Protonated covalent organic framework materials, methods of making and uses thereof

Abstract

The application provides a method for improving hydrogen adsorption performance of a covalent organic framework material, comprising the following steps: providing a covalent organic framework material containing an imine bond, and placing the covalent organic framework material in hydrochloric acid vapor for protonation treatment; the application also provides a protonated covalent organic framework material and a use of the protonated covalent organic framework material as a hydrogen storage medium.

Description

Technical Field

[0001] This invention relates to the technical field of hydrogen storage materials, and in particular to a method for preparing and applying a protonated covalent organic framework material. Background Technology

[0002] With increasing human demand for and use of energy, non-renewable energy sources such as fossil fuels (coal, oil, and natural gas) are becoming increasingly depleted. Large-scale development and utilization of renewable energy has become a crucial component of the energy strategies of countries worldwide. Hydrogen energy, with its abundant sources, environmental friendliness, renewability, and high energy density, is considered the most ideal energy source for the future. One of the biggest technological obstacles to using hydrogen as a fuel is its storage. Hydrogen is gaseous at room temperature and pressure, with a density only 1 / 14 that of air. A car traveling 482.7 km (300 miles) would require approximately 5–13 kg of hydrogen, and at room temperature and pressure, 5 kg of hydrogen occupies a staggering 56 cubic meters of gas. 3 Space is needed. Clearly, the application of hydrogen-powered vehicles requires more practical and feasible hydrogen storage methods.

[0003] Many hydrogen storage methods have been proposed, the most common being compressed gas storage and liquid hydrogen storage. While these methods are easy to implement and technologically mature, they each have their own significant drawbacks. For example, storing and transporting high-pressure gaseous hydrogen in steel cylinders poses certain risks, as hydrogen can dissolve and permeate into the steel walls under high pressure, causing hydrogen embrittlement. This presents a significant safety hazard for long-term hydrogen storage, and the storage capacity is relatively small and the cost is high. Liquid hydrogen has a higher density than gaseous hydrogen, but its storage temperature is -252.8℃, meaning that storage requires a large amount of energy and excellent insulation equipment, thus placing high demands on equipment and increasing costs. Therefore, finding new hydrogen storage materials and methods is extremely important, and porous materials have recently become a major focus of research.

[0004] Adsorption is a phenomenon where a gas partially remains after contact with a solid. It is broadly classified into two categories: chemisorption and physisorption, based on differences in adsorption forces, heat of adsorption, adsorption rate, selectivity, adsorption temperature, and pressure. The main chemical hydrogen storage materials are metal hydrides. Chemisorption is typically related to activation energy, meaning that molecules attracted to the surface must first overcome an energy barrier to bind tightly to the surface. Therefore, chemisorption processes generally have high activation energies, slow desorption processes, and some metal hydrides are even non-renewable. Physicosorption hydrogen storage, on the other hand, involves accumulating gas molecules on the surface of a material without chemically reacting with it. It relies on intermolecular interactions between the gas and the material to adsorb and store the gas. Because the forces between hydrogen molecules and the pore surface of the adsorbent are weak, physisorption hydrogen storage exhibits faster adsorption-desorption kinetics, and commonly operates at lower temperatures and higher pressures. Major physisorption hydrogen storage materials include porous materials such as zeolites, activated carbon, carbon nanotubes, and metal-organic frameworks.

