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Aqueous Phase Acetic Acid Ketonization over Monoclinic Zirconia Qiuxia Cai, Juan A. Lopez-Ruiz, Alan R. Cooper, Jian-guo Wang, Karl O Albrecht, and Donghai Mei ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03298 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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Aqueous Phase Acetic Acid Ketonization over Monoclinic Zirconia Qiuxia Cai1,2, Juan A. Lopez-Ruiz1, Alan R. Cooper1, Jian-guo Wang2,*, Karl O. Albrecht1,*, Donghai Mei1,* 1 2

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, USA

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China

ABSTRACT Heterogeneous catalysis in the aqueous phase is paramount to the catalytic conversion of renewable biomass resources to transportation fuels and useful chemicals. To gain fundamental insights into how the aqueous phase affects catalytic reactions over solid catalysts, vapor and aqueous phase acetic acid ketonization over a monoclinic zirconia (m-ZrO2) catalyst had been comparatively investigated using ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations. The monoclinic zirconia was modeled by the most stable _

ZrO2(111) surface structure. The aqueous phase consisted of 111 explicit water molecules with a _

density of 0.93 g/cm3. The AIMD simulation results reveal that the aqueous phase/ZrO2(111) interface is highly dynamic. At the typical reaction temperature of 550 K, ~67% six-fold coordinated Zr6c Lewis acidic sites are occupied by either water molecules or hydroxyls while all two-fold coordinated O2c sites are protonated as hydroxyls. As a result, it is expected that there _

are limited active sites on the ZrO2(111) surface for acetic acid adsorption in aqueous phase. _

Acetic acid ketonization on the ZrO2(111) surface in both vapor and aqueous phases is assumed to be proceeded via the β-keto acid intermediate. In vapor phase, an alternative LangmuirHinshelwood (LH) mechanism in which the neighboring co-adsorbed acetic acid and di-anion can directly combine together and form the CH3COOHCH2COO* intermediate is identified as

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the more feasible pathway than the traditional C-C coupling step via the combination of acyl and di-anion. In the aqueous phase, our DFT results demonstrate that water molecules actively participate in the deprotonation and protonation steps via Grotthuss proton transfer mechanism. Furthermore, our results suggest that an Eley-Rideal (ER) mechanism pathway for the formation of the β-keto acid intermediate is feasible in the aqueous phase based on the observed energetic analysis. However, the low availability of di-anion is also a key factor that inhibits the ketonization reaction in the aqueous phase. The effects of dynamic aqueous phase on the key surface reaction steps are further confirmed by sampling different reaction configurations from AIMD trajectories. Keywords: Aqueous Phase, Ketonization, Acetic Acid, Zirconia, ab initio Molecular Dynamics, Density Functional Theory


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1. Introduction Bio-oils and aqueous phases derived from biomass include a large amount of oxygenates such as aldehydes, ketones, sugars, phenolic compounds and carboxylic acids.1-5 These biomassderived oxygenates need to be further upgraded to improve the physical and chemical properties of bio-oils by reducing oxygen content. Due to poor volatility, high water solubility and chemical instability of bio-oils, conducting these upgrading processes in aqueous phase at elevated temperature is a promising technology.6-9 With rapidly growing interests for efficient conversion of renewable biomass to hydrocarbon fuels and value-added chemicals, developing catalysts stable in the presence of liquid water is highly desired. Therefore, fundamental understanding of how the aqueous phase affects the heterogeneous reaction over solid catalyst surfaces, particularly on the molecular level, is vital for the successful implementation of aqueous phase catalysis in biomass conversion with high reactivity and selectivity. Compared to heterogeneous catalytic reactions at vapor-solid interfaces that had been widely studied both experimentally and theoretically for decades, our understanding of catalytic reactions at condensed liquid-solid interfaces is relatively poor. This is due to the extreme structural complexity of the liquid-solid interface.10-16 The reactivity and selectivity at the liquid-solid interface will be determined not only by the catalyst structure and active sites that resulted from the strong interaction between the solid surface and aqueous phase, but also by the effectiveness of mass transport of reactants and products in the aqueous phase. Ketonization of carboxylic acids is an efficient process for converting two carboxylic acids molecules into a ketone. The ketones may be further converted to long-chain hydrocarbons via reactions such as aldol condensation.17 Vapor-phase ketonization of carboxylic acids on a series of metal oxide catalysts have been extensively studied.18-26 Among various metal oxides,


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monoclinic zirconia (ZrO2) has been regarded as one of the most efficient catalysts for vapor phase carboxylic acid ketonization.18-19, 23, 27-28 Pulido et al. proposed that acetic acid ketonization over ZrO2 catalysts would be via a β-keto acid (CH3COCH2COO) intermediate.23 This β-keto acid mechanism is referred to as the traditional LH mechanism in Figure 1. The adsorbed acetate (CH3COO), which forms from acetic acid dissociation via the O-H bond scission, is firstly activated by α-hydrogen abstraction of the methyl group to get a di-anion (CH2COO) species. Concurrently, a molecularly adsorbed acetic acid molecule on an adjacent site is dehydroxylated to form an adsorbed acyl (CH3CO) species. The di-anion then reacts with the neighboring acyl forming the β-keto acid intermediate, which is further converted into enolate (CH3COCH2) and carbon dioxide via decarboxylation. Finally, the acetone (CH3COCH3) product is produced by the protonation of enolate. In contrast to the gas phase reaction, condensed phase ketonization has not been extensively studied. Pham et al. studied the aqueous phase ketonization of acetic acid over Ru/TiO2/C catalysts.29 They found that in the presence of water, the competitive adsorption between water and acetic acid on the high active Ti3+ Lewis acid site has a negative effect on the ketonization reaction rate. Introduction of a hydrophobic activated carbon support improved activity by protecting the active sites from water blockage. Recently, Lopez-Ruiz et al.30 showed that monoclinic ZrO2 is active for the aqueous condensed phase ketonization of acetic acid at 568 K. However, the reaction rate was about one order of magnitude lower than the vapor phase (water free) ketonization of acetic acid reported by Wang and Iglesia28 over monoclinic ZrO2 at 523K. These results suggest that water molecules likely compete with acetic acid adsorption onto active sites, thereby decreasing the availability of free active sites and surface coverage of acetic acid.8,

29

On the other hand, the presence of water might provide some advantages to the


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ketonization of acetic acid by facilitating certain reaction steps such as protonation and deprotonation via the Grotthuss proton transfer mechanism.31-33 We will demonstrate both advantages and disadvantages of performing the reaction in condensed water in the present work. First principles-based density functional theory (DFT) calculations have been used to investigate the effects of water on catalytic reactions on metal surfaces with explicit treatment of water.6-7, 31, 34-37 A previous DFT study of the water-adsorbate interaction on the Pt(111) surface indicated that larger and more hydrophilic reaction intermediates form more hydrogen bonds with surrounding water molecules, thus influencing the energies and entropies of the aqueous system.35 Desai et al. found the presence of water molecules helps to stabilize charged products derived from the heterolytic O-H bond-breaking of acetic acid over the Pd(111) surface.34 As a result, the acetic acid dissociation shifts from homolytic dissociation in vapor phase to heterolytic dissociation in water.7 Desai and Neurock also found that the presence of liquid water not only affects the overall reaction energies and activation barriers of elementary reaction steps, but also provides new reaction pathways by Grotthuss proton transfer mechanism involving the surrounding water molecules.34 Recently, Yoon et al. studied aqueous phase phenol protonation over Pt and Ni catalysts.31 They found that the liquid water environment significantly influences the stability of the surface bound keto species and appreciably lowers the activation barrier of enol/keto tautomerization reaction. Most recent theoretical work focused on the dynamic nature of interfacial structures of metal oxides under water,38-41 as well as water assisted facile proton transfer between the surface O sites.42-43 To the best of our knowledge, no theoretical studies of aqueous phase effects on catalytic reactions over metal oxides have been reported. In the present work, both ab initio molecular dynamics (AIMD) simulations and DFT calculations were carried out to study the effects of aqueous phase on the ketonization of acetic


