New Membrane Architecture with High Performance: ZIF-8 Membrane

Article pubs.acs.org/cm

New Membrane Architecture with High Performance: ZIF‑8 Membrane Supported on Vertically Aligned ZnO Nanorods for Gas Permeation and Separation Xiongfu Zhang,*,† Yaguang Liu,† Shaohui Li,† Lingyin Kong,† Haiou Liu,† Yanshuo Li,‡ Wei Han,§ King Lun Yeung,§ Weidong Zhu,∥ Weishen Yang,‡ and Jieshan Qiu*,† †

State Key Laboratory of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ‡ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 116023 Dalian, China § Department of Chemical and Biomolecular Engineering & Division of Environment, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ∥ Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China S Supporting Information *

ABSTRACT: A new ZIF-8 membrane architecture with high performance supported on vertically aligned ZnO nanorods was successfully prepared. The vertically aligned, single crystal ZnO nanorods were grown seamlessly from porous ceramic support to form an intermediate support layer for the ZIF-8 membrane. They provide multiple anchorages for the ZIF-8 membrane that are both strong and flexible. The nanorods were activated to induce a uniform nucleation of ZIF nuclei on their surface to initiate and guide the growth of a defect-free ZIF-8 membrane. Single gas permeations and binary separations carried out to investigate the transport properties of these new membrane architectures confirmed that the ZIF-8 membranes were free of defects and stable at a higher temperature (473 K).

1. INTRODUCTION

and application. The difference in the thermal expansion coefficients between the MOF membrane and the ceramic or metal substrate can result in crack formation during the heat treatment, which is considered as another major cause of membrane failure. This work explores a new membrane architecture wherein the ZIF-8 membrane is supportedaway from the substrate on vertically aligned ZnO nanorods. ZnO has a very malleable morphology and has been synthesized in various shapes including rods, rings, belts, and tetrapods.43−46 Single crystals often have a greater strength than amorphous and polycrystalline materials and could provide an extra strength to the composite membrane. Grown seamlessly from the porous ceramic substrate, the ZnO nanorods form a well interlinked, strut-like support layer that could allow greater vertical and lateral expansion of the membrane, lowering the risk of crack formation during heating/cooling operations. Besides serving as an intermediary support structure, the single-crystal ZnO nanorods act as an inorganic linker between the porous

The diverse pore structure and chemistry of the metal−organic frameworks (MOFs) make them attractive membrane materials for tackling not only the classical separation problems such as the separation and purification of hydrogen from mixtures, the removal of CO2, and the separations of alkanes from alkenes, linear from branched hydrocarbons, and of aromatic compounds.1−14 They also promise to play an important role in the new areas of large isomers and chiral and biomolecule separations.15,16 The reproducible preparation of MOF membranes that are free of defects and stable under harsh operation conditions remain an important challenge due to the material’s propensity to grow into undesirably large crystals and thick films.17 There are numerous methods for preparing MOF membranes including the in situ synthesis without17−21 or with substrate modification,22−24 seeded growth,12−15,25−31 and the reactive seeding method 32,33 along with other techniques.11,34−40 The MOF membranes prepared by the seeded methods often have a better quality.13,41,42 However, the MOF nanocrystals used as seeds are difficult to prepare, and seeding is often problematic. Seed attachment to the substrate relies mostly on surface adhesion via polymers or organic linkers, and seed and/or membrane detachments can occur during synthesis © 2014 American Chemical Society

