Articles
DOI: 10.1002/cphc.201500118
Suzuki Coupling Reactions Catalyzed by PdO Dispersed on Dealuminated Y Zeolite in Air under Ambient Conditions Kazu Okumura,*[a] Takumi Mushiake,[b] Yu Matsui,[c] and Akira Ishii[c] Suzuki coupling reactions are performed using PdO loaded on dealuminated Y (USY) zeolite. The reaction between bromobenzene and phenylboronic acid is complete in 15 min at room temperature in air, with a turnover number of 1300. The reaction can be repeated at least five times by using 1 wt % Pd. Inductively coupled plasma analysis does not reveal the dissolution of Pd from products, even if the reaction is repeated up to four times. Pd K-edge extended X-ray absorption fine
structure analysis reveals the presence of molecular-like PdO and a mixture of Pd0–PdO before and after the reaction, respectively. This is probably because Pd stabilized by Al sites is present at the II sites of the Y-type zeolite, as estimated using first-principles calculations. Conversely, Pd species change to PdO clusters after repeated reactions in air using the thermally treated sample.
1. Introduction Suzuki coupling reactions are important in the production of pharmaceuticals, organic electroluminescent devices, and liquid crystals.[1] Numerous Pd complexes have been developed for use in these reactions. However, there are difficulties in the synthesis and purification of these complexes. Another class of catalysts for these coupling reactions—heterogeneous catalysts—can be easily prepared and readily separated from byproducts. The major problem associated with heterogeneous catalysts is low activity, probably because they have an inhomogeneous structure with low dispersion of active sites. Nevertheless, high activity is to be expected if highly dispersed active sites with homogeneous structure are fabricated onto supports. Indeed, quantum dots of Cu and Cu/Pd mixtures with narrow size distributions were found to be good catalysts for Suzuki cross-coupling.[2] In this regard, zeolites have a large surface area and uniform micropores, which can accommodate dispersed metal clusters, leading to a high surface-to-volume ratio. Thus, using zeolites as supports for Pd is expected to lead to high catalytic activity. In fact, we have demonstrated [a] Prof. K. Okumura Department of Applied Chemistry Faculty of Engineering, Kogakuin University 2665-1 Nakano-machi, Hachioji City, Tokyo (Japan) E-mail: [email protected] [b] T. Mushiake Department of Chemistry and Biotechnology Graduate School of Engineering Tottori University, 4-101 Koyama-cho Minami Tottori City, Tottori (Japan) [c] Y. Matsui, Prof. A. Ishii Department of Applied Mathematics and Physics Graduate School of Engineering Tottori University, 4-101 Koyama-cho Minami Tottori City, Tottori (Japan) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201500118.
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a turnover number (TON) of 107 when using Pd/USY zeolite as a catalyst for the reaction of bromobenzene derivatives.[3] In addition, Djakovitch and Koehler reported that Pd–NaY zeolites exhibit a high activity in the Heck reaction of aryl bromides with olefins.[4] Choi et al. have also demonstrated that PdII-exchanged mesoporous NaA zeolite was an active catalyst for the coupling reactions of bulky aryls.[5] Herein, we report the high activity exhibited by PdO loaded on dealuminated Y (ultra-stable Y, USY) zeolite for Suzuki reactions at room temperature in air. The reactions proceeded in H2O with or without EtOH. The optimal conditions for the Pd catalysts are such that the reaction can be performed in air, at room temperature, and using aqueous solvents. Therefore, significant attention has been focused on using these reaction conditions.[6] In fact, Sajiki et al. have reported that Pd/C is active in the Suzuki coupling reaction at room temperature,[7] although extended reaction times are required for completion. Recently, Liu et al. have reported that the reaction using Pd/C can be accelerated by using a mixed solvent of H2O and EtOH.[8] In addition, several research groups have reported that PdO, PdO hydrate, and Pd0 nanoparticles generated by the reduction of PdO act as catalysts for the Suzuki coupling reaction at room temperature.[9] PdO has been applied to other types of coupling reactions.[10] Moreover, Joucla et al. synthesized (E)-stilbenes from chloroiodobenzenes using sequential one-pot Suzuki–Miyaura and Heck reactions using Pd/C or Pd/SiO2.[11] Under such conditions, a significant improvement might be achieved using highly dispersed PdO as the catalyst. In addition to obtaining highly active catalysts, the precise characterization of Pd is possible by using a catalyst with an active center and a homogeneous structure. Changes in this structure can reveal changes to the nature of the active center. Hence, we have used USY zeolite as a support for Pd. As mentioned above, we have observed that the Pd/USY catalyst exhibits high catalytic activity at 383 K with o-xylene as the solvent.[3] Therefore, PdO/USY was used as
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Articles the catalyst for the Suzuki coupling reaction in this study. The Pd species present before and after the reaction were characterized in detail using Pd K-edge X-ray absorption fine structure (XAFS) analysis coupled with transmission electron microscopy (TEM). Moreover, the location of Pd0—generated during reaction with the substrate—in the pores of USY zeolites was estimated using first-principles calculations, assuming that the Pd0 atom was situated on the wall of the zeolite framework.
Table 1. Scope of the Suzuki coupling reaction using PdO/USY.[a]
R
Solvent
Time [h]
Yield [%]
TON
H H H NO2 NO2 CN OH
EtOH/H2O EtOH/H2O H2O EtOH/H2O H2O EtOH/H2O EtOH/H2O
3 0.2 20 1.5 5 0.5 1
82 99 96 99 89 99 87
11 000 1300 130 2500 1000 1300 110
[a] Typical reaction conditions: bromobenzene derivative (0.5 mmol), phenylboronic acid (0.75 mmol), K2CO3 (1.0 mmol), EtOH/H2O (1:1 v/v) or H2O as a solvent, and PdO/USY catalyst; temperature = 298 K. The weight of catalyst used was changed, the amounts of other reagents were constant.
