JP6596567B2 - Carbon dioxide absorbing material and manufacturing method thereof - Google Patents

Description

The present invention relates to a carbon dioxide absorbing material having excellent carbon dioxide absorption characteristics and a method for producing the same.
This application claims priority based on Indian Patent Application No. 2932 / MUM / 2015 (Indian Patent Application No. 2932 / MUM / 2015) filed on August 3, 2015. The contents are incorporated herein by reference.

Since carbon dioxide (CO ₂ ) becomes a global warming gas, there is an urgent need to significantly reduce emissions. Therefore, development of materials and techniques for selectively separating and recovering CO ₂ from exhaust gas discharged from thermal power plants, factories, automobiles and the like is required. So far, for example, from porous materials, chemically absorbing materials introduced with amino groups having CO ₂ absorption ability, metal-organic framework (MOF), carbonaceous materials, alkali metal carbonates, etc. A CO ₂ adsorbing material / absorbing material has been proposed.

Most of these materials are used as CO ₂ absorbing materials in a low temperature environment from room temperature to 200 ° C. or less because of their low heat resistance and their physical adsorption characteristics. On the other hand, among these, the CO ₂ absorbing material made of alkali metal carbonate is expected as a CO ₂ absorbing material that can be used in a high temperature environment because it can be used at a high temperature exceeding 450 ° C. In particular, alkali metal carbonates such as lithium silicate have been reported to have a CO ₂ absorption capacity of 10% to 35% by mass, and great expectations are placed on their practical use (for example, Patent Documents 1 to 3). 3 and non-patent documents 1 and 2).

Japanese Patent No. 3396642 Japanese Patent No. 3591724 Japanese Patent No. 4427498

Journal of Materials Chemistry A, 2014, 2, 12792-12798 Journal of Materials Chemistry A, 2013, 1, 3919-3925

However, this alkali metal carbonate, the absorption rate of CO ₂ 50mg / (g · min) or less, and typically slow and 10mg / (g · min) or less, to process the CO ₂ efficiently A large amount of CO ₂ absorbing material was required. Further, although this alkali metal carbonate is excellent in CO ₂ absorption characteristics in a temperature range of 600 ° C. or higher, there is also a drawback that the CO ₂ absorption capacity in an intermediate temperature range of 200 ° C. or higher and 450 ° C. or lower is extremely low.
The present invention has been made in view of the above problems, and an object thereof is CO ₂ absorption properties provide a more improved carbon dioxide absorbing material. Another object of the present invention is to provide a method for easily producing such a carbon dioxide-absorbing material.

In order to realize the above object, the present invention provides a method for producing a carbon dioxide (CO ₂ ) absorbing material mainly composed of lithium silicate. This manufacturing method includes preparing a sol-like composition in which a Li-Si precursor compound containing a lithium (Li) component and a silicon (Si) component is dispersed in an aqueous solution, and applying electromagnetic waves to the sol-like composition. Irradiation to obtain a gel-like composition, and firing the gel-like composition to obtain lithium silicate containing lithium and silicon.

According to such a configuration, when lithium silicate is obtained by applying a so-called sol-gel method, the time required for gelation can be shortened, and lithium silicate with improved CO ₂ absorption characteristics can be obtained. Thereby, for example, a CO ₂ absorbing material with improved CO ₂ absorption capacity and CO ₂ absorption rate can be provided.

In a preferred embodiment of the method for producing a CO ₂ absorbent material disclosed herein, the sol composition causes a hydrolysis reaction in an aqueous solution containing the lithium (Li) component and the silicon (Si) component. It is characterized by preparing by making it.
With such a configuration, it is possible to stably manufacture a CO ₂ absorbent material having a good quality by applying a known sol-gel method.

In a preferred aspect of the method for producing a CO ₂ absorbent material disclosed herein, the silicon (Si) component is at least one of silica alkoxide, colloidal silica, and fumed silica. With this configuration, it is possible to manufacture a CO ₂ absorbent material that is more easily excellent in CO ₂ absorption performance.

In a preferred embodiment of the method for producing a CO ₂ absorbing material disclosed herein, the sol-like composition further includes a germanium (Ge) component. With this configuration, various CO ₂ absorbing materials having different characteristics such as crystal form, CO ₂ absorption temperature range, CO ₂ absorption capacity, and the like can be manufactured.

In a preferred aspect of the method for producing a CO ₂ absorbing material disclosed herein, the electromagnetic wave is a microwave having a wavelength of 1 mm or more and 1 m or less and a frequency of 300 MHz or more and 300 GHz or less. Here, the total irradiation time of the electromagnetic wave is preferably 1 minute or more and 60 minutes or less.
Even with such a configuration, a CO ₂ absorbent material having excellent CO ₂ absorption performance can be easily produced.

In a preferred embodiment of the method for producing a CO ₂ absorbent material disclosed herein, the lithium silicate is further combined with an alkali carbonate.
With such a configuration, it is possible to manufacture a CO ₂ absorbent material having an increased CO ₂ absorption capacity and CO ₂ absorption rate in a temperature range of 200 ° C. or more and 600 ° C. or less, for example.

In a preferred embodiment of the method for producing a CO ₂ absorbent material disclosed herein, the alkali carbonate contains two or more of a sodium (Na) component, a potassium (K) component, and a lithium (Li) component. Is preferred. For example, it is a preferable aspect that the ratio of each alkali component in the alkali carbonate is Na: 1 mol% or more and 80 mol% or less, K: 1 mol% or more and 70 mol% or less, and Li: 1 mol% or more and 90 mol% or less. obtain. In particular, the alkali carbonate is preferably a eutectic carbonate.
With such a configuration, it is possible to manufacture a CO ₂ absorbent material in which, for example, the CO ₂ absorption capacity and the CO ₂ absorption rate in the temperature range of 200 ° C. or more and 600 ° C. or less are further increased.

In another aspect, the techniques disclosed herein provide a CO ₂ absorbing material. This CO ₂ absorbing material is characterized in that it is a CO ₂ absorbing material mainly composed of lithium silicate manufactured by any one of the manufacturing methods described above.
As a result, a CO ₂ absorbent material with improved CO ₂ absorption characteristics such as CO ₂ absorption capacity and CO ₂ absorption rate is realized.

The CO ₂ absorbing material disclosed herein is mainly composed of lithium silicate, and the CO ₂ absorption rate per unit weight in a temperature range of 500 ° C. to 710 ° C. is 25 mg / (g · min) or more. It can be characterized by being. For example, the CO ₂ gas absorption rate per unit weight in the temperature range of 600 ° C. or more and 710 ° C. or less is preferably 50 mg / (g · min) or more. Thus, according to the technology disclosed herein, a CO ₂ absorbing material having a significantly improved CO ₂ absorption rate can be provided.

In a preferred aspect of the CO ₂ absorbent material disclosed herein, the lithium silicate has a short dimension when a dimension in the longitudinal direction is a and a dimension in one short direction perpendicular to the longitudinal direction is b. The rod-like particles having a direction dimension b of 100 nm or less and an aspect ratio defined by a / b of 2 or more may occupy 50% by number or more.
Such a CO ₂ absorbing material composed of particles having a high aspect ratio is preferable in that aggregation is unlikely to occur even when repeatedly used in a high temperature environment, and excellent CO ₂ absorbing characteristics can be maintained over a long period of time. In addition, in this specification, the dimension of lithium silicate is a value measured by observing with observation means, such as an electron microscope.

In a preferred aspect of the CO ₂ absorbing material disclosed herein, the carbon dioxide-containing material further includes an alkali carbonate, and at least a part of the surface of the lithium silicate is covered with the alkali carbonate. According to such a configuration, it is possible to provide a CO ₂ absorbing material having a stable and greatly increased CO ₂ absorption rate.

As described above, according to the technology disclosed herein, the CO ₂ absorbing material capable of selectively and efficiently absorbing CO ₂ in a medium temperature to a high temperature range (for example, a temperature range of about 200 ° C. to 710 ° C.). Is provided. Such a CO ₂ absorbing material is, for example, a CO ₂ absorption from a high-temperature mixed gas or CO ₂ single-phase gas in a liquid fuel production plant using natural gas as a raw material, a CO ₂ recovery system using a water gas shift reaction, or the like. It can be used particularly useful in

