Article pubs.acs.org/JPCC
Dual-Copper Catalytic Site Formed in CuMFI Zeolite Makes Effective Activation of Ethane Possible Even at Room Temperature Atsushi Itadani,† Hiroe Torigoe,† Takashi Yumura,‡ Takahiro Ohkubo,† Hisayoshi Kobayashi,‡ and Yasushige Kuroda*,† †
Department of Fundamental Material Science, 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: The role of dual-cation sites in zeolites has received a renaissance in chemistry and industry directed toward fixation and activation of various gases; such sites may be expected to be more efficient than a single-cation site. We aimed to clarify the real active centers in the copper-ion-exchanged MFI-type zeolite (CuMFI) for ethane (C2H6). A peculiar feature was found in the appearance of the characteristic IR bands at 2644 and 2582 cm−1 when C2H6 was adsorbed on Cu+ formed in CuMFI. The existence of dual species composed of two Cu+ ions bridging C2H6 was clearly indicated by extended X-ray absorption fine structure (EXAFS) data. Density functional theory calculations gave clear evidence that the two IR bands are distinctly due to C2H6 adsorbed on the dual-Cu+ site and not on a single site; this agrees with the EXAFS data. These data lead us to conclude that the dual-Cu+ site in the CuMFI sample is indispensable for efficient activation of C2H6 through the simultaneous interaction of C2H6 with two Cu+ ions.
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Very recently, we have, for the first time, indicated the existence of dual species composed of Cu+ in CuMFI being bridged by unsaturated hydrocarbons (i.e., ethyne (C2H2) and ethene (C2H4)).29,30 Furthermore, it has been considered that the dual-Cu+ site in CuMFI contributes to extremely strong adsorption of C2H2 and C2H4 at room temperature. The activation of the CC and CC, as well as C−H, bonds in these molecules takes place more efficiently on the dual-Cu+ sites than on the single-Cu+ site in CuMFI. In addition, the dual-cation sites are crucial in the direct decomposition of NO.31−35 In recent years, there has been a renewal of interest in the important role of the dual sites; some interesting reports describe the formation of carbon monoxide (CO) or carbon dioxide (CO2) adsorption complexes on dual-cation sites in zeolites.36−41 However, the pictures that we proposed of the dual-cation site in zeolites for the adsorption of C2H2 and C2H4 are quite different in type from those introduced by other groups for the CO and CO2 adsorption. In the cases of CO and CO2 adsorption, the dual-cation site in zeolite pores is composed of the sites standing opposite to each other in the pore,36−41 whereas for the cases of C2H2 and C2H4, the copper ions on the dual-Cu+ site in CuMFI are located side by side at intervals of about 2.2−3.2 Å.29,30 However, it has been difficult to justify our proposal, because other groups have reported different types of C2H2 and C2H4 adsorbed complexes formed in the copper-ion-exchanged zeolites.42−46 Taking account of the fact that two Cu+ sites in CuMFI positioned side by side have contributed to the direct decomposition of NO, we now
INTRODUCTION Activation of carbon−hydrogen (C−H) and carbon−carbon (C−C) bonds in organic compounds, such as saturated and/or unsaturated hydrocarbons, has, for a long time, been explored by many researchers to convert inexpensive substances to more valuable ones. In strategies for activation, materials containing transition-metal ions have frequently been used as the catalysts. For example, in the field of organometallic compounds, complexes containing rhodium-, ruthenium-, or palladium, although expensive and laborious to prepare, have been found to activate the C−H and C−C bonds in organic materials.1−3 On the other hand, in the field of solid catalysts, attempts have been made to activate the C−H and C−C bonds in organic compounds by utilizing acidic zeolites, as well as their metalion-exchanged forms.4−6 We have recently found that a copperion-exchanged MFI-type zeolite (CuMFI) strongly interacts with CH4 molecules even at room temperature and also that the working site for the activation of the C−H bond in CH4 is the monovalent copper ion (Cu+), which had been formed in the sample through evacuation at high temperatures.7 Additionally, it has been proved that the CuMFI sample exhibits remarkable and surprising features in interactions with relatively inert molecules, such as dinitrogen (N2),8−16 xenon (Xe),17,18 and dihydrogen (H2),19−23 even at room temperature, as well as exhibiting high levels of catalytic activity for the direct decomposition of nitrogen monoxide (NO).24−28 In every case, both the conformation (i.e., binding fashion) of the gaseous molecules adsorbed and the position of the Cu+ sites in the zeolite nanopores play a pivotal role in the adsorption or activation of various gases. © 2012 American Chemical Society
Received: March 8, 2012 Published: April 25, 2012 10680
