EP4593995A1 - Asymmetric hollow carbon fibers including oxidized surface layers - Google Patents

Abstract

An asymmetric hollow carbon fiber may include an outer wall surrounding a hollow interior space, wherein the outer wall includes an oxidized surface layer, a dense separating layer, and a porous support layer. The dense separating layer may be between the oxidized surface layer and the porous support layer. The porous support layer may be between the dense separating layer and the hollow interior space. The oxidized surface layer may have a thickness greater than 0.5 microns. The oxidized surface layer includes oxygen in an amount greater than 11 atomic% of the total atoms of the oxidized surface layer.

Description

ASYMMETRIC HOLLOW CARBON FIBERS INCLUDING OXIDIZED SURFACE LAYERS

CROSS-REFERENCE TO RELATED APPLIC TIONS

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 63/418,724 filed October 24, 2022, the entire disclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

[0002] Embodiments described herein generally relate to gas separation, and more specifically to asymmetric hollow carbon fibers suitable for use in gas separation.

BACKGROUND

[0003] Membranes are widely used for the separation of gases and liquids, including for example, separating acid gases, such as CO2 and H2S from natural gas, and the separation of ethylene and ethane from each other. Gas transport through such membranes is commonly modeled by the sorption-diffusion mechanism. Currently, polymeric membranes are well studied and widely available for gaseous separations due to easy process-ability and low cost. Carbon molecular sieve (CMS) membranes, however, have been shown to have attractive separation performance properties exceeding that of polymeric membranes.

[0004] In gas separation technology, it is desired to make reverse selective/surface flow membranes as reported in the review articles by D. Paul et al. (Progress in Polymer Science, 38, (2013) 740- 766) and Hill et al. (Science, Vol 296, 19 April, 2002). Most of the development in this area has been focused on polymeric membranes. Reverse selective membranes are typically made through the incorporation of specific functional groups which enhance the solubility of one gas over the other during the separation of gas molecules. Synthetically, the process to create such chemical compositions difficult for CMS membranes because it is difficult to incorporate such functional groups into a CMS membrane. For example, a pyrolysis temperature of greater than 500 °C converts most of the polymeric structure to a carbon structure and most of the functional groups that could enhance solubility of gas molecules are thus lost during pyrolysis. As a result, it is difficult to enhance solubility driven gas transport in CMS membranes and membrane separation performance is mostly governed by size based or diffusion based separation mechanisms.

[0005] Accordingly, it would be desirable to produce asymmetric hollow carbon fibers that may, for example be used to produce a reverse selective CMS membrane in a filamentary form factor.

SUMMARY

[0006] Asymmetric hollow carbon fibers may be utilized to produce CMS membranes for use in gas separations. In particular, asymmetric hollow carbon fibers that may be utilized to separate CO2 from N2 are desired.

[0007] According to aspects, an asymmetric hollow carbon fiber may include an outer wall surrounding a hollow interior space. The outer wall may comprise an oxidized surface layer, a dense separating layer, and a porous support layer. The dense separating layer may be between the oxidized surface layer and the porous support layer. The porous support layer may be between the dense separating layer and the hollow interior space. The oxidized surface area may have a thickness greater than 0.5 microns. The oxidized surface layer may comprise oxygen in an amount greater than 11 atomic% of the total atoms of the oxidized surface layer.

[0008] It is to be understood that both the foregoing brief summary and the following detailed description present embodiments of the technology, and are intended to provide an overview or framework for understanding the nature and character of the technology as it is claimed. The accompanying drawings are included to provide a further understanding of the technology, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments and, together with the description, serve to explain the principles and operations of the technology. Additionally, the drawings and descriptions are meant to be merely illustrative, and are not intended to limit the scope of the claims in any manner.

[0009] Additional features and advantages of the described embodiments will be set forth in the detailed description that follows. The additional features and advantages of the described embodiments will be, in part, readily apparent to those skilled in the art from that description or recognized by practicing the described embodiments, including the detailed description that follows as well as the drawings and the claims. BRIEF DESCRIPTION OF THE DRAWING

[0010] Reference will now be made in greater detail to various embodiments, some of which are illustrated in the accompanying drawings, wherein:

[0011] FIG. 1 depicts a cross section of an asymmetric hollow carbon fiber, according to one or more embodiments of the present disclosure.

DET ILED DESCRIPTION

[0012] Embodiments disclosed herein are directed to asymmetric hollow carbon fibers. Now referring to FIG. 1, a cross section of an asymmetric hollow carbon fiber 100 according to embodiments is shown. The asymmetric hollow carbon fiber 100 may comprise an outer wall surrounding a hollow interior space 140. The outer wall may comprise an oxidized surface layer 110, a dense separating layer 120, and a porous support layer 130. The dense separating layer 120 may be between the oxidized surface layer 110 and the porous support layer 130. The porous support layer 130 may be between the dense separating layer 120 and the hollow interior space 140. The oxidized surface layer 110 may have a thickness greater than 0.5 microns. The oxidized surface layer 110 may comprise oxygen in an amount greater than 11 atomic% of the total atoms of the oxidized surface layer 110.

[0013] In embodiments, the hollow carbon fiber 100 may be asymmetric. As used herein, the term “asymmetric” refers to a property of the hollow carbon fiber 100 in which the hollow carbon fiber 100 has at least one relatively more dense layer, which may be the dense separating layer 120 and at least one relatively less dense layer, which may be the porous support layer 130. For instance, in embodiments, one layer of the hollow carbon fiber 100 may be greater than or equal to 2 pm and less than or equal to 6 pm and be more dense than a second layer. The second layer may be thicker than the first layer, such as greater than or equal to 20 pm and less than or equal to 200 pm. In one or more embodiments, the dense separating layer 120 may have a density greater than the density of the porous support layer 130.

[0014] As described herein, in one or more embodiments, the oxidized surface layer 110 may have a thickness of greater than 0.5 microns, such as greater than 0.75 microns, greater than 1.0 microns, greater than 1.25 microns, greater than 1.5 microns, greater than 1.75 microns, greater than 2.0 microns, greater than 2.5 microns, or even greater than 3.0 microns. In some embodiments, the oxidized surface layer 110 may have a thickness of less than 2.0 microns. For example, the oxidized surface layer 110 may have a thickness of from 0.5 microns to 2.0 microns, such as from 0.5 microns to 1.8 microns, from 0.5 microns to 1.6 microns, from 0.5 microns to 1.4 microns, from 0.5 microns to 1.2 microns, from 0.5 microns to 1.0 microns, from 0.5 microns to 0.8 microns, from 0.5 microns to 0.6 microns, from 0.6 microns to 2.0 microns, from 0.6 microns to 1.8 microns, from 0.6 microns to 1.6 microns, from 0.6 microns to 1.4 microns, from 0.6 microns to 1.2 microns, from 0.6 microns to 1.0 microns, from 0.6 microns to 0.8 microns, from 0.8 microns to 2.0 microns, from 0.8 microns to 1.8 microns, from 0.8 microns to 1.6 microns, from 0.8 microns to 1.4 microns, from 0.8 microns to 1.2 microns, from 0.8 microns to 1.0 microns, from 1.0 microns to 2.0 microns, from 1.0 microns to 1.8 microns, from 1.0 microns to 1.6 microns, from 1.0 microns to 1.4 microns, from 1.0 microns to 1.2 microns, from 1.2 microns to 2.0 microns, from 1.2 microns to 1.8 microns, from 1.2 microns to 1.6 microns, from 1.2 microns to 1.4 microns, from 1.4 microns to 2.0 microns, from 1.4 microns to 1.8 microns, from 1.4 to 1.6 microns, from 1.6 microns to 2.0 microns, from 1.6 microns to 1.8 microns, or from 1.8 microns to 2.0 microns. Without being bound by theory, it is believed that an oxidized surface layer 110 having a thickness less than 0.5 microns may not sufficiently facilitate the transport of gas molecules with higher critical condensation temperatures resulting in a nonreverse selective fiber. It is also believed that an oxidized surface layer 110 having a thickness greater than 2.0 microns may negatively impact the gas permeance performance of the fiber.

