Composite Carbon Molecular Sieve Hollow Fiber Membranes

Subscriber access provided by UNIV OF ALABAMA BIRMINGHAM

Separations

Composite Carbon Molecular Sieve Hollow Fiber Membranes: Resisting Support Densification via Silica Particle Stabilization Chen Zhang, Kuang Zhang, Yuhe Cao, and William J. Koros Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02386 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Composite Carbon Molecular Sieve Hollow Fiber Membranes: Resisting Support Densification via Silica Particle Stabilization Chen ZhangÁ, Kuang ZhangÁ, Yuhe Cao, William J. Koros* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta GA USA30332

ACS Paragon Plus Environment

1

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 34

ABSTRACT

Resisting densification of hollow fiber support layers under high-temperature pyrolysis is critical to form carbon molecular sieve hollow fiber membranes with thin separation layer and attractive productivity. In this paper, a new ³VLOLFD SDUWLFOH VWDELOL]DWLRQ´ approach is introduced to form thinskinned composite carbon molecular sieve hollow fiber membranes with excellent resistance to support layer densification. By dispersing small-sized silica particles with low bulk density in the support layer of polymer precursor hollow fibers, composite carbon molecular sieve hollow fiber membranes were formed with highly porous supports. The composite carbon molecular sieve hollow fiber membranes showed very attractive selectivities and productivities higher than monolithic asymmetric carbon molecular sieve hollow fiber membranes formed by the standard sol-gel support stabilization technique.

KEYWORDS carbon molecular sieves, composite CMS hollow fiber membranes, silica particles, nanomaterials, molecular separations


2

Page 3 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction ,Q KLV PRUH WKDQ

\HDUV¶ FDUHHU DV D chemical engineer, Richard Noble has made excellent

contributions to membrane science, most notably synthesis of high-performance zeolite membranes. To enable advanced non-polymeric membrane materials (zeolites, carbon molecular sieves, metal-organic frameworks, etc.) for large-scale membrane separations, the membranes must provide highly-attractive separation performance to justify their typically higher production cost. In this work, we discuss a method to form composite carbon molecular sieve hollow fiber membranes with reduced separation layer thickness and significantly enhanced separation performance.

Molecularly-selective membranes can reduce the energy consumption and CO2 footprints of largescale chemical separations.1-3 Extending membrane-based molecular separations beyond established applications (e.g. desalination4, air separation5-6, natural gas processing7, hydrogen purifications8-9) can be enabled with advanced membranes based on rigid molecular sieves.7, 10-18 Carbon molecular sieves (CMS) are rigid nanoporous molecular sieves formed by controlled pyrolysis of polymeric precursors.12, 19 Based on their roles in molecular transport, the pores in CMS materials are generally divided into two classes-micropores and ultramicropores.20 The micropores comprise three-GLPHQVLRQDO ³FKDPEHUV´ DQG DUH IRUPHG E\ SDcking imperfections between defective graphene plates. Unlike micropores, the ultramicropores are two-dimensional ³ZLQGRZV´ WKDW FRQWURO passage of individual penetrant molecules into and out of each micropore. Ultramicropores are believed to primarily comprise the defects in the graphene plates that form the micropore walls.20 The micropores enable attractive membrane permeabilities by providing


3


Page 4 of 34

large sorption capacities and large penetrant jump lengths. On the other hand, the ultramicropores provide entropically-enabled diffusion selectivities21 and synergistic sorption selectivities.22

CMS membranes have been studied for separation of gas23-27, vapor28-30, liquid31-32, and more recently, supercritical fluid mixtures.33 In a number of examples, CMS membranes show highly attractive separation performance well above existing polymer upper bounds.22-23, 34 Characterizing the pore structure of amorphous CMS, especially ultramicropores, is challenging. Nevertheless, it is the amorphous nature of CMS pore structure that provides combined facile tunability and mechanical flexibility not seen in ordered crystalline molecular sieves. Fundamental knowledge in precursor polymer chemistry and the CMS pore structure formation process is useful to create CMS materials with tunable pore structures and transport properties.20, 35-36 In fact, starting from the same precursor material, an impressively-wide range of membrane separation performance can be obtained in CMS membranes for target molecular separations.22, 28 Such tunability can be achieved by simply optimizing processing variables such as pyrolysis parameters28, pyrolysis environments37, pre-treatments, and post-treatments38.

Equally important to designing membrane materials with attractive separation performance is the ability to translate a membrane material into a scalable geometry with thin and defect-free separation (skin) layers. Such translation is less challenging for polymeric materials than for rigid molecular sieves, including carbon molecular sieves.10,

39-41

A major challenge involved with

translating carbon molecular sieves into the highly-scalable hollow fiber geometry is densification RI WKH KROORZ ILEHU¶V SRURXV support. During high-temperature pyrolysis, the porous support of the precursor hollow fiber membrane is usually densified.42 This densification leads to formation of


4

Page 5 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CMS hollow fiber membranes with poor permeances and separation layer at least one order of magnitude thicker than their precursors. 7KH WHUP ³SHUPHDQFH´ UHIOHFWV WR WKH LQWULQVLF permeability of a selective layer divided by its thickness. The degree of densification depends on, among other factors, the glass transition temperature (Tg) of the precursor. For precursor hollow fibers made from lower Tg polymers (e.g. Matrimid® [Tg = 305 oC]), total support layer densification can occur leading to thick-walled low permeance CMS hollow fiber membranes. For precursor hollow fibers made from higher Tg polymers (e.g. 6FDA/BPDA-DAM [Tg = 424 oC]), the porous support may be only partially densified; however, the skin layer is still much thicker than for the precursor hollow fiber.

