Patent Description:
Membranes are widely used for the separation of gases and liquids, including for example, separating acid gases, such as CO<NUM> and H<NUM>S from natural gas, and the removal of O<NUM> 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 process-ability and low cost. CMS membranes, however, have been shown to have attractive separation performance properties exceeding that of polymeric membranes.

Polyimides have been pyrolyzed to form CMS membranes under many differing conditions. <CIT> discloses pyrolyzing under vacuum and inert gases with trace amounts of oxygen. Other patents describe processes for producing carbon membranes (both asymmetric hollow "filamentary" and flat sheets), and applications for gas separation, include, for example, <CIT>, and <CIT>. Steel and Koros performed a detailed investigation of the impact of pyrolysis temperature, thermal soak time, and polymer composition on the performance of carbon membranes. (<NPL>);<NPL>)). In these works membranes were produced in an air atmosphere at <NUM> Hg pressure.

The impact of pyrolysis atmosphere has been researched. Suda and Haraya disclosed the formation of CMS membranes under different environments. ) Similarly, Geiszler and Koros disclosed the results of CMS fibers produced from pyrolysis of fluorinated polyimide in helium and argon for both O<NUM>/N<NUM> and H<NUM>/N<NUM> separations.

<CIT> discloses a carbon molecular sieve made by pyrolyzing a hollow fiber membrane made of a polyimide polymer. At least <NUM>% of the dianhydride-derived units are derived from 6FDA and at least <NUM>% of the diamine-derived units are derived from DETDA.

<NPL> discloses a method of making a carbon molecular sieve using a polyimide which is a 6FDA/PMDA-TMMDA copolyimide.

<NPL> discloses copolyimides having structural segments with rotational freedom and other structural segments without rotational freedom.

When making asymmetric hollow fibers CMS membranes from polyimides, which have a thin dense separating layer and thick inner porous support structure, it has been difficult to make the hollow fibers without having undesired structural collapse. Structural collapse results in an undesired thicker separating layer resulting in poor permeance of desired permeate gases rendering the fibers commercially impractical. (see <NPL>).

To address this problem, complicated involved methods have been described such as in <CIT>. In this patent, the application of a sol-gel silica that undergoes vinyl cross-linking on the inner porous walls of the polyimide is described to reduce the structural collapse during pyrolysis to form the hollow fiber CMS membrane. Recently, <CIT> describes a separate particular preoxidation of particular polyimides, such as 6FDA/BPDA-DAM, having the stoichiometry shown in Formula <NUM>. Formula <NUM> shows a chemical structure for 6FDA/BPDA-DAM where X and Y are each <NUM> so as to provide a <NUM>:<NUM> ratio. <CHM>
This polyimide after undergoing the pre-oxidation was reported to improve the structural collapse and reduce sticking of the fibers during and after pyrolysis.

It would be desirable to provide a method to make a polyimide membrane that avoids any one of the problems mentioned above. In particular, it would desirable to provide a method that did not involve any further process steps involving heat-treatments or treatments prior to pyrolysis of the polyimide membrane to form the carbon molecular sieve membrane.

A first aspect of the invention is a method of making an asymmetric hollow fiber carbon molecular sieve comprising,.

The method of the invention allows the realization of a CMS asymmetric membrane that has reduced or no structural collapse, which can result in improved combinations of selectivity and permeability for desired gas pairs. Illustratively, the method allows for CMS membrane having good selectivity for similar sized gas molecules (e.g., hydrogen/ethylene; ethylene/ethane; propylene/propane and butylene/butane) while still having higher permeance of the target permeate gas molecule (e.g., hydrogen in gases containing hydrogen/ethylene). That is, the selectivity/permeance characteristics (productivity) are substantially improved relative to CMS asymmetric hollow fiber membranes made using polyimides that do not display the storage modulus behavior above <NUM>.

A process for separating a gas molecule from a gas feed comprised of the gas molecule and at least one other gas molecule can be undertaken, said process comprising.

A gas separating module can be provided comprising a sealable enclosure comprised of: a plurality of asymmetric hollow fiber carbon molecular sieves, comprising at least one asymmetric hollow fiber carbon molecular sieve of the invention, contained within the sealable enclosure; 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.

The gas separation method is particularly useful for separating gas molecules in gas feeds that have very similar molecular sizes such as ethane/ethylene and propane/propylene. It may also be used to separate gases from atmospheric air such as oxygen or separating gases (e.g., methane) in natural gas feeds.

The asymmetric hollow fiber carbon molecular sieve (hollow fiber CMS) is formed from a polyimide hollow fiber having a thin dense layer on the outer surface of the fiber and a thicker porous support layer on the inner surface of the fiber. Desirably, the hollow fibers are substantially defect-free. "Defect-free" is determined to be when the selectivity of a gas pair, typically oxygen (O<NUM>) and nitrogen (N<NUM>), through a hollow fiber membrane is at least <NUM> 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.

