A high-permeability MOF thin film, its preparation method and application

Abstract

This invention discloses a highly permeable MOF thin film, its preparation method, and its application. The preparation method includes the following steps: 1) depositing zinc carbonate nanosheets on the surface of a porous support; 2) calcining the porous support to obtain a porous support with a zinc oxide nanosheet layer deposited on its surface; 3) converting the zinc oxide nanosheet layer into an MOF film in the presence of an organic ligand, thereby obtaining the MOF thin film. This invention first deposits ultrathin zinc carbonate nanosheets on the surface of a porous support, then calcines them to form a loosely arranged, mechanically stable ultrathin zinc oxide nanosheet layer on the surface of the porous support. This layer is used as the zinc source for synthesizing the MOF film. The support is placed in an organic ligand atmosphere to induce the conversion of the zinc oxide nanosheets on the support surface into a MOF film. The resulting MOF film has a densely oriented membrane structure and exhibits no leakage or filling of the pores of the support.

Description

Technical Field

[0001] This invention specifically relates to a high-permeability MOF thin film, its preparation method, and its application. Background Technology

[0002] Metal-organic framework (MOF) materials stand out for their diverse topologies and flexible, customizable host-guest chemistry. This provides a unique platform for MOF-based membranes to fine-tune nanochannels, including pore structure and specific interfacial interactions at the angstrom scale. Consequently, microporous MOF membranes with well-designed pore systems have been extensively studied, exhibiting high selectivity and permeability in most gas separation applications. However, striking a balance between simple, sustainable fabrication processes and the superior performance of the resulting MOF membranes remains an ongoing challenge.

[0003] Metal oxide conversion strategies—synthetic strategies that immobilize metal oxides (such as zinc oxide (ZnO)) on a support as a metal source and then activate them with organic ligands to form a membrane—show promise for the simple preparation of MOF membranes. However, in the prior art, most reported metal oxide layers involve relatively complex preparation procedures, and the oxide layers obtained through these procedures inevitably exhibit a certain degree of porosity and permeability. This results in the incomplete conversion of the dense oxide layer, introducing additional transfer resistance and ultimately leading to low permeability and overall efficiency when the membrane is used for gas separation. Summary of the Invention

[0004] In view of the shortcomings and deficiencies of the prior art, the purpose of this invention is to provide a method for preparing a high-permeability MOF membrane. The MOF membrane prepared by this method does not exhibit pore permeation and has high permeability, which significantly improves the separation effect when used for gas separation.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing a MOF thin film includes the following steps: 1) depositing zinc carbonate nanosheets on the surface of a porous support; 2) calcining the porous support to obtain a porous support with a zinc oxide nanosheet layer deposited on its surface; 3) converting the zinc oxide nanosheet layer into a MOF film in the presence of an organic ligand to obtain the MOF thin film.

[0007] In some embodiments, in step 1), the thickness of the zinc carbonate nanosheets after deposition is 0.5–2 μm.

[0008] In some embodiments, in step 1), the deposition is achieved by vacuum filtration of a dispersion of zinc carbonate nanosheets on a porous support.

[0009] In some embodiments, the molar concentration of zinc carbonate nanosheets in the dispersion is 0.04–0.2 mol·L⁻¹. -1 .

[0010] In some embodiments, the dispersed phase of the dispersion is selected from one or more combinations of water, methanol, ethanol, and acetone.

[0011] In some embodiments, the preparation method further includes a step of preparing zinc carbonate nanosheets by hydrothermal reaction using zinc oxide nanoparticles and sodium bicarbonate aqueous solution as raw materials.

[0012] In some embodiments, the zinc oxide nanoparticles have a particle size of 50–200 nm.

[0013] In some embodiments, the molar ratio of the zinc oxide nanoparticles to sodium bicarbonate is 1:1 to 3.

[0014] In some embodiments, the temperature of the hydrothermal reaction is 60–100°C.

[0015] In some embodiments, the hydrothermal reaction takes 2 to 5 hours.

[0016] In some embodiments, the zinc carbonate nanosheets have a lateral dimension of 1–10 μm and a thickness of 10–50 nm.

[0017] In some embodiments, the preparation method further includes a vacuum drying step prior to the calcination.

[0018] In some embodiments, in step 2), the calcination temperature is 300–500°C.

