Gas separation membrane

Abstract

Provided is a gas separation membrane having good gas separation properties of carbon dioxide from nitrogen and good gas permeability of carbon dioxide. A gas separation membrane which selectively transmits carbon dioxide from a mixed gas and has a thin film of a polydimethylsiloxane derivative that includes a main chain comprising a siloxane bond and a methyl group bonded to a silicon atom included in the siloxane bond, a part of the methyl group being substituted with a substituent X, a molecular descriptor [alpha] MOL of the substituent X calculated on the basis of formula (1) and formula (2) satisfies formula (3) # imgabs0 # (In formula (1), i is a natural number changing from 1 to N; n is the number of atoms except for hydrogen atoms contained in the substituent X; # imgabs 1 # (In formula (2), rAi is the covalent bond distance of each atom excluding the hydrogen atom contained in the substituent X; rC is the covalent bond distance of the sp3 orbital of the carbon atom. # imgabs2 #

Description

Technical Field

[0001] The present invention relates to a gas separation membrane. Background Art

[0002] To achieve carbon neutrality, technologies for absorbing and directly recovering atmospheric carbon dioxide are being explored. Known technologies include chemical absorption and adsorption methods, which use an absorbing liquid and an adsorbent to absorb and adsorb carbon dioxide, and membrane separation methods, which use gas separation membranes to separate carbon dioxide.

[0003] For example, Patent Document 1 discloses a gas-selective composite membrane that selectively allows specific gases to permeate. This gas-selective composite membrane is manufactured by laminating a thin film of a siloxane compound on a porous membrane-like polymer support; subjecting the surface layer of the thin film to a plasma treatment with a non-polymerizing gas; and depositing a plasma-polymerized film on the thin film. Furthermore, the invention discloses that these processes result in a gas-selective composite membrane with strong adhesion between the thin film and the plasma-polymerized film; and that the film has a thickness of 1 μm to 30 μm. Furthermore, the invention discloses the use of this gas-selective composite membrane to selectively permeate gases such as oxygen, hydrogen, and helium, and to recover the separated gases. Furthermore, it is generally believed that the use of such a gas-selective composite membrane can also separate carbon dioxide.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 60-075320 Summary of the Invention

[0007] In the gas permselective composite membrane described in Patent Document 1, two thin films each composed of an organosiloxane compound are laminated on a porous support.

[0008] The present inventors have repeatedly studied and found that organosiloxane compounds have room for improvement in terms of the gas separation properties of carbon dioxide from nitrogen and the gas permeability of carbon dioxide.

[0009] Therefore, a technical problem is to realize a gas separation membrane having excellent gas separation properties for carbon dioxide from nitrogen and excellent gas permeability for carbon dioxide.

[0010] The gas separation membrane involved in the application example of the present invention is a gas separation membrane.

[0011] Selectively permeate and separate carbon dioxide from a mixed gas containing carbon dioxide and nitrogen,

[0012] A film comprising a polydimethylsiloxane derivative including a main chain composed of siloxane bonds and methyl groups bonded to silicon atoms included in the siloxane bonds, wherein a portion of the methyl groups is substituted with a substituent X,

[0013] The molecular descriptor α of the substituent X calculated based on the following formula (1) and formula (2) is: MOL Satisfies the following formula (3),

[0014] [Mathematical formula 1]

[0015]

[0016] In the above formula (1), i is a natural number ranging from 1 to N; N is the number of atoms other than hydrogen atoms contained in the substituent X;

[0017] [Mathematical formula 2]

[0018]

[0019] In the above formula (2), r Ai is the covalent bond distance of each atom except the hydrogen atom contained in the substituent X; r C is the sp of carbon atom 3 Covalent bond distances of orbitals.

[0020] [Mathematical formula 3]

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a cross-sectional view schematically showing a gas separation membrane according to an embodiment.

[0023] Description of Reference Numerals

[0024] 1: gas separation membrane; 3: first layer; 4: second layer; 31: upper surface; X: substituent. DETAILED DESCRIPTION

[0025] Hereinafter, the gas separation membrane of the present invention will be described in detail based on the embodiments shown in the drawings.

[0026] 1. Overview of gas separation membranes

[0027] First, the configuration of the gas separation membrane according to the embodiment will be described.

[0028] Figure 1 is a cross-sectional view schematically showing the gas separation membrane 1 according to the embodiment. Figure 1In FIG, as three axes orthogonal to each other, an X axis, a Y axis, and a Z axis are set and indicated by arrows. In addition, the base end side of the arrow representing each axis is regarded as "negative" and the tip end side is regarded as "positive".

[0029] Figure 1 The gas separation membrane 1 shown has a function of selectively permeating and separating carbon dioxide from a mixed gas containing carbon dioxide and nitrogen. Figure 1 The gas separation membrane 1 shown is a composite membrane having a first layer 3 and a second layer 4. It should be noted that Figure 1 The structure of the gas separation membrane 1 shown is an example, and for example, the first layer 3 may be omitted.

[0030] The average thickness of the second layer 4 is set to be thinner than the average thickness of the first layer 3. Thus, the second layer 4 has good gas separation properties and also has high gas permeability.

