EP4642562A1

EP4642562A1 - Reverse selective carbon membranes and methods of making the same

Info

Publication number: EP4642562A1
Application number: EP24706884.4A
Authority: EP
Inventors: Surendar R. Venna; Junqiang Liu; Derrick Flick; Thomas C. FITZGIBBONS; Li Tang; Nikki J. MONTANEZ; Hali J. MCCURRY; James B. HEARD; Barry B. Fish; Abhishek Roy
Original assignee: Dow Global Technologies LLC
Current assignee: Dow Global Technologies LLC
Priority date: 2023-01-20
Filing date: 2024-01-17
Publication date: 2025-11-05
Also published as: CN120435340A; KR20250138749A; WO2024155660A1

Abstract

A method of making a reverse selective asymmetric hollow fiber carbon membrane may include heating an asymmetric hollow fiber carbon membrane. The method may also include exposing the asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere to form a reverse selective asymmetric hollow fiber carbon membrane. After exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere the reverse selective asymmetric hollow fiber carbon membrane may have a weight that is from -5 wt.% to 20 wt.% less than the weight of the asymmetric hollow fiber carbon membrane.

Description

REVERSE SELECTIVE CARBON MEMBRANES AND METHODS OF MAKING THE SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 63/480,737 filed January 20, 2023, the entire disclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

[0002] Embodiments of the present disclosure generally relate to hollow fiber carbon molecular sieve (CMS) membranes for use in gas separation, and in particular, methods for producing asymmetric hollow fiber CMS membranes with reverse selectivity.

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 removal of O2 from air. 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 processability and low cost. CMS membranes, however, have been shown to have attractive separation performance properties exceeding that of polymeric membranes.

[0004] CMS membranes are typically produced through thermal pyrolysis of polymer precursors. For example, it is known that defect- free hollow fiber CMS membranes can be produced by pyro lyzing cellulose hollow fibers. In addition, many other polymers have been used to produce CMS membranes in fiber and dense film form, among which polyimides have been favored. Polyimides have a high glass transition temperature, are easy to process, and perform better than most other polymeric membranes, even prior to pyrolysis.

[0005] CMS membrane separation properties are primarily affected by the following factors: (1) pyrolysis precursor, (2) pyrolysis temperature, (3) thermal soak time, and (4) pyrolysis atmosphere. For example, increases in both temperature and thermal soak time have been shown to increase the selectivity but decrease permeance for CO2/CH4 separation. In addition, a precursor polymer with a rigid, tightly packed structure tends to lead to a CMS membrane having higher selectivity compared with less rigid precursor polymers.

[0006] 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. However, in the field of CMS membranes, there have not been many reports in the literature regarding the formation of reverse selective asymmetric hollow filamentary CMS 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 is easier for polymeric membranes, than 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.

[0007] Accordingly, it would be desirable to produce asymmetric hollow fiber carbon membranes that are also reverse selective gas transportation membranes.

SUMMARY

[0008] One type of separation application in which CMS membranes may find utility is gas separations. For example, CMS membranes that may be utilized to separate CO2 from 112 CO2 from N2 and ethylene from H2 are desired.

[0009] According to one or more embodiments described herein, a method of making a reverse selective asymmetric hollow fiber carbon membrane may include heating an asymmetric hollow fiber carbon membrane. The method may further include exposing the asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere to form a reverse selective asymmetric hollow carbon fiber membrane. After exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere the reverse selective asymmetric hollow fiber carbon membrane may have a weight that is from -5 wt.% to 20 wt.% less than the weight of the asymmetric hollow fiber carbon membrane.

[0010] 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.

[0011] 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 DRAWINGS

[0012] The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:

[0013] FIGURE (FIG.) 1A is a flow chart that includes steps for making a reverse selective asymmetric hollow fiber carbon membrane, according to one or more embodiments described herein;

[0014] FIG. IB is another flow chart that includes steps for making a reverse selective asymmetric hollow fiber carbon membrane, according to one or more embodiments described herein; [0015] FIG. 2 is a schematic of a system for pyrolysis and oxidation of a hollow fiber CMS membrane in accordance with embodiments described herein;

[0016] FIG. 3 is a chart of temperature on the x-axis and simulated O2 concentration on the y- axis for use with embodiments described herein;

[0017] FIG. 4A is chart depicting the weight change of membranes formed at different pyrolysis temperatures during oxidation of the membranes;

[0018] FIG. 4B is a chart depicting the temperature of oxidation over time for the membranes oxidized in FIG. 4A;

[0019] FIG. 4C is a chart depicting the weight change of membranes during oxidation of the membranes at different peak oxidation temperatures;

[0020] FIG. 5A is a chart depicting the hydrogen permeance of a comparative oxidized fiber;

[0021] FIG. 5B is a chart depicting the carbon dioxide permeance of a comparative oxidized fiber;

[0022] FIG. 5C is a chart depicting the methane permeance of a comparative oxidized fiber;

[0023] FIG. 5D is a chart of hydrogen and carbon dioxide selectivity of a comparative oxidized fiber;

[0024] FIG. 6A is a chart depicting the hydrogen permeance over time of two membranes, according to one or more embodiments described herein; and

[0025] FIG. 6B is a chart depicting the ethylene permeance over time of two membranes, according to one or more embodiments described herein. DETAILED DESCRIPTION

[0026] Specific embodiments of the present application will now be described. The disclosure may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth in this disclosure. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the subject matter to those skilled in the art.

[0027] Now referring to FIG. 1A, a process flow diagram for a method of making a reverse selective asymmetric hollow fiber carbon membrane is depicted. According to one or more embodiments, the method depicted in flow diagram 100 may comprise at least step 110 of heating an asymmetric hollow fiber carbon membrane, step 120 of exposing the heated asymmetric hollow fiber membrane to an oxygen-containing atmosphere, and step 130 of cooling of the reverse selective asymmetric hollow fiber carbon membrane

[0028] As described herein, a reverse selective asymmetric hollow fiber carbon membrane may be produced by the method shown in FIG. 1 A. As used herein, the term “reverse selective” refers to a membrane that allows at least some larger molecules to permeate the membrane at a faster rate than at least some smaller molecules.

[0029] As used herein, the term “asymmetric” refers to a property of a hollow fiber carbon membrane in which the hollow fiber carbon membrane has at least one relatively dense layer and at least one relatively less dense layer. For example, in embodiments, one layer of the hollow fiber CMS membrane may be greater than or equal to 1 pm and less than or equal to 10 pm and may be denser 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.

