Theoretical studies of the interaction of terephthalate anion in MgAl-layered double hydroxides

Abstract

We used first principles calculations based on Density-Functional Theory to investigate the structural and electronic properties of Mg–Al-TA (TA=terephthalate anion) layered double hydroxides for the relations x=NAl/(NAl+NMg) equal to 1/4, 1/3 and 1/2. Value of x=1/2 is unusual in the literature due to the so-called cation avoidance rule; notwithstanding, some experimental results have demonstrated that it is possible to be obtained using TA as counterion. The calculated geometric parameters are in good agreement with available experimental data. The gauge-including projector augmented wave method was applied to compute solid state nuclear magnetic resonance chemical shifts of terephthalate's carbons for structure with x=1/3. A good correlation between the minimum energy for TA rotating angle and the chemical shifts was obtained. The interlayer interactions were discussed on the basis of electronic density differences and Bader charge analysis. Our calculations show that the electron density in the interlayer region does not change significantly while the molar ratio x increases. This result is in line with the Bader analysis. The calculations show that there is a major change in the difference of charge density between water molecules and layer than anion and layer when the molar ratio x increases. This result demonstrates the importance of water molecules in the stabilization of LDHs with higher layer charge. Also, a geometric factor helps in the formation of structures with x=1/2. That is, the area occupied by each positive charge in the layers needs to be large enough to have the TA anions in the interlayer space along the c structural vector.