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

Abstract

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 spent equilibrating with a controlled water vapour pressure have been considered. 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 ²⁷ A1 MAS NMR. It has been found that reconstruction is complete when the sample has been calcined at or below 550 °C, after 24 h rehydration, while equilibration for 3 days is adequate to reconstruct samples previously calcined at 750 °C. Only partial reconstruction is observed after calcination at 1000 °C. However, the NMR results indicate that, in all cases, some of the Al ions remain in tetrahedral sites.