Porous Polyethersulfone-Supported Zeolitic Imidazolate Framework

May 31, 2012 - School of Chemical Engineering, the University of Queensland, Brisbane 4072 Australia ... Citation data is made available by participan...
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Porous Polyethersulfone-Supported Zeolitic Imidazolate Framework Membranes for Hydrogen Separation Lei Ge,† Wei Zhou,† Aijun Du,‡ and Zhonghua Zhu†,* †

School of Chemical Engineering, the University of Queensland, Brisbane 4072 Australia Centre for Computational Molecular Science, Australian, Institute for Bioengineering and Nanotechnology, the University of Queensland

‡

S Supporting Information *

ABSTRACT: ZIF-8 thin layer has been synthesized on the asymmetric porous polyethersulfone (PES) substrate via secondary seeded growth. Continuous and dense ZIF-8 layer, containing microcavities, has good affinity with the PES support. Single gas permeance was measured for H2, N2, CH4, O2, and Ar at different pressure gradients and temperatures. Molecular sieving separation has been achieved for selectively separating hydrogen from larger gases. At 333 K, the H2 permeance can reach ∼4 × 10−7 mol m−2 s−1 Pa−1, and the ideal separation factors of H2 from Ar, O2, N2, and CH4 are 9.7, 10.8, 9.9, and 10.7, respectively. Long-term hydrogen permeance and H2/N2 separation performance show the stable permeability of the derived membranes.

1. INTRODUCTION Energy efficient separation of gases or liquids is vital in industry, especially in fuel processing. In recent decades, high energy costs of traditional gas separation methods (such as adsorption and distillation) have provided prominence to the economically competitive membrane-based gas separation processes.1 Membranes with both high selectivity and high permeability are desired for the industrial scale applications. Novel and high performance materials, including polymer, ceramic, metal, and mixed matrix membranes, are pursued by many researchers. As new microporous hybrid materials, metal−organic frameworks (MOFs), comprised of transition metals and oxides connected by organic linkages to create 1D, 2D, and 3D microporous structures, have been receiving considerable attention due to their remarkable properties, such as high surface area, adsorption affinities, and large diversity in structures and pore size.2,3 Their utilizations in gas adsorption, drug delivery, molecular separation, fuel cells, and catalysis applications have been discovered and evaluated.4−9 Various supported MOFs membranes have been fabricated and investigated in separation applications in the past one decade. The first MOF membrane with a gas separation performance (H2/N2 = 7) superior to Knudsen diffusion was found in a copper-net-supported HKUST-1 membrane.10 The MOF-5 membranes were synthesized by using microwaveinduced rapid seeding and solvothermal secondary growth.11,12 A high ideal selectivity membrane (H2/N2 = 23) was prepared by Taspatsis et al.13 by the second growth of Cu(hfipbb)(H2 hfipbb)0.5 on the porous alumina support. Among them, Zeolite imidazolate frameworks (ZIF), a subclass of MOFs consisting inorganic metal ions or metal clusters coordinated with organic imidazole/imidazolate ligands,4,14 has a sodalite cagelike © 2012 American Chemical Society

structure and pore windows between 0.3−0.36 nm, which can be suitable materials for hydrogen separation. Bux et al.15−17 fabricated the ZIF-8 membranes on a titania support via secondary seeded growth and studied the gas separation performance, of H2/C3H8 and ethene/ethane. Li et al.18 also investigated the gas permeability of alumina-supported ZIF-7 membrane. The H2/CO2 selectivity of 6.5 can be reached due to smaller pore size of ZIF-7. ZIF-22 and ZIF-90 membranes were prepared and their hydrogen separation performances were evaluated by Huang et al.19−21 At 323 K, the hydrogen permeances are higher than 1.6 × 10−7 mol m−2 Pa−1, the mixture separation factors of H2/CO2, H2/O2, H2/N2, and H2/ CH4 are 7.2, 6.4, 6.4, 5.2, respectively. The CO2 selectivity was also investigated on ZIF-822 and SIM-123 membranes. Efficient recovery of bioalcohols was also found in alumina-supported ZIF-8 membrane by pervaporation.24 For membrane application, the compatibility between selective layer and support determines the integrity of the whole membrane and the permeation stability. Eliminating cracks, pinholes, defects, and improving membrane mechanical properties are vital to the separation efficiency and stability of the MOF membranes. Until now, most of the reported MOFs membranes are supported on ceramic; a continuous and defectless layer cannot be formed on the ceramic without support treatment or seeding.19,21,25 For solving the adhesive problem between the MOFs layer and the support layer, flexible polymer support has been utilized due to the favorable interactions between polymer support and organic ligands of MOFs.26,27 Also, the excellent Received: April 12, 2012 Revised: May 19, 2012 Published: May 31, 2012 13264

