Preparation of New Molecular Sieve Material by Selective Leaching Method

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Mesoporous gamma-alumina was prepared by a new synthetic method, i.e. calcining kaolin minerals at around 1000ﾟC to form fine microtexture with dispersing very fine gamma-alumina grains in amorphous silica matrix and selectively leaching amorphous silica properly using KOH solution to evolve uniform mesopores within the pseudomorph particles of the starting materials. Pore structure with very sharp pore size distribution at several nm in size was obtained by this selective leaching method when it was synthesized using kaolinite from Georgia, U.S.A.as the starting material, calcining at 1000ﾟC and selective leaching at 90ﾟC for 1h by 4M KOH solution. The porous gamma-alumina had characteristic properties to show more uniform pore size and larger pore volume than those of various gamma-alumina reported so far. Therefore, the water vapor adsorption-desorption isotherms of the gamma-alumina at room temperature showed very steep increase of adsorption at around relative humidity (RH) of 80% by capillary condensation and reached maximum adsorption of ca.600ml (STP) /g at RH of 90%. On the other hand, desorption of condensed water from mesopores in the gamma-alumina occurred suddenly at RH of 70%. Since the very quick adsorption and desorption of water vapor in the gamma-alumina was understood due to capillary condensation and evaporation owing to the uniform mesopores, mesoporous gamma-alumina with molecular sieve properties was considered to be prepared by the present selective leaching method.