Ionic liquid composition for carbon dioxide separation membrane, carbon dioxide separation membrane retaining said composition, and carbon dioxide concentrator equipped with said carbon dioxide separation membrane

Abstract

The purpose of the present invention is to provide: an ionic liquid composition for a carbon dioxide separation membrane, which can be used to separate carbon dioxide having a high to low partial pressure, especially a partial pressure of 1 kPa or less; a carbon dioxide separation membrane holding said composition; and a carbon dioxide concentration apparatus provided with said carbon dioxide separation membrane. Provided is an ionic liquid composition obtained by combining: an ionic liquid (I) which is an aminium having one or more primary or secondary amino groups in a cation and having an ethylenediamine or propylenediamine skeleton; and an ionic liquid (II) which does not have a primary or secondary amino groups in a cation and in which an anion is an oxo acid anion. By using the ionic liquid composition it is possible to improve the CO2 permeability and CO2 selectivity of the carbon dioxide separation membrane and selectively separate and recover carbon dioxide having a high partial pressure to a low partial pressure of 1 kPa or less.

Description

[Technical Field]

[0001] The present invention relates to an ionic liquid composition for use in a carbon dioxide separation membrane, a carbon dioxide separation membrane holding the composition, and a carbon dioxide concentrating device equipped with the carbon dioxide separation membrane. [Background technology]

[0002] Technology to separate and capture carbon dioxide is necessary for the production of hydrogen and methane from natural gas, and for maintaining closed living environments such as outer space and underwater.From the perspective of reducing greenhouse gas emissions, research is being actively conducted on carbon dioxide separation and capture technology, targeting large emission sources such as thermal power plants and steelworks, and also for the application of carbon dioxide as fertilizer in the agricultural sector. Representative carbon dioxide gas separation technologies include: (1) chemical absorption, which separates carbon dioxide from a mixed gas containing carbon dioxide by selectively absorbing it into a solvent through a chemical reaction, and then recovers it by stripping it from the solvent through a reverse reaction; (2) physical absorption, which physically separates carbon dioxide from a mixed gas containing carbon dioxide by selectively absorbing it into a solvent without using a chemical reaction, and then strips it from the solvent by reducing the pressure to recover it; and (3) membrane separation, which utilizes the partial pressure difference of carbon dioxide across a membrane to supply a gas containing carbon dioxide to be separated from one side of the membrane and selectively transport and separate the carbon dioxide in the gas to be separated to the other side of the membrane.

[0003] Ionic liquids are generally liquid salts composed only of cations and anions, with melting points below room temperature. Ionic liquids have the following characteristics: they are liquid over a wide temperature range, have very low vapor pressure (non-volatile), are flame-retardant, have excellent heat resistance, chemical stability, a wide potential window, high ionic conductivity, and can dissolve a wide variety of chemical species. Therefore, ionic liquids have been the subject of extensive research, including as electrolytes for electrochemical devices, solvents in various processes such as separation and refinement and organic reactions, and functional materials for actuators and lubricants.

[0004] In carbon dioxide separation and recovery technology, ionic liquids have these properties, particularly the non-volatility and low specific heat required for carbon dioxide absorbents, and so their use as carbon dioxide absorbents is being considered. For example, in the chemical absorption method, it has been proposed to use a chemical absorption ionic liquid, such as an ionic liquid having an amino group in the cation, as an absorption liquid (Patent Documents 1, 2, and 3), or to use an ionic liquid as a solvent in a non-aqueous absorption liquid containing an amine compound (Patent Document 4, Non-Patent Document 1). In addition, in the physical absorption method, it has been proposed to use, as the absorption liquid, an ionic liquid with excellent physical absorption properties, such as an ionic liquid using an imidazolium-based cation that does not have an amino group in the cation (Patent Documents 5 to 8).

[0005] On the other hand, since membrane separation methods separate gases by utilizing differences in solubility in and diffusibility through a membrane, it is necessary to improve both the permeability and selectivity of carbon dioxide in order to improve the separation efficiency of carbon dioxide. However, with polymer membranes that have been used as conventional carbon dioxide separation membranes, it is known that increasing the selectivity results in a decrease in permeability, and there has been a limit to the improvement in the permeability and selectivity of carbon dioxide using polymer membranes. To overcome this limit, it is considered effective to use a substance called a carrier, which selectively and reversibly reacts with and transports specific components in the gas to be separated.

[0006] When a liquid membrane containing the carrier is used as a carbon dioxide separation membrane, selective transport based on the chemical reaction between carbon dioxide and the carrier is utilized, which allows for higher selectivity of carbon dioxide separation than conventional polymer membranes, and the permeation rate of carbon dioxide through the membrane is often also fast. However, when a membrane is used with a volatile ordinary solvent, the gas separation function of the membrane is deactivated due to the volatilization loss of the solvent, and therefore it cannot be used for a long period of time. Therefore, it has been proposed to use a liquid membrane containing a nonvolatile ionic liquid as the carrier, instead of a volatile solvent.

[0007] For example, a liquid membrane containing an ionic liquid having an amino group in the cation has been proposed, and the above-mentioned Patent Document 1 describes the use of a porous liquid membrane impregnated with an absorption liquid whose main component is an ionic liquid having a primary amino group in the cation as a carbon dioxide separation membrane. Furthermore, Patent Document 9 discloses a liquid membrane in which an ionic liquid is held in the pores of a porous membrane for the purpose of separating and recovering carbon dioxide, and the ionic liquid is composed of a cation having an amino group, such as 1-(3-aminopropyl)-3-methylimidazolium.

[0008] Carbon dioxide separation membranes using ionic liquids containing imidazolium cations such as 1,3-dialkylimidazolium have also been proposed (Patent Documents 10 and 11, Non-Patent Documents 2 and 3).

[0009] Furthermore, Patent Document 12 proposes a separation membrane suitable for carbon dioxide fertilization in the agricultural field, which includes a porous layer with affinity for an ionic liquid, in which an ionic liquid-containing liquid is held in the pores. The document lists, as the ionic liquid to be used in the separation membrane, an ionic liquid containing a cation selected from ammoniums, imidazoliums, and phosphoniums, and an anion selected from fluorine-containing anions, cyano-group-containing anions, and anions derived from amino acids, and among these, [P 4444 [Pro], etc., with the chemical formula [R3R'P] + A combination of a phosphonium represented by the formula (wherein R is an alkyl group having 2 to 6 carbon atoms, and R' is an alkyl group having 4 to 16 carbon atoms) and an anion derived from proline is considered to be preferred. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-36950 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-55785 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-10760 [Patent Document 4] Japanese Patent Application Laid-Open No. 2017-104775 [Patent Document 5] Japanese Patent Application Laid-Open No. 2006-305544 [Patent Document 6] Japanese Patent Application Laid-Open No. 2009-106909 [Patent Document 7] Special Publication No. 2011-510811 [Patent Document 8] Japanese Patent Application Laid-Open No. 2016-77935 [Patent Document 9] US Patent Publication No. 2014 / 0283839 [Patent Document 10] Japanese Patent Application Laid-Open No. 2010-214324 [Patent Document 11] International Publication No. 2013 / 118776 [Patent Document 12] International Publication No. 2018 / 211945 [Non-patent literature]

[0011] [Non-Patent Document 1] Mitsuo Kanakubo et al., "Solvent Effect on CO2 Absorption of Amine Compounds in Non-Aqueous Solvents," 40th Solution Chemistry Symposium, October 18, 2017 [Non-patent document 2] Takashi Makino,et.,al.International Journal of Membrane Science and Technology,2015,2,14-20 [Non-patent document 3] Kenta Fuji,et.al.,Chemistry Letters,2015, 44, 17-19 Summary of the Invention [Problem to be solved by the invention]

[0012] In order to effectively utilize unused carbon dioxide as a carbon source, technology for highly efficient separation and capture of carbon dioxide is required. Traditionally, development of technology for separating and capturing high-concentration (high partial pressure) carbon dioxide has been the main focus, but in recent years, in anticipation of separation and capture from various carbon dioxide emission sources, there is also a need to develop technology for separating and capturing low-concentration (low partial pressure) carbon dioxide, such as that found in the atmosphere. Therefore, membrane separation using a carrier, which has the potential to achieve high carbon dioxide permeability selectivity as mentioned above, is expected to be one of the promising separation and recovery technologies.

[0013] For example, Patent Document 11 discloses that a carbon dioxide permeable membrane comprising an amino acid ionic liquid and a porous membrane impregnated with the amino acid ionic liquid, in which the amino acid liquid contains 3 to 50 mass % of water, maintains high carbon dioxide permeability and carbon dioxide / nitrogen selectivity even when the carbon dioxide partial pressure is low, and also discloses that the carbon dioxide partial pressure in the mixed gas may be 15 kPa. 4444 The graph shows the results of measurements performed using [Gly] or [emim][Gly] while varying the partial pressure in the range of 2 to 30 kPa. However, no carbon dioxide separation membrane has yet been found that can efficiently separate carbon dioxide at partial pressures as low as 1 kPa or less.

[0014] The present invention has been made in view of the above circumstances, and aims to provide an ionic liquid composition for a carbon dioxide separation membrane that can be used to separate carbon dioxide at a range of partial pressures from high to low, particularly at 1 kPa or less; a carbon dioxide separation membrane that retains the composition in its pores; and a carbon dioxide concentrating device that includes the carbon dioxide separation membrane. [Means for solving the problem]

[0015] In order to efficiently separate and capture carbon dioxide from the atmosphere using a liquid membrane containing an ionic liquid, it is necessary to improve the carbon dioxide permeability as well as the selectivity of carbon dioxide (CO2) over nitrogen (N2) (hereinafter referred to as "CO2 selectivity") under low carbon dioxide partial pressure conditions. In membrane separation, gas permeability is expressed as the product of the gas solubility and the gas diffusion rate. To improve the CO2 permeability and CO2 selectivity in membrane solutions containing ionic liquids as carriers, it is important to increase the solubility of carbon dioxide in the ionic liquid carrier, suppress the solubility of nitrogen, and further increase the diffusion rate of the carrier that has chemically reacted with carbon dioxide.

[0016] Therefore, the inventors attempted to use an ionic liquid using an aminium having one or more primary or secondary amino groups and an ethylenediamine or propylenediamine skeleton, which is described in Patent Document 3 as an ionic liquid having excellent chemical absorption properties, and an ionic liquid used as a diluent described in the document, but found that it was not possible to improve the carbon dioxide permeation selectivity (see Comparative Examples 1 to 8 described below). As a result of further investigation, the researchers found that by using an ionic liquid composition that combines ionic liquid (I), which is an aminium having one or more primary or secondary amino groups in the cation and an ethylenediamine or propylenediamine skeleton, with ionic liquid (II), which has an oxoacid anion in the cation without a primary or secondary amino group, the CO2 permeability and CO2 selectivity of a carbon dioxide separation membrane can be improved, and that even low partial pressure carbon dioxide can be selectively separated and recovered.