[0030] The reverse selective asymmetric hollow fiber carbon membranes formed from the methods described herein may have a weight that is from -5 wt.% to 20 wt.% less than the weight of the asymmetric hollow fiber carbon membrane prior to oxidation. For example, the reverse selective asymmetric hollow fiber carbon membrane may have a weight that is from -5 wt.% to 15 wt.% less than the weight of the asymmetric hollow fiber carbon membrane, such as from -5 wt.% to 10 wt.%, from -5 wt.% to 5 wt.%, from -5 wt.% to 0 wt.%, from 0 wt.% to 20 wt.%, from 0 wt.% to 15 wt.%, from 0 wt.% to 10 wt.%, from 0 wt.% to 5 wt.%, from 5 wt.% to 20 wt.%, from 5 wt.% to 15 wt.%, from 5 wt.% to 10 wt.%, from 10 wt.% to 20 wt.%, from 10 wt.% to 15 wt.%, from 15 wt.% to 20 wt.% less than the weight of the asymmetric hollow fiber carbon membrane, or any combination of these ranges. Without being bound by theory, it is believed that during oxidation of an asymmetric hollow fiber carbon membrane oxygenates may form on the surface of the fibers. Further oxidation may then result in the formation of carbon dioxide from these oxygenates which is released from the membrane as a gas. Oxygenate formation may cause weight gain of a fiber during oxidation and the formation of carbon dioxide may cause weight loss of a fiber. It is believed that balancing oxygenate and carbon dioxide formation may beneficially impact the separation properties of a reverse selective asymmetric hollow fiber carbon membrane, such as membrane stability, membrane permeance, and gas selectiveness compared to reverse selective membranes formed without balancing the formation of oxygenates and carbon dioxide. A change in weight of the asymmetric hollow fiber carbon membrane from -5 wt.% to 20 wt.% during oxidation may indicate a preferred balance between oxygenate formation and carbon dioxide formation.

[0031] Further, it is believed that oxidation of an asymmetric hollow fiber carbon membrane is affected by the interaction of a variety of factors, such as temperature of the membrane during exposure to the oxygen-containing atmosphere, oxygen concentration of the oxygen-containing atmosphere, time of exposure, and the type of oxidant used. It is believed that by measuring the weight change of the asymmetric hollow fiber carbon membrane during oxidation it may be possible to balance these factors to form a reverse selective asymmetric hollow fiber carbon membrane with beneficial membrane properties. For example, at higher temperatures a shorter time of exposure and/or a lower concentration of oxygen may be utilized to achieve a target weight change when compared to oxidation at a lower temperature. Essentially, the weight change of the membrane during oxidation may be utilized to determine ideal oxidation conditions even when performing oxidation under differing conditions, such as, type of oxidant or temperature of oxidation.

[0032] Referring again to FIG. 1, in one or more embodiments, the method of making a reverse selective asymmetric hollow fiber carbon membrane may comprise step 110 of heating an asymmetric hollow fiber carbon membrane at a temperature that is from 200 °C to 1200 °C. For example, the asymmetric hollow fiber carbon membrane may be heated to a temperature that is from 200 °C to 1100 °C, from 200 °C to 1000 °C, from 200 °C to 900 °C, from 200 °C to 800 °C, from 200 °C to 700 °C, from 200 °C to 600 °C, from 200 °C to 500 °C, from 200 °C to 400 °C, from 200 °C to 300 °C, from 300 °C to 1200 °C, from 300 °C to 1100 °C, from 300 °C to 1000 °C, from 300 °C to 900 °C, from 300 °C to 800 °C, from 300 °C to 700 °C, from 300 °C to 600 °C, from 300 °C to 500 °C, from 300 °C to 400 °C, from 400 °C to 1200 °C, from 400 °C to 1100 °C, from 400 °C to 1000 °C, from 400 °C to 900 °C, from 400 °C to 800 °C, from 400 °C to 700 °C, from 400 °C to 600 °C, from 400 °C to 500 °C, from 500 °C to 1200 °C, from 500 °C to 1100 °C, from 500 °C to 1000 °C, from 500 °C to 900 °C, from 500 °C to 800 °C, from 500 °C to 700 °C, from 500 °C to 600 °C, from 600 °C to 1200 °C, from 600 °C to 1100 °C, from 600 °C to 1000 °C, from 600 °C to 900 °C, from 600 °C to 800 °C, from 600 °C to 700 °C, from 700 °C to 1200 °C, from 700 °C to 1100 °C, from 700 °C to 1000 °C, from 700 °C to 900 °C, from 700 °C to 800 °C, from 800 °C to 1200 °C, from 800 °C to 1100 °C, from 800 °C to 1000 °C, from 800 °C to 900 °C, from 900 °C to 1200 °C, from 900 °C to 1100 °C, from 900 °C to 1000 °C, from 1000 °C to 1200 °C, from 1000 °C to 1100 °C, from 1100 °C to 1200 °C, or any combination of these ranges. In some embodiments, the asymmetric hollow fiber carbon membrane may be heated at a temperature that is from 900 °C to 1200 °C. In some embodiments the asymmetric hollow fiber carbon membrane may be heated to a temperature that is from 200 °C to 600 °C. In other embodiments the asymmetric hollow fiber carbon membrane may be heated to a temperature that is from 700 °C to 1000 °C. Without being bound by theory, it is believed that heating the asymmetric hollow fiber carbon membrane to a temperature that is less than 200 °C may not sufficiently allow for the oxidation of the asymmetric hollow fiber carbon membrane. As described herein above, it is believed the temperature range the asymmetric hollow fiber is heated to may be beneficially adapted based on the other oxidation conditions, such as, for example the type of oxidant, to target a specific weight change of the membrane.

[0033] In one or more embodiments, the heating of the asymmetric hollow fiber carbon membrane in step 110 may be done in a furnace. In one or more embodiments the heating of the asymmetric hollow fiber carbon membrane in step 110 may be done in a furnace under an inert atmosphere. In some embodiments, the heating may utilize a controlled inert purge gas atmosphere. For example, an inert gas such as argon may be used as the purge gas atmosphere. Other suitable inert gases may include, but are not limited to, nitrogen, helium, or combinations thereof. [0034] In one or more embodiments, the concentration of oxygen in the furnace during the heating of step 110 may be less than 10 ppm. For example, the concentration of oxygen in the furnace during heating may be less than 9 ppm, less than 8 ppm, less than 7 ppm, less than 6 ppm, less than 5 ppm, less than 4 ppm, less than 3 ppm, less than 2 ppm, or even less than 1 ppm. In some embodiments, the atmosphere of the furnace during heating step 110 may be essentially oxygen free.

