J. Phys. Chem. C 2007, 111, 19043-19051
19043
Dicopper(II)-Dioxygen Complexes in Y Zeolite for Selective Catalytic Oxidation of Cyclohexane under Photoirradiation Ken-ichi Shimizu,* Yoshiaki Murata, and Atsushi Satsuma Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan ReceiVed: August 23, 2007; In Final Form: October 10, 2007
The photooxidation of cyclohexane with hydrogen peroxide is catalytically promoted by copper(II)-exchanged Y zeolite (CuY), resulting in the selective formation of cyclohexanol and cyclohexyl hydroperoxide. Analyses by in-situ UV-vis, in-situ Cu K-edge XAFS, and Raman spectroscopies are used to identify the active species in CuY treated with H2O2 solution. The reaction of H2O2 with isolated Cu(II) ions in Y zeolite yields the bis(µ-oxo)dicopper(II) (complex 1) and the O22-- or HOO--bridged dicopper(II) (complex 2). The complex 2 is partly converted to superoxodicopper(II) (complex 3) at ambient temperature. At higher temperature, the thermal desorption of the bridged dioxygen species in complexes 2 and 3 occurs to yield two monomeric Cu(II) complexes and O2, while the complex 1 remains unchanged up to 473 K. Under photoirradiation the dioxygen species in complexes 2 and 3 are consumed, and the consumption rates increased when cyclohexane is added to the gas mixture, suggesting that the photoexcitation of the complexes is involved in an important step of the selective cyclohexane oxidation.
Introduction Designing artificial catalysts by mimicking catalytic properties of natural metalloenzymes has been a challenging area to chemists and biologists. During the past few decades, bioinorganic chemists synthesized a great number of copper-protein mimics to understand the structure and functional properties of real enzymes.1-12 A multinuclear Cu cluster particulate, which is the active center of methane monooxygenase (pMMO),13 catalyzes alkane hydroxylation, and hence it is a highly attractive target for the design of a bio-inspired catalyst for alkane oxidation. Very recently, Kirillov et al. reported for the first time that homogeneous pMMO-inspired catalysts containing oxygen-bridged multicopper complexes are effective for the oxidation of cyclohexane with hydrogen peroxide.14,15 Generally, the catalytic activity of the model complexes in a homogeneous medium may decrease with time due to ligand oxidation or formation of polymeric complexes. In order to prevent their structural change and to increase their separability, several attempts have been devoted to the development of heterogeneous enzyme-inspired copper oxidation catalysts.16-27 However, in most cases, organic ligands are indispensable in the preparation of the active copper species. Although attempts have been made on the spectroscopic characterization of enzyme-inspired copper oxidation catalysts, very few have focused on the dynamic behavior of active copper species and the reactivity of active oxygen species, and hence the mechanism is not clarified. Groothaert et al. successfully showed that bis(µ-oxo)dicopper(II) complex anchored in ZSM-5 zeolites28,29 oxidized methane into methanol,25,26 demonstrating that biomimetic dicopper active species can be stabilized by an all-inorganic ligand, zeolite. They also showed in-situ UV-vis evidence on the dynamic behavior of the active copper-oxygen species.25,26 However, this complex did not act as a catalytic site (turnover number ) 0.02). In our * Corresponding author. Fax: +81-52-789-3193. E-mail: kshimizu@ apchem.nagoya-u.ac.jp.
previous study, it was found that the O2-bridged multicopper(II) complexes in BEA zeolite, formed during benzene photooxidation with H2O2 over Cu-BEA, catalyze the selective onestep conversion of benzene to diphenols.27 Cyclohexane oxidation is an important commercial reaction to yield cyclohexanol and cyclohexanone, which are important themselves and as intermediates to other chemicals such as adipic acid, nylon-6,6′-polyamide-6 and so on.30 The current commercial process operates with cyclohexane conversion of 4% and 85% selectivity at high temperature using air (above 12 atm) and Co(III) homogeneous catalyst. The oxidation of cyclohexane under milder conditions is a topic of great interest. Many attempts14,15,30-33 have been made to substitute the current process by heterogeneous systems that can catalyze the selective oxidation of alkanes with hydrogen peroxide as oxidant, which only produces water as a byproduct. However, a few examples are reported on the heterogeneous catalytic oxidation at around room temperature.31,33 We describe here how the enzymeinspired dioxygen-bridged multicopper(II) complexes in Y zeolite, formed by the H2O2 treatment of Cu(II)-exchanged Y zeolite, catalyze the selective cyclohexane photooxidation at room temperature. Detailed spectroscopic characterization is shown to investigate the dynamic structural changes and photoreactivity of both copper species and active oxygen species. Experimental Section NaY zeolite (JRC-Z-Y5.6, SiO2/Al2O3 ) 5.6, a reference catalyst of the Catalysis Society of Japan) and HY zeolite (JRCZ-HY5.6, SiO2/Al2O3 ) 5.6) were supplied from the Catalysis Society of Japan. Cu(II)-exchanged Y zeolite (CuY) was prepared by exchanging NaY (10 g) with an aqueous solution of copper(II) acetate (0.04 M, 200 cm-3) at 298 K for 24 h, and adjusting the pH of the solution to 7.0 using a dilute NH4OH solution. The slurry was then centrifuged, washed with deionized water, and dried in air at 373 K for 12 h. The Cu content (2.0
10.1021/jp0767821 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2007
19044 J. Phys. Chem. C, Vol. 111, No. 51, 2007 mmol g-1, Cu/Al ) 0.44) was determined by ICP (Inductively Coupled Plasma). The photocatalytic reaction at room temperature (300 K) was performed in a closed batch reactor under air. The mixture containing CuY (0.01 g, Cu ) 20 µmol), cyclohexane (10 mmol), acetonitrile (20 cm3), and 30% aqueous H2O2 (10 mmol) in a cylindrical flask (100 cm3) made of Pyrex was irradiated for 6 h from the side with a 400 W Xe lamp (Ushio SX-UI500XQ) using a cut filter UTF-50S-34U (Shiguma Koki Co., λ ) 34), which cuts off half light intensity at 340 nm. A small portion of the reaction mixture was periodically sampled from the reactor, and toluene was added to the mixture as an internal standard for GC analysis. Cyclohexanol and cyclohexanone concentrations were measured with GC (DB-1 column) using a flame ionization detector. Cyclohexyl hydroperoxide concentration was measured after reaction with triphenylphosphine to form cyclohexanol.33 The amount of gas-phase CO2 was determined by gas