Gas separation membrane

Abstract

To provide a gas separation membrane excellent in gas separability of carbon dioxide from nitrogen and gas permeability of carbon dioxide.SOLUTION: A gas separation membrane selectively permeating carbon dioxide has a thin film of a derivative in which a part of methyl groups of polydimethylsiloxane is substituted with a substituent X, and a molecular descriptor αMOL of the substituent X calculated from the following formulas (1) and (2) satisfies the following formula (3). [In the above formula (1), i is a natural number from 1 to N. N is the number of atoms excluding hydrogen atoms contained in the substituent X.] [In the above formula (2), rAi is a covalent bond distance of each atom excluding a hydrogen atom contained in the substituent X. rC is the covalent bond distance of the sp3 orbital of the carbon atom.]SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to gas separation membranes. [Background technology]

[0002] To achieve carbon neutrality, technologies are being considered to capture and directly capture carbon dioxide from the atmosphere. Known technologies include chemical absorption and adsorption, which involves absorbing and adsorbing carbon dioxide into an absorbent or adsorbent, and membrane separation, which separates carbon dioxide using a gas separation membrane.

[0003] For example, Patent Document 1 discloses a gas-selective permeable composite membrane that selectively allows specific gases to pass through. This gas-selective permeable composite membrane is manufactured through a process that involves laminating a thin film of a siloxane compound on a film-like polymeric porous support, subjecting the surface layer of the thin film to plasma treatment with a non-polymerizable gas, and depositing a plasma-polymerized membrane on the thin film. It also discloses that these processes result in a gas-selective permeable composite membrane with strong adhesion between the thin film and the plasma-polymerized membrane, and that the thickness of the thin film is 1 μm to 30 μm. It also discloses that this gas-selective permeable composite membrane can selectively allow gases such as oxygen, hydrogen, and helium to pass through, and that the separated gases can be recovered. It is believed that carbon dioxide can also be separated using such a gas-selective permeable composite membrane. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 60-075320 Summary of the Invention [Problem to be solved by the invention]

[0005] In the gas-selective permeable composite membrane described in Patent Document 1, two thin films, each made of an organosiloxane compound, are laminated on a porous support.

[0006] As a result of extensive investigations, the present inventors have found that there is room for improvement in the gas separation properties of organosiloxane compounds for carbon dioxide relative to nitrogen and in the gas permeability of carbon dioxide.

[0007] Therefore, it is an issue to realize a gas separation membrane that has excellent gas separation properties for carbon dioxide from nitrogen and also has excellent gas permeability for carbon dioxide. [Means for solving the problem]

[0008] The gas separation membrane according to the application example of the present invention is A gas separation membrane that selectively permeates and separates carbon dioxide from a mixed gas containing carbon dioxide and nitrogen, a thin film of a polydimethylsiloxane derivative comprising a main chain formed of siloxane bonds and methyl groups bonded to silicon atoms contained in the siloxane bonds, wherein a portion of the methyl groups are substituted with substituents X; The molecular descriptor α of the substituent X calculated based on the following formulas (1) and (2) MOL satisfies the following formula (3).

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[0009] [Figure 1] 1 is a cross-sectional view schematically showing a gas separation membrane according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] The gas separation membrane of the present invention will be described in detail below based on the embodiments shown in the accompanying drawings.

[0011] 1. Overview of gas separation membranes First, the configuration of the gas separation membrane according to the embodiment will be described.

[0012] Fig. 1 is a cross-sectional view schematically showing a gas separation membrane 1 according to an embodiment. In Fig. 1 of the present application, three mutually orthogonal axes are set as an X-axis, a Y-axis, and a Z-axis, each indicated by an arrow. The base end of the arrow indicating each axis is designated as "minus" and the tip end is designated as "plus."

[0013] The gas separation membrane 1 shown in Fig. 1 has the function of selectively permeating and separating carbon dioxide from a mixed gas containing carbon dioxide and nitrogen. The gas separation membrane 1 shown in Fig. 1 is a composite membrane having a first layer 3 and a second layer 4. Note that the configuration of the gas separation membrane 1 shown in Fig. 1 is an example, and for example, the first layer 3 may be omitted.

[0014] The average thickness of the second layer 4 is set to be thinner than the average thickness of the first layer 3. This allows the second layer 4 to have good gas separation properties and high gas permeability.

[0015] In the gas separation membrane 1 shown in Figure 1, the positive side of the Z axis is defined as "up" and the negative side of the Z axis is defined as "down." A mixed gas is supplied to the upper side of the gas separation membrane 1. In addition, in the gas separation membrane 1 of Figure 1, carbon dioxide is selectively permeated from the top to the bottom and separated.

[0016] 1.1.First layer The form of the first layer 3 is not particularly limited, and may be a sheet (flat plate) as shown in FIG. 1, or may be a spiral, tubular, hollow fiber, or other form.

[0017] Polymeric materials can be used as materials for the first layer 3. Examples of polymeric materials include polyolefin resins such as polyethylene and polypropylene, fluorine-containing resins such as polytetrafluoroethylene, polyvinyl fluoride, and polyvinylidene fluoride, polystyrene, cellulose acetate, polyurethane, polyacrylonitrile, polyphenylene oxide, polysulfone, polyethersulfone, polyimide, polyaramid, and organopolysiloxane.

