Covalent organic framework bilayer positively charged nanofiltration membrane, method of making and use thereof

Abstract

The application discloses a covalent organic framework double-layer positively charged nanofiltration membrane and a preparation method and application thereof. The preparation method comprises the following steps: dissolving 1,3,5-benzene tricarboxaldehyde in an organic solvent to obtain a first solution, dissolving triaminoguanidine hydrochloride in an aqueous phase solvent to obtain a second solution; dropping the first solution on the surface of the second solution, sealing and standing, and removing an upper oil phase; diluting a dispersion liquid, mixing the dispersion liquid with a polyethylene imine solution uniformly to obtain a fourth solution; filtering and uniformly dispersing and adhering the fourth solution on the surface of a base film; immersing the base film in a n-heptane solution of benzene tricarbonyl chloride to react, and heating and crosslinking to obtain the covalent organic framework double-layer positively charged nanofiltration membrane. The nanofiltration membrane has a magnesium ion rejection rate of 99.3% for a magnesium chloride solution, a magnesium ion rejection rate of about 98.9% for different mass ratios of magnesium-lithium solutions, a low lithium ion rejection rate which decreases with the increase of the mass ratio of magnesium-lithium, and a more excellent separation effect for the high magnesium-lithium ratio characteristics of Chinese salt lake brine.

Description

Technical Field

[0001] This application belongs to the field of membrane separation materials technology, specifically relating to a covalent organic framework bilayer positively charged nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Nanofiltration membrane separation technology is a novel pressure-driven membrane separation technology that features simple operation, low cost, and environmental friendliness. Due to its unique nanoporous structure and surface charge, it exhibits strong retention of multivalent ions while allowing monovalent ions to pass through, making it widely used in the separation of monovalent and multivalent ions.

[0003] Extracting lithium resources from existing salt lake brines is an important way for my country to supply lithium resources in the future. However, the nanofiltration membranes currently available on the market have poor separation effect on lithium and magnesium ions, which are key components in the lithium extraction process from salt lakes, and cannot meet the needs of the application.

[0004] Traditional polyamide membrane materials have good film-forming properties, but their positive charge is weak and their stability is poor, resulting in poor performance in extracting lithium resources from solutions with a high magnesium-to-lithium ratio. Therefore, the highlight of this invention is to improve traditional polyamide materials and construct a novel double-layer high-performance nanofiltration membrane. Summary of the Invention

[0005] The purpose of this application is to provide a covalent organic framework bilayer positively charged nanofiltration membrane, its preparation method and application, in order to solve the technical problem that the existing nanofiltration membranes have poor separation effect on lithium and magnesium ions in the key process of lithium extraction from salt lakes, and cannot meet the application requirements.

[0006] To achieve the above objectives, one technical solution adopted in this application is:

[0007] A method for preparing a covalent organic framework bilayer positively charged nanofiltration membrane is provided, comprising:

[0008] 1,3,5-pyromellitic methyl ether was dissolved in an organic solvent to obtain a first solution, and triaminoguanidine hydrochloride was dissolved in an aqueous solvent to obtain a second solution.

[0009] The first solution was dropped onto the surface of the second solution, sealed and allowed to stand for incubation to obtain a third solution. The upper oil phase of the third solution was removed to obtain a covalent organic framework nanosheet dispersion.

[0010] After diluting the covalent organic framework nanosheet dispersion, the diluted covalent organic framework nanosheet dispersion was mixed evenly with a polyethyleneimine solution to obtain a fourth solution.

[0011] The fourth solution is filtered and then uniformly dispersed and attached to the surface of the base membrane to obtain a composite membrane.

[0012] The composite membrane was immersed in a heptane solution of pyromellitic methyl chloride and reacted. After the reaction was completed, it was heated for thermal crosslinking to obtain the covalent organic framework bilayer positively charged nanofiltration membrane.

[0013] In one or more embodiments, the organic solvent comprises mesitylene and 1,4-dioxane in a volume ratio of (8-10):1, and the molar concentration of 1,3,5-trimethylbenzaldehyde in the first solution is 0.08-0.12 mol / L.

[0014] In one or more embodiments, the aqueous solvent comprises an acetic acid solution with a mass fraction of 5-6 wt%, and the molar concentration of the triaminoguanidine hydrochloride in the second solution is 0.008-0.012 mol / L.

[0015] In one or more embodiments, the sealed static culture specifically refers to sealing and statically culturing in an incubator at 25°C for 3 days.

[0016] In one or more embodiments, in the step of diluting the covalent organic framework nanosheet dispersion and then mixing the diluted covalent organic framework nanosheet dispersion with a polyethyleneimine solution to obtain a fourth solution, the dilution factor of the covalent organic framework nanosheet dispersion is 8 to 12 times, the volume ratio of the diluted covalent organic framework nanosheet dispersion to the polyethyleneimine solution is (0.2 to 1.5): 5, the concentration of the polyethyleneimine solution is 0.4 to 0.6 g / L, and the molecular weight is 68,000 to 72,000.

