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Amine-functionalized [email protected] as filler in mixed matrix membrane for selective CO2 separation
Separation and Purification Technology 213 (2019) 63–69
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Amine-functionalized MOFs@GO as filler in mixed matrix membrane for selective CO2 separation Mingmin Jia, Yi Feng, Jianhao Qiu, Xiong-Fei Zhang, Jianfeng Yao
T
⁎
College of Chemical Engineering, Jiangsu Key Lab for the Chemistry & Utilization of Agricultural and Forest Biomass, Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China
A R T I C LE I N FO
A B S T R A C T
Keywords: UiO-66-NH2 Graphene oxide Amine functionalized Mixed matrix membrane CO2 separation
UiO-66-NH2@GO nanoparticles were synthesized by growing UiO-66-NH2 on graphene oxide (GO) nanosheets. The obtained nanoparticles were incorporated in polyimide (PI) matrix to improve the CO2 separation performance. The crystalline structure, interaction between UiO-66-NH2 and GO and the dispersion of UiO-66NH2@GO were characterized by various techniques. Owning to the interaction between UiO-66-NH2 and GO, UiO-66-NH2 nanocrystals were well-anchored on GO sheets and the size was tailored. Also, it can be found that the high-aspect GO nanosheets in polymer matrix improved the dispersion of UiO-66-NH2 nanoparticles. The high porosity of UiO-66-NH2@GO and inherent adsorption property to CO2 significantly enhanced the gas separation performance of composite membranes compared to pristine PI membrane. The developed UN@GO-PI mixed matrix membrane with 5 wt% of UiO-66-NH2@GO loading showed excellent and stable CO2/N2 separation performance with the CO2 permeability of 7.28 Barrer and the CO2/N2 selectivity of 52.
1. Introduction Membrane based on gas separation technology has been considered to be a prospective alternative to traditional technologies such as absorption, cryogenic purification and adsorption [1]. Polymer-based membranes always suffer from the trade-off between permeability and selectivity, while inorganic membranes are limited by their frangible and high cost [2,3]. Incorporating nanofillers such as zeolites [4], activated carbons [5], silicas [6], and new classes of porous materials [7,8] into polymer membranes often leads to improvements in gas permeability without sacrifice of selectivity. Furthermore, gas selectivity could be improved simultaneously compared to the pure polymer membranes in some cases [9]. Recently, graphene oxide (GO) has attracted increased attention as a kind of nanofiller in mixed matrix membranes (MMMs), owing to its unique atomic-thick structures with high thermal and mechanical properties, easy surface functionalization, and high aspect ratio (> 1000) [10]. Generally, with high-aspect ratio GO in MMMs, membranes always showed enhanced selectivity and reduced permeability, resulting from inherent adsorption to specific gas molecules and tortuous path of GO for gas diffusion. To overcome this limitation, many approaches were tried such as increasing the oxidation degree [11], organic group functionalization [12–16] and decoration with polymers [10,17–20] to improve adsorption property of CO2 and extend the ⁎
interlayer channels of GO. Apart from above methods, an alternative is to decorate porous nanoparticles on GO nanosheets to enhance gas separation performance of MMMs [21]. In those composite membranes, nanoparticles were well-dispersed by strong steric effect of GO, which also improves the compatibility between nanofillers and polymer matrix [22]. Metal organic frameworks (MOFs) with high porosity, strong affinity toward certain gas molecules, and good thermal stability are emerging as promising candidates for membrane separation [23,24]. Naturedly, MOFs@GO became an excellent candidate as nanofillers in MMMs. Dong et al. synthesized ZIF-8@GO and incorporated it in Pebax matrix. The barrier effect of GO and microporosity of MOF dominated the separation properties of MMMs, in which CO2 permeability and CO2/N2 selectivity were increased by 191% and 174% compared to pure Pebax membrane, respectively. Besides, Castarlenas et al. [25] synthesized UiO-66/GO by direct solvothermal synthesis and incorporated it in glassy polymers polyimide to obtain MMMs for gas separation, where enhanced CO2/CH4 separation performance with a CO2/CH4 selectivity of 51 was obtained. In fact, most literature of MOF@GO-based MMMs focused on the synergistic effect of MOF and GO on improvement of CO2/N2 separation, lacking the study of enhancing the instinct adsorption property of nanofillers. As reported before, amine-functionalization of MOF crystals is an effective strategy for enhancing the CO2 affinity, which offers great potential for fabricating CO2 separation
Corresponding author. E-mail address: [email protected] (J. Yao).
https://doi.org/10.1016/j.seppur.2018.12.029 Received 8 November 2018; Received in revised form 12 December 2018; Accepted 12 December 2018 Available online 13 December 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
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UiO-66-NH2
GO
GO GO
Graphene oxide (GO)
UiO-66-NH2@GO
Fig. 1. The fabrication of UiO-66-NH2 on GO nanosheets to form UiO-66-NH2@GO.
methanol, and then dried at 60 °C overnight. As a comparison, UiO66@GO was fabricated by the similar procedure as above.
MMMs [26–28]. Besides, polyimide (PI) was selected as polymer matrix owning to their high gas selectivity and high chemical, thermal, and mechanical resistance [29]. In this work, we reported a new MMM by using UiO-66-NH2@GO as the nanofiller, in which UiO-66-NH2 nanocrystals were in-situ grown on the surface of GO nanosheets. As shown in Fig. 1, by combining the interaction of organic functional groups and electrostatic interaction between UiO-66-NH2 and GO, the UiO-66-NH2 nanoparticles were wellanchored on GO nanosheets. Meanwhile, the microstructures and fillers-polymer interaction of membranes were systematically characterized and studied. Gas separation results showed that both permeability and selectivity of CO2/N2 improved as compared to the pristine Matrimid membrane.
