A high-flux reverse osmosis composite membrane and a preparation method thereof

Abstract

The application belongs to the technical field of reverse osmosis membrane in water desalination and purification, and discloses a high-flux reverse osmosis composite membrane and a preparation method thereof. The preparation method comprises the following steps: adding a monoamine with a non-planar flexible aliphatic ring structure to a m-phenylenediamine aqueous solution in an interfacial polymerization; on the basis of not changing the rigid main body crosslinking structure of a reverse osmosis membrane separation layer, a non-planar flexible ring structure is introduced, the chain segment arrangement of the membrane material is optimized at a molecular scale, a plurality of sub-nanometer free volumes formed in the membrane material are obtained, and a plurality of water transmission channels are created, so that the water permeation and salt retention of the reverse osmosis membrane are simultaneously improved. The preparation process of the reverse osmosis composite membrane provided by the application is simple, the continuity of the interfacial polymerization process can be maintained, the process is compatible with an industrial process, the cost is low, and the reverse osmosis composite membrane can be used in many fields such as seawater / brackish water desalination, pure water / ultra-pure water preparation, sewage purification and reclaimed water reuse.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of reverse osmosis membrane in water desalination and purification, and particularly relates to a high-flux reverse osmosis composite membrane and a preparation method thereof. BACKGROUND

[0002] As an important membrane separation technology, reverse osmosis technology has been widely used in seawater / brackish water desalination, pure water / ultra-pure water preparation, sewage purification and water reuse, and has become one of the key means to solve the problems of water resource shortage, uneven water distribution and water pollution. A typical reverse osmosis composite membrane is mainly composed of three parts: non-woven fabric, porous support layer and polyamide separation layer. The polyamide separation layer plays a key role in separation, which is prepared by interfacial polymerization of p-phenylenediamine aqueous solution and trimesoyl chloride organic solution on the surface of the porous support layer. Under the action of pressure difference (greater than the osmotic pressure difference on both sides of the membrane), the reverse osmosis membrane selectively permeates water molecules in the raw material liquid and retains other components (including inorganic salt ions, organic molecules, etc.), thereby realizing water desalination and purification.

[0003] Although reverse osmosis membranes have been widely used, their development is still hindered by many factors, among which the lower permeation flux and the trade-off between permeability and rejection rate are major problems that need to be solved in the development of reverse osmosis membranes. Improving the water flux of the membrane while maintaining high salt rejection rate has always been a difficult point and a constantly pursued goal in the research and development of reverse osmosis membranes. Studies (Culp T E, Khara B, Brickey K P, et al. Science, 371:72-75) have shown that in the reverse osmosis separation process, the free volume in the polyamide separation layer provides a diffusion channel for the transmembrane mass transfer of water molecules, and therefore the distribution of free volume is a key factor affecting the water flux of reverse osmosis membranes. The studies also pointed out that an ideal reverse osmosis membrane should contain a large number of and uniformly distributed sub-nanometer free volumes. However, the separation layer of the current reverse osmosis membrane is a fully aromatic polyamide, and the planar rigid benzene ring structure leads to the close packing of polyamide molecular chains, resulting in a low free volume fraction (the proportion of free volume in the total volume of the material) and a lack of sufficient water molecule diffusion channels.

[0004] Adding nanomaterials (such as graphene, MOF, molecular sieve, etc.) to the separation layer is currently the main means to increase the free volume. Nanofillers usually have micropores, and the gap between the nanofillers and the polyamide can serve as an additional water diffusion channel, thereby improving the water flux of the membrane. However, the nanofillers are difficult to disperse uniformly in the separation layer, and the gap between the fillers and the polyamide is prone to evolve into defects, leading to a significant decrease in the salt rejection rate of the reverse osmosis membrane. Therefore, it is still a difficult point in the research and preparation of reverse osmosis membranes to uniformly increase the free volume of the separation layer at the molecular scale and achieve high flux and high rejection.

[0005] The non-planar flexible ring structure can hinder the close packing of polymer chains, which is conducive to the formation of free volume in the membrane material. A typical case is a poly (piperazine amide) nanofiltration membrane. The poly (piperazine amide) nanofiltration membrane is very similar to the polyamide reverse osmosis membrane. They have the same point that they are both prepared by interfacial polymerization between a binary amine and a ternary acid chloride. The difference is that the amine monomer used in the preparation of the reverse osmosis membrane is m-phenylenediamine with a planar rigid structure, and the typical amine monomer used in the preparation of the nanofiltration membrane is piperazine with a non-planar flexible ring structure. Due to the larger free volume fraction, the water flux of the poly (piperazine amide) nanofiltration membrane is much higher than that of the reverse osmosis membrane. On the other hand, the flexible main chain arranged loosely will lead to the free volume size (pore size) in the membrane material being too large, so the separation precision (taking sodium chloride as the interception object) of the nanofiltration membrane is much lower than that of the reverse osmosis membrane. It can be expected that combining the high interception of the reverse osmosis membrane with the high flux of the nanofiltration membrane is an ideal choice for preparing a high-flux reverse osmosis membrane, but directly adding piperazine into the reverse osmosis membrane preparation system using the traditional thinking cannot achieve the expected result. Not only is the membrane flux slightly improved, but the sodium chloride rejection rate also decreases sharply. The reason for the above phenomenon is that the addition of the binary amine monomer piperazine will produce an uncontrollable competitive reaction with m-phenylenediamine. The rigid skeleton structure of the reverse osmosis separation layer is occupied by a large number of flexible ring structures, which changes the main cross-linking structure, so that the rejection rate of monovalent ions by the reverse osmosis membrane cannot be guaranteed.

