Highly selective polyamide nanocomposite membrane for lithium-magnesium separation and preparation method thereof

Abstract

The present invention relates to a highly selective polyamide nanocomposite membrane for lithium-magnesium separation and a preparation method thereof. The polyamide nanocomposite membrane is composed of a lower layer and an upper layer bonded via an amide bond, wherein the lower layer is a porous layer of water-soluble, protonatable, positively charged polyamide dendrimers; the structure of the positively charged polyamide dendrimers consists of an aliphatic core (G0), an aliphatic branching structure (G0 to Gn-1), and an aromatic end-capping structure (Gn); the molecular structure of the polyamide-amine dendrimers is generally as follows; the highly selective polyamide nanocomposite membrane prepared by the present invention has a Li+ rejection rate of between 15% and 15%, and a Mg+ rejection rate of between 15% and 15%. 2+ The rejection rate is above 98%, and the selectivity calculated from it is 40-100, which has excellent lithium-magnesium screening selectivity. + / Mg 2+ The selectivity is better than the currently reported ion-selective composite membrane, indicating that the membrane preparation method of the present invention has significant technological progress. #imgabs0#

Description

Technical Field

[0001] The present invention belongs to the field of high-selectivity membrane separation, and in particular relates to a high-selectivity polyamide nanocomposite membrane for lithium-magnesium separation and a preparation method thereof. Background Art

[0002] Ion selective separation technology is a common demand in the fields of water treatment, resource development, energy storage and conversion. In particular, with the rapid development of energy storage in recent years, the demand for high-purity lithium has increased dramatically, and lithium extraction from salt lake brine is an important way to ensure my country's lithium supply. Nanofiltration technology, represented by polyamide thin-layer composite membrane (TFC), has the advantages of good ion selective separation ability and high efficiency and energy saving. - / SO4 2- 、Li + / Mg 2+ , olefins / alkanes, etc.) has great potential in the separation field. However, when existing commercial polyamide nanocomposite membranes are used to separate mixed solutions with a high magnesium / lithium ratio, since the separation layer is a single interfacial polymerized polyamide dense layer and the bottom is weakly positively charged, it is difficult to perform high-selective screening of lithium and magnesium, resulting in high energy consumption and cost of the separation process. Therefore, according to the ion separation mechanism of the nanomembrane, that is, the Donan effect, it is urgent to design and develop a polyamide nanomembrane with a strong positive charge at the bottom to improve Li + / Mg 2+ Selectivity to meet the rapidly growing market demand.

[0003] Current research generally believes that the selective separation of monovalent / polyvalent ions by nanofiltration membranes mainly depends on the differences in hydration size and valence between different ions. For the polyamide separation layer of existing commercial nanofiltration membranes, not only is its structural uniformity poor and its pore size distribution wide, it is difficult to achieve the separation of Mg with similar hydration radius. 2+ He Li + The high-precision screening of Li+ can be achieved; and the large number of ionizable -COOH groups on its surface and inside make its separation layer as a whole highly negatively charged, and have a strong electrostatic adsorption effect on cations, thereby reducing Li+ + / Mg 2+ Therefore, based on the separation mechanism of nanofiltration membrane, the Li ion selectivity of polyamide nanofiltration membrane can be improved by strengthening size screening and charge repulsion. + / Mg 2+ Selective.

[0004] The strategy to enhance the size screening effect is to start from regulating the uniformity of the polyamide separation layer structure to form a polyamide dense layer with a more uniform pore size distribution, so as to improve its separation accuracy on the angstrom scale. It is mainly achieved by constructing an intermediate layer, regulating the diffusion rate of aqueous phase monomers, regulating the interfacial distribution of aqueous phase monomers, etc. to reduce the spatial heterogeneity of the interfacial polymerization reaction, thereby achieving a narrow distribution of the pore size of the polyamide dense layer. For example, patents CN113262642A and CN112755817A use anionic surfactants (such as sodium dodecyl sulfonate and phosphate diester compounds) to self-assemble a monomolecular network layer at the water-oil interface, regulate the orderly diffusion of amine monomers and the spatial orderliness of the interfacial polymerization reaction, so as to prepare a polyamide nanofiltration membrane with uniform structure and uniform pore size distribution. The retention rate of these high-performance composite nanofiltration membranes for divalent cations is greater than 98.5%, the retention rate for monovalent positive ions is less than 35%, and the pure water flux is as high as 160L m -2 h -1 However, in practical applications, these polyamide membranes are not only susceptible to salt concentration and cross-flow shear, but are also more sensitive to chemical cleaning, and their ion selectivity may be more sensitive to changes in nanochannel size (Ind. Eng. Chem. Res. 2020, 59, 17653).

