Preparation method of Ni-DLMB type MOF film

Abstract

The invention belongs to the technical field of membrane separation materials, and relates to a preparation method of a Ni-DLMB type MOF (Metal Organic Framework) membrane, which specifically comprises the following steps: firstly, preparing a large-size Ni-DLMB crystal, preparing the large-size Ni-DLMB crystal into a submicron seed crystal in a ball milling treatment manner, and then introducing the crystal into the surface of an alpha-Al2O3 carrier by a coating method, so as to obtain the Ni-DLMB type MOF membrane. And then the compact and defect-free Ni-DLMB type MOF membrane prepared by a secondary growth method has relatively good separation performance on a mixture of methanol / methyl tert-butyl ether and methanol / dimethyl carbonate.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane separation materials, and relates to a preparation method of Ni-DLMB type MOF membrane, in particular to a ball-milled seed-assisted secondary growth method for Ni-DLMB type MOF membrane. Background Art

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

[0003] Pervaporation is a new type of membrane separation technology for separating liquid mixtures, which has many advantages such as simple and flexible operation, low energy consumption, high separation efficiency, no introduction of other reagents, no secondary treatment of products, no secondary pollution, and easy scale-up. It can be widely used for dehydration of organic solvents, especially suitable for isomers, heat-sensitive, near-boiling and azeotropic systems that are difficult or impossible to separate by traditional separation methods. It is considered to be a very important and promising high-tech in the current industrial technology transformation.

[0004] Pervaporation membrane materials can be divided into pervaporation organic membranes and pervaporation inorganic membranes. Compared with organic membranes, inorganic membranes have many advantages such as good thermochemical stability, long service life, few vulnerable parts of equipment, low maintenance cost, convenient replacement of membrane modules, high membrane flux, large separation coefficient, no swelling, and solvent corrosion resistance. They have become the preferred membrane materials for pervaporation. Pervaporation inorganic membranes use molecular sieves as the membrane layer material (core separation membrane layer) and utilize their regular pore structures to achieve molecular size-level 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, the Ni atoms along the x-axis coordinate with the β-carboxyl O atoms, while the Ni atoms along the y-axis are connected by α-O atoms. In addition, the O atoms in the metal-oxygen coordination chain along the x-axis are completely derived from L-malic acid, while along the y-axis, the O atoms are alternately provided by L-malic acid and D-malic acid, forming a two-dimensional planar layer with a fes topological structure. Due to the different coordination directions of the Ni atoms and N atoms in the axial positions, the adjacent Ni-N bonds exhibit a torsional angle of 45.2°, resulting in an intertwined structure between the adjacent Bipy molecules along the y-axis direction, forming triangular channels with a supermicroporous window size of 0.3×0.42 nm. Its pore size is between the kinetics of methanol (0.38 nm) and methyl tert-butyl ether (0.62 nm), providing a basis for size sieving, and its surface hydroxyl functional groups can also strengthen the affinity for polar molecules such as methanol through hydrogen bonding and dipole interactions, making it an extremely excellent ideal separation membrane material.

[0006] Currently, MOF zeolite membranes are mainly prepared by the secondary growth method. The secondary growth method is to first coat the surface of the carrier with homogeneous zeolite seeds to form a seed layer, and through the induction of the seeds, the seed layer undergoes a crystallization reaction in the film-forming solution to form a continuous and dense MOF zeolite membrane layer (Modern Chemical Industry, 2018, 38(12): 62-66.). By pre-coating the seeds, the nucleation and growth stages of MOF crystals are separated, controlling the growth rate of the crystals in the membrane layer and realizing the controllable preparation of the microstructure of the MOF membrane layer such as film thickness, crystal grain size, and orientation (Separation and Purification Technology, 2008, 61(2): 175-181.). However, the limitation of this method for preparing Ni-DLMB type MOF zeolite membranes is that when the most critical Ni-DLMB type MOF zeolite seeds are synthesized by traditional solvothermal method, hydrothermal method or interfacial diffusion method, the obtained crystal sizes are generally large, with an average particle size usually between 50-100 μm, and the size distribution range is wide. Secondary growth relies on the active sites on the surface of the seeds to initiate directional growth, while the specific surface area of large-sized Ni-DLMB crystals is small, and the effective growth interface provided by the seeds per unit mass is insufficient, resulting in a decrease in the secondary growth rate or difficulty in starting the growth process. Especially in scenarios where precise control of the crystal hierarchical structure is required, it cannot meet the requirements of interface matching and lattice orientation. Summary of the Invention

[0007] To solve the above problems, the present invention provides a method for preparing a Ni-DLMB type MOF membrane, which is prepared by a ball-milled seed-assisted secondary growth method.

