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Microporous and Mesoporous Materials 96 (2006) 79–83

Structure and morphology of microporous carbon membranematerials derived from poly(phthalazinone ether sulfone ketone)

Bing Zhang a, Tonghua Wang a,*, Shili Liu a, Shouhai Zhang b, Jieshan Qiu a,*,Zhigang Chen c, Huiming Cheng c

a Carbon Research Laboratory, State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology,

158 Zhongshan Road, P.O. Box 49, Dalian, Liaoning 116012, Chinab Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road,

Dalian, Liaoning 116012, Chinac Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road,

Shenyang, Liaoning 110016, China

Received 17 March 2006; received in revised form 19 June 2006; accepted 19 June 2006Available online 28 July 2006

Abstract

A novel polymeric precursor, poly(phthalazinone ether sulfone ketone) (PPESK), was used to prepare microporous carbon mem-branes by carbonization at 950 �C. The structure and morphology of the microporous carbon membrane materials were characterizedby X-ray diffraction, Raman spectrometry, transmission electron microscopy, scanning electron microscopy and nitrogen adsorptiontechniques. The results illustrate that PPESK is a promising carbon membrane precursor materials, which results in well-developedmicroporosity after carbonization treatment. The pore structure of carbon membranes derived from PPESK consists of two kinds ofpores: ultramicropore centering at 0.56 nm and supermicropore centering at 0.77 nm. Graphitic structure and turbostratic carbon coexistin the as-prepared carbon membranes, of which the interlayer d spacing, the microcrystal size La and the stacking height Lc are 0.357,3.91 and 4.39 nm, respectively. For PPESK, the oxidative stabilization prior to the carbonization is beneficial to the preparation of thecarbon membrane with high gas separation performance, which helps to shift the pore size distribution to smaller pore width and toinhibit the growth of crystallites in the carbon matrix.� 2006 Elsevier Inc. All rights reserved.

Keywords: Carbon membrane; Porous carbon; Stabilization; Carbonization; Microstructure

1. Introduction

Nowadays, membrane technology for gas separationhas made a remarkable progress in various areas includingnitrogen and oxygen enrichment, hydrogen recovery andacid gas removal from natural gas. However, there stillexist some key technical challenges faced by current mem-brane technology to enlarge the market for gas separation.The first and immediate challenge is how to achieve higher

1387-1811/$ - see front matter � 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2006.06.025

* Corresponding authors. Fax: +86 411 88993968 (T.H. Wang), Fax:+86 411 88993991 (J.-S. Qiu).

E-mail addresses: wangth@chem.dlut.edu.cn (T. Wang), jqiu@dlut.edu.cn (J. Qiu).

selectivity for the relevant application with at leastequivalent productivity [1]. Although great efforts havebeen made to improve the gas separation performance ofmembranes in the last two decades, for the conventionalpolymeric membranes, it seems difficult to further improvethe membrane’s performance [1,2]. Moreover, polymericmembrane materials cannot be used in harsh environment.Therefore, strong interests have shifted to look for newmembrane materials for gas separation that could providebetter selectivity, better thermal stability and good chemi-cal inertness such as zeolite and carbon membranes.

Carbon membrane, a kind of microporous molecularsieving carbon materials with a filmy shape, is regardedas one of the most promising candidates for gas separation,

80 B. Zhang et al. / Microporous and Mesoporous Materials 96 (2006) 79–83

which exhibits both high permeability and selectivity owingto its unique pore structure [2,3]. Carbon membrane is pri-marily prepared by carbonizing polymeric films at hightemperature in vacuum or inert atmosphere [4]. This kindof carbon molecular sieving material is globally amor-phous, which is formed by disordered stacking of aromat-ically turbostratic carbon. Dislocations and defects in theorientation of aromatic microdomains result in free volumeand microporosity in the carbon matrix. The microporesare usually considered to be nearly slit-shaped and the poremouth dimensions are close to the sizes of gas molecules,which can effectively distinguish some gas molecules froma mixture [5,6].

Studies have shown that the gas separation performanceof carbon membranes is strongly influenced by the choiceof precursor materials [7,8]. Aromatic polyimides are gen-erally accepted as starting materials for fabricating carbonmembranes with good separation properties. However,their high cost and limitation of commercial availabilityhave hampered their wide use in the preparation of carbonmembranes for gas separation [9]. Therefore, it is necessaryto look for a more suitable precursor to fabricate carbonmembranes that are more attractive commercially.

