Infrared and near-infrared spectroscopic study of synthetic hydrotalcites with variable divalent/trivalent cationic ratios

Near-infrared (NIR), X-ray diffraction (XRD) and infrared (IR) spectroscopy have been applied to hydrotalcites of the formula Mg₆ (Fe,Al)₂(OH)₁₆(CO₃)·4H₂O formed by intercalation with the carbonate anion as a function of divalent/trivalent cationic ratio. Such hydrotalcites were found to show variation in the d-spacing attributed to the size of the cation. In the IR (1750–4000 cm⁻¹), the position of all bands except those at approximately 3060 cm⁻¹ shift to higher wavenumbers as the cation ratio increases. Conversely, at wavenumbers below 1000 cm⁻¹, the bands shift to lower wavenumbers as the cation ratio increases. A water bending mode at higher wavenumbers was also observed which indicates that the water is strongly hydrogen bonded. In the NIR spectrum between 8000 and 12,000 cm⁻¹, there is a broad feature which is attributed to electronic bands of the ferrous ion and low intensity sharp bands due to overtones of the OH stretching vibrations. It is also apparent from this region that Fe²⁺ substitutes for Mg²⁺. The intensity of bands at 7750 and 5200 cm⁻¹ increases as the cation ratio increases in the NIR spectrum. Hydrotalcites with a magnesium amount 3 and 4 times greater than that of aluminium and iron combined, in the lower wavenumber region of the NIR spectrum, have very similar spectral profiles. This work has shown that hydrotalcites with different divalent/trivalent ratios can be synthesised and characterised by infrared spectroscopy.