Method for improving the hydrogen storage properties of covalent organic framework compounds and their use for hydrogen storage

Abstract

This invention provides a method for improving the hydrogen storage performance of covalent organic framework compounds, comprising dehydrating and polycondensing an aromatic polyamino monomer and an aromatic polyaldehyde monomer to form a covalent organic framework compound, wherein the aromatic polyamino monomer and / or the aromatic polyaldehyde monomer contains at least one fluorinated aromatic ring, at least one hydrogen atom on the fluorinated aromatic ring is substituted with fluorine, and there are unsubstituted hydrogen atoms on the fluorinated aromatic ring, wherein the covalent organic framework compound has a two-dimensional or three-dimensional structure.

Description

Technical Field

[0001] This invention relates to the technical field of hydrogen storage materials, and in particular to a method for improving the hydrogen storage performance of covalent organic framework compounds and the use of fluorinated covalent organic framework compounds as hydrogen storage media. 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 framework materials 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 framework, connected by strong covalent bonds (such as CC, CO, CO, CO). Among them, 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 MOF materials, COF materials have a 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] However, physical hydrogen storage materials face 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, 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, the BET specific surface area exceeds 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-type physical hydrogen storage materials.

[0009] Because COF materials all contain aromatic ring structures (requiring moderately rigid structural units), these building blocks are difficult to make functional, and functionalization often requires further pore modification. For hydrogen storage properties, the adsorption heat of aromatic ring structures is typically very high (~4 kJ / mol), making it difficult to provide effective adsorption sites. Effective methods are needed to improve the hydrogen storage performance of materials by increasing the adsorption heat of adsorption sites. Summary of the Invention

[0010] As mentioned earlier, combining the properties of physical and chemical hydrogen storage materials is key to developing efficient hydrogen storage materials and also a major challenge in the field of hydrogen storage. Through in-depth research, the inventors unexpectedly discovered that fluorination of specific sites on the aromatic rings of covalent organic framework (COF) structures can significantly increase the heat of hydrogen adsorption and the hydrogen storage capacity of COF compounds.

[0011] According to a first aspect of the present invention, a method for improving the hydrogen storage performance of a covalent organic framework compound is provided, comprising dehydrating and polycondensing an aromatic polyamino monomer with an aromatic polyaldehyde monomer to form a covalent organic framework compound, wherein the aromatic polyamino monomer and / or the aromatic polyaldehyde monomer contains at least one fluorinated aromatic ring, wherein at least one hydrogen atom on the fluorinated aromatic ring is substituted with fluorine, and wherein unsubstituted hydrogen atoms are present on the fluorinated aromatic ring, and the covalent organic framework compound has a two-dimensional or three-dimensional structure.

[0012] According to a second aspect of the present invention, there is provided the use of a fluorinated covalent organic framework compound as a hydrogen storage medium, wherein the fluorinated covalent organic framework compound is formed by dehydration condensation polymerization of an aromatic polyamino monomer and an aromatic polyaldehyde monomer, the aromatic polyamino monomer and / or the aromatic polyaldehyde monomer containing at least one fluorinated aromatic ring, at least one hydrogen atom on the fluorinated aromatic ring being substituted with fluorine, and unsubstituted hydrogen atoms being present on the fluorinated aromatic ring, the fluorinated covalent organic framework compound having a two-dimensional or three-dimensional structure.

[0013] In some embodiments, the fluorinated aromatic ring is selected from benzene ring, naphthalene ring, anthracene ring, phenanthrene ring, or pyrene ring.

[0014] In some embodiments, the fluorinated aromatic ring is a benzene ring.

[0015] In some embodiments, the aromatic polyaldehyde monomer contains one fluorinated aromatic ring having two para-aldehyde groups, and the aromatic polyamino monomer has one of the following structures:

[0016]

[0017]

[0018] Where R is an amino group or a 4-aminophenyl group.

[0019] In some embodiments, the aromatic polyamino monomer contains one fluorinated aromatic ring having two para-amino groups, and the aromatic polyaldehyde monomer has one of the following structures:

[0020]

[0021] Where R is an aldehyde group or a 4-aldehyde phenyl group.

[0022] In some embodiments, the aromatic polyamino monomer is 1,3,5-tris(4-aminophenyl)benzene or tetra(4-aminophenyl)methane.

[0023] In some embodiments, the aromatic polyaldehyde monomer is 2,5-difluoro-terephthalaldehyde.

