WO2021043492A1

WO2021043492A1 - Gas storage material and gas storage system

Info

Publication number: WO2021043492A1
Application number: PCT/EP2020/070421
Authority: WO
Inventors: Christophe LAVENN; Patrick Ginet; Susumu Kitagawa; Nobuhiko HOSONO; Mickaele BONNEAU
Original assignee: L'air Liquide Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude; Kyoto University
Priority date: 2019-09-02
Filing date: 2020-07-20
Publication date: 2021-03-11
Also published as: JP2021037460A

Abstract

To provide a gas storage material and gas separation system capable of regulating the storage pressure and release pressure of a gas. A gas storage material which has two cubic lattice-shaped organometallic complexes composed of metal atoms and at least three kinds of ligands, wherein the two organometallic complexes form an inter-penetrating structure so that one apex portion of a unit cell of one of the organometallic complexes is positioned in a space inside one unit cell of the other organometallic complex.

Description

Gas Storage Material and Gas Storage System

Technical Field

The present invention relates to a gas storage material and gas storage system. Background Art

Gas is generally stored by being compressed or liquefied. The need for pressure regulating devices and double-steel cylinders capable of safely maintaining pressurized gas, for example, has been highlighted as a drawback in such cases. Cylinder shapes and sizes are normally fixed in consideration of the high pressure required to obtain sufficient volume and the attendant safety problems, and cannot be easily adjusted for specific applications. Such limitations pertain to all commercial gases requiring high pressure for such applications as well as to gases or gas mixtures that cannot be safely compressed to such pressures and that require special containers.

For example, acetylene (C2H2) is a highly reactive gas which may explode when pressurized to 0.2 MPa or more, even in the absence of oxygen. This is caused by the exothermic decomposition of C2H2 into C and FF as well as self-cyclization type reactions. Acetylene is thus a gas that cannot be stored at high pressure.

Except for high-grade acetylene, which is usually stored in the gas phase at a pressure under 0.15 MPa (gauge pressure), techniques for dissolving gas (at a pressure of approximately 1.5 MPa) in an organic solvent (acetone or N, N-dimethylformamide) contained in steel pipe cylinders filled with porous calcium silica and fiber glass are available as a practical way to store acetylene (Patent Document 1). The principal application for this type of acetylene storage is welding and cutting. Solvents are expensive for manufacturers, are time-consuming to handle, and may expose consumers to significant safety risks if improperly handled. For safety reasons, it is also essential to avoid problems associated with the use of solvents by limiting such applications, such as limiting the flow rate, which is directly related to cylinder dimensions, or limiting the use of cylinders to the upright position. When acetylene gas flows, solvent contamination is usually approximately 2 to 5%. Because of the dependence of flow rate on cylinder dimensions, low-volume cylinders can have only limited flow rates. The presence of solvents results in several drawbacks. As noted above, the evaporation of solvent associated with the desorption of acetylene (cylinder use) results in significant safety risks for users. In fact, the evaporation of solvent may result in the formation of pockets containing solvent-free (dried) porous material. Under these conditions, the initial storage pressure of acetylene is approximately 1.5 MPa, and the desorbed acetylene can thus form bubbles having a pressure that is higher than the explosion limit (0.2 MPa), resulting in the potential for spontaneous explosion. To limit solvent evaporation and the subsequent risk of explosion, the flow rate of a cylinder while in use is normally limited in direct proportion to the internal volume of the cylinder. The removal of the acetylene from the solvent is an endothermic process, resulting in the subsequent cooling of the cylinder. The desorption of acetylene decreases, as does the resulting flow rate, and the cylinder appears to be spent until the temperature increases (to room temperature), significantly limiting continuous use of the cylinder.

Prior Art Documents Patent Documents

Patent Document 1 : US Patent No. 7,807,259

Summary of the Invention Problem to be Solved by the Invention

Because of the above drawbacks related to storage primarily employing solvents, it would be highly desirable to provide a solution that would allow acetylene to be stored in sufficient volume without the use of solvents. Apart from solvent-based techniques, other acetylene containers are commercially available for acetylene that has been compressed to a pressure of 0.15 MPa. These containers have been shown to be of high purity (no solvent contamination) but have lower storage volume than containers employing solvents.

Adsorbents exhibiting typical adsorption behaviour (IUPAC I type isothermal adsorption profile) offer virtually no advantage in this design because of the extremely low operating pressure range, specifically, a container pressure of preferably less than 0.2 MPa, and a release pressure that is higher than 0.1 MPa, which corresponds to the container outlet pressure. A storage solution allowing a sufficient volume of acetylene to be stored and released at an adjustable, low pressure would thus be highly desirable. Similarly, a solution for storing enough gas at low pressure (less than 3 MPa) would reduce safety risks because of the low pressure, which would be desirable for all gases and gas mixtures.

Metal-Organic Frameworks (MOF), which are also known as Porous Coordination Polymers (PCP), are a type of organic-inorganic hybrid material comprising metal ion- based nodes that form a framework by means of coordination bonds with a variety of organic or organometallic ligands. These materials have garnered increasing interest within the scientific community over the past several years because they are porous and have high volume and specific surface area. MOFs are also highly adjustable, and different materials can be obtained when different organic ligands are used. MOFs also have unique "respiration" or "flexible" structures, and thus exhibit unique adsorption- desorption characteristics, which are characterized primarily by strong adsorption beginning at gate opening pressure (storage pressure) associated with adsorption and desorption hysteresis. This characteristic has critical significance for storage applications because of the strong and rapid changes in the level of adsorption in a small pressure range, which means that these materials can achieve a higher working capacity than materials exhibiting typical Langmuir-type adsorption isotherm profiles.

However, even though this specific adsorption profile is a matter of considerable interest, there has been virtually no research on flexible MOFs, and the adsorption profiles are difficult to adjust.

In light of the problems noted above, an object of the present invention is to provide a gas storage material and gas separation system capable of regulating the storage pressure and release pressure of a gas.