[0017] In one or more embodiments, the step of uniformly dispersing and attaching the fourth solution to the surface of the base film after filtration specifically includes:

[0018] The base membrane is placed in the filtration port of the filtration device, and the fourth solution is taken and filtered through the filtration port so that the fourth solution is evenly dispersed and attached to the surface of the base membrane after filtration.

[0019] The base membrane is a polyethersulfone ultrafiltration membrane.

[0020] In one or more embodiments, in the step of immersing the composite membrane in a heptane solution of trimesoyl chloride for reaction, the concentration of trimesoyl chloride in the heptane solution is 0.15–0.25 g / L, and the reaction time is 60 s.

[0021] In one or more embodiments, the heating temperature in the thermal crosslinking step after the reaction is completed is 55-65°C, and the heating time is 8-12 minutes.

[0022] Another technical solution adopted in this application is:

[0023] A covalent organic framework bilayer positively charged nanofiltration membrane prepared by the preparation method described in any of the above embodiments is provided.

[0024] Another technical solution adopted in this application is:

[0025] This invention provides an application of a covalent organic framework bilayer positively charged nanofiltration membrane prepared by the preparation method described in any of the above embodiments in a lithium extraction process from salt lakes.

[0026] The advantages of this application, which differ from existing technologies, are:

[0027] This application modifies the covalent organic framework with guanidine groups to improve its positive charge, and then introduces the guanidine organic framework onto the surface of a nanofiltration membrane, which effectively improves the positive charge of the nanofiltration membrane and the lithium-magnesium separation effect. At the same time, due to the unique uniform pore structure of the covalent organic framework nanomaterial and the unique hydrogen bond sites in the pores, it makes it more difficult for magnesium ions to pass through, which is conducive to the passage of lithium ions.

[0028] During the growth of polyamide, the covalent organic framework material can be effectively fixed. Since the covalent organic framework material is uniformly mixed with the aqueous monomer polyethyleneimine of polyamide, the covalent organic framework material can be uniformly dispersed and fixed in the polyamide layer, exhibiting good separation selectivity and stability in lithium magnesium ion mixed solution.

[0029] This application presents a covalent organic framework bilayer positively charged nanofiltration membrane that achieves a magnesium ion rejection rate of 99.3% for magnesium chloride solutions. For solutions with different MgCl2 / LiCl mass ratios, the magnesium ion rejection rate remains at a relatively high level of approximately 98.9%, while the lithium ion rejection rate remains at a lower level, gradually decreasing with increasing MgCl2 / LiCl mass ratio. This membrane is particularly suitable for high-Mg content salts in Chinese salt lake brines. 2+ / Li + Its characteristics result in superior separation performance;

[0030] The nanofiltration membrane tested in this application showed a Zeta potential of 37.4 mV, which is higher than that of traditional polyamide membranes, and its pore size, as simulated by molecular dynamics, is 0.66 nm, slightly higher than that of Li. + 0.38nm and Mg 2+ A depth of 0.43 nm can improve the positive charge and size sieving ability of nanofiltration membranes, thereby enhancing lithium-magnesium separation efficiency.

[0031] The nanofiltration membrane of this application incorporates numerous COF nanosheets with guanidine groups. The guanidine group itself contains an N+ with a positive charge, which can still provide a positive charge under alkaline conditions. At the same time, the other amino groups in the guanidine group are relatively closely spaced. This positional relationship can improve the ability of the amino groups to retain protons under alkaline conditions. The guanidine group can continuously provide a positive charge under different pH conditions, ensuring the positive charge of the nanofiltration membrane, thereby ensuring the magnesium-lithium separation performance. It is expected to achieve good results in practical applications in weakly alkaline brine lakes. Attached Figure Description

[0032] Figure 1 This is a schematic flowchart of one embodiment of the preparation method of the covalent organic framework bilayer positively charged nanofiltration membrane of this application;

[0033] Figure 2 These are SEM images of the covalent organic framework bilayer positively charged nanofiltration membranes from Examples 1 to 4;

[0034] Figure 3 These are SEM images of cross-sections of the covalent organic framework bilayer positively charged nanofiltration membranes from Examples 1 to 4.

[0035] Figure 4 These are atomic force microscopy characterization images of the covalent organic framework bilayer positively charged nanofiltration membranes of Examples 1 to 4;

[0036] Figure 5 This is an experimental graph showing the magnesium ion rejection rate in Example 4 of this application;

[0037] Figure 6 This is the Zeta potential data graph for Example 5 of this application;

[0038] Figure 7 This is the Zeta potential data graph for Example 6 of this application;

[0039] Figure 8 This is a test data graph for Example 7 of the effect of this application;

[0040] Figure 9 This is a molecular dynamics simulation diagram of Example 8 of this application. Detailed Implementation

[0041] The present application will now be described in detail with reference to the embodiments shown in the accompanying drawings. However, these embodiments do not limit the present application, and any structural, methodological, or functional modifications made by those skilled in the art based on these embodiments are included within the protection scope of the present application.

