An ultrathin defect-free zifs@go composite film, a preparation method and application thereof

Abstract

The application discloses an ultrathin defect-free ZIFs@GO composite film, a preparation method and application thereof, and comprises the following steps: 1) preparing GO by a modified Hummers method; 2) depositing GO on a polymer base film to prepare a GO composite film; and 3) growing ZIFs coordination polymers on the GO composite film by an interfacial counter-diffusion method to prepare the composite film. The application optimizes the ZIFs synthesis conditions and reduces non-selective defects to improve the separation performance of the ZIFs film. In the reverse diffusion process, GO acts as a barrier to make the precursor diffuse along the interlamellar gap and greatly slow down the diffusion rate; by optimizing the driving force of diffusion in the solution, the ZIFs crystallization rate can be accurately matched, the ZIFs growth content can be effectively reduced, and the generation of intercrystalline defects can be reduced. The GO lamella is rich in oxygen functional groups, which provides sites for the heterogeneous nucleation of ZIFs, especially at the edges and defects of the GO lamella, so that the non-selective defects in the GO film can be effectively reduced; meanwhile, the nanoscale limitation enables the ZIFs heteroepitaxial growth to be carried out along the two-dimensional channel instead of the vertical direction, which is beneficial to the formation of an ultrathin crystal structure and realizes the preparation of the ultrathin defect-free composite film.

Description

Technical Field

[0001] This invention belongs to the field of gas separation membrane technology, and relates to an ultrathin, defect-free ZIFs@GO composite membrane and its preparation method. The obtained ZIFs@GO composite membrane can be used in, but is not limited to, gas separation systems. Background Technology

[0002] Global warming and climate change have attracted widespread attention due to the emission of greenhouse gases (carbon dioxide (CO2), methane, nitrous oxide, ozone, and chlorofluorocarbons, etc.). Simulation studies have shown that a CO2 concentration of 450 μL / L represents a critical point for abrupt climate and environmental changes; therefore, controlling atmospheric CO2 concentration is a crucial strategy for addressing the climate crisis. Membrane separation technology is increasingly used for efficient CO2 capture due to its environmental friendliness, low energy and capital costs, and small equipment footprint. A key challenge in achieving efficient CO2 capture through membrane separation is the preparation of membrane materials that possess both high permeability and high selectivity. Traditional organic polymer membranes are widely used due to their low cost and ease of processing, while novel separation membranes, represented by microporous materials, are also attracting attention due to their ease of membrane formation and nanoscale transport channels.

[0003] Metal-organic frameworks (MOFs) are microporous crystals formed by rigid organic units through metal-organic ligand coordination or hydrogen bonding, possessing regular and definite pore sizes. Due to their diverse structures and functions, MOFs contain unsaturated metal sites on their surfaces that can coordinate with gas molecules for adsorption and separation. They also exhibit good thermal stability, discrete and ordered structures, ultra-low density, and large internal surface area (exceeding 6000 m²). 2 MOFs offer advantages such as ease of synthesis and compatibility. Furthermore, the shape, size, and chemical functionality of the pores within MOF materials can be tuned by selecting suitable linker-metal pairs. Among these, zeolite imidazolate framework (ZIF) materials are finding increasingly widespread application in gas separation membranes. According to Kwon et al.'s article "Heteroepitaxially Grown Zeolitic Imidazolate Framework Membranes with Unprecedented Propylene / Propane Separation Performances," ZIF-67 readily undergoes uniform nucleation in solution but struggles with heterogeneous nucleation on substrate surfaces, making the preparation of ultrathin (<100 nm) ZIF membranes extremely challenging. Therefore, the preparation of defect-free ultrathin ZIF membranes is a key technology for achieving high-performance gas separation. Summary of the Invention

[0004] The purpose of this invention is to provide an ultrathin, defect-free ZIFs@GO composite membrane material, solving the problem of difficulty in preparing ultrathin, defect-free ZIF selective layers in existing technologies. Simultaneously, a method for preparing a defect-free ultrathin ZIFs@GO composite membrane is provided, which is simple to operate and can yield MOF gas separation composite membranes with stable performance and high gas selectivity.

