Preparation of supported asymmetric carbonmolecular 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)�5.3, P(O2)�1.14�10ÿ9 mol/m2 s Pa; �(He/N2)�26.5, P(He)�5.7�10ÿ9 mol/m2 s Pa; �(CO2/CH4)�37.3, P(CO2)�4.0�10ÿ9 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.

polymericmembranes).Theattentionhasbeenfocussed

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

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

Journal of Membrane Science 144 (1998) 105±111

*Corresponding author. Fax: +34 85 297 662.

0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.

P I I S 0 3 7 6 - 7 3 8 8 ( 9 8 ) 0 0 0 3 7 - 4

the former creates serious dif®culties for practical use.

On the other hand, the preparation of effective sup-

ported 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 handi-

cap 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 func-

tion is to provide a physical support to dense skin. This

con®guration yields membranes with a high selectiv-

ity, 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 unsup-

ported asymmetric carbon molecular sieve membranes

by carbonization of polymeric membranes with an

asymmetric structure. However, these authors empha-

size that the microstructure and gas permeation prop-

erties 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 mem-

branes 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 (dia-

meter: 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 mea-

sured 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 poly-

amide±imide resin was carefully deposited on the

surface of a macroporous support by means of a knife.

The support with the intermediate layer was carbo-

nized under vacuum at a temperature of 5508C (heat-

ing 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 poly-

meric 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-methyl-

pyrrolidone (NMP) was deposited over support by

spin coating technique (speed, 1600 rpm). After for-

mation of a thin homogeneous polymeric ®lm (approx.

5 min) it was gelled by immersion at room tempera-

ture into a coagulant bath (acetone or isopropyl alco-

hol) between 30 min and 1 h. The gellated polymeric

layer was dried in air at room temperature and sub-

sequently 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 materi-

als were analysed using a scanning electron micro-

scope (Zeiss DSM 942).

106 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 mem-

brane was attached in a permeation cell (Millipore

high pressure ®lter holder). In contact with the mem-

brane layer, high purity gases supplied from com-

pressed 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 pres-

sure 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 mem-

branes 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) corre-

sponds to the ®nal carbon membrane obtained by

carbonization of the polyimide membrane. The struc-

ture 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. Addition-

ally, 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

detected. This indicates that the asymmetric structure

of polymeric precursor does not undergo any arrange-

ment 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 subse-

quent coating of support by the casting solution and a

very homogeneous polymeric ®lm is obtained. How-

ever, 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 asym-

metric 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 sup-

port'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)�(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,

2.6 AÊ )>CO2 (44, 3.3 AÊ )>O2 (32, 3.46 AÊ )>N2 (28,

3.64 AÊ )>CH4 (16, 3.8 AÊ ). The values in the brackets

A.B. Fuertes, T.A. Centeno / Journal of Membrane Science 144 (1998) 105±111 107

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).

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

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

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 tempera-

ture (activation energy�0). This anomalous behaviour

suggests that adsorption of CO2 into carbon micro-

pores 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 increas-

ing 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 permea-

tion 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 pre-

viously [14], selectivity values around 12 were deter-

mined 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 pre-

paration 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.

Table 1

Separation factors of gas pairs in the asymmetric carbon membrane

Selectivity �(i/j) Temperature (8C) Knudsen separation factor

25 50 75 100 125 150

O2/N2 5.5 4.7 3.9 3.3 2.8 2.5 0.94

He/N2 26.5 20.3 16.4 13.4 11.2 10.2 2.65

CO2/CH4 37.4 22.3 14.3 12.6 8.0 6.7 0.60

CO2/N2 18.7 13.5 10.5 8.5 6.8 5.8 0.80

A.B. Fuertes, T.A. Centeno / Journal of Membrane Science 144 (1998) 105±111 109

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 mechan-

ism. 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 mix-

tures 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 mem-

branes 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 mem-

branes prepared here correspond to an activated

mechanism (molecular sieving), the measured acti-

vated 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 (Con-

tract. 7220-EC/043).

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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

This work Rao and Sircar [4] Hayashi et al. [10]

Membrane characteristics

Type Asymmetric Symmetric Symmetric

No. of coating steps 1 5 3

Precursor Polyimide (BPDA±pPDA) PVDC Polyimide (BPDA±ODA)

Support Porous graphite Porous graphite Porous alumina

Temp. carbonization (8C) 5508C 10008C 8008CMembrane thickness, mm 1.5 2.5 2

Pure gas permabilities and permselectivities (258C)

P(N2), Ba 1 76 0.3

�(O2/N2)a 5.5 No data No data

P(He), Ba 31 31 47

�(He/N2)a 26.5 0.4 100

P(CO2), Ba 18 1055 7.8

�(CO2/CH4)a 37.4 1.65 100

aRatio of pure gas permeabilities.

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