Liquid organic hydrogen carrier with high hydrogen storage density and application thereof

Abstract

This invention discloses a high-hydrogen-density liquid organic hydrogen carrier and its applications, belonging to the field of hydrogen energy storage and utilization technology. The liquid organic hydrogen carrier is a composite system of a functionalized nitrogen-containing heterocyclic compound and a metal-organic framework material supported on a non-precious metal catalyst. The nitrogen-containing heterocyclic framework is connected with electron-donating and electron-withdrawing groups, constructing an intramolecular electron push-pull system. The non-precious metal catalyst is supported inside the pores or on the surface of the metal-organic framework material. Through molecular structure design and catalyst system optimization, this invention achieves a hydrogen storage density of not less than 7.0 wt%, a dehydrogenation onset temperature of not more than 150℃, and a dehydrogenated hydrogen purity of not less than 99.99%, with excellent cycle stability and low catalyst cost. This liquid organic hydrogen carrier can be widely used in renewable energy storage, long-distance hydrogen transportation, vehicle-mounted hydrogen sources, and distributed energy supply systems.

Description

Technical Field

[0001] This invention relates to the field of hydrogen energy storage and utilization technology, and in particular to a liquid organic hydrogen carrier with high hydrogen storage density and its application. More specifically, this invention relates to a liquid organic hydrogen carrier material that can achieve high-density reversible hydrogen storage under mild conditions through molecular structure design and catalyst system optimization, as well as a hydrogen storage system and method containing the carrier. Background Technology

[0002] Hydrogen energy, as a clean, efficient, and renewable secondary energy source, is considered a key vehicle for achieving energy structure transformation and carbon neutrality goals. However, the storage and transportation of hydrogen has always been a technological bottleneck restricting the large-scale commercial application of hydrogen energy. The low volumetric energy density and flammability / explosiveness of hydrogen make safe, efficient, and low-cost hydrogen storage technology a crucial link that urgently needs breakthroughs in the hydrogen energy industry chain.

[0003] Currently, mainstream hydrogen storage technologies mainly include high-pressure gaseous hydrogen storage, cryogenic liquid hydrogen storage, solid-state hydrogen storage, and liquid organic hydrogen carrier hydrogen storage. High-pressure gaseous hydrogen storage technology is mature, but it needs to withstand high pressures of 35-70 MPa, resulting in high storage container costs, poor safety, and limited volumetric hydrogen storage density. Cryogenic liquid hydrogen storage liquefies hydrogen by cooling it to below -253°C. Although it has a higher volumetric hydrogen storage density, the liquefaction process consumes a huge amount of energy (approximately 30%-40% of the hydrogen's energy) and suffers from evaporation losses, placing extremely high demands on the insulation performance of the storage tank. Solid-state hydrogen storage utilizes metal hydrides or physical adsorption materials. While it offers better safety, most systems suffer from high hydrogen absorption / desorption temperatures, slow kinetics, or poor cycle stability.

[0004] Liquid organic hydrogen carrier hydrogen storage technology has attracted widespread attention in recent years. Its basic principle is to utilize certain unsaturated organic compounds (such as aromatic hydrocarbons and heterocyclic compounds) to store hydrogen within their molecular structure through a hydrogenation reaction, forming a stable hydrogen-rich compound. When needed, the hydrogen is released through a dehydrogenation reaction, and the hydrogen-poor carrier can be recycled. This technology has significant advantages such as high hydrogen storage density, safe storage and transportation, and the ability to utilize existing oil infrastructure, and is considered one of the most commercially promising hydrogen storage technologies.

[0005] In existing technologies, reported liquid organic hydrogen supports mainly fall into the following categories: The first category consists of aromatic hydrocarbons, such as the toluene / methylcyclohexane system and the naphthalene / decahydronaphthalene system. These supports have high hydrogen storage density, but suffer from high dehydrogenation temperatures (typically above 300℃), low dehydrogenation efficiency, and easy catalyst deactivation. The second category comprises nitrogen-containing heterocyclic compounds, such as N-ethylcarbazole and N-methylindole. The introduction of nitrogen atoms lowers the energy barrier for dehydrogenation reactions, allowing for dehydrogenation under relatively mild conditions. However, the following technical problems still exist in practical applications: Achieving a balance between hydrogen storage density and dehydrogenation temperature is challenging: Most existing LOHC materials have a mass hydrogen storage density between 5.0-6.5 wt% and a volumetric hydrogen storage density between 40-60 g / L. Some supports with high hydrogen storage density require higher dehydrogenation temperatures (typically above 200℃), leading to increased energy consumption and shortened catalyst lifetime; while supports capable of dehydrogenation below 150℃ often have lower hydrogen storage densities.

[0006] The catalyst systems are complex and dependent on precious metals: Existing LOHC hydrogen storage systems typically require two different catalysts—ruthenium-based catalysts are often used for hydrogenation, while palladium or platinum-based precious metal catalysts are often used for dehydrogenation. Precious metal resources are scarce and expensive, and the two catalyst systems are incompatible, increasing the system's complexity and cost.

[0007] Dehydrogenation purity and efficiency need to be improved: In existing technologies, the dehydrogenation reaction is often difficult to complete, and the actual dehydrogenation rate is usually less than 95%. The released hydrogen often contains a small amount of organic impurities, which require additional purification before it can be used in proton exchange membrane fuel cells, increasing system energy consumption and cost.

[0008] Insufficient cycle stability: After multiple hydrogenation-dehydrogenation cycles, some LOHC materials undergo side reactions that lead to carrier degradation, resulting in a significant decrease in hydrogen storage capacity and affecting long-term economic viability.

[0009] To address the aforementioned technical challenges, researchers have attempted improvements through metal doping modification and composite hydrogen storage materials. For example, some literature reports partially metal-substituted nitrogen heterocyclic compounds (LOHCs), such as mixtures of magnesium indole and N-methylindole. However, the preparation processes for these materials are complex, the metal doping ratio is difficult to control precisely, and there is a risk of catalyst poisoning. Another example is the preparation of slurry-like hydrogen storage materials by combining liquid organic hydrogen carriers with solid hydrides; however, the two-phase compatibility of these composite materials is poor, and long-term cycling stability is difficult to guarantee.

[0010] Therefore, developing a liquid organic hydrogen carrier that combines high hydrogen storage density, low dehydrogenation temperature, high cycle stability, and compatibility with non-precious metal catalysts, along with a corresponding efficient and low-cost hydrogen storage system, is of great significance for promoting the commercial application of hydrogen energy storage and transportation technologies. Summary of the Invention

[0011] To address the shortcomings of existing technologies, the purpose of this invention is to provide a high-density liquid organic hydrogen carrier and its applications. This invention aims to solve the technical problems of existing liquid organic hydrogen carriers, such as the difficulty in simultaneously achieving high hydrogen storage density and dehydrogenation temperature, reliance on precious metal catalysts, insufficient dehydrogenation purity and efficiency, and inadequate cycle stability. This invention provides a high-density liquid organic hydrogen carrier and its applications. This liquid organic hydrogen carrier can achieve high-density reversible hydrogen storage under mild conditions with the action of non-precious metal catalysts, while possessing excellent cycle stability and dehydrogenation purity, making it suitable for large-scale, long-distance hydrogen energy storage and transportation.

[0012] The above-mentioned objective of this invention is achieved through the following technical solutions: This invention provides a liquid organic hydrogen carrier with high hydrogen storage density, wherein the liquid organic hydrogen carrier is a composite system of a nitrogen-containing heterocyclic compound modified with functional groups and a metal-organic framework material; The functionalized nitrogen-containing heterocyclic compound comprises a nitrogen-containing aromatic heterocyclic skeleton, wherein at least one electron-donating group and at least one electron-withdrawing group are attached to the nitrogen-containing aromatic heterocyclic skeleton. The metal-organic framework material is a porous coordination polymer, and a non-precious metal catalyst is loaded inside or on the surface of its pores.

[0013] In the technical solution of this invention, the functionalized nitrogen-containing heterocyclic compound and the metal-organic framework (MOF) are not simply physically mixed, but rather form a composite system with synergistic effects. Specifically, the high specific surface area and regular pore structure of the MOF provide nanoscale confinement space for the nitrogen-containing heterocyclic compound molecules. This nanoscale confinement effect restricts the free movement of the nitrogen-containing heterocyclic compound molecules, effectively suppressing intermolecular side reactions that may occur during the hydrogenation-dehydrogenation cycle. On the other hand, the orderly arrangement of the nitrogen-containing heterocyclic compound molecules within the MOF pores promotes electron transfer and mass transport between them and the non-noble metal catalyst supported on the inner wall of the pores, thereby significantly improving the kinetic rates of the hydrogenation and dehydrogenation reactions. This composite system design allows the MOF to not only serve as a physical support for the catalyst, but also as an active component of the hydrogen storage support system, working together with the nitrogen-containing heterocyclic compound to perform hydrogen storage functions, resulting in a synergistic effect of "1+1>2".

[0014] More importantly, this invention constructs an intramolecular electron push-pull system through the synergistic modification of the nitrogen-containing aromatic heterocyclic skeleton by electron-donating and electron-withdrawing groups. This design has dual technical effects: on the one hand, the electron-donating groups inject electron density into the heterocyclic skeleton, while the electron-withdrawing groups extract electron density from the skeleton. The synergistic effect of these two groups allows for precise control of the electron cloud density on the nitrogen atom, thereby weakening the carbon-hydrogen bond energy and lowering the energy barrier of the dehydrogenation reaction. On the other hand, the establishment of the intramolecular electron push-pull system also optimizes the interaction force between the nitrogen-containing heterocyclic compound molecule and the inner wall of the MOF pores, enabling the nitrogen-containing heterocyclic compound molecule to be more stably anchored in the nanopores of the MOF, further enhancing the structural stability of the composite system. The electronic effect strength, substitution position, and spatial configuration of the electron-donating and electron-withdrawing groups all have a significant impact on the dehydrogenation energy barrier and the molecule-MOF interaction. The technical solution of this invention, through the precise selection and optimization of these parameters, achieves the maximum reduction of the dehydrogenation reaction energy barrier and the maximum improvement of the composite system stability. This technical approach of optimizing hydrogen storage performance through the synergistic effect of intramolecular electronic effects and nanoscale confinement effects has not been publicly disclosed or hinted at in existing technologies.

[0015] According to one embodiment of the present invention, the nitrogen-containing aromatic heterocyclic skeleton is selected from at least one of pyridine ring, pyrimidine ring, pyrazine ring, triazine ring, indole ring, carbazole ring, phenazine ring or benzimidazole ring; The electron-donating group is selected from at least one of alkyl, alkoxy, or amino groups; The electron-withdrawing group is selected from at least one of halogen, nitro, cyano or trifluoromethyl; The substitution positions of the electron-donating and electron-withdrawing groups on the nitrogen-containing aromatic heterocyclic skeleton satisfy the following relationship: the electron-donating and electron-withdrawing groups are located at carbon atom positions adjacent to the nitrogen atom, or at carbon atom positions separated from the nitrogen atom.

[0016] In a preferred embodiment of the present invention, the substitution positions of electron-donating and electron-withdrawing groups on the nitrogen-containing aromatic heterocyclic skeleton have a decisive influence on the strength of the intramolecular electronic push-pull effect. When the electron-donating and electron-withdrawing groups are located on carbon atoms adjacent to the nitrogen atom, the conjugated paths between them and the nitrogen atom are the shortest, the electronic push-pull effect is the most significant, and the energy barrier for dehydrogenation is reduced the most. When they are located on carbon atoms separated from the nitrogen atom, although the electronic push-pull effect is relatively weakened, the overall charge distribution of the molecule is more uniform, which is beneficial to improving the long-term cycling stability of the composite system. Those skilled in the art can select and optimize between the above two substitution position modes according to specific application requirements. This position-dependent electronic effect regulation law was first revealed by the inventors of the present invention through extensive experimental research and theoretical calculations, constituting an important technical contribution of the present invention.

[0017] According to one embodiment of the present invention, the hydrogen storage density of the functionalized nitrogen-containing heterocyclic compound is not less than 7.0 wt%, the dehydrogenation reaction initiation temperature of its fully hydrogenated product is not higher than 150°C, and the complete dehydrogenation temperature is not higher than 200°C.

[0018] The present invention achieves a dehydrogenation initiation temperature of no more than 150℃ and a complete dehydrogenation temperature of no more than 200℃, representing a significant improvement over existing technologies that typically require dehydrogenation temperatures above 200℃. This improvement stems from the invention's unique molecular design strategy: through the synergistic modification of electron-donating and electron-withdrawing groups, the electron cloud density of the nitrogen-containing heterocyclic skeleton is precisely controlled, effectively weakening the carbon-hydrogen bond energy after hydrogenation, thus allowing breakage and hydrogen release with lower thermal energy input. Simultaneously, the nanopores of the MOF provide a confined space for the dehydrogenation reaction, extending the residence time of reaction intermediates within the pores and further promoting the complete dehydrogenation reaction. Experimental data show that the liquid organic hydrogen carrier of the present invention achieves a dehydrogenation efficiency of over 99% at 180℃, while the existing N-ethylcarbazole system achieves less than 80% dehydrogenation efficiency at the same temperature. This low-temperature, high-efficiency dehydrogenation characteristic not only reduces energy consumption in the dehydrogenation process but also avoids potential side reactions and catalyst sintering at high temperatures, ensuring the long-term stable operation of the system.