[0005] To achieve the above-mentioned objectives, the technical solution of the present invention is as follows:

[0006] An ultrathin, defect-free ZIFs@GO composite membrane material includes a ZIFs@GO separation layer and a support layer. The support layer is a polymer membrane serving as the base membrane. The separation layer is a ZIFs-embedded graphene oxide (GO) layer, specifically composed of a graphene oxide layer deposited on the base membrane and thin ZIFs layers embedded within the internal pores of the graphene oxide. In this membrane material, the regular pores within the ZIFs and the interlayer gaps of the GO layers serve as two-dimensional gas transport channels. These two elements work synergistically to form a defect-free structure, resulting in stable membrane performance and high gas selectivity.

[0007] The thickness of the ZIFs@GO composite film material is 20-100 nm.

[0008] A method for preparing an ultrathin, defect-free ZIFs@GO composite membrane material includes the following steps:

[0009] Step 1: Preparation of graphene oxide (GO)

[0010] 1.1) Slowly inject flake graphite and sodium nitrate into concentrated sulfuric acid, stir in an ice-water bath for 0.5-3 hours, then slowly add potassium permanganate, continue stirring for another 0.5-3 hours, and then raise the temperature to 35-70℃ to continue the reaction for 0.5-2 hours. During this reaction, the strong acid penetrates into the graphite interlayers, forming a graphite-strong acid first-order intercalation, which widens the interlayer spacing of the graphite. With the strong acid as a catalyst, the strong oxidant begins to oxidize the graphite from the periphery to the center, introducing oxygen-containing functional groups such as hydroxyl, carboxyl, epoxy, and carbonyl groups at the edges and inside of the flakes.

[0011] 1.2) Add deionized water to the above reaction system for the first time, and continue the reaction at 85-98℃ for 0.5-1.5h. During this reaction, the OH groups in the water molecules... - With HSO4 - Ion exchange occurs. The dissociated HSO4 - It can react with graphite, increasing the interlayer spacing of the graphite sheets. Then, deionized water is added a second time, and the reaction continues for 0.5-3 hours, after which it is allowed to cool to room temperature. During this reaction, the deionized water dilutes the product, preventing graphene oxide agglomeration and facilitating the obtaining of highly dispersed GO nanosheets.

[0012] 1.3) Add 30% hydrogen peroxide to the above reaction system to remove excess potassium permanganate, then wash repeatedly with deionized water and centrifuge until the resulting supernatant is neutral. Finally, disperse by ultrasonication and dry to obtain solid GO.

[0013] In step 1.1), 1.0-3.0g of flake graphite, 0.5-1.5g of sodium nitrate, and 3.0-6.0g of potassium permanganate are added for every 30mL of concentrated sulfuric acid.

[0014] In step 1.2), for every 30 mL of concentrated sulfuric acid added in step 1.1), the first addition of deionized water corresponds to 50-100 mL of deionized water, and the second addition of deionized water corresponds to 50-100 mL of deionized water.

[0015] In step 1.3), for every 30 mL of concentrated sulfuric acid from step 1.1), 10-50 mL of hydrogen peroxide is added.

[0016] Step 2: Preparation of GO composite membrane

[0017] 2.1) At room temperature, the GO obtained in the first step is added to solvent A, and a uniform GO dispersion is obtained by stirring and sonication, and the dispersion is prepared into a concentration of 0.0005-2.0 g / L.

[0018] 2.2) The obtained GO dispersion was subjected to pressure filtration to uniformly stack GO on the surface of the polymer-based membrane at a pressure of 1-10 bar for 0.5-2 h to obtain a wet GO composite membrane;

[0019] 2.3) Place the wet GO composite membrane in an electric heating constant temperature drying oven and leave it at a temperature of 25-40℃ for 15-30 hours to obtain the GO composite membrane.

[0020] In step 2.1), solvent A is one or more of deionized water, methanol, and ethanol.

[0021] In step 2.2), the GO deposition amount on the polymer base film surface is 0.5-10 g / m. 2 The interlayer spacing of the GO stacked structure is 0.5-1.3 nm.

[0022] In step 2.2), the polymer base film in the second step is one of polysulfone, polyethersulfone, and polyacrylonitrile.

