Dehydration and Dehydroxylation of Layered Double Hydroxides: New Insights from Solid-State NMR and FT-IR Studies of Deuterated Samples

The dehydration and dehydroxylation processes (25–375 °C) of a variety of Mg and Al containing layered double hydroxides (LDHs), (Mg_1–<i>x</i>Al_<i>x</i>(OH)₂(A^<i>n</i>–)_{<i>x</i>/<i>n</i>}·<i>y</i>H₂O, where <i>x</i> = 0.197–0.273 and A^<i>n</i>– = NO₃^–, ClO₄^–, Cl^–, or CO₃^2–) were carefully investigated by using high resolution solid-state ¹H and ²⁷Al solid-state NMR, as well as FT-IR spectroscopy on deuterated samples. Dehydration is found to occur at a lower temperature (usually below 150 °C) than dehydroxylation (usually above 150 °C), the latter consisting of two subprocesses. Definitive evidence shows that the dehydroxylation of Mg₂AlOH species start at a lower temperature than Mg₃OH, however, the temperature ranges for the two processes overlap significantly. Furthermore, dehydration and dehydroxylation are affected by both the nature of the charge compensating anions in the interlayer and the ratio of the intralayer cations (Mg/Al). The fact that FT-IR spectra with higher resolution can be obtained on deuterated samples implies this approach can be readily used for in situ study of LDHs