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acid over monoclinic zirconia (m-ZrO2) catalyst following the β-keto acid mechanism.23 Two alternative elementary steps were studied for the formation of the β-keto acid intermediate following a Langmuir-Hinshelwood (LH) and an Eley-Rideal (ER)-type elementary step as shown in Figure 1. The m-ZrO2 catalyst surface was represented by the most stable surface _

structure ZrO2(111) and the aqueous phase was modeled by explicit water molecules. The major objective of the present work is to understand how the dynamic interface of aqueous _

phase/ZrO2(111) surface affects the acetic acid ketonization using DFT calculations.

2. Methodology All calculations were performed using the spin-polarized, gradient-corrected functional of Perdew, Burke, and Ernzerhof (PBE) implemented in the CP2K package (2.4.0 version).44-45 The wave functions were expanded in an optimized double-ζ Gaussian basis set (DZVP).46 An auxiliary plane wave basis set of 360 Ry energy cutoff was used for the electrostatic energy calculation.47 Only the Gamma k-point sampling was used for the system. The convergence of other basis sets like triple-ζ basis set (TZVP) for Zr, C, H, O atoms was tested and confirmed. The van der Waals correction proposed by Grimme was included in all calculations.48 In the _

present work, the zirconia catalyst was modeled by a periodic (2×2) ZrO2(1 11) surface slab (Zr64O128) with four ZrO2 tri-layers. Each ZrO2 tri-layer consists of 16 Zr atoms and 32 O atoms. _

The ZrO2(111) surface was chosen because it was identified as the most stable surface structure _

of m-ZrO2.49-50 The unit cell of ZrO2(111) surface slab has eleven non-equivalent atoms on the surface, including three six-fold coordinated Zr (Zr6c) atoms and one Zr7c atom, as well as one


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O2c, four O3c and two O4c atoms, while the bulk Zr and O atoms are Zr7c and O3c or O4c atoms, respectively. It is expected that the Zr7c atoms and O4c atoms are inert during the reaction due to _

their lack of under-coordination. The ZrO2(111) slabs were separated by a vacuum region (20 Å _

in the z direction) for the vapor phase system. For the aqueous phase/ZrO2(111) system, a total of 111 explicit water molecules were filled in this region to represent the aqueous phase with a density of 0.93 g/cm3. The equilibrated dynamic interface between the aqueous phase and the _

ZrO2( 1 11) surface was obtained using AIMD simulations. The AIMD simulations were performed in the canonical ensemble (NVT) with Nose-Hoover thermostat51-52 at 550 K for a time period of 10 ps with a time step of 0.5 fs. Then the reactant was inserted into the _

equilibrated aqueous phase/ZrO2( 1 11) system with very little fluctuation of the nearest surrounding waters and standard DFT calculation was used to optimize the system. The acetic acid ketonization reaction in vapor and aqueous phases was also explored using DFT calculations. The transition state of each elementary step was identified using climbing image nudged elastic band method (CI-NEB).53-54 The activation barrier (∆Eǂ) and reaction energy (∆E) for each elementary step are calculated as the total energy differences between the transition state and initial state, and between final state and initial state. All water molecules in the aqueous phase are relaxed during the optimization of the initial and final states. The adsorption energy of the adsorbate on the catalyst surface in aqueous phase and aq vap vapor phase ( ∆E adsorbate and ∆E adsorbate ) are calculated by35 aq aq ∆E adsorbate = E adsorbate

/ ZrO 2

aq vac − E ZrO − E adsorbate 2

(1)

vap vap ∆E adsorbate = E adsorbate

/ ZrO 2

vac vac − E ZrO − E adsorbate 2

(2)


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aq where E adsorbate

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_

/ ZrO 2

is the total energy of the adsorbate on the ZrO2(111) surface in the aqueous _

aq phase; E ZrO is the energy of the ZrO2(1 11) surface in the aqueous phase by removing the 2

adsorbate from the simulation system without further optimization, i.e., the coordinates of all vap atoms in the system are fixed; E adsorbate

_

/ ZrO 2

is the total energy of the adsorbate on the ZrO2(111) _

vap surface in the vapor phase; E ZrO is the energy of the optimized ZrO2(111) surface in vaccum, and 2

vac is the energy of the adsorbate in vacuum. E adsorbate

To account for the entropic contribution (∆S) and zero-point energy corrections (∆ZPEC) caused by the presence of the aqueous phase, the Gibbs free reaction energy (∆G) and Gibbs free energy of activation (∆Gǂ) were also calculated by classical thermodynamic method in both vapor and aqueous phases. The detailed calculation approach for the calculation of ∆G and ∆Gǂ are given in the Supporting Information.

3. Results and Discussion _

3.1 The aqueous phase / ZrO2(111) interface The calculations of Gibbs free reaction energy and Gibbs free energy of activation (∆G and ∆Gǂ) were carried out in this work because the studied reaction occurs on the solid catalyst surface in the presence of a liquid condensed phase where large configurational entropy effects would be observed. We found that the Gibbs free reaction energy and Gibbs free energy of activation were largely affected by the configurational entropy, in particular, the vibrational


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entropy of aqueous phase. Considering the inaccuracy derived from the low vibrational frequencies of water molecules in the aqueous phase is persistent and has a profound effect on the Gibbs free reaction energy and Gibbs free energy of activation, we choose to discuss the elementary steps in terms of reaction energetics (∆E and ∆Eǂ). All calculated ∆E, ∆Eǂ, ∆G and ∆Gǂ values are listed in the Tables 1 to 3 and Figures 10 to 12. It is clear that both sets of values would lead the same conclusions and is further discussed in section 3.6. _

Before discussing the interfacial structure of aqueous phase/ZrO2(111) system, we first _

investigated water adsorption, dissociation and proton transfer over the ZrO2(1 11) surface in vapor phase. Water molecularly adsorbs at the Zr6c site in a slightly tilted configuration with an adsorption energy of −125 kJ/mol (Table 1, Figure 2a). Our calculations suggest water dissociation is kinetically favorable due to the low activation energy of 8 kJ/mol. As the O-H bond activates, the H atom moves down to the surface O2c site. At the transition state shown in _

Figure 2a, the O-H distance is 1.21 Å. It is found that water dissociation on the ZrO2(1 11) surface is slightly exothermic with an activation energy of −9 kJ/mol. This suggests that both _

water dissociation and water formation on the ZrO2(111) surface are quite facile. In other words, upon adsorption, water can either exist on the surface in dissociative form or molecular form. With regard to the proton transfer between O2c site and O3c site, as shown in Figure 3a, the proton prefers to stay at the O2c site, and a very high activation energy of 156 kJ/mol is identified _

for the proton transferring from the O2c site to the neighboring O3c site. On the ZrO2(1 11) surface, therefore, it is expected that the O2c site may be occupied by a proton during the ketonization reaction.


