Mechanism of CH4 Activation on a Monomeric Zn2+-Ion Exchanged in

Aug 29, 2013 - The first band can be assigned to the OH stretching vibration associated with the formation of the Brønsted acid site and the latter t...
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Mechanism of CH4 Activation on a Monomeric Zn2+-Ion Exchanged in MFI-Type Zeolite with a Specific Al Arrangement: Similarity to the Activation Site for H2 Akira Oda,† Hiroe Torigoe,† Atsushi Itadani,† Takahiro Ohkubo,† Takashi Yumura,‡ Hisayoshi Kobayashi,‡ and Yasushige Kuroda*,† †

Department of Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima, Kita-ku, Okayama 700-8530, Japan ‡ Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan S Supporting Information *

ABSTRACT: In this work, we used both experimental and density functional theory (DFT) calculation methods to examine the mechanism of CH4 activation taking place on the Zn2+ ion exchanged MFI-type zeolite (ZnMFI). The heterolytic dissociation of CH4 on ZnMFI around 300 K was observed experimentally, causing the appearance of IR bands at 3615, 2930, and 2892 cm−1. The first band can be assigned to the OH stretching vibration associated with the formation of the Brønsted acid site and the latter to the C−H stretching modes ascribable to the −[ZnCH3]+ species. Combining the IR spectroscopy with a DFT calculation, it is apparent that the heterolytic C−H bond dissociation of CH4 has an activation energy of 15 kJ mol−1 and takes place on a monomeric Zn2+ at the M7S2 site. The M7S2 site has a specific Al arrangement in MFI and exhibits a pronounced reactivity for the H−H bond cleavage of H2, even at room temperature. In addition, to our knowledge, we are the first to succeed in explaining the dissociation process of CH4 by applying natural bond orbital (NBO) and interaction localized orbital (ILO) analyses to the present system; the donation interaction from the CH4−σ(C−H) orbital to the Zn−4s orbital triggers the cleavage of the C−H bond of CH4 under mild conditions.

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Transition-metal ions existing on the specific reaction fields in zeolites, microporous crystalline aluminosilicates, exhibit unique adsorption properties and catalytic activities for various gases. The divalent zinc ion (Zn2+), which has a 3d104s0 electron configuration, exchanged in MFI-type zeolite (ZnMFI) has a novel ability to activate CH4. So far, many investigators have reported that the Zn2+ in MFI works as an active center for the conversion of CH4 or various alkanes to valuable substances under mild conditions.9−30 Taking the electron configuration of Zn2+ in MFI into consideration, how it takes a pivotal role and works effectively as a CH4 activation site is a fascinating question, and a thorough understanding of the mechanism of activation of CH4 on ZnMFI is important for the progress of its applied and fundamental chemistry.31 The most significant finding related to the state of the activation site for CH4 in ZnMFI was published in 2004 by Kazansky et al., who were the first to report that the heterolytic CH4 bond dissociation takes place at around room temperature on

INTRODUCTION

Because we are faced with a serious problem related to the limitation of the fossil fuels that support our lifestyle, there is an urgent need to develop new energy and chemical resources. In this situation, abundant and clean methane (CH4) is a material that could be used as an alternative and renewable energy resource, as well as an excellent raw material for production of chemicals. However, the conversion of CH4, as a starting material, to value-added substances is difficult under usual conditions because CH4 has an unusually high C−H bond enthalpy. Therefore, efficient activation of the C−H bond in a CH4 molecule under mild conditions is essential for the next generation, “methane economy”.1,2 So far, various transitionmetal ions supported on solid surfaces have been used to activate the C−H bond in CH4; activation of the C−H bond in CH4 can be achieved when σ- or π-complexes are formed, resulting from electron transfer reactions between CH4 and transition-metal ions.3−8 However, a practicable CH4 activation catalyst functioning under mild conditions and able to meet the needs of the next generation, “methane economy”, has never been developed. © 2013 American Chemical Society

