Method for directly growing highly-oriented metal organic framework film by using liquid-gas phase reaction

Abstract

The invention discloses a method for directly growing a highly-oriented metal organic framework film by using a liquid-gas phase reaction, which is different from the traditional solid-gas reaction (such as ALD or CVD) by using a direct chemical reaction between solvated metal ions on the surface of a substrate and organic ligands in a gas phase on a liquid-gas interface. According to the LVD process, metal ions dissolved in a water phase are directly utilized, an ultrathin water layer is used as a carrier, and continuous supply and uniform distribution of the metal ions on a reaction interface are ensured through liquidity of a liquid phase, so that a continuous nucleation process is supported; the film forming process depends on the accurate balance of the reaction temperature on the water evaporation rate and the ligand vapor consumption kinetics, and methanol is introduced into the reaction system as a structure-directing agent. The methanol molecules have stronger interaction with the crystal face (112) of the KAUST-7 crystal, the surface is stabilized through the generated'solvent end-capping effect ', and the growth of the crystal along the vertical direction of the surface is inhibited, so that the crystal is induced to form high (112) preferred orientation on the surface of the substrate.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterial preparation and membrane separation technology, specifically relating to a method for directly growing highly oriented metal-organic framework membranes using liquid-gas phase reaction. Technical Background

[0002] Metal-organic framework (MOF) membranes, as thin film forms of MOF materials, combine the porous characteristics of MOFs with the continuity and ease of integration of thin films. This overcomes the limitation of MOF powder materials being difficult to directly apply to device integration, further expanding the application scenarios of MOF materials and becoming one of the current research hotspots in materials science. The performance of MOF membranes is closely related to their crystal orientation. Highly oriented MOF membranes can achieve directional alignment of crystal channels, effectively reducing intramembrane grain boundary defects and significantly improving their selectivity, permeability, and stability in applications such as gas separation and catalysis. Therefore, the preparation of highly oriented MOF membranes is one of the key prerequisites for their industrial application.

[0003] Currently, the main methods for preparing MOF films include in-situ solvothermal synthesis, secondary growth, electrochemical deposition, layer-by-layer self-assembly, vapor phase deposition, and vapor phase-assisted synthesis. Among these, in-situ solvothermal synthesis is one of the most commonly used methods. This method involves immersing the substrate in a mixed solution containing metal ions and organic ligands, and reacting under high temperature and high pressure conditions to allow MOF crystals to grow in situ on the substrate surface to form a thin film. However, this method has significant drawbacks: the reaction system is a homogeneous liquid environment, making it difficult to control the nucleation and growth direction of MOF crystals, easily leading to the formation of irregularly oriented polycrystalline films with numerous grain boundary defects; at the same time, the reaction is time-consuming, typically requiring tens of hours or even several days, resulting in low preparation efficiency; and the high temperature and high pressure conditions place high demands on equipment, increasing preparation costs and hindering large-scale production. Secondary growth is the most commonly used preparation method. It involves first loading MOF seeds onto the substrate surface, then performing secondary growth to form a continuous film. While this can improve the film's orientation to some extent, the method is cumbersome. The preparation and loading of seeds require precise control, making it difficult to operate. Furthermore, loose bonding between the seeds and the substrate, and between the seeds and the subsequently grown MOF film, can easily occur, resulting in poor mechanical stability and making it difficult to meet the stringent requirements of practical applications. The overall preparation cycle is also long, limiting production efficiency. This method involves pre-depositing an oriented seed layer on the substrate before epitaxial growth. Electrochemical deposition can be performed at room temperature and pressure, allowing for precise control of film thickness. It is suitable for preparing MOF films on conductive substrates. However, this method requires sophisticated equipment and may introduce impurities that affect film performance. It also struggles to achieve highly oriented crystal growth, limiting its applicability to specific types of MOF films and failing to meet the orientation requirements of various MOF systems. For example, CN 119701652 A discloses a highly stable MOF membrane and its preparation and gas-liquid separation application. First, a dense MIL-140s seed layer is deposited on a porous support; then, it is placed in a precursor liquid containing a metal source and organic ligands. After a certain period of thermodynamic growth, the gaps between the seed layers are gradually closed through the coordination reaction of the metal source and organic ligands, and finally a continuous and well-intercalated MIL-140s membrane layer is formed. Gas (liquid) separation tests show that the MIL-140s membrane has excellent gas-liquid separation performance. Changing the thermodynamic growth temperature can significantly regulate and optimize the microstructure and mesostructure of MIL-140s membranes (such as crystal orientation, membrane thickness, intergranular defects, etc.), thereby altering the sieving channel size and corresponding separation system of the MIL-140s membrane. However, this method has narrow applicability, mainly employing a solvothermal method, which is merely an optimization adjustment of the method. The reaction temperature is mostly high, and a MIL-140A seed layer needs to be prepared. It is only suitable for MIL-140s and similar systems, and its film-forming effect is not applicable to fluorine-containing metal anion systems.

