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 specific to each process, the energy required for purification could be greatly reduced. Advances in the development of inorganic membranes have been quite rapid in recent years. For example, in 1991 the reported CO2/N2 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 cross-sectional diameter) for a selected series of molecules. Hydrogen and helium are smaller and lighter than the others. Structural isomers such as n-C4H10 and i-C4H10 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 CO2 and N2 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, 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.