FIG. 1A is a flowchart showing a method for manufacturing a CO ₂ absorbent material according to an embodiment. FIG. 1B is a flowchart showing a method for producing a CO ₂ absorbent material according to another embodiment. FIG. 1C is a flowchart showing a method for manufacturing a CO ₂ absorbent material according to another embodiment. FIG. 2 is an XRD pattern of a MW-SG sample according to an embodiment. FIG. 3 is a TEM image of (a) an as-made MW-SG sample and (b) a MW-SG sample after CO ₂ absorption / desorption in the examples. FIG. 4 is a dynamic TGA curve of an SG sample produced by a conventional method and an MW-SG sample produced by this technique. FIG. 5 is a CO ₂ isothermal absorption curve of a MW-SG sample according to one embodiment. FIG. 6 is a graph showing the CO ₂ absorption rate of the MW-SG sample according to one embodiment. FIG. 7 is a graph showing cycle characteristics of the MW-SG sample according to one embodiment. FIG. 8 is a dynamic TGA curve of a MW-SG-NKL sample according to another embodiment. FIG. 9 is a CO ₂ isothermal absorption curve of a MW-SG-NKL3 sample according to another embodiment. FIG. 10 is a graph showing the CO ₂ absorption rate of the MW-SG-NKL3 sample according to another embodiment. FIG. 11 shows XRD for (a) a silicon raw material, (b) a lithium raw material, (c) a sol solution before irradiation with electromagnetic waves, and (d) a gel solution after irradiation with electromagnetic waves according to an embodiment. It is a pattern. FIG. 12 is a diagram showing the results of in-situ high-resolution X-ray diffraction analysis of a gel solution dry powder while heat-treating. FIG. 13 is a TEM image of a gel-like solution dry powder heat-treated at (a, b) 473K, (c, d) 673K, (e, f) 773K, (g, h) 1073K. FIG. 14 is an SEM image of the gel solution dry powder heat-treated at 1073K. FIG. 15 is a SEM image illustrating the entirety of the lithium silicate particles according to an embodiment, and (b) is a partially enlarged view thereof. 16A to 16E are TEM images of the lithium silicate particles in FIG. FIG. 17 is a CO ₂ isothermal absorption curve of lithium silicate according to one embodiment. FIG. 18 is an XRD pattern of (a) lithium silicate and (b) germanium-doped lithium silicate according to one embodiment. FIG. 19 is a Raman spectrum of (a) lithium silicate and (b) germanium-doped lithium silicate according to one embodiment. FIG. 20 is an SEM image at low magnification of germanium-doped lithium silicate particles according to one embodiment. FIGS. 21A to 21D are TEM images of different magnifications and observation fields of germanium-doped lithium silicate particles according to an embodiment. FIG. 22 is a dynamic TGA curve of germanium-doped lithium silicate according to one embodiment. FIG. 23 is a CO ₂ isothermal absorption curve at 300 ° C. of germanium-doped lithium silicate according to one embodiment.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings as appropriate. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field. In the present specification, the notation “X to Y” indicating a numerical range means “X or more and Y or less” unless otherwise specified.

1A to 1C are flowcharts showing a method for manufacturing a carbon dioxide (CO ₂ ) absorbent material according to an embodiment. The manufacturing method disclosed here typically manufactures a CO ₂ absorbing material mainly composed of lithium silicate.
As lithium silicate, a compound containing lithium (Li), silicon (Si), and oxygen (O) can be considered. Typically, lithium silicate can be a variety of compounds represented by the general formula: Li _x Si _y O _z , where x, y, and z satisfy x + 4y−2z = 0. . Typically, various types of lithium silicate in a form including a silicate anion in which an arbitrary number of silicate ions are linked and a lithium cation can be considered. As such lithium silicate, typically, lithium orthosilicate (Li ₄ SiO ₄ ), lithium metasilicate (Li ₂ SiO ₃ ), lithium disilicate (Li ₂ Si ₂ O ₅ ), meta trisilicate Lithium acid (Li ₄ Si ₃ O ₈ ), lithium metatetrasilicate (Li ₆ Si ₄ O ₁₁ ), or the like may be used. The lithium silicate is not limited to these examples, for _example, typified by _{Li 8 SiO 6, Li 6 Si} 2 O 7, Li 12 SiO 8 or the like may be a compound. These may be a single phase composed of any one kind, or may be a mixed phase containing any two or more kinds in combination. Lithium silicate, those most essential to have the backbone is a single SiO ₄ tetrahedra or Si linked (e.g. SiO ₄ connecting body), alkali metal elements such as Li in the pores of the tetrahedron Is considered to be ionized. The inventors believe that the SiO ₄ tetrahedron can undergo isomorphous substitution with other tetrahedrons according to the manufacturing method disclosed herein. Such is not particularly limited as substituents, for example, typically, AlO and ₄ tetrahedra, FeO ₄ tetrahedra, GeO ₄ tetrahedra, SnO ₄ tetrahedron or the like is considered.

Accordingly, the lithium silicate disclosed herein may be a compound containing another element (M) in addition to the above Li, Si, and O. Such other elements are not particularly limited as long as they are elements constituting a compound that can exist stably under the conditions of absorption and desorption of CO ₂ . For example, the same group 14 element germanium (Ge), tin (Sn), lead (Pb), etc. as silicon, such as said aluminum (Al) and iron (Fe), may be sufficient. Among these, a germanium (Ge) component is preferable. Such a group 14 element is preferable because it easily substitutes for Si in the crystal structure of lithium silicate and can exist relatively stably. Although the ratio of the other elements contained in lithium silicate is not strictly limited, for example, the ratio of silicon (Si): other elements (M) is about 1: 0.001 to 1: 0.5. It is preferable that the ratio is 1: 0.04 to 1: 0.45. Such lithium silicate can be understood as, for example, various compounds represented by the general formula: Li _x M _y2 Si _y1 O _z . Here, x, y1, y2 and z in the formula are all natural numbers and satisfy y1 + y2 = y and x + 4y−2z = 0.

As a representative example of the CO ₂ absorbent material disclosed herein, from the viewpoint of CO ₂ absorption capacity, it is preferable that lithium orthosilicate is 70 mol% or more, more preferably 80 mol% or more of lithium silicate. Particularly preferably, it is 90 mol% or more, for example, substantially 100 mol%. Hereinafter, as the lithium silicate, the present invention will be described by taking, for example, a case where the lithium orthosilicate is substantially 100 mol% as an example. Note that the composition of the above lithium silicate can be allowed to change depending on the surrounding environment, for example.

Further, “having lithium silicate as a main component” means that lithium silicate accounts for the largest proportion in the compound constituting the CO ₂ absorbing material. Ratio of lithium silicate to total CO ₂ absorbing material, although can not be strictly defined in consideration of the CO ₂ absorbing material in the form of a complex which will be described later, is typically at least 50 wt%, preferably 60 wt% (Particularly preferably 70% by mass or more, for example, 80% by mass or more, 90% by mass or more, 95% by mass or more, substantially 100% by mass). For example, the ratio of the lithium silicate can be calculated based on X-ray diffraction analysis (XRD) analysis.

The manufacturing method of the CO ₂ absorbing material, for example as shown in FIG. 1A, is characterized in that it comprises the following steps (S1) ~ (S3).
(S1) Preparing a sol composition in which a Li—Si precursor compound containing a lithium (Li) component and a silicon (Si) component is dispersed in an aqueous solution.
(S2) Obtaining a gel composition by irradiating the sol composition with electromagnetic waves.
(S3) Obtaining lithium silicate containing lithium and silicon by firing the gel composition.
Although not an indispensable step, as shown in FIG. 1B, the manufacturing method disclosed herein can also implement the following step (S4) following the above step (S3).
(S4) Compounding the obtained lithium silicate with an alkali carbonate.

S1. Preparation of sol-like composition The CO ₂ absorbing material disclosed herein can be preferably adjusted preferably by a wet method in which a sol-like composition containing a Li-Si precursor compound is gelled.
Here, the sol-like composition may be a colloidal aqueous solution containing a Li—Si precursor compound as a dispersoid and at least water as a dispersion medium. The water as the dispersion medium may be distilled water, ion exchange water, pure water, or the like. As long as the dispersion medium is mainly composed of water, a water-soluble low molecular weight organic solvent such as a lower alcohol (methanol, ethanol, butanol, isopropanol), a ketone such as ethylene glycol or acetone may be mixed. The dispersion medium is preferably composed of 100% by mass of water in order to increase the efficiency of hydrolysis described later (that is, the sol composition can be a hydrosol).

  The Li-Si precursor compound as a dispersoid contains a lithium component and a silicon component as raw materials for the above lithium silicate, and the above silicic acid is obtained by a treatment such as a dehydration condensation reaction, a polycondensation reaction, and firing (heating). It can be any compound that can form lithium. Typically, it may be a precipitate in which an aqueous solution of lithium and silicon is precipitated as a hydroxide or the like, and these may be in the form of a hydrate or a hydrated complex. Further, the precipitate such as hydroxide may be dehydrated and condensed as long as it can maintain the sol state. In other words, the Li—Si precursor compound is a sol-like colloidal particle, and its particle size and the like are not strictly limited as long as it is a size capable of maintaining the colloidal solution. For example, the average particle size of the Li—Si precursor compound is typically about 1 nm to 5 μm, preferably about 3 nm to 3 μm, and more preferably about 10 nm to 1 μm. As the average particle diameter, a value measured by a dynamic light scattering method can be adopted.

Such a sol composition may be prepared, for example, by dispersing a separately prepared Li-Si precursor compound in a dispersion medium such as water, or the Li-Si precursor compound is dispersed in advance. The sol composition may be obtained, or may be prepared by using a known dry method such as a combustion method or an arc method, or a wet method such as a sedimentation method or a gel method (including a sol-gel method). Good.
As an example, when preparing a sol composition by a general sol-gel method, in a mixed solution in which a lithium salt and a silica alkoxide can be in a dissolved state, the alkoxide is brought into contact with water to cause hydrolysis. A precursor compound containing the component and the Si component can be formed in this mixed solution. At this time, since the sol composition preferably uses water as a dispersion medium, it is more preferable to prepare the sol composition by adding alkoxide little by little to an aqueous solution in which a lithium salt is dissolved. preferable. For example, the precursor compound can be obtained by adding an alcohol solution of silica alkoxide to this aqueous solution while stirring the aqueous solution in which the lithium salt is dissolved. Stirring can be performed using other means such as, for example, a magnetic stirrer, mechanical stirrer, or ultrasonic stirrer.

