Characterization of Repeatedly Reconstructed Mg−Al Hydrotalcite-like Compounds: Gradual Segregation of Aluminum from the Structure

Calcined hydrotalcite-like compounds (HTlcs) can be reconstructed to their original structure by exposure to an aqueous solution, provided that the original HTlcs contain volatile interlayer anions. In the present study, repetitive treatment consisting of calcination at 400 °C and subsequent reconstruction in an aqueous solution was carried out for Mg−Al−CO 3 HTlcs. Gradual changes in these HTlcs during repetitions of the treatment were observed. The quantity of interlayer carbonate anions decreased as the treatment was repeated. This finding indicates that the HTlcs were not completely reconstructed and that some Al was extracted from the structure during treatments, because the quantity of interlayer anions is proportional to Al content in HTlcs. The extracted Al increased with repetitions of the treatment, and spinel (MgAl 2 O 4 ), whose Al content is much higher than HTlcs, was eventually detected after several repetitions as a consequence of segregation of the extracted Al phase. The spinel obtained by the repetitive treatments was highly crystalline and comparable to spinel obtained by calcination of Mg−Al coprecipitates above 1000 °C.