A method for preparing a Ni-DLMB type MOF membrane

Abstract

The application belongs to the technical field of membrane separation materials, and relates to a preparation method of a Ni-DLMB type MOF membrane. Specifically, first, a large-size Ni-DLMB crystal is prepared, the submicron crystal seeds are prepared by means of ball milling treatment, then the crystal is introduced to the surface of an alpha-Al2O3 carrier by a coating method, and finally, the dense and defect-free Ni-DLMB type MOF membrane prepared by a secondary growth method has good separation performance for a methanol / methyl tert-butyl ether mixture and a methanol / dimethyl carbonate mixture.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation material technology, and relates to a method for preparing Ni-DLMB type MOF membrane, particularly a ball milling seed-assisted secondary growth method for Ni-DLMB type MOF membrane. Background Technology

[0002] With the growth of global energy demand, energy conservation and efficiency improvement in chemical separation technologies have become a research hotspot. Membrane separation technology, due to its advantages such as low energy consumption, no phase change, and continuous operation, is regarded as an important way to achieve green chemistry. The separation of methanol from methyl tert-butyl ether or methanol from dimethyl carbonate is the core step in the purification after etherification reaction in the petrochemical industry. However, the separation systems have small polarity differences and similar boiling points, and traditional distillation methods are energy-intensive and inefficient. There is an urgent need to develop high-performance separation membranes to replace existing processes.

[0003] Pervaporation is a novel membrane separation technology for separating liquid mixtures. It has many advantages, such as simple and flexible operation, low energy consumption, high separation efficiency, no introduction of other reagents, no need for secondary product treatment, no secondary pollution, and easy scale-up. It can be widely used for organic solvent dehydration and is particularly suitable for isomers, thermosensitive, near-boiling and azeotropic systems that are difficult or impossible to separate by traditional separation methods. It is considered to be an extremely important and promising high technology in current industrial technology transformation.

[0004] Pervaporation membrane materials can be divided into organic pervaporation membranes and inorganic pervaporation membranes. Compared with organic membranes, inorganic membranes have many advantages, such as good thermochemical stability, long service life, fewer vulnerable parts, low maintenance costs, convenient membrane module replacement, high membrane flux, large separation coefficient, no swelling, and resistance to solvent corrosion, making them the preferred membrane material for pervaporation. Inorganic pervaporation membranes use molecular sieves as the membrane layer material (core separation membrane layer), utilizing their regular pore structure to achieve molecular-scale separation between different components.

[0005] Ni-DLMB[Ni2(DL-mal)2(Bipy)] (DL-mal = DL-malic acid, Bipy = 4,4'-bipyridine) is a metal-organic framework material with nickel ions as the metal center and DL-malic acid and 4,4'-bipyridine as ligands. In Ni-DLMB, Ni atoms along the x-axis are coordinated with β-carboxyl O atoms, while Ni atoms along the y-axis are connected by α-O atoms. Furthermore, the O atoms in the metal-oxygen coordination chain along the x-axis are entirely derived from L-malic acid, while along the y-axis, O atoms are alternately provided by L-malic acid and D-malic acid, forming a two-dimensional planar layer with a fes topology. Due to the different coordination directions of Ni and N atoms along the axial direction, adjacent Ni-N bonds exhibit a 45.2° twist angle, resulting in an interwoven structure between adjacent Bipy molecules along the y-axis, forming triangular channels with an ultramicropore window size of 0.3 × 0.42 nm. Its pore size lies between that of methanol (0.38 nm) and methyl tert-butyl ether (0.62 nm), providing a basis for size sieving. Furthermore, its surface hydroxyl functional groups can enhance the affinity for polar molecules such as methanol through hydrogen bonding and dipole interaction, making it an extremely excellent and ideal separation membrane material.

