Investigation of Silicalite-1 Crystallization Using Attenuated Total Reflection/Fourier Transform Infrared Spectroscopy

Abstract

The crystallization of silicalite-1 in aqueous sols has been investigated using ATR/FTIR spectroscopy as a function of sol composition. 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 at the sols of high SiO 2 concentrations. These structures have been considered to be the "primary" nanoparticles reported by other researchers, and their IR spectra revealed that their structures are different from that of the silicalite-1 crystals. The evolution of the nanoparticle structure as a function of crystallization time has 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 most of the crystallization. Finally, once the silicalite-1 nuclei are formed, they grow in a suspension of these "intermediate" particles.

Tetrapropylammonium Occlusion in Nanoaggregates of Precursor of Silicalite-1 Zeolite Studied by 1H and 13C NMR

“…Typical molar composition space for silicalite-1 in the system SiO2-TPAOH-H2O occurring either in concentrated media (yellow area) as investigated by Corkery, van Santen, and Cundy et al [12,13,24,59], or dilute systems (blue area) as studied by Davis, Tsapatsis, and Shantz et al [3,22,27,28,41,42,50,51,60,61]. Both domains have also been extensively explored by Nikolakis, Lobo, and Martens et al [1,2,4,10,14,17,20,25,26,62].…”

“…However, the very early stages of zeolite formation lies in the pre-heating stage, at precursor formation. Since their discovery in the mid 1990s many controversial studies have attempted to derive the exact status of the very early nanoparticles of small size in the range a few nanometers [2,3,[12][13][14][15][16][17][18][19][20][21]. The question of whether these nanoparticles contain occluded organic templates or are simply organic-free silica entities has been disputed [9] and a core-shell structure with a negatively charged surface silica core surrounded by a shell of organocations, might seem to be the most accepted description [1,2,10,15,[22][23][24][25][26][27][28][29][30].…”

Investigation of the Mechanism of Colloidal Silicalite‐1 Crystallization by Using DLS, SAXS, and 29Si NMR Spectroscopy

“…In smaller particles, such as the ones observed in clear sols, one must rely on molecular tools, such as 29 Si NMR or IR spectroscopy. [21] Rimer et al developed an indirect method to probe the molecular structure of nanoparticles by measuring dissolution enthalpies and rates. [22] The gathered evidence shows that the crystallization of silicalite-1 cannot be described entirely by classical nucleation and growth theories.…”

Indirect Observation of Structured Incipient Zeolite Nanoparticles in Clear Precursor Solutions

“…[1b, 5,7,12,13] Based on the presence of a band near 550 cm À1 , which is widely accepted to be characteristic of the MFI structure, [14] either crystalline [1b, 12] or amorphous character of the precursor particles has been suggested. [5,7,13] These IR studies were performed on freeze-dried samples, [12,13] extracted precursor particles, [1b, 5] and original sols.…”

“…[19] On the other hand, crystalline Si-O networks, both dense and porous silica structures, are unambiguously distinguishable from amorphous silica by the presence of an IR peak near 1230 cm À1 . [5,7,14,20,21] This peak is indicative of the presence of translational symmetry in the silicon-oxygen network and therefore may serve as an even better indicator for the existence of zeolite subnanoparticles when other crystalline dense silica phases are not present. This is especially important for analyzing zeolite films.…”