Carbon dioxide adsorbent and method for separating carbon dioxide

Abstract

To provide a carbon dioxide adsorbent and a carbon dioxide separation method that have a high effective carbon dioxide adsorption capacity even under the influence of heat and endothermic effects generated during carbon dioxide adsorption and desorption when separating carbon dioxide from a mixed gas containing carbon dioxide. [Solution] A carbon dioxide adsorbent comprising a zeolite having a faujasite-type crystal structure, wherein 70% or more of the cations constituting the zeolite are lithium ions, the Si / Al ratio is 1.5 or higher, and the difference in the Langmuir-Freundlich equilibrium constant (1 / kPa) obtained from the carbon dioxide adsorption isotherm at 0°C and the carbon dioxide adsorption isotherm at 50°C is 0.3 or less.

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide adsorbent and a method for separating carbon dioxide. [Background technology]

[0002] Carbon dioxide is considered a major cause of global warming, and efforts to reduce its emissions are becoming increasingly active worldwide. Therefore, various studies are being actively pursued to enable the capture or storage of carbon dioxide from exhaust gases without releasing it into the atmosphere. Known methods for capturing carbon dioxide include membrane separation and concentration, and chemical absorption methods utilizing the reaction absorption of basic compounds.

[0003] However, currently, both methods have high carbon dioxide capture costs and have not yet reached the stage of widespread adoption. For example, the chemical absorption method requires heating to desorb the absorbed carbon dioxide from the basic compound solution, making it energy-intensive unless it is a large-scale facility equipped with waste heat utilization equipment. Therefore, as a method that can relatively reduce energy costs, the pressure swing adsorption (PSA) method has been proposed for small-scale businesses. The PSA method involves repeatedly performing an adsorption step in which a mixed gas is introduced at a predetermined pressure into an adsorption tower filled with an adsorbent to adsorb a specific gas component, and a desorption step in which the adsorption tower, with the adsorbed gas component, is reduced to a predetermined pressure and the gas component is recovered or discharged. For this reason, it is considered that energy costs can be relatively reduced. Other methods include thermal swing adsorption (TSA), which separates gases by raising the temperature of the desorption step above the temperature of the gas adsorption step and utilizing the difference in adsorption capacity at low and high temperatures, and pressure and thermal swing adsorption (PTSA), which combines the PSA and TSA methods.

[0004] Activated carbon is widely used as an adsorbent, but it has a low effective adsorption capacity for carbon dioxide. For this reason, zeolite adsorbents are commonly used as adsorbents in CO2-PSA methods, for example. Zeolite adsorbents are required to have a large difference between the adsorption capacity at adsorption pressure and the adsorption capacity at desorption pressure (hereinafter also referred to as "effective adsorption capacity"), and to have high adsorption selectivity for carbon dioxide compared to gases such as nitrogen. However, it is said that the carbon dioxide recovery efficiency decreases in the PSA method due to temperature fluctuations caused by gas adsorption and desorption. In the PSA method, the heat of adsorption when carbon dioxide is adsorbed onto the zeolite adsorbent generates heat and raises the temperature. This reduces the carbon dioxide adsorption capacity of the zeolite adsorbent. On the other hand, it is known that the temperature drops due to an endothermic reaction during carbon dioxide desorption, and carbon dioxide cannot be sufficiently desorbed.

[0005] Therefore, in recent years, adsorbents such as FAU-type zeolite, a type of zeolite, and FAU-type zeolite obtained by ion exchange to the Li type have been used (see Patent Document 1 below). Patent Document 1 proposes FAU in which the SiO2 / Al2O3 ratio is 4.5 to 7 (Si / Al ratio is 2.25 to 3.5), 20% to 90% of the cations are sodium ions, and the remainder are lithium ions, etc. This FAU is intended to separate carbon dioxide with a high selectivity that could not be obtained with conventional carbon dioxide separation agents when separating carbon dioxide from a gas containing carbon dioxide and nitrogen using PSA. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-175202 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the carbon dioxide separation agent described in Patent Document 1 had room for improvement in terms of its effective carbon dioxide adsorption capacity under the influence of heat and endothermic heat generated during carbon dioxide adsorption and desorption when separating carbon dioxide from a mixed gas containing carbon dioxide.

[0008] The present invention aims to provide a carbon dioxide adsorbent and a carbon dioxide separation method that have a high effective carbon dioxide adsorption capacity even under the influence of heat and endothermic effects generated during carbon dioxide adsorption and desorption when separating carbon dioxide from a mixed gas containing carbon dioxide. [Means for solving the problem]

[0009] The present invention provides, for example, the following carbon dioxide adsorbent and carbon dioxide separation method. [1] Contains a zeolite having a faujasite-type crystal structure, More than 70% of the cations constituting the zeolite are lithium ions. The Si / Al ratio is 1.5 or higher. A carbon dioxide adsorbent in which the difference in the equilibrium constant (1 / kPa) of the Langmuir-Freundlich equation, obtained from the carbon dioxide adsorption isotherm at 0°C and the carbon dioxide adsorption isotherm at 50°C, is 0.3 or less. [2] The carbon dioxide adsorbent according to [1], wherein the Si / Al ratio is 3.5 or less. [3] The carbon dioxide adsorbent according to [1] or [2], wherein the cation further comprises at least one of an alkali metal other than lithium and an alkaline earth metal. [4] The carbon dioxide adsorbent according to [1] or [2], wherein the cation further comprises one or more ions selected from the group consisting of sodium, potassium, rubidium, cesium, and magnesium. [5] A method for separating carbon dioxide, comprising a separation step of separating carbon dioxide from a mixed gas containing carbon dioxide, wherein the separation step comprises a step of bringing a carbon dioxide adsorbent described in any of [1] to [4] into contact with the mixed gas to adsorb the carbon dioxide onto the carbon dioxide adsorbent. [6] The method for separating carbon dioxide according to [5], wherein the separation step further comprises a step of separating carbon dioxide from the carbon dioxide adsorbent by a pressure swing adsorption method. [Effects of the Invention]

[0010] The present invention provides a carbon dioxide adsorbent and a method for producing carbon dioxide that have a high effective carbon dioxide adsorption capacity even under the influence of heat and endothermic reactions that occur during carbon dioxide adsorption and desorption when separating carbon dioxide from a mixed gas containing carbon dioxide. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a graph showing the X-ray diffraction spectrum of the union-exchanged zeolite powder obtained in Example 1. [Figure 2] Figure 2 is a graph showing the carbon dioxide adsorption isotherms at 0°C and 50°C for Example 1. [Figure 3] Figure 3 is a graph showing the carbon dioxide adsorption isotherms at 0°C and 50°C for Comparative Example 2. [Modes for carrying out the invention]

[0012] <<Carbon dioxide adsorbent>> The carbon dioxide adsorbent of the present invention contains a zeolite having a faujasite-type crystal structure, wherein 70% (mol%) or more of the cations constituting the zeolite are lithium ions, the Si / Al ratio is 1.5 or higher, and the difference in the Langmuir-Freundlich equilibrium constant K(1 / kPa) obtained from carbon dioxide adsorption isotherms at 0°C and 50°C is 0.3 or less.

[0013] The carbon dioxide adsorbent of the present invention has a high effective carbon dioxide adsorption capacity even under the influence of heat generation and heat absorption that occur during the adsorption and desorption of carbon dioxide when separating carbon dioxide from a mixed gas containing carbon dioxide. Further, the carbon dioxide adsorbent of the present invention can also increase the effective adsorption capacity of carbon dioxide as compared with conventionally known X-type zeolite.

[0014] The carbon dioxide adsorbent of the present invention is a zeolite adsorbent and can be used in the PSA method. In the PSA method, in the adsorption step, a mixed gas containing carbon dioxide is supplied and pressurized into a tower filled with an adsorbent to selectively adsorb carbon dioxide. Next, in the desorption step, high-purity carbon dioxide can be recovered by reducing the pressure to a pressure close to vacuum. Here, in order to increase the effective adsorption capacity of carbon dioxide, an adsorbent with a large carbon dioxide adsorption capacity at the adsorption pressure and a small carbon dioxide adsorption capacity at the desorption pressure is required. This effective adsorption capacity is generally the difference in the adsorption capacity of carbon dioxide with different equilibrium pressures in the adsorption isotherm at 25°C.

[0015] However, in the PSA method, usually, when a gas is adsorbed on the adsorbent, the temperature inside the tower rises due to heat generation by the heat of adsorption, resulting in a reduction in the adsorption amount. During desorption, the temperature inside the tower decreases due to an endothermic reaction, reducing the amount of carbon dioxide desorbed. In particular, when carbon dioxide is adsorbed and desorbed using an exhaust gas containing a high concentration of carbon dioxide or a process in which a high concentration of carbon dioxide is circulated, the temperature fluctuation becomes large due to heat generation and heat absorption. When the temperature fluctuation is large, the recovery efficiency (effective adsorption capacity) deteriorates. However, the inventors of the present invention have found that the adsorbent of the present invention can maintain a high effective adsorption capacity even when the temperature fluctuation is large.

[0016] Zeolite is a compound having a TO4 unit (T element is an element other than oxygen constituting the framework) composed of silicon or aluminum, etc., and oxygen as a basic unit. As the zeolite, specifically, crystalline porous aluminosilicates (hereinafter also referred to as "aluminosilicates"), crystalline porous aluminophosphates (ALPO), or crystalline porous silicoaluminophosphates (SAPO), etc. can be mentioned.

[0017] The zeolite is not particularly limited with respect to the constituent elements of the framework structure as long as the effects of the present invention are not impaired. The zeolite may be used alone or in combination of two or more. For example, in the case of ALPO, SAPO, etc. in which many phosphorus atoms are present in the framework, cations tend to be relatively easily reduced, so the interaction between the zeolite and carbon dioxide tends to be suppressed.

[0018] From the viewpoint of the interaction between the zeolite of the present invention and carbon dioxide, the zeolite is preferably an aluminosilicate.

[0019] The zeolite contained in the carbon dioxide adsorbent of the present invention has a faujasite crystal structure. Faujasite refers to a zeolite having a zeolite framework structure classified as "FAU" by the International Zeolite Association. Since the zeolite contained in the carbon dioxide adsorbent of the present invention has a faujasite structure, the volume of the cage that can take in a large amount of carbon dioxide is large, so the adsorption capacity of carbon dioxide can be increased.

[0020] <Si / Al ratio> The Si / Al ratio (molar ratio of Si / Al, that is, the ratio of the number of atoms) of the zeolite contained in the carbon dioxide adsorbent of the present invention is the molar ratio of Si to Al and is 1.5 or more. When the Si / Al ratio is small, the number of cations constituting the zeolite increases and the interaction with carbon dioxide becomes strong, but if it is too small, the space volume of the zeolite is reduced too much. Also, if there are too many cations, it becomes difficult for carbon dioxide to desorb in the desorption process, so the effective adsorption capacity of carbon dioxide decreases.

