WO2025229824A1 - Excipient composed of metal organic structure, and method for producing said excipient - Google Patents

Abstract

Provided are: an excipient composed of a metal organic structure, the excipient having a stabilized form despite no binder component being contained therein, effectively suppressing collapse and powderization resulting from adsorption and desorption, being capable of efficiently expressing the exceptional adsorption performance of a metal organic structure, and being exceptional in terms of storage stability of a gas; and a method for manufacturing the excipient. An excipient according to the present invention is characterized by being molded from a metal organic structure provided with a metal and an organic ligand coordinated to the metal.

Description

Shaped body made of metal organic framework and method for producing the same

The present invention relates to a shaped body made of a metal-organic framework, and more specifically to a shaped body made of a metal-organic framework with excellent gas storage performance, and a method for producing the same.

Substances consisting of a central metal and multidentate organic ligands coordinated thereto, known as metal-organic frameworks (MOFs) or porous coordination polymers (PCPs), are porous three-dimensional structures formed by the accumulation of metal complexes consisting of the central metal and organic ligands. Unlike other porous materials such as zeolites and activated carbons, the pores of metal-organic frameworks have pore sizes and internal pore spaces that can be designed, and many types have been reported based on the combination of metal ions and organic ligands.
Metal organic frameworks have been reported to have pore diameters of about 0.3 nm to about 3 nm and specific surface areas of about 1000 m ² /g to about 2000 m ² /g, and even exceeding 5000 m ² /g. Metal organic frameworks with regular pore diameters and high specific surface areas have the property of being able to adsorb gases within the pores, and various developments are underway for their use as adsorbents.

For example, Patent Document 1 below describes a shaped porous polymer metal complex as a solid adsorbent, which contains a flexible resin made of a flexible porous polymer metal complex and a copolymer of a specific methacrylic acid ester. It also describes that this shaped body is resistant to disintegration and powdering, has excellent gas adsorption performance, and can be used as a gas adsorption material to be stored inside a gas storage device.

The flexible porous polymer metal complex used in Patent Document 1 mentioned above is prone to collapse and powdering when made into a shaped body due to changes in the material structure caused by gas adsorption and desorption. However, Patent Document 1 uses a specific flexible resin that can follow the structural changes of the flexible porous polymer metal complex, thereby effectively preventing the shaped body from collapsing and powdering even after repeated adsorption and desorption.

Patent No. 7278543

However, if a shaped body made of such a metal-organic framework contains a binder component, the organic compound such as a resin that is the binder component may clog the pores of the metal-organic framework, and the excellent adsorption performance inherent to the metal-organic framework may not be fully exhibited.
Furthermore, the shaped body of Patent Document 1 is not limited to use as an adsorbent; it is also desired to have the ability to efficiently retain and store the adsorbed gas within the metal-organic framework.

Therefore, an object of the present invention is to provide a shaped body made of a metal-organic framework, which has a stable form even without containing a binder component, is effectively inhibited from collapsing or powdering due to adsorption/desorption, can efficiently exhibit the excellent adsorption performance of the metal-organic framework, and has excellent storage stability for gas, and a method for producing the same.
Another object of the present invention is to provide a gas storage container containing the above-mentioned shaped body.

The present invention provides a shaped object that is formed from a metal-organic framework that includes a metal and an organic ligand coordinated to the metal.

In the shaped body of the present invention,
(1) The metal organic framework is a metal organic framework having a hydroxyl group;
(2) The organic ligand is a cyclodextrin;
(3) The composition does not contain any organic compounds other than the organic ligand.
(4) Used for gas storage;
is preferred.

The present invention also provides a gas storage container that contains the above-described shaped body.

The present invention further provides a method for producing the above-mentioned shaped body, which is characterized by press-molding the metal-organic framework under conditions of 20 to 200°C and a surface pressure of 4 MPa or more.

In the shaped body of the present invention, by using a specific metal-organic framework as the metal-organic framework, it is possible to maintain its shape with good stability without blending a binder component, and it does not collapse even after repeated adsorption and desorption as in conventional adsorbents, and can be used as an adsorbent for a long period of time.
Furthermore, as is clear from the results of the examples described below, shaped bodies formed under specified pressurized conditions can exhibit superior gas storage performance compared to the powder state before shaping, and are particularly useful as storage materials for gases such as nitrogen, carbon dioxide, or hydrogen.