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performed at 298 K for the sample (ca. 150 mg) evacuated at 873 K. After the first run, the sample was treated at 298 K for 4 h under a reduced pressure of 1.3 mPa, and again the isotherm was measured at 298 K (second adsorption). Quantum chemical calculations were employed on the basis of the hybrid Hartree−Fock/density functional theory (B3LYP) method49−52 using the Gaussian 09 program package.53 The B3LYP method consists of the Slater exchange; the Hartree−Fock exchange; the exchange functional of Becke;49,50 the correlation functional of Lee, Yang, and Parr (LYP);52 and the correlation functional of Vosko, Wilk, and Nusair (VMN).54 We optimized a geometry for ethane binding into Cu cations embedded in a zeolite model with Si, Al, and O, where terminal atoms are bonded to H atoms. Because of limitations of computational resources, we used the 6-311G* basis set55−57 for the Cu cations and ethane, the 6-31G* basis set58−60 for Al atoms and O atoms bound to the substituted Al atom and those coordinated by the Cu atom at the same time, and the 3-21G basis set61−66 for Si, H, and the other O atoms in the zeolite framework. To obtain theoretically the vibrational frequencies for the adsorbed ethane, we truncated an optimized structure to construct Cu−ethane complexes plus their neighboring atoms. In the Cu−ethane complexes, the terminal atoms are bound by H atoms. We performed the vibrational analysis using the Cu−ethane complexes. Since the Cu−ethane complexes are not local minima, there are some imaginary vibrational frequencies. However, we can roughly estimate the vibrational frequencies of the adsorbed ethane and compare between the calculated and experimental results. Generally, theoretical harmonic frequencies overestimate experimental values because of incomplete descriptions of electron correlations and the neglect of mechanical anharmonicity. To compensate for this problem, a uniform scaling factor of 0.962 was applied to the calculated frequencies obtained at the B3LYP level of theory, as described in ref 67. The stabilization energies for the models obtained were corrected by basis set superposition errors using the counterpoise method.68 In addition, the observation of the interaction between the zeolite lattice, including copper ions and ethane, was performed by using the interaction frontier orbital (IFO) developed recently by Kobayashi on the basis of ref 69.
examine the states of active sites in a sample exhibiting the adsorption and activation of C2H6 using multiple experimental methods, such as IR and X-ray absorption fine structure (XAFS) spectrometry and large-scale density functional theory (DFT) calculations. We hope to give some hint for developing new and efficient catalysts for NO decomposition, if the states of the active dual-Cu+ site in CuMFI for small organic molecules composed of CC bonds are clarified.
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EXPERIMENTS AND CALCULATIONS Sodium-form MFI-type zeolite (NaMFI; Si/Al = 11.9) was purchased from the Tosoh Co., Japan. The CuMFI samples were prepared in a mixed aqueous solution of Cu(CH3COO)2 and NH4CH3COO at room temperature from the starting material NaMFI according to our proposed method.7,12−14,18,29 The copper contents in the samples were determined by chelatometric titration. The copper ion-exchange levels of the samples obtained were 75% and 135%, which was estimated by assuming that one divalent copper ion is exchanged for two monovalent sodium ions. These samples are denoted as CuMFI-75 and CuMFI-135, respectively. The NaMFI zeolite was dispersed in an aqueous solution of 0.3 M NH4NO3 at 363 K for 1 h. By repeating this operation 10 times, the ammonium-ion form of the MFI (NH4MFI) sample was prepared. The protonic form of the MFI sample (HMFI) was obtained by the calcination of the NH4MFI sample at 773 K for 4 h under the atmosphere. The CO (99.9%) and C2H6 (99.5%) gases used as the adsorbates were purchased from the GL Sciences Co., Japan. Before the adsorption, every sample was treated at 873 K for 4 h under a reduced pressure of 1.3 mPa. In the case of the CuMFI sample, such a treatment promoted the reduction of divalent copper ions (Cu2+) to Cu+ ions (Cu+/[total amounts of exchanged copper ion] = 0.85 and 0.97 for CuMFI-75 and CuMFI-135, respectively; Figure S1 in the Supporting Information). The Fourier transform infrared (FT-IR) spectra were recorded at room temperature in the wavenumber range of 4000−400 cm−1 on a Digilab FTS4000MXK FT-IR spectrophotometer (Randolph, USA) with an MCT detector kept at the temperature of liquid nitrogen. One hundred forty-four times were accumulated at a spectral resolution of 2 cm−1. A self-supporting disk (ca. 7 mg cm−2) was placed into a sample cell that is capable of in situ treatment and gas introduction.47 The intensity of the band was corrected by normalizing the intensity of the skeletal mode of zeolite at around 2000 cm−1. The Cu K-edge XAFS spectra were collected in transmission mode at the beamline PF-AR NW10A equipped with a doublecrystal monochromator of Si(311) of the Photon Factory in the Institute of Materials Structure Science (KEK, Tsukuba, Japan) under the ring-operating condition of 6.5 GeV and about 50 mA. Calibration of the photon energy was carried out by using the pre-edge peak of a copper foil (8.9788 keV). The X-ray absorption near edge structure (XANES) and the extended Xray absorption fine structure (EXAFS) spectra were recorded at energy intervals of 0.5 and 2−3 eV, respectively. A selfsupporting disk (ca. 50 mg cm−2) was prepared and loaded into an in situ sample cell that was designed as with an IR cell.47 The analysis of the spectral data was employed using the program developed by Maeda.48 The adsorption isotherms of C2H6 were volumetrically measured at 298 K using an adsorption apparatus equipped with an MKS-690 pressure sensor. The first adsorption was
