A comparison of carbon/nanotube molecular sieve membranes with polymer blend carbon molecular sieve membranes for the gas permeation application

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Microporous and Mesoporous Materials 113 (2008) 499–510

A comparison of carbon/nanotube molecular sieve membraneswith polymer blend carbon molecular sieve membranes

for the gas permeation application

Popuri Srinivasa Rao a, Ming-Yen Wey b,*, Hui-Hsin Tseng c, Itta Arun Kumar b,Tzu-Hsiang Weng b

a Environmental Restoration Disaster Reduction (ERDR) Research Center, National Chung Hsing University, Taichung 402, Taiwanb Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan

c Department of Occupational Safety and Health, Chung Shan Medical University, Taichung 402, Taiwan

Received 20 April 2007; received in revised form 7 December 2007; accepted 10 December 2007Available online 23 December 2007

Abstract

A novel polyetherimide (PEI)/multi-wall carbon nanotubes (MWCNTs) composite carbon membrane was prepared by spin coatingmethod using PEI as a precursor. The PEI precursor was also modified by blending with polyvinylpyrolidone (PVP) and its effect on thegas transport property of the resulting carbon membrane was examined. The characteristics of the carbon structures and the gas perme-ation properties of the carbon molecular sieve (CMS) membranes pyrolyzed at 500 �C were investigated. The best performance wasobtained using the PEI/nanotube CMS membrane, where CO2 permeability 1463 Barrer and O2/N2 permselectivity was 24.16 at26 �C. Further the membranes were extensively characterized by field emission scanning electron microscopy (FE-SEM) for surface mor-phology studies and thermal gravimetric analysis (TGA) for thermal properties. X-ray diffraction was employed to characterize the struc-tures. The results indicated that the MWCNTs improve gas diffusivity by increasing the micropore volume, even the MWCNTs were notwell dispersed in CMS membrane.� 2007 Elsevier Inc. All rights reserved.

Keywords: Polyetherimide; Multi-wall carbon nanotubes; Carbon molecular sieve membranes; Gas permeation; Characterization

1. Introduction

Gas separation membranes have been used in a widevariety of applications, such as oxygen or nitrogen enrich-ment, hydrogen recovery, acid gas treatment and naturalgas dehydration [1]. Today, large-scale polymeric mem-brane systems are widely used in different separation pro-cesses. However, inorganic membrane technology israpidly receiving global attention owing to superior separa-tion properties when compared to polymeric membranes.Recently inorganic membranes such as micro and meso-

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.12.008

* Corresponding author. Tel.: +886 4 2285 2455; fax: +886 4 2286 2587.E-mail address: [email protected] (M.-Y. Wey).

porous silica membranes [2], zeolite membranes [3], nano-porous carbon membranes [4], and carbon molecularsieve membranes (CMS membranes) [5] were quickly devel-oped, and they offer outstanding potential for applicationin gas separation. Among these materials, CMS mem-branes have been recognized as attractive gas separationmaterials due to their high flux and selectivity [5–12].CMS membranes are prepared by the carbonization ofsuitable polymer precursor such as polyimide [13–16],poly-acrylonitrile [17], phenolic resin [18,19], polyfurfurylalcohol (PFA) [20,21], etc. In addition, separation proper-ties of CMS membranes can be finely adjusted to a specificseparation task by tailoring the microstructures throughthermal and chemical treatments. The ability to tailor thestructure and properties of CMS membranes holds much

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500 P.S. Rao et al. / Microporous and Mesoporous Materials 113 (2008) 499–510

promise in application of these materials to gas separationproblems.

Now a days researchers are focusing on two approachesin the preparation of CMS membranes i.e., the selection ofa new precursor polymer and the use of blend or compositematerials as the precursor. In general, pyrolysis tempera-ture plays an important role on membrane’s pore structureand consequently on their gas transport properties. It iswell known that microporous (<2 nm) CMS membranescan be prepared by pyrolysis of nonporous dense polymerprecursors. On the other hand, mesoporous (2–50 nm)CMS membranes can be prepared by pyrolysis of polymerprecursors obtained from a polymer blending method andphase inversion technique [15]. The pyrolysis of blendedpolymers led to the creation of two different porous carbonstructures i.e., low carbon yield (thermally labile polymer)and high carbon yield (thermally stable polymer). Conse-quently, larger pores were created, derived from the ther-mally labile polymer in the microporous carbon materialsformed from the thermally stable polymer. Kim et al. suc-cessfully prepared blend microporous CMS membranes ofpolyimide (thermally stable polymer) and polyvinylproli-done (PVP, thermally labile polymer) [22]. They preparedmicroporous membranes that had a high permeabilityand selectivity.