[0019] In some embodiments, the calcination time in step 2) is 2 to 3 hours.

[0020] In some embodiments, in step 2), the thickness of the zinc oxide nanosheet layer is 0.5–2 μm.

[0021] In some embodiments, in step 3), the organic ligand is selected from one or more combinations of imidazole, benzimidazole, 2-methylimidazolium, 4,5-dichloroimidazolium, 5-chlorobenzimidazole, 5-methylbenzimidazole, and 2-aminobenzimidazole.

[0022] In some embodiments, in step 3), the conversion method is selected from one or more combinations of solution heat treatment, steam heat treatment, and electrochemical treatment.

[0023] In some embodiments, the organic ligand exists in solution form, and the concentration of the organic ligand in the solution is 1 to 3 mol / L.

[0024] In some embodiments, step 3) also includes the addition of a ligand modifier selected from one or more combinations of sodium formate, formic acid, acetic acid, and benzoic acid.

[0025] In some embodiments, the porous support is selected from porous alumina or porous silicon carbide.

[0026] In some embodiments, the porous support has a pore size of 0.2–5 μm and a porosity of 30–50%.

[0027] In some embodiments, the porous support is selected from a disc, a single-channel tube, or a multi-channel tube.

[0028] In some embodiments, the MOF is selected from one or more combinations of ZIF-7, ZIF-8, ZIF-62, ZIF-71, ZIF-95, and ZIF-302.

[0029] The present invention also provides MOF films prepared by the above preparation method.

[0030] The present invention also provides the use of the above-mentioned MOF film for gas separation.

[0031] Furthermore, the gas is selected from carbon dioxide / methane, carbon dioxide / nitrogen, ethylene / ethane, propylene / propane, and is used for the separation of hexane isomers.

[0032] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0033] This invention involves first depositing ultrathin zinc carbonate nanosheets onto the surface of a porous support, followed by vacuum drying and calcination. This results in a loosely arranged, mechanically stable layer of ultrathin zinc oxide nanosheets on the porous support surface, which is then used as the zinc source for synthesizing MOF membranes. The substrate is placed in an organic ligand atmosphere to induce the transformation of the zinc oxide nanosheets on the support surface into MOF membranes. The resulting MOF membrane exhibits a densely oriented membrane structure and shows no leakage or filling of the pores within the support.

[0034] The ultrathin structure of zinc oxide nanosheets used in this invention and their loosely packed state on the support surface give them high accessibility and reactivity, enabling complete conversion during the MOF membrane process. Simultaneously, the zinc oxide nanosheets, as a precursor deposition material, prevent pore seepage within the support pores, resulting in a MOF membrane with high gas permeability. In contrast, existing technologies using zinc oxide nanosols or nanoparticles as the zinc source are prone to pore seepage, leading to more membrane defects and less than optimal separation performance. Attached Figure Description

[0035] Figure 1 SEM image of the zinc oxide nanosheets prepared in Example 1;

[0036] Figure 2 The image shows the XRD pattern of zinc oxide nanosheets prepared by conversion in Example 1.

[0037] Figure 3 SEM images of the surface and cross-section of the MOF film prepared in Example 1;

[0038] Figure 4 EDS image of the MOF thin film prepared in Example 1;

[0039] Figure 5 SEM images of the surface and cross-section of the MOF film prepared in Example 2;

[0040] Figure 6 The XRD pattern of the MOF thin film prepared in Example 2;

[0041] Figure 7 The MOF thin film prepared in Example 2 <100> Orientation AFM diagram;

[0042] Figure 8 SEM images of the surface and cross-section of the MOF film prepared in Example 3;

[0043] Figure 9 The XRD pattern of the MOF thin film prepared in Example 3;

[0044] Figure 10 SEM image of the zinc oxide nanosheets prepared in Example 4;

[0045] Figure 11 SEM images of the surface and cross-section of the MOF film prepared in Example 4;

[0046] Figure 12 SEM image of the zinc oxide nanoparticle layer prepared in Comparative Example 1;

[0047] Figure 13 The XRD pattern of the MOF thin film prepared in Comparative Example 1;

[0048] Figure 14 SEM images of the surface and cross-section of the MOF film prepared for Comparative Example 1. Detailed Implementation

[0049] The technical solutions of the present invention will be described in detail below with reference to specific embodiments, so that those skilled in the art can better understand and implement the technical solutions of the present invention, but the present invention is not limited to the scope of the examples described.