[0035] Referring still to the flow chart 100 of FIG. 1, step 120 generally includes exposing the heated asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere. In one or more embodiments, the oxygen-containing atmosphere may comprise one or more of air, oxygen gas, carbon dioxide, or steam. In embodiments, the oxygen-containing atmosphere may comprise one or more of air, oxygen gas, carbon dioxide, or steam in an amount from 100 ppm to 1,000,000 ppm of the oxy gen-containing atmosphere, such as from 100 ppm to 1000 ppm, from 1000 ppm to 5000 ppm, from 5000 ppm to 10,000 ppm, from 10,000 ppm to 25,000 ppm, from 25,000 ppm to 50,000 ppm, from 50,000 ppm to 100,000 ppm, from 100,000 ppm to 250,000 ppm, from 250,000 ppm to 500,000 ppm, from 500,000 ppm to 750,000 ppm, from 750,000 ppm to 1,000,000 ppm, or any combination of these ranges.

[0036] 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. [0037] In some embodiments, the oxygen-containing atmosphere may comprise oxygen in an amount greater than or equal to 1000 ppm, such as greater than or equal to 5000 ppm, greater than or equal to 10,000 ppm, greater than or equal to 15,000 ppm, greater than or equal to 20,000 ppm, greater than or equal to 25,000 ppm, greater than or equal to 30,000 ppm, greater than or equal to 40,000 ppm, greater than or equal to 50,000 ppm, greater than or equal to 75,000 ppm, greater than or equal to 100,000 ppm, greater than or equal to 150,000 ppm, or even greater than or equal to 200,000 ppm of oxygen.

[0038] Referring still to exposure step 120, in one or more embodiments, the asymmetric hollow fiber carbon membrane may be exposed to the oxygen-containing atmosphere for a time period of from 1 minute to 1000 minutes. For example, the asymmetric hollow fiber carbon membrane may be exposed to the oxygen-containing atmosphere for from 1 minute to 900 minutes, from 1 minute to 800 minutes, from 1 minute to 700 minutes, from 1 minute to 600 minutes, from 1 minute to 500 minutes, from 1 minute to 400 minutes, from 1 minute to 300 minutes, from 1 minute to 200 minutes, from 1 minute to 100 minutes, from 100 minutes to 1000 minutes, from 100 minutes to 900 minutes, from 100 minutes to 800 minutes, from 100 minutes to 700 minutes, from 100 minutes to 600 minutes, from 100 minutes to 500 minutes, from 100 minutes to 400 minutes, from 100 minutes to 300 minutes, from 100 minutes to 200 minutes, from 200 minutes to 1000 minutes, from 200 minutes to 900 minutes, from 200 minutes to 800 minutes, from 200 minutes to 700 minutes, from 200 minutes to 600 minutes, from 200 minutes to 500 minutes, from 200 minutes to 400 minutes, from 200 minutes to 300 minutes, from 300 minutes to 1000 minutes, from 300 minutes to 900 minutes, from 300 minutes to 800 minutes, from 300 minutes to 700 minutes, from 300 minutes to 600 minutes, from 300 minutes to 500 minutes, from 300 minutes to 400 minutes, from 400 minutes to 1000 minutes, from 400 minutes to 900 minutes, from 400 minutes to 800 minutes, from 400 minutes to 700 minutes, from 400 minutes to 600 minutes, from 400 minutes to 500 minutes, from 500 minutes to 1000 minutes, from 500 minutes to 900 minutes, from 500 minutes to 800 minutes, from 500 minutes to 700 minutes, from 500 minutes to 600 minutes, from 600 minutes to 1000 minutes, from 600 minutes to 900 minutes, from 600 minutes to 800 minutes, from 600 minutes to 700 minutes, from 700 minutes to 1000 minutes, from 700 minutes to 900 minutes, from 700 minutes to 800 minutes, from 800 minutes to 1000 minutes, from 800 minutes to 900 minutes, from 900 minutes to 1000 minutes, or from any combination of these ranges. [0039] In one or embodiments, the asymmetric hollow fiber carbon membrane may be held at a specific temperature during the exposure to the oxygen-containing atmosphere. In embodiments where the oxygen-containing atmosphere comprises air or oxygen gas, the asymmetric hollow fiber carbon membrane may be held at a temperature that is from 200 °C to 600 °C, such as from 200 °C to 550 °C, from 200 °C to 500 °C, from 200 °C to 450 °C, from 200 °C to 400 °C, from

200 °C to 350 °C, from 200 °C to 300 °C, from 200 °C to 250 °C, from 250 °C to 600 °C, from

250 °C to 550 °C, from 250 °C to 500 °C, from 250 °C to 450 °C, from 250 °C to 400 °C, from

250 °C to 350 °C, from 250 °C to 300 °C, from 300 °C to 600 °C, from 300 °C to 550 °C, from

300 °C to 500 °C, from 300 °C to 450 °C, from 300 °C to 400 °C, from 300 °C to 350 °C, from

350 °C to 600 °C, from 350 °C to 550 °C, from 350 °C to 500 °C, from 350 °C to 450 °C, from

350 °C to 400 °C, from 400 °C to 600 °C, from 400 °C to 550 °C, from 400 °C to 500 °C, from

400 °C to 450 °C, from 450 °C to 600 °C, from 450 °C to 550 °C, from 450 °C to 500 °C, from

500 °C to 600 °C, from 500 °C to 550 °C, from 550 °C to 600 °C, or any combination of these ranges.

[0040] In embodiments, where the oxygen-containing atmosphere comprise carbon dioxide, the asymmetric hollow fiber carbon membrane may held at a temperature that is from 700 °C to 1000 °C, such as from 700 °C to 950 °C, from 700 °C to 900 °C, from 700 °C to 850 °C, from 700 °C to 800 °C, from 700 °C to 750 °C, 750 °C to 1000 °C, from 750 °C to 950 °C, from 750 °C to 900 °C, from 750 °C to 850 °C, from 750 °C to 800 °C, from 800 °C to 1000 °C from 800 °C to 950 °C, from 800 °C to 900 °C, from 800 °C to 850 °C, from 850 °C to 1000 °C from 850 °C to 950 °C, from 850 °C to 900 °C, , from 900 °C to 1000 °C from 900 °C to 950 °C, from 950 °C to 1000 °C, or any combination of these ranges.

[0041] Referring still to the flow chart 100 of FIG. 1, step 130 generally includes allowing the reverse selective asymmetric hollow fiber carbon membrane to cool. In one or more embodiments the membrane may be allowed to cool to a temperature that is less than 50 °C, such as less than 40 °C, or even less than 30 °C. In one or more embodiments, the asymmetric hollow fiber carbon membrane may be exposed to the oxygen-containing atmosphere before being allowed to cool.