Received: January 23, 2014 Revised: February 17, 2014 Published: February 18, 2014 1975

dx.doi.org/10.1021/cm500269e | Chem. Mater. 2014, 26, 1975−1981

Chemistry of Materials

Article

source at an average pulsed current of 0.1 pA was used to sample three randomly selected 200 × 200 μm2 areas to obtain an average spectrum. The individual spectrum was collected for 40 s at an ion flux dosage of ca. 2 × 1011 ions/cm2. Synthesis of ZIF-8 Membranes. The ZIF-8 membranes were grown from a synthesis mixture with a molar ratio of 0.65 HCOONa:1.0 ZnCl2:1.5 Hmim:450 MeOH at 373 K for 5 h. The typical synthesis solution was prepared by dissolving in sequence 1.08 g of zinc chloride, 0.97 g of 2-methylimidazole, and 0.54 g of sodium formate in 80 mL of methanol under constant stirring. The liquid volume was kept at 80 mL for a membrane deposition area of 5.2 cm2. After the solvothermal synthesis, the membrane was removed, cooled to room temperature, and thoroughly washed with methanol to remove any unreacted residues. The membranes were dried in an oven at 303 K for 24 h and stored in a desiccator for later use. The ends of the membrane tubes were sealed with an epoxy resin, pretreated in a vacuum at 323 K for 12 h before gas permeation and separation studies. 2.3. Characterization. The membranes were examined under electron microscopes and analyzed by X-ray diffraction (XRD). Membranes were sectioned into regular 4 mm2 pieces using a low speed diamond saw. The samples were carefully rinsed with ethanol to remove debris from cutting. The sample specimens were examined by the scanning electron microscopy (SEM, NOVA NANOSEM 450 and Quanta 200) with a field emission gun operating at 5−15 kV. XRD measurements were made on a D/max-2400 X-ray diffractometer using Cu Kα radiation in a range of 3−100°. 2.4. Gas Permeation. Gas permeations were measured for single gases including H2 (0.29 nm), CO2 (0.33 nm), N2 (0.36 nm), and CH4 (0.38 nm), while the separation of binary gas mixtures was investigated for H2/CO2, H2/N2, and H2/CH4. The experimental setup shown in Figure S1 consists of a stainless steel module, gas delivery, and analysis systems. Electronic mass flow controllers were used to meter out a precise flow of purified gases. Membrane pressure was regulated using a back-pressure regulator and pressure controller system. Gases from the retentate and permeate outlets were measured by a bubble flow meter and analyzed by a gas chromatograph. The single gas permeation was conducted by pressurizing the feed stream, while keeping the permeate pressure at 1 atm. The gas permeations at transmembrane pressures of 0.06 to 0.12 MPa were measured. The membrane was purged to remove entrained gases prior to each experimental run. The permeance, Pi, as defined by eq 1:

ceramic substrate and the thin polycrystalline MOF membrane. Using a new activation method47 that promotes uniform nucleation, the ZnO nanorods penetrate seamlessly into ZIF-8, thus forming a mechanically strong crystalline bond. This new method was used to prepare supported ZIF-8 tubular membranes that were characterized in detail. Practical issues including membrane reproducibility, permeation, and separation properties are also reported.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Porous α-alumina ceramic tubes (4 mm o.d., 3 mm i.d., and 0.1 μm pore size) purchased from Hyflux Ltd. Co. were cut into 55 mm lengths and washed repeatedly in distilled water and ethanol under sonication to remove dirt and impurities before drying in an oven at 373 K for 5 h. The ceramic tubes were then calcined in a furnace at 823 K for 6 h. The chemicals and reagents used for the membrane preparation include zinc chloride (≥98%), zinc acetate (98.0%), sodium formate dihydrate (≥99.5%), ethylene glycol monomethyl ether (C3H8O2, EGME, 99.0%), and monoethanolamine (C2H7NO, MEA, 99.0%) for depositing the ZnO nanorod layer and zinc nitrate hexahydrate (≥99.0%), hexamethylenetetramine (≥99.0%), and methanol anhydrous (≥99.5%) for the growth of the ZIF-8 membrane. Except for 2-methylimidazole (Hmim, 99%) purchased from Sigma-Aldrich Chemical. Co. Ltd., the rest of the chemicals were supplied by Sinopharm Chemical Reagent Co. Ltd. All chemicals were used without further purifications. 2.2. Preparation of ZIF-8 Membranes. The ZIF-8 membrane preparation involves (1) the deposition and growth of aligned ZnO nanorods on the porous ceramic support tube, (2) the activation of ZnO nanorods for ZIF-8 deposition, and (3) the synthesis of the ZIF8 membrane as illustrated in Scheme 1.

