A solid-state metal organic framework hydrogen storage material and a preparation method thereof

Abstract

The application provides a solid-state metal organic framework hydrogen storage material and a preparation method thereof. The method comprises the following steps: preparing an aqueous solution of a pore structure regulator; preparing a mixed aqueous solution of metal ions containing a main metal ion source and a doped metal ion source; preparing a ligand mixed solution with a pH of 8-11 containing a first organic ligand, a second organic ligand and an alkali regulator; mixing the mixed aqueous solution of metal ions and the aqueous solution of the pore structure regulator to obtain a mixture, and adding the ligand mixed solution dropwise into the mixture to obtain a suspension containing a precipitate; performing solid-liquid separation on the suspension, collecting the solid precipitate and performing washing and drying on the solid precipitate to obtain a metal organic framework precursor; and performing gradient temperature activation treatment on the metal organic framework precursor under a protective atmosphere. The hydrogen storage material has improved hydrogen storage performance, excellent cycle stability, a simple process, controllable cost and is suitable for various hydrogen storage scenes.

Description

Technical Field

[0001] This invention belongs to the field of inorganic functional materials and hydrogen energy storage technology, specifically relating to a solid metal-organic framework hydrogen storage material and its preparation method. Background Technology

[0002] Guided by the "dual carbon" goal, the energy crisis and environmental pollution caused by traditional fossil fuels are becoming increasingly severe, making the development and utilization of clean energy a crucial direction for global energy transformation. Hydrogen energy, with its energy density as high as 142 MJ / kg and combustion product consisting solely of water, is a zero-carbon clean energy source. Its wide availability and high renewability have made it widely recognized as an ideal alternative to fossil fuels, offering irreplaceable application prospects in areas such as vehicle power, distributed energy, and fuel cells. However, the large-scale commercial application of hydrogen energy is limited by efficient, safe, and low-cost hydrogen storage technology. This core bottleneck has become an obstacle preventing the hydrogen energy industry from moving from the laboratory to practical application.

[0003] Currently, developed hydrogen storage technologies are mainly divided into two major technical paths: physical hydrogen storage and chemical hydrogen storage. However, both have insurmountable technical defects. Among physical hydrogen storage technologies, high-pressure gaseous hydrogen storage has become the most widely used hydrogen storage method due to its simple equipment structure and mature operation process. However, this technology has inherent shortcomings: the hydrogen storage density is low (the gravimetric hydrogen storage density is only about 1.0-1.5 wt% under conventional 35 MPa pressure), and hydrogen is prone to leakage under high pressure, posing an explosion risk. At the same time, the compression process has high energy consumption, which does not meet energy-saving requirements. Although cryogenic liquid hydrogen storage can achieve a higher volumetric hydrogen storage density, it requires maintaining an extremely low temperature environment of -253℃. The cost of refrigeration systems and insulation equipment is high, and the hydrogen evaporation loss rate can reach 1-3% per day, resulting in poor long-term storage economics and making it difficult to apply to large-scale civilian scenarios.

[0004] Chemical hydrogen storage technologies include metal hydride hydrogen storage, coordination hydride hydrogen storage, and organic liquid hydrogen storage. Their advantage lies in their high hydrogen storage capacity, but they generally face the problem of harsh hydrogen absorption and desorption conditions: most metal hydrides require temperatures above 300°C to achieve effective hydrogen release, the reaction kinetics are slow, and the reversibility is poor, with significant capacity decay after several hydrogen absorption and desorption cycles; organic liquid hydrogen storage has the characteristics of safe storage and convenient transportation, but the dehydrogenation process requires expensive catalysts, the reaction conversion rate is low, and the product separation is difficult, resulting in high overall usage costs and limiting its application in mobile energy scenarios such as vehicles.

[0005] Metal-organic frameworks (MOFs), a novel class of porous crystalline materials assembled from inorganic metal ions / clusters and organic ligands through coordination bonds, have demonstrated immense potential in gas adsorption and separation due to their unique advantages, including large specific surface area, tunable pore structure, rich topological structures, and controllable surface chemistry. They are considered one of the most promising physical hydrogen storage materials. Compared to traditional porous materials (such as activated carbon and zeolite molecular sieves), MOFs allow for precise control of the metal types in inorganic nodes and the molecular structure of organic ligands, enabling directional design of pore size, pore shape, and surface forces. This targeted optimization of hydrogen adsorption performance provides a new technological path to overcome the bottlenecks in hydrogen storage technology.

[0006] In recent years, the scientific community has developed a variety of MOF materials with hydrogen storage potential, such as NU-1500-Al, MOF-399, MOF-210, DUT-32, and MOF-808. Research data shows that these materials exhibit good hydrogen storage performance under extremely low temperature conditions (77K, liquid nitrogen temperature). However, the hydrogen storage performance of existing MOF hydrogen storage materials degrades significantly under room temperature (25-35℃) and intermediate temperature (35-150℃) conditions, and the gravimetric hydrogen storage density at room temperature is usually less than 2.0% by weight, which cannot meet the basic requirements for hydrogen storage density in practical applications such as on-board hydrogen energy storage.

[0007] A thorough analysis reveals three main reasons for this: First, the interaction between hydrogen molecules and the surface of MOF materials is primarily driven by van der Waals forces, resulting in a low adsorption enthalpy (typically 4-5 kJ / mol). This makes hydrogen molecules highly susceptible to desorption at room and intermediate temperatures, hindering stable adsorption. Second, the pore structure design of existing MOF materials is largely optimized for low-temperature adsorption scenarios, leading to insufficient matching between pore size and hydrogen molecule dynamics diameter. This results in a lack of effective adsorption sites at room temperature, preventing the formation of strong adsorption. Third, the crystal structure stability of some MOF materials is insufficient, making them prone to crystal collapse and pore blockage during repeated hydrogen adsorption / desorption cycles. This results in short cycle life, failing to meet the long-term usage requirements of practical applications. Furthermore, existing MOF hydrogen storage material preparation processes suffer from numerous shortcomings, such as the need for high-temperature and high-pressure reactions, reaction cycles lasting tens of hours, complex component ratio control, and poor batch-to-batch product stability. These factors contribute to the difficulty of large-scale preparation and high production costs, further hindering industrialization.

[0008] To address these technical challenges, researchers have attempted to optimize the hydrogen storage performance of MOF materials through various modification methods. For example, they have introduced nitrogen-containing heterocyclic ligands to enhance the polar interaction between the material surface and hydrogen molecules, or doped with transition metal ions to construct additional adsorption sites, thereby increasing the adsorption enthalpy. Other studies have employed core-shell structure designs to combine MOFs with magnesium-based alloys, leveraging the synergistic effect of physical and chemical adsorption to improve hydrogen storage capacity. However, these improvement schemes still have drawbacks: ligand functionalization may lead to a decrease in the specific surface area of ​​the MOF material, thus reducing the overall hydrogen storage capacity; metal ion doping easily introduces crystal structure defects, affecting the material's cycling stability; and composite modification significantly increases the complexity of the preparation process, raises production costs, and makes it difficult to achieve efficient and reversible hydrogen storage at room temperature.

[0009] In summary, existing hydrogen storage technologies all suffer from insurmountable technical bottlenecks. While MOF materials, as novel hydrogen storage materials, possess potential advantages, they still cannot meet practical application requirements in terms of room temperature / intermediate temperature hydrogen storage capacity, cycle stability, and preparation processes. Therefore, developing a solid-state metal-organic framework hydrogen storage material with a simple preparation process, controllable cost, and excellent room temperature / intermediate temperature hydrogen storage capacity and good cycle stability has become an urgent technical problem to be solved in the field of hydrogen energy storage technology. Summary of the Invention

[0010] Based on the technical problems described above, this invention aims to overcome the technical shortcomings of existing metal-organic framework hydrogen storage materials, such as low hydrogen storage capacity at room temperature / intermediate temperature and poor cycle stability. It also addresses the challenges of stringent reaction conditions, complex component control, and difficulties in large-scale production associated with existing preparation methods. By controlling the proportions of material components and the preparation process, this invention provides a solid-state metal-organic framework hydrogen storage material that achieves a synergistic improvement in high hydrogen storage capacity and excellent cycle stability at room temperature / intermediate temperature. Furthermore, its preparation process is mild and controllable, cost-effective, and suitable for large-scale production to meet the application needs of practical hydrogen storage scenarios such as vehicle-mounted and distributed energy storage.