  When proceeding with the hydrolysis reaction and condensation reaction, an acid such as hydrochloric acid or an alkali such as ammonia is added to the mixed solution as a hydrolysis catalyst for the purpose of adjusting the reaction rate (hydrolysis rate and condensation rate). Also good. In addition, this hydrolysis catalyst can also play the role which controls the pH of a reaction solution and adjusts the primary particle size of the precursor compound formed.

  As the lithium salt, an oxide of lithium or various compounds that can be converted into an oxide by heating can be used. Specific examples include lithium oxides, hydroxides, carbonates, nitrates, sulfates, phosphates, acetates, formates, oxalates, halides, and the like. Preferably, it may be various water-soluble salts. Alternatively, lithium alkoxide can be used.

As the silica alkoxide, various compounds that can be used in the sol-gel method can be used without particular limitation. For example, specifically, methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, 1,2-bistrimethoxysilylethane, silica alkoxide having 1 to 4 alkoxy groups bonded to Si atom, glycidyl A silica alkoxide having a functional group such as a group introduced therein can be preferably used. Of these, alkoxysilane is preferably used, and examples thereof include tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, and 1,2-bistrimethoxysilylethane. Silica alkoxide in which 1 to 4 alkoxy groups are bonded to Si atoms is shown as a preferred example. Particularly preferably, tetraethoxysilane (Si (OC ₂ H ₅ ) ₄ : TEOS, also referred to as tetraethyl orthosilicate), tetramethoxysilane (Si (OCH ₃ ) ₄ : TMOS, also referred to as tetramethyl orthosilicate), methyltrimethoxy Silane. Any one of these may be used alone, or any two or more of them may be used in combination.

  In addition, as a preparation method of sol-like compositions other than the sol-gel method illustrated above, employ | adopting the following method is illustrated, for example. For example, as a silicon component, a nano silica material such as gel method silica (including colloidal silica), precipitated silica, fumed silica (including silica xerogel, silica cryogel, and silica aerogel) is used, and the same as described above. A sol-like composition can be prepared. Although not particularly limited, such a silica material and a lithium component may form a complex with each other in an aqueous solution, or may form a compound by a hydrolysis reaction and / or a dehydration condensation reaction, for example. Therefore, it can be suitably used as the sol composition disclosed herein.

  Moreover, what is necessary is just to prepare so that the said Li-Si precursor compound may contain the said other element, when lithium silicate contains another element (M). For example, when preparing a sol composition by the sol-gel method, another element (M) may be added to the mixed solution. Typically, as illustrated in FIG. 1C, a salt of another element (M) or the like may be added to the mixed solution in the same manner as the lithium component. Thereby, lithium silicate containing Li, Si, O, and M can be manufactured in the same procedure.

S2. Formation of Gel Composition by Electromagnetic Irradiation Next, the prepared sol composition is irradiated with electromagnetic waves to gel the sol composition to obtain a gel composition.
For example, in a sol-like composition prepared by a sol-gel method, generally, a precipitate such as a hydrate formed by hydrolysis may undergo dehydration condensation by subsequent stirring. Since such a condensation reaction proceeds in a chain manner between adjacent Li—Si precursor compounds, the Li—Si precursor compound can form a three-dimensional condensate by continuing stirring. This condensate imparts viscosity to the sol-like composition and changes it into a gel-like composition. Therefore, a gel composition can be naturally obtained by stirring the sol composition for a certain time. In addition, as described above, it is also known that the formation of a gel composition can be promoted by adding a hydrolysis catalyst or adjusting an environmental temperature or the like.

On the other hand, in the technique disclosed here, the gelation is more preferably promoted by irradiating the sol composition with electromagnetic waves. Although the stirring operation is not necessarily required, it is more preferable to irradiate the electromagnetic wave while stirring the sol composition from the viewpoint of uniformly proceeding the gelation of the sol composition. Although the detailed mechanism is not clear, it is considered that irradiation of electromagnetic waves to the sol composition will cause some change in the skeleton structure of the Li-Si precursor compound in the formed gel composition. Can do. Due to such a change in the skeleton structure, the lithium silicate obtained from the gel composition can be obtained as a powder having a special form different from usual. According to the study by the present inventors, due to the action of electromagnetic waves, specificity is manifested in the coupling mode of the above-mentioned SiO ₄ tetrahedron, and the SiO ₄ tetrahedron arrangement (for example, the coupling angle and the degree of coupling) is anisotropic. I think that you can see.

Therefore, the shape of the individual lithium silicate crystal particles constituting the CO ₂ absorbing material is not particularly limited, but is typically not so-called spherical or granular, for example, rod-shaped, plate-shaped, scale-shaped Shape, petal-like, sponge-like and the like can have high shape anisotropy. Based on such a special form of lithium silicate, lithium silicate disclosed herein can be considered as the CO ₂ absorbing properties such as CO ₂ absorption capacity and CO ₂ absorption rate and the like are effectively improved . For example, lithium silicate having a high CO ₂ absorption capacity close to the theoretical value can be produced. Still, the desorption of the absorbed CO ₂ can be performed well. Therefore, in the technique disclosed herein, irradiation with electromagnetic waves can be an indispensable operation for improving CO ₂ absorption characteristics. Moreover, a gel-like composition can also be obtained in a short time by the gelatinization by irradiation of electromagnetic waves. Thereby, it can be said that it is preferable also in that the manufacturing time of the CO ₂ absorbent material can be shortened.

  As the electromagnetic wave, any of ultra-low frequency, long wave, medium wave, short wave, microwave (high frequency), infrared ray, visible ray, ultraviolet ray, X-ray, and gamma ray can be used. However, from the viewpoint of electromagnetic wave irradiation efficiency, it is preferable to employ an electromagnetic wave that can effectively heat the sol composition. From this viewpoint, the microwave is directly absorbed by water molecules such as water as a dispersion medium of the sol-like composition and water molecules such as crystal water and hydrated water contained in the Li-Si precursor compound. Can generate heat and heat the Li-Si precursor compound. This microwave heating (dielectric heating) may be a preferred embodiment because the Li—Si precursor compound can be heated from the inside at high speed and with low energy loss.

  The frequency (wavelength), output, irradiation time, etc. of the microwave are not particularly limited. For example, it can be determined as appropriate so that the amount of energy necessary for gelation of the sol composition to be irradiated can be appropriately supplied. For example, a microwave having a wavelength of 1 mm to 1 m and a frequency of 300 MHz to 300 GHz is used. As for the frequency, it may be more appropriate to use a high frequency generated by a 2.45 GHz band magnetron based on an international standard (may be a high frequency in the 915 MHz band depending on the region). For example, the output is suitably about 300 W to 300 kW from the viewpoint of heating efficiency, preferably about 300 W to 10 kW, more preferably about 300 W to 2000 W, and particularly preferably about 500 W to 1600 W.

  Further, the irradiation time can be adjusted in consideration of the output, the amount of the sol-like composition, the form at the time of irradiation (degree of microwave arrival), and the like. Moreover, for example, the microwave may be continuously irradiated over a predetermined irradiation time, or may be irradiated a plurality of times through an interval. As a preferred form of microwave irradiation, a microwave with a predetermined output is irradiated until the dispersion medium in the sol-like composition boils, and after cooling the sol-like composition, a microwave with a predetermined output is again applied. Irradiation is repeated a predetermined number of times. Here, when the dispersion medium is volatilized by boiling, the dispersion medium may be supplemented as necessary. The microwave irradiation time is, for example, as one example, when producing about 0.2 to 0.5 mol of lithium silicate, the range of 600 W to 1000 W (eg 700 W) at 2.45 GHz is 1 minute in total. Illustrating irradiation for about 20 minutes or less (for example, 4 to 12 minutes).

  The gel-like composition may be a state where the sol-like composition is almost lost in fluidity and cured. Although it is difficult to strictly define the state of the gel, typically, it is (i) a cohesive dispersion system having a composition of at least two components of a dispersoid and a dispersion medium. Thus, it can be defined as (ii) a mechanical behavior having characteristics of a solid, and (iii) a state where both the dispersoid and the dispersion medium are continuously (homogeneously) spread throughout the system. The gel-like composition in the present specification may be a chemical gel in which almost all dispersoids are three-dimensionally linked by a covalent bond through a clear sol-gel transition well known to those skilled in the art. That is, the dispersoid can have substantially the same composition as the target lithium silicate. Such a dispersoid becomes an amorphous (that is, non-crystalline) substance composed of a three-dimensional but discontinuously bonded structure. The dispersoid is impregnated with a dispersion medium to form a gel composition.

S3. Firing of lithium silicate The gel composition thus obtained is dried and fired. Thereby, excess components other than the dispersion medium and lithium silicate are completely removed, and the target solid lithium silicate can be obtained. Even if the gel composition contains a portion where the hydrolysis reaction and dehydration condensation reaction are not completely completed, these reactions are promoted by firing, and the gel composition is typically crystalline. It is transformed into a dense solid.
Drying may be natural drying or drying using a dryer. Firing may be performed using a general heating furnace. Furthermore, drying and baking may be performed in combination. In this case, for example, an air oven furnace can be used, or a supercritical drying method, a spray drying method, a spray granulation method, a spray pyrolysis method, or the like can be used. Any one of these drying and firing methods may be carried out alone or in combination with each other.