[0018] Among these, organopolysiloxane is preferably used as the constituent material of the first layer 3. One molecule of organopolysiloxane has the basic constituent units R 1 SiO 3 / 2 Units expressed as T units, R 2 R 3 SiO 2 / 2 Units expressed by (D units) and R 4 R 5 R 6 SiO 1 / 2 It contains at least the unit (M unit) represented by the following formula: 1 ~R 6 is an aliphatic hydrocarbon or a hydrogen atom. One molecule of organopolysiloxane is composed of a combination of these T units, D units, and M units.

[0019] Specific examples of organopolysiloxanes include polydimethylsiloxane, polymethylphenylsiloxane, polydiphenylsiloxane, polysulfone / polyhydroxystyrene / polydimethylsiloxane copolymer, dimethylsiloxane / methylvinylsiloxane copolymer, dimethylsiloxane / diphenylsiloxane / methylvinylsiloxane copolymer, methyl-3,3,3-trifluoropropylsiloxane / methylvinylsiloxane copolymer, dimethylsiloxane / methylphenylsiloxane / methylvinylsiloxane copolymer, diphenylsiloxane / dimethylsiloxane copolymer terminated with vinyl, polydimethylsiloxane terminated with vinyl, polydimethylsiloxane terminated with H, dimethylsiloxane-methylhydrosiloxane copolymer, etc. These also include forms in which crosslinked reaction products are formed. Furthermore, the constituent material of the first layer 3 may be a composite of one or more of these, or a composite material in which organopolysiloxane is the main component in terms of mass ratio, and other resin components are used in combination.

[0020] Organopolysiloxane has large interatomic distances of about 1.8 angstroms between the Si-O and Si-C bonds that make it up, resulting in a large free volume, which allows for good diffusion of carbon dioxide molecules and good gas permeability to carbon dioxide, making it an effective material for the first layer 3.

[0021] The average thickness of the first layer 3 is preferably set to be thicker than the average thickness of the thin film included in the second layer 4. This provides the first layer 3 with necessary and sufficient mechanical properties as a base layer of the gas separation membrane 1. The difference between the average thickness of the first layer 3 and the average thickness of the second layer 4 is preferably 5 μm or more, and more preferably 30 μm or more.

[0022] The average thickness of the first layer 3 is preferably 1 μm or more and 3000 μm or less, more preferably 5 μm or more and 500 μm or less, and even more preferably 10 μm or more and 150 μm or less, thereby realizing a first layer 3 having necessary and sufficient mechanical properties and sufficient gas permeability.

[0023] The average thickness of the first layer 3 is determined, for example, by observing the cross section of the gas separation membrane 1 under magnification and averaging the thicknesses measured at 10 points on the first layer 3.

[0024] Furthermore, the carbon dioxide gas permeability of the first layer 3 is preferably set to be higher than that of the second layer 4. Specifically, when carbon dioxide is the target gas component, the gas permeation rate of the first layer 3 is higher than the gas permeation rate of the second layer 4. This enables the first layer 3 to mechanically support the second layer 4 while imparting good gas permeability to the gas separation membrane 1.

[0025] High gas permeability means a high carbon dioxide permeation rate. Specifically, when carbon dioxide is supplied to each of the first layer 3 alone and the second layer 4 alone at a total upstream pressure of 5 MPa, the carbon dioxide permeation rate through the first layer 3 should be higher than the carbon dioxide permeation rate through the second layer 4.

[0026] The carbon dioxide permeation rate of the first layer 3 is 1×10 when carbon dioxide is supplied to the upstream side at a total pressure of 5 MPa at a temperature of 40°C. -5 cm 3 (STP) / cm 2 ·sec·cmHg (10 GPU) or more is preferable, and 3×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg (30 GPU) or more is more preferable, 100 GPU or more is even more preferable, and 200 GPU or more is particularly preferable.

[0027] The first layer 3 can be manufactured by a method for manufacturing a sheet or film, or by a method in which a film is formed on a sacrificial layer and then the sacrificial layer is removed.

[0028] 1.2.Second layer The second layer 4 is provided on an upper surface 31 (one surface) of the first layer 3. The second layer 4 has a gas separating property for separating carbon dioxide from nitrogen.

[0029] 1.2.1. Molecular Descriptor α of Substituent X MOL The second layer 4 includes a thin film of a polydimethylsiloxane derivative. The second layer 4 may be composed of this thin film alone, or may include a monomolecular film or monoatomic film provided on the surface of the thin film. An example of the monomolecular film is a coating derived from a silane coupling agent.

[0030] The polydimethylsiloxane derivative is formed by substituting a portion of the methyl groups contained in polydimethylsiloxane with a substituent X. The base polydimethylsiloxane contains a main chain formed by siloxane bonds and methyl groups bonded to silicon atoms contained in the siloxane bonds, and has excellent gas permeability to carbon dioxide. By introducing the substituent X into such a polydimethylsiloxane, the gas separation ability of carbon dioxide relative to nitrogen can be improved. As a result, a gas separation membrane 1 can be realized that has excellent gas separation ability of carbon dioxide relative to nitrogen and gas permeability to carbon dioxide.