2.3. Preparation of UiO-66-NH2@GO composites membrane 0.013–0.26 g of UiO-66-NH2@GO powders were dispersed in 10 g of NMP, followed by sonicating for 30 min and stirring for 1 h. Then, 1.3 g of PI polymer was added into the above dispersion solution containing UiO-66-NH2@GO nanoparticles. The mixed solution was stirred until the full dissolution of the polymer. Prior to casting, the dope solution was kept for 24 h to remove any bubbles dissolved. The as-prepared solution was cast on a clean glass plate using a casting knife with an air gap of 150 μm. The just-cast film was left at 50 °C for 12 h to allow some solvent evaporation, followed by drying at 80 °C for 8 h and 180 °C for 3 h for further solvent evaporation. The as-synthesized membranes were denoted as UN@GO-PI-X, and X (=1, 5, 10 and 20) refers to the mass fraction of UiO-66-NH2@GO in the polymeric matrix. As a comparison, pristine PI membrane, GO-PI-1 (1 wt% of GO in PI), U-NH2-PI5 (5 wt% of UiO-66-NH2 in PI) and U@GO-PI-5 (5 wt% of UiO-66@GO in PI) were fabricated by the similar procedure as above.
2. Materials and methods 2.1. Chemicals Matrimid® 5218 polyimide (PI) was purchased from Alfa Aesar. Zirconium chloride (ZrCl4, 98%) was supplied by Aladdin Industrial Company, China. 2-aminoterephthalic acid (ATA), terephthalic acid (PTA), N,N-dimethyl formamide (DMF), acetic acid (HAc), N-Methyl pyrrolidone (NMP) and methanol were purchased from Sinopharm Chemical Reagent Company. Nature graphite powder (3.85 μm average particle size) was purchased from Qingdao Jinrilai Graphite Company, China. Potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrochloric acid (32 wt%, HCl), sulfuric acid (H2SO4, 98 wt%) and hydrogen peroxide aqueous solution (H2O2, 30 wt%) were purchased from National Pharmaceutical Group Chemical Reagent Co. Ltd., China. All of the materials were used without further purification.
2.4. Characterization X-Ray Diffraction (XRD) was used to analyze the crystalline structure of the samples by using Rigaku Ultima IV with Cu Kα radiation (λ = 0.1542 nm) at 40 kV and 10 mA. Nitrogen adsorption-desorption isotherms were measured at 77 K on an automatic volumetric adsorption apparatus (ASAP 2020). Morphologies of MOFs@GO nanoparticles and membranes were obtained utilizing a JSM-7600F (JEOL Ltd., Japan) scanning electron microscope (SEM) with an operating voltage of 5 kV. Fourier transform infrared (FTIR, Thermo Electron Nicolet-360, USA) was used to analyze the functional groups of UiO-66-NH2, GO and the interaction between UiO-66-NH2@GO and polymers using the KBr wafer technique. Thermal stability of UiO-66-NH2 and UiO-66NH2@GO was investigated by thermogravimetric Analyzer (TGA Q5000-IR, TA Instruments) from 25 to 800 °C with a heating rate of 5 °C/min under N2 atmosphere.
2.2. Preparation of UiO-66-NH2@GO GO was prepared by the Hummers method, and the lateral size of laminar GO nanosheets is 2–5 μm with a thickness of 6–15 nm [30,31]. UiO-66-NH2 was prepared according to our previously reported reference [31]. For UiO-66-NH2@GO, 0.017 g of dry GO powder and 0.191 g of ZrCl4 were added in 81.7 ml of DMF. The mixed solution was sonicated for 30 min and stirred for 1 h for three times. 0.148 g of ATA and 4.83 g of HAc were dissolved in the above solution containing GO and ZrCl4. The solution was then transferred into a 150 ml Teflon-lined stainless steel autoclave and heated at 120 °C for 24 h. The resulting UiO-66-NH2@GO powder was collected and washed with DMF and
2.5. Gas separation experiments Both single-gas and mixed-gas permeation experiments were conducted by Wicke-Kallenbach technique [32]. A flat-sheet permeation cell with an effective area of 2.84 cm2 was used for all tests at room 64
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Fig. 3a presents XRD patterns of UiO-66-NH2, UiO-66@GO and UiO66-NH2@GO composite. The XRD pattern of UiO-66-NH2 is in agreement with those reported elsewhere [35]. The similar diffraction pattern of the composites to UiO-66-NH2 indicated that GO did not disturb the crystallization of UiO-66 and UiO-66-NH2 [21,36]. It should be noted that the peak of GO is not observed in the composite, which should result from the high dispersion of GO in UiO-66@GO and UiO66-NH2@GO [37]. In order to analyze the interaction between GO and UiO-66-NH2, FTIR spectra were conducted (Fig. 3b). For GO, the peaks at 1731, 1624, 1385 and 1092 cm−1 correspond to the stretching vibrations of the oxygen-containing functional groups in GO [37]. For UiO-66-NH2, the absorption peaks at 1657 and 1625 cm−1 prove the existence of –NH2 [31]. It should be noted that the peaks of oxygencontaining functional groups in UiO-66-NH2@GO disappear. One reason should be that the content of GO in the composites was relatively low. Another might be the interaction between oxygen-containing groups and Zr4+ and –NH2 of UiO-66-NH2. The absorption peak at 1625 cm−1 corresponding to the stretching vibrations of N-C]O was enhanced in a degree, resulting from the vibration of non-conjugated carboxylic and amidogen. This phenomenon also confirmed the successful growth of UiO-66-NH2 on the GO sheets [8,38]. As shown in Fig. 3c, BET surface area of UiO-66-NH2@GO (1004 m2/g) is higher than pristine UiO-66-NH2 (736 m2/g), which would show enhanced adsorption capacity to CO2[33,39]. This improvement can be explained that new pores were generated on the interface between graphene oxide layers and MOF ‘blocks’ [37]. The stability of UiO-66-NH2 and UiO-66NH2@GO was further studied by Thermogravimetric analysis (TGA) (Fig. 3d). At the very beginning, UiO-66-NH2 underwent an obvious weight loss below 300 °C because of the removal of physically adsorbed water, guest water and other physical adsorbed solvent (residual DMF in the pores). The observed weight loss at high temperatures (350–600 °C) corresponds to the structure collapse of UiO-66-NH2 [8]. The degradation temperature of UiO-66-NH2 is about 400–500 °C, while that of UiO-66-NH2@GO is 500–580 °C. This phenomenon demonstrates that the composites have better thermal stability than the parent materials UiO-66-NH2 [37].