2. Results and Discussion 2.1. Suzuki Coupling Reaction Catalyzed by PdO/USY Suzuki couplings were performed using Pd loaded on different supports in air at room temperature. Figure 1 a shows the time-course for the conversion of bromobenzene in the reaction between bromobenzene and phenylboronic acid. PdO/ USY exhibited the highest catalytic activity among the tested
Table 1 lists the scope of the reaction between bromobenzene derivatives and phenylboronic acid over PdO/USY. PdO/ USY was active in the reactions using different bromobenzene derivatives such as 4-bromophenol and bromobenzonitrile. Although the reaction proceeded in H2O as the solvent (without the addition of ethanol), a significantly longer reaction time of 20 h was required. Such a long duration may be attributed to the low solubility of bromobenzene derivatives in water, making the reaction sluggish compared to the reaction in a mixture of H2O and ethanol. Table 2 lists the catalytic performance of PdO/USY for the reaction of heterocyclic compounds, including bromopyridine and bromothiophene derivatives. In these cases, the reaction did not proceed at room temperature. However, when the reaction was performed at 353 K, the coupling products were obtained in satisfactory yields. 2.2. Reusability of the Catalyst and Amount of Dissolved Pd
The reusability of a supported catalyst is an essential consideration because of economic and environmental reasons.[12] To investigate the reusability of the catFigure 1. Time-course of the conversion of bromobenzene over a) Pd loaded on different alyst, the reaction was repeated over PdO/USY supports and b) Pd/USY treated under different conditions prior to use. (1.0 mol % Pd). The conversion of bromobenzene [Pd] = 0.075 mol %; temperature = 298 K. reached 99 % in less than 30 min even after repeating the reaction five times (Figure 2 a), albeit the reaction catalysts: the reaction was completed in 15 min, achieving rate decreased with each repetition. The initial turna TON of 1300. In contrast, a significantly lower activity was over frequencies observed at the first, third and fifth runs were achieved over Pd loaded on NaY and activated carbon under less than 98, 18 and 7 min¢1 respectively. For measuring the 0 the same reaction conditions. Although Pd /USY prepared by extent of Pd leaching, the filtered solution and products were dissolved in acetone and water and evaporated to dryness. reduction with H2 at 473 K was also active in the reaction, the The residual solid obtained after thermal treatment in an oven time taken to complete the reaction was significantly longer was dissolved in aqua regia before inductively coupled plasma than that using PdO/USY (Figure 1 b). The reaction over PdO/ (ICP) analysis. The detection limit of Pd was estimated to be USY proceeded without stirring, that is, the solution was trans25 ppb. However, ICP analysis did not reveal the dissolution of parent at the beginning; however, after 15 min, the product Pd, at least for reactions repeated up to four times (Figure 3). was observed as a white solid (Figure S1 in the Supporting InTo confirm if Pd dissolution occurred at a significantly lower formation). The product was easily isolated in a yield of 94 % concentration in the first reaction, the Suzuki coupling was after the addition of water and filtration. performed at a scale 25 times greater than previously perChemPhysChem 2015, 16, 1719 – 1726
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Articles Table 2. Scope of the Suzuki coupling between heterocyclic halides and phenylboronic acid using PdO/USY.[a] Halide
Product
Time [h]
Yield [%]
TON
2
93
1200
6
73
970
2
87
1100
1
99
1300
[a] Typical reaction conditions: heterocyclic halide (0.5 mmol), phenylboronic acid (0.75 mmol), K2CO3 (1.0 mmol), EtOH/H2O (1:1 v/v), and PdO/USY catalyst (Pd loading: 0.4 wt%, 0.010 g); temperature = 353 K.
agreement with a previous report on Pd/C[7] and poly(N-vinyl-2-pyrrolidone)-stabilized Pd.[13] The authors found that Pd0 nanoparticles are stable to metal leaching throughout the reaction. Moreover, it has been reported that the Suzuki coupling reaction between bromobenzene and phenylboronic acid in EtOH/H2O is solely facilitated by nanosized Pd clusters, which were stable to metal leaching.[10] Despite these facts, the possibility for the leached Pd to be the active species could not be ruled out in view of the many reports supporting such a role.[14] For instance, Thathagar et al. demonstrated that the Pd0 atoms or PdII ions leached into solutions were the Figure 2. Repeated reaction between bromobenzene and phenylboronic acid catalyzed by Pd/USY. a) Runs 1–5 and b) runs 6–10. Each reaction was started at the time points inactive species in both Heck and Suzuki coupling reacdicated by arrows. The catalyst was thermally treated in air at 773 K after using it five tions by using a membrane reactor.[15] Furthermore, times. [Pd] = 1.0 mol %; temperature = 298 K. in this case as mentioned above, the reaction proceeded without stirring. Therefore, another possibility might be that the leached Pd returned to the support after the completion of the reaction, as proposed by Djakovitch et al. (deposition mechanism),[16] such that Pd was not detected in the filtered solution and products. The catalyst used for the reactions that were repeated five times was thermally treated in air at 773 K and was used in subsequent reactions. After repeating the reaction six to 10 times, a small amount of Pd (less than 1 % of the Pd present in the catalyst) was detected. The amount of dissolved Pd tended to increase with the increase in the number of repeated reactions (Figure 3). A similar tendency was observed for 1.0 wt % Pd-loaded USY zeolite (Figure S2). 2.3. Characterization of the Pd Species Loaded on Zeolites
Figure 3. Amount of dissolved Pd obtained after repeating the reaction between bromobenzene and phenylboronic acid using 0.4 wt % PdO/USY. The catalyst was thermally treated in air after use in the fifth run.
formed, while the amount of PdO/USY remained constant. For this purpose, the solution was concentrated to 5 mL before ICP analysis. Although an excess amount of catalyst was used, dissolved Pd was not observed in the filtrate together with coupling products. This suggested that the reaction proceeds under heterogeneous conditions. This observation was in ChemPhysChem 2015, 16, 1719 – 1726
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Pd species loaded on USY zeolite before and after the repeated reactions were characterized using Pd K-edge XAFS measurements at the Japan Synchrotron Radiation Research Institute (Figure 4). Initially, the Pd¢O peak was observed at 0.16 nm in the spectrum of the as-prepared 0.4 wt % Pd/USY, the intensity of which was consistent with that of bulk PdO (Figure 4 a), indicating that the valence of Pd was 2 + (calcined). Doublet peaks of Pd¢O¢Pd were observed at 0.22– 0.35 nm in the spectrum of bulk PdO; these peaks were absent
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Figure 4. a) Pd K-edge EXAFS Fourier transforms and b) XANES of Pd/USY, measured before and after the Suzuki coupling reaction between bromobenzene and phenylboronic acid; as-prepared and runs 1–5.
