Theoretical Investigation of Methane Hydroxylation over Isoelectronic

Article pubs.acs.org/IC

Theoretical Investigation of Methane Hydroxylation over Isoelectronic [FeO]2+- and [MnO]+-Exchanged Zeolites Activated by N2O M. Haris Mahyuddin,†,‡ Yoshihito Shiota,† Aleksandar Staykov,§ and Kazunari Yoshizawa*,† †

Institute for Materials Chemistry and Engineering and IRCCS and §International Institute for Carbon-Neutral Energy Research, Kyushu University, Fukuoka 819-0395, Japan ‡ Engineering Physics Research Group, Bandung Institute of Technology, Bandung 40132, Indonesia S Supporting Information *

ABSTRACT: While the most likely structure of the active site in ironcontaining zeolites has been recently identified as [FeO]2+ (Snyder et al. Nature 2016, 536, 317−321), the mechanism for the direct conversion of methane to methanol over this active species is still debatable between the direct-radical-rebound or nonradical (concerted) mechanism. Using density functional theory on periodic systems, we calculated the two reaction mechanisms over two d4 isoelectronic systems, [FeO]2+ and [MnO]+ zeolites. We found that [FeO]2+ zeolites favor the direct-radical-rebound mechanism with low CH4 activation energies, while [MnO]+ zeolites prefer the nonradical mechanism with higher CH4 activation energies. These contrasts, despite their isoelectronic structures, are mainly due to the differences in the metal coordination number and Oα (oxo) spin density. Moreover, molecular orbital analyses suggest that the zeolite steric hindrance further degrades the reactivity of [MnO]+ zeolites toward methane. Two types of zeolite frameworks, i.e., medium-pore ZSM-5 (MFI framework) and small-pore SSZ-39 (AEI framework) zeolites, were evaluated, but no significant differences in the reactivity were found. The rate-determining reaction step is found to be methanol desorption instead of methane activation. Careful examination of the most stable sites hosting the active species and calculation for N2O decomposition over [Fe]2+-MFI and -AEI zeolites were also performed. room temperature with involvement of a CH3• radical, leading to the formation of hydroxy (Fe−OH)α and methoxy (Fe− OCH3)α groups, the latter of which is then hydrolyzed by water to produce methanol.20 At a higher temperature (160 °C), methanol can be directly produced without the preformation of methoxy groups through a quasi-catalytic reaction.21 Recently, an experimental work utilizing Mössbauer spectroscopy and magnetic circular dichroism, a spectroscopic technique capable of probing only the active metal centers, successfully identified a mononuclear square-planar FeIV center with an Oα atom bound to it, [FeO]2+ species, on the β site of BEA zeolite.22 Computational works based on density functional theory (DFT) by Rosa et al.23 and our group24,25 previously predicted the same active species in MFI zeolite and the gas phase, respectively. These [FeO]2+-MFI and bare [FeO]2+ abstract the H atom of methane with different mechanisms (radical and nonradical, respectively) and activation energies (Ea = 6.6 and 4.9 kcal/mol, respectively).23−25 Different active site structures in a small cluster model of MFI zeolite, i.e., [FeO]+-MFI26 and [Fe2(μ-O)]2+MFI27 zeolites, were also predicted by DFT calculations, but

1. INTRODUCTION Microporous aluminosilicate anions (zeolites) exchanged for metal cations or simply metal-exchanged zeolites are attractive catalysts for large-scale industrial processes because of their low production cost and stable active sites. Pioneered by the works of Panov and co-workers,1−4 an iron-exchanged ZSM-5 zeolite (MFI topology) was found to rapidly catalyze the direct oxidation of inert methane into methanol at room temperature with high selectivity. The so-called surface α-oxygen (Oα) that formed on the Fe active center upon activation by N2O is remarkably reactive toward methane and benzene.5−7 The structure of this active species has been extensively investigated and debated. In a high Fe/Al ratio (0.8−1.0), Marturano et al.,8 Battiston et al.,9,10 and Jia et al.11 suggested that an oxygenbridged binuclear iron is formed in the MFI zeolite. However, in a lower Fe/Al ratio (e.g., 0.38), Wood et al. suggested that a mononuclear OFeO core is formed in the N2O-activated FeMFI zeolite.12,13 Jiš́ a and Sklenak, on the other hand, evidenced the formation of two α-FeII cations on two adjacent sixmembered rings (6-MRs) of an Fe-Ferrite (FER) zeolite at low Fe/Al ratios.14,15 These α-FeII cations, upon reaction with N2O, form (FeIII−O•−)α complexes,16−18 which are electronic isomers of FeIVO species.19 Such an [FeO]2+ species in the MFI zeolite was reported to abstract the H atom of methane at © XXXX American Chemical Society

