Preparation of supported asymmetric carbon molecular sieve membranes

Journal of Membrane Science 144 (1998) 105±111

Preparation of supported asymmetric carbon molecular sieve membranes A.B. Fuertes*, T.A. Centeno Instituto Nacional del CarboÂn, CSIC, Apartado 73, 33080-Oviedo, Spain Received 21 May 1997; received in revised form 21 January 1998; accepted 21 January 1998

Abstract Asymmetric carbon membranes were made by casting a solution of 13 wt% polyamic acid in N-methylpyrrolydone (NMP) upon a macroporous carbon support. The polymeric solution was coagulated in a bath of isopropyl alcohol and dried at room temperature and at 1508C in air. The resulting polymer was heat treated under vacuum involving two steps: (i) imidization at 3808C during 1 h (heating rate: 18C/min) and (ii) carbonization at 5508C for 1 h (heating rate: 0.58C/min). The carbon membrane obtained in only one casting step shows an asymmetric structure formed by a dense skin layer with a thickness of around 1 mm and a porous substrate (6 mm thickness) of the same material. The gas permeation results indicate that the gas transport through the membrane occurs according to an activated mechanism (molecular sieving). The selectivity and permeation rate measured at 258C for the O2/N2, He/N2, and CO2/CH4 systems were respectively: (O2/N2)ˆ5.3, P(O2)ˆ 1.1410ÿ9 mol/m2 s Pa; (He/N2)ˆ26.5, P(He)ˆ5.710ÿ9 mol/m2 s Pa; (CO2/CH4)ˆ37.3, P(CO2)ˆ4.010ÿ9 mol/m2 s Pa. # 1998 Elsevier Science B.V. Keywords: Carbon membranes; Molecular sieve; Gas separations; Asymmetric membranes; Polyimide

1. Introduction At the present time, there is a growing interest in the development of gas separation membranes based on materials providing better selectivity, thermal stability and chemical stability than those already existing (i.e. polymeric membranes). The attention has been focussed on materials that exhibit molecular sieve properties: (a) silica materials [1], (b) zeolites [2] and (c) carbon materials [3±5]. Related to the latter case, it is well known that the pyrolysis of certain types of substances (natural or polymeric) leads to carbon materials with a *Corresponding author. Fax: +34 85 297 662. 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(98)00037-4

very narrow micropore distribution below 1 nm [6] which make possible to separate gas pairs with very similar molecular dimensions. Different authors have reported that the controlled carbonization of polymers like polyimide, polyfurfuryl alcohol or PVDC allows to obtain crack-free carbon molecular sieve ®lms [7] which suggest a potential use of such materials in the preparation of carbon membranes. In practice, carbon membranes have been prepared in two main con®gurations: (a) unsupported carbon membranes (¯at membranes, capillary tubes or hollow ®bres) [5,8,9] and (b) supported membranes (¯at or tubular) on a macroporous material [4,10±12]. Both types present some drawbacks. Thus, the brittleness of

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A.B. Fuertes, T.A. Centeno / Journal of Membrane Science 144 (1998) 105±111

