Solvation Effects in the Hydrodeoxygenation of Propanoic Acid over

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Solvation Effects in the Hydrodeoxygenation of Propanoic Acid over a Model Pd(211) Catalyst Sina Behtash, Jianmin Lu, Osman Mamun, Christopher T Williams, John R. Monnier, and Andreas Heyden J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10419 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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The Journal of Physical Chemistry

Solvation Effects in the Hydrodeoxygenation of Propanoic Acid over a Model Pd(211) Catalyst

Sina Behtash, Jianmin Lu, Osman Mamun, Christopher T. Williams, John R. Monnier, and Andreas Heyden*

Department of Chemical Engineering, University of South Carolina, 301 S. Main St., Columbia, South Carolina 29208, USA

______________________________________ *Corresponding author: email: [email protected]

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Abstract The effects of liquid water and 1,4-dioxane on the hydrodeoxygenation of propionic acid over Pd(211) model surfaces have been studied from first principles. A microkinetic model with parameters obtained from density functional theory and implicit solvation models was developed to study the effects of these solvents on the reaction mechanism and kinetic parameters. In the presence of water, dehydrogenated derivatives of propionic acid and propionate are stabilized and a new decarboxylation mechanism involving CH3CCOOH surface species is facilitated, leading to a higher decarboxylation rate. However, stronger adsorption of CO in the presence of liquid water resulted in fewer free sites and an overall lower turnover frequency. In contrast, in the presence of 1,4-dioxane, the most dominant decarboxylation pathway does not involve a dehydrogenated propionate species, but propionate goes through decarboxylation to form CO2 and C2 fragments very similar to the mechanism in the gas phase. Again, in the presence of 1,4dioxane, CO adsorbs more strongly and fewer free sites are available for catalysis, leading to a slightly smaller turnover frequency.

In all reaction environments, we observed that the

decarbonylation mechanism is slightly preferred over the decarboxylation mechanism and that C-OH bond cleavage is the most rate-controlling step followed by α-carbon dehydrogenation steps and (in liquid water) decarboxylation of dehydrogenated derivatives. Comparing solvent effects over Pd(211) with those over Pd(111), we observe that the free site coverage is reduced in the presence of solvents on all Pd surfaces which reduces the activity of Pd(211). In addition, elementary steps that involve a carboxyl/carboxylate group changing its orientation from the surface to the liquid phase, such as the dehydrogenation of propionate, are significantly facilitated such that liquid water increased the activity of Pd(111). Keywords: Solvent effects; organic acid; propionic acid; palladium; Pd(211), stepped surfaces; density functional theory; COSMO; decarbonylation; decarboxylation; microkinetic modeling 2 ACS Paragon Plus Environment

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1. Introduction Catalytic conversion of lipids to second generation biofuels is one potential route for utilization of biomass. Lipid feedstocks contain oxygenates which are often not desired for the current energy infrastructure due to corrosion issues and a lower energy density than conventional hydrocarbon fuels.1-5 However, lipids can be converted to biofuels with very similar properties to conventional fuels (green fuels) through a hydrodeoxygenation (HDO) process that eliminates all oxygen atoms from the feedstock molecules at moderate reaction conditions while minimizing bi-products and waste.6 Conversion of lipids has been investigated using conventional hydrodesulphurization catalysts such as sulfided NiMo/Al2O3 and CoMo/Al2O3.6-8 However, considering the low level of sulfur in biomass and the higher activity of oxygenated feeds versus sulfide feeds, conventional hydrotreating catalysts display a short catalyst lifetime. Also, difficulties in the separation of carbon oxides from the recycle gas have been reported.7-8 To rationally design a new catalyst for the HDO of lipids, it is desirable to first understand the reaction mechanism and kinetics of the HDO of organic acids, esters and ultimately triglycerides which are the main components of lipid feedstocks. We have previously investigated the kinetics and reaction mechanism of propionic acid9-10 and methyl propionate,11 our model acid and ester molecules, under gas-phase conditions; however, as industrial hydrotreatment processes often occur in a complex liquid environment, our understanding of the mechanism cannot be completed without studying solvent effects. In the experimental studies12-18 of the hydrodeoxygenation of organic acids and esters, various solvents such as dodecane, mesitylene, and water have been used as the reaction medium. These studies imply that that the effects of solvents on the HDO of acids and esters can

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be significant. For example, in a study12 on the deoxygenation of ethyl stearate over Pd/C catalysts, it was reported that the formation of n-heptadecane is favored in the presence of dodecane, while in mesitylene stearic acid was identified as the main product. In another study18 on the catalytic deoxygenation of oleic acid (C18) over Pd/C it was reported that the presence of water can change the selectivity towards C17 hydrocarbons by up to 20%. While the reported changes in the selectivity of the products are important, more work remains to be done to explain and characterize the solvent effects on the kinetics and reaction mechanism quantitatively. To investigate the effects of solvents on the HDO of organic acids, we have previously performed a DFT study on the HDO of propionic acid over a Pd(111) model surface. We studied the effects of polar solvents such as water as well as non-polar solvents such as n-octane on the elementary reactions involved in the HDO of propionic acid, and developed a microkinetic model to quantify the effects of solvent molecules on the reaction rates and other kinetic parameters. We found that while non-polar solvents such as octane do not change the kinetics of the HDO of propionic acid, polar solvents such as water can alter the reaction mechanism and kinetics to some degree. In the presence of water, the overall activity was enhanced, and the turnover frequency of the decarboxylation (DCX) pathway, which is not the dominant pathway under gas-phase condition, was significantly increased in the presence of water, and became essentially competitive to the dominant decarbonylation (DCN) mechanism.19-20 The effects of solvents on the importance of the DCX and DCN mechanism can be critical in understanding and explaining the discrepancies in the literature, as there is not a consensus on the dominant mechanism of the HDO of organic acids and esters. Specifically, the DCX was reported 21-22 to be the dominant mechanism for the HDO of stearic, lauric, and capric


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acid while in other studies of the HDO of caprylic acid,23 the DCN was identified to be the dominant HDO mechanism. Next, we identified as another determining factor on the importance of the DCX and DCN mechanism the structure of the Pd surface. In our previous study24 on the HDO of propionic acid over a Pd(211) stepped surface, we found that decarboxylation reactions are competitive to decarbonylation reactions while on a flat terrace surface model the DCX mechanism was two orders of magnitude slower than the DCN mechanism. In this study, we investigated the combined effects of surface structure and solvents on the HDO of propionic acid and the importance of the DCX and DCN mechanisms under various reaction conditions. We present a thorough DFT study of solvent effects on the free energies of surface intermediates and transition states of all elementary steps involved in the HDO of propionic acid over a Pd(211) model surface. We calculated the effects of solvents on the free energies of reaction and activation of various elementary steps, and finally, developed a microkinetic reactor model under various reaction conditions that permits quantifying the solvent effects on the turnover frequencies (TOFs), abundant surface intermediates, reaction orders, and apparent activation barriers. Specifically, we focus on liquid water and liquid 1,4-dioxane as solvents which are typical protic and aprotic polar solvents that somewhat mimic a complex polar reaction environment.

