Investigação estrutural e aplicações de silicatos lamelares do tipo Magadiita a partir de cálculos ab initio baseados na DFT.

Abstract

Na-Magadiite (Na2Si14O28(OH)2·8H2O) is a hydrous layered silicate formed by layers of siloxanes (SiO4) and silanols/silanolates (SiO3OH/SiO3O - ) neutralized by sodium cations hexacoordinated with water molecules. This solid is well known for producing modified materials for use in polluting gas adsorbents and/or catalysts. Its structure has been without structural elucidation for a long time and several proposals have been unsuccessfully suggested in the literature. Thus, the main objective of this thesis is to present the structural investigation Na-magadiite, recently described through the Rietveld refinement of 3D electron diffraction combined with the DFT calculations of this work. The crystallographic data from the experimental refinement were optimized in their geometries and the main peaks in the XRD patterns were maintained after the calculation. The analysis of the interlayer region showed two positions of coordinated water molecules (axial and equatorial). The hydrogen bonds with oxygen atoms in the layer were 1.8 Å and 2.0 Å, respectively, and distinct interactions were confirmed by the charge density. Simulated RMNES spectra corroborated the experimental data regarding the Q3 (-95.0 ppm) and Q4 (-105.0 to -114.0 ppm) sites for the 29Si nucleus; octahedral geometry signal (0.0 ppm) in the 23Na nucleus; and signals related to water molecules (3.0 to 10.0 ppm) and silanol groups (15.0 ppm) in the 1H nucleus. The thermodynamic analysis of three steps of calcination of this solid were performed. The dehydration of the interlayer region showed two distinct line graphics for Gibbs, in which ΔGº < 0 kJ.mol-1 with axial H2O leaving from 32 ºC, while ΔGº < 0 kJ.mol-1 for equatorial H2O molecules from of 150°C. For both, the enthalpy curves showed endothermic reactions. In the second step, the Gibbs curve suggested that zeolitic sodium silicate is thermodynamically more favorable in relation to Si14O28 zeolite during layer condensation. The final step showed ΔGº < 0 kJ.mol-1 to form sodium quartz silicate. In the last part, the cation-exchanged models were evaluated: K-, Ca- and Mg-magadiite. The interlayer region of each optimized material showed water behavior similar to the sodium form, leading to the same XRD pattern peaks at 25º – 30º in 2θ. Bivalent cations showed higher water-layer interaction and lower coordinated bond length compared to K+ and Na+ . Distinct bands related to equatorial and axial water movements in the infrared spectra suggested that, in experimental isotherms of adsorption, the first coordination around the dehydrated cation occurred by axial waters molecules and, only after filling the hydration sphere, equatorial positions are identified in the final structure