10 poly acrylonitrile

ORIGINAL ARTICLE

Development and characterization of polyacrylonitrile

(PAN) based carbon hollow fiber membrane

Syed Mohd Saufi,1

and Ahmad Fauzi Ismail2

AbstractSaufi, S.M. and Ismail, A.F.

Development and characterization of polyacrylonitrile (PAN)

based carbon hollow fiber membraneSongklanakarin J. Sci. Technol., 2002, 24(Suppl.) : 843-854

This paper reports the development and characterization of polyacrylonitrile (PAN) based carbon

hollow fiber membrane. Nitrogen was used as an inert gas during pyrolysis of the PAN hollow fiber mem-

brane into carbon membrane. PAN membranes were pyrolyzed at temperature ranging from 500o

C to

800o

C for 30 minutes of thermal soak time. Scanning Electron Microscope (SEM), Fourier Transform

Infrared Spectroscopy (FTIR) and gas sorption analysis were applied to characterize the PAN based carbon

membrane. Pyrolysis temperature was found to significantly change the structure and properties of carbon

membrane. FTIR results concluded that the carbon yield still could be increased by pyrolyzing PAN mem-

branes at temperature higher than 800o

C since the existence of other functional group instead of CH group.

Gas adsorption analysis showed that the average pore diameter increased up to 800o

C.

Key words : carbon membrane, hollow fiber, gas separation, polyacrylonitrile

1

M.Sc. (Gas Engineering), Lecturer, 2

Ph.D. (Chemical Engineering), Prof., Membrane Research Unit,

Faculty of Chemical Engineering and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310

Skudai, Johor Bahru, Malaysia

Corresponding e-mail : [email protected]

Received, 9 January 2003 Accepted, 10 June 2003

Polyacrylonitrile (PAN)

Saufi, S.M. and Ismail, A.F.

Songklanakarin J. Sci. Technol.

Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech. 844

Membrane technology is becoming moreuseful for separation of gas mixture and offersgreat advantages in its operations (Robeson,1999). Polymeric membranes had been success-fully used for this purpose. However, their perme-ability-selectivity combination is still not up tothe industries satisfaction and their applicationis limited especially related to severe environ-ment such as higher temperature and corrosiveoperation. At present, carbon molecular sievesmembrane (CMSM) has been identified as asolution to this problem (Koros and Mahajan,2000). It offers a balance on permeability-selec-tivity trade off and the separation can be main-tained in the environment that was prohibited bypolymeric membrane previously. Ismail and Davidthoroughly reviewed the advantages of carbonmembrane and the potential application for gasseparation (Ismail and David, 2001).

The interest in developing carbon mem-brane only grow after Koresh and Soffer (Koreshand Soffer, 1987; 1986; 1983) had successfullyprepared apparently crack-free carbon hollowfiber membranes by carbonizing cellulose hollowfibers. CMSM contains narrow constrictions atthe entrance of the micropores that approach themolecular dimensions of gaseous species. Theconstrictions allow the passage of the smallerspecies and restrict the larger ones as ruled ina molecular sieving mechanism (Fuertes andCenteno, 1999). It is believed that CMSM has apore size in the range of 3Å to 6Å (Fuertes andCenteno, 1998) that enhance the discriminationbetween gas molecules of different sizes.

CMSM can be produced by pyrolyzing apolymeric membrane precursor in a controlledcondition and atmosphere. Lately, numerous pre-cursors have been used to form carbon mem-brane such as polyimides and derivatives, poly-acrylonitrile, phenolic resin, poly (vinylidenechloride), poly (furfuryl alcohol), cellulose, phe-nol formaldehyde, polyetherimide and polypyr-rolone. Many researchers used polyimides astheir precursor for producing carbon membrane.Jones and Koros (Jones and Koros, 1994) re-ported that the best carbon membrane, in terms

of both separation and mechanical properties,were produced from the pyrolysis of aromaticpolyimides.

However, polyimides are commercially ex-pensive materials and some can be obtained inlaboratory scale only (Centeno and Fuertes, 1999;Fuertes et al, 1999). Therefore, in order to reducethe cost and time for carbon membrane fabri-cation, other alternative polymer must be consid-ered. Therefore, the objective of this study is toreport the development of PAN based carbonhollow fiber membrane. We have selected PANas a precursor for carbon membrane based onmany reasons as will be discussed in the follow-ing section. PAN is spun using dry-wet spinningprocess to form hollow fiber membrane. Inert gaspyrolysis system is used to pyrolyze the PANmembrane into carbon hollow fiber membrane.

