(588b) Pulse-Field Gradient NMR Study of TAA+-Silica Association in Zeolite Clear-Solution Syntheses

Silicalite-1 syntheses from optically transparent dilute TPAOH / TEOS / H₂O mixtures have been widely studied and are of fundamental importance as a model system for understanding zeolite nucleation. These mixtures have been studied by many labs using methods including small-angle scattering, electron microscopy, calorimetry and various spectroscopies.^[1-6]. Recently, Fedeyko et al. reported formation of silica nanoparticles in base solutions of tetraalkylammonium (TAA) cations above a critical aggregation concentration of silica originally dissolved in solution.^[3] An important, and as yet unresolved question is how the interaction between the silica nanoparticles and the organocation results in zeolite nucleation and crystallization.

We will summarize our efforts in determining the forces between organocation-silica nanoparticles using ¹H Pulse-Field gradient NMR and Two-Dimensional Diffusion-Ordered NMR spectroscopy(DOSY). From measurements of the organocation self-diffusion coefficient one can determine the fraction of the organocations bound to the silica nanoparticle. From this information one can then determine thermodynamic properties for these mixtures as they relate to cation ? nanoparticle interactions. We have found the diffusion coefficient of TPA⁺ decreased significantly in the presence of silica nanoparticles and it decreased inversely with the silica/TPA⁺ ratio. Hence, we could assume a two state model consisting of nanoparticle-bound TPA⁺ in equilibrium with freely diffusing TPA⁺ in bulk solution. Since the chemical exchange between the bound and free state of TPA⁺ is fast compared to the relaxation time scale, the observed diffusivity of TPA⁺ is a population weighted average of the two sites. From this analysis information about the adsorption strength and binding ratio of TPA⁺ versus silica nanoparticles at adsorption saturation can be extracted. Further variation of the association strength of silica nanoparticles and TPA⁺ with electrolyte concentration and organocations with different density charge (N⁺/C) can provide more detailed illustration of the organic-inorganic interaction at the beginning of silicalite-1 crystallization. The NMR results for a series of TAA cations will be discussed in the context of existing literature, and the implications of the findings for zeolite growth will be discussed.

Reference

[1] Davis,.T.M., Drews, T.O., Rmanan, H., et.al., nature materials, 2006, 5, 400-408

[2] Kumar, S., Wang, Z., Penn, R.L., J. Am. Chem. Soc. ,2008, 130, 17284?17286

[3] Fedeyko, J., Rimer, J., Lobo, R. et.al., J. Phys. Chem. B, 2004, 108, 12271-12275

[4] Kinrade, S.D., Knight, C. T. G., Pole, D.L., Inorg. Chem. 1998, 37, 4272-4277

[5] Cheng. C.H., Shanzt, D.F., J.Phys.Chem.B, 2005, 109, 7266-7274

[6] Schoeman, B. J., Microporous Mesoporous Mater. 1998, 22, 9-22