Nanoparticles of Pd on Hybrid Polyoxometalate−Ionic Liquid Material

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Nanoparticles of Pd on Hybrid Polyoxometalate-Ionic Liquid Material: Synthesis, Characterization, and Catalytic Activity for Heck Reaction A. Corma,*,† S. Iborra,† F. X. Llabre´s i Xamena,† R. Monto´n,† J. J. Calvino,‡ and C. Prestipino§,| Instituto de Tecnologı´a Quı´mica UPV-CSIC, UniVersidad Polite´cnica de Valencia, Consejo Superior de InVestigaciones Cientı´ficas, AVda. de los Naranjos, s/n, 46022 Valencia, Spain, Department of Materials Science and Metallurgical Engineering and Inorganic Chemistry, UniVersity of Ca´diz, Repu´blica Saharaui s/n, 11510 Puerto Real, Ca´diz, Spain, European Synchrotron Radiation Facility (ESRF), BP 220, 38043 Grenoble, France, and UniVersite´ de Rennes1, CNRS, UMR 6226, Campus de Beaulieu, 35042 Rennes Cedex, France ReceiVed: February 18, 2010; ReVised Manuscript ReceiVed: April 7, 2010

By partial exchange of surface protons of a polyoxometalate (POM; H5PO40V2Mo10) by an ionic liquid (IL; butylmethylimidazolium, bmim+), a protonic IL-POM ([bmim]4HPO40V2Mo10) was obtained. Further exchange of this material with Pd2+ yields a final solid that HRTEM and EXAFS-XANES techniques show to be formed by very regular nanoparticles of PdO with ∼2 nm diameter, which are supported on 1 µm particles of IL-POM hybrid material. The PdO nanoparticles can be easily reduced to Pd, and the resultant material is an active and selective catalyst for the Heck reaction. The catalyst can be recycled, though metal leaching occurs in polar solvents. This can be an interesting material for performing Pd-catalyzed reactions in nonpolar solvents or for working in gas-phase reactions. 1. Introduction The development of hybrid organic-inorganic nanocomposites has emerged as an important field of investigation in materials chemistry.1 These nanocomposites are regarded as promising materials for overcoming challenges in areas such as energy storage, catalysis, and optoelectronics.1–7 Polyoxometalates (POMs), a family of anionic inorganic metal oxide clusters exhibiting a wide range of topologies and physicochemical properties,8 have attracted special interest as nanoscale components for the preparation of nanocomposites, and in the past few decades, the use of POMs and POM-based compounds as catalysts has become a significant area of research. In particular, POMs have received much attention in the field of acid and oxidation catalysis because of their acidic and redox properties, which can be controlled at the molecular or atomic level.9 Countercations, while often overlooked, can be the determinant for critical aspects of the catalytic (particularly on the redox behavior) and aggregation behavior of POMs.10,11 On the other hand, despite the demonstrated utility of ionic liquids (IL) in the preparation of novel materials,12–16 the generation of nanocomposites combining IL-forming cations with inorganic species has received little attention to date. In this sense, Bourlinos et al.17 have prepared a POM-based ionic liquid by partially replacing the proton ions of a heteropolyacid (H3PW12O40) with bulky PEG-containing quaternary ammonium cations. Also, Rickert et al.18,19 have prepared and characterized a series of novel organic-inorganic hybrid materials based on tetralkylphosphonium polyoxometalate ionic liquids. However, catalytic applications of POM-based IL are scarce. In one of the few catalytic studies with these materials, Chhikara et al.20 * Corresponding author. Phone: (+34) 963877800. Fax: (+34) 963877809. E-mail [email protected]. † Universidad Polite´cnica de Valencia. ‡ University of Ca´diz. § ESRF. | Universite´ de Rennes1.

