A MOF membrane having a MOF structure in a hole and a method for preparing the same

Abstract

The application discloses a MOF membrane with a MOF structure in a hole and a preparation method thereof, and belongs to the technical field of synthesis of MOF materials. A construction strategy of a MOF in a MOF membrane is provided, smaller MOF particles are successfully grown in a larger-aperture MOF membrane hole to control the aperture size, and the molar ratio of raw materials in a synthesis process is adjusted to control the growth in the hole to further control the aperture size of the membrane, so that it is possible to apply the high-stability MOF membrane to different separation processes.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of MOF material synthesis, and particularly relates to a MOF film with a MOF structure in pores and a preparation method thereof. BACKGROUND

[0002] Metal-organic frameworks (MOFs) as a kind of multifunctional porous material, due to the high specific surface area, designable pore size and other properties, have been widely used in catalysis, separation and gas storage and other fields, and thus have attracted great attention from researchers in related fields in the past few decades, and the number of studies related to MOFs is growing. So far, tens of thousands of different MOF structures have been successfully synthesized by using a wide range of metal ions and organic ligands.

[0003] Each MOF has unique chemical and structural properties, which are determined by the included components, metal ions (or clusters), organic linkers and their inherent coordination interactions. These components and structural properties are two key factors that affect the basic properties and subsequent applications of MOFs. Therefore, considering their inherent chemical and structural properties, selecting appropriate metals and organic components is crucial to endow the final MOF with the required properties. Ultimately, using ideal components to produce MOFs with desired structures is the best way to achieve the best MOF with desired properties, tailored for specific purposes. In addition, producing hybrid MOFs with complex components, structures and morphologies can also provide great opportunities to obtain special MOFs with advanced properties and functions.

[0004] Zr-MOFs is a new type of MOFs material, which has variable topological structure and unique performance, and has more organic ligand connection points than other MOF materials. Among them, the high-valence metal ion can be regarded as a hard acid, and the organic ligand can be regarded as a hard base. Based on the hard and soft acid-base theory, the high-valence metal ion and the organic ligand can form a strong metal-ligand bond, so the Zr-MOFs has good stability and can be used for strict size sieving, and is considered as a kind of porous material with great industrial application potential. MOF-808 is a typical Zr-MOFs material, which has excellent stability and high specific surface area, and has been used in the fields of gas storage, adsorption and catalysis. However, due to its large pore size (18.4 Å), this material is difficult to be well applied in most separation processes. At present, the research on MOF-808 material mainly focuses on the optimization of synthesis method and material modification, which are difficult to further effectively control the pore size of MOF-808 film in order to better apply it to different separation processes, so it is necessary to design effective strategies to control the pore size of MOF-808. SUMMARY

[0005] The present application aims to solve the problems in the prior art, and provides a MOF membrane with a MOF-in-pore structure, which is based on a MOF-in-MOF membrane strategy and grows small-pore MOF crystals in a large-pore MOF membrane to effectively regulate the membrane pore size.

[0006] In order to achieve the above technical purpose, the present application is realized by the following technical scheme:

[0007] The MOF membrane with the MOF-in-pore structure comprises a base membrane and a selective separation layer on the surface of the base membrane, the material of the selective separation layer is a first MOF crystal, and a second MOF crystal is distributed in the pore channel of the first MOF crystal.

[0008] The base membrane refers to a porous polymer membrane.

[0009] The first MOF crystal is MOF-808, and the second MOF crystal is MOF-801.

[0010] The preparation method of the MOF membrane with the MOF-in-pore structure comprises the following steps:

[0011] Step 1: preparing a separation layer formed by the first MOF crystal on the surface of the base membrane;

[0012] Step 2: placing the membrane obtained in step 1 in a reaction solution containing a metal source and a ligand for synthesizing the second MOF crystal, taking it out and performing temperature rising hydrothermal synthesis, so that the second MOF crystal is generated in the pore channel of the first MOF crystal;

[0013] Step 3: activating the membrane obtained in step 2.

[0014] The preparation method of the MOF membrane with the MOF-in-pore structure comprises the following steps:

[0015] 1) preparing a MOF-808 membrane and activating it;

[0016] 2) dissolving a zirconium metal source and a fumaric acid ligand in a mixed solvent of water and acetic acid to obtain a MOF-801 raw material solution;

[0017] 3) immersing the activated MOF-808 membrane in the MOF-801 raw material solution prepared in step 2) for a period of time, taking it out, and placing it in an oven to perform a hydrothermal reaction, thereby obtaining a non-activated MOF-801 in MOF-808 membrane;

[0018] 4) washing the non-activated MOF-801 in MOF-808 membrane with deionized water, immersing it in deionized water, heating and drying, thereby obtaining an activated MOF-801 in MOF-808 membrane.