[0024] The beneficial effects of this invention are as follows:

[0025] 1. In the prior art, the modification of hydrogen storage materials is usually carried out by increasing the specific surface area of ​​the material or by doping the material to improve its hydrogen storage performance. This process is complicated and costly. However, the present invention regulates the adsorption induction force of covalent organic framework compounds through simple structural modification, which is beneficial to solving the problem of low hydrogen storage capacity of existing hydrogen storage materials.

[0026] 2. The low heat of adsorption of covalent organic framework compounds in the prior art has hindered their widespread adoption and application as hydrogen storage media. This invention improves the heat of hydrogen adsorption of covalent organic framework compounds by fluorination treatment at specific sites on the aromatic ring of COF, thereby increasing the hydrogen storage capacity of COF compounds and promoting the practical application of covalent organic frameworks in the field of hydrogen storage.

[0027] 3. The method of the present invention is universal and shows good implementation effect on covalent organic framework compounds with different structures and dimensions.

[0028] 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

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0030] Figure 1(a) shows a schematic diagram of the synthesis process of the two-dimensional covalent organic framework compound provided by the present invention.

[0031] Figure 1(b) shows a schematic diagram of the synthesis process of the three-dimensional covalent organic framework compound provided by the present invention.

[0032] Figures 2(a) to (g) show the X-ray diffraction (XRD) patterns of the covalent organic framework compounds provided in Examples 1 to 7 of the present invention.

[0033] Figure 3(a) shows the nitrogen adsorption-desorption isotherm of the two-dimensional covalent organic framework compound provided in Example 1 of the present invention.

[0034] Figure 3(b) shows the nitrogen adsorption-desorption isotherm of the three-dimensional covalent organic framework compound provided in Example 2 of the present invention.

[0035] Figure 4(a) shows the hydrogen adsorption-desorption isotherm of the two-dimensional covalent organic framework compound of the present invention.

[0036] Figure 4(b) shows the hydrogen adsorption-desorption isotherms of the three-dimensional covalent organic framework compounds of the present invention.

[0037] Figure 4(c) shows the hydrogen adsorption-desorption isotherm of the two-dimensional covalent organic framework compound provided in Preparation Example 4 of the present invention.

[0038] Figures 5(a) and (b) show the hydrogen adsorption as a function of pressure (bar) for two-dimensional and three-dimensional covalent organic framework compounds, respectively. Detailed Implementation

[0039] As used in this article, the terms covalent organic framework, covalent organic skeleton, and COF are used interchangeably.

[0040] 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 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.

[0041] 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.

[0042] 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".

[0043] In this invention, the term "fluorinated aromatic ring" refers to the substitution of H on an aromatic ring by F. The substitution of H on a substituent connected to the aromatic ring by F does not fall under the category of "fluorinated aromatic ring" in this invention.

[0044] As described above, the present invention provides a method for improving the hydrogen storage performance of a covalent organic framework compound, comprising dehydrating and polycondensing an aromatic polyamino monomer and an aromatic polyaldehyde monomer to form a covalent organic framework compound, wherein the aromatic polyamino monomer and / or the aromatic polyaldehyde monomer contains at least one fluorinated aromatic ring, at least one hydrogen atom on the fluorinated aromatic ring is substituted with fluorine, and there are unsubstituted hydrogen atoms on the fluorinated aromatic ring, wherein the covalent organic framework compound has a two-dimensional or three-dimensional structure.

[0045] According to the present invention, the aromatic polyamino monomer is an aromatic compound containing two or more amino groups. For example, the aromatic polyamino monomer may be selected from substituted or unsubstituted p-phenylenediamine, substituted or unsubstituted triaminobenzene and other substituted or unsubstituted polyamino aromatic compounds, substituted or unsubstituted diamino heterocyclic compounds, substituted or unsubstituted triamino heterocyclic compounds or substituted or unsubstituted polyamino heterocyclic compounds.

[0046] According to the present invention, the aromatic polyaldehyde monomer is an aromatic compound containing two or more aldehyde groups. For example, the polyaldehyde monomer is selected from substituted or unsubstituted terephthalaldehyde, substituted or unsubstituted biphenyl dicarboxaldehyde, substituted or unsubstituted p-thiophene dicarboxaldehyde and other substituted or unsubstituted polyaldehyde aromatic compounds, substituted or unsubstituted dialdehyde heterocyclic compounds, substituted or unsubstituted trialdehyde heterocyclic compounds or substituted or unsubstituted polyaldehyde heterocyclic compounds.