temperature (25 °C) and 0.3–0.45 MPa. The compositions of the steadystate feed, retentate, and permeate were all tested by gas chromatography (Agilent 7890) equipped with a thermal conductive detector (TCD). Prior to the test, residual gas present in membranes and pipeline was removed by vacuum pump. After system reached steady-state, all gas permeation measurements were performed more than five times. The gas permeability can be calculated using the following equation:
Pi =
LNi AΔP
(1)
where L is the thickness of membrane (cm), Ni is the volume permeate rate of gas (cm3/s) at standard temperature and pressure (STP), A (cm2) is the test area of membrane, and ΔP is the transmembrane pressure (cmHg). The unit of permeability (Pi) is commonly expressed as Barrer (1 Barrer = 10−10 cm3 (STP) cm/(cm2 s cmHg)). The ideal selectivity (separation factor) can be calculated by the following equation:
S(i/j) =
Pi Pj
(2)
3. Results and discussion 3.1. Physicochemical properties of UiO-66-NH2@GO The morphologies of UiO-66-NH2 and the UiO-66-NH2@GO composite were investigated by SEM. As shown in Fig. 2a, UiO-66-NH2 exhibits an octahedral morphology. While, for UiO-66-NH2@GO (Fig. 2b), UiO-66-NH2 nanoparticles were well-anchored on GO layers rather than physically mixed with GO. As a result, the GO nanosheets were well-dispersed in UiO-66-NH2 and covered by UiO-66-NH2. This favorable structure of the UiO-66-NH2@GO composite can effectively avoid the layer stacking of GO [33]. Besides, the particle size of the UiO-66-NH2 on GO nanosheets was reduced to a certain extent as shown in Fig. 2c and d. The reason might be related to the additional space constraints caused by GO during the synthesis of the composites [34].
Fig. 2. SEM images (a, b) and particle size distribution (c, d) of UiO-66-NH2 (a, c) and UiO-66-NH2@GO (b, d). 65
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Fig. 3. XRD patterns (a) and FTIR spectra (b) of GO, UiO-66-NH2, UiO-66@GO and UiO-66-NH2@GO, N2 adsorption-desorption isotherms (c) and TGA curves (d) of UiO-66-NH2 and UiO-66-NH2@GO.
increasing the filler to 20 wt%, the agglomeration of UiO-66-NH2@GO particles can clearly be found (Fig. 4h), possibly owing to the gravity force and the quite different physical property between UiO-66NH2@GO and PI matrix [40]. To investigate the influence of fillers on the arrangement of polymer chains, the crystalline structure of MMMs was studied by XRD (Fig. 5a). Pure PI membrane shows a strong and broad peak at 15.4° due to its amorphous property [41]. For U-NH2-PI-5 MMMs, the peak position of PI almost unchanged. While, after incorporating GO nanosheets in PI matrix, the peak of PI obviously shifted to lower degree (∼14.6°), suggesting enhanced hydrogen bonding between GO nanosheets and PI molecules [11,22]. Meanwhile, by incorporating UiO-66-NH2@GO, the peak of PI also shifted to a lower degree for UiO-66-NH2@GO based MMMs. The characteristic peaks belonging to UiO-66-NH2 and UiO-66NH2@GO could obviously be found in MMMs, indicating that the
3.2. Membrane characterization The cross-section morphologies of resultant membranes were characterized by SEM (Fig. 4). Pure PI membrane shows a homogeneous structure (Fig. 4a). For GO-PI-1 as shown in Fig. 4b, GO nanosheets were well-compatible with PI matrix without any noticeable defects that is similar as reported in literature [11]. For U-NH2-PI-5 (Fig. 4c), it is difficult to achieve well dispersion of filler in polymer matrix and slight aggregation appeared. However, UiO-66@GO and UiO-66NH2@GO nanoparticles homogeneously distributed in the PI matrix with the filler contents of 1 and 5 wt% (Fig. 4d–f). Compared to the UNH2-PI-5 with 5 wt% of UiO-66-NH2, it can be clearly seen that incorporation of GO with strong steric effect could obviously improve the interface compatibility between fillers and the PI matrix. UN@GO-PI-10 also shows a relatively homogeneous layer (Fig. 4g). By further
Fig. 4. SEM images of the cross-section and surface (inset) morphology of pure PI (a), GO-PI-1 (b), U-NH2-PI-5 (c), U@GO-PI-5 (d) UN@GO-PI-1 (e), UN@GO-PI-5 (f), UN@GO-PI-10 (g) and UN@GO-PI-20 (h), the scale bar represents 1 μm in insets. 66
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Fig. 5. XRD patterns (a) and FTIR spectra (b) of pristine PI, GO-PI-1, U-NH2-PI-5, U@GO-PI-5, UN@GO-PI-1, UN@GO-PI-5, UN@GO-PI-10 and UN@GO-PI-20 mixed matrix membranes.
crystalline structure of fillers was not destroyed in membranes preparation procedure. Besides, with increase of UiO-66-NH2@GO loading from 1 to 20 wt%, the corresponding characteristic peaks of UiO-66NH2@GO gradually increase. In order to further characterize and analyze the interaction between inorganic fillers and polymer chains, FTIR spectra were conducted (Fig. 5b). For pristine PI membrane, the absorption peaks at 1785 and 1727 cm−1 are attributed to the asymmetric and symmetric stretching of C]O groups in imides group. The band at 1378 cm−1 corresponds to CeN stretching in the imides group [42,43]. For U-NH2-PI-5, no obvious absorption peak emerged, resulting from that the characteristic peaks of –NH2 in UiO-66-NH2 were covered by Matrimid’s peaks [28]. For GO-PI-1, the characteristic peak at 1092 cm−1 corresponds to the stretching vibrations of the oxygencontaining functional groups of GO, confirming existence of GO in PI matrix. The intensity of such peak was enhanced with increasing UiO66-NH2@GO loading in UiO-66-NH2@GO-based MMMs. Meanwhile, it should be noticed that the peak at 1636 cm−1 assigned to NeC]O group was enhanced with increasing UiO-66-NH2@GO loading, also indicating more GO interacted with PI [8,41].