Received: May 20, 2017

A

DOI: 10.1021/acs.inorgchem.7b01284 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the zeolite database35 was used for building their periodic structures. Optimized unit cells with lattice parameters of a = 20.406 Å, b = 20.142 Å, and c = 13.522 Å for MFI zeolite and a = 13.728 Å, b = 12.715 Å, and c = 18.579 Å for AEI zeolite were used for all calculations. To introduce the [FeO]2+ and [MnO]+ extraframeworks into the zeolites, one and two Si atoms of the zeolites were, respectively, replaced by Al atoms, resulting in a negative charge that compensates for the extraframework cations. Owing to such different numbers of Si → Al replacement, the Fe and Mn atoms in these models differ in CN, as will be further discussed in section 3.1. As suggested in the previous works,22,23,28 the 6-MRs of MFI and AEI zeolites are considered as the locations for [FeO]2+ species, while the zigzag 10-MRs of MFI and 8-MRs of AEI are considered as the locations for [MnO]+ species. In addition to the periodic models, hydrogen-terminated cluster models obtained from the optimized periodic structures were used to calculate MOs and orbital energies. These cluster models (see Figure S1 in the Supporting Information) are composed of four SiO3H2 and two [AlO3H2]− ligands bound to [FeO]2+ species29 and two SiO3H2 and one [AlO3H2]− ligands bound to [MnO]+ species.26 2.2. Computational Methods. Calculations for the periodic structures were performed under the Kohn−Sham formulation38,39 as implemented in the Vienna Ab Initio Simulation Package (VASP).40,41 The projector-augmentedwave method was employed to describe the interaction between ion cores and electrons.42,43 The electron exchange correlation was treated by the generalized gradient approximation based on the Perdew, Burke, and Ernzerhof (PBE) functional.44 The plane-wave basis sets with a cutoff energy of 550 eV were used for all calculations. Brillouin zone sampling was restricted to the Γ point only. The semiempirical Grimme’s D2 method was employed to account for van der Waals (vdW, dispersion) correction.45 All calculations, except for the optimizations of isolated molecules, were spin-polarized with two fixed spin multiplicities, i.e., the quintet and triplet states. However, it is worth noting that DFT calculations are known to produce errors when the relative energies of different spin states are determined.46 The conjugate gradient method was employed to optimize intermediate structures, while the climbing-image nudge elastic band (CI-NEB) method47,48 with a quasi-Newton algorithm was used to locate the transition states (TSs). Four to six NEB images (the maximum distances between images were less than 0.9 Å) generated by the imagedependent pair potential method49 were employed. The geometry optimizations and CI-NEB calculations were considered to be converged when the maximum forces on all atoms were less than 0.03 eV/Å. MO calculations on the cluster models were performed by using the Gaussian 09 program code with the PBE exchange-correlation functional. A careful check using the hybrid B3LYP functional50−52 was done and resulted in similar trends of orbital energies. The Wachters−Hay (6311+G**) basis set53,54 was used for the Fe and Mn atoms, while the D95** basis set was used for the Si, Al, O, C, and H atoms. During calculations, all atoms of the periodic models were allowed to fully relax, while only the terminating H atoms of the cluster models were allowed to fully relax. Atomic charges and spin densities were calculated by using the gridbased Bader analysis algorithm.55,56 All optimized structures and MOs were visualized by using VESTA.57

they are less reactive (Ea = 16.0 and 26.3 kcal/mol, respectively) than the [FeO]2+-MFI23 zeolite cluster when oxidizing methane. Although DFT calculations on zeolite cluster models can estimate reaction energies at a relatively low computational cost, they neglect the important zeolite confinement effects that have significant contributions to the methane activation energy.28,29 Moreover, the α site of MFI zeolite considered in ref 23 is only one of three distinct 6-MR sites possible to host the [FeO]2+ active species. Thus, it is indispensable to calculate an accurate activation energy for methane’s H-atom abstraction (HAA) over [FeO]2+ hosted on the most stable 6-MR site of a fully periodic structure of MFI zeolite and determine the most likely reaction mechanism. A similar d4 electronic oxo species, [MnO]+ cation, is also known to reactively dissociate a C−H bond of methane in the gas phase.30,31 On the basis of DFT calculations, we reported the bare [MnO]+ species to activate methane with a barrier of 9.4 kcal/mol,32,33 which is almost twice higher than that for the bare [FeO]2+ species.24,25 Such different reactivities of the isoelectronic species suggest that the metal’s electronic configuration is not the only factor influencing the reactivity. We are therefore interested in studying the hydroxylation of methane over [FeO]2+ and [MnO]+ zeolites in terms of their molecular orbitals (MOs). For the zeolite host, the mediumpore ZSM-5 (MFI) and small-pore SSZ-39 (AEI) zeolites are chosen because we previously reported that a [Cu2(μ-O)]2+ species in the AEI zeolite activates methane with a lower barrier (10.8 kcal/mol) than that in the MFI zeolite does (16.3 kcal/ mol).34 However, the main reason for such a reactivity difference is difficult to tell clearly because there are two important factors affecting the reactivity of [Cu2(μ-O)]2+ zeolites, namely, the zeolite confinement and ∠Cu−O−Cu angle, where their effects highly depend on the zeolite pore structure. On the contrary, the local structures of [FeO]2+ and [MnO]+ species in zeolites remain the same irrespective of the zeolite structures. Accordingly, by comparing methane activation energies over [FeO]2+-MFI and -AEI as well as [MnO]+-MFI and -AEI zeolites, we can analyze only the confinement effects of MFI and AEI zeolites on methane. In this work, we used DFT on periodic systems to calculate methane hydroxylation over [FeO]2+ and [MnO]+ active species on the most stable sites of MFI and AEI zeolites through two known reaction mechanisms. Moreover, we performed detailed MO analyses of methane’s HAA over the two systems. This paper describes the importance of metal coordination number (CN) in determining the reaction mechanism and the roles of Oα spin density and zeolite steric hindrance on the reactivity toward methane. This paper also clarifies the similarity of confinement effects exerted by MFI and AEI zeolites on methane.