[0019] Further, in the MOF-801 raw material solution prepared in step 2), the molar ratio of the zirconium metal source and the fumaric acid ligand is 1:1-3, preferably 1:2; the concentration of the zirconium metal source is 1-4 mmol / 50 mL, preferably 1 mmol / 50 mL; and the volume ratio of water to acetic acid in the mixed solvent is 3.5-4:1-1.5.

[0020] Further, in step 3), the MOF-808 film is soaked in the MOF-801 raw material solution for 1-3 hours, the hydrothermal reaction temperature is 75-90°C, and the reaction time is 18-24 hours.

[0021] Further, in step 4), the drying temperature is 120-130°C, and the drying time is 12-24 hours.

[0022] Further, in step 1), the MOF-808 film is prepared as follows:

[0023] I. The zirconium metal source is dissolved in a mixed solvent of N,N-dimethylformamide and formic acid, the trimesic acid ligand is dissolved in a mixed solvent of N,N-dimethylformamide and formic acid, and then the zirconium source solution and the fumaric acid solution are added to both sides of the film device, with a film support body in the middle of the device, and the film support body is loaded with MOF-808 seed crystals;

[0024] II. The film device is placed in an oven for heating and reaction, and an unactivated MOF-808 film is obtained;

[0025] III. The unactivated MOF-808 film is subjected to solvent exchange treatment, and an activated MOF-808 film is obtained.

[0026] Further, in step I, the volume ratio of N,N-dimethylformamide to formic acid in the mixed solvent is 3:2, and the film support body is a porous nylon material.

[0027] Further, in step II, the heating reaction temperature is 85-100°C, and the reaction time is 18-20 hours.

[0028] Further, in step III, the solvent exchange treatment process is as follows: the unactivated MOF-808 film is washed with DMF for 3 times, washed with methanol for 5 times, then soaked in a methanol solution, and vacuum dried at 170°C for 12 hours to obtain the activated MOF-808 film.

[0029] Based on the construction strategy of MOF in MOF membrane, the above method is used to prepare a MOF-801 in MOF-808 membrane, which is characterized in that MOF-801 crystals are grown in the membrane pores of the MOF-808 membrane to regulate the membrane pore size, so that the MOF-808 membrane can be applied to different separation processes. For example, as can be seen from the example part, this membrane can be effectively applied in the selective separation of H2 and CO2.

[0030] The beneficial effects of the present application are:

[0031] 1. The present application proposes a construction strategy of MOF in MOF membrane. First, a dense and continuous MOF-808 membrane is prepared by anti-diffusion method, and then MOF-801 particles are grown in the membrane pores of the MOF-808 membrane by hydrothermal reaction method to control the pore size, so that the MOF-808 membrane can be used for different separation processes;

[0032] 2. The present application first soaks the MOF-808 membrane in the MOF-801 raw material liquid for a period of time, and then takes it out for hydrothermal reaction. Compared with the method of soaking the MOF-808 membrane in the MOF-801 raw material liquid and directly carrying out in-situ hydrothermal reaction, this treatment can avoid the growth of a large amount of MOF-801 on the surface of the MOF-808 membrane, ensure that the MOF-801 is successfully grown in the pores, and further realize the successful regulation of the pore size of the MOF-808 membrane;

[0033] 3. The present application controls the growth of MOF-801 in the membrane pores of the MOF-808 membrane by changing the concentration of Zr metal source in the MOF-801 raw material liquid and the molar ratio of Zr metal source and Fum organic ligand, so as to further control the pore size of the MOF-801 in MOF-808 membrane;

[0034] 4. The method disclosed in the present application can effectively regulate the membrane pore size of the MOF-808 membrane, which makes it possible to apply the MOF-808 membrane to different separation processes. Based on the good stability of MOF-808, the MOF-801 in MOF-808 membrane disclosed in the present application has good application prospect and wide application range. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 is a preparation flowchart of MOF-801 in MOF-808 membrane, wherein (1) is a schematic diagram of preparing MOF-808 membrane by anti-diffusion method; (2-3) are schematic diagrams of growing MOF-801 particles in the pores of MOF-808 membrane;