[0005] The strategy for strengthening the charge repulsion effect is to increase the positive charge density on the surface and within the polyamide separation layer, forming an overall positively charged polyamide dense layer to enhance the charge repulsion effect between the nanochannels and the cations. On the one hand, more positively charged groups can be introduced into the polyamide active layer by screening aqueous / oil-phase monomers or doping with positively charged organic / inorganic nanofillers to improve the overall positive charge of the membrane. On the other hand, molecules with positively charged groups can be grafted onto the polyamide membrane surface through surface modification methods (such as surface coating, secondary interfacial polymerization, surface grafting, free radical reactions, etc.) to increase the surface positive charge density of the separation layer. For example, patent CN113694740A uses ionized quaternary ammonium salts as aqueous phase monomers to prepare a polyamide membrane with a high positive charge density inside the separation layer through interfacial polymerization; patent CN105597577A introduces amino-functionalized MOFs through interfacial polymerization to increase the positive charge density inside the polyamide separation layer; patent CN112844046A grafts multifunctional amino compounds onto the surface of a polyamide membrane through a secondary interfacial polymerization reaction to prepare a method to increase the positive charge density on the surface of the separation layer. These methods are limited by the paradox between diffusion rate, compatibility, or layer thickness and permeability, resulting in limited improvement in the overall positive charge of the polyamide separation layer. In addition, these methods mostly focus on the positive charge density on the membrane surface, and few studies have focused on the effects of the positive charge density and structural design on the back of the polyamide separation layer on the ion selectivity and permeability of the polyamide membrane. Summary of the Invention

[0006] The object of the present invention is to provide a polyamide nanomembrane with a strong positive charge in the lower layer of a separation layer, so as to improve the lithium-magnesium screening selectivity. The present invention is studied on the basis of our invention patent (ZL 202010754678.4). The aforementioned invention patent proposes the concept of an asymmetric polyamide separation layer, that is, the separation layer consists of two layers, the upper layer is an interfacial polymerized polyamide dense layer, and the lower layer is a dendritic macromolecular porous layer. The upper layer and the lower layer are bonded by amide bonds to form a structure in which the lower layer is loose and the upper layer is dense. The membrane formed by this structure has a large brine permeation flux and a high desalination retention rate. However, in subsequent studies, it was found that because the lower layer is an aromatic dendritic macromolecular structure and is insoluble in water, the peripheral blocked amine groups cannot be protonated in water, and it is difficult to improve the positive charge of the lower layer of the separation membrane. Combined with the charge repulsion separation principle of nanofiltration membrane, the weakly positively charged lower layer of the polyamide separation layer cannot achieve efficient screening of lithium and magnesium. Therefore, it is urgent to develop a polyamide nanomembrane with a strong positive charge in the lower layer of the separation layer to improve the lithium and magnesium screening selectivity.

[0007] The problem that the aromatic dendrimers in the lower porous layer of the asymmetric polyamide separation layer are difficult to dissolve in water, resulting in the peripheral end-capping amine groups being difficult to protonate and the positive charge being weak is addressed in the present invention. The present invention proposes constructing polyamide dendrimers with an aliphatic core and a branched structure to enhance water solubility, with aromatic amine groups on the periphery to provide diazotization-coupling reactivity and deprotonation capability, thereby enhancing positive charge. Because the polyamide dendrimers designed in the present invention are water-soluble and their peripheral end-capping amine groups are protonable, the resulting porous dendrimer layer (the lower layer of the separation layer) has a strong positive charge. On this porous layer, a polyamide dense layer is prepared by interfacial polymerization, ultimately forming an asymmetric polyamide nanomembrane with a dense upper layer and a strongly positively charged dendrimer porous layer on the lower layer, thereby enhancing lithium-magnesium screening.

[0008] The lower layer of the polyamide nano-membrane is a porous layer of polyamide dendritic macromolecules that are water-soluble, protonatable and positively charged, and the upper layer is a dense layer of interfacially polymerized polyamide.