2. METHODOLOGY 2.1. Models for [FeO]2+ and [MnO]+ Species in MFI and AEI Zeolites. The MFI and AEI zeolite frameworks are composed of units of SiO4 tetrahedra (T) positioned at 12 and 3 distinct T sites, respectively.35 Each of these zeolites has a three-dimensional pore system, where the MFI zeolite has interconnected five-ring units constructing straight 10-ring channels (5.3 × 5.6 Å) in the [010] direction and zigzag 10-ring channels (5.1 × 5.5 Å) in the [100] direction,35,36 while the AEI zeolite has interconnected six-ring units constructing eightring channels (3.8 × 3.8 Å).35,37 One unit cell of the MFI (Si96O192) and AEI (Si48O96) zeolite frameworks retrieved from B


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Inorganic Chemistry

3. RESULTS AND DISCUSSION 3.1. [FeO]2+ and [MnO]+ Species in MFI and AEI Zeolites. As shown in Figure 1, there are three different 6-MR

are energetically most stable and thus used for calculations of the HAA reaction, as discussed in section 3.2. At these most stable sites, as shown in Figure 2a,b, the Fe center is

Figure 2. Selected optimized structures for (a) [FeO]2+-MFI, (b) [FeO]2+-AEI, (c) [MnO]+-MFI, and (d) [MnO]+-AEI zeolites at the corresponding most stable site. Only the extraframeworks and zeolite rings are shown, while the remaining atoms of the zeolites are omitted for clarity. Optimized structures of those at the other T sites are available in the Supporting Information.

coordinated to four framework O (OF) atoms and one Oα atom, forming an Fe CN of 5. The FeOα bond lengths of [FeO]2+-MFI and -AEI zeolites are calculated to be similar (1.619 Å for both) as are their Oα spin densities (0.53 and 0.56, respectively). Within the 10-MRs of the MFI zeolite and the 8-MRs of the AEI zeolite, as shown in Figure 2c,d, there are respectively five (i.e., T1, T2, T3, T10, and T12) and three (i.e., T1, T2, and T3) possible sites for an Al atom to substitute one of the Si atoms. Table 2 shows that among all of these T sites, [MnO]+-

Figure 1. 6-MR sites in (a) MFI and (b) AEI zeolite frameworks. The unit cell of each framework is depicted as a cube.

sites in the MFI zeolite, namely, the α, β, and δ sites,58,59 while there is only one type of 6-MR site in the AEI zeolite. Within these rings, the two substituting Al atoms are separated by two Si atoms,23,29 and this arrangement results in eight [i.e., α(T1/ T7), α(T5/T8), α(T2/T11), β(T4/T10), β(T1/T7), β(T5/ T11), δ(T11/T11), and δ(T7/T12)] and two (i.e., T1/T2 and T3/T3) Al-pair sites for the MFI and AEI zeolites, respectively. Table 1 shows that, among all of these Al-pair sites, the [FeO]2+-MFI zeolite with the Al pair at the δ(T11/T11) site and the [FeO]2+-AEI zeolite with the Al pair at the T1/T2 site

Table 2. Relative Stabilities, Geometrical Parameters, and Oα Spin Density (SD-Oα) of [MnO]+-MFI and -AEI Zeolites Calculated in the Quintet State Al site 10-MR(T1) 10-MR(T2) 10-MR(T3) 10-MR(T10) 10-MR(T12)

Table 1. Relative Stabilities, Geometrical Parameters, and Oα Spin Densities (SD-Oα) of [FeO]2+-MFI and -AEI Zeolites Calculated in the Quintet State Al-pair site α(T1/T7) α(T5/T8) α(T2/T11) β(T4/T10) β(T1/T7) β(T5/T11) δ(T11/T11) δ(T7/T12) 6-MR(T1/T2) 6-MR(T3/T3) a

ΔE (kcal/mol)

Fe−OFa (Å)

[FeO]2+-MFI Zeolite 3.8 2.018−2.220 12.5 2.045−2.168 14.0 2.046−2.167 3.3 1.989−2.083 13.3 1.925−1.990 21.4 2.005−2.140 0.0 2.041−2.046 21.2 1.964−2.193 [FeO]2+-AEI Zeolite 0.0 1.996−2.313 2.1 1.994−2.073

FeOα (Å)

SD-Oα

1.629 1.623 1.623 1.627 1.629 1.623 1.619 1.620

0.65 0.63 0.66 0.62 0.67 0.58 0.53 0.58

1.619 1.618

0.56 0.56

8-MR(T1) 8-MR(T2) 8-MR(T3) a

ΔE (kcal/mol)

Mn−OFa (Å)

MnOα (Å)

SD-Oα

1.645 1.646 1.639 1.642 1.645

0.10 0.09 0.18 0.14 0.12

1.648 1.642 1.641

0.08 0.16 0.17

[MnO]+-MFI Zeolite 4.3 1.998, 2.018 2.2 1.993, 2.025 2.0 1.960, 2.075 0.7 1.976, 2.044 0.0 1.982, 2.033 [MnO]+-AEI Zeolite 2.4 2.009, 2.018 2.4 1.966, 2.080 0.0 1.962, 2.068

OF stands for O atom of the zeolite framework.