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_

The aqueous phase/ZrO2(1 11) system is modeled by a liquid water phase with 111 _

explicit water molecules over the ZrO2( 1 11) surface slab using AIMD simulations. Upon _

equilibration, the highly dynamic interfacial structure of the aqueous phase/ZrO2(111) system was evaluated. Whereas some water molecules remain adsorbed on the catalyst surface, others _

dissociate by reacting with lattice oxygen atoms on the ZrO2( 1 11) surface leading to the formation of hydroxyl groups. As shown in Figure 4, about 67% of the Zr6c sites (statistically averaged) are occupied by either water molecules or hydroxyls and all the O2c active sites are _

occupied by protons at equilibrium, indicating limited availability of active sites over the ZrO2(1 11) surface under reaction conditions. Similar behavior is also shown in the interfacial structure between liquid water and rutile TiO2(110) in a recent work.16 _

To get a detailed picture of water interaction with the ZrO2(111) surface in the aqueous phase, we calculated the water adsorption, dissociation, and proton transfer energies from the O2c _

to the O3c site. The equilibrated aqueous phase/ZrO2(111) configuration was chosen as the initial structure. Compared to the reaction energy of water adsorption on the surface in the vapor phase _

(−125 kJ/mol), we find the interaction of water molecules with the ZrO2(111) surface in the aqueous phase is stronger at −212 kJ/mol (Table 1). The stronger adsorption energy indicates the water molecule interacting with the zirconia surface in the aqueous phase is thermodynamically more favorable than it is in the vapor phase. The dissociation of an adsorbed water molecule in the aqueous phase was studied through two dissociation pathways. The first dissociation pathway is the same as the pathway used in the


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vapor phase, namely the formation of a hydroxyl at the Lewis acid site with migration of the H to the O2c. The second dissociation pathway is mediated by water molecules in the aqueous phase via the Grotthuss proton transfer mechanism. For comparison with the vapor phase, the calculated reaction energies, activation barriers, and the structures along the two pathways are also shown in Figure 2. It was found that the reaction energies and activation barriers for the two pathways are very similar. The same low activation barrier (7 kJ/mol) for water dissociation in both pathways indicate that water dissociation is also facile in the aqueous phase. The greater exothermicity of water dissociation in the aqueous phase compared to the vapor phase, -33 and 9 kJ/mol respectively, implies that the dissociative form of water is more preferable in the aqueous phase than in the vapor phase. However, it is noted that the reverse process, i.e., water formation, is more feasible in the vapor phase compared to the aqueous phase with activation energies of 17 and ~40 kJ/mol, respectively. The proton transfer between two different surface oxygen sites in the aqueous phase was also studied and is shown in Figure 3. Similar to the vapor phase, the direct path for a proton _

transferring from the O2c site to the O3c site on the ZrO2(111) surface is kinetically relevant with an activation barrier of 128 kJ/mol, Figure 3b. The proton is also thermodynamically more stable at the O2c site over the O3c site in aqueous phase. However, with the help of water molecules in aqueous phase, the proton could easily transfer from O2c site to O3c site via Grotthuss mechanism, Figure 3c. The proton at the O2c site moves to the liquid water molecule over the O2c site. At the same time, the proton originally belonging to that water molecule would move to the O3c site. The activation barrier for this indirect proton transfer pathway (Grotthuss mechanism) is only 47 kJ/mol, which is much lower than the activation barrier of 128 kJ/mol for the direct proton transfer pathway. This is consistent with a recent theoretical study that suggested the 11 ACS Paragon Plus Environment

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facile proton transfer between surface oxygen sites on ceria in the aqueous phase is achieved by the aid of liquid water molecules.55 As a consequence, it can be expected that water dissociation/formation as well as proton transfer between different surface oxygen sites over the _

ZrO2(111) surface in the aqueous phase are very facile. The results clearly illustrate the highly _

dynamic nature of the interface of the aqueous phase/ZrO2( 1 11) system, which has been observed in our AIMD simulations. 3.2 Effects of aqueous phase on the reactant adsorption and product desorption Compared to the vapor phase, the existence of a liquid phase over the metal oxide surface creates a complex and dynamic liquid-solid interface where the reaction occurs. In addition to a decrease in availability of free active sites on the catalyst surface, the reactant adsorption and product desorption were observed to be significantly different from those in vapor phase. _

Acetic acid adsorbed onto the ZrO2(1 11) surface in both dissociative and molecular modes. The adsorption of acetic acid in the aqueous phase in both modes is stronger than in the vapor phase. In the vapor phase, as shown in Table 1, the dissociative adsorption of acetic acid is stronger than the molecular adsorption by 97 kJ/mol. This is reflected by the shorter Zr-O bond length (2.10 Å) in the dissociative configuration than in the molecular configuration (2.39 Å), shown in Figure 5a and Figure 5b. Similarly, in the aqueous phase, the Zr-O bond length (2.29 Å) for the dissociative adsorption is shorter than the molecular adsorption (2.45 Å). However, the adsorption energy difference for acetic acid between dissociative and molecular modes in the aqueous phase becomes less pronounced (18 kJ/mol) compared to the vapor phase (97 kJ/mol). Furthermore, the trend of binding strength for acetic acid and water is CH3COOH (dissociative) > H2O > CH3COOH (molecular) in the vapor phase, but they bind with similar 12 ACS Paragon Plus Environment

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strength in the aqueous phase, indicating the competitive adsorption between water and acetic acid in the aqueous phase. _

Acetone adsorption at the Zr6c site on the ZrO2(111) surface in vapor and aqueous phases are shown in Figure 5c and Figure 5f. Unlike acetic acid, the adsorption structures of acetone in both environments are very similar with the same Zr-O bond length of 2.28 Å. The calculated adsorption energies for acetone are −107 and −141 kJ/mol in the vapor and aqueous phases, respectively. This result suggests that acetone can desorb from the surface more easily compared to water and acetic acid, which is particularly important for the aqueous phase acetic acid ketonization as it should not pronouncedly hinder the surface reaction by blocking the active sites. This finding is consistent with results recently reported by Lopez-Ruiz et al.30 in which they provide spectroscopic evidences showing that acetone desorbs more easily than acetic acid on monoclinic ZrO2 in the absence and presence of water. To further understand how the aqueous phase affects the reactant adsorption and product desorption, we also calculated the energy difference by subtracting the adsorption energy in vapor phase from that in aqueous phase as the interaction energy measurement for the species with surrounding aqueous environment. For comparison, here we consider the interaction energies for molecular form of acetic acid and acetone. It is found that the molecular form of acetic acid is more hydrophilic than the acetone on the basis of the calculated interaction energies of −133 and −34 kJ/mol, respectively. In other words, it suggests that acetic acid prefers to stay in the aqueous phase over acetone via the strong hydrogen bonding interaction. In addition, as mentioned before, the limited availability of active sites and competitive adsorption between water and acetic acid would hinder the adsorption of acetic acid on the zirconia catalyst in _

aqueous phase. These findings highlight the difficulties for acetic acid to adsorb on the ZrO2(1 13 ACS Paragon Plus Environment

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11) surface, although it could strongly bond at the surface upon adsorption (Table 1), which was also suggested in the previous work. 29 3.3 Acetic acid ketonization in vapor phase _

