Thermal effects 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 degrees C of the Zn/Al (ratio 2: 1) synthetic hydrotalcites having chloride and carbonate as interlayer anions, of formula [Zn0.65Al0.35(OH)(2)]Cl-0.35 center dot 0.35H(2)O (I) and [Zn0.65Al0.35(OH)(2)](CO3)(0.175)center dot 0.69H(2)O (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 >= 200 degrees C, (ii) the water loss is completed at 110 degrees C, (iii) the Zn2+ segregation process starts to be visible over 110 degrees C, and (iv) the weight loss in the temperature range 500-900 degrees 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 degrees 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 degrees C; and (iv) like M-/Al LDHs, for completely dehydrated II the LDH structure is restored upon rehydration. The evaporation enthalpy of water is Delta H = 46 kJ mol(-1) for I and Delta H = 100.6 kJ mol(-1) for water content < 0.5 mol/mol and Delta H = 69.80 kJ mol(-1) for water content > 0.5 mol/mol for II.