STRUCTURAL STUDIES ON THE ION-EXCHANGED PHASES OF A POROUS TITANOSILICATE, NA2TI2O3SIO4-CENTER-DOT-2H(2)O

A titanosilicate with the ideal formula, Na2Ti2O3SiO4 . 2H(2)O, contai ning unidimensional channels, was synthesized hydrothermally and conve rted to the hydrogen form by acid treatment. The hydrogen form was par tially ion exchanged by sodium ions to obtain a 50% sodium ion exchang ed phase. The crystals of the sodium phase, NaHTi2O3SiO4 . 2H(2)O, ret ain the symmetry and unit cell parameters of the parent disodium compo und, space group P4(2)/mcm, a = b = 7.832(1) Angstrom, c = 11.945(2) A ngstrom, and Z = 4. The sodium ions are located on the ac faces of the crystal while the water molecules occupy the channels. Ion exchange o f the acid form by potassium ions leads to a phase with a maximum pota ssium to proton ratio of about 2, In the acid, H2Ti2O3SiO4 . 1.5H(2)O, and potassium phases, K0.5H1.5Ti2O3SiO4 . 1.5H(2)O and K1.38H0.62Ti2O 3SiO4 . H2O, the a and b axes are doubled while the c-axis dimension i s retained. These doubled dimensions were transformed to a primitive t etragonal cell which has a volume twice that of the parent sodium form . The crystals belong to the space group P4(2)/mbc with a = 11.039(1) Angstrom, c = 11.886(1) Angstrom for the acid phase, a = 11.015(1) Ang strom, c = 12.017(1) Angstrom for the K1.38H0.62 phase, and a = 11.060 4(3) Angstrom, c = 11.9088(3) Angstrom for the K0.5H1.5 phase. The num ber of molecules in the unit cell in these three cases is 8. In the ac id form, the channels are occupied by the water molecules, which are i nvolved in hydrogen bonding among themselves as well as with the frame work 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 potas sium ions are located at the center of the channel and are bonded to t he silicate oxygens. The remaining ions are found near the framework w hich is close to the positions of the sodium ions of the ac faces in t he parent compound. These ions are bonded to both the framework and wa ter oxygen atoms. The titanium atoms in all the phases are octahedrall y coordinated, and they are grouped as clusters of four. These cluster s are linked by the silicate groups along the a and b directions and b y Ti-O-Ti bonds along the c directions. This structural data provide a basis for explaining the observed ion exchange behavior and ion selec tivity.