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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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DOI: 10.1039/C4NR06567K
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Synthesis of stable heterogeneous catalysts by supporting carbonstabilized palladium nanoparticles on MOFs
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Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X DOI: 10.1039/b000000x Cycling instability of catalysts is a persisting challenge in heterogeneous catalysis. In this work, we reported an effective in-situ strategy for preparing ZIF-8 supported carbonstabilized Pd nanoparticles (C@Pd/ZIF-8). The original ZIF8 structure was well preserved after formation of Pd nanoparticles and amorphous carbon. Here, the Pd nanoparticles were encapsulated in carbon matrix with a good dispersion. The as-prepared catalysts showed a better activity and cycling stability in the hydrogenation of C=C containing substrates. C@Pd/ZIF-8 catalysts are reusable without significant loss of activity after 5 times, exhibiting higher cycling stability than those by directly supported Pd nanoparticles on ZIF-8 (Pd/ZIF-8). Metal nanoparticles (NPs) have been applied as highly active catalysts in the field of organic syntheses because the catalytic performances of these NPs, such as oxidation and hydrogenation.1 The heterogeneous catalytic systems with metals (Pd, Pt, Au) have been reviewed by Hutchings,2 and among these noble metals, Pd-based catalysts have been extensively investigated because of the remarkable catalytic activity.3-5 In general, Pd NPs are dispersed onto the porous matrices,6-10 such as mesoporous silica and zeolite, to prepare heterogeneous catalysts. However, the naked Pd NPs supported on porous materials usually suffer from serious leaching or aggregation during the catalytic reactions, which inevitably leads to a loss of catalytic activity and cycling stability.11 To overcome these disadvantages, surface functionalization to supporting matrices was therefore established.12 Although these approaches can alleviate the problem of Pd NPs leaching by strengthening the interactions between the NPs and matrices, surface functionalization of the matrices usually requires multiple procedures, which may also risk a decreased activity due to the passivation of strong ligands to the NPs. As a consequence, it is necessary to find a new strategy for the preparation of heterogeneous Pd-based catalysts with high activity and reusability. Very recently, there has been a rapidly growing attention towards the capsulation of catalytically active NPs inside metalorganic frameworks (MOFs) due to MOFs have well defined porosity, high specific surface area and chemical tunability.13-17 In general, the functional properties of MOFs are mainly dominated by their pores. The tunable pores can be used for the fabrication of metal NPs with controlled sizes, thus circumventing the concerned issue of NPs migration and aggregation. Furthermore, MOFs offer the permanent nanoscaled cavities and open channels, which provide congenital conditions for small This journal is © The Royal Society of Chemistry [year]
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molecules to access, and also exhibit a potential for the use as a template. In this regard, some important pioneer achievements have been made.18-23 Xu’s group reported catalytically active Pd NPs immobilized inside the pores of a typical metal-organic framework, MIL-101, without aggregation of Pd NPs on the external surfaces of framework by using a “double solvents” method.18 The resulting Pd@MIL-101 composites represented the highly active MOF-immobilized metal NPs for catalytic reactions. Tang and co-workers used amino-functionalized IRMOF-3 shell to surround the Pd NPs core via a facile mixed solvothermal method, and the nanocomposites exhibit high activity and excellent stability in the cascade reaction.19 Our previous work also successfully encapsulated a series of presynthesized PVP-modified NPs into a zeolitic imidazolate framework, including Pd NPs, Au NPs and Pt NPs etc.22-24 We found that the restriction of MOFs framework to the encapsulated NPs provided a good size-based selectivity to the reactants. However, in most existing MOF nanocomposite materials, there are few reports focused on reusability of MOF encapsulated NPs catalysts. Considering the great potential applications of these hybrid catalysts in commercial utilizations, it is important to improve the stability of catalysts. An effective approach is to mitigate of migration and agglomeration of NPs by deposition and thermal decomposition of carbon coating on the catalysts surface.25 For example, Jiang and co-workers showed excellent stability for carbon riveted Pt/TiO2-C catalysts based on in-situ carbonization of the glucose, which was ascribed to the unique composite nanoarchitecture.26 In the present investigation, we reported a novel approach for preparing highly stable heterogeneous catalysts by using zeolitic imidazolate framework-8 (ZIF-8), a typical MOF consisting of zinc ions linked by 2-methylimidazole ligands [Zn(MeIM)2; MeIM = 2-methylimidazole], as a porous supporting matrix to disperse the Pd NPs in-situ encapsulated by carbon. Herein, the in-situ formed Pd NPs were stably encapsulated inside the continuous formed carbon, and these carbon-encapsulated Pd NPs were highly dispersed on the surface of the porous ZIF-8 without aggregation. As a result, the high activity and stability of the asprepared ZIF-8 supported carbon-stabilized Pd NPs (C@Pd/ZIF8) catalysts could be expected due to their unique structure configuration. In a series of heterogeneous hydrogenations, the high activity of C@Pd/ZIF-8 catalysts was confirmed, and it also showed considerably improved cycling stability when compared with the catalysts prepared by supporting naked Pd NPs on ZIF-8 (Pd/ZIF-8). The framework of ZIF-8 has a high thermal stability, and well defined three-dimensional interconnected porous structure with high surface area and large pore size.27-29 Hence, the porous ZIF8 crystals were ideal matrices for the convenient preparation of Journal Name, [year], [vol], 00–00 | 1
Nanoscale Accepted Manuscript
Published on 06 January 2015. Downloaded by Gazi Universitesi on 06/01/2015 12:22:06.
