Formation of Spinel from a Hydrotalcite-Like Compound at Low Temperature: Reaction between Edges of Crystallites

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Abstract
The thermal decomposition behavior of hydrotalcite-like compounds (HTlcs) prepared by reconstruction of calcined HTlcs is described. From the results of X-ray diffraction (XRD), it seems that dicarboxylate intercalates of HTlc calcined at 500 °C are completely reconstructed to Mg-Al-CO3 HTlc by exposure to aqueous Na2CO3. However, the Mg-Al-CO3 HTlc reconstructed under particular conditions yields spinel (MgAl2O4) at 400 °C. This temperature is very low, because Mg-Al-CO3 HTlc that has been reported yields spinel at 900 °C after forming a Mg-Al double oxide. The reconstructed Mg-Al-CO3 HTlc that yields spinel at 400 °C is obtained when the following conditions are fulfilled: the crystallites of the starting dicarboxylate intercalates are coagulated tightly and the calcined HTlcs and reconstructed materials are not ground. The Mg-Al-CO3 HTlc reconstructed under these conditions contains only 55–70% of carbonate anions required by stoichiometry. Therefore, we conclude that the transformation of reconstructed Mg-Al-CO3 HTlc to spinel at 400 °C is the result of a reaction occurring between edges of crystallites.