Molecular-Level Pictures of Chemical and Structural Transformations of Mg–Al Layered Double Hydroxide Crystals (Mg/Al = 2) at Elevated Temperatures

Abstract

Mg–Al layered double hydroxides (Mg–Al LDHs) are crystalline compounds with layered structures composed of hydroxide layers and interlayer anions such as CO32–. A detailed understanding of the thermal decomposition behaviors is indispensable for materials design toward promising applications such as CO2 adsorbents. Here, we report that the thermal decomposition behavior of a well-crystallized Mg–Al LDH having an Mg/Al atomic ratio of 2, where all hydroxyl groups experience an identical chemical environment, provides quantitative evidence for and clear molecular-/atomic-level pictures of the multi-step transformation of the crystals at elevated temperature. Thermal decomposition is found to occur in multi-steps of (1) release of the interlayer water, (2) dehydroxylation of just one-third of hydroxyl groups accompanied by the formation of coordinatively unsaturated sites followed by coordination of carbonate to metals, and (3) collapse of the layered structure at a higher temperature. The stepwise structural transformations are not ascribable to different coordination environments of hydroxyl groups. The reason is possibly that the structure after the partial dehydroxylation of the metal hydroxide layers is rather stable. Structural optimization by first-principles DFT calculations and its powder X-ray diffraction simulation supported the interpretations for and the molecular-level pictures of the structural transformations. These results resolve the various interpretations on the structural change of the crystals.