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Non-activation ZnO array as buffering layer to fabricate strongly adhesive metal-organic framework/PVDF hollow fiber membranes
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Non-activation (NA) ZnO array is directly grown on PVDF hollow fiber membrane. The defect-free MOF layers can be synthesized easily on the NA-ZnO array without any activation procedure. The array and MOF layers have strong adhesion with the hollow fiber membrane. The prepared ZIF membranes exhibit excellent gas separation performance.
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Metal-organic frameworks, as a kind of most reported porous materials, have a series of extraordinary features, such as high surface area, fascinating adsorption affinities, facilely tailorable functionality and a wide range of pore sizes. 1 Thanks to those features, MOF materials can be used in various fields, such as gas adsorption and storage, separation, sensors, electronics and catalysis.2 In particular, similar to the zeolite, MOFs can also be grown on the substrate to form a continuous layer, that makes the MOF membranes have been greatly developed in recent years.3 However, because of the poor nucleation, adhesion, intergrowth and substrates, fabrication of high-quality and practical MOF membranes for gas separation is still a big challenge. Various synthetic methods have been reported for fabrication of MOF membranes. 1) In situ growth, that is the growth of the MOF layer on substrate derectly.4 But only a few studies use this method to prepare MOF membranes due to the poor heterogeneous nucleation site on the substrate. 2) Chemical modification, which puts the functional group on the substrate to anchor metal ions or ligand.5,6 This method can provide the sufficient heterogeneous nucleation site for MOF growth and form continous membranes. 3) Seeded growth, this method has been proven to be more versatile for preparing a variety of MOF membranes.7 However, seeded growth method also has its own problem, that is the tender adhesion between the substrate and MOF layer. To strengthen seed crystals anchor on the porous substrates, new seeded methods including the reactive seeding, step-by-step seeding, thermal seeding, and polymer/MOF seeding, etc, were developed.8,9 Dual metal source method is also used to prepare MOF membranes,10 which tactfully exploits the formation mechanism of the MOF. Moreover, this method can effectively omit the processes of seed preparation and deposition. ZnO, as an excellent metal source for MOF membrane growth, has been investigated.11,12 However, if the ZnO is used for growing the ZIF-8 directly, the ligands will This journal is © The Royal Society of Chemistry [year]
react with the Zn 2+ in solution. Large amount of the homogeneous nucleation sites will be formed. The ZnO can’t provide the adequate heterogeneous nucleation sites for continuous MOF membranes. Thus the activation is crucial for the MOF membrane formation.11,12 Certainly, there are many other methods can be used for synthesis of MOF membranes, such as contra-diffusion synthesis, interfacial synthesis, liquid-phase epitaxy and so on.13
Fig. 1 Scheme of chemical structure and morphology for the prepared MOF/PVDF membrane. 60
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The substrate is very important component for separation performance which determines the wide application of MOF membranes. Various substrates were used to support MOF layer. Inorganic substrates such as aluminium oxide, metal net and titanium dioxide were mostly studied.4-8,10 The free standing MOF membrane was also developed.14 However, considering the membrane surface area per volume, manufacturing costs, scale up and long-term mechanical stability, the polymer hollow fiber membrane may be the promising candidate for the MOF membrane substrates. Centrone et al. successfully grew MIL-47 on the PAN polymer surfaces for the first time.15 Brown et al. also prepared ZIF-90 membranes on Torlon hollow fiber.16 Recently, we have fabricated the CuBTC/polyacrylonitrile (PDMS) and CuBTC/polysulfone (PSf) hollow fiber membranes through chemical modification approach and seeded growth, respectively.17,9 Based on the above analysis, herein, we have directly grown the non-activation (NA) ZnO array with multimorphologies on PVDF hollow fiber membrane by using a new synthetic method. The high-quality MOF/PVDF membranes can be synthesized easily on the NA-ZnO array buffering layer (Fig. 1). Unlike the previous studies,11,12 the [journal], [year], [vol], 00–00 | 1
ChemComm Accepted Manuscript
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Wanbin Li, a Qin Meng, b Xiaonian Li, a Congyang Zhang, a Zheng Fan, a Guoliang Zhang,*a
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DOI: 10.1039/C4CC03864A