In our previous study, poly(phthalazinone ether sulfoneketone) (PPESK) has been found to be a very promisingalternative candidate in the preparation of carbon mem-branes for their good gas separation performance [10]. Inaddition, PPESK features lower cost and excellent compre-hensive properties, i.e. good solubility, high char yield andthermal resistance, etc. In order to develop the potential ofthis carbon membrane precursor, it is of very significanceto investigate the microstructure of the PPESK-derivedmicroporous carbon membrane materials in detail. In thepresent work the carbon structure and the pore structureof PPESK-based carbon membranes are mainly addressed.

2. Experimental

2.1. Materials and carbon membrane preparation

The PPESK polymer used in the present study with asulfone/ketone ratio of 1:1 was supplied by Dalian NewPolymer Corporation of China. The typical chemical struc-ture and detailed synthesis procedure of PPESK can befound elsewhere [11].

Freestanding symmetric polymeric membranes wereprepared as below. PPESK was dissolved in N-methyl-2-pyrrolidone (NMP) to form a 15 wt.% solution. After filter-ing and defoaming under vacuum, the solution was cast intoa film form on a horizontal and clean glass plate put on aheating platform at 80 �C for 24 h. Then, the polymericmembranes were placed in a vacuum drier at 100 �C foranother 24 h to further remove the residual solvent. Carbonmembranes were made following two procedures: One wasconducted by directly carbonizing the polymeric PPESKmembranes; the other was performed by an oxidative stabil-ization prior to carbonization. The oxidative stabilization

process was undertaken at 460 �C for 30 min in flowingair at a flowing rate of 200 mL/min. The carbonization con-ditions adopted were: flowing argon of 200 mL/min, heat-ing rate of 1 �C/min, final temperature of 950 �C, and theholding time of 60 min. The carbon membranes made withthe stabilization process was denoted as CM-S, while thecarbon membranes made without stabilization step wasdenoted as CM-D.

2.2. Characterizations

X-ray diffraction (XRD) patterns of membrane sampleswere recorded using D/Max-2400 diffractometer withCuKa (k = 0.154 nm) radiation in a range of diffractionangle 2h from 5� to 60�. Raman spectra were obtained withJobin Yvon Lab Ram HR800 spectroscopy at room tem-perature, and samples were excited using the 632.8 nm lineof a He–Ne laser (1.96 eV). The morphology of carbonmembranes was examined using high-resolution transmis-sion electron microscope (HRTEM) (Philips TECNAIG220) and scanning electron microscope (SEM) (KYKY2800B). The pore size distribution (PSD) of carbon mem-branes was calculated using nitrogen adsorption techniqueby an AUTO SORB-1-MP analyzer at �196 �C.

3. Results and discussion

3.1. Morphology and pore structure of carbon membranes

The morphology of carbon membrane CM-D wasexamined by SEM and HRTEM, of which the results areshown in Fig. 1. The SEM examination, as can be seenfrom Fig. 1a, reveals that no cracks are present in theunsupported carbon membranes, of which the surface isvery dense and uniform. Unfortunately, it is also difficultto find any porous structure due to the relatively low reso-lution of SEM. Nevertheless, the HRTEM could yieldsome information of pore structure of CM-D, as shownin Fig. 1b, in which the ‘‘white’’ shapes looking brightare considered to be the pores of carbon membranes, whilethe ‘‘black’’ regions represent the carbon matrix [12]. Therandomly oriental and intersectant ‘‘white’’ and ‘‘black’’stripes shown in the HRTEM image indicate that the porestructure of carbon membrane is mainly consisted of inter-connective nano-channels that are formed by the disor-dered packing of turbostratic carbon sheets and clusters.The average diameter of nano-channels is estimated toca. 0.5–0.6 nm on the basis of the HRTEM results.

To get more detailed information of pore structure ofcarbon membranes, nitrogen adsorption method was usedto determine the pore size distribution (PSD). As shownin Fig. 2, the pore size of the CM-D membrane measuredby H–K method centers at about 0.62 nm [13], which hasa lasting long tail, indicating the micropore size distribu-tion is multimodel [2]. After decomposing the PSD withtwo hypothetic Gaussian distributions, two pore size distri-butions of ultramicropores and supermicropores, as shown

Fig. 1. Morphology of carbon membranes CM-D: (a) Top view SEMimage inserted with cross-section image and (b) HRTEM image embeddedwith enhanced image for clarity.

