Carbon dioxide adsorbent and method for recovering carbon dioxide

Abstract

To provide a carbon dioxide adsorbent exhibiting superior carbon dioxide adsorption capacity along with superior durability.SOLUTION: A carbon dioxide adsorbent includes a clay mineral having a crystallite size of 10 nm or less in the (060) plane of the crystallite.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a carbon dioxide adsorbent and a carbon dioxide recovery method.

Background Art

[0002] In order to avoid the serious impact on the environment due to global warming, reducing the emissions of greenhouse gases centered on carbon dioxide is a common global issue. Against this background, in recent years, efforts have been promoted for "carbon recycling" that captures carbon dioxide as a carbon resource, recovers it, and reuses (recycles) it in various products and fuels. The development of carbon dioxide adsorbents that can efficiently recover carbon dioxide and carbon dioxide separation and recovery technologies is also underway. As carbon dioxide separation and recovery technologies, amine absorption method (chemical absorption method), physical adsorption method, membrane separation method, etc. are known.

[0003] The amine absorption method is a carbon dioxide separation and recovery technology that utilizes the property of amine-based organic compounds to absorb carbon dioxide by chemical reaction. As a carbon dioxide adsorbent used in the amine absorption method, for example, Patent Document 1 discloses a carbon dioxide separation material containing a polyamine, wherein the polyamine has a hydrogen atom or a functional group bonded to a nitrogen atom and contains a propyl polyamine component having three or more propyl groups bonded to different nitrogen atoms in the molecule, at least one of the propyl groups is a hydroxypropyl group having a hydroxy group, the hydroxypropyl group is bonded to a nitrogen atom constituting a tertiary amine, the hydroxy group is bonded to a secondary carbon atom, and two or more of the propyl groups are unsubstituted isopropyl groups having no hydroxy group. Patent Document 2 also discloses a method for separating carbon dioxide in which a slurry composed of a solid carbon dioxide absorbent and a dispersion medium is brought into contact with a gas containing carbon dioxide to absorb carbon dioxide, and the slurry is heated to desorb carbon dioxide. As the solid carbon dioxide absorbent, an example is polyethylene polyamine supported on a solid carrier.

[0004] In addition, the physical adsorption method is a carbon dioxide separation and recovery technology that adsorbs and desorbs carbon dioxide by utilizing physical adsorption. Patent Document 3 discloses a carbon dioxide recovery method characterized by selectively capturing carbon dioxide in combustion exhaust gas using a carbon dioxide capture material containing clay composed of expandable minerals, and bringing the carbon dioxide desorbed gas into contact to desorb carbon dioxide. Patent Document 3 describes using montmorillonite K10 (trade name, manufactured by Sigma-Aldrich), Kunipia-F (trade name, manufactured by Kunimine Industries Co., Ltd.), and Lucentite (trade name, manufactured by Coop Chemical Co., Ltd.) as the expandable minerals.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] Since the carbon dioxide separation material or carbon dioxide solid absorbent described in Patent Documents 1 and 2 uses an organic amine-based compound as described above, it has poor heat resistance, and its durability is likely to decrease due to repeated absorption and discharge of carbon dioxide. Furthermore, it is known that the absorption and discharge of carbon dioxide by the organic amine-based compound are inhibited by acidic gases such as sulfur oxides (SOx) and nitrogen oxides (NOx) coexisting in the exhaust gas, resulting in a decrease in recovery performance. Therefore, the application of the above amine-based compound to the carbon dioxide recovery method is restricted by the use environment (reaction conditions, etc.). Furthermore, in order to separate the carbon dioxide bonded to the organic amine-based compound, it is necessary to heat the slurry of the organic amine-based compound, which requires a large amount of energy for this heating, resulting in an increase in cost. In the carbon dioxide recovery method described in Patent Document 3, although clay minerals such as montmorillonite and lussite show relatively good carbon dioxide recovery efficiency, further improvement in carbon dioxide recovery efficiency is required.

[0007] An object of the present invention is to provide a carbon dioxide adsorbent that exhibits excellent carbon dioxide adsorption ability and also excellent durability. Another object of the present invention is to provide a carbon dioxide recovery method using the above carbon dioxide adsorbent.

Means for Solving the Problems

[0008] The inventors of the present invention have intensively studied in view of the above problems. As a result, it has been found that by using a clay mineral having a crystallite size within a specific range as a carbon dioxide adsorbent, carbon dioxide can be adsorbed with high efficiency. The present invention has been completed based on these findings and through further studies.