[0031] exist Figure 1 In the gas separation membrane 1 shown, the positive side of the Z axis is referred to as "upper" and the negative side of the Z axis is referred to as "lower". The mixed gas is supplied above the gas separation membrane 1. Figure 1 In the gas separation membrane 1, carbon dioxide is selectively permeated from the upper side to the lower side and separated.

[0032] 1.1. First layer

[0033] The form of the first layer 3 is not particularly limited and can be Figure 1 The shapes shown are sheet (flat plate), spiral, tubular, hollow wire, etc.

[0034] Examples of the material constituting the first layer 3 include polymer materials. Examples of the polymer material 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 ether, polysulfone, polyethersulfone, polyimide, polyaramid, and organopolysiloxane.

[0035] Among them, the constituent material of the first layer 3 is preferably organopolysiloxane. One molecule of organopolysiloxane contains at least R 1 SiO 3 / 2 The unit represented by R (T unit) 2 R 3 SiO 2 / 2 The unit represented by R (D unit) and 4 R 5 R 6 SiO 1 / 2 The unit represented by (M unit) is used as the basic constituent unit. It should be noted that in each unit, R 1 ~R 6A single molecule of organopolysiloxane is composed of the above T units, D units, and M units.

[0036] 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, vinyl-terminated diphenylsiloxane / dimethylsiloxane copolymer, vinyl-terminated polydimethylsiloxane, H-terminated polydimethylsiloxane, and dimethylsiloxane-methylhydrogensiloxane copolymer. It should be noted that 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 comprising an organopolysiloxane as a primary component and other resin components in combination, based on a mass ratio.

[0037] It should be noted that organopolysiloxane has a large interatomic distance of approximately 1.8 angstroms between the Si-O and Si-C bonds, resulting in a large free volume. This allows for good diffusion of carbon dioxide molecules and high gas permeability for carbon dioxide, making it a useful material for the first layer 3.

[0038] 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 ensures that the first layer 3 has sufficient mechanical properties required as the 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 greater, more preferably 30 μm or greater.

[0039] The average thickness of the first layer 3 is preferably 1 μm to 3000 μm, more preferably 5 μm to 500 μm, and even more preferably 10 μm to 150 μm. This allows for a first layer 3 having sufficient mechanical properties and gas permeability.

[0040] The average thickness of the first layer 3 can be obtained, for example, by magnifying and observing a cross section of the gas separation membrane 1 and averaging the thicknesses measured at ten locations in the first layer 3 .

[0041] Furthermore, the gas permeability of carbon dioxide in the first layer 3 is preferably set to be higher than the gas permeability of carbon dioxide in the second layer 4. Specifically, the gas permeation rate of the first layer 3 when carbon dioxide is the target gas component is higher than the gas permeation rate of the second layer 4. Thus, the first layer 3 can mechanically support the second layer 4 while also imparting good gas permeability to the gas separation membrane 1.

[0042] It should be noted that high gas permeability means a high carbon dioxide permeation rate. Specifically, when carbon dioxide is supplied to the first layer 3 and the second layer 4 alone at a total upstream pressure of 5 MPa, the carbon dioxide permeation rate through the first layer 3 is greater than the carbon dioxide permeation rate through the second layer 4.

[0043] The carbon dioxide permeation rate of the first layer 3 is preferably 1×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg (10GPU) or more, more preferably 3×10-5cm 3 (STP) / cm 2 ·sec·cmHg (30 GPU) or more, more preferably 100 GPU or more, and particularly preferably 200 GPU or more.

[0044] The first layer 3 can be produced by a sheet or film production method, or can be produced by forming a film on a sacrificial layer and then removing the sacrificial layer.

[0045] 1.2. Second layer

[0046] The second layer 4 is provided on the upper surface 31 (one surface) of the first layer 3. The second layer 4 has a gas separation property of carbon dioxide from nitrogen.

[0047] 1.2.1. Molecular descriptor α of the substituent X MOL

[0048] The second layer 4 comprises a thin film of a polydimethylsiloxane derivative. The second layer 4 may consist solely of the thin film, or may comprise a monomolecular film or a monoatomic film provided on the surface of the thin film. Examples of the monomolecular film include a film derived from a silane coupling agent.

[0049] A polydimethylsiloxane derivative is formed by substituting a portion of the methyl groups contained in polydimethylsiloxane with a substituent X. The base polydimethylsiloxane comprises a main chain composed of siloxane bonds and methyl groups bonded to the silicon atoms contained in the siloxane bonds, and exhibits excellent carbon dioxide gas permeability. By introducing the substituent X into such a polydimethylsiloxane, the gas separation performance of carbon dioxide from nitrogen can be improved. As a result, a gas separation membrane 1 having excellent carbon dioxide gas separation performance from nitrogen and carbon dioxide gas permeability can be achieved.

[0050] However, there are currently no specific guidelines for selecting substituents X to address these issues. Therefore, the present inventors have conducted extensive research to optimize the molecular structure of polydimethylsiloxane derivatives. They discovered that by selecting substituents X based on the covalent bond distances between atoms, it is possible to achieve a film with excellent gas separation properties for carbon dioxide from nitrogen without inhibiting carbon dioxide permeability, thereby completing the present invention.