[0017] The present invention has been completed based on the above findings, and in order to solve the above problems, the present invention employs the following means. [1] An ionic liquid composition for use in a carbon dioxide separation membrane, Contains an ionic liquid (I) and an ionic liquid (II), The ionic liquid (I) is an aminium having a cation having one or more primary or secondary amino groups and an ethylenediamine or propylenediamine skeleton, The ionic liquid (II) is an ionic liquid composition for use in a carbon dioxide separation membrane, characterized in that the cation does not have a primary or secondary amino group and the anion is an oxo acid anion. [2] The ionic liquid composition for a carbon dioxide separation membrane according to [1], wherein the aminium is one or more selected from 2-aminoethylaminium, 2-(N-hydroxyethylamino)ethylaminium, 3-aminopropylaminium, 3-(N-methylamino)propylaminium, 2-(2-(aminoethyl)amino)ethylaminium, and 2-(2-(2-(aminoethyl)aminoethyl)amino)ethylaminium. [3] The ionic liquid composition for a carbon dioxide separation membrane according to [1] or [2], wherein the anion of the ionic liquid (I) is bis(trifluoromethylsulfonyl)amide. [4] The ionic liquid composition for a carbon dioxide separation membrane according to any one of [1] to [3], wherein the oxoacid anion is one or more selected from the group consisting of carboxylate, phosphate, and phosphonate. [5] The oxoacid anion is acetate, 2-(1-methoxyethyl) Kishi ) propionate, and methylphosphonate. [6] The ionic liquid composition for a carbon dioxide separation membrane according to [4] or [5], wherein the cation of the ionic liquid (II) is one or more selected from the group consisting of 1-ethyl-3-methimidazolium, N,N-diethyl-N-methyl-N-heptylammonium, and N,N-diethyl-N-methyl-N-(6-hydroxyhexyl)ammonium. [7] A carbon dioxide separation membrane characterized by holding the ionic liquid composition for a carbon dioxide separation membrane according to any one of [1] to [6]. [8] The carbon dioxide separation membrane according to [7], characterized in that it comprises an ionic liquid-affinity porous layer that retains the ionic liquid composition for carbon dioxide separation membranes in its pores, and an ionic liquid-non-affinity porous layer. [9] The carbon dioxide separation membrane according to [8], wherein the porous layer with an affinity for an ionic liquid contains an inorganic material.

[10] The carbon dioxide separation membrane according to [9], wherein the inorganic material contains metal oxide particles having an average particle size of 0.001 to 5 μm on a number basis.

[11] The carbon dioxide separation membrane according to any one of [8] to

[10] , wherein the porous layer with an affinity for the ionic liquid has an average thickness of 0.01 to 10 μm.

[12] The carbon dioxide separation membrane according to any one of [7] to

[11] , for separating and concentrating carbon dioxide having a partial pressure of 1 kPa or less.

[13] A carbon dioxide concentrator comprising the carbon dioxide separation membrane according to any one of [7] to

[12] . [Effects of the Invention]

[0018] The ionic liquid composition for a carbon dioxide separation membrane of the present invention can improve CO2 permeability and CO2 selectivity, and can efficiently separate and recover carbon dioxide from high partial pressures to low partial pressures of 1 kPa or less. [Brief explanation of the drawings]

[0019] [Figure 1] A schematic diagram of the device used to measure the CO2 permeability coefficient and N2 permeability coefficient of the CO2 separation membrane. [Figure 2] Graph showing the partial pressure dependence of the CO2 permeability coefficient of a CO2 separation membrane (Comparative Example 1) using [emim][DCA] [Figure 3] Graph showing the partial pressure dependence of the N2 permeability coefficient of a CO2 separation membrane (Comparative Example 1) using [emim][DCA] [Figure 4] Graph showing the partial pressure dependence of CO2 selectivity of a CO2 separation membrane (Comparative Example 1) using [emim][DCA] [Figure 5] Graph showing the partial pressure dependence of the CO2 permeability coefficient of [emim][Tf2N]-based CO2 separation membranes (Comparative Examples 2 to 8) [Figure 6] Graph showing the partial pressure dependence of the N2 permeability coefficient of [emim][Tf2N]-based CO2 separation membranes (Comparative Examples 2 to 8) [Figure 7] Graph showing the partial pressure dependence of CO2 selectivity of [emim][Tf2N]-based CO2 separation membranes (Comparative Examples 2 to 8) [Figure 8] Graph showing the temperature dependence of the CO2 permeability coefficient of [emim][Tf2N]-based CO2 separation membranes (Comparative Examples 2 and 5) [Figure 9] Graph showing the temperature dependence of the N2 permeability coefficient of [emim][Tf2N]-based CO2 separation membranes (Comparative Examples 2 and 5) [Figure 10] Graph showing the temperature dependence of CO2 selectivity of [emim][Tf2N]-based CO2 separation membranes (Comparative Examples 2 and 5) [Figure 11] FIG. 1 shows the composition dependence of the CO2 permeability coefficient of [emim][MeHPO3]-based CO2 separation membranes (Examples 1 to 6, Comparative Examples 8 and 9). [Figure 12] FIG. 1 shows the composition dependence of the N2 permeability coefficient of [emim][MeHPO3]-based CO2 separation membranes (Examples 1 to 6, Comparative Examples 8 and 9). [Figure 13] FIG. 1 shows the composition dependence of CO2 selectivity for [emim][MeHPO3]-based CO2 separation membranes (Examples 1 to 6, Comparative Examples 8 and 9). [Figure 14] FIG. 1 shows the CO2 partial pressure dependence of the CO2 permeability coefficient of [emim][MeHPO3]-based CO2 separation membranes (Examples 1 to 6, Comparative Examples 8 and 9). [Figure 15] FIG. 1 shows the CO2 partial pressure dependence of the N2 permeability coefficient of [emim][MeHPO3]-based CO2 separation membranes (Examples 1 to 6, Comparative Examples 8 and 9). [Figure 16] FIG. 1 shows the CO2 partial pressure dependence of CO2 selectivity for [emim][MeHPO3]-based CO2 separation membranes (Examples 1 to 6, Comparative Examples 8 and 9). [Figure 17] FIG. 1 shows the composition dependence of the CO selectivity and N permeability coefficient of [emim][AcO]-based and [emim][1O2OPrO]-based CO2 separation membranes (Examples 7 to 11, Example 15, Comparative Examples 8, 10, and 11). [Figure 18]FIG. 1 shows the composition dependence of CO selectivity for [emim][AcO]-based and [emim][1O2OPrO]-based CO2 separation membranes (Examples 7 to 11, Example 15, Comparative Examples 8, 10, and 11). [Figure 19] Graph showing the CO2 partial pressure dependence of the CO2 permeability coefficient of [emim][AcO]-based CO2 separation membranes (Examples 8, 12, 13, and Comparative Example 10) [Figure 20] Graph showing the CO2 partial pressure dependence of the N2 permeability coefficient of [emim][AcO]-based CO2 separation membranes (Examples 8, 12, 13, Comparative Example 10) [Figure 21] Graph showing the CO2 partial pressure dependence of CO2 selectivity for [emim][AcO]-based CO2 separation membranes (Examples 8, 12, 13, Comparative Example 10) [Figure 22] 1 shows the composition dependence of the CO2 permeability coefficient of [emim][AcO]-based (amine-modified) CO2 separation membranes (Examples 8, 14, 16-20, Comparative Examples 8, 10, 12). [Figure 23] 1 shows the composition dependence of the N2 permeability coefficient of [emim][AcO]-based (amine-modified) CO2 separation membranes (Examples 8, 14, 16 to 20, Comparative Examples 8, 10, and 12). [Figure 24] [Emim][AcO]-based (amine-modified) CO2 separation membranes (Examples 8, 14, 16-20, Comparative Examples 8, 10, 12) - Figure 1 shows the composition dependence of CO2 selectivity. [Figure 25] 1 shows the composition dependence of the CO2 permeability coefficient of [N1227][AcO]-based and [N1226OH][AcO]-based CO2 separation membranes (Examples 21 and 22, Comparative Examples 8, 13, and 14). [Figure 26] 1 shows the composition dependence of the N2 permeability coefficient of [N1227][AcO]-based and [N1226OH][AcO]-based CO2 separation membranes (Examples 21 and 22, Comparative Examples 8, 13, and 14). [Figure 27] FIG. 1 shows the composition dependence of CO selectivity for [N1227][AcO]-based and [N1226OH][AcO]-based CO separation membranes (Examples 21 and 22, Comparative Examples 8, 13, and 14). DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention provides an ionic liquid composition for use in a carbon dioxide separation membrane, which comprises ionic liquid (I) and ionic liquid (II), wherein the cation of ionic liquid (I) is an aminium having one or more primary or secondary amino groups and an ethylenediamine or propylenediamine skeleton, and the cation of ionic liquid (II) does not have a primary or secondary amino group and the anion is an oxoacid anion.

[0021] The ionic liquid composition for a carbon dioxide separation membrane of the present invention, the carbon dioxide separation membrane holding the composition, and the carbon dioxide concentrating device equipped with the carbon dioxide separation membrane will be described in detail below. However, these are for the purpose of explaining the present invention and do not limit the scope of the present invention. Note that the symbol "~" indicating a range of numerical values includes the numerical values before and after it as the lower and upper limits.

[0022] [Ionic liquid (I)] The ionic liquid (I) in the ionic liquid composition for a carbon dioxide separation membrane of the present invention is an ionic liquid in which the cation is an aminium having one or more primary or secondary amino groups and an ethylenediamine or propylenediamine skeleton.

[0023] (cation) The cation in the ionic liquid (I) of the present invention is an aminium having a primary amino group in which one carbon atom and two hydrogen atoms are bonded to one nitrogen atom, or a secondary amino group in which two carbon atoms and one hydrogen atom are bonded to one nitrogen atom.

[0024] The aminium has at least one ethylenediamine skeleton or propylenediamine skeleton, and when it is an aminium having one ethylenediamine skeleton, it is represented by the following general formula.

[0025] [ka]

[0026] In the formula, R1 and R2 both represent hydrogen atoms, or one represents a hydrogen atom and the other represents a saturated or unsaturated alkyl group which may have a substituent. The alkyl group of R1 or R2 is preferably one having a small number of carbon atoms, such as a methyl group, an ethyl group, or a 2-hydroxyethyl group. R3, R4, and R5 all represent hydrogen atoms, or two of them represent hydrogen atoms and the other represents an optionally substituted saturated or unsaturated alkyl group, or all represent optionally substituted saturated or unsaturated alkyl groups. The alkyl groups may be the same or different, and may be linear or cyclic, branched, or two groups may be bonded to form a ring. Examples of compounds having an ethylenediamine skeleton represented by the above formula 1 include the following 2-aminoethylaminium and 2-(N-hydroxyethylamino)ethylaminium.

[0027] [ka]

[0028] The propylene diamine skeleton is obtained by substituting a propylene group for the ethylene group in the ethylene diamine skeleton described above. Specifically, the ethylene group in the formula 1 described above is substituting a propylene group. The carbon atom of the propylene group in the propylene diamine skeleton may have a substituent such as an alkyl group or heteroalkyl group having a small carbon number. Examples of compounds having a propylenediamine skeleton include the following 3-aminopropylaminium and 3-(N-methylamino)propylaminium.

[0029] [ka]

[0030] The aminium according to the present invention also includes those having two or more ethylenediamine skeletons or propylenediamine skeletons. For example, in the formula 1, when either R1 or R2 is an aminoethyl group, the aminium is diethylenetriaminium, and when either R1 or R2 is a 2-(aminoethyl)aminoethyl group, the aminium is triethylenetetraaminium. Specific examples include the following 2-(2-(aminoethyl)amino)ethylaminium and 2-(2-(2-(aminoethyl)aminoethyl)amino)ethylaminium.

[0031] [ka]

[0032] (anion) The anion in the ionic liquid (I) is not particularly limited, but preferred examples include amides such as bis(trifluoromethylsulfonyl)amide and dicyanamide, sulfonates such as methylsulfonate and trifluoromethylsulfonate, sulfates such as methyl sulfate and ethyl sulfate, halide ions such as fluoride ion, chloride ion, bromide ion and iodide ion, and carboxylates such as trifluoroacetate, and in particular, amide anions such as bis(trifluoromethylsulfonyl)amide (abbreviated as [TfN]) are preferably used.

[0033] Preferable ionic liquids (I) include the following, and among them, [HDAH][Tf2N] is preferably used.

[0034] [ka]

[0035] [Ionic liquid (II)] The ionic liquid (II) in the ionic liquid composition for a carbon dioxide separation membrane of the present invention is an ionic liquid in which the cation does not have a primary or secondary amino group and the anion is an oxo acid anion.

[0036] (oxoacid anion) Examples of oxoacid anions include carboxylates such as acetate, propionate, butanoate, and lactate, phosphates such as dimethyl phosphate, diethyl phosphate, and dibutyl phosphate, phosphonates such as methyl phosphonate, ethyl phosphonate, and butyl phosphonate, sulfates such as methyl sulfate, ethyl sulfate, and octyl sulfate, and sulfonates such as methyl sulfonate and tosylate. Preferably, the oxoacid anion is one or more selected from the following carboxylates, phosphates, and phosphonates.