have synthesized a new material based on imidazoliun cation and tungstate anion (tris(imidazolium)-tetrakis(diperoxotungsto) phosphate(-3), which was active for the oxidation of alcohols with hydrogen peroxide. Recently, Bordoloi et al.21 have reported the synthesis of a imidazolium cation exchanged molybdovanadatephosphoric acid (H5[PMo10V2O40]32.5 H2O) immobilized onto ionic liquid-modified mesoporous silica SBA15, which showed good activity and selectivity for the aerobic oxidation of a variety of alcohols. Also, Lang et al.22 have reported the preparation of a phosphotungstic acid immobilized on ionic liquid-modified polymer as heterogeneous catalyst for the oxidation of alcohols using H2O2 as oxidant. Interestingly, the catalytic potential of IL-POM hybrids containing metal exchanged cations remains yet unexplored. In this work, we intended the preparation of new heterogeneous catalysts based on organic-POM hybrids, in which a partial replacement of the surface protons of the POM (H5PO40V2Mo10) by butylmethylimidazolium (bmim+) allows to obtain a protonic IL-POM ([bmim]4HPO40V2Mo10), in which the remaining proton could then be subsequently exchanged by a metal ion, such as Pd2+. However, an in deep characterization of the material obtained shows that the Pd2+ exchanged composite (theoretically ([bmim]4Pd0.5PO40V2Mo10) was not obtained. Instead, the solid obtained contains highly dispersed PdO spherical nanoparticles of 2-3 nm that form a secondary berrylike cluster structure of about 20 nm. These clusters decorate the surface formed by bulky (ca. 1 µm) globular particles generated by the interaction between the POM and the ionic liquid. We will show that this new material exhibits excellent activity and stability for the Heck coupling reaction, being recyclable several times without appreciable loss of activity. 2. Experimental Section Reagents and solvents were obtained from commercial sources and used without further purification. GC (Varian 3900) and GC-MS (Agillent 5973N) with a 30 m TRB 5 capillary

10.1021/jp1014934  2010 American Chemical Society Published on Web 04/21/2010

Pd on Hybrid Polyoxometalate-Ionic Liquid column were used for product analysis. C and N contents of the solids were determined with a Fisons CHNO elemental analyzer. Mo, P, Pd, and V contents were determined by dissolving the solid in a mixture of HF/HCl/HNO3 conc. (30 mg in ca. 1:1:1 mL), diluting the solution in water (30 mL) and measuring by quantitative atomic adsorption spectroscopy (Varian SpectrAA 10 plus). Water content in the solid products was determined by thermogravimetric analysis in a Mettler Toledo TGA/SDTA 851e by increasing the temperature from 25 to 800 °C at 10°/min heating rate with air flow. Liquid 1H NMR was recorded in a 300 MHz Bruker Avance instrument using DMSO-d6 as solvents and TMS as internal standard. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and high resolution electron microscopy (HREM) images were recorded with a JEOL 2010-FEG microscope operated at 200 kV. The structural resolution of the instrument is 0.19 nm. The digital diffraction pattern (DDP) reported as an inset in the HREM figure corresponds to the logscale power spectrum of the corresponding fast Fourier transform. X-ray absorption experiments at the Pd K Edge were performed at the BM29 beamline23 at the European Synchrotron Radiation Facility (ESRF) in fluorescence mode. The monochromator was equipped with two Si(111) flat crystals and harmonic rejection was achieved using Rh-coated mirrors placed after the monochromator. The fluorescence signal was recorded with a Canberra multichannel Ge 13-element,24 sampling in the range of the Pd KR fluorescence signal and applying corrections for deadtime and lifetime. For the XANES part of the spectra, a sampling step of 0.3 eV has been adopted, while for the EXAFS part the sampling step was 0.025 Å-1, with an integration time of 3 s/point. Extraction of the χ(k) function was performed using Klementev’s programs.25 Phase and amplitudes were calculated with FEFF7 code,26 using as input the structure of PdO solved by X-ray diffraction.27 The k2χ(k) functions were Fourier transformed in the ∆k ) 3.00-12.00 Å-1 interval. The fits were performed in R-space in the ∆R ) 1.00-4.00 Å range. Catalyst Preparation. Synthesis of H5PO40V2Mo10 · 27H2O (POM). The Keggin salt of molybdenum and vanadium was prepared according to the method described by Stobbe-Kreemers et al.28 Briefly, 24.4 g NaVO3 (0.2 mols) was dissolved in 100 mL of boiling deionized water. A solution of 7.1 g Na2HPO4 · 2H2O (0.04 mols) in 100 mL of water was added. After cooling at room temperature and upon addition of 5 mL of concentrated sulphuric acid, the solution turned red. Subsequently, 121 g Na2MoO4 · 2H2O (0.5 mols), dissolved in 200 mL of water, was added to the solution. The reaction mixture was vigorously stirred, while 85 mL of concentrated sulphuric acid was added dropwise. Then the solution was cooled and 500 mL of diethyl ether was added to extract the HPA product. Upon addition of diethyl ether, three layers were formed; the darkest layer contained the HPA-etherate complex. This layer was separated and the diethyl ether was removed in a rotary evaporator. The resulting product was recrystallized from water in a desiccator above concentrated sulphuric acid, giving large orange-red crystals. Atomic absorption: Calcd: 1.4% P, 44% Mo, 4.6% V. Found: 1.8% P, 42% Mo, 4.4% V. TGA: 27 H2O molecules: H5PO40V2Mo10 · 27H2O. Synthesis of [bmim]4HPO40V2Mo10 (POM-IL). A total of 4.23 g H5PO40V2Mo10 · 27H2O (2.5 mmol) was dissolved in 10 mL of water. [bmim]PF6 (10 mmol, bmim )3-butyl-1-methylimidazolinium, 2.88 g) was dissolved in 5 mL of acetone. The colorless ionic liquid solution was added dropwise over a

