Carbon dioxide absorbent

Abstract

To provide a carbon dioxide absorbent which contains an amine compound having a property of being liquid at a normal temperature (25°C) and turning carbonate formed after carbon dioxide absorption into solid, and can suppress a decrease in carbon dioxide absorption due to the formation of the carbonate, and expressing an excellent carbon dioxide absorption.SOLUTION: The present invention relates to a carbon dioxide absorbent which contains an amine compound (A) and an ion liquid (B), wherein the amine compound (A) is a compound which is liquid at 25°C and in which carbonate of the amine compound (A) is solid at 25°C, and the ion liquid (B) consists of a cation represented by the general formula (1), and an anion containing at least one kind selected from the group consisting of a carboxylate anion, a sulfonate anion, and a phosphate anion. In the formula (1), R1 is an aliphatic hydrocarbon group with a carbon number of 1 to 18, and R2 is a hydrogen atom or an aliphatic hydrocarbon group having 1 to 18 carbon atoms.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a carbon dioxide absorbent.

Background Art

[0002] From the perspective of the global warming problem, reduction of carbon dioxide is required. As one of the methods for reducing carbon dioxide, there is a technology (CCS: Carbon dioxide Capture and Storage) for efficiently recovering high-concentration (about 10 to 30% by volume) carbon dioxide from exhaust gas discharged from thermal power plants and the like, and burying and storing it underground or in the sea. As technologies related to the carbon dioxide absorbent used in CCS, for example, the technologies described in Patent Documents 1 and 2 can be mentioned. Patent Document 1 describes a method for recovering carbon dioxide using specific alkanolamines as a carbon dioxide absorbent. Patent Document 2 describes using a carbon dioxide absorbent solution containing a carbon dioxide chemisorbing amine having a nitrogen-hydrogen bond and a tertiary amine solvent not having a nitrogen-hydrogen bond as a carbon dioxide absorbent.

[0003] As described in Patent Documents 1 and 2, many technologies using alkanolamines are known regarding the carbon dioxide absorbent used in CCS. On the other hand, in recent years, a technology (DAC: Direct Air Capture) for directly recovering low-concentration carbon dioxide (about 0.04% by volume) in the air has attracted attention. The carbon dioxide absorbent used in DAC is required to have a higher carbon dioxide absorption capacity than the carbon dioxide absorbent used in CCS.

[0004] Technologies related to DAC are also variously studied. For example, Patent Document 3 discloses an absorbent for absorbing carbon dioxide in the air, which contains an alkylamine substituted with a hydroxy group or an optionally substituted amino group. Patent Document 4 discloses a carbon dioxide absorbent in the air, which contains 4-trifluoromethylbenzylamine. Patent Document 5 discloses a carbon dioxide absorbent containing a polyamine compound having an alicyclic hydrocarbon structure in a predetermined amount or more. Patent Document 6 discloses a carbon dioxide absorbent containing an amine compound having a predetermined heterocyclic structure in a predetermined amount or more.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0006] Patent Document 3 describes that among alkylamines substituted with a hydroxy group or an optionally substituted amino group, monoethanolamine, diethanolamine, 2-(methylamino)ethanol, 2-amino-2-methyl-1-propanol, ethylenediamine, N,N'-dimethylethylenediamine, diethylenetriamine, o-xylylenediamine, m-xylylenediamine, or p-xylylenediamine are particularly preferably used because they have a high carbon dioxide absorption ability. Among the above, for example, m-xylylenediamine and some of its derivatives are liquids at room temperature (25 °C), but the carbonates obtained by absorbing carbon dioxide are solids. However, such carbon dioxide absorbents have a problem that they cannot be applied to existing carbon dioxide recovery devices for liquid processes. In addition, the solidification of the carbonate formed after carbon dioxide absorption slows down the diffusion of carbon dioxide in the carbon dioxide absorbent, which may lead to a decrease in carbon dioxide absorbability.

[0007] An object of the present invention is to provide a carbon dioxide absorbent containing an amine compound that is a liquid at room temperature (25 °C) and has a property that the carbonate formed after carbon dioxide absorption becomes a solid, which suppresses the decrease in carbon dioxide absorbability due to the formation of the carbonate and can exhibit excellent carbon dioxide absorbability.

Means for Solving the Problems

[0008] The present inventors have found that the above problems can be solved by a carbon dioxide absorbent containing an amine compound and an ionic liquid that satisfy predetermined requirements. That is, the present invention relates to the following. [1] A carbon dioxide absorbent containing an amine compound (A) and an ionic liquid (B), wherein the amine compound (A) is a compound that is a liquid at 25 °C and the carbonate of the amine compound (A) is a solid at 25 °C, and the ionic liquid (B) is composed of a cation represented by the following general formula (1) and an anion containing at least one selected from the group consisting of a carboxylate anion, a sulfonate anion, and a phosphate anion.

Chemical formula

Advantages of the Invention

[0009] According to the present invention, even when an amine compound that is liquid at room temperature (25°C) and has the property that the carbonate after carbon dioxide absorption becomes solid is used, it is possible to provide a carbon dioxide absorbent that suppresses a decrease in carbon dioxide absorbability due to the formation of the carbonate and exhibits excellent carbon dioxide absorbability.

Modes for Carrying Out the Invention

[0010] [Definitions] In the present specification, "having good carbon dioxide absorbability" means that the amount of carbon dioxide absorbed when contacted with a gas containing carbon dioxide is large, and "having good carbon dioxide absorbability from the air" means that the amount of absorption of low-concentration (about 0.04% by volume) carbon dioxide in the air is large. Specifically, the carbon dioxide absorbability can be evaluated by the method described in the examples. In the present specification, the description of "XX to YY" means "XX or more and YY or less".