[0008] The ball-milled seed-assisted secondary growth method is a composite process that uses mechanical ball milling technology to control the size and surface modification of initial crystals, prepares small-sized seeds that meet the requirements of secondary growth, and uses these seeds to induce the directional growth of a target metal-organic framework (MOF) film layer on the surface of a substrate. It is a new method for preparing MOF separation membranes with overly large crystal sizes that are difficult to prepare by conventional methods.

[0009] Compared with the conventional hydrothermal secondary growth method, the ball-milled 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, breaks through the bottleneck of uncontrollable seed size in traditional methods, enables the seeds to be stably suspended in the secondary growth solvent and evenly 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 membrane, using the ball-milled seed-assisted secondary growth method. First, large-sized Ni-DLMB crystals are prepared, and uniform submicron-sized Ni-DLMB seeds are obtained through ball milling combined with solvent sedimentation treatment. A uniform and flat seed layer is prepared on the surface of a porous α-Al2O3 carrier by spin coating. Finally, a dense and defect-free Ni-DLMB type MOF membrane is prepared through secondary growth. The steps are as follows:

[0012] (1) Synthesize large-sized Ni-DLMB crystals by hydrothermal method

[0013] Mix malic acid, inorganic metal salt, and 4,4'-bipyridine in an organic solvent and transfer it to a high-pressure reaction kettle. React in an oven. After the reaction is completed, wait for the system to cool naturally to room temperature, centrifuge to collect the product, and dry it after repeatedly washing with methanol to obtain Ni-DLMB crystals.

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

[0015] Add the Ni-DLMB crystals obtained in step (1) and a ball milling medium (zirconia balls) to a ball milling jar according to a mass ratio, and add absolute ethanol for ball milling treatment. After ball milling, disperse the product in ethanol for sedimentation, centrifuge the supernatant, and dry it to obtain Ni-DLMB seeds.

[0016] (3) Coat the Ni-DLMB seed layer

[0017] Disperse the ball-milled Ni-DLMB seeds in ethanol to obtain a Ni-DLMB seed solution; coat the Ni-DLMB seed solution on the surface of a porous α-Al₂O₃ support, and after drying the coated support, obtain a uniform, dense and defect-free Ni-DLMB seed layer.

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

[0019] Mix malic acid, inorganic metal salt, and 4,4'-bipyridine in an organic solvent and transfer it to a high-pressure reaction kettle. Place the porous α-Al₂O₃ support coated with the seed layer into the synthesis solution and transfer it to an oven for secondary growth. After the reaction is completed, wait for the system to cool naturally to room temperature, take out the flaky membrane and slowly rinse it with methanol, and obtain a Ni-DLMB type MOF membrane after drying.

[0020] In the 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 temperature of the hydrothermal reaction is 100 - 150 °C; the time of the hydrothermal reaction is 4 - 48 hours; the drying temperature is 80 - 150 °C; the drying time is 6 - 48 hours.

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

[0022] In the step (3), the mass fraction of the Ni-DLMB seed solution is 0.1 wt% - 0.5 wt%; the shape of the porous α-Al₂O₃ support is tubular, flat, hollow fiber or multi-channel support, preferably flaky; the method for obtaining the seed layer on the surface of the porous α-Al₂O₃ support is impregnation method, hot impregnation method, secondary variable temperature hot impregnation method, vacuum crystal coating method, spraying method, rubbing method or spin coating method, preferably spin coating method; the drying temperature is 80 - 150 °C; 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 the 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 for the secondary growth reaction is 4-24 hours; the temperature for the secondary growth reaction is 120-175 °C; the drying temperature is 80-150 °C; the drying time is 6-48 hours.

