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Highly Selective and Permeable Porous Organic Framework Membrane for CO2 Capture Xue Gao, Xiaoqin Zou,* Heping Ma, Shuang Meng, and Guangshan Zhu* Microporous membranes with pore openings at a molecular level can exhibit size selectivity as molecular sieves, which are promising in membrane-based gas separations.[1] Between microporous materials, zeolites, metal organic frameworks (MOFs) and active carbons are widely used for this purpose. Very recently, porous organic frameworks (POFs or COFs)[2] as a new family member of molecular sieves have been a hot and frontier research theme in materials science and technology, and attracted increasing attentions. POFs are composed of different organic moieties linked by covalent bonds, resulted in diversified structures with controllable pores ranging from 0.3 to 50 nm. The features of highly permanent porosity, exceptionally high thermal stability, and low framework density; make this class of porous materials as an ideal candidate in membrane applications.[3] Microporous POFs owing to their high surface areas and intrinsic polymer characteristics of the entirely covalent bonded networks, possess great advantages over classical inorganic porous counterparts of being easily processed into membranes. Thus, the preparation of POF membranes for gas separation has been proposed [3b-d] since they provide an energy-efficient and reliable technology in separating various gases including air purification, hydrogen recovery and the upgrading of natural gas. CO2 capture or separation from the flue gas or natural gas is of great interests from the energy and environmental perspective.[4] Currently, removal or purification of CO2 from gas streams of power plants and reservoirs is commonly accomplished by cryogenic method or sorption approach using different adsorbents; both of which are costly and inefficient. In order to search an efficient technology with long-term viability for CO2 removal, microporous membranes have been pursued as alternative means because it combines two merits of high CO2 uptake and affinity in porous materials, and operation flexibility with membrane process. Based on the considerations above, an N-rich Schiff based POF (SNW-1)[5] is selected as a potential candidate for membrane fabrication. SNW-1 constructed from industrial chemicals of melamine and di-aldehydes (the chemical structure of a fragment of SNW-1 is shown in Figure 1a), is a nitrogen-rich
X. Gao, Dr. X. Zou, H. Ma, S. Meng, Prof. G. Zhu, State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry Jilin University Changchun, China 130012 Tel/Fax: +86-431-85168331 E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201400020
Adv. Mater. 2014, DOI: 10.1002/adma.201400020
polymer; providing abundant active sites (e.g. amino groups) for CO2 sorption. The obtained SNW-1 exhibits a three-dimensional framework with major pore size falling in the molecular scale (5 Å). Polysulfone (PSF Udel P-3500) is chosen as the appropriate matrix thanks to its glassy and organic nature, high thermal/chemical stability, and commercial availability. SNW-1/PSF membrane is fabricated by filling SNW-1 nanoparticles into PSF matrix using a spin-coating method; and further applied for CO2 capture from gas streams of CO2/CH4 and CO2/N2. The extended aminal network of SNW-1 polymer is built up by forming C-N bonds between two monomers of melamine and terephthalaldehyde, the structure of which (Figure 1a) is verified by 13C and 15N NMR data (Figure S1).[5] Upon polymerization, many pores are generated by an elimination of water molecules; and the porosity is probed by the N2-sorption measurement (Figure 1b). The adsorption isotherms show a steep uptake at low relative pressures, followed by nearly horizontal adsorption and desorption branches at high pressures, typical for microporous materials. The texture data is summarized in Table S1, Supporting Information. The specific Brunauer– Emmet–Teller surface area (SBET) and micropore volume are 821 m2 g−1 and 0.26 cm3 g−1, indicating a high degree of cross linking of monomers; which is in an agreement of the peak disappearance of C O vibration (1690 cm−1) from aldehydes (Figure S2, Supporting Information). To be noted, the major pore size of voids in SNW-1 is calculated to be around 5 Å using the NL-DFT method as seen from the pore size distribution curve (insert picture in Figure 1b); which can be also visualized in a molecular model of a fragment of SNW-1 in Figure 1a. The value of pore size corresponds to a small micropore (3∼20 Å for microporous materials from IUPAC). IR spectrum shows that SNW-1 network bears free N-H groups (Figure S2, Supporting Information). Small pores and available N-H groups in this POF material are beneficial for CO2 recognition, and thus selective diffusion of CO2 (3.3 Å) through the pores would be expected. The morphology and size of as-synthesized SNW-1 particles are studied by SEM (Figure 2a). Spherical shape is observed for as-prepared nanoparticles, and the size of an individual one is about 100 nm. Small particles are superior for obtaining a homogeneous mixture with matrix for further membrane fabrication, which is evidenced by a stable suspension without any visible sedimentation after one month at static condition (see insert picture in Figure 2a). SNW-1/PSF membrane is fabricated by spin coating SNW-1/PSF suspension in CHCl3 onto a macroporous glass frit. The optical picture in Figure 2b shows that the support is entirely covered by SNW-1/PSF layer homogeneously. Insight views of the membrane layer are inspected by SEM. Figure 2c and d display the side views of a particular
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Figure 1. a) Schematic representation of chemical structure and molecular model of a fragment of SNW-1 showing its porous network (drawn by Materials Studio); b) N2 adsorption isotherms of SNW-1 at 77 K and pore size distribution calculated by NL-DFT method (insert).
