ZnO Nanorods Induced Heteroepitaxial Growth of SOD Type Co

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ZnO Nanorods Induced Heteroepitaxial Growth of SOD Type Cobased Zeolitic Imidazolate Framework Membranes for H Separation 2

Pei Nian, Yujia Li, Xiang Zhang, Yi Cao, Hai-ou Liu, and Xiongfu Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17568 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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ACS Applied Materials & Interfaces

ZnO Nanorods Induced Heteroepitaxial Growth of SOD Type Co-based Zeolitic Imidazolate Framework Membranes for H2 Separation Pei Nian, Yujia Li, Xiang Zhang, Yi Cao, Haiou Liu, Xiongfu Zhang* State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China

KEYWORDS: Metal-organic framework membrane, ZnO nanorods, Co-based membrane, Heteroepitaxial Growth, Gas separation

ABSTRACT: Up to now, the fabrication of well-intergrown Co-based ZIFs membranes on porous tubular supports is still a major challenge. We report here a heteroepitaxial growth for preparing well-intergrown Co-based ZIFs (ZIF-67 and ZIF-9) tubular membranes with high performance and excellent thermal stability by employing a thin layer of ZnO nanorods acting as both nucleation centers and anchor sites for the growth of MOF membranes. The results show that well-intergrown Co-ZIF-67 and Co-ZIF-9 membranes, respectively, are successfully achieved on the ZnO nanorods modified porous ceramic tubes. This highly active heteroepitaxial growth may be attributed to the fact that the (Zn,Co) hydroxy double salt (HDS) intermediate produced in situ from ZnO nanorods acts as heteroseeds and enables the uniform growth of Co-

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based membranes. The H2/CO2 selectivity of the as-prepared Co-ZIF-9 tubular membrane could reach about 23.8 and the H2/CH4 selectivity of Co-ZIF-67 tubular membrane is as high as 45.4. Moreover, the membranes demonstrate excellent stability bacause of the ZnO nanorods as linkers between the membrane and substrate.

1. INTRODUCTION Metal-organic frameworks (MOFs) are superb candidates for membranes and have great potential in many separation processes because of their well-defined pore structures, diverse chemistry, and attractive adsorption properties.1-3 In particular, zeolitic imidazolate frameworks (ZIFs) with zeolite topologies, consisting of transition metals (Zn or Co) and imidazolate ligands, a relatively new subfamily of MOFs, are of more attractive interest for membrane-based gas separation because of their exceptional thermal/chemical stabilities and ultra-micropores (less than 0.5 nm).4-6 In fact, some ZIF membranes have exhibited impressive capacities for separating important gas mixtures, including hydrogen purification, CO2 capture, and olefin/paraffin separation.7-10 Co-based ZIF membranes can not only separate some small-molecular gas mixtures, but also be applied in catalytic reaction as membrane reactors due to their small pore size and the presence of the redox catalytic cobalt centers.11-14 Therefore, it is extremely expected that well-intergrown continuous Co-based ZIF membranes, especially tubular membrane, could be achieved. The reason for choosing a tubular substrate is mainly duo to their prominent advantages over planar geometry for facile scaling of separation units in industrial production.15,16 Nowadays, there are two important Co-based ZIFs including ZIF-67 and ZIF-9, which are paid to intense attention. ZIF-67, constructed by cobalt ions and 2-methylimidazole, is of isostructural SOD zeolitic topology and the effective aperture size is ~ 4.0 Å,10,17 while ZIF-9 consisting of cobalt ions and benzimidazole has a SOD zeolitic topology with pore size of 3.0


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Å.11,18 They as membranes are rather appropriate to separate propylene (4.0 Å) from propylene/propane (4.2 Å) mixtures17 and H2 (2.9 Å) from H2/CO2 (3.3 Å) mixtures,11 respectively. There have already been some attempts and progress made in preparing ZIF-67 and ZIF-9 membranes for gas separation,10,19-21 but all the Co-ZIFs membranes achieved are still grown on porous discs as substrates, which greatly limits its industrial applications. For examples, Jeong et al. achieved well-intergrown ZIF-67/ZIF-8 polycrystalline membrane on αAl2O3 disc for the separation of propylene/propane mixtures through the heteroepitaxial growth of ZIF-67 membranes on ZIF-8 seeds.10 Pan et al. also fabricated a zinc-substituted ZIF-67 membrane for the separation of propylene/propane mixtures on α-alumina disc by incorporation of zinc ions into the ZIF-67 framework.19 In addition, Zhou et al. prepared ZIF-67 membrane on the Co(OH)2 nanosheets modified stainless-steel nets via an electro-deposition method.20 As for the synthesis of ZIF-9 membrane, Zhong et al. grew a CNT@IL/ZIF-9 hybrid membrane on αAl2O3 disc exhibited high selectivity for H2/CO2.21 Compared to the synthesis of Zn-based ZIFs membranes, it is more difficult to prepare Co-based ZIFs membranes. The reason is mainly ascribed to the fact that the formation of Co-based ZIF crystals is very fast in the synthesis solution rather than growing their corresponding membranes on the substrates, thereby leading to poor-quality Co-based ZIF membranes. In fact, for the preparation of crystalline MOF membranes, the core issue is to select a feasible synthesis method for obtaining a continuous and well-intergrown membrane. So far, numerous approaches have been employed for fabricating well-intergrown MOF membranes, such as in situ synthesis,22 secondary growth,23-25 organic functional surface modification,26-29 inorganic microstructure surface modification,20,30-38 along with other techniques.39-41 Among them, recently significant attention has been devoted to inorganic surface modifiers, which possess