[0009] The present invention forms a polyamide membrane with an asymmetric double-layer structure, wherein the upper layer is negatively charged and the lower layer is positively charged and the lower layer is positively charged and the polyamide membrane is formed. Figure 1 As shown, 1 is a non-woven fabric, 2 is a polysulfone support layer, 3 is a highly positively charged polyamide dendritic macromolecular porous layer, 4 is a polyamide dense layer, and 5 is a polyamide separation layer. The polyamide membrane prepared by this invention has high flux and high Li + / Mg2+ It is selective, has a simple preparation process, and is easy to promote in industry. It can be used in processes such as lithium extraction from salt lakes / brine, water softening, and heavy metal wastewater treatment.

[0010] The present invention is achieved through the following technical solutions:

[0011] A highly selective polyamide nanomembrane for lithium-magnesium separation comprises a lower layer and an upper layer bonded via amide bonds. The lower layer comprises a porous layer of water-soluble, protonated, positively charged polyamide dendrimers, and the upper layer comprises an interfacially polymerized polyamide dense layer. The porous polyamide dendrimer layer and the dense polyamide layer are referred to as the polyamide separation layer or active layer in the present invention. The porous polyamide dendrimer layer is formed by self-assembly of strongly positively charged dendrimers, while the dense polyamide layer is prepared by interfacial polymerization of an aqueous amine solution and an oily acyl chloride monomer solution within the porous dendrimer layer.

[0012] The specific structure of the water-soluble, protonatable, positively charged polyamide dendrimer consists of a polyamide-amine dendrimer core, a branched structure, and peripheral modified terminal aniline groups. The peripheral modified terminal aniline groups are used to improve the positive charge and self-assembly reactivity of the polyamide dendrimer, and the aliphatic chain structure is used to enhance water solubility.

[0013] The structure of the positively charged polyamide dendrimer proposed in the present invention consists of an aliphatic core (G0), aliphatic branching structures (G1 to Gn-1), and an aromatic end-capping structure (Gn), wherein n is the number of iterations of the dendrimer and ranges from 1 to 7. The general molecular structure formula of the polyamide-amine dendrimer is as follows:

[0014]

[0015] The molecular structure of the core G0 is as follows:

[0016]

[0017] Among them, x is 2-4, y is 2-4, and z is 2-4;

[0018] The branched structures (G1 to Gn-1) are:

[0019] wherein x is 2-4, y is 2-4, z is 2-4; k=1-7, preferably k=4, 5, 6;

[0020] The structure of the end-capping molecule (Gn) is: R3=H, or

[0021] The self-assembly mode of the dendritic macromolecular porous layer with high positive charge density under the separation layer is diazotization-coupling reaction.

[0022] The diazotization-coupling reaction specifically includes:

[0023] 1. Preparation of dendrimer solution: A positively charged polyamide dendrimer composed of the core (G0), branched structures (G1 to Gn-1), and end-capping structure (Gn) is prepared into an aqueous solution with a pH of 1-3;

[0024] 2. Preparation of a strongly positively charged porous layer of polyamide dendrimers: The polysulfone-based membrane is immersed in a polyamide dendrimer solution for 5-9 minutes and a sodium nitrite solution for 30-100 seconds, respectively. Then, a strongly positively charged porous layer of polyamide dendrimers is obtained, which is the lower layer of the polyamide nanomembrane separation layer.

[0025] The polysulfone-based membrane containing the highly positively charged polyamide dendrimer porous layer is exposed to a 0.5-1.5 wt.% aqueous amine solution for 1-5 minutes and a 0.05-0.3 w / v % oil-based acyl chloride solution for 10-180 seconds, respectively, to form a dense polyamide layer, which serves as the upper layer of the polyamide separation layer. Finally, the membrane is washed with n-hexane for 10-60 seconds and then heat-treated at 60°C for 5-10 minutes. The remaining amine groups in the highly positively charged polyamide dendrimer porous layer after the diazotization-coupling reaction can form amide bonds with acyl chloride molecules produced during interfacial polymerization, thereby forming a bilayer asymmetric polyamide separation layer with an upper dense polyamide layer and a lower highly positively charged polyamide dendrimer porous layer.

[0026] The separation layer performance test of the highly selective polyamide nanocomposite membrane prepared by the present invention: when the feed solution is a single salt, the tested salts are MgCl2 and LiCl, the single salt concentration is 2000ppm, and the test pressure is 145psi; when the feed solution is a mixed salt solution, the mixed salt solution consists of 500ppm LiCl and 10000ppm MgCl2, at this time Mg 2+ With Li + The mass ratio is 31.2 and the tested temperature is 25°C.

