426212 Mechanistic Insights on C-C Bond Cleavage, C-O Bond Cleavage, and Hydrogen Insertion in Light Carboxylic Acids Catalyzed By Dispersed Ruthenium Clusters in Aqueous Medium

Wednesday, November 11, 2015: 4:15 PM
355E (Salt Palace Convention Center)
Junnan Shangguan, Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, ON, Canada and Ya-Huei (Cathy) Chin, Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, ON, Canada

Mechanistic Insights on C-C Bond Cleavage, C-O Bond Cleavage, and Hydrogen Insertion in Light Carboxylic Acids Catalyzed by Dispersed Ruthenium Clusters in Aqueous Medium

Junnan Shangguan and Ya-Huei (Cathy) Chin*

Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada.


Reactions of light carboxylic acids with hydrogen form alcohols as alkylating agents, which react with phenolic compounds (e.g., phenol, catechol) to produce substituted aromatics from bio-oil mixtures. Hydrogenation of carboxylic acids requires the initial activation of RC(O)-OH bond (R=CnH2n+1, n1) to form a surface acyl species (RC=O), followed by successive H addition onto its C=O bond, while leaving the carbon backbone intact. This step occurs in parallel with the undesired C-C bond cleavage that forms smaller alkanes (e.g., methane). The reaction network of carboxylic hydrogenation in aqueous phase has been proposed and examined on dispersed transition metal clusters [1-3], but rate dependencies, selectivity trends, site requirements, and the effects of reaction medium have not been rigorously established. Here, we interrogate the initial CH3C(O)-OH bond cleavage to surface acetyl (CH3CO*) with sequential C-H insertion, O-H insertion, and C-C dissociation of surface acetyl during the reactions of acetic acid, one of the simplest and most abundant light organic acids contained in bio-oil, with hydrogen on nanometer-sized ruthenium clusters. We propose a sequence of elementary steps that captures the catalytic sojourn of acetic acid on dispersed Ru clusters in aqueous phase based on kinetic and isotopic evidence and derive from which a kinetic model that captures the rate and selectivity dependence. We describe the catalytic requirements for the initial dissociation of carboxylic acid and proton transfer mechanism at the homogenous-heterogeneous interface in aqueous medium, unlike those found under ultra-high vacuum or in the gas phase. The additional H insertion routes assisted by protons led to the higher H-insertion rates and thus higher selectivities towards the desired alcohol products.

At mild temperature (413-543 K) and H2 pressure (10-60 bar), acetic acid hydrogenation produces ethanol, ethyl acetate, methane and ethane (carbon selectivities 20-90 %, 3-17 %, 4-67 %, and 2-11 %, respectively) from parallel and sequential surface reactions. The Ru cluster surfaces are covered predominantly with hydroxyl (OH*) and acetate (CH3COO*) species, derived from quasi-equilibrated H2O adsorption and dissociation steps (steps 4 and 5, Table 1) and acetic acid dissociation (steps 2 and 3, Table 1), respectively. Chemisorbed acetic acid (CH3COOH*) dissociates through an initial CH3C(O)-OH cleavage and results in an adsorbed acetyl (CH3CO*) and hydroxyl (OH*) species in a kinetically relevant step (step 6, Table 1). This step, the pseudo steady-state treatments of all surface intermediates, and the assumption of OH* and CH3COO* as the most abundant surface intermediates lead to the observed first-order dependence on H2 and CH3COOH at all H2 pressures (10-60 bar) and low CH3COOH concentration (0-0.88 M) for CH3COOH turnovers, but the reaction order on CH3COOH decreases to zero and then to negative values at high CH3COOH concentration (>0.88 M), as shown in Figure 1a. The fate of surface acetyl species (CH3CO*), which either undergo sequential H-insertion, leading to the formation of C2 compounds (ethanol, ethane and ethyl acetate) or C-C cleavage to C1 compound (methane), determines the overall carbon selectivities. The selectivity for the formation of carbon products with two carbon atoms (ethanol, ethyl acetate, ethane) over one carbon atom (methane) is defined as; it increases with increasing H2 pressure as well as CH3COOH concentration. This trend indicates the involvement of two distinct H-insertion steps in the kinetically relevant steps that lead to the formation of C2 compounds: (i) surface adatom (H*) derived from H2 chemisorption (step 1, Table 1), which undergoes C-H insertion onto CH3CO* (step 7, Table 1), and (ii) unbounded and partially charged H atom from CH3COOH* participates in an electrophilic addition onto the oxygen of CH3CO* (step 8, Table 1) through an O-H bond formation. The C-H insertion and O-H formation of CH3CO* are different reaction paths that lead the selectivity parameterto increase with both the H2 pressure and CH3COOH concentration.

The effects of temperature on rates and selectivity are shown in an Arrhenius form in Figure 1b. The apparent barrier for acetic acid activation is 45 kJ/mol; this barrier reflects the barrier for the kinetically relevant step (step 6, Table 1) and the heats of adsorption of H*, OH*, CH3COOH* together with the heat of surface reactions for a set of quasi-equilibrated steps (step 1-5, Table 1). Rate ratios for the formation of C2 over C1 products () decrease with increasing temperature (Figure 1b), because the undesired C-C bond cleavage depends much more sensitively on temperature than the H-insertion step.

In summary, catalytic hydrogenation of acetic acid on dispersed Ru clusters in the aqueous phase forms ethanol via an initial CH3C(O)-OH cleavage of acetic acid followed by two distinct H addition routes. These routes are less sensitive to temperature than the competitive C-C bond cleavage route that evolves methane.


[1] H. Olcay, L. Xu, Y. Xu, G.W. Huber, Aqueous-Phase Hydrogenation of Acetic Acid over Transition Metal Catalysts, ChemCatChem, 2 (2010) 1420-1424.

[2] L. Chen, Y. Li, X. Zhang, Q. Zhang, T. Wang, L. Ma, Mechanistic insights into the effects of support on the reactionpathway for aqueous-phase hydrogenation of carboxylic acidover the supported Ru catalysts, Applied Catalysis A: General, 478 (2014) 117-128.

[3] J. Lee, Y.T. Kim, G.W. Huber, Aqueous-phase hydrogenation and hydrodeoxygenation of biomass-derived oxygenates with bimetallic catalysts, Green Chemistry, 16 (2014) 708-718.


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