Synthesis and morphological control of large titanosilicate ETS-10 crystals

Hydrothermal synthesis of large titanosilicate ETS-10 crystals was investigated at 473 K by adjusting the Na2O/K2O ratio (x/y) and H2O content (z) of synthesis mixtures with molar composition 5.5SiO2:xNa2O:yK2O:1TiO2:zH2O, where x = 1.66–4.18, y = 0.28–2.80, x + y = 4.46, and z = 300–900. Square truncated bipyramidal crystals with the increasing basal width and decreasing aspect ratio (i.e., decreasing combined height of two pyramids divided by the width of the shared base) were grown upon decreasing x/y in mixtures with z = 300, and by increasing z in mixtures with x/y = 2.0. The largest ETS-10 crystals (average dimensions ∼80 × 80 × 22 μm) were obtained using mixture with x/y = 0.6 and z = 300. These are the largest ETS-10 crystals reported to date. The linear dimensions of the stepped island-like structures that are present on the square crystal faces increased concomitantly with the increasing basal width and decreasing ETS-10 aspect ratio. These results suggest that both the replacement of Na ions by K ions in the synthesis mixture and its dilution with water suppressed three- and two-dimensional nucleation of ETS-10. Also, these results suggest a lower concentration of defects along the ⋯Ti–O–Ti–O–Ti⋯ chains in large crystals.