Roles of Water Molecules in Modulating the Reactivity of Dioxygen

Research Article pubs.acs.org/acscatalysis

Roles of Water Molecules in Modulating the Reactivity of DioxygenBound Cu-ZSM‑5 toward Methane: A Theoretical Prediction Takashi Yumura,*,† Yuuki Hirose,† Takashi Wakasugi,† Yasushige Kuroda,‡ and Hisayoshi Kobayashi† †

Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Department of Chemistry, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Kita-ku, Okayama 700-8530, Japan

‡

S Supporting Information *

ABSTRACT: We propose theoretically that the reactivity of O2bound Cu-ZSM-5 toward methane is enhanced by the presence of one water molecule near a dinuclear copper site inside a 10membered ring of the zeolite cavity. The current study employed density functional theory (DFT) calculations with the B3LYP functional to elucidate reaction intermediates during dioxygen activation by Cu-ZSM-5 in the presence of one water molecule attached to a dicopper site. The initial event is the formation of a hydroperoxo species bridged by the dicopper site via an H atom transfer from an attached water to the bound dioxygen. After the formation of the intermediate, the hydroperoxo O−O bond is completely cleaved to form radical oxygen containing intermediates, such as a Cu−O−Cu species bound by two OH groups (HO−Cu−O−Cu−OH), as well as a copper oxyl group containing intermediate (HO−Cu−OH−CuO). The radical oxygen containing intermediates can cleave a methane C−H bond in a homolytic fashion. Examining the barrier for the C−H bond activation obtained from DFT calculations, we found that the two types of intermediates have the power to more effectively cleave methane C−H bonds than the Cu−O−Cu intermediate that has been proposed to be formed in the absence of a water molecule. The current DFT findings propose that O2-bound Cu-ZSM5 in the presence of one water molecule is a potential candidate for catalysts desired for methane to methanol conversion under mild conditions. Recently, techniques for controlling the number of water molecules near the active site of a ZSM-5 zeolite have been developed, and therefore the DFT findings should stimulate experimental efforts for constructing catalysts for direct methane hydroxylation. KEYWORDS: DFT calculations, zeolite, direct methane oxidation, nanometer sized cavity, C−H bond activation

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INTRODUCTION Transition-metal-containing zeolites are very attractive1−18 as potential candidates for catalysts that can mimic the ability of soluble and particulate methane monooxygenases (sMMO and pMMO)19−28 to directly convert methane to methanol under mild conditions. Constructing catalysts desired for the direct oxidation of methane to methanol under ambient conditions has been a challenging issue in modern chemistry.29−33 In this direction, a pioneering paper was published by Panov et al., who investigated that N2O-mediated iron-containing ZSM-5 zeolite can catalyze direct methane oxidation to methanol.1 The catalytic reactions, reminiscent of those in sMMO,19−21 were computationally studied by Shiota and Yoshizawa.2−4 In addition to iron-containing zeolite catalystys, copper-containing ZSM-5 zeolite catalysts (Cu-ZSM-5)5−18 have been developed in analogy to pMMO.22−24 A dinuclear copper site was proposed to be the active site for the dioxygen activation by Cu-ZSM-5. In one proposed mechanism (Scheme 1),5 a dinuclear copper site interacts with dioxygen to form a peroxo intermediate. The dioxygen O−O bond is then completely cleaved, leading to a bis(μ-oxo) dicopper species (Scheme 1a).5,6 The bis(μ-oxo) dicopper species was first reported to be © XXXX American Chemical Society

Scheme 1. Proposed Reaction Mechanisms for the Dioxygen Activation by Cu-ZSM-5 and the Following Conversion of Methane to Methanol: (a) Formation of a Bis(μ-oxo) Dicopper Species via a Peroxo Complex Formed by a Reaction with Dioxygen and a Dicopper Species inside a ZSM-5 Cavity; (b) Peroxo Complex Conversion into a Cu− O−Cu Species Responsible for the Methane to Methanol Conversion

the active site responsible for methane C−H bond activation, according to ref 5. Received: November 3, 2015 Revised: March 4, 2016