the former creates serious dif®culties for practical use. On the other hand, the preparation of effective supported carbon membranes requires that the cycle of polymer deposition±carbonization must be repeated several times in order to obtain an almost crack-free membrane. In this way, Rao and Sircar [12] have developed modules formed by ®ve-coated carbon membranes supported on ¯at macroporous carbon supports, to separate hydrogen±hydrocarbon mixtures. However, this complex procedure constitutes a handicap for the practical utilization of supported carbon molecular sieve membranes. Almost all polymeric membranes used in gas separation are of the asymmetric-type [13]. They are constituted of two structurally distinct layers, one of which is a thin, dense, selective skin layer and the other a thick, macroporous layer whose function is to provide a physical support to dense skin. This con®guration yields membranes with a high selectivity, which simultaneously maintain high permeation rates. This type of con®guration could be transferred to the carbon membranes provided that the polymeric precursor presents an asymmetric structure before carbonization. In this context, Haraya et al. [8] have described the preparation of capillary tubes of unsupported asymmetric carbon molecular sieve membranes by carbonization of polymeric membranes with an asymmetric structure. However, these authors emphasize that the microstructure and gas permeation properties of this type of membrane were dif®cult to control. Taking into account the fact that the cracks in the carbon molecular sieve ®lms usually result from defects existing on the surface of the macroporous support, the presence of a sponge-like structure will lessen the effects upon dense carbon ®lm. In this way, defect-free supported carbon molecular sieve membranes will be achieved more easily. We report here the preparation of a ¯at asymmetric carbon membrane supported on a macroporous carbon support. The existence of an almost defect-free carbon molecular sieve ®lm obtained by only one casting step is suggested by the gas permeation experiments. 2. Experimental Disk-shape macroporous carbon supports (diameter: 35 mm; thickness: 2.2 mm) were obtained by

carbonization (N2 at 8508C) of agglomerated graphite particles (Aldrich no. 7782-42-5) blended with a phenolic resin. The support has a porosity of 30% and a mean pore diameter of around 1 mm, as measured by mercury porosimetry. In order to diminish the presence of defects on the ®nal carbon molecular sieve ®lm, the macroporous support was coated with an intermediate carbon layer. Thus, a paste formed by ®ne graphite particles (Timrex KS6, TIMCAL G‡T) with a mean diameter of 3 mm blended with a polyamide±imide resin was carefully deposited on the surface of a macroporous support by means of a knife. The support with the intermediate layer was carbonized under vacuum at a temperature of 5508C (heating rate, 0.58C/min). Finally, it was ®nely polished until a mirror appearance was achieved. In a second stage, a polymeric membrane with an asymmetric structure, obtained by the phase inversion technique, was deposited over the support. The polymeric precursor was a polyamic acid in solution which after imidization conducts to formation of BPDA± pPDA polyimide with a formula

A 13% solution of poliamic acid in N-methylpyrrolidone (NMP) was deposited over support by spin coating technique (speed, 1600 rpm). After formation of a thin homogeneous polymeric ®lm (approx. 5 min) it was gelled by immersion at room temperature into a coagulant bath (acetone or isopropyl alcohol) between 30 min and 1 h. The gellated polymeric layer was dried in air at room temperature and subsequently subjected to the following treatments: (a) Drying at 1508C during 1 h in air (heating rate: 38C/ min); (b) Imidization under vacuum at 3808C (heating rate, 18C/min) during 1 h; (c) Carbonization under vacuum at 5508C (heating rate, 0.58C/min) for 1 h. The carbonized samples were slowly cooled under vacuum to room temperature. The cross-section structures of the resulting materials were analysed using a scanning electron microscope (Zeiss DSM 942).

A.B. Fuertes, T.A. Centeno / Journal of Membrane Science 144 (1998) 105±111

The permeation rate of pure gases through the carbon membranes was measured by means of a volumetric membrane apparatus. The carbon membrane was attached in a permeation cell (Millipore high pressure ®lter holder). In contact with the membrane layer, high purity gases supplied from compressed gas cylinders are introduced at high pressure. The pressure was measured by a manometer. Vacuum was maintained on the low side of the membrane, and the permeate was collected in an evacuated volume. The variation of pressure was monitored with a pressure transducer (Leybold CM 1000) connected to a computer. The permeation rate of pure gases through the membrane has been estimated from the variation of pressure with time at the low-pressure side of the system. 3. Results and discussion 3.1. Membrane structure SEM microphotographs of the cross-sections of the materials corresponding to the different steps of membranes preparation are shown in Fig. 1. The structure of the polymeric membranes prepared by gellation of polyamic acid and after imidization at 3808C of gelled polyamic acid (polyimide membrane) are seen in Fig. 1(a) and (b), respectively. Fig. 1(c)±(e) corresponds to the ®nal carbon membrane obtained by carbonization of the polyimide membrane. The structure of asymmetric carbon membranes consists in a dense layer with a thickness around 1±1.5 mm and a uniform macroporous matrix (5±6 mm) formed by elongated pores of around 1 mm (Fig. 1(c) and (d)). The interface between the dense skin layer and the porous matrix is very sharp and a graded density is not detected. The top layer is very smooth (Fig. 1(e)), being almost defect-free, but a few defects with a diameter of around 0.5±1.5 mm are observed on the surface by SEM. It is believed that the observed pinholes are related to the presence of small bubbles in the casting solution during the coating. Additionally, a good adherence between the porous matrix and the macroporous carbon support is observed (Fig. 1(c) and (e)). By comparing the structure of polymeric membranes (Fig. 1(a) and (b)) with that obtained after heat treatment (Fig. 1(c)±(e)), no difference is

