CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 63/418,723 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 nanographitic 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] The impact of high pyrolysis temperature, such as greater than 800 °C, has been researched and while increasing pyrolysis temperature has been reported to improve selectivity, it has been accompanied by a corresponding decrease in permeability at high pyrolysis temperatures. It would be desirable to produce an asymmetric hollow carbon fiber suitable for use in a CMS membrane that addresses one or more of the problems presented above such as improving selectivity for select gases, such as ethylene, without substantially decreasing its permeance. It would also be desirable to have such an asymmetric hollow carbon fiber be able to be incorporated into a CMS membrane with a free standing hollow asymmetric fiber form factor

SUMMARY

[0005] According to aspects, an asymmetric hollow carbon fiber may comprise an outer wall surrounding a hollow interior space. The outer wall may comprise a dense separating layer that may have a thickness from 2 microns to 6 microns and a porous support later between the dense separating layer and the hollow interior space. The dense separating layer and the porous support layer may comprise a nanographitic structure with interplanar spacings less than 3.6 angstroms and a basal plane greater than 1.3 nm. The dense separating layer may comprise micropores with radii greater than 4 angstroms.

[0006] 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.

[0007] Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the drawings, the detailed description that follows and the claims.

BRIEF DESCRIPTION OF THE DRAWING

[0008] Reference will now be made in greater detail to various embodiments, some of which are illustrated in the accompanying drawings, wherein:

[0009] 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

[0010] The asymmetric hollow carbon fibers according to embodiments disclosed and described herein may advantageously separate olefins from paraffins. In particular, the asymmetric hollow carbon fibers disclosed herein may advantageously separate ethylene from ethane. Typically, the industrial separation of ethylene from ethane is performed at a scale, such that small improvements in separation performance can have large impacts on the performance of the separation process.

[0011] 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 130. The outer wall may comprise a dense separating layer 110 and a porous support layer 120 between the dense separating layer 110 and the hollow interior space 130. The dense separating layer 110 may have a thickness of from about 2 microns to about 6 microns. The dense separating layer 110 and the porous support layer 120 may comprise a nanographitic structure with interplanar spacings less than 3.6 angstroms and a basal plane greater than 1.3 nm. The dense separating layer 110 may comprise micropores with radii greater than 4 angstroms. Methods for making such fibers and for making carbon molecular sieve membranes comprising such fibers are also disclosed in embodiments. Further, methods for using a carbon molecular sieve membrane comprising such fibers to separate ethylene from a gas feed comprising ethylene and ethane are also disclosed in embodiments.

[0012] Without being bound by theory it is believed that the combination of properties of the asymmetric hollow carbon fibers disclosed herein such as the thickness of the dense separating layer 110, interplanar spacing size, basal plane size, and micropore size are, taken together, associated with high gas selectivity without an accompanying decrease in gas permeance. It is believed that this combination of properties may act cooperatively to provide an improvement in gas selectivity and gas permeance when compared to hollow carbon fibers that do not have the disclosed combination of properties.

[0013] In embodiments, the hollow carbon fibers may be asymmetric. As used herein, the term “asymmetric” refers to a property of the hollow carbon fibers in which the hollow carbon fiber has at least one relatively more dense layer, which may be the dense separating layer 110 and at least one relatively less dense layer, which may be the porous support layer 120. For instance, in embodiments, one layer of the hollow carbon 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 110 may have a density greater than the density of the porous support layer 120.

[0014] In one or more embodiments, the dense separating layer 110 may have a thickness of from about 2 microns to about 6 microns. For example, the dense separating layer 110 may have a thickness of from 2 microns to 6 microns, such as from 2 microns to 5 microns, from 2 microns to 4 microns, from 2 microns to 3 microns, from 3 microns to 6 microns, from 3 microns to 5 microns, from 3 microns to 4 microns, from 4 microns to 6 microns, from 4 microns to 5 microns, or from 5 microns to 6 microns. Without being bound by theory, it is believed that a dense separating layer 110 having a thickness from about 2 microns to about 6 microns, when part of the combination of asymmetric hollow carbon fiber properties as disclosed herein may improve the gas selectivity of the fiber when compared to asymmetric hollow carbon fibers that do not have a dense separating layer 110 having a thickness from about 2 microns to about 6 microns.