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DOI: 10.1021/acscatal.5b02477 ACS Catal. 2016, 6, 2487−2495

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Scheme 2. Proposed Mechanisms for Reactions between Methane and a Cu−O−Cu Species inside a ZSM-5 Cavity: (a) Radical Rebound Mechanism for the Methane to Methanol Conversion by a Cu−O−Cu species inside a ZSM-5 Cavity; (b) Another Reaction between Methane and Two Cu−O−Cu Species To Form Cu−OH−Cu and Cu−OCH3−Cu Species

Since publication of the milestone paper in 2005,5 a detailed mechanism for methane to methanol conversion has been debated by several experimental and computational approaches.7−18 In 2009, an excellent experimental paper was published by Woertink et al., who utilized resonance Raman spectroscopy to identify reaction intermediates during the dioxygen activation by Cu-ZSM-5 at 450 °C.7 The enhanced Raman spectrum together with normal coordination analysis indicated the formation of a bent Cu−O−Cu core in the reaction (Scheme 1b). The findings were further backed up by other experimental analyses,5−9 especially UV−vis spectroscopy measurements. In fact, the UV−vis studies found that the absorption band at 22700 cm−1 originated from the Cu−O−Cu species. More interestingly, the absorption band decays after exposure to methane at 200 °C. This result indicates that the Cu−O−Cu species, generated from the dioxygen activation by Cu-ZSM-5, can activate methane C−H bonds under relatively mild conditions. Similar methane oxidation reactions were observed in other types of copper-containing zeolites, such as mordenite (Cu-MOR) by the same group10 as well as by Alayon et al.12−15 These experimental findings were supported by density functional theory (DFT) calculations via a small zeolite model.7,9 The DFT calculations found that a methane C−H bond is cleaved by a radical oxygen atom bridged by the two copper cations to form a methyl radical, as shown in Scheme 2. When the resultant methyl radical is rebound to the OH group bridged by the dicopper site, methanol can be formed, which is the so-called “radical rebound mechanism” (Scheme 2a). In contrast, there is a different reaction path after the methyl radical formation: the methyl radical moves to another Cu−O− Cu species, and then the methoxy group is formed between the dicopper sites (Scheme 2b). Because the methoxy intermediate would be stabilized, further treatments are required to form methanol, as shown in Scheme 3. One effective procedure for the formation of methanol from the methoxy intermediates was proposed by Alayon et al.12−15 According to refs 12−15, water in the gas phase was introduced after or before methane activation by O2-bound Cu-MOR (Scheme 3b,c). In accord with their proposals,12−15 there are two roles of the introduced water molecules in converting methane to methanol by CuMOR under dioxygen conditions. One is that water helps to desorb methanol from the methoxy group bound to the dicopper site by a competitive adsorption, and the other is that water changes the structure of an active site in a way that will affect its reactivity toward methane. Although the previous experimental papers seem to show the successful conversion of methane to methanol by the O2-bound dicopper site inside a zeolite cavity, dioxygen activation in a peroxo intermediate to form the Cu−O−Cu species requires high-temperature treatments of 450 °C5−17 (see Scheme 3). Energy provided by the high-temperature treatments can be

Scheme 3. Series of Treatments in the Methane to Methanol Conversion by Copper-Containing Zeolites, Proposed Experimentally: (a) Series in O2-Bound Cu-ZSM-5 Reported in Ref 5; (b) Series in O2-Bound Cu-MOR-5 Reported in Ref 12; (c) Series in O2-Bound Cu-MOR-5 Reported in Ref 13a

a

These series consist of dioxygen addition and its activation (denoted by O2), methane addition and its reaction (denoted by CH4), and He and H2O additions (denoted by He and H2O, respectively).

used to break at least two Cu−O and dioxygen O−O bonds in the peroxo intermediate. However, it is dispensable to break the dioxygen O−O bond under mild conditions to mimic the catalytic ability of pMMO that can easily break methane C−H bonds. Considering the drawback in the current processes, together with the positive roles of water molecules in the dioxygen activation, we suppose that water will aid in activating a peroxo intermediate, which facilitates the following methane activation, as given in Scheme 4.34 On the basis of our Scheme 4. Series of Treatments in the Methane to Methanol Conversion by O2-Bound Cu-ZSM-5 in the Presence of One Water Molecule, Proposed in This Study

hypothesis, the present study investigates via the use of DFT calculations whether or not H2O addition following the formation of a peroxo intermediate forms an active site with higher reactivity toward methane.