  The firing conditions are not particularly limited as long as the gel composition can be changed into lithium silicate. For example, it is sufficient if the Li—Si precursor compound in an amorphous state can be crystallized into lithium silicate. Specifically, when the firing temperature is about 500 ° C. or lower, the firing atmosphere is preferably an oxygen-containing atmosphere (typically an air atmosphere) so that the oxidation of the gel composition proceeds appropriately. . When the firing temperature is approximately 500 ° C. or higher (excess), the firing atmosphere may be an oxygen-containing atmosphere (typically an air atmosphere), for example, air, nitrogen gas, carbon dioxide gas, or the like. Two or more mixed gases may be used. The firing temperature is suitably higher than 473 ° C. (preferably 500 ° C. or higher). The firing temperature is more preferably 700 ° C. or higher, and particularly preferably 800 ° C. or higher. Thereby, while being able to remove the excess component originating in the used raw material, highly crystalline lithium silicate can be obtained. The upper limit of the firing temperature is not particularly limited, but can be, for example, 1100 ° C. or lower, 1000 ° C. or lower, typically about 900 ° C. The firing time is not particularly limited, but for example, about 30 minutes to 5 hours is appropriate, 1 hour to 5 hours is more preferable, and 2 hours to 4 hours is particularly preferable.

In the CO ₂ absorbing material realized in this way, lithium silicate is obtained from particles having a typical spherical shape, granular shape or angular shape (primary particles), or particles having a special shape (primary particles). Obtained as a composed powder. This special form is a shape having high anisotropy, and may be, for example, a rod shape, a plate shape, a scale shape, or the like. Moreover, lithium silicate can comprise a powder in the form of secondary particles in which these primary particles are bonded. The secondary particles may be granular, for example, or may be in a special form expressed as, for example, a petal shape or a spongy shape.

  In the case where lithium silicate is composed of spherical particles or square particles having little anisotropy, the average particle diameter thereof is not strictly limited, but is, for example, about 10 nm to 1 μm, 20 nm to 500 nm. It may be about. As such dimensions, for example, an arithmetic average value of equivalent circle diameters measured based on observation with an electron microscope can be adopted.

  Further, for example, typically, lithium silicate is obtained as a powder composed of an aggregate of needle-like (fiber-like) or rod-like (rod-like) particles microscopically (for example, on the order of nanometers). Such rod-like particles refer to crystal forms that have grown greatly in one direction (one dimension). Here, when the dimension in the longitudinal direction of the crystal is a and the dimension in one short direction perpendicular to the longitudinal direction is b, the average dimension b in the short direction is approximately 100 nm or less, preferably 50 nm or less. And particularly preferably 30 nm or less. The aspect ratio defined by a / b is typically 2 or more, preferably 5 or more, for example, 10 or more, and particularly preferably 15 or more. Such dimensions can also be measured based on, for example, observation with an electron microscope (the same applies hereinafter).

Further, the lithium silicate may be a powder made of a plate-like crystal in which the above spherical particles and rod-like particles are integrated together in a slightly macroscopic manner (for example, on the order of micrometers).
Here, the plate shape is expressed as a leaf shape, a mica shape, a thin plate shape, a layer shape, or the like, and means a crystal form that grows greatly in a plane direction (two-dimensional). Typically, it is a relatively flat lamellar crystal. Here, when the dimension in the longitudinal direction in the crystal plane is a, and the dimension in one thickness direction perpendicular to the longitudinal direction is b, the average dimension b in the lateral direction is approximately 10 μm or less, preferably It may be 5 μm or less, particularly preferably 1 μm or less. Moreover, although the dimension of a longitudinal direction is not specifically limited, it is about 50 micrometers or less in general. The aspect ratio defined by a / b for this plate-like crystal is typically 2 or more, preferably 3 or more, for example, 5 or more, and particularly preferably 10 or more. One plate-like crystal may constitute a particle alone, or a plurality of plate crystals may be aggregated (coupled) to constitute one particle. When multiple crystals are assembled, they may be twinned. When the plate-like crystal is an aggregate of a plurality of primary particles, such plate-like crystal may include fine voids in the gaps between the primary particles. The size of such voids is not particularly limited, and may be, for example, a mesopore having a diameter of 2 nm or more and less than 50 nm, or a macropore having a diameter of 50 nm or more.

  Scale-like and petal-like shapes can be understood as a kind of plate-like deformation, and are crystal forms that grow greatly in the plane direction. Among them, the scale-like shape refers to a crystal form in which a plurality of plate-like crystals, for example, are generally aligned in the plane direction and are gathered like fish scales. The petal shape refers to a crystal form in which, for example, a plurality of plate-like crystals are arranged in different plane directions, and the petals made up of a single crystal gather together to form a flower. Such a special shape is considered to be a form in which unique crystal habits are strongly reflected by the action of electromagnetic waves peculiar to the above manufacturing method. The particle size of such particles is not particularly limited, but may be, for example, about 0.1 μm to 20 μm.

  The spongy form is expressed as a sponge form, a honeycomb form, or the like, and has a large bulk as a whole, but is a crystal form including a large number of voids through a thin plate-like wall (wall surface). About this wall part, it can also be understood similarly to said plate-like crystal. The size of the voids contained in the particles is not particularly limited, but may be, for example, mesopores or macropores. It can typically be a macropore. The particle size of such spongy particles is not particularly limited, and may be, for example, about 0.1 μm to 20 μm.

In the lithium silicate disclosed herein, particles having such a characteristic shape may occupy 50% by number or more, for example, 70% by number or more, particularly 80% by number, of the entire particles constituting the lithium silicate. Is preferred. Such a characteristic shape can provide high CO ₂ absorption characteristics close to the theoretical values. In addition to absorbing CO ₂ , desorption can be achieved with high efficiency. Furthermore, even when the absorption and release of CO ₂ at high temperature are repeated, the aggregation of crystal particles can be suppressed and the decrease in CO ₂ absorption ability can be suppressed.

In addition, the CO ₂ absorbing material realized in this way has a CO ₂ absorption rate of about 1.5 to 2 times compared to, for example, lithium silicate manufactured by the known sol-gel method using the same material. It can be as high as twice. Moreover, such CO ₂ absorption ability is expressed from a lower temperature, and can absorb CO _{2 in} a temperature range of about 200 ° C. or more and about 710 ° C., for example. In the temperature range of 450 ° C. or higher, CO ₂ can be absorbed at a higher speed. For example, the CO ₂ absorption rate per unit weight in the temperature range of 500 ° C. or more and 710 ° C. or less may be 20 mg / (g · min) or more, and more preferably 25 mg / (g · min) or more. Further, the CO ₂ gas absorption rate per unit weight in the temperature range of 600 ° C. or more and 710 ° C. or less can be 30 mg / (g · min) or more, more preferably 50 mg / (g · min) or more. . That is, by the technique disclosed herein, a CO ₂ absorbing material capable of exhibiting excellent CO ₂ absorbing ability in a temperature range of about 200 ° C. or higher and about 710 ° C., particularly preferably 450 ° C. or higher and 710 ° C. or lower is realized. Note that the CO ₂ absorbent material disclosed herein begins to release CO ₂ at a temperature of approximately 720 ° C. or higher (eg, 750 ° C. or higher). Therefore, up to the temperature of such CO ₂ release (for example, less than 720 ° C.), CO ₂ can be absorbed by the CO ₂ absorbing material.

S4. Compounding of lithium silicate and alkali carbonate As described above, lithium silicate obtained as described above can be used as it is as a CO ₂ absorbing material, but, for example, compounded with alkali carbonate. By doing so, it is possible to obtain a CO ₂ absorbent material with further improved CO ₂ absorption capacity.
The alkali carbonate can be softened or become a liquid phase in the CO ₂ absorption temperature range of the lithium silicate disclosed herein. The presence of such alkali carbonate is thought to facilitate the transfer of CO ₂ to the lithium silicate. Thereby, the absorption capacity of CO ₂ on the lower temperature side can be greatly increased, and the absorption rate of CO ₂ can also be increased.

  Alkali carbonates include alkali metal carbonates such as sodium (Na), potassium (K), lithium (Li), rubidium (Rb), cesium (Cs), and francium (Fr) components. be able to. These may be any one kind of alkali metal carbonate, or may be a carbonate containing two or more kinds of alkali metals. In the technique disclosed herein, the alkali carbonate preferably contains one or more of a sodium component, a potassium component, and a lithium component, more preferably any two or more, and three. It is particularly preferred.

Further, when the alkali carbonate is a mixed system of three kinds of alkali metal carbonates of Li, K, and Na, these are preferably, for example, solid solution crystals or eutectic crystals. In the case of a solid solution, the ratio of Na, K, Li in the alkali carbonate is preferably as follows. Such a composition may also include a eutectic composition.
Na: 1 mol% to 80 mol% K: 1 mol% to 70 mol% Li: 1 mol% to 90 mol%

When the alkali carbonate is a eutectic, the eutectic point becomes lower, which is preferable because the CO ₂ absorption capacity and the CO ₂ absorption rate can be further improved. In the K-Li carbonate, the eutectic composition is a mixed system in which the molar ratio of K ₂ CO ₃ and Li ₂ CO ₃ is about 55:45. When the alkali carbonate is a K-Li carbonate, the molar ratio of K to Li (K: Li) is preferably in the range of about 60:40 to 40:60 including such a eutectic composition. The eutectic composition of Na-Li carbonate is when the molar ratio of Na ₂ CO ₃ and Li ₂ CO ₃ is about 49:51. Therefore, when the alkali carbonate is Na-Li carbonate, the molar ratio of Na to Li (Na: Li) is preferably in the range of about 55:45 to 45:55 including the eutectic composition. And in Na-K-Li-based carbonate, a eutectic composition may be when approximately 31:35:34 by _Na 2 CO ₃ and _K 2 CO ₃ and _Li 2 CO ₃ and the molar ratio. Therefore, when the alkali carbonate is a Na-K-Li carbonate, the molar ratio of Na, K and Li (Na: K: Li) is about 25 to 35:30 to 40:30 including such a eutectic composition. It is preferable to be in the range of about 40. Among these, the alkali carbonate is particularly preferably a Li—Na—K-based eutectic carbonate.