Fig. 3 Cross-section and surface SEM images of (a) (b) ZIF8/PVDF and (c) (d) ZIF-7/PVDF. 45 5
Fig. 2 Cross-section and surface SEM images of (a) (b) NAZnO/PVDF synthesized at 353 K and (c) (d) NA-ZnO/PVDF synthesized at 423 K. 50
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First, we attempted to grow ZnO array on the membrane using conventional approach, zinc chloride and ammonia were used as precursor and pH regulator, respectively. To obtain the ZnO array and improve the stability, PVDF hollow fiber was ammoniated to from amino group on the membrane surface. However, no ZnO crystal was found to be deposited on the surface (Fig. S1 ESI†). To obtain the NA-ZnO array and make it convenient for MOF membrane synthesis, zinc nitrate hexahydrate, 2-methyl-imidazole and sodium formate were employed. After crystallization, as Fig. 2a and b, NAZnO sheets with the thickness of nano-scale were grown on the membrane surface as array. It can be observed that the nano-sheets NA-ZnO crossed the dense layer of the PVDF hollow fiber, which greatly improved the anchoring strength between the array and substrate. FTIR diffractogram of NAZnO revealed that 2-methyl-imidazole was integrated in the prepared NA-ZnO (Fig. 1 and Fig. S2 ESI†), which should be benefit for the synthesis of the MOF membranes and make the activation step be omitted. Since temperature was an important factor for the morphology of NA-ZnO array, we increased the synthetic temperature to 423 K. As expected, the morphology of the NA-ZnO crystals was changed to rice-body (Fig. 2c and d). The low-magnification SEM images demonstrated the array layer was uniform (Fig. S3 ESI†). As we can see, no matter what the morphology was, a large tract of NA-ZnO array covered on the surface tidily. The XRD patterns showed that the crystalline were ZnO (Fig. S4 ESI†). The adhesion was found to be verified by the ultrasonic method. After the NA-ZnO/PVDF was subjected to ultrasonic treatment for 60 min, both the NA-ZnO arrays can maintain an excellent structure and even no NA-ZnO crystal was found to fall off (Fig. S5a and b ESI†), which demonstrated that the arrays had especially good adhesion.
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After the NA-ZnO array was grown on PVDF hollow fiber successfully, we further grew the ZIF layer on the NA-ZnO array. The NA-ZnO/PVDF was directly immersed into the ZIF precursor solution for crystallization. Like previous studies,12 the activation of ZnO was found to be vital for ZIF-8 membrane formation. But here, because the NA-ZnO array was integrated by 2-methyl-imidazole, the uniform ZIF layer can be prepared without any procedure of the activation of ZnO. After the ZIF-8 was grown by zinc chloride, 2-methylimidazole and sodium formate, the SEM images of the prepared membranes were shown in Fig. 3. The cross-section of the membrane showed that the crystals were anchored to the substrate tightly. The ZIF-8 crystals were well inter-grown and impacted. From the surface SEM image, no cracks, pinholes, or other defects were visible. It revealed that highquality membrane was made. Moreover, the lowmagnification SEM images indicated the ZIF-8 layer uniformly covered on the entirely hollow fiber (Fig. S5c ESI†). In fact, the only difference of precursor solution between NA-ZnO array and ZIF-8 membrane was different metal salt. However, different crystals was obtained, this should be attributed to the water in zinc nitrate hexahydrate. Based on this progress, the ZIF-7/PVDF membrane was also synthesized. To demonstrate that the NA-ZnO array was an excellent nuclei provider, the synthetic time here was shortened to 12 h, although the synthetic time for most reported ZIF membrane was longer than 24 h. The uniform and well inter-growth ZIF-7 layer was well grown on the PVDF hollow fiber. Accordingly, the smaller thickness of ZIF-7/PVDF was the result of the shorter synthetic time. What is more, from the SEM images, the NA-ZnO array layer was not obvious. This may be because the metal oxides as the metal ion source participated in the reaction.18 To check the effect of ZnO array morphology on the preparation of MOF membrane, we have also grown the ZIF-8 and ZIF-7 layer on PVDF hollow fiber with different NA-ZnO arrays, but no big difference of the ZIF/PVDF was observed. The XRD diffractograms indicated that the materials were crystalline and constituted the same phase of the ZIF-8 and the ZIF-7 (Fig. S6 ESI†). For ZIF-8/PVDF, the phase of ZnO was This journal is © The Royal Society of Chemistry [year]
ChemComm Accepted Manuscript
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activation procedure can be thoroughly omitted. Moreover, the array and MOF layer held strong adhesion with the hollow fiber membrane, and the prepared MOF membranes possessed excellent gas separation performance.
