Pore surface fractal analysis of PEG and La(III)-doped mesoporous alumina obtained by the sol-gel method

Towards Macroporous α-Al2O3-Routes, Possibilities and Limitations

This article combines a systematic literature review on the fabrication of macroporous α-Al₂O₃ with increased specific surface area with recent results from our group. Publications claiming the fabrication of α-Al₂O₃ with high specific surface areas (HSSA) are comprehensively assessed and critically reviewed. An account of all major routes towards HSSA α-Al₂O₃ is given, including hydrothermal methods, pore protection approaches, dopants, anodically oxidized alumina membranes, and sol-gel syntheses. Furthermore, limitations of these routes are disclosed, as thermodynamic calculations suggest that γ-Al₂O₃ may be the more stable alumina modification for A_BET > 175 m²/g. In fact, the highest specific surface area unobjectionably reported to date for α-Al₂O₃ amounts to 16-24 m²/g and was attained via a sol-gel process. In a second part, we report on some of our own results, including a novel sol-gel synthesis, designated as mutual cross-hydrolysis. Besides, the Mn-assisted α-transition appears to be a promising approach for some alumina materials, whereas pore protection by carbon filling kinetically inhibits the formation of α-Al₂O₃ seeds. These experimental results are substantiated by attempts to theoretically calculate and predict the specific surface areas of both porous materials and nanopowders.

Kinetic of sintering of polyethilene glycol and lanthanum dopped aluminum oxide obtained by the sol-gel method

Sintering and crystallization of low-density polyethylene glycol (PEG) and lanthanum, La(III)-doped Al2O3 aerogels prepared from aluminum isopropoxide were investigated. The sintering behavior of non-doped and doped aerogels was examined by following the change of specific surface area with isothermal heat-treatment. The specific surface area and crystalline phases of non-doped and PEG+La(III)-doped aerogels were determined, and the effects of dopants on the sintering and crystallization of Al2O3 aerogels are discussed. Isothermal sintering experiments showed that the sintering mechanism of non-doped and PEG+La(III)-doped Al2O3 aerogels is surface diffusion. The specific surface areas of alumina samples decrease rapidly during the initial period of sintering, and more slowly with prolonged sintering time. The change of the porous structure is correlated with the phase transformation of ?-Al2O3 during calcinations of Al2O3 aerogels. The surface area of non-doped Al2O3 aerogels came to about 20 m2g-1 with heat-treatment at 1100?C because of crystallization of ?-Al2O3 after densification. In the case of heattreatment at 1200?C, the largest surface area was observed for PEG+La(III) doped Al2O3 aerogels and the XRD pattern showed only low ordered ?-Al2O3. These indicate that the addition of PEG+La(III) to boehmite sol prevents Al2O3 aerogels from sintering and crystallizing to the ?-Al2O3 phase. Even after 20 h at 1000?C, PEG+La (III)-doped alumina samples maintain a rather good specific surface area (108 m2 g-1) in comparison to the non-doped, containing mainly ?-Al2O3 and minor amounts of ?-Al2O3. Aluminum-oxides with these structural and textural properties are widely used as a coatings and catalyst supports in the field of various catalysis.