[0024] Advantages of the present invention:

[0025] (1) The ball milling pretreatment process skips the complex chemical regulation for preparing submicron-sized seeds. By adjusting the ball milling speed and time, it can quickly adapt to different systems, forming a universal preparation platform. Moreover, the single-machine production capacity of the ball milling equipment can reach the kilogram level, and the performance repeatability between batches is high, significantly reducing the production cost and enhancing the feasibility of industrial scale-up.

[0026] (2) The seeds after ball milling expose more metal coordination active sites. The specific surface area of the small-sized seeds is increased by 3-5 times compared with the traditional seeds, providing sufficient nucleation sites for secondary growth, promoting the oriented epitaxial growth of metal ions and ligands along the seed lattice, and greatly reducing the film layer defect rate. The Ni-DLMB type MOF membrane synthesized by the method of the present invention has a uniform and highly dense structure. Its separation performance is: at a temperature of 25 °C, the flux for 15 wt.% methanol / methyl tert-butyl ether is 0.51 kg·m -2 ·h -1 , and the separation factor is 577; the flux for 10 wt.% methanol / dimethyl carbonate is 0.62 kg·m -2 ·h -1 , and the separation factor is 43, and the performance exceeds that of the commercial PVA membrane.

[0027] (3) The present invention effectively saves the membrane preparation cost, significantly shortens the membrane preparation cycle, fully embodies the concept of green chemistry, and has important industrial promotion and practical application value. Description of the Drawings

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

[0029] Figure 2 It is the scanning electron microscope (SEM) image of the Ni-DLMB seeds prepared by ball milling in Example 1;

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

[0031] Figure 4It is the scanning electron microscope (SEM) image of the Ni-DLMB type MOF membrane synthesized in Example 1; (a) surface, (b) cross-section;

[0032] Figure 5 It is the X-ray diffraction (XRD) pattern of the Ni-DLMB type MOF membrane synthesized in Example 1.

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

[0034] Figure 7 It is the pervaporation performance graph of 10 wt.% methanol / dimethyl carbonate for the Ni-DLMB type MOF membrane synthesized in Example 2. Detailed implementation manners

[0035] The following further illustrates the detailed implementation manners of the present invention in combination with the attached drawings and technical solutions.

[0036] Example 1

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

[0038] First, dissolve 1 mmol of nickel acetate tetrahydrate in 20 ml of methanol, and dissolve 1 mmol of malic acid and 1 mmol of 4,4'-bipyridine in 10 ml of methanol. After ultrasonic treatment of the above solutions in an ultrasonic cleaner for 15 minutes, mix them and transfer them to a reaction kettle after stirring at 30 °C for 2 hours. React at 100 °C for 48 hours. After the reaction is completed, turn off the oven and keep the reaction kettle in the oven until it cools to room temperature. Centrifuge to collect the generated crystals, wash them three times repeatedly with methanol, and then place the collected powder in a vacuum oven at 80 °C and dry for 12 hours;

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

[0040] Load the crystals obtained in step (1) into a ball milling jar. The mass ratio of Ni-DLMB crystals to the ball milling medium is 1:5, and add 5 ml of absolute ethanol; use a high-energy ball mill to perform forward and reverse alternating ball milling at 200 rpm, with each direction lasting for 30 minutes and a cumulative ball milling time of 12 hours. After ball milling, slowly rinse the powder in the jar with 1000 mL of ethanol, transfer it to a precipitation container and fill it with solution. Then perform solvent sedimentation and let it stand for 24 hours. At this time, the large-sized crystals settle to the bottom, and the upper suspension is collected by centrifugation to obtain uniformly dispersed small-sized crystals;

[0041] (3) Coating of Ni-DLMB seed layer

[0042] Weigh the milled crystals and disperse them in ethanol to prepare a seed solution with a mass fraction of 0.1 wt%. Stir and ultrasonically treat the seed solution at room temperature for 12 hours. Take 0.2 mL of the suspension and spin-coat it on the surface of a porous flake-shaped α-Al2O3 support at a speed of 3000 r / min for 60 seconds. The coated support is placed in an 80 °C environment and dried for 12 hours, and then transferred to a desiccator for sealed storage;

