Syntheses, Crystal Structures, and Ion-Exchange Properties of Porous Titanosilicates, HM₃Ti₄O₄(SiO₄)₃·4H₂O (M = H⁺, K⁺, Cs⁺), Structural Analogues of the Mineral Pharmacosiderite

Abstract

Titanosilicate, a structural analogue of the mineral pharmacosiderite, was prepared in its potassium and cesium forms, and the cesium form was ion exchanged to obtain the acid form. The alkali cation derivatives, HM 3 Ti 4 O 4 (SiO 4 ) 3 ·4H 2 O (M = K +, Cs + ), as well as the acid form, H 4 Ti 4 O 4 (SiO 4 ) 3 ·8H 2 O, are isostructural with the mineral. They crystallize in the cubic space group P 4̄3 m with a = 7.8212(2) Å, a = 7.7644(3) Å, a = 7.8214(6) Å, and Z = 1 for the cesium, potassium, and proton phases, respectively. In the structure, four titanium octahedra link to form Ti 4 O 4 cubes around the unit-cell corners, and silicate tetrahedra join the titanium octahedra in all three crystallographic directions to form a three-dimensional framework. These linkages create three-dimensional tunnel systems that are filled by water molecules and exchangeable ions. The charge-neutralizing protons in the acid form bond to the oxygens in the Ti 4 O 4 unit. Among the four protons in this group, three exchange for the cesium or potassium cations and the remaining proton would then be disordered over these four oxygens. Some general observations on the ion-exchange behavior of the proton phase are presented.