Microporous Inorganic Membranes for Gas Separation
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Microporous Inorganic Membranes for Gas Separation Shigeharu Morooka and Katsuki Kusakabe Introduction Microporous inorganic membranes are potentially useful in gas Separation in emerging areas such as catalytic reactors, gasification of coal, molten-carbonate and solid-electrolyte fuel cells, and water decomposition by thermochemical reactions. If the feed or product gases can be separated at elevated temperatures spe cific to each process, the energy required for purification could be greatly reduced. Advances in the development of inor ganic membranes have been quite rapid in recent years. For example, in 1991 the reported C 0 2 / N 2 selectivity at ambient temperature was less than 10, but by 1997 it had improved to approximately 100. The permeation rate and permselectivity of porous inorganic membranes are dependent on the microstructures of membrane/support composites such as pore size and distribution, porosity, tortuosity, and the affinity between permeating species and pore walls. Figure 1 shows the relationship between molecular weight and kinetic diameter (calculated from minimum equilibrium crosssectional diameter 1 ) for a selected series of molecules. Hydrogen and helium are smaller and lighter t h a n the others. Structural isomers such as n-C4H 10 and i-C 4 Hio have the same mass but quite different sizes. Therefore, the control of micropores is of critical importance in these cases. However, the molecular masses and sizes of C 0 2 and N 2 are not greatly different; thus the difference in affinity is important for Separation of these molecules. In order to achieve effective Separation of small-molecule gases, the membrane pores should be smaller than 2 nm. In the case of mesopores or macropores,
MRS BULLETIN/MARCH 1999
gases permeate with low selectivities through these pores. In this article, preparation processes and permeation properties of porous inorganic membranes are reviewed, and permeation mechanisms are discussed. 160
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Table I shows materials, structures, and preparation processes for typical micro porous inorganic membranes. Micro porous membranes are composed largely of amorphous silica, carbon, and zeolites. Silica-based membranes are produced by chemical vapor deposition (CVD), solgel, and pyrolysis techniques. Carbon membranes are produced by pyrolysis of precursor films, and zeolite membranes are formed by hydrothermal synthesis. Although metal oxides, as well as car bon, possess high melting points, they may undergo phase transitions and struc tural changes which result in failure of the membranes at lower temperatures. For example, crystallization of amor phous silica occurs at 873°C, above which a m o r p h o u s silica membranes are not usable. Zeolites may deteriorat
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