Experimental


PAN is one of the versatile polymers thatis widely used for making membranes due to itsgood solvent resistance property. It has beenused as a substrate for nanofiltration (NF) andreverse osmosis (RO) (Kim et al, 2002). Thethermosetting characteristic offered by PANmakes it suitable as a carbon membrane precur-sor. It usually does not liquefy or soften duringany stage of pyrolysis and preserves its morphol-ogy upon the pyrolysis. The general molecularstructure of PAN is shown in Figure 1.

In carbon fiber manufacturing, PAN isrecognized as the most important and promisingprecursor for carbon fiber. It dominates nearly90% of all worldwide sales (Gupta and Harrison,

Figure 1. Molecular structure of polyacrylonitrile

(PAN)


Vol. 24 (Suppl.) 2002 : Membrane Sci. &Tech.


Saufi, S.M. and Ismail, A.F.845

1999). There are numerous PAN fibers advan-tages including a high degree of molecularorientation, higher melting point (PAN fibertends to decompose before its melting point, T

m

of 317-330oC) and a greater yield of the carbon

fiber (Donnet and Bansal, 1984). PAN fibersform a thermally stable, highly oriented mole-cular structure when subjected to a low tempera-ture heat treatment, which is not significantlydisrupted during the carbonization treatment athigher temperatures, meaning that the resultingcarbon fibers have good mechanical properties(Donnet and Bansal, 1984).

On the other hand, it is possible to blendthe PAN with other polymers to alter the finalpore size distribution of carbon membrane.Linkov et al., (Linkov et al, 1994a) preparedPAN hollow fiber membrane by varying the vis-cosity of the precursor solution by blending itwith methyl methacrylate. They also appliedphase inversion by casting PAN with poly (ethyl-ene glycol) (PEG) and poly (vinylpyrrolidone)(PVP) in order to synthesize membranes with 50-400 nm pore sizes. The pore size distribution wasvery narrow in the case of the PAN-PVP precursor(Linkov, 1994b).

One of the main problems of unsupportedcarbon membrane (i.e. hollow fiber) is brittlenessof the carbon structure. We believe that by theapplication of PAN precursors, which is widelyused in the production of high strength carbonfiber, this problem can be minimized. However, inthis case, first task is to develop PAN-based car-bon membrane. Furthermore, some improvementand optimization must be done in the next stagein order to achieve high performance PAN-basedcarbon membrane.

Preparation of carbon membrane

Dry/Wet spinning process

A binary polymer solution consisting of150g PAN (Aldrich 18131-5) and 850g dime-thylformamide (DMF) was prepared for making1000g of spinning solution. Dry/wet-spinning pro-cess was applied in preparing asymmetric PANhollow fiber membranes precursor. Spinning so-

lution and bore fluids were extruded simultane-ously through a spinneret to form a nascenthollow fiber at an ambient temperature. Theinternal surface of hollow fiber was contactedwith a bore fluid and experienced wet phaseseparation in order to form a circular hollowlumen. Water was chosen as bore fluid with flowrate of lml/min (Pesek and Koros, 1994; Clausiand Koros, 2000), which was controlled by high-pressure syringe pump.

The fiber was then directed to a force con-vective chamber, which provides a controlledforce convective environment for inducing dryphase separation through an air gap. The inert gasnitrogen was used to allow the fiber skin forma-tion. The hollow fiber was then immersed intononsolvent (water) coagulation bath for wetseparation and then the washing treatment bath.The coagulation bath temperature was controlledat 14

oC by a refrigeration/heating unit to ensure

rapid solidification, while the washing bath waskept at ambient temperature.

The resulting hollow fibers had an outerdiameter of 600µm with a 300µm inner diam-eter. The fully formed PAN hollow fiber wassubjected to solvent exchange in methanol for 2days and hexane for another 2 days before beingdry in ambient atmosphere.

Inert gas pyrolysis

Inert gas nitrogen was set up for pyrolysisof PAN hollow fiber membrane as shown inFigure 2. PAN hollow fiber bundles were in-serted into a quartz tube and wrapped with stain-less steel wire type 304 (outside diameter3 mm) at both ends. Then, the quartz tube wasinserted into Carbolite wire wound tube furnace(Model CTF 12/65/550) that can be set to a maxi-mum temperature of 1200

oC. The pyrex socket

was connected at the front of quartz tube tochannel the nitrogen gas during pyrolysis andthe other side of quartz tube was located in a fumecupboard to purge the volatile gas evolved. Allthe connection were properly tighten to preventair from entering the quartz tube which couldinterrupt the inert gas pyrolysis process.





