Synthesis and Structural Characterization of a Pure ZnAl(OH)(SO)·2.6HO Layered Double Hydroxide.

The phase purity of a series of ZnAl(OH)SO· nHO layered double hydroxides (ZnAl-LDH) obtained from a reaction of bayerite (Al(OH)) with an excess of zinc(II) sulfate under hydrothermal conditions was investigated as a function of the reaction temperature, the duration of the hydrothermal treatment, and the zinc(II) concentration. The product quality, i.e., crystalline impurities, Al impurities, and bulk Zn:Al ratio, were assessed by powder X-ray diffraction (PXRD), Al MAS NMR, and elemental analysis. Structural characterization of a stoichiometric ZnAl-LDH (120 °C, 9 days, and 2.8 M Zn(II)) showed a well-defined structure of the metal ion layer as evidenced by a single, well-defined Zn environment: i.e., no Zn substitution on the Al sites according to Zn k-edge EXAFS and PXRD. Furthermore, nearly all of the 12 different H atoms in the -OH groups and 4 Al resonances could be assigned using H,Al NMR correlation experiments recorded with ultrafast MAS. The interlayer water content is variable on the basis of thermogravimetric analysis and changes in the H MAS NMR spectra with temperature. A composition of ZnAl(OH)(SO)·2.6HO was obtained from a combination of these techniques and confirmed that ZnAl-LDH is isostructural with the mineral nickelalumite (NiAl(OH)SO·3HO).