Formation of colloidal molecular sieves: influence of silica precursor

The formation of nanosized zeolite with MFI type structure from precursor mixtures utilizing tetraethylorthosilicate (TEOS) and precipitated silica (Sipernat 320) as silica sources was studied with in situ dynamic light scattering (DLS). All measurements were performed with fully concentrated samples, and the DLS data showed that the size of the primary species in the precursor mixtures is strongly dependent on the type of silica source employed. The precursor particles formed in TEOS-containing solution aged for 30 h at room temperature are much smaller (∼2.5 nm radius) than those prepared from precipitated silica (∼45 nm). After heating at 90 °C the amorphous precursor colloids are completely transformed into crystalline zeolite with MFI structure in 10 and 15 h for the TEOS and precipitated silica containing mixtures, respectively. The mean hydrodynamic radii of the crystalline MFI nanoparticles correspond to the size of the amorphous particles formed immediately after preparation of both precursor mixtures. Replacing the TEOS with precipitated silica leads to a change in the particle size distribution in the precursor solution, which influences the final radius of the MFI crystallites.