Thermal conversion of a layered (Mg/Al) double hydroxide to the oxide

Abstract

A new type of experiment involving temperature-variable in situ(one-dimensional)²⁷Al MAS NMR studies, in which the sample is heated and studied directly in the NMR tube, has been developed. In the particular case chosen for study, the thermal conversion of the layered double hydroxide Mg₆Al_3.4(OH)_18.82(CO₃)_1.5(NO₃)_0.364.5H₂O to its corresponding oxide showed a diminution of the octahedral site and the formation of the tetrahedral site as the sample was heated. The experiment clearly showed that a tetrahedral site was formed by heating to 100 °C but the existence of a fivecoordinate site at elevated temperatures remains in doubt. Nutation experiments, however, on the layered double hydroxide showed that there are two types of octahedral environments which implies that the structure is more complex than has been considered previously. On slow heating to the oxide, the intensity of the peaks associated with one of the octahedral sites, in which the aluminium is probably associated with the carbonate, is decreased. Rapid heating in an oven at 450 °C produces a different oxide in which there are two octahedral and two tetrahedral sites. The rate of heating is clearly an important factor in the conversion of the layered double hydroxide (LDH) to the corresponding oxide (LDO).