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RESULTS AND DISCUSSION To examine the interaction of CuMFI with C2H6 at room temperature, we performed IR measurements when exposing the samples to C2H6 gas at room temperature. A free gaseous C2H6 molecule is generally known to have four characteristic modes due to the C−H stretching vibrations in the molecule: the ν1 (symmetry A1g: 2954 cm−1), ν5 (A2u: 2896 cm−1), ν7 (Eg: 2969 cm−1), and ν10 (Eu: 2985 cm−1) modes.70 Moreover, a ν2 C−C stretching vibration mode in C2H6 was observed at 995 cm−1 (A1g).70 Figure 1 shows the IR spectra for the adsorbed C2H6 species on the CuMFI samples with different copper-ionexchange levels at room temperature. In this work, especially, the experiment was conducted under lower C2H6 pressures to minimize the contributions from the interactions of C2H6 with other active sites existing in CuMFI, such as Brønsted acid sites. The figure shows that two types of broad bands at 2644 and 2582 cm−1 are distinctly observed for CuMFI-135 exposed to C2H6 gas under very low pressures (spectra 1−5 in Figure 1A), whereas for CuMFI-75, these two bands have markedly smaller intensities than those for CuMFI-135 (spectra 1−3 in Figure 1B). In addition to the bands at 2644 and 2582 cm−1, some 10681
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and 2582 cm−1 for the adsorbed C2H6 species on Cu+ in CuMFI-135. It is noteworthy that both the intensities and the positions of these bands are hardly altered by the increase in the pressure of C2H6, although other bands increase their intensities while maintaining their positions (spectra 4 and 5 in Figure 1A). A detailed discussion on the assignments of these characteristic bands will be given later, based on the results obtained by applying the large-scale DFT calculations. The adsorbed C2H6 species remained in small amounts on CuMFI after re-evacuation at room temperature (spectrum 6 in Figure 1A and spectrum 4 in Figure 1B); the interaction of Cu+ with C2H6 is a relatively weak interaction, in comparison with that with C2H2.29 Next, we measured the XAFS spectra to obtain structural information for the C2H6 species adsorbed on Cu+ ions in CuMFI. Figure 2 shows the XANES spectra, the Fourier transform of the EXAFS oscillations at the K-edge of the copper-ion exchanged, and the k3-weighted curves as a function of the k vector for CuMFI-135. In Figure 2A, two characteristic bands appear at 8.983 and 8.993 keV for the sample treated at 873 K, which are, respectively, assigned to the 1s−4pπ and 1s− 4pσ electronic transitions of Cu+ in the sample (red line).71−74 The appearance of these bands is explained by considering that the Cu+ ions assume a linear or planar coordination structure.71,72 The intensity of the band at 8.983 keV decreased slightly when the CuMFI-135 sample was exposed to C2H6 gas at room temperature (blue line). The interpretation of this decrease is that the Cu+ species acts as the adsorption site, and the structure around Cu+ in CuMFI-135, which takes a linear or a planar coordination, was deformed through the interaction with C2H6 at room temperature. In a comparison of the intensity of the band at 8.983 keV for the sample treated at 873 K with that for the one re-evacuated at room temperature after the C2H6 adsorption, the spectrum for the latter state almost recovered its original one, indicating that the C2H6 species adsorbed on CuMFI-135 interacts more weakly with Cu+ in the sample than C2H2 does.29 This finding is in good accord with the results from the IR data. Depicted in Figures 2B,C are the EXAFS data and the k3weighted curves as a function of the k vector for the CuMFI135 sample. For the CuMFI-135 sample treated at 873 K (red line in Figure 2B), a band observed at around 1.6 Å (no phaseshift correction) is ascribable to the backscattering from the nearest-neighboring zeolite lattice oxygen atoms. The EXAFS datum obtained was analyzed by the least-squares method using Cu2O as the reference material for the oxygen around the copper (Cu−O), resulting in the coordination number (NCu−O) = 2.6 (±0.4), distance (rCu−O) = 1.95 (±0.01) Å, Debye− Waller factor (Δσ2Cu−O) = 0.008 Å2, and R-factor = 3.3%. It is clear that the results of curve fitting for Cu−O are well reproduced by these parameters, as shown in Figure 2C(a). It was considered from the evaluated coordination number of 2.6 that the Cu+ ions formed in CuMFI through the treatment at 873 K in vacuo are in states interacting with two or three zeolite lattice oxygen atoms. Incorporating this into the theoretical models of the ion-exchange sites in MFI proposed by Sauer’s and Nachtigall’s groups, the exchanged monovalent copper ions in CuMFI-135 appear to be formed on sites, such as I2 (twocoordinate site) and M5, M6, and M7 (three-coordinate site).75−82 In addition to the 1.6 Å band, other bands also appear at around 2.2 and 2.8 Å (no phase-shift correction). We and other groups had already observed these bands;29,83−85 they are caused by backscattering from the copper−copper
Figure 1. IR spectra for the adsorbed C2H6 on (A) CuMFI-135 and (B) CuMFI-75 at room temperature. (A) The sample was exposed to C2H6 gas under the equilibrium pressures of 0.27 (1), 10 (2), 80 (3), 420 (4), and 980 Pa (5), and spectrum 6 was measured at room temperature after the re-evacuation of the sample at room temperature. (B) The sample was exposed to C2H6 gas under the equilibrium pressures of 0.53 (1), 22 (2), and 592 Pa (3), and spectrum 4 was measured at room temperature after the re-evacuation of the sample at room temperature. All spectra were obtained as the difference between the absorption spectrum measured after the adsorption of C2H6 and the reference spectrum for the sample treated at 873 K.
