A DFT Study of Direct Oxidation of Benzene to Phenol by N2O over [Fe

ARTICLE pubs.acs.org/JPCC

A DFT Study of Direct Oxidation of Benzene to Phenol by N2O over [Fe(μ-O)Fe]2þ Complexes in ZSM-5 Zeolite Mehmet Ferdi Fellah,*,† Evgeny A. Pidko,‡ Rutger A. van Santen,*,‡ and Isik Onal*,§ †

Department of Chemical Engineering, Yuzuncu Yil University, Van, 65080, Turkey Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands § Department of Chemical Engineering, Middle East Technical University, Ankara, 06531, Turkey ‡

bS Supporting Information ABSTRACT: Density functional theory (DFT) calculations were carried out in a study of the mechanism of benzene oxidation by N2O to phenol over an extra framework dimeric [FeOFe]2þ species in ZSM-5 zeolite represented by a [Si6Al2O9H14(Fe(μ-O)Fe)] cluster model. The catalytic reactivity of such a binuclear species is compared with that of mononuclear Fe2þ and (FeO)þ sites in ZSM-5 investigated in our earlier works at the same level of theory (J. Phys. Chem. C 2009, 113, 15307; 2010, 114, 12580). The activation energies for the elementary reaction step involved in the benzene hydroxylation over the binuclear and the mononuclear iron sites are comparable. The major difference in the catalytic behavior of the systems considered is related to the ability of Fe3þ-containing sites to promote side reactions leading to the active site deactivation. Regeneration of the active site via the phenol desorption is much less favorable than its dissociation resulting in the formation of very stable grafted phenolate species on both the [Fe(μ-O)Fe]2þ and (FeO)þ sites. In the case of Fe2þ sites such an alternative reaction path does not exist resulting in their stable catalytic performance. Benzene hydroxylation and phenol formation over the binuclear (Fe(μ-O)Fe)2þ sites in ZSM-5 are promoted in the presence of water. These computational findings are consistent with the experimental observations and allow their rationalization at the molecular level.

1. INTRODUCTION Direct oxidation of benzene is more sustainable and a much more attractive alternative for the production of phenol than the current cumene process.1 High-silica ZSM-5 zeolites modified with iron (FeZSM-5) are able to catalyze a highly selective direct benzene oxidation to phenol by N2O.222 Numerous experimental2335 and theoretical3443 studies indicate that the unique oxidation properties of Fe-ZSM-5 catalysts are related to the ability of extra framework iron species stabilized in the zeolite matrix to promote stoichiometric N2O decomposition (eq 1) resulting in the formation of highly reactive extra framework oxygen species (R-oxygen). The subsequent interaction of benzene with the thus formed R-oxygen species leads to its selective oxidation to phenol, which was proved by both experimental4,5,13,22,4446 and theoretical4751 studies. The following elementary reaction steps are proposed to be involved in the catalytic oxidation of benzene by N2O over Fe-ZSM-5 catalyst:2,45,46 N2 O þ ð ÞR f ðOÞR þ N2

ð1Þ

ðOÞR þ C6 H6 f ðC6 H6 OHÞR

ð2Þ

ðC6 H6 OHÞR f ð ÞR þ C6 H5 OH

ð3Þ

r 2011 American Chemical Society

Despite numerous studies devoted to the investigation of the mechanism of benzene oxidation over Fe-ZSM-5, the molecular picture underlying its unique oxidation activity is not clear. Understanding the nature of active sites in Fe-ZSM-5 zeolites is one of the major challenges in the investigation of their catalytic reactivity. Extra framework Fe species in the zeolite micropores can be present as mononuclear,5259 binuclear,5964 oligonuclear cationic species, small oxide clusters or mixed oxide phases combining Fe and Al.6466 A number of experimental techniques such as EXAFS, XANES, FT-IR, EPR, M€ossbauer spectroscopy, and UVvisible spectroscopy have been used so far to unravel the properties of the reactive iron species in Fe-ZSM-5 catalysts. Most experimental studies2,22,32,34,52,6769 concluded on the exclusive role of the exchangeable Fe2þ sites in FeZSM-5 for benzene oxidation to phenol by N2O. An EPR study by Volodin et al.70 has shown that such isolated bivalent iron cations in ZSM-5 zeolites are present in their high spin state (S = 4/2). Two theoretical studies have considered isolated Fe2þ sites Received: February 17, 2011 Revised: April 11, 2011 Published: April 22, 2011 9668

dx.doi.org/10.1021/jp201582s | J. Phys. Chem. C 2011, 115, 9668–9680

The Journal of Physical Chemistry C in Fe-ZSM-5 for catalytic benzene oxidation.51,71 An Fe2þ ion stabilized at the R cation site of ferrierite was used as a model of the active site for nitrous oxide decomposition and the subsequent selective oxidation of phenol.72 Theoretical studies have also considered a monovalent Fe cationic species as the active component in Fe-ZSM-5.47,48,73,74 Benzene oxidation to phenol by N2O was theoretically investigated over mononuclear sites such as [Fe1þ],74,75 [Fe2þ], 51,71 [(FeO)1þ]47,50,73,76 and [(FeO2)1þ]47 in Fe-ZSM-5 zeolite. Ensing et al.77 also used Fe2þ ions coordinated by water molecules for the Fenton oxidation reaction. Besides mononuclear iron species, extra framework cationic sites composed of several iron cations are often proposed as the active sites in Fe-ZSM-5 zeolite. Reactivity of binuclear ([Fe(μ-O)Fe]2þ)43 and ([Fe(μ-O)(μ-OH)Fe]þ)78 sites in N2O decomposition have been previously investigated. Dimeric (Fe(μ-O)Fe)2þ species have also been considered in theoretical studies as the preferred site for CO and NO adsorption79 and as the catalytic site for methane activation.80 Xia et al.81 have experimentally studied benzene oxidation to phenol by N2O at room temperature with Fe-ZSM-5 catalyst. In their study, the UV bands in the range of 500900 nm were assigned to the grafted Fe(III)phenolate complexes on Fe-ZSM-5 catalyst. It has been suggested that the formation of such species is promoted in the presence of dimeric iron species in Fe-ZSM-5. It has also been reported that the rate of phenol formation can substantially be enhanced in the presence of water. Despite numerous experimental and theoretical studies on the benzene oxidation to phenol by N2O with Fe-ZSM-5 catalyst reported so far, the nature of the active component of Fe-ZSM-5 is still under debate. This study aims at the investigation of the catalytic reactivity of dimeric iron sites in ZSM-5 for the oxidation of benzene to phenol formation by nitrous oxide. It is a continuation of our earlier work devoted to the investigation of reactivity of mononuclear Fe2þ, Fe1þ and (FeO)1þ iron species in ZSM-5 zeolite.71,74,76 A comparative analysis of the predicted catalytic reactivities of these different extra framework species is performed with the main goal to understand the qualitative relations between the state of the iron species in zeolitic matrix and their behavior toward benzene oxidation.

