JP7840985B2 - A carbon dioxide absorbing material that exhibits a high CO2 absorption effect in the intermediate temperature range and a method for manufacturing the same. - Google Patents

Description

This invention relates to a carbon dioxide absorbing material exhibiting excellent carbon dioxide absorption characteristics, particularly in the intermediate temperature range, and to a method for producing the same. This application claims priority to Indian Patent Application No. 202121018578, filed on April 22, 2021, the entire contents of which are incorporated herein by reference.

Carbon dioxide ( _CO2 ) is a greenhouse gas, and therefore a significant reduction in its emissions is urgently needed. This necessitates the development of materials and technologies to selectively separate and recover _CO2 from exhaust gases emitted from thermal power plants, factories, automobiles, and other sources. To date, various CO2 adsorption/absorbing materials have been proposed, such as porous materials, chemical absorbents incorporating _amino groups with _CO2 absorption capabilities, metal-organic frameworks (MOFs), carbonaceous materials, magnesium oxide materials, and alkali metal carbonates.

Due to their low heat resistance and physical adsorption properties, most of these materials are used as _CO2 absorbers in low-temperature environments from room temperature to below 200°C. On the other hand, among the above, _CO2 absorbers made of alkali metal carbonates are expected to be usable in high-temperature environments because they can be used at temperatures exceeding 400°C. For example, lithium silicate prepared by irradiating a sol-like composition containing lithium and silicon components with electromagnetic waves can exhibit excellent _CO2 absorption properties in a temperature range of approximately 600°C or higher, and by combining it with alkali carbonates, _{the CO2} absorption properties in the temperature range of approximately 400°C to 600°C are improved (Japanese Patent No. 6596567 and Chemical Engineering Journal 406 (2021) 126731).

Incidentally, in order to reduce the amount of _CO2 emitted into the environment, there is a need for the development of _CO2 absorbing materials that have excellent _CO2 absorption characteristics in an intermediate temperature range (for example, around 200°C to 400°C). In the conventional technology using lithium silicate described above, there is room for further improvement in the _CO2 absorption characteristics in this intermediate temperature range. Furthermore, it has been reported that magnesium oxide-based _CO2 absorbing materials have excellent _CO2 absorption capacity in an intermediate temperature range of 350°C to 450°C (U.S. Patent No. 6,280,503). However, through our own research, we have found that there is room to improve the durability of such _CO2 absorbing materials and to further lower the temperature range of _CO2 absorption and desorption.
This invention has been made in view of the above problems, and aims to provide a carbon dioxide absorbing material with improved _CO2 absorption characteristics. Another object of this invention is to provide a method for producing such a carbon dioxide absorbing material.

To achieve the above objectives, the technology disclosed herein provides a method for producing a carbon dioxide ( _CO₂ ) absorbing material comprising lithium silicate or lithium zincate, alkali carbonate, alkali nitrate, and magnesium carbonate. This production method includes: preparing a first powder comprising lithium silicate particles or lithium zincate particles; mixing the first powder with a second powder comprising alkali carbonate particles and alkali nitrate particles to obtain a powder containing first composite particles in which the alkali carbonate particles and alkali nitrate particles are aggregated on the surface of the lithium silicate particles or lithium zincate particles; mixing a third powder comprising magnesium carbonate particles with the second powder to obtain a powder containing second composite particles in which the alkali carbonate particles and alkali nitrate particles are aggregated on the surface of the magnesium carbonate particles; and mixing the powder containing the first composite particles with the powder containing the second composite particles to obtain a mixed powder containing aggregates having carbon dioxide absorbing capacity formed by the aggregation of the first composite particles and the second composite particles.
According to this configuration, it is possible to manufacture a carbon dioxide absorbing material with improved _CO2 absorption characteristics, particularly in the intermediate temperature range.

Furthermore, in a preferred embodiment of the carbon dioxide absorbing material manufacturing method disclosed herein, when the total amount of the mixed powder is 100 wt%, the total ratio of the first powder and the third powder is 50 wt% to 95 wt%, the ratio of the first powder is 5 wt% to 60 wt%, and the ratio of the third powder is 5 wt% to 60 wt%. With this configuration, it is possible to manufacture a carbon dioxide absorbing material with improved _CO2 absorption capacity, particularly in the intermediate temperature range.

Furthermore, in a preferred embodiment of the carbon dioxide absorbing material manufacturing method disclosed herein, at least one of the alkali carbonate particles and the alkali nitrate particles contains two or more components selected from sodium (Na), potassium (K), and lithium (Li). The second powder may also contain eutectic carbonate/nitrate particles. With this configuration, it is possible to manufacture a carbon dioxide absorbing material with further improved _CO2 absorption capacity, particularly in the intermediate temperature range.

Furthermore, in a preferred embodiment of the carbon dioxide absorbing material manufacturing method disclosed herein, the mixed powder containing the aggregates is further heat-treated. This allows for the production of an activated carbon dioxide absorbing material with improved carbon dioxide absorption properties.

In other respects, the technology disclosed herein provides a carbon dioxide absorbing material. This carbon dioxide absorbing material is a powder capable of absorbing and desorbing carbon dioxide, comprising a second particle formed by the aggregation of a plurality of first particles, wherein the first particles contain an alkali silicate containing at least lithium silicate or an alkali zincate containing at least lithium zincate, an alkali carbonate, an alkali nitrate, and a magnesium-containing compound containing at least magnesium carbonate. With this configuration, the _CO2 absorption reaction by lithium silicate or lithium zincate is promoted in a lower temperature range. This makes it possible to improve the _CO2 absorption capacity, particularly in the intermediate temperature range.

Furthermore, in one preferred embodiment of the carbon dioxide absorbing material disclosed herein, when the total of the silicon (Si) component contained in the alkali silicate and the magnesium (Mg) component contained in the magnesium-containing compound is 100 mol%, the proportion of the silicon (Si) component is 10 mol% or more and 85 mol% or less. With this configuration, the _CO2 absorption capacity in the intermediate temperature range is further improved.

Furthermore, in a preferred embodiment of the carbon dioxide absorbing material disclosed herein, when the total of the zinc (Zn) component contained in the alkali zincate and the magnesium (Mg) component contained in the magnesium-containing compound is set to 100 mol%, the proportion of the zinc (Zn) component is 5 mol% or more and 80 mol% or less. With this configuration, the _CO2 absorption capacity in the intermediate temperature range is further improved.

Furthermore, in a preferred embodiment of the carbon dioxide absorbing material disclosed herein, the first particles have at least one of the following forms: flake-like and rod-like. With this configuration, voids are more easily formed in the second particles, increasing the surface area in contact with _CO2 . As a result, absorption and desorption of _CO2 occur more easily, and the _CO2 absorption characteristics are further improved.

Furthermore, in one preferred embodiment of the carbon dioxide absorbing material disclosed herein, at least one of the alkali carbonate and alkali nitrate contains two or more components selected from sodium (Na), potassium (K), and lithium (Li). In particular, it is preferable that the alkali carbonate and alkali nitrate form a eutectic. This configuration further improves the _CO2 absorption capacity in the intermediate temperature range.

Furthermore, the carbon dioxide absorbing material disclosed herein is also provided as a heat-treated product of the carbon dioxide absorbing material disclosed herein. This enables the realization of an activated carbon dioxide absorbing material with improved _CO2 absorption characteristics.