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detected. This indicates that the asymmetric structure of polymeric precursor does not undergo any arrangement during the carbonization stage. The structure of the macroporous carbon support is important, in order to obtain a crack-free thin ®lm of carbon molecular sieve membrane. In fact, when carbon supports without intermediate layer were coated, the polymeric solution partially slipped in the substrate and defects in the ®nal membrane were present. In order to prevent this, a thin layer (thickness around 10 mm) formed by ®ne graphite particles (mean diameter: 3 mm) was deposited on the carbon support. This intermediate layer favours the subsequent coating of support by the casting solution and a very homogeneous polymeric ®lm is obtained. However, the existence of the intermediate layer is not suf®cient to obtain good symmetric carbon molecular sieve membranes in only one casting step. Thus, as reported previously [14], three casting steps were necessary to obtain an almost defect-free symmetric carbon molecular sieve membrane. In the case of the asymmetric carbon membranes presented here, only one casting step was necessary to reach an almost defect-free membrane, which indicates that the asymmetric structure drastically reduces the presence of defects. The reason of that is probably the fact that the sponge-like structure reduces the effect of the support's own defects on the thin carbon molecular sieve ®lm. 3.2. Permeation measurements The results of gas permeation, shown in Fig. 2, indicate that the transport of gases through the devices described here does not occur according to a Knudsen mechanism. When the permeation of gases takes place under this regime, the permeation rate decreases with gas molecular weight (M) and temperature. On the other hand, the selectivities of gas pairs achieved by Knudsen mechanism are given by (i/j)ˆ(Mj/Mi)1/2. In Fig. 2 the variation of gas permeation rates with temperature is represented. However, in the present work the permeation rate is well correlated with the kinetic diameter of the gas molecules instead of their molecular weights and it increases with temperature. Thus, the permeation values are in the order He (4, Ê )>O2 (32, 3.46 A Ê )>N2 (28, Ê )>CO2 (44, 3.3 A 2.6 A Ê ). The values in the brackets Ê )>CH4 (16, 3.8 A 3.64 A

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Fig. 1. SEM photomicrographs of asymmetric fractured membrane sections: (a) polyamic acid membrane; (b) polyimide membrane; (c), (d) carbon membrane (cross-section); (e) carbon membrane (top view).

A.B. Fuertes, T.A. Centeno / Journal of Membrane Science 144 (1998) 105±111

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two gases [(i/j) being (kinetic diameter)i<(kinetic diameter)j] increases as temperature decreases. In our case, the highest separation factors of O2/N2, He/N2, CO2/CH4 and CO2/N2 systems are achieved at 258C. On the other hand, it is observed that the permeation rate of carbon dioxide does not change with temperature (activation energy0). This anomalous behaviour suggests that adsorption of CO2 into carbon micropores takes place, and therefore, the CO2 transport through the membrane results from a combination of transport in the gas phase (molecular sieving) and surface diffusion of the adsorbed molecules across the micropores. The decrease of adsorption with increasing temperature will be compensated by the increase in gas diffusivity. As a result, the permeation rate hardly changes with temperature. This agrees with the observation that at low temperatures (i.e. <508C), the permeation rate of CO2 increases along the permeation experiments as the pressure into the membrane increases. This is also compatible with the existence of a surface diffusion transport mechanism. The present permselectivity values are slightly lower than those measured for a carbon membrane obtained by a multicoating method. As shown previously [14], selectivity values around 12 were determined for O2/N2 separation through a membrane obtained by coating a carbon support with three layers of the same polyimide used in the present study. This difference indicates the existence of small defects (pinholes). In fact, the existence of a few pinholes with a size between 0.5 and 1.5 mm was detected by scanning electron microscopy. It suggests that the procedure presented here, one-stage method of preparation of asymmetric carbon membranes, needs to be re®ned. Further efforts are on the way to optimize the method of preparation of asymmetric carbon membranes outlined here.