proximately 2, suggesting the formation of very fine Pd clusters. Figure 5 a shows the Pd K-edge extended XAFS (EXAFS) of Pd/USY treated in air after five reactions in succession. Table S2 lists the data obtained from curve-fitting analysis. In this case, doublet Pd¢O¢Pd peaks were clearly observed between 0.22 and 0.35 nm, suggesting that the agglomeration of Pd progresses during the reaction, although the intensity of the Pd¢O¢Pd peaks was lower than that of bulk PdO. Unlike in reactions 1–5, this feature did not change on further use of the catalyst for the repeated reaction: Pd on USY zeolite maintained an oxidation state of 2 + , even after its use for the Suzuki coupling reaction. In agreement with this result, the XANES result of reactions 6–10 was close to bulk PdO, as shown in Figure 5 b.The above EXAFS data revealed that there are two types of Pd. The first
in the spectrum of PdO/USY, indicating the formation of molecular-like PdO on the USY support. This result was supported by the curve-fitting analysis, by which the bond distance and coordination number (CN) of the Pd¢O bond agreed well with those of bulk PdO (Table S1, run 0; Table S2, PdO). Conversely, in the spectrum of 1.0 wt % Pd/USY, small Pd¢O¢Pd peaks were observed at 0.28 and 0.32 nm (Figure S3), suggesting the formation of slightly agglomerated PdO clusters; these clusters can be observed by the comparison of this spectrum with that of bulk PdO. The CNs of the Pd¢O¢Pd peaks observed at 0.28 and 0.32 nm were 1.0 and 2.1, respectively, which were significantly lower than those of bulk PdO (4 and 8, respectively). The small CN of the Pd¢O¢Pd bonds of PdO suggested that the degree of agglomeration of Figure 5. a) Pd K-edge EXAFS Fourier transforms and b) XANES of Pd/USY measured before and after the Suzuki coupling reaction between bromobenzene and phenylboronPdO in 1 wt % PdO/USY is small and that cluster-like ic acid; re-calcined and runs 6–10. The catalyst was thermally treated in air at 773 K after PdO is formed. use in the fifth run. The Pd K-edge XAFS results of PdO/USY used for reactions 1–5 are included in Figure 4 a. The intensity type was a mixture of Pd0 and PdO, whereas the second type of the Pd¢O bond was lower than that of as-prepared PdO/ 0 USY, suggesting that Pd is partially reduced to form Pd . Alterwere PdO clusters, which were observed in the samples calnatively, a small peak appeared at 0.25 nm, which was unamcined in air at 773 K. Consistent with the Pd K-edge XAFS analbiguously assigned to the Pd¢Pd bond of Pd metal by comparysis, the color of the 0.4 wt % Pd/USY catalyst after one to five ison with the spectrum of Pd foil. In agreement with this consecutive reactions and six to 10 was gray and light brown, result, the data obtained from X-ray absorption near-edge characteristic of the metallic Pd and PdO, respectively (Figspectroscopy (XANES) measurements of the used catalyst for ure S5). Generally, the Suzuki coupling reaction is proposed to reactions 1–5 were in between those of PdO and Pd foil (Figproceed with Pd0.[17] However, in contrast, the species collected ure 4 b). The features of the spectra did not change with reafter six to 10 consecutive reactions were characterized as peated use of the catalyst. Although the intensity of the Pd¢ Pd2 + , as clearly evidenced by EXAFS analysis. To gain insight Pd bond of Pd0 slightly increased in comparison with that of into the formation of the partially reduced Pd species, XAFS measurements using PdO/USY treated with different combinathe Pd¢Pd bond of the 0.4 wt % sample, a similar tendency tions of substrates were conducted. For this purpose, various was observed when 1.0 wt % Pd/USY was used for repeated resubstrate combinations were charged with PdO/USY, followed actions: partially reduced Pd was observed (Figure S4). by catalyst separation by filtration and washing with a mixture Table S1 includes the data obtained from curve-fitting analysis. of H2O and EtOH (1:1). Figure 6 shows the radial distribution The CN of the Pd¢O bond was approximately 2, which was half that of PdO, suggesting that the average valence is + 1. function of Pd K-edge EXAFS of the collected samples. When On the other hand, the CN of Pd¢Pd was determined to be apbromobenzene was charged with PdO/USY, only slight changes ChemPhysChem 2015, 16, 1719 – 1726
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Figure 7. TEM image of 1 wt % Pd/USY after eight repeated reactions.
Figure 6. Radial distribution function of Pd K-edge EXAFS for Pd/USY, bulk PdO and Pd foil . a) As-prepared sample, and measured after the addition of b) bromobenzene, c) bromobenzene and phenylboronic acid, d) bromobenzene, phenylboronic acid and K2CO3. Temperature = 298 K; time = 0.5 h.
were observed in the EXAFS data; the same observation was made when a mixture of bromobenzene and phenylboronic acid was charged with PdO/USY. By contrast, when a mixture of bromobenzene, phenylboronic acid, and K2CO3 was charged with PdO/USY, the intensity of the Pd¢O bond markedly decreased, whereas that of the Pd¢Pd bond of Pd0 increased. This observation suggested that the reduction of PdO and simultaneous agglomeration of the resultant Pd0 progressed when a mixture of three substrates was added to PdO/USY. The formation mechanism of Pd0 was not clearly revealed at this stage. However, this suggests that Pd0 is generated because of the reaction with the substrate to form products (biphenyl) on the highly dispersed PdO. Another possibility is that NH4 + present on the USY reduced PdO to give Pd0 in a similar manner to the generation of Pd0 during the calcination of [Pd(NH3)4]2 + in zeolites.[18] Generally, the Suzuki coupling reaction is initiated by the oxidative addition of halogenated aryls on Pd0.[19] Therefore, the initial step probably involves the partial reduction of PdO with bromobenzene in situ, considering that a small Pd¢Pd bond of Pd0 was observed after the addition of bromobenzene at 0.25 nm (phase-shift uncorrected). It is likely that, as the reaction progresses, further reduction of PdO as well as partial growth of Pd occur when K2CO3 is added to the mixture of bromobenzene and phenylboronic acid. 2.4. TEM Observations Figure 7 shows the TEM image of PdO/USY after repeated use (eight times) in the reaction between bromobenzene and phenylboronic acid. The Pd loading in this sample was 1 wt %. The observed aggregate shown was only the zeolite support, and no aggregated Pd particles were observed in the whole region. This suggested that the size of Pd particles is at least ChemPhysChem 2015, 16, 1719 – 1726
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smaller than 1 nm, which was in agreement with EXAFS results: formation of PdO clusters was observed after repeated reaction. In agreement with this result, diffractions caused by metal Pd and PdO were not observed in X-ray diffraction patterns of the repeat samples. 2.5. First-Principles Calculations To determine the possible location of Pd0 in USY zeolite—observed after the reaction was repeated up to five times—in 0.4 wt % samples, first-principles calculations were conducted. The influence of Al (acid sites) on the stabilization of Pd was also examined. It was hypothesized that Pd located in the supercage of USY zeolites might be active species in the Suzuki coupling reaction, whereas Pd in the hexagonal prism and sodalite cage did not participate in the reaction considering that the reactants were bulky compared to the pore size of these structures. Therefore, the calculation was conducted on Pd located at sites II and III, which are situated on the six- and fourmembered rings facing the supercage, respectively. In addition, the location of the proton was considered. We constructed 19 models in which one or two Al atoms were fully substituted with Si (Figures S6 and S7). Figure 8 shows the representative structure of Pd located at sites II and III. First, the adsorption energy of Pd on site II was higher than that on site III; both sites II and III had two Al atoms in the ring. Second, the adsorption energy tended to increase, accompanied by the substitution of Si with Al in the zeolite framework. Considering this tendency, the most stable structure was Pd located at sites II having two Al atoms in the ring. Moreover, calculation of the adsorption energy of Pd in the structures such as that shown in Figure 8 b was undertaken in which distribution of protons were different (Figure S7 p–s). As a result, the average adsorption energy of these structures was determined to be 2.46 eV. Another important feature of the interaction between Pd and zeolite is the electron transfer between the two. To investigate electron transfer, a Bader charge analysis was conducted.[20] Figure 9 shows the charge distribution of Pd and zeolite, calculated based on the model structure shown in Figure 8 b. The adsorption energy of this Pd was the lowest among the calcu-
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Articles Finally, the local density of states (LDOS) for Pd in site II was calculated. Figure 10 a and b show the LDOS of Pd on the six-membered rings composed of six Si atoms and four Si plus two Al atoms corresponding to the structure shown in Figure 8 b, respectively. A comparison of Figures 10 a and b shows that the latter structure exhibited an increase in the LDOS in the d electron of Pd, suggesting that the interaction between Pd and the zeolite wall is enhanced by the substitution of Si with Al in the sixmembered ring of Y-type zeolite. This interaction might contribute to the stabilization of Pd to avoid the sintering of Pd during the reaction, thus extending the lifetime of the active Pd0 species in the Suzuki coupling reaction.
3. Conclusions
Figure 8. Representative model structure of Pd located at the II (a, b) and III (c, d) sites of Y-type zeolite. The values indicate the adsorption energy of Pd.