2. Methods 2.1 Solvation Model Solvents can modify the strength of adsorption of oxygenated hydrocarbon species on a metal surface due to direct adsorbate-solvent interactions and indirect solvent-metal interactions



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that change the electronic structure of the metal and thus modify the metal-adsorbate interaction. In this study, the approximate effect of solvents is investigated with the help of the iSMS method.25 iSMS is a new approach for modeling reactions at metal-liquid interfaces with implicit solvation models. More information about iSMS and a validation of this method has recently been published by some of us.25 Basically, the free energy of an adsorbed intermediate on a liquid

periodic metal slab at the solid-liquid interface, G surface + int ermediate , is defined using a subtraction scheme: liquid vacuum liquid vacuum G surface + int ermediate = G surface + int ermediate + ( G cluster + int ermediate − E cluster + int ermediate ) (1) vacuum where, G surface +int ermediate , is the free energy in the absence of a solvent (plane-wave DFT energy

of the periodic slab model including vibrational contributions to the free energy), liquid G cluster + int ermediate is the free energy of a metal cluster in the liquid (without explicitly considering

vibrational contributions) constructed by removing selected metal atoms from the periodic-slab vacuum model and removing the periodic boundary conditions, and E cluster + int ermediate is the DFT energy

of the same cluster in the absence of the solvent. Combinations of COSMO and COSMO-RS26-27 liquid implicit solvation models have been used to calculate G cluster + int ermediate . COSMO-RS calculations

have been performed using COSMOtherm.28 Thermodynamic properties of the solvents are obtained from the COSMOtherm database, based on results of quantum chemical COSMO calculations at the BP-TZVP level of theory. For all other structures, COSMO-RS input files have been generated from COSMO calculations at the same level of theory. We note that as a first approximation we did not include the solvent degrees of freedom in the reaction coordinate since this would require a micro-solvation approach which is currently still impractical for a large network of elementary steps. 6 ACS Paragon Plus Environment

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2.2 DFT calculations Cluster model DFT calculations at BP86 level of theory29-30 (as required for COSMO-RS calculations) were carried out using TURBOMOLE 6.0.31-33 The Pd(211) cluster model surface has been modeled by a two layered cluster containing 48 Pd atoms shown in Figure S1. These structures were constructed by removal of the periodic boundaries from the periodic slabs that were obtained from our previous plane-wave (VASP)34-35 calculations.10 All adsorbates were represented by all-electron TZVP36-38 basis sets while for Pd we used a relativistic small core potential (ECP) together with a basis set of same quality as the adsorbates for the valence electrons. The Coulomb potential was approximated with the RI-J approximation with auxiliary basis sets.39-41 Single point energy calculations were performed with a self-consistent field energy convergence criterion of 1.0 × 10-6 Ha.

Finally, for each cluster model, energy

calculations on various spin surfaces were performed to identify the lowest energy spin state for further solvation effect calculation. For cluster models in the liquid phase, COSMO calculations were performed on the same spin surface as for the vacuum cluster calculations. The dielectric constant was set to infinity to provide the input for the COSMO-RS calculations. As a first approximation we neglected structural changes of the adsorbates due to solvation effects. For cavity construction, default radii-based cavities were used listed in the supporting information.

2.3 Microkinetic Modeling For surface reactions, the forward rate constant (kfor) of each reaction is calculated as ∆G ‡

k for

k T − = B e k BT h

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where kB is the Boltzmann constant, T denotes the reaction temperature, h is the Planck constant, and ∆G‡ represents the free energy of activation at a specific temperature and reaction environment.

In other words, in the presence of solvents, the free energy of activation

(∆G‡solvent) and free energy of reaction (∆Grxn-solvent) were calculated as, ‡ ‡ ∆GSolvent = ∆GGas + GTS (solv) − GIS (solv) ,

(3)

∆Grxn-solvent = ∆GGas + GFS (solv) − GIS (solv)

(4)

and

where, GIS(solv), GFS(solv), and GTS(solv) are the solvation energies of the initial, final, and transition states, respectively, that were obtained from the difference in energy of the COSMO‡ RS and gas-phase cluster calculations, and ∆GGas and ∆GGas are the free energy of activation and

reaction under gas phase conditions, respectively. The reverse rate constant (krev) is calculated from microscopic reversibility and the thermodynamic equilibrium constant K is given as

k rev =

k for K

(5)

For an adsorption reaction, A(g)+*→A*, the rate of adsorption is given by collision theory with a sticking probability of 1 independent of solvent.

kfor =

N0

1 2πmAk BT

(6)

where N0 is the number of sites per area (1.478×1019 m-2) and mA denotes the molecular weight of A. The desorption rate constant is again given by the equilibrium constant, i.e., equation 5. In the presence of a solvent, the free energy of adsorption for A(g)+*→A* is calculated as,

∆Gads−solvent = ∆Gads− gas + GA* (solv) − GPd (solv) 8 ACS Paragon Plus Environment

(7)

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where ∆Gads− gas is the free energy of adsorption under gas phase conditions and G A* (solv) and

GPd (solv) are as before the solvation free energies of the adsorbed molecule A and the Pd surface immersed in the solvent, respectively. With the forward and reverse rate constants defined, rates of the elementary reactions can be expressed by mean-field rate laws. Considering that some of the adsorbed intermediates occupy multiple active sites, the rate expressions and steady state molecular balance equations are highly nonlinear. To solve the set of steady state differential reactor equations and to obtain the surface coverages of the intermediates, we used the BzzMath library42 developed by BuzziFerraris. No assumptions were made regarding rate-controlling steps. Finally, we note that 1,4dioxane has not been considered explicitly as an adsorbed species in the microkinetic models due to its weak interaction with the Pd surface at 473 K, leading to a low surface coverage that can be neglected in the site balance.

3. Result and Discussion Figure 1 illustrates a schematic of the investigated elementary reactions and intermediates involved in the HDO of propionic acid over Pd(211). Solvents can stabilize or destabilize these intermediates, and consequently, alter the adsorption energies and elementary activation and reaction energies of various steps.

3.1 Solvent Effects on Reaction Intermediate Adsorption Strength In order to quantify the changes in the adsorption strength of various intermediates in the absence and presence of solvents, we considered adsorption processes as, +∗ ↔ ∗ 9 ACS Paragon Plus Environment

(8)


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+∗ ↔ ∗

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(9)

and calculated the effect of a solvent on the free energy of adsorption of intermediates, ∆ , = , − , = [ ∗ − ∗ ] − [ ∗ − ∗ ]

(10)

where, , and , are the free energy of adsorption of a gas molecule of intermediate A in the presence and absence of solvent. ∗ and ∗ are the free energy of adsorbed A in the presence and absence of solvent, and similarly ∗ and ∗ are the free energy of the active sites of the catalyst in the presence and absence of solvent molecules. We note that this procedure permits us to compute changes in adsorption strength relative to a free surface site even for intermediates that only exist on a metal surface, i.e., that are unstable in the gas or solvent phase, which again facilitates understanding the specific effects of solvents on the free energy of any state in the reaction network. Table 1 lists the changes in the adsorption strength of various intermediates involved in the HDO of propionic acid over Pd(211) in the presence of liquid water and 1,4-dioxane. In addition, Table S1 in the supporting information lists comparable changes in adsorption strength due to various solvents on Pd(111) sites from our previous paper.20 For the convenience of comparison of the solvent effects on the adsorption strength of the intermediates, we classified the intermediates in the Figure 1 into five different classes of structurally similar intermediates.

Class I: Propionic acid and its dehydrogenated derivatives

Intermediates such as CH3CHCOOH and CH2CHCOOH are the products of propionic acid dehydrogenation steps which we previously19-20 found to be rate-controlling and influential in the overall kinetics. Solvent-induced changes in the adsorption strength of these dehydrogenated species can result in changes in the activation and reaction energies of various


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dehydrogenation steps, and consequently these species can significantly affect the overall activity.

Liquid water stabilizes adsorbed propionic acid by 0.03 eV on Pd(211) sites.

Dehydrogenated propionic acid species are even more stabilized by liquid water. For example, CH3CHCOOH adsorbs stronger by 0.11 eV, CH2CHCOOH is stabilized by 0.08 eV, and CH3CCOOH adsorbs stronger by 0.12 eV. Overall, these species are less affected by water on Pd(211) sites than on Pd(111) sites. Also, in the presence of liquid water the products of dehydrogenation steps of propionic acid are stabilized more than the reactant (propionic acid) which suggests that dehydrogenation steps are more favored in water. In the presence of 1,4-dioxane, the adsorption strength of propionic acid is increased by 0.07 eV which is larger than in the presence of water. However, the dehydrogenated species are affected less in 1,4-dioxane.

For example, CH3CHCOOH is stabilized by 0.08 eV,

CH2CHCOOH is stabilized by 0.03 eV and CH3CCOOH adsorbs stronger by 0.10 eV. Considering that propionic acid itself was stabilized by 0.07 eV in the presence of 1,4-dioxane, only the dehydrogenation to CH3CCOOH is facilitated by the solvent.

Class II: Propanoyl (CH3CH2CO) and its dehydrogenated derivatives

Propanoyl and its dehydrogenated derivatives are the products of hydroxyl removal from propionic acid and its dehydrogenated derivatives which we previously19-20 found to be also ratecontrolling steps in the HDO of propionic acid. Overall, we find that the adsorption strength of propanoyl and its derivatives is hardly affected by the presence of solvents on Pd sites.