J. Phys. Chem. C, Vol. 114, No. 19, 2010 8829 period of 20 min with vigorous stirring to the orange heteropolyacid solution, giving a light orange suspension. The mixture was stirred for 30 min and filtered through a nylon membrane filter under vacuum. The orange solid was dried under vacuum for 1 h. Yield: 5.44 g (93%). Elem. Anal. Calcd: 16% C, 4.8% N, 2.6% H. Found: 16% C, 5% N, 2.7% H. Atomic absorption measurements gave 41 and 4.0% Mo and V, which corresponds well with the expected 41 and 4.4% Mo and V values, respectively. 1H NMR (DMSO): 0.90 (3H, t), 1.25 (2H, m), 1.77 (2H, q), 2.50 (DMSO), 3.33 (H2O), 3.88 (3H, s), 4.20 (2H, t), 7.70 (1H, s), 7.77 (1H, s), 9.10 (1H, s). Synthesis of Pd0.5H4PO40V2Mo10 (POM-Pd). PdSO4 · 2H2O 125 mg (0.5 mmol) was dissolved in 25 mL of water and added dropwise to H5PO40V2Mo10 2.3 g (1 mmol). The mixture was stirred for 2 h at room temperature under a nitrogen atmosphere. Water was removed by filtration through a nylon membrane filter, and the solid was washed several times with water. The solid was dried under vacuum for 1 h. The Mo, V, and Pd determined by atomic absorption (50% Mo, 5.4% V, 2.6% Pd) correspond fairly well with the theoretical values (53% Mo, 5.6% V, 2.9% Pd). Synthesis of [bmim]4Pd0.5PO40V2Mo10 (POM-IL-Pd). PdSO4 · 2H2O (250 mg, 1 mmol) was dissolved in 5 mL of water and added dropwise to [bmim]4HPO40V2Mo10 (2.3 g, 1 mmol). The mixture was stirred for 2 h at room temperature under a nitrogen atmosphere. Water was removed by filtration through a nylon membrane filter, and the solid was washed several times with water. The solid was dried under vacuum for 1 h. The resultant elemental analysis (16% C, 4.5% N, 2.5% H) corresponds well with the expected values (16% C, 4.7% N, 2.6% H). Elem. Anal. Calcd: 41% Mo, 4.3% V, 2.3% Pd. Found: 39% Mo, 5.2% V, 2.7% Pd. 1H NMR (DMSO): 0.90 (3H, t), 1.28 (2H, m), 1.77 (2H, q), 2.50 (DMSO), 3.33 (H2O), 3.87 (3H, s), 4.19 (2H, t), 5.76 (1H, s), 7.70 (1H, s), 7.77 (1H, s), 9.10 (1H, s). Catalytic Reaction. General Procedure. To the POM-ILPd catalyst (63 mg, 0.04 mmol Pd), iodobencene (0.340 mL, 3 mmol), styrene (0.450 mL, 3.9 mmol), and triethylamine (0.5 mL, 3.5 mmol) in 0.5 mL DMF were added and introduced into the reaction flask with a syringe. The mixture was kept under vigorous stirring at 100 °C for 2 h. Several samples were taken during this time and analyzed by GC. When the reaction was terminated, the mixture was cooled at room temperature. A total of 5 mL of acetone was added to make the filtration easier over a nylon membrane filter. Catalyst was washed with an additional 5 mL of acetone. Solvents were removed from the sample by rotary evaporation, giving large white stilbene crystals. Molar balances were in all cases higher than 98%. Catalyst Reuse. After a normal reaction run, the solid catalyst was separated by filtration, washed with acetone, and dried under vacuum at room temperature for 2 h. The dry solid was weighed and reused. 3. Results and Discussion 3.1. Electron microscopy. Figure 1 shows the electron micrographs obtained with high angle annular dark field scanning transmission electron microscopy (HAADF-STEM; a and c) and high-resolution TEM (HRTEM; b) of the sample POM-IL-Pd. According to the inset in Figure 1a, POM-IL-Pd crystallizes in the form of large globular particles of about 1 × 0.6 µm, which are decorated by smaller particles. A closer inspection of the sample (main body image in Figure 1a and c) revealed that these tiny particles consist of aggregates of about 20 nm diameter formed by very small and uniform spherical

