Thermal catalyst and method for producing thermal catalyst

Abstract

A method for producing a thermal catalyst according to the present disclosure comprises: a step for heating a liquid containing a plurality of solvents in which metal ions or metal cluster ions and organic ligands are dissolved, and synthesizing a metal organic framework (MOF); a step for combusting the MOF at a temperature of 600°C or lower and converting the MOF into a metal oxide having a (111) plane occupying a larger area than the (100) plane thereof; and a step for causing the metal oxide to support metal nanoparticles.

Description

Thermal catalyst and method for producing the same

[0001] FIELD OF THE DISCLOSURE The present disclosure relates to thermal catalysts and methods for making thermal catalysts.

[0002] This disclosure aims to heat and burn a metal-organic framework, typically a porous metal complex, and utilize its framework to produce CO ２ From CH ４ This application claims priority based on Japanese Patent Application No. 2024-124498, filed on July 31, 2024, the contents of which are incorporated herein by reference.

[0003] Metal-organic frameworks (MOFs) are porous materials containing metal ions or metal cluster ions and organic ligands coordinated thereto. Compared to conventional activated carbons and zeolites, they are characterized by a large specific surface area and pore sizes the size of molecules. As a result, they are known to be used for selective gas adsorption and as catalyst supports (see, for example, Patent Documents 1, 2, and 3).

[0004] In Patent Document 1, a composite material that can produce ammonia from nitrogen and hydrogen is combined with an inorganic material such as a metal. Using this catalyst, ammonia can be produced in a short time.

[0005] Patent Document 2 discloses the use of titanium-based MOFs in the fields of gas storage / separation and catalysis.

[0006] In Patent Document 3, a catalyst function is demonstrated by combining an MOF and a metal on a conventional metal oxide support.

[0007] CO ２ Reduction is a global trend, and the catalysts shown above are also CO ２ Recently, CO ２ CO and CH ４It has been proposed that MOFs with large specific surface areas can be used as supports for catalytic metals or as catalysts themselves. MOFs can be synthesized with any crystal structure by adjusting the synthesis process, and it is known that catalytic activity varies depending on the crystal plane (see, for example, Non-Patent Document 1).

[0008] Japanese Patent Application Laid-Open No. 2013-198844 Japanese Patent Application Laid-Open No. 2012-518032 Japanese Patent Application Laid-Open No. 2019-141758

[0009] ACS Catal. 2021.11, 650-658

[0010] However, Patent Documents 1, 2, 3, and Non-Patent Document 1 use organic ligands that are sensitive to heat, which poses a problem with heat resistance. Therefore, all of the prior art documents are limited to use at relatively low temperatures ranging from room temperature to 400°C or less. Furthermore, the catalyst activity (conversion rate) decreases over time in a heated environment, making them unsuitable for long-term catalytic reactions and limiting their use. While high activity at low temperatures is an important parameter for industrial use of catalysts, improving heat resistance so that catalytic activity (conversion rate) improves over time in a heated environment is also important for achieving structural stability and long life of the catalyst.

[0011] That is, an object of the present disclosure is to provide a thermal catalyst that has low-temperature activity and high heat resistance, and a method for producing the thermal catalyst.

[0012] One aspect of the present disclosure for solving the above problems is a method for producing a thermal catalyst, including the steps of: heating a liquid containing a plurality of solvents in which metal ions or metal cluster ions and organic ligands are dissolved to synthesize MOFs (metal-organic frameworks); burning the MOFs at a temperature of 600°C or less to convert the MOFs into metal oxides in which the (111) plane occupies a larger area than the (100) plane; and supporting metal nanoparticles on the metal oxides.

[0013] One aspect of the present disclosure for solving the above problems is a thermal catalyst characterized in that the (111) plane occupies a larger area than the (100) plane, and metal nanoparticles are supported on the metal oxide.

[0014] According to the present disclosure, the MOF framework with a (111) plane, which is catalytically active due to combustion, is low-temperature active CO ２ It is possible to provide a thermal catalyst that is modifiable and has high heat resistance.

