Synthesis of Porous Inorganic Membranes
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rbon Membranes Microporous carbon is widely used for liquid or gas purification because of its strong adsorptive properties and high surface area. It is also used for air Separa tion by pressure swing adsorption (PSA), relying on its adsorptive and molecularsieving properties. From the Standpoint of applications, microporous carbons are classified into activated carbons with pore size 0.8-2 nm, and ultramicroporous carbons or carbon molecular sieves with pores 0.3-0.6 nm. Activated car bons are used because of their strong adsorption properties, while carbon mo lecular sieves are useful on account of
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their molecular-sieving as well as ad sorption properties. Microporous carbons are often classi fied into graphitizing and nongraphitiz ing.2"4 Graphitizing carbons are produced from precursors that melt during heat treatment, forming a mesophase structure. This partially ordered structure is maintained upon solidification and turns into graphite at temperatures above 1400°C.2 Graphitizing carbons are produced from materials such as petroleum pitch, and from polymers such as poly(vinyl Chlo ride) (PVC) and polyimides (PIs). Nongraphitizing carbons, also known as glassy carbons, result from heating polymeric precursors, which are initially crosslinked or become crosslinked before the onset of decomposition. The crosslinked solid lacks the mobility to attain long-range order and, upon heat treatment, turns into a network of randomly oriented platelets or ribbons. Heat ing above 2000°C can cause development of long-range order by coalescence of the ribbonlike elements. Precursors of such carbons include low-rank coals, cellulose, and synthetic polymers such as poly(furfuryl alcohol) (PFA), phenolic resins, and poly(vinylidene Chloride) (PVDC). Car bons formed by decomposition on Fe, Ni, or Co of gas mixtures containing CO, CHj, and other small hydrocarbons have intermediate structure between graphi tizing and nongraphitizing. 5 The carbon literature contains numerous reports dealing with the characterization of porous carbons by adsorption of probe molecules and the effect of the heating protocol on total pore volume and pore size. There is little Information, however, about the relationship between precursor structure and pore structure of carbon. Porous structure aside, the mechanical properties of carbon membranes are of
critical importance in applications. Considerable Information is available about the effect of processing conditions on the mechanical properties of carbon fibers from coal-tar pitch, polyacrylonitrile (PAN), and PI precursors. Carbonization studies of these polymers have shown that flow-induced orientation generated during fiber spinning and drawing persists in subsequent carbonization and is conducive to high tensile and flexural strength. 6 " 9 Alignment of poly mer chains or liquid crystals is expected to favor dense bonding normal to the fiber axis, imparting high tensile strength to the carbon fibers. In nongraphitizing car bons, the spatial pattern of bonding is more random and le
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