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Figure 1. (a) Scanning transmission electron microscopy; (b) high resolution transmission electron microscopy images of sample POM-IL-Pd. The inset in (b) corresponds to the electron diffraction image along [221] of the nanoparticle indicated; (c; left) High resolution detail of the aggregates of ca. 20 nm, composed of small nanoparticles, and (right) height profile along the straight line shown in the image, in which it can be clearly seen that the size of the small nanoparticles is quite uniform, with a diameter comprised between 2-3 nm.

crystallites of about 2-3 nm. Examination of the elemental composition of the sample by X-ray energy dispersive spectroscopy (XEDS) has shown that all the Pd present in the solid is concentrated exclusively in these aggregates, while the bare parts of the support surface, not decorated with the aggregates, only contain V, Mo, and P, and no traces of Pd are detected (see Figure S1 in Supporting Information). In the HRTEM image in Figure 1b, the interference fringes of the nanoparticle are clearly observed, which are regularly spaced by 2.24 Å. According to the ED image shown in the inset and the measured interplanar spacing, this should correspond to a nanoparticle of oxidized palladium, PdO, viewed along [221]. This nanoparticle is representative of the whole sample, although a minor fraction of metallic Pd particles of about the same size was also detected, which can be generated in situ under the reducing conditions of the measurement (treatment under high vacuum and exposure to the electron beam). In summary, according to the electron microscopy study, palladium in sample POM-IL-Pd is in the form of very small and uniform PdO spherical nanoparticles of about 2-3 nm, which agglomerate into clusters of about 20 nm. These clusters decorate the surface of large (ca. 1 µm) globular particles of the support formed by the interaction between the POM and the ionic liquid.

3.2. X-ray Absorption Spectroscopy. The XANES spectrum at the Pd K edge of the fresh POM-IL-Pd sample is shown in Figure 2a, together with the spectrum corresponding to a bulk PdO reference sample (prepared by calcination of Pd(OH)2 at 900 °C). The two spectra are very similar, and they are in both cases representative of Pd ions in a +2 oxidation state. Given the position of the edge, a significant contribution from Pd0 (arising from eventual palladium metal nanoparticles) can be ruled out in sample POM-IL-Pd. The Fourier transform of the EXAFS signal of both samples is also compared in Figure 2b. Also, in this case, the same position of the first shell (Pd-O) and second shell (Pd-Pd) signals is found for the POM-IL-Pd and the reference PdO samples. However, a significant reduction of the intensity of the second shell signal (i.e., of the coordination number) in sample POM-IL-Pd with respect to the reference PdO occurs, indicating that the POM-IL-Pd contains palladium in the form of PdO particles of very small dimension. Notice that for particles larger than about 3-4 nm the differences in intensity of the second shell (Pd-Pd) signal with respect to a bulk PdO reference sample would be hardly appreciated by EXAFS. In agreement with the XANES spectrum, the EXAFS of the POMIL-Pd sample shows no evidence for the presence of metallic palladium.

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Figure 2. (a) XANES spectra at the Pd K edge of the sample POM-IL-Pd (-b-) and of a reference PdO sample (solid line); (b) phase-uncorrected, k3-weighted Fourier transform of the EXAFS spectra of the same two Pd-containing samples; (c) Fourier transform into R space of the k3-weighted experimental signal for sample POM-IL-Pd and of its best fit (solid line). Both the modulus and the imaginary part of the Fourier transforms are reported.