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METHODS OF CALCULATION In this study, we employed the B3LYP35−39 DFT calculations implemented in the Gaussian 09 suite of programs,40 following the method given in our previous papers.41−55 B3LYP calculations have been generally used in chemistry because the calculations can well reproduce the corresponding 2488


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the dihedral angle Cu−O−O−Cu is a key parameter to distinguish the dioxygen bridging modes; the dihedral angles in the cis and trans end-on fashions are 0 and 180°, respectively. Figure 1 shows optimized structures for dioxygen binding to a

experimental data. The structure of the ZSM-5 zeolite is shown in section S1 in the Supporting Information.56 We extracted the red part of the ZSM-5 framework and prepared a Si92O151H66 cluster model as an aluminum-free ZSM-5 model whose terminal Si atoms are saturated by the H atoms, as shown in section S1. The aluminum-free ZSM-5 model (Si92O151H66) was fully optimized by B3LYP DFT calculations. In the optimized geometry, separations between two diametrically opposed silicon atoms within a 10-membered ring are only 0.3% longer than those obtained experimentally. In the current study, we concentrate our discussion on catalytic reactions inside a 10-membered ring of ZSM-5 zeolite. Although some terminal Si atoms deviate from the periodic model, the deviating Si atoms are far from the 10-membered ring of ZSM-5. Thus, the cluster model is appropriate to obtain preliminary results on mechanisms of the catalytic reactions inside a ZSM-5 cavity. Using the ZSM-5 Si92O151H66 cluster, we constructed a Cu-ZSM-5 model including a 10-membered ring (10-MR), in which two Cu cations are located near two Al atoms substituted for two Si atoms. In this Cu-ZSM-5 model, the Al pair is in the third-nearest-neighbor position. The details are given in ref 41. Here, we considered reaction intermediates in the triplet spin state, formed by the reaction between Cu-ZSM-5 and dioxygen in the presence of a water molecule because the ground state of dioxygen is a spin triplet. First, the current study investigated how the presence of a water molecule influences the structural features of the reaction intermediates. For this purpose, we fully optimized geometries for the reaction intermediates in hydrous O2-bound Cu-ZSM-5. As the second issue to discuss in the study, we examined whether or not the intermediates formed in the presence of a water molecule can effectively activate a methane C−H bond. We used the 6-311G* basis set for the Cu cations,57,58 the 6-31G* basis set for the adsorbing dioxygen, water, and four O atoms that are bound to the two substituted Al atoms,59−61 and the 3-21G basis set for Al, Si, H, and the other O atoms in the zeolite framework.62−67 To search for transition states connecting two intermediates, quadratic synchronous transit (QST) approaches were employed. The previous study41 confirmed that the B3LYP calculations can well reproduce the Cu−O separations obtained experimentally and computationally,68,69 Furthermore, ref 42 showed that the B3LYP calculations yielded vibrational frequencies for C−H stretching modes of methane deformed by the interactions with Cu-ZSM-5 that were in good agreement with those obtained from IR measurements. The agreements with the computational and experimental values indicate the reliability of our choices in the current calculations.