MFI and -AEI zeolites with the corresponding Al atom located at the T12 and T3 sites, respectively, are energetically most stable. Moreover, Table S1 in the Supporting Information shows that the 6-MR sites of the MFI and AEI zeolites are energetically unfavorable to host the [MnO]+ extraframework. At these most stable sites (see Figures 2c,d), the Mn center is coordinated to two OF atoms and one Oα atom, forming a Mn CN of 3. The MnOα bond lengths of the [MnO]+-MFI and -AEI zeolites are calculated as 1.645 and 1.641 Å, respectively,

OF stands for the O atom of the zeolite framework. C


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Figure 3. Reaction energy diagrams and optimized ground-state structures of all intermediates and TSs in methane hydroxylation over (a) [FeO]2+MFI with an Al pair at the δ(T11/T11) site and (b) [FeO]2+-AEI with an Al pair at the T1/T2 site. All energies are given in kilocalories per mole. The values in parentheses are relative energies that include dispersion correction. Only the extraframeworks and zeolite rings are shown, while the remaining atoms of the zeolites are omitted for clarity.

Table 3. Geometrical Parameters, Charges, and Spin Densities for All Intermediates and TSs in Methane Hydroxylation over [FeO]2+-MFI and -AEI Zeolites, Calculated in the Quintet State (Structures Shown in Figure 3)a charge (e)/spin density [FeO]2+-MFI

[FeO]2+-AEI

a

reactant comp. TS1(direct) TS1(nonradical) radical int. hydroxo int. TS2(direct) TS2(nonradical) product comp. reactant comp. TS1(direct) TS1(nonradical) radical int. hydroxo int. TS2(direct) TS2(nonradical) product comp.

dFe−O (Å)

dO−C (Å)

dC−H (Å)

dO−H (Å)

dFe−C (Å)

Fe

Oα

C

1.620 1.703 1.797 1.774 1.793 1.794 1.864 2.045 1.621 1.706 1.801 1.775 1.776 1.795 1.851 2.054

3.504 2.541 2.942 2.777 2.529 2.699 2.117 1.463 3.445 2.524 2.761 2.825 2.596 2.760 2.073 1.460

1.096 1.232 2.483 1.766 2.416 2.175 2.391 2.021 1.096 1.258 2.935 1.819 3.096 2.148 2.467 2.017

2.408 1.315 0.972 1.016 0.976 0.978 0.975 0.973 2.375 1.271 0.975 1.011 0.985 0.981 0.981 0.973

4.427 3.877 3.226 4.026 2.073 3.906 2.400 3.128 4.413 3.938 3.113 4.102 2.023 3.973 2.394 3.146

+1.61/3.10 +1.59/3.65 +1.59/3.96 +1.61/3.96 +1.51/3.53 +1.59/3.94 +1.49/3.73 +1.36/3.63 +1.54/3.08 +1.58/3.67 +1.56/3.92 +1.61/3.96 +1.63/3.45 +1.58/3.94 +1.46/3.74 +1.36/3.64

−0.50/0.52 −0.68/0.25 −1.06/0.36 −0.99/0.38 −1.02/0.25 −1.06/0.36 −1.05/0.13 −1.14/0.07 −0.43/0.56 −0.70/0.29 −1.06/0.36 −1.01/0.41 −0.98/0.31 −1.05/0.39 −0.96/0.12 −1.14/0.06

−0.16/0.00 −0.24/−0.29 −0.17/−0.76 −0.25/−0.76 −0.31/−0.19 −0.17/−0.73 −0.19/−0.23 +0.25/0.00 −0.08/0.00 −0.27/−0.32 −0.11/−0.67 −0.26/−0.77 −0.49/−0.19 −0.21/−0.76 −0.22/−0.21 +0.26/0.00

O and H stand for the Oα and abstracted H atoms, respectively.

reaction intermediate, while the nonradical HAA mechanism involves a four-centered TS (H···OM···CH3), leading to the formation of a hydroxo methyl complex (HO−M−CH3) as the reaction intermediate, where M is metal. In this work, we investigate both mechanisms. For both mechanisms, the reaction proceeds initially with the adsorption of methane on the [FeO]2+ active site, termed reactant complex [FeO(CH4)]2+-Z, where Z is zeolite. Methane is anticipated to be adsorbed and diffused through the 10-MR channels of the MFI zeolite and the 8-MR channels of the AEI zeolite because they are the only sites large enough for the access of methane. As shown in Figure 3 (reactant complex), the incoming methane approaches the active site from the side of the Oα atom with ∠Fe−O−C angles of 114.5° and 116.5°

and the Oα spin densities are calculated as 0.12 and 0.17, respectively, which are much smaller than those for the [FeO]2+-MFI and -AEI zeolites. Because a high SD-Oα is usually a good predictor for an efficient C−H bond cleavage of methane,60,61 [FeO]2+ zeolites are expected to be more reactive than [MnO]+ zeolites. 3.2. Mechanisms for Methane Hydroxylation over [FeO]2+ Zeolites and Nitrous Oxide Decomposition over [Fe]2+ Zeolites. There are two known mechanisms for the HAA, namely, the direct-radical (rebound)62−64 and nonradical24 HAA mechanisms. The main difference between the two mechanisms is the TS structure: The direct HAA mechanism involves a radical-like TS (MO···H···CH3•), leading to formation of the methyl radical (MOH···CH3•) as the D


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Inorganic Chemistry

Figure 4. Reaction energy diagrams and optimized ground-state structures of all intermediates and TS3 in the direct N2O decomposition over (a) [Fe]2+-MFI and (b) [Fe]2+-AEI zeolites. All energies and distances are given in kilocalories per mol and angstroms, respectively. The values in parentheses are relative energies that include dispersion correction. Only the extraframeworks and zeolite rings are shown, while the remaining atoms of the zeolites are omitted for clarity.