[0015] In one or more embodiments, the oxidized surface layer 110 may be a continuous layer. For example, the oxidized surface layer 110 may extend around the entire surface of the asymmetric hollow carbon fiber 100 such that the oxidized surface layer 110 completely covers the dense separating layer 120. Without being bound by theory, it is believed that oxidation of the surface layer may enhance solubility driven transport of gas molecules when compared to nonoxidized layers of the fiber. As such, it may be desirable to have the surface layer of the fiber be a continuous oxidized layer to maximize contact between the gas molecules and the oxidized surface layer 110.

[0016] In one or more embodiments, the oxidized surface layer 110 may comprise oxygen in an amount greater than 11 atomic% of the total atoms of the oxidized surface layer 110. For example, the oxidized surface layer 110 may comprise oxygen in an amount greater than 12 atomic% of the total atoms of the oxidized surface layer 110, such as greater than 14 atomic%, greater than 16 atomic%, greater than 18 atomic%, or even greater than 20 atomic% of the total atoms of the oxidized surface layer 110. In some embodiments, the oxidized surface layer 110 may comprise oxygen in an amount greater than 15 atomic% of the total atoms of the oxidized surface layer 110. In one or more embodiments, the oxidized surface layer 110 may comprise oxygen in an amount less than 25% of the total atoms of the oxidized surface layer 110. For example the oxidized surface layer 110 may comprise oxygen in an amount from 15 atomic % to 25 atomic% of the total atoms of the oxidized surface layer 110, such as from 15 atomic% to 24 atomic%, from 15 atomic% to 23 atomic%, from 15 atomic% to 22 atomic%, from 15 atomic% to 21 atomic%, from 15 atomic% to 20 atomic%, from 15 atomic% to 19 atomic%, from 15 atomic% to 18 atomic%, from 15 atomic% to 17 atomic%, from 15 atomic% to 16 atomic%, from 16 atomic% to 25 atomic%, 16 atomic% to 24 atomic%, from 16 atomic% to 23 atomic%, from 16 atomic% to 22 atomic%, from 16 atomic% to 21 atomic%, from 16 atomic% to 20 atomic%, from 16 atomic% to 19 atomic%, from 16 atomic% to 18 atomic%, from 16 atomic% to 17 atomic%, from 17 atomic% to 25 atomic%, 17 atomic% to 24 atomic%, from 17 atomic% to 23 atomic%, from 17 atomic% to 22 atomic%, from 17 atomic% to 21 atomic%, from 17 atomic% to 20 atomic%, from 17 atomic% to 19 atomic%, from 17 atomic% to 18 atomic%, from 18 atomic% to 25 atomic%, 18 atomic% to 24 atomic%, from 18 atomic% to 23 atomic%, from 18 atomic% to 22 atomic%, from 18 atomic% to 21 atomic%, from 18 atomic% to 20 atomic%, from 18 atomic% to 19 atomic%, from 19 atomic% to 25 atomic%, from 19 atomic% to 24 atomic%, from 19 atomic% to 23 atomic%, from 19 atomic% to 22 atomic%, from 19 atomic% to 21 atomic%, from 19 atomic% to 20 atomic%, from 20 atomic% to 25 atomic%, from 20 atomic% to 24 atomic%, from 20 atomic% to 23 atomic%, from 20 atomic% to 22 atomic%, from 20 atomic% to 21 atomic%, from 21 atomic% to 25 atomic%, from 21 atomic% to 24 atomic%, from 21 atomic% to 23 atomic%, from 21 atomic% to 22 atomic%, from 22 atomic% to 25 atomic%, from 22 atomic% to 24 atomic%, from 22 atomic% to 23 atomic%, from 23 atomic% to 25 atomic%, from 23 atomic% to 24 atomic%, or from 24 atomic% to 25 atomic%. Without being bound by theory, it is believed that if the oxidized surface layer 110 comprises oxygen in an amount less than 11 atomic% of the total atoms of the oxidized surface layer 110, the oxidized surface layer 110 may not have sufficiently enhanced gas solubility to facilitate solubility driven transport of gas molecules. It is also believed that if the oxidized surface layer 110 comprises oxygen in an amount greater than 25 atomic% of the total atoms of the oxidized surface layer 110, the gas permeance and gas selectivity of the fiber may be negatively impacted. [0017] In one or more embodiments, the asymmetric hollow carbon fiber 100 may have a G peak and D peak with an intensity ratio of the D peak to the G peak of less than 1.2 as measured using Raman spectroscopy at a Raman excitation wavelength of 532 nm. In some embodiments, the asymmetric hollow carbon fiber 100 may have a G and D peak with an intensity ratio of the D peak to the G peak of at least 1.0 as measured using Raman spectroscopy at a Raman excitation wavelength of 532 nm. For example, the intensity ratio of the D peak to the G peak may be from 1.0 to 1.2, from 1.0 to 1.15, or from 1.15 to 1.2. Without being bound by theory, it is believed that an intensity ratio of the D peak to the G peak of less than 1.2 may be associated with the formation of a reverse selective asymmetric hollow carbon fiber 100 as it may indicate the fiber shifting from a crystalline graphite structure to an amorphous graphite structure.

[0018] In one or more embodiments, the asymmetric hollow carbon fiber 100 may have a D(002) peak of less than 4.0 A in transmission as measured by X-ray scattering at an energy level of 17 keV. For example, the asymmetric hollow carbon fiber 100 may have a D(002) peak of less than 3.9 A, less than 3.8 A, less than 3.7 A, less than 3.6 A, or even less than 3.5 A. In some embodiments the D(002) peak may be at least 3.3 A in transmission. For example, the asymmetric hollow carbon fiber 100 may have a D(002) peak of from 3.3 A to 4.0 A, from 3.3 A to 3.9 A, from 3.3 A to 3.8 A, from 3.3 A to 3.7 A, from 3.3 A to 3.6 A, from 3.3 A to 3.5 A, from 3.3 A to 3.4 A, from 3.4 A to 4.0 A, from 3.4 A to 3.9 A, from 3.4 A to 3.8 A, from 3.4 A to 3.7 A, from 3.4 A to 3.6 A, from 3.4 A to 3.5 A, from 3.5 A to 4.0 A, from 3.5 A to 3.9 A, from 3.5 A to 3.8 A, from 3.5 A to 3.7 A, from 3.5 A to 3.6 A, from 3.6 A to 4.0 A, from 3.6 A to 3.9 A, from 3.6 A to 3.8 A, from 3.6 A to 3.7 A, from 3.7 A to 4.0 A, from 3.7 A to 3.9 A, from 3.7 A to 3.8 A, from 3.8 A to 4.0 A, from 3.8 A to 3.9 A, or from 3.9 A to 4.0 A. Without being bound by theory, it is believed that a D(002) peak of less than 4.0 A as measured by X-ray scattering at an energy level of 17 keV may indicate that the bulk of the fiber has undergone pyrolysis and subsequent carbonization to form an asymmetric hollow carbon fiber 100. In addition, a D(002) peak of less than 4.0 A in transmission may also indicate that oxidation of the fiber has primarily occurred at the surface layer and not throughout the fiber layers. It is believed that if the other fiber layers are oxidized the fiber may not have the desired gas permeation and selectivity characteristics.