[0051] Specifically, in the film of the polydimethylsiloxane derivative, the molecular descriptor α of the substituent X calculated based on the following formula (1) and formula (2) is: MOL The following formula (3) is satisfied.

[0052] [Formula 4]

[0053]

[0054] In the above formula (1), i is a natural number ranging from 1 to N. N is the number of atoms excluding hydrogen atoms contained in the substituent X.

[0055] [Formula 5]

[0056]

[0057] In the above formula (2), r Ai It is the covalent bond distance of each atom except the hydrogen atom contained in the substituent X. C is the sp of the carbon atom 3 Covalent bond distances of orbitals.

[0058] [Formula 6]

[0059]

[0060] By making the molecular descriptor α of the substituent X MOLSatisfying the above formula (3) allows for a second layer 4 with high gas separation properties for carbon dioxide relative to nitrogen. 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 gas separation properties for carbon dioxide relative to nitrogen and excellent carbon dioxide gas permeability can be achieved.

[0061] Molecular descriptor α of the substituent X MOL Also known as HallKierAlpha, it is equivalent to the above formula (2) including the covalent bond distance r Ai Covalent bond distance r C The descriptor α Ai Therefore, the covalent bond distance r Ai A substituent X composed of short atoms easily satisfies the above formula (2).

[0062] In addition, the molecular descriptor α MOL It is preferable to satisfy the following formula (3-1), and it is more preferable to satisfy the following formula (3-2).

[0063] [Formula 7]

[0064]

[0065] [Formula 8]

[0066]

[0067] On the other hand, the molecular descriptor α MOL The lower limit of is not particularly limited, but is preferably -7.00 or higher, and more preferably -6.00 or higher, in consideration of the availability of the constituent materials for achieving the lower limit.

[0068] Here, the molecular descriptor α is listed MOL Calculation example.

[0069] As shown in the above formula (1), the molecular descriptor α MOL Defined by the above formula (2), it is the descriptor α of each atom in the substituent X Ai It should be noted that the calculation of the descriptor α Ai When the hydrogen atom is removed.

[0070] As an example, consider butadiene (C=CC=C). Although butadiene has four carbon atoms, the electron orbitals of the four carbon atoms are all sp 2 Track. Based on sp 2 The covalent bond distance of the orbital is 0.67 Å. On the other hand, based on sp 3 The covalent bond distance of the orbital is 0.77 Å. Therefore, the descriptor α defined by the above formula (2) isAi = (0.67 / 0.77)-1=-0.13. Thus, the molecular descriptor α defined by the above formula (1) is MOL It is (-0.13)×4=-0.52.

[0071] As another example, consider butane (CCCC). Although butane has four carbon atoms, the electron orbitals of the four carbon atoms are all sp 3 Therefore, the descriptor α defined by the above formula (2) is Ai = (0.77 / 0.77)-1=0. Thus, the molecular descriptor α defined by the above formula (1) is MOL It is 0×4=0.

[0072] As shown in the above example, the molecular descriptor α of various substituents X can be calculated. MOL .

[0073] In addition, the following Table 1 lists the covalent bond distances r based on representative electron orbitals.

[0074] [Table 1]

[0075]

[0076] The polydimethylsiloxane derivative is represented by the following general formula (A).

[0077] [Chemical Formula 1]

[0078]

[0079] In the above general formula (A), Me is a methyl group, X is a substituent, and n is the number of repeating units.

[0080] The substituent X in the polydimethylsiloxane derivative replaces a portion of the methyl groups in the polydimethylsiloxane. The ratio (substitution rate) of the methyl groups in the polydimethylsiloxane replaced by the substituent X is not particularly limited, but is preferably 10% to 40% by number, and more preferably 20% to 30% by number. This allows for a gas separation membrane 1 having both excellent carbon dioxide gas separation properties from nitrogen and excellent carbon dioxide gas permeability.

[0081] It should be noted that when the substitution rate is less than the lower limit, the gas separation property may be reduced. On the other hand, when the substitution rate is greater than the upper limit, the gas permeability may be reduced.

[0082] Calculate molecular descriptor α MOL As a molecular model simulating the above-mentioned general formula (A), a molecular model having a structure represented by the following formula (X-0) was used.

[0083] [Chemical Formula 2]

[0084]

[0085] The weight average molecular weight of the polydimethylsiloxane derivative is not particularly limited, but is preferably 1000 to 25000, more preferably 2000 to 15000, and even more preferably 2000 to 8000. This provides a polydimethylsiloxane derivative capable of forming a thin film having sufficient film strength and excellent carbon dioxide gas permeability.

[0086] The weight average molecular weight of the polydimethylsiloxane derivative is a polystyrene-equivalent molecular weight measured by gel permeation chromatography (GPC method).

[0087] As such a substituent X, there is no particular limitation, but preferably a group containing a ring structure such as an aromatic ring or a heterocyclic ring is used. By containing such a ring structure, the covalent bond distance of the atoms of the group can be shortened. Thus, the molecular descriptor α is obtained. MOL Small substituent X.

[0088] Specific examples of the substituent X containing an aromatic ring include a structure represented by the following formula (X-1).

