Structure of the Silica Phase Extracted from Silica/(TPA)OH Solutions Containing Nanoparticles

Abstract

Subcolloidal particles of a few nanometers in diameter are observed during the clear-solution synthesis of silicalite-1. These nanoparticles (3-5 nm) can be synthesized at room temperature starting from tetrapropylammonium (TPA) hydroxide, tetraethyl orthosilicate (TEOS), and water, and they have been reported to have a uniform structure identical to that of zeolite ZSM-5 (called nanoblocks or nanoslabs). To study their structure, we followed the extraction procedure proposed in the literature to obtain a dry powder of the particles. These dried particles were analyzed with powder X-ray diffraction (XRD), solid-state NMR spectroscopy, FTIR spectroscopy, thermogravimetric analysis, and N 2 adsorption isotherms. The results are compared with those obtained for colloidal size silicalite-1, amorphous silica, and the mesoporous silicate SBA-15. To obtain a better idea of the shape and structure of the particles, we conducted simulated annealing modeling to fit the particle shape to the fractions of Q n obtained from the 29 Si MAS NMR spectra. The model structures are in excellent agreement with our NMR data and suggest a poorly defined particle shape, in contrast to previous reports. The XRD patterns of samples with particle sizes in the range of the nanoparticles were simulated using the Debye formula and the SKIP algorithm. These simulations were carried out using structural models of silicalite-1 nanocrystals, the proposed nanoblock structure, and the silica particles derived from simulated annealing. We found no evidence in support of a well-defined MFI-like structure in the extracted material. The particles contain TPA, partly associated with the particles and partly as (TPA)Cl formed by the extraction process. The evidence accumulated here is in disagreement with the well-defined structure of the nanoparticles previously reported.

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].…”

“…The nature of the nanoparticles during zeolite formation by heating has been claimed as being an amorphous-like structure [9][10][11]. 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]. Alternatively, at precursor formation before any heating, light diffusing nanoparticles have been demonstrated as aggregated oligomeric silicates, and not as dense silica particles [4].…”

Hierarchische Nanofertigung: von geformten Zeolithnanopartikeln zu hochleistungsfähigen Trennmembranen

“…[176] Für den letztgenannten Mechanismus gibt es keine eindeutigen und schlüssigen Beweise. [168,[177][178][179][180] Dagegen sind die ersten vier Mechanismen mit dem Befund vereinbar, dass Vorstufen-Nanopartikel (mit einer Größe von 3-5 nm) ohne Merkmale der Silicalit-1-Struktur sowohl in frischen wie auch gealterten TPA-Silica-Lösungen anzutreffen sind. [169][170][171][172]181] M. Tsapatsis und M. A. Snyder Aufsätze Wir beschränken unsere Diskussion auf Silicalit-1 aus klaren Solen, weisen aber darauf hin, dass Silicalit-1 [182] und andere Zeolithe [173,174,183] durch Keimbildung in amorphen Gelen und deren anschließende Umwandlung entstehen können, ähnlich wie dies für nichtzeolithische Systeme diskutiert wurde.…”

Structure Determination of Molecular Sieve Nanoparticles with Electron Microscopy and Powder X-Ray Diffraction

“…Based on the optimized structure, powder X-ray diffraction patterns of nanoparticles of finite size were simulated using a GPU-accelerated algorithm. [5,6] Electron diffraction patterns were also simulated using Multislice method to investigate the effects of small size. [7] …”