[0021] While the Si / Al ratio is not particularly limited as long as it is 1.5 or higher, it is preferably between 1.5 and 3.5. In this case, the zeolite cations and carbon dioxide maintain a moderately strong interaction without the spatial volume of the zeolite decreasing too much, thus maximizing the effective adsorption capacity of carbon dioxide. From the viewpoint of further maximizing the effective adsorption capacity, the Si / Al ratio is preferably 3.35 or lower, more preferably 3.25 or lower, and particularly preferably 3.15 or lower.

[0022] From the viewpoint of increasing the selectivity of carbon dioxide to nitrogen (hereinafter also referred to as "carbon dioxide selectivity"), the Si / Al ratio is preferably 1.75 or higher, more preferably 2.0 or higher, and particularly preferably 2.25 or higher. Furthermore, from the viewpoint of maximizing the effective adsorption capacity of carbon dioxide and carbon dioxide selectivity, the Si / Al ratio is preferably 1.7 to 3.35, more preferably 2.0 to 3.25, and particularly preferably 2.25 to 3.15.

[0023] The Si / Al ratio of the zeolite contained in the carbon dioxide adsorbent of the present invention can be measured by elemental analysis using scanning electron microscope energy-dispersive X-ray spectroscopy (SEM-EDX).

[0024] <Cation> The content of exchange cations and aluminum can be determined by compositional analysis of the zeolite. Examples of methods for compositional analysis include energy-dispersive X-ray spectroscopy (EDX / EDS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0025] Lithium ions are chosen as the cations that make up more than 70% of the cations constituting the zeolite, because they tend to have high effective adsorption capacity and carbon dioxide selectivity even under temperature fluctuations. Lithium ions have a smaller ion size than other alkali metal ions or alkaline earth metal ions (sodium ions, potassium ions, calcium ions, etc.). Therefore, lithium ions can suppress the reduction in the space volume that carbon dioxide can occupy within the zeolite due to cations, thereby increasing the carbon dioxide adsorption capacity in the adsorption process. On the other hand, lithium ions have a higher charge density than other alkali metal ions and alkaline earth metal ions. The oxygen in carbon dioxide molecules is negatively polarized (δ - ) and carbon becomes positively polarized (δ + Since cations and oxygen in the zeolite are strongly attracted to each other by electrostatic interactions, adsorption occurs. However, lithium ions, which have a small ion size and high charge density, tend to repel carbon more readily than other ions. In the desorption process, it is known that when the zeolite is placed at a low temperature due to an endothermic reaction, the molecular motion of carbon dioxide is suppressed and the adsorption capacity of the zeolite increases. However, if there are enough lithium ions with high charge density, carbon dioxide is more easily desorbed, and the effective adsorption capacity of carbon dioxide is thought to increase. Therefore, lithium ions are intended to account for more than 70% of the cations constituting the zeolite.

[0026] The carbon dioxide adsorbent of the present invention has a FAU-type skeleton, and more than 70% of the ion exchange sites within this zeolite contain lithium ions. Here, the ion exchange sites are tetrahedral silicate units (SiO4) and tetrahedral aluminate units ((AlO4)). -This indicates the location of cations that compensate for the negative charge generated on the bonded oxygen with ). FAU-type zeolites have a structure in which cages inside the pores are arranged three-dimensionally from the entrance of the 12-membered oxygen ring, and are constructed from 12-membered oxygen rings, 6-membered oxygen rings, and 4-membered oxygen rings. In the case of ion exchange, cations at ion exchange sites located on the zeolite surface are easily replaced, while cations in cages consisting of 6-membered oxygen rings and 4-membered oxygen rings tend not to be replaced. It is assumed that 60-70% of all ion exchange sites are located on the surface of FAU-type zeolites, and it is thought that the ion exchange sites that mainly contribute to the effects of the present invention are those located on the surface of FAU-type zeolites. Generally, the lithium ion exchange selectivity of FAU-type zeolites is lower than that of sodium and potassium, and it is known that the exchangeability slows down further in exchange processes where the proportion of ion exchange sites exceeds 50%. Therefore, a large amount of lithium compound is required to bring the ratio of lithium ions after lithium ion exchange close to 100%. On the other hand, the price of lithium compounds used for ion exchange has been rising in recent years. Therefore, from an economic standpoint, the preferred range for the lithium ion ratio after lithium ion exchange is 70-90%, more preferably 70-80%, and particularly preferably 70-78%.

[0027] While the cations other than lithium ions are not particularly limited, it is preferable that such cations consist of at least one of alkali metal ions and alkaline earth metal ions. Specific examples of alkali metal ions include sodium ions, potassium ions, rubidium ions, and cesium ions. Examples of alkaline earth metal ions include magnesium ions, calcium ions, strontium ions, and barium ions. In this specification, cations that remain without ion exchange are also referred to as "residual ions." Divalent ions composed of alkaline earth metal ions generally have a strong interaction with carbon dioxide and are not easily desorbed at the desorption pressure. However, magnesium ions, like lithium ions, have a small ionic size and a high charge density, so they are more likely to repel the positively polarized (δ + ) carbon of carbon dioxide compared to other ions, making it easier to desorb carbon dioxide at the desorption pressure and tending to increase the effective adsorption capacity. From the perspective of increasing the carbon dioxide adsorption capacity, as cations other than lithium ions that make up zeolite, it is preferable that their ionic size is small. Among them, one or more ions selected from the group consisting of sodium ions, potassium ions, rubidium ions, cesium ions, and magnesium ions are preferable, one or more of sodium ions, potassium ions, and magnesium ions are more preferable, and at least one of sodium ions and magnesium ions is more preferable.

[0028] The adsorption capacity of carbon dioxide in zeolite can be determined from the adsorption isotherm. The adsorption isotherm can be quantified by the Langmuir adsorption equation. However, the Langmuir equation holds when the adsorbate molecules are adsorbed in a monolayer on the adsorption sites on the surface of the adsorbent. When there are many cation species in the adsorbent of this embodiment, the surface of the adsorbent is likely to be a heterogeneous surface, so the accurate value of the adsorption capacity cannot be obtained. Therefore, when there are many cation species in the adsorbent, usually the value of the adsorption capacity can be obtained from the following Langmuir-Freundlich equation. q=(Q×K×p n ) / (1+K×p n )

[0029] [[ID=十七]] Here, q is the adsorption capacity of carbon dioxide (mol / kg), Q is the saturated adsorption capacity (mol / kg), K is the equilibrium constant (1 / kPa), p is the pressure (kPa), and n is a constant characterizing the heterogeneity and is in the range of 0 < n < 1.

[0030] In carbon dioxide adsorption and desorption, for example, the effective adsorption capacity can be calculated from the difference between the carbon dioxide adsorption capacity determined from the partial pressure of carbon dioxide during adsorption and the carbon dioxide adsorption capacity determined from the partial pressure of carbon dioxide during desorption, based on the adsorption isotherm at 25°C. The larger the saturation adsorption capacity Q in the Langmuir equation and the smaller the equilibrium constant K, the steeper the slope of the isotherm (a line showing the relationship between pressure and adsorption capacity under constant temperature conditions), and therefore the greater the effective adsorption capacity. However, there is a trade-off relationship between the saturation adsorption capacity Q and the equilibrium constant K, so a zeolite with appropriately balanced values ​​for these two is required. Generally, the larger the value of the equilibrium constant K, the stronger the interaction with carbon dioxide, and the smaller the value, the weaker the interaction. Therefore, a carbon dioxide adsorbent with a balanced equilibrium constant K is desired, where the value of the equilibrium constant K is not too small to adequately adsorb carbon dioxide in the adsorption process under high pressure, and where the value of the equilibrium constant K is not too large to easily desorb carbon dioxide in the desorption process under low pressure.

[0031] While it is possible to maximize effective adsorption capacity if there are no temperature fluctuations during carbon dioxide adsorption and desorption and the equilibrium constant K is at an appropriate value, temperature fluctuations inevitably occur in reality. Therefore, even under temperature fluctuations, a high effective adsorption capacity can be maintained in carbon dioxide adsorbents by ensuring that the equilibrium constant K is close to the value of the equilibrium constant K when there are no temperature fluctuations during carbon dioxide adsorption and desorption. In the carbon dioxide adsorbent of the present invention, the difference in the equilibrium constant K(1 / kPa) of the Langmuir-Freundlich equation, which is determined from carbon dioxide adsorption isotherms at 0°C and 50°C, is 0.3 or less.

[0032] From the viewpoint of increasing the effective adsorption capacity of carbon dioxide, the difference in the equilibrium constant K(1 / kPa) is preferably 0.25 or less, and more preferably 0.2 or less.

[0033] In the carbon dioxide adsorbent of the present invention, the Langmuir-Freundlich equilibrium constant K, which can be determined from the adsorption isotherms at 0°C and 50°C, is not particularly limited, but from the viewpoint of increasing the effective adsorption capacity, it is preferable that it be between 0.0010 (1 / kPa) and 0.5 (1 / kPa). Thus, if the equilibrium constant K is not too large, the interaction between the adsorbent and carbon dioxide does not become strong, so the adsorption capacity does not increase easily, especially under desorption pressure, and carbon dioxide is easily desorbed. On the other hand, if the equilibrium constant K is not too small, the interaction with carbon dioxide becomes strong, so carbon dioxide can be adsorbed sufficiently.

[0034] In the carbon dioxide adsorbent of the present invention, from the viewpoint of increasing the effective adsorption capacity of carbon dioxide, it is more preferable that the equilibrium constant K determined from the adsorption isotherms at 0°C and 50°C is 0.0025 (1 / kPa) or more and 0.4 (1 / kPa) or less, respectively, and even more preferable that the equilibrium constant K is 0.0050 (1 / kPa) or more and 0.3 (1 / kPa) or less.

[0035] Furthermore, the equilibrium constant K determined from the adsorption isotherm at 0°C is preferably 0.5 (1 / kPa) or less, preferably 0.4 (1 / kPa) or less, more preferably 0.3 (1 / kPa) or less, and particularly preferably 0.25 (1 / kPa) or less. The equilibrium constant K determined from the adsorption isotherm at 0°C is preferably 0.1 (1 / kPa) or more.

[0036] Furthermore, the equilibrium constant K determined from the adsorption isotherm at 50°C is preferably 0.001 (1 / kPa) or higher, more preferably 0.0025 (1 / kPa) or higher, even more preferably 0.0050 (1 / kPa) or higher, and particularly preferably 0.0075 (1 / kPa) or higher. The equilibrium constant K determined from the adsorption isotherm at 50°C is preferably 0.2 (1 / kPa) or lower.