Although not necessarily limited to this example, the average particle size of the alkali carbonate used for the composite is typically about 50 nm to 5 μm, and preferably 100 nm to 1 μm. 200 nm to 500 nm is particularly preferable.
The proportion of the alkali carbonate complexed with lithium silicate is suitably, for example, 100 parts by mass of lithium silicate and 1 to 50 parts by mass of the alkali carbonate, and 5 parts by mass. The amount is preferably 40 parts by mass or less and particularly preferably 10 parts by mass or more and 30 parts by mass or less.

At the time of compounding, after sufficiently mixing lithium silicate and alkali carbonate, for example, the two can be integrated by firing under the above firing conditions. Thus, lithium silicate and an alkali carbonate can be produced CO ₂ absorbing material is complexed.

The CO ₂ absorbing material realized in this way is, for example, one whose CO ₂ absorption capacity is increased by about twice or more compared to lithium silicate produced by the known sol-gel method using the same material. It can be. Moreover, such CO ₂ absorption ability is expressed from a lower temperature, and can absorb CO _{2 in} a temperature range of about 200 ° C. or more and about 710 ° C., for example. In particular, the CO ₂ absorption capacity from a temperature range of 300 ° C. or higher can be greatly improved. Therefore, for example, in the temperature range of 400 ° C. or higher, particularly 450 ° C. or higher, CO ₂ can be absorbed at a higher speed. For example, the CO ₂ absorption rate per unit weight in the temperature range of 450 ° C. or more and 650 ° C. or less may be 40 mg / (g · min) or more. Further, the CO ₂ absorption rate per unit weight in the temperature range of 500 ° C. or more and 650 ° C. or less can be 50 mg / (g · min) or more, more preferably 80 mg / (g · min) or more, and particularly preferably 100 mg. / (G · min) or more. Further, the CO ₂ gas absorption rate per unit weight in the temperature range of 600 ° C. or more and 650 ° C. or less can be 100 mg / (g · min) or more, preferably 150 mg / (g · min) or more, more preferably It may be 200 mg / (g · min) or more. That is, by the technique disclosed herein, a CO ₂ absorbing material capable of exhibiting excellent CO ₂ absorbing ability in a temperature range of about 200 ° C. or higher and about 710 ° C., particularly preferably 350 ° C. or higher and 710 ° C. or lower is realized.

Hereinafter, the manufacturing method of the CO ₂ absorbent material disclosed herein will be described with reference to specific embodiments. However, it is not intended that the present invention be limited to the following examples.

Reference example. Sol-gel method Lithium orthosilicate was synthesized using lithium nitrate (LiNO ₃ , manufactured by Alfa Aesar) and colloidal silica (manufactured by Sigma-Aldrich) as starting materials.
First, 15.3 g of LiNO ₃ was dissolved in 225 mL of distilled water to prepare a 1M lithium nitrate aqueous solution. Then, while this lithium nitrate aqueous solution was stirred at room temperature at a constant rate, 25% ammonium hydroxide (manufactured by SD Fine-Chem Limited) was slowly added until the pH reached 8 to perform hydrolysis. Subsequently, 3.3 g of colloidal silica was added dropwise to the reaction aqueous solution while stirring was continued, and stirring was continued for 1 hour to obtain a sol composition. The amount of colloidal silica added is such that the molar ratio of Li: Si is 4: 1 with respect to the amount of lithium nitrate. In this sol composition, it is considered that the precursor of lithium silicate is mainly precipitated as a hydroxide in the form of a hydrated complex. This sol-like composition was further cured at room temperature for 24 hours, dehydrated and condensed, dried at 110 ° C., and calcined at 800 ° C. for 3 hours to obtain powdered lithium orthosilicate. The lithium orthosilicate thus obtained is denoted as SG.

1-1. Production of Lithium Orthosilicate Lithium orthosilicate was synthesized according to the technique disclosed herein using lithium nitrate (LiNO ₃ , Alfa Aesar) and colloidal silica (Sigma Aldrich) as starting materials.
First, as in the above Reference Example, hydrolysis was performed by adding 25% ammonium hydroxide to pH 8 while stirring an aqueous lithium nitrate solution at room temperature. Colloidal silica was dropped into the reaction aqueous solution and stirred for 1 hour using a magnetic stirrer to obtain a sol composition in which the lithium silicate precursor was dispersed in the aqueous solution. Next, a dehydration condensation reaction was performed by irradiating the sol composition with 700 W electromagnetic waves at 2.45 GHz. A microwave oven was used for electromagnetic wave irradiation, and irradiation for 2 minutes was performed 5 times for a total of 10 minutes. Specifically, after the sol composition is boiled by irradiating with electromagnetic waves for 2 minutes, a rest period is provided to cool the sol composition to room temperature, and the amount of water evaporated by boiling is compensated to the initial amount. Was repeated five times. The dehydrated and condensed gel-like composition (reactant) was dried at 110 ° C. and calcined in an air atmosphere at 800 ° C. for 3 hours to obtain a powder. The powder sample thus obtained is denoted as MW-SG.

1-2. XRD
The powder sample (MW-SG) obtained as described above was subjected to X-ray diffraction analysis (XRD) analysis. For sample identification, X-ray small angle / wide angle diffraction setup using Cu Kα rays (λ = 0.154 nm) (Xenocs, France, Xeuss SAXS / WAXS system, 2θ = 4 ° -36 ° and Netherlands, Eindhoven, manufactured by PANalytical, X'pert Pro diffractometer, 2θ = 10 ° to 90 °) was used. The obtained XRD pattern is shown in FIG.
From this XRD pattern, the obtained powder has high crystallinity, and most of the diffraction peaks are nos. It was found to correspond to the lithium orthosilicate phase (Li ₄ SiO ₄ ) of 37-1472. Some diffraction peaks were attributed to lithium metasilicate (Li ₂ SiO ₃ , JCPDS card No. 29-0828), which is the reaction of lithium orthosilicate to absorb CO ₂ in the atmosphere at room temperature. It was thought that this was caused by
Li ₄ SiO ₄ + CO ₂ → Li ₂ SiO ₃ + Li ₂ CO ₃

1-3. TEM observation In addition, the powder sample (MW-SG) was observed with a transmission electron microscope (TEM). For TEM observation, Tecnai G2 manufactured by FEI, The Netherlands was used. The obtained TEM image is shown in FIG. FIG. 3A shows the observation result of the MW-SG sample immediately after production, and FIG. 3B shows the observation result of the MW-SG sample after evaluation of the CO ₂ absorption characteristics described later.
As a result of TEM observation, MW-SG has a rod-like particle aggregated to form a powder, and the dimension (diameter) in the width direction of the rod is approximately 100 nm or less, typically 50 nm or less. The average diameter in MW-SG of a) was about 20 nm. Further, it was confirmed that the aspect ratio of the rod-shaped particles was surely larger than 2 and was approximately 5 or more and approximately 10 or more. In addition, as shown in FIG. 3B, the MW-SG sample showed no significant change in form before and after heating in the evaluation of the CO ₂ absorption characteristics, and it was found that aggregation due to heating hardly occurred. .

1-4. CO ₂ absorption characteristics <dynamic absorption characteristics>
The carbon dioxide absorption characteristics of the powder sample (MW-SG) were evaluated as dynamic absorption characteristics with the ambient temperature changed. Specifically, thermogravimetric analysis (TGA) measures the amount of detachable gas generated by absorbing CO ₂ as a probe molecule in a predetermined amount of MW-SG sample and continuously increasing the temperature of the sample. It was evaluated by. The measurement was performed by the N ₂ purge method, using 100% CO ₂ gas as an absorption gas, and measuring at a temperature rising rate of 20 ° C./min in a temperature range of 100 ° C. to 800 ° C. The obtained dynamic TGA curve is shown in FIG. In FIG. 4, the dynamic TGA curve about SG sample produced by the conventional sol-gel method was also shown for the comparison.

MW-SG prepared by the technique disclosed herein begins to absorb CO ₂ from a lower temperature than conventional SG, and the CO ₂ absorption temperature range is about 450 ° C. to 710 ° C. In addition, it was confirmed that CO ₂ emission occurred at about 750 ° C. or higher. The CO ₂ absorption capacity of the MW-SG was found to have about twice near the absorption amount of the SG in exceed 200 mg / g, a dynamic absorption. Further, it was confirmed that the CO ₂ absorption rate was faster than SG from the rise of the TGA curve, and the CO ₂ absorption performance was excellent.