Class III: Propionate (CH3CH2COO) and its dehydrogenated derivatives



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Propionate and its dehydrogenated derivatives are the products of O-H bond dissociation in propionic acid and its dehydrogenated derivatives. These intermediates eventually go through C-C bond cleavages to form CO2 and C2 fragments, and consequently, are essential in the DCX mechanism. CH3CH2COO is not affected significantly by the presence of liquid water on Pd sites since it adsorbs through the carboxylate group that is pointing away from the solvent. However, further dehydrogenated products such as CH3CHCOO and CH3CCOO are stabilized significantly on Pd(211) by 0.16 and 0.26 eV, respectively. This stabilization is similar, though slightly stronger than the stabilization on Pd(111) sites and can be explained by the carboxylate group now pointing in the direction of the solvent. In the presence of 1,4-dioxane, the trends are again similar to the ones in the class I products. Propionate is relative to liquid water more stabilized in 1,4-dioxane by 0.05 eV, while CH3CHCOO and CH3CCOO are significantly stabilized in 1,4-dioxane but less than in water by 0.09 eV and 0.14 eV, respectively. Overall in both solvents, dehydrogenation of propionate is significantly facilitated on Pd sites.

Class IV: Ethane and its dehydrogenated derivatives

C2 hydrocarbon fragments are the products of C-C bond cleavages in propanoyl and propionate. Generally, these intermediates are not affected significantly by the presence of liquid 1,4-dioxane. Only CH2C is somewhat stabilized on Pd(211) by 0.05 eV in 1,4-dioxane and 0.08 eV in liquid water (hardly any stabilization was found on Pd(111)). In contrast, CH3C which also interacts strongly with the surface is not affected by the solvents which can be understood by difference in adsorption configurations of these intermediates; CH3C adsorbs on a hollow site while CH2C is adsorbed on a bridge site. Otherwise, the weaker adsorbed CH3CH3,


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CH3CH2, and CH2CH2 species are destabilized in liquid water on Pd(211) by 0.07, 0.04, and 0.03 eV, respectively, probably due to a hydrophobic effect.

Class V: Small species such as OH, CO, H2O, CO2, and H

In the presence of liquid water, COOH, OH, and H2O are significantly stabilized on Pd(211) by 0.12, 0.10, and 0.12 eV, respectively. This stabilization is similar to Pd(111) sites, although OH is significantly stronger stabilized on Pd(211). In the presence of 1,4-dioxane, these species are also stabilized, however, less strongly by 0.09, 0.05, and 0.08 eV, respectively. CO2 which was hardly effected by water on Pd(111), is significantly stabilized by both water and 1,4-dioxane by 0.07 eV. The stronger stabilization of CO2 on Pd(211) can be rationalized by the stronger adsorption on the unsaturated Pd(211) sites. Next, of the two high surface coverage species, hydrogen and CO, the adsorption strength of H is hardly affected, while CO is stabilized by 0.08 eV in water and 0.07 eV in 1,4-dioxane. This observation is similar to our previous observation on Pd(111) sites, which suggested that CO poisoning could be more prevalent in the presence of solvents.

[Figure 1 Here]

[Table 1 Here] 3.2 Solvent Effects on Reaction Rate Parameters Effects of solvents on the free energies of activation and reaction of all elementary steps involved in the HDO of propionic acid over Pd(211) are illustrated in Table 2 for a reaction temperature of 473 K. Table S2 lists corresponding free energies for Pd(111) from our previous



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study.19-20

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For ease of comparison, water solvent effects on free energies of reaction and

activation barriers are presented for Pd(111) in brackets [ ] next to the corresponding values of the Pd(211) surface model. We previously24 identified reaction 2 (CH3CH2COOH* + 2*⇌ CH3CHCOOH** + H*), reaction 6 (CH3CHCOOH** + 2*⇌ CH2CHCOOH*** + H*), reaction 11 (CH2CHCOOH*** + *⇌ CH2CHCO*** + OH*), and reaction 29 (CH3CH2COO** ⇌ CH3CH2* + CO2*) to be the rate-controlling steps on Pd(211) in the vapor phase. In the presence of water, the α-carbon dehydrogenation reaction 2 (CH3CH2COOH* + 2*⇌ CH3CHCOOH** + H*) becomes more exergonic by 0.10 eV [as shown in Table S2, reaction 2 is similarly more exergonic by 0.09 eV in the presence of water on the Pd(111) surface model]; also, its activation barrier decreases by 0.06 eV [decrease by 0.08 on Pd(111)]. The β-carbon dehydrogenation reaction 6 (CH3CHCOOH** + 2*⇌ CH2CHCOOH*** + H*) is not affected significantly since both the reactant and product are stabilized similarly [reaction 6 becomes less exothermic by 0.04 eV in the presence of water on Pd(111)]. (CH2CHCOOH*** + *⇌

Next, the C-OH bond cleavage reaction 11

CH2CHCO*** + OH*) becomes more exergonic by 0.04 eV;

however, the activation barrier increases by 0.02 eV [on Pd(111) the activation barrier is reduced by 0.06 eV while there is no change in reaction free energy]. Finally, the free energy of reaction 29 (CH3CH2COO** ⇌ CH3CH2* + CO2*) decreases significantly by 0.05 eV [in contrast, on Pd(111) the reaction becomes more endergonic by 0.04 eV in liquid water; this difference can be understood by the stronger adsorption of CO2 on Pd(211) in the presence of solvents]. Also, its activation barrier is slightly decreased by 0.01 eV [0.06 eV]. Overall, the rate-controlling steps in the presence of water are slightly facilitated.


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In the presence of 1,4-dioxane, the dehydrogenation of the α-carbon of propionic acid (reaction 2) is slightly inhibited and the free energy of reaction of this step is increased by 0.03 eV. Further dehydrogenation of CH3CHCOOH to CH2CHCOOH (reaction 6) has also become less exergonic by 0.05 eV. Also, the activation barrier is increased by 0.02 eV. However, reaction 11 (CH2CHCOOH*** + *⇌ CH2CHCO*** + OH*) is facilitated and has become less endergonic by 0.04 eV (the activation barrier is also lowered by 0.02 eV). Finally, reaction 29 (CH3CH2COO** ⇌

CH3CH2* + CO2*) is not significantly affected by 1,4-dioxane.

Considering that liquid water and 1,4-dioxane accelerate some elementary reactions but slow down others, we developed mean-field microkinetic models for the HDO of propionic acid over our Pd(211) model surface in the presence of various solvents.

[Table 2 Here]

3.3 Microkinetic Modeling We choose a reaction temperature for our microkinetic models of 473 K. Also, the partial pressures of propionic acid and H2 were set to 0.01 and 0.2 bar which agrees well with the reaction conditions of our experimental collaborators43 and our previous vapor phase study.24 The partial pressures of H2O, CO2, and CO were set to 0.001 bar which corresponds to approximately 10% conversion.

We note that our results and conclusions are relatively

insensitive to the reaction conditions. Also, since our model does not contain a water-gas shift model, the product partial pressures had to be fixed in the calculation of turnover frequencies. A method similar to Grabow et al.44 was used for determining coverage dependent adsorption energies of CO and H. More details about the lateral interactions used in the microkinetic model



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for this study can be found in our previous paper.24 All DFT-derived rate constants for various elementary reactions considered in our microkinetic model of the HDO of propionic acid over Pd(211) are listed in Table 3. In the following, we briefly review our previous results in the absence of solvents.24 The overall turnover frequency (TOF) was calculated to be 6.20×10-8 s-1, which is relatively low, however, low TOFs are unfortunately common for computational studies on model metal surfaces due to the difficulty in describing lateral interactions correctly in mean-field models and due to the general difficulty in identifying the most relevant active site structure responsible for the experimentally observed catalysis.45-46 The sum of the TOFs of the DCN pathways is 5.35×10-8 s-1 which suggests that the DCN mechanism is the dominant mechanism and slightly more favored than DCX pathways with a TOF of 8.47×10-9 s-1.