Means for Solving the Problem

As a result of extensive research, the inventors perfected the present invention upon discovering that the above object could be achieved by adopting the following configuration. One embodiment of the present invention relates to a gas storage material which has two cubic lattice-shaped organometallic complexes composed of metal atoms and at least three kinds of ligands, wherein the two organometallic complexes form an inter-penetrating structure so that one apex portion of a unit cell of one of the organometallic complexes is positioned in a space inside one unit cell of the other organometallic complex.

This gas storage material has cubic lattice-shaped organometallic complexes as basic structures, and therefore has greater flexibility than zeolites and activated carbon. An inter-penetrating structure is also formed, so that cells of one of the organometallic complexes alternately fit into spaces inside cells of the other organometallic complex. An adsorbate gas is taken into spaces in the inter-penetrating structure (referred to below as "gas intake spaces"). Prior to gas adsorption (at atmospheric pressure), the two organometallic complexes have a flat folded arrangement, where they are stable, in terms of energy, due to ligand p^~p stacking interactions, for example (a rhomboid arrangement in which, viewing the unit cells from the side, pairs of opposing corners are very close to each other). In other words, the gas intake space is at its lowest. Meanwhile, when gas begins to be pressurized and the gas pressure increases to the point where the stabilizing energy breaks down, the cells of the two organometallic complexes rise up and begin to separate from each other (a rectangular arrangement in which, viewing the unit cells from the side, the pairs of opposing corners that had been very close to each other are now separated from each other). The gas intake spaces begin to enlarge or expand. At the stage where the gas pressure has further increased and the size of the gas intake spaces has become larger than the size of the gas molecules, the gas begins to be taken up into the gas intake spaces. The pressure at this time is the storage pressure. As the gas continues to be pressurized, the change in the size of the gas intake spaces peaks, and no more gas is taken up. The intake amount changes rapidly from the start to the end of gas intake; this series of events corresponds to gate opening behaviour. When the gas is then depressurized, the gas begins to be released from the gas intake spaces. However, the cell structure of the complexes is stabilized as a result of the gas being packed into the gas intake spaces, and the amount of gas that is released therefore gradually decreases until the gas pressure has fallen to a certain level. When the gas pressure has continued to fall and reaches a pressure where the stabilization by the packing effect breaks down, the gas is rapidly released from the gas intake spaces. The pressure at this time is the release pressure. The state that prevailed prior to gas intake will theoretically be reached as the gas pressure continues to fall further. The series of events during the drop in pressure corresponds to gate release behaviour. One feature of the gate opening/release behaviour is the presence of a hysteresis type adsorption-desorption curve.

Figure 1A schematically illustrates typical adsorption behaviour (IUPAC type I isothermal adsorption profile), and Figure IB schematically illustrates a hysteresis type adsorption-desorption curve. The adsorption pressure (storage pressure: P2) is similar in both the adsorption-desorption curve for typical adsorption behaviour (IUPAC I type isothermal adsorption profile) and the hysteresis type adsorption-desorption curve. However, when the gas pressure falls from P2 to Pi, virtually no gas is desorbed in the former curve, whereas almost all of the adsorbed gas is desorbed in the latter curve. The value obtained by subtracting the desorbed amount from the adsorbed amount corresponds to the working capacity (adsorption capacity) that can be utilized within the working pressure range, and the gas storage material exhibiting hysteresis type adsorption- desorption behaviour can thus have a substantial working capacity in a working pressure range comparable to the conventional range. The storage pressure and release pressure can be regulated in this gas storage material, thus making it possible to set a working capacity, working pressure range, and working temperature suited to the intended gas.

The positions of the two organometallic complexes relative to each other (specifically, the size of the gas intake spaces) can vary according to the size of the unit cells. The organometallic complexes also contain at least three kinds of ligands, and the proportions in which the ligands are present can be changed to control the flexibility (deformability) or the physical or chemical properties, for example, of the organometallic complexes. The structure of the organometallic complexes per se can thus be distorted (such as a quadrangular prism shape in which the relative positions of the top surface and bottom surface of a cubic shape are displaced in parallel, resulting in shear deformation), allowing the size and shape of the gas intake spaces to be changed. In this gas storage material, a specific ligand design can be used to control the deformation of the complexes per se caused by the distance between cells (complex adjacency), the size of the cells, or the content of heterogeneous atoms in the inter penetrating structure of the cubic lattice-shaped organometallic complexes, thereby making it possible to regulate the storage pressure and release pressure and to achieve efficient gas storage performance.

[0017]

In a preferred embodiment, in each of the organometallic complexes, assuming that the apex portions of the unit cells are at the centre of an orthogonal coordinate system having an x-axis, a y-axis and a z-axis, at least one of the metal atoms is present at the centre, at least two kinds of planar lattice-forming ligands are coordinated, two in the x-axis direction and two in the y-axis direction, relative to the at least one metal atom, forming a planar lattice structure, and two pillar ligands different from the planar lattice-forming ligands are coordinated in the z-axis direction relative to the one metal atom, forming a cubic lattice structure in which the planar lattice structure is layered in the z-axis direction. Organometallic complexes having an inter-penetrating structure are commonly formed by a procedure in which a planar lattice structure (xy plane) is formed, and the planar lattice structure is then layered using pillar ligands in the thicknesswise direction (z-axis direction). At least two kinds of ligands can be used as the planar lattice-forming ligands of the planar lattice structure so as to modify the structure or the physical or chemical properties of the planar lattice structure to the extent desired and to then efficiently regulate the storage pressure and release pressure of gas in the organometallic complex as a whole.

In a preferred embodiment, two metal atoms are present in the centre, a dicarboxylic acid ion ligand A and a dicarboxylic acid ion ligand B that is different from the dicarboxylic acid ion ligand A are coordinated, as the planar lattice forming ligands, to the two metal atoms so as to form a paddle-wheel unit, and pyridine derivative ligands are used as the pillar ligands.