Weiqiang Zhou,a Binghua Zou,c Weina Zhang,b Danbi Tian,c Wei Huang,*a Fengwei Huo*ab
Nanoscale
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supported carbon-capsulated NPs catalysts by simple thermal treatment. On the basis of the above idea, glucose (carbon source) and Pd salts were simultaneously introduced into the porous channel of ZIF-8, and then converted to the corresponding carbon and metallic NPs by thermal reduction treatment. Fig. 1 is the schematic illustration showing the preparation procedures of C@Pd/ZIF-8 catalysts. First, the as-prepared ZIF-8 was impregnated with the mixture of glucose and PdCl2, and then, the intermediate material of glucose-Pd2+/ZIF-8 was simply treated by thermal reduction in Ar/H2 flow. Based on the TGA curve analysis (Fig. S1, ESI†), the glucose is stable up to ~175 oC, while a sharp weight loss is observed when the temperature is beyond 200 oC. The weight loss of glucose reaches 42% at 300 o C. For ZIF-8 (Fig. S2, ESI†), a weight-loss of 8.7% is observed below 300 oC, corresponding to the removal of water molecules residual in the porous structure of ZIF-8. In the range of 300-400 o C, a long plateau is shown. Almost no weight loss associated to the decomposition of ZIF-8 is observed when the temperature is at 300 oC. These results suggest that the structure of ZIF-8 can be well maintained at 300 oC, while the glucose can be carbonized. Consequently, we chose the temperature of 300 oC for the thermal reduction of glucose-Pd2+/ZIF-8. In this reduction process, the Pd2+ was in-situ reduced as Pd NPs, and simultaneously, the glucose was decomposed to carbon. It is noted that the growth of Pd NPs should be strictly controlled by the surrounding carbon, as well as the pores of ZIF-8, which therefore provide good dispersion and high stability. Actually, the following TEM (Fig. 3) results confirm this prediction. Based on acid digestion and ICP-AES analysis (Perkin-Elmer Optima 2100DV), the loading of noble metals in C@Pd/ZIF-8 and Pd/ZIF-8 is about 2.7% and 2.9%, respectively.
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which presumably attribute to the low loading of Pd and/or small particle size of Pd NPs.30 Similar to the C@Pd/ZIF-8, Pd/ZIF-8 catalysts show high crystal peaks of ZIF-8 in its XRD pattern while the peaks related to Pd is still absence. To further prove that the ZIF-8 structure was well retained in C@Pd/ZIF-8, we conducted the analyses of Fourier transform infrared (FT-IR) spectrum. As shown in the spectrum of pure ZIF-8 (Fig. S5, ESI†), almost all of the peaks are in good agreement with those of reported ZIF-8.31,32 The peaks at 3136 cm-1 and 2929 cm-1 are attributed to the stretching vibrations of C-H bonds in the methyl group and imidazole ring, respectively. The peak at 1584 cm-1 can be ascribed to the C=N stretch mode of imidazole ring of ZIF-8, and the peaks in the range of 6501500 cm-1 are associated with the ring stretching or bending. The spectrum of C@Pd/ZIF-8 catalysts shows the identical peaks at 3136, 2930 and 1584 cm-1 with pure ZIF-8. The FT-IR results are consistent with the above XRD analysis, confirming the presence of ZIF-8 structure in the catalysts after thermal treatment.
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Fig. 2 SEM images and particle size distribution of the as-prepared ZIF-8 (a, b), Pd/ZIF-8 (c, d) and C@Pd/ZIF-8 (e, f).
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Fig. 1 Schematic illustration of synthesis of C@Pd/ZIF-8 catalysts.
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Based on N2 adsorption-desorption isotherms (Fig. S3, ESI†), the BET surface area for ZIF-8, Pd/ZIF-8 and C@Pd/ZIF-8 was 1383 m2 g-1, 1336 m2 g-1 and 929 m2 g-1, respectively. As expected, C@Pd/ZIF-8 has the lower surface area than pure ZIF8 due to the carbon residual inside the pores. The pore size distributions of C@Pd/ZIF-8 and Pd/ZIF-8 are mainly at ~2 nm, and it will be large enough for hydrogenation reactions of substrates within the cavity. The crystal structure of the assynthesized ZIF-8, C@Pd/ZIF-8 and Pd/ZIF-8 composites were characterized by powder X-ray diffraction (PXRD) (Fig. S4, ESI†). It can be observed that the diffraction peaks of synthesized ZIF-8 at 2θ values of 7.33o, 10.38o, 12.71o, 14.68o and 16.43o are well matched with those of simulated XRD patterns of ZIF-8,29 which are perfectly indexed to (011), (002), (112), (022) and (013) crystal planes, respectively. Compared with ZIF-8, the C@Pd/ZIF-8 catalysts also show the identical characteristic peaks of (011), (002), (112), (022) and (013) planes. These results suggest that the well preservation of ZIF-8 structure in the C@Pd/ZIF-8 catalysts. Additionally, no obvious diffractions were detected for Pd species from the XRD patterns of C@Pd/ZIF-8, This journal is © The Royal Society of Chemistry [year]
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The SEM images (Fig. 2) show morphology and size distribution of the as-prepared ZIF-8, Pd/ZIF-8 and C@Pd/ZIF-8 samples. As seen in Fig. 2a, c and e, it is observed that the single particles in the three samples have the same shapes but different sizes. Based on the size distribution analyses (Fig. 2b, d and f), the average sizes of pure ZIF-8, Pd/ZIF-8 and C@Pd/ZIF-8 are about 165 nm, 175 nm and 210 nm, respectively. It is worth noting that the size of C@Pd/ZIF-8 particles is larger than that of the synthesized ZIF-8. It may attribute to amorphous carbon which formed by the decomposition of glucose on the surface of ZIF-8. However, the size of Pd/ZIF-8 particle is close to that of pure ZIF-8 due to the absence of carbon. Based on TEM analysis, the distribution of Pd NPs is well dispersed, and no serious aggregation of the Pd NPs is observed. TEM images of Pd/ZIF-8 and C@Pd/ZIF-8 catalysts are shown in Fig. 3a and d, and the corresponding particle size distributions are presented in Fig. 3c and f. As shown in Fig. 3a-c, the Pd NPs in Pd/ZIF-8 are dispersed in ZIF-8 crystal, and the corresponding particle sizes are around 1.7±0.6 nm. In Fig. 3f, the Pd particles in C@Pd/ZIF-8 exhibited larger size with average diameter of 2.7±0.4 nm, showing a narrow distribution. It can be attributed to the stabilization of carbon towards the Pd particles. Moreover, the formation of the carbon could restrain the excessive growth of the Pd particles during the reduction. The formed carbon could provide enough steric hindrance to prevent the aggregation of Pd nuclei when the Pd2+ was reduced by H2. Subsequently, highresolution TEM (HRTEM) observation was carried out (Fig. 3b Journal Name, [year], [vol], 00–00 | 2
Nanoscale Accepted Manuscript
DOI: 10.1039/C4NR06567K
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and e). It can be seen that small Pd particles with observed lattice are embedded in the catalysts. The small particle size may explain that there is no obvious peak of Pd in the XRD patterns of C@Pd/ZIF-8. Considering the uniform dispersion and small particle size of Pd NPs, high catalytic activity of the C@Pd/ZIF-8 catalysts can be expected in the hydrogenation reactions.