[0015] In one or more embodiments, the dense separating layer 110 and the porous support layer 120 may comprise a nanographitic structure. As used in the present disclosure the term “nanographitic” refers to a structure comprising graphite crystals less than 100 nm in size. In one or more embodiments the nanographitic structure may have interplanar spacings less than 3.6 angstroms (A). As used in the present disclosure the term “interplanar spacings” refers to the distance between the planes of carbon atoms that form graphite crystals. For example, the nanographitic structure may have interplanar spacings less than 3.5 A, less than 3.4 A, or even less than 3.35 A. In some embodiments, the nanographitic structure may have interplanar spacings from 3.3 A to 3.6 A. For example, the nanographitic structure may have interplanar spacings 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 3.6 A, from 3.4 A to 3.5 A, or from 3.5 A to 3.6 A.

[0016] The basal plane of the nanographitic structure may, according to one or more embodiment be greater than 1.3 nm. As used in the present disclosure the term “basal plane” refers to the size of the planes of carbon atoms that form graphite crystals. For example, the basal plane of the nanographitic structure may be greater than 1.5 nm, greater than 5 nm, greater than 10 nm, greater than 20 nm, greater than 30 nm, greater than 40 nm, or even greater than 50 nm. In some embodiments the basal plane may be less than 100 nm. For example, the basal plane of the nanographitic structure may be from 1.3 nm to 100 nm, from 1.3 nm to 75 nm, from 1.3 nm to 50 nm, from 1.3 nm to 40 nm, from 1.3 nm to 30 nm, from 1.3 nm to 20 nm, from 1.3 nm to 10 nm, from 1.3 nm to 5 nm, from 1.3 nm to 1.5 nm, from 1.5 nm to 100 nm, from 1.5 nm to 75 nm, from

1.5 nm to 50 nm, from 1.5 nm to 40 nm, from 1.5 nm to 30 nm, from 1.5 nm to 20 nm, from 1.5 nm to 10 nm, from 1.5 nm to 5 nm, from 5 nm to 100 nm, from 5 nm to 75 nm, from 5 nm to 50 nm, from 5 nm to 40 nm, from 5 nm to 30 nm, from 5 nm to 20 nm, from 5 nm to 10 nm, from 10 nm to 100 nm, from 10 nm to 75 nm, from 10 nm to 50 nm, from 10 nm to 40 nm, from 10 nm to 30 nm, from 10 nm to 20 nm, from 20 nm to 100 nm, from 20 nm to 75 nm, from 20 nm to 50 nm, from 20 nm to 40 nm, from 20 nm to 30 nm, from 30 nm to 100 nm, from 30 nm to 75 nm, from 30 nm to 50 nm, from 30 nm to 40 nm, from 40 nm to 100 nm, from 40 nm to 75 nm, from 40 nm to 50 nm, from 50 nm to 100 nm, from 50 nm to 75 nm, or from 75 nm to 100 nm.

[0017] Without being bound by theory it is believed that having a nanographitic structure with a basal plane greater than 1.3 nm and interplanar spacings less than 3.6 A may be associated with asymmetric hollow carbon fibers with improved gas selectivity and gas permeance when compared to asymmetric hollow carbon fibers without such a nanographitic structure.

[0018] As disclosed herein, in one or more embodiments, the dense separating layer 110 may comprise micropores. In one or more embodiments, the micropores may have radii greater than 4 A as determined by small angle x-ray scattering. For example, the micropores may have radii greater than 4.2 A, greater than 4.4 A, greater than 4.6 A, greater than 4.8 A, greater than 5.0 A, greater than 5.2 A, greater than 5.4 A, greater than 5.6 A, greater than 5.8 A, or even greater than 6.0 A. In one or more embodiments, the micropores may have radii less than 10 A as determined by small angle x-ray scattering. For example, the micropores may have radii from 4.2 A to 10 A, from 4.2 A to 9.5 A, from 4.2 A to 9 A, from 4.2 A to 8.5 A, from 4.2 A to 8 A, from 4.2 A to 7.5 A, from 4.2 A to 7 A, from 4.2 A to 6.5 A, from 4.2 A to 6 A, from 4.2 A to 5.5 A, from 4.2 A to 5 A, from 4.2 A to 4.5 A, from 4.5 A to 10 A, from 4.5 A to 9.5 A, from 4.5 A to 9 A, from 4.5 A to 8.5 A, from 4.5 A to 8 A, from 4.5 A to 7.5 A, from 4.5 A to 7 A, from 4.5 A to