In a preferred embodiment, the dicarboxylic acid ion ligand A and dicarboxylic acid ion ligand B are each independently represented by any of the following formulae (la) through (lc), and have mutually different structures. Chemical Formula 1

COO coo- COO

(1a) (1b) (1c)

(In the formula, R¹ is a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a halogen atom, an alkanoyl group, a hydroxyalkyl group, an aryl group, an aryloxy group, an aralkyl group, a carboxy group, a cyano group, an amino group, or a nitro group. nl is an integer of 0 to 4, n2 is an integer of 0 to 3, and n3 is an integer of 0 to 4.

When there are more than one R¹, R¹ may be the same as or different from each other.) In a preferred embodiment, the pyridine derivative ligands are represented by any of the following formulae (2a) through (2e).

Chemical Formula 2

(2d) (2e) (In the formula, R² is the same as R¹ in formulae (la) through (lc). n4 and n5 are each independently an integer of 0 to 4.

When there are more than one R², R² may be the same as or different from each other.) The above-mentioned ligands are used as the planar lattice-forming ligands and the pillar ligands to allow the storage pressure and release pressure of the gas storage material to be adjusted more efficiently. Among them, the dicarboxylic acid ion ligands and pyridine derivative ligands represented by the above formulae are preferred in the interests of the size of the gas intake space (unit cell size), affinity for gas, ease of gas storage material synthesis, and starting material availability. These ligands are used to allow the storage pressure and release pressure to be tailored to the intended gas and to ensure efficient gas storage.

In a preferred embodiment, heterogeneous ligands other than the main ligands, which have the lowest molecular weight among the planar lattice-forming ligands, are present in a total amount of 1 mol% to 70 mol%. Ensuring that the heterogeneous ligands are present in a total amount within the above range (the amount of one heterogeneous ligand when there is only one heterogeneous ligand) will ensure sufficiently effective structural modification while allowing the inherent gas storage performance of the organometallic complexes to be maintained.

In one embodiment, the gas storage material is suitable for storing gas for which the explosion limit at 25°C in a non-oxidizing atmosphere is 0.2 MPa. The storage pressure and release pressure can be adjusted according to the intended gas in this embodiment, making it suitable for storing gas that is difficult to handle at high pressure.

In one embodiment, the gas may be acetylene. The gas storage material has a high adsorption capacity at low pressure, thus allowing even explosive acetylene to be stored safely and efficiently.

In one embodiment, the present invention relates to a gas storage system for storing one or more kinds of gas, comprising the gas storage material, a pressurization/depressurization mechanism for increasing or reducing the gas pressure, and a control unit for controlling the pressure of the pressurization/depressurization mechanism, wherein the pressure for storing the gas in the gas storage material and the pressure for releasing the gas from the gas storage material are controlled by changing the kind of ligands that form the organometallic complexes of the gas storage material.

In this gas storage system, the storage pressure and release pressure of the gas storage material can be regulated simply by changing the design of the ligands, enabling more efficient gas storage. It is thus possible to construct a gas storage system that is tailor- made for the intended gas.

Brief Description of the Drawings

Figure 1A schematically illustrates typical adsorption behaviour (IUPAC type I isothermal adsorption profile).

Figure IB schematically illustrates a hysteresis type adsorption-desorption curve.

Figure 2 schematically illustrates the inter-penetrating structure of the organometallic complexes in a gas storage material according to one embodiment of the present invention.

Figure 3 schematically illustrates an example of a paddle-wheel type metal-organic nodule structure found in organometallic complexes forming the gas storage material. Figure 4 schematically illustrates changes in storage pressure and release pressure.

Figure 5 A shows X-ray diffraction images of powdered Zn-CAT-Al incorporating 5 mol%, 10 mol%, 15 mol%, and 20 mol% of 2-aminoterephthalic acid (NFE-bdc).

Figure 5 B shows X-ray diffraction images of powdered Zn-CAT-Al incorporating 5 mol%, 10 mol%, 15 mol%, and 20 mol% of 2-nitroterephthalic acid (NCk-bdc).

Figure 5 C shows X-ray diffraction images of powdered Zn-CAT-Al incorporating 5 mol%, 10 mol%, 15 mol%, and 20 mol% of tetrafluoroterephthalic acid (F4-bdc).

Figure 6A shows isothermal adsorption curves for the adsorption (represented by solid symbols and *) and desorption (represented by hollow symbols and x) of acetylene at 273 K in the single-ligand complex of Synthesis Example 1 and in the heterogeneous- ligand complexes in Synthesis Examples 2 through 5.

Figure 6B shows isothermal adsorption curves for the isothermal adsorption (represented by solid symbols and *) and desorption (represented by hollow symbols and x) of carbon dioxide at 194.7 K in the single-ligand complex of Synthesis Example 1 and in the heterogeneous-ligand complexes in Synthesis Examples 2 through 5.

Figure 6 C shows isothermal adsorption curves for the adsorption (represented by solid symbols and *) and desorption (represented by hollow symbols and x) of oxygen at 90.2 K in the single-ligand complex of Synthesis Example 1 and in the heterogeneous-ligand complexes in Synthesis Examples 2 through 5.

Figure 7 A shows isothermal adsorption curves for the adsorption (represented by solid symbols and *) and desorption (represented by hollow symbols and x) of acetylene at 273 K in the single-ligand complex of Synthesis Example 1 and in the heterogeneous- ligand complexes in Synthesis Examples 6 through 9.

Figure 7 B shows isothermal adsorption curves for the isothermal adsorption (represented by solid symbols and *) and desorption (represented by hollow symbols and x) of carbon dioxide at 194.7 K in the single-ligand complex of Synthesis Example 1 and in the heterogeneous-ligand complexes in Synthesis Examples 6 through 9.

Figure 7 C shows isothermal adsorption curves for the adsorption (represented by solid symbols and *) and desorption (represented by hollow symbols and x) of oxygen at 90.2 K in the single-ligand complex of Synthesis Example 1 and in the heterogeneous-ligand complexes in Synthesis Examples 6 through 9.