In the present work, we assume the acetic acid ketonization reaction over the ZrO2(111) surface follows the reaction mechanism via a β-keto acid intermediate, which is illustrated in Figure 1. The LH-type β-keto acid mechanism has been generally accepted for the ketonization of carboxylic acids over metal oxide catalysts.23, 56-58 In addition to the typical C-C coupling reaction between co-adsorbed acyl (CH3CO*) and di-anion (CH2COO*) species, our calculations show the C-C coupling step could also occur between co-adsorbed acetic acid and di-anion. In this section, we will discuss each elementary step in the acetic acid ketonization mechanism in the vapor phase. The calculated reaction energies (∆E) and activation energies (∆Eǂ) are summarized in Table 2 and the corresponding structures of initial state (IS), transition state (TS) and final state (FS) for the acetic acid ketonization in the vapor phase are shown in Figure 6. The calculated Gibbs free energy profile is shown in Figure 7. _

The ketonization reaction on the ZrO2(111) surface is initialized with an adsorbed acetic acid molecule at the Lewis acidic Zr6c site. We find that the deprotonation of dissociatively adsorbed acetic acid is facile. The calculated activation energy for the O-H bond scission of acetic acid is only 13 kJ/mol. The formed acetate (CH3COO*) then reacts with a lattice O2c atom forming a surface hydroxyl and di-anion via the C-H bond scission (α-hydrogen abstraction). The α-hydrogen abstraction is endothermic with the reaction energy of +46 kJ/mol and a kinetically relevant activation energy of 125 kJ/mol. As shown in Figure 6b, the methyl group of


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the acetate first tilts down to the surface oxygen as the C-H bond becomes activated. At the transition state, this C-H distance is 1.53 Å. At the same time, it is expected there is another molecularly adsorbed acetic acid in the proximity of the di-anion species. The adjacent acetic acid is dehydroxylated to form the acyl intermediate via the C-O bond scission. Compared to the _

α-hydrogen abstraction step, the dehydroxylation of acetic acid over the ZrO2(111) surface has a lower activation energy (44 kJ/mol) which is shown by the reaction pathway in Figure 6c. An early transition state for the dehydroxylation step is identified with the broken C-O bond distance of 1.46 Å, which is slightly longer than the original C-O bond length of 1.38 Å for the initial state. The surface di-anion species can react with the neighboring acyl leading to the formation of the β-keto acid intermediate via the C-C bond coupling. The C-C distance decreases from 4.40 Å at the initial state to 3.00 Å at the transition state (Figure 6d). At the final state, the formed C_

C bond length is 1.52 Å. The formed β-keto acid intermediate is bound at the ZrO2(111) surface via only three Zr-O bonds. Similar to the results previously reported by Pulido et al., the C-C coupling step has the highest activation energy (184 kJ/mol) of all steps, although this step is thermodynamically favorable with a reaction energy of −140 kJ/mol. The β-keto acid intermediate then decarboxylates into enolate (CH3COCH2*) and gaseous CO2 via C-C bondbreaking (Figure 6e). This decarboxylation step is endothermic with a reaction energy of +95 kJ/mol; the activation energy of this step is 110 kJ/mol. Unlike the hydrogenation reaction over the metal surface, which generally involves a hydrogen atom, the protonation of enolate into _

acetone over the ZrO2(111) surface could be achieved by a surface hydroxyl. In this scenario, the protonation is accompanied by the O-H bond-breaking and the C-H bond formation. Our calculation shows that the final protonation step is thermodynamically neutral with a low


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activation energy of 55 kJ/mol. Because the C-C coupling of the acyl and the di-anion in the LH mechanism has a high activation energy, an alternative C-C coupling step was identified in the present work. In this alternative LH mechanism, the di-anion could directly combine with a neighboring co-adsorbed acetic acid, instead of the acyl intermediate as proposed in the traditional LH mechanism, and form the CH3COOHCH2COO* intermediate. Surprisingly, it is found that this C-C coupling step is both thermodynamically and kinetically more feasible than the traditional LH mechanism. The calculated activation energy is only 27 kJ/mol and the reaction energy is −139 kJ/mol. The formed CH3COOHCH2COO* intermediate then decomposes into the β-keto acid intermediate and water by the aid of a surface hydroxyl. Although this decomposition step is kinetically relevant with an activation energy of 120 kJ/mol, this alternative pathway leading to the formation of the β-keto acid intermediate is still much more favorable over the C-C coupling step described in the traditional LH mechanism (184 kJ/mol). The alternative LH mechansim has also been proposed in recent computational modeling studies.59-60 Wang and Iglesia found that the monodentate carboxylate would act as the precursor to the reactive 1-hydroxy enolate (CH2COOH) intermediates while the bidentate carboxylate, which is followed by the formation of di-anion species (CH2COO) is only the spectator.59 However, the monodentate carboxylate is much less stable than the bidentate carboxylate on the monoclinic zirconia surface. It is believed that the most stable bidentate carboxylate will firstly be converted into the monodentate configuration, then proceed the α-H abstraction step before the C-C coulping with the adsorbed acetic acid. In the gas phase, our calculated Gibbs free energies of activation for the α-H abstraction of the bidentate configuration is 122 kJ/mol at 550 K, which is qualitatively consistent with the reported Gibbs free energies of activation for the α16 ACS Paragon Plus Environment

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H abstraction of the monodentate and bidentate configurations are ~105 and ~150 kJ/mol at 523 K, respectively.59 In summary, similar to previous computational works that have studied the ketonization of acetic acid following the traditional LH mechanism,23, 56-58 our results suggest that the β-keto acid formation elementary step has the highest activation energy in the vapor phase over the _

ZrO2(1 11) surface. However, the C-C bond formation elementary step in the alternative LH mechanism has a lower activation energy compared to the traditional LH mechanism, 27 and 184 kJ/mol, respectively. Therefore, we conclude that the α-hydrogen abstraction is the elementary _

step with the highest activation energy for acetic acid ketonization over the ZrO2(111) surface in the vapor phases, and it proceeds as follows: CH 3COOH + CH 3COOH → CH 3COO + H + CH 3COOH → CH 2COO + 2 H + CH 3COOH → CH 3COOHCH 2COO + 2 H → CH 3COCH 2COO + H 2O + H → CH 3COCH 2 + CO 2 + H 2O + H → CH 3COCH 3 + CO 2 + H 2O 3.4 Acetic acid ketonization in aqueous phase The water molecules in the aqueous phase play several roles during the ketonization of acetic acid. Water molecules limit surface availability by blocking active sites, form surface hydroxyl groups, and may also directly participate in the surface reactions via Grotthuss proton transfer mechanism. As shown in Table 1, the Gibbs free energy of water adsorption is −156 kJ/mol. Considering the highly dynamic nature of the liquid-solid interface, any reaction that involves the adsorption of a water molecule on the catalyst surface will greatly aid the ∆G of the elementary step, and any step involving the desorption of water from the catalyst surface will