[0006] Currently, MOF molecular sieve membranes are mainly prepared using the secondary growth method. The secondary growth method involves first coating homogeneous molecular sieve seed crystals onto the surface of a support to form a seed layer. Through the induction effect of the seed crystals, the seed layer undergoes a crystallization reaction in the film-forming solution to form a continuous and dense MOF molecular sieve membrane (Modern Chemical Industry, 2018, 38(12):62-66.). By pre-coating the seed crystals, the nucleation and growth phases of MOF crystals are separated, controlling the growth rate of crystals in the membrane layer and achieving controllable preparation of MOF membrane microstructures such as membrane thickness, crystal size, and orientation (Separation and purification technology, 2008, 61(2):175-181.). However, the limitation of this method in preparing Ni-DLMB type MOF molecular sieve membranes is that when the most critical Ni-DLMB type MOF molecular sieve seed crystals are synthesized by traditional solvothermal, hydrothermal, or interfacial diffusion methods, the resulting crystals are generally large, with an average particle size usually between 50-100 μm and a wide size distribution range. Secondary growth relies on the active sites on the seed surface to initiate directional growth. However, large-size Ni-DLMB crystals have a small specific surface area, and the effective growth interface provided per unit mass of seed is insufficient, resulting in a reduced secondary growth rate or difficulty in initiating the growth process. This is especially true in scenarios requiring precise control of the crystal hierarchy, where the requirements for interface matching and lattice orientation cannot be met. Summary of the Invention

[0007] To address the above problems, this invention provides a method for preparing Ni-DLMB type MOF films, which employs a ball milling seed-assisted secondary growth method.

[0008] Ball milling seed-assisted secondary growth is a composite process that uses mechanical ball milling technology to control the size and modify the surface of initial crystals to prepare small-sized seed crystals that meet the requirements for secondary growth. These seed crystals are then used to induce the directional growth of target metal-organic framework (MOF) films on the substrate surface. This is a novel method for preparing MOF separation membranes with excessively large crystal sizes that are difficult to fabricate using conventional methods.

[0009] Compared with conventional hydrothermal secondary growth methods, the ball milling seed-assisted secondary growth method of the present invention, through the synergistic effect of mechanical ball milling and surface modification, does not rely on conventional processes with complex chemical conditions. It breaks through the bottleneck of uncontrollable seed size in traditional methods, so that the seed can be stably suspended in the secondary growth solvent and uniformly loaded on the carrier, and can actively induce the directional epitaxial growth of metal ions and ligands along the seed lattice.

[0010] The technical solution of the present invention:

[0011] A method for preparing a Ni-DLMB type MOF film employs a ball milling seed-assisted secondary growth method. First, large-sized Ni-DLMB crystals are prepared. Then, uniform submicron-sized Ni-DLMB seed crystals are obtained through ball milling combined with solvent precipitation. A uniform and smooth seed layer is then prepared on the surface of a porous α-Al₂O₃ support using spin coating. Finally, a dense, defect-free Ni-DLMB type MOF film is prepared through secondary growth. The steps are as follows:

[0012] (1) Hydrothermal synthesis of large-size Ni-DLMB crystals

[0013] Malic acid, inorganic metal salt, and 4,4'-bipyridine were mixed in an organic solvent and transferred to a high-pressure reactor. The reaction was carried out in an oven. After the reaction was completed, the system was allowed to cool naturally to room temperature. The product was collected by centrifugation, washed repeatedly with methanol, and dried to obtain Ni-DLMB crystals.

[0014] (2) Preparation of Ni-DLMB seeds by ball milling

[0015] The Ni-DLMB crystals obtained in step (1) and the ball milling media (zirconia balls) were added to the ball milling jar at a mass ratio, and anhydrous ethanol was added for ball milling. After ball milling, the product was dispersed in ethanol to settle, and the supernatant was centrifuged and dried to obtain Ni-DLMB seed crystals.

[0016] (3) Coating with Ni-DLMB seed layer

[0017] The ball-milled Ni-DLMB seeds were dispersed in ethanol to obtain Ni-DLMB seed solution; the Ni-DLMB seed solution was coated on the surface of a porous α-Al2O3 support, and the seed-coated support was dried to obtain a uniform, dense and defect-free Ni-DLMB seed layer.

[0018] (4) Preparation of Ni-DLMB type MOF membrane

[0019] Malic acid, inorganic metal salt, and 4,4'-bipyridine were mixed in an organic solvent and transferred to a high-pressure reactor. A porous α-Al₂O₃ support coated with a seed layer was placed into the synthesis solution and transferred to an oven for secondary growth. After the reaction was completed, the system was allowed to cool naturally to room temperature. The sheet-like membrane was then removed, slowly rinsed with methanol, and dried to obtain a Ni-DLMB type MOF membrane.

