TY - JOUR
T1 - The Role of Protons and Hydrides in the Catalytic Hydrogenolysis of Guaiacol at the Ruthenium Nanoparticle-Water Interface
AU - Shangguan, Junnan
AU - Hensley, Alyssa J.R.
AU - Gradiski, Matthew V.
AU - Pfriem, Niklas
AU - McEwen, Jean Sabin
AU - Morris, Robert H.
AU - Chin, Ya Huei Cathy
N1 - Publisher Copyright:
Copyright © 2020 American Chemical Society.
PY - 2020/10/16
Y1 - 2020/10/16
N2 - The mechanistic roles of free hydronium ions, surface hydrides, and interfacial protons during guaiacol hydrodeoxygenation (HDO) on ruthenium nanoparticles have been established. As guaiacol adsorbs on Ru, it loses its strong aromaticity and undergoes a rapid H-shift from its hydroxyl to meta carbons (in relation to its hydroxyl group), causing adsorbed enol and keto surface isomers to exist in chemical equilibrium. HDO occurs via a hydridic H-adatom (H*) attack on the enol, followed by a kinetically relevant C-O bond rupture step, during which water shuttles the hydroxyl proton, enabling its intramolecular attack on the methoxy, evolving to a highly charged [Ru(s)-(C6H5O-)···(H+)···OCH3]† transition state. The competing hydrogenation (HYD) begins with a rapid H∗ attack on the keto form, before a second, kinetically relevant H∗ attack without proton involvement. Water, despite shifting the thermodynamics toward the more polar surface keto, promotes HDO to a much greater extent than HYD, because of its dual catalytic roles in reducing the activation free energies - (i) it mobilizes the hydroxyl proton of partially saturated guaiacol (Brønsted acid) and functions cooperatively with the Ru metal surface (base) in rupturing the C-O bond and stabilizing the resulting cationic carbon-ring fragment and (ii) water layers solvate the charged [Ru(s)-(C6H5O-)···(H+)···OCH3]† transition state. Free hydronium ions do catalyze a separate homogeneous enol-keto isomerization, but this reaction is kinetically unrelated to HDO catalysis. This mechanistic picture explains the strong effects of a polar protic solvent in hydrodeoxygenation, highlighting the requirements of surface hydrides and interfacial protons acting in tandem to complete a HDO turnover and the cooperative role of the protic solvent and the metal surface in breaking the aromaticity and preferentially stabilizing charged transition states.
AB - The mechanistic roles of free hydronium ions, surface hydrides, and interfacial protons during guaiacol hydrodeoxygenation (HDO) on ruthenium nanoparticles have been established. As guaiacol adsorbs on Ru, it loses its strong aromaticity and undergoes a rapid H-shift from its hydroxyl to meta carbons (in relation to its hydroxyl group), causing adsorbed enol and keto surface isomers to exist in chemical equilibrium. HDO occurs via a hydridic H-adatom (H*) attack on the enol, followed by a kinetically relevant C-O bond rupture step, during which water shuttles the hydroxyl proton, enabling its intramolecular attack on the methoxy, evolving to a highly charged [Ru(s)-(C6H5O-)···(H+)···OCH3]† transition state. The competing hydrogenation (HYD) begins with a rapid H∗ attack on the keto form, before a second, kinetically relevant H∗ attack without proton involvement. Water, despite shifting the thermodynamics toward the more polar surface keto, promotes HDO to a much greater extent than HYD, because of its dual catalytic roles in reducing the activation free energies - (i) it mobilizes the hydroxyl proton of partially saturated guaiacol (Brønsted acid) and functions cooperatively with the Ru metal surface (base) in rupturing the C-O bond and stabilizing the resulting cationic carbon-ring fragment and (ii) water layers solvate the charged [Ru(s)-(C6H5O-)···(H+)···OCH3]† transition state. Free hydronium ions do catalyze a separate homogeneous enol-keto isomerization, but this reaction is kinetically unrelated to HDO catalysis. This mechanistic picture explains the strong effects of a polar protic solvent in hydrodeoxygenation, highlighting the requirements of surface hydrides and interfacial protons acting in tandem to complete a HDO turnover and the cooperative role of the protic solvent and the metal surface in breaking the aromaticity and preferentially stabilizing charged transition states.
KW - Ru catalysts
KW - aqueous phase
KW - aryl ether bond
KW - hydrodeoxygenation
KW - hydrogenolysis
KW - intramolecular proton transfer
KW - solvent effect
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U2 - 10.1021/acscatal.0c01963
DO - 10.1021/acscatal.0c01963
M3 - Article
AN - SCOPUS:85095598845
SN - 2155-5435
VL - 10
SP - 12310
EP - 12332
JO - ACS Catalysis
JF - ACS Catalysis
IS - 20
ER -