[0027] The highly selective polyamide nanocomposite membrane for lithium-magnesium separation of the present invention has a porous layer formed by diazotization-coupling reaction of water-soluble, protonable, positively charged polyamide dendrimers, and has a significant positive charge. Its zeta potential at pH = 7 is 10-20 mV. This shows that the present invention has prepared a highly positively charged lower layer of the separation layer.

[0028] The beneficial effects of the present invention are:

[0029] (1) At present, no research on increasing the positive charge of the lower layer of the separation layer has been reported, and the polyamide dendrimer porous layer with high positive charge density in the present invention is completely different from the role of the inorganic / organic intermediate layer in the literature. It can participate in the interfacial polymerization reaction and stably combine with the interfacial polymerization polyamide dense layer through amide bonds to form a separation layer with the polyamide dense layer; therefore, the separation layer of the polyamide membrane prepared by the patent of the present invention has asymmetry in structure and charge, which is conducive to the improvement of permeability and selectivity.

[0030] (2) The present invention can control the positive charge of the lower layer of the separation layer based on structural design. The polyamide dendrimers can not only provide abundant positively charged groups to significantly increase the overall positive charge of the polyamide separation layer, but also provide charged nanochannels to further enhance the coordinated effect of charge repulsion and dielectric repulsion.

[0031] (3) The polyamide membrane with a high positive charge density in the lower layer of the polyamide separation layer prepared by the present invention exhibits excellent membrane separation performance. When tested under the condition of a single salt feed solution (single salt concentration of 2000 ppm, single salts MgCl2 and LiCl, respectively, and test pressure of 145 psi), the MgCl2 rejection rate is 93.0-98.3%, and the water flux is 224.8-335.9 L·m -2 ·h -1 The LiCl rejection rate is 23.6-49.6%, and the water flux is 289.6-400.7 L·m -2 ·h -1 The specific operation of using the prepared highly selective polyamide nanomembrane for lithium-magnesium separation is as follows: first, a mixed brine solution of 500ppm LiCl and 10000ppm MgCl2 is prepared to simulate the composition of a salt lake. 2+ With Li + The mass ratio of the prepared polyamide membrane was 31.2. Under cross-flow conditions, 25 ° C, and an operating pressure of 145 psi, the prepared polyamide membrane was continuously filtered for 1 hour, and the Li content in the permeate and the raw material solution was tested by ion chromatography. + and Mg 2+ Concentration, according to the following formula:

[0032]

[0033] Calculate the ion screening selectivity. The highly selective polyamide nano-membrane prepared by the present invention has a Li+ rejection rate of -15% to 15% and a Mg 2+ The rejection rate is above 98%, and the selectivity calculated from it is between 40-100, which has excellent lithium-magnesium screening selectivity. + / Mg 2+The selectivity is better than the currently reported ion-selective composite membrane, indicating that the membrane preparation method of the present invention has significant technical progress. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Asymmetric polyamide nanocomposite membrane structure with high positive charge density under the polyamide separation layer (wherein 1 is non-woven fabric, 2 is polysulfone support layer, 3 is highly positively charged polyamide dendritic macromolecular porous layer, 4 is polyamide dense layer, and 5 is polyamide separation layer)

[0035] Figure 2 Schematic diagram of the molecular structure of the polyamide separation layer in Example 4 of the present invention. As shown in the figure, the positively charged polyamide dendrimers are connected by azo bonds (-N=N-) to form a loose porous layer. The porous layer is connected to the interfacial polymerized polyamide dense layer through amide bonds (-CO-NH-), and the two form an asymmetric structure.

[0036] Figure 3 The surface morphology of the polyamide separation layer of Example 1 of the present invention shows a clear nano-stripe structure on the surface

[0037] Figure 4 The cross-sectional morphology of the polyamide separation layer of Example 1 of the present invention shows a distinct upper and lower layer structure, with the upper layer being a dense polyamide layer and the lower layer being a positively charged porous layer.

[0038] Figure 5 The Zeta potential diagram of the 128-aniline-terminated polyamide dendrimer prepared in Example 1 of the present invention shows that the surface of the macromolecular particles exhibits obvious positive charge, indicating that the peripheral amino groups can be protonated and exhibit positive charge.

