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Role of keto-enol tautomerization in the copper-catalyzed hydrogenation of ketones Jenoff E. De Vrieze, Joris W. Thybaut, and Mark Saeys ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00279 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 23, 2019
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Role of keto-enol tautomerization in the coppercatalyzed hydrogenation of ketones Jenoff E. De Vrieze, Joris W. Thybaut, Mark Saeys* Laboratory for Chemical Technology, Technologiepark 125, B-9052 Ghent, Belgium Graphical Abstract (TOC)
Abstract Hydrogenation of alkoxy intermediates is often rate-limiting in the hydrogenation of carbonyl groups over copper catalysts. Using first principles microkinetic simulations with coveragedependent kinetic parameters, we find that an enol hydrogenation pathway, i.e., not passing through alkoxy-intermediates, can become dominant, provided the difference in stability between the adsorbed ketone and enol tautomers is below 30 kJmol-1. In this scenario, the surface is covered with spectator alkoxides. When water is added, surface alkoxide species can be protonated by surface water or by hydroxyl groups, and oxygen or hydroxyl hydrogenation becomes rate-limiting. We illustrate this change in mechanism for acetol hydrogenation, where the dominant reaction pathway shifts from enol, to aldehyde, and ultimately to ketone hydrogenation with increasing water:acetol feed ratio. Keywords: Hydrogenation, copper, microkinetic modeling, DFT, coverage effects, acetol, ketones
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1. Introduction Promoted copper catalysts are emerging as catalysts for the hydrogenation of aldehyde and ketone compounds.1–4 Their superior selectivity and low material cost make them an attractive alternative to the conventional platinum and palladium catalysts in the hydrogenation of carbonyl functionalities.5,6 Further optimization of these materials is hampered by a lack of insight in the reaction mechanism and in the nature of the active site(s), which both are intensely debated.7,8 A single site mechanism is often hypothesized, in which the hydrogenation activity originates from metallic copper.9,10 Others attribute the high hydrogenation activity and selectivity to a synergetic effect between metallic copper and a second active site, which can be either a different copper species, Cu+,11,12 or an active site located on the support13,14. In the hydrogenation of carbonyl compounds, typically, two main pathways are considered depending on which atom is hydrogenated first, Figure 1 (blue lines).15–17 In the alkoxy pathway, the carbon atom is hydrogenated first, while in the alkyl pathway, the oxygen atom is hydrogenated first. Via keto-enol tautomerization, additional pathways open up.18 In the case of acetone hydrogenation, 2-hydroxypropylene can be formed through this keto-enol tautomerization. This opens up 2 additional pathways, depending on which carbon atom is hydrogenated first.19 Using first principles microkinetics, we showed that the enol pathways do not contribute to the hydrogenation of acetone over copper, and that the alkoxy pathway is dominant under typical hydrogenation conditions.20 Similar to Fischer-Tropsch Synthesis on cobalt21 and alcohol oxidation on gold22, hydrogenation of the oxygen atom in the alkoxy intermediate can be facilitated by proton transfer from hydroxyl species and adsorbed water molecules, in the presence of water.20 In the case of alpha-hydroxy ketones, such as acetol, additional keto-enol pathways open up, as illustrated in Figure 1.23 First, acetol can undergo hydrogenation via the traditional pathways, i.e., the alkoxy and the alkyl pathway, Figure 1 (blue lines). Hydrogenation of the oxygen atom, 1 ACS Paragon Plus Environment
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Figure 1(dashed lines) can occur via direct hydrogenation or via proton transfer from surface hydroxyl species and adsorbed water molecules. Hydrogenation of the carbon atoms, Figure 1 (solid lines), only occurs via direct hydrogenation.20 Alternatively, the 1-enol, 2-enol and aldehyde species can be formed via keto-enol tautomerization, Figure 1(dotted lines). Ketoenol tautomerization can be considered fast and quasi-equilibrated.24 The 1-enol and 2-enol species can be hydrogenated via two pathways, depending on which carbon atom is hydrogenated first. In the enol pathways, the difficult hydrogenation of alkoxy species is not required. The aldehyde tautomer can be hydrogenated, similar to acetol, via an alkoxy and via an alkyl pathway, Figure 1 (orange lines). In total, 8 different pathways are possible for acetol hydrogenation.
Figure 1. Pathways for the hydrogenation of acetol: direct pathways (blue) and via keto-enol tautomerism (dotted lines): 2-hydroxypropanaldehyde (orange), 1,2-dihydroxypropylene (green) and 2,3-hydroxypropylene (red). Hydrogenation of a carbon atom (solid lines) occurs via direct hydrogenation, hydrogenation of an oxygen atom (dashed lines) can occur via direct hydrogenation or via proton transfer from surface hydroxyl species or from adsorbed water.
