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

• DOI: 10.1016/j.jcat.2016.04.024
• Publication Date: 2016-06-07T19:01:43Z
• Authors: Junnan Shangguan; Mariefel V. Olarte; Ya-Huei (Cathy) Chin
• Keywords: Bond cleavage

Abstract Catalytic pathways for acetic acid (CH 3 COOH) 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. CH 3 COOH 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 2–17% 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 CH 3 COOH activation is the kinetically-relevant step, during which CH 3 C(O) OH bond cleaves on a metal site pair at Ru cluster surfaces nearly saturated with adsorbed hydroxyl (OH ∗ ) and acetate (CH 3 COO ∗ ) intermediates, forming an adsorbed acetyl (CH 3 CO ∗ ) and hydroxyl (OH ∗ ) species. Acetic acid turnover rates increase proportionally with both H 2 (10–60 bar) and CH 3 COOH concentrations at low CH 3 COOH concentrations ( 3 COOH concentration and the CH 3 COO ∗ coverages increase and the vacant Ru sites concomitantly decrease. Beyond the initial CH 3 C(O) OH bond activation, sequential H-insertions on the surface acetyl species (CH 3 CO ∗ ) lead to C 2 products and their derivative (ethanol, ethane, and ethyl acetate) while the competitive C C bond cleavage of CH 3 CO ∗ 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 δ+ (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 δ+ ) shuffling during H-insertion steps in the aqueous phase, unlike those in the vapor phase, during the hydrogenation of acetic acid on Ru clusters.