Metal organic framework

Abstract

To improve the thermal stability of an MIL-101 based metal-organic framework.SOLUTION: When a metal-organic framework having only a ligand having two or more functional groups for coordination as a ligand is specified as a basic structure MOF based on a result of powder XRD analysis, the basic structure MOF of the metal-organic framework is MIL 101. In a thermogravimetric curve (TG curve), a ratio (Rtg / Rc) is 0.40 or more and 0.90 or less, where Rtg is a ratio of a mass loss amount Δ mtg of the metal-organic framework during a temperature rise from a collapse start temperature to 500 °C. to a mass mtg of the metal-organic framework at the collapse start temperature, and Rc is a ratio of a theoretical value Δ mc of a mass loss amount due to structural collapse of a basic structure MOF having a mass mc to the mass mc of the basic structure MOF after coordinated water is removed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Metal-organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are complex crystals formed by the coordination of 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 other gases, or as catalytic materials. Various types of metal-organic frameworks have been known (see, for example, Patent Documents 1 and 2). For example, Patent Document 1 discloses metal-organic frameworks as various porous metal complexes, including MIL-101(Cr) and MIL-101(Fe). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-196677 [Patent Document 2] Japanese Patent Publication No. 2022-22982 Summary of the Invention [Problem to be solved by the invention]

[0004] When a metal-organic framework is used as an adsorbent for water vapor or other gases, it is expected to be applied to an adsorptive separation system for water vapor or other gases using processes such as pressure swing adsorption (PSA) or thermal swing adsorption (TSA). When a metal-organic framework is applied to the above-mentioned adsorptive separation system, the durability of the metal-organic framework and the system including the same can be improved by increasing the thermal stability of the metal-organic framework. However, since metal-organic frameworks contain organic substances, they generally have relatively low thermal stability. As an example of improving the thermal stability of a metal-organic framework, for example, Patent Document 2 discloses that the thermal stability is improved by the zinc ion of a zinc-based metal-organic framework being in a tetracoordinated form with a specific ligand ion. In contrast, the MIL-101-based metal-organic framework is a metal-organic framework that adsorbs a relatively large amount of water and requires relatively low raw material costs for production, and is therefore expected to see expanded use. However, its thermal stability has not been fully studied, and further improvement in thermal stability has been desired. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms. (1) According to one embodiment of the present disclosure, there is provided a metal-organic framework, wherein, when a metal-organic framework having only ligands having two or more functional groups for coordination among the ligands constituting the metal-organic framework is identified as a basic structure MOF based on the results of powder XRD analysis of the metal-organic framework, the basic structure MOF is MIL-101, and a collapse onset temperature at which the metal-organic framework structurally collapses is found in a thermogravimetric curve (TG curve) measured in a dry air stream at a heating rate of 5°C / min over a heating temperature range of 25 to 500°C. where Rtg is a ratio of a mass loss Δmtg of the metal-organic framework during a temperature rise from the collapse onset temperature to 500°C to a mass mtg of the metal-organic framework at that temperature, and Rc is a ratio of a theoretical value Δmc of a mass loss due to structural collapse of the basic structure MOF having a mass mc to a mass mc of the basic structure MOF after coordinated water is removed, then the ratio (Rtg / Rc) of the ratio Rtg to the ratio Rc is 0.40 or more and 0.90 or less. According to this metal-organic framework, in a metal-organic framework having a basic structure MOF of MIL-101, when Rtg is the ratio of the mass loss Δmtg to the mass mtg of the metal-organic framework at the collapse onset temperature, and Rc is the ratio of the theoretical value Δmc of the mass loss to the mass mc of the basic structure MOF after coordinated water is removed, the ratio (Rtg / Rc) is set to 0.40 or more and 0.90 or less. Such a value of the ratio (Rtg / Rc) indicates that the state of the ligands in the metal-organic framework has changed compared to the basic structure MOF, and by setting the value of the ratio (Rtg / Rc) within the above range, the regularity of the crystal structure and the particle shape of the metal-organic framework can be changed, thereby increasing the thermal stability of the metal-organic framework. (2) The metal-organic framework of the above form may have a monocarboxylic acid as part of the ligand. With this configuration, in the metal-organic framework, terephthalic acid, which is a ligand shared with the basic structure MOF MIL-101, and the monocarboxylic acid have the same functional group as the functional group for coordination. Therefore, crystal growth via each ligand can proceed through a common process, simplifying the manufacturing process of the metal-organic framework. (3) In the metal organic framework of the above embodiment, the monocarboxylic acid may be formic acid. In this configuration, since formic acid has a relatively small acid dissociation constant pKa compared to other monocarboxylic acids such as acetic acid, the ion concentration of the monocarboxylic acid in the solution increases during synthesis of the metal organic framework, making it possible to reduce the amount of monocarboxylic acid used as a raw material. (4) In the metal-organic framework of the above embodiment, the basic structure MOF may be MIL-101(Cr). With this configuration, MIL-101(Cr) is a metal-organic framework that adsorbs a relatively large amount of water and is produced using relatively inexpensive raw materials, so that a metal-organic framework having these advantages can be obtained. (5) The metal-organic framework of the above embodiment may further comprise a ligand having a higher spontaneous ignition temperature than the ligand of the basic structure MOF. With such a configuration, it is possible to further increase the collapse onset temperature of the metal-organic framework and further improve the thermal stability. (6) In the metal organic framework of the above embodiment, a diffraction peak corresponding to a compound represented by the general formula MeOOH (wherein Me is a metal element constituting the metal organic framework) may be included in the synchrotron X-ray diffraction pattern of the metal organic framework. With this configuration, a metal organic framework with better thermal stability can be obtained. The present disclosure can be realized in various forms other than those described above, for example, in the form of a method for manufacturing a metal-organic framework, or a humidity control device such as a heat pump or a desiccant air conditioning system that includes a metal-organic framework. [Brief explanation of the drawings]