2. SURFACE MODEL AND CALCULATION METHOD All calculations in this study were performed in the framework of density functional theory82 with a hybrid B3LYP8385 exchange-correlation functional using the Gaussian 03 suite of program.86 The 6-31G(d,p) basis set was used for all atoms in molecular models considered in this study. Similar to our previous studies71,74,76 a cluster modeling approach was used to simulate a representative portion of ZSM-5 zeolite stabilizing the extra framework iron species. Previous studies in iron-modified ZSM-5 and ferrierite zeolites by Kachurovskaya et al.51,72 revealed only minor quantitative differences between the results obtained using cluster and periodic modeling approach. In addition, cluster modeling approach has been widely used to create a qualitative molecular-level picture of CO and NO adsorption on different iron sites of Fe-ZSM-5.79 Very similar structural and energetic properties of the adsorption complexes were obtained by using small 5T and large 83T cluster models.79 A more important issue is that DFT calculations on cluster or periodic zeolite models generally predict very similar reactivity trends.79,8789 In this study, an 8T cluster representing two adjacent 5T rings from the wall of ZSM-5 zeolite was used. The starting geometry

ARTICLE

Figure 1. Optimized geometry of (FeOFe)-ZSM-5 cluster (distance values in units of angstroms).

of the cluster model corresponded to the experimental XRD structure of ZSM-5.90 The initial cluster model contained 8 Si and 9 O atoms. Two Al atoms were introduced at the T9 and T12 lattice positions in the adjacent 5T rings to generate distant framework anionic sites resulting in a [Si6Al2O9H14]2 cluster model. The negative charge of the cluster was compensated by the extra framework reactive [FeOFe]2þ complex. The resulting cluster model is shown in Figure 1. The dangling bonds of the terminal silicon/aluminum atoms of all clusters were terminated with H atoms. All atoms of the cluster, except terminating H atoms, and the reactant and product molecules were kept relaxed. Terminating H atoms were kept fixed to orient in the TO direction of the next Si site. All energies obtained from the DFT calculations used for the estimation of the reaction heats and activation barriers were corrected for the zero-point energy obtained from frequency calculations performed on all the optimized structures. Vibrational analysis was also performed to obtain vibrational frequencies for reactions on Fe-ZSM-5 clusters. Time-dependent DFT calculations were performed to calculate UVvis and Raman spectra for the comparison with the experimental data. Mulliken population analysis91 was used to analyze atomic charges and atomic spin densities. Natural bond orbital (NBO)92 analysis was used to analyze the electronic configurations of iron centers. The convergence criteria involving gradients of maximum force, root-mean-square (rms) force, maximum displacement, and rms displacement in Gaussian’03 software were 0.000450, 0.000300, 0.001800, and 0.001200, respectively. The computed ÆS2æ values confirmed that the spin contamination was very small (0.29% after annihilation). The ferromagnetic coupling was assumed for dimeric iron complexes. Previous studies have shown that that antiferromagnetic coupling of the two iron atoms has no significant effect on the N2O decomposition reaction mechanism on the FeOFe site of Fe-ZSM-5.43 The following computational strategy was employed in this study. At the initial stage, a set of single-point energy (SPE) calculations was performed to determine the spin state of the iron-containg cluster model. SPEs were calculated with different values of spin multiplicity (SM) for each cluster system, and the lowest energy spin state was used in the subsequent calculations. Geometry optimization of reaction intermediates and adsorption complexes was performed as follows. The adsorbate molecule was first located in the vicinity of the iron site of the cluster at a selected distance followed by a scan of a potential surface along 9669

dx.doi.org/10.1021/jp201582s |J. Phys. Chem. C 2011, 115, 9668–9680

The Journal of Physical Chemistry C

ARTICLE

the potential reaction coordinate for the particular reaction step. The resulting energy profiles were used to construct the initial models for the transition state (TS) structures and equilibrium geometries (EG), i.e. reaction intermediates. The corresponding guess structure after the energy profile was obtained for the reaction step, i.e. the geometry with the minimum energy on the energy profile, was then reoptimized by means of EG calculations to obtain the final geometry for the particular reaction step. In this reoptimization calculation, the reaction coordinate was not fixed. Additionally, the geometry with the highest energy from the energy profile was taken as the input geometry for the TS geometry calculations. Starting from these geometries, the TS structures were calculated using the synchronous quasi-Newtonian method of optimization, QST3.93 The relative energies of the TS and EG structures (ΔE) were calculated as ΔE ¼ Esystem ðEcluster þ Eadsorbate Þ where Esystem is the calculated energy of the optimized TS or EG structure, Ecluster is the total energy of the optimized cluster model, and Eadsorbate is the total energy of the adsorbent (N2O or C6H6).

3. RESULTS 3.1. Optimization of Cluster and Reactant Molecules. The lowest energy electronic structure of the initial [Fe(μ-O)Fe]ZSM-5 cluster corresponds to the SM value of 9 meaning there are 8 unpaired electrons. This implies that both iron sites in the reactive [FeOFe]2þ complex are present in their formal þ3 oxidation high-spin (S = 4/2) state. This spin state was used in all subsequent calculations. The optimized geometry of the (FeOFe)-ZSM-5 cluster is shown in Figure 1. SiO distances of cluster range from 1.55 to 1.68 Å in line with the experimentally determined values (1.551.65 Å).90 The iron centers in the binuclear [Fe(μ-O)Fe]2þ complex are equivalent. The optimized FeO1 distances are 1.803 and 1.806 Å. The optimized linear N2O molecule has the NN bond length of 1.134 Å, and the NO bond is 1.192 Å. The optimized CC and CH bond lengths in gaseous C6H6 molecule are 1.397 and 1.082 Å, respectively. The theoretical values correspond well with the structural parameters determined experimentally.94,95 3.2. N2O Decomposition on the [Fe(μ-O)Fe]2þ Site. We assume the following sequence of elementary reaction steps for the decomposition of N2O over the binuclear [Fe(μ-O)Fe]2þ site in ZSM-5 and the formation of the reactive oxygenated species.

dissociation of the adsorbed N2O resulting in the formation of gaseous N2 and an extra framework terminal oxygen species (step 2). Coordination of N2O to one of the iron centers in the binuclear site is exothermic (ΔE = 31 kJ/mol). Subsequent N2 dissociation (step 2) proceeds with an activation barrier of 75 kJ/mol and shows a reaction energy of 78 kJ/mol. The NO bond in the TS structure (1.355 Å) is strongly elongated compared to the free N2O molecule. At the next step, step 3, a second N2O molecule coordinates to the oxygenated binuclear iron site ([OFe(μO)Fe]2þ) and subsequently dissociates at step 4, resulting in the formation of the [OFe(μ-O)FeO]2þ complex and N2 molecule. The presence of an additional terminal oxygen ligand at one of the iron centers apparently has only a minor influence on the ability of the adjacent iron site to coordinate N2O. The calculated N2O adsorption energy at step 3 equals 33 kJ/mol. The dissociation of the second N2O molecule (step 4) is less favored kinetically and thermodynamically compared to the first oxygenation reaction (step 2). The calculated activation and reaction energies for step 4 are equal to 93 and 45 kJ/mol, respectively. The NO distance in the TS structure of step 4 is 1.419 Å. The energy profile for step 4 obtained by the coordinate driving calculation in Gaussian’03 is shown in Figure S4 in the Supporting Information to illustrate the computational methodology used. 3.3. Oxidation of Benzene to Phenol on the [OFe(μ-O)FeO]-ZSM-5 Cluster. We consider benzene oxidation to take place over the fully oxygenated binuclear iron complex formed at step 4 considered above. The following elementary reaction steps 516 are considered to be involved in different potential reaction paths for the oxidation of benzene to phenol and water-assisted benzene oxidation. Step 5, benzene adsorption on [OFe(μ-O)FeO]: ½OFeðμ -OÞFeO þ C6 H6 f ½OFeðμ -OÞFe OC6 H6

ð8Þ Step 6, hydrogen transfer and phenol formation: ½OFeðμ -OÞFe O C6 H6 f ½OFeðμ -OÞFe C6 H5 OH

Step 7, phenol desorption:

½OFeðμ -OÞFe C6 H5 OH f ½OFeðμ -OÞFe þ C6 H5 OH

ð10Þ Step 60 , keto-tautomer of phenol (KTP) formation from adsorbed benzene:

Step 1, first N2O adsorption: ½Feðμ -OÞFe þ N2 O f ½Feðμ -OÞFe N2 O

½OFeðμ -OÞFe O C6 H6 f ½OFeðμ -OÞFe OC6 H6

ð4Þ

ð11Þ

Step 2, N2 dissociation: ½Feðμ -OÞFe N2 O f ½Feðμ -OÞFeO þ N2

Step 8, The formation Fe-phenolate and μ-OH from KTP:

ð5Þ

½OFeðμ -OÞFe OC6 H6 f ½OFeðμ -OHÞFe OC6 H5

Step 3, second N2O adsorption:

ð12Þ

½Feðμ -OÞFeO þ N2 O f ½N2 O Feðμ -OÞFeO Step 4, N2 dissociation:

ð9Þ

Step 9, phenol formation from grafted species:

ð6Þ

½OFeðμ -OHÞFe OC6 H5 f ½OFeðμ -OÞFe C6 H5 OH

ð13Þ

½N2 O Feðμ -OÞFeO f ½OFeðμ -OÞFeO þ N2

Step 90 , Fe-phenolate and FeOH formation from step 8:

ð7Þ

½OFeðμ -OHÞFe OC6 H5 f ½HOFeðμ -OÞFe OC6 H5

Decomposition of the first N2O molecule on [Fe(μ-O)Fe]2þ involves the initial molecular adsorption (step 1) followed by the

ð14Þ 9670



ARTICLE

Figure 3. Equilibrium geometry for step 60 (distance values in units of angstroms).