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unique advantages, such as high affinity with the substrate, high activity for the MOF membrane growth, high thermal stability, eco-friendly, and easy fabrication.42 For instance, Qiu et al. successfully fabricated Ni-based and Cu-based MOF membranes on Ni30 and Cu nets31 by a twin metal source technique. Zhang et al.32 fabricated ZIF-8 membranes on ZnO nanorods modified tubular substrates, where the ZnO layer served as an intermediate layer to facilitate the nucleation and growth of ZIF-8 membrane. Wang et al.36 directly synthesized ZIF-8 membrane on the ZnO-Al2O3 composite hollow fiber, which shows excellent hydrogen permeance and good reproducibility. In addition, Liu et al.37 prepared ZnAl-CO3 layered double hydroxide (LDH) layer on α-Al2O3 substrate to promote the heterogeneous nucleation of ZIF-8 crystals and result in uniform ZIF-8 membrane. These homologous metals or metal oxides acted as the nucleation centers for the preparation of continuous MOF membranes. Besides, very recently, heteroexpitaxial growth strategy was also used to grow MOF membranes with improved microstructures, thereby leading to excellent separation performances. Jeong et al.10 and coworkers first reported a well-intergrown ZIF-67/ZIF-8 polycrystalline membrane by heteroepitaxially growing ZIF-67 on ZIF-8 seeds layer, which exhibited unprecedentedly high propylene/propane separation. Similarly, Peng et al.43grew MIL-110 nanorods on porous substrate as heteroseeds to grow continuous HKUST-1 membrane. Although such progress have been made based on the above methods in preparing MOF membranes on porous disc supports, there are few reports on growing Co-based ZIF membranes on porous tubular substrates. Thus, it is essential to develop an available and universal surface modifier with excellent reactivity and stability for preparing Co-based ZIF membranes. Herein, inspired by the heteroepitaxially grown MOF membranes, we present an efficient methodology for preparing well-intergrown and stable Co-based ZIF tubular membranes by


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introducing ZnO nanorods layer on porous tubular substrates as induction sites for membrane growth. Up to now, there has been no report on the successful growth of Co-ZIF membranes on porous tubular substrates by ZnO induced heteroepitaxial growth. This protocol employs the readily available and malleable ZnO as heterogeneous nucleation bars for the growth of Co-ZIF67 or Co-ZIF-9 membrane. As shown in the preparation scheme of Figure 1, a thin layer of ZnO nanorod arrays was first grown on the porous α-Al2O3 tube by a hydrothermal process, and then the ZnO nanorods acting as both seeds and linkers induced forming a continuous Co-ZIF-67 or Co-ZIF-9 membrane in its corresponding precursor solution by a solvethermal growth.

Figure 1. Schematic illustration of the synthesis of Co-ZIFs (ZIF-67, ZIF-9) membranes via ZnO nanorods induced heteroepitaxial growth.

2. MATERIALS AND METHODS Materials. The chemicals used in the membrane synthesis include the cobalt nitrate hexahydrate (Co(NO3)2 ∙6H2O, ≥99.0%), zinc nitrate hexahydrate (Zn(NO3)2 ∙6H2O≥99.0%), zinc acetate (Zn(Ac)2 ∙ 2H2O 98.0%), ethylene glycol monomethyl ether (C3H8O2, EGME, 99.0%), monoethanolamine (C2H7NO, MEA, 99.0%), hexamethylenetetramine (C6H12N4 ≥ 99.0%)


