Organic metal structure, method for producing same, gas adsorption material using same, and method for producing gas adsorption material

Abstract

The present invention provides a method for producing an organic metal structure, the method using a crystallization device that includes a reactant introduction zone, a crystallization zone, a circulation zone, and a recovery zone. In the reactant introduction zone, an aqueous solution that contains metal ions and an aqueous solution that contains an organic ligand which is capable of coordinating with the metal ions are simultaneously and continuously added from different nozzles that are arranged on a flow passage, and are immediately mixed with each other with use of a mixing mechanism. In the crystallization zone, the metal ions and the organic ligand are reacted to grow a crystal of a metal organic structure while generating a crystal nucleus of the metal organic structure. In the circulation zone, a slurry which contains crystals of the metal organic structure is circulated to a reactant introduction step. In the recovery zone, some of the slurry is extracted continuously or at predetermined time intervals.

Description

Metal-organic framework, method for producing same, gas adsorption material using same, and method for producing gas adsorption material

[0001] The present invention relates to a metal-organic framework and a method for producing the same, and also to a gas adsorption material using the metal-organic framework and a method for producing the same.

[0002] Metal-organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are porous materials with a highly ordered lattice structure formed by metal ions and organic ligands. Taking advantage of the highly ordered lattice structure of MOFs, MOFs are expected to be used in applications such as gas storage materials for hydrogen and other gases, carbon dioxide concentration and separation materials, heterogeneous catalysts, conductive materials, and magnetic materials.

[0003] MOFs can be synthesized, for example, by a solution method in which metal ions and organic ligands self-assemble in a liquid (see, for example, Non-Patent Document 1). In this case, the solution is placed in a sealed container and maintained at a temperature higher than the evaporation temperature of the solvent, causing the solution to evaporate and increase its own pressure. MOFs can be synthesized under high pressure using the fact that the molecular volume of the MOF is smaller than the molecular volume of the solute as a driving force (called hydrothermal synthesis in the case of aqueous solvents and solvothermal synthesis in the case of organic solvents). MOFs can also be synthesized by continuously supplying and stirring a solution of metal ions dissolved in an organic solvent and water and a solution of organic ligands dissolved in an organic solvent into a stirring tank, or by mixing them in a tubular microreactor.

[0004] In batch reactions, whether using hydrothermal or solvothermal synthesis, the concentrations of metal ions and organic ligand ions in the solution are high at the beginning of MOF synthesis, and the solute concentration inevitably decreases as the reaction progresses. Since maximum supersaturation is achieved initially when metal ions and organic ligand ions are mixed, the crystal growth rate should also be maximized. However, in reality, the driving force, supersaturation, is consumed by the nucleation rate, resulting in the precipitation of numerous microcrystalline particles. Furthermore, as crystallization progresses, the solute concentration decreases and supersaturation disappears, ultimately resulting in almost no crystal growth. As a result, the MOF production rate per unit volume and per unit time is extremely low. This is the same in continuous reactions such as tubular microreactors, where the solute concentration in the reaction field changes from high to low despite backmixing of the solution.

[0005] In contrast, continuous stirring and reaction in a stirred tank is an excellent way to maintain constant reaction conditions (e.g., solute concentration, temperature, or pH) once a steady state is reached. However, no method has been shown to date for controlling the nucleation rate and linear growth rate of MOF crystals within a predetermined range. Crystallizers that continuously supply reactants to a stirred tank and continuously remove the product are common. In particular, so-called MSMPR (Mixed Suspension Mixed Product Removal) crystallizers, in which the entire stirred tank maintains approximately the same concentration, cannot maintain a high degree of supersaturation of the reaction solution because the MOF reaction occurs simultaneously with the dropwise addition, causing the reaction solution in the apparatus to rapidly convert into MOF fine particles. As a result, the resulting MOF crystals are composed of many fine particles and have a low bulk density.

[0006] In addition, when crystallizing ionic reactants, pH is generally an important parameter, and there are reports that MOFs are also crystallized in buffer solutions to maintain a constant pH. However, this has the disadvantage of increasing the number of ionic species in the reaction field, making the reaction field a more complex system.

[0007] An organic ligand is an organic substance containing two or more carbon atoms, typically four or more, and having at least two carboxylic acid groups. It is stable at room temperature and pressure at a relative humidity of 50%. Examples include fumaric acid, terephthalic acid, trimesic acid, triazole, and oxalic acid. To synthesize an MOF, the organic substance and metal salt must be ionic, and the metal must be coordinated to the carboxylic acid group of the organic substance. To simultaneously dissolve these two ions, organic solvents such as DMF (dimethylformamide), DEF (diethylformamide), and NMP (N-methyl-2-pyrrolidone) are commonly used. However, these organic solvents are more expensive than water. Furthermore, unless the organic solvent is completely separated by washing from the produced MOF, it cannot be used in fields such as pharmaceuticals and food.

[0008] As an alternative method, Patent Document 1 proposes a method for producing MOFs in a short time by applying centrifugal force and shear force to a mixture of raw materials, namely, a metal ion donor, a multidentate ligand, and a solvent.

[0009] US Patent Application Publication No. 2022 / 0220129

[0010] "Fundamentals of Materials Science", No. 7, "Fundamentals of Porous Coordination Polymers (PCP) / Metal-Organic Frameworks (MOF)", Sigma-Aldrich Japan, September 2012, Internet<URL:https: / / www.sigmaaldrich.com / JP / ja / campaigns / materials-science-basic>

[0011] In hydrothermal synthesis and solvothermal processes, as the temperature increases, water or solvent evaporates, increasing its own pressure. In the absence of crystals, the solute concentration is high but the pressure is low, so supersaturation gradually increases before nucleation occurs. When a certain level of supersaturation is reached, nucleation occurs suddenly, resulting in the production of many small crystals. Subsequently, as the pressure increases, the supersaturation level increases. By this time, the solute concentration decreases, and there is no more solute for crystal growth. As a result, many small crystals are obtained. MOFs composed solely of such small crystals have a low bulk density, and when packed into an apparatus, the amount of MOF per unit volume of the apparatus decreases, resulting in a decrease in adsorption capacity. To solve this problem, a method has been proposed to increase particle size by maintaining supersaturation at high temperature and high self-pressure for a long period of time, whereby fine particles dissolve and coarse particles grow, a phenomenon known as Ostwald ripening. However, this method requires a long time, resulting in an industrial issue of extremely low production rate per unit time.

[0012] In the case of a tubular microreactor, the crystals and the reaction solution move together inside the device, but the phenomenon is the same as in the batch hydrothermal synthesis method and the solvothermal method, and the above-mentioned phenomenon results in MOFs with small particle sizes, which reduces the amount of adsorption per unit volume.

[0013] When MOFs are used as adsorbents as described above, a high degree of crystallinity and high particle packing are important for improving performance in the application.

[0014] Therefore, an object of the present invention is to provide an MOF and a method for producing the same, as well as a gas adsorption material using an MOF and a method for producing the same, which can overcome the drawbacks of the prior art described above.

[0015] While MOFs can be produced using the various manufacturing methods described above, the MOFs produced by these methods have small particle sizes and low bulk densities, resulting in poor packing properties. Furthermore, the pore structure of a porous substance is one-dimensional or multidimensional, and the longer the pore structure, the faster the adsorption and desorption rate tends to be due to the capillary effect. In other words, the higher the crystallinity, the faster the adsorption and desorption rate. Therefore, the requirements for the crystalline structure of porous substances such as MOFs are a wide particle size distribution, allowing smaller crystal particles to fill the spaces between the crystal particles, thereby increasing the bulk density, and a high degree of crystallinity, which increases the adsorption and desorption rate. It is also clear that the fewer impurities a MOF contains, the more stable its adsorption and desorption.

[0016] To satisfy these requirements, a method for growing an MOF with a wide particle size distribution, i.e., containing large crystal particles and a mixture of large and small crystal particles, and simultaneously having a high degree of crystallinity, may be employed. The inventors have made this possible through extensive investigation of the shape of the apparatus for producing such MOFs and repeated experiments. Furthermore, the inventors have discovered a method for washing to remove impurities that can be carried out in large quantities in a short time using a small amount of solvent. Furthermore, by granulating the resulting metal-organic framework, they have discovered an industrially useful gas adsorption material and a suitable method for producing the gas adsorption material.

[0017] The crystallization phenomenon of MOFs due to metathesis reactions in aqueous systems is characterized by the fact that there is almost no heat of crystallization. The metathesis reaction here refers to the reaction of MX where M is the metal of the MOF, O is the organic ligand, and X and Y are the counter ions. ｌ +O ｍ Y ｎ →MO ｍ +X ｌ Y ｎ As a compound of MOF (MO ｍ ) and a counter ion compound (X ｌ Y ｎ) is a reaction that produces a compound of formula (III). A surprising feature of this commonly used metathesis reaction is that it generates almost no heat of crystallization when used with MOFs. Generally, in crystallization, when the reactant solution changes phase to the product crystals and solidifies, heat is generated in the amount of heat of dissolution. One possibility is that if the phase change of the counter ion in the metathesis reaction were sodium sulfate, this would be because the dissolution and precipitation would be an endothermic reaction at temperatures above 33°C. However, the concentration of the metathesis reaction in MOFs is in a range that is significantly more dilute than the solubility of sodium sulfate, so no phase change would occur, and this possibility is ruled out.

