Preparation method of nanofiltration membrane with gradient distribution of structure density, product and use thereof

Abstract

The application relates to a preparation method of a nanofiltration membrane with gradient distribution of structural density, a product and an application thereof, and belongs to the field of membrane separation technology. The application is designed based on the multilevel structure of a polymer, a planar amine / non-planar electrolyte amine composite monomer is designed, and a diffusion rate of the composite monomer pair is adjusted by adding a diffusion promoting / inhibiting agent; the nanofiltration membrane with gradient distribution of structural density is constructed by fine regulation of the diffusion rate between the composite monomers and the difference of intrinsic structures. The nanofiltration membrane with gradient distribution of structural density prepared by the above method has the advantages of large water flux, high ion screening precision and strong long-term service stability.

Description

Technical Field

[0001] This invention belongs to the field of nanofiltration membrane technology, specifically a method for preparing nanofiltration membranes with a structural density gradient distribution, as well as the product and its applications. Background Technology

[0002] Material separation and purification are essential processes in industrial production, and membrane separation technology, which can further reduce energy consumption in separation processes, has become a research hotspot in recent years. Nanofiltration membranes, a type of separation membrane between ultrafiltration and reverse osmosis, can effectively retain solutes in the 200-1000 Da range, exhibiting high retention rates and low operating pressures, and can be used in water treatment, food processing, and pharmaceutical industries. The performance of nanofiltration membranes mainly depends on the structure and properties of their active layer, which is typically composed of polyamide and formed on a porous support layer via interfacial polymerization. Interfacial polymerization is a simple, efficient, and controllable membrane fabrication method that utilizes the polymerization reaction of two immiscible monomer solutions at the interface of a porous support layer to form an ultrathin polymer membrane. Nanofiltration membranes prepared by interfacial polymerization possess good mechanical strength, chemical stability, and separation performance, but also have some drawbacks, such as a rough active layer surface, wide pore size distribution, and low charge density. These drawbacks can affect the retention selectivity and water flux of the nanofiltration membrane.

[0003] To improve the performance of nanofiltration membranes, several methods have been proposed, such as changing monomer concentration, adding diffusion modifiers, adjusting pH, and controlling reaction time. Among these, adding diffusion modifiers is a commonly used method, which can adjust the separation performance of nanofiltration membranes by altering the structure and properties of the active layer. For example, adding dendritic polyamine compounds can increase the crosslinking density and charge density of the active layer, improving the rejection rate and antifouling ability of the nanofiltration membrane, and enhancing its selectivity for cations. However, existing additive methods also have some drawbacks, such as difficulty in controlling additive concentration, incomplete reaction between additives and monomers, and damage to the porous support layer by the additives. Summary of the Invention

[0004] In view of the above-mentioned problems in the prior art, the purpose of this invention is to design and provide a technical solution for preparing a nanofiltration membrane with a structural density gradient distribution, as well as its products and applications. This method can effectively improve the structure and properties of the active layer and enhance the ion selectivity and water flux of the nanofiltration membrane.

[0005] The method for preparing a nanofiltration membrane with a structural density gradient distribution is characterized by comprising the following steps:

[0006] 1) Prepare non-planar electrolyte amine monomer A with a stereostructure by quaternization or sulfonation;

[0007] 2) A polymer porous membrane is sequentially contacted with a first-phase solution and a second-phase solution to form a functional layer. The first-phase solution consists of a complex amine monomer, a diffusion regulator, and a solvent. The complex amine monomer is a non-planar electrolyte amine monomer A with a rigid three-dimensional structure and a planar amine monomer B. The content of non-planar electrolyte amine monomer A in the first-phase solution is 0.1-2.9 wt%, the content of planar amine monomer B is 0.1-2.9 wt%, the content of diffusion regulator is 0.05-1.5 wt%, and the balance is solvent. By designing and screening the complex amine monomer and combining it with the diffusion regulator, the intrinsic structure and diffusion rate of the complex amine monomer are finely controlled to synergistically construct a nanofiltration membrane with a structural density gradient distribution. Figure 1 );

[0008] The second phase solution consists of acyl chloride compounds and alkane solvents, and is post-processed to obtain a nanofiltration membrane.

[0009] The method for preparing a nanofiltration membrane with a structural density gradient distribution is characterized in that, in step 2), the non-planar electrolyte amine monomer A is at least one of a quaternary ammonium salt or sulfonate containing an amino-terminal group and having an aromatic ring as the main structure with a stereostructure.