Figure 1 is a flowchart illustrating the method for manufacturing a _CO2 absorbing material according to the first embodiment. Figure 2 is a flowchart illustrating the method for manufacturing a _CO2 absorbing material according to the second embodiment. Figure 3A is an FE-SEM image of a _CO2 absorbing material (LSME) according to one embodiment before _CO2 absorption. Figure 3B is an FE-SEM image of a _CO2 absorbing material (LSME) according to one embodiment before _CO2 absorption. Figure 3C is an FE-SEM image of a _CO2 absorbing material (LSME) according to one embodiment before _CO2 absorption. Figure 3D is a TEM image of a _CO2 absorbing material (LSME) according to one embodiment before _CO2 absorption. Figure 4 shows the dynamic TGA curve of a _CO2 absorbing material (LSME) according to one embodiment. Figure 5 shows the _CO2 isothermal adsorption curve of a _CO2 absorbing material (LSME) according to one embodiment. Figure 6 is a graph showing the maximum _CO2 absorption capacity of a _CO2 absorbing material (LSME) according to one embodiment at various temperatures. Figure 7 is a graph showing the absorption and desorption of _CO2 by a pressure swing process under constant temperature conditions of 300°C in a _CO2 absorbing material (LSME) according to one embodiment. Figure 8 is a graph showing the absorption and desorption of _CO2 by a pressure swing process accompanied by temperature changes in a _CO2 absorbing material (LSME) according to one embodiment. Figure 9 is a graph showing the absorption and desorption of _CO2 in a _CO2 absorbing material (LSME) according to one embodiment, under a 20% _CO2 gas atmosphere. Figure 10 is a graph showing the cycle characteristics of a _CO2 absorbing material (LSME) according to one embodiment. Figure 11 shows the XRD patterns of a _CO2 absorbing material (LSME) according to one embodiment, before _CO2 absorption, after _CO2 absorption, and after _CO2 desorption. Figure 12 shows the FT-IR patterns of a _CO2 absorbing material (LSME) according to one embodiment, before _CO2 absorption, after _CO2 absorption, and after _CO2 desorption. Figure 13 is a graph showing the relationship between the mixing ratio of LS in a _CO2 absorbing material (LSME) according to one embodiment and the maximum _CO2 absorption capacity obtained from the dynamic TGA curve at each mixing ratio. Figure 14A is an FE-SEM image of the _CO2 absorbing material (LZME) according to the second embodiment before _CO2 absorption. Figure 14B is an FE-SEM image of the _CO2 absorbing material (LZME) according to the second embodiment before _CO2 absorption. Figure 14C is an FE-SEM image of the _CO2 absorbing material (LZME) according to the second embodiment before _CO2 absorption. Figure 15 shows the dynamic TGA curve of the _CO2 absorbing material (LZME) according to the second embodiment. Figure 16 is a graph showing the absorption and desorption of _CO2 by a pressure swing process under constant temperature conditions of 350°C for the _CO2 absorbing material (LZME) according to the second embodiment. Figure 17 is a graph showing the relationship between the LZ mixing ratio of the _CO2 absorbing material (LZME) according to the second embodiment and the maximum _CO2 absorption capacity obtained from the dynamic TGA curve at each mixing ratio.

The following describes preferred embodiments of the technology disclosed herein, with reference to the drawings as appropriate. Matters other than those specifically mentioned herein, but necessary for implementing the technology disclosed herein, can be understood as design matters for those skilled in the art based on the prior art. The technology disclosed herein can be implemented based on the content disclosed herein and common technical knowledge in the art. In this specification, the notation "X to Y" indicating a numerical range means "X or greater and Y or less," and includes "greater than X and less than Y."

Figures 1 and 2 are flow diagrams illustrating a method for producing a carbon dioxide ( _CO₂ ) absorbent material according to one embodiment. The production method disclosed herein typically produces a _CO₂ absorbent material comprising lithium silicate or lithium zincate, an alkali carbonate, an alkali nitrate, and a magnesium compound containing at least magnesium carbonate.

The method for producing the _CO2 absorbing material disclosed herein is characterized by including, for example, the following (A) to (D), as shown in Figures 1 and 2.
(A) Prepare a first powder containing lithium silicate particles or lithium zincate particles.
(B) Mix the first powder with a second powder containing alkali carbonate particles and alkali nitrate particles to obtain a powder containing first composite particles in which the alkali carbonate particles and alkali nitrate particles are aggregated on the surface of the lithium silicate particles or lithium zincate particles.
(C) Mix a third powder containing magnesium carbonate particles with the second powder to obtain a powder containing second composite particles in which the alkali carbonate particles and alkali nitrate particles are aggregated on the surface of the magnesium carbonate particles.
(D) Mix the powder containing the first composite particles and the powder containing the second composite particles to obtain a mixed powder containing aggregates having carbon dioxide absorbing capacity formed by the aggregation of the first composite particles and the second composite particles.
Although not a mandatory step, the manufacturing method disclosed herein may also include step (E) below, following step (D) above.
(E) Heat-treat the mixed powder containing the obtained aggregates.
The above-mentioned (A) to (D) are merely a list of requirements included in the carbon dioxide absorption material manufacturing method disclosed herein, and do not limit the process to being carried out in the order of (A), (B), (C), and (D). For example, the process may be carried out in the order of (C), (A), (B), and (D), or (A) and (C) may be carried out simultaneously.
The following describes a _CO2 absorbing material containing lithium silicate as the first embodiment.

<First Embodiment>
(A) Preparation of the first powder containing lithium silicate particles The first powder containing lithium silicate particles prepared here is a component that can absorb and desorb carbon dioxide. Typically, the proportion of lithium silicate particles to the total first powder is 90 wt% or more, preferably 95 wt% or more, and may be 100 wt%.
Lithium silicate is typically produced by the following reaction equation:
Li ₄ SiO ₄ +CO ₂ → Li ₂ SiO ₃ +Li ₂ CO ₃
The reaction shown can absorb _CO2 from the atmosphere. Furthermore, since this reaction is reversible, lithium silicate can desorb _CO2 .

Lithium silicate can be a compound containing lithium (Li), silicon (Si), and oxygen (O). Typically, lithium silicate can be a compound represented by the general formula: Li _x Si _y O _z , where x, y, and z are positive real numbers satisfying x + 4y - 2z = 0. Typically, various forms of lithium silicate can be considered, including silicate anions consisting of any number of silicate ions linked together and lithium cations. Typical examples of such lithium silicate may be lithium orthosilicate (Li ₄ SiO ₄ ), lithium metasilicate (Li ₂ SiO ₃ ), lithium disilicate (Li ₂ Si ₂ O ₅ ), lithium metatrisilicate (Li ₄ Si ₃ O ₈ ), lithium metatetrasilicate (Li ₆ Si ₄ O ₁₁ ), etc. Furthermore _, lithium silicate is not limited to these examples, _and may include compounds such as _Li₂SiO₆ , _{Li₂Si₂O₃} , _{and Li₁₂SiO₄} . These may be single phases consisting of any one of these, or multiphases containing a combination of two or _more of these. Most lithium silicates essentially have a framework of _SiO₄ tetrahedra or linked elements of Si (e.g., _SiO₄ links), and it is thought that alkali metal elements such as Li are ions occupying the voids in this tetrahedron.

The lithium silicate disclosed herein may be a compound containing other elements (M) in addition to the above-mentioned Li, Si, and O. Such other elements are not particularly limited as long as they constitute a compound that can stably exist under _CO2 absorption and desorption conditions. For example, they may be aluminum (Al), iron (Fe), or germanium (Ge), tin (Sn), lead (Pb), etc., which are group 14 elements like silicon. Such group 14 elements are preferred because they can easily substitute for Si in the crystal structure of lithium silicate and can exist relatively stably. Although the proportion of other elements contained in lithium silicate is not strictly limited, for example, the molar ratio of silicon (Si) to other elements (M) is preferably about 1:0.001 to 1:0.5, and more preferably 1:0.04 to 1:0.45.

As a typical example of the _CO2 absorbing material disclosed herein, from the viewpoint of _CO2 absorption capacity, it is preferable that lithium orthosilicate constitutes 70 mol% or more of the lithium silicate, more preferably 80 mol% or more, and particularly preferably 90 mol% or more, for example, substantially 100 mol%. Hereinafter, the teachings will be explained using, for example, the case in which lithium orthosilicate constitutes substantially 100 mol% as lithium silicate. It should be noted that the composition of the above lithium silicate may change depending on, for example, the surrounding environment.

The first powder containing lithium silicate may be a commercially available product or may be manufactured by conventionally known methods, but preferably it is manufactured by a method encompassing the following (A1) to (A3), as shown in Figure 1.
(A1) Prepare 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.
(A2) To obtain a gel-like composition by irradiating a sol-like composition with electromagnetic waves.
(A3) To obtain lithium silicate containing lithium and silicon by calcining the gel-like composition.
Although (A1) to (A3) are shown in Figures 1 and 2, it is not intended to indicate that these are essential steps in the carbon dioxide absorption material manufacturing method disclosed herein.

(A1) Preparation of sol-like composition The first powder containing lithium silicate particles can generally be preferably prepared by a wet method of gelling a sol-like composition containing a Li-Si precursor compound.
Here, the sol-like composition may be a colloidal aqueous solution containing a Li-Si precursor compound as the dispersed phase and at least water as the dispersion medium. The water used as the dispersion medium may be distilled water, deionized water, pure water, etc. As long as the dispersion medium is mainly water, it may also contain water-soluble, low-molecular-weight organic solvents such as lower alcohols (methanol, ethanol, butanol, isopropanol, etc.), ethylene glycol, acetone, or other ketones. The dispersion medium is preferably 100% by mass of water to improve the efficiency of the hydrolysis described later (i.e., the sol-like composition may be a hydrosol).