chromatography. The amount of H2O2 was analyzed by UV-vis absorption at 420 nm using TiO(SO4) as pigment.34 For a characterization of CuY interacting with H2O2, CuY was soaked in 2 M H2O2(aq)/CH3CN solution (H2O2/Cu ) 100/ 1) and dried in air at room temperature (300 K) for 5 min, and the obtained powder, named CuY-H-300, was investigated by various spectroscopic measurements. Note that the CuY-H-300 showed about 1.3 times larger weight than untreated CuY, indicating that the treated sample contains a large amount of H2O and CH3CN, most probably in the micropore of zeolite. Therefore, the results of solid-state characterization at 300 K adopted in this study should reflect the structure of copper species in liquid-phase oxidation condition in which the copper active site should be solvated by H2O and CH3CN. Temperature-programmed desorption (TPD) of O2 from CuYH-300 (0.05 g) was carried out using TPD equipment (BEL JAPAN). After evacuation for 15 min, followed by purging in a He flow at 300 K for 15 min, TPD was performed under He flow (60 cm3 min-1) at a reduced pressure (200 Torr) by raising the temperature to 500 K at a heating rate of 10 K min-1, and the outlet gases were analyzed by the mass spectrometer. Cu K-edge X-ray absorption spectra were obtained in a transmission mode at the BL01B1 in the SPring-8. The storage ring was operated at 8 GeV. A Si(111) single crystal was used to obtain a monochromatic X-ray beam. A self-supported wafer form of the sample (60 mg) with about 7 mm diameter was placed in a quartz in-situ flow cell35,36 in a He flow (200 cm3 min-1) under atmospheric pressure. For in-situ XAFS measurements of the H2O2-treated CuY, the sample was immediately placed in the in-situ cell, followed by purging with a He flow for 5 min at 300 K, and by stepwise heating in a He flow. The H2O2-treated CuY heated at 473 K is named CuY-H-473, for example. Analyses of the X-ray absorption near-edge structures (XANES) and the extended X-ray absorption fine structure (EXAFS) were performed using the REX version 2.3 program (RIGAKU). For XANES analysis, the energy was defined by assigning the first inflection point of the Cu foil spectrum to 8980.3 eV. The Fourier transformation of the k3-weighted EXAFS oscillation from k space to r space was performed over the range 2.0-13.5 Å-1 to obtain a radial distribution function. The inversely Fourier filtered data were analyzed with a common curve-fitting method in the k range of 3.5-13.5 Å-1. For the curve-fitting analysis, the empirical phase shift and amplitude functions for Cu-Cu and Cu-O shells were extracted from the data for Cu powder and Cu2O measured at each temperature, respectively.
Shimizu et al. Diffuse reflectance UV-vis spectra were recorded with a UV-vis spectrometer (JASCO V-550) equipped with an insitu flow cell.27,36 A diffuse reflectance sample cell is connected with a gas flow system. The light source is directed to the center of an integrating sphere by optical fiber. Reflectance was converted to pseudo-absorbance using the Kubelka-Munk function. BaSO4 was used to collect a background spectrum. A flow of He was fed to the sample (50 mg) at a flow rate of 100 cm3 min-1, and UV-vis spectra were recorded in a temperature range of 323-673 K. The change in the spectra of the H2O2-treated CuY by heating (Figure 8) was studied as follows; the CuY-H-300 sample was immediately placed in the in-situ cell, followed by purging in a He flow for 5 min at 300 K, and by heating at 323, 373, 423, and 473 K in He flow for 10 min. The change in the spectra of the CuY-H-300 after photoirradiation at 300 K (Figure 10) was studied as follows. After purging the CuY-H-300 in the in-situ cell in He flow for 5 min, the sample was photoirradiated (hν > 340 nm) at 300 K under a flow of He or cyclohexane(13%)/He. The cyclohexane(13%)/He gas mixture was introduced to the in-situ cell by passing the carrier gas (He) to a saturator. The light beam was passed through a UTF-50S-34U filter (Shiguma Koki Co.) and a fine stainless mesh to adjust the irradiation intensity. After irradiation for 5 min, the spectrum was measured under He flow without exposure to air, and then the sample was photoirradiated again for the next 5 min. Raman spectra, with a resolution of 3 cm-1, were collected at room temperature with a NRS-1000 Raman spectrometer equipped with a microscope and a 532.3 nm laser as an excitation source. Two scans were accumulated, and each scan required 120 s. The change in the spectra of the H2O2-treated CuY by heating (Figure 6) was studied as follows. The CuYH-300 sample (60 mg) was immediately placed in the Pyrex tube, followed by purging in a He flow at 300 K, and by heating at 323, 373, 423, and 473 K in a He flow for 10 min. After each treatment, a small portion of the sample powder was placed on a glass plate under ambient condition, and a Raman spectrum was measured ex-situ. The change in the spectra of the H2O2treated CuY under photoirradiation at 300 K (Figure 10) was studied adopting the same treatment condition (gas composition and photoirradiation method) as the UV-vis experiments in Figure 9. After the treatment, a small portion of the sample powder was placed on a glass plate for ex-situ Raman measurement. For the freshly prepared H2O2-treated CuY (t ) 0 min in Figure 11), Raman spectra of four different samples were measured, and errors for the Raman band area at 829 and 1050 cm-1 were estimated to be 15% and 21%, respectively. ESR spectra were measured by an X-band JEOL JES-TE200 spectrometer at a microwave power level (1.0 mW) at which microwave power-saturation of the signals did not occur. The magnetic field was calibrated with a JEOL NMR FIELD METER ES-FC5. ESR spectra of the samples in the Suprasil quartz tube were measured at 77 K. Results Photocatalytic Hydroxylation of Cyclohexane. The conversions of cyclohexane to cyclohexanol, cyclohexanone, cyclohexyl hydroperoxide (ROOH), and CO2 are shown in Figure 1 as a function of photoirradiation time. When a mixture of a cyclohexane and aqueous H2O2 in acetonitrile containing a catalytic amount of CuY (0.2 mol %) was photoirradiated through cut-filter (>340 nm), cyclohexanol and cyclohexyl hydroperoxide were selectively formed. ROOH is initially the predominant product with 68% selectivity (2 h), and the
Dicopper(II)-Dioxygen Complexes in Y Zeolite
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Figure 1. Yields of (b) cyclohexanol, (2) cyclohexanone, (9) cyclohexyl hydroperoxide, and ([) CO2; (O) total yield of oxygenates; and (+) conversion of H2O2 during photooxidation of cyclohexane with H2O2 on CuY zeolite.
