Research Article pubs.acs.org/acscatalysis
Understanding Selective Hydrogenation of α,β-Unsaturated Ketones to Unsaturated Alcohols on the Au25(SR)18 Cluster Runhai Ouyang and De-en Jiang* Department of Chemistry, University of California, 501 Big Springs Road, Riverside, California 92521, United States ABSTRACT: The Au25(SR)18 cluster can catalyze 100% selective hydrogenation of α,β-unsaturated ketones to unsaturated alcohols. However, the mechanism remains a mystery. Here we unravel the underlying mechanism by using first-principles density functional theory calculations with benzalacetone as a substrate. We find that the Au25(SR)18 cluster cannot directly activate either H2 or benzalacetone separately. Instead, starting with coadsorption of H2 and benzalacetone on Au25(SR)18, H2 heterolytically cleaves to the substrate and to a surface Au atom of the cluster, followed by the facile transfer of H from the Au atom to the partially hydrogenated substrate. In this mechanism, CO and CC hydrogenations have barriers of 0.99 and 1.12 eV, respectively, in agreement with the experimentally observed selectivity toward unsaturated alcohol. In addition, we show that the ethanol solvent can further stabilize the partially hydrogenated intermediate of CO hydrogenation via a hydrogen bond, leading to a smaller H2 cleavage energy (0.90 eV). Hence, the heterolytic cleavage of H2 on the Au25 nanocluster favors the more polar CO bond of benzalacetone, leading to selective formation of unsaturated alcohol. This work reveals that the weak interaction between H2 and the Au cluster, the formation of a Au hydride, and the polar solvent are responsible for the high selectivity of the α,βunsaturated ketone hydrogenation to the corresponding unsaturated alcohol over the Au25 nanocluster. KEYWORDS: gold nanocluster, density functional theory, benzalacetone, solvent effects, hydrogenation
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INTRODUCTION Selective hydrogenation of α,β-unsaturated carbonyl compounds to the corresponding unsaturated alcohols is an important reaction in the production of fine chemicals and pharmaceuticals.1 This reaction is challenging especially for ketones because the hydrogenation of the CC bond is usually both thermodynamically and kinetically favored over the hydrogenation of the CO bond.2,3 Although conventional heterogeneous metal catalysts such as Pd, Pt, and Ru can readily catalyze the selective hydrogenation of the CO bond of α,β-unsaturated aldehydes to unsaturated alcohols with high activity and selectivity, selective hydrogenation of α,βunsaturated ketones to unsaturated alcohols is difficult due to the extra steric hindrance,4 and the final products are always saturated ketones.5,6 In contrast, nanoscale Au catalysts have been shown to be promising in the selective hydrogenation of α,β-unsaturated ketones to unsaturated alcohols.7−10 Milone et al. observed the first example of selective hydrogenation of unconstrained α,βunsaturated ketones to unsaturated alcohols by molecular hydrogen using Au supported on Fe2O3 as a catalyst in ethanol solvent; they obtained a selectivity higher than 60%.11 Further, they found that the activity and selectivity were greatly influenced by the reactant structure and the nature of support but were less affected by the size and morphology of the Au particle.12−14 For example, on the Au/Fe2O3 catalyst, the © XXXX American Chemical Society
hydrogenation selectivity to unsaturated alcohols was higher than 60% for benzalacetone and 4-methyl-3-penten-2-one but only 15% for the 3-penten-2-one.12 In addition, Ide et al. conducted experiments of selective hydrogenation of α,βunsaturated ketones and aldehydes over various supported metal catalysts.6 Rojas et al. studied selective hydrogenation of benzalacetone over Au/SiO2 and Ir/SiO2 and found a selectivity of 46% to unsaturated alcohol on Au but a very low selectivity on Ir.15 Several recent theoretical studies looked into the mechanism of hydrogenation for α,β-unsaturated carbonyl compounds (especially, aldehydes such as acrolein) on metal surfaces. On the surfaces of traditional hydrogenation catalysts such as Ni,16 Pd,17,18 and Pt,19−21 the α,β-unsaturated carbonyl compounds can become activated via strong chemical interaction of the CC and/or CO group with the metal surface. Dissociation of an H2 molecule on these metals is facile; adsorbed atomic H atoms then participate in the subsequent hydrogenation reaction. Hydrogenation of the CC bond was found to be kinetically favored in comparison to that of the CO group, and the saturated carbonyls were the dominant products.16,19 In contrast to these metals, adsorptions of both the H2 molecule Received: July 18, 2015 Revised: September 27, 2015
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the calculations that involved radicals. The barriers were obtained using the climbing image elastic band (CI-NEB) method.42 van der Waals interactions were included using the DFT-D3 method of Grimme.43,44 Methyl was used for the R of the Au25(SR)18 catalyst for the purpose of simplifying the calculations. Atomic charges of the Au25(SR)18 catalyst were analyzed by Bader’s atom-in-molecules method.
