27Al and 25Mg solid-state magic-angle spinning nuclear magnetic resonance study of hydrotalcite and its thermal decomposition sequence

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Abstract
Solid-state 27Al and 25Mg MAS NMR and X-ray powder diffraction in combination with thermal analysis indicates that synthetic hydrotalcite decomposes thermally in at least three weight-loss steps. The first step, at 250–280 °C, principally involves the loss of the interlayer water, but the appearance of some tetrahedral Al in the 27Al NMR spectra suggests that this is accompanied by the onset of Al dehydroxylation. The second step, at 300–400 °C, is accompanied by a rapid increase in the proportion of tetrahedral Al to a maximum at 400 °C, indicating full Al dehydroxylation of the Al regions. The third step, above 400 °C, represents loss of interlayer carbonate and dehydroxylation of Mg and is accompanied by the appearance of poorly crystalline MgO in both the X-ray and 25Mg NMR spectra. In the range ca. 400–900 °C, we suggest that the phase assemblage consists of intergrown microdomains of poorly crystalline MgO, possibly containing substituent Al3+ and vacancies, and a phase with NMR characteristics similar to spinel-based aluminas such as γ-Al2O3. By 1200 °C, migration of Al from the MgO phase and of Mg into the spinel has occurred, resulting in the appearance of crystalline pure MgO and MgAl2O4 with an inversion parameter of ca. 0.3.

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