Preparation and characterization of Al-rich Zn-Al hydrotalcite-like compounds

Hydrotalcite-like compounds, described by the formula [Zn1-xAlx(OH)2][(CO3)x/2 · nH2O], were prepared by coprecipitation methods at 80°C and characterized by bulk chemical analysis, X-ray powder diffraction (XRD), nuclear magnetic resonance (NMR), and scanning-transmission electron microscopy (STEM). An x value of 0.33 was previously assumed to be an upper limit, but recently, Al-rich hydrotalcite-like compounds have been prepared with x as large as 0.44 by hydrothermal synthesis. In the Zn-Al system, Al-rich hydrotalcite was synthesized at normal pressure by coprecipitation. Zn-Al hydrotalcite-like compounds were obtained in the range of x = 0.3 to 0.4. An Al-rich hydrotalcite-like compound with x = 0.44 was formed in mixtures containing large amounts of a poorly crystalline Zn-Al phase. A continuous contraction of the hydrotalcite-like structure occurred as x increased, both the a and c lattice parameters decreasing for x values as large as 0.44. This study illustrates the advantages of using quantitative analytical electron microscopy with high spatial resolution to complement conventional (and bulk) characterization techniques for correlating structural and compositional characteristics of finely divided materials.