[0006] Through the above analysis, the problems and defects of the prior art are:

[0007] (1) In the prior art, the water flux and salt rejection rate of the reverse osmosis membrane cannot be improved simultaneously.

[0008] (2) In the prior art, due to the trade-off effect between selectivity and permeability, it is a technical problem in the field to greatly improve the permeation flux of the reverse osmosis membrane while maintaining high salt rejection. The traditional method for improving the flux of the reverse osmosis membrane, such as reducing the cross-linking degree of the polyamide separation layer, reducing the thickness of the separation layer, adding nanofillers, etc., will all lead to a decrease in salt rejection. Since the prior art does not use a monamine monomer reaction strategy to uniformly introduce non-planar flexible ring structures into the rigid main cross-linking structure of the separation layer, more water transport channels are created at the molecular level, and non-selective defects are avoided, so the prior art cannot effectively break through the trade-off effect between selectivity and permeability.

[0009] (3) The prior art does not combine the advantages of traditional reverse osmosis membrane materials and traditional nanofiltration membrane materials, so the reverse osmosis membrane obtained is difficult to have both high flux and high rejection, and the process of improving the flux of the existing reverse osmosis membrane is complicated and costly. SUMMARY

[0010] In order to overcome the problems in the related art, the present disclosure provides a high-flux reverse osmosis membrane and a preparation method.

[0011] The technical solution is as follows: a preparation method of a high-flux reverse osmosis composite membrane comprises the following steps:

[0012] By adding a monamine with a non-planar flexible aliphatic ring structure in the aqueous solution of m-phenylenediamine in interfacial polymerization, the non-planar flexible ring structure is introduced without changing the rigid main body crosslinking structure of the separation layer of the reverse osmosis membrane, the chain segment arrangement of the membrane material is optimized at the molecular scale, the multiple sub-nanometer free volumes formed in the membrane material are obtained, and multiple water transport channels are created, thereby simultaneously improving the water permeation and salt rejection of the reverse osmosis membrane.

[0013] In one embodiment, the monamine comprises one of cyclohexylamine, piperidine, 3-methylpiperidine, and 4-methylpiperidine.

[0014] In one embodiment, the preparation method of the high-flux reverse osmosis membrane comprises the following steps:

[0015] S1, dissolving m-phenylenediamine and a monamine with a non-planar flexible aliphatic ring structure in water and fully stirring to obtain an aqueous solution containing m-phenylenediamine and monamine monomers;

[0016] S2, immersing a porous base membrane in the aqueous solution prepared in step S1 for 1-5 min to uniformly adsorb m-phenylenediamine and the monamine on the surface of the base membrane, and storing the aqueous solution in the pores of the base membrane, and then blowing off or rolling off the residual aqueous solution on the surface of the porous base membrane by a wind knife or a roller;

[0017] S3, dissolving trimesoyl chloride in an organic solvent and fully stirring to obtain a trimesoyl chloride organic phase solution;

[0018] S4, slowly pouring the trimesoyl chloride organic phase solution prepared in step S3 onto the upper surface of the membrane obtained in step S2 to initiate interfacial polymerization, so that m-phenylenediamine and the monamine react with trimesoyl chloride in the organic phase to form a polyamide separation layer containing a rigid crosslinked skeleton of full aromatic and a non-planar flexible ring side chain, and the reaction lasts for 10-120 s;

[0019] S5, performing heat treatment on the membrane obtained in step S4 at 50-80°C for 2-30 min, and then immersing in deionized water to obtain a high-flux reverse osmosis membrane.

[0020] In one embodiment, the monamine with a flexible aliphatic ring structure in step S1 is at least one of cyclohexylamine, piperidine, 3-methylpiperidine, and 4-methylpiperidine, which is determined by a combination of molecular simulation and experiments.

[0021] In one embodiment, the mass concentration of the intermediate phenylenediamine in step S1 is 0.1% to 5% and the mass concentration of the monamine having a flexible aliphatic ring structure is 0.1% to 5% by membrane performance evaluation and formulation optimization.

[0022] In one embodiment, the porous base in step S2 can be one of a polysulfone ultrafiltration membrane, a polyethersulfone ultrafiltration membrane, a polyimide ultrafiltration membrane, a polyacrylonitrile ultrafiltration membrane, a polyethylene ultrafiltration membrane or a polypropylene ultrafiltration membrane.

[0023] In one embodiment, the organic solvent in step S3 is at least one of hexane, cyclohexane, n-heptane, toluene, xylene, isopar G, isopar E, isopar H, isopar L or isopar M.

[0024] The mass concentration of the trimesoyl chloride organic phase solution in step S3 is 0.01% to 2% by membrane performance evaluation and formulation optimization.

[0025] In one embodiment, the soaking time in deionized water in step S5 is 10 to 60 min by membrane performance evaluation and formulation optimization.

[0026] Another object of the present application is to provide a high-flux reverse osmosis composite membrane prepared according to the preparation method of the high-flux reverse osmosis composite membrane, which is essentially a cross-linked polyamide material, different from the traditional reverse osmosis membrane material, the polyamide material proposed in the present application not only contains a rigid wholly aromatic cross-linked skeleton structure, but also contains a non-planar flexible cyclic side chain. The wholly aromatic cross-linked skeleton structure can ensure the compactness and stability of the separation layer, so that the membrane has excellent salt retention performance; the non-planar flexible cyclic side chain with uniform molecular size distribution not only optimizes the arrangement of the molecular skeleton to reduce defect generation, but also generates more sub-nanometer scale free volumes to provide more diffusion channels for water molecules to cross the membrane, thereby realizing the simultaneous improvement of water permeability and salt retention performance.