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anhydrous methanol (≥99.5%) and N,N’-dimethylformamide (DMF, ≥99.5%) were provided by Sinopharm Chemical reagent Co. Ltd. Benzimidazole (Bim, ≥99.0%) and 2-methylimidazole (Hmim, 99.0%) were purchased from Sigma-Aldrich Chemical. Zinc oxide nanoparticles (cal. 50 ± 10 nm) were supplied by Beijing Nachen S&T Ltd. Porous α-Al2O3 tubes (O.D. 4 mm, I.D. 3 mm, with average 100 nm pore diameter) were provided by Hyflux Co. Ltd. The tubes were cut into 55 mm in lengths and ultrasonically washed with deionized water, and then dried at 150 ℃ for 6h. Preparation of ZnO Nanorods. The ZnO nanorods on the porous α-Al2O3 tube were prepared by a hydrothermal synthesis according to our previous reports.32,33 In a typical procedure, firstly, a thin Zn-based sol layer was dip-coated on the inner surface of the tube. For synthesis of the Znbased sol, 5g Zn(Ac)2·2H2O was first dispersed in 30 mL of EGME and stirred at 70 ℃ for 1h. Then 1.3 mL of MEA was dropwise added to the above solution and stirred at 25 ℃ for 12 h to get a stable Zn-based sol. After that, the α-Al2O3 tube was coated with the sol and dried at 100 ℃ for 1 h. This coating process was repeated two times and followed by calcining at 400 ℃ for 2h to get a thin ZnO nanoparticles layer supported on the tube. Secondly, the ZnO nanorods were grown on the seeded α-Al2O3 tube as follows: 1.1 g hexamethylenetetramine and 2.3 g Zn(NO3)2·6H2O were dissolved in 80 mL water and stirred for 0.5h to achieve a synthesis solution. Then the seeded tube was vertically placed in the synthetic solution for crystallization at 100 ℃ for 6 h. Finally, the tube was washed by deionized water and dried at 80 ℃ for 24 h. Preparation of ZIF-67 Membrane. The ZIF-67 membrane was grown heteroepitaxially on the ZnO nanorods layer under solvothermal conditions. The synthesis solution was prepared according to the following procedure as reported elsewhere, with minor modifications.10 Briefly,


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0.22 g of Co(NO3)2·6H2O and 4.54 g of Hmim were dissolved in a mixture of 5 ml of methanol and 35 ml of deionized water, respectively. Two solutions were blended for 2 min and poured into a 100 ml autoclave containing a ZnO nanorods modified tube. The autoclave was kept in an oven at 120 ℃ for 24 h. Afterwards, the ZIF-67 membrane with purple was rinsed with fresh methanol. Then the as-synthesized ZIF-67 membrane was placed in 30 mL methanol and kept for 2 days for solvent exchange, the sample was replenished with fresh methanol every 12h. Finally, the membrane was dried in a vacuum oven at 60 ℃ for 12 h before characterizations and testing. Preparation of ZIF-9 membrane. Firstly, the ZnO nanorods grown tube was immersed in a solution with 0.5 M Bim and DMF at 60 ℃ for 1 h for activation. The purpose is the implantation of the ligand (Bim) to the ZnO nanorods for favoring the nucleation and growth of ZIF-9 membrane. Subsequently, the ZIF-9 membrane was grown heteroepitaxially on the activated ZnO nanorods layer under solvothermal conditions. In a typical synthesis solution,21 Co(NO3)2·6H2O (0.3g) was dissolved in 60 mL of DMF and a solid of Bim (0.52 g) was added after constant stirring. The activated ZnO nanorods modified tube was placed vertically into the mother solution and kept at 120 ℃ for 24h. After the solvothermal synthesis, the ZIF-9 membrane with dark purple was taken out and washed with fresh methanol. Then the solvent exchange process for the as-prepared ZIF-9 membrane was the same as the ZIF-67 membrane mentioned above. Characterization. Powder X-ray diffraction (PXRD) patterns were recorded in the 2θ range of 3-60 º using a D/max-2400 X-ray diffractometer with Cu Kα radiation. The membrane morphology was checked by scanning electron microscopy (FEI Nova NanoSEM 450 and


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Quanta 450) operating at 3 and 20 keV, respectively. Energy dispersive X-ray spectrometer (EDX) and line scanning analyses were operated with 20 keV of acceleration and 10 mm of working distance. Infrared spectra were collected using a Bruker EQUINOX55 FT-IR spectrometer. Evaluation of Membrane Separating Performance. Gas permeation of the prepared membranes was carried out with different kinetic diameters of gases H2 (0.29 nm), CO2 (0.33 nm), N2 (0.36 nm) and CH4 (0.38 nm) using a home-made gas permeation apparatus.32-34 The membrane was sealed in a permeation module with silicone O-rings. For the permeation test for single gases, the feed stream was pressurized and controlled with the exactitude manometer, while keeping the permeate pressure at 1 bar. The gas flux was measured by a soap bubble flow meter. The data was recorded until a constant permeance lasted at least 2 h. The ideal selectivity was the value of the ratio for different gas permeances. The binary gas mixture permeation measurements were performed with the Wicke-Kallenbach technique. A 50:50 mixture of gas was applied to the feed side, and pressure at both feed side and permeate side were 1 bar. An online gas chromatograph (GC7890T) was used to measure the gas composition. The separation factor of H2 over other gases α H2/j was defined as the molar fractions of the components (H2, j) in the permeate side, divided by the molar fractions of the components (H2, j) in the feed side, as shown in the following equation:

α H /j= 2

y x

H2,Perm H2,Feed

y x

j,Perm

j,Feed

3. RESULTS AND DISCUSSION 3.1 Formation and Characterization of Co-based ZIFs Membranes on ZnO Nanorods.


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As reported in our previous work,32 ZnO nanorods acting as seeds and linkers could successfully induce forming a high-performance Zn-ZIF (ZIF-8) membrane because the metal oxide, which is the same metal source as the MOF to be grown, greatly favors the formation of its corresponding MOF membrane. Thus, it is important to pre-introduce a thin layer of ZnO nanorods on the surface of the porous ceramic tube. In this work, based on the heteroepitaxial growth of Co-ZIF67 membrane on ZnO nanorods, a layer of ZnO nanorods was first grown on the substrate to act as heteroseeds and anchoring sites for subsequent growth of Co-ZIF-67 membrane. Figure 2a and b showed the ZnO nanorods were aligned vertically with hexagonal cross-section of 200-250 nm in diameter and 2-2.5 µm in length. It can be observed that the ZnO nanorods uniformly covered the entire surface of the tube to serve as an intermediate layer for the formation of ZIF67 membrane. Meanwhile, the ZnO nanorods layer can also modify some large defects of the substrate and favor growing a dense membrane. From the cross-sectional image, it can be seen obviously that the ZnO nanorods grew seamlessly on the substrate, revealing a strong adhesion with the porous tube. Then, the ZnO nanorods modified tube was immersed into the ZIF-67 precursor solution for crystallization and membrane growth. Initially, after the short reaction for 1h, as shown in Figure 2c, a layer of unknown phase with flocculent species was uniformly formed over the ZnO nanorods. The flocculent species might be the metastable phase produced at the initial stage, as the degree of crystallization was very low and no peak was detected in XRD. The cross-section indicates that the ZnO nanorods partly dissolved and intergrew together (Figure 2d). The flocculent species were subsequently incubated into bulk crystals after 2h (Figure S1). When the reaction time was prolonged to 24h, a well-intergrown and defect-free polycrystalline ZIF-67 membrane consisting of well-formed prismatic crystals with a thickness of around 3µm was produced by heteroepitaxially growth on the ZnO nanorods (Figure 2e and


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f). It was found that the ZIF-67 polycrystalline crystals grew with the interseptal space between the ZnO nanorods, and even packed the ZnO nanorods, forming a uniform and continuous ZIF67 layer. The XRD analysis detected the ZIF-67 layer grown on the ZnO nanorods. The peaks of both ZIF-67 structure phase and ZnO structure phase existed in the XRD pattern, indicating the ZnO nanorods has not completely dissolved during the heteroepitaxial growth (Figure 2g). Simultaneously, the EDX line scan analysis was conducted along the cross-sectional of the ZIF67 membrane marked with a red solid line (Figure 2h). As expected, a cobalt-rich region of ca. 2 µm in thickness was clearly seen from the top membrane to the ZnO region, demonstrating the uniform membrane was, indeed, Co-ZIF-67 rather than Zn-ZIF-8 structure. The zinc-rich region with ca. 3µm in depth was located under the Co-ZIF-67 layer, demonstrating that the ZnO nanorods only slightly dissolved and most of them are still embedded in the support at one end and connected the continuous ZIF-67 layer at another end, thus acting as excellent linkers. The EDX result revealed that the ZnO nanorods partly penetrated into the ZIF-67 layer, thus the ZnO nanostructure could allow the expansion of the membrane, preventing the cracks possibly produced during heating/cooling processes. Thus, this approach makes it feasible to achieve a reproducible and stable ZIF-67 membrane.


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Figure 2. SEM images of ZnO nanorods grown on the α-Al2O3 tube (a, b); ZIF-67 membrane on ZnO nanorods modified tube at 120 ℃ for 1h (c, d) and 24h (e, f); XRD patterns of the samples (g); EDX elemental profiles of the cross-section of the ZIF-67 membrane grown for 24 h (h).

In this work, it was found that both the induced activity of ZnO nanorods for the heteroepitaxial growth of Co-ZIF-67 membrane and strong attachment of ZnO nanorods layer on ceramic tube were extremely critical for achieving a well-intergrown ZIF-67 membrane. Without ZnO nanorods layer, it is impossible to form high-quality ZIF-67 membranes under the current synthesis conditions, only a few flake–like particles attached on the subatrate (Figure S2a). However, the powders precipitated simultaneously in the solution were determined to be pure ZIF-67 phase with polyhedral shape of about 1 µm (Figure S3), which was in good agreement