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Fig. 3 TEM images and Pd particle size distribution of Pd/ZIF-8 (a-c) and C@Pd/ZIF-8 (d-f). Table 1 Catalytic activity of C@Pd/ZIF-8 and Pd/ZIF-8 for the heterogeneous hydrogenation of olefinsa
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Table 2 Comparison of the cycling stability of the C@Pd/ZIF-8 and Pd/ZIF-8 catalysts in heterogeneous hydrogenationa
Conversion (%) Substrates
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Substrate conversion (%)
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Reaction conditions: 5.0 µmol Pd catalyst, 2.0 MPa H2, 80 oC, 30 min, and substrate 2.0 mmol.
Pd catalysts have been widely used for the catalytic hydrogenation of olefins.33-35 Hence, the catalytic activity of C@Pd/ZIF-8 catalysts was evaluated in hydrogenation of unsaturated compounds, including styrene, 1-heptene, cyclohexene, cyclooctene and quinoline. The results obtained by using C@Pd/ZIF-8 and Pd/ZIF-8 as catalysts are shown in Table 1. Pd/ZIF-8 catalysts exhibit comparable conversion yield in hydrogenation of olefins compared with the C@Pd/ZIF-8. For example, the conversion yield of styrene catalyzed by Pd/ZIF-8 is 98.5%, which is slightly higher than that of C@Pd/ZIF-8 (95.1%). The cycling stabilities of C@Pd/ZIF-8 and Pd/ZIF-8 in the hydrogenation of styrene, 1-heptene and cyclohexene were This journal is © The Royal Society of Chemistry [year]
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95.1b (98.5c)
75.2 (77.8)
47.3 (44.9)
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72.7 (30.6)
42.2 (5.1)
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68.3 (9.7)
39.7 (0)
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also conducted under the same conditions. As shown in Table 2, the C@Pd/ZIF-8 catalysts reused 5 times without significant loss of the catalytic activity. In the hydrogenations of styrene, 1heptene and cyclohexene, the corresponding conversion yields in the 5th run are 85.3%, 66.9% and 37.5%, respectively, which account for the corresponding activity in 1-run by 89.70%, 88.96% and 79.28%, respectively. It should be also noted that these values are lower than their actual values because there is some loss of catalysts during recycling. The TEM image of the reused C@Pd/ZIF-8 catalyst shows that the mean diameter of Pd NPs is about 3 nm, the same as the fresh catalyst, indicating that no agglomeration of NPs occurs during the reaction (Fig. S6, ESI†). On the contrary, the conversion yields catalyzed by Pd/ZIF-8 have decreased drastically during the recycles. In the 4th run, the conversion yields of styrene, 1-heptene and cyclohexene are only 25.5%, 9.7% and 0, respectively. It can be explained that the Pd particles supported by pure ZIF-8 may suffer serious aggregating or leaching during the reactions, confirmed by the TEM observation (Fig. S7, ESI†), which accounts for the catalyst deactivation. We also conducted the cycling stability experiments of C@Pd/ZIF-8 and commercial Pd/Carbon (Sigma-Aldrich), the results demonstrated that the commercial Pd/C catalyst exhibited poor recycling stability compared with C@Pd/ZIF-8 (Table S1, ESI†). In contrast, the Pd NPs in C@Pd/ZIF-8 can avoid aggregating and leaching due to the efficient stabilization of amorphous carbon, exhibiting satisfied cycling stability and high activity.
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Reaction conditions: 5.0 µmol Pd catalyst, 2.0 MPa H2, 80 oC, 30 min, and substrate 2.0 mmol. b Results obtained from the C@Pd/ZIF-8 catalyst. c Results obtained from the Pd/ZIF-8 catalyst.
In conclusion, we have demonstrated an effective strategy for preparing the heterogeneous catalysts of ZIF-8 supported the carbon-stabilized Pd NPs. For the synthesis, the introduced glucose is a key factor to achieve the desired catalyst structure, providing the continuous in-situ formed carbon to stabilize the Pd NPs. Due to the efficient stabilization of carbon matrices, the Pd NPs with controlled size can be well dispersed onto the ZIF-8. The as-prepared C@Pd/ZIF-8 has exhibited high activity in hydrogenation of olefins. It can be used 5 times without significant loss of the catalytic activity, showing a satisfied reusability.