(Metal-organic structure)
The shaped body of the present invention comprises a metal-organic structure comprising a metal and an organic ligand coordinated to the metal. While a known metal-organic structure that has been conventionally used in adsorbents can be used, a metal-organic structure having a hydroxyl group is particularly preferred.
As described in the aforementioned Patent Document 1, flexible porous polymer metal complexes undergo a change in skeletal structure and an increase in volume as they adsorb and desorb adsorbed gases. Therefore, when shaped into a molded body, stress is applied, cracks occur, and the body collapses (pulverizes). In the present invention, by using a specific metal organic framework having hydroxyl groups, even such a metal organic framework can be made into a shaped body that does not collapse even when gas is adsorbed and desorbed, without containing a binder component. In other words, the hydroxyl groups of the metal organic framework used in the present invention form bonds and themselves function as a binder, thereby promoting the formation of a shaped body. Furthermore, the hydroxyl groups act as a cushion, making it possible to follow volume changes and effectively suppressing the collapse of the shaped body.
Furthermore, a shaped body made of a metal-organic framework having hydroxyl groups generates new grain boundaries, and the hydroxyl groups present at the grain boundary interfaces make it possible to capture polar gases such as carbon dioxide gas and gas molecules with molecular diameters equal to or smaller than the pore diameter of the grain boundary interfaces, resulting in superior gas storage performance compared to unshaped bodies.

The organic ligand constituting such a metal organic framework having a hydroxyl group may be any organic ligand that forms a coordinate bond with the metal. As the organic ligand, a compound having a functional group that forms a coordinate bond with the metal can be used.
In one embodiment, a compound having a structure capable of encapsulating a guest molecule can be used as the organic ligand, such as a cyclodextrin-based compound.
Examples of cyclodextrin compounds include α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin.

On the other hand, the organic ligand itself does not necessarily have to have the function of being able to encapsulate guest molecules. Any substance may be used as long as the metal organic framework as a whole has the function of being able to encapsulate guest molecules. Examples of such organic ligands include compounds having a monocyclic or polycyclic skeleton (e.g., hydroxytrimesic acid, etc.).
In the present invention, the organic ligand is preferably a compound having a structure capable of encapsulating a guest molecule, and in particular, a cyclodextrin-based compound can be preferably used.

Furthermore, the metal constituting the metal organic framework may be any metal capable of forming a coordinate bond with the organic ligand, and is not limited thereto. Examples include metal ions such as Li, Na, K, Rb, Be, Mg, Ca, Sr, Ba, Sc, Y, Ti, Ar, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Al, Ga, In, Tl, Si, Ge, Sn, Pb, As, Sb, and Bi, and at least one can be selected from these. Among these, alkali metal ions are preferred, and potassium ions are particularly preferred.

In a metal-organic framework (CD-MOF) in which the organic ligand is a cyclodextrin-based compound and the metal is potassium, a metal-organic framework in which 0.05 to 0.80 moles of cyclodextrin-based compound are bonded to 1 mole of potassium ion, more preferably 0.10 to 0.20 moles, is preferred in terms of demonstrating excellent storage performance when made into a shaped body.

As shown in Table 1 below, the metal organic framework having hydroxyl groups used in the shaped body of the present invention has the characteristics of having a mass transfer coefficient in the range of 0.010 to 0.300 sec ⁻¹ and a surface diffusion coefficient in the range of 0.010 to 0.200 sec ⁻¹ in nitrogen gas, and therefore has gas storage performance and higher gas release performance than zeolite and metal organic frameworks not having hydroxyl groups.
Furthermore, for carbon dioxide gas, compared to zeolite and metal organic frameworks that do not have hydroxyl groups, the material has the characteristics of a mass transfer coefficient in the range of 0.001 to 0.160 sec ^-1 and a surface diffusion coefficient in the range of 0.001 to 0.015 sec ^-1 , and therefore has high gas storage performance.
Furthermore, compared to a powder state, the molded shaped body has new grain boundaries, which are thought to result in a better mass transfer coefficient and surface diffusion coefficient, and therefore has excellent gas storage performance.