[0010] The method for preparing a nanofiltration membrane with a structural density gradient distribution is characterized in that, in step 2), the non-planar electrolyte amine monomer A is obtained by quaternization or sulfonation treatment of a non-planar polyamine (monomer A precursor) with a three-dimensional structure.

[0011] The method for preparing a nanofiltration membrane with a structural density gradient distribution is characterized in that the quaternization or sulfonation treatment is carried out by: using a quaternization reagent or its solution to perform a quaternization reaction, wherein the quaternization reagent is at least one of 2,3-epoxypropyltrimethylammonium chloride, benzyl chloride, iodomethane, and iodoethane; and using a sulfonation reagent or its solution to perform a sulfonation reaction, wherein the sulfonation reagent is at least one of 1,3-propanesulfonic acid lactone, dimethyl sulfate, and chlorosulfonic acid.

[0012] The method for preparing a nanofiltration membrane with a structural density gradient distribution is characterized in that: the non-planar polyamine with a stereostructure (non-planar electrolyte amine monomer A precursor) is at least one of 2,6,14-triaminotriptene, 2,3,6,7,14,15-hexaaminotriptene, tetra(4-aminophenyl)methane, 5,5',6,6'-tetraamino-3,3,3',3'-tetramethyl-1,1'-spirobisindane, and 9,9'-spirobis[fluorene]-2,2',7,7'-tetraamine.

[0013] The method for preparing a nanofiltration membrane with a structural density gradient distribution is characterized in that, in step 2), the planar amine monomer B is at least one of ethylenediamine, diethylenetriamine, 1,2,3-propanetriamine, p-phenylenediamine, and m-phenylenediamine.

[0014] The method for preparing a nanofiltration membrane with a structural density gradient distribution is characterized in that, in step 2), the diffusion regulator is at least one of polycarboxylate betaine methacrylate, sulfonic acid polyethyleneimine, sodium dodecyl sulfonate, and polyvinyl alcohol.

[0015] The nanofiltration membrane prepared by this invention includes a nonwoven fabric layer and an ultrafiltration membrane support layer. The ultrafiltration membrane support layer is characterized by sequentially contacting a first phase solution and a second phase solution to form a polyamide separation layer. The polyamide separation layer is a polyamide separation layer with a gradient structural density distribution, comprising a high structural density layer far from the support layer and a low structural density layer close to the support layer. The average pore size of the low structural density layer is such that the proportion of pores larger than the average pore size to the total number of pores is less than or equal to 10%.

[0016] The nanofiltration membrane prepared by the present invention is characterized in that: the thickness ratio of the high structure density layer to the low structure density layer is greater than or equal to 0.1 and less than or equal to 9; the average pore size of the high structure density layer is greater than or equal to 0.5 nm and less than or equal to 0.8 nm.

[0017] Application of the nanofiltration membrane prepared by this invention in precise ion separation.

[0018] This invention, through extensive experimentation, reveals that the ideal structural characteristics of a high-precision ion separation membrane are: 1) sub-nanometer-sized pore channels with narrow pore size distribution to ensure concentration gradient differences between monovalent and multivalent ions at angstrom-level precision; 2) a stable pore channel structure to resist the influence of external factors on the polymer structure; 3) high porosity and suitable pore length to mitigate the decrease in the diffusion rate of monovalent ions caused by the close packing and cross-linking of polymer chains; and 4) specific charge properties to provide additional Donnan repulsion effects. The multi-level structure of the polymer (short-range structure, long-range structure, and aggregated structure) collectively determines the precise ion separation performance of the polymer membrane. The short-range structure is the chemical structure of a single molecule, affecting the hydrophilicity / phobicity and charge-carrying properties of the membrane material. The long-range structure includes the flexibility and rigidity of a single molecule, its morphology and size, and specific conformations, affecting the intramolecular pore size and distribution of the membrane material and the structural stability of the polymer chains.

[0019] The aforementioned method for preparing a nanofiltration membrane with a gradient structural density distribution is the first to propose a multi-level polymer structure design. It utilizes planar / non-planar composite amine monomers and employs interfacial polymerization to construct a nanofiltration membrane with sub-nanometer ion-sieving channels possessing specific charges. This enhances pore size sieving and Donnan repulsion coupling, enabling the nanofiltration membrane to achieve precise ion separation at the angstrom level. Simultaneously, due to the different migration rates of the non-planar rigid polyamine electrolyte and the planar amine compound in the first-phase solution, a functional layer with a gradient structural density distribution is formed, consisting of a high-structural-density layer far from the support layer and a low-structural-density layer near the support layer. This reduces mass transfer resistance across the membrane and further improves water flux. The nanofiltration membrane prepared by this invention exhibits ultra-high selective separation of monovalent / divalent ions and high water flux, thereby achieving efficient and precise ion separation. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the composition of the nanofiltration membrane of the present invention;

