Gas separation membrane

Abstract

The purpose of the present invention is to provide a gas separation membrane having superior gas separation performance to conventional gas separation membranes.　A gas separation membrane according to the present invention has an intermediate layer on a polymer membrane substrate, and has separation layers on the intermediate layer, said intermediate layer having a non-porous structure, said separation layers being silica-based layers, and said silica-based separation layers having different chemical compositions or layer structures in the thickness direction.

Description

Technical Field

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

Background Art

[0002] A gas separation membrane that allows a specific gas to permeate and separate from a mixed gas is known.

[0003] For example, Patent Document 1 proposes a gas separation membrane having a resin layer containing a compound having a siloxane bond.

[0004] Further, as the gas separation membrane, one having a silica-based separation layer on a porous base material is known.

[0005] As a method for forming the silica-based separation layer, in addition to a film forming method using a sol-gel method, a film forming method by an atmospheric pressure plasma CVD (Chemical Vapor Deposition) method is known. The atmospheric pressure plasma CVD method has advantages such as being capable of forming a film at normal temperature and normal pressure, and being capable of forming a large-area film by continuous processing because no vacuum equipment is required.

[0006] For example, Patent Document 2 is a method for producing a gas separation filter in which a separation layer is formed on a porous base material by an atmospheric pressure plasma chemical vapor deposition method, wherein a mixed gas of nitrogen and argon is introduced as a discharge gas into a discharge section to generate atmospheric pressure plasma, a volatile organosilicon compound is introduced below the discharge section to be mixed with the atmospheric pressure plasma, the separation layer is formed on the porous base material, and the nitrogen content in the discharge gas is 5.0% by volume or less. A method for producing a gas separation filter characterized by the above is proposed.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

[0008] However, conventional gas separation membranes have insufficient gas separation performance, and there has been a need to develop gas separation membranes with superior gas separation performance compared to conventional ones.

[0009] This invention has been made in view of the above problems, and aims to provide a gas separation membrane that has superior gas separation performance compared to conventional gas separation membranes. [Means for solving the problem]

[0010] The present invention relates to a gas separation membrane having an intermediate layer on a polymer film substrate, and a separation layer on the intermediate layer, The aforementioned intermediate layer has a non-porous structure. The separation layer is a silica-based separation layer. The present invention relates to a gas separation membrane in which the silica-based separation layer has different chemical composition or layer structure in the thickness direction.

[0011] Preferably, the silica-based separation layer has a higher proportion of inorganic structure in the thickness direction from the intermediate layer side to the other side.

[0012] Preferably, the silica-based separation layer has a higher content of Si atoms and oxygen atoms and a lower content of carbon atoms in the thickness direction, moving from the intermediate layer side to the other side.

[0013] It is preferable that the ratio of oxygen atoms to Si atoms on the surface of the silica-based separation layer, O / Si, is less than 1.7.

[0014] The silica-based separation layer is preferably a multilayer structure.

[0015] Preferably, the silica-based separation layer comprises at least a first separation layer laminated on the intermediate layer and a second separation layer laminated on the first separation layer, wherein the content of Si atoms and oxygen atoms in the second separation layer is higher than the content of Si atoms and oxygen atoms in the first separation layer, and the content of carbon atoms in the second separation layer is lower than the content of carbon atoms in the first separation layer.

[0016] Preferably, the ratio of the number of carbon atoms to the number of Si atoms in the first separation layer, C / Si, is 1.5 or more and 1.8 or less, and the ratio of the number of carbon atoms to the number of Si atoms in the second separation layer, C / Si, is 1.0 or more and less than 1.5.

[0017] The raw material for the silica-based separation layer is preferably a volatile organosilicon compound.

[0018] The intermediate layer preferably contains a polysiloxane.

[0019] The polysiloxane is preferably polydimethylsiloxane.

[0020] The gas separation membrane preferably has a CO2 transmission rate of 90 GPU or more at 25°C.

