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

Mg-Al layered double hydroxides (LDHs) with CO32- interlayer anions are promising CO2 adsorbents. Here, we analyzed the quantitative gas evolution behaviors of Mg-Al LDH particles with different Mg/Al ratios during the multistep chemical/structural transformations at elevated temperatures. The Mg/Al molar ratio strongly affects the behavior: the transformation changes from two apparent steps to three steps depending on the Mg/Al ratio. The transformation occurs in essentially the same way as that observed for large Mg-Al LDH crystals: (1) release of the interlayer water, (2) partial dehydroxylation of the hydroxyl layers followed by coordination of carbonate ions to the metals, and (3) collapse of the layered structure. We provide a molecular/atomic level picture of the structure in each step of the transformation by first-principles density functional theory (DFT) calculation. The structurally optimized model and reexamination of experimental data showed that step (1) results in a large decrease in the interlayer distance of the LDH from ∼7.6 to ∼6.7 Å (a decrease of ∼0.9 Å) and the possible explanation is the waving of the metal hydroxide layers. This study provides a comprehensive understanding of the structural changes of LDHs with various Mg/Al ratios to resolve the various interpretations in the literature.