Figure 2. Nitrogen gas pyrolysis system

The PAN membrane was subjected to ther-mostabilization process in air at 250

oC for 30

min at heating rate of 5oC/min before the pyro-

lysis was conducted. Thermostabilization is nec-essary in order to cross-link the PAN chains andto prepare a structure that can withstand hightemperature process. This can also ensure both themolecules and the fibrillar orientation will not beeliminated during the final heat treatment process.

Before the pyrolysis process started, theinert gas needed to be purged into the pyrolysissystem to remove the unwanted air or oxygen.This was to prevent the oxidation from occurringduring high temperature pyrolysis process. Then,the precursor was heated to a required pyrolysistemperature in the range of 500-800

oC and main-

tained at that temperature for 30 min by settingup the temperature control systems. The heatingrate was set at 3

oC/min and nitrogen gas flow rate

was maintained at 200 cm3/min. The resulting

carbon membrane was cooled down to ambienttemperature in an inert gas atmosphere.

Characterization of carbon membrane

Scanning electron microscopy (SEM)

Scanning Electron Microscopy (SEM) hasbeen used to investigate the resultant membranemorphology. Images of fiber surface, skin layerstructure and cross sections of membrane pre-

pared under different carbonization condition canbe viewed clearly. Before passing through SEM,the membrane samples had to go through the goldcoating process. After that, the samples wereimaged and photographed by employing a scan-ning electron microscope (SEMEDAX; XL40:PW6822/10) with potentials of 10kV in achievingmagnification ranging from 500x to 5000x.

Fourier transform infrared spectroscopy

(FTIR)

Fourier Transform Infrared Spectroscopy(FTIR) is a very useful tool to detect the exist-ence of functional groups in a membrane. TheFTIR results display changes of functional groupsand elements in the membranes when they areheated from room temperature to pyrolysis temp-erature.

Gas sorption analysis

Nova 1000 Gas Sorption Analyzer, Quanta-chrome Corporation was used to determine theaverage pore size and micropore volume of car-bon membrane. The sample was pretreated in avacuum at 400

oC for about 2 hours. The heated

sample cell was immersed into liquid nitrogen tomaintain the temperature at 77 K before testing.Nitrogen gas was used as an analysis gas.





Results and Discussion

Effects of pyrolysis temperature on the mor-

phology of PAN-based carbon membrane

Changing the high temperature treatmentparameter can vary the permeation characteristicsof a carbon membrane. This is in contrast topolymeric membranes, where such variationsfrequently involve the synthesis of a new mem-brane material (Koresh and Soffer, 1986). Thisunique characteristic is one of the advantages ofcarbon membrane for gas separation.

From SEM image in Figure 3, the structureof PAN carbon membrane changes greatly bet-ween the inner and outside diameter of the fiber(i.e. middle region) compared to PAN precursor.For the PAN-based carbon membrane, the outerselective skin is clearly separated from the innerwall by the larger macrovoid between it. Mean-while for PAN membrane, the pores from the innerdiameter are likely connected to the outer diame-ter pores. This structure could be a reason for theimbalance of permeability-selectivity in polymericmembrane, where the higher selectivity give thelower permeability to gas separation. On the otherhand, the larger macrovoid presented in carbonmembrane contributes to a higher permeabilitywithout interrupting the higher selectivity. Higher

selectivity is determined by the pore constrictionat the outer surface of the carbon fiber. Therefore,carbon materials not only have the ability to per-form molecular sieving but also may allow aconsiderably higher flux of the penetrant throughthe material (Steel and Koros, 2003).

Pyrolysis temperature will greatly changethe morphology and structure of PAN membrane.For PAN precursor, the membrane must be pyro-lyzed at a minimum temperature of 400

oC (David,

2001). Figure 4 shows the variations of thePAN-based carbon membrane structure with thepyro-lysis temperature ranging from 500-800

oC.