6.5 A, from 4.5 A to 6 A, from 4.5 A to 5.5 A, from 4.5 A to 5 A, from 5 A to 10 A, from 5 A to

9.5 A, from 5 A to 9 A, from 5 A to 8.5 A, from 5 A to 8 A, from 5 A to 7.5 A, from 5 A to 7 A, from 5 A to 6.5 A, from 5 A to 6 A, from 5 A to 5.5 A, from 5.5 A to 10 A, from 5.5 A to 9.5 A, from 5.5 A to 9 A, from 5.5 A to 8.5 A, from 5.5 A to 8 A, from 5.5 A to 7.5 A, from 5.5 A to 7 A, from 5.5 A to 6.5 A, from 5.5 A to 6 A, from 6 A to 10 A, from 6 A to 9.5 A, from 6 A to 9 A, from 6 A to 8.5 A, from 6 A to 8 A, from 6 A to 7.5 A, from 6 A to 7 A, from 6 A to 6.5 A, from 6.5 A to 10 A, from 6.5 A to 9.5 A, from 6.5 A to 9 A, from 6.5 A to 8.5 A, from 6.5 A to 8 A, from 6.5 A to 7.5 A, from 6.5 A to 7 A, from 7 A to 10 A, from 7 A to 9.5 A, from 7 A to 9 A, from 7 A to 8.5 A, from 7 A to 8 A, from 7 A to 7.5 A, from 7.5 A to 10 A, from 7.5 A to 9.5 A, from 7.5 A to 9 A, from 7.5 A to 8.5 A, from 7.5 A to 8 A, from 8 A to 10 A, from 8 A to 9.5 A, from 8 A to 9 A, from 8 A to 8.5 A, from 8.5 A to 10 A, from 8.5 A to 9.5 A, from 8.5 A to 9 A, from 9 A to 10 A, from 9 A to 9.5 A, or from 9.5 A to 10 A. Without being bound by theory it is believed that asymmetric hollow carbon fibers comprising a dense separating layer 110 comprising micropores with radii greater than 4 A as determined by small angle may improve the gas selectivity of asymmetric hollow carbon fibers when compared to asymmetric hollow fibers that do not comprise micropores with radii greater than 4 A.

[0019] In one or more embodiments, the asymmetric hollow carbon fibers may have a nanocrystalline graphite structure. As used in the present disclosure the term “nanocrystalline graphite” refers to a fiber that has transitioned from an amorphous or non-crystalline carbon structure to a crystalline graphite structure. The transition from amorphous carbon to crystalline graphite may be determined using Raman Spectroscopy. Raman spectroscopy uses light to measure molecular vibrations and these vibrations can be used to determine characteristics such as molecular structure and crystallinity. Without being bound by theory, it is believed that as a carbon fiber transitions from an amorphous structure to a nanocrystalline structure the intensity ratio of the Raman D peak to the Raman G peak initially increases followed by a decrease in the intensity ratio. It is believed that this “turn-over” of the intensity ratio may be used to determine if an asymmetric hollow carbon fiber 100 has transitioned from an amorphous carbon structure to a nanocrystalline graphite structure.

[0020] In one or more embodiments, the asymmetric hollow carbon fiber 100 may comprise less than 50% pyrrole based nitrogen as a percentage of the total amount of nitrogen in the fiber. For example, the asymmetric hollow carbon fiber 100 may comprise less 45% pyrrole based nitrogen as a percentage of the total amount of nitrogen in the fiber, such as less than 40%, less than 35%, less than 30%, less than 25%, less than 20%, less than 15%, or even less than 10% pyrrole based nitrogen as a percentage of the total amount of nitrogen in the fiber. Without being bound by theory it is believed that a pyrrole based nitrogen percentage of less than 50 % of the total amount of nitrogen in the fiber may indicate that the fiber has been formed via pyrolysis at a higher temperature compared to a fiber that has a pyrrole based nitrogen percentage of greater than 50 % based on the total amount of nitrogen in the fiber.

