Stability and transformation kinetics of 3R_1 and 3R_2 polytypes of Mg-Al layered double hydroxides

摘要

Two different polytypes of Mg-Al layered double hydroxide with rhombohedral symmetry can be synthesized either by calcination-rehydration or by hydrothermal synthesis. The structural differences of these polytypes, namely 3R_1 and 3R_2 could induce distinct characteristic performances, making it essential to study their thermal stability regimes and transformation kinetics. The results show that transformations of both polytypes are feasible. The transition temperature is at 110 ℃ with polytype 3R_1 being stable at lower temperatures and polytype 3R_2 at higher temperatures. The transformation kinetic of polytype 3R_1 into 3R_2 is inhibited by the presence of carbonate ions in the interlayer. In their absence, polytype 3R_1 transforms faster into 3R_2 than vice versa. Furthermore, the transformation of polytype 3R_1 into 3R_2 is accompanied by the formation of brucite, indicating the lower Mg-Al ratio in polytype 3R_2. The transformation is solvent mediated and no solid state transformation was observed.