[0036] Figure 2are SEM images of porous nylon support with different seed loadings, wherein, a subfigure is SEM image of porous nylon support without loading seed; b-e subfigures are SEM images of porous nylon support with 0.15 mg, 0.30 mg, 0.45 mg, 0.60 mg MOF-808 seed loadings, respectively;

[0037] Figure 3 are SEM images of MOF-808 membranes prepared by counter-diffusion growth of MOF-808 on porous nylon supports with different MOF-808 seed loadings, wherein, a-b subfigures are surface and cross-sectional morphology images of MOF-808 membrane (denoted as M1) prepared by counter-diffusion growth of MOF-808 on porous nylon support without loading seed, respectively; c-d subfigures are surface and cross-sectional morphology images of MOF-808 membrane (denoted as M2) prepared by counter-diffusion growth of MOF-808 on porous nylon support with 0.15 mg seed loading, respectively; e-f subfigures are surface and cross-sectional morphology images of MOF-808 membrane (denoted as M3) prepared by counter-diffusion growth of MOF-808 on porous nylon support with 0.30 mg seed loading, respectively; g-h subfigures are surface and cross-sectional morphology images of MOF-808 membrane (denoted as M4) prepared by counter-diffusion growth of MOF-808 on porous nylon support with 0.45 mg seed loading, respectively; i-j subfigures are surface and cross-sectional morphology images of MOF-808 membrane (denoted as M5) prepared by counter-diffusion growth of MOF-808 on porous nylon support with 0.60 mg seed loading, respectively;

[0038] Figure 4 are XRD images of MOF-808 membrane and MOF-808 powder generated in counter-diffusion reaction, wherein, a subfigure is XRD image of MOF-808 membrane; b subfigure is XRD image of MOF-808 membrane prepared by counter-diffusion growth of MOF-808 on porous nylon support with different MOF-808 seed loadings;

[0039] Figure 5 are SEM images of MOF-808 membranes prepared by counter-diffusion growth at different temperatures, wherein, a-b subfigures are surface and cross-sectional morphology images of MOF-808 membrane prepared by counter-diffusion growth at 80℃, respectively; c-d subfigures are surface and cross-sectional morphology images of MOF-808 membrane prepared by counter-diffusion growth at 85℃, respectively; e-f subfigures are surface and cross-sectional morphology images of MOF-808 membrane prepared by counter-diffusion growth at 90℃, respectively; g-h subfigures are surface and cross-sectional morphology images of MOF-808 membrane prepared by counter-diffusion growth at 95℃, respectively;

[0040] Figure 6 is N2 adsorption / desorption isotherm of MOF-808 powder prepared by counter-diffusion method at 77 K;

[0041] Figure 7 H2, CO2 gas performance test graphs of the porous nylon support and MOF-808 membrane;

[0042] Figure 8 In a, b small graphs are SEM graphs of the surface of the MOF-801 in MOF-808 membrane prepared in Example 1, c, d small graphs are SEM graphs of the cross section of the MOF-801 in MOF-808 membrane prepared in Example 1;

[0043] Figure 9 In a, b small graphs are SEM graphs of the surface and cross section of the MOF-808 membrane used in Example 1, c, d small graphs are SEM graphs of the surface and cross section of the MOF-801 in MOF-808 membrane prepared in Example 1 by growing MOF-801 in the pores of the MOF-808 membrane;

[0044] Figure 10 In a small graph is an XRD comparison graph before and after growing MOF-801 in the pores of the MOF-808 membrane; b small graph is a water contact angle test graph, wherein Nylon: porous nylon support; MOF-808: MOF-808 membrane; 801 in 808: MOF-801 in MOF-808 membrane;

[0045] Figure 11 In a small graph is the N2 adsorption / desorption isotherm at 77 K of the MOF-801 in MOF-808 membrane generated under different concentrations of Zr metal source when the molar ratio of Zr metal source to Fum organic ligand is 1:1; b small graph is a pore size and specific surface area statistical graph of the MOF-801 in MOF-808 membrane generated under different concentrations of Zr metal source when the molar ratio of Zr metal source to Fum organic ligand is 1:1, 1 Zr corresponds to the adsorption curve of 1 eq Zr in a graph, representing the use of a metal source with a concentration of 1 mmol / 50 mL, and the explanation of 2 Zr, 3 Zr, 4 Zr is the same; c small graph is the N2 adsorption / desorption isotherm at 77 K of the MOF-801 in MOF-808 membrane generated under different concentrations of Zr metal source when the molar ratio of Zr metal source to Fum organic ligand is 1:2; d small graph is a pore size and specific surface area statistical graph of the MOF-801 in MOF-808 membrane generated under different concentrations of Zr metal source when the molar ratio of Zr metal source to Fum organic ligand is 1:2, 1 Zr corresponds to the adsorption curve of 1 eq Zr in c graph, representing the use of a metal source with a concentration of 1 mmol / 50 mL, and the explanation of 4 Zr is the same;

