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

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 CO and CC 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 CC 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 (<0.83 M), but decrease from first to zero order as the CH 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 CC bond cleavage of CH 3 CO* causes the eventual methane formation. The instantaneous carbon selectivities towards 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 towards C 2 products decrease with increasing temperature, because of higher observed barriers for CC 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.

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces

“…Parallel studies have resulted in similar results for CO-H 2 reactions on smaller Ru particles (Ru/SiO 2 , 1.8 and 3.5 nm) and Co particles (Co/SiO 2 ) . Similar CO* coverage effects were also observed previously during CO* oxidation − and oxygenate decarbonylation ,, studies on metal surfaces at high adsorbate coverages, indicating the findings herein address a larger problem within catalysis.…”

“…This thermodynamic formalism is appropriate for descriptions of chemical reaction dynamics on densely covered surfaces using activities instead of concentration in the rate equation and describes the effects of CO pressure (and CO* coverage) on rates and infrared spectra; it is also consistent with the theoretical treatments reported here. This thermodynamic formalism accounts for co-adsorbate interactions in the context of nonideal thermodynamic treatments of reactivity over the entire range of surface coverages that prevail in the practice of CO hydrogenation. − ,,, Such descriptions also provide fundamental insights into the dynamics of the many other reactions that occur on nearly saturated surfaces, such as CO oxidation, − NO reduction, − C–C, or C–O hydrogenolysis, ,, and oxygen reduction at electrode surfaces. − …”

Hollow Structure and Electron Promotion Effect of Mesoporous Pd/CeO2 Catalyst for Enhanced Catalytic Hydrogenation

“…Noble metal nanoparticles (NPs) have been extensively applied in many fields of catalysis owing to their high catalytic activity, especially catalytic hydrogenation . It is well known that noble metal NPs with high surface energy tend to aggregate to form large nanoparticles during catalytic reaction, thereby leading to loss of catalytic activity.…”

Hydrotreating of Guaiacol and Acetic Acid Blends over Ni2P/ZSM-5 Catalysts: Elucidating Molecular Interactions during Bio-Oil Upgrading

“…On the other hand, ethane is produced from the complete hydrodeoxygenation of acetic acid, as it has been found previously using different catalysts. 42,43…”