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penalize the ∆G of the elementary step. For these reasons, the discussion of the elementary steps _

involved in the aqueous phase ketonization of acetic acid over a ZrO2(111) surface is also done in terms of ∆E and ∆Eǂ. The results are listed in Table 3 and the corresponding structures of initial state (IS), transition state (TS) and final state (FS) are shown in Figure 8. The calculated Gibbs free energy profile is shown in Figure 9. Similar to the vapor phase, acetic acid ketonization in the aqueous phase begins with an _

adsorbed acetic acid molecule over the ZrO2( 1 11) surface. We find that the acetate is spontaneously generated from the deprotonation of adsorbed acetic acid with no activation barrier. To achieve the C-C coupling, the di-anion needs to be formed from acetate via the αhydrogen abstraction step. In the aqueous phase, our calculations show the α-hydrogen abstraction of acetate is thermodynamically unfavorable and kinetically difficult with a high activation energy of 178 kJ/mol. Similar to the vapor phase α-hydrogen abstraction, the C-C backbone of acetate tilts down to the surface when the C-H bond is broken. At the transition state shown in Figure 8b, the C-H distance is 1.47 Å. Compared to the vapor phase, the α-hydrogen abstraction step in the aqueous phase is more thermodynamically inhibited with a reaction energy of +128 kJ/mol compared to +46 kJ/mol in the vapor phase. As discussed in section 3.1, about 67% of the Zr sites are occupied by OH groups. Therefore, we also studied the α-hydrogen abstraction step in the aqueous phase, where the acetate intermediate loses a H atom and a surface OH group receives the H atom via the Grotthuss proton transfer mechanism (Figure 8c). Instead of directly moving to an empty surface oxygen atom, the detaching α-hydrogen atom from the methyl group of acetate moves toward a neighboring water molecule. Simultaneously, one of the H atoms in the water molecule leaves and recombines with a second water molecule 18 ACS Paragon Plus Environment

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closer to a surface hydroxyl group at the Zr6c site. Then, the hydrogen atom leaves the second water molecule and recombines with the surface hydroxyl to form a new water molecule. The calculated activation energy for the proton mediated α-hydrogen abstraction step in the aqueous phase is lowered from 178 to 106 kJ/mol and the reaction energy is lowered from +128 to +97 kJ/mol. This result suggests that the presence of water molecules inhibits the direct pathway of α-hydrogen abstraction step in the aqueous phase compared to the vapor phase. Although the Grotthuss proton transfer mechanism allows for a more energetically favorable pathway involving different surface sites by using neighboring water molecules as a proton transfer media, the reverse reaction, i.e., the protonation of di-anion, has a very low activation barrier of 9 kJ/mol, indicating that the di-anion will be unstable in the aqueous phase. This is also consistent with the previous work.61 Our calculation results suggest that the low availability of di-anion is also a key factor that inhibits the ketonization reaction in the aqueous phase. The acyl intermediate is obtained from the dehydroxylation of co-adsorbed acetic acid. As shown in Figure 8d, a neighboring unoccupied Zr6c site is required for the adsorption of the acetic acid. As the C-O bond is activated, the C-O bond distance extends from 1.35 Å in the initial state to 1.91 Å at the transition state. In the final state, the C-O bond is completely broken with the C-O distance of 3.02 Å. The calculated activation energy and reaction energy are +125 and +70 kJ/mol, respectively. Compared to the vapor phase, the dehydroxylation of acetic acid in the aqueous phase is both thermodynamically and kinetically more unfavorable. Furthermore, the reverse reaction that converts the acyl intermediate into acetic acid is both thermodynamically _

and kinetically more favorable. Therefore, it is expected that acyl intermediates on the ZrO2(111) surface in the aqueous environment are low. After these two neighboring intermediates are formed, the β-keto acid intermediate will form by the C-C coupling reaction (Figure 8e). Our 19 ACS Paragon Plus Environment

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results suggest that the C-C coupling step is thermodynamically favorable with a reaction energy of −174 kJ/mol. However, it has a high activation energy of +184 kJ/mol, which is very similar to the activation energy obtained for the vapor phase. The formed β-keto acid intermediate further converts to enolate and CO2 via C-C bond-breaking with an activation energy of 112 kJ/mol (Figure 8f). The activation energy results suggest that both the C-C coupling and the decomposition of β-keto acid are similar in vapor and aqueous phases. Finally, the protonation step of the enolate is shown in Figure 8g. The H atom is produced by the O-H bond scission of the surface hydroxyl group where the H atom is binding to a O2c surface atom. Then the H atom will move toward the C atom of the enolate in the upright direction. At the transition state, the CH distance is 1.43 Å and the activation energy is +77 kJ/mol. Like the α-hydrogen abstraction step, we also explore the enolate protonation in the aqueous phase via Grotthuss proton transfer mechanism (Figure 8h), in which the H atom of surface hydroxyl group is binding to an O3c surface atom. Compared to direct protonation, our calculation results show that the activation energy is lowered from 77 kJ/mol for the direct protonation to 36 kJ/mol for water mediated protonation. Furthermore, the enolate protonation in the aqueous phase becomes much more thermodynamically favorable with a reaction energy of −81 kJ/mol by using neighboring water molecules as the facile proton transfer media. We conclude that the Grotthuss proton transfer mechanism may play a major role in both deprotonation and protonation elementary steps. Similar to the vapor phase, the alternative LH mechanism in the condensed phase directly combines co-adsorbed acetic acid with a di-anion to form the CH3COOHCH2COO* surface intermediate is also investigated. The reaction energy of this alternative C-C coupling step is -74 kJ/mol and the activation energy is 47 kJ/mol, indicating that it is both more thermodynamically and kinetically feasible than the C-C coupling elementary step of the traditional LH mechanism. 20 ACS Paragon Plus Environment

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The formed CH3COOHCH2COO* intermediate then decomposes into β-keto acid and water (Figure 8j) by the aid of surface hydroxyl with an activation energy of 52 kJ/mol and a low reaction energy of −4 kJ/mol, which suggests that the reverse process is also very facile. Similar to the vapor phase results, the β-keto acid formation elementary steps have the highest activation energies in the traditional LH mechanism. However, the formation of acyl intermediate is inhibited in aqueous phase, the activation energy increases from 44 kJ/mol in the vapor phase to 125 kJ/mol in the aqueous phase. The alternative LH mechanism does not require the formation of acyl intermediate to get the β-keto acid and the activation energy for the C-C bond formation is lowered significantly. In summary, these results suggest that whereas the highest activation energies for the ketonization of acetic acid in the aqueous phase in the traditional LH mechanism is C-C coupling step forming β-keto acid intermediate, the alternative LH mechanism lowered the activation energy for the C-C bond formation from 184 to 47 kJ/mol by allowing for direct coupling of the di-anion intermediate with a co-adsorbed acetic acid. The highest activation energies in the alternative LH mechanism are the α-hydrogen abstraction and decarboxylation of β-keto acid intermediate into enolate. In summary, the aqueous phase ketonization reaction will be slower than vapor phase ketonization reaction because of the following reasons: 1) a lower availability of sites due to water and hydroxyl coverage, and 2) lower stability and surface coverage of key surface reactive intermediates such as the di-anion. 3.5 Evaluating an Eley-Rideal-type mechanism in aqueous phase _

As mentioned above, the availability of active sites over the ZrO2(111) surface in the aqueous phase is limited. Considering that a LH-type mechanism needs two acetic acids to adsorb at neighbor active sites on the surface and that water and hydroxyl groups are blocking 21 ACS Paragon Plus Environment

