Oxidation of Ethylbenzene to Acetophenone with N-Doped Graphene

Article pubs.acs.org/JPCC

Oxidation of Ethylbenzene to Acetophenone with N‑Doped Graphene: Insight from Theory Chiara Ricca,†,‡ Frédéric Labat,*,‡ Nino Russo,† Carlo Adamo,‡,§ and Emilia Sicilia*,† †

Dipartimento di Chimica, Università della Calabria, Cubo 14C Via P. Bucci, 87036 Arcavacata di Rende (CS), Italy Institut de Recherche de Chimie Paris, CNRS École Nationale Supérieure de Chimie de Paris, Chimie ParisTech, 11 rue P. et M. Curie, F-75231 Paris Cedex 05, France § Institut Universitaire de France, 103 Bd Saint-Michel, F-75005 Paris, France ‡

ABSTRACT: A detailed theoretical study of the selective oxidation of ethylbenzene to acetophenone over nitrogen-doped graphene was carried out using methods rooted in density functional theory and two different models, based on periodic and cluster approaches. A comparison of these two models not only allows for an adequate choice of the cluster size, to have a more realistic model, but also clearly shows the local nature of the investigated reaction. The whole reaction pathway was characterized in terms of both intermediate structures and associated energies. The identified mechanism, in which the OOH radical is the active peroxide species that drives the reaction, is thermodynamically favorable due to the overall high exothermicithy. It is also kinetically favored since the formation of the hydroperoxyl radical and its decomposition in other active oxygen species are associated with low activation energies. Both results clearly underline the high catalytic activity of nitrogendoped graphene toward this reaction.

1. INTRODUCTION The development of new catalytic routes for the selective oxidation of hydrocarbons into the corresponding oxygencontaining desired products such as alcohols, aldehydes, ketones, epoxides, and carboxylic acids is one of the most important and at the same time challenging reactions of the chemical industry. Recently, the selective oxidation of ethylbenzene to acetophenone has become a topic of particular interest because acetophenone is the raw material for the production of perfumes, pharmaceuticals, resins, alcohol, esters, and aldehydes. Acetophenone can be synthesized in various ways. Traditionally it is obtained by means of Friedel−Craft acylation of aromatic hydrocarbons with acyl halides or acid anhydrides or through their oxidation with inorganic oxidants (like permanganate or dichromate).1−4 Unfortunately, these reactants are expensive or produce hazardous and toxic byproducts. Commercially, acetophenone is produced from the liquid-phase oxidation of ethylbenzene with oxygen as the oxidant and Co(OAc)2·4H2O as catalyst,1 but in the last years, a lot of other different catalytic systems have been proposed for this process. In particular, heterogeneous and homogeneous catalysts of different metallic species are known: NiAl hydrocalcite,5 Co(III) complexes with pyridine ligands,6 alumina-supported potassium dichromate,2 and molecular sieves of nanoporous aluminum phosphate doped with Ce7 or Mn8 could be used with oxygen as oxidant; MCM-41 molecular sieves with Mn and Co,9 mixed metallic oxides, and manganese nanocatalysts10 could be used with TBHP (tert-butyl hydroperoxide) as oxidant; and 8-quinolato manganese(III) complexes11 and cobalt tungstate12 could be utilized with H2O2 as oxidant. © 2014 American Chemical Society

Unfortunately, as the majority of these catalysts requires acidic solvents or contains metallic species, the processes are not only expensive but also environmentally unfriendly. As a consequence, a great deal of interest has been concentrated on the development of efficient, selective, environmentally friendly, cheap, reusable, and metal-free catalysts, which allow the selective oxidation of C−H bonds under mild conditions. Carbon-based materials have been particularly studied in the last two decades because of their unusual physical, chemical, and mechanical properties. In particular, graphene, the singleatom-thick two-dimensional carbon sheet with the same structure as the individual layers in graphite, has been shown to possess remarkable properties that could allow its use in several areas such as catalysis.13,14 Doping of graphene represents an efficient way to tune the properties of the material and to extend its possible applications. For example, nitrogen-doped graphene could be used in a lot of different applications like electrochemical devices, field-effect transistors, ultracapacitors, biomedical sensors, and as a photocatalyst.15 The most studied application of doped graphene is its use as a catalyst for the oxygen reduction reaction (ORR) in fuel cells.16−27 Recently, Gao et al.26 proposed nitrogen-doped graphene as a superior catalyst for the selective oxidation of C−H benzylic bonds, like for the oxidation of ethylbenzene to acetophenone, with oxygen or tert-butyl hydroperoxide (TBHP) as oxidant. The experimental findings show that the catalytic activity of Received: March 3, 2014 Revised: May 19, 2014 Published: May 20, 2014 12275