B. Zhang et al. / Microporous and Mesoporous Materials 96 (2006) 79–83 81

in Fig. 2, can be clearly observed, in which the peak posi-tions locate at 0.56 and 0.77 nm, respectively, which corre-

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00.00

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Por

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Pore width (nm)

BET surface area: 456.9 cm2/gMax. pore volume: 0.301 cm3/gMedian pore diameter: 0.62 nm

Experimental results Fitting results Gaussian decomposition

Fig. 2. Plot of pore size distribution of carbon membrane material CM-Dmeasured with nitrogen adsorption.

spond to the relatively narrow constrictions and largecavities in the proposed ‘‘bottlenecks’’ pore model. Forcarbon membranes derived from polyimide, Steel et al.reported similar results [2].

The BET surface area and the maximum pore volumefor CM-D are 456.9 cm2/g and 0.301 cm3/g, respectively.These results further evidenced that the CM-D carbonmembranes have well-developed microporosity.

3.2. Carbon structure of carbon membranes

XRD patterns of the microporous carbon membranesand the polymeric precursor PPESK are shown in Fig. 3.A broad peak in the XRD patterns can be seen at 19.4�for PPESK, indicating that PPESK is an amorphouspolymeric material [14]. For the as-prepared microporousCM-D membrane, the peak is shifted from 19.4� to 24.9�owing to the diffraction of (002) plane in graphite. In addi-tion, a new diffraction peak appears at 43.7� for carbonmembranes, which is a fingerprint peak for graphite dueto the (100) diffraction. This implies that graphitic micro-crystals have been formed in the carbon matrix [15]. Theinterlayer spacing of aromatic layers (d002) calculated fromthe (002) peak position by means of Bragg’s equation is0.357 nm, which is close to the value of ideal graphite(0.335 nm). The crystallite stacking height Lc is calculatedto be 4.39 nm, which is obtained from the full width at halfmaximum (FWHM) of the (002) peaks using Scherrer’sequation [16]. The results suggest that the crystallites withgraphitized structure are formed in the microporous car-bons prepared by directly carbonizing PPESK at 950 �C.In other words, the microporous carbons obtained fromPPESK by direct carbonization tend to form an orderedand graphite-like carbon structure.

The XRD technique is a fundamental and necessarymethod for studying the carbon stacking structure, but itis not sensitive for carbon materials with low crystallinityin terms of quantitative evaluation of the stacking structure(i.e. in-plane size La from the (100) diffraction peak). In

10 20 30 40 50 60

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PPESK 0.456 - CM-D 0.357 4.39 CM-S 0.374 2.56

Line

Fig. 3. XRD patterns of PPESK and as-synthesized carbon membranematerials.

800 1000 1200 1400 1600 1800

Gaussian decomposition Raman spectrum Fitting results

G peak

Inte

nsity

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Raman shift (cm-1)

D peak

Fig. 4. Raman spectrum of carbon membrane CM-D.

Fig. 5. HRTEM image of carbon membranes CM-S.

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00.00

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BET surface area: 650.8 cm2/gMax. pore volume: 0.310 cm3/gMedian pore diameter: 0.58 nm

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Fig. 6. Plot of pore size distribution of carbon membrane material CM-Smeasured with nitrogen adsorption.

82 B. Zhang et al. / Microporous and Mesoporous Materials 96 (2006) 79–83

addition, the La value can also be obtained by Ramanspectrometry that is more convenient due to its indirectLa measurement from the intensity of Raman D-band (dis-ordered or amorphous structures) and G-band (graphiticstructures) using an empirical equation, La = 4.4(IG/ID)[17]. With the Raman data, the graphite mole fraction incarbon materials can also be evaluated by an empiricalequation, XG = IG/(ID + IG) [18]. For CM-D carbon mem-brane, the Raman spectrum can be decomposed intotwo Gaussian-type peaks, of which the D-band is at1328.6 cm�1 and the G-band is at 1606.6 cm�1, as shownin Fig. 4, and the calculated crystal size La of CM-Dcarbon membrane is 3.91 nm with the XG value being0.47. The La value of 3.91 nm is close to the La value(4.04 nm) of carbon materials prepared from graphitizablepolyimide at 1300 �C [19], implying that PPESK is moreeasily graphitized in comparison to polyimide. The detailedgraphitization behavior of PPESK membranes will be fur-ther investigated at temperatures over 2000 �C in ourfuture study.

The results presented above have demonstrated thatPPESK is an easily graphitizable polymer, which tends toform ordered graphitic structure. However, the calculatedXG value indicates that the graphitization degree of thePPESK-derived microporous carbon membranes preparedat 950 �C is not very high, suggesting that the amorphousand graphitic structures may co-exist in the carbonmembranes.

