Metal organic framework

Abstract

This metal organic framework comprises a first ligand that has a benzene ring and has two or more functional groups for coordination, and a second ligand that does not contain a benzene ring and contains one functional group for coordination. The ratio of the water vapor adsorption level per unit mass of the metal organic framework at a relative pressure of 0.95 to the water vapor adsorption level per unit mass of a basic structure MOF at a relative pressure of 0.95 is 84% or greater in a water vapor isotherm measured at 298K when a metal organic framework having only the first ligand as a ligand is identified as the basic structure MOF on the basis of the results of powder XRD analysis of the metal organic framework.

Description

metal organic framework

[0001] The present disclosure relates to metal-organic frameworks.

[0002] Metal organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are complex crystals formed by coordinate bonds between metal ions and organic ligands. They have a highly regular porous coordination network structure formed by the metal ions and organic ligands, and contain numerous nanospaces within. By utilizing these nanospaces, MOFs can be used as materials for occluding and separating various substances such as water vapor and gases, or as catalytic materials. Various types of metal organic frameworks have been known in the past. For example, Patent Document 1 discloses metal organic frameworks as various porous metal complexes containing MIL-101(Cr) and MIL-101(Fe).

[0003] Japanese Patent Application Laid-Open No. 2015-196677

[0004] When a metal-organic framework is used as an adsorbent for gases or water vapor, it is expected that it will be applied to an adsorption separation system for gases or water vapor using processes such as pressure swing adsorption (PSA) or temperature swing adsorption (TSA). When a metal-organic framework is applied to the above-mentioned adsorption separation system, it becomes possible to increase the throughput of the system and make the device more compact by increasing the adsorption rate of gases or water vapor by the metal-organic framework. However, the adsorption rate of gases and water vapor in metal-organic frameworks has not been sufficiently studied, and further improvement of the adsorption rate has been desired.

[0005] The present disclosure can be realized in the following aspects. (1) According to one aspect of the present disclosure, there is provided a metal-organic framework comprising: a first ligand having a benzene ring and two or more functional groups for coordination; and a second ligand having no benzene ring and one functional group for coordination. When a metal-organic framework having only the first ligand as a ligand is identified as a basic structure MOF based on the results of powder XRD analysis of the metal-organic framework, in a water vapor adsorption isotherm measured at 298 K, the ratio of the water vapor adsorption amount per unit mass of the metal-organic framework at a relative pressure of 0.95 to the water vapor adsorption amount per unit mass of the basic structure MOF at a relative pressure of 0.95 is 84% ​​or more. According to this metal-organic framework, a basic MOF having a first ligand having a benzene ring as a ligand is partially substituted with a second ligand having no benzene ring and one functional group for coordination. By partially replacing the first ligand with a second ligand occupying less space in the three-dimensional crystal structure, new micropores are generated within the metal-organic framework that contribute to the diffusivity and fluidity of gases and water vapor, thereby increasing the adsorption rate of gases and water vapor. As a result, the throughput of a system including the metal-organic framework can be increased, and devices including the metal-organic framework can be made more compact. (2) In the metal-organic framework of the above embodiment, the first ligand may have two or more carboxy groups as the functional group for coordination. With this configuration, a metal-organic framework having a ligand having a carboxy group is generally synthesized by hydrothermal synthesis, and therefore can be made into a metal-organic framework that is resistant to decomposition in water and has relatively high stability. (3) In the metal organic framework of the above embodiment, the second ligand may have a carboxy group as the functional group for coordination. With this configuration, the first ligand and the second ligand have the same functional group as the functional group for coordination. Therefore, crystal growth via the first ligand and crystal growth via the second ligand can be performed in a common step, thereby simplifying the manufacturing process of the metal organic framework.Furthermore, since the first ligand and the second ligand have the same functional group as the functional group for coordination, a good coordinate bond can be formed with the metal ion contained in the metal organic framework. (4) In the metal organic framework of the above embodiment, the first ligand and the second ligand may have the same functional group as the functional group for coordination. With this configuration, crystal growth via the first ligand and crystal growth via the second ligand can be performed in a common step, thereby simplifying the manufacturing process of the metal organic framework. Furthermore, since the first ligand and the second ligand have the same functional group as the functional group for coordination, a good coordinate bond can be formed with the metal ion contained in the metal organic framework. (5) In the metal organic framework of the above embodiment, the basic structure MOF may have an MIL-type crystal structure. With this configuration, since metal-organic frameworks having an MIL-type crystal structure are generally synthesized by hydrothermal synthesis, they can be relatively stable metal-organic frameworks that are resistant to decomposition by water. (6) In the metal-organic framework of the above embodiment, the basic structure MOF may be MIL-101. With this configuration, MIL-101 is a metal-organic framework that adsorbs a relatively large amount of water and requires relatively low raw material costs for production, so that a metal-organic framework having these advantages can be obtained. (7) In the metal-organic framework of the above embodiment, in a pore size distribution calculated by applying the NLDFT method to a nitrogen adsorption isotherm measured at 77 K, the differential pore volume of pores having a pore diameter of 1.46 nm may be larger than the differential pore volume of pores having a pore diameter of 1.11 nm. With this configuration, due to the replacement of a part of the first ligands in the basic structure MOF with the second ligand, new micropores that contribute to the diffusivity and fluidity of gases and water vapor are generated within the metal-organic framework, and a metal-organic framework having a higher adsorption rate for gases and water vapor can be obtained. (8) In the metal-organic framework of the above form, in X-ray diffraction analysis using synchrotron radiation, the ratio (Y / X) of the diffraction peak intensity Y of the (111) plane to the diffraction peak intensity X of the (311) plane may be less than 0.42.With this configuration, the adsorption rate of gases and water vapor in the metal-organic framework can be increased. (9) In the metal-organic framework of the above embodiment, in a spectrum obtained by Kubelka-Munk transformation of a diffuse reflectance spectrum obtained by ultraviolet-visible spectroscopy, the ratio (β / α) of the strongest peak intensity β in a wavelength range of 515 nm or more and less than 750 nm to the strongest peak intensity α in a wavelength range of 250 nm or more and less than 400 nm may be 0.045 or less. With this configuration, the adsorption rate of gases and water vapor in the metal-organic framework can be increased. The present disclosure can be realized in various forms other than those described above, and can be realized in the form of, for example, a method for manufacturing a metal-organic framework, or a humidity control device such as a heat pump or a desiccant air-conditioning system equipped with a metal-organic framework.

[0006] 1 is a schematic diagram showing the planar structure of MIL-101(Cr). 2 is a schematic diagram showing MIL-101(Cr) in which a portion of the ligands are substituted with formic acid. 3 is a flowchart outlining a method for producing a metal organic framework. 4 is an explanatory diagram showing XRD charts of each sample side by side. 5 is an explanatory diagram showing an SEM image of the "0FA" sample. 6 is an explanatory diagram showing an SEM image of the "20FA" sample. 7 is an explanatory diagram showing an SEM image of the "30FA" sample. 8 is an explanatory diagram showing an SEM image of the "40FA" sample. 9 is an explanatory diagram showing the TG curves of each sample. 10 is an explanatory diagram showing the relationship between the "formic acid blending ratio x" and the "formic acid substitution ratio x'". 11 is an explanatory diagram showing the water vapor adsorption isotherm per unit mass measured at 298 K. 12 is an explanatory diagram showing the relationship between the amount of adsorbed water vapor and time for each sample. 13 is an explanatory diagram showing the measurement conditions for measuring a nitrogen adsorption isotherm. 14 is an explanatory diagram showing the nitrogen adsorption isotherm per unit mass measured at 298 K. An explanatory diagram showing the results of calculating the pore distribution by applying the NLDFT method. An explanatory diagram showing the relationship between the differential pore volume when the pore diameters are 1.11 nm and 1.46 nm. An explanatory diagram showing the BET specific surface area and the NLDFT total pore volume together. An explanatory diagram showing the synchrotron X-ray diffraction charts of each sample lined up. An explanatory diagram showing the synchrotron X-ray diffraction charts of each sample superimposed. An explanatory diagram showing the results of calculating the diffraction intensity ratio (Y / X). An explanatory diagram showing the UV-Vis spectrum after Kubelka-Munk transformation. An explanatory diagram showing the values ​​of "γ / α" and "β / α".