[0042] Referring now to FIG. IB, as shown in flow chart 102, in one or more embodiments, the method of making a reverse selective asymmetric hollow fiber carbon membrane may include an additional cooling step 140 prior to exposure step 120. In one or more embodiments, the furnace may be allowed to cool to a temperature that is greater than or equal to 500 °C before the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere. For example, the furnace may be allowed to cool to a temperature of greater than or equal to 550 °C, greater than or equal to 600 °C, greater than or equal to 650 °C, greater than or equal to 700 °C, greater than or equal to 750 °C, greater than or equal to 800 °C, greater than or equal to 850 °C, greater than or equal to 900 °C, greater than or equal to 950 °C, greater than or equal to 1000 °C, greater than or equal to 1050 °C, greater than or equal to 1100 °C, or even greater than or equal to 1150 °C. In some embodiments, the furnace may be allowed to cool to a temperature that is from 500 °C to 700 °C, such as from 500 °C to 675 °C, from 500 °C to 650 °C, from 500 °C to 625 °C, from 500 °C to 600 °C, from 500 °C to 575 °C, from 500 °C to 550 °C, from 500 °C to

525 °C, from 525 °C to 700 °C, from 525 °C to 675 °C, from 525 °C to 650 °C, from 525 °C to

625 °C, from 525 °C to 600 °C, from 525 °C to 575 °C, from 525 °C to 550 °C, from 550 °C to

700 °C, from 550 °C to 675 °C, from 550 °C to 650 °C, from 550 °C to 625 °C, from 550 °C to

600 °C, from 550 °C to 575 °C, from 575 °C to 700 °C, from 575 °C to 675 °C, from 575 °C to

650 °C, from 575 °C to 625 °C, from 575 °C to 600 °C, from 600 °C to 700 °C, from 600 °C to

675 °C, from 600 °C to 650 °C, from 600 °C to 625 °C, from 625 °C to 700 °C, from 625 °C to

675 °C, from 625 °C to 650 °C, from 650 °C to 700 °C, from 650 °C to 675 °C, from 675 °C to

700 °C, or any combination of these ranges.

[0043] In one or more embodiments, the concentration of oxygen in the oxygen-containing atmosphere may increase as the temperature of the furnace decreases during the cooling of the furnace. In embodiments, the concentration of oxygen in the furnace may increase by from 2.0 ppm to 3.0 ppm per a 1 °C drop in the temperature of the furnace. For example, the concentration of oxygen in the furnace may increase by from 2.0 ppm to 2.9 ppm per a 1 °C drop in the temperature of the furnace, such as from 2.0 ppm to 2.8 ppm, from 2.0 ppm to 2.7 ppm, from 2.0 ppm to 2.6 ppm, from 2.0 ppm to 2.5 ppm, from 2.0 ppm to 2.4 ppm, from 2.0 ppm to 2.3 ppm, from 2.0 ppm to 2.2 ppm, from 2.0 ppm to 2.1 ppm, from 2.1 ppm to 3.0 ppm, from 2.1 ppm to 2.9 ppm, from 2.1 ppm to 2.8 ppm, from 2.1 ppm to 2.7 ppm, from 2.1 ppm to 2.6 ppm, from 2.1 ppm to 2.5 ppm, from 2.1 ppm to 2.4 ppm, from 2.1 ppm to 2.3 ppm, from 2.1 ppm to 2.2 ppm, from 2.2 ppm to 3.0 ppm, from 2.2 ppm to 2.9 ppm, from 2.2 ppm to 2.8 ppm, from 2.2 ppm to 2.7 ppm, from 2.2 ppm to 2.6 ppm, from 2.2 ppm to 2.5 ppm, from 2.2 ppm to 2.4 ppm, from 2.2 ppm to 2.3 ppm, from 2.3 ppm to 3.0 ppm, from 2.3 ppm to 2.9 ppm, from 2.3 ppm to 2.8 ppm, from 2.3 ppm to 2.7 ppm, from 2.3 ppm to 2.6 ppm, from 2.3 ppm to 2.5 ppm, from 2.3 ppm to 2.4 ppm, from 2.4 ppm to 3.0 ppm, from 2.4 ppm to 2.9 ppm, from 2.4 ppm to 2.8 ppm, from 2.4 ppm to 2.7 ppm, from 2.4 ppm to 2.6 ppm, from 2.4 ppm to 2.5 ppm, from 2.5 ppm to 3.0 ppm, from 2.5 ppm to 2.9 ppm, from 2.5 ppm to 2.8 ppm, from 2.5 ppm to 2.7 ppm, from 2.5 ppm to 2.6 ppm, from 2.6 ppm to 3.0 ppm, from 2.6 ppm to 2.9 ppm, from 2.6 ppm to 2.8 ppm, from 2.6 ppm to 2.7 ppm, from 2.7 ppm to 3.0 ppm, from 2.7 ppm to 2.9 ppm, from 2.7 ppm to 2.8 ppm, from 2.8 ppm to 3.0 ppm, from 2.8 to 2.9 ppm, or from 2.9 ppm to 3.0 ppm. In some embodiments, the concentration of oxygen in the furnace may increase by from 2.3 ppm to 2.5 ppm per a 1 °C drop in the temperature of the furnace. In some embodiments, the concentration of oxygen in the furnace may increase at a rate such that the maximum concentration of oxygen in the furnace is reached when the furnace has cooled to a temperature that is from 100 °C to 200 °C.

[0044] In one or more embodiments, the oxygen-containing atmosphere may enter the furnace via diffusion. In other embodiments, the oxygen-containing atmosphere may enter the furnace by being injected into the furnace through a gas inlet.

[0045] In one or more embodiments, a pressure differential may be created across the asymmetric hollow fiber carbon membrane during the exposure to the oxygen-containing atmosphere such that the oxygen-containing atmosphere is pulled across a cross-section of the asymmetric hollow fiber carbon membrane. In some embodiments, the pressure differential may be created by adding an oxygen-containing gas at one end of the furnace and flowing the gas through the furnace to exit at the opposite end of the furnace.

[0046] Having described a process for making a reverse selective asymmetric hollow fiber carbon membrane, a system 200 that may be utilized with the methods of the present disclosure is provided in FIG. 2. Flow control device 10 is first set to an “on” configuration, meaning that fluid is permitted to flow through gas conduit 12 from purge gas reservoir 14 to furnace 16, which contains the hollow fiber carbon membrane (not shown). During the heating step 110, most or even all of the fluid entering furnace 16 originates from purge gas reservoir 14. After heating the furnace 16 may be allowed to cool. When the temperature within furnace 16 decreases to the desired oxidation initiation temperature, flow control device 10 reduces the flow of the fluid from the purge gas reservoir 14. In embodiments where the oxygen-containing atmosphere may enter the furnace by being injected through a gas inlet, flow control device 18 may increase the flow of the fluid from the oxidant reservoir 20 into furnace 16 via gas conduit 22. In the embodiment shown in FIG. 2 conduit 12 and conduit 22 both feed into inlet 24. The oxidant reservoir 20 may contain a premixed volume of oxidant and carrier gas, such as, for example, air (oxygen gas as oxidant and nitrogen gas as carrier) or a mixture of oxygen gas (oxidant) and argon (carrier). In embodiments where the oxygen-containing atmosphere may enter the furnace via diffusion, flow control device 10 may be used to reduce or stop the flow of fluid from the purge gas reservoir 14 into the furnace. An oxygen-containing atmosphere, for example air, may then enter the furnace via conduit 28 and outlet 26 via diffusion.