When making the polyimide hollow fiber, conventional procedures known in the art may be used (see, for example <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT> and <CIT>). Exemplary methods include coextrusion procedures such as a dryjet/wet-quench 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 the hollow fibers.

To make the polyimide hollow fiber, a dope solution is prepared for the spinning process where the dope solution is comprised of a polyimide and solvents. When making a hollow fiber, typically the dope solution is a mixture of solvents that solubilize the polyimide and a second solvent that does not solubilize (or solubilizes to a limited extent) the polyimide, but is soluble with the solvent that solubilizes the polyimide are used. Exemplary solvents that are useful to solubilize the polyimide include polar aprotic solvents such as N-Methyl-<NUM>-pyrrolidone (NMP), tetrahydrofuran (THF), dimethylacetamide (DMAc) and dimethylformamide (DMF). Exemplary solvents that do not solubilize the polyimide, but are soluble with the solvents that do solubilize the polyimide include methanol, ethanol, water, and <NUM>-propanol. To facilitate the practical formation of the hollow fiber, generally, the polyimide needs to be dissolved in an amount of at least about <NUM>% to <NUM>% by weight of the dope solution. Desirably the amount of polyimide solubilized is at least <NUM>%, <NUM>%, <NUM>% or <NUM>%. Such dope solution consists of both non-volatile solvents (e.g., NMP) and volatile solvents (e.g., THF and ethanol). The evaporation of the volatile solvents (boiling point <<NUM>) in the air gap promotes the formation of a dense skin layer on the outer surface of the fiber and thus creates the asymmetric fiber structure.

The polyimide is a polyimide that has a storage modulus minimum at a temperature greater than <NUM> that is less than the storage modulus at a temperature of <NUM>, but no more than ten times less measured using dynamic mechanical thermal analysis from <NUM> to a temperature where the polyimide carbonizes. Without being bound in any way, the storage modulus minimum above <NUM> may be or could be correlated or attributed with the polyimide undergoing glass transition or the like prior to carbonizing. The temperature where the polyimide carbonizes (temperature where the polyimide starts to decompose and form carbon in a non-oxidizing atmosphere) may vary, but in general the temperature is above <NUM> and inevitably will carbonize at a temperature at or above <NUM> or <NUM>. The polyimide preferably is a thermoplastic.

The dynamic mechanical thermal analysis is performed using a thin film sample of the polyimide having general dimensions that are <NUM> long, <NUM> wide, and <NUM> thick. The samples are kept under N<NUM> purge during the measurements. The films are first heated to <NUM> and equilibrated at this temperature for <NUM> minutes. Thereafter temperature is ramped to <NUM> at the rate of <NUM>/minute, and finally to <NUM> at the rate of <NUM>/minute. The oscillation frequency is set at <NUM> rad/s and the strain amplitude is set at <NUM>%. An exemplary dynamic mechanical thermal analyzer that may be used is RSA III rheometer from TA Instruments, New Castle, DE.

The aromatic polyimides are a reaction product of a dianhydride and a diamine, which is understood to proceed by forming a polyamic acid intermediate that is subsequently ring-closed to form the polyimide by chemical and/or thermal dehydration. The dianhydride is comprised of an aromatic dianhydride having no rotational freedom within the dianhydride, which means that only one aromatic ring is present or there are no single bonds between aromatic moieties, which would allow the aromatic rings to rotate in relation to each other, and a dianhydride that has rotational freedom within the dianhydride. The dianhydride having no rotational freedom is <NUM>,<NUM>,<NUM>,<NUM>-naphthalene tetracarboxylic dianhydride (NTDA), benzoquinonetetracarboxylic dianhydride or combinations thereof. Examples of a dianhydride having rotational freedom include benzophenone-<NUM>,<NUM>',<NUM>,<NUM>'-tetracarboxylic dianhydride (BTDA), <NUM>,<NUM>'-[<NUM>,<NUM>,<NUM>-trifluoro-<NUM>-(trifluoromethyl)ethylidene]-<NUM>,<NUM>-isobenzofurandione (6FDA) and <NUM>,<NUM>',<NUM>,<NUM>'-biphenyl tetracarboxylic dianhydride (BPDA). Combinations of aromatic dianhydrides are contemplated.

The diamine used to make the polyimide has no rotational freedom. The diamine has no rotational freedom and, in particular, the aromatic diamine has only one aromatic ring. Examples of diamines having no rotational freedom include <NUM>,<NUM>,<NUM>-trimethyl-<NUM>,<NUM>-phenylenediamine (DAM), <NUM>,<NUM>-diaminobenzoic acid (DABA), <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>,<NUM>-phenylenediamine (durene), dimethyl-<NUM>,<NUM>-diaminodiphenyl-thiophene-<NUM>,<NUM>'-dioxide (DDBT), meta-phenylenediamine (m-PDA), para-phenylenediamine, and <NUM>,<NUM>-diaminotoluene (<NUM>,<NUM>-DAT).