[0007] A. Structure of the Metal Organic Framework: The metal organic framework of this embodiment includes a first ligand having a benzene ring and two or more functional groups for coordination, and a second ligand having no benzene ring and one functional group for coordination.

[0008] Here, the metal-organic framework is a porous metal complex obtained by utilizing the self-organization of metal ions and organic ligands, and is a crystalline material that forms a highly regular array structure. The presence of countless nanospaces within the array structure gives it excellent properties as an adsorbent. The metal-organic framework generally includes an organic ligand (corresponding to the first ligand in this embodiment) having two or more functional groups for coordination, which are arranged to surround the metal ions and form coordinate bonds with the metal ions. This allows the crystal structure to grow three-dimensionally via the organic ligand, forming a steric crystal structure in which each constituent element is regularly arranged three-dimensionally. In other words, the "functional group for coordination" can be said to be a functional group involved in crystal growth. The metal organic framework of the present embodiment further includes a second ligand having only one functional group for coordination in addition to a first ligand having two or more functional groups for coordination. This suppresses three-dimensional crystal growth and makes one-dimensional crystal growth more likely to proceed, thereby causing a change in the regularity of the crystal structure.

[0009] Furthermore, in the metal-organic framework of this embodiment, the first ligand has a benzene ring, but the second ligand does not. That is, the second ligand is an atomic group that occupies a smaller space in the three-dimensional structure in which the constituent elements are three-dimensionally arranged than the first ligand. That is, in the metal-organic framework of this embodiment, a portion of the first ligand, which is essential for forming a three-dimensional structure in which the constituent elements are three-dimensionally arranged regularly, is replaced with a second ligand that occupies a smaller space in the three-dimensional crystal structure. By adopting such a configuration, the metal-organic framework of this embodiment increases the adsorption rate of gases and water vapor. The effect of improving the adsorption rate is thought to be achieved by the fact that the replacement of the larger spatially-occupying first ligand with the smaller second ligand, as described above, generates new micropores in the metal-organic framework, thereby improving the diffusivity and fluidity of adsorbates such as water vapor within the metal-organic framework.

[0010] The first ligand included in the metal organic framework of this embodiment is not particularly limited as long as it has a benzene ring and two or more functional groups for coordination. For example, it can be a carboxylic acid having a carboxy group as the functional group for coordination, or an amine having an amino group. In particular, it is preferable that the functional group for coordination has two or more carboxy groups. For example, an aromatic dicarboxylic acid such as terephthalic acid or an aromatic tricarboxylic acid such as trimesic acid can be suitably used. Since a metal organic framework including a ligand having a carboxy group is generally synthesized by hydrothermal synthesis, using a ligand having a carboxy group as the ligand can result in a metal organic framework that is resistant to decomposition in water and has relatively high stability.

[0011] The second ligand in the metal organic framework of this embodiment is not particularly limited as long as it does not contain a benzene ring and contains one functional group for coordination, but as described above, from the viewpoint that it occupies a smaller space in the three-dimensional crystal structure than the first ligand, it is desirable that it does not contain a cyclic hydrocarbon skeleton and that it has a linear hydrocarbon skeleton. If it has a linear hydrocarbon skeleton, the number of carbon atoms in the hydrocarbon skeleton is desirably 6 or less, more desirably 5 or less, and even more desirably 4 or less.

[0012] Furthermore, the second ligand is preferably a monocarboxylic acid having a carboxy group as a functional group for coordination. Examples of monocarboxylic acids used as the second ligand include formic acid and acetic acid. Among these, formic acid is preferably used. Since formic acid has a relatively small acid dissociation constant pKa compared to other monocarboxylic acids such as acetic acid, during the synthesis of the metal organic framework, the ion concentration of the monocarboxylic acid in the solution increases, making it easier for the synthesis reaction to proceed. Therefore, the amount of monocarboxylic acid used as a raw material can be reduced.

[0013] Furthermore, it is desirable that the functional group for coordination possessed by the second ligand is the same functional group as the functional group for coordination possessed by the first ligand. For example, when a dicarboxylic acid is used as the first ligand, it is desirable to use a monocarboxylic acid as the second ligand. This enables crystal growth via the first ligand and crystal growth via the second ligand to be performed in a common step, thereby simplifying the manufacturing process of the metal-organic framework. Furthermore, since the first ligand and the second ligand have the same functional group as the functional group for coordination, good coordinate bonding can be achieved with the metal ions contained in the metal-organic framework.

[0014] The metal ion as the complex metal constituting the metal organic framework of the present embodiment may be appropriately selected in consideration of the combination with the ligand depending on the application of the metal organic framework (for example, the application of gas occlusion and separation, the type of target gas, etc.), and is not particularly limited. For example, a zirconium ion (Zr ４＋ ), zinc ions (Zn ２＋ ), copper ions (Cu ２＋), aluminum ions (Al ３＋ ), iron ions (Fe ３＋ ), cobalt ions (Co ３＋ ), nickel ions (Ni ２＋ ), magnesium ions (Mg ２＋ ), chromium ions (Cr ３＋ ), manganese ions (Mn ２＋ ) can be at least one selected from the following.

[0015] Further, below, a metal-organic framework will be described assuming that a "metal-organic framework having only a first ligand as a ligand" is identified as a "basic structure MOF" based on the results of powder XRD analysis of the metal-organic framework. To identify the above-mentioned basic structure MOF for a metal-organic framework, powder XRD analysis is performed on the metal-organic framework of interest, and the basic structure MOF can be identified by comparing the obtained XRD chart with the XRD pattern in the CIF file (data obtained by X-ray structure analysis, and XRD pattern included in crystal structure data registered in accordance with the Common Information Format for Crystallography (CIF)).

[0016] In the metal-organic framework of this embodiment, which is obtained by adding a second ligand in addition to a first ligand to the composition of a basic structure MOF, the peak pattern of the basic structure MOF is maintained in the XRD pattern up to a certain range of the substitution ratio of the second ligand to the first ligand. At this time, in the XRD pattern of the metal-organic framework, depending on the type of second ligand contained in the metal-organic framework and the amount of the second ligand added, peaks that are not included in the XRD pattern of the basic structure MOF may appear. However, even in such cases, the peak pattern of the XRD pattern of the basic structure MOF is maintained. Therefore, the metal-organic framework of this embodiment can be identified as a basic structure MOF by XRD analysis.

[0017] Note that the metal organic framework contains metal ions together with ligands. Generally, metal organic frameworks having the same ligands and in which the constituent elements are arranged with the same regularity will have similar XRD patterns, making it difficult to identify the metal ions contained in the metal organic framework from the results of XRD analysis. Even when the metal organic framework contains a second ligand in addition to a first ligand, the XRD pattern may not change compared to the basic structure MOF, making it difficult to identify the second ligand from the results of XRD analysis of the metal organic framework. Furthermore, as will be described later, when the metal organic framework of this embodiment contains a halogen, it is also difficult to identify the halogen contained in the metal organic framework from the results of XRD analysis. Therefore, when identifying the metal ions or halogens contained in the metal organic framework and the corresponding basic structure MOF, or when identifying the second ligand of the metal organic framework, the metal ions, halogens, and second ligands may be identified by ion chromatography analysis, nuclear magnetic resonance (NMR) analysis, or ICP emission spectroscopy.