[0021] The gas separation membrane preferably has a CO2 / CH4 transmittance ratio of 15 or more at 25°C. [Effects of the Invention]

[0022] The gas separation membrane of the present invention has silica-based separation layers with different chemical compositions or layer structures in the thickness direction, and exhibits superior gas separation performance (particularly the CO2 / CH4 permeability ratio) compared to conventional gas separation membranes. Furthermore, the gas separation membrane of the present invention also has the characteristic of high gas permeability (particularly the CO2 permeability). If the silica-based separation layer has a continuously or stepwise increasing proportion of inorganic structure from the intermediate layer side to the other side in the thickness direction, a gas separation membrane with even better effects can be obtained. [Brief explanation of the drawing]

[0023] [Figure 1] This is a schematic diagram of a separation layer film-forming apparatus used in an atmospheric pressure plasma CVD method. [Modes for Carrying Out the Invention]

[0024] Hereinafter, embodiments of the present invention will be described.

[0025] [Gas Separation Membrane] The gas separation membrane of the present invention has an intermediate layer on a polymer membrane base material, and a silica-based separation layer on the intermediate layer, wherein the intermediate layer has a non-porous structure, and the silica-based separation layer has a different chemical composition or layer structure in the thickness direction.

[0026] The polymer membrane base material (support) is not particularly limited as long as it can support the intermediate layer, but is preferably a porous base material. Examples of the material for forming the polymer membrane base material include various materials such as polysulfone, polyaryl ether sulfone such as polyether sulfone, polyester, polyamide, polyimide, silicone, silicone rubber, polyurethane, polyethylene, polypropylene, polystyrene, polycarbonate, polyether ether ketone, polyphenylene oxide, polyacrylonitrile, polyvinyl fluoride, and polyvinylidene fluoride. Among them, polysulfone or polyaryl ether sulfone is particularly preferably used from the viewpoint of chemical, mechanical and thermal stability. The thickness of the polymer membrane base material is usually about 50 to 500 μm, preferably 100 to 200 μm, but is not limited thereto. The polymer membrane base material may be reinforced by lining with woven fabric, non-woven fabric or the like.

[0027] When the polymer film substrate is a porous substrate, the porous substrate may have a symmetrical or asymmetrical structure, but an asymmetrical structure is preferable from the viewpoint of achieving both the support function of the intermediate layer and the smoothness of the intermediate layer-forming surface. The average pore size of the intermediate layer-forming surface of the porous substrate is preferably 0.01 to 0.5 μm.

[0028] The gas separation membrane of the present invention has an intermediate layer between the polymer membrane substrate and the silica-based separation layer, from the viewpoint of improving gas separation performance by making the film thickness of the silica-based separation layer uniform.

[0029] Since the intermediate layer has a non-porous structure, it can improve the smoothness of the silica-based separation layer and thus improve gas separation performance.

[0030] Examples of materials for forming the intermediate layer include polysiloxanes such as polydimethylsiloxane, polydiethylsiloxane, polymethylphenylsiloxane, and polydimethyldiphenylsiloxane; fluororesins such as polytetrafluoroethylene; epoxy resins such as polyethylene oxide; polyimide resins; polysulfone resins; polyacetylene resins such as polytrimethylsilylpropyne and polydiphenylacetylene; and polyolefin resins such as polymethylpentene. Of these, polysiloxane is preferred, and polydimethylsiloxane is more preferred, from the viewpoint of improving gas separation performance and gas permeability.

[0031] The intermediate layer may contain nanoparticles such as silica particles, titania particles, and alumina particles.

[0032] The thickness of the intermediate layer is typically 0.5 to 10 μm, and is preferably 0.5 to 5 μm, more preferably 1 to 3 μm, from the viewpoint of improving gas separation performance and gas permeability.

[0033] The silica-based separation layer is a layer that has the function of separating gases with different molecular sizes by allowing gases with smaller molecular sizes to permeate from a mixed gas containing two or more gases with different molecular sizes, while inhibiting the permeation of gases with larger molecular sizes.

[0034] The silica-based separation layer is characterized by having different chemical compositions or layer structures in the thickness direction. Preferably, the silica-based separation layer has a continuously or stepwise increasing proportion of inorganic structures in the thickness direction from the intermediate layer side to the other side, and more preferably, in the thickness direction, the content of Si atoms and oxygen atoms increases continuously or stepwise from the intermediate layer side to the other side, and the content of carbon atoms decreases continuously or stepwise.