[0019] In one or more embodiments, a plurality of asymmetric hollow carbon fibers may be combined to form a carbon molecular sieve membrane. In some embodiments, the carbon molecular sieve membrane may comprise channels that form between the oxidized surface layers of adjacent fibers. In some embodiments, the carbon molecular sieve membrane may not comprise an external support.

[0020] In embodiments, the asymmetric membrane may be an entity composed of an extremely thin, dense skin over a thick porous substructure, which may be of the same or different material as that of the dense skin layer. In embodiments, the asymmetric membrane may be fabricated in a single step by phase inversion, or the thin layer may be coated on the pre -prepared porous support using a dip coating method. These layers in the asymmetric membranes may be created physically by coating or created by chemical modification. The asymmetric membrane may be in the form of a hollow fiber configuration. In embodiments, the asymmetric membrane may contain a third layer of the same or different material as needed to enhance the membrane performance.

[0021] In one or more embodiments, the method of making the asymmetric hollow carbon fiber 100 may include providing a polymeric precursor fiber. A polymeric precursor fiber may be provided by forming a precursor polymer into hollow fibers using conventional methods. For example, co-extrusion procedures including processes such as a dry-jet wet spinning process (in which an air gap exists between the tip of the spinneret and the coagulation or quench bath) or a wet spinning process (with zero air-gap distance) may be used to from polymeric precursor fibers.

[0022] The polymeric precursor fiber may be any useful polymer for making asymmetric hollow carbon fibers. In some embodiments, the polymeric precursor fiber may be a polyimide. The polyimide may be a conventional or fluorinated polyimide. In some embodiments, polyimides used to form the polymeric precursor fiber may contain at least two different moieties selected from 2, 4, 6-trimethyl- 1,3 -phenylene diamine (DAM), oxydianaline (ODA), dimethyl-3,7- diaminodiphenyl-thiophene-5,5’-dioxide (DDBT), 3, 5 -diaminobenzoic acide (DABA), 2.3,5,6- tetramethyl- 1 ,4-phenylene diamine (durene), meta-phenylenediamine (m-PDA), 2,4- diaminotolune (2,4-DAT), tetramethylmethylenedianaline (TMMDA), 4,4’ -diamino 2,2’- biphenyl disulfonic acid (BDSA); 5,5’-[2,2,2-trifluoro-l-(trifluoromethyl)ethylidene]-l,3- isobenzofurandion (6FDA), 3, 3 ’,4, 4 ’-biphenyl tetracarboxylic dianhydride (BPD A), pyromellitic dianhydride (PMDA), 4,4 ’-oxy diphthalic anhydride (ODPA), 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA), and benzophenone tetracarboxylic dianhydride (BTDA). In some embodiments, the two different moieties are two or more of 6FDA, BPDA and DAM. [0023] In one or more embodiments, the polyimide polymeric precursor may be, 6FDA/BPDA-DAM. 6FDA/BPDA-DAM may be synthesized via thermal or chemical processes from a combination of three monomers: DAM, 6FDA, and BPD A, each commercially available, for example, from Sigma- Aldrich Corporation. Formula 1 below shows a representative structure for 6FDA/BPDA-DAM, with a potential for adjusting the ratio between X and Y to tune polymer properties. As used in examples below, a 1 : 1 ratio of component X and component Y may also abbreviated as 6FDA/BPDA(1 :1)-DAM.

Formula 1. Chemical structure of 6FDA/BPDA-DAM

[0024] In one or more embodiments, the polyimide polymeric precursor may be 6FDA- DAM. 6FDA-DAM lacks BPDA such that Y equals zero in Formula 1 above. Formula 2 below shows a representative structure for this polyimide.

Formula 2. Chemical structure of 6FDA-DAM

[0025] In one or more embodiments, the polyimide polymeric precursor may be MATRIMID™ 5218 (Huntsman Advanced Materials), a commercially available polyimide that is a copolymer of 3,3’,4,4’-benzo-phenonetetracarboxylic acid dianhydride and 5(6)-amino-l-(4’- aminophenyl)- 1 ,3, 3 -trimethylindane (BTDA-DAPI).

[0026] In one or more embodiments, the polyimide polymeric precursor may be 6FDA/PMDA-DAM as shown in formula 3 below.

Formula 3. Chemical structure of 6FDA/PMDA-DAM

[0027] In one or more embodiments, the polymeric precursor hollow fibers are substantially defect-free. As used in the present disclosure the term “defect-free” means that the selectivity of a gas pair, typically oxygen (O2) and nitrogen (N2), through a polymeric precursor hollow fiber membrane is at least 90 percent of the selectivity for the same gas pair through a dense film prepared from the same composition as that used to make the polymeric precursor hollow fiber membrane. For example, a 6FDA/BPDA(1 :1)-DAM polymer has an intrinsic O2/N2 selectivity (also known as “dense film selectivity”) of 4.1.

[0028] Pyrolysis conditions influence asymmetric hollow carbon fiber 100 physical properties and, accordingly, are chosen with care. In one or more embodiments, the precursor fibers may be pyrolyzed in a furnace. Any suitable supporting means for holding the fibers within the furnace may be used during the pyrolysis including sandwiching between two metallic wire meshes or using a stainless steel mesh plate in combination with stainless steel wires and as described by US Pat. No. 8,709,133 at col. 6 line 58 to col. 7, line 4, which is incorporated by reference.

