(122g) Investigation of the Effect of Synthesis Sol Composition on Silicalite-1 Crystallization Using ATR / FTIR Spectroscopy | AIChE

(122g) Investigation of the Effect of Synthesis Sol Composition on Silicalite-1 Crystallization Using ATR / FTIR Spectroscopy


Nikolakis, V. - Presenter, University of Delaware
Patis, A. - Presenter, FORTH/ ICEHT

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.


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