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affinities were proved by growing MOF on the polymer27 and fabricating the MOF/polymer mixed matrix membrane.28−31 Polymer-supported ZIF-8 membrane was first prepared on a porous nylon membrane by contra-diffusion.32 The measured permeance is much higher than ceramic-supported ZIF membrane; yet the H2/N2 selectivity was only slightly higher than Knudsen selectivity. The contra-diffusion synthesis is timeconsuming (24−72 h), and the derived ZIF-8 thin layer is estimated to be not dense enough to provide as high selectivity as ceramic-supported ones. Better performance can be expected by fabricating a denser ZIF layer on the polymer support via short time secondary seeded growth. In this study, an asymmetric polyethersulfone film fabricated by the phase transition method is used as the support for growing a ZIF-8 thin layer by second growth. The self-prepared PES film presents a pore size gradient and the side with a similar pore size to ZIF-8 nanocrystals was seeded with crystallized ZIF-8, with the particles uniformly trapped in the pore cages. By comparing to membranes from seedless growth, a dense layer of ZIF-8 can be formed on the seeded PES support by applying secondary growth at high temperature. The ZIF-8 layer and PES support are well attached and compacted together. It shows that the asymmetric membranes have been successfully fabricated. One side of the membrane is covered with micropore-sized ZIF-8 (0.34 nm) as the gas selective layer, and, on the other side of the membrane, the PES layer with micrometer pores was acted as the membrane support. The physical chemistry properties of the ZIF-8 from the as-obtained supported membrane were studied, which confirmed the dense layer structure and compatibility between the ZIF layer and the support. High hydrogen permeance and molecular sieved selectivity were observed; the pressure and temperature effects on permeation are also investigated.

h. After hydrothermal synthesis, the membranes were washed with fresh methanol and degassed for 2 h at 150 °C. For comparing, ZIF-8 membrane growth without seeding was also conducted under same hydrothermal condition. 2.5. Characterization. The X-ray diffraction spectra (XRD) of the supported ZIF-8 membranes were obtained with a Bruker Advanced X-ray Diffractometer (40 kV, 30 mA) with Cu Kα (λ = 0.15406 nm) radiation at a scanning rate of 1°/min from 5° to 90°. Attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) spectra were obtained using a Nicolet 5700 ART spectrometer with an average of 64 scans in the range 400−3800 cm−1 to obtain information for the ZIF-8-supported membranes. Regarding the characterization equipment, a JEOL JSM 6300 operated at 5 kV was used for scanning electron microscopy (SEM), and JEOL 6610 coupling with an Oxford SDD energy dispersive X-ray spectrometer (EDS) was used for elemental analysis. High-resolution transmission electron microscopy (HRTEM) was performed on a JEOL JEM-2100 microscope with accelerating voltages of 200 kV. The samples were dispersed by sonication in a mixture of ethanol and isopropanol and then deposited on a holey carbon TEM grid and dried. N2 physisorption isotherms of the samples were obtained using a Micromeritics TriStar 3020 at −196 °C, after degassing samples for 24 h at 150 °C. The corresponding specific surface areas (Sg) were calculated by the langmuir equation at relative pressure (P/Pθ) between 0.005 and 0.05. Total pore volumes (Vp) were evaluated at relative pressures (P/Pθ) close to unity, and the micropore volume was gained from the Dubinin− Radushkevich (DR) method by taking the data points in the range of P/P0 of 0.01−0.3. Pore size distributions (PSD) were calculated from desorption branches of the isotherms using the Horvath−Kawazoe (HK) method. 2.6. Permeation Test. The variable feed pressure and the constant volume permeation system were used to test pure-gas permeation fluxes at room temperature, with the permeation apparatus shown in Figure S1 of the Supporting Information. The membranes were held under vacuum for approximately 5 min to achieve a steady state before the exposing to the selected gas at a specific pressure. The permeation coefficient is calculated using the following equation:

2. EXPERIMENTAL SECTION 2.1. Materials. Polyethersulfone (PES) was purchased from Radel A-300, Solvay Advanced Polymers. Analytical grade Nmethyl-2-pyrrolidone (NMP), 2-methylimidazole (Hmim), zinc nitrate hexahydrate, and methanol were supplied by Sigma-Aldrich. 2.2. Synthesis of Porous PES Support. The porous pure PES membranes were fabricated by the phase transition method. The PES was dissolved into NMP solution (weight ratio = 1:4) and degassed at room temperature. The solution was cast onto clean and dry glass plates at room temperature. The nascent membranes were immersed into a water bath for 48 h at 25 °C. The derived membranes were dried at 170 °C under vacuum for 6 h. 2.3. Synthesis of ZIF-8 Nanocrystals. The ZIF-8 nanocrystals were prepared as reported previously.33 A solution of Zn(NO3)2·6H2O (0.2933 g) in 20 mL methanol is rapidly added into a solution of Hmim (0.649 g) in 20 mL methanol under stirring. The mixture slowly turned to milky color and was kept stirring for 1 h. Then the nanocrystals were separated by centrifugation and washing with methanol. The products are dried at 40 °C in air. 2.4. Growth of ZIF-8 Thin Film on PES Support. ZIF-8 membranes were prepared on the PES supports by secondary seeded growth.22 Before synthesis, one side of the porous PES support was carefully rubbed with the dry ZIF-8 seeds. Then the supports were placed in a Teflon lined stainless steel autoclave and filled with the synthesis solution as described above. The hydrothermal reaction was conducted at 90 °C for 6

P=

273.15 × 1010 VL dp P0 × 76 760AT dt 14.7

(1)

where P is the permeation coefficient in Barrer (1 barrer = 1 × 10−10 cm3 (STP) cm cm−2 s−1 cmHg−1), A is the effective area of the membrane (cm2), T is the absolute temperature (K), V is the dead-volume of the downstream chamber (cm3), L is the membrane thickness (cm), P0 is the feed pressure (psi), and dp/dt is the steady rate of pressure increase in the downstream side (mmHg s−1). Values and error bars reported in the tables and figures are based on measurements of three different membrane samples via standard deviation calculation. The ideal selectivity for two gases is determined as:

α=

PA PB

(2)

where PA and PB are the permeation coefficients of pure gas A and B, respectively. For mixed-gas permeation, Ar was used as the sweep gas and a calibrated gas chromatograph (Shimadzu GC-8A) was used to 13265

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measure the gas concentrations. The separation factor αi,j of a binary mixture permeation (1:1) is defined as the quotient of the molar ratios of the components i and j in the permeate divided by the quotient of molar ratio of components i and j in the retentate,34 as shown below: αi , j =

yi ,permeate /yj ,permeate yi ,retentate /yj ,retentate

(3)

3. RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction pattern of the synthesized ZIF-8 membrane and simulated structure diagram of ZIF-8.

Figure 2. FTIR ATR spectra of synthesized ZIF-8 powder (a), pure porous PES support (b), and PES-supported ZIF-8 membrane by secondary seeded growth (c).