[0020] In step (1), the inorganic metal salt is nickel acetate tetrahydrate, nickel carbonate, or nickel sulfate hexahydrate; the organic solvent is methanol or ethanol; the molar ratio of malic acid, inorganic metal salt, and 4,4'-bipyridine is (1-3):(1-3):1, preferably (1.5-2.5):(1.5-2.5):1; the hydrothermal reaction temperature is 100-150℃; the hydrothermal reaction time is 4-48 hours; the drying temperature is 80-150℃; and the drying time is 6-48 hours.

[0021] In step (2), the mass ratio of the Ni-DLMB crystal to the ball milling media is 1:5-1:20; the ball milling speed is 200-500 rpm; the ball milling time is 6-12 hours; and the settling time is 6-24 hours.

[0022] In step (3), the mass fraction of the Ni-DLMB seed solution is 0.1wt%-0.5wt%; the porous α-Al2O3 support is in the shape of a tube, plate, hollow fiber or porous channel support, preferably sheet-like; the method for obtaining the seed layer on the surface of the porous α-Al2O3 support is impregnation, hot impregnation, secondary variable temperature hot impregnation, vacuum coating, spraying, wiping or spin coating, preferably spin coating; the drying temperature is 80-150℃; the drying time is 6-48 hours.

[0023] In step (4), the inorganic metal salt is nickel acetate tetrahydrate, nickel carbonate, or nickel sulfate hexahydrate; the organic solvent is methanol or ethanol; the molar ratio of malic acid to inorganic metal salt and 4,4'-bipyridine is (1-3):(1-3):1, preferably (1.5-2.5):(1.5-2.5):1; the time of the secondary growth reaction is 4-24 hours; the temperature of the secondary growth reaction is 120-175℃; the drying temperature is 80-150℃; and the drying time is 6-48 hours.

[0024] The beneficial effects of this invention are:

[0025] (1) The ball milling pretreatment process skips the complex chemical regulation of preparing submicron seed crystals. By adjusting the ball milling speed and time, it can be quickly adapted to different systems, forming a universal preparation platform. Moreover, the single-machine capacity of the ball milling equipment can reach the kilogram level, and the performance repeatability between batches is high, which significantly reduces production costs and improves the feasibility of industrial scale-up.

[0026] (2) After ball milling, the seed crystals expose more metal coordination active sites. The specific surface area of ​​the small-sized seed crystals is 3 to 5 times higher than that of traditional seed crystals, providing sufficient nucleation sites for secondary growth. This promotes the directional epitaxial growth of metal ions and ligands along the seed crystal lattice, significantly reducing the defect rate of the film. The Ni-DLMB type MOF membrane synthesized by the method of this invention has a uniform and highly compact structure. Its separation performance is as follows: at a temperature of 25℃, the flux for 15wt.% methanol / methyl tert-butyl ether is 0.51 kg·m³. -2 ·h -1 The separation factor was 577; the flux for 10 wt.% methanol / dimethyl carbonate was 0.62 kg·m³. -2 ·h -1 With a separation factor of 43, its performance surpasses that of commercial PVA films.

[0027] (3) This invention effectively saves film-making costs, significantly shortens the film-making cycle, fully embodies the concept of green chemistry, and has important industrial promotion and practical application value. Attached Figure Description

[0028] Figure 1 This is a scanning electron microscope (SEM) image of the large-size Ni-DLMB crystal synthesized in Example 1;

[0029] Figure 2 The image shows a scanning electron microscope (SEM) image of the Ni-DLMB seed crystals prepared by ball milling in Example 1.

[0030] Figure 3 The image shows the X-ray diffraction (XRD) pattern of the Ni-DLMB crystal in Example 1.

[0031] Figure 4These are scanning electron microscope (SEM) images of the Ni-DLMB type MOF film synthesized in Example 1; (a) surface, (b) cross-section;

[0032] Figure 5 The image shows the X-ray diffraction (XRD) pattern of the Ni-DLMB type MOF film synthesized in Example 1.