The polyimide is the reaction product of a combination of dianhydrides with some having rotational freedom and some not having rotational freedom and a diamine having no rotational freedom and in particular an aromatic diamine that has only one aromatic ring. A polyimide not according to the present invention is exemplified by the polyimide 6FDA/PMDA-DAM as represented in below Formula <NUM>:
<CHM>
where X and Y represent the mole fraction of each dianhydride used to make the polyimide with X and Y adding up to <NUM> and n represents an integer representing the number of repeat units and n may be any value to realize the weight average molecular weight described herein. Desirably, Y is from <NUM>, <NUM> or <NUM> to <NUM>, <NUM> or <NUM>. Each of the monomers used to make 6FDA/PMDA-DAM is commercially available for example from Sigma-Aldrich Co. Louis, MO or TCI America, Portland, OR.

Generally, the polyimide of the invention has a molecular weight sufficient to form a polyimide fiber having the requisite strength to be handled and subsequently pyrolyzed, but not so high that it becomes impractical to dissolve to make a dope solution able to form the hollow fiber. Typically, the weight average (Mw) molecular weight of the polyimide is <NUM> to <NUM> kDa, but desirably the molecular weight of <NUM> to <NUM> kDa. Polymer molecular weight may be controlled by stoichiometry of dianhydride to diamine monomers, monomer purity, as well as use of monofunctional endcapping agents such as monoamines (i.e., aniline, <NUM>-ethynylaniline) and monoanhydrides (i.e., phthalic anhydride, succinic anhydride, maleic anhydride).

After the dope solution is formed, the solution is shaped into a hollow fiber as described above. After shaping, the solvents may be exchanged with other solvents (such as methanol and hexane) to prevent, for example, pore collapse, and the solvents are further removed by any convenient method such as application of heat, vacuum, flowing gases or combination thereof and include those known in the art.

After removing the solvent, the formed hollow polyimide fiber is pyrolyzed to form the asymmetric hollow fiber carbon molecular sieve. The hollow polyimide fibers may be pyrolyzed 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 <NUM> millibar). <CIT> and co-pending <CIT> describe a suitable heating method for pyrolysis of the polyimide fibers to form the CMS hollow fibers. A pyrolysis temperature of between about <NUM> to about <NUM> may advantageously be used. The pyrolysis temperature may be adjusted in combination with the pyrolysis atmosphere to tune the performance properties of the resulting CMS hollow fiber membrane. For example, the pyrolysis temperature may be <NUM> or more. Optionally, the pyrolysis temperature is maintained between about <NUM> and about <NUM>. The pyrolysis soak time (i.e., the duration of time at the pyrolysis temperature) may vary (and may include no soak time) but advantageously is between about <NUM> hour to about <NUM> hours, alternatively from about <NUM> hours to about <NUM> hours, alternatively from about <NUM> hours to about <NUM> hours. An exemplary heating protocol may include starting at a first set point of about <NUM>, then heating to a second set point of about <NUM> at a rate of about <NUM> per minute, then heating to a third set point of about <NUM> at a rate of about <NUM> per minute, and then a fourth set point of about <NUM> at a rate of about <NUM> per minute. The fourth set point is then optionally maintained for the determined soak time. After the heating cycle is complete, the system is typically allowed to cool while still under vacuum or in a controlled atmosphere.

In one embodiment the pyrolysis utilizes a controlled purge gas atmosphere during pyrolysis in which low levels of oxygen are present in an inert gas. 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 combinations thereof. By using any suitable method such as a valve, the inert gas containing a specific concentration of oxygen may be introduced into the pyrolysis atmosphere. For example, the amount of oxygen in the purge atmosphere may be less than about <NUM> ppm (parts per million) O<NUM>/Ar. Alternatively, the amount of oxygen in the purge atmosphere may be less than <NUM> ppm O<NUM>/Ar. Embodiments include pyrolysis atmospheres with about <NUM> ppm, <NUM> ppm, or <NUM> ppm O<NUM>/Ar.

After pyrolyzing, the CMS membrane that has formed is cooled to a temperature where no further pyrolysis occurs. Generally, this is a temperature where no decomposition products would be evolved from the precursor polymer and may vary from polymer to polymer. Generally, the temperature is <NUM> or less and typically the temperature is taken as <NUM>, <NUM> or essentially typical ambient temperatures (<NUM> to <NUM>). The cooling may be at any useful rate, such as passively cooling (e.g., turning off the power to furnace and allowing to cool naturally). Alternatively, it may be desirable to more rapidly cool such as using known techniques to realize faster cooling such as removing insulation, or using cooling fans or employment of water cooled jackets.

