Towards Macroporous α-Al(2)O(3)—Routes, Possibilities and Limitations

This article combines a systematic literature review on the fabrication of macroporous α-Al(2)O(3) with increased specific surface area with recent results from our group. Publications claiming the fabrication of α-Al(2)O(3) with high specific surface areas (HSSA) are comprehensively assessed and critically reviewed. An account of all major routes towards HSSA α-Al(2)O(3) 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-Al(2)O(3) may be the more stable alumina modification for A(BET) > 175 m(2)/g. In fact, the highest specific surface area unobjectionably reported to date for α-Al(2)O(3) amounts to 16–24 m(2)/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-Al(2)O(3) seeds. These experimental results are substantiated by attempts to theoretically calculate and predict the specific surface areas of both porous materials and nanopowders.