional desalination membranes and has excellent industrial desalination application value.

[0096] It should be understood that the various forms of processes shown above can be used to rearrange, add, or delete steps. For example, the steps described in this application can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution of this application can be achieved, and this is not limited herein.

[0097] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "a plurality of" means two or more, unless otherwise explicitly specified.

[0098] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A Janus-structured composite desalination membrane, characterized in that, The composite desalination membrane includes a porous support substrate and a Janus hierarchical separation layer with vertical heterogeneous features constructed on the surface of the porous support substrate. The Janus hierarchical separation layer comprises, from bottom to top: a hydrophobic nanochannel layer with directional low-friction water transport characteristics, and a dense zwitterionic hydrophilic network layer covering the surface of the hydrophobic nanochannel layer. The hydrophobic nanochannel layer is formed by in-situ orientation growth of a crystalline or semi-crystalline rigid porous framework material, and the inner walls of the pores of the porous framework material are modified with low surface energy functional groups to construct a low-friction internal local hydrophobic nano-microenvironment; the dense zwitterionic hydrophilic network layer covers the surface of the hydrophobic nanochannel layer, and its three-dimensional cross-linked network permeates and fills the gaps of the non-intrinsic microscopic boundary defects of the hydrophobic nanochannel layer, forming a hetero-interpenetrating structure with the hydrophobic nanochannel layer.

2. The Janus-structured composite desalination membrane according to claim 1, characterized in that, The porous support membrane includes polysulfone, polyethersulfone, polyacrylonitrile, polyimide, or polyvinylidene fluoride ultrafiltration membrane.

3. The Janus-structured composite desalination membrane according to claim 1, characterized in that, The crystalline or semi-crystalline rigid porous framework materials include covalent organic framework materials, metal-organic framework materials, hydrogen-bonded organic framework materials, porous polymer networks, or supramolecular organic frameworks.

4. The Janus-structured composite desalination membrane according to claim 1, characterized in that, The low surface energy modified functional groups include fluoroalkyl, perfluoroaryl, trifluoromethyl, highly branched alkyl, or organosiloxane groups.

5. The Janus-structured composite desalination membrane according to claim 1, characterized in that, The dense zwitterionic hydrophilic network layer is generated by interfacial polycondensation reaction of zwitterionic modified polyamine or zwitterionic modified polyol with polyacrylamide monomer. The dense zwitterionic hydrophilic network layer contains any one or more of the following structures: sulfonic acid inner salt, carboxylic acid inner salt, phosphate inner salt or trimethylamine N oxide.

6. A method for preparing a Janus-structured composite desalination membrane as described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1: Contact the porous support substrate with a first solution containing a first framework precursor and a second framework precursor for 5-10 minutes, so that the framework precursor is directionally adsorbed onto the surface and surface micropores of the porous support substrate. The first solution contains 0.6-0.9 wt% of the first framework precursor and 0.5-0.7 wt% of the second framework precursor. S2: Subsequently, it is introduced into a second solution containing a catalyst and a surfactant, and in-situ crystallized and grown at 20~80℃ for 5~60 min. A directional, ordered network assembly is achieved through covalent bonds, coordination bonds, or supramolecular forces, generating a hydrophobic nanochannel layer in situ on the surface of the porous support substrate. The substrate is then washed and dried. The mass fraction of the catalyst in the second solution is 0.02~0.4wt%, and the mass fraction of the surfactant is 0.2~0.35wt%. S3: Recoat with an aqueous solution for 1-5 min; the aqueous solution contains zwitterionic modified polyamine or zwitterionic modified polyol and a surface tension modifier, the surface tension modifier guiding the zwitterionic precursor small molecules to directionally penetrate into the intrigued boundary defects and grain boundary pinholes of the underlying hydrophobic nanochannel layer; the mass fraction of the zwitterionic modified polyamine or zwitterionic modified polyol in the aqueous solution is 0.5-2.0 wt%, and the mass fraction of the surface tension modifier is 0.05-0.5 wt%. S4: Remove excess aqueous solution, introduce an organic phase solution containing polyacrylamide chloride monomer into the membrane surface for a secondary interfacial polymerization reaction for 20-60 s, and after the reaction, heat-cur and dry at 60-100℃ for 5-10 min to construct a dense zwitterionic hydrophilic network layer in situ above the hydrophobic nanochannel layer; the mass fraction of the polyacrylamide chloride monomer in the organic phase solution is 0.01wt%-0.2wt%.

7. The preparation method according to claim 6, characterized in that, In step S1, one of the first skeletal precursor and the second skeletal precursor is a polyamine monomer containing low surface energy groups, and the other is a polyaldehyde monomer; in step S2, the catalyst is an acid catalyst.

8. The preparation method according to claim 7, characterized in that, The polyamine monomer containing low surface energy groups includes 2,2-bis(4-aminophenyl)hexafluoropropane, 2,5-diaminotrifluorotoluene, or 1,3,5-tris(2,3,5,6-tetrafluoro)benzene; the polyaldehyde monomer includes pyromellitic methyl methacrylate, 2,4,6-tricarboxymethyl phloroglucinol, terephthalaldehyde, or 2,3,5,6-tetrafluoro-terephthalaldehyde; the acid catalyst includes acetic acid, hydrochloric acid, or trifluoromethanesulfonic acid; and the surfactant includes sodium dodecyl sulfonate.

9. The preparation method according to claim 6, characterized in that, In step S3, the surface tension modifier includes sodium dodecyl sulfate; the zwitterionic modified polyamine includes polyethyleneimine modified with sulfonic acid inner salt, polyethyleneimine modified with carboxylic acid inner salt, or polyethyleneimine modified with trimethylamine N-oxide; the zwitterionic modified polyol includes N-methyldiethanolamine N-oxide or N-benzyldiethanolamine N-oxide; in step S4, the polyacrylamide chloride monomer includes pyromellitic acid trimethylol chloride, isophthaloyl chloride, or 1,3,5-benzenetrisulfonyl chloride.

10. The application of a Janus-structured composite desalination membrane as described in any one of claims 1 to 5 in seawater desalination, brackish water desalination, and saline-alkali water desalination.

Images