[0042] The demand for lithium resources is increasing due to the development of the new energy vehicle industry. China's salt lakes are rich in lithium resources, with approximately 70% of the lithium resources existing in the salt lake water. Compared with traditional ore extraction, extracting lithium from salt lake brine has the advantages of low cost and relatively simple operation.

[0043] Therefore, extracting lithium resources from existing salt lake brines is an important way for my country to supply lithium resources in the future. However, the nanofiltration membranes currently available on the market have poor separation effect on lithium and magnesium ions, which are key points in the lithium extraction process from salt lakes, and cannot meet the application requirements.

[0044] Traditional polyamide membrane materials have good film-forming properties, but their positive charge is weak and their stability is poor, resulting in poor performance in extracting lithium resources from solutions with high magnesium-to-lithium ratios. Therefore, the applicant has improved traditional polyamide materials to construct novel nanomembranes to achieve efficient separation of lithium and magnesium ions, breaking through the bottlenecks of traditional polyamide nanomembranes.

[0045] Specifically, the applicant has developed a covalent organic framework bilayer positively charged nanofiltration membrane; please refer to [link to relevant documentation]. Figure 1 , Figure 1 This is a schematic flowchart of one embodiment of the preparation method of the covalent organic framework bilayer positively charged nanofiltration membrane of this application.

[0046] As shown in the figure, the preparation method of this bilayer nanofiltration membrane includes:

[0047] S100. Dissolve 1,3,5-pyromellitic methyl ether in an organic solvent to obtain a first solution, and dissolve triaminoguanidine hydrochloride in an aqueous solvent to obtain a second solution.

[0048] First, 1,3,5-pyromellitic aldehyde dissolved in an organic solvent was used as a raw material for preparing covalent organic frameworks. At the same time, triaminoguanidinium hydrochloride containing guanidine groups was introduced and dissolved in an aqueous solvent to prepare guanidine ion covalent organic frameworks.

[0049] In one embodiment, the organic solvent may include mesitylene and 1,4-dioxane in a volume ratio of (8-10):1, and the molar concentration of 1,3,5-trimethylbenzaldehyde in the first solution is 0.08-0.12 mol / L.

[0050] In other embodiments, other organic solvents can also be selected, and the molar concentration of 1,3,5-pyromellitic aldehyde can also be selected according to the actual working conditions to achieve the dissolution of 1,3,5-pyromellitic aldehyde, thus achieving the effect of this embodiment.

[0051] In one embodiment, the aqueous solvent may include an acetic acid solution with a mass fraction of 5-6 wt%, and the molar concentration of triaminoguanidine hydrochloride in the second solution may be 0.008-0.012 mol / L. In other embodiments, other aqueous solvents may also be used, and the molar concentration of triaminoguanidine hydrochloride may be selected based on actual operating conditions, all of which can achieve the effect of this embodiment.

[0052] S200. The first solution is dropped onto the surface of the second solution, sealed and allowed to stand for incubation to obtain the third solution. The upper oil phase of the third solution is removed to obtain a covalent organic framework nanosheet dispersion.

[0053] The first solution of the oil phase is dropped onto the surface of the second solution of the aqueous phase. During the sealed static incubation process, 1,3,5-trimethylbenzenealdehyde in the oil phase forms positively charged covalent organic framework nanosheets with small pores. At the same time, the covalent organic framework at the water-oil interface can react with triaminoguanidine hydrochloride to generate guanidine covalent organic framework nanosheets. Thus, guanidine groups are grafted onto the covalent organic framework to improve its positive charge, resulting in a dispersion of covalent organic framework nanosheets.

[0054] In one embodiment, sealed static incubation can be performed by incubating the nanosheets in a 25°C incubator for 3 days. In other embodiments, the incubation temperature, incubation equipment, and incubation time can be adjusted based on actual needs to achieve the synthesis of guanidine-based covalent organic framework nanosheets and thus achieve the effects of this embodiment.

[0055] S300. After diluting the covalent organic framework nanosheet dispersion, mix the diluted covalent organic framework nanosheet dispersion with the polyethyleneimine solution to obtain the fourth solution.

[0056] In one embodiment, the dilution factor of the covalent organic framework nanosheet dispersion can be 8 to 12 times.

[0057] In one embodiment, the volume ratio of the covalent organic framework nanosheet dispersion to the polyethyleneimine solution can be (0.2–1.5):5, the concentration of the polyethyleneimine solution can be 0.4–0.6 g / L, and the molecular weight can be 68,000–72,000.

[0058] S400. After filtering the fourth solution, it is uniformly dispersed and attached to the surface of the base membrane to obtain a composite membrane.