[0037] [ka]

[0038] In the formula, R1 and R2 represent unsubstituted or saturated or unsaturated alkyl groups substituted with a hydroxyl group or a heteroatom.

[0039] Particularly preferred carboxylates include acetate (abbreviated as [AcO]), 2-(1-methoxyethyl) Kishi ) propionate (abbreviated as [1O2OPrO]), and as a phosphonate, methyl phosphonate (abbreviated as [MeHPO3]), etc. are used.

[0040] (cation) The cation in the ionic liquid (II) of the present invention is not particularly limited as long as it does not have a primary or secondary amino group. Examples thereof include imidazoliums such as 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, and 1-octyl-3-methylimidazolium; N-butyl-N,N,N-trimethylammonium; N,N,N,N,N-tetrabutylammonium; N-(2-hydroxyethyl)-N,N,N-trimethylammonium; N,N-diethyl-N-methyl-N-heptylammonium; and N,N-diethyl-N-methyl-N-(6 ammoniums such as N-methylpyridinium, N-ethylpyridinium, and N-butylpyridinium; pyrrolidiniums such as N,N-dimethylpyrrolidinium, N-methyl-N-ethylpyrrolidinium, and N-methyl-N-butylpyrrolidinium; phosphoniums such as tetrabutylphosphonium, triethyloctylphosphonium, tributyloctylphosphonium, and trihexyltetradecylphosphonium; and particularly, imidazoliums having an alkyl group on the side chain, such as 1,3-dialkylimidazolium, are preferably used. In dialkylimidazoliums having alkyl groups on their side chains, shortening the alkyl groups on the side chains reduces the gas solubility due to physical absorption, thereby improving the selectivity for nitrogen and the like (see Comparative Examples 3 and 4 described below). Therefore, 1-ethyl-3-methylimidazolium (abbreviated as [emim]) is particularly preferably used. Representative examples of the ionic liquid (II) include the following ionic liquids:

[0041] [ka]

[0042] [Ratio of both ionic liquids] The preferable mixing ratio of the two ionic liquids for improving the permeation selectivity of carbon dioxide varies depending on the combination of the two ionic liquids. For example, when the ionic liquid (I) is [HDAH][TfN] and the ionic liquid (II) is [emim][AcO], the permselectivity can be improved when the ionic liquid (I) is in the range of 5 to 40 mol % relative to the total amount of both ionic liquids. Furthermore, when the ionic liquid (I) is [HDAH][TfN] and the ionic liquid (II) is [emim][MeHPO], the permeation selectivity can be improved when the ionic liquid (I) is in the range of 5 to 80 mol% relative to the total amount of both ionic liquids.

[0043] [Carbon dioxide separation membrane] The carbon dioxide separation membrane of the present invention is not particularly limited as long as it can retain the ionic liquid composition for carbon dioxide separation of the present invention (hereinafter simply referred to as "ionic liquid composition"). For example, when the carbon dioxide separation membrane has a porous layer, the ionic liquid composition is retained in the pores of the porous layer by, for example, impregnating the membrane with the ionic liquid composition. Furthermore, when the carbon dioxide separation membrane has a porous layer containing inorganic material particles, the ionic liquid composition is similarly retained in the pores of the porous layer by, for example, impregnating the membrane with the ionic liquid composition.

[0044] A preferred carbon dioxide separation membrane includes an ionic liquid-affinity porous layer (C) that holds an ionic liquid composition (A) in its pores, and an ionic liquid-non-affinity porous layer (B). The porous layer (C) having an affinity for an ionic liquid may contain an inorganic material, for example, metal oxide particles having an average particle size of 0.001 to 5 μm on a number basis. The average thickness of the porous layer (C) having an affinity for the ionic liquid is preferably 0.01 to 10 μm. Each layer will be described below.

[0045] [Porous layer with no affinity for ionic liquid (B)] The porous layer with no affinity for ionic liquids (B) has a large number of pores (micropores or voids) inside, and its surface (which may include the surfaces (or wall surfaces) of the internal voids) is usually hydrophobic (relatively hydrophobic to the porous layer with an affinity for ionic liquids (C)). The voids may or may not include closed pores, but at least include interconnected pores (or through-pores) that are interconnected in the thickness direction. The porous layer with no affinity for ionic liquids (B) (the material that constitutes the porous layer with no affinity for ionic liquids (B) or the components that form the porous layer with no affinity for ionic liquids) may contain a resin (e.g., a thermoplastic resin) as the main component (for example, 50% by weight or more, preferably 70% by weight or more, and more preferably 90% by weight or more (substantially 100% by weight) of the entire porous layer with no affinity for ionic liquids (B)). Because of its excellent formability, the ionic liquid non-affinity porous layer (B) is usually a porous film (porous film, microporous film or microporous film) made of a thermoplastic resin.

[0046] Examples of thermoplastic resins include polyolefin resins, polyester resins (e.g., polyalkylene arylate resins such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate), polycarbonate resins (e.g., bisphenol polycarbonate resins such as bisphenol A polycarbonate resin, bisphenol F polycarbonate resin, and bisphenol S polycarbonate resin), polyamide resins (e.g., aliphatic polyamide resins such as polyamide 6 and polyamide 66), polysulfone resins (e.g., polysulfone and polyethersulfone), fluororesins, and cellulose derivatives. These thermoplastic resins may be used alone or in combination of two or more.

[0047] Of these thermoplastic resins, polyolefin resins, fluororesins, and cellulose derivatives (particularly polyolefin resins, fluororesins, and polyolefin resins from the viewpoint of easy availability) are preferred, and among these, polyα-C2-3 olefin resins such as polyethylene resins and polypropylene resins (particularly polyethylene resins), and fluororesins such as PTFE and PVDF (particularly PVDF) are preferred.

[0048] These thermoplastic resins may contain conventional additives. Examples of conventional additives include stabilizers such as heat stabilizers, antioxidants, and ultraviolet absorbers, as well as preservatives, disinfectants, plasticizers, lubricants, colorants, viscosity modifiers, leveling agents, surfactants, and antistatic agents. These additives may be used alone or in combination of two or more. The proportion of the additives may be, for example, 50 parts by weight or less, preferably 30 parts by weight or less (e.g., 0.01 to 30 parts by weight), and more preferably 10 parts by weight or less (e.g., 0.1 to 10 parts by weight) per 100 parts by weight of the resin.

[0049] The method for preparing such a porous membrane of a thermoplastic resin is not particularly limited, and it may be prepared by a conventional method, for example, a method utilizing phase separation of a resin solution, a method of stretching a resin film, or a method of irradiating a resin film with high-energy rays such as alpha rays.

[0050] Furthermore, the ionic liquid non-affinity porous layer (B) may be subjected to a conventional surface treatment (for example, the treatment described in JP-A-6-9810, i.e., a treatment of adhering a crosslinked product derived from an ethylenically unsaturated monomer having a fluorinated alkyl group) in order to adjust the wettability (or contact angle) with the ionic liquid composition (A).

[0051] As the ionic liquid non-affinity porous layer (B), commercially available products may be used, such as "Seapore" manufactured by Ube Maxell, Ltd., "Eupore" manufactured by Ube Industries, Ltd., and "Durapel" manufactured by Merck Millipore.

[0052] The average thickness of the porous layer (B) having no affinity for an ionic liquid may be, for example, 1 to 200 μm, preferably 5 to 150 μm, and more preferably 10 to 130 μm.

[0053] The pore size (average pore size or average pore size) of the ionic liquid-incompatible porous layer (B) may be selected from a wide range, for example, from 0.001 to 10 μm (e.g., 0.01 to 5 μm), and may be, for example, 0.001 to 1 μm (e.g., 0.005 to 0.5 μm), preferably 0.01 to 0.4 μm (e.g., 0.03 to 0.35 μm), and more preferably 0.05 to 0.3 μm (e.g., 0.07 to 0.25 μm). If the pore size is too small, gas permeability may be reduced. If the pore size is too large, the ionic liquid composition (A) may permeate and may not be retained in the carbon dioxide separation membrane (ionic liquid-containing laminate). In this specification and claims, the pore size (average pore size or average pore size) can be measured by a conventional method such as mercury intrusion porosimetry.

[0054] The porosity (porosity or porosity) of the ionic liquid-incompatible porous layer (B) may be selected from a wide range, for example, from 1 to 90% (e.g., 10 to 80%), depending on the manufacturing method of the porous layer, and may be, for example, from 20 to 85%, preferably from 30 to 80%, and more preferably from 40 to 75%. If the porosity is too low, gas permeability may be reduced. If the porosity is too high, the ionic liquid composition (A) may permeate and may not be retained in the carbon dioxide separation membrane (ionic liquid-containing laminate). In this specification and claims, the porosity (porosity or porosity) refers to the volume ratio of voids in either one of the porous layers (the entire ionic liquid-incompatible porous layer (B) or the entire ionic liquid-compatible porous layer (C)). This can be measured by the method described in the Examples below.

[0055] The interconnected pore ratio of the ionic liquid non-affinity porous layer (B) may be, for example, 50% or more, preferably 70% or more, and more preferably 90% or more (e.g., substantially 100%). In this specification and claims, the interconnected pore ratio represents the volume ratio of interconnected pores to voids in the porous layer, and may be calculated from a cross-sectional image observed with a scanning electron microscope (SEM) or the like.

[0056] The contact angle of the ionic liquid non-affinity porous layer (B) with the ionic liquid composition (A) may be, for example, 90° or more (e.g., 90 to 150°), preferably 95° or more (e.g., 95 to 148°), and more preferably 100° or more (e.g., 100 to 145°). If the contact angle is too small, the ionic liquid-containing liquid (A) may pass through and may not be retained. In this specification and claims, the contact angle can be measured by the conventional method described above.

[0057] [Porous layer (C) with affinity for ionic liquid (or second porous layer (C))] The ionic liquid-affinitive porous layer (C) has a large number of pores (pores or voids) inside, and its surface (which may include the surfaces (or wall surfaces) of the internal voids) is usually hydrophilic (relatively hydrophilic to the ionic liquid-non-affinitive porous layer (B)). The voids may or may not include closed pores, but at least include interconnected pores (or through pores) that interconnect in the thickness direction. The porous layer (C) with an affinity for ionic liquid (materials constituting the porous layer (C) with an affinity for ionic liquid, or components forming the porous layer (C) with an affinity for ionic liquid) may contain, as a main component, an organic material such as the resin described in the section on the porous layer (B) without an affinity for ionic liquid. However, in terms of excellent formability and mechanical properties, it is preferable that the porous layer (C) with an affinity for ionic liquid contains, as a main component, an inorganic material in a proportion of, for example, 50% by weight or more, preferably 70% by weight or more, and more preferably 90% by weight or more (substantially 100% by weight) of the entire porous layer (C) with an affinity for ionic liquid. Therefore, the porous layer (C) with an affinity for ionic liquid is preferably made of an inorganic material, as a main component, in a proportion of, for example, 50% by weight or more, preferably 70% by weight or more, and even more preferably 90% by weight or more (substantially 100% by weight) of the porous layer (C) without an affinity for ionic liquid. Although the resin described in Section B) may be hydrophilized (e.g., a hydrophilized PTFE porous membrane, a hydrophilized PVDF porous membrane, etc.), it is usually a porous membrane (porous membrane, porous membrane, or microporous membrane) formed from an inorganic material. When the ionic liquid-affinity porous layer (C) is formed from an inorganic material, the ionic liquid-containing laminate is endowed with the rigidity inherent in inorganic materials, making the carbon dioxide separation membrane (ionic liquid-containing laminate) easy to handle even if it is thin, and effectively improving its handleability. In particular, this is preferred because it not only effectively suppresses swelling or gelation, which can cause a decrease in gas permeability, but also improves dimensional stability.

