Pre-treatment of raw materials for the hydrothermal synthesis of hydrotalcite-like compounds

Hydrotalcite-like compounds are layered materials with anionic exchange and adsorption properties. Alternatively, this material is synthesized by hydrothermal treatment of magnesium oxide (MgO) and aluminium trihydroxide (ATH). Since this synthesis deals with solid raw materials, and the reaction proceeds through a solution mediated dissolution–precipitation process, it is a challenge to enhance the reactivity of the raw materials. This research aims to investigate the effect of four different pre-treatments: blending, ultrasound treatment, and wet grinding on the conversion rate. In experiments at 80 °C, only 3R1 polytype HTlc was formed, while a mixture of 3R1 and 3R2 polytypes was found in the experiments at 170 °C. All the pre-treatment techniques cause a decrease in particle size of the raw materials whereby wet grinding induced the strongest effect. Experimentally the fastest conversions at both synthesis temperatures were achieved by wet grinding. A reaction mechanism is proposed which explains the observed conversions as a function of the particles size of the reactants, the exposure time of the MgO to water and the reaction temperature. Two important parameters to be taken into account in the synthesis are thus exposure to water prior to the reaction and the particle size of the raw materials.