Mechanism of Structure Direction in the Synthesis of Pure-Silica Zeolites. 2. Hydrophobic Hydration and Structural Specificity

Abstract

Several syntheses of zeolites that employ organic structure-directing agents are investigated via ^1H-^(29)Si CP MAS NMR spectroscopy. Inorganic-organic interactions in the tetrapropylammonium- and 1,6-hexanediamine-mediated syntheses of pure-silica ZSM-5 (Si-ZSM-5) are compared; these results are contrasted to those from the 1,6-hexanediamine-mediated synthesis of pure-silica ZSM-48 (Si-ZSM-48), in which the diamine serves as a pore-filling agent. Synthesis gels containing tetramethylammonium, tetraethylammonium, tetrabutylammonium, tetrapentylammonium, and tetraethanolammonium in lieu of tetrapropylammonium are evaluated by ^1H-^(29)Si CP MAS NMR to examine the significance of hydrophobic hydration behavior in structure direction. Finally, the effects of sodium, the silica source, and the substitution of D_2O for H_2O on the kinetics of nucleation and crystallization are discussed in terms of their mechanistic implications. A modified mechanism of structure direction in zeolite synthesis is proposed for which the formation of inorganic-organic composite structures is initiated by overlap of the hydrophobic hydration spheres of the inorganic and organic components, with subsequent release of ordered water to establish favorable intermolecular van der Waals interactions.