[0058] Examples of inorganic materials include metal oxides such as Group 4A metal oxides (e.g., titanium oxide, zirconium oxide, etc.), Group 5A metal oxides (vanadium oxide, etc.), Group 6A metal oxides (molybdenum oxide, tungsten oxide, etc.), Group 7A metal oxides (manganese oxide, etc.), Group 8 metal oxides (nickel oxide, iron oxide, etc.), Group 1B metal oxides (copper oxide, etc.), Group 2B metal oxides (zinc oxide, etc.), Group 3B metal oxides (aluminum oxide, indium oxide, etc.), Group 4B metal oxides (silicon oxide, tin oxide, etc.), and Group 5B metal oxides (antimony oxide, etc.).

[0059] These metal oxides can be used alone or in combination of two or more. Among these metal oxides, from the viewpoints of affinity (or hydrophilicity) with the ionic liquid composition (A), ease of preparation of a dispersion (or slurry) due to specific gravity, and ease of availability, Group 3B metal oxides such as aluminum oxide and Group 4B metal oxides such as silicon oxide (particularly Group 3B metal oxides such as aluminum oxide) are preferred.

[0060] The inorganic material (or metal oxide) may be in the form of particles. The average particle size of the inorganic material (or metal oxide) may be, for example, 0.001 to 10 μm (e.g., 0.01 to 5 μm), preferably 0.1 to 3 μm (e.g., 0.3 to 2 μm), and more preferably 0.5 to 1.5 μm (e.g., 0.8 to 1.2 μm), based on the number of particles. In this specification and claims, the average particle size can be measured by the method described in the Examples below.

[0061] The shape of the particles is not particularly limited, and examples thereof include spherical (or nearly spherical), ellipsoidal, polygonal (such as polygonal pyramidal, cuboidal, or rectangular parallelepiped), plate-like, rod-like, and irregular shapes, but the particles are usually irregular in many cases. Furthermore, the inorganic material may or may not be surface-treated to improve dispersibility.

[0062] When the porous layer (C) with an affinity for an ionic liquid is prepared using a particulate inorganic material (or metal oxide), the gaps (voids) between the particles allow the gas permeability of the porous layer (C) with an affinity for an ionic liquid to be adjusted to a high level, effectively preventing a decrease in gas permeability even in the laminate structure. Furthermore, perhaps due to the rigidity of the porous layer (C) with an affinity for an ionic liquid, the encapsulated ionic liquid-containing liquid (A) is unlikely to ooze out even when it comes into contact with the surface of the carbon dioxide separation membrane (for example, the side of the porous layer (C) with an affinity for the ionic liquid in the carbon dioxide separation membrane). This makes it easier to stably maintain the ionic liquid composition (A) in a liquid state and effectively prevents the surface of the carbon dioxide separation membrane (ionic liquid-containing laminate) from becoming sticky.

[0063] The porous layer (C) having affinity for an ionic liquid may be subjected to a conventional surface treatment (e.g., treatment with a silane coupling agent) in order to adjust the wettability (or contact angle) with the ionic liquid composition (A).

[0064] The average thickness of the porous layer (C) having an affinity for an ionic liquid can be selected, for example, from the range of 0.01 to 100 μm (e.g., 0.03 to 70 μm), and may be, for example, 0.05 to 50 μm (e.g., 0.1 to 30 μm), preferably 0.5 to 20 μm (e.g., 1 to 15 μm), and more preferably 1 to 10 μm (e.g., 2 to 7 μm). If the average thickness is too large, the weight of the carbon dioxide separation membrane (ionic liquid-containing laminate) may increase.

[0065] The pore size (average pore size or average pore diameter) of the porous layer (C) with an affinity for an ionic liquid may be, for example, 0.001 to 10 μm (e.g., 0.01 to 5 μm). If the pore size is too small, not only will the amount of ionic liquid composition (A) that can be retained decrease, but gas permeability may also decrease. If the porous layer (C) with an affinity for an ionic liquid is formed from an inorganic material (e.g., metal oxide particles), it appears that gas permeability can be easily adjusted to a high level.

[0066] The porosity (porosity or porosity) of the ionic liquid-affinity porous layer (C) may be selected from a wide range of, for example, 1 to 90% (e.g., 10 to 80%), and may be, for example, 5 to 70% (e.g., 10 to 60%), preferably 15 to 50% (e.g., 20 to 45%), and more preferably 25 to 40% (e.g., 30 to 35%). If the porosity is too small, not only will the amount of ionic liquid composition (A) that can be retained decrease, but gas permeability may also decrease. If the porosity is too large, the ionic liquid-containing liquid (A) may not be stably retained.

[0067] The interconnected pore ratio of the porous layer (C) having an affinity for an ionic liquid may be, for example, 50% or more, preferably 70% or more, and more preferably 90% or more (for example, substantially 100%).

[0068] The contact angle of the porous layer (C) with the ionic liquid composition (A) may be, for example, less than 90° (e.g., 0° or more and less than 90°), preferably 85° or less (e.g., 15 to 85°), and more preferably 80° or less (e.g., 30 to 80°). If the contact angle is too large, it may become difficult to retain the ionic liquid composition (A).

[0069] The difference in contact angle with the ionic liquid composition (A) between the porous layer without affinity for ionic liquid (B) and the porous layer with affinity for ionic liquid (C) may be, for example, 10° or more (e.g., 15 to 55°), preferably 20° or more (e.g., 25 to 50°), and more preferably 30° or more (e.g., 30 to 45°). If the difference in contact angles is too small, it may be difficult to stably retain the ionic liquid composition (A). On the other hand, if the difference in contact angles is too large, when the basis weight of the ionic liquid composition (A) is low, it may not spread flat (or in the plane direction) inside the porous layer with affinity for ionic liquid.

[0070] [Carbon dioxide separation membrane (ionic liquid-containing laminate) and its manufacturing method] The carbon dioxide separation membrane (ionic liquid-containing laminate) of the present invention may comprise a laminate (ionic liquid-free laminate) comprising an ionic liquid-non-affinity porous layer (B) and an ionic liquid-affinity porous layer (C), and may further comprise a step (impregnation step) of impregnating the pores of the ionic liquid-affinity porous layer (C) with a liquid (or impregnation liquid) containing the ionic liquid composition (A).

[0071] The impregnation liquid may consist solely of the ionic liquid composition (A), or may be a mixed liquid (solution or dispersion) obtained by mixing the ionic liquid composition (A) with a solvent (or dispersion medium). From the viewpoint of facilitating the reduction of the equivalent membrane thickness of the ionic liquid composition (A), the impregnation liquid is preferably a mixed liquid. In this specification and claims, the term "equivalent membrane thickness" refers to the membrane thickness when a liquid membrane having the same area as the carbon dioxide separation membrane (ionic liquid-containing laminate) is formed using the ionic liquid composition (A) contained in the porous layer.

[0072] The solvent (or dispersion medium) is preferably a solvent with higher volatility than the ionic liquid composition (A), and examples thereof include water, alcohols (lower alcohols such as methanol, ethanol, isopropanol, butanol, and cyclohexanol), ketones (acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone), esters (methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl formate, and ethyl formate), ethers (diethyl ether, dioxane, and tetrahydrofuran), aliphatic hydrocarbons (hexane and the like), alicyclic hydrocarbons (cyclohexane and the like), aromatic hydrocarbons (benzene and the like), halogenated hydrocarbons (dichloromethane and dichloroethane), cellosolves (methyl cellosolve and ethyl cellosolve), cellosolve acetates, and amides (dimethylformamide and dimethylacetamide). These solvents can be used alone or in combination of two or more. Among these solvents, aqueous solvents (or water-soluble solvents) such as water and alcohols (e.g., C2-6 alkanols such as methanol) are commonly used. The concentration of the ionic liquid composition (A) in the impregnation liquid may be, for example, 0.001 to 100% by weight, preferably 0.01 to 50% by weight (e.g., 0.05 to 30% by weight), and more preferably 0.1 to 10% by weight (e.g., 0.1 to 8% by weight).

[0073] The method for impregnating with the impregnation liquid is not particularly limited, and may be, for example, a method in which the impregnation liquid is injected under pressure. Specifically, in a laminate (a laminate not containing an ionic liquid) comprising a porous layer with no affinity for ionic liquid (B) and a porous layer with an affinity for ionic liquid (C), the surface of the laminate on the side of the porous layer with an affinity for ionic liquid (C) (or the outermost layer of the laminate) may be brought into contact with the impregnation liquid, and the opposite side (the side of the porous layer with no affinity for ionic liquid (B)) may be depressurized (or suctioned from the opposite side) to impregnate with the impregnation liquid. By such a method, the carbon dioxide separation membrane (ionic liquid-containing laminate) of the present invention can be easily and efficiently formed. Furthermore, when the above-mentioned mixed liquid is used as the impregnation liquid, the carbon dioxide separation membrane (ionic liquid-containing laminate) may be prepared by volatilizing the solvent (or dispersion medium) after the impregnation step. By removing the solvent (or dispersion medium), the equivalent membrane thickness of the ionic liquid-containing liquid (A) can be easily adjusted, and thinning is also easy. There are no particular restrictions on the method for volatilizing the solvent; volatilization can be achieved by heating and / or reducing the pressure as appropriate, depending on the boiling point and vapor pressure of the solvent.

[0074] In the carbon dioxide separation membrane (ionic liquid-containing laminate) of the present invention, the content of ionic liquid composition (A) preferably does not exceed 100 parts by volume per 100 parts by volume of voids inside the porous layer with an affinity for the ionic liquid (C). In other words, the carbon dioxide separation membrane (ionic liquid-containing laminate) preferably does not have a second ionic liquid-containing layer adjacent to the porous layer with an affinity for the ionic liquid (C) that contains (or retains) ionic liquid composition (A) and that contains ionic liquid composition (A) that does not fit into the voids. Therefore, from the viewpoint of improving the carbon dioxide permeation rate and handleability of the carbon dioxide separation membrane (ionic liquid-containing laminate), the porous layer (C) with affinity for an ionic liquid may contain 100 parts by volume or less of ionic liquid composition (A) relative to 100 parts by volume of the internal voids. This can be selected from the range of, for example, 0.1 to 99 parts by volume (e.g., 1 to 90 parts by volume), and may contain, for example, 3 to 80 parts by volume (e.g., 5 to 70 parts by volume), preferably 10 to 50 parts by volume (e.g., 15 to 45 parts by volume), and more preferably 20 to 40 parts by volume (e.g., 25 to 35 parts by volume). If the amount of ionic liquid composition (A) is too large, handleability may be reduced.

[0075] In the carbon dioxide separation membrane (ionic liquid-containing laminate) of the present invention, the equivalent membrane thickness of the ionic liquid composition (A) may be, for example, 0.01 to 5 μm (e.g., 0.05 to 3 μm), preferably 0.1 to 2 μm (e.g., 0.15 to 1.5 μm), and more preferably 0.2 to 1 μm (e.g., 0.2 to 0.7 μm). If the equivalent membrane thickness is too large, the permeation rate may decrease.

[0076] A laminate comprising a porous layer with no affinity for ionic liquid (B) and a porous layer with an affinity for ionic liquid (C) (an ionic liquid-free laminate) can be prepared, for example, by directly or indirectly laminating (or forming) the porous layer with an affinity for ionic liquid (C) on one surface of the porous layer with no affinity for ionic liquid (B). The method for laminating (or forming) the porous layer with an affinity for ionic liquid (C) is not particularly limited and may be, for example, pressure bonding, heat fusion, or adhesion with an adhesive or pressure-sensitive adhesive. When the porous layer with an affinity for ionic liquid (C) is formed from an inorganic material, it may be formed using a conventional method, such as sintering a powdered inorganic material. However, from the viewpoint of easy and efficient formation of the desired porous layer and improved handleability, it may also be formed by a method including a coating step of applying a dispersion (or slurry) of particulate (or powdered) inorganic material in a dispersion medium and drying the resulting coating.

[0077] Examples of the dispersion medium include the same solvents (or dispersion media) as exemplified in the section on the impregnation liquid. These dispersion media can be used alone or in combination of two or more. Of these dispersion media, water is usually used. When water is used as the dispersion medium, a small amount of alcohol such as isopropanol (for example, 0.01 to 10 parts by weight, preferably 0.1 to 2 parts by weight, per 100 parts by weight of the inorganic material) may be added as needed to improve the coatability of the ionic liquid-non-affinity porous layer (B).