[0018] The driving force behind crystallization is the degree of supersaturation. Since the degree of supersaturation depends primarily on temperature and concentration, heat generated by crystallization reduces the degree of supersaturation at the crystal growth interface. However, since MOF crystallization generates almost no heat of crystallization, the degree of supersaturation can be accurately controlled as long as the concentration can be properly controlled. Next, in this invention, the crystal growth rate of MOFs was measured, and it was found to be proportional to the degree of supersaturation. However, considering the large particles obtained, the crystal growth rate was surprisingly low. Furthermore, it was found that increasing the degree of supersaturation increases the crystal growth rate. This phenomenon is often observed when the solubility of the product is low; nucleation occurs before the growth occurs, and supersaturation is not consumed by the growth. However, in the case of porous materials, the apparent linear growth rate increases due to the inclusion of water in the pores, and this is often not always the case. However, the present inventors have discovered for the first time that, as a unique phenomenon in MOFs, which are porous materials, the balance between the nucleation rate and the crystal growth rate is unexpectedly dominated by the nucleation rate, and that a high crystal growth rate cannot be obtained unless the degree of supersaturation is quite high.

[0019] Therefore, in the present invention, we have devised a new apparatus format for obtaining coarse MOF crystals by taking advantage of the newly discovered relationship between the linear growth rate and nucleation rate of MOFs. Specifically, a reactant introduction zone and a crystallization zone are provided in part of a crystallizer. In the reactant introduction zone, a solution containing metal ions and a solution containing organic ligands are rapidly introduced and mixed to achieve a high degree of supersaturation. Crystal particles contained in a suspended slurry are passed through and mixed with this highly supersaturated solution, causing crystals to grow epitaxially on the surfaces of the crystal particles, resulting in the production of crystals with large crystallite sizes and large particle sizes.

[0020] On the other hand, there is a certain correlation between the rotation speed of the agitator for mixing in the reactant introduction zone and the nucleation rate. That is, increasing the rotation speed of the agitator to improve mixing increases the nucleation rate and tends to increase the number of small MOF crystal particles. We devised a method to produce coarse MOF crystals by growing crystals in the crystallization zone in accordance with the growth rate of the MOF, or by attaching the generated fine particles to the crystal surface, thereby incorporating excess nuclei onto the crystal surface and reducing the number of nuclei.

[0021] It is possible to use a so-called multi-stage impeller, which is a coaxial impeller for transporting the reaction liquid, separate from the stirring blades in the reactant introduction zone. This makes it possible to separately control the stirring state and transport speed in the reactant introduction zone. In the present invention, when a multi-stage impeller was tested, no significant effect was observed.

[0022] By forming coarse crystals from MOFs, the specific surface area of ​​the crystals was reduced, making it possible to reduce the amount of mother liquor adhering to the crystal surface, thereby making it easier to wash the crystals.

[0023] Furthermore, one method for separating MOF crystals from the mother liquor is to use a large centrifugal force. Although the exact mechanism behind this is unclear, the inventors have newly discovered that this method improves the washability of the crystals and reduces the amount of washing liquid required.

[0024] The present invention provides an organometallic structure comprising a crystal containing an organic ligand and a metal ion, wherein the organic ligand is a dianion of fumaric acid or a dianion of terephthalic acid, and the metal ion is an aluminum ion, a zinc ion, or an iron (III) ion, and the tap density is 0.3 g / mL or more and 0.9 g / mL or less.

[0025] The present invention also provides a gas adsorption material comprising a granulated mixture of the above-described metal-organic framework as a gas adsorption component and a phenolic resin as a shape-maintaining component.

[0026] The present invention also provides a method for producing a metal-organic framework using a crystallization apparatus equipped with a reactant introduction zone, a crystallization zone, a circulation zone, and a recovery zone, wherein in the reactant introduction zone, an aqueous solution containing metal ions and an aqueous solution containing an organic ligand capable of coordinating to the metal ions are simultaneously and continuously added from different nozzles installed in a flow path and immediately mixed using a mixing mechanism; in the crystallization zone, the metal ions and the organic ligands are reacted to grow crystals of the metal-organic framework while simultaneously generating crystal nuclei of the metal-organic framework; in the circulation zone, a slurry containing crystals of the metal-organic framework is circulated to a reactant introduction step; and in the recovery zone, a portion of the slurry is withdrawn continuously or at predetermined time intervals.

[0027] The present invention further provides a continuous washing method for the metal-organic framework produced by the above method, the continuous washing method comprising a washing step of washing the metal-organic framework with a washing liquid to wash and remove counter anions for dissolving the metal ions constituting the metal-organic framework and counter cations for dissolving the organic ligands, the washing step comprising a first washing step, a solid-liquid separation step, and a second washing step, the liquid subjected to the first washing step being separated into a solid fraction and a liquid fraction in the solid-liquid separation step, the solid fraction being further washed in the second washing step, and the liquid fraction being used as a part of the washing liquid in the first washing step.

[0028] The present invention further provides a method for producing a gas adsorption material, comprising the steps of: mixing the metal-organic framework produced by the method described above with a thermosetting phenolic resin to obtain a mixture; adding water to the mixture and tumbling to granulate to obtain granules; separating the granules that have grown to a predetermined particle size according to particle size; and subjecting the granules that do not reach the predetermined particle size to the granulation step again, while heating the granules that have reached the predetermined particle size at a first temperature to remove the water, and further heating them to a second temperature to cure the thermosetting phenolic resin.

[0029] Fig. 1 is a schematic diagram showing an apparatus preferably used in carrying out the production method of the present invention. Fig. 2 is a schematic diagram showing an example of a reaction apparatus preferably used in carrying out the production method of the present invention. Figs. 3(a) and 3(b) are optical microscope photographs of crystals of the MOF of the present invention. Fig. 4 is a graph showing the results of a nitrogen adsorption / desorption test of the spherical granules of Example 4. Fig. 5 is a graph showing the results of a carbon dioxide adsorption test of the metal-organic framework of Example 1 and the spherical granules of Example 4. Fig. 6 is a schematic diagram showing a continuous washing apparatus preferably used in carrying out the production method of the present invention.

[0030] The present invention will be described below based on a preferred embodiment with reference to FIG. 1 . The present invention relates to a method for producing a metal-organic framework (MOF) composed of crystals containing organic ligands and metal ions. The present invention is a continuous method for producing an MOF. For this purpose, the production method preferably uses a crystallizer equipped with a reactant introduction zone A, a crystallization zone B, a circulation zone C, and a recovery zone D. The production method using this crystallizer can be broadly divided into the following steps (1) to (4). (1) Reactant Introduction Step: In this step, in the reactant introduction zone A, an aqueous solution M containing metal ions and an aqueous solution L containing an organic ligand capable of coordinating to the metal ions are simultaneously and continuously added from different nozzles installed in the flow channel, and then immediately mixed using a mixing mechanism E. (2) Crystallization Step: In this step, in the crystallization zone B, the metal ions and the organic ligands are reacted to grow crystals of the metal-organic framework, and simultaneously generate crystal nuclei of the metal-organic framework. (3) Crystal Circulation Step: In this step, in the circulation zone C, a slurry containing crystals of the metal-organic framework is circulated to the reactant introduction step. That is, the metal-organic framework obtained by the reaction is smoothly transferred to the reactant introduction step, and the crystals and solution are circulated. (4) Crystal Recovery Step: In this step, in the recovery zone D, a portion of the slurry is withdrawn continuously or at predetermined time intervals. Note that in this production method, additional steps may be performed, as necessary, before step (1), between steps (1) and (2), between steps (2) and (3), between steps (3) and (4), and / or after step (4). By combining the steps shown here, it is possible to produce MOFs with large particle sizes despite the phenomenon newly discovered by the inventors, namely, a slow growth rate of MOFs and a high nucleation rate, thereby broadening the particle size distribution of MOFs and thereby increasing the bulk density of MOFs. At the same time, the present invention makes it possible to easily achieve a high degree of supersaturation in the crystallization process, which allows for extremely high MOF production rates per unit volume and per unit time, making it possible to inexpensively produce MOFs. Each step will be explained below.

[0031] [Reactant Introduction Step] MOFs are substances formed from metal ions and organic ligands. Therefore, in this step, metal ions and organic ligands, which are raw materials for the MOF, are first prepared. The metal ions are prepared in the form of an aqueous solution. The organic ligands are also prepared in the form of an aqueous solution. One of the features of this production method is that the MOF is produced in water rather than in an organic solvent. Producing MOFs in water has the advantages of not requiring complex production equipment, allowing for a high degree of freedom in production conditions, and reducing the environmental impact.

[0032] As the metal ion, an appropriate one can be used depending on the specific application of the target MOF. When the application of the MOF is, for example, gas storage or gas separation, metal ions such as aluminum ions, iron ions, copper ions, zinc ions, zirconium ions, chromium ions, and cobalt ions can be used. Of the various metal ions described above, aluminum ions, iron ions, chromium ions, and cobalt ions are generally hexacoordinate. Copper ions and zinc ions are generally tetracoordinate. Depending on the type of solvent and pH, zinc ions may form clusters of four zinc nuclei and oxygen ions to form hexavalent hexacoordinate ions, but this zinc tetranuclear cluster ion is a type of zinc ion. Zirconium ions are tetracoordinate and octacoordinate, and, like zinc ions, four tetracoordinate nuclei may form clusters to form hexacoordinate ions. Of the various metal ions described above, aluminum ions, iron ions, zinc tetranuclear clusters, and copper ions are preferably used because they are more likely to produce MOFs with high gas storage and gas separation performance.