Figure 2. TPD curves of the mass signal due to O2 (m/z ) 32) for CuY-H-300 (solid line) and HY after H2O2 treatment.
TABLE 1: Oxidation of Cyclohexane with H2O2 by Various Catalystsa yield of products (%) entry
catalyst
1 2 3 4
CuY no catalyst CuY CuYe
hν H2O2 conv. mol % (nm) -olb -onec ROOHd total (%) 0.2 0 0.2 0.2
>340 >340 dark >340
8.0 0 0 0
1.2 0 0 0
5.5 0 0 0
14.7 0 0 0
100 22 8
a Condition: cyclohexane (10 mmol), H2O2 (10 mmol), T ) 300 K, t ) 6 h, catalyst amount ) 0.01 g. b Cyclohexanol. c Cyclohexanone. d Cyclohexyl hydroperoxide. e H2O2 ) 0 mmol.
Figure 3. In-situ Cu K-edge XANES spectra in a flow of He.
selectivity decreased with time (37% after 6 h). The cyclohexanol selectivity increased from 25% (2 h) to 54% (6 h), suggesting that ROOH is an intermediate in the formation of cyclohexanol. The reaction results after 6 h are summarized in Table 1. Relatively small amounts of cyclohexanone (8.2% selectivity after 6 h) and only a trace amount of gaseous CO2 (1.3% selectivity after 6 h) were also produced. After 6 h of photoirradiation, the total yield of oxygenates (cyclohexanol, cyclohexanone, and ROOH) was 14.7%, and H2O2 conversion was 100% (entry 1). The oxidation of cyclohexane with H2O2 under photoirradiation (>340 nm) did not proceed (entry 2). When the cyclohexane oxidation by H2O2 with CuY was performed in the dark (entry 3) or when the photooxidation of cyclohexane by CuY was performed in the absence of H2O2 in air (entry 4), no oxygenates were produced. Therefore, the presence of H2O2 as oxidant and CuY as well as photoirradiation are essential for the oxidation of cyclohexane. H2O2 conversion detected in the absence of catalyst and in the dark (Table 1) may be due to a photochemical reaction and H2O2 decomposition by CuY, respectively. Under the standard condition (entry 1), the turnover number (TON) based on Cu ions, which is estimated as a minimum TON value, was 74, indicating that the reaction proceeds catalytically. From these results, it is concluded that the photocatalytic oxidation of cyclohexane with H2O2 to the oxygenates proceeds over CuY. Spectroscopic Characterization of Cu Complexes. When CuY was soaked with the reaction mixture, cyclohexane/ H2O2(aq)/CH3CN solution, or H2O2(aq)/ CH3CN solution, its color changed from light blue to brown, suggesting that the reaction of CuY with H2O2 results in a structural change of the copper species. Various spectroscopic characterizations of CuY treated with H2O2 were performed to investigate the catalyst structure. Figure 2 shows a TPD curve of O2 from the H2O2treated CuY (Cu-H-300). Desorption of O2 occurred at higher temperature for CuY-H-300 (in a temperature range of 300450 K with a maximum at 386 K) than H2O2-treated HY (in a temperature range of 300-380 K with a maximum at 342 K).
Untreated CuY showed no O2 desorption peak below 500 K (not shown). These results indicate that O2 desorption from CuYH-300 is not due to a desorption or decomposition of physisorbed H2O2 or thermal reduction of Cu(II) in CuY. This suggests a strong interaction of oxygen species and copper sites. In order to clarify the interaction of the oxygen species and copper sites and their molecular structures, several spectroscopic characterizations are shown below as a function of temperature. The local structure and oxidation states of Cu species are studied by X-ray absorption near-edge structures (XANES) at the Cu K-edge. It is established that the energy position of the K-edge depends on the copper ion valence state; it shifts to higher energy with an increase in the oxidation number of copper.28,29 Figure 3 shows the XANES spectra of CuY and the CuY-H-300 which were immediately placed in the in-situ cell, followed by purging in a He flow for 5 min at 300 K, and by heating in a He flow at 473 K (CuY-H-473). After these treatments, the positions of the absorption edge did not markedly change, and no peak at 8981 eV due to Cu(I) species appeared, indicating no change in the copper oxidation state (Cu2+). The intensity of the 1s-4p transition peaks in the post-edge region is lower for CuY-H-300 than for CuY, indicating a change in the local structure of Cu(II) species after the H2O2 treatment. There are isosbestic points at 8988 and 9005 eV in the serical XANES spectra, indicating a direct change of the isolated Cu(II) ions to certain Cu(II) species. The k3-weighted Cu K-edge EXAFS and the Fourier transforms (FT) for CuY and the H2O2-treated CuY samples are shown in Figure 4, parts A and B. In the FT for the untreated CuY, the first shell due to Cu-O bonds is observed and its EXAFS is featureless at higher distances, indicating that Cu(II) ions in CuY are present as a monomeric complex as reported in previous EXAFS studies on Cu(II) exchanged zeolites.27,28 The key feature in EXAFS data in CuY-H-300 is two new peaks appearing at R ) 2.36 and 2.95 Å (phase shift uncorrected), suggesting the presence of atoms in the second coordination sphere. The result of the curve-fitting analysis is shown in Table
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Figure 4. (A) k3-weighted Cu K-edge EXAFS spectra (solid lines) and the best fit derived from curve-fitting analysis (dashed lines), and (B) Fourier transforms for (a) untreated CuY and H2O2-treated CuY samples heated stepwise at (b) 300 K, (c) 323 K, (d) 373 K, (e) 423 K, and (f) 473 K under a flow of He. The spectra were measured in-situ after 10 min of purging at each temperature.
