DFT Study of Direct Methanol Oxidation to Formaldehyde by N2O on

Article pubs.acs.org/JPCC

DFT Study of Direct Methanol Oxidation to Formaldehyde by N2O on the [Fe]2+−ZSM-5 Zeolite Cluster Mehmet Ferdi Fellah*,† and Isik Onal*,‡ †

Department of Chemical Engineering, Bursa Technical University, Bursa, 16200, Turkey Department of Chemical Engineering, Middle East Technical University, Ankara, 06531, Turkey

‡

ABSTRACT: The mechanistic pathways of direct oxidation of methanol to formaldehyde by N2O were theoretically investigated by means of density functional theory (DFT) over an extra framework species in ZSM-5 zeolite represented by a [Si6Al2O9H14]2−[Fe]2+ cluster model. The catalytic reactivity of these species is compared with that of mononuclear Fe1+ and (FeO)1+ sites in ZSM-5 investigated in our earlier work at the same level of theory (J. Catal. 2011, 282, 191). The formation of the grafted species including methoxy on the [Fe]2+ site was calculated to be thermodynamically more stable than on the [FeO]1+ site and less stable than on the [Fe]1+ site. The order of activation barrier values of a critical step, proton transfer from grafted methoxy to form formaldehyde and water, on these sites is as follows: [Fe]1+ > [Fe]2+ ≫ [FeO]1+. The calculated vibrational frequencies for grafted species on the iron site on the surface are in good agreement with the experimental values. selective oxidation of benzene to phenol. The α-form of oxygen (extra framework oxygen) formed by decomposition of N2O plays an important role in the direct oxidation of benzene on Fe−ZSM-5 that is confirmed by both experimental11−16 and theoretical17−21 literature. According to both experimental22−34 and theoretical33−42 reports, Fe−ZSM-5 is an active catalyst for the stoichiometric decomposition of N2O to form α-oxygen. The following reaction is proposed for the decomposition of N2O to form α-oxygen.15,16,43

1. INTRODUCTION Methanol is usually used as a feedstock (or as a solvent) for bulk organic chemicals (primarily formaldehyde).1 40% of the produced methanol is utilized to form formaldehyde on iron molybdenum oxide catalysts. Formaldehyde is one of the world’s most important industrial and research chemicals mainly due to its relatively low cost and high purity. It is also an intermediate chemical for a variety of chemical reactions.2 Formaldehyde is currently produced directly from methanol via two different methods such as dehydrogenation or oxidative dehydrogenation on Ag or Cu catalysts and oxidation with Fe-containing MoO3 catalysts. These processes have two fundamental disadvantages: high operating temperatures (573−923 K) and production of several byproducts such as carbon monoxide, carbon dioxide, dimethyl ether, methyl formate, and formic acid.2 There has been significant research activity to develop new processes for producing formaldehyde. In the literature, there are a few theoretical studies3−6 on methanol oxidation to formaldehyde using small iron-oxo species. Liang et al.7 have investigated several reactions of methanol oxidation to formaldehyde on [FeO]1+−ZSM-5 modeled as [(SiH3)2AlO2(OH)2(FeO)] in their theoretical study where direct oxidation of methane on Fe-ZSM-5 zeolite has been studied. The only theoretical study on methanol oxidation by N2O on ZSM-5 has been conducted by Fellah8 using [(SiH3)4AlO4(Fe)1+ or (FeO)1+] cluster models. No other investigation concerning methanol oxidation to formaldehyde by N2O on Fe−ZSM-5 zeolite has been found in either theoretical or experimental literature. Fe-exchanged ZSM-5 catalysts have good catalytic activities for a number of reactions. It has been experimentally reported9,10 that [Fe,Al]MFI gives good performance in the © 2012 American Chemical Society

N2O + ()α → (O)α + N2

(1)