[0006] [Figure 1] An explanatory diagram showing TG curves of a metal-organic framework and a basic structure MOF. [Figure 2] 1 is a flowchart showing an outline of a method for producing a metal-organic framework. [Figure 3] FIG. 1 is an explanatory diagram showing XRD charts of each sample side by side. [Figure 4] FIG. 1 is an explanatory diagram showing an SEM image of the "0FA" sample. [Figure 5] An explanatory diagram showing an SEM image of a "20FA" sample. [Figure 6] An explanatory diagram showing an SEM image of a "30FA" sample. [Figure 7] An explanatory diagram showing an SEM image of a "40FA" sample. [Figure 8] An explanatory diagram showing an SEM image of a "50FA" sample. [Figure 9] An explanatory diagram showing an SEM image of a "60FA" sample. [Figure 10] FIG. 2 is an explanatory diagram showing the TG curves measured for each sample. [Figure 11] An explanatory diagram showing "Rtg", "Rc", and "Rtg / Rc". [Figure 12] FIG. 1 is an explanatory diagram showing the results of synchrotron X-ray diffraction of each sample. DETAILED DESCRIPTION OF THE INVENTION

[0007] A. Metal-organic framework composition: The metal-organic framework of the present embodiment is designated as MIL-101 when, based on the results of powder XRD analysis of the metal-organic framework, a metal-organic framework having only ligands that have two or more functional groups for coordination among the ligands constituting the metal-organic framework is identified as a "basic structure MOF." Here, the metal-organic framework is a porous metal complex obtained by utilizing the self-assembly of metal ions and organic ligands. It is a crystalline material that forms a highly ordered array structure, and exhibits excellent properties as an adsorbent due to the innumerable nanospaces present within the array structure. To identify the above-described basic structure MOF for a metal-organic framework, powder XRD analysis is performed on the target metal-organic framework, and the obtained XRD chart is compared with the XRD pattern in a CIF file (data obtained by single-crystal X-ray structural analysis and included in crystal structure data registered in the Common Information Format for Crystallography (CIF)). The relationship between the metal-organic framework of the present embodiment and the basic structure MOF is further described below.

[0008] Metal-organic frameworks generally have, as organic ligands, ligands having two or more functional groups for coordination (hereinafter also referred to as "first ligands") as described above. However, the metal-organic framework of this embodiment further includes a ligand having one functional group for coordination (hereinafter also referred to as "second ligands"). That is, the metal-organic framework of this embodiment is a compound in which a part of the first ligands in the basic MOF structure is substituted with a second ligand. In the metal-organic framework, the first ligands arranged so as to surround the metal ions form coordinate bonds with the metal ions, causing the crystal structure to grow three-dimensionally via the first ligands, forming a steric crystal structure in which each constituent element is regularly arranged three-dimensionally. Therefore, the "functional group for coordination" can be said to be a functional group involved in crystal growth. The metal organic framework of this embodiment further includes a second ligand having only one functional group for coordination, which suppresses three-dimensional crystal growth and facilitates one-dimensional crystal growth, thereby partially changing the regularity of the crystal structure. The basic structure MOF described above is specified as a metal organic framework having only the first ligand as a ligand, and has a highly regular crystal structure.

[0009] 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 substitution ratio of the second ligand to the first ligand. In this case, depending on the type of second ligand contained in the metal-organic framework and the amount of the second ligand added, peaks not included in the XRD pattern of the basic structure MOF may appear in the XRD pattern of the metal-organic framework. 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.