[0046] Figure 12The graphs show the H2 and CO2 gas performance of the MOF-801 in MOF-808 membrane synthesized using the MOF-808 membrane in Example 1 and the MOF-801 membrane synthesized with different molar ratios of Zr metal source and Fum organic ligand as MOF-801 reactants. In the graphs, 801-808 refer to MOF-801 in MOF-808 membranes. The concentration of the Zr metal source used to prepare MOF-801 is 4 mmol / 50 mL. Detailed Implementation

[0047] The following embodiments further illustrate the content of the present invention, but should not be construed as limiting the present invention. Any modifications and substitutions made to the methods, steps, or conditions of the present invention without departing from the essence of the invention are within the scope of the present invention.

[0048] MOF-808 membranes were prepared by reverse diffusion method to explore the optimal conditions for membrane fabrication.

[0049] 1) Dissolve 363 mg of zirconium oxychloride octahydrate (ZrOCl2·8H2O) in a mixed solvent of DMF / formic acid (45 / 30 mL), and dissolve 1182 mg of trimesic acid (BTC) in a mixed solvent of DMF / formic acid (45 / 30 mL). ， C9H6O6 was dissolved in a mixed solvent of DMF / formic acid (45 / 30 mL). The zirconium source solution and the trimesic acid solution were added to the left and right sides of the U-shaped glass membrane device, respectively. The device was separated in the middle by a porous nylon support. Different amounts of MOF-808 seed crystals (0, 0.15, 0.3, 0.45 and 0.60 mg, respectively) were loaded onto the porous nylon support by filtration of 100 mL of deionized water containing different amounts of MOF-808 powder.

[0050] 2) Place the membrane fabrication apparatus in an oven and react at 95°C for 18 h. During the membrane fabrication process, the MOF particles formed in the solution are also collected for characterization.

[0051] 3) The prepared sample was first rinsed with DMF 3 times, then washed with methanol 5 times, and then soaked in methanol solution. Finally, it was vacuum dried at 170℃ for 12 h to obtain the activated MOF-808 membrane or MOF-808 powder.

[0052] Performance testing of MOF-808 membrane:

[0053] 1) Effect of loading different amounts of seed crystals on the support on MOF-808 film

[0054] Porous nylon supports possess advantages such as good thermal and chemical stability, low cost, and low transport resistance. They also exhibit some tolerance to the DMF solvent used in the experiment and are inexpensive. Therefore, they are used as porous supports for growing MOF-808 membranes. Figure 2 (Small image in middle a).

[0055] Directly growing MOF-808 films on unseeded porous nylon supports using the reverse diffusion method cannot form a complete film layer; only a small number of MOF-808 particles appear on the surface of the porous nylon support. Figure 3 (small images in ab).

[0056] However, by loading MOF-808 seeds onto a porous nylon support and then growing MOF-808 films via reverse diffusion, a dense and continuous MOF-808 film layer can be grown. It can be assumed that the addition of MOF-808 seeds can overcome the bottleneck of nucleation reaction between Zr metal clusters (Secondary building units, SBU) and BTC organic ligands, reduce the difficulty of crystal nucleation, and enable the formation of a dense film layer. Figure 3 The small figure in the middle shows the morphology of MOF-808 films synthesized under different seed loading amounts on the porous nylon support. It can be seen from the figure that the seed loading amount on the porous nylon support has no significant effect on the morphology of the MOF-808 film.

[0057] XRD characterization revealed that both the MOF-808 film grown by reverse diffusion (with a seed loading of 0.45 mg) and the powder generated during the process exhibited characteristic peaks of MOF-808. The MOF-808 film also contained peaks of the porous nylon support at 15–30 °C. Figure 4 (See small image a). Meanwhile, we also tested the XRD patterns of MOF-808 films synthesized with different seed loadings and found that they all exhibited the same MOF-808 characteristic peaks (…). Figure 4 (See small figure b). In the case of no seed loading, no film was formed, so the peak shape is different from the characteristic peak of MOF-808. This can be attributed to incomplete crystal nucleation reaction.