Vila et al.23 studied acetol hydrogenation over copper catalysts using isotope labeling and showed that, under glycerol hydrogenolysis conditions, the enol pathways potentially have a 2 ACS Paragon Plus Environment
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significant contribution to the formation of propylene glycol. This is a surprising observation given the rather low activity of copper in the hydrogenation of C=C bonds.25 In this work, we present a first principles microkinetic model to investigate the competition between the traditional hydrogenation pathways20 and the ones involving tautomerization23 under typical hydrogenation conditions. In addition, the effect of water on the dominant reaction pathway is investigated. Acetol hydrogenation is selected as a model reaction, because it is the rate-limiting step in the hydrogenolysis of glycerol,26 the main side product in biodiesel production, and a model reaction for the hydrogenation of alpha-hydroxy ketones. 2. Computational methods Gibbs free energies and thermodynamic parameters were calculated using the VdW-DF2 functional,27,28 as implemented in the Vienna ab initio simulation package (VASP),29,30 and a plane-wave basis set with a cutoff kinetic energy of 400 eV. The VdW-DF2 functional provides an accurate description of the adsorption of CO, CH3O and CH3OH on Cu(111) and on Pt(111).20 The selection of the Cu(111) facet as the surface model was motivated by the structure-insensitivity of copper-catalyzed hydrogenation31–33 and glycerol hydrogenolysis. Hydrogenation reactions are in general structure-insensitive because the reaction occurs over a single atom center.34 The Cu(111) surface was modeled as a five-layer slab using a p(4x4) unit cell, with an optimized lattice constant of 3.74 Å that is slightly larger than the experimental lattice, 3.62 Å35. The adsorbates and the top two layers of the copper slab were relaxed, the bottom three layers were fixed at bulk positions. Repeated slabs were separated by a 15 Å vacuum layer. A 3 x 3 x 1 Monkhorst-Pack grid was used to sample the Brillouin zone. Kinetic coefficients were determined via transition state theory. Transition states were located with the climbing image nudged elastic band (ci-NEB)36,37 method and subsequently refined with the dimer38 method. Enthalpies and entropies for surface intermediates and transition states 3 ACS Paragon Plus Environment
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were calculated from frequencies for the complete structure. The enthalpy and entropy for gas phase molecules was calculated by combining the electronic energy, the zero-point energy (ZPE) and contributions from translational, rotational and vibrational partition functions. The calculated kinetic coefficients were combined into a microkinetic model to simulate an ideal plug flow reactor. Eq. (1) describes the mass balances for the gas phase species. ∂𝐶𝑖
∂𝐶𝑖 + 𝑢𝑠 = 𝜌𝑏𝑒𝑑𝐶𝑡𝑅𝑤,𝑖 ∂𝑡 ∂𝑧
(1)
with 𝐶𝑖, the concentration of species i in mol m-3, 𝑢𝑠 the superficial velocity in m s-1, 𝜌𝑏𝑒𝑑 the density of the catalyst bed in kgcat m-3, 𝐶𝑡 the total site concentration in mol kgcat-1 and 𝑅𝑤,𝑖 the net formation rate of species i in s-1. A typical active site concentration of 1 mmol kgcat-1 was used.26 Eq. (2) describes the mass balance for the surface intermediates. ∂𝜃𝑖 ∂𝑡
= 𝑅𝑤,𝑖
(2)
Instead of solving the steady-state equations, the transient equations were integrated until steady state to ensure smooth mathematical convergence. The set of transient equations was solved using the DASPK39 solver as implemented in an in-house FORTRAN code. A central differencing scheme was applied to discretize the axial reactor coordinate. Simulations with the as-calculated kinetic parameters show surface coverages of over 95%. This is inconsistent with the coverage that was used for the calculations of the kinetic parameters, 1/16 ML. Coverage effects typically exhibit a strong influence on the kinetic parameters40,41 and on the predicted activity20,42. To account for coverage effects in the microkinetic simulations, we implemented the approach proposed by Jorgensen and Grönbeck42 and refined in our previous study.20 First, a sensitivity analysis was performed (Figure S6) to identify the sensitivity of the microkinetic model to the stability of each reaction intermediate. Subsequently, the destabilization of each of reaction intermediates j, by each species i with a 4 ACS Paragon Plus Environment
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significant surface coverage (>1%) was calculated by gradually increasing the coverage of species i around reaction intermediate j in a p(3x3) Cu(111) unit cell. The resulting coverage corrections were fitted to power law expressions and the different corrections are combined to calculate the total coverage correction of species j, Eq. (3). 𝑛
𝛿𝐺𝑗({𝜃𝑖}∀𝑖) = 𝛿𝐺𝑗, 𝑖𝑛𝑡𝑟𝑎(𝜃𝑗) +
∑𝛿𝐺
𝑗, 𝑖,𝑖𝑛𝑡𝑒𝑟(𝜃𝑖)
(3)
𝑖≠𝑗
Coverage corrections for the transition states were calculated using a Bronsted-Evans-Polanyi relation, Eq. (4). 𝑛𝑟𝑒𝑎𝑐𝑡
𝛿𝐺𝑇𝑆,𝑘({𝜃𝑖}∀𝑖) = 𝛼𝑘
∑ 𝛿𝐺 ({𝜃 } 𝑗
𝑖 ∀𝑖
𝑛𝑝𝑟𝑜𝑑
) + (1 ― 𝛼𝑘) ∑ 𝛿𝐺𝑙({𝜃𝑖}∀𝑖)
𝑗=1
(4)
𝑙=1
with 𝛿𝐺𝑇𝑆,𝑘 the coverage correction for the transition state of reaction k, 𝛼𝑘 the transfer coefficient for reaction k and 𝛿𝐺𝑗({𝜃𝑖}) the coverage-correction for species i, Eq. (3). Similar to the acetone hydrogenation model, a transfer coefficient of 0.5 was applied.20 3. Results and Discussion Reaction pathways for the hydrogenation of acetol As shown in Figure 1, several hydrogenation pathways are possible for the hydrogenation of alpha-hydroxy ketones such as acetol. An overview of the preferred adsorption sites for reactants, products and intermediates is shown in Figure S1. The transition states for the acetol, the aldehyde, the 1-enol and the 2-enol pathways are shown in Figure S2 to Figure S5. Oxygen atoms are hydrogenated either via direct hydrogenation, or via proton transfer from surface hydroxyl species or adsorbed water molecules. Carbon atoms are only hydrogenated directly, i.e., by surface hydrogen, since proton transfer from surface hydroxyl and water species has barriers above 150 kJmol-1.20 In total, 21 elementary reaction steps were considered.