3.3. Effects of oxidative stabilization on carbon and pore

structure

The PPESK polymer is a kind of thermoplastic resin,which would go through a melting or softening stage dur-ing carbonization. However, the melting stage is unfavor-able for the preparation of porous carbon materialsbecause it would destroy the homogeneously microporousstructure evolved due to the removal of functional groupsin the form of volatiles or gases during carbonization[18]. An oxidative stabilization process prior to carboniza-

tion is normally recommended to prevent thermoplasticpolymers from melting stage at high temperatures, whichwould form cross-linked structure and help to maintainthe morphology and structure of precursors in the resultantmicroporous carbons [20–22].

Fig. 5 is the typical HRTEM image of carbon mem-brane CM-S prepared from PPESK via the two-step stabil-ization and carbonization process. It can be seen clearlythat for CM-S membrane, no large ordered domain withparallel graphitic sheets is present. The pore structure ofCM-S is almost consisted of the disordered packing ofturbostratic carbon sheets or clusters.

The PSD profile of CM-S (as shown in Fig. 6) is similar tothat of CM-D, but in comparison to the CM-D membrane,the PSD of CM-S (see Fig. 6) has shifted towards smallerpore size (see Fig. 2). The two kinds of PSD in CM-S are stilldiscernable, but it is hard to be decomposed by Gaussiandistribution due to the incompleteness of PSD in the lower

Table 1Gas permeation of carbon membranes from PPESK via stabilization and carbonization (measured at 30 �C)a

Sample code Permeability (Barrerb) Selectivity

H2 CO2 O2 N2 H2/N2 CO2/N2 O2/N2

PPESK 2.02 0.73 0.49 0.38 5.30 1.90 1.30CM-650 102.9–133.8 83.3–86.1 10.2–12.5 0.59–0.67 156.8–172.0 127.5–141.2 14.1–18.5CM-800 44.1–51.7 27.6–34.2 4.4–4.5 0.16–0.18 275.6–278.5 150.4–213.8 23.8–27.5CM-950 5.9–6.4 1.2–2.1 1.3–2.4 0.55–0.65 9.9–10.1 2.1–3.7 2.3–2.9

a Taken from our previous study [10].b 1 Barrer = 10�10 cm3(STP) cm cm�2 s�1 cm Hg�1.

B. Zhang et al. / Microporous and Mesoporous Materials 96 (2006) 79–83 83

range of pore width. Compared with the CM-D membrane,the mean pore width of CM-S becomes smaller and reducesto 0.58 nm, while the BET surface area and maximum porevolume increase to 650.8 cm2/g and 0.310 cm3/g, respec-tively. This suggests that the CM-S membrane have smallerpore width and larger amount of micropores than the CM-D membrane.

The effects of stabilization on the carbon structure canbe inferred by the XRD results shown in Fig. 3. TheCM-S membrane has a relatively larger interlayer spacing(0.374 nm) and lower crystallite stacking height (2.56 nm)than the CM-D membrane, implying that the oxidativestabilization helps to inhibit the growth of microcrystalsduring carbonization and to form carbon membrane withlarger amount of pore structure.

Table 1 lists the primary gas permeation of CM-Sreported in our previous work [10], showing that theCM-S has an excellent gas separation capability. Unfortu-nately, the gas permeability of CM-D is not yet obtainedbecause the large area defect-free carbon membranes aredifficult to be formed because it is easy to cling to the sup-port during the carbonization step. Obviously, an oxidativestabilization process prior to carbonization for PPESK ispreferred in order to fabricate carbon membrane with sat-isfactory gas separation performance.

4. Conclusions

Poly(phthalazinone ether sulfone ketone) (PPESK) isa promising precursor for fabrication of microporouscarbon membranes. The carbon membranes derived fromPPESK possess a large amount of microporosity with twokinds of pore size distributions: ultramicropore centeringat 0.56 nm and supermicropore centering at 0.77 nm. Gra-phitic structure and turbostratic carbon coexist in the car-bon matrix with the interlayer d spacing, microcrystal sizeLa and the stacking height Lc being 0.357, 3.91 and4.39 nm, respectively. Oxidative stabilization process priorto carbonization lowers the mean pore width and increasesthe maximum pore volume, which would be beneficial toenhance the gas permeability and selectivity of carbon

membranes. The stabilization step helps to inhibit thegrowth of crystallites in carbon membranes during carbon-ization, resulting in the increase in the interlayer d spacingand the decease in the microcrystal stacking height Lc.

Acknowledgments

This work was partly financially supported by theNational Natural Science Foundation of China (No.20276008), the National Basic Research Program of China(G2003CB615806), the Program for New Century Excel-lent Talents in Universities of China (No. NCET-04-0274).

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