Figure 1. Optimized structures for O2-bound Cu-ZSM-5 in the presence or absence of water molecules. The number of contained water molecules is (a) 0, (b) 1, (c) 2, (d) 3, and (e) 6. Key parameters in the optimized structures for anhydrous and hydrous O2-bound CuZSM-5 are given in Table 1.

dicopper site of ZSM-5 in the presence or absence of water molecules, where the numbers of water molecules are 0−3 and 6. Key geometrical parameters in the optimized structures are given in Table 1. The two monovalent copper cations are bound to framework oxygen atoms near the substituted Al atoms (section S1 in the Supporting Information). In the Al pair configuration, the dicopper separation is 2.54 Å. The dicopper site is responsible for the dioxygen binding and the activation of the O−O bond. In the absence of water molecules, dioxygen binds to the dicopper site of Cu-ZSM-5 in a trans end-on fashion (Cu−O−O−Cu). In the dioxygen binding fashion, the Cu−O−O−Cu dihedral angle is 142.5°. Then, the copper separation is lengthened to 3.96 Å. Note that the bridging fashion is different from that obtained in the previous study (side-on or cis end-on fashion)41 because different spin states were considered for the calculations.70 The addition of water molecules to the O2-bound Cu-ZSM-5 stabilizes the O2/Cu2 structures well. In fact, the O2-bound CuZSM-5 structure in the presence of one water molecule is stable by 102.9 kcal/mol relative to the dissociation limit toward CuZSM-5, dioxygen, and one water molecule. Furthermore, Figure 1 and Table 1 show that the number of water molecules inside a 10-membered ZSM-5 cavity influences the dioxygen bridging mode. When one water molecule binds to the O2-bound dicopper active site (Cu−O−O−Cu−OH2), the dioxygen

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RESULTS AND DISCUSSION First, let us discuss how dioxygen binding in a dicopper site inside a 10-membered ring of ZSM-5 is influenced by the addition of water molecules. Chart 1 shows the typical fashions of dioxygen binding with the two copper atoms: the cis end-on, trans end-on, and side-on fashions. With the end-on fashions, Chart 1. Three Types of Binding Modes of Dioxygen to a Dicopper Site

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labela

Cu···Cu

O−O

Cu−O(O2)b

Cu−O(w)c

Cu−O−O−Cud

0 1 2 2 3 6

A B C D E F

3.96 3.86 3.64 3.83 3.65 3.72

1.37 1.38 1.40 1.36 1.37 1.38

1.79/1.80 1.80/1.87 1.82/1.88 1.84/1.85 1.83/1.87 1.85/1.86

2.01 1.99 2.00/2.14 1.96/2.14 1.96/1.98

142.5 130.9 126.8 106.0 104.7 100.5

a

The optimized structures for O2-bound Cu-ZSM-5 with or without water molecules can be found in Figure 1. These geometries can be distinguished by the labels in Figure 1. Calculated total energies in the optimized structures are given in section S2 in the Supporting Information. b These bonds (in Å) indicate separation between a copper cation and an oxygen atom of the bound dioxygen. cThese bonds (in Å) indicate separation between a copper cation and the water oxygen atom. dThe dihedral Cu−O−O−Cu angle (in deg).

Figure 2. Optimized structures for reaction intermediates and transition states during dioxygen activation by Cu-ZSM-5 in the presence of one water molecule. Optimized bond lengths are given in Å.

bridging fashion is trans end-on, similar to that in the absence of water molecules. This result indicates that there is enough room to allow a dioxygen molecule to bind to the dicopper site in a trans end-on fashion. In this structure, one copper cation to which a water molecule binds is coordinated by four oxygen atoms and the other cation is three-coordinated. When two water molecules approach the O2-bound dicopper active site, there are two types of optimized structures. In one structure, one water molecule binds to a copper cation, and at the same time, the other water molecule is weakly bound to the attached water molecule through hydrogen bonds (Cu−O−O−Cu− OH2···OH2, Figure 1c). However, Figure 1d has both water molecules binding to each copper cation (H2O−Cu−O−O− Cu−OH2). Interestingly, the Cu−O−O−Cu−OH2···OH2 structure is 1.3 kcal/mol more stable than the H2O−Cu−O− O−Cu−OH2 structure, despite a smaller number of coordination bonds between the copper cation and the water molecule’s oxygen atom. Due to limitation of the inner ZSM-5 space, the binding of two water molecules to the dicopper site substantially changes the dioxygen binding fashion due to repulsions between the water molecules and the bound dioxygen. In fact, the dihedral Cu−O−O−Cu angle in the