In the dominant DCN

mechanism, propionic acid goes through dehydrogenation of both α- and β-carbon prior to removal of the hydroxyl and carbonyl groups: CH3CH2COOHCH3CHCOOHCH2CHCOOH CH2CHCOCH2CH+COCH3CH3+ CO. In contrast, in the dominant DCX mechanism, propionic acid does not go through dehydrogenation steps but follows a sequence of CH3CH2COOHCH3CH2COO+H CH3CH2+CO2 CH3CH3+CO2 steps. Finally, the most abundant surface intermediates were identified to be CO, H, and free sites with surfaces coverages of 52.1%, 42.9% and 5.0%, respectively. Next, we will discuss the effects of solvents on the TOFs and kinetic parameters, and for the convenience of direct comparison with gas-phase values, we report the correspondent gasphase numbers in parenthesis [ ] next to the liquid water and 1,4-dioxane values.

[Table 3 (Rate Constants) here]


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Liquid Water Effects:

Water itself is one of the products of our reaction network as well as a relevant reaction environment. In the presence of liquid water, calculations were carried out in the same reaction environment, i.e., at 473 K and chemical potentials corresponding to the same gas phase partial pressures of the gas-phase study described above (except for water). For the water chemical potential we assumed gas-liquid equilibrium and computed the corresponding water partial pressure as, L xwater f water = ywaterPtotal = Pwater

(11)

L where xwater is the mole fraction of water in the solvent mixture, f water is the fugacity of pure

water at 473 K, ywater is the mole fraction of water in the vapor, Ptot is the total pressure of the system, and Pwater denotes the partial pressure of water. Assuming the solution is dilute, the mole fraction of water in solution ( xwater ) is 1. Next, the fugacity of pure water at 473 K can be L obtained from a steam table47 and is f water =14.17 bar, i.e., under these conditions the partial

pressure of water is calculated to be 14.17 bar and this value was used in all microkinetic simulations for the HDO of propionic acid in liquid water. The overall turnover frequency was calculated to be 4.63×10-8 s-1 which is slightly smaller than the gas-phase TOF [6.20×10-8 s-1]. In the most dominant pathway, propionic acid follows dehydrogenation steps of the α- and β-carbon to form CH2CHCOOH. CH2CHCOOH loses the hydroxyl group to form CH2CHCO.

Next,

The hydroxyl group gets

hydrogenated to form water and the CH2CHCO species decomposes to a C2 fragment and CO.



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This pathway is identical to the dominant pathway in the absence of water. A schematic of the TOFs of the important reaction pathways in the presence of water is shown in Figure 2. The most abundant surface intermediates are CO with a surface coverage of 55.1% [52.1%], adsorbed hydrogen with a coverage of 40.7% [42.9%], and free sites with a surface coverage of 3.0 % [5.0%]. We note that the coverages of both H2O and OH were increased significantly to 0.8% and 0.3%, respectively, while their coverages in the absence of water were negligible (θwater=[4.8×10-6 %] and θOH=[3.2×10-6 %]). As discussed in the previous sections, dehydrogenation of propionic acid and propionate are significantly facilitated in the presence of water. Our microkinetic modeling results confirm this and in the dominant DCX mechanism, propionic acid goes through two dehydrogenation steps to form CH3CCOOH which goes through C-C bond cleavage to form CH3C and COOH: CH3CH2COOHCH3CHCOOHCH3CCOOHCH3C+COOHCH3CH3+CO2.

We note

that in the absence of water, the dominant DCX mechanism does not proceed via dehydrogenation of propanoic acid but formation of propionate which goes directly through decarboxylation

to

form

CH3CH2

and

CO2:

CH3CH2COOHCH3CH2COO+H

CH3CH2+CO2CH3CH3+CO2. While this pathway is not dominant anymore in the presence of water, it is still competitive to the dominant pathway with a TOF of 2.16×10-9 s-1 which is only slightly smaller than the TOF of the dominant DCX pathway in water (1.09×10-8 s-1). Considering that the overall TOF of the DCX mechanism (sum of the TOFs of all DCX pathways) has increased significantly in liquid water, the ratio of the overall TOF of DCX pathways to the overall TOF of the DCN pathways increased from 0.16 in the vapor phase to 0.66 in liquid water. In other words, the DCX and DCN pathways are essentially competitive in water.


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Finally, we note that while the rate-controlling reactions, as well as DCX pathways, are facilitated in water, the overall turnover frequency is slightly lower than in the vapor-phase. This observation can be explained by the stronger CO adsorption strength, a higher CO coverage, and lower free site coverage in liquid water. As shown in Table 2, many surface intermediates occupy more than one site, the elementary rate expressions become functions of the second, third or fourth power of the free site coverage, and consequently, the free site coverage has a strong effect on the turnover frequency.

[Figure 2 Here (Pd 211 –Gas vs. Pd211-Water)]

Liquid Dioxane Effects:

In the presence of 1,4-dioxane, the overall TOF is calculated to be 4.59×10-8 s-1 which is also slightly lower than the TOF in the absence of solvents [6.20×10-8 s-1]. The DCN mechanism is identified to be again the dominant mechanism with an overall TOF of 2.91×10-8 s-1, and the dominant reaction pathway is similar to the one in the gas-phase: CH3CH2COOH CH3CHCOOHCH2CHCOOHCH2CHCOCH2CH+COCH3CH3. Similar to the liquid water case, the DCX mechanism is facilitated in the presence of 1,4-dioxane. The ratio of the DCX to DCN TOFs is increased from 0.16 in the vapor phase to 0.58 in 1,4-dioxane. Also, the DCX pathway proceeding via dehydrogenation of propionate: CH3CH2COOHCH3CH2COO CH3CHCOOCH3CCOOCH3C+CO2CH3CH3+CO2, TOF=1.45×10-9 s-1, is facilitated. However, the most dominant DCX mechanism remains to be identical to the gas-phase where propionate directly goes through decarboxylation: CH3CH2COOHCH3CH2COO+H CH3CH2+CO2 CH3CH3+CO2 with the TOF of 9.51×10-9 s-1. Finally, the most abundant



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surface intermediates are H with the coverage of 41.9% [42.9%] and CO with a coverage of 54.6% [52.1%]. Again, the free site coverage is reduced to 3.5% [5.0%] and accordingly, the overall turnover frequency is reduced in the presence of 1,4-dioxane.

[Figure 3 Here (Pd 211-Gas vs. Pd 211-Dioxane)]

3.4 Apparent activation barrier, reaction orders, and sensitivity analysis Apparent activation barriers were computed in the temperature range of 423 to 523 K in all reaction environments.  ∂ ln( r )  Ea = RT 2    ∂T  p i

(12)

Next, the reaction order with respect to hydrogen was calculated at 473 K in a partial pressure range of 0.05 to 0.4 bar. Similarly, the reaction order of propionic acid and CO were calculated at 473 K in a pressure range of 0.005 to 0.1 bar and 0.0001 to 0.1 bar, respectively.

 ∂ ln(r ) 

 α i =  ∂ ln( p ) i T , p 

(13) j ≠i

Finally, Campbell’s degrees of rate and thermodynamic control,48-52 XRC and XTRC, were used to determine rate controlling steps and intermediates in the mechanism. Rate controlling steps and intermediates are those transition states and intermediates that most strongly influence the reaction rate and are potential activity descriptors.

X RC,i =

ki r

 ∂r    ,  ∂ki  K i , k j ≠ ki

X TRC, n

     1  ∂r =  0  r   − Gn      ∂ RT     Gm0 ≠n ,Gi0 ,TS 


(14)

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where r is the overall rate of reaction, ki is the forward rate constant for step i, Ki is the equilibrium constant for step i, R is the ideal gas constant, T denotes the reaction temperature, and Gn0 is the free energy of adsorbate n. In the following, we summarize our previous results of the HDO of propionic acid over Pd(211) in the absence of solvents.24 Our model predicted an apparent activation energy of 0.86 eV. The reaction order with respect to propionic acid, CO and H2 were predicted to be +1.0, 0.0, and -3.2, respectively. The C-OH bond dissociation step in reaction 11 (CH2CHCOOH*** + * ⇌ CH2CHCO*** + OH*) was identified to be rate controlling with XRC = 0.92.

The

dehydrogenation of the α-carbon of propionic acid (reaction 2) and the β-carbon of CH3CHCOOH (reaction 6) were also partially rate controlling with XRC = 0.12 and 0.15, respectively.