A breakthrough in formation of thin-skinned CMS hollow fiber membranes with practical permeances was made by Bhuwania and co-workers43 through in situ sol-gel reactions. By soaking the precursor hollow fibers in vinyltrimethoxysilane (VTMS)/hexane solution followed by moisture-induced hydrolysis and condensation, a crosslinked silica network was formed around the ³VWUXWV´ FRPSULVLQJ WKH SUHFXUVRU KROORZ ILEHU¶V porous support layer. This sol-gel ³VWDELOL]DWLRQ´ PHWKRG was effective to resist support densification, and successfully created monolithic asymmetric CMS hollow fibers with substantially thinner skin layer and increased permeance. The sol-gel method involves contacting the precursor fiber skin layer with VTMS monomer molecules, which may diffuse into and through the precursor fiber skin layer. Residual VTMS molecules on the fiber skin layer can also form a silica layer on the final CMS hollow fiber surface as well as on the support struts. In some cases, such a silica layer could add non-selective mass transfer resistance to permeation, thereby compromising permeances and selectivities of the CMS hollow fiber membranes. Accordingly, it is attractive to develop alternative stabilization


5


Page 6 of 34

methods that can effectively resist support layer densification with minimum potential impacts on membrane transport properties.

In this work, we report a proof-of-concept for a new approach to resist support densification in CMS hollow fiber membranes without relying on sol-gel stabilization. This new technique enables resistance to support densification by dispersing pre-formed silica particles in the precursor hollow fiber¶V SRURXV support layer prior to pyrolysis. Two commercially-available candidate silica particles were chosen based on the results of syringe extrusion tests. Composite precursor hollow fiber membranes were formed using the selected silica particles, followed by pyrolysis into composite CMS hollow fiber membranes. The membranes were evaluated for CO2/CH4 separation, and the results were compared with monolithic asymmetric CMS hollow fiber membranes formed by sol-gel stabilization.

2. Materials and methods 2.1 Materials Matrimid® 5218 polyimide (density 1.2 g/cm3) was obtained from Huntsman Corporation. US3448 silica nanoparticles (average particle diameter ~ 15 nm, with a proprietary hydrophobic silane treatment) were obtained from US Research Nanomaterials, Inc. Non-hydrophobized silica

nanoparticles were less convenient to disperse, which made it challenging to form high-quality hollow fiber membranes. The bulk density of US3448 silica particles is 0.056 g/cm3. In addition, C803 silica particles (average particle diameter ~ 3.8 µm) were obtained from W.R. Grace and Company. The bulk density of C803 silica particles is 0.07 g/cm3. Finally, S5505 silica


6

Page 7 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanoparticles (average particle diameter ~ 60 nm) were obtained from Sigma-Aldrich with a bulk density of 0.326 g/cm3.

2.2 Understanding silica particle stabilization by syringe extrusion tests 2.2.1 Formation of syringe-extruded non-hollow precursor fibers Clearly, many nanoparticles can be considered for the general strategy illustrated here, and only two cases are shown here based on silica to clarify the fundamental issues. To allow focus on the fundamental of densification prevention, first, non-hollow fibers were used via simple syringe extrusion. The silica particles were dried in a vacuum oven at 180 °C overnight to remove moisture. The polymer powders were dried in a convection oven at 110 °C overnight. The dried silica particles (11.4 wt%) were dispersed in N-methyl-pyrrolidone (NMP). A sonication bath was used to assist the dispersion until no visible agglomerates were seen. To avoid formation and settling of large particles agglomerates, a NMP solution containing about 10 wt% of the total polymer was first added slowly to the silica particle dispersion. The remaining solvent and dried polymer powders were then added to the dispersion. The dispersions comprise 25.5 wt% polymer, 8.5 wt% silica particles, and 66 wt% NMP. The dispersions were rolled on a rolling mixer before being transferred into 10 cc hypodermic syringes (BD Luer-LokTM Tip). The dispersions were extruded from the syringes into a warm water bath (~50 oC) to form non-hollow precursor fibers. After the syringe extrusion, the non-hollow precursor fibers were soaked in DI water baths for 3 days to remove residual solvent. The fibers were then solvent exchanged in glass containers with three separate 20 min methanol baths followed by three separate 20 min hexane baths and dried under vacuum at 75 °C for 3 hours.


7


Page 8 of 34

2.2.2 Formation of non-hollow carbon molecular sieve fibers by pyrolysis Non-hollow carbon molecular sieve fibers were formed by controlled pyrolysis following procedures described elsewhere.44 The non-hollow precursor fibers described above were placed on a wired stainless steel mesh plate (McMaster Carr, Robbinsville, NJ) in a quartz tube (National Scientific Company, GE Type 214 quartz tubing, Quakertown, PA, USA), and then loaded into a pyrolysis furnace (Thermocraft, Inc., model 23-24-1ZH, Winston-Salem, NC, USA). The entire system was purged with ultra-high purity (UHP) argon for at least 12 hours until O2 level in the system dropped below 1 ppm. Pyrolysis was performed using the heating protocol below under continuous purge of ultra-high purity argon (200 cc/min). After the heating protocol was completed, the furnace was naturally cooled down to room temperature. Heating protocol:

1)

50 oC to 250 oC (13.3 oC /min)

2)

250 oC to 535 oC (3.85 oC /min)

3)

535 oC to 550 oC (0.25 oC /min)

4)

Thermal soak at 550 oC for 120 min

5)

Natural cooling down to room temperature

2.3 Formation of precursor hollow fiber membranes 2.3.1 Formation of monolithic precursor hollow fiber membranes Monolithic Matrimid® SUHFXUVRU KROORZ ILEHU PHPEUDQHV ZHUH IRUPHG XVLQJ WKH ³GU\-jet/wet TXHQFK´ WHFKQLTXH. The spinning dope composition and spinning parameters of the monolithic precursor hollow fiber membranes were identical with those described in the literature.45 After


8

Page 9 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


removal from the take-up drum, the monolithic precursor fibers were soaked sequentially in at least four separate water baths for 3 days to remove residual solvents, and then solvent-exchanged with sequential 1 hour baths of methanol and hexane. After air-drying in a fume hood for 1 hour, the fibers were dried in a vacuum oven at 75 °C for ~3 hours.

