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A study on the catalytic properties of properly activated hydrotalcite (HT) with special attention to the nature and amount of active sites present in this solid base catalyst has been undertaken. Only a small fraction (5%) of the available basic sites in the rehydrated calcined HT is active in liquid-phase aldol condensations. These sites exhibit high catalytic activity and are most likely localized at the edges of the HT-platelets. Besides a high activity, these modified HTs also show a high selectivity. No further condensation products other than diacetone alcohol (DAA) in the acetone self-condensation could be observed. Initial results with the citral-acetone condensation show that even at 273 K this reaction is catalyzed by modified HTs with a conversion of 65% and a selectivity of 90%, when the citral concentration is not too high (∼1 wt.%). At higher citral concentrations, no reaction is observed indicating a negative order in citral concentration.

The thermal decomposition of hydrotalcites (HTs) with different interlayer anions in the 298–523 K region has been investigated by using transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X‐ray diffraction (XRD), and IR, 27Al MAS‐NMR and X‐ray absorption near‐edge structure (XANES) spectroscopy. The thermal stability of the HT with interlayer oxalate was remarkably higher than that of HTs with interlayer carbonate; the onset temperatures for decomposition were 523 K and 473 K, respectively. It is proposed that the basicity of the interlayer anion is the key factor in the onset of dehydroxylation of the brucite‐like layers: the lower the basicity, the more thermally stable the HT compound. After heat treatment at 723 K, small Mg(Al)O domains (≈5 nm) within the HT crystallites cause broadening of the XRD reflections. The electron diffraction pattern consists of spots and rings, due to nonrandomly oriented crystalline material. Formation of disordered bonds, caused by nonideal packing between the decomposing adjacent cation layers in the (1 1 1) direction, could explain the expanded d value in the resulting MgO‐like phase observed with XRD and electron diffraction. The orientation of the Mg(Al)O domains in the heat‐treated material may be crucial for the so‐called “memory effect” of HTs.

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The changes in the layered structure of Mg ± Al hydrotalcite (Mg/ Al ˆ 2) during heat treatment have been investigated by using in situ XAFS simultaneously at the Mg and Al K-edges. The development of unique in situ instrumentation allowed the coordination environments at both the Mg and Al centers to be monitored as a function of the temperature and heat treatment. The results of this study show that the hydrotalcite structure is highly flexible, and should lead to the further development of hydrotalcites as new solid basic catalysts. Moreover, the Mg and Al cations in the cation layers show different behavior as a function of temperature. The coordination of some octahe-dral Al ions decreases already at a temperature of 425 K, whereas the coordination about Mg does not show any modification at this temperature. However , hydrotalcite treated at 425 K, followed by cooling down to room temperature resulted in a complete reversal to the original octahedral Al coordination. It is proposed that AlÀOH bond breakage occurs at % 425 K, without the evolution of H 2 O. This bond is restored after cooling to room 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 H 2 O molecules and the changes are hence irreversible without the presence of excess water. Heat treatment 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 Al and Mg centers of hydrotalcite to a distance of 15 Š, to which XAFS spectroscopy is sensitive.