H2O−Cu−O−O−Cu−OH2 structure is 106.0°, diminished from that in the Cu−O−O−Cu structure in the absence of water molecules. Such repulsion interactions cannot operate in the Cu−O−O−Cu−OH2···OH2 structure, whose dioxygen bridging fashion is kept to trans end-on. Due to the presence or absence of the repulsive interactions, the energy differences between the two optimized structures for O2-bound Cu-ZSM-5 containing two water molecules are reasonable. When the number of attached water molecules is 3 or 6 inside a limited space of ZSM-5, two water molecules bind to both copper cations, and additional water molecules are weakly bound to two water molecules attached to copper cations through hydrogen bonds. In this situation, the dioxygen binding fashion is similar to that in H2O−Cu−O−O−Cu− OH2. As shown in Figure 1, we found two types of water molecules: water molecules that are coordinatively bound to copper cations and water molecules that are weakly bound to the water molecules attached to copper cations. Because the hydrogen bonds are weak relative to the Cu−O coordination bond, water molecules that do not directly bind to copper cations are easily removed by the He treatments with lower energy before the methane insertion in Scheme 4. However, the 2490


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Figure 3. Potential energy diagram for the dioxygen activation by Cu-ZSM-5 in the presence of one water molecule. Energies relative to the dissociation limit toward Cu-ZSM-5, dioxygen, and one water molecule are given in kcal/mol. A triplet surface energy was considered.

Figure 4. Changes in calculated spin densities on reaction intermediates during the dioxygen activation by Cu-ZSM-5 in the presence of one water molecule.

migration from the attached water molecule forms a hydroperoxo species bridged by the two copper cations plus one OH group (Cu−O−OH−Cu−OH). The Cu−O−OH−Cu−OH species is converted to an HO−Cu−O−Cu−OH species through a transition state where the H atom migrates within the hydroperoxo species (intra H atom migration). Another event in this transition state is that the hydroperoxo O−O bond is significantly cleaved by the intra H atom migration (the hydroperoxo O−O separation is 2.17 Å). The intra H atom migration within the hydroperoxo species together with its O− O bond activation requires an activation energy of 31.9 kcal/ mol. The activation barrier is similar to that in ref 71. It is important to note that the HO−Cu−O−Cu−OH species has a Cu−O−Cu core, whose structure is similar to those reported in ref 7 as well as to that of a corresponding synthetic model.72 At the final step, the HO−Cu−O−Cu−OH species transforms to the HO−Cu−OH−CuO species via a transition state where the H atom of an OH group migrates to the bridged oxygen atom. An energy of 15.7 kcal/mol is required to carry out this process. Figure 3 shows four local minima and three transition states connecting two local minima on the potential energy surface of this reaction. From Figure 3, we can see that all reaction intermediates are energetically stable. More importantly, Figure 3 shows that the most energy consuming process is the intra H atom migration within the hydroperoxo species, concomitant

He treatments cannot remove two water molecules coordinatively bound to the copper cations. In this situation, the Cu− O−O−Cu−OH2 and H2O−Cu−O−O−Cu−OH2 structures should be considered. As mentioned above, the H2O−Cu−O− O−Cu−OH2 structure is unstable relative to the Cu−O−O− Cu−OH2···OH2 structure by 1.3 kcal/mol, and thus the conversion from H2O−Cu−O−O−Cu−OH2 to Cu−O−O− Cu−OH2···OH2 can proceed easily by a further loweredtemperature treatment. Thus, we will concentrate our discussion on reaction pathways for dioxygen activation in Cu-ZSM-5 in the presence of one water molecule and on the following methane activation. Dioxygen Activation by Cu-ZSM-5 in the Presence of One Water Molecule. In this section, we discuss how dioxygen activation takes place in a dicopper active site in the presence of one water molecule inside a ZSM-5 cavity. Figure 2 shows the reaction pathway obtained from DFT calculations, and Figure 3 displays its energy profile. As shown in Figure 2, the reaction starts from the Cu−O−O−Cu−OH2 species. After the formation of the Cu−O−O−Cu−OH2 species, an H atom of the attached water molecule migrates to an O atom of the bridged dioxygen via a transition state. The activation energy for this process was calculated to be 25.0 kcal/mol. In the transition state, the O−H bond is lengthened to 1.46 Å, and the newly formed O−H bond length is 1.08 Å. The H atom 2491