Figure 8 A shows isothermal adsorption curves for the adsorption (represented by solid symbols and *) and desorption (represented by hollow symbols and x) of acetylene at 273 K in the single-ligand complex of Synthesis Example 1 and in the heterogeneous- ligand complexes in Synthesis Examples 10 through 13.

Figure 8 B shows isothermal adsorption curves for the isothermal adsorption (represented by solid symbols and *) and desorption (represented by hollow symbols and x) of carbon dioxide at 194.7 K in the single-ligand complex of Synthesis Example 1 and in the heterogeneous-ligand complexes in Synthesis Examples 10 through 13. Figure 8 C shows isothermal adsorption curves for the isothermal adsorption (represented by solid symbols and *) and desorption (represented by hollow symbols and x) of carbon dioxide at 90.2 K in the single-ligand complex of Synthesis Example 1 and in the heterogeneous-ligand complexes in Synthesis Examples 10 through 13. Mode for Implementing the Invention

Embodiments of the present invention are described below with reference to the drawings. The embodiments described below illustrate examples of the present invention. The present invention is not in any way limited to the following embodiments, and includes various modifications executed within a range that does not alter the essence of the present invention. The configurations described below do not all necessarily comprise configurations essential to the present invention. In some or all of the drawings, parts that are not required for the explanations may be omitted, and parts may be expanded or reduced in scale in order to facilitate the explanation.

Gas Storage Material

Figure 2 schematically illustrates the inter-penetrating structure of the organometallic complexes in a gas storage material according to an embodiment of the present invention. The gas storage material of the present embodiment has two cubic lattice shaped organometallic complexes (dark-coloured lattice and light- coloured lattice), corresponding to what are called adaptive metal-organic frameworks (also known as a flexible MOFs or gate opening MOFs). The two organometallic complexes form an inter-penetrating structure so that one apex portion of a unit cell of one of the organometallic complexes is positioned in a space inside one unit cell of the other organometallic complex. In other words, the gas storage material belongs to the MOF family in which two elements are linked (CAT: Catenated MOF). A CAT is a structure in which two independent three-dimensional cubic lattice-shaped frameworks penetrate each other. Flexible frameworks can exhibit different types of flexibility.

In this embodiment, the focus is on adjusting and controlling the properties of the metal- organic framework, which is a hybrid material composed of metal ions and organic ligands. Although the primary focus in this embodiment is on adjusting the gas adsorption properties of porous MOFs, the embodiment is also applicable to the modification of one or more material properties (such as the light-emission, colour, colour-changing properties, catalytic activity, mechanical properties, hydrophilic/hydrophobic properties, stability in solvents, thermal stability, compatibility with molecules containing polymers/oligomers/binders, the size of the particles that are formed, and the reaction kinetics of formation). The resulting material may be used directly in the intended application, may be moulded, or may be subjected to other physical or chemical processes such as heating, solvent treatment, addition of additives, ion exchange, ligand exchange, surface reaction, ion encapsulation, mechanical grinding, or photographic processes.

The conditions under which the gas storage material is used typically involve normal atmospheric conditions (such as, but not limited to, 0.1 MPa or 298 K), but may be different depending on the application. The storage level is defined as the amount of gas that is loaded in the gas storage material at low temperature and/or high pressure, and the residual level corresponds to the remaining amount of gas that is held by the gas storage material at the working temperature and pressure. The working capacity (adsorption capacity) corresponds to the difference between the amount of gas that is stored or loaded in the gas storage material and the amount of stored gas that is left over in the gas storage material. The working capacity thus corresponds to the total amount of gas that can be used (stored) per unit of gas storage material (one storage-release cycle).

The gas storage material of the present embodiment involves advances in MOF phase in addition to adsorption phenomena, where the inter-penetrating structure noted above enables further increases in the gas intake space capacity that contributes to the rapid gas absorption. A high working capacity is therefore produced, with virtually no absorbed gas left over under the conditions of use. The residual level of gas that has been adsorbed by the gas storage material under the conditions of use (298 K or 0.1 MPa), defined as the volume ratio of remaining gas versus the volume of gas storage material, is preferably 40 v/v% or less, is more preferably 20 v/v% or less, and is even more preferably so low as to be negligible. The working capacity of the gas storage material (the value obtained by subtracting the lowest gas storage level after the release process from the greatest gas storage level after the loading process) after one storage- release cycle of the intended gas is preferably 50 v/v% or more, more preferably 60 v/v% or more, and even more preferably 70 v/v% or more. The working pressure can be a pressure that is preferably between vacuum conditions (such as 0.001 MPa) and 30 MPa, and more preferably between 0.1 MPa (atmospheric pressure) and 10 MPa. The lower limit of the working pressure (vacuum conditions) should preferably range from 0.08 to 0.18 MPa when the gas is acetylene, for example, and should preferably range from 0.08 to 0.35 MPa for other gases. The working temperature is preferably -40°C to 150°C, and more preferably 10°C to 30°C.

The independent organometallic complexes (frameworks) typically comprise metal centres (preferably transition metals), planar lattice-forming ligands coordinated, perpendicularly to each other within a plane, to the metal centres, and pillar ligands coordinated perpendicularly to the plane relative to the metal centres, thereby forming a cubic lattice-shaped structure.

In a preferred embodiment, two metal atoms are present in the centre, a dicarboxylic acid ion ligand A and a dicarboxylic acid ion ligand B that is different from the dicarboxylic acid ion ligand A are coordinated, as the planar lattice forming ligands, to the two metal atoms so as to form a paddle-wheel unit, and pyridine derivative ligands are used as the pillar ligands. Figure 3 schematically illustrates an example of a paddle-wheel type organometallic nodule structure found in organometallic complexes forming a gas storage material. In one potential complex having metal-metal bonds (MM bonds), a plane is formed, where four carboxylic acid ion groups are coordinated to two metal ions (Zn) in the x-axis direction and y-axis direction, and oxygen (0) surrounds the metal ions. The z-axis direction is occupied by the nitrogen (N) of two pyridine derivative ligands. In the present specification, this type of node structure is defined as a CAT-A type. The above dicarboxylic acid ion ligands A and B, pillar ligands, or metal atoms can be combined to obtain geometric configurations of organometallic complexes having a variety of shapes (such as metal-carboxylic acid ion paddle-wheels).

