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

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.

  *cathy.chin@utoronto.ca

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=C_nH_2n+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.

References  

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[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.