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DOI: 10.1039/C4CC03864A
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a Institute of Oceanic and Environmental Chemical Engineering, College of Chemical Engineering and Material Science, Zhejiang University of Technology, Hangzhou 310014, China. Fax/Tel:+86 571 88320863; Email:[email protected] b Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China. † Electronic Supplementary Information (ESI) available: Experimental details, FTIR, and SEM of the membrane. See DOI: 10.1039/b000000x/ 1 (a) N. Stock, S. Biswas, Chem. Rev. 2012, 112, 933; (b) R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O’Keeffe and O. M. Yaghi, Science 2008, 319, 939; (c) N. Stock and S. Biswas, Chem. Rev., 2012, 112, 933. 2 (a) D. Bradshaw, A. Garai and J. Huo, Chem. Soc. Rev., 2012, 41, 2344; (b) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T. H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724; (c) S. Sorribas, P. Gorgojo, C. Téllez, J. Coronas, and A. G. Livingston, J. Am. Chem. Soc., 2013, 135, 15201. 3 (a) D. Zacher, O. Shekhah, C. Wöll and R. A. Fischer, Chem. Soc. Rev., 2009, 38, 1418; (b) J. Gascon and F. Kapteijn, Angew. Chem. Int. Ed., 2010, 49, 1530. 4 Y. Liu, Z. Ng, E. A. Khan, H. K. Jeong, C. B. Ching and Z. Lai, Microporous Mesoporous Mater., 2009, 118, 296. 5 S. Hermes, F. Schröder, R. Chelmowski, C. Wöll and R. A. Fischer, J. Am. Chem. Soc., 2005, 127, 13744. 6 A. Huang, H. Bux, F. Steinbach and J. Caro, Angew. Chem. Int. Ed., 2010, 49, 4958. 7 R. Ranjan and M. Tsapatsis, Chem. Mater., 2009, 21, 4920. 8 (a) Y. Hu, X. Dong, J. Nan, W. Jin, X. Ren, N. Xu and Y. M. Lee, Chem. Commun., 2011, 47, 737; (b) J. Nan, X. Dong, W. Wang, W. Jin and N. Xu, Langmuir, 2011, 27, 4309; (c) V. V. Guerrero, Y. Yoo, M. C. McCarthy and H. K. Jeong, J. Mater. Chem., 2010, 20, 3938; (d) Y. S. Li, H. Bux, A. Feldhoff, G. L. Li, W. S. Yang and J. Caro, Adv. Mater., 2010, 22, 3322. 9 W. Li, G. Zhang, C. Zhang, Q. Meng, Z. Fan and C. Gao, Chem. Commun., 2014, 50, 3214. 10 (a) S. Zhou, X. Zou, F. Sun, H. Ren, J. Liu, F. Zhang, N. Zhao and G. Zhu, Int. J. Hydrogen. Energ., 3013, 38, 5338; (b) H. Guo, G. Zhu, I. J. Hewitt and S. Qiu, J. Am. Chem. Soc., 2009, 131, 1646; (c) Y. Yue, Z. A. Qiao, X. Li,A. J. Binder, E. Formo, Z. Pan, C. Tian, Z. Bi and S. Dai, Cryst. Growth Des., 2013, 13, 1002. 11 X. Dong, K. Huang, S. Liu, R. Ren, W. Jin and Y. S. Lin, J. Mater. Chem., 2012, 22, 19222. 12 (a) X. Zhang, Y. Liu, L. Kong, H. Liu, J. Qiu, W. Han, L. T. Weng, K. L. Yeung and W. Zhu, J. Mater. Chem. A, 2013, 1, 10635; (b) X. Zhang, Y. Liu, S. Li, L. Kong, H. Liu, Y. Li, W. Han, K. L. Yeung, W. Zhu, W. Yang and J. Qiu, Chem. Mater. 2014, 26, 1975. 13 (a) J. Yao, D. Dong, D. Li, L. He, G. Xu, H. Wang, Chem. Commun. 2011, 47, 2559; (b) R. Ameloot, F. Vermoortele, W. Vanhove, M. B. J. Roeffaers, B. F. Sels and D. E. De Vos, Nat. Chem., 2011, 3, 382; (c) O. Shekhah, R. Swaidan, Y. Belmabkhout, M. Plessis, T. Jacobs, L. J. Barbour, I. Pinnau and M. Eddaoudi, Chem. Commun., 2014, 50, 2089. 14 Y. Mao, L. Shi, H. Huang, W. Cao, J. Li, L. Sun, X. Jin and X. Peng, Chem. Commun., 2013, 49, 5666. 15 A. Centrone, Y. Yang, S. Speakman, L. Bromberg, G. C. Rutledge and T. A. Hatton, J. Am. Chem. Soc., 2010, 132, 15687. 16 A. J. Brown, J. R. Johnson, M. E. Lydon, W. J. Koros, C. W. Jones, and S. Nair, Angew. Chem. Int. Ed., 2012, 51, 10615. 17 W. Li, Z. Yang, G. Zhang, Z. Fan, Q. Meng, C. Shen, C and Gao, J. Mater. Chem. A, 2014, 2, 2110. 18 W. Zhan, Q. Kuang, J. Zhou, X. Kong, Z. Xie and L. Zheng, J. Am. Chem. Soc., 2013, 135, 1926-1933. 19 (a) A. Huang, Y. Chen, N. Wang, Z. Hu, J. Jiang and J. Caro, Chem. Commun., 2012, 48, 10981; (b) Z. Xie, J. Yang, J. Wang, J. Bai, H. Yin, B. Yuan, J. Lu, Y. Zhang, L. Zhou and C. Duan, Chem. Commun., 2012, 48, 5977.
