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New membrane architecture: ZnO@ZIF-8 mixed matrix membrane exhibiting superb H2 permselectivity and excellent stability
Inorganic Chemistry Communications 48 (2014) 77–80
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
New membrane architecture: ZnO@ZIF-8 mixed matrix membrane exhibiting superb H2 permselectivity and excellent stability Yaguang Liu a, Shaohui Li a, Xiongfu Zhang a,⁎, Haiou Liu a, Jieshan Qiu a, Yanshuo Li c, King Lun Yeung b a b c
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China Department of Chemical and Biomolecular Engineering, the Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
a r t i c l e
i n f o
Article history: Received 5 July 2014 Received in revised form 23 August 2014 Accepted 28 August 2014 Available online 29 August 2014
a b s t r a c t A new ZnO@ZIF-8 mixed matrix membrane was achieved by in-situ growth of ZIF-8 crystals within well-aligned ZnO nanorods supported on a porous α-Al2O3 ceramic tube. The ZnO nanorods act as both seed sites for ZIF-8 growth and strut-like frameworks for the connection of the membrane with the alumina substrate. The resulting membrane exhibits superb H2 selective permeation performance and excellent stability. © 2014 Elsevier B.V. All rights reserved.
Keywords: Metal organic frameworks ZnO nanorods Film Membrane Gas separation
Metal-organic framework (MOF) structure and chemistry gave raise to many unique and interesting properties that have promising applications in chemical conversion, gas separation and storage [1,2]. MOFs with their well-defined pores and tunable chemistry are superb candidates for membranes [3–7]. Indeed, MOF membranes have been successfully prepared by different synthesis strategies [5,6]. However, low-defect MOF membranes remain a challenge due to the poor nucleation and growth of MOF thin films. Poor intergrowth and brittleness can also compromise membrane performance [8–11]. It is not uncommon in preparing MOF membranes to modify the support in order to improve film deposition and growth [12–15]. Seeded growth method has proven to be versatile for preparing a large variety of MOF membranes [16–18]. High quality seeds and MOF nanoparticles are commonly used, and polymers are often used to adsorb and assemble the seeds on the support. The reactive seeding method was developed to avoid the use of polymers that may interfere with the membrane transport. Membranes of ZIF-78 and ZIF-71 on ZnO disks and MIL-53 on alumina disks were prepared by the reactive seeding method for selective separation [19–21]. Although great progress has been made for MOF preparation, indicating that metal oxides can seed and promote the formation of MOF structure in many morphologies [22–24], to the best of our knowledge, there is no report on forming a continuous membrane within ZnO nanorods for separation. ⁎ Corresponding author. E-mail address: [email protected] (X. Zhang).
http://dx.doi.org/10.1016/j.inoche.2014.08.023 1387-7003/© 2014 Elsevier B.V. All rights reserved.
This work explores a new membrane architecture, ZnO@ZIF-8 hybrid membrane, wherein well-aligned ZnO nanorods are used to anchor and reinforce the ZIF-8 metal-organic framework membrane on a porous ceramic support. ZIF-8 was nucleated uniformly on the surface of the nanorods, and induced to grow radially outward from the rods in a direction parallel to the support until it intergrown with the neighboring crystals forming a membrane. The scheme of the membrane process is illustrated in Fig. 1. The new membrane was prepared on hollow fiber tubes (HFT, 4 mm o.d., 3 mm i.d.) of 100 nm pore size. Each membrane measured 60 mm in length. In brief, the well-aligned ZnO nanorods were grown seamlessly from the support (Fig. 2a) by hydrothermal synthesis. For the sample shown in Fig. 2a, a uniform growth was obtained over the entire substrate. Fig. 2b shows that the ZnO nanorods measure 3 ± 0.5 μm in length and have a hexagonal cross-section of about 200 nm. A uniform nucleation on the surface of the ZnO nanorods results in a uniform radial growth of ZIF-8 by an in-situ synthesis from a synthesis solution with a molar composition of 2 HCOONa:1.0 ZnCl2:3 Hmim:360 EtOH for 6 h at 353 K. The ZnO@ZIF-8 mixed matrix membrane architecture was shown in Fig. 2c & d. Unlike conventional membranes, the enormous surface area of the nanorods provides a solid anchor for the membrane. The nanorods remained after membrane synthesis and can be seen from the membrane cross-sections (cf. Fig. 2d), EDXS composition profile (cf. Fig. 2e) and X-ray diffraction (cf. Fig. 2f). The seamless intergrowth between neighboring ZIF-8 crystals is clearly evident in Fig. 2d and indeed, common defects such as pinholes and cracks were absent. This could be the reason for the excellent gas permeation of these membranes.
