(570at) Pulsed-Field Gradient NMR Investigations of Zeolite Nucleation and Growth

Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy was used to study the organocation-silica interactions in the synthesis of zeolites from optically transparent reaction mixtures. PFG-NMR offers a powerful tool to examine these interactions by permitting determination of the diffusion coefficients of the cations. In this work, we used methyltripropylammonium (MTPA+), ethyltriproprylammonium (ETPA+), and tetrapropylammonium (TPA+) cations as structure directing agents (SDAs) to investigate their influence in the silicalite-1 formation. We also used 4,4'-trimethylenebis(dimethylpiperidinium) cation as a SDA because, although it is known that this cation selectively forms ZSM-12, our previous work showed that ZSM-12 cannot be made from optically transparent mixtures with this cation. Our PFG-NMR experiments showed that in the presence of silica the diffusion coefficients of the tetraalkylammonium organocations decreased compared with those of the cations in water alone. This decrease is attributed to a reduction in the cation mobility while the cations interact with silica nanoparticles. However, the diffusion coefficients of 4,4'-trimethylenebis(dimethylpiperidinium) cation did not show a significant change in the presence of silica and we believe this is due to a lack of interactions between this cation and the silica nanoparticles. We measured the diffusion coefficients as a function of the organocation concentration to determine the fraction of organocation bound to the silica through a model that considers the cation in either a free or a bound state. The resulting adsorption isotherms were fit with the Langmuir model, from which the adsorption constants and the derived thermodynamic parameters of the system were obtained. The results suggest that the energetics of the systems changes depending on the organocation identity and the cation/silica ratio. Ongoing work in our lab includes studying these mixtures using in-situ NMR experiments to analyze the effect of temperature on the zeolite formation. We believe that these results will help development of a more general approach to making high-silica zeolites from optically transparent mixtures containing silica nanoparticles.