Porous Polybenzimidazole Membrane Supports for Composite Membranes

Abstract

The present invention provides highly permeable and porous polybenzimidazole membranes, methods of making them, and their application as a high-performance membrane support for gas separation composite membranes. The polybenzimidazole membranes are bonded to a fabric substrate.

Claims

1. A method of making a flat sheet of a membrane suitable for membrane support, comprising: providing a fabric sheet; applying a coating solution to the fabric sheet; wherein the solution comprises a polybenzimidazole in an aprotic polar solvent to form a PBI-coated fabric; passing the PBI-coated fabric into an aqueous coagulation bath to form a porous PBI coated composite wherein the coagulation bath is at a temperature of at least 45° C.; and rinsing and drying the porous PBI composite.

2. The method of claim 1 wherein the fabric is a non-woven fabric.

3. The method of claim 1 wherein the solvent is DMAc.

4. The method of claim 1 wherein the membrane is produced in a roll-to-roll process.

5. (canceled)

6. The method of claim 1 further comprising an evaporation period of 3 to 15 seconds, or 4 to 10 seconds, or 10 to 60 seconds prior to immersing in the coagulation bath.

7. The method of claim 1 wherein the coating is applied to a thickness of 20 to 500 μm, such as by setting a 20 to 500 μm gap during knife casting.

8-11. (canceled)

12. The method of claim 1 wherein the non-woven fabric is selected from polyester, polyethylene, polypropylene, or polyetherether ketone non-woven fabrics.

13-14. (canceled)

15. The method of claim 1 wherein the porous membranes have 98% of the pores by number are less than 50 nm in diameter, or less than 40 nm in diameter, or less than 20 nm in diameter, or less than 15 nm in diameter, when tested under a scanning electron microscope, followed by an imaging processing.

16. The method of claim 1 wherein the porous membranes have a CO2 permeance of greater than 4 kGPU or greater than 7 kGPU or greater than 26 kGPU or greater than 85 kGPU or greater than 171 kGPU or greater than 260 kGPU.

17. A porous PBI membrane support or a porous PBI membrane supported composite membrane made by the method of claim 1.

18. A porous PBI membrane, comprising: a fabric layer; a PBI layer bonded to the fabric layer; and further characterizable by: a) wherein the porous PBI membrane comprises a CO.sub.2 permeance in the range of 50 to 260 kGPU, or 20 to 50 kGPU, or 100 to 400 kGPU, or a N.sub.2 permeance in the range of 50 to 300 kGPU, or 20 to 50 kGPU, or 100 to 500 kGPU, preferably a gas (CO.sub.2 or N.sub.2) permeance of at least 200 kGPU; b) wherein the PBI layer comprises a pore size of less than 50 nm, and a surface porosity of at least 8%; or c) wherein the PBI layer comprises finger-like pores observed from the cross-section under microscope, and wherein at least 50 vol % of the pores in the PBI layer have an aspect ratio of at least 2 (or at least 3 or at least 5), wherein aspect ratio is defined as maximum length divided by average width (diameter) of each pore, and where length is perpendicular to the surface of the fabric layer.

19. (canceled)

20. The porous PBI membrane of claim 18 wherein the PBI layer has a surface porosity of at least 5%.

21. (canceled)

22. The porous PBI membrane of claim 18 wherein the PBI layer has a solvent resistance such that, if soaked in chloroform, tetrahydrofuran, or acetone for 1 hour, the soaked and then dried membrane comprises a gas permeance at least 90% its permeance measured before the solvent soaking.

23. The porous PBI membrane of claim 18 comprising a CO.sub.2 selective top layer and wherein the composite membrane has a CO.sub.2 permeance of at least 1000 GPU or at least 3000 GPU and a CO.sub.2/N.sub.2 selectivity of at least 8 or at least 25 at 25° C. and a feed pressure of 1 atm.

24. The porous PBI membrane of claim 18 comprising a CO.sub.2 selective layer, wherein the CO.sub.2 selective layer has a thickness in the range of 100 to 500 nm and wherein the composite membrane has β.sub.CO2 of 39 to 64% or at least 10%.

