(704d) On the Mechanisms of Ethane Dehydrogenation on Isolated Fe/SiO2

Sustainability imperatives have spurred the development of isolated metal catalysts for a variety of reactions of economic importance. Ethane dehydrogenation to ethylene is one such pivotal reaction that has garnered renewed interest owing to the shale gas revolution. Commercial chromium- and platinum-based catalysts are toxic and expensive, underscoring the need for employing earth-abundant metal catalysts. Transition metals grafted on amorphous SiO­₂ supports, such as Fe@am-SiO₂, emerge as promising candidates for their activity and selectivity for non-oxidative ethane dehydrogenation. Its active site and C-H activation mechanism is debated due to the structural and compositional heterogeneity of the amorphous support. Atomistic understanding of these materials is key to designing novel catalysts tailored for C-H activation chemistry.

Herein, we employed electronic structure calculations to elucidate the nature of the active site and identify dominant reaction pathways for ethane dehydrogenation on Fe@am-SiO₂. We considered Fe centers in oxidation states 2+ and 3+, and coordination environments consisting of silanol (SiOH) and silanolate (SiO^–) ligands. We report that high-spin (quintet) Fe d⁶ centers paired with basic silanolate ligands activate the C-H bond heterolytically and that the ensuing β-hydride elimination of the metal-alkyl intermediate by the metal requires spin-crossing into the triplet spin state. We further provide evidence that σ-metathesis, namely ligand exchange between ethane and the metal-hydride is not a viable catalytic pathway as it is energetically demanding. On Fe(+3), we propose a new redox mechanism, in which the Fe d⁵ active site is reduced to d⁶ by silanolate. Upon heterolytic C-H cleavage, Fe reoxidizes to d⁵ by donating an electron back to the silanolate which then sets the stage for the β-hydride elimination over the low-spin state (quartet) of Fe d⁵. We further show that the proposed redox mechanism is energetically competitive with the heterolytic C-H activation mechanism previously identified for other transition metals.