Received: July 3, 2013 Revised: August 21, 2013 Published: August 29, 2013 19525

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specified in the captions to the figures, spectra were measured in the atmosphere of CH4 or H2. Measurements of H2 Adsorption Isotherms. Prior to the measurement of H2 adsorption, the sample (ca. 150 mg) was at first evacuated at 873 K for 4 h under a reduced pressure of 1.3 mPa. On this sample, the first adsorption isotherm of H2 was measured at 298 K using a volumetric adsorption apparatus equipped with an MKS Baratron pressure sensor, type 390. After the first adsorption measurement, the sample was reevacuated at 298 K for 4 h under a reduced pressure of 1.3 mPa, and then the second adsorption isotherm was measured at 298 K. The measurements of H2 adsorption were also performed for the ZnMFI sample pretreated at 473 K with CH4 under the pressure of Pe = 13.3 kPa to get information about the state of the CH4 activation site functioning in this sample. In this measurement, the CH4-treated sample was first evacuated at 298 K, followed by the first adsorption measurement of H2 at 298 K. The second H2 adsorption experiment was carried out again at 298 K for the sample reevacuated at 298 K after the first adsorption measurement. Computational Methodology (Calculation Method). In the present study, we used one of the most successful hybrid Hartree−Fock/DFT method, so-called the B3LYP functional, implemented in the Gaussian 03 and 09 program packages.33 A ZnAl2Si9O20H20 cluster was used as a candidate model which was truncated from a Al2Si90O151H67 cluster, to represent the M7 site in the MFI-type zeolite crystalline structure.32 This model was terminated by H atoms. Only a Zn atom was optimized in this model, and the peripheral part was fixed to the MFI-type zeolite crystalline structure, because the full optimization leads to a deformation of the cluster shape due to the small size or its smallness. The optimized structure, which is hereafter denoted as Zn2+-M7S2, is shown in Figure 1. In the

monomeric Zn2+ incorporated in MFI.21 However, the mechanism and actual working site for the C−H bond activation taking place on ZnMFI has never been clarified, and hitherto, there has been no meaningful knowledge through which the pivotal role of Zn2+ in MFI can be understood. Under such circumstances, we have recently succeeded in specifying the location of the Zn2+ in MFI that exhibits the potential for H−H bond cleavage in an H2 molecule at room temperature and found that an unprecedented reversible redox process, involving formation of a stable atomic Zn0, takes place on a particular site.32 On the basis of that fundamental work, we planned to use H2 as a probe molecule for examining the state of the CH4 activation site in ZnMFI. Density functional theory (DFT) calculations, combined with experimental techniques, are the key methods we have applied recently, to identify the reaction species and to elucidate the CH4 activation mechanism. In the present study, we aimed to understand the state of the Zn2+ ion in an MFI zeolite acting as an active center for CH4 through both experimental and theoretical analyses. To our knowledge, we have been the first to clarify the dissociative CH4 adsorption process that takes place on the Zn2+ ion of a ZnMFI, which is positioned on the site with a specific M7 site with a S2 configuration (M7S2) having a specific configuration of Al atoms in the MFI.

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EXPERIMENTS AND CALCULATIONS Materials. The sodium-form MFI-type zeolite (NaMFI) sample (ca. 5 mg) with an Si/Al ratio of 11.9, which had been purchased from Tosoh Co. Japan, was dispersed in an aqueous solution of 0.3 M Zn(NO3)2 with stirring at room temperature for 1 h. This operation was repeated 10 times to obtain the ZnMFI sample. The ZnMFI sample made in this way was washed thoroughly with distilled water and dried at room temperature. The ion-exchange level of the present sample was evaluated to be 95%, which was estimated by assuming that one divalent zinc ion is exchanged for two monovalent sodium ions, by the chelatometric titration method. The H2 (purity of 99.99%) and CH4 gases (purity of 99.99%) used as the adsorbates were purchased from GL Sciences Company. Measurements of Infrared (IR) Spectra. The sample (ca. 7 mg) was pressed under a pressure of 100 kg cm−2 into a disk of diameter of 10 mm and then set in an in situ cell with KRS-5 windows and was at first evacuated at 873 K in vacuo. IR spectra were recorded at room temperature using a Digilab FTS-4000MXK FTIR spectrophotometer equipped with an MCT detector kept at the temperature of liquid nitrogen; the conditions of both accumulation times of 128 and the numerical resolution of 2 cm−1 (the resolution in apparatus is set to 4 cm−1) were used for recording the spectra. In the experiment utilizing H2 as a probe molecule (Figure 3) for getting the information about the state of the site functioning as the CH4 activation in the MFI sample, the sample evacuated at 873 K was first treated at 473 K with CH4 under the pressure of Pe = 13.3 kPa; the chemisorption of CH4 takes place in this stage, accompanying with the formation of −[ZnCH3]+ and −OH. After that treatment, this sample was treated with H2 under an in situ condition at various temperatures: 300, 373, and 473 K. All spectra shown in the figures are represented as the difference spectra between the respective samples and the reference state, the 873 K evacuated sample. Except for the data

Figure 1. Model for the Zn2+-M7S2 site in ZnMFI used in this study. This site is made up of the components of the ZnAl2Si9O20H20 cluster.