[0089] [Chemical Formula 3]

[0090]

[0091] In the above formula (X-1), * represents a bond to a Si atom contained in the polydimethylsiloxane derivative. The same applies to other formulae described below.

[0092] Specific examples of heterocycles include saturated or unsaturated 5-membered or 6-membered heterocycles. Examples of heteroatoms contained in the heterocycles include nitrogen atoms, sulfur atoms, and oxygen atoms. The number of heteroatoms contained in a heterocycle is preferably one or more and three or less.

[0093] Among these, heterocycles in which a nitrogen atom is used as the heteroatom (nitrogen-containing heterocycle) are preferably used. Substituents X containing nitrogen-containing heterocycles can particularly reduce the molecular descriptor αA. This contributes to achieving a second layer 4 with particularly high gas separation properties for carbon dioxide relative to nitrogen.

[0094] Examples of the heterocycle in which the hetero atom is a nitrogen atom (nitrogen-containing heterocycle) include diazetidine, pyrazolidinone, imidazolidine, pyrazoline, imidazoline, pyrazole, imidazole, triazole, tetrazole, oxazole, isoxazole, isothiazole, oxadiazole, thiadiazole, thiazolidinedione, oxazolidinone, hydantoin, piperazine, pyridazine, pyrazine, triazine, morpholine, oxazine, thiomorpholine, thiazine, cytosine, thymine, uracil, thiomorpholine dioxide, tetrahydropyrrolopyrrole, dihydropyrrolopyrrole, furopyrrole, thienopyrrole, indazole, benzimidazole, azaindole, pyrazolopyrrole, pyrimidine, purine, benzisoxazole, benzisothiazole, benzoxazole, benzothiazole, benzothiadiazole, adenine, guanine, quinoxaline, phthalazine, quinazoline, quinoline, naphthyridine, pyridopyrimidine, pyridopyrazine, pteridine, benzoxazine, phenazine, phenoxazine, phenothiazine, diazepine, thiazepine, pyrrole, pyridine, pyrimidine, triazine, indoline, indole, isoindole, indolizine, tetrahydroquinoline, dihydroquinoline, quinoline, isoquinoline, quinolinone, isoquinolinone, carbazole, acridine, etc.

[0095] In addition, the substituent X preferably contains a ring condensation structure or a ring aggregate structure. By containing such a structure, the covalent bond distance of the atoms of the group can be further shortened. Thus, the molecular descriptor α is obtained. MOL The substituent X is specially optimized. In addition, the separation performance parameters described below can be optimized. As a result, a gas separation membrane 1 having particularly high gas separation performance is obtained.

[0096] Specific examples of the aromatic ring having a ring-condensed structure include indene, naphthalene, azulene, fluorene, phenanthrene, anthracene, tetracene, pyrene, and triphenylene.

[0097] Specific examples of the aromatic ring having a ring aggregate structure include biphenyl, terphenyl, 2-phenylnaphthalene, and 1,2′-binaphthyl.

[0098] Specific examples of the substituent X including an aromatic ring having a ring-condensed structure and a ring-aggregated structure include a structure represented by the following formula (X-2).

[0099] [Chemical Formula 4]

[0100]

[0101] Specific examples of the heterocycle having a ring-condensed structure include indole, benzofuran, indolizine, chromene, quinoline, purine, indazole, and carbazole.

[0102] Specific examples of the substituent X containing a heterocyclic ring having a ring-condensed structure include structures represented by the following formula (X-3) or formula (X-4).

[0103] [Chemical Formula 5]

[0104]

[0105] [Chemical Formula 6]

[0106]

[0107] Specific examples of the heterocycle having a ring aggregate structure include 2,3'-bifuran and 2,2'-bipyrazin-6-yl.

[0108] Specific examples of the substituent X containing a heterocyclic ring having a ring aggregate 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).

[0109] [Chemical Formula 7]

[0110]

[0111] [Chemical Formula 8]

[0112]

[0113] [Chemical Formula 9]

[0114]

[0115] [Chemical Formula 10]

[0116]

[0117] [Chemical Formula 11]

[0118]

[0119] [Chemical Formula 12]

[0120]

[0121] [Chemical Formula 13]

[0122]

[0123] Specific examples of the substituent X containing a heterocyclic ring having a ring-condensed structure and a ring-aggregated structure include structures represented by formula (X-12), formula (X-13), and formula (X-14).

[0124] [Chemical Formula 14]

[0125]

[0126] [Chemical Formula 15]

[0127]

[0128] [Chemical Formula 16]

[0129]

[0130] Furthermore, the substituent X preferably comprises a keto group, more preferably a cyclic keto group. Thus, the molecular descriptor α is obtained. MOL The substituent X is specially optimized. In addition, the separation performance parameters described below can be optimized. As a result, a gas separation membrane 1 having particularly high gas separation performance is obtained.

[0131] Examples of the substituent X containing a keto group include structures represented by the above-mentioned formula (X-8), formula (X-13), formula (X-14), and the like.

[0132] It should be noted that these groups may further have a substituent. Examples of the substituent include the groups exemplified above.

[0133] Furthermore, the substituent X may contain an optional 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-.

[0134] 1.2.2. Separation performance parameters

[0135] The gas separation property of carbon dioxide from nitrogen is also referred to as "carbon dioxide separation ability". MOL When the above formula (3) is satisfied, it is considered that the second layer 4 has a good carbon dioxide separation ability.