[0035] The ratio of oxygen atoms to Si atoms on the surface of the silica-based separation layer, O / Si, is preferably less than 1.7, more preferably 1.65 or less, and even more preferably 1.60 or less, from the viewpoint of improving gas separation performance and gas permeability. The lower limit of O / Si is usually 1.25 or higher, preferably 1.30 or higher, and more preferably 1.40 or higher.

[0036] The silica-based separation layer may be a single layer or a multilayer structure of two or more layers, but from the viewpoint of improving gas separation performance and gas permeability, and from the viewpoint of ease of manufacture, a multilayer structure of two or more layers is preferred, and more preferably a two-layer or three-layer structure. Specifically, the silica-based separation layer has at least a first separation layer laminated on the intermediate layer and a second separation layer laminated on the first separation layer, wherein the content of Si atoms and oxygen atoms in the second separation layer is higher than the content of Si atoms and oxygen atoms in the first separation layer, and the content of carbon atoms in the second separation layer is lower than the content of carbon atoms in the first separation layer. Furthermore, if the silica-based separation layer has a third or more separation layer on the second separation layer, the content of Si atoms and oxygen atoms in each of the third or more separation layers increases in stages, and the content of carbon atoms decreases in stages.

[0037] From the viewpoint of improving gas separation performance and gas permeability, the ratio of the number of carbon atoms to the number of Si atoms in the first separation layer, C / Si, is preferably 1.5 or more and 1.8 or less, more preferably 1.5 or more and 1.7 or less, and the ratio of the number of carbon atoms to the number of Si atoms in the second separation layer, C / Si, is preferably 1.0 or more and less than 1.5, more preferably 1.2 or more and 1.4 or less.

[0038] The thickness of the silica-based separation layer (total thickness in the case of a multilayer structure) is not particularly limited, but from the viewpoint of improving gas separation performance and gas permeability, it is preferably 0.01 to 2 μm, more preferably 0.1 to 1.5 μm, and even more preferably 0.5 to 1.2 μm.

[0039] The gas separation membrane of the present invention preferably has a CO2 transmission rate of 90 GPU or more at 25°C, more preferably 95 GPU or more, even more preferably 100 GPU or more, even more preferably 130 GPU or more, even more preferably 150 GPU or more, even more preferably 200 GPU or more, and even more preferably 250 GPU or more.

[0040] The gas separation membrane of the present invention preferably has a CO2 transmission rate of 90 GPU or more at 150°C, more preferably 100 GPU or more, even more preferably 150 GPU or more, even more preferably 200 GPU or more, even more preferably 250 GPU or more, and even more preferably 300 GPU or more.

[0041] Furthermore, the gas separation membrane of the present invention preferably has a CO2 / CH4 transmittance ratio of 15 or more at 25°C, more preferably 30 or more, even more preferably 35 or more, and even more preferably 40 or more.

[0042] Furthermore, the gas separation membrane of the present invention preferably has a CO2 / CH4 transmittance ratio of 9 or higher at 150°C, more preferably 9.5 or higher, even more preferably 10 or higher, and even more preferably 12 or higher.

[0043] [Method for manufacturing gas separation membranes] The intermediate layer can be formed by known methods, for example, by applying a composition containing an intermediate layer forming material onto the polymer film substrate and drying it.

[0044] The silica-based separation layer can be formed on the intermediate layer, for example, by atmospheric pressure plasma CVD.

[0045] The separation layer deposition apparatus used in atmospheric pressure plasma CVD can be a known separation layer deposition apparatus; for example, the separation layer deposition apparatus 1 shown in Figure 1 can be used.

[0046] The raw material for the silica-based separation layer is not particularly limited as long as it can be chemically vapor-deposited by atmospheric pressure plasma. Examples include volatile organosilicon compounds such as hexamethyldisiloxane, trimethylethoxysilane, and methyltriethoxysilane. From the viewpoint of improving gas separation performance and gas permeability, hexamethyldisiloxane is preferred.

