Nanoporous Ionic Organic Networks: Stabilizing and Supporting

Letter pubs.acs.org/NanoLett

Nanoporous Ionic Organic Networks: Stabilizing and Supporting Gold Nanoparticles for Catalysis Pengfei Zhang,† Zhen-An Qiao,† Xueguang Jiang,§ Gabriel M. Veith,‡ and Sheng Dai*,†,§ †

Chemical Sciences Division and ‡Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: Nanoporous ionic organic networks (PIONs) with a high ionic density (three cation−anion pairs per unit) have been synthesized by a facile SN2 nucleophilic substitution reaction. Owing to the electrostatic and steric effect, those ionic networks with porous channels can stabilize and support gold (Au) nanoparticles (NPs) in 1−2 nm. The Au@PION hybrid materials used as a heterogeneous catalyst were highly active, selective, and stable in the aerobic oxidation of saturated alcohols. KEYWORDS: Gold nanoparticles, gold catalysis, porous polyelectrolyte, porous poly(ionic liquid)s, aerobic oxidation

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thermal activation, however, can potentially induce catalytic instability in SLIP systems. In this effort, we attempt to advance IL-based homogeneous Au stabilization by developing nanoporous ionic organic networks (PIONs) with extremely high ionic density (three ion pairs per unit) that act as solid stabilizers and at the same time as support materials for heterogeneous Au nanocatalysts. Actually, ionic organic polymers for metallic NPs have already been achieved by pioneering groups, and the most popular method was synthesis of cationic or anionic monomers followed by free radical polymerization.32−39 Those ionic organic polymers as capping ligands for Au NPs worked in a homogeneous manner (both Au precursors and ligands were dissolved), because the weak interaction between the Au precursor and the solid material made stabilization by heterojunction very difficult. The design of intensively ionized porous polymers that have a strong interaction with an Au precursor is of great interest. The cationic backbone of the current PION was constructed by a benzyl SN2 nucleophilic substitution reaction between 1,3,5-tris(bromomethyl)benzene and bipyridines (Scheme 1a). The essence of this synthetic strategy is effectively realizing ionization and polymerization in one process. By coupling intrinsically ionic complexation and accessible anion exchange with porous channels, such an exceptional family of porous polymers could serve as both stabilizers and supports for Au NPs (Scheme 1b).40−42 In addition, we showed that the Au@PION hybrid materials are active and stable in the aerobic oxidation of saturated alcohols. The synthesis of PIONs started with a nucleophilic substitution between commercial 1,3,5-tris(bromomethyl)benzene (A1) and 1,2-bis(4-pyridyl)ethylene (B1) in the presence of a hard template. Owing to the benzylic rotation,

uring the past decade, the possibility of modulating the physicochemical properties of ionic liquids (ILs) by an appropriate combination of cations and anions allied to their supramolecular organization enables the ILs-mediated preparation of metallic nanoparticles (NPs).1−5 Meanwhile, gold (Au) NPs with increased fractions of edges and corners have been recognized as surprisingly active and extraordinary effective catalysts, which has generated a highly popular research topic on the frontier between homogeneous and heterogeneous catalysis.6−14 Indeed, ILs have been found to be very good media for generating highly dispersed Au NPs without extra stabilizing molecules; very small Au NPs can be synthesized in various ILs (e.g., BmimBF4, BmimCl, BmimPF6) by chemical reduction.15−28 The ILs can form a protective layer around Au ions initially, and those weakly coordinating ions will bind to NP surfaces to give rise to a classic, ion-based, DLVO (Dergaugin−Landau−Verwey−Overbeek)-type Coulombic repulsion via both electronic and steric protection for Au NPs. Catalysis by Au NPs trapped in ILs that run in biphase/ homogeneous/semiheterogeneous modes has been demonstrated to be successful; however, important issues, such as complete catalyst recycling and separation, are still unresolved. More recently, a new technology, supported ionic liquid phase (SILP) catalysis, has emerged as an attractive alternative for the immobilization of catalytically active metal NP/IL composites.29−31 In SILP catalysts, a thin film of ILs containing the metal species is confined on the surface of a porous support. SILP catalysts combine the advantages of ILs (e.g., a structuredirecting role) with those of supported materials (e.g., high porosity or large surface area) for heterogeneous catalysis, and a variety of reactions have been studied in which SILP catalysts proved to be more active and selective than common systems.29−31 Several drawbacks associated with multicomponent SILP systems, such as the leaching of ILs in contact with solvents and the detachment of ILs from metal NPs under © XXXX American Chemical Society