for occluded solvents (e.g. CHCl3, H2O), followed by a plateau up to 673 K; indicating that SNW-1/PSF membrane possesses a better thermal stability in comparison to neat SNW-1. Molecular recognition of SNW-1 toward CO2 was evaluated by gas sorption measurements. Figure 3a presents adsorptiondesorption isotherms of CO2, CH4 and N2 at 298 K. From the isotherm for CO2, it can be found that SNW-1 exhibits high sorption capacity for CO2 with an uptake of 50 cm3 g−1 at P/P0 = 1.0; suggesting that SNW-1 is a CO2-philic material. Parallel measurements of CH4 and N2 sorption have been carried out as well (Figure 3a). In contrast to the high uptake for CO2, small amounts of CH4 and N2 are adsorbed in SNW-1 under the same conditions (10 and 5 cm3 g−1 at P/P0 = 1.0 for CH4 and N2 respectively). The poor adsorption ability of SNW-1 for CH4 and N2 seems reasonable, because both molecules are difficult to be polarized as seen from their respective polarizabilities (26.3 × 10−25 cm3 for CO2 vs. 26.0 × 10−25 cm3 for CH4 and 17.6 × 10−25 cm3 for N2).[6] To prove this hypothesis, an experimental evidence is provided by the isosteric heats of adsorption (Qst) for CO2, N2 and CH4 calculated from temperature-dependent isotherms using Clausius-Clapeyron equation.[7] As seen from Figure S5, the Qst value of CO2 (35 kJ mol−1 at low uptakes) is much higher than those of CH4 (25 kJ mol−1) and N2 (5 kJ mol−1). Higher Qst means stronger interaction between CO2 and SNW-1 framework, which is in agreement with their respective polarizabilities. Further, the sorption affinity of CO2 over CH4 or N2 is also demonstrated by the index of adsorption selectivity calculated from single-component adsorption isotherms using ideal adsorption solution theory (IAST).[8] Figure 3b shows the evolution of adsorption selectivities in function of bulk pressure of binary gas mixtures of CO2/CH4 and CO2/N2. SNW-1 shows favorably high selectivities (αs) for CO2 in both systems. Typically, a highest αs of 15 is observed for Figure 2. a) SEM image of as-synthesized SNW-1 nanoparticles, and its suspension in CHCl3 CO2 over CH4; and the αs value of CO2 over (insert picture of PSF (left) and SNW-1/PSF (12wt%, right) solutions after one month at static condition); b) optical photos of pure PSF (left) and SNW-1/PSF membrane casted on the sup- N2 is as high as 50 (with deviations of 3%), port (right); c,d) the cross-sectional SEM views of SNW-1/PSF membrane (12wt%) prepared probably due to very low uptake for N2 at 298 K. The high sorption capacity and selectivity via cryofracturing. membrane with 12wt.% SNW-1 at low and high magnifications. The membrane layer is composed of SNW-1 particles and PSF polymer (Figure 2c), and a thickness of 15 µm can be determined. With a close look, it can be found that SNW-1 nanoparticles are embedded into PSF matrix (Figure 2d), rendering an excellent adhesion of SNW-1 with PSF. Other membranes with varied SNW-1 contents were also prepared in the same manner, and their surface features were investigated as well (Figure S3). The chemical compositions and thermal stabilities of a series of SNW-1/PSF membranes were examined by elemental (Table S2) and TG analyses (Figure S4), respectively. With more SNW-1 in the membrane, more N-H groups are available; which is consistent with gradual increases in their corresponding IR absorbance (Figure S2). TG curve of SNW-1/ PSF membrane shows a small weight loss of ∼10% at 453 K
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of SNW-1 toward CO2 can be explained with the following factors, namely small pores with a size of 5 Å and available N-H groups in the skeleton. Small pores in SNW-1 are presumably to have large excess adsorption density, resulted in highly packed CO2 molecules in the pores via Van der Waals interaction.[9] Meanwhile, the presence of N-H moieties in the SNW-1 network could create strong and directed interactions (e.g. hydrogen-bonding, high polarizability) between N-H groups and CO2 molecules, resulted in the preferential CO2 adsorption.[6] The results obtained from gas adsorption measurements and IAST calculations provide us a hint on unique CO2 separation characteristics in gas mixtures if SNW-1 is processed into a membrane. For realizing the feasibility of CO2 capture from gas streams, SNW-1/PSF membrane was prepared and the permeation tests were carried out. The permeation results of single gases (CO2, CH4, N2) at room temperature (298 K) are summarized in Table S3, Supporting Information. The membrane exhibits highly selective permeation of CO2 with ideal separation factors of 27 and 29 for CO2/CH4 and CO2/N2 respectively (Table S3).
Adv. Mater. 2014, DOI: 10.1002/adma.201400020
These values are close to their sorption selectivities (Figure 3b) and greatly exceed the corresponding Knudsen constants (ca. 0.6 for CO2/CH4 and 0.8 for CO2/N2), suggesting that the separating ability of the membrane is largely dependent on the adsorption selectivity of SNW-1 toward CO2. Moreover, gas separation studies of binary gases (CO2/CH4, CO2/N2) with an equal volume ratio were performed in order to evaluate the membrane performance in capturing CO2 from mixture gas streams. Figure 4 presents the separation results of different membranes with varied SNW-1 contents. For both mixtures of CO2/CH4 and CO2/N2, CO2 permeabilities show an increasing trend with more SNW-1 nanoparticles introduced into the membranes; indicating SNW-1 facilitates the permeation of CO2 owing to its high porosity and specific sorption affinity. The separation factors are increasing gradually with increasing SNW-1 contents and reach maxima at a content of 12wt.%. This expected behavior is due to more selective permeation of CO2 through the membrane when denser CO2-philic N-H sites are present in SNW-1/PSF membrane. A lower separation factor is also observed for the membrane with a content of 15wt.% SNW-1. The decreased separating ability is probably because of an occurrence of some cracks or big interfacial holes between SNW-1 particles and PSF matrix, the reason of which is supported by a rapid drop in Young’s modulus (Figure S6) when
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SNW-1 content is increased to 15wt.%.[10] It is worth mentioning that SNW-1/PSF membrane (12wt.% SNW-1) shows very high separation factors of 34 and 40 for CO2/CH4 and CO2/ N2 respectively, which are higher than their corresponding ideal ones (27 and 29). This finding can be interpreted by competitive adsorption [11] between gases, in couple with molecular sieving effect[12] (hindered diffusion of gas molecules in relatively small pores). That is to say, CO2 are preferentially adsorbed with subsequent permeation through the membrane in binary mixtures of CO2/CH4 and CO2/N2 due to specific molecular recognition of SNW-1 toward CO2 although the self-diffusivity of CO2 is smaller than those of CH4 and N2. Once CO2 molecules (3.3 Å) are penetrating through the pores (5 Å), the diffusion of CH4 (3.8 Å) or N2 molecules (3.64 Å) are blocked to a large extent. Additionally, SNW-1/PSF membranes display improved CO2 permeabilities if taking pure PSF polymer as a reference, evidenced by a highest value of 22.4 barrer. Compared with other reported PSF-based membranes (Figure S7), it can be found that both separation factor and permeability of SNW-1/ PSF membrane greatly surpasses its counterparts and are approaching to the upper bound of Robeson’s plot.[13] The high separation efficiency guarantees the appropriate application in CO2 capture or purification from other permanent gases using this type of POF membrane. To address the thermal stability of SNW-1/PSF membrane, permeations at different temperatures (from 288 to 348 K) were evaluated (Figure S8, Supporting Information). CO2 permeabilities in both mixtures are increasing with elevating temperature, indicating an activated diffusion process. The apparent activation energies (Eact) for CO2, N2, CH4 are obtained by fitting the experimental data using an Arrhenius equation.[14] The calculated values of Eact for CO2, N2, CH4 are 7.3, 11.9, 12.2 kJ mol−1. The Eact trend gives a good correlation of their corresponding molecular sizes (CO2: 3.3 Å, N2: 3.64 Å, CH4: 3.8 Å), pointing out the pore-size effect plays another important role in gas diffusion as well as permeation through the membrane. A decrease in separation factor (a product of sorption and diffusion selectivities)[15] is probably because of a greater reduction in CO2 sorption capacity of SNW-1 at higher temperatures in comparison to those of N2 and CH4 (Figures 3a and S5a, Supporting Information). However, these separation factors are still significantly larger than their Knudsen constants. Meanwhile, the reproducibility and robustness of the membrane was also examined (Figure S9, Supporting Information). SNW-1/PSF membrane retains the separation factor and permeability at a high value over a week period, suggesting that this membrane has excellent stability. In conclusion, the fabrication of SNW-1/PSF POF membrane is exemplified by introducing nano-sized SNW-1 particles with intrinsically small micropores into PSF matrix via spin-coating its suspension onto a porous support. Gas sorption measurements as well as IAST calculation have demonstrated that SNW-1 exhibits high sorption capacity and affinity toward CO2 owing to its high micro-porosity and functional amino groups. As-prepared membrane is further applied for CO2 separation. Gas separation results reveal that SNW-1/PSF membrane shows an excellent separation performance including high separation factor (34 and 40 for CO2/CH4 and CO2/N2) and enhanced CO2 permeability (22.4 barrer). Besides, TG analysis, 4
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repetitive uses at high temperatures, and long-time tests prove that SNW-1/PSF membrane is stable and robust; showing its potentiality in the practical application of CO2 capture.