[0010] Note that metal organic frameworks contain metal ions along with ligands. Generally, metal organic frameworks with 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 a 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, nuclear magnetic resonance (NMR), or ICP emission spectroscopy.

[0011] As described above, the metal organic framework of this embodiment has the basic structure MOF specified as above as MIL-101. MIL-101 has terephthalic acid as the first ligand and is represented by the following composition formula (1).

[0012] [ka]

[0013] (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.)

[0014] The terephthalic acid contained as the first ligand in the metal-organic framework of this embodiment is a dicarboxylic acid having two carboxy groups as functional groups for coordination (functional groups involved in crystal growth). 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 structural MOF of MIL-101 is particularly preferable as a metal-organic framework used for adsorption and separation of water vapor.

[0015] The second ligand, which is included in the metal-organic framework of this embodiment and has one functional group for coordination, can be selected from various ligands, but it is preferable to use a monocarboxylic acid, for example. In this way, if the first and second ligands have the same functional group for coordination, crystal growth via the first ligand and crystal growth via the second ligand can be performed in a common process, simplifying the metal-organic framework production process. When a monocarboxylic acid is used as the second ligand, it is preferable to use formic acid, for example. Formic acid has a relatively small acid dissociation constant pKa compared to other monocarboxylic acids such as acetic acid. Therefore, during the synthesis of the metal-organic framework, the ion concentration of the monocarboxylic acid in the solution is increased, thereby reducing the amount of monocarboxylic acid used as a raw material. Furthermore, it is preferable that the second ligand does not have a cyclic hydrocarbon group, but has a linear hydrocarbon group. When a linear hydrocarbon group is present, the hydrocarbon backbone group preferably has 6 or less carbon atoms, more preferably 5 or less carbon atoms, and even more preferably 4 or less carbon atoms.

[0016] The metal ions 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 are not particularly limited. The metal ions contained in MIL-101 may be trivalent metal ions, and for example, chromium ions (Cr 3+ ), aluminum ions (Al 3+ ), iron ions (Fe 3+) can be at least one selected from the following.

[0017] Furthermore, the metal-organic framework of this embodiment exhibits the following properties in a thermogravimetric curve (TG curve) measured in a dry air stream at a heating rate of 5°C / min over a temperature range of 25-500°C. Specifically, Rtg is the ratio of the mass loss Δmtg of the metal-organic framework during heating from the collapse onset temperature to 500°C to the mass mtg of the metal-organic framework at the collapse onset temperature of the metal-organic framework of this embodiment. Furthermore, Rc is the ratio of the theoretical mass loss Δmc of a basic structure MOF with mass mc due to the loss of ligands to the mass mc after coordinated water is removed. In this case, the ratio of the ratio Rtg to the ratio Rc (Rtg / Rc) is preferably 0.40 or more, more preferably 0.48 or more, from the viewpoint of ensuring the adsorption performance of the metal-organic framework. Furthermore, the ratio (Rtg / Rc) is preferably 0.90 or less, more preferably 0.79 or less, from the viewpoint of enhancing the thermal stability of the metal-organic framework. Here, the "collapse onset temperature" refers to the temperature at which the metal-organic framework starts to collapse structurally as the temperature rises. Specifically, it is the temperature at which a reaction starts in which the first ligand, the second ligand, and further halogens contained in the metal-organic framework are lost as the temperature of the metal-organic framework rises. In other words, the "collapse onset temperature" can be said to be the temperature at which the thermal decomposition of the metal-organic framework starts. Furthermore, the point at which the temperature rises from the collapse onset to 500°C is set as the point at which the above-mentioned structural collapse reaction in the metal-organic framework ends.

[0018] FIG. 1 is an explanatory diagram showing an example of a TG curve of the metal-organic framework (hereinafter also referred to as "MOF") of this embodiment, measured under the above conditions. In FIG. 1, the TG curve of the MOF of this embodiment is shown by a dashed line, and the point at which the MOF of this embodiment starts to collapse is indicated by an arrow, and the mass of the MOF at that time (mtg) is also shown. Note that when the metal-organic framework is heated, the water coordinated within the metal-organic framework is released before the metal-organic framework starts to collapse. Furthermore, in FIG. 1, the point at which the MOF of this embodiment is heated to 500°C is indicated by an arrow, and the mass of the MOF at that time (mtg 500 Therefore, the mass loss Δmtg of the MOF during the temperature rise from the collapse initiation temperature to 500°C is expressed as "Δmtg = mtg - mtg 500 Therefore, the ratio Rtg of the mass loss Δmtg to the mass mtg at the collapse initiation temperature of the MOF of the embodiment is expressed as "Rtg = Δmtg / mtg". In the TG curve of FIG. 1, the vertical axis indicates the mass loss rate TG [%], but in FIG. 1, the measured value of the mass [g] of the MOF at the collapse initiation point indicated by the arrow is shown in parentheses as (mtg), and the measured value of the mass [g] of the MOF at the point indicated by the arrow where the MOF is heated to 500°C (at the end of structural collapse) is shown in parentheses as (mtg 500 )