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ChemComm Accepted Manuscript
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almost all disappeared, which was agreed with the SEM images. The ZIF/PVDF was further subjected to ultrasonic treatment for 60 min. No ZIF crystal was found to be exfoliated from the PVDF hollow fiber, except some corner of the ZIF crystals were destroyed (Fig. S5d ESI†), which demonstrated the ZIF layers clung to the substrate strongly. Besides ZIF-8 and ZIF-7, we also attempted to synthesize CuBTC (known as HKUST-1) on the NA-ZnO/PVDF. The results showed that well impacted CuBTC membrane can be obtained (Fig. S7 ESI†). Gas permeation data for H2, CO2 and N2 were collected at room temperature and 1 bar using self-made gas permeation equipment.9 Before gas permeation, the ZIF membranes were dried under vacuum. The results revealed that the ZIF membranes exhibited remarkably higher H2 permeances compared with other gases N2 and CO2. The ideal separation factors (ISF) of H2/N2 and H2/CO2 were much larger than the values of Knudsen diffusion (3.74 and 4.69), and reached 18.14, 16.29 and 20.27, 18.43 for ZIF-8/PVDF and ZIF7/PVDF, respectively. Because the aperture of ZIF-7 (0.3 nm) was between the kinetic diameters of H2 (0.29 nm) and CO2 (0.33 nm) and smaller than that of ZIF-8 (0.34 nm), the ZIF7/PVDF held the superior ISF. Moreover, the ISF of prepared membranes were larger than most of reported ZIF membranes (Tab. S1). In addition, the H2 permeances of the membranes obtained were also excellent, 20.14 and 23.54 ×10 −7 mol s−1 m−2 Pa−1 for ZIF-8/PVDF and ZIF-7/PVDF, respectively, which were larger than most ZIF membranes (below than 5 ×10 −7 mol s−1 m−2 Pa−1), except the ZIF-95 (24.6 ×10 −7 mol s−1 m−2 Pa−1) and ZIF-8 (573.0 ×10 −7 mol s−1 m−2 Pa−1) membranes reported by Huang et al. and Xie et al., respectively.19 The larger H2 permeance of the ZIF-7/PVDF should be attributed to the thinner ZIF layer. Similar to the reported studies, as a result of the well-known lattice flexibility of ZIFs, the larger size gases such as CO2 (0.33 nm) and N2 (0.36) can also pass through the smaller aperture ZIF membranes.19a In summary, we have reported a new method to fabricate strongly adhesive MOF membranes by using non-activation ZnO array as buffering layer. The NA-ZnO array layers with diversity of morphologies were successfully grown on the PVDF hollow fiber substrate. The NA-ZnO array layer had strong adhesion with PVDF membrane. NA-ZnO array was proved to be an outstanding buffering layer for synthesis of crack-free and uniform MOF membranes without any activation procedure. The fabricated MOF/PVDF membranes had commendable hollow fiber structures and exhibited excellent H2 permselectivity. For ZIF-7 membrane, the ideal separation factors for H2/N2 and H2/CO2 were 20.37 and 18.43, respectively, and the permeance of H2 can reach as high as 23.54 ×10 −7 mol s−1 m−2 Pa−1. These properties recommend that NA-ZnO array should be excellent buffering layer for fabricating MOF membranes, and the prepared ZIF/PVDF membranes will be the promising candidates for industrial hydrogen separation. We thank for financial support the National Natural Science Foundation of China (Nos. 21236008 and 21176226).
ChemComm
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DOI: 10.1039/C4CC03864A
The table of contents: Non-activation ZnO array is grown on PVDF membrane to synthesize defect-free MOF membranes which exhibit excellent gas separation performance. 5
Title: Non-activation ZnO array as buffering layer to fabricate strongly adhesive metal-organic
ChemComm Accepted Manuscript
Published on 19 June 2014. Downloaded by University of Waikato on 14/07/2014 07:28:41.
framework/PVDF hollow fiber membranes
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This journal is © The Royal Society of Chemistry [year]
ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: G. zhang, Q. Meng, X. Li, C. Zhang, Z. Fan and W. Li, Chem. Commun., 2014, DOI: 10.1039/C4CC03864A.