[0011] [Carbon Dioxide Absorbent] The carbon dioxide absorbent of the present invention is a carbon dioxide absorbent containing an amine compound (A) and an ionic liquid (B), wherein the amine compound (A) is a liquid at 25°C and the carbonate of the amine compound (A) is a solid at 25°C, and the ionic liquid (B) consists of a cation represented by the following general formula (1) and an anion containing at least one selected from the group consisting of a carboxylate anion, a sulfonate anion, and a phosphate anion. [Chemical formula] (In formula (1), R 1 is an aliphatic hydrocarbon group having 1 to 18 carbon atoms, and R 2 is a hydrogen atom or an aliphatic hydrocarbon group having 1 to 18 carbon atoms.) By having the above configuration, the carbon dioxide absorbent of the present invention is a liquid at room temperature (25°C) and, even when using an amine compound having the property that the carbonate after carbon dioxide absorption becomes a solid, suppresses the decrease in carbon dioxide absorbability due to the formation of the carbonate and exhibits excellent carbon dioxide absorbability.

[0012] The reason why the carbon dioxide absorbent of the present invention exhibits the above effects is not clear, but it is considered as follows. The amine compound (A) used in the carbon dioxide absorbent of the present invention is a liquid at 25°C and its carbonate is a solid at 25°C. Since the amine compound (A) becomes a solid carbonate when absorbing carbon dioxide, the diffusion of carbon dioxide in the carbon dioxide absorbent slows down as the formation of the carbonate progresses, resulting in a decrease in carbon dioxide absorbability. However, the carbon dioxide absorbent of the present invention contains a specific ionic liquid (B), so that at least a part of the carbonate of the amine compound (A) dissolves in the ionic liquid (B) to prevent surface solidification of the carbon dioxide absorbent. Therefore, it is considered that the decrease in carbon dioxide absorbability due to the surface solidification can be suppressed. When an ionic liquid not corresponding to the ionic liquid (B) is used, it is presumed that the carbon dioxide absorbent solidifies from the surface as the amine compound (A) absorbs carbon dioxide, thereby slowing down the diffusion of carbon dioxide and decreasing the carbon dioxide absorbability. In addition, since the ionic liquid (B) has no boiling point, when carbon dioxide is desorbed from the carbon dioxide absorbent after carbon dioxide absorption by heating or the like, volatilization hardly occurs, and it is considered that the repeatability of the carbon dioxide absorbent is further improved.

[0013] <amine compound (A)> The amine compound (A) is an amine compound that is liquid at 25°C and whose carbonate is solid at 25°C. Here, the amine compound that is liquid at 25°C refers to an amine compound having a melting point of 25°C or lower, and the carbonate that is solid at 25°C refers to a carbonate having both a melting point and a decomposition temperature exceeding 25°C.

[0014] The amine compound (A) only needs to have at least one amino group, but from the viewpoint of improving carbon dioxide absorbability, it is preferable that the number of amino groups in the amine compound (A) is 2 or more. The amino groups referred to here include all of primary to tertiary amino groups. In this specification, the "primary amino group" is an amino group having two hydrogen atoms on the nitrogen atom, that is, it means a -NH 2 group. From the viewpoints of improving carbon dioxide absorbability and ease of desorption of carbon dioxide after carbon dioxide absorption, the number of amino groups in the amine compound (A) is more preferably 2 or more and 6 or less, still more preferably 2 or more and 4 or less, even more preferably 2 or more and 3 or less, and even more preferably 2.

[0015] From the viewpoint of improving carbon dioxide absorbability, the amine compound (A) preferably has at least a primary amino group. This is because the primary amino group has less steric hindrance and thus easily absorbs carbon dioxide to form a carbonate. From the viewpoint of improving carbon dioxide absorbability, the ratio of the primary amino group in all the amino groups in the amine compound (A) is preferably 35 mol% or more, more preferably 40 mol% or more, still more preferably 50 mol% or more, even more preferably 60 mol% or more, even more preferably 70 mol% or more, even more preferably 80 mol% or more, and 100 mol% or less.

[0016] From the viewpoint of improving carbon dioxide absorbability, the total amine value of the amine compound (A) is preferably 600 mgKOH / g or more, more preferably 800 mgKOH / g or more, and is preferably 1,200 mgKOH / g or less. Specifically, the total amine value can be determined by the method described in the examples.

[0017] From the viewpoint of further improving the desorbability of carbon dioxide and the recyclability, the maximum carbon dioxide dissociation temperature of the amine compound (A) measured by the following method is preferably 180°C or lower, more preferably 160°C or lower, still more preferably 140°C or lower. The lower limit of the maximum carbon dioxide dissociation temperature is not particularly limited, but is, for example, 40°C or higher. (Method) The amine compound (A) having absorbed carbon dioxide is heated from 23°C to 250°C at a heating rate of 10°C / min, and the temperature at which the endothermic amount associated with the desorption of carbon dioxide becomes maximum is measured, and this temperature is defined as the maximum carbon dioxide dissociation temperature. Here, the amine compound (A) having absorbed carbon dioxide can be prepared, for example, by allowing 5 mmol of the amine compound (A) to stand in air at 23°C and 50% RH for 24 hours.