Furthermore, CD-MOF suitable for use in the shaped article of the present invention has a pore volume of 0.300 to 0.600 ^cm /g, preferably 0.350 to 0.550 ^cm /g, and an average pore diameter of 1.300 to 1.600 nm, preferably 1.400 to 1.550 nm. The pore volume, pore diameter, and average pore diameter are measured by the methods described in the Examples below.

(Method for producing metal-organic structure)
The metal organic framework capable of being molded into a shaped body of the present invention can be produced by, but is not limited to, the following method.
That is, after preparing a first aqueous solution containing metal ions and a cyclic organic compound that is an organic ligand capable of coordinating to the metal ions, a second solution containing an organic solvent is added to this first aqueous solution to produce a metal organic framework in which the organic ligand is coordinated to the metal ions.
In the first aqueous solution, the metal compound that supplies the metal ions is not limited to, but examples thereof include metal hydroxides, inorganic halide salts such as chloride salts, and inorganic acid salts such as nitrates and acetates. Specifically, when the metal ions are alkali metal ions such as potassium ions, it is preferable to use alkali metal hydroxides, and when the metal ions are Zn ions or Fe ions, it is preferable to use inorganic acid salts.

In the first aqueous solution, the amount of organic ligand per mole of metal ion is preferably in the range of 0.05 to 50 moles, more preferably in the range of 0.1 to 40 moles, and more preferably in the range of 0.125 to 30 moles.
When the metal ion is an alkali metal ion, the amount of organic ligand per mole of metal ion is preferably in the range of 0.05 to 5 mol, particularly preferably in the range of 0.125 to 0.80 mol. When the metal ion is a potassium ion and the organic ligand is a cyclodextrin compound, as mentioned above, the amount is preferably in the range of 0.05 to 0.80 mol, particularly in the range of 0.125 to 0.375 mol when the organic ligand is α-cyclodextrin, and in the range of 0.125 to 0.750 mol when the organic ligand is γ-cyclodextrin.

The amount of water in the first aqueous solution is preferably in the range of 50 to 5,000 mol per mol of metal ion, and when the metal ion is an alkali metal ion, the amount of water is preferably in the range of 80 to 200 mol, particularly preferably in the range of 100 to 150 mol, per mol of metal ion.
The molar ratio of the organic ligand to water (organic ligand:water) is preferably in the range of 1:50 to 1:2000, and particularly when the organic ligand is a cyclodextrin-based compound, it is desirable that the ratio be in the range of 1:100 to 1:1300.

The second solution to be added to the first aqueous solution prepared as described above is preferably a solution capable of lowering the pH of the first aqueous solution, and is preferably an organic solvent capable of deprotonating the coordination bond site of the organic ligand. The organic ligand is deprotonated and a coordination bond is formed with the metal ion, thereby generating a metal organic framework.
Examples of such organic solvents include at least one solvent selected from alcoholic solvents such as methanol, ethanol, 1-propanol, and 1-butanol, ketone solvents such as acetone, and aprotic solvents such as N,N-dimethylformamide. Among these, alcoholic solvents are preferred, and methanol is particularly suitable.
The organic solvent is preferably added in an amount of 3 to 1,000 mol, particularly 5 to 500 mol, per mol of metal ion. In particular, when the metal ion is potassium ion and the organic ligand is α-cyclodextrin, the organic solvent is preferably added in an amount of 10 to 400 mol, preferably 50 to 300 mol, more preferably 100 to 200 mol, per mol of potassium ion. When the organic ligand is γ-cyclodextrin, the organic solvent is preferably added in an amount of 3 to 100 mol, preferably 5 to 50 mol, more preferably 10 to 30 mol, per mol of potassium ion.

The second solution is preferably added to the first aqueous solution over a period of 1 to 20 minutes, preferably 10 to 15 minutes, while stirring the first aqueous solution. Adding the second solution all at once may result in cloudiness, potentially preventing the desired metal-organic framework from being obtained. However, adding the second solution over a period of time within the above range allows nuclei to form and allows the growth of a crystal structure, making it easier to obtain the desired metal-organic framework. The addition method is not limited to these, but examples include a method of adding the solution at a constant rate using a dropping funnel, a method of adding a fixed amount at regular intervals, and a method of adding the solution by gradually changing the amount added.
After the addition of the second solution is completed, the mixed solution is preferably stirred at room temperature for 5 to 50 hours, preferably 10 to 30 hours. The produced metal-organic framework is isolated by filtration or the like, washed with an organic solvent such as methanol as needed, and then dried to remove the organic solvent and water, thereby obtaining a powder of the metal-organic framework.