[0136] The carbon dioxide separation capacity is determined by the gas used with carbon dioxide in the mixed gas. The present inventors have diligently studied an indicator that can be used to evaluate the carbon dioxide separation capacity when selectively separating carbon dioxide from a mixture of carbon dioxide and nitrogen. They have discovered that the carbon dioxide separation capacity can be quantified using a separation performance parameter based on the activity coefficient of nitrogen and the activity coefficient of carbon dioxide in the second layer 4.

[0137] 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 set to γ 2 CO2 In addition, the separation performance parameter of the second layer 4 at 25°C is set to ξ2=ln(γ 2 N2） -ln(γ 2 CO2In this case, the second layer 4 preferably satisfies 0.56<ξ2. By making the separation performance parameter ξ2 satisfy such a relationship, it can be evaluated that the second layer 4 has a high carbon dioxide separation capability.

[0138] The activity coefficient γ of nitrogen in the second layer 4 2 N2 represents the degree of non-ideality of nitrogen molecules in the second layer 4. In addition, the activity coefficient γ of carbon dioxide in the second layer 4 is 2 CO2 Indicates the degree of non-ideality of carbon dioxide molecules in the second layer 4. The larger the activity coefficient, the greater the non-ideality. If the non-ideality of carbon dioxide molecules is smaller than that of nitrogen molecules, it can be considered that the second layer 4 has 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 ).

[0139] The molecular descriptor α of the aforementioned substituent X MOL When the above formula (3) is satisfied, the probability that the separation performance parameter ξ2 of the second layer 4 satisfies 0.56<ξ2 increases, and a good carbon dioxide separation capability can be obtained.

[0140] It should be noted that the second layer 4 preferably satisfies 0.80 ≤ ξ2, and more preferably satisfies 0.90 ≤ ξ2. It should be noted that if the separation performance parameter ξ2 of the second layer 4 is less than the lower limit, the relative affinity of carbon dioxide molecules for nitrogen molecules in the second layer 4 may decrease. Consequently, 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 specifically set. Considering the availability of constituent materials for achieving it, it is preferably ξ2 ≤ 2.00, and more preferably ξ2 ≤ 1.80.

[0141] 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 set to γ 1 CO2 In addition, the separation performance parameter of the first layer 3 at 25°C is set to ξ1=ln(γ 1 N2 )-ln(γ 1 CO2 ). In this case, the first layer 3 and the second layer 4 satisfy ξ1<ξ2.

[0142] By ensuring that the separation performance parameters ξ1 and ξ2 satisfy this relationship, the second layer 4 has a higher affinity for carbon dioxide molecules relative to nitrogen molecules than the first layer 3. This improves the gas separation performance of carbon dioxide relative to nitrogen in the second layer 4. On the other hand, the first layer 3 has a lower affinity for carbon dioxide molecules than the second layer 4. Therefore, even if the pressure increases downstream of the gas separation membrane 1, the probability of the carbon dioxide phase, after permeating the gas separation membrane 1, permeating back into the first layer 3 can be reduced.

[0143] 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. This can particularly improve the gas separation performance of carbon dioxide from nitrogen.

[0144] It should be noted that the upper limit of ξ2-ξ1 does not need to be particularly set, but considering the availability of constituent materials for achieving it, ξ2-ξ1≤1.20 is preferred, and ξ2-ξ1≤1.00 is more preferred.

[0145] Here, the COSMO-RS method is described. The following three references (1) to (3) serve as the basic literature for the COSMO-RS method.

[0146] (1) Klamt, AJ Phys. Chem. 99, 2224 (1995).

[0147] (2) Klamt, A.; Jonas, V.; Burger, T.; Lohrenz, JCJ Phys. Chem. A102, 5074 (1998).

[0148] (3) Eckert, F. and A. Klamt, AIChE Journal, 48, 369 (2002).

[0149] The COSMO-RS method is a calculation method that uses statistical mechanics of liquids to calculate the chemical potential μ of molecules based on the surface screening charge σ of molecules calculated by quantum chemistry, and determines the values ​​of various equilibrium physical properties. The COSMO-RS method can calculate the activity coefficient γ mentioned above. 1 N2 , γ 1 CO2 , γ 2 N2 , γ 2 CO2 .

[0150] It should be noted that when calculating the activity coefficient of the molecule, the effects of the molecular volume and the molecular surface area need to be added to the calculation. These effects are represented by the free volume combinatorial term of Elbro. Regarding the free volume combinatorial term of Elbro, it is described in the following reference (4).

[0151] (4) Elbro, H. S.; Fredenslund, A.; Rasmussen, P. A. Macromolecules 23, 4707(1990).

[0152] The free volume corresponds to the volume of the gaps in the polymer, that is, in the first layer 3 and the second layer 4, where the low molecules, that is, nitrogen molecules or carbon dioxide molecules, can move.

[0153] Let the free volume of an arbitrary molecule i be v i F , and let the volume of the core of molecule i be v i * , and let the volume of molecule i in the system be v i When, the free volume v i F is represented by the following formula (a).

