PREPARATION AND CHARACTERIZATION OF A1-RICH Zn-A1 HYDROTALCITE-LIKE COMPOUNDS

Hydrotalcite-like compounds, described by the formula (Zn, xAlx(OH)2) ((CO3)x/2 �9 nHzO), were prepared by coprecipitation methods at 80~ 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, #p-rich hydrotalcite was synthesized at normal pressure by coprecipitation. Zn-#P hydrotalcite- like compounds were obtained in the range of x = 0.3 to 0.4. An #P-rich hydrotalcite-like compound with x = 0.44 was formed in mixtures containing large amounts of a poorly crystalline Zn-A1 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 quan- titative analytical electron microscopy with high spatial resolution to complement conventional (and bulk) characterization techniques for correlating structural and compositional characteristics of finely divided materials.