bands were distinctly observed in the wavenumber region of 2990−2880 cm−1 for both CuMFI samples. The bands, except the ones at 2644, 2582, and 2989 cm−1, were also seen in the adsorption of C2H6 onto NaMFI and HMFI samples treated at 873 K (Supporting Information, Figure S2). Taking into account the amount of Cu+ in the present sample, it can be considered that the appearance of the characteristic bands at 2644 and 2582 cm−1, and also of the band at 2989 cm−1, can be ascribed to the formation of the species associated with the interaction of C2H6 with Cu+ in CuMFI at room temperature; other bands found in the CuMFI samples were hardly distinguishable from those in NaMFI and HMFI because the positions of the bands observed were almost the same in these samples. Therefore, we focus particularly on the bands at 2644 10682
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Figure 2. (A) XANES spectra and (B) Fourier transform of the EXAFS oscillations at the K edge of the copper ion in CuMFI-135 obtained under various conditions: evacuation at 873 K (red line); exposure to C2H6 gas of 129 Pa (blue line) and 1.59 kPa (purple line) at room temperature; and re-evacuation at room temperature (dotted green line). The black line in (B) represents the spectrum for CuMFI-75 evacuated at 873 K. The transform of the data (k3χ(k) vs k) into real space is performed over a k range of 2−15 Å−1. (C) The k3-weighted curves as a function of k for CuMFI-135 evacuated at 873 K (a, Cu−O; b, Cu−Cu; c, Cu−Cu) and exposed to C2H6 of 129 Pa (d, Cu−O and Cu−C; e, Cu−C) and of 1.59 kPa (f, Cu−O and Cu−C; g, Cu−C): (red curve) experimental data and (dotted blue curve) best-fit spectra.
(±0.03) Å, Δσ2Cu−Cu = 0.021 Å2, and R-factor = 3.3%; for the 2.8 Å band, NCu−Cu = 1.0 (±0.5), rCu−Cu = 3.29 (±0.03) Å, Δσ2Cu−Cu = 0.019 Å2, and R-factor = 13.9%. As can be seen from the results of curve fitting assuming the Cu−Cu contribution (Figure 2C(b and c)), in the present case, the former band is well-fitted. Thus, we adopted these analyzed
(Cu−Cu) species in the CuMFI sample. Thus, we considered that the Cu+ ions formed in CuMFI by the treatment at 873 K in vacuo are present at similar intervals, indicating the existence of the dual-Cu+ sites. In practice, the data obtained by fitting the Cu−Cu pair using copper metal as a reference were as follows: for the 2.2 Å band, NCu−Cu = 1.0 (±0.3), rCu−Cu = 2.63 10683
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Scheme 1. Structural Model for C2H6 Adsorbed on Dual-Cu+ Site in CuMFI Evaluated from EXAFS Data (Unit: Å)
structures (both statically and dynamically) of the adsorption species formed. From these data, we assumed the existence of an interaction between the paired Cu+ species and C2H6 and analyzed the EXAFS data by applying the least-squares method, utilizing scattering parameters obtained from Cu2O and [KCu(CN)2] as the reference materials for the oxygen and the carbon around the copper (Cu−O and Cu−C), respectively.86 For the CuMFI sample that was exposed to C2H6 gas of low equilibrium pressure at room temperature (blue line in Figure 2B), the number of carbons around the copper and their distances were evaluated to be, respectively, NCu−C = 0.8 (±0.4) and rCu−C = 2.94 (±0.04) Å (Δσ2Cu−C = 0.003 Å2) by fitting a Cu−C (R-factor = 6.1%) for the characteristic band at around 2.5 Å. As for the band observed at around 1.6 Å, the coordination numbers and the distances of the oxygen and the carbon atoms around the copper ion were found to be NCu−O = 2.6 (±0.4), rCu−O = 1.95 (±0.03) Å, and Δσ2Cu−O = 0.024 Å2 and NCu−C = 1.0 (±0.2), rCu−C = 1.98 (±0.01) Å, and Δσ2Cu−C = 0.005 Å2, respectively, by fitting two shells composed of Cu−O and Cu−C (R-factor = 2.6%). In addition, for the band at 2.5 Å observed in the CuMFI sample that was exposed to C2H6 gas at higher pressure and room temperature (purple line in Figure 2B), the values evaluated were almost the same as those obtained for the sample exposed to C2H6 gas at lower pressure (NCu−C = 1.0 (±0.5), rCu−C = 2.93 (±0.03) Å, Δσ2Cu−C = 0.004 Å2, R-factor = 8.7%). The analyzed values for the band at 1.6 Å by considering the twoshell model were NCu−O = 2.6 (±0.3), rCu−O = 1.96 (±0.01) Å, and