[0015] FIG. 1(a) shows the NH3 crystals obtained by the method according to the first embodiment, in which the (111) plane appears preferentially. ２ 1(b) shows an SEM image of the surface of NH-MIL-125 and a schematic diagram of the crystal structure. ２ 1(c) shows an SEM image of the surface of NH-MIL-125 and a schematic diagram of the crystal structure. ２ 1(a) to 1(c) show SEM images of the surface of NH-MIL-125 and a schematic diagram of the crystal structure. ２ 1(a) to 1(c) are XRD patterns of NH-MIL-125 obtained by the method described in the first embodiment. ２ 1 shows an XRD pattern of the metal oxide obtained by the method described in embodiment 1 after burning MIL-125. ２ 1 is a TEM image of a thermal catalyst in which metal nanoparticles are supported by an impregnation method on a metal oxide obtained by burning MIL-125. ２ and CO conversion in Example 1 ２ From CH ４ 1 is a graph showing the selectivity and the CO ２ Conversion of CO ２ From CH ４ 7(a) is an SEM image of a foam usable in the third embodiment, and FIG. 7(b) is a cross-sectional view schematically showing one cross section of the foam whose SEM image is shown in FIG. 7(a).

[0016] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that the scope of the present disclosure is not limited to the following embodiments and can be modified as desired within the scope of the technical concept of the present disclosure.

[0017] First Embodiment A method for producing a thermal catalyst according to the first embodiment includes a step of synthesizing a MOF (metal organic framework) by heating a liquid containing a plurality of solvents in which metal ions or metal cluster ions and organic ligands are dissolved (MOF synthesis step), a step of burning the MOF at a temperature of 600°C or less to convert the MOF into a metal oxide in which the (111) plane occupies a larger area than the (100) plane (conversion step), and a step of supporting metal nanoparticles on the metal oxide (supporting step). The thermal catalyst produced by this step is a metal oxide in which the (111) plane of the combustion product of the MOF (metal organic framework) occupies a larger area than the (100) plane, and is characterized in that metal nanoparticles are supported. That is, the thermal catalyst according to the first embodiment is a metal oxide in which the (111) plane occupies a larger area than the (100) plane, and is supported with metal nanoparticles.

[0018] One aspect of the first embodiment will be specifically described below.

[0019] (MOF synthesis step: MOF synthesis step) ２ and hydrogen to form CH ４ CO by the Sabatier reaction to obtain ２ The reforming of NH3 requires a temperature of 350°C or higher. The catalyst disclosed herein has been developed to provide a thermal catalyst that exhibits catalytic activity at 350°C or lower and has heat resistance that is resistant to thermal degradation over time, and utilizes MOFs. The MOF disclosed herein is a Ti-based MOF that has a three-dimensional structure, and examples of such MOFs include MIL-22, MIL-25, MIL-91, MIL-125, and NH3. ２ -MIL-125, MIL-167, COK-69, PCN-22, PCN-415, NH ２ Examples of Ti-based MOFs include PCN-415, PCN-416, Ti-CAT-5, Ti-ATA, and MTM-1, but the Ti-based MOFs are not limited to these. ２ -MIL-125 is particularly preferred because of its stable structure.

[0020] This NH ２For -MIL-125, titanium chloride, tetraisopropyl orthotitanate, or tetrabutyl orthotitanate is preferably used as the metal salt, and tetraisopropyl orthotitanate is preferably used to synthesize the (111) face. The organic ligand is 2-aminoterephthalic acid (NH ２ -BDC) is used. These metal salts and organic ligands are dissolved in a solvent to produce a solution containing multiple solvents in which metal ions or metal cluster ions and organic ligands are dissolved. This solvent is composed of methanol and dimethylformamide (DMF), and is mixed in a volume ratio of 1 part methanol to 5.0 to 20.0 parts dimethylformamide, preferably 5.0 to 10.0, to produce an MOF synthesis solution.

[0021] A specific example of the synthesis method is to first synthesize 2-aminoterephthalic acid (NH ２ 0.54 g of methyl methacrylate (BDC) was mixed with 1 ml of methanol and 9 ml of dimethylformamide (DMF), and 1 mmol / L of tetraisopropyl orthotitanate was added. The resulting MOF mixture was then stirred. This mixing ratio corresponds to the mixing ratio in Example 1 described below. To create a catalyst with low-temperature activity, defects may be introduced into the MOF structure. To introduce defects, a minute amount of acid can be added to the MOF mixture. This acid can be selected from hydrochloric acid, nitric acid, sulfuric acid, acetic acid, carbonic acid, oxalic acid, and phosphoric acid, with acetic acid being particularly preferred. Acetic acid is added in an amount ranging from 0.01 to 1 ml to the above solvent (methanol and dimethylformamide mixture), and the amount of acetic acid is preferably in the range of 0.4 to 0.6 ml relative to the amount of solvent. The mixed solution containing the metal salt and organic ligand was placed in a pressure vessel and heated to a temperature ranging from 100 to 300°C, preferably 150°C. Heating temperatures higher than 300°C may result in the formation of many fine crystals, while heating temperatures lower than 100°C may prevent the synthesis of MOFs. The heating time is in the range of 3 to 300 hours, preferably 12 to 48 hours. The pressure inside the pressure vessel may be, for example, 1 to 10 atmospheres, preferably 1 to 2 atmospheres.