[0021] Figure 2 This is a diagram showing the membrane pore structure distribution of Comparative Example 1 and Example 3 of the present invention;

[0022] In the figure: 1-polyamide separation layer, 2-ultrafiltration membrane support layer, 3-nonwoven fabric layer, 4-polyamide separation layer with structural density gradient distribution. Detailed Implementation

[0023] The present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards; if there is no corresponding national standard, they are performed according to general standard requirements or general methods.

[0024] Comparative Example 1

[0025] Polysulfone solution was directly coated onto nonwoven fabric using a doctor blade and then immersed in a water bath to form a porous polysulfone base membrane. A first-phase solution containing a complex amine monomer was uniformly applied to the surface of the porous polysulfone base membrane and allowed to contact for 3 minutes. The first-phase solution consisted of 3 wt% piperazine, 0.05 wt% polyvinyl alcohol, and the balance water. Excess residual solution on the surface of the porous polysulfone base membrane was removed using an air knife. Then, a second-phase solution containing trimesoyl chloride was uniformly applied. The second-phase solution consisted of 0.3 wt% trimesoyl chloride and the balance n-hexane. The first and second-phase solutions were allowed to contact for 1 minute at 20°C to form a functional layer. The membrane was then heat-treated at 85°C for 10 minutes to obtain a nanofiltration membrane. The functional layer of the prepared nanofiltration membrane was a uniform functional layer, without upper and lower surface layers, and the thickness of the functional layer was 192 nm.

[0026] Example 1

[0027] Polysulfone solution was directly coated onto nonwoven fabric using a doctor blade and then immersed in a water bath to form a porous polysulfone substrate membrane. The precursor of polyamine compound A, 2,6,14-triaminotriptene, was added to an aqueous solution of 2,3-epoxypropyltrimethylammonium chloride to prepare a triaminotriptene quaternary ammonium salt electrolyte. A first-phase solution containing the complex amine compound was uniformly applied to the surface of the porous polysulfone substrate membrane and allowed to contact for 3 minutes. The first-phase solution consisted of 0.1 wt% triaminotriptene quaternary ammonium salt electrolyte and 2... The membrane consists of 0.9 wt% 1,2,3-propanetriamine, 0.05 wt% polyvinyl alcohol, and the balance water. Excess residual solution on the surface of the polysulfone porous base membrane is removed by using an air knife. Then, a second phase solution containing pyromellitic chloride, consisting of 0.3 wt% pyromellitic chloride and the balance n-hexane, is uniformly applied. The first and second phase solutions are brought into full contact at 20°C for 1 min to form a functional layer. The membrane is then heat-treated at 85°C for 10 min to obtain a nanofiltration membrane.

[0028] Example 2

[0029] Example 2 was carried out in the same manner as Example 1, except that the content of 2,6,14-triaminotriptene was changed to 1.0 wt% and the content of 1,2,3-propanetriamine was changed to 2.0 wt%.

[0030] Example 3

[0031] Example 3 was carried out in the same manner as Example 1, except that the content of 2,6,14-triaminotriptene was changed to 2.0 wt% and the content of 1,2,3-propanetriamine was changed to 1.0 wt%.

[0032] Example 4

[0033] Example 4 was carried out in the same manner as Example 1, except that the content of 2,6,14-triaminotriptene was changed to 2.9 wt% and the content of 1,2,3-propanetriamine was changed to 0.1 wt%.

[0034] Example 5

[0035] Example 5 was carried out in the same manner as Example 3, except that the precursor of polyamine electrolyte A was changed from 2,6,14-triaminotriptene to 2,3,6,7,14,15-hexaaminotriptene.

[0036] Example 6

[0037] Example 6 was carried out in the same manner as Example 5, except that the precursor of the nonplanar electrolyte amine monomer A was changed from 2,3,6,7,14,15-hexaaminotriptene to tetra(4-aminophenyl)methane.

[0038] Example 7

[0039] Example 7 was carried out in the same manner as Example 3, except that the planar amine monomer B was changed from 1,2,3-propanetriamine to ethylenediamine.

[0040] Example 8

[0041] Example 8 was carried out in the same manner as Example 3, except that the diffusion regulator was changed from polyvinyl alcohol to polycarboxylate betaine methacrylate.