[0031] However, no specific guidelines are known for selecting the type of substituent X that will solve the above problems. Therefore, the present inventors conducted extensive research to optimize the molecular structure of polydimethylsiloxane derivatives. They discovered that by selecting the substituent X based on the covalent bond distance between each atom, it is possible to achieve a thin film that exhibits good carbon dioxide gas separation properties relative to nitrogen without impairing carbon dioxide gas permeability, and thus completed the present invention.

[0032] Specifically, in a thin film of a polydimethylsiloxane derivative, the molecular descriptor α of the substituent X calculated based on the following formulas (1) and (2) is MOL satisfies the following formula (3).

[0033]

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[0034]

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[0035]

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[0036] Molecular descriptor α of substituent X MOL By satisfying the above formula (3), a second layer 4 with high carbon dioxide gas separation properties relative to nitrogen can be realized. Furthermore, such a substituent X is unlikely to inhibit the excellent carbon dioxide gas permeability of polydimethylsiloxane. Therefore, a gas separation membrane 1 with excellent carbon dioxide gas separation properties relative to nitrogen and excellent carbon dioxide gas permeability can be realized.

[0037] Molecular descriptor α of substituent X MOL is also called Hall-Kier Alpha, and is the covalent bond distance r C Covalent bond distance r to Ai Descriptor α containing the ratio of Ai Therefore, the covalent bond distance r Ai Substituent X composed of atoms with a short length tends to satisfy the above formula (2).

[0038] Also, the molecular descriptor α MOL preferably satisfies the following formula (3-1), and more preferably satisfies the following formula (3-2).

[0039]

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[0040]

number

[0041] On the other hand, the molecular descriptor α MOL Although there are no particular restrictions on the lower limit, taking into consideration the availability of constituent materials that can achieve this, it is preferably −7.00 or more, and more preferably −6.00 or more.

[0042] where the molecular descriptor α MOL Here is an example of calculation: As shown in the above formula (1), the molecular descriptor α MOL is the descriptor α of each atom in the substituent X, as defined in the above formula (2). Ai The descriptor α Ai In calculating the above, hydrogen atoms are excluded.

[0043] As an example, consider the case of butadiene (C=CC=C). Butadiene has four carbon atoms, but the electron orbitals of the four carbon atoms are all sp 2 sp orbital. 2 The distance of the covalent bond by the orbital is 0.67 Å. On the other hand, sp 3 The covalent bond distance due to the orbital is 0.77 Å. Therefore, the descriptor α defined in the above formula (2) Ai is (0.67 / 0.77)-1=-0.13. Then, the molecular descriptor α defined by the above formula (1) MOL becomes (-0.13)×4=-0.52.

[0044] As another example, consider the case of butane (CCCC). Butane has four carbon atoms, but the electron orbitals of the four carbon atoms are all sp 3 Therefore, the descriptor α defined in the above formula (2) Ai is (0.77 / 0.77)-1=0. Then, the molecular descriptor α defined by the above formula (1) MOL becomes 0×4=0.

[0045] As in the above example, the molecular descriptors α for various substituents X are MOL can be calculated. The covalent bond distances r due to representative electron orbitals are listed in Table 1 below.

[0046] [Table 1]

[0047] The polydimethylsiloxane derivative is represented by the following general formula (A).

[0048] [ka] [In the above general formula (A), Me is a methyl group, X is a substituent X, and n is the number of repeating units.]

[0049] The substituent X in the polydimethylsiloxane derivative replaces a portion of the methyl groups of the polydimethylsiloxane. The ratio (substitution rate) of the methyl groups of the polydimethylsiloxane substituted with the substituent X is not particularly limited, but is preferably 10% to 40% in number, and more preferably 20% to 30%. This allows for the realization of a gas separation membrane 1 that exhibits both good carbon dioxide separation properties relative to nitrogen and good carbon dioxide gas permeability.

[0050] If the substitution rate is below the lower limit, the gas separation ability may be reduced, whereas if the substitution rate is above the upper limit, the gas permeability may be reduced.

[0051] Molecular Descriptor α MOL As a molecular model for calculating the above, a molecular model having a structure represented by the following formula (X-0) is used as a model simulating the above general formula (A).

[0052] [ka]

[0053] The weight-average molecular weight of the polydimethylsiloxane derivative is not particularly limited, but is preferably from 1,000 to 25,000, more preferably from 2,000 to 15,000, and even more preferably from 2,000 to 8,000. This makes it possible to obtain a polydimethylsiloxane derivative that can form a thin film having sufficient film strength and excellent gas permeability to carbon dioxide.

[0054] The weight average molecular weight of the polydimethylsiloxane derivative is a polystyrene-equivalent molecular weight measured by gel permeation chromatography (GPC method).

[0055] Although there is no particular limitation on such a substituent X, groups containing ring structures such as aromatic rings and heterocyclic rings are preferably used. By containing such ring structures, the covalent bond distance of the atoms in the group can be shortened. This allows the molecular descriptor α MOL A substituent X with a small value is obtained.

[0056] Specific examples of the substituent X containing an aromatic ring include structures represented by the following formula (X-1).

[0057] [ka]

[0058] In the above formula (X-1), * represents a bond bonded to a Si atom contained in the polydimethylsiloxane derivative. The same applies to other formulas described below.

[0059] Specific examples of heterocycles include saturated or unsaturated 5- or 6-membered heterocycles. Examples of heteroatoms contained in heterocycles include nitrogen atoms, sulfur atoms, and oxygen atoms. The number of heteroatoms contained in one heterocycle is preferably 1 to 3.