[0050] Example 1

[0051] This embodiment provides a MOF thin film, the specific preparation steps of which are as follows:

[0052] Zinc carbonate nanosheets were prepared by hydrothermal reaction using zinc oxide nanoparticles (100 nm in diameter) and sodium bicarbonate aqueous solution as raw materials. The molar ratio of zinc oxide nanoparticles to sodium bicarbonate was 1:1, the reaction temperature was 100 °C, and the reaction time was 2 h, resulting in zinc carbonate nanosheets (5 μm in lateral dimension and 25 nm in thickness).

[0053] Disperse 0.01 g of the above zinc carbonate nanosheets in 2 mL of water (0.04 mol·L⁻¹). -1 A layer of zinc carbonate nanosheets, approximately 1 μm thick, is deposited onto a porous alumina ceramic sheet via vacuum filtration.

[0054] Alumina ceramic sheets with deposited zinc carbonate nanosheets were calcined in a muffle furnace at 35°C for 2 hours to form loosely packed zinc oxide nanosheets.

[0055] Figure 1 and Figure 2 The SEM and XRD images of the obtained zinc oxide nanosheets are shown. It can be seen that a pure phase zinc oxide nanosheet layer is formed on the porous alumina ceramic sheet, with a thickness of ~1 μm.

[0056] 2.4283 g of 2-methylimidazole was ultrasonically dissolved in 16 mL of deionized water to prepare a ligand solution. Subsequently, an alumina ceramic sheet with a ~1 μm thick zinc oxide nanosheet layer was placed in a polytetrafluoroethylene reactor containing the above ligand solution and placed in an 80°C forced-air drying oven for 6 hours to promote the conversion of the zinc oxide nanosheets into a ZIF-8 membrane, followed by natural cooling. The ZIF-8 membrane obtained by heat treatment with this 2-methylimidazole ligand solution was rinsed three times alternately with deionized water and methanol, and then immersed in fresh methanol for 12 hours to remove residual water molecules from the pores of the ZIF-8 membrane. Finally, the ZIF-8 membrane was dried at room temperature for 12 hours before structural characterization and gas permeation testing.

[0057] Figure 2 and Figure 3XRD patterns and surface and cross-sectional SEM images of the ZIF-8 film prepared by the conversion of zinc oxide nanosheets are shown. It can be seen that the zinc oxide nanosheet layer has been completely converted into a highly crystalline ZIF-8 film. The film surface is dense and defect-free, with a thickness of 1 μm. Figure 4 The EDS diagrams shown indicate that the ZIF-8 membrane exhibits no porosity or permeation. This ZIF-8 membrane demonstrates excellent separation performance for propylene / propane (C3H6 / C3H8), with a permeation rate of approximately 200 GPU for propylene and a selectivity of approximately 80% for propylene / propane separation. This performance surpasses that of most ZIF-8 membranes prepared using zinc oxide conversion.

[0058] Example 2

[0059] The zinc oxide nanosheets were prepared in the same manner as in Example 1.

[0060] 4.105 g of 2-methylimidazole was ultrasonically dissolved in 50 mL of deionized water to prepare a ligand solution. 0.1 mL of sodium formate was added to this ligand solution. Then, the aforementioned alumina ceramic sheet (working electrode) with deposited zinc oxide nanosheets and the graphite electrode (reference electrode) were placed together in a 50 mL electrolytic cell. The prepared ligand solution was poured into the electrolytic cell, ensuring that the solution covered the porous alumina ceramic sheet. A constant current of 0.5 mA was then applied, and the reaction time was 60 min. The applied constant current promoted the deprotonation of the ligands and attacked the zinc oxide nanosheets, causing point-to-point conversion to form a ZIF-8 membrane. After the conversion, the membrane cleaning, pore activation, and drying steps were the same as in Example 1.

[0061] Figure 5 and Figure 6 SEM and XRD images of the surface and cross-section of the ZIF-8 film prepared by electrochemically driven conversion of zinc oxide nanosheets are shown. Unlike the film prepared in Example 1, the electrochemically synthesized ZIF-8 film exhibits... <100> With channel orientation and a film thickness of only 500nm, there is also no porosity. Figure 7 show <100> The inter-growth of oriented ZIF-8 crystals results in a relatively large membrane surface roughness. This ZIF-8 membrane exhibits excellent separation performance for ethylene / ethane (C2H4 / C2H6), with a permeation rate of approximately 130 GPU for ethylene and a selectivity of approximately 8.2 for propylene / propane. This performance surpasses that of most MOF membranes used for ethylene / ethane separation.