2.3.2 Formation of VTMS-treated monolithic precursor hollow fiber membranes as controls VTMS-treated monolithic precursor hollow fiber membranes were formed following procedures described in the literature.43 After the monolithic precursor hollow fibers were spun, they were soaked in 10 wt% VTMS/hexane solution for 24 hours. After the solution was drained, the fibers were stored in a glove bag saturated with water vapor for another 24 hours. The fibers were then dried in a vacuum oven at 150 oC for 12 hours prior to pyrolysis.

2.3.3 Formation of composite precursor hollow fiber membranes with only silica particles &RPSRVLWH SUHFXUVRU KROORZ ILEHU PHPEUDQHV ZHUH IRUPHG XVLQJ WKH ³GU\-MHW ZHW TXHQFK´ technique with a composite spinneret. The sheath spinning dope was free of silica particles, and was extruded from the sheath (outside) channel of the composite spinneret. The core spinning dope comprising silica particles were extruded from the core (inside) channel of the spinneret. Compositions of the sheath and core spinning dope are summarized in Table 1. Composition of the sheath spinning dope was identical to the spinning dope of the monolithic precursor hollow fiber membranes.45 Composition of the core spinning dope was identical with the dispersion used for syringe extrusion tests. The bore fluid comprised 90 wt% NMP and 10 wt% water. After removal from the take-up drum, the composite precursor fibers were soaked sequentially in at least four separate water baths for 3 days to remove residual solvents, and then solvent-exchanged with


9


Page 10 of 34

sequential 1 hour baths of methanol and hexane. After air-drying in a fume hood for 1 hour, the fibers were dried in a vacuum oven at 75 °C for ~3 hours.

Table 1. Spinning dope compositions of composite precursor hollow fiber membranes. Sheath dope (wt%)45

Core dope (wt%)

Matrimid®

26.2

25.5

N-methyl-pyrrolidone (NMP)

53

66

Tetrahydrofuran

5.9

N/A

Ethanol

14.9

N/A

Silica particles (US3448 or C803)

N/A

8.5

Component

Table 2. Spinning parameters of composite precursor hollow fiber membranes. Spinning parameter

Value

Dope temperature (oC)

65

Quench bath temperature (oC)

50

Sheath dope flow rate (cc/hour)

18

Core dope flow rate (cc/hour)

180

Bore fluid flow rate (cc/hour)

60

Air gap (cm)

10

Fiber take-up rate (m/min)

5

2.4 Formation of carbon molecular sieve hollow fiber membranes by pyrolysis Monolithic thick-walled CMS hollow fiber membranes without stabilization, monolithic asymmetric CMS hollow fiber membranes with sol-gel stabilization, and composite CMS hollow fiber membranes with silica particle stabilization were formed using the same pyrolysis set-up and heating protocol as described in 2.2.2. Following formation of the CMS hollow fiber membranes,


10

Page 11 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


membrane modules were constructed using the Scotch-WeldTM DP-100 epoxy resin (3M Corporation) based on procedures described elsewhere.46

2.5 Characterizations and gas permeation measurements Morphology of the precursor hollow fibers and CMS hollow fibers was characterized using a LEO 1550 field emission scanning electron microscope (SEM). Single-gas CO2 and CH4 permeation of the monolithic and composite CMS hollow fiber membranes was studied using the constantvolume method under 50 psia feed pressure at 35 oC (vacuum downstream). More details of the single-gas permeation measurements can be found in the literature.47

3. Results and discussion 3.1 Resisting substrate densification by silica particle stabilization Syringe extrusion tests are fast and cost-effective methods to study fiber substrate and skin layer properties without spinning actual hollow fiber membranes, which consumes a much larger amount of materials. Figure 1 shows morphologies of non-hollow Matrimid® precursor fibers made by the syringe extrusion tests, as well as non-hollow CMS fibers pyrolyzed at 550 oC under ultra-high purity Argon. Without silica particles, the porous structure of non-hollow precursor fibers (Figure 1A) was totally densified during pyrolysis (Figure 1B). This is consistent with the results reported in literature using hollow fibers.42 Figure 1 C shows morphologies of non-hollow precursor fibers made with S5505 silica particles. The S5505 silica particles were not effective to resist substrate densification during pyrolysis, and the porous structure was totally densified during pyrolysis (Figure 1D). Figure 1 E & G shows morphologies of non-hollow precursor fibers made with US3448 and C803 silica particles, respectively. It was interesting to see that US3448 and


11


Page 12 of 34

C803 silica particles were very effective to resist substrate densification during pyrolysis, which enabled formation of non-hollow CMS fibers (Figure 1 F & H) with very open and porous structure.