25. The porous PBI membrane of claim 18 comprising a CO.sub.2 selective layer, wherein the CO.sub.2 selective layer comprises poly(ethylene oxide terephthalate)-poly(butylene terephthalate).

26. The porous PBI membrane of claim 18 comprising a CO.sub.2 selective layer, comprising a CO.sub.2 permeance as a function of the thickness of the CO.sub.2 selective layer of about 12.6 kGPU at a CO.sub.2 selective layer thickness of 100 nm to about 5000 GPU at a CO.sub.2 selective layer thickness of 450 nm or any value along a line between these points.

27. A porous PBI membrane supported composite membrane or a composite membrane intermediate, comprising: a fabric layer; a PBI layer bonded to the fabric layer, wherein the porous PBI layer comprises any characteristics of claim 18; and a top layer coated on the PBI layer so that the porous PBI layer is sandwiched between the fabric layer and top layer.

28-33. (canceled)

34. The porous PBI membrane supported composite membrane or a composite membrane intermediate of claim 18 wherein the top layer comprises polydimethylsiloxane attached to the PBI layer.

35. A method of separating a component of a fluid mixture comprising passing the fluid mixture in contact with the membrane of claim 18.

36. (canceled)

Description

BRIEF DESCRIPTION OF THE FIGURES

[0033] FIG. 1 Prior Art. Schematic illustration of a thin film composite (TFC) membrane comprised of a porous membrane support providing mechanical reinforcement and a dense selective layer performing gas separation. The membrane support consists of a porous polymer layer and a non-woven fabric layer. An optional gutter layer and an optional protective layer are sometimes required to improve membrane separation performance.

[0034] FIG. 2. (a) Repeating unit of PBI or poly(2,2′-(m-phenylene)-5,5′-bisbenzimidazole). Illustrations of (b) a porous PBI membrane comprised of a porous PBI layer and a non-woven fabric substrate and (c) a general procedure to prepare a porous PBI membrane.

[0035] FIG. 3 Surface and cross-sectional SEM images of porous PBI membranes, respectively: (a, ax) PBI-S1, (b, bx) PBI-S2, (c, cx) PBI-S3, (d, dx) PBI-S4, (e, ex) PBI-S5, and (f, fx) PBI-S6. The notations denote the used PBI concentration in wt. % and the water bath temperature in ° C., respectively.

[0036] FIG. 4 Schematic illustration of surface pore size and porosity determination via integrating the SEM imaging technique with an imaging processing by ImageJ.

[0037] FIG. 5 (a) Effect of solvent soaking on CO.sub.2 permeance of PBI-S6 and a commercial porous support (PAN-S). Surface morphology of PAN-S before (b) and after (c) soaking in acetone for 1 hour.

[0038] FIG. 6 (a) Effect of thermal exposure temperature on CO.sub.2 permeance of PBI-S6 and PAN-S. Surface morphology of porous supports after thermal exposure: (b) PAN-S at 100° C. and (c) PBI-S6 at 200° C.

[0039] FIG. 7 Cross-sectional SEM micrographs of PDMS/PBI-S6 TFC membranes fabricated using PDMS solution at concentration of (a) 0.5, (b) 1.0 and (c) 2.0%, respectively. (d) Comparison of CO.sub.2/N.sub.2 separation permeance of PDMS/PBI-S6 two-layer TFC membranes with the most permeable PDMS TFC membranes reported in the literature (Refs a-f). The dash lines are to guide the eye. PSF: polysulfone and PAN: polyacrylonitrile.

[0040] FIG. 8: Schematic illustrations of a gas separation composite membrane comprising of a porous support and a selective layer with thickness of l. The composite membrane suffers a geometric restriction and pore penetration effect that increase gas diffusion pathway distance.

[0041] FIG. 9 Schematic illustrations and cross-sectional SEM image of a multi-layer TFC membrane comprised of a Polyactive selective layer, a PDMS gutter layer, and a PBI-S6 membrane support.