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with the regular morphology of ZIF-67 crystals (Figure S2b). This may be attributed to the fact that for the formation of ZIF-67, homogeneous nucleation and growth in the solution is much faster than heterogeneous one on the substrate,10 resulting in only producing ZIF-67 powders in solution rather than ZIF-67 layer on a naked alumina substrate. Once Co2+ solution was added and mixed with Hmim solution, ZIF-67 crystals were immediately formed as shown in Figure S4. Therefore, it is difficult to achieve continuous and dense ZIF-67 membranes on porous tubular substrates without the aid of a layer of ZnO nanorods. In order to check the universality and superiority of our proposed heteroepitaxial synthesis strategy in fabricating Co-MOF membranes, we also choose ZIF-9 as candidate to prepare ZIF-9 membrane by using this heteroepitaxial growth. Similarly, without ZnO nanorods layer, the surface of the substrate was covered with a layer of loose ZIF-9 crystals which are poorintergrown as shown in Figure 3a. The ZIF-9 layer of about 10 µm in thickness has visible gaps between the layer and the substrate, showing the weak interaction between the membrane and the substrate. In comparison, the membrane crystallized better and more continuous on a layer of ZnO nanorods modified alumina support as shown in Figure 3c. The ZIF-9 crystals were uniform and intergrown well, demonstrating that ZnO nanorods could be used as efficient nucleation sites for the heteroepitxial growth of ZIF-9 membrane. It is worth noting that different from the preparation of the above ZIF-67 membrane, here, the activation of ZnO nanorods in a DMF solution containing Bim ligands play a key role in forming a dense ZIF-9 membrane as shown in Figure 4. After the activation of the ZnO nanorods with the Bim solution, there were many spherical particles (~ 500 nm) produced (Figure 4a and b), standing on top of each of the ZnO nanorods. As we reported previously,32 these small particles were similar to “ZIF-like” nucleis and can act as the heteroseeds for ZIF-9 growth, leading to the formation of a well-


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intergrown and defect-free ZIF-9 membrane with 12 µm as shown in Figure 4c and d. The composition change with reaction time was shown by EDX scan line analysis (Figure 4e, Figure S5). It showed that the molar ratio of Co to Zn increased with the time and then reached an equillibrium after 24 h, demonstrating the growth of ZIF-9 membrane had ceased due to the absence of enough nutrient. Figure 4f showed an EDX line along the cross-section of ZIF-9 membrane marked with a red solid line. Indeed, a cobalt-rich region of ca.12 µm was clearly seen from the top membrane to the ZnO region. This cobalt-rich thichness was right similar to that of the well-intergrown ZIF-9 membrane, demonstrating that the uniform membrane was CoZIF-9 rather than Zn-ZIF-7 structure phase.

Figure 3. SEM images of the ZIF-9 membrane synthesized on the naked α-Al2O3 tube using in situ growth (a, b); ZIF-9 membrane synthesized on the ZnO nanorods modified α-Al2O3 tube (c, d).


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Figure 4. SEM images of ZnO nanorods after activation by the Bim solution (a,b); ZIF-9 membrane grown on the ZnO nanorods activated with Bim (c,d); Change in Zn/Co molar ratio of the ZIF-9 membrane during the reaction for 48h based on EDX results (e); EDX elemental profiles of the cross-section of the ZIF-9 membrane grown on ZnO nanorods modified tube (f).

The structure of the achieved ZIF-9 membrane was further characterized by PXRD as shown in Figure 5. Interestingly, we found that the orientation of ZIF-9 membrane on the ZnO nanorods modified substrate was obviously different from those of random ZIF-9 powder crystals simulated and the ZIF-9 membrane grown on the bare substrate. From the XRD pattern of the membrane on the ZnO nanorods as shown in Figure 5a and b, the three main peaks at 7.1, 7.7, and 16.3°, corresponding to (01-1), (-120), and (122) crystal planes, respectively, matched well with simulated ones from ZIF-9 single-crystal data, indicating the membrane is ZIF-9 phase. However, the peak of (-120) planes for the membrane decreased to almost invisible, while the (122) peak increased obviously, compared with those of simulated ZIF-9 crystals and the


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membrane grown on the bare substrate as shown in Figure 5b and d, suggesting that the ZIF-9 membrane preferentially grew along (122) plane on the ZnO modified substrate. The propagation of (122) plane through the ZIF-9 structure was shown in Figure S6. For further analyze the orientation of ZIF-9 membrane, orientation indices (OIs) were calculated for the three main diffractions (Table S1). It is believed that if the OI value for any crystal plane surpasses 1, which was deemed preferentially oriented parallel to the support.44 In this work, the ZIF-9 membrane grown on the ZnO nanorods modified substrate had an OI (122) value of 2.65 which are much larger than 1, indicaing the crystals are oriented with its (122) planes parallel to the substrate. The similar orientation growth was also observed in ZIF-67 membranes grown on ZnO nanorods modofied substrates (Figure S7). Similarly, the peak intensity of the (011) planes decreased obviously, whereas the (112) peak increased when the ZIF-67 crystals grew on the ZnO nanorods modified substrates, indicating that the ZIF-67 membrane preferentially grows along the (112) plane on the ZnO nanorods modified substrate. The orientation change may be attributed to the preferentially c-oriented ZnO nanorods which have the highest relative intensity for the (002) diffraction peak (Figure S8). Therefore, addition of highly c-oriented ZnO nanorods layer in preparing MOFs membranes could alter the crystal orientation from the random orientation to a preferred one. The preferentially oriented growth manner favors the intergrowth of the MOF crystals and thus reduces the defects produced during the membrane formation. Moreover, the directional growth of MOF crystals is significant for their corresponding membrane for gas permeation performance.38 Therefore, the c-oriented ZnO nanorods assisted strategy might be meanningful for fabricating the MOFs with 1D channels to achieve a membrane with highly efficient molecule permeation property.


