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

Mg-Al layered double hydroxides (Mg-AlLDHs) arecrystalline compounds with layered structures composed of hydroxidelayers and interlayer anions such as CO3 (2-). A detailed understanding of the thermal decomposition behaviorsis indispensable for materials design toward promising applicationssuch as CO2 adsorbents. Here, we report that the thermaldecomposition behavior of a well-crystallized Mg-Al LDH havingan Mg/Al atomic ratio of 2, where all hydroxyl groups experience anidentical chemical environment, provides quantitative evidence forand clear molecular-/atomic-level pictures of the multi-step transformationof the crystals at elevated temperature. Thermal decomposition isfound to occur in multi-steps of (1) release of the interlayer water,(2) dehydroxylation of just one-third of hydroxyl groups accompaniedby the formation of coordinatively unsaturated sites followed by coordinationof carbonate to metals, and (3) collapse of the layered structureat a higher temperature. The stepwise structural transformations arenot ascribable to different coordination environments of hydroxylgroups. The reason is possibly that the structure after the partialdehydroxylation of the metal hydroxide layers is rather stable. Structuraloptimization by first-principles DFT calculations and its powder X-raydiffraction simulation supported the interpretations for and the molecular-levelpictures of the structural transformations. These results resolvethe various interpretations on the structural change of the crystals.

Automated summary

This study reports the thermal decomposition behavior of a well-crystallized Mg-Al LDH with an Mg/Al atomic ratio of 2, revealing a multi-step transformation process at elevated temperatures. The steps include the release of interlayer water, dehydroxylation accompanied by the formation of unsaturated sites, coordination of carbonate ions to metals, and collapse of the layered structure at higher temperatures.