[0027] In combination with all the above technical solutions, the present application has the following advantages and positive effects:

[0028] First, in view of the technical problems existing in the prior art and the difficulty in solving the problems, the technical solutions of the present application are closely combined with the technical solutions to be protected, the results and data in the research and development process, and the like, and the technical problems solved by the technical solutions are analyzed in detail and deeply. Some creative technical effects brought about after solving the problems are described as follows:

[0029] The application adds monobasic amine (including but not limited to cyclohexylamine, piperidine, 3-methylpiperidine, 4-methylpiperidine) with a non-planar flexible aliphatic ring structure to the m-phenylenediamine aqueous solution polymerized at the interface, without changing the rigid main crosslinking structure of the separation layer of the reverse osmosis membrane, uniformly introducing the non-planar flexible ring structure, optimizing the chain arrangement of the membrane material at the molecular scale, and then uniformly forming more sub-nanometer free volumes in the membrane material, so that the water flux and salt retention rate of the reverse osmosis membrane are simultaneously improved, the water permeation flux of the obtained reverse osmosis membrane reaches 31.3 L·m -2 ·h -1 ·MPa -1 , which is 3 times that of the traditional reverse osmosis membrane (Comparative Example 1), and the sodium chloride retention rate is 99.5%, reaching the highest standard of reverse osmosis membranes-seawater desalination reverse osmosis membranes.

[0030] Due to the trade-off effect between selectivity and permeability, it is a technical problem in the field to greatly improve the permeation flux of the reverse osmosis membrane while maintaining high salt retention. The traditional method for improving the flux of the reverse osmosis membrane, such as reducing the crosslinking degree of the polyamide separation layer, reducing the thickness of the separation layer, adding nanofillers, etc., will all lead to a decrease in salt retention rate. The application uses a monobasic amine monomer reaction strategy to uniformly introduce a non-planar flexible ring structure into the rigid main crosslinking structure of the separation layer, creating more water transport channels at the molecular level and avoiding the formation of non-selective defects, effectively breaking the trade-off effect between selectivity and permeability.

[0031] Secondly, from the perspective of the product as a whole or the technical solution to be protected by the application, the technical effects and advantages of the technical solution to be protected by the application are described as follows:

[0032] The application first proposes a monobasic amine monomer-based reverse osmosis membrane preparation strategy, forming a high-flux reverse osmosis membrane with a “rigidity and flexibility” characteristic, and the separation layer thereof contains a fully aromatic rigid backbone and a non-planar flexible ring side chain. The rigid backbone structure ensures the separation precision of the reverse osmosis membrane, and the flexible side chain optimizes the molecular chain arrangement to form more and uniformly distributed sub-nanometer free volumes, effectively improving the flux of the reverse osmosis membrane. The application realizes the organic combination of the advantages of traditional reverse osmosis membrane materials and traditional nanofiltration membrane materials, and obtains a reverse osmosis membrane with high flux and high retention, and the preparation method is simple, does not need to change the existing reverse osmosis membrane preparation process, is low in cost, and is good in compatibility.

[0033] Compared with the prior art, the advantages of the application further include:

[0034] (1) In this invention, a monoamine with a non-planar flexible aliphatic ring structure is added to a traditional interfacial polymerization system (aqueous phase: m-phenylenediamine solution, oil phase: trimesoyl chloride solution). The monoamine can participate in the interfacial polymerization process and react with the acyl chloride to form amide bonds. However, since the monoamine has only one reactive functional group, it forms a capped end after the reaction. Therefore, the participation of the monoamine does not change the cross-linked backbone structure formed by the acyl chloride and m-phenylenediamine, but only serves as a side link into the main backbone structure. The non-planar flexible aliphatic ring structure optimizes the molecular chain arrangement of the membrane material, giving the separation layer more and more uniform free volume and increasing water diffusion channels. At the same time, compared with the traditional method of adding nanofillers, the introduction of the flexible ring structure in this invention is achieved at the molecular scale and does not produce non-selective defects, thereby achieving a double increase in water flux and salt rejection rate. Taking cyclohexylamine as an example, the reaction process of the mixed aqueous solution of m-phenylenediamine and cyclohexylamine with the organic phase solution of trimesoyl chloride, as well as the three-dimensional diagram and the diagram of the planar and flexible non-planar structures during cross-linking are shown in the figure. Figure 2 As shown.

[0035] (2) This invention introduces a monoamine with a non-planar flexible aliphatic ring structure into the reverse osmosis membrane preparation process. Without changing the rigid main cross-linked structure of the reverse osmosis membrane, a non-planar flexible ring structure is uniformly introduced to optimize the molecular chain arrangement of the membrane material, form more sub-nanometer free volumes, and make the free volume distribution more uniform, thereby achieving a simultaneous increase in reverse osmosis membrane permeate flux and salt rejection rate, breaking the constraint effect between permeability and rejection rate.