Experimental Section Chemicals: Commercially available glassy polysulfone (PSF) Udel P-3500, melamine and terephthalaldeyde were purchased from Acros, Alfa Aesar and Sigma-Aldrich, respectively. Anhydrous dimethyl sulfoxide was obtained via distillation. Chloroform (CHCl3) was used as received without any further purification. Macroporous glass frit supports (25 mm in diameter, pore size 2–4 µm, Taixing City, Science and Education Equipment Co. Ltd., China) were used as substrates in our experiments. Pretreatment of Substrates: Macroporous glass frit supports were polished with SiC sandpapers (2000 mesh) to obtain smooth surfaces. And then the supports were cleaned under ultrasonic condition in deionized water for several times and dried at 393 K. Synthesis of SNW-1: A flame-dried Schlenk flask with a condenser and a magnetic stirring bar was charged with melamine (313 mg, 2.485 mmol), terephthalaldehyde (500 mg, 3.728 mmol) and dimethyl sulfoxide (DMSO, 15.5 ml). After degassing with argon bubbles, the mixture was heated to 453 K for 72 h under an inert atmosphere. After reaction and cooling down the reactor to room temperature, the precipitated SNW-1 powder was isolated via filtration over a Büchner funnel and washed with excess N,N-dimethylformamide (DMF) and tetrahydrofurane (THF). The powder was further purified by sequential Soxhlet extraction with methanol, THF and methanol; and dried under vacuum at 393 K overnight. Finally, off-white powders were obtained in 55% yield. Fabrication of SNW-1/PSF Membrane: Spin-coating method was employed in the membrane preparation. Pure PSF dense membranes were made from a PSF-CHCl3 solution. Typically, PSF beads were first degassed at 393 K for 10 h under vacuum to remove adsorbed substances. Then, 0.4 g PSF was dissolved in 4 ml CHCl3 and stirred overnight. For the fabrication of SNW-1/PSF membrane, an additional step of well dispersing SNW-1 nanoparticles in the solvent is required. The typical procedure involves separately dissolving PSF and SNW-1 in CHCl3; subsequently mixing 20% of the required PSF solution with SNW-1 solution under vigorous stirring, followed by ultrasonic treatment for 30 minutes. After ultrasonication, the residual PSF solution was added in the mixture above with additional stirring for 1 day in order to obtain a homogeneous solution. For all membranes with different SNW-1 contents, the solvent percentages of solvent to SNW-1/PSF (88 to 12 wt%) in the mixture were identical, allowing good viscosity. Subsequently, as-prepared SNW-1/PSF solutions were spin-coating on macroporous glass frits or casted on flat glass plates. After naturally drying, the membranes were subjected to a further thermal treatment at 393 K under vacuum in order to remove the residual solvent. Characterizations: Solid-state 13C CP/MAS NMR measurement was carried out on a Bruker Avance III model 400 MHz NMR spectrometer at a MAS rate of 5 kHz. 13C CP MAS chemical shifts are referenced to the resonances of adamantane (C10H16) standard (dCH2 = 38.4). The 15N {1H} CP MAS spectrum was recorded on a Bruker Avance III 400 WB spectrometer equipped with a 4 mm standard bore CP MAS probe head whose X channel was tuned to 40.55 MHz for 15N and the other channel was tuned to 400.18 MHz for broad band 1H decoupling, using a magnetic field of 9.39 T at 297 K. The dried and finely powdered sample was packed in the ZrO2 rotor closed with Kel-F cap which was spun at 5 kHz rate. The experiment was conducted at a contact time of 2 ms. A total of 10000 scans were recorded with 3 s recycle delay for each sample. All 15N CP MAS chemical shifts are referenced to the resonances of α-glycine (C2H2NO2) standard (d = 31.0). Thermogravimetric analysis (TG) was carried out on a Netzch Sta 449c thermal analyzer with a heating rate of 10 K min−1 under air atmosphere. Mechanical properties of membranes were carried out at
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[3]
[4]
[5] [6] [7]
[8] [9] [10]
Supporting Information Additional figures and tables are provided.
[11]
Acknowledgements We are grateful to the financial supports from National Basic Research Program of China (973 Program, grant nos. 2012CB821700), Major International (Regional) Joint Research Project of NSFC (grant nos. 21120102034), and NSFC (grant nos. 20831002).
