Thermodynamic study of alkali metal ion exchanges on a manganese dioxide with hexagonal structure

Ion-exchange equilibria of alkali metal ions and protons of a manganese dioxide material have been studied thermodynamically. The material was synthesized by pyrolysis from a mixture of manganese carbonate and potassium tert-butoxide with content of 40% on a molar basis, at 530 °C. The chemical composition of the material is MnO2.09 · 0.40H2O, and its crystal structure belongs to a hexagonal system with lattice constants a = 22.14 ± 0.03 Å and c = 4.900 ± 0.002 Å, which has not been reported in JCPDS. The ion-exchange titration curve showed that the material exhibited a property of weak acid, and the maximum uptakes were 3.67 meq Li/g, 2.59 meq Na/g, 2.54 meq K/g, 2.18 meq Rb/g, and 2.10 meq Cs/g. The plot of the corrected selectivity coefficient vs. fractional exchange (Kiellandʹs plot) was almost straight for Li+H+, Rb+H+, and Cs+H+ exchanges, but curved for Na+H+ and K+H+ exchanges. The selectivity order at infinitesimal exchange is Li < Na < Cs ⪡ K < Rb at 30~60 °C. The pore structure in the material is expected to fit with K+ and Rb+. The negative Gibbs free energy change for K+H+ and Rb+H+ exchanges gDGideal exch0(K) = −22 kJ/mol and ΔGidealexech0 (Rb) = −27 kJ/mol) showed that these exchange reactions were preferable in the present material. The entropy change was large for K+/H+ and Rb+/H+ exchanges (ΔS0hyp exch(K) = 282 J/K · mol and Δ0hyp exch(Rb) = 458 J/K · mol), indicating high selectivities for K+ and Rb+ in the present material. The small ΔS0hyp exch for Cs+/H+ exchange may express the size misfit of the ion with the opening Of the exchange Site.