Phenyl Ring Transfer Mechanism of Styrene Selective Oxidation to

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

A Phenyl Ring Transfer Mechanism of Styrene Selective Oxidation to Phenyl Acetaldehyde on Gold Catalysts from Density Functional Theoretical Studies Xiaomei Zhao, Pu Wang, Zhongyun Ma, and Yong Pei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08790 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on January 4, 2019

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The Journal of Physical Chemistry

A Phenyl Ring Transfer Mechanism of Styrene Selective Oxidation to Phenyl Acetaldehyde on Gold Catalysts from Density Functional Theory (DFT) Studies Xiaomei Zhaoǁ, Pu Wangǁ, Zhongyun Ma* and Yong Pei* Department of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applicationics of Ministry of Education, Key Laboratory for Green Organic Synthesis and Application of Hunan Province, Xiangtan University, Hunan Province, Xiangtan 411105, People’s Republic of China

Abstract Olefin selective oxidation is one of the most important reactions in the modern chemical industry. In this work, we systematically studied the reaction mechanism of styrene selective oxidation to benzaldehyde, styrene epoxide and phenyl acetaldehyde on various model O-atom pre-adsorbed gas-phase gold clusters AuNq (N = 16, 28, 34, 55; q = 0, ±1) and flat Au(111) surface by means of density functional theory (DFT) calculations. We proposed a new reaction channel, namely, the styrene first interacts with the oxygen−adsorbed gold catalysts to form a five-membered ring known as oxametallacycle intermediate (OMME), then the phenyl group linked to the β-C atom transfers to the α-C atom and forming the phenyl acetaldehyde. This new mechanism is different from the hydrogen atom transfer mechanism proposed previously. The DFT calculation results showed the new reaction pathway for the formation of phenyl acetaldehyde is energetically feasible over the gold clusters as well as the Au(111) surface, and exhibits high selectivity on the positively charged and neutral gold clusters, while on the anionic gold clusters it has no outstanding selectivity. This work provided new mechanistic insights into the styrene selective oxidation and the effect of charge state of gold clusters.

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1. Introduction As one of the vital important industrial reactions, the olefin selective oxidation is pivotal in a wide range of chemistry industrial applications. The styrene epoxide products are versatile intermediates in the organic synthesis of fine chemicals and pharmaceuticals. At present, the main catalysts used are nanostructures composed of noble metals such as silver, platinum and palladium or alloy nanostructures of these metals.1–3 Ever since Nakatsuji4 reported that the reducible metal-oxide-supported Au nanoclusters showed high activity for olefin oxidation at low temperature, the selectively catalytic oxidation reaction of olefin on gold nanocatalysts have received great attentions. Many researchers have found that the size and support of subnanometer gold clusters can significantly affect the catalytic activities of nanogold catalysts5–13. In order to understand the origin of catalytic performance more clearly, a large number of experimenters were devoted to the study of the catalytic activity of gold nanoclusters with different sizes and supports. Haruta et al.13 showed that the size and shape of gold nanoclusters and the properties of substrate support significantly affect the catalytic activity of gold nanoparticles (NPs) in propylene oxidations. For example, for Au/TiO2, only hemi-spherical Au NPs with size of 2.0~5.0 nm were active for propylene oxide synthesis, whereas spherical Au NPs larger than 5.0 nm in diameter favored the complete combustion of propylene to CO2, and Au clusters smaller than 2.0 nm led to propylene hydrogenation. The studies suggested that the hemispherical particles with diameters smaller than 4.0 nm produce propylene oxide with selectivity higher than 90% at low temperature. Turner et al.14 proved that the size of gold clusters is one of the important factors affecting its activity and catalytic selectivity. They studied the selective catalytic oxidation of Au55/BN or Au55/SiO2 (1.5~1.6 nm) system or Au/SiO2( > 3.0 nm), and found that gold nanoparticles with a mean size of 2 nm had a certain activity for styrene oxidation, while the gold nanoclusters with size larger than ~3.0 nm are completely inactive to styrene oxidation using the dioxygen as the sole oxidation. The results displayed that the conversion of styrene is about 20%, the main product of oxidation is aldehyde (82%), and the selectivity of epoxide is very low (14%) when gold nanoparticles supported on inert materials (such as BN and SiO2). Patil et al. and Li et al.15−19 studied that the catalytic activity of the gold clusters and the selectivity of the epoxides from styrene were improved when gold