[0036] (3) The reverse osmosis composite membrane provided by this invention has a simple preparation process, can maintain the continuity of the interfacial polymerization process, has good compatibility with industrial processes, and is inexpensive. The prepared reverse osmosis membrane maintains a NaCl rejection rate of over 99.5% and a flux of up to 31.3 L·m -2 ·h -1 ·MPa -1 (Under the same NaCl rejection rate, the flux of a traditional reverse osmosis membrane is only 10.0 L·m -2 ·h -1 ·MPa -1 It can be used in many fields such as seawater / brackish water desalination, pure water / ultrapure water preparation, sewage purification and reclaimed water reuse, and has broad industrial application prospects. Attached Figure Description

[0037] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure.

[0038] Figure 1 This is a flowchart of the preparation method of the high-flux reverse osmosis membrane provided in the embodiments of the present invention;

[0039] Figure 2 This invention provides a three-dimensional diagram and a diagram of planar and flexible non-planar structures during the reaction process of a mixed aqueous solution of m-phenylenediamine and cyclohexylamine with an organic phase solution of pyromellitic trimethylolpropionate (PMT).

[0040] Figure 3 This is an electron microscope image of the reverse osmosis composite membrane prepared in Comparative Example 1 provided in this embodiment of the invention. Figure 3 (a) and (b) are electron microscope images of the film surface at different magnifications, respectively.

[0041] Figure 4 These are electron microscope images of the reverse osmosis composite membranes prepared in Examples 1-4 of this invention. Figure 4 (a) and (b) are electron microscope images of the film surface in Examples 1-4 at different magnifications;

[0042] like Figure 5 The figure shown is a three-dimensional view of the free volume distribution of the polyamide network in Comparative Example 1, Comparative Example 3, and Examples 1-4, wherein, Figure 5 In the figures, a represents the free volume distribution of the polyamide network in Comparative Example 1; b represents the free volume distribution of the polyamide networks in Examples 1-4; c represents the free volume distribution of the polyamide network formed by adding aniline in Comparative Example 3; and d represents the free volume distribution of the polyamide network formed by adding n-hexylamine in Comparative Example 3. Detailed Implementation

[0043] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0044] I. Explanation of the Implementation Example:

[0045] This invention provides a method for preparing a high-flux reverse osmosis composite membrane, comprising the following steps:

[0046] By adding a monoamine with a non-planar flexible aliphatic ring structure to the aqueous solution of m-phenylenediamine polymerized at the interface, and without changing the rigid main cross-linking structure of the reverse osmosis membrane separation layer, a non-planar flexible ring structure is introduced to optimize the chain segment arrangement of the membrane material at the molecular scale, thereby obtaining multiple sub-nanometer free volumes formed within the membrane material and creating multiple water transport channels to simultaneously carry out water permeation and salt retention of the reverse osmosis membrane.

[0047] Specifically, such asFigure 1 As shown, the high-flux reverse osmosis membrane and its preparation method provided in this embodiment of the invention include the following steps:

[0048] S101, dissolve m-phenylenediamine and a monoamine with a non-planar flexible aliphatic ring structure in water and stir thoroughly to obtain an aqueous solution containing both m-phenylenediamine and monoamine monomers.

[0049] S102, the porous base membrane is immersed in the aqueous solution prepared in step S101 for 1 to 5 minutes to allow the m-phenylenediamine and monoamine to be uniformly adsorbed on the surface of the base membrane, while the aqueous solution is stored inside the pores of the base membrane. Then the remaining aqueous solution on the surface of the porous base membrane is dried by air knife or roller.

[0050] S103, dissolve pyromellitic chloride in an organic solvent and stir thoroughly to obtain a pyromellitic chloride organic phase solution;

[0051] S104, the pyromellitic trimethylolpropionic acid chloride organic phase solution prepared in step S103 is slowly poured onto the surface of the membrane obtained in step S102 to initiate an interfacial polymerization reaction, which lasts for 10-120 seconds. During this process, m-phenylenediamine and monoamine monomers diffuse into the organic phase and then react with pyromellitic trimethylolpropionic acid chloride in the organic phase. The difference is that the reaction of di-m-phenylenediamine and ternary pyromellitic trimethylolpropionic acid chloride will form a cross-linked polyamide network, while the monoamine will form a capped end after the reaction. Therefore, it can only be grafted onto the cross-linked backbone in the form of side chains to form a polyamide separation layer containing a rigid cross-linked backbone with full aromatics and a non-planar flexible cyclic side chain.

[0052] S105, the membrane obtained in step S104 is heat-treated at 50-80°C for 2-30 minutes, and then soaked in deionized water to obtain the final polyamide reverse osmosis composite membrane.

[0053] Furthermore, by combining molecular simulation and experimentation, it is determined that the monoamine with a flexible aliphatic ring structure in step S101 is preferably at least one of cyclohexylamine, piperidine, 3-methylpiperidine, and 4-methylpiperidine.

[0054] Furthermore, through membrane performance evaluation and formulation optimization, the mass concentration of intermediate phenylenediamine in step S101 was determined to be 0.1%–5%, and the mass concentration of the monoamine with a flexible aliphatic ring structure was determined to be 0.1%–5%.

[0055] Furthermore, numerous experimental tests have shown that the method proposed in this invention is applicable to various porous membranes. In step S102, the porous membrane can be one of polysulfone ultrafiltration membrane, polyethersulfone ultrafiltration membrane, polyimide ultrafiltration membrane, polyacrylonitrile ultrafiltration membrane, polyethylene ultrafiltration membrane, or polypropylene ultrafiltration membrane.

[0056] Further, in step S103, the organic solvent is at least one of hexane, cyclohexane, n-heptane, toluene, xylene, isopar G, isopar E, isopar H, isopar L, or isopar M.