[0078] If necessary, a small amount (for example, 0.01 to 10 parts by weight, preferably 0.1 to 2 parts by weight per 100 parts by weight of the inorganic material) of a binder (e.g., water-soluble resins such as carboxymethyl cellulose or its salts (e.g., sodium salts), hydroxyalkyl cellulose (e.g., hydroxyethyl cellulose, hydroxypropyl cellulose), and methyl cellulose; or a latex such as styrene-butadiene rubber latex) may be added. While a binder is not always necessary, it often makes it easier to prepare a porous layer (C) with a large thickness that has affinity for the ionic liquid.

[0079] The concentration of the inorganic material in the dispersion is, for example, 0.1 to 50% by weight, preferably 1 to 30% by weight, and more preferably 3 to 20% by weight (for example, 5 to 15% by weight) based on the total weight of the dispersion.

[0080] The coating method is not particularly limited, and includes conventional methods such as a roll coater method, an air knife coater method, a blade coater method, a rod coater method, a reverse coater method, a bar coater method, a comma coater method, a dip-squeeze coater method, a die coater method, a gravure coater method, a microgravure coater method, a silk screen coater method, a dip method, a spray method, and a spinner method. Of these methods, the bar coater method is widely used. If necessary, the dispersion (or coating liquid) may be applied multiple times.

[0081] In the coating step, after the dispersion is cast or coated, the dispersion medium is evaporated to dry the coating. The drying temperature can usually be selected depending on the boiling point of the dispersion medium, and may be, for example, 50 to 150°C, preferably 80 to 120°C, and more preferably 90 to 110°C.

[0082] The carbon dioxide separation membrane of the present invention (or an ionic liquid-free laminate comprising a porous layer with no affinity for ionic liquid (B) and a porous layer with an affinity for ionic liquid (C)) may have a two-layer structure consisting of the porous layer with no affinity for ionic liquid (B) and the porous layer with an affinity for ionic liquid (C), or may have a multilayer structure of three or more layers (e.g., a 3- to 5-layer structure) including another layer (or a third layer) such as the support layer. The third layer is not particularly limited as long as it is gas permeable, and examples thereof include the support layer [e.g., a net (or mesh) made of metal (stainless steel, etc.) or a resin], an adhesive or pressure-sensitive adhesive layer, etc. These third layers may be used alone or in combination of two or more. From the viewpoint of gas permeability, the carbon dioxide separation membrane (ionic liquid-containing laminate) of the present invention preferably has a two- to three-layer structure (particularly a two-layer structure). From the viewpoint of effective retention or immobilization of the ionic liquid composition (A), it is preferable that the porous layer with no affinity for ionic liquid (B) and the porous layer with an affinity for ionic liquid (C) are formed adjacent to each other.

[0083] The carbon dioxide separation membrane (ionic liquid-containing laminate) of the present invention obtained in this manner has excellent gas permeability and can be suitably used, for example, in the agricultural field as a carbon dioxide separation membrane (carbon dioxide concentration membrane) for fertilizing plants. The carbon dioxide separation membrane of the present invention is usually used with the ionic liquid-affinity porous layer (C) containing ionic liquid composition (A) on the gas supply side (supply side or upstream side) and the opposite side (the ionic liquid-non-affinity porous layer (B) side) on the permeation side (or downstream side).

[0084] The carbon dioxide permeability coefficient, nitrogen permeability coefficient and carbon dioxide selectivity can be measured by the method described in the examples below.

[0085] [Carbon dioxide concentrator equipped with a carbon dioxide separation membrane] The carbon dioxide concentrating apparatus of the present invention is equipped with the carbon dioxide separation membrane. The shape of the carbon dioxide separation membrane is not particularly limited, and may be, for example, a flat membrane, a spiral shape formed by winding a flat membrane, or a hollow fiber membrane. These shapes may be used alone or in combination of two or more. The carbon dioxide separation membrane is usually used to form a membrane module (concentration unit or separation unit) together with a support material for supporting or fixing the carbon dioxide separation membrane. The material and shape of the support material are not particularly limited as long as they do not inhibit gas permeation, and are selected appropriately depending on the shape of the carbon dioxide separation membrane. Furthermore, the concentration unit may include one carbon dioxide separation membrane, or may include two or more carbon dioxide separation membranes.

[0086] In addition to the concentration unit, the carbon dioxide concentrator of the present invention often further includes an intake unit for supplying a gas component containing carbon dioxide (e.g., atmospheric air) to the carbon dioxide separation membrane. The intake unit can supply a gas component containing carbon dioxide to the carbon dioxide separation membrane by generating a pressure difference between the upstream side (or gas supply side) and downstream side (permeation side) of the concentration unit. The intake unit is not particularly limited as long as it can generate the pressure difference, and may be disposed either upstream or downstream of the concentration unit. Specifically, it may be, for example, an air compressor disposed upstream of the concentration unit or a pump (e.g., a diaphragm pump) disposed downstream.

[0087] The carbon dioxide concentrator of the present invention can operate (or run) as long as it includes at least the concentration unit and the intake unit, so the device configuration (or design) can be simplified and it can be easily made smaller. Furthermore, the permeation rate (carbon dioxide permeation rate) of the carbon dioxide concentration membrane is high, so carbon dioxide can be effectively or efficiently concentrated (or enriched) even at a relatively low differential pressure. Therefore, even a small intake unit with low intake capacity can operate smoothly (without any problems). [Example]

[0088] The present invention will be specifically described below based on examples and comparative examples. Although the description will focus on the separation of low partial pressure carbon dioxide, the examples are intended to illustrate preferred examples of the present invention, and do not in any way limit the application of the present invention to the separation of high partial pressure carbon dioxide.

[0089] [Method for preparing carbon dioxide separation membrane] A carbon dioxide separation membrane (hereinafter referred to as "CO2 separation membrane") was prepared by the following method. A hydrophilized PTFE filter (manufactured by Merck Millipore) used as the substrate was washed with ethanol, acetone, and ultrapure water, and then the filter was dried under reduced pressure at 70°C for 12 hours using a vacuum dryer. A predetermined amount of ionic liquid was added dropwise to the filter, and the filter was heated to 40°C and evacuated with a vacuum pump for 12 hours to impregnate the ionic liquid. Excess ionic liquid was wiped off from the filter so that the ionic liquid occupied 95-100% of the pore volume of the filter, and the resulting membrane was used as a CO2 separation membrane. Alumina-coated filters (manufactured by Daicel) and titania-coated filters (manufactured by Daicel) were also prepared using the same method.

[0090] [Evaluation method for carbon dioxide separation membranes] The CO2 permeability coefficient and N2 permeability coefficient of the CO2 separation membrane were determined using the apparatus shown in FIG. In the figure, 1 is a CO2 / N2 standard gas cylinder, 2 is an N2 gas cylinder, 3 is an Ar gas cylinder, 4 to 6 are mass flow controllers, 7 is a CO2 separation membrane, 8 is a separation membrane holder, 9 is an oven, 10 and 11 are thermo-hygrometers, 12 and 14 are traps, 13 is a chiller, 15 and 16 are back pressure valves, 17 and 20 are soap film flow meters, 18 is a gas chromatograph (TCD-GC), and 19 is a CO2 concentration meter.

[0091] First, the CO2 separation membrane 7 was sandwiched between two PTFE filters (manufactured by Advantec) and set in a separation membrane holder 8. After starting temperature control using an oven 9, a CO2 / N2 mixed gas (CO2 composition: 0.04 mol%, 0.10 mol%, 0.50 mol%, 1.00 mol%) at atmospheric pressure was supplied to the supply bypass at a predetermined flow rate, and high-purity argon (99.99995 mol% or higher) at atmospheric pressure was supplied to the permeation bypass at a predetermined flow rate. The supply gas passing through the bypass was analyzed using a gas chromatograph (Shimadzu Corporation, GC-8A) 18 and a CO2 concentration meter (Vaisala, GMP343) 19, confirming that the CO2 composition of the supply gas matched the target values (0.04 mol%, 0.10 mol%, 0.50 mol%, 1.00 mol%). The permeation gas passing through the bypass was analyzed using the same method, confirming that components other than argon contained in the permeation gas were below the detection limit. Thereafter, the feed gas and permeate gas were supplied to the separation membrane holder 8, and the gas permeation test was initiated. The composition of the permeate gas was analyzed at regular intervals, and when the CO2 and N2 compositions remained constant for at least one hour, it was deemed that a steady state had been reached. The feed gas and permeate gas were analyzed using a gas chromatograph 18 and a CO2 concentration meter 19, and the flow rates of the gases were measured using soap film flow meters 17 and 20 (Horiba, Ltd. SF-1U). From the CO2 composition, N2 composition, and flow rate, the flow rates of CO2 and N2 permeating the CO2 separation membrane per unit time (cm 3 These values were multiplied by the separation membrane thickness (cm) and then the separation membrane area (cm 2 ) by the gas partial pressure difference (cmHg) between the supply side and the permeation side to obtain the CO2 permeability coefficient and the N2 permeability coefficient. In the examples and comparative examples, the unit of the permeability coefficient is Barrer (cm 3 cm / cm 2 ·s·cmHg×10 10 ) was used as the standard. The CO2 selectivity was defined as the ratio of the CO2 permeability coefficient to the N2 permeability coefficient.

[0092] [Synthesis of ionic liquids] (Synthesis Example 1: Synthesis of 3-aminopropylaminium bis(trifluoromethylsulfonyl)amide ([APAH][TfN])) A methanol solution of bis(trifluoromethylsulfonyl)imide (Kanto Chemical, hereafter referred to as HTf2N) (methanol was from Wako Pure Chemical Industries) was slowly added dropwise to a methanol solution of 3-aminopropylamine (manufactured by Aldrich). After the addition was complete, the mixture was stirred overnight at room temperature to obtain a methanol solution containing [APAH][Tf2N] shown in the following formula. The methanol was distilled off under reduced pressure, and the mixture was further dried in vacuo at 50°C for 30 hours to obtain [APAH][Tf2N].

[0093] [ka]

[0094] (Synthesis Example 2: Synthesis of 3-(N-methylamino)propylaminium bis(trifluoromethylsulfonyl)amide ([MAPAH][TfN])) [MAPAH][Tf2N] shown in the following formula was obtained in the same manner as in Synthesis Example 1, except that 3-(N-methylamino)propylamine (manufactured by Aldrich) was used as the raw material.

[0095] [ka]

[0096] (Synthesis Example 3: Synthesis of 3-(N,N-dimethylamino)propylaminium bis(trifluoromethylsulfonyl)amide ([DMAPAH][TfN])) [DMAPAH][Tf2N] shown in the following formula was obtained in the same manner as in Synthesis Example 1, except that 3-(N,N-dimethylamino)propylamine (manufactured by Aldrich) was used as the raw material.

[0097] [ka]

[0098] (Synthesis Example 4: Synthesis of 2-(N-hydroxyethylamino)ethylaminium bis(trifluoromethylsulfonyl)amide ([HDAH][TfN])) [HDAH][Tf2N] shown in the following formula was obtained in the same manner as in Synthesis Example 1, except that 2-(N-hydroxyethylamino)ethylamine (manufactured by Aldrich) was used as the raw material.

[0099] [ka]

[0100] (Synthesis Example 5: Synthesis of 2-aminoethylaminium bis(trifluoromethylsulfonyl)amide ([EDAH][TfN])) [EDAH][Tf2N] shown in the following formula was obtained in the same manner as in Synthesis Example 1, except that 2-aminoethylamine (manufactured by Nacalai Tesque) was used as the raw material.

[0101] [ka]

[0102] (Synthesis Example 6: Synthesis of 2-(2-(aminoethyl)amino)ethylaminium bis(trifluoromethylsulfonyl)amide ([DETAH][TfN])) [DETAH][Tf2N] shown in the following formula was obtained in the same manner as in Synthesis Example 1, except that 2-(2-(aminoethyl)amino)ethylamine (manufactured by TCI) was used as the raw material.