After cooling, the CMS hollow fiber membrane may be subjected to a further treatment, for example, to make the fiber more stable or improve particular permeance/selectivity for particular gases. Such further treatments are described in pending provisional <CIT>.

The method enables the formation of an asymmetric hollow fiber carbon molecular sieve CMS that has a wall that is defined by an inner surface and outer surface of said fiber and the wall has an inner porous support region (support layer) extending from the inner surface to an outer microporous region (separation layer) that extends from the inner porous support region to the outer surface. Surprisingly, it has been discovered when the aromatic polyimide has the aforementioned storage modulus characteristic, structural collapse of the inner porous support region may be avoided and the outer microporous separation layer may be tailored to be desirably thin in absence of any pretreatment of the polyimide fiber, for example, as described in PCT Publ. <CIT> or incorporation of an inorganic gel such as described in <CIT> described previously. Avoidance of structural collapse, generally, means that the corresponding separation layer in the asymmetric polyimide fiber when pyrolyzed to make the hollow fiber CMS, the separation layer in the hollow fiber CMS has a thickness that is within <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% of the corresponding separation layer of the polyimide fiber. Illustratively, if the polyimide separation layer is <NUM> micrometers, the corresponding CMS fiber separation layer thickness may be <NUM>, <NUM>, <NUM>, <NUM> or <NUM> micrometers. Desirably the CMS separation layer may be essentially (-within <NUM>% or less) the same thickness as the corresponding polyimide separation layer.

Typically, the outer separation layer has a thickness of at most <NUM>% of the wall extending from the inner surface to the outer surface. The outer separation layer typically has a thickness of <NUM> micrometers to <NUM> micrometers, desirably <NUM> micrometers to <NUM> micrometers, more desirably <NUM> to <NUM> micrometer. Herein, microporous shall mean pores <<NUM> in diameter; mesoporous shall mean <NUM>-<NUM> in diameter and macroporous shall mean ><NUM> in diameter. The microstructure of the separation layer in CMS is generally characterized with microporous pores. The support layer is generally characterized by a microstructure where the pores are microporous, macroporous or both.

The gas permeation properties of a membrane can be determined by gas permeation experiments. Two intrinsic properties have utility in evaluating the 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 (<NUM> Barrer=<NUM>-<NUM> [cm<NUM> (STP) cm]/[cm<NUM> s cmHg], calculated as the flux (ni) divided by the partial pressure difference between the membrane upstream and downstream (Δpi), and multiplied by the thickness of the membrane (l).

Another term, "permeance", is defined herein as productivity of asymmetric hollow fiber membranes and is typically measured in Gas Permeation Units (GPU) (<NUM> GPU=<NUM>-<NUM> [cm<NUM> (STP)]/[cm<NUM> s cmHg]), determined by dividing permeability by effective membrane separation layer thickness. <NUM> GPU = <NUM> × <NUM>-<NUM> mol. Pa-<NUM> <MAT>.

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 unit less ratio.

In a particular embodiment, the asymmetric hollow CMS membrane produced by the method enables a carbon hollow fiber CMS membrane that has a permeance of at least <NUM> GPU for a target gas molecule (permeate) and a selectivity of at least <NUM>. In particular embodiments the permeate/retentate gas molecule pairs may be ethylene/ethane, propylene/propane, butylene/butane, hydrogen/ethylene, methane/carbon dioxide, methane/water, oxygen/nitrogen, or methane/hydrogen sulfide. Illustratively, the feed gas generally is comprised of at least <NUM>% of the permeate gas molecule (e.g., ethylene or propylene) and <NUM>% of the retentate gas molecule (e.g., ethane or propane).

In a particular embodiment the CMS membrane produced has a permeance of at least <NUM> GPU for propylene (permeate) and a selectivity of at least <NUM> for propylene/propane. Desirably, in this embodiment the permeance is at least <NUM>, <NUM> or even <NUM> GPU for propylene. Likewise, in this embodiment the selectivity is at least <NUM>, <NUM>, <NUM> or even <NUM> for propylene/propane. In another particular embodiment, the CMS membrane produced has a permeance of at least <NUM> GPU for ethylene (permeate) and a selectivity of at least <NUM> ethylene/ethane. Desirably, in this embodiment the permeance is at least <NUM>, <NUM>, <NUM> or even <NUM> GPU for ethylene. Likewise, in this embodiment the selectivity is at least <NUM>, <NUM> or even <NUM> for ethylene/ethane. In a further embodiment, the CMS membrane produced has a permeance of at least <NUM> GPU for butylene (permeate) and a selectivity of at least <NUM> butylene/butane. Desirably, in this embodiment the permeance is at least <NUM>, <NUM>, <NUM> or even <NUM> GPU for butylene. Likewise, in this embodiment the selectivity is at least <NUM>, <NUM> or even <NUM> for butylene/butane.