[0018] From the viewpoint of suppressing the weight loss due to heating during the desorption of carbon dioxide and further improving the recyclability, the maximum endothermic temperature of the amine compound (A) measured by the following method is preferably 150°C or higher, more preferably 160°C or higher, still more preferably 170°C or higher, and even more preferably 180°C or higher. (Method) The amine compound (A) is heated from 23°C to 350°C at a heating rate of 10°C / min, and the temperature at which the endothermic amount associated with the volatilization of the amine compound (A) becomes maximum is measured, and this temperature is defined as the maximum endothermic temperature of the amine compound (A).

[0019] From the perspective that the amine compound (A) is liquid at 25°C and from the perspective of improving carbon dioxide absorbability, it is preferably a low-molecular compound. Specifically, the molecular weight of the amine compound (A) is preferably less than 1,000, more preferably 800 or less, still more preferably 600 or less, even more preferably 500 or less, and even more preferably 300 or less. Also, from the perspective of improving the repeatability of use of the carbon dioxide absorbent, the molecular weight of the amine compound (A) is preferably 70 or more, more preferably 90 or more, and still more preferably 100 or more.

[0020] From the perspective of improving carbon dioxide absorbability and from the perspective of improving the repeatability of use of the carbon dioxide absorbent, the amine compound (A) preferably has a ring structure. Since the amine compound having a ring structure is less likely to undergo an oxidation reaction, even when heat treatment is performed to desorb carbon dioxide, it is difficult for an oxidation reaction or weight loss to occur. Further, since the amine compound having a ring structure is less likely to absorb moisture, the amount of energy required to volatilize water, which is necessary when desorbing carbon dioxide, is small, and it is considered that carbon dioxide is easily desorbed.

[0021] The amine compound (A) preferably has only one of the above ring structures. That is, the amine compound (A) is preferably a monocyclic compound.

[0022] From the perspective of improving carbon dioxide absorbability and from the perspective of availability, the above ring structure is preferably a 5-membered ring or a 6-membered ring, and more preferably a 6-membered ring.

[0023] Examples of the above ring structure include an alicyclic hydrocarbon structure, an aromatic hydrocarbon structure, and a heterocyclic structure. These ring structures may have a substituent, and examples of the substituent include an alkyl group having 1 to 8 carbon atoms which may have a hydroxy group or a cyano group. In this specification, the alicyclic hydrocarbon structure refers to a saturated or unsaturated ring structure composed of carbon and hydrogen that does not have aromaticity. Specific examples of the alicyclic hydrocarbon structure include a cyclopropane ring, a cyclobutane ring, a cyclopentane ring, a cyclohexane ring, a cycloheptane ring, a cyclooctane ring, a norbornane ring, an adamantane ring, and the like. Among the above alicyclic hydrocarbon structures, it is preferably at least one selected from the group consisting of a cyclopentane ring and a cyclohexane ring, and more preferably a cyclohexane ring.

[0024] In this specification, the aromatic hydrocarbon structure refers to a ring structure composed of carbon and hydrogen having aromaticity. Specific examples of the aromatic hydrocarbon structure include a benzene ring, a naphthalene ring, an anthracene ring, and the like, and preferably a benzene ring.

[0025] In this specification, the heterocyclic structure refers to a ring structure containing a hetero atom in the ring. Examples of the heterocyclic structure include a heterocyclic structure containing a nitrogen atom and an oxygen atom as hetero atoms in the ring structure, a heterocyclic structure containing only a nitrogen atom as a hetero atom in the ring structure, a heterocyclic structure containing only an oxygen atom as a hetero atom in the ring structure, a heterocyclic structure containing a nitrogen atom and a sulfur atom as hetero atoms in the ring structure, a heterocyclic structure containing only a sulfur atom as a hetero atom in the ring structure, and the like.

[0026] From the viewpoints of improving carbon dioxide absorbability and economy, the ring structure is preferably at least one selected from the group consisting of an alicyclic hydrocarbon structure and an aromatic hydrocarbon structure, more preferably at least one selected from the group consisting of a cyclohexane ring and a benzene ring, and still more preferably contains a benzene ring.

[0027] When both cis and trans isomers can be taken as in the case of a cyclohexane ring, the ring structure may be any of a cis isomer, a trans isomer, and a mixture of a cis isomer and a trans isomer.

[0028] As the amine compound (A), from the viewpoint of improving carbon dioxide absorbability and improving the recyclability of the carbon dioxide absorbent, more preferably, it is a compound represented by the following general formula (A-1), which is a liquid at 25°C and whose carbonate is a solid at 25°C. H 2 N-(CH 2 ) m -X-(CH 2 ) n -NH 2 (A-1) (In the formula, X is a ring structure which may have a substituent, m is a number from 0 to 8, and n is a number from 1 to 8.) X is preferably a benzene ring, a cyclohexane ring, or a norbornane ring which may have a substituent, more preferably a benzene ring or a cyclohexane ring which may have a substituent, and still more preferably a benzene ring. The substituent is an alkyl group having 1 to 8 carbon atoms which may have a hydroxy group or a cyano group, preferably an alkyl group having 1 to 4 carbon atoms, and more preferably a methyl group. m is preferably from 0 to 4, more preferably from 0 to 2, and still more preferably 1, and n is preferably from 1 to 4, more preferably from 1 to 2, and still more preferably 1.