[0033] Figure 6 The graph shows the pervaporation stability test performance of the Ni-DLMB type MOF membrane synthesized in Example 2 with 15 wt.% methanol / methyl tert-butyl ether.

[0034] Figure 7 The graph shows the pervaporation performance of the Ni-DLMB type MOF membrane synthesized in Example 2 with 10 wt.% methanol / dimethyl carbonate. Detailed Implementation

[0035] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0036] Example 1

[0037] (1) Preparation of large-size Ni-DLMB crystals

[0038] First, dissolve 1 mmol nickel acetate tetrahydrate in 20 ml methanol, and dissolve 1 mmol malic acid and 1 mmol 4,4'-bipyridine in 10 ml methanol. Place the above solutions in an ultrasonic cleaner and sonicate for 15 minutes. Then mix them and stir at 30°C for 2 hours. Transfer the mixture to a reaction vessel and react at 100°C for 48 hours. After the reaction is complete, turn off the oven and keep the reaction vessel in the oven until it cools to room temperature. Centrifuge to collect the generated crystals. Wash the crystals three times with methanol. Place the collected powder in an 80°C vacuum oven and dry for 12 hours.

[0039] (2) Preparation of Ni-DLMB seeds by ball milling

[0040] The crystals obtained in step (1) were loaded into a ball mill jar with a Ni-DLMB crystal to ball milling medium mass ratio of 1:5, and 5 ml of anhydrous ethanol was added. The jar was then milled alternately in both directions at 200 rpm using a high-energy ball mill, with each direction lasting 30 minutes, for a total of 12 hours. After milling, the powder in the jar was slowly rinsed with 1000 mL of ethanol, transferred to a precipitation container, and filled with the solution. Solvent sedimentation was then performed, and the jar was allowed to stand for 24 hours. At this time, large crystal particles settled to the bottom, and the upper suspension was collected by centrifugation to obtain uniformly dispersed small-sized crystals.

[0041] (3) Applying Ni-DLMB seed layer

[0042] Weigh the ball-milled crystals and disperse them in ethanol to prepare a 0.1 wt% seed solution. Stir the seed solution at room temperature and sonicate for 12 hours. Take 0.2 mL of the suspension and spin-coat it onto the surface of a porous sheet-like α-Al₂O₃ support at 3000 r / min for 60 seconds. Dry the coated support at 80 °C for 12 hours, then transfer it to a desiccator and seal it for storage.

[0043] (4) Preparation of Ni-DLMB type MOF membrane

[0044] 1 mmol nickel acetate tetrahydrate, 1 mmol malic acid, and 1 mmol 4,4'-bipyridine were added to 30 mL of methanol and stirred at room temperature for 1 hour. The mixture was then transferred to a high-pressure reactor, and a porous α-Al₂O₃ sheet-like support coated with a seed layer was placed with its seed layer facing the bottom of the reactor using a support. The reactor was then crystallized in a 120°C oven for 24 hours. After the reaction was complete, the system was allowed to cool naturally to room temperature. The sheet-like membrane was then removed and slowly rinsed with methanol. Subsequent treatment steps included washing the sheet-like membrane with fresh anhydrous methanol, changing the solvent every 12 hours, repeating this process at least three times to thoroughly remove residual organic ligands from the membrane layer and pores. After washing, the membrane was dried in a 150°C vacuum oven for 24 hours.

[0045] Example 2

[0046] (1) Preparation of large-size Ni-DLMB crystals

[0047] First, dissolve 2 mmol of nickel carbonate in 20 ml of methanol, and dissolve 2 mmol of malic acid and 1 mmol of 4,4'-bipyridine in 10 ml of methanol. Place the above solutions in an ultrasonic cleaner and sonicate for 15 minutes. Then mix them and stir at 30°C for 2 hours. Transfer the mixture to a reaction vessel and react at 120°C for 24 hours. After the reaction is complete, turn off the oven and keep the reaction vessel in the oven to cool to room temperature. Centrifuge to collect the generated crystals. Wash the crystals three times with methanol. Place the collected powder in an 80°C vacuum oven and dry for 12 hours.