The most popular precursor of the reported CMS mem-branes is related to polyimide due to its high thermal stabil-ity and excellent gas separation performance [23–25].Polyimides are insoluble in organic solvents and are castinto membranes in their polyamic acid form. Polyetheri-mide (PEI) is an exception and dissolves in some solventspermitting direct casting. PVP has been primarily used toincrease casting solution viscosity and to assist other spin-ning parameters for reverse osmosis and pervaporation[4,26]. It has also been used to improve selectivity of vaporpermeation membranes [27]. In addition to that PVPincreases the selectivity in composite membranes by reduc-ing the relative transport rate through large pores ordefects at the membrane surface.

Though great progress has been made in the field ofCMS membranes, it is not uncommon that a strongtrade-off relationship exists between the permeability andselectivity. To solve this challenging task, here we proposea simple strategy to incorporate carbon nanotubes into themembranes that could significantly improve the gas fluxwithout losing the selectivity of membranes. Since the dis-covery of carbon nanotubes in 1991, great interest has beengenerated in talking advantage of their remarkable proper-ties, besides applications in electronic and structural mate-rials. Carbon nanotubes have also been proposed for use inchemical applications, including gas separation, purifica-tion and storage.

In this communication, we report a different approach inpreparation of CMS membranes by incorporating multiwall carbon nanotubes (MWCNT) into the polymerprecursor PEI in order to improve the permeation perfor-mance of membranes. Further the membrane’s perfor-

mance for CO2, O2 and N2 gas molecules is comparedwith PEI and PEI/PVP blend CMS membranes in termsof permeability and selectivity. All the membranes wereextensively characterized by FTIR, FE-SEM, XRD andTGA to evaluate the surface morphology and thermalanalysis of CMS membranes.

2. Experimental

2.1. Materials

PEI (Mw. of repeated unit = 592 g mol�1) and PVP(Mw = 40,000 g mol�1) were used to prepare membranes,purchased from Aldrich Co. MWCNTs manufactured byCVD process (supplied by Desun Nano Company, Tai-wan) whose purity of greater than 95% were used for themodification of a PEI membrane. The MWCNTs were intubular shape composed of six-membered carbon rings,with 10–20 nm of diameter, 5–15 lm of the length and116 m2/g of surface area. Raw MWCNTs were washedwith acid mixture, concentrated sulfuric acid (H2SO4)and nitric acid (HNO3) (3:1 by volume) for surface modifi-cation and easy dispersion in the organic solvents. Onegram of nanotubes was added to 100 ml of the acid mixturein round bottom flask, and refluxed at 80 �C for 4 h. Oncooling the mixture was washed with double distilled wateruntil getting neutrality of water. The MWCNTs were sep-arated from water by filtration and dried under vacuumat 60 �C for further use. Solvent N-methyl-2-pyrolidone(NMP) was purchased from Mallinckrodt Chemicals Co.,USA.

2.2. Membrane preparation

PEI-based CMS membranes were prepared by spincoating technique using porous alumina disk as support.Fifteen percent of PEI polymer solution was prepared bydissolving 15 g of PEI in 85 ml of NMP solvent at 80 �C.Five percent of PVP solution was also prepared by dissolv-ing 5 g of PVP in 95 ml NMP. Both the polymers PEI andPVP were mixed in 3:1 ratio and stirred vigorously for 2 hto form homogeneous polymer solution. Porous a-aluminadisks of average pore size: 0.14 lm, diameter: 2.3 mm,porosity: 40–48%, were used as the membrane support.The supports were coated by PEI and PEI/PVP solutionwith spin technique of 5000 rpm for 15 s. After coating,the membranes with support were kept in Isopropyl alco-hol (IPA)–water (1:1 ratio) coagulating bath for 3 h. Thenthe membranes were dried in an air and subsequently donepyrolysis in horizontal quartz furnace. Fig. 1 represents thestructural representation of PEI and PVP.