[0004] It is evident that existing atomic layer deposition (ALD), chemical vapor deposition (CVD), or gel vapor deposition (GVD) methods typically rely on the reaction between a solid precursor layer (such as metal oxides) and ligand vapor. For MOF systems where solid oxide films cannot be easily formed at certain metal nodes (such as fluorine-containing metal anion systems), these methods are difficult to apply. Similarly, for other MOF film fabrication processes, they are complex and have limited applicability. Therefore, it is necessary to develop a fabrication method that requires no seed crystal assistance, has a simple process, and allows for precise control of crystal orientation. Summary of the Invention

[0005] To address the above problems, this invention proposes a method for directly growing highly oriented metal-organic framework films using a liquid-gas phase reaction, the specific steps of which are as follows:

[0006] The method involves using liquid-vapor deposition (LVD) to directly grow a continuous MOF film with (112) crystal orientation on a porous substrate. The core of this process lies in the interfacial reaction between a solvated metal ion liquid layer retained on the substrate surface and ligand molecules in the gas phase. By adjusting the ambient temperature and precursor concentration, the water evaporation and crystal nucleation growth processes can be precisely balanced, thereby avoiding random orientation and inducing the crystals to align along a specific direction.

[0007] Furthermore, the method includes interfacial liquid layer construction, gas-phase ligand reaction, and synergistic regulation of growth steps;

[0008] Furthermore, the interface liquid layer is constructed by immersing a porous substrate in an aqueous solution of a precursor containing a metal source, and after removal, retaining a liquid interface layer containing saturated metal ions on the substrate surface and in the pores.

[0009] Furthermore, the gas-phase ligand reaction involves placing a substrate with a liquid interface layer in a reactor containing organic ligand vapor and structure-directing agent vapor;

[0010] Furthermore, the synergistic growth is achieved by controlling the reaction temperature to balance the evaporation rate of liquid water and the diffusion consumption rate of ligands, thereby directly inducing MOF crystals to grow along a specific crystal orientation at the liquid-gas interface, forming a continuous and dense oriented thin film.

[0011] Furthermore, when the prepared MOF membrane is a KAUST-7 membrane, i.e. NbOFFIVE-1-Ni, the metal source is selected from salts containing fluorinated metal anions, NiNbOF5; the organic ligand is pyrazine; the structure directing agent is a polar alcohol; the polar alcohol is methanol; and the porous substrate is an α-Al2O3 ceramic substrate.

[0012] Furthermore, when the MOF membrane is a KAUST-7 membrane, in the construction of the interfacial liquid layer, the concentration of the precursor aqueous solution is 0.01 g / mL to 0.03 g / mL; the mass ratio of the organic ligand to the structure directing agent is 1:1.5-1.7; and the reaction temperature in step (3) is 60℃ to 80℃.

[0013] The mixed vapor of the gas-phase ligand reaction is generated by the volatilization of a liquid pyrazine-methanol mixture under heat in a closed environment.

[0014] Furthermore, the structure-directing agent generates a solvent-sealing effect through strong interaction with a specific crystal plane, thereby kinetically inhibiting the vertical growth of that crystal plane; when preparing the KAUST-7 film, the methanol vapor preferentially adsorbs onto the (112) crystal plane, inducing the formation of a highly (112) oriented ultrathin film, and the thickness of the prepared MOF film is controlled between 200 and 500 nm.