[0019] While the above ratio Rtg is an actual measured value based on the TG curve, the ratio Rc, as mentioned above, is the ratio of the theoretical value Δmc of the mass loss due to the loss of ligands from a basic structure MOF with mass mc to the mass mc after the coordinated water is lost. The basic structure MOF with mass mc at the start of the collapse corresponds to the state in which water is lost in the composition formula shown in formula (1). The theoretical value of the mass when such a basic structure MOF with mass mc undergoes structural collapse and the reaction of losing the first and second ligands, as well as halogens contained in the metal-organic framework, is completed is mc col In this case, the theoretical mass loss Δmc of the basic structure MOF from the collapse start temperature to the end of collapse is expressed as Δmc = mc - mc colTherefore, the ratio Rc of the theoretical mass loss Δmc to the mass mc at the collapse starting temperature of the basic structure MOF is expressed as "Rc = Δmc / mc". Note that the theoretical mass (mc col ) represents the mass when the entire amount of MIL-101(Cr) having a mass mc reacts to form Cr2O3, for example, when the basic structure MOF is MIL-101(Cr). As described above, Rc for the basic structure MOF is calculated as a theoretical value, but for the sake of convenience in explaining the effects of the MOF of the embodiment, FIG. 1 also shows the TG curve measured for a metal organic framework having the composition of the basic structure MOF.

[0020] The above theoretical values ​​for the basic structure MOF will be further explained below. In the following, an example will be explained in which the basic structure MOF is MIL-101(Cr). For example, the composition formula of MIL-101(Cr) containing Cl as a halogen is expressed by the following formula (2), and MIL-101(Cr) in a dehydrated state immediately before the structure of this MIL-101(Cr) collapses is expressed by the following formula (3).

[0021] [ka] [ka]

[0022] From the molecular weight of MIL-101(Cr) expressed by the above formula (2), the mass per mole of MIL-101(Cr) is 735.5 [g / mol], and the mass per mole of dehydrated MIL-101(Cr) expressed by formula (3) is 699.5 [g / mol]. When such a basic structure MOF collapses into Cr2O3, (3 / 2) moles of Cr2O3 are produced from 1 mole of MIL-101(Cr). The mass of Cr2O3 in this case is 228 g. In other words, the theoretical mass mc when a basic structure MOF (MIL-101(Cr)) with a mass mc = 699.5 g collapses into Cr2O3 is colTherefore, the theoretical mass loss Δmc of the basic structure MOF is 699.5g - 228g = 471.5g. Therefore, the ratio Rc is 471.5 / 699.5 = 0.674.

[0023] For the MOF of the embodiment, when the ratio Rtg based on the measured value and the ratio Rc based on the theoretical value are determined as described above, the ratio of the ratio Rtg to the ratio Rc (Rtg / Rc) falls within the aforementioned numerical range. In the metal-organic framework of the embodiment, the substitution ratio of a portion of the first ligands in the basic structure MOF to the second ligand increases to a certain extent, and as shown in FIG. 1, the mass loss until the end of collapse when the structure collapses with increasing temperature decreases, and the value of the ratio (Rtg / Rc) falls below the upper limit of the above numerical range. Accordingly, the collapse onset temperature increases, improving the thermal stability of the metal-organic framework. One possible reason why the mass loss during structure collapse is reduced by the addition of the second ligand compared to the basic structure MOF is that the molecular weight of the second ligand is smaller than the molecular weight of the first ligand. When the content of the second ligand increases to a certain extent and the mass loss during structural collapse becomes smaller than that of the basic structure MOF, the regularity of the crystal structure and particle shape of the metal-organic framework change. It is believed that such changes increase the thermal stability of the entire metal-organic framework and raise the collapse onset temperature. MIL-101 is known to have octahedral crystal particle shapes. However, regardless of the type of metal ion or the type of added second ligand, the regularity of the crystal structure and the shape of the crystal particle can change when the content of the second ligand increases to a certain extent. Therefore, it is believed that the thermal stability of the metal-organic framework can be improved by setting the ratio (Rtg / Rc), which reflects the degree of addition of the second ligand, within the above-mentioned range. In the TG curve shown in Figure 1, the collapse onset temperature can be determined as the temperature at which the peak rises in the DTA curve. This is because the temperature at which the peak rises in the DTA curve is thought to be the temperature at which the formation of metal oxides such as Cr2O3 begins due to the structural collapse of the metal-organic framework. Furthermore, when the metal organic framework is a molded body, the above-mentioned TG curve may be measured after pulverizing the metal organic framework into powder.

