title: Kinetic and Mechanistic Assessment of Alkanol/Alkanal Decarbonylation and Deoxygenation Pathways on Metal Catalysts.
abstract: This study combines theory and experiment to determine the kinetically relevant steps and site requirements for deoxygenation of alkanols and alkanals. These reactants deoxygenate predominantly via decarbonylation (C-C cleavage) instead of C-O hydrogenolysis on Ir, Pt, and Ru, leading to strong inhibition effects by chemisorbed CO (CO*). C-C cleavage occurs via unsaturated species formed in sequential quasi-equilibrated dehydrogenation steps, which replace C-H with C-metal bonds, resulting in strong inhibition by H2, also observed in alkane hydrogenolysis. C-C cleavage occurs in oxygenates only at locations vicinal to the C═O group in RCCO* intermediates, because such adjacency weakens C-C bonds, which also leads to much lower activation enthalpies for oxygenates than hydrocarbons. C-O hydrogenolysis rates are independent of H2 pressure and limited by H*-assisted C-O cleavage in RCHOH* intermediates on surfaces with significant coverages of CO* formed in decarbonylation events. The ratio of C-O hydrogenolysis to decarbonylation rates increased almost 100-fold as the Ir cluster size increased from 0.7 to 7 nm; these trends reflect C-O hydrogenolysis reactions favored on terrace sites, while C-C hydrogenolysis prefers sites with lower coordination, because of the relative size of their transition states and the crowded nature of CO*-covered surfaces. C-O hydrogenolysis becomes the preferred deoxygenation route on Cu-based catalysts, thus avoiding CO inhibition effects. The relative rates of C-O and C-C cleavage on these metals depend on their relative ability to bind C atoms, because C-C cleavage transitions states require an additional M-C attachment.
authors.affiliation: Department of Chemical and Biomolecular Engineering and ‡Division of Chemical Sciences, E. O. Lawrence Berkeley National Laboratory, University of California at Berkeley , Berkeley, California 94720, United States.
authors.forename: Elif I
authors.initials: EI
authors.lastname: Gürbüz
authors.affiliation: Department of Chemical and Biomolecular Engineering and ‡Division of Chemical Sciences, E. O. Lawrence Berkeley National Laboratory, University of California at Berkeley , Berkeley, California 94720, United States.
authors.forename: David D
authors.initials: DD
authors.lastname: Hibbitts
authors.affiliation: Department of Chemical and Biomolecular Engineering and ‡Division of Chemical Sciences, E. O. Lawrence Berkeley National Laboratory, University of California at Berkeley , Berkeley, California 94720, United States.
authors.forename: Enrique
authors.initials: E
authors.lastname: Iglesia