[0005] Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are both rapidly developing porous crystalline materials in recent years. MOFs are primarily composed of nitrogen- and oxygen-containing porous organic ligands of aromatic acids or bases, which hybridize with inorganic metal centers through coordination bonds to form a three-dimensional network structure, hence they are also known as porous coordination polymers (PCPs). Due to their similarity to the pore structure of zeolites, but with a more flexible framework, they are also called "soft zeolites." The first generation of MOF materials was synthesized in the mid-1990s. At this time, the pore structure of MOF materials still required the support of guest molecules; if the guest molecules were removed, the framework would collapse, resulting in an unstable pore structure. Subsequently, researchers began assembling anionic, cationic, and neutral ligands into coordination polymers, synthesizing a new generation of MOF materials. The organic ligands in these MOF materials are mainly carboxyl-containing organic anionic ligands, sometimes mixed with nitrogen-containing heterocyclic organic neutral ligands. This generation of MOF materials overcomes the shortcomings of the previous generation. When guest molecules are introduced or removed, or when certain external stimuli (such as pressure) are applied, the framework structure of the material changes to some extent but does not collapse. Covalent organic frameworks (COFs) are a new type of framework structure material synthesized in recent years, which can have one-dimensional, two-dimensional, and three-dimensional crystal structures. These materials contain only organic structural units in their frameworks, connected by strong covalent bonds (such as CC, CO, BO). COF-6, COF-8, and COF-10 have a layered two-dimensional structure similar to graphite. The other three materials, COF-102, COF-105, and COF-108, are three-dimensional materials formed by introducing triangular and tetrahedral nodes. These materials have large porosity and specific surface area, good thermal stability, and are easy to functionalize. Compared with MOFs, COFs have lower crystal density, making them promising for more effective applications in gas storage. Meanwhile, the covalent bonds connecting the COF building blocks are more stable than the coordinate bonds in MOFs, giving the material higher stability and potential for further modification.

[0006] Currently, for physical hydrogen storage materials, MOF / COF materials mainly improve hydrogen storage performance by increasing their specific surface area and pore volume.

[0007] For example, MOF-5 is a typical example among many MOF compounds, whose framework [Zn4O(bdc)3] is composed of Zn4O(-COO)6 units and terephthalate bdc. 2-A three-dimensional network with a PCU topology is formed by interconnected components. Patent document WO2005003622A1 discloses a hydrogen storage container with added MOF-5 material. Under a pressure of 3 bar, the container with added MOF-5 has a hydrogen storage weight that is 1.46 times greater than that without added MOF-5.

[0008] For physical hydrogen storage materials, there is a trade-off between high specific surface area and material stability. High specific surface area MOF / COF materials are often prepared using highly reversible reactions (such as boric acid-based COFs; three-dimensional boric acid COFs have a BET of ~5000), which also means they have a stronger tendency to decompose (poor chemical stability). Secondly, pore collapse is also an unavoidable problem for high specific surface area MOF / COF materials. For example, BET specific surface areas exceeding 5000 m²... 2 The activation process of MOFs with a specific surface area of ​​7200 g is often quite complex, requiring methods such as supercritical CO2. This demonstrates that excessively high specific surface areas present problems in terms of both chemical stability and pore structure collapse. Furthermore, even current MOF materials with a specific surface area of ​​7200 g cannot meet the hydrogen storage density requirements for hydrogen storage systems set by the U.S. Department of Energy. Therefore, finding solutions beyond increasing the BET specific surface area is a worthwhile direction to explore for MOF / COF-based physical hydrogen storage materials. Further adsorption thermal regulation based on high specific surface area COF materials is a good starting point, but how to achieve adsorption thermal regulation at the material level requires systematic exploration.

[0009] Combining the properties of physical and chemical hydrogen storage materials is key to developing efficient hydrogen storage materials and represents a major challenge in the field. To address the hydrogen storage characteristics required for hydrogen storage and transportation, current scientific research suggests combining the high adsorption heat of chemical hydrogen storage materials with the low adsorption heat of porous physical hydrogen storage materials. This involves adjusting the internal pore environment of porous materials to enhance their interaction with hydrogen molecules. Summary of the Invention

[0010] To address the aforementioned technical problems, the inventors have discovered that significantly increasing the hydrogen adsorption heat and hydrogen storage capacity of covalent organic framework (COF) compounds can be achieved through protonation treatment of COFs. By selecting imine-linked COFs with excellent stability and treating them with hydrochloric acid vapor to protonate the imine bonds, the adsorption heat of the imine sites is increased, thereby enhancing the hydrogen storage capacity of the COF.

[0011] A first aspect of the present invention provides a protonated covalent organic framework material, wherein the covalent organic framework material has a structure as shown in formula (I):

[0012] in,

[0013] R1 and R2 are each independently selected from H, C1 to C6 alkyl, methoxy, and ethoxy groups.

[0014] A is a six-membered aromatic ring or a heterocyclic aromatic ring.

[0015]

[0016] J is a protonable site, and

[0017] One or more Js have been protonated.