[0039] Figure 6 Zeta potential plot of the porous layer of amino-terminated polyamide dendrimers prepared in Example 1 of the present invention and a pristine polysulfone ultrafiltration membrane. The plot shows that at pH 7, the pristine polysulfone ultrafiltration membrane has a zeta potential of -36.1 mV, while the porous layer of amino-terminated dendrimers has a zeta potential of 12.3 mV. DETAILED DESCRIPTION

[0040] The technical solution of the present invention is further explained and illustrated below in conjunction with specific embodiments and test examples so that those skilled in the art can fully understand the present invention. However, this explanation and illustration does not further limit the technical solution of the present invention. The technical solutions obtained by simple numerical replacement and conventional adjustment based on the present invention fall within the scope of protection of the present invention.

[0041] Example 1

[0042] Preparation of a highly positively charged porous polyamide dendrimer layer: A polysulfone-based membrane (PSF) was immersed in a 0.08 w / v% solution of water-soluble, protonable, positively charged polyamide dendrimers at pH 1 for 5 minutes, followed by immersion in a 0.5 w / v% sodium nitrite solution for 60 seconds to form a highly positively charged porous polyamide dendrimer layer. The specific structure of the water-soluble, protonable, positively charged polyamide dendrimer is as follows:

[0043]

[0044] The molecular formula of the core is:

[0045]

[0046] Among them, x is 2, y is 2, and z is 2;

[0047] The branched structure is:

[0048] Where, x is 2, y is 2, z is 2, and k=4;

[0049] The end-capping molecular structure is: R3=H

[0050] The water-soluble, protonatable, positively charged dendrimer is a 1,28-aniline-terminated polyamide dendrimer, and its molecular structure is as follows:

[0051] Preparation of a polyamide nanocomposite membrane with a strongly positively charged lower layer: A polysulfone-based membrane with a porous layer of polyamide dendrimers assembled on its surface was immersed in a 0.8 w / v% piperazine aqueous solution for 2 minutes. Excess solution was removed with a rubber roller. Next, a 0.1 w / v% solution of trimesoyl chloride in n-hexane was reacted for 20 seconds to form a dense polyamide layer. This resulted in an asymmetric polyamide separation layer consisting of a dense polyamide upper layer and a porous dendrimer layer with a high positive charge density. After washing with n-hexane for 30 seconds, the membrane was heat-treated at 60°C for 5 minutes. Finally, the resulting polyamide nanocomposite membrane with a strongly positively charged lower layer was subjected to performance testing. Figure 3 This is the surface morphology of the polyamide nanocomposite membrane prepared in Example 1 of the present invention, and its surface presents a clear nano-stripe structure. Figure 4 This is the cross-sectional morphology of the polyamide nano-separation layer obtained in Example 1 of the present invention. The cross-sectional view shows a distinct upper and lower layer structure, wherein the upper layer is a dense polyamide layer and the lower layer is a positively charged porous layer. Figure 5 The Zeta potential diagram of the 128-aniline-terminated polyamide dendrimer prepared in Example 1 of the present invention shows that the surface of the macromolecular particles exhibits obvious positive charge, indicating that the peripheral amino groups can be protonated and exhibit positive charge. Figure 6 The zeta potential diagram of the surface of the amino-terminated dendritic macromolecule porous layer and the original polysulfone ultrafiltration membrane prepared in Example 1 of the present invention shows that when pH = 7, the zeta potential value of the original polysulfone ultrafiltration membrane surface is -36.1 mV, while the zeta potential value of the amino-terminated dendritic macromolecule porous layer is 12.3 mV.

[0052] Example 2

[0053] Preparation of a highly positively charged porous polyamide dendrimer layer: A polysulfone-based membrane (PSF) was immersed in a 0.08 w / v% solution of water-soluble, protonable, positively charged polyamide dendrimers at pH 2 for 5 minutes, followed by immersion in a 0.5 w / v% sodium nitrite solution for 60 seconds to form a highly positively charged porous polyamide dendrimer layer. The specific structure of the water-soluble, protonable, positively charged polyamide dendrimer is as follows:

[0054] The molecular formula of the core is:

[0055] The molecular formula of the core is:

[0056]

[0057] Among them, x is 3, y is 3, and z is 3;

[0058] The branched structure is:

[0059] Where, x is 2, y is 2, z is 2, and k=5;

