Ab Initio Simulation of Changes in Geometry, Electronic Structure, and Gibbs Free Energy Caused by Dehydration of Hydrotalcites Containing Cl- and CO32- Counteranions

This ab initio study was performed to better understand the correlation between intercalated water molecules and layered double hydroxides (LDH), as well as the changes that occur by the dehydration process of Zn-Al hydrotalcite-like compounds containing Cl- and CO32- counterions. We have verified that the strong interaction among intercalated water molecules, cointercalated anions, and OH groups from hydroxyl layers is reflected in the thermal stability of these compounds. The Zn2/3Al1/3(OH)(2)Cl-1/3 center dot 2/3H(2)O hydrotalcite loses all the intercalated water molecules around 125 degrees C, while the Zn2/3Al1/3(OH)(2)(CO3)(1/6)center dot 4/6H(2)O compound dehydrates at about 175 degrees C. These values are in good agreement with experimental data. The interlayer interactions were discussed on the basis of electron density difference analyses. Our calculation shows that the electron density in the interlayer region decreases during the dehydration process, inducing the migration of the Cl anion and the displacement of the hydroxyl layer from adjacent layers. Changes in these compound structures occur to recover part of the hydrogen bonds broken due to the removal of water molecules. It was observed that the chloride ion had initially a lower Lowdin charge (Cl-0.43), which has increased its absolute value (Cl-0.58) after the water molecules removal, while the charges on carbonate ions remain invariant, leading to the conclusion that the Cl anion can be more influenced by the amount of water molecules in the interlayer space than the CO32- anion in hydrotalcite-like compounds.