[0024] To enhance the thermal stability of a metal-organic framework by adding a second ligand, it is desirable that the spontaneous ignition temperature of the second ligand be higher than that of the first ligand. The structural collapse of a metal-organic framework accompanying an increase in temperature is considered a type of combustion reaction. While a simple elimination reaction would be considered an endothermic reaction, the structural collapse reaction accompanied by mass loss is an exothermic reaction overall. Therefore, by substituting some of the ligands with ligands having a higher spontaneous ignition temperature, it is possible to increase the collapse onset temperature of the metal-organic framework and improve its thermal stability. For example, the spontaneous ignition temperature of formic acid, which can be used as a second ligand, is known to be 520°C, which is higher than the spontaneous ignition temperature of 496°C of terephthalic acid, which constitutes MIL-101.

[0025] Furthermore, in order to enhance the thermal stability of the metal-organic framework, it is desirable that the X-ray diffraction pattern of the metal-organic framework contains a diffraction peak corresponding to a compound represented by the general formula MeOOH (where Me is a metal element constituting the metal-organic framework). With such a configuration, the above-mentioned compounds, which are oxyhydroxides of the metal elements constituting the metal-organic framework, such as CrOOH and AlOOH, can generally function as flame retardants (Shu-ting Liang et al., Ceramics International 40 (2014) 4367-4373; A Laachachi et al., Polymer Degradation and Stability 94 (2009) 1373-1378). Therefore, a metal-organic framework with superior thermal stability can be obtained. It is believed that the above-mentioned compounds, which are oxyhydroxides of the metal elements constituting the metal-organic framework, undergo a decomposition reaction, which is an endothermic reaction, when the temperature of the metal-organic framework is increased, thereby exhibiting a flame retardant effect. Furthermore, when the temperature of the metal organic framework is increased, the metal organic framework and the compound form a composite, and the concentration of the metal organic framework, which is a combustible substance, in the composite is diluted by the compound, which is also thought to improve the thermal stability of the metal organic framework.

[0026] B. Method for producing metal-organic frameworks: 2 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.

[0027] 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 in which the basic structure MOF is MIL-101(Cr), it is sufficient to prepare metal salts, chlorides, oxides, etc. that generate trivalent metal ions such as chromium. In addition, in this embodiment, terephthalic acid is used as the first ligand to produce a metal organic framework in which the basic structure MOF is MIL-101.

[0028] The compounding ratio of the first ligand to the second ligand can be appropriately determined depending on the type (composition) of the metal-organic framework to be synthesized so that the addition of the second ligand results in an appropriate deformation that improves the thermal stability of the basic structure MOF. For example, consider a case in which a metal-organic framework is produced in which the basic structure MOF is MIL-101(Cr) and the monocarboxylic acid formic acid is used as the second ligand. Here, when the compounding amount (charge amount) of the first ligand in the production of a basic structure MOF (MIL-101(Cr)) containing only the first ligand (terephthalic acid) as the ligand is taken as 100%, the ratio (molar ratio) of the amount of the first ligand reduced in accordance with the amount of the second ligand 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 compounding amount of the first ligand." The "substitution ratio (molar ratio) of the blending amount of the first ligand" is preferably 35% or more, and more preferably 40% or more. The "substitution ratio (molar ratio) of the blending amount of the first ligand" can be, for example, 60% or less.

[0029] Examples of the solvent that can be used include water, methanol, ethanol, N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), and mixtures thereof, and may be appropriately selected depending on the types of other raw materials including the first and second ligands.

[0030] After step T100, solvothermal synthesis is carried out using these raw materials (step T110). For example, a metal organic framework having an MIL-type crystal structure, such as the metal organic framework of this embodiment, is generally suitable for hydrothermal synthesis using water as a solvent. Here, a carboxylic acid is used as the first ligand, and the carboxyl group, which is the functional group for coordination, is deprotonated to form "-CO2 - " coordinates with the metal ion, and a crystal structure grows, synthesizing a metal-organic framework.

[0031] 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, are dissolved in the solid component recovered in step T120 (step T130). When terephthalic acid is used as the first ligand and the metal-organic framework is synthesized by hydrothermal synthesis using water as the solvent, as in this embodiment, 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 as an unnecessary component in the solid 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.

[0032] 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 a metal-organic framework with increased purity. The solid component recovered in step T140 is dried to remove the solvent (step T150), thereby obtaining a metal-organic framework.