[0047] Referring still to FIG. 2, during both heating and oxidation, gases from the furnace 16 may pass through outlet 26 and may eventually be vented via conduit 28. Conveniently, these gases may be analyzed using oxygen sensor 30 to allow proper control of the process, including gas composition during the pyrolysis. Though embodiments of the present disclosure have been described in the context of system 200 other systems suitable for use with the embodiments described herein are contemplated.

[0048] As used herein, the term “conduit” includes, but is not limited to, casings, liners, pipes, tubes, coiled tubing, and mechanical structures with interior voids.

[0049] As used herein, the term “reservoir” includes any container of any size capable of containing a fluid, whether in liquid or gaseous form. Exemplary reservoirs include, but are not limited to, gas cylinders, holding tanks, bladders, inflatable membranes (such as a balloon), drums, and bottles.

[0050] As used herein, the term “flow control device” includes, but is not limited to, a ball valve, a butterfly valve, a choke valve, a diaphragm valve, a gate valve, a globe valve, a knife valve, a needle valve, a pinch valve, a piston valve, a plug valve, a solenoid valve, and a spool valve.

[0051] In one or more embodiments, the method of making a reverse selective asymmetric hollow fiber carbon membrane may further comprise allowing the reverse selective asymmetric hollow fiber carbon membrane to cool to a temperature of less than 50 °C. For example, the reverse selective asymmetric hollow carbon fiber may be a allowed to cool to a temperature less than 45 °C, such as less than 40 °C, less than 35 °C, less than 30 °C, less than 25 °C, or the reverse selective asymmetric hollow carbon fiber may even be allowed to cool to less than 20 °C. [0052] In one or more embodiments, the method of making a reverse selective asymmetric hollow fiber carbon membrane may further comprise forming an asymmetric hollow fiber carbon membrane. In embodiments, forming an asymmetric hollow fiber carbon membrane may comprise providing a polymeric precursor, heating the polymeric precursor to a pyrolysis temperature of from 800 °C to 1200 °C, pyrolyzing the polymer precursor at the pyrolysis temperature to form an asymmetric hollow fiber carbon membrane, and allowing the asymmetric hollow fiber carbon membrane to cool to a temperature that is less than 50 °C.

[0053] As described herein, in one or more embodiments the method of making a reverse selective asymmetric hollow fiber carbon membrane may comprise providing a polymeric precursor. The polymeric precursor may be any useful polymer for making hollow fiber carbon membranes, such as polyimides for example. When a polyimide is used, the polyimide may be a conventional or fluorinated polyimide. In embodiments, the polymeric precursor may comprise a polymer comprising monomers Ax, By, and Cz, where X, Y, and Z are the mole fraction of each of A, B, and C, respectively, present in the polymer. In embodiments, X + Y + Z = 1. In other embodiments, X + Y + Z < 1, and other monomers are present in the polymer.

[0054] Each of A, B, and C is a monomer selected from the group consisting of 2,4,6- trimethyl-l,3-phenylene diamine (DAM); oxydianaline (ODA); dimethyl-3,7-diaminodiphenyl- thiophene-5, 5 '-dioxide (DDBT); 3,5-diaminobenzoic acid (DABA); 2.3,5,6-tetramethyl-l,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, d'biphenyl tetracarboxylic dianhydride (BPD A); pyromellitic dianhydride (PMDA); 1 ,4,5,8- naphthalene tetracarboxylic dianhydride (NTDA); 4,4'-oxydiphthalic anhydride (ODPA); 5(6)- amino-l-(4’-aminophenyl)-l,3,3-trimethylindane (DAPI); and 3,3 ',4, 4 '-benzophenone tetracarboxylic dianhydride (BTDA). In embodiments, polyimides may contain at least two different moieties selected from DAM; ODA; DDBT; DABA; durene; m-PDA; 2,4-DAT; TMMDA; BDSA; 6FDA; BPD A; PMDA; NTDA; and BTDA.

[0055] In embodiments, A is a monomer selected from the group consisting of 6FDA, ODPA, and BTDA; B is DAM; and C is a monomer selected from the group consisting of BPDA and PMDA. In embodiments, A is 6FDA; B is DAM; and Z is 0. In embodiments, the polyimide may be MATRIMID™ 5218 (Huntsman Advanced Materials), a commercially available polyimide in which A is BTDA; B is DAPI; and Z is 0.

[0056] In embodiments, the polyimide may comprise, consist essentially of, or consist of 6FDA/BPDA-DAM, as shown in formula (1), which may be synthesized via thermal or chemical processes from a combination of three commercially available monomers: DAM; 6FDA, and BPDA. In embodiments of formula (1), X + Y may be from 0.1 to 0.9, and Z may be from 0.1 to 0.9. In embodiments of formula (1), X + Y may be from 0.1 to 1, and Z may be from 0 to 0.9. In embodiments of formula (1), X may be 0 and Y + Z may be 1. In embodiments, X and Z may be from 0.25, 0.3, or 0.4 to 0.9, 0.8, or 0.75. In embodiments, X + Y is 0.5 and Z is 0.5. Formula (2) below shows a representative structure for 6FDA/BPDA-DAM, with a potential for adjusting the ratio between X and Z to tune polymer properties. In embodiments, a 1 : 1 ratio of X to Z may also abbreviated as 6FDA/BPDA(1 :1)-DAM.

[0057] In embodiments, the polyimide may be formed by the reaction of a diamine with a dianhydride. In such embodiments, at least one of A, B, and C, is a diamine, and at least one other of A, B, and C, is a dianhydride. In embodiments, the total diamine and the total dianhydride may be in a molar ratio of diamine to dianhydride of greater than or equal to 49:51 to 51 :49. In embodiments, the diamine and dianhydride may be in a molar ratio of diamine to dianhydride of about 50:50. [0058] In embodiments, more than one dianhydride may be used with one diamine. In such embodiments using two dianhydrides, dianhydride 1 and dianhydride 2, the molar ratio of dianhydride 1 to dianhydride 2 may be greater than or equal to 20:80 and less than or equal to 80:20. For example, this molar ratio may be greater than or equal to 25:75 and less than or equal to 75:25, greater than or equal to 30:70 and less than or equal to 70:30, greater than or equal to 35:65 and less than or equal to 65:35, greater than or equal to 40:60 and less than or equal to 60:40, or even greater than or equal to 45:55 and less than or equal to 55:45. In embodiments, the molar ratio of dianhydride 1 to dianhydride 2 may be about 50:50. In embodiments, one dianhydride may be used with more than one diamines. In such embodiments using two diamines, diamine 1 and diamine 2, the molar ratio of diamine 1 to diamine 2 may be greater than or equal to 20:80 and less than or equal to 80:20. For example, this molar ratio may be greater than or equal to 25:75 and less than or equal to 75:25, greater than or equal to 30:70 and less than or equal to 70:30, greater than or equal to 35:65 and less than or equal to 65:35, greater than or equal to 40:60 and less than or equal to 60:40, or even greater than or equal to 45:55 and less than or equal to 55:45. In embodiments, the molar ratio of diamine 1 to diamine 2 may be about 50:50.