[0062] Example 3

[0063] The zinc oxide nanosheets were prepared in the same manner as in Example 1.

[0064] 3.105 g of 2-methylimidazole powder was ultrasonically dissolved in 15 mL of methanol to prepare a ligand solution. The methanol solution was then transferred to a 100 mL polytetrafluoroethylene (PTFE) reactor. The prepared alumina ceramic sheet with deposited zinc oxide nanosheets was placed on a custom-made support with the nanosheets facing down, and the support was then placed in the reactor. Ligand vapor heat treatment was performed at 150 °C, during which ligand molecules were uniformly generated under the action of methanol vapor, promoting MOF film formation. After 6 hours of reaction, the film was cleaned, the pores were activated, and the film was dried as in Example 1.

[0065] Figure 8 and Figure 9 SEM and XRD images of the surface and cross-section of the MOF membrane prepared by ligand vapor-driven conversion of zinc oxide nanosheets are shown. Unlike the membrane prepared in Example 1, the vapor thermal conversion of zinc oxide nanosheets produced a continuous and dense ZIF-L membrane with a thickness of approximately 4 μm. The ZIF-L membrane prepared by this method exhibits high compactness and excellent separation performance for carbon dioxide / nitrogen (CO2 / N2), with a carbon dioxide permeation rate of approximately 300 GPU and a carbon dioxide / nitrogen separation selectivity of approximately 20.

[0066] Example 4

[0067] The preparation of zinc carbonate nanosheets is the same as in Example 1.

[0068] 0.1 g of zinc carbonate nanosheets were dispersed in 50 mL of deionized water and deposited on the inner wall of a porous alumina ceramic tube (13 cm) through vacuum filtration to form a layer of zinc carbonate nanosheets.

[0069] Alumina ceramic sheets with deposited zinc carbonate nanosheets were calcined in a muffle furnace at 350°C for 2 hours to form loosely packed zinc oxide nanosheets.

[0070] Figure 10 A pure-phase zinc oxide nanosheet layer with a thickness of ~2 μm was formed on the inner wall of a porous alumina ceramic tube.

[0071] The outer surface of the alumina tube with zinc oxide nanosheets deposited on its inner wall was sealed with PTFE tape to prevent the ligand solution from seeping into the support from the outside. The alumina tube was then placed vertically in a PTFE reactor filled with the ligand solution and subjected to a solvothermal reaction in an 80°C forced-air drying oven for 6 hours. The ligand solution was prepared by dissolving 500 mmol of 2-methylimidazole in 200 mL of an ethanol / water (1:2) mixture. After membrane conversion, the membrane cleaning, pore activation, and drying procedures were the same as in Example 1.

[0072] Figure 11SEM images of the surface and cross-section of the tubular ZIF-8 membrane prepared by zinc oxide nanosheet conversion are shown. The images reveal a dense, defect-free membrane surface with a thickness of 3 μm, exhibiting no porosity or permeation. This tubular ZIF-8 membrane demonstrates excellent separation performance for propylene / propane (C3H6 / C3H8), with a permeation rate of approximately 100 GPU for propylene and a selectivity of approximately 130 for propylene / propane separation.

