A hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework film, a preparation method and applications thereof

Abstract

The application provides a hydrophobic-hydrophilic heterogeneous two-dimensional metal organic framework film, a preparation method and application. The hydrophobic-hydrophilic heterogeneous two-dimensional metal organic framework film comprises a hydrophobic MOFs layer and a hydrophilic MOFs layer, and can be used for water body photothermal desalination. The preparation method of the film comprises four steps of preparing a hydrophilic MOFs nanosheet colloidal solution, alkalization modification, hydrophobic modification and solidification into a film. The hydrophobic-hydrophilic heterogeneous two-dimensional metal organic framework film can fully meet the light-heat conversion capacity, water transmission capacity and water evaporation capacity required by solar light-heat water evaporation, and can realize efficient light-heat seawater desalination treatment.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of seawater desalination, and relates to a seawater desalination membrane material, in particular to a hydrophobic-hydrophilic heterogeneous two-dimensional metal organic framework membrane, a preparation method and application thereof. BACKGROUND

[0002] The scarcity of fresh water resources is one of the serious problems currently faced by the world. Currently, the fresh water resources available for direct human use are less than 0.36%. In remote island areas and some coastal areas, seawater desalination has become the only method to solve the shortage of fresh water resources. Photo-thermal seawater desalination is one of the most promising technologies. The main principle of this method is to collect abundant and clean solar energy, use materials with photo-thermal conversion capability to heat and evaporate seawater, and thus achieve the purpose of seawater desalination. Compared with multi-stage flash evaporation, ion exchange and reverse osmosis seawater desalination methods, the photo-thermal seawater desalination method has low equipment requirements and small energy demand, and is a low-cost seawater desalination treatment method.

[0003] A suitable membrane material is a key technical point that determines the efficiency of photo-thermal seawater desalination. A Chinese patent with application publication number CN115945074A discloses a heat-stable ceramic-based MXene composite membrane for photo-thermal desalination. The composite membrane uses a ceramic-based hollow fiber tubular membrane or a flat plate membrane as a carrier, and a crosslinking agent is introduced to modify the outer surface of the ceramic substrate to increase its bonding force with MXene. A heat-stable ceramic-based MXene composite membrane is prepared by vacuum filtration. A Chinese patent with application publication number CN111346576A discloses a graphene-based hydrogel that can be used for photo-thermal desalination water treatment. The patent uses graphene oxide, dopamine hydrochloride and tris(hydroxymethyl) aminomethane hydrochloride as raw materials, and a graphene-based hydrogel is prepared by a one-step method.

[0004] The main defects in the above-mentioned prior art are:

[0005] First, in terms of materials, the ceramic-based MXene composite membrane and the graphene-based hydrogel in the above-mentioned patents do not have pores in the synthesis process of the sheet-shaped MXene material and the sheet-shaped graphene oxide material. The channels for gas dissipation and water transmission are only formed by the stacking of sheets, and the processing efficiency is low when used for seawater desalination.

[0006] Second, in terms of actual application, the ceramic-based MXene composite membrane and the graphene-based hydrogel in the above-mentioned patents are hydrophilic substances, which have poor durability and are prone to salt crystal deposition in saltwater evaporation applications.

[0007] Thirdly, in the preparation aspect, the MXene raw material in the Chinese patent with the application publication number CN115945074A needs to be etched by hydrofluoric acid to selectively etch the Al layer in Ti3AlC2, and the hydrofluoric acid has strong corrosiveness and volatility; and the graphene oxide raw material in the Chinese patent with the application publication number CN111346576A is generally prepared by the Hummers method, and the concentrated sulfuric acid used in the method has strong corrosiveness and strong oxidizing property. The above technical processes are complex and have certain danger. SUMMARY

[0008] In view of the defects and deficiencies of the prior art, the purpose of the present application is to provide a hydrophilic-hydrophobic heterogeneous two-dimensional metal organic framework film, a preparation method and application, and to solve the technical problem that the seawater desalination treatment efficiency of the composite film material in the prior art needs to be further improved.

[0009] To solve the above technical problems, the present application adopts the following technical solutions:

[0010] A preparation method of a hydrophilic-hydrophobic heterogeneous two-dimensional metal organic framework film, the method specifically comprises the following steps:

[0011] Step one, preparing a hydrophilic MOFs nanosheet colloidal solution:

[0012] Step 1.1, preparing a central metal ion solution:

[0013] The metal salt and the surfactant are dispersed in the organic solvent by ultrasonic to prepare the central metal ion solution, wherein the mass ratio of the metal salt to the surfactant is (4-8):1.