[0058] 2) Subsequent tests were conducted on MOF-808 films prepared using porous nylon supports with a seed loading of 0.45 mg. The main focus was on the effect of different reaction temperatures on MOF-808 film formation. The initial raw material preparation process and the assembly process of the reaction solution and porous nylon support remained unchanged. The film-forming apparatus, consisting of the assembled zirconium source solution, fumaric acid solution, and porous nylon support, was placed in an oven and reacted at 80, 85, 90, and 95 °C for 18 h, respectively. During the film formation process, the MOF particles formed in the solution were also collected for characterization. The prepared samples were first washed three times with DMF, then washed five times with methanol, and then immersed in a methanol solution. Finally, they were vacuum dried at 170 °C for 12 h to obtain activated MOF-808 films or MOF-808 particles.

[0059] refer to Figure 5 The MOF-808 film grown by reverse diffusion at 80℃ exhibits significant intergranular defects on its surface, with most of the film consisting of a large accumulation of MOF-808 particles. The film thickness is less than 1 μm and discontinuous. However, as the temperature increases, the intergranular defects on the surface of the MOF-808 film grown by reverse diffusion at 85℃ begin to decrease, resulting in a distinct film layer. When the temperature continues to rise to 90℃, the film surface displays a clear MOF-808 crystal structure without obvious intergranular defects. At 95℃, the film surface is dense and defect-free, with a uniform and continuous film layer. This demonstrates that temperature can promote the nucleation reaction of Zr metal clusters SBU and BTC organic ligands, and increasing the temperature allows them to react and form a continuous film layer.

[0060] 3) Based on the above tests, the MOF-808 film tested in the subsequent tests was prepared based on a porous nylon support with a seed loading of 0.45 mg and a back diffusion temperature of 95℃; the MOF-808 powder was also prepared under the corresponding conditions.

[0061] BET analysis of the MOF-808 powder generated during the reverse diffusion process showed a specific area of ​​2429 m². 2 g -1 The average pore size is 19.4 Å. Figure 6 )

[0062] H2 and CO2 gas flux tests were performed on the MOF-808 membrane. It was found that compared to a porous nylon support with a pore size of 100 nm, the prepared MOF-808 membrane exhibited an order-of-magnitude decrease in CO2 and H2 gas flux, from 2.14 × 10⁻⁶. 8 It decreased to 7.82×10 6 GPU, and selectivity was increased from 1.50 to 4.23 ( Figure 7It can be assumed that H2 and CO2 undergo free molecular diffusion on the macroporous (100 nm) porous nylon support, so the selectivity is approximately 1. However, on the MOF-808 membrane with a pore size of 19.4 Å, Knudsen diffusion occurs, so its selectivity is approximately the ideal selectivity of Knudsen diffusion for CO2 and H2 (4.7). Example 1

[0063] This embodiment discloses a method for preparing MOF-801 in MOF-808 membranes, specifically including the following steps:

[0064] 1) Dissolve 363 mg ZrOCl2·8H2O in a mixed solvent of DMF / formic acid (45 / 30 mL), and dissolve 1182 mg BTC ligand in a mixed solvent of DMF / formic acid (45 / 30 mL). Add the zirconium source solution and the pyromellitic acid solution to the left and right sides of the U-shaped glass membrane device, respectively. The device is separated in the middle by a porous nylon support. The porous nylon support is loaded with 0.45 mg MOF-808 seed crystals by filtration of a deionized aqueous solution containing MOF-808 powder.

[0065] 2) Place the membrane-forming device in an oven and react at 95°C for 18 hours;

[0066] II. Hydrothermal Preparation of MOF-801 in MOF-808 Membranes

[0067] 1) After the reaction is complete, the MOF-808 membrane taken out from the U-shaped glass membrane-making device is washed with methanol 5 times, and then washed with deionized water 3 times to remove the residual solvent.

[0068] 2) Dissolve ZrOCl2·8H2O and fumaric acid in water / acetic acid ( V 水 : V 乙酸 In the mixed solvent (3.7:1.3), the concentration of Zr metal source is 1 mmol / 50 mL, and the molar ratio of Zr metal source to Fum organic ligand is 1:1.