[0009] The above problems of the present invention have been solved by the following means. 〔1〕 A carbon dioxide adsorbent containing a clay mineral having a crystallite size of 10 nm or less on the (060) plane of the crystallite. 〔2〕 The carbon dioxide adsorbent according to 〔1〕 above, wherein the ratio of the amount of leached cations of the clay mineral to the cation exchange capacity of the clay mineral is 3 or more. 〔3〕 The carbon dioxide adsorbent according to 〔1〕 or 〔2〕 above, wherein the amount of leached cations of the clay mineral is 120 meq / 100 g or more. 〔4〕 The carbon dioxide adsorbent according to any one of 〔1〕 to 〔3〕 above, wherein the clay mineral is smectite. 〔5〕 The carbon dioxide adsorbent according to 〔4〕 above, wherein the smectite is stibnite and / or hectorite. 〔6〕 A carbon dioxide recovery method for recovering carbon dioxide using the carbon dioxide adsorbent according to any one of 〔1〕 to 〔5〕 above. 〔7〕 The carbon dioxide recovery method according to the above [6], wherein the recovery of the carbon dioxide is carried out by humidifying the carbon dioxide adsorbent under room temperature conditions. 〔8〕 The carbon dioxide recovery method according to the above [6] or [7], wherein the adsorption and desorption of carbon dioxide are repeatedly carried out using the carbon dioxide adsorbent.

Advantages of the Invention

[0010] According to the carbon dioxide adsorbent of the present invention and the carbon dioxide recovery method of the present invention, carbon dioxide can be stably and efficiently adsorbed or recovered.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0012] Hereinafter, preferred embodiments of the present invention will be specifically described, but the present invention is not limited to the following embodiments except as defined in the present invention.

[0013] [Carbon Dioxide Adsorbent] The carbon dioxide adsorbent according to the first embodiment of the present invention (hereinafter, also referred to as "the adsorbent of the present invention") has a crystallite size (hereinafter, "L" on the (060) plane of the crystallites. 060(also referred to as “”).) contains a clay mineral having a crystallite size of 10 nm or less. Since the adsorbent of the present invention has the clay mineral with the above-mentioned specific crystallite size and thus has excellent carbon dioxide adsorption ability, the adsorbent of the present invention can be suitably used as a member for carbon dioxide adsorption, concentration, and recovery in, for example, a carbon dioxide adsorption concentrator or a carbon dioxide recovery device. Therefore, the adsorbent of the present invention contributes to the effective utilization of resources and can be an important material in promoting carbon neutrality with substantially zero carbon dioxide emissions and carbon recycling that reuses carbon resources.

[0014] Generally, since clay minerals are excellent in heat resistance and durability, the above-mentioned carbon dioxide adsorption and the like can be stably performed under various conditions. For example, the organic amine-based compounds used in the conventional amine absorption method are easily decomposed by acidic gases such as sulfur oxides (SOx) and nitrogen oxides (NOx), and it is known that the compounds deteriorate and decompose due to repeated use over a long period, resulting in a decrease in recovery performance. In contrast, since the adsorbent of the present invention has high durability against acidic gases such as sulfur oxides and nitrogen oxides, it can exhibit stable and high carbon dioxide adsorption (adsorption stability) even when used repeatedly over a long period. The form of the adsorbent of the present invention is not particularly limited, and it can have various shapes such as pellet shape, honeycomb shape, film shape, powder shape (particle shape), etc.

[0015] (Clay mineral) In the adsorbent of the present invention, the clay mineral functions as a carbon dioxide adsorbing component (active ingredient) for adsorbing carbon dioxide. The type of clay mineral used in the adsorbent of the present invention is not particularly limited as long as it satisfies the above crystallite size. Examples of such clay minerals include smectite, and it is preferably one or more selected from montmorillonite, beidellite, nontronite, saponite, hectorite, sauconite, and stibnite, more preferably hectorite and / or stibnite, and even more preferably stibnite. Further, the above smectite may be natural smectite or synthetic smectite, and synthetic smectite is preferred. The interlayer cation (metal ion present between crystal layers) of these smectites is preferably sodium ion (that is, the smectite is preferably sodium-type smectite). Here, in the case of XX-type smectite (smectite in which the main interlayer cation is XX ion), the ratio (molar basis) of XX ion in the total interlayer cations is 50% or more, preferably 60% or more, more preferably 70% or more, further preferably 80% or more, and even more preferably 90% or more. Also, substantially all of the interlayer cations may be XX ions.

[0016] In the adsorbent of the present invention, the water content of the clay mineral is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less.