MFI and -AEI zeolites, i.e., −0.68 (−0.24) and −0.70 (−0.27), respectively, are more negative than those for the corresponding reactant complex, i.e., −0.50 (−0.16) and −0.43 (−0.08), respectively, indicating charge transfer between Oα···H···C atoms, which is consistent with the definition of the H-atomtransfer reaction.65 The H atom abstracted via TS1(nonradical), on the other hand, is covalently bound to the Oα atom, while the methyl, instead of coordinating with the Fe center, is separated away from it (dFe−C > 3 Å) and forms a planar structure that has strong radical character [highly negative Catom spin densities of −0.76 and −0.67 respectively for TS1(nonradical) of [FeO]2+-MFI and -AEI zeolites]. Moreover, the separated C···H bond length of this unexpected TS structure is calculated to be more than 2.4 Å, which is too long for TS1(nonradical). This suggests that a four-centered TS structure is not favorable for [FeO]2+-MFI and -AEI zeolites because of weak interactions between the Fe and C atoms, as discussed below. TS1(direct) and TS1(nonradical) lead to formation of the methyl radical [FeOH···CH3•]2+-Z and hydroxo [HO−Fe− CH3]2+-Z intermediates, respectively. The [FeOH···CH3•]2+MFI and -AEI radical intermediates are by 4.0 and 4.2 kcal/mol, respectively, higher in energy than the corresponding reactant complex, resulting in slightly endothermic HAA reactions, which agree with the 4.3 kcal/mol endothermic reaction by the [FeO]2+-CHA zeolite.29 Such a slightly endothermic reaction is expected due to the radical formation and absence of the Fe−C bond (large Fe···C distances of 4.026 and 4.102 Å, respectively). In contrast, the [HO−Fe−CH3]2+-MFI and -AEI hydroxo intermediates are by 3.4 and 6.2 kcal/mol, respectively, lower in energy than the corresponding reactant complex, resulting in slightly exothermic HAA reactions. Such slight stabilizations due to Fe−C bond formation suggest that the Fe−C interactions are weak. In Table 3, the Fe atomic spin densities of the [FeOH···CH3•]2+-MFI and -AEI radical intermediates (3.96 for both) are higher than those of the corresponding reactant complex (3.10 and 3.08, respectively), indicating changes of the oxidation states of Fe from FeIVO to FeIII−O•−, which is highly reactive.22 Let us now focus on the direct HAA mechanism in the quintet ground state (blue lines in Figure 3). The product (methanol) complex is formed via a second TS (TS2), where

and is adsorbed preferably in the quintet state of [FeO]2+-MFI and -AEI zeolites with adsorption energies of Eads = −1.3 and −0.2 kcal/mol, respectively. If the dispersion correction is taken into account, these values are strengthened to −6.3 and −5.3 kcal/mol, respectively, because of the nonpolar nature of methane. One H atom of methane is then abstracted via a first TS (TS1). Figure 3 shows two TS1s in the quintet ground state, i.e., TS1 of the direct-radical HAA mechanism (blue lines) and TS1 of the nonradical HAA mechanism (green lines). C−H bond activation energies (E aTS1 ) for TS1(direct) and TS1(nonradical) of the [FeO]2+-MFI zeolite, which are = measured from the reactant complex to TS1, are ETS1(d) a = 9.3 kcal/mol, respectively, while 6.8 kcal/mol and ETS1(n) a = 6.7 kcal/mol those for the [FeO]2+-AEI zeolite are ETS1(d) a = 7.7 kcal/mol, respectively. As indicated by and ETS1(n) a parenthesized values in Figure 3, the dispersion correction insignificantly changes these activation energies because both the reactant complex and TS1 levels are lowered by similar values, compared to magnitudes of energy.34 The lower ETS1(d) a values, suggest that the direct HAA mechanism is the ETS1(n) a energetically more preferable, as is also observed in experivalues are quite similar ments.20 Interestingly, these ETS1(d) a despite the difference in the zeolite frameworks hosting the active species and even comparable with those for the [FeO]2+CHA29 and -MFI(α site)23 zeolites (6.0 and 6.6 kcal/mol, respectively). Because it is known that zeolite confinement contributes to reduction of the methane activation energy,28,29 these similar ETS1(d) values suggest that MFI, AEI, and CHA a zeolites, despite their different pore structures, exert similar confinement effects on methane. This confirms our previous work,34 showing that AEI, CHA, AFX, and MFI zeolites lower the activation energy of methane by similar magnitudes of energy, but [Cu2(μ-O)]2+-AEI, -CHA, -AFX, and -MFI zeolites differ in reactivity toward methane because of the differences in the ∠Cu−O−Cu angle. The separated C···H (and approaching Oα···H) distances for TS1(direct) of [FeO]2+-MFI and -AEI zeolites, as shown in Table 3, are calculated to be 1.232 (1.315) and 1.258 Å (1.271 Å), respectively, which agree with those for [FeO]2+-BEA.22 At these TSs, the FeOα bond is found to be elongated to 1.706 and 1.703 Å, respectively. The atomic charges of the Oα (and C) atoms for TS1(direct) of [FeO]2+E


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Inorganic Chemistry

Figure 5. Reaction energy diagrams and optimized ground-state structures of all intermediates and TSs in methane hydroxylation over (a) [MnO]+MFI with the Al atom at the 10-MR(T12) site and (b) [MnO]+-AEI with the Al atom at the 8-MR(T3) site. All energies are given in kilocalories per mole. The values in parentheses are relative energies that include dispersion correction. Only the extraframeworks and zeolite rings are shown, while the remaining atoms of the zeolites are omitted for clarity.