[0014] Step 1.2, preparing an organic ligand solution:

[0015] The compound containing the porphyrin structure is dispersed in the organic solvent by ultrasonic to prepare the organic ligand solution.

[0016] Step 1.3, preparing a MOFs nanosheet colloidal solution:

[0017] The central metal ion solution prepared in step 1.1 is mixed with the organic ligand solution prepared in step 1.2, and then heated to 100-150 DEG C for 8-24 h; and then centrifuged and washed to prepare the MOFs nanosheet colloidal solution.

[0018] Step 1.4, centrifugal purification:

[0019] The MOFs nanosheet colloidal solution prepared in step 1.3 is ultrasonically dispersed in water, and then centrifuged at a high speed of 6500-10000 rpm for 20-40 min, and the supernatant is discarded; the precipitate is resuspended and then centrifuged at a low speed of 2500-5000 rpm for 20-40 min, and the upper liquid is sucked to obtain a hydrophilic MOFs nanosheet colloidal solution.

[0020] Step two, alkalization modification:

[0021] The hydrophilic MOFs nanosheet colloidal solution prepared in step 1.4 is mixed with an alkaline solution, and after reaction in a reaction container under a nitrogen atmosphere for 8-24 h, the product obtained by the reaction is centrifuged, and the supernatant is discarded, and then the precipitate is resuspended to obtain an alkalized MOFs nanosheet colloidal solution.

[0022] Step three, hydrophobic modification:

[0023] The hydrophobic modifier is ultrasonically dissolved in an organic solvent, and then slowly added to the alkalized MOFs nanosheet colloidal solution prepared in step two, and then stirred and mixed uniformly, and then centrifuged, and after centrifugation, the supernatant is discarded, and then the precipitate is resuspended to obtain a hydrophobic MOFs nanosheet colloidal solution.

[0024] Step four, film solidification:

[0025] The hydrophobic MOFs nanosheet colloidal solution prepared in step three is stacked in parallel on a porous membrane substrate to obtain a hydrophobic two-dimensional MOFs film; and then the hydrophilic MOFs nanosheet colloidal solution prepared in step 1.4 is stacked in parallel on the hydrophobic two-dimensional MOFs film, and after drying, a film formed on the porous membrane substrate is a hydrophilic-hydrophobic heterogeneous two-dimensional metal organic framework film.

[0026] The present application also has the following technical features:

[0027] Specifically, in step 1.1, the metal salt is selected from copper salt, iron salt and zinc salt.

[0028] Specifically, in step 1.1, the surfactant is one or more of pyrazine, formic acid and benzoic acid.

[0029] Specifically, in steps 1.1, 1.2 and step three, the organic solvent is one or more of N,N-dimethylformamide, anhydrous ethanol, acetone, isopropanol and acetonitrile.

[0030] Specifically, in step 1.2, the compound containing a porphyrin structure is tetra(4-carboxyphenyl)porphyrin.

[0031] Specifically, in step two, the alkaline solution is a sodium hydroxide solution or a potassium hydroxide solution.

[0032] Specifically, in step three, the hydrophobic modifier is a fluorosilane such as tridecafluorooctyltriethoxysilane or heptadecafluorodecyltrimethoxysilane.

[0033] Specifically, in step four, the porous membrane substrate is a polyvinylidene fluoride membrane substrate or a cellulose ester membrane substrate.

[0034] The present invention also protects a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane prepared by the above-described preparation method. The hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane includes a hydrophobic MOF layer and a hydrophilic MOF layer. The hydrophobic MOF layer is prepared by a colloidal solution of hydrophobic MOF nanosheets, and the hydrophilic MOF layer is prepared by a colloidal solution of hydrophilic MOF nanosheets. The thickness of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane is 3 to 10 micrometers.

[0035] This invention also protects the application of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane described above in photothermal desalination of water.

