ABSTRACT:The initial molecular steps of the acid-catalyzed silica sol-gel process departing from tetraethylorthosilicate (TEOS) were investigated by in situ 29 Si NMR and UV-Raman spectroscopy. The use of a substoichiometric H 2 O:TEOS molar ratio (rvalue 0.2-1.2) slowed the silicate oligomerization reaction and allowed unraveling the initial steps of silica condensation. Molecular modeling confirmed Raman signal and 29 Si NMR shift assignment. A comprehensive listing of all Raman and 29 Si NMR assignments is provided, including unique Raman assignments of cyclosilicates and the linear tetramer. The combination of experiment and modeling allowed an analysis of the reaction kinetics. The derived kinetic model and the experimental observation both revealed that the H 2 O: TEOS molar ratio had a strong influence on the reaction kinetics but not on the reaction pathways. The multianalytical approach led to development of an oligomerization scheme. As dominant oligomerizations, chain growth, cyclodimerization, and branching were identified. Under the investigated conditions, chains did not grow longer than pentamer, and ring sizes were limited to 6-rings. Chains of 4 Si atoms and 4-rings were abundant species. Branched rings and chains were formed by attachment of dimers and trimers. Gelation proceeded from branched 4-rings and branched chains with limited hydroxyl functionalities.
Ein Durchbruch: Das mikroporöse metall‐organische Gerüst MIL‐47 ist ein ausgezeichnetes Adsorbens für die Trennung von C8‐Alkylarenen wie Ethylbenzol, meta‐Xylol und para‐Xylol. Das Anwendungspotenzial von MIL‐47, mit seiner hohen Aufnahmekapazität und seiner Hydrophobie, wurde mit Durchbruch‐ und chromatographischen Experimenten nachgewiesen (siehe Bild).
Amorphous microporous silica (AMS) materials with variation in microporosity were prepared using an acid-catalyzed sol−gel procedure departing from tetraethylorthosilicate. The silicas were fined to specific particle
sizes by crushing and sieving. AMS materials were loaded with 10 wt % ibuprofen either by uptake of molten
ibuprofen or by adsorption of ibuprofen from a methylene chloride solution. DSC analysis of ibuprofen-loaded AMS confirmed the molecular dispersion of ibuprofen on the silica material. In vitro ibuprofen release
from AMS carrier was investigated in simulated intestinal fluid and in a dissolution medium simulating the
gastrointestinal tract with simulated gastric fluid followed by simulated intestinal fluid. The release of ibuprofen
molecules from the AMS silica carrier is governed by diffusion. The diffusivity of ibuprofen in the investigated
series of AMS samples was in the range 10-14−10-11 m2 s-1. By adapting the porosity and particle size of
AMS, the release could be evenly spread over periods from 3 to 100 h. This flexibility of AMS opens
perspectives for designing tailor-made controlled release formulations.
The hydrothermal crystallizations of two zeolite topologies (FAU and LTA) have been studied by simultaneous UV-Raman spectroscopy and X-ray diffraction in a home built setup. A wide angle X-ray diffractometer has been redesigned and combined with Raman components. The results revealed, despite similar structures of the starting gels, different aluminosilicate species evolved in the two systems, prior to emerging Bragg scattering. Based on this the sodalite cage could be ruled out as a common building unit for both frameworks.
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