[0069] 3) After washing, the MOF-808 membrane is immersed in the solution prepared in step 2) for 3 h, then removed and placed in an oven to react at 80℃ for 20 h to obtain unactivated MOF-801 in MOF-808 membrane;

[0070] 4) Dissolve ZrOCl2·8H2O with different amounts of fumaric acid in water / acetic acid ( V 水 : V 乙酸In a mixed solvent of 3.7:1.3, MOF-808 powder was soaked in the solution for 3 h, then removed and placed in an oven to react at 80 °C for 20 h to obtain unactivated MOF-801 in MOF-808 powder;

[0071] 5) The unactivated MOF-801 in MOF-808 membrane and the unactivated MOF-801 in MOF-808 powder were washed three times with deionized water and then soaked in deionized water solution. Finally, they were vacuum dried at 120℃ for 12 h to obtain the activated MOF-801 in MOF-808 membrane or powder. Example 2

[0072] The difference between Example 2 and Example 1 lies only in step 2): dissolving ZrOCl2·8H2O and fumaric acid in water / acetic acid. V 水 : V 乙酸 In the mixed solvent (3.7:1.3), the concentration of Zr metal source is 2 mmol / 50 mL, and the molar ratio of Zr metal source to Fum organic ligand is 1:1. Example 3

[0073] The difference between Example 3 and Example 1 lies only in step 2): dissolving ZrOCl2·8H2O and fumaric acid in water / acetic acid. V 水 : V 乙酸 In the mixed solvent (3.7:1.3), the concentration of Zr metal source is 3 mmol / 50 mL, and the molar ratio of Zr metal source to Fum organic ligand is 1:1. Example 4

[0074] The difference between Example 4 and Example 1 lies only in step 2): dissolving ZrOCl2·8H2O and fumaric acid in water / acetic acid. V 水 : V 乙酸 In the mixed solvent (3.7:1.3), the concentration of Zr metal source was 4 mmol / 50 mL, and the molar ratio of Zr metal source to Fum organic ligand was 1:1. Example 5

[0075] The difference between Example 5 and Example 1 lies only in step 2): dissolving ZrOCl2·8H2O and fumaric acid in water / acetic acid. V 水 : V 乙酸In the mixed solvent (3.7:1.3), the concentration of Zr metal source is 1 mmol / 50 mL, and the molar ratio of Zr metal source to Fum organic ligand is 1:2. Example 6

[0076] The difference between Example 6 and Example 1 lies only in step 2): dissolving ZrOCl2·8H2O and fumaric acid in water / acetic acid. V 水 : V 乙酸 In the mixed solvent (3.7:1.3), the concentration of Zr metal source was 4 mmol / 50 mL, and the molar ratio of Zr metal source to Fum organic ligand was 1:2.

[0077] Comparative Example

[0078] The difference between this comparative example and Example 1 is that the MOF-808 membrane was directly immersed in the MOF-801 feed solution to grow MOF-801 particles by a traditional in-situ hydrothermal reaction (without using the method of taking the membrane out of the immersion solution after immersion and then carrying out the hydrothermal reaction). The in-situ hydrothermal reaction temperature was also 80°C, and the reaction time was 20 h.

[0079] Relevant performance tests:

[0080] Tests revealed that the MOF-801 grown in situ using the hydrothermal method in the comparative example generated a new membrane layer on top of the original MOF-808 membrane, exhibiting a structure similar to a bilayer membrane. Figure 8 (small images c and d), and the surface has a large number of MOF-801 particles ( Figure 8 (See figures a and b in the middle). The film thickness has increased significantly compared to the original MOF-808 film, from ~2 μm to ~3 μm. When using the in-situ hydrothermal method, most of the MOF-801 is preferentially generated in the reaction solution rather than in the pores of the MOF-808 film, resulting in a large amount of MOF-801 growing on the surface of the MOF-808 film and even forming a bilayer film structure.

[0081] In this embodiment, a method of first soaking and then taking out the membrane for hydrothermal reaction is adopted. When the MOF-808 membrane is soaked in the MOF-801 feed solution, the MOF-801 feed solution is adsorbed into the MOF-808 pores. Then, the membrane with the feed solution adsorbed after soaking for a period of time is taken out separately for reaction, and MOF-801 crystals can be grown in the pores.

[0082] refer to Figure 9 The surface morphology and thickness of the membrane prepared by the method used in Example 1 did not change significantly compared to the MOF-808 membrane.

[0083] XRD tests were performed on the MOF-801 in MOF-808 membrane prepared in Example 1, and the results are referenced. Figure 10 In the small figure (a), it was found that after growing MOF-801 in the pores of the MOF-808 membrane, the first peak in the double peak at ~8° showed a significant increase, which corresponds to the characteristic peak of MOF-801, indicating the successful introduction of MOF-801.