<Isothermal adsorption characteristics>
Next, the isothermal absorption characteristic of carbon dioxide in the powder sample (MW-SG) was evaluated by changing the measurement temperature in various ways. The measurement temperature was set every 50 ° C. within the range of 400 ° C. to 700 ° C. In the isothermal absorption test, 100% CO ₂ gas was used, and the MW-SG sample was heated to a predetermined measurement temperature at a heating rate of 10 ° C./min, and then held for 2 hours (7200 seconds) in a 100% CO ₂ stream. The CO ₂ absorption capacity at the temperature was measured. The obtained isothermal absorption curve is shown in FIG.
As can be seen from FIG. 5, the higher the temperature, the faster the CO ₂ absorption rate. For example, when the measurement temperature is 700 ° C., the equilibrium is reached in about 15 minutes. Further, when the measurement temperature was as low as 400 ° C., it was found that the CO ₂ absorption was performed although the CO ₂ absorption amount was reduced.

The absorption rate was calculated from the rising slope of the isothermal absorption curve in FIG. Specifically, the slope of the isothermal absorption curve for 2 minutes from the start of measurement was taken as the absorption rate. The results are shown in FIG.
As is apparent from FIG. 6, in the temperature range of about 650 ° C. to 700 ° C., the CO ₂ absorption rate is 50 mg / (g · min) or more, for example, at 700 ° C., it is 100 mg / (g · min) or more. I understood.

1-5. Cycle characteristics The cycle characteristics of CO ₂ absorption when absorption and desorption of carbon dioxide were repeatedly performed at a high temperature of 600 ° C. or higher for the powder sample (MW-SG) were evaluated. Specifically, using a TGA apparatus, the MW-SG sample is heated to a predetermined CO ₂ absorption temperature of 700 ° C. at a rate of 10 ° C./min, and held at this absorption temperature for 20 minutes under a CO ₂ stream. Te was absorbed CO _2. Next, the MW-SG sample was heated to 800 ° C., and CO ₂ was desorbed by maintaining the MW-SG sample at this desorption temperature in a N ₂ stream. From the second cycle of CO ₂ absorption temperature of 600 ° C., After cooling the MW-SG samples down to 600 ° C., it was then held under 20 minutes CO ₂ stream to absorb CO _2. Such absorption and desorption of CO ₂ were performed 10 times in total, and the change in the weight of the MW-SG sample at that time was examined. The results are shown in FIG.
As is clear from FIG. 7, it was confirmed that MW-SG had stable cycle characteristics with no variation in absorption characteristics even after repeated absorption and desorption of CO ₂ .

2-1. Preparation of alkaline carbonate / lithium orthosilicate composite First, a powdery MW-SG sample was prepared in the same manner as described above. Further, carbonates of sodium (Na), potassium (K) and lithium (Li) are added in the order of mass ratio (Na: K: Li) (1) (10:30:60), (2) ( 32:37:31) and (3) (31:45:24). These are the molar percentages of (Na: K: Li) and are equal to (1) 8.5: 19.5: 72, (2) 31:27:42, (3) 31:35:34. In addition, the alkali carbonate of (3) is a composition corresponding to the eutectic carbonate of Na—K—Li. These are mixed with alkali carbonate at a rate of 15 to 20 parts by mass with respect to 100 parts by mass of the MW-SG sample, and the mixture is heated to 800 ° C. at a rate of temperature increase of 1 ° C./min. Baked. The composite of alkali carbonate and lithium orthosilicate thus obtained was designated as MW-SG-NKL1 to NKL3 according to the alkali composition.

2-2. CO ₂ absorption characteristics <dynamic absorption characteristics>
The carbon dioxide absorption characteristics of the obtained alkali carbonate / lithium orthosilicate composite (MW-SG-NKL1 to NKL3) were set as dynamic absorption characteristics with the ambient temperature changed in the same manner as in the above 1-4 column. evaluated. As a result, the obtained dynamic TGA curve is shown in FIG. For reference, FIG. 8 also shows the results for SG prepared by a conventional sol-gel method and MW-SG to which no alkali carbonate is added.
It was found that MW-SG-NKL1 to NKL3 complexed with alkali carbonate all increased CO ₂ absorption capacity in dynamic absorption by about 1.5 to 3 times compared to MW-SG. . In addition, a change was also seen in the CO ₂ absorption characteristics, and the amount of CO ₂ absorption at a lower temperature range of about 300 ° C. to 650 ° C. increased, and it was confirmed that the absorption rate at this low temperature range was greatly increased. It was done. This tendency was most noticeable for MW-SG-NKL3 in which a eutectic carbonate of Na, K, Li was combined. It is presumed that these alkali carbonates were softened or liquefied at 400 ° C. to 500 ° C., and this molten carbonate improved the absorption and diffusibility of CO ₂ at lower temperatures.

<Isothermal absorption characteristics>
Therefore, isothermal absorption characteristics of carbon dioxide were performed on MW-SG-NKL3 in the same manner as described in the above 1-4 column. Incidentally, MW-SG-NKL3, as shown in FIG. 8, 200 ° C. or higher, and typically begin to absorb CO ₂ from about 300 ° C., the CO ₂ absorption capacity at 700 ° C. CO ₂ absorption at 650 ° C. Decrease than capacity. Therefore, the test was performed every 350C in the temperature range from 350C to 650C. Further, the CO ₂ absorption rate was calculated from the slope of the curve for 2 minutes from the start of measurement of the obtained isothermal absorption curve. As for these results, the isothermal absorption curve is shown in FIG. 9, and the CO ₂ absorption rate is shown in FIG. About FIG. 10, the result about MW-SG to which SG produced by the conventional sol-gel method and the alkali carbonate were not added was also shown for reference.

As shown in FIG. 9, it was found that the MW-SG-NKL3 also increased in CO ₂ absorption rate as the temperature increased, and reached equilibrium in about 5 to 10 minutes when the measurement temperature was 650 ° C., for example. Also, it was found that CO ₂ absorption can be performed better when the measurement temperature is as low as 350 ° C. than when MW-SG is at 400 ° C.

As shown in FIG. 10, the CO ₂ absorption rate of MW-SG-NKL3 combined with alkali eutectic carbonate is not limited to the SG sample prepared by the conventional sol-gel method in all temperature ranges. It was confirmed that the value was significantly higher than that of the SG sample. The CO ₂ absorption rate of MW-SG-NKL3 rises rapidly to 26.7 mg / (g · min) at 400 ° C. to 450 ° C., and is higher than 450 ° C. at which the alkali eutectic carbonate softens and melts. It was found that It was found that the CO ₂ absorption rate of MW-SG-NKL3 at 600 ° C. was about 4 times or more that of MW-SG and about 5 times or more at 650 ° C.
As is apparent from FIG. 10, in the temperature range of about 600 ° C. to 650 ° C., the CO ₂ absorption rate is 200 mg / (g · min) or more, for example, a level reaching about 350 mg / (g · min) at 650 ° C. I found out that

From the above, the technology disclosed herein provides a CO ₂ absorbing material with an expanded CO ₂ absorption temperature range and improved CO ₂ absorption capacity and CO ₂ absorption rate, and a method for producing the same. In the above embodiment, an example in which colloidal silica is used as the silicon component is shown. Although a specific example is not shown, the present inventors use fumed silica instead of the colloidal silica or use a sol-like composition prepared by a sol-gel method using TEOS as a starting material. It has been confirmed that a CO ₂ absorbing material excellent in CO ₂ absorbing ability can be obtained as in the above examples. Therefore, those skilled in the art can prepare a sol composition by various methods other than those shown in the above examples.

3-1. Confirmation of Effect of Electromagnetic Irradiation In 1-1 above, lithium orthosilicate (Li ₄ SiO ₄ ) was synthesized using a sol-gel method using lithium nitrate (LiNO ₃ ) as a lithium raw material and colloidal silica as a silicon raw material. In the sol-gel reaction, gelation by hydrolysis was promoted by irradiating the sol-like reaction solution with electromagnetic waves.
Therefore, XRD analysis is performed on (a) colloidal silica and (b) lithium nitrate used as raw materials, (c) a sol solution before irradiation with electromagnetic waves, and (d) a gel solution after irradiation with electromagnetic waves. Thus, the constituent phases of each material were examined. In addition, about (c) sol-form or (d) gel-form solution, it analyzed about the powder obtained by drying each solution. The results are shown in FIG.

  As is clear from FIG. 11, (a) colloidal silica used as a raw material is amorphous, and (b) lithium nitrate is crystalline. Then, in the reaction solution (c) which has become a sol after the start of hydrolysis, it can be confirmed that although a crystalline lithium nitrate is still present, a diffraction peak due to the (101) plane of LiOH appears. . In the (d) reaction solution after the treatment with electromagnetic waves, the diffraction peak from the (101) plane of LiOH is relatively greatly increased, so that the hydrolysis is remarkably accelerated by the irradiation of electromagnetic waves. It was confirmed clearly. These results clearly show that partial or total substitution of lithium salt to LiOH can be a suitable route for producing high performance lithium silicate and is therefore disclosed in this embodiment. ing.

3-2. Phase change due to calcination Next, the dry powder of the gel solution after irradiation with the electromagnetic wave was subjected to heat treatment in air, and in-situ high-resolution X-ray diffraction (high-resolution X-ray diffraction) The change in crystal phase of this dry powder was followed by subjecting it to a diffraction (HTXRD) analysis. HTXRD analysis was performed at 746K, 773K, 1073K, 1176K, 1273K. The results of HTXRD analysis are shown in FIG. The powder sample after the heat treatment at an appropriate temperature corresponds to the powder sample (MW-SG) obtained in 1-1. In the following description, it is assumed that the heat treatment is performed in the air unless otherwise specified.