and the substrate on the extended Au surfaces were found to be weak.19,22 The activation energy of H2 is usually too high to take place on Au surfaces, except at the low-coordinated site.23,24 Several studies have shown that the weak interaction of reactants with the Au surface plays a key role in determining the selectivity difference.8,22 How this selectivity changes for nanometer and subnanometer gold nanoclusters would be an interesting question to answer. A recent breakthrough in the selective hydrogenation of α,βunsaturated ketones to unsaturated alcohols was made by Jin and workers.25,26 They studied the selective hydrogenation of benzalacetone to unsaturated alcohol over atomically precise thiolate-protected Au nanoclusters such as Au25(SR)18 (−SR is a thiolate group).25 Surprisingly, all the gold nanocluster catalysts that they tested showed 100% selectivity toward unsaturated alcohols. Moreover, these atomically precise Au catalysts maintained their identity after the reaction.25,27−29 Unfortunately, the underlying mechanism responsible for the 100% selectivity toward unsaturated alcohol remains elusive. These nanoclusters can also catalyze many other reactions with or without a support.30−32 One prominent feature of the Aun(SR)m catalyst for the selective hydrogenation reaction as observed from experiments is that, though the conversion depends on the specific ligand length and type, cluster size, charge state, and support, the selectivity are surprisingly insensitive to these factors.25,28,29 The geometric and electronic properties of the Au25(SR)18 cluster have been extensively studied. The Au25(SR)18 cluster is comprised of an icosahedral Au13 core protected by six RS− Au−SR−Au−SR staple motifs and has the discrete moleculelike electronic structure.33 This magic cluster has been found to be very stable,34,35 and its electronic structure can be well understood according to the concept of superatom complexes.34 The delocalized electrons primarily populate the icosahedral Au core, whereas the exterior Au atoms of the staple motifs have a positive charge due to the electron transfer to the S atoms of the ligands.36−38 In addition, the Au25(SR)18 structure has two facets, where three Au atoms from the staple motifs are well exposed for reactant access.26,28,31,33 Whether or not H2 activation and selective CO binding on the Au25(SR)18 cluster is responsible for its 100% selectivity toward the unsaturated alcohol is an interesting question to answer. The well-defined compositions and structures of the atomically precise gold nanoclusters provide a valuable platform to reveal the underlying mechanism. To that end, herein we use density functional theory (DFT) calculations to gain atomiclevel insights into the mechanism of the selective hydrogenation over the atomically precise Au25(SR)18 catalyst.
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RESULTS AND DISCUSSION Our first hypothesis was that direct H2 activation is the ratelimiting step. To test this hypothesis, we examined adsorption and activation of H2 on the Au25(SR)18 cluster. Direct H2 Activation on the Au25(SR)18 Cluster. The Au25(SR)18 cluster surface has two open facets, as shown in Figure 1, where three surface gold atoms in three separate
Figure 1. Side view (a) and top view (b) of the open facets of the Au25(SCH3)18 cluster. Color code: yellow, core Au; green, surface Au; magenta, S; gray, C. H atoms are not shown. Dashed red triangles indicate the open facets.