In another preferred embodiment, the dicarboxylic acid ion ligand A and dicarboxylic acid ion ligand B are each independently represented by any of the following formulae (la) through (lc), and have mutually different structures. Chemical Formula 3 coo- coo cocr

(1a) (1b) (1c)

When there are more than one R¹, R¹ may be the same as or different from each other.) Examples of alkyl groups include methyl, ethyl, and propyl groups.

Examples of alkoxy groups include methoxy and ethoxy groups. Examples of halogen atoms include fluorine, chlorine, bromine, and iodine atoms. Examples of alkanoyl groups include methylcarbonyl and ethylcarbonyl groups. Examples of hydroxyalkyl groups include hydroxymethyl and hydroxy ethyl groups. Examples of aryl groups include phenyl groups. Examples of aryloxy groups include phenoxy groups.

Examples of aralkyl groups include benzyl and phenethyl groups.

In a preferred embodiment, the pyridine derivative ligands are represented by any of the following formulae (2a) through (2e). Chemical Formula 4

(2d) (2e)

(In the formula, R² is the same as R¹ in formulae (la) through (lc). n4 and n5 are each independently an integer of 0 to 4.

In the gas storage material of the present embodiment, organometallic complexes containing at least three different kinds of heterogeneous ligands can be fabricated, with nearly no changes in the structure of the resulting organometallic complexes, to thereby control the storage pressure and release pressure as well as the temperature at which the pressure is produced. Because of the size and property limitations of organometallic complexes, the main ligands are typically ligands having a low molecular weight or degree of substitution with substituents, for example, and the heterogeneous ligands are ligands having a relatively higher molecular weight or degree of substitution with substituents. Or the main ligands may be unmodified (unsubstituted) ligands, and the heterogeneous ligands may be modified (substituted) ligands. When the molecular weight is the same and the order of the bonds between atoms is different, compounds having a higher order of bonds are used as the main ligands. Typically, the organometallic complex (I) contains planar lattice-forming ligands and pillar ligands as constituent ligands, and can contain the main ligands and heterogeneous ligands as the planar lattice-forming ligands. The present embodiment can include other embodiments, such as embodiments in which the organometallic complex (II) contains pillar ligands and planar lattice-forming ligands as constituent ligands, and contains heterogeneous ligands and the main ligands as the pillar ligands, and embodiments in which the organometallic complex (III) contains pillar ligands and planar lattice-forming ligands as constituent ligands, and contains the main ligands as the planar lattice-forming ligands and the heterogeneous ligands as the pillar ligands. Modes of substitution for the main ligands include (1) modes in which the framework atoms (primarily carbon) forming the main ligand are substituted with other atoms, (2) modes in which hydrogen atoms on the framework atoms of main ligands are substituted with other functional groups, or (3) combinations of (1) and (2). Combining main ligands and heterogeneous ligands as the ligands constituting the organometallic complex will not change the mode of adsorption. A heterogeneous ligand-containing organometallic complex that contains heterogeneous ligands in addition to the main ligands (also referred to below as "heterogeneous ligand complex") can therefore be fabricated to control the storage pressure and release pressure (at a fixed temperature), without changing the working capacity (adsorption level) of the gas storage material. As the gate opening (storage) and gate closing (release) behaviours shift, the working capacity can be adjusted with a high degree of sophistication in keeping with the target pressure and temperature regions, even though the total working capacity is still the same. Organometallic complexes containing only main ligands are also referred to as "single ligand complexes".

As examples of heterogeneous ligands and main ligands, compounds represented by the following formulae (4a) through (4f) are suitable for heterogeneous ligands when terephthalic acid represented by the following formula (3) (1,4-benzenedicarboxylic acid) is used as the main ligand in planar lattice-forming ligands.

Chemical Formula 5

Chemical Formula 6

In the gas storage agent of the present embodiment, heterogeneous ligands can be used to regulate the shift in storage pressure (gate opening pressure) and release pressure (gate closing pressure). Figure 4 schematically illustrates changes in storage pressure and release pressure. The shift involves moving from the pressure P2 (solid line in Figure 4) of an organometallic complex of only main ligands toward a higher pressure P₃ (dotted line in Figure 4), designated as a positive pressure shift, or toward a lower pressure Pi (dashed line in Figure 4), designated as a negative shift. Whether the shift is positive or negative is determined by the selection of the heterogeneous ligands that are introduced. The range of the shift is also controlled by the amount of the heterogeneous ligands that are introduced.

In the present embodiment, no particular limitations are imposed on the total amount of heterogeneous ligands other than the main ligands, which have the lowest molecular weight among the planar lattice-forming ligands, but the lower limit is preferably 1 mol%, more preferably 2 mol%, and even more preferably 3 mol%. The upper limit of the total amount is preferably 70 mol%, more preferably 50 mol%, and even more preferably 30 mol%. Ensuring that the heterogeneous ligands are present in a total amount within the above range (the amount of one heterogeneous ligand when there is only one heterogeneous ligand) will ensure sufficiently effective modification of the structure of the organometallic complexes by the heterogeneous ligands while allowing the inherent gas storage performance of the organometallic complexes to be maintained. Examples of the effects of heterogeneous ligands on gas adsorption include, but are not particularly limited to, one or more of the following: modifying the state of equilibrium of the heterogeneous ligand complexes in the gate closing configuration, modifying the state of equilibrium of the heterogeneous ligand complexes in the gate opening configuration, and modifying the intensity or nature of the the guest (gas)- organometallic complex interactions. The principal effects of ligand substitution on gas adsorption can result from (i) the electron-withdrawing or electron-donating ability, and (ii) the stabilization of the gated closing phase. As a whole, these modifications are greatly dependent on ligand selection. Gate opening adsorption is a cooperative phenomenon, and gate opening adsorption and gate closing therefore depend on the chemical properties of microcrystals in their entirety.