[0017] In the present invention or in this specification, "crystallite" means the smallest unit particle portion that can be regarded as a single crystal among crystal grains. In the adsorbent of the present invention, L 060 The content of the clay mineral with L of 10 nm or less can be appropriately set according to the use, form, etc. of the adsorbent of the present invention. In the adsorbent of the present invention, L 060The content of clay minerals with a size of 10 nm or less is preferably 10% by mass or more, more preferably 20% by mass or more, still more preferably 30% by mass or more, still more preferably 40% by mass or more, and can also be 50% by mass or more, may be 60% by mass or more, may be 70% by mass or more, may be 80% by mass or more, and may be 90% by mass or more. Also, all of the adsorbent of the present invention is L 060 may be clay minerals with a size of 10 nm or less. The adsorbent of the present invention is L 060 may contain clay minerals with a size exceeding 10 nm. In this case, among the clay minerals contained in the adsorbent of the present invention, the proportion of L 060 clay minerals with a size of 10 nm or less is preferably 10% by mass or more, more preferably 20% by mass or more, still more preferably 30% by mass or more, still more preferably 40% by mass or more, and can also be 50% by mass or more, may be 60% by mass or more, may be 70% by mass or more, may be 80% by mass or more, and may be 90% by mass or more. It is also preferable that all of the clay minerals contained in the adsorbent of the present invention are L 060 clay minerals with a size of 10 nm or less. In addition, examples of components other than clay minerals that can be contained in the adsorbent of the present invention include other components described later.

[0018] -Crystallite size- By setting the L 060 of the clay mineral to 10 nm or less, the carbon dioxide adsorption amount can be increased compared to clay minerals with a size exceeding 10 nm. L 060 It is presumed that the reason for the increase in carbon dioxide adsorption ability by setting the L 060 to 10 nm or less is as follows. It is known that in clay minerals, the planar part of the crystal layer (tetrahedral sheet) composed of silicon dioxide tetrahedrons is low-polarity, while the terminal part of the crystal layer (layer side surface) has many polar groups and is more polar than the planar part. When the crystallite size of the clay mineral is small, the proportion of the terminal part of the crystal layer per unit weight increases, and it is considered that the adsorption amount of carbon dioxide, which is a polar molecule, increases. From the viewpoint of further improving the carbon dioxide adsorption ability, the clay mineral is L 060It is preferably 9 nm or less, more preferably 8 nm or less, and even more preferably 7 nm or less.

[0019] - Method for Measuring Crystallite Size - In the present invention or this specification, the crystallite size can be the average value of the crystallite sizes calculated according to the following Scherrer's (Equation 1) from the full width at half maximum of the diffraction peak of the (060) plane of the crystallites measured by the X-ray diffraction (XRD) method (the arithmetic mean value of the calculated values obtained by measuring multiple times). D hkl =(K·λ) / (βcosθ) (Equation 1) In the above (Equation 1), D hkl is the "size of the crystallite (crystallite size)" in the direction perpendicular to the (hkl) plane of the crystallite, K is the Scherrer constant, λ is the wavelength of the X-ray used in the XRD method, β is the broadening (full width at half maximum) of the diffraction X-ray peak of the (hkl) plane, and θ is the Bragg angle of the diffraction X-ray.

[0020] The amount of carbon dioxide adsorbed at the end portion of the crystal layer is considered to vary depending on the type of polar group present at the end portion of the crystal layer. From the viewpoint of further improving the carbon dioxide adsorption ability, it is preferable that a metal alkoxide structure (-OM, where M is a metal atom, preferably an alkali metal atom, and more preferably a sodium atom) is present at the end portion of the crystal layer. Although a hydroxyl group (-OH) also exists at the end portion of the crystal layer, since the metal alkoxide structure (-OM) has stronger ionic bonding and greater polarity than the hydroxyl group (-OH), it is considered that a larger amount of carbon dioxide can be adsorbed. At least a part of the above metal alkoxide structure is preferably a sodium alkoxide structure (-ONa), and the ratio of the sodium alkoxide structure in the total number of the above metal alkoxide structures is preferably 50% or more, more preferably 70% or more, even more preferably 80% or more, and preferably 90% or more.

[0021] (Relationship between Cation Exchange Capacity and Leached Cation Amount) The relationship between the cation exchange capacity (CEC) of a clay mineral and the amount of leached cations (total amount of cations present in the intercrystalline layer and the terminal part of the crystal layer) (Lc: Leaching cation quantity) can be evaluated as the ratio value of the amount of leached cations to the cation exchange capacity (Lc / CEC). That is, when a metal alkoxide structure (-OM) is present at the terminal part of the crystal layer of the clay mineral, the value of Lc / CEC becomes large (a value greater than 1), and when there is almost no metal alkoxide structure (-OM) at the terminal part of the crystal layer, the value of Lc / CEC becomes small (a value closer to 1). In the adsorbent of the present invention, the value of Lc / CEC in the clay mineral is preferably 3 or more. When "the value of Lc / CEC is 3 or more", it can be evaluated that the molar amount of cations present at the terminal part of the crystal layer of the clay mineral is 2 times or more the molar amount of cations between the crystal layers. From the viewpoint of further improving the carbon dioxide adsorption ability of the adsorbent of the present invention, Lc / CEC is more preferably 4 or more, and even more preferably 5 or more. In addition, the Lc / CEC can be 20 or less, may be 15 or less, or may be 10 or less.