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many of the ZrO2 surface, we conducted the ER mechanism where just one acetic acid molecule adsorbs on the surface to form a di-anion species and another acid molecule in the condensed phase nearby could react with the adsorbed di-anion species. Compared to the vapor phase, the condensed phase is characteristic of slower diffusion and, as shown in section 3.2, acetic acid molecules prefer to stay solvated because of the strong hydrogen bonding interaction. Therefore, we speculate that a solvated acetic acid molecule will remain long enough near a surface dianion species to initiate an ER type mechanism, Figure 8k. However, we speculate that this scenario is less likely to occur in the vapor phase operation because the acetic acid molecules move significantly faster compared to the aqueous phase and are not in proximity to the surface di-anion species for sufficient time to initiate the ER type mechanism. For these reasons, we speculate that ER mechanism is more likely to occur in the aqueous phase and focused on studying the ER mechanism in the aqueous phase only. Figures 8k and 8l show the formation of the β-keto acid intermediate via the ER mechanism. The C-C coupling elementary step has an activation energy of 27 kJ/mol and a reaction energy of −103 kJ/mol, indicating that it is more energetically preferred than the C-C coupling step explored in the traditional and alternative LH mechanisms (184 and 47 kJ/mol, respectively). The formed CH3COHOHCH2COO* intermediate then decomposes into the β-keto acid and water by the aid of surface hydroxyl with an activation energy of 50 kJ/mol and a reaction energy of −12 kJ/mol. This suggests the formation and decomposition of the β-keto acid intermediate are both more thermodynamically and kinetically feasible compared to the LH mechanisms discussed above. Regardless if the reaction proceeds via LH or ER type mechanism, these results suggest that low availability of surface sites due to high water and hydroxyl surface coverage and the low stability and surface coverage of the di-anion specie are responsible for the 22 ACS Paragon Plus Environment

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lower reaction rate observed in the aqueous phase compared to the vapor phase. Therefore, we _

conclude that the acetic acid ketonization over the ZrO2(111) surface in the aqueous phase may proceed as follows: CH 3COOH + CH 3COOH ( l ) → CH 3COO + H + CH 3COOH ( l ) → CH 2COO + 2H + CH 3COOH ( l ) → CH 3COHOHCH 2COO + H → CH 3COCH 2COO + H 2O + H → CH 3COCH 2 + CO2 + H 2O + H → CH 3COCH 3 + CO2 + H 2O

3.6 Verification of the configurational effect of aqueous phase on key elementary steps Compared with the modeling of heterogeneous reactions at the vapor-solid interface, the biggest challenge for correctly and accurately describing surface reactions over the solid catalyst in the aqueous phase is due to the complexity of the liquid-solid interface. To show the configurational effects of water molecules on the activation energy, reaction energy, Gibbs free energy of activation and Gibbs free reaction energy, we studied the configuration sampling for three reactions: α-hydrogen abstraction via proton transfer; C-C coupling in the alternative LH mechanism; and C-C coupling in the ER mechanism. For each reaction, on the basis of the corresponding initial state used in section 3.4 and 3.5, we carried out another series of AIMD simulation runs at 550 K for an equilibration time period of 3 ps with the time step of 0.5 fs. The atoms of the reactants and ZrO2 catalyst were fixed while the environmental water molecules were allowed to relax during the AIMD simulations. Then another 1500 steps of AIMD runs were performed for sampling. Starting from the configuration at 3 ps, another 3 configurations were picked every 500 steps and DFT calculations were used to optimize these structures. We calculated the activation energy, reaction energy, Gibbs free energy of activation, Gibbs free reaction energy for the same reaction step using these optimized structures with different aqueous environments as initial states. Figures 10-12 show the calculated initial, transition, final


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structures and the energetic results for the three reaction steps. The corresponding results discussed in section 3.4 and 3.5 were chosen as reference. We first investigated the α-hydrogen abstraction via proton transfer in four initial configurations that were sampled at 6000, 6500, 7000 and 7500 steps in the AIMD trajectory. Figure 10 shows five different structure sets of initial, transition and final states of this elementary reaction step. It is found that although the moving paths of the α-hydrogen are different, the calculated Gibbs free energies of activation corresponding to these five reaction configurations are quite close, with an averaged value of 102±12 kJ/mol. Furthermore, the C-H distances in the five final states are similar, with the calculated Gibbs free reaction energies of 95±9 kJ/mol. The finding here suggests that the configuration of the water molecules does not have a significant effect on the kinetic and thermodynamic behavior of this reaction. The C-C coupling step is one of the primary elementary steps in the acetic acid ketonization reaction over the zirconia catalyst. This work postulates two new mechanisms for the C-C coupling elementary step following an alternative LH mechanism and an ER mechanism. First, we studied the effect of configuration sampling on the C-C coupling via the alternative LH mechanism. Figure 11 shows five different sets of configurations with both _

reaction intermediates adsorbed on the ZrO2(1 11) surface in which the C-C distances in the initial state configurations are different, ranging from 3.58 to 4.00 Å. At the transition states, the forming C-C bond distances are quite different and the Gibbs free energies of activation range from 37 to 77 kJ/mol. The Gibbs free reaction energies range from −30 to 5 kJ/mol. Although the results shows the strong configuration effect of the aqueous environment, the conclusion is still valid that this reaction step is both kinetically and thermodynamically feasible as mentioned in section 3.4. To further confirm the kinetic behavior of this reaction step, thermodynamic 24 ACS Paragon Plus Environment

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integration method based on constrained AIMD simulations was used to calculate the free energy difference (∆F). The details of the thermodynamic integration method is given in the Supporting Information. The initial and final configurations are taken from the structures in section 3.4 and C-C bond distance is chosen as the reaction coordinate, the calculated free energy of activation is 44 kJ/mol at the constrained C-C distance of 2.50 Å, which is consistent with the conclusion that this step is kinetically feasible. Lastly and most importantly, to verify the novel ER reaction path identified in this work, we studied the sampling configuration effect on the C-C coupling reaction via the ER mechanism. Figure 12 shows five different sets of configurations with different C-C distances in the initial state configurations ranging from 2.95 to 3.42 Å. Compared to the alternative LH mechanism, lower activation energies are found and the averaged value of Gibbs free energies of activation is 30±5 kJ/mol, indicating that the kinetic behavior of this reaction step is not affected by the changing configuration of water molecules. Although the Gibbs free reaction energies have a strong fluctuation, the conclusion in section 3.5 that this step is both more thermodynamically and kinetically feasible compared to the LH mechanisms still hold true. In summary, the different configurations of water molecules in the aqueous phase result in the change of energetics in these elementary steps, even when water molecules are not actively participating in the reaction. The results shown in this section also reveal that the description of the elementary steps discussed in sections 3.4 and 3.5 are still valid. 4. Conclusions The acetic acid ketonization reaction over a monoclinic zirconia catalyst in both vapor and condensed aqueous phases were investigated using AIMD simulations and DFT calculations. In the presence of the condensed aqueous phase, about 67% surface Zr6c active sites on the


