Reconstruction of layered double hydroxides from calcined precursors: a powder XRD and Al-27 MAS NMR study

The reconstruction of layered double hydroxides with the hydrotalcite-like structure from material previously calcined at increasing temperatures has been studied. Both the calcination temperature and the time the samples spe nt equilibrating with a controlled water vapour pressure have been consider ed. The crystallinity of the samples and the crystalline phases formed have been assessed by powder X-ray diffraction, while the local environment of the Al ions has been probed by Al-27 MAS NMR. It has been found that recons truction is complete when the sample has been calcined at or below 550 degr ees C, after 24 h rehydration, while equilibration for 3 days is adequate t o reconstruct samples previously calcined at 750 degrees C. Only partial re construction is observed after calcination at 1000 degrees C. However, the NMR results indicate that, in all cases, some of the Al ions remain in tetr ahedral sites.