[0059] The fourth solution contains covalent organic framework nanosheets and polyethyleneimine. The fourth solution is filtered and uniformly dispersed and attached to the surface of the base membrane, which can form a covalent organic framework layer with uniformly dispersed polyethyleneimine on the base membrane, thus obtaining a composite membrane.

[0060] In one embodiment, the method of uniformly dispersing and attaching the fourth solution to the surface of the base membrane after filtration can be to place the base membrane at the filtration port of the filtration device and then filter the fourth solution through the filtration port.

[0061] In one embodiment, the base membrane may be a polyethersulfone ultrafiltration membrane.

[0062] S500: The composite membrane is immersed in a heptane solution of trimesoyl chloride and reacted. After the reaction is completed, it is heated for thermal crosslinking to obtain a covalent organic framework bilayer positively charged nanofiltration membrane.

[0063] After forming a covalent organic framework layer with uniformly dispersed polyethyleneimine on the surface of the base film, the composite film can be immersed in a heptane solution of trimesoyl chloride for reaction. Based on the interfacial polymerization reaction, the polyethyleneimine polymerizes to grow a polyamide layer. During the growth of the polyamide layer, the covalent organic framework is fixed, which effectively solves the problem of poor film-forming properties of the covalent organic framework and obtains relatively uniform pores on the polyamide layer.

[0064] Understandably, the guanidinyl covalent organic framework introduced into the covalent organic framework bilayer positively charged nanofiltration membrane effectively improves the positive charge of the nanofiltration membrane. Since magnesium ions have more positive charge than lithium ions, the nanofiltration membrane has a greater retention effect on magnesium ions than on lithium ions, thus improving the lithium-magnesium separation effect.

[0065] Meanwhile, due to the unique uniform pore structure of covalent organic framework nanomaterials and the unique hydrogen bond sites within the pores, magnesium ions have greater difficulty passing through, which is beneficial for lithium ions to pass through.

[0066] In addition, covalent organic framework materials can be effectively fixed during the growth of polyamide. Since the covalent organic framework materials are uniformly mixed with the aqueous monomer polyethyleneimine of polyamide, the covalent organic framework materials can be uniformly dispersed and fixed in the polyamide layer, exhibiting good separation selectivity and stability in lithium magnesium ion mixed solution.

[0067] The technical solution of this application will be explained in further detail below with reference to specific embodiments.

[0068] Example 1:

[0069] A covalent organic framework bilayer positively charged nanofiltration membrane is prepared using the following steps:

[0070] (1) Dissolve 0.2 mmol of 1,3,5-trimethylbenzaldehyde in 20 mL of organic solvent to obtain the first solution, and sonicate for 0.5 h until completely dissolved. The organic solvent is a mixed solution of trimethylbenzylene and 1,4-dioxane in a volume ratio of 9:1.

[0071] (2) Dissolve 0.2 mmol of triaminoguanidine hydrochloride in an aqueous solvent to obtain a second solution, wherein the aqueous solvent is a 5.4 wt% acetic acid solution and the molar concentration of triaminoguanidine hydrochloride in the second solution is 0.01 mol / L.

[0072] (3) The first solution was slowly added dropwise to the surface of the second solution, and the solution was sealed and placed in a biochemical incubator at 25°C for 3 days to obtain the third solution. After removing the oil phase on the upper layer of the third solution, a covalent organic framework nanosheet dispersion was obtained.

[0073] (4) Dilute the covalent organic framework nanosheet dispersion with deionized water to 10 times the original concentration, then take 0.2 mL of the diluted solution and mix it with 0.5 mL of polyethyleneimine solution to obtain the fourth solution, wherein the concentration of the polyethyleneimine solution is 0.5 g / L and the molecular weight is 70000.

[0074] (5) Place the 5cm diameter polyethersulfone ultrafiltration membrane in the filtration port of the vacuum filtration flask device, and filter the fourth solution through the filtration port to obtain the composite membrane.

[0075] (6) The composite membrane was immersed in a 0.2 g / L solution of pyromellitic chloride in n-heptane for 60 s and then removed and placed in an oven at 60 °C for thermal crosslinking for 10 min to obtain a covalent organic framework double-layer positively charged nanofiltration membrane.

[0076] Example 2:

[0077] A covalent organic framework bilayer positively charged nanofiltration membrane is prepared in a manner that is basically the same as in Example 1, except that in step (4) of this example, the fourth solution is obtained by mixing 0.4 mL of diluent with 0.5 mL of polyethyleneimine solution.

[0078] Example 3:

[0079] A covalent organic framework bilayer positively charged nanofiltration membrane is prepared in a manner that is basically the same as in Example 1, except that in step (4) of this example, the fourth solution is obtained by mixing 0.8 mL of diluent with 0.5 mL of polyethyleneimine solution.

[0080] Example 4:

[0081] A covalent organic framework bilayer positively charged nanofiltration membrane is prepared in a manner that is basically the same as in Example 1, except that in step (4) of this example, the fourth solution is obtained by mixing 1.5 mL of diluent with 0.5 mL of polyethyleneimine solution.