[0011] Specifically, according to one aspect of the present invention, a method for preparing a solid metal-organic framework hydrogen storage material is provided, comprising the following steps:

[0012] (1) Prepare an aqueous solution of a pore structure regulator, wherein the pore structure regulator is selected from one or more of acetic acid, formic acid and propionic acid;

[0013] (2) Prepare an aqueous solution of mixed metal ions containing a main metal ion source and a doped metal ion source, wherein the main metal ion source is a mixture of zinc nitrate and nickel nitrate in a weight ratio of 1.5:1 to 5:1, and the doped metal ion source is cobalt nitrate, iron nitrate or a mixture thereof;

[0014] (3) Prepare a ligand mixture with a pH of 8-11 containing a first organic ligand, a second organic ligand and a base adjuster, wherein the first organic ligand is selected from one or more of pyromellitic acid, 1,4-terephthalic acid and 4,4'-biphenyldicarboxylic acid, the second organic ligand is selected from one or more of triethylenediamine, 4,4'-bipyridine and ethylenediamine, and the base adjuster is selected from one or more of sodium hydroxide, potassium hydroxide and lithium hydroxide;

[0015] (4) The aqueous solution of the metal ion mixture is mixed with the aqueous solution of the pore structure regulator to obtain a mixture, and the ligand mixture is added dropwise to the mixture to carry out a coordination reaction to obtain a suspension containing precipitate;

[0016] (5) The suspension is subjected to solid-liquid separation, the solid precipitate is collected and washed and dried to obtain the metal-organic framework precursor;

[0017] (6) Under a protective atmosphere, the metal-organic framework precursor is subjected to gradient temperature activation treatment to obtain the solid metal-organic framework hydrogen storage material, wherein:

[0018] Based on the total weight of the main metal ion source, doped metal ion source, first organic ligand, second organic ligand and pore structure modifier as 100%, the main metal ion source accounts for 30-80%, the doped metal ion source accounts for 1.5-17%, the first organic ligand accounts for 8-40%, the second organic ligand accounts for 4.8-25%, and the pore structure modifier accounts for 0.8-10%.

[0019] According to certain preferred embodiments of the present invention, the doped metal ion source is a mixture of cobalt nitrate and iron nitrate in a weight ratio of 2:1 to 5:1.

[0020] According to certain preferred embodiments of the present invention, the main metal ion source accounts for 85-93% and the doped metal ion source accounts for 7-12% based on the total weight of the main metal ion source and the doped metal ion source being 100%.

[0021] According to certain preferred embodiments of the present invention, the weight ratio of the first organic ligand and the second organic ligand is 1.5:1 to 1.8:1.

[0022] According to certain preferred embodiments of the present invention, the solvent used to prepare the aqueous solution of the pore structure regulator, the aqueous solution of the metal ion mixture, and the ligand mixture is a mixed solvent of deionized water and an alcohol solvent in a weight ratio of 3:1 to 10:1, wherein the alcohol solvent is selected from one or more of ethanol, methanol, and isopropanol.

[0023] According to certain preferred embodiments of the present invention, the first organic ligand is a mixture of pyromellitic acid and 1,4-terephthalic acid in a weight ratio of 1:2 to 2:1.

[0024] According to certain preferred embodiments of the present invention, the second organic ligand is a mixture of triethylenediamine and ethylenediamine in a weight ratio of 1:1 to 3:1.

[0025] According to certain preferred embodiments of the present invention, in step (1), deionized water and ethanol in a weight ratio of 3:1-10:1 are mixed, heated to 25-40°C, and the pore structure regulator is added under a stirring rate of 200-300 r / min to prepare an aqueous solution of the pore structure regulator.

[0026] According to certain preferred embodiments of the present invention, in step (2), the main metal ion source and the doped metal ion source are added to a mixed solvent of deionized water and ethanol at a weight ratio of 3:1 to 10:1 at a stirring rate of 200-300 r / min, and then ultrasonically treated for 15-30 min at a power of 300-500W and a frequency of 40kHz.

[0027] According to certain preferred embodiments of the present invention, in step (3), under the condition of stirring rate of 300-400 r / min, a first organic ligand and a second organic ligand are sequentially added to a mixed solvent of deionized water and ethanol at a weight ratio of 3:1-10:1 at 30-45°C, followed by ultrasonic treatment and the addition of an alkali adjuster to adjust the pH to 8-11.

[0028] According to certain preferred embodiments of the present invention, in step (4), the aqueous solution of the metal ion mixture is mixed with the aqueous solution of the pore structure regulator to obtain a mixture, and then the mixture is heated to 35-50°C, and the ligand mixture is added dropwise to the mixture at a dropping rate of 1-3 mL / min. After the addition is completed, the mixture is stirred at a stirring rate of 300-400 r / min for 2-4 hours.

[0029] According to certain preferred embodiments of the present invention, in step (4), after adding and stirring, the temperature is further raised to 55-70°C and allowed to stand at a constant temperature for 8-16 hours.

[0030] According to certain preferred embodiments of the present invention, in step (5), the solid-liquid separation is centrifugal separation, with a centrifugal speed of 8000-10000 r / min and a time of 10-15 min.

[0031] According to certain preferred embodiments of the present invention, in step (5), the washing is to wash the solid precipitate 3-5 times with deionized water and ethanol respectively.

[0032] According to certain preferred embodiments of the present invention, in step (5), the drying is vacuum drying, the drying temperature is 60-80℃, the vacuum degree is 0.08-0.09 MPa, and the drying time is 8-12 hours.

[0033] According to certain preferred embodiments of the present invention, in step (6), the protective atmosphere is a nitrogen atmosphere.

[0034] According to certain preferred embodiments of the present invention, the gradient temperature activation process includes:

[0035] In the first stage, the temperature is increased to 120-150℃ at a rate of 2-5℃ / min and held at a constant temperature for 2-3 hours.

[0036] In the second stage, the temperature is increased to 180-220℃ at a rate of 1-3℃ / min and held at a constant temperature for 3-5 hours.

[0037] According to another aspect of the present invention, a solid metal-organic framework hydrogen storage material is provided, which is prepared according to the method described above.

[0038] According to certain preferred embodiments of the present invention, the specific surface area of ​​the solid metal-organic framework hydrogen storage material is 2200-2500 m². 2 / g, with an average pore size of 0.9-2.2 nm.

[0039] According to certain preferred embodiments of the present invention, the solid metal-organic framework hydrogen storage material has a hydrogen storage density of 3.0-3.7 wt% at 25°C and 30 bar.

[0040] According to certain preferred embodiments of the present invention, the solid metal-organic framework hydrogen storage material has a hydrogen storage density of 2.0-2.5 wt% at 80°C and 30 bar.

[0041] According to certain preferred embodiments of the present invention, the solid metal-organic framework hydrogen storage material has a hydrogen storage density of 1.1-1.5 wt% at 120°C and 30 bar.

[0042] According to certain preferred embodiments of the present invention, the solid metal-organic framework hydrogen storage material retains a hydrogen storage capacity of greater than or equal to 88% after 2000 hydrogen adsorption / desorption cycles at 25°C and 30 bar.

[0043] Compared with existing technologies, the beneficial effects of this invention are as follows: First, the hydrogen storage performance at room temperature and medium temperature is significantly improved. Through dual-ligand synergy and metal ion doping, the interaction with hydrogen is enhanced, and the hydrogen storage density far exceeds that of existing MOF materials, meeting the needs of practical applications. Second, the hydrogen adsorption and desorption conditions are mild and the kinetic performance is excellent. The adsorption enthalpy is in the optimal range of physical adsorption, and hydrogen can be rapidly adsorbed and desorbed over a wide temperature and pressure range. The adsorption equilibrium is fast, the desorption rate is high, and the energy consumption is low. Third, the cycle stability is good. The pore structure regulator and gradient activation process reduce crystal defects. After multiple hydrogen adsorption and desorption cycles, the hydrogen storage capacity decays little, and the structure remains intact. Fourth, the preparation process is simple and controllable. It can be completed at room temperature and pressure. The reaction time is short, the raw materials are readily available, the product purity is high, the batches are stable, the space yield is high when the production scale is large, and the cost is controllable. Fifth, the structural design is highly flexible. By adjusting the proportion of each component, specific surface area, and other parameters, it can be adapted to different hydrogen storage scenarios such as vehicle-mounted and distributed energy storage. Attached Figure Description

[0044] The accompanying drawings are provided in this specification to more clearly explain the technical solutions of the present invention; however, the art is not limited thereto.