Figure 2. (a) TS and (b) EG for step 5 (distance values in units of angstroms).

Step 10, water adsorption on Fe with phenolate: ½HOFeðμ -OÞFe OC6 H5 þ H2 O f ½HOFeðμ -OÞFe H2 O OC6 H5

ð15Þ

Step 11, phenol formation from phenolate and water molecules: ½HOFeðμ -OÞFe H2 O OC6 H5 f ½HOFeðμ -OÞFe HO C6 H5 OH

ð16Þ

Step 12, phenol desorption: ½HOFeðμ -OÞFe HO C6 H5 OH f ½HOFeðμ -OÞFe HO þ C6 H5 OH

ð17Þ Figure 4. (a) TS and (b) EG for step 8 (distance values in units of angstroms).

Step 13, hydrogen transfer to μ-O: ½HOFeðμ -OÞFe HO f ½HOFeðμ -OHÞFeO

Step 15, water desorption:

ð18Þ

½H2 O Feðμ -OÞFeO f ½Feðμ -OÞFeO þ H2 O

Step 14, water formation:

ð20Þ

½HOFeðμ -OHÞFeO f ½H2 O Feðμ -OÞFeO

Step 16, oxygen desorption:

ð19Þ

½Feðμ -OÞFeO f ½Feðμ -OÞFe þ ð1=2ÞO2 9671

ð21Þ




At the initial stage of the oxidation reaction benzene molecule adsorbs to the fully oxygenated [OFe(μ-O)FeO]2þ site (step 5). This reaction step is exothermic (ΔE = 49 kJ/mol) and shows an activation barrier of 42 kJ/mol. The corresponding TS structure and the resulting C6H6 adsorption complex are depicted in Figure 2. Proton transfer from the activated CH bond of the adsorbed benzene to the terminal O ligand of the extra framework iron cluster results in the formation of an adsorbed phenol molecule (step 6). This reaction step is strongly favored thermodynamically (ΔE = 193 kJ/mol) and faces a relatively low activation barrier (Eact = 61 kJ/mol). Subsequent desorption of phenol (step 7) is endothermic (ΔE = 80 kJ/mol) and results in the formation of a partially oxygenated [Fe(μ-O)FeO]2þ site. Alternatively to the hydroxylation reaction (step 6) the activated benzene complex with [OFe(μ-O)FeO]2þ formed at step 5 can undergo an isomerization reaction (step 60 ) resulting in the formation of an adsorbed keto-tautomer of phenol (KTP, Figure 3). This is a barrierless reaction and shows a very high exothermicity (ΔE = 162 kJ/mol). At the next stage one of the H atoms of the CH2 moiety in the thus formed KTP complex is transferred to the bridging O ligand of the binuclear iron site resulting in the formation of a grafted Fe-phenolate species and μ-OH group bridging two Fe centers (step 8). The transition state structure for this reaction step and the resulting reaction product are shown in Figure 4. This proton

ARTICLE


transfer reaction shows a very low activation barrier of 4 kJ/mol and substantially stabilizes the system by 79 kJ/mol. Subsequent proton transfer from the bridging hydroxyl group in the [OFe(μ-OH)FeOC6H5]2þ complex to the phenolate moiety results in phenol formation (step 9). This reaction step is endothermic (ΔE = 48 kJ/mol) but shows a rather low activation barrier (Eact = 62 kJ/mol). Phenol leaves the reactive site via the step 7 discussed above (ΔE = 80 kJ/mol). Another alternative reaction path for the transformation of [OFe(μ-OH)FeOC6H5]2þ species is the hydrogen transfer from the bridging position to the terminal oxygen species on Fe1 site (step 90 ). Because of the higher basicity of the terminal O ligand compared to that of the bridging one, this reaction is favored thermodynamically (ΔE = 55 kJ/mol) and proceeds with an activation barrier of 73 kJ/mol. Figure 5 depicts the optimized structures of the transition state and the resulting [HOFe(μ-O)FeOC6H5]2þ complex. There is no direct reaction path for further transformation of [HOFe(μ-O)FeOC6H5]2þ resulting in C6H5OH formation. We propose that the interaction with an external proton-donor can result in the decomposition of the grafted phenolate species and yield phenol. Coordination of a water molecule (step 10) at the Fe2 site of [HOFe(μ-O)FeOC6H5]2þ species is exothermic (ΔE = 82 kJ/mol) and does not alter significantly the electronic structure of the iron sites. Subsequent hydrolysis of the phenolate moiety results in the formation of a coordination complex of phenol with a hydroxylated iron 9672



ARTICLE

Table 1. Mulliken Atomic Charge and Spin Density Values and Electron Configurations of Iron Atoms of EGs for Steps steps/cluster

iron atom

Mulliken atomic charge

Mulliken spin density

electron configuration

Fe1

þ0.815

3.814

Fe2

þ0.809

3.795

[core]4S(0.28)3d(6.15)4p(0.03)

step 2

Fe1

þ0.900

3.791

[core]4S(0.26)3d(6.14)4p(0.02)

(Fe(μ-O)FeO)

Fe2

þ1.024

3.295

[core]4S(0.26)3d(5.91)4p(0.03)4d(0.01)

step 3

Fe1

þ0.925

3.777

[core]4S(0.28)3d(6.12)4p(0.02)

step 1

[core]4S(0.29)3d(6.15)4p(0.03)

Fe2

þ1.005

3.318

[core]4S(0.26)3d(5.91)4p(0.03)4d(0.01)

step 4

Fe1

þ0.997

3.258

[core]4S(0.27)3d(5.91)4p(0.02)4d(0.01)

(OFe(μ-O)FeO) step 5

Fe2 Fe1

þ1.159 þ0.996

3.281 3.275

[core]4S(0.27)3d(5.88)4p(0.02)4d(0.01) [core]4S(0.27)3d(5.91)4p(0.02)4d(0.01)

Fe2

þ1.052

4.110

[core]4S(0.25)3d(5.79)4p(0.03)5p(0.01)

Fe1

þ0.963

3.351

[core]4S(0.26)3d(5.91)4p(0.03)4d(0.01)

Fe2

þ0.880

3.797

[core]4S(0.26)3d(6.10)4p(0.02)5p(0.01)

Fe1

þ0.947

3.330

[core]4S(0.26)3d(5.92)4p(0.03)4d(0.01)

Fe2

þ0.884

3.799

[core]4S(0.27)3d(6.08)4p(0.02)

step 8

Fe1

þ1.528

2.718

[core]4S(0.27)3d(6.17)4p(0.03)

step 9

Fe2 Fe1

þ1.888 þ0.962

4.104 3.353

[core]4S(0.25)3d(5.83)4p(0.03) [core]4S(0.26)3d(5.91)4p(0.03)4d(0.01)

Fe2

þ0.871

3.797

[core]4S(0.25)3d(6.11)4p(0.02)5p(0.01)

Fe1

þ0.904

2.935

[core]4S(0.31)3d(5.95)4p(0.02)5p(0.01)

Fe2

þ1.113

4.117

[core]4S(0.25)3d(5.81)4p(0.03)

