SYNTHESES, CRYSTAL-STRUCTURES, AND ION-EXCHANGE PROPERTIES OF POROUS TITANOSILICATES, HM(3)TI(4)O(4)(SIO4)(3)CENTER-DOT-4H(2)O (M=H+, K+, CS+), STRUCTURAL ANALOGS OF THE MINERAL PHARMACOSIDERITE

Titanosilicate, a structural analogue of the mineral pharmacosiderite, was prepared in its potassium and cesium forms, and the cesium form w as ion exchanged to obtain the acid form. The alkali cation derivative s, HM(3)Ti(4)O(4)(SiO4)(3) .4H2O (M = K+, Cs+), as well as the acid fo rm, H4Ti4O4(SiO4)(3) . 8H(2)O, are isostructural with the mineral. The y crystallize in the cubic space group <P(4)over bar 3m> with a = 7.82 12(2) Angstrom, a = 7.7644(3) Angstrom, a = 7.8214(6) Angstrom, and Z = 1 for the cesium, potassium, and proton phases, respectively. In the structure, four titanium octahedra link to form Ti4O4 cubes around th e unit-cell corners, and silicate tetrahedra join the titanium octahed ra in all three crystallographic directions to form a three-dimensiona l framework. These linkages create three-dimensional tunnel systems th at are filled by water molecules and exchangeable ions. The charge-neu tralizing protons in the acid form bond to the oxygens in the Ti4O4 un it. Among the four protons in this group, three exchange for the cesiu m or potassium cations and the remaining proton would then be disorder ed over these four oxygens. Some general observations on the ion-excha nge behavior of the proton phase are presented.