Highly Efficient Nanocatalysts Supported on Hollow Polymer

Jan 3, 2008 - Hollow polymer nanospheres were employed in the fabrication of noble metal-supported catalysts via a supercritical route. In this method...
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J. Phys. Chem. C 2008, 112, 774-780

Highly Efficient Nanocatalysts Supported on Hollow Polymer Nanospheres: Synthesis, Characterization, and Applications Shiding Miao, Chengliang Zhang, Zhimin Liu,* Buxing Han, Yun Xie, Sujiang Ding, and Zhenzhong Yang* Beijing National Laboratory for Molecular Sciences, Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China ReceiVed: August 16, 2007; In Final Form: October 25, 2007

Hollow polymer nanospheres were employed in the fabrication of noble metal-supported catalysts via a supercritical route. In this method, the metal precursors were first adsorbed on the polymer support in a supercritical CO2-ethanol solution, followed by H2 reduction, generating metal (Pd, Rh, Pt)/polymer composites. The resultant Pd/polymer nanospheres were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy equipped with energy dispersive spectroscopy, and X-ray photoelectron spectroscopy analysis. It was indicated that the Pd nanoparticles with a size of about 5 nm were uniformly attached to the surface of the polymer spheres. The activities of the Pd/polymer composites for the allyl alcohol hydrogenation and Heck reaction were also investigated. The catalyst exhibited a high activity and stability in these two reactions.

1. Introduction Heterogeneous catalysts have been widely investigated and employed since they are easily recovered and regenerated as compared to homogeneous catalysts.1 Nanoparticle (NP)-supported catalysts have attracted much interest due to their high surface area and improved activity. In particular, polymersupported and/or stabilized NP catalysts have exhibited high efficiency and found many promising applications in catalysis fields.2 In this kind of catalyst, the NPs were generally stabilized via electrostatic, steric, and electrosteric means or through the use of ligands.3 Poly(N-vinyl-2-pyrrolidone) (PVP) is the most used polymer for NP stabilization because it fulfills both steric and ligand requirements. Recently, diverse polymers including polystyrene beads,4 polyurea,5 polyacrylonitrile and/or poly(acrylic acid),6 multilayer polyelectrolyte films,7 and polysilane micelles with cross-linked shells8 have been used as supports for NP catalysts.9 The methods for synthesis of the polymersupported NPs include impregnation,10 sol-gel methods,11 organometallic deposition,12 sonochemical methods,13 layer-bylayer deposition,14 and so on.15 While the polymer-based nanocatalysts have been extensively investigated, it is still desirable to develop this kind of catalyst with the aim that polymer supports can be designed with unique structures and functions, which may lead to some unusual properties of the designed catalysts. In recent years, polymer hollow latex spheres have been paid great attention owing to their special structures and potential applications in many fields.16 Especially, their low density renders them stably suspensible in certain solutions without sedimentation even decorated with metal nanoparticles; moreover, they have high surface areas and changeable surface properties, which endow them applications in catalysis.17 Supercritical fluids (SCFs), featured with a low viscosity, high diffusivity, and near zero surface tension, have been successfully applied in the synthesis of nanocomposites, and some special * Corresponding authors. E-mail: (Z.L.) [email protected] and (Z.Y.) [email protected].