[12]
[13] Received: January 2, 2014 Published online:
[1] a) M. Yu, R. D. Noble, J. L. Falconer, Acc. Chem. Res. 2011, 44, 1196; b) J. Caro, M. Noack, Microporous Mesoporous Mater. 2008, 115, 215; c) M. Shah, M. C. McCarthy, S. Sachdeva, A. K. Lee, H. K. Jeong, Ind. Eng. Chem. Res. 2012, 51, 2179. [2] a) X. Q. Zou, H. Ren, G. S. Zhu, Chem. Commun. 2013, 49, 3925; b) A. Thomas, Angew. Chem. Int. Ed. 2010, 49, 8328; c) N. B. McKeown, P. M. Budd, Macromolecules 2010, 43, 5163; d) A. I. Cooper, Adv. Mater. 2009, 21, 1291; e) P. Kaur, J. T. Hupp, S. T. Nguyen, ACS Catal. 2011, 1, 819; f) A. P. Côté, A. I. Benin, N. W. Ockwig, M. O’Keeffe, A. J. Matzger, O. M. Yaghi, Science 2005, 310, 1166; g) F. Vilela, K. Zhang, M. Antonietti, Energy
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[14] [15] [16]
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room temperature on a test machine (SHIMADZU AG-1). Membranes (5 mm wide and 5 cm long) were fixed into the testing station and subjected to an extension rate of 2 mm min−1. Each membrane was tested several times to get an average report value. SEM images were obtained using a JEOL JSM-6700F scanning electron microscope, and all the samples were prepared via the cryofracturing method. The FT-IR spectra were collected on a Bruker IF66F/V Fourier transform infrared spectrometer with KBr pellets in the range of 4000-400 cm−1. Elemental analyses (C, H, N) were carried out on a Perkin-Elmer 240 analyzer. The surface area, micropore volume, and pore size of SNW-1 are calculated from N2 adsorption isotherms using BET, t-plot, and NL-DFT methods respectively. Prior to the measurement, the sample was degassed at 393 K for 24 h; and N2 sorption isotherms were subsequently recorded at 77 K on a Micromeritics ASAP 2010M analyzer. Adsorption isotherms of N2, CO2 and CH4 were also measured at 273 and 298 K. Gas Separations: Prior to gas separation, membranes were dehydrated in a vacuum oven at 393 K and 10−5 Pa overnight to remove adsorbed water. Then, SNW-1/PSF membrane was fixed in a membrane chamber and sealed with xylene resistant fluorocarbon O-rings and connected to a gas flow system.[16] Binary gas mixtures of CO2/CH4 and CO2/N2 were introduced into the membrane module in the feed side, and permeate was carried by argon sweep gas. The feed and permeate gases were analyzed by on-line gas chromatography (GC-450, BRUKER).
Environ. Sci. 2012, 5, 7819; h) Q. Chen, M. Luo, P. Hammershøj, D. Zhou, Y. Han, B. W. Laursen, C. G. Yan, B. H. Han, J. Am. Chem. Soc. 2012, 134, 6084; i) Y. Luo, B. Li, W. Wang, K. Wu, B. Tan, Adv. Mater. 2012, 24, 5703. a) R. Babarao, S. Dai, D. Jiang, Langmuir 2011, 27, 3451; b) N. Du, M. M. D. Cin, I. Pinnau, A. Nicalek, G. P. Robertson, M. D. Guiver, Macromol. Rapid Commun. 2011, 32, 631; c) X. Zhu, C. Tian, S. M. Mahurin, S. H. Chai, C. Wang, S. Brown, G. M. Veith, H. Luo, H. Liu, S. Dai, J. Am. Chem. Soc. 2012, 134, 10478; d) M. Carta, R. M. Evans, M. Croad, Y. Rogan, J. C. Jansen, P. Bernardo, F. Bazzarelli, N. B. McKeown, Science 2013, 339, 303. a) Y. S. Bae, R. Q. Snurr, Angew. Chem. Int. Ed. 2011, 50, 11586; b) Q. Wang, J. Luo, Z. Zhong, A. Borgna, Energy Environ. Sci. 2011, 4, 42. M. G. Schwab, B. Fassbender, H. W. Spiess, A. Thomas, X. Feng, K. Müllen, J. Am. Chem. Soc. 2009, 131, 7216. D. M. D’Alessandro, B. Smit, J. R. Long, Angew. Chem. Int. Ed. 2010, 49, 6058. a) D. Farrusseng, C. Daniel, C. Gaudillère, U. Ravon, Y. Schuurman, C. Mirodatos, D. Dubbeldam, H. Frost, R. Q. Snurr, Langmuir 2009, 25, 7383; b) R. C. Lochan, M. H. Gordon, Phys. Chem. Chem. Phys. 2006, 8, 1357. a) A. L. Mayers, J. M. Prausnitz, AIChE J. 1965, 11, 121; b) B. Liu, B. Smit, J. Phys. Chem. C 2010, 114, 8515. K. Mosher, Y. Lin, J. Wilcox, M. S. Thesis, Stanford University 2011. M. J. C. Ordonez, K. J. Balkus, J. P. Ferraris, I. H. Musselman, J. Membr. Sci. 2010, 361, 28. a) E. G. Pérez, J. B. Parra, C. O. Ania, A. G. Sánchez, J. M. van Baten, R. Krishna, D. Dubbeldam, S. Calero, Adsorption 2007, 13, 469; b) X. Zou, F. Zhang, S. Thomas, G. Zhu, V. Valtchev, S. Mintova, Chem. Eur. J. 2011, 17, 12076. a) S. Li, J. L. Falconer, R. D. Noble, Adv. Mater. 2006, 18, 2601; b) Y. S. Li, F. Y. Liang, H. Bux, A. Feldhoff, W. S. Yang, J. Caro, Angew. Chem. Int. Ed. 2010, 49, 548; c) S. Aguado, C. H. Nicolas, V. M. Baslé, C. Nieto, H. Amrouche, N. Bats, N. Audebrand, D. Farrusseng, New J. Chem. 2011, 35, 41. a) B. Zornoza, A. M. Joaristi, P. S. Crespo, C. Tellez, J. Coronas, J. Gascon, F. Kapteijn, Chem. Commun. 2011, 47, 9522; b) B. Zornoza, B. Seoane, J. M. Zamaro, C. Téllez, J. Coronas, ChemPhysChem 2011, 12, 2781; c) H. B. T. Jeazet, C. Staudt, C. Janiak, Chem. Commun. 2012, 48, 2140; d) B. Zornoza, S. Irusta, C. Téllez, J. Coronas, Langmuir 2009, 25, 5903; e) J. Ahn, W. J. Chung, I. Pinnau, M. D. Guiver, J. Membr. Sci. 2008, 314, 123; f) S. Kim, L. Chen, J. K. Johnson, E. Marand, J. Membr. Sci. 2007, 294, 147; g) B. Zornoza, O. Esekhile, W. J. Koros, C. Téllez, J. Coronas, Sep. Purif. Technol. 2011, 77, 137. Y. Li, F. Liang, H. Bux, W. Yang, J. Caro, J. Membr. Sci. 2010, 354, 48. H. Bux, C. Chmelik, J. M. Baten, R. Krishna, J. Caro, Adv. Mater. 2010, 22, 4741. a) X. Yin, G. Zhu, W. Yang, Y. Li, G. Zhu, R. Xu, J. Sun, S. Qiu, R. Xu, Adv. Mater. 2005, 17, 2006; b) X. Q. Zou, P. Bazin, F. Zhang, G. S. Zhu, V. Valtchev, S. Mintova, ChemPlusChem 2012, 77, 437.