1.5 g of modified MWCNTs was dispersed in 98.5 ml ofNMP solvent to prepare carbon nanotube suspension. TheMWCNTs suspension was sonnicated 2 h for a better dis-persion of nanotubes in NMP. One milliliter of this nano-tube solution was mixed with required amount of 15% PEIpolymer solution in 1:40 ratio and stirred constantly for 3 h

Fig. 1. Chemical structure of: (a) polyetherimide (PEI), and (b) poly(vinylpyrolidone) (PVP).

P.S. Rao et al. / Microporous and Mesoporous Materials 113 (2008) 499–510 501

at 80 oC. Thus the prepared PEI/MWCNTs dispersion wascoated on an alumina support by above procedure. Thecoated membranes were kept in IPA–water bath. Themembranes obtained by above process were carbonizedusing high temperature tube furnace.

2.3. Carbonization procedure

Membranes were pyrolyzed under vacuum (<0.05mm Hg) in a quartz tube furnace. A schematic of the pyro-lysis furnace was shown in Fig. 2. A 2 in. ID, 33 in. longquartz tube with a glass end cap was used for pyrolysis ina high temperature muffle furnace (supplied by C.F. ElectricHeating, Taiwan) equipped with four heating elements tominimize the axial and radial temperature gradients. Thesamples were kept in middle of the quartz tube whichresided in a furnace. The quartz tube was sealed with a glassend with a stopcock and a Viton O-ring, held by a pinch-type clamp. ANC-602 Programmable temperature control-

Fig. 2. Schematic representatio

ler was used to control the precise temperature–time proto-col. A standardized protocol was followed carefully toobtain the carbonized membranes. The tube was evacuatedto pressures of 0.05 mm Hg, as measured by McLeod’sgauge on the furnace, prior to starting the heating cycle.At this point, the valve connecting the vacuum gauge tothe tube was closed and the pyrolysis cycle was started. Apyrolysis temperature programme consisting of two stepswas used for the preparation of the CMS membranes. Inthe first step, the precursor in a tubular furnace was heatedto 150 �C for 3 h (soaking time is 1 h) with the heating rate1 �C min�1 to remove excess solvent under air stream of50 ml min�1. In second step, the temperature was increasedto 500 �C for carbonization with heating rate 0.5 �C min�1

and hold at final temperature for 1 h under vacuum. Thefurnace was allowed to cool gradually to room temperaturebefore samples were removed from the furnace and placedin a dessicator. Thus, the obtained carbon membranes wereused as CMS membranes for gas permeation.

n of carbonization set-up.


2.4. Gas permeation measurement

The permeation characteristics of the CMS membraneswere measured by a system showed in Fig. 3. A standardvacuum time-lag method was used to measure the singlegas permeabilities at a feed pressure of 152 cm Hg. Allmeasurements were carried out at 26 �C. CO2 (0.33 nm),O2 (0.346 nm) and N2 (0.364 nm) were tested as penetrantsusing Millipore 47 mm disc gas permeation cell; the valuesin the parentheses are the kinetic diameter of the gas. Forthe permeation experiments, the purity of used gases wasgreater than 99.99%. The CMS membranes were maskedusing impermeable aluminum tape with a predeterminedarea (4 cm2) and then epoxy sealing was applied at theinterface between the tape and the CMS membranes toinhibit any gas leak. Leak detection tests were performedin all chambers prior to any experiments. Each time themembrane changed or replaced, it was exposed to vacuumfor 12 h before performing permeation experiments. Thedriving force for the system was provided by a turbomolecular vacuum pump (model GVD-050A, purchasedfrom ULVAC Inc., JAPAN).

The pressure was measured by using pressure sensorsand recorded with the help of Visidaq Builder computersoftware. Both the feed and permeate sides of the mem-brane cell were evacuated (<10�5 Torr) prior to each mea-surement. The permeability coefficients were expressed inBarrer (1 Barrer = 1 � 10�10 cm3 (STP) cm cm�2 s�1 cmHg�1).

Hence the permeation rate (P) can be calculated by

P ¼ dpdt

� �V :T 0

A:DP� L

T :P 0

ð1Þ

Fig. 3. Schematic laboratory set

where V is the volume of gas permeation in unit time (cm3),dp/dt is the rate of pressure rise in the steady state, DP isthe pressure difference in the membrane side, A representsarea of the membrane (cm2) and L represents the mem-brane thickness (cm), P0 being 76 cm Hg and T0 being273 K, and T is the measured temperature (K).

