Syntheses of Disordered and Al-Rich Hydrotalcite-Like Compounds

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 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"><mml:msub><mml:mi>P</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:mi>H</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:msub><mml:mi/><mml:mn>2</mml:mn></mml:msub></mml:mrow><mml:mi>O</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mn>100</mml:mn><mml:mspace width="thickmathspace"/><mml:mi>M</mml:mi><mml:mi>P</mml:mi><mml:mi>a</mml:mi></mml:math> 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.