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Ni-ZIF-8-PEBA for high performance CO2 separation
Journal of Membrane Science 560 (2018) 38–46
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Mixed-matrix membranes based on Zn/Ni-ZIF-8-PEBA for high performance CO2 separation ⁎
T
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Xinru Zhanga,b, Tao Zhanga, Yonghong Wanga,b, , Jinping Lia,b, Chengcen Liua, Nanwen Lic, , Jiayou Liaoc a
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan 030024, Shanxi, China c State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Mixed-matrix membranes Zn/Ni-ZIF-8 CO2 separation Asymmetric adsorption/desorption Separation mechanism
Mixed-matrix membranes (MMMs) for CO2/N2 separation were prepared by incorporating nickel-substituted ZIF-8 (Zn/Ni-ZIF-8) into a polyether-block-amide (PEBA2533) polymer, in which the Zn and Ni ions in Zn/NiZIF-8 have different affinities for CO2 based on π-complexation; these MMMs exhibited exceptional molecular sieving properties compared to ZIF-8-PEBA MMMs. Scanning electron microscopy revealed that Zn/Ni-ZIF-8 nanoparticles were small and uniformly distributed within the PEBA, and attenuated total reflectance Fouriertransform infrared spectroscopy revealed good interfacial compatibility between the Zn/Ni-ZIF-8 and PEBA; consequently these MMMs exhibited excellent mechanical properties. The 10% Zn/Ni-ZIF-8-PEBA MMM showed a high CO2 permeability of 321 Barrer, and a CO2/N2 selectivity of 42.8 at 2 bar; these values were much higher than those of the 10% ZIF-8-PEBA MMM (266 Barrer and 33.8, respectively). We assume that different metal-ion affinities towards CO2 resulted in the asymmetric adsorption/desorption behaviour of these MMMs, which led to a significantly higher permselectivity for the Zn/Ni-ZIF-8-PEBA MMM. The MMM loaded with 10% Zn/Ni-ZIF-8 exhibited long-term CO2-separation-performance stability under mixed-gas conditions, with a CO2 permeability of 282 Barrer and a CO2/N2 selectivity of 42.7. Moreover, the permselectivity of the 10% Zn/Ni-ZIF-8-PEBA MMM under pure-gas condition surpassed Robeson's upper bound reported in 2008.
1. Introduction The development of industry and population growth have turned people's attentions to the issues of climate change and global warming [1]. Mitigating the atmospheric CO2-emission issue and employing effective methods for CO2 trapping are urgent priorities. CO2 is commonly removed from gas mixtures by chemical absorption [2] and cryogenic separation [3], as well as other methods [4–6]. Recently, membrane separation technology has offered an effective method for the removal of CO2 from flue gases [7–10], which is a consequence of its high efficiency, adaptability, operational simplicity, low energy consumption, low cost, and environmental friendliness [11,12]. Membranes are conventionally divided into inorganic membranes, polymer organic membranes, and organic-inorganic hybrid membranes according to their structure [13]. Inorganic membranes have the advantages of resisting harsh chemical cleaning, high temperature and wear resistance, high chemical stabilities and long lifetimes, and autoclavabilities [14]. These outstanding properties make inorganic ⁎
membranes good candidates for use in gas-separation applications. However, as a result of their costs and fragile structures, they are difficult to prepare on the large scale. Polymer materials such as polyimide [15], polyamide [16], poly(ethylene glycol) [17], poly(dimethyl siloxane) [18], poly(vinyl alcohol) [19], poly(ethylene oxide) [20], poly (vinylimidazolium) [21], and poly(ether block amide) [22] have been extensively used to prepare polymeric membranes for CO2 separation, and have received considerable attention because of their excellent mechanical performance, reproducibilities, and relatively economical processing capabilities. However, gas permeability and selectivity are restricted by the trade-off introduced by Robeson [23]. Inorganic particles have been embedded into polymeric matrices to produce mixed-matrix membranes (MMMs) [24] that can deliver both high permeability and selectivity due to the advantages of the two phases involved. Specifically, inorganic particles disrupt molecularchain packing, where a relaxed membrane structure is beneficial for gas transport, leading to enhanced permeability. Furthermore, the adapted inorganic-particle structure not only improved the affinity for the
Corresponding author at: College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China. Corresponding author. E-mail addresses: [email protected] (Y. Wang), [email protected] (N. Li).
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https://doi.org/10.1016/j.memsci.2018.05.004 Received 15 January 2018; Received in revised form 30 April 2018; Accepted 3 May 2018 Available online 04 May 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.