The Li-Si precursor compound as a dispersed phase contains lithium and silicon components, which are the raw materials for the lithium silicate described above. It can be various compounds capable of forming the lithium silicate through treatments such as dehydration condensation, polycondensation, and calcination (heating). Typically, it may be a precipitate formed from a water-soluble solution of lithium and silicon, which may be in the form of a hydrate or hydrated complex. Furthermore, the precipitate, such as the hydroxide, may undergo dehydration condensation to the extent that it can maintain a sol state. In other words, this Li-Si precursor compound is a colloidal particle exhibiting a sol state, and its particle size is not strictly limited as long as it can maintain a 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. Such an average particle size can be determined by dynamic light scattering.

Such a sol-like composition may be prepared, for example, by dispersing a separately prepared Li-Si precursor compound in a dispersion medium such as water, or by obtaining a sol-like composition in which the Li-Si precursor compound is pre-dispersed, or by using a known dry method such as a combustion method or arc method, or a wet method such as a sedimentation method or gel method (including the sol-gel method).
As an example, when preparing a sol-like composition by a general sol-gel method, a precursor compound containing Li and Si components can be formed in a mixed solution in which the lithium salt and silica alkoxide can be dissolved. This is achieved by bringing the alkoxide into contact with water to induce hydrolysis. In this case, since water is preferably used as the dispersion medium for the sol-like composition, it is more preferable to prepare the sol-like composition by adding the alkoxide little by little to an aqueous solution in which the lithium salt is dissolved. For example, a precursor compound can be obtained by adding an alcohol solution of silica alkoxide to an aqueous solution in which the lithium salt is dissolved while stirring the aqueous solution. Stirring can be carried out using other means such as a magnetic stirrer, a mechanical stirrer, or an ultrasonic stirrer.

To facilitate these hydrolysis and condensation reactions, an acid such as hydrochloric acid or an alkali such as ammonia may be added to the mixed solution as a hydrolysis catalyst to adjust the reaction rate (hydrolysis rate and condensation rate). Such a hydrolysis catalyst may also play a role in controlling the pH of the reaction solution and adjusting the primary particle size of the formed precursor compound.

As lithium salts, lithium oxides and various compounds that can be converted into oxides upon heating can be used. Specifically, examples include lithium oxides, hydroxides, carbonates, nitrates, sulfates, phosphates, acetates, formates, oxalates, and halides. Preferably, various water-soluble salts can be used. Lithium alkoxides can also 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 alkoxides in which 1 to 4 alkoxy groups are bonded to a Si atom, and silica alkoxides into which functional groups such as glycidyl groups have been introduced can be preferably used. Among these, the use of alkoxysilanes is preferred, and as examples of preferred silica alkoxides, such as tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, and 1,2-bistrimethoxysilylethane, in which 1 to 4 alkoxy groups are bonded to a Si atom are shown. Particularly preferred are tetraethoxysilane (Si( _OC2H5 ) ₄ :TEOS, also known as tetraethyl orthosilicate), tetramethoxysilane (Si _{( OCH3} ) ₄ :TMOS, also known as tetramethyl orthosilicate), and methyltrimethoxysilane. Any one of these may be used alone, or two or more may be used in combination.

Furthermore, as an example of a method for preparing sol-like compositions other than the sol-gel method exemplified above, the following methods can be employed. For example, a sol-like composition can be prepared in the same manner as above using nanosilica materials such as gel-processed silica (including colloidal silica), precipitated silica, or fumed silica (including silica xerogel, silica cryogel, and silica aerogel) as the silicon component. While not particularly limited, such silica materials and lithium components can be suitably used as the sol-like compositions disclosed herein by forming complexes with each other in aqueous solutions, or by forming compounds through hydration reactions and/or dehydration condensation reactions.

(A2) Formation of gel-like composition by electromagnetic wave irradiation Next, the prepared sol-like composition is irradiated with electromagnetic waves to gel it and obtain a gel-like composition. Irradiating the sol-like composition with electromagnetic waves is intended to more favorably promote gelation. Stirring is not necessarily required, but from the viewpoint of uniformly promoting gelation of the sol-like composition, it is more preferable to irradiate the sol-like composition with electromagnetic waves while stirring it. Note that even without electromagnetic wave irradiation, a gel-like composition can be obtained by continuously stirring the sol-like composition.

Any of the following electromagnetic waves can be used: very low frequency, long wave, medium wave, short wave, microwave (high frequency), infrared, visible light, ultraviolet, X-ray, and gamma ray. However, from the viewpoint of electromagnetic wave irradiation efficiency, it is preferable to use electromagnetic waves that can effectively heat the sol-like composition. In this respect, microwaves can be directly absorbed by water molecules such as water as the dispersion medium of the sol-like composition, or water molecules such as crystal water and water of hydration contained in the Li-Si precursor compound. These water molecules generate heat, which can heat the Li-Si precursor compound. This microwave heating (dielectric heating) is a preferred embodiment because it allows for high-speed and selective heating of the Li-Si precursor compound from within while minimizing energy loss.

The frequency (wavelength), power output, and irradiation time of the microwaves are not particularly limited. For example, they can be appropriately determined to supply the amount of energy necessary for the sol-like composition to gel. For instance, a microwave with a wavelength of 1 mm to 1 m and a frequency of 300 MHz to 300 GHz is often used. Regarding the frequency, it may be more appropriate to use high-frequency waves generated by a magnetron in the 2.45 GHz band, based on international standards (high-frequency waves in the 915 MHz band may also be used in some regions). Furthermore, regarding the power output, from the viewpoint of heating efficiency, a power output of approximately 300 W to 300 kW is appropriate, preferably 300 W to 10 kW, more preferably 300 W to 2000 W, and particularly preferably 500 W to 1600 W.

Furthermore, the irradiation time can be adjusted considering factors such as the output power, the amount of the sol-like composition, and the form of irradiation (degree of microwave penetration). Microwave irradiation may also be performed continuously for a predetermined duration, or multiple times with intervals in between. A preferred form of microwave irradiation involves irradiating the sol-like composition with microwaves of a predetermined output power until the dispersion medium boils, then cooling the sol-like composition, and repeating this process a predetermined number of times. If the dispersion medium volatilizes due to boiling, the dispersion medium may be replenished as needed. For example, when producing approximately 0.2 to 0.5 moles of lithium silicate, a microwave irradiation time of 600 W to 1000 W (e.g., 700 W) at 2.45 GHz is exemplified, for a total of 1 minute to 20 minutes (e.g., 4 to 12 minutes).

(A3) Calcination of lithium silicate The gel-like composition obtained in this way is dried and calcined. This completely removes the dispersion medium and excess components other than lithium silicate, and the desired solid lithium silicate can be obtained. Even if the gel-like composition contains areas where hydrolysis and dehydration condensation reactions have not been completely completed, these reactions are accelerated by calcination, and the gel-like composition is transformed into a typically crystalline, dense solid. This yields the first powder containing lithium silicate particles.
Drying may be done by natural drying or by using a drying oven. Firing may be done using a general heating furnace. Furthermore, drying and firing may be combined. In this case, for example, an air oven may be used, or supercritical drying, spray drying, spray granulation, or spray pyrolysis may be employed. These drying and firing methods may be performed individually or in combination.

The firing conditions are not particularly limited as long as the gel-like composition can be converted into lithium silicate. For example, any conditions that allow the amorphous Li-Si precursor compound to crystallize into lithium silicate are acceptable. Specifically, when the firing temperature is approximately 500°C or lower, it is preferable to use an oxygen-containing atmosphere (typically an atmospheric atmosphere) for the firing to allow the oxidation of the gel-like composition to proceed favorably. When the firing temperature is approximately 500°C or higher (exceeding this), the firing atmosphere may be an oxygen-containing atmosphere (typically an atmospheric atmosphere), or it may be, for example, air, nitrogen gas, carbon dioxide gas, or a mixture of two or more of these gases. The firing temperature should be higher than 473°C (preferably 500°C or higher). A firing temperature of 700°C or higher is more preferable, and 800°C or higher is particularly preferable. This allows for the removal of excess components derived from the raw materials used, and enables the acquisition of highly crystalline lithium silicate. There is no particular upper limit to the firing temperature, but it can be, for example, 1100°C or less, 1000°C or less, and typically around 900°C. There is no particular limit to the firing time, but for example, 30 minutes to 5 hours is appropriate, 1 hour to 5 hours is more preferable, and 2 hours to 4 hours is particularly preferable.

The lithium silicate particles contained in the first powder produced in this manner may be particles formed by the aggregation of particles having special morphologies. Such special morphologies may be highly anisotropic shapes, such as rod-shaped, plate-shaped, flake-shaped, and petal-shaped. Such lithium silicate particles have higher _CO2 absorption characteristics than lithium silicate particles produced by known sol-gel methods. Therefore, in this embodiment as well, by using lithium silicate particles produced by (A1) to (A3), a carbon dioxide absorbing material with improved _CO2 absorption characteristics can be produced.