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_

ZrO2(111) surface are occupied by either water molecules or hydroxyls and all the O2c active sites are protonated, thus decreasing the availability of free surface active sites for acetic acid adsorption. Furthermore, although acetic acid strongly adsorbs at the zirconia surface, the hydrophilic nature of acetic acid hinders its adsorption in aqueous phase due to the strong hydrogen bonding interaction, increasing the propensity of the acetic acid to remain solvated. Our DFT calculations suggest that water molecules in the aqueous phase hinder the ketonization reaction by blocking active sites, but actively participate in the elementary reaction steps involving protonation or deprotonation via Grotthuss proton transfer mechanism. An alternative LH elementary step was identified for the formation of the β-keto acid intermediate with lower activation energy compared to the previously postulated traditional LH mechanism. Because of the high coverage of water and hydroxyls in the aqueous phase, we investigated an energetically favorable ER elementary step for the formation of the β-keto acid with lower activation barrier in than the traditional and alternative LH mechanisms. Whereas the alternative LH and ER mechanisms provide a more energetically favorable route for the C-C bond formation, the αhydrogen abstraction elementary step has a similarly high activation energy in both vapor and aqueous phase even after taking into consideration the Grotthuss proton transfer mechanism. However, the reverse reaction, the protonation of the di-anion, is enhanced in the aqueous phase compared to vapor phase. Regardless of the reaction mechanism, our results suggest that low availability of surface sites due to high water and hydroxyl surface coverage and the low stability and surface coverage of the di-anion specie are responsible for the lower reaction rate observed in the aqueous phase compared to the vapor phase. Configuration sampling studies show that the energetics of the key elementary steps are affected by the configuration of water molecules in the aqueous phase, even when water molecules are not actively participating in the reaction. 26 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications. Details on the calculations of Gibbs free energies and thermodynamic integration method for free energy calculation. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (D. Mei). *E-mail: [email protected] (J.G. Wang). *E-mail: [email protected] (K.O. Albrecht)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Acknowledgements This work was financially supported by the United States Department of Energy through the Energy Efficiency and Renewable Energy (EERE) Bioenergy Technologies Office (BETO) 27 ACS Paragon Plus Environment

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and performed at the Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for DOE by Battelle Memorial Institute. Computing time was granted by a user proposal at the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) and by the National Energy Research Scientific Computing Center (NERSC). EMSL is a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at PNNL. D. Mei and Q. Cai also acknowledge the support from the US DOE, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences.

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(43) Tocci, G.; Michaelides, A., J. Phys. Chem. Lett. 2014, 5, 474-480. (44) Perdew, J. P.; Burke, K.; Ernzerhof, M., Phys. Rev. Lett. 1996, 77, 3865-3868. (45) VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J., Comp. Phys. Commun. 2005, 167, 103-128. (46) VandeVondele, J.; Hutter, J., J. Chem. Phys. 2007, 127, 114105. (47) Lippert, G.; Hutter, J.; Parrinello, M., Mol. Phys. 1997, 92, 477-487. (48) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., J. Chem. Phys. 2010, 132, 154104. (49) Christensen, A.; Carter, E. A., Phys. Rev. B 1998, 58, 8050-8064. (50) Piskorz, W.; Grybos, J.; Zasada, F.; Cristol, S.; Paul, J.-F.; Adamski, A.; Sojka, Z., J. Phys. Chem. C 2011, 115, 24274-24286. (51) Nosé, S., J. Chem. Phys. 1984, 81, 511-519. (52) Bussi, G.; Donadio, D.; Parrinello, M., J. Chem. Phys. 2007, 126, 014101. (53) Henkelman, G.; Uberuaga, B. P.; Jonsson, H., J. Chem. Phys. 2000, 113, 9901-9904. (54) Mills, G.; Jonsson, H.; Schenter, G. K., Surf. Sci. 1995, 324, 305-337. (55) Farnesi Camellone, M.; Negreiros Ribeiro, F.; Szabová, L.; Tateyama, Y.; Fabris, S., J. Am. Chem. Soc. 2016, 138, 11560-11567. (56) Pacchioni, G., ACS Catal. 2014, 4, 2874-2888. (57) Pham, T. N.; Sooknoi, T.; Crossley, S. P.; Resasco, D. E., ACS Catal. 2013, 3, 2456-2473. (58) Renz, M., Eur. J. Org. Chem. 2005, 979-988. (59) Wang, S.; Iglesia, E., J. Phys. Chem. C 2017, 121, 18030-18046. (60) Ignatchenko, A. V.; McSally, J. P.; Bishop, M. D.; Zweigle, J., Mol. Catal. 2017, 441, 35-62. (61) Ignatchenko, A. V.; DeRaddo, J. S.; Marino, V. J.; Mercado, A., Appl. Catal. A Gen. 2015, 498, 1024.


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ACS Catalysis

Table 1. DFT calculated energy difference (∆E), zero-point energy correction (∆ZPEC), entropic contribution (T∆S) and Gibbs free energy of adsorption (∆G) for the adsorption of water, acetic _

acid and acetone on the ZrO2(111) surface in the vapor phase and the condensed aqueous phase (T=550K). All energies are in kJ/mol. Vapor phase H2O CH3COOH(molecular) CH3COOH(dissociative) CH3COCH3

Aqueous phase

∆E

∆H

∆ZPEC

T∆S

∆G

∆E

∆H

∆ZPEC

T∆S

∆G

–125 –87 –184 –107

–115 –80 –179 –105

14

–50 –46 –34 –64

–65 –34 –145 –41

–212 –220 –238 –141

–199 –212 –228 –129

15

–43 –60 –40 –56

–156 –152 –188 –73

3 2 3

9 6 11

Table 2. DFT calculated reaction energies and activation energies for all elementary steps in acetic acid _

ketonization on ZrO2(111) surface in the vapor phase at T=550 K. All energies are in kJ/mol. Step

Elementary step: Traditional LH mechanism

∆Eǂ

∆E

∆Hǂ

∆H

∆ZPEC

T∆Sǂ

T∆S

∆Gǂ

∆G –13

1

CH3COOH* + * → CH3COO* + H*

13

–22

6

–20

1

–11

–7

17

2

CH3COO* + * → CH2COO* + H*

125

46

103

41

-3

–19

–14

122

55

3

CH3COOH* + * → CH3CO* + OH*

44

–20

36

–27

-7

–16

–15

52

–12

4

CH3CO* + CH2COO* → CH3COCH2COO* + *

184

–140

168

–139

2

13

0

155

–139

5

CH3COCH2COO* + * → CH3COCH2* + CO2*

110

95

104

94

-2

14

13

90

81

6

CH3COCH2* + H* → CH3COCH3* + *

55

7

41

5

-2

–4

6

45

–1

Elementary step: Alternative LH mechanism 7

CH3COOH* + CH2COO* → CH3COOHCH2COO* + *

27

–139

15

–133

7

–8

-8

23

–125

8

CH3COOHCH2COO* + H* → CH3COCH2COO* + H2O*

120

16

103

9

-9

8

12

95

–3


ACS Catalysis 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

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Table 3. DFT calculated reaction energies and activation energies for elementary steps in acetic acid _

ketonization on the ZrO2(111) surface in the condensed aqueous phase at T=550 K. All energies are in kJ/mol. (step 2a and 6a denote the Grotthuss proton transfer mechanism) Step

Elementary step: Traditional LH mechanism

∆Eǂ

∆E

∆Hǂ

∆H

∆ZPEC

T∆Sǂ

T∆S

∆Gǂ

∆G

1

CH3COOH* + * → CH3COO* + H*

~0

–50

~0

–48

5

~0

–12

~0

–36

2

CH3COO* + * → CH2COO* + H*

178

128

161

124

–3

–6

–5

167

129

2a

CH3COO* + OH* → CH2COO* + H2O*

106

97

86

91

–6

-12

-1

98

92

3

CH3COOH* + * → CH3CO* + OH*

125

70

107

65

–7

–13

5

120

60

4

CH3CO* + CH2COO* → CH3COCH2COO* + *

184

–174

164

–167

15

12

–27

152

–140

5

CH3COCH2COO* + * → CH3COCH2* + CO2*

112

84

102

77

-13

16

27

86

50

6


77

–1

59

–1

–2

10

8

49

–9


36

–81

13

–90

–14

5

21

8

–111

6a

Elementary step: Alternative LH mechanism 7

CH3COOH* + CH2COO* → CH3COOHCH2COO* +*

47

-74

46

-61

21

-19

-30

65

-30

8

CH3COOHCH2COO* + H* → CH3COCH2COO* + H2O*

52

-4

20

-2

~0

18

5

2

-7

9

CH3COOH (l) + CH2COO* + H2O* → CH3COHOHCH2COO* + OH*

27

–103

13

–94

14

–16

–18

29

–76

10

CH3COHOHCH2COO* → CH3COCH2COO* + H2O (l)

50

–12

28

–20

–12

21

14

7

–34

Elementary step: ER mechanism


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ACS Catalysis

Figure 1. Reaction mechanisms for acetic acid ketonization in the vapor and the condensed aqueous phase via the β-keto acid intermediate.