[0015] Furthermore, by changing the metal source in the above method to prepare oriented Zn2(bim)4 films or (Co / Zn)2(bim)4 films, films with high (002) orientation characteristics can be obtained;

[0016] The metal source is zinc nitrate; the organic ligand is pyrazine; the structure directing agent is ethanol; the mass ratio of organic ligand to structure directing agent is 1:1.25-1.5; the reaction temperature is 165-180℃; and the reaction time is 10-16 hours.

[0017] (Co / Zn)₂(bim)₄ films were prepared using zinc nitrate and cobalt nitrate as the orientation metal sources; pyrazine was used as the organic ligand; and ethanol or methanol was used as the structure directing agent. The mass ratio of the organic ligand to the structure directing agent in highly (002) oriented films was 1:1.25-1.5, thus obtaining highly (002) oriented films. The reaction temperature was 165-180℃, and the reaction time was 10-16 hours.

[0018] Technical Principles

[0019] The preparation method involved in this invention is based on a novel liquid-gas deposition (LVD) strategy. Its core principle lies in utilizing the direct chemical reaction between solvated metal ions on the substrate surface and organic ligands in the gas phase at the liquid-gas interface. Unlike traditional solid-gas reactions (such as ALD or CVD), the LVD process directly utilizes metal ions dissolved in the aqueous phase. The ultrathin water layer on the substrate surface not only acts as a carrier for metal ions but also ensures a continuous supply and uniform distribution of metal ions at the reaction interface through the fluidity of the liquid phase, thereby supporting a continuous nucleation process. The film formation process depends on a precise balance between the reaction temperature and the rate of water evaporation and the kinetics of ligand vapor consumption. By controlling the reaction temperature, a critical interfacial water layer can be maintained, preventing the precipitation of metal ions into random particles due to excessively rapid water evaporation (e.g., excessively high temperature) or discontinuous film formation due to insufficient reaction kinetics (e.g., excessively low temperature). Methanol is introduced into the reaction system as a structure-directing agent. Methanol molecules interact more strongly with the (112) crystal plane of KAUST-7 crystals, stabilizing the surface through the resulting "solvent capping effect" and kinetically inhibiting crystal growth along the direction perpendicular to the plane, thereby inducing a highly (112) preferred orientation of the crystal on the substrate surface. Other orientation films are obtained by adjusting the metal source and structure directing agent to obtain the corresponding orientation films.

[0020] Technical effect

[0021] Compared with the prior art, the preparation method provided by this invention has the following significant technical advantages:

[0022] Significantly simplified process (no seed crystal required): It enables in-situ growth of highly oriented MOF films on porous substrates, completely eliminating the need for pre-depositing an oriented seed crystal layer. This overcomes the stringent requirements of traditional "secondary growth methods" on the morphology, uniformity, and epitaxial growth conditions of the seed layer, significantly shortening the preparation cycle and reducing process complexity.

[0023] Ultrathin and continuous dense membrane: By synergistically adjusting the concentration of metal precursors and the reaction temperature, an ultrathin continuous membrane with a thickness of only about 200 nm can be obtained. This ultrathin structure, while ensuring density, significantly reduces the transport resistance of gas molecules and significantly improves the permeation flux. Highly consistent orientation: The prepared KAUST-7 membrane exhibits a highly uniform (112) crystal orientation. XRD tests show that it has almost no other impurity peaks besides the (112) peak. This high degree of orientation consistency ensures the high reproducibility and stability of membrane separation performance. Wide precursor universality: Due to the use of a liquid-phase metal supply mode, this method can prepare MOF systems in which metal nodes are difficult to exist in the form of solid oxide layers. This expands the material selection range of oriented MOF membranes. Excellent separation performance and stability: Experimental verification shows that the obtained (112) oriented membrane exhibits a separation factor of up to 52.7 in H2 / CO2 separation, with H2 flux remaining stable above 285 GPUs, and has excellent long-term operating stability (no significant performance degradation after 100 hours of continuous operation), meeting the requirements of industrial applications for high selectivity and durability. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the crystal structure of KAUST-7;