TABLE 2: Cu K-Edge EXAFS Analysis catalysts
shell
CNa
Rb (Å)
σ2 c (Å2)
Rfd (%)
CuY CuY-H-300
O O Cu Cu O Cu Cu O Cu Cu O Cu Cu O Cu
5.2 5.5 1.4 1.8 4.3 1.1 1.4 5.0 1.3 0.7 5.4 1.2 0.4 5.5 1.3
1.92 1.92 2.83 3.29 1.93 2.83 3.28 1.92 2.83 3.30 1.92 2.83 3.30 1.92 2.91
0.087 0.089 0.080 0.001 0.078 0.080 0.001 0.087 0.085 0.001 0.095 0.080 0.001 0.095 0.080
3.1 3.1
CuY-H-323 CuY-H-373 CuY-H-423 CuY-H-473
1.2 1.4 1.9 3.3
a Coordination number. b Bond distance. c Debye-Waller factor. d Residual factor.
2. For CuY-H-300, a simulated oscillation obtained using curvefitting with Cu-O and Cu-Cu parameters extracted from Cu2O and Cu foil, respectively, fit well with the experimental one (Figure 4A). The best fit gives 1.4 and 1.8 Cu atoms being bridged at Cu-Cu distances of 2.83 and 3.29 Å, respectively (Table 2).37 The EXAFS feature did not change after CuY-H300 was purged with He for 30 min at 300 K (result not shown). When the CuY-H-300 sample was heated in a He flow from 323 to 473 K, the EXAFS feature of Cu-Cu interaction gradually changed with the temperature. EXAFS parameters for these samples (Table 2) are plotted as a function of temperature in Figure 5A. The coordination numbers of the first Cu-Cu shell (at R ) 2.83-2.91 Å) did not markedly change with temperature. Whereas, those of the second Cu-Cu shell (at R ) 3.28-3.30 Å) decreased from 1.8 to 0 as the temperature increased from 300 to 473 K. From the Cu-Cu distances in Table 2 (2.83-3.30 Å), these Cu-Cu features can be assigned to Cu-O-Cu interactions. Inspecting the Cu-Cu distances of copper-dioxygen complexes in the literature (Table 3),1-12,28,29 the two Cu-Cu features with distances of 2.83-2.91 and 3.283.30 Å could be tentatively assigned to multicopper(II) complexes bridged by oxygen species. Raman spectroscopic experiments were performed to identify the oxygen species. The Raman spectra of the H2O2-treated CuY samples are shown in Figure 6. Treating the catalyst with H2O2
Figure 5. Charges in (A) the coordination numbers for (O) the first Cu-Cu shell and (b) the second Cu-Cu shell, and the Cu-Cu distances for (0) the first Cu-Cu shell and (9) the second Cu-Cu shell (from Table 2); and (B) changes in the UV-vis band height at 400 nm (from Figure 3) and Raman bands area (from Figure 6) at (O) 829 cm-1 and (0) 1050 cm-1 as a function of temperature for H2O2treated CuY.
produced a broad band at 879 cm-1 due to H2O238 and a shoulder at around 829 cm-1. When the H2O2-treated CuY was exposed to a flow of He at 300 K, the band due to H2O2 (879 cm-1) disappeared and the band at 829 cm-1 was clearly observed. Simultaneously, a band at 1050 cm-1 assignable to the superoxide ion (O2-)8,9 appeared, and its intensity increased with time. The band at 829 cm-1 is assignable to the peroxide ion (O22-)39-41 or hydroperoxide ion (HOO-).6-8 No bands in this range appeared in the spectra of the H2O2-treated HY zeolite and untreated CuY (not shown). The CuY-H-300 sample was heated stepwise from 323 to 773 K in He flow for 10 min. After each treatment, ex-situ Raman spectra (Figure 6) were measured under ambient condition. Temperature dependence of the relative area of the band at 829 and 1050 cm-1 is plotted in Figure 5B. The intensity of the bands due to O22- or HOO- (829 cm-1) and O2- (1050 cm-1) decreased with temperature, and the former band disappeared at lower temperature (423 K) than the latter band (473 K). The diffuse reflectance UV-vis spectrum of the CuY-H-300 measured under He flow showed a new unresolved band around 250-600 nm with a maximum at 400 nm (Figure 7). The band in this range is close to the O-to-Cu2+ charge-transfer transition of several copper-oxygen complexes reported in the literature (Table 3).1-12,27-29 The CuY-H-300 sample was heated stepwise from 323 to 773 K in a He flow for 10 min, and the spectrum (Figure 8) was measured at each temperature. Note that the spectrum of CuY collected at each temperature under a flow of He was subtracted from each spectrum. Temperature dependence of the band intensity at 400 nm is plotted in Figure 5B. The intensity of the band at 400 nm, which is tentatively assigned to copper-oxygen complexes, decreases with temperature. At 473 K, a band at 300 nm and a broad band around 430 nm were observed. Figure 9 shows ESR spectra of CuY and CuY-H-300. The spectrum of CuY showed an ESR signal with anisotropic g
Dicopper(II)-Dioxygen Complexes in Y Zeolite
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TABLE 3: Spectroscopic Properties of Copper(II)-Oxygen Complexes in Y Zeolite and Representative Copper(II) Complexes
a Cu-Cu interatomic distance. b Wavelength of maximum absorption for the LMCT band in UV-vis spectra. c Wavenumber of maximum absorption for the νO-O band in Raman spectra.
Figure 8. Changes in the in-situ UV-vis spectra of H2O2-treated CuY after exposure to a flow of He at 300 K for 5 min, and then being heated stepwise at 323, 373, 423, and 473 K. The spectrum of CuY at each temperature was subtracted from each spectrum.