The CMS membranes are particularly suitable for separating gases that are similar in size such as described above, which involves feeding a gas feed containing a desired gas molecule and at least one other gas molecule through the CMS membrane. The flowing of the gas results in a first stream having an increased concentration of the desired gas molecule and, a second stream having an increased concentration of the other gas molecule. The process may be utilized to separate any of the aforementioned gas pairs and in particular is suitable for separating ethylene and ethane or propylene and propylene. When practicing the process, 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 produced by the method of the invention that are contained within the sealable enclosure. The sealable enclosure has 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.

The CMS of Comparative Example <NUM> was made using 6FDA:BPDA-DAM (<NUM>:<NUM>) polymer. The 6FDA:BPDA-DAM was acquired from Akron Polymer Systems, Akron, OH. Gel permeation chromatography was performed to evaluate the molecular weight. Tosoh TSKgel Alpha-M columns were used with <NUM>/min eluent of dimethylformamide (DMF) with <NUM>/L lithium nitrate. Waters <NUM> separation module/Viscotek TDA <NUM> interface/ Waters <NUM> RI detector was used as the detector and was at <NUM>. The polymer was dissolved in DMF at <NUM> wt%, and the sample injection volume was <NUM>µL. Agilent PEO/PEG EasiCal standards was used for calibration. The polymer had a weight average molecular weight (Mw) of <NUM> kDa and polydispersity index (PDI) of <NUM>. The polymer was dried under vacuum at <NUM> for <NUM> hours and then a dope was formed. The dope was made by mixing the 6FDA:BPDA-DAM polymer with solvents and compounds in Table <NUM> and roll mixed in a Qorpak™ glass bottle sealed with a polytetrafluoroethylene (TEFLON™) cap and a rolling speed of <NUM> revolutions per minute (rpm) for a period of about <NUM> weeks to form a homogeneous dope.

The homogeneous dope was loaded into a <NUM> milliliter (mL) syringe pump and allowed to degas overnight by heating the pump to a set point temperature of <NUM> using a heating tape.

Bore fluid (<NUM> wt% NMP and <NUM> wt% water, based on total bore fluid weight) was loaded into a separate <NUM> syringe pump and then the dope and bore fluid were coextruded through a spinneret operating at a flow rate of <NUM> milliliters per hour (mL/hr) for the dope, and <NUM>/hr for the bore fluid, filtering both the bore fluid and the dope in line between delivery pumps and the spinneret using <NUM> and <NUM> 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 <NUM>.

After passing through a five centimeter (cm) air gap, the nascent fibers that were formed by the spinneret were quenched in a water bath (<NUM>) and the fibers were allowed to phase separate. The fibers were collected using a <NUM> meter (m) diameter polyethylene drum passing over TEFLON guides and operating at a take-up rate of <NUM> meters per minute (m/min).

The fibers were cut from the drum and rinsed at least four times in separate water baths over a span of <NUM> hours. The rinsed fibers in glass containers and effect solvent exchange three times with methanol for <NUM> minutes and then hexane for <NUM> minutes before recovering the fibers and drying them under argon purge at a set point temperature of <NUM> for two hours.

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 ¼ inch (<NUM>) (outside diameter, OD) stainless steel tubing. Each tubing end was connected to a ¼ inch (<NUM>) stainless steel tee; and each tee was connected to ¼ inch (<NUM>) female and male NPT tube adapters, which were sealed to NPT connections with epoxy. The membrane modules were tested using a constant pressure permeation system. Argon was used as sweep gas in the permeate side. The flow rate of the combined sweep gas and permeate gas was measured by a Bios Drycal flowmeter, while the composition was measured by gas chromatography. The flow rate and composition were then used for calculating gas permeance. The selectivity of each gas pair as a ratio of the individual gas permeance was calculated. The mixed gas feed used for precursor defect-free property examination was <NUM> mol% CO<NUM>/<NUM> mol% N<NUM>. The permeation unit was maintained at <NUM>, and the feed and permeate/sweep pressures were kept at <NUM> and <NUM> psig, respectively.

In addition, the polyimide was cast into a film and cut into pieces of having dimensions that are <NUM> long, <NUM> wide, and <NUM> thick and dynamic mechanical thermal analysis was performed on the film as follows. DMTA was carried out on the polyimide films in tension mode using a RSA III rheometer from TA Instruments. The films were kept under a N<NUM> purge during the measurements. The films were first heated to <NUM> and equilibrated at this temperature for <NUM> minutes. Thereafter temperature was ramped to <NUM> at the rate of <NUM>/minute, and finally to <NUM> at the rate of <NUM>/minute. The oscillation frequency was set at <NUM> rad/s and strain amplitude at <NUM>%. The results of the DMTA tests are shown in <FIG> and in Table <NUM>.