Fe1

þ0.866

2.937

[core]4S(0.34)3d(5.95)4p(0.03)5p(0.01)

step 6 step 60

step 90 step 10

Fe2

þ1.165

4.147

[core]4S(0.26)3d(5.77)4p(0.02)

step 11

Fe1

þ0.941

2.989

[core]4S(0.29)3d(5.95)4p(0.02)5p(0.01)

step 13

Fe2 Fe1

þ1.068 þ0.721

4.173 3.806

[core]4S(0.27)3d(5.74)4p(0.03)5p(0.01) [core]4S(0.28)3d(6.10)4p(0.04)5p(0.01)

step 14 step 17

Fe2

þ1.118

3.225

[core]4S(0.27)3d(5.89)4p(0.02)4d(0.01)

Fe1

þ0.860

3.808

[core]4S(0.27)3d(6.11)4p(0.02)

Fe2

þ1.002

3.325

[core]4S(0.26)3d(5.91)4p(0.03)4d(0.01)

Fe1

þ0.869

3.835

[core]4S(0.28)3d(6.13)4p(0.03)

Fe2

þ1.022

4.137

[core]4S(0.26)3d(5.79)4p(0.04)5p(0.01)

step 18

Fe1

þ0.806

3.817

[core]4S(0.29)3d(6.15)4p(0.03)

step 180

Fe2 Fe1

þ0.771 þ0.827

3.802 3.820

[core]4S(0.26)3d(6.15)4p(0.03) [core]4S(0.29)3d(6.15)4p(0.03)

step 20 step 21

Fe2

þ0.757

3.766

[core]4S(0.30)3d(6.10)4p(0.03)

Fe1

þ0.894

3.861

[core]4S(0.26)3d(6.12)4p(0.03)

Fe2

þ0.732

3.708

[core]4S(0.32)3d(6.11)4p(0.04)

Fe1

þ0.806

3.817

[core]4S(0.29)3d(6.15)4p(0.03)

Fe2

þ0.771

3.802

[core]4S(0.26)3d(6.15)4p(0.03)

(Fe(μ-O)Fe)

Fe1

þ0.831

3.812

[core]4S(0.30)3d(6.15)4p(0.03)

(HOFe(μ-O)FeOH)

Fe2 Fe1

þ0.832 þ0.971

3.801 2.877

[core]4S(0.31)3d(6.15)4p(0.03) [core]4S(0.31)3d(5.96)4p(0.02)5p(0.01)

Fe2

þ1.094

4.191

[core]4S(0.26)3d(5.75)4p(0.03)5p(0.01)

complex ([HOFe(μ-O)Fe(OH)(HOC6H5)]2þ phenol formation (step 11)). A proton transfer from the coordinated H2O to the OC6H5 moiety is endothermic (ΔE = 39 kJ/mol) and faces a barrier of 51 kJ/mol. The reaction product and the transition state structure are shown in Figure 6. At the next step, step 12, phenol leaves the hydroxylated iron species with a desorption energy of 71 kJ/mol. The optimized structure of the resulting ([HOFe(μ-O)FeOH]2þ complex is shown in Figure S9 in Supporting Information. To regenerate oxygenation activity of the extra framework binuclear iron site the removal of water that has been used as a cocatalyst at the previous reaction steps is necessary. This involves a

number of proton-transfer steps within the extra framework iron complex. First, a proton from the terminal OH group at the Fe1 site migrates to the bridging μ-O ligand (step 13, Eact = 114 kJ/mol; ΔE = 43 kJ/mol). Recombination of the bridging hydroxyl moiety with the terminal one at the Fe2 site (step 14) results in the formation of molecular H2O. This step is substantially more favorable compared to the initial proton transfer to the bridging O ligand. The calculated activation and reaction energies are 35 and 27 kJ/mol, respectively. The release of H2O from the binuclear iron site proceeds through a desorption step 15 with a reaction energy of 85 kJ/mol. 3.4. Oxidation of Benzene to Phenol on the [Fe(μ-O)FeO]-ZSM-5 Cluster. All of the phenol formation paths 9673



ARTICLE

Table 2. Mulliken Atomic Charge and Spin Density Values and Electron Configurations of Iron Atoms of TSs for Steps steps

iron atom

Mulliken atomic charge

Mulliken spin density

step 2

Fe1

þ0.861

3.827

[core]4S(0.28)3d(6.14)4p(0.03)

Fe2

þ1.035

4.033

[core]4S(0.27)3d(5.88)4p(0.03)5p(0.01)

Fe1

þ1.054

4.052

[core]4S(0.26)3d(5.85)4p(0.03)5p(0.01)

Fe2

þ1.140

3.294

[core]4S(0.27)3d(5.88)4p(0.02)4d(0.01)

Fe1

þ0.924

3.336

[core]4S(0.26)3d(5.91)4p(0.03)4d(0.01)

Fe2

þ1.090

3.484

[core]4S(0.26)3d(5.92)4p(0.02)

step 6

Fe1

þ0.943

3.341

[core]4S(0.26)3d(5.91)4p(0.03)4d(0.01)

step 8

Fe2 Fe1

þ0.883 þ0.948

3.785 3.314

[core]4S(0.27)3d(6.08)4p(0.02) [core]4S(0.26)3d(5.91)4p(0.03)4d(0.01)

Fe2

þ0.882

3.793

[core]4S(0.26)3d(6.09)4p(0.02)5p(0.01)

step 9

Fe1

þ0.986

3.320

[core]4S(0.26)3d(5.90)4p(0.02)4d(0.01)

Fe2

þ0.829

3.775

[core]4S(0.32)3d(6.07)4p(0.02)

step 90

Fe1

þ0.938

4.074

[core]4S(0.28)3d(5.85)4p(0.03)5p(0.01)

Fe2

þ0.929

3.470

[core]4S(0.29)3d(6.03)4p(0.03)

step 11

Fe1

þ0.956

2.977

[core]4S(0.29)3d(5.94)4p(0.02)5p(0.01)

step 13

Fe2 Fe1

þ1.055 þ0.931

4.180 2.889

[core]4S(0.28)3d(5.73)4p(0.02)5p(0.01) [core]4S(0.29)3d(5.98)4p(0.03)5p(0.01)

Fe2

þ0.985

4.085

[core]4S(0.28)3d(5.84)4p(0.03)5p(0.01)

step 14

Fe1

þ0.792

3.835

[core]4S(0.28)3d(6.10)4p(0.03)5p(0.01)

Fe2

þ1.088

3.276

[core]4S(0.27)3d(5.89)4p(0.02)4d(0.01)

Fe1

þ0.863

3.796

[core]4S(0.28)3d(6.15)4p(0.03)

Fe2

þ0.843

2.898

[core]4S(0.31)3d(6.00)4p(0.02)

step 18

Fe1

þ0.828

3.817

[core]4S(0.29)3d(6.15)4p(0.03)

step 20

Fe2 Fe1

þ0.766 þ0.834

3.774 3.831

[core]4S(0.30)3d(6.10)4p(0.04) [core]4S(0.28)3d(6.14)4p(0.03)

Fe2

þ0.785

3.760

[core]4S(0.32)3d(6.09)4p(0.03)5p(0.01)

step 21

Fe1

þ0.866

3.838

[core]4S(0.27)3d(6.14)4p(0.03)

Fe2

þ0.755

3.772

[core]4S(0.33)3d(6.08)4p(0.03)

step 4 step 5

step 17

Step 20, Fe-phenolate and μ-OH formation from KTP:

presented in section 3.3 result in the formation of a partially oxygenated [Fe(μ-O)FeO]2þ species. The terminal O ligand in this structure can also be considered as the reactive site for benzene oxidation. By analogy with the case of the fully oxygenated binuclear iron site, we considered the following elementary steps 1721 involved in the direct benzene to phenol oxidation over [Fe(μ-O)FeO]2þ.