[0023] The preparation mechanism of this step is as follows: A GO dispersion is placed above a polymer-based membrane. Under pressure, solvent water permeates through the polymer-based membrane, while GO nanosheets are trapped on the membrane surface, forming a GO stacked layer. Increasing the oxidation temperature and reaction time increases the degree of GO oxidation, leading to an increase in oxygen-containing functional groups on the GO surface and electrostatic repulsion between GO layers, thus increasing the GO interlayer spacing. Furthermore, increasing the filtration pressure and extending the filtration time results in a more compact GO layer packing, thereby reducing the GO interlayer spacing.

[0024] Step 3: Preparation of ZIFs@GO composite membrane

[0025] 3.1) At room temperature, the metal salt is added to solvent B and dispersed by stirring to form a uniform metal salt solution, which is then prepared into a dispersion of 0.1-1.5 g / L; the organic ligand is added to solvent C and dispersed by stirring to form a uniform organic ligand solution, which is then prepared into a dispersion of 0.1-18 g / L.

[0026] 3.2) Using the GO composite membrane obtained in the second step as the interface, the GO layer side of the GO composite membrane is an organic ligand solution, and the base membrane side is a metal salt solution. ZIFs are grown in the surface pores of the GO layer using a reverse diffusion method. After being placed at a temperature of 25-50℃ for 6-24 hours, a wet ZIFs@GO material composite membrane is obtained. The mass ratio of the metal salt to the organic ligand is 1:1-1:12.

[0027] 3.3) Place the ZIFs@GO composite film in an electric thermostatic drying oven at 25-50℃ for 15-30h to obtain the ZIFs@GO composite film with a thickness of 20-100nm.

[0028] In step 3.1), solvent B is one or more of deionized water, methanol, and ethanol, and solvent C is one or more of deionized water, methanol, and ethanol.

[0029] In step 3.1), the metal salt is selected from zinc nitrate hexahydrate and cobalt nitrate hexahydrate. The organic ligand is selected from dimethylimidazole and benzimidazole.

[0030] In step 3.2), the ZIFs are one of ZIF-7, ZIF-8, and ZIF-67.

[0031] The reaction mechanism of this step is as follows: During the reverse diffusion process, the GO layer acts as a barrier, separating the two precursors (i.e., the organic ligand and the metal ion) on both sides. Therefore, the precursors only meet each other through spontaneous diffusion along the lamellar channels of GO. Furthermore, the nucleation of ZIFs occurs at the chelation sites between the metal ion and the oxygen-containing functional groups of GO, particularly at the edges and defects of the GO lamellars, effectively reducing homogeneous reactions in the bulk solution. In addition, the ZIF crystals grown between the GO layers form a defect-free, dense, selective layer, further slowing down the diffusion rate of the two precursors. Once a continuous defect-free ZIF layer is formed, the diffusion of the two precursors is blocked. This "self-constraining" mechanism ensures the formation of ultrathin ZIFs@GO composite layers, thus enabling highly efficient gas separation performance.

[0032] This invention employs a one-step in-situ method based on the concept of reverse diffusion, introducing a GO membrane to regulate the diffusion rates of two precursors, optimizing ZIF synthesis conditions, reducing non-selective defects, and improving the separation performance of the ZIF membrane. During reverse diffusion, GO acts as a barrier, causing the precursors to diffuse along the interlamellar gaps and significantly slowing down the diffusion rate. By optimizing the driving force of diffusion in solution, the ZIF crystallization rate can be precisely matched, effectively reducing the ZIF growth content and decreasing the formation of intergranular defects. GO lamellars have abundant oxygen-containing functional groups, providing sites for heterogeneous nucleation of ZIFs, especially at the edges and defects of GO lamellars, effectively reducing non-selective defects within the GO membrane. Simultaneously, the nanoscale constraint allows the heteroepitaxial growth of ZIFs to proceed along two-dimensional channels rather than vertically, which is beneficial for the formation of ultrathin crystal structures and the preparation of ultrathin, defect-free composite membranes.

[0033] An ultrathin, defect-free ZIFs@GO composite membrane material for use, but not limited to, gas separation systems.

