WO2017022249A1 - Carbon dioxide-absorbing material and method of producing same - Google Patents

Abstract

The herein disclosed technology provides a method of producing a carbon dioxide-absorbing material that exhibits improved CO₂ absorption characteristics. This production method is a method of producing a carbon dioxide-absorbing material in which a major component thereof is lithium silicate, and includes the following steps: preparing a sol composition in which an Li-Si precursor compound containing a lithium (Li) component and a silicon (Si) component is dispersed in an aqueous solution; obtaining a gel composition by exposing the sol composition to electromagnetic waves; and calcining the gel composition to obtain a lithium silicate containing lithium and silicon. As for a carbon dioxide-absorbing material, at least a part of a surface of the lithium silicate may be covered with a alkali carbonate. At least a part of the surface of this lithium silicate may be covered by an alkali carbonate.

Description

CARBON DIOXIDE-ABSORBING MATERIAL AND METHOD OF PRODUCING SAME

The present invention relates to a carbon dioxide-absorbing material that has excellent carbon dioxide absorption characteristics and to a method of producing same.
The present application claims priority based on Indian Patent Application No. 2932/MUM/2015 filed on August 3, 2015, and the contents of that application are incorporated in their entirety in this Description by reference.

Carbon dioxide (CO₂) is a greenhouse gas and major reductions in its emissions are thus a pressing concern. The development is therefore required of materials and technologies that can selectively separate and recover CO₂ from the exhaust gases emitted from, for example, automobiles and fossil fuel-fired power plants and factories. CO₂-adsorbing materials/absorbing materials composed of, for example, the following have been introduced to date: porous materials; CO₂ absorption-capable chemical absorbents that incorporate, for example, the amino group; metal-organic frameworks (MOFs); carbonaceous materials; and alkali metal carbonates.

Almost all of these materials are used as CO₂-absorbing materials in low-temperature environments, i.e., from room temperature to 200°C, due to their low heat resistance and their physical absorption characteristics. Among the preceding, on the other hand, CO₂-absorbing materials composed of alkali metal carbonates can be used at high temperatures in excess of 450°C and as a consequence there are expectations for their use as CO₂-absorbing materials capable of use in high-temperature environments. In particular, it has been reported that alkali metal carbonates such as lithium silicate have a CO₂ absorption capacity of 10 mass% to as much as 35 mass% and there are thus strong expectations for their entry into practical applications (refer, for example, to Patent Literature 1, Patent Literature 2, Patent Literature 3, and Non Patent Literature 1 and Non Patent Literature 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, these alkali metal carbonates have had a slow CO₂ absorption rate of not more than 50 mg/(g×min) and typically not more than 10 mg/(g×min), and this has required large amounts of CO₂-absorbing material in order to efficiently treat CO₂. In addition, while these alkali metal carbonates have had excellent CO₂ absorption characteristics in the high-temperature region of 600°C and above, a problem with them has been their very low CO₂ absorption capacity in the intermediate temperature region of, for example, from 200°C to 450°C.
The present invention was pursued considering these problems, and an object of the present invention is to provide a carbon dioxide-absorbing material that exhibits CO₂ absorption characteristics that have been further improved. An additional object of the present invention is to provide a simple and convenient method of producing this carbon dioxide-absorbing material.

In order to achieve these objects the present invention provides a method of producing a carbon dioxide (CO₂)-absorbing material in which a major component thereof is lithium silicate. This production method encompasses preparing a sol composition in which an Li-Si precursor compound containing a lithium (Li) component and a silicon (Si) component is dispersed in an aqueous solution; obtaining a gel composition by exposing the sol composition to electromagnetic waves; and calcining this gel composition to obtain a lithium silicate that contains lithium and silicon.

The use of this construction for obtaining a lithium silicate by application of the so-called sol-gel method makes it possible to shorten the time required for gelation and to obtain a lithium silicate that has improved CO₂ absorption characteristics. This then makes it possible to provide a CO₂-absorbing material that exhibits, for example, an improved CO₂ absorption capacity and an improved CO₂ absorption rate.

A preferred aspect of the herein disclosed method of producing a CO₂-absorbing material is characterized in that the sol composition is prepared by carrying out a hydrolysis reaction in an aqueous solution containing the lithium (Li) component and the silicon (Si) component.
This construction makes possible the stable production of a high-quality CO₂-absorbing material through the application of a known sol-gel method.

A preferred aspect of the herein disclosed method of producing a CO₂-absorbing material is characterized in that the silicon (Si) component is at least one selected from silica alkoxides, colloidal silicas, and fumed silicas. This construction makes it possible to more easily produce a CO₂-absorbing material that exhibits an excellent CO₂ absorption performance.

A preferred aspect of the herein disclosed method of producing a CO₂-absorbing material is characterized in that the sol composition additionally contains a germanium (Ge) component. This construction makes possible the production of a variety of CO₂-absorbing materials having different properties, e.g., crystal form, CO₂ absorption temperature region, CO₂ absorption capacity, and so forth.

A preferred aspect of the herein disclosed method of producing a CO₂-absorbing material is characterized in that the electromagnetic waves are microwaves having a wavelength from 1 mm to 1 m and a frequency from 300 MHz to 300 GHz. The total exposure time to the electromagnetic waves here is preferably from 1 minute to 60 minutes.
This construction also makes possible the simple and convenient production of a CO₂-absorbing material that exhibits an excellent CO₂ absorption performance.

A preferred aspect of the herein disclosed method of producing a CO₂-absorbing material characteristically further includes forming a composite of the lithium silicate with an alkali carbonate.
This construction makes possible the production of a CO₂-absorbing material that exhibits, for example, an increased CO₂ absorption capacity and CO₂ absorption rate in the temperature range from 200°C to 600°C.

In a preferred aspect of the herein disclosed method of producing a CO₂-absorbing material, the aforementioned alkali carbonate preferably contains two or more from among a sodium (Na) component, a potassium (K) component, and a lithium (Li) component. For example, in a preferred aspect the proportion of respective alkali components in the alkali carbonate is Na : from 1 mol% to 80 mol%, K : from 1 mol% to 70 mol%, and Li : from 1 mol% to 90 mol%. This alkali carbonate is particularly preferably a eutectic carbonate.
This construction makes possible the production of a CO₂-absorbing material that exhibits, for example, an even greater increase in the CO₂ absorption capacity and the CO₂ absorption rate in the temperature range from 200°C to 600°C.

In another aspect the herein disclosed art provides a CO₂-absorbing material. This CO₂-absorbing material is characteristically a CO₂-absorbing material in which the major component is a lithium silicate that has been produced by any of the hereabove-described production methods.
This serves to realize a CO₂-absorbing material that exhibits improved CO₂ absorption characteristics such as, for example, the CO₂ absorption capacity and CO₂ absorption rate.

The herein disclosed CO₂-absorbing material is characterized in that it has lithium silicate as its major component and has a CO₂ absorption rate per unit weight in the temperature range from 500°C to 710°C of at least 25 mg/(g×min). For example, the CO₂ gas absorption rate per unit weight in the temperature range from 600°C to 710°C is also preferably at least 50 mg/(g×min). A CO₂-absorbing material having a substantially improved CO₂ absorption rate is provided by such a herein disclosed art.

In a preferred aspect of the herein disclosed CO₂-absorbing material, when a is the dimension in a long direction and b is the dimension in a short direction orthogonal to the long direction, the dimension b in this short direction is not more than 100 nm and the aspect ratio given by a/b is at least 2, at least 50 number% of the total lithium silicate are rod-shaped particles.
The CO₂-absorbing material composed of particles having such a high aspect ratio is preferred because it resists aggregation even when used repetitively in a high-temperature environment and thus can maintain excellent CO₂ absorption characteristics on a long term basis. In this Description, the dimensions of the lithium silicate refer to values measured by observation with an observation means such as an electron microscope.

A preferred aspect of the herein disclosed CO₂-absorbing material is characterized in that an alkali carbonate is additionally incorporated therein and at least a part of the surface of the lithium silicate is covered by this alkali carbonate. This construction makes it possible to provide a CO₂-absorbing material having a CO₂ absorption rate that is stably and substantially increased.

The herein disclosed art as described in the preceding provides a CO₂-absorbing material that can selectively and efficiently absorb CO₂ in medium-temperature to high-temperature regions (for example, the temperature range from about 200°C to 710°C). Such a CO₂-absorbing material is particularly useful for the absorption of CO₂ from high-temperature mixed gases and CO₂ single-phase gases in, for example, liquid fuel production plants sourced from natural gas, CO₂ recovery systems that utilize the water-gas shift reaction, and so forth.

