Activation and transfer of reactive hydrogen intermediates are ubiquitous steps that occur in hydrogenation and hydrodeoxygenation reactions in petro-chemical processing, biomass conversion, and pharmaceutical syntheses. At the molecular scale level, hydrogen activation may involve the formation of hydride, hydrogen adatom, or proton intermediates, which exhibit distinct reactivities in the hydrogen transfer step. We describe and compare the hydrogen activation and transfer mechanism during the reduction of linear aldehydes and ketones (CnH2nO, n=3-6) on transition metal clusters in the vapor phase and condensed phase with polar protic solvents. The charges of the reactive hydrogen intermediates differ and as a result the reactions occur via mechanistically distinct routes. Vapor phase hydrogenation of aldehydes and ketones on transition metal cluster surfaces proceeds via the alkoxy pathway, during which a H* adatom insert initially onto the carbon in a quasi-equilibrated step followed by kinetically relevant 2nd H* adatom insertion step that completes the catalytic cycles. In the presence of polar protic solvent, H* adatoms become solvated as hydrodium ions, stabilized by the solvent molecules as Zundel or Eigen cationic aggregates. These large hydrodium ion clusters selectively attack the oxygen atom of the carbonyl functional groups, forming stable, charge intermediates, before the second hydrogen addition step onto the carbon atom in a kinetically relevant step. These distinct mechanisms lead to different kinetic requirements of these reactions, when occur in the vapor phase and in the condensed phase with polar protic solvent. The ability to alter the charge of the reactive hydrogen provides the opportunity to tune the selectivities of hydrogenation and hydrodeoxygenation reactions.

Identity of reactive hydrogen species and their transfer during the hydrogenation of aldehydes and ketones

255th National Meeting (2018)

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