[0082] Comparative Example 1:

[0083] A nanofiltration membrane is prepared using the following steps:

[0084] A covalent organic framework bilayer positively charged nanofiltration membrane is prepared using the following steps:

[0085] (1) Dissolve 0.2 mmol of 1,3,5-trimethylbenzaldehyde in 20 mL of organic solvent to obtain the first solution, and sonicate for 0.5 h until completely dissolved. The organic solvent is a mixed solution of trimethylbenzylene and 1,4-dioxane in a volume ratio of 9:1.

[0086] (2) Dissolve 0.2 mmol of triaminoguanidine hydrochloride in an aqueous solvent to obtain a second solution, wherein the aqueous solvent is a 5.4 wt% acetic acid solution and the molar concentration of triaminoguanidine hydrochloride in the second solution is 0.01 mol / L.

[0087] (3) The first solution was slowly added dropwise to the surface of the second solution, and the solution was sealed and placed in a biochemical incubator at 25°C for 3 days to obtain the third solution. After removing the oil phase on the upper layer of the third solution, a covalent organic framework nanosheet dispersion was obtained.

[0088] (4) Dilute the covalent organic framework nanosheet dispersion with deionized water to 10 times the original concentration, place a 5 cm diameter polyethersulfone ultrafiltration membrane in the vacuum filtration flask, and then take 0.7 mL of the diluted solution and filter it through the filtration port to obtain a nanofiltration membrane.

[0089] Comparative Example 2:

[0090] A nanofiltration membrane is prepared using the following steps:

[0091] (1) Dissolve 0.2 mmol of 1,3,5-trimethylbenzaldehyde in 20 mL of organic solvent to obtain the first solution, and sonicate for 0.5 h until completely dissolved. The organic solvent is a mixed solution of trimethylbenzylene and 1,4-dioxane in a volume ratio of 9:1.

[0092] (2) Dissolve 0.2 mmol of triaminoguanidine hydrochloride in an aqueous solvent to obtain a second solution, wherein the aqueous solvent is a 5.4 wt% acetic acid solution and the molar concentration of triaminoguanidine hydrochloride in the second solution is 0.01 mol / L.

[0093] (3) The first solution was slowly added dropwise to the surface of the second solution, and the solution was sealed and placed in a biochemical incubator at 25°C for 3 days to obtain the third solution. After removing the oil phase on the upper layer of the third solution, a covalent organic framework nanosheet dispersion was obtained.

[0094] (4) Dilute the covalent organic framework nanosheet dispersion with deionized water to 10 times the original concentration, then take 0.2 mL of the diluted solution and mix it with 0.5 mL of polyethyleneimine solution to obtain the fourth solution, wherein the concentration of the polyethyleneimine solution is 0.5 g / L and the molecular weight is 70000.

[0095] (5) Place the 5cm diameter polyethersulfone ultrafiltration membrane at the filtration port of the vacuum filtration flask device, and filter the fourth solution through the filtration port to obtain a nanofiltration membrane.

[0096] Comparative Example 3:

[0097] A nanofiltration membrane is prepared using the following steps:

[0098] (1) Place a polyethersulfone ultrafiltration membrane with a diameter of 5 cm into the filtration port of the vacuum filtration flask device, take 0.7 mL of polyethyleneimine solution and filter it through the filtration port to obtain a composite membrane. The concentration of the polyethyleneimine solution is 0.5 g / L and the molecular weight is 70000.

[0099] (2) The composite membrane was immersed in a 0.2 g / L solution of pyromellitic chloride in n-heptane for 60 s and then removed and placed in an oven at 60 °C for thermal crosslinking for 10 min to obtain a nanofiltration membrane.

[0100] Example 1: Characterization Analysis

[0101] The surfaces of the covalent organic framework bilayer positively charged nanofiltration membranes prepared in Examples 1 to 4 were observed using high-resolution emission scanning electron microscopy (SEM). SEM images of the membrane products with a scale bar of 500 nm were obtained. Figure 2 , Figure 2 These are SEM images of the covalent organic framework bilayer positively charged nanofiltration membranes from Examples 1 to 4.

[0102] The covalent organic framework bilayer positively charged nanofiltration membranes prepared in Examples 1 to 4 were cut into thin strips and quenched in liquid nitrogen. SEM observation was then performed to obtain SEM cross-sectional images of the membrane products at a scale of 500 nm. Figure 3 , Figure 3 These are SEM images of cross-sections of covalent organic framework bilayer positively charged nanofiltration membranes from Examples 1 to 4.

[0103] The covalent organic framework bilayer positively charged nanofiltration membranes prepared in Examples 1 to 4 were fixed on a glass surface. Microcantilever deformation was induced by the minute van der Waals forces generated when a probe swept across the sample. Sample roughness information was obtained by analyzing the deformation changes of the microcantilever. Figure 4 , Figure 4 These are atomic force microscopy characterization images of the covalent organic framework bilayer positively charged nanofiltration membranes of Examples 1 to 4.