Figure 6. Raman spectra measured at room temperature (300 K) for (a) CuY soaked with H2O2 (slurry), which was then exposed to a flow of He for (b) 5 min and (c) 30 min at 300 K, and was then heated stepwise at (d) 323 K, (e) 373 K, (f) 423 K, and (g) 473 K under a flow of He for 10 min and (h) H2O2-treated HY after purging with He for 5 min. The spectra were measured under ambient condition (exsitu).
Figure 9. ESR spectra of CuY and CuY-H-300 at 77 K.
Figure 7. In-situ diffuse reflectance UV-vis spectra of CuY and CuY-H-300 at 300 K.
values (g⊥ ) 2.076, g| ) 2.390), characteristic for hydrated Cu(II) ions present as isolated species in zeolite.42 The ESR intensity due to the isolated Cu(II) species decreased after the H2O2 treatment; the intensity of CuY-H-300 was 17% of the value for untreated CuY. It is known for dimeric Cu(II) species that ESR signal intensity decreases with temperature.43 For example, Kodera et al.8 reported that a bis(µ-hydroxo)dicopper(II) complex was ESR-silent at 77 K, because of the strong antiferromagnetic coupling between two Cu(II) ions. Taking into account the XANES result, the ESR result in Figure 9 suggests that after the H2O2-treatment the majority of Cu(II) species became ESR-silent species such as dimeric Cu(II) species.
Photoreactivity of the Complexes. The photoreactivity of the copper-oxygen complex in Y zeolite to cyclohexane vapor was studied by in-situ UV-vis experiments. Figure 10, parts A and B, shows UV-vis spectra of CuY-H-300 as a function of exposure time to flowing He and cyclohexane(13%)/He gas mixture, respectively. The time course of the band height at 400 nm is plotted in Figure 10C. The spectrum did not markedly change after exposing CuY-H-300 to a cyclohexane/He flow for 20 min in the dark (not shown). When the sample was photoirradiated (hν > 340 nm) in flowing He or cyclohexane/ He under photoirradiation (hν > 340 nm), the intensity of the band due to the copper-oxygen complex decreased with time, indicating that the complex was consumed by a photoreaction. The rate of consumption was larger in the presence of the cyclohexane vapor in a flow. This suggests that the copperoxygen complex is photoreactive toward cyclohexane. The photoreactivity of oxygen species to cyclohexane was studied by Raman experiments. Figure 11, parts A and B, shows Raman spectra of CuY-H-300 as a function of exposure time to flowing He and cyclohexane(13%)/He, respectively. The time
19048 J. Phys. Chem. C, Vol. 111, No. 51, 2007
Figure 10. Charges in the in-situ UV-vis spectra of H2O2-treated CuY at 300 K as a function of photoirradiation time in a flow of (A) He or (B) cyclohexane(13%)/He. The spectrum of CuY was subtracted from each spectrum. (C) Changes in the relative band height at 400 nm.
course of the relative band intensity is plotted in Figure 11C. The spectrum did not markedly change after exposure to a cyclohexane/He flow for 20 min in the dark (not shown). When the sample was photoirradiated (hν > 340 nm) in flowing He or cyclohexane/He under photoirradiation (hν > 340 nm), the intensity of the band due to O2- (1050 cm-1) and O22- or HOO(829 cm-1) decreased with time. The rate of decrease was larger in the presence of the cyclohexane vapor in a flow. This observation provides direct evidence that the oxygen species are photoreactive toward cyclohexane. Discussion Identification of Dicopper(II)-Dioxygen Complexes. The O2-TPD result (Figure 2) suggests the copper-oxygen interaction in H2O2-treated CuY. A combination of ESR and XANES results shows that the majority of the isolated Cu(II) species in CuY became ESR-silent species, such as dimeric Cu(II) species, after the H2O2 treatment. The EXAFS result of CuY-H-300 gives the average structural information of the majority species, that is the ESR-silent species in the samples. According to the results summarized in Table 3 and Figure 5, we will propose that at least three different dicopper-dioxygen complexes, 1, 2, and 3, are formed by the reaction of Cu(II) ions in CuY zeolite with H2O2. In the research area of copper-enzyme inspired oxidation catalysts,1-12,23,26-29 the active-site structure is usually identified by the position of the Raman band due to O-O stretching, the position of UV-vis absorption due to oxygen-to-copper charge transfer, and the Cu-Cu distances. Most of the reported dioxygen-copper complexes are homogeneous complexes using organic polydentate ligands. Therefore, the structure of organic ligand-free complexes, 1, 2, and 3 cannot be deduced by direct comparison with a homogeneous analogous compound. Instead, comparisons can be made with similar homogeneous compounds
Shimizu et al.
Figure 11. Raman spectra measured under ambient condition as a function of photoirradiation time. H2O2-treated CuY was first exposed to a flow of He for 30 min, and then it was photoirradiated in a flow of (A) He or (B) cyclohexane(13%)/He. (C) Time course of the relative area of the band at (O,b) 829 cm-1 and (0,9) 1050 cm-1 in a flow of (closed symbols) He or (open symbols) cyclohexane(13%)/He.