[0042] FIG. 10 Chemical structure of some polybenzimidazoles with different R groups that can be used in the invention. The R groups displayed from left to right: m-phenylene, pyridine, diphenyl sulfone, (perfluoropropane-2,2-diyl)dibenzene, bis(trifluoromethyl)biphenyl (BTBP), perfluorocyclobutyldibenzene (PFCB), and phenylindane.

DETAILED DESCRIPTION OF THE INVENTION

Example 1: Porous PBI Membrane Supports

[0043] The present invention provides a facile and scalable method to prepare flat-sheet porous PBI membranes for membrane support application in TFC membranes. PBI is used as a membrane material due to its exceptional chemical and thermal stability. FIG. 2a shows the chemical structure of Celazole® PBI (poly(2,2′-(m-phenylene)-5,5′-bisbenzimidazole)), which is commercially available in forms of powder and solution. The invented porous PBI membranes comprise a non-woven fabric and a porous PBI layer as shown in FIG. 2b. The non-woven fabric provides mechanical support as well as allows the realization of continuous production in a roll-to-roll process. A chemical/thermal-resistant polyphenylene sulfide (PPS) non-woven fabric is used as the substrate in this invention. A phase inversion method is employed to fabricate porous PBI membranes. The phase inversion method involving polymer precipitation by water is the most important technique to prepare microporous membranes in both laboratory and industry. In this process, a liquid polymer solution is precipitated into two phases: a polymer-rich phase that forms the matrix of the membrane and a polymer-lean phase that forms the membrane pores.

a. Membrane Fabrication and Characterization

[0044] Membrane Fabrication: As displayed in FIG. 2c, porous PBI membranes are prepared in the following steps. A PBI solution at a desirable concentration was firstly cast on PPS non-woven fabrics using a casting knife with a gap setting of 150 μm and a knife speed of 4 cm s.sup.−1 at 20-23° C. and relative humidity of 60% (the ambient condition at our laboratory). An evaporation period of 5 seconds was allowed before immersing the PBI/PPS non-woven fabrics into a water quench bath set at a given temperature (20-60° C.). After 1 hour, the resulting membrane was rinsed twice using deionized water to remove residual dimethylacetamide (DMAc). The PBI membrane was finally obtained after drying at 23° C. in a fume hood for 24 hours. Water quench bath temperature and PBI solution concentration were varied to achieve different pore structures in this invention. Table 1 summarizes sample identifications, and their fabrication conditions including PBI solution property and water bath temperature. Solution casting of a polymer on a non-woven fabric requires a sufficient viscosity to prevent solution penetration to the backside of the fabric. The viscosity of a polymer solution is tied to the polymer concentration, and it decreases when the solution is diluted. The viscosities of PBI solutions were determined using an Anton Paar MCR 302 rheometer at a 50 mm-diameter cone/plate sample stage with a 1 mm sample gap in a rotational mode at a shear rate of 0.1-100 s.sup.−1 and 25° C. The obtained viscosities are summarized in Table 1. For example, 15 wt. % PBI has a viscosity of 1450±13 cP at 25° C., compared with 260±20 cP for 10 wt. % PBI. The selected PPS non-woven fabric can hold PBI solution as dilute as 10 wt. %.

TABLE-US-00001 TABLE 1 Porous PBI membrane fabrication conditions and characterizations. Water bath CO.sub.2 Pore size Surface PBI DMAc Viscosity temp. permeance range* porosity Sample ID (wt. %) (wt. %) (25° C., cP) (° C.) (kGPU) (dia., nm) (%) PBI-S1 15.0 85.0 1450 ± 13 20  3.8 ± 0.6 5-13 2.8 ± 0.5 PBI-S2 15.0 85.0 1450 ± 13 40  7.3 ± 1.0 5-15 4.2 ± 0.6 PBI-S3 15.0 85.0 1450 ± 13 60 26 ± 3 5-18 6.2 ± 0.9 PBI-S4 12.5 87.5 820 ± 4 60 85 ± 7 5-25 8.6 ± 1.0 PBI-S5 11.0 89.0 440 ± 3 60 171 ± 12 5-33 13 ± 2  PBI-S6 10.0 90.0 286 ± 2 60 260 ± 20 5-42 20 ± 2  *98% of the surface pores are within the range.