[0034] The beneficial effects of this invention are:

[0035] (1) Two-dimensional ultrathin GO sheets can serve as growth interfaces for ZIFs and promote heterogeneous nucleation of ZIF crystals. In addition, oxygen-containing functional groups at GO sheet defects chelate with the metal source, providing sites for precise growth of ZIFs and controlling crystal distribution. The targeted growth of ZIF crystals reverses the modification of GO defects, thereby improving the separation selectivity of GO.

[0036] (2) The “self-regulation” mechanism in the reverse diffusion method effectively controls the growth content of MOF crystals during the reaction process, which can ensure that ZIFs crystals are grown in GO sheets while maintaining the ultrathin structure of GO film.

[0037] (3) The method of the present invention can easily prepare ZIFs@GO composite membrane materials at room temperature by adding water and ethanol. The reaction solvent used has low toxicity and is an environmentally friendly reaction. Attached Figure Description

[0038] Figure 1 This is a scanning electron microscope image of ZIF-67@GO from Example 3.

[0039] Figure 2 These are the Fourier transform infrared spectra of ZIF-67@GO, ZIF-67, and GO from Example 3.

[0040] Figure 3 These are gas separation performance diagrams for the examples and comparative examples. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that while the technical solutions of this invention are described in detail below through embodiments, the scope of protection of this invention is not limited thereto.

[0042] Example 1: The method of the present invention includes the following steps:

[0043] Step 1: Preparation of graphene oxide (GO)

[0044] 1.1) Slowly add 3.0g of flake graphite and 1.5g of sodium nitrate into 30mL of concentrated sulfuric acid. Stir in an ice-water bath for 0.5h, then slowly add 3.0g of potassium permanganate and continue stirring for 0.5h. Then raise the temperature to 50℃ and continue the reaction for 0.5h.

[0045] 1.2) Add 100 mL of deionized water to the above reaction system for the first time, and raise the temperature to 85 °C and continue the reaction for 1 h; then add 50 mL of deionized water for the second time, and continue the reaction for 0.5 h; finally, let it stand and cool to room temperature.

[0046] 1.3) Add 50 mL of 30% hydrogen peroxide to the above reaction system to remove excess potassium permanganate, then wash repeatedly with deionized water and centrifuge until the resulting supernatant is neutral. Finally, disperse by ultrasonication and dry to obtain solid GO.

[0047] Step 2: Preparation of GO composite membrane

[0048] 2.1) At room temperature, the GO obtained in the first step was added to the solvent ethanol, and a uniform GO dispersion was obtained by stirring and sonication, and a dispersion of 1.0 g / L was prepared.

[0049] 2.2) The obtained GO dispersion was subjected to pressure filtration to uniformly stack GO on the surface of a polyacrylonitrile-based membrane at a pressure of 1 bar for 0.5 h to obtain a wet GO composite membrane; wherein the deposition amount of GO on the polymer-based membrane was 10 g / m³. 2 The interlayer spacing of GO sheets is 1.3 nm;

[0050] 2.3) Place the wet GO composite membrane in an electric thermostatic drying oven and leave it at 40°C for 30 hours to obtain the GO composite membrane.

[0051] Step 3: Preparation of ZIFs@GO composite membrane

[0052] 3.1) At room temperature, zinc nitrate hexahydrate metal salt was added to ethanol solvent and stirred to form a uniform metal salt solution, which was then prepared into a 0.1 g / L dispersion. Benzimidazole organic ligand was added to ethanol solvent and stirred to form a uniform organic ligand solution, which was then prepared into a 0.1 g / L dispersion.

[0053] 3.2) Using the GO composite membrane obtained in the second step as the interface, the GO layer side of the GO composite membrane is an organic ligand solution, and the base membrane side is a metal salt solution. ZIF-7 is grown in the surface pores of the GO layer using a reverse diffusion method. After being placed at 50°C for 24 hours, a wet ZIFs@GO material composite membrane is obtained. The mass ratio of the metal salt to the organic ligand is 1:1.

[0054] 3.3) The ZIFs@GO composite film was placed in an electric thermostatic drying oven at 50°C for 15 hours to obtain the ZIFs@GO composite film with a thickness of 100 nm.