[0047] The discharge gas cylinder 2 is filled with discharge gas (such as N2, Air, or O2), and the carrier gas cylinder 3 is filled with carrier gas (such as Ar). By passing the carrier gas through a bubbler 4 containing liquid volatile organosilicon compounds, a carrier gas containing volatile organosilicon compounds is obtained. Then, a mixed gas, which is a mixture of the carrier gas containing volatile organosilicon compounds and the discharge gas, is supplied to the plasma generator 5. The volatile organosilicon compounds are decomposed by atmospheric pressure plasma generated in the discharge section 6, and a silica-based separation layer is formed on a polymer film substrate 7 having an intermediate layer by chemical vapor deposition. If the polymer film substrate 7 having an intermediate layer is moved while deposition is performed during the formation of the silica-based separation layer, a silica-based separation layer of uniform thickness can be obtained. In addition, deposition may be performed multiple times (multiple cycles) to form a silica-based separation layer with a thicker film.

[0048] Examples of methods for forming silica-based separation layers with different chemical compositions or layer structures in the thickness direction include the following:

[0049] (1) A method for continuously or stepwise changing the composition of the discharge gas. By continuously or gradually changing the composition of the discharge gas, silica-based separation layers can be formed in which the chemical composition or layer structure differs continuously or gradually in the thickness direction.

[0050] For example, when forming a silica-based separation layer with a three-layer structure consisting of a first separation layer, a second separation layer, and a third separation layer, N2 is used as the discharge gas when forming the first separation layer, Air is used as the discharge gas when forming the second separation layer, and O2 is used as the discharge gas when forming the third separation layer. Alternatively, a discharge gas containing N2 and O2 may be used, and the mixing ratio of N2 and O2 may be changed when forming each of the first, second, and third separation layers.

[0051] By continuously or stepwise changing the composition of the discharge gas from oxygen-lean to oxygen-rich, a silica-based separation layer can be formed in which the proportion of inorganic structure increases continuously or stepwise from the intermediate layer side to the other side in the thickness direction. Specifically, a silica-based separation layer can be formed in which the content of Si atoms and oxygen atoms increases continuously or stepwise from the intermediate layer side to the other side in the thickness direction, and the content of carbon atoms decreases continuously or stepwise. This silica-based separation layer has superior gas separation performance and gas permeability.

[0052] (2) A method for continuously or stepwise changing the concentration of volatile organosilicon compounds in a mixed gas. By continuously or stepwise changing the concentration of volatile organosilicon compounds in the mixed gas, silica-based separation layers can be formed in which the chemical composition or layer structure differs continuously or stepwise in the thickness direction.

[0053] For example, when forming a silica-based separation layer with a two-layer structure consisting of a first separation layer and a second separation layer, the concentration of volatile organosilicon compounds is increased (e.g., 125-800 ppm) when forming the first separation layer, and decreased (e.g., 50-100 ppm) when forming the second separation layer.

[0054] By continuously or stepwise reducing the concentration of volatile organosilicon compounds in the mixed gas, a silica-based separation layer can be formed in which the proportion of inorganic structure continuously or stepwise increases from the intermediate layer side to the other side in the thickness direction. Specifically, a silica-based separation layer can be formed in which the content of Si atoms and oxygen atoms continuously or stepwise increases from the intermediate layer side to the other side in the thickness direction, and the content of carbon atoms continuously or stepwise decreases. This silica-based separation layer has superior gas separation performance and gas permeability.

[0055] After forming the silica-based separation layer on the intermediate layer, heat treatment (annealing) may be performed.

[0056] The gas separation membrane of the present invention is not limited in any way in terms of its shape. That is, any conceivable membrane shape is possible, such as a flat membrane, cylindrical membrane, or spiral element. [Examples]

[0057] The present invention will be described below with reference to examples, but the present invention is not limited in any way by these examples.

[0058] Example 1 A coating solution containing polydimethylsiloxane (PDMS) was applied to a porous polysulfone support (CF-30K, manufactured by Nitto Denko Corporation) by spin coating and dried to form an intermediate layer (thickness: 3 μm). Using a separation layer deposition apparatus, a silica-based separation layer (total thickness: 1.2 μm) with a two-layer structure consisting of a first separation layer (thickness: 400 nm) and a second separation layer (thickness: 800 nm) was deposited on the intermediate layer by atmospheric pressure plasma CVD under the manufacturing conditions described in Table 2 to produce a gas separation membrane.