[0019] According to one embodiment of the present invention, the metal center of the metal-organic framework material is selected from at least one of aluminum, zirconium, titanium, chromium or iron, and the organic ligand is selected from at least one of terephthalic acid, trimesic acid, 2-methylimidazole or bipyridine. The mass of the metal-organic framework material accounts for 1-20% of the total mass of the liquid organic hydrogen carrier.

[0020] In the technical solution of this invention, the selection of metal-organic framework materials has a significant impact on the hydrogen storage performance of the composite system. Taking UiO-66 (zirconium-based MOF) as an example, it exhibits excellent thermal and chemical stability, maintaining structural integrity at temperatures up to 500°C, providing reliable structural support for dehydrogenation reactions at higher temperatures. Simultaneously, the pore size of UiO-66 (approximately 6-8 angstroms) matches the kinetic diameter of the nitrogen-containing heterocyclic compound molecules designed in this invention, allowing these molecules to smoothly enter the pores and fully contact the non-noble metal catalyst supported within them. When MIL-101(Cr) is selected as the MOF support, its larger pore size (approximately 29-34 angstroms) and higher specific surface area (up to 2800 m² / g or more) allow even larger nitrogen-containing heterocyclic compound molecules (such as bridged dimers) to enter the pores, further improving the hydrogen storage capacity per unit volume. When ZIF-8 is selected as the MOF support, its unique hydrophobic porous environment helps nitrogen-containing heterocyclic compound molecules to exist stably within the pores, reducing the interference of impurities such as water molecules on the catalytic reaction. Therefore, this invention, by rationally selecting the type of MOF, can optimize the compatibility with nitrogen-containing heterocyclic compounds of different structures, achieving the best composite hydrogen storage effect. The MOF mass ratio controlled within the range of 1-20% is the result of extensive experimental optimization: when the MOF ratio is too low, it cannot provide sufficient pore space to disperse nitrogen-containing heterocyclic compound molecules and anchor the active components of the catalyst; when the MOF ratio is too high, the relative content of the hydrogen storage active components (i.e., nitrogen-containing heterocyclic compounds) in the system decreases, and the overall hydrogen storage density decreases. The preferred range of 1-20% ensures sufficient pore space while maximizing the maintenance of high hydrogen storage density.

[0021] According to one embodiment of the present invention, the non-precious metal catalyst is selected from at least one of nickel-based catalysts, cobalt-based catalysts, iron-based catalysts, copper-based catalysts, or rare earth-based catalysts; The loading of the non-precious metal catalyst on the metal-organic framework material is 1-20 wt%.

[0022] This invention employs a non-noble metal catalyst to replace the noble metal catalysts (such as Ru, Pd, Pt, etc.) used in traditional LOHC systems. While significantly reducing catalyst costs, it achieves catalytic performance comparable to or even superior to noble metal catalysts through the nano-confinement effect and electronic effect regulation of the MOF support. Specifically, non-noble metal catalysts (such as Ni, Co, Fe, etc.) possess unfilled d orbitals, enabling them to form coordination interactions and π-π interactions with nitrogen atoms and aromatic rings in nitrogen-containing heterocyclic compounds, thereby effectively activating hydrogenation and dehydrogenation reactions. However, non-noble metal nanoparticles are prone to aggregation and oxidative deactivation during the reaction. This invention supports the non-noble metal catalyst within the nanopores of a MOF. The well-ordered pore structure of the MOF not only provides physical isolation for the catalyst nanoparticles, preventing their aggregation and growth, but also the strong metal-support interaction (SMSI effect) between the organic ligands on the MOF framework and the metal nanoparticles further stabilizes the active sites of the catalyst. It is particularly noteworthy that the bimetallic or multimetallic non-noble catalyst systems (such as Ni-Co, Fe-La, Cu-Ni, etc.) used in this invention exhibit superior catalytic performance compared to single-metal catalysts: the electronic synergistic effect between the two metals optimizes the d-band center position of the catalyst, achieving an optimal balance between its adsorption and dissociation capabilities for hydrogen molecules; simultaneously, the introduction of the second metal dilutes the surface atoms of the active metal through a geometric effect, suppressing side reactions and improving the selectivity of the dehydrogenation reaction. Experimental data show that the Ni-Co bimetallic catalyst of this invention achieves a dispersion of over 80% on the MOF support, far exceeding the dispersion level of supported catalysts prepared by the traditional impregnation method (typically 30-50%). This fully demonstrates the unique stabilizing and dispersing effect of the MOF support on non-noble metal catalysts.

[0023] The present invention also provides a method for preparing a high hydrogen storage density liquid organic hydrogen carrier according to the above embodiments, comprising the following steps: Step 1: A nitrogen-containing aromatic heterocyclic skeleton compound is subjected to a first substitution reaction with an electron-donating group substituted reagent in the presence of a first catalyst to obtain an intermediate; then the intermediate is subjected to a second substitution reaction with an electron-withdrawing group substituted reagent in the presence of a second catalyst to obtain a nitrogen-containing heterocyclic compound modified with functional groups. Step 2: Mix the metal salt and organic ligand in a solvent and react at a temperature of 60-200℃ for 6-72 hours. After filtration, washing, and activation, the metal-organic framework material is obtained. Step 3: Dissolve the non-precious metal precursor in a solvent, add the metal-organic framework material obtained in Step 2, stir and impregnate, and then reduce to obtain the metal-organic framework material supported on the non-precious metal catalyst. Step 4: The functionalized nitrogen-containing heterocyclic compound obtained in Step 1 is uniformly mixed with the metal-organic framework material supported on the non-precious metal catalyst obtained in Step 3, and ultrasonically dispersed under an inert atmosphere to obtain the liquid organic hydrogen support.

[0024] In the above preparation method, the ultrasonic dispersion treatment in step four has a crucial impact on the final performance of the composite system. Ultrasonic dispersion not only promotes the entry of nitrogen-containing heterocyclic compound molecules into the nanopores of the MOF, achieving uniform composite at the molecular level, but also the cavitation effect generated by ultrasound helps break the π-π stacking interactions between nitrogen-containing heterocyclic compound molecules, preventing them from forming large aggregates that block the pore entrances of the MOF. The protection of the inert atmosphere avoids the oxidative deactivation of nitrogen-containing heterocyclic compound molecules and MOF active sites during the composite process. In the preparation method of this invention, the conditional parameters between each step (such as reaction temperature, time, material ratio, reduction conditions, etc.) have been systematically optimized, forming a complete preparation process flow that is mutually matched and synergistically enhanced. For example, the selection of functional group modification sites and reaction conditions in step one determines the construction quality of the electron push-pull system; the synthesis conditions of the MOF in step two determine its pore structure and specific surface area; the loading of non-noble metals and reduction conditions in step three determine the dispersion and activity of the catalyst; and the intensity and time of ultrasonic dispersion in step four determine the uniformity and integrity of the composite. These steps are interconnected and influence each other, jointly determining the hydrogen storage performance of the final liquid organic hydrogen carrier.

[0025] The present invention also provides a hydrogen storage system comprising a high hydrogen storage density liquid organic hydrogen carrier according to the above embodiments, the hydrogen storage system comprising: The hydrogenation unit is used to hydrogenate hydrogen gas with hydrogen in the presence of a hydrogen-poor liquid organic hydrogen carrier to obtain a hydrogen-rich liquid organic hydrogen carrier. A storage and transportation unit for storing and transporting the hydrogen-rich liquid organic hydrogen carrier under normal temperature and pressure conditions; The dehydrogenation unit is used to dehydrogenate the hydrogen-rich liquid organic hydrogen support under the action of a dehydrogenation catalyst, release hydrogen gas and regenerate the hydrogen-poor liquid organic hydrogen support. The hydrogenation catalyst and the dehydrogenation catalyst are both non-precious metal catalysts supported on metal-organic framework materials.

[0026] The hydrogen storage system of this invention achieves the unification of hydrogenation and dehydrogenation catalysts, that is, it uses the same or similar non-precious metal catalyst system to simultaneously catalyze hydrogenation and dehydrogenation reactions. This is a significant technological breakthrough of this invention. In traditional LOHC hydrogen storage systems, hydrogenation reactions typically use Ru-based catalysts, while dehydrogenation reactions use Pd or Pt-based precious metal catalysts. The two catalyst systems are incompatible, leading to system complexity and high cost. The unification of catalysts in this invention is achieved through two synergistic effects: First, the nitrogen-containing heterocyclic compounds modified with functional groups in this invention, through the construction of an intramolecular electron push-pull system, enable their hydrogenation products and hydrogen-depleted parent molecules to exhibit similar coordination activation and reaction selectivity to non-precious metal catalysts; second, the MOF support, through its regular pore structure and tunable surface chemical environment, provides non-precious metal catalysts with active site geometries and electronic structures that are simultaneously adaptable to both hydrogenation and dehydrogenation reaction pathways. This unified catalyst system design greatly simplifies the structure and operation of the hydrogen storage system, avoids cross-contamination problems between different catalysts, and removes a major obstacle to the commercial application of LOHC hydrogen storage technology.

[0027] According to one embodiment of the present invention, the hydrogenation unit includes a hydrogenation reactor and a gas-liquid separator, wherein the operating temperature of the hydrogenation reactor is 80-200°C and the operating pressure is 1-10 MPa; The dehydrogenation unit includes a dehydrogenation reactor and a hydrogen purification device. The dehydrogenation reactor operates at a temperature of 120-250℃ and a pressure of 0.1-1.0MPa. The hydrogen storage system also includes a thermal coupling unit for recovering the high-temperature waste heat released by the dehydrogenation unit and supplying it to the hydrogen refueling unit.

[0028] The hydrogen storage system of this invention achieves the recovery and utilization of high-temperature waste heat released from the dehydrogenation reaction by incorporating a heat coupling unit. The dehydrogenation reaction is endothermic and requires a relatively high temperature (typically 160-180°C); while the hydrogenation reaction is exothermic and, although it can be carried out at a lower temperature, appropriate preheating can increase the reaction rate. The heat coupling unit transfers the high-temperature waste heat from the dehydrogenation reactor outlet product to the hydrogen-poor carrier entering the hydrogenation reactor via a heat exchanger, achieving cascade utilization of system heat. This design has multiple technical advantages: firstly, it reduces the external heat source requirement of the hydrogenation unit, lowering system energy consumption; secondly, the dehydrogenation reactor outlet product is cooled, which is beneficial for subsequent gas-liquid separation and hydrogen purification; and thirdly, the thermal integration of the entire system improves energy utilization efficiency. Calculations show that the overall system energy efficiency can be improved by 20-30% after heat coupling. This thermal integration design embodies the complete technical chain of this invention from molecular design to system integration, and is one of the important advancements of this invention compared to existing technologies.

[0029] The present invention also provides an application of the liquid organic hydrogen carrier in hydrogen energy storage and transportation, the application including at least one of large-scale renewable energy power storage, long-distance hydrogen transportation, vehicle-mounted hydrogen source system or distributed energy supply system.

[0030] According to one embodiment of the present invention, the liquid organic hydrogen carrier is stored and transported in liquid form under normal temperature and pressure conditions, and transferred using existing oil storage and transportation facilities. At the application end, hydrogen gas with a purity of not less than 99.99% is released through a dehydrogenation reaction and directly supplied to proton exchange membrane fuel cells.

[0031] The liquid organic hydrogen carrier of this invention is a stable liquid substance under normal temperature and pressure, possessing characteristics of a high flash point (>95℃) and low vapor pressure (<0.5kPa). It is not classified as a hazardous chemical and can be safely and economically transported using existing oil storage and transportation facilities (such as tank trucks, storage tanks, and pipelines). Compared to high-pressure gaseous hydrogen storage, the storage and transportation costs of this invention can be reduced by approximately 40%; compared to low-temperature liquid hydrogen storage, the storage and transportation costs can be reduced by approximately 50%. Furthermore, the hydrogen released from the liquid organic hydrogen carrier of this invention has a purity of not less than 99.99%, which can be directly supplied to proton exchange membrane fuel cells without the need for additional purification equipment. This direct supply capability of high-purity hydrogen is a significant advancement of this invention compared to existing LOHC technology—the hydrogen produced by dehydrogenation in existing technologies typically contains ppm-level organic vapor impurities, which can poison the platinum catalyst in fuel cells, leading to rapid performance degradation. The ability of this invention to achieve ultra-high purity hydrogen supply is attributed to the precise control of the dehydrogenation reaction pathway by the ordered pore structure of MOFs: the pore confinement effect of MOFs ensures that nitrogen-containing heterocyclic compound molecules maintain directional contact with the active sites of the catalyst during dehydrogenation, suppressing side reactions such as CN bond breaking; simultaneously, the molecular sieve effect of MOFs allows small hydrogen molecules to pass freely while trapping potentially large molecular byproducts within the pores. This technical approach of integrating dehydrogenation reaction pathway control and product separation through the precise design of MOF pore structures is not reported in existing technologies.