Well-dispersed PdO on USY zeolite was found to be the precursor to active species in the Suzuki coupling reaction between bromobenzene derivatives and phenylboronic acid in aqueous solutions at room temperature. We found that the valence and structure of Pd on the Y-type zeolite support was significantly dependent on the treatment in the repeated reaction, that is, a mixture of Pd0 and PdO was generated after the catalytic reaction. Dissolution of Pd was not detected after the catalyst was used. In the Pd/USY catalyst, Pd0 species were suggested to be located at the II site (six-membered rings) of the Y-type zeolite based on first-principles calculations. Pd0 was
Figure 9. Charge distribution calculated on the basis of the model structure shown in Figure 8 b.
lated model structures. Bader charge analysis conducted on this structure revealed that Pd is slightly negatively charged (¢0.0178 eV), which might promote the reaction, as envisaged for the coordinated Pd complexes with strongly s-donating ligands such as trialkylphosphines.[21] ChemPhysChem 2015, 16, 1719 – 1726
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Figure 10. LDOS for Pd on the six-membered rings composed of a) six Si atoms and b) four Si and two Al atoms corresponding to the model structure shown in Figure 8 b.
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Articles further stabilized by the presence of Al in the six-membered ring. This feature significantly changed after thermal treatment in air, followed by the use of the catalyst in subsequent reactions. Cluster-like PdO was observed in the air-calcined samples even after the catalyst was used. This suggested that PdO itself probably acts as the active species, unlike in typical cases reported.
Experimental and Calculations Sample Preparation and Catalytic Reaction The NH4 + form of USY zeolite (HSZ-341NHA, Tosoh, Tokyo, Japan) was used as the support for Pd. Pd (0.4 wt %) was introduced into USY zeolite by an ion-exchange method using an aqueous solution (3.8 Õ 10-4 M) of Pd(NH3)4Cl2 (Sigma–Aldrich) for 4 h at 343 K. Pd was loaded on NaY zeolite (HSZ-320NAA, Tosoh), HY zeolite and activated carbon (Wako Co.) for comparison purposes. HY zeolite was prepared by the ion-exchange reaction of NaY (HSZ-320NAA, Tosoh) with aq. NH4NO3 (1 m) three times at 353 K, followed by thermal treatment in air at 773 K. In typical conditions, bromobenzene (0.5 mmol), phenylboronic acid (0.75 mmol), K2CO3 (1.0 mmol), EtOH/H2O (1:1 v/v) or H2O, and PdO/USY catalyst (0.075 or 1 mol %) were added to a flask, and the solution was vigorously stirred using a magnetic stirrer. The reaction was performed at room temperature (298 K) under atmospheric conditions. The products were analyzed using a gas chromatograph (GC-2010, Shimadzu) equipped with a capillary column (InertCap 1) and a flame ionization detector. Tridecane was used as the internal standard. To measure the amount of leached Pd, the solution was separated by filtration. The solid was washed repeatedly with water and acetone. These washings were combined with the filtrate and the solution evaporated to dryness. The residue was thermally treated in air at 773 K. Then, the remaining solid was dissolved in aqua regia (ca. 5 mL). The concentration of Pd in the obtained solution was measured by ICP analysis.
Pd K-edge XAFS Measurements and Data Analysis Pd K-edge XAFS data were obtained from synchrotron radiation experiments performed at the BL01B1 station with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, SPring8; proposal No. 2012B1121, 2013B1067). Pd K-edge XAFS data were collected in the quick mode; a Si(111) monochromator was continuously moved from 4.758 to 4.408 for 5 min. The beam size was 5 mm (horizontal) Õ 0.8 mm (vertical) at the sample position. The catalyst collected after the reaction was thoroughly rinsed with EtOH and H2O alternately. The sample was dried in an oven at 323 K. Then, it was pressed into a wafer prior to XAFS measurement. In EXAFS analysis, oscillations were extracted by a spline smoothing method.[22] The Fourier transformation of the k3-weighted EXAFS oscillations and k3c(k) from the k space to the r space was conducted over the range of 25–130 nm¢1. The inversely Fourier-filtered data were analyzed in the k range between 25 and 130 nm¢1 by a typical curve-fitting method. In the curve-fitting analysis, the empirical phase-shift and amplitude functions of Pd¢ O and Pd¢Pd were extracted from the data obtained for PdO and Pd foils, respectively. EXAFS data were analyzed using the REX program (Rigaku, Tokyo, Japan). ChemPhysChem 2015, 16, 1719 – 1726
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TEM Measurements The TEM image was acquired using a JEOL JEM2010 microscope with an acceleration voltage of 200 kV. The specimen was prepared by the crushing method, and the TEM observations were carried out at room temperature.
First-principles Calculations In this study, all calculations were conducted using a first-principles calculation technique based on density functional theory within a local density approximation by using the Perdew and Zunger parameterization[23] as the term-exchange correlation with a cutoff energy of 500 eV. We used the Vienna ab initio simulation package, a first-principle calculation code with high precision using the projector augmented wave method. For the sampling of the Brillouin zone, a 4 Õ 4 Õ 4 Monkhorst–Pack grid was used,[24] which was obtained by the convergence results from energy versus k-points. Figure 11 shows the unit cell of Y-type zeolite used in this calcula-
Figure 11. The unit cell of Y-type zeolite used for first-principles calculations.
tion. The lattice constant was 1.7343 nm. The calculation was performed under periodic boundary conditions. Pd was introduced into two types of ion-exchange sites (II and III), which are adjacent to the supercage. Si atoms were partially substituted by Al atoms to reveal the influence of H + on the stabilization of Pd. The location of H + generated by the substitution of Si with Al was also considered in the calculation. The adsorption energy Ebind for the adsorbed atom on the surface was defined as [Eq. (1)]: E ¼ E USY þE Pd ¢E total
ð1Þ
where E is the total energy of the relaxed sulfur-surface system, and Etotal, EUSY and EPd represent the total energy of the relaxed bare surface, energy of the zeolite and energy of Pd, respectively. Using this relationship, binding energy is positive if the total energy decreases when the Pd atom is brought from infinite distance and placed onto the surface.
Acknowledgements This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 24560946, 2012-2015 and (A), 70183001, 2011-2015.