[0104] like Figure 1 As shown, the nanofiltration membrane surface has uniformly distributed small pores, i.e., uniformly dispersed covalent organic frameworks. However, as the doping amount of the covalent organic framework exceeds a certain limit, larger pores appear on the membrane surface, such as... Figure 1 In Example 4, larger pores can affect the retention of magnesium ions.

[0105] like Figures 2 to 3 As shown, with the increase of covalent organic framework doping, the film thickness tends to increase, and the surface roughness of the film also tends to increase.

[0106] Example 2: Nanofiltration performance test

[0107] Nanofiltration performance was tested using a cross-flow filtration device. The covalent organic framework bilayer positively charged nanofiltration membranes prepared in Examples 1 to 4 were used as filter membranes, and a MgCl2 solution with a concentration of 1000 ppm was selected as the filtrate. After pre-pressurization at 4 bar for 0.5 h, the magnesium ion concentration of the permeate was measured, and the pure water flux and magnesium ion rejection rate were calculated. The results are shown in the table below.

[0108]

[0109]

[0110] As shown above, the nanofiltration membranes of Examples 1 to 4 all exhibit excellent MgCl2 retention performance while ensuring high pure water flux. In Examples 1 to 3, the MgCl2 retention performance gradually improves with the increase of covalent organic framework doping. This is mainly because the guanidine covalent organic framework nanomaterial effectively improves the positive charge of the nanofiltration membrane. At the same time, the unique uniform pore structure of the covalent organic framework nanomaterial and the unique hydrogen bond sites within the pores make it more difficult for magnesium ions to pass through, which is conducive to the passage of lithium ions. The MgCl2 retention rate of Example 3 reaches 99.3%.

[0111] However, when the doping amount of the covalent organic framework increases to a certain limit, such as the decrease in MgCl2 retention rate in Example 4, it is mainly because the excessive doping amount forms larger pores, which affects the retention of MgCl2.

[0112] Example 3: Performance Test of Mixed Salt Separation

[0113] The covalent organic framework bilayer positively charged nanofiltration membrane prepared in Example 3 was used as the filter membrane. Solutions with different MgCl2 / LiCl mass ratios of 2000 ppm were selected as filtrates. After pre-pressurization at 4 bar for 0.5 h, the permeate was taken to detect the ion concentration, and the ion rejection rate was calculated. The data in the table below are obtained.

[0114]

[0115]

[0116] As shown in the table above, the covalent organic framework bilayer positively charged nanofiltration membrane prepared in Example 3 maintained a high magnesium ion rejection rate of approximately 98.9% under different mass ratios of MgCl2 / LiCl solution, while maintaining a low lithium ion rejection rate. The lithium ion rejection rate gradually decreased with increasing MgCl2 / LiCl solution mass ratio. This is particularly relevant for high Mg content in salt lake brines in my country. 2+ / Li + It has the characteristics of superior separation effect.

[0117] Example of effect 4:

[0118] Nanofiltration membranes prepared in Comparative Examples 1 to 3 and Example 3 were used as filter membranes, and a solution of 2000 ppm MgCl2 / LiCl with a mass ratio of 10 was selected as the filtrate. After pre-pressurization at 4 bar for 0.5 h, the ion concentration of the permeate was measured, and the magnesium ion rejection rate was calculated. Figure 5 .

[0119] Please see Figure 5 , Figure 5 This is an experimental graph showing the magnesium ion rejection rate of Example 4 of this application. As shown in the figure, Comparative Example 1 has the lowest magnesium ion rejection rate. This is mainly because it only has covalent organic framework nanosheets on the base film. Since the covalent organic framework nanosheets have poor film-forming properties, they cannot form a film, which affects the magnesium ion rejection rate.

[0120] The lithium ion rejection rate of Comparative Example 2 was slightly better than that of Comparative Example 1. This was mainly because Comparative Example 2 added polyethyleneimine solution compared to Comparative Example 1. Polyethyleneimine played a certain role in rejection. However, since polyethyleneimine did not react with the n-heptane solution of trimesoyl chloride to form a polyamide layer, the rejection effect and the fixation effect on the covalent organic framework were both poor, which affected the magnesium ion rejection rate.

[0121] Comparative Example 3 used only a polyethyleneimine solution, which was reacted with a heptane solution of trimesoyl chloride to generate a polyamide layer. Its retention effect on magnesium ions was weaker than that of Example 3. This was mainly because the guanidinyl covalent organic framework introduced into the covalent organic framework bilayer positively charged nanofiltration membrane effectively improved the positive charge of the nanofiltration membrane and improved the lithium-magnesium separation effect. At the same time, due to the unique uniform pore structure of the covalent organic framework nanomaterial and the unique hydrogen bond sites in the pores, it is more difficult for magnesium ions to pass through, which is conducive to the passage of lithium ions.