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Hydrogenation through the ketone and the aldehyde pathway follows similar mechanisms, exhibiting energy profiles similar to that in acetone hydrogenation,20 Figure 2. It is to be noted that the adsorbed ketone tautomer is about 15 kJmol-1 more stable than the aldehyde tautomer. When surface hydrogen is used as the hydrogenating species, Figure 2A and Figure 2D, a stable alkoxy intermediate is formed with a low reaction barrier both in the ketone and the aldehyde pathway. Hydrogenation of these alkoxy intermediates with surface hydrogen is highly activated. Hydrogenation of the oxygen atom in the first step, forming an alkyl intermediate, is also highly activated. The resulting 1- and 2-alkyl intermediates are unstable and can be hydrogenated to propylene glycol with a relatively low barrier. Hydrogen as a hydrogenating agent
Hydroxyl as a hydrogenating agent
Water as a hydrogenating agent
Figure 2. Electronic energy profiles for the hydrogenation of acetol to propylene glycol via the acetol pathway (top row) and aldehyde pathway (bottom row) on Cu(111) with surface hydrogen (left column), surface hydroxyl (middle column) and adsorbed water species (right column) as a hydrogenating species. The transition states for the acetol pathway are presented in Figure S2AH, the transition states for the aldehyde pathway are presented in Figure S3A-G.
In the presence of water, hydroxyl species can be formed on the copper surface, either through direct dissociation, or via spillover from the support. When such hydroxyl species are available, hydrogenation of the oxygen atom is facilitated by proton transfer in both the ketone and the aldehyde pathway, Figure 2B and Figure 2E. Hydrogenation of the carbon atom to form an alkoxy intermediate is still preferred. Proton transfer from surface hydroxyl species lowers the 6 ACS Paragon Plus Environment
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activation barrier to the alkyl intermediate, but the limited stability of the alkyl intermediates still results in a significant barrier. Proton transfer from surface hydroxyl species drastically lowers the barrier for hydrogenation of the alkoxy intermediates, and in the aldehyde pathway, the transition state energy even falls below the energy of the products, propylene glycol and O*, at infinite distance, Figure 2E. Since co-adsorbed O* and propylene glycol are lightly more stable, a transition state was still located relative to the co-adsorbed initial state, as indicated by the purple dotted line in Figure 2E. Similar to surface hydroxyl species, proton transfer from adsorbed water molecules reduces the barrier to hydrogenate the oxygen atom of the carbonyl group, see Figure 2C and Figure 2F. In the case of proton transfer from adsorbed water, the relatively limited stability of the alkyl intermediates results in the alkoxy pathways being favored. Though proton transfer facilitates the conversion of alkoxy intermediates, the importance of these pathways will depend on the availability of hydroxyl and water species on the surface, and on the rate of O* or OH* hydrogenation.20 Hydrogenation of alkoxy intermediates is not required in the pathway via the enol tautomers, i.e., via the 1-enol and 2-enol species, see Figure 3. Both enol intermediates are significantly less stable than adsorbed ketone and aldehyde. In the gas-phase, the 1-enol and 2-enol tautomers are 25 and 36 kJmol-1 less stable than acetol, respectively. Also their adsorption energies on Cu(111), -19 and -13 kJmol-1, are higher than the adsorption energy for acetol, -40 kJmol-1. However, the high activation barriers for hydrogenation of the oxygen atom no longer have to be surmounted. As shown in Figure 3A, two routes are available for the 1-enol pathway: via the 2-alkyl and via the 1-alkyl species. Both intermediates have similar stabilities and are formed with a similar barrier of 60 kJmol-1. Hydrogenation of the 2-alkyl species has a slightly lower barrier than hydrogenation of the 1-alkyl intermediate.
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Similarly, two routes are available for the 2-enol pathway, Figure 3B. Interestingly, the barrier for the first hydrogenation step of the 3-alkyl (transition states in Figure S5A) route is significantly higher than that for the other carbon-atom hydrogenation steps (transition states in Figure S4A, S4C and S5C). A similar barrier was calculated for acetone hydrogenation, and can be related to the different transition state geometry for this step to form a primary alkyl species without hydroxyl substituents. 20
Figure 3. Electronic energy profiles for the hydrogenation of acetol to propylene glycol via the 1-enol pathway (A) through the 1-alkyl (dotted line) and 2-alkyl (solid line) intermediates and via the 2-enol pathway (B) through the 2-alkyl (solid line) and 3-alkyl (dotted line) intermediate. The transition states for the 1-enol pathway are shown in Figure S4, the transition states for the 2-enol pathway in Figure S5.