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Journal Name, [year], [vol], 00–00 | 3
Nanoscale Accepted Manuscript
DOI: 10.1039/C4NR06567K
Nanoscale
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Acknowledgements
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We acknowledge financial support from National Key Basic Research Program of China (973) (2015CB932200), Natural Science Foundation of Jiangsu Province (BM2012010), and Foundation for distinguished Young Scholars of Jiangsu Province (BK20140044).
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Key Laboratory of Flexible Electronics (KLOFE) & Institue of Advanced Materials (IAM), National Jiangsu Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China. Email: [email protected] b School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Fax: +65-6790 9081; Tel: +65-6316 8921; E-mail: [email protected] c College of Science, Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China. † Electronic Supplementary Information (ESI) available: The experiment details, TGA, N2 adsorption-desorption measurements, XRD and FT-IR results of synthesized samples. See DOI: 10.1039/b000000x/ 1 Y. Chen, L. Bai, C. Zhou, J. Lee and Y. Yang, Chem. Commun., 2011, 47, 6452. 2 G. J. Hutchings, Chem. Commun., 2008, 44, 1148. 3 Y. W. Kim, M. J. Kim, Bull. Korean Chem. Soc., 2010, 31, 1368. 4 H. Mao, J. Ma, Y. Liao, S. Zhao and X. Liao, Catal. Sci. Technol., 2013, 3, 1612. 5 H. Mao, H. Yu, J. Chen and X. Liao, Sci. Rep., 2013, 3, 2226. 6 R. J. White, R. Luque, V. L. Budarin, J. H. Clark and D. J. Macquarrie, Chem. Soc. Rev., 2009, 38, 481. 7 C. M. Crudden, M. Sateesh and R. Lewis, J. Am. Chem. Soc., 2005, 127, 10045. 8 T. R. Felthouse and J. A. Murphy, J. Catal., 1986, 98, 411. 9 E. W. Ping, R. Wallace, J. Pierson, T. F. Fuller and C. W. Jones, Microporous Mesoporous Mater., 2010, 132, 174. 10 H. Mao, S. Peng, H. Yu, J. Chen, S. Zhao and F. Huo, J. Mater. Chem. A., 2014, 2, 5847. 11 G. Yang, G. Gao, C. Wang, C. Xu and H. Li, Carbon., 2008, 46, 747. 12 Y. S. Chun, J. Y. Shin, C. E. Song and S. Lee, Chem. Commun, 2008, 8, 942. 13 O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim, Nature., 2003, 423, 705. 14 H. Liu, Y. Liu, Y. Li, Z. Tang and H. Jiang, J. Phys. Chem. C., 2010, 114, 13362. 15 J. A. Johnson, S. Chen, T. C. Reeson, Y. Chen, X. Zeng and J. Zhang, Chem. Eur. J., 2014, 20, 7632. 16 J. Juan-Alcaniz, J. Gascon and F. Kapteijn, J. Mater. Chem., 2012, 22, 10102. 17 B. Li, Y. Zhang, D. Ma, T. Ma, Z. Shi and S. Ma, J. Am. Chem. Soc., 2014, 136, 1202. 18 M. Yadav, A. Aijaz and Q. Xu, Funct. Mater. Lett., 2012, 5, 1250039. 19 M. Zhao, K. Deng, L. He, Y. Liu, G. Li, H. Zhao and Z. Tang, J. Am. Chem. Soc., 2014, 136, 1738. 20 S. Hermes, M. Schroter, R. Schmid, L. Khodeir, M. Muhler, A. Tissler, R. W. Fischer and R. A. Fischer, Angew. Chem. Int. Ed., 2005, 44, 6237.
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21 S. Hermes, D. Zacher, A. Baunemann, C. Woll and R. A. Fischer, Chem. Mater., 2007, 19, 2168. 22 G. Lu, S. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Qi, Y. Wang, X. Wang, S. Han, X. Liu, J. S. DuChene, H. Zhang, Q. Zhang, X. Chen, J. Ma, S. Loo, W. D. Wei, Y. Yang, J. T. Hupp and F. Huo, Nat. Chem., 2012, 4, 310. 23 W. Zhang, G. Lu, C. Cui, Y. Liu, S. Li, W. Yan, C. Xing, Y. R. Chi, Y. Yang and F. Huo, Adv. Mater., 2014, 26, 4056. 24 W. Zhang, G. Lu, S. Li, Y. Liu, H. Xu, C. Cui, W. Yan, Y. Yang and F. Huo, Chem. Commun., 2014, 50, 4296. 25 Z. Jiang, Z. Wang, D. Gu and E. S. Smotkin, Chem. Commun., 2010, 46, 6998. 26 Z. Jiang, Z. Wang, Y. Chu, D. Gu and G. Yin, Energy Environ. Sci., 2011, 4, 728. 27 J. Zhuang, C. Kuo, L. Chou, D. Liu, E. Weerapana and C. Tsung, ACS Nano, 2014, 8, 2812. 28 Q. Song, S. K. Nataraj, M. V. Roussenova, J. C. Tan, D. J. Hughes, W. Li, P. Bourgoin, M. A. Alam, A. K. Cheetham, S. A. AlMuhtaseb and E. Sivaniah, Energy Environ. Sci., 2012, 5, 8359. 29 K. S. Park, Z. Ni, A. P. Cote, J. Y. Choi, R. Huang, F. J. UribeRomo, H. K. Chae, M. O’Keeffe and O. M. Yaghi, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 10186. 30 M. Sabo, A. Henschel, H. Frode, E. Klemm and S. Kaskel, J. Mater. Chem., 2007, 17, 3827. 31 J. Cravillon, S. Munzer, S. Lohmeier, A. Feldhoff, K. Huber and M. Wiebcke, Chem. Mater., 2009, 21, 1410. 32 U. P. N. Tran, K. K. A. Le and N. T. S. Phan, ACS Catal., 2011, 1, 120. 33 Y. Lan, M. Zhang, W. Zhang and L. Yang, Chem. Eur. J., 2009, 15, 3670. 34 C. Chen, G. Lv, X. Huang, X. Liao, W. Zhang and B. Shi, Bull. Korean Chem. Soc., 2012, 33, 403. 35 C. M. Cirtiu, A. F. Dunlop-Briere and A. Moores, Green Chem., 2011, 13, 288.