For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description to be read in connection with the accompanying drawing, wherein:
FIG. 1A is a flow diagram that shows the method of producing a CO₂-absorbing material according to an embodiment; FIG. 1B is a flow diagram that shows the method of producing a CO₂-absorbing material according to another embodiment; FIG. 1C is a flow diagram that shows the method of producing a CO₂-absorbing material according to another embodiment; FIG. 2 is the XRD pattern of the MW-SG sample according to an embodiment; FIG. 3(a) is the TEM image of the MW-SG sample in the as-prepared condition for the example, and FIG. 3(b) is the TEM image of the MW-SG sample after CO₂ absorption and desorption; FIG. 4 contains the dynamic TGA curves for the SG sample prepared by a conventional method and the MW-SG sample prepared by the herein disclosed art; FIG. 5 contains CO₂ isothermal absorption curves for the MW-SG sample according to an embodiment; FIG. 6 is a graph that shows the CO₂ absorption rate for the MW-SG sample according to an embodiment; FIG. 7 is a graph that shows the cycle characteristics of the MW-SG sample according to an embodiment; FIG. 8 contains the dynamic TGA curves of MW-SG-NKL samples according to another embodiment; FIG. 9 contains the CO₂ isothermal absorption curves of an MW-SG-NKL3 sample according to another embodiment; FIG. 10 is a graph of the CO₂ absorption rate for the MW-SG-NKL3 sample according to another embodiment; FIG. 11 contains XRD patterns for (a) the silicon starting material, (b) the lithium starting material, (c) the sol solution prior to exposure to electromagnetic waves, and (d) the gel solution post-exposure to electromagnetic waves, according to an embodiment; FIG. 12 is a diagram that shows the results of high-resolution X-ray diffraction analysis in-situ while carrying out a heat treatment on the dry powder from the gel solution; FIG. 13 contains TEM images of the dry powder from the gel solution, which has been heat treated at (a, b) 473 K, (c, d) 673 K, (e, f) 773 K, and (g, h) 1073 K; FIG. 14 contains SEM images of the dry powder from the gel solution, which has been heat treated at 1073 K; FIG. 15 is an SEM image that gives an example of a whole lithium silicate particle according to an embodiment, and FIG. 15(b) is an enlargement of a portion thereof; FIGS. 16(a) to (e) are TEM images of the lithium silicate particle of FIG. 15; FIG. 17 is the CO₂ isothermal absorption curve of a lithium silicate according to an embodiment; FIG. 18 contains the XRD pattern of (a) a lithium silicate and (b) a germanium-doped lithium silicate according to an embodiment; FIG. 19 contains the Raman spectra of (a) a lithium silicate and (b) a germanium-doped lithium silicate according to an embodiment; FIG. 20 is an SEM image at low magnification of germanium-doped lithium silicate particles according to an embodiment; FIGS. 21(a) to (d) are TEM images at different magnifications and observation fields of germanium-doped lithium silicate particles according to an embodiment; FIG. 22 contains the dynamic TGA curves of germanium-doped lithium silicate according to an embodiment; and FIG. 23 is the CO₂ isothermal absorption curve at 300°C of a germanium-doped lithium silicate according to an embodiment.

Preferred embodiments of the present invention are described in the following with reference to the drawings as appropriate. Moreover, matters required for the execution of the present invention but not particularly described in this Description can be understood as design matters for the individual skilled in the art based on the conventional art in the pertinent field. The present invention can be implemented based on the contents disclosed in this Description and the common general technical knowledge in the pertinent field. With reference to phrases of the form "X to Y" that indicate a numerical value range in this Description, these mean "at least X and not more than Y" unless specifically indicated otherwise.

FIGS. 1A to 1C are flow diagrams that show the method of producing the carbon dioxide (CO₂)-absorbing material according to an embodiment. The herein disclosed production method typically produces a CO₂-absorbing material in which the major component is lithium silicate.
Lithium silicate can be thought of as a compound that contains lithium (Li) and silicon (Si) and oxygen (O). Lithium silicate can typically be a variety of compounds represented by the general formula Li_xSi_yO_z where x, y, and z in this formula satisfy x + 4y - 2z = 0. It can typically be regarded as individual lithium silicate species of a form that contains a lithium cation and a silicate anion in which arbitrary numbers of silicate ions are connected. This lithium silicate may typically be lithium orthosilicate (Li₄SiO₄), lithium metasilicate (Li₂SiO₃), lithium disilicate (Li₂Si₂O₅), lithium metatrisilicate (Li₄Si₃O₈), lithium metatetrasilicate (Li₆Si₄O₁₁), and so forth. Lithium silicate is not limited to these examples and may, for example, be a compound as represented by Li₈SiO₆, Li₆Si₂O₇, Li₁₂SiO₈, and so forth. These may be a single phase composed of any single species or may be a mixed phase containing a combination of any two or more species. For the most part, lithium silicate is thought essentially to have a skeleton (for example, an SiO₄ chain) in which SiO₄ tetrahedra are connected together in various ways, with an alkali metal element such as Li inserted as the ion in the vacant spaces of these tetrahedra. The present inventors believe that, in accordance with the herein disclosed production method, the SiO₄ tetrahedra can undergo isomorphic substitution with other tetrahedra. For example, typically the AlO₄ tetrahedron, FeO₄ tetrahedron, GeO₄ tetrahedron, SnO₄ tetrahedron, and so forth can be considered for such a substituent, although there is no particular limitation thereon.

Thus, the herein disclosed lithium silicate may be a compound that contains another element (M) in addition to the Li, Si, and O cited above. There are no particular limitations on this other element as long as it is an element that forms a compound capable of being present in a stable manner under the conditions of CO₂ absorption and desorption. Examples are aluminum (Al) and iron (Fe) as referenced above and elements from the same Group 14 as silicon, e.g., germanium (Ge), tin (Sn), and lead (Pb). A germanium (Ge) component is preferred among these. These Group 14 elements are preferred because they readily substitute for Si in the crystalline structure of lithium silicate and can be present in a relatively stable manner. The proportion of the other element present in the lithium silicate is not strictly limited, but, for example, the silicon (Si) : other element (M) ratio is preferably approximately 1 : 0.001 to 1 : 0.5 and is more preferably 1 : 0.04 to 1 : 0.45. Such a lithium silicate, for example, can be understood as various compounds represented by the general formula Li_xM_y2Si_y1O_z. The x, y1, y2, and z in this formula are each natural numbers and satisfy y1 + y2 = y and x + 4y - 2z = 0.

In a representative example of the herein disclosed CO₂-absorbing material, and viewed from the standpoint of the CO₂ absorption capacity, lithium orthosilicate is preferably at least 70 mol% of the lithium silicate, more preferably at least 80 mol%, and particularly preferably at least 90 mol%, and, for example, is desirably substantially 100 mol%. In the following the present invention is described, for example, using the case of substantially 100 mol% lithium orthosilicate for the lithium silicate as an example. A change in the composition of the lithium silicate as a function, for example, of the surrounding environment and so forth, may be allowed.

The specification of lithium silicate as the "major component" means that lithium silicate occupies the largest proportion in the compounds that constitute the CO₂-absorbing material. The proportion of lithium silicate in the CO₂-absorbing material cannot be strictly prescribed when one considers the CO₂-absorbing material that takes the form of the composite described below, but is typically at least 50 mass% and preferably is at least 60 mass% (particularly preferably at least 70 mass%, for example, at least 80 mass%, at least 90 mass%, at least 95 mass%, and substantially 100 mass%). The proportion of lithium silicate can be determined, for example, based on X-ray diffraction analysis (XRD) as an example.

The method of producing this CO₂-absorbing material characteristically contains, for example, the following steps (S1) to (S3) as shown in FIG. 1A.
(S1) Preparing a sol composition in which an 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 exposing the sol composition to electromagnetic waves.
(S3) Calcining this gel composition to obtain a lithium silicate that contains lithium and silicon.
While not an essential step, the following step (S4) may also be carried out in the herein disclosed production method subsequent to step (S3), as shown in FIG. 1B.
(S4) Forming a composite of the obtained lithium silicate with an alkali carbonate.

S1. Preparation of the sol composition
The herein disclosed CO₂-absorbing material can in general be advantageously produced using a wet method in which a sol composition containing an Li-Si precursor compound is gelled.
This sol composition can be a colloidal aqueous solution that contains the Li-Si precursor compound as a dispersed material and at least water as the dispersion medium. The water used as the dispersion medium may be distilled water, ion-exchanged water, pure water, and so forth. As long as water is the major component, a water-soluble, low-molecular-weight organic solvent, e.g., lower alcohols (e.g., methanol, ethanol, butanol, and isopropanol) and ketones such as acetone, and so forth, may be mixed into this dispersion medium. The dispersion medium is preferably 100 mass% water (i.e., the sol composition can be a hydrosol) in order to raise the efficiency of hydrolysis, infra.

The Li-Si precursor compound forming the dispersed material can be any compound that contains the lithium component and silicon component serving as the starting materials for the lithium silicate, and that can form this lithium silicate through processes such as a dehydration condensation reaction or polycondensation reaction and calcination (heating). It can typically be a precipitate provided by the precipitation of the water-soluble lithium and silicon solutes as, for example, the hydroxide, and this may take the form of a hydrate or hydrated complex. In addition, the precipitate, e.g., the hydroxide, may undergo dehydration condensation as long as the sol state can still be maintained. Stated differently, this Li-Si precursor compound is a colloidal particle in sol form and its particle diameter and so forth are not strictly limited as long as it is a size that can maintain a colloidal solution. For example, the average particle diameter of the Li-Si precursor compound is typically approximately 1 nm to 5 μm and can be preferably approximately 3 nm to 3 μm and more preferably approximately 10 nm to 1 μm. The value measured by a dynamic light scattering method can be used for this average particle diameter.

This sol composition can be prepared, for example, by dispersing the separately prepared Li-Si precursor compound in the dispersion medium, e.g., water; or by acquiring a sol composition in which the Li-Si precursor compound has already been dispersed; or by using a known dry method, e.g., a combustion method or arc method, or wet method, e.g., a precipitation method or gel method (including the sol-gel method).
Taking as an example the production of the sol composition by a general sol-gel method, hydrolysis is produced by contacting the alkoxide with water in a mixed solution in which a lithium salt and a silica alkoxide can assume a dissolved state, thereby forming a precursor compound containing the Li component and Si component in this mixed solution. Here, since the dispersion medium for the sol composition is preferably water, the sol composition is more preferably produced by the addition of the alkoxide in small portions to an aqueous solution provided by the dissolution of the lithium salt. The precursor compound is obtained, for example, by stirring an aqueous solution of the lithium salt and adding silica alkoxide alcohol solution to the aqueous solution. Stirring may be carried out using magnetic or mechanical stirrers or other methods of mixing like ultrasonic mixing could be carried out.