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 or in a bundle containing multiple hollow fibers contacting each other. 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 <NUM> standard cubic centimeters per minute (sccm)). Prior to pyrolyzing the furnace was purged of oxygen by evacuating and then purging the tube furnace for a minimum of six hours to reduce the oxygen level to less than <NUM> ppm. All of the fibers were preheated to <NUM> at a ramp rate of <NUM>/min, then heated to <NUM> at a ramp rate of <NUM>/min, followed by heating to <NUM> at a ramp rate of <NUM>/min, and to <NUM> at <NUM>/min, finally soak at <NUM> for <NUM> hours. After the soak time, the furnace was shut off, cooled under the flowing argon (passively cooled), which typically cooled in about <NUM> to <NUM> hours.

After cooling the fibers were left to sit under the inert gas stream for <NUM> hours to allow the newly formed CMS to stabilize. The fibers after pyrolysis were stuck together and had to be carefully separated prior to any gas separation testing. The asymmetric polyimide hollow fiber prior to pyrolyzing and after pyrolyzing to form the asymmetric hollow fiber CMS are shown in <FIG> scanning electron micrographs. From the micrograph it is readily apparent that the separation layer of the CMS hollow fiber is substantially greater than the separation of the corresponding polyimide layer indicating significant collapse of the porous support structure.

For module making and permeation tests, the fibers that were separated on the mesh prior to pyrolysis were used. Afterwards they were removed from the furnace and potted into modules as described above. The modules were allowed at least <NUM> hours to set before being loaded into the permeation testing system for initial tests. All permeation tests were determined using a <NUM>:<NUM> mixture of propylene and propane, or ethylene and ethane, or hydrogen and ethylene in a constant pressure system described above with <NUM> psig (<NUM> kPa) upstream and downstream at <NUM> psig ((<NUM> kPa) argon purge at <NUM>. The stage cut was maintained at less than <NUM>%. For stable performance, the membranes were allowed to sit in the lab for at least <NUM> months and tested for stable performance. For each test, the permeation was run multiple hours and most of time more than <NUM> hours. The permeance and selectivity results are shown in Table <NUM>.

A 6FDA/PMDA-DAM polyimide having a mole ratio of 6FDA/PMDA of <NUM>/<NUM> (<NUM>%/<NUM>%) was made as follows. Into a <NUM> neck <NUM> flask with a slow N2 sweep, <NUM> grams of <NUM>-methyl-<NUM>-pyrrolidinone (<NUM> grams), toluene (<NUM>) were loaded and stirred with a magnetic stirring bar. Toluene was distilled from the mixture into a Dean-Stark type trap and drained. The apparatus was cooled to room temperature while stirring. The Dean-Stark type trap was removed and the flask was placed under positive N2. Vacuum sublimed pyromellitic dianhydride (<NUM> grams, <NUM> mol), vacuum sublimed <NUM>,<NUM>'-(hexafluoroisopropylidene)diphthalic anhydride (<NUM> grams, <NUM> mmol), vacuum sublimed maleic anhydride (<NUM> gram, <NUM> mmol), and <NUM>,<NUM>,<NUM>-trimethyl-m-phenylene diamine (<NUM> grams, <NUM> mol) were added to the flask with <NUM> of dry <NUM>-methyl-<NUM>-pyrrolidinone used to rinse down monomers. After ~<NUM> hours reacting under overhead stirring, the inherent viscosity of polyamic acid was <NUM> dL/g (<NUM>/dL, <NUM>. 0oC, <NUM>-methyl-<NUM>-pyrrolidinone). To the stirred polyamic acid solution dry <NUM>-picoline (<NUM>) was injected with a solution of <NUM>-methyl-<NUM>-pyrrolidinone (<NUM>) and acetic anhydride (<NUM>) added dropwise over ~<NUM> hours with stirring continuing overnight. The polyimide product was isolated by precipitation in stirred methanol (~<NUM>) with polyimide being collected by filtration and subsequently washed four times with fresh methanol. The polyimide product was dried to a constant weight in a ~100oC vacuum oven with a recovered yield of <NUM> grams. Inherent viscosity of the polyimide was <NUM> dL/g (<NUM>/dL, <NUM>. 0oC, <NUM>-methyl-<NUM>-pyrrolidinone). Using the same GPC conditions, the polymer Mw was found to be <NUM> kDa and PDI was <NUM>.

The polyimide of this example was formed into hollow fibers as described in Comparative Example <NUM> except that the dope composition was as shown in Table <NUM> and the following differences. The dope and bore fluid flow rates were both <NUM>/hr. The spinneret temperature was <NUM>, and the quench bath temperature was <NUM>. The air gap was <NUM> and the take-up rate was <NUM>/min. Likewise the polyimide was cast into films and DMTA was performed with the results shown in <FIG> and Table <NUM>. After forming the polyimide fibers they were pyrolyzed as described in Comparative Example <NUM>. The fibers had an inner porous support layer that displayed no collapsing of the structure and a distinct separation region of about <NUM> micrometers as shown in <FIG> compared to the corresponding separation region in the polyimide fiber from which it was made (<FIG>). In addition, the fibers, after being pyrolyzed, did not stick together at all and were easily separated despite being bundled and contacting each other during pyrolysis.