[0045] Discussion on surface pore morphology and gas permeance: The higher permeance and higher surface porosity with sub-50 nm pores are preferred in a high-performance porous membrane support. As displayed in FIGS. 3a-f, PBI membranes with tunable pore structure were achieved by varying PBI solution concentration and water quench bath temperature. Surface pore size and porosity were determined by the SEM imaging processing method as described above. The porous PBI membranes show nano-size pores and a surface porosity as high as 20% as summarized in Table 1. FIGS. 3ax-fx present cross-sectional SEM micrographs of the PBI membranes at their near-surface region. They show a finger-like structure comprising a microporous skin layer and a macrovoid sublayer. Their microporous skin layer thickness (150-320 nm), measured as the distance from the top surface to the tip of a finger structure, is marked in the micrographs. The skin layer with fine surface pores can provide a smooth surface for thin film coating, and the pillar-like macrovoid walls in the sublayer offer major mechanical support. As summarized in Table 1, increasing water bath temperature from 20 to 60° C. increases membrane gas permeance, pore size, and surface porosity. Decreasing PBI concentration from 15 to 10 wt. % can further improve membrane gas permeance, pore size, and surface porosity. For example, PBI S3 membrane, prepared using a 15 wt. % PBI solution and a 60° C. water bath, showed 26 kGPU CO.sub.2 permeance, pore diameter less than 18 nm, and 6.2% surface porosity. Using a diluted 10 wt. % PBI solution, the produced PBI S6 membrane exhibited 260 kGPU CO.sub.2 permeance, pore diameter less than 42 nm, and 20% surface porosity. The PBI membranes are characterized by high-permeance, nano-sized pore, and high-porosity.

b. Solvent Resistance

[0046] The fabrication of TFC membranes primarily relies on solution-coating techniques, and the chemical stability of a porous support thus becomes an important parameter to be considered. We evaluated solvent resistance of the porous PBI membranes (e.g., PBI-S6) by monitoring changes of their gas permeance before and after soaking in a solvent for 1 hour. This treatment simulates the solvent exposure history of a support when applied with a coating solution during the TFC membrane manufacturing. Prior to gas permeation tests on soaked samples, solvent was slowly evaporated in a fume hood for 2 hours, followed by vacuuming for 16 hours to completely remove the solvent at 23° C. In this evaluation, a commercial polyacrylonitrile porous support (PAN-S) was selected as a benchmark because it is among the most chemically stable porous supports available in the market. FIG. 5a shows that CO.sub.2 permeance of PBI-S6 was slightly affected by the common solvents used for thin film coating, including hexane, ethanol, tetrahydrofuran (THF), chloroform, and acetone. Its gas permeance maintained at about 260 kGPU, demonstrating its excellent solvent resistance to the selected solvents. Although PAN-S was stable in hexane and ethanol, its gas permeance dropped after soaking in THF, chloroform, or acetone. Especially in acetone, PAN-S only managed to recover 46 out of 144 kGPU after the soaking treatment. The permeance drop in PAN-S was presumably caused by solvent swelling. Even though PAN cannot be dissolved by the selected solvents, it would be swollen by solvents and thus pore morphology might be changed after drying. As shown in FIG. 5b-c, PAN-S becomes less porous after soaking in acetone than its pristine state. This comparison proves the outstanding solvent resistance of the PBI porous membranes.