[0033] C. Use of metal-organic frameworks as adsorbents: 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.

[0034] (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. The adsorption heat pump has a configuration combining an adsorber configured as a heat exchanger filled with an adsorbent and an evaporator that evaporates water by the adsorption force of the adsorbent, or a configuration combining an adsorber configured as a heat exchanger filled with an adsorbent and a condenser that condenses water vapor desorbed from the adsorbent using exhaust heat to turn it into water. In such an adsorption heat pump, the metal organic framework of this embodiment can be used as the adsorbent included in the adsorbent.

[0035] (C-2) Desiccant air conditioning system: The metal organic framework of the present embodiment can be used as an adsorbent that adsorbs water vapor in a desiccant air conditioning system. The 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 to release moisture and humidify the air in the room or the like. In such a desiccant air conditioning system, the metal organic framework of the present embodiment can be used as the adsorbent that constitutes the dehumidifying rotor.

[0036] According to the metal-organic framework of the present embodiment configured as described above, in the metal-organic framework whose basic structure MOF is MIL-101, a portion of the terephthalic acid ligand is substituted with a second ligand, thereby setting the aforementioned ratio (Rtg / Rc) within the aforementioned range. This changes the regularity of the crystal structure and particle shape of the metal-organic framework, thereby improving the thermal stability of the metal-organic framework. As described above, MIL-101 is a metal-organic framework that adsorbs a relatively large amount of water, and the raw material costs for its production are relatively low. It has properties that make it preferable for a metal-organic framework used for adsorption and separation of water vapor. However, MIL-101 has a relatively low thermal decomposition temperature (for example, about 300°C for MIL-101(Cr)), which makes it susceptible to degradation by heat treatment, and its applications are likely to be limited. The metal organic framework of this embodiment has a basic structure MOF of MIL-101, and by incorporating a second ligand and satisfying the requirement related to the ratio (Rtg / Rc) described above, the thermal decomposition temperature of the metal organic framework increases and the thermal stability improves. [Example]

[0037] <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 manufacturing method shown in Figure 2. The prepared metal-organic frameworks are represented by the composition formula (4) below.

[0038] [ka]

[0039] In the above formula (4), x represents the "proportion (molar ratio) of the amount of the first ligand reduced by the amount of the second ligand (formic acid)" when the "amount (charge) of the first ligand when producing the basic structure MOF (MIL-101(Cr))" is taken as 100%, and corresponds to the "substitution ratio 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 (4) (where x = 0). Six types of samples were prepared with the "substitution ratio (molar ratio) x of the first ligand" set to 0%, 20%, 30%, 40%, 50%, and 60%. The samples with the above-mentioned "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 twice the molar ratio of the above-mentioned molar ratio x%.

[0040] When preparing each of the above samples, chromium nitrate (Cr(NO3)3), terephthalic acid, formic acid, and hydrochloric acid (HCl) were used as raw materials. When preparing a sample with the "substitution ratio (molar ratio) of the first ligand blending amount" of x%, the blending amounts [mmol] of the raw materials were (Cr(NO3)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 raw materials.

[0041] 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. Subsequently, 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. Subsequently, 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.

[0042] <Powder X-ray diffraction> Powder X-ray diffraction patterns were obtained for each of the samples "0FA," "20FA," "30FA," "40FA," "50FA," and "60FA" prepared as described above using the following method. First, each sample was pretreated by evacuating it at 125°C for at least 6 hours. Powder XRD analysis was then 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.

[0043] Figure 3 shows the XRD charts of the "0FA," "20FA," "30FA," "40FA," "50FA," and "60FA" samples side by side. Figure 3(A) shows the results for the low-angle region, and Figure 3(B) shows the results for the high-angle region. As shown in Figure 3, the XRD patterns of each sample contained peak patterns that matched the peak patterns in the XRD chart of the CIF file of MIL-101(Cr). Based on the XRD patterns, it was confirmed that the basic structure MOF could be identified as MIL-101(Cr). Furthermore, the XRD patterns of the "40FA," "50FA," and "60FA" samples contained peak patterns that matched the peak patterns in the XRD chart of the CIF file of CrOOH.

[0044] <Observation of Particle Shape> For each of the prepared samples of "0FA", "20FA", "30FA", "40FA", "50FA", and "60FA", the particles were observed by SEM images. Prior to the observation using a scanning electron microscope (SEM), each of the above samples was pretreated by evacuation at 125°C for 6 hours or more.

