Hidróxidos duplos lamelares com potencial aplicação no agronegócio por cálculos ab initio

Abstract

Layered double hydroxides (LDH) or hydrotalcite-like compounds introduces as its main feature, a large surface area, thermal stability and high anion exchange capacity. These characteristics enable the use of HDL in various applications, in catalysis, medicine,photochemistry and environmental. This compounds have been attracting a considerable attention from the academy and the industry. In this work, LDHs that can be applied in agribusiness were studied. LDHs models were built with intercalated anions that are nutrients for plants, monohydrogen phosphate (HPO4 2-), nitrate (NO3 -) and also, another LDHs intercalated with the herbicides 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2- methylphenoxyacetic (MCPA). The release of nutrients is proposed by the anion exchange reaction for carbonate (CO3 2-), which studies have shown to be a thermodynamically favored reaction. However, for LDHs intercalated with herbicides, only the geometry and the electronic structure were studied. Regarding the layer composition, the layers Mg2Fe, Mg2Al, and Zn2Al were studied, all of them with a 2:1 molar fraction. For the LDH Mg2Fe due to the presence of an open shell atom (Fe), studies were carried out involving, restricted, unrestricted and DFT + U calculations. The HPO4 2- anion can be intercalated in the LDH with different stable orientations which can cause differences in the basal spacing. The stability of the different orientations was evaluated, and a phase diagram for the dehydration process of the most stable structure was constructed. The minerals motukoreaite and natroglaucocerinite that are hydrotalcite-like compounds were also studied. The stability of these materials was evaluated in two different polytypes 1H and 3R. Thermodynamics for the cation exchange reaction was also investigated. In these minerals, in addition to the intercalated anion, also have a intercalated hexahydrate cation. The cation exchange of potassium (K+) by sodium (Na+) was thermodynamically supported. This is the first work using DFT to perform cationic exchange in LDH, besides that K+ is also a nutrient for plants. The study of the energy and nature of the bandgap for LDH was performed using LDHs with layer composition of Mg2Al,Zn2Al intercalated with NO3 -, chloride (Cl-), hydroxyl (OH-) and CO3 2- anions. The correction of the Kohn-Sham states was done using the GW method, which showed both corrections, for the electronic bandgap, and for the band energy. The HSE06 hybrid functional, also used in DFT calculations, showed an improvement in terms of the gap energies, compared to the PW91 functional. However, there is no difference in the positioning of the bands. The layer composition and the intercalated anions, both affect the bandgap energy in LDHs.