[0018] As described above, the metal-organic framework of this embodiment can be constructed using various combinations of first ligands, second ligands, and metal ions, but the basic structure MOF can be constructed, for example, from one type of first ligand and one type of metal. Examples of the metal-organic framework of this embodiment include metal-organic frameworks having a ligand having a benzene ring and two or more carboxy groups, such as metal-organic frameworks having MIL-101, HKUST-1, or UiO-66-BDC as the basic structure MOF. Furthermore, in the metal-organic framework of this embodiment, the basic structure MOF preferably has, for example, an MIL-type crystal structure. This is because metal-organic frameworks having an MIL-type crystal structure are generally synthesized by hydrothermal synthesis and are therefore resistant to decomposition in water and have relatively high stability. Specifically, those having a cubic crystal structure, such as MIL-100 series having trimesic acid as the first ligand and MIL-101 series having terephthalic acid as the first ligand, are desirable. Among these, MIL-101 is desirable. MIL-101 is represented by the following composition formula (1).

[0019]

[0020] (wherein Me is a metal element, and X represents an element or atomic group that can be changed depending on the raw material used, and can be, for example, a halogen such as Cl or F.)

[0021] MIL-101 is known as a metal-organic framework that adsorbs a relatively large amount of water, and the raw material cost for its production is relatively low. Therefore, a metal-organic framework having a basic structure MOF of MIL-101 is particularly preferable as a metal-organic framework to be used for adsorbing and separating water vapor.

[0022] FIG. 1 shows the structure of MIL-101(Cr) as an example of a basic structure MOF, in which metal ions (Cr ３＋ 2 is a schematic diagram showing, in a plane, how terephthalic acid, a ligand, is arranged to surround and coordinate bond with MIL-101(Cr). As an example of the metal-organic framework of this embodiment, FIG. 2 is a schematic diagram showing, in the same manner as FIG. 1, the configuration of a metal-organic framework in which some of the ligands of MIL-101(Cr) are substituted with formic acid, a second ligand. In FIG. 2, the anion of formic acid used for substitution is shown surrounded by a dashed circle.

[0023] As shown in Figure 1, in MIL-101(Cr) having only the first ligand as a ligand, terephthalic acid having two carboxyl groups, which are functional groups for coordination, links metal ions (metal clusters), forming a three-dimensional structure in which the constituent elements are regularly arranged three-dimensionally. In contrast, in MIL-101(Cr) in which some of the ligands are substituted with formic acid, the formic acid has only one functional group for coordination, and therefore the links between the metal ions (metal clusters) are interrupted at the formic acid, as shown in Figure 2. Furthermore, since formic acid, which is the second ligand without a benzene ring, occupies a smaller space in the crystal structure than terephthalic acid, which is the first ligand with a benzene ring, in the metal-organic framework of the present embodiment, by substituting some of the first ligand with the second ligand, new micropores that contribute to the diffusivity and fluidity of gases and water vapor are generated within the metal-organic framework, and the adsorption rate of gases and water vapor is increased.

[0024] Furthermore, the metal organic framework of this embodiment has a water vapor adsorption isotherm measured at 298 K, and the relative pressure (P / P ０ : P is pressure (Pa), P ０ is a saturated vapor pressure (Pa)) of 0.95, the ratio of the water vapor adsorption amount per unit mass of the metal organic framework of this embodiment when the relative pressure is 0.95 to the water vapor adsorption amount per unit mass when the saturated vapor pressure (Pa) is 0.95 (hereinafter also referred to as the "ratio of water vapor adsorption amount per unit mass to the basic structure MOF at 298 K") is 84% ​​or more. The above "ratio of water vapor adsorption amount per unit mass to the basic structure MOF at 298 K" is preferably 85% or more, more preferably 90% or more, and even more preferably 95% or more.

[0025] The metal-organic framework of this embodiment further includes a second ligand added to the basic MOF composition, and the addition of such a second ligand changes the water vapor adsorption amount per unit mass when the metal-organic framework is used as an adsorbent. As described above, the addition of the second ligand can improve the gas and water vapor adsorption rate, but as the amount of the second ligand added becomes excessive, the water vapor adsorption amount per unit mass of the metal-organic framework tends to decrease. The above-described feature based on the water vapor adsorption isotherm serves as an index for eliminating a state in which the addition of the second ligand reduces the water vapor adsorption amount per unit mass, thereby hindering the effect of improving the performance of the adsorbent (improving the water vapor adsorption amount per unit mass) due to the improved adsorption rate caused by the addition of the second ligand.

[0026] Furthermore, in the metal-organic framework of this embodiment, when the basic structure MOF is MIL-101, it is desirable that the metal-organic framework have the following characteristic feature: in the pore size distribution calculated by applying the NLDFT method from the nitrogen adsorption isotherm measured at 77 K, the differential pore volume of pores having a pore diameter of 1.46 nm is larger than the differential pore volume of pores having a pore diameter of 1.11 nm. As described above, in the metal-organic framework of this embodiment, some of the first ligands having a benzene ring are replaced with second ligands that occupy a smaller space in the crystal structure. In this way, it is thought that the replacement of some of the first ligands with second ligands and the loss of the atomic group containing the benzene ring contained in the first ligand newly generates micropores having a pore diameter of about 1.46 nm as described above. It is thought that the generation of new micropores having a pore diameter of about 1.46 nm improves gas flow (gas diffusibility and fluidity) within the metal-organic framework and increases the adsorption rate of gas and water vapor. Therefore, the above-mentioned characteristic point related to the pore volume can be an index for determining whether the addition of the second ligand has generated new micropores in the metal-organic framework, thereby enhancing gas diffusibility.

[0027] Furthermore, as described above, the metal organic framework of this embodiment further includes a second ligand, which causes a partial change in the regularity of the crystal structure, and this may result in a change in the results of X-ray diffraction analysis. For example, when the basic structure MOF is MIL-101, such a change can be observed in the "ratio (Y / X) of the diffraction peak intensity Y of the (111) plane to the diffraction peak intensity X of the (311) plane" in X-ray diffraction analysis using synchrotron radiation. In a metal organic framework in which a second ligand is added to MIL-101, the ratio (Y / X) in X-ray diffraction analysis using synchrotron radiation is preferably less than 0.42, and more preferably less than 0.14. Furthermore, the ratio (Y / X) is preferably greater than 0.043.

[0028] Furthermore, the metal organic framework of the present embodiment may further include a second ligand, causing a partial change in the regularity of the crystal structure, which may also cause a change in the results of the diffuse reflectance spectrum (UV-Vis spectrum) obtained by ultraviolet-visible spectroscopy. For example, when the basic structure MOF is MIL-101, such a change can be observed in the "ratio (β / α) of the strongest peak intensity β in the wavelength range of 515 nm or more and less than 750 nm to the strongest peak intensity α in the wavelength range of 250 nm or more and less than 400 nm" in the spectrum obtained by Kubelka-Munk transformation (transformed using a K-M function). In the metal organic framework in which a second ligand is added to MIL-101, the ratio (β / α) in the spectrum obtained by Kubelka-Munk transformation of the UV-Vis spectrum is preferably 0.045 or less, and more preferably 0.040 or less.

[0029] B. Method for Producing a Metal-Organic Framework: Figure 3 is a flowchart showing an outline of the method for producing a metal-organic framework of this embodiment. When producing the metal-organic framework of this embodiment, first, raw materials for synthesizing the metal-organic framework are prepared and weighed (step T100). Specifically, raw materials including a compound serving as a metal ion source constituting the metal-organic framework, a first ligand, a second ligand, and a solvent are weighed. The types and combinations of these raw materials may be appropriately selected depending on the composition of the metal-organic framework to be produced.

