Understanding Alkane Dehydrogenation through Alcohol Dehydration on γ-Al ₂ O ₃

Nonoxidative dehydrogenation of alkanes is an important chemical reaction, as it can be used for production of olefins, commonly used as building blocks for a range of plastics and chemicals. Several metal oxides, including γ-Al ₂ O ₃ , have exhibited dehydrogenation activity, showing promise as candidates for the upgrade of alkanes. In this work we use computational chemistry methods to investigate the mechanism of ethane, propane, n-butane, and i-butane dehydrogenation pathways on strong Lewis acid-base sites of γ-Al ₂ O ₃ . On the basis of our calculations, it was shown that a concerted dehydrogenation mechanism is energetically preferred with the formation of a carbenium-ion-like transition state. An alkane dehydrogenation model was developed on the basis of methodology previously applied in structure-activity relations (SAR) for alcohol dehydration on metal oxides. The carbenium ion stability (CIS), shown to be a descriptor in alcohol dehydration, was used as a quantitative descriptor in alkane dehydrogenation and was found to correlate with the calculated activation energy barriers for the hydrocarbons in question. Increased hydrocarbon substitution (branching) was found to decrease the calculated reaction barriers, on the basis of the CIS at the transition state. Importantly, SARs developed for alcohol dehydration on various metal oxides were found to accurately capture the catalytic activity trends in alkane dehydrogenation, accounting for catalyst acid-base surface properties and the CIS of intermediates at the transition state. These results highlight that identifying the appropriate physicochemical descriptors of catalysts can accurately describe a different set of reactions (alcohol dehydration vs alkane dehydrogenation) as long as these reactions progress with similar transition states. Such models can accelerate the discovery of highly active metal oxide catalysts for the production of olefins.