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/C4CC03864A
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Non-activation ZnO array as buffering layer to fabricate strongly adhesive metal-organic framework/PVDF hollow fiber membranes
5
10
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Non-activation (NA) ZnO array is directly grown on PVDF hollow fiber membrane. The defect-free MOF layers can be synthesized easily on the NA-ZnO array without any activation procedure. The array and MOF layers have strong adhesion with the hollow fiber membrane. The prepared ZIF membranes exhibit excellent gas separation performance.
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Metal-organic frameworks, as a kind of most reported porous materials, have a series of extraordinary features, such as high surface area, fascinating adsorption affinities, facilely tailorable functionality and a wide range of pore sizes. 1 Thanks to those features, MOF materials can be used in various fields, such as gas adsorption and storage, separation, sensors, electronics and catalysis.2 In particular, similar to the zeolite, MOFs can also be grown on the substrate to form a continuous layer, that makes the MOF membranes have been greatly developed in recent years.3 However, because of the poor nucleation, adhesion, intergrowth and substrates, fabrication of high-quality and practical MOF membranes for gas separation is still a big challenge. Various synthetic methods have been reported for fabrication of MOF membranes. 1) In situ growth, that is the growth of the MOF layer on substrate derectly.4 But only a few studies use this method to prepare MOF membranes due to the poor heterogeneous nucleation site on the substrate. 2) Chemical modification, which puts the functional group on the substrate to anchor metal ions or ligand.5,6 This method can provide the sufficient heterogeneous nucleation site for MOF growth and form continous membranes. 3) Seeded growth, this method has been proven to be more versatile for preparing a variety of MOF membranes.7 However, seeded growth method also has its own problem, that is the tender adhesion between the substrate and MOF layer. To strengthen seed crystals anchor on the porous substrates, new seeded methods including the reactive seeding, step-by-step seeding, thermal seeding, and polymer/MOF seeding, etc, were developed.8,9 Dual metal source method is also used to prepare MOF membranes,10 which tactfully exploits the formation mechanism of the MOF. Moreover, this method can effectively omit the processes of seed preparation and deposition. ZnO, as an excellent metal source for MOF membrane growth, has been investigated.11,12 However, if the ZnO is used for growing the ZIF-8 directly, the ligands will This journal is © The Royal Society of Chemistry [year]
react with the Zn 2+ in solution. Large amount of the homogeneous nucleation sites will be formed. The ZnO can’t provide the adequate heterogeneous nucleation sites for continuous MOF membranes. Thus the activation is crucial for the MOF membrane formation.11,12 Certainly, there are many other methods can be used for synthesis of MOF membranes, such as contra-diffusion synthesis, interfacial synthesis, liquid-phase epitaxy and so on.13
Fig. 1 Scheme of chemical structure and morphology for the prepared MOF/PVDF membrane. 60
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The substrate is very important component for separation performance which determines the wide application of MOF membranes. Various substrates were used to support MOF layer. Inorganic substrates such as aluminium oxide, metal net and titanium dioxide were mostly studied.4-8,10 The free standing MOF membrane was also developed.14 However, considering the membrane surface area per volume, manufacturing costs, scale up and long-term mechanical stability, the polymer hollow fiber membrane may be the promising candidate for the MOF membrane substrates. Centrone et al. successfully grew MIL-47 on the PAN polymer surfaces for the first time.15 Brown et al. also prepared ZIF-90 membranes on Torlon hollow fiber.16 Recently, we have fabricated the CuBTC/polyacrylonitrile (PDMS) and CuBTC/polysulfone (PSf) hollow fiber membranes through chemical modification approach and seeded growth, respectively.17,9 Based on the above analysis, herein, we have directly grown the non-activation (NA) ZnO array with multimorphologies on PVDF hollow fiber membrane by using a new synthetic method. The high-quality MOF/PVDF membranes can be synthesized easily on the NA-ZnO array buffering layer (Fig. 1). Unlike the previous studies,11,12 the [journal], [year], [vol], 00–00 | 1
ChemComm Accepted Manuscript
Published on 19 June 2014. Downloaded by University of Waikato on 14/07/2014 07:28:41.
