Preparation of New Molecular Sieve Material by Selective Leaching Method

采用一种新的合成方法制备介孔γ -氧化铝，即在1000℃左右的温度下煅烧高岭土矿物，使极细的γ -氧化铝颗粒分散在无定形二氧化硅基体中，形成细小的微观结构，并用KOH溶液选择性地浸出无定形二氧化硅，使原料的假晶状颗粒内形成均匀的介孔。以美国乔治亚州的高岭石为原料，在1000‰的温度下煅烧，在90‰的温度下用4M的KOH溶液选择性浸出1h，得到了孔径分布在几nm处非常清晰的孔结构。该多孔γ -氧化铝具有比目前报道的各种γ -氧化铝更均匀的孔径和更大的孔体积的特点。因此，室温下γ -氧化铝的水蒸气吸附-解吸等温线在相对湿度（RH）为80%左右时，毛细冷凝的吸附量急剧增加，在相对湿度（RH）为90%时，吸附量达到最大，为ca.600ml (STP) /g。另一方面，在相对湿度为70%时，凝结水从γ -氧化铝的介孔中突然解吸。由于具有均匀的介孔结构，水蒸气在γ -氧化铝中的吸附和解吸速度非常快，可以通过毛细管冷凝和蒸发来实现，因此可以认为采用选择性浸出法制备具有分子筛性质的介孔γ -氧化铝。

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.