[0154] [Mathematical formula 9]

[0155]

[0156] The volume v i * of the core of molecule i in the above formula (a) is obtained by quantum chemical calculation. On the other hand, the volume v i of molecule i in the system is i represented by the following formula (b) using the molecular weight M i of molecule i and the density ρ

[0157] [Mathematical formula 10]

[0158]

[0159] In the above formula (b), N A is the Avogadro constant.

[0160] When calculating the behavior of the low molecules in the polymer, molecule i is regarded as a polymer. And, the molecular weight M i of the polymer is unified to 10000, and the density ρ i is unified to <1.00 g / cc>, and the activity coefficient at a calculation temperature of 25 °C is calculated.

[0161] Thermodynamic property estimation software is used for calculations using the COSMO-RS method. Examples of such software include BIOVIA COSMOtherm 2022 manufactured by Dassault Systemes. In parameterization, BP_TZVPD_FINE_20.ctd is used.

[0162] The molecular model used in calculating the activity coefficient of the molecule has a structure represented by the above-mentioned general formula (A) and has a substitution rate of the substituent X set to 25%.

[0163] The average thickness of the thin film included in the second layer 4 is not particularly limited, but is preferably 1 nm or more and 100 nm or less, more preferably 5 nm or more and 90 nm or less, and even more preferably 30 nm or more and 80 nm or less. Thus, in the second layer 4, the gas permeability of carbon dioxide can be improved while ensuring the gas separation of carbon dioxide from nitrogen. It should be noted that if the average thickness of the thin film included in the second layer 4 is less than the lower limit, the constituent material of the second layer 4 may reduce the gas separation performance. On the other hand, if the average thickness of the thin film included in the second layer 4 is greater than the upper limit, the constituent material of the second layer 4 may reduce the gas permeability of carbon dioxide in the second layer 4.

[0164] The average thickness of the thin film included in the second layer 4 is obtained as the average value of thicknesses measured at 10 locations of the thin film included in the second layer 4 by, for example, enlarging and observing a cross section of the gas separation membrane 1 .

[0165] 1.3. Other components

[0166] While the gas separation membrane 1 according to the embodiment has been described above, an optional layer may be provided on at least one of the downstream side of the first layer 3 and between the first layer 3 and the second layer 4. For example, a porous layer composed of a porous material may be provided on the downstream side of the first layer 3. The porous layer preferably has higher gas permeability and higher rigidity than the first layer 3. This further enhances the rigidity of the gas separation membrane 1, contributing to improved shape retention and durability of the gas separation membrane 1.

[0167] Examples of the constituent material of the porous layer include polymer materials, ceramic materials, and metal materials. The constituent material of the porous layer may also be a composite material of these materials and other materials.

[0168] Examples of the polymer material 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 ether, polysulfone, polyethersulfone, polyimide, and polyaramid.

[0169] Examples of ceramic materials include alumina, cordierite, mullite, silicon carbide, and zirconia, and examples of metal materials include stainless steel.

[0170] The average thickness of the porous layer is not particularly limited, but is preferably 1 μm to 3000 μm, more preferably 5 μm to 500 μm, and even more preferably 10 μm to 150 μm. This allows the porous layer to have sufficient rigidity to support the first layer 3 and the second layer 4 .

[0171] The average thickness of the porous layer is the average value of thicknesses measured at 10 locations in the porous layer in the stacking direction. The thickness of the porous layer can be measured using, for example, a thickness gauge.

[0172] The average pore size of the porous layer is preferably 0.1 μm or less, more preferably 0.01 μm to 0.09 μm, and even more preferably 0.01 μm to 0.07 μm. This can prevent the first layer 3 from falling out to the downstream side of the porous layer.

[0173] The average pore size of the porous layer is measured using a through-pore diameter evaluation device. Examples of the through-pore diameter evaluation device include a Palm Porometer manufactured by PMI.

[0174] The porosity of the porous layer is preferably 20% to 90%, more preferably 30% to 80%. This allows the porous layer to have both good gas permeability and sufficient rigidity. The porosity of the porous layer is measured using the aforementioned through-pore diameter evaluation device.

[0175] It should be noted that, in the gas separation membrane 1 described above, the gas separation ratio of carbon dioxide to nitrogen is preferably 10 or greater, and more preferably 15 or greater. Thus, a gas separation membrane 1 suitable for separating carbon dioxide from the atmosphere is obtained. The gas separation ratio of carbon dioxide to nitrogen is calculated as the ratio of the carbon dioxide permeation rate to the nitrogen permeation rate.

[0176] In addition, the aforementioned first layer 3 may be a porous layer as described above.

[0177] 2. Method for manufacturing gas separation membrane

[0178] The gas separation membrane 1 is produced, for example, by forming a film of a raw material of the second layer 4 on the upper surface 31 of the first layer 3 .

[0179] As the film-forming method of the raw material of the second layer 4, for example, various liquid phase film-forming methods such as dipping method, dropping method, inkjet method, dispenser method, spray method, screen printing method, coater coating method, spin coating method, and gas phase film-forming methods such as plasma CVD method and plasma polymerization method can be cited.