When carrying out this hydrolysis reaction and the condensation reaction, an acid such as hydrochloric acid or a base such as ammonia may be added to the mixed solution as a hydrolysis catalyst for the purpose of adjusting the reaction rates (hydrolysis rate and condensation rate). This hydrolysis catalyst may also function to adjust the primary particle diameter of the resulting precursor compound through control of the pH of the reaction solution.

Lithium oxide and the various compounds that can form the oxide by heating can be used as the lithium salt. Specific examples here are the oxide, hydroxide, carbonate, nitrate, sulfate, phosphate, acetate, formate, oxalate, halide, and so forth of lithium. The various water-soluble salts are preferred. An alkoxide of lithium may also be used.

Usable as the silica alkoxide are, without particular limitation, the various compounds that can be used in the sol-gel method. For example, in specific terms the use is preferred of methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, 1,2-bistrimethoxysilylethane, silica alkoxide in which 1 to 4 alkoxy groups are bonded to the Si atom, silica alkoxide that contains a functional group such as the glycidyl group, and so forth. Among the preceding, the use of an alkoxysilane is preferred, and, for example, silica alkoxides in which 1 to 4 alkoxy groups are bonded to the Si atom, as typified by tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, and 1,2-bistrimethoxysilylethane, are preferred examples. Particularly preferred are tetraethoxysilane (Si(OC₂H₅)₄ : TEOS, also referred to as tetraethyl orthosilicate), tetramethoxysilane (Si(OCH₃)₄ : TMOS, also referred to as tetramethyl orthosilicate) and trimethoxy(methyl)silane. A single one of any of these may be used by itself or a combination of any two or more may be used.

The following techniques, for example, are examples of methods that may be used to produce the sol composition besides the sol-gel method provided above as an example. For example, the sol composition may be produced as described above using a silica nanomaterial, e.g., a gel-method silica (including colloidal silica), a precipitation-method silica, or a fumed silica (including silica xerogels, silica cryogels, and silica aerogels), as the silicon component. The herein disclosed sol composition provided, for example, by such a silica material and the lithium component forming a complex with each other in aqueous solution and/or forming a compound by, for example, a hydrolysis reaction and/or a dehydration condensation reaction, can be favorably used although this is not a particular limitation.

When the lithium silicate contains another element (M), production may be carried out so that the aforementioned Li-Si precursor compound contains this additional element. For example, when the sol composition is prepared by the sol-gel method, the other element (M) may be added to the mixed solution. Typically, and as exemplified in FIG. 1C, for example, a salt of the other element (M) may be added to the mixed solution just as for the lithium component. By doing this, a lithium silicate containing Li, Si, O, and M can be produced by the same procedure as described in the following.

S2. Formation of the gel composition by exposure to electromagnetic waves
A gel composition is then obtained by gelling the sol composition by exposing the prepared sol composition to electromagnetic waves.
For example, by continuously stirring, a dehydration condensation can generally be produced at the precipitate, e.g., the hydrate formed by hydrolysis, in the sol composition prepared by the sol-gel method. By continuing to stir, the Li-Si precursor compound can form a three-dimensional condensate due to the chain development of this condensation reaction between neighboring Li-Si precursor compounds. This condensate imparts viscosity to the sol composition and causes conversion to the gel composition. Thus, the gel composition can be obtained as a matter of course by stirring the sol composition for a prescribed period of time. In addition, it is also known that the formation of the gel composition can be accelerated by the addition of a hydrolysis catalyst as described above and/or by adjusting, for example, the ambient temperature.

In contrast to this, this gelation is even more favorably accelerated in the herein disclosed art by exposing the sol composition to electromagnetic waves. While the stirring process is not necessarily required, when viewed in terms of inducing the uniform development of gelation of the sol composition, the exposure to electromagnetic waves is more preferably carried out while stirring the sol composition. While the detailed mechanism is not clear, it can be thought that exposure of the sol composition to electromagnetic waves brings about some change in the framework structure of the Li-Si precursor compound for the gel composition that is formed. In addition, due to this change in the framework structure, the lithium silicate obtained from this gel composition can be obtained as a powder that has a special form different from the usual. According to investigations by the present inventors, it is thought that, due to the action of the electromagnetic waves, special features are exhibited in the connection regime for the SiO₄ tetrahedra and anisotropy is then seen in the arrangement of the SiO₄ tetrahedra (for example, the connection angle and the degree of connection).

Accordingly, the shape of the individual crystalline lithium silicate particles that constitute the CO₂-absorbing material, while not being particularly limited, is typically not so-called spherical to granular, but can assume a highly anisotropic shape referred to as, for example, rod-shaped, plate-shaped, scale-shaped, petal-shaped, and sponge-shaped. It can be concluded that the CO₂ absorption characteristics, i.e., the CO₂ absorption capacity, the CO₂ absorption rate, and so forth, of the herein disclosed lithium silicate are effectively improved based on this special form of the lithium silicate. For example, a lithium silicate can be produced that is provided with a high CO₂ absorption capacity that approaches the theoretical value. Desorption of the absorbed CO₂ can also be carried out favorably. Accordingly, the exposure to electromagnetic waves is an indispensable operation for improving the CO₂ absorption characteristics in the herein disclosed art. In addition, the gel composition can also be obtained in a shorter period of time through the promotion of gelation by exposure to electromagnetic waves. This is also preferred with regard to achieving a shortening of the time for production of the CO₂-absorbing material.

Any of the following may be used as the electromagnetic waves: ultralow frequencies, long waves, medium waves, short waves, microwaves (high frequency), infrared radiation, visible light, ultraviolet radiation, x-rays, and gamma radiation. However, the use of electromagnetic waves that can effectively heat the sol composition is preferred from the standpoint of the efficiency of electromagnetic wave exposure. Viewed from this perspective, microwaves are directly absorbed by the water used as the dispersion medium for the sol composition and by the water molecules of, for example, the water of hydration and water of crystallization present in the Li-Si precursor compound, and these water molecules generate heat and can heat the Li-Si precursor compound. The use of this microwave heating (dielectric heating) is a preferred aspect because this can rapidly and selectively heat the Li-Si precursor compound from within while suppressing energy losses.

The microwaves are not particularly limited with regard to their frequency (wavelength) or the output, exposure time, and so forth. For example, these can be determined as appropriate so as to enable the suitable supply of the amount of energy necessary for the gelation of the sol composition that is the exposure target. For example, microwaves can be used that have a wavelength from 1 mm to 1 m and a frequency from 300 MHz to 300 GHz. With regard to the frequency, based on international standards the use of magnetron-generated high frequencies in the 2.45 GHz band may be more appropriate (or high frequencies in the 915 MHz band depending on the region). In addition, for example, with regard to the output, approximately 300 W to 300 kW is appropriate from the standpoint of the heating efficiency while approximately 300 W to 10 kW is preferred, approximately 300 W to 2000 W is more preferred, and approximately 500 W to 1600 W is particularly preferred.

The exposure time can be adjusted considering the output, the amount of the sol composition, the configuration during exposure (degree of microwave penetration), and so forth. Exposure to the microwaves may be carried out, for example, continuously over a prescribed exposure time or may be carried out a plurality of times with an interposed interval. In a preferred embodiment for microwave exposure, the following is repeated a prescribed number of times: exposure of the sol composition to microwaves at a prescribed output until the dispersion medium boils; then cooling of the sol composition; and then re-exposure to microwaves at a prescribed output. Here, when, for example, the dispersion medium is volatilized due to boiling, the dispersion medium may be replenished as necessary. As an example of the microwave exposure time, exposure is carried out, for example, to produce approximately 0.2 to 0.5 mol lithium silicate, for a total of approximately 1 minute to 20 minutes (for example, 4 to 12 minutes) to a 600 W to 1000 W (for example, 700 W) at 45 GHz.

The gel composition is a consolidated state provided by an almost complete loss of fluidity by the sol composition. This gel state is difficult to define rigorously, but it typically can be defined as a state (i) that is a cohesive disperse system that has a composition of at least two components that are at least a dispersed material and a dispersion medium, (ii) that exhibits a mechanical behavior that has the characteristics of a solid, and (iii) in which both the dispersed material and dispersion medium continuously (and homogeneously) extend throughout the system as a whole. In this Description, the gel composition can be a chemical gel in which, going through a clear sol-gel transition as is well known to those skilled in the art, roughly all of the dispersed material is three-dimensionally connected by covalent bonding. That is, the dispersed material has approximately the same composition as the lithium silicate target. This dispersed material forms an amorphous (i.e., not crystalline) material composed of a matrix that is three dimensional but not continuously bonded. In addition, the gel composition has a structure in which the dispersion medium remains impregnated in this dispersed material.

S3. Calcination of the lithium silicate
The thusly obtained gel composition is dried and calcined. This can provide, through the complete removal of the dispersion medium and excess components that are not the lithium silicate, the intended solid lithium silicate. Even if sites are present in the gel composition where the hydrolysis reaction and dehydration condensation reaction are not entirely completed, these reactions will be accelerated by the calcination and the gel composition will then typically be converted into a compact crystalline solid.
The drying may be the natural drying and may be carried out using a dryer. The calcination may be carried out using a general heating furnace. The drying and calcination may, furthermore, be carried out in combination. When combining the drying and calcination, for example, it could be carried out by using an air oven and furnace or other methods like super critical drying, spray drying, spray granulation or spray pyrolysis. These means for drying and calcining could be carried out alone or in combination.