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

[0044] Add 1 mmol of nickel acetate tetrahydrate, 1 mmol of malic acid, and 1 mmol of 4,4'-bipyridine to 30 mL of methanol and stir at room temperature for 1 hour. Subsequently, transfer the mixture to a high-pressure reactor, place the porous α-Al2O3 flake support coated with the seed layer facing the bottom of the reactor, and crystallize it in an oven at 120 °C for 24 hours. After the reaction is completed, wait for the system to cool naturally to room temperature, take out the flake membrane and rinse it slowly with methanol. The subsequent treatment steps are as follows: Wash the flake membrane with fresh anhydrous methanol, change the solvent every 12 hours, and repeat at least 3 times to thoroughly remove the residual organic ligands in the membrane layer and pores; After the washing is completed, place the membrane in a vacuum oven at 150 °C and dry it for 24 hours.

[0045] Example 2

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

[0047] First, dissolve 2 mmol of nickel carbonate in 20 ml of methanol, dissolve 2 mmol of malic acid and 1 mmol of 4,4'-bipyridine in 10 ml of methanol, put the above solutions into an ultrasonic cleaner and ultrasonically treat them for 15 minutes, then mix them and stir at 30 °C for 2 hours, and then transfer them into a reaction kettle. React at 120 °C for 24 hours. After the reaction is completed, turn off the oven and keep the reaction kettle in the oven until it cools to room temperature. Centrifuge to collect the generated crystals, wash them repeatedly with methanol three times, and then place the collected powder in a vacuum oven at 80 °C and dry it for 12 hours;

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

[0049] Load the crystals obtained in step (1) into a ball milling jar. The mass ratio of Ni-DLMB crystals to the ball milling medium is 1:10, and add 10 ml of anhydrous ethanol. Use a high-energy ball mill to perform forward and reverse alternating ball milling at 350 rpm, with each direction lasting for 30 minutes and a cumulative ball milling time of 8 hours. After the ball milling is completed, slowly rinse the powder in the jar with 1000 mL of ethanol, transfer it to a precipitation container and fill it with solution. Then perform solvent sedimentation and let it stand for 12 hours. At this time, the large particles of crystals settle to the bottom, and the upper suspension is collected by centrifugation to obtain uniformly dispersed small-sized crystals;

[0050] (3) Coating with Ni-DLMB seed layer

[0051] Weigh the milled crystals and disperse them in ethanol to prepare a seed solution with a mass fraction of 0.3 wt%. Stir the seed solution at room temperature and ultrasonically treat it for 12 hours. Take 0.2 mL of the suspension and spin-coat it on the surface of a porous flaky α-Al2O3 support at a speed of 3000 r / min for 60 seconds. The coated support is placed in an 80°C environment and dried for 12 hours, and then transferred to a desiccator for sealed storage;

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

[0053] Add 2 mmol of nickel carbonate, 2 mmol of malic acid, and 1 mmol of 4,4'-bipyridine to 30 mL of methanol and stir at room temperature for 1 hour. Then transfer the mixture to a high-pressure reaction kettle, place the porous α-Al2O3 flaky support coated with the seed layer facing the bottom of the reaction kettle, and crystallize it in an oven at 150°C for 12 hours. After the reaction is completed, wait for the system to cool naturally to room temperature, take out the flaky membrane and rinse it slowly with methanol. The subsequent treatment steps are as follows: Wash the flaky membrane with fresh anhydrous methanol, change the solvent every 12 hours, and repeat at least 3 times to thoroughly remove the residual organic ligands in the membrane layer and pores; After the washing is completed, place the membrane in a vacuum oven at 150°C and dry it for 24 hours.

[0054] Example 3

[0055] (1) Preparation of large-sized 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. After ultrasonic treatment of the above solutions in an ultrasonic cleaner for 15 minutes, mix them and stir at 30°C for 2 hours, then transfer them to a reaction kettle and react at 150°C for 6 hours. After the reaction is completed, turn off the oven and keep the reaction kettle in the oven until it cools to room temperature. Centrifuge to collect the generated crystals, wash them repeatedly with methanol three times, and then place the collected powder in a vacuum oven at 80°C and dry it for 12 hours;