Construction of a coverage-dependent microkinetic model A microkinetic model was constructed based on the energy profiles in Figure 2 and Figure 3, combined with the hydroxyl/water regeneration steps: the hydrogenation of surface oxygen, of surface hydroxyl and the disproportionation of two hydroxyl species. The hydrogenation steps have activation barrier of 118 and 130 kJmol-1, respectively. Disproportionation has a lower barrier of 62 kJmol-1. An overview of the adsorption and keto-enol tautomerization rate coefficients is provided in Table 1. Table 1. Adsorption and tautomerization rate and equilibrium coefficients at 200°C for the gasphase molecules in the model. The adsorption rate coefficients are calculated using collision theory and the equilibrium coefficients by DFT (Eq. S1, Eq. S2). The rate coefficient for H2 dissociation on Cu(111) was calculated using transition state theory (Eq. S3). Similar to Figure 2, the ketone is in blue, 1-enol in green, 2-enol in red, and aldehyde in orange. Reaction
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂(𝑔) + ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂
ΔH (kJ mol-1) ∗
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻(𝑔) + ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻 ∗
ΔS (J mol-1 K-1)
Kads (-) ―4
―40
―145
6.5 ∙ 10
―49
―154
2.6 ∙ 10 ―3
k+ (s-1 Pa-1)
k- (s-1)
2
8.6 ∙ 10
1.3 ∙ 1011
8.4 ∙ 102
3.3 ∙ 1010
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(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂 ∗ ⇌ 𝐶𝐻2𝐶(𝑂𝐻)𝐶𝐻2𝑂𝐻 ∗
63
1
1.1 ∙ 10 ―7
1.1 ∙ 106
1.0 ∙ 1013
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂 ∗ ⇌ 𝐶𝐻3𝐶(𝑂𝐻)𝐶𝐻𝑂𝐻 ∗
46
―6
8.4 ∙ 10 ―6
8.4 ∙ 107
1.0 ∙ 1013
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂 ∗ ⇌ 𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻𝑂 ∗
22
―2
4.0 ∙ 10 ―3
4.0 ∙ 1010
1.0 ∙ 1013
―7
―3
1.0 ∙ 104
𝐻2(𝑔) + 2 ∗ ⇌ 2𝐻
∗
𝐻2𝑂(𝑔) + ∗ ⇌ 𝐻2𝑂 ∗
―6
―134
4.2 ∙ 10
―21
―104
9.0 ∙ 10 ―4
4.2 ∙ 10
1.7 ∙ 103
1.9 ∙ 1011
As keto-enol tautomerization can be considered a fast step, the rate coefficients for the exothermic direction were fixed at 1013 s-1; the corresponding forward rate coefficients were calculated by imposing thermodynamic consistency. An overview of the kinetic coefficients for the surface reactions is provided in Table 2. The pre-exponential factors for hydrogenation with surface hydrogen are all close to the typical value of 1013 s-1. Due to the higher entropic penalty to reach the transition state, proton transfer from surface hydroxyl species and from adsorbed water molecules have lower pre-exponential factors, in the range of 1010 s-1 and 107 s-1, respectively. For reactions where the transition state free energy falls below the free energy of the reactants at infinite separation, e.g., proton transfer from adsorbed water molecules to the 1-alkoxy species, the rate coefficient was set at 1013 s-1. The corresponding rate coefficient for the reverse reaction was again determined via thermodynamic consistency.
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Table 2. Pre-exponential factors, activation enthalpies and rate coefficients at 200 °C for the surface reactions in the microkinetic model as determined via transition state theory (Eq. S3). Similar to Figure 2, ketone is in blue, 1-enol in green, 2-enol in red, and aldehyde in orange. Reaction
A+ (s-1)
Ea+ (kJ mol-1)
A- (s-1)
Ea- (kJ mol-1)
k+ (s-1)
k- (s-1)
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂 ∗ + 𝐻 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂 ∗ + ∗
5.0 ∙ 1011
62
5.6 ∙ 1012
120
7.6 ∙ 104
3.0 ∙ 10 ―1
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂𝐻 ∗ + 𝐻 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + ∗
5.6 ∙ 1013
63
4.8 ∙ 1011
171
5.8 ∙ 106
7.1 ∙ 10 ―8
𝐶𝐻2𝐶(𝑂𝐻)𝐶𝐻2𝑂𝐻 ∗ + 𝐻 ∗ ⇌ 𝐶𝐻2𝐶𝐻(𝑂𝐻)𝐶𝐻2𝑂𝐻 ∗ + ∗