[0030] As the metal ion source, metal salts such as nitrates, chlorides, oxides, etc. that generate the target metal ions can be used. For example, when producing a metal organic framework having a basic structure MOF of MIL-101(Cr), a metal salt, chloride, oxide, etc. that generates a trivalent metal ion such as chromium can be prepared. As the first ligand, for example, the above-mentioned aromatic dicarboxylic acids including terephthalic acid or aromatic tricarboxylic acids including trimesic acid can be suitably used. Furthermore, as the second ligand, for example, a monocarboxylic acid such as formic acid or acetic acid can be suitably used.

[0031] The compounding ratio of the first ligand to the second ligand may be appropriately set according to the type (composition) of the metal-organic framework to be synthesized, so that the addition of the second ligand causes an appropriate deformation in the crystal structure of the basic structure MOF that brings about the effect of improving the adsorption rate. For example, consider the case of producing a metal-organic framework in which the basic structure MOF is MIL-101(Cr), and in which terephthalic acid, a dicarboxylic acid, is used as the first ligand and formic acid, a monocarboxylic acid, is used as the second ligand. Here, when the blending amount (charge amount) of the first ligand when producing a basic structure MOF (MIL-101(Cr)) containing only the first ligand (terephthalic acid) as a ligand is taken as 100%, the ratio (molar ratio) of the blending amount of the first ligand reduced according to the blending amount of the second ligand in order to partially replace it with the second ligand (formic acid) in the metal-organic framework of this embodiment is referred to as the "substitution ratio of the blending amount of the first ligand." Such a "substitution ratio (molar ratio) of the blending amount of the first ligand" is preferably 10% or more, more preferably 15% or more, and even more preferably 20% or more. Furthermore, it is preferably less than 40%, more preferably 38% or less, and even more preferably 35% or less.

[0032] The above-mentioned "amount (charge amount) of the first ligand when producing the basic structure MOF (MIL-101(Cr))" is a value set based on the ratio (molar ratio) of the first ligand to the metal ion source compound in the composition formula of the desired metal-organic framework to be produced. In step T100, the amounts of the raw material reagents can be appropriately adjusted, taking into consideration, for example, the purity of the raw material reagents used, so that a metal-organic framework of the desired composition can be obtained with sufficient purity.

[0033] As the solvent, for example, water, methanol, ethanol, N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), or a mixture thereof can be used, and may be appropriately selected depending on the types of other raw materials including the first and second ligands, etc.

[0034] After step T100, solvothermal synthesis is carried out using these raw materials (step T110). For example, the metal organic framework having the above-mentioned MIL-type crystal structure is generally suitable for hydrothermal synthesis using water as a solvent. For example, when a carboxylic acid is used as the first ligand, the carboxyl group, which is a functional group for coordination, is deprotonated to form "-CO ２ － " coordinates with the metal ion, and a crystal structure grows, synthesizing a metal-organic framework.

[0035] After the synthesis reaction is completed, the liquid containing the synthesized metal-organic framework is filtered (step T120) to recover a solid component containing the metal-organic framework. Then, components other than the metal-organic framework, specifically, remaining insoluble raw materials, in the solid component recovered in step T120 are dissolved (step T130). For example, when terephthalic acid is used as the first ligand and the metal-organic framework is synthesized by hydrothermal synthesis using water as the solvent, the solubility of terephthalic acid in water is relatively low. Therefore, the remaining terephthalic acid that is not used in the synthesis reaction can be recovered in the solid component as an unnecessary component. In such a case, by using an appropriate solvent such as dimethylformamide (DMF), the remaining insoluble raw materials can be dissolved without dissolving the metal-organic framework.

[0036] After dissolving the remaining insoluble raw materials, the solution containing the remaining insoluble raw materials is filtered (step T140), thereby recovering a solid component containing the metal-organic framework with increased purity. The solid component recovered in step T140 is dried to remove the solvent (step T150), thereby obtaining the metal-organic framework.

[0037] C. Use example of metal-organic framework as adsorbent: The metal-organic framework of the present embodiment can be used, for example, as a water vapor adsorbent in a humidity control device. An example of a device including the metal-organic framework of the present embodiment as a water vapor adsorbent will be described below.

[0038] (C-1) Heat Pump: The metal organic framework of this embodiment can be used, for example, as a moisture absorbent included in an adsorption heat pump. An adsorption heat pump has a configuration that combines an adsorber configured as a heat exchanger filled with an adsorbent with an evaporator that evaporates water using the adsorption power of the adsorbent, or a configuration that combines an adsorber configured as a heat exchanger filled with an adsorbent with a condenser that condenses water vapor desorbed from the adsorbent using exhaust heat to form water. In such an adsorption heat pump, the metal organic framework of this embodiment can be used as the adsorbent included in the adsorbent.

[0039] (C-2) Desiccant Air Conditioning System: The metal-organic framework of this embodiment can be used as an adsorbent that is provided in a desiccant air conditioning system and adsorbs water vapor. A desiccant air conditioning system is an apparatus that includes a dehumidifying rotor made of an adsorbent, removes moisture from air such as outside air, and supplies air with an appropriately adjusted humidity to a room or the like. A rotor-shaped adsorbent is provided, and a portion of the rotor adsorbs moisture from air such as outside air. The portion of the rotor that adsorbs moisture moves as the rotor rotates, and is heated at its destination, thereby releasing moisture and humidifying the air in the room or the like. In such a desiccant air conditioning system, the metal-organic framework of this embodiment can be used as the adsorbent that constitutes the dehumidifying rotor.

[0040] According to the metal-organic framework of the present embodiment configured as described above, in a basic structure MOF including a first ligand having a benzene ring as a ligand, a portion of the first ligand is replaced with a second ligand that does not include a benzene ring and includes one functional group for coordination. By replacing a portion of the first ligand with a second ligand that occupies less space in the three-dimensional crystal structure in this manner, new micropores that contribute to the diffusivity and fluidity of gases and water vapor are generated within the metal-organic framework, thereby increasing the adsorption rate of gases and water vapor. As a result, it is possible to increase the throughput in a system including the metal-organic framework and to make the device including the metal-organic framework more compact.

[0041] <Preparation of Metal-Organic Frameworks> As metal-organic frameworks, MIL-101(Cr) containing only terephthalic acid as a ligand and metal-organic frameworks in which formic acid, a second ligand, was added to MIL-101(Cr) at various ratios were prepared based on the preparation method shown in Fig. 3. The prepared metal-organic frameworks are represented by the composition formula (2) below.

[0042]

[0043] In the above formula (2), x represents the "proportion (molar ratio) of the amount of the first ligand reduced in accordance with the amount of the second ligand" when the second ligand (formic acid) is added, assuming that the "amount (charge) of the first ligand when producing the basic structure MOF (MIL-101(Cr))" is 100%, and corresponds to the "substitution ratio of the amount of the first ligand" described above. The "amount of the first ligand when producing the basic structure MOF" is a value set based on the ratio (molar ratio) of the first ligand to the metal ion source compound in the composition formula (2) (where x = 0). Six types of samples were prepared, with the "substitution ratio (molar ratio) x of the amount of the first ligand" being 0%, 20%, 30%, 40%, 50%, and 60%. The samples with the "substitution ratio (molar ratio) x of the amount of the first ligand" of 0%, 20%, 30%, 40%, 50%, and 60% are also referred to as "0FA," "20FA," "30FA," "40FA," "50FA," and "60FA," respectively. The "0FA" sample is a sample with the same composition as the basic structure MOF. Note that, when the amount of the dicarboxylic acid first ligand is reduced by an amount equivalent to the above-mentioned molar ratio x% relative to the basic structure MOF, the amount of the monocarboxylic acid second ligand is set to an amount equivalent to a molar ratio twice the above-mentioned molar ratio x%.

