Synthesis and Structural Characterization of a Pure ZnAl₄(OH)₁₂(SO₄)·2.6H₂O Layered Double Hydroxide

The phase purity of a series of ZnAl₄(OH)₁₂SO₄· nH₂O layered double hydroxides (ZnAl₄-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 ZnAl₄-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 ZnAl₄(OH)₁₂(SO₄)·2.6H₂O was obtained from a combination of these techniques and confirmed that ZnAl₄-LDH is isostructural with the mineral nickelalumite (NiAl₄(OH)₁₂SO₄·3H₂O).