Microporous and Mesoporous Materials 26 (1998) 23–26

Carbon molecular sieve membranes from polyetherimide

A.B. Fuertes *, T.A. CentenoInstituto Nacional del Carbon, CSIC, Apartado 73, 33080-Oviedo, Spain

Received 16 March 1998; received in revised form 8 June 1998; accepted 19 June 1998

Abstract

A carbon molecular sieve membrane with a high gas permselectivity for permanent gas pairs(O2–N2, N2–He, CO2–CH4) is obtained by carbonization of a commercial polyimide (polyetherimide). The membraneis obtained in only one casting step, and it is constituted by a thin microporous carbon film of around 3 mm inthickness which is supported on a macroporous carbon substrate. © 1998 Elsevier Science B.V. All rights reserved.

Keywords: Carbon molecular sieve; Gas separation; Membrane; Polyetherimide

1. Introduction sors of CMSMs are either very expensive commer-cial materials or available only on the laboratoryscale. In this context, one polyimide-based materialCarbon molecular sieve membranes (CMSMs)

are very promising materials in the field of gas which can be used economically is a polyetherim-ide, which is manufactured by General Electricseparation, both in terms of separation properties

(permeability and selectivity) and stability (ther- ( UltemA 1000). This material has been used toprepare polymeric membranes [8–10] because ofmal and chemical ). CMSMs produced by pyrolysis

of different polymeric materials (polyimide-based, its superior strength and chemical resistance.The main objective of this paper is to evaluatePVDC, phenolic resin, polyfurfuryl alcohol, etc.)

are characterized by a very narrow microporosity the use of polyetherimide as a precursor to preparecarbon molecular sieve membranes. In the present(~3–6 A) which allows discrimination between

gas molecules of different size. As reported in the study we describe a method for the preparation ofsupported carbon molecular sieve membranes fromliterature, polymers like polyimides are excellent

precursors to obtain carbon molecular sieve films a polyetherimide, and we present their structuresand gas permeation properties.[1], and have been used extensively in the prepara-

tion of CMSMs with different configurations, e.g.hollow fibers [2], supported membranes [3–5],capillary tubes [6 ] and unsupported carbon flat

2. Experimentalmembranes [7]. Most polyimides used as precur-

Porous carbon disks (diameter 35 mm, thickness2.5 mm, porosity 30%, mean pore size ~1 mm)* Corresponding author. Fax: +34 85 297662;

E-mail: abefu@mail.incar.csic.es obtained by agglomeration of fine graphite par-

1387-1811/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved.PII: S1387-1811 ( 98 ) 00204-2

24 A.B. Fuertes, T.A. Centeno / Microporous and Mesoporous Materials 26 (1998) 23–26

ticles with a phenolic resin were used as mem- it presents a symmetric sponge-like structure witha thickness above 30 mm formed by interconnectedbrane supports. The method of preparation ofcavities. On the other hand, Fig. 1(b) shows athese carbon supports has been described else-cross-section of the supported carbon membranewhere [4,5]. A polyetherimide purchased fromachieved by carbonization of the supported poly-Polysciences Inc. was used as the CMSM precur-meric film. Two different parts can be distinguishedsor. The chemical formula of polyetherimide is

in this microphotograph: (1) the carbon film andA small quantity of dope solution containing(2) the porous carbon support. The former is a15 wt% of polyetherimide in 1-methyl-dense, thin layer having a symmetric structure2-pyrrolidone was spread on the finely polishedaround 3 mm in thickness. Comparison of Fig. 1(a)surface of a carbon support by means of the spinand (b) suggests that the polyetherimide passescoating technique (spin velocity 5000 rpm, spinthrough a plastic stage during carbonization. Thetime ~10 s), resulting in a thin film of the poly-plastic properties of polyetherimide were con-meric solution on the support. The coated carbonfirmed by observing the carbonization of the poly-was immediately immersed into a coagulant bathmeric sample by an optical microscope attachedconsisting of isopropanol at room temperatureto a small furnace. The plastic behavior implies a(~20°C) for approximately 1 h. The gelled poly-rearragement of the matter during the carboniza-meric layer was dried in air at room temperaturetion. As a result, the structure of the porousand carbonized under vacuum (<0.01 mbar) bypolymeric layer is completely changed into a denseheating up to 1073 K (heating rate 0.5 K min−1)and uniform carbon film after heat treatment. Anyfor 1 h. The carbonized samples were cooled slowlytraces of the original polymeric structure or bub-in vacuum to room temperature. The permeationbles formed during the plastic stage in the carbon-rate of pure gases through the carbon membranesization were detected in the carbon film formed.was measured by means of a volumetric membrane

As described previously, the method proposedapparatus at temperatures between 293 and 423 K.here involves several key processes which provideThe carbon membrane was attached in a permea-an almost defect-free CMSM in only one castingtion cell, into which high-purity gases suppliedstep. Thus, the rapid coagulation of polymer pre-from compressed gas cylinders were introduced atvents the infiltration of solution into the poroushigh pressure in contact with the selective mem-carbon substrate, and an excellent polymeric filmbrane layer. The permeated gas was collected in ais achieved. On the other hand, the plastic mattercalibrated volume previously evacuated andformed during polymer carbonization has a highconnected to a low pressure transducer.viscosity, and does not penetrate into the porouscarbon support. In this way, the effects of supportdefects upon the selective carbon layer are lessened.