Additionally, C-C bond scission in the DCX pathway, reaction 29

(CH3CH2COO** ⇌ CH3CH2* + CO2*), was identified to be rate controlling with XRC = 0.12. Finally, the degree of thermodynamic rate control of H* was calculated to be -6.08 implying that adsorbed hydrogen atoms inhibited the reaction significantly. The XTRC of CO was 0.08. This small and positive XTRC of CO can be explained by our lateral interaction model between H and CO where an increase in the coverage of CO leads to a decrease in the coverage of hydrogen on the surface which results overall in a slightly higher TOF (because the number of free sites increases).

Liquid Water

In the presence of liquid water, our model predicts an apparent activation energy of 1.91 eV which is significantly higher than the calculated barrier in the absence of solvents [0.86 eV]. This high apparent activation barrier in the presence of water can be explained by the Pd(211)



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step edges being more crowded in liquid water; and increasing the temperature does not only increase the elementary rate constant, but also increases the free site coverage which together results in a high apparent activation energy. The reaction orders with respect to propionic acid, hydrogen, and CO were calculated to be 1.00 [1.00], -2.7 [-3.2], and -0.04 [0.00], respectively. These results can be explained by the low acid coverage and relatively high H and CO coverage. While the CO reaction order is slightly negative, it is less negative than the hydrogen reaction order since a higher CO coverage reduces the H coverage due to strong lateral interactions. Finally, our sensitivity analysis predicts that reaction 11, which is the removal of OH from CH2CHCOOH, remains the most rate-controlling reaction with an XRC of 0.63 [0.92] in the presence of liquid water. Next, the dehydrogenation of the α-carbon of propionic acid, reaction 2 (CH3CH2COOH* + 2*⇌ CH3CHCOOH** + H*), as well as reaction 6, which is the dehydrogenation of the β-carbon of CH3CHCOOH, are the rate-controlling dehydrogenation steps with an XRC of 0.10 [0.11] and 0.07 [0.15], respectively. Clearly, the influence of these DCN reactions on the overall kinetics and TOF has decreased in the presence of water. However, new DCX pathways are facilitated in the presence of water and become partially ratecontrolling. For example, reaction 7 (CH3CHCOOH** + 2*→ CH3CCOOH*** + H*) has become rate-controlling with an XRC of 0.30 [0.00]. Similarly, reaction 33 (CH3CHCOO*** ⇌ CH3CH** + CO2*) has become partially rate-controlling with an XRC of 0.08 [0.00], reaction 36 (CH3CCOOH*** → CH3C* + COOH**) and reaction 38 (CH3CCOO*** ⇌ CH3C* + CO2* + *) have also become influential and both have an XRC of 0.03 [0.00], and reaction 29 (CH3CH2COO** ⇌ CH3CH2* + CO2*) remains somewhat rate-controlling with an XRC of 0.06 [0.12]. Finally, our model predicts an XTRC of approximately 0.0 for CO and -6.08 for adsorbed


Page 23 of 40

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hydrogen in the presence of water which are essentially equivalent to our model results in the absence of water.

Liquid 1,4-Dioxane

In the presence of 1,4-dioxane, our model predicts an apparent activation energy of 1.55 eV which is somewhat in between the vapor-phase and liquid water phase results. The reaction order with respect to propionic acid, CO and H2 were predicted to be +1.0 [+1.0], -0.11[0.0], and -2.39 [-3.2], respectively. The trend in the changes in the reaction orders is similar to our simulations in liquid water, where the reaction order of CO has become slightly negative, and the reaction order of H2 is less negative. The negative reaction orders indicate that the surface is highly covered by H and CO and only a small amount of free sites is available. In the presence of 1,4-dioxane, removal of the hydroxyl group in CH2CHCOOH (reaction 11) remains the most rate-controlling step with an XRC of 0.62 which is lower than in the vapor phase and very similar to the XRC in liquid water.

Similar to our gas-phase simulations,

dehydrogenation of the α-carbon of propionic acid, reaction 2, and the dehydrogenation of CH3CHCOOH to CH2CHCOOH (reaction 6), are both partially rate-controlling with an XRC of 0.09 for both reactions. Next, reaction 29 (CH3CH2COO** ⇌ CH3CH2* + CO2*) is also playing an important role in the overall activity with an XRC of 0.22. As in the vapor phase (and unlike the water phase), the direct C-CO2 bond dissociation is the preferred decarboxylation pathway in liquid 1,4-dioxane. C-CO2 bond dissociation of dehydrogenated propionate, reaction 33 (CH3CHCOO*** ⇌ CH3CH** + CO2*), is only of minor importance with an XRC of 0.03 in 1,4-dioxane. Finally, our model predicts an XTRC of approximately 0.0 for CO and -6.12 for



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Page 24 of 40

adsorbed hydrogen in the presence of 1,4-diocane which are essentially the same as in liquid water and the gas phase.

3.5 Pd(211) versus Pd(111) We previously19-20 studied the effects of solvents on the HDO of propionic acid over Pd(111) flat surfaces. Table S1 and S2 in the supporting information present the effects of solvents on the adsorption strength of the intermediates and overall reactions involved in the HDO of propionic acid over Pd(111) flat surfaces in a similar format to the one used in this paper. Overall, we observed a very similar trend in the effects of polar solvents and specially water on propionate and its dehydrogenated derivatives on both Pd(111) and Pd(211) model surfaces. For example, on flat surfaces, the CH3CHCOO and CH3CCOO surface species are stabilized by -0.15 and -0.21 eV in the presence of water. By stabilizing CH3CHCOO and CH3CCOO, dehydrogenation steps such as reaction 30 (CH3CH2COO** + 2* → CH3CHCOO*** + H*) and 34 (CH3CHCOO*** + * → CH3CCOO*** + H*) are facilitated. According to Table S2, in the presence of water, reaction 30 becomes less endergonic by 0.17 eV and it activation barrier is decreased by 0.24 eV.

Similarly, reaction 34 becomes more

exothermic by 0.08 eV and the activation barrier of this step is lowered by 0.10 eV in water. Consequently, on flat surfaces DCX pathways that involve a dehydrogenation step of propionate (Reaction 30 and 34) were facilitated by two to three orders of magnitude, and the overall TOF increased by one order of magnitude (acceleration of some rate controlling steps also played a role here). We note that on flat surfaces, the dominant DCX mechanism involves dehydrogenation steps of propionate, and consequently, liquid water which facilitates these steps significantly


Page 25 of 40

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increased the TOF of the DCX mechanism as well as the overall TOF. In contrast, on Pd(211) steps and in the absence of solvents, the dominant mechanism does not involve any dehydrogenation prior to decarboxylation (CH3CH2COOHCH3CH2COO+H CH3CH2+ CO2CH3CH3+CO2), and therefore, the effects of water in facilitating dehydrogenation steps does not lead to significant changes in the TOF of the DCX pathways. However, from a mechanistic point of view, the presence of water leads to dehydrogenation pathways of propanoic acid and propionate becoming the preferred decarboxylation routes. Furthermore, on all Pd surfaces we find the surface to be more crowded in liquid water, i.e., the free site coverage is decreased by a few percent, primarily due to a stronger CO adsorption of 0.08 eV. The consequences of a lower free site coverage―which has the potential to significantly reduce the TOF since some rate controlling steps are 2nd or 3rd order in free site coverage―are most pronounced for step sites. On Pd(111) the free sites coverage was predicted to be between 30%-40% and a change in free site coverage of a few percent represents only a relative small change in free site coverage and consequently only a small effect on the rate of the rate-controlling steps, i.e., other solvent effects are more important. In contrast, for Pd(211) steps, the free site coverage is only about 5% in the absence of solvents and a change to 3% in the presence of water represents a significant relative change in free site coverage and consequently a significant change in the rate of these rate-controlling steps.

4. Conclusion The solvent effects of water and 1,4-dioxane have been studied for the hydrodeoxygenation of propionic acid over a Pd(211) model surface and compared to solvent effects over Pd(111). An implicit solvation scheme and microkinetic modeling tools have been



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Page 26 of 40

used to investigate the effects of solvents on the adsorption strength of various surface intermediates, solvent effects on elementary reaction rate parameters, and solvent effects on overall kinetic rate parameters and the reaction mechanism.

