Nanocrystalline mesoporous γ-alumina powders "UPMC1 material" gathers thermal and chemical stability with high surface area

Abstract

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-Al 2O3 particles of around 6 nm has occurred, which has modified the network characteristics (i.e., porosity, 0.34 cm 3·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. © 2006 American Chemical Society.