In the case of pure penetrant gas, the ideal separationfactor of pure gas A/B (aA/B) is defined as the ratio of per-meation rate of A to that of B, which can be expressed by

aA=B ¼ P A=P B ð2Þ

2.5. Characterization of carbon membranes

2.5.1. Field emission scanning electron microscope (FE-SEM)

SEM photographs were taken with JSM 5600 ScanningElectron Microscope (Japan) to examine the morphologyand surface structure of the CMS membranes at therequired magnification at room temperature. The mem-branes were deposited on brass hold and sputtered with athin coat of gold under vacuum. Acceleration voltage usedwas 20 kV with the secondary electron image as a detector.

2.5.2. Fourier transform infrared (FTIR) studies

FTIR spectra of PEI membrane and its carbonizedforms were recorded on a JASCO-4100 FTIR-ATR spec-trometer over the wave range of 4000–400 cm�1.

2.5.3. Thermogravimetric analysis (TGA)

The weight loss of polymer precursor during pyrolysiswas characterized by thermogravimetric analysis with aTGA Seiko SSC 5000 thermogravimetric analyzer. The

-up of gas permeation unit.


analysis was carried out with a ramp of 10 �C min�1 at thetemperature ranging from 50 to 600 �C. The purge gas N2

and its flow rate was controlled at 50 ml min�1.

2.5.4. Wide-angle X-ray diffraction (WAXD) analysis

X-ray diffraction analysis was performed to qualitativelymeasure the ordered dimensions and interchain spacing ofcarbon membranes with Rigaku RU H3R (Japan) X-raydiffractometer at room temperature. The d-spacing valuesare interpreted as the average chain spacing in a CMSMmatrix. Accordingly a small piece of sample film was firstsecured onto a holder. The measurement was completedin a scan range of 2h = 20–60o with step increment of0.02o. Ni-filtered Cu Ka radiation with a wavelength ofk = 1.5418 A was used in the experiments. The average d

spacing was determined on the basis of Bragg’s law

nk ¼ 2d sinh ð3Þwhere d is the dimension spacing, h is the diffraction angle, ı

is the X-ray wavelength, and n is an integral number(1,2,3,...).

3. Results and discussion

3.1. FE-SEM of carbon membrane

The structure of the carbon membranes were examinedby means of a FE-SEM. Fig. 4 shows the images of topview and cross-sections of the carbon membranes fabri-cated according to the conditions listed in Sections 2.2and 2.3. As can be seen with PEI as precursor, PEI-basedcarbon membrane (Fig. 4a) has some microporous struc-ture on the surface of the carbon layer; however, compar-ing to cross-section of PEI-based carbon membrane(Fig. 4e), the porous structure was not penetrate insidethe carbon layer. The results show that, with high amplifi-cation ratio, it is easy to found that there are some defectsbut only on the surface of the carbon membrane; therefore,the PEI-based carbon membrane still have the molecularsieving ability for gases. While blending PEI with PVP,due to the different thermal properties, it can be found thatthe size of microporous structure was reduced (Fig. 4b).The Fig. 4c shows a top view of PEI/nanotube-based car-bon membrane. White spots indicate the nanotubes andblack surface indicate the carbon domains. This gives aclear image of nanotube in the CMS membrane matrix.The results show that, when carbon nanotube was incorpo-rated into the PEI solution and after carbonization, thecarbon layer and carbon nanotube have a good contactand reduce the pore size also.

SEM microphotograph of cross-section of supportedcarbon molecular sieve membranes obtained after carbon-ization of a supported PEI-based film is shown in Fig. 4e–g. Two different parts can be distinguished in the SEMimages, the carbon layer and the macroporous aluminasupport. All of the carbon films are dense layer having asymmetric structure and it has a thickness around 3.9 lm

(for PEI), 3.5 lm (for PEI/PVP) and 11.2 lm (for PEI/nanotube), respectively.

3.2. FTIR spectra analysis

Fig. 5 shows the typical FTIR spectra of PEI, PEI/PVPblend membranes of both polymer and carbonized mem-branes along with PEI/nanotube composite membrane.Fig. 5a shows a prominent peak at 1107 cm�1 indicatethe acetal (C–O–C) linkage of PEI precursor. Peaks aroundat 1780 and 1371 cm�1 indicating characteristic of C@Ostretching and C–N stretching were observed (Fig. 5b).The sharp peak appeared at 1647 cm�1 was due to aro-matic C@C group (Fig. 5b). These peaks are elongated inFig. 5b indicates the blending of PEI with PVP. All thesepeaks were disappeared in all CMS membranes (Fig. 5c–e) after pyrolysis at 500 oC. This gives the support of car-bonization of all the membranes with PEI precursor.