[0021] In one or more embodiments, the dense separating layer 110 of the asymmetric hollow carbon fiber 100 may comprise oxygen in an amount greater than 5 atomic% of the total amount of atoms of the dense separating layer 110. In one or more embodiments, the dense separating layer 110 of the asymmetric hollow carbon fiber 100 may comprise oxygen in an amount less than 15 atomic% of the total amount of atoms of the dense separating layer 110. For example, the asymmetric hollow carbon fiber 100 may comprise oxygen in an amount of from 5 atomic% to 15 atomic% of the total atoms of the dense separating layer 110, such as from 5 atomic% to 12.5 atomic%, from 5 atomic% to 10 atomic%, from 5 atomic % to 7.5 atomic%, from 7.5 atomic% to 15 atomic%, from 7.5 atomic% to 12.5 atomic%, from 7.5 atomic% to 10 atomic%, from 10 atomic% to 15 atomic%, from 10 atomic% to 12.5 atomic%, or from 12.5 atomic% to 15 atomic%. Without being bound by theory, it is believed that an oxygen content of from 5 atomic% to 15 atomic% of the total atoms of the dense separating layer may be associated with improved gas permeance and selectivity of the fiber when compared to fibers with oxygen content less than 5 atomic% or greater than 15 atomic%.

[0022] 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 indicates that the bulk of the fiber carbonized to form an asymmetric hollow carbon fiber 100. [0023] In one or more embodiments, a plurality of asymmetric hollow carbon fibers may be combined to form a carbon molecular sieve membrane. In embodiments, the carbon molecular sieve membrane may comprise channels that form between the dense separating layers 110 of adjacent fibers. In embodiments, the carbon molecular sieve membrane may not comprise an external support.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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 BPDA, 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

[0028] 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

[0029] 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). [0030] 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

[0031] 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.

[0032] One of the ways asymmetric hollow carbon fibers according to embodiments disclosed and described herein and CMS membranes using the same are produced is by pyrolysis of polymeric hollow fiber at a temperature greater than 900 degrees Celsius (°C). Conventional knowledge was that increasing pyrolysis temperature results in a corresponding increase in selectivity, but increasing pyrolysis temperature results in a decrease of permeability. For instance, Koros et al. (Adv.Mater.2017, 29, 1701631) reports synthesis of ultra selective CMS dense membrane by pyrolyzing MATRIMID based precursor at high temperature (900 °C). This trend is consistent with Pinnau et al. (https://doi.Org/10.1016/j.memsci.2019.05.020), which reports a decrease in membrane permeability with increase in membrane selectivity for selected gas pairs. Thus, conventional knowledge is that permeability and ultra high selectivity are inversely related. Further, membranes formed using these conditions have typically only been in a dense film form factor.

[0033] However, an unexpected anomaly in the above trend (i.e., the inverse relationship between permeability and selectivity) was observed where a modest increase in permeability with a step change in selectivity was observed for fibers pyrolyzed at certain temperatures. This allows synthesis of high selective CMS asymmetric membranes without sacrificing too much of permeability when CMS membranes are formed under certain pyrolysis conditions. 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.

[0034] Pyrolysis conditions will now be described. Any suitable supporting means for holding the hollow fiber CMS membranes 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.

[0035] 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. According to embodiments, the pyrolysis temperature of method for manufacturing hollow fiber carbon membrane may be greater than or equal to 900 °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. In embodiments, the pyrolysis temperature may be 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 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 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 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 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 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 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 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 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 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 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, or greater than or equal to 900 °C and less than or equal to 925 °C. At pyrolysis temperatures below 900 °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. Without being bound by theory, it is believed that pyrolysis temperatures over 900 °C may form asymmetric hollow carbon fibers that transition from an amorphous carbon structure to a nanocrystalline graphite structure.