The entire SEM images were at 800X magnifica-tion. It can be observed that the amount of theouter pores skin was increased with the increasingpyrolysis temperature. During the pyrolysis, by-products of different volatilities are evolved andgenerate pores through the polymeric matrix(Steel, 2000). Therefore, as the pyrolysis temp-erature increases the amount of by-products alsoincreases which enhances the pore formation.However, at relatively higher pyrolysis tempera-ture, the pore structure becomes tighter whichgives an increase of gas selectivity (Geiszler andKoros, 1996). Densification of the porous carbonmatrix occurs at high pyrolysis temperature,which can be identified by an increase of shrink-

(b) PAN Based Carbon Membrane(a) PAN Membrane

Figure 3. Cross section of PAN membrane and PAN based carbon membrane





Figure 4. Carbon membrane at a different pyrolysis temperature

age factor as shown in Table 1.

Effects of pyrolysis temperature on the func-

tional groups evolution during pyrolysis

Generally, carbon membrane is known byits amorphous porous structure created by theevolution of gases generated during pyrolysis ofthe polymeric precursor. Though the membrane isamorphous in nature, one still finds carbon mem-brane subdomains, where the structure of thepolymeric precursor is still recognizable. Thissubdomain structure determines, in part, the dif-ferences that are found in the performance ofcarbon membrane derived from various poly-meric precursors. The extent of this subdomainstructure also depends on the pyrolysis conditions(Sedigh et al, 1999).

The mechanism of carbon membrane for-mation from a PAN is a complex process andvarious reaction schemes have been proposed tooccur especially by researchers involved in carbonfiber production. During oxidative stabilization

Table 1. Percentage of shrinkage based on the

variations of inside diameter of the fiber.

Pyrolysis Inside

temperature diameter (µµµµµm)

PAN membrane 300 -CM 500

oC 249 17.0%

CM 600oC 216 28.0%

CM 700oC 202 32.7%

CM 800oC 204 32.0%

% Shrinkage





process, cyclization and oxidation taking place toform a ladder polymer as shown in Figure 5(David and Ismail, 2003). This ladder structureis thermally stable and ready to withstand hightemperature of pyrolysis process.

Pyrolysis of PAN membrane involvesmany process such as chain scission, chain break-down, by-products evolution or cross linking de-pending on heat treatment history. The principalscission products releases from PAN precursor inthe temperature ranging from 400 to 1000

oC are

hydrogen cyanide (HCN), ammonia (NH3) and

nitrogen (N2). Depending on the extent of pre-oxi-

dation, various amounts of water (H2O), carbon

monoxide (CO) and carbon dioxide (CO2) were

also formed. In addition, small quantities of hy-drogen (H

2) and methane (CH

4) were released

during the carbonization process (Lewin andPreston, 1983). Figure 6 shows the proposedmechanism of the stabilization chemistry andcarbonization of PAN-based carbon fiber.

Figure 7 and Figure 8 show the spectrum ofpyrolyzed PAN membrane at different pyrolysistemperatures. Meanwhile Table 2 gives a summa-ry of FTIR results for PAN membrane pyrolyzedusing nitrogen pyrolysis system. The originalPAN molecule consists of functional groups suchas methyl (CH

3) and nitrile (C≡N). During oxida-

tion, numerous new transition structures wereformed, such as ketones, aldehydes, carboxilycacid etc. as in Figure 6. After carbonization, allof these transition compounds were expected toevolve as volatilities and only carbon and hydro-gen atoms remain. However, 100% conversion ofpolymer to carbon is not achieved due to the

Figure 5. Structure of ladder polymer from PAN

Table 2. Summary of FTIR results for PAN mem-

branes pyrolyzed at different temperature

Pyrolysis Functional Frequency

temperature group (cm-1

)

C≡N 2216.42500

oC C-N 1265.90

C=C 1595.42

C≡N 2224.05C=N 1649.36C-N 1265.77

600oC C=C 1539.65

N-H 3445.29C=O (Aldehydes) 1739.53

C=O (Ketones) 1701.79

C=N 1653.23C-N 1021.84

700oC C=C 1540.69

N-H 3442.13C=O (Ketones) 1701.01

C-H 2924.63

C=N 1649.58C-N 1021.39

800oC C=C 1541.42

N-H 3442.97C-H 2923.64

existence of other compounds.The C=C groups exist in all pyrolyzed

PAN membrane. This group is formed by aroma-tization process that occurred during thermosta-bilization of PAN membrane. The nitrile groups(C≡N) disappear when PAN membrane is pyrolyzedat temperature of 700

oC. Aldehydes and ketones

can be detected at pyrolysis temperature of600

oC and aldehydes disappeared when reaching

700oC. A ketone group disappeared at 800

oC

spectrum but nitrogen atom can still be found atthis spectrum. It can be concluded that pyrolysistemperature at 800

oC seems not enough to re-

move all non-carbons atoms (i.e. N, O, etc) inPAN membrane because of the existence of otherfunctional group instead of CH group as detectedby FTIR spectrum.





