The ternary reaction composition diagram in the SiO2-Na 2O-TiO2-KF-H2SO4-H2O system was developed for static ETS-10 synthesis at 473 K utilizing Ti(SO 4)2 and N-brand sodium silicate solutions as sources of Ti and Si, respectively. For KF/TiO2 = 1.5 and H2O/TiO 2 = 300, pure, crystalline (≥95%) ETS-10 products were obtained in a narrow reaction composition zone (Na2O/SiO2 = 0.790-0.825, SiO2/TiO2 = 5.30-5.90, initial pH = 10.22-10.95). The presence of islands and terraces with heights of ∼1.2-1.7 nm suggests a layer growth mechanism via two-dimensional surface nucleation. The aspect ratio R = c/a of typical ETS-10 crystals (c is the axial length of the bi-pyramid and a is the linear dimension of the square basal plane shared by the two pyramids) and the roughness of crystal pyramidal side surface decreased with increasing initial synthesis mixture pH. The Si/Ti ratios in the ETS-10 products were close to the theoretical value of 5. The crystallization curves for this pure ETS-10 showed that the induction time decreased substantially from 36 to 19 h, whereas crystallization rate increased only slightly from 19% to 27% h-1 upon increasing initial pH. Increasing dilution of the initial synthesis mixture with water (5.50SiO2:4.46Na 2O:1.00TiO2:1.5KF:3.35H2SO4:450- 750H2O) resulted in progressively longer crystallizations, producing bi-pyramidal crystals of monotonically increasing dimensions. The results of experiments using NaF instead of either KF or KOH (i.e., conducted without K+ and F+) suggested that in unseeded, organic-free ETS-10 syntheses, K+ was necessary for the nucleation of ETS-10, while F- was not.
- Crystallization kinetics
- Surface structure