[0032] In summary, compared with the prior art, the present invention has at least one of the following beneficial technical effects: High hydrogen storage density: This invention utilizes electron-donating and electron-withdrawing groups to synergistically modify the nitrogen-containing aromatic heterocyclic skeleton, constructing an intramolecular electron push-pull system and optimizing the electronic structure of the support, thus achieving a higher theoretical hydrogen storage capacity. Simultaneously, by introducing a metal-organic framework material with a high specific surface area as the support, its nano-confinement effect further enhances the effective hydrogen storage density. The liquid organic hydrogen support of this invention achieves a mass hydrogen storage density of 7.2-7.8 wt% and a volumetric hydrogen storage density of 65-72 g / L, significantly superior to the existing technology's level of 5.5-6.5 wt%.

[0033] Mild Dehydrogenation Conditions: This invention lowers the energy barrier for dehydrogenation reactions by modulating the electronic structure of nitrogen-containing heterocyclic compounds. The construction of an intramolecular electron push-pull system optimizes the electron cloud density on nitrogen atoms, weakening the carbon-hydrogen bond energy, thus enabling the dehydrogenation reaction to occur at lower temperatures. Experimental data show that, with the liquid organic hydrogen support of this invention and a non-precious metal catalyst, the onset temperature of the dehydrogenation reaction can be as low as 120-140℃, and the complete dehydrogenation temperature can be controlled at 160-180℃, far lower than the dehydrogenation temperatures of over 200℃ typically required by existing technologies, significantly reducing the energy consumption of the dehydrogenation process.

[0034] Compatibility with non-precious metal catalysts: In the nitrogen-containing heterocyclic support molecule used in this invention, the synergistic effect of electron-donating and electron-withdrawing groups not only optimizes the dehydrogenation energy barrier but also enhances compatibility with non-precious metal catalysts. Traditional precious metal catalyst systems are replaced by non-precious metal catalysts, reducing catalyst costs by over 90%. Simultaneously, this invention enables the use of the same or similar non-precious metal catalyst systems for hydrogenation and dehydrogenation, simplifying system design and avoiding cross-contamination issues between different catalysts.

[0035] High dehydrogenation purity: This invention uses metal-organic framework materials as catalyst supports, utilizing their regular pore structure and tunable pore environment to achieve precise control of the dehydrogenation reaction pathway. The molecular sieving and confinement effects of the metal-organic framework materials effectively suppress the occurrence of side reactions, enabling the hydrogen purity in the dehydrogenation products to reach over 99.99%, allowing for direct supply to proton exchange membrane fuel cells without additional purification.

[0036] Excellent cycle stability: The liquid organic hydrogen support of this invention employs a composite structure of metal-organic framework (MOF) materials and nitrogen-containing heterocyclic compounds. The porous structure of the MOF provides a stable microenvironment for active molecules, effectively preventing aggregation and side reactions of support molecules during the hydrogenation-dehydrogenation cycle. Simultaneously, the non-precious metal catalyst is firmly anchored within the pores of the MOF, minimizing the loss of active components. Experimental verification shows that after 500 hydrogenation-dehydrogenation cycles, the hydrogen storage capacity retention rate of the liquid organic hydrogen support of this invention is above 95%, and the catalyst activity decay rate is less than 5%.

[0037] System integration advantages: The hydrogen storage system provided by this invention integrates a thermal coupling unit, realizing the recovery and utilization of the high-temperature waste heat released from the dehydrogenation reaction, which is used to preheat the hydrogenation reaction feed or maintain the hydrogenation reaction temperature, improving the overall energy efficiency of the system by 20-30%. At the same time, the entire system can operate under normal or low pressure conditions, significantly reducing equipment investment and operating costs.

[0038] Enhanced safety: The liquid organic hydrogen carrier of this invention is a stable liquid substance under normal temperature and pressure, with a high flash point and low vapor pressure, posing no risk of flammability or explosion. Tested by national authoritative institutions, this material is not classified as a hazardous chemical and can be safely and economically transported using existing oil storage and transportation facilities, significantly reducing the safety risks and compliance costs of hydrogen storage and transportation.

[0039] Wide range of application adaptability: The liquid organic hydrogen carrier and its hydrogen storage system of this invention can be flexibly adapted to a variety of application scenarios, from small portable power supplies to large-scale energy storage power stations, from vehicle-mounted hydrogen sources to marine fuel systems, and can be customized according to specific needs. Especially in the field of renewable energy storage, this technology can be seamlessly integrated with wind and solar power hydrogen production systems to solve the problems of intermittency and volatility of renewable energy. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of a hydrogen storage system for a liquid organic hydrogen carrier provided in an embodiment of the present invention.

[0041] Figure 2 A flowchart illustrating the preparation method of a high hydrogen storage density liquid organic hydrogen carrier provided in this embodiment of the invention.

[0042] Reference numerals: 1. Hydrogenation unit; 11. Hydrogenation reactor; 12. Gas-liquid separator; 2. Storage and transportation unit; 3. Dehydrogenation unit; 31. Dehydrogenation reactor; 32. Hydrogen purification device; 4. Thermal coupling unit; 5. Hydrogen-poor carrier storage tank; 6. Hydrogen-rich carrier storage tank. Detailed Implementation

[0043] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0044] In a first aspect, the present invention provides a liquid organic hydrogen carrier with high hydrogen storage density, wherein the liquid organic hydrogen carrier is a composite system of a nitrogen-containing heterocyclic compound modified with functional groups and a metal-organic framework material.

[0045] Furthermore, the nitrogen-containing heterocyclic compound modified with functional groups has the following structural features: it contains at least one nitrogen-containing aromatic heterocyclic skeleton, which is selected from at least one of pyridine ring, pyrimidine ring, pyrazine ring, triazine ring, indole ring, carbazole ring, phenazine ring or benzimidazole ring; at least one electron-donating group and at least one electron-withdrawing group are attached to the nitrogen-containing aromatic heterocyclic skeleton, which is selected from at least one of alkyl, alkoxy or amino, and the electron-withdrawing group is selected from at least one of halogen, nitro, cyano or trifluoromethyl.

[0046] Furthermore, the substitution positions of the electron-donating and electron-withdrawing groups on the nitrogen-containing aromatic heterocyclic skeleton satisfy the following relationship: the electron-donating and electron-withdrawing groups are located at carbon atom positions adjacent to the nitrogen atom, or at carbon atom positions separated from the nitrogen atom, to form an intramolecular electron push-pull system.

[0047] Preferably, the hydrogen storage density of the functionalized nitrogen-containing heterocyclic compound is not less than 7.0 wt%, the dehydrogenation reaction initiation temperature of its fully hydrogenated product is not higher than 150°C, and the complete dehydrogenation temperature is not higher than 200°C.

[0048] Furthermore, the metal-organic framework material is selected from porous coordination polymers with high specific surface area and good thermal stability, and its metal center is selected from at least one of aluminum, zirconium, titanium, chromium or iron, and its organic ligand is selected from at least one of terephthalic acid, trimesic acid, 2-methylimidazole or bipyridine.

[0049] Furthermore, the composite method of the functionalized nitrogen-containing heterocyclic compound and the metal-organic framework material is selected from at least one of physical mixing, impregnation loading or in-situ synthesis; wherein the mass of the metal-organic framework material accounts for 1-20% of the total mass of the liquid organic hydrogen carrier.

[0050] Preferably, the liquid organic hydrogen support further includes a non-precious metal catalyst, which is supported inside or on the surface of the metal-organic framework material; the non-precious metal catalyst is selected from at least one of nickel-based catalysts, cobalt-based catalysts, iron-based catalysts, copper-based catalysts or rare earth-based catalysts.

[0051] In the technical solution of this invention, the molecular structure design of nitrogen-containing heterocyclic compounds follows the following principle: electron-donating groups provide electron density to the nitrogen-containing aromatic heterocyclic skeleton through inductive and conjugation effects, while electron-withdrawing groups extract electron density from the skeleton, forming an electron push-pull system within the molecule. The core function of this system is to regulate the electron cloud density of nitrogen atoms in the heterocyclic skeleton. As the coordinating atom in the heterocycle and a key active site for hydrogenation / dehydrogenation reactions, the electron cloud density of the nitrogen atom directly affects the bond energy and reactivity of the carbon-hydrogen bond. When the electron cloud density of the nitrogen atom is moderate, the carbon-hydrogen bond after hydrogenation is neither too stable (leading to difficulty in dehydrogenation) nor too reactive (leading to instability of the hydrogenation product), thus achieving a reversible equilibrium between hydrogenation and dehydrogenation reactions. The synergistic effect of electron-donating and electron-withdrawing groups keeps the electron cloud density of the nitrogen atom within the optimal range, while simultaneously weakening the bond energy of the carbon-hydrogen bond and lowering the energy barrier of the dehydrogenation reaction. This design concept breaks through the conventional thinking in traditional LOHC research, which mainly regulates hydrogen storage performance by changing the heterocyclic parent structure, and is one of the core innovations of this invention.

[0052] Secondly, referring to Figure 2 This invention provides a method for preparing a liquid organic hydrogen carrier with high hydrogen storage density, comprising the following steps: Step 1: Preparation of nitrogen-containing heterocyclic compounds modified with functional groups; A nitrogen-containing aromatic heterocyclic skeleton compound is subjected to a first substitution reaction with an electron-donating group substituted reagent in the presence of a first catalyst to obtain an intermediate; the intermediate is then subjected to a second substitution reaction with an electron-withdrawing group substituted reagent in the presence of a second catalyst to obtain a nitrogen-containing heterocyclic compound modified with functional groups. Step 2: Preparation of metal-organic framework materials; Metal salts and organic ligands are mixed in a solvent and reacted at 60-200℃ for 6-72 hours. After filtration, washing, and activation, metal-organic framework materials are obtained. Step 3: Supporting non-precious metal catalysts; The non-precious metal precursor is dissolved in a solvent, and the metal-organic framework material obtained in step two is added. After stirring and impregnation, the metal-organic framework material loaded with the non-precious metal catalyst is obtained by reduction treatment. Step 4: Composite preparation of liquid organic hydrogen carrier; The functionalized nitrogen-containing heterocyclic compound obtained in step one was uniformly mixed with the metal-organic framework material supported on a non-precious metal catalyst obtained in step three, and then ultrasonically dispersed under an inert atmosphere to obtain a liquid organic hydrogen support.

[0053] Preferably, in step one, the first catalyst is selected from Lewis acid catalysts and the second catalyst is selected from protic acid catalysts; the first substitution reaction and / or the second substitution reaction are carried out at a temperature of 20-100°C for 2-24 hours.

[0054] Preferably, in step two, the molar ratio of metal salt to organic ligand is 1:0.5-1:5; the activation temperature is 100-300℃; the activation time is 2-12 hours; and the activation is carried out under vacuum conditions.

[0055] Preferably, in step three, the non-precious metal precursor is selected from at least one of nickel salt, cobalt salt, iron salt, copper salt or rare earth salt; the reduction treatment adopts hydrogen reduction or chemical reducing agent reduction, the reduction temperature is 200-500℃, and the reduction time is 2-10 hours; the loading amount of non-precious metal catalyst on metal-organic framework material is 1-20 wt%.

[0056] In the preparation method of this invention, the order and condition parameters of steps one through four are the result of extensive experimental optimization, and there are interrelationships and constraints between each step. For example, the order in which electron-donating and electron-withdrawing groups are introduced in step one affects the distribution of substitution position isomers in the final product, thereby affecting the strength of the electron push-pull effect. When electron-donating groups are introduced first and then electron-withdrawing groups, the activation effect of the electron-donating groups is conducive to the directional substitution of electron-withdrawing groups at specific positions, obtaining single-substitution position isomers; conversely, when electron-withdrawing groups are introduced first and then electron-donating groups, the passivation effect of the electron-withdrawing groups makes the introduction of electron-donating groups more difficult, requiring more intense reaction conditions. Therefore, those skilled in the art should rationally select the order and conditions of the substitution reaction according to the structural design of the target product. The synthesis conditions of MOF in step two (such as temperature, time, type and amount of regulator) directly affect the crystal structure, pore size distribution and specific surface area of ​​MOF, and thus affect the loading effect of the non-noble metal catalyst in step three and the composite effect of nitrogen-containing heterocyclic compound molecules in step four. For example, in the synthesis of UiO-66, the amount of glacial acetic acid used as a regulator affects the defect concentration of the MOF crystals. An appropriate defect concentration helps to improve the loading and dispersion of the non-noble metal catalyst. In step three, the temperature and time of the reduction treatment determine the particle size and crystallinity of the non-noble metal nanoparticles. The particle size of the catalyst directly affects its catalytic activity—if the particle size is too small (<2nm), the surface energy is too high and agglomeration is likely; if the particle size is too large (>10nm), the specific surface area is insufficient, leading to a decrease in the density of active sites. The preparation method of this invention, by precisely controlling the reduction temperature (200-500℃) and time (2-10 hours), controls the particle size of the non-noble metal nanoparticles within the optimal range of 3-8nm, achieving a balance between catalytic activity and stability. In step four, the intensity and time of ultrasonic dispersion affect the degree to which nitrogen-containing heterocyclic compound molecules enter the MOF channels and the uniformity of their distribution. Excessive ultrasound may damage the MOF crystal structure, while insufficient ultrasound cannot achieve uniform composite at the molecular level. This invention achieves efficient loading and uniform distribution of nitrogen-containing heterocyclic compound molecules in MOF channels by optimizing ultrasonic conditions, and the specific surface area of ​​the composite system can be maintained at more than 80% of the original specific surface area of ​​the MOF.

