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Gas Separation Properties of Carbon Membranes formed by Pyrolysis of Polyimide

Masaji MATSUDA, Hiroyuki SUDA, Hiroshi YANAGISHITA, Naostugu ITOH, Kenji HARAYA and Toshikatu HAKUTA
J. of NIMC. Vol.4, No.6, pp.223-230(1996)

We have investigated the formation of gas separation carbon membrane with high thermal and chemical stability.Flat type carbon membranes were obtained by pyrolysis of Kapton polyimide film at 950Ž under oxygen free atmosphere (heating rate: 80, 270, 800Ž/hr). The gas permeability measured at 35Ž was shown to be in the order of H2> He„CO2„ O2„ N2 for all of the prepared membranes. The permeability became lower with decreasing the heating rate. Whereas the gas permselectivity (the ratio of permeabilities for chosen gas pairs) became higher. Each permselectivity for the membrane heat-treated at 80Ž / hr was 4700(H2), 2800(He), 208(Ne), 122(CO2), 36(O2), respectively, when that of N2 was regarded as unity. The extremely high gas permselectivity was suggested to originate from its molecular sieving effect, by which the membrane can discriminate the smaller molecules from the bigger ones.
We investigated a preparation method of capillary type carbon membrane (O. D. 1.5mm) for practical application. The method consists of a formation step of an asymmetric polyamic acid (PAA) membrane and a pyrolysis step. The PAA membrane with an asymmetric structure was prepared by a phase inversion technique, using a PAA solution in dimethylacetamide for casting. The dried PAA capillary membrane was imidized to Kapton type polyimide, and then pyrolysed under vacuum. The obtained carbon membrane had an asymmetric structure consisting of dense surface layer and several types of pores inside. Although the membrane with thicker surface dense layer obtained in ethanol coagulant showed lower gas permeance, it had similar permselectivity as that of the flat type membrane. Whereas the membrane with thinner surface dense layer obtained in water coagulant showed higher gas permeance but lower permselectivity. The present method will enable us to prepare an asymmetric capillary type carbon membranes both with high gas permeability and permselectivity, if we can control the formation step of the PAA membrane.

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