[0103] [ka]

[0104] (Synthesis Example 7: Synthesis of 2-(2-(2-(aminoethyl)aminoethyl)amino)ethylaminium bis(trifluoromethylsulfonyl)amide ([TETAH][TfN])) [TETAH][Tf2N] shown in the following formula was obtained in the same manner as in Synthesis Example 1, except that 2-(2-(2-(aminoethyl)aminoethyl)amino)ethylamine (manufactured by Sigma-Aldrich) was used as the raw material.

[0105] [ka]

[0106] (Synthesis Example 8: Synthesis of 1-ethyl-3-methylimidazolium 3-(2-methoxyethoxy)propionate ([emim][1O2OPrO])) N-Ethylimidazole (Sigma-Aldrich), dimethyl carbonate (Sigma-Aldrich), and methanol (Wako Pure Chemical Industries) were reacted at 120 °C for 1 day to obtain a methanol solution of 1-ethyl-3-methylimidazolium methyl carbonate ([emim][CHOCO2]). The content of [emim][CHOCO2] in the methanol solution was determined by NMR (Bruker Avance 400). An equal amount of 3-(2-methoxyethoxy)propionate (Koei Chemical Industry) was slowly added dropwise to the [emim][CHOCO2] methanol solution and the reaction was allowed to proceed at room temperature for 1 day. To remove unreacted materials, the mixture was evaporated under reduced pressure and extracted with toluene to obtain crude [emim][1O2OPrO]. The volatile components were then removed by drying under reduced pressure at 40 °C for 1 week to obtain [emim][1O2OPrO], as shown below.

[0107] [ka]

[0108] (Synthesis Example 9: N,N-diethyl-N-methyl-N-heptylammonium acetate ([N 1227 Synthesis of ][AcO]) Iodomethane was added dropwise to a solution of N,N-diethyl-N-heptylamine in acetonitrile at 40°C, and the mixture was stirred for 21 hours. The reaction solution was concentrated under reduced pressure, and the concentrate was washed with toluene. The washed concentrate was dried under reduced pressure to obtain N,N-diethyl-N-methyl-N-heptylammonium iodide. This N,N-diethyl-N-methyl-N-heptylammonium iodide was dissolved in methanol, and silver(I) oxide was added in portions. After stirring at around 25°C for 12 hours or more, the mixture was filtered, and the residue was washed with methanol to obtain a filtrate. Acetic acid was added dropwise to the obtained filtrate at around 20°C, and the mixture was stirred. This solution was concentrated and dried under reduced pressure to obtain the compound [N 1227 [AcO] was obtained.

[0109] [ka]

[0110] (Synthesis Example 10: N,N-diethyl-N-methyl-N-(6-hydroxyhexyl)ammonium acetate ([N 1226OH Synthesis of "[AcO]" Iodomethane was added dropwise to a solution of N,N-diethyl-N-(6-hydroxyhexyl)amine in acetonitrile at 40°C, followed by stirring at around 25°C. The reaction solution was concentrated under reduced pressure, and the concentrate was washed with toluene. The washed concentrate was dried under reduced pressure to obtain N,N-diethyl-N-methyl-N-(6-hydroxyhexyl)ammonium iodide. This N,N,N-diethyl-N-methyl-N-(6-hydroxyhexyl)ammonium iodide was dissolved in methanol, and silver(I) oxide was added in portions. After stirring for 12 hours or more at around 25°C, the mixture was filtered, and the residue was washed with methanol to obtain a filtrate. Silver(I) oxide was again added to this filtrate, stirred, and then filtered. Acetic acid was added dropwise to the obtained filtrate at around 20°C, followed by stirring. This solution was concentrated and dried under reduced pressure to obtain the compound [N 1226OH [AcO] was obtained.

[0111] [ka]

[0112] [Ionic liquids other than those mentioned above] In addition to the ionic liquids obtained in Synthesis Examples 1 to 10 above, the following commercially available ionic liquids were used. (1) 1-ethyl-3-methylimidazolium dicyanamide ([emim][DCA], manufactured by Aldrich)

[0113] [ka]

[0114] (2) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([emim][TfN], manufactured by Iolitec)

[0115] [ka]

[0116] (3) 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([omim][TfN], manufactured by Iolitec)

[0117] [ka]

[0118] (4) 1-ethyl-3-methylimidazolium methylphosphonate ([emim][MeHPO], manufactured by Kanto Chemical Co., Ltd.)

[0119] [ka]

[0120] (5) 1-ethyl-3-methylimidazolium acetate ([emim][AcO], manufactured by Aldrich)

[0121] [ka]

[0122] [Comparative Examples 1 to 8] We investigated a CO2 separation membrane prepared using a mixture of an ionic liquid (I) having an aminium with one or more primary or secondary amino groups and an ethylenediamine or propylenediamine skeleton, and a diluent whose anion is [Tf2N] ([emim][Tf2N] or [omim][Tf2N] described in Patent Document 3).

[0123] (Comparative Example 1) Based on the above-mentioned CO2 separation membrane preparation method, a CO2 separation membrane was prepared by impregnating a hydrophilized PTFE filter with 1-ethyl-3-methylimidazolium dicyanamide ([emim][DCA]). This was designated Comparative Example 1. The gas permeability coefficient of Comparative Example 1 was measured using four CO2 / N2 mixed gases with different CO2 compositions (0.04 mol%, 0.10 mol%, 0.50 mol%, and 1.00 mol%) at a temperature of 40°C, a feed gas flow rate of 100 ml / min, and a permeation gas flow rate of 20 ml / min, and the dependence of the gas permeability coefficient on CO2 partial pressure was investigated. The obtained CO2 permeability coefficient, N2 permeability coefficient, and CO2 selectivity on CO2 partial pressure are shown in Figures 2, 3, and 4.

[0124] (Comparative Example 2) A CO2 separation membrane was prepared in the same manner as in Comparative Example 1, except that the ionic liquid impregnated into the hydrophilized PTFE filter was changed to the anion 1-ethyl-3-merimidazolium bis(trifluoromethylsulfonyl)amide ([emim][Tf2N]). This was designated Comparative Example 2.

[0125] (Comparative Examples 3 to 8) CO separation membranes were prepared in the same manner as in Comparative Example 1, except that the ionic liquid used to impregnate the hydrophilized PTFE filter was a mixture of 3-aminopropylaminium bis(trifluoromethylsulfonyl)amide ([APAH][TfN]) obtained in Synthesis Example 1, 3-(N-methylamino)propylaminium bis(trifluoromethylsulfonyl)amide ([MAPAH][TfN]) obtained in Synthesis Example 2, bis(trifluoromethylsulfonyl)amide ([DMAPAH][TfN]) obtained in Synthesis Example 3, or 2-(N-hydroxyethylamino)ethylaminium bis(trifluoromethylsulfonyl)amide ([HDAH][TfN]) obtained in Synthesis Example 4 with either [emim][TfN] or 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([omim][TfN]), as shown in Table 1 below. These were designated Comparative Examples 3 to 8.

[0126] [Table 1]

[0127] The gas coefficients of Comparative Examples 2 to 8 were measured under the same conditions as Comparative Example 1, and the dependence on CO2 partial pressure was investigated. The CO2 partial pressure dependence of the obtained CO2 permeability coefficient, N2 permeability coefficient, and CO2 selectivity is shown in Figures 5, 6, and 7. In the figure, □: Comparative Example 2, ■: Comparative Example 3, ▲: Comparative Example 4, ◇: Comparative Example 5, ◆: Comparative Example 6, ◯: Comparative Example 7, ●: Comparative Example 8 In Figures 5 and 6, the plots of ■ (Comparative Example 3) and ◆ (Comparative Example 6) overlap. In addition, in FIG. 7, plots other than ▴ (Comparative Example 4) and ◯ (Comparative Example 7) overlap.

[0128] In order to examine the temperature dependency, in Comparative Examples 2 and 5, the temperature was changed to 80° C. and measurements were carried out under the same conditions as in Comparative Example 1. The temperature dependence of the obtained CO2 permeability coefficient, N2 permeability coefficient, and CO2 selectivity is shown in Figures 8, 9, and 10. In the figure, □: Comparative Example 2, ■: Comparative Example 5

[0129] [Summary] 5 and 7, the CO2 permeability coefficient and CO2 selectivity were lower in Comparative Example 3 (■), Comparative Example 5 (◇), and Comparative Example 6 (◆) than in Comparative Example 2 (□) which used [emim][Tf2N]. This indicates that mixing [emim][Tf2N] with [APAH][Tf2N], [MAPAH][Tf2N], or [DMAPAH][Tf2N] is not effective in improving the CO2 permeation selectivity of the separation membrane. 8 and 10, even when the temperature was raised from 40° C. to 80° C., the CO 2 permeation selectivity of Comparative Example 5 (■) remained lower than that of Comparative Example 2 (□). 5 and 7, Comparative Example 4 (▲), which used [omim][Tf2N] with an elongated alkyl chain of the imidazolium cation instead of [emim][Tf2N] in Comparative Example 3 (■), showed a further decrease in CO2 permeability coefficient and CO2 selectivity compared to Comparative Example 3. Furthermore, Comparative Example 4 showed an increase in N2 permeability coefficient compared to Comparative Example 3. On the other hand, as shown in Figures 5 and 7, the CO2 permeability coefficient and CO2 selectivity of Comparative Example 7 (◯) were higher than those of Comparative Example 2 (□) and Comparative Example 8 (●). 2 and 3, Comparative Example 1 using [emim][DCA] had lower CO2 permeability coefficient and N2 permeability coefficient than Comparative Examples 2, 3, 4, 5, and 6 shown in Figures 5 and 6. On the other hand, Figure 4 shows that the CO2 selectivity of Comparative Example 1 was higher than those of Comparative Examples 2 to 6.

[0130] As described above, we found that mixing [emim][Tf2N] with [HDAH][Tf2N], which has both a hydroxyl group and an amino group in the cation, improved CO2 permselectivity compared to using either ionic liquid alone. However, the CO2 permselectivity obtained was still not sufficient.

[0131] Therefore, instead of [emim][TfN], we investigated the use of an ionic liquid (II) having an oxo acid anion (hydrogen-bond accepting ionic liquid) (see Patent Document 4 and Non-Patent Document 1) that has a superior solubility of the product after CO absorption compared to the above-mentioned [emim][TfN] in the solvent of the chemical absorption solution.

[0132] [Examples 1 to 6, Comparative Example 9] In Examples 1 to 6 and Comparative Example 9, 1-ethyl-3-methylimidazolium methylphosphonate ([emim][MeHPO3]) having methylphosphonate, a type of oxo acid anion, as the anion was used as the ionic liquid (II).

[0133] Example 1 A liquid (20 mol% [HDAH][Tf2N]) was prepared by mixing the [HDAH][Tf2N] and the [emim][MeHPO3]. The resulting mixture was impregnated into a hydrophilized PTFE filter in the same manner as above to prepare a CO2 separation membrane, which was designated Example 1.

[0134] (Examples 2 to 6, Comparative Example 9) CO2 separation membranes were prepared in the same manner as in Example 1, except that the composition of [HDAH][Tf2N] in Example 1 was changed as shown in Table 2 below, and were designated Examples 2 to 5 and Comparative Example 9. In addition, a CO2 separation membrane was prepared in the same manner as in Example 1, except that [HDAH][Tf2N] in Example 1 was changed to [APAH][Tf2N], and was designated Example 6. Table 2 also includes the aforementioned Comparative Example 8 (composition 100 mol%).