Δσ2Cu−O = 0.023 Å2 for Cu−O and NCu−C = 1.0 (±0.3), rCu−C = 1.97 (±0.02) Å, and Δ(σ2Cu−C) = 0.006 Å2 for Cu−C, respectively (R-factor = 2.5%). For the C2H6 species adsorbed on CuMFI-135, the results of the curve fitting fitted well for both Cu−O and Cu−C (Figure 2C(d−g)). These results indicate that the dual species consisting of Cu+ in CuMFI bridged by C2H6 are formed preferentially when the CuMFI sample is exposed to C2H6 gas at low pressure and room temperature (Scheme 1). Combining the results obtained from EXAFS and IR data, it is reasonable to deduce that the observed IR bands at 2644 and 2582 cm−1 are caused by the C2H6 species adsorbed on the dual-Cu+ sites in CuMFI. With further increases in the C2H6 pressure, the C2H6 molecule seems not to be adsorbed on the dual-Cu+ site in CuMFI because there was no change in the coordination number between copper and carbon, as shown from the analysis data for the band at 2.5 Å; also, no increase in the intensities of IR bands was observed at 2644 and 2582 cm−1, although the additional sites, for example, a single-Cu+ site and Brønsted acid site, in the sample are considered to act as adsorption sites for C2H6 at higher pressures. The band at 2.5 Å disappeared when the sample was re-evacuated at room temperature, and the band at 1.6 Å almost recovered its original intensity after evacuation (dotted green line). The XAFS measurements also showed that the interaction of Cu+ with C2H6 is not as strong as that observed in the C2H2 adsorption system.29 We have recently incorporated a large-scale DFT calculation in our research.7,17,18,29,30,87 To certify the adsorption model evaluated from the EXAFS and IR data, we applied this method to the present system to justify our evaluated model. In this study, we used a large-scale model calculation in the B3LYP functional to investigate interactions between a hydrocarbon and CuMFI, instead of periodic boundary condition (PBC) calculations. The advantage of using a cluster model in the B3LYP calculation is that we can compare between
Figure 3. Optimized structure of C2H6 adsorbed on the dual-Cu+ site in CuMFI obtained at the B3LYP level of theory. The two copper ions are located at the position near two five-membered rings forming the main channel. Each bond length (Å) is summarized in Table 1.
data to construct the model related to the dual-Cu+ site in CuMFI. In contrast, the novel EXAFS bands arising from the dual-Cu+ sites found in CuMFI-135 were scarcely observed for CuMFI-75 (black line in Figure 2B), which also supports the existence of large numbers of the dual-copper sites, which are formed in the samples having a higher exchange capacity for copper ions. Because the nonstoichiometrically copper-ionexchanged zeolites exhibit higher levels of catalytic activity for the direct decomposition of NO than do samples with low exchange levels,31−35 such Cu-pair species formed in zeolites are suggested to work as efficient sites for NO decomposition. What should be noticed at this stage is the appearance of a distinct band at around 2.5 Å (no phase-shift correction) and a simultaneous decrease in intensity of the band at around 1.6 Å, with a slight increase in its width, when the CuMFI sample was treated with C2H6 gas at room temperature (blue line in Figure 2B). The band at 2.5 Å hardly changes in its intensity and position even if the pressure of C2H6 increases further, whereas the band at 1.6 Å decreases in its intensity (purple line in Figure 2B). These facts are interpreted by considering that the value of the Debye−Waller factor for Cu−O (1.6 Å band) evaluated after treating the CuMFI-135 sample with C2H6 is larger than that before the treatment; this is because of the irregularity of 10684
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Figure 4. Optimized structures of C2H6 adsorbed on the single-Cu+ site in CuMFI obtained at the B3LYP level of theory. (A) The copper ion is present at the intersection between a straight and a zigzag channel and binds to two framework oxygen atoms. (B, C) The copper ion is located above a five-membered or six-membered ring on a wall of a straight channel, respectively. Each bond length (Å) is summarized in Table 2.