½Feðμ -OÞFe OC6 H6 f ½Feðμ -OHÞFe OC6 H5 ð26Þ Step 21, phenol formation from grafted species: ½Feðμ -OHÞFe OC6 H5 f ½Feðμ -OÞFe C6 H5 OH ð27Þ

Step 17, benzene adsorption on [Fe(μ-O)FeO]: ½Feðμ -OÞFeO þ C6 H6 f ½Feðμ -OÞFe O C6 H6 Step 18, phenol formation from adsorbed benzene:

electron configuration

ð22Þ

½Feðμ -OÞFe O C6 H6 f ½Feðμ -OÞFe C6 H5 OH ð23Þ Step 19, phenol desorption: ½Feðμ -OÞFe C6 H5 OH f ½Feðμ -OÞFe þ C6 H5 OH ð24Þ 0

Step 18 , keto-tautomer of phenol (KTP) formation from adsorbed benzene: ½Feðμ -OÞFe O C6 H6 f ½Feðμ -OÞFe OC6 H6 ð25Þ

Similar to the case of the fully oxygenated cluster discussed above, at the initial step of benzene oxidation over [Fe(μ-O)FeO]2þ site, benzene coordinates to the terminal O ligand with one of its carbon atoms (step 17) via an activated adsorption process. This step is slightly exothermic (ΔE = 19 kJ/mol) and shows a rather high activation energy (102 kJ/mol). The activation adsorption of benzene is followed by an H transfer from the phenyl ring to the coordinated oxygen ligands resulting in the formation of adsorbed phenol (step 18). This process is highly exothermic (ΔE = 172 kJ/mol) and has an activation energy of 85 kJ/mol. The electronic properties of the iron centers in the respective structures are very similar (Tables 1 and 2). Similar to the situation considered in section 3.3, a reaction path competing with the direct phenol formation is identified. The adsorbed benzene isomerizes (step 180 ) to a keto-tautomer of phenol without an activation barrier and with a very high exothermicity (ΔE = 144 kJ/mol). The KTP species further decomposes yielding a grafted Fe-phenolate moiety and a 9674




bridging OH group (step 20) with a negligible activation energy (Eact = 2 kJ/mol) and high exothermicity (ΔE = 90 kJ/mol). Phenol is formed via the proton transfer from the bridging OH group to the phenolate moiety (step 21, Eact and ΔE are, respectively, 63 and 62 kJ/mol). The TS structure and the resulting adsorption complex with phenol are shown in Figure 7. No significant changes are observed in the electronic structure of the extra framework iron atoms involved in the transition states and reaction intermediates formed along the KTP reaction path (steps 1721, Tables 1 and 2). The catalytic cycle is closed via phenol desorption (step 19, ΔE = 75 kJ/mol).

4. DISCUSSION Benzene oxidation to phenol by N2O has been theoretically investigated on the (FeOFe)2þ site of the ZSM-5 cluster in this study to allow comparing the reactivity of such species with our earlier computational results71,74,76 obtained for Fe1þ-, Fe2þ- and (FeO)1þ-containing cluster models. The SM number of 9 computed in this study for the cluster is the same as the SM number used for the iron dimer cluster in the literature.26,34,43,79,80 Tables 1 and 2 report the Mulliken atomic charges, spin densities, and electron configurations of iron atoms of the cluster for all steps for EG and TS geometries, respectively. As can be seen from these tables, the iron atoms have a high Mulliken spin density, which indicates that the unpaired electrons are localized on iron atoms. The Mulliken spin densities of iron atoms of the cluster after the reactions are somewhat higher than those of the initial FeOFe-ZSM-5 cluster. The small

ARTICLE

increase in spin density is also reflected in the Mulliken atomic charges, which show a more positive character on iron atoms in the cluster during the reactions in comparison to the initial FeOFe-ZSM-5 cluster. It has been suggested that the shift of the band position of 455 cm1 for the calcined catalyst to 526 cm1 for the hightemperature treated Fe-ZSM-5 catalyst was related to a change of the FeOFe angle from ∼150 to ∼130 in an experimental study52 on benzene hydroxylation. Kurtz96 reported a relation between IR bands and FeOFe angles for (μ-O)di-iron complexes, where the bands near 570 and 450 cm1 were obtained for 120 and 150, respectively. In present study, the FeOFe angles and their related frequencies for the [HOFe(μO)FeOH]-ZSM-5 and [Fe(μ-O)Fe]-ZSM-5 clusters were computed as 145, 485 cm1 and 117, 585 cm1, respectively. These values are in line with the experimental values mentioned above. Moreover, the FeOFe angles of the cluster for all steps considered in this study increased after the reactions. An increase in the FeOFe angle after a reaction is a generally observed trend.26,79,80 Prior to the benzene oxidation the [Fe(μ-O)Fe] site promotes the N2O decomposition resulting in the formation of [Fe(μ-O)FeO] and [OFe(μ-O)FeO] complexes. The thus formed terminal extra framework oxygen species are the reactive centers for the subsequent oxidation of benzene to phenol. A comparison of the activation energy barriers for both N2O decomposition and benzene oxidation to phenol cycle steps on the [Fe(μ-O)Fe]2þZSM-5 cluster with the available literature data is given in Table 3. N2O decomposition reactions occur via two consecutive steps: N2O adsorption and its subsequent dissociation on the cluster. These reactions take place in a similar manner on the alternative Fe2þ-ZSM-571 and (FeO)1þ-ZSM-5 clusters,76 whereas a direct dissociative adsorption has been previously observed for the Fe1þZSM-5 cluster.41 Panov et al.33 have shown that the molecular adsorption of N2O precedes its decomposition over a Fe-ZSM-5 catalyst containing 0.056 wt % Fe. The activation barriers for first and second N2O molecule decomposition reactions are somewhat smaller than previous theoretical values34,43 reported on the same cluster. The difference with the results presented here (Table 3) is probably due to the differences in the computational methodologies used and more specifically to the differences in basis sets used for the description of iron atoms. Figure 8 shows a summary of the calculated reaction energy diagram for the benzene oxidation to phenol over the fully oxygenated binuclear iron complex ([OFe(μ-O)FeO]). Activation barriers for the respective reaction steps involved in the benzene oxidation to phenol are also tabulated in Table 3. Steps 512 include benzene adsorption, phenol formation from both adsorbed benzene and grafted complexes formed on the cluster, keto-tautomer of phenol (KTP) formation and phenol desorption reactions. The most critical reaction here is the formation of KTP which was originally proposed in a theoretical study by Kachurovskaya et al.72 The formation of KTP intermediate (step 60 ) is strongly favored and proceeds without an activation barrier while its consecutive reaction (step 6, phenol formation from adsorbed benzene) has a higher activation barrier of 61 kJ/mol. The KTP molecule readily interacts with the extra framework iron species resulting in the formation of a grafted phenolate intermediate (step 8) with an almost negligible lower activation energy (4 kJ/mol). The activation barriers for the formation of phenol via steps 6 and 9, representing the phenol formation directly from the adsorbed benzene and from the phenolate and 9675



ARTICLE

Table 3. Comparison of Activation Barriers for All Steps with Available Literature Data (Values in Units of kJ/mol) iron sites of Fe-ZSM-5 ([Fe(μ-O)Fe]2þ)a

steps step 1, N2O adsorption

0b

0 d

step 2, N2 dissociation

e

75, 97, 115

step 3, N2O adsorption

0

step 4, N2 dissociation

93, 43,d 116e

steps 5 and 17, benzene adsorption

42, 102

steps 6 and 18, phenol formation from adsorbed benzene

61, 85

steps 60 and 180 , KTP formation

0

steps 8 and 20, phenolate and μ-OH formation steps 9 and 21, phenol formation from grafted complexes