To form carbon molecular sieve, the polymer precursor is usually heated above the polymer¶V glass transition temperature, so the polymer translates from a relatively rigid glassy material to a soft rubbery material with mobile polymer chains. This mobilization can lead to densification of the pore structure.48 The results shown in Figure 1 F & H suggest that silica particles can resist such densification, possibly by inhibiting the motion of the polymer chains as the material translates into the rubbery state. Comparing Figure 1 D/F/H further suggests that a higher silica particle/polymer volume ratio in the precursor fibers may be desirable to achieve effective resistance to densification. The silica particle/polymer weight ratios were identical in the three samples (0.33); however, the S5505 silica particles have much higher bulk density (0.326 g/cm3) than the US3448 (0.056 g/cm3) and C803 (0.07 g/cm3) silica particles. As a result, the silica particle/polymer volume ratio in S5505 silica particle-loaded precursor fibers (1.2) may be much lower than precursor fibers loaded with US3448 (7.2) and C803 silica particles (5.8). This is presumably why S5505 silica particles were not as effective as US3448 and C803 silica particles to resist substrate densification during pyrolysis (Figure 1D); however, we prefer not to speculate at this time.


12

Page 13 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Cross-sectional scanning electron microscopy of syringe-extruded non-hollow Matrimid® precursor fibers and pyrolyzed CMS non-hollow fibers. (A) non-hollow precursor fiber without silica particles; (B) non-hollow CMS fiber without silica particles showing substrate densification; (C) non-hollow precursor fiber with S5505 silica particles; (D) non-hollow CMS fiber with S5505 silica particles showing substrate densification; (E) non-hollow precursor fiber with US3448 silica particles; (F) non-hollow CMS fiber with US3448 silica particles showing resistance to substrate densification; (G) non-hollow precursor fiber with C803 silica particles; (H) non-hollow CMS fiber with C803 silica particles showing resistance to substrate densification.


13


Page 14 of 34

3.2 Formation of composite CMS hollow fiber membranes stabilized by silica particles Since US3448 or C803 silica particles were shown to be excellent stabilizing agents by syringe extrusion tests, they were chosen to form actual composite precursor hollow fiber membranes. As noted in section 2.3.3, the composite precursor hollow fiber membranes comprise both a sheath layer and a core layer. The silica particles were only dispersed in the fiber core layer. Spinning dope compositions and spinning parameters can be found in Table 1 and 2, respectively. The composite precursor hollow fiber membranes were pyrolyzed into composite CMS hollow fiber membranes at 550 oC under continuous flow of ultra-high purity Argon. The S5505 silica particles were not used to form composite hollow fiber membranes as syringe extrusion tests suggest that they are not effective to resist substrate densification during pyrolysis. Morphologies of monolithic precursor hollow fiber membranes without stabilization are shown in Figure 2 A & B. As expected, without stabilization, the asymmetry of the precursor hollow fiber membrane was lost under pyrolysis with totally densified substrate (Figure 2 C & D). As a result, the skin layer thickness of the monolithic thick-walled CMS hollow fiber membrane was identical to fiber wall thickness (~46 P


14

Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Cross-sectional scanning electron microscopy of Matrimid® precursor hollow fiber membranes and CMS hollow fiber membranes. (A) & (B) monolithic precursor hollow fiber membranes without silica particles; (C) & (D) monolithic thick-walled CMS hollow fiber membranes without silica particles showing substrate densification; (E) & (F) composite precursor hollow fiber membranes with US3448 silica particles; (G) & (H) composite CMS hollow fiber membranes with US3448 silica particles showing resistance to support layer densification; (I) & (J) composite precursor hollow fiber membranes with C803 silica particles; (K) & (L) composite CMS hollow fiber membranes with C803 silica particles showing resistance to support layer densification; (M) image showing good mechanical flexibility of composite precursor hollow fiber membranes with US3448 silica particles; (N) image showing good mechanical flexibility of composite CMS hollow fiber membranes with US3448 silica particles.


15


Page 16 of 34

Figure 2 E-H shows morphologies of the composite precursor hollow fiber membranes and composite CMS hollow fiber membranes stabilized by US3448 silica particles. The outer diameter (OD) of the composite precursor hollow fiber membranes was ~400 µm. The support layer of the composite CMS hollow fiber membranes remains highly-open during pyrolysis without densification. This is consistent with the results of syringe extrusion tests shown by Figure 1 F. The composite CMS fiber has a thin and dense skin layer ~ 3-4 P (Figure 2H), which is thinner than the typical skin layer of monolithic asymmetric CMS hollow fiber membranes made by solgel stabilization (~ 5-

P .43 The OD of the composite CMS hollow fiber membranes was ~240

µm. Figure 2 I-L shows morphologies of the composite precursor hollow fiber membranes and composite CMS hollow fiber membranes stabilized by C803 silica particles. It was interesting to see that a dense skin layer was not observed on the composite CMS hollow fiber membranes stabilized with C803 silica particles, and the porous support was exposed at the fiber surface (Figure 2 L). The large C803 particles possibly caused poor adhesion between the sheath and core layer of the composite precursor hollow fibers. This is presumably responsible for detachment of the dense skin layer as a result of large thermal stress during high-temperature pyrolysis. Macrovoids can be seen in the support of the composite precursor hollow fiber membranes, and these macro-voids are translated into the support of the composite CMS hollow fiber membranes. The macro-voids were formed possibly due to slow take-up rates (5 min/min) of the precursor hollow fibers. Previous studies49 suggest that the macro-voids may be eliminated by increasing the precursor hollow fiber take-up rates.