[0022] The amount of leached cations (Lc) of the clay mineral is preferably 120 meq (milliequivalent) / 100 g or more, more preferably 140 meq / 100 g or more, and even more preferably 160 meq / 100 g or more. There is no particular limitation on the cation exchange capacity (CEC) of the clay mineral. Since there is a slight correlation between the crystallite size and the CEC, the CEC is preferably 100 meq / 100 g or less, more preferably 70 meq / 100 g or less, and even more preferably 40 meq / 100 g or less.

[0023] - Method for measuring cation exchange capacity - The method for measuring the cation exchange capacity can be measured in accordance with the Japan Bentonite Industry Association Standard Test Method JBAS - 106 - 77 (Method for Measuring the Cation Exchange Capacity (CEC) of Bentonite (Powder)). Specifically showing the procedure, it becomes the following (Step 1-1) to (Step 1-4). (Step 1-1) Stir the clay mineral in an aqueous ammonium acetate solution to perform cation exchange of all the cations between the crystal layers of the clay mineral with ammonium ions, and extract (release from the clay mineral) all the cations present in the terminal portions of the crystal layers of the clay mineral. (Step 1-2) Remove the cations between the crystal layers after cation exchange and the metal ions extracted from the terminal portions of the crystal layers, etc. by alcohol washing. (Step 1-3) Add an aqueous potassium chloride solution to extract the ammonium ions between the crystal layers of the cation-exchanged clay mineral. (Step 1-4) Measure the amount of extracted ammonium ions using an ammonium ion electrode. By this method, the cations present between the crystal layers of the clay mineral (sodium ions are the main cations in the case of sodium-type clay minerals) can be quantified.

[0024] - Method for measuring the amount of leached cations - By partially changing the procedure of the above-mentioned Japan Bentonite Industry Association Standard Test Method JBAS-106-77 (Method for Measuring the Cation Exchange Capacity (CEC) of Bentonite (Powdery)), the amount of leached cations can be measured. Specifically showing the procedure, it becomes the following (Step 2-1), (Step 2-2). (Step 2-1) Stir the clay mineral in an aqueous ammonium acetate solution to perform cation exchange of all the cations between the crystal layers of the clay mineral with ammonium ions, and extract (release from the clay mineral) all the cations present in the terminal portions of the crystal layers of the clay mineral. (Step 2-2) Collect the supernatant after the above stirring, and measure the amount of cations (cations other than ammonium ions) in the collected solution using an atomic absorption spectrometer or the like. By this method, it is possible to quantify the total amount of cations present between the crystal layers of the clay mineral (sodium ions are the main cations in the case of sodium-type clay minerals) and each cation present in the terminal portions of the crystal layers and the like.

[0025] (Other components) The adsorbent of the present invention may contain other components as long as the target function of the above-mentioned clay mineral is not impaired. Examples of such other components include adsorbents for making carbon dioxide more easily adsorbable (including adsorption assisting materials), materials used for molding the clay mineral (shaping materials), and the like. Examples of the adsorbent include activated carbon, activated alumina, zeolite, molecular sieves, silica gel, and the like. Examples of the shaping material include lubricants, resins, binders, and the like. Furthermore, amine compounds and calcium-based compounds, etc., which are known for their selective adsorption ability (absorption ability) of carbon dioxide, can also be used in combination.

[0026] [Carbon dioxide recovery method] The second embodiment of the present invention is a carbon dioxide recovery method (hereinafter, also referred to as "the recovery method of the present invention"). The recovery method of the present invention can apply a known carbon dioxide recovery method except for using the adsorbent of the present invention. For example, in addition to the adsorbent of the present invention, a device having a container filled with the adsorbent, a pipe for ventilating a gas containing carbon dioxide and the like, a supply pump (pressure pump), etc., can adsorb and recover carbon dioxide on the adsorbent. The configuration of such a device can be appropriately designed according to the type, flow rate, and scale of the target gas for recovering (removing) carbon dioxide. In this specification, the carbon dioxide-containing gas to be recovered (removed) of carbon dioxide is also referred to as "raw material gas". Examples of general raw material gases include exhaust gas and the like. In addition, in the recovery method of the present invention, the "recovery" of carbon dioxide may mean recovering the adsorbent itself having adsorbed carbon dioxide, or recovering carbon dioxide by desorbing (detaching) carbon dioxide from the adsorbent having adsorbed carbon dioxide as described later.