[0036] The beneficial technical effects of this invention compared to the prior art are as follows:

[0037] (I) The hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane of the present invention, in addition to the gas escape channels and water transport channels formed by the stacking of hydrophobic and hydrophilic MOFs layers, also possesses high porosity and abundant hydrophilic channels within the hydrophilic MOFs layer itself, which can selectively prevent salt ion diffusion. Furthermore, the porphyrin-based metal-organic framework combines the advantages of both porphyrin and metal-organic frameworks, exhibiting super-strong light absorption and rapid energy conversion capabilities across the entire visible spectrum. Therefore, this hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane can fully meet the photothermal conversion capacity, water transport capacity, and water evaporation capacity required for solar-powered hot water evaporation, thereby enabling highly efficient solar-powered seawater desalination.

[0038] (II) The hydrophilic and hydrophobic heterogeneous two-dimensional metal-organic framework membrane of the present invention has a layered structure. Since the layers are tightly stacked together, it has a continuous transport channel, which can effectively reduce heat loss in the vertical direction and also has good mechanical properties. At the same time, the large-scale nanosheet structure ensures sufficient heat in the horizontal direction.

[0039] (III) The hydrophilic and hydrophobic heterogeneous two-dimensional metal-organic framework membrane of the present invention has excellent durability, stability and self-cleaning ability, and is suitable for practical industrial applications.

[0040] (IV) The method for preparing the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane of the present invention does not require the use of highly corrosive and highly oxidizing reagents, the process is safe and reliable, and the raw materials and reagents required for preparation are readily available. The reaction conditions are mild and the production cost is low, making it suitable for large-scale production. Attached Figure Description

[0041] Figure 1 This is a SEM image of a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane.

[0042] Figure 2 This is a schematic diagram of a photothermal desalination device.

[0043] The labels in the diagram represent: 1-Seawater storage tank, 2-Freshwater collection tank, 3-Condensation plate, 4-Hydrophilic and hydrophobic heterogeneous two-dimensional metal-organic framework membrane, and 5-Polystyrene foam.

[0044] The technical solution of the present invention will be further described below with reference to the embodiments. Detailed Implementation

[0045] In this invention:

[0046] MOFs refer to metal-organic framework materials, also known as metal-organic skeleton compounds.

[0047] It should be noted that, unless otherwise specified, all raw materials used in this invention are those known in the art.

[0048] Following the above technical solutions, specific embodiments of the present invention are given below. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention.

[0049] Example 1:

[0050] This embodiment provides a method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane, which specifically includes the following steps:

[0051] Step 1: Preparation of hydrophilic MOF nanosheet colloidal solution:

[0052] Step 1.1, Prepare the central metal ion solution:

[0053] Copper nitrate trihydrate and pyrazine were ultrasonically dispersed in a mixed solution of N,N-dimethylformamide and anhydrous ethanol to prepare a central metal ion solution with a concentration of 0.674 g / L. The mass ratio of copper nitrate trihydrate to pyrazine was 6:1, and the volume ratio of N,N-dimethylformamide to anhydrous ethanol was 4:1. In this embodiment, pyrazine can inhibit the longitudinal growth of metal-organic frameworks.

[0054] Step 1.2, Preparation of organic ligand solution:

[0055] Tetra(4-carboxyphenyl)porphyrin was ultrasonically dispersed in N,N-dimethylformamide solution to prepare an organic ligand solution with a concentration of 0.5 g / L.

[0056] Step 1.3, Preparation of MOF nanosheet colloidal solution:

[0057] The central metal ion solution obtained in step 1.1 was mixed with the organic ligand solution obtained in step 1.2, and then heated to 120°C for 16 h. After centrifugation and washing, a colloidal solution of MOF nanosheets was obtained; the volume ratio of metal ion solution to organic ligand solution was 2:1.

[0058] Step 1.4, centrifugal purification:

[0059] The MOF nanosheet colloidal solution obtained in step 1.3 was ultrasonically dispersed in 50 mL of deionized water and centrifuged at 8000 rpm for 30 min to remove small flakes and impurities from the supernatant. The precipitate was resuspended and then centrifuged at 4000 rpm for 30 min. The upper liquid was then collected to obtain the hydrophilic MOF nanosheet colloidal solution with a concentration of 0.05 g / L.

[0060] Step 2, Alkali modification:

[0061] The hydrophilic MOF nanosheet colloidal solution obtained in step 1.4 was mixed with 0.6 mol / L NaOH solution and reacted with nitrogen gas in a three-necked flask for 12 h. Then, the mixture was centrifuged at 4000 rpm for 30 min to remove the remaining alkaline solution in the supernatant. The precipitate was then resuspended to obtain the alkaline MOF nanosheet colloidal solution.