[0033] Aqueous solutions of metal ions can be obtained, for example, by dissolving water-soluble metal compounds in water. Examples of water-soluble metal compounds include water-soluble salts, halides, and hydroxides. Specifically, when the metal ion is, for example, aluminum ion, aluminum sulfate or aluminum chloride can be used as the water-soluble metal compound. When the metal ion is, for example, iron ion, iron(II) chloride, iron(III) chloride, and iron(II) sulfate can be used as the water-soluble metal compound. When the metal ion is, for example, copper ion, copper(II) sulfate and copper(II) chloride can be used as the water-soluble metal compound. When the metal ion is, for example, zinc ion, zinc sulfate can be used as the water-soluble metal compound. When the metal ion is, for example, cobalt ion, cobalt(II) sulfate and cobalt(II) chloride can be used as the water-soluble metal compound.

[0034] The concentration of metal ions in the aqueous metal ion solution is preferably 0.1 mol / L or more in terms of increasing the production rate per unit volume per unit time. From the viewpoint of making this advantage even more pronounced, the concentration of metal ions in the aqueous metal ion solution is more preferably 0.15 mol / L or more, and even more preferably 0.3 mol / L or more. Furthermore, the concentration of metal ions in the aqueous metal ion solution is preferably 1.8 mol / L or less in terms of not being significantly different from the concentration of the organic ligand. From the viewpoint of making this advantage even more pronounced, the concentration of metal ions in the aqueous metal ion solution is more preferably 1.0 mol / L or less, and even more preferably 0.9 mol / L or less.

[0035] The pH of the aqueous solution of the metal ion is not particularly limited as long as no precipitate containing the metal is formed in the solution. When the pH of the aqueous solution of the organic ligand described below is basic, it is preferable to set the pH of the aqueous solution of the metal ion to 4 or less from the viewpoint of neutralizing the pH of the solution obtained by mixing the two aqueous solutions.

[0036] The organic ligand used to produce an MOF together with a metal ion is not particularly limited in type, as long as it can coordinate to the metal ion. Suitable examples of organic ligands include nitrogen donor ligands and oxygen donor ligands. Examples of nitrogen donor ligands include imidazole, 2,2'-bipyridyl, 3,3'-bipyridyl, 4,4'-bipyridyl, 2,2'-bipyrazine, 2,2'-bipyrimidyl, and 1,4-di(4-pyridyl)benzene. These nitrogen donor ligands can be used alone or in combination of two or more. Examples of oxygen donor ligands include anions of polybasic organic acids. The polybasic organic acid may be an aliphatic polybasic organic acid or an aromatic polybasic organic acid. Specific examples include dianions of dibasic organic acids such as oxalic acid, fumaric acid, terephthalic acid, phthalic acid, and isophthalic acid, and trianions of tribasic organic acids such as trimesic acid. The hydrogen atoms bonded to the aromatic rings in the aromatic polybasic organic acids may be substituted with various functional groups such as hydroxyl groups, amino groups, and nitro groups. These oxygen donor ligands may be used alone or in combination of two or more.

[0037] The type of organic ligand to be used may be appropriately selected depending on the specific application of the MOF. However, from the viewpoint of easily obtaining a MOF with a high bulk density, it is preferable to use an oxygen donor ligand, and it is particularly preferable to use a dianion of a dibasic organic acid.

[0038] The concentration of the organic ligand in the aqueous solution of the organic ligand is preferably 0.3 mol / L or more in order to maximize the production rate per unit volume per unit time. From the viewpoint of making this advantage even more pronounced, the concentration of the organic ligand in the aqueous solution of the organic ligand is more preferably 0.5 mol / L or more, and even more preferably 0.6 mol / L or more. Furthermore, the concentration of the organic ligand in the aqueous solution of the organic ligand is preferably 0.9 mol / L or less in order to be equal to or less than the saturated solubility of the organic ligand. From the viewpoint of making this advantage even more pronounced, the concentration of the organic ligand in the aqueous solution of the organic ligand is more preferably 0.85 mol / L or less, and even more preferably 0.8 mol / L or less.

[0039] An aqueous solution containing an organic ligand can be prepared, for example, by dissolving a polybasic organic acid in water or a salt of a polybasic organic acid in water. When the solubility of a polybasic organic acid is low, the polybasic organic acid can be made more soluble by adding the polybasic organic acid to water whose pH has been adjusted to 9 or higher with a basic substance such as sodium hydroxide (in this case, the aqueous solution is an aqueous solution of a salt of the polybasic acid).

[0040] Depending on the type of polybasic acid or polybasic acid salt, a water-soluble organic solvent may be contained in the aqueous solution containing the organic ligand. The water-soluble organic solvent is added to increase the solubility of the polybasic acid or polybasic acid salt. Examples of water-soluble organic solvents include monohydric alcohols such as ethanol, methanol, and isopropyl alcohol; glycols such as ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, butylene glycol, and hexylene glycol; mono- or diethers of the above glycols with lower alcohols such as methanol, ethanol, propanol, and butanol; esters of the above glycols with lower fatty acids; and polyhydric alcohols such as glycerin and sorbitol. These water-soluble organic solvents can be used alone or in combination of two or more.

[0041] It is preferable that the aqueous solution containing metal ions and the aqueous solution containing organic ligands are added simultaneously to the reaction vessel, as this allows for easy production of MOFs with high bulk density. Examples of simultaneous addition of both aqueous solutions to the reaction vessel include a reactor having a bottom and a wall connected thereto, in which an aqueous solution containing metal ions and an aqueous solution containing organic ligands are separately supplied, and a tubular reactor in which an aqueous solution containing metal ions is supplied from one end and an aqueous solution containing organic ligands is supplied from the other end, and the two aqueous solutions meet countercurrently in the reaction vessel. From the viewpoint of MOF productivity and ease of reaction control, it is preferable to employ a reactor having a bottom and a wall connected thereto, in which an aqueous solution containing metal ions and an aqueous solution containing organic ligands are separately supplied.

[0042] It is preferable that the aqueous solution containing metal ions and the aqueous solution containing organic ligands are added continuously to the reaction vessel, as this facilitates the production of a MOF with a high bulk density. In this specification, "continuous addition" includes not only the continuous addition of both aqueous solutions, but also the intermittent addition of both aqueous solutions to the extent that they can be considered to be added continuously. For example, the intermittent addition of both aqueous solutions with an interval of up to 600 seconds is included in the "continuous addition" category.

[0043] The aqueous solution containing metal ions and the aqueous solution containing organic ligands are preferably added so that the number of moles of metal ions in the aqueous solution containing metal ions and the number of moles of organic ligands in the aqueous solution containing organic ligands are approximately stoichiometric to the desired MOF.

[0044] The combination of metal ions and organic ligands for producing a MOF can be appropriately selected depending on the specific application of the MOF. Preferred combinations include, for example, combinations of hexacoordinate metal ions such as aluminum ions, iron ions, chromium ions, zinc tetranuclear clusters, and cobalt ions with bidentate ligands such as dibasic organic acids, and combinations of tetracoordinate metal ions such as copper ions and zinc ions with tridentate ligands such as tribasic organic acids.

[0045] A specific combination of a metal ion and an organic ligand is, for example, a combination in which the metal ion is an aluminum ion and the organic ligand is a fumaric acid dianion. When this combination is adopted, the fumaric acid dianion is preferably added in an amount of 0.9 mol to 1.1 mol, more preferably 0.95 mol to 1.05 mol, and even more preferably 0.98 mol to 1.03 mol per mol of aluminum ion.

[0046] When the metal ion is an aluminum ion and the organic ligand is a dianion of terephthalic acid, the dianion of terephthalic acid is preferably added in an amount of 0.9 mol to 1.1 mol, more preferably 0.95 mol to 1.05 mol, and even more preferably 0.98 mol to 1.03 mol relative to 1 mol of aluminum ion.

[0047] When the metal ion is an iron(III) ion and the organic ligand is a dianion of terephthalic acid, the dianion of terephthalic acid is preferably added in an amount of 0.9 mol to 1.1 mol, more preferably 0.95 mol to 1.05 mol, and even more preferably 0.98 mol to 1.03 mol relative to 1 mol of iron(III) ion.

[0048] When the metal ion is a zinc(II) ion and the organic ligand is a fumaric acid dianion, the amount of the fumaric acid dianion added is preferably 0.9 mol or more and 1.1 mol or less, more preferably 0.95 mol or more and 1.05 mol or less, and even more preferably 0.98 mol or more and 1.03 mol or less, per 4 mol of zinc(II) ion.

[0049] The introduction of the aqueous solution containing metal ions and the aqueous solution containing organic ligands into the reaction vessel is preferably carried out so that the pH of the reaction solution is maintained at a specific value. For example, when producing an MOF consisting of aluminum fumarate crystals, it is preferable to maintain the pH of the reaction solution at 3 to 6, more preferably 3 to 5, and even more preferably 3 to 4.5. When producing an MOF consisting of zinc fumarate crystals, it is preferable to maintain the pH of the reaction solution at 5 to 9, more preferably 6 to 9, and even more preferably 6.5 to 8.5. When producing an MOF consisting of aluminum terephthalate crystals, it is preferable to maintain the pH of the reaction solution at 3 to 7, more preferably 3.5 to 7, and even more preferably 3.5 to 6. When producing an MOF consisting of iron (III) terephthalate crystals, it is preferable to maintain the pH of the reaction solution at 3 to 7, more preferably 4.5 to 6.5, and even more preferably 5 to 6.