than pristine PI membrane was ascribed to the inherent adsorption of GO nanosheets and the reduced mobility of polymer chains in the presence of GO nanosheets [7]. While, the UiO-66@GO MMM (U@GOPI-5) showed little enhanced permeability and selectivity compared to pure PI membrane, resulting from high porosity of UiO-66 and welldispersion of fillers in polymer matrix. MMMs with the incorporation of 5 wt% UiO-66-NH2 (U-NH2-PI-5) exhibited higher permeability but lower selectivity (CO2 permeability of 8.45 Barrer and CO2/N2 ideal selectivity of 13). The increased permeability was mainly attributed to the existence of UiO-66-NH2 with a large pore size (0.6 nm) [28], but some agglomeration of UiO-66-NH2 formed in polymer matrix reduced the selectivity. Gas separation performance was enhanced with the well-dispersed nanofillers. The inherent adsorption of both UiO-66-NH2 and GO to CO2 would improve the permeability of CO2, and the tortuous channels supplied by GO restrict the transport of gas molecule with larger sizes, resulting in enhanced CO2/N2 selectivity. However, too many UiO-66-NH2@GO nanoparticles in polymer matrix (e.g. UN@GO-PI-10 and UN@GO-PI-20) lead to more membrane defects and give rise to the low CO2/N2 selectivity. In order to study the intrinsic adsorption property to specific gas molecules of UN@GO-PI MMMs, the single gas and mixed gas separation results at different feeding pressure (0.3–0.45 MPa) on the UN@GO-PI-5 MMMs were conducted (Fig. 6). For single gas separation (Fig. 6a), the permeability of CO2 and N2 decreased slightly with the increase of feed pressure, which is the actual behavior for glassy polymer [45]. It confirms that CO2 and N2 do not plasticize the polymer matrix and thus the hydrostatic effect played a dominated role [46]. For mixed gas separation (Fig. 6b), the reduced permeability of CO2 was mainly attributed to a part of N2 molecules occupied the transport channels. Meanwhile, as the pressure increasing, the permeability of CO2 enhanced a little because more adsorption potential occupied by CO2 molecules. With the increase of CO2 and N2 pressure, the gas separation of UN@GO-PI-5 MMMs reached a good balance. The above results prove that the as-prepared UN@GO-PI-5 MMMs showed a good structural stability with the pressure increase. In order to further understand the gas-transport through UN@GOPI-5 MMMs, the influence of permeating temperature (25–65 °C) on gas permeation was also investigated. As shown in Table 2, the permeability of CO2 and N2 increased with the permeating temperature, attributed to the enhancement of gas diffusion [46]. While, CO2 permeability is less sensitive to the temperature variation, resulting from the inherent adsorption property to CO2 [47]. The stability of UN@GO-PI-5 MMM was also examined by immersing in water at 25 °C for 24–72 h (Table 3), followed by the vacuum drying at 100 °C for 24 h. It can be seen from Table 2 that UN@GO-PI-5 MMM maintained stable gas separation performance even soaked in water for 72 h, also supporting its high stability. Combined with the satisfied separation performances, UN@GO-PI MMMs are very
3.3. Gas separation performance The gas separation performance of UiO-66-NH2@GO MMMs was strengthened with the interaction between UiO-66-NH2 and GO, and the enhanced adsorption ability of fillers to CO2 [44]. As shown in Table 1, the MMMs incorporated with UiO-66-NH2@GO exhibited enhanced permeability and selectivity. Clearly, the permeability of CO2 and CO2/N2 selectivity for MMMs with 5 wt% UiO-66-NH2@GO reached the maximum values (CO2 permeability of 7.28 Barrer and CO2/N2 ideal selectivity of 52.0). Pure PI membrane only shows a CO2 permeability of 2.28 Barrer with a CO2/N2 ideal selectivity of 28.9. MMM doped with GO nanosheets (GO-PI-1) exhibited lower permeability but higher selectivity compared to other MMMs because there is no pore on GO sheets, and the gas molecules can only permeate through the edges of GO sheets. The higher permeability of CO2 (3.15 Barrer) Table 1 Gas permeability and selectivity on pristine PI and mixed-matrix membranes at 25 °C and 3 bar. Sample
PCO2 (Barrer)
PN2 (Barrer)
αCO2/N2
Pure PI GO-PI-1 U@GO-PI-5 U-NH2-PI-5 UN@GO-PI-1 UN@GO-PI-5 UN@GO-PI-10 UN@GO-PI-20
2.28 3.15 3.56 8.45 3.12 7.28 7.33 18.1
0.079 0.049 0.10 0.65 0.11 0.14 0.24 9.03
28.9 64.3 35.6 13.0 29.2 52.0 30.5 2.0
67
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Fig. 6. Single (a) and mixed gas separation (b) results of UN@GO-PI-5 at different feeding pressure. Table 2 The separation performance of UN@GO-PI-5 MMMs at different temperatures. Temperature (°C)
PCO2 (Barrer)
PN2 (Barrer)
SCO2/N2
25 45 55 65
7.28 9.12 10.3 11.5
0.14 0.26 0.37 0.48
52.0 35.1 27.8 24.0
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Table 3 The separation performance of UN@GO-PI-5 MMMs after soaking in water for 0–72 h. Time (h)
PCO2 (Barrer)
PN2 (Barrer)
SCO2/N2
0 24 48 72
7.28 8.03 7.08 7.43
0.14 0.14 0.13 0.14
52.0 57.4 54.5 53.1
attractive for CO2/N2 separation in potential industrial application. 4. Conclusion In summary, we reported a high-performance MMMs for efficient separation of CO2/N2 by taking advantage of novel UiO-66-NH2@GO composite filler. Various characterization results confirmed that the UiO-66-NH2@GO fillers exhibited good adhesion with the polymer matrix. The gas separation performance was significantly enhanced compared to pure PI membrane and other composite membranes. UN@GO-PI-5 exhibited a high and stable CO2/N2 separation performance (CO2 permeability: 7.28 Barrer, CO2/N2 selectivity: 52) at 25 °C with the feed pressure of 0.3 MPa, resulting from the highly-dispersed UiO-66-NH2@GO fillers in PI membrane and the instinct adsorption of UiO-66-NH2 and GO to CO2 molecules. Acknowledgement The authors are grateful for the financial supported by Natural Science Key Project of the Jiangsu Higher Education Institutions, China (15KJA220001), Jiangsu Province Six Talent Peaks Project, China (2016-XCL-043), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). YF thanks the financial support of the Natural Science Foundation of Jiangsu Province Youth Fund, China (BK20170919). References [1] J. Yao, H. Wang, Zeolitic imidazolate framework composite membranes and thin films: synthesis and applications, Chem. Soc. Rev. 43 (2014) 4470–4493. [2] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) 390–400. [3] B. Seoane, J. Coronas, I. Gascon, M. Etxeberria Benavides, O. Karvan, J. Caro, F. Kapteijn, J. Gascon, Metal-organic framework based mixed matrix membranes: a
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Amine-functionalized MOFs@GO as filler in mixed matrix membrane for selective CO2 separation Mingmin Jia, Yi Feng, Jianhao Qiu, Xiong-Fei Zhang, Jianfeng Yao
T
⁎
College of Chemical Engineering, Jiangsu Key Lab for the Chemistry & Utilization of Agricultural and Forest Biomass, Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China
A R T I C LE I N FO
A B S T R A C T
Keywords: UiO-66-NH2 Graphene oxide Amine functionalized Mixed matrix membrane CO2 separation
UiO-66-NH2@GO nanoparticles were synthesized by growing UiO-66-NH2 on graphene oxide (GO) nanosheets. The obtained nanoparticles were incorporated in polyimide (PI) matrix to improve the CO2 separation performance. The crystalline structure, interaction between UiO-66-NH2 and GO and the dispersion of UiO-66NH2@GO were characterized by various techniques. Owning to the interaction between UiO-66-NH2 and GO, UiO-66-NH2 nanocrystals were well-anchored on GO sheets and the size was tailored. Also, it can be found that the high-aspect GO nanosheets in polymer matrix improved the dispersion of UiO-66-NH2 nanoparticles. The high porosity of UiO-66-NH2@GO and inherent adsorption property to CO2 significantly enhanced the gas separation performance of composite membranes compared to pristine PI membrane. The developed UN@GO-PI mixed matrix membrane with 5 wt% of UiO-66-NH2@GO loading showed excellent and stable CO2/N2 separation performance with the CO2 permeability of 7.28 Barrer and the CO2/N2 selectivity of 52.