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ZnO rod growth
Porous alumina
ZIF-8 growth
ZnO nanorods
Fig. 1. Preparation scheme for the ZnO@ZIF-8 mixed matrix membrane on porous α-Al2O3 tube.
Our previous work [25,26] has shown that ZIF-8 crystals mainly grow on the upper ends of the ZnO nanorods, resulting in forming a type of strut-like ZIF-8 membrane through activation process, followed by regrowth. Here, an in-situ synthesis without activation, was used to make ZIF-8 crystals mainly grow within the frameworks of the ZnO nanorods, thus leading to a mixed matrix membrane with excellent stability as the following permeation results. Five membranes grown from separate synthesis batches were prepared and their gas permeances were measured and shown in Table 1. Four of the five membranes have comparable H2 permeance of (41 ± 8) × 10−9 mol·m−2·s−1·Pa−1. All four membranes have very similar H2/N2, H2/CO2 and H2/CH4 permselectivities indicating good membrane reproducibility. M3 membrane has significantly higher flux possibly due to defective end sealing. The gas permeance decreases rapidly with increasing size of the diffusing gas molecule (i.e., H2, 0.29 nm; CO2, 0.33 nm; N2, 0.36 nm; CH4, 0.38 nm). Framework flexibility and pore distortion in ZIF-8 explain the permeation of N 2 and CH 4 gases that are larger than ZIF-8 pores (ca. 0.34 nm). However, their high permselectivities (i.e., H2/N2 = 13; H2/CH4 = 16) and the absence
a
Table 1 Results of single gas permeation for five membranes obtained from different batches at the same condition. Membrane
H2 permeancea
H2/CO2
H2/N2
H2/CH4
M1 M2 M3 M4 M5 Average
32.86 41.19 97.27 52.22 39.02 41.32
5.8 5.6 5.4 5.4 6.5 5.8
13.1 13.6 10.1 12.4 13.5 13.2
16.6 15.2 11.5 15.3 15.8 15.7
a
H2 permeance/∗10−9 mol·m−2·s−1·Pa−1. The average is calculated except for M3.
of viscous/Poiseuille flow suggest that the membranes are free of defects. Binary gas separation measurements were carried out for M1 membrane at 303 K. The H2 permeance from H2:CO2, H2:N2 and H2: CH4 binary gas mixtures was 27.0 × 10− 9 mol·m− 2·s− 1·Pa− 1 at a transmembrane pressure of 0.05 MPa. Separation selectivities for H2/CO2, H2/N2 and H2/CH4 were respectively 3.2, 10.4 and 12.6, lower than the measured permselectivity values of 5.8, 13.1 and 16.6. This is not unusual as diffusion in mixtures can often lead to lower flux and selectivities in porous membranes. Furthermore, the sorption of gases can distort the ZIF-8 framework and change the motion of the organic linkers altering the interaction and mobility of the gases in the pores [27]. The new membrane architecture is suitable for the preparation of ZIF mixed matrix membranes and has the advantages of greater rigidity and stability. Fig. 3 plots the results of single gas permeation experiments carried out on M1 membrane. Measurements were made as the membrane was heated from 303 K to 423 K and cooled back to 303 K. It can be observed from the plot that the process is reversible. Temperature has the least effect on H2/N2 and H2/CH4 permselectivities, while H2/CO2 increases from 5.8 to 12.4 for 303 K to 423 K. This phenomenon
c
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4 m
d c b a 10
20
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40
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2 (degree) Fig. 2. SEM images of the samples: a, b: ZnO nanorods grown on the substrate; c, d: ZIF-8 membrane grown within the ZnO nanorods; e: EDX elemental analysis of the hybrid membrane cross-section; and f: XRD patterns of the samples (g: magnification of the membrane sample c).