[0050] As described above, the present production method is a continuous method, which has the advantage that it is easy to maintain a constant pH of the reaction solution. In contrast, in a batch production method, the pH of the reaction solution is likely to change as the reaction proceeds, making it difficult to maintain the pH of the reaction solution at an optimum value from the start to the end of the reaction.

[0051] In order to adjust the pH of the reaction solution to fall within the above-mentioned preferred range, it is preferable to supply a basic compound such as sodium hydroxide to the reaction vessel simultaneously with supplying the metal ions and organic ligand to the reaction vessel, which can be achieved, for example, by adding a basic substance to the aqueous solution containing the organic ligand.

[0052] From the viewpoint of appropriately controlling the pH of the reaction solution and successfully obtaining the desired MOF, the molar ratio of the basic substance to the organic ligand supplied to the reaction vessel is preferably a specific value. For example, when the organic ligand is fumaric acid, the metal ion is aluminum ion, and the basic substance is sodium hydroxide, the molar ratio (basic substance / organic ligand) is preferably 2.5 to 4.5, more preferably 2.6 to 4.4, and even more preferably 2.7 to 3.8. Furthermore, when the organic ligand is fumaric acid, the metal ion is zinc ion, and the basic substance is sodium hydroxide, the molar ratio (basic substance / organic ligand) is preferably 7 to 10, more preferably 7.5 to 9.5, and even more preferably 7.5 to 8.5. Furthermore, when the organic ligand is terephthalic acid, the metal ion is aluminum ion, and the basic substance is sodium hydroxide, the molar ratio (basic substance / organic ligand) is preferably 2.5 or more and 4 or less, more preferably 2.6 or more and 4 or less, and even more preferably 2.8 or more and 3.5 or less. Furthermore, when the organic ligand is terephthalic acid, the metal ion is iron (III) ion, and the basic substance is sodium hydroxide, the molar ratio (basic substance / organic ligand) is preferably 2 or more and 4 or less, more preferably 2.4 or more and 3.8 or less, and even more preferably 2.6 or more and 3.3 or less. The number of moles of the basic substance used in calculating the molar ratio (basic substance / organic ligand) also includes the number of moles of the basic substance necessary for neutralizing the organic ligand.

[0053] FIG. 2 shows a schematic diagram of an example of a reaction apparatus suitable for use in the present production method. However, the present invention is not limited to this example, as long as it includes four steps equivalent to those of the present invention. The reaction apparatus 10 shown in the figure is a crystallization apparatus known as a draft tube baffle (hereinafter also referred to as "DTB") type. The reaction apparatus 10 includes a reaction vessel 13 having a bottom 11 and a wall 12 connected to the bottom 11. A cylindrical draft tube 14 is disposed within the reaction vessel 13. The draft tube 14 is disposed within the reaction vessel 13 so that its axial direction is parallel to the depth direction of the reaction vessel 13. The cross-sectional shape of the draft tube 14 may be, for example, circular, elliptical, or polygonal. The draft tube 14 is open at both the top and bottom ends. The lower end 14a of the draft tube 14 is located a predetermined distance from the bottom of the reaction vessel 13.

[0054] The reaction apparatus 10 is equipped with a first supply pipe 15 for supplying an aqueous solution containing metal ions into the reaction vessel 13. The reaction apparatus 10 also is equipped with a second supply pipe 16 for supplying an aqueous solution containing organic ligands into the reaction vessel 13. Both the first supply pipe 15 and the second supply pipe 16 are disposed inside the draft tube 14. Both the first supply pipe 15 and the second supply pipe 16 are disposed such that their lower ends 15a, 16a are located higher than the lower end 14a of the draft tube 14. The vicinity of the lower ends 15a, 16a of the first supply pipe 15 and the second supply pipe 16 corresponds to the reactant introduction zone described above.

[0055] The reactor 10 is equipped with a means for stirring the liquid. Examples of stirring means include stirring by rotating an impeller, stirring by circulating the liquid using a pump, and stirring by blowing air bubbles. FIG. 2 shows a stirring means using the rotation of an impeller 17. The impeller 17 is connected to the lower end of a drive shaft 18, the upper end of which is connected to a drive source 19 such as a motor installed outside the reaction vessel 13. When driven by the drive source 19, the drive shaft 18 and the impeller 17 connected thereto rotate around the drive shaft, thereby generating a downward or upward flow. The area around the impeller 17 corresponds to the crystallization zone described above. In this case, it is preferable that the sum of the volumes of the reactant introduction zone and the crystallization zone of the reactor 10 be 30% or less of the volume of the crystallizer, from the viewpoint of easily obtaining coarse crystals by increasing the circulation frequency. From the viewpoint of making this advantage more pronounced, the sum of the volumes of the reactant introduction zone and the crystallization zone is preferably 10% or more and 30% or less, more preferably 15% or more and 30% or less, and even more preferably 20% or more and 30% or less, of the volume of the crystallizer.

[0056] In this step, an aqueous solution of metal ions and an aqueous solution of organic ligands are mixed in a reaction vessel to react with each other to produce the desired MOF crystals. This reaction is described with reference to FIG. 2.

[0057] The reaction between the metal ions and the organic ligands in this step is preferably carried out so that the reaction product formed in the mixture of the two aqueous solutions remains supersaturated. This increases the bulk density of the reaction product, MOF. As used herein, the term "supersaturated" refers to a state in which the concentration of the reaction product, MOF, in the reaction solution is higher than its saturated solubility.

[0058] In this production method, a reactor 10 consisting of the DTB-type crystallizer shown in FIG. 2 is used to achieve a supersaturated MOF. In the reactor 10 shown in FIG. 2, an aqueous solution containing metal ions and an aqueous solution containing organic ligands are supplied to the draft tube 14 via a first supply pipe 15 and a second supply pipe 16 disposed within the draft tube 14, where the two aqueous solutions are mixed to produce a reaction solution. At this time, the rotation direction of the impeller 17 is controlled to generate a downward flow of the reaction solution within the draft tube 14. The above-mentioned supersaturated state is likely to occur within the draft tube 14, particularly between the lower ends 15a, 16a of the first supply pipe 15 and the second supply pipe 16 and the impeller 17. The supersaturated state may occur locally or throughout the entire reaction solution.

[0059] When the aqueous solution containing metal ions and the aqueous solution containing organic ligands are supplied into the reaction vessel 13, one or both of the aqueous solutions may be supplied in a heated state in order to promote the reaction. The heating temperature may be, for example, 40° C. or higher, preferably 60° C. or higher, and more preferably 70° C. or higher. The heating temperature may be, for example, 95° C. or lower, preferably 90° C. or lower, and more preferably 85° C. or lower.

[0060] Regardless of whether the aqueous solution containing metal ions and / or the aqueous solution containing organic ligands is heated, the reaction can be carried out in an open system, i.e., under atmospheric pressure. This is advantageous in terms of simplifying the reaction apparatus 10 and facilitating reaction control. However, this does not prevent the production method from being carried out in a closed system, for example, under autogenous pressure exceeding 1 atmosphere.

[0061] As the reaction solution flows downward through the draft tube 14, the metal ions react with the organic ligands to form MOF crystal nuclei, and crystals grow around the crystal nuclei. The generated MOF crystals flow together with the reaction solution from the lower end 14a of the draft tube 14 toward the bottom 11 of the reaction vessel 13, where they collide with the bottom 11, causing their flow direction to change upward. The reaction solution containing the MOF crystals flows upward along the outer surface of the draft tube 14 and reaches the upper end 14b of the draft tube 14. The region where this flow occurs corresponds to the circulation zone described above. The liquids circulating within the crystallizer are an aqueous solution containing metal ions, an aqueous solution containing organic ligand ions, and a slurry in which the generated crystals of the metal-organic framework are suspended. These liquids are mixed together and circulate within the crystallizer. As described above, a downward flow occurs within the draft tube 14, and the reaction liquid (which contains MOFs) that reaches the upper end 14b of the draft tube 14 is drawn into the draft tube 14 and flows downward within the draft tube 14. As described above, an aqueous solution containing metal ions and an aqueous solution containing organic ligands are supplied to the draft tube 14 via the first supply pipe 15 and the second supply pipe 16, and therefore the MOFs contained in the reaction liquid react with the newly supplied metal ions and organic ligands, causing growth of MOF crystals and aggregation of the crystals, resulting in an increase in the particle size of the MOFs.

[0062] According to this production method, the reaction solution circulates between the inside and outside of the draft tube 14, allowing the simultaneous and continuous generation of MOF crystal nuclei and crystal growth. As a result, MOFs with a wide particle size distribution, ranging from large to small particles, are produced. In MOFs with a wide particle size distribution, small particles tend to fill between large particles, resulting in a high bulk density. The degree of circulation of the reaction solution can be controlled by the rotation speed of the impeller 17. Generally, the higher the rotation speed of the impeller 17, the higher the circulation rate of the reaction solution. In order to circulate the reaction solution vertically within the reaction vessel 13, it is preferable to adjust the volume of the reaction solution so that the liquid level L of the reaction solution is located above the upper end 14b of the draft tube 14.