Journal Name, [year], [vol], 00–00 | 4
Nanoscale Accepted Manuscript
DOI: 10.1039/C4NR06567K
Nanoscale Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: W. Zhou, B. Zou, W. Zhang, D. Tian, W. Huang and F. Huo, Nanoscale, 2015, DOI: 10.1039/C4NR06567K.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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DOI: 10.1039/C4NR06567K
Cite this: DOI: 10.1039/c0xx00000x
COMMUNICATION
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Synthesis of stable heterogeneous catalysts by supporting carbonstabilized palladium nanoparticles on MOFs
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Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X DOI: 10.1039/b000000x Cycling instability of catalysts is a persisting challenge in heterogeneous catalysis. In this work, we reported an effective in-situ strategy for preparing ZIF-8 supported carbonstabilized Pd nanoparticles (C@Pd/ZIF-8). The original ZIF8 structure was well preserved after formation of Pd nanoparticles and amorphous carbon. Here, the Pd nanoparticles were encapsulated in carbon matrix with a good dispersion. The as-prepared catalysts showed a better activity and cycling stability in the hydrogenation of C=C containing substrates. C@Pd/ZIF-8 catalysts are reusable without significant loss of activity after 5 times, exhibiting higher cycling stability than those by directly supported Pd nanoparticles on ZIF-8 (Pd/ZIF-8). Metal nanoparticles (NPs) have been applied as highly active catalysts in the field of organic syntheses because the catalytic performances of these NPs, such as oxidation and hydrogenation.1 The heterogeneous catalytic systems with metals (Pd, Pt, Au) have been reviewed by Hutchings,2 and among these noble metals, Pd-based catalysts have been extensively investigated because of the remarkable catalytic activity.3-5 In general, Pd NPs are dispersed onto the porous matrices,6-10 such as mesoporous silica and zeolite, to prepare heterogeneous catalysts. However, the naked Pd NPs supported on porous materials usually suffer from serious leaching or aggregation during the catalytic reactions, which inevitably leads to a loss of catalytic activity and cycling stability.11 To overcome these disadvantages, surface functionalization to supporting matrices was therefore established.12 Although these approaches can alleviate the problem of Pd NPs leaching by strengthening the interactions between the NPs and matrices, surface functionalization of the matrices usually requires multiple procedures, which may also risk a decreased activity due to the passivation of strong ligands to the NPs. As a consequence, it is necessary to find a new strategy for the preparation of heterogeneous Pd-based catalysts with high activity and reusability. Very recently, there has been a rapidly growing attention towards the capsulation of catalytically active NPs inside metalorganic frameworks (MOFs) due to MOFs have well defined porosity, high specific surface area and chemical tunability.13-17 In general, the functional properties of MOFs are mainly dominated by their pores. The tunable pores can be used for the fabrication of metal NPs with controlled sizes, thus circumventing the concerned issue of NPs migration and aggregation. Furthermore, MOFs offer the permanent nanoscaled cavities and open channels, which provide congenital conditions for small This journal is © The Royal Society of Chemistry [year]
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molecules to access, and also exhibit a potential for the use as a template. In this regard, some important pioneer achievements have been made.18-23 Xu’s group reported catalytically active Pd NPs immobilized inside the pores of a typical metal-organic framework, MIL-101, without aggregation of Pd NPs on the external surfaces of framework by using a “double solvents” method.18 The resulting Pd@MIL-101 composites represented the highly active MOF-immobilized metal NPs for catalytic reactions. Tang and co-workers used amino-functionalized IRMOF-3 shell to surround the Pd NPs core via a facile mixed solvothermal method, and the nanocomposites exhibit high activity and excellent stability in the cascade reaction.19 Our previous work also successfully encapsulated a series of presynthesized PVP-modified NPs into a zeolitic imidazolate framework, including Pd NPs, Au NPs and Pt NPs etc.22-24 We found that the restriction of MOFs framework to the encapsulated NPs provided a good size-based selectivity to the reactants. However, in most existing MOF nanocomposite materials, there are few reports focused on reusability of MOF encapsulated NPs catalysts. Considering the great potential applications of these hybrid catalysts in commercial utilizations, it is important to improve the stability of catalysts. An effective approach is to mitigate of migration and agglomeration of NPs by deposition and thermal decomposition of carbon coating on the catalysts surface.25 For example, Jiang and co-workers showed excellent stability for carbon riveted Pt/TiO2-C catalysts based on in-situ carbonization of the glucose, which was ascribed to the unique composite nanoarchitecture.26 In the present investigation, we reported a novel approach for preparing highly stable heterogeneous catalysts by using zeolitic imidazolate framework-8 (ZIF-8), a typical MOF consisting of zinc ions linked by 2-methylimidazole ligands [Zn(MeIM)2; MeIM = 2-methylimidazole], as a porous supporting matrix to disperse the Pd NPs in-situ encapsulated by carbon. Herein, the in-situ formed Pd NPs were stably encapsulated inside the continuous formed carbon, and these carbon-encapsulated Pd NPs were highly dispersed on the surface of the porous ZIF-8 without aggregation. As a result, the high activity and stability of the asprepared ZIF-8 supported carbon-stabilized Pd NPs (C@Pd/ZIF8) catalysts could be expected due to their unique structure configuration. In a series of heterogeneous hydrogenations, the high activity of C@Pd/ZIF-8 catalysts was confirmed, and it also showed considerably improved cycling stability when compared with the catalysts prepared by supporting naked Pd NPs on ZIF-8 (Pd/ZIF-8). The framework of ZIF-8 has a high thermal stability, and well defined three-dimensional interconnected porous structure with high surface area and large pore size.27-29 Hence, the porous ZIF8 crystals were ideal matrices for the convenient preparation of Journal Name, [year], [vol], 00–00 | 1
Nanoscale Accepted Manuscript
Published on 06 January 2015. Downloaded by Gazi Universitesi on 06/01/2015 12:22:06.