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Figure 5. Potential energy diagram for the methane C−H bond activation by HO−Cu−O−Cu−OH inside a ZSM-5 cavity. Energies relative to the dissociation limit toward the methane complex are given in kcal/mol. The triplet surface energy was considered.

Figure 6. Potential energy diagram for the methane C−H bond activation by HO−Cu−OH−CuO inside a ZSM-5 cavity. Energies relative to the dissociation limit toward the methane complex formed in this structure are given in kcal/mol. The triplet surface energy was considered.

site, being less reactive than a Cu−O−Cu species, which was proposed in ref 7, as shown in Section S3 in the Supporting Information. Basically, similar spin densities can be found in Cu−O−OH−Cu−OH, although the spin density on the O atom attached by an H atom from H2O decreases from 0.42 to 0.11. After the intra H atom migration within the hydroxo moiety concomitant with its O−O bond cleavage, we found striking changes in terms of spin density populations; significant spin density was only found on the bridged oxygen atom in HO−Cu−O−Cu−OH (0.9) and on the terminal oxygen atom in HO−Cu−OH−CuO (1.1), indicating the formation of a radical oxyl group bound by a copper cation.74,75 Instead, the spin densities disappear on the copper atom bound by the oxyl group in Figure 4. The changes in the spin densities can also be understood from section S4 in the Supporting Information. Similar radical oxygen atoms bound to the copper cation can be seen in ref 7, as well as in [CuO]+ in the gas phase reported by Schwarz et al.,76,77 whose existence was previously predicted by the DFT computations of Shiota and

with its O−O bond activation (TS2). However, TS2 is energetically stable relative to the dissociation limit toward Cu-ZSM-5, dioxygen, and water. Therefore, the entire reaction is downhill, suggesting that the dioxygen activation by CuZSM-5 in the presence of one water molecule can proceed. Here, let us discuss the characteristics of the reaction intermediates formed after the dioxygen activation by Cu-ZSM5 in the presence of one water molecule. Figure 4 displays changes in calculated spin densities on the reaction intermediates. The first Cu−O−O−Cu−OH2 intermediate has copper cations with spin densities of 0.61 and 0.43, being larger than those in Cu(I)-ZSM-5. Alternately, spin densities of oxygen atoms of bound dioxygen (0.42 and 0.39) are smaller than those in free dioxygen. These results indicate that some 3d electrons of both Cu(I) cations transfer to singly occupied orbitals of dioxygen (π* orbitals) to activate its O−O bond.73 As a result of the electron transfer, formal charges of the copper cations become 2. The changes in spin density distributions indicate the formation of peroxo species in the dinuclear copper 2492


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Table 2. Barriers (Ea in kcal/mol) for the Activation of the Methane C−H Bond by Radical Oxygen Atom Containing Intermediates Formed in Anhydrous and Hydrous O2-Bound Cu-ZSM-5 Ea

HO−Cu−O−Cu−OHa

HO−Cu−OH−CuOa

Cu−O−Cub

Cu−O−Cuc

10.5

7.9

30.1

18.5

a

The reaction intermediates can be seen in Figures 5 and 6. bThe optimized geometries for a Cu−O−Cu species inside a ZSM-5 cavity were obtained in the current study. See section S5 in the Supporting Information. cThe activation energy obtained in ref 7, using a small ZSM-5 model, included zero-point vibrational energy in the activation energy. A detailed discussion on the comparison between our values and other values in terms of the activation energy is given in section S5 in the Supporting Information.