[0060] Of these, heterocycles preferably include those in which the heteroatom is a nitrogen atom (nitrogen-containing heterocycle). In the substituent X containing a nitrogen-containing heterocycle, the molecular descriptor α A This contributes to realizing the second layer 4 having particularly high gas separation properties for carbon dioxide from nitrogen.

[0061] Examples of heterocycles (nitrogen-containing heterocycles) in which the heteroatom is a nitrogen atom include diazetidine, pyrazolidone, imidazolindine, pyrazoline, imidazoline, pyrazole, imidazole, triazole, tetrazole, oxazole, isoxazole, isothiazole, oxadiazole, thiadiazole, thiazolidinedione, oxazolidone, hydantoin, piperazine, pyridazine, pyrazine, triazine, morpholine, oxazine, thiomorpholine, thiazine, cytosine, thymine, uracil, thiomorpholine dioxide, tetrahydropyrrolopyrrole, dihydropyrrolopyrrole, furopyrrole, thienopyrrole, indazole, benzimidazole, aza Examples of such compounds include indole, pyrazolopyrimidine, purine, benzisoxazole, benzisothiazole, benzoxazole, benzthiazole, benzothiazazole, adenine, guanine, quinoxaline, fusalazine, quinazoline, cinnoline, naphthilidine, pyridopyrimidine, pyridopyrazine, pteridine, benzoxazine, phenazine, phenoxazine, phenothiazine, diazepine, thiazepine, pyrrole, pyridine, pyrimidine, triazine, indoline, indole, isoindole, indolizine, tetrahydroquinolinoline, dihydroisoquinolinoline, quinoline, isoquinoline, quinolinone, isoquinolinone, carbesol, and acridine.

[0062] Furthermore, the substituent X preferably contains a ring condensation structure or a ring assembly structure. By containing these structures, the covalent bond distance between the atoms of the group can be shortened. This allows the molecular descriptor α MOL In addition, the separation performance parameters described below can be optimized. As a result, a gas separation membrane 1 with particularly high gas separation properties can be obtained.

[0063] Specific examples of aromatic rings having a condensed ring structure include indene, naphthalene, azulene, fluorene, phenanthrene, anthracene, naphthacene, pyrene, and triphenylene.

[0064] Specific examples of aromatic rings having a ring assembly structure include biphenyl, terphenyl, 2-phenylnaphthyl, and 1,2'-binaphthyl.

[0065] Specific examples of the substituent X containing an aromatic ring having a fused ring structure or an assembled ring structure include structures represented by the following formula (X-2).

[0066] [ka]

[0067] Specific examples of heterocycles having a fused ring structure include indole, benzofuran, tindolizine, chromene, quinoline, purine, indazole, and carbazole.

[0068] Specific examples of the substituent X containing a heterocycle having a fused ring structure include structures represented by the following formula (X-3) or formula (X-4).

[0069] [ka]

[0070] [ka]

[0071] Specific examples of heterocycles having a ring assembly structure include 2,3'-bifuran and 2,2'-bipyrazin-6-yl.

[0072] Specific examples of the substituent X containing a heterocycle having a ring assembly structure include structures represented by formula (X-5), formula (X-6), formula (X-7), formula (X-8), formula (X-9), formula (X-10), and formula (X-11).

[0073] [ka]

[0074] [ka]

[0075] [ka]

[0076] [ka]

[0077] [ka]

[0078] [ka]

[0079] [ka]

[0080] Specific examples of the substituent X containing a heterocycle having a ring condensation structure or ring assembly structure include structures represented by formula (X-12), formula (X-13), and formula (X-14).

[0081] [ka]

[0082] [ka]

[0083] [ka]

[0084] Furthermore, the substituent X preferably contains a ketone group, more preferably a cyclic ketone group. This allows the molecular descriptor α MOL In addition, the separation performance parameters described below can be optimized. As a result, a gas separation membrane 1 with particularly high gas separation properties can be obtained.

[0085] Examples of the substituent X containing a ketone group include structures represented by the above formula (X-8), formula (X-13), formula (X-14), and the like.

[0086] These groups may further have a substituent, and examples of the substituent include the groups exemplified above.

[0087] The substituent X may contain any divalent group between the above-mentioned group and the Si atom. Examples of the divalent group include -NH-CO-, -NH-, -O-, -S-, -CO-, and -O-CO-.

[0088] 1.2.2. Separation performance parameters The gas separation ability of carbon dioxide from nitrogen is also called "carbon dioxide separation ability." Molecular descriptor α of substituent X MOL However, when the above formula (3) is satisfied, the second layer 4 can be said to have good carbon dioxide separation ability.

[0089] Carbon dioxide separation capacity depends on the gas used together with carbon dioxide in the gas mixture. The present inventors have conducted extensive research into an index that can be used to evaluate carbon dioxide separation capacity when selectively separating carbon dioxide from a gas mixture of carbon dioxide and nitrogen. They have found that carbon dioxide separation capacity can be quantified by an index called a separation performance parameter based on the activity coefficient of nitrogen and the activity coefficient of carbon dioxide in the second layer 4.