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[11] L. Joucla, G. Cusati, C. Pinel, L. Djakovitch, Adv. Synth. Catal. 2010, 352, 1993 – 2001. [12] R. N. Baig, R. S. Varma, Chem. Commun. 2013, 49, 752 – 770. [13] A. F. Lee, P. J. Ellis, I. J. Fairlamb, K. Wilson, Dalton Trans. 2010, 39, 10473 – 10482. [14] M. B. Thathagar, J. E. ten Elshof, G. Rothenberg, Angew. Chem. Int. Ed. 2006, 45, 2886 – 2890; Angew. Chem. 2006, 118, 2952 – 2956; J. G. De Vries, Dalton Trans. 2006, 421 – 429. [15] A. V. Gaikwad, A. Holuigue, M. B. Thathagar, J. E. ten Elshof, G. Rothenberg, Chem. Eur. J. 2007, 13, 6908 – 6913. [16] L. Djakovitch, K. Kçhler, J. G. de Vries in Nanoparticles and Catalysis (Ed.: D. Astruc), Wiley-VCH, Weinheim, 2008, pp. 303 – 348. [17] C. Amatore, A. Jutand, Acc. Chem. Res. 2000, 33, 314 – 321. [18] W. Sachtler, Catal. Today 1992, 15, 419 – 429. [19] N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457 – 2483. [20] G. Henkelman, A. Arnaldsson, H. Jûnsson, Comput. Mater. Sci. 2006, 36, 354 – 360. [21] E. Galardon, S. Ramdeehul, J. M. Brown, A. Cowley, K. K. Hii, A. Jutand, Angew. Chem. Int. Ed. 2002, 41, 1760 – 1763; Angew. Chem. 2002, 114, 1838 – 1841. [22] D. Koningsberger, J. van Zon, H. van’t Blik, D. Sayers, J. Chem. Phys. 1985, 82, 5742 – 5754. [23] J. P. Perdew, A. Zunger, Phys. Rev. B 1981, 23, 5048. [24] H. J. Monkhorst, J. D. Pack, Phys. Rev. B 1971, 13, 5188.
Received: March 4, 2015 Published online on March 27, 2015
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DOI: 10.1002/cphc.201500118
Suzuki Coupling Reactions Catalyzed by PdO Dispersed on Dealuminated Y Zeolite in Air under Ambient Conditions Kazu Okumura,*[a] Takumi Mushiake,[b] Yu Matsui,[c] and Akira Ishii[c] Suzuki coupling reactions are performed using PdO loaded on dealuminated Y (USY) zeolite. The reaction between bromobenzene and phenylboronic acid is complete in 15 min at room temperature in air, with a turnover number of 1300. The reaction can be repeated at least five times by using 1 wt % Pd. Inductively coupled plasma analysis does not reveal the dissolution of Pd from products, even if the reaction is repeated up to four times. Pd K-edge extended X-ray absorption fine
structure analysis reveals the presence of molecular-like PdO and a mixture of Pd0–PdO before and after the reaction, respectively. This is probably because Pd stabilized by Al sites is present at the II sites of the Y-type zeolite, as estimated using first-principles calculations. Conversely, Pd species change to PdO clusters after repeated reactions in air using the thermally treated sample.
1. Introduction Suzuki coupling reactions are important in the production of pharmaceuticals, organic electroluminescent devices, and liquid crystals.[1] Numerous Pd complexes have been developed for use in these reactions. However, there are difficulties in the synthesis and purification of these complexes. Another class of catalysts for these coupling reactions—heterogeneous catalysts—can be easily prepared and readily separated from byproducts. The major problem associated with heterogeneous catalysts is low activity, probably because they have an inhomogeneous structure with low dispersion of active sites. Nevertheless, high activity is to be expected if highly dispersed active sites with homogeneous structure are fabricated onto supports. Indeed, quantum dots of Cu and Cu/Pd mixtures with narrow size distributions were found to be good catalysts for Suzuki cross-coupling.[2] In this regard, zeolites have a large surface area and uniform micropores, which can accommodate dispersed metal clusters, leading to a high surface-to-volume ratio. Thus, using zeolites as supports for Pd is expected to lead to high catalytic activity. In fact, we have demonstrated [a] Prof. K. Okumura Department of Applied Chemistry Faculty of Engineering, Kogakuin University 2665-1 Nakano-machi, Hachioji City, Tokyo (Japan) E-mail: [email protected] [b] T. Mushiake Department of Chemistry and Biotechnology Graduate School of Engineering Tottori University, 4-101 Koyama-cho Minami Tottori City, Tottori (Japan) [c] Y. Matsui, Prof. A. Ishii Department of Applied Mathematics and Physics Graduate School of Engineering Tottori University, 4-101 Koyama-cho Minami Tottori City, Tottori (Japan) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201500118.
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a turnover number (TON) of 107 when using Pd/USY zeolite as a catalyst for the reaction of bromobenzene derivatives.[3] In addition, Djakovitch and Koehler reported that Pd–NaY zeolites exhibit a high activity in the Heck reaction of aryl bromides with olefins.[4] Choi et al. have also demonstrated that PdII-exchanged mesoporous NaA zeolite was an active catalyst for the coupling reactions of bulky aryls.[5] Herein, we report the high activity exhibited by PdO loaded on dealuminated Y (ultra-stable Y, USY) zeolite for Suzuki reactions at room temperature in air. The reactions proceeded in H2O with or without EtOH. The optimal conditions for the Pd catalysts are such that the reaction can be performed in air, at room temperature, and using aqueous solvents. Therefore, significant attention has been focused on using these reaction conditions.[6] In fact, Sajiki et al. have reported that Pd/C is active in the Suzuki coupling reaction at room temperature,[7] although extended reaction times are required for completion. Recently, Liu et al. have reported that the reaction using Pd/C can be accelerated by using a mixed solvent of H2O and EtOH.[8] In addition, several research groups have reported that PdO, PdO hydrate, and Pd0 nanoparticles generated by the reduction of PdO act as catalysts for the Suzuki coupling reaction at room temperature.[9] PdO has been applied to other types of coupling reactions.[10] Moreover, Joucla et al. synthesized (E)-stilbenes from chloroiodobenzenes using sequential one-pot Suzuki–Miyaura and Heck reactions using Pd/C or Pd/SiO2.[11] Under such conditions, a significant improvement might be achieved using highly dispersed PdO as the catalyst. In addition to obtaining highly active catalysts, the precise characterization of Pd is possible by using a catalyst with an active center and a homogeneous structure. Changes in this structure can reveal changes to the nature of the active center. Hence, we have used USY zeolite as a support for Pd. As mentioned above, we have observed that the Pd/USY catalyst exhibits high catalytic activity at 383 K with o-xylene as the solvent.[3] Therefore, PdO/USY was used as
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Articles the catalyst for the Suzuki coupling reaction in this study. The Pd species present before and after the reaction were characterized in detail using Pd K-edge X-ray absorption fine structure (XAFS) analysis coupled with transmission electron microscopy (TEM). Moreover, the location of Pd0—generated during reaction with the substrate—in the pores of USY zeolites was estimated using first-principles calculations, assuming that the Pd0 atom was situated on the wall of the zeolite framework.
Table 1. Scope of the Suzuki coupling reaction using PdO/USY.[a]
R
Solvent
Time [h]
Yield [%]
TON
H H H NO2 NO2 CN OH
EtOH/H2O EtOH/H2O H2O EtOH/H2O H2O EtOH/H2O EtOH/H2O
3 0.2 20 1.5 5 0.5 1
82 99 96 99 89 99 87
11 000 1300 130 2500 1000 1300 110
[a] Typical reaction conditions: bromobenzene derivative (0.5 mmol), phenylboronic acid (0.75 mmol), K2CO3 (1.0 mmol), EtOH/H2O (1:1 v/v) or H2O as a solvent, and PdO/USY catalyst; temperature = 298 K. The weight of catalyst used was changed, the amounts of other reagents were constant.