[0048] (2) Preparation of Ni-DLMB seeds by ball milling

[0049] The crystals obtained in step (1) were loaded into a ball mill jar with a Ni-DLMB crystal to ball milling medium mass ratio of 1:10, and 10 ml of anhydrous ethanol was added. The jar was then milled alternately in both directions at 350 rpm using a high-energy ball mill, with each direction lasting 30 minutes, for a total of 8 hours. After milling, the powder in the jar was slowly rinsed with 1000 mL of ethanol, transferred to a precipitation container, and filled with the solution. Solvent sedimentation was then performed, and the jar was allowed to stand for 12 hours. At this time, large crystal particles settled to the bottom, and the upper suspension was collected by centrifugation to obtain uniformly dispersed small-sized crystals.

[0050] (3) Applying Ni-DLMB seed layer

[0051] Weigh the ball-milled crystals and disperse them in ethanol to prepare a 0.3 wt% seed solution. Stir the seed solution at room temperature and sonicate for 12 hours. Take 0.2 mL of the suspension and spin-coat it onto the surface of a porous sheet-like α-Al₂O₃ support at 3000 r / min for 60 seconds. Dry the coated support at 80 °C for 12 hours, then transfer it to a desiccator and seal it for storage.

[0052] (4) Preparation of Ni-DLMB type MOF membrane

[0053] 2 mmol of nickel carbonate, 2 mmol of malic acid, and 1 mmol of 4,4'-bipyridine were added to 30 mL of methanol and stirred at room temperature for 1 hour. The mixture was then transferred to a high-pressure reactor, and a porous α-Al₂O₃ sheet-like support coated with a seed layer was placed with its seed layer facing down. The mixture was then crystallized in a 150°C oven for 12 hours. After the reaction was complete, the system was allowed to cool naturally to room temperature. The sheet-like membrane was then removed and slowly rinsed with methanol. Subsequent treatment steps included washing the sheet-like membrane with fresh anhydrous methanol, changing the solvent every 12 hours, repeating this process at least three times to thoroughly remove residual organic ligands from the membrane layer and pores. After washing, the membrane was dried in a 150°C vacuum oven for 24 hours.

[0054] Example 3

[0055] (1) Preparation of large-size Ni-DLMB crystals

[0056] First, dissolve 3 mmol of nickel sulfate hexahydrate in 20 ml of methanol, and dissolve 3 mmol of malic acid and 1 mmol of 4,4'-bipyridine in 10 ml of methanol. Place the above solutions in an ultrasonic cleaner and sonicate for 15 minutes. Then mix them and stir at 30°C for 2 hours. Transfer the mixture to a reaction vessel and react at 150°C for 6 hours. After the reaction is complete, turn off the oven and keep the reaction vessel in the oven to cool to room temperature. Centrifuge to collect the generated crystals. Wash the crystals three times with methanol. Place the collected powder in an 80°C vacuum oven and dry for 12 hours.

[0057] (2) Preparation of Ni-DLMB seeds by ball milling

[0058] The crystals obtained in step (1) were loaded into a ball mill jar with a Ni-DLMB crystal to ball milling medium mass ratio of 1:20, and 20 ml of anhydrous ethanol was added. The jar was then milled alternately in both directions at 500 rpm using a high-energy ball mill, with each direction lasting 30 minutes, for a total of 6 hours. After milling, the powder in the jar was slowly rinsed with 1000 mL of ethanol, transferred to a precipitation container, and filled with the solution. Solvent sedimentation was then performed, and the jar was allowed to stand for 6 hours. At this point, large crystal particles settled to the bottom, and the upper suspension was collected by centrifugation to obtain uniformly dispersed small-sized crystals.

[0059] (3) Applying Ni-DLMB seed layer

[0060] Weigh the ball-milled crystals and disperse them in ethanol to prepare a 0.5 wt% seed solution. Stir the seed solution at room temperature and sonicate for 12 hours. Coat the seed layer using a hot-dip immersion method: First, seal both ends of the tubular carrier with polytetrafluoroethylene plugs and place it in an oven at 120°C for preheating for at least 4 hours. Quickly remove the preheated tubular carrier and vertically immerse it in the seed suspension for 20 seconds. Slowly lift the carrier, remove the sealing plugs, and place it in an oven at 80°C to dry and cure for 12 hours.

