Characterization of semicrystalline titanium(iv) phosphates and their selectivity of cations and lithium isotopes

Abstract

A semicrystalline layered titanium phosphate (scTiOP), which showed the chemical properties of a cation exchanger, was synthesized from a mixed solution of titanyl sulfate and ammonium dihydrogenphosphate solutions under mild conditions. Based on the results of ³¹P MAS NMR measurements, infrared measurements, thermogravimetry–differential thermal analysis, ICP atomic emission spectroscopic analysis and powder X-ray diffraction analysis, the ion exchanger was identified as Ti₂O₃(H₂PO₄)₂·2H₂O. Ion exchangers with varying degrees of condensation of the dihydrogenphosphate groups, and consequently having three-dimensional cross-linked structures with minutely controlled ion exchange sites, were obtained by the thermal treatment of scTiOP at different temperatures. The ion exchangers obtained by thermal treatment at temperatures from 200 to 600 °C showed a high affinity toward sodium ion among the alkali metal ions, whereas scTiOP and the ion exchanger treated at 100 °C showed no specific selectivity of any alkali metal ion. The single-stage separation factor, S, for the lithium isotopes was a slow increasing function of the temperature of the thermal treatment up to 600 °C, and the maximum S value at 25 °C was 1.028. The occurrence of a high affinity to sodium ion, the increase in S value and the development of a three dimensional –O–P–O– network seemed mutually correlated.