Wanbin Li, a Qin Meng, b Xiaonian Li, a Congyang Zhang, a Zheng Fan, a Guoliang Zhang,*a
ChemComm
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DOI: 10.1039/C4CC03864A
Fig. 3 Cross-section and surface SEM images of (a) (b) ZIF8/PVDF and (c) (d) ZIF-7/PVDF. 45 5
Fig. 2 Cross-section and surface SEM images of (a) (b) NAZnO/PVDF synthesized at 353 K and (c) (d) NA-ZnO/PVDF synthesized at 423 K. 50
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First, we attempted to grow ZnO array on the membrane using conventional approach, zinc chloride and ammonia were used as precursor and pH regulator, respectively. To obtain the ZnO array and improve the stability, PVDF hollow fiber was ammoniated to from amino group on the membrane surface. However, no ZnO crystal was found to be deposited on the surface (Fig. S1 ESI†). To obtain the NA-ZnO array and make it convenient for MOF membrane synthesis, zinc nitrate hexahydrate, 2-methyl-imidazole and sodium formate were employed. After crystallization, as Fig. 2a and b, NAZnO sheets with the thickness of nano-scale were grown on the membrane surface as array. It can be observed that the nano-sheets NA-ZnO crossed the dense layer of the PVDF hollow fiber, which greatly improved the anchoring strength between the array and substrate. FTIR diffractogram of NAZnO revealed that 2-methyl-imidazole was integrated in the prepared NA-ZnO (Fig. 1 and Fig. S2 ESI†), which should be benefit for the synthesis of the MOF membranes and make the activation step be omitted. Since temperature was an important factor for the morphology of NA-ZnO array, we increased the synthetic temperature to 423 K. As expected, the morphology of the NA-ZnO crystals was changed to rice-body (Fig. 2c and d). The low-magnification SEM images demonstrated the array layer was uniform (Fig. S3 ESI†). As we can see, no matter what the morphology was, a large tract of NA-ZnO array covered on the surface tidily. The XRD patterns showed that the crystalline were ZnO (Fig. S4 ESI†). The adhesion was found to be verified by the ultrasonic method. After the NA-ZnO/PVDF was subjected to ultrasonic treatment for 60 min, both the NA-ZnO arrays can maintain an excellent structure and even no NA-ZnO crystal was found to fall off (Fig. S5a and b ESI†), which demonstrated that the arrays had especially good adhesion.
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After the NA-ZnO array was grown on PVDF hollow fiber successfully, we further grew the ZIF layer on the NA-ZnO array. The NA-ZnO/PVDF was directly immersed into the ZIF precursor solution for crystallization. Like previous studies,12 the activation of ZnO was found to be vital for ZIF-8 membrane formation. But here, because the NA-ZnO array was integrated by 2-methyl-imidazole, the uniform ZIF layer can be prepared without any procedure of the activation of ZnO. After the ZIF-8 was grown by zinc chloride, 2-methylimidazole and sodium formate, the SEM images of the prepared membranes were shown in Fig. 3. The cross-section of the membrane showed that the crystals were anchored to the substrate tightly. The ZIF-8 crystals were well inter-grown and impacted. From the surface SEM image, no cracks, pinholes, or other defects were visible. It revealed that highquality membrane was made. Moreover, the lowmagnification SEM images indicated the ZIF-8 layer uniformly covered on the entirely hollow fiber (Fig. S5c ESI†). In fact, the only difference of precursor solution between NA-ZnO array and ZIF-8 membrane was different metal salt. However, different crystals was obtained, this should be attributed to the water in zinc nitrate hexahydrate. Based on this progress, the ZIF-7/PVDF membrane was also synthesized. To demonstrate that the NA-ZnO array was an excellent nuclei provider, the synthetic time here was shortened to 12 h, although the synthetic time for most reported ZIF membrane was longer than 24 h. The uniform and well inter-growth ZIF-7 layer was well grown on the PVDF hollow fiber. Accordingly, the smaller thickness of ZIF-7/PVDF was the result of the shorter synthetic time. What is more, from the SEM images, the NA-ZnO array layer was not obvious. This may be because the metal oxides as the metal ion source participated in the reaction.18 To check the effect of ZnO array morphology on the preparation of MOF membrane, we have also grown the ZIF-8 and ZIF-7 layer on PVDF hollow fiber with different NA-ZnO arrays, but no big difference of the ZIF/PVDF was observed. The XRD diffractograms indicated that the materials were crystalline and constituted the same phase of the ZIF-8 and the ZIF-7 (Fig. S6 ESI†). For ZIF-8/PVDF, the phase of ZnO was This journal is © The Royal Society of Chemistry [year]
ChemComm Accepted Manuscript
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activation procedure can be thoroughly omitted. Moreover, the array and MOF layer held strong adhesion with the hollow fiber membrane, and the prepared MOF membranes possessed excellent gas separation performance.