[0047] According to a preferred embodiment of the present invention, the aromatic polyaldehyde monomer contains one fluorinated aromatic ring, the fluorinated aromatic ring having two para-aldehyde groups, and the aromatic polyamino monomer has one of the following structures:

[0048]

[0049] Where R is an amino group or a 4-aminophenyl group.

[0050] In another preferred embodiment, the aromatic polyamino monomer contains one fluorinated aromatic ring having two para-amino groups, and the aromatic polyaldehyde monomer has one of the following structures:

[0051]

[0052] Where R is an aldehyde group or a 4-aldehyde phenyl group.

[0053] According to the present invention, the two-dimensional or three-dimensional structure refers to a structure having periodically repeating two-dimensional or three-dimensional crystal structural units.

[0054] The two-dimensional structure can be a two-dimensional sheet-like structure with horizontally arranged polygonal structural units having internal holes. The polygons can be, for example, equilateral triangles or irregular triangles, quadrilaterals, pentagons, hexagons, or combinations thereof, and have internal holes. Considering chemical structural stability, the polygons can be, for example, equilateral triangles, regular quadrilaterals, regular pentagons, regular hexagons, or combinations thereof, or for example, regular pentagons, regular hexagons, or combinations thereof, or for example, regular hexagons.

[0055] The three-dimensional structure can be any three-dimensional crystal structure existing in an overlapping stacked structure, an interlaced stacked structure, a unidirectional stacked structure, or a random stacked structure, and it has pores inside.

[0056] According to the present invention, the dehydration condensation polymerization of aromatic polyamino monomers and aromatic polyaldehyde monomers to form covalent organic framework compounds can be any dehydration condensation reaction well known in the art. For example, the dehydration condensation polymerization is carried out in a mixed solvent with polyamino monomers and polyaldehyde monomers as reactants, 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. The catalyst can be selected from any catalyst well known in the art; for example, the catalyst can 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. 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.

[0057] According to the present invention, at least one hydrogen atom on the fluorinated aromatic ring is substituted with fluorine, and the fluorinated aromatic ring contains unsubstituted hydrogen atoms. Preferably, the fluorinated aromatic ring is a fluorinated benzene ring. More preferably, the fluorinated aromatic ring is a fluorinated benzene ring, and the aromatic polyaldehyde monomer contains the fluorinated benzene ring, while the aromatic polyamino monomer does not contain the fluorinated benzene ring.

[0058] According to the present invention, the fluorinated aromatic ring has two para-aldehyde groups or two para-amino groups attached to it, and at least one hydrogen atom is substituted by fluorine, and the fluorinated aromatic ring contains unsubstituted hydrogen atoms; preferably, the fluorinated aromatic ring has two para-aldehyde groups attached to it, and at least one hydrogen atom is substituted by fluorine, and the fluorinated aromatic ring contains unsubstituted hydrogen atoms; more preferably, the fluorinated aromatic ring has two para-aldehyde groups attached to it, and one, two, or three hydrogen atoms are substituted by fluorine. Most preferably, the fluorinated aromatic ring has two para-aldehyde groups attached to it, and the fluorinated aromatic ring is fluorinated.

[0059] The inventors unexpectedly discovered that COF compounds containing monofluorofluorinated, difluorofluorinated, and trifluorofluorinated aromatic rings exhibit better hydrogen storage performance compared to COF compounds containing perfluorinated aromatic rings and unfluorinated COF compounds. In particular, compared to COF compounds containing perfluorofluorinated aromatic rings, COF compounds containing polyfluorofluorinated aromatic rings simultaneously possess significantly higher specific surface area and higher hydrogen storage capacity. Furthermore, to improve hydrogen storage capacity, the fluorination site should be located on the aromatic ring (e.g., a benzene ring), and not on other substituents attached to the aromatic ring (e.g., a benzene ring).

[0060]

[0061] Formula I shows a portion of the molecular structural unit of the two-dimensional COF compound in the preparation example of the present invention, wherein the substituent R on the benzene ring 1 To R 4 The samples were independently selected from H, OCH3, and F. The inventors conducted theoretical research and listed the dipole moments (molecular polarization indices) under different substituent conditions in Table 1. Table 3 shows that the introduction of monofluorinated, fluorinated, and trifluorinated substituents increased the polarity of the substitution sites on the benzene ring. The inventors discovered that introducing strongly polar groups at specific sites on the aromatic ring can regulate the pore environment of the COF to increase its interaction with hydrogen molecules, which is an effective strategy to improve the material's adsorption heat, thereby enhancing the hydrogen storage performance of the COF material.