[0044] When preparing each of the above samples, chromium nitrate (Cr(NO ３ ) ３ ), terephthalic acid, formic acid, and hydrochloric acid (HCl) were used. When preparing a sample in which the "substitution ratio (molar ratio) of the blending amount of the first ligand" is x%, the blending amount [mmol] of each raw material is (Cr(NO ３ ) ３: terephthalic acid : formic acid : HCl) = (8.0 : 8.0 × (1 − x / 100 ): 8.0 × ( x / 100 ) × 2 : 3.95). In step T100, 42 mL of pure water was further added to the above-mentioned raw materials.

[0045] In step T110, the raw material prepared in step T100 was placed in a 100 mL container made of PTFE (polytetrafluoroethylene), which was then sealed in a stainless steel container and subjected to hydrothermal synthesis at 220°C for 8 hours. Thereafter, in the filtration step of step T120, washing with methanol was performed three times. In step T130, the solid component recovered in step T120 was stirred in N,N-dimethylformamide (DMF) at room temperature for 12 hours or more to dissolve the remaining insoluble raw material, terephthalic acid. Thereafter, in the filtration step of step T140, washing with methanol was performed three times. In step T150, the solid component recovered in step T140 was dried at 80°C for 12 hours or more to obtain each sample, which was a metal-organic framework.

[0046] For "0FA," "20FA," "30FA," and "40FA," samples with different washing conditions were further prepared by changing only steps T130 and T140. Specifically, instead of the above-described steps T130 and T140, the recovered solid component was washed with 300 mL of methanol, followed by centrifugation at 3,500 to 200,000 rpm, which was repeated twice to prepare samples. These samples for "0FA," "20FA," "30FA," and "40FA" with different washing conditions were used to obtain diffuse reflectance spectra (UV-Vis spectra) by ultraviolet-visible spectroscopy, as described below.

[0047] <Powder X-ray Diffraction> Powder X-ray diffraction patterns were obtained for each of the samples "0FA," "20FA," "30FA," and "40FA" prepared as described above by the following method. First, each sample was pretreated by evacuating it at 125°C for at least 6 hours. Then, powder XRD analysis was performed using an X-ray diffractometer (MiniFlex600, manufactured by Rigaku Corporation). XRD was measured using a CuKα radiation source, with a tube voltage of 40 kV and a tube current of 15 mA.

[0048] Figure 4 is an explanatory diagram showing the XRD charts of the samples "0FA," "20FA," "30FA," and "40FA" side by side. As shown in Figure 4, the XRD patterns of each sample match the peak patterns of the XRD chart of the CIF file of MIL-101(Cr). Based on the XRD patterns, it was confirmed that the basic structure MOF can be identified as MIL-101(Cr).

[0049] <Observation of particle shape> The particles of each of the prepared samples "0FA", "20FA", "30FA", and "40FA" were observed using SEM images. Prior to observation using a scanning electron microscope (SEM), each of the above samples was pretreated by evacuating to a vacuum at 125°C for 6 hours or more.

[0050] Figures 5 to 8 are explanatory diagrams showing SEM images of samples "0FA," "20FA," "30FA," and "40FA," respectively. As shown in Figure 5, the primary particle shape of "0FA," which is a MOF with a basic structure, is octahedral. Furthermore, as shown in Figures 6 to 8, by adding a second ligand, the crystal particles were deformed so that the octahedral vertices became rounded, resulting in crystal particles with a truncated octahedral shape. The degree of deformation of the particle shape and the proportion of particles with a truncated octahedral shape increased with increasing amounts of the second ligand. Furthermore, in the "40FA" sample, as shown in Figure 8, "particles ND," which are aggregates of elongated rod-like particles (nanorod-like particles), were observed, which are particles with a particle shape completely different from an octahedral shape. Such nanorod-shaped particles are thought to be by-products whose crystal structure and particle shape are significantly different from those of MIL-101, resulting from the addition of a second ligand, which makes it easier for the growth of a one-dimensional crystal structure to proceed.

[0051] <Derivation of Formic Acid Substitution Amount> Each of the above-described samples is specified by the blending amount of formic acid, which is the second ligand. The results of determining the substitution amount of formic acid in each of these samples (x when the composition of each sample is expressed by formula (2)) from measured values ​​will be described below. In the following description, with regard to the proportion of formic acid, the above-mentioned "substitution ratio (molar ratio) x of the blending amount of the first ligand" based on the blending amount of the raw materials in each sample set using formula (2) will also be referred to as "formic acid blending ratio x." Furthermore, the "proportion (molar ratio) of formic acid derived as x in formula (2)" derived based on measured values ​​as described below will also be referred to as "formic acid substitution ratio x'." An attempt was made to derive the "formic acid substitution ratio x'" for "0FA," "20FA," "30FA," and "40FA." However, for "40FA," which had the highest blending amount of second ligand and suffered significant deformation of the crystal structure and particle shape, even when the "formic acid substitution ratio x'" was calculated in the same manner as for the other samples, the value exceeded 100%, and no reasonable result was obtained. Therefore, the results for "0FA," "20FA," and "30FA" will be explained below.

[0052] FIG. 9 is an explanatory diagram showing the TG curves obtained by performing thermogravimetry (TG) on each sample. First, each of the above samples was pretreated by treating overnight at 125°C. Thereafter, TG measurement was performed under conditions of an air atmosphere, an air supply rate of 150 mL / min, and a temperature rise rate of 5°C / min. As shown in FIG. 9, in the TG curves corresponding to all of the samples, a trailing edge (the point at which the rate of decrease in the TG value becomes maximum) was observed around 300°C (280 to 290°C), and a rapid decrease in mass was observed at the trailing edge. This rapid decrease in mass is thought to indicate that the first and second ligands, etc., are released from each sample, which is a metal-organic framework, causing structural collapse. For each sample, the mass loss from the trailing edge to 400°C was measured, and the "mass change Δm ｘ 9, as an example, the mass change amount Δm ０ Due to the structural collapse caused by the heating, each metal organic framework shown in formula (2) is dehydrated to chromium (III) oxide (Cr ２ O３ ) was generated, and calculations were performed to determine the "substitution ratio x' of formic acid."

[0053] 10A and 10B are explanatory diagrams showing the relationship between the "formic acid blending ratio x" and the "formic acid substitution ratio x'". FIG. 10A shows the relationship between the "formic acid blending ratio x" and the "mass change Δm ｘ Specific values ​​for "theoretical value Δm ｘ [g]" represents the total amount of the metal organic framework represented by formula (2) in a dehydrated state, which is then dehydrated to form chromium (III) oxide (Cr ２ O ３ ) occurs, and indicates the calculated value of the mass change when the value of x in formula (2) is "the blending ratio x of formic acid" and the mass of the metal organic framework in a dehydrated state before the reaction is 1 g. ｘ "x'" is a value calculated as described above based on the TG curve shown in FIG. 9, and indicates the value when the mass of the metal organic framework in a dehydrated state before the reaction is 1 g. "Substitution ratio x' of formic acid" is the value obtained when the entire amount of the metal organic framework in a dehydrated state represented by formula (2) is structurally collapsed and chromium (III) oxide (Cr ２ O ３ The mass loss when the mass loss occurs is the mass loss Δm ｘ The "formic acid substitution ratio x'" is considered to represent the true value of x in formula (2) that represents the metal organic framework obtained when the metal organic framework of formula (2) is synthesized by blending raw materials at "formic acid blending ratio x". As shown in FIG. 10 , the "theoretical value Δm ｘ " and "Mass change Δm ｘ One of the reasons for this is that the metal-organic framework could not be completely dehydrated before the temperature rise that caused the structure to collapse, resulting in a mass change of Δm ｘ '" includes the amount of dehydration, and "theoretical value Δm ｘ It is thought that the value was larger than that of

[0054] As shown in FIG. 10A, the larger the blending ratio of the second ligand, the smaller the "mass change amount Δm" obtained from the TG curve. ｘ This is thought to indicate that, because the second ligand has a smaller molecular weight than the first ligand, the amount of mass loss due to elimination of the ligand decreased as the blending ratio of the second ligand increased.