2. Results and Discussion 2.1. Suzuki Coupling Reaction Catalyzed by PdO/USY Suzuki couplings were performed using Pd loaded on different supports in air at room temperature. Figure 1 a shows the time-course for the conversion of bromobenzene in the reaction between bromobenzene and phenylboronic acid. PdO/ USY exhibited the highest catalytic activity among the tested
Table 1 lists the scope of the reaction between bromobenzene derivatives and phenylboronic acid over PdO/USY. PdO/ USY was active in the reactions using different bromobenzene derivatives such as 4-bromophenol and bromobenzonitrile. Although the reaction proceeded in H2O as the solvent (without the addition of ethanol), a significantly longer reaction time of 20 h was required. Such a long duration may be attributed to the low solubility of bromobenzene derivatives in water, making the reaction sluggish compared to the reaction in a mixture of H2O and ethanol. Table 2 lists the catalytic performance of PdO/USY for the reaction of heterocyclic compounds, including bromopyridine and bromothiophene derivatives. In these cases, the reaction did not proceed at room temperature. However, when the reaction was performed at 353 K, the coupling products were obtained in satisfactory yields. 2.2. Reusability of the Catalyst and Amount of Dissolved Pd
The reusability of a supported catalyst is an essential consideration because of economic and environmental reasons.[12] To investigate the reusability of the catFigure 1. Time-course of the conversion of bromobenzene over a) Pd loaded on different alyst, the reaction was repeated over PdO/USY supports and b) Pd/USY treated under different conditions prior to use. (1.0 mol % Pd). The conversion of bromobenzene [Pd] = 0.075 mol %; temperature = 298 K. reached 99 % in less than 30 min even after repeating the reaction five times (Figure 2 a), albeit the reaction catalysts: the reaction was completed in 15 min, achieving rate decreased with each repetition. The initial turna TON of 1300. In contrast, a significantly lower activity was over frequencies observed at the first, third and fifth runs were achieved over Pd loaded on NaY and activated carbon under less than 98, 18 and 7 min¢1 respectively. For measuring the 0 the same reaction conditions. Although Pd /USY prepared by extent of Pd leaching, the filtered solution and products were dissolved in acetone and water and evaporated to dryness. reduction with H2 at 473 K was also active in the reaction, the The residual solid obtained after thermal treatment in an oven time taken to complete the reaction was significantly longer was dissolved in aqua regia before inductively coupled plasma than that using PdO/USY (Figure 1 b). The reaction over PdO/ (ICP) analysis. The detection limit of Pd was estimated to be USY proceeded without stirring, that is, the solution was trans25 ppb. However, ICP analysis did not reveal the dissolution of parent at the beginning; however, after 15 min, the product Pd, at least for reactions repeated up to four times (Figure 3). was observed as a white solid (Figure S1 in the Supporting InTo confirm if Pd dissolution occurred at a significantly lower formation). The product was easily isolated in a yield of 94 % concentration in the first reaction, the Suzuki coupling was after the addition of water and filtration. performed at a scale 25 times greater than previously perChemPhysChem 2015, 16, 1719 – 1726
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Product
Time [h]
Yield [%]
TON
2
93
1200
6
73
970
2
87
1100
1
99
1300
[a] Typical reaction conditions: heterocyclic halide (0.5 mmol), phenylboronic acid (0.75 mmol), K2CO3 (1.0 mmol), EtOH/H2O (1:1 v/v), and PdO/USY catalyst (Pd loading: 0.4 wt%, 0.010 g); temperature = 353 K.
agreement with a previous report on Pd/C[7] and poly(N-vinyl-2-pyrrolidone)-stabilized Pd.[13] The authors found that Pd0 nanoparticles are stable to metal leaching throughout the reaction. Moreover, it has been reported that the Suzuki coupling reaction between bromobenzene and phenylboronic acid in EtOH/H2O is solely facilitated by nanosized Pd clusters, which were stable to metal leaching.[10] Despite these facts, the possibility for the leached Pd to be the active species could not be ruled out in view of the many reports supporting such a role.[14] For instance, Thathagar et al. demonstrated that the Pd0 atoms or PdII ions leached into solutions were the Figure 2. Repeated reaction between bromobenzene and phenylboronic acid catalyzed by Pd/USY. a) Runs 1–5 and b) runs 6–10. Each reaction was started at the time points inactive species in both Heck and Suzuki coupling reacdicated by arrows. The catalyst was thermally treated in air at 773 K after using it five tions by using a membrane reactor.[15] Furthermore, times. [Pd] = 1.0 mol %; temperature = 298 K. in this case as mentioned above, the reaction proceeded without stirring. Therefore, another possibility might be that the leached Pd returned to the support after the completion of the reaction, as proposed by Djakovitch et al. (deposition mechanism),[16] such that Pd was not detected in the filtered solution and products. The catalyst used for the reactions that were repeated five times was thermally treated in air at 773 K and was used in subsequent reactions. After repeating the reaction six to 10 times, a small amount of Pd (less than 1 % of the Pd present in the catalyst) was detected. The amount of dissolved Pd tended to increase with the increase in the number of repeated reactions (Figure 3). A similar tendency was observed for 1.0 wt % Pd-loaded USY zeolite (Figure S2). 2.3. Characterization of the Pd Species Loaded on Zeolites
Figure 3. Amount of dissolved Pd obtained after repeating the reaction between bromobenzene and phenylboronic acid using 0.4 wt % PdO/USY. The catalyst was thermally treated in air after use in the fifth run.
formed, while the amount of PdO/USY remained constant. For this purpose, the solution was concentrated to 5 mL before ICP analysis. Although an excess amount of catalyst was used, dissolved Pd was not observed in the filtrate together with coupling products. This suggested that the reaction proceeds under heterogeneous conditions. This observation was in ChemPhysChem 2015, 16, 1719 – 1726
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Pd species loaded on USY zeolite before and after the repeated reactions were characterized using Pd K-edge XAFS measurements at the Japan Synchrotron Radiation Research Institute (Figure 4). Initially, the Pd¢O peak was observed at 0.16 nm in the spectrum of the as-prepared 0.4 wt % Pd/USY, the intensity of which was consistent with that of bulk PdO (Figure 4 a), indicating that the valence of Pd was 2 + (calcined). Doublet peaks of Pd¢O¢Pd were observed at 0.22– 0.35 nm in the spectrum of bulk PdO; these peaks were absent
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Figure 4. a) Pd K-edge EXAFS Fourier transforms and b) XANES of Pd/USY, measured before and after the Suzuki coupling reaction between bromobenzene and phenylboronic acid; as-prepared and runs 1–5.
