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DFT Study of Layered Double Hydroxides with Cation Exchange Capacity: (A⁺(H₂O)₆)[M₆²⁺Al₃(OH)₁₈(SO₄)₂]·6H₂O (M²⁺ = Mg, Zn and A⁺ = Na, K)

Moraes, Pedro Ivo R., Wypych, Fernando, Leitão, Alexandre A.
Journal of physical chemistry 2019 v.123 no.15 pp. 9838-9845
aluminum, anions, cation exchange, cation exchange capacity, cations, hydroxides, magnesium, physical chemistry, potassium, sodium, sulfates, thermodynamics, zinc
Layered double hydroxides (LDH) equivalent to the motukoreaite and natroglaucocerinite mineral structures were investigated by ab initio calculations as well as the capacity to exchange their cations without removing the intercalated sulfate. Due to the M²⁺/M³⁺ molar ratio in all mineral structures, the layer domains contained three positive charges ([M₆²⁺M₃³⁺(OH)₁₈]³⁺), which, after intercalation with two sulfate anions, built a negatively charged compound ([M₆²⁺M₃³⁺(OH)₁₈](SO₄)₂)⁻, and these compounds were intercalated with hydrated sodium cations. Structures with different layer compositions (Mg/Al and Zn/Al) and with different intercalated cations (Na⁺ and K⁺) were constructed. In all cases, the 3R polytype was the most stable. A thermodynamic study of the cation exchange reaction was performed, and the sodium cations were replaced by potassium, maintaining the intercalated sulfate. LDHs are traditionally anion exchangers, but the studied formulations can also exchange cations, which opens new perspectives for their applications.