Synthesis of titanosilicate ETS-4 with controlled morphology and investigation of its crystallization kinetics

ETS-4 crystals with controlled morphology were hydrothermally synthesized at 448 K under static conditions by changing their pH through variation of the amount of sulfuric acid added (x=3.3-4.4). The overall molar composition was 3.6SiO₂ : 1TiO₂ : 5.5Na₂O : xH₂SO₄ : 230.2H₂O. An increase of the initial pH from 11.50 (x=4.4) to 12.90 (x=3.3) resulted in a change of morphology of the ETS-4 product from spherulitic particles of submicron crystallites to monolithic crystals with rectangular prism morphology (average dimensions ∼1 × 15 × 2 μm). With the increase of initial pH, the polycrystalline particles resulted in progressively fewer and larger crystallites. The presence of growth spirals on the {1 0 0} surfaces of monolithic crystals suggested that high alkalinities resulted in low supersaturation of the (titano)silicate species, producing monolithic crystals with the largest dimension coinciding with linear -OTiOTiO- chains running in the [0 1 0] direction. Crystallization kinetics of ETS-4 was found to depend on the alkalinity of the synthesis mixture. The induction period decreased from ∼30 to ∼4 h when the initial pH increased from 11.50 to 12.90. The crystallization rate increased somewhat upon increase of the initial pH from 11.50 to 12.60 (x=3.6) only to decrease substantially upon further increase of the initial pH. These results indicate a complex growth process of ETS-4 spherulites, including both branching and spiral growth mechanisms.