Weiqiang Zhou,a Binghua Zou,c Weina Zhang,b Danbi Tian,c Wei Huang,*a Fengwei Huo*ab
Nanoscale
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supported carbon-capsulated NPs catalysts by simple thermal treatment. On the basis of the above idea, glucose (carbon source) and Pd salts were simultaneously introduced into the porous channel of ZIF-8, and then converted to the corresponding carbon and metallic NPs by thermal reduction treatment. Fig. 1 is the schematic illustration showing the preparation procedures of C@Pd/ZIF-8 catalysts. First, the as-prepared ZIF-8 was impregnated with the mixture of glucose and PdCl2, and then, the intermediate material of glucose-Pd2+/ZIF-8 was simply treated by thermal reduction in Ar/H2 flow. Based on the TGA curve analysis (Fig. S1, ESI†), the glucose is stable up to ~175 oC, while a sharp weight loss is observed when the temperature is beyond 200 oC. The weight loss of glucose reaches 42% at 300 o C. For ZIF-8 (Fig. S2, ESI†), a weight-loss of 8.7% is observed below 300 oC, corresponding to the removal of water molecules residual in the porous structure of ZIF-8. In the range of 300-400 o C, a long plateau is shown. Almost no weight loss associated to the decomposition of ZIF-8 is observed when the temperature is at 300 oC. These results suggest that the structure of ZIF-8 can be well maintained at 300 oC, while the glucose can be carbonized. Consequently, we chose the temperature of 300 oC for the thermal reduction of glucose-Pd2+/ZIF-8. In this reduction process, the Pd2+ was in-situ reduced as Pd NPs, and simultaneously, the glucose was decomposed to carbon. It is noted that the growth of Pd NPs should be strictly controlled by the surrounding carbon, as well as the pores of ZIF-8, which therefore provide good dispersion and high stability. Actually, the following TEM (Fig. 3) results confirm this prediction. Based on acid digestion and ICP-AES analysis (Perkin-Elmer Optima 2100DV), the loading of noble metals in C@Pd/ZIF-8 and Pd/ZIF-8 is about 2.7% and 2.9%, respectively.
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which presumably attribute to the low loading of Pd and/or small particle size of Pd NPs.30 Similar to the C@Pd/ZIF-8, Pd/ZIF-8 catalysts show high crystal peaks of ZIF-8 in its XRD pattern while the peaks related to Pd is still absence. To further prove that the ZIF-8 structure was well retained in C@Pd/ZIF-8, we conducted the analyses of Fourier transform infrared (FT-IR) spectrum. As shown in the spectrum of pure ZIF-8 (Fig. S5, ESI†), almost all of the peaks are in good agreement with those of reported ZIF-8.31,32 The peaks at 3136 cm-1 and 2929 cm-1 are attributed to the stretching vibrations of C-H bonds in the methyl group and imidazole ring, respectively. The peak at 1584 cm-1 can be ascribed to the C=N stretch mode of imidazole ring of ZIF-8, and the peaks in the range of 6501500 cm-1 are associated with the ring stretching or bending. The spectrum of C@Pd/ZIF-8 catalysts shows the identical peaks at 3136, 2930 and 1584 cm-1 with pure ZIF-8. The FT-IR results are consistent with the above XRD analysis, confirming the presence of ZIF-8 structure in the catalysts after thermal treatment.
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Fig. 2 SEM images and particle size distribution of the as-prepared ZIF-8 (a, b), Pd/ZIF-8 (c, d) and C@Pd/ZIF-8 (e, f).
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Fig. 1 Schematic illustration of synthesis of C@Pd/ZIF-8 catalysts.