[0029] Precursor polymers may be pyrolyzed to form the hollow fiber CMS membranes (i.e., carbonize the precursor polymer) under various inert gas purge or vacuum conditions (e.g. a pressure less than or equal to 0.1 millibar). U.S. Pat. No. 6,565,631 describes a heating method for pyrolysis of polymeric fibers to form hollow fiber CMS membranes, and is incorporated herein by reference. For either polymeric films or fibers, the pyrolysis temperature may be greater than or equal to 800 °C and less than or equal to 1200 °C. The pyrolysis temperature may be adjusted in combination with the pyrolysis atmosphere to tune the performance properties of the resulting hollow fiber CMS membrane. For example, the pyrolysis temperature may be 1000 °C or more. Optionally, the pyrolysis temperature may be greater than or equal to 900 °C and less than or equal to 1000 °C. In embodiments, the pyrolysis temperature may be greater than or equal to 825 °C and less than or equal to 1200 °C, greater than or equal to 850 °C and less than or equal to 1200 °C, greater than or equal to 875 °C and less than or equal to 1200 °C, greater than or equal to 900 °C and less than or equal to 1200 °C, greater than or equal to 925 °C and less than or equal to 1200 °C, greater than or equal to 950 °C and less than or equal to 1200 °C, greater than or equal to 975 °C and less than or equal to 1200 °C, greater than or equal to 1000 °C and less than or equal to 1200 °C, greater than or equal to 1025 °C and less than or equal to 1200 °C, greater than or equal to 1050 °C and less than or equal to 1200 °C, greater than or equal to 1075 °C and less than or equal to 1200 °C, greater than or equal to 1100 °C and less than or equal to 1200 °C, greater than or equal to 1125 °C and less than or equal to 1200 °C, greater than or equal to 1150 °C and less than or equal to 1200 °C, greater than or equal to 1175 °C and less than or equal to 1200 °C, greater than or equal to 800 °C and less than or equal to 1175 °C, greater than or equal to 825 °C and less than or equal to 1175 °C, greater than or equal to 850 °C and less than or equal to 1175 °C, greater than or equal to 875 °C and less than or equal to 1175 °C, greater than or equal to 900 °C and less than or equal to 1175 °C, greater than or equal to 925 °C and less than or equal to 1175 °C, greater than or equal to 950 °C and less than or equal to 1175 °C, greater than or equal to 975 °C and less than or equal to 1175 °C, greater than or equal to 1000 °C and less than or equal to 1175 °C, greater than or equal to 1025 °C and less than or equal to 1175 °C, greater than or equal to 1050 °C and less than or equal to 1175 °C, greater than or equal to 1075 °C and less than or equal to 1175 °C, greater than or equal to 1100 °C and less than or equal to 1175 °C, greater than or equal to 1125 °C and less than or equal to 1175 °C, greater than or equal to 1150 °C and less than or equal to 1175 °C, greater than or equal to 800 °C and less than or equal to 1150 °C, greater than or equal to 825 °C and less than or equal to 1150 °C, greater than or equal to 850 °C and less than or equal to 1150 °C, greater than or equal to 875 °C and less than or equal to 1150 °C, greater than or equal to 900 °C and less than or equal to 1150 °C, greater than or equal to 925 °C and less than or equal to 1150 °C, greater than or equal to 950 °C and less than or equal to 1150 °C, greater than or equal to 975 °C and less than or equal to 1150 °C, greater than or equal to 1000 °C and less than or equal to 1150 °C, greater than or equal to 1025 °C and less than or equal to 1150 °C, greater than or equal to 1050 °C and less than or equal to 1150 °C, greater than or equal to 1075 °C and less than or equal to 1150 °C, greater than or equal to 1100 °C and less than or equal to 1150 °C, greater than or equal to 1125 °C and less than or equal to 1150 °C, greater than or equal to 800 °C and less than or equal to 1125 °C, greater than or equal to 825 °C and less than or equal to 1125 °C, greater than or equal to 850 °C and less than or equal to 1125 °C, greater than or equal to 875 °C and less than or equal to 1125 °C, greater than or equal to 900 °C and less than or equal to 1125 °C, greater than or equal to 925 °C and less than or equal to 1125 °C, greater than or equal to 950 °C and less than or equal to 1125 °C, greater than or equal to 975 °C and less than or equal to 1125 °C, greater than or equal to 1000 °C and less than or equal to 1125 °C, greater than or equal to 1025 °C and less than or equal to 1125 °C, greater than or equal to 1050 °C and less than or equal to 1125 °C, greater than or equal to 1075 °C and less than or equal to 1125 °C, greater than or equal to 1100 °C and less than or equal to 1125 °C, greater than or equal to 800 °C and less than or equal to 1100 °C, greater than or equal to 825 °C and less than or equal to 1100 °C, greater than or equal to 850 °C and less than or equal to 1100 °C, greater than or equal to 875 °C and less than or equal to 1100 °C, greater than or equal to 900 °C and less than or equal to 1100 °C, greater than or equal to 925 °C and less than or equal to 1100 °C, greater than or equal to 950 °C and less than or equal to 1100 °C, greater than or equal to 975 °C and less than or equal to 1100 °C, greater than or equal to 1000 °C and less than or equal to 1100 °C, greater than or equal to 1025 °C and less than or equal to 1100 °C, greater than or equal to 1050 °C and less than or equal to 1100 °C, greater than or equal to 1075 °C and less than or equal to 1100 °C, greater than or equal to 800 °C and less than or equal to 1075 °C, greater than or equal to 825 °C and less than or equal to 1075 °C, greater than or equal to 850 °C and less than or equal to 1075 °C, greater than or equal to 875 °C and less than or equal to 1075 °C, greater than or equal to 900 °C and less than or equal to 1075 °C, greater than or equal to 925 °C and less than or equal to 1075 °C, greater than or equal to 950 °C and less than or equal to 1075 °C, greater than or equal to 975 °C and less than or equal to 1075 °C, greater than or equal to 1000 °C and less than or equal to 1075 °C, greater than or equal to 1025 °C and less than or equal to 1075 °C, greater than or equal to 1050 °C and less than or equal to 1075 °C, greater than or equal to 800 °C and less than or equal to 1050 °C, greater than or equal to 825 °C and less than or equal to 1050 °C, greater than or equal to 850 °C and less than or equal to 1050 °C, greater than or equal to 875 °C and less than or equal to 1050 °C, greater than or equal to 900 °C and less than or equal to 1050 °C, greater than or equal to 925 °C and less than or equal to 1050 °C, greater than or equal to 950 °C and less than or equal to 1050 °C, greater than or equal to 975 °C and less than or equal to 1050 °C, greater than or equal to 1000 °C and less than or equal to 1050 °C, greater than or equal to 1025 °C and less than or equal to 1050 °C, greater than or equal to 800 °C and less than or equal to 1025 °C, greater than or equal to 825 °C and less than or equal to 1025 °C, greater than or equal to 850 °C and less than or equal to 1025 °C, greater than or equal to 875 °C and less than or equal to 1025 °C, greater than or equal to 900 °C and less than or equal to 1025 °C, greater than or equal to 925 °C and less than or equal to 1025 °C, greater than or equal to 950 °C and less than or equal to 1025 °C, greater than or equal to 975 °C and less than or equal to 1025 °C, greater than or equal to 1000 °C and less than or equal to 1025 °C, greater than or equal to 800 °C and less than or equal to 1000 °C, greater than or equal to 825 °C and less than or equal to 1000 °C, greater than or equal to 850 °C and less than or equal to 1000 °C, greater than or equal to 875 °C and less than or equal to 1000 °C, greater than or equal to 900 °C and less than or equal to 1000 °C, greater than or equal to 925 °C and less than or equal to 1000 °C, greater than or equal to 950 °C and less than or equal to 1000 °C, greater than or equal to 975 °C and less than or equal to 1000 °C, greater than or equal to 800 °C and less than or equal to 975 °C, greater than or equal to 825 °C and less than or equal to 975 °C, greater than or equal to 850 °C and less than or equal to 975 °C, greater than or equal to 875 °C and less than or equal to 975 °C, greater than or equal to 900 °C and less than or equal to 975 °C, greater than or equal to 925 °C and less than or equal to 975 °C, greater than or equal to 950 °C and less than or equal to 975 °C, greater than or equal to 800 °C and less than or equal to 950 °C, greater than or equal to 825 °C and less than or equal to 950 °C, greater than or equal to 850 °C and less than or equal to 950 °C, greater than or equal to 875 °C and less than or equal to 950 °C, greater than or equal to 900 °C and less than or equal to 950 °C, greater than or equal to 925 °C and less than or equal to 950 °C, greater than or equal to 800 °C and less than or equal to 925 °C, greater than or equal to 825 °C and less than or equal to 925 °C, greater than or equal to 850 °C and less than or equal to 925 °C, greater than or equal to 875 °C and less than or equal to 925 °C, greater than or equal to 900 °C and less than or equal to 925 °C, greater than or equal to 800 °C and less than or equal to 900 °C, greater than or equal to 825 °C and less than or equal to 900 °C, greater than or equal to 850 °C and less than or equal to 900 °C, greater than or equal to 875 °C and less than or equal to 900 °C, greater than or equal to 800 °C and less than or equal to 875 °C, greater than or equal to 825 °C and less than or equal to 875 °C, greater than or equal to 850 °C and less than or equal to 875 °C, greater than or equal to 800 °C and less than or equal to 850 °C, greater than or equal to 825 °C and less than or equal to 850 °C, or greater than or equal to 800 °C and less than or equal to 825 °C. At pyrolysis temperatures below 800 °C the desired selectivity may not be obtainable, and at temperatures above 1200 °C the structure of the CMS membrane may be compromised. It is envisioned that the range of acceptable pyrolysis temperatures may be greater than or equal to any of the temperatures described herein and less than or equal to any of the temperatures described herein.