c. Thermal Stability

[0047] Thermal treatment is often employed to evaporate solvents and promote thin film formation in TFC membrane fabrication. Especially in the large-scale production involving a continuous roll-to-roll process, heating units must be used to rapidly dry or cure the membranes. We examined the thermal stability of the PBI support by studying the dependence of gas permeance on thermal exposure temperature. Prior to gas permeance measurement, each sample was baked in an oven at a given temperature for 1 hour. This process simulates a potential thermal heat treatment used to remove solvents during the thin film coating. FIG. 6a shows that CO.sub.2 permeance of PBI-S6 was maintained at 260 kGPU from 23 to 100° C., whereas PAN-S can only withstand thermal exposure up to 75° C., and it almost lost its porous feature at 100° C. (FIG. 6b) with a low CO.sub.2 permeance of 5 kGPU. Although gas permeance of PBI-S6 declined at 125-200° C., it still presented 155 kGPU after thermal annealing at 200° C. FIG. 6c evidences thermal treatment at 200° C. shrank pore size to 8-37 nm on PBI-S6 and decreased surface to about 12%, thus leading to the drop in gas permeance. Nevertheless, the CO.sub.2 permeance of PBI-S6 annealed at 200° C. was still higher than that (144 kGPU) of pristine PAN-S. This sought-after thermal stability also expands the porous PBI membrane's application to high-temperature membrane separations, for example, being a porous support for the TFC membranes used in the pre-combustion CO.sub.2 capture (H.sub.2/CO.sub.2 separation) operated at 150° C. or above.

Example 2. Porous PBI Membranes as a Membrane Support for Two-Layer TFC Membranes

[0048] Herein, we evaluated the porous PBI membranes' performance as a porous support in practical TFC membranes, that is, the capability of providing a suitable surface for the formation of defect-free thin films without introducing too much gas transport resistance. To examine the PBI membranes' supporting performance, a thin polydimethylsiloxane (PDMS) layer is applied on PBI-S6 to form a traditional two-layer TFC membrane. Rubbery PDMS is chosen as a coating material because it has been widely utilized in industrial gas and vapor separations, and more importantly, its thin film permeance is stable with time. PBI-S6 is selected for this demonstration because it shows the highest gas permeance and surface porosity among the invented membranes.

[0049] Membrane fabrication and characterizations: PDMS-based two-layer TFC membranes were fabricated on a PBI-S6 support using a knife casting method. First, a PBI-S6 support of 5.0×7.5 cm was taped on a glass substrate and immersed in water for 5 minutes. This water pre-wetting process is to reduce PDMS solution pore penetration during the coating. Second, excess water on the support membrane surface was gently removed using flowing nitrogen, immediately followed by applying a PDMS/hexane solution using a casting knife with a blade clearance 50 μm above the support membrane. Finally, the PDMS composite membrane was cross-linked and dried in an oven at 100° C. for 1 hour. The concentration of PDMS prepolymer in hexane was varied from 0.5, 1.0, to 2.0 wt. % to achieve PDMS layers with different thicknesses. The PDMS prepolymer comprises 86.2 wt. % vinyl-PDMS (Dehesive® 944), 8.6 wt. % Crosslinker V24, and 5.2 wt. % Catalyst OL. The resulting PDMS layer thickness was determined by cross-sectional SEM, performed with a FEI Quanta™ 600F scanning electron microscope (Thermo Fisher Scientific, OR, USA). Pure-gas permeances of CO.sub.2 and N.sub.2 across the PDMS/PBI-S6 two-layer TFC membranes were determined using a constant pressure/variable volume method at 25° C. as described in Gas Permeance Measurement of Glossary.

[0050] Results and discussion on PDMS/PBI two-layer TFC membranes: Using a traditional and scalable knife casting method, 100 to 450 nm thick defect-free PDMS thin films (FIGS. 7a-c), can be easily prepared on top of PBI-S6, demonstrating that the surface morphology of the porous PBI membrane is suitable for producing thin films as thin as 100 nm. FIG. 7d compares CO.sub.2 permeance of the PDMS/PBI-S6 TFC membranes with state-of-the-art PDMS membranes supported by commercial PAN or PSF supports {Refs: (a) Nanoscale, 8 (2016) 8312; (b) J. Membr. Sci., 499 (2016) 191; (c) J. Membr. Sci., 541 (2017) 367; (d) Energy Environ. Sci., 9 (2016) 434; (e) Sep. Purif. Technol., 239 (2020) 116580; (f) ACS Appl. Mater. Interfaces, 7 (2015) 15481.}. At a similar PDMS thickness, PBI-S6 supported membranes provided much higher permeance than that of PAN and PSF supported ones, proving that PBI-S6 imposed less gas transport resistance and thus is a much better membrane support than those commercial PAN and PSF supports. It is also noticeable that our 100 nm-thick PDMS/PBI-S6 membrane showed the highest CO.sub.2 permeance (i.e., 12600 GPU) among the reported PDMS composite membranes.