[0090] Specifically, the activity coefficient of nitrogen in the second layer 4 calculated by the COSMO-RS method is γ 2 N2 The activity coefficient of carbon dioxide in the second layer 4 is γ 2 CO2 The separation performance parameter of the second layer 4 at 25°C is ξ2 = ln(γ 2 N2 )-ln(γ 2 CO2 ) In this case, the second layer 4 preferably satisfies 0.56<ξ2. When the separation performance parameter ξ2 satisfies this relationship, the second layer 4 can be evaluated as having high carbon dioxide separation ability.

[0091] Activity coefficient of nitrogen in the second layer 4 γ 2 N2 represents the degree of non-ideality of nitrogen molecules in the second layer 4. Also, the activity coefficient of carbon dioxide in the second layer 4, γ 2 CO2 represents the degree of non-ideality of carbon dioxide molecules in the second layer 4. The larger the activity coefficient, the greater the non-ideality. When the non-ideality of carbon dioxide molecules is smaller than that of nitrogen molecules, the second layer 4 can be considered to have a higher affinity for carbon dioxide molecules than for nitrogen molecules. Therefore, the separation performance parameter of the second layer 4 at 25°C is defined as ξ2 = ln(γ 2 N2 )-ln(γ 2 CO2 ) is defined as

[0092] Molecular descriptor α of the aforementioned substituent X MOLHowever, when the above formula (3) is satisfied, the separation performance parameter ξ2 of the second layer 4 is more likely to satisfy 0.56<ξ2, and good carbon dioxide separation performance can be obtained.

[0093] The second layer 4 preferably satisfies 0.80≦ξ2, and more preferably 0.90≦ξ2. If the separation performance parameter ξ2 of the second layer 4 is below the lower limit, the relative affinity of carbon dioxide molecules to nitrogen molecules in the second layer 4 may decrease. As a result, the gas separation performance of carbon dioxide from nitrogen in the second layer 4 may decrease. On the other hand, the upper limit of the separation performance parameter ξ2 does not need to be particularly set, but considering the availability of constituent materials that can achieve it, it is preferable that ξ2≦2.00, and more preferably ξ2≦1.80.

[0094] In addition, the activity coefficient of nitrogen in the first layer 3 calculated by the COSMO-RS method is γ 1 N2 The activity coefficient of carbon dioxide in the first layer 3 is γ 1 CO2 The separation performance parameter of the first layer 3 at 25°C is ξ1 = ln(γ 1 N2 )-ln(γ 1 CO2 ) In this case, it is preferable that the first layer 3 and the second layer 4 satisfy ξ1<ξ2.

[0095] When the separation performance parameters ξ1 and ξ2 satisfy this relationship, the affinity of carbon dioxide molecules to nitrogen molecules is higher in the second layer 4 than in the first layer 3. This results in higher gas separation performance for carbon dioxide from nitrogen in the second layer 4. On the other hand, because the affinity of carbon dioxide molecules to the first layer 3 is lower than in the second layer 4, even if the pressure increases downstream of the gas separation membrane 1, the probability that carbon dioxide that has permeated the gas separation membrane 1 will permeate in the opposite direction to the first layer 3 can be reduced.

[0096] Furthermore, the first layer 3 and the second layer 4 preferably satisfy 0.10≦ξ2−ξ1, more preferably 0.30≦ξ2−ξ1, and even more preferably 0.50≦ξ2−ξ1, thereby enabling the gas separation performance of carbon dioxide relative to nitrogen to be particularly improved.

[0097] The upper limit of ξ2-ξ1 does not need to be specifically set, but taking into consideration the availability of constituent materials to achieve it, it is preferable that ξ2-ξ1≦1.20, and more preferably ξ2-ξ1≦1.00.

[0098] Here, the COSMO-RS method will be explained. The COSMO-RS method is based on the following three references (1) to (3). (1) Klamt, AJ Phys. Chem. 99, 2224 (1995). (2) Klamt, A.; Jonas, V.; Burger, T.; Lohrenz, JCJ Phys. Chem. A 102, 5074 (1998). (3) Eckert, F. and A. Klamt, AIChE Journal, 48, 369 (2002).

[0099] The COSMO-RS method is a calculation method that calculates the chemical potential μ of molecules by statistical mechanics for liquids based on the surface screening charge σ of molecules obtained by quantum chemical calculations, and determines various equilibrium physical properties. The COSMO-RS method calculates the activity coefficient γ 1 N2 , γ 1 CO2 , γ 2 N2 , γ 2 CO2 can be calculated.

[0100] When calculating the activity coefficient of a molecule, the effects of molecular volume and molecular surface area must be taken into account. These effects are expressed by Elbro's free volume combinatorial term. The Elbro free volume combinatorial term is described in the following reference (4). (4) Elbro, HS; Fredenslund, A.; Rasmussen, PA Macromolecules 23, 4707(1990).

[0101] The free volume corresponds to the volume of gaps in the polymer, that is, in the first layer 3 and the second layer 4, through which small molecules, that is, nitrogen molecules and carbon dioxide molecules, can move.

[0102] The free volume of any molecule i is v i F Let the volume of the hard core of molecule i be v i * The volume of molecule i in the system is v i Then, the free volume v i F is expressed by the following formula (a).