The calcination conditions should be able to convert the gel composition to the lithium silicate, but are not otherwise particularly limited. For example, these should be conditions that can crystallize the Li-Si precursor compound, which resides in an amorphous state, into the lithium silicate. Specifically, when the calcination temperature is made approximately 500°C or less, an oxygen-containing atmosphere (typically an air atmosphere) is preferably used for the calcination atmosphere so as to favorably develop the oxidation of the gel composition. In addition, when the calcination temperature is made in 500°C or higher (or in excess of 500°C), the calcination atmosphere may be an oxygen-containing atmosphere, typically an air atmosphere, or may be, for example, air, nitrogen gas, carbon dioxide gas, or a mixed gas of two or more of the preceding. The calcination temperature is suitably a temperature higher than 473°C (preferably at least 500°C). The calcination temperature is more preferably at least 700°C and is particularly preferably at least 800°C. This makes it possible to remove the excess components that originate with the starting materials used and to obtain a highly crystalline lithium silicate. The upper limit on the calcination temperature is not particularly limited, but, for example, it can be not more than 1100°C, not more than 1000°C, and is typically about 900°C. There are no particular limitations on the calcination time, but, for example, approximately 30 minutes to 5 hours is suitable while 1 hour to 5 hours is more preferred and 2 hours to 4 hours is particularly preferred.

The lithium silicate in the thusly realized CO₂-absorbing material is obtained as a powder composed of particles (primary particles) having a typical spherical, granular, or rectangular form or particles (primary particles) having a special form. This special form can be a highly anisotropic shape, for example, rod shapes, plate shapes, scale shapes, and so forth. In addition, the lithium silicate can be composed of a powder in the form of secondary particles provided by the bonding of these primary particles. These secondary particles, for example, may be granular or, for example, may have a special form presenting as a petal shape or sponge shape.

When the lithium silicate is composed of, for example, cubic particles or spherical particles having a low anisotropy, their average particle diameter is not strictly limited, but may be, for example, from about 10 nm to 1 mm or from about 20 nm to 500 nm. This dimension can be acquired, for example, as the arithmetic average value of the equivalent circle diameters measured based on electron microscopic observation.

For example, the lithium silicate is typically obtained in the form of a powder composed microscopically (for example, in the order of nanometer) of aggregates of needle-shaped (fiber-shaped) or bar-shaped (rod-shaped) particles. These bar-shaped particles refer to a crystalline form that has grown largely in one direction (one dimensional). Using a for the dimension in the long direction of this crystal and b for the dimension in a short direction orthogonal to this long direction, the average dimension b in the short direction is not more than approximately 100 nm and can be preferably not more than 50 nm and particularly preferably not more than 30 nm. In addition, the aspect ratio given by a/b is typically at least 2 and can be preferably at least 5, for example, at least 10, and particularly preferably at least 15. These dimensions can also be measured, for example, based on electron microscopic observation (the same applies in the following).

Considered at a somewhat more macroscopic level (for example, in the order of micrometer), the lithium silicate may also be a powder composed of plate-shaped crystals provided by the aggregation of the aforementioned spherical particles or rod-shaped particles into a single body.
This plate shape is also expressed by, for example, leaf shape, mica shape, thin plate shape, and lamellar, and refers to a crystalline form which has grown largely in the planar directions (two dimensional). This is typically a relatively flat, thin plate-shaped crystal. Here, using a for the dimension in the long direction within the crystal face and using b for the dimension in a thickness direction orthogonal to this long direction, the average dimension b in the short direction is not more than approximately 10 mm and can preferably be not more than 5 mm and particularly preferably not more than 1 mm. The dimension in the long direction, while not being particularly limited, is not more than approximately 50 mm. The aspect ratio given by a/b for this plate-shaped crystal is typically at least 2 and can be preferably at least 3, for example, at least 5, and particularly preferably at least 10. A single plate-shaped crystal may by itself constitute a particle, or a plural number of plate-shaped crystals may be aggregated (bonded) to form a single particle. When a plurality of crystals are aggregated, these may be twin crystals. When a plate-shaped crystal is an aggregate of a plurality of primary particles, microvoids may be encompassed within the gaps between the primary particles in such a plate-shaped crystal. The dimensions of these voids are not particularly limited, but, for example, they may be mesopores with a diameter of approximately at least 2 nm and less than 50 nm or macropores with a diameter of at least 50 nm.

The scale shape and petal shape can also be understood as a type of modification of the plate shape and are crystalline forms that have grown largely in the planar direction. Between the two, scale shape refers, for example, to a crystalline form in which a plurality of plate-shaped crystals are aggregated, with the plane directions approximately aligned, like fish scales as it were. Petal shape refers, for example, to a crystalline form in which a plurality of plate-shaped crystals are aggregated with different plane directions such that petals, as it were, composed of a single crystal form a flower. It is also thought that these special shapes are forms for which the specific crystal habits are strongly reflective of the action of the electromagnetic waves that is a characteristic feature of the aforementioned production method. The particle size of such particles is not particularly limited, but, for example, can be approximately 0.1 mm to 20 mm.

The sponge shape is also expressed by spongiform, honeycomb, and so forth, and is a crystalline form that has a large bulk as a whole, but which contains a large number of voids with interposed thin plate-like wall elements (wall surfaces). These wall elements can also be understood just as for the plate-shaped crystals described in the preceding. The size of the voids present within the particles is not particularly limited, and these may be, for example, mesopores or macropores. They typically can be macropores. The particle diameter of the sponge-shaped particle is not particularly limited, but, for example, can be approximately 0.1 mm to 20 mm.

For the herein disclosed lithium silicate, particles having these characteristic shapes preferably can account for at least 50 number%, for example, at least 70 number% and particularly 80 number%, of the total particles constituting the lithium silicate. These characteristic shapes make it possible to provide high CO₂ absorption characteristics approaching theoretical values. Not only a highly efficient CO₂ absorption, but also a highly efficient desorption can be realized. In addition, even in the case of repetitive CO₂ absorption and release at high temperatures, crystal particle aggregation can be suppressed and a decline in the CO₂ absorption capacity can be inhibited.

The thusly realized CO₂-absorbing material can have, for example, a CO₂ absorption rate that is increased by approximately 1.5- to 2-times over that of a lithium silicate produced by the known sol-gel method using the same materials. Moreover, this CO₂ absorption capacity is expressed from lower temperatures and, for example, makes possible CO₂ absorption in a temperature range from approximately 200°C to 710°C. In addition, CO₂ can be absorbed at faster speeds in the temperature range at and above 450°C. For example, the CO₂ absorption rate per unit weight in the temperature range from 500°C to 710°C can be at least 20 mg/(g×min) and more preferably can be at least 25 mg/(g×min). The CO₂ gas absorption rate per unit weight in the temperature range from 600°C to 710°C can be at least 30 mg/(g×min) and more preferably can be at least 50 mg/(g×min). That is, the herein disclosed art realizes a CO₂-absorbing material that can exhibit an excellent CO₂ absorption capacity in the temperature region of approximately 200°C to 710°C and particularly preferably 450°C to 710°C. The herein disclosed CO₂-absorbing material begins to release CO₂ at a temperature of about 720°C or higher (for example, at 750°C or higher). Accordingly, this CO₂-absorbing material can engage in CO₂ absorption up to this CO₂ release temperature (for example, below 720°C).

S4. Composite formation between the lithium silicate and an alkali carbonate
The thusly obtained lithium silicate may be used as a CO₂-absorbing material as such as described above, but, for example, through composite formation with an alkali carbonate can be made into a CO₂-absorbing material that has a CO₂ absorption capacity that is increased still further.
The alkali carbonate can soften or form a liquid phase in the CO₂ absorption temperature region of the herein disclosed lithium silicate. It is thought that the presence of this alkali carbonate supports a smoother movement of the CO₂ to the lithium silicate. By doing this, the CO₂ absorption capacity can be substantially broadened on the lower temperature side and the CO₂ absorption rate can also be increased.

The carbonate of an alkali metal, e.g., a sodium (Na) component, potassium (K) component, lithium (Li) component, rubidium (Rb) component, cesium (Cs) component, or francium (Fr) component, can be considered for the alkali carbonate. This may be the carbonate of any single species of alkali metal or may be a carbonate that contains two or more species of alkali metals. In the herein disclosed art, the alkali carbonate preferably contains any one or more of the sodium component, potassium component, and lithium component and more preferably contains any two or more thereof and particularly preferably contains all three.

When the alkali carbonate is a mixed system of the three alkali metal carbonates of Li, K, and Na, this is preferably, for example, a solid-solution crystal or a eutectic. In the case of a solid solution, the proportions of the Na, K, and Li in the alkali metal carbonate are preferably as given below. These compositions can also include eutectic compositions.
Na : from 1 mol% to 80 mol%
K : from 1 mol% to 70 mol%
Li : from 1 mol% to 90 mol%

The alkali carbonate is preferably a eutectic since, due to the lower eutectic point, additional improvements in the CO₂ absorption capacity and CO₂ absorption rate can be obtained. For K-Li system carbonates, the eutectic composition is a mixed system in which the molar ratio between K₂CO₃ and Li₂CO₃ is approximately 55 : 45. When the alkali carbonate is a K-Li system carbonate, the molar ratio (K : Li) between the K and Li is preferably in the range of approximately 60 : 40 to 40 : 60, which includes this eutectic composition. The eutectic composition for Na-Li system carbonates occurs when the molar ratio between Na₂CO₃ and Li₂CO₃ is approximately 49 : 51. Thus, when the alkali carbonate is an Na-Li system carbonate, the molar ratio (Na : Li) between the Na and Li is preferably in the range of approximately 55 : 45 to 45 : 55, which includes this eutectic composition. For the Na-K-Li system carbonates, the eutectic composition can be when the molar ratio among Na₂CO₃, K₂CO₃, and Li₂CO₃ is approximately 31 : 35 : 34. Thus, when the alkali carbonate is an Na-K-Li system carbonate, the molar ratio (Na : K : Li) among the Na, K, and Li is preferably in the range of approximately 25 to 35 : 30 to 40 : 30 to 40, which includes this eutectic composition. Among the preceding, the alkali carbonate is particularly preferably an Li-Na-K system eutectic carbonate.