Scheme 1. Preparation Scheme for ZnO Nanorods Supported ZIF-8 Membrane

Growth of Aligned ZnO Nanorods. Vertically aligned ZnO nanorods were grown on the inner wall of the porous ceramic tube by a hydrothermal process. A thin layer of ZnO seeds was slip-casted on the surface. The seeds were prepared by dissolving 8.3 g of Zn(Ac)2 in 48 mL of EGME under constant stirring at 343 K. A total of 2.4 mL of MEA was added into the solution dropwise and the mixture was aged at room temperature for 10 h to obtain a stable sol containing 14 wt % ZnO seeds. The seeded tube was dried at 373 K for 1 h. It was necessary to repeat the seeding procedure twice before calcining in air at 673 K for 2 h at a heating rate of 1 K·min−1 to obtain a thin layer of ZnO seeds. The ZnO nanorods were grown under hydrothermal conditions from a synthesis mixture containing 4 mmol of hexamethylenetetramine dissolved in 40 mL of deionized distilled water with 2 mmol of Zn(NO3)2·6H2O added after aging to obtain a clear solution. The hydrothermal synthesis was carried out in a 100 mL Teflon-lined autoclave at 373 K for 6 h, and the prepared tubes were recovered after synthesis and washed with an excess amount of deionized, distilled water to remove the unreacted reactants. Activation of ZnO Nanorods. The prepared tubes were carefully wrapped with Teflon tape to prevent ZIF-8 deposition on the outer tube surface. The tubes were then immersed in an activation solution containing 0.5 M Hmim at 323 K for 2.5 h. The purpose is to create nucleation sites on the surface of the ZnO nanorods for the deposition and growth of ZIF-8 MOF. The activation process was monitored, and TOF-SIMS measurements were made to identify the nature of the nuclei formed during the activation process. An ION-TOF GmbH TOF-SIMS V spectrometer equipped with 25 keV Bi3+ cluster ion

Pi =

Ni ΔPA i

(1)

where Ni is the permeate rate of component i (mol·s−1), ΔPi is the trans-membrane pressure difference of i component (Pa), and A is the membrane area (m2). The ideal separation factor is calculated as the ratio of permeance Pi and Pj in eq 2: αi / j =

Pi Pj

(2)

Binary gas separation was carried out for equimolar gas mixtures. Nitrogen was used as the sweep gas. The volumetric flow rates of the feed and permeate streams were kept constant at 100 and 50 sccm, respectively. The feed and permeate streams were also kept at the same pressure of 101 kPa. The gas composition was analyzed by an online gas chromatograph (GC7890T), and the separation factor αij was calculated from eq 3:

αij =

yi /yi xj/xj

(3)

where xi and xj are the molar fractions of components i and j in the retentate stream which is similar to the feed component, while, yi and yj are the molar ratio of components i and j in the permeate side. 1976



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3. RESULTS AND DISCUSSION 3.1. Growth of ZnO Nanorods. The aligned ZnO nanorods were grown on the surface of the porous ceramic tube as shown in Figure 1. The ZnO nanorods were aligned

Figure 2. SEM images of the sample grown on the porous ceramic tube without activation: (a) low magnification, (b) high magnification.