[0057] Furthermore, through membrane performance evaluation and formulation optimization, the mass concentration of the pyromellitic methyl chloride organic phase solution in step S103) was determined to be 0.01% to 2%.

[0058] Furthermore, through membrane performance evaluation and formulation optimization, the soaking time in deionized water in step S105 was determined to be 10–60 min.

[0059] Through the above embodiments, this invention adds a monoamine with a non-planar flexible aliphatic ring structure to a traditional interfacial polymerization system (aqueous phase: m-phenylenediamine solution, oil phase: trimesoyl chloride solution). The monoamine can participate in the interfacial polymerization process, reacting with the acyl chloride to form amide bonds. However, since the monoamine has only one reactive functional group, it forms a capped end after the reaction. Therefore, the participation of the monoamine does not change the cross-linked backbone structure formed by the acyl chloride and m-phenylenediamine; it only serves as a side link to the main backbone structure. The non-planar flexible aliphatic ring structure optimizes the molecular chain arrangement of the membrane material, giving the separation layer more and more uniform free volume, increasing water diffusion channels. Simultaneously, compared with the traditional method of adding nanofillers, the introduction of the flexible ring structure in this invention is achieved at the molecular scale and does not produce non-selective defects, thereby achieving a dual increase in water flux and salt rejection rate. Taking cyclohexylamine as an example, the reaction process of the mixed aqueous solution of m-phenylenediamine and cyclohexylamine with the organic phase solution of trimesoyl chloride, as well as the three-dimensional diagram and the diagrams of the planar and flexible non-planar structures during cross-linking are shown below. Figure 2 As shown, high-flux reverse osmosis composite membranes are essentially cross-linked polyamide materials. Unlike traditional reverse osmosis membrane materials, they not only contain a rigid, fully aromatic cross-linked backbone structure, but also non-planar, flexible cyclic side chains.

[0060] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0061] II. Evidence of the relevant effects of the embodiments:

[0062] Experimental conditions: In the following examples and control examples, the prepared reverse osmosis composite membrane was also tested. Specifically, after pre-pressurization for 2 hours at 1.5 MPa, 25°C, and 7 LPM, its rejection rate for 2000 mg / L sodium chloride and its permeate flux were tested. The permeate flux of the composite membrane was measured in L·m³. -2 ·h -1 ·MPa -1 .

[0063] Comparative Example 1: No monoamine was added; only m-phenylenediamine and trimesoyl chloride were used in the reaction.

[0064] This comparative example 1 provides a method for preparing a reverse osmosis composite membrane, comprising the following steps:

[0065] (1) Dissolve m-phenylenediamine in water to obtain an aqueous solution with a mass concentration of 2%;

[0066] (2) Immerse the polysulfone ultrafiltration membrane in the aqueous solution prepared in step (1), soak for 5 minutes and remove it. The residual aqueous solution on the membrane surface is dried by air knife.

[0067] (3) Dissolve pyromellitic chloride in n-hexane to obtain an organic phase solution of pyromellitic chloride with a mass fraction of 0.1%;

[0068] (4) Slowly pour the organic phase solution obtained in step (3) onto the upper surface of the membrane obtained in step (2) to initiate the interfacial polymerization reaction, continue for 60s to form a cross-linked polyamide separation layer, and pour off the remaining pyromellitic trimethylol chloride organic phase solution from above the membrane.

[0069] (5) The membrane obtained in step (4) is heat-treated at 60°C for 2 minutes, and then soaked in deionized water for 30 minutes to obtain a polyamide reverse osmosis composite membrane.

[0070] Analysis: The permeate flux of the obtained polyamide reverse osmosis membrane is 10.2 L·m⁻¹. -2 ·h -1 ·MPa -1 The retention rate for sodium chloride is 98%.

[0071] Comparative Example 2: Adding a non-planar flexible aliphatic ring structure to the preparation of reverse osmosis membranes Diamine (piperazine and cyclohexanediamine)

[0072] Piperazine molecular formula;

[0073]

[0074] The molecular formula of cyclohexanediamine;

[0075]

[0076] (1) Dissolve m-phenylenediamine and piperazine or cyclohexanediamine in water to obtain an aqueous solution, wherein the mass concentration of m-phenylenediamine is 2% and the mass concentration of piperazine or cyclohexanediamine is 1% respectively; the remaining steps are the same as those in the previous comparative example.

[0077] Water production flux (piperazine as additive): 9.9 L·m -2 ·h -1 ·MPa -1The retention rate for sodium chloride was 48.4%.

[0078] Water production flux (with cyclohexanediamine as additive): 20.5 L·m -2 ·h -1 ·MPa -1 The retention rate for sodium chloride is 70.5%.

[0079] Analysis: Compared with Control Example 1, the salt rejection performance of both reverse osmosis composite membranes in Control Example 2 decreased. Analysis revealed that the diamino groups of piperazine or cyclohexanediamine caused uncontrollable competition with the intermediate-phenylenediamine in the reverse osmosis system, altering the main cross-linking structure and forming larger micropores. This resulted in an inability to guarantee the rejection rate of monovalent ions by the reverse osmosis membrane.

[0080] Compare with Example 3: Add with Planar rigid aromatic ring Monoamines (aniline) with a flexible structure Fatty chain-like structure Name Monoamine (n-hexylamine)

[0081] The molecular formula of aniline is:

[0082]

[0083] Hexylamine molecular formula:

[0084]

[0085] (1) Dissolve m-phenylenediamine and aniline or n-hexylamine in water to obtain an aqueous solution, wherein the mass concentration of m-phenylenediamine is 2% and the mass concentration of piperazine or cyclohexylenediamine is 1% respectively; the remaining steps are the same as in the previous comparative example.

