Trends in C 2 −C 4 Alcohols Dehydration Catalyzed by the γ‐Alumina (100) Surface: The Isobutanol Puzzle

ABSTRACT The monomolecular dehydration of alcohols into olefins catalyzed by the (100) surface of γ‐alumina is investigated by DFT simulations. The transition states (TS) and mean energy paths are determined for C 2 to C 4 alcohols through the intrinsic reaction coordinate (IRC) method. The reaction occurs through a non‐ideal E2‐type mechanism: the C α ─O bond stretching takes place before the C β ─H β stretching along IRC. Quantitative structural and energetic trends are recovered: primary alcohols are the least reactive with the lower (respectively larger) C α ─O (respectively C β ─H β ) stretching, while secondary and tertiary are the most reactive with the larger (respectively lower) C α ─O (respectively C β ─H β ) stretching. However, the calculated dehydration kinetic constant of isobutanol underestimates the experimental one. By analyzing structural and charge descriptors along the IRC path and at TS, we identify that the TS of isobutanol dehydration is subject to steric hindrance between the two methyl groups present on and the (100) surface, which destabilizes the TS during the elimination. This destabilization is confirmed by the Bader charge analysis of the TS carbocationic alkyl moiety. We finally question the relevance of the Al site located on the (100) surface for catalyzing the dehydration of alcohols containing hindered atom.