[0029] Preferable specific examples of the amine compound (A) include at least one selected from the group consisting of metaxylylenediamine, isophoronediamine, norbornanediamine, 1,3-bis(aminomethyl)cyclohexane, and 1,4-bis(aminomethyl)cyclohexane. From the viewpoint of improving carbon dioxide absorbability, it preferably contains at least one selected from the group consisting of metaxylylenediamine, isophoronediamine, 1,3-bis(aminomethyl)cyclohexane, and 1,4-bis(aminomethyl)cyclohexane, more preferably contains at least one selected from the group consisting of metaxylylenediamine, isophoronediamine, and 1,3-bis(aminomethyl)cyclohexane, and still more preferably contains at least one selected from the group consisting of metaxylylenediamine and 1,3-bis(aminomethyl)cyclohexane.

[0030] <Ionic liquid (B)> The carbon dioxide absorbent of the present invention contains an ionic liquid (B) composed of a cation represented by the following general formula (1) and an anion containing at least one selected from the group consisting of a carboxylate anion, a sulfonate anion, and a phosphate anion.

Chemical formula

[0031] In the present specification, the ionic liquid refers to a liquid composed of an anion and a cation having a melting point of 35°C or lower under atmospheric pressure. The melting point is determined from the heat quantity change with respect to the reference during DSC measurement under atmospheric pressure at a temperature rising rate of 10°C / min in the range of -150°C to 100°C using a differential scanning calorimeter. From the viewpoint of improving the carbon dioxide absorbability of the carbon dioxide absorbent, the melting point of the ionic liquid (B) is preferably 25°C or lower, more preferably 20°C or lower.

[0032] In the general formula (1), examples of the aliphatic hydrocarbon group for R 1 and R 2 include an alkyl group having 1 to 18 carbon atoms and an alkenyl group having 2 to 18 carbon atoms. Among these, the aliphatic hydrocarbon groups for R 1 and R 2 are preferably an alkyl group having 1 to 18 carbon atoms, more preferably an alkyl group having 1 to 8 carbon atoms, still more preferably an alkyl group having 1 to 4 carbon atoms, even more preferably an alkyl group having 1 to 3 carbon atoms, and even more preferably a methyl group or an ethyl group. R 1 and R 2 The aliphatic hydrocarbon groups in may be the same as or different from each other. In the general formula (1), from the viewpoint of improving the solubility of the carbonate of the amine compound (A) and from the viewpoint of electrochemical stability, R 1 is preferably an ethyl group, a propyl group, or a butyl group, more preferably an ethyl group, and R 2 is preferably a hydrogen atom or a methyl group, more preferably a methyl group.

[0033] Specific examples of the cation represented by the general formula (1) include at least one selected from the group consisting of 1,3-dimethylimidazolium, 1-methyl-3-propylimidazolium, 1-methyl-3-isopropylimidazolium, 1-ethyl-3-methylimidazolium, 1,3-diethylimidazolium, 1-ethyl-3-propylimidazolium, 1-ethyl-3-isopropylimidazolium, 1-propyl-3-isopropylimidazolium, 1,3-dipropylimidazolium, 1-butyl-3-methylimidazolium, 1-butyl-3-ethylimidazolium, 1-butyl-3-propylimidazolium, 1-butyl-3-isopropylimidazolium, and 1,3-dibutylimidazolium. Among these, from the viewpoint of improving the solubility of the carbonate of the amine compound (A) and from the viewpoint of electrochemical stability, the cation represented by the general formula (1) preferably contains at least one selected from the group consisting of 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium, and more preferably contains 1-ethyl-3-methylimidazolium.

[0034] The anion constituting the ionic liquid (B) contains at least one selected from the group consisting of a carboxylic acid anion, a sulfonic acid anion, and a phosphoric acid anion. More specifically, the anion constituting the ionic liquid (B) includes an anion obtained by removing at least one proton from the acid compounds exemplified below.

[0035] As the carboxylic acid in the carboxylic acid anion, any of monocarboxylic acid, polycarboxylic acid, and hydroxycarboxylic acid can be used. From the viewpoint of improving the solubility of the carbonate of the amine compound (A), the carboxylic acid is preferably a low-molecular compound. Specifically, the molecular weight of the carboxylic acid is preferably less than 1,000, more preferably 800 or less, still more preferably 600 or less, even more preferably 500 or less, even more preferably 300 or less, and even more preferably 200 or less. Also, from the viewpoint of improving the recyclability of the carbon dioxide absorbent, the molecular weight of the carboxylic acid is preferably 45 or more, more preferably 55 or more. Also, from the viewpoints of improving the solubility of the carbonate of the amine compound (A) and improving the recyclability of the carbon dioxide absorbent, the number of carbon atoms of the carboxylic acid is preferably 1 to 24, more preferably 2 to 18, still more preferably 2 to 8, and even more preferably 2 to 6.

[0036] Examples of the monocarboxylic acid include saturated or unsaturated aliphatic monocarboxylic acids and aromatic monocarboxylic acids that do not have a hydroxy group and in which some hydrogen atoms may be halogenated. Examples of the saturated aliphatic monocarboxylic acid include formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, lauric acid, myristic acid, stearic acid, behenic acid, trifluoroacetic acid, and the like. Examples of the unsaturated aliphatic monocarboxylic acid include acrylic acid, methacrylic acid, oleic acid, linoleic acid, and the like. Examples of the aromatic monocarboxylic acid include benzoic acid, cinnamic acid, naphthoic acid, and the like.

[0037] Examples of the polycarboxylic acid include 2- to 4-valent polycarboxylic acids that do not have a hydroxy group and in which some hydrogen atoms may be halogenated. Examples of the saturated aliphatic polycarboxylic acid include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and the like. Examples of the unsaturated aliphatic polycarboxylic acid include maleic acid, fumaric acid, itaconic acid, and the like. Examples of the aromatic polycarboxylic acid include phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, pyromellitic acid, and the like.