[0051] As shown in FIG. 8, a composite membrane is made of a porous support overlaid with a selective layer. The porous support would inevitably impose additional gas transport resistance due to a geometric restriction and pore penetration effect. The geometric restriction occurs because gas can only diffuse through the selective layer where a pore is present. A limited pore accessibility would increase gas diffusion pathway distance and thus the gas transport resistance. The pore penetration happens because dilute coating solution tends to intrude into the support pores before it solidifies as a thin film. Compared to the nominal thickness (l), the gas diffusion pathway through a plugged pore (l.sub.p) becomes longer, leading to an increase in gas transport resistance. The effect of a porous support on gas permeance through a composite membrane can be characterized by membrane permeance efficiency (β.sub.A):

[00002]${\beta }_A={\left(\frac{P}{l}\right)}_{\exp }/{\left(\frac{P}{l}\right)}_{\mathrm{ideal}}$

where (P.sub.A/l).sub.exp is the experimentally determined gas permeance and (P.sub.A/l).sub.ideal is the ideal permeance without any restrictions from the support membrane. A higher β.sub.A value indicates the porous support impose less gas transport resistance. Table 2 compares CO.sub.2 permeance efficiency value (β.sub.CO2) of the PDMS/PBI-S6 TFC membranes in this invention with state-of-the-art composite membranes. The selective layer is made thinner and thinner to improve the overall gas permeance, leading to a decrease in the β.sub.A value. This can be ascribed to an amplified geometric restriction on a thinner selective layer: a typical porous support has surface porosity no more than 20%, so gas penetrants primarily enter into the selective layer away from the pore region (FIG. 8); the ratio of gas diffusion pathway distance to selective layer thickness (l.sub.g/l) increases significantly as decreasing the thickness l, resulting in a relatively higher transport resistance. Consequently, our PDMS/PBI-S6 TFC membranes presented β.sub.CO2 value of 64.3% at 450 nm but then decreased to 39.4% at 100 nm (Table 2). Nevertheless, the PDMS/PBI-S6 TFCs exhibit much higher β.sub.CO2 values than those PAN and PSF supports based TFC membranes at any thickness ranges. For example, at 100-210 nm, our TFC has β.sub.CO2 values varying from 39.4-59.1%, compared to 14.4-37.5% observed on the reported TFCs in the literature. This comparison reconfirms that our PBI supports outperform those popular commercial porous supports, by approximately doubling the permeance efficiency in most cases.

TABLE-US-00002 TABLE 2 Fabrication and test conditions, gas separation properties, and CO.sub.2 permeance efficiency (β.sub.CO2) of PDMS-based two-layer TFC membranes. CO.sub.2 Test Feed Coating PDMS perm. CO.sub.2/N.sub.2 temp. pressure β.sub.CO2 Substrate technique (l, nm) (GPU) select. (° C.) (bar) (%) Ref. 1 PBI-S6 Knife casting 450 4570 11.7 25 1.0 64.3 This 2 210 9000 11.6 25 1.0 59.1 work 3 100 12600 11.5 25 1.0 39.4 4 PAN Spin coating 400 2860 9.0 35 3.5 30.2 a 5 PAN Spin coating 350 4050 9.0 35 3.5 37.3 b 6 PAN Dip coating 230 5140 10.6 25 2.0 36.9 c 7 PAN Spin coating 190 2880 10.0 35 3.4 14.4 d 8 PSF Knife casting 120 10000 10.5 25 0.2 37.5 e 9 PSF Knife casting 100 6000 8.0 25 1.0 18.8 f Refs: a Nanoscale, 8 (2016) 8312; b J. Membr. Sci., 499 (2016) 191; c J. Membr. Sci., 541 (2017) 367; d Energy Environ. Sci., 9 (2016) 434; e Sep. Purif. Technol., 239 (2020) 116580; f ACS Appl. Mater. Interfaces, 7 (2015) 15481.