[0062] Table 1

[0063] Serial Number <![CDATA[R 1 ]]> <![CDATA[R 2 ]]> <![CDATA[R 3 ]]> <![CDATA[R 4 ]]> Dipole moment (eV) 1 H H H H 0.37684808 2 F H H H 0.39489081 3 F H F H 0.41059401 4 F F F H 0.40033184 5 F F F F 0.38501040 6 H <![CDATA[OCH3]]> H <![CDATA[OCH3]]> 0.38234260

[0064] 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.

[0065] The raw materials or reagents used in the following preparation examples are either commercially available or self-made.

[0066] [Preparation Example 1]

[0067] Synthesis of the two-dimensional fluorinated covalent organic framework compound TPB-DFTP-COF

[0068] In a mixed solvent of o-dichlorobenzene and n-butanol (o-DCB + n-BuOH, 1 mL, 1:1 volume ratio), 0.1 mmol of 1,3,5-tris(4-aminophenyl)benzene (TPB) and 0.15 mmol of 2,5-difluoroterephthalaldehyde (DFTP) were added to dissolve them in the mixed solvent to obtain a mixture. Acetic acid (6 mol / L, 0.1 mL) was added to the mixture, and the temperature was raised to 120 °C and maintained for 3 days. After filtration, washing, and purification of the reaction product, a fluorinated two-dimensional COF was obtained, named TPB-DFTP-COF. The schematic diagram of its synthesis process is shown in Figure 1(a).

[0069] [Preparation Example 2]

[0070] Synthesis of three-dimensional fluorinated covalent organic framework compound 3D-F-COF

[0071] In a mixed solvent of dioxane and 1,3,5-trimethylbenzene (1 mL, 1:1 volume ratio), tetrakis(4-aminophenyl)methane (0.1 mmol) and 2,5-difluoroterephthalaldehyde (0.2 mmol) were added and dissolved to obtain a mixture. Acetic acid was added to the mixture, and the mixture was heated to 120 °C and reacted for 3 days. The reaction product was centrifuged and purified by tetrahydrofuran extraction to obtain the fluorinated three-dimensional COF, named 3D-F-COF. The schematic diagram of its synthesis process is shown in Figure 1(b).

[0072] [Preparation Example 3 (Comparative)]

[0073] Synthesis of the two-dimensional methoxylated covalent organic framework compound TPB-DMTP-COF

[0074] In a mixed solvent of o-dichlorobenzene and n-butanol (1 mL, 1:1 volume ratio), 0.1 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.15 mmol of 2,5-dimethoxytetraphenyldialdehyde (DMTP) were added and dissolved to obtain a mixture. Acetic acid was added to the mixture, and the temperature was raised to 120 °C and maintained for 3 days. The reaction product was filtered, washed, and purified to obtain a two-dimensional methoxylated COF, named TPB-DMTP-COF. A schematic diagram of its synthesis process is shown in Figure 1(a).

[0075] [Preparation Example 4 (Comparative)]

[0076] Synthesis of the two-dimensional perfluorinated covalent organic framework compound TPB-TFTP-COF

[0077] In a mixed solvent of o-dichlorobenzene and n-butanol (1 mL, 1:1 volume ratio), 0.1 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.15 mmol of 2,3,5,6-tetrafluoro-terephthalaldehyde were added and dissolved to obtain a mixture. Acetic acid 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 a two-dimensional perfluorinated COF, named TPB-TFTP-COF.

[0078] [Preparation Example 5 (Comparative)]

[0079] Synthesis of two-dimensional unsubstituted covalent organic framework compounds

[0080] In a mixed solvent of o-dichlorobenzene and n-butanol (1 mL, 1:1 volume ratio), 0.1 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.15 mmol of terephthalaldehyde were added and dissolved to obtain a mixture. Acetic acid 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 a two-dimensional fluorine-free COF, named 2D-F-free-COF.

[0081] [Preparation Example 6 (Comparative)]

[0082] Synthesis of 3D-MeO-COF, a three-dimensional methoxylated covalent organic framework compound

[0083] In a mixed solvent of dioxane and 1,3,5-trimethylbenzene (1 mL, 1:1 volume ratio), tetrakis(4-aminophenyl)methane (0.1 mmol) and 2,5-dimethoxyterephthalaldehyde (0.2 mmol) were added and dissolved to obtain a mixture. Acetic acid 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 centrifuged and purified by tetrahydrofuran soxhlet extraction to obtain the fluorinated three-dimensional methoxylated COF, named 3D-MeO-COF, as shown in Figure 1(b).

