Thermal Effect on Mixed Metal (Zn/Al) Layered Double Hydroxides: Direct Modeling of the X-Ray Powder Diffraction Line Shape Through Molecular Dynamics Simulations

The effects of thermal treatment in the range 25-200 °C of the Zn/Al (ratio 2:1) synthetic hydrotalcites having chloride and carbonate as interlayer anions, of formula [Zn0.65Al0.35(OH)2]Cl0.35 · 0.35H2O (I) and [Zn0.65Al0.35(OH)2](CO3)0.175 · 0.69H2O (II), were studied using an integrated X-ray powder diffraction (XRPD)-molecular dynamics (MD) approach. The main novelty of this study consists in calculating the XRPD pattern with several MD structure models of the selected series instead of using a single structure. This was needed to match the experimental XRPD curve whose line shapes contain all the information useful to describe the polycrystalline material. The thermal characterization showed in the case of I that (i) the stability of the dehydrated LDH structure is maintained up to temperature g 200 °C, (ii) the water loss is completed at 110 °C, (iii) the Zn2+ segregation process starts to be visible over 110 °C, and (iv) the weight loss in the temperature range 500-900 °C is attributable to ZnCl2 evaporation, while in the case of II that (i) the dehydrated LDH structure is unstable; (ii) for temperatures up to 150 °C, the amount of intrinsic interlayer water is unaffected in the bulk, (iii) the loss of typical brucite-like structure occurs in the range 150-180 °C; and (iv) like Mg/Al LDHs, for completely dehydrated II the LDH structure is restored upon rehydration. The evaporation enthalpy of water is ΔH ) 46 kJ mol-1 for I and ΔH ) 100.6 kJ mol-1 for water content < 0.5 mol/mol and ΔH ) 69.80 kJ mol-1 for water content > 0.5 mol/mol for II