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Figure 5. XRD patterns of ZIF-9 membrane on ZnO nanorods modified ceramic tube (a,b); ZIF9 membrane on naked tube (c,d); Magnified XRD patterns for ZIF-9 membranes with 2θ angle between 6° and 18° (b,d).

3.2. Mechanism of the Heteroepitaxial Growth of Co-based ZIFs Membranes. According to the principle that the metal oxide which has the same metal source as the MOF to be grown favors the growth of its conressponding membrane, ZnO in the form of nanorods or ultrathin layers introduced onto the surface of a porous substrate can successfully induce the formation of a continuous Zn-based ZIF membranes.32-34 However, up to date, there is no report on ZnO induced heteroepitaxial growth of Co-based ZIF-membranes. Here, we for the first time achieved continuous Co-ZIF-67 and Co-ZIF-9 membranes, respectively, by using this ZnO nanorods induced heteroepitaxial growth strategy. In order to further understand this heteroepitaxial strategy, a membrane growth mechanism was proposed and discussed as shown schematically in Figure 6. We hypothesized that during the membrane growth, the ZnO nanorods first react with Co2+ in the synthesis solution to form (Zn, Co) hydroxy nitrate salts (HDSs) which then drives the Co-MOF formation. This was also confirmed by Paesons et al.’s work in which (Zn,Cu) HDS formed in situ from ZnO particles enabled rapid growth of HKUST-


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1.45,46 The HDSs are layered compounds consisting of cationic sheets connected by inorganic/organic interlamellar anions,47 and easily formed by reaction of one divalent metal oxide with another different divalent cation. These materials possess high reaction activity and excellent anion exchangeability.48 Our experimental results indicated that a layer of (Zn,Co) HDS was easily produced after the ZnO nanorods grown on the glass support were treated with the ZIF-67 precursor solution (denoted as ZIF-67-M) without containing any ligand (Hmim) at 120 ℃ for 24h as shown in Figure 7a. The nanosheets consisted of regular nanoflakes, typically 3-4 µm in length and 50-80 nm in thickness, were cross-linked and grew vertically on the glass substrate (Figure 7a, Figure S9a). This morphology was similar to that observed at the initial stage of growing ZIF-67 membrane on ZnO nanorods (Figure 2c). Elemental mapping showed that Zn and Co elements were homogeneously distributed throught the whole nanosheet layer (Figure 7c, Figure S9c). When the ZnO nanorods were treated with ZIF-9 precursor solution (denoted as ZIF-9-M) without containing any ligand (Bim) at 120 ℃ for 24 h as shown in Figure 7b, there were no nanoflakes and only some substances formed and unevenly attached on the ZnO nanorods, in which both of Zn and Co were also detected on the ZIF-9-M sample by the EDX analysis (Figure 7d, Figure S9d). The morphology discrepancy between ZIF-67-M and ZIF-9-M might be due to the reason that ZnO nanorods have different solubility in the two kinds of different precursor solutions, which was in accordance with previous results.49


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Figure 6. Schematic illustration of proposed mechanism for the heteroepitaxial growth of the Co-ZIFs (ZIF-67, ZIF-9) membrane on ZnO nanorods layer.

Figure 7. SEM images of ZnO nanorods grown on glass substrates treated with ZIF-67 (a) and ZIF-9 (b) precursor solutions without containing ligands (Hmim or Bim), respectively; EDX mapping images of the corresponding red square marked in parts a and b (c,d).

The XRD patterns of the two samples derived ZIF-67-M and ZIF-9-M matched well with the (Zn, Co) hydroxy nitrate as shown in Figure 8a and b. Furthermore, Figure 8c and d showed the FTIR spectra collected from ZnO nanorods and after sequential exposure to Co(NO3)2 and ligand solutions. After the ZnO nanorods layer was treated with ZIF-67-M solution and ZIF-9-M


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solution, respectively, the negative-going modes confirmed the appearance of NO3- (1360 and 1420 cm-1) and distinct O-H group modes(~3300-3620 cm-1), indicating the formation of hydroxy nitrate.50,51 But the sample treated with ZIF-9 precursor solution gave weak absorption peaks of O-H, which was consistent with the SEM results (Figure 7b). After the ZIF-67-M sample was subsequently exposed to the Hmim solution, it was seen from Figure 8c that the peaks for NO3- and O-H diminished, while both the C-H stretching vibration from imidazole ring (~3136 cm-1) and C-H stretching vibration of the methyl (~2927 cm-1) from the Hmim ligand appeared. In addition, the Co-N vibration (~425 cm-1) appeared, revealing the incorporation of Co into the structure of ZIF-67. For the sample of ZIF-9-M, after the exposure to Bim, the C=C stretching vibration of the benzene ring (~1456 cm-1), C-H bending vibration of orthodisubstituted benzene (~745 cm-1) which is contributed from Bim ligand and Co-N vibration (~425 cm-1) appeared (Figure 8d), demonstrating the formation of ZIF-9 after the exposure of the ZIF-9-M to Bim solution.52 These changes clearly reveal that the anion exchange process easily occurs in the (Zn,Co) HDS, and thus favors the formation of Co-based ZIFs, which futher supports our proposed reaction process, that is, ZnO nanorods first react with Co2+ in the synthesis solution to form (Zn, Co) hydroxy nitrate salt (HDS), followed by anion exchange between NO3- and OH- in the HDSs and ligands. Then the exchanged HDSs drive the growth of Co-based ZIFs, thus leading to formation of continuous and dense Co-based ZIF membranes.


