Preparation of ion exchangers in the hydrogen form from M1+xTi2P3−xSixO12 (M = Li, Na) crystals and glass-ceramics and their characterization

Abstract

Inorganic ion exchangers in the hydrogen form were prepared by two different methods and their properties were investigated. The first method was a conventional one; the precursors, LiTi₂(PO₄)₃ and Na_1+xTi₂P_3¬xSi_xO₁₂ (x = 0, 1, 1.5), were prepared by solid-phase reactions and the corresponding ion exchangers in the hydrogen form, HTP(Li) and HTPS(Na) (x = 0, 1, 1.5), were obtained by acid treatment of the precursors. In the other method, Na_1+xTi₂P_3¬xSi_xO₁₂–β-Ca₃(PO₄)₂ (x = 0, 1) glass-ceramics were first formed by the crystallization of Na₂O–CaO–TiO₂–P₂O₅–(SiO₂) glasses and then sodium ions in the Na_1+xTi₂P_3¬xSi_xO₁₂ (x = 0, 1) phases were displaced by protons and the β-Ca₃(PO₄)₂ phase was dissolved out by acid treatments to obtain ion exchangers in the hydrogen form, gcHTPS(Na) (x = 0, 1). The leaching ability of sodium ions from the precursors, the specific surface areas and the ion exchange rates were higher for ion exchangers prepared via glass-ceramics than for the corresponding ion exchangers prepared conventionally. The ion exchangers prepared did not show any specific affinity toward Group 1 metal ions except for HTPS(Na) (x = 1) and gcHTPS(Na) (x = 0), for which a slight affinity toward sodium and/or lithium ions was observed. HTPS(Na) (x = 1) and gcHTPS(Na) (x = 1) were found to be slightly ⁶Li-specific; the single-stage separation factor, S, for the lithium isotopes was 1.002 and 1.004 under basic conditions at 25 °C respectively. In contrast to the other ion exchangers, HTP(Li) has a layered structure with an interlayer distance of 10.0 Å. The S value on HTP(Li) was 1.012 under basic conditions at 25 °C. A strong feature of this system is the fast ion exchange rate; the monovalent ion–monovalent ion exchange equilibrium was attained within three minutes at 25 °C.