N2O decomposition over the [Fe]+-MFI zeolite cluster in the sextet state.70 The dissociating N−O (and approaching FeO) bond lengths are calculated to be 1.467 (1.824) and 1.491 (1.808) Å respectively for TS3 of [Fe]2+-MFI and -AEI zeolites. After dissociation, the resultant N2 molecule is weakly adsorbed on the [FeO]2+-MFI and -AEI zeolite surfaces with only 1.0 and 0.5 kcal/mol of energy, respectively, required to desorb it. 3.3. Mechanism for Methane Hydroxylation over [MnO]+ Zeolites. For the reaction over [MnO]+-MFI and -AEI zeolites, we consider only the nonradical HAA mechanism because the geometry optimizations for the [MnOH···CH3•]+MFI and -AEI radical intermediates cannot converge to the minimum required accuracy and eventually go to the neighboring local minima, which is the hydroxo intermediate. Such an unfavorable formation of the radical intermediate is due to strong interactions between the Mn and C atoms, as indicated by the formation of a low-lying hydroxo intermediate in Figure 5. This is consistent with Pauling’s rules71 defining that the electrostatic bond strength for a given cation coordinated to each anion is inversely proportional to the cation CN. Because the Mn center of the [MnO]+ zeolites has a lower CN (3) than the Fe center of the [FeO]2+ zeolites does (CN = 5), it is expected for Mn−C interactions to be stronger than Fe−C interactions. Thus, the CN of active metal sites would determine the reaction pathways. The [MnO(CH4)]+-MFI and -AEI reactant complexes, as shown in Figure 5, are preferably formed in the quintet state with dMn−C = 3.994 and 3.916 Å, respectively, which are by ∼0.4 Å shorter than the dFe−C value of the [FeO(CH4)]2+-MFI and -AEI reactant complexes in the same spin state, causing the methane adsorption energies to be slightly lower (−1.7 and −0.9 kcal/mol or −5.3 and −6.3 kcal/mol, respectively, if dispersion correction is taken in account). Subsequently, one H atom of methane is abstracted via a four-centered TS1 [H··· OMn···CH3]+-Z, where the C···H distance for TS1(nonradical) of [MnO]+-MFI and -AEI zeolites in the quintet state is elongated to 1.326 and 1.333 Å, respectively, while the Mn···C

the OH group rotates away from the methyl radical to initiate the HO−CH3 rebound. The activation energies for this second half-reaction, measured from the radical intermediate to = 3.7 and 2.0 kcal/mol respectively TS2(direct), are ETS2(d) a = for [FeO]2+-MFI and -AEI zeolites, which agree with ETS2(d) a 1.9 kcal/mol for the [FeO]2+-CHA zeolite.29 The formed [Fe(CH3OH)]2+-MFI and -AEI product complexes are lowlying, showing strong adsorption of methanol on the Fe center with Fe−O bond lengths of 2.045 and 2.054 Å, respectively. Figure 3 shows that to desorb methanol from the [Fe]2+-MFI and -AEI zeolite surfaces, desorption energies of 19.3 and 18.5 kcal/mol, respectively, are required. These rather high energies are consistent with the experimental work of Parfenov et al.,66 reporting that methanol desorption, which requires a higher temperature (>200 °C) than methane activation does, is the rate-determining step in methane hydroxylation by the Fe-MFI zeolite. Along the reaction, the ground state remains in the quintet state without the occurrence of spin inversion,67 which agrees with the previous works.22,23,29 The overall reaction over [FeO]2+-MFI and -AEI zeolites in the quintet state is exothermic by 16.5 and 20.4 kcal/mol, respectively. To regenerate the Oα atom, [Fe]2+-MFI and -AEI zeolites are reactivated by reacting them with an N2O molecule. As shown in Figure 4, nitrous oxide is initially adsorbed on [Fe]2+-MFI and -AEI zeolites in a η1-O manner with Fe···O distances (and Fe−O−N angles) in the quintet ground state of 2.315 (131.2°) and 2.207 Å (126.6°), respectively, and adsorption energies of −3.6 and −2.4 kcal/mol or −9.5 and −7.3 kcal/mol, respectively, if dispersion correction is taken into account. Subsequently, N2O is decomposed into a N2 molecule and an O atom via a third TS (TS3) with activation energies of ETS3 = a = 15.9 and 17.1 kcal/mol 15.1 and 16.9 kcal/mol or ETS3(vdw) a respectively for TS3 of [Fe]2+-MFI and -AEI zeolites, which agree very well with the experimental (Fe-MFI)68 and previous DFT-calculated ([Fe]2+-MFI at the δ site)69 values of 16.5 and 14.1 kcal/mol, respectively. For comparison, our group previously reported an activation energy of 18.8 kcal/mol for F


Article

Inorganic Chemistry

Table 4. Geometrical Parameters, Charges, and Spin Densities for All Intermediates and TSs in Methane Hydroxylation over [MnO]+-MFI and -AEI Zeolites, Calculated in the Quintet State (Structures Shown in Figure 4)a charge (e)/spin density [MnO]+-MFI

[MnO]+-AEI

a

reactant comp. TS1(nonradical) hydroxo int. TS2(nonradical) product comp. reactant comp. TS1(nonradical) hydroxo int. TS2(nonradical) product comp.

dMn−O (Å)

dO−C (Å)

dC−H (Å)

dO−H (Å)

dMn−C (Å)

