MIR and NIR spectroscopy of sol–gel hydrotalcites with various trivalent cations

Three different hydrotalcites were synthesized from magnesium ethoxide, and aluminium, gallium and indium acetylacetonate, using the sol–gel technique. The colloid suspensions initially obtained were gelled and separated by centrifugation. XRD diffraction patterns confirmed that all solids thus obtained possessed a hydrotalcite structure. The resulting hydrotalcites were characterized by mid-infrared (MIR) and near-infrared (NIR) spectroscopies. The two types of spectra were found not to depend on the synthetic medium or trivalent metal used and were thus quite similar. The MIR spectra for the three solids included a strong band at 3500–3000 cm⁻¹ due to stretching vibrations of the different types of O–H groups in them. The signal at about 1370 cm⁻¹ observed for all solids indicates that the sole interlayer anion present was carbonate. The NIR spectra exhibited the bands for the first and second overtone of the O–H stretching vibration in addition to various combination bands.