A new complete, representative model for the hydroxylated surface of amorphous silica is presented and characterized by means of periodic DFT calculations. This model accounts for the experimentally encountered ring size distribution, Si−O−Si and O−Si−O angles, silanols density, and distribution (isolated, associated, geminals). Properties such as NMR shifts, dehydrogenation energies, OH vibrational frequencies, and the interaction with water are investigated. The results are compared with former experimental and theoretical results. This new representative model for this complex surface would probably help the investigation of its reactivity toward amino acids or other organic molecules, opening new perspectives in the understanding of the chemistry of amorphous materials.
The designed elaboration of alumina sub-micrometric spherical powder that combines 3D ordered
mesoporosity of high accessibility, nanocrystalline structure, and thermal stability up to 900 °C is reported.
The strategy used to elaborate such new materials labeled “UPMC1” involves specific block-copolymer
templating, aluminum sol−gel chemistry, tuned aerosol generation (spray drying), and sequential thermal
treatments that allow designing of a whole set of mesoporous catalytic supports by adjusting ceramization
conditions between 700 and 900 °C. When calcination temperature reaches 700 °C, the network remains
amorphous and displays structural features of highly porous materials (i.e., porosity, 0.56 cm3·g-1; surface
area, 403 m2·g-1; well-calibrated pore diameter, 13 nm). After 30 min at 900 °C, crystallization into
γ-Al2O3 particles of around 6 nm has occurred, which has modified the network characteristics (i.e.,
porosity, 0.34 cm3·g-1; surface area, 134 m2·g-1; well-calibrated pore diameter, 12.5 nm) without destroying
the mesostructure. Both amorphous and crystalline final materials present the remarkable properties of
mesoporous materials with the unique amphoteric properties of the γ-alumina surface (40% of tetragonal
acid sites) that have great potential application in catalysis, in environment, and as an adsorbent. The
present work points out that ordered mesoporosity has the ability to stabilize materials with amorphous
or metastable crystalline structure at higher temperatures than what is observed for nonordered mesoporous
analogous systems. Such a phenomenon is discussed on the basis of extensive materials characterization
mainly based on TEM, XRD, and 29Al high-resolution solid-state NMR.
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