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nanoclusters supported on active materials, such as alkaline earth oxides (Al2O3, Ga2O3, In2O3 and Tl2O3), alkaline metal oxides (MgO, CaO), transition metal oxides (TiO2, Cr2O3, MnO2, Fe2O3, Co3O4, NiO, CuO, ZnO, Y2O3, ZrO2, La2O3 and U3O8). However, Wang et al.20 reported that gold nanogold clusters with sizes of 20 nm to 150 nm also had a high catalytic activity for the oxidation of styrene. It was found that Au+ /PN exhibited staggering catalytic activity for the oxidation of styrene when positively charged Au+ adsorbed on the PN support. They believed that there was a certain charge transfer between the gold clusters and the supports, make the gold clusters be more positively charged, and thus could effectively promote the dissociation of oxygen molecules. In addition, there are some researchers reported that the effect of hydrogen and water on activity and selectivity.21–24 Wang et al.21 proposed that the addition of H2O to the reaction solvent was found to drastically affect the styrene product selectivity. Their calculations showed that a high selectivity of 90% to benzaldehyde was achieved in an apolar solvent, and a selectivity of 95% to styrene epoxide was attained in a polar solvent. They explained that the reason is the strong electronic perturbation of Au-catalyst by the N-doped carbon support interaction and it controlled the reaction selectivity in styrene oxidation. Density functional theory (DFT) calculations have been widely used to investigate the reaction mechanism of styrene oxidation on gold catalysts25–33. Xue et al.25 studied the reaction mechanism for the selective oxidation on an atomic oxygen covering Au (111) surface by the DFT calculations. The process of reaction includes two steps: forming the oxametallacycle intermediate (OMME, i.e., oxygen-metal-metal-ethyl) and then producing the products. It was found that styrene can be easily epoxidized by O atoms, and the OMME plays a key role in the reaction, where OMME is a ring structures including a –C–C–O– framework and two Au atoms. The calculated energy barriers together with the analysis of reaction rates of different reaction channels indicated that the formation of the epoxide product is favorable over aldehyde, in good agreement with previous experimental observations. Gao et al.32 studied the mechanism of styrene epoxidation on Au38 and Au55 clusters stimulated by the experimental discovery of shape size threshold effects of catalytic activity of gold nanoclusters toward styrene oxidation. They have found the key step of styrene oxidation was not the direct dissociation of the O2 molecule. Instead, the O2 was



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activated via the formation of an OMME on the gold clusters. After that, the O–O bond scission led to the formation of styrene epoxide. A similar mechanism was also proposed by Gao et al. using the Au34Cu12 as a model catalyst.33 Numerous experiments and theoretical studies have showed that the styrene oxidation over gold catalysts may produce several products, including benzaldehyde, styrene epoxide, phenyl acetaldehyde, and some secondary oxidation products, such as acetophenone, benzoic and benzeneacetic acid, and some combustion products. For the formation of benzaldehyde and epoxide, the reaction mechanisms have been extensively studied. Xue et al.25 proposed the mechanisms of benzaldehyde and epoxide formation, which can be expressed as Path 1a, 1b, 2a and 2b as shown in Scheme 1. We considered there are two kinds of C atoms in styrene, one is α-C, which is attached to the phenyl group, and the adjacent one is β-C. The Path 1a is the β-C of styrene attack O atom to form a ring oxametallacycle intermediate of Au–O–C(β)–C–Au (named as the OMME1), then another O atom transfer near

and

attack

the

α-C

atom

to

form

the

oxametallacycle

intermediate

of

Au–O–C(α)–C(β)–O–Au (named as the OMME3) after the formation of the ring OMME1, finally the scission of the C–C bond lead to the formation of benzaldehyde. The Path 1b is the cyclization reaction of α-C atom and O atom after the formation of the ring OMME1, and leads to the formation of styrene epoxide. The Path 2a is the α-C of styrene attack O atom to form a ring oxametallacycle intermediate of Au–O–C(α)–C–Au (named as the OMME2), then form the same intermediate OMME3 by another O atom transfer near and attack the β-C atom, final the scission of the C–C bond leads to the formation of benzaldehyde. The Path 2b is the formation of styrene epoxide by the cyclization reaction of β-C atom and O atom of OMME2. As for the phenyl acetaldehyde product, the well-accepted mechanism reported previously, the mechanism of styrene selective oxidation to phenyl acetaldehyde on gold catalysts was proposed by Friend et al.26 and Wang et al.20 it can be defined as Path 1c. The Path 1c is the styrene firstly interacts with the oxygen-adsorbed gold clusters to form the intermediate OMME1, and then the formation of phenyl acetaldehyde through the migration of hydrogen atom from the β-C atom to the α-C after formation of OMME1. Path 2a and 2b lead to the formation of benzaldehyde and epoxide and the two reaction pathways have been


