Precise identification and characterization of catalytically active sites on the surface of γ-alumina.

γ-alumina is one of the oldest and most important commercial catalytic materials with high surface area and stability. These attributes enabled its use as the first commercial large-scale heterogeneous catalyst for ethanol dehydration. Despite progress in materials characterization and over a hundred years of active research, the nature of the specific sites on the surface of γ-alumina which are responsible for its unique catalytic properties has remained obscure and controversial. Using combined infrared spectroscopy, electron microscopy and solid-state nuclear magnetic resonance measurements we identify the octahedral, amphoteric (O) 5 Al(VI)-OH sites on the (100) segments of massively restructured (110) facets on typical rhombus-platelet γ-alumina as well as the (100) segments of irrational surfaces (invariably always present in all γ-alumina samples) responsible for its unique catalytic activity. Such (O) 5 Al(VI)-OH sites are also present on the macroscopically defined (100) facets of γ-alumina with elongated/rod-like geometry. The exact mechanism by which these sites lose -OH groups upon thermal dehydroxylation resulting in coordinatively unsaturated penta-coordinate Al +3 O 5 sites is clarified. These coordinatively unsaturated penta-coordinate Al sites demonstrate new chemistry, producing well-defined thermally stable Al-carbonyl complexes. Our findings contribute to the understanding of the nature of coordinatively unsaturated Al sites on the surface of γ-alumina and their role as catalytically active sites.