(B) Preparation of powder containing the first composite particles By mixing the first powder containing lithium silicate particles obtained in (A) above with a second powder containing alkali carbonate particles and alkali nitrate particles, a powder containing the first composite particles can be obtained in which alkali carbonate particles and alkali nitrate particles are aggregated on the surface of lithium silicate particles. The alkali carbonate and alkali nitrate may soften or become liquid in the _CO2 absorption temperature range of the carbon dioxide absorbing material disclosed herein. The presence of such alkali carbonate and alkali nitrate is thought to facilitate the transfer of _CO2 to the lithium phase. This makes it possible to significantly increase the _CO2 absorption capacity at lower temperatures and to increase the _CO2 absorption rate.

As alkali carbonates, carbonates of alkali metals such as sodium (Na), potassium (K), lithium (Li), rubidium (Rb), cesium (Cs), and francium (Fr) can be considered. These may be carbonates of only one alkali metal, or carbonates containing two or more alkali metals.

As alkaline nitrates, nitrates of alkali metals such as sodium (Na), potassium (K), lithium (Li), rubidium (Rb), cesium (Cs), and francium (Fr) can be considered. These may be nitrates of only one alkali metal, or nitrates containing two or more alkali metals.

In the technologies disclosed herein, at least one of the alkali carbonate and alkali nitrate preferably contains one or more of the following components: sodium, potassium, and lithium; more preferably two or more; and particularly preferably three.

When at least one of the alkali carbonate and alkali nitrate is a mixed system containing two or more of Li, K, and Na, it is preferable that they are mixed in proportions such that they form a solid solution crystal or a eutectic. In the case of a solid solution, the proportions of Na, K, and Li in the alkali carbonate are preferably as follows. Such a composition may also include a eutectic composition.
Na: 1 mol% or more and 80 mol% or less K: 1 mol% or more and 70 mol% or less Li: 1 mol% or more and 90 mol% or less

When alkaline carbonates and alkaline nitrates are eutectic, the eutectic point is lower, which is preferable because it can further improve the _CO2 absorption capacity and _CO2 absorption rate. For the mixing ratio to form a K-Li eutectic carbonate/nitrate, the molar ratio of K to Li (K:Li) is preferably in the range of 60:40 to 40:60. For the mixing ratio to form a Na-Li eutectic carbonate/nitrate, the molar ratio of Na to Li (Na:Li) is preferably in the range of 55:45 to 45:55. For the mixing ratio to form a K-Na eutectic carbonate/nitrate, the molar ratio of K to Na (K:Na) is preferably in the range of 65:35 to 75:25 or 35:65 to 25:75. Furthermore, for the mixing ratio that forms the Na-K-Li eutectic carbonate/nitrate, the molar ratio of Na, K, and Li (Na:K:Li) is preferably in the range of approximately 25-35:30-40:30-40.

While not necessarily limited to this example, the average particle size of alkali carbonate particles and alkali nitrate particles is typically appropriate to be between 50 nm and 5 μm, preferably between 100 nm and 1 μm, and particularly preferably between 200 nm and 500 nm. Such average particle size can be measured by laser diffraction/scattering.

The second powder is typically a powder containing alkali carbonate powder and alkali nitrate powder, or typically a powder in which these are mixed. The mixing method is not particularly limited, but may include, for example, mixing with a mortar and pestle, or using an electric mixing device such as a ball mill. Such mixing may be carried out in a dry state, or a dispersion medium may be used. Examples of dispersion mediums include lower alcohols (methanol, ethanol, butanol, isopropanol, etc.), water, acetone, etc. After mixing, drying may be carried out as needed. Such drying method may be the same as the drying method described in (A3) above. Such drying can be carried out, for example, at a temperature of room temperature to 150°C.

The second powder may be heat-treated before mixing with the first powder to form a eutectic carbonate/nitrate (ECN), which is a eutectic of alkaline carbonate and alkaline nitrate. That is, the second powder may contain eutectic carbonate/nitrate particles as alkaline carbonate particles and alkaline nitrate particles. The heat treatment can be carried out, for example, in a temperature range of 200°C to 650°C, preferably in the range of 350°C to 500°C. The heat treatment time can be, for example, 30 minutes to 3 hours.

The method for mixing the first powder and the second powder may be the same as the method for mixing the alkali carbonate powder and alkali nitrate powder described above. The drying method is also the same. This allows for obtaining a powder containing first composite particles in which alkali carbonate particles and alkali nitrate particles are aggregated on the surface of lithium silicate particles.

(C) Preparation of powder containing the second composite particle By mixing the second powder and the third powder containing magnesium carbonate particles, a powder containing the second composite particle can be obtained in which alkali carbonate particles and alkali nitrate particles are aggregated on the surface of the magnesium carbonate particles. Magnesium carbonate is a component that can improve the _CO2 absorption capacity of the carbon dioxide absorbing material disclosed herein at low temperatures.

As the magnesium carbonate particles contained in the third powder, for example, hydrate particles such as magnesium carbonate hydrate, magnesium carbonate hydroxide hydrate, and magnesium carbonate hydroxide pentahydrate can be used.
While not particularly limited, the average particle diameter of magnesium carbonate particles is typically suitable to be between 50 nm and 5 μm, preferably between 100 nm and 1 μm, and particularly preferably between 200 nm and 500 nm. This average particle diameter can be measured by laser diffraction/scattering.

The method for mixing the second powder and the third powder may be the same as the method for mixing the alkali carbonate powder and alkali nitrate powder described above. The drying method is also the same. This allows for obtaining a powder containing second composite particles in which alkali carbonate particles and alkali nitrate particles are aggregated on the surface of magnesium carbonate particles.

(D) Preparation of aggregates of the first composite particles and the second composite particles By mixing the powder containing the obtained first composite particles with the powder containing the second composite particles, a mixed powder containing aggregates having carbon dioxide absorbing capacity formed by the aggregation of the first composite particles and the second composite particles (i.e., the carbon dioxide absorbing material disclosed herein) can be obtained. The mixing method may be the same as the mixing method for alkali carbonate powder and alkali nitrate powder described above. The drying method may also be the same.

The mixed powder (carbon dioxide absorbing material) obtained in this way contains a first powder, a second powder, and a third powder. When the total amount of the mixed powder is 100 wt%, it is preferable to blend the first powder and the third powder so that their combined ratio is 50 wt% to 95 wt%, more preferably 60 wt% to 80 wt%, and even more preferably 65 wt% to 75 wt%. Furthermore, the ratio of the first powder to the total amount of the mixed powder is preferably 5 wt% to 60 wt%. Also, the ratio of the third powder is preferably 5 wt% to 60 wt%. Furthermore, if the first powder contains lithium silicate particles, the ratio of the first powder is more preferably 20 wt% to 50 wt%, and even more preferably 30 wt% to 50 wt%. By using these ratios, it is possible to produce a carbon dioxide absorbing material with improved _CO2 absorption capacity. In addition, the remaining proportion of the mixed powder after the combined proportion of the first powder and the third powder is typically the proportion of the second powder.

(E) Heat treatment of mixed powder containing aggregates The obtained mixed powder (carbon dioxide absorbing material) may be used as is, or it may be heat-treated before use. Heat treatment is preferable because it can change the components contained in the mixed powder into an activated state in which carbon dioxide absorption properties are improved. For example, by heat treatment, as described above, alkali carbonate and alkali nitrate can form a solid solution or eutectic, so that high _CO2 absorption properties can be exhibited in a lower temperature range. Also, by heat treatment, magnesium carbonate can be changed to magnesium oxide. Magnesium oxide exhibits _CO2 absorption capacity in a lower temperature range than lithium silicate, so _CO2 absorption properties are improved in a lower temperature range (especially in the intermediate temperature range).
The heat treatment conditions are preferably carried out in a temperature range of 300°C to 650°C, and more preferably in a temperature range of 350°C to 500°C. Even if such heat treatment is not performed, the carbon dioxide absorbing material disclosed herein can change to an activated state with use, as its operating temperature range is typically 100°C to 650°C.

The mixed powder (carbon dioxide absorbing material) obtained as described above contains second particles, which are aggregates of multiple first particles. The first particles contain at least an alkali silicate containing lithium silicate, an alkali carbonate, an alkali nitrate, and a magnesium-containing compound containing at least magnesium carbonate.
The lithium silicate contained in the first powder described above may form salts with alkaline components other than lithium (e.g., potassium, sodium, etc.) that can be found in alkali carbonates and alkali nitrates, thereby forming alkali silicates. Furthermore, the magnesium carbonate contained in the third powder described above may form other magnesium salts such as magnesium nitrate when mixed with alkali nitrates. In addition, since magnesium carbonate can be converted to magnesium oxide through heat treatment or under the operating temperature range of the carbon dioxide absorbent material disclosed herein, the first particles may further contain magnesium oxide.