[0042] Example 9

[0043] Example 9 was carried out in the same manner as Example 3, except that the diffusion modifier was changed from polyvinyl alcohol to sulfonic acid-type polyethyleneimine.

[0044] The nanofiltration membrane prepared by the above method includes a nonwoven fabric layer 3, an ultrafiltration membrane support layer 2, and an ultrafiltration membrane support layer 2. The ultrafiltration membrane support layer 2 is sequentially contacted with a first phase solution and a second phase solution to form a polyamide separation layer 1. The polyamide separation layer 1 is a polyamide separation layer 4 with a gradient distribution of structural density, comprising a high structural density layer far from the support layer and a low structural density layer close to the support layer. The average pore size of the low structural density layer is such that the proportion of pores larger than the average pore size to the total number of pores is less than or equal to 10%. The thickness ratio of the high structural density layer to the low structural density layer is greater than or equal to 0.1 and less than or equal to 9. The average pore size of the high structural density layer is greater than or equal to 0.5 nm and less than or equal to 0.8 nm.

[0045] The following experimental data further demonstrates the beneficial effects of the present invention.

[0046] Determination of the active layer thickness and the ratio of high to low structure density layers in nanofiltration membranes: Nanofiltration membrane sheets were encapsulated in resin, fixed in a sample holder, and TEM samples were prepared using the thin-film slicing method. After staining, TEM observations were performed, and the overall thickness of the active layer was measured using the TEM images. The pore size and pore size distribution of the nanofiltration membrane were tested using positron annihilation assays. The thicknesses of the high and low structure density layers were calculated, and the ratio of the high to low structure density layer thicknesses was further calculated.

[0047] Testing of water flux and rejection rate (MgSO4 rejection rate) of nanofiltration membranes: Nanofiltration membranes prepared in Comparative Example 1 and Examples 1 to 5 were used to test water flux and desalination rate under the following operating conditions: operating pressure 0.5 MPa, influent MgSO4 concentration of 2000 ppm aqueous solution, and operating time 60 min. Various parameters of the nanofiltration membranes are shown in Table 1 below.

[0048] Table 1. Parameter test table for nanofiltration membranes in Comparative Example 1 and Examples 1-8

[0049]

[0050] The test results in Table 1 show that the nanofiltration membrane prepared by this invention has higher water flux and higher rejection rate. Comparison of Examples 1 to 5 and Comparative Example 1 shows that the nanofiltration membrane prepared by interfacial polymerization using planar / non-planar composite amine monomers as polyamine monomers has a significantly reduced functional layer thickness, a smaller average pore size, and a significantly reduced proportion of pores larger than the average pore size, indicating that the nanofiltration membrane prepared by this invention has a more uniform pore size distribution. Comparison of Examples 1 to 4 shows that increasing the content of the three-dimensional non-planar electrolyte amine monomer A and reducing the content of the planar amine monomer B gradually increases the thickness of the high-structure-density layer, decreases the thickness of the low-structure-density layer, and gradually increases the water flux. Comparison of Examples 3 and Examples 5-8 shows that similar effects can be achieved using non-planar electrolyte monomers and planar amine monomer B with similar structures.

[0051] Ion selectivity test of nanofiltration membrane: Nanofiltration membranes prepared in Comparative Example 1 and Examples 1 to 8 were tested for water flux and ion selectivity under the following operating conditions: operating pressure 2.0 MPa, feed water a mixed aqueous solution of MgSO4 32000 ppm and Li2SO4 3000 ppm, and operating time 60 min. The water flux and ion selectivity of the nanofiltration membrane were then tested. + / Mg 2+ Select the separation factor S(Li) + / Mg 2+ As shown in Table 2.

[0052] Table 2. Ion selectivity test results for nanofiltration membranes in Comparative Example 1 and Examples 1-5.

[0053]

[0054] As can be seen from the test structures in Table 2, the nanofiltration membrane prepared by the method of this invention has higher water flux and Li + / Mg 2+ By selecting a separation factor, the goal of simultaneously improving the ion selectivity and water flux of nanofiltration membranes can be achieved, thereby realizing efficient and precise ion separation.

[0055] In traditional methods, the performance of nanofiltration membranes is limited by factors such as wide pore size distribution and surface roughness, thus restricting their separation efficiency. This invention utilizes interfacial polymerization with planar / non-planar amines as composite monomers to construct sub-nanometer ion sieving channels with specific charges, forming a functional layer with a gradient distribution of structural density, thereby improving the ion selectivity and water flux of the nanofiltration membrane. Experimental verification shows that the nanofiltration membrane prepared by this invention exhibits ultra-high separation performance and water flux in the field of ion separation, providing an efficient and precise solution for ion separation processes. This method is simple to operate, requires no modification to existing equipment, and is economically cost-effective.

