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

Abstract

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 3.6SiO 2 : 1TiO 2 : 5.5Na 2O : xH 2SO 4 : 230.2H 2O. 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.