Comparative Structural, thermodynamic and electronic analyses of Zn-Al-An- hydrotalcite-like compounds (An-=Cl-, F-, Br-, OH-, CO32- or NO3-): An ab initio study

We have investigated the influence of different anions intercalated in the layered double hydroxides (LDHs). The main aim of this work is to understand the layer-anion and the intermolecular interactions in hydrotalcite-like compounds with F-, Br-, OH-, CO32- and NO3-. This ab initio study describes the thermodynamics and structural modifications derived from the anion exchange on Zn2/3Al1/3(OH)(2)Cl-1/3 center dot 2/3H(2)O LDH. When the Cl- anion is exchanged by F-, Br- or OH-, any significant structural variation on the material is not observed. Even so, the more strongly bound anions are those with smaller ionic radii, which result in a decreased spacing between the hydroxyl layers. The CO32- and the NO3- anions have identical symmetry in free state, but their behavior as interlayer anions in LDHs is very different. After the optimization, the carbonate anion kept the orientation parallel to the hydroxide layers. This configuration is energetically more favorable because the three oxygen atoms of the CO32- anion can properly interact with hydroxyl groups of hydroxide layers by forming hydrogen bonds. Unlike the carbonate, the nitrate ion has its molecular plane tilted to a direction that forms an angle with c vector, which is less than 90 degrees. The orientation of the NOT anion appears to vary depending on the amount of co-intercalated water molecules. The anion-exchange selectivity in the LDH follows the sequence: NO3-<Br-<Cl-<F-<OH-<CO32-, obtained by the calculation of the Gibbs free energy. (C) 2011 Elsevier B.V. All rights reserved.