4, 2 62,63

0

step 9 , Fe-phenolate and FeOH formation

73

step 10, water adsorption on Fe with phenolate

0 51

steps 7, 12, 19, phenol desorption

80, 71, 75

step 13, hydrogen transfer of FeOH to μ-O

114 35

step 15, water desorption step 16, oxygen desorption

85 293

Fe2þ

Fe1þ

0c b

f

113, 129

83c

19,g 10,h 12 f 243, g,i 157j,i

20

b

77

b

0

c

41

89,k,l 73,m,l 100n,l c

69b 158,b 190,o,p 215,n 141q

step 11, phenol formation from phenolate and water molecules

step 14, water formation

(FeO)1þ

82,b 94,b 85,o,p 84,n 84q

203,k 130,m 252o

126c

24,k 85,m 19o

experimental 126r

benzene oxidation to phenol by N2O on Fe-ZSM-5 catalyst a

b

This study. Fellah et al. (Gaussian/DFT/B3LYP/Cluster). Fellah et al.71 (GAUSSAIN/DFT/B3LYP/Cluster). d Hansen et al.43 TURBOMOLE/ DFT/B3LYP/Cluster). e Hansen et al.34 TURBOMOLE/DFT/B3LYP/Cluster). f Heyden et al.38 (TURBOMOLE/DFT/B3LYP/Cluster). g Fellah and Onal41 (Gaussian/DFT/B3LYP/Cluster). h Yoshizawa et al.40 (Gaussian/DFT/B3LYP/Cluster). i O2 molecule was formed directly by second N2O decomposition. j Ryder et al.39 (Gaussian/DFT/B3LYP/Cluster). k Fellah and Onal74 (Gaussian/DFT/B3LYP/Cluster). l Benzene adsorption occurs via CH bond activation of benzene on the active site. m Yoshizawa et al.48 (Gaussian/DFT/B3LYP/Cluster). n Yoshizawa et al.50 Gaussian/ ONIOM/QM:DFT/B3LYP-MM:UFF/total 2084 atoms). o Ryder et al.47 (Gaussian/DFT/B3LYP/Cluster). p Reaction occurs on the (FeO2) center of the ZSM-5cluster. q Yoshizawa et al.73 (Gaussian/DFT/B3LYP/Cluster r Panov et al.14 (648698 K, feed gas concentration of 3080 mol % benzene, 1.55.0 mol % N2O and 04.7 mol % phenol). 76

c

Figure 8. Relative energy profile for benzene oxidation to phenol on (OFeOFeO) site (steps 615).

hydroxyl groups, respectively, are very close to each other (61 and 62 kJ/mol, respectively).

Xia et al.81 have reported that the addition of water increases phenol production in the room-temperature oxidation of 9676



ARTICLE

benzene by N2O over Fe-ZSM-5. Thus, we also considered the potential role of water in benzene oxidation over the binuclear iron site. In particular we investigated the water-promoted decomposition of the grafted Fe-phenolate and FeOH species to phenol to simulate the experimental study by Xia et al.,81 where water was added to the reactant stream after the formation of the grafted complexes on the catalyst. This is similar to the hydrolysis of methoxy and hydroxyl on the Fe1þ-ZSM-5 cluster.75 As can be seen from Table 3 and Figure 8, addition of water has decreased the activation barrier of phenol formation by 10 kJ/mol with respect to other water-free phenol formation reaction paths (steps 6 and 9). In addition, Xia et al.81 have reported UVvisible and Raman spectra of a Fe-ZSM-5 catalyst treated with benzene at room temperature. In their study, a UVvis band at 340 nm and a broad band in the range of 500900 nm were assigned to the grafted Fe(III)phenolate complexes. Ferric phenolate complexes were usually characterized by the absorption bands in the range 450900 nm,97 The TD-DFT computed UVvis spectra for the structure containing Fephenolate and FeOH reveal absorption bands at 383 nm and in the range of 450900 nm, respectively, which is in line with the experimental results. The Table 4. Raman Bands for the Complex Including FePhenolate and FeOH (EG of Step 90 )

a

this study

other theoreticala

experimentala

648

645

643

FeO

911

881

896

FeO and CdC

1010 1104

1010 1105

990 1149

phenyl ring breath CH

1279

1254

1228

CO

1491

1480

1475

CdC

1616

1619

1580

CdC

1631

1630

1607

CdC

Xia et al.81

band assignment

Raman bands calculated for the geometry shown in Figure 5b are compared with the available experimental results81 in Table 4. Our theoretical predictions are in perfect agreement with the experimental data. Futhermore, Xia et al.81 have also studied the high-temperature oxygen evolution from Fe-ZSM-5. The formation of oxygen was observed only at temperatures above 800 K which is consistent with the reaction energy of 293 kJ/mol calculated for such a process (step 16) from the extra framework binuclear iron complex. All of the phenol formation paths (steps 1721) over [OFe(μ-O)FeO]2þ resulted in the formation of a partially oxygenated [Fe(μ-O)FeO]2þ species, which can also act as the reactive center for benzene oxidation. The corresponding DFT-computed reaction energy diagram for the benzene oxidation to phenol over the [Fe(μ-O)FeO]-ZSM-5 cluster is shown in Figure 9. Activation barriers for benzene oxidation to phenol cycle steps on the [Fe(μ-O)FeO]2þ site are summarized in Table 3. These steps are similar to the steps 512 considered for the [OFe(μ-O)FeO] site. Activation barriers of these steps are comparable with each other except those of steps 5 and 17, involving the initial activated benzene adsorption. The activation energy calculated for step 17 (102 kJ/mol) in the case of the partially oxygenated species is much higher than the value of 42 kJ/mol computed for step 5 that takes place over the [OFe(μ-O)FeO]2þ site. This is probably due to the fact that the iron atoms of the TS for step 5 have higher spin densities than those of the iron atom of the TS for step 17. This is indicative of a higher positive charge on the iron atom in the fully oxygenated complex compared. It should be noted that the benzene adsorption and KTP formation reactions (steps 5, 60 , 17 and 180 ) on both [OFe(μO)FeO] and [Fe(μ-O)FeO] sites cause a substantial loss of aromaticity of the benzene ring as is evidenced by the geometrical perturbations of the adsorbed molecules in the corresponding structures. The aromaticity of the hydrocarbon moiety is restored during hydrogen transfer reactions (steps 6, 8, 9, 90 , 11, 18, 20

Figure 9. Relative energy profile for benzene oxidation to phenol on (FeOFeO) site (steps 1727). 9677


The Journal of Physical Chemistry C and 21) resulting in either the phenolate formation or the phenol adsorption complex. This effect is contributing to the high exothermicity of the respective chemical reactions. In summary, the essential result of this study is that the interaction of the oxygenated binuclear [OFe(μ-O)FeO]2þ and [Fe(μ-O)FeO]2þ sites with benzene to produce phenol is more energetically favored than involving the mononuclear iron sites in ZSM-5. The major difference between these two sites and the (FeO)2þ species resulting from the N2O dissociation over the exchangeable Fe2þ cations is that in the case of the binuclear clusters a reaction alternative to the phenol desorption is present that leads to the formation of the highly stable grafted OH and phenolate species. A similar path also exists for the (OFeO)1þ species originating from the oxygenation of isolated FeOþ species. We propose that the formation of grafted phenolates results in a rapid deactivation of the catalytic iron sites. Only in the case of isolated Fe2þ such a deactivation path is not present resulting in their stable catalytic activity. This high catalytic stability of the exchangeable Fe2þ sites is in line with experimental observation.2022,81 Grafted phenolates are easily formed with a very low activation barrier after the formation of ketotautomer of phenol molecule that is produced without a barrier. The overall activation energy for the formation of phenol and the regeneration of the active site from the phenolate state is substantially higher compared to the direct route. In the presence of water its decomposition is facilitated and therefore the poisoning effect is diminished. In the presence of water the activation barrier for the formation of phenol decreases in line with the experimental results by Xia et al.81 The combination of the current theoretical results and those presented in our previous studies in refs 71, 74, and 76 clearly indicate the unique role of mononuclear iron(II) sites on Fe-ZSM-5 catalyst in the catalytic oxidation of benzene to phenol by N2O.