[0022] When the ratio of dimethylformamide (DMF) to methanol in the solvent is high, the (111) plane appears preferentially on the surface of the resulting MOF. Figure 1(a) shows the NH ２ Fig. 1(b) shows an SEM image of the surface of NH-MIL-125 and a schematic diagram of its crystal structure. ２ 1(c) shows an SEM image of the surface of NH-MIL-125 and a schematic diagram of the crystal structure. ２ 1(a), 1(b), and 1(c) are SEM images and schematic diagrams of the crystal structure of MIL-125. Figures 1(a), 1(b), and 1(c) show samples prepared in the same manner, except that the volume ratio of dimethylformamide (DMF) to methanol in the solvent was adjusted. The SEM images show that the crystal structure appears different between the (111) and (100) planes. As shown in Figure 1, when the MOF crystal surface is composed only of (111) planes, each crystal exhibits a bipyramidal structure. On the other hand, when composed only of (100) planes, each crystal has a cylindrical structure. Furthermore, when (111) and (100) planes are mixed, the vertex perpendicular to the base of the bipyramid changes to a square, and this plane becomes the (100) plane. The other side surfaces are (111) planes, and the crystal must be synthesized so that the area of ​​these (111) planes is larger than the area of ​​the (100) planes.

[0023] Next, the synthesized NH ２ The XRD pattern of -MIL-125 is shown in Figure 2, which shows that it has a crystalline structure. ２ The XRD pattern of MIL-125 is shown in Fig. 1. MOFs with a (100) plane and MOFs with a mixture of (100) and (111) planes can be obtained by adjusting the volume ratio of methanol to dimethylformamide (DMF) to less than 5.

[0024] (Conversion step: conversion step) The MOF obtained by the synthesis described above is combusted at a temperature of 600°C or less to convert the MOF into metal oxide 1. In the metal oxide obtained by the conversion step, the (111) plane occupies a larger area than the (100) plane. As described above, by the conversion step, the MOF framework with the (111) plane, which has high catalytic activity, is converted into a low-temperature active CO ２ By combusting MOFs with many (111) planes, it is possible to obtain a metal oxide in which the (111) planes occupy a larger area than the (100) planes, as described above. Thus, the crystal structure of the MOF is typically inherited by the metal oxide after combustion. For example, the MOF is heated at a temperature of 300°C to 600°C, preferably 400°C to 500°C, in an air atmosphere for at least one hour. The heating time in the conversion step can be, for example, 5 hours or less. By such heating, the anatase phase of metal oxide 1 can be obtained. For example, NH ２ By heating MIL-125 as described above, TiO ２ Such an anatase phase of TiO ２ Metal oxide 1 such as NH ２ The above conditions are met so that the plane orientation of the MOFs such as MIL-125 is maintained. Figure 3 shows the NH that became the metal oxide 1 after the heat treatment of the MOFs shown in Figures 1(a) to 1(c). ２ 3 shows the XRD pattern of MIL-125. All of the metal oxides 1 whose XRD patterns are shown in Figure 3 have the same structural pattern as the anatase phase. Table 1 shows the TG analysis results of the MOFs whose SEM images are shown in Figures 1(a), 1(c), and 1(b) from top to bottom. ２ -MIL-125 (111) surface is NH ２ Since the weight loss is smaller than that of the (100) face of MIL-125, the amount of carbon remaining in the structure is larger in the (111) face than in the (100) face by 5 to 10 wt % or less.

[0025]