[0027] The adsorbent of the present invention increases the carbon dioxide adsorption amount by repeating the adsorption and desorption of carbon dioxide, and then converges to a certain high range. Therefore, it is also preferable to use the adsorbent of the present invention in the recovery method of the present invention after undergoing pre-adsorption and desorption of carbon dioxide a plurality of times (preferably 1 time, more preferably 2 times or more, still more preferably 3 times or more). Usually, the carbon dioxide adsorption amount becomes stable by pre-adsorption and desorption about 3 to 10 times. The conditions for pre-adsorption and desorption may be the same as the conditions in the recovery method of the present invention.

[0028] (Shape of the adsorbent of the present invention) The shape of the adsorbent of the present invention used in the recovery method of the present invention is not particularly limited. From the viewpoint of further improving the carbon dioxide recovery rate (carbon dioxide adsorption rate of the adsorbent of the present invention) in the recovery method of the present invention, it is preferable to adopt a shape that does not obstruct the flow of the raw material gas as much as possible and increases the contact efficiency between the raw material gas and the adsorbent. More specifically, it is more preferable to form the adsorbent of the present invention into a pellet shape, a honeycomb shape, a film shape, etc. and use it. In addition, when adopting a flow-through contact method such as a fluidized bed, the adsorbent of the present invention can also be used in a powder form.

[0029] (Raw material gas) The carbon dioxide concentration in the raw material gas is not particularly limited, and raw material gases with a low concentration of carbon dioxide in the order of ppm (volume basis) to a high concentration of several% to several tens of volume% can be used. In addition, there is no particular limitation on other gases that may coexist in the raw material gas. For example, gases such as nitrogen, oxygen, water vapor, and argon that are abundant in air may be included. In addition, nitrogen oxides (NOx), sulfur oxides (SOx), etc. contained in exhaust gases from industrial boilers, etc. may be included. Since the adsorbent of the present invention used in the recovery method of the present invention is less affected by the above nitrogen oxides, sulfur oxides, etc. than amine-based compounds, carbon dioxide can be stably adsorbed and recovered. In addition, the proportion of water vapor in the raw material gas is preferably 20 mol% or less, more preferably 10 mol% or less, and still more preferably 5 mol% or less.

[0030] (Reaction conditions) In addition, in the recovery method of the present invention, the temperature of the raw material gas when the adsorbent of the present invention comes into contact with the raw material gas is not particularly limited. For example, in a carbon dioxide recovery method such as the above amine absorption method, the temperature of the raw material gas is cooled to near room temperature (45°C or lower) and then transferred to an absorption tower filled with an amine-based compound. On the other hand, in the recovery method of the present invention, as described above, since an adsorbent containing a specific clay mineral as an adsorbent component is used, it has better heat resistance than the above amine-based compound. Therefore, the temperature of the raw material gas brought into contact with the adsorbent of the present invention may be below zero degree, or may be a high temperature of, for example, 100°C or higher. Also, the pressure when the adsorbent of the present invention comes into contact with the raw material gas is not particularly limited. From the viewpoint of further improving the carbon dioxide adsorption rate in the recovery method of the present invention, it is preferable to bring them into contact under pressurized conditions. The relative humidity when the adsorbent of the present invention comes into contact with the raw material gas is preferably less than 40%, more preferably less than 30%, still more preferably less than 20%, still more preferably less than 10%, still more preferably less than 6%, and still more preferably less than 3%, although it also depends on the carbon dioxide concentration in the raw material gas.

[0031] (Desorption of carbon dioxide) In the recovery method of the present invention, when the carbon dioxide adsorption amount of the adsorbent of the present invention reaches near saturation or the adsorption efficiency deteriorates, the carbon dioxide adsorbed on the adsorbent of the present invention is desorbed (detached). By desorbing carbon dioxide in this way, carbon dioxide can be separated from the adsorbent and recovered (or removed). The method for desorbing carbon dioxide is not particularly limited, and examples include a method of subjecting the adsorbent of the present invention to heat treatment, reduced pressure treatment, or ultrasonic treatment, a method of applying vibration, and a method of contacting with a carbon dioxide desorption gas or the like. Further, as shown in the examples described later, carbon dioxide can also be desorbed at room temperature only by humidification (humidification treatment) without heating. The method of desorbing carbon dioxide by humidification is particularly preferable because it requires lower energy costs than the method of desorbing carbon dioxide by heating or reduced pressure. The relative humidity when desorbing carbon dioxide is preferably 40% or more, more preferably 50% or more. Also, the temperature (room temperature) when desorbing carbon dioxide varies depending on seasons and the like, but is usually in the range of 5 to 50 °C, may be 10 to 40 °C, or may be 15 to 35 °C.

[0032] As described above, since the adsorbent of the present invention is excellent in durability, in the recovery method of the present invention, the adsorbent of the present invention can be repeatedly used to adsorb and desorb carbon dioxide. The number of repeated uses may be 50 or more, 100 or more, or 1000 or more.