[0086] Water production flux (with aniline as an additive): 9.2 L·m -2 ·h -1 ·MPa -1 The retention rate for sodium chloride was 92.7%.

[0087] Water production flux (with cyclohexanediamine as additive): 20.0 L·m -2 ·h - ·MPa -1 The retention rate for sodium chloride was 85.3%.

[0088] Analysis: Compared to Control Example 2, the salt rejection performance of the two reverse osmosis composite membranes decreased less when aniline and n-hexylamine with monoamino functional groups were used as additives. This is because the monoamino functional group does not replace the main cross-linking structure formed by trimesoyl chloride and m-phenylenediamine, thus resulting in a smaller decrease in the sodium chloride rejection rate. However, the planar aromatic ring structure cannot increase the additional free volume, thus failing to improve the permeate flux of the reverse osmosis membrane; while the aliphatic chain structure of n-hexylamine improved the water flux, it could not guarantee the sodium chloride rejection rate.

[0089] Example 1: Addition of a monoamine (cyclohexylamine) with a non-planar flexible aliphatic ring structure

[0090] Cyclohexylamine molecular formula:

[0091]

[0092] This embodiment 1 provides a method for preparing a high-flux reverse osmosis composite membrane, comprising the following steps:

[0093] (1) Dissolve m-phenylenediamine and cyclohexylamine in water to obtain an aqueous solution, wherein the mass concentration of m-phenylenediamine is 2% and the mass concentrations of cyclohexylamine are 0.5%, 1%, 2%, 3%, and 4%, respectively;

[0094] (2) Immerse the polysulfone ultrafiltration membrane in the aqueous solution prepared in step (1), soak for 5 minutes and remove it. The residual aqueous solution on the membrane surface is dried by air knife.

[0095] (3) Dissolve pyromellitic chloride in n-hexane to obtain an organic phase solution of pyromellitic chloride with a mass fraction of 0.1%;

[0096] (4) Slowly pour the organic phase solution obtained in step (3) onto the upper surface of the membrane obtained in step (2) to initiate the interfacial polymerization reaction, continue for 60s to form a cross-linked polyamide separation layer, and pour off the remaining pyromellitic trimethylol chloride organic phase solution from above the membrane.

[0097] (5) The membrane obtained in step (4) is heat-treated at 60°C for 2 minutes, and then soaked in deionized water for 30 minutes to obtain a polyamide reverse osmosis composite membrane.

[0098] The separation performance of a series of reverse osmosis composite membranes prepared by the above method was tested, and they were recorded as Examples 1-1 to 1-5 according to the mass concentrations of cyclohexylamine of 0.5%, 1%, 2%, 3%, and 4%, respectively. The test results are shown in Table 1.

[0099] Table 1. Separation performance test results of Example 1 and Control Example 1

[0100] Cyclohexylamine concentration (%) Water production flux LMH Sodium chloride rejection (%) Comparative Example 1 Example 1-1 0 10.2 98.0 Example 1-2 0.5 20.4 98.8 Example 1-3 1.0 31.3 99.5 Example 1-4 2.0 28.2 99.2 Example 1-5 3.0 37.7 97.7 Figure 3 4.0 27.4 93.4

[0101] From the test results of Example 1 and Control Example 1 in Table 1 above, it can be seen that the permeate flux and sodium chloride rejection rate of the reverse osmosis composite membrane prepared after adding cyclohexylamine to the aqueous solution are significantly improved; regarding salt rejection performance, when the amount of cyclohexylamine added is no more than 2%, the sodium chloride rejection rate of the reverse osmosis membrane is maintained above 98.8%, and when the amount added is 1%, the flux of the prepared reverse osmosis composite membrane can reach 31.3 L·m -2 ·h -1 ·MPa -1 The sodium chloride retention rate was maintained at 99.5%.

[0102] Figure 4 This is an electron microscope image of the reverse osmosis composite membrane prepared in Comparative Example 1 provided in this embodiment of the invention;

[0103] Figure 3 These are electron microscope images of the membrane surface of the reverse osmosis composite membranes prepared in Examples 1-4 of this invention; wherein, Figure 4 Images (a) and (b) are electron microscope images of the reverse osmosis composite membrane prepared in Comparative Example 1 at different magnifications. Figure 3 Images (a) and (b) are electron microscope images of the reverse osmosis composite membranes prepared in Examples 1-4 at different magnifications, for comparison. Figure 4 and Figure 5 It can be seen that the addition of cyclohexylamine increases the number of leaf-like structures on the membrane surface, thereby increasing the membrane filtration area.

[0104] like Figure 5 The figure shown is a three-dimensional view of the free volume distribution of the polyamide network in Comparative Example 1, Comparative Example 3, and Examples 1-4, wherein, Figure 5 In the figures, a represents the free volume distribution of the polyamide network in Comparative Example 1; b represents the free volume distribution of the polyamide networks in Examples 1-4; c represents the free volume distribution of the polyamide network formed by adding aniline in Comparative Example 3; and d represents the free volume distribution of the polyamide network formed by adding n-hexylamine in Comparative Example 3.

