AL-27 AND MG-25 SOLID-STATE MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE STUDY OF HYDROTALCITE AND ITS THERMAL-DECOMPOSITION SEQUENCE

Solid-state Al-27 and Mg-25 MAS NMR and X-ray powder diffraction in co mbination with thermal analysis indicates that synthetic hydrotalcite decomposes thermally in at least three weight-loss steps. The first st ep, at 250-280-degrees-C, principally involves the loss of the-interla yer water, but the appearance of some tetrahedral Al in the Al-27 NMR spectra suggests that this is accompanied by the onset of Al dehydroxy lation. The second step, at 300-400-degrees-C, is accompanied by a rap id increase in the proportion of tetrahedral Al to a maximum at 400-de grees-C, indicating full Al dehydroxylation of the Al regions. The thi rd step, above 400-degrees-C, represents loss of interlayer carbonate and dehydroxylation of Mg and is accompanied by the appearance of poor ly crystalline MgO in both the X-ray and Mg-25 NMR spectra. In the ran ge ca. 400-900-degrees-C, we suggest that the phase assemblage consist s of intergrown microdomains of poorly crystalline MgO, possibly conta ining substituent Al3+ and vacancies, and a phase with NMR characteris tics similar to spinel-based aluminas such as 7-Al2O3. By 1200-degrees -C, migration of Al from the MgO phase and of Mg into the spinel has o ccurred, resulting in the appearance of crystalline pure MgO and MgAl2 O4 with an inversion parameter of ca. 0.3.