nanostructures have been prepared.18 Supercritical (SC) CO2 is the most commonly used SCF since it possesses an easily accessible critical point (Tc ) 31.1 °C and Pc ) 7.38 MPa)19 and is nontoxic, non-flammable, and chemically inert. In addition, SC CO2 is able to swell many polymers, so it has been proven to be a good medium for the synthesis of polymer-based composites.20 It may also provide new routes for the preparation of polymer-supported NP catalysts. Polystyrene hollow latex spheres (PHLS) with functional shells were synthesized in our previous work.21 Besides the previously mentioned properties, they were highly cross-linked and well-dispersible in water and some organic solvents (such as ethanol, benzene, etc.); meanwhile, the shell could be easily swollen by media including SC CO2, dimethylformamide (DMF), and benzene, which endow these polymer spheres many potential applications.22 Herein, we employed this PHLS in the synthesis of NP catalysts and reported a method to fabricate a PHLS-supported nanocatalyst. In this method, the metal precursors were first adsorbed on the polymer support in a SC CO2ethanol solution, followed by H2 reduction, and noble metal (Pd, Pt, and Rh) NPs were successfully supported on the PHLS support, achieving PHLS-supported nanocatalysts. As an example, the as-prepared Pd-PHLS catalyst was characterized with different techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), etc. Moreover, the resultant Pd-PHLS composites were used to catalyze allyl alcohol hydrogenation and Heck reactions, respectively, and the activity for these two reactions was investigated. 2. Experimental Procedures 2.1. Materials. Iodobenzene and allyl alcohol were supplied by Acros. PdCl2 (AR grade), anhydrous ethanol, N,N-dimethylformamide, acrylonitrile, divinylbenzene, and triethylamine were purchased from Beijing Chemical Reagent Co. Hydrogen (99.99%) and CO2 (99.99%) were provided by Beifen Co. The polymer hollow nanospheres used in this work were synthesized

10.1021/jp076596v CCC: $40.75 © 2008 American Chemical Society Published on Web 01/03/2008

Nanocatalysts Supported on Polymer Nanospheres based on our previous work.21 Typically, hollow latex cages consisting of linear polystyrene (MW ) 125 000 Da) and poly(methyl methacrylate) (MW ) 125 000 Da), purchased from Rhom and Haas Co., were swollen by an acrylonitrile/divinylbenzene mixture, and then polymerization of the swelling monomers was initiated to form an interpenetrating network shell of PHLSs. The average diameter and shell thickness of the polymer spheres were about 500 and about 50 nm, respectively. 2.2. Synthesis of Catalyst. To synthesize the PHLS-supported Pd catalyst, 0.6 g of PHLS, 2.0 mL of H2PdCl4 aqueous solution (containing 0.057 mmol of Pd, prepared by dissolving PdCl2 in a HCl aqueous solution), and 8.0 mL of ethanol were loaded in a 30 mL stainless steel high-pressure autoclave with an inner Teflon lining. After being sealed, the autoclave was immersed into a water bath of 40 °C, and CO2 was compressed into the autoclave up to 12 MPa. The sealed system was kept for 6 h under stirring before CO2 was released. Subsequently, the autoclave was charged with H2 (2.0 MPa) and maintained at 100 °C for about 30 min. Finally, the autoclave was cooled down and depressurized. The solids were filtered out, washed with ethanol, and finally dried under vacuum at 60 °C for 4 h, yielding 0.58 g of product. The mass content of palladium was 1.0%, calculated based on the initial amounts of PHLS and Pd. This sample was denoted as Pd-PHLS-SCF. For comparison, the PHLS-supported Pd catalysts were also prepared in ethanol and aqueous solutions under similar conditions in the absence of SC CO2, and the resulting samples were named Pd-PHLSethanol and Pd-PHLS-water, respectively. Using similar supercritical procedures, Rh and Pt nanoparticle-supported PHLS composites were also prepared, respectively. The contents of Rh and Pt on PHLS were 1.25 and 3.64 wt %, respectively, based on the initial amounts of the corresponding precursors, and the two samples were denoted as Rh-PHLS-SCF and PtPHLS-SCF. 2.3. Catalyst Characterization. XPS was collected on an ESCALab220i-XL spectrometer at a pressure of about 3 × 10-9 mbar using Al KR (hν ) 1486.6 eV) radiation as the exciting source, which was operated at 15 kV and 20 mA. C1s binding energy was set to 284.6 eV as an energy calibration. XRD analysis was preformed on an X-ray diffracometer (SW, X’PERT) with nickel filtered Cu KR radiation (λ ) 1.54060 Å) operated at 40 kV and 10 mA. SEM images were taken on a JEOL JSM-4300 field emission scanning electron microscope at a voltage of 15 kV. High-resolution TEM (HRTEM) micrographs were taken on a transmission electron microscope (Philips, Tecnai F30) equipped with an energy-dispersive X-ray spectrometer and an ultrahigh-resolution pole piece that could result in a point resolution of 0.17 nm at an operating voltage of 300 kV, and the images were electronically captured using a CCD camera. 2.4. Allyl Alcohol Hydrogenation. The reactions were carried out in a 20 mL stainless-steel autoclave equipped with a magnetic stirrer. In a typical experiment, 10.0 mg of Pd-PHLSSCF nanocatalyst and 2.19 g of allyl alcohol were loaded into the autoclave, and the air inside the autoclave was replaced by H2. After charging H2 up to the desired pressure, the reaction mixture was stirred (300 rpm) at the desired temperature. After the appropriate reaction time, the autoclave was cooled to 0 °C quickly in an ice-water bath, and it was depressurized slowly. The catalyst was then separated from the solution by high-rate centrifugation (15 000 rpm). The clear solution was analyzed by GC and GC-MS. GC data were recorded on a HP 4890 GC system. GC-MS data were obtained by using an APEXII series

