Investigating the role of the different metals in hydrotalcite Mg/Al-based adsorbents and their interaction with acidic sorbate species

The role of aluminum in the structure of hydrotalcite-based sorbents was investigated with 27Al-NMR and XPS with respect to the chemisorption mechanism of different sorbate species, such as CO2, H2O and H2S, at elevated temperatures. Sorbent materials with different ratios of Al/Mg were studied in order to elucidate the influence of the material composition on the Al coordination. Two different tetrahedrally and octahedrally coordinated Al species were detected. It was found that their quantitative appearance depends on the ratio of Al/Mg within the sorbent material. A higher Mg content leads to a higher distortion and to the formation of more tetrahedrally coordinated Al with a high chemical shift. Whereas CO2 and H2O adsorption does not seem to influence well-coordinated Al, as observed with NMR, chemisorption of H2S showed a clear interaction with Al leading to the formation of an additional tetrahedrally coordinated site with a high chemical shift. XPS showed that a reversible metal sulfate complex was formed on the sorbent, which was reversible upon exposure to H2O.