While not necessarily being limited to or by this example, the average particle diameter of the alkali carbonate used for composite formation typically is suitably from approximately 50 nm to 5 mm and is preferably 100 nm to 1 mm and is particularly preferably 200 nm to 500 nm.
In addition, the proportion of the alkali carbonate made into the composite with the lithium silicate is, for example, suitably from 1 mass parts to 50 mass parts, preferably from 5 mass parts to 40 mass parts, and particularly preferably from 10 mass parts to 30 mass parts using 100 mass parts for the lithium silicate.

During composite formation, after the lithium silicate and alkali carbonate have been thoroughly mixed, the two can be made into a single body by, for example, calcination under the previously described calcination conditions. By doing this, a CO₂-absorbing material can be produced in which the lithium silicate and alkali carbonate are made into a composite.

The thusly realized CO₂-absorbing material can have a CO₂ absorption capacity that is increased approximately to at least twice that, for example, of the lithium silicate produced by the known sol-gel method using the same materials. In addition, the CO₂ absorption capacity is realized from lower temperatures, making possible the absorption of CO₂ in the temperature region, for example, of approximately 200°C to 710°C. In particular, the CO₂ absorption capacity in the temperature range from and above 300°C can be substantially improved. Accordingly, CO₂ can be absorbed at an even higher speed, for example, in the temperature range at and above 400°C and in particular at and above 450°C. For example, the CO₂ absorption rate per unit weight in the temperature range from 450°C to 650°C can be at least 40 mg/(g×min). In addition, the CO₂ absorption rate per unit weight in the temperature range from 500°C to 650°C can be at least 50 mg/(g×min) and can be more preferably at least 80 mg/(g×min) and particularly preferably at least 100 mg/(g×min). Moreover, the CO₂ gas absorption rate per unit weight in the temperature range from 600°C to 650°C can be at least 100 mg/(g×min) and can be preferably at least 150 mg/(g×min) and more preferably at least 200 mg/(g×min). That is, the herein disclosed art realizes a CO₂-absorbing material that can exhibit an excellent CO₂ absorption capacity in the temperature region approximately from 200°C to 710°C and particularly preferably from 350°C to 710°C.

Specific embodiments are provided and described below for the herein disclosed method of producing a CO₂-absorbing material. However, there is no intent to limit the present invention to or by the following examples.

Reference Example. The Sol-Gel Method
Lithium orthosilicate was synthesized from lithium nitrate (LiNO₃, Alfa Aesar) and colloidal silica (Sigma-Aldrich) starting materials.
First, a 1 M aqueous lithium nitrate solution was initially prepared by dissolving 15.3 g of LiNO₃ in 225 mL of distilled water. While stirring this aqueous lithium nitrate solution at a prescribed speed at room temperature, hydrolysis was performed by the gradual addition of 25% ammonium hydroxide (S.D. Fine-Chem Limited) until the pH reached 8. Then, while continuing to stir, 3.3 g of colloidal silica was added dropwise to this aqueous reaction solution and stirring was then continued for 1 hour to obtain a sol composition. The amount of colloidal silica addition is an amount that provides an Li : Si molar ratio of 4 : 1 with reference to the amount of the lithium nitrate. It is thought that the precursor for lithium silicate is precipitated in this sol composition mainly as a hydroxide having the form of a hydrated complex. This sol composition was subjected to dehydration condensation by ageing for an additional 24 hours at room temperature followed by drying at 110°C and calcination for 3 hours at 800°C to yield lithium orthosilicate in powder form. The thusly obtained lithium orthosilicate is designated SG.

1-1. Production of lithium orthosilicate
Lithium orthosilicate was synthesized according to the herein disclosed art from lithium nitrate (LiNO₃, Alfa Aesar) and colloidal silica (Sigma-Aldrich) starting materials.
First, proceeding as in the Reference Example, hydrolysis was carried out by the addition of 25% ammonium hydroxide to pH 8 while stirring the aqueous lithium nitrate solution at room temperature. Colloidal silica was added dropwise to this aqueous reaction solution and stirring was carried out for 1 hour to obtain a sol composition in which the lithium silicate precursor was dispersed in the aqueous solution. A dehydration condensation reaction was run by exposing this sol composition to a 700 W electromagnetic wave at 2.45 GHz. A microwave oven was used to carry out exposure to the electromagnetic wave, and exposure was performed five times for 2 minutes each for a total of 10 minutes. Specifically, the following was carried out five times: boiling the sol composition by exposure to the electromagnetic wave for 2 minutes; then providing a rest period and cooling the sol composition to room temperature; and replacing the amount of water volatilized by boiling back to the initial amount. The gel composition (reaction product) provided by the dehydration condensation was dried at 110°C and calcined for 3 hours under atmospheric conditions at 800°C to yield powder. The thusly obtained powder sample is designated MW-SG.

1-2. XRD
X-ray diffraction (XRD) analysis was performed on the powder sample (MW-SG) obtained as described above. Small-angle X-ray scattering/wide-angle diffraction set-ups (Xeuss SAXS/WAXS system from Xenocs, France, 2q = 4° to 36° and X'pert Pro diffractometer, from PANalytical, Eindhoven, the Netherlands, 2q = 10° to 90°) were used to characterize the samples using the Cu Ka line (l = 0.154 nm). The obtained XRD pattern is given in FIG. 2.
This XRD pattern demonstrated that the obtained powder had a high crystallinity and that almost all of the diffraction peaks were in agreement with the lithium orthosilicate phase (Li₄SiO₄) of JCPDS Card No. 37-1472. While some of the diffraction peaks could be attributed to lithium metasilicate (Li₂SiO₃, JCPDS Card No. 29-0828), this was thought to be due to the absorption of atmospheric CO₂ by the lithium orthosilicate at room temperature and the occurrence of the following reaction.
Li₄SiO₄ + CO₂ → Li₂SiO₃ + Li₂CO₃

1-3. TEM observations
The powder sample (MW-SG) was also observed with a transmission electron microscope (TEM). A Tecnai G2, FEI, the Netherlands, was used for the TEM observation. The resulting TEM images are given in FIG. 3. FIG. 3(a) shows the result for observation of the MW-SG sample immediately after production and FIG. 3(b) shows the result for observation of the MW-SG sample after the evaluation of the CO₂ absorption characteristics that is described below.
According to the results of the TEM observations, the MW-SG formed a powder through the aggregation of rod-shaped particles; the dimension (diameter) in the width direction of the rods was approximately not more than 100 nm and was typically not more than 50 nm; and the average diameter for the MW-SG in FIG. 3(a) was approximately 20 nm. In addition, the aspect ratio of the rod-shaped particles was shown to be clearly larger than 2 and was roughly at least 5 and approximately 10 or more. As shown in FIG. 3(b), it was found for the MW-SG sample that no substantial change in morphology was seen between before and after the heating in the evaluation of the CO₂ absorption characteristics and that, for example, heating-induced aggregation was suppressed.

1-4. The CO₂ absorption characteristics
< The dynamic absorption characteristics >
The carbon dioxide absorption characteristics of the powder sample (MW-SG) were evaluated in terms of dynamic absorption characteristics based on changing the ambient temperature. Specifically, the evaluation was performed by thermogravimetric analysis (TGA) by absorbing CO₂ as the probe molecule onto a prescribed amount of the MW-SG sample and measuring the amount of gas absorption/desorption produced by the continuous rise in the sample temperature. The measurements were carried out by the N₂ purge method using 100% CO₂ gas for the absorption gas in the temperature region from 100°C to 800°C at a rate of temperature rise of 20°C/min. The obtained dynamic TGA curve is shown in FIG. 4. For comparison, a dynamic TGA curve is also given in FIG. 4 for the SG sample produced by the conventional sol-gel method.

It was shown that the MW-SG produced by the herein disclosed art began to absorb CO₂ at a lower temperature than did the SG produced by a conventional method, and had a CO₂ absorption temperature range of approximately 450°C to 710°C with CO₂ release being produced at about 750°C and above. In addition, it was demonstrated that the CO₂ absorption capacity of the MW-SG exceeded 200 mg/g and that it has absorbed about twice that of the SG within the duration of the dynamic absorption run. It was also demonstrated to have, beginning with the rise in the TGA curve, a CO₂ absorption rate faster than that of the SG and was thus demonstrated to have an excellent CO₂ absorption performance.

< The isothermal absorption characteristics >
The isothermal absorption characteristics for carbon dioxide of the powder sample (MW-SG) were then evaluated at several different measurement temperatures. The measurement temperatures were set at each 50°C in the range from 400°C to 700°C. 100% CO₂ gas was used in the isothermal absorption test, and the CO₂ absorption capacity at the indicated temperatures was measured by heating the MW-SG sample to the prescribed measurement temperature at a rate of temperature rise of 10°C/min and thereafter holding for 2 hours (7200 seconds) under a current of 100% CO₂. The obtained isothermal absorption curves are given in FIG. 5.
As may be understood from FIG. 5, the CO₂ absorption rate was faster at higher temperatures, and, for example, equilibrium was reached in about 15 minutes at a measurement temperature of 700°C. In addition, when the measurement temperature was low at 400°C, even though the amount of CO₂ absorption was reduced, the development of CO₂ absorption was still indicated.