dx.doi.org/10.1021/jp502179n | J. Phys. Chem. C 2014, 118, 12275−12284

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boundaries. On the other side, the small size of the cluster and its “molecular” nature make this type of calculation much less expensive from a computational point of view. In fact, very large, and consequently computationally expensive, slabs would be required to describe the local changes in the active sites within the periodic approach. Therefore, the use of the cluster approach greatly reduces the computational costs, especially in the search of transition states, allowing at the same time an adequate description of local properties such as adsorption energies, geometries, and vibrational frequencies.

graphene can be tuned by nitrogen doping, the activity and yield of the desired product both being dependent on graphitic dopant content. Results appear to be comparable to or even better than those of traditional metal catalysts. The reaction is carried out in the aqueous phase at mild temperature conditions, and the catalyst can be easily recovered by filtration and reused without significantly affecting its performance. To attempt to explain why such nitrogen-doped catalysts appear to be so active for the C−H activation, the authors performed a series of energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) experiments to characterize the catalyst samples. There are at least four different locations of N dopant in the carbon atom framework named pyridinic, pyrrolic, quaternary, graphitic and N-oxide (Figure 1). Experiments showed that graphitic nitrogen dopant

2. COMPUTATIONAL DETAILS The reaction mechanism of the ethylbenzene oxidation to acetophenone over N-doped graphene was studied by means of both finite (i.e., cluster) and infinite (i.e., periodic) models, based on both the density functional theory and Gaussian-type orbitals. Cluster calculations were carried out with the GAUSSIAN09 code.28 Owing to the key role dispersion forces play when dealing with aromatic systems like those involved in our study, the dispersion-corrected hybrid density functional ωB97X-D29 was used. For C, N, O, and H atoms, the 6-31G** basis sets were used.30 Nitrogen-doped graphene was modeled through polycyclic aromatic hydrocarbons (PAHs), made up by fused hexagonal rings with delocalized π electrons and terminated with C−H bonds. For each cluster model, one C atom was replaced with one N atom. Only nitrogen graphitic sites were taken into account because experimental data shows improved catalytic activity of nitrogen-doped graphene toward selective ethylbenzene oxidation to be correlated to this particular nitrogen configuration. To identify the cluster model that better reproduces the properties of nitrogen-doped graphene outlined in ref 26, geometry optimization was performed on cluster models of different size, topology, and edge type (arm-chair or zigzag). For instance, models with one N atom and an even number of H atoms, ranging in size from 46 to 126 atoms, have been considered in their doublet state. To identify minima and transition states, no symmetry and geometry restrictions were imposed during optimization. For each transition state, vibrational analysis was performed in the harmonic approximation to verify its saddle point character; it was also carefully checked that the vibrational mode associated with the imaginary frequency corresponded to the correct movement of involved atoms. IRC calculations were performed to assess the proper connection between transition states and corresponding minima. For all the open shell species, ⟨S2⟩ values were checked to verify that spin contamination was negligible. Periodic calculations were carried out with the CRYSTAL31 code, which allows us, through a standard linear combination of atomic orbitals (LCAO) approach, to self-consistently solve both the Hartree−Fock (HF) and Kohn−Sham (KS) equations. The PBE (Perdew−Burke−Ernzerhof)32 and the PBE033 (a parameter-free functional mixing 25% of HF exchange in a PBE scheme) functionals corrected for dispersion according to the Grimme D234 scheme were selected. Such dispersion-corrected functionals will be indicated as PBE-D and PBE0-D, respectively. A modified 6-21G* basis set (m-621G*),35 with α3sp set to 0.24, was used for all C, N, and O atoms. A hexagonal (7 × 7) supercell of 98 atoms and with a of 17.205 Å was chosen to simulate nitrogen-doped graphene.