In the presence of water,

dehydrogenated derivatives of propionic acid and propionate are significantly stabilized and consequently, in the dominant decarboxylation mechanism, propionic acid and propionate are dehydrogenated to form CH3CCOOH and CH3CHCOO prior to any C-C bond dissociation. While the decarboxylation rate increased significantly in water, making this pathway competitive to the dominant decarbonylation pathway, the overall turnover frequency is hardly affected by liquid water. Due to a stronger CO adsorption in water, the coverage of CO is increased and the free site coverage decreased which negatively affects the overall reaction rate. In the presence of 1,4-dioxane, dehydrogenation of propionate was also facilitated but not to the same extent as in water, and consequently, the dominant decarboxylation pathway remains the same as in the absence of solvents, i.e., propionate directly goes through a C-C bond dissociation to form CH3CH2 and CO2. Next, CO is also stabilized in the presence of 1,4dioxane and consequently, the coverage of free sites is decreased, leading to a slightly lower turnover frequency than in the gas-phase. In all reaction environments, we observe that C-OH bond cleavage is the most rate-controlling step (a key reaction step in the decarbonylation) followed by α- and β-carbon dehydrogenation and C-CO2(H) bond cleavage (a key step in the decarboxylation). Comparing the solvents effects on the hydrodeoxygenation of propionic acid over Pd(211) with those over Pd(111), we conclude that on all Pd surfaces solvent effects reduce the free site coverage which potentially reduces the activity of Pd and accelerates some elementary reactions such as the dehydrogenation of propionate which involves a product species that is


Page 27 of 40

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strongly stabilized by polar solvents since the carboxylate group is now directed towards the solvent (the reactant species adsorbs through the carboxylate group).

Overall, it is very

challenging to predict solvent effects in heterogeneous catalysis since solvents affect various surface species and elementary steps and only microkinetic modeling permits understanding the overall effect of the complex interplay of these elementary steps. For example, while solvent effects on elementary steps clearly suggested that polar solvents accelerate decarboxylation rates relative to decarbonylation rates, only microkinetic modeling permitted us to predict that solvents such as water reduce the overall activity of Pd step edges while they increase the activity of Pd terrace sites.

5. Acknowledgement We gratefully acknowledge financial support from the National Science Foundation (CHE-1153012) and in part by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical

Sciences

Division

under

Contract

DE-FG02-11ER16268

(DE-SC0007167).

Computational resources have been provided by the National Energy Research Scientific Computing Center (NERSC) which is supported by the Office of Science of the U.S. Department of Energy and in part by XSEDE under grant number TG-CTS090100. Finally, computing resources from the USC NanoCenter and USC’s High Performance Computing Group are gratefully acknowledged.

Supporting Information Figure illustrating the Pd(211) cluster model. Effect of liquid water, n-butanol, and n-octane on the adsorption strength of various intermediates and reaction free energies of various elementary



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steps over Pd(111) at 473 K. Cavity radii of various elements used in COSMO-RS calculations. This material is available free of charge via the Internet at http://pubs.acs.org.


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21. Rozmyslowicz, B.; Maki-Arvela, P.; Tokarev, A.; Leino, A. R.; Eranen, K.; Murzin, D. Y., Influence of Hydrogen in Catalytic Deoxygenation of Fatty Acids and Their Derivatives over Pd/C. Ind. Eng. Chem. Res. 2012, 51, 8922-8927. 22. Ford, J. P.; Immer, J. G.; Lamb, H. H., Palladium Catalysts for Fatty Acid Deoxygenation: Influence of the Support and Fatty Acid Chain Length on Decarboxylation Kinetics. Top. Catal. 2012, 55, 175-184. 23. Boda, L.; Gyorgy, O.; Solt, H.; Ferenc, L.; Valyon, J.; Thernesz, A., Catalytic Hydroconversion of Tricaprylin and Caprylic Acid as Model Reaction for Biofuel Production from Triglycerides. Appl. Catal. A 2010, 374, 158-169. 24. Behtash, S.; Lu, J.; Williams, C. T.; Monnier, J. R.; Heyden, A., Effect of Palladium Surface Structure on the Hydrodeoxygenation of Propanoic Acid: Identification of Active Sites. J. Phys. Chem. C 2015, 119, 1928-1942. 25. Faheem, M.; Suthirakun, S.; Heyden, A., New Implicit Solvation Scheme for Solid Surfaces. J. Phys. Chem. C 2012, 116, 22458-22462. 26. Klamt, A., Conductor-Like Screening Model for Real Solvents - a New Approach to the Quantitative Calculation of Solvation Phenomena. J. Phys. Chem. 1995, 99, 2224-2235. 27. Klamt, A.; Jonas, V.; Burger, T.; Lohrenz, J. C. W., Refinement and Parametrization of Cosmo-Rs. J. Phys. Chem. A 1998, 102, 5074-5085. 28. Klamt, A., Cosmo-Rs: From Quantum Chemistry to Fluid Phasethermodynamics and Drug Design; Elsevier Science, 2005. 29. Becke, A. D., Density-Functional Exchange-Energy Approximation with Correct AsymptoticBehavior. Phys. Rev. A 1988, 38, 3098-3100. 30. Perdew, J. P., Density-Functional Approximation for the Correlation-Energy of the Inhomogeneous Electron-Gas. Phys. Rev. B 1986, 33, 8822-8824. 31. Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C., Electronic-Structure Calculations on Workstation Computers - the Program System Turbomole. Chem. Phys. Lett. 1989, 162, 165-169. 32. Treutler, O.; Ahlrichs, R., Efficient Molecular Numerical-Integration Schemes. J. Chem. Phys. 1995, 102, 346-354. 33. Schwenk, C. F.; Rode, B. M., Influence of Electron Correlation Effects on the Solvation of Cu2+. J. Am. Chem. Soc. 2004, 126, 12786-12787. 34. Kresse, G.; Hafner, J., Abinitio Molecular-Dynamics for Liquid-Metals. Phys. Rev. B 1993, 47, 558561. 35. Kresse, G.; Furthmuller, J., Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. 36. Weigend, F.; Haser, M.; Patzelt, H.; Ahlrichs, R., Ri-Mp2: Optimized Auxiliary Basis Sets and Demonstration of Efficiency. Chem. Phys. Lett. 1998, 294, 143-152. 37. Weigend, F.; Ahlrichs, R., Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297-3305. 38. Weigend, F., Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057-1065. 39. Eichkorn, K.; Treutler, O.; Ohm, H.; Haser, M.; Ahlrichs, R., Auxiliary Basis-Sets to Approximate Coulomb Potentials (Vol 240, Pg 283, 1995). Chem. Phys. Lett. 1995, 242, 652-660. 40. Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R., Auxiliary Basis Sets for Main Row Atoms and Transition Metals and Their Use to Approximate Coulomb Potentials. Theor. Chem. Acc. 1997, 97, 119124. 41. Von Arnim, M.; Ahlrichs, R., Performance of Parallel Turbomole for Density Functional Calculations. J. Comp. Chem. 1998, 19, 1746-1757.


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42. Buzzi-Ferraris, G., “BzzMath: Numerical libraries in C++”, Politecnico di Milano: www.chem.polimi.it/homes/gbuzzi. Accessed: 1/13/2016. 43. Lugo-Jose, Y. K.; Monnier, J. R.; Williams, C. T., Gas-Phase, Catalytic Hydrodeoxygenation of Propanoic Acid, over Supported Group Viii Noble Metals: Metal and Support Effects. Appl. Catal. A 2014, 469, 410-418. 44. Grabow, L. C.; Hvolbaek, B.; Norskov, J. K., Understanding Trends in Catalytic Activity: The Effect of Adsorbate-Adsorbate Interactions for Co Oxidation over Transition Metals. Top. Catal. 2010, 53, 298310. 45. Grabow, L. C.; Mavrikakis, M., Mechanism of Methanol Synthesis on Cu through Co2 and Co Hydrogenation. ACS Catal. 2011, 1, 365-384. 46. Lu, J.; Heyden, A., Theoretical Investigation of the Reaction Mechanism of the Hydrodeoxygenation of Guaiacol over a Ru(0 0 0 1) Model Surface. J. Catal. 2015, 321, 39-50. 47. Smith, J. M.; Van Ness, H. C.; Abbott, M. M., Introduction to Chemical Engineering Thermodynamics; McGraw-Hill, 2005. 48. Kozuch, S.; Shaik, S., A Combined Kinetic-Quantum Mechanical Model for Assessment of Catalytic Cycles: Application to Cross-Coupling and Heck Reactions. J. Am. Chem. Soc. 2006, 128, 33553365. 49. Kozuch, S.; Shaik, S., Kinetic-Quantum Chemical Model for Catalytic Cycles: The Haber-Bosch Process and the Effect of Reagent Concentration. J. Phys. Chem. A 2008, 112, 6032-6041. 50. Campbell, C. T., Micro- and Macro-Kinetics: Their Relationship in Heterogeneous Catalysis. Top. Catal. 1994, 1, 353-366. 51. Campbell, C. T., Finding the Rate-Determining Step in a Mechanism - Comparing Dedonder Relations with the "Degree of Rate Control". J. Catal. 2001, 204, 520-524. 52. Stegelmann, C.; Andreasen, A.; Campbell, C. T., Degree of Rate Control: How Much the Energies of Intermediates and Transition States Control Rates. J. Am. Chem. Soc. 2009, 131, 8077-8082.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TABLES: Table 1. Effect of liquid water and 1,4-dioxane on the adsorption strength of various intermediates in the HDO of propionic acid over a Pd(211) model surface at a temperature of 473 K. ∆(∆G) is the difference in adsorption free energy of intermediate A in the absence ( +∗ ↔ ∗ and presence of solvent +∗ ↔ ∗ ). Water