[0018] A second aspect of the present invention provides a method for improving the hydrogen adsorption performance of a covalent organic framework material, comprising the following steps: protonating the covalent organic framework material containing imine bonds with hydrochloric acid vapor.

[0019] Another aspect of the invention includes the use of the above-described protonated covalent organic framework material or the covalent organic framework material obtained according to the above method as a hydrogen storage medium.

[0020] The beneficial effects of this invention are as follows:

[0021] 1. In the prior art, the modification of hydrogen storage materials is usually achieved by increasing the specific surface area of ​​the material or by doping the material to improve its hydrogen storage performance. This process is complex and costly. However, the present invention improves the hydrogen adsorption performance of imine-type covalent organic frameworks through a simple protonation treatment, which is beneficial to solving the problem of low hydrogen storage capacity of existing hydrogen storage materials.

[0022] 2. The low adsorption heat of existing covalent organic framework compounds hinders their widespread adoption and application as hydrogen storage media. This invention proposes a simple protonation treatment method to improve the hydrogen storage capacity of porous physical hydrogen storage materials by increasing their adsorption heat, which is conducive to promoting the practical application of covalent organic frameworks in the field of hydrogen storage.

[0023] 3. The method of the present invention is universal and shows good implementation effect on covalent organic frameworks with different structures.

[0024] 4. Existing methods for modifying hydrogen storage materials are limited to milligram-level laboratory scales, while the method of this invention can achieve scale-up preparation, which is beneficial for industrial applications. Attached Figure Description

[0025] Figure 1 A schematic diagram illustrating the preparation process of the protonated covalent organic framework material according to the present invention is shown.

[0026] Figure 2 The X-ray diffraction (XRD) patterns of the covalent organic framework material before and after protonation treatment are shown.

[0027] Figure 3 The infrared spectra of covalent organic framework materials before and after protonation treatment are shown.

[0028] Figure 4 The hydrogen adsorption-desorption isotherms of the covalent organic framework material before and after protonation treatment are shown.

[0029] Figure 5 The graphs showing the hydrogen adsorption amount of the covalent organic framework material before and after protonation treatment versus pressure (bar) are presented. Detailed Implementation

[0030] As used in this article, the terms covalent organic framework, covalent organic skeleton, and COF are used interchangeably.

[0031] The terms “protonation” and “acidification” are used interchangeably, referring to the binding of a proton (or a positively charged hydrogen ion) to a protonable site in a compound.

[0032] The term "protonable site" refers to an electron-donating group in a compound that can bind to a proton in an acid and undergo protonation.

[0033] Furthermore, unless otherwise defined, it should be understood that all terms used in this specification have the same meaning as commonly understood by those skilled in the art.

[0034] To make the technical solutions and advantages of the present invention clearer and easier to understand, the present invention will be clearly and completely described below through specific embodiments and in conjunction with the accompanying drawings. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0035] It should also be noted that, in this invention, the terms "exemplary" or "for example" are used to indicate that something is being described as an example, illustration, or illustration. Any implementation or design described as "exemplary" or "for example" in this invention should not be construed as being more preferred or advantageous than other implementations or design options. Specifically, the use of terms such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0036] In this invention, "at least one" means one or more, and "more than one" means two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone, where A and B can be singular or plural. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, at least one of a, b, or c can represent: a, b, c, a and b, a and c, b and c, a and b and c, where a, b, and c can be single or multiple. It is worth noting that "at least one" can also be interpreted as "one or more items".

[0037] As described above, the present invention provides a protonated covalent organic framework material having the structure shown in formula (I), wherein J is a protonable site. The protonable site refers to any group capable of protonation. Preferably, J is an imine bond, and the C atom of each imine bond is connected to a benzene ring containing R1 and R2 in formula (I).

[0038] According to the protonated covalent organic framework material of the present invention, wherein protonation is achieved by contacting the covalent organic framework material with hydrochloric acid vapor. The hydrochloric acid vapor refers to hydrogen chloride gas that is substantially free of water molecules or contains no water molecules, obtained after the volatilization of concentrated hydrochloric acid.