[0026] (Step of supporting metal nanoparticles: Supporting step) The TiO obtained from this MOF２ The metal oxide 1, such as SiO2, can be supported by metal nanoparticles 2, for example, by impregnation. Specifically, a metal salt containing the metal used in the catalyst and the metal oxide 1 obtained from MOFs are stirred in an alcohol solvent, and the MOFs are burned in an evaporator to deposit the metal salt that will form the metal nanoparticles 2 on the surface of the resulting metal oxide 1. The resulting metal oxide 1 is then annealed in a hydrogen atmosphere (10% concentration or higher) at 100-400°C, preferably 250-350°C, for 0.5-6 hours, preferably 1-2 hours, for hydrogen reduction. The stirring of the metal salt containing the metal used in the catalyst and the metal oxide 1 obtained from MOFs can be carried out for, for example, 1 hour or more. Examples of alcoholic solvents that can be used include methanol, ethanol, and isopropanol. The amount of metal salt in the catalyst may be in the range of 1-30 wt % by weight, preferably 3-10 wt %, based on the weight ratio of the metal. Figure 4 shows a TEM image of the metal oxide 1 to which the metal nanoparticles 2 have been attached using the above-described method. Figure 4 shows the NH ２ The (111) surface of MIL-125 was annealed at 200°C for 1 hour to form TiO ２ 4 is a TEM image showing the results of the above-mentioned method, including an enlarged view of a portion thereof, and it can be seen from the appearance that the crystalline structure is maintained. As can be seen in FIG. 4, the metal nanoparticles 2 are finely dispersed over a wide area. The metal species of the metal nanoparticles 2 attached by the above-mentioned method is a metal compound containing one or more of Pt, Pd, Fe, Co, Sn, Cu, Ni, Ru, Zn, Au, and Ag, and in particular CO ２ Conversion of CO ２ From CH ４ In order to increase the selectivity to Ru, Ru and Ru metal compounds are preferred. The average particle size of the metal nanoparticles 2 is preferably 1 to 100 nm, and more preferably 1 to 10 nm.

[0027] The thermal catalyst according to the first embodiment obtained in this manner is a combustion product of MOFs, a metal oxide in which the (111) plane occupies a larger area than the (100) plane, and metal nanoparticles are supported. The thermal catalyst according to the first embodiment is synthesized by controlling the MOF structure to have a structure advantageous for catalytic activity, and by utilizing the framework after MOF combustion, it exhibits high low-temperature activity and heat resistance. The fact that the (111) plane occupies a larger area than the (100) plane can be confirmed by observing the shape using an SEM. In the metal oxide of this embodiment, when the crystal surface is composed only of (100) planes, each crystal has a cylindrical structure. When composed only of (111) planes, each crystal is a bipyramid. When (100) planes and (111) planes are mixed, the crystal becomes a bipyramid with a flat diagonal angle without a vertex. Therefore, in this embodiment, when the (111) plane occupies a larger area than the (100) plane in the metal oxide, this means that when the thermal catalyst is observed with an SEM, the following (i) or (ii) is satisfied. The area can be calculated by assuming that any one side is formed symmetrically. (i) When cylindrical shapes and bipyramidal shapes are mixed, either the number of cylindrical shapes in the field of view is less than 50%, or the number of bipyramidal shapes in the field of view is 50% or more. (ii) When (111) planes and (100) planes are mixed in one solid, the areas are compared, and in SEM observation, the area of ​​the (111) plane is larger than the area of ​​the (100) plane in at least 5 out of 10. It is desirable that the metal oxide 1 contained in the thermal catalyst is composed of a metal oxide 1 having a (111) plane, i.e., all of the metal oxides whose crystal structure can be confirmed are composed of bipyramids.

[0028] In the first embodiment, the metal oxide is NH ２ -It is preferable that it is a combustion product of MIL-125, and the metal oxide is TiO containing C (carbon). ２ It is desirable that ２ - TiO made by MIL-125 ２It has been confirmed that the thermal catalyst using the above improves catalytic activity (conversion rate) over time and has excellent heat resistance. It can be confirmed that the metal oxide 1 in the thermal catalyst is a combustion product of Ti-based MOF by XRD pattern and carbon detection by EDS elemental analysis. The combustion product of Ti-based MOF is anatase type TiO ２ or rutile type TiO ２ The pattern is typically anatase TiO ２ The EDS elemental analysis shows a carbon peak. ２ The same applies to the combustion products of MIL-125.

[0029] In the thermal catalyst of the first embodiment, the metal nanoparticles are Ru or an alloy containing Ru, ２ From CH ４ It is particularly desirable for the selectivity to

[0030] The thermal catalyst thus obtained is heated to produce CO ２ and hydrogen gas mixture to efficiently reduce CO ２ CH ４ Furthermore, the thermal catalyst according to this embodiment is active at low temperatures and also exhibits heat resistance. For example, by heating the thermal catalyst, CO ２ The mixture gas ratio of hydrogen and N was 1:4. ２ Inert gases such as Ar, He, etc. are used and supplied to a heated catalyst at a GHSV (Gasous Hourly Space Velocity), preferably at a flow rate of 24,000 to 40,000 ml / (g [= catalyst weight]·Hr). ２ CH ４ When the mixed gas was supplied at the above-mentioned mixed gas ratio, NH ２ -TiO of metal oxide 1 obtained from MIL-125 (111) surface ２ Regarding the catalytic performance of CO ２ From CH ４ The selectivity to NH was 100% between 150°C and 350°C, and activity was also obtained at temperatures below 200°C. ２ The catalyst obtained from the MIL-125(100) surface was２ From CH ４ The selectivity to NH was 100% between 150°C and 350°C, and activity was observed at temperatures above 200°C. ２ - Compared with the MIL-125 (111) surface, NH ２ The MIL-125 (100) surface is inferior in terms of low temperature activity. Figure 5 shows the results.