[0038] Examples of the hydroxycarboxylic acid include aliphatic hydroxycarboxylic acids such as glycolic acid, lactic acid, and tartaric acid; and aromatic hydroxycarboxylic acids such as salicylic acid and mandelic acid.

[0039] From the viewpoint of improving the solubility of the carbonate of the amine compound (A), the carboxylic acid in the carboxylate anion is preferably an aliphatic monocarboxylic acid having no hydroxy group and in which some hydrogen atoms may be halogenated, more preferably a saturated aliphatic monocarboxylic acid having no hydroxy group and in which some hydrogen atoms may be halogenated, still more preferably containing at least one selected from the group consisting of acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, and trifluoroacetic acid, and even more preferably containing at least one selected from the group consisting of acetic acid and trifluoroacetic acid.

[0040] Examples of the sulfonic acid in the sulfonate anion include organic sulfonic acids represented by RSO 3 H, and alkylbenzene sulfonic acids represented by RC 6 H 4 SO 3 H. In RSO 3 H and RC 6 H 4 SO 3 H, R is preferably an alkyl group having 1 to 18 carbon atoms, more preferably an alkyl group having 1 to 12 carbon atoms. Specific examples of the organic sulfonic acid (RSO 3 H) include methanesulfonic acid, ethanesulfonic acid, propanesulfonic acid, butanesulfonic acid, and the like. Specific examples of the alkylbenzene sulfonic acid (RC 6 H 4 SO 3 H) include p-toluenesulfonic acid, dodecylbenzenesulfonic acid, and the like.

[0041] As the phosphoric acid in the phosphate anion, in addition to phosphoric acid, alkyl phosphate esters represented by ROPO 3 H 2 and alkyl ether phosphate esters represented by RO(CH 2 CH 2 O) n PO 3 H 2 etc. can be mentioned. In ROPO 3 H 2 and RO(CH 2 CH 2 O) n PO 3 H 2 , R is preferably an alkyl group having 1 to 18 carbon atoms, more preferably an alkyl group having 1 to 12 carbon atoms.

[0042] From the viewpoint of improving the solubility of the carbonate of the amine compound (A), the anion constituting the ionic liquid (B) preferably contains at least one selected from the group consisting of carboxylic acid anions and sulfonic acid anions, more preferably an anion obtained by removing one proton from an aliphatic monocarboxylic acid that does not have a hydroxy group and some of whose hydrogen atoms may be halogenated, and an anion obtained by removing one proton from an organic sulfonic acid represented by RSO 3 H (where R is preferably an alkyl group having 1 to 18 carbon atoms, more preferably an alkyl group having 1 to 12 carbon atoms), more preferably an anion obtained by removing one proton from a saturated aliphatic monocarboxylic acid that does not have a hydroxy group and some of whose hydrogen atoms may be halogenated, and an anion obtained by removing one proton from methanesulfonic acid, even more preferably at least one selected from the group consisting of acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, trifluoroacetic acid, and methanesulfonic acid, and even more preferably an anion obtained by removing one proton from at least one selected from the group consisting of acetate ion (CH 3 COO - ), trifluoroacetate ion (CF 3 COO - ), and methanesulfonate ion (CH 3 SO 3- It contains at least one selected from the group consisting of

[0043] The ionic liquid (B) more preferably contains at least one selected from the group consisting of ionic liquids represented by the following structural formulas (B-1) to (B-4), still more preferably contains at least one selected from the group consisting of ionic liquids represented by the following structural formulas (B-1), (B-3), and (B-4), and still more preferably contains 1-ethyl-3-methylimidazolium acetate represented by the following structural formula (B-1).

Chemical formula

[0044] <Content> From the viewpoint of improving carbon dioxide absorbability, the ratio of the content of the amine compound (A) to the total amount of the amine compound (A) and the ionic liquid (B) in the carbon dioxide absorbent is preferably 0.02 to 0.80 as the mass ratio [(A) / {(A)+(B)}], more preferably 0.04 to 0.50, still more preferably 0.05 to 0.40, still more preferably 0.06 to 0.30, and still more preferably 0.08 to 0.20. When the mass ratio [(A) / {(A)+(B)}] is 0.02 or more, the carbon dioxide absorbability derived from the amine compound (A) is improved, and when it is 0.80 or less, the solubility of the carbonate of the amine compound (A) in the ionic liquid (B) is good, so the carbon dioxide absorbability is improved.

[0045] From the viewpoint of improving carbon dioxide absorbability, the content of the amine compound (A) in the carbon dioxide absorbent is preferably 1% by mass or more, more preferably 3% by mass or more, still more preferably 5% by mass or more, still more preferably 8% by mass or more, still more preferably 10% by mass or more, and preferably 90% by mass or less, more preferably 80% by mass or less, still more preferably 60% by mass or less, still more preferably 50% by mass or less, and still more preferably 30% by mass or less.

[0046] Further, from the viewpoint of improving carbon dioxide absorption performance, the total amount of the amine compound (A) and the ionic liquid (B) in the carbon dioxide absorbent is preferably 50% by mass or more, more preferably 60% by mass or more, still more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, even more preferably 98% by mass or more, and 100% by mass or less.