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Figure 1. IR spectra of the pristine hollow latex cages (a) and the as-synthesized PHLS (b).

(HP 5890 GC equipped with an FT-ICR mass selective detector). The catalyst was washed with absolute ethanol 3 times and dried under vacuum at 60 °C for 4 h prior to being reused in the next run. 2.5. Heck Reaction. Iodobenzene (5.0 mmol), methyl acrylate (7.5 mmol), triethylamine (Et3N, 7.5 mmol), N,N-dimethylformamide (DMF, 10.0 mL), and the Pd-PHLS-SCF composite (10.6 mg, iodobenzene/Pd ) 5000) were added into a Schleck flask equipped with a constant temperature oil bath of 100 °C, and the mixture was stirred. After 1 h, sampling was performed every 6 min, and the samples were analyzed by GC using dodecane as an internal standard. Conversion was determined from the amount of consumed iodobenzene. The products were also analyzed by GC-MS at some typical conditions. To check their stability, the catalyst was recovered, washed thoroughly, and dried before being reused in the next run. 3. Results and Discussion 3.1. Characterization of Pd/Polymer Composites. Figure 1 exhibits the FTIR spectra of the pristine hollow latex cages and the PHLS synthesized in this work. On the spectrum of the PHLS, the characteristic peaks at around 2243 and 1731 cm-1 and the three peaks between 1450 and 1600 cm-1 are assigned to the --CN, CdO bond stretching, and phenyl framework stretching, respectively.23 The IR analysis displays that the shell of PHLS has a desirable amount of CdO and -CN groups, which may facilitate further adsorption of other species (such as metal ions) and in situ growth of nanoparticles within the shell. Therefore, this PHLS could act as a candidate of catalyst supports, and it might open a new pathway for the design of polymer-supported catalysts. In this work, a noble metal salt (such as PdCl2, RhCl3, and H2PtCl6) that is soluble in ethanol was used as precursor, and it was first adsorbed by PHLS in ethanol with the aid of SC CO2, followed by H2 reduction, generating noble metal nanoparticle decorated PHLS composites. With PdCl2 as a precursor, some Pd/PHLS composites were prepared under different conditions. As an example, these Pd-PHLS composites were examined in detail. Figure 2a illustrates a SEM image of the product (denoted as Pd-PHLS-SCF), which clearly shows that the PHLS support exhibited a regular spherical shape with a diameter of about 500 nm. TEM observations gave more information about the microstructure of the product. As illustrated in Figure 2b, the polymer support showed a hollow cavity, and numerous monodispersed nanoparticles were uniformly decorated on the PHLS. The particle size was mainly in the range of 4 to ∼6 nm. Through TEM observation, it was found that every polymer sphere was decorated with uniform nanoparticles and that almost all the nanoparticles were attached

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Figure 2. (a) SEM image of Pd-PHLS-SCF, (b) TEM image of Pd-PHLS-SCF, (c) HRTEM image of Pd-PHLS-SCF, (d) EDS profile of PdPHLS-SCF, (e) TEM image of Pd-PHLS-ethanol, and (f) TEM image of Pd-PHLS-water.