[0122] In addition, it is understandable that the magnesium ion rejection rates of Comparative Example 1, which uses a covalent organic framework alone, and Comparative Example 3, which uses a polyamide layer alone, are both weaker than those of Example 3, which uses both a covalent organic framework and a polyamide layer. The embodiments of this application effectively improve the magnesium ion rejection rate through the synergistic effect of the guanidine covalent organic framework and the polyamide layer.

[0123] Example 5: Charge Performance Analysis

[0124] The surface Zeta potentials of the nanofiltration membranes prepared in Comparative Examples 1 to 3 and Example 3 were measured respectively to obtain... Figure 6 .

[0125] Please see Figure 6 , Figure 6 This is a Zeta potential data graph for Example 5 of this application. As shown in the figure, the Zeta potential of the simple guanidine covalent organic framework membrane prepared in Comparative Example 1 is 24 mV, which is slightly lower than the liquid Zeta potential of the guanidine COF nanosheets. This is because the guanidine COF membrane uses a polyethersulfone ultrafiltration membrane as a substrate, and the surface of the polyethersulfone ultrafiltration membrane itself is negatively charged, which reduces the positive charge of the guanidine COF membrane.

[0126] Compared to Comparative Example 1, Comparative Example 2 added polyethyleneimine. After the addition of polyethyleneimine, the amino density on the surface decreased, resulting in the positive charge of Comparative Example 2 being slightly lower than that of Comparative Example 1.

[0127] Comparative Example 3 used only a polyethyleneimine solution, which reacted with a heptane solution of trimesoyl chloride to generate a polyamide layer. The polyamide layer had the lowest positive charge. This is because polyethyleneimine, as the aqueous monomer of the polyamide layer, had most of its amino groups reacting with trimesoyl chloride to form a polyamide crosslinking network, which was consumed, resulting in a reduction in the number of protonable amino groups. At the same time, the unreacted acyl chloride groups hydrolyzed to generate carboxyl groups, further reducing the positive charge on the membrane surface.

[0128] Example 3 further reacted with trimesoyl chloride to form a polyamide crosslinking network compared to Comparative Example 2. Some of the amino groups on the polyethyleneimine and guanidine covalent organic framework were also consumed in the reaction, resulting in a slight decrease in the surface positive charge density compared to Comparative Example 2. However, the surface positive charge density of the film in Example 3 still reached 1.68 mC·m. -2 It is 1.4 times that of Comparative Example 3, which confirms that introducing a guanidine covalent organic framework into a conventional polyamide film can effectively increase the surface positive charge density of the composite nanofiltration membrane.

[0129] Example of effect 6:

[0130] The zeta potential of the nanofiltration membrane surface in Comparative Example 3 and Example 3 was measured at different pH values ​​to obtain... Figure 7 .

[0131] Please see Figure 7 , Figure 7This is a Zeta potential data graph for Example 6 of this application. As shown in the figure, the surface Zeta potential of the nanofiltration membrane in Example 3 is consistently higher than that in Comparative Example 3. When the pH is less than 7, the difference in surface Zeta potential between the nanofiltration membrane in Example 3 and the nanofiltration membrane in Comparative Example 3 remains at approximately 7 mV; when the pH is greater than 7, the difference in surface Zeta potential between the nanofiltration membrane in Example 3 and the nanofiltration membrane in Comparative Example 3 rapidly increases to approximately 14 mV.

[0132] This is because the amino groups on the polyamide surface have a strong protonation ability under acidic conditions, but under alkaline conditions, most of the amino groups on the polyamide surface quickly lose protons, leading to a rapid decrease in the number of positively charged groups on the membrane surface and a corresponding rapid decrease in the positive charge density on the membrane surface. At pH = 8.08, the nanofiltration membrane surface of Comparative Example 3 is electrically neutral; as the pH continues to increase, the nanofiltration membrane surface of Comparative Example 3 becomes negatively charged.

[0133] Compared to the nanofiltration membrane of Comparative Example 3, the nanofiltration membrane of Example 3 incorporates many COF nanosheets with guanidinium groups. The guanidinium group itself contains an N+ with a positive charge, which can still provide a positive charge under alkaline conditions. At the same time, the other amino groups in the guanidinium group are relatively close together, and this positional relationship can improve the ability of the amino groups to retain protons under alkaline conditions.

[0134] As can be seen from the above, the guanidinium group can continuously provide positive charge under different pH conditions, ensuring the positive charge of the nanofiltration membrane, thereby ensuring the magnesium-lithium separation performance.

[0135] The nanofiltration membrane of Example 3 exhibits a significantly higher surface positive charge density than that of Comparative Example 3 under all pH conditions, but this advantage is even more pronounced under alkaline conditions. The higher surface positive charge density of Example 3 can enhance the Donnan effect of the membrane, thereby improving its magnesium-lithium separation capability. Considering that brine solutions from salt lakes are generally weakly alkaline in practical applications, the nanofiltration membrane of Example 3 is expected to achieve good results in real-world applications.