Figure 3a shows that the activation process did not alter the macro- and microscale structures of the deposited ZnO nanorod layer. Closer examination of the nanorod surface at high magnification (Figure 3b) reveals the formation of surface protuberances (25 ± 5 nm) along the entire length of the rods that were not seen on the original, untreated ZnO nanorods (Figure 1b). These protuberances are believed to be nuclei formed by the reaction of Hmim and the ZnO surface. X-ray diffraction of the sample, however, displayed only diffraction lines belonging to the porous ceramic tube (α-Al2O3) and ZnO from the nanorod layer (Figure 3e). No ZIF-8 phase could be detected by XRD. The Hmim contains nitrogen, and energy dispersive X-ray spectroscopy (EDXS) was used to determine the presence of a nitrogen element in the sample. Figure 3c shows that the ZnO nanorod layer is rich in elemental nitrogen, but not the supporting ceramic. This is consistent with the proposed reaction of Hmim and ZnO nanorods. As the reaction occurred on the surface, TOF-SIMS was used for the analysis. TOFSIMS detected fragments from Hmim as well as larger fragments that are consistent with the structural building unit (SBU) of ZIF-8 including Zn(L)4, Zn(L)2, ZnL, and ZnLL′ and ZnL′ as shown by the spectrum in Figure 3d. This confirmed that the reaction did occur between Hmim and the ZnO surface with the formation of Zn(Hmim)y species on the surface and possibly also embryonic ZIF-8 nuclei as indicated by the protuberances on the nanorods. It is clear that the activation process occurred uniformly on the nanorod layer (Figure 3c) with the formation of uniform “ZIF-like” nuclei along the length of the nanorods (Figure 3b), resulting in a uniform nucleation on the nanorods’ surface. The activation process is sensitive to the solvent used as solvent affects the solubility, pH, and reactivity. The Hmim solution in water not only was more alkaline (i.e., pH = 10) but also promoted the dissolution of ZnO resulting in a bare surface after treatment. The Hmim solution in ethanol was less alkaline (i.e., pH = 8), but ZIF-8 formation was faster in ethanol; thus ZIF-8 particle formation occurred. This is also in agreement with the results reported in the previous references.48,49 3.3. ZIF-8 Membrane Preparation. Figure 4 shows that a well-intergrown layer of ZIF-8 film was deposited and grown on the surface of the activated ZnO nanorods. The polycrystalline film is around 6 μm thick and consists of intergrown prismatic ZIF-8 crystals as shown in Figure 4a and c. A higher magnification image of the surface in Figure 4b shows wellformed crystal facets that are relatively free from surface deposits. XRD analysis also detected a ZIF-8 structure phase. Figure 4c shows that there is a sharp interface between the porous ceramic tube and the ZnO nanorods layer, and also

Figure 1. SEM images of ZnO nanorods grown on the porous ceramic tube after 6 h of hydrothermal synthesis: (a, b) Top view, (c, d) cross section, (b, d) a high magnification view of the sample, showing the detailed structure of the support and ZnO nanorods layer.

vertically with each nanorod having a hexagonal cross-section of roughly 200 ± 30 nm and length of ca. 3 ± 0.5 μm. They were uniformly deposited over the entire inner surface of the porous ceramic tubes to serve as an intermediate support layer for the ZIF-8 membrane. The nanorods are closely packed, and a porosity of about 30−40% was estimated from the top view images of the sample shown in Figure 1a and b. The nanorods are aligned within 30° of the vertical axis as shown in Figure 1c. A high magnification image of the cross-section in Figure 1d shows that the ZnO nanorods grew seamlessly from the porous alumina support, indicating a strong adhesion to the ceramic. Intergrowths between neighboring nanorods are apparent in the sample (Figure 1c and d) and could have occurred either during the growth of the nanorods or during the subsequent heat treatment. This provided a planar interconnectivity between the nanorods, giving the layer greater strength. Single gas permeances with respect to the kinetic diameter of the gases were measured for the bare porous ceramic tube and after ZnO nanorod deposition (see Figure S4). It is indicated that Knudsen diffusion is the dominant transport mechanism in both bare and ZnO-deposited ceramic tubes. Calculations indicated that the ZnO layer is responsible for 38% of the flow resistance across the ZnO-deposited ceramic tube. 3.2. Activation of ZnO Nanorods. The activation of the ZnO nanorods with 2-methylimidazole (Hmim) solution is crucial for ZIF-8 deposition. Without activation, only a few large ZIF-8 crystals were grown on the ZnO nanorods without forming a film as shown in Figure 2, suggesting poor nucleation of ZIF-8 on the surface of the ZnO nanorods. A sharp interface can be seen in Figure 2 where the ZIF-8 crystal sat on top of the ZnO nanorods. There is no evidence of ZIF-8 intrusion into the ZnO nanorod layer. Therefore, the ZnO nanorods were activated by immersion in a neutral methanol solution containing the Hmim organic ligand to generate nuclei to seed the deposition and growth of the ZIF-8 membrane. 1977