adsorption systems including CH4 on Zn2+-M7S2 site, only the central Zn + CH4 moiety was optimized as the case of the free cluster model described above. We used the 6-311G* basis set for the Zn atom and adsorbing CH4 molecule, the 6-31G* basis set for two Al atoms and the O atoms that are bound to the two substituted Al atoms and simultaneously coordinated by the Zn atom, and the 3-21G basis set for the Si, H, and the other O atoms in the zeolite framework. We have confirmed previously that B3LYP calculations with the same basis set can reproduce experimental IR vibrational frequencies of methane under the interactions with CuMFI well.34,35 After optimizations of the CH4 molecule adsorbed on Zn2+MFI, we obtained their stabilization energies corrected for basis set superposition errors (BSSEs) by using the counterpoise method.36 A vibrational analysis was performed for the 19526

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optimized structure to compare with IR spectra obtained experimentally. Generally, theoretical harmonic frequencies overestimate experimental values due to incomplete descriptions of electron correlations and neglecting mechanical anharmonicity. To compensate the problem, a uniform scaling factor of 0.962 was used on calculated frequencies with the B3LYP level of theory.37 In this work, the bond fission and formation are pursued along the whole reaction path. Usually the frontier electron theory is applied, where the orbitals for isolated subsystems are adopted as the reference orbital sets. However, for the latter half of the reaction, the isolated subsystems are no longer a good reference. Furthermore we are interested in the local atom−atom interactions which are responsible for the bond fission and formation. Usually such bond alternation interactions are dispersed among many occupied MO’s in the canonical form. We conceived the unique localized orbitals named interaction localized orbitals (ILOs).38 The ILO is built by the unitary transformation of the occupied MO’s (not the subsystem orbitals). In the present work, the criterion localizing within CH4 moiety, that is, four C−H bonds, is adopted. The ILO is ranked in the decreasing order of localization strength, and the orbital interactions responsible for the fission of CH3− H bond and the formation of Zn−CH3 and O−H bonds are concentrated within only a few ILOs. A more detailed explanation for the ILO method is also given in the Supporting Information: part 4. The NBO (natural bond orbital) analysis method was also employed for analyzing the bonding features between Zn2+ in MFI and CH4. NBO analysis is also used to estimate the atomic charge instead of the usual Mulliken population. This method has been integrated in the Gaussian 03 and 09 program packages.39

Figure 2. IR spectra in the reaction processes of ZnMFI with CH4 at various temperatures. ZnMFI was treated with CH4 at (1) 298 K, (2) 323 K, (3) 373 K, and (4) 473 K, respectively. The spectrum (5) was measured after re-evacuation at 298 K of the sample which was treated with CH4 at 473 K. Spectra are shown (a) in the region between 4000 and 2600 cm−1, (b) in the OH vibration region, and (c) in the C−H vibration region, respectively.

systems observed for HMFI-CH4, as well as ZnO−CH4.41,42 More striking was the appearance of an OH band at 3615 cm−1, accompanying the bands at 2930 and 2892 cm−1. These features can be interpreted in terms of the dissociative adsorption of CH4; the band at 3615 cm−1 can be assigned to the O−H stretching mode of the Brønsted acid site, and the latter two bands at 2930 and 2892 cm−1 to the C−H stretching modes of a −[ZnCH3]+ species formed in the MFI.21 It is noteworthy that an increase in reaction temperature to 473 K brings about a concurrent increase in the band intensities observed at 3615, 2930, and 2892 cm−1 (spectra 2−4). After the re-evacuation at room temperature of the sample treated with CH4 at 473 K (spectrum 5), the absorption bands at 3003 and 2810 cm−1, which result from molecularly adsorbed CH4 on Zn2+, became completely attenuated, whereas the absorption bands at 3615, 2930, and 2892 cm−1, which result from dissociated CH4, were unchanged. These IR data, as shown in Figure 2, suggest that heterolytic CH4-bond dissociation is accelerated through the treatment at higher temperatures, indicating that some activation process exists in the reaction. Taking account of these data and the mechanism of H2dissociation, a reasonable assumption about the reaction mechanism is as follows; the dissociative adsorption of CH4 takes place on a monomeric Zn2+ exchanged in MFI, even at room temperature, according to the following reaction:

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RESULTS AND DISCUSSION Methane is well-known as an important candidate molecule for energy generation and a feasible carbon feedstock for the future, although it is an inert molecule. Taking account of both the specific H2 activation features induced by ZnMFI, even at room temperature, as found in our preceding work,32 and the similarity in magnitude of bond enthalpies between H−H and C−H, it seems that there is a potential to use ZnMFI as an effective catalyst for CH4 activation. There are several papers that describe the dissociation reactions of these molecules taking place on ZnMFI, including dissociation at room temperature.13,15,16,27 To determine the possibility of CH4 dissociation under mild conditions, first, we examined the ability of ZnMFI to activate CH4 at around room temperature. Figure 2 shows infrared (IR) spectra for ZnMFI exposed to CH4 gas (Pe = 13.3 kPa) at various temperatures after evacuating ZnMFI at 873 K in vacuo. When a ZnMFI sample was exposed to a CH4 gas at room temperature, broad and sharp bands were observed at 3003 and 2810 cm−1, respectively (spectrum 1 in Figure 2a and c). These bands can be assigned to the ν3 and ν1 C−H stretching vibration modes of CH4 physically adsorbed onto the Zn2+ in the MFI zeolite.21,40 For information, the changing processes of IR spectrum observed in the physisorption of CH4 are also given in Figure S1-1. This behavior is very specific in the ZnMFI-CH4 adsorption system; the band intensity due to the ν1 band is stronger compared to that for ν3 band, in spite of the former band being ascribable to the symmetric stretching vibration in the original mode. Such nature is quite different from the nature observed for the

ZA −OL −Zn 2 +−OL −Z B + CH4 → ZA −OL −[ ZnCH3]+ + H+−OL −Z B

(1)

where ZA and ZB represent the zeolite lattice including an Al atom: and OL means the lattice oxygen atom. The proposed mechanism can be interpreted as the dissociative adsorption of CH4 caused by a specific reaction field emanating from the Zn2+ 19527

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Figure 3. (a) IR spectra of ZnMFI in the reaction with H2 at various temperatures at: (1) 298 K, (2) 373 K, and (3) 473 K, respectively. (a-1) In the OH stretching region and (a-2) in the ZnH stretching region. All spectra were measured after evacuation of a residual H2 gas at 298 K. (b) Changes in the IR spectra of ZnMFI treated with CH4 during the reaction with H2 at various temperatures: (1) 298 K, (2) 373 K, and (3) 473 K. (b-1) in the OH stretching region, (b-2) in the CH stretching region being ascribable to the formed −[ZnCH3] species, and (b-3) in the ZnH stretching region. In this experiment, the ZnMFI sample evacuated at 873 K was at first treated with CH4, followed by evacuation at 298 K. After that, the sample was successively reacted with H2 at respective temperatures.

Figure 3a), indicating that the dissociative adsorption of H2 occurs on a monomeric Zn2+ in MFI: reaction 2.

ion exchanged on the MFI-type zeolite: an interaction with a monomeric Zn2+ in MFI. For the present ZnMFI sample, it has already been proven that there are at least three dominant types of Zn2+ species in the sample: (1) a monomeric Zn2+ acting as an H2 dissociative adsorption site, (2) a dimeric [Zn−O−Zn]2+ species, which scarcely functions as an H2-activation site, and (3) monomeric Zn2+ that does not act as a dissociative H2 adsorption site.32 It is only the Zn2+ exchanged in the M7S2 site that functions as the activation site for H2: the monomeric Zn2+ species on the site with a specific configuration of Al atoms.32 On the basis of such findings, we used H2 as a probe molecule to obtain information about the CH4 activation site. We further examined the H2adsorption properties of the sample before and after covering some sites with CH4; IR spectroscopy of the sample exposed to H2 (Pe = 13.3 kPa) was conducted at various temperatures before and after treatment with CH4 at 473 K under an equilibrium pressure of 13.3 kPa. The resultant data are shown in Figure 3a and b. When the sample without CH 4 pretreatment was exposed to H2 at room temperature, IR bands were observed distinctly at 3615 cm−1 and 1933 cm−1, which were assigned to the OH species formed as the Brønsted acid site and the −[ZnH]+ species, respectively (spectrum 1 in

ZA −OL −Zn 2 +−OL −Z B + H 2 → ZA −OL −[ ZnH]+ + H+−OL −Z B

(2)

The intensities of these bands increased with an increase in treatment temperature, because the chemical reaction accelerates at higher temperatures (spectra 2 and 3 in Figure 3a). By contrast, any characteristic IR band resulting from a −[ZnH]+ species was scarcely observed (spectrum 1 in Figure 3b) with the sample treated with CH4 at 473 K, even after exposure to H2 at room temperature. This finding indicates that the adsorbed CH4 species completely hinders the dissociative adsorption of H2 because the site working as the dissociative adsorption site in ZnMFI for H2 is completely covered by the adsorbed CH4 species. After treatment at 373 K in H2 gas, the IR band at 1933 cm−1, which results from the −[ZnH]+ species, is observed as a weak band, and simultaneously the intensities of the bands at 2930 and 2892 cm−1, which result from the −[ZnCH3]+ species, are slightly attenuated (spectrum 2 in Figure 3b). The intensity of the IR absorption band at 1933 cm−1 increased as the temperature used for H2 treatment was 19528