consisting of Cu(II) ions and organic polydentate ligands, whose spectroscopic characteristics may be slightly different from that of the copper-dioxygen species stabilized by an all-inorganic ligand, zeolite. Table 3 summarizes the Cu-Cu distance (from EXAFS) and Raman and UV-vis features of complexes 1, 2, and 3 in comparison with the representative homogeneous complexes in the literature. The spectroscopic results of the bis(µ-oxo)dicopper(II) complex stabilized by ZMS-5 zeolite28,29 are also listed in Table 3. The Raman results (Figure 6) of H2O2treated CuY show the presence of the two oxygen species at 300 K. The band at 829 cm-1 assignable to peroxide ion (O22-) or hydroperoxide ion (HOO-) first appeared, and then a band at 1050 cm-1 assignable to superoxide ion (O2-) appeared. The former species was thermally less stable than the latter one (Figures 5B and 6). These results suggest that the reaction of H2O2 with Cu(II) ions in Y zeolite yields a peroxide (O22-) or hydroperoxide (HOO-) species, which is then thermally converted to the O2- species. Since these dioxygen species did not form on the H2O2-treated HY zeolite, Cu(II) sites play an important role in the formation of these oxygen species. EXAFS results give structural information of Cu species. Studies on the copper-enzyme mimics have established that Cu-Cu separation distance is one of the key fingerprints for identifying the structure of a multicopper-dioxygen complex (Table 3).1-9 Although Cu-Cu coordination numbers of CuY-H-300 (1.4 and 1.8) may suggest the presence of large clusters, such as trinuclear complexes, we assume the mono- and dicopper complexes because the UV-vis feature of CuY-H-300, having a band at 400 nm, is clearly different from those of the trinuclear copper complex.1,4 EXAFS analysis (Table 2, Figure 5A) shows that
Dicopper(II)-Dioxygen Complexes in Y Zeolite
J. Phys. Chem. C, Vol. 111, No. 51, 2007 19049
SCHEME 1
there are two Cu peaks assignable to two Cu-O-Cu distances (2.83-2.91 and 3.28-3.30 Å). The bis(µ-oxo)dicopper(II) complex formed in O2-activated Cu-ZSM-5 has been characterized by a Cu-Cu distance of 2.87 Å and UV-vis bands at 330 and 440 nm.28,29 When the H2O2-treated CuY was heated at 473 K, where the Cu-O-Cu EXFAS contribution at 3.283.30 Å and the Raman bands due to dioxygen species are absent, the Cu-O-Cu distance of 2.91 Å and UV-vis bands at 300 and 430 nm were observed. These features are very similar to the spectroscopic characteristics of the bis(µ-oxo)dicopper(II) complex. Therefore, we attribute the complex 1 to the bis(µoxo)dicopper(II) complex. Groothaert et al.28 reported that the reaction of bis(µ-oxo)dicopper species in ZSM-5 with water yielded the bis(µ-hydroxo)dicopper(II) complex having a UVvis band at 330 nm. The distance 2.91 Å and a UV-vis band at 400 nm of CuY-H-473 could exclude a possible assignment of the complex 1 to the hydroxide-bridged bis(µ-hydroxo)dicopper(II) complex having a Cu-O-Cu distance of 2.983.08 Å.2b However, we cannot exclusively rule out a possibility that the Cu-O-Cu distance of 2.83 Å observed for H2O2treated CuY samples heated below 423 K, having a UV-vis band around 330 nm, can be assigned to the bis(µ-hydroxo)dicopper(II) complex, which will be produced by the reaction of residual water with bis(µ-oxo)dicopper(II). The coordination number of the first Cu-Cu feature (2.832.91 Å) does not depend on temperature in the range of 300473 K (Figure 5A), indicating that the amount of the complex 1 does not change in this temperature range. On the other hand, the amount of two other types of species decreases with temperature. As shown in Figure 5B, Raman band intensities for O22- or HOO- (829 cm-1) and O2- (1050 cm-1), UV-vis band intensity for the charge transfer from oxygen to copper (400 nm), and the coordination number for the longer Cu-OCu feature (3.28-3.30 Å) in EXAFS spectra decreased with temperature and these features were absent at 473 K. Taking into account the O2-TPD result that thermal desorption of O2 occurs in the temperature range of 300-460 K, the above result suggests that two types of dicopper-dioxygen complexes other than the complex 1 are present in the sample and their concentration decreases with temperature because of their thermal decomposition. The complexes characterized by Raman bands at 829 and 1050 cm-1 are named complexes 2 and 3, respectively. At 423 K, where only one Raman band at 1050 cm-1 is observed, the sample showed the Cu-O-Cu distance of 3.30 Å and UV-vis bands at 400 nm. These Raman and UV-vis features are very similar to those of the superoxodicopper(II) complex.8,9 Although there are no literature data of the Cu-O-Cu distance of this complex, the Cu-O-Cu EXAFS feature (3.30 Å) indicates the O2--bridged dicopper structure and excludes the possibility of the end-on superoxomonocopper complex.12 Therefore, we attribute the complex 3 to the superoxodicopper(II) complex. This complex was ob-