[0055] In Example 1, the ZIF-7 crystals in the ZIF-7@GO composite film are uniformly distributed within the GO sheets, with most of the ZIF-7 crystals having a size of approximately 800 nm. Furthermore, there is no obvious phase interface between ZIF-7@GO and polyacrylonitrile, indicating a tight bond between the selective layer and the base film.

[0056] Example 2: The method of the present invention includes the following steps:

[0057] Step 1: Preparation of graphene oxide (GO)

[0058] 1.1) Slowly add 1.5g of flake graphite and 1.0g of sodium nitrate into 30mL of concentrated sulfuric acid. Stir in an ice-water bath for 3h, then slowly add 5.0g of potassium permanganate. Continue stirring for 3h, then raise the temperature to 70℃ and continue the reaction for 2h.

[0059] 1.2) Add 75 mL of deionized water to the above reaction system for the first time, and continue the reaction at 90 °C for 1.5 h; add 75 mL of deionized water a second time, and continue the reaction for 3 h; finally, let it stand and cool to room temperature.

[0060] 1.3) Add 25 mL of 30% hydrogen peroxide to the above reaction system to remove excess potassium permanganate, then wash repeatedly with deionized water and centrifuge until the resulting supernatant is neutral. Finally, disperse by ultrasonication and dry to obtain solid GO.

[0061] Step 2: Preparation of GO composite membrane

[0062] 2.1) At room temperature, the GO obtained in the first step was added to the solvent methanol, and a uniform GO dispersion was obtained by stirring and sonication, which was prepared into a dispersion of 2.0 g / L.

[0063] 2.2) The obtained GO dispersion was subjected to pressure filtration to uniformly stack GO on the surface of the polysulfone-based membrane at a pressure of 10 bar for 2 hours to obtain a wet GO composite membrane; wherein the deposition amount of GO on the polymer-based membrane was 0.5 g / m³. 2 The interlayer spacing of GO sheets is 0.5 nm.

[0064] 2.3) Place the wet GO composite membrane in an electric thermostatic drying oven and leave it at 25°C for 15 hours to obtain the GO composite membrane.

[0065] Step 3: Preparation of ZIFs@GO composite membrane

[0066] 3.1) At room temperature, zinc nitrate hexahydrate metal salt was added to methanol solvent and dispersed by stirring to form a uniform metal salt solution, which was prepared into a 1 g / L dispersion; dimethylimidazole organic ligand was added to methanol solvent and dispersed by stirring to form a uniform organic ligand solution, which was prepared into an 8 g / L dispersion.

[0067] 3.2) Using the GO composite membrane obtained in the second step as the interface, the GO layer side of the GO composite membrane is an organic ligand solution, and the base membrane side is a metal salt solution. ZIF-8 is grown in the surface pores of the GO layer using a reverse diffusion method. After being placed at 30°C for 12 hours, a wet ZIFs@GO material composite membrane is obtained. The mass ratio of the metal salt to the organic ligand is 1:8.

[0068] 3.3) The ZIFs@GO composite film was placed in an electric thermostatic drying oven at 30°C for 30 hours to obtain the ZIFs@GO composite film with a thickness of 80 nm.

[0069] In Example 2, the ZIF-8 crystals in the ZIF-8@GO composite film are uniformly distributed within the GO sheets, with most ZIF-8 crystals measuring approximately 500 nm in size. Furthermore, there is no obvious phase interface between ZIF-8@GO and polysulfone, indicating a tight bond between the selective layer and the base film.

[0070] Example 3: The method of the present invention includes the following steps:

[0071] Step 1: Preparation of graphene oxide (GO)

[0072] 1.1) Slowly add 1.0g of flake graphite and 0.5g of sodium nitrate into 30mL of concentrated sulfuric acid. Stir in an ice-water bath for 2h, then slowly add 6.0g of potassium permanganate. Continue stirring for 2h, then raise the temperature to 35℃ and continue the reaction for 2h.

[0073] 1.2) Add 50 mL of deionized water to the above reaction system for the first time, and continue the reaction at 98 °C for 0.5 h; then add 100 mL of deionized water for the second time, and continue the reaction for 1 h; finally, let it stand and cool to room temperature.

