Vibrational Spectroscopy and MAS NMR Studies of A Cryptomelane Family of Mixed (7Li+, 2 H+) Form Manganic Acids

Abstract

ABSTRACTCryptomelane-type manganic acids (CMAs) synthesized by a soft-chemical redox process of KMnO4 with MnSO4 in H2SO4 and α-MnO2s by a decomposition-oxidation process of (CH3)3COK and MnCO3 at 530°C were studied by using the vibrational spectroscopy and 7Li- and 2H-MAS NMR. The infrared absorption bands of the α-MnO2s at 706cm-1 were not changed irrespective of the sort of alkali cations and their uptake amount, while large shifts of this band were observed on the CMA exchanged by alkali cations. Hence, cation-exchange sites of these solids will occupy crystallographically different positions. 7Li-MAS NMR spectra of these manganic acids showed different two chemical shifts depending on their synthetic routes. These findings suggest that different ion-exchange sites are formed at a local level in the 2×2 type tunnel, strongly depending on their synthetic conditions. These are crucial for the control of the cation affinity. These details at a molecular level cannot be obtained by using the conventional XRD analyses.