1,4-Dioxane

∆(∆G) / eV

∆(∆G) / eV

CH3CH2COOH

-0.03

-0.07

CH3CHCOOH

-0.11

-0.08

CH2CHCOOH

-0.08

-0.03

CH3CCOOH

-0.12

-0.10

CHCHCOOH

-0.13

-0.08

CH3CH2CO

0.02

-0.02

CH3CHCO

-0.02

-0.04

CH2CHCO

-0.02

-0.03

CH3CCO

0.00

-0.01

CHCHCO

-0.02

-0.01

CH3CH2COO

0.01

-0.05

CH3CHCOO

-0.16

-0.09

CH3CCOO

-0.26

-0.14

CH3CH3

0.07

0.01

CH3CH2

0.04

0.00

CH3CH

-0.01

-0.02

CH2CH2

0.03

0.00

CH3C

0.01

0.00

CH2CH

-0.01

-0.01

CHCH

0.00

0.00

CH2C

-0.08

-0.05

COOH

-0.12

-0.09

H

-0.02

-0.01

CO

-0.08

-0.07

H 2O

-0.12

-0.08

CO2

-0.07

-0.07

OH

-0.10

-0.05

Adsorbed species


Page 32 of 40

Page 33 of 40


1 2 3 4 Table 2. Reaction free energies in eV for all elementary reaction steps in the hydrodeoxygenation of propionic acid 5 over a Pd(211) model surface at a temperature of 473 K in various reaction environments. 6 7 8 Gas Water 1,4-Dioxane Reaction # 9 ‡ ‡ ∆Grxn ∆G ∆Grxn ∆G ∆Grxn ∆G‡ 10 CH3CH2COOH* + 3*→ CH3CH2CO*** + OH* 0.17 1.18 0.11 1.15 0.18 1.20 1 11 CH3CH2COOH* + 2*→ CH3CHCOOH** + H* -0.11 0.55 -0.20 0.49 -0.13 0.56 2 12 CH3CH2CO*** → CH3CH2* + CO* + * -0.80 0.57 -0.86 0.42 -0.85 0.54 3 13 CH CH CO*** → CH CHCO** + H* -0.14 0.82 -0.19 0.75 -0.18 0.81 4 3 2 3 14 CH CHCOOH** + *→ CH CHCO** + OH* 0.14 1.19 0.12 1.27 0.13 1.23 5 3 3 15 CH3CHCOOH** + 2*→ CH2CHCOOH*** + H* -0.11 0.68 -0.10 0.66 -0.07 0.69 6 16 CH CHCOOH** + 2*→ CH CCOOH*** + H* 0.29 0.74 0.26 0.74 0.27 0.75 7 3 3 17 CH CHCO** + *→ CH CH** + CO* -0.93 0.76 -1.00 0.77 -0.97 0.77 8 3 3 18 CH3CHCO** + 2*→ CH3CCO*** + H* -0.27 0.64 -0.27 0.65 -0.26 0.67 9 19 -0.12 0.41 -0.14 0.33 -0.12 0.37 10 CH3CHCO** + 2*→ CH2CHCO*** + H* 20 0.12 0.86 0.08 0.88 0.08 0.85 21 11 CH2CHCOOH*** + *→ CH2CHCO*** + OH* 22 0.04 0.61 -0.03 0.57 -0.02 0.57 12 CH2CHCOOH*** + *→ CHCHCOOH*** + H* 23 CH CCOOH*** + *→ CH CCO*** + OH* -0.43 0.74 -0.41 0.82 -0.39 0.83 13 3 3 24 CH CCO***→ CH C* + CO* + * -1.15 0.54 -1.23 0.50 -1.21 0.50 14 3 3 25 CH CHCO*** + *→ CH CH*** + CO* -0.89 0.57 -0.96 0.59 -0.95 0.57 15 2 2 26 -0.10 0.52 -0.12 0.48 -0.09 0.52 16 CH2CHCO*** + 2*→ CHCHCO**** + H* 27 -0.01 1.12 -0.01 1.14 0.01 1.15 17 CHCHCOOH*** + 2*→ CHCHCO**** + OH* 28 -1.01 0.94 -1.07 0.98 -1.06 0.96 18 CHCHCO**** → CHCH*** + CO* 29 0.21 0.77 0.23 0.77 0.21 0.77 19 CHCH*** + H*→ CH2CH*** + * 30 -0.23 0.54 -0.17 0.57 -0.20 0.56 20 CH2CH*** + H* → CH2CH2** + 2* 31 -0.40 0.65 -0.49 0.65 -0.45 0.65 21 CH2CH***→ CH2C** + H* 32 -0.01 1.03 0.10 1.07 0.05 1.06 22 CH2C** + H*→ CH3C* + 2* 33 0.09 0.76 0.11 0.77 0.10 0.78 23 CH2CH*** + H* → CH3CH** + 2* 34 0.50 0.94 0.50 0.94 0.49 0.93 24 CH3C* + H* → CH3CH** 35 CH3CH** + H* → CH3CH2* + 2* 0.27 0.70 0.33 0.78 0.30 0.75 25 36 -0.02 0.61 0.04 0.62 0.01 0.63 26 CH3CH2* + H* → CH3CH3* + * 37 CH3CH2* + 2* → CH2CH2** + H* -0.58 0.19 -0.56 0.12 -0.57 0.15 27 38 -0.45 0.38 -0.44 0.40 -0.45 0.39 28 CH3CH2COOH* + 2* → CH3CH2COO** + H* 39 CH CH COO** → CH CH * + CO * 0.36 1.52 0.31 1.51 0.34 1.52 29 3 2 3 2 2 40 CH CH COO** + 2* → CH CHCOO*** + H* 0.63 1.13 0.44 1.09 0.58 1.14 30 3 2 3 41 0.28 0.93 0.21 0.93 0.27 0.95 31 CH3CHCOOH** + * → CH3CHCOO** + H* 42 CH CHCOOH** + * → CH CH ** + COOH* 0.10 1.27 0.08 1.25 0.07 1.28 32 3 3 43 -0.54 0.80 -0.46 0.81 -0.54 0.80 33 CH3CHCOO*** → CH3CH** + CO2* 44 -0.06 0.71 -0.18 0.60 -0.13 0.66 34 CH3CHCOO*** + * → CH3CCOO*** + H* 45 -0.07 1.02 -0.23 0.90 -0.13 1.00 46 35 CH3CCOOH*** + * → CH3CCOO*** + H* 47 -0.69 0.72 -0.68 0.70 -0.69 0.72 36 CH3CCOOH*** → CH3C* + COOH** 48 0.12 0.97 0.07 0.90 0.04 0.91 37 CH2CHCOOH*** + * → CH2CH*** + COOH* 49 -0.88 0.69 -0.69 0.89 -0.81 0.79 38 CH3CCOO*** → CH3C* + CO2* + * 50 COOH** → CO * + H* -0.35 0.56 -0.33 0.55 -0.34 0.57 39 2 51 COOH** → CO* + OH* -0.89 0.53 -0.95 0.63 -0.91 0.59 40 52 0.07 0.94 0.07 1.00 0.08 0.99 41 OH* + H* → H2O* + * 53 54 55 56 57 58 59 33 60 ACS Paragon Plus Environment