The major challenge or debate in understanding the activity of Fe/ZSM-5 catalysts is about the nature of the active sites in the ZSM-5 zeolite.44 Extra-framework Fe species in the zeolite micropores can be present as mono-, bi-, or oligonuclear cationic species, neutral iron oxide species, or mixed oxide phases combining Fe and Al. Several experimental studies have utilized extended X-ray absorption fine structure, highresolution X-ray absorption near-edge spectroscopy, Fourier transform infrared spectroscopy, electron spin resonance, electron paramagnetic resonance, X-ray diffraction, Mössbauer spectroscopy, and UV−visible spectroscopy and have reported that iron sites of Fe−ZSM-5 range from mononuclear sites45−53 and binuclear structures52,54−58 to small iron oxide or oligomeric clusters.58−60 On the basis of an experimental61 XAFS study, it was reported that FeO complexes where Fe atoms are in the 3+ and 4+ oxidation states exist in the Fe− ZSM-5 catalyst. Recently, a computational study investigated Received: March 11, 2012 Revised: May 30, 2012 Published: June 5, 2012 13616

dx.doi.org/10.1021/jp302340g | J. Phys. Chem. C 2012, 116, 13616−13622

The Journal of Physical Chemistry C

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the direct benzene oxidation to phenol by N2O on Fe2+−ZSM5 zeolite represented as a [Si6Al2O9H14Fe] cluster.44 The aim of this study is to analyze the catalytic reactivity on the [Fe]2+ site in ZSM-5 zeolite for the catalytic oxidation of methanol to formaldehyde formation by nitrous oxide. Density functional theory (DFT) calculations with the B3LYP formalism using 6-31G(d,p) as a basis set for all atoms are utilized to obtain energy profiles, equilibrium geometries, and transition state geometries.

2. SURFACE MODEL AND CALCULATION METHOD All calculations in this study were based on DFT62 as implemented in the Gaussian suite of programs.63 Becke’s three-parameter hybrid method64,65 involving the Lee, Yang, and Parr correlation functional B3LYP formalism66 was utilized to take into account the exchange and correlation. The 6-31G(d,p) basis set was used for all atoms including the iron atom. In the present study, a cluster modeling approach was used to simulate a representative portion of ZSM-5 zeolite stabilizing the extra framework iron species. Only minor quantitative differences between the results obtained using the cluster and periodic modeling approach have been reported in previous studies on iron-modified ZSM-5 and ferrite zeolites by Kachurovskaya et al.21,67 Furthermore, the cluster modeling approach has been widely used to create a qualitative molecularlevel picture of CO and NO adsorption on different iron sites of Fe−ZSM-5.68 Very similar structural and energetic properties of the adsorption complexes were obtained by using small 5T and large 83T ZSM-5 cluster models. For example, adsorption energies for CO adsorption were found as −43.8 and −39.7 kcal/mol for small and large Fe−ZSM-5 clusters. The adsorption geometries for the two types of calculations seemed to be very similar with that Fe−C and C−O distances were 1.778, 1.159 and 1.811, 1.142 Å, respectively.68 It should be also importantly noted that DFT calculations on cluster or periodic zeolite models generally predict very similar reactivity trends.68−71 The ZSM-5 cluster which was cut from inside a ZSM-5 channel in this study was constructed by using the Cartesian coordinates reported by Lermer et al. in an XRD study.72 In this study, an 8T cluster representing two adjacent 5T rings from the wall of ZSM-5 zeolite was used. The initial cluster model contained eight Si and nine O atoms. Two Al atoms were placed in different positions (T9 and T12 lattice positions) of each ring to obtain distant anionic sites in double 5T rings of the ZSM-5 cluster modeled as [Si6Al2O9H14]2−. The [Fe]2+ site used to obtain a neutral cluster is located on oxygen atoms of a ring. The resulting cluster model is shown in Figure 1. The dangling bonds of the terminal silicon and/or 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. Only terminating H atoms were kept fixed to orient in the T−O direction of the next Si site. Energy profile, equilibrium geometry (EG), and transition state (TS) calculations were performed for the determination of the activation barriers and relative reaction energies. All energy values and energy differences in this study include zero-point energy (ZPE) corrections, which were obtained using frequency calculations at a temperature of 298 K because no experimental thermochemistry data are available for methanol oxidation to formaldehyde by N2O on the Fe−ZSM-5 catalyst. Some error which is termed as spin contamination may be introduced into the calculations during the cluster calculations using spin multiplicity (SM). This spin contamination