[0030] The pyrolysis soak time (i.e., the duration of time at the pyrolysis temperature) may vary (and may include no soak time) but may be, for example, greater than or equal to 1 hour and less than or equal to 10 hours, greater than or equal to 2 hours and less than or equal to 8 hours, greater than or equal to 4 hours and less than or equal to 6 hours. An exemplary heating protocol may include: (1) starting at a first set point of about 50 °C; (2) heating to a second set point of about 250 °C at a rate of about 13.3 °C per minute; (3) heating to a third set point of about 535 °C at a rate of about 3.85 °C per minute; (4) heating to a fourth set point of about 550 °C to 700 °C at a rate of about 0.25 °C per minute. The fourth set point may then be maintained for the determined soak time.

[0031] As noted above, the precursor polymers may be pyrolyzed under various inert gas purge or vacuum conditions. In embodiments, the precursor polymers may be pyrolyzed under vacuum at low pressures (e.g. less than or equal to 0.1 millibar). In embodiments, the pyrolysis utilizes a controlled inert purge gas atmosphere. By way of example, an inert gas such as argon is used as the purge gas atmosphere. Other suitable inert gases include, but are not limited to, nitrogen, helium, or any combination thereof.

[0032] After pyrolyzing, the asymmetric hollow carbon fiber 100 that has formed may be cooled to temperature around ambient such as below 50 °C. The cooling may be at any useful rate, such as passively cooling (e.g., turning off the power to furnace and allowing to cool naturally). Alternatively, it may be desirable to more rapidly cool such as using known techniques to realize faster cooling such as cooling fans or employment of water cooled jackets or opening the furnace to the surrounding environment.

[0033] In one or more embodiments, the asymmetric hollow carbon fiber 100 may be exposed to an oxygen-containing atmosphere during the cooling of the furnace. In one or more embodiments, the oxygen-containing atmosphere may comprise oxygen in an amount from 100 ppm to 1000 ppm. For example, the oxygen containing atmosphere may comprise oxygen in an amount from 100 ppm to 900 ppm, from 100 ppm to 800 ppm, from 100 ppm to 700 ppm, from 100 ppm to 600 ppm, from 100 ppm to 500 ppm, from 100 ppm to 400 ppm, from 100 ppm to 300 ppm, from 100 ppm to 200 ppm, from 200 ppm to 1000 ppm, from 200 ppm to 900 ppm, from 200 ppm to 800 ppm, from 200 ppm to 700 ppm, from 200 ppm to 600 ppm, from 200 ppm to 500 ppm, from 200 ppm to 400 ppm, from 200 ppm to 300 ppm, from 300 ppm to 1000 ppm, from 300 ppm to 900 ppm, from 300 ppm to 800 ppm, from 300 ppm to 700 ppm, from 300 ppm to 600 ppm, from 300 ppm to 500 ppm, from 300 ppm to 400 ppm, from 400 ppm to 1000 ppm, from 400 ppm to 900 ppm, from 400 ppm to 800 ppm, from 400 ppm to 700 ppm, from 400 ppm to 600 ppm, from 400 ppm to 500 ppm, from 500 ppm to 1000 ppm, from 500 ppm to 900 ppm, from 500 ppm to 800 ppm, from 500 ppm to 700 ppm, from 500 ppm to 600 ppm, from 600 ppm to 1000 ppm, from 600 ppm to 900 ppm, from 600 ppm to 800 ppm, from 600 ppm to 700 ppm, from 700 ppm to 1000 ppm, from 700 ppm to 900 ppm, from 700 ppm to 800 ppm, from 800 ppm to 1000 ppm, from 800 ppm to 900 ppm, or from 900 ppm to 1000 ppm.

[0034] In one or more embodiments, the asymmetric hollow carbon fiber 100 may be exposed to the oxy gen-containing atmosphere during cooling at a temperature from 600 °C to 300 °C. For example, the asymmetric hollow carbon fiber 100 may be exposed to the oxygencontaining atmosphere during cooling at a temperature from 600 °C to 350 °C, from 600 °C to 400 °C, from 600 °C to 450 °C, from 600 °C to 500 °C, from 600 °C to 550 °C, from 550 °C to

300 °C, from 550 °C to 350 °C, from 550 °C to 400 °C, from 550 °C to 450 °C, from 550 °C to

500 °C, from 500 °C to 300 °C, from 500 °C to 350 °C, from 500 °C to 400 °C, from 500 °C to

450 °C, from 450 °C to 300 °C, from 450 °C to 350 °C, from 450 °C to 400 °C, from 400 °C to

300 °C, from 400 °C to 350 °C, or from 350 °C to 300 °C.

[0035] In one or more embodiments, a pressure differential may be created across the asymmetric hollow carbon fiber 100 during the exposure to the oxygen-containing atmosphere. The pressure differential may be such that the oxygen-containing atmosphere is pulled across the cross-section of the fiber starting from the oxidized surface layer 110 to the hollow interior space 140. In some embodiments, the pressure differential may be created by adding an oxygencontaining gas at one end of the furnace and flowing the gas through the furnace to exit at the opposite end.

[0036] The asymmetric hollow carbon fiber 100 may be exposed to the oxygen-containing atmosphere for greater than or equal to 0.5 hours and less than or equal to 24 hours, greater than or equal to 1 hours and less than or equal to 24 hours, greater than or equal to 1.5 hours and less than or equal to 24 hours, greater than or equal to 2.5 hours and less than or equal to 24 hours, greater than or equal to 3.5 hours and less than or equal to 24 hours, greater than or equal to

4.5 hours and less than or equal to 24 hours, greater than or equal to 5.5 hours and less than or equal to 24 hours, greater than or equal to 6.5 hours and less than or equal to 24 hours, greater than or equal to 7.5 hours and less than or equal to 24 hours, greater than or equal to 8.5 hours and less than or equal to 24 hours, greater than or equal to 9.5 hours and less than or equal to 24 hours, greater than or equal to 10.5 hours and less than or equal to 24 hours, greater than or equal to

11.5 hours and less than or equal to 24 hours, greater than or equal to 12.5 hours and less than or equal to 24 hours, greater than or equal to 13.5 hours and less than or equal to 24 hours, greater than or equal to 14.5 hours and less than or equal to 24 hours, greater than or equal to 15.5 hours and less than or equal to 24 hours, greater than or equal to 16.5 hours and less than or equal to 24 hours, greater than or equal to 17.5 hours and less than or equal to 24 hours, greater than or equal to 18.5 hours and less than or equal to 24 hours, greater than or equal to 19.5 hours and less than or equal to 24 hours, greater than or equal to 20.5 hours and less than or equal to 24 hours, greater than or equal to 21.5 hours and less than or equal to 24 hours, greater than or equal to