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DOI: 10.1039/C4CC03864A
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a Institute of Oceanic and Environmental Chemical Engineering, College of Chemical Engineering and Material Science, Zhejiang University of Technology, Hangzhou 310014, China. Fax/Tel:+86 571 88320863; Email:[email protected] b Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China. † Electronic Supplementary Information (ESI) available: Experimental details, FTIR, and SEM of the membrane. See DOI: 10.1039/b000000x/ 1 (a) N. Stock, S. Biswas, Chem. Rev. 2012, 112, 933; (b) R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O’Keeffe and O. M. Yaghi, Science 2008, 319, 939; (c) N. Stock and S. Biswas, Chem. Rev., 2012, 112, 933. 2 (a) D. Bradshaw, A. Garai and J. Huo, Chem. Soc. Rev., 2012, 41, 2344; (b) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T. H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724; (c) S. Sorribas, P. Gorgojo, C. Téllez, J. Coronas, and A. G. Livingston, J. Am. Chem. Soc., 2013, 135, 15201. 3 (a) D. Zacher, O. Shekhah, C. Wöll and R. A. Fischer, Chem. Soc. Rev., 2009, 38, 1418; (b) J. Gascon and F. Kapteijn, Angew. Chem. Int. Ed., 2010, 49, 1530. 4 Y. Liu, Z. Ng, E. A. Khan, H. K. Jeong, C. B. Ching and Z. Lai, Microporous Mesoporous Mater., 2009, 118, 296. 5 S. Hermes, F. Schröder, R. Chelmowski, C. Wöll and R. A. Fischer, J. Am. Chem. Soc., 2005, 127, 13744. 6 A. Huang, H. Bux, F. Steinbach and J. Caro, Angew. Chem. Int. Ed., 2010, 49, 4958. 7 R. Ranjan and M. Tsapatsis, Chem. Mater., 2009, 21, 4920. 8 (a) Y. Hu, X. Dong, J. Nan, W. Jin, X. Ren, N. Xu and Y. M. Lee, Chem. Commun., 2011, 47, 737; (b) J. Nan, X. Dong, W. Wang, W. Jin and N. Xu, Langmuir, 2011, 27, 4309; (c) V. V. Guerrero, Y. Yoo, M. C. McCarthy and H. K. Jeong, J. Mater. Chem., 2010, 20, 3938; (d) Y. S. Li, H. Bux, A. Feldhoff, G. L. Li, W. S. Yang and J. Caro, Adv. Mater., 2010, 22, 3322. 9 W. Li, G. Zhang, C. Zhang, Q. Meng, Z. Fan and C. Gao, Chem. Commun., 2014, 50, 3214. 10 (a) S. Zhou, X. Zou, F. Sun, H. Ren, J. Liu, F. Zhang, N. Zhao and G. Zhu, Int. J. Hydrogen. Energ., 3013, 38, 5338; (b) H. Guo, G. Zhu, I. J. Hewitt and S. Qiu, J. Am. Chem. Soc., 2009, 131, 1646; (c) Y. Yue, Z. A. Qiao, X. Li,A. J. Binder, E. Formo, Z. Pan, C. Tian, Z. Bi and S. Dai, Cryst. Growth Des., 2013, 13, 1002. 11 X. Dong, K. Huang, S. Liu, R. Ren, W. Jin and Y. S. Lin, J. Mater. Chem., 2012, 22, 19222. 12 (a) X. Zhang, Y. Liu, L. Kong, H. Liu, J. Qiu, W. Han, L. T. Weng, K. L. Yeung and W. Zhu, J. Mater. Chem. A, 2013, 1, 10635; (b) X. Zhang, Y. Liu, S. Li, L. Kong, H. Liu, Y. Li, W. Han, K. L. Yeung, W. Zhu, W. Yang and J. Qiu, Chem. Mater. 2014, 26, 1975. 13 (a) J. Yao, D. Dong, D. Li, L. He, G. Xu, H. Wang, Chem. Commun. 2011, 47, 2559; (b) R. Ameloot, F. Vermoortele, W. Vanhove, M. B. J. Roeffaers, B. F. Sels and D. E. De Vos, Nat. Chem., 2011, 3, 382; (c) O. Shekhah, R. Swaidan, Y. Belmabkhout, M. Plessis, T. Jacobs, L. J. Barbour, I. Pinnau and M. Eddaoudi, Chem. Commun., 2014, 50, 2089. 14 Y. Mao, L. Shi, H. Huang, W. Cao, J. Li, L. Sun, X. Jin and X. Peng, Chem. Commun., 2013, 49, 5666. 15 A. Centrone, Y. Yang, S. Speakman, L. Bromberg, G. C. Rutledge and T. A. Hatton, J. Am. Chem. Soc., 2010, 132, 15687. 16 A. J. Brown, J. R. Johnson, M. E. Lydon, W. J. Koros, C. W. Jones, and S. Nair, Angew. Chem. Int. Ed., 2012, 51, 10615. 17 W. Li, Z. Yang, G. Zhang, Z. Fan, Q. Meng, C. Shen, C and Gao, J. Mater. Chem. A, 2014, 2, 2110. 18 W. Zhan, Q. Kuang, J. Zhou, X. Kong, Z. Xie and L. Zheng, J. Am. Chem. Soc., 2013, 135, 1926-1933. 19 (a) A. Huang, Y. Chen, N. Wang, Z. Hu, J. Jiang and J. Caro, Chem. Commun., 2012, 48, 10981; (b) Z. Xie, J. Yang, J. Wang, J. Bai, H. Yin, B. Yuan, J. Lu, Y. Zhang, L. Zhou and C. Duan, Chem. Commun., 2012, 48, 5977.