[0045] Figure 1 A flowchart illustrating the preparation process of a solid metal-organic framework hydrogen storage material according to the present invention is shown.

[0046] Figure 2 A scanning electron microscope (SEM) image of the solid metal-organic framework hydrogen storage material 1 prepared in Example 1 is shown;

[0047] Figure 3 The X-ray diffraction (XRD) patterns of the solid metal-organic framework hydrogen storage materials 1-4 prepared in Examples 1-4 are shown. Detailed Implementation

[0048] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. It will be understood that other embodiments may be implemented without departing from the scope or spirit of the invention. Therefore, the following detailed description is non-limiting.

[0049] Unless otherwise specified, all figures used in this specification to represent characteristic dimensions, quantities, and physical properties should be understood to be modified by the term "about" in all cases. Therefore, unless otherwise stated, the numerical parameters listed in the foregoing specification are approximations, and those skilled in the art can appropriately modify these approximations to obtain the desired properties using the teachings disclosed herein.

[0050] As mentioned above, the main technical problems faced in the field of hydrogen storage include: existing metal-organic framework (MOF) hydrogen storage materials have low hydrogen storage capacity at room temperature and intermediate temperature, hydrogen absorption and desorption depend on harsh conditions, have poor kinetic performance, and are prone to crystal structure collapse and pore blockage during repeated cycling, resulting in poor cycling stability. Meanwhile, the preparation of existing MOF hydrogen storage materials suffers from drawbacks such as harsh reaction conditions, complex component control, poor batch-to-batch product stability, and difficulty in large-scale production. Furthermore, existing modification and optimization schemes for hydrogen storage technologies often lead to a decrease in material specific surface area, the introduction of structural defects, and increased production costs, failing to achieve efficient and reversible hydrogen storage at room temperature and thus failing to meet the practical application requirements such as vehicle-mounted applications. This invention aims to solve one or more of the above technical problems.

[0051] Specifically, according to one aspect of the present invention, a method for preparing a solid metal-organic framework hydrogen storage material is provided, comprising the following steps:

[0052] (1) Prepare an aqueous solution of a pore structure regulator, wherein the pore structure regulator is selected from one or more of acetic acid, formic acid and propionic acid;

[0053] (2) Prepare an aqueous solution of mixed metal ions containing a main metal ion source and a doped metal ion source, wherein the main metal ion source is a mixture of zinc nitrate and nickel nitrate in a weight ratio of 1.5:1 to 5:1, and the doped metal ion source is cobalt nitrate, iron nitrate or a mixture thereof;

[0054] (3) Prepare a ligand mixture with a pH of 8-11 containing a first organic ligand, a second organic ligand and a base adjuster, wherein the first organic ligand is selected from one or more of pyromellitic acid, 1,4-terephthalic acid and 4,4'-biphenyldicarboxylic acid, the second organic ligand is selected from one or more of triethylenediamine, 4,4'-bipyridine and ethylenediamine, and the base adjuster is selected from one or more of sodium hydroxide, potassium hydroxide and lithium hydroxide;

[0055] (4) The aqueous solution of the metal ion mixture is mixed with the aqueous solution of the pore structure regulator to obtain a mixture, and the ligand mixture is added dropwise to the mixture to carry out a coordination reaction to obtain a suspension containing precipitate;

[0056] (5) The suspension is subjected to solid-liquid separation, the solid precipitate is collected and washed and dried to obtain the metal-organic framework precursor;

[0057] (6) Under a protective atmosphere, the metal-organic framework precursor is subjected to gradient temperature activation treatment to obtain the solid metal-organic framework hydrogen storage material, wherein:

[0058] Based on the total weight of the main metal ion source, doped metal ion source, first organic ligand, second organic ligand and pore structure modifier as 100%, the main metal ion source accounts for 30-80%, the doped metal ion source accounts for 1.5-17%, the first organic ligand accounts for 8-40%, the second organic ligand accounts for 4.8-25%, and the pore structure modifier accounts for 0.8-10%.

[0059] Figure 1 The flowchart illustrating the preparation process of a solid-state metal-organic framework hydrogen storage material according to the present invention is shown, which includes the following steps:

[0060] (1) Preparation of an aqueous solution of the pore structure modifier;

[0061] (2) Prepare a mixed aqueous solution of metal ions containing a main metal ion source and a doped metal ion source;

[0062] (3) Prepare a ligand mixture containing a first organic ligand, a second organic ligand, and an alkali adjuster with a pH of 8-11;

[0063] (4) Mix an aqueous solution of metal ions with an aqueous solution of a pore structure modifier to obtain a mixture, and add a ligand mixture dropwise to the mixture to obtain a suspension containing precipitate;

[0064] (5) The suspension is subjected to solid-liquid separation, the solid precipitate is collected, washed and dried to obtain the metal-organic framework precursor;

[0065] (6) Under a protective atmosphere, the metal-organic framework precursor is activated by gradient heating.

[0066] Specifically, the preparation method for solid-state metal-organic framework (MOF) hydrogen storage materials of the present invention aims to solve the technical problems of low room temperature / intermediate temperature hydrogen storage capacity, poor cycle stability, and stringent preparation processes and complex component control of existing MOF hydrogen storage materials. By screening the types of each reaction component, optimizing the component ratio, and improving the preparation process parameters, the pore structure, adsorption sites, and framework stability of the metal-organic framework hydrogen storage material are synergistically optimized. The following details the steps, component selection, and preferred implementation scheme of the preparation method of the present invention, combined with technical principles and experimental data.

[0067] In step (1) of the preparation method according to the present invention, the pore structure regulator is selected from one or more of acetic acid, formic acid and propionic acid, and an aqueous solution is prepared by a mixture of deionized water and alcohol solvent, preferably the pore structure regulator is dissolved at 25-40°C and 200-300 r / min stirring rate.

[0068] Acetic acid, formic acid, and propionic acid are all low-carbon chain carboxylic acids, belonging to weak organic acids. The carboxyl groups in their molecular structures can undergo weak coordination with metal ions. In subsequent coordination reactions, this weak coordination can moderately regulate the coordination reaction rate between metal ions and organic ligands, avoiding rapid crystal growth due to excessively fast coordination reactions, which would lead to products with uneven pore structures and numerous crystal defects. At the same time, low-carbon chain carboxylic acids have moderate thermal stability and can be completely removed during subsequent gradient temperature activation processes, leaving no impurities in the MOF framework. After removal, they can be directionally formed into a pore structure with uniform size and regular distribution, achieving control over the specific surface area and average pore size of the material.

[0069] According to certain technical solutions of the present invention, the solvent for preparing the pore structure regulator solution is preferably a mixture of deionized water and an alcohol solvent (one or more of ethanol, methanol, and isopropanol) in a weight ratio of 3:1 to 10:1. This mixed solvent system has both hydrophilic and lipophilic properties, which can ensure the full dissolution of low-carbon chain carboxylic acids and improve the compatibility with subsequent aqueous solutions of metal ions and ligand mixtures, avoiding inhomogeneity of the reaction system due to solvent phase separation. Among them, ethanol is the optimal choice of alcohol solvent due to its good solubility, moderate volatility, easy removal, and low cost.

[0070] Furthermore, in this invention, the amount of pore structure modifier added is 0.8-10% based on the total weight of the main metal ion source, doped metal ion source, first organic ligand, second organic ligand, and pore structure modifier, which is 100%. If the amount added is less than 0.8%, effective control of the pore structure cannot be achieved, and the specific surface area and pore size distribution of the material are difficult to meet the requirements for hydrogen storage performance. If the amount added is greater than 10%, the excessive carboxylic acid will compete with the organic ligand for the coordination sites of metal ions, resulting in insufficient MOF framework construction, decreased crystal structure stability, and even pore blockage.