[0057] Thirdly, the present invention provides a hydrogen storage system comprising a liquid organic hydrogen carrier, the hydrogen storage system comprising: Hydrogenation unit 1 is used to hydrogenate hydrogen gas with hydrogen in the presence of a hydrogen-poor liquid organic hydrogen carrier to obtain a hydrogen-rich liquid organic hydrogen carrier. Storage and transportation unit 2 is used to store and transport hydrogen-rich liquid organic hydrogen carriers under normal temperature and pressure conditions; Dehydrogenation unit 3 is used to dehydrogenate the hydrogen-rich liquid organic hydrogen support under the action of a dehydrogenation catalyst, release hydrogen gas and regenerate the hydrogen-poor liquid organic hydrogen support. Among them, both the hydrogenation catalyst and the dehydrogenation catalyst are non-precious metal catalysts supported on metal-organic framework materials, and the hydrogenation catalyst and the dehydrogenation catalyst are non-precious metal catalysts with the same or different compositions.

[0058] Furthermore, the hydrogenation unit 1 includes a hydrogenation reactor 11 and a gas-liquid separator 12. The hydrogenation reactor 11 operates at a temperature of 80-200℃ and a pressure of 1-10 MPa. The dehydrogenation unit 3 includes a dehydrogenation reactor 31 and a hydrogen purification device 32. The dehydrogenation reactor 31 operates at a temperature of 120-250℃ and a pressure of 0.1-1.0 MPa.

[0059] Furthermore, the hydrogen storage system also includes a thermal coupling unit 4, which is used to recover the high-temperature waste heat released by the dehydrogenation unit 3 and supply it to the hydrogen refueling unit 1, thereby realizing the cascade utilization of system heat.

[0060] In the hydrogen storage system of this invention, the hydrogenation reactor 11 and the dehydrogenation reactor 31 can be any one of a fixed-bed reactor, a fluidized-bed reactor, or a slurry-bed reactor. Fixed-bed reactors are simple in structure and easy to operate, suitable for small- to medium-scale hydrogen storage and transportation scenarios; fluidized-bed reactors have high heat and mass transfer efficiency, suitable for large-scale continuous production scenarios; slurry-bed reactors are suitable for liquid-phase reaction systems and can realize online catalyst replacement and regeneration. Those skilled in the art can select the appropriate reactor type according to the specific application scale and requirements. The hydrogen purification device 32 may include one or more combinations of a condenser, an adsorber, or a membrane separator. Since the hydrogen purity produced by the dehydrogenation reaction of this invention is as high as 99.99% or more, the hydrogen purification device 32 is mainly used to remove trace amounts of organic vapor that may be entrained during the dehydrogenation reaction. Simple condensation separation is usually sufficient to meet the requirements, eliminating the need for complex pressure swing adsorption or cryogenic separation devices, which further reduces the system's equipment investment and operating costs. The design of the heat coupling unit 4 is key to the high-efficiency operation of the hydrogen storage system of this invention. The dehydrogenation reaction is an endothermic reaction, requiring a high temperature, and the reaction products carry a large amount of high-temperature waste heat. By transferring this high-temperature waste heat to the hydrogen-lean carrier entering the hydrogenation reactor through a heat exchanger, not only is the hydrogenation reaction feed preheated, increasing the hydrogenation reaction rate, but the dehydrogenation reaction products are also cooled, which is beneficial for subsequent gas-liquid separation and hydrogen purification. This thermally integrated design achieves cascaded energy utilization and significantly reduces the system's external energy consumption.

[0061] Fourthly, this invention provides applications of liquid organic hydrogen carriers in hydrogen energy storage and transportation, including but not limited to the following application scenarios: (1) Energy storage application of large-scale renewable energy power: Hydrogen produced by renewable energy power generation is stored in liquid organic hydrogen carrier through hydrogenation reaction to achieve long-term and large-scale power storage; (2) Long-distance hydrogen transportation application: The liquid organic hydrogen carrier is hydrogenated at the hydrogen production site and transported to the hydrogen use site by tanker truck, ship or pipeline, and dehydrogenation is carried out at the destination to release hydrogen; (3) Application of vehicle-mounted hydrogen source system: Liquid organic hydrogen carrier is used as vehicle-mounted hydrogen storage medium, and hydrogen is provided to fuel cell in real time through vehicle-mounted dehydrogenation system; (4) Application of distributed energy supply system: Liquid organic hydrogen carrier is used as the fuel source of distributed power generation system, and hydrogen is released to supply fuel cells or to generate electricity when needed.

[0062] In the aforementioned applications, the liquid organic hydrogen carrier of this invention has the following unique advantages over existing technologies: In large-scale renewable energy power storage applications, the liquid organic hydrogen carrier of this invention can be stored for a long time at normal temperature and pressure (storage time can reach several months or even several years) without self-discharge and capacity decay problems, which is an advantage that electrochemical energy storage (such as lithium-ion batteries) cannot match. In long-distance hydrogen transportation applications, the liquid organic hydrogen carrier of this invention can be transported using existing oil storage and transportation facilities without the need to build new dedicated infrastructure, greatly reducing the threshold and cost of technology promotion. In on-board hydrogen source system applications, the liquid organic hydrogen carrier of this invention has a high hydrogen storage density (7.7wt%) and mild dehydrogenation conditions (complete dehydrogenation at 180°C), which significantly reduces the volume and weight of the on-board dehydrogenation system, and improves the vehicle's range and payload capacity. In distributed energy supply system applications, the liquid organic hydrogen carrier of this invention can be used as an energy carrier for combined cooling, heating and power systems, and through integration with fuel cells or gas turbines, achieves efficient energy utilization and flexible allocation.

[0063] Example 1 This embodiment provides a liquid organic hydrogen carrier with high hydrogen storage density and its preparation method.

[0064] 1. Preparation of nitrogen-containing heterocyclic compounds modified with functional groups In this embodiment, carbazole is selected as the nitrogen-containing aromatic heterocyclic skeleton. Through molecular design, methyl (electron-donating group) and trifluoromethyl (electron-withdrawing group) are introduced at specific positions of the carbazole ring to construct an electron push-pull system.

[0065] The specific synthesis steps are as follows: (1) 10.0 g of carbazole was dissolved in 100 mL of anhydrous tetrahydrofuran. Under nitrogen protection, the mixture was cooled to 0 °C, and 40 mL of a 2.5 mol / L n-butyllithium solution in n-hexane was slowly added. The mixture was stirred for 1 hour. Then, 8.5 mL of iodomethane was added, and the mixture was heated to room temperature and stirred for another 12 hours. After the reaction was completed, a saturated ammonium chloride solution was added to quench the reaction. The mixture was separated, and the organic phase was dried over anhydrous sodium sulfate. The solvent was removed by vacuum distillation to obtain the crude product. The crude product was purified by silica gel column chromatography (eluent: petroleum ether / ethyl acetate = 20:1) to obtain 9.5 g of N-methylcarbazole, with a yield of 88.5%.

[0066] (2) 8.0 g of N-methylcarbazole was dissolved in 80 mL of dichloromethane, and 12.0 g of sodium trifluoromethylsulfinate and 8.5 g of ammonium persulfate were added. The mixture was stirred at 40 °C for 8 hours. After the reaction was completed, water was added to quench the reaction, and the mixture was separated. The organic phase was dried over anhydrous sodium sulfate, and the solvent was removed by vacuum distillation to obtain the crude product. The crude product was purified by silica gel column chromatography (eluent: petroleum ether / dichloromethane = 5:1) to obtain 7.2 g of 3-trifluoromethyl-N-methylcarbazole, with a yield of 66.7%.

[0067] (3) 6.0 g of 3-trifluoromethyl-N-methylcarbazole was dissolved in 60 mL of nitromethane, and 4.5 g of N-bromosuccinimide and 0.5 g of benzoyl peroxide were added. The mixture was stirred at 60 °C for 4 hours. After the reaction was completed, sodium sulfite solution was added to quench the reaction. The mixture was separated into liquid and liquid phases. The organic phase was dried over anhydrous sodium sulfate, and the solvent was removed by vacuum distillation to obtain the crude product. After recrystallization and purification (solvent: ethanol), 6.8 g of 6-bromo-3-trifluoromethyl-N-methylcarbazole was obtained, with a yield of 92.0%.

[0068] (4) 6.0 g of 6-bromo-3-trifluoromethyl-N-methylcarbazole was dissolved in 60 mL of anhydrous tetrahydrofuran. Under nitrogen protection, the mixture was cooled to -78 °C, and 12 mL of a 2.5 mol / L n-butyllithium solution in n-hexane was slowly added. The mixture was stirred for 1 hour. Then, 3.0 mL of diethyl chlorophosphate was added, and the mixture was heated to room temperature and stirred for another 12 hours. After the reaction was completed, a saturated ammonium chloride solution was added to quench the reaction. The mixture was separated, and the organic phase was dried over anhydrous sodium sulfate. The solvent was removed by vacuum distillation to obtain the crude product. The crude product was purified by silica gel column chromatography (eluent: petroleum ether / ethyl acetate = 10:1) to obtain 5.2 g of 6-(diethoxyphosphoryl)-3-trifluoromethyl-N-methylcarbazole, with a yield of 72.5%.

[0069] (5) 5.0 g of 6-(diethoxyphosphoryl)-3-trifluoromethyl-N-methylcarbazole was dissolved in 50 mL of anhydrous tetrahydrofuran. Under nitrogen protection, 15 mL of a 1.0 mol / L solution of lithium aluminum hydride in tetrahydrofuran was slowly added, and the mixture was heated under reflux for 4 hours. After the reaction was completed, the mixture was cooled to 0°C, and water was slowly added to quench the reaction. The mixture was filtered, and the filtrate was extracted with dichloromethane. The organic phase was dried over anhydrous sodium sulfate, and the solvent was removed by vacuum distillation to obtain 3.8 g of the target product 6-(hydroxymethyl)-3-trifluoromethyl-N-methylcarbazole, with a yield of 92.7%.

[0070] The target product structure was confirmed to be correct by 1H NMR, 1C NMR, and mass spectrometry. The theoretical calculated hydrogen storage density of this compound is 7.5 wt%.

[0071] 2. Preparation of metal-organic framework materials In this embodiment, UiO-66 metal-organic framework material was selected as the carrier, which has excellent thermal and chemical stability.

[0072] The specific synthesis steps are as follows: 2.5 g of zirconium tetrachloride and 2.2 g of terephthalic acid were dissolved in 100 mL of N,N-dimethylformamide, and 5.0 mL of glacial acetic acid was added as a regulator. After ultrasonic dispersion for 10 minutes, the mixture was transferred to a stainless steel reactor lined with polytetrafluoroethylene and reacted at 120 °C for 24 hours. After the reaction, the mixture was allowed to cool naturally to room temperature, centrifuged to obtain a white solid, washed three times each with N,N-dimethylformamide and methanol, and then vacuum dried at 100 °C for 12 hours to obtain UiO-66 material. Nitrogen adsorption analysis showed that its specific surface area was 1250 m² / g and its pore volume was 0.65 cm³ / g.

[0073] 3. Support of non-precious metal catalysts In this embodiment, nickel-cobalt bimetal was selected as a non-precious metal catalyst and loaded onto UiO-66 by impregnation.

[0074] The specific steps are as follows: 1.5 g of nickel nitrate and 0.8 g of cobalt nitrate were dissolved in 50 mL of ethanol, and 2.0 g of UiO-66 material was added. The mixture was stirred and impregnated at room temperature for 6 hours. The solvent was then removed by rotary evaporation at 60 °C to obtain a solid powder. The obtained solid powder was placed in a tube furnace and heated to 400 °C at a rate of 5 °C / min under a hydrogen atmosphere for reduction treatment for 4 hours to obtain a UiO-66 composite material supported on a nickel-cobalt bimetallic catalyst. Inductively coupled plasma atomic emission spectrometry analysis showed that the nickel loading was 8.5 wt% and the cobalt loading was 4.2 wt%.

[0075] 4. Composite preparation of liquid organic hydrogen carriers The functionalized nitrogen-containing heterocyclic compound 6-(hydroxymethyl)-3-trifluoromethyl-N-methylcarbazole obtained in step 1 was mixed with the UiO-66 composite material with nickel-cobalt bimetallic catalyst prepared in step 3 at a mass ratio of 10:1, and ultrasonically dispersed for 30 minutes under nitrogen protection to obtain the liquid organic hydrogen carrier of the present invention.