Fig. 2. Modification of permeation rate of carbon membrane with temperature in an Arrhenius plot.

correspond to the molecular weight and the kinetic diameter of the gases tested here. Additionally, from the change of gas permeation with temperature, estimates were obtained for the apparent activation energies: He, 1.6 kJ/mol; CO2, 0 kJ/mol; O2, 3.1 kJ/mol; N2, 9.8 kJ/mol; CH4, 14.5 kJ/mol. These values increase with the kinetic diameter of gas molecules. On the other hand, as indicated in Table 1, the permselectivities of different gas pairs are higher than those predicted from a Knudsen transport mechanism. All these observations suggest that gas transport through the carbon membrane occurs according to a molecular sieving mechanism (activated diffusion) instead of Knudsen mechanism as observed for gas diffusion in carbon molecular sieves and zeolites [15]. As a consequence of the change in activation energy with molecular size, the gas separation factor of different gas pairs changes with temperature. As shown in Table 1, the gas separation factor between Table 1 Separation factors of gas pairs in the asymmetric carbon membrane Selectivity (i/j)

O2/N2 He/N2 CO2/CH4 CO2/N2

Temperature (8C)

Knudsen separation factor

25

50

75

100

125

150

5.5 26.5 37.4 18.7

4.7 20.3 22.3 13.5

3.9 16.4 14.3 10.5

3.3 13.4 12.6 8.5

2.8 11.2 8.0 6.8

2.5 10.2 6.7 5.8

0.94 2.65 0.60 0.80

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Table 2 Comparative analysis between characteristics of supported carbon molecular sieve membranes obtained in this work with those found in the literature

Membrane characteristics Type No. of coating steps Precursor Support Temp. carbonization (8C) Membrane thickness, mm

This work

Rao and Sircar [4]

Hayashi et al. [10]

Asymmetric 1 Polyimide (BPDA±pPDA) Porous graphite 5508C 1.5

Symmetric 5 PVDC Porous graphite 10008C 2.5

Symmetric 3 Polyimide (BPDA±ODA) Porous alumina 8008C 2

76 No data 31 0.4 1055 1.65

0.3 No data 47 100 7.8 100

Pure gas permabilities and permselectivities (258C) 1 P(N2), Ba 5.5 (O2/N2)a P(He), Ba 31 26.5 (He/N2)a 18 P(CO2), Ba 37.4 (CO2/CH4)a a

Ratio of pure gas permeabilities.

Results reported in the literature refer mainly to unsupported carbon molecular sieve membranes (¯at membranes, hollow ®bres or capillary tubes), and only a few studies deal the preparation of supported carbon molecular sieve membranes. A comparative analysis between the characteristics of the supported carbon membrane described here with those obtained by other authors is given in Table 2. The results corresponding to the ®ve-coated membrane prepared by Rao et al. [4] and Rao and Sircar [12] indicate that the gas transport through it occurs by selective adsorption of the more adsorbed species instead of a molecular sieve mechanism. It also exhibits high permeability values but very low selectivities, as estimated from the ratio of pure gas permeabilities. This kind of carbon membrane has been designed to separate H2/hydrocarbon gas mixtures instead of gas permanent mixtures. On the other hand, the three-coated membrane prepared by Hayashi et al. [10,11] shows comparable results with those reported in this work for one-coated asymmetric carbon membrane. 4. Conclusions Our results show that ¯at carbon supported membranes with an asymmetric structure can be obtained by using the phase inversion technique. It leads, in