3.3. Thermogravimetric analysis

Weight change during the heat treating of PEI, PEI/PVP and PEI/MWCNTs membranes were evaluated bythermogravimetric and differential thermogravimetric anal-ysis curves in Fig. 6a and b. Fig. 6a shows the weight losscurves of the samples under N2 atmosphere in the range150–600 oC with heating rate of 10 oC min�1. From theweight loss curve of PEI, it can be observed that the PEIbegins weight loss at 440 oC and followed by final decom-position that began slowly round 535 oC after heating pro-cess. From the TGA data, PEI showed a major weight lossof 25.3% between 440 and 535 �C and total weight loss at500 �C was 20.1%. The curve of PEI/PVP exhibits twomajor weight loss stages at temperatures about 393 and436 �C followed by the final decomposition at 540 �C.Thermal decomposition of organic polymers is character-ized by the breaking of main chains [28] and different ther-mal degradation routes of PEI and PVP may lead to thecreation of different porous CMS membrane structure. Inthe case of PEI/PVP, the first weight loss-rate peak wascaused by the breaking of PVP and the second one wasdue to the splitting of main chain. As shown in Fig. 6a,PEI was stable up to 540 �C, while PVP decomposed max-imum when the temperature reached to 440 �C. The ther-mal decomposition behavior of PEI and PEI/PVP was ingood agreement with the results reported by Kim et al.[22]. Major weight loss observed in PEI/PVP blend was27.6% in between 390 and 540 �C. PEI/MWCNTs mem-brane shows a one-step weight loss at temperature 480 �Cand final decomposition begin around 535 �C as shown inFig. 6a. The major weight loss of PEI/MWCNTs mem-brane was 39.5% at 535 �C and 21.3% in between 480and 535 �C. An increase in the thermal degradation tem-perature is observed in the PEI/MWCNTs compositemembrane. The decomposition of all membranes beginsaround 400–450 �C and the sample weight decreases signif-icantly after the temperature 480 �C. This was due to the

Fig. 4. FE-SEM microphotographs of membranes: top view of: (a) PEI-based, (b) PEI/PVP-based, (c) PEI/nanotube-based carbon membrane, and (d)nanotube embedded in carbon matrix; cross section of (e) PEI-based, (f) PEI/PVP-based, (g) PEI/nanotube-based carbon membrane, and (h) nanotubeembedded in carbon matrix.


evolution of CO, CO2 and CH4, which caused a cleavage ofbenzene ring of the PEI precursor. The different weight lossbehaviors of PEI, PEI/PVP and PEI/MWCNTs mem-

branes, as observed in the TGA data, provide the possibil-ity to get CMSM with different pore structures during thepyrolysis.

Fig. 5. FTIR spectra of polymeric membranes: (a) PEI, (b) PEI/PVP, and CMS membranes (c) PEI, (d) PEI/PVP, (e) PEI/nanotube.


The differential thermal analysis (DTA) of PEI, PEI/PVP and PEI/MWCNTs membranes with temperaturewere shown in Fig. 6b. The maximum weight-loss ratesoccur at temperatures from 430 to 560 �C. PEI was morethermally stable and decomposes at higher temperaturethan PVP as shown in Fig. 6b. The differentiated weightloss curve of the PVP showed a maximum at 423 �C. Thedifferentiated weight loss curve of the PEI showed a maxi-mum at 532 �C. The PEI/MWCNTs membrane decom-posed slowly and showed a maximum differential weightloss at 540 �C. However, the peak intensity was shifted toright side after incorporating nanotubes into the membranematrix indicates the increase of thermal stability of themembranes. The distinct variation in the intensities of theDTG curves was an indication of the weight loss to heatingrate during the pyrolysis process. Pyrolysis at high temper-

ature with low heating rate and long soak time is thus nec-essary to minimize the pores which are amenable to gaspermeation. However, there is a need to diminish the pos-sibility of the formation of cracks in the resulting carbonmembrane at higher temperature.