[0071] According to step (2) of the preparation method of the present invention, a bimetallic ion system consisting of a main metal ion source and a doped metal ion source is constructed. The main metal ion source is a mixture of zinc nitrate and nickel nitrate in a weight ratio of 1.5:1 to 5:1, and the doped metal ion source is cobalt nitrate, iron nitrate, or a mixture thereof. A homogeneous aqueous solution of mixed metal ions is prepared by a combination of ultrasonic treatment and stirring.

[0072] The main metal ion source was a mixture of zinc nitrate and nickel nitrate, rather than a single metal ion source, because zinc and nickel ions have a significant synergistic effect: Zn 2+ Ni has an ionic radius of 0.74 Å, a stable ionic configuration, and can form strong coordination bonds with the carboxyl groups of organic ligands. It is the core ion for constructing the rigid framework of MOFs, ensuring the structural stability of the material; 2+As a transition metal ion, its d orbitals contain unpaired electrons, enabling it to form weak dipole-dipole interactions with hydrogen molecules. This increases the adsorption enthalpy between the material and hydrogen molecules, expanding the number of hydrogen adsorption sites and addressing the problem of excessively rapid desorption at room temperature caused by the low adsorption enthalpy resulting from van der Waals forces alone in existing MOF materials. Furthermore, nitrate ions are readily soluble anions that can be completely removed during subsequent washing, leaving no residue in the product and preventing impurities from affecting hydrogen storage performance.

[0073] According to the technical solution of the present invention, the weight ratio of zinc nitrate to nickel nitrate is 1.5:1-5:1. This ratio range represents the optimal balance between framework stability and hydrogen storage adsorption sites. If the ratio of zinc nitrate is lower than 1.5:1, i.e., the proportion of nickel ions is too high, the rigidity of the MOF framework will be insufficient, and the framework will easily collapse during repeated hydrogen adsorption and desorption cycles. If the ratio of zinc nitrate is higher than 5:1, i.e., the proportion of nickel ions is too low, it cannot provide sufficient hydrogen adsorption sites, and the hydrogen storage capacity of the material will decrease significantly.

[0074] According to the technical solution of the present invention, the doping metal ion source is selected from cobalt nitrate, ferric nitrate, or a mixture thereof, both of which are transition metal ions. 2+ Fe 3+ Cobalt and iron ions have more unpaired electrons, enabling them to form stronger dipole interactions with hydrogen molecules. This introduces additional hydrogen adsorption sites into the MOF framework, further enhancing the material's hydrogen storage capacity. Simultaneously, cobalt and iron ions have ionic radii similar to zinc and nickel ions, allowing them to partially replace the main metal ions in the framework without disrupting the MOF's crystal structure. The preferred doping metal ion source in this invention is a mixture of cobalt nitrate and iron nitrate in a weight ratio of 2:1 to 5:1. The synergistic doping effect of these two ions is more significant than that of single doping, forming more types of adsorption sites in the framework and further improving the hydrogen storage capacity.

[0075] In terms of doping ratio, this invention preferably uses a total weight of 100% for the main metal ion source and the doped metal ion source, with the main metal ion source accounting for 85-93% and the doped metal ion source accounting for 7-12%. Furthermore, according to the technical solution of this invention, the total amount of doped metal ion source added is 1.5-17% of the total weight of the main metal ion source, the doped metal ion source, the first organic ligand, the second organic ligand, and the pore structure modifier. This range matches the aforementioned ratio, balancing the requirements of improved hydrogen storage capacity and framework stability.

[0076] In the preparation of the mixed aqueous solution of metal ions, the metal salt is first fully dissolved at 25-40℃ and a stirring rate of 200-300 r / min, and then ultrasonically treated for 15-30 min at a power of 300-500W and a frequency of 40kHz. The limited stirring rate and temperature ensure the full dissolution of the metal salt and avoid local agglomeration; while ultrasonic treatment helps to achieve uniform dispersion of metal ions. The ultrasonic power of 300-500W can generate sufficient cavitation effect to break the local aggregation of metal ions, so that zinc, nickel, cobalt / iron ions form a homogeneous ionic system in the mixed solvent. This ensures the uniform coordination of each metal ion with the organic ligand in the subsequent coordination reaction and avoids the non-uniform pore structure caused by ion agglomeration.

[0077] According to step (3) of the preparation method of the present invention, a dual-ligand system of a first organic ligand + a second organic ligand was constructed, and the pH of the ligand mixture was limited to 8-11. pH was controlled by an alkaline regulator, and the preparation process parameters of the ligand mixture were optimized, laying the foundation for the full conduct of subsequent coordination reactions. This design solves the problem that existing MOF materials using only a single ligand cannot simultaneously achieve both framework stability and adsorption performance. Through the synergistic effect of the dual ligands, both the construction of the rigid MOF framework and the introduction of more hydrogen adsorption sites are ensured.

[0078] According to the technical solution of the present invention, the first organic ligand is selected from one or more of pyromellitic acid, 1,4-terephthalic acid, and 4,4'-biphenyldicarboxylic acid. These ligands are all aromatic polycarboxylic acid ligands, containing multiple carboxyl groups in their molecular structure, capable of forming multi-point coordination with metal ions, and are the core ligands for constructing the rigid framework of MOFs. The conjugated structure of the aromatic ring endows the ligand with good stability, ensuring that the MOF framework does not easily collapse during repeated hydrogen adsorption and desorption cycles. Furthermore, different ligands have different carboxyl group numbers and molecular sizes, allowing for the control of the pore structure size of the MOF through combination. Preferably, the first organic ligand of the present invention is a mixture of pyromellitic acid and 1,4-terephthalic acid in a weight ratio of 1:2 to 2:1. Pyromellitic acid contains four carboxyl groups, exhibiting strong coordination ability and capable of constructing a dense three-dimensional network framework. 1,4-terephthalic acid contains two carboxyl groups, has a moderate molecular size, and can form more porous structures in the framework. The synergistic combination of the two can achieve a balance between framework stability and pore structure richness, increasing the specific surface area of ​​the material.

[0079] According to the technical solution of the present invention, the second organic ligand is selected from one or more of triethylenediamine, 4,4'-bipyridine, and ethylenediamine. These ligands are nitrogen-containing heterocyclic or aliphatic amine ligands. The nitrogen atom in the molecular structure contains lone pairs of electrons, which can coordinate with metal ions, acting as auxiliary ligands to participate in the construction of the MOF framework and further optimize the pore structure distribution. Furthermore, the lone pairs of electrons in the nitrogen atom can form hydrogen bonds with hydrogen molecules, enhancing the interaction between the material and hydrogen molecules, increasing adsorption sites, and solving the problem of excessively low adsorption enthalpy of hydrogen molecules in existing MOF materials. Preferably, the second organic ligand of the present invention is a mixture of triethylenediamine and ethylenediamine in a weight ratio of 1:1 to 3:1. The heterocyclic structure of triethylenediamine has good stability and can enhance the rigidity of the framework, while the aliphatic chain structure of ethylenediamine is more flexible and can introduce more hydrogen bond adsorption sites. The synergistic effect of the two achieves a dual improvement in structural stability and adsorption performance.

[0080] Preferably, the weight ratio of the first organic ligand to the second organic ligand is 1.5:1 to 1.8:1, which represents the optimal balance between framework construction and adsorption performance. Furthermore, according to the technical solution of the present invention, the amount of the first organic ligand added is limited to 8-40% of the total weight of the main metal ion source, the doped metal ion source, the first organic ligand, the second organic ligand, and the pore structure modifier; the amount of the second organic ligand added is 4.8-25% of the total weight of the main metal ion source, the doped metal ion source, the first organic ligand, the second organic ligand, and the pore structure modifier. This range matches the weight ratio of the two ligands, ensuring an appropriate coordination ratio between the ligand and the metal ion, preventing insufficient framework construction due to insufficient ligand, and preventing residual ligand clogging the pores due to excessive ligand.