[0103]

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[0104] In the above equation (a), the volume of the hard core of molecule i, v i * is calculated by quantum chemical calculation. On the other hand, the volume of molecule i in the system, v i is the molecular weight M of molecule i i and density ρ i Using this, it is expressed by the following formula (b).

[0105]

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[0106] N in the above formula (b) A is Avogadro's number. When calculating the behavior of small molecules in a polymer, we consider molecule i as a polymer. Then, the molecular weight M i is 10000, and density ρ i are unified to 1.00 g / cc, and the activity coefficient at a temperature of 25°C is calculated.

[0107] Thermodynamic property prediction software such as BIOVIA COSMOtherm 2022 from Dassault Systèmes can be used for calculations using the COSMO-RS method. Parameterization can be performed using the BP_TZVPD_FINE_20.ctd file.

[0108] The molecular model used for calculating the molecular activity coefficient is a molecular model having a structure represented by the above general formula (A) with a substitution rate of the substituent X of 25%.

[0109] The average thickness of the thin film contained in the second layer 4 is not particularly limited, but is preferably 1 nm to 100 nm, more preferably 5 nm to 90 nm, and even more preferably 30 nm to 80 nm. This allows the second layer 4 to enhance its carbon dioxide gas permeability while ensuring the gas separation ability of carbon dioxide from nitrogen. If the average thickness of the thin film contained in the second layer 4 is below the lower limit, the gas separation ability may be reduced depending on the material of the second layer 4. On the other hand, if the average thickness of the thin film contained in the second layer 4 exceeds the upper limit, the gas permeability of carbon dioxide in the second layer 4 may be reduced depending on the material of the second layer 4.

[0110] The average thickness of the thin film included in the second layer 4 is determined, for example, by observing the cross section of the gas separation membrane 1 at an enlarged scale and averaging the thicknesses measured at 10 points on the thin film included in the second layer 4.

[0111] 1.3.Other Configurations The gas separation membrane 1 according to the embodiment has been described above, but an optional layer may be provided in at least one of the following locations: downstream of the first layer 3, and between the first layer 3 and the second layer 4. For example, a porous layer made of a porous material may be provided downstream of the first layer 3. The porous layer preferably has higher gas permeability and higher rigidity than the first layer 3. This can further increase the rigidity of the gas separation membrane 1, contributing to improving the shape retention and durability of the gas separation membrane 1.

[0112] Examples of materials for the porous layer include polymeric materials, ceramic materials, metal materials, etc. The material for the porous layer may also be a composite material containing these materials and other materials.

[0113] Examples of polymeric materials include polyolefin resins such as polyethylene and polypropylene, fluorine-containing resins such as polytetrafluoroethylene, polyvinyl fluoride and polyvinylidene fluoride, polystyrene, cellulose acetate, polyurethane, polyacrylonitrile, polyphenylene oxide, polysulfone, polyethersulfone, polyimide, and polyaramid.

[0114] Examples of ceramic materials include alumina, cordierite, mullite, silicon carbide, zirconia, etc. Examples of metal materials include stainless steel, etc.

[0115] The average thickness of the porous layer is not particularly limited, but is preferably from 1 μm to 3000 μm, more preferably from 5 μm to 500 μm, and even more preferably from 10 μm to 150 μm, so that the porous layer has the necessary and sufficient rigidity to support the first layer 3 and the second layer 4.

[0116] The average thickness of the porous layer is the average value of thicknesses measured at 10 points on the porous layer in the stacking direction. The thickness of the porous layer can be measured using, for example, a thickness gauge.

[0117] The average pore size of the porous layer is preferably 0.1 μm or less, more preferably 0.01 μm or more and 0.09 μm or less, and even more preferably 0.01 μm or more and 0.07 μm or less, which can prevent the first layer 3 from leaking out to the downstream side of the porous layer.

[0118] The average pore size of the porous layer is measured using a through-pore size evaluation device, such as a Perm Porometer manufactured by PMI.

[0119] The porosity of the porous layer is preferably 20% or more and 90% or less, and more preferably 30% or more and 80% or less. This allows the porous layer to have both good gas permeability and sufficient rigidity. The porosity of the porous layer is measured using the through-pore diameter evaluation device described above.

[0120] The gas separation membrane 1 described above preferably has a gas separation ratio of carbon dioxide to nitrogen of 10 or more, more preferably 15 or more. This makes it possible to obtain a gas separation membrane 1 suitable for separating carbon dioxide from the atmosphere. The gas separation ratio of carbon dioxide to nitrogen is determined as the ratio of the permeation rate of carbon dioxide to the permeation rate of nitrogen. Furthermore, the first layer 3 may be a porous layer as described above.

[0121] 2. Gas separation membrane manufacturing method The gas separation membrane 1 is produced, for example, by forming a membrane of the raw material of the second layer 4 on the upper surface 31 of the first layer 3.

[0122] Examples of methods for forming a film from the raw material of the second layer 4 include various liquid phase film forming methods such as immersion, dropping, inkjet, dispenser, spraying, screen printing, coater application, and spin coating, and gas phase film forming methods such as plasma CVD and plasma polymerization.

[0123] Of these, the inkjet method is preferably used. In the inkjet method, ink is ejected and fixed while an inkjet head is moved relative to the upper surface 31. A liquid containing the raw material for the second layer 4 is used as the ink. By using the inkjet method, it is possible to fix the desired amount of raw material at the desired position with a high probability. Therefore, it is possible to form a second layer 4 with a high coverage even if the film thickness is thin. As a result, it is possible to efficiently produce a gas separation membrane 1 that has high gas separation properties for carbon dioxide from nitrogen and also has excellent gas permeability for carbon dioxide.