Figure 1. Bonding configurations for nitrogen atoms in N-doped graphene: (a) pyridinic, (b) pyrrolic, (c) graphitic, and (d) N-oxide.

plays a pivotal role for the observed remarkable activity in the C−H oxidation reaction. The catalytic activity considerably increases with the increase of nitrogen at graphitic sites, which seems to behave like an electronic promoter, stimulating the chemical reactivity of the neighboring carbon atoms, without directly taking part in the C−H bond activation reaction. Graphitic N atoms induce a charge and spin density redistribution on adjacent C atoms, promoting their reactivity. Those carbon atoms with the highest charge and spin density values appear to become the ideal active sites for the adsorption of oxygen species, especially peroxide species, that are believed to be the active intermediates in the reaction mechanism, which is still unknown. With the support of the information coming from experiments about the process of oxidation of ethylbenzene to acetophenone with oxygen as oxidant7 and the properties of the catalysts,26 in this work, we aim at investigating the relationships between the electronic properties and the catalytic activity of nitrogen-doped graphene toward the selective oxidation of ethylbenzene to acetophenone. The active sites on the surface of the catalysts and the interaction between the peroxide radicals and the nitrogen-doped graphene as well as a plausible reaction mechanism were studied in the framework of density functional theory by using both periodic and cluster approaches. The surface of a heterogeneous catalyst, like nitrogen-doped graphene, could be simulated through finite or infinite models. The use of a combined approach allows us to make use of the advantages of both formalisms, to get a deeper insight into the structure, properties, and reactivity of the material. The main advantage of the periodic approach is the more realistic description of the system, avoiding the typical drawbacks of small clusters related to artificial (cluster) 12276



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shown in Figure 2. They are all planar radicals with a C2v symmetry axis along the C−N bond. To choose the most appropriate model, Mulliken charges and spin densities on the N atom and the ortho, meta, and para C atoms, as well as the adsorption energy of the OOH radical on the C atom with the highest spin density, were calculated. The OOH radical is, indeed, the simplest of the family of peroxide radicals, which are believed to be the main intermediates in the selective oxidation of C−H benzylic bonds catalyzed by nitrogen-doped graphene. These results were compared with data coming from periodic calculations to assess their reliability. Also previous calculations by Gao et al.26 based on PBE and plane wave (PW) basis sets were taken into consideration. Concerning the outcomes of periodic calculations, results reported in Table 1 show that the introduction of nitrogen in graphitic positions of the graphene network (Figure 3) determines a local redistribution of charges and spin densities on C atoms adjacent to N. In each case, the N atom, which has one more electron than C and is more electronegative, bears a considerable negative charge. The ortho C atoms have, instead, the largest compensating positive charge, while charges on meta and para C atoms are close to zero. Spin density due to the unpaired electron introduced with nitrogen doping is mainly delocalized onto the ortho C and N atoms. From a comparison of the values calculated for the two 1.02% and 3.06% doping amounts, it is clear how the charge and spin density redistribution is much more noteworthy when the doping amount increases: Co* atoms in ortho to three N atoms bear much higher charges and spin density values than the Co atoms in ortho to one N. As the sites on the nitrogen-doped graphene surface with high values of charge and spin density are believed to be the active sites for the reaction, the results obtained for the two doping amounts can explain the experimental observation that the efficiency and selectivity of the oxidation reaction increase with the increase of the doping percentage. There is experimental evidence that active oxygen species, in particular peroxide species, are involved in the reaction mechanism of ethylbenzene oxidation catalyzed by nitrogendoped graphene. For this reason, for the doping amount of 1.02%, we evaluated the adsorption energy of the OOH radical on C atoms with higher charge and spin density, which are assumed to be the preferred adsorption sites (Figure 4). The OOH radical is chemisorbed on the Co carbon with an adsorption energy of 30.44 and 25.83 kcal/mol using PBE0-D and PBE-D, respectively (Table 2). With both methods, the Co moves out of the plane of graphene by 0.40 Å after the formation of the Co−O bond (1.47 Å for PBE0-D and 1.50 Å for PBE-D). This is in excellent agreement with previously published data.25 When going to cluster data, Mulliken charge distribution agrees with the periodic results (Table 1): the N atom always bears a significant negative charge, while the compensating positive charge is localized mainly on the ortho C atoms (Co1 and Co2). With the substitution of one C atom with one N, four of the five valence electrons of N are placed in the σ and π orbitals, as for carbon, and there is one extra electron, whose additional charge is localized mainly on N that is more electronegative than C. Reproduction of the spin density behaviour required, instead, many attempts in order to select the appropriate model. The majority of cluster models showed values of spin close to zero even on N and the Co atoms. For only two clusters, C41NH16