1. Introduction Membrane based on gas separation technology has been considered to be a prospective alternative to traditional technologies such as absorption, cryogenic purification and adsorption [1]. Polymer-based membranes always suffer from the trade-off between permeability and selectivity, while inorganic membranes are limited by their frangible and high cost [2,3]. Incorporating nanofillers such as zeolites [4], activated carbons [5], silicas [6], and new classes of porous materials [7,8] into polymer membranes often leads to improvements in gas permeability without sacrifice of selectivity. Furthermore, gas selectivity could be improved simultaneously compared to the pure polymer membranes in some cases [9]. Recently, graphene oxide (GO) has attracted increased attention as a kind of nanofiller in mixed matrix membranes (MMMs), owing to its unique atomic-thick structures with high thermal and mechanical properties, easy surface functionalization, and high aspect ratio (> 1000) [10]. Generally, with high-aspect ratio GO in MMMs, membranes always showed enhanced selectivity and reduced permeability, resulting from inherent adsorption to specific gas molecules and tortuous path of GO for gas diffusion. To overcome this limitation, many approaches were tried such as increasing the oxidation degree [11], organic group functionalization [12–16] and decoration with polymers [10,17–20] to improve adsorption property of CO2 and extend the ⁎
interlayer channels of GO. Apart from above methods, an alternative is to decorate porous nanoparticles on GO nanosheets to enhance gas separation performance of MMMs [21]. In those composite membranes, nanoparticles were well-dispersed by strong steric effect of GO, which also improves the compatibility between nanofillers and polymer matrix [22]. Metal organic frameworks (MOFs) with high porosity, strong affinity toward certain gas molecules, and good thermal stability are emerging as promising candidates for membrane separation [23,24]. Naturedly, MOFs@GO became an excellent candidate as nanofillers in MMMs. Dong et al. synthesized ZIF-8@GO and incorporated it in Pebax matrix. The barrier effect of GO and microporosity of MOF dominated the separation properties of MMMs, in which CO2 permeability and CO2/N2 selectivity were increased by 191% and 174% compared to pure Pebax membrane, respectively. Besides, Castarlenas et al. [25] synthesized UiO-66/GO by direct solvothermal synthesis and incorporated it in glassy polymers polyimide to obtain MMMs for gas separation, where enhanced CO2/CH4 separation performance with a CO2/CH4 selectivity of 51 was obtained. In fact, most literature of MOF@GO-based MMMs focused on the synergistic effect of MOF and GO on improvement of CO2/N2 separation, lacking the study of enhancing the instinct adsorption property of nanofillers. As reported before, amine-functionalization of MOF crystals is an effective strategy for enhancing the CO2 affinity, which offers great potential for fabricating CO2 separation
Corresponding author. E-mail address: [email protected] (J. Yao).
https://doi.org/10.1016/j.seppur.2018.12.029 Received 8 November 2018; Received in revised form 12 December 2018; Accepted 12 December 2018 Available online 13 December 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
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UiO-66-NH2
GO
GO GO
Graphene oxide (GO)
UiO-66-NH2@GO
Fig. 1. The fabrication of UiO-66-NH2 on GO nanosheets to form UiO-66-NH2@GO.
methanol, and then dried at 60 °C overnight. As a comparison, UiO66@GO was fabricated by the similar procedure as above.
MMMs [26–28]. Besides, polyimide (PI) was selected as polymer matrix owning to their high gas selectivity and high chemical, thermal, and mechanical resistance [29]. In this work, we reported a new MMM by using UiO-66-NH2@GO as the nanofiller, in which UiO-66-NH2 nanocrystals were in-situ grown on the surface of GO nanosheets. As shown in Fig. 1, by combining the interaction of organic functional groups and electrostatic interaction between UiO-66-NH2 and GO, the UiO-66-NH2 nanoparticles were wellanchored on GO nanosheets. Meanwhile, the microstructures and fillers-polymer interaction of membranes were systematically characterized and studied. Gas separation results showed that both permeability and selectivity of CO2/N2 improved as compared to the pristine Matrimid membrane.
2.3. Preparation of UiO-66-NH2@GO composites membrane 0.013–0.26 g of UiO-66-NH2@GO powders were dispersed in 10 g of NMP, followed by sonicating for 30 min and stirring for 1 h. Then, 1.3 g of PI polymer was added into the above dispersion solution containing UiO-66-NH2@GO nanoparticles. The mixed solution was stirred until the full dissolution of the polymer. Prior to casting, the dope solution was kept for 24 h to remove any bubbles dissolved. The as-prepared solution was cast on a clean glass plate using a casting knife with an air gap of 150 μm. The just-cast film was left at 50 °C for 12 h to allow some solvent evaporation, followed by drying at 80 °C for 8 h and 180 °C for 3 h for further solvent evaporation. The as-synthesized membranes were denoted as UN@GO-PI-X, and X (=1, 5, 10 and 20) refers to the mass fraction of UiO-66-NH2@GO in the polymeric matrix. As a comparison, pristine PI membrane, GO-PI-1 (1 wt% of GO in PI), U-NH2-PI5 (5 wt% of UiO-66-NH2 in PI) and U@GO-PI-5 (5 wt% of UiO-66@GO in PI) were fabricated by the similar procedure as above.