Typically, when an electron-withdrawing group is introduced into one of the main ligands to form a heterogeneous ligand (such as when a nitro group ( N O ₂ ^— ) is introduced into terephthalic acid (benzenedicarboxylic acid (bdc)), the substituent withdraws electrons from aromatic rings and oxygen-intermetallic ionic bonds (O-M bonds). As a result, when NCh-bdc is added, O-M bonds contribute less and the heterogeneous ligand complex has higher energy compared with organometallic complexes containing bdc as the single ligand. As a result, the stabilization of the heterogeneous ligand complex by the guest-organometallic complex interaction contributes more to lowering the phase transition energy barrier. The energy available for transition depends on the chemical potential of the gas/organometallic complex in cases of gas adsorption, so when an electron- withdrawing group such as N0₂ is used to partially substitute the ligands of the organometallic complex, lowering the energy barrier leads to a drop in the gate-opening pressure for a given gas.

Similarly, in cases where a substituent such as NH2 is partially introduced, (i) the electron-donating ability and (ii) the ability to donate hydrogen bonds must be taken into consideration. The electron-donating ability increases the O-M bond strength, so that heterogeneous ligand complexes have a lower energy level compared with the organometallic complexes containing a single ligand. Hydrogen bonds between aromatic rings and NH2 also further contribute to the stabilization of the organometallic complex prior to gas adsorption. Accordingly, the phase transition energy barrier is higher. The available energy resides in the chemical potential of the gas/organometallic complex (is derived from the gas-organometallic complex interaction), so when hydrogen bond-donating groups are used with electron-donating properties such as those of N H 2 to partially substitute ligands in organometallic complexes, the higher energy barrier results in a higher gate opening pressure for a given gas.

In the present embodiment, the metal atoms are preferably transition metal atoms, more preferably Group 4 transition metal atoms, and even more preferably Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn. Of these, the metal atoms are preferably Cu or Zn. Metals such as the above can be used as the metal atoms that form the organometallic complex to allow cubic lattice-shaped organometallic complexes to be efficiently and easily produced and to easily regulate gas storage pressure and release pressure.

The way in which heterogeneous ligands are contained in the two organometallic complexes is not particularly limited; for example, in cases where main ligands and heterogeneous ligands are contained, examples include the following: (a) one of the organometallic complexes contains only main ligands, and the other organometallic complex contains only heterogeneous ligands; (b) one of the organometallic complexes contains main ligands and heterogeneous ligands, and the other organometallic complex contains only main ligands; (c) one of the organometallic complexes contains main ligands and heterogeneous ligands, and the other organometallic complex contains only heterogeneous ligands; and (d) both of the two organometallic complexes contain both main ligands and heterogeneous ligands. Configuration (d) is preferred in the interests of ease of synthesis of the organometallic complexes and the uniformity of the properties of the two complexes.

Examples of gases that can be stored by the gas storage material in this embodiment include, but are not limited to, hydrogen, oxygen, methane, acetylene, ethylene, ethane, carbon dioxide, carbon monoxide, nitrogen, nitric oxide, nitrogen oxides, noble gases, halogen gases, or any mixture thereof. Among these, the gas storage material is suitable for storing gas for which the explosion limit at 25°C in a non-oxidizing atmosphere is 0.2 MPa. The storage pressure and release pressure can be adjusted according to the intended gas in this embodiment, making it suitable for storing gas that is difficult to handle at high pressure. Acetylene is an example of such an explosive gas.

Method for Producing Gas Storage Material

The method for producing the gas storage material can include, but is not particularly limited to, methods that are well known in the production of MOFs. Specific examples include one-pot synthesis methods (such as self-assembly methods, solvothermal methods, microwave irradiation methods, ionothermal methods, and high throughput methods), stepwise synthesis methods (such as methods for precursor complexes of organometallic node structures, complex-ligand methods, in-situ sequential synthesis methods, and post-synthesis modification methods), sonochemical synthesis methods, and mechanochemical synthesis methods.

In an example of a method of production employing a self-assembly method (a one-pot synthesis method), a metal salt (such as a metal nitrate) serving as the metal centre and planar lattice-forming ligands serving as the planar lattice structures (main ligands and heterogeneous ligands) are mixed in a solvent. A mixture containing pillar ligands and solvent can be added to a mixture containing complexes having planar lattice structures, and the resulting mixture can be allowed to react either at room temperature or while heated to produce a gas storage material in which cubic lattice-shaped organometallic complexes penetrate each other.

Examples of solvents that can be used for dissolving the ligands or metal salt include, but are not particularly limited to, cyclic or acyclic amide-based solvents such as dimethylformamide (DMF) or N-methylpyrrolidone, alcohol-based solvents such as methanol or ethanol, ketone-based solvents such as acetone, aromatic-based solvents such as toluene, and water. The reaction temperatures are preferably 25°C to 150°C, and more preferably 70°C to 120°C. The reaction time is preferably 2 to 72 hours, and more preferably 6 to 48 hours. The target gas storage material can be produced by collecting the product of the reaction by filtration or centrifugation, for example, washing the product with a solvent noted above, and then drying the product.

Gas Storage System

In one embodiment, the present invention relates to a gas storage system for storing one or more kinds of gas, comprising the gas storage material, a pressurization/depressurization mechanism for increasing or reducing the gas pressure, and a control unit for controlling the pressure of the pressurization/depressurization mechanism, wherein the pressure for storing the gas in the gas storage material and the pressure for releasing the gas from the gas storage material are controlled by changing the proportions of the metal atoms that form the organometallic complexes of the gas storage material.

Known pressurization/depressurization mechanisms and control units (not illustrated) can be used in conjunction with each other to control the gas pressure. Examples of pressurization/depressurization mechanisms that can be used include pressurizing pumps and depressurizing (vacuum) pumps. The control unit preferably controls temperature and flow rate, for example, in addition to the mixed gas pressure. Known computing devices such as CPUs or MPUs can be used as the control unit.