When the total of the silicon (Si) component (typically Si contained in alkali silicates) and the magnesium (Mg) component (e.g., Mg contained in magnesium salts and magnesium oxide) in the carbon dioxide absorbing material disclosed herein is taken as 100 mol%, the proportion of the silicon component is preferably 5 mol% to 85 mol%, more preferably 20 mol% to 75 mol%, and even more preferably 35 mol% to 75 mol%. With such proportions, the _CO2 absorption capacity is improved, especially in the intermediate temperature range (e.g., 200°C to 400°C). The proportions of the composition of the carbon dioxide absorbing material can be calculated, for example, based on X-ray diffraction analysis (XRD).

The morphology of the first particles is not particularly limited, but can be spherical, granular, rod-shaped, flake-shaped, etc., although flake-shaped is preferred. When flake-shaped first particles aggregate, they easily form voids with each other, increasing the surface area in contact with _CO2 . This is preferable because it improves the _CO2 absorption characteristics. For example, plate-shaped lithium silicate particles can be produced by the methods A1 to A3 described above. By using such lithium silicate particles, flake-shaped first particles can be obtained. Note that flake-shaped refers to a crystalline form that has grown significantly in the plane direction (two dimensions). Typically, it is a relatively flat, thin plate-like crystal. Here, when the dimension in the longitudinal direction within the crystal plane is a, and the dimension in the thickness direction perpendicular to this longitudinal direction is b, the average dimension b in the thickness direction is generally 500 nm or less, preferably 300 nm or less, and particularly preferably 100 μm or less. Furthermore, the average dimension a in the longitudinal direction is not particularly limited, but is generally about 5 μm or less, preferably 1 μm or less, and more preferably 500 μm or less. 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. It is preferable that particles having such characteristic morphology account for 50-something percent or more of the total first particles, for example 70-something percent or more, and particularly 80-something percent or more. The first particles may be particles formed by the aggregation of multiple even smaller particles. Therefore, the first particles may contain fine voids between such small particles. The morphology and number ratio of the first particles can be analyzed, for example, by electron microscopy observation using a field emission scanning electron microscope (FE-SEM).

The average particle diameter of the second particle is not particularly limited, but for example, it may be between 1 μm and 20 μm, and between 5 μm and 15 μm. The second particle is formed by the aggregation of multiple first particles, typically with voids between them. The average particle diameter of the second particle can be measured by laser diffraction/scattering.

As described above, a carbon dioxide absorbing material with improved _CO2 absorption characteristics can be obtained. Furthermore, in a second embodiment of the technology disclosed herein, a method for producing a carbon dioxide absorbing material is provided in which a first powder containing lithium zincate particles is used instead of a first powder containing lithium silicate particles.

<Second Embodiment>
In the second embodiment, the first powder containing lithium zincate particles is used instead of lithium silicate particles. As lithium zincate, compounds containing lithium (Li), zinc (Zn), and oxygen (O) can be considered. Typically, lithium zincate can be various compounds represented by the general formula LixZnyOz, where x, y, and z are positive real numbers satisfying x + 2y - 2z = 0. Typically, various forms of lithium zincate can be considered, comprising a zincate anion consisting of any number of zincate ions and a lithium cation. Examples of such lithium zincate may be _Li₆ZnO₄ , _Li₄ZnO₃ , _etc. These may be single phases consisting of any one of these, or multiphases containing a combination of two or more of _these .

Lithium zincate typically absorbs _CO₂ to form other LixZnOz phases (2z = x + 2y; when x = 0, the compound formed is ZnO; y and z are always greater than 0) depending on the degree of carbonation. A typical reaction is represented by the following equation:
Li ₆ ZnO ₄ +CO ₂ → Li ₄ ZnO ₃ +Li ₂ CO ₃
As shown in the diagram, since this reaction is reversible, lithium zincate can desorb _CO2 . For this reason, lithium _zincate preferably has a high proportion of _Li6ZnO4 , for example, 70 mol% or more, more preferably 80 mol% or more, particularly preferably 90 mol% or more, and for example, substantially 100 mol% is desirable.

Lithium zincate may be commercially available or manufactured according to conventionally known methods. A preferred manufacturing method is to follow the lithium silicate manufacturing methods described in (A1) to (A3) above, for example, by using a zinc salt instead of colloidal silica, which is the Si source used in the lithium silicate manufacturing method. For example, zinc nitrate can be suitably used as the zinc salt. This allows for the production of a first powder containing lithium zincate particles. Then, by similarly carrying out (A) to (D) above using this first powder, a carbon dioxide absorbing material according to the second embodiment can be manufactured. Note that (E) above may also be carried out in the same manner as in the first embodiment.

The carbon dioxide absorbing material obtained in this manner contains a first powder, a second powder, and a third powder, and the preferred proportions of each powder in the carbon dioxide absorbing material containing lithium zincate are the same as in the first embodiment described above. However, regarding the ratio of the first powder to the third powder containing lithium zincate, the proportion of the first powder is preferably 5 wt% to 25 wt%. By using such a ratio, it is possible to produce a carbon dioxide absorbing material with improved _CO2 absorption capacity, especially in the intermediate temperature range.

The carbon dioxide absorbing material according to the second embodiment includes second particles, which are aggregates of a plurality of first particles, similar to the first embodiment described above. The first particles contain an alkali zincate containing at least lithium zincate, an alkali carbonate, an alkali nitrate, and a magnesium-containing compound containing at least magnesium carbonate.
Lithium zincate can form salts with alkaline components other than lithium (e.g., potassium, sodium, etc.) that may be present in alkali carbonates and alkali nitrates, potentially becoming alkali zincates.

In the second embodiment, the first particles can take on various forms such as spherical, granular, rod-shaped, or flake-shaped. Among these, the presence of rod-shaped first particles is preferable, as it can create larger voids than those formed by the aggregation of spherical or granular particles. This increases the surface area in contact with _CO2 , thereby improving the _CO2 absorption characteristics. A rod-shaped particle refers to a crystalline form that has grown significantly in one direction (one dimension). Here, if the length of the crystal is a and the length of one of its short sides perpendicular to the length is b, the average length b in the short side is generally 500 nm or less, preferably 300 nm or less, and particularly preferably 200 nm or less. The aspect ratio defined by a/b is typically 2 or more, and may be, for example, 5 or more. Such dimensions can be measured, for example, based on electron microscope observation.

The average particle diameter of the second particles in the second embodiment is not particularly limited, but for example, it may be between 1 μm and 20 μm, and between 5 μm and 15 μm. The second particles are formed by the aggregation of multiple first particles, typically with voids between them. This average particle diameter can be measured by laser diffraction/scattering.

In the carbon dioxide absorbing material according to the second embodiment, when the total of the zinc (Zn) component (typically Zn contained in alkali zincate) and the magnesium (Mg) component (typically Mg contained in magnesium salts and magnesium oxide) is 100 mol%, the proportion of the zinc component is preferably 5 mol% to 80 mol%, more preferably 5 mol% to 40 mol%, and even more preferably 5 mol% to 25 mol%. With such proportions, the _CO2 absorption capacity is particularly improved in the intermediate temperature range (e.g., 200°C to 400°C). The proportions of the composition of the carbon dioxide absorbing material can be calculated, for example, based on XRD.

The carbon dioxide absorbing material obtained as described above may have carbon dioxide absorbing capacity in a temperature range of, for example, 100°C to 650°C. In particular, it has a more suitable _CO2 absorption capacity in the intermediate temperature range of 150°C to 450°C, and even more so in the intermediate temperature range of 200°C to 400°C. Furthermore, the carbon dioxide absorbing material disclosed herein can selectively absorb _{CO2 .} Therefore, it can be used, for example, for the recovery of _CO2 from blast furnaces and basic oxygen furnaces in the steel industry, the recovery of _CO2 from high-temperature flue gases in power plants, and for improving the efficiency of water-gas shift reactions for hydrogen production by removing CO2. In addition, the carbon dioxide absorbing material disclosed herein can desorb absorbed carbon dioxide by temperature changes called temperature swing processes or by changes in the partial pressure of the gas in the atmosphere called pressure swing processes. As a result, the carbon dioxide absorbing material disclosed herein can also be used to produce pure _CO2 gas. The produced _CO2 gas can be used, for example, for industrial or food purposes. It can also be used as a raw material for chemical substances such as CO and methane. In particular, the carbon dioxide absorbing material disclosed herein can desorb absorbed _CO2 at high temperatures of, for example, 450°C or higher, and is therefore suitable for use in the conversion to methane by a thermal catalyst.