Mn

Oα

C

1.643 1.709 1.787 1.845 1.999 1.641 1.707 1.792 1.874 2.009

3.786 2.627 2.879 1.934 1.460 4.292 2.652 2.943 1.907 1.458

1.098 1.326 2.955 2.373 2.022 1.099 1.333 3.091 2.284 2.014

2.721 1.363 0.972 0.966 0.975 3.197 1.391 0.976 0.982 0.987

3.994 2.218 1.989 2.807 3.055 3.916 2.171 1.989 2.761 3.104

+1.41/3.72 +1.41/3.70 +1.41/3.84 +1.10/4.21 +0.81/3.92 +1.40/3.71 +1.41/3.65 +1.42/3.84 +1.11/4.21 +0.82/3.92

−0.78/0.13 −0.84/0.16 −1.18/0.10 −1.14/0.01 −1.14/0.04 −0.76/0.14 −0.84/0.20 −1.09/0.09 −1.02/0.01 −1.10/0.05

−0.14/0.01 −0.48/−0.03 −0.44/−0.13 −0.13/−0.32 +0.22/0.00 −0.14/0.01 −0.47/−0.02 −0.48/−0.13 −0.14/−0.31 +0.24/0.00

O and H stand for the Oα and abstracted H atoms, respectively.

Figure 6. Selected MOs and orbital energies for α and β spins of (a) the reactant complex and (b) TS1(direct) of the [FeO]2+-MFI zeolite cluster, calculated in the quintet state. The H-terminations are omitted for clarity.

respectively for HAA on [MnO]+-MFI and -AEI zeolites in the quintet state). The MnOα bonds of [HO−Mn−CH3]+-MFI and -AEI hydroxo intermediates are elongated to 1.787 and 1.792 Å, respectively (see Table 4). The CH3 and OH ligands are then recombined via TS2 to form a product complex of methanol, [Mn(CH3OH)]+-Z. The activation energy for this recombination is calculated to be 50.2 and 45.3 kcal/mol respectively for TS2s of [MnO]+-MFI and -AEI zeolites in the quintet state. These high barriers are due to the formation of MnI, which is a rather unstable oxidation state for manganese. Previously, our group suggested that, to avoid the formation of such unstable species, N2O decomposition possibly occurs in the hydroxo intermediate when the concentration of N2O is high.73 For the same reason, the [Mn(CH3OH)]+-MFI and -AEI product complexes also lie in high levels (8.6 and 6.6 kcal/ mol, respectively). The formed methanol molecule is bound to the Mn atom through its O atom with Mn−O bond lengths of 1.999 and 2.009 Å, respectively. This adsorbed methanol molecule requires 28.4 and 30 kcal/mol of energy to be desorbed from [Mn]+ -MFI and -AEI zeolite surfaces, respectively. The overall reaction over [MnO]+-MFI and -AEI zeolites is endothermic by 37.0 and 36.6 kcal/mol, respectively.

distance is shortened to 2.218 and 2.171 Å, respectively (see Table 4), indicating initiation of the Mn−C bond. The methane = 12.4 and 13.7 activation energies are calculated to be ETS1(n) a kcal/mol, which are respectively higher by 5.6 and 7.0 kcal/mol values for [FeO]2+-MFI and -AEI zeolites in the than the ETS1(d) a same spin state, as expected from the smaller spin densities at the Oα atom of [MnO]+ zeolites (see Table 2). However, = 12.9 kcal/mol interestingly, they are comparable with ETS1(n) a calculated for the [FeO]+-MFI zeolite using the same periodic structure and methods.28 Moreover, Michel and Baerends reported that HAA of methane over the equally charged [FeO(H2O)5]2+ and [MnO(H2O)5]2+ complexes in the gas phase requires similar activation energies of 2.2 and 2.3 kcal/ mol, respectively.72 These results suggest that the metal−oxo charge, which is related to the metal CN and spin density of the Oα atom, seems to play a significant role particularly in methane activation by iron- and manganese-containing complexes. After abstraction, the H atom is covalently bound to the Oα atom, while the methyl moiety is strongly bound to the Mn atom to form a low-lying hydroxo complex, [HO−Mn−CH3]+Z, which results in an exothermic HAA (22.6 and 21.6 kcal/mol G


Article

Inorganic Chemistry 3.4. MO Analyses. To gain a better understanding of the C−H bond cleavage mechanism, we calculate the α- and β-spin MOs and their energies for the reactant complexes and TS1s of [FeO]2+-MFI and [MnO]+-MFI zeolite clusters in the quintet state. The MOs for the [FeO(CH4)]2+-MFI reactant complex in Figure 6a shows that the Fe and Oα atoms form antibonding 3σ* (dz2 − pz), 2πx* (dxz − px), and 2πy* (dyz − py) orbitals as well as bonding 3σ (dz2 − pz), 1πx (dxz + px), and 1πy (dyz + py) orbitals at lower levels (not shown),74,75 where the FeOα bond is the z axis. On the other hand, the δxy and δx2 − y2 orbitals are purely nonbonding, although it seems that the δx2−y2 orbital lobes directed along the axes slightly overlap with the σ lone pairs of the OF atoms.23 This figure also shows that the highest occupied molecular orbital (HOMO, σC−H-donor orbital) of methane interacts with the unoccupied 3σ*(α) and 2πx*(β) orbitals of the Oα atom to find a compromise position between the vertical (pz and σC−H overlap) and the horizontal (px and σC−H overlap).23 This is the reason for the bent Fe−O−C structure of the reactant complex (∠Fe−O−C ≈ 115°). Upon activation of methane via TS1(direct), as shown in Figures 6b, the 3σ*(α)-acceptor orbital is aligned toward the methyl radical and interacts more strongly with the σC−H-donor orbital to undergo charge transfer between them, causing the FeOα bond to elongate and the 3σ*(α) level to increase from −5.64 to −5.40 eV. This slight increase is apparently the reason for the low activation energy of methane over [FeO]2+-zeolites. The Mn and Oα atoms of the [MnO(CH4)]+-MFI reactant complex, as shown in Figure 7a, form antibonding 2πx* (dxy −