Two different doping amounts were tested to mimic experimental values.26 The 1.02% amount was simulated by substituting one C atom with one N atom, while the 3.06% amount was obtained by replacing three C atoms with the same number of N atoms. Only the graphitic configuration was examined like for cluster calculations. 61 k points were used for the sampling of the IBZ (irreducible Brillouin zone) of this supercell. The smearing of the Fermi surface was applied, with the temperature smearing parameter set to 0.001 hartree. For the evaluation of the Coulomb and exchange series, an extraf ine integration scheme was considered. For the periodic calculations, the adsorption of the hydroperoxyl radical OOH only onto the carbon atoms in ortho to nitrogen, which are considered to be the most stable adsorption sites, was studied according to ref 26. For all the studied systems, charge and spin density on each atom were estimated by using the Mulliken population analysis.36 Finally, for both cluster and periodic calculations, all the adsorption energies were computed subtracting the total energy of the combined adsorbate/graphene system from the sum of the total energies of graphene and of the isolated adsorbate molecule, Eads = Egraphene + Eadsorbate − Eadsorbate/graphene.

3. RESULTS AND DISCUSSION A possible pathway for the oxidation of ethylbenzene to acetophenone over N-doped graphene with O2 as the oxidant was probed according to hypotheses reported in the literature.7 The assumed mechanism, as illustrated in Scheme 1, entails the Scheme 1. Possible Reaction Mechanism for the Oxidation of Ethylbenzene to Acetophenone over Nitrogen-Doped Graphene with Oxygen as Oxidant

adsorption of both oxygen and ethylbenzene (EB) on the surface of the catalyst and the abstraction of a H atom from the CH2 group of EB by O2, with the formation of the phenylethyl (PE) and the hydroperoxyl (OOH) radicals. According to ref 26 such radicals should be the active intermediates of the process. In the second step, PE reacts with the hydroperoxyl radical to form 1-phenylethanol (PEtOH) and atomic oxygen. Afterward, PEtOH interacts with atomic oxygen to form the corresponding radical (POH) together with the hydroxyl radical. Finally, the POH radical transfers one H atom to the OH radical, forming water and acetophenone. Like EB, all the aromatic species formed through the mechanism could be stabilized by the π−π interactions with the graphene sheet. This hypothesis will be verified in the following, after a careful selection of the cluster model. 3.1. Selection of a Cluster Model for N-Doped Graphene. The structures of the models used to simulate nitrogen-doped graphene by using the cluster approach are 12277



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Figure 2. Structures of the studied clusters: (a) C31NH14, (b) C39NH16, (c) C41NH16, (d) C47NH18, (e) C53NH20, (f) C65NH20, (g) C98NH24, and (h) C99NH26. (i) Labeling scheme of the equivalent C atoms with respect to the C2v symmetry axis along the Co1−N bond of each cluster. Carbon atoms as gray and nitrogen atoms as blue spheres.