(Shaped object and its manufacturing method)
The shaped body of the present invention is an important feature in that it comprises the above-mentioned metal organic framework, preferably a metal organic framework having a hydroxyl group, more preferably CD-MOF, and does not contain any organic compounds other than the organic ligands that constitute the metal organic framework. By not containing any organic compounds other than the organic ligands, there is no risk of the pores of the shaped body and the metal organic framework being blocked.
The shaped body of the present invention may be composed of a mixture of a metal organic framework and an inorganic binder component. Examples of the inorganic binder component include known components such as alumina. When a metal organic framework and an inorganic binder component are used to produce the shaped body of the present invention, the amount of the inorganic binder component is typically 40% by mass or less, preferably 4% by mass or less, and more preferably 2% by mass or less, relative to the total amount of the metal organic framework and the inorganic binder component. When the amount of the inorganic binder component is within the above range, it is possible to achieve both shape retention and gas storage performance of the shaped body. Furthermore, when the shaped body is composed of a metal organic framework having a hydroxyl group, it is possible to achieve shape retention and better gas storage performance of the shaped body without containing an inorganic binder component.

The form of the shaped body can be any shape, such as tablet, pellet, or granule, but what is important in the present invention is that the metal-organic framework made of powder is solidified by pressing at 20-200°C and a surface pressure of 4 MPa or more. In particular, in the case of CD-MOF, it is preferable that it is pressed at 10-180°C and a surface pressure of 4-100 MPa, and more preferably at a surface pressure of 4-60 MPa. Shaped bodies solidified under these conditions are able to possess gas storage properties that could not be achieved in powder form.

The shaped object of the present invention has excellent adsorption and storage properties for carbon dioxide gas, nitrogen gas, hydrogen gas, etc., and can be suitably used as an adsorption and storage material for these gases. In particular, it has excellent adsorption and storage properties for carbon dioxide gas. Furthermore, the shaped object of the present invention can release gas in a short period of time.
The shaped body of the present invention can be formed into a predetermined shape and filled into a column or the like to be used as an adsorbent, but it is more suitable to fill it into a pressure vessel or the like and use it as part of a gas storage container.

As a method for storing these gases, the shaped body of the present invention can be brought into contact with the gas under pressure to adsorb the gas, thereby storing the gas in the shaped body.
The pressure conditions are preferably in the range of 0.01 to 8.00 MPa for 0.01 to 24 hours. The stored gas can be released (desorbed) by heating to a temperature equal to or higher than the adsorption temperature. The shaped body after release can be reused, and the gas can be stored by contacting it with the gas again.

In order to explain the present invention in more detail, examples carried out by the inventors will be described below.

(Measurement method)
[Pore volume/pore diameter]
The pore diameter and pore volume of the metal organic framework were calculated by measuring the nitrogen gas adsorption isotherm at liquid nitrogen temperature by a multipoint method using a BELSORP MAX II type manufactured by Microtrackbell, and then calculating the pore diameter and pore volume by MP calculation.

[Evaluation of gas mass transfer coefficient and surface diffusion coefficient]
The mass transfer coefficient and surface diffusion coefficient of nitrogen gas of the metal organic framework were calculated by measuring the gas adsorption rate at liquid nitrogen temperature by a multipoint method using a BELSORP MAX II type manufactured by Microtrackbell Co., Ltd. The results are shown in Table 1.
The mass transfer coefficient and surface diffusion coefficient of _CO2 gas of the gas adsorbent were calculated by measuring the gas adsorption rate at 298 K and in the measurement pressure range of 0 to 100 kPa using a BELSORP MAX II type manufactured by Microtrackbell Co., Ltd. The results are shown in Table 1.
The mass transfer coefficient is a value related to the amount of substance gas passing through a unit area per unit time, and is a value that varies depending on the measurement conditions. The surface diffusion coefficient is also a value related to the amount of substance gas passing through a unit area per unit time, and is a value specific to the object being measured, rather than a value that varies depending on the measurement conditions.
For reference, Table 1 also shows the mass transfer coefficients and surface diffusion coefficients of zeolite (Shilton MT-8000, manufactured by Mizusawa Industrial Chemicals Co., Ltd.) and ZIF-8 (manufactured by Sigma-Aldrich), a metal organic framework having no hydroxyl groups.