Received: August 30, 2014 Revised: January 18, 2015

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Scheme 1. (a) Synthetic Routes to Nanoporous Ionic Organic Networks (PIONs), NMP: N-Methyl-2-pyrrolidinone; (b) a Controllable Route to Au@PION Hybrid Materials

the ionic networks themselves are not sufficiently rigid to prevent intermolecular packing; therefore, silica NPs (particle size: 12 nm) were introduced for porosity control. Pink to yellow powders at high yield (>90%) were obtained after the silica was dissolved in hydrofluoric acid. The Fourier transform infrared spectra of PION-1 with SiO2 (PION-1@SiO2) showed a huge peak around 870−1280 cm−1 related to the template; this peak disappeared after the removal of silica, but the representative sorption peaks for PION-1 were preserved (Figure S1 in the Supporting Information). The successful removal of silica was also confirmed by the thermogravimetric analysis of PION-1@SiO2 and PION-1 in air, in which very little residue was observed for PION-1 at 800 °C (Figure S2 in the Supporting Information). X-ray diffraction (XRD) measurements of PION-1 exhibited a broad reflection around 21°, which suggests its amorphous state (Figure S3 in the Supporting Information). The backbones of ionic networks were characterized by solid-state 13C CP/MAS (cross-polarization magic angle spinning) NMR (Figure 1a). The attribution of those peaks was justified by solution 13C NMR studies of monomers and model molecules (Figures S4−S7 in the Supporting Information). The broad carbon signals at around d ≈ 136 ppm in the NMR spectra are in agreement with responses of model molecules for the phenyl ring and can be ascribed to the carbon atoms of the phenyl rings of PIONs (Figure S4 in the Supporting Information). Peak A of PION-2 and peaks A and B in PION-1 and PION-3 should be induced by the carbon atoms (C-2, C-4, and C-6 positions) in the pyridinium ring bearing a positive charge. In PION-3, carbon atom A (C-4 position) close to the azo group showed a higher chemical shift than carbon atom B (Figure S6 in the Supporting Information);

Figure 1. (a) Solid-state13C CP/MAS NMR analysis of PIONs; (b) SEM and (c) TEM images of a PION-1 sample.

whereas in the PION-1 sample, carbon atom B (C-4 position) next to the vinyl group showed a lower chemical shift than carbon atom A (Figure S7 in the Supporting Information). The carbon atoms (C-3 and C-5 positions) in the pyridinium ring illustrated lower chemical shifts compared with other carbons in the ring, especially the PION-3 sample (116 ppm for the B