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clearly studied. In this work, we proposed a new mechanism to understand the phenyl acetaldehyde formation, as the Path 2c illustrated in Scheme 1, which is the styrene first interacts with the oxygen-adsorbed gold catalysts to form the intermediate OMME2, then the phenyl group of the α-C atom transfers to the β-C atom and form the phenyl acetaldehyde. It is known that the dissociation of O2 is difficult on Au catalysts and the O2 dissociation is considered to be a rate-controlling step under several conditions.29,34-42 For the smaller sized clusters, an even-odd size effect had been found for O2 activation over AuN– clusters with N is less than 20.36-38 DFT calculations carried out by Roldán et al. indicated a critical size of feasible O2 dissociation over cuboctahedral Au38 cluster.34,35 Although the direct O2 activation and dissociation is relatively hard on gold catalysts, elementary steps important in olefin oxidation can be studied by depositing atomic oxygen on the Au surface using other means. For instance, the pre-oxidized metal surfaces or nanoparticles have been widely used as model catalysts to offer insight into elementary oxidation processes, including CO and olefin oxidation.26, 43-45 Deng et al.26 investigated the selective oxidation mechanism of styrene on an atomic oxygen atom covered Au(111) surface. They also pointed out that extended Au surfaces are capable of promoting selective hydrocarbon oxidation once the oxygen atoms are seeded on the surface.44 On the theoretical aspect, Lin et al.23 investigated the styrene oxidation with O2 as the sole oxidant on different sized gold clusters and studied a different reaction mechanism for styrene-selective oxidation on these nanoclusters in the presence of a H2 and O2 mixture. They found the presence of H2 significantly facilitates the O2 activation and the atomic O atoms were released from the interactions of H2 and O2 on gold clusters, which explained the promotion oxidation of olefin on gold nanoclusters in O2/H2 mixture. Chen et al. studied the ethylene epoxidation on O atoms-precovered Au nanoparticle and Au(111) surface.45 The Au nanoparticle was found to be intrinsically much more selective for ethylene epoxidation than the Au(111) surface. Recently, Staykov et al. studied aerobic oxidation of alkanes on O-atom pre-covered icosahedral gold nanoparticle Au55 (Au55-O).46 On the basis of these aforementioned observations, we chose O atom pre-covered Au clusters and Au(111) as our research models. We have systematically studied the reaction pathways of styrene selective oxidation to benzaldehyde, styrene epoxide, and phenyl acetaldehyde over



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different sized and charged model gold catalysts AuNq (N = 16, 28, 34, 55; q = 0, ±1) clusters, as well as model Au(111) surface by means of the density functional theory (DFT) computations. The calculated results showed that the newly proposed reaction channel (Path 2c) for the phenyl acetaldehyde formation is energetically feasible on the different sized and charged gold clusters, as well as the bulk Au(111) surface. In addition, the new channel exhibited high selectivity on the positively charged and neutral gold clusters by comparing the energy barriers of TS2-Path 2 over different charged gold clusters, while it is not dominant over negatively charged gold clusters. Scheme1. The possible mechanisms of styrene selective oxidation on atomic oxygen covered gold catalyst.

2. Computational Method and Details In this work, the gold nanocluster models used are the most stable structures determined by the joint photoelectron spectroscopy experiment (PES) measurement and density functional theory (DFT) calculations.47–49 The geometric structures of intermediates and transition

states

were

optimized

using

either

a


restricted

or

unrestricted

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Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional implemented in Dmol3 6.1.50-53 The DFT-based relativistic semi-core pseudopotential (DSPP) and double numerical plus d-functions (DND) basis sets were used to calculate the energy of structural optimization and transition state search. The reaction pathway of the styrene oxidation was calculated using the combination of a linear synchronous transit (LST) and quadratic synchronous transit (QST) algorithm with conjugated gradient optimization method. The convergence criterion of the geometric optimization was set to be 1.0×10–5 hartree for energy change, 2.0×10–3 hartree/Å for the gradient, and 3.0×10–3 Å for the displacement, respectively. The smearing parameter was set to be 0.001 Hartree in geometric optimization. It should be noted that the calculation of the study did not consider the spin flip effect. We also re-optimized all reactants, intermediate, transition and products included in various reaction paths using the DFT(PBE)-D method. The dispersion energy is expressed within the method of Tkatchenko and Scheffler (TS),54 which is formally identical to that of DFT-D2 method. After applying the dispersion energy corrections, an obvious increase of the styrene adsorption energy is seen on various AuN clusters. However, the overall trend of energy barrier heights and the major conclusions of the relatively higher selectivity of styrene transfer pathway are consistent with the PBE results. As shown in Table S4 in the Supporting Information, we also displayed the O2 dissociation adsorption energy of oxygen atoms on differently charged and sized gold clusters calculated by two methods. The dissociation adsorption energies are in range of –0.57 and 0.72 eV (by DFT calculations) and –0.52 and 0.67 eV (by DFT-D) calculations. The results indicate that the O2 dissociation adsorption energies are sensitive to the cluster size. In the present cluster models, when the gold cluster size is beyond Au34, the O2 dissociation adsorption becomes unfavorable in comparison to the molecular O2 adsorption. This critical cluster size is consistent with previous theoretical studies.32,34,35 The model of Au(111) surface was chosen as the unit cell of p(5×5×4) with the corresponding coverage of 0.04 monolayer (ML) O atoms. The periodic slabs consist of four metallic layers. The thickness of vacuum layer is 30 Å and the bottom two Au-atom layers are fixed at the optimized positions. The real-space global cutoff radius was set to be 4.5 Å. The structure optimizations and transition-state search were carried out with a 3×1×1 k-point grid.