[0036] 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 24 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 at a rate of about 0.25 °C per minute. The fourth set point may then be maintained for the determined soak time.

[0037] Precursor polymers may be carbonized under various inert gas purge or vacuum conditions, preferably under inert gas purge conditions, for the vacuum pyrolysis, preferably at low pressures (e.g. less than 0.1 millibar). In embodiments, the pyrolysis utilizes a controlled inert purge gas atmosphere with a small amount of oxidant, such as oxygen. Thus, in one or more embodiments, the pyrolysis atmosphere comprises an inert gas and oxygen. In embodiments, the inert gas is selected from the group consisting of nitrogen, helium, argon, or combinations thereof. In one or more embodiments, the inert purge gas is argon, thus the pyrolysis atmosphere, in embodiments, comprises argon and oxygen. It has been found that it is possible to further increase the permeability of membranes that are pyrolyzed at high temperatures (e.g., greater than or equal to 900 °C) by conducting the pyrolysis in trace amount of oxygen in the inert gas mixture. In contrast, conventional knowledge holds that the presence of oxygen decreases permeability for membranes pyrolyzed at temperature of 550 °C or greater (see U.S. Patent Application Publication No. 2013/030592).

[0038] In one or more embodiments, the purge gas atmosphere may comprise oxygen in an amount less than 40 ppm. In some embodiments, the purge gas atmosphere may comprise oxygen in an amount less than 10 ppm. In embodiments, the purge gas atmosphere may comprise at least 5 ppm of oxygen. For example, the purge gas atmosphere may comprise oxygen in an amount from 5 ppm to 40 ppm, from 5 ppm to 35 ppm, from 5 ppm to 30 ppm, from 5 ppm to 25 ppm, from 5 ppm to 20 ppm, from 5 ppm to 15 ppm, from 5 ppm to 10 ppm, from 10 ppm to 40 ppm, from 10 ppm to 35 ppm, from 10 ppm to 30 ppm, from 10 ppm to 25 ppm, from 10 ppm to 20 ppm, from 10 ppm to 15 ppm, from 15 ppm to 40 ppm, from 15 ppm to 35 ppm, from 15 ppm to 30 ppm, from 15 ppm to 25 ppm, from 15 ppm to 20 ppm, from 20 ppm to 40 ppm, from 20 ppm to 35 ppm, from 20 ppm to 30 ppm, from 20 ppm to 25 ppm, from 25 ppm to 40 ppm, from 25 ppm to 35 ppm, to 25 ppm to 30 ppm, from 30 ppm to 40 ppm, from 30 ppm to 35 ppm, or from 35 ppm to 40 ppm.

[0039] After pyrolysis, the hollow fiber CMS membrane that has formed is cooled to temperature near room temperature, such as less than or equal to 50 °C. The cooling may be at any useful rate, such as passively cooling (e.g., turning off the power to the furnace and allowing to cool naturally). Alternatively, it may be desirable to more rapidly cool such as by using known techniques to realize faster cooling. Known techniques include, but are not limited to, cooling fans or employment of water cooled jackets or opening the furnace to the surrounding environment.

[0040] 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 dense separating layers 110 of adjacent fibers. In some embodiments, the carbon molecular sieve membrane may not comprise an external support. In some embodiments, the carbon molecular sieve membrane may be formed by pyrolyzing a plurality of polymeric precursor fibers, as described above.

[0041] In one or more embodiments, a method of separating ethylene from a gas feed comprising ethylene and ethane 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 ethylene and a second stream having an increased concentration of ethane. 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.

[0042] According to an aspect, either alone or in combination with any other aspect an asymmetric hollow carbon fiber includes an outer wall surrounding a hollow interior space. The outer wall comprises a dense separating layer having a thickness from 2 microns to 6 microns and a porous support layer between the dense separating layer and the hollow interior space. The dense separating layer and the porous support layer comprise a nanographitic structure with interplanar spacings less than 3.6 angstroms and a basal plane greater than 1.3 nm. The dense separating layer comprises micropores with radii greater than 4 angstroms.