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The synthetic methodology of supporting Au NPs on PION1 involves ion exchange and chemical reduction. In brief, the AuCl4− ions were first introduced as counterions of the cationic backbone (AuCl4@PION-1), and the mixture was then reduced by a freshly prepared aqueous solution of NaBH4. The formation of an AuCl4@PION-1 intermediate was confirmed by X-ray photoelectron spectroscopy (XPS) measurement to have an Au 4f7/2 binding energy of around 85.2 eV for Au3+ and a Cl− response at ∼199 eV (Figure 2b). An immediate color change from pink to deep red−brown occurred with vigorous gas evolution. After the reaction mixture was rapidly stirred for 2 h, the solids were separated by centrifugation (labeled Au@ PION-1). As shown in the scanning transmission electron microscope (STEM) and TEM images, the Au NPs of 2 wt % Au@PION-1 were quite small (mean size: 2.2 nm) and homogeneously dispersed on the polymer support (Figure 2c− e). A typical XRD pattern of the as-prepared Au composites showed a broad reflection at 2θ = 38° assigned to (111) reflection of the cubic (fcc) gold lattice (Figure 2f). Actually, the initial incorporation of [AuCl4]− anion into the ionic backbone has been suggested by XPS data. Since the cationic backbone could interact with [AuCl4]− anions before reduction, a small size for the Au NPs could be expected, considering the electrostatic repulsion from the highly charged structure and the steric effect of the cross-linked network. In general, Au NPs/clusters are stabilized by soluble organic ligands, which often enwrap the Au clusters densely and thereby deactivate the catalytic activity to a degree.13,14,32 Such an open structure for PION-1 would expose a high fraction of accessible atoms on the surfaces of the Au NPs, which should be beneficial for heterogeneous catalysis. The selective oxidation of chemically inert saturated alcohols to their corresponding aldehydes or ketones using molecular oxygen is a profound process in both organic synthesis and the chemical industry.45−47 To probe the catalytic activity of the Au@PION-1 material, aerobic oxidation of cyclohexanol to cyclohexone, a key step for Nylon-6 and Nylon-66 production, was initially selected as a model reaction. The catalytic results are summarized in Table 1. Optimization of the reaction solvents indicated that toluene was a preferable medium for this process (entries 1−4, Table 1). In the presence of 2 wt % Au@ PION-1 catalyst, cyclohexanol was oxidized completely (conv.: >99%) with high selectivity (sel.: >99%) to cyclohexone in 20 h, whereas blank runs without a catalyst or with only polymer support did not give rise to any detectable products (entries 5− 7, Table 1). Comparable oxidation was also carried out under an argon atmosphere, and a moderate cyclohexanol conversion (60.4%) was observed, which revealed a possible reaction pathway via O2-asisted dehydrogenation process. It should be emphasized that no inorganic bases were needed in this Aupromoted oxidation. On the basis of these encouraging results, Au@PION-1 was then extended to the oxidation of some other saturated alcohols. Cyclopentanol, cyclooctanol, 1-octanol, 2-octanol, and 2-adamantanol with steric hindrance were oxidized smoothly with the corresponding aldehyde or ketone as main products (entries 9−13, Table 1). The recovery and reuse of Au@PION1 was then investigated by cyclohexanol oxidation for 6 h. After the first run, the catalyst was recovered by centrifugation, and then it was carefully transferred into a reactor by the reaction solvent. The catalytic activity of Au@PION-1, with respect to cyclohexanol conversion, was basically maintained over five runs (Figure 3a). The catalyst recovery by weight was 84% after

monomer, Figure S6 in the Supporting Information). The signals at approximately d = 62−66 ppm can be assigned to the benzylic carbon atom next to the charged unit. On a macroscopic scale, interconnected polymer spheres with surface wrinkles and holes are observed in the scanning electron microscopy (SEM) image of PION-1.They have their origin probably in the polymerization-induced phase separation (Figure 1b, Figure S8). As shown in the transmission electron microscopy (TEM) image, the polymer spheres feature an abundant inner porosity, fabricated by silica scaffolds (Figure 1c, Figure S9). The TEM analysis was also helpful in excluding the presence of excess silica nanoparticles. The N2 sorption measurements of PIONs showed their BET surface areas of 107−132 m2/g, and the significant N2 uptake at a high P/Po range, together with the pore size distribution, suggested that the porosity was dominated by large mesopores and macropores (Figure 2a, Figure S10). Compared with neutral porous

Figure 2. (a) N2 sorption isotherms of PION samples at 77 K. For clarity, the isotherm curves were offset by 50 cm3/g for PION-2 and 100 cm3/g for PION-1; (b) XPS measurement for AuCl4@PION-1 sample; (c) STEM and (d) TEM images of 2 wt % Au@PION-1 sample; (e) particle size distribution of Au NPs; (f) XRD patterns of fresh 2 wt % Au@PION-1 and recovered sample.

polymers, it seems that those PIONs had low surface areas; we consider that this nucleophilic substitution reaction-mediated polymerization may immerse only part of the space of the hard template.43,44 This hypothesis was studied by controlled synthesis of PION-1 in the presence of an ordered mesoporous silica template (KIT-6) (Note S1, Supporting Information). In turn, the ionic structure indeed provides a natural platform for the variability and immobilization of anions, and the PION-1 could be regarded, in one view, as a well-defined nanoreactor for stabilizing and supporting Au NPs. C