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

value which is related with the unpaired electrons of the system must be negligible.73 The computed values in this study confirmed that the spin contamination was very small (max 0.25% after annihilation). This shows that unpaired electrons of the system do not change during the catalytic cycle reactions. Vibrational analysis was also performed to obtain vibrational frequencies and Gibbs free energies. All frequency values were scaled by 0.961374 to reproduce experimental fundamentals. Time-dependent DFT calculations were performed to calculate UV−vis spectra. Mulliken population analysis75 was utilized to obtain Mulliken atomic charges and Mulliken atomic spin densities. Natural bond orbital (NBO)76 analysis was used to obtain electronic configurations of iron atoms. The convergence criteria involving gradients of maximum force, rootmean-square (rms) force, maximum displacement, and rms displacement in Gaussian software were 0.000450, 0.000300, 0.001800, and 0.001200, respectively. The computational strategy employed in this study is as follows. Initially, the correct SM of the system consisting of clusters and adsorbing molecules was determined by single point energy (SPE) calculations. SPEs were calculated with different SM numbers for each cluster system, and the SM number that corresponds to the lowest SPE was accepted as the correct SM. The cluster and the adsorbing molecules, N2O and CH3OH, were then fully optimized geometrically by means of EG calculations. The adsorbing molecule was first located over the active site of the cluster at a selected distance, and a coordinate driving calculation was performed by selecting a reaction coordinate to obtain the variation of the relative energy with a decreasing reaction coordinate to get an energy profile as a function of the selected reaction coordinate distance. These energy profiles are also utilized to find TS and final EGs by using the geometry that has the highest energy and the geometry that has the final minimum energy, respectively. SPE calculations were performed where necessary by locating the adsorbing molecule in the vicinity of the catalytic cluster. Coordinate driving calculations resulted in an energy profile. The resulting relative energies for the cluster and reactant molecule complex were plotted against the reaction coordinate. The relative energy was defined with the following formula ΔE = ESystem − (ECluster + EAdsorbate)

where ESystem is the calculated energy of the optimized geometry containing the cluster and the adsorbing molecule 13617



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(TS or EG); ECluster is the energy of the cluster; and EAdsorbate is that of the adsorbing molecule. After the energy profile was obtained for the reaction step, the geometry with the minimum energy on the energy profile was reoptimized by means of EG calculations to obtain the optimized 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 obtained by using the Berny Algorithm77 implemented in Gaussian software.

Step 7: Desorption of formaldehyde (or water) CH 2O−(or H 2O)−FeO−ZSM5 → FeO−ZSM5 + CH 2O (or H 2O)

where FeOxygen or FeO represents iron atom located oxygen atoms of a ring of the double 5T ZSM-5 cluster. The first and second reactions (steps 1 and 2) of the catalytic cycle for oxidation of methanol to formaldehyde by N2O on the Fe2+−ZSM-5 cluster are the adsorption and dissociation of the N2O molecule resulting in an FeO−O site on the ZSM-5 cluster. These steps were investigated in our previous study.44 The methanol molecule adsorbs on the Fe center of the [FeO−O] site at the next reaction (step 3). This exothermic reaction step (ΔE = −62 kJ/mol) occurs without an activation barrier. The corresponding optimized geometry is shown in Figure 2.