The absorption rate was calculated from the slope of the rise in the isothermal absorption curve in FIG. 5. Specifically, the absorption rate was taken to be the slope of the isothermal absorption curve for the 2 minutes after the start of the measurement. These results are given in FIG. 6.
As is clear from FIG. 6, it was demonstrated that the CO₂ absorption rate in the temperature range of approximately 650°C to 700°C was at least 50 mg/(g×min); for example, it was at least 100 mg/(g×min) at 700°C.

1-5. The cycle characteristics
The cycle characteristics in CO₂ absorption were evaluated by subjecting the powder sample (MW-SG) to repetitive carbon dioxide absorption and desorption at a high temperature of at least 600°C. Specifically, using a TGA instrument, the MW-SG sample was heated at a rate of 10°C/min to 700°C, which was the prescribed CO₂ absorption temperature, and CO₂ absorption was then carried out at this absorption temperature by holding in a CO₂ current for 20 minutes. The MW-SG sample was then heated to 800°C and CO₂ desorption was performed by holding in an N₂ current at this desorption temperature. 600°C was used for the CO₂ absorption temperature beginning with the second cycle: the MW-SG sample was cooled to 600°C and CO₂ absorption was then performed by holding for 20 minutes in a CO₂ current. This CO₂ absorption and desorption was carried out a total of 10 times, and the weight changes in the MW-SG sample during this sequence were checked. The results are given in FIG. 7.
As is clear from FIG. 7, even during consecutive and repeated CO₂ absorption and desorption, no variability was seen in the absorption characteristics of the MW-SG and it could thus be confirmed to have stable cycle characteristics.

2-1. Preparation of alkali carbonate and lithium orthosilicate composites
MW-SG samples in powder form were first prepared in the same manner as described above. In addition, the carbonates of sodium (Na), potassium (K), and lithium (Li) were mixed so as to provide the following mass ratios: (1) (10 : 30 : 60), (2) (32 : 37 : 31), and (3) (31 : 45 : 24) in the order (Na : K : Li). These are equal to (Na : K : Li) mol% of (1) 8.5 : 19.5 : 72, (2) 31 : 27 : 42, and (3) 31 : 35 : 34. The alkali carbonate in (3) is the blend corresponding to the Na-K-Li eutectic carbonate. These were mixed in the proportion of 15 to 20 mass parts of the alkali carbonate per 100 mass parts of the MW-SG sample and the mixture was heated to 800°C at a ramp rate of 1°C/min and calcination was performed for 3 hours at 800°C. The thusly obtained alkali carbonate/lithium orthosilicate composites were designated MW-SG-NKL1 to -NKL3 in accordance with the alkali blend.

2-2. The CO₂ absorption characteristics
< The dynamic absorption characteristics >
Operating as in section 1-4 above, the carbon dioxide absorption characteristics of the obtained alkali carbonate × lithium orthosilicate composites (MW-SG-NKL1 to -NKL3) were evaluated through the dynamic absorption characteristics based on changing the ambient temperature. The dynamic TGA curves obtained as a result are shown in FIG. 8. For reference, FIG. 8 also contains the results for SG, which was prepared by a conventional sol-gel method, and for MW-SG, which lacked the addition of alkali carbonate.
It was shown that the MW-SG-NKL1 to -NKL3 composites with alkali carbonate in all instances had a CO₂ absorption capacity that was increased about 1.5- to 3-times that of the MW-SG within the duration of dynamic absorption run. In addition, differences in the CO₂ absorption characteristics were also seen: the amount of CO₂ absorption in the lower temperature region from about 300°C to 650°C was shown to be larger and the absorption rate in this low temperature region was also shown to be substantially increased. This trend was seen most significantly with MW-SG-NKL3, in which the Na, K, Li eutectic carbonate was formed into the composite. This is believed to occur because at 400°C to 500°C these alkali carbonates soften or convert to a liquid phase and the CO₂ absorption and diffusion characteristics in the lower temperature region are improved by this melted carbonate.

< The isothermal absorption characteristics >
Operating as in section 1-4 above, the isothermal absorption characteristics for carbon dioxide were therefore evaluated on MW-SG-NKL3. As shown in FIG. 8, MW-SG-NKL3 begins to absorb CO₂ at above 200°C and typically from about 300°C, and its CO₂ absorption capacity at 700°C is smaller than its CO₂ absorption capacity at 650°C. Due to this, the test was run at each 50°C in the temperature range from 350°C to 650°C. In addition, the CO₂ absorption rate was calculated using the obtained isothermal absorption curve from the slope of the curve for the two minutes after the beginning of the measurement. The results for the isothermal absorption curves are shown in FIG. 9 and the results for the CO₂ absorption rate are shown in FIG. 10. For reference, FIG. 10 also contains the results for SG, which was prepared by a conventional sol-gel method, and for MW-SG, which lacked the addition of alkali carbonate.

As shown in FIG. 9, the CO₂ absorption rate was also faster at higher temperatures for MW-SG-NKL3, and, for example, it was shown that equilibrium was reached in about 5 to 10 minutes when the measurement temperature was 650°C. In addition, it was shown that even at the low measurement temperature of 350°C, a better CO₂ absorption occurred than was the case for MW-SG at 400°C.

As seen in FIG. 10, it was shown that, over the entire temperature region, the value of the CO₂ absorption rate for MW-SG-NKL3, which was a composite with a eutectic alkali carbonate, was substantially higher certainly than for the SG sample, which was prepared by a conventional sol-gel method, but also than for the MW-SG sample. The CO₂ absorption rate of this MW-SG-NKL3 was shown to have undergone a sharp increase at 400°C-450°C to 26.7 mg/(g×min) and to present even higher values at above 450°C, where the eutectic alkali carbonate undergoes softening and melting. The CO₂ absorption rate by MW-SG-NKL3 was shown to be at least about 4-times that for MW-SG at 600°C and at least about 5-times at 650°C.
As is clear from FIG. 10, it was shown that the CO₂ absorption rate in the temperature range of about 600°C to 650°C was at least 200 mg/(g×min); for example, at 650°C it was at a level that reached to about 350 mg/(g×min).

In accordance with the preceding, the herein disclosed art provides a CO₂-absorbing material that exhibits an expanded CO₂ absorption temperature range, an improved CO₂ absorption capacity, and an improved CO₂ absorption rate, and also provides a method of producing this CO₂-absorbing material. The example of the use of colloidal silica as the silicon component is given in the examples described above. Although specific examples are not provided, the present inventors have confirmed that a CO₂-absorbing material having the same excellent CO₂ absorption performance as in the preceding examples is obtained using fumed silica in place of this colloidal silica and using a sol composition prepared by the sol-gel method using TEOS as a starting material. Accordingly, the individual skilled in the art will be able to prepare the sol composition using various procedures besides those shown in the preceding examples.

3-1. Confirmation of the effect of exposure to electromagnetic waves
Lithium orthosilicate (Li₄SiO₄) was synthesized in 1-1 above using the sol-gel method and use lithium nitrate (LiNO₃) as the lithium starting material and colloidal silica as the silicon starting material. The gelation due to hydrolysis in the sol-gel reaction was accelerated by exposing the sol-form reaction solution to electromagnetic waves.
XRD analysis was then carried out on the (a) colloidal silica and (b) lithium nitrate used as starting materials and on (c) the sol solution prior to exposure to electromagnetic waves and (d) the gel solution post-exposure to electromagnetic waves in order to check the constituent phases of each of these materials. In the case of the (c) sol solution and (d) the gel solution, the analysis was run on the powder obtained by drying the particular solution. The results are shown in FIG. 11.

As is clear from FIG. 11, the (a) colloidal silica used as a starting material is amorphous while the (b) lithium nitrate is crystalline. In addition, in the case of the (c) reaction solution that had become a sol due to the start of hydrolysis, it can be seen that, while crystalline lithium nitrate is present, a diffraction peak originating with the (101) plane of LiOH has appeared. In the case of the (d) reaction solution that has been treated with electromagnetic waves, the diffraction peak from the (101) plane of LiOH has undergone a relatively large increase and, based on this, it could be clearly confirmed that hydrolysis is substantially accelerated by exposure to electromagnetic waves. These results clearly reveal that the partial or full substitution of Lithium salt to LiOH may be a suitable way to produce high performance Lithium silicate and therefore form a part of this embodiment.

3-2. Phase changes due to calcination
Then, the dry powder from the gel solution that had been exposed to electromagnetic waves as described above was subjected to a heat treatment in air and the changes in the crystalline phases in the dry powder were monitored by submission to high-resolution X-ray diffraction (HTXRD) analysis in-situ. The HTXRD analysis was carried out at 746 K, 773 K, 1073 K, 1176 K, and 1273 K. The results of this HTXRD analysis are given in FIG. 12. After the heat treatment at the applicable temperature, the powder sample corresponds to the powder sample (MW-SG) obtained in 1-1, supra. In the description that follows, the heat treatment was carried out in air unless specifically indicated otherwise.

The following could be confirmed from the HTXRD analysis as shown in FIG. 12.
i. At 746 K, the dry powder is entirely amorphous.
ii. At 773 K, the presence of Li₂SiO₃ in the dry powder can be seen. Accordingly, the nucleation (crystallization) of Li₂SiO₃ is produced in the temperature range from above 746 K to less than 773 K.
iii. At 1073 K, the dry powder is a mixed phase of Li₂SiO₃ (lithium metasilicate) and Li₄SiO₄ (lithium orthosilicate), but is essentially composed of Li₄SiO₄.
iv. In heat treatment up to 1073 K, the diffraction intensities from the (110) plane and (011) plane of Li₄SiO₄ increase accompanying the rise in the temperature, while the diffraction intensity from the (111) plane of Li₂SiO₃ declines, and it is thought based on this that the production of Li₄SiO₄ proceeds through the consumption of Li₂SiO₃.
v. Accompanying the rise in the heat treatment temperature, the peaks shift slightly to the left, but this is thought to be caused by the growth stress induced in the crystal due to the volume expansion that accompanies the phase change from Li₂SiO₃ to Li₄SiO₄.
vi. While the temperature was raised to 1273 K, no change at all is seen in the constituent phases of the powder at above 1073 K. Thus, it could be confirmed that, for the powder sample (MW-SG) with the composition indicated above, stable phases, e.g., Li₄SiO₄ and so forth, are obtained by heating to not more than 1273 K.