[0124] Prior to the formation of the second layer 4, the upper surface 31 of the first layer 3 may be subjected to an activation treatment. The activation treatment is not particularly limited as long as it is a treatment that activates the upper surface 31. Examples of the activation treatment include a method of irradiating the upper surface 31 with energy rays, a method of heating the upper surface 31, a method of exposing the upper surface 31 to plasma or corona, and a method of exposing the upper surface 31 to ozone gas. Examples of the energy rays include infrared rays, ultraviolet rays, and visible light.

[0125] 3. Applications of gas separation membranes The gas separation membrane 1 according to the embodiment can be used for separating and capturing carbon dioxide from a mixed gas containing carbon dioxide and nitrogen, separating and purifying carbon dioxide, etc. In particular, it is effective to use the gas separation membrane 1 in a technology for separating and capturing carbon dioxide contained in the atmosphere (direct air capture (DAC)).

[0126] 4. Effects of the above embodiment As described above, the gas separation membrane 1 according to the embodiment is a gas separation membrane that selectively permeates and separates carbon dioxide from a gas mixture containing carbon dioxide and nitrogen, and has a thin film of a polydimethylsiloxane derivative. This thin film has a main chain formed of siloxane bonds and methyl groups bonded to silicon atoms contained in the siloxane bonds, with some of the methyl groups being substituted with substituents X. In the gas separation membrane 1, the molecular descriptor α of the substituent X, calculated based on the following formulas (1) and (2), isMOL satisfies the following formula (3).

[0127]

number

[0128]

number

[0129]

number

[0130] With this configuration, a gas separation membrane 1 having good gas separation properties for carbon dioxide relative to nitrogen and good gas permeability for carbon dioxide can be obtained.

[0131] In the gas separation membrane 1 according to the embodiment, the substituent X contains an aromatic ring. According to this configuration, the covalent bond distance of the atom of the substituent X can be shortened. This reduces the molecular descriptor α MOL A substituent X with a small value is obtained.

[0132] In the gas separation membrane 1 according to the embodiment, the substituent X contains a nitrogen-containing heterocycle. According to this configuration, the covalent bond distance of the atom of the substituent X can be shortened. This reduces the molecular descriptor α MOL A substituent X with a small value is obtained.

[0133] In the gas separation membrane 1 according to the embodiment, the substituent X includes a fused ring structure or a ring assembly structure.

[0134] This configuration can further shorten the covalent bond distance of the atom of the substituent X. This results in a gas separation membrane 1 with particularly high gas separation properties.

[0135] In the gas separation membrane 1 according to the embodiment, the substituent X includes a ketone group. With this configuration, a gas separation membrane 1 with particularly high gas separation properties can be obtained.

[0136] Furthermore, the gas separation membrane 1 according to the embodiment has a first layer 3 which has a thicker average thickness than the thin film and has high gas permeability to carbon dioxide, and a second layer 4 which includes a thin film provided on one side of the first layer 3.

[0137] With this configuration, the first layer 3 can mechanically support the second layer 4 while imparting good gas permeability to the gas separation membrane 1.

[0138] In addition, in the gas separation membrane 1 according to the embodiment, the average thickness of the thin film is 1 nm or more and 100 nm or less.

[0139] With this configuration, the gas permeability of the second layer 4 to carbon dioxide can be increased while ensuring the gas separation property of carbon dioxide from nitrogen.

[0140] Although the gas separation membrane according to the present invention has been described above based on the preferred embodiment, the present invention is not limited to this.

[0141] For example, the gas separation membrane of the present invention may be one in which each part of the above embodiment is replaced with a component having a similar function, or one in which any component is added to the above embodiment. [Example]

[0142] Next, specific examples of the present invention will be described. 5. Fabrication of Gas Separation Membranes 5.1. Example 1 First, a PDMS sheet was prepared as the first layer. The PDMS sheet was a 30 μm thick sheet made of unsubstituted polydimethylsiloxane. Next, one side of the PDMS sheet was subjected to plasma activation.

[0143] Next, solutions of polydimethylsiloxane derivatives shown in Table 2 were prepared. The substitution rate of the substituent X in the polydimethylsiloxane derivatives was set to 25%.

[0144] Next, the obtained solution was ejected onto the first layer by an inkjet method and then dried at 80° C. As a result, a thin film (second layer) made of a polydimethylsiloxane derivative was formed on the first layer, and a gas separation membrane was obtained.

[0145] 5.2. Examples 2 to 14 A gas separation membrane was obtained in the same manner as in Example 1, except that the constituent material of the second layer was changed as shown in Table 2 or Table 3.

[0146] 5.3. Comparative Example 1 The formation of the second layer was omitted, and the gas separation membrane of Comparative Example 1 was formed using only the first layer.

[0147] 5.4. Comparative Example 2 A gas separation membrane was obtained in the same manner as in Example 1, except that unsubstituted polydimethylsiloxane was used as the constituent material of the second layer.