To obtain further information on the size of the PdO particles in the sample POM-IL-Pd, we have further analyzed the EXAFS part of the spectrum. The analysis by EXAFS spectroscopy affords element specific information regarding interatomic distances, structural disorder and number and type of neighboring atoms at a given distance of the absorber atom.29 The main parameter reflecting the particle size is the average coordination number, which can be determined, in principle, up to the fifth coordination shell of the absorber atom. The relationship between the particle size and the average coordination number is based on the fact that a fraction of the absorber atoms are located at the surface of the particle. These atoms will have lower coordination numbers than the atoms located at the bulk of the particle, and the fraction of such surface atoms will increase as the particle size decreases. This relationship relies on an accurate description of the shape of the particles and their size distribution, and in absence of this information, the results should be taken as indicative. In the case of metallic particles, several precedents can be found in the literature in which this approach has been used to determine particle size: From the pioneering works by Lytle and co-workers,30–41 to the successive works by Frenkel and co-workers,42,43 in which detailed functional dependencies between the coordination number and the shape and size distribution of metallic nanoparticles were accurately described. However, as we have already shown by XAFS and HRTEM, the POM-IL-Pd sample does not contain metallic, but metal oxide particles. Consequently, the functional relationships derived for metal can not be applied, so we have searched for a new functional relationship for metal oxide nanoparticles. To do this, we have developed an ad hoc software in which the average coordination number for the different paths (single and multiple scattering) contributing to the EXAFS signal is obtained empirically for a series of metal oxide clusters of increasing size. This approach and the software developed present the advantages of being independent of the structure of the material under study and that it can also be applied to metal oxides and not only to metals. The software consist mainly in a modification of PATH routine as implemented in FEFF6 code.44 The procedure for inferring the average size of the PdO particles has been as follows. We have assumed that the PdO

particles in sample POM-IL-Pd are spherical (as we have confirmed by HRTEM analysis, vide infra) and that all the particles have the same size. We have then generated a series of clusters of increasing size with the ATOMS code,45 that have been cut from the structure solved by X-ray diffraction for PdO.27 For each cluster, the average coordination number around Pd has been calculated considering all the Pd ions of the particle; that is, the function average coordination number versus particle size, versus R, has been generated. The calculus was extended to two paths: second shell Pd-Pd and third shell Pd-Pd. The calculus has also been applied to the first shell, Pd-O, but it has not been taken into account for the evaluation of the particle size, because it is not reasonable to hypothesize a coordinative unsaturation of Pd2+ under ambient conditions. In other words, we assume that all Pd atoms in the particles will be surrounded by four oxygen atoms.46 The values of the average coordination number have also been obtained from the fit47 of the experimental EXAFS spectrum of the reference PdO and the POM-IL-Pd samples and compared to those calculated for the generated clusters. From this comparison, a direct estimation on the size of the PdO particles in sample POM-ILPd can be obtained. The results obtained for the EXAFS analysis of both the reference PdO and the POM-IL-Pd samples are summarized in Table 1. The good quality of the fit of the POMIL-Pd sample signal can be seen in Figure 2c. According to the results in Table 1, a reduction of the average coordination number of Pd in sample POM-IL-Pd with respect to the reference PdO from 4.00 to 3.09 in Pd-Pd second shell and from 8.00 to 4.20 in Pd-Pd third shell occurs. On the contrary, and in agreement with our approximation, there is no significant reduction in the average coordination number in the Pd-O first shell (from 4.00 to 3.96). The decrease in the average coordination number is more marked for the Pd-Pd third shell than for the second shell, because the former is more sensitive to small long-range variations. Therefore, the estimation of the particle size from the analysis of the Pd-Pd third shell is expected to be more accurate. The values of of 3.09 and 4.20 for Pd-Pd second and third shells obtained for sample POM-IL-Pd have then been compared with those calculated for the clusters of increasing size, and the results obtained are shown in Figure 3.

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TABLE 1: Analysis of the EXAFS Spectra of the Reference PdO and POM-IL-Pd Samplesa

a

R ( 0.02 (Å)

( 10%

D - W ( 10% (Å2)

∆E ( 2 (eV)

PdO Reference Pd-O 1sh (2.0176) Pd-Pd 2sh (3.03) Pd-Pd 3sh (3.42)

2.02 3.06 3.44

4.00 4.00 8.00

0.0040 0.0060 0.0096

7.23 7.23 7.22

Sample POM-IL-Pd Pd-O 1sh (2.0176) Pd-Pd 2sh (3.03) Pd-Pd 3sh (3.42)

2.03 3.05 3.46

3.96 3.09 4.20

0.0042 0.0068 0.0110

6.31 6.31 6.31

The values between brackets in the first column correspond to the distances reported for the structure of PdO solved by X-ray diffraction.

Figure 3. Functional relationship between calculated coordination number of second shell (-9-) and third shell (-2-) vs particle size for PdO particles of spherical shape. Bold lines represent the values obtained for the fits of the second and third shells for sample POMIL-Pd.