[0136] Example 7:

[0137] The retention performance of nanofiltration membranes of Comparative Example 3 and Example 3 for neutral polyethylene glycol (PEG) of different molecular weights was tested to calculate the average pore size of the nanofiltration membranes, thereby describing the size sieving capability of the composite nanofiltration membranes. Figure 8 .

[0138] Please see Figure 8 , Figure 8This is a test data graph for Example 7 of this application. As shown in the figure, compared with the polyamide membrane of Comparative Example 3, the nanofiltration membrane of Example 3, which is doped with a certain amount of guanidine covalent organic framework nanosheets, has a stronger ability to retain neutral PEG molecules and exhibits stronger size sieving ability. This may be because the guanidine covalent organic framework nanosheets themselves have a relatively small pore structure and contain many amino groups, which can also undergo amidation reactions with TMC. Therefore, adding an appropriate amount of guanidine COF nanosheets can improve the crosslinking degree of the PA-COF-3 membrane.

[0139] Example 8:

[0140] Molecular dynamics analysis was performed on the nanofiltration membrane of Example 3, and the results were obtained. Figure 9 .

[0141] Please see Figure 9 , Figure 9 This is a molecular dynamics simulation diagram of Example 8 of this application. As shown in the figure, the nanofiltration membrane of Example 3 simulates a pore size of 0.66 nm, slightly larger than that of Li. + 0.38nm and Mg 2+ A depth of 0.43 nm can improve the positive charge and size sieving ability of nanofiltration membranes, thereby enhancing lithium-magnesium separation.

[0142] The foregoing description of this disclosure is provided to enable any person skilled in the art to implement or use this disclosure. Various modifications to this disclosure will be apparent to those skilled in the art, and the general principles applicable herein can be applied to other variations without departing from the scope of this disclosure. Therefore, this disclosure is not limited to the examples and designs described herein, but is consistent with the widest scope of the principles and novel features disclosed herein.

Claims

1. A method for preparing a covalent organic framework bilayer positively charged nanofiltration membrane, characterized in that, include: 1,3,5-Pyromellitic methyl ether is dissolved in an organic solvent to obtain a first solution, wherein the organic solvent comprises mesitylene and 1,4-dioxane in a volume ratio of (8~10):1, and the molar concentration of 1,3,5-pyromellitic methyl ether in the first solution is 0.08~0.12 mol / L. Triaminoguanidine hydrochloride is dissolved in an aqueous solvent to obtain a second solution, wherein the aqueous solvent comprises an acetic acid solution with a mass fraction of 5~6 wt%, and the molar concentration of triaminoguanidine hydrochloride in the second solution is 0.008~0.012 mol / L. The first solution was dropped onto the surface of the second solution, sealed and allowed to stand for incubation to obtain a third solution. The upper oil phase of the third solution was removed to obtain a covalent organic framework nanosheet dispersion. After diluting the covalent organic framework nanosheet dispersion, the diluted covalent organic framework nanosheet dispersion was mixed evenly with a polyethyleneimine solution to obtain a fourth solution. The dilution factor of the covalent organic framework nanosheet dispersion was 8 to 12 times, the volume ratio of the covalent organic framework nanosheet dispersion to the polyethyleneimine solution was (0.2 to 1.5): 5, the concentration of the polyethyleneimine solution was 0.4 to 0.6 g / L, and the molecular weight was 68,000 to 72,000. The fourth solution is filtered and then uniformly dispersed and attached to the surface of the base membrane to obtain a composite membrane. The composite membrane is immersed in a heptane solution of trimesoyl chloride and reacted. After the reaction is completed, it is heated for thermal crosslinking to obtain the covalent organic framework double-layer positively charged nanofiltration membrane. The concentration of trimesoyl chloride in the heptane solution is 0.15~0.25 g / L, and the reaction time is 60 s.

2. The preparation method according to claim 1, characterized in that, The sealed static culture specifically refers to the culture being placed in a sealed static incubator at 25°C for 3 days.

3. The preparation method according to claim 1, characterized in that, The step of filtering the fourth solution and then uniformly dispersing and attaching it to the surface of the base film specifically includes: The base membrane is placed in the filtration port of the filtration device, and the fourth solution is taken and filtered through the filtration port so that the fourth solution is evenly dispersed and attached to the surface of the base membrane after filtration. The base membrane is a polyethersulfone ultrafiltration membrane.

4. The preparation method according to claim 1, characterized in that, The heating temperature in the thermal crosslinking step after the reaction is completed is 55~65℃, and the heating time is 8~12min.

5. A covalent organic framework bilayer positively charged nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 4.

6. The application of a covalent organic framework bilayer positively charged nanofiltration membrane prepared by any one of the preparation methods described in claims 1 to 4 in the lithium extraction process from salt lakes.

Images