Unique structural properties of the Mg-Al hydrotalcite solid base catalyst: An in situ study using Mg and Al K-edge XAFS during calcination and rehydration

The changes in the layered structure of Mg-Al hydrotalcite (Mg/Al = 2) duri ng heat treatment have been investigated by using in situ XAFS simultaneous ly at the Mg and Al K-edges. The development of unique in situ instrumentat ion allowed the coordination environments at both the Mg and Al centers to be monitored as a function of the temperature and heat treatment. The resul ts of this study show that the hydrotalcite structure is highly flexible, a nd should lead to the further development of hydrotalcites as new solid bas ic catalysts. Moreover, the Mg and Al cations in the cation layers show dif ferent behavior as a function of temperature. The coordination of some octa hedral Al ions decreases already at a temperature of 425 K, whereas the coo rdination about Mg does not show any modification at this temperature. Howe ver, hydrotalcite treated at 425 K, followed by cooling down to room temper ature resulted in a complete reversal to the original octahedral Al coordin ation. It is proposed that Al-OH bond breakage occurs at approximate to 425 K, without the evolution of H2O, This bond is restored after cooling to roo m temperature. The actual dehydroxylation of hydrotalcite commences between 425 and 475 K, as indicated by a change in coordination of both the Mg and Al centers. This is accompanied by the evolution of H2O molecules and the changes are hence irreversible without the presence of excess water. Heat t reatment at 725 K leads to the development of an MgO-like phase (octahedral Mg) and a mixed octahedral/tetrahedral Al phase. A subsequent rehydration at room temperature entirely restores the original coordination about the A l and Mg centers of hydrotalcite to a distance of 15 Angstrom, to which XAF S spectroscopy is sensitive.