[0037] <<Method for manufacturing carbon dioxide adsorbent>> Next, we will explain the method for producing carbon dioxide adsorbents.

[0038] The carbon dioxide adsorbent of the present invention can be manufactured by a manufacturing method comprising a hydrothermal synthesis step of synthesizing an union-exchanged zeolite using a mixed gel by hydrothermal synthesis, and an ion exchange step of ion-exchanging the cations of the zeolite obtained in the hydrothermal synthesis step to a desired cation type.

[0039] The method for producing a carbon dioxide adsorbent may further include a separation step after the hydrothermal synthesis step in which the solid product is separated from the liquid containing water. The method for producing a carbon dioxide adsorbent may further include a washing step in which the solid separated in the separation step is washed with water or a predetermined solvent. Furthermore, the method for producing a carbon dioxide adsorbent may further include a drying step in which the solid separated in the separation step is dried, if necessary. The method for producing a carbon dioxide adsorbent may also include a calcination step in which the ion-exchanged zeolite is calcined, if necessary.

[0040] (1) Hydrothermal synthesis process (Mixed gel) The above-mentioned mixed gel is a mixture containing a Si source, an Al source, and water, and optionally containing an alkali metal source, an alkaline earth metal source, and / or an organic structure modifier.

[0041] (Si source) The Si source is a raw material compound used to introduce silicon atoms into the zeolite. While there are no particular limitations on the Si source as long as it is commonly used, examples include sodium silicate, colloidal silica, fumed silica, silica sol, and ethyl silicate. These compounds may be used individually or in combination of two or more. Of these, sodium silicate is preferred because it tends to yield zeolites with a high degree of crystallinity.

[0042] (Al source) The Al source is a raw material compound used to introduce aluminum atoms into the zeolite. While there are no particular limitations on the Al source as long as it is commonly used, examples include sodium aluminate, aluminum sulfate, aluminum nitrate, aluminum acetate, aluminum hydroxide, aluminum oxide, aluminum chloride, aluminum alkoxide, boehmite, and metallic aluminum. These compounds may be used individually or in combination of two or more. Of these, sodium aluminate, aluminum sulfate, aluminum nitrate, aluminum hydroxide, aluminum alkoxide, and boehmite are preferred, with sodium aluminate being more preferred, as they tend to yield zeolites with a high degree of crystallinity.

[0043] (Alkali metal sources and alkaline earth metal sources) Examples of alkali metal sources include alkali metal hydroxides, carbonates, bicarbonates, acetates, sulfates, and nitrates. These compounds may be used individually or in combination of two or more. Of these, alkali metal hydroxides are preferred because they tend to yield zeolites with a high degree of crystallinity. Examples of alkaline earth metal sources include hydroxides, carbonates, bicarbonates, acetates, sulfates, and nitrates of alkaline earth metals. These compounds may be used individually or in combination of two or more. Of these, at least one of the alkaline earth metal hydroxides is preferred because it tends to yield zeolites with a high degree of crystallinity.

[0044] Examples of alkali metals or alkaline earth metals include Li, Na, K, Rb, Cs, Mg, Ca, Sr, and Ba. These may be used individually or in combination of two or more. Among alkali metals and alkaline earth metals, at least one of Na and K is preferred, with Na being more preferred, from the viewpoint of facilitating the formation of FAU-type skeleton crystals.

[0045] Specific alkali metal sources include sodium hydroxide, sodium acetate, sodium sulfate, sodium nitrate, sodium carbonate, and sodium bicarbonate, when the alkali metal is sodium (Na).

[0046] (Organic structure directing agent) Organic structure-controlling agents are compounds that promote crystallization of mixed gels into a faujasite-type crystalline structure during hydrothermal synthesis. In zeolite crystallization, organic structure-controlling agents can be used as needed. Any type of organic structure-controlling agent that can form the desired FAU-type zeolite structure is acceptable. Examples of organic structure-controlling agents include amines and quaternary ammonium salts. These may be used individually or in combination of two or more. From a cost perspective, it is preferable not to use organic structure-controlling agents in the production of FAU-type zeolite.

[0047] (seed crystal) Seed crystals may be added to the mixed gel. The seed crystals are zeolites with the same crystal structure as the zeolite being manufactured.

[0048] Seed crystals, when added to a mixed gel, act as nuclei that serve as starting points for crystal growth, thereby promoting crystallization. From the viewpoint of promoting crystallization, a large amount of seed crystals is preferable; however, if too much seed crystal is added, the effect of promoting crystallization slows down, while manufacturing costs increase. In the synthesis of FAU, sufficient crystal growth generally occurs even without seed crystals, but it is desirable to shorten the manufacturing time by promoting crystal growth. Therefore, the amount of seed crystals added is preferably 0.01 parts by mass to 30 parts by mass, more preferably 0.1 parts by mass to 20 parts by mass, and particularly preferably 0.5 parts by mass to 10 parts by mass, per 100 parts by mass of solid content (Si source, Al source, alkali metal source, alkaline earth metal source, etc., excluding water).

[0049] (Preparation process of mixed gel) The above-described preparation step for the mixed gel may include, for example, a mixing step of mixing a Si source, an Al source, and water, and optionally an alkali metal source, an alkaline earth metal source, an organic structure-regulating agent, and / or seed crystals, either all at once or in multiple stages, and a maturation step of the mixture obtained in the mixing step.

[0050] The mixing step may involve mixing a Si source, an Al source, and water, as well as, if necessary, an alkali metal source, an alkaline earth metal source, an organic structure modifier, and / or seed crystals, either in one step or in multiple steps.

[0051] The order of mixing in multiple stages is not limited and can be selected as appropriate depending on the conditions being used. Multi-stage mixing can be performed with or without stirring. When stirring, the stirring method is not particularly limited as long as it is a commonly used stirring method. Specific examples of stirring methods include impeller stirring, vibration stirring, oscillating stirring, and centrifugal stirring. In the case of blade agitation, the rotational speed of the blade is not particularly limited as long as it is a commonly used rotational speed, but for example, it can be between 1 rpm and less than 2000 rpm. The temperature of the mixing process is not particularly limited as long as it is within the range of commonly used temperatures, but examples include -20°C to less than 80°C. The duration of the mixing process is not particularly limited and can be appropriately selected depending on the temperature during the mixing process. For example, the duration of the mixing process can range from more than 0 minutes to 1000 hours or less.

[0052] The maturation process can be carried out either by standing or by stirring. When stirring is performed during the maturation process, the stirring method is not particularly limited as long as it is a commonly used stirring method. However, specific examples of stirring methods include using impeller stirring, vibratory stirring, oscillating stirring, and centrifugal stirring. In the case of blade agitation, the rotational speed of the blade is not particularly limited as long as it is a commonly used rotational speed, but for example, it can be between 1 rpm and less than 2000 rpm. The temperature during the maturation process is not particularly limited as long as it is within the range of commonly used temperatures, but examples include a range of -20°C to less than 80°C. The maturation period is not particularly limited and can be selected as appropriate depending on the maturation temperature, but examples of maturation periods include periods of more than 0 minutes and less than or equal to 1000 hours.

[0053] (Hydrothermal synthesis temperature) In the hydrothermal synthesis step, preferably, the mixed gel obtained in the preparation step is held at a predetermined hydrothermal synthesis temperature for a predetermined time, either stirred or left standing, to hydrothermally synthesize the union-exchanged zeolite. The hydrothermal synthesis temperature is preferably 70°C to 130°C. The hydrothermal synthesis temperature is not particularly limited as long as it is a temperature that is generally used, but it is preferably 75°C or higher from the viewpoint of shortening the synthesis time and being economically superior in the production of zeolite. From the viewpoint of suppressing the formation of zeolite having a skeleton different from the FAU type skeleton, the hydrothermal synthesis temperature is more preferably 80°C or higher, and even more preferably 85°C or higher. From the viewpoint of suppressing the formation of zeolite having a skeleton different from the FAU type skeleton, the hydrothermal synthesis temperature is more preferably 120°C or lower, even more preferably 115°C or lower, and even more preferably 110°C or lower. The temperature for hydrothermal synthesis can be constant or it can be changed in stages.

[0054] (Hydrothermal synthesis time) The time for hydrothermal synthesis is not particularly limited as long as it is a commonly used time, and can be appropriately selected depending on the hydrothermal synthesis temperature. From the viewpoint of forming the FAU skeleton, the time for hydrothermal synthesis is preferably 1 hour or more, and more preferably 2 hours or more. From the viewpoint of obtaining highly crystalline FAU-type zeolite, the time for hydrothermal synthesis is even more preferably 3 hours or more. From the standpoint of economic efficiency in manufacturing zeolite, the hydrothermal synthesis time is preferably 10 days or less, more preferably 5 days or less, and even more preferably 1 day or less.

[0055] (container) In the hydrothermal synthesis process, the container for the mixed gel is not particularly limited as long as it is a commonly used container. However, if the pressure inside the container increases at a predetermined temperature, or if the mixed gel is subjected to pressurization by a gas that does not inhibit crystallization, the container is preferably a pressure-resistant container.

[0056] (Stirring or standing) The hydrothermal synthesis process may be carried out by standing or stirring. In the hydrothermal synthesis process, preferred methods for stirring the mixed gel include rotating a cylindrical pressure vessel containing the mixed gel with respect to a horizontally positioned rotating shaft such that the intermediate portion of its outer surface along the longitudinal direction intersects with the rotating shaft, and rotating a cylindrical pressure vessel containing the mixed gel with respect to a horizontally positioned rotating shaft such that the intermediate portion of its outer surface along the longitudinal direction is fixed to the rotating shaft via a support member perpendicular to the rotating shaft.

[0057] (2) Separation process The separation process involves separating the solid product from the liquid containing water after the hydrothermal synthesis process. The separation method is not particularly limited as long as it is a general method. Suitable separation methods include filtration, decantation, spray drying (rotary spray, nozzle spray, ultrasonic spray, etc.), drying using a rotary evaporator, vacuum drying, freeze-drying, or natural drying. Filtration or decantation is typically used.

[0058] (3) Drying process The drying process is the process of drying the solids that were separated in the separation process. The drying temperature for drying the separated solid is not particularly limited as long as it is a typical drying temperature, but it is usually between room temperature and 150°C or below. The atmosphere used for drying is not particularly limited as long as it is a commonly used atmosphere, but typically, an air atmosphere, an inert gas atmosphere such as nitrogen or argon, or an inert gas atmosphere with oxygen added is used.

