A contribution to understanding the mechanism of crystallization of silicalite-1 in heterogeneous systems (hydrogels)

Solid phase (gel) separated from freshly prepared hydrogel having the batch molar composition: 2.5Na2O–8TPABr–60SiO2–800H2O as well as the solids drawn off the reaction mixture, having the same batch molar composition, during its hydrothermal treatment at 170 °C were analyzed by different methods such as powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), light microscopy (LM), scanning-electron microscopy (SEM), transmission electron microscopy (TEM), electron diffraction (ED) and atomic force microscopy (AFM). Analysis of the obtained results have shown that the freshly prepared gel represents a “hierarchical structure” in which the largest individual gel aggregates (having the size 200 nm–1000 nm, or more) are composed of smaller particles having the size in the range 40 nm–80 nm, which represent aggregates of ⩽10 nm particles. Heating of the hydrogel causes structural changes in the gel matrix; 36T units of MFI structure present in the freshly prepared gel gradually transforms into fully crystalline nuclei of silicalite-1. The nuclei formed inside the gel particles and/or small silicalite-1 nanocrystals formed by a limited grow of nuclei in the gel matrix, can grow only after their release from the gel dissolved during the crystallization, i.e. when they are in full contact with the liquid phase. Growth of silicalite-1 crystals in heterogeneous systems takes place by incorporation of small, partially crystalline 10 nm particles and/or their aggregates (10 nm ⩽ D ⩽ 40 nm) onto the surface of growing silicalite-1 crystals.