[0062] Step 3, hydrophobic modification:

[0063] The tridecafluorooctyltriethoxysilane was ultrasonically dissolved in anhydrous ethanol, and then the alkalized MOF nanosheet colloidal solution prepared in step two was slowly added. The mixture was stirred and mixed, and then centrifuged at 10,000 rpm for 10 min to remove unmodified nanosheets and impurities from the supernatant. The precipitate was resuspended to obtain the hydrophobic MOF nanosheet colloidal solution with a concentration of 0.12 g / L. The mass ratio of tridecafluorooctyltriethoxysilane, alkalized MOF nanosheet colloidal solution and anhydrous ethanol was 2:1:1.

[0064] Step 4, curing into a film:

[0065] Take 24 mL of the hydrophobic MOF nanosheet colloidal solution prepared in step 3, and use a vacuum filtration device to stack it in parallel onto a polyvinylidene fluoride (PVDF) membrane substrate with a pore size of 0.22 μm and a diameter of 43 mm to prepare a hydrophobic two-dimensional MOF membrane; then take 48 mL of the hydrophilic MOF nanosheet colloidal solution prepared in step 1.4, and use a vacuum filtration device to stack it in parallel onto the hydrophobic two-dimensional MOF membrane, and then dry it for 24 hours. The membrane formed on the PVDF membrane substrate is a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane.

[0066] Example 2:

[0067] This embodiment presents a hydrophilic-phobic heterogeneous two-dimensional metal-organic framework membrane prepared using the method of Example 1, and its structure is characterized as follows: Figure 1 As shown, this hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane comprises a hydrophobic MOF layer and a hydrophilic MOF layer. The hydrophobic MOF layer is prepared from a colloidal solution of hydrophobic MOF nanosheets, and the hydrophilic MOF layer is prepared from a colloidal solution of hydrophilic MOF nanosheets.

[0068] In this embodiment, the thickness of the membrane is calculated based on the mass of the hydrophilic MOF nanosheet colloidal solution, the mass of the hydrophobic MOF nanosheet colloidal solution, and the diameter of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane. The thickness of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane is calculated to be 4 micrometers.

[0069] In this embodiment, a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane was peeled off from a polyvinylidene fluoride (PVDF) membrane substrate, and its desalination performance was then tested. The specific testing procedure was as follows: The hydrophilic layer of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane was bonded to polystyrene foam, and the polystyrene foam was placed in a 250 mL beaker containing NaCl solution (200 g, 3.5 wt%). The beaker was placed on an electronic balance, and a brine evaporation test was conducted under the illumination of a sunlight simulator. The mass change of the brine was recorded in real time using the electronic balance. The brine evaporation rate of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane was finally calculated to be 3.52 ± 0.09 kg / (m²). 2 ·h).

[0070] Example 3:

[0071] This embodiment describes the application of the hydrophilic and hydrophobic heterogeneous two-dimensional metal-organic framework membrane of Example 2 in photothermal desalination of water bodies. This application is realized by using a photothermal desalination device.

[0072] like Figure 2As shown, the photothermal desalination device includes a seawater storage tank (1) and a freshwater collection tank (2). A condensing plate (3) is installed above the seawater storage tank (1) and the freshwater collection tank (2). The condensing plate (3) is inclined. The higher side of the condensing plate (3) is fixed on the seawater storage tank (1), and the lower side of the condensing plate (3) is fixed on the freshwater collection tank (2). The raw water to be desalinated is added into the seawater storage tank (1). Then, the hydrophilic layer of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane (4) is attached to the surface of the polystyrene foam (5). The polystyrene foam (5) with the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane (4) is placed into the raw water to be desalinated. Finally, the photothermal desalination device is placed in the sunlight.

[0073] In this embodiment, the desalination principle of the photothermal desalination device is as follows: Salt water enters the hydrophilic MOFs layer through the pores on the buoyancy layer. This layer has abundant hydrophilic channels, which ensures a sufficient supply of salt water during the evaporation process and selectively prevents the diffusion of salt ions. Then, the salt water enters the hydrophobic MOFs layer, which has efficient photothermal conversion characteristics and thermal conductivity, concentrating the heat at the interface between the hydrophilic and hydrophobic layers. The converted heat rapidly evaporates the water in the pores of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane, and the abundant pores provide a rapid escape path for the steam. Finally, the steam is condensed to obtain fresh water.