[0056] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a nanofiltration membrane with a structural density gradient distribution, characterized in that... Includes the following steps: 1) Prepare non-planar electrolyte amine monomers with stereoconfiguration through quaternization or sulfonation reactions; 2) The polymer ultrafiltration membrane is sequentially contacted with a first-phase solution and a second-phase solution to form a separation functional layer. The first-phase solution consists of planar / non-planar composite amine monomers, a diffusion modifier, and a solvent. The composite amine monomers are a non-planar electrolyte amine monomer A with a three-dimensional configuration and a planar amine monomer B with a two-dimensional configuration. The content of non-planar electrolyte amine monomer A in the first-phase solution is 0.1-2.9 wt%, the content of planar amine monomer B is 0.1-2.9 wt%, the content of diffusion modifier is 0.05-1.5 wt%, and the balance is solvent. The second phase solution consists of acyl chloride compounds and alkane solvents, and is post-processed to obtain a nanofiltration membrane.

2. The method for preparing a nanofiltration membrane with a structural density gradient distribution as described in claim 1, characterized in that... In step 2): the non-planar electrolyte amine monomer A is at least one of a quaternary ammonium salt or sulfonate containing an amino-terminal group and having an aromatic ring as the main structure with a stereostructure.

3. The method for preparing a nanofiltration membrane with a structural density gradient distribution as described in claim 1, characterized in that... In step 2): the non-planar electrolyte amine monomer A is prepared by quaternization or sulfonation reaction of a non-planar polyamine with a stereostructure.

4. The method for preparing a nanofiltration membrane with a structural density gradient distribution as described in claim 3, characterized in that... The specific method of the quaternization or sulfonation treatment is as follows: quaternization reaction is carried out using a quaternization reagent or its solution, wherein the quaternization reagent is at least one of 2,3-epoxypropyltrimethylammonium chloride, benzyl chloride, iodomethane, and iodoethane; sulfonation reaction is carried out using a sulfonation reagent or its solution, wherein the sulfonation reagent is at least one of 1,3-propanesulfonic acid lactone, dimethyl sulfate, and chlorosulfonic acid.

5. The method for preparing a nanofiltration membrane with a structural density gradient distribution as described in claim 3, characterized in that: The stereochemically structured nonplanar polyamine is a precursor of nonplanar electrolyte amine monomer A, and the stereochemically structured nonplanar polyamine is at least one of 2,6,14-triaminotriptene, 2,3,6,7,14,15-hexaaminotriptene, tetra(4-aminophenyl)methane, 5,5',6,6'-tetraamino-3,3,3',3'-tetramethyl-1,1'-spirobisindene, and 9,9'-spirobis[fluorene]-2,2',7,7'-tetraamine.

6. The method for preparing a nanofiltration membrane with a structural density gradient distribution as described in claim 1, characterized in that... In step 2): the planar amine monomer B with a two-dimensional configuration is at least one of ethylenediamine, diethylenetriamine, 1,2,3-propanetriamine, p-phenylenediamine and m-phenylenediamine.

7. The method for preparing a nanofiltration membrane with a structural density gradient distribution as described in claim 1, characterized in that... In step 2), the diffusion regulator is at least one of polycarboxylate betaine methacrylate, sulfonic acid polyethyleneimine, sodium dodecyl sulfonate, and polyvinyl alcohol.

8. A nanofiltration membrane prepared by any one of the nanofiltration membrane preparation methods of claims 1-7, comprising a nonwoven fabric layer and an ultrafiltration membrane support layer, characterized in that... The ultrafiltration membrane support layer is sequentially contacted with the first phase solution and the second phase solution to form a polyamide separation layer. The polyamide separation layer is a polyamide separation layer with a gradient distribution of structural density, which includes a high structural density layer far away from the support layer and a low structural density layer close to the support layer. The average pore size of the low structural density layer is such that the proportion of pores larger than the average pore size to the total number of pores is less than or equal to 10%.

9. The nanofiltration membrane as described in claim 8, characterized in that: The thickness ratio of the high structure density layer to the low structure density layer is greater than or equal to 0.1 and less than or equal to 9; the average pore size of the high structure density layer is greater than or equal to 0.5 nm and less than or equal to 0.8 nm.

10. The application of the nanofiltration membrane prepared by any one of the nanofiltration membrane preparation methods in the precise separation of ions.

Images