Physico-chemical Properties of Synthetic Hydrotalcites in Relation to Composition

Ahstraet--Hydrotalcite solid solutions were prepared by coprecipitation followed by hydrothermal treatment between 150 ~ and 250~C. Based on the structural formula [Mgl_xAlx(OH)~lX+l(COz)x/2 9 mH20] x-, pure solid solutions were formed in the range Al/(Al + Mg) = 0.2 to 0.33, where m = (1 3x/2). Maximum crystallite size was achieved by hydrothermal treatment between 180 ~ and 200~ with x = 0.337 to 0.429. Crystal strain was also minimized at these values of x. The adsorption c~pacity for Naphthol Yellow S increased as x increased and reached a maximum (1.56 x 10 6 moles/m 2) when x = 0.287, a value eight times larger than that of Mg(OH)~. A weak endothermic DTA peak at about 350~ is probably due to the loss of structural water in the main layer of the structure. On calcination between 400 ~ and 700~ only periclase was detected, probably containing A1 in solid solution. Hydration of the calcined product resulted in the reconstruction of the original hydrotalcite structure. Key Words--Crystal strain, Hydrotalcite, Solid solution, Synthesis, Thermal stability.