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Highly Selective and Permeable Porous Organic Framework Membrane for CO2 Capture Xue Gao, Xiaoqin Zou,* Heping Ma, Shuang Meng, and Guangshan Zhu* Microporous membranes with pore openings at a molecular level can exhibit size selectivity as molecular sieves, which are promising in membrane-based gas separations.[1] Between microporous materials, zeolites, metal organic frameworks (MOFs) and active carbons are widely used for this purpose. Very recently, porous organic frameworks (POFs or COFs)[2] as a new family member of molecular sieves have been a hot and frontier research theme in materials science and technology, and attracted increasing attentions. POFs are composed of different organic moieties linked by covalent bonds, resulted in diversified structures with controllable pores ranging from 0.3 to 50 nm. The features of highly permanent porosity, exceptionally high thermal stability, and low framework density; make this class of porous materials as an ideal candidate in membrane applications.[3] Microporous POFs owing to their high surface areas and intrinsic polymer characteristics of the entirely covalent bonded networks, possess great advantages over classical inorganic porous counterparts of being easily processed into membranes. Thus, the preparation of POF membranes for gas separation has been proposed [3b-d] since they provide an energy-efficient and reliable technology in separating various gases including air purification, hydrogen recovery and the upgrading of natural gas. CO2 capture or separation from the flue gas or natural gas is of great interests from the energy and environmental perspective.[4] Currently, removal or purification of CO2 from gas streams of power plants and reservoirs is commonly accomplished by cryogenic method or sorption approach using different adsorbents; both of which are costly and inefficient. In order to search an efficient technology with long-term viability for CO2 removal, microporous membranes have been pursued as alternative means because it combines two merits of high CO2 uptake and affinity in porous materials, and operation flexibility with membrane process. Based on the considerations above, an N-rich Schiff based POF (SNW-1)[5] is selected as a potential candidate for membrane fabrication. SNW-1 constructed from industrial chemicals of melamine and di-aldehydes (the chemical structure of a fragment of SNW-1 is shown in Figure 1a), is a nitrogen-rich
X. Gao, Dr. X. Zou, H. Ma, S. Meng, Prof. G. Zhu, State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry Jilin University Changchun, China 130012 Tel/Fax: +86-431-85168331 E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201400020
Adv. Mater. 2014, DOI: 10.1002/adma.201400020
polymer; providing abundant active sites (e.g. amino groups) for CO2 sorption. The obtained SNW-1 exhibits a three-dimensional framework with major pore size falling in the molecular scale (5 Å). Polysulfone (PSF Udel P-3500) is chosen as the appropriate matrix thanks to its glassy and organic nature, high thermal/chemical stability, and commercial availability. SNW-1/PSF membrane is fabricated by filling SNW-1 nanoparticles into PSF matrix using a spin-coating method; and further applied for CO2 capture from gas streams of CO2/CH4 and CO2/N2. The extended aminal network of SNW-1 polymer is built up by forming C-N bonds between two monomers of melamine and terephthalaldehyde, the structure of which (Figure 1a) is verified by 13C and 15N NMR data (Figure S1).[5] Upon polymerization, many pores are generated by an elimination of water molecules; and the porosity is probed by the N2-sorption measurement (Figure 1b). The adsorption isotherms show a steep uptake at low relative pressures, followed by nearly horizontal adsorption and desorption branches at high pressures, typical for microporous materials. The texture data is summarized in Table S1, Supporting Information. The specific Brunauer– Emmet–Teller surface area (SBET) and micropore volume are 821 m2 g−1 and 0.26 cm3 g−1, indicating a high degree of cross linking of monomers; which is in an agreement of the peak disappearance of C O vibration (1690 cm−1) from aldehydes (Figure S2, Supporting Information). To be noted, the major pore size of voids in SNW-1 is calculated to be around 5 Å using the NL-DFT method as seen from the pore size distribution curve (insert picture in Figure 1b); which can be also visualized in a molecular model of a fragment of SNW-1 in Figure 1a. The value of pore size corresponds to a small micropore (3∼20 Å for microporous materials from IUPAC). IR spectrum shows that SNW-1 network bears free N-H groups (Figure S2, Supporting Information). Small pores and available N-H groups in this POF material are beneficial for CO2 recognition, and thus selective diffusion of CO2 (3.3 Å) through the pores would be expected. The morphology and size of as-synthesized SNW-1 particles are studied by SEM (Figure 2a). Spherical shape is observed for as-prepared nanoparticles, and the size of an individual one is about 100 nm. Small particles are superior for obtaining a homogeneous mixture with matrix for further membrane fabrication, which is evidenced by a stable suspension without any visible sedimentation after one month at static condition (see insert picture in Figure 2a). SNW-1/PSF membrane is fabricated by spin coating SNW-1/PSF suspension in CHCl3 onto a macroporous glass frit. The optical picture in Figure 2b shows that the support is entirely covered by SNW-1/PSF layer homogeneously. Insight views of the membrane layer are inspected by SEM. Figure 2c and d display the side views of a particular
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Figure 1. a) Schematic representation of chemical structure and molecular model of a fragment of SNW-1 showing its porous network (drawn by Materials Studio); b) N2 adsorption isotherms of SNW-1 at 77 K and pore size distribution calculated by NL-DFT method (insert).