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ChemComm Accepted Manuscript
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almost all disappeared, which was agreed with the SEM images. The ZIF/PVDF was further subjected to ultrasonic treatment for 60 min. No ZIF crystal was found to be exfoliated from the PVDF hollow fiber, except some corner of the ZIF crystals were destroyed (Fig. S5d ESI†), which demonstrated the ZIF layers clung to the substrate strongly. Besides ZIF-8 and ZIF-7, we also attempted to synthesize CuBTC (known as HKUST-1) on the NA-ZnO/PVDF. The results showed that well impacted CuBTC membrane can be obtained (Fig. S7 ESI†). Gas permeation data for H2, CO2 and N2 were collected at room temperature and 1 bar using self-made gas permeation equipment.9 Before gas permeation, the ZIF membranes were dried under vacuum. The results revealed that the ZIF membranes exhibited remarkably higher H2 permeances compared with other gases N2 and CO2. The ideal separation factors (ISF) of H2/N2 and H2/CO2 were much larger than the values of Knudsen diffusion (3.74 and 4.69), and reached 18.14, 16.29 and 20.27, 18.43 for ZIF-8/PVDF and ZIF7/PVDF, respectively. Because the aperture of ZIF-7 (0.3 nm) was between the kinetic diameters of H2 (0.29 nm) and CO2 (0.33 nm) and smaller than that of ZIF-8 (0.34 nm), the ZIF7/PVDF held the superior ISF. Moreover, the ISF of prepared membranes were larger than most of reported ZIF membranes (Tab. S1). In addition, the H2 permeances of the membranes obtained were also excellent, 20.14 and 23.54 ×10 −7 mol s−1 m−2 Pa−1 for ZIF-8/PVDF and ZIF-7/PVDF, respectively, which were larger than most ZIF membranes (below than 5 ×10 −7 mol s−1 m−2 Pa−1), except the ZIF-95 (24.6 ×10 −7 mol s−1 m−2 Pa−1) and ZIF-8 (573.0 ×10 −7 mol s−1 m−2 Pa−1) membranes reported by Huang et al. and Xie et al., respectively.19 The larger H2 permeance of the ZIF-7/PVDF should be attributed to the thinner ZIF layer. Similar to the reported studies, as a result of the well-known lattice flexibility of ZIFs, the larger size gases such as CO2 (0.33 nm) and N2 (0.36) can also pass through the smaller aperture ZIF membranes.19a In summary, we have reported a new method to fabricate strongly adhesive MOF membranes by using non-activation ZnO array as buffering layer. The NA-ZnO array layers with diversity of morphologies were successfully grown on the PVDF hollow fiber substrate. The NA-ZnO array layer had strong adhesion with PVDF membrane. NA-ZnO array was proved to be an outstanding buffering layer for synthesis of crack-free and uniform MOF membranes without any activation procedure. The fabricated MOF/PVDF membranes had commendable hollow fiber structures and exhibited excellent H2 permselectivity. For ZIF-7 membrane, the ideal separation factors for H2/N2 and H2/CO2 were 20.37 and 18.43, respectively, and the permeance of H2 can reach as high as 23.54 ×10 −7 mol s−1 m−2 Pa−1. These properties recommend that NA-ZnO array should be excellent buffering layer for fabricating MOF membranes, and the prepared ZIF/PVDF membranes will be the promising candidates for industrial hydrogen separation. We thank for financial support the National Natural Science Foundation of China (Nos. 21236008 and 21176226).
ChemComm
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DOI: 10.1039/C4CC03864A
The table of contents: Non-activation ZnO array is grown on PVDF membrane to synthesize defect-free MOF membranes which exhibit excellent gas separation performance. 5
Title: Non-activation ZnO array as buffering layer to fabricate strongly adhesive metal-organic
ChemComm Accepted Manuscript
Published on 19 June 2014. Downloaded by University of Waikato on 14/07/2014 07:28:41.
framework/PVDF hollow fiber membranes
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