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Reaction conditions: alcohol 1 mmol, anisole 1 mmol (Internal Standard), 2% Au@PION-1 10 mg, solvent 3 mL, O2 1 atm, TOF = [reacted mol alcohol]/[(total mol gold) × (reaction time)]. The TOFs were measured after the first 1 h of reaction, TON = [reacted mol alcohol]/[total mol gold]. Conv., conversion; Sel., selectivity. The solvent-free oxidations were performed in 30 mmol alcohols. Superscripted letters indicate the following: b, the molar ratio between alcohol and gold; c, blank oxidation without catalysts; d, blank oxidation with PION-1 only; e, reaction in argon atmosphere; f, control oxidation with Au@TiO2 catalyst.

the mean diameter of the Au NPs was 2.9 nm, which was a little bigger than the size of the pristine catalyst (Figure 3b). In consideration of the nice dispersity of PION in alcohols and current base-free catalysis, solvent-free oxidation of cyclohexanol by atmospheric oxygen was subsequently studied. With the molar ratio of alcohol to Au at ∼240 000, oxidation at 140 °C afforded a 24.4% cyclohexanol conversion in 40 h; however, a controlled reaction without catalyst did not result in any detectable products (conv.: 0.2%) (entries 14−16, Table 1). A remarkable turnover frequency (TOF = 2064 h−1) and turnover number (TON = 58 560) for the current Au nanocatalyst were achieved, which could be comparable with those for other noble metal catalysts in the literatures (Au@ hydrotalcite: TOF = 56 h−1, TON = 207; Au@meso-SiO2: TOF = 119 h−1; Au@ammonium salts of hyperbranched polystyrene: TOF = 2 h−1, TON = 50; Au−Pd@layered double hydroxide: TOF = 4 h−1, TON = 60; Pd@Al2O3: TOF = 75 h−1, TON = 1200).40−42,48−51 Considering that the results reported above were obtained under different reaction conditions, control oxidation with a reference catalyst (Au@ TiO2) from the World Gold Council was then tested under exactly the same conditions (entries 15 and 17, Table 1). The active Au@TiO2 resulted in a slightly higher TOF (2517 h−1) value compared with the Au@PION-1 catalyst (TOF = 2064 h−1). However, it seems that more byproducts (sel.: 72%) formed in the presence of Au@TiO2, and the overoxidation of cyclohexanone was possibly caused by the active O2•− radical.

Figure 3. (a) Recycling runs for the aerobic oxidation of cyclohexanol by Au@PION-1 catalysts; reaction condition: cyclohexanol 1 mmol, anisole 1 mmol (Internal Standard), 2 wt % Au@PION-1 (recovered), toluene 3 mL, O2 1 atm, 6 h. Conv., conversion for cyclohexanol; Sel., selectivity for cyclohexanone; (b) STEM image of recovered 2 wt % Au@PION-1 sample after five runs.

five runs, which is understandable in such small-scale test reactions. The liquid phase of the reaction mixture was also collected by hot filtration after the first run and analyzed by inductively coupled plasma mass spectrometry. A very low amount of dissolved gold (∼0.1% of the total gold) was detected in the solution at the end of the reaction. Moreover, after the Au@PION-1 catalyst was removed from the reaction solution after 6 h (Conv.:37%), the supernatant did not show any further reactivity over the next 20 h. The crystalline state of the Au NPs in the recovered catalyst, based on the corresponding XRD pattern, did not change after five runs (Figure 2f). A STEM image of the reused catalyst indicated that D