[0060] The end-capping molecular structure is: R3=H

[0061] Preparation of a polyamide nanocomposite membrane with a strongly positively charged lower layer: A polysulfone-based membrane with a porous layer of polyamide dendrimers assembled on its surface was immersed in a 1 w / v% piperazine aqueous solution for 3 minutes. Excess solution was removed with a rubber roller. Next, a 0.15 w / v% solution of trimesoyl chloride in n-hexane was reacted for 20 seconds to form a dense polyamide layer. This resulted in an asymmetric polyamide separation layer consisting of a dense polyamide upper layer and a porous dendrimer layer with a high positive charge density. After washing with n-hexane for 40 seconds, the membrane was heat-treated at 60°C for 5 minutes. Finally, the resulting polyamide nanocomposite membrane with a strongly positively charged lower layer was subjected to performance testing.

[0062] Example 3

[0063] Preparation of a highly positively charged porous polyamide dendrimer layer: A polysulfone-based membrane (PSF) was immersed in a 0.01 w / v% solution of water-soluble, protonable, positively charged polyamide dendrimers at pH 3 for 5 minutes, followed by immersion in a 0.5 w / v% sodium nitrite solution for 90 seconds to form a highly positively charged porous polyamide dendrimer layer. The specific structure of the water-soluble, protonable, positively charged polyamide dendrimer is as follows:

[0064] The molecular formula of the core is:

[0065]

[0066] Among them, x is 3, y is 3, and z is 3;

[0067] The branched structure is:

[0068] Where, x is 2, y is 2, z is 2, and k=6;

[0069] The end-capping molecular structure is:

[0070] Preparation of a polyamide nanocomposite membrane with a strongly positively charged lower layer: A polysulfone-based membrane with a porous layer of polyamide dendrimers assembled on its surface was immersed in a 1 w / v% piperazine aqueous solution for 3 minutes. Excess solution was removed with a rubber roller. Next, a 0.15 w / v% solution of trimesoyl chloride in n-hexane was reacted for 20 seconds to form a dense polyamide layer. This resulted in an asymmetric polyamide separation layer consisting of a dense polyamide upper layer and a porous dendrimer layer with a high positive charge density. After washing with n-hexane for 40 seconds, the membrane was heat-treated at 60°C for 5 minutes. Finally, the resulting polyamide nanocomposite membrane with a strongly positively charged lower layer was subjected to performance testing.

[0071] Example 4

[0072] Preparation of a highly positively charged porous polyamide dendrimer layer: A polysulfone-based membrane (PSF) was immersed in a 0.08 w / v% solution of water-soluble, protonable, positively charged polyamide dendrimers at pH 2 for 5 minutes, followed by immersion in a 0.5 w / v% sodium nitrite solution for 60 seconds to form a highly positively charged porous polyamide dendrimer layer. The specific structure of the water-soluble, protonable, positively charged polyamide dendrimer is as follows:

[0073] The molecular formula of the core is:

[0074] The molecular formula of the core is:

[0075]

[0076] Among them, x is 2, y is 2, and z is 2;

[0077] The branched structure is:

[0078] Where, x is 2, y is 2, z is 2, and k=3;

[0079] The end-capping molecular structure is: R3=H

[0080] Preparation of a polyamide nanocomposite membrane with a strong positive charge on the lower layer of the separation layer: immerse a polysulfone-based membrane with a porous layer of polyamide dendritic macromolecules assembled on the surface in a 1w / v% piperazine aqueous solution for 3 minutes, and remove excess solution with a rubber roller. Then, a 0.15w / v% n-hexane solution of trimesoyl chloride is reacted for 20 seconds to form a polyamide dense layer; an asymmetric polyamide separation layer is obtained, which is composed of a polyamide dense layer on the upper layer and a dendritic macromolecule porous layer with a high positive charge density on the lower layer. After washing with n-hexane solvent for 40 seconds, place at 60°C and heat-treat for 5 minutes. Finally, the performance of the obtained polyamide nanocomposite membrane with a strong positive charge on the lower layer of the polyamide separation layer is tested. Figure 2 As shown, positively charged polyamide dendrimers are connected by azo bonds (-N=N-) to form a loose porous layer, and the porous layer is connected to an interfacial polymerized polyamide dense layer via amide bonds (-CO-NH-), and the two form an asymmetric structure.