Adsorption energies of the hydroperoxyl radical computed for different adsorption sites of the selected C41NH16 cluster showed an almost linear dependence of the adsorption energy of this radical on the spin density of the adsorption site (see Figure 6), as already observed in refs 20 and 23. All these observations confirm that the spin density redistribution due to doping is responsible for the ability of nitrogen-doped graphene to adsorb peroxide-like radicals. 3.2. Reaction Mechanism. Figure 7 shows the computed energy profiles for the probed mechanism calculated by using both cluster and PBC approaches. For a more rigorous comparison of the data, PBE0-D single-point cluster calculations on ωB97X-D optimized geometries were also carried out since this latter functional is not currently available in the CRYSTAL code used in this work. According to what was already pointed out in the literature,39 the overall reaction is calculated to be extremely exothermic by both the cluster and the PBC approaches, −71.7 kcal/mol with ωB97X-D/6-31G** for instance, and thus thermodynamically accessible. On the selected cluster, we first studied the adsorption of the oxidant O2 at the ωB97X-D/6-31G** level. For these calculations only the triplet state of oxygen was considered, which is the ground state for O2. There are two possible modes of adsorption for oxygen on the N-doped graphene surface: the side-on or Yeager model and the end-on or Pauling model.19 Calculations show that oxygen is physisorbed in diatomic side-on modality, preferentially on the ortho Co1 atom of the cluster, with the O−O bond axis parallel to the Co1−N bond (see Figure 5(b)). The molecule is at about 3 Å from the surface of the catalyst, and the adsorption energy is 2.93 kcal/mol. The interaction does not cause a distortion of the geometry: the cluster is still planar, even if the N atom is slightly pushed under the plane of the surface as a consequence of the repulsion between the O and N

and C98NH24, the spin density values were calculated to be substantially different from zero. In both cases, spin density is delocalized mainly on N and ortho carbon Co1. At the ωB97X-D level, spin density values are 0.260 on Co1 and 0.099 on N for C41NH16 and 0.184 on Co1 and 0.100 on N for C98NH24. Positive values of spin density are also calculated on the Cp1 carbon atoms of these structures. A closer look at the structure of these clusters reveals that they have the same topology, but in C98NH24 the N atom is farther from the edge of the cluster (see Figure 2). There is, therefore, a dependency of spin density distribution on the topology of the cluster and on the position of the doping atom. In particular, for fixed doping atom position and cluster topology, increasing the cluster size causes a decrease in the effect of spin density redistribution. This observation is in agreement with the study of Yu et al.37 concerning the properties of triangular graphene clusters doped with nitrogen and/or boron. Electronic properties appear to be strongly dependent on the edge type and the position of the doping atoms. Adsorption of the OOH radical on the Co1 carbon atom of each cluster was investigated (Figure 5(a)). The adsorption geometry is the same for all the models: the OOH radical forms a bond with the Co1 atom, which loses the sp2 hybridation and moves out of the plane of the simulated graphene, forming a tetrahedral structure. The adsorption energies instead vary considerably with the type of cluster. In particular, for clusters with the same topology, when the size increases, the spin density on the Co1 decreases together with, consequently, the adsorption energy.20,38 From the analysis of the topology and size of the clusters that better reproduce the charge and spin density distribution and the adsorption energy of the probe species computed by using the periodic approach, C41NH16 turned out to be a suitable model to simulate the surface of nitrogen-doped graphene by the cluster approach. 12278


sp

so* sm

sN so

qp

qo* qm

qN qo

−0.15 +0.10 +0.13 +0.21 +0.13 +0.03 −0.02 −0.05 +0.09 +0.01

−0.02 −0.01 −0.02 +0.01 +0.01 +0.03 +0.11

−1.00 +0.40 +0.40

−1.00 +0.27 +0.27

+0.02 +0.04 +0.02 +0.10 +0.05 +0.10 +0.18 +0.18

1.01% C98NH24 PBE0-D m-6-21G*

1.01% C98NH24 ωB97X-D 6-31G**

clusters

−0.14 +0.01 −0.02 +0.17 −0.02

+0.039 −0.017 −0.026 +0.046 +0.070 +0.099 +0.26 −0.01

−0.94 +0.29 +0.32

2.38% C41NH16 ωB97X-D 6-31G**

−0.01 −0.02 −0.00 +0.12 −0.02

−0.02 +0.05 −0.06 −0.03 −0.02 +0.09 +0.21

−0.99 +0.38 +0.39

2.38% C41NH16 PBE0-D m-6-21G*

+0.04

−0.00

+0.07 +0.04

+0.00

+0.01

−0.73 +0.25

1.02% PBE-D m-6-21G*

+0.08

−0.03

+0.11

+0.38 +0.02

+0.09 +0.13

−0.01

−0.00 +0.10 +0.06

+0.72 +0.02

−0.70 +0.24

3.06% PBE-D m-6-21G*

PBC

+0.01

−0.81 +0.30

1.02% PBE0-D m-6-21G*

+0.18

+0.56 +0.03

+0.11 +0.17

−0.01

+0.77 +0.01

−0.79

3.06% PBE0-D m-6-21G*

+0.08

+0.03

+0.17

+0.00

+0.06

−0.81 +0.21

1.04% PBE/PW [24]