[0055] 10(B) is a graph showing the relationship between the "formic acid blending ratio x" and the "formic acid substitution ratio x'" described above. As shown in FIG. 10(B), the "formic acid substitution ratio x'" increased nonlinearly with an increase in the "formic acid blending ratio x."

[0056] <Measurement of Water Vapor Adsorption Isotherm> For each of the prepared samples "0FA", "20FA", "30FA", "40FA", "50FA", and "60FA", a water vapor adsorption isotherm per unit mass was measured using Belsorp MAX (manufactured by Microtrac-Bell Co., Ltd.). First, each of the above samples was placed in a dedicated sample cell, and pre-treated by evacuating at 125°C for 6 hours or more. At this time, the amount of metal organic framework put into the sample cell was set to an amount that would be about 100 mg after the above pre-treatment. Then, under a temperature condition of 298 K (25°C), the relative pressure (P / P ０ : P is pressure (Pa), P ０ The adsorption isotherms were measured on the adsorption and desorption sides in the saturated vapor pressure (Pa) range of 0 to 0.95. The relative pressure change was 0.05, or the adsorption amount change per unit mass was 200 cm. ３ g －１ When measuring the water vapor adsorption isotherm, equilibrium was determined when the pressure change was less than 0.3% at 25°C for 300 seconds.

[0057] FIG. 11 is an explanatory diagram showing the results of the water vapor adsorption isotherm per unit mass measured at 298 K. FIG. 11(A) shows the obtained water vapor adsorption isotherm. As shown in FIG. 11(A), the water vapor adsorption isotherms per unit mass of "0FA" and "20FA" were almost the same. Furthermore, when comparing "20FA," "30FA," "40FA," "50FA," and "60FA," which contained a second ligand, the water vapor adsorption amount per unit mass decreased as the proportion of the second ligand increased in the relative pressure range of 0.6 or higher.

[0058] Furthermore, based on the water vapor adsorption isotherm per unit mass in Figure 11, the ratio of the water vapor adsorption amount per unit mass for other metal organic frameworks at a relative pressure of 0.95 to the water vapor adsorption amount per unit mass for "0FA" corresponding to the basic structure MOF at a relative pressure of 0.95 was calculated. Figure 11(B) shows the water vapor adsorption isotherm per unit mass of other metal organic frameworks at a relative pressure of 0.95. ０ 11(B) is an explanatory diagram showing the water vapor adsorption amount per unit mass when the ratio of the second ligand to the water vapor adsorption amount is 0.95, and the ratio of each sample to "0FA" for the water vapor adsorption amount. Note that FIG. 11(B) shows the results for "0FA," "20FA," "30FA," and "40FA." As shown in FIG. 11(B), the ratio decreased as the blending ratio of the second ligand increased. The ratio for "40FA" was 83%, while the ratios for "20FA" and "30FA" were 84% or higher.

[0059] <Evaluation of Adsorption Rate> Thermogravimetry (TG) was performed on each of the prepared samples "0FA", "20FA", "30FA", and "40FA" to examine the relationship between the amount of water vapor adsorption and time, and evaluate the water vapor adsorption rate.

[0060] FIG. 12 is an explanatory diagram showing the relationship between the amount of water vapor adsorbed for each sample and time. FIG. 12(A) shows the entire measurement from start to finish, and FIG. 12(B) shows an enlarged view of a portion of FIG. 12(A). Here, the horizontal axis represents time, and the vertical axis represents the ratio of the amount of water vapor adsorbed to the metal-organic framework and temperature. The ratio of the amount of water vapor adsorbed to the metal-organic framework refers to the ratio of the mass of water vapor adsorbed to each sample (m-HO) to the mass of each sample used for adsorption (m-MOF). The mass of each sample (m-MOF) is the value when each sample is heated and held at 125°C and the mass increase / decrease reaches equilibrium. The mass of water vapor adsorbed to each sample (m-HO) is the value obtained by subtracting the mass of each sample (m-MOF) at the start of the water vapor adsorption reaction from the mass of each sample containing water vapor after the water vapor adsorption reaction has started.

[0061] When performing TG measurement, each sample was first pretreated by treating it overnight at 125°C. Then, aggregated primary particles were removed by passing it through a 100 μm mesh sieve. Subsequently, TG measurement was performed under conditions of a nitrogen atmosphere, a nitrogen supply rate of 100 mL / min, and a temperature decrease rate of 10°C / min, where the temperature was decreased from 125°C to 25°C. Before being supplied to the TG apparatus, the nitrogen used in the TG measurement was first humidified using a bubbler adjusted to 25°C, and then further conditioned using a glass tube for a water trap adjusted to 13°C. The mass of each sample used in the measurement was adjusted to 2.3 to 2.4 mg.

[0062] As shown in Figures 12(A) and 12(B), the slope of the graph for "20FA" and "30FA" was greater than that for "0FA," confirming an improvement in the adsorption speed. In Figure 12(B), the range in which the slope of the graph for "20FA" and "30FA" increases is indicated by a double-headed arrow. In contrast, the slope of the graph for "40FA" was smaller than that for "0FA," confirming a decrease in the adsorption speed.

[0063] It should be noted that when adsorption is performed, the rate-limiting factors for mass transfer accompanying adsorption are known to be extra-particle diffusion rate-limiting (the rate of movement in the gas boundary film outside the particles of the adsorbent) and intra-particle diffusion rate-limiting (the diffusion rate within the particles of the adsorbent), and it is considered important to evaluate the adsorption performance of an adsorbent based on the intra-particle diffusion rate. Conventional knowledge regarding the measurement of the adsorption rate of water vapor in a metal-organic framework includes the following: in order to observe the intra-particle diffusion rate-limiting, it is desirable that the mass of the metal-organic framework used is relatively small; MIL-101(Cr) has three types of pores with different sizes, namely, a middle cage with a diameter of approximately 29 Å, a large cage with a diameter of approximately 34 Å, and a micropore with a diameter of approximately 8 Å; and when the mass of the metal-organic framework is relatively large, the change in relative humidity (ΔP / P ０ It has been reported that by suppressing the mass of the metal-organic framework, after the middle cage is filled with water vapor, the diffusion and adsorption of water vapor in the large cage can be observed as being limited by intra-particle diffusion (K. Yanagita et al., J. Phys. Chem. C 123 (2019) 387-398). Prior to the TG measurements shown in Figures 12(A) and 12(B), preliminary experiments in which the mass of the metal-organic framework was changed showed that the error increased as the mass of the metal-organic framework was reduced, making measurement difficult (data not shown). Therefore, the mass of the metal-organic framework that allows good measurement was set to 2.3 to 2.4 mg, as described above, and the TG measurements shown in Figures 12(A) and 12(B) were performed. In Figures 12(A) and 12(B), differences in the adsorption rate between samples occurred after a certain amount of time had passed since the start of the measurement. This is thought to indicate that after the middle cage is filled with water vapor, the water vapor diffuses and adsorbs in the large cage, and is thought to represent the result of evaluating the rate-limiting behavior of intragranular diffusion.