5. CONCLUSIONS The elementary reaction steps involved in the catalytic benzene oxidation to phenol by N2O over binuclear iron sites in Fe-ZSM-5 represented by a [Si6Al2O9H14(Fe(μ-O)Fe)] cluster model have been investigated by means of DFT calculations. According to the results obtained in this study and our previous theoretical results,71,74,76 the isolated Fe2þ sites in ZSM-5 are the preferred active species for benzene oxidation to phenol by N2O as compared to (FeO)1þ, Fe1þ and [Fe(μ-O)Fe]2þ complexes in ZSM-5. When (FeO)1þ and [Fe(μ-O)Fe]2þ are considered as the reactive center, the formation of very stable grafted phenolates is preferred over the regeneration of the active sites via phenol desorption in the course of the catalytic reaction. Grafted phenolate species is easily formed with a very low activation barrier after the formation of keto-tautomer of phenol molecule produced without a barrier. Addition of water promoted the decomposition of the grafted phenols via the phenol formation. Our theoretical studies on catalytic activity of different sites in Fe-ZSM-5 show that the mononuclear iron(II) sites are responsible for the catalytic reactivity of Fe-ZSM-5 in the oxidation of benzene to phenol by N2O. ’ ASSOCIATED CONTENT

bS

Supporting Information. Several illustrations including EG and TS geometries for steps 1, 2, 3, 4, 6, 9, 10, 13, 14, 17, 18,

ARTICLE

180 and 20. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*M.F.F.: phone, þ90 432 225 1024; e-mail, mff[email protected]. I.O.: phone, þ90 312 210 2639; fax, þ90 312 210 2600; e-mail, [email protected]. R.A.v.S.: phone, þ31 40 247 3082; fax, þ31 40 245 5054; e-mail, [email protected].

’ ACKNOWLEDGMENT The numerical calculations reported in this paper were performed at TUBITAK ULAKBIM, High Performance and Grid Computing Center (TR-Grid e-Infrastructure). This study was also supported by CENG HPC System of METU. ’ REFERENCES (1) Wallace, J.; , Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Wiley: Hoboken, NJ, 2005; Vol. 18, p 747. (2) Panov, G. I. CATTECH 2000, 4, 18–32. (3) Panov, G. I.; Kharitonov, A. S.; Sobolev, V. I. Appl. Catal., A 1993, 98, 1–20. (4) Kharitonov, A. S.; Aleksandrova, T. N.; Panov, G. I.; Sobolev, V. I.; Sheveleva, G. A.; Paukshtis, E. A. Kinet. Catal. 1994, 35, 270–278. (5) Sobolev, V. I.; Kharitonov, A. S.; Paukshtis, Ye. A.; Panov, G. I. J. Mol. Catal. 1993, 84, 117–124. (6) Sobolev, V. I.; Dubkov, K. A.; Paukshtis, E. A.; Pirutko, L. V.; Rodkin, M. A.; Kharitonov, A. S.; Panov, G. I. Appl. Catal., A 1996, 141, 185–192. (7) Sheveleva, G. A.; Kharitonov, A. S.; Panov, G. I.; Sobolev, V. I.; Razdobarova, N. L.; Paukshtis, Y. A.; Romannikov, V. N. Petrol. Chem. 1993, 33, 516–524. (8) Kharitonov, A. S.; Sheveleva, G. A.; Panov, G. I.; Sobolev, V. I.; Paukshtis, Ye. A.; Romannikov, V. N. Appl. Catal., A 1993, 98, 33–43. (9) Sheveleva, G. A.; Panov, G. I.; Kharitonov, A. S.; Romannikov, V. N.; Vostrikova, L. A. Sib. Khem. Zh. 1992, 3, 93–96. (10) Panov, G. I.; Sheveleva, G. A.; Kharitonov, A. S.; Romannikov, V. N.; Vostrikova, L. A. Appl. Catal., A 1992, 82, 31–36. (11) Panov, G. I.; Kharitonov, A. S.; Sobolev, V. I. Appl. Catal., A 1993, 98, 1–20. (12) Panov, G. I.; Sheveleva, G. A.; Kharitonov, A. S.; Romannikov, V. N.; Vostrikova, L. A. Appl. Catal. 1992, 82, 31–36. (13) Volodin, A. M.; Bolshov, V. A.; Panov, G. I. J. Phys. Chem. 1994, 98, 7548–7550. (14) Ivanov, A. A.; Chernyavsky, V. S.; Gross, M. J.; Kharitonov, A. S.; Uriarte, A. K.; Panov, G. I. Appl. Catal., A 2003, 249, 327–343. (15) Pirutko, L. V.; Chernyavsky, V. S.; Uriarte, A. K.; Panov, G. I. Appl. Catal., A 2002, 227, 143–157. (16) Pillai, K. S.; Jia, J.; Sachtler, W. M. H. Appl. Catal., A 2004, 264, 133–139. (17) Waclaw, A.; Nowinska, K.; Schwieger, W. Appl. Catal., A 2004, 270, 151–156. (18) Yuronov, I.; Bulushev, D. A.; Renken, A.; Kiwi-Minsker, L. Appl. Catal., A 2007, 319, 128–136. (19) Perathoner, S.; Pino, F.; Centi, G.; Giordano, G.; Katovic, A.; Nagy, J. B. Top. Catal. 2003, 23, 125–136. (20) Hensen, E. J. M.; Zhu, Q.; Janssen, R. A. J.; Magusin, P. C. M. M.; Kooyman, P. J.; van Santen, R. A. J. Catal. 2005, 233, 123–135. (21) Hensen, E. J. M.; Zhu, Q.; van Santen, R. A. J. Catal. 2005, 233, 136–146. (22) Hensen, E. J. M.; Zhu, Q.; Liu, P. H.; Chao, K. J.; van Santen, R. A. J. Catal. 2004, 226, 466–470. 9678


The Journal of Physical Chemistry C (23) Kapteijn, F.; Rodreiguez-Mirasol, J.; Moulijn J. Appl. Catal. B 1996, 9, 25–64. (24) Kapteijn, F.; Marb, G.; Rodriguez-Mirasol, J.; Moulijn, J. A. J. Catal. 1997, 167, 256–265. (25) El-Malki, E. M.; van Santen, R. A.; Sachtler, W. M. H. J. Catal. 2000, 196, 212–223. (26) Wood, B. R.; Reimer, J. A.; Bell, A. T.; Janicke, M. T.; Ott, K. C. J. Catal. 2004, 224, 148–155. (27) Zhu, Q.; Mojet, B. L.; Janssen, E. J. M.; van Grondelle, J.; Magusin, P. C. M. M.; van Santen, R. A. Catal. Lett. 2002, 81, 205–212. (28) Pirngruber, G. D.; Roy, P. K.; Prins, R. J. Catal. 2007, 246, 147–157. (29) Pirngruber, G. D.; Luechinger, M.; Roy, P. K.; Cecchetto, A.; Smirniotis, P. J. Catal. 2004, 224, 429–440. (30) Groen, J. C.; Br€uckner, A.; Berrier, E.; Maldonado, L.; Moulijin, J. A.; Ramirez, J. P. J. Catal. 2006, 243, 212–216. (31) da Cruz, R. S.; Mascarenhas, A. J.S.; Andrade, H. M. C. Appl. Catal., B 1998, 18, 223–231. (32) Dubkov, K. A.; Ovannesyan, N. S.; Shteinman, A. A.; Starokon, E. V.; Panov, G. I. J. Catal. 2002, 207, 341–352. (33) Panov, G. I.; Uriarte, A. K.; Rodkin, M. A.; Sobolev, V. I. Catal. Today 1998, 41, 365–385. (34) Hansen, N.; Heyden, A.; Bell, A. T.; Keil, F. J. J. Catal. 2007, 248, 213–225. (35) Heyden, A.; Bell, A. T.; Keil, F. J. J. Catal. 2005, 233, 26–35. (36) Yakovlev, A. L.; Zhidomirov, G. M.; van Santen, R. A. Catal. Lett. 2001, 75, 45–48. (37) Yakovlev, A. L.; Zhidomirov, G. M.; van Santen, R. A. J. Phys. Chem. B 2001, 105, 12297–12302. (38) Heyden, A.; Peters, B.; Bell, A. T.; Keil, F. J. J. Phys. Chem. B 2005, 109, 1857–1873. (39) Ryder, J. A.; Chakraborty, A. K.; Bell, A. T. J. Phys. Chem. B 2002, 106, 7059–7064. (40) Yoshizawa, K.; Yumura, T.; Yoshihito, Y.; Yamabe, T. Bull. Chem. Soc. Jpn. 2000, 73, 29–36. (41) Fellah, M. F.; Onal, I. Catal. Today 2007, 137, 410–417. (42) Yoshizawa, K.; Yumura, T.; Shiota, Y.; Yamabe, T. Bull. Chem. Soc. Jpn. 2000, 73, 29–36. (43) Hansen, N.; Heyden, A.; Bell, A. T.; Keil, F. J. J. Phys. Chem. C 2007, 111, 2092–2101. (44) Chernyavsky, V. S.; Pirutko, L. V.; Uriarte, A. K.; Kharitonov, A. S.; Panov, G. I. J. Catal. 2007, 245, 466–469. (45) Parmon, V. N.; Panov, G. I.; Uriarte, A.; Noskov, A. S. Catal. Today 2005, 100, 115–131. (46) Dubkov, K. A.; Sobolev, V. I.; Talsi, E. P.; Rodkin, M. A.; Watkins, N. H.; Shteinman, A. A.; Panov, G. I. J. Mol. Catal. A 1997, 123, 155–161. (47) Ryder, J. A.; Chekraborty, A. K.; Bell, A. T. J. Catal. 2003, 220, 84–91. (48) Yoshizawa, K.; Shiota, Y.; Kagawa, Y.; Yamabe, T. J. Phys. Chem. B 2000, 104, 734–740. (49) Yoshizawa, K.; Shiota, Y.; Yamabe J. Am. Chem. Soc. 1999, 121, 147–153. (50) Shiota, Y.; Suzuki, K.; Yoshizawa, K. Organometallics 2006, 25, 3118–3123. (51) Kachurovskaya, N. A.; Zhidomirov, G. M.; Hensen, E. J. M.; van Santen, R. A. Catal. Lett. 2003, 86, 25–31. (52) Sun, K.; Xia, H.; Feng, Z.; van Santen, R. A.; Hensen, E.; Li, C. J. Catal. 2008, 254, 383–396. (53) Battiston, A. A.; Bitter, J. H.; de Groot, F. M. F.; Overweg, A. R.; Stephan, O.; van Bokhoven, J. A.; Kooyman, P. J.; van der Spek, C.; Vanko, G.; Koningsberger, D. C. J. Catal. 2003, 213, 251–271. (54) Zecchina, A.; Rivallan, M.; Berlier, G.; Lamberti, C.; Ricciardi, G. Phys. Chem. Chem. Phys. 2007, 9, 3483–3499. (55) Krishna, K.; Makkee, M. Catal. Today 2006, 114, 23–30. (56) Long, R. Q.; Yang, R. T. Catal. Lett. 2001, 74, 201–205. (57) Schwidder, M.; Kumar, M. S.; Klementiev, K.; Pohl, M. M.; Br€ukner, A.; Gr€uert, W. J. Catal. 2005, 231, 314–330.