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Based on N2 adsorption-desorption isotherms (Fig. S3, ESI†), the BET surface area for ZIF-8, Pd/ZIF-8 and C@Pd/ZIF-8 was 1383 m2 g-1, 1336 m2 g-1 and 929 m2 g-1, respectively. As expected, C@Pd/ZIF-8 has the lower surface area than pure ZIF8 due to the carbon residual inside the pores. The pore size distributions of C@Pd/ZIF-8 and Pd/ZIF-8 are mainly at ~2 nm, and it will be large enough for hydrogenation reactions of substrates within the cavity. The crystal structure of the assynthesized ZIF-8, C@Pd/ZIF-8 and Pd/ZIF-8 composites were characterized by powder X-ray diffraction (PXRD) (Fig. S4, ESI†). It can be observed that the diffraction peaks of synthesized ZIF-8 at 2θ values of 7.33o, 10.38o, 12.71o, 14.68o and 16.43o are well matched with those of simulated XRD patterns of ZIF-8,29 which are perfectly indexed to (011), (002), (112), (022) and (013) crystal planes, respectively. Compared with ZIF-8, the C@Pd/ZIF-8 catalysts also show the identical characteristic peaks of (011), (002), (112), (022) and (013) planes. These results suggest that the well preservation of ZIF-8 structure in the C@Pd/ZIF-8 catalysts. Additionally, no obvious diffractions were detected for Pd species from the XRD patterns of C@Pd/ZIF-8, This journal is © The Royal Society of Chemistry [year]
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The SEM images (Fig. 2) show morphology and size distribution of the as-prepared ZIF-8, Pd/ZIF-8 and C@Pd/ZIF-8 samples. As seen in Fig. 2a, c and e, it is observed that the single particles in the three samples have the same shapes but different sizes. Based on the size distribution analyses (Fig. 2b, d and f), the average sizes of pure ZIF-8, Pd/ZIF-8 and C@Pd/ZIF-8 are about 165 nm, 175 nm and 210 nm, respectively. It is worth noting that the size of C@Pd/ZIF-8 particles is larger than that of the synthesized ZIF-8. It may attribute to amorphous carbon which formed by the decomposition of glucose on the surface of ZIF-8. However, the size of Pd/ZIF-8 particle is close to that of pure ZIF-8 due to the absence of carbon. Based on TEM analysis, the distribution of Pd NPs is well dispersed, and no serious aggregation of the Pd NPs is observed. TEM images of Pd/ZIF-8 and C@Pd/ZIF-8 catalysts are shown in Fig. 3a and d, and the corresponding particle size distributions are presented in Fig. 3c and f. As shown in Fig. 3a-c, the Pd NPs in Pd/ZIF-8 are dispersed in ZIF-8 crystal, and the corresponding particle sizes are around 1.7±0.6 nm. In Fig. 3f, the Pd particles in C@Pd/ZIF-8 exhibited larger size with average diameter of 2.7±0.4 nm, showing a narrow distribution. It can be attributed to the stabilization of carbon towards the Pd particles. Moreover, the formation of the carbon could restrain the excessive growth of the Pd particles during the reduction. The formed carbon could provide enough steric hindrance to prevent the aggregation of Pd nuclei when the Pd2+ was reduced by H2. Subsequently, highresolution TEM (HRTEM) observation was carried out (Fig. 3b Journal Name, [year], [vol], 00–00 | 2
Nanoscale Accepted Manuscript
DOI: 10.1039/C4NR06567K
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and e). It can be seen that small Pd particles with observed lattice are embedded in the catalysts. The small particle size may explain that there is no obvious peak of Pd in the XRD patterns of C@Pd/ZIF-8. Considering the uniform dispersion and small particle size of Pd NPs, high catalytic activity of the C@Pd/ZIF-8 catalysts can be expected in the hydrogenation reactions.
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Fig. 3 TEM images and Pd particle size distribution of Pd/ZIF-8 (a-c) and C@Pd/ZIF-8 (d-f). Table 1 Catalytic activity of C@Pd/ZIF-8 and Pd/ZIF-8 for the heterogeneous hydrogenation of olefinsa
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Table 2 Comparison of the cycling stability of the C@Pd/ZIF-8 and Pd/ZIF-8 catalysts in heterogeneous hydrogenationa
Conversion (%) Substrates
C@Pd/ZIF-8
Pd/ZIF-8
95.1
98.5
75.2
77.8
47.3
44.9
24.7
19.6
Substrate conversion (%)
Cycle
21.8
17.7
N
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20
25
Reaction conditions: 5.0 µmol Pd catalyst, 2.0 MPa H2, 80 oC, 30 min, and substrate 2.0 mmol.
Pd catalysts have been widely used for the catalytic hydrogenation of olefins.33-35 Hence, the catalytic activity of C@Pd/ZIF-8 catalysts was evaluated in hydrogenation of unsaturated compounds, including styrene, 1-heptene, cyclohexene, cyclooctene and quinoline. The results obtained by using C@Pd/ZIF-8 and Pd/ZIF-8 as catalysts are shown in Table 1. Pd/ZIF-8 catalysts exhibit comparable conversion yield in hydrogenation of olefins compared with the C@Pd/ZIF-8. For example, the conversion yield of styrene catalyzed by Pd/ZIF-8 is 98.5%, which is slightly higher than that of C@Pd/ZIF-8 (95.1%). The cycling stabilities of C@Pd/ZIF-8 and Pd/ZIF-8 in the hydrogenation of styrene, 1-heptene and cyclohexene were This journal is © The Royal Society of Chemistry [year]
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95.1b (98.5c)
75.2 (77.8)
47.3 (44.9)
2
94.5 (72.8)
74.4 (54.1)
45.1 (20.3)
3
92.7 (49.3)
72.7 (30.6)
42.2 (5.1)
4
87.2 (25.5)
68.3 (9.7)
39.7 (0)
5
85.3
66.9
37.5
a
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also conducted under the same conditions. As shown in Table 2, the C@Pd/ZIF-8 catalysts reused 5 times without significant loss of the catalytic activity. In the hydrogenations of styrene, 1heptene and cyclohexene, the corresponding conversion yields in the 5th run are 85.3%, 66.9% and 37.5%, respectively, which account for the corresponding activity in 1-run by 89.70%, 88.96% and 79.28%, respectively. It should be also noted that these values are lower than their actual values because there is some loss of catalysts during recycling. The TEM image of the reused C@Pd/ZIF-8 catalyst shows that the mean diameter of Pd NPs is about 3 nm, the same as the fresh catalyst, indicating that no agglomeration of NPs occurs during the reaction (Fig. S6, ESI†). On the contrary, the conversion yields catalyzed by Pd/ZIF-8 have decreased drastically during the recycles. In the 4th run, the conversion yields of styrene, 1-heptene and cyclohexene are only 25.5%, 9.7% and 0, respectively. It can be explained that the Pd particles supported by pure ZIF-8 may suffer serious aggregating or leaching during the reactions, confirmed by the TEM observation (Fig. S7, ESI†), which accounts for the catalyst deactivation. We also conducted the cycling stability experiments of C@Pd/ZIF-8 and commercial Pd/Carbon (Sigma-Aldrich), the results demonstrated that the commercial Pd/C catalyst exhibited poor recycling stability compared with C@Pd/ZIF-8 (Table S1, ESI†). In contrast, the Pd NPs in C@Pd/ZIF-8 can avoid aggregating and leaching due to the efficient stabilization of amorphous carbon, exhibiting satisfied cycling stability and high activity.