[0064] <Measurement of Nitrogen Adsorption Isotherm> For each of the prepared samples "0FA", "20FA", "30FA", and "40FA", a nitrogen adsorption isotherm per unit mass was measured using Belsorp MAX (manufactured by Microtrac-Bell Corporation). First, each of the above samples was placed in a dedicated sample cell, and pre-treated by evacuating at 125°C for 6 hours or more. At this time, the amount of metal-organic framework put into the sample cell was set to an amount that would be about 100 mg after the above pre-treatment. Then, under a temperature condition of 77 K (-196°C), the relative pressure (P / P ０ : P is pressure (kPa), P ０ The adsorption isotherms on the adsorption and desorption sides were measured in the range of 0 to 1.0 (101.325 (kPa)).

[0065] FIG. 13 shows the measurement conditions for measuring the nitrogen adsorption isotherm, specifically, the measurement pressure during adsorption (P / P ０ ) and the measured pressure during desorption (P / P ０ ) is shown in FIG. 13. ０ ) or the change in adsorption amount per unit mass is 50 cm ３ g －１ When measuring the nitrogen adsorption isotherm, equilibrium was determined when the pressure change was less than 0.3% at 77 K for 300 seconds.

[0066] Fig. 14 is an explanatory diagram showing the nitrogen adsorption isotherm per unit mass measured at 77 K. Fig. 14(B) shows the results shown in Fig. 14(A) with the relative pressure (P / P ０ ) on a logarithmic scale.

[0067] FIG. 15 is an explanatory diagram showing the results of calculating the pore distribution by applying the NLDFT method to the nitrogen adsorption isotherm described above. Analysis by the NLDFT method was performed using the analysis software BELMaster (Version 7.3.2.0), and the analysis settings were: curve interpolation "cubic spline curve", model "Cylinder", adsorbent "Metal Oxide", data selection "adsorption", fitting method "Tikhonov regularization", and pore size definition "Solid Definition". In FIG. 15, the horizontal axis shows the pore diameter D, and the vertical axis shows the value (dV / dD) obtained by differentiating the pore volume with the pore diameter. FIG. 15(A) shows the pore diameter range from 0 to 10 nm, and FIG. 15(B) shows an enlarged view of the pore diameter range from 1 to 2 nm.

[0068] As previously mentioned, MIL-101(Cr) is known to have three types of pores with different sizes: a middle cage with a diameter of approximately 29 Å, a large cage with a diameter of approximately 34 Å, and a micropore with a diameter of approximately 8 Å (K. Yanagita et al., J. Phys. Chem. C 123 (2019) 387-398). In Figure 15(A), for each of the "0FA," "20FA," "30FA," and "40FA" samples, a peak (indicated by a white star) indicating a pore thought to correspond to a middle cage with a diameter of approximately 29 Å (2.9 nm) and a peak (indicated by a black star) indicating a pore thought to correspond to a large cage with a diameter of approximately 34 Å (3.4 nm) are observed.

[0069] Furthermore, in FIG. 15(B), a peak is observed at a pore diameter of 1.11 nm in "0FA," which is MIL-101(Cr). This peak is thought to correspond to the micropores with a diameter of approximately 8 Å among the three types of pores known in the art. Furthermore, in FIG. 15(B), in "20FA," "30FA," and "40FA," which contain a second ligand, the differential pore volume of pores with a pore diameter greater than 1.11 nm is increased relative to the differential pore volume of pores with a pore diameter of 1.11 nm. In particular, the degree of increase in differential pore volume is large at a pore diameter of approximately 1.46 nm. In FIG. 15(B), the areas corresponding to pore diameters of 1.11 nm and 1.46 nm are indicated by hatching.

[0070] 16 is an explanatory diagram showing the differential pore volume (dV / dD) value of pores having a pore diameter of 1.11 nm (hereinafter also referred to as "P1.11"), the differential pore volume (dV / dD) value of pores having a pore diameter of 1.46 nm (hereinafter also referred to as "P1.46"), and the ratio value (P1.46 / P1.11) of these values, which are calculated based on the pore diameter distribution of FIG. 15. As shown in FIG. 16, unlike "0FA", in "20FA", "30FA" and "40FA" in which a second ligand was blended, the differential pore volume of pores having a pore diameter of 1.46 nm was larger than the differential pore volume of pores having a pore diameter of 1.11 nm. At this time, the larger the blending ratio of the second ligand in the sample, the larger the value of (P1.46 / P1.11). This is thought to indicate that by substituting a portion of the first ligand (terephthalic acid) having a benzene ring with the second ligand (formic acid) which occupies a smaller space in the crystal structure, new micropores that can contribute to the diffusivity and fluidity of gases and water vapor are generated within the metal-organic framework.

[0071] FIG. 17 is an explanatory diagram showing the BET specific surface area calculated by analysis using the nitrogen adsorption isotherm shown in FIG. 14 for each sample, and the total pore volume (also referred to as "NLDFT total pore volume") calculated by applying the NLDFT method to the nitrogen adsorption isotherm. As shown in FIG. 17, compared to "0FA", which is a basic structure MOF, each sample containing a second ligand had a reduced BET specific surface area and NLDFT total pore volume. In particular, "40FA" showed a large decrease in BET specific surface area and NLDFT total pore volume, which is thought to result in a decrease in performance as an adsorbent for gases and water vapor. Therefore, from the perspective of obtaining the effect of improving the adsorption rate by incorporating a second ligand while ensuring performance as an adsorbent in a metal-organic framework, it was confirmed that "20FA" and "30FA" are desirable among the samples shown in FIG. 17.

[0072] <X-ray diffraction using synchrotron radiation> X-ray diffraction measurements using synchrotron radiation were performed on each of the prepared samples, "0FA," "20FA," "30FA," "40FA," "50FA," and "60FA." X-ray diffraction measurements were performed on the BL5S2 beamline at the Aichi Synchrotron Light Center. Each sample was packed into a Lindemann glass capillary with a diameter of 1.0 mm and measured. The measurements were performed under conditions of a wavelength of 1.0 Å and an exposure time of 16 minutes (8 minutes x 2 times).

[0073] As an example, Fig. 18 is an explanatory diagram showing X-ray diffraction charts of "0FA," "20FA," and "30FA" lined up side by side, and Fig. 19 is an explanatory diagram showing X-ray diffraction charts of "0FA," "20FA," and "30FA" superimposed on each other. Figs. 18 and 19 also show an X-ray diffraction chart based on the CIF file data for MIL-101(Cr), which is a basic structure MOF.

[0074] When comparing "20FA" and "30FA" with the basic structure MOF, as shown by the dashed line in FIG. 18, relatively large changes in peak intensity and the like were observed, particularly in the range of diffraction angle 2θ of 0.5 to 2.5°. Such changes were particularly noticeable in the diffraction peak intensities of the (111) and (311) planes, as shown in FIG. 19. Specifically, with regard to the diffraction peak intensity of the (111) plane, the peak intensity of the metal-organic framework to which a second ligand was added tended to be lower than that of the basic structure MOF. Furthermore, with regard to the diffraction peak intensity of the (311) plane, the peak intensity of the metal to which a second ligand was added tended to be higher than that of the basic structure MOF. Note that, as shown in FIG. 19, for example, in the diffraction peak of the (111) plane, not only the peak intensity but also the peak position changes to some extent. Therefore, the peak position of each plane in the metal-organic framework to which a second ligand was added is identified as the peak closest to the peak position based on the data in the CIF file for the basic structure MOF.