[0081] The pH of the ligand mixture is a crucial parameter affecting the completeness of the coordination reaction. This invention limits it to 8-11. The alkaline regulator is selected from one or more of sodium hydroxide, potassium hydroxide, and lithium hydroxide. The carboxyl group of the first organic ligand has a low ionization constant, resulting in incomplete dissociation under neutral or acidic conditions, hindering sufficient coordination with metal ions. However, under alkaline conditions of 8-11, the carboxyl group can completely dissociate into carboxylate ions, significantly enhancing its coordination ability with metal ions and enabling a complete coordination reaction to construct the MOF framework. Simultaneously, the weakly alkaline environment of 8-11 prevents the hydrolysis of metal ions to form hydroxide precipitates, avoiding impurity formation. If the pH is below 8, the carboxyl group dissociates incompletely, the coordination reaction is insufficient, and the MOF framework has numerous defects, significantly reducing the material's specific surface area and hydrogen storage capacity. If the pH is above 11, the solution is too alkaline, causing metal ions to hydrolyze, generating precipitates such as zinc hydroxide and nickel hydroxide, preventing the formation of the MOF framework with the organic ligand.

[0082] Sodium hydroxide, potassium hydroxide, and lithium hydroxide are all strong bases that can quickly regulate pH. Furthermore, their cations are alkali metal ions, which can be completely removed during subsequent washing processes and will not remain in the product. Therefore, they are the optimal alkali regulators suitable for the system of this invention.

[0083] The preparation of the ligand mixture was carried out at 30-45℃ and a stirring rate of 300-400 r / min. The first organic ligand and the second organic ligand were added sequentially, dispersed by stirring, and then sonicated. Finally, an alkali adjuster was added to adjust the pH. The temperature of 30-45℃ and the stirring rate of 300-400 r / min ensured sufficient dispersion of the organic ligands in the mixed solvent, preventing ligand aggregation. The sequential addition of the first and second organic ligands avoided phase separation due to differences in solubility, ensuring the homogeneity of the ligand system. Sonication further broke up local aggregation of the ligands, forming a homogeneous dispersion system in the mixed solvent, creating conditions for subsequent uniform coordination with metal ions.

[0084] In the preparation method of the present invention, step (4) coordination reaction is an important step in the formation of MOF crystals. The present invention achieves slow and uniform growth of MOF crystals by optimizing parameters such as reaction feeding method, dropping acceleration rate, reaction temperature and time, and obtains metal-organic framework precursors with regular pore structure and high crystallinity.

[0085] In this invention, a feeding method is adopted in which an aqueous solution of metal ions and an aqueous solution of a pore structure modifier are mixed first, and then the ligand mixture is added dropwise. This method ensures thorough mixing of the metal ions and the pore structure modifier. The structure modifier forms weak coordination with the metal ions in advance, laying the foundation for subsequent directional regulation of the pore structure. If the ligand mixture is mixed with the pore structure modifier first, the carboxyl groups of the ligands will compete with the carboxyl groups of the pore structure modifier, resulting in a decrease in the regulation effect of the pore structure modifier. The preferred dropping rate of the ligand mixture is 1-3 mL / min. This low-speed dropping method ensures that the ligand mixture enters the metal ion system slowly and uniformly, allowing the coordination reaction to proceed uniformly throughout the system, and the MOF crystals to grow slowly, forming a product with a regular pore structure and high crystallinity.

[0086] After the addition is complete, the mixture is stirred at 35-50℃ and 300-400 r / min for 2-4 hours. This stage is the initial coordination reaction stage. The mild reaction temperature and suitable stirring rate can ensure the full coordination of metal ions and organic ligands to form the crystal nucleus of MOF crystal. Preferably, the temperature is then raised to 55-70℃ and kept at a constant temperature for 8-16 hours. This stage is the crystal growth stage. The increase in temperature can improve the crystal growth rate and make the crystal nucleus grow into a complete MOF crystal. The static reaction can avoid the mechanical damage to the crystal caused by stirring and ensure the integrity and crystallinity of the crystal.

[0087] Step (5) of the present invention obtains a high-purity, low-impurity metal-organic framework precursor from the coordination reaction suspension through solid-liquid separation, washing, and drying operations.

[0088] Preferably, the present invention uses centrifugation as the solid-liquid separation method, with a centrifugation speed of 8000-10000 r / min and a time of 10-15 min. These centrifugation parameters can generate sufficient centrifugal force to cause the solid precipitate in the suspension to settle quickly and completely, achieving effective solid-liquid separation; at the same time, this speed will not cause mechanical damage to the MOF crystal, ensuring the integrity of the crystal structure.

[0089] Regarding washing, this invention employs a method of washing the solid precipitate 3-5 times sequentially with deionized water and ethanol. The main function of deionized water is to remove soluble salt impurities adsorbed on the precipitate surface, such as unreacted nitrate ions and cations from alkali regulators. Ethanol, as an organic solvent, possesses both hydrophilic and lipophilic properties, effectively removing both residual deionized water and unreacted organic ligands. Furthermore, ethanol's high volatility reduces time costs for subsequent vacuum drying. The preferred number of washing cycles is 3-5. Centrifugation is performed after each washing to ensure thorough separation of the washing solution and the precipitate, thereby improving the washing effect.

[0090] This invention employs a vacuum drying method, with a drying temperature of 60-80℃, a vacuum degree of 0.08-0.09 MPa, and a drying time of 8-12 hours. Vacuum drying avoids the oxidation of the MOF precursor by oxygen in the air during atmospheric pressure drying. Simultaneously, under negative pressure, the boiling point of the solvent decreases, enabling rapid solvent removal at lower temperatures and preventing the decomposition of the MOF precursor due to high-temperature drying.

[0091] Step (6) of this invention includes gradient temperature activation of the metal-organic framework precursor under a protective atmosphere. This is a crucial step in transforming the precursor into a solid metal-organic framework hydrogen storage material with a porous structure and high hydrogen storage performance. By optimizing the activation atmosphere, heating rate, activation temperature, and isothermal time, this invention achieves the gentle removal of solvents and weak coordination modifiers from the precursor, while simultaneously improving the crystallinity of the MOF crystals and constructing a stable porous hydrogen storage structure.

[0092] Nitrogen is preferably used as the protective atmosphere in this invention.

[0093] Preferably, in this invention, a two-stage gradient temperature activation treatment is adopted, specifically: in the first stage, the temperature is increased to 120-150°C at a rate of 2-5°C / min and held at a constant temperature for 2-3 hours; in the second stage, the temperature is increased to 180-220°C at a rate of 1-3°C / min and held at a constant temperature for 3-5 hours. The advantage of gradient heating lies in achieving stepwise removal of solvent and pore structure modifier, avoiding pore collapse caused by rapid solvent evaporation: The first stage, at 120-150℃, is a low activation temperature, mainly removing physically adsorbed solvent molecules (deionized water and ethanol) from the MOF precursor. The slow heating rate of 2-5℃ / min ensures that solvent molecules are removed slowly and uniformly from the pores, preventing damage to the pore structure due to internal pressure generated by rapid solvent evaporation; The second stage, at 180-220℃, is a high activation temperature, mainly removing pore structure modifier molecules weakly coordinated with metal ions, as well as a small amount of chemically adsorbed solvent molecules. In this stage, the heating rate is further reduced to 1-3℃ / min, ensuring gentle removal of the pore structure modifier. At the same time, the pore structure formed after removal can rearrange at this temperature, improving the regularity of the pore structure.

[0094] According to another aspect of the present invention, a solid metal-organic framework hydrogen storage material is provided.

[0095] The solid-state metal-organic framework hydrogen storage material prepared by the above method in this invention achieves dual optimization in both microstructure and hydrogen storage performance, with a specific surface area of ​​2200-2500 m². 2 / g, with an average pore size of 0.9-2.2 nm. This structural parameter is optimal for hydrogen molecule adsorption within the range of 2200-2500 nm. 2 The high specific surface area of ​​ / g provides a large number of adsorption sites for hydrogen molecules, while the average pore size of 0.9-2.2 nm is slightly larger than the kinetic diameter of hydrogen molecules (0.289 nm). This ensures that hydrogen molecules diffuse rapidly in the pores, enabling rapid hydrogen adsorption and desorption, and also allows hydrogen molecules to form multilayer adsorption in the pores, significantly improving the hydrogen storage capacity.