In the gas storage system of this embodiment, the storage pressure and release pressure of the gas storage material can be adjusted simply by changing the type and proportion of the ligands that are used, enabling more efficient gas storage. It is thus possible to construct a gas storage system that is tailor-made for the intended gas.

In the gas storage material and gas storage system illustrated thus far, the gas is stored in a solid adsorbent (storage material). The present invention thus allows containers to be handled more safely, in any orientation, than when gas is stored in liquid form or is dissolved in solvent. The absence of a solvent is also useful for the purposes of higher gas purity.

Examples

The present invention is illustrated in greater detail by, but is not limited to, the following examples, as other examples are possible within the scope of the present invention.

All chemical substances and solvents were purchased as commercial grade products and were used without being further refined. The following are abbreviations for components used in the examples.

Planar lattice-forming ligands: Main ligand

1.4-benzenedicarboxylic acid/terephthalic acid: bdc (compound represented by formula (3) above)

Planar lattice-forming ligands: Heterogeneous ligands

2-aminoteterephthalic acid: NH₂-bdc (compound represented by formula (4a) above) 2-nitroterephthalic acid: NO ₂ -bdc (compound represented by formula (4b) above) Tetrafluoroterephthalic acid: F₄-bdc (compound represented by formula (4c) above)

2.5-pyridinedicarboxylic acid: py-bdc (compound represented by formula (4e) above)

Pillar ligand 4,4-Pyridine: bpy

Synthesis of Gas Storage Material

Synthesis Example 1: Synthesis of single-ligand complex

Zinc (II) nitrate (2 equivalents; 0.2 mol) was dissolved in the minimum amount of dimethylformamide (DMF), and the resulting mixture was added to a separate solution of bdc (2 equivalents; 0.2 mol) dissolved as the main ligand dissolved in DMF. The mixture was heated in a thermostatic oil bath set to 100°C. An ethanol solution of bpy (1 equivalent; 0.1 mol) as a pillar ligand was then added dropwise to the mixture. The total amount of solvent was 1 L, and the solvent composition was ethanol (40% by volume) and DMF (60% by volume). After the dropwise addition of the solution, the mixture was stirred for approximately 5 minutes to 1 hour at 100°C. 24 to 48 hours after the start of the reaction, the reaction mixture was cooled to room temperature, and the precipitate was centrifuged off and washed three times with DMF and three times with ethanol to remove unreacted species. The resulting powder was dried for several hours at reduced pressure, giving a single-ligand complex (sometimes indicated as "Zn-CAT- Al").

The yield was approximately 98%.

Synthesis Examples 2 to 14: Synthesis of heterogeneous ligand complexes Heterogeneous ligand complexes were synthesized under the same conditions as the single-ligand complex. The proportions of desired heterogeneous ligands were controlled during synthesis. In typical synthesis, such as when a modified equivalent of bdc (indicated as R-bdc) is introduced as a heterogeneous ligand in a proportion of 10 mol %, zinc (II) nitrate (2 equivalents; 0.2 mol) is dissolved in the minimum amount of DMF, and the resulting mixture is added to a solution of bdc and R-bdc (R-bdc/[R- bdc+bdc]=10 mol%, total of 2 equivalents) in DMF to prepare a mixture. A mixture was prepared accordingly, where the heterogeneous ligands were added in the proportions shown in Table 1. The mixture was then heated in a thermostatic oil bath set to 100°C. An ethanol solution of bpy (1 equivalent) as the pillar ligand was then added dropwise to the mixture. The total amount of solvent was 1 L, and the solvent composition was ethanol (40% by volume) and DMF (60% by volume). After the dropwise addition of the solution, the mixture was stirred for approximately 5 minutes to 1 hour at 100°C (controlled in thermostatic bath). 24 to 48 hours after the start of the reaction, the reaction mixture was cooled to room temperature, and the precipitate was centrifuged off and washed three times with DMF and three times with ethanol to remove unreacted species. The resulting powder was dried for several hours at reduced pressure, giving heterogeneous ligand complexes. The yield was approximately 60 to 98%. Assessment and Results

All materials were characterized by powder X-ray diffraction (pXRD), thermogravimetric analysis (TGA), CO2 gas adsorption at 195 K, and C2H2 adsorption at 195 K and 273 K.

Powder X-Ray Diffraction (pXRD) pXRD was carried out with a Rigaku SmartLab X-Ray diffractometer (45 kV, 20 mA) using CuKa radiation. pXRD data was recorded at a scanning speed of 5 min in 0.01°steps from 3°to 60°(2Q).

Figure 5 A shows X-ray diffraction images of powdered Zn-CAT-Al incorporating 5 mol%, 10 mol%, 15 mol%, and 20 mol% of 2-aminoterephthalic acid (NFE-bdc). Figure 5 B shows X-ray diffraction images of powdered Zn-CAT-Al incorporating 5 mol%, 10 mol%, 15 mol%, and 20 mol% of 2-nitroterephthalic acid (NCk-bdc). Figure 5 C shows X-ray diffraction images of powdered Zn-CAT-Al incorporating 5 mol%, 10 mol%, 15 mol%, and 20 mol% of tetrafluoroterephthalic acid (F4-bdc). It was thus demonstrated that the heterogeneous ligand complex corresponded to the phase of the single-ligand complex (mixture of dry and wet phases). A few other peaks were observed, but were attributed to the crystallographic positions of substituents.

Adsorption Characteristics

Isothermal gas adsorption was carried out using a volume adsorption apparatus BELsorp-MAX (BEL Japan, Inc.) equipped with a temperature-controlling cryostat, that was operated under the control of BEL-cryo software.

Analysis was typically performed using 100 mg samples. Prior to adsorption analysis, all samples were degassed in vacuo for at least 6 hours at 423 K to remove guest molecules (solvent), and were reactivated between the different adsorption analyses. Figures 6A through 6C, 7A through 7C, and 8A through 8C show isothermal adsorption curves for the adsorption (represented by solid symbols and *) and desorption (represented by hollow symbols and x) of acetylene, carbon dioxide, and oxygen at each temperature in the single-ligand complex of Synthesis Example 1 and in the heterogeneous-ligand complexes in Synthesis Examples 2 through 13. Table 1 shows the storage pressure and release pressure of acetylene and carbon dioxide when adsorbed and desorbed by changing the content of the heterogeneous ligands in the heterogeneous ligand complexes.

