Ructural Studies on the Ion-Exchanged Phases of a Porous Titanosilicate, Na2Ti2O3SiO4·2H2O
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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 K1.38H0.62Ti2O3SiO 4·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.
author list (cited authors)
Poojary, M. M., Bortun, A. I., Bortun, L. N., & Clearfield, A.