3. Results and discussion Additionally, as a result of matter displacementsduring the plastic stage, a uniform, thin, dense

3.1. Membrane structure carbon film is obtained from the thick, porouspolymeric layer, and the pinholes existing in the

In Fig. 1(a), an SEM microphotograph of a precursor film are eliminated: as a consequence,they are not transferred to the carbon membrane.cross-section of the gelled polymeric film is shown:

25A.B. Fuertes, T.A. Centeno / Microporous and Mesoporous Materials 26 (1998) 23–26

Fig. 2. Modification of gas permeability of the CMSM withtemperature. [1 Barrer=10−10 cm3 (STP) cm cm−2 s−1cmHg−1].

nism. This means that the microporosity of thecarbon film is very narrow and, as a consequence,it can discern between gas molecules depending ontheir molecular size. From the permselectivity data,the micropore size seems to be below 1 nm. Thepermselectivity values of gas pairs (calculated fromthe ratio of pure gas permeabilities) at differenttemperatures are reported in Table 1. Accordingto the change in activation energy with the size ofgas molecules indicated above, the permselectivi-ties increase as the temperature decreases. Thus,for this kind of material, the highest separationselectivity is reached at the lowest temperatures.(b)

(a)

A comparison between the performance of theFig. 1. Scanning electron microphotographs of (a) a polymericCMSM described here with known gas separationfilm and (b) the supported carbon molecular sieve membrane.polymeric membranes can be made in terms ofcombined selectivity (a)–permeability (P). In thiscontext, Koros [12] has plotted data of a(O2–N2)3.2. Permeation measurementsversus P(O2) for numerous polymeric membranes,and the values corresponding to CMSM reportedAs shown in Fig. 2, where the variation ofhere are above the upper limit. Compared to otherpermeability of different gases with temperature is

plotted, the gas permeability increases as theTable 1kinetic diameter of the gas molecule decreases,Permselectivity values of various gas pairs at differentfrom methane (s=3.8 A) to helium (s=2.6 A).temperaturesMoreover, the activation energies are in the same

range as those obtained for carbon molecular Temperature (°C) O2–N2 He–N2 CO2–CH4 CO2–N2sieves [11], and show a clear dependence on the25 7.4 121 25 15kinetic diameter. All these facts indicate that the100 6.7 42 31 14transport of gas molecules through the carbon film150 5.1 20 20 9.1

takes place according to a molecular sieve mecha-

26 A.B. Fuertes, T.A. Centeno / Microporous and Mesoporous Materials 26 (1998) 23–26

supported CMSMs, the carbon membrane pre- for the preparation of defect-free carbonmembranes.sented in this work shows similar values to those

reported by Hayashi et al. [3] for an alumina-supported CMSM obtained from a laboratory-

Acknowledgementsynthesized polyimide (BPDA-ODA).

Financial support of the ECSC Programme(contract no. 7220-EC/043) is acknowledged.

4. ConclusionsReferences

The results obtained in this work show that thepolyetherimide constitutes a very interesting poly- [1] H. Hatori, Y. Yamada, M. Shiraishi, H. Nakata, S.

Yoshitomi, Carbon 30 (1992) 719.meric precursor to prepare CMSMs.[2] C.W. Jones, W.J. Koros, Carbon 32 (1994) 1419.Carbonization of the polymeric layer coated on a[3] J. Hayashi, H. Mizuta, M. Yamamoto, K. Kusakabe, S.

macroporous carbon support leads to the forma- Morooka, Ind. Eng. Chem. Res. 35 (1996) 4364.[4] T.A. Centeno, A.B. Fuertes, Spanish Patent 9 701 038,tion of a thin microporous carbon film which

1997.possesses a thickness of around 3 mm. The carbon[5] A.B. Fuertes, T.A. Centeno, J. Membrane Sci. 144film formed exbits molecular sieving properties,

(1998) 105.and as a consequence it can discern between gas [6 ] K. Haraya, H. Suda, H. Yanagishita, S. Matsuda,

J. Chem. Soc., Chem. Commun. (1995) 1781.molecules with very similar molecular sizes. Thus,[7] H. Suda, K. Haraya, J. Chem. Phys. B101 (1997) 3988.the permselectivity of the membrane for the[8] K. Kneifel, K.V. Peinemann, R. Waldemann, US PatentO2–N2 system was 7.4 at 25°C.

4 933 085, 1990.The results obtained show that by using the [9] B. Bikson, J.T. Macheras, J.K. Nelson, US Patent

5 181 940, 1993.phase inversion technique during the preparation[10] J.T. Macheras, B. Bikson, J.K. Nelson, US Patentstage, an almost defect-free carbon membrane can

5 443 728, 1995.be obtained in only one casting step. This is an[11] J. Karger, D.M. Ruthven, Diffusion in Zeolites and Other

important improvement with respect to the Microporous Solids, Wiley, New York, 1992, p. 419.[12] W.J. Koros, Chem. Eng. Prog. October (1995) 68.multicoating methods described in the literature