for occluded solvents (e.g. CHCl3, H2O), followed by a plateau up to 673 K; indicating that SNW-1/PSF membrane possesses a better thermal stability in comparison to neat SNW-1. Molecular recognition of SNW-1 toward CO2 was evaluated by gas sorption measurements. Figure 3a presents adsorptiondesorption isotherms of CO2, CH4 and N2 at 298 K. From the isotherm for CO2, it can be found that SNW-1 exhibits high sorption capacity for CO2 with an uptake of 50 cm3 g−1 at P/P0 = 1.0; suggesting that SNW-1 is a CO2-philic material. Parallel measurements of CH4 and N2 sorption have been carried out as well (Figure 3a). In contrast to the high uptake for CO2, small amounts of CH4 and N2 are adsorbed in SNW-1 under the same conditions (10 and 5 cm3 g−1 at P/P0 = 1.0 for CH4 and N2 respectively). The poor adsorption ability of SNW-1 for CH4 and N2 seems reasonable, because both molecules are difficult to be polarized as seen from their respective polarizabilities (26.3 × 10−25 cm3 for CO2 vs. 26.0 × 10−25 cm3 for CH4 and 17.6 × 10−25 cm3 for N2).[6] To prove this hypothesis, an experimental evidence is provided by the isosteric heats of adsorption (Qst) for CO2, N2 and CH4 calculated from temperature-dependent isotherms using Clausius-Clapeyron equation.[7] As seen from Figure S5, the Qst value of CO2 (35 kJ mol−1 at low uptakes) is much higher than those of CH4 (25 kJ mol−1) and N2 (5 kJ mol−1). Higher Qst means stronger interaction between CO2 and SNW-1 framework, which is in agreement with their respective polarizabilities. Further, the sorption affinity of CO2 over CH4 or N2 is also demonstrated by the index of adsorption selectivity calculated from single-component adsorption isotherms using ideal adsorption solution theory (IAST).[8] Figure 3b shows the evolution of adsorption selectivities in function of bulk pressure of binary gas mixtures of CO2/CH4 and CO2/N2. SNW-1 shows favorably high selectivities (αs) for CO2 in both systems. Typically, a highest αs of 15 is observed for Figure 2. a) SEM image of as-synthesized SNW-1 nanoparticles, and its suspension in CHCl3 CO2 over CH4; and the αs value of CO2 over (insert picture of PSF (left) and SNW-1/PSF (12wt%, right) solutions after one month at static condition); b) optical photos of pure PSF (left) and SNW-1/PSF membrane casted on the sup- N2 is as high as 50 (with deviations of 3%), port (right); c,d) the cross-sectional SEM views of SNW-1/PSF membrane (12wt%) prepared probably due to very low uptake for N2 at 298 K. The high sorption capacity and selectivity via cryofracturing. membrane with 12wt.% SNW-1 at low and high magnifications. The membrane layer is composed of SNW-1 particles and PSF polymer (Figure 2c), and a thickness of 15 µm can be determined. With a close look, it can be found that SNW-1 nanoparticles are embedded into PSF matrix (Figure 2d), rendering an excellent adhesion of SNW-1 with PSF. Other membranes with varied SNW-1 contents were also prepared in the same manner, and their surface features were investigated as well (Figure S3). The chemical compositions and thermal stabilities of a series of SNW-1/PSF membranes were examined by elemental (Table S2) and TG analyses (Figure S4), respectively. With more SNW-1 in the membrane, more N-H groups are available; which is consistent with gradual increases in their corresponding IR absorbance (Figure S2). TG curve of SNW-1/ PSF membrane shows a small weight loss of ∼10% at 453 K
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of SNW-1 toward CO2 can be explained with the following factors, namely small pores with a size of 5 Å and available N-H groups in the skeleton. Small pores in SNW-1 are presumably to have large excess adsorption density, resulted in highly packed CO2 molecules in the pores via Van der Waals interaction.[9] Meanwhile, the presence of N-H moieties in the SNW-1 network could create strong and directed interactions (e.g. hydrogen-bonding, high polarizability) between N-H groups and CO2 molecules, resulted in the preferential CO2 adsorption.[6] The results obtained from gas adsorption measurements and IAST calculations provide us a hint on unique CO2 separation characteristics in gas mixtures if SNW-1 is processed into a membrane. For realizing the feasibility of CO2 capture from gas streams, SNW-1/PSF membrane was prepared and the permeation tests were carried out. The permeation results of single gases (CO2, CH4, N2) at room temperature (298 K) are summarized in Table S3, Supporting Information. The membrane exhibits highly selective permeation of CO2 with ideal separation factors of 27 and 29 for CO2/CH4 and CO2/N2 respectively (Table S3).
Adv. Mater. 2014, DOI: 10.1002/adma.201400020
These values are close to their sorption selectivities (Figure 3b) and greatly exceed the corresponding Knudsen constants (ca. 0.6 for CO2/CH4 and 0.8 for CO2/N2), suggesting that the separating ability of the membrane is largely dependent on the adsorption selectivity of SNW-1 toward CO2. Moreover, gas separation studies of binary gases (CO2/CH4, CO2/N2) with an equal volume ratio were performed in order to evaluate the membrane performance in capturing CO2 from mixture gas streams. Figure 4 presents the separation results of different membranes with varied SNW-1 contents. For both mixtures of CO2/CH4 and CO2/N2, CO2 permeabilities show an increasing trend with more SNW-1 nanoparticles introduced into the membranes; indicating SNW-1 facilitates the permeation of CO2 owing to its high porosity and specific sorption affinity. The separation factors are increasing gradually with increasing SNW-1 contents and reach maxima at a content of 12wt.%. This expected behavior is due to more selective permeation of CO2 through the membrane when denser CO2-philic N-H sites are present in SNW-1/PSF membrane. A lower separation factor is also observed for the membrane with a content of 15wt.% SNW-1. The decreased separating ability is probably because of an occurrence of some cracks or big interfacial holes between SNW-1 particles and PSF matrix, the reason of which is supported by a rapid drop in Young’s modulus (Figure S6) when
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SNW-1 content is increased to 15wt.%.[10] It is worth mentioning that SNW-1/PSF membrane (12wt.% SNW-1) shows very high separation factors of 34 and 40 for CO2/CH4 and CO2/ N2 respectively, which are higher than their corresponding ideal ones (27 and 29). This finding can be interpreted by competitive adsorption [11] between gases, in couple with molecular sieving effect[12] (hindered diffusion of gas molecules in relatively small pores). That is to say, CO2 are preferentially adsorbed with subsequent permeation through the membrane in binary mixtures of CO2/CH4 and CO2/N2 due to specific molecular recognition of SNW-1 toward CO2 although the self-diffusivity of CO2 is smaller than those of CH4 and N2. Once CO2 molecules (3.3 Å) are penetrating through the pores (5 Å), the diffusion of CH4 (3.8 Å) or N2 molecules (3.64 Å) are blocked to a large extent. Additionally, SNW-1/PSF membranes display improved CO2 permeabilities if taking pure PSF polymer as a reference, evidenced by a highest value of 22.4 barrer. Compared with other reported PSF-based membranes (Figure S7), it can be found that both separation factor and permeability of SNW-1/ PSF membrane greatly surpasses its counterparts and are approaching to the upper bound of Robeson’s plot.[13] The high separation efficiency guarantees the appropriate application in CO2 capture or purification from other permanent gases using this type of POF membrane. To address the thermal stability of SNW-1/PSF membrane, permeations at different temperatures (from 288 to 348 K) were evaluated (Figure S8, Supporting Information). CO2 permeabilities in both mixtures are increasing with elevating temperature, indicating an activated diffusion process. The apparent activation energies (Eact) for CO2, N2, CH4 are obtained by fitting the experimental data using an Arrhenius equation.[14] The calculated values of Eact for CO2, N2, CH4 are 7.3, 11.9, 12.2 kJ mol−1. The Eact trend gives a good correlation of their corresponding molecular sizes (CO2: 3.3 Å, N2: 3.64 Å, CH4: 3.8 Å), pointing out the pore-size effect plays another important role in gas diffusion as well as permeation through the membrane. A decrease in separation factor (a product of sorption and diffusion selectivities)[15] is probably because of a greater reduction in CO2 sorption capacity of SNW-1 at higher temperatures in comparison to those of N2 and CH4 (Figures 3a and S5a, Supporting Information). However, these separation factors are still significantly larger than their Knudsen constants. Meanwhile, the reproducibility and robustness of the membrane was also examined (Figure S9, Supporting Information). SNW-1/PSF membrane retains the separation factor and permeability at a high value over a week period, suggesting that this membrane has excellent stability. In conclusion, the fabrication of SNW-1/PSF POF membrane is exemplified by introducing nano-sized SNW-1 particles with intrinsically small micropores into PSF matrix via spin-coating its suspension onto a porous support. Gas sorption measurements as well as IAST calculation have demonstrated that SNW-1 exhibits high sorption capacity and affinity toward CO2 owing to its high micro-porosity and functional amino groups. As-prepared membrane is further applied for CO2 separation. Gas separation results reveal that SNW-1/PSF membrane shows an excellent separation performance including high separation factor (34 and 40 for CO2/CH4 and CO2/N2) and enhanced CO2 permeability (22.4 barrer). Besides, TG analysis, 4
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repetitive uses at high temperatures, and long-time tests prove that SNW-1/PSF membrane is stable and robust; showing its potentiality in the practical application of CO2 capture.