[0061] (4) Preparation of Ni-DLMB type MOF membrane

[0062] 3 mmol of nickel sulfate hexahydrate, 3 mmol of malic acid, and 1 mmol of 4,4'-bipyridine were added to 30 mL of methanol and stirred at room temperature for 1 hour. The mixture was then transferred to a high-pressure reactor, and a porous tubular α-Al₂O₃ support coated with a seed layer was placed in the reactor using a support. The reactor was then crystallized in a 150°C oven for 4 hours. After the reaction was complete, the system was allowed to cool naturally to room temperature. The tubular membrane was then removed and slowly rinsed with methanol. Subsequent treatment steps included washing the tubular membrane with fresh anhydrous methanol, changing the solvent every 12 hours, repeating this process at least three times to thoroughly remove residual organic ligands from the membrane layer and pores. After washing, the membrane was dried in a 150°C vacuum oven for 24 hours.

[0063] Depend on Figure 1 As shown, the Ni-DLMB crystal morphology before ball milling is a large columnar structure with a size of approximately 100 μm. After ball milling, the crystals... Figure 2 As shown, the crystals undergo significant changes; the original large-sized columnar structure is destroyed, transforming into uniform granular particles with a size reduced to approximately 200-300 nm. The fine, uniform, and well-dispersed nature of the ball-milled crystals makes them ideal as small seed crystals for MOF film growth. These small seed crystals provide uniform nuclei for MOF film growth, contributing to the formation of a more uniform and dense film structure on the substrate and improving the overall film performance. Further observation of its XRD pattern reveals… Figure 3As shown, the Ni-MOF powder before treatment is in good agreement with the simulated spectrum, which clearly confirms the correctness of the synthesized phase.

[0064] Depend on Figure 4 As shown, the surface of the Ni-DLMB type MOF film is formed by a dense stack of uniformly sized, regularly geometrically stacked crystals. The crystal interfaces are tightly bonded and free of obvious defects, indicating that the optimized process effectively controlled the crystal growth process and achieved uniform control of the microstructure. The film reveals a gradient structure: the upper layer consists of columnar crystals oriented perpendicular to the substrate, with highly consistent grain orientation, confirming that the fabrication process achieved precise cross-scale control of the film structure. Figure 5 As shown, through comparative analysis with the simulated XRD spectrum, it was found that the characteristic diffraction peak positions of the thin film completely match the theoretical phase, indicating that the expected phase structure was successfully synthesized.

[0065] The obtained Ni-DLMB type MOF membrane was subjected to permeation testing using a pervaporation testing device to evaluate its compactness and organic solvent separation performance. The specific operation was as follows: the obtained Ni-DLMB type MOF membrane was sealed in a membrane module and immersed in a constant temperature water bath, with the vacuum level on the permeate side dynamically maintained by a vacuum pump. The permeate vapor was condensed and collected in a liquid nitrogen cold trap, and the permeate flux was accurately determined using micro-mass analysis. The component concentrations of the feed solution and permeate products were quantitatively analyzed by gas chromatography. Figure 6 As shown, during continuous operation from 0 to 72 hours, the separation factor of the Ni-DLMB MOF membrane remained consistently around 570 with minimal fluctuations; the permeation flux remained stable at approximately 0.5 kg·m⁻²·h⁻¹. This result indicates that the Ni-DLMB MOF membrane exhibits excellent temporal stability, with neither the separation factor nor the permeation flux showing significant decline during long-term operation. This reflects the stability of its structure during prolonged pervaporation, without significant damage or performance degradation due to continuous molecular permeation and mass transfer processes. Figure 7 As shown, the membrane flux for 10 wt.% methanol / dimethyl carbonate is 0.62 kg·m³. -2 ·h -1 With a separation factor of 43, the Ni-DLMB type MOF membrane exhibits excellent separation performance.