The method for manufacturing the _CO2 absorbing material disclosed herein will be described below with reference to specific embodiments. However, the technology disclosed herein is not intended to be limited to the following examples.

1-1. Preparation of the first powder containing lithium silicate (LS) Lithium silicate (lithium orthosilicate) was synthesized using lithium nitrate ( _LiNO₃ , manufactured by Alfa Aesar) and colloidal silica (manufactured by Sigma-Aldrich) as starting materials, according to the technology disclosed herein.
First, 11.5 g of lithium nitrate ( _LiNO₃ ) was dissolved in distilled water, and hydrolysis was carried out by adding a 25% ammonia solution to this lithium nitrate aqueous solution while stirring at room temperature until the pH reached 8. Colloidal silica (enough to contain 2.5 g of _SiO₂ ) was added dropwise to this reaction aqueous solution, and the mixture was stirred at room temperature for 1 hour to obtain a sol-like composition in which the lithium silicate precursor was dispersed in the aqueous solution. Next, a dehydration condensation reaction was carried out on this sol-like composition by irradiating it with an electron beam at 2.45 GHz and 700 W. A microwave oven was used for electron beam irradiation, and the irradiation was performed five times for 2 minutes each, for a total of 10 minutes. Specifically, the sol-like composition was irradiated with an electron beam for 2 minutes to boil, a rest period was provided, and the sol-like composition was cooled to room temperature, and the amount of water evaporated by boiling was replenished to the initial amount. This process was repeated five times. The dehydration condensed gel-like composition (reactant) was dried at 110°C to 150°C and calcined at 800°C in an air atmosphere for 3 hours to obtain a powder. The powder sample obtained in this way is denoted as LS.

1-2. Preparation of Alkali Carbonate/Nitrate/Lithium Silicate Composite Particles _{K₂CO₃ powder} , _KNO₃ powder, and _NaNO₃ powder were prepared and mixed using a mortar and pestle to a mass ratio of _K₂CO₃ : _KNO₃ : _NaNO₃ = 24 : ₄₅ : 31. This mass ratio is a formulation that allows these to form eutectic carbonate/nitrate. Next, this mixture was dried in an air oven at 80°C for 30 minutes. The powder obtained in this way will be referred to as eutectic carbonate/nitrate (ECN) below. Then, 0.42 g of ECN and 1 g of the obtained LS powder were dispersed in isopropanol (IPA), mixed using a mortar and pestle, and dried in an air oven at 80°C for 30 minutes to obtain a powder. The powder sample obtained in this way will be referred to as LS + ECN.

1-3. Preparation of Magnesium Carbonate/Alkali Carbonate/Nitrate/Lithium Silicate Composite _Particles 1 g of magnesium carbonate hydroxide hydrate (( _MgCO₃ ) _₄・Mg(OH) _₂・_xH₂O , manufactured by Sigma-Aldrich, product number: 227668; hereinafter, this powder will also be referred to as "MG") as the source of MgCO₃, and 0.42 g of ECN were dispersed in IPA and mixed using a mortar and pestle. Then, the mixture was dried in an air oven at 80°C for 30 minutes to obtain a powder. The powder sample thus obtained will be referred to as MG + ECN. The magnesium carbonate hydroxide hydrate used contained 24 wt% Mg.
0.5 g of LS+ECN powder and 0.5 g of MG+ECN powder were dispersed in IPA, mixed using a mortar and pestle, and then dried in an air oven at 80°C for 30 minutes to obtain a powder. The powder sample obtained in this way is denoted as LSME. The weight ratio of this LSME was LS:MG:ECN = 35:35:30. Furthermore, the molar ratio of the Si component to the total Si and Mg components in the LSME was 44%.

1-4. FE-SEM and TEM Observation The obtained LSME powder samples were observed using FE-SEM. For FE-SEM observation, a Hitachi High-Technologies Corporation SU8230 was used. The obtained FE-SEM images are shown in Figures 3A to 3C. In addition, the LSME powder samples were observed using a transmission electron microscope (TEM). For TEM observation, a 300kV high-resolution TEM (HR-TEM): Tecnai G ² 30 S-Twin manufactured by FEI GmbH in the Netherlands was used. The obtained TEM image is shown in Figure 3D.
FE-SEM observation revealed that the LSME powder sample consisted of aggregated flake-like particles forming particles with a diameter of approximately 5 μm or more. In these flake-like particles, the average length in the longitudinal direction within the crystal plane was approximately 500 nm or less, and the average length in the thickness direction was approximately 100 nm or less. Furthermore, the aspect ratio between the average length in the longitudinal direction and the average length in the thickness direction was consistently 2 or greater, and generally 5 or greater. TEM observation also revealed that these flake-like particles were further formed by aggregates of generally spherical particles.

1-5. _CO2 Absorption Characteristics <Dynamic Absorption Characteristics>
The carbon dioxide absorption characteristics of the obtained LSME powder samples were evaluated as dynamic absorption characteristics under varying ambient temperature. Specifically, a predetermined amount of _CO2 as a probe molecule was adsorbed onto the LSME powder sample, and the amount of adsorbed and desorbed gases produced by continuously increasing the sample temperature was measured by thermogravimetric analysis (TGA). 100% _CO2 gas was supplied at a rate of 50 ml/min, and measurements were performed in the temperature range of 100°C to 650°C at a heating rate of 10°C/min. The weight change of the powder sample was measured, and the _CO2 absorption capacity (mg/g) per gram of powder sample was determined. The obtained dynamic TGA curve is shown in Figure 4. A Perkin Elmer STA6000 was used for these measurements.

As shown in Figure 4, the LSME powder sample was confirmed to have _CO2 absorption capacity in the temperature range of 100°C to 650°C. Furthermore, it exhibited excellent _CO2 absorption capacity in the temperature range of 150°C to 450°C while increasing the temperature. It was also confirmed to have superior _CO2 absorption capacity in the temperature range of 200°C to 400°C.
Furthermore, it was confirmed that when the temperature rose above 450°C, the absorbed _CO2 was desorbed. This confirmed that the LSME powder sample can desorb _CO2 gas through a temperature swing process (i.e., a temperature change without a change in the partial pressure of _CO2 gas).

<Isothermal Absorption Characteristics>
Next, the isothermal absorption characteristics of carbon dioxide in the LSME powder sample were evaluated by TGA while varying the measurement temperature. The measurement temperatures were set to 200°C, 250°C, and 300°C. 100% _CO2 gas was used for the isothermal absorption test. The LSME powder sample was heated to the predetermined measurement temperature at a heating rate of 10°C/min, and then held under a 100% CO2 _gas stream for 60 minutes to measure the _CO2 absorption capacity at that temperature. The obtained isothermal absorption curve is shown in Figure 5. After 60 minutes, the 100% _CO2 gas was replaced with 100% _N2 gas, and the temperature was raised to 350°C to desorb the _CO2 gas.
As can be seen from Figure 5, even at 200°C, _{the CO2} absorption capacity reached 44 mg/g or more (i.e., 1 mmol/g or more) in about 10 minutes, and further increased to 50 mg/g or more in about 20 minutes. It was also confirmed that in the temperature range of 200°C to 300°C, the _CO2 absorption rate and _CO2 absorption capacity improved as the temperature increased.

Next, the _CO2 absorption capacity of the LSME powder sample at predetermined temperatures was measured by TGA. Specifically, the temperatures were set to 100°C, 150°C, 200°C, 250°C, 300°C, 325°C, 350°C, 375°C, and 400°C, and the _CO2 gas absorption capacity after 60 minutes was measured. Four trials were performed at each temperature, and the average values and error bars are shown in Figure 6.
As shown in Figure 6, the _CO2 gas absorption capacity was 40 mg/g or more in the temperature range of 200°C to 400°C, and 44 mg/g or more (i.e., 1 mmol/g or more) in the temperature range of 200°C to 375°C. Furthermore, it was confirmed that an excellent _CO2 absorption capacity of 100 mg/g or more was achieved, particularly in _the temperature range of 250°C to 350°C.

<Pressure Swing>
The absorption and desorption characteristics of _CO2 gas in LSME powder samples were evaluated by TGA (Total Gas Aggregation) by changing the partial pressure of the gas in the atmosphere (pressure swing). Specifically, the LSME powder sample was placed at 300°C and 100% _CO2 gas was supplied at a rate of 50 ml/min for 50 minutes. Then, while maintaining 300°C, the 100% _CO2 gas was replaced with 100% _N2 gas. The obtained isothermal absorption curves are shown in Figure 7.
As shown in Figure 7, after the supplied gas was changed from _CO₂ to _N₂ , it was confirmed that the LSME powder sample desorbed _CO₂ gas under a constant temperature of 300°C. This confirmed that the LSME powder sample can absorb and desorb _CO₂ through pressure swing under a constant temperature.