After the fibers were removed from the furnace, they were potted into modules as described above. The membranes were aged for at least <NUM> months and tested. For each test, the permeation was run multiple times to ensure repeatable stable results and typically were run more than <NUM> hours. The results are shown in Table <NUM>.

From the results it is readily apparent that the hydrogen and propylene permeance was substantially higher for Example <NUM> yet the selectivity was at least the same when paired with ethylene and propane respectively.

A 6FDA/PMDA-DAM polyimide having a mole ratio of 6FDA/PMDA of <NUM>/<NUM> (<NUM>%/<NUM>%) was made as follows. Into a <NUM> neck <NUM> flask with a slow N2 sweep, <NUM> of <NUM>-methyl-<NUM>-pyrrolidinone, toluene (<NUM>) were loaded and stirred with a magnetic stirring bar. Toluene was distilled from the mixture into a Dean-Stark type trap and drained. The apparatus was cooled to room temperature while stirring. The Dean-Stark type trap was removed and the flask was placed under positive N2. Vacuum sublimed pyromellitic dianhydride (<NUM> grams, <NUM> mmol), vacuum sublimed <NUM>,<NUM>'-(hexafluoroisopropylidene)diphthalic anhydride (<NUM> grams, <NUM> mmol), and <NUM>,<NUM>,<NUM>-trimethyl-m-phenylene diamine (<NUM> grams, <NUM> mmol) were added to the flask. After ~<NUM> hours reacting under overhead stirring, the inherent viscosity of polyamic acid was <NUM> dL/g (<NUM>/dL, <NUM>, <NUM>-methyl-<NUM>-pyrrolidinone). To the stirred polyamic acid solution in injected dry <NUM>-picoline (<NUM>) with a solution of <NUM>-methyl-<NUM>-pyrrolidinone (<NUM>) and acetic anhydride (<NUM>) added dropwise over ~<NUM> hours with stirring continuing overnight. The polyimide product was isolated by precipitation in stirred methanol (~<NUM>) with polyimide being collected by filtration and subsequently washed three times with fresh methanol. The polyimide product was dried to a constant weight in a ~<NUM> vacuum oven with a recovered yield of <NUM> grams. Inherent viscosity of the polyimide was <NUM> dL/g (<NUM>/dL, <NUM>, <NUM>-methyl-<NUM>-pyrrolidinone). Likewise the polyimide was cast into films and DMTA was performed with the results shown in <FIG> and Table <NUM>. This polyimide is expected to perform in the same manner as the polyimide of Example <NUM>.

A first 6FDA/PMDA-DAM polyimide having a mole ratio of 6FDA/PMDA of <NUM>/<NUM> (<NUM>%/<NUM>%) was made as follows. Into a <NUM> neck <NUM> flask with a slow N<NUM> sweep, <NUM> grams of <NUM>-methyl-<NUM>-pyrrolidinone and toluene (<NUM>) were loaded and stirred with a magnetic stirring bar. Toluene was distilled from the mixture into a Dean-Stark type trap and drained. The apparatus was cooled to room temperature while stirring. The Dean-Stark type trap was removed and the flask was placed under positive N<NUM>. Vacuum sublimed pyromellitic dianhydride (<NUM> grams, <NUM> mol), vacuum sublimed <NUM>,<NUM>'-(hexafluoroisopropylidene)diphthalic anhydride (<NUM> grams, <NUM> mmol), vacuum sublimed maleic anhydride (<NUM> gram, <NUM> mmol), and <NUM>,<NUM>,<NUM>-trimethyl-m-phenylene diamine (<NUM> grams, <NUM> mol) were added to the flask. After ~<NUM> hours reacting under overhead stirring, the inherent viscosity of polyamic acid was <NUM> dL/g (<NUM>/dL, <NUM>, <NUM>-methyl-<NUM>-pyrrolidinone). To the stirred polyamic acid solution dry <NUM>-picoline (<NUM>) was injected with a solution of <NUM>-methyl-<NUM>-pyrrolidinone (<NUM>) and acetic anhydride (<NUM>) added dropwise over ~<NUM> hours with stirring continuing <NUM> hours. The polyimide product was isolated by precipitation in stirred methanol (~<NUM>) with polyimide being collected by filtration and subsequently washed four times with fresh methanol. The polyimide product was dried to a constant weight in a ~<NUM> vacuum oven with a recovered yield of <NUM> grams. Inherent viscosity of the polyimide was <NUM> dL/g (<NUM>/dL, <NUM>, <NUM>-methyl-<NUM>-pyrrolidinone). Using the same GPC condition, the Mw was measured to be <NUM> kDa and PDI was <NUM>.