[0059] (4) Ion exchange process The ion exchange process is a step in which cations of un-ion-exchanged zeolite obtained in the hydrothermal synthesis process, separation process, or drying process are exchanged for desired cations. The alkali metal ion source or alkaline earth metal ion source used for ion exchange is not particularly limited as long as it is water-soluble, and includes, for example, halides such as LiCl, NaCl, KCl, RbCl, CsCl, MgCl2, CaCl2, SrCl2, BaCl2, nitrates such as LiNO3, NaNO3, KNO3, RbNO3, CsNO3, Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, Ba(NO3)2, carbonates such as Li2CO3, Na2CO3, K2CO3, Rb2CO3, Cs2CO3, MgCO3, CaCO3, SrCO3, BaCO3, CH3COOLi, CH3COONa, CH3COOK, CH3COORb, CH3COOCs, (CH3COO Acetates such as 2Mg, (CH3COO)2Ca, (CH3COO)2Sr, (CH3COO)2Ba, sulfates such as Li2SO4, Na2SO4, K2SO4, Rb2SO4, Cs2SO4, MgSO4, CaSO4, SrSO4, BaSO4, or organic acid salts obtained by changing the halogen ions, nitrate ions, carbonate ions, acetate ions, sulfate ions contained in the halides, nitrates, carbonates, acetates, and sulfates to bicarbonate ions, phosphate ions, or hydrogen phosphate ions, etc., or hydroxides obtained by changing the halogen ions, nitrate ions, carbonate ions, acetate ions, and sulfates contained in the halides, nitrates, carbonates, acetates, and sulfates to hydroxyl groups, etc., can be used. The alkali metal ion source and the alkaline earth metal ion source may be used individually or in combination of two or more types. Ion exchange can be carried out by contacting the alkali metal ion source and the alkaline earth metal ion source with the un-ion-exchanged zeolite obtained in the hydrothermal synthesis step, separation step, or drying step.

[0060] The temperature of ion exchange is not particularly limited as long as it is within the range of typical ion exchange temperatures, but it is usually between room temperature and 100°C or below.

[0061] Ion exchange can be carried out with or without stirring. When stirring, the stirring method is not particularly limited as long as it is a commonly used stirring method. Specific examples of stirring methods include impeller stirring, vibration stirring, oscillating stirring, and centrifugal stirring. In the case of blade agitation, the rotational speed of the blade is not particularly limited as long as it is a commonly used rotational speed, but for example, it can be between 1 rpm and less than 2000 rpm.

[0062] Furthermore, methods to increase the ratio of desired cations after ion exchange from cations in un-ion-exchanged zeolite to desired cations include increasing the concentration of the exchange solution used for ion exchange and increasing the number of ion exchange cycles. In the carbon dioxide adsorbent of the present invention, it is preferable that 70% or more of the cations present in the zeolite are lithium ions, and the remainder are at least one of alkali metal ions and alkaline earth metal ions. In the case of ion exchange, cations at ion exchange sites located on the surface of FAU-type zeolite are easily replaced. From the viewpoint that the effects of the present invention are greater due to the contribution of ions present on the zeolite surface than ions present in the FAU-type zeolite cage, for example, when retaining lithium and potassium ions in an un-ion-exchanged (sodium-type) zeolite, it is preferable to perform potassium ion exchange first, followed by lithium ion exchange. Alternatively, when using an exchange solution containing lithium ions and potassium ions, it is preferable to increase the concentration of lithium ions. In particular, from the viewpoint of efficiently exchanging cations on the zeolite surface for lithium ions, it is preferable to perform ion exchange in the final step of ion exchange using an exchange solution containing only lithium ions as the cation species.

[0063] (5) Firing process The firing process involves firing the ion-exchange zeolite as needed.

[0064] (Firing temperature) The firing temperature is not particularly limited as long as it is a temperature commonly used for firing, but if it is desired to remove the organic structural modifier, it is preferable to have a temperature of 300°C or higher, and more preferably 350°C or higher, as this reduces the proportion of the organic structural modifier remaining. A firing temperature of 400°C or higher is even more preferable because it shortens the firing time and is economically advantageous when manufacturing zeolite. The firing temperature is preferably less than 600°C, more preferably 570°C or lower, and even more preferably 550°C or lower, as this tends to preserve the crystallinity of the zeolite.

[0065] (Baking time) The firing time is not particularly limited as long as it is sufficient to remove the organic structural modifier, and can be appropriately selected depending on the firing temperature. However, since this tends to reduce the proportion of remaining organic structural modifier, it is preferable to fire for 0.5 hours or more, more preferably 1 hour or more, and even more preferably 3 hours or more. The firing time is preferably 20 days or less, more preferably 10 days or less, and even more preferably 7 days or less, as this tends to preserve the crystallinity of the zeolite.

[0066] (Firing atmosphere) The firing atmosphere is not particularly limited as long as it is an atmosphere commonly used for firing, but typically an air atmosphere, an inert gas atmosphere such as nitrogen or argon, or an inert gas atmosphere with added oxygen are used.

[0067] The carbon dioxide adsorbent of the present invention can be used in any form depending on the application. Such forms include powder and molded articles. Specific shapes of the molded body include at least one selected from the group consisting of spherical, ellipsoidal, disc-shaped, cylindrical, polyhedral, amorphous, and petal-shaped.

[0068] When the carbon dioxide adsorbent of the present invention is used in powder form, the carbon dioxide adsorbent of the present invention comprises a granular base material and a coating layer provided on the base material, wherein the coating layer may be made by drying a slurry obtained by mixing a solvent such as water or alcohol with an ion-exchange zeolite.

[0069] When the carbon dioxide adsorbent of the present invention is used in the form of a molded body, the carbon dioxide adsorbent comprises an ion exchange zeolite and a binder. Examples of binders include silica, alumina, kaolin, atabulgite, montmorillonite, bentonite, allophane, and ceviolite. These compounds may be used individually or in combination of two or more. Molded articles can be formed using any molding method. Examples of molding methods include rolling granulation, stirring granulation, press molding, extrusion molding, injection molding, casting, and sheet molding.

[0070] <<Method for separating carbon dioxide>> The present invention relates to a method for separating carbon dioxide, which includes a separation step of separating carbon dioxide from a mixed gas containing carbon dioxide, wherein the separation step includes a step of bringing a carbon dioxide adsorbent into contact with a mixed gas containing carbon dioxide to adsorb carbon dioxide onto the carbon dioxide adsorbent.

[0071] According to the carbon dioxide separation method of the present invention, when the carbon dioxide adsorbent is brought into contact with a mixed gas containing carbon dioxide, the carbon dioxide in the mixed gas is adsorbed by the carbon dioxide adsorbent, thereby separating carbon dioxide from the mixed gas. The carbon dioxide adsorbent has a high effective carbon dioxide adsorption capacity even under the influence of heat and heat generated during the adsorption and desorption of carbon dioxide when separating carbon dioxide from a mixed gas. In other words, according to the carbon dioxide separation method of the present invention, even when high temperatures occur during carbon dioxide adsorption, the adsorption capacity under high pressure is equivalent to or better than that of conventional zeolite adsorbents, and even when low temperatures occur during carbon dioxide desorption, the adsorption capacity under low pressure tends to be significantly reduced compared to conventional products. Therefore, carbon dioxide can be efficiently separated from a mixed gas.

[0072] In the above separation process, methods for separating carbon dioxide from a mixed gas containing carbon dioxide can be used, such as the PSA method, the temperature swing method (TSA method), or the pressure-temperature swing method (PTSA method). In the above separation process, from the viewpoint of improving carbon dioxide recovery efficiency, it is preferable to use the PSA method as a method for separating carbon dioxide from a mixed gas containing carbon dioxide. In this case, carbon dioxide can be effectively separated from the mixed gas using the PSA method. Furthermore, while a significant amount of energy is consumed in the depressurization process during the PSA method, if the effective adsorption capacity is equivalent to that of conventional zeolites, the carbon dioxide adsorbent of the present invention can reduce the degree of depressurization, thereby reducing energy costs.

[0073] The above mixed gas consists of carbon dioxide and other dissimilar gases. Such heterogeneous gases are not particularly limited, but the present invention can be more effective when they include heterogeneous gases having lower polarity than carbon dioxide (low-polarity gases). Here, polarity refers to the charge imbalance that can be determined from the polarizability, dipole moment, and quadrupole moment that occur within a molecule or between atoms, and can be considered as the strength of electrostatic interaction. The heterogeneous gases may consist of at least one type, or two or more types. Examples of different gases include nitrogen, oxygen, hydrogen, hydrocarbons, and low-polarity gases such as noble gases.

[0074] Examples of mixed gases include combustion exhaust gases. In combustion exhaust gases, generally, excluding carbon dioxide, the majority consists of nitrogen and oxygen, which have low polarity.

[0075] Since the carbon dioxide adsorbent of the present invention can increase the effective adsorption capacity of carbon dioxide over a wide pressure range, a mixed gas containing carbon dioxide at a wide concentration range from low to high can be used as the mixed gas.

[0076] The carbon dioxide separation method of the present invention is suitable for separating carbon dioxide from a mixed gas containing a high concentration of carbon dioxide. This is because when the mixed gas contains a high concentration of carbon dioxide, the temperature fluctuation range inside the tower filled with carbon dioxide adsorbent tends to increase. Furthermore, even when the carbon dioxide concentration in the mixed gas is low, the carbon dioxide adsorbent of the present invention can be suitably used as the adsorbent in the subsequent stage, for example, in a multi-stage PSA method where the carbon dioxide in the mixed gas is concentrated by a carbon dioxide adsorbent in the preceding stage (e.g., zeolite or activated carbon), and the carbon dioxide is separated and recovered by a carbon dioxide adsorbent in the subsequent stage. Moreover, even in a single-stage PSA method, the carbon dioxide adsorbent of the present invention can be suitably used in a single-stage PSA method that has a process of recovering the mixed gas (washing gas) that has not reached the desired carbon dioxide concentration generated in the initial stages of desorption when separating and recovering carbon dioxide in the mixed gas, and re-adsorbing the high-concentration carbon dioxide in the washing gas onto the carbon dioxide adsorbent. Furthermore, the carbon dioxide separation method of the present invention is usually applied to mixed gases that require concentration, for example, mixed gases with a carbon dioxide content of 99% by volume or less. The carbon dioxide separation method of the present invention is preferably applied when the mixed gas contains 10% to 99% by volume of carbon dioxide, more preferably 20% to 99% by volume, and particularly 30% to 99% by volume.

[0077] Here, the cleaning gas process is a process in which a portion of the desorbed carbon dioxide gas is recycled as cleaning gas to the adsorption tower after adsorption has finished. In the desorption process, highly polar carbon dioxide is desorbed relatively slowly, while less polar nitrogen and other elements are desorbed relatively quickly. Therefore, to recover high-purity carbon dioxide, it is common to recover the gas from the later stages of the desorption process. The gas from the early stages of the desorption process contains a large amount of impurities such as nitrogen, and is therefore reused as cleaning gas. However, even the gas from the early stages of the desorption process contains a high concentration of carbon dioxide because the carbon dioxide is concentrated by the adsorbent.