3-3. Investigation of particle morphologies
The dry powder from the gel solution after exposure to electromagnetic waves as described above was subjected to a heat treatment for about 3 hours at different temperatures and the morphology of the powder sample was then observed using a TEM. The heat treatment temperatures were 473 K, 673 K, 773 K, and 1073 K. The results are given in a to h in FIG. 13. In FIG. 13, a and b are TEM images of the dry powder from the gel solution, after heat treatment at 473 K; c and d are TEM images after heat treatment at 673 K; e and f are TEM images after heat treatment at 773 K; and g and h are TEM images after heat treatment at 1073 K. Images are given at two different magnifications for each of these heat-treatment temperatures. The powder heat treated for 3 hours at 1073 K corresponds to the powder sample (MW-SG) prepared in 1-1 above.

As shown in a and b in FIG. 13, the powder heat treated at 473 K was seen to be composed of microfine spherical particles. It is thought that this spherical powder shape derives from the colloidal silica. As shown in c and d, the presence of spherical particles could also still be seen in the powder heat treated at 673 K.
Here, it is thought that the spherical particles observed for 673 K are composed of an amorphous silica phase based on the fact that LiNO₃, the presence of which is seen in the XRD pattern in FIG. 11(d) in the dry powder prior to heat treatment, has a melting point of approximately 528 K and based on the fact that, in accordance with the HTXRD analytical results in FIG. 12, this dry powder undergoes crystallization at above 746 K. Accordingly, it was ascertained that the amorphous silica phase in the colloidal silica used as the silica source in these examples is not completely eliminated (decomposed) in the reaction solution by hydrolysis and exposure to electromagnetic waves, and that it undergoes drying and is supplied to calcination while retaining its spherical form. It is thought that, even during drying and calcination, silica particle aggregation is satisfactorily inhibited up to 673 K due to a thorough wetting of the surfaces of these silica particles by the melted lithium salt.

With regard to the powder that has been heat treated at 773 K, as shown in e and f in FIG. 13 a complete transformation has occurred and nanofibrous particles were observed. The HTXRD analytical results in FIG. 12 demonstrate that an Li₂SiO₃ phase is produced at 773 K. Based on this, it is presumed that these nanofibrous particles are composed of Li₂SiO₃ crystals. With regard to the formation of these nanofibrous crystals, considering that up to 673 K the amorphous silica particles are present in the molten lithium salt without aggregating, it is thought that the highly anisotropic nanofibrous crystals are formed because at temperatures above 673 K the spherical silica is arranged with a special regularity in the molten lithium salt and because the SiO₄ tetrahedra are bonded at a prescribed bond angle. Thus, Li₂SiO₃ crystal nucleation is produced at amorphous silica particles that are coated by the lithium salt in a molten state and reside in a one-dimensional arrangement, and it is thought that this favorably expedites the formation of the nanofibrous crystalline particles.

As shown in g and h of FIG. 13, for the powder that has been heat treated at 1073 K, it was found that the nanofibrous crystalline particles were converted into particles having a nanorod shape (this can be a nanoroll shape). It is demonstrated that these nanorod-shaped particles are Li₄SiO₄ and are formed by the expansion and unification of the Li₂SiO₃ nanofibers accompanying the phase change.

The powder heat treated at 1073 K (corresponds to the MW-SG sample of FIG. 3(a)) was then observed with a scanning electron microscope (SEM). An EVO18, Special Edition (Carl Zeiss AG, Germany) was used for the SEM observation, and the observations were carried out at an acceleration voltage of 20 kV. These results are given in A (approximately 2600X) and B (approximately 5000X) of FIG. 14.
As shown in FIG. 14, when observed more macroscopically in the order of micrometer (for example, approximately 1000X to 5000X), the nanorod-shaped particles were found to have assumed the form of plate-shaped crystals or a petal shape formed by their assembly.

4-1. Production of a lithium-rich silicon-lithium composite compound
A lithium silicate was prepared and a carbon dioxide-absorbing material was obtained proceeding as in 1-1 above. However, in the present example, the amount of colloidal silica added to the aqueous reaction solution after hydrolysis was reduced to make a lithium-rich blend in which, using the amount of colloidal silica addition, Li : Si was 8 : 1 as the molar ratio. The electromagnetic wave exposure conditions were exposure a total of 5 times for 2 minutes to electromagnetic waves at 2.45 GHz and 700 W. The obtained gel composition was dried in a 150°C oven and heated for 3 hours at 500°C in air and then calcined for 30 minutes at 800°C in an N₂ atmosphere.
The condition of the particles constituting the obtained lithium silicate powder was observed by SEM and TEM, and these results are given in FIG. 15 and FIG. 16, respectively.

FIG. 15(a) contains the SEM image (approximately 5000X) of a whole lithium silicate particle (primary particle), while FIG. 15(b) contains an enlarged view (approximately 12500X) of a portion thereof. As is clear from FIG. 15(a), the lithium-rich lithium silicate was shown to have a sponge-shaped crystal form rather than a petal shape in the order of micrometer. In this SEM image, the wall surfaces of the lithium silicate crystal, which have formed so as to surround voids, are clearly demonstrated to have a thickness that is suitably thin relative to the dimensions (width) of the voids and to have a thickness that is thinner than the plate-shaped crystals seen for the lithium orthosilicate. In addition, it is also shown that the crystal wall surfaces are bonded to each other at smaller intervals. The voids that can be observed in this figure were shown to be macropores having diameters of approximately 50 nm to 500 nm.
Thus, it was confirmed that the morphology of the lithium silicate particles that are formed can be substantially changed in the herein disclosed production method by changing the composition of the lithium silicate. In addition, the proportion of the Li to the Si is unusually large in this lithium silicate. Due to this, the crystals are structured with a configuration in which Li is present in large amounts at the perimeter of the chain of SiO₄ tetrahedra that forms the framework. That is, this lithium silicate can be understood to be a silicon-lithium composite compound that is constituted of a core portion for which the main component thereof is silicon (Si) and a shell portion for which the main component thereof is lithium (Li) present so as to cover the surface of the core portion. It can be concluded that such a lithium silicate fits well with the lithium silicate production mechanism described in 3-3 above.

A lithium-rich lithium silicate having the chemical composition given by Li₈SiO₆ is disclosed in Non Patent Literature 2. This Li₈SiO₆, while being provided with a high CO₂ absorption capacity, presents the problem of having an inferior CO₂ desorption performance. This Li₈SiO₆ is synthesized by a solid-phase reaction method and is composed of very large particles exceeding 50 mm (refer, for example, to FIG. 2). Moreover, even when these very large particles are microscopically observed in the order of 1 mm, the characteristic crystal forms as herein disclosed cannot be confirmed. Given this, it is hypothesized that the reason for the preceding is that the specific surface area of the Li₈SiO₆ crystal is quite small and the CO₂ desorption reaction then does not proceed adequately. For example, it is thought that the occurrence of the following reaction field for CO₂ desorption is poorly supported in this Li₈SiO₆ crystal.
Li₂SiO₃ + Li₂CO₃ → Li₄SiO₄ + CO₂

FIGS. 16(a) to (e) are TEM images, for different fields and at different magnifications, of the same lithium silicate particles as in FIG. 15. As shown in FIG. 16, the lithium silicate particles having a sponge-shaped form at the micrometer level were found to be formed by the aggregation of primary particles having sizes of approximately 20 nm to several hundred nm. Thus, the wall surfaces of the sponge-shaped bodies were found to be aggregates of even finer particles. In addition, it could also be confirmed that a plurality of these particles are bonded with the formation of voids and thus that the wall surface itself is a porous structure. It was found that the voids contained in this wall surface are micropores or mesopores having diameters of approximately not more than 50 nm. Based on this, it was found that the herein disclosed lithium silicate particles incorporate relatively large voids (macropores) within the particle due to the sponge shape and also incorporate smaller voids (mesopores and micropores) in their wall surfaces. It is thought that the high CO₂ absorption characteristics of this lithium silicate, see below, are realized through such a porous structure.

4-2. The CO₂ absorption characteristics
The carbon dioxide (CO₂) absorption characteristics of the lithium-rich lithium silicate obtained as described above were examined using thermogravimetric analysis. A nitrogen gas purge was first carried out in the thermogravimetric analysis. Specifically, the sample was heated to 800°C at a ramp rate of 10°C/min under an N₂ current at a flow rate of 49 mL/min; this was followed by holding for 20 minutes at 800°C and then dropping the temperature to 710°C and holding for 10 minutes. The N₂ gas was subsequently switched over to 100% CO₂ gas and the amount of CO₂ gas absorption at 710°C was measured. The obtained isothermal absorption curve is given in FIG. 17.
As shown in FIG. 17, the CO₂ absorption capacity of the lithium silicate of the present example was seen to reach to approximately 840 mg/g. This is a very high value equal to approximately 19 mmol CO₂/g when converted to a molar basis. Given that the absorption capacity measured at 650°C on the powder (MW-SG) with the Li₄SiO₄ composition referenced in 2-2 above was approximately 350 mg/g, it was then confirmed that an approximately 2-fold to even a 3-fold increase in the absorption capacity is made possible through differences in the composition and crystal structure. When the composition of the lithium silicate produced in the present example is theoretically calculated from the CO₂ absorption capacity, it is thought to correspond to approximately Li₁₂SiO₈ or 6(Li₂O)SiO₂.

