Transformation Mechanism of Magnesium and Aluminum Precursor Solution into Crystallites of Layered Double Hydroxide

Layered double hydroxides (LDHs), members of a family of two-dimensional anionic clay with flexibility in composition, have found a wide variety of applications in industry, including as additives in polymers, as precursors to magnetic materials, in biology and medicine, in catalysis, and in environmental remediation. A detailed understanding of the mechanism of the LDH formation should gain deep insight on the synthetic methodologies of the material and further allow the properties of the resulting LDH to be tailored to specific applications. Herein, we report a systematic investigation of the formation mechanism of the typical MgAl-LDH by urea precipitation method from a magnesium and aluminum precursor salt solution. It is revealed that, at the first stage of the synthesis, amorphous colloidal hydroxide aluminum is formed from the aluminum precursor salt solution. Then, the amorphous hydroxides are transformed into the crystallites of oxide-hydroxide aluminum boehmite gamma-AlOOH, accompanying the continuous incorporation of surrounding Mg2+ into the sheet of the lamellar gamma-AlOOH, leading to the charge imbalance of the sheet, which destroys the hydrogen bonds existing between the sheets. Subsequently, the carbonate ions in the solution are intercalated into the interlayer galleries by an electrostatic interaction for balancing the sheet charge, resulting in an initial LDH phase with alveolate-like structure. Finally, the main layers stack to build a three-dimensional network with the positive charge being balanced by the carbonate ions arranged in the hydrated interlayer galleries, and the integrated plate-like structure of LDH is formed. Throughout the above-mentioned processes, the incorporation of magnesium ions into the sheet of the lamellar boehmite can play a primary role for the formation of LDH crystallites.