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

Abstract

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.

Surface Base Sites of MgO Covered with Al2O3: XANES Analysis of Al and Mg K-edges

“…The peak at 1565 eV is a typical fraction of AlO 4 tetrahedra [16,23]. The AlO 6 octahedra generally show two resonance peaks above the absorption edge at 1568 and 1572 eV, whereas the AlO 4 tetrahedra show the absorption edge at 1566 eV, as reported by Bokhoven et al [22]. The spectral patterns of Al 2 O 3 /MgO catalysts in the low Al 3?…”

“…in octahedral sites gave two prominent peaks at 1567 and 1572 eV at Al K-edge XANES; the intensity of the peak at 1572 eV being larger. The feature of XANES can be attributed to the distorted octahedral AlO 6 local structure [22]. In contrast, sodium aluminate contains a three-dimensional framework of corner-linked AlO 4 tetrahedra.…”

Local Structure Analysis of MgB₂ Thin Films by Polarized XAFS

“…The sturcture of 10 nm film in the horizontal direction is similar to that of the diopsite compound [7]. On the other hand, in the vertical direction the structure is close to spinel structure, N =4 [8]. As seen here, the 10 nm MgB 2 film has different structure from the hexagonal MgB 2 (50 nm film in Fig.…”

Thermal decomposition and structural reconstruction effect on Mg–Fe-based hydrotalcite compounds

“…This phenomenon has been observed in Mg-Al and Zn-Al HTs [17], and it is, particularly, well-known in Mg-Albased HTs [18][19][20]. Total reconstruction occurs only in a given temperature range where the thermal decomposition is performed.…”