The particle size can be estimated from this graphic by determining the diameter of the calculated cluster that gives approximately the same average coordination number as that determined experimentally from the fit of the EXAFS signal, that is, the intersection between the functional versus particle diameter and the horizontal lines. As it can be observed, slightly different results for the particle size are obtained using the fits of the Pd-Pd second and third shell: 2 and 1.3 nm, respectively. This discrepancy can have two different origins, and probably both of them are contributing to the observed differences. (i) The particles are not perfectly spherical and (ii) there is a distribution of particles of different sizes. Note that the comparison for the estimation of the particle size is made with the average coordination number, which accounts for all the particles of the sample. If the shape of these particles is not spherical, the effect on the second and third shell fittings will be different, and the same will be observed for particles of different sizes. In any case, the two values of 2.0 and 1.3 nm obtained for both fits can be considered as the upper and lower limits of the particle size dimensions, that is, most particles of the sample POM-IL-Pd will have a dimension that will be comprised between these two values. Moreover, the real situation is probably closer to the value of 1.3 nm obtained from the fit of the Pd-Pd third shell signal, because as we have commented above, this provides a more accurate estimation of the particle size. We can see that there is a good agreement between the particle size estimated from the EXAFS analysis and that measured by HRTEM, which give a particle size between 2 and 3 nm diameter.

Figure 4 shows a comparison of the XANES and EXAFS parts of the spectra, at the Pd K edge, obtained for samples POM-IL-Pd, both fresh and after being used in two catalytic cycles (POM-IL-Pdused) in the Heck coupling of iodobenzene and styrene and for the sample prepared without the ionic liquid, POM-Pd. For comparison, the spectrum obtained for the PdO and Pd metal references is also included. As it can be clearly observed in Figure 4, both the XANES and EXAFS spectra of the sample POM-IL-Pdused are practically undistinguishable from those of metallic palladium. In other words, the Pd2+ ions initially present in the fresh POM-IL-Pd sample are gradually reduced to Pd0 during the catalytic reaction. After two catalytic cycles, the amount of Pd2+ in the catalyst is so small that it is out of the detection limits by X-ray absorption spectroscopies. As a reference, we have analyzed the local environment of palladium in the binary compound POM-Pd. Results in Figure 4 show that it is different from that observed for fresh and used POM-IL-Pd. From the position of the Pd K edge, the oxidation state of Pd in POM-Pd is +2, as in the fresh POM-IL-Pd sample. However, the EXAFS part of the spectrum does not show the Pd-Pd second and third shells expected for palladium oxide, that is, palladium ions are isolated, so this rules out the occurrence of both PdO (as in POM-IL-Pd) and palladium metal (as in POM-IL-Pdused) particles. The presence of a Pd-O first shell EXAFS signal in the POM-Pd sample indicates that isolated Pd2+ ions are placed near the oxygen atoms of the polyoxometalate, as can be expected for an electrostatic POM5-Pd2+ interaction (where POM5- stands for the Keggin type [PO40V2Mo10]5- polyoxometalate units). In summary, the XANES and EXAFS data suggest that Pd2+ cations in sample POM-Pd act as charge compensating counterions for the negatively charged POM5- units, while in POM-IL-Pd, nanoparticles of PdO decorate the surface of ∼1 µm particles formed by ionic liquid exchanged POM. 3.3. Catalytic Activity. POM-IL-Pd was tested as a catalyst for the Heck coupling of iodobenzene and styrene using triethylamine as base under milder reaction conditions (100 °C and 100 ppm Pd) than those usually reported in the literature48 and using DMF as a solvent. The reaction affords trans-stilbene (3; Scheme 1) with high conversion (93%) and 100% selectivity within a 2 h reaction time, while a TON of 15000 was calculated for this reaction. In Figure 5, the kinetic behavior of POM-ILPd is plotted and compared with (a) POM-Pd catalyst that has been prepared by exchanging POM with Pd and is lacking the IL moiety; (b) a homogeneous catalyst such as Pd-acetate; and (c) the POM exchanged with IL (POM-IL), which is lacking Pd. As can be observed in Figure 5, the catalytic activity of POM-IL-Pd is superior to those shown by POM-Pd and the homogeneous catalyst, while, without palladium (POM-IL), as expected, the Heck coupling is not accomplished. This behavior has to be attributed to the fact that in POM-IL-Pd, Pd is in the

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Figure 4. (a) XANES spectra at the Pd K edge of samples (from bottom to top): PdO, POM-IL-Pd, POM-Pd, POM-IL-Pdused, and Pd metal; (b) phase-uncorrected, k3-weighted Fourier transform of the EXAFS spectra of the same samples. Both the modulus and the imaginary part of the Fourier transforms are reported.