[0073] Comparative Example 1

[0074] The process is essentially the same as in Example 1, except that zinc oxide sol is deposited on the surface of an alumina support instead of zinc carbonate nanosheets, and the zinc oxide sol is calcined at 350°C for 2 hours to obtain a dense zinc oxide layer (with a thickness of ~1 μm). This layer is then used as the zinc source for the same MOF film growth steps as in the example. Figure 12 The SEM images show obvious porosity in the support, with zinc oxide particles filling the pores. The support coated with the zinc oxide particle layer was placed in a ligand solution for a conversion reaction. The resulting membrane underwent the same post-processing steps as in Example 1 and was then used for propylene / propane separation performance testing. This is in contrast to the ZIF-8 membrane prepared using zinc oxide nanosheets in Example 1. Figure 2 In contrast, the ZIF-8 film prepared using zinc oxide nanoparticles treated with ligands for 6 hours in Comparative Example 1 still exhibited characteristic peaks of zinc oxide, indicating that the zinc oxide particles were not completely converted. Figure 13 ). Figure 14 The surface structure of the prepared ZIF-8 membrane is similar to that of Example 1, but the cross-section is significantly different, exhibiting obvious ZIF-8 porosity permeation. This inadvertently increases the actual thickness of the membrane layer, resulting in increased mass transfer resistance. Consequently, the membrane's permeation rate for propylene is only 25 GPU, and the propylene / propane selectivity is approximately 60%, indicating the presence of a small number of grain boundary defects in the membrane. Both the membrane performance and separation performance are inferior to those of Example 1.

[0075] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing MOF thin films, characterized in that: The preparation method includes the following steps: 1) depositing zinc carbonate nanosheets on the surface of a porous support, wherein the porous support is selected from porous alumina or porous silicon carbide; 2) calcining the porous support to obtain a porous support with a zinc oxide nanosheet layer deposited on its surface; 3) converting the zinc oxide nanosheet layer into an MOF film in the presence of an organic ligand to obtain the MOF film; wherein the organic ligand is selected from one or more combinations of imidazole, benzimidazole, 2-methylimidazolium, 4,5-dichloroimidazolium, 5-chlorobenzimidazole, 5-methylbenzimidazole and 2-aminobenzimidazole.

2. The method for preparing MOF thin films according to claim 1, characterized in that: In step 1), the thickness of the zinc carbonate nanosheets after deposition is 0.5~2 μm; and / or, in step 1), the deposition is achieved by vacuum filtration of a dispersion of zinc carbonate nanosheets on a porous support; the molar concentration of zinc carbonate nanosheets in the dispersion is 0.04~0.2 mol·L⁻¹. -1 ; and / or, the dispersed phase of the dispersion is selected from one or more combinations of water, methanol, ethanol, and acetone.

3. The method for preparing MOF thin films according to claim 1, characterized in that: The preparation method further includes a step of preparing zinc carbonate nanosheets by hydrothermal reaction using zinc oxide nanoparticles and sodium bicarbonate aqueous solution as raw materials; wherein the particle size of the zinc oxide nanoparticles is 50~200 nm; and / or, the molar ratio of the zinc oxide nanoparticles to sodium bicarbonate is 1:1~3; and / or, the temperature of the hydrothermal reaction is 60~100℃; and / or, the time of the hydrothermal reaction is 2~5 h; and / or, the lateral dimension of the zinc carbonate nanosheets is 1~10 μm and the thickness is 10~50 nm.

4. The method for preparing MOF thin films according to claim 1, characterized in that: The preparation method further includes a vacuum drying step before calcination; and / or, in step 2), the calcination temperature is 300~500℃. o C; and / or, in step 2), the calcination time is 2-3 h; and / or, in step 2), the thickness of the zinc oxide nanosheets is 0.5-2 μm.

5. The method for preparing MOF thin films according to claim 1, characterized in that: In step 3), the organic ligand exists in solution form, and the concentration of the organic ligand in the solution is 1~3 mol / L; and / or, in step 3), a ligand modifier is also added, wherein the ligand modifier is selected from one or more combinations of sodium formate, formic acid, acetic acid, and benzoic acid.

6. The method for preparing MOF thin films according to claim 1, characterized in that: In step 3), the conversion method is selected from one or more combinations of solution heat treatment, steam heat treatment, and electrochemical treatment.

7. The method for preparing MOF thin films according to claim 1, characterized in that: The porous support has a pore size of 0.2~5 μm and a porosity of 30~50%; and / or, the porous support is selected from one of a disc, a single-channel tube, or a multi-channel tube.

8. The method for preparing MOF thin films according to claim 1, characterized in that: The MOF is selected from one or more combinations of ZIF-7, ZIF-8, ZIF-62, ZIF-71, ZIF-95, and ZIF-302.

9. A MOF thin film prepared by the method of any one of claims 1-8.

10. Use of the MOF film of claim 9 for gas separation.

11. The use according to claim 10, characterized in that: The gas is selected from carbon dioxide / methane, carbon dioxide / nitrogen, ethylene / ethane, propylene / propane, and hexane isomers.

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