[0059] Elemental analysis of the silica-based separation layer and evaluation of the gas permeability of the gas separation membrane were performed using the method described below. The results are shown in Table 1.

[0060] In Example 1, the silica-based separation layer had a higher content of Si atoms and oxygen atoms in the second separation layer than in the first separation layer. Conversely, the carbon atom content was lower in the second separation layer than in the first separation layer. In other words, the silica-based separation layer in Example 1 had a higher proportion of inorganic structures in the second separation layer than in the first separation layer.

[0061] Examples 2-4 A gas separation membrane was manufactured in the same manner as in Example 1, except that the manufacturing conditions described in Table 2 were used for the formation of the silica-based separation layer.

[0062] Elemental analysis of the silica-based separation layer and evaluation of the gas permeability of the gas separation membrane were performed using the method described below. The results are shown in Table 1.

[0063] In the silica-based separation layers of Examples 2-4, the content of Si atoms and oxygen atoms was higher in the second separation layer than in the first separation layer. Conversely, the content of carbon atoms was lower in the second separation layer than in the first separation layer. In other words, in the silica-based separation layers of Examples 2-4, the proportion of inorganic structures was higher in the second separation layer than in the first separation layer.

[0064] Comparative Example 1 Using a separation layer deposition apparatus, a silica-based separation layer (total thickness: 1.2 μm) with a two-layer structure consisting of a first separation layer (thickness: 400 nm) and a second separation layer (thickness: 800 nm) was fabricated on a porous polysulfone support (CF-30K, manufactured by Nitto Denko Corporation) by atmospheric pressure plasma CVD under the manufacturing conditions described in Table 2 to produce a gas separation membrane.

[0065] Elemental analysis of the silica-based separation layer and evaluation of the gas permeability of the gas separation membrane were performed using the method described below. The results are shown in Table 1.

[0066] In Comparative Example 1, the silica-based separation layer had a higher content of Si atoms and oxygen atoms in the second separation layer than in the first separation layer. Conversely, the carbon atom content was lower in the second separation layer than in the first separation layer. In other words, the silica-based separation layer of Comparative Example 1 had a higher proportion of inorganic structures in the second separation layer than in the first separation layer.

[0067] Comparative Example 2 A coating solution containing polydimethylsiloxane (PDMS) was applied to a porous polysulfone support (CF-30K, manufactured by Nitto Denko Corporation) by spin coating and dried to form an intermediate layer (thickness: 3 μm). Subsequently, the surface of the formed intermediate layer was irradiated with plasma under the conditions described in Table 2 to modify the surface of the intermediate layer and form a separation layer, thereby manufacturing a gas separation membrane.

[0068] The gas permeability of the gas separation membrane was evaluated using the method described below. The results are shown in Table 1.

[0069] In Comparative Example 2, the intermediate layer containing the separation layer had a higher content of Si atoms and oxygen atoms at the surface of the separation layer than at the center of the intermediate layer. Conversely, the content of carbon atoms was lower at the surface of the separation layer than at the center of the intermediate layer. In other words, in Comparative Example 2, the intermediate layer containing the separation layer had a higher proportion of inorganic structure at the surface of the separation layer than at the center of the intermediate layer.

[0070] Comparative Examples 3 and 4 A coating solution containing polydimethylsiloxane (PDMS) was applied to a porous polysulfone support (CF-30K, manufactured by Nitto Denko Corporation) by spin coating and dried to form an intermediate layer (thickness: 5 μm). Using a separation layer deposition apparatus, a single-layer silica-based separation layer (thickness: 1.2 μm) was deposited on the intermediate layer by atmospheric pressure plasma CVD under the manufacturing conditions described in Table 2 to produce a gas separation membrane.

[0071] Elemental analysis of the silica-based separation layer and evaluation of the gas permeability of the gas separation membrane were performed using the method described below. The results are shown in Table 1.

[0072] The silica-based separation layers of Comparative Examples 3 and 4 had the same content of Si atoms, oxygen atoms, and carbon atoms in the thickness direction.

