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** Jeroen A. van Bokhoven,* Jules C. A. A. Roelofs, Krijn P. de Jong, and Diek C. Koningsberger [a] Abstract: The changes in the layered structure of Mg ± Al hydrotalcite (Mg/ Al  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 coordi- nation 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 develop- ment of hydrotalcites as new solid basic catalysts. Moreover, the Mg and Al cations in the cation layers show differ- ent behavior as a function of temper- ature. The coordination of some octahe- dral Al ions decreases already at a temperature of 425 K, whereas the co- ordination about Mg does not show any modification at this temperature. How- ever, hydrotalcite treated at 425 K, fol- lowed by cooling down to room temper- ature resulted in a complete reversal to the original octahedral Al coordination. It is proposed that Al OH bond break- age 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 sub- sequent rehydration at room temper- ature entirely restores the original coor- dination about the Al and Mg centers of hydrotalcite to a distance of 15 , to which XAFS spectroscopy is sensitive. Keywords: EXAFS spectroscopy ´ heterogeneous catalysis ´ hydrotal- cite ´ solid bases Introduction Hydrotalcites (HTs) and structures derived therefrom have many applications that are related to their structural proper- ties. [1] Recently, modified HTs have proven to be active in the coupling of various ketones and aldehydes in base-catalyzed aldol condensation reactions, [2, 3] such as the condensation of citral and acetone to yield pseudoionone, which is an industrially important intermediary for the production of vitamin A. Usage of a solid base catalyst instead of the currently applied homogeneous alkaline bases makes reuse of the catalyst possible, leading to a reduction of waste streams. Furthermore, modified HTs display high activity already at low temperatures. These properties make HT-like compounds promising catalysts for commercial use. The structure and the related activity of HT-like com- pounds are strongly related to the applied heat treatment. [3, 4] A calcination step at high temperature (723 ± 773 K) followed by a rehydration step at room temperature yields a highly active catalyst for the catalytic reactions mentioned above. An understanding of the processes leading to an active catalyst, demands detailed knowledge of the structural changes in HT during heat treatment. Hydrotalcite, [Mg 6 Al 2 (OH) 16 ](CO 3 ) ´ 4H 2 O, is a layered double hydroxide (LDH). LDH structures are related to that of brucite, Mg(OH) 2 , in which the Mg cations occupy the centers of hydroxy octahedra. These octahedra are joined along their edges, forming a layered structure, composing hexagonal platelets. [1] In hydrotalcite some of the Mg 2 ions are replaced by Al 3 ions, inducing a net positive charge in the cation layers. Charge-balancing anions (usually CO 3 2 ) and water molecules are present in the interlayers. X-ray diffraction (XRD), 27 Al MAS NMR, IR, thermal gravimetric analysis (TGA), and CO 2 temperature program- [a] Dr. J. A. van Bokhoven, J. C. A. A. Roelofs, Prof. K. P. de Jong, Prof. D. C. Koningsberger Debye Institute, Department of Inorganic Chemistry and Catalysis Utrecht University Sorbonnelaan 16, 3508 TB Utrecht (The Netherlands) Fax: ( 31) 302536763 E-mail : j.a.vanbokhoven@chem.uu.nl [**] XAFS  X-ray absorption fine structure. FULL PAPER  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 0947-6539/01/0706-1258 $ 17.50+.50/0 Chem. Eur. J. 2001, 7 , No. 6 1258