[0105] from ​ As can be seen, compared with the conventional reverse osmosis membrane material of the control example, the addition of cyclohexylamine with a non-planar flexible aliphatic ring structure increases the free volume fraction of the polyamide membrane material from 10.32% to 11.03%, providing more diffusion channels for water molecules to transfer across the membrane. On the other hand, the addition of aniline with a planar structure reduces the free volume fraction of the polyamide membrane material to 8.61%, while the addition of n-hexylamine with a chain structure has almost no effect on the free volume fraction of the membrane material.

[0106] Example 2:

[0107] Piperidine molecular formula:

[0108]

[0109] This embodiment 2 provides a method for preparing a high-flux reverse osmosis composite membrane, including the following steps:

[0110] (1) Dissolve m-phenylenediamine and piperidine in water to obtain an aqueous solution, wherein the mass concentration of m-phenylenediamine is 2% and the mass concentration of piperidine is 1.5%;

[0111] (3) Dissolve pyromellitic chloride in cyclohexane to obtain an organic phase solution of pyromellitic chloride with a mass fraction of 0.15%;

[0112] (4) Slowly pour the organic phase solution obtained in step (3) onto the upper surface of the membrane obtained in step (2) to initiate the interfacial polymerization reaction, continue for 30 seconds to form a cross-linked polyamide separation layer, and pour off the remaining pyromellitic trimethylol chloride organic phase solution from above the membrane.

[0113] The remaining steps remain unchanged (Example 1).

[0114] Results: The reverse osmosis composite membrane prepared by the above method was tested for separation performance. Its permeate flux was 21.2 LMH and its sodium chloride rejection rate was 99.3%.

[0115] Example 3:

[0116] 3-Methylpiperidine molecular formula:

[0117]

[0118] This embodiment 3 provides a method for preparing a high-flux reverse osmosis composite membrane, comprising the following steps:

[0119] (1) Dissolve m-phenylenediamine and 3-methylpiperidine in water to obtain an aqueous solution, wherein the mass concentration of m-phenylenediamine is 2% and the mass concentration of 3-methylpiperidine is 1%.

[0120] (3) Dissolve pyromellitic chloride in n-hexane to obtain an organic phase solution of pyromellitic chloride with a mass fraction of 0.15%;

[0121] (4) Slowly pour the organic phase solution obtained in step (3) onto the upper surface of the membrane obtained in step (2) to initiate the interfacial polymerization reaction, continue for 90s to form a cross-linked polyamide separation layer, and pour off the remaining pyromellitic trimethylol chloride organic phase solution from above the membrane.

[0122] The remaining steps remain unchanged (Example 1).

[0123] Results: The reverse osmosis composite membrane prepared by the above method was tested for separation performance. Its permeate flux was 20.2 LMH and its sodium chloride rejection rate was 99.5%.

[0124] Example 4:

[0125] 4-Methylpiperidine molecular formula:

[0126]

[0127] This embodiment 4 provides a method for preparing a high-flux reverse osmosis composite membrane, comprising the following steps:

[0128] (1) Dissolve m-phenylenediamine and 4-methylpiperidine in water to obtain an aqueous solution, wherein the mass concentration of m-phenylenediamine is 2% and the mass concentration of 4-methylpiperidine is 1%.

[0129] (3) Dissolve trimesoyl chloride in n-hexane to obtain a 0.15% (w / w) organic phase solution of trimesoyl chloride.

[0130] (5) The membrane obtained in step (4) is heat-treated at 60°C for 4 minutes, and then soaked in deionized water for 30 minutes to obtain a polyamide reverse osmosis composite membrane.

[0131] The remaining steps remain unchanged (Example 1).

[0132] Results: The reverse osmosis composite membrane prepared by the above method was tested for separation performance. Its permeate flux was 21.7 LMH and its sodium chloride rejection rate was 99.4%.

[0133] Example 5:

[0134] This embodiment 5 provides a method for preparing a high-flux reverse osmosis composite membrane, comprising the following steps:

[0135] (1) Dissolve m-phenylenediamine, cyclohexylamine, and piperidine in water to obtain an aqueous solution, wherein the mass concentration of m-phenylenediamine is 2.0%, the mass concentration of cyclohexylamine is 1%, and the mass concentration of piperidine is 0.5%.

[0136] (3) Dissolve pyromellitic chloride in isopar E to obtain an organic phase solution with a mass fraction of 2.0%; the remaining steps remain unchanged (Example 1).

[0137] Results: The reverse osmosis composite membrane prepared by the above method was tested for separation performance. Its permeate flux was 24.7 LMH and its sodium chloride rejection rate was 99.2%.

[0138] Example 6:

[0139] This embodiment 5 provides a method for preparing a high-flux reverse osmosis composite membrane, comprising the following steps:

[0140] (1) Dissolve m-phenylenediamine, cyclohexylamine, and piperidine in water to obtain an aqueous solution, wherein the mass concentration of m-phenylenediamine is 2.0%, the mass concentration of cyclohexylamine is 1%, and the mass concentration of piperidine is 0.5%.

[0141] (2) Immerse the polyimide ultrafiltration membrane in the aqueous solution prepared in step (1), soak for 5 minutes and remove it. The residual aqueous solution on the membrane surface is dried by rollers.

[0142] The remaining steps remain unchanged (Example 1).

[0143] Results: The separation performance of the reverse osmosis composite membrane prepared by the above method was tested, and its permeate flux was 25.7 LMH, and the sodium chloride rejection rate was 99.1%.

[0144] The parts not mentioned above can be achieved by drawing on existing technologies.