[0045] Figures 4 to 9 are explanatory diagrams showing SEM images of the samples of "0FA", "20FA", "30FA", "40FA", "50FA", and "60FA" respectively. As shown in Figure 4, the particle shape of the primary particles of "0FA", which is the basic structure MOF, is an octahedral shape. And as shown in Figures 5 to 9, by incorporating the second ligand, the crystal particles deformed so that the vertices of the octahedral shape became rounded, resulting in crystal particles with a truncated octahedral shape. The degree of such particle shape deformation and the proportion of particles with a truncated octahedral shape increased as the amount of the second ligand incorporated increased. Also, as shown in Figures 7 to 9, in each of the samples of "40FA", "50FA", and "60FA", as particles with a particle shape completely different from the octahedral shape, an aggregate of elongated rod-shaped particles (nanorod-shaped particles) designated as "Particle ND" was observed. Such nanorod-shaped particles are considered to be by-products with a crystal structure and particle shape significantly different from those of MIL-101, or CrOOH observed by XRD, which are generated because the addition of the second ligand makes it easier for, for example, the growth of a one-dimensional crystal structure to proceed. "40FA", "50FA", and "60FA" in which nanorod-shaped particles were observed are samples with enhanced thermal stability compared to "0FA", as will be described later.

[0046] <TG-DTA Measurement> Thermogravimetric-differential thermal analysis (TG-DTA) measurements were performed on each of the "0FA," "20FA," "30FA," "40FA," "50FA," and "60FA" samples to obtain TG and DTA curves. Specifically, each sample was first pretreated at 125°C overnight. TG-DTA measurements were then performed in an air atmosphere under the following conditions: air supply rate 150 mL / min, heating rate 5°C / min, and temperature range 25-500°C. The sample weight was adjusted to approximately 10 mg after dehydration. The sample containers used were made of aluminum oxide (Al2O3).

[0047] Fig. 10 is an explanatory diagram showing the TG curves obtained by performing TG-DTA measurements on each sample. As shown in Fig. 10, the TG curves corresponding to all samples showed the collapse onset point explained based on Fig. 1 around 300°C (280 to 320°C). Fig. 10 also shows a DTA curve, and here, the rise temperature of the peak where the peak value in the DTA curve is maximum is taken as the collapse onset temperature in the TG curve.

[0048] As shown in Figure 10, the "20FA" and "30FA" samples showed TG curves similar to those of the "0FA" sample. That is, the collapse onset temperature and TG value at 500 °C were almost the same. In contrast, the TG curves of the "40FA," "50FA," and "60FA" samples showed a smaller absolute value of the TG value at 500 °C, a smaller mass loss (Δmtg) after structural collapse, and a higher collapse onset temperature compared to the "0FA" sample. This higher collapse onset temperature confirmed that the "40FA," "50FA," and "60FA" samples have higher thermal stability than the "0FA" sample, which corresponds to the basic structure MOF.

[0049] 1, the ratio of the ratio Rtg to the ratio Rc (Rtg / Rc) was calculated for each sample based on the results of the TG-DTA measurement. The ratio Rtg was calculated based on the TG curve in FIG. 10 by dividing the mass mtg of the metal-organic framework at the collapse onset temperature for each sample and the mass mtg when heated to 500°C. 500 ) was measured, and the mass loss Δmtg of each sample during the temperature rise from the collapse onset temperature to 500°C was calculated, and the mass loss Δmtg was calculated as the ratio of the mass loss Δmtg to the mass mtg. Note that because the basic structure MOF is MIL-101(Cr), the ratio Rc of the theoretical mass loss Δmc due to the loss of ligands of the basic structure MOF with mass mc to the mass mc after the coordinated water is released is 0.674 as mentioned above.

[0050] Figure 11 is an explanatory diagram showing the "Rtg ratio," "Rc ratio," and "Rtg / Rc" values ​​for each sample. As shown in Figure 11, the "40FA," "50FA," and "60FA" samples, which were found to have improved thermal stability, had "Rtg / Rc" values ​​in the range of 0.40 to 0.90.

[0051] <Synchrotron X-ray diffraction> Synchrotron radiation X-ray diffraction measurements were performed on each of the prepared samples, "0FA," "20FA," "30FA," "40FA," "50FA," and "60FA." Specifically, each sample was sealed in a glass capillary with an inner diameter of 1.0 mm, and then irradiated with synchrotron radiation X-rays with a wavelength of 1.0 Å. The exposure time was 8 minutes x 2 times, for a total of 16 minutes, and the diffraction pattern was measured.