[0135] [Table 2]

[0136] The gas permeability coefficients of Examples 1 to 6 and Comparative Example 9 were measured using the four types of CO2 / N2 mixed gases with different CO2 compositions at a temperature of 40°C and a supply gas flow rate of 100 mlThe composition dependence and CO2 partial pressure dependence were investigated. The composition dependence of the obtained CO permeability coefficient, N permeability coefficient, and CO selectivity at a CO partial pressure of 0.04 kPa (CO composition 0.04 mol%) is shown in Figures 11, 12, and 13. Note that Comparative Example 8 (composition 100 mol%) was also included in these figures. In the figure, □: Examples 1 to 5, Comparative Example 8, Comparative Example 9, ■: Example 6, Comparative Example 9

[0137] The CO2 partial pressure dependence of the CO2 permeability coefficient, N2 permeability coefficient, and CO2 selectivity are shown in Figures 14, 15, and 16. Comparative Example 8 (composition 100 mol%) was also included in these figures. In the figure, ◆: Example 1, ■: Example 2, ◇: Example 3, △: Example 4, ▲: Example 5, ◯: Example 6, □: Comparative Example 8, ●: Comparative Example 9 In FIG. 15, the plots of ◆ (Example 1), ◇ (Example 3), and ▲ (Example 5) overlap.

[0138] [Summary] As shown in Figures 11 and 13, the CO2 separation membranes (Examples 1 to 5) (□) that mixed [HDAH][Tf2N] and [emim][MeHPO3] had an improved CO2 permeability coefficient and maintained an almost constant N2 permeability coefficient compared to the CO2 separation membranes (Comparative Example 8) (□) that used only [HDAH][Tf2N] and the CO2 separation membranes (Comparative Example 9) (■) that used only [emim][MeHPO3], resulting in a significant improvement in CO2 selectivity. 14 and 16, the CO2 separation membrane (Example 1) (◆) with a mixing ratio of 20:80 and the CO2 separation membrane (Example 5) (▲) with a mixing ratio of 80:20 showed improved CO2 permeability coefficients and CO2 selectivity as the CO2 partial pressure decreased, and furthermore, under CO2 partial pressure conditions up to 1 kPa, they exhibited higher CO2 permeability coefficients and CO2 selectivity than Comparative Example 8 (□) and Comparative Example 9 (●). Note that Comparative Examples 8 and 9 did not show the CO2 partial pressure dependence seen in Examples 2 and 5, and the CO2 permeability coefficient, N2 permeability coefficient, and CO2 selectivity were almost constant. Furthermore, as can be seen from Figures 11 and 13, and the above-mentioned Figures 5 and 7, the CO2 separation membrane (Example 6) (■) made by mixing [APAH][Tf2N] and [emim][MeHPO3] showed a higher CO2 permeability coefficient and CO2 selectivity than the CO2 separation membrane (Comparative Example 2) (◆), the CO2 separation membrane (Comparative Example 3) made by mixing [APAH][Tf2N] and [emim][Tf2N] (■), and the CO2 separation membrane (Comparative Example 9) (■) made by using only [emim][MeHPO3]. Such an improvement effect was not observed in the CO2 separation membrane (Comparative Example 3) (■) which was a mixture of [emim][Tf2N], an ionic liquid with weak hydrogen bond acceptance, and [APAH][Tf2N].

[0139] As described above, it has been revealed that CO2 separation membranes using an ionic liquid composition that mixes an aminium having an amino group (regardless of whether it has a hydroxyl group or not) with [emim][MeHPO3], which uses methylphosphonate, a type of oxoacid anion, as the anion, have improved CO2 permeation selectivity compared to CO2 separation membranes using each ionic liquid alone.

[0140] Therefore, to verify whether similar excellent effects could be obtained with ionic liquids containing other oxo acid anions, we investigated the use of 1-ethyl-3-methylimidazolium acetate ([emim][AcO]) or 1-ethyl-3-methylimidazolium 3-(2-methoxyethoxy)propionate ([emim][1O2OPrO]) as ionic liquid (II) instead of [emim][MeHPO3].

[0141] [Examples 7 to 11, Comparative Example 10] In Examples 7 to 11 and Comparative Example 10, acetate, a type of oxo acid anion, was used as the ionic liquid (II).

[0142] Example 7 The [HDAH][Tf2N] was used as ionic liquid (I), and a liquid ([HDAH][Tf2N] 5 mol%) was prepared by mixing this with [emim][AcO] as ionic liquid (II). The resulting mixture was impregnated into a hydrophilized PTFE filter in the same manner as above to prepare a CO2 separation membrane, which was designated Example 7.

[0143] (Examples 8 to 11, Comparative Example 10) CO2 separation membranes were prepared in the same manner as in Example 7, except that the composition (mol%) of [HDAH][Tf2N] in Example 7 was changed as shown in Table 3 below, and were designated as Examples 8 to 11 and Comparative Example 10. Table 2 also includes the aforementioned Comparative Example 8 (100 mol%).

[0144] [Table 3]

[0145] The gas permeability coefficients of Examples 7 to 11 and Comparative Example 10 were measured at a temperature of 40°C and a gas flow rate of 100 ml The composition dependence of the CO2 permeability coefficient, N2 permeability coefficient, and CO2 selectivity was investigated. The composition dependence of the obtained CO2 permeability coefficient and N2 permeability coefficient at a CO2 partial pressure of 0.04 kPa (CO2 composition 0.04 mol%) is shown in Figure 17. The figure also includes Comparative Example 8 (composition 100 mol%). In the figure, □: CO2 permeability coefficients of Examples 7 to 11, Comparative Example 8, and Comparative Example 10, ■: N2 permeability coefficients of Examples 7 to 11, Comparative Example 8, and Comparative Example 10 The composition dependency of CO2 selectivity is indicated by ◇ in Figure 18. The graph also includes Comparative Example 8 (composition 100 mol%) (♦).

[0146] Furthermore, for Example 8 and Comparative Example 10, the gas permeability coefficient was measured at 40°C using the four CO2 / N2 mixed gases with different CO2 compositions, and the dependence of the CO2 permeability coefficient, N2 permeability coefficient, and CO2 selectivity on the CO2 partial pressure was investigated. In Comparative Example 10, as described above, the supply side gas flow rate was 100 ml In Example 8, the measurement was performed at a feed gas flow rate of 400 ml / min, and the permeation gas flow rate was 200 ml / min. The CO2 partial pressure dependence of the obtained CO2 permeability coefficient, N2 permeability coefficient, and CO2 selectivity is shown in Figures 19, 20, and 21. In the figure, ■: Example 8, ◆: Comparative Example 10 As shown in Figure 19, the reason why increasing the gas flow rate in Example 8 improved the CO2 permeability coefficient compared to Figure 17 is because the increase in gas flow rate reduced the CO2 partial pressure on the permeation side, increasing the difference in CO2 partial pressure between the supply side and the permeation side. However, for the separation membrane of Comparative Example 10, which has a low CO2 permeability coefficient, the CO2 permeability coefficient does not change even when the permeation gas flow rate is increased.

[0147] [Examples 12 and 13] In Examples 12 and 13, differences in the substrates used were investigated.

[0148] Example 12 A CO separation membrane was prepared as Example 12 in the same manner as in Example 8, except that the mixed solution of [HDAH][TfN] and [emim][AcO] ([HDAH][TfN] 10 mol%) used in Example 8 was impregnated into an alumina-coated filter.

[0149] Example 13 A CO2 separation membrane was prepared as Example 13 in the same manner as in Example 12, except that the mixed solution was impregnated into a titania-coated filter.

[0150] The gas permeability coefficients of Examples 12 and 13 were measured using the four CO2 / N2 mixed gases with different CO2 compositions at a supply gas flow rate of 400 ml / min and a permeation gas flow rate of 200 ml / min, and the dependence on CO2 partial pressure was examined. The CO partial pressure dependence of the obtained CO permeability coefficient, N permeability coefficient, and CO selectivity is shown in Figures 19, 20, and 21. Note that Example 8 and Comparative Example 10 (100 mol%) were also included in these figures. In the figure, ■: Example 8, ◇: Example 12, ◯: Example 13, ◆: Comparative Example 10

[0151] [Example 14] [APAH][TfN] was used instead of [HDAH][TfN] used in Example 8, and a liquid (10 mol% of [APAH][TfN]) was prepared by mixing [APAH][TfN] with [emim][AcO] as ionic liquid (II). The resulting mixture was impregnated into a hydrophilized PTFE filter in the same manner as above to prepare a CO2 separation membrane, which was designated Example 14.

[0152] The gas permeability coefficient of Example 14 was measured at a temperature of 40° C., a feed gas flow rate of 400 ml / min, and a permeation gas flow rate of 40 ml / min, and the composition dependency was examined. The composition dependence of the obtained CO2 permeability coefficient, N2 permeability coefficient, and CO2 selectivity at a CO2 partial pressure of 0.04 kPa (CO2 composition 0.04 mol%) is shown by triangles in Figures 22, 23, and 24. In these figures, the above-mentioned Example 8 ([HDAH][Tf2N] composition 10 mol%) and Comparative Example 8 ([HDAH][Tf2N] composition 100 mol%) are also included and are indicated by squares.

[0153] [Example 15, Comparative Example 11] In Example 15 and Comparative Example 11, 3-(2-methoxyethoxy)propionate, a type of oxoacid anion, was used as the ionic liquid (II), as shown in Table 4. The table also includes Comparative Example 8.

[0154] Example 15 A CO separation membrane was prepared as Example 15 in the same manner as in Example 14, except that a mixture of [HDAH][TfN] and the [emim][1O2OPrO] obtained in Synthesis Example 8 ([HDAH][TfN] 10 mol%) was used.

[0155] (Comparative Example 11) A CO2 separation membrane was prepared as Comparative Example 11 in the same manner as in Example 15, except that only [emim][1O2OPrO] was used.

[0156] [Table 4]

[0157] The gas permeability coefficients of Example 15 and Comparative Example 11 were measured at 40°C to examine the composition dependency. In Example 15, the supply gas flow rate was 400 ml In Comparative Example 11, the measurement was performed at a feed gas flow rate of 100 ml / min. ml / min, and the permeation gas flow rate was 20 ml / min. The composition dependence of the CO2 permeability coefficient and N2 permeability coefficient at a CO2 partial pressure of 0.04 kPa (CO2 composition 0.04 mol%) is shown in FIG. In the figure, ◇: CO2 permeability coefficients of Example 15 and Comparative Examples 8 and 11, ◆: N2 permeability coefficients of Example 15 and Comparative Examples 8 and 11 The plot of the N2 permeability coefficient for Example 15 (◆) overlaps with that for Example 8 (■). The composition dependence of the obtained CO2 selectivity is shown by the ♦ mark in FIG.

[0158] [Summary] 17 and 18, it was found that the CO2 separation membranes using an ionic liquid mixture of [HDAH][Tf2N] and [emim][AcO] (Examples 7 to 10) exhibited higher CO2 permeability coefficients and CO2 selectivities than the CO2 separation membranes using each pure ionic liquid (Comparative Examples 8 and 10). Furthermore, the CO2 separation membrane using an ionic liquid mixture of [HDAH][Tf2N] and [emim][AcO] (Example 11) also showed superior CO2 permeability selectivity to the CO2 separation membrane using only [HDAH][Tf2N] (Comparative Example 8). As can be seen from Figures 19 and 21, this significant improvement effect was observed even when the membrane substrate was changed from the hydrophilized PTFE filter (Example 8) to an alumina-coated filter (Example 12) and a titania-coated filter (Example 13). Furthermore, as shown in Figures 22 and 24, even when the [HDAH][Tf2N] in Example 8 (□) was changed to [APAH][Tf2N] (Example 14) (△), a higher CO2 permeability coefficient and CO2 selectivity were observed than the CO2 separation membrane using only [emim][AcO] (Comparative Example 10) (■). Furthermore, as shown in Figures 17 and 18, when the [emim][AcO] in Example 8 (■, ◇) was changed to [emim][1O2OPrO] (Example 15) (◆, ◆), the CO2 permeation selectivity of Example 15 decreased compared to Example 8, but it still exhibited a higher CO2 permeability coefficient and CO2 selectivity than the CO2 separation membrane using only [HDAH][Tf2N] (Comparative Example 8) (◇, ◆), the CO2 separation membrane using only [emim][AcO] (Comparative Example 10) (□, ◇), and the CO2 separation membrane using only [emim][1O2OPrO] (Comparative Example 11) (◆, ◆). Furthermore, compared to Example 1 (□ in Figures 11 and 13), Example 8 (□ in Figure 17, ◇ in Figure 18) and Example 15 (◇ in Figure 17, ◆ in Figure 18) showed higher CO2 permeability coefficients and CO2 selectivities. This demonstrates that a composition using an ionic liquid with carboxylate (acetate and 3-(2-methoxyethoxy)propionate) as the anion can provide a CO2 separation membrane with superior CO2 permeability selectivity than a composition using an ionic liquid with phosphonate (methylphosphonate) as the anion.