[0059] In embodiments, the polymeric precursor membranes as produced, but not pyrolyzed, are substantially defect-free. “Defect-free” means that selectivity of a gas pair through a 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 precur membrane. By way of illustration, a 6FDA/BPDA(1 :1)-DAM polymer has an O2/N2 selectivity (also known as “dense film selectivity”) of 4.1.

[0060] In embodiments, the precursor polymers may be formed into hollow fibers. Conventional procedures to make these may be used. For example, coextrusion procedures including 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 make hollow fibers.

[0061] 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.

[0062] Pyrolysis conditions influence hollow fiber CMS membrane physical properties. 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.

[0063] 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 polymeric 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. Without being bound by theory, it is believed that at pyrolysis temperatures below 800 °C the desired gas selectivity may not be obtainable. However, it is also believed that 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.

[0064] 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.

[0065] 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.

[0066] In one or more embodiments, 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, purging with a gas having a lower temperature than the hollow fiber CMS membrane, or opening the furnace to the surrounding environment.

[0067] In one or more embodiments, a method of separating ethylene from a gas feed comprising ethylene and hydrogen comprises flowing the gas feed through a reverse selective asymmetric hollow fiber carbon membrane made by the methods as described herein and separating the gas feed into a first stream having an increased concentration of ethylene and a second stream having an increased concentration of hydrogen. In some embodiments, the reverse selective asymmetric hollow fiber carbon membrane is desirably fabricated into a module comprising a sealable enclosure comprised of a plurality of carbon membranes that is comprised of at least one reverse selective asymmetric hollow fiber carbon membrane that is made using the methods described herein 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.

[0068] In a first aspect of the present disclosure, a method of making a reverse selective asymmetric hollow fiber carbon membrane may comprise heating an asymmetric hollow fiber carbon membrane. The method may also comprise exposing the asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere to form a reverse selective asymmetric hollow fiber carbon membrane. After exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere the reverse selective asymmetric hollow fiber carbon membrane may have a weight that is from -5 wt.% to 20 wt.% less than the weight of the asymmetric hollow fiber carbon membrane. [0069] A second aspect of the present disclosure may include the first aspect where the reverse asymmetric hollow fiber carbon membrane has a weight that is from -5 wt.% to 5 wt.% less than the weight of the asymmetric hollow fiber carbon membrane.

[0070] A third aspect of the present disclosure may include any previous aspect or combination of aspects, where the oxygen-containing atmosphere comprises one or more of air, oxygen gas, carbon dioxide, or steam.

[0071] A fourth aspect of the present disclosure may include any previous aspect or combination of aspects, where the method further comprises heating the asymmetric hollow fiber carbon membrane at a temperature that is from 600 °C to 1200 °C in a furnace under an inert atmosphere, allowing the furnace to cool to a temperature that is greater than or equal to 500 °C, and exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere during the cooling of the furnace to form a reverse selective asymmetric hollow fiber carbon membrane.

[0072] A fifth aspect of the present disclosure may include the fourth aspect where the asymmetric hollow fiber carbon membrane is heated to a temperature that is from 900 °C to 1200 °C before being exposed to the oxygen-containing atmosphere.

[0073] A sixth aspect of the present disclosure may include the fourth or fifth aspects where the concentration of oxygen in the oxygen-containing atmosphere increases as the temperature of the furnace decreases during the cooling of the furnace.

[0074] A seventh aspect of present disclosure may include the fourth to sixth aspects where the concentration of oxygen in the furnace increases by from 2 ppm to 3 ppm per a 1 °C drop in the temperature of the furnace.

[0075] An eighth aspect of the present disclosure may include any previous aspect or combination of aspects where the oxygen-containing atmosphere enters the furnace via diffusion or where the oxygen-containing atmosphere is injected into the furnace through a gas inlet.

[0076] A ninth aspect of the present disclosure may include the first to third aspects where the asymmetric hollow fiber carbon membrane is heated to a temperature greater than or equal to 550 °C and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for a time period of from 1 minute to 1000 minutes before being allowed to cool.

[0077] A tenth aspect of the present disclosure may include any previous aspect or combination of aspects, where the concentration of oxygen in the oxygen-containing atmosphere is from 100 ppm to 1000 ppm

[0078] An eleventh aspect of the present disclosure may include the first to third aspects, where the oxygen-containing atmosphere comprises air, the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere at a temperature from 200 °C to 600 °C, and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for from 1 minute to 1000 minutes.

[0079] A twelfth aspect of the present disclosure may include the eleventh aspect, where the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere at a temperature from 250 °C to 500 °C, and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for from 1 minutes to 1000 minutes.

[0080] A thirteenth aspect of the present disclosure may include the first to third aspects, where the oxygen-containing atmosphere comprises carbon dioxide, the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere at a temperature from 700 °C to 1000 °C, and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for from 1 minute to 1000 minutes.

[0081] A fourteenth aspect of the present disclosure may include any previous aspect or combination of aspects, where the method further comprises creating a pressure differential across the asymmetric hollow fiber carbon membrane during the exposure to the oxygen-containing atmosphere such that the oxygen-containing atmosphere is pulled across a cross-section of the asymmetric hollow fiber carbon membrane.

[0082] A fifteenth aspect of the present disclosure may include any previous aspect or combination of aspects, where the method further comprises providing a polymeric precursor, heating the polymeric precursor to a pyrolysis temperature from 800 °C to 1200 °C, pyrolyzing the polymer precursor at the pyrolysis temperature to form an asymmetric hollow fiber carbon membrane, and allowing the asymmetric hollow fiber carbon membrane to cool to a temperature that is less than 50 °C.

TEST METHODS

Gas Permeance and Selectivity

[0083] 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) .

[0084] 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.

[0085] 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. EXAMPLES

[0086] 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.