[0031] Also, FIG. ２ From CH ４ The thermal catalyst of the present disclosure shows that the conversion rate and selectivity to CO ２ From CH ４ The conversion rate of NH3 was improved. With conventional thermal catalysts, the conversion rate decreases as the catalytic reaction time passes. ２ -CO of thermal catalyst made by burning MIL-125 (111) surface ２ From CH ４ The conversion rate to TiO of metal oxide 1 produced by heating was characterized by an increase in conversion rate over time. The reason for this is not clear, but it is thought that the MOF has a carbon skeleton. ２ It contains carbon, which is thought to be the cause of this.

[0032] Second Embodiment As another method for supporting the metal nanoparticles 2 shown in the first embodiment, a precipitation method may be used. That is, the supporting step can be performed by a precipitation method in addition to an impregnation method. In the supporting step using the precipitation method, the metal oxide 1, such as TiO, obtained by burning the MOF in the conversion step is ２ The metal salt is placed in a mixed solution of one or more of water, methanol, and ethanol and stirred. The metal atoms in the metal salt are added in a range of 1 to 30% by weight, preferably 3 to 10% by weight, of the metal oxide. Also, sodium borohydride (NaBH ) is added in an amount that can supply a hydrogen molar concentration of 10 times or more the molar weight of the metal in the metal salt. ４Metal nanoparticles 2 can also be reduced and precipitated on the surface of the catalyst support by adding metal oxide 1 to a mixture of one or more of water, methanol, and ethanol in which metal nanoparticles 2 are dissolved, and stirring the mixture. After precipitation, metal oxide 1 carrying metal nanoparticles 2 is washed using filtration or an evaporator. A thermal catalyst prepared in this manner can also achieve performance similar to that of the first embodiment.

[0033] Third Embodiment Because the thermal catalysts prepared in the first and second embodiments are powders, they are typically molded into spheres by applying heat or pressure. However, gas may not penetrate the molded body, potentially resulting in a lower-than-expected catalytic reaction. This embodiment is implemented to overcome this issue. One method for achieving this is to form an MOF in a porous body 3, such as a foam, as shown in Figure 7. Figure 7(a) is an SEM image of the foam, and Figure 7(b) is a cross-sectional view schematically illustrating a cross section of the foam whose SEM image is shown in Figure 7(a). A specific method for forming a thermal catalyst by performing a MOF synthesis process on either or both the surface and interior of a porous body, followed by annealing to form a metal oxide 1, and then attaching metal nanoparticles 2, is as follows.

[0034] As the metal salt, titanium chloride, tetraisopropyl orthotitanate, or tetrabutyl orthotitanate was used, and NH ２ -MIL-125 (111) plane, that is, NH in which the (111) plane appears preferentially ２ To synthesize MIL-125, it is preferable to use tetraisopropyl orthotitanate. The organic ligand is 2-aminoterephthalic acid (NH ２ -BDC) is used. These metal salts and organic ligands are dissolved in a solvent. NH ２ To synthesize MIL-125, the solvent is a MOF synthesis solution consisting of methanol and dimethylformamide (DMF), with the volume ratio of methanol to dimethylformamide being 1:5.0 to 20.0, preferably 5.0 to 10.0.

[0035] Defects may be introduced into the MOF structure in order to create a catalyst active at low temperatures. To introduce defects into the MOF structure, a minute amount of acid is added to the MOF synthesis solution. This acid is selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, acetic acid, carbonic acid, oxalic acid, and phosphoric acid, with acetic acid being particularly preferred. To prepare the synthesis solution, acetic acid is added in an amount ranging from 0.1 to 10 wt % relative to the solvent, preferably 4 to 6 wt % relative to the solvent amount. To utilize the entire porous body, the entire porous body 3 must be immersed in the synthesis solution. Similar ratios can be used when using other acids.