[0096] In terms of hydrogen storage performance, the hydrogen storage material of this invention has a hydrogen storage density of 3.0-3.7 wt% at 25°C and 30 bar, which is much higher than the hydrogen storage density of less than 2.0 wt% of existing MOF materials at room temperature, meeting the basic requirements for hydrogen storage density in practical application scenarios such as on-board hydrogen energy storage. Under intermediate temperature conditions, the material still maintains excellent hydrogen storage performance, with a hydrogen storage density of 2.0-2.5 wt% at 80°C and 30 bar, and 1.1-1.5 wt% at 120°C and 30 bar, solving the problem of severe degradation of hydrogen storage performance of existing MOF materials at intermediate temperatures.

[0097] In terms of cycle stability, the hydrogen storage material of this invention maintains a hydrogen storage capacity retention of greater than or equal to 88% after 2000 hydrogen adsorption / desorption cycles at 25°C and 30 bar, far exceeding the level of existing MOF materials that exhibit significant capacity decay after only a few cycles. This excellent cycle stability is attributed to the material's regular pore structure and stable crystal framework. The directional regulation of the pore structure modifier and the gradient temperature activation process reduce crystal defects, ensuring that the pore and framework structures remain intact during repeated hydrogen adsorption / desorption cycles, preventing pore blockage and framework collapse.

[0098] The solid metal-organic framework hydrogen storage material of the present invention also has the advantage of strong structural design flexibility. By adjusting the types and ratios of each component, the specific surface area and average pore size of the material can be changed in a targeted manner to meet the needs of different hydrogen storage scenarios such as vehicle-mounted and distributed energy storage. For example, to meet the need for high volumetric hydrogen storage capacity in vehicle-mounted hydrogen storage, the proportion of zinc ions can be appropriately increased and the average pore size can be reduced; to meet the need for high cycle stability in distributed energy storage, the proportion of cobalt-iron mixed doping can be appropriately increased and the ratio of dual ligands can be optimized.

[0099] In summary, the preparation method of the solid metal-organic framework hydrogen storage material of the present invention achieves synergistic improvement of material structure and hydrogen storage performance through component screening and ratio optimization and process parameter control. The prepared material has excellent room temperature / medium temperature hydrogen storage capacity and outstanding cycle stability. Moreover, the preparation process is mild and controllable and can be completed at room temperature and pressure, which has broad application prospects in the field of hydrogen energy storage.

[0100] The present invention will now be described in more detail with reference to embodiments. It should be noted that these descriptions and embodiments are intended to facilitate understanding of the present invention and are not intended to limit the invention.

[0101] Example

[0102] In this invention, unless otherwise specified, all reagents used are commercially available products and are used directly without further purification. Furthermore, "%" refers to "weight %" and "parts" refers to "parts by weight".

[0103] Table 1 below lists specific information about the raw materials used in the embodiments and comparative examples of the present invention.

[0104]

[0105] Table 2 below lists specific information about the experimental equipment used in the embodiments and comparative examples of the present invention.

[0106]

[0107] Performance testing methods

[0108] (I) Test method for gravimetric hydrogen storage density at 25℃ and 30 bar

[0109] The gravimetric hydrogen storage density of the solid metal-organic framework hydrogen storage materials prepared in the following examples and comparative examples was tested at 25°C and 30 bar according to the method described below.

[0110] Referring to GB / T 38336-2019 "Test Method for Hydrogen Absorption and Desorption Performance of Metal Hydride Hydrogen Storage Materials", the above tests were conducted using the volumetric method. Specifically, in a closed, constant-temperature, and constant-pressure test system, the pressure and volume changes of hydrogen were measured, and the hydrogen absorption capacity of the material was calculated using the ideal gas law, thereby obtaining the gravimetric hydrogen storage density.

[0111] Specifically, 1.0 ± 0.001 g of the solid metal-organic framework hydrogen storage material sample prepared in each embodiment and comparative example was placed in a vacuum drying oven and dried under vacuum at 60°C and 0.08 MPa for 2 hours to remove adsorbed moisture and gas from the sample surface. After cooling to room temperature, the sample was transferred to the sample container of the hydrogen storage testing system and sealed. Then, high-purity hydrogen gas (99.999%) was introduced into the testing system to 30 bar, all valves were closed, and the system was kept at a constant temperature for 1 hour. Subsequently, the sample container was placed in a 25°C constant temperature water bath (constant temperature water bath pot), and the vacuum pump was turned on to degas the sample container until the vacuum degree inside the sample container was ≤ 1 × 10⁻⁶. -3 The system was degassed at 30 bar for 2 hours, and the vacuum valve was closed after degassed. Then, high-purity hydrogen was introduced into the test system to stabilize the system pressure at 30 bar and maintain a constant temperature of 25°C. The pressure changes of the system during hydrogen absorption were recorded until the pressure no longer changed (hydrogen absorption equilibrium), and the equilibrium time was 6 hours.

[0112] Then, the number of hydrogen moles absorbed by the sample is calculated according to the ideal gas law PV = nRT. Combining the molar mass of hydrogen (2.016 g / mol) and the sample mass, the gravimetric hydrogen storage density is calculated using the following formula:

[0113]

[0114] Where: ω is the gravimetric hydrogen storage density (wt%), m 氢气 m represents the mass of hydrogen absorbed (g). 样品 The mass of the sample is (g).

[0115] In the above tests, the same sample was tested in parallel three times, and the arithmetic mean was taken as the test result. The relative standard deviation (RSD) was ≤2.0%.

[0116] (II) Test method for gravimetric hydrogen storage density at 80℃ and 30 bar

[0117] The gravimetric hydrogen storage density of the solid metal-organic framework hydrogen storage materials prepared in the following examples and comparative examples was tested at 80°C and 30 bar.

[0118] The principle, instruments, sample pretreatment, data calculation, and precision requirements of this test are consistent with the corresponding content of the test method under the detailed conditions of 25℃ and 30bar described above. Only the isothermal conditions are changed. The specific differences are as follows:

[0119] Temperature control device: Place the sample container in an oil bath (instead of a water bath), and the temperature fluctuation of the oil bath should be ≤ ±0.1℃;

[0120] Temperature equilibrium: After degassing, heat the oil bath to 80°C and keep it at that temperature for 30 minutes to allow the temperature inside the sample container to reach equilibrium before performing the hydrogen absorption test.

[0121] Hydrogen absorption equilibrium: Maintain a constant temperature of 80°C during hydrogen absorption until the system pressure no longer changes. The equilibrium time is 8 hours.

[0122] The same sample was tested in parallel three times, and the arithmetic mean was taken as the test result. The relative standard deviation (RSD) was ≤2.5%.

[0123] (III) Test method for gravimetric hydrogen storage density at 120℃ and 30 bar

[0124] The gravimetric hydrogen storage density of the solid metal-organic framework hydrogen storage materials prepared in the following examples and comparative examples was tested at 120°C and 30 bar.

[0125] The principle, instruments, sample pretreatment, data calculation, and precision requirements of this test are consistent with the test method detailed above under the conditions of 25℃ and 30bar. The specific adjustments are as follows:

[0126] Temperature and pressure control device: A high-temperature and high-pressure constant temperature reactor is used as the sample vessel to replace the conventional sample vessel. The temperature control method is a jacketed oil bath constant temperature, and the temperature fluctuation is ≤ ±0.1℃.

[0127] Degassing and temperature control: After the sample container is degassed, the temperature is raised to 120°C and kept at that temperature for 1 hour to allow the temperature to reach equilibrium.

[0128] Hydrogen absorption test: High-purity hydrogen gas was introduced until the system pressure stabilized at 30 bar, and the temperature was kept constant at 120°C. Pressure changes were recorded, and the hydrogen absorption equilibrium time was 10 hours.

[0129] The same sample was tested in parallel three times, and the arithmetic mean was taken as the test result. The relative standard deviation (RSD) was ≤2.0%.