Example 3. Porous PBI Membranes as a Membrane Support for Multi-Layer TFC Membranes

[0052] Industrial membranes often use multi-layer TFC membranes with an intermediate gutter layer (<1000 nm) between the selective layer and the porous membrane support to achieve high permeance for gas separation. (Kattula et al., Designing ultrathin film composite membranes: the impact of a gutter layer. Scientific Report 5, 15016 (2015)) A gutter layer plays at least two roles in achieving high-performance TFC membranes when employed. First, it prevents dilute coating solutions of the selective layer material from penetrating into the bottom porous support. The pore penetration of low-permeability selective layer material would block support pores and significantly increase mass transfer resistance. Second, it provides a smoother surface than the bare porous support. A smooth surface forms a continuous defect-free ultra-thin selective layer to boost membrane permeance. In the meantime, a gutter layer would inevitably impose additional mass transfer resistance to the resulting TFC membrane, so favorable gutter layers must be formed from highly permeable materials to avoid any significant transport resistance. PDMS has been the most used gutter layer material due to its high gas permeability and stable thin-film performance, and thereby it is employed as the gutter layer material in this example. We evaluated the porous PBI membranes' performance as a porous support in a multi-layer TFC membrane for CO.sub.2/N.sub.2 separation (or post-combustion carbon capture). The selective layer of the multi-layer TFC membrane uses poly(ethylene oxide terephthalate)-poly(butylene terephthalate) because it is a widely-employed CO.sub.2-selective membrane material and commercialized under the tradename of Polyactive™ by PolyVation BV, Netherlands.

[0053] Membrane fabrication and characterizations: To fabricate a Polyactive/PDMS/PBI-S6 multi-layer TFC membrane for CO.sub.2/N.sub.2 separation, a 100 nm-thick PDMS gutter layer was firstly coated onto a PBI-S6 membrane support by knife casting a 0.5% PDMS coating solution following the coating method described in Example 2. A 2×2 cm coupon cut from the resulting PDMS/PBI-S6 two-layer membrane was then mounted onto a glass disc with a diameter of 3.8 cm by taping the coupon's four edges. Afterwards, 0.25 mL Polyactive™ solution (0.5 wt. % in tetrahydrofuran solvent) was spin-coated (1000 rpm for 1 minute) on the PDMS/PBI-S6 two-layer structure. Finally, a Polyactive/PDMS/PBI-S6 multi-layer TFC membrane was obtained by drying the spin-coated sample at 50° C. under vacuum for 4 hours. The Polyactive layer thickness was determined as 30 nm by an Alpha-SE ellipsometer (J.A. Woollam Co., Lincoln, Nebr.) and then confirmed by cross-sectional SEM characterization performed with a FEI Quanta™ 600F scanning electron microscope (Thermo Fisher Scientific, OR, USA). Pure-gas permeances of CO.sub.2 and N.sub.2 across the Polyactive/PDMS/PBI-S6 multi-layer TFC membranes were determined using a constant pressure/variable volume method at 25° C. as described in Gas Permeance Measurement of Glossary.

TABLE-US-00003 TABLE 3 Comparison of CO.sub.2/N.sub.2 separation permeance of Polyactive/PDMS/PBI-S6 multi-layer TFC membranes with the best-performing Polyactive- based multi-layer TFC membrane reported in the literature. Membrane Selective layer CO.sub.2 perm. CO.sub.2/N.sub.2 support Gutter layer thickness (nm) (GPU) select. Reference PBI-S6 PDMS 30 3100 ± 200 41 ± 2 This work Commercial Metal-organic 80 2100 33 Liu et al. ACS PAN frameworks Nano 12 (2018) 11591 Customized PDMS 80 1330 52 Yave et al., PAN Nanotechnology 21 (2010) 395301 Customized PDMS 45 1780 60 Yave et al., Energy PAN Environ. Sci. 4 (2011) 4656