3. RESULTS 3.1. Optimization of Cluster. The optimized geometry for the [Fe]2+−ZSM-5 cluster was obtained for the neutral charge cluster, and the SM corresponding to the lowest SPE was determined to be 5 meaning that there are four unpaired electrons. The SM number was also determined as 5 for the cluster including N2O and methanol molecules. This value was used for all reaction studies. It means that the number of unpaired electrons was kept constant throughout all calculations with an even number of electrons to avoid spin transitions along the reaction path. The optimized geometry of cluster is depicted in Figure 1. Si−O distances of a cluster range from 1.55 to 1.68 Å. The corresponding distances reported earlier in the experimental literature are between 1.55 and 1.65 Å.72 3.2. Direct Oxidation of Methanol to Formaldehyde by N2O on the [Fe]2+−ZSM-5 Cluster. The following elementary reaction steps 1−7 are considered to be involved in different potential reaction paths for the oxidation of methanol oxidation to formaldehyde by N2O on the [Fe]2+−ZSM-5 cluster: Step 1: N2O adsorption

Figure 2. Final equilibrium geometry for methanol adsorption on Fe−ZSM-5 (step 3) (distance values in units of angstroms).

N2O + FeOxygen−ZSM5 → (FeO−ON2)−ZSM5 (2)

Proton transfer (step 4) from the OH group of adsorbed methanol to the O atom of the extra framework iron cluster results in the formation of adsorbed grafted species such as hydroxyl and methoxy. This step is strongly favored thermodynamically (ΔE = −142 kJ/mol) and faces a relative activation barrier of 43 kJ/mol. TS and EG of this step are depicted in Figure 3. Subsequent proton transfer from the CH3 group of adsorbed methoxy to the adsorbed hydroxyl group in the extra framework iron cluster results in formaldehyde and water formation (step 5). This reaction step is exothermic (ΔE = −95 kJ/mol) and shows a very high activation barrier (Eact = 122 kJ/ mol). TS and EG for this step are represented in Figure 4. Following these reaction steps, formaldehyde formed on the clusters or water will desorb from the surface. If formaldehyde first desorbs from the surface, the desorption energy value is calculated to be 85 kJ/mol. The desorption energy is 92 kJ/mol if water desorbs first from the surface. ΔG values for formaldehyde and water desorption from the surface are computed as 32 and 46 kJ/mol, respectively. When formaldehyde is absent on the surface, the desorption energy value for water is computed as 160 kJ/mol.

Step 2: N2O dissociation (FeO−ON2)−ZSM5 → (FeO−O)−ZSM5 + N2 (3)

Step 3: Methanol adsorption (FeO−O)−ZSM5 + CH3OH → (O−FeO−OHCH3)−ZSM5 (4) Step 4: Proton transfer from the OH of MetOH to form methoxy species (O−FeO−OHCH3)−ZSM5 → (HO−FeO−OCH3)−ZSM5 (5) Step 5: Proton transfer from grafted methoxy to form formaldehyde and water

(HO−FeO−OCH3)−ZSM5 → (H 2O−FeO−OCH 2)−ZSM5 Step 6: Desorption of water (or formaldehyde)

(6)

4. DISCUSSION In this study, catalytic oxidation of methanol to formaldehyde by N2O on the [Fe]2+ site in ZSM-5 zeolite has been theoretically investigated using a cluster model. It should be noted that very similar structural and energetic properties of the

(H 2O−FeO−OCH 2)−ZSM5 → (FeO−OCH 2 or H 2O)−ZSM5 + H2O (or CH 2O)

(8)

(7) 13618



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Figure 4. (a) TS and (b) EG for proton transfer from CH3 of methoxy to form formaldehyde and water on Fe−ZSM-5 (step 5) (distance values in units of angstroms).

Figure 3. (a) TS and (b) EG for proton transfer from the OH to form grafted species on Fe−ZSM-5 (step 4) (distance values in units of angstroms).