1.6 ∙ 1012
110
2.5 ∙ 101
4.3 ∙ 101
5
Direct hydrogenation of the carbon atom
98
5.3 ∙ 1013
𝐶𝐻2𝐶(𝑂𝐻)𝐶𝐻2𝑂𝐻 + 𝐻 ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂𝐻 + ∗
11
7.7 ∙ 10
61
14
1.7 ∙ 10
79
1.4 ∙ 10
3.1 ∙ 105
𝐶𝐻2𝐶𝐻(𝑂𝐻)𝐶𝐻2𝑂𝐻 ∗ + 𝐻 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + ∗
6.0 ∙ 1012
74
3.4 ∙ 1011
188
4.0 ∙ 104
6.6 ∙ 10 ―10
𝐶𝐻3𝐶(𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + 𝐻 ∗ ⇌ 𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + ∗
2.4 ∙ 1012
63
1.0 ∙ 1014
57
2.4 ∙ 105
4.7 ∙ 107
𝐶𝐻3𝐶(𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + 𝐻 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂𝐻 ∗ + ∗
1.4 ∙ 1012
63
1.5 ∙ 1014
61
1.5 ∙ 105
2.7 ∙ 107
11
∗
∗
∗
∗
∗
∗
𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻𝑂𝐻 + 𝐻 ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻 + ∗
3.5 ∙ 1013
71
7.4 ∙ 10
𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻𝑂 ∗ + 𝐻 ∗ ⇌ 𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻2𝑂 ∗ + ∗
2.2 ∙ 1011
57
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂 ∗ + 𝐻 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂𝐻 ∗ + ∗
8.0 ∙ 1012
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂 ∗ + 𝐻 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + ∗
182
5
5.7 ∙ 10
6.4 ∙ 10 ―9
2.2 ∙ 1013
140
1.3 ∙ 105
7.2 ∙ 10 ―3
124
1.7 ∙ 1015
79
1.6 ∙ 10 ―1
3.4 ∙ 106
4.1 ∙ 1013
129
6.8 ∙ 1012
132
2.4 ∙ 10 ―1
1.6 ∙ 10 ―2
𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻𝑂 ∗ + 𝐻 ∗ ⇌ 𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + ∗
2.5 ∙ 1012
131
1.6 ∙ 1014
102
9.6 ∙ 10 ―3
8.9 ∙ 102
𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻2𝑂 ∗ + 𝐻 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + ∗
1.0 ∙ 1014
134
1.5 ∙ 1012
133
1.7 ∙ 10 ―1
3.2 ∙ 10 ―3
100
1.0 ∙ 1013 𝑎
0
6.1 ∙ 10 ―2
1.0 ∙ 1013
Direct hydrogenation of the oxygen atom
Hydrogenation of the oxygen atom via proton transfer from surface OH species
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂 ∗ + 𝑂𝐻 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂𝐻 ∗ + 𝑂 ∗ ∗
∗
∗
6.8 ∙ 109 ∗
7.6 ∙ 10
62
4.7 ∙ 10
8
1.2 ∙ 10
5.7 ∙ 1010
𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻𝑂 ∗ + 𝑂𝐻 ∗ ⇌ 𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + 𝑂 ∗
5.3 ∙ 1010
100
1.3 ∙ 1013
15
4.3 ∙ 10 ―1
2.9 ∙ 1011
𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻2𝑂 ∗ + 𝑂𝐻 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + 𝑂 ∗
1.8 ∙ 1014
58
1.0 ∙ 1013 𝑎
0
7.6 ∙ 107
1.0 ∙ 1013
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂 + 𝑂𝐻 ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻 + 𝑂
11
11
5
Hydrogenation of the oxygen atom via proton transfer from adsorbed water
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂 ∗ + 𝐻2𝑂 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝑂𝐻 ∗ + 𝑂𝐻 ∗
1.6 ∙ 107
83
3.6 ∙ 1012
42
1.1 ∙ 10 ―2
7.5 ∙ 107
(𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂 ∗ + 𝐻2𝑂 ∗ ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + 𝑂𝐻 ∗
4.4 ∙ 1010
25
7.3 ∙ 1012
33
7.6 ∙ 107
1.5 ∙ 109
𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻𝑂 ∗ + 𝐻2𝑂 ∗ ⇌ 𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻𝑂𝐻 ∗ + 𝑂𝐻 ∗
8.6 ∙ 107
39
5.7 ∙ 1012
16
3.9 ∙ 103
1.1 ∙ 1011
1.5 ∙ 10
38
13
1.0 ∙ 10
5.6 ∙ 1013
∗
∗
∗
𝐶𝐻3𝐶𝐻(𝑂𝐻)𝐶𝐻2𝑂 + 𝐻2𝑂 ⇌ (𝐶𝐻3)(𝐶𝐻2𝑂𝐻)𝐶𝐻𝑂𝐻 + 𝑂𝐻
∗
13 𝑎
1.0 ∙ 10
0
14
Regeneration of the adsorbed hydroxyl/water species 𝐻2𝑂 ∗ + ∗ ⇌ 𝐻 ∗ + 𝑂𝐻 ∗
4.1 ∙ 1010
125
4.2 ∙ 1013
130
6.6 ∙ 10 ―4
2.0 ∙ 10 ―1
𝐻2𝑂 ∗ + 𝑂 ∗ ⇌ 2𝑂𝐻 ∗
1.8 ∙ 1010
0.3
3.0 ∙ 1012
60
7.6 ∙ 105
2.0 ∙ 10 ―1
𝐻 ∗ + 𝑂 ∗ ⇌ 𝑂𝐻 ∗
1.9 ∙ 1013
118
5.1 ∙ 1012
175
1.6 ∙ 100
2.2 ∙ 10 ―7
a
The pre-exponential factor of the exothermic step of a non-activated reaction was fixed at 1.0 ∙ 1013, the kinetic
coefficient of the endothermic step was calculated via thermodynamic consistency.