Y. Liu et al. / Inorganic Chemistry Communications 48 (2014) 77–80 20
50
20
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423K 25
H2/CH4 25
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5
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Perm eanance/10 -9 m olm -2 s -1 Pa -1
50
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1
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0
0
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Fig. 3. Single gas permeation as a function of temperature: permeance (left) and permselectivity (right).
is observed in different literatures [28,29] including our previous work [26]. The permeances of all gases decreased with increasing temperatures. This might be related to the changes in the energetics of C_C bonds that govern the stretching of the methyl-imidazole ring and therefore the size of the pore aperture. And this is also proved by the in-situ IR results at different temperatures (Fig. S11). The peaks at 1350–1500 cm−1 are assigned to the stretching of two kinds of C_C bonds of the methyl-imidazole ring and have an obvious change with temperature, while there is no significant change for C_N bond at 1584 cm−1. As shown in Fig. S12, both the C_C bonds (a and b) have the most important effect on the aperture of ZIF-8. It is generally accepted that configurational diffusion is the predominant transport mechanism [26,30] as the size of the diffusing molecule (i.e., CO2) approaches that of the pore opening (i.e., ZIF-8). Therefore, any perturbation in pore diameter caused by temperature or gas sorption is expected to cause a large change in the gas permeation. This could explain the greater temperature sensitivity of CO2 permeance, implying that the new membrane architecture is free of defects. In conclusion, a new ZnO@ZIF-8 mixed matrix membrane was successfully conceptualized and fabricated by in-situ growth of ZIF-8 crystals within well-aligned ZnO nanorods. The ZnO nanorods can act as both nucleation sites and anchors for the formation of a dense and robust ZnO@ZIF-8 mixed matrix membrane. The resulting membrane possesses highly selective H2 permeation from small gases (CO2, N2 and CH4) due to its molecular sieving function presented by the pore of the membrane. It is worth noting that the permeance of CO2 through the membrane obviously decreases with the temperature, implying that this membrane is free of defects. The present synthesis strategy may open up a new route for preparing new MOF membrane architecture and also be used to prepare metal oxide@MOF mixed matrix films which can serve as possible semiconducting multifunction [31].
Acknowledgment We gratefully acknowledge the financial support by the National Natural Science Foundation of China (Nos. 21476039, 21076030, 21036006), the Natural Science Foundation of Liaoning Province (No. 201202027) and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130041110022).
Appendix A. Supplementary Data Electronic Supplementary Information (ESI) available: The details of synthesis, characterization and gas permeation of the membrane.
Supplementary data to this article can be found online at http://dx.doi. org/10.1016/j.inoche.2014.08.023.