[0063] From the perspective of successfully obtaining MOF with a wide particle size distribution, in this step, it is preferable to adjust the generation rate of crystal nuclei of MOF generated by the reaction of metal ions and organic ligands. From this perspective, the nucleation rate of MOF crystals is 1×10 5 particles / (h·m ３ ) or more, preferably 5×10 ５ particles / (h·m ３ ) or more, more preferably 1×10 ６ particles / (h·m ３ ) or more. Also, the nucleation rate of MOF crystals is preferably 1×10 １６ particles / (h·m ３ ) or less, preferably 5×10 １５ particles / (h·m ３ ) or less, more preferably 1×10 １４ particles / (h·m ３ ) or less. The control of the nucleation rate of MOF crystals is achieved, for example, by adjusting the rotation speed of the impeller 17, the diameter of the draft tube 14, and the concentration and supply rate of the aqueous solution containing metal ions and the aqueous solution containing organic ligands.

[0064] The nucleation rate of MOF crystals is measured by the following method. In a continuous crystallization apparatus, when a steady state is achieved, the production rate P / ρcV´, the nucleation rate Fv´, the linear growth rate (dl / dθ) av , the particle size l ｐ ＊ and the particle size distribution m and the crystal suspension density (1 - ε i ) are known to satisfy the following relational expressions (Japan Chemical Industry Technical Report Creative, 1, No. 5 (2004) Internet <URL:https: / / www.nippon-chem.co.jp / dcms_media / other / cre2004-2.pdf). In the following formula, V´ is the apparatus volume, ρ ｃ is the density of the crystal, k ａ is the surface shape factor of the crystal, k ｖ is the volume shape factor of the crystal. m is the slope of the particle size distribution line dotted on the Rosin-Rammler diagram, l p ＊represents the particle size characteristic number when the product crystals are plotted on a Rosin-Rammler diagram.

[0065]

[0066] Here, there are six variables and two equations, so the degree of freedom is four. Therefore, if four of these variables can be measured, the remaining two variables can be calculated. The production rate P / ρcV' can be measured from the mass of crystals obtained from the crystallizer. The particle size l and particle size distribution m can be measured by observing the crystals under a microscope or by using a particle size distribution measuring device. The suspension density (1-ε i For the crystallization method, it is possible to measure the ratio of the volume of the slurry to the volume of the crystals by sampling the slurry in the crystallizer. As a result, the nucleation rate and linear growth rate can be calculated by these two equations in the same way as the actual measurements.

[0067] In this step, in addition to or instead of controlling the nucleation rate of MOF crystals as described above, controlling the linear growth rate of MOF crystals is also advantageous from the viewpoint of successfully obtaining MOFs with a wide particle size distribution. From this viewpoint, the linear growth rate of MOF crystals is preferably 1 μm / h or more, more preferably 10 μm / h or more, and even more preferably 80 μm / h or more. Furthermore, the linear growth rate of MOF crystals is preferably 200 μm / h or less, more preferably 150 μm / h or less, and even more preferably 100 μm / h or less. Control of the linear growth rate of MOF crystals can be achieved, for example, by adjusting the rotation speed of the impeller 17, the diameter of the draft tube 14, and the concentrations and supply rates of the aqueous solution containing metal ions and the aqueous solution containing organic ligands.

[0068] The linear growth rate of crystals of a MOF is measured in a similar manner to the measurement of the nucleation rate described above.

[0069] [Crystal Recovery Step] In this step, MOF crystals precipitated in the reaction solution are removed from the reaction vessel 13. The crystals are removed through the outlet pipe 20 provided in the reaction apparatus 10. One end of the outlet pipe 20 is located inside the reaction vessel 13, and the other end is located outside the reaction vessel 13. Although not shown, the outlet pipe 20 may be equipped with a pump and / or an on-off valve for removing the reaction solution. The region where the crystals are removed through the outlet pipe 20 corresponds to the recovery zone described above. The MOF crystals contained in the reaction solution are withdrawn from the reaction vessel 13 together with the reaction solution through the outlet pipe 20. The reaction solution containing the MOF crystals is preferably withdrawn continuously or at predetermined time intervals. When the reaction solution is withdrawn at predetermined time intervals, the withdrawal intervals may or may not be constant. In either case, the withdrawal interval is preferably 0.5 to 10 minutes, from the viewpoint of maintaining a constant volume of the crystallizer, although this depends on the volume of the reaction vessel 13 and the production amount of the MOF. To further enhance this advantage, the reaction liquid withdrawal interval is more preferably 1 to 5 minutes, and even more preferably 1 to 2 minutes.

[0070] The amount of reaction liquid withdrawn is desirably controlled so that the MOF production reaction in the reaction vessel 13 becomes steady. Specifically, it is desirably withdrawn so that the amount of MOFs produced in the reaction vessel 13 and the amount of MOFs withdrawn from the outlet pipe 20 are balanced.

[0071] To produce MOFs with large crystal grain sizes, it is advantageous to increase the number of times the crystals pass through the reaction solution introduction zone and the crystallization zone. The number of times X [times / min] passes through the zones is determined by the discharge flow rate Q [m ３ / min] is the sum of the volumes of the reaction solution introduction zone A and the crystallization zone B, V [m ３ X = Q / V (3) Each time the crystal passes through the zone, it grows at a linear growth rate G [μm] depending on the degree of supersaturation and the residence time in the zone. The value multiplied by the number of passes X is the particle size L. total[μm / min]. total =G×X (4 formula) Particle size L total Since is proportional to the number of passes, the longer the residence time in the crystallizer, the larger the particle size, but because nucleation occurs simultaneously with crystal growth, the crystal size distribution becomes broader, and the average crystal size in the entire apparatus does not necessarily correlate with the residence time in the entire apparatus. In any case, the longer the residence time in the entire apparatus, the more likely it is that coarse crystals will be obtained, so the longer the residence time in the entire apparatus, the wider the particle size distribution.

[0072] Thus, controlling the residence time of the reaction solution during withdrawal is also advantageous from the perspective of successfully obtaining MOFs with a broad particle size distribution. From this perspective, it is preferable to withdraw MOF crystals from the reaction vessel 13 so that the residence time in the crystallization apparatus, including the reactant introduction zone, crystallization zone, circulation zone, and recovery zone, is 3 minutes or more, and it is particularly preferable to withdraw MOF crystals from the reaction vessel 13 so that the residence time is 5 minutes or more, and particularly 10 minutes or more. From the same perspective, it is preferable to withdraw MOF crystals from the reaction vessel 13 so that the residence time is 30 minutes or less, and it is particularly preferable to withdraw MOF crystals from the reaction vessel 13 so that the residence time is 25 minutes or less, and particularly 20 minutes or less. In a MOF reaction, the residence time of the entire apparatus is determined by dividing the volume of the entire apparatus by the sum of the flow rates of the metal component and the organic ligand component in the metathesis reaction.

[0073] [Washing Step] The MOF crystals extracted from the reaction vessel 13 are subjected to a washing step as needed. In the washing step, washing is mainly carried out using water. For example, the reaction solution containing the MOF crystals is extracted from the reaction vessel 13, and the reaction solution is placed in a Buchner funnel or the like to filter the water, thereby obtaining a cake-like MOF crystal. The crystals can then be washed with water to remove impurities. Alternatively, the MOF crystals can be washed by elutriation. Alternatively, a washing method using a liquid cyclone can be employed.

[0074] In particular, subjecting the organometallic structure produced by the above-described method to the following continuous washing method is advantageous because it reduces the amount of washing solution. This continuous washing method includes a washing step in which the organometallic structure is washed with a washing solution to wash away counter anions for dissolving the metal ions constituting the organometallic structure and counter cations for dissolving the organic ligands. Examples of this washing solution include water and mixed solvents of water and water-soluble organic solvents. Examples of water-soluble organic solvents include water-soluble lower aliphatic alcohols. Examples of water-soluble lower aliphatic alcohols that are suitable for use include aliphatic monohydric alcohols having from 1 to 4 carbon atoms, and specific examples include methanol, ethanol, and isopropyl alcohol.

[0075] The washing step preferably includes a first washing step, a solid-liquid separation step, and a second washing step. In these steps, it is preferable that the liquid subjected to the first washing step is separated into a solid fraction and a liquid fraction in the solid-liquid separation step, and the solid fraction is further washed in the second washing step, and the liquid fraction is used as part of the washing liquid in the first washing step. This makes it possible to reduce the amount of washing liquid, as described above. In particular, by repeating the washing step and the solid-liquid separation step multiple times and using the liquid fraction obtained in the solid-liquid separation step as part of the washing liquid in the previous washing step, the amount of washing liquid can be further reduced. There are no particular limitations on the means for separating the solid fraction and the liquid fraction in the solid-liquid separation step. For example, solid-liquid separation using centrifugal force makes it possible to efficiently separate the solid fraction and the liquid fraction.