16

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3 illustrates the formation processes of monolithic thick-walled CMS hollow fiber membranes without stabilization, monolithic asymmetric CMS hollow fiber membranes with solgel stabilization, and composite CMS hollow fiber membranes stabilized with US3448 silica particles. Both the sol-gel and silica particles approaches use silica to resist substrate densification during pyrolysis. In the sol-gel method, the silica is formed within the substrate around the polymer support struts by in situ crosslinking VTMS monomers. In addition to crosslinked silica inside the porous substrate, the sol-gel method can lead to formation of a thin porous silica layer on top of the hollow fiber surface causing non-selective mass transfer resistance. In the silica particle method, however, pre-formed silica particles are dispersed in the precursor fiber support in the space between the polymer struts. Since no VTMS monomers are used in the silica particle method, the skin layer of the CMS hollow fiber is free of silica.


17


Page 18 of 34

Figure 3. Schematic representing formation processes of CMS hollow fiber membranes. (a) monolithic thick-walled CMS hollow fiber membranes without stabilization; (b) monolithic asymmetric CMS hollow fiber membranes with sol-gel stabilization; (c) composite CMS hollow fiber membranes stabilized with US3448 silica particles. Color captions: Light blue represents polymer precursor. Light grey represents carbon molecular sieve. Light purple represents in situ formed crosslinked silica or pre-formed silica particles.


18

Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.3 Separation performance of composite CMS hollow fiber membranes stabilized by silica particles Separation performance of the composite CMS hollow fiber membranes was evaluated with CO2/CH4 single-gas permeation at 35 oC. The composite CMS hollow fiber membranes stabilized with US3448 silica particles showed very attractive CO2 permeance (~164 GPU) and CO2/CH4 ideal selectivities (~55). Both US3448 and C803 silica particles were effective to resist substrate densification in the composite CMS hollow fiber membranes. However, the composite CMS hollow fiber membranes made with C803 silica particles had poor CO2/CH4 ideal selectivities (~1.1-1.2) and much higher CO2 permeance (~15000 GPU). These results indicate that the composite CMS hollow fiber membranes made with C803 silica particles were highly-defective. This is clearly consistent with the membrane morphology (Figure 2 L) that no dense skin is formed on the hollow fiber surface. The particle diameter (~ 3.8 µm) of C803 silica is comparable with the compRVLWH KROORZ ILEHU PHPEUDQH¶V VNLQ layer (~ 3-4 µm). As a result, any C803 silica particles embedded at the hollow fiber sheath-core interface may damage the skin layer during hightemperature pyrolysis (Figure 2L). On the other hand, the US3448 silica particles (~ 15 nm) are much smaller and have minimal effects on formation of the dense skin layer. Although some agglomerates were observed for the US3448 silica particles (Figure 2H), they did not compromise selectivities of the composite CMS hollow fiber membranes.


19


Page 20 of 34

Figure 4. Comparing single-gas CO2 and CH4 permeation results (35 oC) of monolithic thickwalled CMS hollow fiber membranes without stabilization, monolithic asymmetric CMS hollow fiber membranes formed by sol-gel stabilization, and composite CMS hollow fiber membranes stabilized by US3448 silica particles. The CMS hollow fiber membranes were all pyrolyzed at 550 o

C using Matrimid® as the precursor material. 1 GPU=3.348×10-10 mol/m2ÂVÂ3D

Separation performance of the composite CMS hollow fiber membranes stabilized with US3448 silica particles was compared with monolithic CMS hollow fiber membranes without stabilization and monolithic CMS hollow fiber membranes with sol-gel stabilization (Figure 4). The CMS hollow fiber membranes were tested for permeation following identical storage history/conditions after membrane formation to minimize the effects of physical aging on performance comparisons. The CO2 permeance (~164 GPU) of the composite CMS hollow fiber membrane was dramatically increased by 580 % over the monolithic thick-walled CMS hollow fiber membranes without stabilization (~24 GPU). The higher permeance of the silica-particle-stabilized fibers reflect prevention of support densification during pyrolysis, resulting in substantially thinner CMS hollow fiber skin layer (~3-4 m vs ~46 m). The CO2/CH4 ideal selectivities (~55) of the composite


20

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CMS hollow fiber membranes were within ~90% of the intrinsic selectivity (~62) measured from monolithic thick-walled CMS hollow fiber membranes without stabilization. Figure 4 also shows separation performance of the monolithic asymmetric CMS hollow fiber membranes made by solgel stabilization. Both the sol-gel and silica particle methods are able to resist support layer densification; however, the composite CMS hollow fiber membranes with silica particle stabilization provide more attractive separation performance under the studied conditions. The composite CMS hollow fiber membranes showed ~80% higher CO2 permeance (164 vs 92 GPU) and ~25% higher CO2/CH4 ideal selectivities (55 vs 44). The higher permeance was partially due to thinner skin layer (~3-4 m vs ~5-6 m) of the composite CMS hollow fiber membranes. For monolithic asymmetric CMS hollow fibers formed by sol-gel stabilization, the precursor hollow fibers were soaked in VTMS solution prior to pyrolysis. Analysis by X-ray photoelectron spectroscopy showed that a thin layer of silica was present on the CMS fiber surface.43 This residual silica layer may add additional non-selective mass transfer resistance to permeation for CO2 vs CH4, thereby reducing both CO2 permeance and CO2/CH4 selectivity.

