Abstract:
Hydrocalumite is a layered double hydroxide (LDH) that is finding increased application
in numerous scientific fields. Typically, this material is produced through environmentally polluting
methods such as co-precipitation, sol-gel synthesis and urea-hydrolysis. Here, the hydrothermal
green (environmentally friendly) synthesis of hydrocalumite (CaAl-OH) from Ca(OH)2 and
Al(OH)3 in water and the parameters that influence its formation are discussed. The parameters
investigated include the reaction temperature, reaction time, molar calcium-to-aluminium ratio,
the morphology/crystallinity of reactants used, mixing and the water-to-solids ratio. Hydrocalumite
formation was favoured in all experiments, making up between approximately 50% and 85% of
the final crystalline phases obtained. Factors that were found to encourage higher hydrocalumite
purity include a low water-to-solids ratio, an increase in the reaction time, sufficient mixing,
the use of amorphous Al(OH)3 with a high surface area, reaction at an adequate temperature and,
most surprisingly, the use of a calcium-to-aluminium ratio that stoichiometrically favours katoite
formation. X-ray diffraction (XRD) and Rietveld refinement were used to determine the composition
and crystal structures of the materials formed. Scanning electron microscopy (SEM) was used to
determine morphological differences and Fourier-transform infrared analysis with attenuated total
reflectance (FTIR-ATR) was used to identify possible carbonate contamination, inter alia. While the
synthesis was conducted in an inert environment, some carbonate contamination could not be avoided.
A thorough discussion on the topic of carbonate contamination in the hydrothermal synthesis of
hydrocalumite was given, and the route to improved conversion as well as the possible reaction
pathway were discussed.
