Si／Al比对水热法合成皂石结构的影响

Structural Characteristics of Hydrothermally Synthesized Saponites with Different Si/Al Ratios

水热法合成的皂石,其结构特性除受到时间、温度、pH值等参数的影响外,还与铝的占位与配位形式有着直接的联系。作为一个特殊的骨架离子,铝对皂石物相的纯度、结构稳定性以及表面反应性等都起着至关重要的作用。采用水热法,固定初始物料中镁的添加量,硅与铝的添加量之和为一定值,改变Si/Al比(摩尔比),制备了一系列皂石,探究铝元素在皂石中的存在形式和微结构环境。X射线衍射分析表明:铝的添加量对皂石的结晶度有着重要的影响,当Si/Al比为5.43～12.30时,皂石结晶良好。29Si、27Al NMR谱研究表明:在皂石的形成过程中,铝离子和镁离子分别优先进入四面体和八面体晶格中,当Si/Al比为2.33～1.80时,铝离子进入八面体中的含量明显增多;合成皂石片层中普遍存在SiIV悬键,并出现Q2Si(OAl)信号,这与天然蒙皂石的NMR谱存在明显的区别。

A series of saponites were synthesized with fixed contents of magnesium, certain amounts of silicon and aluminum and various Si/Al ratios in a starting gel by a hydrothermal method. The influences of parameters （i.e., chemical composition, crystallization time, pH value and temperature） on the structure of saponite in the synthesis process were investigated. Aluminum as a special skeleton cation could affect the morphology, textural and surface properties. The X-ray diffraction patterns showed that the well-crystallized saponite could be obtained in the Si/Al ratio range from 5.43-12.30. The 29Si and 27Al NMR spectra suggested that aluminum and magnesium were prior to enter tetrahedral and octahedral sites, respectively. The incorporation of aluminum cation in six-fold coordination was favored in the Si/Al ratio range of 2.33-1.80. Q2 Si（OAl） was observed in the cases of high contents of Si4＋, corresponding to Si located at the edges of saponite. This was different from natural clay minerals, i.e., Q2 Si（OAl） is hardly detected in the natural smectites.