[0078] While there are no particular limitations on the temperature during carbon dioxide adsorption and desorption, the PSA method is generally used under conditions exposed to ambient temperature. However, the endothermic and exothermic effects during gas adsorption and desorption of the carbon dioxide adsorbent cause significant temperature fluctuations in the adsorbent and the adsorption tower. For example, in a laboratory-level PSA, if 15g of a NaX-type zeolite molded body (product name: F-9HA, manufactured by Tosoh Corporation) is packed into an adsorption tower and adsorption and desorption (desorption in this case is performed under reduced pressure down to 6kPa) is repeated under room temperature of 20°C by passing a mixed gas consisting of 10 vol% carbon dioxide and 90 vol% nitrogen, the tower temperature fluctuates to over 30°C during the adsorption process and below 15°C during the desorption process. Furthermore, if the same molded body is used and adsorption and desorption (desorption in this case is performed under reduced pressure down to 6kPa) is passed through it under room temperature of 20°C, the tower temperature fluctuates to over 35°C during the adsorption process and below 5°C during the desorption process. Temperature fluctuations tend to increase with scale-up, higher carbon dioxide concentrations, and desorption under reduced pressure closer to a vacuum. The adsorbent of the present invention exhibits a higher effective adsorption capacity than conventional zeolites under conditions where temperature fluctuations in the adsorption tower are large. For this reason, the adsorbent of the present invention is preferably applied when the temperature fluctuation range in the adsorption tower during the adsorption-desorption process of the PSA method is 10°C or more, more preferably 20°C or more, and particularly preferably 30°C or more. On the other hand, if the temperature fluctuation range in the adsorption tower is too large, the effective adsorption capacity of carbon dioxide tends to decrease significantly. Therefore, the adsorbent of the present invention is preferably applied when the temperature fluctuation range in the adsorption tower during the adsorption-desorption process of the PSA method is 100°C or less, more preferably 90°C or less, and even more preferably 80°C or less.

[0079] Adsorption isotherms are generally used to evaluate the performance of adsorbents. Effective adsorption capacity is calculated from the difference between the adsorption capacity at equilibrium pressure during adsorption and the adsorption capacity at equilibrium pressure during desorption, using an adsorption isotherm with a constant temperature. Therefore, this effective adsorption capacity does not take into account the effect of adsorption heat. As a result, there is often a large difference between the effective adsorption capacity obtained from the adsorption isotherm and the actual amount of carbon dioxide recovered by the PSA method, and a performance comparison that takes adsorption heat into account is necessary. When a general zeolite adsorbent is used by filling a PSA for small-scale businesses, for example, if several kilograms to several hundred kilograms of NaX-type zeolite molded bodies are filled per adsorption column, and the carbon dioxide concentration in the mixed gas is 90 vol%, the concentration of nitrogen (a different gas) is 10 vol%, the pressure during adsorption is at atmospheric pressure (101.325 kPa), and the pressure during desorption is 6.3 kPa or less, and assuming that the mixed gas temperature and ambient temperature are both 20-25°C, the column temperature during the adsorption process is expected to be approximately 50°C, and the column temperature during the desorption process is expected to be approximately 0°C. Therefore, in this invention, adsorption isotherms at 0°C and 50°C are used to evaluate the performance of the adsorbent. [Examples]

[0080] The present invention will be described in detail below, but the present invention is not limited to the following examples.

[0081] <Composition analysis> Zeolite powder or molded zeolite was used as the sample after being ground in an agate mortar. Elemental analysis of the powder samples was performed using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS, instrument name: JCM-6000Plus, manufactured by JEOL Ltd.). From the quantitative analysis of elements such as cations, Si, and Al obtained, the Si / Al ratio of the sample and the ratio of target cations in the cations were determined. The ratio of target cations in the cations was calculated based on the following formula. Ratio of target cation to total cation (%) = Percentage of target cation (number of atoms) × Valence of target cation / Percentage of aluminum (number of atoms) × 100

[0082] SEM-EDS is a technique that uses a detector attached to a scanning electron microscope to detect characteristic X-rays generated by electron beam irradiation and obtain elemental information of a sample. When characteristic X-rays enter the detector, a number of electron-hole pairs equivalent to the energy of the characteristic X-rays are generated, and the element can be identified by measuring this number. However, EDS has extremely low accuracy in detecting lithium, which has a small atomic number. Therefore, in this example and comparative example, the ratio of lithium ions in the cation was calculated based on the following formula. Percentage of lithium ions in a cation (%) =[Aluminum content (number of atoms) - {(Content of cation 1 other than lithium ions (number of atoms) × Valence of cation 1) + (Content of cation 2 other than lithium ions (number of atoms) × Valence of cation 2) + (Content of cation 3 other than lithium ions (number of atoms) × Valence of cation 3) + ...}] / Aluminum content (number of atoms) × 100 However, the expression enclosed in { and} in the above formula represents the sum of all types of cations other than lithium ions. For example, as in Example 8, when magnesium ions and sodium ions are present as cations other than lithium, the ratio of lithium ions can be calculated using the following formula. =[Aluminum content (number of atoms) %) - {(Magnesium ion content (number of atoms) %) × 2) + (Sodium ion content (number of atoms) %) × 1)}] / Aluminum content (number of atoms) × 100

[0083] <Identification of crystal structure> Zeolite powder was prepared as a sample (powder sample) by grinding it in an agate mortar. The powder sample was then measured using an X-ray diffractometer (instrument name: benchtop XRD Aeris, manufactured by Malvern Panalytical). The measurement conditions were as follows. Line source: Cu line Acceleration voltage / current: 40kV / 15mA Measurement mode: CONTINUOUS Measurement range: 2~50° (0.02° / step)

[0084] <Langmuir-Freundlich equilibrium constant K> The Langmuir-Freundlich equilibrium constant K at 0 °C and 50 °C was determined from the adsorption isotherms at 0 °C and 50 °C. At this time, the pressure range of the adsorption isotherm was set to 0 kPa or more and 105 kPa or less. For example, in Comparative Example 1, the Langmuir-Freundlich equilibrium constant K at 0 °C is 0.60 (1 / kPa), and the Langmuir-Freundlich equilibrium constant K at 50 °C is 0.15 (1 / kPa). Based on the following equation, when the adsorption capacity q is plotted against the pressure, a plot equivalent to the adsorption isotherm can be obtained. [Measurement temperature 0 °C] Saturation adsorption capacity Q = 6.61 (mol / kg) Equilibrium constant K = 0.60 (1 / kPa) Constant n = 0.54 (-) q=(6.61×0.60×p 0.54 ) / (1+0.60×p 0.54 ) (q: Carbon dioxide adsorption capacity, p: Pressure) [Measurement temperature 50 °C] Saturation adsorption capacity Q = 6.60 (mol / kg) Equilibrium constant K = 0.15 (1 / kPa) Constant n = 0.57 (-) q=(6.60×0.15×p 0.57 ) / (1+0.15×p 0.57 ) (q: Carbon dioxide adsorption capacity, p: Pressure)

[0085] Example 1 38.3 g of pure water, 25.9 g of an aqueous sodium hydroxide solution (NaOH, manufactured by Fujifilm Wako Pure Chemical Corporation) prepared to a concentration of 20%, and 0.9 g of sodium aluminate (NaAlO2, manufactured by Fujifilm Wako Pure Chemical Corporation) were added to a Teflon (registered trademark) beaker, and these were stirred with a magnetic stirrer until the solution became transparent to obtain an Al source solution. In a separate Teflon beaker, 25.6 g of pure water and 9.4 g of 38% sodium silicate aqueous solution (Na2SiO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added, and these were stirred with a magnetic stirrer until homogeneous to obtain the Si source solution. Then, while stirring the Si source solution, the Al source solution was added and stirred for 4 hours to prepare a mixed gel.

[0086] To the above mixed gel, sodium-type FAU (product name: HSZ-320NAA, manufactured by Tosoh Corporation) was added as a seed crystal at a ratio of 1.0 part by mass per 100 parts by mass of the solid content of the mixed gel (NaAlO2, NaOH, Na2SiO3). The mixture was then stirred and placed in a stainless steel autoclave containing a Teflon cylindrical container. Without stirring, hydrothermal synthesis was performed at 90°C for 16 hours to obtain a crystallized slurry. The obtained crystallized slurry was filtered through filter paper, the solid remaining on the filter paper was washed with pure water, and the solid was dried at 110°C to obtain 1.9 g of white union-exchanged zeolite powder.

[0087] The obtained union-exchanged zeolite powder was subjected to X-ray diffraction (XRD) to measure its X-ray diffraction spectrum. The results are shown in Figure 1. From the X-ray diffraction spectrum shown in Figure 1, it was confirmed that a diffraction pattern of a single phase of FAU-type zeolite was obtained. No diffraction peaks originating from impurities such as LTA-type zeolite were observed. Elemental analysis of the obtained union-exchanged zeolite powder was performed using scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX), and it was confirmed that the Si / Al ratio was 1.58. Furthermore, 100 mol% of the cations constituting the union-exchanged zeolite were sodium. FAU-type zeolite in which 100 mol% of the cations are sodium may be referred to as "Na-type FAU" below.

[0088] A 0.5 mol / kg aqueous solution of lithium chloride (LiCl, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was prepared by dissolving 2.2 g of lithium chloride in 100 g of pure water. This solution was mixed with 1 g of Na-type FAU obtained as described above, and the resulting mixture was stirred at 60°C for 3 hours to perform ion exchange between sodium ions and lithium ions to obtain the 1st ion exchange solution. After filtering this 1st ion exchange solution through filter paper, the solid remaining on the filter paper was washed with pure water and dried at 110°C for 2 hours to obtain FAU-type zeolite (hereinafter also referred to as "1st Li-exchanged FAU") which had undergone one ion exchange with lithium ions. To increase the lithium ion ratio after lithium ion exchange to a predetermined value, the 1st Li-exchanged FAU was mixed with 100 g of a 0.5 mol / kg lithium chloride aqueous solution and stirred at 60°C for 3 hours to perform ion exchange and obtain a 2nd ion-exchanged solution. This 2nd ion-exchanged solution was then filtered through filter paper, the solid remaining on the filter paper was washed with pure water, and dried at 110°C for 2 hours to obtain a FAU-type zeolite (hereinafter also referred to as "2nd Li-exchanged FAU") that had undergone two lithium ion exchanges. Subsequently, the obtained 2nd Li-exchanged FAU was mixed with 100 g of a 0.5 mol / kg lithium chloride aqueous solution and ion exchange was performed by stirring at 60°C for 3 hours to obtain a 3rd ion-exchanged solution. This 3rd ion-exchanged solution was then filtered through filter paper, the solid remaining on the filter paper was washed, and the solution was dried at 110°C for 2 hours. In this way, a total of three lithium ion exchanges were performed, yielding a FAU-type zeolite that had undergone three lithium ion exchanges (hereinafter also referred to as "3rdLi-exchanged FAU"). In this manner, a carbon dioxide adsorbent consisting of 3rdLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent using SEM-EDX confirmed that the Si / Al ratio was 1.58. Furthermore, the proportion of lithium ions after lithium ion exchange was 86.6%, and the proportion of sodium ions, the residual ions after lithium ion exchange, was 13.4%. In other words, 86.6 mol% of the cations constituting the zeolite was lithium.