[0076] [MOF Obtained by the Present Production Method] The MOF obtained by the present production method is typically represented by the general formula ML ｘ A ｙIn the general formula, M represents a metal ion, L represents an organic ligand, and A represents an anion other than the organic ligand represented by L. Details of M and L are as described above. The combination of M and L may be appropriately selected depending on the specific application of the MOF. For example, a dianion of fumaric acid or a dianion of terephthalic acid may be selected as M, and an aluminum ion, a zinc ion, or an iron (III) ion may be selected as L. In the general formula, the anion represented by A may be a hydroxide ion, a sulfate ion, a chloride ion, or an oxide ion (O ２－ ) is one or more selected from the following.

[0077] In the general formula, x represents the number of moles of organic ligand L per mole of metal ion M. For example, when M is an aluminum ion or an iron (III) ion and L is a dianion of fumaric acid or a dianion of terephthalic acid, x is preferably 0.85 to 1.15, more preferably 0.88 to 1.12, and even more preferably 0.9 to 1.1. When M is a zinc ion and L is a dianion of terephthalic acid, x is preferably 0.6 to 0.9, more preferably 0.65 to 0.85, and even more preferably 0.7 to 0.8. When M is a zinc ion and L is a dianion of fumaric acid, x is preferably 0.20 to 0.30, more preferably 0.22 to 0.28, even more preferably 0.23 to 0.27, and particularly preferably 0.24 to 0.26.

[0078] In the general formula, y represents the number of moles of anion A per mole of metal ion M. For example, when M is an aluminum ion, L is a dianion of fumaric acid or a dianion of terephthalic acid, and A is a hydroxide ion, y is preferably 0.7 to 1.2, more preferably 0.8 to 1.2, and even more preferably 0.9 to 1.1. When M is an iron(III) ion, L is a dianion of fumaric acid or a dianion of terephthalic acid, and A is a hydroxide ion, y is preferably 0.8 to 1.2, more preferably 0.88 to 1.12, and even more preferably 0.9 to 1.1. When M is a zinc ion, L is a dianion of terephthalic acid, and A is an oxide ion, and x is 3 / 4, y is preferably 0.20 or more and 0.30 or less, more preferably 0.22 or more and 0.28 or less, even more preferably 0.23 or more and 0.27 or less, and particularly preferably 0.24 or more and 0.26 or less. When M is a zinc ion, L is a dianion of fumaric acid, and A is a hydroxide ion and an oxide ion, the MOF has the composition formula ZnL ｘ (OH) ｙ１ O ｙ２ (wherein L represents a dianion of fumaric acid.) In this case, when x is 1 / 4, y1 is preferably 0.8 or more and 1.2 or less, more preferably 0.88 or more and 1.15 or less, and even more preferably 0.9 or more and 1.1 or less. Furthermore, y2 is preferably 0.20 or more and 0.30 or less, more preferably 0.22 or more and 0.28 or less, even more preferably 0.23 or more and 0.27 or less, and particularly preferably 0.24 or more and 0.26 or less.

[0079] According to this production method, MOFs having a wide particle size distribution ranging from large to small particle sizes and high crystallinity can be easily obtained. The broad particle size distribution of the MOFs obtained by this production method can be confirmed by measurement using a particle size distribution analyzer, and can also be evaluated by tap density. When evaluating the particle size distribution of an MOF using tap density, the higher the tap density, the wider the MOF's particle size distribution can be evaluated. Tap density is the mass per unit volume when the sample volume no longer changes after filling a container with naturally falling powder and applying a tapping impact to the container. This can be measured in accordance with JIS K 5101-12-2:2004. Specifically, tap density can be measured using, for example, a DUAL AUTOTAP (manufactured by Yuasa Ionics Co., Ltd.). The tap density of the MOF measured by this method is preferably 0.3 g / cm. ３ 0.9g / cm or more ３ or less, more preferably 0.4 g / cm ３ 0.9g / cm or more ３ More preferably 0.5 g / cm or less ３ 0.8g / cm or more ３ More preferably, it is 0.6 g / cm or less. ３ 0.7g / cm or more ３ As described above, since the MOF obtained by this production method has a large tap density, when it is used as, for example, an adsorbent for carbon dioxide, the adsorbent will have a high adsorption capacity for carbon dioxide per unit volume.

[0080] The MOFs obtained by this production method are characterized by a wide particle size distribution and a large average particle size. Specifically, the average particle size of the MOFs obtained by this production method is preferably 10 μm or more and 150 μm or less, more preferably 20 μm or more and 100 μm or less, and even more preferably 30 μm or more and 100 μm or less. The average particle size of the MOF is measured by the following method. In cases where the number of MOF particles of 1 μm or more is predominant, as in the present invention, the particle size distribution can be measured by image analysis using a Keyence optical microscope, Model VHX-7000. Alternatively, measurements can also be made using a Horiba, Ltd. laser scattering particle size distribution analyzer, Model LA-960. Since the evaluation criteria are different, the absolute values ​​of particle size cannot be compared, but it is possible to evaluate the size of particle size using the average particle size measured by the same method without any problems. In either case, it is useful to calculate the nucleation rate and linear growth rate if the Risin-Rammler grain size characteristic number L* and slope m are determined in addition to the so-called average grain size (domain size).

[0081] Furthermore, the MOF obtained by this production method has a large BET specific surface area despite its large average particle size. Specifically, the BET specific surface area of ​​the MOF obtained by this production method is 740 m ２ / g or more 1200m ２ / g or less, and ２ / g or more 1150m ２ / g or less is more preferable, and ２ / g or more 1150m ２ / g or less is even more preferable. The BET specific surface area of ​​MOFs is measured by the following method. 10 mg to 20 mg of a sample was placed in a measurement vessel and maintained at 573 K for 1 hour under vacuum evacuation, performing a pretreatment step to remove deposits such as water. After evacuating the measurement vessel to 77 K, nitrogen was gradually introduced into the vessel to adsorb the nitrogen onto the MOFs. The amount of nitrogen adsorbed onto the MOFs was measured, and then the vessel was depressurized to measure the amount of nitrogen desorbed. This allowed the relationship between the amount of nitrogen adsorbed and desorbed and the change in pressure to be investigated, and the BET specific surface area to be calculated.

[0082] Furthermore, the MOF obtained by this production method is characterized by its high degree of crystallinity. Specifically, the crystallinity of the MOF obtained by this production method is high, at 100% or more, particularly 150% or more, and especially 200% or more. The crystallinity of the MOF is measured by the following method. The MOF is analyzed by powder X-ray diffraction analysis, and the height of the highest peak appearing approximately near 2θ = 10°, i.e., the first peak, is determined. For a commercially available MOF manufactured by Sigma-Aldrich (CAS No.: 1370461-06-5), the ratio to the height of a similar peak is calculated, and the crystallinity can be calculated with the reference height set to 100%. Furthermore, due to the high degree of crystallinity, the MOF obtained by this production method exhibits small changes in the angle of the diffraction peak observed by XRD even when water is repeatedly adsorbed and desorbed, and at the same time, small changes in the height of the diffraction peak are also observed.

[0083] [Gas Adsorption Material] The MOF obtained by the present production method is suitably used, for example, as a gas adsorption material. Accordingly, the present invention provides use of the MOF obtained by the present production method as a gas adsorption material. In particular, the present invention provides use of a metal-organic framework consisting of aluminum fumarate crystals and having a tap density of 0.3 g / mL or more and 0.9 g / mL or less as a gas adsorption material.

[0084] The gas adsorption material is preferably composed of a granule of a mixture of MOFs as a gas adsorption component and a phenolic resin as a shape-retaining component, and preferably includes the steps of: Step A: mixing MOFs and a thermosetting phenolic resin to obtain a mixture; Step B: adding water to the mixture and tumbling to obtain granules; Step C: separating the granules that have grown to a predetermined particle size according to particle size; and Step D: subjecting the granules that have not reached the predetermined particle size to a granulation step again, heating the granules that have reached the predetermined particle size at a first temperature to remove the water, and further heating them to a second temperature to cure the thermosetting phenolic resin.

[0085] The type of thermosetting phenolic resin used in step A is not particularly limited, and any known thermosetting phenolic resin can be used. When water is added in step B, the water acts as a flocculant, causing the granules to grow like a snowball during tumbling granulation, thereby enabling efficient granulation. From this perspective, it is preferable to add atomized water. A mist generator such as a sprayer can be used to atomize the water. In step C, the granules can be separated according to particle size by, for example, sieving. The first temperature used in step D may be any temperature capable of removing water contained in the granules, and can be set, for example, between 80°C and 130°C. The second temperature for curing the thermosetting phenolic resin is higher than the first temperature and, although it depends on the type of thermosetting phenolic resin, can generally be set between 130°C and 200°C.

[0086] In order to ensure that the gas adsorbent material obtained in this manner has a strength sufficient for practical use, its crushing strength is preferably 20 N or more, more preferably 30 N or more, and even more preferably 50 N or more. From the same perspective, the rotational strength of the gas adsorbent material when evaluated using a rolling device is preferably 3% or less, more preferably 2% or less, and even more preferably 1% or less. The crushing strength of the gas adsorbent material is measured according to Japan Powder Process Industry Association Standard G001, Method for measuring the crushing strength of granulated products (JIS Z 8841:1993). The rotational strength of the gas adsorbent material is measured according to Japan Powder Process Industry Association Standard G002, Method for measuring the rotational strength of granulated products (JIS Z 8841:1993).