[0057] (2) Milling to prepare Ni-DLMB seeds

[0058] Load the crystals obtained in step (1) into a ball milling jar. The mass ratio of Ni-DLMB crystals to the ball milling medium is 1:20, and 20 ml of absolute ethanol is added. Use a high-energy ball mill to perform forward and reverse alternating ball milling at 500 rpm, with each direction lasting for 30 minutes and the cumulative ball milling time being 6 hours. After ball milling, slowly rinse the powder in the jar with 1000 mL of ethanol, transfer it to a precipitation container and fill it with solution. Then perform solvent sedimentation and let it stand for 6 hours. At this time, the large crystal particles settle to the bottom, and the upper suspension is collected by centrifugation to obtain uniformly dispersed small-sized crystals;

[0059] (3) Coating the Ni-DLMB seed layer

[0060] Weigh the ball-milled crystals and disperse them in ethanol to prepare a seed solution with a mass fraction of 0.5 wt%. Stir and ultrasonically treat the seed solution at room temperature for 12 hours. Use the thermal impregnation method to coat the seed layer: First, seal both ends of the tubular carrier with a polytetrafluoroethylene plug, place it in an oven at 120 °C, and preheat it for at least 4 hours. Quickly take out the preheated tubular carrier and vertically immerse it in the seed suspension for 20 seconds, slowly lift the carrier, remove the sealing plug, and place it in an oven at 80 °C for drying and curing for 12 hours;

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

[0062] Add 3 mmol of nickel sulfate hexahydrate, 3 mmol of malic acid, and 1 mmol of 4,4'-bipyridine to 30 mL of methanol and stir at room temperature for 1 hour. Then transfer the mixture to a high-pressure reaction kettle, place the porous tubular α-Al2O3 carrier coated with the seed layer in the reaction kettle using a bracket, and crystallize it in an oven at 150 °C for 4 hours. After the reaction is completed, wait for the system to cool naturally to room temperature, take out the tubular membrane and slowly rinse it with methanol. The subsequent treatment steps are: Wash the tubular membrane with fresh absolute methanol, change the solvent every 12 hours, and repeat at least 3 times to thoroughly remove the residual organic ligands in the membrane layer and pores; After washing, place the membrane in a vacuum oven at 150 °C for drying for 24 hours.

[0063] As Figure 1 shown, the morphology of the Ni-DLMB crystals before ball milling treatment is a relatively large columnar structure with a size of about 100 μm. After ball milling treatment, as Figure 2 shown, the crystals change significantly. The original large-sized columnar structure is destroyed and transformed into uniform granular shapes with the size reduced to about 200 - 300 nm. The small size, uniformity, and good dispersibility of the ball-milled crystals make them very suitable as small seeds required for the growth of the MOF membrane. These small seeds can provide uniform crystal nuclei for the growth of the MOF membrane, help form a more uniform and dense membrane layer structure on the substrate, and improve the overall performance of the membrane. Further observe its XRD pattern, as Figure 3As shown, the Ni-MOF powder before treatment basically matches the simulated spectrum, clearly confirming the correctness of the synthesized phase.

[0064] As Figure 4 shown, the surface of the Ni-DLMB type MOF membrane is formed by the dense stacking of regular geometric crystals with uniform sizes. The crystal interfaces are tightly bonded and there are no obvious defects, indicating that the optimized process effectively regulates the crystal growth process and realizes the control of the microstructure uniformity. It reveals that the membrane has a gradient structure feature: the upper layer is composed of columnar crystals oriented perpendicular to the substrate, and the grain orientations are highly consistent, confirming that the preparation process realizes the cross-scale precise regulation of the membrane layer structure. As Figure 5 shown, through the comparative analysis with the simulated XRD spectrum, it is found that the characteristic diffraction peak positions of the thin film are completely consistent with the theoretical phase, indicating that the expected phase structure has been successfully synthesized.