[0074] 1.3) Add 10 mL of 30% hydrogen peroxide to the above reaction system to remove excess potassium permanganate, then wash repeatedly with deionized water and centrifuge until the resulting supernatant is neutral. Finally, disperse by ultrasonication and dry to obtain solid GO.

[0075] Step 2: Preparation of GO composite membrane

[0076] 2.1) At room temperature, the GO obtained in the first step was added to the solvent deionized water, and a uniform GO dispersion was obtained by stirring and sonication, and a dispersion of 0.0005 g / L was prepared.

[0077] 2.2) The obtained GO dispersion was subjected to pressure filtration to uniformly stack GO on the surface of the polyethersulfone membrane at a pressure of 5 bar for 1 hour to obtain a wet GO composite membrane; wherein the deposition amount of GO on the polymer membrane was 3 g / m³. 2 The interlayer spacing of GO sheets is 0.8 nm.

[0078] 2.3) Place the wet GO composite membrane in an electric thermostatic drying oven and leave it at 30°C for 24 hours to obtain the GO composite membrane.

[0079] Step 3: Preparation of ZIFs@GO composite membrane

[0080] 3.1) At room temperature, cobalt nitrate hexahydrate metal salt was added to deionized water solvent and dispersed by stirring to form a uniform metal salt solution, which was prepared into a dispersion of 1.5 g / L; dimethylimidazole organic ligand was added to deionized water solvent and dispersed by stirring to form a uniform organic ligand solution, which was prepared into a dispersion of 18 g / L.

[0081] 3.2) Using the GO composite membrane obtained in the second step as the interface, the GO layer side of the GO composite membrane is an organic ligand solution, and the base membrane side is a metal salt solution. ZIF-67 is grown in the surface pores of the GO layer using a reverse diffusion method. After being placed at 25°C for 6 hours, a wet ZIFs@GO material composite membrane is obtained. The mass ratio of the metal salt to the organic ligand is 1:12.

[0082] 3.3) The ZIFs@GO composite film was placed in an electric thermostatic drying oven at 25°C for 24 hours to obtain the ZIFs@GO composite film with a thickness of 20 nm.

[0083] In Example 3, ZIF-67 crystals in the ZIF-67@GO composite film are uniformly distributed within the GO sheets, with most of the ZIF-67 crystals having a size of approximately 300 nm.

[0084] like Figure 1 As shown in the cross-sectional SEM image, the ultrathin ZIF-67@GO selective layer and the porous polymer substrate constitute the composite separation membrane. The ZIF-67@GO selective layer has a thickness of 20 nm, and there is no obvious phase interface between ZIF-67@GO and the polyethersulfone substrate membrane, indicating that the selective layer and the substrate membrane are tightly bonded.

[0085] like Figure 2 As shown, the FT-IR spectra of the GO film are at 3400 and 1716 cm⁻¹. -1 Characteristic peaks appear at 3133, 1580, and 1174 cm⁻¹, which are attributed to stretching vibration peaks of the -OH and C=O functional groups. Based on the FT-IR spectrum of the ZIF-67 film, characteristic peaks are observed at 3133, 1580, and 1174 cm⁻¹. -1 The three absorption peaks at 423 cm⁻¹ represent the stretching vibration peaks of the CH, C=N, and CN bonds in the imidazole ring, respectively; simultaneously, the peak at 423 cm⁻¹... -1 The characteristic peak at 1716 cm⁻¹ is attributed to the bending vibration of the Co-N coordination interaction. Furthermore, due to the coordination interaction between the cobalt ion and the carboxyl group, the peak at 1716 cm⁻¹ in GO... -1 The C=O peak at 1060 cm⁻¹ is significantly reduced in the ZIF-67@GO composite membrane. -1 The appearance of the absorption peak is related to the Co-O bonds formed between the ZIF-67 nanosheets and the GO sheets. Therefore, Figure 2 The characteristic peaks shown fully demonstrate the successful growth of ZIF-67 crystals in the GO stacked layers.