3.4. X-ray diffraction (XRD)

XRD can be used as a qualitative measure of the interchain spacing differences between carbon materials. Car-bon materials with different regular, ordered dimensionsand amorphous morphologies should exhibit differentXRD spectra profiles. Fig. 7 shows the XRD patterns ofthe carbon membrane derived from PEI, PEI/PVP andPEI/MWCNTs composite carbon membranes. The shiftof 00 2 peak in the profiles with different membranes was

0

40

80

120

160

200

150 200 250 300 350 400 450 500 550 600 650

Temperature (oC)

ug/m

in.

PEI

PEI/PVP

PEI/nanotube

0

10

20

30

40

50

60

70

80

90

100

Wei

ght r

emai

n (%

).

PEI

PEI/PVP

PEI/nanotube

PEI

PEI/PVPPEI/nanotube

a

b

PVP

PEI

PEI/nanotube

Fig. 6. (a) TGA and (b) DTA curves of PEI, PEI/PVP and PEI/nanotube membranes.


examined and the average values of the interplanar spacing(d0 0 2) were estimated. The d-spacing can be visualized asthe average spacing between the centers of the chains inthe molecular matrix and it is calculated using Bragg equa-tion. For traditional carbon membranes, a broad weak(002) peak is present, which can be attributed to the tur-bostratic structure with randomly oriented graphitic car-bon layers. It can be seen that the spectrum of the PEIshows a broad reflection at 2h being 19.97� (d-spacing being3.39 A) and a sharp reflection at 44.34� (d-spacing being2.04 A) which is the same d-spacing as the (100) plane ingraphite. The spectra of PEI/PVP CMSM is relatively dis-persed and diffused in comparison to that of PEI CMSM.The reflection at 2h being 17.88� (d-spacing being 3.43 A)can be observed for PEI/PVP CMSM, while reflection at

24.96� (d-spacing being 3.68 A) for PEI/nanotube compos-ite CMSM. This result demonstrates the carbon–carbonspacing, which is recognized by d-spacing in graphiteplanes. Hypothetically, the conjugate aromatic graphiticstructure forms and CMSMs exhibit the graphite natureof high selectivity. The appearances of weak peak reflectionat 2h being 44.34� (d-spacing being 2.04 A), 42.35� (d-spac-ing being 2.06 A) and 43.98� (d-spacing being 2.05 A) forPEI, PEI/PVP, and PEI/MWCNTs CMS membranes,respectively, represent the existence of carbon–carbonspacing on graphitic planes, which indicates that the mate-rial has rigid aromatic graphitic planes with more orderedand better packed structure [22]. The addition of the nano-tubes into the carbon membrane reveals slightly differencesin the d-spacing value when pyrolyzed at the same condi-

Inte

nsity

(c)

(b)

(a)

0 10 20 30 40 50 602 Theta

Fig. 7. XRD pattern of: (a) PEI, (b) PEI/PVP and (c) PEI/nanotube membranes.


tion. The interlayer distance can be considered as a diffu-sional path for gas molecules through the carbon mem-branes, which is helpful to evaluate the microstructure ofthe carbon membranes [29]. From the XRD data, the d-spacing values are increasing from PEI to PEI/PVP andagain increase from PEI/PVP to PEI/MWCNTs.

3.5. Gas permeation properties of the PEI-based and PEI/

PVP-based CMS membranes

Gas permeation properties were carried out to investi-gate the molecular sieve capabilities of the PEI, PEI/PVPCMS membranes by measuring the pure component gaspermeability through the membranes using CO2, N2 andO2 gases. The main aim of this work was to study the influ-ence of membrane blending on the gas permeation proper-ties of CMS membranes. The precursor PEI exhibited apermeability that increased in the order CO2 > O2 > N2.In addition, the selectivity of CO2/O2, O2/N2 and CO2/N2 increased through the membranes in the order CO2/N2 > CO2/O2 > O2/N2. The permeation flux of CMS mem-branes decreased and the selectivity increased with increas-ing molecular size. The permeability of gas species for thecarbon membranes derived from PEI/PVP blend and theresults were compared with those of the PEI derived car-

Table 1Gas permeabilites and permselectivities measured at 26 �C for the PEI, PEI/P

Membrane Permeability (Barrer)

P(CO2) P(O2) P(N2)