SCHEME 1

form of very small and uniform PdO spherical nanoparticles of 2-3 nm, which agglomerate into clusters of 20 nm. These clusters are supported on 1 µm particles of POM-IL hybrid support. This structural composition affords a highly dispersed Pd catalyst which results to be more active than POM-Pd where Pd is in the form of exchanged Pd2+ in the POM structure. The POM-IL-Pd is also more active than the homogeneous Pd+2OOCCH3. The difference in activity between POM-IL-Pd and POM-Pd is even more remarkable when a less reactive halobenzene such as bromobenzene is used (Table 2, entries 5 and 6). In addition, when methyl acrylate was used as reactant

(entry 7), the catalytic activity was also very high, giving 83% conversion and 100% selectivity of the corresponding transcinnamate (5; Scheme 1). Study of the Influence of the SolWent and Leaching of the Metal. The Heck coupling between styrene and iodobenzene using POM-IL-Pd and POM-Pd was performed using solvents of different polarities. As can be observed in Table 3, for both catalysts, the highest activity was obtained with the most polar solvent DMF, while with apolar solvents such as toluene and o-xylene the TOFs obtained were lower in spite of using a higher reaction temperature. The reaction was also performed in the

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Figure 5. Time conversion plot for Heck reaction between iodobenzene (3 mmol) and styrene (3.9 mmol) in DMF at 100 °C in the presence of different catalysts: (9) POM-IL-Pd; (2) POM-Pd; (b) Pd-acetate; (*) POM-IL.

absence of solvent, which is an interesting option to reduce the E factor, however, activity was sensibly lower and temperature has to be increased to achieve high performances. As can be observed in Table 3 (entries 5-8), in all cases, POM-Pd gives lower catalytic activity than POM-IL-Pd. It has been reported49,50 that the use of polar solvents and, particularly, DMF causes a partial leaching of Pd metal from the solid to the solution, while no palladium occurs in toluene.49,51,52 Then, the higher activity observed with DMF could be associated with leached Pd species. We have checked this point by performing experiments with DMF and toluene and filtering out the solid at the reaction temperature at ∼50% of the final conversion. As can be observed in Figure 6b, while toluene does not promote the leaching of the metal, a slight leaching is observed with DMF (Figure 6a) because the yield is slightly increased when the solid was removed from the reaction medium. However, the contribution of the Pd leached to the final yield is less than 10%, which does not justify the differences in activity observed between both solvents. In this

sense, it is important to note that the leaching of Pd2+ to the solvent is more marked for the catalyst POM-Pd than for the POM-IL-Pd, thus, reflecting the higher stabilizing ability of the ternary POM-IL-Pd compound to retain Pd. According to the ICP analysis, the catalyst POM-Pd loses ∼75% of the initial Pd2+ after two catalytic runs (while the fresh catalyst contains 2.6 wt % Pd, the material recovered after two catalytic cycles contains only 0.7 wt % Pd). Under the same conditions, the ternary compound POM-IL-Pd loses ∼30% of the initial Pd2+ (from 2.7 wt % Pd in the fresh material to 1.8 wt % Pd in the used catalyst). Study of the Stability of the POM-IL-Pd Catalyst. To study the stability of POM-IL-Pd material, the catalyst was submitted to five consecutive runs. As can be observed in Figure 7, the catalytic activity was practically maintained up to the third cycle, while in the fourth and fifth runs a loss in activity of 17 and 40%, respectively, is observed. Additionally, the SEM analysis showed that the characteristics of the sample changed dramatically after the catalytic use. Figure S2 (Supporting Information) shows the backscatter SEM image obtained for a POM-IL-Pdused sample used in four consecutive catalytic runs. In this image, palladium (the heaviest element) is observed as white spots. Clearly, after the catalytic reaction, the former PdO nanoparticles have been reduced and aggregated to form large metallic palladium particles. Further proof of this comes from the XEDS analysis and comparison with the fresh sample shown in Figure S2 and is in accordance with XANES and EXAFS spectra of the POM-IL-Pdused presented in Figure 4. For comparison purposes, the stability of POM-Pd was also tested by performing several reuses. As can be observed in Figure 7, the catalytic activity is practically maintained only during two consecutive cycles, while a notable loss in activity (70%) is already observed during the fourth run. The SEM image of the POM-Pd sample after its use in two consecutive cycles showed no aggregation of Pd metallic particles, as occurred in the case of POM-IL-Pdused, which indicates that Pd2+ remains as exchanged metal in the POM structure. However,