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Figure 3. (a) Cross-sectional and (b) top SEM views of the ZnO nanorods after activation by a Hmim solution, (c) EDXS line plot for nitrogen (N) along the cross-section of sample, (d) TOF-SIMS spectrum taken from the same sample, and (e) X-ray diffractogram of the sample (iii) with bare ceramic support (i), ZnO (ii), and ZIF-8 (iv) for reference.

Besides having superior strength, the single crystal nanorod layer also provides greater vertical and lateral degrees of freedom to compensate for the mismatch in the expansion coefficients of the ZIF-8 membrane and ceramic tube. This prevents defects from forming and propagating into cracks. It was therefore believed that the ZIF-8 membrane should exhibit better thermal properties (see Figure 6b and Table S1). 3.4. Membrane Gas Permeation and Separation. Single gas permeation and binary gas separation were investigated for the ZIF-8 membranes for hydrogen (H2), carbon dioxide (CO2), nitrogen (N2), and methane (CH4) gases. Single Gas Permeation. The single gas permeances of H2, CO2, N2, and CH4 through the prepared ZIF-8 membrane are plotted in Figure 5 according to the kinetic diameter of the permeating molecules. Hydrogen has the highest permeance among the four gases investigated, and the permeance decreases rapidly with increasing the kinetic diameter of the permeating gases. Molecular sieving is observed for N2 and CH4, which display ideal selectivities of H2/N2 = 11.1 and H2/ CH4 = 12.6, respectively, higher than their corresponding Knudsen selectivity values of 3.7 and 2.8, respectively. The kinetic diameter of CO2 (0.33 nm) is comparable to the ZIF-8 window opening size (ca. 0.34 nm), and the ideal selectivity of H2/CO2 is 5.2, close to its Knudsen selectivity value of 4.7. The single gas permeances of H2, CO2, N2, and CH4 are plotted in Figure 6a with respect to the trans-membrane pressure difference. The permeance plots also show that viscous/ Poiseuille flow is absent, and the supported ZIF-8 membrane is free of cracks and defects. Recently, Battisti et al.50 used the molecular simulation to investigate the gas separation proper-

Figure 4. SEM images of the ZIF-8 membrane grown on the porous ceramic tube modified by ZnO nanorods after Hmim activation: (a, b) top view, (c, d) cross section, (b, d) a high magnification view of the sample, showing the detailed structure of the support, ZnO nanorods, and ZIF-8 layer.

between the ZnO nanorods layer and the ZIF-8 film. Examining the membrane at the higher magnification (Figure 4d) reveals that at the one end, the nanorods grew seamlessly from the porous support while at the other end, the nanorods penetrated seamlessly into the ZIF-8 film creating multiple anchorages of strong molecular bonds on a molecular level. 1978



Article

H2/CH4 remain relatively unchanged. Configurational diffusion has been proposed as the main transport mechanism in molecular sieves such as zeolites when the diameter of a permeating molecule approaches that of the pore opening, where subtle changes in pore diameter could cause a large change in gas diffusion.60,61 Framework flexibility and pore distortion could explain the observed permeation behavior. Indeed, Battisti et al.50 asserted that an explicit modeling of the framework flexibility is critical in obtaining reliable gas-diffusion properties in these small-pore materials, particularly when the size of the permeating molecule is close to size of the pore opening. Recently, Kumari et al.62 made a direct observation of the temperature induced structural transformation in ZIF-8. It was observed that the weakening of the Zn−N bond and swinging of the methyl-imidazole ring occurred at 150 and 300 K, opening the aperture for greater accessibility. This could possibly explain the higher flux obtained for the ZIF-8 membrane at room temperature. Gas adsorption can distort the framework and change the motion of organic linkers, thus altering the gas-adsorbent interaction and gas mobility in the pores.59 Pressure is shown in both the experiment and computation to also cause framework deformation and reorientation of the imidazolate linkers,59,63−66 leading to a subtle change of the membrane pore. It is also clear from the experiments (Table S1) that the membranes were stable under high temperature operation and remained defect-free after repeated heating to 473 K. ZIF-8 membranes grown directly on a ceramic substrate tend to deteriorate after experiencing repeated heating and cooling. This suggests that the vertically aligned ZnO nanotube layer prevents crack formation due to a mismatch between the thermal expansion of the membrane and the ceramic support. The reproducibility of membrane preparation and performance is of the utmost importance for its successful commercial application.42,47 Table 1 reports the single gas permeances of