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2. Experimental
targeted gas, but also supplied diffusion channels to match the kinetic diameters of the gas molecules [25]. Exploiting defect-free MMMs is challenging because MMM fabrication is confronted by polymer-filler incompatibility issues. The formations of non-selective voids, a sieve-in cage morphology, polymer rigidification, and pore blockages are due to poor polymer-filler interactions that result in the deterioration of membrane performance [26]. Hence, the judicious selection of the polymer and inorganic filler is crucial for MMM development. Over the years, zeolites, silica, and metal-organic frameworks (MOFs), as inorganic MMM fillers, have been shown to improve membrane performance [27]. Zeolitic imidazolate frameworks (ZIFs), a subclass of MOF that is composed of inorganic metal clusters coordinated with organic imidazole ligands, have been extensively used as membranes in gas-separation applications. ZIF-8 [28–32], a member of this family with sodalite topology, is highly porous and thermally stable. Moreover, owing to the unique pore size (0.34 nm) and structure of this microporous material, molecular-sieving behaviour of ZIF-8 for CO2 has been extensively investigated in recent years. In addition, ZIF-8 is suitable for addition to gas-separating polymer-membrane materials due to its flexible framework [33–35]. PEBA block copolymers, which are synthesized from dicarboxylic-acid-terminated aliphatic polyamides and polyoxyalkylene glycols, exhibit excellent CO2-separation performance [36]. Hägg et al. [30] developed ZIF-8-PEBA MMMs for CO2/N2 separation; these MMMs were two-layer membranes composed of a polymer layer with a few dispersed inorganic particles, and an inorganic layer. The permeability was found to increase with increasing amounts of inorganic filler, while selectivity decreased slightly in single-gas experiments due to increments in the ZIF-8 selective-particle content. Mosleh et al. [37] synthesized a ZIF-8 suspension that was directly incorporated into a PEBA matrix in order to improve the gas-separation performance of the membrane. The CO2 and N2 permeabilities of the ZIF-8-PEBA MMM loaded with 30% ZIF-8 were increased by 111% and 99%, respectively. However, to the best of our knowledge, research on ZIF-8-PEBA MMMs is still in an exploratory stage. In recent years, many researchers have demonstrated that π-complexation between transition-metal ions and CO2 effectively enhances CO2 permeability and selectivity in the dry state [38,39]. Nickel-substituted ZIF-8 (Zn/Ni-ZIF-8) [40], in which nickel ions are in situ incorporated to the ZIF-8 skeleton without altering the ZIF-8 structure, combines molecular sieving with the π-complexation mechanism of transition-metal ions for CO2 separation. In Zn/Ni-ZIF-8, zinc and nickel ions exhibit different affinities towards CO2 due to their inherent characteristics that include the numbers of virtual orbitals, charges, and ionic radii, among others, resulting in the asymmetric adsorption of CO2-gas molecules. Moreover, Zn/Ni-ZIF-8 has an intersecting 3-D network, with a very high number of pores, a high surface area, and is compatible with polymers that are potential MMM fillers for CO2 separation [41]. Herein, we first employed Zn/Ni-ZIF-8 as a filler during the preparation of MMMs with PEBA block copolymers for high-performance gas-separation membranes. The effects of Zn/Ni-ZIF-8 loading on the mechanical properties and microstructure were investigated. The influences of filler loading and feed pressure on gas-permeation performance were also studied, and the asymmetric adsorption/desorption transport mechanism of these MMMs for CO2 is discussed on a preliminary basis by investigating the effects of different MMM Ni-ion combination pathways on gas-permeation performance of the as-prepared membranes. We found that Zn/Ni-ZIF-8 was uniformly dispersed in these MMMs, and that Zn/Ni-ZIF-8 exhibited good compatibility with the PEBA in these MMMs. As a consequence, a Zn/Ni-ZIF-8-PEBA MMM exhibited good CO2 permselectivity when compared to the ZIF-8-PEBA MMM, with long-term stability over 150 h.