The calculation parameters are the same as that for calculating gold nanocluster structure, except that the convergence criterion for the displacement was set to 3.0×10–3 Å. 3. Results and discussion To illustrate the mechanism of styrene selective oxidation on Au catalysts, we first discussed the adsorption of two-oxygen atoms and styrene on gold nanoclusters AuNq (N = 16, 28, 34, 55; q = 0, ±1), and followed by the reaction pathways of styrene oxidation. Then, we verified the newly discovered mechanism by calculating the catalytic oxidation of styrene on the Au (111) surface with the pre-covered O atoms.

3.1 Adsorption of Styrene on Gold Nanoclusters On the basis of aforementioned discussions, we neglected the dissociation process of molecular oxygen on gold nanoclusters, and pre-adsorbed directly two oxygen atoms on the AuNq (N = 16, 28, 34, 55; q = 0, ±1) clusters, then adsorbed the styrene in the adjacent sites. The adsorption energies of styrene are displayed in Table 1 (Table S1 in the Supporting Information summarized the adsorption energies of styrene on various gold clusters calculated by DFT-D method). From Table 1, the adsorption energies of styrene vary from –0.45 eV to –1.27 eV. It can be found that the adsorption energies of the styrene molecule on AuN+ clusters are much larger than that on AuN and AuN– clusters, which are in range of –0.86 to –1.27eV. This indicated that the adsorption of styrene on AuN+ clusters is relatively stronger. The larger binding energies of styrene on the positively charged AuN+ clusters can be understood from the σ-π bonding between the C=C bond and Au atom. It is well known that the coordination between C=C bond and transition metal atoms via the formation of σ-donation and π back-donation bond. For the adsorption styrene on gold nanoclusters, the Au-6s orbital interacts with the occupied π orbital of C=C bond to form an σ-donation bond, by which the charge is transferred from C=C bond to Au atom. At the same time, the occupied Au-5d orbital (such as dxy) interacts with the anti-bonding π orbital to form π back-donation bond, and the gold atom back-donates the electrons to the anti-bonding π orbital of C=C bond. As the positively charged clusters process stronger electronic affinity than the neutral and negatively charged clusters, which draw more electrons from styrene and


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to form stronger σ-donation bond than that on neutral and negatively charged clusters while less is back-donated to the C=C anti-π orbital, which leads to stronger σ-donation bonding between C=C and positively charged cluster and hence larger binding energy.

Table 1. Adsorption energies of styrene on different charged and sized gold clusters calculated at the level of PBE/DND method. All the energies are in unit of eV. The PBE-D adsorption energies are displayed in the Table S1 in Supporting Information. Styrene Adsorption Energy N AuN+

AuN

AuN–

16

–1.27

–0.83

–0.45

28

–1.25

–0.88

–0.59

34

–1.21

–0.83

–0.58

55

–0.86

–0.61

–0.53

2. Pathways of Styrene Selective Oxidation on AuNq (N = 16, 28, 34, 55; q = 0, ±1) Clusters On the basis of the adsorption mode of styrene on gold clusters, we proposed two competitive reaction pathways, i.e. Path 1 and Path 2, which lead to three products, benzaldehyde, styrene epoxide and phenyl acetaldehyde. The details of the reaction pathways are shown in Scheme 1. The Path 1a and Path 2a are the pathways of styrene oxidation to benzaldehyde, the Path 1b and 2b to epoxide, and Path 1c and Path 2c to phenyl acetaldehyde. These reaction pathways consist of two essential steps: the formation of OMME intermediates, and the production of the products. In the first step, the α-C of styrene attacks O atom to form OMME1 in the Path 1, while the β-C attacks O atom to form OMME2 in the Path 2. It was proposed that the formation of OMME intermediate plays a key role in the styrene oxidation on atomic O-covered Au(111)26, and the formation of OMME intermediate was considered as a rate-determining step during styrene oxidation process. Therefore, there is a transition state in the first step, which is labeled as TS1 in Scheme 1. Then, we will determine which pathway has higher conversion efficiency for styrene oxidation by comparing the reaction energy barriers of TS1-Path1 and TS1-Path2. To understand the mechanism of styrene selective oxidation on gold nanoclusters,



we studied the two pathways of styrene oxidation on AuN+, AuN and AuN− (N = 16, 28, 34 and 55), respectively.

