Mechanistic insights on C-O and C-C bond activation and hydrogen insertion during acetic acid hydrogenation catalyzed by ruthenium clusters in aqueous medium

Catalytic pathways for acetic acid (CH3COOH) and hydrogen (H-2) reactions on dispersed Ru clusters in the aqueous medium and the associated kinetic requirements for C-O and C-C bond cleavages and hydrogen insertions are established from rate and isotopic assessments. CH3COOH reacts with H-2 in steps that either retain its carbon backbone and lead to ethanol, ethyl acetate, and ethane (47-95%, 1-23%, and 21-7% carbon selectivities, respectively) or break its C-C bond and form methane (1-43% carbon selectivities) at moderate temperatures (413-523 K) and H-2 pressures (10-60 bar, 298 K). Initial CH3COOH activation is the kinetically-relevant step, during which CH3C(O)-OH bond cleaves on a metal site pair at Ru cluster surfaces nearly saturated with adsorbed hydroxyl (OH*) and acetate (CH3COO*) intermediates, forming an adsorbed acetyl (CH3CO*) and hydroxyl (OH*) species. Acetic acid turnover rates increase proportionally with both H-2 (10-60 bar) and CH3COOH concentrations at low CH3COOH concentrations (<0.83 M), but decrease from first to zero order as the CH3COOH concentration and the CH3COO* coverages increase and the vacant Ru sites concomitantly decrease. Beyond the initial CH3C(O)-OH bond activation, sequential H-insertions on the surface acetyl species (CH3CO*) lead to C-2 products and their derivative (ethanol, ethane, and ethyl acetate) while the competitive C-C bond cleavage of CH3CO* causes the eventual methane formation. The instantaneous carbon selectivities toward C-2 species (ethanol, ethane, and ethyl acetate) increase linearly with the concentration of proton-type H delta+ (derived from carboxylic acid dissociation) and chemisorbed H*. The selectivities toward C-2 products decrease with increasing temperature, because of higher observed barriers for C-C bond cleavage than H-insertion. This study offers an interpretation of mechanism and energetics and provides kinetic evidence of carboxylic acid assisted proton-type hydrogen (H delta+) shuffling during H-insertion steps in the aqueous phase, unlike those in the vapor phase, during the hydrogenation of acetic acid on Ru clusters. (C) 2016 Elsevier Inc. All rights reserved.