[0180] Among them, the inkjet method is preferably used. The inkjet method is a method of ejecting ink to fix it while moving the inkjet head relative to the upper surface 31. The ink uses a liquid containing the raw material of the second layer 4. By using the inkjet method, a target amount of raw material can be fixed at a target position with a high probability. Therefore, even if the film thickness is thin, a second layer 4 with a high coverage rate can be formed. As a result, a gas separation membrane 1 with high gas separation properties of carbon dioxide relative to nitrogen and excellent gas permeability of carbon dioxide can be efficiently manufactured.

[0181] It should be noted that, before forming 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 activates the upper surface 31. Examples of the activation treatment include irradiating the upper surface 31 with energy rays, heating the upper surface 31, exposing the upper surface 31 to plasma or corona, and exposing the upper surface 31 to ozone gas. Examples of energy rays include infrared rays, ultraviolet rays, and visible light.

[0182] 3. Application of gas separation membranes

[0183] The gas separation membrane 1 according to the embodiment is used for separating and recovering carbon dioxide from a mixed gas containing carbon dioxide and nitrogen, separating and purifying carbon dioxide, etc. In particular, the use of the gas separation membrane 1 is effective in recovering carbon dioxide contained in the atmosphere (direct air recovery (DAC)).

[0184] 4. Effects of the Implementation Method

[0185] 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 mixed gas containing carbon dioxide and nitrogen, and comprises a thin film of a polydimethylsiloxane derivative. The thin film comprises a polydimethylsiloxane derivative having a main chain composed of siloxane bonds and methyl groups bonded to silicon atoms contained in the siloxane bonds, wherein a portion of the methyl groups is substituted with a substituent X. Furthermore, in the gas separation membrane 1, the molecular descriptor α of the substituent X calculated based on the following formulas (1) and (2) is: MOL The following formula (3) is satisfied.

[0186] [Mathematical formula 11]

[0187]

[0188] In the above formula (1), i is a natural number ranging from 1 to N. N is the number of atoms excluding hydrogen atoms contained in the substituent X.

[0189] [Mathematical formula 12]

[0190]

[0191] In the above formula (2), r Ai It is the covalent bond distance of each atom except the hydrogen atom contained in the substituent X. C is the sp of the carbon atom 3 Covalent bond distances of orbitals.

[0192] [Mathematical formula 13]

[0193]

[0194] According to such a configuration, a gas separation membrane 1 having excellent gas separation performance of carbon dioxide from nitrogen and excellent gas permeability of carbon dioxide is obtained.

[0195] Furthermore, in the gas separation membrane 1 according to the above embodiment, the substituent X includes an aromatic ring.

[0196] According to such a structure, the covalent bond distance of the atoms of the substituent X can be shortened. Thus, the molecular descriptor α is obtained. MOL Small substituent X.

[0197] In the gas separation membrane 1 according to the above embodiment, the substituent X includes a nitrogen-containing heterocyclic ring.

[0198] According to such a structure, the covalent bond distance of the atoms of the substituent X can be shortened. Thus, the molecular descriptor α is obtained. MOL Small substituent X.

[0199] In the gas separation membrane 1 according to the above embodiment, the substituent X includes a ring-condensed structure or a ring-aggregated structure.

[0200] According to such a configuration, it is possible to further shorten the covalent bond distance between atoms of the substituent X. Thus, a gas separation membrane 1 having particularly high gas separation performance is obtained.

[0201] In the gas separation membrane 1 according to the above embodiment, the substituent X includes a keto group.

[0202] According to such a configuration, a gas separation membrane 1 having particularly high gas separation performance is obtained.

[0203] The gas separation membrane 1 according to the embodiment includes the first layer 3 having an average thickness thicker than the thin film and high carbon dioxide gas permeability, and the second layer 4 including the thin film provided on one surface of the first layer 3 .

[0204] With such a configuration, the first layer 3 can mechanically support the second layer 4 while imparting good gas permeability to the gas separation membrane 1 .

[0205] Furthermore, in the gas separation membrane 1 according to the above embodiment, the average thickness of the thin film is 1 nm or more and 100 nm or less.

[0206] According to such a configuration, the gas permeability of carbon dioxide can be improved while ensuring the gas separation performance of carbon dioxide from nitrogen in the second layer 4 .

[0207] As mentioned above, the gas separation membrane according to the present invention has been described based on preferred embodiments, but the present invention is not limited thereto.

[0208] For example, the gas separation membrane according to the present invention may be replaced with components having the same functions as those in the above-described embodiments, or an arbitrary component may be added to the above-described embodiments.

[0209] Example

[0210] Next, specific examples of the present invention will be described.

[0211] 5. Fabrication of gas separation membranes

[0212] Example 1

[0213] 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 a plasma treatment as an activation treatment.

[0214] Next, solutions of the polydimethylsiloxane derivatives shown in Table 2 were prepared. The substitution rate of the substituent X in the polydimethylsiloxane derivatives was set to 25%.

[0215] Next, the obtained solution was ejected onto the first layer by an inkjet method and then dried at 80° C. Thus, a thin film (second layer) composed of a polydimethylsiloxane derivative was formed on the first layer, thereby obtaining a gas separation membrane.

[0216] 5.2. Examples 2 to 14

[0217] 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.