[0043] A second aspect of the present disclosure includes any previous aspect or combination of aspects, where the asymmetric hollow carbon fiber has a nanocrystalline graphite structure.

[0044] A third aspect f the present disclosure includes any previous aspect or combination of aspects, where the fiber comprises less than 50% pyrrole based nitrogen as a percentage of the total amount of nitrogen in the fiber.

[0045] A fourth aspect of the present disclosure includes any previous aspect or combination of aspects, where the dense separating layer comprises oxygen in an amount greater than 5 atomic% of the total atoms of the dense separating layer. [0046] A fifth aspect of the present disclosure includes any previous aspect or combination of aspects, where the dense separating layer comprises oxygen in an amount less than 15 atomic% of the total atoms of the dense separating layer.

[0047] A sixth aspect of the present disclosure includes any previous aspect or combination of aspects, where the fiber comprises oxygen in an amount less than 5 atomic% of the total atoms of the fiber.

[0048] 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.

[0049] An eighth aspect of the present disclosure includes any previous aspect or combination of aspects, where a carbon molecular sieve membrane comprises 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 dense separating layers of adjacent fibers.

[0050] A ninth aspect of the present disclosure includes any previous aspect or combination of aspects, where the carbon membrane does not comprise an external support.

[0051] A tenth aspect of the present disclosure includes a method of separating ethylene from a gas feed comprising ethylene and ethane, the method comprising providing the 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 ethylene and a second stream having an increased concentration of ethane.

[0052] An eleventh aspect includes a method of making the asymmetric hollow carbon fibers of the first through seven aspects, the method comprising preparing a polymeric precursor fiber, and pyrolyzing the polymeric precursor fiber at a temperature greater than 900 °C under a low-oxygen atmosphere to form an asymmetric hollow carbon fiber.

[0053] A twelfth aspect of the present disclosure includes any previous aspect or combination of aspects, where the pyrolysis of the polymeric precursor occurs in an atmosphere comprising an inert gas and oxygen. [0054] A thirteenth aspect of the present disclosure includes any previous aspect or combination of aspects, where the atmosphere comprises less than 40 ppm of oxygen.

[0055] A fourteenth aspect of the present disclosure includes any previous aspect or combination of aspects, where the atmosphere comprises less than 10 ppm of oxygen.

[0056] 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

[0057] 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 [cm ³ (STP) cm]/[cm ² s 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) .

[0058] 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 [cm ³ (STP)]/[cm ² s cmHg]), determined by dividing permeability by effective membrane separation layer thickness.

[0059] 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

[0060] Raman Spectroscopy was performed on the carbon molecular sieve hollow carbon fibers 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

[0061] 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

[0062] 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

[0063] 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

[0064] 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

[0065] 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

[0066] 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.

[0067] 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.

[0068] 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).

[0069] 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.

[0070] 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.

[0071] The precursor fibers were pyrolized in a pyrolysis chamber having an oxygen content at room temperature (with Ar as the inert purge gas) kept between 5-10 ppm whereas for sample 3, oxygen content was raised to 30 ppm by introducing a premixture of 30 ppm oxygen and Ar. After the membranes were pyrolyzed, single fiber module was fabricated and tested for C2H4/C2H6 gas pair permeance. The pyrolyzed and/or oxidized CMS hollow fibers were potted in the stainless-steel casing to test the gas separation performance. 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 were monitored and controlled by pressure transducers. In these experiments, the single-fiber CMS fiber modules were 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. Table 2

[0072] Table 2 shows, an initial decrease in ethylene permeability as pyrolysis temperature increased up to 800 °C (see, e.g. Comparative Examples A-C). At pyrolysis temperatures greater than 800 °C, however, the permeability of ethylene increased corresponding with a sharp increase in the selectivity of the ethylene/ethane gas pair as well (see, e.g. Samples 1-3). This increase in both permeability and selectivity corresponds with the transition from an amorphous carbon state (Am) to a nanocrystalline graphite structure (Nc) as indicated by the I(D)/I(G) ratio “turnover” from Comparative Example C to Samples 1-3.

[0073] 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.