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 > 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.