staple motifs are well exposed with little steric hindrance for reactants to access. The open facets have been proposed as active sites.26,28,31,33 We examined the adsorption of a single H atom, an H2 molecule, and the two dissociated H atoms on the open facet of the Au25(SR)18 cluster, in order to understand the activation of H2 on the cluster. We found that the adsorption of the H2 molecule is very weak with an adsorption energy of −0.19 eV, with respect to the H2 molecule in gas phase, and is insensitive to the adsorption site. Figure 2a shows an adsorption geometry of H2 at the open facet. We found that dissociative adsorption of H2 is thermodynamically uphill by
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METHODS Density functional theory calculations were performed using the Vienna ab initio simulation package (VASP).39 The core− valence electron interaction was described by projector augmented wave (PAW),40 and the wave function was expanded by plane waves with a cutoff energy of 400 eV. The Perdew−Burke−Ernzerhof form of the generalized gradient approximation was used for electron exchange and correlation.41 Geometry relaxations were carried out using the conjugate-gradient algorithm with a criterion that all the residual force components on each atom are less than 0.03 eV/ Å. A box size of 30 × 30 × 30 Å was employed to minimize interaction between periodic images; Γ point only was used to sample the k space. Spin polarization was turned on for all of
Figure 2. Minimum-energy path of H2 (blue spheres) dissociation on a surface Au atom of the Au25(SR)18 nanocluster: (a) initial state (IS); (b) transition state (TS); (c) final state (FS). 6625
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ACS Catalysis 0.89 eV with respect to the gas-phase H2: the two dissociated H atoms are bound next to a common staple Au atom (Figure 2c). To elucidate if H2 activation is likely by the Au25(SR)18 cluster, we obtained the minimum-energy path for the H2 activation by a surface gold atom. The transition-state structure is shown in Figure 2b; we found that the barrier is as high as 2.14 eV. This high activation barrier indicates that direct activation of the H2 molecule on the Au25(SR)18 cluster is unlikely, given the experimental reaction temperature at 0 °C.25 Thus, we need to seek alternative hypotheses for the selective hydrogenation of benzalacetone to unsaturated alcohol over the Au25(SR)18 cluster. Hydrogenation of Benzalacetone with Heterolytically Cleaved H2. Our second hypothesis was that the substrate (benzalacetone) helps H2 activation by directly receiving a H atom. To test this hypothesis, we first examined the adsorption of the substrate. The optimized structure and adsorption energy of benzalacetone on the Au25(SR)18 cluster are shown in Figure 3. Both the flat and vertical adsorptions are mainly via van der
Figure 4. Hydrogenation routes with the H atom adding to different positions of the benzalacetone. m1−m4 represent addition to the β carbon, α carbon, the carbonyl carbon, and the oxygen atom, respectively. The values above the arrows are activation energies (eV). Abbreviations: SK, saturated ketone; UA, unsaturated alcohol.
Figure 3. Adsorption structure and energy of benzalacetone on the Au25(SR)18 catalyst: (a) flat adsorption; (b) vertical adsorption.
Waals interactions. Flat adsorption (Figure 3a) is favored in comparison to the vertical adsorption (Figure 3b) due to the larger interaction area of the flat configuration with the nanocluster. Thus, the phenyl ring on the substrate directs the adsorption mode of the benzalacetone in a flat-lying fashion that facilitates the subsequent hydrogenation reaction. We next examined heterolytic cleavage of H2 to Au and benzalacetone. Before the cleavage takes place, H2 and benzalacetone coadsorb on the Au25(SR)18 catalyst. Then the H 2 molecule dissociates: one H atom goes to the benzalacetone, and another H atom goes to a surface gold atom. The cleaved H atom can add to benzalacetone’s β carbon, α carbon, the carbonyl carbon, or the carbonyl oxygen atom, leading to four different routes, designated as m1−m4, respectively, in Figure 4. We mapped out the minimum-energy paths for the four routes: the activation barriers are shown in Figure 4, while the transition-state structures are shown in Figure 5. Below we discuss the four routes in detail. We first examine the CC bond hydrogenation. In the m1 route, one H atom is added to the β carbon of the benzalacetone (Figure 5a), forming a radical with a transition state of 1.12 eV. This partially hydrogenated radical is, however, not stable, and draws up the H atom adsorbed on gold with only 0.01 eV to form the saturated ketone. In the m2 mechanism, the cleaved H atom is added to the α carbon (Figure 5b), forming a radical slightly more stable (−0.1 eV) than the m1 counterpart but with a similar activation energy of 1.21 eV. The subsequent addition of the H adsorbed on gold to the radical leading to the saturated ketone is facile with a barrier
Figure 5. Transition-state structures corresponding to routes (a) m1, (b) m2, (c) m3, and (d) m4. H atoms cleaved from the H2 are shown as blue spheres.