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Figure 8. (a) XRD patterns of ZnO nanorods grown on glass substrates treated with ZIF-67 and ZIF-9 precursor solutions without containing ligands (Hmim or Bim), respectively; (b) XRD patterns of (Zn,Co) HDS (red) derived from ZnO powder (black); (c) FTIR spectra for ZnO nanorods (black), after sequential exposure to Co(NO3)3 (red) and Hmim (blue) solution, and the ZIF-67 membrane (purple); (d) FTIR spectra for ZnO nanorods (black), after sequential exposure to Co(NO3)3 (red) and Bim ( dark blue) solution, and the ZIF-9 membrane (violet).

From the above discussion, it can be confirmed that a (Zn, Co) HDS intermediate formed in situ from ZnO layer, indeed, enables the heteroepitaxial growth of Co-based MOF membranes because this intermediate can not only act as the heterogeneous nucleation sites by introducing Co2+ to the substrate, but also has excellent anion exchange ability, which greatly favors the formation of uniform Co-based MOF membranes. In order to further demonstrate the universality of ZnO layer assisted heteroepitaxial growth of MOF membranes, we synthesized


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other three types of MOF membranes including Cu-based CuBDC53 and HKUST-131 membranes, and Ni-based USO-2-Ni54,55 on the ZnO nanorods modified substrates. (Figure S10S12). It is clearly seen that all the three kinds MOF membranes derived from ZnO nanorods modified substrates were more continuous and denser than those obtained by in situ method. Therefore, it is no doubt that the ZnO layer enables the assisted fabriacation of the non-Zn based MOF membranes due to the formation of (Zn, M2+) HDSs, like (Zn,Co) HDS, (Zn,Cu) HDS and (Zn, Ni) HDS, which could provide heterogeneous nucleation sites and anchoring bars. As demonstrated above, ZnO nanorods are an excellent modification layer for a porous tubular substrate and enable the heteroepitaxial growth of MOF membranes because of their high affinity with substrate and superior reactivity for crystallization of continuous MOF membranes. It is believed that the ZnO assisted growth strategy developed here could be employed to prepare various MOFs and their membranes since the conversion is well manipulated. 3.3. Gas Permeation Performance of the Co-ZIFs Membranes. The gas permeation properties of the as-prepared Co-ZIFs (ZIF-67 and ZIF-9) tubular membranes were evaluated by using home-made gas permeation setup. The single gas permeantion properties of ZIF-67 and ZIF-9 membranes, respectively, at 25 ℃ and 0.1 MPa as a function of the kinetic diameters of molecule are shown in Figure 9a and Figure S13. Both H2 permeances of the two kinds membrane are much higher than the other gases, and there is a obvious cut-off between H2 and other larger gases. In the case of ZIF-67 membrane, the H2 permeance is 22.75×10-8 mol m-2 s-1 Pa-1 and the ideal selectivity of H2/CO2, H2/N2 and H2/CH4 are 8.6, 21.8 and 45.4, respectively, which are superior to Knudsen diffusion coefficients (4.7, 3.7 and 2.8 for H2/CO2, H2/N2 and H2/CH4, respectively). For ZIF-9 membrane, the H2


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permeance is 18.48×10-8 mol m-2 s-1 Pa-1, while the ideal selectivity of H2/CO2 is as high as 23.8, indicating the ZIF-9 membrane has an efficient separation performance for H2/CO2. The high reproducibility of membrane preparation and performance is an effective method to reduce the cost of membrane preparation in industrial production. Table S2 and S3 list the single gas permeances of the membranes obtained in separate batches. Among them, 9 of 10 membranes had a comparable H2 permeances with a mean value of 22×10-8 mol m-2 s-1 Pa-1 for ZIF-67 and 18×10-8 mol m-2 s-1 Pa-1 for ZIF-9 membranes, respectively. Moreover, the ideal selectivities of the prepared membranes are also similar, showing the good reproducibility of the ZnO nanorods induced heteroepitaxial growth of Co-ZIF membranes. The separation performance of some binary mixtures for the prepared membranes were also measured as shown in Table S4-S5. The gas permeances and mixture selectivities have a slight decrease compared to those of corresponding single gases due to the competitive adsorption of gases. These results above show that the Co-based ZIF membranes prepared by ZnO induced heteroepitaxial growth are dense without any pinholes and cracks. The stability of the membrane is another crucial for its successful commercial application. Figure 9b and c show the long-time separation performances of the Co-based membranes for binary mixtures of H2/CH4 and H2/CO2 with the temperature change from 30 to 150 ℃, respectively. It is clearly indicated that the ZIF-67 membrane gives a slight decrease for H2 permeance from 15.65×10-8 to 12.71×10-8 mol m-2·s-1·Pa-1 and a little increase for H2/CH4 separation selectivity from 41.8 to 48.2. This trend can be explained by different behaviors of H2 and CH4 in the relative values of adsorption and diffusion towards the ZIF-67 membrane with increasing the temperature.56,57 Similar results are also obtained in the ZIF-9 membrane for binary mixtures of H2/CO2 with the temperature change, and the Co-based ZIF membranes achieved in this work could exhibit exceeding stability during the long-term


