Structure of Zeolite A (LTA) Surfaces and the Zeolite A/Water Interface
Structure of Zeolite A (LTA) Surfaces and the Zeolite A/Water Interface
复制标题
沸石 A (LTA) 表面的结构和沸石 A/水界面
DOI：
10.1021/jp909355e
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发表时间：
2010
期刊：
影响因子：
--
通讯作者：
Gren W
中科院分区：
文献类型：
--
作者：
Gren W
原子模拟研究了硅质和铝硅酸钠（Na-A）形式的沸石LTA的表面结构和稳定性。首先，采用静态晶格最小化方法对表面结构进行优化。这些模拟预测了{100}的单4环终止和{111}的双4环终止对硅质LTA同样稳定。在框架中加入铝离子使{100}相对于{111}表面稳定。这种表面稳定性变化的一个后果是，预测的平衡形态从纯硅质的球形变为立方。然后将LTA板浸入水中并使用分子动力学进行模拟。硅质LTA具有疏水区，而铝硅酸盐LTA的水密度位于不同的晶体位置。沸石表面显示出在表面附近施加显著的水有序。这反过来又影响了水的扩散率。水的扩散系数与水的结构有关，这导致扩散系数具有明显的各向异性。还发现水的存在增加了{100}表面的表面稳定性。最后，我们发现Na+离子浸出到溶液中，在表面吸附位点之间迁移并穿过6环和8环，从而在LTA表面形成弥漫性钠层，这也对沸石表面附近的原子传输具有重要意义。
Atomistic simulations were used to investigate the surface structure and stability of siliceous and sodium aluminosilicate (Na-A) forms of the zeolite LTA. First, the surface structures were optimized with static lattice minimization. These simulations predict that the single 4-ring termination of {100} and the double 4-ring of {111} are equally stable for siliceous LTA. The inclusion of aluminum ions into the framework stabilizes the {100} relative to the {111} surface. One consequence of this change in surface stability is that the predicted equilibrium morphology changes from spherical for purely siliceous to cubic. Slabs of LTA were then immersed in water and simulated using molecular dynamics. The siliceous LTA was found to have hydrophobic regions, whereas in the aluminosilicate the water density resides at distinct crystallographic sites. The zeolite surfaces were shown to impose significant water ordering near the surfaces. This, in turn, affects the water diffusivity. The diffusivity of water is correlated with water structure, which leads to clear anisotropy in the diffusion coefficient. The presence of water is also found to increase the surface stability of the {100} surfaces. Finally, we found that Na+ions leach into the solution, migrating between surface adsorption sites and moving through the 6- and 8-rings, hence forming a diffuse sodium layer above the LTA surface, which also has important implications for atom transport near zeolite surfaces.