Molecular simulation, application, synthesis and characterization of layered double hydroxide in search of anionic clays
Layered double hydroxides (LDH) belong to a large class of anionic and basic clays, also known as Hydrotalcite-like compounds (HTlc). They are composed of positively charged brucite-like [Mg(OH)2] layers with trivalent cations substituting for divalent cations at the centers of octahedral sites of hydroxide sheets whose vertexes contain hydroxide ions, and each OH 2 group is shared by three octahedral cations and points toward the interlayer regions. Synthesis of Layered Double Hydroxide (LDHs) of Mg/Al-NO 3-1 and Mg/Al-CO3-2 of molar ratio of Mg/Al (3:1) with composition Mg1-xAlx-NO 3 (where X = M (III)/(M (III) + M (II)) = 1/3 in the starting solution) were prepared by a coprecipitation method under different synthesis temperatures. Their structural and textual properties were examined. It is found from the results that the increase of the synthesis temperature will increase the interlayer spacing of the layered compounds; the higher temperature activates more anions and water molecules to enter into the interlayer, and the interlayer spacing increases gradually with the temperature, but at room temperature, the sample shows the best crystallinity. The samples have been characterized by X-ray diffraction (XRD) and Fourier transforms infrared spectroscopy (FTIR). The X-ray spectra showed that basal spacing for synthesized Mg/Al-NO3-1 and Mg/Al-CO 3-2 was between 7.84 and 7.47A°, respectively. The expansion of the layered structure was observed to accommodate the surfactant anion between the interlayers. Molecular simulations of Mg/Al Layered Double Hydroxide with an interlayer NO3-1 were performed to better understand the structure of layered double hydroxides and their hydration behavior. A set of models with variable numbers of interlayer water molecules was investigated, with the assumption of no constraints on the movements of any atoms or on the geometry of the simulation super cells. Crystallographic parameters and two components of the hydration energy v were calculated. One of these components is related to the interaction of water molecules with the rest of the structure and is controlled primarily by formation of a hydrogen-bonding network in the interlayer. The other is related to expansion of the host structure itself and reflects decreasing electrostatic interactions as the c-axis expands upon swelling. The dependence of these two energy components on the degree of hydration provides useful insight into the nature of hydrotalcite swelling behavior.
Baki, Musa, "Molecular simulation, application, synthesis and characterization of layered double hydroxide in search of anionic clays" (2008). ETD Collection for University of Texas, El Paso. AAI1456733.