[0047] <Other components> The carbon dioxide absorbent of the present invention can appropriately contain components other than the amine compound (A) and the ionic liquid (B) as long as the effects of the invention are not impaired. Examples of such components include amine compounds other than component (A), deterioration inhibitors, antifoaming agents, antioxidants, desiccants (such as magnesium sulfate and molecular sieves) for removing moisture, and the like.

[0048] In addition, from the viewpoint of further improving carbon dioxide absorption performance, the content of the amine compound (A) in all the amine compounds contained in the carbon dioxide absorbent is preferably 50% by mass or more, more preferably 60% by mass or more, still more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, even more preferably 98% by mass or more, and 100% by mass or less.

[0049] From the viewpoint of further improving carbon dioxide absorption performance, the water content in the carbon dioxide absorbent is preferably 10% by mass or less, more preferably 5% by mass or less, still more preferably 1% by mass or less, even more preferably 0.5% by mass or less, even more preferably 0.1% by mass or less, even more preferably 0.01% by mass or less, and it is even more preferable that it is substantially free of water. Here, "substantially free of water" means that water is not intentionally added, and it does not exclude the presence of a small amount of water as an impurity.

[0050] <Method for preparing carbon dioxide absorbent> The method for preparing the carbon dioxide absorbent is not particularly limited. For example, it can be prepared by blending an amine compound (A) and an ionic liquid (B) and mixing them using a known apparatus.

[0051] Since the carbon dioxide absorbent of the present invention has good carbon dioxide absorption from the air, it can be suitably used for the technology (DAC) of directly absorbing carbon dioxide in the air. In addition, the carbon dioxide absorbent of the present invention can be suitably used, for example, when recovering low-concentration carbon dioxide of 0.01% by volume or more and 1% by volume or less.

[0052] <Method for recovering carbon dioxide> The method for recovering carbon dioxide in the present invention is characterized by using the above-mentioned carbon dioxide absorbent. According to the method, the amount of carbon dioxide absorbed from the gas containing carbon dioxide can be improved. In addition, carbon dioxide can be recovered with lower energy, and the carbon dioxide absorbent can be repeatedly used well.

[0053] The method for recovering carbon dioxide preferably includes a step (absorption step) of bringing the above-mentioned carbon dioxide absorbent into contact with a gas containing carbon dioxide to absorb carbon dioxide into the carbon dioxide absorbent.

[0054] (Absorption step) The absorption step is a step of bringing the above-mentioned carbon dioxide absorbent into contact with a gas containing carbon dioxide to absorb carbon dioxide into the carbon dioxide absorbent. The method of bringing the carbon dioxide absorbent into contact with the gas can be selected according to the form of the carbon dioxide absorbent and is not particularly limited. For example, the carbon dioxide absorbent and the gas containing carbon dioxide can be brought into contact by passing the gas containing carbon dioxide through the carbon dioxide absorbent, spraying the carbon dioxide absorbent into the gas containing carbon dioxide, or installing the carbon dioxide absorbent in the gas containing carbon dioxide.

[0055] The gas containing carbon dioxide is not particularly limited, and examples thereof include air, flue gas from thermal power plants, exhaust gas from steelworks, exhaust gas from cement plants, exhaust gas from chemical plants, biogas, natural gas, and the like. From these gases, it is particularly required to recover carbon dioxide with energy savings, and the present invention is particularly effective. The concentration of carbon dioxide in the gas, the pressure of the gas, and the temperature of the gas are not particularly limited, and the method of the present invention can be applied to gases under a wide range of conditions. In addition, the gas containing carbon dioxide may contain acidic gases other than carbon dioxide. Examples of the acidic gas include CO, NOx, SOx in exhaust gas, formaldehyde generated in methanol fuel power generation, and other hydrogen chloride, hydrogen sulfide, and the like. When the gas containing carbon dioxide contains acidic gases other than carbon dioxide, it is preferable to combine known processes for removing other acidic gases. Specifically, there are embodiments in which the carbon dioxide recovery method of the present invention is applied to a gas containing acidic gases other than carbon dioxide, or embodiments in which other acidic gases are removed from the gas containing acidic gases other than carbon dioxide by known means and then the carbon dioxide recovery method of the present invention is applied.

[0056] In the absorption step, the temperature at which the carbon dioxide absorbent is brought into contact with the gas containing carbon dioxide is preferably 0°C or higher and less than 60°C, more preferably 20°C or higher and less than 60°C, and even more preferably 30°C or higher and less than 60°C from the viewpoint of improving the carbon dioxide absorption amount.

[0057] More preferably, the carbon dioxide recovery method includes an absorption step of bringing the carbon dioxide absorbent into contact with the gas containing carbon dioxide to absorb carbon dioxide in the carbon dioxide absorbent, and a desorption step of desorbing carbon dioxide from the carbon dioxide absorbent that has absorbed carbon dioxide in the absorption step, and the desorption step includes at least one step selected from the group consisting of the following (I) to (III). By this method, carbon dioxide can be separated and recovered from the gas containing carbon dioxide. (I) A step of subjecting the carbon dioxide absorbent that has absorbed carbon dioxide to reduced pressure conditions (II) Step of bringing an inert gas not containing carbon dioxide into contact with the carbon dioxide absorbent that has absorbed carbon dioxide (III) Step of heating the carbon dioxide absorbent that has absorbed carbon dioxide

[0058] (Desorption step) The desorption step is a step of desorbing carbon dioxide from the carbon dioxide absorbent that has absorbed carbon dioxide in the absorption step. As a method for desorbing carbon dioxide from the carbon dioxide absorbent that has absorbed carbon dioxide, preferably, a method including at least one step selected from the group consisting of the above (I) to (III) is mentioned. The steps of (I) to (III) may be combined in two or more.