2. Materials and methods 2.1. Chemicals Matrimid® 5218 polyimide (PI) was purchased from Alfa Aesar. Zirconium chloride (ZrCl4, 98%) was supplied by Aladdin Industrial Company, China. 2-aminoterephthalic acid (ATA), terephthalic acid (PTA), N,N-dimethyl formamide (DMF), acetic acid (HAc), N-Methyl pyrrolidone (NMP) and methanol were purchased from Sinopharm Chemical Reagent Company. Nature graphite powder (3.85 μm average particle size) was purchased from Qingdao Jinrilai Graphite Company, China. Potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrochloric acid (32 wt%, HCl), sulfuric acid (H2SO4, 98 wt%) and hydrogen peroxide aqueous solution (H2O2, 30 wt%) were purchased from National Pharmaceutical Group Chemical Reagent Co. Ltd., China. All of the materials were used without further purification.
2.4. Characterization X-Ray Diffraction (XRD) was used to analyze the crystalline structure of the samples by using Rigaku Ultima IV with Cu Kα radiation (λ = 0.1542 nm) at 40 kV and 10 mA. Nitrogen adsorption-desorption isotherms were measured at 77 K on an automatic volumetric adsorption apparatus (ASAP 2020). Morphologies of MOFs@GO nanoparticles and membranes were obtained utilizing a JSM-7600F (JEOL Ltd., Japan) scanning electron microscope (SEM) with an operating voltage of 5 kV. Fourier transform infrared (FTIR, Thermo Electron Nicolet-360, USA) was used to analyze the functional groups of UiO-66-NH2, GO and the interaction between UiO-66-NH2@GO and polymers using the KBr wafer technique. Thermal stability of UiO-66-NH2 and UiO-66NH2@GO was investigated by thermogravimetric Analyzer (TGA Q5000-IR, TA Instruments) from 25 to 800 °C with a heating rate of 5 °C/min under N2 atmosphere.
2.2. Preparation of UiO-66-NH2@GO GO was prepared by the Hummers method, and the lateral size of laminar GO nanosheets is 2–5 μm with a thickness of 6–15 nm [30,31]. UiO-66-NH2 was prepared according to our previously reported reference [31]. For UiO-66-NH2@GO, 0.017 g of dry GO powder and 0.191 g of ZrCl4 were added in 81.7 ml of DMF. The mixed solution was sonicated for 30 min and stirred for 1 h for three times. 0.148 g of ATA and 4.83 g of HAc were dissolved in the above solution containing GO and ZrCl4. The solution was then transferred into a 150 ml Teflon-lined stainless steel autoclave and heated at 120 °C for 24 h. The resulting UiO-66-NH2@GO powder was collected and washed with DMF and
2.5. Gas separation experiments Both single-gas and mixed-gas permeation experiments were conducted by Wicke-Kallenbach technique [32]. A flat-sheet permeation cell with an effective area of 2.84 cm2 was used for all tests at room 64
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Fig. 3a presents XRD patterns of UiO-66-NH2, UiO-66@GO and UiO66-NH2@GO composite. The XRD pattern of UiO-66-NH2 is in agreement with those reported elsewhere [35]. The similar diffraction pattern of the composites to UiO-66-NH2 indicated that GO did not disturb the crystallization of UiO-66 and UiO-66-NH2 [21,36]. It should be noted that the peak of GO is not observed in the composite, which should result from the high dispersion of GO in UiO-66@GO and UiO66-NH2@GO [37]. In order to analyze the interaction between GO and UiO-66-NH2, FTIR spectra were conducted (Fig. 3b). For GO, the peaks at 1731, 1624, 1385 and 1092 cm−1 correspond to the stretching vibrations of the oxygen-containing functional groups in GO [37]. For UiO-66-NH2, the absorption peaks at 1657 and 1625 cm−1 prove the existence of –NH2 [31]. It should be noted that the peaks of oxygencontaining functional groups in UiO-66-NH2@GO disappear. One reason should be that the content of GO in the composites was relatively low. Another might be the interaction between oxygen-containing groups and Zr4+ and –NH2 of UiO-66-NH2. The absorption peak at 1625 cm−1 corresponding to the stretching vibrations of N-C]O was enhanced in a degree, resulting from the vibration of non-conjugated carboxylic and amidogen. This phenomenon also confirmed the successful growth of UiO-66-NH2 on the GO sheets [8,38]. As shown in Fig. 3c, BET surface area of UiO-66-NH2@GO (1004 m2/g) is higher than pristine UiO-66-NH2 (736 m2/g), which would show enhanced adsorption capacity to CO2[33,39]. This improvement can be explained that new pores were generated on the interface between graphene oxide layers and MOF ‘blocks’ [37]. The stability of UiO-66-NH2 and UiO-66NH2@GO was further studied by Thermogravimetric analysis (TGA) (Fig. 3d). At the very beginning, UiO-66-NH2 underwent an obvious weight loss below 300 °C because of the removal of physically adsorbed water, guest water and other physical adsorbed solvent (residual DMF in the pores). The observed weight loss at high temperatures (350–600 °C) corresponds to the structure collapse of UiO-66-NH2 [8]. The degradation temperature of UiO-66-NH2 is about 400–500 °C, while that of UiO-66-NH2@GO is 500–580 °C. This phenomenon demonstrates that the composites have better thermal stability than the parent materials UiO-66-NH2 [37].