PEI 52.8 11.6 3.0PEI/PVP 63.8 21.4 4.6PEI/nanotube 1463.0 723.7 30.0

bon membrane pyrolyzed at 500 �C (see Table 1). Consid-ering increase in the gas permeability for the carbonmembranes with PVP, the permeability of gases showedrelatively higher values compared with PEI membrane.From XRD results, this is probably due to increase in porestructure of the membranes after adding PVP, however, thepermeability notably increased up to about 5–8 times com-pared with the PEI derived carbon membrane. It isassumed that the PEI carbon membrane prepared withPVP provided an enhanced diffusional pathway for thegas species in the domain of the thermally labile polymer[30]. Compared to the PEI CMS membrane, the selectivi-ties of CO2/O2, O2/N2 and CO2/N2 gases of the PEI/PVPmembranes either were the less reduced or were increased.Fig. 8 shows the typical gas permeability of the PEI/PVPmembrane. The gas permeabilities of the selected gaseswere in the order CO2 > O2 > N2. Generally, the perme-abilities of smaller gases through microporous membranessuch as CMS membranes are concomitant with the order ofthe kinetic diameter of gas molecules. The correlation ofthe permeability of gases through the PEI/PVP membraneswith their kinetic diameters indicates that the permeationrate was primarily dependent upon diffusion, and thussize-dependent selectivity dominates the separation proper-ties of the PEI/PVP membranes.

VP and PEI/nanotube carbon membranes

Selectivity

P(CO2)/P(O2) P(CO2)/P(N2) P(O2)/P(N2)

4.5 17.5 3.93.0 13.7 4.62.0 48.8 24.2

1

10

100

1000

10000

3.25 3.3 3.35 3.4 3.45 3.5 3.55 3.6 3.65 3.7

Kinetic gas diameter (oA)

Perm

eabi

lity

× 1

0-10 (c

m3

(ST

P) c

m c

m-2

s-1 cm

Hg-1

) PEI

PEI+PVP

PEI+Nanotube

CO2

O2

N2

Fig. 8. Gas permeability vs. kinetic diameter of gas molecules.


For polymers, permanent gases permeate throughmicropores called as free volumes. Since the microporesizes in polymer are very small, fluctuations due to ther-mally induced vibrations can significantly affect averagemicropore size and hence selectivity. The frequency andamplitude of these fluctuations can be affected by the rigid-ity of the polymer chains. On the other hand, CMS mem-branes not only have the ability to carry out molecularsieving but also may allow considerably high gas perme-ability through the material. This is because the structureof the CMS membranes consists of many pores with slit-like shapes and the CMS membranes have larger pore sizedistribution. Hence, the gas permeabilities of CMS mem-branes are higher than those of polymeric membranes(although the d-spacing of polymers is generally larger thanthose of CMS materials). However, the difference in d spac-ing among CMS membranes can give useful informationon the gas permeability and selectivity of CMS membranes.

3.6. Gas permeation through PEI/carbon nanotube-based

CMS membranes

Pure gas permeation rates of CMS membrane of PEI/nanotube for single gases are shown in Table 1. It can beseen that the permeation rate of CO2, O2, and N2, increasesafter incorporation of nanotubes into the carbon mem-brane. Correspondingly, O2/N2 selectivity enhanced from3.9 to 24.2 according to Table 1. Similar enhancement isobserved for the selectivity CO2/N2, after nanotubes beingadded in PEI solution. The permeabilities of all gases showthat the permeation rates through these membranes arecorrelated with the kinetic diameter instead of the molecu-

lar weight of gas molecules and decrease as the molecularsize of gas increase (see Fig. 8). The fact that much higherseparation factors than those expected from Knudsen diffu-sion are achieved confirms that the pores of the preparedcarbon membrane are of molecular dimensions. The aboveresults indicate the appearance of a molecular sieving effectin CMSM. The permeability of CMSM increases with theaddition of nanotubes to the precursor, which indicatesthat the final carbon structure form more porous or lesscompact structures in the carbon matrix (as shown inFig. 7).

