Ni-, Mg- and Co-containing hydrotalcite-like materials with a sheet-like morphology: synthesis and characterization

A new synthesis of hydrotalcite-like materials having a composition [M²⁺_1−xAl³⁺_x](OH)₂ × A⁻_x × mH₂O, where M²⁺ is magnesium, nickel or cobalt and A⁻ is a monocarboxylic anion, is described. The synthesis involves reacting a source of the divalent cation with pseudoboehmite and a monocarboxylic acid in an aqueous slurry at 95°C for 6 h. The reactive source of magnesium is either magnesium oxide or hydroxide and the reactive source for nickel and cobalt are the respective hydroxides. This synthesis method results in the formation of hydrotalcites with a unique sheet-like morphology with the sheets having breadth to thickness ratio in excess of 100, in contrast to the previously known hexagonal hydrotalcites which have a breadth to thickness ratio between 5 and 10. The phrase ‘sheet-like’ in this work describes the macroscopic morphology of the hydrotalcite crystallites. A key finding was that the presence of monocarboxylic acid was necessary for the formation of sheet morphology. It was also demonstrated that sheet and hexagonal morphologies readily interconvert via a common mixed oxide postcursor.