Structure of Zeolite A (LTA) Surfaces and the Zeolite A/Water Interface

Abstract

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.