[Gas storage vessel evaluation]
For the gas storage container evaluation, a shaped or unshaped form of the metal-organic framework described below was introduced into a pressure holding container equipped with a pressure gauge and made of stainless ^steel with an internal volume of 154 cm3, and the container was pressurized to an internal pressure of 0.8 MPa, and nitrogen gas or carbon dioxide gas was filled until the introduced pressure and the internal pressure of the container became the same value, and the change in internal pressure after the valve was opened was measured.

Preparation and Evaluation of Gas Adsorbents
Example 1
Pure water was added to a 150 ml container, and potassium hydroxide was then added to the container and dissolved at room temperature. γ-Cyclodextrin was then added and dissolved at room temperature. The pH of the resulting solution was 13.98. Next, while stirring this solution with a stirrer, methanol was added dropwise over a period of approximately 15 minutes. After the addition, the solution was stirred for 24 hours to obtain a suspension containing a solid product. The pH of the solution after the addition of methanol was 13.85. The solid product was filtered from the resulting suspension, and the isolated solid product was washed with methanol. After washing, the solid product was dried overnight at 50°C to obtain metal organic framework A according to Example 1. The molar ratio of each component was potassium ion:γ-cyclodextrin:water:methanol = 1.00:0.12:20.37:2.86.
The pore diameter of the obtained metal organic framework A was 0.5 to 2.0 nm, and the pore volume at an average pore diameter of 1.508 nm was 0.538 cm ³ /g.
The metal-organic framework A (0.5 g) was added to a metal for a Φ10 hot press machine, and pressure was applied in the press machine at 25.0° C. and 0.5 MPa for 5 minutes, thereby producing a shaped body B.
Also, shaped body B (10 g) was filled into a pressure-retaining vessel, which was then filled with a fill gas to produce a gas storage vessel.

(Comparative Example 1)
The metal organic framework A (unshaped body C) was filled into a pressure holding vessel, and then a fill gas was filled therein to produce a gas storage vessel.

(Gas storage container evaluation results)
Nitrogen gas storage results:
Although the amount of change in internal pressure after a certain period of time was the same, filling with shaped body B enabled gas to be released in a shorter time and improved the decrease in internal pressure compared to filling with unshaped body C. Furthermore, the shaped body did not undergo any change in shape before and after gas filling.
Carbon dioxide gas storage results:
Compared to filling with unshaped body C, filling with shaped body B allowed for gas release in a shorter time and improved the decrease in internal pressure. Furthermore, as the amount of carbon dioxide gas adsorbed increased by molding into a shaped body, the amount of gas released also increased, and the amount of change in internal pressure also increased compared to filling with unshaped body C. Furthermore, the shaped body did not undergo any change in shape before and after gas filling.
From the above, it became clear that even with the same CD-MOF, the shaped body has better gas storage and gas release properties than the powder form, and therefore, after the valve of the pressure vessel is opened, the stored filled gas can be released, suppressing a decrease in the pressure inside the vessel.

The shaped object of the present invention has excellent gas storage properties, and is particularly excellent in adsorption and storage properties for gases such as carbon dioxide, nitrogen, and hydrogen, and therefore can be suitably used as a gas storage container.

Claims (7)

A shaped object characterized by being formed from a metal-organic framework comprising a metal and an organic ligand coordinated to the metal. The shaped body according to claim 1, wherein the metal organic framework is a metal organic framework having a hydroxyl group. The shaped body according to claim 1, wherein the organic ligand is cyclodextrin. The shaped body according to claim 1, which does not contain any organic compounds other than the organic ligand. The shaped object described in any one of claims 1 to 4, which is used for gas storage. A gas storage container comprising the shaped body described in claim 5. 2. The method for producing a shaped body according to claim 1, wherein the metal-organic framework is pressure-molded under conditions of 20 to 200° C. and a surface pressure of 4 MPa or more.