[0074] In this embodiment, the polystyrene foam not only has a low density but also has a heat insulation function, further preventing the upper solar heat from being conducted to the water body below the polystyrene foam.

[0075] In this embodiment, desalinated water was finally obtained in the freshwater collection tank, indicating that the photothermal desalination device can achieve desalination of seawater.

[0076] Comparative Example 1:

[0077] This comparative example provides a method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane. The method is basically the same as that in Example 1, except that in the central metal ion solution in step 1.1, the mass ratio of copper nitrate trihydrate to pyrazine is 3:1.

[0078] In this comparative example, the final hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane includes a hydrophobic MOF layer and a hydrophilic MOF layer. The hydrophobic MOF layer is prepared by a colloidal solution of hydrophobic MOF nanosheets, and the hydrophilic MOF layer is prepared by a colloidal solution of hydrophilic MOF nanosheets.

[0079] In this comparative example, the desalination performance of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane was tested. The specific testing process was exactly the same as in Example 1. Finally, the brine evaporation rate of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane was calculated to be 2.97±0.09 kg / (m2·h), which was lower than that in Example 1. This is because the reduced number of copper ions resulted in the failure to completely replace the hydrogen ions in the organic ligands, thereby affecting the photothermal desalination efficiency of the two-dimensional MOF membrane.

[0080] Comparative Example 2:

[0081] This comparative example provides a method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane. The method is basically the same as that in Example 1, except that in step 1.4, anhydrous ethanol is used instead of deionized water, and the concentration of the hydrophilic MOF nanosheet colloidal solution is 0.05±0.02 g / L.

[0082] In this comparative example, the final hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane includes a hydrophobic MOF layer and a hydrophilic MOF layer. The hydrophobic MOF layer is prepared by a colloidal solution of hydrophobic MOF nanosheets, and the hydrophilic MOF layer is prepared by a colloidal solution of hydrophilic MOF nanosheets.

[0083] In this comparative example, the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane was peeled off from the polyvinylidene fluoride membrane substrate, and then the desalination performance of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane was tested. However, because the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane prepared in this comparative example will wrinkle after drying and its mechanical strength will decrease, the illumination of the xenon lamp will be uneven, and the photothermal desalination efficiency cannot be accurately calculated.

[0084] Comparative Example 3:

[0085] This comparative example provides a method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane. The method is basically the same as that in Example 1, except that the speed of high-speed centrifugation in step 1.4 is different.

[0086] In this comparative example, step 1.4 is as follows: The MOF nanosheet colloidal solution obtained in step 1.3 is ultrasonically dispersed in 50 mL of deionized water, centrifuged at 6000 rpm for 30 min to remove small flakes and impurities from the supernatant, the precipitate is resuspended and then centrifuged at 4000 rpm for 30 min, and the upper liquid is collected to obtain the hydrophilic MOF nanosheet colloidal solution.

[0087] In this comparative example, the insufficient rotation speed of the high-speed centrifugation resulted in incomplete removal of smaller nanosheets and impurities from the MOF nanosheet solution. After stacking the nanosheets into a film, numerous microporous defects appeared on the film surface, leading to a decrease in the mechanical properties of the MOF film.

[0088] Comparative Example 4:

[0089] This comparative example provides a method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane. The method is basically the same as that in Example 1, except that the speed of low-speed centrifugation in step 1.4 is different.

[0090] In this comparative example, step 1.4 is as follows: The MOFs nanosheet colloidal solution obtained in step 1.3 is ultrasonically dispersed in 50 mL of deionized water, centrifuged at 8000 rpm for 30 min to remove small flakes and impurities, and then centrifuged at 2000 rpm for 30 min. The upper liquid is then collected to obtain the hydrophilic MOFs nanosheet colloidal solution.

[0091] In this comparative example, due to the excessively low speed of centrifugation, the supernatant after centrifugation contained not only monolayer MOF nanosheets but also unrestricted MOF particles and larger impurities, which affected the film-forming performance of the MOF nanosheet colloidal solution and ultimately failed to obtain a solidified hydrophilic-phobic heterogeneous two-dimensional metal-organic framework membrane.