Figure 3. XRD profiles of the PHLS (curve a) and the Pd-PHLSSCF composites (curve b).

onto PHLS. This suggests that the interaction between the nanoparticles and the polymer support was strong enough. The HRTEM image (Figure 2c) displays that the typical spherical crystallites with a diameter of ∼5 nm were well-adhered on the shell of the PHLS; moreover, it seems that the nanoparticles were partially embedded in the polymer shell, implying the firm anchoring of the particles on the polymer support. The aligned lattice spacing of the nanoparticles was measured to be about 2.25 Å, which can be indexed to a fcc structure of metallic palladium with d111 ) 2.246 Å.24 EDS analysis (Figure 2d) demonstrates that the composites contained elements C, N, O, and Pd and that there was no detectable Cl, indicating that the Pd precursor was converted into metallic Pd during the reduction process. The as-prepared Pd-PHLS-SCF composites were also examined by XRD, which exhibited similar XRD patterns to the virgin PHLS, as shown in Figure 3. On the XRD patterns of the Pd-PHLS-SCF composites, there is a very weak diffraction peak around 40.1° assigned to Pd(0), which suggests that the presence of Pd on the composites and the particle size was very small. XPS spectroscopy is an important technique to determine the oxidation state of anchored particles. The survey spectrum of the Pd-PHLS-SCF composites (Figure 4a) also shows that no element chlorine was detectable, consistent with the EDS result. The XPS spectrum of Pd 3d can be deconvolved into two main

Figure 4. XPS spectra of sample Pd-PHLS-SCF: (a) survey spectrum and (b) high-resolution XPS Pd 3d spectrum in Pd-PHLS-SCF.

doublet peaks, as illustrated in Figure 4b. The doublet with a binding energy at 335.5 eV (assigned to Pd 3d5/2) and 341.3 eV (assigned to Pd 3d3/2) can be indexed to the Pd(0) state. The other set of peaks at 337.2 eV (assigned to Pd 3d5/2) and 342.4 eV (assigned to Pd 3d3/2) is ascribed to the Pd oxidation state. Calculating the integration areas of these two doublets using XPSPEAK software, it can be estimated that the atomic ratio of metallic Pd/Pd(II) was about 59:41. These results suggest that the palladium species on PHLS was Pd(0) and the Pd oxidation state. Since there was no detectable chlorine in the composites, the oxidation form of Pd is possibly palladium oxide,25 which might result from the surface oxidation of

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Figure 5. TEM image (a) and high-resolution Rh 3d XPS spectrum (b) of Rh-PHLS-SCF and TEM image (c) and high-resolution Pt 4f XPS spectrum (d) of Pt-PHLS-SCF.

palladium nanoparticles when they are exposed to air. A similar phenomenon was also reported by others.26 All XRD, XPS, TEM, and EDS analyses confirm that Pd nanoparticles of small sizes were decorated onto the PHLS support with the aid of SC CO2. For comparison, we also performed two control experiments to load Pd onto the PHLS in ethanol and aqueous solutions, respectively, under similar conditions in the absence of SC CO2. Figure 2e,f illustrates the TEM images of the Pd-PHLS composites prepared in an ethanol solution (Pd-PHLS-ethanol) and in an aqueous solution (PdPHLS-water). It can be observed that Pd particles were also anchored on the PHLS in these solutions; however, the particle size was much larger than that in Pd-PHLS-SCF, and the size distribution was also much wider. For Pd-PHLS-ethanol composites, the particles tended to aggregate, although the particle size was relatively small, and many aggregates seemed to be clusters on the polymer surface (Figures 2e). For Pd-PHLSwater composites, larger particle aggregates with sizes from 20 to 100 nm were randomly decorated on the PHLS support (Figures 2f); moreover, some particles were free of the PHLS support. The previous results suggest that the property of the media played an important role in the fabrication of the Pd-PHLS composites. It is known that SCFs possess a lower density, higher diffusivity, lower viscosity, and near zero surface tension as compared to liquid solutions. Under the experimental conditions to prepare the Pd-PHLS-SCF composites, the CO2ethanol mixture reached a supercritical state, which thus possessed the properties of SCFs. The dissolved solute can easily reach the surface of the suspended polymer spheres and be adsorbed due to the unusual properties of the fluid.27 Moreover, the SC CO2-ethanol mixture can also swell the polymer substrates considerably.28 This might provide the precursor molecules with the chance to enter the swollen shells of the PHLS with the aid of the SC mixture.22 Thus, in the SC CO2ethanol solution, most precursor molecules could be adsorbed by the polymer support. Following hydrogen reduction, the