crystals show characteristic infrared bands at around 1585 (C N stretch), 1145, and 995 cm−1 (C−N stretch), are in well agreement with previous reports.33,37 Compared to the spectra of pure PES, the observed additional peaks from PES-supported ZIF-8 membrane are all assigned to ZIF-8. The morphological and pore structure features of ZIF-8 crystals are examined by HRTEM (part a of Figure 3) and N2-adsorption isotherm (part b of Figure 3), sharp hexagonal crystals with homogeneous of 50−100 nm are observed. ZIF-8 nanocrystals show type I isotherm indicating the microporous structure. The Langmuir surface area of the ZIF-8 crystals is 1580 m2/g, and the micropore volume is 0.88 cm3/g calculated by the DR method in the pressure range of P/P0 of 0.01−0.3. For membrane application, the selective layer of ZIF-8 should be continuous and dense to avoid the unselective gas leakage. The selective layer and the support are preferably to be integrated intimately. By seeding and rubbing, the ZIF-8 crystals are fully filled in the pores of PES support surface, as can be seen in Figure 4. Because the nanocrystals possess a similar size to the pore size of one side of PES support (100− 200 μm), ZIF-8 seeds can be fixed in the surface of porous PES support. These seeds can provide nucleation sites for growing crystal into dense membrane during solvothermal process and ensure the adhesivity between ZIF-8 layer and PES support. Figure 5 shows the SEM morphologies and EDS spectrum of ZIF-8 membranes from seeded growth. High porosity and asymmetric PES support, with a thickness of around 130 μm, can be seen in Figure S2 of the Supporting Information. After secondary growth, the PES polymer substrate is covered by a uniform film with a thickness of ∼7.2 μm (part a of Figure 5) estimated by EDS mapping (accumulated with red and blue color of zinc and nitrogen). No visible interface between ZIF-8 layer and PES support can be seen in SEM. A transition layer of about 2−3 μm is observed between ZIF-8 and PES layers, which is formed during seeding and secondary growth. This layer confirms that high organic functionalized ZIF-8 crystals have excellent adherence to the polymer substrate. In contrast, ZIF-8 membrane grown on PES support without seeding shows poor adhesivity between ZIF-8 layer and polymer support, huge gap between two layers and porous ZIF-8 film can be observed (Figure S3 of the Supporting Information). EDS spectra show the composition of both layers, zinc signal can be clearly observed in the upper layer (ZIF-8), and more sulfur can be

Figure 1. (a) X-ray diffractogram of PES-supported ZIF-8 membrane (red, observed; blue, simulated; green, difference between observed and simulated), (b) the structure diagram of ZIF-8 (light blue, ZnN4 tetrahedra; blue, N; red, C; green, H).

ZIF-8 consists of ZnN4 clusters linked by 2-methylimidazole and processes large pores (11.6 Ǻ ) connected through small apertures (3.4 Ǻ ). A high degree of crystallinity can be observed in part a of Figure 1 and indexed to the cubic I4̅3m space group with all the peaks matched with those of ZIF-8 reported previously.15,22,33 On the basis of the ZIF-8 structure data,35 the observed pattern and the simulated pattern show high similarity, which further confirms the formation of pure crystalline ZIF-8 (Table S1 of the Supporting Information). The phase of porous support is not observed in XRD spectra due to the amorphous structure of glassy polyethersulfone.36 The FTIR spectra of ZIF-8 crystals, PES support, and PESsupported ZIF-8 membrane are shown in Figure 2. The ZIF-8 13266

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Figure 3. TEM image (a) and nitrogen sorption isotherms, (b) of assynthesized ZIF-8 crystals.

found in the down PES layer in part b of Figure 5. The magnified surface image shows that the crystals of ZIF-8 are intensely compacted together forming a dense layer without

Figure 5. Cross-section SEM images of (a) EDS mapping of PESsupported ZIF-8 membrane (red, zinc; green, carbon; blue, nitrogen; gray, sulfur); (b) EDS spectrum of ZIF-8 layer and PES substrate; and (c) top view of magnified ZIF-8 layer.

cracks and pinholes (part c of Figure 5). The crystals are between 80 and 120 nm of size and spherical-like shaped resulting from a shape transform from hexagonal structure due to surface energy minimization by SEM electron beam.38 The gas permeance of PES-supported ZIF-8 membranes from seeded growth and seedless growth were measured by the variable feed pressure and constant volume permeation method, and the results are shown in Figure 6. Clearly, the permeance of hydrogen is much higher than those of other

