Structural Model Proposition and Thermodynamic and Vibrational Analysis of Hydrotalcite-Like Compounds by DFT Calculations

Abstract

We propose a model to calculate the electronic structure of hydrotalcite-like compounds by using periodic boundary conditions and ab initio density functional theory (DFT). The proposed method to build up layered double hydroxides (LDHs) was tested for Zn2/3Al1/3(OH)2Cl1/3·2/3H2O, Zn2/3Al1/3(OH)2(CO3)1/6·4/6H2O, and Mg2/3Al1/3(OH)2(CO3)1/6·4/6H2O with 3R1 polytype. In the model, the occupation of cationic sites in hydroxide layers is ordered and the interlayer anions and water molecules form a film between the layers. Direct comparison with experimental structural parameters shows good agreement. The a parameter is close to that in brucite for all three LDHs. The c parameter is smaller (about 1 Å) for the LDHs with CO32− as a consequence of its strong interaction with hydroxide layers. Those interactions were evidenced by the difference of density and vibrational analysis. The intercalated water molecules have small mobility and interact strongly with one another and with interlayer anions and hydroxide layers. These interactions cause the downshift in the calculated vibrational wavenumbers of water O−H stretching modes below 3420 cm−1 and are consistent with the reported infrared spectra of hydrotalcite-like compounds. The calculated formation enthalpies for LDHs with carbonate are in agreement with the previously reported trend. The biggest difference between theoretical and experimental values is 2 kcal/mol. The calculated formation Gibbs energies are negative. The zero point energy is important to evaluate ΔH, but the formation entropy does not affect the Gibbs free energy significantly.