[0075] 20 is an explanatory diagram showing the results of calculating the ratio (Y / X) of the diffraction peak intensity Y of the (111) plane to the diffraction peak intensity X of the (311) plane for each of the samples "0FA," "20FA," "30FA," "40FA," "50FA," and "60FA" based on the results of X-ray diffraction analysis using synchrotron radiation. As shown in FIG. 20, it was confirmed that by adding a second ligand to the basic structure MOF MIL-101, the value of the ratio (Y / X) becomes smaller than the measured value of 0.43 for the basic structure MOF and the value of 0.42 based on the data in the CIF file.

[0076] <UV-Vis Diffuse Reflectance Measurement> The prepared samples "0FA," "20FA," "30FA," and "40FA" were subjected to spectral measurement by UV-Vis diffuse reflectance. The measurement was performed using a UV-3100PC ultraviolet-visible-near-infrared analytical photometer (manufactured by Shimadzu Corporation) under the following conditions, in a wavelength range of 250 nm to 750 nm. Scan speed: medium, sampling pitch: 1.0, auto-sampling pitch: disabled, measurement mode: single, photometric value: reflectance, slit width: (20), light source switching wavelength: 300 nm, detector switching wavelength: 880 nm, S / R switching: standard, detector lock: automatic, slit program: standard.

[0077] Each sample was prepared as follows. First, magnesium sulfate and each sample were mixed in a mortar to prepare a mixed powder for evaluation. Then, after filling a sample holder with magnesium sulfate and compacting it, the surface of the compact was scraped to form a depression, and the mixed powder was filled into the depression for evaluation. As a background, a sample holder containing only a magnesium sulfate compact was also prepared and evaluated. The obtained diffuse reflectance spectrum was converted to a spectrum with wavelength [nm] on the horizontal axis and intensity [a.u.] on the vertical axis using Kubelka-Munk transformation.

[0078] FIG. 21 is an explanatory diagram showing a UV-Vis spectrum after Kubelka-Munk transformation. FIG. 21(A) shows the entire UV-Vis spectrum, and FIG. 21(B) shows the intensity range of 0.00 to 0.02 [a.u.] with the vertical axis expanded. In FIG. 21(A), the strongest peak in the wavelength range of 250 nm or more and less than 400 nm is shown as "MAX250-400," and the peak intensity of such peak is referred to as "I(MAX250-400)" or "α." In FIG. 21(B), the strongest peak in the wavelength range of 400 nm or more and less than 515 nm is shown as "MAX400-515," and the peak intensity of such peak is referred to as "I(MAX400-515)" or γ. In addition, in FIG. 21(B), the strongest peak in the wavelength range of 515 nm or more and less than 750 nm is shown as "MAX515-750", and the peak intensity of such a peak is written as "I(MAX515-750)" or β.

[0079] Fig. 22 is an explanatory diagram showing the results of calculating the ratio (γ / α) and the ratio (β / α) for each of the samples "0FA," "20FA," "30FA," and "40FA." As shown in Fig. 22, it was confirmed that the ratio (β / α) for "20FA" and "30FA," whose performance as adsorbents was improved by adding a second ligand to the basic structure MOF MIL-101, was 0.045 or less.

[0080] The present disclosure is not limited to the above-described embodiments, and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted.

[0081] The present disclosure can also be realized in the following forms. [Application Example 1] A metal-organic framework comprising: a first ligand having a benzene ring and two or more functional groups for coordination; and a second ligand not including a benzene ring and including one functional group for coordination, wherein, when a metal-organic framework having only the first ligand as a ligand is identified as a basic structure MOF based on a result of powder XRD analysis of the metal-organic framework, a ratio of the water vapor adsorption amount per unit mass when the relative pressure of the metal-organic framework is 0.95 to the water vapor adsorption amount per unit mass when the relative pressure of the basic structure MOF is 0.95 is 84% ​​or more in a water vapor adsorption isotherm measured at 298 K. [Application Example 2] The metal-organic framework according to Application Example 1, wherein the first ligand has two or more carboxy groups as the functional groups for coordination. [Application Example 3] The metal-organic framework according to Application Example 1 or 2, wherein the second ligand has a carboxy group as the functional group for coordination. [Application Example 4] The metal-organic framework according to any one of Application Examples 1 to 3, wherein the first ligand and the second ligand have the same functional group as the functional group for coordination. [Application Example 5] The metal-organic framework according to any one of Application Examples 1 to 4, wherein the basic structure MOF has an MIL-type crystal structure. [Application Example 6] The metal-organic framework according to any one of Application Examples 1 to 5, wherein the basic structure MOF is MIL-101. [Application Example 7] The metal organic structure according to Application Example 6, wherein in a pore size distribution calculated by applying an NLDFT method to a nitrogen adsorption isotherm measured at 77 K, the differential pore volume of pores having a pore diameter of 1.46 nm is larger than the differential pore volume of pores having a pore diameter of 1.11 nm.[Application Example 8] The metal-organic structure according to Application Example 6, wherein in an X-ray diffraction analysis using synchrotron radiation, a ratio (Y / X) of a diffraction peak intensity Y of the (111) plane to a diffraction peak intensity X of the (311) plane is less than 0.42. [Application Example 9] The metal-organic structure according to Application Example 6, wherein in a spectrum obtained by Kubelka-Munk transformation of a diffuse reflectance spectrum obtained by ultraviolet-visible spectroscopy, a ratio (β / α) of an strongest peak intensity β in a wavelength range of 515 nm to less than 750 nm to an strongest peak intensity α in a wavelength range of 250 nm to less than 400 nm is 0.045 or less.

Claims

1. A metal organic framework comprising: a first ligand having a benzene ring and two or more functional groups for coordination; and a second ligand having no benzene ring and one functional group for coordination, wherein, when a metal organic framework having only the first ligand as a ligand is identified as a basic structure MOF based on the results of powder XRD analysis of the metal organic framework, in a water vapor adsorption isotherm measured at 298 K, the ratio of the water vapor adsorption amount per unit mass when the relative pressure of the metal organic framework is 0.95 to the water vapor adsorption amount per unit mass when the relative pressure of the basic structure MOF is 0.95 is 84% ​​or more.

2. A metal organic framework according to claim 1, wherein the first ligand has two or more carboxy groups as functional groups for coordination.

3. A metal organic framework according to claim 1 or 2, wherein the second ligand has a carboxy group as the functional group for coordination.

4. A metal organic framework according to any one of claims 1 to 3, characterized in that the first ligand and the second ligand have the same functional group as the functional group for coordination.

5. A metal organic framework according to any one of claims 1 to 4, characterized in that the basic structure MOF has an MIL-type crystal structure.

6. A metal organic framework according to any one of claims 1 to 5, characterized in that the basic structure MOF is MIL-101.

7. The metal-organic structure according to claim 6, wherein in a pore size distribution calculated by applying the NLDFT method to a nitrogen adsorption isotherm measured at 77 K, the differential pore volume of pores having a pore diameter of 1.46 nm is larger than the differential pore volume of pores having a pore diameter of 1.11 nm.

8. The metal organic structure according to claim 6, wherein in an X-ray diffraction analysis using synchrotron radiation, the ratio (Y / X) of the diffraction peak intensity Y of the (111) plane to the diffraction peak intensity X of the (311) plane is less than 0.

42.

9. The metal organic structure according to claim 6, wherein in a spectrum obtained by Kubelka-Munk transformation of a diffuse reflectance spectrum obtained by ultraviolet-visible spectroscopy, the ratio (β / α) of the strongest peak intensity β in a wavelength range of 515 nm or more and less than 750 nm to the strongest peak intensity α in a wavelength range of 250 nm or more and less than 400 nm is 0.045 or less.

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