temperature (25 °C) and 0.3–0.45 MPa. The compositions of the steadystate feed, retentate, and permeate were all tested by gas chromatography (Agilent 7890) equipped with a thermal conductive detector (TCD). Prior to the test, residual gas present in membranes and pipeline was removed by vacuum pump. After system reached steady-state, all gas permeation measurements were performed more than five times. The gas permeability can be calculated using the following equation:
Pi =
LNi AΔP
(1)
where L is the thickness of membrane (cm), Ni is the volume permeate rate of gas (cm3/s) at standard temperature and pressure (STP), A (cm2) is the test area of membrane, and ΔP is the transmembrane pressure (cmHg). The unit of permeability (Pi) is commonly expressed as Barrer (1 Barrer = 10−10 cm3 (STP) cm/(cm2 s cmHg)). The ideal selectivity (separation factor) can be calculated by the following equation:
S(i/j) =
Pi Pj
(2)
3. Results and discussion 3.1. Physicochemical properties of UiO-66-NH2@GO The morphologies of UiO-66-NH2 and the UiO-66-NH2@GO composite were investigated by SEM. As shown in Fig. 2a, UiO-66-NH2 exhibits an octahedral morphology. While, for UiO-66-NH2@GO (Fig. 2b), UiO-66-NH2 nanoparticles were well-anchored on GO layers rather than physically mixed with GO. As a result, the GO nanosheets were well-dispersed in UiO-66-NH2 and covered by UiO-66-NH2. This favorable structure of the UiO-66-NH2@GO composite can effectively avoid the layer stacking of GO [33]. Besides, the particle size of the UiO-66-NH2 on GO nanosheets was reduced to a certain extent as shown in Fig. 2c and d. The reason might be related to the additional space constraints caused by GO during the synthesis of the composites [34].
Fig. 2. SEM images (a, b) and particle size distribution (c, d) of UiO-66-NH2 (a, c) and UiO-66-NH2@GO (b, d). 65
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Fig. 3. XRD patterns (a) and FTIR spectra (b) of GO, UiO-66-NH2, UiO-66@GO and UiO-66-NH2@GO, N2 adsorption-desorption isotherms (c) and TGA curves (d) of UiO-66-NH2 and UiO-66-NH2@GO.
increasing the filler to 20 wt%, the agglomeration of UiO-66-NH2@GO particles can clearly be found (Fig. 4h), possibly owing to the gravity force and the quite different physical property between UiO-66NH2@GO and PI matrix [40]. To investigate the influence of fillers on the arrangement of polymer chains, the crystalline structure of MMMs was studied by XRD (Fig. 5a). Pure PI membrane shows a strong and broad peak at 15.4° due to its amorphous property [41]. For U-NH2-PI-5 MMMs, the peak position of PI almost unchanged. While, after incorporating GO nanosheets in PI matrix, the peak of PI obviously shifted to lower degree (∼14.6°), suggesting enhanced hydrogen bonding between GO nanosheets and PI molecules [11,22]. Meanwhile, by incorporating UiO-66-NH2@GO, the peak of PI also shifted to a lower degree for UiO-66-NH2@GO based MMMs. The characteristic peaks belonging to UiO-66-NH2 and UiO-66NH2@GO could obviously be found in MMMs, indicating that the
3.2. Membrane characterization The cross-section morphologies of resultant membranes were characterized by SEM (Fig. 4). Pure PI membrane shows a homogeneous structure (Fig. 4a). For GO-PI-1 as shown in Fig. 4b, GO nanosheets were well-compatible with PI matrix without any noticeable defects that is similar as reported in literature [11]. For U-NH2-PI-5 (Fig. 4c), it is difficult to achieve well dispersion of filler in polymer matrix and slight aggregation appeared. However, UiO-66@GO and UiO-66NH2@GO nanoparticles homogeneously distributed in the PI matrix with the filler contents of 1 and 5 wt% (Fig. 4d–f). Compared to the UNH2-PI-5 with 5 wt% of UiO-66-NH2, it can be clearly seen that incorporation of GO with strong steric effect could obviously improve the interface compatibility between fillers and the PI matrix. UN@GO-PI-10 also shows a relatively homogeneous layer (Fig. 4g). By further
Fig. 4. SEM images of the cross-section and surface (inset) morphology of pure PI (a), GO-PI-1 (b), U-NH2-PI-5 (c), U@GO-PI-5 (d) UN@GO-PI-1 (e), UN@GO-PI-5 (f), UN@GO-PI-10 (g) and UN@GO-PI-20 (h), the scale bar represents 1 μm in insets. 66
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Fig. 5. XRD patterns (a) and FTIR spectra (b) of pristine PI, GO-PI-1, U-NH2-PI-5, U@GO-PI-5, UN@GO-PI-1, UN@GO-PI-5, UN@GO-PI-10 and UN@GO-PI-20 mixed matrix membranes.
crystalline structure of fillers was not destroyed in membranes preparation procedure. Besides, with increase of UiO-66-NH2@GO loading from 1 to 20 wt%, the corresponding characteristic peaks of UiO-66NH2@GO gradually increase. In order to further characterize and analyze the interaction between inorganic fillers and polymer chains, FTIR spectra were conducted (Fig. 5b). For pristine PI membrane, the absorption peaks at 1785 and 1727 cm−1 are attributed to the asymmetric and symmetric stretching of C]O groups in imides group. The band at 1378 cm−1 corresponds to CeN stretching in the imides group [42,43]. For U-NH2-PI-5, no obvious absorption peak emerged, resulting from that the characteristic peaks of –NH2 in UiO-66-NH2 were covered by Matrimid’s peaks [28]. For GO-PI-1, the characteristic peak at 1092 cm−1 corresponds to the stretching vibrations of the oxygencontaining functional groups of GO, confirming existence of GO in PI matrix. The intensity of such peak was enhanced with increasing UiO66-NH2@GO loading in UiO-66-NH2@GO-based MMMs. Meanwhile, it should be noticed that the peak at 1636 cm−1 assigned to NeC]O group was enhanced with increasing UiO-66-NH2@GO loading, also indicating more GO interacted with PI [8,41].