Table 1. Computed Mulliken Atomic Charges (q, (e)) and Spin Densities (s) of Carbon Atoms in ortho (o and o*), meta (m), and para (p) Positions of N in N-Doped (7 × 7) Graphene (PBC Columns) and of Carbon Atoms in ortho (o, in Order of Appearance o1 and o2), meta (m, in Order of Appearance m1, m2, and m3) and para (p, in Order of Appearance p1 and p2) Positions of N in C41NH16 and C98NH20 (Cluster Columns)

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Figure 3. Labeling scheme and spin density plots of N-doped graphene with doping amounts of 1.02 and 3.06%. Nitrogen atoms as blue spheres. Isosurface values of 0.0005.

Figure 4. Optimized structure of the OOH radical adsorbed on 1N-graphene obtained at the PBE0-D level.

Table 2. Computed Structural Parameters of the OOH Radical Adsorbed on the Co1 Carbon Atom of Clusters C41NH16 and C98NH24 and on the ortho Carbon Atom (Co) of the 1N-Graphenea cluster

Δz, Co dN−Co dCo−O1 dO1−O2 dO2−H dH−Cm dH−Cp aN−Co−O1 aCo−O1 −O2 aO1−O2−H aCo−O1−O2−H Eads

PBC

C98NH24 1.01% ωB97X-D/6-31G**

C98NH24 1.01% PBE0-D/m-6-21G*

C41NH16 2.38% ωB97X-D/6-31G**

C41NH16 2.38% PBE0-D/m-6-21G*

1.02% PBE-D/m-6-21G*

1.02% PBE0-D/m-6-21G*

[24] PBE/PW

0.39 1.46 1.46 1.42 0.97 2.73 2.90 108.6 109.8 101.6 94.0 11.30

0.39 1.46 1.46 1.42 0.97 2.73 2.90 108.6 109.8 101.7 94.0 11.53

0.40 1.46 1.46 1.42 0.97 2.70 2.87 108.9 109.8 101.7 91.8 29.98

0.40 1.46 1.46 1.42 0.97 2.70 2.87 108.9 109.8 101.7 91.8 29.52

0.40 1.48 1.50 1.48 0.98 2.59 2.73 108.4 110.8 101.4 65.8 25.83

0.40 1.47 1.47 1.43 0.97 2.52 2.63 107.7 110.3 101.6 77.3 30.44

0.42 1.60

23.52

Distances (d) in Å, angles (a) in degrees, and adsorption energies (Eads) in kcal/mol. Δz,Co is the upshift of the Co atom with respect to the basal plane formed by the three Co atoms. a

atoms. The change from the monatomic end-on absorption mode in which O2 is adsorbed on pristine graphene, as reported in refs 18 and 40, to the diatomic side-on mode on nitrogendoped graphene could be a consequence of the charge and spin density redistribution due to doping, which influences the

adsorption ability of oxygen and reactive intermediates like peroxide species.18,40,41 After the adsorption of the oxidant on the catalyst surface, its reaction with ethylbenzene (EB) was studied, and the optimized geometries and most important structural features 12280



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Figure 5. ωB97X-D/6-31G** optimized structures for the OOH radical and the O2 molecule when adsorbed on to the Co1 C atom of C41NH16.

