(589d) 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  

[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.