Cálculos de estrutura e propriedades de novos materiais formados por gibbsita e lítio

Abstract

Layered double hydroxides (LDHs) have attracted considerable interest due to their potential applications as selective adsorbents, hosts for drugs and biomolecules, precursors for ceramics and nanocomposite materials. An important aspect in the LDH synthesis is the nature of the cations. Since the preparation of a new compound is based on the isomorphic substitution of a metal in an oxidation state by another in a different (or in the same) oxidation state, the difference between the radius can not be very large. It is known from the literature that a family of LDH is structurally related to the gibbsite (Al(OH)3). It is a hydrargilite formed by a hydroxyl layer (OH-) with aluminum ions (Al3+) occupying two-thirds of the octahedral sites. The alternance between occupied site and vacancy creates a small deformation of the octahedron. As a result, the vacancy is slightly wider compared to the occupied site. The vacancies of the gibbsite layer have the diameter comparable to small cations such as Li1+, without large deformations, once the atomic radius of Li1+ and Al3+ are close. In this case, the generated charge is counterbalanced by anions and water molecules in the interlayer space. This is the only example of LDH formed by 1+ and 3+ cations. The stacking structure of these materials based on electrostatic interaction between the layers allows the interlayer anion diffusion. The understanding of the influence of different interlayer anions in the LDH structure have great importance to help on theoretical planning for technological applications, since the intercalation behavior is important in many processes. The present work discusses the structural modifications derived from anion exchange on Li- Al-Cl formed by the treatment of the gibbsite with lithium chloride (LiCl) in solution, by ab initio calculations. The Cl− ion in the LDH precursor was exchanged for a series of anions, F−, Br−, OH−, I−, NO3 − , CO3 −2 and SO4 −2. The XRD, NMR and IR spectra were evaluated, also the charge density difference and projections density state were evaluated. The Gibbs energy and the enthalpy for the formation reactions of the [LiAl2(OH)6]Cl.nH2O were also calculated.