[0092] Comparative Example 5:

[0093] This comparative example provides a method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane. The method is basically the same as that in Example 1, except that the speed of low-speed centrifugation in step 1.4 is different.

[0094] In this comparative example, step 1.4 is as follows: The MOFs nanosheet colloidal solution obtained in step 1.3 is ultrasonically dispersed in 50 mL of deionized water, centrifuged at 8000 rpm for 30 min to remove small flakes and impurities from the supernatant, the precipitate is resuspended and then centrifuged at 5500 rpm for 30 min, and the upper liquid is collected to obtain the hydrophilic MOFs nanosheet colloidal solution.

[0095] In this comparative example, the excessively high rotation speed of the low-speed centrifugation resulted in the presence of too many small single-layer nanosheets in the MOF colloidal solution, which affected the film-forming performance of the MOF nanosheet colloidal solution and ultimately failed to obtain a solidified hydrophilic-phobic heterogeneous two-dimensional metal-organic framework membrane.

[0096] Comparative Example 6:

[0097] This comparative example provides a method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane. The method is basically the same as that in Example 1, except that in step four, a nylon membrane with a pore size of 0.22 micrometers and a diameter of 43 millimeters is used instead of a polyvinylidene fluoride membrane.

[0098] In this comparative example, the final hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane includes a hydrophobic MOF layer and a hydrophilic MOF layer. The hydrophobic MOF layer is prepared by a colloidal solution of hydrophobic MOF nanosheets, and the hydrophilic MOF layer is prepared by a colloidal solution of hydrophilic MOF nanosheets.

[0099] In this comparative example, since the membrane cannot be detached from the nylon membrane substrate, desalination performance testing and practical application in photothermal brine evaporation are not possible.

[0100] Comparative Example 7:

[0101] This comparative example provides a method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane. The method is basically the same as that in Example 1, except that in step four, the volumes of the hydrophobic MOF nanosheet colloidal solution and the hydrophilic MOF nanosheet colloidal solution are 12 mL and 24 mL, respectively.

[0102] In this comparative example, the final hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane includes a hydrophobic MOF layer and a hydrophilic MOF layer. The hydrophobic MOF layer is prepared by a colloidal solution of hydrophobic MOF nanosheets, and the hydrophilic MOF layer is prepared by a colloidal solution of hydrophilic MOF nanosheets.

[0103] In this comparative example, the thickness of the membrane was calculated based on the mass of the hydrophilic MOF nanosheet colloidal solution, the mass of the hydrophobic and hydrophilic MOF nanosheet colloidal solution, and the diameter of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane. The thickness of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane was calculated to be 2 micrometers.

[0104] In this comparative example, the membrane was too thin to detach from the polyvinylidene fluoride membrane substrate, thus preventing desalination performance testing and its application in actual photothermal brine evaporation.

[0105] Comparative Example 8:

[0106] This comparative example presents a method for preparing a hydrophilic two-dimensional metal-organic framework membrane, which specifically includes the following steps:

[0107] Step 1: Preparation of hydrophilic MOF nanosheet colloidal solution:

[0108] In this comparative example, step one is exactly the same as step one in Example 1.

[0109] Step 4, curing into a film:

[0110] Take 96 mL of the hydrophilic MOF nanosheet colloidal solution prepared in step one, and use a vacuum filtration device to stack it in parallel on a polyvinylidene fluoride membrane substrate with a pore size of 0.22 μm and a diameter of 43 mm to prepare a hydrophilic two-dimensional metal-organic framework membrane.

[0111] In this comparative example, the final hydrophilic two-dimensional metal-organic framework membrane consists only of a hydrophilic MOF layer, which is prepared from a hydrophilic MOF nanosheet colloidal solution.

[0112] In this comparative example, the thickness of the membrane was calculated based on the mass of the hydrophilic MOF nanosheet colloidal solution and the diameter of the hydrophilic two-dimensional metal-organic framework membrane. The calculated thickness of the two-dimensional MOF membrane was 4 micrometers. When this hydrophilic two-dimensional metal-organic framework membrane is used for actual photothermal brine evaporation, salt crystals are easily deposited on its surface, leading to a decrease in desalination efficiency.