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the M7S2 site in a η2 binding fashion (Zn−1H and Zn−2H bond lengths are similar: 2.05 and 2.04 Å, respectively): model A. In the succeeding step (model B), the carbon atom at CH4 approaches the Zn2+-ion exchanged on the M7S2 site, resulting in bond formation with a bond-length of Zn−C (2.05 Å), which is shorter than that for Zn−1H or Zn−2H (2.23 and 2.11 Å, respectively). This finding tells us that the interaction between Zn and C atoms becomes predominant, causing deformation of the Td symmetry of CH4. Simultaneously, the formation of the OL−1H bond and the weakening of the C− 1H bond occurs. After formation of the Zn−C bond, further elongation of the C−H bond in CH4 was observed and was accompanied by the formation of Zn−C and OL−1H bonds (models C and D). In the final stage, these species are transformed into the −[ZnCH3]+ and −OH species (model E); the C−H bond in CH4 was easily dissociated in the present model, which was supported by the experimental findings in this work. For the final species, the bond distance between the zinc and carbon atoms of the formed −[ZnCH3]+ species was calculated to be 1.93 Å, which is in fair agreement with the datum obtained by using X-ray absorption analysis of dimethyl zinc.44,45 In addition, the values of vibrational modes for the formed species, namely, the −[ZnCH3]+ and −OH species, were calculated to be 2933, 3009, and 3655 cm−1. These values are consistent with the experimentally obtained data that are shown in Figure 2. In the present system, the most striking aspect is that the activation barrier for the reaction path is only +15 kJ mol−1. This value is low enough to give rise to the dissociative adsorption of CH4 at around room temperature, in comparison with the general bond energy for the C−H bond, which is reported to be 412 kJ mol−1. In addition, the stabilization energy in the final state was calculated to be −179 kJ mol−1 by comparing with the reference state composed of both the CH4 molecule and Zn2+-M7S2 site model. It is noteworthy that the evaluated stabilization energy of CH4 is not so high as that of H2 (−213 kJ mol−1).32 The relationship between the stabilization energies of CH4 and H2 gives a good account of the fact that the exothermic substitution reaction with H2 is observed in the sample treated with CH4 as shown in Figure 3. As a result, it has become apparent that the present experimental data related to the dissociative adsorption of CH4 are successfully reproduced by adopting the same model Zn2+M7S2 as that proposed for the H2 dissociative adsorption site reported in our previous work.32 The question that we must consider next is how interactions between CH4 and a monomeric Zn2+ on M7S2 lead to weakening of the C−H bond in CH4. To discuss the C−H bond activation mechanism, we have to mention the electronic state of CH4 and interactions leading to the C−H bond activation in CH4 from the viewpoints of the commonly known frontier orbital theory. The molecular orbital diagram of a free CH4 molecule is shown in Figure 5. The diagram tells us that a free CH4 molecule having eight electrons is very stable because of the occupation of the low-lying 1a1 and 3-fold degenerate 1t2 MOs. The 1t2 HOMO and 2a1 LUMO have remarkable bonding and antibonding properties with respect to the C−H bonds, respectively, and the HOMO−LUMO gap is calculated to be 12.2 eV.46 Therefore, it was expected that great difficulty would be encountered in activating the C−H bond in CH4. In principle, the C−H activation would be established through the electron donation to the LUMO (i.e., CH4−σ*(C−H) orbital) from an electron donating species such as an appropriate metal ion with a low oxidation number and/or electron transfer from

raised up to 473 K, accompanied by complete attenuation of the IR bands at 2930 and 2892 cm−1.43 Thus, as the treatment temperature of the sample covered with CH4 is raised, the site that acts as the H2 dissociation site is restored (Figure 3b, spectra 2 and 3). Moreover, these interesting phenomena were also supported by the measurement of the H2 adsorption isotherms at 298 K for the samples with or without CH4 treatment at 473 K (Figure S1-2). These findings provide reliable evidence that the dissociative adsorption of CH4 or H2 takes place at the same site, which is on the monomeric Zn2+ positioned on the M7S2 site. As a result, it was established that the monomeric Zn2+ having the potential for H−H bond cleavage of H2 also works as a CH4 dissociative adsorption site at above 323 K. To verify further our proposal that the monomeric Zn2+ exchanged on the M7S2 site in ZnMFI that works as the activation center for the H−H bond-cleavage also acts as the dissociative adsorption center for CH4, we applied density functional theory (DFT) analyses to CH4 activation on the monomeric Zn2+ positioned on the M7S2 site having a specific configuration of Al atoms in MFI.32 As a result, it was proven that the dissociative adsorption of CH4 takes place on this site as shown in Figure 4. In the activation process of CH4, the changes in both structures (models A−E) and reaction energies in the CH4−[Zn2+−M7S2] system are represented in Figure 4. Detailed structural parameters for the respective models A−E are provided in Table 1. In the first step (model A), the CH4 molecule is apparently molecularly adsorbed on Zn2+ existing in