[0084] Water contact angle tests were performed on MOF-801 in MOF-808 membranes. The porous nylon support itself is hydrophilic; after growing the MOF-808 membrane on its surface, the hydrophobicity of MOF-808 resulted in a water contact angle of 117.75°. However, after growing MOF-801 within the pores of the MOF-808 membrane, the contact angle decreased to 100.42°. Figure 10 (See small image b). This is because MOF-801 is hydrophilic, so the water contact angle decreases after MOF-801 is grown inside the pores of the MOF-808 membrane.

[0085] In Examples 1-4, the molar ratio of Zr metal source and Fum organic ligand as reactants for preparing MOF-801 was 1:1. However, as the reactant concentration gradually increased, the pore size and specific surface area of ​​the MOF-808 membrane decreased from 1 mmol / 50 mL to 4 mmol / 50 mL. When the reactant concentration was 1 mmol / 50 mL, the pore size decreased significantly to 10.72 Å, and the specific surface area decreased significantly to 475 m². 2 g -1 When the concentration was increased to 2 mmol / 50 mL, the pore size further decreased to 8.31 Å, while the specific surface area no longer changed significantly; when the concentration was further increased, the pore size no longer decreased significantly, and the specific surface area no longer showed a significant trend of change. Figure 11 (Small images a and b)

[0086] To explore whether the pore size could be further reduced, Examples 5-6 attempted to set the molar ratio of the Zr metal source and the Fum organic ligand in the preparation of MOF-801 to 1:2, and increased the Zr metal source concentration from 1 mmol / 50 mL to 4 mmol / 50 mL. The results are shown below. Figure 11 In the small image of cd, at the same Zr metal source concentration (1 mmol / 50 mL), the pore size of the MOF-808 membrane further decreased to 7.64 Å. When the Zr metal source concentration was further increased, the pore size no longer decreased significantly. When the Zr metal source concentration was 4 mmol / 50 mL, the pore size of the MOF-808 membrane was adjusted to 7.02 Å.

[0087] The reason why the pore size no longer changes significantly when the reactant concentration is gradually increased to a certain value under a fixed reactant ratio is likely because the pore space of the MOF-808 membrane is limited. After reaching a certain reactant concentration, the pore space is nearly saturated with adsorbed reactants. Therefore, further increasing the concentration does not significantly increase the number of MOF-801 particles grown within the pores, hence the pore size no longer changes drastically. The reason why increasing the molar ratio of Zr metal source to Fum organic ligand to 1:2 has a more significant effect on reducing the pore size of MOF-801 is likely because excess Fum organic ligand is beneficial to MOF-801 growth. This is because each Zr6O4(OH)4(-CO2) in the MOF-801 crystal structure... 12 The metal cluster has 12 Fum organic ligand binding sites, and its structure is analogous to the 'kiss ball number problem' in three-dimensional space, which is the most compact packing mode of SBUs.

[0088] The flux of MOF-801 in MOF-808 membranes prepared under different molar ratios of Zr metal source and Fum organic ligand was tested with CO2 and H2 gases, and it was found that the flux of MOF-801 in MOF-808 membranes was reduced by an order of magnitude compared with that of MOF-808 membranes. Figure 12 Using an equimolar ratio of Zr metal source and Fum organic ligand as MOF-801 reactants, the flux of the prepared MOF-801 in MOF-808 membrane was reduced to 6.93 × 10⁻⁶. 5 The GPU selectivity was 4.28, which did not show a particularly significant improvement. This is because the membrane pore size was 8.27 Å at this point. Figure 11 The flux of the MOF in MOF membrane (as shown in the small figure) is much greater than that of CO2 (3.3 Å) and H2 (2.8 Å), indicating that Knudsen diffusion still dominates the diffusion mechanism. However, when the molar ratio of the Zr metal source to the Fum organic ligand increases to 1:2, the flux of the prepared MOF in MOF membrane decreases to 2.16 × 10⁻⁶. 4 GPU, selectivity 5.59. At this point, the membrane pore size is 7.02 Å ( Figure 11 (See the small figure in the middle d), diffusion is still dominated by Knudsen diffusion. However, the selectivity breaks through the Knudsen diffusion selectivity. This is because MOF materials have unique interaction forces with CO2, which gives them a high adsorption capacity for CO2 and allows H2 to pass through preferentially, thus exhibiting a certain degree of selectivity.

