The elucidation of Silicalite-1 crystallization mechanism has been the subject of numerous investigations[1-10]. This zeolite is usually crystallized from alkaline sols containing siliceous species and tetrapropylammonium cations, and its nucleation and growth take place in the presence of a suspension of nanoparticles having sizes less than ~10 nm. The nanoparticle structure and role in crystallization are fundamental problems that have also received a lot of attention. These nanoparticles are pre-organized inorganic/organic composites and their structure is most likely different than that of silicalite-1 crystals.

We have investigated the crystallization of silicalite-1 crystals in aqueous sols using Attenuated Total Reflection / Fourier Transform Infrared (ATR/FTIR) spectroscopy[10]. The effect of SiO2, and tetrapropylammonium hydroxide (TPAOH) concentration on the structure of the intermediate species as a function of time has been studied and will be presented. The compositions studied have been selected because they enable the acquisition of spectra with high signal/noise ratio and because they have been studied the past.The existence of oligomeric siliceous species has been verified for all compositions after the end of tetraethoxysilane (TEOS) hydrolysis. On the other hand, more condensed and highly crosslinked siliceous structures have been observed only when the SiO2 concentration was above a certain limit. These structures have been considered to be the “primary” nanoparticles reported by other researchers, and their IR spectra revealed that their structure is different than that of the silicalite-1 crystals. It has also been found that TPAOH content affects the structure and connectivity of both oligomeric and nanoparticle siliceous species. Increasing the TPAOH content resulted in a decrease of the SiO2 connectivity of the siliceous nanoparticles as well as to a broader distribution of the oligomeric siliceous species.

The evolution nanoparticle structure as a function of crystallization time has also been monitored. It has been found that upon heating of the synthesis sols, the “primary” nanoparticles transform to a new form that has an increased number of SiOSi bonds. These “intermediate” particles do not have the structure of silicalite-1 and they are present for the most part of crystallization. Finally once the silicalite-1 nuclei are formed they grow in a suspension of these “intermediate” particles. This transformation was more pronounced in the diluted sols than in the sols with low water content. Finally, our results will be analyzed in the context of the crystallization mechanisms proposed in the literature.

References

[1] T.M. Davis, T.O. Drews, H. Ramanan, C. He, J. Dong, M.A. Katsoulakis, E. Kokkoli, A.V. McCormick, R.L. Penn, M. Tsapatsis, Mechanistic principles of nanoparticle evolution to zeolite crystals, Nature Materials 5 (2006) 400-408

[2] J.M. Fedeyko, J.D. Rimer, R.F. Lobo, D.G. Vlachos, Spontaneous formation of silica nanoparticles in basic solutions of small tetraalkylammonium cations, Journal of Physical Chemistry B 108 (2004) 12271-12275

[3] V.Nikolakis, E. Kokkoli, M. Tirrell, M. Tsapatsis, D.G. Vlachos, Zeolite growth by addition of subcolloidal particles: Modeling and experimental validation, Chemistry of Materials 12 (2000) 845-853

[4] S.Y Yang, and.A. Navrotsky, Study on synthesis of TPA-silicalite-1 from initially clear solutions of various base concentrations by in situ calorimetry, potentiometry and SAXS, Chemistry of Materials 16 (2004) 3682-3687

[5] S.L. Burkett, and M.E. Davis, Mechanisms of structure direction in the synthesis of TPA/Si-ZSM-5, Journal of Physical Chemistry 98 (1994) 4647-4653

[6] C.E.A. Kirschhock, R. Ravishankar, L. Van Looveren, P.A. Jacobs, J.A. Martens, Mechanism of transformation of precursors into nanoslabs in the early stages of MFI and MEL zeolite formation from TPAOH-TEOS-H2O and TBAOH-TEOS-H2O mixtures, J. Phys. Chem B. 103 (1999) 4972-4978

[7] J..M. Fedeyko, D.G. Vlachos, and R.F. Rimer, Langmuir 21 (2005) 5197-5206

[8] B..J. Shöeman, Analysis of the nucleation and growth of TPA-silicalite-1 at elevated temperatures with the emphasis on colloidal stability, Microporous and Mesoporous Materials 22 (1998) 9-22

[9] B.J. Shöeman, A high temperature in situ laser light-scattering study of the initial stage in the crystallization of TPA-silicalite-1, Zeolites 18 (1996) 97-105

[10]A. Patis, V. Dracopoulos, and V. Nikolakis, Investigation of Silicalite-1 Crystallization Using Attenuated Total Reflection/Fourier Transform Infrared Spectroscopy, J. Phys. Chem C, 111 (2007)17478 - 17484