[0039] In this document, the term "substantially free of water molecules" means that gaseous water molecules account for less than 7% of the volume of the mixture of hydrogen chloride gas and gaseous water molecules, preferably less than 3% of the volume, and more preferably less than 1% of the volume.

[0040] According to the protonated covalent organic framework material of the present invention, A in the structure shown in formula (Ⅰ) is a benzene ring or a triazine ring.

[0041] According to the protonated covalent organic framework material of the present invention, R1 and R2 in the structure shown in formula (I) can each be independently selected from -H, alkyl, hydroxyalkyl, alkoxy; preferably, R1 and R2 can each be independently selected from -H, C1 to C6 alkyl, C1 to C6 hydroxyalkyl, C1 to C6 alkoxy; more preferably, R1 and R2 can each be independently selected from -H, methyl, ethyl, isopropyl, isobutyl, tert-butyl, methoxy, ethoxy, hydroxyisopropyl and hydroxyethyl; most preferably, R1 and R2 are methoxy.

[0042] The present invention also provides a method for improving the hydrogen adsorption performance of covalent organic framework materials, comprising the following steps: protonating the covalent organic framework material containing imine bonds with hydrochloric acid vapor.

[0043] The protonation treatment can be achieved by placing the covalent organic framework material in hydrochloric acid vapor or by introducing hydrochloric acid vapor into the covalent organic framework material, as long as the covalent organic framework material and the hydrochloric acid vapor come into contact for an appropriate time. For example, at room temperature and pressure, a certain concentration of concentrated hydrochloric acid can be placed in a desiccator containing color-changing silica balls, so that the volatilized hydrogen chloride gas comes into contact with the covalent organic framework material. For example, the concentrated hydrochloric acid has a concentration of more than 20% by weight; preferably, it has a concentration of 36% to 38% by weight; more preferably, it has a concentration of 37% by weight.

[0044] In the method of this invention, the hydrochloric acid vapor refers to hydrogen chloride gas that is substantially free of water molecules or contains no water molecules, obtained after the volatilization of concentrated hydrochloric acid. In a preferred embodiment of this invention, the water molecules in the hydrogen chloride gas obtained after the volatilization of concentrated hydrochloric acid are substantially adsorbed by the desiccant; therefore, the hydrochloric acid vapor is essentially a hydrogen chloride gas free of water molecules. The desiccant can be a commonly used desiccant in the art that does not react with hydrogen chloride gas. For example, the desiccant is selected from any one of calcium chloride, silica gel, silicon tetrachloride, phosphorus pentoxide, or concentrated sulfuric acid. Through in-depth research, the inventors unexpectedly discovered that the protonated covalent organic framework material obtained by protonation treatment using hydrochloric acid vapor according to this invention has a larger pore volume and specific surface area, as well as better gas adsorption performance. If hydrochloric acid aqueous solution is used to treat COF, the reaction process is too violent. On the one hand, this leads to a large number of hydrogen chloride and water molecules within the COF pores, causing a sharp decrease in the material's pore volume and specific surface area, and a significant reduction in gas adsorption performance. On the other hand, water molecules are adsorbed within the COF pores and are difficult to remove. If the COF is dried, both water and hydrogen chloride molecules will be removed, thus negating the protonation effect. Therefore, it is advantageous to use hydrochloric acid vapor containing a small amount of water molecules or essentially no water molecules for protonation treatment, which is within the scope of this invention.

[0045] In the method of this invention, the protonation treatment time is preferably 30 to 180 minutes; more preferably 45 to 120 minutes; for example, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 52 minutes, 54 minutes, 56 minutes, 58 minutes, 60 minutes, 62 minutes, 64 minutes, 66 minutes, 68 minutes, 70 minutes, 72 minutes, 74 minutes, 76 minutes, 78 minutes, 80 minutes, 85 minutes, 90 minutes, 95 minutes, 100 minutes, 105 minutes, 110 minutes, 115 minutes, 120 minutes, 130 minutes, 140 minutes, 150 minutes, 160 minutes, and 170 minutes. Those skilled in the art can adjust the protonation treatment time within the above range according to the COF material used to avoid insufficient protonation due to too short a treatment time and potential structural damage to the COF material due to too long a treatment time.