Next, the LSME powder sample was placed at 350°C and 100% _CO2 gas was supplied at a rate of 50 ml/min for 60 minutes. After that, the temperature was changed to 300°C, and the 100% _CO2 gas was replaced with 100% _N2 gas. The resulting _CO2 absorption curve is shown in Figure 8.
As shown in Figure 8, it was confirmed that _CO2 can be desorbed even when the _CO2 desorption temperature (300°C) is lower than the _CO2 absorption temperature (350°C). Also, as can be seen from Figure 6, the CO2 absorption capacity is larger at the _CO2 desorption temperature (300°C) than at the _CO2 absorption temperature (350°C). Even in such cases, it was confirmed that _CO2 gas can be desorbed by a pressure swing that changes _the supplied _CO2 gas to _N2 gas.

<Absorption characteristics for 20% _CO2 gas>
An LSME powder sample was placed under a gas stream consisting of 20% _CO₂ gas and 80% _N₂ gas. The temperature was set to 250°C and maintained for 60 minutes. After that, the temperature was increased to 350°C and the gas was switched to 100% _N₂ gas. The resulting _CO₂ absorption curve is shown in Figure 9.
As shown in Figure 9, the LSME powder sample was found to be able to absorb _CO2 even when the _CO2 content was 20%, and its _CO2 absorption capacity at 250°C was confirmed to be 44 mg/g or more (i.e., 1 mmol/g or more).

1-6. Cycle Characteristics The cycle characteristics of _CO2 absorption were evaluated when _CO2 gas absorption and desorption were repeatedly performed on an LSME powder sample at a high temperature of 300°C or higher. Specifically, using a TGA apparatus, the LSME powder sample was heated to 300°C and held at this temperature under a 100% _CO2 gas flow for 60 minutes to absorb _CO2 gas. Next, the LSME powder sample was heated to 350°C and held at this desorption temperature under a 100% _N2 gas flow for 30 minutes to desorb _CO2 gas. This absorption/desorption cycle was repeated four times, and the weight change of the LSME powder sample was examined. The results are shown in Figure 10.
As is clear from Figure 10, the LSME powder sample showed no variation in its absorption characteristics even after repeated absorption and desorption of _CO2 , confirming that it possesses stable cycle characteristics.

1-7. XRD
X-ray diffraction analysis (XRD) was performed on LSME powder samples. For this analysis, a small-angle/wide-angle X-ray scattering setup (Xeuss SAXS/WAXS system, 2θ = 15° to 36°, manufactured by Xenocs, France) using Cu Kα rays (λ = 0.154 nm) was employed.
For this analysis, LSME powder samples were used before _CO2 absorption, after _CO2 absorption, and after _CO2 desorption (regeneration). For the first sample, the LSME powder sample before _CO2 absorption or desorption was used for XRD analysis (pre-absorption sample). For the second sample, 100% _CO2 gas was supplied to the LSME powder sample at 300°C for 60 minutes to absorb _CO2 , and then the sample was cooled to room temperature while the _CO2 gas was still supplied. This sample was carefully removed and used for XRD (post-absorption sample). For the third sample, _CO2 absorption was performed at 300°C for 60 minutes, then the temperature was raised to 350°C, and the supply of 100% CO2 gas was changed to ₁₀₀ % _N2 gas to desorb the _CO2 gas. This sample was cooled to room temperature under _N2 gas supply, carefully removed, and used for XRD analysis (regeneration sample). The obtained XRD patterns are shown in Figure 11.

Phase identification of the XRD peaks _{was performed} based on the following PDF (Pair Distribution Function) reference numbers: _Li₄SiO₄ : _00-020-0637 , _Li₂SiO₃ : 00-015-0519, _Li₂CO₃ : 00-022-1141, MgCO₃: 00-008-0479, _Mg ( _NO₃ ) _{₂ :} 00-019-0765, K₂Mg( _CO₃ ) _₂ : 01-075-1725, _KNO₃ : 01-071-1558, _NaNO₃ : 00-036-1474. As shown in Figure 11, after _CO2 absorption, in addition to _{the Li2SiO3} peak, the peaks of carbonate phases such as _Li2CO3 , _{MgCO3 ,} and _K2Mg ( _CO3 ) ₂ became stronger. Furthermore, the proportion of these carbonate peaks decreased significantly after _CO2 desorption. In addition, the proportion of _{the Li4SiO4} peak area increased both before and after _{CO2 gas} absorption. From this, it is considered that the LSME powder sample can absorb _CO2 gas by reacting with _{CO2 gas} to synthesize _Li2SiO3 (where the _{Li2SiO3 phase} is also formed), _MgCO3 , and _K2Mg ( _CO3 ) ₂ . Furthermore, considering that complete conversion from the _{Li₄SiO₄ 4-} phase to the _{Li₂SiO₃ -} phase and _{Li₂CO₃ -} phase due to CO₂ absorption (and, more importantly, the formation of the _{Li₄SiO₄ 4-} layer due to _{CO₂ desorption} ) is hardly achieved at low temperatures around 300°C, it is thought that the MgCO₃ _- phase and _K₂Mg ( _CO₃ ) _₂- phase formed in LSME help to form pathways through which _CO₂ diffuses into and out of the lithium phase. Therefore, the _CO₂ absorption and desorption performance of LSME at low temperatures is unique, resulting from the synergistic effect of the material combination. A similar mechanism is thought to be present in the case of LZME, which will be discussed later.

1-8. FT-IR
LSME powder samples were analyzed by Fourier transform infrared spectroscopy (FT-IR). A Bruker αE FT-IR spectrophotometer was used for this analysis. The LSME powder samples used were the same as those analyzed by XRD described above; that is, LSME powder samples obtained under the same conditions as the samples analyzed by XRD, LSME powder samples obtained after _CO2 gas absorption at 300°C, and LSME powder samples obtained after _CO2 desorption at 350°C. The obtained FT-IR patterns _are shown in Figure 12.
As shown in Figure 12, the FT-IR pattern after _CO2 absorption showed peaks at 1435 ^cm⁻¹ (the middle dotted line of the three dotted lines located between approximately 1300 ^cm⁻¹ and 1500 ^cm⁻¹ ) and 892 ^cm⁻¹ (the left dotted line of the two dotted lines located between approximately 800 ^cm⁻¹ and 900 ^cm⁻¹ ). On the other hand, the FT-IR patterns before _CO2 absorption and after _CO2 desorption showed almost no peaks at these positions. The peaks at these positions appear based on the asymmetric stretching vibration and out ^- of-plane bending vibration of carbonate ions ( _CO₃²⁻ ). Therefore, it was confirmed that LSME can retain at least a portion of the _CO2 gas as carbonate ions. The peaks at the dotted lines other than those mentioned above are considered to be unidentate carbonates and are thought to originate from _CO2 attached to the surface of the LSME powder sample.

1-9. Evaluation of the proportion of LS in LSME Of the above 1-3, powder samples were prepared by changing the mixing ratio of LS + ECN powder and MG + ECN powder so that the proportion of LS ranged from 0 wt% to 70 wt%. Note that when LS is 0 wt%, it means that the sample consists only of MG + ECN powder, and when LS is 70 wt%, it means that the sample consists only of LS + ECN powder. In all proportions, the proportion of ECN is 30 wt%, and the remaining proportion is the proportion of magnesium hydroxide carbonate (MG) powder mentioned above.
The dynamic absorption characteristics were evaluated using the prepared powder sample in the same manner as described in sections 1-5 above. The value of the maximum _CO2 absorption capacity was then determined from the obtained dynamic TGA curve. Figure 13 shows a graph illustrating the relationship between the proportion of LS and the resulting maximum _CO2 absorption capacity.
As shown in Figure 13, it was confirmed that _CO2 absorption capacity is present when the proportion of LS is between 5 wt% and 60 wt%. At this time, the proportion of the Si component to the total of the Si and Mg components contained in LSME (hereinafter, this proportion will also be called the "proportion of Si component") corresponds to 5 mol% to 85 mol%. Furthermore, when the proportion of LS is approximately between 20 wt% and 50 wt% (the proportion of Si component is approximately between 20 mol% and 75 mol%), it exhibits an improved _CO2 absorption capacity. Moreover, when the proportion of LS is approximately between 30 wt% and 50 wt% (the proportion of Si component is approximately between 35 mol% and 75 mol%), it was confirmed that it exhibits a superior _CO2 absorption capacity compared to MG + ECN alone (LS is 0 wt%) and LS + ECN alone (LS is 70 wt%).