TABLE 1: Results of Catalytic Hydrogenation of Allyl Alcohola entry

catalyst

t (h)

1 2d 3d 4d 5d 6e 7f 8 9 10g 113h

Pd-PHLS-SCF Pd-PHLS-SCF Pd-PHLS-SCF Pd-PHLS-SCF Pd-PHLS-SCF Pd-PHLS-SCF Pd-PHLS-SCF Pd-PHLS-ethanol Pd-PHLS-water Pd/C Pd/Al2O3

8.0 8.0 8.6 8.8 9.3 10.5 13.5 32.5 42.0 18.5

conversion % selectivity (%)b TOFc 98.8 99.2 99.8 99.9 98.3 99.8 96.5 94.2 81.7 97.5

92.6 94.9 93.6 93.6 94.1 97.6 96.8

4938 4960 4644 4540 4250 3803 2858 1160

91.5

2108 1300

a Reaction conditions: T ) 30 °C and PH2 ) 1.0 MPa; the reactions were carried out in the absence of solvent unless specially noted. b Selectivity to 1-propanol; the byproduct is propanal. c Turnover frequency (TOF) was measured as mol of consumed allyl alcohol per mol of Pd per hour. d Pd-PHLS-SCF was reused 2, 3, 4, and 5 times in entries 2 to ∼5 followed by entry 1. e Solvent was a methanol/water (4:1) mixture. f Solvent was water. g Commercial Pd/C (5%) catalyst was provided by BaoJi RuiKe Co. h Data reported in ref 32. The reaction was conducted in a methanol/water (4:1) mixture under the same conditions.

precursor molecules adsorbed on the PHLS were reduced to Pd(0), which nucleated and formed nanoparticles on the PHLS support. However, in the cases of water and ethanol as solvents, only some of the precursor molecules could be adsorbed on the polymer support since the precursor had a larger solubility in water or ethanol than in the SC CO2-ethanol solution, which rendered more nanoparticles to be formed in water or ethanol after H2 reduction. Therefore, the produced nanoparticles aggregated on the surfaces of PHLS to form Pd-PHLS composites. Similarly, the as-prepared PHLS was also applied to support other noble metal nanoparticles including Pt and Rh via a SC fluid-assisted route. Figure 5 displays the TEM images of RhPHLS-SCF (Rh mass content, 1.25%) and Pt-PHLS-SCF (Pt mass content 3.64%) composites and their corresponding core

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Figure 6. Dependence of TOF on recycled runs preformed for allyl alcohol hydrogenation catalyzed by the Pd-PHLS-SCF catalyst.