As shown in FIG. 12, the following could be confirmed from the HTXRD analysis.
• At 746K, the dry powder is completely amorphous.
In 773 K ·, the presence of _Li 2 SiO ₃ can be confirmed in a dry powder. Thus, nucleation (crystallization) of Li ₂ SiO ₃ occurs in the temperature range above 746K and below 773K.
In 1073 K ·, dry powder is a mixed phase of _Li 2 SiO ₃ (lithium metasilicate) and _Li 4 SiO ₄ (lithium orthosilicate), essentially is composed of _Li 4 SiO _4.
-With heat treatment up to 1073K, the diffraction intensity from the (110) plane and (011) plane of Li ₄ SiO ₄ increases with increasing temperature, and the diffraction intensity from the (111) plane of Li ₂ SiO ₃ decreases. Therefore, it is considered that the production of Li ₄ SiO ₄ proceeds by consuming Li ₂ SiO ₃ .
・ The peak slightly shifts to the left as the heat treatment temperature rises. This is due to growth stress introduced into the crystal due to volume expansion accompanying phase change from Li ₂ SiO ₃ to Li ₄ SiO ₄ . it is conceivable that.
-Although the temperature was raised to 1273K, when it exceeded 1073K, no change was observed in the constituent phases of the powder. That is, for the powder sample (MW-SG) having the above composition, it was confirmed that a stable phase such as Li ₄ SiO ₄ was obtained by heating at 1273 K or less.

3-3. Confirmation of particle form Further, the dried powder of the gel solution after irradiation with the electromagnetic wave was subjected to heat treatment at various temperatures for about 3 hours, and the form of the powder sample was observed using TEM. The heat treatment temperatures were 473K, 673K, 773K, and 1073K. The results are shown in FIGS. In FIG. 13, a and b are 473K, c and d are 673K, e and f are 773K, and g and h are TEM images of the gel-like solution dry powder heat-treated at 1073K, which are different at each heat treatment temperature. Two magnification images are shown. Note that the powder heat-treated at 1073 K for 3 hours corresponds to the powder sample (MW-SG) prepared in 1-1.

As shown in FIGS. 13a and 13b, it was confirmed that the powder heat-treated at 473K was composed of fine spherical particles. This spherical particle shape is considered to be derived from colloidal silica. Further, as shown in c and d, the presence of spherical particles could be confirmed in the powder heat-treated at 673K.
Here, the melting point of LiNO ₃ that is confirmed to exist in the dry powder before heat treatment in the XRD pattern of FIG. 11 (d) is about 528K. From the result of HTXRD analysis in FIG. Therefore, it is considered that the spherical particles observed at 673K are composed of an amorphous silica phase. Therefore, the amorphous silica phase in the colloidal silica used as the silica source in this example is not completely lost (decomposed) by hydrolysis and electromagnetic wave irradiation in the reaction solution, and keeps a spherical shape. It was confirmed that it was dried and used for firing. Since the surface of the silica particles is well wetted with the molten lithium salt, it is considered that the aggregation of the silica particles is suitably suppressed up to 673K even during drying and firing.

As shown in e and f of FIG. 13, in the powder heat-treated at 773 K, the nanofiber-like particles were clearly observed. From the result of HTXRD analysis in FIG. 12, it is known that a Li ₂ SiO ₃ phase is generated at 773K. Therefore, the nano-fibrous particles are determined to consist of _Li 2 SiO ₃ crystals. In consideration of the fact that amorphous silica particles existed in the molten lithium salt up to 673K, the formation of such nanofibrous crystals is special in that the spherical silica is in the molten lithium salt at temperatures above 673K. It is considered that nanofiber-like crystals with high anisotropy were formed by arranging with regularity and combining the SiO ₄ tetrahedron with a predetermined bond angle. That is, nucleation of Li ₂ SiO ₃ crystals occurs in amorphous silica particles that are coated with a molten lithium salt and are in a one-dimensional array, which favorably promotes the formation of nanofiber-like crystal particles. It is thought that.

As shown in g and h of FIG. 13, in the powder heat-treated at 1073 K, it was found that the nanofiber-like crystal particles were changed to nanorod-like (can be nano-roll-like) particles. These nanorod-shaped particles are Li ₄ SiO ₄ , and it can be seen that Li ₂ SiO ₃ nanofibers were formed by expansion and integration with phase change.

Therefore, the powder heat-treated at 1073K (corresponding to the MW-SG sample in FIG. 3A) was observed with a scanning electron microscope (SEM). For SEM observation, EVO18, Special Edition manufactured by Carl Zeiss, Germany was used, and observed at an acceleration voltage of 20 kV. The results are shown in A (about 2600 times) and B (about 5000 times) in FIG.
As shown in FIG. 14, the nanorod-like particles exhibit a plate-like crystal or a petal-like shape in which the nanorod-like particles are aggregated when observed macroscopically (for example, about 1000 to 5000 times). Was confirmed.

4-1. Preparation of lithium-rich silicon-lithium composite compound In the same manner as in 1-1 above, lithium silicate was prepared to obtain a carbon dioxide-absorbing material. However, in this example, the amount of colloidal silica added to the reaction aqueous solution after hydrolysis is decreased, and the amount of colloidal silica added is a lithium-rich blend in which the molar ratio of Li: Si is 8: 1. It was. Moreover, the electromagnetic wave irradiation conditions were such that the electromagnetic wave of 2.45 GHz and 700 W was irradiated for a total of 5 times for 2 minutes. The obtained gel composition was dried in an oven at 150 ° C., heated in the atmosphere at 500 ° C. for 3 hours, and then baked in an N ₂ atmosphere at 800 ° C. for 30 minutes.
The morphology of the particles constituting the obtained lithium silicate powder was observed with SEM and TEM, and the results are shown in FIGS. 15 and 16, respectively.

(A) of FIG. 15 is a SEM image (about 5000 times) of the entire lithium silicate particles (primary particles), and (b) is a partially enlarged view (about 12500 times). As is clear from FIG. 15 (a), it was found that lithium-rich lithium silicate has a spongy crystal form rather than a petal form in the micrometer tor order. In this SEM image, the wall surface of the lithium silicate crystal formed so as to surround the void is sufficiently thin with respect to the dimension (width) of the void, and is thicker than the plate-like crystal found in lithium orthosilicate. You can see that it is thin. It can also be seen that the crystal wall surfaces are bonded to each other at a smaller interval. It was found that the voids that can be confirmed in this figure are macropores having a diameter of about 50 nm to 500 nm.
Thus, according to the manufacturing method disclosed here, it was confirmed that the form of the lithium silicate particles to be formed greatly changes by changing the composition of lithium silicate. Further, this lithium silicate has a very large ratio of Li to Si. Therefore, a crystal is formed in a state where a large amount of Li is present around the Si tetrahedron connected body serving as a skeleton. That is, this lithium silicate is composed of a silicon-containing core part composed mainly of silicon (Si) and a shell part composed mainly of lithium (Li) present so as to cover the surface of the core part. It can be understood that it is a lithium composite compound. It can be said that such lithium silicate is well suited to the lithium silicate production mechanism described in 3-3 above.

Non-Patent Document 2 discloses lithium-rich lithium silicate having a chemical composition represented by Li ₈ SiO ₆ . Although this Li ₈ SiO ₆ has a high CO ₂ absorption capacity, there is a problem that it is inferior in CO ₂ desorption. This Li ₈ SiO ₆ is synthesized by a solid phase reaction method and is composed of huge particles exceeding 50 μm (see, for example, FIG. 2). Further, even when these huge particles are observed finely on the order of 1 μm, the characteristic crystal form as disclosed herein cannot be confirmed. As a result, the specific surface area of the Li ₈ SiO ₆ crystal is extremely small, and it is expected that the reason is that the CO ₂ desorption reaction does not proceed appropriately. For example, it is considered that the following reaction field for desorption of CO ₂ is difficult to prepare for this Li ₈ SiO ₆ crystal.
Li ₂ SiO ₃ + Li ₂ CO ₃ → Li ₄ SiO ₄ + CO ₂

FIGS. 16A to 16E are TEM images obtained by observing the same lithium silicate particles as in FIG. 15 with different fields of view and magnifications. It was found that the lithium silicate particles having a spongy form on the micrometer order are formed by aggregating primary particles of about 20 nm to several 100 nm as shown in FIG. In other words, the wall surface of the spongy body was found to be a collection of finer particles. It was also confirmed that a plurality of these particles are bonded with voids and the wall surface itself has a porous structure. It was found that the voids included in the wall surface are mesopores or micropores having a diameter of about 50 nm or less. For this reason, the lithium silicate particles disclosed herein contain relatively large voids (macropores) in the particles due to the sponge shape, and more minute voids (mesopores and micropores) in the wall surface. I found out. Such a porous structure is considered to realize the high CO ₂ absorption characteristics of the lithium silicate described later.