[0046] According to the method of the present invention, the covalent organic framework material containing imine bonds is prepared by reacting polyamino monomers and polyaldehyde monomers in a mixed solvent under the catalysis of a catalyst. Typically, the reaction is carried out at 100 to 150°C, preferably at 100 to 130°C, more preferably at 110°C, 112°C, 114°C, 116°C, 118°C, or 120°C, and most preferably at 120°C.

[0047] According to the method of the present invention, the polyamino monomer is a compound containing two or more amino groups. For example, the polyamino monomer may be selected from ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine and other polyene polyamines, p-phenylenediamine, triaminobenzene and other polyamino aromatic compounds, diamino heterocyclic compounds, triamino heterocyclic compounds, or polyamino heterocyclic compounds. Preferably, the polyamino compound is 1,3,5-tris(4-aminophenyl)benzene.

[0048] According to the method of the present invention, the polyaldehyde monomer is a compound containing two or more aldehyde groups, for example, the polyaldehyde monomer is selected from substituted or unsubstituted terephthalaldehyde, substituted or unsubstituted biphenylaldehyde, and substituted or unsubstituted p-thiophenecarboxaldehyde. Preferably, the polyaldehyde monomer is substituted terephthalaldehyde. More preferably, the polyaldehyde monomer is 2,5-dimethoxyterephthalaldehyde.

[0049] According to the method of the present invention, the catalyst may be selected from any catalyst well known in the art, for example, the catalyst may be selected from one or more of formic acid, acetic acid, p-toluenesulfonic acid, oxalic acid, lactic acid, hydrochloric acid, sulfuric acid, and pyrrolidine; preferably, the catalyst is selected from acetic acid.

[0050] According to the method of the present invention, the mixed solvent can be selected from any mixed solvent well known in the art. For example, the mixed solvent can be any one of ethylene glycol + cyclohexane, mesitylene + dioxane, n-butanol + dioxane, o-dichlorobenzene + n-butanol, and mesitylene + n-butanol. In the mixed solvent, the volume ratio of the former to the latter is 9:1 to 1:9; for example, 5:1 to 1:5, 3:1 to 1:3; preferably, the volume ratio of the two liquids is 1:1.

[0051] The preferred conditions of the present invention will be further described below with reference to the embodiments and the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0052] All raw materials or reagents used in the following examples are commercially available or self-made.

[0053] Example 1

[0054] (Preparation of imine-type covalent organic frameworks)

[0055] In a 1 mL, 1:1 volume ratio mixture of o-dichlorobenzene (o-DCB) and n-butanol (BuOH), 0.1 mmol of 1,3,5-tris(4-aminophenyl)benzene (TPB) and 0.15 mmol of 2,5-dimethoxyterephthalaldehyde (DMTP) were added to dissolve the substances in the mixed solvent to obtain a mixture. Acetic acid (6 mol / L, 0.1 mL) was added to the mixture, and the mixture was heated to 120 °C and maintained at this temperature for 3 days. The reaction product was filtered, washed, and purified to obtain an imine-type covalent organic framework, named DMTP-TPB-COF. See also... Figure 1 .

[0056] (Preparation of protonated imine-type covalent organic frameworks)

[0057] Protonated imine-type covalent organic frameworks were prepared by treatment with hydrochloric acid vapor. A 37% by weight hydrochloric acid solution was placed in a desiccator containing color-changing silica balls at room temperature and pressure. The prepared DMTP-TPB-COF was then placed in the desiccator for 60 minutes to obtain the protonated COF, named H@DMTP-TPB-COF. See also Figure 1 .

[0058] Effect Measurement

[0059] (Crystal structure analysis and chemical composition determination)

[0060] Powder crystal X-ray diffraction (XRD) is used to analyze the crystal structure of materials, while infrared spectroscopy is used to analyze their chemical composition. For example... Figure 2 As shown, the powder crystal X-ray diffraction (XRD) results of DMTP-TPB-COF and H@DMTP-TPB-COF confirm that protonation treatment has no significant effect on the crystal structure of COF materials. (Reference) Figure 3 It can be seen that the imine bond was successfully protonated after the protonation treatment.