Figure 4. Cross-section SEM image of porous PES support with ZIF-8 seed trapped in the pores. 13267

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structure (Figure 5 and Figure S2 of the Supporting Information), it can be deduced that the more compact ZIF8 layer is synthesized on the porous PES substrate via trapping ZIF-8 crystals into the surface pore of polymer support and secondary seeded growth. On the other hand, similar gas permeances of Ar, O2, N2, and CH4 are observed, which is in line with references.18,19 While in the real membrane, the nonzero permeances of large gases can be attributed to a certain influence of the nonsize-selective gas transport through the grain boundaries of the polycrystalline ZIF layer.18 Therefore, the gas permeation processes become much more complex. In other words, small molecular gas (e.g., H2) will fill up the microcavities and pass through the ZIF-8 pores, which follow the molecular sieve diffusion. Whereas larger gases (e.g., CH4) will permeate through the intercrystal pores and are adsorbed by organic linker molecular interactions, this process may be controlled by the adsorption−diffusion model.18,21 The pressure-dependent permeance of the PES-supported ZIF-8 membrane is presented in Figure 7. With the increment

Figure 6. Single-gas permeances of different gases and separation factors for H2 over other gases on the ZIF-8 membranes derived from seeded growth (a) and seedless growth (b) at 333 K as a function of kinetic diameter.

gases, in accordance with the previous reports on ZIF membrane with similar pore size.16,19,20 As for seeded growth membrane, the H2 permeance can reach ∼4 × 10−7 mol m−2 s−1 Pa−1 at 333 K, and the ideal separation factors of H2 from Ar, O2, N2, and CH4 are 9.7, 10.8, 9.9, and 10.7, respectively. The mixture separation factors of H2/CH4, H2/N2, H2/O2, and H2/Ar are also shown in Table S2 of the Supporting Information, and all these selectivities exceed the corresponding Knudsen selectivities. The molecular sieve performance is then deduced because the selectivity is higher than Knudsen theoretic selectivity (H2/Ar = 2.8, H2/O2 = 3.7, H2/N2 = 4, H2/CH4 = 4.47). Furthermore, compared with the gas permeation data of ZIF series membranes from literature, the PES-supported ZIF-8 membranes showed a comparable gas permeation performance, as shown in Table S2 of the Supporting Information. Also, the permeance of secondary seeded growth ZIF-8 membrane on PES in this study is lower than that of nylon-supported ZIF-8 membrane synthesized from contra-diffusion (PH2 = 10−6∼10−5 mol m−2 s−1 Pa−1),32 whereas the selectivity is two more times larger in this study than nylon-supported membrane. On the other hand, as can be seen in part b of Figure 5, PES-supported ZIF-8 membrane grown without seeding shows much higher permeance (PH2 = 1.73 × 10−5 mol m−2 s−1 Pa−1) but lower hydrogen selectivity (H2/N2 = 3.6). This poor separation performance of membrane derived from seedless growth can be attributed to the less compact and noncontinuous ZIF-8 layer. Upon the basis of the separation performances (Figure 6) and the membrane

Figure 7. Permeances of different gases through PES-supported ZIF-8 membrane as a function of the pressure gradient at 333 K.

of pressure gradient, the hydrogen permeance slightly decreases, whereas that of other gases increases resulting in a hydrogen selectivity drop with pressure, which is in accordance with ZIF-22 membrane19 reported previously. Generally speaking, the effective diffusivity increases with increasing pressure and surface coverage leading to the total permeance increment with pressure. However, microporous metal−organic frameworks, including ZIF-8, have been reported as promising hydrogen storage materials.35,39,40 At higher pressure, more cage windows of ZIF-8 are filled by adsorbed molecular hydrogen. High occupancy-dependent diffusion resistance is generated as a result of decreasing the jumping frequency of gas moleculars in neighboring cages of ZIF-8 structure.41,42 Therefore, in our study, a slight decrease of hydrogen permeance was observed with increasing pressure gradient. The temperature dependence on the PES-supported ZIF-8 membrane is shown in Figure 8. The hydrogen permeance increased from 2.74 × 10−7 to 4.39 × 10−7 mol m−2 s−1 Pa−1, whereas negative activation of the larger gases (N2, O2, Ar) was observed when elevating temperature from 293 to 353 K, strongly indicating an activated transport mechanism of the microporous membrane. The molecular sieving or activated diffusion, described by eq 4, is governed by the apparent 13268