(a) Styrene oxidation pathways of Path 1 on gold catalysts.

(b)

Styrene oxidation pathways of Path 2 on gold catalysts.

Figure 1. Schematical illustration of reaction intermediate and transition states in the reaction Path 1 and Path 2.

We first investigated the styrene oxidation process over AuN+ (N= 16, 28, 34 and 55) clusters. Figure 1 provides a more detailed summary of the different reaction pathways.


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Structures of reactants, products, intermediates, and transition states during the reaction are shown, especially the structures of TS2 in different pathways. Table 2 and Table 3 list the energy barriers of styrene selective oxidation along Path 1 and Path 2 on various AuN+ clusters (Table S2 and S3 in the Supporting Information give the energy barriers of all steps in styrene catalytic oxidation on various gold clusters calculated by DFT+D method), and Figure 2 is the snapshots of optimized reaction intermediate, transition state, and energies of elementary reaction steps. As shown in Table 2 and 3, the energy barriers of TS1-Path 1 on Au16+, Au28+, Au34+ and Au55+ clusters are in the range of 0.79 – 1.27 eV, and the energy barriers of TS1-Path 2 are in the range of 0.62 – 1.47 eV. The difference of the energy barriers for the two reaction paths is not obvious, which indicated the competition of Path 1 and Path 2 to form OMME intermediates. However, we noted that the energy barriers of the TS1-Path 1 on Au55+ are substantially lower than the smaller size gold clusters. The decreased energy barriers of TS1-Path 1 on Au55+ cluster suggested this gold cluster has higher conversion efficiency for styrene oxidation with respect to the smaller ones. The second step of styrene selective oxidation is the formation of benzaldehyde, styrene epoxide or phenyl acetaldehyde products. This step goes through different transition states, which are denoted as TS2 and TS3 as emphasized in Figure 2. Table 2 and Table 3 list the energy barriers of TS2 and TS3 for Path 1 and Path 2, which may enable us to judge which pathway is energetically more favorable to form selective products. For the formation of benzaldehyde and epoxide, the reaction mechanisms have been extensively studied. Here we focus majorly on the phenyl acetaldehyde formation channels. A well-known mechanism of phenyl acetaldehyde formation involves an H-atom transfer process (from the β-C atom to α-C atom in C=C double bond) as shown in Scheme 1 and Figure 1 (TS2-Path 1c). Our proposed new reaction channel Path 2c in this work describes a different way to form phenyl acetaldehyde started from the OMME2 intermediate that the phenyl groups is moved from the α-C atom to the β-C atom (TS2-Path 1c). In this work, we mainly discuss the universality and selectivity of the Path 2c for the phenyl acetaldehyde formation.



(a) Styrene oxidation pathways of Path 1 on AuN+ clusters.

(b) Styrene oxidation pathways of Path 2 on AuN+ clusters. Figure 2. The two reaction pathways of styrene oxidation on Au16+, Au28+, Au34+ and Au55+ clusters. The black line (Path a) represents styrene reacting with O atoms to generate the benzaldehyde and formaldehyde; the green line (Path b) indicates the pathway leads to generate the epoxides; the red line (Path c) represents the formation of phenyl acetaldehyde. The TS1, TS2, TS3 are the transition state of elemental steps, the P1 and P2 are the products, the OM1, OM2, OM3 are the intermediate OMME in different reaction steps, and the values in parentheses represent the relative energy of corresponding structures calculated by the DFT-D method.


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Table 2. Reaction energy barriers of the Path 1 on various AuN+, AuN and AuN– clusters calculated by the PBE/DND method. The energy barriers of corresponded reaction steps calculated by the PBE-D method are given in the Table S2 in the Supporting Information. All the energies are in eV.