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[22] S. Tanaka, K. Kida, T. Nagaoka, T. Ota, Y. Miyake, Mechanochemical dry conversion of zinc oxide to zeolitic imidazolate framework, Chem. Commun. 49 (2013) 7884–7886. [23] Y. Yue, Z. Qiao, X. Li, A.J. Binder, E. Formo, Z. Pan, C. Tian, Z. Bi, S. Dai, Nanostructured zeolitic imidazolate frameworks derived from nanosized zinc oxide precursors, Cryst. Growth Des. 13 (2013) 1002–1005. [24] R. Wu, X. Qian, X. Rui, H. Liu, B. Yadian, K. Zhou, J. Wei, Q. Yan, X. Feng, Y. Long, L. Wang, Y. Huang, Zeolitic imidazolate framework 67-derived high symmetric porous Co3O4 hollow dodecahedra with highly enhanced lithium storage capability, Small 10 (2014) 1932–1938. [25] X. Zhang, Y. Liu, L. Kong, H. Liu, J. Qiu, W. Han, L. Weng, K. Yeung, W. Zhu, A simple and scalable method for preparing low-defect ZIF-8 tubular membranes, J. Mater. Chem. A 1 (2013) 10635–10638. [26] X. Zhang, Y. Liu, S. Li, L. Kong, H. Liu, Y. Li, W. Han, K. Yeung, W. Zhu, W. Yang, J. Qiu, New membrane architecture with high performance: ZIF-8 membrane supported
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on vertically aligned ZnO nanorods for gas permeation and separation, Chem. Mater. 26 (2014) 1975–1981. C.O. Ania, E. García-Pérez, M. Haro, J.J. Gutiérrez-Sevillano, T. Valdés-Solís, J.B. Parra, S. Calero, Understanding gas-induced structural deformation of ZIF-8, J. Phys. Chem. Lett. 3 (2012) 1159–1164. A.J. Brown, J.R. Johnson, M.E. Lydon, W.J. Koros, C.W. Jones, S. Nair, Continuous polycrystalline zeolitic imidazolate framework-90 membranes on polymeric hollow fibers, Angew. Chem. Int. Ed. 51 (2012) 10615–10618. Z. Zhao, X. Ma, Z. Li, Y.S. Lin, Synthesis, characterization and gas transport properties of MOF-5 membranes, J. Membr. Sci. 382 (2011) 82–90. P. Weisz, Zeolites—new horizons in catalysis, Chem. Tech. 3 (1973) 498–505. G. Lu, J.T. Hupp, Metal-organic frameworks as sensors: a ZIF-8 based Fabry–Pérot device as a selective sensor for chemical vapors and gases, J. Am. Chem. Soc. 132 (2010) 7832–7833.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
New membrane architecture: ZnO@ZIF-8 mixed matrix membrane exhibiting superb H2 permselectivity and excellent stability Yaguang Liu a, Shaohui Li a, Xiongfu Zhang a,⁎, Haiou Liu a, Jieshan Qiu a, Yanshuo Li c, King Lun Yeung b a b c
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China Department of Chemical and Biomolecular Engineering, the Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
a r t i c l e
i n f o
Article history: Received 5 July 2014 Received in revised form 23 August 2014 Accepted 28 August 2014 Available online 29 August 2014
a b s t r a c t A new ZnO@ZIF-8 mixed matrix membrane was achieved by in-situ growth of ZIF-8 crystals within well-aligned ZnO nanorods supported on a porous α-Al2O3 ceramic tube. The ZnO nanorods act as both seed sites for ZIF-8 growth and strut-like frameworks for the connection of the membrane with the alumina substrate. The resulting membrane exhibits superb H2 selective permeation performance and excellent stability. © 2014 Elsevier B.V. All rights reserved.
Keywords: Metal organic frameworks ZnO nanorods Film Membrane Gas separation
Metal-organic framework (MOF) structure and chemistry gave raise to many unique and interesting properties that have promising applications in chemical conversion, gas separation and storage [1,2]. MOFs with their well-defined pores and tunable chemistry are superb candidates for membranes [3–7]. Indeed, MOF membranes have been successfully prepared by different synthesis strategies [5,6]. However, low-defect MOF membranes remain a challenge due to the poor nucleation and growth of MOF thin films. Poor intergrowth and brittleness can also compromise membrane performance [8–11]. It is not uncommon in preparing MOF membranes to modify the support in order to improve film deposition and growth [12–15]. Seeded growth method has proven to be versatile for preparing a large variety of MOF membranes [16–18]. High quality seeds and MOF nanoparticles are commonly used, and polymers are often used to adsorb and assemble the seeds on the support. The reactive seeding method was developed to avoid the use of polymers that may interfere with the membrane transport. Membranes of ZIF-78 and ZIF-71 on ZnO disks and MIL-53 on alumina disks were prepared by the reactive seeding method for selective separation [19–21]. Although great progress has been made for MOF preparation, indicating that metal oxides can seed and promote the formation of MOF structure in many morphologies [22–24], to the best of our knowledge, there is no report on forming a continuous membrane within ZnO nanorods for separation. ⁎ Corresponding author. E-mail address: [email protected] (X. Zhang).
http://dx.doi.org/10.1016/j.inoche.2014.08.023 1387-7003/© 2014 Elsevier B.V. All rights reserved.