Table 1

Gate-opening pressure Gate-closing pressure

Ligand (storage pressure) (release pressure) content (Ego, kPa) (P_gc, kPa)

(%mol) C0₂, 194.7 K C₂H₂, 273 K C0₂, 194.7 K C₂H₂, 273 K

Synthesis 0 (pure Zn-

Example 1 CAT-A1) 2.5 45.4 1.5 16.7

N0₂-bdc

Synthesis

Example 2 5 %mol 1.3 31.7 1.0 15.8

Synthesis

Example 3 10 %mol 1.0 23.3 0.8 13.2

Synthesis

Example 4 15 %mol 0.6 17.3 0.6 11.2

Synthesis

Example 5 20 %mol 0.5 12.1 0.4 8.8

NH₂-bdc

Synthesis

Example 6 5 %mol 3.0 48.1 1.5 17.1

Synthesis

Example 7 10 %mol 3.4 51.7 1.6 18.2

Synthesis

Example 8 15 %mol 4.4 55.7 1.8 19.2

Synthesis

Example 9 20 %mol 5.6 60.9 2.0 21.2

F4-bdc

Synthesis

Example 10 5 %mol 5.1 45.3 1.3 17.0 Synthesis

Example 11 10 %mol 2.5 45.9 1.3 16.2 Synthesis

Example 12 15 %mol 3.0 47.7 1.3 16.9 Synthesis

Example 13 20 %mol 3.8 50.0 1.3 17.3 py-bdc

Synthesis

Example 14 10 %mol 40.9 13.8

The shift in the gate opening pressure of acetylene at 273 K and of carbon dioxide at 194.7 K was correlated to the uptake of the heterogeneous ligands, and to the properties of the heterogeneous ligands that had been selected, in the structure of the heterogeneous ligand complexes. Similarly, the gate closing behaviour was also affected in the same manner as the gate opening behaviour and was similarly correlated to the uptake of heterogeneous ligands and to the properties of the heterogeneous ligands that had been selected. When organometallic complexes have multiple gate opening behaviours, the heterogeneous ligand complexes can affect one or more of the gate adsorption behaviours by the same or different methods. Adsorbents can thus be adjusted by preparing heterogeneous ligand complexes so as to adapt to the actual conditions of use that have been set as targets.

This method allows flexible organometallic complexes to be adapted to target values by adjusting the gas release pressure. In particular, this technique allows material to be modified so as to prevent pressure from increasing in a manner that is dangerous or to bring about various advantages with regard to the use thereof. This is also a technique that is suitable for use in applications at temperatures ranging from ultra-low to ambient temperatures (up to 150°).

Significant but limited change in gas adsorption can also be observed depending on the size of the functional groups incorporated into the heterogeneous ligand complexes as well as the affinity of the functional groups for the gas molecules that are adsorbed. This change can correspond to loss or increase in volume depending on the main ligands and heterogeneous ligands, and can affect the density of the gas storage material or the available pore volume.

Claims

1. Gas storage material which has two cubic lattice-shaped organometallic complexes composed of metal atoms and at least three kinds of ligands, wherein the two organometallic complexes form an inter-penetrating structure so that one apex portion of a unit cell of one of the organometallic complexes is positioned in a space inside one unit cell of the other organometallic complex.

2. Gas storage material according to Claim 1, wherein in each of the organometallic complexes, assuming that the apex portions of the unit cells are at the centre of an orthogonal coordinate system having an x-axis, a y-axis and a z-axis, at least one of the metal atoms is present at the centre, at least two kinds of planar lattice-forming ligands are coordinated, two in the x-axis direction and two in the y-axis direction, relative to the at least one metal atom, forming a planar lattice structure, and two pillar ligands different from the planar lattice-forming ligands are coordinated in the z-axis direction to the one metal atom, forming a cubic lattice structure in which the planar lattice structure is layered in the z-axis direction.

3. Gas storage material according to Claim 2, wherein two metal atoms are present in the centre, a dicarboxylic acid ion ligand A and a dicarboxylic acid ion ligand B that is different from the dicarboxylic acid ion ligand A are coordinated, as the planar lattice forming ligands, to the two metal atoms so as to form a paddle-wheel unit, and pyridine derivative ligands are used as the pillar ligands.

4. Gas storage material according to Claim 3, wherein the dicarboxylic acid ion ligand A and dicarboxylic acid ion ligand B are each independently represented by any of the following formulae (la) through (lc), and have mutually different structures.

Chemical Formula 1

(1a) il b) (1c)

When there are more than one R¹, R¹ may be the same as or different from each other.)

5. Gas storage material according to Claim 3 or 4, wherein the pyridine derivative ligands are represented by any of the following formulae (2a) through (2e).

Chemical Formula 2

When there are more than one R², R² may be the same as or different from each other.)

6. Gas material according to any of Claims 2 through 5, wherein heterogeneous ligands other than the main ligands, which have the lowest molecular weight among the planar lattice-forming ligands, are present in a total amount of 1 mol% to 70 mol%.

7. Gas storage material according to any of Claims 1 through 6, which is used to store gas for which the explosion limit at 25°C in a non-oxidizing atmosphere is 0.2 MPa.

8. Gas storage material according to Claim 7, wherein the gas is acetylene.

9. Gas storage system for storing one or more kinds of gas, comprising the gas storage material according to any of Claims 1 through 8, a pressurization/depressurization mechanism for increasing or reducing the gas pressure, and a control unit for controlling the pressure of the pressurization/depressurization mechanism, wherein the pressure for storing the gas in the gas storage material and the pressure for releasing the gas from the gas storage material are controlled by changing the kind of ligands that form the organometallic complexes of the gas storage material.