[0036] The material of the porous body 3, such as a foam, is either a metal, a ceramic, or a resin. In the case of a metal, a metal selected from the group consisting of Ag, Cu, Ti, Al, and Fe, or an alloy containing a metal included in this group as a main component, is preferred, as long as an MOF can be synthesized on the surface. Here, an alloy containing a metal included in this group as a main component refers to an alloy in which the sum of the contents of Ag, Cu, Ti, Al, and Fe is 50% by mass, and may contain 50% by mass or more of any one metal included in this group. Examples of such alloys include stainless steel (Steel Use Stainless: SUS). Preferred ceramics are carbides or metal oxides such as silicon carbide, alumina, and titanium oxide, but are not limited to these as long as an MOF can be synthesized on the surface. Resins may be polyurethane-based, polystyrene-based, olefin-based, polyvinyl chloride-based, phenol-based, or other materials. In addition to foams, pumice and other materials may also be used as porous bodies, as long as an MOF can be synthesized on their surfaces.

[0037] To form a MOF on either or both the surface and interior (interior of the porous body), it is necessary to first form metal ions or metal cluster ions on the surface of the porous body. Therefore, the porous body is first washed with acid or alkali and then immersed in a solution containing metal ions or metal clusters. After forming metal ions or metal cluster ions on the surface of the porous body in this way, the porous body is immersed in the MOF synthesis solution described above, whereby MOFs are synthesized starting from the metal ions or metal cluster ions on the surface of the foam. Alternatively, a silane coupling agent may be used to form a MOF on either or both the surface and interior of the porous body. In the MOF synthesis process of the third embodiment, it is desirable to form a MOF at least on the surface of the porous body, and more desirable to form it on the surface and interior of the porous body. The surface of the porous body refers to a region less than 1 μm from the outermost surface of the porous body, and the interior of the porous body refers to a region 1 μm or more from the outermost surface of the porous body.

[0038] When a foam is used as the porous body, the opening diameter of the foam is between 1 μm and 1 mm, preferably 200 μm or less, as in the foam shown in FIG. 7 , and the average porosity is preferably 80% or more. To ensure that the MOF synthesis solution penetrates fully into the foam, a vacuum pump is used to draw a vacuum to 1 / 10 or less of atmospheric pressure, preferably 100 Pa or less, allowing the solution to penetrate into the foam. With the solution permeating the foam, the pressure vessel containing the foam is heated to a temperature range of 100°C to 300°C, preferably 150°C, and pressurized to synthesize the MOF. The temperature of the pressure vessel during MOF synthesis is preferably around 150°C, for example, in the range of 125°C to 175°C. The pressure inside the pressure vessel can be, for example, 1 to 10 atmospheres, preferably 1 to 2 atmospheres. The heating time is preferably 3 to 300 hours, and synthesis is preferably completed over 12 to 48 hours.

[0039] By doing so, NH is released into either or both the surface and the interior of the porous body such as a foam. ２In particular, MOFs with a (111) surface are preferentially formed by adjusting the MOF synthesis solution. ２ A porous body having a MOF such as TiO-MIL-125 formed on its surface is heated in an air atmosphere at a temperature of 300 to 600°C, preferably 400 to 500°C, for at least one hour. The heating time can be, for example, 3 hours or less. By the conversion step, TiO of metal oxide 1 is formed on either or both of the surface and the interior of the porous body. ２ The anatase phase of the catalyst is obtained. The remaining steps are the same as those in the first and second embodiments. In the supporting step, metal nanoparticles 2 are formed on metal oxide 1 formed on the surface and / or the interior of a porous body, and the resulting thermal catalyst can have the same properties as those in the first embodiment. Thus, even in a thermal catalyst production method in which the MOF synthesis step uses porous body 3 as a catalyst support and synthesizes MOFs on at least one of the surface and the interior of the porous body of the catalyst support, a thermal catalyst that can solve the above-mentioned problems can be obtained. Furthermore, according to the third embodiment, the catalytic reaction area can be further increased. Therefore, the reaction rate can be increased by using such a thermal catalyst.

[0040] <Embodiment 4> In Embodiment 4, the foam described in Embodiment 3 is replaced with a honeycomb-structured porous body (honeycomb porous body). The cell density of the honeycomb porous body is 600 cpsi or less (cpsi: unit indicating the number of cells per square inch). The cell density of the honeycomb porous body is, for example, 100 cpsi or more. The honeycomb-structured porous body is a porous body having a cross-sectional shape in which hexagons are periodically arranged. For example, the honeycomb porous body may have a cross-sectional shape in which regular hexagons are packed tightly together with no gaps, penetrate in a direction perpendicular to the cross-section, and have a configuration in which the areas surrounded by the sides of the regular hexagons are hollow. The material of the honeycomb porous body may be any of the materials exemplified for the porous body 3 such as the foam, but an oxide of an inorganic material such as a metal oxide is preferable, specifically titanium oxide, zinc oxide, cerium oxide, silica (SiO ２ ) is desirable.

