Microporous titanosilicate AM-2: Ion-exchange and thermal stability

Hydrothermally synthesized microporous titanosilicate AM-2 (K2TiSi3O9 · H2O) was used for ion-exchange experiments with various mono- and divalent cations (Na+, Cs+, Mn2+, Ca2+, Sr2+). The exchange process was monitored with chemical analysis of the exchange solution, and the exchanged structures were refined from powder X-ray diffraction data using the Rietveld method. Dehydration was studied by thermo-gravimetric analysis and powder XRD in order to determine thermal stability. The experiments show that the kinetics of the exchange reaction at 90 °C depends on the ionic radius and is faster for small cations (e.g., Mn) than for large ones (e.g., Cs). The cation and H2O arrangement in the cavities depends on the valence of the exchanged cation: Monovalent cation positions can clearly be distinguished from H2O sites, whereas bivalent cations share their positions with H2O molecules, leading to a high degree of disorder among channel occupants. AM-2 (K2TiSi3O9 · H2O) was able to withstand dehydration by lowering its symmetry from orthorhombic (space group P212121) to monoclinic (space group P21) above 250 °C, and back to orthorhombic upon rehydration at room temperature. The Na-exchanged structure was the only one to show monoclinic symmetry (space group P21/c) at room temperature induced by ion-exchange Na → K. All other structures showed with increasing temperature an irreversible degradation, either gradual or stepwise, to X-ray amorphous. The maximum thermal stability depends on size and valence of the exchanged cation, with smaller size and higher valence causing faster degradation. The X-ray amorphous products recrystallise above 700–750 °C, leading to new phases structurally not related to AM-2.