[0148] 5.5. Comparative Example 3 A gas separation membrane was obtained in the same manner as in Example 1, except that the second layer was made of a polydimethylsiloxane derivative obtained by introducing a substituent containing an amino group into PDMS. The substitution rate of this substituent was 25%.

[0149] In each of the above examples and comparative examples, the constituent materials of the second layer (base and substituent X), molecular descriptor α MOLThe average thickness, separation performance parameter ξ2, and ξ2-ξ1 are shown in Tables 2 and 3. In Tables 2 and 3, "○" indicates that the substituent X contains a fused ring structure, a ring assembly structure, or a ketone group, respectively. The constituent materials of the first layer, the average thickness, and the separation performance parameter ξ1 are also shown in Tables 2 and 3.

[0150] 6. Evaluation of gas separation membranes The gas separation membranes of each example and each comparative example were evaluated as follows.

[0151] 6.1.Gas Permeability A test sample was prepared by cutting a circle with a diameter of 5 cm from each of the gas separation membranes of each example and comparative example. Next, a gas mixture of carbon dioxide and nitrogen in a volume ratio of 13:87 was supplied to the upstream side of the test sample using a gas permeability measuring device. The total pressure on the upstream side was adjusted to 5 MPa, the partial pressure of carbon dioxide to 0.65 MPa, the flow rate to 500 mL / min, and the temperature to 40°C. The gas components that permeated the test sample were then analyzed by gas chromatography.

[0152] Next, from the analysis results, the gas permeation rate R of carbon dioxide through the gas separation membrane CO2 Next, the gas permeation rate R was calculated for a gas separation membrane in which the formation of the second layer was omitted (gas separation membrane composed of only the first layer: Comparative Example 1). CO2 The gas permeation rate R calculated for the gas separation membranes of each example and each comparative example is based on CO2 The degree to which the CO2 permeability was reduced was calculated as the "CO2 permeability reduction rate." The CO2 permeability reduction rate is the ratio of the reduction to the aforementioned standard. The calculated CO2 permeability reduction rate was then compared with the following evaluation criteria to relatively evaluate the gas permeability of the gas separation membrane. The evaluation results are shown in Tables 2 and 3.

[0153] A: The CO2 permeability reduction rate is less than 20% B: CO2 permeability reduction rate is more than 20% but less than 30% C: CO2 permeability reduction rate is over 30%

[0154] 6.2.Gas Separation From the above analysis results, the nitrogen gas permeation rate R N2 Next, the nitrogen gas permeation rate R N2 Carbon dioxide gas transmission rate R CO2 Ratio R CO2 / R N2 The ratio R CO2 / R N2 The gas separation properties of the gas separation membranes were evaluated relative to one another based on the following evaluation criteria. The evaluation results are shown in Tables 2 and 3.

[0155] A: Ratio R CO2 / R N2 is particularly larger than that of Comparative Example 1 B: Ratio R CO2 / R N2 is larger than Comparative Example 1 (smaller than A) C: Ratio R CO2 / R N2 is equivalent to Comparative Example 1 D: Ratio R CO2 / R N2 is smaller than Comparative Example 1

[0156] [Table 2]

[0157] [Table 3]

[0158] As is clear from Tables 2 and 3, the gas separation membranes of each Example showed a small decrease in gas permeability relative to the reference (small CO2 permeability reduction rate) and had higher carbon dioxide gas separation properties than the comparative examples. In particular, it was found that gas separation properties could be improved when a group containing a nitrogen-containing heterocycle, a group containing a ring-fused or ring-assembled structure, a group containing a ketone group, or the like was used as the substituent X. [Explanation of symbols]

[0159] 1...gas separation membrane, 3...first layer, 4...second layer, 31...upper surface, X...substituent

Claims

1. A gas separation membrane that selectively permeates and separates carbon dioxide from a mixed gas containing carbon dioxide and nitrogen, a thin film of a polydimethylsiloxane derivative comprising a main chain formed of a siloxane bond and methyl groups bonded to silicon atoms contained in the siloxane bond, wherein a portion of the methyl groups are substituted with a substituent X; The molecular descriptor α of the substituent X calculated based on the following formulas (1) and (2) ＭＯＬ A gas separation membrane characterized in that: [Equation 1] [In the above formula (1), i is a natural number varying from 1 to N, and N is the number of atoms excluding hydrogen atoms contained in the substituent X.] [Equation 2] [In the above formula (2), r Ａｉ is the covalent bond distance of each atom excluding hydrogen atoms contained in the substituent X. Ｃ is the sp of the carbon atom ３ is the covalent bond distance of the orbital.] [Equation 3]

2. The gas separation membrane according to claim 1 , wherein the substituent X contains an aromatic ring.

3. The gas separation membrane according to claim 1 , wherein the substituent X comprises a nitrogen-containing heterocycle.

4. 4. The gas separation membrane according to claim 1, wherein the substituent X comprises a fused ring structure or a ring assembly structure.

5. 4. The gas separation membrane according to claim 1, wherein the substituent X comprises a ketone group.

6. a first layer having an average thickness greater than that of the thin film and having a high gas permeability to carbon dioxide; a second layer including the thin film provided on one surface of the first layer; The gas separation membrane according to any one of claims 1 to 3, comprising:

7. 4. The gas separation membrane according to claim 1, wherein the thin film has an average thickness of 1 nm or more and 100 nm or less.

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