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 CO 3 2−. A detailed understanding of the thermal decomposition behaviors is indispensable for materials design toward promising applications such as CO 2 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 multistep 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.

Comprehensive Analysis of the Chemical and Structural Transformations of Mg–Al–CO3 Layered Double Hydroxides with Different Mg/Al Ratios at Elevated Temperatures

“…The gas evolution rate profiles of H 2 O and CO 2 from the LDH particle samples upon heating are shown in Figure . For LDH­[Mg/Al = 2] (Figure a), distinct three-step evolution of H 2 O was observed at 503, 619, and 669 K. Three-step evolution of H 2 O was also observed for the well-crystallized LDH (Mg/Al = 2) large particle sample in our previous study, in which the origin of water evolution was elucidated with quantitative evidence . The first H 2 O evolution step (step (1)) is due to the desorption of the interlayer water molecules .…”

“…In this study, we obtained the optimized structures (structures I and II). Structure III is the same as that calculated by CASTEP 2020 in our previous study …”

“…In the structure of LDH with Mg/Al = 2, each hydroxyl group coordinates to two Mg ions and one Al ion. That is, all of the hydroxide ions have an identical chemical environment (hereafter, this is denoted Mg 2 Al–OH) . In contrast, in the structure of the LDH with Mg/Al = 3, some of the hydroxyl groups coordinate to three Mg ions; that is, hydroxyl groups with different chemical environments (Mg 2 Al–OH and Mg 3 –OH) coexist.…”

“…Structure I corresponds to the fully hydrated LDH sample at room temperature. The amount of interlayer water for structure I was set to the experimentally determined value for the fully hydrated, well-developed large crystal sample of Mg–Al LDH (Mg/Al = 2) . Structure II corresponds to Mg–Al LDH after the complete removal of interlayer H 2 O molecules.…”

“…The reason for this is possibly that the structure after the partial dehydroxylation of the metal hydroxide layers is rather stable. Gaseous CO 2 is only evolved in step (3) …”