5-1. Production of a germanium-doped lithium silicate
A lithium silicate was prepared and a carbon dioxide-absorbing material was obtained in accordance with 1-1 above. However, in the present example, in order to produce a germanium (Ge)-containing lithium silicate, as shown in the flow chart in FIG. 1C germanium tetrachloride (GeCl₄) was also used as a germanium source and introduction into the lithium silicate was carried out by dissolving this germanium tetrachloride in the aqueous lithium nitrate solution. The amount of addition of the germanium tetrachloride was a proportion that provided Si : Ge = 1 : 0.1826 as the molar ratio. In addition, the aqueous reaction solution in which all the starting materials were mixed was subjected to ultrasound stirring for 5 minutes followed by exposure to electromagnetic waves. Using a microwave oven for the exposure to electromagnetic waves, the dehydration condensation reaction was promoted by exposure for 4 minutes to electromagnetic waves at 2.45 GHz and 700 W. The composition of the thusly prepared lithium silicate is Li₄Ge_0.15Si_0.846O.

5-2. XRD analysis
XRD analysis was performed on the germanium-doped lithium silicate obtained as described above, and the obtained XRD pattern is given in FIG. 18(b). For reference, the XRD pattern is given in FIG. 18(a) for the lithium silicate with the composition Li₄SiO₄ shown in FIG. 2.
As shown in FIG. 18, peaks for Li₄Ge₅O₁₂ were weakly detected along with peaks for Li₄SiO₄ in the XRD pattern (b) for the germanium-doped lithium silicate. In addition, the Li₄SiO₄ peaks were shifted somewhat to the low angle side. Based on this, it was confirmed that Ge had been introduced into the crystalline structure of Li₄SiO₄.

5-3. Raman analysis
Raman spectroscopic analysis was performed on the germanium-doped lithium silicate, and the obtained Raman spectrum is given in FIG. 19(b). For reference, the Raman spectrum is also given in FIG. 19(a) for the lithium silicate with the composition Li₄SiO₄.
The three peaks at 784, 823, and 862 cm^-1 seen only in FIG. 19(b) were in good agreement with the vibration frequencies for the valence electrons of the GeO₄ tetrahedron and were results that were in good support of the results of the XRD analysis. It was also confirmed by the Raman spectrum that a material having the composition given by the general formula Li₄Ge_0.15Si_0.846O₄ had been formed by the substitution of Ge for Si in the crystal structure.

5-4. Morphology of the germanium-doped lithium silicate
The morphology of the particles constituting the germanium-doped lithium silicate obtained as described above was observed by SEM and TEM, and these results are given in FIG. 20 and FIG. 21, respectively.
FIG. 20 is an SEM image of the germanium-doped lithium silicate particles. It could be confirmed for the germanium-doped lithium silicate that, just as for the lithium silicate in FIG. 14, petal-shaped lithium silicate crystals had formed through the combination of a plurality of plate-shaped crystals. With the germanium-doped lithium silicate of the present example, a single one of the plate-shaped crystals was found to have a thickness of not more than about 5 mm and dimensions in the planar direction of around 10 mm or more, wherein formation occurred through the assembly of relatively thin plate-shaped crystals.
FIGS. 21(a) to (d) are TEM images of the germanium-doped lithium silicate particles, wherein the images are at different magnifications and locations of observation. These TEM images show a condition in which the single plate-shaped crystals observed in the SEM image in FIG. 20 have grown largely along a certain direction.

5-5. Dynamic absorption characteristics
The carbon dioxide absorption characteristics of the germanium-doped lithium silicate obtained as described above were evaluated, as in 1-4 above, as the dynamic absorption characteristics at varying ambient temperatures. The dynamic TGA curves obtained as a result are given in FIG. 22.
In the present example, three additional germanium-doped lithium silicates having different levels of Ge doping were prepared as described above in 5-1 and Li₄GeO₄ was prepared for reference, and their dynamic CO₂ absorption characteristics were examined and evaluated in the same manner. These results and the results for Li₄SiO₄ are given in FIG. 22. In the additionally prepared germanium-doped lithium silicates, the molar ratio between Si and the doped Ge was 1 : 0.4470, 1 : 0.083, and 1 : 0.04 as Si : Ge.

As shown in FIG. 22, it was demonstrated that the doping of Ge into the lithium silicate brought about an approximately 1.2-fold increase in the amount of CO₂ absorption in dynamic absorption in comparison to that for Li₄SiO₄ and Li₄GeO₄. In addition, changes in the CO₂ absorption characteristics were seen due to the doping with Ge: the amount of CO₂ absorption was found to be sharply increased in the low temperature region of about 300°C to 500°C, and the absorption rate in this low temperature region was found to have undergone a large increase. It was confirmed that the amount of CO₂ absorption also increased as the proportion of Ge doping relative to Si, as the molar ratio, increased from 0, but that the amount of CO₂ absorption shifted to a reduction when the Ge molar ratio exceeded approximately 0.1826.

5-6. Isothermal absorption characteristics
The isothermal absorption characteristics for carbon dioxide were then examined at a low temperature of 300°C using the germanium-doped lithium silicate with Si : Ge = 1 : 0.1826 as the molar ratio that was prepared in 5-1 as described above. The isothermal absorption curve obtained as a result is shown in FIG. 23.
As shown in FIG. 23, the germanium-doped lithium silicate did not have a very large absorption capacity per se, but was found to have a CO₂ absorption characteristic whereby CO₂ was rapidly absorbed even at the low temperature of 300°C.

Specific examples of the present invention are described in detail in the preceding, but these are nothing more than examples and do not limit the scope of the claims. Various and diverse modifications and alterations to the specific examples provided above as examples are included in the art described in the claims.

Claims (19)

1. A method of producing a carbon dioxide-absorbing material that comprises a lithium silicate as a major component thereof,
   the method comprising:
   preparing a sol composition comprising an Li-Si precursor compound containing a lithium (Li) component and a silicon (Si) component, and an aqueous solution dispersing the Li-Si precursor compound;
   exposing the sol composition to electromagnetic waves to obtaining a gel composition; and
   calcining this gel composition to obtain a lithium silicate that contains lithium and silicon.
   
2. The production method according to claim 1, wherein the sol composition is prepared by a hydrolysis reaction of the lithium (Li) component and the silicon (Si) component in an aqueous solution.
   
3. The production method according to claim 2, wherein the silicon (Si) component is at least one selected from the group consisting of silica alkoxides, colloidal silicas, and fumed silicas.
   
4. The production method according to any one of claims 1 to 3, wherein the sol composition additionally contains a germanium (Ge) component.
   
5. The production method according to any one of claims 1 to 4, wherein the electromagnetic waves are microwaves having a wavelength from 1 mm to 1 m and a frequency from 300 MHz to 300 GHz.
   
6. The production method according to any one of claims 1 to 5, wherein a total time of exposure to the electromagnetic waves is from 1 minute to 60 minutes.
   
7. The production method according to any one of claims 1 to 6, further comprising forming a composite of the lithium silicate with an alkali carbonate.
   
8. The production method according to claim 7, wherein the alkali carbonate comprises two or more from among a sodium (Na) component, a potassium (K) component, and a lithium (Li) component.
   
9. The production method according to claim 7 or 8, wherein the proportion of each alkali component in the alkali carbonate is as follows:
   Na : from 1 mol% to 80 mol%,
   K : from 1 mol% to 70 mol%, and
   Li : from 1 mol% to 90 mol%.
   
10. The production method according to any one of claims 7 to 9, wherein the alkali carbonate is a eutectic carbonate.
    
11. A carbon dioxide-absorbing material comprising a silicon-lithium composite compound comprising:
    a core portion comprising silicon as a major component thereof; and
    a shell portion comprising lithium as a major component thereof and covering a surface of at least a part of the core portion.
    
12. A carbon dioxide-absorbing material comprising a powder having a lithium silicate as a major component thereof, wherein
    individual particles constituting the powder have at least one shape selected from the group consisting of a rod shape, a plate shape, a scale shape, and a petal shape.
    
13. The carbon dioxide-absorbing material according to claim 12, wherein the lithium silicate is given by the general formula Li_xSi_yO_z where the x, y, and z in the formula satisfy x + 4y - 2z = 0.
    
14. The carbon dioxide-absorbing material of claim 12 or 13, which additionally contains at least one of sodium and potassium wherein
    a portion of the lithium in the lithium silicate is replaced by the sodium and potassium.
    
15. The carbon dioxide-absorbing material according to any one of claims 12 to 14, further comprising germanium, wherein
    a portion of the silicon in the lithium silicate is replaced by the germanium.
    
16. The carbon dioxide-absorbing material according to any one of claims 11 to 15, which has a CO₂ absorption rate per unit weight of at least 25 mg/(g × min) in a temperature range from 500°C to 710°C.
    
17. The carbon dioxide-absorbing material according to any one of claims 11 to 16, which has a CO₂ gas absorption rate per unit weight of at least 50 mg/(g × min) in a temperature range from 600°C to 710°C.
    
18. The carbon dioxide-absorbing material according to any one of claims 11 to 17, wherein at least 50 number% of the total lithium silicate are rod-shaped particles for which, when a is the dimension in a long direction and b is the dimension in a short direction orthogonal to the long direction, the dimension b in this short direction being not more than 100 nm and the aspect ratio given by a/b being at least 2.
    
19. The carbon dioxide-absorbing material according to any one of claims 11 to 18, further comprising an alkali carbonate, the alkali carbonate covering at least a part of a surface of the lithium silicate.

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