ESCA and thermodynamic studies of alkali metal ion exchange reactions on an α-MnO2 phase with the tunnel structure
A manganic acid cation-exchange material with the 2×2 type tunnel structure was synthesized by decomposition of a mixture of 80 mol% MnCO3 and 20 mol% (CH3)3COK at 530°C, followed by leaching out any other residual ionic impurities using 1M HNO3. Its chemical composition was represented by H1.7Mn8O15.01. The theoretical ion exchange capacity was 2.49 mmol g−1, assuming that all the H+s are exchangeable. The binding energies of Mn 2p1/2 and Mn 2p3/2 levels were not shifted upon alkali cation exchange of this material. This is clear evidence that no redox process is associated with the cation exchange. The plot of the corrected selectivity coefficient in logarithmic scale vs. the charge fraction M was linear in a small range of the charge fraction up to 0.1. This suggests that each cation resides in a similar chemical environment at this concentration level within the tunnel. The slope was −40 for Li+/H+ exchange, −46 for Na+/H+ exchange, −108 for K+/H+ exchange, −72 for Rb+/H+ exchange, and −60 for Cs+/H+ exchange. The corrected selectivity coefficient at infinitesimal exchange increased in the order Li<Na<CsK≈Rb at 30–60°C, indicating that the largest hydrated cation is the least selective and cations having the same dimension as the tunnel size are the most selective. Thermodynamic functions were evaluated for the hypothetical ion-exchange reaction with the least steric hindrance.