1 2 3 Table 2. (Continued) 4 Note: * symbolizes a free site. 5 6 Reaction # 7 8 42 CH3CH2COOH+*→CH3CH2COOH* 9 43 CH3CH3*→ CH3CH3 + * 10 44 CH2CH2**→ CH2CH2 + 2* 11 45 H2O + * → H2O* 12 46 CO2 + * → CO2* 13 47 CHCH + * → CHCH* 14 48 CO + * → CO* 15 49 H2 + 2* → 2H* 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Gas ∆Grxn 0.41 0.56 -0.42 0.28 0.56 -1.36 -1.29 -0.48

Page 34 of 40

Water

∆G‡ N/A N/A N/A N/A N/A N/A N/A N/A

∆Grxn 0.38 0.63 -0.38 0.16 0.48 -1.37 -1.37 -0.52



1,4-Dioxane ∆Grxn 0.33 0.56 -0.41 0.21 0.49 -1.35 -1.35 -0.50


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Equilibrium and forward rate constants for the elementary steps in the HDO of propionic acid over a Pd(211) model surface at a temperature of 473 K in various reaction environments. Reaction # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Gas

Water -1

Keq

kforward (s ) -2

1.56×10 1.39×101 3.38×108 2.94×101 3.31×10-2 1.35×101 7.39×10-4 7.79×109 8.20×102 1.99×101 4.88×10-2 3.91×10-1 3.67×104 1.92×1012 3.40×109 1.14×101 1.42 5.27×1010 5.68×10-3 2.37×102 1.89×104 1.24 8.67 4.95×10-6 1.48×10-3 1.48 1.60×106 6.85×104 1.55×10-4 1.91×10-7 9.46×10-4 9.13×10-2 5.48×105 4.64 5.94 2.50×107 5.88×10-2 2.62×109 5.67×103 2.82×109 1.78×10-1

2.61 1.36×107 8.07×106 1.60×104 2.02 6.04×105 1.43×105 7.77×104 1.40×106 3.91×108 7.16×103 3.43×106 1.28×105 1.83×107 7.41×106 3.10×107 1.04×101 8.52×102 5.71×104 1.74×107 1.14×106 1.15×102 7.23×104 1.00×103 3.08×105 2.92×106 8.66×1010 9.69×108 6.63×10-4 9.52 1.08×103 2.76×10-1 3.00×104 2.68×105 1.22×102 1.95×105 4.65×102 4.87×105 1.10×107 2.28×107 9.05×102

1,4-Dioxane -1

Keq

kforward (s ) -2

6.80×10 1.26×102 1.47×109 1.00×102 5.08×10-2 1.06×101 1.54×10-3 4.34×1010 8.20×102 3.25×101 1.30×10-1 2.18 2.25×104 1.37×1013 1.89×1010 1.86×101 1.42 2.30×1011 3.48×10-3 5.44×101 1.72×105 8.34×10-2 5.31 4.95×10-6 3.40×10-4 3.40×10-1 9.80×105 5.36×104 5.29×10-4 2.02×10-5 5.27×10-3 1.49×10-1 7.70×104 8.81×101 3.01×102 1.96×107 2.01×10-1 2.48×107 3.47×103 1.23×1010 1.78×10-1

5.45 5.93×107 3.20×108 8.91×104 2.84×10-1 9.87×105 1.43×105 6.08×104 1.10×106 2.78×109 4.38×103 9.15×106 1.80×104 4.88×107 4.54×106 8.27×107 6.37 3.19×102 5.71×104 8.33×106 1.14×106 4.31×101 5.66×104 1.00×103 4.33×104 2.28×106 4.82×1011 5.93×108 8.47×10-4 2.54×101 1.08×103 4.51×10-1 2.35×104 3.98×106 2.32×103 3.19×105 2.59×103 3.60×103 1.41×107 1.96×106 2.08×102


kforward (s-1)

Keq -2

1.22×10 2.27×101 1.15×109 7.84×101 3.84×10-2 5.06 1.21×10-3 2.08×1010 6.42×102 1.99×101 1.30×10-1 1.70 1.38×104 8.37×1012 1.48×1010 8.92 8.69×10-1 1.80×1011 5.68×10-3 1.14×102 6.44×104 2.85×10-1 6.78 6.33×10-6 7.09×10-4 7.09×10-1 1.25×106 6.85×104 2.53×10-4 6.51×10-7 1.21×10-3 1.91×10-1 5.48×105 2.58×101 2.59×101 2.50×107 4.19×10-1 4.70×108 4.44×103 4.61×109 1.39×10-1

1.60 1.06×107 1.68×107 2.04×104 7.57×10-1 4.73×105 1.12×105 6.08×104 6.71×105 1.04×109 9.15×103 9.15×106 1.41×104 4.88×107 7.41×106 3.10×107 4.98 5.22×102 5.71×104 1.07×107 1.14×106 5.51×101 4.43×104 1.28×103 9.03×104 1.79×106 2.31×1011 7.58×108 6.63×10-4 7.45 6.61×102 2.16×10-1 3.00×104 9.14×105 1.99×102 1.95×105 2.03×103 4.19×104 8.61×106 5.23×106 2.65×102


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

Table 3. (Continued) Reaction # 42 43 44 45 46 47 48 49

Gas

Water -1

Keq

kforward (s ) -5

4.61×10 1.47×10-6 2.70×104 9.46×10-4 1.18×10-6 3.18×1014 5.33×1013 1.20×105

7

8.04×10 1.26×108 1.31×108 1.63×108 1.09×108 1.36×108 1.31×108 4.89×108

1,4-Dioxane -1

Keq

kforward (s ) -5

9.47×10 1.81×10-7 1.26×104 1.93×10-2 7.09×10-6 3.51×1014 3.79×1014 3.20×105

7

8.04×10 1.26×108 1.31×108 1.63×108 1.09×108 1.36×108 1.31×108 4.89×108


kforward (s-1)

Keq -4

2.82×10 8.98×10-7 2.57×104 6.41×10-3 6.08×10-6 2.84×1014 2.32×1014 1.96×105

8.04×107 1.26×108 1.31×108 1.63×108 1.09×108 1.36×108 1.31×108 4.89×108

Page 37 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURES:

Figure 1. Network of elementary reaction steps considered in the hydrodeoxygenation of propionic acid over Pd(211). Elementary reactions involved in the DCX mechanism are shown with blue color arrows, DCN reactions are illustrated with red color arrows, and those reaction involved in both mechanisms such as dehydrogenation reactions and removal of the hydrocarbon pool are shown with gray color arrows. Figure adapted from Ref. [19,20].



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Figure 2. TOFs (s-1) for various elementary steps in the HDO of propionic acid over a Pd(211) model surface in the presence of liquid water at a temperature of 473 K and a chemical potential corresponding to a propionic acid gas phase pressure of 0.01 bar and a hydrogen partial pressure of 0.2 (numbers inside the square brackets [ ] are the corresponding TOFs in the absence of solvent). All other reaction conditions are given in section 3.3. The elementary reactions which are involved in the DCX mechanism are shown with the blue color arrows, DCN reactions are illustrated with the red color arrows, and those reactions which are involved in both of the mechanisms such as, dehydrogenation of propionic acid and its derivatives, and removal of hydrocarbon pool are shown with the gray color arrows. The reactions that are involved in the most dominant pathway are illustrated with a double-line arrow.


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Figure 3. TOFs (s-1) for various elementary steps in the HDO of propionic acid over a Pd(211) model surface in the presence of liquid 1,4-dioxane at a temperature of 473 K and a chemical potential corresponding to a propionic acid gas phase pressure of 0.01 bar and a hydrogen partial pressure of 0.2 (numbers inside the square brackets [ ] are the corresponding TOFs in the absence of solvent). All other reaction conditions are given in section 3.3. The elementary reactions which are involved in the DCX mechanism are shown with the blue color arrows, DCN reactions are illustrated with the red color arrows, and those reactions which are involved in both of the mechanisms such as, dehydrogenation of propionic acid and its derivatives, and removal of hydrocarbon pool are shown with the gray color arrows. The reactions that are involved in the most dominant pathway are illustrated with a double-line arrow.



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Solvation Effects in the Hydrodeoxygenation of Propanoic Acid over

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