Synthesis and Structural Characterization of a Pure ZnAl ₄ (OH) ₁₂ (SO ₄ )·2.6H ₂ O Layered Double Hydroxide

The phase purity of a series of ZnAl 4(OH) 12SO 4· nH 2O layered double hydroxides (ZnAl 4-LDH) obtained from a reaction of bayerite (Al(OH) 3) 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), 27Al MAS NMR, and elemental analysis. Structural characterization of a stoichiometric ZnAl 4-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 1H atoms in the -OH groups and 4 27Al resonances could be assigned using 1H, 27Al NMR correlation experiments recorded with ultrafast MAS. The interlayer water content is variable on the basis of thermogravimetric analysis and changes in the 1H MAS NMR spectra with temperature. A composition of ZnAl 4(OH) 12(SO 4)·2.6H 2O was obtained from a combination of these techniques and confirmed that ZnAl 4-LDH is isostructural with the mineral nickelalumite (NiAl 4(OH) 12SO 4·3H 2O).