[0073] Comparative Example 5 A gas separation membrane was manufactured in the same manner as in Comparative Example 1, except that a sulfonated polyethersulfone NF membrane (Nitto Denko Corporation, NTR-7410) was used as the support instead of a porous polysulfone support (Nitto Denko Corporation, CF-30K), and the silica-based separation layer was fabricated under the manufacturing conditions described in Table 2.

[0074] Elemental analysis of the silica-based separation layer and evaluation of the gas permeability of the gas separation membrane were performed using the method described below. The results are shown in Table 1.

[0075] In Comparative Example 5, the silica-based separation layer had a higher content of Si atoms and oxygen atoms in the second separation layer than in the first separation layer. Conversely, the carbon atom content was lower in the second separation layer than in the first separation layer. In other words, the silica-based separation layer of Comparative Example 5 had a higher proportion of inorganic structures in the second separation layer than in the first separation layer.

[0076] [Measurement and evaluation methods] (Elemental analysis of silica-based separation layer) Elemental analysis of the silica-based separation layer was performed using X-ray electron spectroscopy. A silicon wafer was used as the substrate, and under the same conditions as for film formation on the porous support, the concentration of hexamethyldisiloxane was varied to form a silica-based separation layer. The elemental composition of the surface of the formed silica-based separation layer was then quantified.

[0077] (Gas permeability) The gas permeability of the obtained gas separation membrane was evaluated using pure gases (CO2, CH4). The upstream side of the gas separation membrane was pressurized to atmospheric pressure to a slight pressure (100-110 kPaG), the permeation side with a known volume was evacuated, and then the membrane was disconnected from the exhaust system. The gas permeability was determined from the rate of pressure increase (using the pseudo-steady-state method).

[0078] [Table 1]

[0079] [Table 2] [Industrial applicability]

[0080] The gas separation membrane of the present invention can be used as a separation membrane that allows a specific gas to pass through and separate from a gas mixture. [Explanation of symbols]

[0081] 1: Separation layer film forming equipment 2: Discharge gas cylinder 3: Carrier gas cylinder 4: Bubbler 5: Plasma generator 6:Discharge part 7: Polymer film substrate having an intermediate layer

Claims

1. A gas separation membrane having an intermediate layer on a polymer film substrate, and a separation layer on the intermediate layer, The aforementioned intermediate layer has a non-porous structure. The separation layer is a silica-based separation layer. The silica-based separation layer is a gas separation membrane in which, in the thickness direction, the content of Si atoms and oxygen atoms increases from the intermediate layer side to the other side, and the content of carbon atoms decreases.

2. The gas separation membrane according to claim 1, wherein the ratio of oxygen atoms to Si atoms on the surface of the silica-based separation layer, O / Si, is less than 1.

7.

3. The gas separation membrane according to claim 1, wherein the silica-based separation layer has a multilayer structure.

4. The gas separation membrane according to claim 3, wherein the silica-based separation layer comprises at least a first separation layer laminated on the intermediate layer and a second separation layer laminated on the first separation layer, the content of Si atoms and oxygen atoms in the second separation layer is higher than the content of Si atoms and oxygen atoms in the first separation layer, and the content of carbon atoms in the second separation layer is lower than the content of carbon atoms in the first separation layer.

5. The gas separation membrane according to claim 4, wherein the ratio of the number of carbon atoms to the number of Si atoms in the first separation layer, C / Si, is 1.5 or more and 1.8 or less, and the ratio of the number of carbon atoms to the number of Si atoms in the second separation layer, C / Si, is 1.0 or more and less than 1.

5.

6. The gas separation membrane according to claim 1, wherein the raw material for the silica-based separation layer is a volatile organosilicon compound.

7. The gas separation membrane according to claim 1, wherein the intermediate layer contains polysiloxane.

8. The gas separation membrane according to claim 7, wherein the polysiloxane is polydimethylsiloxane.

9. CO2 at 25°C ２ The gas separation membrane according to claim 1, wherein the transmittance is 90 GPU or more.

10. CO2 at 25°C ２ / CH ４ The gas separation membrane according to claim 1, wherein the transmittance ratio is 15 or more.

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