proximately 2, suggesting the formation of very fine Pd clusters. Figure 5 a shows the Pd K-edge extended XAFS (EXAFS) of Pd/USY treated in air after five reactions in succession. Table S2 lists the data obtained from curve-fitting analysis. In this case, doublet Pd¢O¢Pd peaks were clearly observed between 0.22 and 0.35 nm, suggesting that the agglomeration of Pd progresses during the reaction, although the intensity of the Pd¢O¢Pd peaks was lower than that of bulk PdO. Unlike in reactions 1–5, this feature did not change on further use of the catalyst for the repeated reaction: Pd on USY zeolite maintained an oxidation state of 2 + , even after its use for the Suzuki coupling reaction. In agreement with this result, the XANES result of reactions 6–10 was close to bulk PdO, as shown in Figure 5 b.The above EXAFS data revealed that there are two types of Pd. The first
in the spectrum of PdO/USY, indicating the formation of molecular-like PdO on the USY support. This result was supported by the curve-fitting analysis, by which the bond distance and coordination number (CN) of the Pd¢O bond agreed well with those of bulk PdO (Table S1, run 0; Table S2, PdO). Conversely, in the spectrum of 1.0 wt % Pd/USY, small Pd¢O¢Pd peaks were observed at 0.28 and 0.32 nm (Figure S3), suggesting the formation of slightly agglomerated PdO clusters; these clusters can be observed by the comparison of this spectrum with that of bulk PdO. The CNs of the Pd¢O¢Pd peaks observed at 0.28 and 0.32 nm were 1.0 and 2.1, respectively, which were significantly lower than those of bulk PdO (4 and 8, respectively). The small CN of the Pd¢O¢Pd bonds of PdO suggested that the degree of agglomeration of Figure 5. a) Pd K-edge EXAFS Fourier transforms and b) XANES of Pd/USY measured before and after the Suzuki coupling reaction between bromobenzene and phenylboronPdO in 1 wt % PdO/USY is small and that cluster-like ic acid; re-calcined and runs 6–10. The catalyst was thermally treated in air at 773 K after PdO is formed. use in the fifth run. The Pd K-edge XAFS results of PdO/USY used for reactions 1–5 are included in Figure 4 a. The intensity type was a mixture of Pd0 and PdO, whereas the second type of the Pd¢O bond was lower than that of as-prepared PdO/ 0 USY, suggesting that Pd is partially reduced to form Pd . Alterwere PdO clusters, which were observed in the samples calnatively, a small peak appeared at 0.25 nm, which was unamcined in air at 773 K. Consistent with the Pd K-edge XAFS analbiguously assigned to the Pd¢Pd bond of Pd metal by comparysis, the color of the 0.4 wt % Pd/USY catalyst after one to five ison with the spectrum of Pd foil. In agreement with this consecutive reactions and six to 10 was gray and light brown, result, the data obtained from X-ray absorption near-edge characteristic of the metallic Pd and PdO, respectively (Figspectroscopy (XANES) measurements of the used catalyst for ure S5). Generally, the Suzuki coupling reaction is proposed to reactions 1–5 were in between those of PdO and Pd foil (Figproceed with Pd0.[17] However, in contrast, the species collected ure 4 b). The features of the spectra did not change with reafter six to 10 consecutive reactions were characterized as peated use of the catalyst. Although the intensity of the Pd¢ Pd2 + , as clearly evidenced by EXAFS analysis. To gain insight Pd bond of Pd0 slightly increased in comparison with that of into the formation of the partially reduced Pd species, XAFS measurements using PdO/USY treated with different combinathe Pd¢Pd bond of the 0.4 wt % sample, a similar tendency tions of substrates were conducted. For this purpose, various was observed when 1.0 wt % Pd/USY was used for repeated resubstrate combinations were charged with PdO/USY, followed actions: partially reduced Pd was observed (Figure S4). by catalyst separation by filtration and washing with a mixture Table S1 includes the data obtained from curve-fitting analysis. of H2O and EtOH (1:1). Figure 6 shows the radial distribution The CN of the Pd¢O bond was approximately 2, which was half that of PdO, suggesting that the average valence is + 1. function of Pd K-edge EXAFS of the collected samples. When On the other hand, the CN of Pd¢Pd was determined to be apbromobenzene was charged with PdO/USY, only slight changes ChemPhysChem 2015, 16, 1719 – 1726
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Figure 7. TEM image of 1 wt % Pd/USY after eight repeated reactions.
Figure 6. Radial distribution function of Pd K-edge EXAFS for Pd/USY, bulk PdO and Pd foil . a) As-prepared sample, and measured after the addition of b) bromobenzene, c) bromobenzene and phenylboronic acid, d) bromobenzene, phenylboronic acid and K2CO3. Temperature = 298 K; time = 0.5 h.
were observed in the EXAFS data; the same observation was made when a mixture of bromobenzene and phenylboronic acid was charged with PdO/USY. By contrast, when a mixture of bromobenzene, phenylboronic acid, and K2CO3 was charged with PdO/USY, the intensity of the Pd¢O bond markedly decreased, whereas that of the Pd¢Pd bond of Pd0 increased. This observation suggested that the reduction of PdO and simultaneous agglomeration of the resultant Pd0 progressed when a mixture of three substrates was added to PdO/USY. The formation mechanism of Pd0 was not clearly revealed at this stage. However, this suggests that Pd0 is generated because of the reaction with the substrate to form products (biphenyl) on the highly dispersed PdO. Another possibility is that NH4 + present on the USY reduced PdO to give Pd0 in a similar manner to the generation of Pd0 during the calcination of [Pd(NH3)4]2 + in zeolites.[18] Generally, the Suzuki coupling reaction is initiated by the oxidative addition of halogenated aryls on Pd0.[19] Therefore, the initial step probably involves the partial reduction of PdO with bromobenzene in situ, considering that a small Pd¢Pd bond of Pd0 was observed after the addition of bromobenzene at 0.25 nm (phase-shift uncorrected). It is likely that, as the reaction progresses, further reduction of PdO as well as partial growth of Pd occur when K2CO3 is added to the mixture of bromobenzene and phenylboronic acid. 2.4. TEM Observations Figure 7 shows the TEM image of PdO/USY after repeated use (eight times) in the reaction between bromobenzene and phenylboronic acid. The Pd loading in this sample was 1 wt %. The observed aggregate shown was only the zeolite support, and no aggregated Pd particles were observed in the whole region. This suggested that the size of Pd particles is at least ChemPhysChem 2015, 16, 1719 – 1726
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smaller than 1 nm, which was in agreement with EXAFS results: formation of PdO clusters was observed after repeated reaction. In agreement with this result, diffractions caused by metal Pd and PdO were not observed in X-ray diffraction patterns of the repeat samples. 2.5. First-Principles Calculations To determine the possible location of Pd0 in USY zeolite—observed after the reaction was repeated up to five times—in 0.4 wt % samples, first-principles calculations were conducted. The influence of Al (acid sites) on the stabilization of Pd was also examined. It was hypothesized that Pd located in the supercage of USY zeolites might be active species in the Suzuki coupling reaction, whereas Pd in the hexagonal prism and sodalite cage did not participate in the reaction considering that the reactants were bulky compared to the pore size of these structures. Therefore, the calculation was conducted on Pd located at sites II and III, which are situated on the six- and fourmembered rings facing the supercage, respectively. In addition, the location of the proton was considered. We constructed 19 models in which one or two Al atoms were fully substituted with Si (Figures S6 and S7). Figure 8 shows the representative structure of Pd located at sites II and III. First, the adsorption energy of Pd on site II was higher than that on site III; both sites II and III had two Al atoms in the ring. Second, the adsorption energy tended to increase, accompanied by the substitution of Si with Al in the zeolite framework. Considering this tendency, the most stable structure was Pd located at sites II having two Al atoms in the ring. Moreover, calculation of the adsorption energy of Pd in the structures such as that shown in Figure 8 b was undertaken in which distribution of protons were different (Figure S7 p–s). As a result, the average adsorption energy of these structures was determined to be 2.46 eV. Another important feature of the interaction between Pd and zeolite is the electron transfer between the two. To investigate electron transfer, a Bader charge analysis was conducted.[20] Figure 9 shows the charge distribution of Pd and zeolite, calculated based on the model structure shown in Figure 8 b. The adsorption energy of this Pd was the lowest among the calcu-
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Articles Finally, the local density of states (LDOS) for Pd in site II was calculated. Figure 10 a and b show the LDOS of Pd on the six-membered rings composed of six Si atoms and four Si plus two Al atoms corresponding to the structure shown in Figure 8 b, respectively. A comparison of Figures 10 a and b shows that the latter structure exhibited an increase in the LDOS in the d electron of Pd, suggesting that the interaction between Pd and the zeolite wall is enhanced by the substitution of Si with Al in the sixmembered ring of Y-type zeolite. This interaction might contribute to the stabilization of Pd to avoid the sintering of Pd during the reaction, thus extending the lifetime of the active Pd0 species in the Suzuki coupling reaction.