Experimental Section Chemicals: Commercially available glassy polysulfone (PSF) Udel P-3500, melamine and terephthalaldeyde were purchased from Acros, Alfa Aesar and Sigma-Aldrich, respectively. Anhydrous dimethyl sulfoxide was obtained via distillation. Chloroform (CHCl3) was used as received without any further purification. Macroporous glass frit supports (25 mm in diameter, pore size 2–4 µm, Taixing City, Science and Education Equipment Co. Ltd., China) were used as substrates in our experiments. Pretreatment of Substrates: Macroporous glass frit supports were polished with SiC sandpapers (2000 mesh) to obtain smooth surfaces. And then the supports were cleaned under ultrasonic condition in deionized water for several times and dried at 393 K. Synthesis of SNW-1: A flame-dried Schlenk flask with a condenser and a magnetic stirring bar was charged with melamine (313 mg, 2.485 mmol), terephthalaldehyde (500 mg, 3.728 mmol) and dimethyl sulfoxide (DMSO, 15.5 ml). After degassing with argon bubbles, the mixture was heated to 453 K for 72 h under an inert atmosphere. After reaction and cooling down the reactor to room temperature, the precipitated SNW-1 powder was isolated via filtration over a Büchner funnel and washed with excess N,N-dimethylformamide (DMF) and tetrahydrofurane (THF). The powder was further purified by sequential Soxhlet extraction with methanol, THF and methanol; and dried under vacuum at 393 K overnight. Finally, off-white powders were obtained in 55% yield. Fabrication of SNW-1/PSF Membrane: Spin-coating method was employed in the membrane preparation. Pure PSF dense membranes were made from a PSF-CHCl3 solution. Typically, PSF beads were first degassed at 393 K for 10 h under vacuum to remove adsorbed substances. Then, 0.4 g PSF was dissolved in 4 ml CHCl3 and stirred overnight. For the fabrication of SNW-1/PSF membrane, an additional step of well dispersing SNW-1 nanoparticles in the solvent is required. The typical procedure involves separately dissolving PSF and SNW-1 in CHCl3; subsequently mixing 20% of the required PSF solution with SNW-1 solution under vigorous stirring, followed by ultrasonic treatment for 30 minutes. After ultrasonication, the residual PSF solution was added in the mixture above with additional stirring for 1 day in order to obtain a homogeneous solution. For all membranes with different SNW-1 contents, the solvent percentages of solvent to SNW-1/PSF (88 to 12 wt%) in the mixture were identical, allowing good viscosity. Subsequently, as-prepared SNW-1/PSF solutions were spin-coating on macroporous glass frits or casted on flat glass plates. After naturally drying, the membranes were subjected to a further thermal treatment at 393 K under vacuum in order to remove the residual solvent. Characterizations: Solid-state 13C CP/MAS NMR measurement was carried out on a Bruker Avance III model 400 MHz NMR spectrometer at a MAS rate of 5 kHz. 13C CP MAS chemical shifts are referenced to the resonances of adamantane (C10H16) standard (dCH2 = 38.4). The 15N {1H} CP MAS spectrum was recorded on a Bruker Avance III 400 WB spectrometer equipped with a 4 mm standard bore CP MAS probe head whose X channel was tuned to 40.55 MHz for 15N and the other channel was tuned to 400.18 MHz for broad band 1H decoupling, using a magnetic field of 9.39 T at 297 K. The dried and finely powdered sample was packed in the ZrO2 rotor closed with Kel-F cap which was spun at 5 kHz rate. The experiment was conducted at a contact time of 2 ms. A total of 10000 scans were recorded with 3 s recycle delay for each sample. All 15N CP MAS chemical shifts are referenced to the resonances of α-glycine (C2H2NO2) standard (d = 31.0). Thermogravimetric analysis (TG) was carried out on a Netzch Sta 449c thermal analyzer with a heating rate of 10 K min−1 under air atmosphere. Mechanical properties of membranes were carried out at
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Adv. Mater. 2014, DOI: 10.1002/adma.201400020
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[3]
[4]
[5] [6] [7]
[8] [9] [10]
Supporting Information Additional figures and tables are provided.
[11]
Acknowledgements We are grateful to the financial supports from National Basic Research Program of China (973 Program, grant nos. 2012CB821700), Major International (Regional) Joint Research Project of NSFC (grant nos. 21120102034), and NSFC (grant nos. 20831002).