ARTICLE

(58) Heinrich, F.; Schmidt, E.; Menzel, M.; Gr€unert, W. J. Catal. 2002, 212, 157–172. (59) Brandenberger, S.; Kr€ocker, O.; Tissler, A.; Althoff, R. Appl. Catal., B 2010, 95, 348–357. (60) Sobolev, V. I.; Panov, G. I.; Kharitonov, A. S.; Romannikov, V. N.; Volodin, A. M.; Ione, K. G. J. Catal. 1993, 139, 435–443. (61) Panov, G. I.; Sobolev, V. I.; Dubkov, K. A.; Parmon, V. N.; Ovanesyan, N. S.; Shilov, A. E.; Shteinman, A. A. React. Kinet. Catal. Lett. 1997, 61, 251–258. (62) Ovanesyan, N. S.; Steinman, A. A.; Sobolev, V. I.; Dubkov, K. A.; Panov, G. I. Kinet. Katal. 1998, 38, 863. (63) Xia, H.; Sun, K.; Liu, Z.; Feng, Z.; Ying, P.; Li, C. J. Catal. 2010, 270, 103–109. (64) Kumar, M. S.; Schwidder, M.; Gr€unert, W.; Br€uckner, A. J. Catal. 2004, 227, 384–397. (65) Joyner, R.; Stockenhuber, M. J. Phys. Chem. B 1999, 103, 5963–5976. (66) Schwidder, M.; Kumar, M. S.; Br€uckner, A.; Gr€unert, W. Chem. Commun. 2005, 805–807. (67) Ramirez, J. P.; Kapteijn, F.; Br€uckner, A. J. Catal. 2003, 218, 234–238. (68) Kubanek, P.; Wichterlov, B.; Sobalık, Z. J. Catal. 2002, 211, 109–118. (69) Xin, H.; Koekkoek, A.; Yang, Q.; van Santen, R. A.; Li, C.; Hensen, E. J. M. Chem. Commun. 2009, 7590–7592. (70) Volodin, A. M.; Zhidomirov, G. M.; Dubkov, K. A.; Hensen, E. J. M.; van Santen, R. A. Catal. Today 2005, 110, 247–254. (71) Fellah, M. F.; Van Santen, R. A.; Onal, I. J. Phys. Chem. C 2009, 113, 15307–15313. (72) Kachurovskaya, N. A.; Zhidomirov, G. M.; Hensen, E. J. M.; van Santen, R. A. J. Phys. Chem. B 2004, 108, 5944–5950. (73) Yoshizawa, K.; Shiota, Y.; Kamachi, T. J. Phys. Chem. B 2003, 107, 11404–11410. (74) Fellah, M. F.; Onal, I. Turk. J. Chem. 2009, 33, 333–345. (75) Fellah, M. F.; Onal, I. J. Phys. Chem. C 2010, 114, 3042– 3051. (76) Fellah, M. F.; Onal, I.; Van Santen, R. A. J. Phys. Chem. C 2010, 114, 12580–12589. (77) Ensing, B.; Buda, F.; Bl€ochl, P.; Baerends, E. J. Angew. Chem., Int. Ed. 2001, 40, 2893–2895. (78) Guesmi, H.; Berthomieu, D.; Kiwi-Minsker, L. J. Phys. Chem. C 2008, 112, 20319–20328. (79) Fellah, M. F. J. Phys. Chem. C 2011, 115, 1940–1951. (80) Kurnaz, E.; Fellah, M. F.; Onal, I. Microporous Mesoporous Mater. 2011, 138, 68–74. (81) Xia, H.; Sun, K.; Sun, K.; Feng, Z.; Li, W. X.; Li, C. J. Phys. Chem. C 2008, 112, 9001–9005. (82) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, A1133–A1138. (83) Becke, A. D. Phys. Rev. B 1988, 38, 3098–3100. (84) Becke, A. D.; Roussel, M. R. Phys. Rev. A 1989, 39, 3761–3767. (85) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (86) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian Inc.: Wallingford, CT, 2004. 9679



ARTICLE

(87) Rozanska, X.; Saintigny, X.; van Santen, R. A.; Hutschka, F. J. Catal. 2001, 202, 141–155. (88) Rozanska, X.; van Santen, R. A.; Hutschka, F.; Hafner, J. J. Am. Chem. Soc. 2001, 123, 7655–7667. (89) Vos, A. M.; Rozanska, X.; Schoonheydt, R. A.; van Santen, R. A.; Hutschka, F.; Hafner, J. J. Am. Chem. Soc. 2001, 123, 2799–2809. (90) Lermer, H.; Draeger, M.; Steffen, J; Unger, K. K. Zeolites 1985, 5, 131–134. (91) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833–1840. (92) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, version 3.1; Theoretical Chemistry Institute, University of Wisconsin: Madison, WI. (93) Peng, C.; Schlegel, H. B. Isr. J. Chem. 1993, 33, 449–454. (94) Teffo, J. L.; Chedin, A. J. Mol. Spectrosc. 1989, 135, 389–409. (95) Andzelm, J.; Wimmer, E. J. Chem. Phys. 1992, 96, 1280–1300. (96) Kurtz, D. M. Chem. Rev. 1990, 90, 585–606. (97) Carrano, C. J.; Carrano, M. W.; Sharma, K.; Backes, G.; Loehr, J. S. Inorg. Chem. 1990, 29, 1865–1870.

9680


A DFT Study of Direct Oxidation of Benzene to Phenol by N2O over [Fe

Recommend Documents