22.5 hours and less than or equal to 24 hours, greater than or equal to 23.5 hours and less than or equal to 24 hours, greater than or equal to 0.5 hours and less than or equal to 23 hours, greater than or equal to 0.5 hours and less than or equal to 22 hours, greater than or equal to 0.5 hours and less than or equal to 21 hours, greater than or equal to 0.5 hours and less than or equal to 20 hours, greater than or equal to 0.5 hours and less than or equal to 19 hours, greater than or equal to 0.5 hours and less than or equal to 18 hours, greater than or equal to 0.5 hours and less than or equal to 17 hours, greater than or equal to 0.5 hours and less than or equal to 16 hours, greater than or equal to 0.5 hours and less than or equal to 15 hours, greater than or equal to 0.5 hours and less than or equal to 14 hours, greater than or equal to 0.5 hours and less than or equal to 13 hours, greater than or equal to 0.5 hours and less than or equal to 12 hours, greater than or equal to 0.5 hours and less than or equal to 11 hours, greater than or equal to 0.5 hours and less than or equal to 10 hours, greater than or equal to 0.5 hours and less than or equal to 9 hours, greater than or equal to 0.5 hours and less than or equal to 8 hours, greater than or equal to 0.5 hours and less than or equal to 7 hours, greater than or equal to 0.5 hours and less than or equal to 6 hours, greater than or equal to 0.5 hours and less than or equal to 5 hours, greater than or equal to 0.5 hours and less than or equal to 4 hours, greater than or equal to 0.5 hours and less than or equal to 3 hours, greater than or equal to 0.5 hours and less than or equal to 2 hours, or even greater than or equal to 0.5 hours and less than or equal to 1 hours. [0037] Without intending to be bound by any particular theory, it is believed that oxidation of the asymmetric hollow carbon fiber 100 enhances its ability to separate components of a mixture stream. This may be because the oxidation helps to create a porous surface and increases polarity of that surface. As a result, transport of larger gas molecules may be enhanced due to increased solubility and a pore blocking transport mechanism.

[0038] The fibers pyrolyzed using these conditions have been found to have the physical properties as described above. Thus, while the methods described herein may be utilized to produce the asymmetric hollow carbon fibers of the present disclosure it is contemplated that other methods may be utilized to produce asymmetric hollow carbon fibers having the properties described herein.

[0039] In one or more embodiments, a CMS membrane may comprise a plurality of asymmetric hollow carbon fibers as described herein. In some embodiments, the CMS membrane may comprise channels that form between the oxidized surface layers of adjacent fibers. In some embodiments, the CMS membrane may not comprise an external support. In some embodiments, the CMS membrane may be formed by pyrolyzing a plurality of polymeric precursor fibers, as described above. In one or more embodiments, the CMS membrane may be a reverse selective gas transportation membrane. As used in the present disclosure the term “reverse selective” refers to a membrane that allows at least some larger molecules to permeate the membrane faster than at least some smaller molecules.

[0040] In one or more embodiments, a method of separating carbon dioxide from a gas feed comprising carbon dioxide and nitrogen comprises providing a CMS membrane comprising asymmetric hollow carbon fibers as described herein and flowing the gas feed through the channels of the CMS membrane to produce a first stream having an increased concentration of carbon dioxide and a second stream having an increased concentration of nitrogen. In some embodiments, the CMS membrane is desirably fabricated into a module comprising a sealable enclosure comprised of a plurality of carbon molecular sieve membranes that is comprised of at least one carbon molecular sieve membrane comprising the asymmetric hollow carbon fibers of the present disclosure that are contained within the sealable enclosure. The sealable enclosure having an inlet for introducing a gas feed comprised of at least two differing gas molecules; a first outlet for permitting egress of a permeate gas stream; and a second outlet for egress of a retentate gas stream.

[0041] According to an aspect, either alone or in combination with any other aspect, an asymmetric hollow carbon fiber may comprise an outer wall surrounding a hollow interior space. The outer wall comprises an oxidized surface layer, a dense separating layer, and a porous support layer. The dense separating layer is between the oxidized surface layer and the porous support layer. The porous support layer is between the dense separating layer and the hollow interior space. The oxidized surface layer comprises oxygen in an amount greater than 11 atomic% of the total atoms of the oxidized surface layer.

[0042] A second aspect of the present disclosure includes any previous aspect or combination of aspects, where the oxidized surface layer has a thickness of less than 2 microns.

[0043] A third aspect of the present disclosure includes any previous aspect or combination of aspects, where the oxidized surface layer is a continuous layer.

[0044] A fourth aspect of the present disclosure includes any previous aspect or combination of aspects, where the oxidized surface layer comprises oxygen in an amount greater than 15 atomic% of the total atoms of the surface layer.

[0045] A fifth aspect of the present disclosure includes any previous aspect or combination of aspects, where the oxidized surface layer comprises oxygen in an amount less than 25 atomic% of the total atoms of the surface layer.

[0046] A sixth aspect of the present disclosure includes any previous aspect or combination of aspects, where the fiber has a Raman G and D peak with an intensity ratio of the D peak to the G peak of less than 1.2 at a Raman excitation wavelength of 532 nm.

[0047] A seventh aspect of the present disclosure includes any previous aspect or combination of aspects, where the fiber has a D(002) peak of less than 4.0 A in transmission as measured by X-ray scattering at an energy level of 17 keV.

[0048] An eighth aspect of the present disclosure includes any previous aspect or combination of aspects, where a carbon molecular sieve membrane comprising a plurality of the asymmetric hollow carbon fibers of any of the previous aspects and the carbon molecular sieve membrane comprises channels that form between oxidized surface layers of adjacent fibers.

[0049] A ninth aspect of the present disclosure includes any previous aspect or combination of aspects, where the carbon molecular sieve membrane is a reverse selective gas transportation membrane.

[0050] A tenth aspect of the present disclosure includes a method of separating carbon dioxide from a gas feed comprising carbon dioxide and nitrogen, the method comprising providing the providing the asymmetric hollow fiber carbon molecular sieve membrane of the eighth or ninth aspects and flowing the gas feed through the channels of the carbon molecular sieve membrane to produce a first stream having an increased concentration of carbon dioxide and a second stream having an increased concentration of nitrogen.

[0051] An eleventh aspect of the present disclosure includes any previous aspect or combination of aspects, where a method for making the asymmetric hollow carbon fibers of the first through seventh membranes comprises providing a polymeric precursor asymmetric hollow fiber and heating the polymeric precursor asymmetric fiber at a temperature greater than 800 °C. The polymeric precursor asymmetric hollow fiber undergoes pyrolysis to form an asymmetric hollow carbon fiber during the heating. The heating is performed in a furnace with an inert atmosphere. The method further comprises allowing the furnace to cool to a temperature from 100 °C to 600 °C, exposing the asymmetric hollow carbon fiber to an oxygen-containing atmosphere during the cooling of the furnace, and cooling the asymmetric hollow carbon fiber.

[0052] A twelfth aspect of the present disclosure includes any previous aspect or combination of aspects, where the oxygen-containing atmosphere comprises oxygen in an amount from 100 ppm to 1000 ppm.

[0053] A thirteenth aspect of the present disclosure includes any previous aspect or combination of aspects, where the fiber is exposed to the oxygen-containing atmosphere during cooling at a temperature between 600 °C to 300 °C.

[0054] A fourteenth aspect of the present disclosure includes any previous aspect or combination of aspects, where the method further comprises creating a pressure differential across the asymmetric hollow carbon fiber during the exposure to the oxygen-containing atmosphere such that the oxygen-containing atmosphere is pulled across the cross-section of the fiber starting from the oxidized surface layer to the hollow interior space.

[0055] A fifteenth aspect of the present disclosure includes any previous aspect or combination of aspects, where the polymeric precursor is a polyimide.

TEST METHODS

Gas Permeance and Selectivity

[0056] The gas permeation properties of a membrane can be determined by gas permeation experiments. Two intrinsic properties have utility in evaluating separation performance of a membrane material: its “permeability,” a measure of the membrane’s intrinsic productivity; and its “selectivity,” a measure of the membrane’s separation efficiency. One typically determines “permeability” in Barrer (1 Barrer=10'¹⁰ [cm³ (STP) cm]/[cm²s cmHg], calculated as the flux (n^) divided by the partial pressure difference between the membrane upstream and downstream (Ap^), and multiplied by the thickness of the membrane (Z) .