[12]
[13] Received: January 2, 2014 Published online:
[1] a) M. Yu, R. D. Noble, J. L. Falconer, Acc. Chem. Res. 2011, 44, 1196; b) J. Caro, M. Noack, Microporous Mesoporous Mater. 2008, 115, 215; c) M. Shah, M. C. McCarthy, S. Sachdeva, A. K. Lee, H. K. Jeong, Ind. Eng. Chem. Res. 2012, 51, 2179. [2] a) X. Q. Zou, H. Ren, G. S. Zhu, Chem. Commun. 2013, 49, 3925; b) A. Thomas, Angew. Chem. Int. Ed. 2010, 49, 8328; c) N. B. McKeown, P. M. Budd, Macromolecules 2010, 43, 5163; d) A. I. Cooper, Adv. Mater. 2009, 21, 1291; e) P. Kaur, J. T. Hupp, S. T. Nguyen, ACS Catal. 2011, 1, 819; f) A. P. Côté, A. I. Benin, N. W. Ockwig, M. O’Keeffe, A. J. Matzger, O. M. Yaghi, Science 2005, 310, 1166; g) F. Vilela, K. Zhang, M. Antonietti, Energy
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[14] [15] [16]
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room temperature on a test machine (SHIMADZU AG-1). Membranes (5 mm wide and 5 cm long) were fixed into the testing station and subjected to an extension rate of 2 mm min−1. Each membrane was tested several times to get an average report value. SEM images were obtained using a JEOL JSM-6700F scanning electron microscope, and all the samples were prepared via the cryofracturing method. The FT-IR spectra were collected on a Bruker IF66F/V Fourier transform infrared spectrometer with KBr pellets in the range of 4000-400 cm−1. Elemental analyses (C, H, N) were carried out on a Perkin-Elmer 240 analyzer. The surface area, micropore volume, and pore size of SNW-1 are calculated from N2 adsorption isotherms using BET, t-plot, and NL-DFT methods respectively. Prior to the measurement, the sample was degassed at 393 K for 24 h; and N2 sorption isotherms were subsequently recorded at 77 K on a Micromeritics ASAP 2010M analyzer. Adsorption isotherms of N2, CO2 and CH4 were also measured at 273 and 298 K. Gas Separations: Prior to gas separation, membranes were dehydrated in a vacuum oven at 393 K and 10−5 Pa overnight to remove adsorbed water. Then, SNW-1/PSF membrane was fixed in a membrane chamber and sealed with xylene resistant fluorocarbon O-rings and connected to a gas flow system.[16] Binary gas mixtures of CO2/CH4 and CO2/N2 were introduced into the membrane module in the feed side, and permeate was carried by argon sweep gas. The feed and permeate gases were analyzed by on-line gas chromatography (GC-450, BRUKER).
Environ. Sci. 2012, 5, 7819; h) Q. Chen, M. Luo, P. Hammershøj, D. Zhou, Y. Han, B. W. Laursen, C. G. Yan, B. H. Han, J. Am. Chem. Soc. 2012, 134, 6084; i) Y. Luo, B. Li, W. Wang, K. Wu, B. Tan, Adv. Mater. 2012, 24, 5703. a) R. Babarao, S. Dai, D. Jiang, Langmuir 2011, 27, 3451; b) N. Du, M. M. D. Cin, I. Pinnau, A. Nicalek, G. P. Robertson, M. D. Guiver, Macromol. Rapid Commun. 2011, 32, 631; c) X. Zhu, C. Tian, S. M. Mahurin, S. H. Chai, C. Wang, S. Brown, G. M. Veith, H. Luo, H. Liu, S. Dai, J. Am. Chem. Soc. 2012, 134, 10478; d) M. Carta, R. M. Evans, M. Croad, Y. Rogan, J. C. Jansen, P. Bernardo, F. Bazzarelli, N. B. McKeown, Science 2013, 339, 303. a) Y. S. Bae, R. Q. Snurr, Angew. Chem. Int. Ed. 2011, 50, 11586; b) Q. Wang, J. Luo, Z. Zhong, A. Borgna, Energy Environ. Sci. 2011, 4, 42. M. G. Schwab, B. Fassbender, H. W. Spiess, A. Thomas, X. Feng, K. Müllen, J. Am. Chem. Soc. 2009, 131, 7216. D. M. D’Alessandro, B. Smit, J. R. Long, Angew. Chem. Int. Ed. 2010, 49, 6058. a) D. Farrusseng, C. Daniel, C. Gaudillère, U. Ravon, Y. Schuurman, C. Mirodatos, D. Dubbeldam, H. Frost, R. Q. Snurr, Langmuir 2009, 25, 7383; b) R. C. Lochan, M. H. Gordon, Phys. Chem. Chem. Phys. 2006, 8, 1357. a) A. L. Mayers, J. M. Prausnitz, AIChE J. 1965, 11, 121; b) B. Liu, B. Smit, J. Phys. Chem. C 2010, 114, 8515. K. Mosher, Y. Lin, J. Wilcox, M. S. Thesis, Stanford University 2011. M. J. C. Ordonez, K. J. Balkus, J. P. Ferraris, I. H. Musselman, J. Membr. Sci. 2010, 361, 28. a) E. G. Pérez, J. B. Parra, C. O. Ania, A. G. Sánchez, J. M. van Baten, R. Krishna, D. Dubbeldam, S. Calero, Adsorption 2007, 13, 469; b) X. Zou, F. Zhang, S. Thomas, G. Zhu, V. Valtchev, S. Mintova, Chem. Eur. J. 2011, 17, 12076. a) S. Li, J. L. Falconer, R. D. Noble, Adv. Mater. 2006, 18, 2601; b) Y. S. Li, F. Y. Liang, H. Bux, A. Feldhoff, W. S. Yang, J. Caro, Angew. Chem. Int. Ed. 2010, 49, 548; c) S. Aguado, C. H. Nicolas, V. M. Baslé, C. Nieto, H. Amrouche, N. Bats, N. Audebrand, D. Farrusseng, New J. Chem. 2011, 35, 41. a) B. Zornoza, A. M. Joaristi, P. S. Crespo, C. Tellez, J. Coronas, J. Gascon, F. Kapteijn, Chem. Commun. 2011, 47, 9522; b) B. Zornoza, B. Seoane, J. M. Zamaro, C. Téllez, J. Coronas, ChemPhysChem 2011, 12, 2781; c) H. B. T. Jeazet, C. Staudt, C. Janiak, Chem. Commun. 2012, 48, 2140; d) B. Zornoza, S. Irusta, C. Téllez, J. Coronas, Langmuir 2009, 25, 5903; e) J. Ahn, W. J. Chung, I. Pinnau, M. D. Guiver, J. Membr. Sci. 2008, 314, 123; f) S. Kim, L. Chen, J. K. Johnson, E. Marand, J. Membr. Sci. 2007, 294, 147; g) B. Zornoza, O. Esekhile, W. J. Koros, C. Téllez, J. Coronas, Sep. Purif. Technol. 2011, 77, 137. Y. Li, F. Liang, H. Bux, W. Yang, J. Caro, J. Membr. Sci. 2010, 354, 48. H. Bux, C. Chmelik, J. M. Baten, R. Krishna, J. Caro, Adv. Mater. 2010, 22, 4741. a) X. Yin, G. Zhu, W. Yang, Y. Li, G. Zhu, R. Xu, J. Sun, S. Qiu, R. Xu, Adv. Mater. 2005, 17, 2006; b) X. Q. Zou, P. Bazin, F. Zhang, G. S. Zhu, V. Valtchev, S. Mintova, ChemPlusChem 2012, 77, 437.
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