[0084] [Preparation Example 7 (Comparative)]

[0085] Synthesis of COF-300, a three-dimensional unsubstituted covalent organic framework compound

[0086] In a mixed solvent of dioxane and 1,3,5-trimethylbenzene (1 mL, 1:1 volume ratio), tetrakis(4-aminophenyl)methane (0.1 mmol) and terephthalaldehyde (0.2 mmol) were added and dissolved to obtain a mixture. Acetic acid was added to the mixture, and the mixture was heated to 120 °C and reacted for 3 days. The reaction product was centrifuged and purified by tetrahydrofuran Soxhlet extraction to obtain a fluorinated three-dimensional COF, named COF-300, as shown in Figure 1(b).

[0087] Table 2 lists the properties of the compounds obtained in Examples 1 to 7, and their crystal structures were analyzed using powder crystal X-ray diffraction (XRD). The results are shown in [reference needed]. Figures 2(a) to 2(g) .

[0088] Table 2

[0089]

[0090]

[0091] Referring to Figures 2(a) and 2(c), a comparison of the powder crystal X-ray diffraction results of TPB-DMTP-COF and TPB-DFTP-COF shows that the crystal structure of the fluorine-containing two-dimensional covalent organic framework compounds is not significantly different from that of the methoxy-containing two-dimensional covalent organic framework compounds. Referring to Figures 2(b) and 2(f), a comparison of the powder crystal X-ray diffraction results of 3D-F-COF and 3D-MeO-COF further demonstrates that the introduction of fluorine groups has no significant impact on the crystal structure of the three-dimensional covalent organic framework compounds.

[0092] Figure 2(e) shows the X-ray diffraction pattern of the two-dimensional fluorine-free covalent organic framework compound obtained in Preparation Example 5, in which there are no obvious crystal characteristic peaks, indicating that 2D-F-free-COF has poor crystallinity.

[0093] Example 1

[0094] (BET specific surface area determination and hydrogen storage capacity determination)

[0095] The BET specific surface area and hydrogen storage capacity of a portion of the obtained covalent organic framework compounds were determined using a gas adsorption instrument.

[0096] The atmospheric pressure gas adsorption instrument used was a BELSORP-maxⅡ manufactured by MicrotracBEL.

[0097] The high-pressure hydrogen adsorption instrument used was the HPVA-100 high-pressure volumetric analyzer manufactured by Microlithics Instruments Corporation.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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 3.

[0102] Table 3

[0103]

[0104] As shown in Figures 4(a)-(c), the inventors discovered that, for both two-dimensional and three-dimensional COF compounds, the specific surface area of ​​fluorinated COFs did not change significantly compared to methoxylated and perfluorinated COFs, while the hydrogen adsorption capacity increased substantially. This demonstrates that the improved hydrogen adsorption performance of COF compounds obtained through fluorination via fluorination is not due to an increase in specific surface area, but rather to the improvement in hydrogen adsorption performance brought about by the introduction of fluorinated substituents. Therefore, fluorination with fluorinated substituents is a simple, universally applicable, and effective strategy for improving the hydrogen storage performance of COF compounds.

[0105] Referring to Figures 5(a) and (b), the hydrogen storage capacity of two-dimensional and three-dimensional covalent organic framework compounds is significantly improved under high-pressure hydrogen storage conditions through the fluorination treatment method of the present invention.

[0106] The inventors of this invention performed simulation calculations on the adsorption energy of the material obtained by the method of this invention, and the results are listed in Table 4 to evaluate the effect of the method of this invention on the hydrogen adsorption performance of the material. The simulation calculations were performed using the built-in module CASTEP of Materials Studio (commercially available from Accelrys, Inc., USA). The simulation results show that the fluorination treatment method by introducing a fluorinated substituent can significantly improve the adsorption energy.

[0107] Table 4

[0108]

[0109] Industrial applicability

[0110] The covalent organic framework compounds obtained by the processing method of the present invention can store hydrogen at a practical level, which makes the utilization of hydrogen easier and will have more universal practical value with the advent of the hydrogen society.

Claims

1. The use of fluorinated covalent organic framework compounds as hydrogen storage media, wherein the fluorinated covalent organic framework compound is formed by the dehydration condensation polymerization of aromatic polyamino monomers and aromatic polyaldehyde monomers. The aromatic polyamino monomers thereon are 1,3,5-tris(4-aminophenyl)benzene or tetra(4-aminophenyl)methane; The aromatic polyaldehyde monomer is 2,5-difluoro-terephthalaldehyde.

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