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(6) Zhao, P. X.; Li, N.; Astruc, D. Coord. Chem. Rev. 2013, 257, 638− 665. (7) Jin, R. C. Nanoscale 2010, 2, 343−362. (8) Tsunoyama, H.; Ichikuni, N.; Sakurai, H.; Tsukuda, T. J. Am. Chem. Soc. 2009, 131, 7086−7093. (9) Kojima, K.; Chikama, K.; Ishikawa, M.; Tanaka, A.; Nishikata, T.; Tsutsumi, H.; Igawa, K.; Nagashima, H. Chem. Commun. 2012, 48, 10666−10668. (10) Carrettin, S.; Guzman, J.; Corma, A. Angew. Chem., Int. Ed. 2005, 44, 2242−2245. (11) Hervés, P.; Pérez-Lorenzo, M.; Liz-Marzán, L. M.; Dzubiella, J.; Lu, Y.; Ballauff, M. Chem. Soc. Rev. 2012, 41, 5577−5587. (12) Taketoshi, A.; Haruta, M. Chem. Lett. 2014, 43, 380−387. (13) Stratakis, M.; Garcia, H. Chem. Rev. 2012, 112, 4469−4506. (14) Liu, X.; He, L.; Liu, Y.; Cao, Y. Acc. Chem. Res. 2014, 47, 793− 805. (15) Starkey-Ott, L.; Finke, R. G. Coord. Chem. Rev. 2007, 251, 1075−1100. (16) Mertens, P. G. N.; Poelman, H.; Ye, X.; Vankelecom, I. F. J.; Jacobs, P. A.; De Vos, D. E. Catal. Today 2007, 122, 352−360. (17) Vignolle, J.; Tilley, T. D. Chem. Commun. 2009, 46, 7230−7232. (18) Stefanescu, D.; Glueck, D.; Siegel, R.; Wasylishen, R. J. Cluster Sci. 2008, 19, 445−458. (19) Stefanescu, D. M.; Glueck, D. S.; Siegel, R.; Wasylishen, R. E. Langmuir 2004, 20, 10379−10381. (20) Jewell, A. D.; Sykes, E. C. H.; Kyriakou, G. ACS Nano 2012, 6, 3545−3552. (21) Dolamic, I.; Knoppe, S.; Dass, A.; Bürgi, T. Nat. Commun. 2012, 3, 798. (22) Sun, K.-Q.; Hong, Y.-C.; Zhang, G.-R.; Xu, B.-Q. ACS Catal. 2011, 1, 1336−1346. (23) Huang, J.; Jiang, T.; Gao, H.; Han, B.; Liu, Z.; Wu, W.; Chang, Y.; Zhao, G. Angew. Chem., Int. Ed. 2004, 43, 1397−1399. (24) Fei, Z.; Zhao, D.; Pieraccini, D.; Ang, W.; Geldbach, T.; Scopelliti, R.; Chiappe, C.; Dyson, P. J. Organometallics 2007, 26, 1588−1598. (25) Shana, J.; Tenhu, H. Chem. Commun. 2007, 4580−4598. (26) Zhang, H.; Cui, H. Langmuir 2009, 25, 2604−2612. (27) Tang, S.; Baker, G. A.; Zhao, H. Chem. Soc. Rev. 2012, 41, 4030−4066. (28) Tatumia, R.; Fujihara, H. Chem. Commun. 2005, 83−85. (29) Jakuttis, M.; Schönweiz, A.; Werner, S.; Franke, R.; Wiese, K.; Haumann, M.; Wasserscheid, P. Angew. Chem., Int. Ed. 2011, 50, 4492−4495. (30) Ha, H.; Duc, D.; Dao, T.; Le, M. T.; Riisager, A.; Fehrmann, R. Catal. Commun. 2012, 25, 136−141. (31) Mehnert, C.; Cook, R.; Dispenziere, N.; Afeworki, M. J. Am. Chem. Soc. 2002, 124, 12932−12933. (32) Biondi, I.; Laurenczy, G.; Dyson, P. J. Inorg. Chem. 2011, 50, 8038−8045. (33) Sahiner, N.; Turhan, T.; Lyon, L. A. Energy 2014, 66, 256−263. (34) Zhang, Y.; Quek, X.-Y.; Wu, L.; Guan, Y.; Hensen, E. J. J. Mol. Catal. A: Chem. 2013, 379, 53−58. (35) Zhao, Q.; Zhang, P.; Antonietti, M.; Yuan, J. Y. J. Am. Chem. Soc. 2012, 134, 11852−11855. (36) Fischer, S.; Schmidt, J.; Strauch, P.; Thomas, A. Angew. Chem., Int. Ed. 2013, 52, 12174−12178. (37) Zhang, Q.; Zhang, S.; Li, S. Macromolecules 2012, 45, 2981− 2988. (38) Zhao, H.; Wang, Y.; Wang, R. Chem. Commun. 2014, 50, 10871−10874. (39) Tan, M.; Zhang, Y.; Ying, J. Y. Adv. Synth. Catal. 2009, 351, 1390−1394. (40) He, D.; Shi, H.; Wu, Y.; Xu, B. Q. Green Chem. 2007, 9, 849− 851. (41) Mitsudome, T.; Noujima, A.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Adv. Synth. Catal. 2009, 351, 1890−1896. (42) Yan, X.; Wang, X.; Tang, Y.; Ma, Y. G.; Zou, S.; Li, R.; Peng, X. G.; Dai, S.; Fan, J. Chem. Commun. 2013, 49, 7274−7276.