The possibility of using carbon nanotubes as membranefiller for gas separation has been recognized for some time.The first examinations of this idea used for moleculardynamics simulations of gas transport inside single wallednanotubes [31,32]. These simulations predicted that thetransport of gases inside nanotubes is orders of magnitudefaster than in any other known materials with nanometerscale. The rapid transport rates exist because the walls ofnanotubes are much smoother than other materials. Theseexperiments show that carbon nanotube membranes canhave spectacularly high fluxes. In the present case, PEI/MWCNTs CMS membrane also increases the permeabilityof single gas molecules rapidly compared to other twoCMS membranes. Interestingly and strikingly, the trade-off relationship between the permeability and selectivity isnot obvious after adding nanotubes. In the present study,the XRD results indicate that the remained substitutednanotubes between the interlayer of carbon matrix providethe enhanced suitable interlayer spacing for separating gasmolecules with similar size. These phenomena improvedthe gas permeabilities and selectivities comparing with their

Table 2Comparison of PEI/nanotube CMS membrane performance with the literature

Membrane CT (�C) a PT (�C) b Permeability (Barrer) Selectivity References

PCO2 PO2 PN2 PCO2/ PN2 PO2/ PN2

PI/PVPc 550 25 – 630.0 – 20.0 10.0 [22]PPOd 650 25 218.2 54.7 – – 11.4 [29]TMSPPOe 650 25 529.0 125.0 – – 10.0 [29]Kapton PIf 900 � 0.8 0.3 0.1 60.9 19.7 [33]6FDA/PMDA–TMM DA-co PIg 550 35 187.3 45.0 11.7 – 3.8 [34]P84h 600 25 276.4 72.2 7.8 – – [35]P84-MAg2 700 25 284.9 77.0 8.4 – – [35]Kapton PI 600 35 1820.6 383.1 – 22.2 4.7 [36]Polypyrrolone 500 35 2970.7 674.0 – 40.0 9.1 [37]PEI/nanotube 500 26 1463.0 723.6 30.0 48.8 24.2 Present work

a Carbonization temperature.b Permeation temperature.c Polyvinylpyrolidone.d Polyphenylene oxide.e Trimethyl polyphenylene oxide.f Polyimide.g Copoly(4,40-methylenebis(2,6-dimethyl)-2,2-bis(3,4-dicarboxylphenyl) hexafluoro propane/pyromellitic) diimide.h Commercial polyimide.


precursors. The results presented in Table 2 are promisingin terms of permeability and selectivity compared with theresults of organic and traditional carbon membranes pre-sented by various researchers [22,29,33–37].

Fig. 9 shows the trade-off relationship between the O2

permeability and the O2/N2 selectivity of the CMS mem-branes together with the reference data [29,36,38]. In thispresent study, We prepared CMS membranes successfullyby incorporating the nanotubes and also polymer blendmethod to permeate of small diameter gas molecules(CO2, O2 and N2) and their performance was improved

1

10

100

1 10 1

Oxygen permeability×10-10 (cm

O2/

N2

Per

mse

lect

ivity

PEI

PEI+PVP

PEI+nanotube

Polymeric membrane upper boun

Fig. 9. O2 permeability and O2/N2 selectivity

when compared to PEI CMS membrane. The CMS mem-branes derived from polyetherimide containing a thermallylabile polymer, which exhibited microphase separationbehavior, are superior candidate materials for improvingthe gas permeation performance. On the other hand,PEI/MWCNTs carbon membrane prepared in this studyexhibited an excellent performance comparable to that ofthe carbon membranes derived from commercial Kaptonpolyimide [36]. In addition, the performance of this mem-brane exceeded the upper bound of the conventional poly-meric membranes [38].

00 1000 10000

3 (STP) cm cm-2 s-1 cmHg-1 )

Kapton Polyimide membrane (36)

d (38)

of CMS membranes derived from PEI.


4. Conclusions

PEI/MWCNTs composite membranes with excellent gasseparation performance were prepared by incorporatingmulti-wall carbon nanotubes into a PEI polymer matrix.The results of this membrane were compared with PEIand PEI/PVP blending CMS membranes. Although theSEM images of the synthesized membranes suggested thatthe composite membranes were not uniform due to thenanotubes are not well dispersed. The gas permeabilitywas still increased drastically after adding MWCNT tothe polymer precursor due to the abundant microchannelsbrought by nanotubes. Therefore, MWCNTs offered afavorable effect on increasing gas permeability by decreas-ing the gas diffusion resistance. We are convinced that thenovel membrane material presented here has great poten-tial for application in gas separation.

Acknowledgments

The authors are thankful to the Environmental Restora-tion Disaster Reduction Research Center (sponsored byMinistry of Education, Taiwan), National Chung HsingUniversity for providing fellowship to carry out experimen-tal work in Department of Environmental Engineering.The authors gratefully acknowledge the financial supportfrom National Science Council (NSC 95-2221-E-040-007-).

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