used as simplified expressions for the 10-membered ring of CuMFI in which both the dual-Cu+ site and the single-Cu+ site in the sample are positioned. Here, we consider that, for the dual-Cu+ site in the CuMFI, two Cu+ ions are located near two five-membered rings forming the main channel; the so-called M7 site consisted of the six-membered ring, as shown in Figure 3. Moreover, each single-Cu+ site is positioned on the intersection of the straight and zigzag channels (I2 site: Figure 4A), on the main channels associated with the five-membered (M5) and six-membered (M6) rings in CuMFI (Figure 4B,C), respectively.87 The details of the location of Cu+ in MFI have also been described elsewhere,7,17,18,29,30,75−82,87−89 and we have already proposed these three types of adsorption sites, by which the adsorption phenomena observed for O2 and Xe gases are explained quite naturally.17,18,87 For the C2H6 adsorption
experimental and computational results, because B3LYP methods are known to well reproduce experimental data, especially those relevant to interactions between a transitionmetal cation and a hydrocarbon. However, the PBC programs that we have cannot perform B3LYP calculations. To model properly an MFI cavity, we constructed a large cluster model with AlnSi92−nO151H66 (n = 1 or 2),17,18,30,87 and then we put a hydrocarbon inside a 10-membered cavity of the model. Figure 3 shows the optimized structure of the C2H6 species adsorbed on two Cu+ ions in CuMFI (i.e., as the model for the dual species). For comparison with the dual site, the systems composed of a single Cu+ species as the adsorption site on which the C2H6 species is adsorbed are shown in Figure 4. In these calculations, the Cu2−Al2Si90O151H66 and Cu−AlSi91O151H66 clusters, including a larger model for CuMFI, are 10685
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Table 1. Structural Information on C2H6 Binding into DualCu Cations Embedded in MFI and on Free C2H6 Gas (Unit: Å) C2H6 adsorbed on dual-Cu+ site
free C2H6 gas
1.126 1.102 1.091 1.119 1.090 1.089 1.533 2.660 2.164 3.245 2.777 2.397 1.696 1.966 2.916 3.448 4.151 3.673 2.899 2.747 3.861 1.723 3.315 2.760
1.094 1.094 1.094 1.094 1.094 1.094 1.529
C1−H1 C1−H2 C1−H3 C2−H4 C2−H5 C2−H6 C1−C2 Cu1−Cu2 Cu1−C1 Cu1−C2 Cu2−C1 Cu2−C2 Cu1−H1 Cu1−H2 Cu1−H3 Cu1−H4 Cu1−H5 Cu1−H6 Cu2−H1 Cu2−H2 Cu2−H3 Cu2−H4 Cu2−H5 Cu2−H6
Table 4. Calculated Harmonic Frequencies of C2H6 Adsorbed on Single-Cu+ Site in CuMFI (Unit: cm−1) ν1 ν2 ν5 ν7 ν10 a
model A
model B
model C
1.131 1.093 1.093 1.092 1.093 1.089 1.536 2.223 2.692 1.673 2.488 3.182 2.960 2.556 3.727
1.132 1.100 1.089 1.092 1.089 1.091 1.525 2.210 3.403 1.670 2.073 2.836 3.443 4.157 4.022
1.098 1.093 1.097 1.089 1.094 1.091 1.522 2.545 3.662 2.084 3.231 2.147 3.578 4.327 4.412
Table 3. Calculated Harmonic Frequencies of C2H6 Adsorbed on Dual-Cu+ Site in CuMFI and of Free C2H6 Gas (Unit: cm−1) ν1 ν2 ν5 ν7 ν10
C2H6 adsorbed on dual-Cu+ site
free C2H6 gas
2657.19 971.16 2579.94 2974.13 3018.27
2916.05 964.94 2916.06 2958.24 2983.17
model B
model C
2933.28 944.41 2948.69 2972.36 3010.05
2942.89 982.84
2868.41 983.08
a
2995.74 3017.45
a
2964.92 3015.94
Not detected.
hydrogen atoms, respectively, are bound to the Cu+ ion. The detailed structural parameters of the adsorption model presented are listed in Tables 1 and 2. As can be seen from the optimized dual type of the structural model (Figure 3), two (H1, H2) and one (H4) hydrogen in C2H6 are bonded to the respective copper ions, expressed as Cu1 and Cu2. This adsorption model of C2H6 on CuMFI has not previously been reported, whereas for C2H6 adsorbed on three models (models A, B, and C) of single-Cu+ sites in CuMFI (Figure 4 and Table 2), part of the structure obtained in this work was similar to that obtained by assuming the small-cluster models of C2H6 on Cu+ in CuMFI that were reported by Pidko and Kazansky.44 The resultant distance between two copper ions in MFI for the dual site of the optimized structure (Cu1−Cu2: 2.660 Å) is in good agreement with one of the Cu−Cu distances evaluated from the analysis of the EXAFS spectrum (2.63 Å). We think that another Cu−Cu distance of 3.29 Å obtained experimentally is too long to be formed the dual-C2H6 species in CuMFI, taking into account the molecular size of C2H6. On the basis of the dual or single copper sites mentioned above, the vibrational frequencies were evaluated, and the results are summarized in Tables 3 and 4. Among the frequencies obtained from calculation, the ν1 and ν5 C−H vibration modes of the C2H6 species adsorbed on the dual-Cu+ site in CuMFI show lower frequency values (i.e., 2657.19 (ν1) and 2579.94 (ν5) cm−1), although other C−H modes of adsorbed C2H6 give the bands in the higher-wavenumber region, as well as the frequencies of the free C2H6 molecule (Tables 3 and 4). In addition, for the C−H modes of adsorbed C2H6 on the singleCu+ site in CuMFI, such bands at lower frequencies were scarcely observed (Table 4). Taking into account the results of IR and EXAFS spectra for the CuMFI samples with low and/or high exchange levels, we assigned the IR bands at 2644 and 2582 cm−1 obtained experimentally to ν1 and ν5 C−H modes, respectively, of the C2H6 species adsorbed on the dual-Cu+ site in CuMFI. This assignment is completely