[0089] The carbon dioxide adsorbent obtained as described above was pretreated, and then the adsorption isotherms of carbon dioxide at 0°C and 50°C were measured using a constant-volume gas adsorption method with carbon dioxide and nitrogen gas as the adsorption gases. As pretreatment, the carbon dioxide adsorbent was left to stand at 350°C for 2 hours under reduced pressure of 0.01 kPa or less. A gas adsorption amount measuring device (Belsorp miniII, manufactured by Microtrac-Bel Co., Ltd.) was used to measure the adsorption isotherms. The measured carbon dioxide adsorption isotherms are shown in Figure 2. In Figure 2, the horizontal axis "Pressure" represents the partial pressure of carbon dioxide. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 1.

[0090] Example 2 In a Teflon® beaker, 37.6 g of pure water, 23.1 g of a 20% sodium hydroxide aqueous solution (NaOH, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 0.9 g of sodium aluminate (NaAlO2, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added. These were then stirred with a magnetic stirrer until the solution became clear to obtain an Al source solution. In a separate Teflon beaker, 25.1 g of pure water and 13.4 g of 38% sodium silicate aqueous solution (Na2SiO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added, and these were stirred with a magnetic stirrer until homogeneous to obtain the Si source solution. A mixed gel was prepared by adding an Al source solution to a Si source solution while stirring the Si source solution, and then stirring the resulting mixture for 4 hours. Except for using the mixed gel prepared as described above, 1.9 g of white Na-type FAU was obtained in the same manner as in Example 1.

[0091] The obtained Na-type FAU was analyzed by XRD to measure its X-ray diffraction spectrum, confirming that it exhibited a single-phase diffraction pattern of FAU-type zeolite. No diffraction peaks originating from impurities such as LTA-type zeolite were observed. Elemental analysis of the obtained Na-type FAU using SEM-EDX confirmed a Si / Al ratio of 1.72. Furthermore, the sodium ion ratio in the cation was 100%.

[0092] The resulting Na-type FAU was subjected to lithium ion exchange a total of three times in the same manner as in Example 1, and a carbon dioxide adsorbent consisting of the final 3rdLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent was performed by SEM-EDX, and it was confirmed that the Si / Al ratio was 1.74. Furthermore, the proportion of lithium ions in the cation after lithium ion exchange was 84.0%, and the proportion of sodium ions, which are the residual ions in the cation after lithium ion exchange, was 16.0%. In other words, 84.0 mol% of the cation constituting the zeolite was lithium.

[0093] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 1.

[0094] Example 3 In a Teflon beaker, 36.4 g of pure water, 18.3 g of a 20% sodium hydroxide aqueous solution (NaOH, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 0.9 g of sodium aluminate (NaAlO2, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added. These were then stirred with a magnetic stirrer until the solution became clear to obtain an Al source solution. In a separate Teflon beaker, 24.3 g of pure water and 20.2 g of 38% sodium silicate aqueous solution (Na2SiO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added, and these were stirred with a magnetic stirrer until homogeneous to obtain the Si source solution. A mixed gel was prepared by adding an Al source solution to a Si source solution while stirring the Si source solution, and then stirring the resulting mixture for 4 hours. Except for using the mixed gel prepared as described above, 1.9 g of white Na-type FAU was obtained in the same manner as in Example 1.

[0095] The obtained Na-type FAU was analyzed by XRD to measure its X-ray diffraction spectrum, confirming that it exhibited a single-phase diffraction pattern of FAU-type zeolite. No diffraction peaks originating from impurities such as LTA-type zeolite were observed. Elemental analysis of the obtained Na-type FAU using SEM-EDX confirmed a Si / Al ratio of 1.93. Furthermore, the sodium ion ratio in the cation was 100%.

[0096] The resulting Na-type FAU (1 g) was subjected to a total of three lithium ion exchanges in the same manner as in Example 1, and a carbon dioxide adsorbent consisting of the final 3rdLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent was performed by SEM-EDX, confirming that the Si / Al ratio was 1.90. Furthermore, the proportion of lithium ions in the cation after lithium ion exchange was 80.5%, and the proportion of sodium ions, which are the residual ions in the cation after lithium ion exchange, was 19.5%.

[0097] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 1.

[0098] Example 4 In a Teflon beaker, 0.02 g of pure water, 3.5 g of a 20% sodium hydroxide aqueous solution (NaOH, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 0.2 g of sodium aluminate (NaAlO2, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added. These were then stirred with a magnetic stirrer until the solution became clear to obtain Al source solution 1. In a separate Teflon beaker, 2.2 g of 38% sodium silicate aqueous solution (Na2SiO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, and while stirring with a magnetic stirrer, Al source solution 1 was added. After stirring for 10 minutes, stirring was stopped and the mixture was allowed to stand and mature for 24 hours to obtain a fluid, creamy seed mixed gel.

[0099] In a Teflon beaker, 16.2 g of pure water, 3.4 g of a 20% sodium hydroxide aqueous solution (NaOH, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 1.7 g of sodium aluminate (NaAlO2, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added. These were then stirred with a magnetic stirrer until the solution became clear to obtain Al source solution 2. In a separate Teflon beaker, 18.6 g of 32% sodium silicate aqueous solution (Na2SiO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, and while stirring with a magnetic stirrer, Al source solution 2 was added. These were stirred for 10 minutes to obtain a fluid, creamy base mixed gel.

[0100] 40 g of base mixed gel was mixed with 2.4 g of seed mixed gel, stirred for 30 minutes, then stopped stirring and allowed to stand for 24 hours to mature, thereby obtaining a mixed gel. This mixed gel was placed in a stainless steel autoclave containing a Teflon cylindrical container, and a crystallized slurry was obtained by hydrothermal synthesis at 100°C for 7 hours without stirring. The resulting crystallized slurry was filtered through filter paper, the solid remaining on the filter paper was washed with pure water, and then dried at 110°C to obtain 5.0 g of white Na-type FAU.

[0101] The obtained Na-type FAU was subjected to X-ray diffraction spectroscopy. The X-ray diffraction spectrum confirmed the presence of a single-phase FAU-type zeolite diffraction pattern. No diffraction peaks originating from impurities such as LTA-type zeolite were observed. Elemental analysis of the obtained Na-type FAU using SEM-EDX confirmed a Si / Al ratio of 2.03. Furthermore, the sodium ion ratio in the cation was 100%.

[0102] The resulting 1 g of Na-type FAU was subjected to a total of three lithium ion exchanges in the same manner as in Example 1, and a carbon dioxide adsorbent consisting of the final 3rdLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent was performed by SEM-EDX, and it was confirmed that the Si / Al ratio was 2.10. Furthermore, the proportion of lithium ions in the cation after lithium ion exchange was 76.8%, and the proportion of sodium ions, which are the residual ions in the cation after lithium ion exchange, was 23.2%.

[0103] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 1.

[0104] Example 5 In a Teflon beaker, 0.1 g of pure water, 2.9 g of a 20% sodium hydroxide aqueous solution (NaOH, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 0.3 g of sodium aluminate (NaAlO2, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added. These were then stirred with a magnetic stirrer until the solution became clear to obtain Al source solution 1. In a separate Teflon beaker, 2.6 g of 38% sodium silicate aqueous solution (Na2SiO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, and while stirring with a magnetic stirrer, Al source solution 1 was added. After stirring for 10 minutes, stirring was stopped and the mixture was allowed to stand and mature for 24 hours to obtain the Seed mixed gel. 16.0 g of pure water and 2.3 g of sodium aluminate (NaAlO2, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added to a Teflon beaker, and these were stirred with a magnetic stirrer until the solution became clear to obtain Al source solution 2. In a separate Teflon beaker, 21.8 g of 32% sodium silicate aqueous solution (Na2SiO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, and while stirring with a magnetic stirrer, Al source solution 2 was added. These were stirred for 10 minutes to obtain a base mixed gel. Except for using the seed mixed gel prepared as described above as the seed mixed gel and the base mixed gel prepared as described above as the base mixed gel, 6.6 g of white Na-type FAU powder was obtained in the same manner as in Example 4.

[0105] The obtained Na-type FAU was subjected to X-ray diffraction spectroscopy. The X-ray diffraction spectrum confirmed the presence of a single-phase FAU-type zeolite diffraction pattern. No diffraction peaks originating from impurities such as LTA-type zeolite were observed. Elemental analysis of the obtained Na-type FAU using SEM-EDX confirmed a Si / Al ratio of 2.26. Furthermore, the sodium ion ratio in the cation was 100%.

[0106] The resulting Na-type FAU was subjected to lithium ion exchange a total of three times in the same manner as in Example 1, and a carbon dioxide adsorbent consisting of the final 3rdLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent was performed by SEM-EDX, and it was confirmed that the Si / Al ratio was 2.31. Furthermore, the ratio of lithium ions in the cation after lithium ion exchange was 72.0%, and the ratio of sodium ions, which are the residual ions in the cation after lithium ion exchange, was 28.0%.

[0107] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 2.

[0108] Example 6 In a Teflon beaker, 1.4 g of pure water, 2.3 g of a 20% sodium hydroxide aqueous solution (NaOH, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 0.2 g of sodium aluminate (NaAlO2, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added. These were then stirred with a magnetic stirrer until the solution became clear to obtain Al source solution 1. In a separate Teflon beaker, 2.7 g of 38% sodium silicate aqueous solution (Na2SiO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, and while stirring with a magnetic stirrer, Al source solution 1 was added. After stirring for 10 minutes, stirring was stopped and the mixture was allowed to stand and mature for 24 hours to obtain the Seed mixed gel. In a Teflon beaker, 20.3 g of pure water, 4.9 g of a 20% sodium hydroxide aqueous solution (NaOH, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 1.7 g of sodium aluminate (NaAlO2, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added. These were then stirred with a magnetic stirrer until the solution became clear to obtain Al source solution 2. In a separate Teflon beaker, 33.1 g of 32% sodium silicate aqueous solution (Na2SiO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, and while stirring with a magnetic stirrer, Al source solution 2 was added and the mixture was stirred for 10 minutes to obtain a base mixed gel. Except for using the seed mixed gel prepared as described above as the seed mixed gel and the base mixed gel prepared as described above as the base mixed gel, 7.0 g of white Na-type FAU was obtained in the same manner as in Example 4.