A second 6FDA/PMDA-DAM polyimide having a mole ratio of 6FDA/PMDA of <NUM>/<NUM> (<NUM>%/<NUM>%) was made as follows. Into a <NUM> neck <NUM> flask with a slow N<NUM> sweep, <NUM> grams of <NUM>-methyl-<NUM>-pyrrolidinone and toluene (<NUM>) were loaded and stirred with a magnetic stirring bar. Toluene was distilled from the mixture into a Dean-Stark type trap and drained. The apparatus was cooled to room temperature while stirring. The Dean-Stark type trap was removed and the flask was placed under positive N<NUM>. Vacuum sublimed pyromellitic dianhydride (<NUM> grams, <NUM> mol), vacuum sublimed <NUM>,<NUM>'-(hexafluoroisopropylidene)diphthalic anhydride (<NUM> grams, <NUM> mmol), vacuum sublimed maleic anhydride (<NUM> gram, <NUM> mmol), and <NUM>,<NUM>,<NUM>-trimethyl-m-phenylene diamine (<NUM> grams, <NUM> mol) were added to the flask. After ~<NUM> hours reacting under overhead stirring, the inherent viscosity of polyamic acid was <NUM> dL/g (<NUM>/dL, <NUM>, <NUM>-methyl-<NUM>-pyrrolidinone). To the stirred polyamic acid solution dry <NUM>-picoline (<NUM>) was injected with a solution of <NUM>-methyl-<NUM>-pyrrolidinone (<NUM>) and acetic anhydride (<NUM>) added dropwise over ~<NUM> hours with stirring continuing <NUM> hours. The polyimide product was isolated by precipitation in stirred methanol (~<NUM>) with polyimide being collected by filtration and subsequently washed four times with fresh methanol. The polyimide product was dried to a constant weight in a ~<NUM> vacuum oven with a recovered yield of <NUM> grams. Inherent viscosity of the polyimide was <NUM> dL/g (<NUM>/dL, <NUM>, <NUM>-methyl-<NUM>-pyrrolidinone). Using the same GPC condition, the Mw was measured to be <NUM> kDa and PDI was <NUM>.

The first and second polyimide of this Example <NUM> were blended in <NUM>/<NUM> wt% to form asymmetric hollow fibers. The blended polyimide of the fiber had an Mw of <NUM> kDa and PDI of <NUM>. The polyimide was formed into hollow fibers as described in Example <NUM> except that the dope composition was as shown in Table <NUM> and the following noted differences. The dope and bore fluid flow rates are both <NUM>/hr. The spinneret temperature was <NUM>, and the quench bath temperature was at <NUM>. The air gap was <NUM> and take-up rate was <NUM>/min.

The CMS fibers were obtained by pyrolyzing the polymer fibers as described in Comparative example <NUM> except for the following differences. The heating protocol was as follows: preheat to <NUM>/min, <NUM>/min; heat to <NUM>/min, <NUM>/min; heat to <NUM>, <NUM>/min, heat to <NUM>, <NUM>/min; soak at <NUM> for <NUM> hours. As in Example <NUM>, the hollow CMS fibers were not stuck together and were easily separated. <FIG> are scanning electron micrographs of the hollow polyimide fiber prior to pyrolyzing and the corresponding CMS fiber made therefrom. From the figures, it can readily be seen that the separation region is essentially the same in both polyimide fiber and the CMS fiber indicating essentially no structural collapse.

Claim 1:
A method of making an asymmetric hollow fiber carbon molecular sieve comprising,
(i) providing a dope solution comprised of a polyimide and a solvent, wherein the polyimide has a storage modulus minimum at a temperature greater than <NUM> that is less than the storage modulus at a temperature of <NUM>, but no more than ten times less, wherein the storage modulus minimum is measured using dynamic mechanical thermal analysis as described herein from <NUM> to a temperature where the polyimide carbonizes,
(ii) shaping the dope solution to form a hollow fiber;
(iii) removing the solvent from the hollow fiber to form a polyimide hollow fiber; and
(iv) heating the polyimide hollow fiber in an atmosphere that is non-oxidizing to form the asymmetric hollow fiber carbon molecular sieve;
wherein the polyimide is the reaction product of a dianhydride and a diamine, wherein the diamine has no rotational freedom, and the dianhydride is comprised of an aromatic dianhydride that has no rotational freedom within the dianhydride and a dianhydride that has rotational freedom within the dianhydride;
wherein no rotational freedom means that only one aromatic ring is present or there are no single bonds between aromatic rings that would allow the aromatic rings to rotate in relation to each other and rotational freedom means that there are single bonds between aromatic rings that would allow the aromatic rings to rotate in relation to each other; and
wherein the dianhydride that has no rotational freedom is naphthalenetetracarboxylic dianhydride, benzoquinonetetracarboxylic dianhydride, or combination thereof.