Example 1 - Oxidant Concentration Calibration

[0087] The relationship between the increases in the average O2 concentration in the furnace with the decrease in temperature is determined using the ideal gas law, PV = nRT, where P is pressure, V is volume, n is the number of moles of the O2, R is the ideal gas constant, and T is temperature. In the calculations, the volume is constant and the pyrolysis temperature is 925 °C. Initially, 100 % argon gas is used as the purge gas. After reaching the pyrolysis temperature, the furnace is allowed to cool naturally. When the temperature reaches 575 °C, gas purging is stopped, and the O2 concentration induced by the air back-diffusion inside the furnace is determined.

[0088] FIG. 3 provides a chart of temperature on the x-axis and simulated O2 concentration on the y-axis. As is evident from the chart, the concentration of O2 increases linearly as the temperature decreases. Further, this relationship may be used to determine an expected concentration of O2 when performing pyrolysis, reheating, and oxidation.

Example 2- Formation of Asymmetric Hollow Fiber Carbon Membranes

[0089] 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

[0090] 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.

[0091] 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, fdtering 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.

[0092] 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).

[0093] 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. [0094] 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. Mixed gas permeation tests were performed in a constant-pressure system maintained at 35 °C and 52 psig pressure drop. The gas was fed onto the shell side of the module. Helium was fed to the bore side of the fiber as a sweep gas. The gas permeated through the membranes along with the Helium sweep gas was analyzed using gas chromatography. Permeate flow rate, permeate composition, area of the membranes and pressure drop across the membranes were used to calculate the gas permeance as previously mentioned. The selectivity of each gas pair as a ratio of the individual gas permeance was calculated.

[0095] 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. The pyrolysis temperature was the highest temperature the fibers were heated too during pyrolysis.

Example 3 - Weight Change of Carbon Membranes from Continuous Oxidation under Air

[0096] In Example 3, three asymmetric hollow fiber carbon membranes formed in Example 2 were oxidized by heating the membranes under a continuous air purge at 30 standard cubic centimeters per minute (seem). Two of the membranes were oxidized at 400 °C, one of the 400 °C oxidized membranes was formed by pyrolysis at 550 °C and the other 400 °C oxidized membrane was formed by pyrolysis at 925 °C. The membranes were placed in a quartz tube furnace and the temperature was first raised to 150 °C at a 10 °C/minute (min) ramp rate and held for 30 minutes to remove pre-adsorbed species, then the temperature was raised to 400 °C at a 1 °C/min ramp rate and held at 400 °C for 8 hours. The temperature of the furnace over time during oxidation is shown in FIG. 4B. [0097] Thermogravimetric analysis (TGA) was performed during the oxidation of the membranes and the results are shown in FIG. 4A. As shown in FIG. 4A, the 925 °C pyrolyzed membrane has 81 % residue after oxidation at 400 °C for 8 hours, while the 550 °C pyrolyzed membrane only has 34 % residue, and as a result, weaker mechanical strength. It is believed that the different oxidation performance is caused by the different graphitization degree of the 550 °C and the 925 °C pyrolyzed membranes. A higher temperature of pyrolysis is desirable to obtain mechanically strong fibers after oxidation. Further, as shown in FIG. 4 A, the weight loss of the membranes is almost linear after reaching the final oxidation temperature so it may be possible to use a shorter time of oxidation to improve the weight loss of the fiber and therefore improve the mechanical strength of the membrane.

[0098] A second membrane formed by pyrolysis at 925 °C was oxidized under a continuous air purge at 30 seem and an oxidation temperature of 300 °C. The membrane was placed in a quartz tube furnace and the temperature was first raised to 150 °C at a 10 °C/min ramp rate and held for 30 minutes to remove pre-adsorbed species. The temperature was then raised to 300 °C at a 1 °C/min ramp rate and held at 300 °C for 8 hours.

[0099] TGA was performed during the oxidation of the membrane and the results are shown in FIG. 4C comparing the weight change of the two 925 °C pyrolyzed membranes during oxidation at 300 °C and 400 °C. The 300 °C oxidized membrane showed an initial slight increase in weight followed by a slight weight loss. It is believed the initial weight gain may be due to oxygenate formation on the surface of the carbon membrane and the later weight loss may be due to formation of carbon dioxide.

Example 4 - Permeation Performance of Membranes Oxidized under Continuous Air

[0100] In Example 4, seven fiber samples pyrolyzed at 925 °C in Example 2, were oxidized in a quartz tube furnace under a constant 300 seem of air at various temperatures. Comparative Samples A-D were oxidized by first raising the temperature to 350 °C at a 1 °C/min ramp rate, and then were held at 350 °C for 8 hours. Samples 1 and 2 were oxidized by first raising the temperature to 300 °C at a 1 °C/min ramp rate and then were held at 300 °C for 8 hours. Comparative Sample E was oxidized by first raising the temperature to 250 °C at a 1 °C/min ramp rate and then was held at 250 °C for 8 hours. [0101] The seven oxidized fibers were tested for permeance performance by sealing a single fiber in Sawagelok housing. The module was then connected to a permeation unit at 35 °C. Two mixed gas tests were done sequentially. First an equimolar feed of H2/CO2/CH4 at a total 300 seem rate was used to pressure up the shell side of the hollow fiber to 52 pounds per square inch gauge (psig). The bore side of the hollow fiber was continuously purged by 25 seem of He. The permeate was carried by the He purge to a gas chromatographer (GC) for compositional analysis. The CCh/C Utest was done for about 3-15 hours. Then an equimolar feed of H2/C2H4 at a total 200 seem rate was used to pressure up the shell side of the hollow fiber to 52 psig. The bore side of the hollow fiber was continuously purged by 25 seem of He. The permeate was carried by the He purge to GC for compositional analysis. The H2/C2H4 test was done for about 3-15 hours. The results of the permeation testing were reported in Table 2.

Table 2

[0102] As shown in Table 2 the Comparative Samples A-C were not stable enough to perform permeation testing and broke before results could be obtained. Comparative Sample D had a very fast permeance drop in the first 2-3 hours of testing as shown in FIGS. 5A-5D. Samples 1 and 2 showed good reverse selectivity as indicated by a C2H4/H2 ratio of greater than 5 for both samples. Further, both Samples 1 and 2 had very high C2H4 permeance (greater than 1500 GPU). This indicates that an oxidation temperature of 300 °C under these conditions achieved fibers with good reverse selective performance.

Example 5 - Permeance Performance of Membranes Oxidized during Cooling [0103] In Example 5, a 925 °C pyro lyzed membrane from Example 2, was reheated to a temperature of 925 °C in a quartz tube furnace under an Ar gas purge stream. The furnace was then allowed to cool and when the furnace had cooled to a temperature of 575 °C the purge gas stream was stopped and air was allowed to diffuse back into the furnace until fully cool to produce Sample 3. The simulated rate of air diffusal can be seen in FIG. 3. FIG. 2 shows a schematic depiction of the furnace setup used to oxidize the sample in Example 5.

[0104] 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 Tab, Inc., Chicago, IE) 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 Tabs, 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 3.