[0087] Although the present invention has been described above based on its preferred embodiments, the present invention is not limited to these embodiments. For example, in the above embodiment, the rotation of the impeller 17 causes a downward flow of the reaction liquid in the draft tube 14. However, instead, the reaction may be caused by causing an upward flow of the reaction liquid in the draft tube 14.

[0088] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to these examples.

[0089] Example 1 In this example, an MOF made of aluminum fumarate was continuously produced using the reaction apparatus 10 shown in FIG. 2 . Aluminum sulfate was dissolved in water to obtain an aluminum sulfate aqueous solution having a pH of 3.1 and a concentration of 0.8 mol / L. Furthermore, fumaric acid and sodium hydroxide in an amount three times the molar amount of fumaric acid were dissolved in water to obtain a fumaric acid aqueous solution having a pH of 14.1 and a concentration of 0.75 mol / L. An aluminum sulfate aqueous solution and a fumaric acid aqueous solution were simultaneously and continuously supplied to a reaction vessel 13 having a volume of 3.3 L. The supply rate of the aluminum sulfate aqueous solution was 0.07 L / min. The supply rate of the fumaric acid aqueous solution was 0.15 L / min. The liquid temperatures of both the aluminum sulfate aqueous solution and the fumaric acid aqueous solution were 80°C. The pH of the reaction solution was approximately constant at 3.6. The reaction between aluminum ions and fumarate ions was controlled by the rotation speed of the impeller 17. The nucleation rate and linear growth rate measured by the above-mentioned method are shown in Table 1 below. The MOFs produced in the reaction vessel 13 by the reaction were continuously withdrawn from the reaction vessel 13 together with the reaction liquid through the outlet pipe 20. The amount of MOF withdrawn was adjusted to be balanced with the amount of MOF produced in the reaction vessel 13. The residence time of the reaction liquid measured by the above-mentioned method is shown in Table 2 below. Other conditions are as shown in Table 2. The tap density, average particle size, BET specific surface area, and degree of crystallinity of the obtained MOF were also measured by the above-mentioned methods. The results are shown in Table 2 below. Optical microscope photographs of the obtained MOF crystals are shown in Figures 3(a) and (b). The crystals were translucent, and the surfaces of the small crystals were somewhat uneven. The overall shape was a rounded ellipsoid. However, in calculating the volume shape factor and surface area shape factor, they were approximately regarded as spherical, and the calculations were performed using the above-mentioned method. The production rate P / ρcV' was calculated from the mass of the MOF crystals obtained from the crystallizer, and the crystal density and volume of the crystallizer. The particle size l and particle size distribution m were measured by observing the crystals under a microscope or by using a particle size distribution measuring device. The suspension density (1-ε iFor Run 1, the slurry in the crystallizer was sampled, and the ratio of the slurry volume to the crystals was measured. The nucleation rate and linear growth rate were calculated using Equations 1 and 2 and are shown in Table 1. Table 1 lists the experimental results under various conditions. Runs 1 to 3 show the results of experiments in which the reaction vessel volume was 3.3 L and the residence time was varied. Runs 4 to 9 show the results of experiments in which the reaction vessel volume was 10 L and the residence time was varied. Runs 8 and 9 showed different concentrations of fumaric acid and aluminum sulfate. The sum of the volumes of the reactant introduction zone and the crystallization zone in Runs 1 to 3 was 18% of the crystallizer volume. The sum of the volumes of the reactant introduction zone and the crystallization zone in Runs 4 to 9 was 25% of the crystallizer volume. Table 2 summarizes the results in Table 1. Each value in Table 2 is the average value from Run 1 to Run 9.

[0090] Example 2 In this example, an MOF made of aluminum terephthalate was continuously produced using the reaction apparatus 10 shown in FIG. 2 . Aluminum sulfate was dissolved in water to obtain an aluminum sulfate aqueous solution having a pH of 3.1 and a concentration of 0.8 mol / L. Terephthalic acid and sodium hydroxide were dissolved in water to obtain a terephthalic acid aqueous solution having a pH of 14.2 and a concentration of 0.75 mol / L. An aluminum sulfate aqueous solution and a terephthalic acid aqueous solution were simultaneously and continuously supplied to a 10-L reaction vessel 13. The aluminum sulfate aqueous solution was supplied at a rate of 0.21 L / min. The fumaric acid aqueous solution was supplied at a rate of 0.45 L / min. The liquid temperatures of both the aluminum sulfate aqueous solution and the terephthalic acid aqueous solution were both 80°C. The pH of the reaction solution was approximately constant at 5.8. The reaction between aluminum ions and terephthalic acid ions was controlled by the rotation speed of the impeller 17. The nucleation rate and linear growth rate measured using the above methods were as shown in Table 1 below. The MOFs produced in the reaction vessel 13 by the reaction were continuously withdrawn from the reaction vessel 13 together with the reaction liquid through the outlet pipe 20. The amount of MOFs withdrawn was finely adjusted so that it was balanced with the amount of MOFs produced in the reaction vessel 13 and the liquid level in the crystallizer remained constant. The residence time of the reaction liquid measured by the above-mentioned method was as shown in Table 2 below. The tap density, average particle size, BET specific surface area, and degree of crystallinity of the obtained MOFs were measured by the above-mentioned methods. The results are shown in Table 2 below. In this example, the sum of the volumes of the reactant introduction zone and the crystallization zone was 25% of the volume of the crystallizer.

[0091] Example 3 In this example, a MOF made of zinc fumarate was continuously produced using the reaction apparatus 10 shown in Figure 2. Zinc sulfate was dissolved in water to obtain a zinc chloride aqueous solution having a pH of 4.1 and a concentration of 0.4 mol / L. Furthermore, fumaric acid and sodium hydroxide, eight times the molar amount of fumaric acid, were dissolved in water to obtain a fumaric acid aqueous solution having a pH of 12.6 and a concentration of 0.1 mol / L. Aqueous zinc chloride solutions and aqueous fumaric acid solutions were simultaneously and continuously supplied to a 10-L reaction vessel 13. The supply rate of the aqueous zinc chloride solution was 0.3 L / min. The supply rate of the aqueous fumaric acid solution was 0.3 L / min. The liquid temperatures of both the aqueous zinc chloride solution and the aqueous fumaric acid solution were 80°C. The pH of the reaction solution was approximately constant at 7.5. The reaction between zinc ions and fumarate ions was controlled by the rotation speed of the impeller 17. The nucleation rate and linear growth rate measured using the above methods were as shown in Table 2 below. The MOFs produced in the reaction vessel 13 by the reaction were continuously withdrawn from the reaction vessel 13 together with the reaction liquid through the outlet pipe 20. The amount of MOFs withdrawn was adjusted to be balanced with the amount of MOFs produced in the reaction vessel 13. The residence time of the reaction liquid measured by the above-mentioned method was as shown in Table 2 below. The tap density, average particle size, BET specific surface area, and degree of crystallinity of the obtained MOFs were measured by the above-mentioned methods. The results are shown in Table 2 below. In this example, the sum of the volumes of the reactant introduction zone and the crystallization zone was 25% of the volume of the crystallizer.

[0092] Reference Example 1 In this reference example, an MOF composed of aluminum fumarate was produced batchwise. Aluminum sulfate was dissolved in water to obtain an aluminum sulfate aqueous solution with a pH of 3 and a concentration of 0.32 mol / L. Furthermore, fumaric acid and sodium hydroxide were dissolved in water to obtain a fumaric acid aqueous solution with a pH of 14 and a concentration of 0.63 mol / L. One liter of aluminum sulfate aqueous solution was placed in a 3.3 L reaction vessel and stirred with a stirring blade. 1.1 L of fumaric acid aqueous solution was added over 1 hour. The liquid temperatures of both the aluminum sulfate aqueous solution and the fumaric acid aqueous solution were set to 80°C. After the addition was completed, the reaction solution was further stirred with an impeller to produce an MOF. The reaction time was 1 hour. The pH of the reaction solution was 3 at the start of the reaction and 3.6 at the end of the reaction. The tap density, average particle size, BET specific surface area, and crystallinity of the resulting MOF were measured using the methods described above. The results are shown in Table 2 below.

[0093]

[0094]

[0095] As is clear from the results shown in Tables 1 and 2, the MOFs obtained in each example had a high tap density, a large average particle size, a high BET specific surface area, and a high degree of crystallinity. Also, in Reference Example 1, which employed a batch process, an MOF with a high tap density was obtained, but the residence time of the reaction liquid was long, and it took a long time to synthesize the MOF.

[0096] Example 4: An experiment was conducted to granulate the MOF obtained in Example 1 to produce a gas adsorption material. A mixture was obtained by powder-mixing an MOF made of aluminum fumarate with a thermosetting phenolic resin (Bellpearl S890, manufactured by Air Water Performance Chemicals). The thermosetting phenolic resin content in the mixture was as shown in Table 3. The mixture was then placed in a 70 cm diameter tumbling granulator and mixed. A small amount of water was added to the mixture using a spray bottle to gradually granulate the mixture. The granules were sieved to remove particles of a predetermined particle size, and particles smaller than this were returned to the mixture. The granules that reached a diameter of 3 mm were heated at a first temperature to remove water and further heated to a second temperature to cure the thermosetting phenolic resin. The crushing strength and rotational strength of the resulting spherical granules were measured according to Japanese Industrial Standards JIS Z 8841:1993 and JIS Z 8841:1993. The results are shown in Table 3. Furthermore, the nitrogen gas adsorption / desorption amounts at 77 K and the carbon dioxide gas adsorption amounts at 20°C of the obtained spherical granules were measured based on the gas adsorption method. The nitrogen gas adsorption / desorption amounts were measured using a Bellsorp MAX manufactured by Nippon Bell Co., Ltd. The carbon dioxide gas adsorption amount at 20°C was measured using an iSORB manufactured by Anton Paar. These results are shown in Figures 4 and 5. For comparison, Figure 5 also shows the carbon dioxide gas adsorption amount of the MOF produced in Example 1.