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Reaction conditions: 5.0 µmol Pd catalyst, 2.0 MPa H2, 80 oC, 30 min, and substrate 2.0 mmol. b Results obtained from the C@Pd/ZIF-8 catalyst. c Results obtained from the Pd/ZIF-8 catalyst.
In conclusion, we have demonstrated an effective strategy for preparing the heterogeneous catalysts of ZIF-8 supported the carbon-stabilized Pd NPs. For the synthesis, the introduced glucose is a key factor to achieve the desired catalyst structure, providing the continuous in-situ formed carbon to stabilize the Pd NPs. Due to the efficient stabilization of carbon matrices, the Pd NPs with controlled size can be well dispersed onto the ZIF-8. The as-prepared C@Pd/ZIF-8 has exhibited high activity in hydrogenation of olefins. It can be used 5 times without significant loss of the catalytic activity, showing a satisfied reusability.
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Journal Name, [year], [vol], 00–00 | 3
Nanoscale Accepted Manuscript
DOI: 10.1039/C4NR06567K
Nanoscale
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Acknowledgements
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We acknowledge financial support from National Key Basic Research Program of China (973) (2015CB932200), Natural Science Foundation of Jiangsu Province (BM2012010), and Foundation for distinguished Young Scholars of Jiangsu Province (BK20140044).
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Notes and references 65
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Key Laboratory of Flexible Electronics (KLOFE) & Institue of Advanced Materials (IAM), National Jiangsu Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China. Email: [email protected] b School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Fax: +65-6790 9081; Tel: +65-6316 8921; E-mail: [email protected] c College of Science, Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China. † Electronic Supplementary Information (ESI) available: The experiment details, TGA, N2 adsorption-desorption measurements, XRD and FT-IR results of synthesized samples. See DOI: 10.1039/b000000x/ 1 Y. Chen, L. Bai, C. Zhou, J. Lee and Y. Yang, Chem. Commun., 2011, 47, 6452. 2 G. J. Hutchings, Chem. Commun., 2008, 44, 1148. 3 Y. W. Kim, M. J. Kim, Bull. Korean Chem. Soc., 2010, 31, 1368. 4 H. Mao, J. Ma, Y. Liao, S. Zhao and X. Liao, Catal. Sci. Technol., 2013, 3, 1612. 5 H. Mao, H. Yu, J. Chen and X. Liao, Sci. Rep., 2013, 3, 2226. 6 R. J. White, R. Luque, V. L. Budarin, J. H. Clark and D. J. Macquarrie, Chem. Soc. Rev., 2009, 38, 481. 7 C. M. Crudden, M. Sateesh and R. Lewis, J. Am. Chem. Soc., 2005, 127, 10045. 8 T. R. Felthouse and J. A. Murphy, J. Catal., 1986, 98, 411. 9 E. W. Ping, R. Wallace, J. Pierson, T. F. Fuller and C. W. Jones, Microporous Mesoporous Mater., 2010, 132, 174. 10 H. Mao, S. Peng, H. Yu, J. Chen, S. Zhao and F. Huo, J. Mater. Chem. A., 2014, 2, 5847. 11 G. Yang, G. Gao, C. Wang, C. Xu and H. Li, Carbon., 2008, 46, 747. 12 Y. S. Chun, J. Y. Shin, C. E. Song and S. Lee, Chem. Commun, 2008, 8, 942. 13 O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim, Nature., 2003, 423, 705. 14 H. Liu, Y. Liu, Y. Li, Z. Tang and H. Jiang, J. Phys. Chem. C., 2010, 114, 13362. 15 J. A. Johnson, S. Chen, T. C. Reeson, Y. Chen, X. Zeng and J. Zhang, Chem. Eur. J., 2014, 20, 7632. 16 J. Juan-Alcaniz, J. Gascon and F. Kapteijn, J. Mater. Chem., 2012, 22, 10102. 17 B. Li, Y. Zhang, D. Ma, T. Ma, Z. Shi and S. Ma, J. Am. Chem. Soc., 2014, 136, 1202. 18 M. Yadav, A. Aijaz and Q. Xu, Funct. Mater. Lett., 2012, 5, 1250039. 19 M. Zhao, K. Deng, L. He, Y. Liu, G. Li, H. Zhao and Z. Tang, J. Am. Chem. Soc., 2014, 136, 1738. 20 S. Hermes, M. Schroter, R. Schmid, L. Khodeir, M. Muhler, A. Tissler, R. W. Fischer and R. A. Fischer, Angew. Chem. Int. Ed., 2005, 44, 6237.
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Nanoscale Accepted Manuscript
DOI: 10.1039/C4NR06567K