than pristine PI membrane was ascribed to the inherent adsorption of GO nanosheets and the reduced mobility of polymer chains in the presence of GO nanosheets [7]. While, the UiO-66@GO MMM (U@GOPI-5) showed little enhanced permeability and selectivity compared to pure PI membrane, resulting from high porosity of UiO-66 and welldispersion of fillers in polymer matrix. MMMs with the incorporation of 5 wt% UiO-66-NH2 (U-NH2-PI-5) exhibited higher permeability but lower selectivity (CO2 permeability of 8.45 Barrer and CO2/N2 ideal selectivity of 13). The increased permeability was mainly attributed to the existence of UiO-66-NH2 with a large pore size (0.6 nm) [28], but some agglomeration of UiO-66-NH2 formed in polymer matrix reduced the selectivity. Gas separation performance was enhanced with the well-dispersed nanofillers. The inherent adsorption of both UiO-66-NH2 and GO to CO2 would improve the permeability of CO2, and the tortuous channels supplied by GO restrict the transport of gas molecule with larger sizes, resulting in enhanced CO2/N2 selectivity. However, too many UiO-66-NH2@GO nanoparticles in polymer matrix (e.g. UN@GO-PI-10 and UN@GO-PI-20) lead to more membrane defects and give rise to the low CO2/N2 selectivity. In order to study the intrinsic adsorption property to specific gas molecules of UN@GO-PI MMMs, the single gas and mixed gas separation results at different feeding pressure (0.3–0.45 MPa) on the UN@GO-PI-5 MMMs were conducted (Fig. 6). For single gas separation (Fig. 6a), the permeability of CO2 and N2 decreased slightly with the increase of feed pressure, which is the actual behavior for glassy polymer [45]. It confirms that CO2 and N2 do not plasticize the polymer matrix and thus the hydrostatic effect played a dominated role [46]. For mixed gas separation (Fig. 6b), the reduced permeability of CO2 was mainly attributed to a part of N2 molecules occupied the transport channels. Meanwhile, as the pressure increasing, the permeability of CO2 enhanced a little because more adsorption potential occupied by CO2 molecules. With the increase of CO2 and N2 pressure, the gas separation of UN@GO-PI-5 MMMs reached a good balance. The above results prove that the as-prepared UN@GO-PI-5 MMMs showed a good structural stability with the pressure increase. In order to further understand the gas-transport through UN@GOPI-5 MMMs, the influence of permeating temperature (25–65 °C) on gas permeation was also investigated. As shown in Table 2, the permeability of CO2 and N2 increased with the permeating temperature, attributed to the enhancement of gas diffusion [46]. While, CO2 permeability is less sensitive to the temperature variation, resulting from the inherent adsorption property to CO2 [47]. The stability of UN@GO-PI-5 MMM was also examined by immersing in water at 25 °C for 24–72 h (Table 3), followed by the vacuum drying at 100 °C for 24 h. It can be seen from Table 2 that UN@GO-PI-5 MMM maintained stable gas separation performance even soaked in water for 72 h, also supporting its high stability. Combined with the satisfied separation performances, UN@GO-PI MMMs are very
3.3. Gas separation performance The gas separation performance of UiO-66-NH2@GO MMMs was strengthened with the interaction between UiO-66-NH2 and GO, and the enhanced adsorption ability of fillers to CO2 [44]. As shown in Table 1, the MMMs incorporated with UiO-66-NH2@GO exhibited enhanced permeability and selectivity. Clearly, the permeability of CO2 and CO2/N2 selectivity for MMMs with 5 wt% UiO-66-NH2@GO reached the maximum values (CO2 permeability of 7.28 Barrer and CO2/N2 ideal selectivity of 52.0). Pure PI membrane only shows a CO2 permeability of 2.28 Barrer with a CO2/N2 ideal selectivity of 28.9. MMM doped with GO nanosheets (GO-PI-1) exhibited lower permeability but higher selectivity compared to other MMMs because there is no pore on GO sheets, and the gas molecules can only permeate through the edges of GO sheets. The higher permeability of CO2 (3.15 Barrer) Table 1 Gas permeability and selectivity on pristine PI and mixed-matrix membranes at 25 °C and 3 bar. Sample
PCO2 (Barrer)
PN2 (Barrer)
αCO2/N2
Pure PI GO-PI-1 U@GO-PI-5 U-NH2-PI-5 UN@GO-PI-1 UN@GO-PI-5 UN@GO-PI-10 UN@GO-PI-20
2.28 3.15 3.56 8.45 3.12 7.28 7.33 18.1
0.079 0.049 0.10 0.65 0.11 0.14 0.24 9.03
28.9 64.3 35.6 13.0 29.2 52.0 30.5 2.0
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Fig. 6. Single (a) and mixed gas separation (b) results of UN@GO-PI-5 at different feeding pressure. Table 2 The separation performance of UN@GO-PI-5 MMMs at different temperatures. Temperature (°C)
PCO2 (Barrer)
PN2 (Barrer)
SCO2/N2
25 45 55 65
7.28 9.12 10.3 11.5
0.14 0.26 0.37 0.48
52.0 35.1 27.8 24.0
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Table 3 The separation performance of UN@GO-PI-5 MMMs after soaking in water for 0–72 h. Time (h)
PCO2 (Barrer)
PN2 (Barrer)
SCO2/N2
0 24 48 72
7.28 8.03 7.08 7.43
0.14 0.14 0.13 0.14
52.0 57.4 54.5 53.1
attractive for CO2/N2 separation in potential industrial application. 4. Conclusion In summary, we reported a high-performance MMMs for efficient separation of CO2/N2 by taking advantage of novel UiO-66-NH2@GO composite filler. Various characterization results confirmed that the UiO-66-NH2@GO fillers exhibited good adhesion with the polymer matrix. The gas separation performance was significantly enhanced compared to pure PI membrane and other composite membranes. UN@GO-PI-5 exhibited a high and stable CO2/N2 separation performance (CO2 permeability: 7.28 Barrer, CO2/N2 selectivity: 52) at 25 °C with the feed pressure of 0.3 MPa, resulting from the highly-dispersed UiO-66-NH2@GO fillers in PI membrane and the instinct adsorption of UiO-66-NH2 and GO to CO2 molecules. Acknowledgement The authors are grateful for the financial supported by Natural Science Key Project of the Jiangsu Higher Education Institutions, China (15KJA220001), Jiangsu Province Six Talent Peaks Project, China (2016-XCL-043), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). YF thanks the financial support of the Natural Science Foundation of Jiangsu Province Youth Fund, China (BK20170919). References [1] J. Yao, H. Wang, Zeolitic imidazolate framework composite membranes and thin films: synthesis and applications, Chem. Soc. Rev. 43 (2014) 4470–4493. [2] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) 390–400. [3] B. Seoane, J. Coronas, I. Gascon, M. Etxeberria Benavides, O. Karvan, J. Caro, F. Kapteijn, J. Gascon, Metal-organic framework based mixed matrix membranes: a
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