[0086] Comparative Example 1: The method of the present invention includes the following steps:

[0087] Step 1: Preparation of graphene oxide (GO)

[0088] 1.1) Slowly add 1.0g of flake graphite and 0.5g of sodium nitrate into 30mL of concentrated sulfuric acid. Stir in an ice-water bath for 2h, then slowly add 6.0g of potassium permanganate. Continue stirring for 2h, then raise the temperature to 35℃ and continue the reaction for 2h.

[0089] 1.2) Add 50 mL of deionized water to the above reaction system for the first time, and continue the reaction at 98 °C for 0.5 h; then add 100 mL of deionized water for the second time, and continue the reaction for 1 h; finally, let it stand and cool to room temperature.

[0090] 1.3) Add 10 mL of 30% hydrogen peroxide to the above reaction system to remove excess potassium permanganate, then wash repeatedly with deionized water and centrifuge until the resulting supernatant is neutral. Finally, disperse by ultrasonication and dry to obtain solid GO.

[0091] Step 2: Preparation of GO composite membrane

[0092] 2.1) At room temperature, the GO obtained in the first step was added to the solvent deionized water, and a uniform GO dispersion was obtained by stirring and sonication, and a dispersion of 0.0005 g / L was prepared.

[0093] 2.2) The obtained GO dispersion was subjected to pressure filtration to uniformly stack GO on the surface of the polyethersulfone membrane at a pressure of 2 bar for 1 hour to obtain a wet GO composite membrane; wherein the deposition amount of GO on the polyethersulfone membrane was 300 mg / m³. 2 The interlayer spacing of GO sheets is 0.8 nm.

[0094] 2.3) Place the wet GO composite membrane in an electric thermostatic drying oven and leave it at 30°C for 24 hours to obtain the GO composite membrane.

[0095] like Figure 3 As shown, the ZIFs@GO composite membranes prepared in the above embodiments were subjected to H2 and CO2 permeation performance tests at 1 bar and 298 K. Comparative examples (1-3) and Comparative Example 1 show that the H2 flux of the ZIFs@GO composite membranes in the examples is all higher than 1.7 × 10⁻⁶. -7 mol m -2 s -1 Pa -1 Furthermore, the ideal selectivity of H2 / CO2 increased by nearly 2-9 times compared to Comparative Example 1. ZIFs grown in the GO stacked structure using the reverse diffusion method exhibited lower intergranular defects. In addition, ZIFs could effectively repair defects in the GO surface layers without significantly reducing gas flux, achieving the goal of improving separation performance while maintaining a certain gas flux. Therefore, this improved reverse diffusion synthesis method can be used for the preparation of various MOF ultrathin molecular sieve membranes.

[0096] The above-described embodiments are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A method for preparing an ultrathin, defect-free ZIFs@GO composite membrane material, characterized by comprising the following steps: Step 1: Preparation of GO using the modified Hummers method 1.1) Slowly inject flake graphite and sodium nitrate into concentrated sulfuric acid, stir in an ice-water bath for 0.5-3 h, then slowly add potassium permanganate, continue stirring for 0.5-3 h, and then raise the temperature to 35-70 ℃ and continue the reaction for 0.5-2 h. 1.2) Add deionized water to the above reaction system for the first time, and continue the reaction at 85-98℃ for 0.5-1.5h. Then add deionized water for the second time, continue the reaction for 0.5-3h, and let it stand and cool to room temperature. 1.3) Add hydrogen peroxide to the above reaction system to remove excess potassium permanganate, then wash repeatedly with deionized water and centrifuge until the resulting supernatant is neutral; finally, disperse by ultrasonication and dry to obtain GO; Step 2: Preparation of GO composite membrane 2.1) At room temperature, the GO obtained in the first step is added to solvent A, and a uniform GO dispersion is obtained by stirring and sonication, and the dispersion is prepared into a concentration of 0.0005-2.0 g / L. 2.2) The obtained GO dispersion was subjected to pressure filtration to uniformly stack GO on the surface of the polymer-based membrane at a pressure of 1-10 bar for 0.5-2 h to obtain a wet GO composite membrane. The GO deposition amount on the surface of the polymer-based membrane was 0.5-10 g / m³. 2 ,in, The interlayer spacing of the GO stacked structure is 0.5-1.3 nm; 2.3) The wet GO composite membrane was placed in an electric thermostatic drying oven and allowed to stand to obtain the GO composite membrane; Step 3: Preparation of ZIFs@GO composite membrane 3.1) At room temperature, the metal salt is added to solvent B and dispersed by stirring to form a uniform metal salt solution, which is then prepared as a dispersion of 0.1-1.5 g / L. The organic ligand is added to solvent C and dispersed by stirring to form a uniform organic ligand solution, which is then prepared into a dispersion of 0.1-18 g / L. 3.2) Using the GO composite membrane obtained in the second step as the interface, the GO layer side of the GO composite membrane is an organic ligand solution, and the base membrane side is a metal salt solution. ZIFs are grown in the surface pores of the GO layer using a reverse diffusion method. After being placed at a temperature of 25-50 °C for 6-24 h, a wet ZIFs@GO material composite membrane is obtained. The mass ratio of the metal salt to the organic ligand is 1:1-1:

12. 3.3) Place the ZIFs@GO composite membrane in an electric thermostatic drying oven and let it stand to obtain the ZIFs@GO composite membrane.

2. The method for preparing an ultrathin, defect-free ZIFs@GO composite membrane material according to claim 1, characterized in that, in step 1.1), 1.0-3.0 g of flake graphite, 0.5-1.5 g of sodium nitrate, and 3.0-6.0 g of potassium permanganate are added for every 30 mL of concentrated sulfuric acid; in step 1.2), 50-100 mL of deionized water is added for every 30 mL of concentrated sulfuric acid in step 1.1), and 50-100 mL of deionized water is added for the second addition; in step 1.3), 10-50 mL of hydrogen peroxide with a mass fraction of 30% is added for every 30 mL of concentrated sulfuric acid in step 1.1).

3. The method for preparing an ultrathin, defect-free ZIFs@GO composite film material according to claim 1, characterized in that, In step 2.1), solvent A is one or more of deionized water, methanol, and ethanol; in step 2.2), the polymer base film is one of polysulfone, polyethersulfone, and polyacrylonitrile.

4. The method for preparing an ultrathin, defect-free ZIFs@GO composite film material according to claim 1, characterized in that, In step 2.3), the temperature of the constant temperature drying oven is 25-40℃ and the time is 15-30 h; in step 3.3), the temperature of the constant temperature drying oven is 25-50℃ and the time is 15-30 h.

5. The method for preparing an ultrathin, defect-free ZIFs@GO composite film material according to claim 1, characterized in that, In step 3.1), the metal salt is selected from zinc nitrate hexahydrate and cobalt nitrate hexahydrate, and the organic ligand is selected from dimethylimidazole and benzimidazole; in step 3.2), the ZIFs are selected from ZIF-7, ZIF-8 and ZIF-67.

6. The method for preparing an ultrathin, defect-free ZIFs@GO composite film material according to claim 1, characterized in that, In step 3.1), solvent B is one or more of deionized water, methanol, and ethanol, and solvent C is one or more of deionized water, methanol, and ethanol.

7. An ultrathin, defect-free ZIFs@GO composite membrane, wherein the ultrathin, defect-free ZIFs@GO composite membrane is obtained by any one of the preparation methods described in claims 1-6, comprising a ZIFs@GO separation layer and a support layer; characterized in that, The supporting layer is a polymer film, serving as the base film; the separating layer is a ZIFs-embedded GO layer, which is composed of a GO layer deposited on the base film and a ZIFs thin layer embedded in the internal pores of the GO.

8. The ultrathin, defect-free ZIFs@GO composite film according to claim 7, characterized in that, In the membrane material, the regular pores within the ZIFs and the interlayer gaps of the GO serve as two-dimensional gas transport channels, and the two work together to form a defect-free structure.

9. The ultrathin, defect-free ZIFs@GO composite film according to claim 7, characterized in that, The thickness of the ZIFs@GO composite film material is 20-100 nm.

10. An application of the ultrathin, defect-free ZIFs@GO composite membrane material according to any one of claims 7-9, characterized in that, Used in gas separation systems.

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