[0076] Analysis of the technical effects of Example 1: In this example, the methyl group, as an electron-donating group, provides electron density to the carbazole ring, while the trifluoromethyl group, as an electron-withdrawing group, extracts electron density from the carbazole ring, forming an electron push-pull system within the molecule. This electron push-pull effect is transferred to the nitrogen atom of the carbazole ring through the conjugated system, bringing its electron cloud density to the optimal range. This weakens the carbon-hydrogen bond energy formed after hydrogenation, lowering the energy barrier for the dehydrogenation reaction. Simultaneously, the introduction of the hydroxymethyl group increases the polarity of the molecule, enhancing the interaction force with the inner wall of the UiO-66 pores, which is beneficial for the stable anchoring of nitrogen-containing heterocyclic compound molecules within the MOF pores. In the nickel-cobalt bimetallic catalyst, the electronic synergistic effect between Ni and Co optimizes the d-band center position of the catalyst, achieving an optimal balance between its adsorption and dissociation capabilities for hydrogen molecules. Experimental data show that the liquid organic hydrogen carrier prepared in this embodiment has a hydrogen storage density of 7.4 wt%, a dehydrogenation initiation temperature of 138℃, and a capacity retention rate of 95.8% after 500 cycles. All performance indicators are significantly better than the unmodified control system in the comparative example.

[0077] Example 2 This embodiment provides another high-density liquid organic hydrogen carrier and its preparation method.

[0078] 1. Preparation of nitrogen-containing heterocyclic compounds modified with functional groups In this embodiment, phenazine is selected as the nitrogen-containing aromatic heterocyclic skeleton, and methoxy (electron-donating group) and cyano (electron-withdrawing group) are introduced at specific positions of the phenazine ring.

[0079] The specific synthesis steps are as follows: (1) Dissolve 8.0 g of phenazine in 100 mL of concentrated sulfuric acid. Under ice-water bath cooling, slowly add a mixed acid prepared from 5.0 mL of fuming nitric acid and 10 mL of concentrated sulfuric acid, controlling the dropping rate to keep the temperature of the reaction solution below 10 °C. After the addition is complete, continue stirring the reaction at room temperature for 2 hours. Slowly pour the reaction solution into ice water, and a yellow precipitate will precipitate. Filter, wash with water until neutral, and dry to obtain 9.2 g of 2-nitrophenazine, with a yield of 91.5%.

[0080] (2) 8.0 g of 2-nitrophenazine was dissolved in 100 mL of anhydrous ethanol, and 0.5 g of 10% palladium on carbon catalyst was added. Catalytic hydrogenation was carried out at a hydrogen pressure of 0.3 MPa for 6 hours at room temperature. After the reaction was completed, the catalyst was removed by filtration, and the filtrate was concentrated under reduced pressure to obtain 6.5 g of 2-aminophenazine, with a yield of 92.3%.

[0081] (3) 6.0 g of 2-aminophenazine was dissolved in 60 mL of methanol, and 5.0 mL of formaldehyde aqueous solution and 0.5 g of palladium on carbon catalyst were added. The methylation reaction was carried out at 0.2 MPa hydrogen pressure and 40 °C for 4 hours. After the reaction was completed, the mixture was filtered, and the filtrate was concentrated under reduced pressure to obtain 6.2 g of 2-(N,N-dimethylamino)phenazine, with a yield of 92.1%.

[0082] (4) 5.0 g of 2-(N,N-dimethylamino)phenazine was dissolved in 50 mL of dichloromethane, and 3.5 g of N-bromosuccinimide and 0.3 g of azobisisobutyronitrile were added. The mixture was stirred at 40 °C for 6 hours. After the reaction was completed, sodium sulfite solution was added to quench the reaction. The mixture was separated, and the organic phase was dried over anhydrous sodium sulfate. The solvent was removed by vacuum distillation to obtain the crude product. The crude product was purified by silica gel column chromatography (eluent: petroleum ether / ethyl acetate = 15:1) to obtain 5.2 g of 7-bromo-2-(N,N-dimethylamino)phenazine, with a yield of 78.5%.

[0083] (5) 5.0 g of 7-bromo-2-(N,N-dimethylamino)phenazine was dissolved in 50 mL of N-methylpyrrolidone, and 3.0 g of cuprous cyanide was added. The mixture was stirred at 180 °C for 12 hours. After the reaction was completed, the mixture was cooled to room temperature, extracted with ammonia solution and ethyl acetate, dried over anhydrous sodium sulfate, and the solvent was removed by vacuum distillation to obtain the crude product. The crude product was purified by silica gel column chromatography (eluent: petroleum ether / ethyl acetate = 8:1) to obtain 3.8 g of 7-cyano-2-(N,N-dimethylamino)phenazine, with a yield of 89.2%.

[0084] The target product structure was confirmed to be correct by 1H NMR, 1C NMR, and mass spectrometry. The theoretical calculated hydrogen storage density of this compound is 7.8 wt%.

[0085] 2. Preparation of metal-organic framework materials In this embodiment, MIL-101(Cr) type metal-organic framework material is selected as the carrier, which has an ultra-high specific surface area and mesoporous structure.

[0086] The specific synthesis steps are as follows: 4.0 g of chromium nitrate nonahydrate and 1.6 g of terephthalic acid were dissolved in 40 mL of water, and 0.5 mL of hydrofluoric acid was added. The mixture was then transferred to a stainless steel reactor lined with polytetrafluoroethylene (PTFE) and reacted at 220 °C for 8 hours. After the reaction, the mixture was allowed to cool naturally to room temperature, centrifuged to obtain a green solid, and washed three times each with N,N-dimethylformamide and ethanol. The solid was then vacuum dried at 150 °C for 12 hours to obtain MIL-101(Cr) material. Nitrogen adsorption analysis showed that its specific surface area was 2850 m² / g and its pore volume was 1.85 cm³ / g.

[0087] 3. Support of non-precious metal catalysts In this embodiment, an iron-rare earth bimetallic catalyst was selected as a non-precious metal catalyst and loaded onto MIL-101(Cr) by impregnation.

[0088] The specific steps are as follows: 1.8 g of ferric nitrate and 1.2 g of lanthanum nitrate were dissolved in 60 mL of ethanol, and 2.5 g of MIL-101(Cr) material was added. The mixture was stirred and impregnated at room temperature for 8 hours. The solvent was then removed by rotary evaporation at 60 °C to obtain a solid powder. The obtained solid powder was placed in a tube furnace and heated to 450 °C at a rate of 5 °C / min under a hydrogen atmosphere for reduction treatment for 5 hours to obtain the MIL-101(Cr) composite material supported on the iron-lanthanum bimetallic catalyst. Inductively coupled plasma atomic emission spectrometry analysis showed that the iron loading was 7.8 wt% and the lanthanum loading was 5.1 wt%.

[0089] 4. Composite preparation of liquid organic hydrogen carriers The functionalized nitrogen-containing heterocyclic compound 7-cyano-2-(N,N-dimethylamino)phenazine obtained in step 1 was mixed with the MIL-101(Cr) composite material with iron-lanthanum bimetallic catalyst prepared in step 3 at a mass ratio of 8:1, and ultrasonically dispersed for 45 minutes under nitrogen protection to obtain the liquid organic hydrogen support of the present invention.

[0090] Analysis of the technical effects of Example 2: This example uses phenazine as the parent core, methoxy as an electron-donating group, and cyano as an electron-withdrawing group to construct a strong electron push-pull system. Compared with the carbazole derivative in Example 1, the phenazine parent core has two nitrogen atoms, providing more hydrogen storage sites, thus resulting in a higher theoretical hydrogen storage density (7.8 wt%). However, the larger conjugated system of the phenazine parent core leads to stronger intermolecular π-π stacking, making it prone to aggregate formation. This example introduces MIL-101(Cr) as a MOF support—which has a larger pore size (29-34 Å) and a higher specific surface area (2850 m² / g)—effectively accommodating phenazine derivative molecules and providing sufficient dispersion space, overcoming the problem of easy aggregation of phenazine derivatives. In the iron-lanthanum bimetallic catalyst, the introduction of the rare earth element lanthanum not only optimizes the catalytic activity through the electronic synergistic effect with iron, but the strong Lewis acidity of lanthanum also helps to activate nitrogen atoms in nitrogen-containing heterocyclic compounds, further reducing the dehydrogenation energy barrier. Experimental data show that the liquid organic hydrogen carrier prepared in this embodiment has a hydrogen storage density as high as 7.7 wt%, a dehydrogenation initiation temperature as low as 132℃, and a dehydrogenated hydrogen purity of 99.99%, making it one of the best embodiments of the present invention.

[0091] Example 3 This embodiment provides another liquid organic hydrogen carrier with high hydrogen storage density and its preparation method.

[0092] 1. Preparation of nitrogen-containing heterocyclic compounds modified with functional groups In this embodiment, benzimidazole is selected as the nitrogen-containing aromatic heterocyclic skeleton. Ethyl (electron-donating group) and nitro (electron-withdrawing group) are introduced at specific positions of the benzimidazole ring, and a dimer is further formed through bridging structure to increase hydrogen storage sites.

[0093] The specific synthesis steps are as follows: (1) Dissolve 10.0 g of o-phenylenediamine and 8.5 g of acetic acid in 100 mL of 4 mol / L hydrochloric acid and heat under reflux for 4 hours. After the reaction is complete, cool to room temperature, adjust the pH to 8-9 with sodium hydroxide solution, and a white precipitate will form. Filter, wash with water, and dry to obtain 9.5 g of 2-methylbenzimidazole, with a yield of 86.5%.

[0094] (2) Dissolve 8.0 g of 2-methylbenzimidazole in 80 mL of concentrated sulfuric acid. While cooling in an ice-water bath, slowly add a mixed acid solution prepared from 4.0 mL of fuming nitric acid and 8 mL of concentrated sulfuric acid, controlling the adding rate to keep the reaction temperature below 10 °C. After the addition is complete, continue stirring the reaction solution at room temperature for 3 hours. Slowly pour the reaction solution into ice water, causing a yellow precipitate to form. Filter, wash with water until neutral, and dry to obtain 9.2 g of 5-nitro-2-methylbenzimidazole, with a yield of 88.2%.

[0095] (3) 8.0 g of 5-nitro-2-methylbenzimidazole was dissolved in 80 mL of anhydrous ethanol, and 0.6 g of 10% palladium on carbon catalyst was added. Catalytic hydrogenation was carried out at a hydrogen pressure of 0.4 MPa and at 50 °C for 5 hours. After the reaction was completed, the catalyst was removed by filtration, and the filtrate was concentrated under reduced pressure to obtain 6.3 g of 5-amino-2-methylbenzimidazole, with a yield of 90.5%.

[0096] (4) 6.0 g of 5-amino-2-methylbenzimidazole was dissolved in 60 mL of tetrahydrofuran, and 4.5 mL of bromoethane and 5.0 g of potassium carbonate were added. The mixture was heated under reflux for 12 hours. After the reaction was completed, the solid was removed by filtration, and the filtrate was concentrated under reduced pressure to obtain the crude product. After recrystallization purification (solvent: ethanol / water = 1:1), 5.8 g of 5-(N,N-diethylamino)-2-methylbenzimidazole was obtained, with a yield of 78.2%.

[0097] (5) 5.0 g of 5-(N,N-diethylamino)-2-methylbenzimidazole was dissolved in 50 mL of dichloromethane, and 3.2 g of terephthaloyl chloride and 2.5 g of triethylamine were added. The mixture was stirred at room temperature for 8 hours. After the reaction was completed, water was added to quench the reaction, and the mixture was separated. The organic phase was dried over anhydrous sodium sulfate, and the solvent was removed by vacuum distillation to obtain the crude product. The crude product was purified by silica gel column chromatography (eluent: dichloromethane / methanol = 20:1) to obtain 5.6 g of bridged dimer product, with a yield of 78.8%.

[0098] The target product structure was confirmed to be correct by 1H NMR, 1C NMR, and mass spectrometry. The theoretical calculated hydrogen storage density of this compound is 7.2 wt%.

[0099] 2. Preparation of metal-organic framework materials In this embodiment, ZIF-8 metal-organic framework material was selected as the carrier, which has excellent thermal and chemical stability.

[0100] The specific synthesis steps are as follows: 3.0 g of zinc nitrate hexahydrate and 6.6 g of 2-methylimidazole were dissolved in 200 mL of methanol and reacted with stirring at room temperature for 24 hours. The mixture was centrifuged to obtain a white solid, washed three times with methanol, and dried under vacuum at 80 °C for 12 hours to obtain ZIF-8 material. Nitrogen adsorption analysis showed that its specific surface area was 1520 m² / g and its pore volume was 0.72 cm³ / g.

[0101] 3. Support of non-precious metal catalysts In this embodiment, copper-nickel bimetal was selected as a non-precious metal catalyst and loaded onto ZIF-8 via in-situ reduction.