Table 1. Mulliken Atomic Charges, Spin Densities, and Electronic Configurations of Iron Atoms for EG Structures of Various Steps

several adsorption complexes have been achieved in both small 5T and large 83T ZSM-5 cluster models,68 as mentioned in Section 2. Tables 1 and 2 report the Mulliken atomic charges and spin densities and electron configurations of iron atoms of the cluster for all of the steps with EG and TS geometries, respectively. As can be seen from these tables, the iron atom has a high spin density indicating that the unpaired electrons are localized on the iron atom. Furthermore, the atomic charges of the iron atom for the clusters after a reaction are somewhat higher than those of the initial ZSM-5 clusters, showing a more positive character on the iron atom in the cluster during a reaction in comparison to the initial ZSM-5 clusters. A summary of the calculated reaction energy diagram (including ZPE corrections) for the oxidation of methanol to formaldehyde by N2O over the sites of [Fe]2+ in ZSM-5 zeolite has been shown in Figure 5. Activation and desorption barriers for the respective reaction steps involved in the oxidation of methanol to formaldehyde are also tabulated in Table 3 which includes other theoretical activation barriers as well. Prior to the methanol oxidation, the [Fe] site promotes the N2O decomposition resulting in the formation of the [FeO−O] site. The formed extra framework oxygen species on the ZSM-5 clusters are the reactive centers for the following oxidation of methanol to formaldehyde. Decomposition of the N2O molecule on the [Fe]2+ site has been investigated on the same cluster model and discussed in our previous theoretical study.44 Additionally, another possible step for the dissociation of adsorbed N2O is the reaction which produces N + NO. Because of its high activation barrier value (439 kJ/mol)

cluster/steps

charge

spin density

electron configuration

[Fe]2+−ZSM-5 [Fe−O]2+−ZSM-5 Step 3 Step 4 Step 5

0.79 0.89 1.05 1.10 0.84

3.818 3.822 3.299 4.166 3.838

[core]4s0.223d6.144p0.03 [core]4s0.273d5.914p0.034d0.015p0.01 [core]4s0.273d5.854p0.404d0.01 [core]4s0.253d5.764p0.364d0.015p0.01 [core]4s0.253d6.094p0.305p0.01

Table 2. Mulliken Atomic Charges, Spin Densities, and Electronic Configurations of Iron Atoms for TS Structures of Steps 4 and 5 steps

charge

spin density

electron configuration

Step 4 Step 5

1.03 1.08

3.741 4.138

[core]4s0.283d5.774p0.384d0.015p0.01 [core]4s0.263d5.754p0.334d0.015p0.01

obtained in this study by energy profile calculation, this reaction is not a probable step on the Fe−ZSM-5 cluster. This is consistent with experimental observations.15,16,43 The most critical reaction for the methanol oxidation is the activation of the O−H bond of methanol since the dissociation energy of that bond is 435 kJ/mol.6 In this study, the activation barrier value for the proton transfer from OH of adsorbed methanol on the [FeO]2+ site to form a grafted species (step 4) is calculated to be 43 kJ/mol. This value is substantially lower than the value of dissociation energy of the O−H bond of methanol. Additionally, the activation barrier value for step 4 is 13619