[0057] Another term, “permeance,” is defined herein as productivity of asymmetric hollow fiber membranes and is typically measured in Gas Permeation Units (GPU) (1 GPU=10'⁶ [cm³ (STP)]/[cm²s cmHg]), determined by dividing permeability by effective membrane separation layer thickness.

[0058] Finally, “selectivity” is defined herein as the ability of one gas’s permeability through the membrane or permeance relative to the same property of another gas. It is measured as a unitless ratio. Raman Spectroscopy

[0059] Raman Spectroscopy was performed on the CMS hollow carbon fiber membranes in a side on side geometry with a ThermoScientific Almega DXR MicroRaman spectrometer in a 180 degree backscatter geometry. A 20x microscope objective with a 0.3 NA was utilized with a 532 nm excitation source. A CCD detector was used to collect the data. The MicroRaman system was interfaced with a computer system that controlled both the high resolution grating, roughly 4 cm'¹ resolution, and the laser power via neutral density filters through the OMNIC software package. Raman spectra were plotted and peak fitting was performed with established macros within the IgorPro software package. Two peaks with Lorentzian shapes were used to fit the D and G peaks. A linear background was used and fit between 800 and 2200 cm'¹.

X-ray Scattering

[0060] X-ray scattering was performed at the Dupont-Northwestern-Dow Collaborative Access Team (DND-CAT) beamline 5-ID-D, part of the advanced photon source at Argonne National Lab. The X-ray energy was set to 17 keV (A, = 0.729 A). Small and wide angle X-ray scattering of previously carbonized fibers were measured at the same beamline in air using a transmission geometry. Wide angle X-ray scattering was plotted and analyzed with JADE while the SAXS data was plotted, and analyzed using the IRENA SAS macro within IgorPro. Rg was calculated using the unified fit algorithm while crystallite size was calculated using the Schere equation.

Scanning Electron Microscopy

[0061] Scanning electron microscopy (SEM) was performed using a Hitachi SU-70 FE- SEM with a secondary electron detector. 3 kV accelerating voltage and am emission current of 16000 nA were used to collect the images in field free mode. Samples were prepared by breaking the fibers while submerged in pentane solvent. The use of pentane solvent allowed for clean fracture of the fibers. No final polishing was performed on the cross sections before mounting them with carbon tape to an aluminum stub.

X-Ray Photoelectron Spectroscopy [0062] XPS spectra were obtained by irradiating the fibers with X-rays while simultaneously measuring the binding energy and current of photoelectrons that escape from the top 0 nm to 10 nm of the fiber. All elements have unique binding energies and the elemental peak areas are used to determine the surface composition. XPS is sensitive to all elements except hydrogen and helium. The resultant surface composition is forced to 100 %. The XPS spectra were obtained using a Thermo K- Alpha XPS. The X-ray source was Monochromatic Al Ka at 72 Watts (12 kV, 6 mA). The analyzer pass energy was 200 eV (surveys: 50 msec, 1 eV/step, 5 scans), 80 eV (quantitation scans: 50 msec, 0.15 eV/step, 5 scans), and 20 eV (high resolution C and N scans: 50 msec, 0.10 eV/step, 15 scans). The take-off angle was 90°. The analysis area diameter for the fibers was 100 pm or 50 pm depending on the fiber diameter. The analysis area diameter for ground samples was 400 pm. No flood gun was required except for low temperature annealed samples which required standard flood gun settings. The data was processed using Thermo Avantage software with Thermo’s modified XPS sensitivity factors. Data were collected from 5 data points along a fiber or across ground powders.

EXAMPLES

[0063] The various embodiments of the present disclosure will be further clarified by the following examples. The examples are illustrative in nature and should not be understood to limit the subject matter of the present disclosure.

CMS Membrane Preparation

[0064] The CMS membranes were made using 6FDA-BPDA-DAM polymer. The 6FDA- BPDA-DAM was acquired from Akron Polymer Systems, Akron, OH. The polymer was dried under vacuum at 110 °C for 24 hours and then a dope was formed. The dope was made by mixing the 6FDA-BPDA-DAM polymer with solvents and compounds in Table 1 and roll mixed in a Qorpak™ glass bottle sealed with a polytetrafluoroethylene (TEFLON™) cap and a rolling speed of 5 revolutions per minute (rpm) for a period of about 3 weeks to form a homogeneous dope.

Table 1 - Dope Formulation

NMP = N-Methyl-2-pyrolidone; THF = Tetrahydrofuran

[0065] The homogeneous dope was loaded into a 500 milliliter (mL) syringe pump and the dope was allowed to degas overnight by heating the pump to a set point temperature of 50 °C to 60 °C using a heating tape.

[0066] Bore fluid (85 wt% NMP and 15 wt% water, based on total bore fluid weight) was loaded into a separate 100 mL syringe pump and then the dope and bore fluid were co-extruded through a spinneret operating at a flow rate for of 180 milliliters per hour (mL/hr) for the dope; 60 mL/hr bore fluid, filtering both the bore fluid and the dope in line between delivery pumps and the spinneret using 40 pm and 2 pm metal filters. The temperature was controlled using thermocouples and heating tape placed on the spinneret, dope filters and dope pump at a set point temperature of 70 °C.

[0067] After passing through a fifteen centimeter (cm) air gap, the nascent fibers that were formed by the spinneret were quenched in a water bath (50 °C) and the fibers were allowed to phase separate. The fibers were collected using a 0.32 meter (M) diameter polyethylene drum passing over TEFLON guides and operating at a take-up rate of 30 meters per minute (M/min).

[0068] The fibers were cut from the drum and rinsed at least four times in separate water baths over a span of 48 hours. The rinsed fibers in glass containers and effect solvent exchange three times with methanol for 20 minutes and then hexane for 20 minutes before recovering the fibers and drying them under vacuum at a set point temperature of 110 °C for one hour or drying under vacuum at 75 °C for 3 hours.

[0069] Prior to pyrolyzing the fibers, a sample quantity of the above fibers (also known as “precursor fibers”) were tested for skin integrity. One or more hollow precursor fibers were potted into a % inch (0.64 cm) (outside diameter, OD) stainless steel tubing. Each tubing end was connected to a % inch (0.64 cm) stainless steel tee; and each tee was connected to % inch (0.64 cm) female and male NPT tube adapters, which were sealed to NPT connections with epoxy. Pure gas permeation tests were performed in a constant-pressure system maintained at 35 °C. For each permeation test, the entire system and leak rate was determined to ensure that the leakage was less than 1 percent of the permeation rate of the slowest gas. After evacuating, the upstream end was pressurized (end closest to feed source) of the tube with feed gas (e.g. pure oxygen or pure nitrogen) while keeping the downstream end (end furthest from feed source) under vacuum. The pressure rise was recorded in a constant, known downstream volume over time using LABVIEW software (National Instruments, Austin, TX) until reaching steady state. The permeance of each gas was determined through the membrane by the rate of pressure rise, the membrane area and the pressure difference across the membrane. The selectivity of each gas pair as a ratio of the individual gas permeance was calculated.