Examples

[0033] Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited thereto. Note that “%” and “parts” described in the following preparation examples are all based on mass.

[0034] As the clay minerals of Example 1, Comparative Example 1 and Comparative Example 2, commercially available products were used respectively. The details are as follows. In addition, the interlayer cation of any clay mineral was sodium ion. · Example 1: Smeton ST (trade name, stibnite, manufactured by Kunimine Industries Co., Ltd.) · Comparative Example 1: Kunipia-F (trade name, montmorillonite, manufactured by Kunimine Industries Co., Ltd.) · Comparative Example 2: Smeton SWM (trade name, hectorite, manufactured by Kunimine Industries Co., Ltd.)

[0035] <Preparation Example> As the clay minerals in Examples 2 and 3, synthetic stibnite and synthetic hectorite were used respectively. Preparation examples of each synthetic smectite are shown below. The raw materials of the synthetic clay used in each preparation example were as follows. · Magnesium sulfate (manufactured by Yakushi Kasei Co., Ltd.) · Concentrated sulfuric acid (98%) (manufactured by Toho Kogyo Co., Ltd.) · 25% aqueous sodium hydroxide solution (manufactured by Toho Kogyo Co., Ltd.) · Sodium silicate No. 3 (manufactured by Seishin Shoko Co., Ltd.) · Lithium hydroxide (manufactured by Yakushi Kasei Co., Ltd.)

[0036] (Synthetic stibnite of Example 2) Each aqueous solution used for the preparation of the synthetic stibnite of Example 2 was prepared as follows. Each aqueous solution was stirred for 30 minutes or more to confirm that there was no insoluble matter. · 450 parts of sodium silicate No. 3 was diluted with 2800 parts of distilled water to obtain an aqueous sodium silicate solution. · 361 parts of magnesium sulfate was dissolved in 550 parts of distilled water to obtain an aqueous magnesium sulfate solution. · 548 parts of 25% aqueous sodium hydroxide solution was diluted with 1280 parts of distilled water to obtain Aqueous Sodium Hydroxide Solution 1. · 38 parts of 25% aqueous sodium hydroxide solution was diluted with 50 parts of distilled water to obtain Aqueous Sodium Hydroxide Solution 2. The synthesis of stibnite was carried out according to the following procedure. To the total amount of the above sodium silicate aqueous solution, 73 parts by mass of concentrated sulfuric acid stock solution was added in a short time to obtain a uniform aqueous solution. To the obtained aqueous solution, the total amount of the above magnesium sulfate aqueous solution was added to obtain a uniform aqueous solution. Further, the total amount of the above sodium hydroxide aqueous solution 1 was added to precipitate a white turbid gel-like substance. The precipitate was collected by vacuum filtration using Advantec filter paper 4A to collect the solid content, 3000 parts of water was added thereto, stirred and washed, and the operation of vacuum filtration again with the same filter paper was performed 3 times for washing. 750 parts of distilled water was added to the obtained washed gel-like substance to obtain a white turbid dispersion. To this dispersion, the total amount of the above sodium hydroxide aqueous solution 2 was added, and hydrothermal synthesis treatment was carried out at 225 °C for 6 hours in a 2 L autoclave. The obtained gel-like substance was dried at 105 °C and then pulverized to obtain the clay mineral of Example 2 (synthetic sodium-type stibnite) in which the interlayer cation composed of stibnite is sodium ion.

[0037] (Synthetic hectorite of Example 3) Each aqueous solution used for the preparation of the synthetic hectorite of Example 3 was prepared as follows. Each aqueous solution was stirred for 30 minutes or more to confirm that there was no insoluble matter. · 57 parts of concentrated sulfuric acid was diluted with 1500 parts of distilled water to obtain a sulfuric acid aqueous solution. · 100 parts of sodium silicate No. 3 was diluted with 2000 parts of distilled water to obtain a sodium silicate aqueous solution. · 250 parts of magnesium sulfate was dissolved in 550 parts of distilled water to obtain a magnesium sulfate aqueous solution. · 120 parts of 25% sodium hydroxide aqueous solution was diluted with 1500 parts of distilled water to obtain a sodium hydroxide aqueous solution. · 3.7 parts of lithium hydroxide was dissolved in 50 parts of distilled water to obtain a lithium hydroxide aqueous solution. The synthesis of hectorite was carried out according to the following procedure. The total amount of the above sodium silicate aqueous solution was added to the total amount of the above concentrated sulfuric acid aqueous solution in a short time to obtain a uniform aqueous solution. Next, the above magnesium sulfate aqueous solution was added to obtain a uniform aqueous solution. The above sodium hydroxide aqueous solution was added thereto to precipitate a white turbid gel-like substance. The precipitate was collected by vacuum filtration using Advantec filter paper 4A to collect the solid content, 3000 parts of water was added thereto, and the mixture was stirred and washed, and the operation of vacuum filtration with the same filter paper was performed three times for washing. 750 parts of distilled water was added to the obtained washed gel-like substance to obtain a white turbid dispersion. The total amount of the above lithium hydroxide aqueous solution was added to this dispersion, and hydrothermal synthesis treatment was performed at 225 ° C for 6 hours in a 2L autoclave. The obtained gel-like substance was dried at 105 ° C and then pulverized to obtain the clay mineral (synthetic sodium-type hectorite) of Example 3 in which the interlayer cation composed of hectorite was sodium ion.