4. Summary and Conclusions ,Q WKLV ZRUN ZH GLVFXVVHG D ³VLOLFD SDUWLFOH VWDELOL]DWLRQ´ PHWKRG WR FUHDWH composite carbon molecular sieve hollow fiber membranes with attractive separation performance. Thin skin layer was formed in composite CMS hollow fiber membranes by resisting support densification using silica particles dispersed in the support layer of Matrimid® composite precursor hollow fibers. We found that using small-sized silica particles with low bulk density was crucial to form defect-free CMS hollow fiber membranes with excellent resistance to support layer densification. Under


21


Page 22 of 34

single-gas permeation measurements, the composite CMS hollow fiber membranes showed very attractive CO2/CH4 ideal selectivities (~55) and CO2 permeance (~164 GPU) for natural gas purification. It is clear that both the silica particle and sol-gel stabilization methods are viable strategies to overcome support layer densification during pyrolysis, and more attractive separation performance can be expected via optimizing both methods. High pressure CO2/CH4 mixture permeation measurements would be useful to assess the stability of the composite CMS hollow fiber membranes under aggressive natural gas feed conditions.

The silica particle stabilization method could potentially be extended to thermally-rearranged polymers50-51 and other materials that rely on heat treatment to provide good properties. As noted in the beginning of this article, highly attractive separation performance is required to justify the higher production cost of carbon molecular sieve membranes. Clearly, further increase in the separation performance of composite CMS hollow fiber membranes can be enabled by forming the precursor hollow fiber sheath layer using advanced polymer precursors (e.g. 6FDApolyimides35, polymers of intrinsic microporosity41,

52

, etc.). These advanced polymers are

typically more expensive than the commercially-available polyimide (Matrimid®) discussed in this work. Nevertheless, since the sheath layer only comprises a small (less than 10%, Table 2) portion of the entire hollow fiber, increase in membrane material cost is expected to be minimal.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]


22

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Author Contributions Á& = DQG . = FRQWULEXWHG HTXDOO\ to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT W.J.K. acknowledges financial support from Office of Basic Energy Science of the U.S. Department of Energy (Grant DE-FG02-04ER15510). REFERENCES 1.

Koros, W. J.; Lively, R. P., Water and beyond: Expanding the spectrum of large-scale

energy efficient separation processes. Aiche Journal 2012, 58 (9), 2624-2633. 2.

Sholl, D. S.; Lively, R. P., Seven chemical separations to change the world. Nature 2016,

532 (7600), 435-437. 3.

Galizia, M.; Chi, W. S.; Smith, Z. P.; Merkel, T. C.; Baker, R. W.; Freeman, B. D., 50th

Anniversary Perspective: Polymers and Mixed Matrix Membranes for Gas and Vapor Separation: A Review and Prospective Opportunities. Macromolecules 2017, 50 (20), 7809-7843. 4.

Werber, J. R.; Osuji, C. O.; Elimelech, M., Materials for next-generation desalination and

water purification membranes. Nature Reviews Materials 2016, 1, 16018. 5.

Baker, R. W., Membrane Technology and Applications. Wiley: West Sussex, England, UK,

2004.


23


6.

Page 24 of 34

Minelli, M.; Sarti, G. C., Elementary prediction of gas permeability in glassy polymers.

Journal of Membrane Science 2017, 521, 73-83. 7.

Li, S. G.; Falconer, J. L.; Noble, R. D., Improved SAPO-34 membranes for CO2/CH4

separations. Advanced Materials 2006, 18 (19), 2601-2603. 8.

Ekiner, O. M.; Vassilatos, G., Polyaramide hollow fibers for hydrogen/methane separation

² spinning and properties. Journal of Membrane Science 1990, 53 (3), 259-273. 9.

Zhu, L.; Swihart, M. T.; Lin, H., Unprecedented size-sieving ability in polybenzimidazole

doped with polyprotic acids for membrane H2/CO2 separation. Energy & Environmental Science 2018, 11 (1), 94-100. 10.

Baker, R. W.; Low, B. T., Gas Separation Membrane Materials: A Perspective.

Macromolecules 2014, 47 (20), 6999-7013. 11.

Lin, J. Y. S., Molecular sieves for gas separation. Science 2016, 353 (6295), 121-122.

12.

Koros, W. J.; Zhang, C., Materials for next-generation molecularly selective synthetic

membranes. Nat Mater 2017, 16 (3), 289-297. 13.

Kwon, H. T.; Jeong, H.-K., In Situ Synthesis of Thin Zeolitic±Imidazolate Framework ZIF-

8 Membranes Exhibiting Exceptionally High Propylene/Propane Separation. Journal of the American Chemical Society 2013, 135 (29), 10763-10768. 14.

Agrawal, K. V.; Topuz, B.; Pham, T. C. T.; Nguyen, T. H.; Sauer, N.; Rangnekar, N.;

Zhang, H.; Narasimharao, K.; Basahel, S. N.; Francis, L. F.; Macosko, C. W.; Al-Thabaiti, S.; Tsapatsis, M.; Yoon, K. B., Oriented MFI Membranes by Gel-Less Secondary Growth of Sub-100 nm MFI-Nanosheet Seed Layers. Advanced Materials 2015, 27 (21), 3243-3249. 15.

Paul, D. R., Creating New Types of Carbon-Based Membranes. Science 2012, 335 (6067),

413-414.


24

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


16.

Huang, L.; Zhang, M.; Li, C.; Shi, G., Graphene-Based Membranes for Molecular

Separation. The Journal of Physical Chemistry Letters 2015, 6 (14), 2806-2815. 17.

Cacho-Bailo, F.; Etxeberría-Benavides, M.; David, O.; Téllez, C.; Coronas, J., Structural

Contraction of Zeolitic Imidazolate Frameworks: Membrane Application on Porous Metallic Hollow Fibers for Gas Separation. ACS Applied Materials & Interfaces 2017, 9 (24), 20787-20796. 18.

Petropoulos, J. H.; Papadokostaki, K. G.; Minelli, M.; Doghieri, F., On the role of

diffusivity ratio and partition coefficient in diffusional molecular transport in binary composite materials, with special reference to the Maxwell equation. J. Membr. Sci. 2014, 456, 162-166. 19.