[0081] Example 5

[0082] Preparation of a highly positively charged porous polyamide dendrimer layer: A polysulfone-based membrane (PSF) was immersed in a 0.08 w / v% solution of water-soluble, protonable, positively charged polyamide dendrimers at pH 2 for 5 minutes, followed by immersion in a 0.5 w / v% sodium nitrite solution for 60 seconds to form a highly positively charged porous polyamide dendrimer layer. The specific structure of the water-soluble, protonable, positively charged polyamide dendrimer is as follows:

[0083] The molecular formula of the core is:

[0084] The molecular formula of the core is:

[0085]

[0086] Among them, x is 2, y is 2, and z is 2;

[0087] The branched structure is:

[0088] Where, x is 2, y is 2, z is 2, and k=5;

[0089] The end-capping molecular structure is: R3=H

[0090] Preparation of a polyamide nanocomposite membrane with a strongly positively charged lower layer: A polysulfone-based membrane with a porous layer of polyamide dendrimers assembled on its surface was immersed in a 1 w / v% aqueous solution of tris(2-aminoethyl)amine for 3 minutes. Excess solution was removed with a rubber roller. Next, a 0.15 w / v% solution of trimesoyl chloride in n-hexane was reacted for 20 seconds to form a dense polyamide layer. This resulted in an asymmetric polyamide separation layer consisting of a dense polyamide upper layer and a porous dendrimer layer with a high positive charge density. After washing with n-hexane for 40 seconds, the membrane was heat-treated at 60°C for 5 minutes. Finally, the resulting polyamide nanocomposite membrane with a strongly positively charged lower layer was subjected to performance testing.

[0091] Example 6

[0092] Preparation of a highly positively charged porous polyamide dendrimer layer: A polysulfone-based membrane (PSF) was immersed in a 0.08 w / v% solution of water-soluble, protonable, positively charged polyamide dendrimers at pH 2 for 5 minutes, followed by immersion in a 0.5 w / v% sodium nitrite solution for 60 seconds to form a highly positively charged porous polyamide dendrimer layer. The specific structure of the water-soluble, protonable, positively charged polyamide dendrimer is as follows:

[0093] The molecular formula of the core is:

[0094] The molecular formula of the core is:

[0095]

[0096] Among them, x is 2, y is 2, and z is 2;

[0097] The branched structure is:

[0098] Where, x is 2, y is 2, z is 2, and k=5;

[0099] The end-capping molecular structure is: R3=H

[0100] Preparation of a polyamide nanocomposite membrane with a strongly positively charged lower layer: A polysulfone-based membrane with a porous layer of polyamide dendrimers assembled on its surface was immersed in a 1 w / v% piperazine aqueous solution for 3 minutes. Excess solution was removed with a rubber roller. Next, a 0.09 w / v% solution of 1,2,3,4-cyclobutanetetracarbonyl chloride in n-hexane was reacted for 20 seconds to form a dense polyamide layer. This resulted in an asymmetric polyamide separation layer consisting of a dense polyamide layer on top and a porous dendrimer layer with a high positive charge density on the bottom. After washing with n-hexane for 40 seconds, the membrane was heat-treated at 60°C for 5 minutes. Finally, the resulting polyamide nanocomposite membrane with a strongly positively charged lower layer was subjected to performance testing.

[0101] Example 7

[0102] Preparation of a highly positively charged porous polyamide dendrimer layer: A polysulfone-based membrane (PSF) was immersed in a 0.08 w / v% solution of water-soluble, protonable, positively charged polyamide dendrimers at pH 2 for 5 minutes, followed by immersion in a 0.5 w / v% sodium nitrite solution for 60 seconds to form a highly positively charged porous polyamide dendrimer layer. The specific structure of the water-soluble, protonable, positively charged polyamide dendrimer is as follows:

[0103] The molecular formula of the core is:

[0104] The molecular formula of the core is:

[0105]

[0106] Among them, x is 2, y is 2, and z is 2;

[0107] The branched structure is:

[0108] Where, x is 2, y is 2, z is 2, and k=4;

[0109] The end-capping molecular structure is: R3=H

[0110] Preparation of a polyamide nanocomposite membrane with a strongly positively charged lower layer: A polysulfone-based membrane with a porous layer of polyamide dendrimers assembled on its surface was immersed in a 1 w / v% piperazine aqueous solution for 3 minutes. Excess solution was removed with a rubber roller. Next, a 0.12 w / v% solution of 1,2,3,4-cyclopentanetetraoyl chloride in n-hexane was reacted for 20 seconds to form a dense polyamide layer. This resulted in an asymmetric polyamide separation layer consisting of a dense polyamide upper layer and a porous dendrimer layer with a high positive charge density. After washing with n-hexane for 40 seconds, the membrane was heat-treated at 60°C for 5 minutes. Finally, the resulting polyamide nanocomposite membrane with a strongly positively charged lower layer was subjected to performance testing.