different from that reported by another group.44 In this study, we succeeded in optimizing the structures of C2H6 adsorbed on CuMFI using the large-scale DFT calculation method and also in evaluating the vibrational frequencies of adsorbed C2H6 based on the dualtype model; in our calculation, the confinement effect of MFI zeolite was taken into account to some extent. Pidko and Kazansky also reported the bands in the lower frequencies (i.e., 2652 and 2565 cm−1) for an C2H6 molecule adsorbed on a single Cu+ site from the DFT calculation.44 In contrast, in our case, two lower frequencies were observed only for the C2H6 species adsorbed on the dual-Cu+ sites and were scarcely found on a single-Cu+ site in CuMFI. Therefore, the difference seems to be because of the difference in the scale of the DFT calculation technique and/or because of the difference in the effect of the pore wall in MFI on the conformation of C2H6 adsorbed on Cu+. From the results of the calculated
Table 2. Structural Information on C2H6 Binding into Single-Cu Cation Embedded in MFI (Unit: Å) C1−H1 C1−H2 C1−H3 C2−H4 C2−H5 C2−H6 C1−C2 Cu−C1 Cu−C2 Cu−H1 Cu−H2 Cu−H3 Cu−H4 Cu−H5 Cu−H6
model A
onto the Cu+ site in CuMFI, we must consider two types of binding between hydrogen[s] in C2H6 and Cu+: η1 and η2 bindings. Here, the notations η1 and η2 mean that one and two 10686
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Figure 5. Contour maps of orbitals that characterize the C2H6−dual-Cu+ site interaction in CuMFI. The colors represent the difference of the signs (+ (green) and − (red)) of the wave function, and the orbitals shown in the figure are arranged qualitatively on an index of the strength of interaction.
frequencies, other IR bands observed in the experiment were assigned to the C2H6 species adsorbed on a single-Cu+ site and/or a dual-Cu+ site in CuMFI, as well as on Na+ or H+ sites existing in CuMFI, although it is difficult to distinguish the C2H6 adsorbed on these sites in CuMFI or the zeolite exchanged with Na+ or H+. In addition, in the present work, we could not consider the energies for the adsorption systems exhibiting such a weak interaction with C2H6, in comparison with O2 and C2H2. To clarify the characteristic nature of bonding between the dual-Cu+ site in CuMFI and C2H6, we observed the interaction between the zeolite lattice, including Cu+ ions and C2H6, by applying the interaction frontier orbital (IFO).69 Figure 5 illustrates the resultant contour map of orbitals that characterize the interaction between the zeolite lattice, including two Cu+ ions and C2H6 (the top of figure), simply the C2H6 molecule (the middle figure), and the zeolite lattice itself, including two Cu+ ions (the bottom figure: small cluster models cut out in Figure 3), respectively. As can be seen from this picture and also from Table 1, the copper ion denoted as Cu1 is indicated to interact preferentially with the carbon written as C2, not with C1, and also the copper ion Cu2 with hydrogen H4 in C2H6. Furthermore, the copper ion marked as Cu1 shows the interaction with hydrogen[s] represented by H1 [and H2] in C2H6. The interaction between the Cu+ ion and the C2H6
molecule was triggered by the overlap of the s and dσ orbitals of Cu+ with the σ orbital of C2H6. By applying the IFO method, we found that the back-donation contributed dominantly toward the bond between the dual-Cu+ site and C2H6 since the contribution of the donation had weak intensity and thus was not so important. In this system, electrons were transferred from the σ orbital of Cu1 to the antibonding orbital of Cu1− H1 and from each σ orbital of Cu1 and Cu2 to the antibonding orbital of C2−H4, their antibonding nature leading to the activation of C2H6. For the interaction of C2H6 with the singleCu+ sites in CuMFI (Figure 6, the top figures are small cluster models truncated from the larger cluster model shown in Figure 4); the contribution of the back-donation from the s and dσ orbitals of Cu+ to the σ antibonding orbital of C2H6 (the middle figures) was also found to be larger than that of the donation from the σ orbital of C2H6 to the s orbital of Cu+ (the bottom figure). The C1−H1, C1−H2, and C2−H4 bonds in the C2H6 molecule adsorbed on the dual-Cu+ site in CuMFI are longer than those of the corresponding C−H bonds in the free gaseous C2H6 molecule (Table 1). In comparison with the respective bond lengths for the C2H6 molecule adsorbed on the single-Cu+ sites, the C2−H4 bond is particularly longer than that for a single-Cu+ site (Tables 1 and 2). These figures prove clearly that the C2H6 molecule is considerably activated by its 10687
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Figure 6. Contour maps of orbitals that characterize the interaction between the zeolite lattice, including one Cu+ ion and C2H6. (A) The copper ion is present at the intersection between a straight and a zigzag channel and binds to two framework oxygen atoms. (B, C) The copper ion is located above a five-membered or six-membered ring on a wall of a straight channel, respectively. The colors represent the difference of the signs (+ (green) and − (red)) of the wave function, and the orbitals shown in the figure are arranged qualitatively on an index of the strength of interaction.
