Towards Macroporous α-Al2O3—Routes, Possibilities and Limitations

Abstract

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 ABET > 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.

Non-aqueous cross hydrolysis: an epoxide-free sol-gel route toward highly porous alumina monoliths

“…Phase evaluation by XRD revealed moderately crystalline γ-Al 2 O 3 for the samples calcined at 600°C, and pure α-Al 2 O 3 for those calcined at 1200°C. Both results are in accordance with the expected phase compositions 2 [12].…”

“…Consequently, the specific surface area determined by the BET method also decreases from 422 m 2 /g for the γ-Al 2 O 3 monolith to 11 m 2 /g for the α-Al 2 O 3 monolith. For porous alumina, these porosity values are already among the highest ones achieved to date [12]. Ongoing studies concentrate on the optimization of the procedure with special focus on the gel formation as a key point of the synthesis.…”

Sol–Gel Encapsulation of ZnAl Alloy Powder with Alumina Shell

“…Boehmite is known to form γ-Al 2 O 3 when calcined at 300-500 • C, δ-Al 2 O 3 when calcined at 700-800 • C, θ-Al 2 O 3 at 900-1000 • C, before transforming into α-Al 2 O 3 (corundum) above 1000 • C [22][23][24][25].…”

“…The presence of boehmite indicates that the boehmite-to-γ(Al 2 O 3 ) phase transformation was not complete under the calcination conditions used in this study (1 h at 500 • C). Some studies used an annealing temperature as high as 600 • C and an annealing time as long as 20 h for this conversion [23,24]. by a decrease in the zinc concentration) is evident in alumina-coated areas compared to areas where the core alloy is exposed (despite its own native oxide layer revealed by XRD, as mentioned in Section 3.1).…”

Трансформация Осажденного Гидроксида Алюминия В Низкотемпературный Оксид

“…Процессам получения гидроксидов и оксидов алюминия и их взаимным переходам посвящено огромное число публикаций [5,6]. Это объясняется большим разнообразием оксидно-гидроксидных форм Al, обладающими различными свойствами даже при одинаковом химическом составе.…”