served in a temperature range of 300 to 423 K. Finally, the complex 2 is characterized by the Raman band at 829 cm-1, the Cu-O-Cu distance of 3.28-3.30 Å, and the UV-vis band at 400 nm. The literature data on the model complexes show that the hydroperoxo-copper(II) complexes6-8 have a Raman band in the range 861-892 cm-1 and the Cu-O-Cu distance of 2.95-3.11 Å, while the bent-type (µ-η2:η2-peroxo)dicopper(II) complexes3 have a band around 750 cm-1 and the CuO-Cu distance of 3.2-3.4 Å. The observed distance excludes the possibilities of the planar-type (µ-η2:η2-peroxo)dicopper(II) (3.6 Å)2 and a trans-µ-1,2-binding mode of the peroxide (4 Å).2 Thus, we attribute the complex 2 to the hydroperoxodicopper(II) or the bent-type (µ-η2:η2-peroxo)dicopper(II). The Raman band position of the complex 2 (829 cm-1) is not consistent with that of the model complex with organic ligands (750 cm-1), but this fact does not exclude the possible assignment of the complex 2 to the bent-type (µ-η2:η2-peroxo)dicopper(II) because the band position at 829 cm-1 is rather close to those of O22- adsorbed on metal oxides (884 cm-1 for manganese oxide39 and 831-883 cm-1 for CeO240,41). The Raman result in Figure 6 indicates that the complex 2 (spectrum a, 829 cm-1) is first produced in the freshly prepared H2O2treated CuY, which is then converted to the complex 3 (spectra b and c, 829 cm-1) at 300 K. In our previous study, a copperoxygen complex characterized by the Cu-O-Cu distance of 3.24 Å, the Raman band at 829 cm-1, and the UV-vis band at 370 nm was observed in the H2O2-treated Cu-BEA zeolite, and we tentatively assigned it to the bent-type (µ-η2:η2-peroxo)dicopper(II) complex. A different active-site structure identified in the present study, especially complexes 1 and 3, may be due to a different Cu coordination environment, Cu content, and the steric constraints caused by a different zeolite framework. Taking the above structural model into account, the formation and the thermal reactions of copper-oxygen complexes in H2O2treated CuY are presented in Scheme 1. The reaction of H2O2 with isolated Cu(II) ions in Y zeolite yields the bis(µ-oxo)dicopper(II) (complex 1) and the O22-- or HOO--bridged dicopper(II) (complex 2), that is, the hydroperoxodicopper(II) or the bent-type (µ-η2:η2-peroxo)dicopper(II). The complex 2 is converted to superoxodicopper(II) (complex 3) at ambient temperature. At higher temperature, the thermal desorption of the bridged dioxygen species in complexes 2 and 3 occurs to yield two monomeric Cu(II) complexes and gas-phase O2, while the complex 1 remains unchanged. Photoreactivity of Dicopper(II)-Dioxygen Complexes. The photoreactivity of the copper-oxygen complex in Y zeolite toward cyclohexane was studied by Raman and in-situ UVvis experiments under photoirradiation (hν > 340 nm) in a flow of He or cyclohexane(13%)/He. Raman results (Figure 11) show that complexes 2 (829 cm-1) and 3 (1050 cm-1) are consumed by a photoreaction, and the rate of decrease was larger in the presence of the cyclohexane. A similar set of experiments using
19050 J. Phys. Chem. C, Vol. 111, No. 51, 2007 SCHEME 2: Proposed Mechanism of Cyclohexane Photooxidation with H2O2 by CuY Catalyst
Shimizu et al. Dr. K. Okumura of Tottori University for his help in in-situ XAFS experiment. References and Notes
in-situ UV-vis spectroscopy (Figure 9) gave basically the same results; the rate of decrease in the band assignable to complexes 2 and 3 (400 nm) was larger in the presence of the cyclohexane. These results indicate that complexes 2 and 3 are decomposed under photoirradiation (hν > 340 nm), and the rate of decomposition is higher in the presence of cyclohexane. It follows that active oxygen species on complexes 2 and 3, that is, O22- or HOO- and O2- can be consumed by the reaction with cyclohexane under photoirradiation (hν > 340 nm). The wavelength used for the cyclohexane photooxidation (340 nm) is included in the wavelength range of the LMCT band of complexes 2 and 3 (Figure 7), suggesting the involvement of the photoexcited complex in the cyclohexane oxidation. Although we have no experimental evidence on the photoreactivity of bis(µ-oxo)dicopper(II) (complex 1), we cannot exclude a possible involvement of this species in the catalytic cycle. A proposed mechanism of cyclohexane photooxidation with H2O2 by CuY is shown in Scheme 2. The catalytic cycle includes the following steps: (1) the reaction of Cu(II) ions with H2O2 to yield the dicopper(II)-dioxygen complexes 2 and 3, (2) the reaction of the photoexcited complexes with cyclohexane to yield Cu(II) ions and cyclohexyl hydroperoxide, which can be an intermediate in cyclohexanol formation. Due to the strong σ donation to the coppers, the bridging dioxygen species can be slightly electrophilic in nature. Upon irradiation, the ligand-toCu(II) electron-transfer process could yield more electrophilic dioxygen species which is favorable for electrophilic attack on a C-H bond of cyclohexane. Conclusion The photooxidation of cyclohexane with hydrogen peroxide is catalytically promoted by copper(II)-exchanged Y zeolite (CuY), resulting in the selective formation of cyclohexanol and cyclohexyl hydroperoxide. The reaction of H2O2 with isolated Cu(II) ions in Y zeolite yields the bis(µ-oxo)dicopper(II) (complex 1) and the O22-- or HOO--bridged dicopper(II) (complex 2), that is, the bent-type (µ-η2:η2-peroxo)dicopper(II) or the hydroperoxodicopper(II). A part of the complex 2 is converted to superoxodicopper(II) (complex 3) at room temperature. At higher temperature, the thermal desorption of the bridged dioxygen species in complexes 2 and 3 occurs to yield two monomeric Cu(II) complexes and O2, while the complex 1 remains unchanged up to 473 K. Under photoirradiation the dioxygen species in complexes 2 and 3 can be involved in the important step of the cyclohexane oxidation. The results demonstrate a possibility of enzyme-inspired heterogeneous catalysis for green catalytic oxidation of alkane under a mild condition. Acknowledgment. The X-ray absorption experiment was performed with the approval of the Japan Synchrotron Radiation Research Institute (Proposal No. 2006A1040). The authors thank