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corresponding Knudsen selectivity. Temperature-dependent permeation results demonstrate temperature-activated permeation of the smaller gases and the negative activation of the larger gases, which strongly indicates a molecular sieving transport mechanism. In future work, the thickness and microstructure of the flexible membranes should be optimized by synthesis conditions to improve the hydrogen separation performance. So far, compared to ceramic-supported ZIF membranes in the references, the simplified synthesis condition (no pretreatment of support with covalent linker and no binder adding for seeding) and the promising permeation performance have already shown the good potential of growing MOF membranes on polymer support.

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ASSOCIATED CONTENT

* Supporting Information S

Figure 8. Permeances of different gases through PES-supported ZIF-8 membrane as a function of the temperature at 0.5 bar.

Drawing of the gas separation setup, figures for SEM image of PES support and seedless ZIF-8 membrane, figures for permeation stability, XRD confinement data, and ZIF membrane performance comparison, binary gas separation performances are presented. These are all referred to in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

activation energy, Ea, which can be calculated from an Arrhenius plot of the temperature versus permeance data,43 Jx = −D0K 0e−Ea / RT

dp dx

■

(4) −2 −1

where Jx is the flux (mol m s ) through the membrane, D0 is a temperature-independent proportionality constant associated with mobility energy (Em), K0 is a temperature-independent proportionality constant associated with the isosteric heat of adsorption, Ea is an apparent activation energy (kJ mol−1), R is the gas constant (J mol−1 K−1), and T is the absolute temperature (K). For some molecular sieve membranes, it has been reported that Ea is generally positive for smaller gases (e.g., He and H2), and either close to zero or negative for the larger gases (e.g., CO2, N2, CO).44−46 In this study, the apparent activation energy for H2 is 6.3 ± 1.4 kJ mol−1, which is similar to ZIF-90 (∼4.8 kJ mol−1),21 ZIF-22 (∼8.5 kJ mol−1),19 ZIF-7 (∼11.9 kJ mol−1),47 and Cu3(BTC)2 (∼9.8 kJ mol−1)10 membranes in the literature. However, the permeation of larger gas molecules will tend to decrease with temperature. The effect of the negative Ea (Ar, −2.3 ± 0.4 kJ mol−1; O2, −3.7 ± 0.9 kJ mol−1; N2, −4.2 ± 0.4 kJ mol−1) may be attributed to gas adsorption.44 In this case, the observed hydrogen selectivity upon other gases slightly increases during heating. In addition, the permeation stability of PES-supported ZIF-8 membrane was also examined. Both H2 permeance and H2/N2 selectivity of the membrane remain unchanged for a period of 60 h at room temperature (Figure S4 of the Supporting Information) indicating the stability of PES-supported ZIF-8 membrane.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by Australian Research Council (ARC) discovery project is greatly appreciated. The first author also likes to acknowledge the support from IPRS (Endeavour International Postgraduate Research Scholarship, Australia), UQRS (University of Queensland Research Scholarship).

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REFERENCES

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4. CONCLUSIONS ZIF-8 membranes were prepared by secondary seeded growth on the asymmetric porous polyethersulfone substrate. Compared to seedless growth, the ZIF-8 film from the seeding procedure shows good compatibilities with the polymer support due to trapping ZIF-8 crystals into the surface pores of polymer support and the favorable interactions between organic ligands of ZIF-8 and PES. The asymmetric porous polymer support with pore size similar to the ZIF-8 seed is suitable for growing compact ZIF-8 layers thus improving the integrity of the whole membrane. The membranes display H2 permeance as high as ∼4 × 10−7 mol m−2 s−1 Pa−1 with the separation selectivity around 10 at 333 K at 150 KPa feed pressure. The binary gas selectivity also exceeds the 13269

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3035105 | J. Phys. Chem. C 2012, 116, 13264−13270