[0041] In the thermal catalyst of the fourth embodiment, metal oxide 1 carrying metal nanoparticles 2 is present on either or both the surface and the interior (inside the porous body) of the honeycomb porous body. The thermal catalyst of the fourth embodiment also achieves the same effects as the first embodiment. Furthermore, the thermal catalyst of the fourth embodiment can increase the catalytic reaction area. Therefore, by using such a thermal catalyst, the reaction rate can be increased.

[0042] [Example 1] In Example 1, a thermal catalyst corresponding to the first embodiment was produced. The specific procedure is as follows. First, in the MOF synthesis step, 2-aminoterephthalic acid (NH ２ 1 ml of methanol and 9 ml of dimethylformamide (DMF) were added to 0.54 g of (-BDC), and 1 mmol / L of tetraisopropyl orthotitanate was added and stirred. The mixed solution was then placed in a pressure vessel and heated at a temperature of 150°C, a pressure of 1-2 atm, and a heating time of 24 hours to form NH ２ -MIL-125 was synthesized.

[0043] Then, as a conversion step, NH ２ -MIL-125 was heated at 500°C for 1 hour or more in an air atmosphere to combust it, and TiO was obtained as a metal oxide. ２ Next, as a supporting step, a metal oxide TiO was formed by an impregnation method. ２ Specifically, the metal salt Ru chloride hydrate and the metal oxide TiO obtained in the conversion step were supported on the catalyst. ２ By stirring the above, the metal oxide TiO ２ The metal salt Ru chloride hydrate was applied to the surface of the catalyst. The metal oxide to which the metal salt was applied was then annealed in an evaporator under a hydrogen atmosphere at 200°C for 1 hour to perform hydrogen reduction, thereby producing a thermal catalyst.

[0044] Comparative Example 1 A thermal catalyst was prepared in the same manner as in Example 1, except that 4.6 ml of dimethylformamide (DMF) was used per 1 ml of methanol in the MOF synthesis step.

[0045] Comparative Example 2 A thermal catalyst was prepared in the same manner as in Example 1, except that 2.0 ml of dimethylformamide (DMF) was used per 1 ml of methanol in the MOF synthesis step.

[0046] <Analysis of MOF> In Example 1, Comparative Example 1, and Comparative Example 2, the MOF (NH ２ -MIL-125) was analyzed. First, the crystal structure was confirmed by scanning electron microscopy, and the SEM images shown in Figures 1(a), 1(c), and 1(b) were obtained from Example 1, Comparative Example 1, and Comparative Example 2. It was confirmed from the SEM images that the MOF obtained in Example 1 is an MOF exhibiting a bipyramidal crystal structure in which the (111) plane appears preferentially. In addition, it was confirmed that the MOF obtained in Comparative Example 1 has a structure in which each crystal has a quadrangular vertex (this plane is the (100) plane) perpendicular to the base of the bipyramid, and that the (111) plane and the (100) plane are mixed. It was confirmed that the MOF obtained in Comparative Example 2 exhibits a crystal structure in which each crystal is cylindrical and the (100) plane appears preferentially. When (111) and (100) planes are mixed in one solid, the areas are compared and SEM observation shows that the area of ​​the (111) plane is larger than that of the (100) plane in less than 5 out of 10 MOFs. Therefore, it can be said that the (100) plane occupies a larger area than the (111) plane in the MOF of Comparative Example 2. Furthermore, these structural features were also inherited by the metal oxide after combustion in the conversion process. Next, X-ray diffraction was performed, and the XRD pattern shown in Figure 2 was obtained, confirming that both had a crystalline structure.

[0047] <TG Analysis> In Example 1, Comparative Example 1, and Comparative Example 2, the MOF(NH ２TG analysis was performed on the sample (MIL-125). The TG analysis was performed by measuring the weight change of the sample with respect to temperature using a thermogravimetric analyzer (manufactured by Rigaku, model number: Thermo Plus EVO2 TG8121). Figure 4 shows the weight loss rate obtained by TG analysis. The TG analysis showed that the weight loss rate of the (111) plane corresponding to Example 1 was 65%, which was smaller than the weight loss rate of the other plane orientations corresponding to Comparative Examples 1 and 2, and it can be determined that more carbon remains. <Analysis of Metal Oxide> Furthermore, X-ray diffraction was performed on the metal oxide obtained after the conversion process using the same method as the measurement before the conversion process, and the XRD pattern shown in Figure 3 was obtained. As can be seen from Figure 3, both of them were TiO ２ The anatase phase of TiO ２ was confirmed to be formed.