Next, microkinetic simulations were performed for a typical acetol space time of 500 kgcat s molacetol-1 and 200 °C. For a total pressure of 7.5 MPa and a H2:acetol inlet molar ratio of 2, the surface is completely covered with alkoxy species. In the presence of water, at a total pressure of 10 MPa, a H2:acetol inlet ratio of 2 and a H2O:acetol inlet ratio of 1, the surface is covered with 66% hydroxyl species and 34% atomic oxygen. In both cases, the prediction of a completely covered catalyst surface is inconsistent with the coverage used for the calculation 10 ACS Paragon Plus Environment
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of the kinetic parameters. These physically unrealistic coverages result from the overestimated stability of the surface species in the low coverage calculations. To account for coverage effects in the microkinetic model, we used the method proposed by Jorgensen and Grönbeck42 and refined in our previous publication20. First, a sensitivity analysis is performed to determine how strongly the simulation results depend on the stability of each individual species, Figure S6. For a range of typical conditions (200 °C, 5 MPa H2, 2.5 MPa acetol and both with and without 2.5 MPa H2O) the microkinetic model is insensitive to the stability of the enol species, the alkyl species and propylene glycol. This, however, is a limitation of the applied method. After implementation of the presented coverage corrections, the microkinetic model becomes sensitive to the stability of the 1-enol species and its corresponding alkyls. However, the destabilization of these species is limited and thus not taken into account. Second, for each species to which the model is sensitive, the effect of coverage on its stability is determined. This is done by calculating the binding energy of each of those species while gradually increasing the coverage of each of the dominant species, i.e., species with a coverage above 0.1 ML. The dominant species are oxygen, hydroxyl, and both alkoxy species. For each coverage, several configurations are investigated to identify the most stable co-adsorption structure. The effect of each dominant species on each species to which the model is sensitive, is shown in Figure S7 to Figure S10. The corresponding co-adsorption structures are shown in Figure S11 to Figure S42. In total, about 1200 structures were evaluated to identify the most stable co-adsorption structures. Third, the calculated (de)stabilization corrections, i.e., the decrease or increase in Gibbs free binding energy as a function of coverage, is fitted to 2-parameter power laws, Table 3. Note that not all interactions are destabilizing. Interactions between water molecules and surface oxygen and hydroxyl species are attractive, due to the formation of hydrogen bridges. The interactions of acetol and
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ACS Catalysis
aldehyde with oxygen and hydroxyl are also slightly attractive. Destabilization of the transition states is estimated via a Brønsted-Evans-Polanyi relationship with a transfer coefficient of 0.5.20 Table 3. Intraspecies and interspecies repulsion correction functions to account for the effect of the dominant species, O, OH, and both alkoxy intermediates, on the sensitive species. (kJ mol-1)
Oxygen
Hydroxyl
2-Alkoxy
1-Alkoxy
Hydrogen
193𝜃𝑂3.44
704𝜃𝑂𝐻3.22
22𝜃2 ― 𝑎𝑙𝑘1.36
135𝜃1 ― 𝑎𝑙𝑘2.12
249𝜃𝑂𝐻2.04
812𝜃2 ― 𝑎𝑙𝑘1.96
7954𝜃1 ― 𝑎𝑙𝑘3.73
266𝜃2 ― 𝑎𝑙𝑘1.46
240𝜃1 ― 𝑎𝑙𝑘1.73
Oxygen
(
974 𝜃𝑂 ―
3.5
)
1 16
Hydroxyl
374𝜃𝑂2.49
Water
―67𝜃𝑂1.20
―183𝜃𝑂𝐻1.60
232𝜃2 ― 𝑎𝑙𝑘1.61
278𝜃1 ― 𝑎𝑙𝑘1.75
Acetol
―125𝜃𝑂1.22
―91𝜃𝑂𝐻2.21
571𝜃2 ― 𝑎𝑙𝑘1.68
154𝜃1 ― 𝑎𝑙𝑘1.07
Aldehyde
―178𝜃𝑂2.10
―152𝜃𝑂𝐻2.06
907𝜃2 ― 𝑎𝑙𝑘2.03
76𝜃1 ― 𝑎𝑙𝑘0.98
2-Alkoxy
360𝜃𝑂1.78
454𝜃𝑂𝐻2.18
1-Alkoxy
4322𝜃𝑂4.32
486𝜃𝑂𝐻2.29
(
32 𝜃𝑂𝐻 ―
1.25
)
1 16
(
796 𝜃2 ― 𝑎𝑙𝑘 ―
1
2.45
409𝜃1 ― 𝑎𝑙𝑘1.29
)
16
631𝜃2 ― 𝑎𝑙𝑘1.49
(
5384 𝜃1 ― 𝑎𝑙𝑘 ―
3.42
)
1 16
Reaction path analysis for typical hydrogenation conditions: dry conditions For a typical acetol hydrogenation conditions - a space time of 500 kgcat s molacetol-1, a H2:ketone inlet molar ratio of 2, 7.5 MPa and 200°C26 - the coverage-dependent microkinetic model predicts a turnover frequency (TOF) of 0.002 s-1, an acetol conversion of 0.3% and a 2-alkoxy coverage of 30%. Other coverages fall below 1%. The predicted TOF falls within the experimentally observed range of 10-3 – 10-1.26 Increased the conversion up to 10%, by decreasing the space-time does not affect the dominant pathways, the rate-determining steps or surface coverage. Other coverages are below 1%. Coverage correction factors are below 80 kJ mol-1 for all intermediates. Under these conditions, two pathways are competitive in the hydrogenation of acetol to propylene glycol, Figure 4. 16% of the propylene glycol is obtained by direct hydrogenation of acetol via the stable 2-alkoxy intermediate. Due to the high stability of the 2-alkoxy intermediate, the second step in this pathway is rate-limiting. Hydrogenation through the 2-alkyl intermediate is not favored because of the high barrier to reach this intermediate. The dominant acetol hydrogenation pathway, at 84%, is however via the 1-enol 12 ACS Paragon Plus Environment
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tautomer. The 1-enol tautomer is mainly hydrogenated through the 2-alkyl species, which is slightly more stable than the 1-alkyl species. In this pathway, hydrogenation of the 1-enol species is rate-limiting.