[0109] The obtained Na-type FAU was subjected to X-ray diffraction spectroscopy. The X-ray diffraction spectrum confirmed the presence of a single-phase FAU-type zeolite diffraction pattern. No diffraction peaks originating from impurities such as LTA-type zeolite were observed. Elemental analysis of the obtained Na-type FAU using SEM-EDX confirmed a Si / Al ratio of 2.50. Furthermore, the sodium ion ratio in the cation was 100%.

[0110] The resulting Na-type FAU was subjected to lithium ion exchange a total of three times in the same manner as in Example 1, and a carbon dioxide adsorbent consisting of the final 3rdLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent was performed by SEM-EDX, and it was confirmed that the Si / Al ratio was 2.51. Furthermore, the proportion of lithium ions in the cation after lithium ion exchange was 76.8%, and the proportion of sodium ions, which are the residual ions in the cation after lithium ion exchange, was 23.2%.

[0111] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 2.

[0112] Example 7 We prepared a Na-type FAU (product name: HSZ-320NAA, manufactured by Tosoh Corporation) with a Si / Al ratio of 3.03 and a sodium ion ratio of 100% in the cation. Then, lithium ion exchange was performed on this Na-type FAU a total of three times in the same manner as in Example 1, and a carbon dioxide adsorbent consisting of the final 3rdLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent was performed by SEM-EDX, and it was confirmed that the Si / Al ratio was 3.04. Furthermore, the ratio of lithium ions in the cation after lithium ion exchange was 70.4%, and the ratio of sodium ions, which are the residual ions in the cation after lithium ion exchange, was 29.6%.

[0113] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 2.

[0114] Example 8 100 g of a mixed aqueous solution with a lithium chloride concentration of 0.5 mol / kg and a magnesium chloride concentration of 0.05 mol / kg was mixed with 1 g of Na-type FAU (Si / Al ratio 1.93) powder synthesized in the same manner as in Example 3, and the mixture was stirred at 60°C for 3 hours to perform ion exchange and obtain the 1st ion exchange solution. This 1st ion exchange solution was filtered through filter paper, the solid remaining on the filter paper was washed with pure water, and then dried at 110°C for 2 hours to obtain a FAU-type zeolite (hereinafter also referred to as "1st MgLi exchange FAU") in which ion exchange between lithium ions and magnesium ions had been performed once. Then, in order to increase the ratio of magnesium ions and lithium ions after ion exchange to a predetermined value, the 1st MgLi-exchanged FAU was mixed again with 100 g of the above mixed aqueous solution and ion exchange was performed by stirring at 60°C for 3 hours to obtain the 2nd ion-exchanged solution. Then, after filtering this 2nd ion exchange solution through filter paper, the solid remaining on the filter paper was washed with pure water and dried at 110°C for 2 hours to obtain a FAU-type zeolite (hereinafter also referred to as "2nd MgLi-exchanged FAU") which had undergone two ion exchanges with lithium ions and magnesium ions. Subsequently, the obtained 2nd MgLi-exchanged FAU was mixed with 100 g of a 0.5 mol / kg lithium chloride aqueous solution and ion exchange was performed by stirring at 60°C for 3 hours to obtain a 3rd ion-exchanged solution. This 3rd ion-exchanged solution was then filtered through filter paper, and the solid remaining on the filter paper was washed with pure water and dried at 110°C for 2 hours. In this way, a total of three ion exchanges were performed, resulting in a FAU-type zeolite (hereinafter also referred to as "3rdMgLi-exchanged FAU") in which three ion exchanges with lithium ions and two ion exchanges with magnesium ions were carried out. In this manner, a carbon dioxide adsorbent consisting of 3rdMgLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent using SEM-EDX confirmed that the Si / Al ratio was 1.95. Furthermore, the proportion of lithium ions in the cation after ion exchange was 79.0%, the proportion of magnesium ions in the cation after ion exchange was 12.0%, and the proportion of sodium ions, which were the residual ions in the cation after ion exchange, was 9.0%.

[0115] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 2.

[0116] Comparative Example 1 A Na-type FAU (product name: F-9, manufactured by Tosoh Corporation) with a Si / Al ratio of 1.39, a lithium ion ratio of 0% in the cation, and a sodium ion ratio of 100% in the cation was prepared and used as a carbon dioxide adsorbent.

[0117] For this carbon dioxide adsorbent, the adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C were measured, respectively, in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 3.

[0118] Comparative Examples 2-7 The Na-type FAU synthesized in Examples 1 to 6 were prepared and used as carbon dioxide adsorbents in Comparative Examples 2 to 7. For these carbon dioxide adsorbents, the adsorption isotherms of carbon dioxide and nitrogen at 0°C and 50°C were measured, respectively, in the same manner as in Example 1. Figure 3 is a graph showing the carbon dioxide adsorption isotherms at 0°C and 50°C for Comparative Example 2. In Figure 3, the horizontal axis, "Pressure," represents the partial pressure of carbon dioxide. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbed gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbed gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 3 or Table 4.

[0119] Comparative Example 8 A Na-type FAU (product name: HSZ-320NAA, manufactured by Tosoh Corporation) with a Si / Al ratio of 3.03, a lithium ion ratio of 0% in the cation, and a sodium ion ratio of 100% in the cation was prepared and used as a carbon dioxide adsorbent.

[0120] For this carbon dioxide adsorbent, the adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C were measured, respectively, in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 4.

[0121] Comparative Example 9 We prepared a Na-type FAU (product name: F-9, manufactured by Tosoh Corporation) with a Si / Al ratio of 1.39 and a sodium ion ratio of 100% in the cation. Then, lithium ion exchange was performed on this Na-type FAU a total of three times in the same manner as in Example 1, and a carbon dioxide adsorbent consisting of the final 3rdLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent was performed by SEM-EDX, and it was confirmed that the Si / Al ratio was 1.38. Furthermore, the proportion of lithium ions in the cation after lithium ion exchange was 73.4%, and the proportion of sodium ions, which are the residual ion 1 in the cation after lithium ion exchange, was 26.6%.

[0122] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 5.

[0123] Comparative Example 10 1 g of Na-type FAU, synthesized in the same manner as in Example 3, with a Si / Al ratio of 1.93 and a sodium ion ratio of 100% in the cation, was mixed with 100 g of a 0.04 mol / kg lithium chloride aqueous solution. The mixture was stirred at 60°C for 3 hours to perform ion exchange and obtain an ion-exchange solution. This ion-exchange solution was then filtered through filter paper, and the solid remaining on the filter paper was washed with pure water and dried at 110°C for 2 hours. As a result, a carbon dioxide adsorbent consisting of Li-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent using SEM-EDX confirmed that the Si / Al ratio was 1.93. Furthermore, the proportion of lithium ions in the cation after lithium ion exchange was 33.9%, and the proportion of sodium ions, which are the residual ions in the cation after lithium ion exchange, was 66.1%.

[0124] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 5.

[0125] Comparative Example 11 1 g of Na-type FAU, synthesized in the same manner as in Example 3, with a Si / Al ratio of 1.93 and a sodium ion ratio of 100% in the cation, was mixed with 100 g of a 0.1 mol / kg lithium chloride aqueous solution. The mixture was stirred at 60°C for 3 hours to perform ion exchange and obtain the first ion-exchanged solution. This first ion-exchanged solution was then filtered through filter paper, the solid remaining on the filter paper was washed with pure water, and the mixture was dried at 110°C for 2 hours to exchange the sodium ions, which are the cations, for lithium ions to obtain the first Li-exchanged FAU. Then, in order to increase the lithium ion ratio after lithium ion exchange to a predetermined value, the 1st Li-exchanged FAU was mixed again with 100 g of a 0.1 mol / kg lithium chloride aqueous solution, and ion exchange was performed by stirring at 60°C for 3 hours. After filtering through filter paper, the solid remaining on the filter paper was washed with pure water and dried at 110°C for 2 hours. In this manner, lithium ion exchange was performed a total of two times to obtain 2ndLi-exchanged FAU. As a result, a carbon dioxide adsorbent consisting of 2ndLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent was performed using SEM-EDX, and it was confirmed that the Si / Al ratio was 1.93. Furthermore, the proportion of lithium ions in the cation after lithium ion exchange was 55.8%, and the proportion of sodium ions, which are the residual ions in the cation after lithium ion exchange, was 44.2%.

[0126] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 5.

[0127] Comparative Example 12 1 g of Na-type FAU, synthesized in the same manner as in Example 3, with a Si / Al ratio of 1.93 and a sodium ion ratio of 100% in the cation, was mixed with 100 g of a 0.1 mol / kg lithium chloride aqueous solution. The mixture was stirred at 60°C for 3 hours to perform ion exchange and obtain the first ion-exchanged solution. This first ion-exchanged solution was then filtered through filter paper, the solid remaining on the filter paper was washed with pure water, and the mixture was dried at 110°C for 2 hours to exchange the sodium ions, which are the cations, for lithium ions to obtain the first Li-exchanged FAU. Then, in order to increase the lithium ion ratio after lithium ion exchange to a predetermined value, the 1st Li-exchanged FAU was mixed again with 100 g of a 0.1 mol / kg lithium chloride aqueous solution, and ion exchange was performed by stirring at 60°C for 3 hours. After filtering through filter paper, the solid remaining on the filter paper was washed with pure water, dried at 110°C for 2 hours, and the 2nd Li-exchanged FAU was obtained. Subsequently, the obtained 2nd Li-exchanged FAU was mixed with 100 g of a 0.1 mol / kg lithium chloride aqueous solution and ion exchange was performed by stirring at 60°C for 3 hours to obtain a 3rd ion-exchanged solution. This 3rd ion-exchanged solution was then filtered through filter paper, the solid remaining on the filter paper was washed with pure water, and dried at 110°C for 2 hours. In this manner, a total of three lithium ion exchanges were performed, and a carbon dioxide adsorbent consisting of 3rdLi-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent using SEM-EDX confirmed that the Si / Al ratio was 1.93. Furthermore, the proportion of lithium ions in the cation after lithium ion exchange was 65.8%, and the proportion of sodium ions, which are the residual ions in the cation after lithium ion exchange, was 34.2%.

[0128] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 6.

