Mechanisms of structure direction in zeolite synthesis

The mechanisms by which the geometries of organic structure-directing agents are translated into the product pore architectures in the synthesis of pure-silica and aluminosilicate zeolites are investigated by numerous spectroscopic techniques and variations in synthesis gel composition. For the tetrapropylammonium- and 1,6-hexanediamine-mediated syntheses of pure-silica ZSM-5 (Si-ZSM-5), 1H-29Si CP MAS NMR is performed between the protons of the organic species and the silicon atoms of the zeolite framework precursors in a deuterated synthesis medium to probe the interactions between the organic and inorganic components. The origin of structural specificity in the synthesis of pure-silica zeolites in the presence of structure-directing agents is attributed to the formation of favorable intermolecular van der Waals interactions within inorganic-organic composite species that form the key components in zeolite self-assembly. Investigation of the 1H-29Si CP MAS NMR profiles of silicate gels containing tetraalkylammonium cations that do not induce the formation of a crystalline zeolite product suggest the significance of hydrophobic hydration of the organic component in the formation of the inorganic-organic composite structures that is essential to the synthesis of pure-silica zeolites. For the syntheses of the hexagonal (EMT) and cubic (FAU) polymorphs of the aluminosilicate zeolite faujasite in the presence of 18-crown-6 and 15-crown-5, respectively, a combination of NMR and vibrational spectroscopic techniques and variations in the synthesis compositions are used to elucidate the structure-directing roles of the crown ethers. Sodium/crown ether complexes facilitate and direct the assembly of sodium-templated extended aluminosilicate structures via ion-dipole interactions to form the EMT and FAU products. Thus, for the synthesis of Si-ZSM-5 and the synthesis of EMT and FAU, two different mechanisms of structure direction and self-assembly via the formation of extended inorganic or inorganic-organic composite species are proposed.