This work explores a new membrane architecture, ZnO@ZIF-8 hybrid membrane, wherein well-aligned ZnO nanorods are used to anchor and reinforce the ZIF-8 metal-organic framework membrane on a porous ceramic support. ZIF-8 was nucleated uniformly on the surface of the nanorods, and induced to grow radially outward from the rods in a direction parallel to the support until it intergrown with the neighboring crystals forming a membrane. The scheme of the membrane process is illustrated in Fig. 1. The new membrane was prepared on hollow fiber tubes (HFT, 4 mm o.d., 3 mm i.d.) of 100 nm pore size. Each membrane measured 60 mm in length. In brief, the well-aligned ZnO nanorods were grown seamlessly from the support (Fig. 2a) by hydrothermal synthesis. For the sample shown in Fig. 2a, a uniform growth was obtained over the entire substrate. Fig. 2b shows that the ZnO nanorods measure 3 ± 0.5 μm in length and have a hexagonal cross-section of about 200 nm. A uniform nucleation on the surface of the ZnO nanorods results in a uniform radial growth of ZIF-8 by an in-situ synthesis from a synthesis solution with a molar composition of 2 HCOONa:1.0 ZnCl2:3 Hmim:360 EtOH for 6 h at 353 K. The ZnO@ZIF-8 mixed matrix membrane architecture was shown in Fig. 2c & d. Unlike conventional membranes, the enormous surface area of the nanorods provides a solid anchor for the membrane. The nanorods remained after membrane synthesis and can be seen from the membrane cross-sections (cf. Fig. 2d), EDXS composition profile (cf. Fig. 2e) and X-ray diffraction (cf. Fig. 2f). The seamless intergrowth between neighboring ZIF-8 crystals is clearly evident in Fig. 2d and indeed, common defects such as pinholes and cracks were absent. This could be the reason for the excellent gas permeation of these membranes.
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ZnO rod growth
Porous alumina
ZIF-8 growth
ZnO nanorods
Fig. 1. Preparation scheme for the ZnO@ZIF-8 mixed matrix membrane on porous α-Al2O3 tube.
Our previous work [25,26] has shown that ZIF-8 crystals mainly grow on the upper ends of the ZnO nanorods, resulting in forming a type of strut-like ZIF-8 membrane through activation process, followed by regrowth. Here, an in-situ synthesis without activation, was used to make ZIF-8 crystals mainly grow within the frameworks of the ZnO nanorods, thus leading to a mixed matrix membrane with excellent stability as the following permeation results. Five membranes grown from separate synthesis batches were prepared and their gas permeances were measured and shown in Table 1. Four of the five membranes have comparable H2 permeance of (41 ± 8) × 10−9 mol·m−2·s−1·Pa−1. All four membranes have very similar H2/N2, H2/CO2 and H2/CH4 permselectivities indicating good membrane reproducibility. M3 membrane has significantly higher flux possibly due to defective end sealing. The gas permeance decreases rapidly with increasing size of the diffusing gas molecule (i.e., H2, 0.29 nm; CO2, 0.33 nm; N2, 0.36 nm; CH4, 0.38 nm). Framework flexibility and pore distortion in ZIF-8 explain the permeation of N 2 and CH 4 gases that are larger than ZIF-8 pores (ca. 0.34 nm). However, their high permselectivities (i.e., H2/N2 = 13; H2/CH4 = 16) and the absence
a
Table 1 Results of single gas permeation for five membranes obtained from different batches at the same condition. Membrane
H2 permeancea
H2/CO2
H2/N2
H2/CH4
M1 M2 M3 M4 M5 Average
32.86 41.19 97.27 52.22 39.02 41.32
5.8 5.6 5.4 5.4 6.5 5.8
13.1 13.6 10.1 12.4 13.5 13.2
16.6 15.2 11.5 15.3 15.8 15.7
a
H2 permeance/∗10−9 mol·m−2·s−1·Pa−1. The average is calculated except for M3.
