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[R2-P-07] Mg2+ and Ba2+ ion-exchange of GTS-type Na titanosilicate and its structural characterization
Keywords:Na-GTS, Ion-exchange, Powder XRD
GTS-type microporous titanosilicates have a three-dimensional tunnel-type structure. In this structure, four TiO6 octahedra linked by edge-sharing form a Ti4O16 cluster; the clusters are linked through SiO4 tetrahedra to form a three-dimensional framework with an interconnected pore system involving cavities of 8-ring channels, occupied by the alkali-metal ions and adsorbed water molecules. It is reported that Na-GTS, Na4[(TiO)4(SiO4)3]·6H2O, crystallizes in a rhombohedral phase with space group R3m; its unit cell [a = 7.8123 (6) Å, α = 88.794 (9) °] is very close to cubic cell and often described as a pseudocubic cell. In this study, we focused on Mg2+ and Ba2+, which have the different ionic size from and the same valence as the radioactive elements Sr2+. We investigated the effects of ionic size and valence on the Mg2+- and Ba2+-exchange properties of Na-GTS.
We have prepared the ion-exchanged forms, Na4(1-x)M2x[(TiO)4(SiO4)3]･yH2O (M: Mg, Ba), by treating Na-GTS with MgCl2 and BaCl2 aqueous solutions (CM = 0 ~ 7.9 mol/L) at 25 °C for 24 hours. The maximum ion-exchange rates of Mg and Ba were x = 0.76 and 0.86, respectively. We evaluated the unit-cell parameters from the powder X-ray diffraction (XRD) by a profile fitting method assuming a rhombohedral unit-cell and the H2O content (y) by TG. The increase in the ion-exchange rate (x) decreases the unit-cell volumes (V) and the H2O contents (y) of the Ba2+-exchanged samples, but increases those of the Mg2+-exchanged samples. The increase in V of the Mg2+-exchanged samples can be due to the increase in the y value; it can be brought about by the increase in the free space in the pores due to the exchange of smaller divalent Mg2+ with larger monovalent Na+. The decrease in V of Ba2+-exchanged samples can be explained by the interpretation that the decrease in the number of exchangeable-cations due to the exchange of divalent Ba2+ with monovalent Na+ yields the decrease in the number of water molecules coordinating to it. In these samples, the cation distribution of Mg2+ and Ba2+ in the cavities will be discussed from the comparison of the observed and simulated XRD patterns.
We have prepared the ion-exchanged forms, Na4(1-x)M2x[(TiO)4(SiO4)3]･yH2O (M: Mg, Ba), by treating Na-GTS with MgCl2 and BaCl2 aqueous solutions (CM = 0 ~ 7.9 mol/L) at 25 °C for 24 hours. The maximum ion-exchange rates of Mg and Ba were x = 0.76 and 0.86, respectively. We evaluated the unit-cell parameters from the powder X-ray diffraction (XRD) by a profile fitting method assuming a rhombohedral unit-cell and the H2O content (y) by TG. The increase in the ion-exchange rate (x) decreases the unit-cell volumes (V) and the H2O contents (y) of the Ba2+-exchanged samples, but increases those of the Mg2+-exchanged samples. The increase in V of the Mg2+-exchanged samples can be due to the increase in the y value; it can be brought about by the increase in the free space in the pores due to the exchange of smaller divalent Mg2+ with larger monovalent Na+. The decrease in V of Ba2+-exchanged samples can be explained by the interpretation that the decrease in the number of exchangeable-cations due to the exchange of divalent Ba2+ with monovalent Na+ yields the decrease in the number of water molecules coordinating to it. In these samples, the cation distribution of Mg2+ and Ba2+ in the cavities will be discussed from the comparison of the observed and simulated XRD patterns.