Claims

1. A method for preparing a Ni-DLMB type MOF membrane, characterized in that, A ball milling-assisted secondary growth method was employed to first prepare large-sized Ni-DLMB crystals. Uniform submicron-sized Ni-DLMB seeds were then obtained through ball milling combined with solvent precipitation. A uniform and smooth seed layer was prepared on the surface of a porous α-Al₂O₃ support using spin coating, dip coating, vacuum coating, spray coating, or wiping coating methods. Finally, a dense and defect-free Ni-DLMB-type MOF film was prepared through secondary growth. The steps are as follows: (1) Hydrothermal synthesis of large-size Ni-DLMB crystals Malic acid, inorganic metal salt, and 4,4'-bipyridine were mixed in an organic solvent and transferred to a high-pressure reactor. The reaction was carried out in an oven. After the reaction was completed, the system was allowed to cool naturally to room temperature. The product was collected by centrifugation, washed repeatedly with methanol, and dried to obtain Ni-DLMB crystals. (2) Preparation of Ni-DLMB seed crystals by ball milling The Ni-DLMB crystals obtained in step (1) and the ball milling media were added to the ball milling jar at a mass ratio, and anhydrous ethanol was added for ball milling. After ball milling, the product was dispersed in ethanol to settle, and the supernatant was centrifuged and dried to obtain Ni-DLMB seed crystals. (3) Coating with Ni-DLMB seed layer The ball-milled Ni-DLMB seeds were dispersed in ethanol to obtain Ni-DLMB seed solution; the Ni-DLMB seed solution was coated on the surface of a porous α-Al2O3 support; after the seed-coated support was dried, a uniform, dense and defect-free Ni-DLMB seed layer was obtained. (4) Preparation of Ni-DLMB type MOF membrane Malic acid, inorganic metal salt, and 4,4'-bipyridine were mixed in an organic solvent and transferred to a high-pressure reactor. A porous α-Al2O3 support coated with a seed layer was placed in the synthesis solution and transferred to an oven for secondary growth. After the reaction was completed, the system was allowed to cool naturally to room temperature. The sheet-like membrane was then removed and slowly rinsed with methanol. After drying, a Ni-DLMB type MOF membrane was obtained.

2. The method for preparing a Ni-DLMB type MOF film according to claim 1, characterized in that, In step (1), the inorganic metal salt is nickel acetate tetrahydrate, nickel carbonate, or nickel sulfate hexahydrate; the organic solvent is methanol or ethanol; the molar ratio of malic acid, inorganic metal salt, and 4,4'-bipyridine is (1-3):(1-3):1; the hydrothermal reaction temperature is 100-150℃; the hydrothermal reaction time is 4-48 hours; the drying temperature is 80-150℃; and the drying time is 6-48 hours.

3. The method for preparing a Ni-DLMB type MOF membrane according to claim 1, characterized in that, In step (2), the mass ratio of the Ni-DLMB crystal to the ball milling media is 1:5-1:20; the ball milling speed is 200-500 rpm; the ball milling time is 6-12 hours; and the settling time is 6-24 hours.

4. The method for preparing a Ni-DLMB type MOF membrane according to claim 1, characterized in that, In step (3), the mass fraction of Ni-DLMB seed solution is 0.1wt%-0.5wt%; the shape of the porous α-Al2O3 support is tubular, flat or hollow fiber; the drying temperature is 80-150℃; and the drying time is 6-48 hours.

5. The method for preparing a Ni-DLMB type MOF film according to claim 1, characterized in that, In step (4), the inorganic metal salt is nickel acetate tetrahydrate, nickel carbonate, or nickel sulfate hexahydrate; the organic solvent is methanol or ethanol; the molar ratio of malic acid to inorganic metal salt and 4,4'-bipyridine is (1-3):(1-3):1; the reaction time for secondary growth is 4-24 hours; the reaction temperature for secondary growth is 120-175℃; the drying temperature is 80-150℃; and the drying time is 6-48 hours.

6. The method for preparing a Ni-DLMB type MOF film according to claim 1, characterized in that, In step (1), the molar ratio of malic acid, inorganic metal salt, and 4,4'-bipyridine is (1.5-2.5):(1.5-2.5):

1.

7. The method for preparing a Ni-DLMB type MOF film according to claim 1, characterized in that, In step (4), the molar ratio of malic acid to inorganic metal salt and 4,4'-bipyridine is (1.5-2.5):(1.5-2.5):1.

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