level XPS spectra. From these images, it can be observed that there exists well-dispersed Rh (Figure 5a) and Pt (Figure 5c) nanoparticles on the PHLS with few larger clusters. Figure 5b,d shows the XPS spectra of Rh3d of Rh-PHLS-SCF and Pt4f of Pt-PHLS-SCF, respectively. The spectrum in Figure 5b displays two main doublet peaks. One doublet with a binding energy at 307.2 eV (Rh 3d5/2) and 312.0 eV (Rh 3d3/2) is attributed to Rh(0), and the other with a binding energy at 308.5 eV (Rh 3d5/2) and 313.3 eV (Rh 3d3/2) is indexed to the Rh oxidation state.29 On the basis of the integration areas of these two doublets, it can be concluded that the main species in Rh-PHLSSCF were Rh(0) and Rh oxide. The Pt 4f XPS spectrum in Figure 5d was deconvolved into three doublet peaks, which were assigned to Pt(0) (Pt 4f7/2, 71.4 eV and Pt 4f5/2, 74.7 eV), the Pt(II) chemical state (Pt 4f7/2, 72.5 and Pt 4f5/2, 75.8), and the Pt(IV) species (Pt 4f7/2, 74.6 and Pt 4f5/2, 77.9), respectively,30 which also indicates that Pt in the Pt-PHLS-SCF composites existed in the zerovalent and oxidation states. 3.2. Activity for Allyl Alcohol Hydrogenation of Pd-PHLSSCF. It is well-known that Pd is effective in catalyzing hydrogenation reactions.31 In this work, we selected the hydrogenation of allyl alcohol as a model reaction to study the catalytic activity of the Pd-PHLS-SCF catalyst. Owing to the unusual structure, the catalyst could be homogeneously dispersed in alcohol and alcohol aqueous solutions to form stable suspensions. The allyl alcohol hydrogenation reactions were performed under different conditions as summarized in Table 1. The as-prepared Pd-PHLS-SCF catalyst (Table 1, entry 1) exhibited a much higher activity as compared to the commercially available Pd/C catalyst (Table 1, entry 10). It showed the highest efficiency under the solvent-free condition, and its turnover frequency (TOF) reached 4960 mol of allyl alcohol per mol of Pd per hour (Table 1, entry 2). The catalyst was reused for the other four recycles (Table 1, entries 2 to ∼5), and the activity remained at the same level with a slight decrease (as shown in Figure 6), demonstrating that the as-prepared PdPHLS-SCF catalyst is rather stable for allyl alcohol hydrogenation.

The high efficiency of the catalyst might be partly owed to the good dispersibility of Pd-PHLS-SCF in allyl alcohol solution. Figure 7a shows the optical image of the suspension of allyl alcohol with the Pd-PHLS-SCF catalyst, which was taken after the suspension was kept still for 6 h. There was nearly no sedimentation of the heterogeneous catalyst. The good stability of the Pd-PHLS-SCF was also supported by TEM observation for the Pd-PHLS-SCF catalyst used 5 times. As shown in Figure 7b,c, the polymer support remained in its original structure, and the nanoparticles attached firmly onto the PHLS support with a nearly unchanged particle size. However, the particles appeared to aggregate slightly, which may account for the slight loss of the activity of the used catalyst. The Pd-PHLS-SCF catalyst also showed a high activity for allyl alcohol hydrogenation preformed in a methanol/water solution (Table 1, entry 6) and in water (Table 1, entry 7). Therefore, it is conclusive that the as-prepared Pd-PHLS-SCF catalyst is suitable to hydrogenations performed in alcohol and/ or alcohol aqueous solutions. The activity of the Pd-PHLSethanol and Pd-PHLS-water catalysts was also investigated (Table 1, entries 8 and 9). Although these catalysts were active for allyl alcohol hydrogenation, their activities were much lower than that of the Pd-PHLS-SCF catalyst. This means that the particle size played an important role in the hydrogenation of allyl alcohol. 3.3. Activity for the Heck Reaction of Pd-PHLS-SCF. As described previously, the Pd-PHLS-SCF composite is suitable for the hydrogenation performed in alcohol and aqueous solutions. Pd is also an active catalyst for Heck reactions. Since the as-prepared PHLS can tolerate many organic solvents with their microstructure preserved, in this work, the catalytic activity of the Pd-PHLS-SCF catalyst for Heck reactions was also checked using the model reaction of iodobenzene and methyl acrylate. The reaction of iodobenzene (1.0 equiv) and methyl acrylate (1.5 equiv) with the molar ratio of iodobenzene/Pd ) 5000 was performed at 100 °C in DMF containing triethylamine (Et3N, 1.5 equiv). The results are listed in Table 2. The freshly synthesized Pd-PHLS-SCF catalyst exhibited a very high activity; however, the activity decreased when the catalyst was reused, as illustrated in Figure 8. Although the activity of the catalyst gradually decreased with recycling times, the catalyst still displayed a much higher activity after reuse (Table 2, entries 2 to ∼5), as compared to the catalysts reported in the literature (Table 2, entries 6 to 9). The loss of activity probably resulted from the leaching of Pd species in the reaction process.37 From Figure 8, we know that activity loss was more significant from the first to the second entry. However, the activity decreased slowly as the catalyst was reused for another 3 times. To explain this phenomenon, we examined the morphology of the used catalyst. Different from the original structure, the used catalyst exhibited little change in morphology.