significant rearrangement in the electron configuration of [H··· OMn···CH3]+-MFI zeolite, where the singly occupied 2πy*(α) orbital becomes unoccupied at a higher level of −3.81 eV, swapping over the 2πx*(α) orbital, which now becomes singly occupied at a lower level of −5.13 eV. Such an orbital swap makes the 2πy*(α) orbital activated as the lowest vacant acceptor orbital. However, this orbital is directed to the top, while the σC−H-donor orbital of the approaching methane is directed to the side (see Figure 7b), which makes their interactions inefficient. Moreover, this side approach is the only possible way for methane to interact with the active site because the top approach is hindered by the zeolite ring surrounding the [MnO]+ active site (see Figure 2c,d). As pointed out by Michel and Baerends for the [MnO(H2O)5]2+ complex,72 such inefficient interactions between the donor and acceptor orbitals reduce the MnO oxidative reactivity toward methane. Thus, this concludes that the lower reactivity of [MnO]+ zeolites, compared to [FeO]2+ zeolites, is not only attributed to the lower spin density of the Oα atom but also to the zeolite steric hindrance.

4. CONCLUSIONS Using DFT calculations on fully periodic systems, we have calculated the direct conversion of methane on methanol over d4 isoelectronic [FeO]2+ and [MnO]+ species in ZSM-5 (MFI) and SSZ-39 (AEI) zeolites. The most stable zeolite sites possible to host the active species have been carefully examined, and two known reaction pathways, namely, the direct HAA path via a radical-like TS and the nonradical HAA path via a four-centered TS, have been considered. The computational results show that because of the high Fe CN, both reaction pathways are possible for [FeO]2+-MFI and -AEI zeolites, with the direct HAA path more favorable. In contrast, only the nonradical path is possible for [MnO]+-MFI and -AEI zeolites because of the low Mn CN, leading to strong Mn−C interactions, which disfavor the formation of the methyl radical. Despite the isoelectronic structures of [FeO]2+ and [MnO]+ zeolites, the C−H bond cleavage of methane over [FeO]2+ zeolites through either the direct or nonradical path requires lower activation energies than that over [MnO]+ zeolites, mainly because of the higher spin density at the Oα atom of [FeO]2+ zeolites. Moreover, MO analyses suggest that the zeolite steric effects hinder the donor orbital of methane from efficient interactions with the acceptor orbital of MnO species, which becomes an additional reason for the lower reactivity of [MnO]+ zeolites. The present work also shows that [FeO]2+MFI and -AEI zeolites, where the local structure of the active site remains the same irrespective of the zeolite host, activate methane with similarly low activation energies. This confirms our previous work,34 suggesting that MFI and AEI zeolites contribute similar confinement effects to the lowered activation energy of methane.

Figure 7. Selected MOs and orbital energies for α and β spins of (a) the reactant complex and (b) TS1(nonradical) of the [MnO]+-MFI zeolite cluster, calculated in the quintet state. The H-terminations are omitted for clarity.

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px), 2πy* (dx2−y2 − py), and 3σ* (dyz − pz) orbitals, while the δz2 and δxz orbitals are substantially nonbonding, where the Mn Oα bond is the y axis. Furthermore, Figure 7a shows that the σC−H-donor orbital (HOMO) of methane interacts with the singly occupied 3σ*(α) orbital of the Oα atom, causing the methane to be adsorbed perpendicularly to the MnOα bond (∠Mn−O−C = 85°). Upon activation of methane via TS1(nonradical), as shown in Figure 7b, the σC−H orbital of the methyl moiety overlaps with the 3σ*(α) orbital of Mn to initiate a strong Mn−C bond, causing the 3σ*(α) orbital to stabilize at −5.58 eV. More importantly, this figure shows a

ASSOCIATED CONTENT

S Supporting Information *

[The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01284. FeO]2+- and [MnO]+-MFI cluster models, relative energies and optimized structures of [FeO]2+ and [MnO]+ zeolites with the Al atoms at various T sites, and optimized structures and geometrical parameters for H


Article

Inorganic Chemistry

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the triplet state of intermediates and TSs in methane hydroxylation over [FeO]2+ and [MnO]+ zeolites (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 81-92-8022529. Fax: 81-92-802-2528. ORCID

M. Haris Mahyuddin: 0000-0002-8017-7847 Kazunari Yoshizawa: 0000-0002-6279-9722 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KAKENHI Grants JP24109014, JP15K13710, and JP17H03117 from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), the MEXT Projects of “World Premier International Research Center Initiative”, “Integrated Research Consortium on Chemical Sciences”, and “Elements Strategy Initiative to Form Core Research Center”, and JST-CREST Grant JPMJCR15P5. M.H.M. gratefully acknowledges an Indonesia Endowment Fund for Education, the Ministry of Finance of Indonesia, for scholarship support.

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