[0070] The hollow fibers were pyrolyzed to form the CMS membranes by placing the precursor fibers on a stainless steel wire mesh plate each of them bound separately to the plate using stainless steel wire. The combination of hollow fibers and mesh plate were placed into a quartz tube that sits in a tube furnace. The fibers were pyrolyzed under an inert gas (argon flowing at a rate of 200 standard cubic centimeters per minute (seem)). The precursor fibers were pyrolyzed in a pyrolysis chamber having an oxygen content at room temperature less than 10 ppm. Argon was used as the inert purge gas. After the pyrolysis, the pyrolysis chamber was allowed to cool. For samples that underwent air exposure the gas line to supply purging argon gas was discontinued at various cooling temperatures (i.e., 375 °C, 485 °C, and 575 °C) to enable the air flow into the furnace from the exhaust line at the outlet of the furnace. After the membranes were pyrolyzed and cooled, a single fiber module was fabricated and tested for CO2/N2 gas pair permeance.

[0071] In order to evaluate the method of producing asymmetric hollow fiber CMS membranes disclosed herein, six samples were prepared. The pyrolyzed and/or oxidized CMS hollow fibers are encased in a stainless-steel casing, thereby forming a membrane module for further testing. The membrane module is housed in an oven (Quincy Lab, Inc., Chicago, IL) with temperature control. The test gas flow rates are controlled by mass flow controllers (Brooks Instrument, Hatfield, PA), and pressures are monitored and controlled by pressure transducers. In these experiments, the single-fiber CMS fiber modules are maintained under constant upstream pressure at 35 °C. Argon was used as the sweep gas to carry the permeate to the downstream flowmeter and gas chromatograph (GC). A Maxum II process GC (Siemens, Munich, Germany) is used to measure the composition of the permeate & sweep mixture, and a Mesalabs Bios Drycal flowmeter (Mesa Labs, Inc., Butler, NJ) is used for the permeate flow rate measurement. The volumetric flow rate from the Bios DryCal flowmeter and the composition from the GC were used to analyze the permeance and selectivity of the fibers in the test gas system. The results of the tests are shown in Table 2.

[0072] In Table 2, the pyrolysis temperature is the temperature the polymeric precursor was pyrolyzed at during the final stage of the temperature ramp up. The air exposure temperature is the temperature of the furnace when the Cb/Ar gas stream was introduced to the furnace. I(D)/I(G) is the intensity ratio of the Raman G and D peaks of the fibers using the Raman Spectroscopy procedure above. D(002) is a peak obtained from x-ray scattering procedure described above. The %O (surface) is the atomic% of oxygen on the surface of the fiber as measured using the X-ray photoelectron spectroscopy procedure provided above. Finally, the microporous skin thickness is the thickness of the oxidized layer of the fiber in microns which was determined using scanning electron microscopy using the procedure provided above.

Table 2

[0073] Table 2 indicates that Samples 1 and 2 are significantly more permeable to CO2 than Comparative Example A, which was produced using the same procedure as Samples 1 and 2, except that Comparative Example A did not undergo air exposure. This indicates that high pyrolysis temperature alone is not responsible for the observed improvement in CO2 permeability. Table 2 further indicates that Samples 1 and 2 are significantly more permeable than Comparative Example B, which was exposed to air at a lower temperature than Samples 1 and 2. This indicates that high air exposure temperature is also important to membrane performance. In addition, Comparative examples C and D had significantly lower permeability than either Sample 1 or Sample 2. This indicates that high pyrolysis temperatures is important to achieve high membrane permeability. Put simply, a combination of high pyrolysis temperature combined with oxidation at high temperature results in the formation of reverse selective membranes with desirable permeability and selectivity.

[0074] Table 2 also indicates that a combination of certain structural features is associated with the formation of reverse selective membranes. A microporous skin as determined using Scanning Electron Microscopy was only observed for Samples 1 and 2 indicating an association between a microporous skin and reverse selectivity. In addition to surface porosity, Samples 1 and 2 have a combination of I(D)/I(G) of less than 1.2, a D(002) peak less than 4.0 A in transmission, and high surface oxygen content. These structural characteristics are also associated with the formation of reverse selective membranes.

[0075] It should be apparent to those skilled in the art that various modifications can be made to the described embodiments without departing from the spirit and scope of the claimed subject matter. Thus, it is intended that the specification cover modifications and variations of the described embodiments provided such modifications and variations come within the scope of the appended claims and their equivalents.

Claims

1. An asymmetric hollow carbon fiber, comprising an outer wall surrounding a hollow interior space, wherein the outer wall comprises an oxidized surface layer, a dense separating layer, and a porous support layer, the dense separating layer is between the oxidized surface layer and the porous support layer, the porous support layer is between the dense separating layer and the hollow interior space, the oxidized surface layer has a thickness greater than 0.5 microns, and the oxidized surface layer comprises oxygen in an amount greater than 11 atomic% of the total atoms of the oxidized surface layer.

2. The asymmetric hollow carbon fiber of claim 1, wherein the oxidized surface layer has a thickness of less than 2 microns.

3. The asymmetric hollow carbon fiber of any of the previous claims, wherein the oxidized surface layer is a continuous layer.

4. The asymmetric hollow carbon fiber of any of the previous claims, wherein the oxidized surface layer comprises oxygen in an amount greater than 15 atomic% of the total atoms of the surface layer.

5. The asymmetric hollow carbon fiber of any of the previous claims, wherein the oxidized surface layer comprises oxygen in an amount less than 25 atomic% of the total atoms of the surface layer.

6. The asymmetric hollow carbon fiber of any of the previous claims, wherein the fiber has a Raman G and D peak with an intensity ratio of the D peak to the G peak of less than 1.2 at a Raman excitation wavelength of 532 nm.

7. The asymmetric hollow carbon fiber of any of the previous claims, wherein the fiber has a D(002) peak of less than 4.0 A in transmission as measured by X-ray scattering at an energy level of 17 keV.

8. A carbon molecular sieve membrane comprising, a plurality of the asymmetric hollow carbon fibers of any of the previous claims, and wherein the carbon molecular sieve membrane comprises channels that form between oxidized surface layers of adjacent fibers.

9. The carbon molecular sieve membrane of claim 8, wherein the membrane is a reverse selective gas transportation membrane.

10. A method of separating carbon dioxide from a gas feed comprising carbon dioxide and nitrogen, the method comprising: providing the asymmetric hollow fiber carbon molecular sieve membrane of claims 8 or 9; and flowing the gas feed through the channels of the carbon molecular sieve membrane to produce a first stream having an increased concentration of carbon dioxide and a second stream having an increased concentration of nitrogen.

11. A method of making the asymmetric hollow carbon fiber of claims 1-7, the method comprising: providing a polymeric precursor asymmetric hollow fiber; heating the polymeric precursor asymmetric hollow fiber at a temperature greater than 800 °C, wherein the polymeric precursor asymmetric hollow fiber undergoes pyrolysis to form an asymmetric hollow carbon fiber, and the heating is performed in a furnace with an inert atmosphere; allowing the furnace to cool to a temperature from 100 °C to 600 °C; exposing the asymmetric hollow carbon fiber to an oxygen-containing atmosphere during the cooling of the furnace; and cooling the asymmetric hollow carbon fiber.

12. The method of claim 11, wherein the oxygen-containing atmosphere comprises oxygen in an amount from 100 ppm to 1000 ppm.

13. The method of claims 11 or 12, wherein the fiber is exposed to the oxygen-containing atmosphere during cooling at a temperature between 600 °C to 300 °C.

14. The method of claims 11-13, further comprising creating a pressure differential across the asymmetric hollow carbon fiber during the exposure to the oxygen-containing atmosphere such that the oxygen-containing atmosphere is pulled across the cross-section of the fiber starting from the oxidized surface layer to the hollow interior space.

15. The method of claims 11-14, wherein the polymeric precursor is a polyimide.