Hydrotalcites with an extended Al³⁺-substitution: Synthesis, simultaneous TG-DTA-MS study, and their CO₂ adsorption behaviors

A maximum Al³⁺-substitution has been demonstrated to be 45 mole% of (M + Al) in the brucite layer of hydrotalcites. The chemical composition of the highly substituted hydrotalcites can be typically represented by [M_0.55Al_0.45(OH)₂] [(CO₃)_0.225 · 0.50H₂O] where M = Mg, Ni, Zn, and Co. It showed the small lattice parameters of a₀ 3.05–2.98 A in the hexagonal lattice, which corroborates Al³⁺-substitution in the brucite layer. The simultaneous thermal analyses (TG and DTA) and mass spectrometry (MS) study have been performed. The highly Al³⁺-substituted hydrotalcites also showed quite different isotherms for the CO₂ adsorption. These materials adsorbed CO₂ gas by removing water within the interlayer and showed the selectivity for CO₂ adsorption: Cu–Al ∼Zn—Al < Co—Al < Mg—Al < Ni—Al. The Mg—Al and Co—Al hydrotalcite-like compounds showed a doubled amount of CO₂ by removing carbonate ions within the interlayer.