Energy barrier of TS1



Styrene oxidation on Au16+ / Au16 / Au16–

1.26 / 1.53 / 0.35

1a

0.41 / 0.79 / 0.65

0.69 / 0.78 / 0.26

1b

0.68 / 0.81 / 1.04

-

1c

0.65 / 1.06 / 1.00

-


1.27 / 1.09 / 0.45

1a

0.51 / 0.91 / 1.27

0.22 / 0.06 / 0.86

1b

0.61 / 0.88 / 1.08

32 -

1c

0.51 / 0.97 / 0.91

-


Path 1

1.20 / 1.46 / 1.06

1a

0.45 / 0.80 / 0.84

0.61 / 0.73 / 0.53

1b

0.88 / 0.87 / 0.55

-

1c

0.39 / 0.78 / 0.69

-


0.79 / 0.77 / 0.66

1a

1.56 / 1.30 / 1.00

0.24 / 0.11 / 0.14

1b

0.49 / 0.82 / 0.82

-

1c

0.63 / 0.60 / 1.06

-

Table 3. Reaction energy barriers of the Path 2 on various AuN+, AuN and AuN– clusters calculated by the PBE/DND method. The energy barriers of corresponded reaction steps calculated by the PBE-D method are given in the Table S2 in the Supporting Information. All the energies are in eV.





1.17 / 1.48 / 0.48

2a

0.53 / 0.68 / 0.75

0.01 / 0.80 / 0.50

2b

0.89 / 1.37 / 1.25

-

2c

0.31 / 0.52 / 0.73

-

Styrene oxidation on Au28+ / Au28 / Au28– 1.47 / 0.93 / 0.46

2a

1.24 / 1.19 / 1.27


0.53 / 0.19 / 0.12


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2b

1.03 / 1.21 / 1.65

-

2c

0.52 / 0.52 / 1.25

-


Path 2

1.09 / 0.46 / 0.82

2a

0.76 / 0.77 / 0.80

0.13 / 0.40 / 0.38

2b

1.03 / 1.20 / 1.30

-

2c

0.35 / 0.74 / 0.79

-


0.62 / 0.64 / 0.54

2a

0.80 / 0.86 / 0.98

0.27/ 0.05 / 0.30

2b

0.77 / 1.10 / 0.85

-

2c

0.39 / 0.63 / 0.96

-

The red line of Figure 2 (b) are the pathways of Path 2c on AuN+, in which the phenyl of β-C transfer to the α-C after the intermediate OMME2 formed, and finally phenyl acetaldehyde will formed through TS2. In Table 2, the calculated energy barriers of TS2-Path 1c are in the range of 0.39 – 0.65 eV on Au16+, Au28+, Au34+ and Au55+ clusters, the energy barriers are not the lowest among three branched paths (Paths 1a – 1c), which mean that the styrene selectively oxidized to phenyl acetaldehyde through Path 1c has no advantage. In Table 3, the energy barriers of TS2-Path 2c on various positively charged gold clusters are in the range of 0.31 – 0.52 eV. Interestingly, we found the energy barriers for all of the TS2-Path 2c are lower than that of the TS2-Path 2a and Path 2b, it indicated that the formation of phenyl acetaldehyde through Path 2c is very favorable in the reaction Path 2. We concluded that the positively charged gold clusters of various sizes are not only capable of oxidizing styrene to form phenyl acetaldehyde through Path 2c, and it is more selective than the Path 1c proposed previously. For the understanding of the styrene catalytic oxidation over neutral AuN (N = 16, 28, 34, 55) clusters, we investigated the same processes of the styrene selective oxidation. The energy barriers of the transition states of the reaction steps in Path 1a-1c and Path 2a-2c are summarized in Table 2 and 3, respectively. The snapshots of reaction intermediate, transition state and energies of elementary reaction steps of the two paths are summarized in Figure S1 in the Supporting Information. The red line of Figure S1 is the reaction pathway of the phenyl acetaldehyde formation. It can be found that the new channel is also able to form phenyl