[0054] Results and discussion on Polyactive/PDMS/PBI multi-layer TFC membranes: An ultrathin defect-free Polyactive layer was successfully fabricated on top of a PDMS gutter layer coated PBI-S6 membrane support. As shown in FIG. 9, a dense coating with a thickness of 130 nm deposited on the PBI-S6 membrane support. The dense coating comprises a 100 nm PDMS gutter layer and a 30 nm Polyactive selective layer as measured by cross-sectional SEM and optical ellipsometry. Gas separation performance of the resulting Polyactive/PDMS/PBI-S6 multi-layer TFC membranes was evaluated for CO.sub.2/N.sub.2 separation or post-combustion carbon capture. According to a recent technology economic analysis on the membrane CO.sub.2/N.sub.2 separation (Alex Zoelle et al., Performance and Cost Sensitivities for Post-Combustion Membrane Systems, 2018 NETL CO2 Capture Technology Project Review Meeting, accessed on Jun. 25, 2021, https://www.osti.gov/servlets/purl/1592464), the membrane process becomes profitable for post-combustion carbon capture when CO.sub.2 permeance is greater than 1000 GPU. The cost of CO.sub.2/N.sub.2 separation (or carbon capture cost) significantly decreases as increasing CO.sub.2 permeance in a permeance range of 1000-4000 GPU when CO.sub.2/N.sub.2 selectivity is maintained above 25. As summarized in Table 3, the Polayacitve/PDMS/PBI-S6 multi-layer TFC membranes exhibit an averaged CO.sub.2 permeance of 3100±200 GPU and high CO.sub.2/N.sub.2 selectivity of 41±2. This obtained CO.sub.2 permeance of 3100 GPU is 48% higher than the highest permeance (i.e., 2070 GPU) previously achieved for Polyactive-based multi-layer TFC membranes, as reported by Liu et al. in ACS Nano 12 (2018) 11591, suggesting the superior separation performance of the fabricated Polyactive/PDMS/PBI-S6 membranes. The membranes of this invention also show essentially higher CO.sub.2 permeance than those (1330-1780 GPU) of the Polyactive-based multi-layer TFC membranes on PAN membrane supports coated with a PDMS gutter layer. (Yave et al., Nanotechnology 21 (2010) 395301 and Yave et al., Energy & Environmental Science 4 (2011) 4656). The high CO.sub.2permeance has been achieved for the multi-layer membranes of this example because ultrahigh-permeance PDMS gutter layer (i.e., a record-high CO.sub.2 permeable of 12600 GPU as determined in Example 2) provides a smooth coating surface with no significant mass transfer resistance, while the formation of such a PDMS gutter layer can be ultimately ascribed to the use of the highly permeable PBI-S6 membrane support with nano-sized pores and high surface porosity.

TABLE-US-00004 TABLE 4 Physical properties of porous PBI membranes in this invention and the references Membrane N.sub.2 permeance Pore size Surface description (1000 GPU) (dia., nm) porosity (%) Reference NETL PBI-S6 300 ± 28 <42 20 ± 2  This invention NETL PBI-S4 94 ± 8 <25 8.6 ± 1.0 This invention Microporous PBI N/A 200-700 N/A U.S. Pat. No. 5,091,087 membrane Microporous PBI 9.6-32.2  50-1000 N/A U.S. Pat. No. 6,623,639 membrane U.S. Pat. No. 6,986,844 Microporous PBI N/A >100 N/A Takigawa et al., Separation membrane Science and Technology, 27, 3, 325-339 (1992) PBI porous support 170 ~100 21 Weigelt et al., Membranes 2019, 9, 51 PBI nanofiltration N/A 0.696 N/A Wang et al., Journal of membrane Membrane Science 281 (2006) 307-315 PBI nanofiltration N/A 0.67-0.70 N/A Tashvigh et al. Journal of membrane Membrane Science 572 (2019) 580-587