The higher selectivity of Au@PION-1 (sel.: 95%) was interesting and reasonable because previous work suggested that pyridinium/imidazolium cation can stabilize the O2•− radical; therefore, the PION-1 with high pyridinium ring density might decrease the O2•− radical-induced overoxidation to some extent.52 In summary, we have presented a facile manner of polymerization for charged nanoporous organic materials using a nucleophilic substitution reaction. PIONs with a highly ionized backbone exhibited an unexpected talent for the control of Au NPs and therefore endowed PIONs with dual roles (stabilization and support). The Au NPs trapped in PIONs afforded outstanding performance in the aerobic oxidation of saturated alcohols. For example, the Au@PION hybrid materials enabled the selective oxidation of cyclohexanol to cyclohexanone with exceptional activity (TOF = 2064 h−1). Moreover, the accessible anion exchange character, complexing action, and porous channels of PIONs may be extended to control various metal clusters/NPs/ions, for example, copper specie anions can be incorporated by ionic interaction, which results in active catalysts for the selective oxidation of phenol (Note S2, Supporting Information). We believe that the current strategy for ionic polymers will inspire many more designs of functional organic or inorganic−organic materials for catalysis at the nanoscale.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental details, supplementary figures, tables, and note. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.F.Z. and S.D. were supported as part of the Fluid Interface Reactions, Structures, and Transport (FIRST) Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. ERKCC61. Z.A.Q. and X.G.J. were supported by Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy. GMV was supported by U.S. Department of Energy’s Office of Basic Energy Science, Division of Materials Sciences and Engineering.

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REFERENCES

(1) Kauling, A.; Ebeling, G.; Morais, J.; Pádua, A.; Grehl, T.; Brongersma, H.; Dupont, J. Langmuir 2013, 29, 14301−14306. (2) Itoh, H.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 2004, 126, 3026− 3027. (3) Redel, E.; Walter, M.; Thomann, R.; Hussein, L.; Krüger, M.; Janiak, C. Chem. Commun. 2010, 46, 1159−1161. (4) Kim, K.; Demberelnyamba, D.; Lee, H. Langmuir 2004, 20, 556− 560. (5) Yavuz, M. S.; Cheng, Y.; Chen, J.; Cobley, C. M.; Zhang, Q.; Rycenga, M.; Xie, J.; Kim, C. H.; Song, K. H.; Schwartz, A. G.; Wang, L. V.; Xia, Y. N. Nat. Mater. 2009, 12, 935−939. E

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Nano Letters (43) Wu, D.; Xu, F.; Xu, F.; Sun, B.; Fu, R.; He, H.; Matyjaszewski, K. Chem. Rev. 2012, 112, 3959−4015. (44) Wilke, A.; Yuan, J. Y.; Antonietti, M.; Weber, J. ACS Macro Lett. 2012, 1, 1028−1031. (45) Davis, S. E.; Ide, M. S.; Davis, R. J. Green Chem. 2013, 15, 17− 45. (46) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362−365. (47) Chan-Thaw, C. E.; Villa, A.; Prati, L.; Thomas, A. Chem.Eur. J. 2011, 17, 1052−1057. (48) Gao, L.; Nishikata, T.; Kojima, K.; Chikama, K.; Nagashima, H. Chem.Asian J. 2013, 8, 3152−3163. (49) Shi, Y.; Yang, H.; Zhao, X.; Cao, T.; Chen, J.; Zhu, W.; Yu, Y.; Hou, Z. Catal. Commun. 2012, 18, 142−146. (50) Wu, H.; Zhang, Q.; Wang, Y. Adv. Synth. Catal. 2005, 347, 1356−1360. (51) Umpierre, A. P.; Jesús, E.; Dupont, J. ChemCatChem 2011, 3, 1413−1418. (52) Islam, M.; Imase, T.; Okajima, T.; Takahashi, M.; Niikura, Y.; Kawashima, N.; Nakamura, Y.; Ohsaka, T. J. Phys. Chem. A 2009, 113, 912−916.

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