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acetaldehyde on various neutral gold nanoclusters. As shown in Table 2 and 3, the energy barriers of TS1-Path 1 are in a narrow range of 0.77 – 1.53 eV on Au16, Au28, Au34 and Au55 clusters, and the energy barriers of TS1-Path 2 ranges from 0.46 to 1.48 eV on the different neutral gold clusters. It indicated that the possibility for the styrene selective oxidation on neutral gold nanoclusters may be around 50%. Interestingly, we noted the TS1-Path 1 and TS1-Path 2 both have relatively lower energy barriers on the Au55 cluster, which is similar to that on positively charged gold clusters. The further examination of size-dependent catalytic activity of gold clusters indicated that the energy barriers and adsorption energies of reactants are not strongly dependent on the clusters sizes, even though we found that the adsorption of styrene becomes weaker as increased the sizes of gold clusters. For the second step of phenyl acetaldehyde formation, we mainly discussed the energy barriers of TS2-Path 1c and TS2-Path 2c on various AuN clusters. For the Path 1c of phenyl acetaldehyde formation, the energy barriers of TS2 are in the range of 0.60 to 1.06 eV as displayed in Table 2. They are not the lowest one relative to that of the Path 1a and Path 1b, that is to say, there is no superiority through Path 1c to form phenyl acetaldehyde. From Table 3, the energy barriers of TS2-Path 2c are in a narrow range of 0.52 - 0.74 eV, and the energy barriers of the new reaction channel is the lowest in the three pathways. It also illustrated the new channel is easier to form phenyl acetaldehyde compared to Path 1c. Finally, we calculated the reaction pathways of styrene catalytic oxidation over anionic gold clusters (Au16–, Au28–, Au34– and Au55–). The energy barriers of TS1, TS2 and TS3 in Path 1 and Path 2 are calculated, as shown in Table 2 and 3. The snapshots and transition states of reaction pathways on negatively gold clusters are given in Figure S2 in the Supporting Information. For anionic clusters, the energy barriers of TS1-Path 1 are ranging from 0.35 to 1.06 eV, and the energy barriers of TS1-Path 2 are in the range of 0.46 – 0.82 eV. We found the energy barriers of TS1 on anionic gold clusters are lower than that on positively and neutral charged gold clusters, it suggested that the reaction rate of styrene oxidation is significantly increased on the anionic gold clusters. The red line of Figure S2 described the new pathway of phenyl acetaldehyde formation, it showed that the new channel enable to form phenyl acetaldehyde on anionic gold clusters as well. In Table 2, the energy barriers of TS2-Path 1c are ranging in 0.69 – 1.06 eV, it is not



Page 16 of 25

the lowest ones in all reaction channels. In comparison to the energy barriers of TS2-Path 1c, the energy barriers of TS2-Path 2c are also not the lowest in comparison to that of TS2-Path 2a and 2b. In addition, we noted the energy barriers of TS2-Path 2c on anionic gold clusters are higher than positively charged and neutral gold clusters, which indicated that the Path 2c of phenyl acetaldehyde formation on anionic gold clusters has no outstanding selectivity. Besides, Table 4 summarizes the bond lengths of Cα–Cβ in the transition states (TS2-Path 2c) of phenyl acetaldehyde formation of the new channel on the different charged and sized gold clusters. We found the Cα−Cβ bonds of TS2 are in the range of 1.44–1.46 Å on various gold clusters, it can be confirmed that the transition states of the phenyl acetaldehyde formation are reasonable.

Table 4.

The bond lengths of Cα–Cβ in the transition states (TS2-Path 2c) of phenyl acetaldehyde

formation of the new channel on the different charged and sized gold clusters calculated at the PBE/DND level of theory. All the distances are in unit of Å.

AuN

Cα–Cβ bond length in the transition states (TS2-Path 2c) AuN+

AuN

AuN–

Au16

1.45

1.46

1.45

Au18

1.45

1.46

1.46

Au34

1.44

1.45

1.46

Au55

1.45

1.45

1.45

On the basis of the discussions above, the mechanism of phenyl acetaldehyde formation can be understood from Path 1c and Path 2c. By comparing the styrene selective oxidation pathways on different charged and sized gold clusters, the results showed that the new channel (Path 2c) for the formation of phenyl acetaldehyde is feasible for all gold clusters. Through comparative analysis of the energy barriers of TS2-Path 2c on AuN+, AuN and AuN– clusters, the Path 2c on the positively AuN+ and neutral AuN clusters have the higher selectivity, but has the lower selectivity on the anionic AuN–.

3. The New Channel of Phenyl Acetaldehyde Formation on the Au (111) Surface The above studies compared the selectivity of the new reaction channel and other known


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catalytic oxidation pathways on different size and charged gold nanoclusters. In order to verify the feasibility of this new reaction channel, we continued to study the catalytic oxidation of styrene on the bulk gold surface by DFT method. In these calculations, we chosen the unit cell of p(5×5×4) as the calculated model. Since the model is large, we only shown a small part of gold atoms in the reaction pathways, as shown in Figure 3. We first studied the adsorption of styrene on Au(111) surface, and still ignored the dissociation process of oxygen molecule and pre-adsorbed two oxygen atoms on the Au(111) surface, then the styrene is adsorbed at the adjacent sites. There are also two modes for the adsorption of styrene on Au(111) surface, denoted as the α-C and β-C adsorption mode. Through comparing the energies of the two kinds of adsorption modes, we find the α-C adsorption mode has lower adsorption energy (–0.61 eV). From the bond length analysis, the d(C = C) of the gas phase styrene is 1.344 Å, and the bond length of C=C was increased to 1.378 Å when styrene is adsorbed on the Au(111) surface. It suggested the C=C bond of styrene has been activated in the process of styrene adsorption.