[0025] Figure 2 A schematic diagram of the preparation of oriented KAUST-7 films by LVD method;

[0026] Figure 3 (a, b) electron micrographs and (c) XRD images of the (112) oriented KAUST-7 film;

[0027] Figure 4 (a) Mixed gas separation experiment, (b) Long-term performance stability test and (c) Temperature-dependent performance test of (112) oriented KAUST-7 membrane;

[0028] Figure 5 Electron microscopy (EM) images and XRD patterns of Zn2(bim)4 films (a, b, c) and (CoZn)2(bim)4 films (d, e, f) prepared by LVD method. Detailed Implementation

[0029] Example 1

[0030] Precursor preparation: Prepare a 0.01 g / mL NiNbOF5 aqueous solution. Substrate treatment: Immerse the α-Al₂O₃ ceramic substrate in the above solution for 1 hour to ensure adsorption saturation on both the interior and surface. Gas-phase reaction: Transfer the treated substrate to a reaction vessel containing a mixture of 2 g pyrazine and 4 mL methanol. Film growth: Place the reaction vessel at 60°C and maintain static conditions for 24 hours. Post-treatment: After the reaction, wash the surface with deionized water and dry.

[0031] Example 2

[0032] As in Example 1, the NiNbOF5 aqueous solution was 0.01 g / mL. When the reaction was carried out at the NiNbOF5 aqueous solution of 0.01 g / mL, the liquid layer evaporated too quickly, resulting in the inability to form a film. However, at 80 °C, the evaporation rate of water was reduced to a certain extent, and discontinuous plate-like crystals were formed.

[0033] Example 3

[0034] The aqueous solution of NiNbOF5 is 0.03 g / mL. When reacted at 60℃, it will cause local burst nucleation, forming randomly arranged crystals that lose their orientation.

[0035] Example 4

[0036] The substrate was immersed in an ethanol solution of zinc nitrate (3 g / 5 mL of a mixture of pyrazine and ethanol) for 1 hour. It was then placed in a reactor containing benzimidazole vapor and reacted at 170 °C for 12 hours. SEM showed the formation of a dense stack of two-dimensional nanosheets, and XRD confirmed its (002) preferred orientation.

[0037] Example 5

[0038] Using the same steps as in Example 4, the metal source was replaced with an equal mass ratio of zinc nitrate and cobalt nitrate, while other conditions remained unchanged. The resulting mixed metal MOF film also exhibited high (002) orientation characteristics.

[0039] according to Figure 3 It can be seen that the KAUST-7 film covers the substrate and is continuous and uninterrupted, with a strong diffraction peak only at 16.9°, which confirms that the film has a high (112) orientation characteristic. Scanning electron microscopy (SEM) shows that the film is continuous and has a thickness of about 200 nm. Figure 4 The KAUST-7 membrane separation performance experiment showed the separation performance of H2 / CO2, H2 / CO2, and H2 / CO2. The H2 flux was consistently maintained above 285 GPUs, and the separation factor of H2 / CO2 reached 52.7. The performance was stable over a long period of time with little decay, demonstrating good separation performance.

[0040] Similarly, using the same method, Zn2(bim)4 films and (CoZn)2(bim)4 films prepared by changing the metal source also showed good preparation results, such as... Figure 5 a, b, and c show that the Zn2(bim)4 film has good stable formation, uniformity, and high (002) orientation characteristics; the corresponding d, e, and f also have high (002) orientation characteristics, indicating that this method is suitable for the fabrication of MOF films of other metals, without the need for seed crystal preparation, with a simple preparation process and good industrial application value.

Claims

1. A method for directly growing highly oriented metal-organic framework films using liquid-gas phase reaction, characterized in that, The method involves using a liquid-gas deposition process to directly grow a continuous MOF film with oriented crystal planes, such as (112) oriented crystal planes and (002) oriented crystal planes, on a porous substrate. The core of the process is to achieve an interfacial reaction between the solvated metal ion liquid layer retained on the substrate surface and the ligand molecules in the gas phase. By adjusting the ambient temperature and precursor concentration, the process of water evaporation and crystal nucleation growth can be precisely balanced, thereby avoiding random orientation and inducing the crystals to align in a specific direction.