[0130] (iv) Test method for hydrogen storage capacity retention rate after 2000 hydrogen absorption and desorption cycles at 25℃ and 30 bar

[0131] The hydrogen storage capacity retention rate of the solid metal-organic framework hydrogen storage materials prepared in the following examples and comparative examples was tested after 2000 hydrogen absorption and desorption cycles at 25°C and 30 bar.

[0132] Specifically, the above tests were conducted in accordance with GB / T 38336-2019, "Test Method for Hydrogen Absorption and Desorption Performance of Metal Hydride Hydrogen Storage Materials". The test principle for the hydrogen storage capacity retention rate, which characterizes the cyclic stability of the material, is as follows: Under conditions of 25℃ / 30bar, the sample is subjected to continuous hydrogen absorption and desorption cycle tests. The amount of hydrogen absorbed in each cycle is recorded, and the ratio of the amount of hydrogen absorbed after 2000 cycles to the initial amount absorbed is calculated; this ratio represents the hydrogen storage capacity retention rate. The specific steps are as follows:

[0133] Sample pretreatment and initial hydrogen absorption test: Perform sample pretreatment and initial hydrogen absorption test according to the 25℃ / 30bar gravimetric hydrogen storage density test method described in detail above, and record the gravimetric hydrogen storage density ω1 (weight %) of the initial hydrogen absorption. After the initial hydrogen absorption equilibrium is reached, connect the sample container to the vacuum system, open the vacuum valve to release hydrogen, and release hydrogen until the vacuum degree inside the sample container is ≤1×10⁻⁶. -3 The hydrogen absorption and desorption cycle is completed at 2 bar for 2 hours. Repeat the above hydrogen absorption and desorption steps for continuous cycle testing, maintaining a constant temperature of 25°C, an absorption pressure of 30 bar, and a desorption vacuum degree ≤1×10⁻⁶. -3 The gravimetric hydrogen storage density was recorded for each cycle of hydrogen absorption. Finally, after 2000 hydrogen absorption and desorption cycles, a hydrogen absorption test was performed according to standard methods, and the gravimetric hydrogen storage density ω was recorded at this point. 2000 (weight%);

[0134] The formula for hydrogen storage capacity retention rate is:

[0135]

[0136] Where η is the hydrogen storage capacity retention rate (weight %), ω 2000 ω1 represents the gravimetric hydrogen storage density (wt%) after 2000 cycles, and ω1 represents the initial gravimetric hydrogen storage density (wt%).

[0137] Three sets of 2000-cycle tests were performed on the same sample, with the relative deviation of hydrogen absorption in a single cycle ≤2.0%. Calculate the average value of the results of the three sets of 2000-cycle tests.

[0138] (v) Methods for determining specific surface area and average pore size

[0139] The specific surface area and average pore size of the solid metal-organic framework hydrogen storage materials prepared in the following examples and comparative examples were measured.

[0140] Specifically, referring to GB / T 19587-2017 "Determination of Specific Surface Area of ​​Solid Materials by Gas Adsorption BET Method" and GB / T21650.2-2019 "Determination of Pore Size Distribution and Porosity of Solid Materials by Gas Adsorption Method - Part 2: Mesopores and Macropores", the specific surface area and average pore size of solid metal-organic framework hydrogen storage materials were determined using an ASAP 2460 specific surface area and pore size analyzer from McMurray Technology, Inc.

[0141] Example 1 (E1)

[0142] The specific steps of Example 1 are as follows:

[0143] (1) Preparation of an aqueous solution of the pore structure modifier:

[0144] Take 60g of deionized water and 20g of anhydrous ethanol in a weight ratio of 3:1 and place them in a 250mL three-necked flask. Place the three-necked flask in a constant temperature water bath and heat it to 25℃. Turn on the digital display constant speed stirrer and adjust the stirring speed to 200 r / min. Slowly add 0.8g of glacial acetic acid and continue stirring for 15min until the acetic acid is completely dissolved to obtain an aqueous solution of pore structure regulator for later use.

[0145] (2) Preparation of a mixed aqueous solution of metal ions:

[0146] Weigh 18g of zinc nitrate and 12g of nickel nitrate at a weight ratio of 1.5:1. Select 1.5g of cobalt nitrate as the doping metal ion source. Add the above raw materials to a 250mL beaker containing 60g of deionized water and 20g of anhydrous ethanol (weight ratio 3:1). Place the beaker in a 25℃ constant temperature water bath and stir at 200 r / min until the solid is completely dissolved. Then place the beaker in an ultrasonic cleaner and ultrasonically treat it for 15min at a power of 300W and a frequency of 40kHz to obtain a homogeneous aqueous solution of mixed metal ions for later use.

[0147] (3) Preparation of ligand mixture:

[0148] Weigh 8g of the first organic ligand, pyromellitic acid, and 4.8g of the second organic ligand, triethylenediamine, and add them to a 500mL three-necked flask containing 60g of deionized water and 20g of anhydrous ethanol (weight ratio 3:1). Place the three-necked flask in a 30℃ constant temperature water bath, adjust the stirring speed to 300 r / min, and stir until the solid is initially dispersed. Then, sonicate for 15min. Subsequently, slowly add 1 mol / L sodium hydroxide aqueous solution. Monitor the pH of the solution in real time with a pH meter and adjust it to 8 to obtain the ligand mixture for later use.

[0149] (4) Preparation of suspension by coordination reaction:

[0150] The above-mentioned aqueous solution of mixed metal ions and aqueous solution of pore structure regulator were transferred to a 1000 mL three-necked flask and mixed thoroughly. The constant temperature water bath was heated to 35 °C and stirred at 200 r / min. Then, the ligand mixture was added dropwise to the mixture through a constant pressure dropping funnel at a dropping rate of 1 mL / min. After the addition was completed, the stirring rate was increased to 300 r / min and stirred continuously for 2 h. Then, the constant temperature water bath was heated to 55 °C, the stirring was turned off, and the reaction was allowed to stand at a constant temperature for 8 h to obtain a suspension containing a large amount of white precipitate.

[0151] (5) Solid-liquid separation and precursor preparation:

[0152] The suspension was transferred to centrifuge tubes and placed in a high-speed refrigerated centrifuge. The centrifugation speed was adjusted to 8000 r / min, and the centrifugation was carried out for 10 min. The supernatant was discarded, and the solid precipitate was collected. The solid precipitate was washed three times each with deionized water and anhydrous ethanol, and centrifuged at 8000 r / min for 10 min after each wash. The washed solid precipitate was placed in a vacuum drying oven and dried at 60℃ and 0.08 MPa for 8 h to obtain the metal-organic framework precursor.

[0153] (6) Gradient temperature activation treatment:

[0154] The metal-organic framework precursor was placed in a quartz boat in a tube furnace. High-purity nitrogen (99.999% purity) was introduced into the tube furnace as a protective atmosphere at a flow rate of 50 mL / min. After the atmosphere in the tube furnace was completely replaced, gradient temperature activation was performed: in the first stage, the temperature was increased to 120℃ at a rate of 2℃ / min and held at that temperature for 2 hours; in the second stage, the temperature was increased to 180℃ at a rate of 1℃ / min and held at that temperature for 3 hours. After activation, the furnace was cooled to room temperature to obtain solid metal-organic framework hydrogen storage material 1.

[0155] In the formulation of Example 1, based on the total weight of the main metal ion source, doped metal ion source, first organic ligand, second organic ligand and pore structure modifier as 100%, the main metal ion source accounts for 66.5%, the doped metal ion source accounts for 3.3%, the first organic ligand accounts for 17.8%, the second organic ligand accounts for 10.6%, and the pore structure modifier accounts for 1.8%.

[0156] The solid metal-organic framework hydrogen storage material 1 prepared in Example 1 was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) patterns. Figure 2 A scanning electron microscope (SEM) image of the solid metal-organic framework hydrogen storage material 1 prepared in Example 1 is shown. Figure 3The X-ray diffraction (XRD) patterns of the solid metal-organic framework hydrogen storage material 1 prepared in Example 1 and the solid metal-organic framework hydrogen storage materials 2-4 prepared in Examples 2-4 are shown.