Figure 5. Plots of single gas permeances for H2, CO2, N2, and CH4 and ideal selectivities of H2 to others as a function of the kinetic diameter at 303 K and 0.1 MPa.

Figure 6. Plots of single gas permeances for H2, CO2, N2, and CH4 in the ZIF-8 membrane as a function of (a) trans-membrane pressure difference and (b) temperature.

ties of ZIFs, and their results showed that the zero-pressure adsorption selectivities in the Henry region were H2/CO2 = 0.07, H2/N2 = 0.27, and H2/CH4 = 0.11 while the diffusion selectivities were H2/CO2 = 14.0, H2/N2 = 19.6, and H2/CH4 = 30.0 for ZIF-8 at 298 K. On the basis of their molecular simulation results, the ideal selectivities of H2/CO2 = 1.0, H2/ N2 = 5.9, and H2/CH4 = 3.3 were calculated for a ZIF-8 membrane. Similar molecular simulation results were also reported by Krishna and van Baten51 and Bux et al.52 Structural flexibility is crucial for the proper description of gas diffusion in ZIF-8. This is particularly true for the diffusion of CH4 and CO2.53 Hu et al.54 introduced a new force field model for ZIF-8 with structural flexibility that could very well predict its crystallographic properties including structural transitions, as well as its mechanical, thermophysical, and transport properties. They predicted the diffusion of CH4 and CO2 that have commensurate size as the ZIF-8 aperture connecting the large cavities. Their predicted CH4/CO2 selectivity value of 0.37 is comparable to our experimental value of 0.41 at 303 K. Single gas permeances measured at different temperatures are summarized to investigate the permeation and thermal stability of the as-prepared ZIF-8 membrane as shown in Figure 6b and Table S1. Each gas permeation experiment lasted for 5 days, and the measurements were reproducible. The permeances of H2, CO2, N2, and CH4 were lower at higher temperatures as also observed by other researchers.26,27,55−59 CO2 experienced the largest change in permeance, suffering a decrease of more than 80% from 3.1 × 10−8 to 0.5 × 10−8 mol·m−2·s−1·Pa−1 when the temperature was increased from 303 to 473 K, resulting in a large increase in H2/CO2 ideal selectivity from 5.2 to 10.5. On the other hand, the ideal selectivities of H2/N2 and

Table 1. Single Gas Permeances of ZIF-8 Membranes at 303 K and 0.1 MPa permeance (× 10−8 mol·m−2·s−1· Pa−1)

ideal selectivity

membranes

H2

H2/ CO2

HFM1 HFM2 HFM3 HFM4 HFM5 HFM6 HFM7 HFM8 HFM9 HFM10

19.9 20.2 20.8 20.3 20.3 17.6 16.7 15.9 17.5 18.1

5.8 5.8 5.3 5.5 5.5 5.3 5.4 5.2 5.1 5.3

H2/ N2

H2/ CH4

9.5 9.7 9.1 9.8 10.2 10.1 9.6 11.1 9.1 10.7

11.1 12.8 10.2 11.2 11.1 11.0 12.8 12.6 11.3 11.6

the membranes prepared in separate batches over a period of half a year. All 10 membranes had a comparable ZIF-8 membrane thickness of 6 ± 0.5 μm and their H2 permeances are comparable with a mean value of (18.8 ± 1.6) × 10−8 mol· m−2·s−1·Pa−1. Furthermore, their ideal selectivities are also similar, indicating the high reproducibility of the new membrane architecture and preparation method. These membranes have comparable or better performance compared 1979