[0159] [Examples 16 to 20, Comparative Example 12] The effects of ionic liquids (I) other than [HDAH][TfN] and [APAH][TfN] were investigated using 2-aminoethylaminium bis(trifluoromethylsulfonyl)amide ([EDAH][TfN]) obtained in Synthesis Example 5 above, 2-(2-(aminoethyl)amino)ethylaminium bis(trifluoromethylsulfonyl)amide ([DETAH][TfN]) obtained in Synthesis Example 6 above, and 2-(2-(2-(aminoethyl)aminoethyl)amino)ethylaminium bis(trifluoromethylsulfonyl)amide ([TETAH][TfN]) obtained in Synthesis Example 7 above.

[0160] Example 16 [EDAH][Tf2N] and [emim][AcO] were mixed to prepare a liquid as shown in the following Table 5. Table 5 also includes Example 14 and Comparative Example 10. The resulting mixture was impregnated into a hydrophilized PTFE filter in the same manner as above to prepare a CO2 separation membrane, which was designated Example 16.

[0161] (Examples 17 to 20, Comparative Example 12) CO2 separation membranes were prepared in the same manner as in Example 16, except that [EDAH][Tf2N] in Example 16 was changed as shown in Table 5 below, and were designated as Examples 17 to 20 and Comparative Example 12.

[0162] [Table 5]

[0163] The gas permeability coefficients of Examples 16 to 20 and Comparative Example 12 were measured at a temperature of 40° C., a feed gas flow rate of 400 ml / min, and a permeation gas flow rate of 200 ml / min, and the composition dependency was investigated. The composition dependence of the obtained CO2 permeability coefficient, N2 permeability coefficient, and CO2 selectivity at a CO2 partial pressure of 0.04 kPa (CO2 composition 0.04 mol%) is shown in the above-mentioned FIGS. These figures also include Example 8 (composition 10 mol%) and Comparative Example 8 (composition 100 mol%), which used the above-mentioned [HDAH][TfN], and Example 14 (composition 10 mol%) and Comparative Example 10 (composition 0 mol%), which used the above-mentioned [APAH][TfN]. In the figure, square indicates Example 8, Comparative Example 8, triangle indicates Example 14, black lines indicate Example 16, Comparative Example 10, diamonds indicate Examples 17 to 19, and diamonds indicate Example 20, Comparative Example 12.

[0164] [Summary] 22 and 24, the CO2 separation membranes (Examples 16 to 20) made of an ionic liquid composition containing [EDAH][Tf2N], [DETAH][Tf2N], or a mixture of [TETAH][Tf2N] and [emim][AcO] showed higher CO2 permeability coefficients and CO2 selectivities than the CO2 separation membrane (Comparative Example 10) made of an ionic liquid composition containing only [emim][AcO] or the CO2 separation membrane (Comparative Example 12) made of an ionic liquid composition containing only [TETAH][Tf2N]. When comparing Examples 8 and 14, the CO2 permeability coefficient of [HDAH][Tf2N] (Example 8) was the highest, decreasing in the order of [DETAH][Tf2N] (Example 18), [TETAH][Tf2N] (Example 20), [APAH][Tf2N] (Example 14), and [EDAH][Tf2N] (Example 16). As described above, it has been found that the ionic liquid (I) of the present invention can improve the CO2 permeation selectivity of a CO2 separation membrane by changing the molecular structure of its aminium.

[0165] [Examples 21 to 22, Comparative Examples 13 and 14] Regarding the effect of cations other than [emim] in ionic liquid (II), N,N-diethyl-N-methyl-N-heptylammonium acetate ([N 1227 [AcO]), or N,N-diethyl-N-methyl-N-(6-hydroxyhexyl)ammonium acetate ([N 1226OH [AcO]) was used for the study.

[0166] (Examples 21 and 22) As shown in Table 6 below, [HDAH][Tf2N] and [N 1227 ][AcO] or [N 1226OH A liquid mixture of [AcO] and [AcO] was prepared, and the resulting mixture was impregnated into a hydrophilized PTFE filter in the same manner as described above to prepare a CO2 separation membrane, which was designated as Examples 21 and 22, respectively.

[0167] (Comparative Examples 13 and 14) In Example 21 or 22, [N 1227 ][AcO]Also[N 1226OH CO2 separation membranes were prepared in the same manner as in Examples 21 and 22, except that only [AcO] was used, and these were designated as Comparative Examples 13 and 14. Table 6 also includes the above-mentioned Comparative Example 8 (composition 100 mol%).

[0168] [Table 6]

[0169] The gas permeability coefficients of Examples 21 and 22 and Comparative Examples 13 and 14 were measured at a temperature of 40°C, a feed gas flow rate of 400 ml / min, and a permeation gas flow rate of 20 to 40 ml / min, to examine the composition dependency. The composition dependence of the obtained CO permeability coefficient, N permeability coefficient, and CO selectivity at a CO partial pressure of 0.04 kPa (CO composition 0.04 mol%) is shown in Figures 25, 26, and 27. Note that these figures also include Comparative Example 8 ([HDAH][TfN] composition 100 mol%). In the figure, square: Example 21, black: Example 22, diamond: Comparative Example 13, diamond: Comparative Example 14, triangle: Comparative Example 8

[0170] [Summary] As can be seen from Figures 25 and 27, [HDAH][Tf2N] and [N 1227 ][AcO] or [N 1226OH The CO2 separation membrane (Example 21 or Example 22) made of an ionic liquid composition containing [N 1227 ][AcO] or [N 1226OHThe CO2 permeability coefficient and CO2 selectivity were higher than those of a CO2 separation membrane made of an ionic liquid composition containing only [HDAH][TfN] (Comparative Example 13 or Comparative Example 14) or a CO2 separation membrane made of an ionic liquid composition containing only [HDAH][TfN] (Comparative Example 8). As described above, it was found that in ionic liquid (II) having an oxo acid anion, the CO2 permselectivity of a CO2 separation membrane can be improved even when the molecular structure of the cation is ammonium.

[0171] [Summary] We have demonstrated that CO2 separation membranes using a mixture of two ionic liquids (I) and (II) exhibit higher CO2 permeability coefficients and CO2 selectivities than CO2 separation membranes using each pure ionic liquid. Ionic liquid (I) is an ionic liquid containing an aminium cation with a primary amino group and / or a secondary amino group. Ionic liquid (II) is a hydrogen-bond-accepting ionic liquid with a cation lacking a primary or secondary amino group and an oxoacid anion such as carboxylate, phosphinate, or phosphonate, exhibiting excellent CO2 solubility and solubility of the product after CO2 absorption. When the composition of ionic liquid (I) is preferably in the range of 5 mol% to 80 mol%, and particularly preferably in the range of 5 mol% to 40 mol%, a greater improvement in CO2 permeation selectivity is observed. On the other hand, it is clear from the examples and comparative examples that the excellent effects of improving carbon dioxide permeability and permeation selectivity of the present invention cannot be achieved when using ionic liquids without oxoacid anions, such as [emim][TfN]. That is, in the ionic liquid composition of the present invention, by using an aminium-based ionic liquid (I) having an amino group and an ionic liquid (II) having an oxoacid anion such as carboxylate, phosphate, or phosphonate and having excellent hydrogen bond accepting properties, not only can the solubility of carbon dioxide be increased, but also the diffusion rate of the carrier that has chemically reacted with carbon dioxide can be improved, and as a result, the carbon dioxide permeability and permeation selectivity of the carbon dioxide separation membrane can be significantly improved. [Industrial Applicability]

[0172] The present invention enables efficient separation and recovery of carbon dioxide ranging from high partial pressures to low partial pressures of 1 kPa or less. This technology can be used not only in processes for separating and recovering high-pressure carbon dioxide emitted from biogas production facilities, biomass power generation facilities, incinerators, chemical plants, steel plants, and power plants, but also for rapid and energy-efficient separation and recovery of low-pressure carbon dioxide of 1 kPa or less contained in the atmosphere and in air in environments where normal ventilation is difficult, such as high-rise buildings and enclosed work spaces. This allows unused carbon dioxide to be utilized, for example, to promote the growth of plants and algae. Furthermore, since carbon dioxide can be removed from a space without ventilation, the heat energy of the space is not wasted outside the space, thereby reducing the energy required for air conditioning. [Explanation of symbols]

[0173] 1: CO2 / N2 standard gas cylinder 2: N2 gas cylinder 3: Ar gas cylinder 4~6: Mass flow controller 7:CO2 separation membrane 8: Separation membrane holder 9: Oven 10, 11: Thermohygrometer 12, 14: Trap 13: Chiller 15, 16: Back pressure valve 17, 20: Soap film flowmeter 18: Gas chromatograph (TCD-GC) 19:CO2 concentration meter

Claims

1. An ionic liquid composition for use in a carbon dioxide separation membrane, Contains an ionic liquid (I) and an ionic liquid (II), The ionic liquid (I) is an aminium having a cation having one or more primary or secondary amino groups and an ethylenediamine or propylenediamine skeleton, The ionic liquid (II) is an ionic liquid composition for use in a carbon dioxide separation membrane, characterized in that the cation of the ionic liquid (II) does not have a primary or secondary amino group and the anion is an oxo acid anion.

2. The ionic liquid composition for a carbon dioxide separation membrane according to claim 1, wherein the aminium is one or more selected from 2-aminoethylaminium, 2-(N-hydroxyethylamino)ethylaminium, 3-aminopropylaminium, 3-(N-methylamino)propylaminium, 2-(2-(aminoethyl)amino)ethylaminium, and 2-(2-(2-(aminoethyl)aminoethyl)amino)ethylaminium.

3. 3. The ionic liquid composition for a carbon dioxide separation membrane according to claim 1, wherein the anion of the ionic liquid (I) is bis(trifluoromethylsulfonyl)amide.

4. 4. The ionic liquid composition for a carbon dioxide separation membrane according to claim 1, wherein the oxoacid anion is one or more selected from the group consisting of carboxylate, phosphate, and phosphonate.

5. 5. The ionic liquid composition for a carbon dioxide separation membrane according to claim 4, wherein the oxoacid anion is one or more selected from acetate, 2-(1-methoxyethoxy)propionate, and methylphosphonate.

6. 6. The ionic liquid composition for a carbon dioxide separation membrane according to claim 4 or 5, wherein the cation of the ionic liquid (II) is one or more selected from the group consisting of 1-ethyl-3-methylimidazolium, N,N-diethyl-N-methyl-N-heptylammonium, and N,N-diethyl-N-methyl-N-(6-hydroxyhexyl)ammonium.

7. A carbon dioxide separation membrane characterized by retaining the ionic liquid composition for a carbon dioxide separation membrane according to any one of claims 1 to 6.

8. The carbon dioxide separation membrane according to claim 7, characterized in that the carbon dioxide separation membrane comprises an ionic liquid-affinity porous layer in which the ionic liquid composition for carbon dioxide separation membranes is retained in pores, and an ionic liquid-non-affinity porous layer.

9. The carbon dioxide separation membrane according to claim 8 , wherein the porous layer with an affinity for an ionic liquid contains an inorganic material.

10. The carbon dioxide separation membrane according to claim 9, wherein the inorganic material contains metal oxide particles having an average particle size of 0.001 to 5 μm on a number basis.

11. The carbon dioxide separation membrane according to any one of claims 8 to 10, wherein the porous layer with an affinity for an ionic liquid has an average thickness of 0.01 to 10 µm.

12. The carbon dioxide separation membrane according to any one of claims 7 to 11, for separating and concentrating carbon dioxide having a partial pressure of 1 kPa or less.

13. A carbon dioxide concentrating device comprising the carbon dioxide separation membrane according to any one of claims 7 to 12.

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