Yoshizawa.78 In fact, the calculated spin density of the oxygen atom of [CuO]+ (1.6), whose value is consistent with those obtained in refs 78−80, is close to that of the oxyl radical in HO−Cu−OH−CuO. Methane Activation by Reaction Intermediates with a Radical Oxygen Atom. As discussed in the previous section, the HO−Cu−O−Cu−OH and HO−Cu−OH−CuO species formed after the dioxygen activation have a radical oxygen atom. The radical oxygen atom in both active species has the potential ability to activate a C−H bond of methane, as exemplified in Scheme 2a. In this section, we discuss the energetics for the methane C−H bond activation by the HO− Cu−O−Cu−OH and HO−Cu−OH−CuO species, as shown in Figures 5 and 6, respectively. At the initial stage of the reaction, methane and the radical oxygen atom come into contact inside a ZSM-5 nanospace (methane complex); the shortest separation between the radical O atom and an H atom of methane is 4.29 Å for the HO−Cu−O−Cu−OH species (Figure 5) or 2.81 Å (Figure 6) for the HO−Cu−OH−CuO species. After that, a methane C−H bond is activated by the radical oxygen atom in a transition state with a linear structure. In the transition state, a methane C−H bond was lengthened to ∼1.23 Å, in comparison with those in free methane (1.01 Å). The barrier for the C−H bond activation was calculated to be 10.5 kcal/mol for the HO−Cu−O−Cu−OH case and 7.9 kcal/ mol for the HO−Cu−OH−CuO case. The C−H bond activation results in forming a methyl radical inside a ZSM-5 nanospace (radical intermediate).81 The radical intermediate is 11.0 kcal/mol more stable than the methane complex for the HO−Cu−O−Cu−OH case, and the two intermediates are energetically identical for the HO−Cu−OH−CuO case. The energy differences can be reproduced by empirically dispersion corrected DFT (B97-D)82 calculations.83 where van der Waals interactions can be described. Note that random phase approximations (RPA) are necessary to correctly describe van der Waals interactions between alkanes inside a zeolite cavity, according to refs 84 and 85. However, the RPA methods cannot be applied for large-scale systems such as our models due to their unfavorable scaling with system size.84 One promising alternative is empirical dispersion-corrected DFT calculations,82 because ref 84 reported that such dispersioncorrected DFT calculations can reproduce RPA results in terms of interactions between alkanes and H-zeolites. Here, let us make a comparison between the different radical oxygen containing intermediates (HO−Cu−O−Cu−OH and HO−Cu−OH−CuO) in terms of their reactivity toward methane. Their barriers for the methane C−H bond activation (Ea) are given in Table 2, together with the corresponding activation energies in the Cu−O−Cu species inside a ZSM-5 cavity. Detailed information on the methane activation by the Cu−O−Cu species can be found in section S5 in the Supporting Information. Table 2 shows that the Ea values in HO−Cu−O−Cu−OH and HO−Cu−OH−CuO are lower

than those in the Cu−O−Cu cases (30.1 kcal/mol for our model and 18.5 kcal/mol for ref 7).86 These results indicate that the radical oxygen atoms generated from the dioxygen activation by Cu-ZSM-5 in the presence of one water molecule have the power to easily activate methane C−H bonds, in comparison with the anhydrous case. In other words, one water molecule near an active site of O2-bound Cu-ZSM-5 can strengthen its reactivity toward methane. Thus, the DFT findings clearly suggest that introducing one water molecule near the active site of O2-bound Cu-ZSM-5 can help to lower the energy required for the methane C−H bond activation. We found similar importance of one water molecule in enhancing the reactivity of O2-bound Cu-ZSM-5 containing a trinuclear copper site, which has been recently proposed in ref 18. See the detailed discussion in section S6 in the Supporting Information. It should be noted that the number of water molecules attached to copper cations is sensitive in differentiating the reactivity of O2-bound Cu-ZSM-5. In fact, we considered reaction pathways in H2O−Cu−O−O−Cu−OH2 in Figures S6 and S7 (section S7 in the Supporting Information), although the structure is unstable by 1.3 kcal/mol relative to the Cu−O−O−Cu−OH2··· OH2 structure. According to DFT calculations, such radicalcontaining intermediates cannot be generated in the reaction starting from H2O−Cu−O−O−Cu−OH2. This result also indicates that controlling the number of water molecules near the active site of O2-bound Cu-ZSM-5 is key to modulating its reactivity toward methane.