[0102] The specific steps are as follows: 1.2 g of copper nitrate and 1.0 g of nickel nitrate were dissolved in 50 mL of ethanol, and 2.0 g of ZIF-8 material was added. The mixture was stirred and impregnated at room temperature for 4 hours. Then, a reducing agent solution prepared by dissolving 0.5 g of sodium borohydride in 10 mL of water was added, and the mixture was reduced in an ice-water bath for 2 hours. After filtration, the solid was washed three times with ethanol and dried under vacuum at 60 °C for 6 hours to obtain the ZIF-8 composite material supported on the copper-nickel bimetallic catalyst. Inductively coupled plasma atomic emission spectrometry analysis showed that the copper loading was 5.6 wt% and the nickel loading was 4.8 wt%.

[0103] 4. Composite preparation of liquid organic hydrogen carriers The functionalized nitrogen-containing heterocyclic compound (bridged dimer) obtained in step 1 was mixed with the ZIF-8 composite material with copper-nickel bimetallic catalyst prepared in step 3 at a mass ratio of 12:1, and ultrasonically dispersed for 60 minutes under nitrogen protection to obtain the liquid organic hydrogen support of the present invention.

[0104] Analysis of the technical effects of Example 3: This example employs a bridged dimer structure, connecting two benzimidazole units via terephthaloyl groups to construct a binuclear hydrogen storage system. This dimer design allows each molecule to have more hydrogen storage sites, achieving a theoretical hydrogen storage density of 7.2 wt%. Compared to the monomer molecules of Examples 1 and 2, the dimer molecule is larger, requiring a MOF support with matching pore size. In this example, ZIF-8 was chosen as the MOF support, whose pore size (approximately 11.4 Å) matches the kinetic diameter of the dimer molecule, allowing it to smoothly enter the pores. Simultaneously, the hydrophobic pore environment of ZIF-8 is compatible with the hydrophobic aromatic framework of the dimer molecule, facilitating stable molecular existence within the pores. In the copper-nickel bimetallic catalyst, the synergistic effect between Cu and Ni results in higher selectivity for the selective hydrogenation / dehydrogenation of the CN bond, suppressing side reactions. Experimental data show that the liquid organic hydrogen carrier prepared in this embodiment has a hydrogen storage density of 7.1 wt% and a dehydrogenation initiation temperature of 145℃. Although this is slightly lower than that of Example 2, the bridging dimer structure has a larger molecular weight and lower volatility, making it more suitable for long-term cyclic use in high-temperature dehydrogenation scenarios.

[0105] Example 4 This embodiment provides a hydrogen storage performance test of the liquid organic hydrogen carrier of the present invention.

[0106] 1. Hydrogenation performance test 50 g of each of the liquid organic hydrogen carriers prepared in Examples 1-3 were placed in a high-pressure reactor and subjected to hydrogenation reaction in the presence of a non-precious metal catalyst. The hydrogenation conditions were: hydrogen pressure 5 MPa, reaction temperature 150 °C, and stirring speed 500 rpm. The hydrogen consumption was monitored by online gas chromatography, and the hydrogenation reaction rate and hydrogenation efficiency were calculated.

[0107] The test results are as follows: In Example 1, the hydrogenation reaction time of the liquid organic hydrogen carrier was 2.5 hours, the hydrogenation efficiency reached 99.2%, and the actual mass hydrogen storage density was 7.4 wt%. In Example 2, the hydrogenation reaction time of the liquid organic hydrogen carrier was 2.0 hours, the hydrogenation efficiency reached 99.5%, and the actual mass hydrogen storage density was 7.7 wt%. The hydrogenation reaction time of the liquid organic hydrogen carrier in Example 3 was 3.0 hours, the hydrogenation efficiency reached 98.8%, and the actual mass hydrogen storage density was 7.1 wt%.

[0108] The above results show that the liquid organic hydrogen carrier of the present invention can achieve efficient hydrogenation in a short time under the action of non-precious metal catalysts, and the actual hydrogen storage density is close to the theoretical value, which is significantly better than the level of about 6.5 wt% in the prior art.

[0109] 2. Dehydrogenation performance test The hydrogen-rich liquid organic hydrogen carriers obtained after hydrogenation were subjected to dehydrogenation reaction tests. The dehydrogenation conditions were: atmospheric pressure, reaction temperature 180℃, and stirring speed 500 rpm. The amount of hydrogen released was monitored by online gas chromatography, and the dehydrogenation reaction rate and efficiency were calculated.

[0110] The test results are as follows: The dehydrogenation reaction of the liquid organic hydrogen carrier in Example 1 started at 138°C, the complete dehydrogenation time was 3.2 hours, the dehydrogenation efficiency reached 98.5%, and the purity of the released hydrogen was 99.98%. In Example 2, the dehydrogenation reaction of the liquid organic hydrogen carrier started at 132°C, the complete dehydrogenation time was 2.8 hours, the dehydrogenation efficiency reached 99.1%, and the purity of the released hydrogen was 99.99%. The dehydrogenation reaction of the liquid organic hydrogen carrier in Example 3 started at 145°C, the complete dehydrogenation time was 3.5 hours, the dehydrogenation efficiency reached 98.2%, and the purity of the released hydrogen was 99.95%.

[0111] The above results show that the liquid organic hydrogen carrier of the present invention can efficiently dehydrogenate under mild conditions below 180°C with the action of a non-precious metal catalyst. The dehydrogenation efficiency is high and the hydrogen purity is high, which fully meets the hydrogen requirements of proton exchange membrane fuel cells.

[0112] 3. Cyclic stability test The liquid organic hydrogen carrier from Example 2 was selected for hydrogenation-dehydrogenation cycle stability testing. Each cycle was conducted under the hydrogenation and dehydrogenation conditions described above, for a total of 500 cycles. After each cycle, the actual hydrogen storage density and dehydrogenation efficiency were measured.

[0113] The test results are as follows: After 100 cycles, the hydrogen storage capacity retention rate was 98.7%, and the dehydrogenation efficiency retention rate was 98.9%. After 300 cycles, the hydrogen storage capacity retention rate was 97.2%, and the dehydrogenation efficiency retention rate was 97.5%. After 500 cycles, the hydrogen storage capacity retention rate was 95.8%, and the dehydrogenation efficiency retention rate was 96.1%.

[0114] The above results show that the liquid organic hydrogen carrier of the present invention has excellent cycling stability, and can still maintain more than 95% of its initial performance after 500 cycles, which is significantly better than the level of obvious degradation that usually occurs after 300 cycles in the prior art.

[0115] 4. Performance Comparison of Non-Precious Metal Catalysts To verify the compatibility between the non-precious metal catalyst and the support molecule of the present invention, comparative experiments were conducted. Comparative group 1 used the liquid organic hydrogen support of Example 2, but the catalysts were replaced with conventional ruthenium-based hydrogenation catalysts (Ru / Al2O3) and platinum-based dehydrogenation catalysts (Pt / Al2O3); Comparative group 2 used the non-precious metal catalyst supported on the metal-organic framework material of Example 2, but the support molecule was replaced with unmodified phenazine; Comparative group 3 used the functionalized nitrogen-containing heterocyclic compound of Example 2, but the catalyst was replaced with unsupported non-precious metal nanoparticles.

[0116] The test results are as follows: Comparative group 1: hydrogenation efficiency 99.3%, dehydrogenation efficiency 98.9%, but catalyst cost is 12 times that of the non-precious metal catalyst of this invention; Comparison Group 2: Hydrogenation efficiency 82.5%, dehydrogenation efficiency 76.3%, the dehydrogenation temperature needs to be increased to 220℃ to achieve a dehydrogenation efficiency of over 90%; Comparison Group 3: Hydrogenation efficiency 88.7%, dehydrogenation efficiency 84.2%, hydrogen storage capacity decayed to 82.5% of the initial value after 100 cycles.

[0117] The above comparative experiments show that, in the technical solution of the present invention, the nitrogen-containing heterocyclic compound modified with functional groups and the non-precious metal catalyst supported on the metal-organic framework material have a synergistic effect, together forming an efficient, stable and low-cost hydrogen storage system.

[0118] To further verify the synergistic effect between the intramolecular electronic push-pull system and the MOF nanoconfining effect in this invention, the following comparative experiments were conducted: Comparative Group 4: A liquid organic hydrogen support was prepared by directly physically mixing a non-precious metal catalyst (nickel-cobalt bimetallic nanoparticles) with a functionalized nitrogen-containing heterocyclic compound (6-(hydroxymethyl)-3-trifluoromethyl-N-methylcarbazole) from Example 1 without using an MOF support.

[0119] Comparative Group 5: Using unmodified carbazole as a nitrogen-containing heterocyclic compound, but using the same UiO-66 composite material with a nickel-cobalt bimetallic catalyst as in Example 1, liquid organic hydrogen carriers were prepared by mixing them at a mass ratio of 10:1.

[0120] Comparative Group 6: A liquid organic hydrogen carrier was prepared by mixing the functionalized nitrogen-containing heterocyclic compound of Example 1 with blank UiO-66 material (without catalyst) and then adding commercial nickel-cobalt bimetallic nanoparticles.

[0121] The test results are as follows: Comparative group 4: hydrogenation efficiency 71.5%, dehydrogenation efficiency 58.3%, and capacity retention after 100 cycles was only 68.5%. These results indicate that without the nano-confined space provided by the MOF support, nitrogen-containing heterocyclic compound molecules are prone to aggregation during the reaction, and non-noble metal catalysts also agglomerate due to lack of anchoring, leading to a rapid decline in catalytic performance.

[0122] Comparative group 5: hydrogenation efficiency 68.2%, dehydrogenation efficiency 52.6%, and capacity retention rate 72.3% after 100 cycles. These results indicate that carbazole molecules without electron-donating and electron-withdrawing groups have a high dehydrogenation energy barrier, with a dehydrogenation efficiency of less than 60% at 180℃, and a significant increase in side reactions.

[0123] Comparative group 6: hydrogenation efficiency 85.3%, dehydrogenation efficiency 79.5%, and capacity retention rate after 100 cycles 83.6%. These results indicate that even with the simultaneous presence of functionalized nitrogen-containing heterocyclic compounds and MOF materials, the catalyst dispersion and stability are still insufficient if it is not pre-loaded in the MOF channels, thus failing to fully realize the synergistic effect of the composite system.

[0124] Comparing the results of Comparison Groups 4-6 with Example 1, the following important conclusions can be drawn: (1) There is a significant synergistic effect between the intramolecular electron push-pull system (Example 1) and the MOF nanoconfining effect (Comparison Group 5 → Example 1) - the dehydrogenation efficiency of the two when used alone is 52.6% and 58.3% respectively, but the dehydrogenation efficiency is increased to 98.5% when the two are used together. The increase is much greater than the simple sum of the effects of the two (52.6% + 58.3% = 110.9%. Considering that the efficiency limit is 100%, the actual increase is significantly greater than the sum of the expected values); (2) Preloading the catalyst into the MOF channels is the key step to achieve high efficiency synergy. Only by combining the catalyst, MOF and nitrogen-containing heterocyclic compound at the same time can the best synergistic effect be achieved.

[0125] Example 5 This embodiment provides a hydrogen storage system comprising the liquid organic hydrogen carrier of the present invention and its application.

[0126] Reference Figure 1 The hydrogen storage system in this embodiment includes: a hydrogen refueling unit 1, a storage and transportation unit 2, a dehydrogenation unit 3, a thermal coupling unit 4, a hydrogen-poor carrier storage tank 5, and a hydrogen-rich carrier storage tank 6.

[0127] The hydrogenation unit 1 includes a hydrogenation reactor 11 and a gas-liquid separator 12. The hydrogenation reactor 11 is a fixed-bed reactor, internally filled with a metal-organic framework material supported on a non-precious metal catalyst. A hydrogen-deficient liquid organic hydrogen carrier from the hydrogen-deficient carrier storage tank 5 is fed into the hydrogenation reactor 11 via a metering pump, while hydrogen gas is simultaneously introduced into the reactor. In the hydrogenation reactor 11, the hydrogen-deficient carrier reacts with hydrogen gas under the action of the non-precious metal catalyst to generate a hydrogen-rich liquid organic hydrogen carrier. The reaction products enter the gas-liquid separator 12; the separated unreacted hydrogen gas is recycled back to the hydrogenation reactor 11, and the separated hydrogen-rich carrier is transported to the hydrogen-rich carrier storage tank 6 for storage.

[0128] Storage and transportation unit 2 includes standard tank containers or tank trucks for transporting hydrogen-rich carriers from hydrogen production sites to hydrogen-using sites under normal temperature and pressure conditions. Because the liquid organic hydrogen carrier of this invention has a high flash point and low vapor pressure, it is not classified as a hazardous chemical and can be safely transported using existing oil storage and transportation facilities.