[0052] FIG. 12 is an explanatory diagram showing the results of synchrotron X-ray diffraction for each sample. FIG. 12(A) shows the results in the low-angle region, and FIG. 12(B) shows the results in the high-angle region. As shown in FIG. 12, the XRD patterns of the "40FA," "50FA," and "60FA" samples showed peak patterns that matched the peak pattern of the XRD chart of the CrOOH CIF file. Therefore, CrOOH was present in the "40FA," "50FA," and "60FA" samples, and it is believed that this CrOOH contributes to improving the thermal stability of the metal-organic framework. Specifically, when the temperature of the metal-organic framework increases, the reaction of CrOOH, which is an oxyhydroxide of the metal elements that make up the metal-organic framework, proceeds as shown in the following formula (5), and it is believed that CrOOH exerts a flame-retardant effect. Reaction (5) is known to be an endothermic reaction in which 2CrOOH thermally decomposes to produce Cr2O3 (Shu-ting Liang et al., Ceramics International 40 (2014) 4367-4373). In addition, when the temperature of the metal-organic framework increases, the metal-organic framework and CrOOH form a composite, and the concentration of the metal-organic framework, which is a combustible substance, is diluted by CrOOH within the composite. This is also thought to have contributed to improving the thermal stability of the metal-organic framework.

[0053] [ka]

[0054] 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.

[0055] The present disclosure can also be realized in the following forms. [Application example 1] A metal-organic framework, When a metal organic framework having only ligands having two or more functional groups for coordination among the ligands constituting the metal organic framework is identified as a basic structure MOF based on the results of powder XRD analysis of the metal organic framework, the basic structure MOF is MIL-101, In a thermogravimetric curve (TG curve) measured in a dry air stream at a temperature rise rate of 5°C / min in a temperature rise range of 25 to 500°C, Rtg is defined as a ratio of a mass loss amount Δmtg of the metal-organic framework during a temperature rise from the collapse onset temperature to 500°C to a mass mtg of the metal-organic framework at the collapse onset temperature at which the metal-organic framework structurally collapses; Regarding the basic structure MOF, when the ratio of the theoretical value Δmc of the mass loss due to the structural collapse of the basic structure MOF having mass mc to the mass mc after the coordinated water is removed is defined as Rc, The ratio of the ratio Rtg to the ratio Rc (Rtg / Rc) is 0.40 or more and 0.90 or less. Metal-organic structures. [Application example 2] The metal organic framework according to Application Example 1, It is characterized by having a monocarboxylic acid as a part of the ligand. Metal-organic structures. [Application example 3] The metal organic framework according to Application Example 1 or 2, The monocarboxylic acid is formic acid. Metal-organic structures. [Application example 4] The metal organic framework according to any one of Application Examples 1 to 3, The basic structure MOF is MIL-101(Cr). Metal-organic structures. [Application example 5] The metal organic framework according to any one of Application Examples 1 to 4, The MOF further comprises a ligand having a higher spontaneous ignition temperature than the ligand of the basic structure MOF. Metal-organic structures. [Application Example 6] The metal organic framework according to any one of Application Examples 1 to 5, The synchrotron X-ray diffraction pattern of the metal organic framework contains a diffraction peak corresponding to a compound represented by the general formula MeOOH (wherein Me is a metal element constituting the metal organic framework). Metal-organic structures.

Claims

1. A metal-organic framework, When a metal organic framework having only ligands having two or more functional groups for coordination among the ligands constituting the metal organic framework is identified as a basic structure MOF based on the results of powder XRD analysis of the metal organic framework, the basic structure MOF is MIL-101, In a thermogravimetric curve (TG curve) measured in a dry air stream at a temperature rise rate of 5°C / min in a temperature rise temperature range of 25 to 500°C, Rtg is defined as a ratio of a mass loss amount Δmtg of the metal-organic framework during a temperature rise from the collapse onset temperature to 500°C to a mass mtg of the metal-organic framework at the collapse onset temperature at which the metal-organic framework structurally collapses; Regarding the basic structure MOF, when the ratio of the theoretical value Δmc of the mass loss due to the structural collapse of the basic structure MOF having a mass mc to the mass mc after the coordinated water is removed is defined as Rc, The ratio of the ratio Rtg to the ratio Rc (Rtg / Rc) is 0.40 or more and 0.90 or less. Metal-organic structures.

2. The metal-organic framework according to claim 1, It is characterized by having a monocarboxylic acid as a part of the ligand. Metal-organic structures.

3. The metal organic framework according to claim 2, The monocarboxylic acid is formic acid. Metal-organic structures.

4. The metal-organic framework according to claim 1, The basic structure MOF is MIL-101(Cr). Metal-organic structures.

5. The metal-organic framework according to claim 1, The MOF further comprises a ligand having a higher spontaneous ignition temperature than the ligand of the basic structure MOF. Metal-organic structures.

6. The metal-organic framework according to claim 1, The synchrotron X-ray diffraction pattern of the metal organic framework contains a diffraction peak corresponding to a compound represented by the general formula MeOOH (wherein Me is a metal element constituting the metal organic framework). Metal-organic structures.

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