Chen, Y. D.; Yang, R. T., PREPARATION OF CARBON MOLECULAR-SIEVE

MEMBRANE AND DIFFUSION OF BINARY-MIXTURES IN THE MEMBRANE. Industrial & Engineering Chemistry Research 1994, 33 (12), 3146-3153. 20.

Rungta, M.; Wenz, G. B.; Zhang, C.; Xu, L.; Qiu, W.; Adams, J. S.; Koros, W. J., Carbon

molecular sieve structure development and membrane performance relationships. Carbon 2017, 115, 237-248. 21.

Singh, A.; Koros, W. J., Significance of Entropic Selectivity for Advanced Gas Separation

Membranes. Industrial & Engineering Chemistry Research 1996, 35 (4), 1231-1234. 22.

Zhang, C.; Koros, W. J., Ultraselective Carbon Molecular Sieve Membranes with Tailored

Synergistic Sorption Selective Properties. Adv. Mater. 2017, 10.1002/adma.201701631 (10.1002/adma.201701631). 23.

Qiu, W.; Zhang, K.; Li, F. S.; Zhang, K.; Koros, W. J., Gas Separation Performance of

Carbon Molecular Sieve Membranes Based on 6FDA-mPDA/DABA (3:2) Polyimide. ChemSusChem 2014, 7 (4), 1186-1194.


25


24.

Page 26 of 34

Salinas, O.; Ma, X. H.; Litwiller, E.; Pinnau, I., Ethylene/ethane permeation, diffusion and

gas sorption properties of carbon molecular sieve membranes derived from the prototype ladder polymer of intrinsic microporosity (PIM-1). Journal of Membrane Science 2016, 504, 133-140. 25.

Rungta, M.; Zhang, C.; Koros, W. J.; Xu, L. R., Membrane-based ethylene/ethane

separation: The upper bound and beyond. Aiche Journal 2013, 59 (9), 3475-3489. 26.

Salinas, O.; Ma, X. H.; Wang, Y. G.; Han, Y.; Pinnau, I., Carbon molecular sieve

membrane from a microporous spirobisindane-based polyimide precursor with enhanced ethylene/ethane mixed-gas selectivity. RSC Adv. 2017, 7 (6), 3265-3272. 27.

Swaidan, R.; Ma, X. H.; Litwiller, E.; Pinnau, I., High pressure pure- and mixed-gas

separation of CO2/CH4 by thermally-rearranged and carbon molecular sieve membranes derived from a polyimide of intrinsic microporosity. Journal of Membrane Science 2013, 447, 387-394. 28.

Steel, K. M.; Koros, W. J., An investigation of the effects of pyrolysis parameters on gas

separation properties of carbon materials. Carbon 2005, 43 (9), 1843-1856. 29.

Ma, X. L.; Lin, Y. S.; Wei, X. T.; Kniep, J., Ultrathin carbon molecular sieve membrane

for propylene/propane separation. Aiche Journal 2016, 62 (2), 491-499. 30.

Ma, X.; Williams, S.; Wei, X.; Kniep, J.; Lin, Y. S., Propylene/Propane Mixture Separation

Characteristics and Stability of Carbon Molecular Sieve Membranes. Industrial & Engineering Chemistry Research 2015, 54 (40), 9824-9831. 31.

Koh, D.-Y.; McCool, B. A.; Deckman, H. W.; Lively, R. P., Reverse osmosis molecular

differentiation of organic liquids using carbon molecular sieve membranes. Science 2016, 353 (6301), 804-807. 32.

Lively, R. P.; Sholl, D. S., From water to organics in membrane separations. Nat Mater

2017, 16 (3), 276-279.


26

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


33.

Zhang, C.; Wenz, G. B.; Mayne, J. M.; Williams, P. J.; Liu, G.; Koros, W. J., Purification

of aggressive supercritical natural gas using carbon molecular sieve hollow fiber membranes. Ind. Eng. Chem. Res. 2017, under review. 34.

Robeson, L. M., The upper bound revisited. Journal of Membrane Science 2008, 320 (1±

2), 390-400. 35.

Fu, S.; Wenz, G. B.; Sanders, E. S.; Kulkarni, S. S.; Qiu, W.; Ma, C.; Koros, W. J., Effects

of pyrolysis conditions on gas separation properties of 6FDA/DETDA:DABA(3:2) derived carbon molecular sieve membranes. Journal of Membrane Science 2016, 520, 699-711. 36.

Liu, J.; Calverley, E. M.; McAdon, M. H.; Goss, J. M.; Liu, Y.; Andrews, K. C.; Wolford,

T. D.; Beyer, D. E.; Han, C. S.; Anaya, D. A.; Golombeski, R. P.; Broomall, C. F.; Sprague, S.; Clements, H.; Mabe, K. F., New carbon molecular sieves for propylene/propane separation with high working capacity and separation factor. Carbon 2017, 123, 273-282. 37.

Kiyono, M.; Williams, P. J.; Koros, W. J., Effect of pyrolysis atmosphere on separation

performance of carbon molecular sieve membranes. Journal of Membrane Science 2010, 359 (12), 2-10. 38.

Wenz, G. B.; Koros, W. J., Tuning carbon molecular sieves for natural gas separations: A

diamine molecular approach. AIChE Journal 2017, 63 (2), 751-760. 39.

Baker, R. W., Future directions of membrane gas separation technology. Industrial &

Engineering Chemistry Research 2002, 41 (6), 1393-1411. 40.

