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Thermal evolution of Mg-Al-CO₃ hydrotalcites

¹ Department of Mineralogy, Sofia University, 15 Tzar Osvoboditel, Sofia 1000, ² Institute of Inorganic Chemistry, Bulgarian Academy of Sciences, Academic G. Bonchev Str. bl. 11, Sofia 1113, and ³ Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Academic G. Bonchev Str. bl. 107, Sofia 1113, Bulgaria

^* E-mail: nadia5{at}mail.bg

(Received 9 July 2003; revised 4 January 2004)

The thermal decomposition of hydrotalcite (HT), with chemical composition Mg_1–xAl_x(OH)₂(CO₃)_x/2.(1–3x/2)H₂O, (0.20 < x 0.33), is a complex sequence of dehydration, dehydroxylation and decarbonization and leads to the formation of a series of metaphases: HT-D (dehydrated HT), HT-B (partially dehydroxylated HT) and MO (mixed oxides with periclase-like structure). The evolution of water and CO₂ in natural and synthetic hydrotalcites (a Mg/Al ratio between 2:1 and 3.7:1), heated to 800°C, was investigated by differential thermal analysis, thermogravimetry and evolved gas analysis. At least six endothermic and two exothermic effects were established by computer-aided resolving of the curves. The formation of each HT metaphase was related to the release of a discrete number of water molecules depending on the Al content in the samples and each appeared as a corresponding endothermic peak in the DTA curves. The exothermic processes associated with the crystallization of HT-B and MO metaphases were specified by decomposition of DTA curves. The evolution of CO₂ during the thermal decomposition of the carbonate groups was found to be different for the samples studied. The preservation of CO₃ even at high temperatures was established for synthetic samples with a high Al content. The release of volatile H₂O and CO₂ (which comprise ~40% of the sample mass) provokes fine cracking both along and across the layers.

KEYWORDS: hydrotalcite, DTA, TG, evolving gas analysis

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