[0048] <Analysis of Thermal Catalyst> The thermal catalyst prepared in Example 1 was observed with a transmission electron microscope, and the TEM image shown in Figure 4 was obtained. As shown in Figure 4, it was confirmed that metal nanoparticles 2 were formed (supported) on the surface of bipyramidal metal oxide 1. (Analysis of Conversion Rate and Selectivity) The thermal catalysts prepared in Example 1 and Comparative Example 2 were heated, and CO ２ When a gas containing CO ２ Conversion rate of CO ２ From CH ４ The selectivity to CO was measured. ２ A mixed gas containing nitrogen as an inert gas as a carrier gas, with a mixed gas ratio of 1:4 and hydrogen, was supplied to the heated thermal catalyst at a flow rate of 36,000 ml / (g [= catalyst weight]·Hr) using GHSV (Gas Hourly Space Velocity). The conversion rate and selectivity were measured by gas analysis using GC (Gas Chromatograph). Two types of measurements were obtained: values ​​obtained when the thermal catalyst was heated for 15 minutes at temperatures in 25°C intervals within the temperature range of 150°C to 450°C, and values ​​obtained at each time point when the catalyst was heated at 225°C for 0 to 56 hours. Figure 5 shows the CO2 concentration when the thermal catalysts of Example 1 and Comparative Example 2 were used. ２ Conversion of CO２ From CH ４ 6 is a graph showing the selectivity of CO 2 when the catalyst of Example 1 is used. In the figure, the graph shown as (111 plane) is Example 1, and the graph shown as (100 plane) is Comparative Example 2. The selectivity showed a roughly constant value regardless of the temperature. ２ Conversion of CO ２ From CH ４ 1 is a graph showing the change in selectivity over time.

[0049] 5, it was confirmed that the thermal catalyst having a large number of (111) planes such as in Example 1 has higher catalytic activity in all temperature ranges compared to the thermal catalyst having a large number of (100) planes such as in Comparative Example 2. In other words, it can be said that the thermal catalyst of Example 1 exhibits low-temperature activity. Furthermore, it was confirmed from FIG. 6 that the conversion rate of the thermal catalyst of Example 1 improves over time in a heated environment. In other words, it can be said that the thermal catalyst of Example 1 has high heat resistance. This characteristic is contrary to that of conventional thermal catalysts, where the conversion rate decreases over the course of catalytic reaction time.

[0050] 1: Metal oxide, 2: Metal nanoparticles, 3: Porous material

Claims

1. A method for producing a thermal catalyst, comprising the steps of: heating a liquid containing multiple solvents in which metal ions or metal cluster ions and organic ligands are dissolved to synthesize a metal-organic framework (MOF); burning the MOF at a temperature of 600°C or less to convert the MOF into a metal oxide in which the (111) plane occupies a larger area than the (100) plane; and supporting metal nanoparticles on the metal oxide.

2. The MOF is NH ２ -MIL-125, and the metal oxide is TiO containing C (carbon). ２ The method for producing a thermal catalyst according to claim 1, characterized in that 3. A method for producing a thermal catalyst according to claim 1 or 2, characterized in that the step of synthesizing the MOF includes a step of using a porous body as a catalyst support and synthesizing the MOF on at least one of the surface and the interior of the porous body of the catalyst support.

4. The method for producing a thermal catalyst according to claim 3, wherein the porous body has a honeycomb structure with a cell density of 600 cpsi or less.

5. A method for producing a thermal catalyst according to claim 1 or 2, characterized in that the metal nanoparticles are Ru or an alloy containing Ru.

6. A thermal catalyst characterized by being a metal oxide in which the (111) plane occupies a larger area than the (100) plane and in which metal nanoparticles are supported.

7. The metal oxide is an anatase type TiO containing C (carbon). ２ The thermal catalyst according to claim 6, characterized in that:

8. The metal oxide is NH ２ A thermal catalyst according to claim 6 or 7, characterized in that it is the combustion product of MIL-125.

9. The thermal catalyst according to claim 6 or 7, characterized in that the metal nanoparticles are Ru or an alloy containing Ru.

10. A thermal catalyst according to claim 6 or 7, comprising a porous body, the metal oxide being present on at least one of the surface and the interior of the porous body.

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