of 0.05 eV. Therefore, the m1 and m2 mechanisms of hydrogenation of the CC double bond have similar probabilities. We next look at the hydrogenation of the CO bond. In the m3 mechanism, we considered the route that the cleaved H atom is added to the carbonyl carbon (Figure 5c). We found this route highly unfavorable: the resulting radical is very unstable and the added H in fact recombines spontaneously with the other H adsorbed on gold, forming back to the initial state (H2 and benzalacetone), which is much lower in energy than the final state (2.48 eV). In the m4 mechanism, the cleaved H atom adds to the carbonyl oxygen atom of benzalacetone (Figure 5d) and the resulting radical is stabilized via conjugation with the phenylethenyl group (0.19 eV more stable than the transition state of m1). The barrier of the H2 heterolytic cleavage was calculated to be 0.99 eV, and the subsequent transfer of the H adsorbed on gold to the partially hydrogenated radical forming the unsaturated alcohol product is facile with a small barrier of 0.10 eV. Hence, the m4 route via an attack on the carbonyl oxygen atom is preferred when the CO group is hydrogenated. 6626
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Next we examined how ethanol may assist the hydrogenation of the CO bond via a hydrogen bond. We considered both indirect (ethanol assisted, Figure 7) and direct (ethanol
Comparing CC and CO hydrogenations, we found that the hydrogenation of the CO bond has a lower barrier (0.99 eV via the m4 path) in comparison to that of the CC bond (1.12 eV via the m1 path). This finding is consistent with the selective hydrogenation of the CO bond observed experimentally on the Au25(SR)18 catalyst.25 In our modeling, we simplified the experimental R group C2H4Ph to CH3. Since the m4 route is the key step for hydrogenation of benzalacetone on Au25(SR)18, we tested the ligand effect on this mechanism and found that the reaction barrier changes only by 0.01 eV from R = CH3 to R = C2H4Ph on the Au25(SR)18 catalyst, indicating that our simplification of C2H4Ph to CH3 is reasonable and that access to the open facets of the cluster (Figure 1) is not hindered by the R groups. Why is the CO bond rather than the CC bond preferred in activating H2, and what is the role of the Au atom? To answer these questions, we analyzed the atomic charges on the Au25 cluster and the interaction in the coadsorption state. We found that the surface gold atoms have an average charge of +0.134 |e| and that the positively charged Au atom and the polar CO bond together polarize the H2 bond by forming a complex: Auδ+···Hδ‑−Hδ+···Oδ‑Cδ+, leading to heterolytic cleavage of H2. The CC bond is of course much less polar in benzalacetone and hence has a higher activation energy for H2 cleavage. Influence of the Ethanol Solvent. It has been reported previously that protic solvents such as ethanol and water had significant effects on the activity and selectivity of the gold nanocluster catalysts.8,45−49 In fact, the heterolytic cleavage of H2 to a Au complex and an ethanol solvent molecule, forming Au hydride and EtOH2+, have been reported.45,47 In Jin et al.’s experiment of benzalacetone hydrogenation on the Au25(SR)18 catalyst, the reactions were performed in a solvent mixture of ethanol and toluene.25 Therefore, our hypothesis was that the protic solvent can further facilitate hydrogenation of benzalacetone on the Au25(SR)18 catalyst. To test this hypothesis, we first computed the interactions of ethanol with Au25(SR)18 and benzalacetone: the optimized structure and interaction energy are shown in Figure 6. We found that the
Figure 7. Ethanol-assisted H2 cleavage via the m4 route: the initial (a), transition (b), and intermediate (c) states.
Figure 8. Ethanol-mediated H2 cleavage via the m4 route: the initial (a), transition (b), and intermediate (c) states.
mediated, Figure 8) pathways. We tested both one and two molecules of ethanol next to the active site and found that they have similar activation energies. Figure 7a shows two ethanol molecules adsorbed on the Au cluster and close to benzalacetone, with the first ethanol forming a hydrogen bond with S of the Au25(SR)18 (consistent with the −0.164 |e| charge on the S atom) and the second ethanol hydrogenbonded to the first one. From this configuration H2 cleavage has a barrier of 0.90 eV: Figure 7b,c shows the structure and the relative energy of the transition state and the partially hydrogenated intermediate, respectively. The ethanol molecules help stabilize the intermediate and lower the barrier of H2 cleavage slightly. The subsequent addition of the second H from the surface gold atom to the substrate radical forming the unsaturated alcohol also has a small barrier of 0.18 eV relative to the intermediate state, leading to the stable final state of unsaturated alcohol, which is further stabilized by the polar protic solvent. We also explored the effect of solvent on the m1 route of the CC hydrogenation and found no change in the activation energy. In the ethanol-mediated process, the substrate (benzalacetone) is hydrogen-bonded via the CO bond to one ethanol molecule (Et1) which is hydrogen-bonded to another ethanol (Et2), as shown in Figure 8a. The H2 molecule is then heterolytically cleaved: one H goes to a surface Au atom, and the other goes to Et2 to form [Et2-H]+, which then transfers its hydrogen-bonded H to Et1; next, Et1 transfers its hydrogenbonded H to the substrate. This proton-relay process leading to partial hydrogenation of the CO bond has an activation energy of 0.94 eV (Figure 8b). The partially hydrogenated substrate (Figure 8c) can then diffuse to the surface of the Au25 cluster to receive the second H atom, which is a facile process.