of the intercepted minima and transition states are shown in Figure 8. The first minimum corresponds to the adduct formed by O2 and EB molecules. The aromatic ring of EB is parallel to the surface of the cluster at a distance of about 3 Å, and the oxygen molecule is still placed in a side-on modality, as in the case of the isolated O2 molecule; however, while in the latter case the O−O bond distance is 1.21 Å (Figure 5(b)) and the O−O stretching frequency is around 1702 cm−1, in the presence of EB, the O−O distance of the oxidant is longer (1.29 Å, see Figure 8(a)) and the O−O stretching observed at around 1300 cm−1. Therefore, on the basis of O−O bond distance and stretching frequency, in the case of the coadsorption of EB, O2 resembles a superoxo like species. This mode of adsorption of oxidant, with the elongated O−O bond distance, could facilitate the formation of the OOH radical and thus affect positively the whole process, in line with the low activation barriers that can be observed in Figure 7. The transition state (TS1), which connects the first minimum with the next one, is characterized by an imaginary frequency of 1937i cm−1 that is mainly associated with the stretching of the C−H benzylic bond. Indeed, in this step, O2 abstracts a H atom from the methylene group, forming phenylethyl (PE) and OOH radicals. The barrier for TS1 is of 19.5 kcal/mol. In the next minimum, the PE and OOH radicals are both adsorbed on ortho C sites. Both these carbon atoms are above the surface of the cluster and changed their hybridization. The

Figure 7. Potential energy surfaces for the probed reaction mechanism. Energies are in kcal/mol and relative to the groundstate reactants.

bond length of the C−O bond between the OOH radical and the C atom of graphene is 1.51 Å, while the length of the C−C bond between the ortho C of the cluster and the C atom of the PE radical is 1.62 Å. In particular, the OOH radical, which should be the active species on the catalyst surface, is adsorbed on the ortho carbon atom with the highest spin density (Co1). In the second step of the reaction, the OH group of the OOH radical is transferred to PE, with the formation of 1phenylethanol (PEtOH). The transition state TS2 associated with this step has an imaginary frequency of 878i cm−1, and the corresponding vibrational mode is characterized by the stretching of the O−O bond of the hydroperoxide radical. The barrier for TS2 is 3.6 kcal/mol. PEtOH and atomic oxygen are formed and adsorbed on the nitrogen-doped graphene surface. The phenyl ring of the aromatic alcohol is placed parallel to the surface of the cluster at a distance of about 3 Å, while the O atom is adsorbed on the pyramidalized Co1 atom, with the Co1−O bond length being 1.37 Å. In the third step of the mechanism, one H atom of PEtOH is transferred to oxygen, with breaking of a C−H bond and thus the formation of the organic POH and OH radicals,

Figure 6. Adsorption energy (Eads) of the OOH radical as a function of the spin density (a) and charge (b) of the adsorption site on C41NH16. Labels refer to Figure 2. 12281



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et al.24 in the case of the oxygen reduction reaction on N-doped graphene was not taken into account. The energy profile obtained at the PBE0-D//ωB97X-D level with the cluster approach does not show any significant qualitative change with respect to the PES at the ωB97X-D level, with the exception of the energy of TS3. Indeed, the overall reaction is exothermic by 70.4 kcal/mol, and the fourth step is again the rate-determining step. All the computed barriers are lower than the corresponding ones computed at the ωB97X-D level, with a value of 14.8 kcal/mol for TS4. Nearly all of the minima of the examined mechanism have been intercepted by using the PBC approach. The main geometrical characteristics of these minima are in agreement with the features already outlined by the analysis of the cluster data. The oxygen-containing radical species (OOH, O, and OH) are absorbed on the ortho C of N-doped graphene, which is always pyramidalized, while all the aromatic species are at a distance of about 3 Å from the plane of graphene. A general qualitative agreement between the different structural models is found on the energetic reaction, thus confirming the local nature of the reaction mechanism and the appropriateness of using a (large) cluster to model the infinite graphene sheet. Nevertheless, if this agreement is quantitative for the last intermediate and the products, a significant energy difference is found for the first two localized minima (PE + OOH and PEtOH + O). It is difficult to rationalize such a difference, but it could be either attributed to a larger contribution of the dispersion corrections in the periodic model or to its greater structural rigidity, which somehow stabilizes the concerned reaction intermediates.

Figure 8. ωB97X-D/6-31G** optimized geometries of (a) minima and (b) transition states of the proposed mechanism. Bonds lengths are in angstroms. The corresponding PBC data at the PBE0-D level are reported in parentheses.

