Structure-Activity Relationships in Alkane Dehydrogenation on γ-Al₂O₃: Site-Dependent Reactions

A promising route to produce olefins, the building blocks for plastics and chemicals, is the nonoxidative dehydrogenation of alkanes on metal oxides, taking advantage of the Lewis acid-base surface functionalities of the oxides. However, how alkane dehydrogenation activity depends on the strength of surface acid-base site pairs is still elusive. In this work, we provide fundamental insights into the reaction mechanisms of propane dehydrogenation on different facets of γ-Al₂O₃ and develop structure-activity relationships, using density functional theory calculations and first-principles molecular dynamics simulations. We identified the binding energy of dissociated H₂ as an activity descriptor for alkane dehydrogenation. Interestingly, a volcano relationship between catalytic activity and dissociative H₂ binding energy was discovered for propane dehydrogenation, unraveling a site-dependent catalytic behavior on γ-Al₂O₃, with a concerted surface mechanism being energetically preferred to a sequential one on the most active sites. We demonstrated that although surface hydration, in general, blocks strong Lewis acid-base pairs on the (110) γ-Al₂O₃ surface, the presence of hydroxyl groups (on neighboring to strong Lewis sites) can enhance the propane dehydrogenation activity of a "defect site pair" (Al^III-O^III) of the metastable surface. Moreover, we performed ab initio metadynamics simulations of the most active site on γ-Al₂O₃ to examine the hydrogen formation and surface dynamics under dehydrogenation reaction conditions. Metadynamics simulations demonstrated that the poisoning of active sites by hydrogen adsorption is unlikely under experimental conditions. The developed relationships can be utilized to screen metal oxide surfaces and accelerate the discovery of active catalysts for alkane conversion to olefins.