3. Conclusions
Figure 8. Representative model structure of Pd located at the II (a, b) and III (c, d) sites of Y-type zeolite. The values indicate the adsorption energy of Pd.
Well-dispersed PdO on USY zeolite was found to be the precursor to active species in the Suzuki coupling reaction between bromobenzene derivatives and phenylboronic acid in aqueous solutions at room temperature. We found that the valence and structure of Pd on the Y-type zeolite support was significantly dependent on the treatment in the repeated reaction, that is, a mixture of Pd0 and PdO was generated after the catalytic reaction. Dissolution of Pd was not detected after the catalyst was used. In the Pd/USY catalyst, Pd0 species were suggested to be located at the II site (six-membered rings) of the Y-type zeolite based on first-principles calculations. Pd0 was
Figure 9. Charge distribution calculated on the basis of the model structure shown in Figure 8 b.
lated model structures. Bader charge analysis conducted on this structure revealed that Pd is slightly negatively charged (¢0.0178 eV), which might promote the reaction, as envisaged for the coordinated Pd complexes with strongly s-donating ligands such as trialkylphosphines.[21] ChemPhysChem 2015, 16, 1719 – 1726
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Figure 10. LDOS for Pd on the six-membered rings composed of a) six Si atoms and b) four Si and two Al atoms corresponding to the model structure shown in Figure 8 b.
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Articles further stabilized by the presence of Al in the six-membered ring. This feature significantly changed after thermal treatment in air, followed by the use of the catalyst in subsequent reactions. Cluster-like PdO was observed in the air-calcined samples even after the catalyst was used. This suggested that PdO itself probably acts as the active species, unlike in typical cases reported.
Experimental and Calculations Sample Preparation and Catalytic Reaction The NH4 + form of USY zeolite (HSZ-341NHA, Tosoh, Tokyo, Japan) was used as the support for Pd. Pd (0.4 wt %) was introduced into USY zeolite by an ion-exchange method using an aqueous solution (3.8 Õ 10-4 M) of Pd(NH3)4Cl2 (Sigma–Aldrich) for 4 h at 343 K. Pd was loaded on NaY zeolite (HSZ-320NAA, Tosoh), HY zeolite and activated carbon (Wako Co.) for comparison purposes. HY zeolite was prepared by the ion-exchange reaction of NaY (HSZ-320NAA, Tosoh) with aq. NH4NO3 (1 m) three times at 353 K, followed by thermal treatment in air at 773 K. In typical conditions, bromobenzene (0.5 mmol), phenylboronic acid (0.75 mmol), K2CO3 (1.0 mmol), EtOH/H2O (1:1 v/v) or H2O, and PdO/USY catalyst (0.075 or 1 mol %) were added to a flask, and the solution was vigorously stirred using a magnetic stirrer. The reaction was performed at room temperature (298 K) under atmospheric conditions. The products were analyzed using a gas chromatograph (GC-2010, Shimadzu) equipped with a capillary column (InertCap 1) and a flame ionization detector. Tridecane was used as the internal standard. To measure the amount of leached Pd, the solution was separated by filtration. The solid was washed repeatedly with water and acetone. These washings were combined with the filtrate and the solution evaporated to dryness. The residue was thermally treated in air at 773 K. Then, the remaining solid was dissolved in aqua regia (ca. 5 mL). The concentration of Pd in the obtained solution was measured by ICP analysis.
Pd K-edge XAFS Measurements and Data Analysis Pd K-edge XAFS data were obtained from synchrotron radiation experiments performed at the BL01B1 station with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, SPring8; proposal No. 2012B1121, 2013B1067). Pd K-edge XAFS data were collected in the quick mode; a Si(111) monochromator was continuously moved from 4.758 to 4.408 for 5 min. The beam size was 5 mm (horizontal) Õ 0.8 mm (vertical) at the sample position. The catalyst collected after the reaction was thoroughly rinsed with EtOH and H2O alternately. The sample was dried in an oven at 323 K. Then, it was pressed into a wafer prior to XAFS measurement. In EXAFS analysis, oscillations were extracted by a spline smoothing method.[22] The Fourier transformation of the k3-weighted EXAFS oscillations and k3c(k) from the k space to the r space was conducted over the range of 25–130 nm¢1. The inversely Fourier-filtered data were analyzed in the k range between 25 and 130 nm¢1 by a typical curve-fitting method. In the curve-fitting analysis, the empirical phase-shift and amplitude functions of Pd¢ O and Pd¢Pd were extracted from the data obtained for PdO and Pd foils, respectively. EXAFS data were analyzed using the REX program (Rigaku, Tokyo, Japan). ChemPhysChem 2015, 16, 1719 – 1726
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TEM Measurements The TEM image was acquired using a JEOL JEM2010 microscope with an acceleration voltage of 200 kV. The specimen was prepared by the crushing method, and the TEM observations were carried out at room temperature.
First-principles Calculations In this study, all calculations were conducted using a first-principles calculation technique based on density functional theory within a local density approximation by using the Perdew and Zunger parameterization[23] as the term-exchange correlation with a cutoff energy of 500 eV. We used the Vienna ab initio simulation package, a first-principle calculation code with high precision using the projector augmented wave method. For the sampling of the Brillouin zone, a 4 Õ 4 Õ 4 Monkhorst–Pack grid was used,[24] which was obtained by the convergence results from energy versus k-points. Figure 11 shows the unit cell of Y-type zeolite used in this calcula-
Figure 11. The unit cell of Y-type zeolite used for first-principles calculations.
tion. The lattice constant was 1.7343 nm. The calculation was performed under periodic boundary conditions. Pd was introduced into two types of ion-exchange sites (II and III), which are adjacent to the supercage. Si atoms were partially substituted by Al atoms to reveal the influence of H + on the stabilization of Pd. The location of H + generated by the substitution of Si with Al was also considered in the calculation. The adsorption energy Ebind for the adsorbed atom on the surface was defined as [Eq. (1)]: E ¼ E USY þE Pd ¢E total
ð1Þ
where E is the total energy of the relaxed sulfur-surface system, and Etotal, EUSY and EPd represent the total energy of the relaxed bare surface, energy of the zeolite and energy of Pd, respectively. Using this relationship, binding energy is positive if the total energy decreases when the Pd atom is brought from infinite distance and placed onto the surface.
Acknowledgements This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 24560946, 2012-2015 and (A), 70183001, 2011-2015.
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Received: March 4, 2015 Published online on March 27, 2015
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