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to the ZIF-8 membranes reported in the literature (see Table S2). Binary Gas Separation. The separations of equimolar binary mixtures of H2/CO2, H2/N2, and H2/CH4 were measured at 303 K and reported in Table 2. Nitrogen was used as the sweep

Schematic diagram of the gas permeation apparatus; porous aalumina tube, surface, and cross-section; EDXS of the membrane cross-section; single gas permeances of different gases through the porous ceramic tube before and after the deposition of the ZnO layer; XRD patterns of the samples; single gas permeances through the ZIF-8 membrane at different temperatures; and a comparison of single gas permeances between the ZIF-8 membranes prepared in this work and the membranes from the literature. This information is available free of charge via the Internet at http://pubs.acs.org/

performances of ZIF-8 membrane mixed gases

permeances 10−8 mol· m−2·s−1·Pa−1 gas i/j H2/ CO2 H2/ N2 H2/ CH4

■

permeances 10−8 mol· m−2·s−1·Pa−1

Knudsen constant

i

j

ideal selectivity

i

j

separation factor

4.7

20.5

3.9

5.2

15.8

3.4

4.6

3.7

20.5

2.0

10.3

16.9

2.1

8.2

2.8

20.5

2.8

11.4

16.7

1.7

9.8

ASSOCIATED CONTENT

S Supporting Information *

Table 2. Separation Properties of Some Binary Gas Mixtures Employed W−K Model through the Membrane Measured at 303 K single gas

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: (X.Z.) [email protected]. *E-mail: (J.Q.) [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21076030, 21036006), the Natural Science Foundation of Liaoning Province (No. 201202027), and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130041110022).

gas in the permeate side for H2/CO2 or H2/CH4 mixtures, while methane was used for the H2/N2 mixture. Separation was done at a trans-membrane pressure difference of 0.1 MPa with the permeate stream kept at the ambient pressure. The separation selectivities at 303 K for H2/N2 and H2/CH4 are 8.2 and 9.8, respectively. These are higher than their corresponding Knudsen values of 3.7 and 2.8, demonstrating good molecular sieving function. The H2 permeance was ca. 16.7 × 10−8 mol m−2 s−1 Pa−1. The H2/CO2 separation is 4.6 and is close to the Knudsen value of 4.7. It is interesting to note that the single gas permeance data and the separation results are comparable. Li et al.25 also reported the similarity between the calculated ideal selectivity and separation selectivity for a ZIF-7 membrane. This is probably due to the weak interactions between the permeating gases at low pressures as predicted by a molecular simulation study.50

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REFERENCES

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4. CONCLUSION In summary, the current work describes a new membrane architecture and preparation. Single crystal ZnO nanorods were grown seamlessly from the substrate to create a wellintergrown, porous support layer of good vertical and lateral strengths. The strut-like structure is believed to allow for greater vertical and lateral movements that could compensate for the mismatch in the expansion coefficients of the membrane and the support. The nanorods also serve as an inorganic linker for the growth of a defect-free ZIF-8 membrane. A simple activation method using a Hmim-methanol solution results in a uniform nucleation of “ZIF-like” seeds along the length of the nanorods that seed the growth of a well-intergrown, polycrystalline ZIF-8 membrane layer. The phenomenon of the obvious increase of the ideal selectivity of H2/CO2 with temperature further implies that the prepared ZIF-8 membrane is relatively defect-free. Indeed, the gas permeation and separation results for the membrane are very encouraging, and this kind of novel composite ZIF-8 membrane holds promise for gas separation under harsh conditions. Additional studies using other gases and at elevated pressures are currently being pursued in the laboratory. 1980



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New Membrane Architecture with High Performance: ZIF-8 Membrane

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