Synthesis and Thermal Decomposition of Mn-Al Layered Double Hydroxides

In this paper, the synthesis and thermal decomposition behavior of hydrotalcite-like Mn-Al layered double hydroxide (LDH) have been investigated. First, the Mn-Al LDH was synthesized by the coprecipitation method using various anions such as Cl⁻, CO²⁻₃, NO⁻₃, SO²⁻₄ or dicarboxylic acids (DCA). The single phase of the Mn-Al LDH was obtained when Cl⁻, NO⁻₃ or DCA was used as a guest anion. In the case of CO²⁻₃ or SO²⁻₄, the solid products included MnCO₃ or shigaite as a by-product. The crystallinity of the Cl/Mn-Al LDH was greatly influenced by a drying temperature and that the crystallinity of the Cl/Mn-Al LDH dried at room temperature was found to rise about 6 times in comparison with that dried at 333 K. The DCA/Mn-Al LDH was found to have an expanding LDH structure, supporting that the LDH basal layers were bridged by the intercalated DCA anion. Then, the thermal decomposition of the DCA/Mn-Al LDH has been examined, and the intercalated DCA was found to be decomposed at lower temperature than DCA itself. The oxidation number of Mn ion rose with increasing the heat treatment temperature and was +2.70 with crystallizing Mn₃O₄ after being heated at 973 K. The thermal decomposition of guest DCA was thought to be accelerated by the strong catalytic action of Mn ion in the host hydroxide basal layers.