Table 3

[0105] As Table 3 indicates, Sample 3 which was reheated and exposed to air during the cooling process has significantly higher permeance for both CO2 and ethylene when compared to Comparative Sample F which was not reheated nor exposed to air. Table 3 further indicates that Sample 3 has significantly higher selectivity of ethylene to H2 when compared to Comparative Sample F. A selectivity of ethylene to H2 of greater than 1 indicates that ethylene is permeating more than H2 through the membrane which indicates that sample 3 is a reverse selective membrane. Example 6 - Permeance Performance of Membranes Oxidized with Oxygen Soak

[0106] In Example 6, a 925 °C pyrolyzed membrane from Example 2 was oxidized under a continuous concentration of oxygen. The membrane was first reheated to 575 °C in a quartz tube furnace. Once the furnace had reached temperature 905 ppm of oxygen gas was added to the furnace and that concentration was maintained at temperature for 2 hours to produce Sample 4. Sample 4 was then tested using the permeation testing method of Example 5 and the results are shown in Table 4.

Table 4

As Table 4 indicates, sample 4 which was reheated and exposed to 905 ppm of O2 for 2 hours has significantly higher permeance for both CO2 and ethylene when compared to Comparative Sample F which was not reheated nor exposed to air. Table 4 further indicates that Sample 4 has significantly higher selectivity of ethylene to H2 when compared to Comparative Sample F. A selectivity of ethylene to H2 of greater than 1 indicates that ethylene is permeating more than H2 through the membrane which indicates that sample 4 is a reverse selective membrane.

Example 7 - Permeance Performance of Membranes Oxidized with Carbon Dioxide

[0107] In Example 7, a 925 °C pyrolyzed membrane from Example 2 was oxidized under carbon dioxide. The membrane was first reheated to 800 °C in a quartz tube furnace. Once the furnace had reached temperature carbon dioxide gas was added to the furnace and that concentration was maintained at temperature for 8 hours to produce Sample 5. Sample 5 as then tested using the permeation testing method of Example 5 and the results are shown in Table 5. Table 5

[0108] As Table 5 indicates, sample 5 which was exposed to CO2 at 800 °C for 8 hours has significantly higher permeance for both CO2 and ethylene when compared to Comparative Sample F which was not exposed to CO2 Table 5 also indicates that Sample 5 has significantly higher selectivity of ethylene to H2 when compared to Comparative Sample F. A selectivity of ethylene to H2 of greater than 1 indicates that ethylene is permeating more than H2 through the membrane which indicates that sample 5 is a reverse selective membrane.

[0109] 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 modification and variations come within the scope of the appended claims and their equivalences.

[0110] It is noted that one or more of the following claims and the detailed description utilize the terms “where” or “wherein” as a transitional phrase. For the purposes of defining the present technology, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.”

[0111] It should be understood that any two quantitative values assigned to a property may constitute a range of that property, and all combinations of ranges formed from all stated quantitative values of a given property are contemplated in this disclosure. Where multiple ranges for a quantitative value are provided, these ranges may be combined to form a broader range, which is contemplated in the embodiments described herein.

Claims

1. A method of making a reverse selective asymmetric hollow fiber carbon membrane, the method comprising: heating an asymmetric hollow fiber carbon membrane; exposing the asymmetric hollow fiber carbon membrane to an oxygen-containing atmosphere to form a reverse selective asymmetric hollow fiber carbon membrane; wherein after exposing the asymmetric hollow fiber carbon membrane to the oxygencontaining atmosphere the reverse selective asymmetric hollow fiber carbon membrane has a weight that is from -5 wt.% to 20 wt.% less than the weight of the asymmetric hollow fiber carbon membrane.

2. The method of claim 1, wherein the reverse asymmetric hollow fiber carbon membrane has a weight that is from -5 wt.% to 5 wt.% less than the weight of the asymmetric hollow fiber carbon membrane.

3. The method of claims 1 and 2, wherein the oxygen-containing atmosphere comprises one or more of air, oxygen gas, carbon dioxide, or steam.

4. The method of any preceding claim, further comprising heating the asymmetric hollow fiber carbon membrane at a temperature that is from 600 °C to 1200 °C in a furnace under an inert atmosphere; allowing the furnace to cool to a temperature that is greater than or equal to 500 °C; and exposing the asymmetric hollow fiber carbon membrane to the oxygen-containing atmosphere during the cooling of the furnace to form a reverse selective asymmetric hollow fiber carbon membrane.

5. The method of claim 4, wherein the asymmetric hollow fiber carbon membrane is heated to a temperature that is from 900 °C to 1200 °C before being exposed to the oxy gen-containing atmosphere.

6. The method of claims 4 and 5, wherein the concentration of oxygen in the oxygencontaining atmosphere increases as the temperature of the furnace decreases during the cooling of the furnace.

7. The method of any of claims 4-6, wherein the concentration of oxygen in the furnace increases by from 2 ppm to 3 ppm per a 1 °C drop in the temperature of the furnace.

8. The method of any preceding claim, wherein the oxy gen-containing atmosphere enters the furnace via diffusion or where the oxygen-containing atmosphere is injected into the furnace through a gas inlet.

9. The method of any of claims 1-3, wherein: the asymmetric hollow fiber carbon membrane is heated to a temperature greater than or equal to 550 °C; and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for a time period of from 1 minute to 1000 minutes before being allowed to cool.

10. The method of any preceding claim, wherein the concentration of oxygen in the oxygencontaining atmosphere is from 100 ppm to 1000 ppm.

11. The method of any of claims 1-3, wherein:

The oxygen-containing atmosphere comprises air; the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere at a temperature from 200 °C to 600 °C; and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for from 1 minute to 1000 minutes.

12. The method of claim 11, wherein: the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere at a temperature from 250 °C to 500 °C; and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for from 1 minutes to 1000 minutes.

13. The method of any of claims 1-3, wherein

The oxygen-containing atmosphere comprises carbon dioxide; the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere at a temperature from 700 °C to 1000 °C; and the asymmetric hollow fiber carbon membrane is exposed to the oxygen-containing atmosphere for from 1 minute to 1000 minutes.

14. The method of any preceding claim, further comprising creating a pressure differential across the asymmetric hollow fiber carbon membrane during the exposure to the oxygencontaining atmosphere such that the oxygen-containing atmosphere is pulled across a crosssection of the asymmetric hollow fiber carbon membrane.

15. The method of any preceding claim, further comprising: providing a polymeric precursor; heating the polymeric precursor to a pyrolysis temperature from 800 °C to 1200 °C; pyrolyzing the polymer precursor at the pyrolysis temperature to form an asymmetric hollow fiber carbon membrane; and allowing the asymmetric hollow fiber carbon membrane to cool to a temperature that is less than 50 °C.