(1) Solomon, E. I.; Chen, P.; Metz, M.; Lee, S.; Palmer, A. E. Angew. Chem., Int. Ed. 2001, 40, 4571. (2) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. ReV. 2004, 104, 1013. (3) Pidcock, E.; Obias, H. V.; Abe, M.; Liang, H.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 1999, 121, 1299. (4) Cole, A. P.; Root, D. E.; Mukherjee, P.; Solomon, E. I.; Stack, T. D. P. Science 1996, 273, 1848. (5) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Wang, X.; Young, V. G., Jr.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1996, 118, 11555. (6) Battaini, G.; Monzani, E.; Perotti, A.; Para, C.; Casella, L.; Santagostini, L.; Gullotti, M.; Dillinger, R.; Na¨ther, C.; Tuczek, F. J. Am. Chem. Soc. 2003, 125, 4185-4198. (7) Itoh, K.; Hayashi, H.; Furutachi, H.; Matsumoto, T.; Nagatomo, S.; Tosha, T.; Terada, S.; Fujinami, S.; Suzuki, M.; Kitagawa, T. J. Am. Chem. Soc. 2005, 127, 5212. (8) Kodera, M.; Tachi, Y.; Hirota, S.; Katayama, K.; Shimakoshi, H.; Kano, K.; Fujisawa, K.; Moro-oka, Y.; Naruta, Y.; Kitagawa, T. Chem. Lett. 1998, 389. (9) Mahroof-Tahir, M.; Karlin, K. D. J. Am. Chem. Soc. 1992, 114, 7599. (10) Wada, A.; Harata, M.; Hasegawa, K.; Jitsukawa, K.; Masuda, H.; Mukai, M.; Kitagawa, T.; Einaga, H. Angew. Chem., Int. Ed. 1998, 37, 798. (11) Chen, P.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 10177. (12) Weitzer, M.; Schindler, S.; Brehm, G.; Hoemann, E.; Jung, B.; Kaderli, S.; Zuberbuhler, A. D. Inorg. Chem. 2003, 42, 1800. (13) Chan, S. I.; Chen, K. H.-C.; Yu, S. S.-F.; Chen, C.-Li.; Kuo, S. S.-J. Biochemistry 2004, 43, 4421. (14) Kirillov, A. M.; Kopylovich, M. N.; Kirillova, M. V.; Haukka, M.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L. Angew. Chem., Int. Ed. 2005, 44, 4345. (15) Kirillov, A. M.; Kopylovich, M. N.; Kirillova, M. V.; Karabach, E. Y.; Haukka, M.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L. AdV. Synth. Catal. 2006, 348, 159. (16) Kervinen, K.; Bruijnincx, P. C. A.; Beale, A. M.; Mesu, J. G.; van Koten, G.; Klein Gebbink, R. J. M.; Weckhuysen, B. M. J. Am. Chem. Soc. 2006, 128, 3208. (17) Grommen, R.; Manikandan, P.; Gao, Y.; Shane, T.; Shane, J. J.; Schoonheydt, R. A.; Weckhuysen, B. M.; Goldfarb, D. J. Am. Chem. Soc. 2000, 122, 11488. (18) Mesu, J. G.; Visser, T.; Beale, A. M.; Soulimani, F.; Weckhuysen, B. M. Chem. Eur. J. 2006, 12, 7167. (19) Baute, D.; Arieli, D.; Neese, F.; Zimmermann, H.; Weckhuysen, B. M.; Goldfarb, D. J. Am. Chem. Soc. 2004, 126, 11733. (20) Chavan, S. S. D.; Ratnasamy, P. J. Catal. 2000, 192, 286. (21) Srinisivas, D.; Sivasanker, S. Catal. SurV. Asia 2003, 7, 121. (22) Ganesan, R.; Viswanathan, B. J. Phys. Chem. B 2004, 108, 7102. (23) Lee, C.-H.; Wong, S.-H.; Lin, T.-S.; Mou, C.-Y. J. Phys. Chem. B 2005, 109, 775. (24) Wong, S.-T.; Lee, C.-H.; Lin, T.-S.; Mou, C.-Y. J. Catal. 2004, 228, 1. (25) Groothaert, M. H.; Smeets, P. J.; Sels, B. F.; Jacobs, P. A.; Schoonheydt, R. A. J. Am. Chem. Soc. 2005, 127, 1394. (26) Smeets, P. J.; Groothaert, M. H.; Schoonheydt, R. A. Catal. Today 2005, 110, 303. (27) Shimizu, K.; Maruyama, R.; Hatamachi, T.; Kodama, T. J. Phys. Chem. C 2007, 111, 6440. (28) Groothaert, M. H.; van Bokhoven, J. A.; Battiston, A. A.; Weckhuysen, B. M.; Schoonheydt, R. A. J. Am. Chem. Soc. 2003, 125, 7629. (29) Groothaert, M. H.; Lievens, K.; van Bokhoven, J. A.; Battiston, A. A.; Weckhuysen, B. M.; Pierloot, K.; Schoonheydt, R. A. Chem. Phys. Chem. 2003, 4, 626. (30) Schuchardt, U.; Cardoso, D.; Sercheli, R.; Pereira, R.; da Cruz, R. S.; Guerreiro, M. C.; Mandelli, D.; Spinaca’, E. V.; Pires, E. L. Appl. Catal., A: Gen. 2001, 211, 1. (31) Mizuno, N.; Nozaki, C.; Kiyoto, I.; Misono, M. J. Am. Chem. Soc. 1998, 120, 9267. (32) Spinace´, E. V.; Pastore, H. O.; Schuchardt, U. J. Catal. 1995, 157, 631. (33) Ebitani, K.; Ide, M.; Mitsudome, T.; Mizugaki, T.; Kaneda, K. Chem. Commun. 2002, 690. (34) Ishihara, T.; Ohura, Y.; Yoshida, S.; Hata, Y.; Nishiguchi, H.; Takita, Y. Appl. Catal., A 2005, 291, 215.
Dicopper(II)-Dioxygen Complexes in Y Zeolite (35) Okumura, K.; Yoshino, K.; Kato, K.; Niwa, M. J. Phys. Chem. B 2005, 109, 12380. (36) Shimizu, K.; Sugino, K.; Kato, K.; Yokota, S.; Okumura, K.; Satsuma, A. J. Phys. Chem. C 2007, 111, 1683. (37) For CuY-H-300, the inverse Fourier transform of the peak in a range of R ) 2.10-3.30 Å in Figure 4B gives the EXAFS oscillation due to Cu-M (M ) Cu or Al). It is known that the magnitude of an EXAFS envelope function is high in the region of relatively high k when the surrounding atoms are heavy atoms. For CuY-H-300, the maximum of the envelope is observed around 7.7 Å-1 (data not shown). This value is very close to the envelope for the Cu-Cu shell (R ) 1.38-2.85 Å) of Cu foil, but it is larger than that for the Cu-Al shell (R ) 1.93-2.67 Å) of CuAl2O4
J. Phys. Chem. C, Vol. 111, No. 51, 2007 19051 (around 6.1 Å-1). This result indicates that the Fourier filtered EXAFS oscillation due to Cu-M (R ) 2.10-3.30 Å) for CuY-H-300 is not due to Cu-Al contribution but due to Cu-Cu contribution. (38) Dissanayake, D. P.; Lunsford, J. H. J. Catal. 2003, 214, 113. (39) Li, W.; Oyama, S. T. J. Am. Chem. Soc. 1998, 120, 9047. (40) Pushkarev, V. V.; Kovalchuk, V. I.; d’Itri, J. L. J. Phys. Chem. B 2004, 108, 5341. (41) Li, C.; Domen, K.; Maruya, K.; Onishi, T. J. Am. Chem. Soc. 1989, 111, 7683. (42) Sass, C. E.; Kevan, L. J. Phys. Chem. 1988, 92, 5192. (43) Chavan, S.; Srinivas, D.; Ratnasamy, P. J. Catal. 2000, 192, 286.