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operation, even after a series of temperature changes. This excellent thermal stability of the membranes formed by ZnO induced heteroepitaxial growth results from the special microstucture of the membrane, that is, the ZnO nanorod arrays could provide some freedom for the different expansion coefficients between the MOF membrane and substrate during heating/cooling processes, avoiding the defects from propagating cracks. To demonsrate the comprehensive separation performance of ZnO induced Co-based ZIF membranes (ZIF-67 and ZIF-9) in this work, the plots of H2/CO2 selectivity versus H2 performance are summarized in Figure 9d. It is seen that the H2/CO2 separation results of both our achieved Co-based ZIF membranes exceeds the trade-off line of polymeric membranes. Our Co-ZIFs membranes formed by ZnO nanorods induced growth possess a good balance between selectivity and permeance, showing that the membranes are high-quality and have the advantages for H2 purification. Moreover, the membranes prepared by our strategy own excellent thermal stability.


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Figure 9. (a) Single gas transport behaviours through the ZIF-67 membrane at 25 ℃ and 0.1MPa as a function of the kinetic diameter. The inset shows the ideal selectivity of H2 over CO2, N2 and CH4. (b) H2/CH4 binary gases selectivity of the ZIF-67 membrane versus test time at different temperature. (c) H2/CO2 binary gases selectivity of the ZIF-9 membrane versus test time at different temperature. (d) Comparison of the H2/CO2 separation performances of the prepared ZIF-9 (red) and ZIF-67 (blue) membranes with the 2008 Robeson upper bound of polymeric membranes (blue line) and other membranes reported in the previous studies (black), such as MOF nanosheets,8 CuBTC/MIL-100,58 ZIF-8/rGO,59 zeolite,60 ZIF-7,61 NiAl-CO3LDH,62 ZIF8/GO,63 ZnO/ZIF-8,32

ZIF-22,26 ZIF-9@P84,11

ZIF-9@IL@CNTs,21

Co(OH)2/ZIF-67,20

CuBTC,31 and NH2-MIL-53.64

4. CONCLUSIONS In summary, we successfully prepared well-intergrown Co-based ZIFs (ZIF-67 and ZIF-9) membranes by the heteroepitaxial growth on ZnO nanorods modified ceramic tubes, displaying


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high H2 separation performance and excellent stability. The strongly attachment of ZnO nanorods layer on substrate was found to be essential to heteroepitaxially grown ZIF-67 and ZIF-9 membranes, as the (Zn,Co) hydroxy double salt (HDS) intermediate formed in situ from ZnO layer acted as heteroseeds and anchors enabling the heteroepitaxial growth of Co-based membranes. In addition, a similar synthetic strategy had been applied to the preparation of CuBDC, CuBTC and USO-2-Ni membranes, demonstrating the excellent universality of this synthesis strategy. The resulting ZIF-9 membrane exhibited a high ideal selectivity of 23.8 for H2/CO2. The ZIF-67 membrane also showed excellent hydrogen separation performance with H2 permeance of 22.75×10-8 mol m-2s-1Pa-1 and H2/CH4 ideal selectivity up to 45.4. Moreover, both of the membranes demonstrated excellent stability bacause the ZnO nanorods acted as linkers between the membrane and substrate, which can prevent defects from propagating cracks. Therefore, the proposed heteroepitaxial growth Co-based MOF membranes by ZnO nanorods induced strategy is simple, efficient, and also can be employed to prepare various MOF membranes, as long as the metal source of the MOF membrane could react with ZnO to form corresponding (Zn, M2+) hydroxy nitrate salt HDS. ASSOCIATED CONTENT Supporting Information Electronic Supplementary Information (ESI) avaiable: Additional XRD, SEM, and EDS analyses on the ZIF powders and ZIFs membrane; and gas separation performance of the ZIF membrane reported in the literature (PDF). AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected] (X. Zhang) ; Tel./Fax: +86-411-84986155. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (Nos. 21476039, 21076030) is gratefully acknowledged. REFERENCES (1)

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