TABLE 2: Results for the Heck Reaction of Iodobenzene or Bromobenzene (3 mmol) with Styrene or Methyl Acrylate (3.9 mmol) and Triethylamine (3.6 mmol) in 0.5 mL of DMF As Solvent, In the Presence of 0.01 wt % of Palladiuma entry

catalyst

halobenzene (1)

olefin (2; 4)

PhX conversion (%)

TOF mmols prod · mmols Pd-1 · min-1

1 2 3 4 5 6 7

POM-IL-Pd POM-Pd POM-IL Pd acetate POM-IL-Pd POM-Pd POM-IL-Pd

iodobenzene iodobenzene iodobenzene iodobenzene bromobenzene bromobenzene iodobenzene

styrene styrene styrene styrene styrene styrene methyl acrylate

93 79 0 70 49 18 83

200 174 0 156 26 5 190

a

Reactions were performed at 100 °C for 2 h.

TABLE 3: Results for the Heck Reaction of Iodobenzene (3 mmol) with Styrene (3.9 mmol) and Triethylamine (3.6 mmol) in 0.5 mL of Solvent, In the Presence of 0.01 wt % of Palladiuma

a

entry

catalyst

solvent

temperature (°C)

yield (%)

TOF mmols prod · mmols Pd-1 · min-1

1 2 3 4 5 6 7 8

POM-IL-Pd POM-IL-Pd POM-IL-Pd POM-IL-Pd POM-Pd POM-Pd POM-Pd POM-Pd

DMF o-xylene toluene

100 130 110 140 100 130 110 140

93 76 65 87 79 50 50 74

200 42.1 94.7 231.6 174 36.8 63.2 184.2

DMF o-xylene toluene

Reactions were developed for 2 h. Reaction time ) 30 min.

Pd on Hybrid Polyoxometalate-Ionic Liquid

J. Phys. Chem. C, Vol. 114, No. 19, 2010 8835

Figure 6. Time yield plot for the Heck reaction of iodobenzene (3 mmol) with styrene (3.9 mmol) and triethylamine (3.6 mmol) in the presence of POM-IL-Pd (140 mg, 0.01 wt % of palladium). Reaction (a) was performed in DMF at 100 °C. After 10 min, the catalyst was filtered at the reaction temperature and the reaction was quickly continued in the same conditions (full line curve). Reaction (b) was performed in toluene at 110 °C. After 30 min, the catalyst was filtered at the reaction temperature and the reaction was quickly continued in the same conditions (full line curve).

References and Notes

Figure 7. Yield at 1 h reaction time for the Heck reaction of iodobenzene (3 mmol) and styrene (3.9) in DMF at 140 °C in the presence of POM-IL-Pd (140 mg): (1) fresh catalyst; (2-5) reused catalyst.

as commented above, the elemental analysis of the sample indicates that a strong leaching of the Pd occurs during the reactions. Conclusions Nanoparticles of Pd can be obtained on POM exchanged with ionic liquids. Characterization of the materials by HRTEM and EXAFS-XANES techniques clearly demonstrate that, during the synthesis procedure, 2 nm regular particles of PdO are formed, which are supported on the hybrid material formed by polyoxometalate exchanged with ionic liquid. The PdO can easily be reduced to metal Pd nanoparticles. The material is active and selective for the Heck reaction, especially when working within a polar solvent (DMF). However, upon repeated catalytic cycles, metal leaching occurs. The fact that very regular and small nanoparticles of Pd can be formed by the preparation method presented here can offer catalytic possibilities for reactions catalyzed by Pd in liquid phase and that do not require polar solvents or, even better, for gas-phase Pd-catalyzed reactions. Acknowledgment. The authors thank the Direccio´n General de Investigacio´n Cientı´fica y Te´cnica of Spain (Project MAT200614274-C02-01) for funding. Supporting Information Available: High angle annular dark field scanning transmission electron microscopy (HAADFSTEM) of the sample PMO-IL-Pd (Figure S1) and BSE SEM images of the fresh and used samples and corresponding XEDS analysis (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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