Figure 4. Reaction path analysis for the hydrogenation of acetol to propylene glycol under dry conditions. The numbers indicate the fraction of each component consumed in a particular reaction step. Dotted arrows indicate reaction steps with a fraction below 0.5% while green arrows indicate reaction steps with a fraction above 50%. Conditions: 500 kgcat s molacetol-1, 200 °C, 7.5 MPa, inlet H2:acetol:H2O molar ratio 2:1:0.
The aldehyde pathway does not contribute significantly, less than 0.5%, to the hydrogenation activity. While the hydrogenation barriers for the aldehyde pathway are similar to the barriers for the acetol pathway, (Figure 2) the 17 kJmol-1 lower stability of the aldehyde tautomer and the larger coverage correction factor for the 1-alkoxy intermediate limit the flux through this pathway. The 2-enol pathway is not shown in Figure 4 or any of the reaction path analyses below because it contributes to an extent below 1%. Again, this can be explained in terms of the limited stability of the 2-enol intermediate, Figure 2. The balance between the 1-enol and the acetol pathway is affected by the inlet molar H2:ketone ratio, Figure S43; an increase favors the 1-enol hydrogenation pathway. This can be related to the alkoxy coverage, which reduces with H2 pressure. An increase in temperature favors the acetol pathway Figure S44, which is caused by the higher activation barrier for the 13 ACS Paragon Plus Environment
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hydrogenation of the alkoxy intermediate (Table 2) and decreases the contribution of the 1-enol pathway. The effective activation energy under these conditions is 49 kJmol-1, Figure S45. Effect of water on the dominant reaction path Hydrogenation of ketones and aldehydes is often performed in the presence of water, either as part of the feed, or from water formation during reaction, e.g., by glycerol dehydration. The presence of water drastically impacts on the hydrogenation reaction mechanism, see Figure 5A, and the reaction kinetics, Figure S46. Three regimes can be identified as a function of the inlet molar H2O:acetol ratio. At low water pressures, the 1-enol pathway dominates and the acetol and aldehyde pathways are severely hampered by the high stability of the alkoxy intermediates (Figure 2A and Figure 2D).
Figure 5. Effect of the water to acetol feed flow ratio on the dominant reaction pathway for the hydrogenation of acetol (A). The contribution indicates the fraction of acetol consumed in each particular pathway. A detailed reaction path analysis of points I, II and III is shown in Figure 6, 8 and 9 respectively. The effect of the water to acetol feed flow ratio on the coverage of the most important surface species (B). Conditions: 500 kgcat s molacetol-1, 200 °C, pH2 = 5 MPa, pacetol = 2.5 MPa.
When water is introduced to the feed, see Figure 5A, proton transfer pathways open up where stable alkoxy intermediates are converted by proton transfer from adsorbed water and hydroxyl species (Figure 2). The change in dominant reaction path goes hand in hand with the change in 14 ACS Paragon Plus Environment
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surface coverage, from a 30% 2-alkoxy-covered surface, to a surface covered by a mixture of oxygen, hydroxyl and alkoxy species, Figure 5B. Both the acetol and aldehyde pathway are important in this regime, as illustrated in the reaction path analysis for a H2O:acetol inlet molar ratio of 0.4 (Figure 6). The addition of water slightly increases the TOF from 0.002 s-1 to 0.003 s-1 (Figure S47). Under these conditions, 61% is hydrogenated via the acetol-alkoxy pathway and 36% via the aldehyde/1-alkoxy pathway. The alkoxy intermediates are converted via proton transfer from surface hydroxyl species. The relative importance of the aldehyde and acetol pathway is somewhat sensitive to the coverage corrections, since the aldehyde reactant is more stabilized by the oxygen species than the acetol reactant (Table 3).