of viscous/Poiseuille flow suggest that the membranes are free of defects. Binary gas separation measurements were carried out for M1 membrane at 303 K. The H2 permeance from H2:CO2, H2:N2 and H2: CH4 binary gas mixtures was 27.0 × 10− 9 mol·m− 2·s− 1·Pa− 1 at a transmembrane pressure of 0.05 MPa. Separation selectivities for H2/CO2, H2/N2 and H2/CH4 were respectively 3.2, 10.4 and 12.6, lower than the measured permselectivity values of 5.8, 13.1 and 16.6. This is not unusual as diffusion in mixtures can often lead to lower flux and selectivities in porous membranes. Furthermore, the sorption of gases can distort the ZIF-8 framework and change the motion of the organic linkers altering the interaction and mobility of the gases in the pores [27]. The new membrane architecture is suitable for the preparation of ZIF mixed matrix membranes and has the advantages of greater rigidity and stability. Fig. 3 plots the results of single gas permeation experiments carried out on M1 membrane. Measurements were made as the membrane was heated from 303 K to 423 K and cooled back to 303 K. It can be observed from the plot that the process is reversible. Temperature has the least effect on H2/N2 and H2/CH4 permselectivities, while H2/CO2 increases from 5.8 to 12.4 for 303 K to 423 K. This phenomenon
c
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d c b a 10
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2 (degree) Fig. 2. SEM images of the samples: a, b: ZnO nanorods grown on the substrate; c, d: ZIF-8 membrane grown within the ZnO nanorods; e: EDX elemental analysis of the hybrid membrane cross-section; and f: XRD patterns of the samples (g: magnification of the membrane sample c).
Y. Liu et al. / Inorganic Chemistry Communications 48 (2014) 77–80 20
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Fig. 3. Single gas permeation as a function of temperature: permeance (left) and permselectivity (right).
is observed in different literatures [28,29] including our previous work [26]. The permeances of all gases decreased with increasing temperatures. This might be related to the changes in the energetics of C_C bonds that govern the stretching of the methyl-imidazole ring and therefore the size of the pore aperture. And this is also proved by the in-situ IR results at different temperatures (Fig. S11). The peaks at 1350–1500 cm−1 are assigned to the stretching of two kinds of C_C bonds of the methyl-imidazole ring and have an obvious change with temperature, while there is no significant change for C_N bond at 1584 cm−1. As shown in Fig. S12, both the C_C bonds (a and b) have the most important effect on the aperture of ZIF-8. It is generally accepted that configurational diffusion is the predominant transport mechanism [26,30] as the size of the diffusing molecule (i.e., CO2) approaches that of the pore opening (i.e., ZIF-8). Therefore, any perturbation in pore diameter caused by temperature or gas sorption is expected to cause a large change in the gas permeation. This could explain the greater temperature sensitivity of CO2 permeance, implying that the new membrane architecture is free of defects. In conclusion, a new ZnO@ZIF-8 mixed matrix membrane was successfully conceptualized and fabricated by in-situ growth of ZIF-8 crystals within well-aligned ZnO nanorods. The ZnO nanorods can act as both nucleation sites and anchors for the formation of a dense and robust ZnO@ZIF-8 mixed matrix membrane. The resulting membrane possesses highly selective H2 permeation from small gases (CO2, N2 and CH4) due to its molecular sieving function presented by the pore of the membrane. It is worth noting that the permeance of CO2 through the membrane obviously decreases with the temperature, implying that this membrane is free of defects. The present synthesis strategy may open up a new route for preparing new MOF membrane architecture and also be used to prepare metal oxide@MOF mixed matrix films which can serve as possible semiconducting multifunction [31].
Acknowledgment We gratefully acknowledge the financial support by the National Natural Science Foundation of China (Nos. 21476039, 21076030, 21036006), the Natural Science Foundation of Liaoning Province (No. 201202027) and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130041110022).
Appendix A. Supplementary Data Electronic Supplementary Information (ESI) available: The details of synthesis, characterization and gas permeation of the membrane.
Supplementary data to this article can be found online at http://dx.doi. org/10.1016/j.inoche.2014.08.023.
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