FULL PAPER DOI: 10.1002/ejic.200600580 Thermal Evolution of a MgAl Hydrotalcite-Like Material Intercalated with Hexaniobate Daniel Carriazo, [a] Cristina Martín, [a] and Vicente Rives* [a] Keywords: Layered double hydroxides / Polyoxometalates / Nb-Mg-Al mesoporous oxides / Intercalations A MgAl hydrotalcite-like material intercalated with hexa- niobate has been prepared by the anion exchange method from the corresponding nitrate precursor; the sample and the oxides obtained upon its calcination were characterized by element chemical analysis, powder X-ray diffraction, thermal analyses (thermogravimetric and differential), nitrogen ad- sorption-desorption isotherms at –196 °C, transmission elec- tron microscopy, and FT-IR and UV/Vis spectroscopy. The re- sults show the formation of a microporous hydrotalcite-type solid with a gallery height of 7.2 Å where the H 3 Nb 6 O 19 5– anions are oriented with their C 3 axes perpendicular to the layers. This material preserves its layered structure upon cal- Introduction Layered Double Hydroxides (LDHs), also known as hydrotalcite-like compounds or anionic clays, are lamellar materials generally described by the empirical formula M 2+ 1–x M 3+ x (OH) 2 (A m– ) x/m ·nH 2 O, where A m– is the anion balancing the net positive charge of the brucite-like lay- ers [1–3] developed by a partial M 2+ /M 3+ substitution. A large number of compounds with this structure have been synthesized in the last decades varying the nature and the molar ratio of the cations in the layers and the nature of the interlayer anions. These compounds have shown their suitability in different fields, acting as anion scavengers in wastewater, biomolecule reservoirs or as polymer additives to improve its properties. [4–6] Nevertheless, there is no doubt that it is in the field of catalysis where they find the widest number of applications, as catalysts, catalyst precursors or supports. [7] These last applications are mainly due to the fact that calcination of the mentioned materials gives rise to systems constituted by different phases (mixed metal ox- ides) which usually show higher stability, larger surface de- velopment and longer life-time than others prepared by conventional methods. Polyoxometalates (POMs) are anionic compounds with a cluster structure formed by linked oxygen octahedra coordi- nating transition metals (V, Mo, W, isopolyoxometalates), although in many cases a heteroatom (P, Si) in a tetrahe- [a] Departamento de Química Inorgánica, Universidad de Salam- anca, 37008 Salamanca, Spain © 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2006, 4608–4615 4608 cination up to 400 °C; calcination above this temperature causes decomposition of the hexaniobate and the layered structure collapses, giving rise to amorphous mesoporous sol- ids (Nb-Mg-Al-O) with a large specific surface area (157 m 2 g –1 ). At 800 °C crystallization of Mg 4 Nb 2 O 9 takes place. FT-IR studies on the acid-basic properties carried out by pyridine and 2-propanol adsorption, showed that all solids obtained through hydrotalcite calcination present Lewis-type acid and Brönsted-type basic sites. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) drally coordinated unit is included in the structure (hetero- polyoxometalates). [8] These compounds have also deserved much attention because of the multiple applications they find in many different fields, such as medicine, materials science and catalysis. [9–11] POMs containing niobium have been scarcely studied and, contrary to those containing V, Mo or W, are synthesized under basic pH conditions. [12] The best-known niobium POM is hexaniobate (Nb 6 O 19 8– ), which has the Lindqvist structure, and is obtained through alkaline fusion of Nb 2 O 5 . Niobium compounds are exten- sively used in many heterogeneous catalytic processes where they can act as catalyst supports or as the active phase. [13] In order to modify the textural properties and to develop acid and electron acceptor centres in the hydrotalcite basic structure and thereby improve its catalytic properties, many authors have reported the intercalation of different oxo- or polyoxometalates between the LDH layers, [14,15] giving rise to very selective catalysts in some oxidation reactions such as dehydrogenation or alkene epoxidation. [16–18] In a pre- vious work Evans et al. reported the intercalation of some hexametalates with the Lindqvist structure [V 2 W 4 O 19 4– and Nb x W 6–x O 19 (x+2)– where x = 2, 3, 4] within the interlayers of hydrotalcite, [19] but a study of the intercalation of hexa- niobate is lacking in the literature, and so herein we report the synthesis of a new MgAl LDH intercalated with hexa- niobate (H 3 Nb 6 O 19 5– ) prepared by anion exchange, starting from a nitrate LDH (MgAl-NO 3 ) precursor. Its characteri- zation, as well as of the oxides obtained upon its calci- nation, by element chemical analyses, Powder X-ray Dif- fraction (PXRD), Thermogravimetric (TG) and Differential