[0089] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. However, the above description is merely a specific embodiment of the present invention, and the technical features of the present invention are not limited thereto. Any other embodiments derived by those skilled in the art without departing from the technical solution of the present invention should be covered within the patent scope of the present invention.

Claims

1. A method for preparing a MOF membrane with a porous MOF structure, characterized in that, The MOF membrane with a pore MOF structure includes a base membrane and a selective separation layer on the surface of the base membrane. The material in the selective separation layer is a first MOF crystal, and a second MOF crystal is also distributed in the pores of the first MOF crystal. The preparation method includes the following steps: Step 1: Prepare a separation layer formed by the first MOF crystal on the surface of the base film; Step 2: Place the membrane obtained in Step 1 into a reaction solution containing a metal source and ligands for synthesizing the second MOF crystal. After taking it out, perform hydrothermal synthesis by heating to generate the second MOF crystal in the pores of the first MOF crystal. Step 3: Activate the membrane obtained in Step 2.

2. The method for preparing a MOF membrane with a porous MOF structure according to claim 1, characterized in that, The base film refers to a porous polymer membrane; the first MOF crystal is MOF-808, and the second MOF crystal is MOF-801.

3. The method for preparing a MOF membrane with a porous MOF structure according to claim 1, characterized in that, Includes the following steps: 1) Prepare and activate the MOF-808 membrane; 2) Dissolve the zirconium metal source and fumaric acid ligand in a mixed solvent of water and acetic acid to obtain MOF-801 feed solution; 3) After immersing the activated MOF-808 membrane in the MOF-801 feed solution prepared in step 2) for a period of time, remove it and place it in an oven for hydrothermal reaction to obtain unactivated MOF-801 in MOF-808 membrane; 4) Wash the unactivated MOF-801 in MOF-808 membrane with deionized water and immerse it in deionized water, then heat and dry it to obtain the activated MOF-801 in MOF-808 membrane.

4. The method for preparing a MOF membrane with a porous MOF structure according to claim 3, characterized in that, In step 2), the molar ratio of zirconium metal source to fumaric acid ligand in the prepared MOF-801 raw material solution is 1:1~3, and the concentration of zirconium metal source is 1~4 mmol / 50 mL; the volume ratio of water to acetic acid in the mixed solvent is 3.5~4:1~1.

5.

5. The method for preparing a MOF membrane with a porous MOF structure according to claim 3, characterized in that, In step 3), the MOF-808 membrane is immersed in the MOF-801 feed solution for 1 to 3 hours, the hydrothermal reaction temperature is 75 to 90°C, and the reaction time is 18 to 24 hours.

6. The method for preparing a MOF membrane with a porous MOF structure according to claim 3, characterized in that, In step 4), the heating and drying temperature is 120~130℃, and the drying time is 12~24 h.

7. The method for preparing a MOF membrane with a porous MOF structure according to claim 3, characterized in that, In step 1), the preparation process of the MOF-808 membrane is as follows: I. Dissolve the zirconium metal source in a mixed solvent of organic solvent and formic acid, and dissolve the trimesic acid ligand in a mixed solvent of organic solvent and formic acid. Then add the zirconium source solution and trimesic acid solution to both sides of the membrane device. The device is separated in the middle by a membrane support, and MOF-808 seed crystals are loaded on the membrane support. II. Place the membrane fabrication apparatus in an oven and heat it to react, obtaining an unactivated MOF-808 membrane; III. Solvent exchange treatment is performed on the unactivated MOF-808 membrane to obtain an activated MOF-808 membrane.

8. The method for preparing a MOF membrane with a porous MOF structure according to claim 7, characterized in that, In step I, the volume ratio of organic solvent to formic acid in the mixed solvent is 3:2, and the membrane support is made of porous nylon; in step II, the heating reaction temperature is 85-100℃ and the reaction time is 18-20 h. In step III, the solvent exchange process is as follows: the unactivated MOF-808 membrane is rinsed with DMF 3 times, washed with methanol 5 times, then soaked in methanol solution, and vacuum dried at 170℃ for 12 h to obtain the activated MOF-808 membrane.

9. A MOF membrane having a pore-in-MOF structure, characterized in that, It is obtained by the preparation method described in any one of claims 1-7.

10. The application of the MOF membrane with a pore-in-MOF structure as described in claim 9 in the selective separation of gas mixtures.

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