[0061] (BET specific surface area determination and hydrogen storage capacity determination)

[0062] The BET specific surface area and hydrogen storage capacity of the obtained covalent organic framework compounds were determined using a gas adsorption instrument.

[0063] The atmospheric pressure gas adsorption instrument used was a BELSORP-maxⅡ manufactured by MicrotracBEL.

[0064] The high-pressure hydrogen adsorption instrument used was the HPVA-100 high-pressure volumetric analyzer manufactured by Microlithics Instruments Corporation.

[0065] The BET specific surface area is determined as follows: it is determined by the N2 adsorption isotherm at 77 K, and the BET (Brunauer-Emmett-Teller) equation is used to calculate the surface area of ​​the material.

[0066] The method for determining the hydrogen storage capacity at 77K-atmospheric pressure is as follows: using an atmospheric pressure gas adsorption analyzer, the adsorption isotherm of hydrogen is obtained by the dynamic volume method.

[0067] The method for determining the hydrogen storage capacity at 77K high pressure is as follows: using a high-pressure gas adsorption analyzer, the high-pressure adsorption isotherm of hydrogen is obtained by static volume method.

[0068] The measured BET specific surface area, hydrogen storage capacity at 77K (atmospheric pressure), and hydrogen storage capacity at 77K (high pressure, 80 bar) are shown in Table 1.

[0069] Table 1

[0070]

[0071] The inventors discovered that after protonation treatment, the specific surface area of ​​the COF decreased to some extent due to the introduction of protons. However, surprisingly, due to the protonation of the imine bonds, the adsorption heat of the imine sites increased, and the hydrogen storage capacity of the COF significantly improved. (Reference) Figure 4 Atmospheric pressure hydrogen adsorption-desorption isotherm and Figure 5 The hydrogen adsorption capacity-pressure curves show that, under both atmospheric and high pressure conditions, the hydrogen adsorption capacity of H@DMTP-TPB-COF is significantly higher than that of DMTP-TPB-COF. This demonstrates that protonation treatment improves the hydrogen adsorption performance of covalent organic framework materials, representing a simple and universally effective strategy for enhancing the hydrogen storage performance of covalent organic frameworks.

Claims

1. A protonated covalent organic framework material, wherein the covalent organic framework material has a structure as shown in formula (I): Equation (Ⅰ) in, R1 and R2 are each methoxy groups. A is a benzene ring. J is a protonable site and J is an imine bond, and One or more Js have been protonated; Each of the imine bonds has a C atom attached to a benzene ring containing R1 and R2 in formula (Ⅰ).

2. A method for improving the hydrogen adsorption performance of covalent organic framework materials, comprising the following steps: Protonation of covalent organic framework materials containing imine bonds using hydrochloric acid vapor; wherein the protonation site is an imine bond; The covalent organic framework material containing imine bonds has the structure shown in formula (I): Equation (Ⅰ) in, R1 and R2 are each independently selected from H, C1 to C6 alkyl, methoxy, and ethoxy groups. A is a six-membered aromatic ring or a heterocyclic aromatic ring. J is an imine bond, and the C atom of each imine bond is connected to a benzene ring containing R1 and R2 in formula (Ⅰ).

3. The method of claim 2, wherein A is a benzene ring or a triazine ring.

4. The method of claim 2, wherein R1 and R2 are methoxy groups.

5. The method of claim 2, wherein the protonation treatment takes 30 to 180 minutes.

6. The method of claim 2, wherein the covalent organic framework material containing imine bonds is prepared by reacting polyamino monomers and polyaldehyde monomers in a mixed solvent under the catalysis of a catalyst.

7. The method of claim 6, wherein the reaction is carried out at 100 to 150 °C.

8. The method of claim 2, wherein the hydrochloric acid vapor is obtained by volatilizing concentrated hydrochloric acid.

9. The method of claim 2, wherein the hydrochloric acid vapor refers to hydrogen chloride gas that is substantially free of water molecules or contains no water molecules, obtained by drying concentrated hydrochloric acid after volatilization with a desiccant.

10. Use of the protonated covalent organic framework material as described in claim 1 or the covalent organic framework material obtained by the method according to any one of claims 2 to 9 as a hydrogen storage medium.

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