Figure 4. (a) Changes in stabilization energies in the reaction process evaluated by the DFT calculation method utilizing the M7S2 model: ZA−OL−Zn2+−OL−ZB + CH4 → ZA−OL−[ZnCH3]+ + H+−OL−ZB. (b) In the changing stages (A, B, C, D, and E), the structural data are given: energy, structure, and bond distance in the respective stages. 19529

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Table 1. Structural Information on Respective Models from A to E Which Are Shown in Figure 4b bond distance (Å) and bond angle (degree)

model A

model B (TS)

model C

model D

model E

Zn−C Zn−1H Zn−2H Zn−3H Zn−4H OL−1H C−1H C−2H C−3H C−4H angle (1H−C−2H) angle (3H−C−4H) angle (Zn−C−1H) angle (Zn−C−2H) angle (Zn−C−3H) angle (Zn−C−4H)

2.17 2.05 2.04 3.11 2.68 2.24 1.12 1.11 1.10 1.09 125 111 69 69 143 106

2.05 2.23 2.11 3.00 2.66 1.42 1.33 1.11 1.10 1.09 138 110 79 77 138 112

2.01 2.38 2.23 2.84 2.68 1.17 1.60 1.11 1.10 1.09 147 110 82 86 130 117

1.94 2.56 2.33 2.71 2.65 1.00 1.85 1.10 1.10 1.09 149 109 85 96 124 119

1.93 3.74 2.47 2.52 2.56 0.97 4.11 1.09 1.09 1.09 162 110 66 106 110 113

densities of Zn−3d and −4s AOs during the CH4 activation process are shown in Figure 6. It is clear from this figure that

Figure 5. MO diagram of a free CH4 molecule.

HOMO (i.e., CH4−σ(C−H) orbital) to the metal ion through a σ-complex formation.1,7,35,47 Therefore, in view of the frontier orbital theory, we can categorize interactions between CH4 and the metal ion leading an increase or decrease in electron densities of frontier orbitals of CH4 into two types of interactions: the donation and back-donation interactions. The “donation” refers to the electron transfer from CH4−σ(C−H) to vacant or singly occupied orbitals on the metal ion, while the “back-donation” from occupied orbitals of the metal ion to CH4−σ*(C−H) orbitals. Here, a pivotal role of a monomeric Zn2+ on the M7S2 site lowering the energy barrier in the CH4 activation process is an open question, because divalent zinc cation is categorized as a relatively inert metal ion and barely shows any tendency to accept and/or donate electrons through interaction with other molecules. Usually the population in terms of subsystem MOs is evaluated to see the change of electron density during reactions. However, the subsystem MOs are not a good reference in the latter stage of reactions because the electronic structure at the starting considerably changes in the latter stage. According to the discussion up to here and also below, the main interactions with CH4 occur through the Zn atom in an MFI unit. Therefore, it is entirely fair in the present system to say in the atomic orbital (AO) basis that the populations on five Zn−3d AOs and single Zn−4s AO are simply considered as an indication of the backdonation to CH4 and the donation from CH4, respectively. In the evaluation processes, we applied the NBO analysis method48 for the present system and also used the NPA39 instead of the Mulliken’s method. The changes in the electron

Figure 6. Changes in electron densities of Zn−3d and Zn−4s atomic orbitals for the respective states during the reaction path: ZA−OL− Zn2+−OL−ZB + CH4 → ZA−OL−[ZnCH3]+ + H+−OL−ZB.

the electron density on the Zn−4s AO monotonically increases in the course of CH4 activation, whereas that on the Zn−3d AOs decreases only slightly. These results account reasonably for the donation interaction (CH4−σ(C−H) → Zn−4s) playing the dominant role in this system. Moreover, the stabilization energy resulting from the donation interactions (i.e., Edonation) between CH4−σ(C−H) and Zn−4s orbitals in the model B (TS) was calculated to be 202.5 kJ mol−1 by summing up four kinds of the leading terms.48 While, on the other hand, the back-donation interaction between Zn−3d and CH4−σ*(C−H) orbitals in the model B (TS) barely contributed toward the activation, and the stabilization energy resulting from the back-donation interactions (i.e., Ebackdonation) was calculated to be 10.9 kJ mol−1. Therefore, it is not the backdonation interaction, but the donation interaction between CH4−σ(C−H) and Zn−4s orbitals, that works as the triggering factor for the cleavage of the C−H bond in CH4 in the present system. At this stage, it is meaningful to compare the above behavior of Zn2+ with that for the case involving monovalent copper cations (Cu+), because both ions have same electronic configuration: 3d104s0. Actually, it is well-known that a Cu+ exchanged in zeolite functions as the active site in hydrogen- or hydrocarbon-activation processes, as highlighted by many 19530