ACS Catalysis 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 34 of 48

(a) ∆Eǂ = 8 kJ/mol; ∆E= –9 kJ/mol

(b) ∆Eǂ= 7 kJ/mol; ∆E= –31 kJ/mol

(c) ∆Eǂ= 7 kJ/mol; ∆E= –33 kJ/mol

_

Figure 2. Water adsorption and dissociation on the ZrO2(111) surface in the vapor and condensed aqueous phases. The numbers in the structures of initial state (IS, left), transition state (TS, middle), and final state (FS, right) are the bond-breaking or the bond-forming distances (in Å). (a) vapor phase; (b) condensed aqueous phase; (c) condensed aqueous phase via Grotthuss proton transfer mechanism. The O, H and Zr atoms are represented in red, white and light blue, respectively.


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ACS Catalysis

(a) ∆E ǂ = 156 kJ/mol; ∆E= +81 kJ/mol

(b) ∆E ǂ = 128 kJ/mol; ∆E= +91 kJ/mol

(c) ∆E ǂ = 47 kJ/mol; ∆E= +39 kJ/mol

_

Figure 3. The proton transfer between the O2c and O3c sites on the ZrO2(111) surface in (a) the vapor phase; (b) the condensed aqueous phase; (c) the condensed aqueous phase via Grotthuss proton transfer mechanism. The numbers in the figures are the bond-breaking or the bond-forming distances (in Å). The same color scheme used in Figure 2 is applied.


ACS Catalysis 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

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_

Figure 4. A snapshot of the condensed aqueous phase/ZrO2(111) interface from the AIMD simulation. The same color scheme used in Figure 2 is applied.


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ACS Catalysis

(a)

(b)

(c)

(d)

(e)

(f)

_

Figure 5. The optimized adsorption structures of acetic acid and acetone on the ZrO2(111) surface in the vapor and condensed aqueous phases. (a) molecular acetic acid in vapor phase; (b) dissociative acetic acid in vapor phase; (c) acetone in vapor phase; (d) molecular acetic acid in aqueous phase; (e) dissociative acetic acid in aqueous phase; (f) acetone in aqueous phase. The C atom is in gray. The same color scheme of O, H, Zr atoms used in Figure 2 is applied.


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(a) CH3COOH* + * → CH3COO* + H*

(b) CH3COO* + * → CH2COO* + H*

(c) CH3COOH* + * → CH3CO* + OH*

(d) CH3CO* + CH2COO* → CH3COCH2COO* + *

(e) CH3COCH2COO* + * → CH3COCH2* + CO2*

(f) CH3COCH2* + H* → CH3COCH3* + *


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ACS Catalysis

(g) CH3COOH* + CH2COO* → CH3COOHCH2COO* + *

(h) CH3COOHCH2COO* + H* → CH3COCH2COO* + H2O*

Figure 6. Structures of initial state (IS, left), transition state (TS, middle), and final state (FS, right) for _

all elementary steps in acetic acid ketonization on the ZrO2(111) surface in the vapor phase. The numbers in the figures are the bond-breaking or the bond-forming distances (in Å). The same color scheme used in Figure 5 is applied.


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_

Figure 7. Gibbs free energy profile of acetic acid ketonization on the ZrO2(111) surface in vapor phase.


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ACS Catalysis

(a) CH3COOH* + * → CH3COO* + H*

(b) CH3COO* + * → CH2COO* + H*

(c) CH3COO* + OH* → CH2COO* + H2O*

(d) CH3COOH* + * → CH3CO* + OH*

(e) CH3CO* + CH2COO* → CH3COCH2COO* + *


ACS Catalysis 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

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(f) CH3COCH2COO* + * → CH3COCH2* + CO2*

(g) CH3COCH2* + H* → CH3COCH3* + *

(h) CH3COCH2* + H* → CH3COCH3* + *

(i) CH3COOH* + CH2COO* → CH3COOHCH2COO* + *

(j) CH3COOHCH2COO* + H* → CH3COCH2COO* + H2O(l)


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ACS Catalysis

(k) CH3COOH(l)+CH2COO*+H2O* → CH3COHOHCH2COO*+OH*

(l) CH3COHOHCH2COO* → CH3COCH2COO* + H2O (l)

Figure 8. Structures of initial state (IS, left), transition state (TS, middle), and final state (FS, right) for _

elementary steps in the acetic acid ketonization on the ZrO2(111) surface in the condensed aqueous phase. α-H abstraction step without and with Grotthuss proton transfer mechanism are shown in (b) and (c), respectively. Enolate protonation step without and with Grotthuss proton transfer mechanism are shown in (g) and (h), respectively. The numbers in the figures are the bond-breaking or the bond-forming distances (in Å). The same color scheme used in Figure 5 is applied.


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_

Figure 9. Gibbs free energy profile of acetic acid ketonization on the ZrO2(111) surface in aqueous phase.


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ACS Catalysis

∆Eǂ

∆E

∆Gǂ

∆G

Reference

106

97

98

92

6000

117

104

100

90

6500

119

109

114

104

7000

108

103

102

100

7500

121

100

98

89

Steps

IS

TS

FS

Figure 10. Sampling configuration structures taken from the AIMD trajectory for the α-H abstraction via _

proton transfer on the ZrO2(111) surface in aqueous phase.


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Page 46 of 48

∆Eǂ

∆E

∆Gǂ

∆G

Reference

47

-74

65

-30

6000

48

-38

61

-14

6500

93

4

71

5

7000

48

-45

37

-26

7500

70

-14

77

-9

Steps

IS

TS

FS

Figure 11. Sampling configuration structures taken from the AIMD trajectory for the C-C coupling step _

via alternative LH mechanism on the ZrO2(111) surface in the condensed aqueous phase.


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ACS Catalysis

∆Eǂ

∆E

∆Gǂ

∆G

Reference

27

-103

29

-76

6000

31

-45

34

-29

6500

32

-95

31

-68

7000

32

-37

25

-15

7500

30

-64

33

-45

Steps

IS

TS

FS

Figure 12. Sampling configuration structures taken from the AIMD trajectory for the C-C coupling step _

via ER mechanism on the ZrO2(111) surface in the condensed aqueous phase.


ACS Catalysis 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

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TOC Graphic


Aqueous-Phase Acetic Acid Ketonization over ... - ACS Publications

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