[0129] Comparative Example 13 1 g of Na-type FAU, synthesized in the same manner as in Example 4, with a Si / Al ratio of 2.03 and a sodium ion ratio of 100% in the cation, was mixed with 100 g of a 0.1 mol / kg magnesium chloride aqueous solution. These were stirred at 60°C for 3 hours to perform ion exchange and obtain the 1st ion-exchanged solution. This 1st ion-exchanged solution was then filtered through filter paper, the solid remaining on the filter paper was washed with pure water, and dried at 110°C for 2 hours to exchange the sodium ions, which are the cation, for magnesium ions to obtain the 1st Mg-exchanged FAU. Then, in order to increase the ratio of magnesium ions after magnesium ion exchange to a predetermined value, the 1st Mg-exchanged FAU was mixed again with 100 g of a 0.1 mol / kg magnesium chloride aqueous solution and ion exchange was performed by stirring at 60°C for 3 hours. The resulting 2nd ion-exchanged solution was filtered through filter paper, the solid remaining on the filter paper was washed with pure water, and dried at 110°C for 2 hours. In this way, a total of two magnesium ion exchanges were performed, and a carbon dioxide adsorbent consisting of the 2nd Mg-exchanged FAU was obtained. Elemental analysis of the obtained carbon dioxide adsorbent using SEM-EDX confirmed that the Si / Al ratio was 2.10. Furthermore, the proportion of lithium ions in the cation after magnesium ion exchange was 0.0%, the proportion of magnesium ions in the cation after magnesium ion exchange was 70.0%, and the proportion of sodium ions, which are the residual ions in the cation after magnesium ion exchange, was 30.0%.

[0130] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 6.

[0131] Comparative Example 14 1 g of Na-type FAU (product name: HSZ-320NAA, manufactured by Tosoh Corporation), with a Si / Al ratio of 3.03 and a sodium ion ratio of 100% in the cation, was mixed with 100 g of a 0.1 mol / kg potassium chloride aqueous solution. The resulting mixture was stirred at 60°C for 3 hours to perform ion exchange and obtain the first ion-exchanged solution. This first ion-exchanged solution was then filtered through filter paper, and the solid remaining on the filter paper was washed with pure water. The solution was dried at 110°C for 2 hours to exchange the sodium ions, which are the cation, for potassium ions to obtain the first K-exchanged FAU. Then, in order to increase the ratio of potassium ions after potassium ion exchange to a predetermined value, the 1st K-exchanged FAU was mixed again with 100 g of 0.1 mol / kg potassium chloride aqueous solution and ion exchange was performed by stirring at 60°C for 3 hours. The resulting 2nd ion-exchanged solution was filtered through filter paper, the solid remaining on the filter paper was washed with pure water, and dried at 110°C for 2 hours. In this manner, potassium ion exchange was performed a total of two times to obtain a carbon dioxide adsorbent. Elemental analysis of the obtained carbon dioxide adsorbent using SEM-EDX confirmed that the Si / Al ratio was 3.00. Furthermore, the proportion of lithium ions in the cation after potassium ion exchange was 0.0%, the proportion of potassium ions in the cation after potassium ion exchange was 93.4%, and the proportion of sodium ions, which are the residual ions in the cation after potassium ion exchange, was 6.6%.

[0132] The carbon dioxide adsorbent obtained as described above had its adsorption isotherms for carbon dioxide and nitrogen at 0°C and 50°C measured in the same manner as in Example 1. Then, the equilibrium constant K(K1) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 0°C using carbon dioxide as the adsorbent gas, the equilibrium constant K(K2) of the Langmuir-Freundlich equation was determined from the adsorption isotherm at 50°C using carbon dioxide as the adsorbent gas, and the difference between them, ΔK (=K1-K2), was calculated. The results are shown in Table 6.

[0133] <Rating> (1) Effective adsorption capacity of carbon dioxide and nitrogen The effective adsorption capacity A for carbon dioxide was determined as follows, assuming a PSA (Pressure Sequestration) washing process in which a mixed gas containing carbon dioxide (90 vol%) and nitrogen (10 vol%) is adsorbed at atmospheric pressure and desorbed at a reduced pressure of 6.3 kPa. Specifically, the partial pressures of the mixed gas containing carbon dioxide (90 vol%) and nitrogen (10 vol%) at atmospheric pressure are 91.2 kPa and 10.1 kPa, respectively. The effective adsorption capacity A when carbon dioxide is adsorbed at 50°C and 91.2 kPa and desorbed at 0°C and 6.3 kPa was calculated from the adsorption isotherms of carbon dioxide gas at 0°C and 50°C based on the following formula. The results are shown in Tables 1 to 6. Effective adsorption capacity of carbon dioxide A =[Adsorption capacity of carbon dioxide at 50°C and 91.2kPa] - [Adsorption capacity of carbon dioxide at 0°C and 6.3kPa]

[0134] Furthermore, nitrogen is known to desorb at a significantly faster rate than carbon dioxide. Therefore, assuming that the effect of temperature reduction caused by carbon dioxide desorption is small, the effective adsorption capacity B of nitrogen was calculated from the nitrogen adsorption isotherm at 50°C based on the following formula. The results are shown in Tables 1-6. Effective nitrogen adsorption capacity B =[Nitrogen adsorption capacity at 50°C and 10.1kPa] - [Nitrogen adsorption capacity at 50°C and 6.3kPa]

[0135] (2) Carbon dioxide selectivity The ratio of the effective adsorption capacity of carbon dioxide (A) to the effective adsorption capacity of nitrogen (B) was used as an indicator of "carbon dioxide selectivity" and was calculated based on the following formula. The results are shown in Tables 1 to 6. Carbon dioxide selectivity = [Effective adsorption capacity of carbon dioxide A] ÷ [Effective adsorption capacity of nitrogen B]

[0136] (3) Effective adsorption capacity of carbon dioxide due to differences in temperature fluctuation range (ΔT) To compare the difference in effective carbon dioxide adsorption capacity due to differences in the temperature fluctuation range (ΔT), i.e., the difference in the difference between the adsorption temperature and the desorption temperature, an Arrhenius plot was created from the measured temperature and the carbon dioxide adsorption capacity during adsorption and desorption. The carbon dioxide adsorption capacity at the target temperature was calculated, and the effective carbon dioxide adsorption capacity at temperature fluctuation range (ΔT) = 0°C (adsorption 25°C - desorption 25°C), at ΔT = 10°C (adsorption 30°C - desorption 20°C), and at ΔT = 30°C (adsorption 40°C - desorption 10°C) was also calculated. The results are shown in Tables 1 to 6. In addition, Tables 1-6 also show the effective carbon dioxide adsorption capacity at ΔT = 50°C (adsorption 50°C - desorption 0°C). Here, the effective carbon dioxide adsorption capacity at ΔT = 50°C (adsorption 50°C - desorption 0°C) is the same value as the effective carbon dioxide adsorption capacity A.

[0137] [Table 1]

[0138] [Table 2]

[0139] [Table 3]

[0140] [Table 4]

[0141] [Table 5]

[0142] [Table 6]

[0143] Comparing the examples in Table 1 or Table 2 with the comparative examples in Tables 3 to 6, it can be seen that even with the same Si / Al ratio, ion exchange of more than 70% of the cations with lithium ions reduces the difference in the equilibrium constants of the Langmuir-Freundlich equation obtained from the adsorption isotherms at 0°C and 50°C, indicating a higher effective adsorption capacity A for carbon dioxide.

[0144] Furthermore, comparing Example 1 with Comparative Example 9, it can be seen that when the Si / Al ratio is less than 1.5, the difference in the equilibrium constant of the Langmuir-Freundlich equation becomes larger, resulting in a lower effective adsorption capacity A for carbon dioxide and insufficient desorption of carbon dioxide.

[0145] Comparing Example 3 with Comparative Examples 10-12, it can be seen that when ion exchange by lithium ions is insufficient (when the ratio of lithium ions in the cation is less than 70%), the effective adsorption capacity A of carbon dioxide is low, and the desorption of carbon dioxide is insufficient. The results from Comparative Examples 13 and 14 show that even when the Si / Al ratio is high, if the cation species accounting for more than 70% of the cations are different from lithium ions, the effective adsorption capacity A of carbon dioxide tends to be lower.

[0146] Tables 1-3 compare Examples 1, 4, and 6 with Comparative Examples 1, 2, 5, and 7. It was confirmed that even when the temperature fluctuation range ΔT is 0°C, the carbon dioxide adsorbents in the examples have a higher effective adsorption capacity for carbon dioxide than those in the comparative examples. Furthermore, the effective adsorption capacity of the examples remains higher as the temperature fluctuation range increases. Specifically, at a temperature fluctuation range ΔT = 50°C, Example 1 was found to have an effective adsorption capacity of 50% or more higher than that of Comparative Example 1. [Industrial applicability]

[0147] The carbon dioxide adsorbent of the present invention can be used, for example, as a zeolite adsorbent that can separate and recover carbon dioxide from a mixed gas containing carbon dioxide, such as combustion exhaust gas. Furthermore, the carbon dioxide adsorbent of the present invention contributes to improving the efficiency of carbon dioxide recovery and increasing the purity of carbon dioxide as a zeolite adsorbent in the PSA method. In particular, it can be suitably used in adsorption of mixed gases containing high concentrations of carbon dioxide or in processes in which high concentrations of carbon dioxide are adsorbed, thus greatly contributing to the recovery of carbon dioxide, which is considered a major cause of global warming.

Claims

1. It contains a zeolite having a faujasite-type crystal structure, More than 70% of the cations constituting the zeolite are lithium ions. The Si / Al ratio is 1.5 or higher. A carbon dioxide adsorbent in which the difference in the equilibrium constant (1 / kPa) of the Langmuir-Freundlich equation, obtained from the carbon dioxide adsorption isotherm at 0°C and the carbon dioxide adsorption isotherm at 50°C, is 0.3 or less.

2. The carbon dioxide adsorbent according to claim 1, wherein the Si / Al ratio is 3.5 or less.

3. The carbon dioxide adsorbent according to claim 1, wherein the cation further comprises at least one of an alkali metal ion other than lithium and an alkaline earth metal ion.

4. The carbon dioxide adsorbent according to claim 1, wherein the cation further comprises one or more ions selected from the group consisting of sodium, potassium, rubidium, cesium, and magnesium.

5. A method for separating carbon dioxide, comprising a separation step of separating carbon dioxide from a mixed gas containing carbon dioxide, wherein the separation step includes a step of contacting a carbon dioxide adsorbent described in claim 1 or 2 with a mixed gas containing carbon dioxide to adsorb carbon dioxide onto the carbon dioxide adsorbent.

6. The method for separating carbon dioxide according to claim 5, wherein the separation step further includes a step of separating carbon dioxide from the carbon dioxide adsorbent by a pressure swing adsorption method.

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