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Figure 7. MOs depicted for the dominant interactions between CH4 and Zn2+-M7S2 for the respective structural models (A, B, C, and D) in the reaction path by applying the ILO analysis method: ZA−OL−Zn2+−OL−ZB + CH4 → ZA−OL−[ZnCH3]+ + H+−OL−ZB.

groups.35,49−51 In such processes, the Cu+ exchanged in zeolite shows a significant back-donation nature. The clear difference observed in the bond nature between Zn2+ and Cu+ in zeolite can be explained reasonably by considering the difference in nuclear charges. From Cu+ to Zn2+, the nuclear charge increases by 1, but the number of electrons remains the same. Therefore, both the core and valence electrons are attracted more strongly by the nucleus for Zn2+, and the orbital energies for Zn2+(3d) and Zn2+(4s) become lower than those for Cu+(3d) and Cu+(4s) electrons. This conclusion was verified by calculation for the 3d orbital (HOMO) energy levels of Zn2+ and Cu+ exchanged in zeolite, respectively (Figure SI-3). So in Zn2+ exchanged zeolite, the back-donation is particularly unexpected because of the filled 3d orbital positioned at a far lower energy level, compared with the level of the empty orbital of CH4, whereas the donation interaction from CH4 to Zn(4s) will occur because of the proximity of the empty 4s orbital to the HOMO of CH4. Therefore, compared to Cu+, Zn2+ exchanged in zeolite has a tendency to work as better as an electron acceptor in the CH4 activation process. We will advance the arguments toward how donation interactions between Zn(4s) and CH4−σ(C−H) orbitals occur in the CH4 activation process. To answer this question, we investigated molecular orbitals contributing to donation interactions in models A−D (where the C−1H bond distance of CH4 adsorbed on Zn2+ changes from 1.12 Å to 1.85 Å): Figure 1 and Table 1, including the TS geometry by applying the interaction localized orbital (ILO) analysis to respective models. This method is able to focus directly on the orbitals operating in the bond alternation range and provides a compact picture of them. The dominant interactions between CH4 and Zn2+ in the M7S2 site are represented by the four ILOs, as shown in Figure 7, for the models from A to D. We can depict the bond-dissociation processes well by focusing on the changes in the C−H bond nature through the interaction with Zn2+, together with the interaction with the

lattice O atom. In the present system, the interactions are well and compactly visualized in the four sets of contour maps describing the dominant interactions. The four contour maps (MO#A-1−MO#A-4) for model A correspond to the initial step for the C−H bond activation. Even in model A, we can see the Zn−C bonding overlap (i.e., MO#A-3) between the CH4−σ(C−H) orbital localized in C−2py and the Zn−4s orbital which is hybridized with a small amount of the 3d orbital (4s:3d = 0.74:0.26). A similar interaction nature has also been reported for a system composed of Co−CH4 complexes.52 On the other hand, other CH4−σ(C−H)x and CH4−σ(C−H)z orbitals barely interact with the Zn−4s orbital because of lower orbital-overlapping as a result of the difference in the direction of the orbital protrusion in this state: MO#A-1 and MO#A-2. As a result, it is found that only one type of donation interaction is recognizable in the model A. Turning now to the next stage, that is, model B (TS), the bonding orbital in MO#B-3 is clearly indicative of an overlap between the Zn−4s and C−2py orbitals. The extent of the overlap is greater than that found in model A (MO#A-3), indicating the progression of the bond cleavage in CH4. Moreover, the bonding nature in the interaction between OL− 1H and Zn−C appears in MO#B-1. In this state, the bond formation between (C)−H and OL (i.e., MO#B-1, MO#B-4) can be described as follows. (1) We can clearly see both bonding and antibonding overlaps between the OL−2px and C−2px orbitals in CH4−σ(C−H), which is hybridized with the CH4−σ*(C−H) orbital to some extent. (2) It is noteworthy that both bonding OL−1H and antibonding CH4−σ*(C−H) orbitals by nature barely overlap with Zn−3d and −4s orbitals. In the successive step, model C, the OL−1H bonding orbital (i.e., MO#C-1) is dominantly localized in the OL−2px orbital, and the bond distance between OL−1H is 1.17 Å (Table 1). The localization can be explained by considering that OL−2px orbitals are stabilized because of the formation of an OL−H ionic bond. Simultaneously, the OL−H antibonding orbital is 19531