2-1. Preparation of Lithium Zinc Oate (LZ) In the procedure described in 1-1 above, colloidal silica was replaced with zinc nitrate hexahydrate {Zn( _NO₃ ) _₂ · _6H₂O , manufactured by Merck} to prepare lithium zinc oxide (LZ). Specifically, 11.5 g of lithium nitrate ( _LiNO₃ ) and 8.3 g of zinc nitrate hexahydrate were dissolved in distilled water, and hydrolysis was carried out by adding 25% ammonia solution to this lithium nitrate aqueous solution while stirring at room temperature until the pH reached 8. By stirring at room temperature for 1 hour, a sol-like composition in which the lithium zinc oxide precursor was dispersed in the aqueous solution was obtained. The subsequent procedure was carried out in the same manner as described in 1-1 above.
Next, the same procedure was performed in step 1-2 above, with LS replaced by LZ, to obtain LZ+ECN powder. Furthermore, in step 1-3 above, the LS+ECN powder was changed to LZ+ECN powder, and the mixture was changed to 0.15 g of LZ+ECN powder and 0.6 g of MG+ECN powder. The LZME powder sample was prepared in the same manner except for these changes. The LZME powder sample obtained in this way has a weight ratio of LZ:MG:ECN = 14:56:30.

2-2. FE-SEM Observation The obtained LZME powder sample was observed using FE-SEM in the same manner as described in 1-4 above. The obtained FE-SEM images are shown in Figures 14A to 14C.
FE-SEM observation revealed that the LZME powder sample contained rod-shaped particles, and that multiple rod-shaped particles aggregated to form particles with a diameter of approximately 5 μm or more. The average length of these rod-shaped particles in the longitudinal direction within the crystal plane was approximately 1 μm or less. Furthermore, the aspect ratio between the average length in the longitudinal direction and the average length in the thickness direction was reliably observed to be 2 or more.

2-3. _CO2 Absorption Characteristics <Dynamic Absorption Characteristics>
The _CO2 absorption characteristics of the obtained LZME powder sample were evaluated as dynamic absorption characteristics under varying ambient temperature, in the same manner as described in sections 1-5 above. The resulting dynamic TGA curve is shown in Figure 15.
As shown in Figure 15, the LZME powder sample was confirmed to have _CO2 absorption capacity in the temperature range of 100°C to 450°C. Furthermore, with increasing temperature, it was confirmed to have excellent _CO2 absorption capacity in the temperature range of 200°C to 400°C, and in particular, excellent CO2 gas absorption capacity of 300 mg/g or more in the temperature range of 300°C to 400°C. In addition, it was confirmed that _{CO2 gas} desorption occurred when the temperature exceeded 420°C due to the temperature increase. This confirmed that the LZME powder sample can have _CO2 gas desorbed by a temperature swing process.

<Pressure Swing>
The absorption and desorption characteristics _of the obtained LZME powder sample were evaluated by TGA under pressure swing. Specifically, the LZME powder sample was placed at 350°C and supplied with 100% _CO2 gas at a rate of 50 ml/min for 60 minutes. Then, while maintaining 350°C, the 100% _CO2 gas was replaced with 100% _N2 gas. The obtained isothermal absorption curve is shown in Figure 16.
As shown in Figure 16, it was confirmed that the LZME powder sample desorbed _CO2 gas after 60 minutes at a constant temperature of 350°C (i.e., after the supplied gas was changed from _CO2 to _N2 ). This confirmed that the LZME powder sample can absorb and desorb _CO2 by pressure swing at a constant temperature.

2-4. Evaluation of the proportion of LZ in LZME Powder samples were prepared by changing the mixing ratio of LZ + ECN powder and MG + ECN powder as described in 2-1 above, so that the proportion of LZ ranged from 0 wt% to 70 wt%. When LZ is 0 wt%, it means that the sample consists only of MG + ECN powder, and when LZ is 70 wt%, it means that the sample consists only of LZ + ECN powder. In all ratios, the proportion of ECN is 30 wt%, and the remaining proportion is the proportion of magnesium hydroxide carbonate (MG) as described above.
The dynamic absorption characteristics were evaluated using the prepared powder sample in the same manner as described in sections 1-5 above. The value of the maximum _CO2 gas absorption capacity was then determined from the obtained dynamic TGA curve. Figure 17 shows a graph illustrating the relationship between the LZ ratio and the resulting maximum _CO2 absorption capacity.
As shown in Figure 17, the presence of LZ results in a superior _CO2 absorption capacity compared to MG + ECN alone (0 wt%). For example, when the proportion of LZ is between 5 wt% and 60 wt%, it exhibits a superior _CO2 absorption capacity. The proportion of Zn relative to the sum of the Zn and Mg components (hereinafter also referred to as "the proportion of Zn") in this case corresponds to approximately 5 mol% to 80 mol%. Furthermore, it was confirmed that an even superior _CO2 gas absorption capacity is obtained when the proportion of LZ is between 5 wt% and 40 wt% (the proportion of Zn is approximately between 5 mol% and 40 mol%). Moreover, it was confirmed that an even superior CO2 gas absorption capacity is obtained when the proportion of LZ is between 5 wt% and ₂₅ wt% (the proportion of Zn is approximately between 5 mol% and 25 mol%).

The above describes specific examples of the present invention in detail, but these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples illustrated above.

Claims (12)

A method for manufacturing a carbon dioxide absorbing material,
Prepare a first powder containing lithium silicate particles or lithium zincate particles.
The first powder is mixed with a second powder containing alkali carbonate particles and alkali nitrate particles to obtain a powder containing first composite particles in which the alkali carbonate particles and alkali nitrate particles are aggregated on the surface of the lithium silicate particles or the lithium zincate particles.
A third powder containing magnesium carbonate particles is mixed with the second powder to obtain a powder containing second composite particles in which the alkali carbonate particles and alkali nitrate particles are aggregated on the surface of the magnesium carbonate particles, and
A powder containing the first composite particles and a powder containing the second composite particles are mixed to obtain a mixed powder containing aggregates having carbon dioxide absorbing capacity formed by the aggregation of the first composite particles and the second composite particles.
A method for producing a carbon dioxide absorbing material, including the above. When the total amount of the mixed powder is 100 wt%,
The total ratio of the first powder and the third powder is 50 wt% or more and 95 wt% or less.
The proportion of the first powder is 5 wt% or more and 60 wt% or less.
The proportion of the third powder is 5 wt% or more and 60 wt% or less.
A method for producing a carbon dioxide absorbing material according to claim 1. A method for producing a carbon dioxide absorbing material according to claim 1 or 2, wherein at least one of the alkali carbonate particles and the alkali nitrate particles contains two or more components selected from sodium (Na), potassium (K), and lithium (Li). The method for producing a carbon dioxide absorbing material according to claim 3, wherein the second powder comprises eutectic carbonate/nitrate particles. Furthermore, the method for producing a carbon dioxide absorbing material according to any one of claims 1 to 4, comprising heat-treating the mixed powder containing the aggregates. A powder capable of absorbing and desorbing carbon dioxide,
It includes a second particle which is made up of multiple first particles,
The first particle is,
Alkali silicate containing at least lithium silicate or alkali zincate containing at least lithium zincate,
Alkaline carbonates and,
Alkaline nitrates and,
A carbon dioxide absorbing material containing a magnesium-containing compound, which includes at least magnesium carbonate. The carbon dioxide absorbing material according to claim 6, wherein when the total of the silicon (Si) component contained in the alkali silicate and the magnesium (Mg) component contained in the magnesium-containing compound is taken as 100 mol%, the proportion of the silicon (Si) component is 10 mol% or more and 85 mol% or less. The carbon dioxide absorbing material according to claim 6, wherein when the total of the zinc (Zn) component contained in the alkali zincate and the magnesium (Mg) component contained in the magnesium-containing compound is taken as 100 mol%, the proportion of the zinc (Zn) component is 5 mol% or more and 80 mol% or less. The carbon dioxide absorbing material according to any one of claims 6 to 8, wherein the first particles have at least one of the forms of flakes or rods. The carbon dioxide absorbing material according to any one of claims 6 to 9, wherein at least one of the alkali carbonate and the alkali nitrate contains two or more components selected from sodium (Na), potassium (K), and lithium (Li). The carbon dioxide absorbing material according to claim 10, wherein the alkali carbonate and the alkali nitrate form a eutectic. A carbon dioxide absorbing material that is a heat-treated product of the carbon dioxide absorbing material described in any one of claims 6 to 11.