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methoxy groups on the Fe−ZSM-5 catalyst reported in an experimental study by Wood et al.78 In this study, corresponding frequency values are calculated to be 2962, 2955, and 2888 cm−1 for grafted species including methoxy and hydroxyl on the Fe−ZSM-5 cluster. The stretching frequency of O−H for surface hydroxyl is computed as 3711 cm−1 for the same step. The band at 3673 cm−1 was assigned to the hydroxyl group bonded to the extra framework Fe where the study was done on the Fe/Al-MFI catalyst,78 based on similar bands on FeOOH due to OH coordinated Fe.79,80 It has also been reported that a series of bands were observed at lower energies with the frequencies of 1460, 1434, and 1413 cm−1 which were then assigned to methoxy CH-bending vibrations.78 The equivalent vibrational frequencies are computed as 1444, 1442, and 1428 cm−1 for the equilibrium geometry of Step 5 including grafted species in this study. These bands are also in the region of the spectrum projected for methoxy bending vibrations of C−H.81 Consequently, theoretical frequencies calculated for surface grafted species in the present study are in good agreement with the experimental data mentioned above for methoxy and hydroxyl species on the Fe/Al-MFI catalyst.78 Additionally, it has been also reported by Wood et al.78 that the intensities of the O−H bands are much greater than those of the corresponding methoxy C−H bands. A parallel observation has been previously reported for the interaction of methanol with ZSM-5.82 In the same way, IR intensities for the O−H stretching frequencies are significantly greater than those of C−H stretching frequencies of surface methoxy species in the present study. It should be also mentioned that there could be an interaction between methanol or formaldehyde and the aluminum or the silanol groups of ZSM-5. In several experimental studies,78,82 some vibrational frequencies have been assigned to the silanol-bound methoxy species which could have a significant influence on the reactivity. Other vibrational frequencies of equilibrium geometries for catalytic cycle steps are tabulated in Table 4. The TD-DFT computed UV−vis spectra for the structure containing Fe− methoxy and Fe−OH reveal absorption bands at max. 555 nm in the range of 800−400 nm on the [Fe]2+ site. Figure 6 shows the summary energy diagram (including ZPE corrections) for the oxidation of methanol to formaldehyde by N2O over the [Fe]2+ site used in this study and other iron sites8 such as [FeO]1+ and [Fe]2+ in ZSM-5 zeolite clusters. As can be seen from Table 3 and Figure 6, the essential result of this study and our previous theoretical study8 is that the order of activation barrier values of step 5, proton transfer from grafted methoxy to form formaldehyde and water, on these sites is [Fe]1+ > [Fe]2+ ≫ [FeO]1+. The reason for this might be explained by the fact that the formation of the grafted OH and methoxy (OCH3) species on the [Fe]2+ site proved to be more thermodynamically stable than on the [FeO]1+ site8 and less stable than on the [Fe]1+ site.8 Accordingly it can be stated that the use of the Fe exchanged zeolite cluster could facilitate the oxidation at lower temperatures compared to Cu catalyst.83 A theoretical study83 where partial oxidation of methanol to formaldehyde was studied periodically on the copper (110) surface reports that the activation barriers for the reaction of proton transfer of the OH group of adsorbed methanol and the step of proton transfer from CH3O to form formaldehyde were 70 and 160 kJ/mol, respectively. The corresponding values are computed to be 43 and 122 kJ/mol, respectively, in this study. Moreover, there seems to be a good consistency between the overall heat of reaction values for direct oxidation of methanol

Figure 5. Relative energy profile (including ZPE corrections) for direct methanol oxidation to formaldehyde on Fe2+−ZSM-5 (steps 1−7).

Table 3. Activation and Desorption Barriers for Catalytic Cycle Steps for Methanol Oxidation to Formaldehyde by N2O on Fe−ZSM-5 Zeolite Clusters activation barriers, kJ/mol steps Step 1, N2O adsorption Step 2, N2O dissociation Step 3, methanol adsorption Step 4, proton transfer from the OH of MetOH to form methoxy species Step 5, proton transfer from grafted methoxy to form formaldehyde and water Step 6, desorption of water (or formaldehyde) Step 7, desorption of formaldehyde (or water)

[Fe]2+ site

[Fe]1+ sitea

[FeO]1+ sitea

0 83 0 43

18b 0 23

0 113 0 72

122

155

24

85 160

81 127

43c 101d

a Fellah.8 bSteps 1 and 2 occur simultaneously. cFormaldehyde first desorbs from the surface. dWater second desorbs from the surface.

