Abstract Hydrotalcite-like compounds, [Mg 1-x Al x (OH) 2 ] x+ [xX − ·n H 2 O], where X − = ½CO 3 2− or OH − , were prepared by hydrothermal syntheses at PH2O=100MPa and T = 100°–350°C. Starting materials were MgO, γ-Al 2 O 3 , H 2 O, and MgC 2 O 4 ·2H 2 O. The synthesis depended on temperature, pressure, the Al/(Al + Mg) ratio x, and the CO 2 content of the starting material. Previously an Al content of x = 0.33 was thought to be the upper limit in these double-layer compounds, but by using pressure the Al-content was increased to x = 0.44. Up to x = 0.33, a 0 decreased linearly to about 3.04 Å, but for x ≥0.33, a 0 remained nearly constant at this value. For the synthesized products the layer thickness c’ varied between 7.40 and 7.57 Å in contrast to the natural phases wherein c’ varies from 7.60 to 7.80 Å. At higher temperatures CO 2 -free syntheses, i.e., those without Mg-oxalate, resulted in a disordered hydrotalcite-like phase. The transition temperature between the ordered and the disordered hydrotalcite-like phase depended on the Al-content, x.

H.-H. Lohse

Publications