[0097] As shown in Figure 4, the nitrogen gas adsorption / desorption curves of the spherical granules of Example 4 show a typical Type IV curve according to the IUPAC classification. Figure 5 shows the CO sorption / desorption curves of the MOFs of Example 1 and the spherical granules of Example 4 at 20°C. ２ These MOFs and spherical granules have high adsorption capacities from 0 bar to 2 bar, and also have high adsorption capacities in the pressure range of 2 bar or more. Such isothermal adsorption characteristics make them particularly suitable materials for PSA (pressure swing adsorption) and also suitable for TSA (temperature swing adsorption). In addition, the characteristic that the adsorption capacity increases even after 6 bar is due to the CO ２ As shown in FIG. 5, the CO ２ The adsorption amount of CO２ The amount of adsorption was reduced by about 5% compared to the amount of adsorption, and this reduction was approximately the same as the amount of thermosetting phenolic resin added when producing the spherical granules in Example 4.

[0098] [Example 5] Spherical granules were obtained in the same manner as in Example 4, except that the addition rate of the thermosetting phenolic resin in Example 4 was changed as shown in Table 3. The obtained spherical granules were subjected to the same measurements as in Example 4. The results are shown in Table 3.

[0099] [Examples 6 and 7] In Example 4, the addition rate of the thermosetting phenolic resin was as shown in Table 3. The granulated particle diameter was also as shown in the same table. Except for these, spherical granules were obtained in the same manner as in Example 4. The obtained spherical granules were subjected to the same measurements as in Example 4. The results are shown in Table 3.

[0100] [Comparative Example 2] Granules were produced by extrusion molding. Specifically, MOF (tap density: 0.33 g / cm) made of aluminum fumarate was used. ３ A small amount of kaolin and carbon powder as a lubricant were added to the powder (average particle size 10.2 μm), and the mixture was extruded through a hole approximately 3 mm in diameter and cut into pieces 3 to 4 mm long to produce granules. The resulting granules were subjected to the same measurements as in Example 4. The results are shown in Table 3.

[0101]

[0102] As is clear from the results shown in Table 3, the granules obtained in each example had sufficient strength for practical use.

[0103] Example 8 Using the continuous washing apparatus shown in FIG. 6 , the counter ions (here, sodium sulfate) of the MOF crystals extracted from the reaction vessel 13 shown in FIG. 2 were washed with water. In the first washing step 41, a slurry containing the MOF crystals extracted from the reaction vessel 13 and the washing water sent from the second washing step were mixed and stirred. Next, the slurry from the first washing step was pumped to a hydrocyclone 31 by a pump 30, and the slurry containing the large particles and mother liquor was sent to a second washing step 42. In the second washing step 42, washing water (pure water) was added and mixed and stirred. The slurry was pumped to a hydrocyclone 34 by a pump 33, and the large particles and mother liquor were extracted as products. The fine particles and mother liquor were recycled to the second washing step 42. The obtained MOF crystals were placed in pure water, and the amount of impurities was determined by measuring the electrical conductivity of the liquid. The electrical conductivity was set to 20 mS / m or less. As a result, the amount of washing liquid required was 500 kg, and the same washing effect was obtained with 50% of the amount of water required for washing using filtration or elutriation.

[0104] Comparative Example 3: The counter ions of the MOF crystals extracted from the reaction vessel 13 shown in Figure 2, in this case sodium sulfate, were washed with water by elutriation. 200 kg of pure water was added to 25 kg of slurry containing MOF crystals, and the mixture was stirred and mixed. After allowing to stand, the crystals settled after about 2 hours, and the mixture separated into a crystal layer and a clear mother liquor. The clear mother liquor was removed by pumping, and 200 kg of pure water was added again, followed by stirring and mixing. The amount of impurities in the crystals was determined by measuring the electrical conductivity. This procedure was repeated until the impurity content was below the predetermined level, specifically, until the electrical conductivity reached 20 mS / m or less. The procedure was repeated five times until the electrical conductivity reached 20 mS / m or less. Therefore, the amount of pure water used for washing was 1,000 kg.

[0105] According to the present invention, MOFs with high crystallinity and high bulk density can be easily produced.

Claims

1. An organometallic structure comprising a crystal containing an organic ligand and a metal ion, wherein the organic ligand is a fumaric acid dianion or a terephthalic acid dianion, the metal ion is an aluminum ion, a zinc ion or an iron (III) ion, and the tap density is 0.3 g / mL or more and 0.9 g / mL or less.

2. The metal-organic structure according to claim 1, wherein said crystal is an aluminum fumarate crystal.

3. A gas adsorption material comprising a granulated mixture of the organometallic structure according to claim 1 or 2 as a gas adsorption component and a phenolic resin as a shape-retaining component.

4. The gas adsorption material according to claim 3, which has a crushing strength of 20 N or more and a rotational strength of 3% or less when evaluated using a rolling device.

5. A method for producing a metal-organic framework using a crystallization apparatus equipped with a reactant introduction zone, a crystallization zone, a circulation zone and a recovery zone, comprising: in the reactant introduction zone, an aqueous solution containing metal ions and an aqueous solution containing an organic ligand capable of coordinating to the metal ions are simultaneously and continuously added from different nozzles installed in a flow path, and immediately mixed using a mixing mechanism; in the crystallization zone, the metal ions and the organic ligands are reacted to grow crystals of the metal-organic framework and simultaneously generate crystal nuclei of the metal-organic framework; in the circulation zone, a slurry containing crystals of the metal-organic framework is circulated to a reactant introduction step; and in the recovery zone, a part of the slurry is withdrawn continuously or at predetermined time intervals.

6. The nucleation rate of the crystals is 1×10 ５ piece / (h・m ３ ) or more 1 x 10 １６ piece / (h・m ３ 6. The method according to claim 5, wherein the two aqueous solutions are added so that the total amount of the aqueous solution is equal to or less than 100% by mass.

7. The method according to claim 5 or 6, wherein the two aqueous solutions are added so that the linear growth rate of the crystal is 1 μm / h or more and 200 μm / h or less.

8. The method according to claim 5 or 6, wherein the crystals are withdrawn from the crystallizer so that the residence time of the crystallizer, including the reactant introduction zone, the crystallization zone, the circulation zone and the recovery zone, is 3 minutes or more and 30 minutes or less.

9. The process according to claim 5 or 6, wherein the crystallizer is of the draft tube baffle type.

10. The method according to claim 9, wherein in said draft tube baffle type crystallizer, the sum of the volumes of said reactant introduction zone and said crystallization zone is 30% or less of the volume of said crystallizer.

11. The method according to claim 5 or 6, wherein the metal ion is an aluminum ion, the organic ligand is a dianion of fumaric acid, and 0.9 moles or more and 1.1 moles or less of the dianion of fumaric acid are added per mole of the aluminum ion.

12. The method according to claim 5 or 6, wherein the metal ion is an aluminum ion, the organic ligand is a dianion of terephthalic acid, and 0.9 moles or more and 1.1 moles or less of the dianion of terephthalic acid are added per mole of the aluminum ion.

13. The method according to claim 5 or 6, wherein the metal ion is an iron (III) ion, the organic ligand is a dianion of terephthalic acid, and 0.9 moles or more and 1.1 moles or less of the dianion of terephthalic acid are added per mole of the iron (III) ion.

14. The method according to claim 5 or 6, wherein the metal ion is a zinc (II) ion, the organic ligand is a dianion of fumaric acid, and 0.9 moles or more and 1.1 moles or less of the dianion of fumaric acid are added per 4 moles of the zinc (II) ion.

15. A method for continuously washing the metal-organic framework produced by the method according to claim 5, comprising a washing step of washing the metal-organic framework with a washing liquid to wash and remove a counter anion for dissolving the metal ion constituting the metal-organic framework and a counter cation for dissolving the organic ligand, the washing step comprising a first washing step, a solid-liquid separation step and a second washing step, the liquid subjected to the first washing step being separated into a solid fraction and a liquid fraction in the solid-liquid separation step, the solid fraction being further washed in the second washing step, and the liquid fraction being used as a part of the washing liquid in the first washing step.

16. The continuous washing method according to claim 15, wherein the solid-liquid separation step is performed using centrifugal force.

17. A method for producing a gas adsorption material, comprising the steps of: mixing the metal-organic framework produced by the method according to claim 5 with a thermosetting phenolic resin to obtain a mixture; adding water to the mixture and tumbling granulating to obtain a granulated material; separating the granulated material that has grown to a predetermined particle size according to particle size; and subjecting the granulated material that does not reach the predetermined particle size to the granulation step again, heating the granulated material that has reached the predetermined particle size at a first temperature to remove the water, and further heating it to a second temperature to harden the thermosetting phenolic resin.

Images