[0218] Comparative Example 1

[0219] The formation of the second layer was omitted, and the first layer alone was used as the gas separation membrane of Comparative Example 1.

[0220] Comparative Example 2

[0221] 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.

[0222] Comparative Example 3

[0223] A gas separation membrane was obtained in the same manner as in Example 1 except that a polydimethylsiloxane derivative having an amino group-containing substituent introduced into PDMS was used as a constituent material of the second layer. The substitution rate of the substituent was set to 25%.

[0224] The constituent materials (base and substituent X) of the second layer in the above embodiments and comparative examples, the molecular descriptor α MOL The average thickness, separation performance parameters ξ2 and ξ2-ξ1 are shown in Tables 2 and 3. It should be noted that "○" in Tables 2 and 3 indicates that the substituent X contains a ring-fused structure, a ring-aggregated structure, or a ketone group. Furthermore, the constituent materials, average thickness, and separation performance parameter ξ1 of the first layer are shown in Tables 2 and 3.

[0225] 6. Evaluation of gas separation membranes

[0226] The gas separation membranes of the Examples and Comparative Examples were evaluated as follows.

[0227] 6.1. Gas permeability

[0228] The gas separation membranes from each example and comparative example were cut into circular pieces with a diameter of 5 cm to prepare test samples. Next, using a gas permeability measurement device, a mixed gas consisting of carbon dioxide and nitrogen at a volume ratio of 13:87 was supplied upstream of the test samples. The upstream total pressure was adjusted to 5 MPa, the carbon dioxide partial pressure to 0.65 MPa, the flow rate to 500 mL / min, and the temperature to 40°C. The gas composition after permeating the test samples was then analyzed by gas chromatography.

[0229] Next, the carbon dioxide permeation rate R in the gas separation membrane is calculated based on the analysis results. CO2 Next, the gas permeation rate R calculated for the gas separation membrane in which the second layer is omitted (the gas separation membrane consisting of only the first layer: Comparative Example 1) is used. CO2 When the gas permeation rate R is calculated for the gas separation membranes of each embodiment and each comparative example,CO2 The degree of reduction was defined as the "CO2 permeability reduction rate." The CO2 permeability reduction rate represents the ratio of the reduction magnitude to the aforementioned criteria. The calculated CO2 permeability reduction rate was then compared against the following evaluation criteria to perform a relative evaluation of the gas permeability of the gas separation membrane. The evaluation results are shown in Tables 2 and 3.

[0230] A: CO2 permeability reduction rate is less than 20%;

[0231] B: CO2 permeability reduction rate exceeds 20% and is less than 30%;

[0232] C: CO2 permeability reduction rate exceeds 30%.

[0233] 6.2. Gas Separation

[0234] Based on the above analysis results, the nitrogen gas permeation rate R in the gas separation membrane is calculated. N2 Next, the carbon dioxide gas permeation rate R CO2 Gas permeation rate R relative to nitrogen N2 The ratio R CO2 / R N2 Next, the ratio R CO2 / R N2 The gas separation performance of the gas separation membranes was relatively evaluated against the following evaluation criteria. The evaluation results are shown in Tables 2 and 3.

[0235] A: Ratio R CO2 / R N2 Much larger than Comparative Example 1.

[0236] B: Ratio R CO2 / R N2 Larger than Comparative Example 1 (but smaller than A).

[0237] C: Ratio R CO2 / R N2 Same as Comparative Example 1.

[0238] D: Ratio R CO2 / R N2 Smaller than Comparative Example 1.

[0239] [Table 2]

[0240]

[0241] [Table 3]

[0242]

[0243] Tables 2 and 3 demonstrate that the gas separation membranes of each Example exhibit minimal decrease in gas permeability relative to the standard (smaller CO2 permeability reduction rate) and exhibit higher carbon dioxide gas separation performance compared to the Comparative Examples. In particular, it was found that the use of a nitrogen-containing heterocyclic group, a ring-fused structure or ring-aggregate structure, or a ketone group as the substituent X improves gas separation performance.

Claims

1. A gas separation membrane, characterized in that: Selectively permeate and separate carbon dioxide from a mixed gas containing carbon dioxide and nitrogen, A film comprising a polydimethylsiloxane derivative comprising a main chain composed of siloxane bonds and methyl groups bonded to silicon atoms contained in the siloxane bonds, wherein a portion of the methyl groups is substituted with a substituent X, The molecular descriptor α of the substituent X calculated based on the following formula (1) and formula (2) is: MOL Satisfies the following formula (3), In the formula (1), i is a natural number ranging from 1 to N; N is the number of atoms other than hydrogen atoms contained in the substituent X; In the formula (2), r Ai is the covalent bond distance of each atom except the hydrogen atom contained in the substituent X; r C is the sp of carbon atom 3 The covalent bond distance of the orbital, 。 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 includes a nitrogen-containing heterocyclic ring.

4. The gas separation membrane according to any one of claims 1 to 3, wherein The substituent X includes a ring-condensed structure or a ring-aggregated structure.

5. The gas separation membrane according to any one of claims 1 to 3, wherein The substituent X comprises a keto group.

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

7. The gas separation membrane according to any one of claims 1 to 3, wherein The average thickness of the thin film is 1 nm or more and 100 nm or less.

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