Sanders, D. E.; Smith, Z. P.; Guo, R. L.; Robeson, L. M.; McGrath, J. E.; Paul, D. R.;

Freeman, B. D., Energy-efficient polymeric gas separation membranes for a sustainable future: A review. Polymer 2013, 54 (18), 4729-4761.


27


41.

Page 28 of 34

Jue, M. L.; Breedveld, V.; Lively, R. P., Defect-free PIM-1 hollow fiber membranes.

Journal of Membrane Science 2017, 530, 33-41. 42.

Xu, L.; Rungta, M.; Koros, W. J., Matrimid® derived carbon molecular sieve hollow fiber

membranes for ethylene/ethane separation. Journal of Membrane Science 2011, 380 (1±2), 138147. 43.

Bhuwania, N.; Labreche, Y.; Achoundong, C. S. K.; Baltazar, J.; Burgess, S. K.; Karwa,

S.; Xu, L.; Henderson, C. L.; Williams, P. J.; Koros, W. J., Engineering substructure morphology of asymmetric carbon molecular sieve hollow fiber membranes. Carbon 2014, 76, 417-434. 44.

Ning, X.; Koros, W. J., Carbon molecular sieve membranes derived from Matrimid (R)

polyimide for nitrogen/methane separation. Carbon 2014, 66, 511-522. 45.

Clausi, D. T.; Koros, W. J., Formation of defect-free polyimide hollow fiber membranes

for gas separations. J. Membr. Sci. 2000, 167 (1), 79-89. 46.

Jones, C. W.; Koros, W. J., Carbon molecular sieve gas separation membranes-I.

Preparation and characterization based on polyimide precursors. Carbon 1994, 32 (8), 1419-1425. 47.

Zhang, C.; Zhang, K.; Xu, L.; Labreche, Y.; Kraftschik, B.; Koros, W. J., Highly scalable

ZIF-based mixed-matrix hollow fiber membranes for advanced hydrocarbon separations. AIChE Journal 2014, 60 (7), 2625-2635. 48.

Xu, L. Ph.D. Dissertation, Georgia Institute of Technology, 2012.

49.

Husain, S.; Koros, W. J., Macrovoids in Hybrid Organic/Inorganic Hollow Fiber

Membranes. Industrial & Engineering Chemistry Research 2009, 48 (5), 2372-2379. 50.

Robeson, L. M.; Dose, M. E.; Freeman, B. D.; Paul, D. R., Analysis of the transport

properties of thermally rearranged (TR) polymers and polymers of intrinsic microporosity (PIM) relative to upper bound performance. Journal of Membrane Science 2017, 525, 18-24.


28

Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


51.

Park, H. B.; Han, S. H.; Jung, C. H.; Lee, Y. M.; Hill, A. J., Thermally rearranged (TR)

polymer membranes for CO2 separation. Journal of Membrane Science 2010, 359 (1-2), 11-24. 52.

McKeown, N. B.; Budd, P. M., Polymers of intrinsic microporosity (PIMs): organic

materials for membrane separations, heterogeneous catalysis and hydrogen storage. Chem. Soc. Rev. 2006, 35 (8), 675-683. Table of Contents Graphics


29


Figure 1. Scanning electron microscopy of syringe-extruded non-hollow Matrimid® precursor fibers and pyrolyzed CMS non-hollow fibers. (A) non-hollow precursor fiber without silica particles; (B) non-hollow CMS fiber without silica particles; (C) non-hollow precursor fiber with S5505 silica particles; (D) non-hollow CMS fiber with S5505 silica particles; (E) non-hollow precursor fiber with US3448 silica particles; (F) non-hollow CMS fiber with US3448 silica particles; (G) non-hollow precursor fiber with C803 silica particles; (H) nonhollow CMS fiber with C803 silica particles. 318x480mm (300 x 300 DPI)


Page 30 of 34

Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Scanning electron microscopy of Matrimid® precursor hollow fiber membranes and CMS hollow fiber membranes. (A) & (B) monolithic precursor hollow fiber membranes without silica particles; (C) & (D) monolithic thick-walled CMS hollow fiber membranes without silica particles; (E) & (F) composite precursor hollow fiber membranes with US3448 silica particles; (G) & (H) composite CMS hollow fiber membranes with US3448 silica particles; (I) & (J) composite precursor hollow fiber membranes with C803 silica particles; (K) & (L) composite CMS hollow fiber membranes with C803 silica particles. 424x369mm (96 x 96 DPI)



Figure 3. Schematic representing formation processes of CMS hollow fiber membranes. (a) monolithic thickwalled CMS hollow fiber membranes without stabilization; (b) monolithic asymmetric CMS hollow fiber membranes with sol-gel stabilization; (c) composite CMS hollow fiber membranes stabilized with US3448 silica particles. Color captions: Light blue represents polymer precursor. Light grey represents carbon molecular sieve. Light purple represents in situ formed crosslinked silica or pre-formed silica particles. 163x146mm (300 x 300 DPI)


Page 32 of 34

Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Comparing single-gas CO2 and CH4 permeation results (35 oC) of monolithic thick-walled CMS hollow fiber membranes without stabilization, monolithic asymmetric CMS hollow fiber membranes formed by sol-gel stabilization, and composite CMS hollow fiber membranes stabilized by US3448 silica particles. The CMS hollow fiber membranes were all pyrolyzed at 550 oC using Matrimid® as the precursor material. 1 GPU=3.348×10-10 mol/m2∙s∙Pa. 124x102mm (300 x 300 DPI)



35x15mm (300 x 300 DPI)


Page 34 of 34

Composite Carbon Molecular Sieve Hollow Fiber Membranes

Recommend Documents