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CONCLUSION Using density functional theory (DFT) with the B3LYP functional, we considered reaction mechanisms for dioxygen activation and methane C−H bond activation by Cu-ZSM-5 in the presence of one water molecule. DFT calculations found that the reactivity of O2-bound Cu-ZSM-5 toward methane is strengthened by the presence of one water molecule near its active site. When one water molecule exists near the dicopper active site of O2-bound Cu-ZSM-5, the reaction proceeds in the following manner. The first step is the formation of a hydroperoxo species via an H atom migration from the one water attached to a copper cation to dioxygen. Next, an intra H atom migration within the hydroperoxo group together with its O−O bond activation leads to a Cu−O−Cu species attached to two OH groups (HO−Cu−O−Cu−OH) that can finally be converted into HO−Cu−OH−CuO via an H atom migration from an attached OH group. From the viewpoint of the energetics in the DFT calculations, the reactions can proceed easily. Interestingly, the HO−Cu−O−Cu−OH and HO−Cu− OH−CuO intermediates commonly have a radical oxygen atom that can effectively activate a methane C−H bond in a homolytic manner. In fact, barriers for the methane C−H bond activation were calculated to be 10.5 and 7.9 kcal/mol for HO− Cu−O−Cu−OH and HO−Cu−OH−CuO, respectively. The calculated barriers are lower than those in anhydrous Cu−O− 2493


Research Article

ACS Catalysis

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Cu species that were reported previously. Thus, DFT calculations newly found that introducing one water molecule near the active site of O2-bound Cu-ZSM-5 can facilitate the methane C−H bond activation. Conclusively, we suggested from DFT calculations that controlling the number of water molecules near the active site of O2-bound Cu-ZSM-5 is key to modulating its reactivity toward methane. Techniques for controlling the number of water molecules near the active site inside ZSM-5 have been developed according to ref 87. Thus, our current DFT results should stimulate experimental efforts for constructing catalyst-based O2-bound Cu-ZSM-5 effective for methane to methanol conversion under mild conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02477. Model of ZSM-5 zeolite and its Cu-ZSM-5 model containing a dinuclear copper active site with an Al pair being the third nearest neighbor, calculated total energies of optimized geometries for O2-bound Cu-ZSM-5 with or without a water molecule in Figure 1, properties of an O2-bound dicopper site inside a ZSM-5 cavity, in comparison with a Cu−O−Cu species, spin densities on reactive intermediates during the dioxygen activation by Cu-ZSM-5 in the presence of one water molecule, activation of a methane C−H bond by the Cu−O−Cu species inside a ZSM-5 cavity, influence of one water molecule in the reactivity of O2-bound Cu-ZSM-5 containing a trinuclear copper site, and the reaction pathway in H2O−Cu−O−O−Cu−OH2 obtained from B3LYP calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.Y.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was partially supported by a Grant-in-Aid for Young Scientists (B) from the Japan Society for the Promotion of Science (JSPS) (T.Y. at Kyoto Institute of Technology, no. 26790001) and by a Grant-in-Aid for Scientific Research on the Innovative Area “Stimuli-responsive Chemical Species for the Creation of Fundamental Molecules” (No. 2408) from the Ministry of Education, Culture, Sports, Science and Technology, Government of Japan (MEXT) (T.Y. at Kyoto Institute of Technology, no. 15H00941).

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REFERENCES

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Roles of Water Molecules in Modulating the Reactivity of Dioxygen

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