Figure 6. Proposed mechanism of the stabilization chemistry and carbonization of

PAN based carbon fiber (Fitzer et al, 1986)

Effects of pyrolysis temperature on the porous

structure of carbon membrane

The mechanism of gas permeation and up-take through porous solids is closely related tointernal surface area, dimension of the pores andsurface properties of the solid, rather than to thebulk properties of the solid as in the case withpolymers (Koresh and Soffer, 1987). Therefore, itis important to study and analyze the porousstructure in carbon membrane. The porous struc-ture in carbon membrane contains pore or ap-ertures that approach the molecular dimension ofthe diffusing gas molecules (Jones and Koros,1994; Lafyatis et al, 1991). According to theIUPAC, three different types of pore have beendefined; macropores that are larger than 50 nm;mesopores, between 2 and 50 nm in size; andfinally, micropores, that are smaller than 2 nm.

Generally, the pore size of carbon mem-brane is non-homogeneous, comprised of rela-tively wide openings with a few constrictions(Koresh and Soffer, 1980). The pores are varyingin size and dimensions depending on the mor-phology of the organic precursor and the chem-istry of pyrolysis. The idealized structure ofcarbon material is shown in Figure 9. The poremouth “d”, often referred to as an ultramicropore(<10Å) (Suda and Haraya, 1997; Centeno andFuertes, 1999), which allows molecular sieving ofthe penetrate molecules. The larger micropores,“D” of the material (6-20Å) may allow for diffu-sion of gas molecules through the carbon material(Steel, 2000).

The extent and nature of the porous struc-ture and the surface area will be largely dependenton the nature and morphology of the precursor





Figure 7. FTIR spectrum of pyrolyzed PAN membrane at (a) 500o

C (b) 600o

C

(a) 500o

C

(b) 600o

C

polymer and the history of its heat treatment(Donnet and Bansal, 1984). The imperfect stack-ing between the microfibrils of graphite basalplane in carbon fiber gives rise to empty spacesbetween the microfibrils that form pores orvoids. Generally, low temperature carbon fibershave small but numerous pores that are distri-buted throughout the carbonized material (Donnetand Bansal, 1984).

This is in agreement with the resultsshown in Table 3. It can be observed that theaverage pore diameter increases with increasingthe pyrolysis temperature up to 800

oC due to

the expelling of the carbon atoms as carbon mon-

oxides (David, 2001). However, at a very hightemperature, pore size start to decrease andeventually the pores will shrink and collapseowing to progressive annealing (Suda andHaraya, 1997; Centeno and Fuertes, 1999). TheBET surface area is increased as the pyrolysistemperature reaches the maximum to a value of103 m

2/g, and then decreases rapidly. This trend

also agrees with the results reported by Lee andTsai (Lee and Tsai, 2001).

Conclusion

Based on the results presented, it can be





Figure 8. FTIR spectrum of pyrolyzed PAN membrane at (a) 700o

C and (b) 800o

C

(a) 700o

C

(b) 800o

C

Table 3. Micropores properties of PAN membrane pyrolyzed at different pyrolysis

temperature.

Pyrolysis Average pore Micropore volume BET surface area

temperature diameter (Å) (cc/g) (m2

/g)

500oC 11.19 0.039 108.29

600oC 10.20 0.032 95.47

700oC 12.43 0.040 102.98

800oC 13.13 0.034 85.66





Figure 9. Idealized structure of carbon material

(Steel, 2000)

concluded that PAN membranes can be success-fully pyrolyzed into carbon membrane usingnitrogen gas pyrolysis system. Pyrolysis temp-erature influences the resultant carbon membraneby altering the structure and pore properties of themembrane. FTIR results concluded that the car-bon yield still could be increased by pyrolyzingPAN membranes at temperatures higher than800

oC because of the existence of other func-

tional group instead of CH group. Gas adsorptionanalysis showed that the average pore diameterincreased up to 800

oC. This result encourages

further research on higher pyrolysis temperaturesince it has been hypothesized that the pore dia-meter will shrink at relatively higher temperature.

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10 poly acrylonitrile

Documents

scanning electron

based carbon

pyrolyzed

carbon membrane

temperature

hollow fiber

carbon fiber

functional