interaction with the dual-Cu+ site in CuMFI, in comparison with a single-Cu+ site. Since the activation of alkanes enables us to produce the corresponding carboxylic acid by adding CO (in the case of C2H6, propionic acid), there seems to be a potential for the utilization of CuMFI as an efficient activation catalyst for C2H6 at room temperature, although palladium−copper catalysts have previously been reported to activate C2H6 under relatively mild conditions (e.g., 353 K).90,91
Finally, we evaluated the ratio of the number of Cu+ ions associated with the dual site existing in CuMFI to the total number of Cu+ ions in the sample for giving the information on the amounts of dual sites. Given in Figure 7 are the adsorption isotherms of C2H6 at 298 K on the CuMFI samples with different exchange capacities. For CuMFI-135, the ratio of the number of irreversibly adsorbed C2H6 molecules to the total number of Cu+ ions in CuMFI was 0.13 based on the data both of the irreversibly adsorbed amount of C2H6 evaluated to be 10688
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method was reproduced well by the calculation based on the model that the C2H6 species was adsorbed on the dual-Cu+ site, but not on the single-Cu+ sites in CuMFI. 4. The existence of the dual-Cu+ sites in the samples is essential for the efficient activation of C2H6, as well as C2H2 and C2H4, at room temperature. 5. It will be interesting to examine whether the resemblance described here can be extended to establishing detailed information on the decomposition reaction of NO in the zeolite system utilizing these organic molecules as the probe, eventually leading to the design of efficient NO decomposition catalysts.
ASSOCIATED CONTENT
* Supporting Information
Figure 7. Adsorption isotherms of C2H6 recorded at 298 K on CuMFI-135 (filled red circle, open red circle) and CuMFI-75 (filled green square, open green square). The filled and open symbols represent the first and second adsorptions, respectively.
S
Figure S1: adsorption isotherms of CO at 298 K on the CuMFI samples. Figure S2: IR spectra for the adsorbed C2H6 species on NaMFI and HMFI at room temperature. This material is available free of charge via the Internet at http://pubs.acs.org.
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2.25 cm3 (S.T.P.) g−1 at a pressure of 600 Pa and of the number of Cu+ ions formed in the sample, which was estimated from the CO adsorption data shown in Figure S1 (Supporting Information). In contrast to this sample, the irreversibly adsorbed C2H6 was hardly observed for the CuMFI-75 sample; only a part of the Cu+ in the excessively copper-ion-exchanged CuMFI sample was found to contribute to the efficient activation of C2H6 at room temperature. Recently, some authors have suggested that the dual-copper site in copper-ionexchanged zeolites contributes to the efficient decomposition of NO.92−94 The copper−copper distance they reported is similar to that for the presented dual-Cu+ site in CuMFI. As described above, the dual site is dominantly formed in samples having exchange capacities exceeding 100%, concomitant with the increase in the activity for the NO decomposition reaction; this is the same tendency observed in the C2H6 adsorption. Therefore, it seems reasonable to say that the existence of the dual site is indispensable in both phenomena. It is thus evident that the use of these organic molecules (C2H6 and C2H2) as probe molecules permits the identification of the active dualCu+ site (i.e., Cu+ pair) in CuMFI for NO decomposition. Thus, studies using C2H6 as a probe molecule will give meaningful information on these active sites.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support was provided by the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 21655021). A.I. acknowledges the financial support from the Okayama Foundation for Science and Technology and from the Kinki-chihou Hatsumei Center (KHC). In addition, H.T. would like to express her thanks for the financial support in a doctoral course from the Japan Society for the Promotion of Science (the Research Fellowship for Young Scientists, DC1). The XAFS measurement was performed under the proposals of PF-PAC (Nos. 2009G591 and 2010G693). We thank Prof. M. Nomura, Dr. H. Nitani, and Dr. A. Koyama of KEK in Tsukuba for their kind assistance in measuring the XAFS spectra. Finally, our special thanks are due to Prof. P. Nachtigall at Charles University in Prague for his helpful and kind discussion at Okayama.
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CONCLUSIONS In this study, we aimed at investigating in detail the interaction between Cu+ ions in CuMFI and a C2H6 molecule at room temperature. Several critical new findings of the states of the real active centers in CuMFI for the efficient adsorption of C2H6, as well as of small organic molecules containing CC bonds at room temperature, are listed as follows. 1. We found experimentally that the C2H6 species adsorbed on the Cu+ sites in CuMFI causes two characteristic IR bands at 2644 and 2582 cm−1. 2. We obtained structural information on the C2H6 species adsorbed on CuMFI from the analysis of the EXAFS data; a peculiar backscattering at around 2.2 Å (without phase correction) attributed to the Cu+−Cu+ interaction was found. 3. The results of large-scale DFT calculations justify the usefulness of the dual type of the species composed of C2H6 and the sites of the dual-type Cu+ ions. Two IR bands were observed at 2644 and 2582 cm−1; as well, the distance between copper ions evaluated by the EXAFS
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