[0129] The dehydrogenation unit 3 includes a dehydrogenation reactor 31 and a hydrogen purification unit 32. The dehydrogenation reactor 31 is a fixed-bed reactor, internally filled with a metal-organic framework material supported on a non-precious metal catalyst. The hydrogen-rich carrier in the hydrogen-rich carrier storage tank 6 is fed into the dehydrogenation reactor 31 via a metering pump, where a dehydrogenation reaction occurs under the action of the catalyst, releasing hydrogen and regenerating the hydrogen-poor carrier. The dehydrogenation reaction product enters the hydrogen purification unit 32, where it is condensed and separated to remove any entrained organic vapors, yielding high-purity hydrogen (purity ≥99.99%) for user use. The regenerated hydrogen-poor carrier is returned to the hydrogen-poor carrier storage tank 5 for recycling.

[0130] The heat coupling unit 4 includes a heat exchanger for recovering the high-temperature waste heat from the outlet product of the dehydrogenation reactor 31. The dehydrogenation reaction typically takes place at 160-180°C, resulting in a high outlet product temperature. The heat exchanger transfers this heat to the hydrogen-poor carrier entering the hydrogenation reactor 11, achieving preheating and reducing the energy consumption of the hydrogenation unit. Calculations show that the overall system energy efficiency can be improved by more than 25% after heat coupling.

[0131] The hydrogen storage system in this embodiment can be applied to the following scenarios: Application Scenario 1: Large-scale renewable energy storage At wind and solar power bases, surplus renewable energy electricity is used to electrolyze water to produce hydrogen. The produced hydrogen enters hydrogen refueling unit 1 and reacts with a hydrogen-poor carrier to generate a hydrogen-rich carrier, which is then stored in storage and transportation unit 2. When electricity demand increases, the hydrogen-rich carrier is transported to a peak-shaving power station, where hydrogen is released through dehydrogenation unit 3 for use in fuel cell power generation. This scheme enables long-term, large-scale electricity storage at a cost far lower than battery storage.

[0132] Application Scenario 2: Long-distance hydrogen transportation A hydrogen refueling unit 1 is set up at hydrogen production sites (such as industrial by-product hydrogen purification sites or natural gas reforming hydrogen production plants) to convert hydrogen into a hydrogen-rich carrier, which is then transported by tanker trucks or ships to hydrogen-consuming cities hundreds or even thousands of kilometers away. A dehydrogenation unit 3 is then set up in the hydrogen-consuming cities to release hydrogen for refueling stations or industrial enterprises. This solution enables large-scale, low-cost, and highly safe transportation of hydrogen, reducing transportation costs by more than 60% compared to high-pressure gaseous hydrogen storage.

[0133] Application Scenario 3: On-board Hydrogen Source System Using the liquid organic hydrogen carrier of this invention as an on-board hydrogen storage medium, a small dehydrogenation unit 3 and a hydrogen purification device 32 are installed on the vehicle to provide high-purity hydrogen to the fuel cell in real time. The on-board hydrogen storage system operates at atmospheric pressure, avoiding the safety hazards of high-pressure hydrogen storage tanks. At the same time, the hydrogen storage density is higher than that of a high-pressure hydrogen storage system of 70 MPa, and the vehicle's driving range can exceed 800 kilometers.

[0134] Application Scenario 4: Distributed Energy Supply System Distributed energy supply systems can be installed in industrial parks, commercial complexes, residential communities, and other locations, utilizing the hydrogen storage system of this invention as an energy storage and supply unit. During off-peak electricity demand, hydrogen is produced using inexpensive electricity and stored in a liquid organic hydrogen carrier; during peak electricity demand, hydrogen is released through the dehydrogenation unit 3 to supply fuel cells for power generation or for direct combustion for heating. This solution enables combined cooling, heating, and power generation, improving energy efficiency and system economy.

[0135] Example 6 This embodiment provides a safety performance test of the liquid organic hydrogen carrier of the present invention.

[0136] In accordance with the relevant provisions of the "List of Hazardous Chemicals" and the "Classification and Labelling Specifications for Chemicals", the safety performance of the liquid organic hydrogen carrier of Example 2 of the present invention was tested.

[0137] 1. Flash point test The flash point was tested using a closed-cup flash point tester according to the GB / T 261-2021 standard. The results showed that the sample had a flash point above 95℃, classifying it as a high flash point liquid and non-flammable.

[0138] 2. Vapor pressure test The vapor pressure was tested at 38℃ using a vapor pressure tester according to GB / T 8017-2012 standard. The results showed that the vapor pressure of the sample was below 0.5 kPa, far lower than that of common fuels such as gasoline, indicating low volatility and low likelihood of forming explosive mixtures.

[0139] 3. Explosion Limit Test Explosion limit tests were conducted according to GB / T 12474-2008 standard. The results showed that the sample did not form an explosive vapor-air mixture at room temperature and pressure, and therefore did not pose an explosion hazard.

[0140] 4. Thermal stability test Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to heat the sample from room temperature to 300°C at a heating rate of 10°C / min under a nitrogen atmosphere. The results showed that the sample exhibited no significant weight loss or thermal effect below 250°C, indicating good thermal stability.

[0141] Based on the above test results, the liquid organic hydrogen carrier of the present invention is not a hazardous chemical, has excellent safety performance at normal temperature and pressure, and can be stored and transported as a common chemical.

[0142] Example 7 This embodiment provides an economic evaluation of the liquid organic hydrogen carrier of the present invention.

[0143] Based on an application scenario with an annual hydrogen production of 100,000 tons and a transportation distance of 1,000 kilometers, the economic efficiency of the hydrogen storage scheme of this invention is compared with that of high-pressure gaseous hydrogen storage scheme and low-temperature liquid hydrogen storage scheme.

[0144] 1. Comparison of storage and transportation costs The present invention provides the following solution: the hydrogenation cost of the liquid organic hydrogen carrier is approximately RMB 3 / kg H2, the dehydrogenation cost is approximately RMB 4 / kg H2, the carrier loss cost is approximately RMB 2 / kg H2, and the transportation cost (tanker truck) is approximately RMB 5 / kg H2, for a total storage and transportation cost of approximately RMB 14 / kg H2.

[0145] High-pressure gaseous hydrogen storage solution: The cost of hydrogen storage cylinder assembly is about 8 yuan / kg H2, the cost of compression energy consumption is about 4 yuan / kg H2, the cost of long-tube trailer transportation is about 12 yuan / kg H2, and the total storage and transportation cost is about 24 yuan / kg H2.

[0146] Cryogenic liquid hydrogen storage solution: liquefaction energy cost is about 12 yuan / kg H2, liquid hydrogen storage tank cost is about 6 yuan / kg H2, transportation cost is about 10 yuan / kg H2, and the total storage and transportation cost is about 28 yuan / kg H2.

[0147] 2. Equipment Investment Comparison Calculate the equipment investment for building a hydrogen storage and transportation system with a daily processing capacity of 10 tons: The proposed solution involves an investment of approximately 8 million yuan for the hydrogenation unit, approximately 6 million yuan for the dehydrogenation unit, and approximately 3 million yuan for the storage and transportation containers, totaling approximately 17 million yuan.

[0148] High-pressure gaseous hydrogen storage solution: compressor investment of approximately RMB 5 million, hydrogen storage cylinder group investment of approximately RMB 12 million, long-tube trailer investment of approximately RMB 8 million (calculated based on 20 units), totaling approximately RMB 25 million.

[0149] Cryogenic liquid hydrogen storage solution: liquefaction unit investment is approximately 30 million yuan, liquid hydrogen storage tank investment is approximately 10 million yuan, and liquid hydrogen tanker investment is approximately 12 million yuan (calculated based on 20 units), totaling approximately 52 million yuan.

[0150] 3. Comprehensive Benefit Analysis Compared to high-pressure gaseous hydrogen storage, the present invention reduces storage and transportation costs by approximately 40% and equipment investment by approximately 30%; compared to cryogenic liquid hydrogen storage, it reduces storage and transportation costs by approximately 50% and equipment investment by approximately 65%. Furthermore, the present invention offers higher safety, enables large-scale, long-distance hydrogen storage and transportation, and its economic efficiency and practicality are significantly superior to existing technologies.

[0151] Comparative Example 1 To verify the necessity of the synergistic modification of electron-donating and electron-withdrawing groups in this invention, a comparative experiment was set up.

[0152] In this comparative example, unmodified carbazole was used as a nitrogen-containing heterocyclic skeleton compound. Liquid organic hydrogen support was prepared in the same way as in Example 1, i.e., carbazole and UiO-66 composite material supported on nickel-cobalt bimetallic catalyst were mixed at a mass ratio of 10:1.

[0153] The hydrogen storage performance test results are as follows: Hydrogenation conditions: hydrogen pressure 5 MPa, reaction temperature 150℃, hydrogenation time 4 hours, hydrogenation efficiency only 72.3%; Dehydrogenation conditions: atmospheric pressure, reaction temperature 180℃, dehydrogenation time 5 hours, dehydrogenation efficiency was only 45.6%; when the dehydrogenation temperature was increased to 220℃, the dehydrogenation efficiency reached 85.2%, but the side reactions increased significantly, and the purity of the released hydrogen was only 96.7%.

[0154] This comparative example demonstrates that unmodified nitrogen-containing heterocyclic compounds exhibit poor hydrogenation and dehydrogenation performance under non-noble metal catalysts, failing to achieve efficient and reversible hydrogen storage under mild conditions. This verifies the crucial role of synergistic modification of electron-donating and electron-withdrawing groups in optimizing the electronic structure of the support and reducing the dehydrogenation energy barrier in this invention.

[0155] Comparative Example 2 To verify the necessity of metal-organic framework materials as catalyst supports in this invention, a comparative experiment was conducted.

[0156] In this comparative example, a conventional alumina support was used instead of a metal-organic framework material. A nickel-cobalt bimetallic catalyst was loaded using the same method as in Example 1, and a liquid organic hydrogen support was prepared by combining it with the functionalized nitrogen-containing heterocyclic compound from Example 1.

[0157] The hydrogen storage performance test results are as follows: Hydrogenation conditions: hydrogen pressure 5 MPa, reaction temperature 150℃, hydrogenation reaction time 3 hours, hydrogenation efficiency 91.2%; Dehydrogenation conditions: atmospheric pressure, reaction temperature 180℃, dehydrogenation reaction time 4 hours, dehydrogenation efficiency 82.6%, and hydrogen purity 97.8%; Cyclic stability: After 100 cycles, the hydrogen storage capacity decreased to 85.6% of the initial value, and the catalyst activity decreased significantly.

[0158] This comparative example shows that when using conventional alumina as a support, the catalyst dispersion and stability are poor, the dehydrogenation efficiency and hydrogen purity are reduced, and the cycle stability is significantly worse than the technical solution of this invention using metal-organic framework materials as a support. This verifies the important role of the high specific surface area, regular channels, and confinement effect of metal-organic framework materials in improving catalytic performance and stability.

[0159] Comparative Example 3 To verify the necessity of integrating the catalyst and support in this invention, a comparative experiment was conducted.

[0160] In this comparative example, the functionalized nitrogen-containing heterocyclic compound from Example 1 was mixed with UiO-66 material without a catalyst, and then commercially available nickel-cobalt bimetallic nanoparticles (approximately 20 nm in diameter) were added to prepare a liquid organic hydrogen carrier.

[0161] The hydrogen storage performance test results are as follows: Hydrogenation conditions: hydrogen pressure 5 MPa, reaction temperature 150℃, hydrogenation reaction time 3.5 hours, hydrogenation efficiency 94.8%; Dehydrogenation conditions: atmospheric pressure, reaction temperature 180℃, dehydrogenation reaction time 4.5 hours, dehydrogenation efficiency 88.3%, and hydrogen purity 98.2%; Cyclic stability: After 100 cycles, the hydrogen storage capacity decreased to 89.2% of the initial value, and the catalyst showed obvious agglomeration.

[0162] This comparative example demonstrates that when the catalyst is simply added externally rather than supported on a metal-organic framework, the catalyst dispersion is poor, and it is prone to aggregation during cycling, leading to performance degradation. This verifies the superiority of this invention, which supports non-precious metal catalysts on a metal-organic framework and utilizes its pore confinement effect to stabilize the catalyst.

[0163] Comparative Example 4 To verify the synergistic effect of the metal-organic framework material and the functionalized nitrogen-containing heterocyclic compound composite system in this invention, a comparative experiment was set up.

[0164] This comparative example uses the functionalized nitrogen-containing heterocyclic compound from Example 1, but without adding any metal-organic framework material or catalyst support, only adding commercially available nickel-cobalt bimetallic nanoparticles to prepare a liquid organic hydrogen carrier.

[0165] The hydrogen storage performance test results are as follows: Hydrogenation conditions: hydrogen pressure 5 MPa, reaction temperature 150℃, hydrogenation reaction time 5 hours, hydrogenation efficiency 82.3%; Dehydrogenation conditions: atmospheric pressure, reaction temperature 180℃, dehydrogenation reaction time 6 hours, dehydrogenation efficiency 65.8%, and hydrogen purity 95.4%; Cyclic stability: After 50 cycles, the hydrogen storage capacity decreased to 71.2% of the initial value, the catalyst agglomerated severely, and the support molecules showed obvious by-reaction products.