2. The method for directly growing highly oriented metal-organic framework films using liquid-gas phase reaction as described in claim 1, characterized in that, The method includes interfacial liquid layer construction, gas-phase ligand reaction, and synergistic regulation of growth steps; The interface liquid layer is constructed by immersing a porous substrate in an aqueous solution of a precursor containing a metal source, and after removal, retaining a liquid interface layer containing saturated metal ions on the substrate surface and in the pores. The gas-phase ligand reaction involves placing a substrate with a liquid interface layer in a reactor containing organic ligand vapor and structure-directing agent vapor. The synergistic growth is achieved by controlling the reaction temperature to balance the evaporation rate of liquid water and the diffusion consumption rate of ligands, thereby directly inducing MOF crystals to grow along a specific crystal orientation at the liquid-gas interface, forming a continuous and dense oriented thin film.

3. The method for directly growing highly oriented metal-organic framework films using liquid-gas phase reaction as described in claim 1, characterized in that, The mixed vapor of the gas-phase ligand reaction is generated by the heating and volatilization of a liquid pyrazine-methanol mixture in a closed environment; the structure directing agent generates a solvent-sealing effect through strong interaction with a specific crystal facet, thereby kinetically inhibiting the vertical growth of that crystal facet.

4. A method for directly growing highly oriented metal-organic framework films using liquid-gas phase reaction as described in any one of claims 1-2, characterized in that, When the prepared MOF membrane is a KAUST-7 membrane, also known as NbOFFIVE-1-Ni, the metal source is selected from salts containing fluorinated metal anions, NiNbOF5; the organic ligand is pyrazine; the structure directing agent is a polar alcohol; the polar alcohol is methanol; and the porous substrate is an α-Al2O3 ceramic substrate.

5. The method for directly growing highly oriented metal-organic framework films using liquid-gas phase reaction as described in claim 3, characterized in that, When the MOF membrane is a KAUST-7 membrane, in the construction of the interfacial liquid layer, the concentration of the precursor aqueous solution is 0.01 g / mL to -0.03 g / mL; the mass ratio of organic ligand to structure directing agent is 1:1.5-1.

7.

6. The method for directly growing highly oriented metal-organic framework films using liquid-gas phase reaction as described in claim 3, characterized in that, The reaction temperature of the method is 60°C to 80°C, and the mixture is kept statically at 60°C for 24 hours.

7. The method for directly growing highly oriented metal-organic framework films using liquid-gas phase reaction as described in claim 3, characterized in that, In the preparation of KAUST-7 film, the methanol vapor is preferentially adsorbed on the (112) crystal plane, inducing the formation of a highly (112) oriented ultrathin film, and the thickness of the prepared MOF film is controlled between 200 and 500 nm.

8. The method for directly growing highly oriented metal-organic framework films using liquid-gas phase reaction as described in claim 3, characterized in that, The above method can prepare oriented Zn2(bim)4 films or (Co / Zn)2(bim)4 films by changing the metal source and structure guiding agent, and can obtain highly (002) films.

9. The method for directly growing highly oriented metal-organic framework films using liquid-gas phase reaction as described in claim 7, characterized in that, When oriented Zn2(bim)4 film, the metal source is zinc nitrate; the organic ligand is pyrazine; the structure directing agent is ethanol; the mass ratio of organic ligand to structure directing agent is 1:1.25-1.5; the reaction temperature is 165-180℃; and the reaction time is 10-16 hours.

10. The method for directly growing highly oriented metal-organic framework films using liquid-gas phase reaction as described in claim 7, characterized in that, A (Co / Zn)₂(bim)₄ membrane was prepared using zinc nitrate as the orientation metal source; pyrazine as the organic ligand; and ethanol or methanol as the structure directing agent. The mass ratio of the organic ligand to the structure directing agent in a highly (002) oriented membrane was 1:1.25-1.5, thus obtaining a highly (002) oriented membrane. The reaction temperature was 165-180℃, and the reaction time was 10-16 hours.

Images