Estudo teórico de hidróxidos lamelares com potencial aplicação tecnológica e ambiental

Abstract

The layered double hydroxide (LDH) or hydrotalcite-like compounds have been widely studied due to many applications they have. In catalysis field these compounds can act as catalyst, catalyst support or precursor for supports and catalysts. It is well known that the support morphology has larger influence on the dispersion, activity and selectivity of catalysts. In this case, detailed knowledge about support effects and catalytic sites requires understanding the LDH structure. However, because of the inherent structural disorder, complete structure determination using X-ray diffraction is rarely possible. The IR analysis present broad bands, which makes their assignment ambiguous. Due to the complexity of the LDH and the difficult of acquiring experimental data, we proposed a model to simulate structural features of the hydrotalcite-like compounds with 3R1 polytypes using periodic boundary conditions and ab initio density functional theory (DFT) calculations. The structural information of hydrotalcite from experiments were used to verify the validity of theoretical model. Our results showed good agreements to the powder X-ray patterns. The infrared spectroscopy, enthalpy and Gibbs free energy, also was calculated. The IR calculations showed that intercalated water molecules are highly ordered due the strong intermolecular interactions with other water molecules, interlayer anions and layer hydroxyls. These interaction is reflected by the presence of downshift vibrational frequencies of OH-stretching of interlayer water. The loss of degeneracy of the v3 and v4 vibrational modes are observed, indicating the reduction of the carbonate anion symmetry. The thermodynamic and structural modifications derived from anion exchange on a LDH containing Zn-Al-Cl was also analyzed. The Cl- ion to the LDH precursor was exchanged for a series of anions as F-, Br-, OH-, NO3-, ClO4-, CO32- and SO42-. The resulted show that intercalation of LDH with different anions affect their gap energy and basicity of brucite-like layer. The enthalpy and Gibbs free energy, density of charge, density of state (DOS) and projected density of state (PDOS) were evaluated for all substituted materials.