4. CONCLUSIONS In this paper, a theoretical study of the selective oxidation of ethylbenzene to acetophenone over nitrogen-doped graphene was carried out using DFT techniques and a combined periodic and cluster approach. The use of both approaches allowed us to obtain information that was compared and integrated to achieve a more complete description of the phenomena, combining the advantages of both models. The periodic approach was used to characterize the active sites on the surface of nitrogen-doped graphene, by means of the study of the charge and spin density redistribution induced by the doping of graphene with N atoms in the graphitic configuration. Carbon atoms in ortho to N were found to show the largest compensating positive charge and the spin density due to the unpaired electron is mainly delocalized here in order to counterbalance the strong electron affinity of nitrogen. To select the cluster model which best reproduces the properties of the catalyst surface, the charge and spin density values on N and the C atoms closest to the dopant, as well as the adsorption energy of the hydroperoxyl radical on the C atom with the highest spin density computed by using the periodic approach, were compared with the analogous data obtained from cluster calculations. A strong dependence of the spin density distribution on the cluster topology and size and on the distance of the N atom from the borders was observed. The cluster C41NH16 was found to be the most suitable for the modeling of nitrogen-doped graphene. Adsorption of the OOH radical on different carbon atoms of the cluster was also studied. A linear dependence of the adsorption energy of the radical on the spin density of the adsorption site was confirmed. Hence, the change in the

overcoming the activation barrier for the transition state TS3. The imaginary frequency associated with TS3 is 582i cm−1, and the corresponding vibrational mode is characterized by the stretching of the C−H bond of PEtOH. In the structure of TS3, the H atom which is transferred is at a distance of 1.46 Å from the O atom and of 1.18 Å from the C atom of PEtOH. The length of the Co1−O bond is 1.40 Å. The very low barrier (0.3 kcal/mol) associated with TS3 is smaller than that of TS1, confirming that the alcohol oxidation is faster then ethylbenzene oxidation. The formed organic radical POH is at a distance of about 3 Å from the surface of the cluster, whereas the OH radical is adsorbed on the Co1 atom with a Co1−O distance of 1.42 Å. Once again, the adsorption site moves out of the plane of nitrogen-doped graphene. In the fourth and last step, the H atom of the alcoholic group of POH is transferred to the OH species, forming acetophenone and water, overcoming the barrier corresponding to the transition state TS4. The Co1−O bond length of the OH radical is 2.1 Å in the TS4 structure, and the pyramidalization of the Co1 carbon is no longer evident. The imaginary frequency associated with TS4 is of 9778i cm−1 and corresponds to the vibration mode associated with the stretching of the O−H bond of POH and to the simultaneous stretching of the C−O bond of POH and the Co1−O bond between the OH radical and the cluster. This step is the rate-determining step of the whole process because the corresponding calculated barrier of 21.8 kcal/mol is the highest barrier along the examined pathway. However, it is worth underlining that only a qualitative description of the mechanism was carried out. The influence of bulk water, indeed, that could affect this ratelimiting step by assisting the proton transfer as observed by Yu 12282



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electronic properties of graphene caused by the doping is crucial to explain the reactivity of the catalyst and in particular its ability to absorb the species involved in the reaction mechanism, like the hydroperoxyl radical, but also neutral species like oxygen. Although O2 weakly interacts with nitrogen-doped graphene, the diatomic side-on mode of adsorption, with the oxygen O−O axis parallel to the C−N bond, could be explained as a consequence of the charge and spin density redistribution owing to the introduction of the nitrogen dopant. A possible catalytic pathway for the selective oxidation of ethylbenzene to acetophenone over nitrogen-doped graphene was also examined. This mechanism, in which the OOH radical is the active peroxide species that drives the reaction, was fully studied by using the cluster approach. The computed energy profile for this pathway shows that - the process is thermodynamically favorable due the overall computed high exothermicity; - formation of the hydroperoxyl radical and its decomposition in other active oxygen species are associated with small activation energy barriers, which are compatible with the high catalytic activity of nitrogendoped graphene toward this process. In conclusion, in the framework of the complexity of benzylic C−H bond activation mediated by N-doped graphene, this work gives further insight into the characteristics of the active sites on the surface of the catalyst and produces information on the mechanistic details of this process that are still unknown.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +33 (0)144276728. E-mail: [email protected]. *Tel.: +39 0984492048. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Oxidation of Ethylbenzene to Acetophenone with N-Doped Graphene

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