[0145] Experimental results show that:

[0146] This invention introduces a non-planar flexible aliphatic ring structure into an aqueous solution of m-phenylenediamine undergoing interfacial polymerization (including but not limited to cyclohexylamine, piperidine, 3-methylpiperidine, and 4-methylpiperidine). This uniformly introduces a non-planar flexible ring structure without altering the rigid cross-linked structure of the reverse osmosis membrane separation layer. This optimizes the chain segment arrangement of the membrane material at the molecular scale, resulting in a greater uniform formation of sub-nanometer free volume within the membrane material. Consequently, it simultaneously improves the permeate flux and salt rejection rate of the reverse osmosis membrane, achieving a water permeate flux of 31.3 L·m⁻¹. -2 ·h -1 ·MPa -1 It is 3 times that of traditional reverse osmosis membranes (Comparative Example 1), and its sodium chloride rejection rate is 99.5%, reaching the highest standard of reverse osmosis membranes—the standard of seawater desalination reverse osmosis membranes.

[0147] Due to the trade-off effect between selectivity and permeability, significantly increasing the permeation flux of reverse osmosis membranes while maintaining high salt rejection remains a technical challenge in this field. Traditional methods for increasing reverse osmosis membrane flux, such as reducing the crosslinking degree of the polyamide separation layer, reducing the separation layer thickness, and adding nanofillers, all lead to a decrease in salt rejection. This invention utilizes a monoamine monomer reaction strategy to uniformly introduce a non-planar flexible ring structure into the rigid crosslinked structure of the separation layer, creating more water transport channels at the molecular level, avoiding the formation of non-selective defects, and effectively overcoming the trade-off effect between selectivity and permeability.

[0148] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention and within the spirit and principles of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a high-flux reverse osmosis composite membrane, characterized in that, This preparation method involves adding a monoamine with a non-planar flexible aliphatic ring structure to an aqueous solution of m-phenylenediamine that is polymerized at the interface. Without changing the rigid main cross-linking structure of the reverse osmosis membrane separation layer, a non-planar flexible ring structure is introduced to optimize the chain segment arrangement of the membrane material at the molecular scale, thereby obtaining a multi-sub-nanometer free volume formed in the membrane material and creating multiple water transport channels, which simultaneously improves the water permeability and salt rejection of the reverse osmosis membrane. The method for preparing the reverse osmosis membrane includes the following steps: S1, dissolve m-phenylenediamine and a monoamine with a non-planar flexible aliphatic ring structure in water and stir thoroughly to obtain an aqueous solution containing both m-phenylenediamine and monoamine monomers. S2, immerse the porous base membrane in the aqueous solution prepared in step S1 for 1 min to 5 min, so that m-phenylenediamine and monoamine are uniformly adsorbed on the surface of the base membrane, while the aqueous solution is stored inside the pores of the base membrane. Then, dry the remaining aqueous solution on the surface of the porous base membrane by blowing with an air knife or rolling with rollers. S3, dissolve pyromellitic chloride in an organic solvent and stir thoroughly to obtain a pyromellitic chloride organic phase solution; S4. The pyromellitic trimethylolpropionate organic phase solution prepared in step S3 is slowly poured onto the upper surface of the membrane obtained in step S2 to initiate interfacial polymerization, so that the m-phenylenediamine and monoamine react with the pyromellitic trimethylolpropionate in the organic phase for 10s to 120s to form a polyamide separation layer containing a rigid cross-linked backbone with full aromatics and a flexible cyclic side chain without a planar structure. S5. The membrane obtained in step S4 is heat-treated at 50℃~80℃ for 2min~30min, and then soaked in deionized water to obtain a high-flux reverse osmosis membrane. In step S1, the flexible aliphatic ring structure is determined by a combination of molecular simulation and experimental methods, selecting at least one of piperidine, 3-methylpiperidine, and 4-methylpiperidine. The mass concentration of intermediate phenylenediamine in step S1 was determined to be 0.1%–5% through membrane performance evaluation and formulation optimization, and the mass concentration of monoamine with flexible aliphatic ring structure was determined to be 0.1%–5%.

2. The method for preparing the high-flux reverse osmosis composite membrane according to claim 1, characterized in that, In step S2, the porous base is one of polysulfone ultrafiltration membrane, polyethersulfone ultrafiltration membrane, polyimide ultrafiltration membrane, polyacrylonitrile ultrafiltration membrane, polyethylene ultrafiltration membrane, or polypropylene ultrafiltration membrane.

3. The method for preparing the high-flux reverse osmosis composite membrane according to claim 1, characterized in that, In step S3, the organic solvent is at least one of hexane, cyclohexane, n-heptane, toluene, xylene, isopar G, isopar E, isopar H, isopar L, or isopar M. The mass concentration of the pyromellitic methyl chloride organic phase solution in step S3 was determined to be 0.01%–2% through membrane performance evaluation and formulation optimization.

4. The method for preparing the high-flux reverse osmosis composite membrane according to claim 1, characterized in that, The soaking time in deionized water in step S5 was determined to be 10 min to 60 min through membrane performance evaluation and formulation optimization.

5. A high-flux reverse osmosis composite membrane prepared by the preparation method of the high-flux reverse osmosis composite membrane according to any one of claims 1-4, wherein the high-flux reverse osmosis composite membrane is made of cross-linked polyamide material.

6. The high-flux reverse osmosis composite membrane according to claim 5, characterized in that, This high-flux reverse osmosis composite membrane contains a rigid, fully aromatic cross-linked backbone structure and non-planar, flexible cyclic side chains.

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