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position, accompanied by the recovery of a −[ZnCH3]+ species with C3v symmetry. We concluded that a monomeric Zn2+ positioned on the M7S2 site plays a pivotal role in the CH4 activation process; a monomeric Zn2+ initiates C−H bond activation by working as an electron acceptor reagent for an approaching CH4 molecule.

delocalized into the C−2px orbital resulting in elongation of C−1H bond of CH4 (Table 1). The C−2px component made up of the combined orbital between 2px orbital of OL and σ* orbital of C−1H concurrently overlaps with a Zn−4s orbital, and as a consequence, a new Zn−C bond is formed (i.e., MO#C-4). At this stage, the overlap between the C−2py and Zn−4s orbitals in the MO#C-3 remains unchanged as is observed in MO#B-3. As a consequence, the two types of Zn− C bonding orbitals (MO#C-3 and MO#C-4) contributing to the donation interactions are operating in model C. Taking account of the bond-cleavage process, it can be considered that C−H bond activation is efficiently facilitated by a newly developed donation interaction (MO#C-4). The distorted structure in the ZnCH3 moiety in model C is unstable because of substantial steric hindrance; the Zn−C−2H angle is calculated to be 86.2°, which is smaller than the 1H−C−2H angle of the free CH4 molecule by 23°. Therefore, rearrangement of the −ZnCH3 moiety commences to reduce the steric hindrance and brings about a more stable form, changing the structure from model C to D. As shown in MO#D-4, the rearrangement of ZnCH3 moiety leads to a strong overlap between Zn−4s and C−2px orbitals. On the other hand, the level of overlap between C−2py and Zn−4s becomes considerably less because of the rearrangement of the ZnCH3 moiety, and then the Zn−C bonding orbital (MO#C-3) changes to the orbital having a nonbonding nature (MO#D-3) with Zn. Finally, both [ZnCH3]+ having C3v symmetry and a −OH species are formed. The ILO analysis clearly indicated the bond alternation process continues from Zn2+ and a CH4 molecule toward [ZnCH3]+ and OH, and both Zn−4s and CH4−σ(C−H) orbitals play a role. Altogether it is clear that the Zn2+ in the M7S2 site acts mainly as an electron acceptor in the CH4 activation process.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures and tables: Figure S1 shows the IR spectra of ZnMFI in the adsorption process of CH4 at 300 K. Adsorption isotherms of H2 on ZnMFI measured at 298 K before and after CH4 treatment, as well as the adsorption values evaluated the isotherms, are given in Figure S2 and Table S1. The difference in MO energy diagram between Cu+ and Zn2+ positioned on the cluster models are presented in Figure S3. In part 4, the detailed explanation for the analysis method related to ILO (interaction localized orbital) is also given. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-86-251-7844. Fax: +81-86-251-7853. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was provided by Japan Society of Promotion Science (Grants-in-Aid for Challenging Exploratory Research: No. 21655021). Mr. O.A. and Dr. H.T. acknowledge financial support from Japan Society for the Promotion of Science (Research Fellowship for Young Scientists, DC1).

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CONCLUSIONS In this study, we aimed to clarify the CH4 activation mechanism occurring on Zn2+ exchanged in MFI, which has the electronic state of a d10 structure. Irrespective of the expected chemical inertness of such kinds of ions, we found unusual CH4 activation and succeeded in obtaining new and insightful information for the mechanism of CH4 activation, and its site in the MFI. Effects we clarified in this study are described below. 1. The dissociative adsorption of CH4 takes place on monomeric Zn2+ in the M7S2 site in MFI, which has the ability to cleave the bond in H2. 2. A monomeric Zn2+ on the M7S2 site in MFI acts as an electron acceptor in the CH4 activation process. The donation interaction from the CH4−σ(C−H) to the Zn−4s orbital triggers activation of CH4, leading to C−H bond cleavage in CH4, whereas the back-donation interaction between Zn−3d and CH4−σ*(C−H) orbitals barely contributes to the present C−H bond activation. 3. In the first step of CH4 activation, the interaction between OL and H−C and concurrent donation interactions between CH4−σ(C−H) and Zn−4s orbitals initiate the process. In the next stage, two types of donation interactions between CH4−σ(C−H) and Zn−4s orbitals take place, resulting in the C−H bond elongation in CH4, and followed by the operation of double Zn−C interactions (Cpy−4s and Cpx−4s), accompanied with the simultaneous formation of the OL−H species. Finally, steric hindrance caused by the distortion of the −[ZnCH3]+ moiety is completely resolved by eliminating one type of donation interaction, and then the Zn2+ changes its

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