Structural Studies on the Ion-Exchanged Phases of a Porous Titanosilicate, Na₂Ti₂O₃SiO₄·2H₂O

Abstract

A titanosilicate with the ideal formula, Na2Ti2O3SiO4·2H2O, containing unidimensional channels, was synthesized hydrothermally and converted to the hydrogen form by acid treatment. The hydrogen form was partially ion exchanged by sodium ions to obtain a 50% sodium ion exchanged phase. The crystals of the sodium phase, NaHTi2O3SiO4·2H2O, retain the symmetry and unit cell parameters of the parent disodium compound, space group P42/mcm, a = b = 7.832(1) Å, c = 11.945(2) Å, and Z = 4. The sodium ions are located on the ac faces of the crystal while the water molecules occupy the channels. Ion exchange of the acid form by potassium ions leads to a phase with a maximum potassium to proton ratio of about 2. In the acid, H2Ti2O3SiO4·1.5H2O, and potassium phases, K0.5H1.5Ti2O3SiO4·1.5H2O and K1.38H0.62Ti2O3SiO4·H2O, the a and b axes are doubled while the c-axis dimension is retained. These doubled dimensions were transformed to a primitive tetragonal cell which has a volume twice that of the parent sodium form. The crystals belong to the space group P42/mbc with a = 11.039(1) Å, c = 11.886(1) Å for the acid phase, a = 11.015(1) Å, c = 12.017(1) Å for the K1.38H0.62 phase, and a = 11.0604(3) Å, c = 11.9088(3) Å for the K0.5H1.5 phase. The number of molecules in the unit cell in these three cases is 8. In the acid form, the channels are occupied by the water molecules, which are involved in hydrogen bonding among themselves as well as with the framework oxygens. In the K0.5H1.5 phase, all the K+ ions are in the center of the tunnel. For the K1.38H0.62 phase, about 35% of the total potassium ions are located at the center of the channel and are bonded to the silicate oxygens. The remaining ions are found near the framework which is close to the positions of the sodium ions of the ac faces in the parent compound. These ions are bonded to both the framework and water oxygen atoms. The titanium atoms in all the phases are octahedrally coordinated, and they are grouped as clusters of four. These clusters are linked by the silicate groups along the a and b directions and by Ti−O−Ti bonds along the c directions. This structural data provide a basis for explaining the observed ion exchange behavior and ion selectivity.

Optical properties and local structure of Eu3+-doped synthetic analogue of the microporous titanosilicate mineral sitinakite

“…These lattice changes and the estimated higher symmetry space group in Eu 3+ -doped sitinakite suggested a different distribution of Eu 3+ cations included in the pore system when compared with sodium sites in sitinakite. A similar effect was also observed in the hydrogen converted as well as in the K + , Cs + and Sr 2+ exchanged forms of titanium and titano-niobium sitinakite [8,9]. The Rietveld refinement suggested that after K exchange, one cation site is located at the center of cavity while the other site is near the framework [8].…”

Cesium and strontium exchange by the framework potassium titanium silicate K3HTi4O4(SiO4)3·4H2O

“…This IEC value is high and comparable to that found for Na2Ti203SiO4.2H20. 8 The experimental curves showing the dependencies of alkali metal ions uptake by the KTS-H in 0.1M MC1-MOH (M=Li, Na, K, Cs) aqueous solutions are presented in Fig. 2a.…”

Synthesis and characterization of inorganic sorbents and their application to sorption of radionuclides

“…The synthesis was performed under considerably soft conditions (under air pressure from 3-5 to 0.3-0.4 mPa and at 130-200 • C for 12-24 hours) in comparison with the well-known method by Poojary et al [2]. Changes in the reaction conditions resulted in the product yield increase from 30 to 250-300 g/l of initial reagents.…”

“…The Nb substitution increases sorption capacity towards Cs + ions and decreases Sr 2+ selectivity. This effect is attributed to higher coordination number, an increase in the unit cell volume and change in water / Na + population [2]. Among challenging materials are sodium titanate (NaTi 2 O 5 H), titanium silicates with pharmocosiderite structure, sodium nonatitanate (Na 4 Ti 9 O 20 ·xH 2 O), and heteropolymeta-…”