4-2. CO ₂ absorption characteristics The carbon dioxide (CO ₂ ) absorption characteristics of the lithium-rich lithium silicate obtained above were examined by thermogravimetric analysis. In thermogravimetric analysis, first, nitrogen gas purge was performed. Specifically, the sample was heated to 800 ° C. at a rate of temperature increase of 10 ° C./min under a N ₂ air flow of 49 ml / min, held at 800 ° C. for 20 minutes, and then the temperature was lowered to 710 ° C. Hold for 10 minutes. Thereafter, the N ₂ gas was switched to 100% CO ₂ gas, and the amount of CO ₂ gas absorbed at 710 ° C. was measured. The obtained isothermal absorption curve is shown in FIG.
As shown in FIG. 17, it was confirmed that the CO ₂ absorption capacity of the lithium silicate of this example reaches about 840 mg / g. This is equivalent to about 19 mmol CO ₂ / g in terms of mole, which is a remarkably high value. In addition, since the absorption capacity measured at 650 ° C. for a powder of Li ₄ SiO ₄ (MW-SG) having the composition shown in the above 2-2 was about 350 mg / g, the absorption capacity was about 350 mg / g. It was confirmed that the absorption capacity could be increased by 2 to 3 times. In addition, when the composition of the lithium silicate produced in this example is theoretically calculated from the CO ₂ absorption capacity, it is considered that it roughly corresponds to Li ₁₂ SiO ₈ or 6 (Li ₂ O) SiO ₂ .

5-1. Preparation of germanium-doped lithium silicate Lithium silicate was prepared according to 1-1 above, and a carbon dioxide absorbing material was obtained. However, in this example, in order to produce lithium silicate containing germanium (Ge), as shown in the flowchart of FIG. 1C, germanium chloride (GeCl ₄ ) is further used as a germanium source, and this germanium nitrate is used as a lithium nitrate aqueous solution. And dissolved in lithium silicate. The amount of germanium nitrate added was such that Si: Ge had a molar ratio of 1: 0.1826. Moreover, the reaction aqueous solution which mixed all the raw materials was irradiated with electromagnetic waves after ultrasonic stirring for 5 minutes. A microwave oven was used for the irradiation of electromagnetic waves, and 700 W of electromagnetic waves were irradiated at 2.45 GHz for 4 minutes to promote the dehydration condensation reaction. The composition of lithium silicate produced in this way is Li ₄ Ge _0.15 Si _0.846 O ₄ .

5-2. XRD analysis The germanium-doped lithium silicate obtained as described above was subjected to an XRD analysis, and the obtained XRD pattern is shown in FIG. For reference, FIG. 18A also shows an XRD pattern for the lithium silicate having the Li ₄ SiO ₄ composition shown in FIG.
As shown in FIG. 18, (b) a slight Li ₄ Ge ₅ O ₁₂ peak was identified along with the Li ₄ SiO ₄ peak in the XRD pattern of lithium silicate doped with germanium. Further, the peak of Li ₄ SiO ₄ was slightly shifted to the low angle side. From this, it was confirmed that Ge was introduced into the crystal structure of Li ₄ SiO ₄ .

5-3. Raman analysis Further, Raman spectroscopy analysis was performed on germanium-doped lithium silicate, and the obtained Raman spectrum was shown in FIG. For reference, FIG. 19A also shows a Raman spectrum of lithium silicate having a Li ₄ SiO ₄ composition.
The three peaks at 784, 823, and 862 cm ⁻¹ seen only in FIG. 19 (b) are in good agreement with the vibration frequency of the valence electrons of the GeO ₄ tetrahedron, which is a result that well supports the result of the XRD analysis. . The Raman analysis also confirmed that a material having a composition represented by the general formula: Li ₄ Ge _0.15 Si _0.846 O ₄ was formed by replacing Si with Ge in the crystal structure. .

5-4. Form of germanium-doped lithium silicate The form of the particles constituting the germanium-doped lithium silicate obtained above was observed with SEM and TEM, and the results are shown in FIGS. 20 and 21, respectively.
FIG. 20 is an SEM image of germanium-doped lithium silicate particles. It was confirmed that the germanium-doped lithium silicate formed a petal-like lithium silicate crystal by combining a plurality of plate-like crystals in the same manner as the lithium silicate of FIG. In the germanium-doped lithium silicate of this example, a single plate-like crystal has a thickness of about 5 μm or less and a surface dimension of about 10 μm or more, and a relatively thin plate-like crystal is assembled. It was found that it was configured.
21A to 21D are TEM images of germanium-doped lithium silicate particles, which are different in magnification and observation location. From this TEM image, it can be seen that one plate-like crystal observed in the SEM image of FIG. 20 grows greatly along a predetermined direction.

5-5. Dynamic Absorption Characteristics The carbon dioxide absorption characteristics of the germanium-doped lithium silicate obtained above were evaluated as dynamic absorption characteristics with the ambient temperature changed in the same manner as in 1-4 above. As a result, the obtained dynamic TGA curve is shown in FIG.
In this example, as in the case of the above 5-1, three types of germanium-doped lithium silicates with different Ge doping amounts are further prepared, and for reference, Li ₄ GeO ₄ is also prepared. Similarly, dynamic CO ₂ absorption characteristics were examined and evaluated. These results and the results for Li ₄ SiO ₄ are shown together in FIG. In the newly prepared germanium-doped lithium silicate, the molar ratio of Si to doped Ge was 1: 0.4470, 1: 0.083, 1: 0.04 as Si: Ge. .

As shown in FIG. 22, by doping Ge lithium _silicate, as compared to the Li 4 SiO ₄ and _Li 4 GeO _4, the CO2 absorption capacity in a dynamic absorption is increased as 1.2 times I understood. Further, the change in the CO ₂ absorption properties seen in doping Ge, CO ₂ absorption at low temperature region of about 300 ° C. to 500 ° C. is rapidly many, also large absorption rate in this low temperature range It was confirmed that it was increased. It is confirmed that the CO ₂ absorption increases as the molar ratio of Ge to Si increases from 0, but the CO ₂ absorption starts to decrease when the molar ratio of Ge exceeds about 0.1826. It was.

5-6. Isothermal Absorption Characteristics Therefore, the isothermal absorption characteristics of carbon dioxide at a low temperature of 300 ° C. were examined using germanium-doped lithium silicate having a molar ratio of Si: Ge prepared in 5-1 above of 1: 0.1826. As a result, the obtained isothermal absorption curve is shown in FIG.
As shown in FIG. 23, it is confirmed that germanium-doped lithium silicate has a CO ₂ absorption characteristic that absorbs CO ₂ quickly even at a low temperature of 300 ° C. although the absorption capacity itself is not so much. It was done.

  Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

Claims (18)

A method for producing a carbon dioxide-absorbing material mainly composed of lithium silicate,
Providing a sol-like composition in which a Li-Si precursor compound containing a lithium (Li) component and a silicon (Si) component is dispersed in an aqueous solution;
Irradiating the sol composition with electromagnetic waves to obtain a gel composition; and
Calcining the gel composition to obtain lithium silicate containing lithium and silicon;
The manufacturing method of the carbon dioxide absorption material including this.   The method according to claim 1, wherein the sol composition is prepared by causing a hydrolysis reaction in an aqueous solution containing the lithium (Li) component and the silicon (Si) component.   The manufacturing method according to claim 2, wherein the silicon (Si) component is at least one of silica alkoxide, colloidal silica, and fumed silica.   The manufacturing method according to claim 1, wherein the sol-like composition further includes a germanium (Ge) component.   The manufacturing method according to any one of claims 1 to 4, wherein the electromagnetic wave is a microwave having a wavelength of 1 mm to 1 m and a frequency of 300 MHz to 300 GHz.   The manufacturing method according to any one of claims 1 to 5, wherein a total irradiation time of the electromagnetic wave is 1 minute or more and 60 minutes or less.   Furthermore, the manufacturing method of any one of Claims 1-6 including compounding the said lithium silicate with alkali carbonate.   The said alkali carbonate is a manufacturing method of Claim 7 containing 2 or more types of a sodium (Na) component, a potassium (K) component, and a lithium (Li) component. The ratio of each alkali component in the alkali carbonate is
Na: 1 mol% or more and 80 mol% or less,
K: 1 mol% or more and 70 mol% or less, and Li: 1 mol% or more and 90 mol% or less,
The manufacturing method according to claim 7 or 8, wherein   The manufacturing method according to claim 7, wherein the alkali carbonate is a eutectic carbonate. A powder mainly composed of lithium silicate,
The individual particles constituting the powder are carbon dioxide-absorbing materials having at least one form selected from the group consisting of rods, plates, scales, and petals. The carbon dioxide-absorbing material according to claim 11 , wherein the lithium silicate is represented by the general formula: LixSiyOz, wherein x, y, and z satisfy x + 4y-2z = 0. Furthermore, including at least one of sodium and potassium,
The carbon dioxide absorbing material according to claim 11 or 12 , wherein a part of the lithium in the lithium silicate is substituted with the sodium and potassium. The carbon dioxide absorbing material according to claim 11 , further comprising germanium, wherein a part of the silicon in the lithium silicate is substituted with the germanium. The lithium silicate has a longitudinal dimension of a and a lateral dimension perpendicular to the longitudinal direction of b, the lateral dimension b being 100 nm or less, and a / The carbon dioxide-absorbing material according to any one of claims 11 to 14 , wherein rod-shaped particles having an aspect ratio defined by b of 2 or more occupy 50% by number or more of the whole. In addition, it contains an alkali carbonate,
The carbon dioxide absorbing material according to any one of claims 11 to 15 , wherein at least a part of a surface of the lithium silicate is covered with the alkali carbonate. The carbon dioxide absorbing material according to any one of claims 11 to 16 , wherein a CO ₂ absorption rate per unit weight in a temperature range of 500 ° C or higher and 710 ° C or lower is 25 mg / (g · min) or higher. The carbon dioxide absorbing material according to any one of claims 11 to 17 , wherein a CO ₂ gas absorption rate per unit weight in a temperature range of 600 ° C or higher and 710 ° C or lower is 50 mg / (g · min) or higher. .