Figure 6. Optimized structure of ethanol solvent interactions with (a) Au25(SR)18 and (b) benzalacetone. Dashed lines indicate the hydrogen bonds. Interaction energies are also given.
ethanol molecule could interact with the Au25(SR)18 catalyst via a hydrogen bond between the hydroxyl of the ethanol and the S atom of the catalyst, with an interaction energy of −0.36 eV (Figure 6a). The ethanol could interact with the benzalacetone substrate via a hydrogen bond between the hydroxyl of the ethanol and the O of the benzalacetone, with an interaction energy of −0.39 eV (Figure 6b). 6627
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ACS Catalysis Overall Reaction Profiles and the Catalytic Cycle. We compare the energy profiles of the likely pathways of CO vs CC hydrogenation in Figure 9. Without solvent assist, the
Figure 10. Catalytic cycle of the selective hydrogenation of the benzalacetone to the unsaturated alcohol over the Au25(SR)18 catalysts. The Au25(SR)18 is represented by the RS−Au−SR−Au−SR motif of the open facets, in which the Au atom serves as the catalyst activation center.
Figure 9. Energy profiles of selective hydrogenation of benzalacetone to unsaturated alcohol (UA) and saturated ketone (SK) via solventfree and solvent-assisted routes. The values in this figure indicate the activation energies of the corresponding transition states.
m4 route of CO hydrogenation to unsaturated alcohol (UA) has a barrier of 0.99 eV; with ethanol’s help as in Figure 7’s mechanism, this barrier is further lowered to 0.90 eV. The C C hydrogenation to saturated ketone (SK) is 1.12 eV via the m1 route. If we use the Arrhenius equation and assume a similar pre-exponential factor, we estimate that the selectivity of UA/SK is about 99.5% at 273 K without the solvent assist. With the solvent assist, the selectivity is further closer to 100%. In addition, with a low barrier of 0.90 eV, the reaction should take place at 273 K at a reasonable rate. This result agrees very well with the experimental fact that the selective hydrogenation of benzalacetone to unsaturated alcohol can take place with 100% selectivity in the ethanol solvent even at a low temperature such as 273 K. The catalytic cycle of the solvent-assisted selective hydrogenation is shown in Figure 10. Before the introduction of H2, the benzalacetone and solvent molecules physically adsorb on the Au25(SR)18 catalyst. Then heterolytic cleavage of the added H2 takes place between the surface Au atom of the open facet of the Au25(SR)18 and the O atom of the benzalacetone substrate. This process is facilitated by the assistance of the adjacent ethanol solvent molecule via hydrogen bonding between the ethanol and the −OH group of the partially hydrogenated benzalacetone, with a barrier of 0.90 eV. The second H adsorbed on the Au atom then is transferred to the partially hydrogenated radical and forms the unsaturated alcohol. The catalytic cycle then restarts after desorption of the unsaturated alcohol and adsorption of a new benzalacetone molecule. The solvent-assisted mechanism of hydrogenation on the Au25(SR)18 catalyst is very different from that on the conventional heterogeneous metal catalysts reported.8,48−51 On those metal catalysts, the dissociation of the H2 molecule is facile and atomic hydrogen atoms are already formed on the metal surface before hydrogenation of the unsaturated carbonyl compounds chemically adsorbed on the metal catalyst surface takes place. The solvent-assisted mechanism in the present work is different in that both the H2 and α,β-unsaturated
ketone reactants are not preactivated but the reaction can take place via a synergistic pathway involving H2, the substrate, and the Au25(SR)18 catalyst.
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CONCLUSIONS The mechanism of selective hydrogenation of benzalacetone to unsaturated alcohol over the atomically precise Au25(SR)18 catalyst has been revealed using first-principles density functional calculations. H2 was found to be heterolytically cleaved to the carbonyl O atom of the substrate and to a surface Au atom of the cluster; the intermediate of the CO hydrogenation is stabilized via a hydrogen bond between the hydroxyl group of the partially hydrogenated substrate and solvent molecules. The overall activation energy of the solventassisted pathway is 0.90 eV, lower than the solvent-free pathways of CO hydrogenation (0.99 eV) or CC hydrogenation (1.12 eV). This computational finding explains the experimentally observed 100% selectivity for CO hydrogenation of benzalacetone over the Au25 nanocluster. This work shows that the interaction between the catalyst and the reactant, the formation of the gold hydride, and the polar solvent are responsible for the high selectivity of the selective hydrogenation of α,β-unsaturated ketones to unsaturated alcohols on the Au25(SR)18 cluster.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for D.J.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the University of California, Riverside. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract DE-AC0205CH11231. 6628
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