A time resolved, in-situX-ray diffraction study of the de-intercalation of anions and lithium cations from [LiAl2(OH)6]nX·qH2O (X = Cl−, Br−, NO3−, SO42−)

Abstract

The layered double hydroxides (LDHs), [LiAl₂(OH)₆]_nX·qH₂O (Xⁿ⁻ = Cl⁻, Br⁻, NO₃⁻, SO₄²⁻) spontaneously lose both Li⁺ cations and Xⁿ⁻ anions to give crystalline Al(OH)₃ on stirring in water. The kinetics of this topochemical de-intercalation reaction have been studied using time-resolved, in-situ energy dispersive X-ray powder diffraction. The temperature dependence of the rate of the de-intercalation reaction yields activation energies of 94 ± 15 kJ mol⁻¹ for mono-dispersed [LiAl₂(OH)₆]Cl·1.5H₂O crystallites (particle size 5–10 µm) and 117 ± 15 kJ mol⁻¹ for poly-dispersed [LiAl₂(OH)₆]Cl·1.5H₂O crystallites (particle size 1–30 µm). The rate of ion de-intercalation was found to decrease as the H₂O/LDH ratio decreased. The rate of the de-intercalation has also been shown to be dependent on the interlayer anion, with the reaction rates NO₃⁻ > Cl ⁻ > Br⁻. In the case of SO₄²⁻ rapid de-intercalation only goes to 40% completion.