Characterization of Layered Double Hydroxides(Mg-Al-CO3 systems) and Rehydration Reaction of Their Calcined Products in Aqueous Chromate Solution

摘要

Layered double hydroxides (Mg-Al-CO3 systems, LDH), which are hydrotalcite-like anionic clay minerals, having different Mg2+ to Al3+ ratio were synthesized by coprecipitation method. The subsequent products were characterized by the following methods; elemental analysis, X-ray powder diffraction, thermal analysis (DSC and TGA), FT-IR and 27Al-MAS NMR. X-ray powder patterns showed that the products formed were layered structure materials. Two heat absorption peaks were observed around 20 ?-280?é (surface water and interlayer water) and 280?-500?é (water from lattice hydroxide and carbon dioxide from interlayer carbonate) in DSC diagrams, and they were quantitatively analyzed by TGA diagrams (in case LDH4 16.2% and 28.6% respectively). FT-IR spectra indicate that the interlayer carbonate ions occupied symmetrical sites Between two adjacent layers in a parallel direction. 27Al-MAS NMR spectra show only single resonance (8.6 ppm) of the octahedrally coordinated aluminum similar magnesium. When LDH4 was calcined at 560?é for 3 hours in air, its layered structure was destroyed giving a mixed metal oxide. However it readily became rehydrated in aqueous chromate solution to its original structure.