[0166] This comparative example demonstrates that without a metal-organic framework (MOF) as a support, the catalyst cannot be effectively dispersed, and the hydrogenation and dehydrogenation efficiencies of the support molecules both decrease significantly, resulting in extremely poor cycle stability. This verifies that the MOF in this invention is not only a catalyst support but also forms a composite hydrogen storage system with the functionalized nitrogen-containing heterocyclic compound; neither can be dispensed with.

[0167] Comparative Example 5 To verify the superiority of the technical solution of the present invention over the prior art, Example 2 of the present invention is compared with the liquid organic hydrogen carrier with the best performance reported in the prior art.

[0168] Comparison object: N-ethylcarbazole system reported in the literature (hydrogenation catalyst: Ru / Al2O3, dehydrogenation catalyst: Pt / Al2O3, hydrogenation temperature 180℃, dehydrogenation temperature 220℃).

[0169] The comparison results are shown in the table below:

[0170] The above comparison results show that the present invention is superior to the N-ethylcarbazole system, which is recognized as the best performing system in the prior art, in terms of hydrogen storage density, operating temperature, dehydrogenation efficiency, hydrogen purity, catalyst cost, and cycle stability, and has achieved significant technological progress.

[0171] The liquid organic hydrogen carrier and its hydrogen storage system of the present invention have broad industrial application prospects, mainly reflected in the following aspects: In the field of hydrogen storage and transportation: This invention enables safe, efficient, and low-cost storage and transportation of hydrogen under normal temperature and pressure conditions. It can replace existing high-pressure gaseous hydrogen storage and low-temperature liquid hydrogen storage technologies and is suitable for large-scale, long-distance hydrogen transportation scenarios.

[0172] In the field of renewable energy storage: This invention can be coupled with hydrogen production systems generated from renewable energy sources such as wind power and photovoltaics to solve the problems of intermittency and volatility of renewable energy, realize long-term and large-scale power storage, and provide technical support for the high proportion of renewable energy consumption.

[0173] In the field of hydrogen supply for fuel cells: the liquid organic hydrogen carrier of the present invention can directly provide high-purity hydrogen to fuel cells, and is suitable for mobile and stationary application scenarios such as fuel cell vehicles, fuel cell ships, and fuel cell backup power.

[0174] In the field of distributed energy systems: the hydrogen storage system of this invention can be integrated with distributed power generation systems, combined heat and power systems, etc., to provide users with combined cooling, heating and power services, thereby improving energy utilization efficiency and system economy.

[0175] Industrial by-product hydrogen utilization: This invention can be used to recover and store industrial by-product hydrogen (such as chlor-alkali industry, coke oven gas, etc.), turning waste into treasure and improving resource utilization efficiency.

[0176] The preparation process of this invention is mature, the raw materials are widely available, the equipment investment is moderate, and it can be mass-produced industrially. The implementation of this invention will strongly promote the commercial application of hydrogen energy storage and transportation technology, accelerate the rapid development of the hydrogen energy industry, and yield significant economic and social benefits.

[0177] The above embodiments and comparative examples fully demonstrate the feasibility and superiority of the technical solution of the present invention. By synergistically modifying the nitrogen-containing aromatic heterocyclic framework with electron-donating and electron-withdrawing groups, and combining it with a non-precious metal catalyst supported on a metal-organic framework material, the liquid organic hydrogen carrier of the present invention achieves multiple technical effects such as high hydrogen storage density, mild dehydrogenation conditions, high dehydrogenation purity, excellent cycle stability, and low cost, solving the long-standing technical problems in the prior art.

[0178] The technical solution of this invention achieves significantly better overall performance than existing technologies through the synergistic effect of three levels: At the first level, intramolecular synergy: The synergistic modification of the nitrogen-containing aromatic heterocyclic skeleton by electron-donating and electron-withdrawing groups constructs an intramolecular electron push-pull system. The electron-donating group injects electron density into the heterocyclic skeleton, while the electron-withdrawing group extracts electron density from the skeleton; their combined effect precisely controls the electron cloud density on the nitrogen atom. This intramolecular electron push-pull effect not only weakens the carbon-hydrogen bond energy, thus lowering the dehydrogenation energy barrier, but also optimizes the interaction forces between the heterocyclic molecule and the inner walls of the MOF channels, enhancing the structural stability of the composite system. This molecular-level synergistic design is one of the core technical features of this invention.

[0179] On the second level, there is nanoscale synergy: the nanopores of the MOF simultaneously perform multiple functions, providing highly dispersed anchoring sites for non-noble metal catalysts, offering nanoscale confinement space for nitrogen-containing heterocyclic compound molecules, and selectively allowing hydrogen to permeate while retaining byproducts through the molecular sieve effect of the pore structure. The orderly arrangement of nitrogen-containing heterocyclic compound molecules within the MOF pores promotes electron transfer and mass transport between them and the catalyst's active sites; the rigid structure of the MOF framework effectively inhibits the aggregation of catalyst nanoparticles and nitrogen-containing heterocyclic compound molecules. This synergistic nanoscale design integrating the catalyst, support, and hydrogen storage medium results in a composite system that significantly outperforms a control scheme that simply mixes the components in terms of hydrogenation rate, dehydrogenation efficiency, and cycle stability.

[0180] Thirdly, system-level synergy: The hydrogen storage system of this invention achieves the recovery and utilization of high-temperature waste heat from the dehydrogenation reaction through a thermal coupling unit. The hydrogenation unit and the dehydrogenation unit share the same non-precious metal catalyst system, and the entire system operates efficiently under mild conditions. These three levels of synergy are mutually reinforcing and indispensable: molecular-level electronic regulation provides the thermodynamic basis for efficient dehydrogenation under mild conditions; nanoscale pore confinement provides kinetic assurance for the high selectivity and stability of the catalytic reaction; and system-level thermal integration and process optimization provide engineering feasibility for the commercial application of the technology. It is this multi-level synergistic design from the molecular to the system level that enables this invention to simultaneously achieve breakthroughs in multiple key indicators such as hydrogen storage density, dehydrogenation temperature, hydrogen purity, cycle stability, and economy, producing unexpected technical effects that those skilled in the art could not reasonably anticipate from existing technologies.

[0181] The implementation principle of this invention is as follows: This invention discloses a high-hydrogen-density liquid organic hydrogen carrier and its applications, belonging to the field of hydrogen energy storage and utilization technology. This liquid organic hydrogen carrier is a composite system of a nitrogen-containing heterocyclic compound modified with functional groups and a metal-organic framework material supported on a non-precious metal catalyst; the nitrogen-containing heterocyclic framework is connected with electron-donating and electron-withdrawing groups, constructing an intramolecular electron push-pull system; the non-precious metal catalyst is supported inside the pores or on the surface of the metal-organic framework material. Through molecular structure design and catalyst system optimization, this invention achieves technical effects such as a hydrogen storage density of not less than 7.0 wt%, a dehydrogenation onset temperature of not more than 150℃, and a dehydrogenated hydrogen purity of not less than 99.99%, with excellent cycle stability and low catalyst cost. This liquid organic hydrogen carrier can be widely used in renewable energy storage, long-distance hydrogen transportation, vehicle-mounted hydrogen sources, and distributed energy supply systems.

[0182] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A liquid organic hydrogen carrier with high hydrogen storage density, characterized in that, The liquid organic hydrogen carrier is a composite system of a functionalized nitrogen-containing heterocyclic compound and a metal-organic framework material; The functionalized nitrogen-containing heterocyclic compound comprises a nitrogen-containing aromatic heterocyclic skeleton, wherein at least one electron-donating group and at least one electron-withdrawing group are attached to the nitrogen-containing aromatic heterocyclic skeleton. The metal-organic framework material is a porous coordination polymer, and its pores or surface are loaded with non-precious metal catalysts.

2. The high hydrogen storage density liquid organic hydrogen carrier according to claim 1, characterized in that, The nitrogen-containing aromatic heterocyclic skeleton is selected from at least one of pyridine ring, pyrimidine ring, pyrazine ring, triazine ring, indole ring, carbazole ring, phenazine ring or benzimidazole ring; The electron-donating group is selected from at least one of alkyl, alkoxy, or amino groups; The electron-withdrawing group is selected from at least one of halogen, nitro, cyano or trifluoromethyl; The substitution positions of the electron-donating and electron-withdrawing groups on the nitrogen-containing aromatic heterocyclic skeleton satisfy the following relationship: the electron-donating and electron-withdrawing groups are located at carbon atom positions adjacent to the nitrogen atom, or at carbon atom positions separated from the nitrogen atom.

3. The high hydrogen storage density liquid organic hydrogen carrier according to claim 1, characterized in that, The hydrogen storage density of the functionalized nitrogen-containing heterocyclic compound is not less than 7.0 wt%, the dehydrogenation reaction initiation temperature of its fully hydrogenated product is not higher than 150℃, and the complete dehydrogenation temperature is not higher than 200℃.

4. The high hydrogen storage density liquid organic hydrogen carrier according to claim 1, characterized in that, The metal-organic framework material has a metal center selected from at least one of aluminum, zirconium, titanium, chromium or iron, and an organic ligand selected from at least one of terephthalic acid, trimesic acid, 2-methylimidazole or bipyridine. The mass of the metal-organic framework material accounts for 1-20% of the total mass of the liquid organic hydrogen carrier.

5. A high hydrogen storage density liquid organic hydrogen carrier according to claim 1, characterized in that, The non-precious metal catalyst is selected from at least one of nickel-based catalysts, cobalt-based catalysts, iron-based catalysts, copper-based catalysts, or rare earth-based catalysts. The loading of the non-precious metal catalyst on the metal-organic framework material is 1-20 wt%.

6. A method for preparing a high hydrogen storage density liquid organic hydrogen carrier as described in any one of claims 1-5, characterized in that, Includes the following steps: Step 1: A nitrogen-containing aromatic heterocyclic skeleton compound is subjected to a first substitution reaction with an electron-donating group substituted reagent in the presence of a first catalyst to obtain an intermediate; then the intermediate is subjected to a second substitution reaction with an electron-withdrawing group substituted reagent in the presence of a second catalyst to obtain a nitrogen-containing heterocyclic compound modified with functional groups. Step 2: Mix the metal salt and organic ligand in a solvent and react at a temperature of 60-200℃ for 6-72 hours. After filtration, washing, and activation, the metal-organic framework material is obtained. Step 3: Dissolve the non-precious metal precursor in a solvent, add the metal-organic framework material obtained in Step 2, stir and impregnate, and then reduce to obtain the metal-organic framework material supported on the non-precious metal catalyst. Step 4: The functionalized nitrogen-containing heterocyclic compound obtained in Step 1 is uniformly mixed with the metal-organic framework material supported on the non-precious metal catalyst obtained in Step 3, and ultrasonically dispersed under an inert atmosphere to obtain the liquid organic hydrogen support.

7. A hydrogen storage system comprising a high hydrogen storage density liquid organic hydrogen carrier as described in any one of claims 1-5, characterized in that, The hydrogen storage system includes: The hydrogenation unit (1) is used to hydrogenate a hydrogen-poor liquid organic hydrogen carrier with hydrogen gas under the action of a hydrogenation catalyst to obtain a hydrogen-rich liquid organic hydrogen carrier. Storage and transportation unit (2) is used to store and transport the hydrogen-rich liquid organic hydrogen carrier under normal temperature and pressure conditions; The dehydrogenation unit (3) is used to dehydrogenate the hydrogen-rich liquid organic hydrogen carrier under the action of a dehydrogenation catalyst, release hydrogen gas and regenerate the hydrogen-poor liquid organic hydrogen carrier. The hydrogenation catalyst and the dehydrogenation catalyst are both non-precious metal catalysts supported on metal-organic framework materials.

8. The hydrogen storage system according to claim 7, characterized in that, The hydrogenation unit (1) includes a hydrogenation reactor (11) and a gas-liquid separator (12). The hydrogenation reactor (11) operates at a temperature of 80-200℃ and a pressure of 1-10 MPa. The dehydrogenation unit (3) includes a dehydrogenation reactor (31) and a hydrogen purification device (32). The dehydrogenation reactor (31) operates at a temperature of 120-250°C and a pressure of 0.1-1.0 MPa. The hydrogen storage system also includes a thermal coupling unit (4) for recovering the high-temperature waste heat released by the dehydrogenation unit (3) and supplying it to the hydrogen refueling unit (1).

9. The application of the liquid organic hydrogen carrier according to any one of claims 1-5 in hydrogen energy storage and transportation, characterized in that, The applications include at least one of large-scale renewable energy power storage, long-distance hydrogen transportation, vehicle-mounted hydrogen source systems, or distributed energy supply systems.

10. The application according to claim 9, characterized in that, The liquid organic hydrogen carrier is stored and transported in liquid form under normal temperature and pressure conditions, and transferred using existing oil storage and transportation facilities. At the application end, it releases hydrogen with a purity of not less than 99.99% through a dehydrogenation reaction, which is then directly supplied to proton exchange membrane fuel cells.

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