[0038] <Experimental Example 1> The following experiments were conducted on the clay minerals of each of Examples 1 to 3, and Comparative Examples 1 and 2. The results are shown in Table 1 below.

[0039] (Measurement of crystallite size) Regarding each clay mineral of Examples 1 to 3, and Comparative Examples 1 and 2, the diffraction peak of the (060) plane of the crystallite was measured by an X-ray diffractometer (Ultima IV, manufactured by Rigaku Corporation). From the full width at half maximum of the peak, the crystallite size was calculated according to the following Scherrer's (Equation 1). The above calculated value was obtained by measuring 10 times for each clay mineral, and the arithmetic mean of the 10 calculated values was taken as the crystallite size of each clay mineral. D hkl =(K·λ) / (βcosθ) (Equation 1) In the above (Equation 1), D hkl is the "crystallite size" in the direction perpendicular to the (hkl) plane of the crystallite, K is the Scherrer constant, λ is the wavelength of the X-ray used in the XRD method, β is the broadening (full width at half maximum) of the diffraction X-ray peak of the (hkl) plane, and θ is the Bragg angle of the diffraction X-ray. In the above (Equation 1), hkl was taken as (060).

[0040] (Measurement of cation exchange capacity) The CEC of the clay minerals of each of Examples 1 to 3 and Comparative Examples 1 and 2 was measured by the above-mentioned measurement method in accordance with the Japan Bentonite Industry Association Standard Test Method JBAS-106-77 (Method for measuring the cation exchange capacity (CEC) of bentonite (powder)).

[0041] (Measurement of the amount of leaching cations) The ammonium acetate aqueous solution described in section 2.3 was obtained according to the Japan Bentonite Industry Association standard test method JBAS-106-77 (method for measuring the cation exchange capacity (CEC) of bentonite (powdered)). 10 ml of the obtained aqueous solution was taken with a syringe and filtered through a 0.45 μm simple filter to obtain a sample solution. The sample solution was diluted 25 times to prepare a 50 ml measurement solution, and the cations (other than ammonium ions) were quantified using a microwave plasma atomic emission spectrometer (4100MP-AES, Agilent Technologies) to determine Lc. Note that almost all of the detected cations were sodium ions, so the value will be referred to as Lc(Na) below.

[0042] (Measurement of carbon dioxide adsorption amount) The outline of the apparatus used to measure the amount of carbon dioxide adsorption is shown in Figure 1. 1 g of clay mineral 1 was placed in reactor 2 (stainless steel portable reactor, TVS-N2-10, manufactured by Taiatsu Glass Industry Co., Ltd.), and carbon dioxide gas (CO2: 99.9% by volume or more) was sent from carbon dioxide gas cylinder 4 through gas sending pipe 3 into the reactor while adjusting the pressure detected by pressure gauge 6 with pressure regulator 5, and reacted (adsorbed) for 15 minutes. The gas other than carbon dioxide in the reactor was air, and the introduction pressure of carbon dioxide gas was 0.1 MPa. The amount of adsorbed carbon dioxide was quantified using the amount of sodium hydrogen carbonate as a standard. A plurality of mixtures with different mixing ratios of sodium hydrogen carbonate and clay minerals were prepared, and a calibration curve was created in advance from the absorbance data obtained using a Fourier transform infrared absorption spectrometer (IR Spirit, manufactured by Shimadzu Corporation). Similarly, absorbance data was taken for the clay minerals after the carbon dioxide adsorption, and based on the previously created calibration curve, the amount of carbon dioxide adsorbed by the clay minerals was determined.

[0043]

Table 1

[0044] As shown in Table 1 above, the clay minerals of Comparative Examples 1 and 2 both had a crystallite size L 060 exceeding 10 nm, and neither showed any significant carbon dioxide adsorption ability. In contrast, the clay minerals of Examples 1 to 3 all had a crystallite size L 060 of 10 nm or less, a carbon dioxide adsorption amount of 10 mass% or more, and showed excellent carbon dioxide adsorption ability.