[0059] (I) In the step of subjecting the carbon dioxide absorbent that has absorbed carbon dioxide to reduced pressure conditions (hereinafter also referred to as "step (I)"), from the viewpoint of improving the separation and recovery efficiency of carbon dioxide, the reduced pressure conditions are preferably 10 kPa or less, more preferably 5 kPa or less, and still more preferably 1 kPa or less. Also, from the viewpoint of suppressing the volatilization of the amine compound (A) in the carbon dioxide absorbent, it is preferably 0.1 kPa or more. In step (I), the temperature when subjecting the carbon dioxide absorbent to reduced pressure conditions is not particularly limited, but from the viewpoint of suppressing the volatilization of the amine compound (A) in the carbon dioxide absorbent, it is preferably less than 50°C, more preferably 45°C or less. Also, from the viewpoint of improving the separation and recovery efficiency of carbon dioxide, it is preferably 0°C or more, more preferably 10°C or more.

[0060] (II) In the step of bringing an inert gas not containing carbon dioxide into contact with the carbon dioxide absorbent that has absorbed carbon dioxide (hereinafter also referred to as "step (II)"), by reducing the partial pressure of carbon dioxide, the desorption of carbon dioxide can be promoted. Examples of the inert gas not containing carbon dioxide include nitrogen, helium, argon, etc. Among them, from the viewpoint of improving the separation and recovery efficiency of carbon dioxide, the inert gas not containing carbon dioxide is preferably at least one selected from the group consisting of nitrogen and argon. In step (II), as a method for bringing an inert gas containing no carbon dioxide into contact with the carbon dioxide absorbent, the same method as the contact method described in the absorption step can be mentioned. In step (II), the temperature at which an inert gas containing no carbon dioxide is brought into contact with the carbon dioxide absorbent is not particularly limited, and the heating defined in step (III) in step (II) may be carried out simultaneously, or a temperature below room temperature may be used. From the viewpoint of suppressing the volatilization of the amine compound (A) in the carbon dioxide absorbent, the temperature is preferably less than 50°C, more preferably 45°C or less. Also, from the viewpoint of improving the separation and recovery efficiency of carbon dioxide, it is preferably 0°C or higher, more preferably 10°C or higher.

[0061] (III) The heating temperature in the step of heating the carbon dioxide absorbent that has absorbed carbon dioxide (hereinafter also referred to as "step (III)") is preferably 50°C or higher and 120°C or lower, more preferably 55°C or higher and 110°C or lower, and even more preferably 60°C or higher and 100°C or lower, from the viewpoint of improving the separation and recovery efficiency of carbon dioxide. The heating in step (III) can be carried out by a known method using an apparatus equipped with a heating means. Examples of the heating method include heating by steam or a heat medium, hot air heating, electromagnetic wave heating, ultrasonic heating, induction heating, etc.

[0062] The carbon dioxide absorbent and carbon dioxide separated in the desorption step are recovered individually and can be reused.

[0063] The carbon dioxide recovered by the above method can be used for agricultural applications such as enhanced oil recovery methods and plant factories; industrial gas applications such as beverages and welding; chemical synthesis applications; carbon dioxide storage (CCS) applications. Also, the recovered carbon dioxide may be concentrated before being used for these applications.

Examples

[0064] Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to the scope of the examples. In these examples, various measurements and evaluations were carried out by the following methods.

[0065] (Total amine value of the amine compound) The total amine value was measured by the following method in accordance with JIS K7237-1995. (1) 0.1 g of the amine compound was dissolved in 20 mL of acetic acid. (2) The solution obtained in (1) above was titrated with a 0.1 N perchloric acid - acetic acid solution using an automatic potentiometric titrator ("AT-610" manufactured by Kyoto Electronics Industry Co., Ltd.) to determine the total amine value.

[0066] (Maximum dissociation temperature of carbon dioxide (CO 2 ) in the amine compound) A carbon dioxide concentration meter and a petri dish were placed in an openable desiccator (inner dimensions: 370 mm × 260 mm × 272 mm). The amine compound (5 mmol) was added to the petri dish in the desiccator, the lid was immediately closed, and the amine compound in the desiccator was allowed to stand for 24 hours in an air environment of 23°C and 50% RH. The initial concentration of carbon dioxide was adjusted to approximately 400 ppm. Next, the amine compound was taken out of the desiccator to obtain an amine compound that had absorbed carbon dioxide. The DSC measurement was performed on the amine compound that had absorbed carbon dioxide as follows to measure the maximum dissociation temperature of carbon dioxide in the amine compound. First, differential scanning calorimetry was performed on the amine compound under the conditions of a measurement temperature range of 23 to 250°C, a heating rate of 10°C / min, and a nitrogen atmosphere using a differential thermogravimetric analyzer ("DTG-60" manufactured by Shimadzu Corporation). From the DSC curve obtained thereby, the temperature at which the endothermic amount associated with the desorption of carbon dioxide became maximum was specified, and that temperature was defined as the maximum dissociation temperature of carbon dioxide in the amine compound.

[0067] (Maximum endothermic temperature of the amine compound) DSC measurement was performed on the amine compound as follows to measure the maximum endothermic temperature of the amine compound. First, differential scanning calorimetry was performed on the amine compound using a differential thermal gravimetric analyzer ("DTG-60" manufactured by Shimadzu Corporation) under the conditions of a measurement temperature range of 23 to 350°C, a heating rate of 10°C / min, and a nitrogen atmosphere. From the DSC curve obtained thereby, the temperature at which the endothermic amount associated with the volatilization of the amine compound becomes maximum was specified, and this temperature was defined as the maximum endothermic temperature of the amine compound.