[0157] Furthermore, tests on the specific surface area and average pore size of the solid metal-organic framework hydrogen storage material 1 prepared in Example 1 showed that the specific surface area of ​​this material was 2200 m². 2 / g, and the average pore size is 0.9 nm.

[0158] Furthermore, the solid metal-organic framework hydrogen storage material 1 was tested according to the test methods described above for gravimetric hydrogen storage density at 25°C and 30 bar, gravimetric hydrogen storage density at 80°C and 30 bar, gravimetric hydrogen storage density at 120°C and 30 bar, and hydrogen storage capacity retention rate after 2000 hydrogen absorption and desorption cycles at 25°C and 30 bar. The results are shown in Table 4 below.

[0159] Examples 2-12 (E2-E12) and Comparative Examples 1-5 (CE1-CE5)

[0160] Examples 2-12 (E2-E12) and Comparative Examples 1-5 (CE1-CE5) were prepared in a manner similar to that of Example 1 to prepare solid metal-organic framework hydrogen storage materials 2-12 and comparative solid metal-organic framework hydrogen storage materials 1-5, the only difference being that the component types, ratios and / or reaction conditions were changed as shown in Table 3 below.

[0161] In Comparative Example 1, no doped metal ion source was added. In Comparative Example 2, only zinc nitrate was used as the main metal ion source. In Comparative Example 3, no second organic ligand was added. In Comparative Example 4, no pore structure modifier was used. In Comparative Example 5, the pH of the ligand mixture was adjusted to 7.

[0162] In addition, the solid metal-organic framework hydrogen storage materials prepared in Examples 2-12 (E2-E12) were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that they had similar microstructures and crystal diffraction patterns to the solid metal-organic framework hydrogen storage materials prepared in Example 1.

[0163] In addition, based on the test methods described above for specific surface area, average pore size, gravimetric hydrogen storage density, and hydrogen storage capacity retention rate after 2000 hydrogen adsorption / desorption cycles, solid metal-organic framework hydrogen storage materials 2-12 and comparative solid metal-organic framework hydrogen storage materials 1-5 were tested, and the results are shown in Table 4 below.

[0164]

[0165]

[0166] As shown in Table 4 above, the solid metal-organic framework hydrogen storage materials prepared in Examples 1-12 of the present invention exhibit excellent performance in terms of hydrogen storage density, cycle stability and pore structure parameters. All indicators are significantly better than those in Comparative Examples 1-5, which verifies the rationality and superiority of the formulation design and preparation process of the present invention.

[0167] Specifically, the materials in Examples 1-12 achieved a hydrogen storage density of 3.0-3.7 wt% at 25°C / 30 bar, and maintained a hydrogen storage density of 2.0-2.5 wt% and 1.1-1.5 wt% at intermediate temperatures of 80°C and 120°C. After 2000 hydrogen adsorption / desorption cycles, the hydrogen storage capacity retention rate was ≥88%, and the specific surface area was 2200-2500 m². 2 / g, with an average pore size of 0.9-1.8 nm, and a pore structure adapted to the adsorption requirements of hydrogen molecules. Among them, Examples 10-12, which are doped with composite metals and compounded with two ligands, have better performance, and Example 11 has a room temperature hydrogen storage density of 3.7 wt% and a cycle retention rate of up to 95%.

[0168] On the other hand, the performance of the materials in Comparative Examples 1-5 all showed a significant decline. Comparative Example 1, with no doped metal ions, Comparative Example 2, using a single main metal ion source, and Comparative Example 3, without the addition of a second organic ligand, all resulted in a scarcity of adsorption sites, with room temperature hydrogen storage densities below 2.0 wt% and cycle retention rates not exceeding 70%. Comparative Example 4, without the addition of a pore structure modifier, suffered from deteriorated pore structure parameters, leading to a significant decrease in hydrogen storage and cycling performance. Comparative Example 5, with the pH of the ligand mixture adjusted to 7, resulted in incomplete coordination reactions, numerous structural defects in the material, and all performance indicators were far lower than those of the examples. These results confirm that the present invention, through synergistic main metal doping, dual organic ligand coordination, the introduction of a pore structure modifier, and the control of pH and process parameters, achieves synergistic optimization of the pore structure and adsorption performance of the hydrogen storage material, which is key to obtaining excellent room temperature / intermediate temperature hydrogen storage performance and cycling stability.

[0169] Obviously, those skilled in the art can make various modifications and variations to this disclosure without departing from the spirit and scope of this disclosure. Therefore, if such modifications and variations fall within the scope of this invention, this disclosure is also intended to include such modifications and variations.

Claims

1. A method for preparing a solid metal-organic framework hydrogen storage material, characterized in that, Includes the following steps: (1) Prepare an aqueous solution of a pore structure regulator, wherein the pore structure regulator is selected from one or more of acetic acid, formic acid and propionic acid; (2) Prepare an aqueous solution of mixed metal ions containing a main metal ion source and a doped metal ion source, wherein the main metal ion source is a mixture of zinc nitrate and nickel nitrate in a weight ratio of 1.5:1 to 5:1, and the doped metal ion source is a mixture of cobalt nitrate and iron nitrate in a weight ratio of 2:1 to 5:

1. (3) Prepare a ligand mixture with a pH of 8-11 containing a first organic ligand, a second organic ligand and a base adjuster, wherein the first organic ligand is a mixture of pyromellitic acid and 1,4-terephthalic acid in a weight ratio of 1:2-2:1, the second organic ligand is a mixture of triethylenediamine and ethylenediamine in a weight ratio of 1:1-3:1, and the base adjuster is selected from one or more of sodium hydroxide, potassium hydroxide and lithium hydroxide; (4) The aqueous solution of the metal ion mixture is mixed with the aqueous solution of the pore structure regulator to obtain a mixture, and the ligand mixture is added dropwise to the mixture to carry out a coordination reaction to obtain a suspension containing precipitate; (5) The suspension is subjected to solid-liquid separation, the solid precipitate is collected and washed and dried to obtain the metal-organic framework precursor; (6) Under a protective atmosphere, the metal-organic framework precursor is subjected to gradient temperature activation treatment to obtain the solid metal-organic framework hydrogen storage material, wherein: Based on the total weight of the main metal ion source, doped metal ion source, first organic ligand, second organic ligand and pore structure modifier as 100%, the main metal ion source accounts for 30-80%, the doped metal ion source accounts for 1.5-17%, the first organic ligand accounts for 8-40%, the second organic ligand accounts for 4.8-25%, and the pore structure modifier accounts for 0.8-10%.

2. The method for preparing solid metal-organic framework hydrogen storage material according to claim 1, characterized in that, With the total weight of the main metal ion source and the doped metal ion source being 100%, the main metal ion source accounts for 85-93%, and the doped metal ion source accounts for 7-12%.

3. The method for preparing solid metal-organic framework hydrogen storage material according to claim 1, characterized in that, The weight ratio of the first organic ligand to the second organic ligand is 1.5:1 to 1.8:

1.

4. The method for preparing a solid metal-organic framework hydrogen storage material according to claim 1, characterized in that, The solvents used to prepare the aqueous solution of the pore structure regulator, the aqueous solution of the metal ion mixture, and the ligand mixture are a mixture of deionized water and an alcohol solvent in a weight ratio of 3:1 to 10:1, wherein the alcohol solvent is selected from one or more of ethanol, methanol, and isopropanol.

5. The method for preparing a solid metal-organic framework hydrogen storage material according to claim 1, characterized in that, In step (4), after adding and stirring, the temperature is raised to 55-70°C and kept at a constant temperature for 8-16 hours.

6. The method for preparing a solid metal-organic framework hydrogen storage material according to claim 1, characterized in that, The gradient temperature activation process includes: In the first stage, the temperature is increased to 120-150℃ at a rate of 2-5℃ / min and held at a constant temperature for 2-3 hours. In the second stage, the temperature is increased to 180-220℃ at a rate of 1-3℃ / min and held at a constant temperature for 3-5 hours.

7. A solid-state metal-organic framework hydrogen storage material, characterized in that, The solid metal-organic framework hydrogen storage material is prepared by the method according to any one of claims 1-6.

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