[0065] The obtained Ni-DLMB type MOF membrane is subjected to a permeation test using a pervaporation test device to evaluate the denseness and organic solvent separation performance of the prepared Ni-DLMB type MOF membrane. The specific operation is as follows: The obtained Ni-DLMB type MOF membrane is sealed in a membrane module and then immersed in a constant temperature water bath. The vacuum degree on the permeation side is dynamically maintained by a vacuum pump. After the permeated vapor is condensed and trapped by a liquid nitrogen cold trap, the permeation flux is accurately measured by combining with the micro mass analysis method. The component concentrations of the feed liquid and the permeate are quantitatively analyzed by gas chromatography. As Figure 6 shown, during the continuous operation process from 0 to 72 hours, the separation factor of the Ni-DLMB type MOF membrane always remains at about 570, with a small fluctuation range; the permeation flux is stable at about 0.5 kg·m-2·h-1. This result shows that during the long-term operation of the Ni-DLMB type MOF membrane, neither the separation factor nor the permeation flux shows a significant decline, demonstrating excellent time stability. It reflects that its structure remains stable during the long-term pervaporation process and does not undergo obvious damage or performance degradation due to the continuous molecular permeation and mass transfer processes. As Figure 7 shown, the flux of the membrane for 10 wt.% methanol / dimethyl carbonate is 0.62 kg·m -2 ·h -1 , and the separation factor is 43, showing the excellent separation performance of the Ni-DLMB type MOF membrane.

Claims

1. A preparation method of Ni-DLMB type MOF membrane, characterized in that, Using the ball-milling seed-assisted secondary growth method, large-sized Ni-DLMB crystals are first prepared. Uniform submicron-sized Ni-DLMB seeds are obtained through ball milling combined with solvent sedimentation treatment. A uniform and flat seed layer is prepared on the surface of a porous α-Al2O3 support by spin coating. Finally, a dense and defect-free Ni-DLMB type MOF membrane is prepared through secondary growth. The steps are as follows: (1) Synthesize large-sized Ni-DLMB crystals by hydrothermal method Mix malic acid, inorganic metal salt, and 4,4'-bipyridine in an organic solvent and transfer it to a high-pressure reaction kettle. React in an oven. After the reaction is completed, wait for the system to cool naturally to room temperature. Centrifuge to collect the product, wash it repeatedly with methanol, and then dry it to obtain Ni-DLMB crystals; (2) Prepare Ni-DLMB seeds by ball milling Add the Ni-DLMB crystals obtained in step (1) and the ball-milling medium to the ball-milling tank according to the mass ratio, and add absolute ethanol for ball-milling treatment. After ball milling, disperse the product in ethanol for sedimentation. Centrifuge the supernatant and then dry it to obtain Ni-DLMB seeds; (3) Coat the Ni-DLMB seed layer Disperse the ball-milled Ni-DLMB seeds in ethanol to obtain a Ni-DLMB seed solution. Coat the Ni-DLMB seed solution on the surface of the porous α-Al2O3 support. After drying the coated support, a uniform, dense, and defect-free Ni-DLMB seed layer is obtained; (4) Prepare the Ni-DLMB type MOF membrane Mix malic acid, inorganic metal salt, and 4,4'-bipyridine in an organic solvent and transfer it to a high-pressure reaction kettle. Place the porous α-Al2O3 support coated with the seed layer into the synthesis solution and transfer it to an oven for secondary growth. After the reaction is completed, wait for the system to cool naturally to room temperature. Take out the flaky membrane and slowly rinse it with methanol, and then dry it to obtain the Ni-DLMB type MOF membrane.

2. The preparation method of a Ni-DLMB type MOF membrane 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 temperature of the hydrothermal reaction is 100 - 150 °C; the time of the hydrothermal reaction is 4 - 48 hours; the drying temperature is 80 - 150 °C; the drying time is 6 - 48 hours.

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

4. The preparation method of a Ni-DLMB type MOF membrane according to claim 1, characterized in that, In the step (3), the mass fraction of the Ni-DLMB seed solution is 0.1 wt% - 0.5 wt%; the shape of the porous α-Al2O3 support is tubular, flat, hollow fiber or multi-channel support; the method for obtaining the seed layer on the surface of the porous α-Al2O3 support is impregnation method, thermal impregnation method, secondary variable temperature impregnation method, vacuum crystal coating method, spraying method, rubbing method or spin coating method; the drying temperature is 80 - 150 °C; the drying time is 6 - 48 hours.

5. The preparation method of a Ni-DLMB type MOF membrane according to claim 1, characterized in that, In the 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 the inorganic metal salt and 4,4'-bipyridine is (1 - 3):(1 - 3):1; the time of the secondary growth reaction is 4 - 24 hours; the temperature of the secondary growth reaction is 120 - 175 °C; the drying temperature is 80 - 150 °C; the drying time is 6 - 48 hours.

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

1.

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

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