[0068] (Carbon dioxide absorbability of the carbon dioxide absorbent) In an air environment of 23°C and 50% RH, a carbon dioxide concentration meter and a petri dish were placed in an openable and closable desiccator (inner dimensions: 370 mm × 260 mm × 272 mm). Then, the carbon dioxide absorbent was added to the petri dish in the desiccator in an amount such that the number of amino groups in the amine compound contained in the carbon dioxide absorbent was 10 mmol (in this example, 5 mmol as the amine compound), the door was immediately closed, and the carbon dioxide absorbent was allowed to stand in the desiccator. The initial carbon dioxide concentration in the desiccator was adjusted to 390 to 600 ppm. Immediately after allowing the carbon dioxide absorbent to stand, the carbon dioxide concentration (ppm) in the desiccator was measured for 24 hours at 1-minute intervals using a CO 2 concentration meter ("CO 2 ·Temperature·Humidity Data Logger TR-76Ui" manufactured by Tian Di Co., Ltd.). The amount of carbon dioxide reduction (ppm) was determined from the difference between the initial carbon dioxide concentration and the carbon dioxide concentration after 24 hours, and the carbon dioxide absorbability was evaluated. The larger the amount of carbon dioxide reduction, the higher the carbon dioxide absorbability of the carbon dioxide absorbent.

[0069] (Appearance of the carbon dioxide absorbent after carbon dioxide absorption) In the same manner as described above, the carbon dioxide absorbent was allowed to stand in a desiccator in which the initial carbon dioxide concentration was adjusted to 390 to 600 ppm, and after 24 hours (after carbon dioxide absorption), the appearance at 25°C was visually observed. In Table 1, those that were uniform liquids without precipitation of solids were denoted as "liquid", those in which liquid and solid coexisted were denoted as "liquid + solid", and those that were completely solidified were denoted as "solid".

[0070] Examples 1 to 3 (Preparation and Evaluation of Carbon Dioxide Absorbent) An amine compound and an ionic liquid shown in Table 1 were mixed to prepare a carbon dioxide absorbent. Using the obtained carbon dioxide absorbent, carbon dioxide absorbability and appearance evaluation were performed by the above method. The results are shown in Table 1.

[0071] Comparative Examples 1 to 3 Evaluation was performed in the same manner as in the examples, except that the components of the carbon dioxide absorbent in Example 1 were changed to those described in Table 1. The results are shown in Table 1.

[0072]

Table 1

[0073] In the examples and comparative examples described in Table 1, the following were used as the amine compound and the ionic liquid. (Amine Compound) ·MXDA: Meta-xylylenediamine, manufactured by Mitsubishi Gas Chemical Company, Inc., melting point 14°C, total amine value 824 mgKOH / g, maximum carbon dioxide dissociation temperature 135.5°C, maximum endothermic temperature 183.5°C (Ionic Liquid) ·1-Ethyl-3-methylimidazolium acetate: An ionic liquid composed of a cation and an anion having the following structure, manufactured by Sanyo Chemical Industries, Ltd.

Chemical Formula

Chemical Formula

Chemical Formula

Chemical formula

[0074] From Table 1, it can be seen that the carbon dioxide absorbent of this example is a liquid at room temperature (25°C) and exhibits excellent carbon dioxide absorption compared to the case of the amine compound (A) alone (Comparative Example 1). As shown in Comparative Example 2, the ionic liquid (B) alone does not show carbon dioxide absorption. Also, as shown in Comparative Example 3, it can be seen that the effects of the present invention cannot be obtained even when using an ionic liquid that does not correspond to component (B).

Industrial applicability

[0075] According to the present invention, even when using an amine compound that is a liquid at room temperature (25°C) and has the property that the carbonate after carbon dioxide absorption becomes a solid, it is possible to provide a carbon dioxide absorbent that suppresses the decrease in carbon dioxide absorption due to the formation of the carbonate and exhibits excellent carbon dioxide absorption.

Claims

1. A carbon dioxide absorbent containing an amine compound (A) and an ionic liquid (B), wherein the amine compound (A) is a compound that is liquid at 25°C and the carbonate of the amine compound (A) is solid at 25°C, and the ionic liquid (B) consists of a cation represented by the following general formula (1) and an anion containing at least one selected from the group consisting of a carboxylate anion, a sulfonate anion, and a phosphate anion. A carbon dioxide absorbent. 【Chemical Formula 1】 (In formula (1), R １ is an aliphatic hydrocarbon group having 1 to 18 carbon atoms, and R ２ is a hydrogen atom or an aliphatic hydrocarbon group having 1 to 18 carbon atoms.)

2. The carbon dioxide absorbent according to Claim 1, wherein the amine compound (A) has a ring structure.

3. The carbon dioxide absorbent according to Claim 2, wherein the amine compound (A) contains at least one selected from the group consisting of metaxylylenediamine, isophoronediamine, 1,3-bis(aminomethyl)cyclohexane, and 1,4-bis(aminomethyl)cyclohexane.

4. The carbon dioxide absorbent according to any one of Claims 1 to 3, wherein the ratio of the content of the amine compound (A) to the total amount of the amine compound (A) and the ionic liquid (B) in the carbon dioxide absorbent is 0.02 to 0.80 as a mass ratio [(A) / {(A)+(B)}].

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