TABLE 2: Results of Heck Reactions Catalyzed by Pd-PHLS-SCF Nanocomposite

a

entry

catalyst

t (h)

conversion %a

yieldb

TOFc

1 2d 3d 4d 5d 6 (ref 33) 7 (ref 34) 8 (ref 35) 9 (ref 36)

Pd-PHLS-SCF Pd-PHLS-SCF Pd-PHLS-SCF Pd-PHLS-SCF Pd-PHLS-SCF Pd(0)/PANI PS-supported soluble palladacycle Pd/SiO2 Pd/diatomite

1.2 1.7 1.8 2.5 2.6 24 8 1.0 0.67

∼100 ∼100 ∼100 ∼100 ∼100 83 99 100 100

98.8 98.8 98.2 98.1 98.6 78 99

4167 2941 2778 2000 1923

b

c

95

6.3 355 1440

Conversion was calculated from converted iodobenzene. Yield of (E)-methylcinnamate. TOF was calculated as mol of converted iodobenzene per mol of Pd per hour. d Pd-PHLS-SCF was reused 2, 3, 4, and 5 times in entries 2 to ∼5 followed by entry 1.

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Figure 7. (a) Optical image of the suspension of allyl alcohol with Pd-PHLS-SCF (left) and pure allyl alcohol (right); TEM (b) and HRTEM (c) images of Pd-PHLS-SCF used in allyl alcohol hydrogenation 5 times; and (d) TEM image of Pd-PHLS-SCF used in the Heck reaction 5 times.

mixture, and the swollen polymer support tended to recover its original structure. The Pd species decorated on the polymer support were probably redistributed, and some were embedded within the polymer shell, which led to the morphology change of the catalyst. As the catalyst was reused, the leaching of the Pd species from the reused catalyst occurred more slowly as compared to that from the fresh catalyst since some Pd species were embedded in the polymer shell. Thus, the reused catalyst showed a decreased activity. However, during the reaction process, the leached Pd species were adsorbed by the polymer support, so the catalyst remained high in activity and was more stable after being recycled several times. Figure 8. Dependence of TOF on recycled runs preformed for a Heck reaction catalyzed by the Pd-PHLS-SCF catalyst.

As shown in Figure 7d, although the used catalyst still remained in a hollow spherical structure, the surface of the polymer support became rough; moreover, the contrast between the polymer support and the nanoparticles became weak. It seems that the Pd nanoparticles were embedded into the shells of the PHLS. The morphology change of the catalyst was caused by a swelling of the polymer support with the reaction mixture during the reaction process. It was reported that the leached Pd species were active for the Heck reaction.37 Since most of the Pd nanoparticles were decorated on the surface of the polymer support as the fresh catalyst was used, the leaching of the Pd species occurred easily and rapidly as the reaction proceeded. Most of the leached Pd species could be adsorbed on the polymer support because the polymer was rich in -CN groups. After reaction, the catalyst was separated from the reaction

4. Conclusion In summary, we successfully applied functional polymer hollow latex spheres to prepare metal nanoparticle-supported nanocomposites via a SC CO2 assisted route. The unusual properties of the supercritical solution rendered the produced metal nanoparticles (Pd, Pt, and Rh) to be distributed uniformly on the polymer substrate. These composites are dispersible in aqueous and some organic solutions, which have promising applications in catalysis fields. As an example, the resultant PdPHLS-SCF catalyst displayed a high catalytic activity and stability for the hydrogenation of allyl alcohol and Heck reactions. This work may open a new way to prepare polymerbased catalysts with a high efficiency. Acknowledgment. This work was financially supported by the Ministry of Science and Technology of China (973 Project,

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