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Hydrotalcites are anionic clays whose structure is lamellar. They are constituted by pairs of octahedral divalent and trivalent cations. The substitution of the divalent by the trivalent cations generates a charge excess which has to be compensated by anions such as carbonates, nitrates or iodine among others. The location of the trivalent cations is uncertain as it may depend on the synthesis method. Although hydrotalcites may be synthesized through conventional precipitation from the corresponding salts the obtained cation distribution may be altered in presence of microwave irradiation. In hydrotalcites synthesized in presence of microwave irradiation, magnesium and aluminum are not homogeneously distributed: Aluminum is selectively retained in the particle core. If zinc or nickel are included in the synthesis mixture, zinc, which is a large cation, remains in the pore mouths generating lattice strain. Most of it, then, forms zinc oxide crystallites. Instead, nickel is able to reach the inner part of the hydrotalcite particle. With calcinations, 450 degrees C/4 h, hydrotalcite is never fully destroyed. (c) 2006 Elsevier B.V. All rights reserved. |
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