Figure 6. Reaction path analysis for the hydrogenation of acetol to propylene glycol. The numbers indicate the fraction of each component consumed in the particular reaction step. Dotted arrows indicate reaction steps with a fraction below 0.5% while green arrows indicate reaction steps with a fraction above 50%. Conditions: 500 kgcat s molacetol-1, 200 °C, 8.5 MPa, inlet H2:acetol:H2O molar ratio 2:1:0.4
In the absence of water, hydrogenation of the stable alkoxide species is rate limiting in both the acetol and aldehyde pathway, Figure 4. When water is introduced, proton transfer from surface hydroxyl species and from adsorbed water shifts the rate-limiting step to the regeneration of the hydroxyl/water species. Since proton transfer from hydroxyl species dominates (>99.5%), surface oxygen needs to be converted back to hydroxyl to close the catalytic cycle. Two 15 ACS Paragon Plus Environment
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ACS Catalysis
pathways are available for this step, Figure 7. Surface oxygen can be hydrogenated by surface hydrogen with a barrier of 118 kJmol-1 (Table 2) or converted to two hydroxyl species via reaction with adsorbed water. The latter reaction is quasi-equilibrated. To close the catalytic cycle, one of the hydroxyl species needs to be hydrogenated by surface hydrogen. The barrier for this step, 130 kJmol-1, is slightly higher than the oxygen hydrogenation barrier. The slightly lower barrier and the almost equal coverage of oxygen and hydroxyl species make direct oxygen hydrogenation the dominant regeneration pathway under these conditions. However, both reactions are rate controlling, as confirmed by the sensitivity of the TOF to both barriers.
Figure 7. Routes for the regeneration of the hydroxyl species in the hydrogenation of acetol in the presence of water. Solid arrows represent pathways with a fraction above 0.5%. Conditions: 500 kgcat s molacetol-1, 200 °C, 8.5 MPa, inlet H2:acetol:H2O molar ratio 2:1:0.4.
When the water pressure is increased to a H2O:ketone inlet molar ratio around 1, which is typical for glycerol hydrogenolysis conditions, the TOF further increases to 0.03 s-1, the conversion increases to 2% and a third kinetic regime emerges, dominated by the acetol pathway. The increase in water pressure increases the hydroxyl coverage to 30% and decreases the oxygen coverage to 5% (Figure 5B). A detailed reaction path analysis is shown in Figure 8. The increase in hydroxyl coverage shifts the mechanism further away from the 1-enol pathway. The balance between the acetol and the aldehyde pathway is sensitive to the coverage corrections. Adsorbed oxygen atoms stabilize the aldehyde species more than the acetol species, while surface hydroxyl groups stabilize the acetol species more for these coverages (Table 3). As such, as the hydroxyl coverage increases with increasing water partial pressure and the
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oxygen coverage decreases, the acetol intermediate is increasingly more stabilized compared to the aldehyde, shifting the dominant hydrogenation pathway in its favor.
Figure 8. Reaction path analysis for the hydrogenation of acetol to propylene glycol. The numbers indicate the fraction of each component consumed in the particular reaction step. Dotted arrows indicate reaction steps with a fraction below 0.5% while green arrows indicate reaction steps with a fraction above 50%. Conditions: 500 kgcat s molacetol-1, 200 °C, 100 bar, inlet H2:acetol:H2O molar ratio 2:1:1.
Hydrogenation of the alkoxy intermediates via proton transfer requires the regeneration of the hydroxyl groups. Again, two pathways are available (Figure 8). Due to the higher hydroxyl coverage, the dominant regeneration mechanism changes and the dominant path becomes hydrogenation of a hydroxyl species by surface hydrogen. While this reaction is more activated than the direct hydrogenation of surface oxygen, the increased hydroxyl coverage shifts the relative rate of these steps. 4. Conclusions A coverage-dependent first principles microkinetic model was constructed for the hydrogenation of alpha-hydroxy ketones on copper terraces. Acetol hydrogenation was used as a probe reaction because of its importance in glycerol hydrogenolysis. Eight distinct pathways were considered in the microkinetic model. Acetol can either be hydrogenated directly or undergo tautomerization to 1-enol, 2-enol or aldehyde prior to hydrogenation. Microkinetic 17 ACS Paragon Plus Environment
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simulations under typical (water-free) hydrogenation conditions show that the 1-enol hydrogenation pathway dominates (84%). The acetol and aldehyde hydrogenation pathways are hampered by the pronounced stability of the alkoxy species. Hydrogenation of the alkoxy intermediate is rate-limiting in the acetol pathway, hydrogenation of the 1-enol species in the enol pathway. When water is introduced, additional proton transfer pathways become possible for the conversion of the stable alkoxy intermediates. This dramatically changes the relative surface coverages and, hence, the kinetic regime. At low water pressures, the aldehyde, acetol and 1-enol pathways are competitive, but the aldehyde and acetol pathways gain importance with increasing water pressure. At high water pressures (in our model for H2O:acetol inlet molar ratios above 0.4), the surface is saturated with hydroxyl species, and the mechanism is dominated by the acetol pathway. Hydrogenation of the hydroxyl group by surface hydrogen closes the catalytic cycle and becomes the rate-limiting step. Corresponding Author* Mark Saeys:
[email protected] Associated content Adsorption structures, transition states, estimation of the coverage effects, the effect of process conditions and confirmation of the rate-limiting steps is provided in the supporting information. Acknowledgements The research presented in this publication received funding from the European Reasarch Council under the European Union’s Seventh Framework Program (FP7/2007-2013) / ERC grant agreement n° 615457 and from the Flemish Research Foundation (FWO) under the Odysseus program, FWO G0E5714N.
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