Claims

1. A method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane, characterized in that, The method specifically includes the following steps: Step 1: Preparation of hydrophilic MOF nanosheet colloidal solution: Step 1.1, Prepare the central metal ion solution: A central metal ion solution is prepared by ultrasonically dispersing a metal salt and a surfactant in an organic solvent, wherein the mass ratio of the metal salt to the surfactant is (4-8):

1. Step 1.2, Preparation of organic ligand solution: Organic ligand solutions were prepared by ultrasonically dispersing compounds containing porphyrin structures in an organic solvent. Step 1.3, Preparation of MOF nanosheet colloidal solution: The central metal ion solution obtained in step 1.1 was mixed with the organic ligand solution obtained in step 1.2; the mixture was heated to 100-150℃ and reacted for 8-24 hours; then the mixture was centrifuged and washed to obtain a MOF nanosheet colloidal solution. Step 1.4, centrifugal purification: The MOF nanosheet colloidal solution obtained in step 1.3 was ultrasonically dispersed in water and centrifuged at a high speed of 6500-10000 rpm for 20-40 min. The supernatant was discarded, the precipitate was resuspended, and then centrifuged at a low speed of 2500-5000 rpm for 20-40 min. The upper liquid was then collected to obtain the hydrophilic MOF nanosheet colloidal solution. Step 2, Alkali modification: The hydrophilic MOF nanosheet colloidal solution obtained in step 1.4 is mixed with an alkaline solution and reacted in a reaction vessel under a nitrogen atmosphere for 8–24 h. After centrifuging the product obtained from the reaction, the supernatant is discarded, and the precipitate is resuspended to obtain an alkaline MOF nanosheet colloidal solution. Step 3, hydrophobic modification: The hydrophobic modifier is ultrasonically dissolved in an organic solvent, and then slowly added to the alkalized MOF nanosheet colloidal solution prepared in step two. After stirring and mixing, the mixture is centrifuged. After centrifugation, the supernatant is discarded, and the precipitate is resuspended to obtain the hydrophobic MOF nanosheet colloidal solution. The hydrophobic modifier is fluorosilane. Step 4, curing into a film: The hydrophobic MOF nanosheet colloidal solution obtained in step 3 is stacked in parallel on a porous membrane substrate to prepare a hydrophobic two-dimensional MOF membrane; then the hydrophilic MOF nanosheet colloidal solution obtained in step 1.4 is stacked in parallel on the hydrophobic two-dimensional MOF membrane, and after drying, the membrane formed on the porous membrane substrate is a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane.

2. The method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane as described in claim 1, characterized in that, In step 1.1, the metal salt is selected from copper salt, iron salt and zinc salt.

3. The method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane as described in claim 1, characterized in that, In step 1.1, the surfactant is one or more of pyrazine, formic acid and benzoic acid.

4. The method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane as described in claim 1, characterized in that, In steps 1.1, 1.2, and 3, the organic solvent is one or more of N,N-dimethylformamide, anhydrous ethanol, acetone, isopropanol, and acetonitrile.

5. The method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane as described in claim 1, characterized in that, In step 1.2, the compound containing the porphyrin structure is tetrakis(4-carboxyphenyl)porphyrin.

6. The method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane as described in claim 1, characterized in that, In step two, the alkaline solution is either a sodium hydroxide solution or a potassium hydroxide solution.

7. The method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane as described in claim 1, characterized in that, In step three, the hydrophobic modifier is tridecafluorooctyltriethoxysilane or heptadecafluorodecyltrimethoxysilane.

8. The method for preparing a hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane as described in claim 1, characterized in that, In step four, the porous membrane substrate is a polyvinylidene fluoride membrane substrate or a cellulose ester membrane substrate.

9. A hydrophilic-phobic heterogeneous two-dimensional metal-organic framework membrane prepared by the preparation method according to any one of claims 1 to 8, characterized in that, The hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane includes a hydrophobic MOF layer and a hydrophilic MOF layer. The hydrophobic MOF layer is prepared by a colloidal solution of hydrophobic MOF nanosheets, and the hydrophilic MOF layer is prepared by a colloidal solution of hydrophilic MOF nanosheets. The thickness of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane is 3 to 10 micrometers.

10. The application of the hydrophilic-hydrophobic heterogeneous two-dimensional metal-organic framework membrane as described in claim 9 in photothermal desalination of water.

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