Figure 3. Top view (a) and side view (b) of the Au(111) model structures. Blue and red spheres represent the gold and oxygen atoms of the styrene oxidation reaction, respectively.

There are also two steps in styrene catalytic oxidation pathways on Au(111) surface, that is the first formation of OMME, and then production of one of the three products. The snapshots of reaction intermediate, transition state and energies of elementary reaction steps of Path 1 and Path 2 are summarized in Figure 4. In the first reaction step, the energy barrier of TS1–Path 1 is 1.07 eV, and the energy barrier of TS1-Path 2 is 0.87 eV. The second step is to form benzaldehyde, epoxide or phenyl acetaldehyde starting from the OMME. The red lines in Figure 4 described the pathway of phenyl acetaldehyde formation via either H-atom transfer or the phenyl group transfer process. In comparison to the gold



nanoclusters studied in this work, on the Au(111) surface, in the reaction channel Path 1, the TS2-Path 1b (0.78 eV) is lower than the TS2-Path 1a and 1c, which suggested that the formation of epoxide product is the energy most favorable channel, while in the reaction channel Path 2, the TS2-Path 2a (0.34 eV) is lower than the TS2-Path 2b and 2c, means that the benzaldehyde formation is the lowest energy reaction channel. Of note that the conventional H-atom transfer process in the reaction channel Path 1c is the highest energy channel among three reaction pathways of reaction Path 1. For comparison, in the reaction channel Path 2, the phenyl transfer process (Path 2c) is energetically competitive to that of Path 2a. It indicates that the suggested new reaction channel is also slightly favorable on the bulk gold surface.

(a)Path 1a – 1c

(b)Path 2a – 2c Figure 4. The oxidation reaction pathways of the two reaction paths on Au(111) surface. The black line (Path 1a and Path 2a) represents styrene reacting with O atoms to generate the benzaldehyde and formaldehyde; the green line (Path 1b and Path 2b) indicates the pathway leads to generate the


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epoxides; the red line (Path 1c and Path 2c) represents the formation of phenyl acetaldehyde. The TS1, TS2 and TS3 are the transition state of elemental steps, the P1 and P2 are the products, the OM1, OM2 and OM3 are the intermediate OMME of the basic steps. The values in parentheses represent the relative energy of corresponding structures calculated by the DFT-D method.

Conclusions The present DFT study has simulated the experimental process of the oxidation of styrene on gold clusters and Au(111) surface with pre-covered O atoms. The calculated results showed that there are two steps of styrene catalytic oxidation pathways on gold clusters and Au(111) surface, i.e., first formed a ring intermediates OMME, and then produced benzaldehyde, expoxide and phenyl acetaldehyde. For the phenyl acetaldehyde product, we proposed a new channel of the phenyl acetaldehyde formation through studying the styrene selective oxidation on different size and charged gold catalysts and Au(111) surface. The calculated results suggested that the new channel of the phenyl acetaldehyde formation is feasible for the different sized and charged gold clusters and Au(111) surface. In addition, the new channel exhibited high selectivity on the positively charged and neutral gold clusters, while on the anionic gold clusters it has no outstanding selectivity.

Supporting Information The snapshots of reaction intermediate, transition state and energies of elementary reaction steps of the two paths on neutral and negatively gold clusters are given in the Supporting Information. The Figure S1 is the reaction pathway of the styrene oxidation on Au16, Au28, Au34 and Au55 clusters, and the Figure S2 is reaction pathways of styrene oxidation on Au16–, Au28–, Au34– and Au55– clusters. Table S1 summarizes the adsorption energies of styrene on different charged and sized gold clusters calculated by DFT-D method. Table S2 is the reaction energy barriers of the Path 1 on various AuN+, AuN and AuN– clusters calculated by DFT-D method. Table S3 is the reaction energy barriers of the Path 2 on various AuN+, AuN and AuN– clusters calculated by DFT-D method. Table S4 is the dissociation adsorption energy of O2 on differently charged and sized gold clusters calculated by DFT and DFT-D methods.

This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Tel.: 86-731-58298910.

ORCID Zhongyun Ma: 0000-0002-8727-8608 Yong Pei: 0000-0003-0585-2045 Author Contributions ǁ

X. Z. and P. W. contributed equally. 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.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21773201, 21373176, 21503182) and the project of innovation team of the ministry of education (IRT_17R90).

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TOC Figure


Phenyl Ring Transfer Mechanism of Styrene Selective Oxidation to

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