[0045] <Experimental Example 2> (Carbon Dioxide Adsorption-Desorption Test-1) Using the clay mineral (stibnite) of Example 1, the carbon dioxide adsorption amount after the adsorption operation was measured in the same manner as in Experimental Example 1 above, except that the introduction pressure of carbon dioxide gas was set to 0.5 MPa and the adsorption time was set to 24 hours (first cycle). The results are shown in Figure 2. The black circles shown in Figure 2 represent the carbon dioxide adsorption amount in the clay mineral after the carbon dioxide adsorption operation (before desorption). The carbon dioxide adsorption amount in the first cycle was approximately 27 mass%, which was almost the same as the carbon dioxide adsorption amount of the clay mineral of Example 1 in Experimental Example 1 above. Next, the clay mineral of Example 1 that had adsorbed carbon dioxide in the first cycle was allowed to stand for 24 hours in an atmosphere of room temperature (25°C) and 50% relative humidity using a thermo-hygrostat (STC-V, manufactured by Sampratech Co., Ltd.) to desorb carbon dioxide. The amount of carbon dioxide adsorbed remaining in the clay mineral after the desorption operation (after desorption) was measured in the same manner as above. The results are shown in Figure 2. The white circles shown in Figure 2 represent the amount of carbon dioxide adsorbed (remaining amount) in the clay mineral after desorption of carbon dioxide. The amount of carbon dioxide adsorbed (remaining amount) in the first cycle was approximately 1% by mass, indicating that carbon dioxide could be efficiently separated and recovered by the above desorption operation.

[0046] The adsorption and desorption consisting of the above adsorption operation and desorption operation was defined as one cycle, and the same operation was carried out up to the 45th cycle (the adsorption and desorption was repeated 45 times). The amounts of carbon dioxide adsorbed before and after desorption in each cycle are shown in Figure 2. Even after repeating the adsorption and desorption 45 times, the clay mineral of Example 1 maintained a stable carbon dioxide adsorption property. Furthermore, it was shown that the amount of carbon dioxide adsorbed increased from the first cycle to the third cycle and then converged to a substantially constant high range.

[0047] (Carbon Dioxide Adsorption-Desorption Test - 2) Instead of carbon dioxide gas, a nitrogen-based carbon dioxide mixed gas containing sulfur oxides and nitrogen oxides (a four-component mixed standard gas, NO: 90 - 110 ppm (volume basis), SO2: 90 - 110 ppm (volume basis), CO2: 11.7 - 14.3 volume%, the balance being N2, manufactured by Sumitomo Seika Chemicals Co., Ltd.) (hereinafter also simply referred to as the mixed gas) was used, and an adsorption-desorption test was carried out in the same manner as the above carbon dioxide adsorption-desorption test - 1 except that the adsorption and desorption of carbon dioxide was repeated 68 times. The results are shown in Figure 3.

[0048] Even when using a mixed gas instead of carbon dioxide gas, the carbon dioxide adsorption amount before desorption in the first cycle was about 23% by mass, indicating that carbon dioxide can be selectively adsorbed effectively. Also, since the proportion of carbon dioxide in the mixed gas is 11.7 - 14.3% by volume, it is presumed that the carbon dioxide partial pressure is lower compared to carbon dioxide gas, but it was inferred that it does not affect the carbon dioxide adsorption amount under the introduction pressure condition of about 0.5 MPa. Also, even when the adsorption / desorption cycle was repeated 68 times using the mixed gas, it was found that the viscous mineral of Example 1 stably exhibited high carbon dioxide adsorption. This was considered to be because the clay mineral of Example 1 has high durability against acidic gases.

Explanation of Symbols

[0049] 1 Clay mineral 2 Reactor 3 Gas supply pipe 4 Carbon dioxide gas cylinder 5 Pressure regulator 6 Pressure gauge

Claims

1. A carbon dioxide adsorbent containing a clay mineral having a crystallite size of 10 nm or less on the (060) plane of the crystallite.

2. The carbon dioxide adsorbent according to Claim 1, wherein the value of the ratio of the amount of leached cations of the clay mineral to the cation exchange capacity of the clay mineral is 3 or more.

3. The carbon dioxide adsorbent according to Claim 2, wherein the amount of leached cations of the clay mineral is 120 meq / 100 g or more.

4. The carbon dioxide adsorbent according to Claim 3, wherein the clay mineral is smectite.

5. The carbon dioxide adsorbent according to Claim 4, wherein the smectite is stibnite and / or hectorite.

6. A carbon dioxide recovery method using the carbon dioxide adsorbent according to any one of Claims 1 to 5.

7. The carbon dioxide recovery method according to Claim 6, wherein the recovery of the carbon dioxide is performed by humidifying the carbon dioxide adsorbent under room temperature conditions.

8. The carbon dioxide recovery method according to Claim 7, wherein the adsorption and desorption of carbon dioxide are repeatedly performed using the carbon dioxide adsorbent.

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