only one step, to ¯at asymmetric carbon membranes having almost defect-free thin skin layer. Gas transport through the asymmetric carbon membranes prepared here correspond to an activated mechanism (molecular sieving), the measured activated energies of different pure gases being: N2, 9.8 kJ/mol; O2, 3.1 kJ/mol; He, 1.6 kJ/mol; CH4, 14.5 kJ/mol; CO2, 0 kJ/mol. The selectivity and permeation rate values obtained for the new membrane show that it has good gas separation capabilities. The most relevant result obtained in this investigation is the fact that the preparation of an effective ¯at carbon membrane can be carried out in only one casting step, avoiding complex and unpractical multicoating methods. Acknowledgements The authors would like to gratefully acknowledge the ®nancial support from ECSC Programme (Contract. 7220-EC/043). References [1] R.S.A. de Lange, Microporous sol±gel derived ceramic membranes for gas separation, Ph.D. Dissertation, University of Twente, 1993.

A.B. Fuertes, T.A. Centeno / Journal of Membrane Science 144 (1998) 105±111 [2] E.R. Geus, M.J. Exter, H. van Bekkum, Synthesis and characterization of zeolite (MFI) membranes on porous ceramic supports, J. Chem. Soc. Faraday Trans. 88 (1992) 3101. [3] J.E. Koresh, A. Sofer, Molecular sieve carbon permeselective membrane. Part I. Presentation of a new device for gas mixture separation, Sep. Sci. Technol. 18 (1983) 723. [4] M.B. Rao, S. Sircar, C. Golden, Gas separation by adsorbent membranes, US Patent no. 5 104 425, 1992. [5] C.W. Jones, W.J. Koros, Carbon molecular sieve gas separation membranes ± I. Preparation and characterization based on polyimide precursors, Carbon 32 (1994) 1419. [6] G.M. Jenkins, K. Kawamura, Polymeric Carbons ± Carbon fibre, Glass and Char, Cambridge University Press, Cambridge, 1976. [7] H. Hatori, Y. Yamada, M. Shiraishi, H. Nakata, S. Yoshitomi, Carbon molecular sieve films from polyimide, Carbon 30 (1992) 719. [8] K. Haraya, H. Suda, H. Yanagishita, S. Matsuda, Asymmetric capillary membrane of a carbon molecular sieve, J. Chem. Soc., Chem. Commun. (1995) 1781.

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[9] H. Suda, K. Haraya, Gas permeation through micropores of carbon molecular sieve membranes derived from kapton polyimide, J. Chem. Phys. B. 101 (1997) 3988. [10] J. Hayashi, H. Mizuta, M. Yamamoto, K. Kusakabe, S. Morooka, Separation of ethane/ethylene and propane/propylene systems with carbonized BPDA±pp0 ODA polyimide membrane, Ind. Eng. Chem. Res. 35 (1996) 4176. [11] J. Hayashi, H. Mizuta, M. Yamamoto, K. Kusakabe, S. Morooka, Simultaneous improvement of permeance and permeselectivity of 3,30 ,4,40 -biphenyltetracarboxylic dianhydride-4,40 -oxydianiline polyimide membrane by carbonization, Ind. Eng. Chem. Res. 35 (1996) 4364. [12] M.B. Rao, S. Sircar, Performance and pore characterisation of nanoporous carbon membranes for gas separation, J. Membr. Sci. 110 (1996) 109. [13] R.E. Kesting, A.K. Fritzsche, Polymeric Gas Separation Membranes, Wiley, New York, 1993. [14] T.A. Centeno, A.B. Fuertes, Procedure for preparation of carbon membranes, Spanish Patent no. 9 701 038, 1997. [15] J. KaÈrger, D.M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, Wiley, New York, 1992.