[0111] The separation performance of the polyamide membrane prepared by the present invention is shown in Table 1

[0112]

[0113] a Test conditions: 2000 ppm MgCl2 or LiCl aqueous solution was used as the test solution. The prepared polyamide membrane was continuously filtered for 1 h at 25°C under an operating pressure of 145 psi to test the performance.

[0114] b Test conditions: 500ppm LiCl and 10000ppm MgCl2 mixed salt solution was used as the test solution. The prepared polyamide membrane was continuously filtered for 1 hour at 145psi operating pressure and 25℃. + and Mg 2+ The retention rate was calculated and the ion screening selectivity was calculated; it can be seen from Table 1 that under the mixed salt solution test conditions, the prepared high selectivity polyamide nanomembrane for lithium-magnesium separation has a high Mg 2+ Retention rate (more than 98%) and lower Li + Retention rate, Li of polyamide nanomembranes prepared in Examples 2, 4, 6, and 7 + The retention rate is negative, and the selectivity (Li + / Mg 2+ ) is 44.72-97.13, indicating that the nanomembrane obtained in the present invention has application potential in lithium-magnesium separation and can achieve high-selective screening of lithium and magnesium.

[0115] The technical solutions disclosed and proposed by the present invention can be implemented by those skilled in the art by drawing on the content of this document and appropriately changing the conditions, routes, and other aspects. Although the methods and preparation techniques of the present invention have been described through preferred embodiments, it is obvious that those skilled in the art can modify or recombine the methods and technical routes described herein without departing from the content, spirit, and scope of the present invention to achieve the ultimate preparation technology. It is particularly important to point out that all similar substitutions and modifications that are obvious to those skilled in the art are considered to be included in the spirit, scope, and content of the present invention.

Claims

1. A highly selective polyamide nanomembrane for lithium-magnesium separation, characterized in that: It is composed of a lower layer and an upper layer connected by amide bonds. The lower layer is a porous layer of water-soluble, protonated, positively charged dendritic macromolecules, and the upper layer is a dense layer of interfacially polymerized polyamide. The specific structure of the water-soluble, protonatable, positively charged polyamide dendrimer is composed of a polyamide-amine dendrimer molecular structure consisting of an aliphatic core G0, aliphatic branching structures G1 to Gn-1, and an aromatic end-capping structure Gn. The general molecular structure formula of the polyamide-amine dendrimer is as follows: Where n is the iteration number of the dendrimer, and n ranges from 1 to 7; The molecular structure of the core G0 is as follows: Among them, x is 2-4, y is 2-4, and z is 2-4; The branched structures from G1 to Gn-1 are: Where x is 2-4, y is 2-4, z is 2-4; k = 1-7; The structure of the capping molecule Gn is: R3=H, or The lower layer of the polyamide separation layer is a porous layer formed by diazotization-coupling reaction of water-soluble, protonable and positively charged polyamide dendrimer macromolecules.

2. The method for preparing a highly selective polyamide nanofilm for lithium-magnesium separation according to claim 1, wherein: The self-assembly mode of the dendritic macromolecular porous layer with high positive charge density under the separation layer is diazotization-coupling reaction.

3. The method for preparing a highly selective polyamide nanofilm for lithium-magnesium separation according to claim 1, wherein: k＝4，5，6。 4. The method for preparing a highly selective polyamide nanofilm for lithium-magnesium separation according to claim 2, wherein: The specific steps include: 1) Preparation of dendrimer solution: using the positively charged polyamide dendrimer composed of the aforementioned aliphatic core G0, aliphatic branching structures G1 to Gn-1, and aromatic end-capping structure Gn, to prepare an aqueous solution with a pH of 1-3; 2) Preparation of a strongly positively charged polyamide dendrimer porous layer: The polysulfone-based membrane was immersed in a polyamide dendrimer solution for 5-9 minutes and a sodium nitrite solution for 30-100 seconds, respectively, and then a strongly positively charged polyamide dendrimer porous layer was obtained.

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