somewhat higher than the value of 23 kJ/mol obtained on the [FeO]1+−ZSM-5 cluster.8 The difference between the results presented in this study and the results reported in ref 8 is probably due to the differences in the formal charges of iron atoms located on ZSM-5 zeolite. In the next reaction (step 5), the activation of grafted species to form formaldehyde and water on the surface has required a high energy barrier (Eact = 122 kJ/mol). The activation barrier of this step is relatively lower than the value (165 kJ/mol) for formaldehyde and water formation from grafted species by using the 3T [FeO]1+−ZSM5 cluster7 and the value of 155 kJ/mol obtained on the 5T [FeO]1+−ZSM-5 cluster.8 The reason for this difference between the activation barriers of step 6 (formaldehyde formation from the grafted species) for the [Fe]2+ site and [FeO]1+ site8 in ZSM-5 zeolite might be that in transition state modes which have lower activation barriers the distance of H−O is much greater than those of other modes having higher activation barriers. After the formation of formaldehyde and water on the surface, formaldehyde desorbs first with a desorption energy value of 85 kJ/mol since water desorption from the surface has a higher desorption energy (92 kJ/mol). Moreover, Gibbs free energy changes which were calculated for desorption of formaldehyde and water from the surface [H2O− Fe−OCH2] (+31 and +46 kJ/mol, respectively) also indicate this order of desorption. Finally, water desorption has a desorption value of 160 kJ/mol. The pair of bands at 2921 and 2824 cm−1 was assigned to the asymmetric and symmetric C−H stretching for Fe-bound 13620



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than on the [Fe]1+ site. It can be concluded that the order of activation barrier values of proton transfer from grafted methoxy to form formaldehyde and water on these sites is [Fe]1+ > [Fe]2+ ≫ [FeO]1+. The vibrational frequencies for grafted species on the iron site on the surface are in good agreement with the experimental values.

Table 4. Vibrational Frequencies of Equilibrium Geometries for Catalytic Steps on the Fe−ZSM-5 Cluster step Step 3

Step 4

Step 5

CH2O−ZSM-5

H2O−ZSM-5

frequency type O−H H of OH C−O Fe−O O−H C−O H of OH H of H2O

stretching rocking stretching stretching stretching stretching rocking stretching

H of CH2O H of CH2O C−O H of H2O H of CH2O H of CH2O H of CH2O H of CH2O H of CH2O C−O H of CH2O H of CH2O H of CH2O H of H2O

symmetric stretching antisymmetric stretching stretching scissoring scissoring wagging rocking symmetric stretching antisymmetric stretching stretching scissoring wagging rocking stretching

H of H2O

scissoring

frequencya 3626 1067 908 902 3712 1108 592 3639 3411 2911 3023 1676 1602 1459 1169 1222 2870 3005 1665 1477 1207 1216 3678 3286 1578

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AUTHOR INFORMATION

Corresponding Author

*E-mail: mff[email protected], [email protected] (M. F. Fellah); [email protected] (I. Onal). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The numerical calculations reported in this paper were performed at TUBITAK ULAKBIM, High Performance and Grid Computing Center (TR-Grid e-Infrastructure).

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REFERENCES

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a

Frequency values were scaled by 0.9613 to reproduce experimental fundamentals.73

Figure 6. Relative energy profile (including ZPE corrections) for direct methanol oxidation to formaldehyde on M−ZSM-5 (where M = [Fe]1+, [Fe−O]1+, and [Fe]2+ sites) (steps 1−7).

to formaldehyde by N2O on these sites of ZSM-5 clusters. The vibrational frequencies for grafted species on the surface are also in line with the experimental values.

5. CONCLUSIONS The elementary reaction steps involved in the catalytic methanol oxidation to formaldehyde by N2O over the iron site in ZSM-5 zeolite represented by a [Si6Al2O9H14]2−[Fe]2+ cluster model have been investigated by means of DFT calculations. According to the results obtained in this study and in our previous theoretical results, the formation of the grafted species including methoxy on the [Fe]2+ site was thermodynamically more stable than on the [FeO]1+ site and less stable 13621



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DFT Study of Direct Methanol Oxidation to Formaldehyde by N2O on

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