2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 98%) were received from Alfa Aesar, and 2-methylimidazole (MeIm, 99%) was obtained from Aladdin. PEBA polymer (grade 2533) was provided by Arkema Inc., France. Methanol (MeOH, 98%) was purchased from the Sinopharm Chemical Reagent Co., Ltd., and N,N-dimethylacetamide (DMAc) was purchased from the Tianjin Damao Chemical Reagent Plant. N2 and CO2, with purities of 99.999%, and the mixed gas (CO2/N2 = 20/80 vol%) were provided by the Shanxi Special Gases Company. All chemicals were used as received without further treatment. 2.2. Synthesis of Zn/Ni-ZIF-8 Zn/Ni-ZIF-8 was synthesized by a solvothermal method [40]. The synthesis solution with molar ratio of Zn2+:Ni2+:MeIm = 1:1:16 was prepared as follows. Specifically, Zn(NO3)2·6H2O (1.188 g), Ni (NO3)2·6H2O (1.172 g), and MeIm (5.296 g) were dissolved separately in 100 mL methanol (designated as solution A, solution B and solution C respectively). The solution A and B were added into solution C rapidly and continuously stirred at 25 °C for 24 h. A violet powder was acquired by centrifugation at 10,000 rpm for 10 min; this powder was washed with methanol three times to remove guest molecules from the pores of the material, after which it was dried under vacuum at 80 °C. For comparison, ZIF-8 was prepared by dissolving Zn(NO3)2·6H2O (1.188 g) and MeIm (2.648 g) in methanol; the molar ratio of two reactants in this solution was 1:8. ZIF-8 was synthesized from this solution using a similar process to that used for the preparation of Zn/NiZIF-8 [41]. The yields of Zn/Ni-ZIF-8 and ZIF-8 particles, based on the limiting zinc reactant, were about 57% and 48%, respectively. 2.3. MMM fabrication To prepare the Zn/Ni-ZIF-8-PEBA MMMs, PEBA2533 was added to N,N-dimethylacetamide (DMAc) such that a 6% PEBA solution was obtained. At the same time, specific amounts of Zn/Ni-ZIF-8 were dispersed in DMAc with sonication for 2 h, to produce Zn/Ni-ZIF-8 suspensions. Each suspension was separately added to the aforementioned PEBA solution, and stirred for 8 h at 67 °C. The mixture was then poured onto a clean glass plate, and the cast membrane was placed in a vacuum oven at 30 °C for 24 h, and then dried at 50 °C for about another 24 h. The temperature was then gradually increased to 70 °C and maintained at this temperature for 24 h to remove residual solvent. Thereafter, each membrane was peeled from the glass plate and dried at 60 °C to constant weight prior to characterization or gas-permeation performance testing. For comparison, pure PEBA membrane and ZIF-8-PEBA MMMs were fabricated using similar procedures. In addition, a 5% Zn-Ni-MeIm-PEBA MMM was prepared in order to investigate the effect of the Ni-ion-combination pathway on gas-separation performance, as follows. Zn(NO3)2·6H2O (12 mg), Ni (NO3)2·6H2O (11.6 mg), and MeIm (6.5 mg) were added to DMAc (3 g) and sonicated for 2 h to produce a homogeneous dispersion. This dispersion was then added to a 6% PEBA solution in DMAc and stirred for 8 h at 67 °C. The Zn-Ni-MeIm-PEBA MMM was then fabricated in a similar manner to that described for the fabrication of the Zn/Ni-ZIF-8PEBA MMMs. The dry thicknesses of all membranes used for gas separation was 30 ± 3 µm. X% filler-PEBA refers to the percentage (X) of the filler in the PEBA matrix. 2.4. Characterization The crystal structure of Zn/Ni-ZIF-8 was characterized by X-ray diffraction (XRD, Shimadzu, XRD-6000, Japan) at a scanning rate of 39
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4° min−1 and a 5–65° angular range. Attenuated total internal reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy of the MMMs was conducted using a Nicolet 6700 spectrometer (Thermo Scientific, USA) over the 650–4000 cm−1 range at a resolution of 4 cm−1. N2 adsorption-desorption isotherms were acquired at 77 K in order to determine surface areas and pore volume by the BrunauerEmmett-Teller (BET) method using a JW-BK122W surface area and micropore size analyser (JWGB Sci & Tech Co., Ltd., Japan). Before analysis, each sample was outgassed under vacuum at 180 °C for 8 h. The morphological structures of the Zn/Ni-ZIF-8 and Zn/Ni-ZIF-8-PEBA MMMs were examined by scanning electron microscopy (SEM) (Hitachi, SU8010, Japan) at 3 kV, together with energy-dispersive X-ray spectroscopy (EDXS) for the analysis of elemental composition and content. The CO2-adsorption isotherms of ZIF-8 and Zn/Ni-ZIF-8 were acquired in the 0–100 kPa range at 298 K on a NOVA 1200e adsorption analyser (Quantachrome, USA). Samples were degassed at 300 °C and 10−2 Pa for 10 h prior to any experiment.
Fig. 1. CO2-adsorption isotherms for ZIF-8 and Zn/Ni-ZIF-8 nanoparticles at 298 K.
2.5. Mechanical properties
and permeate, respectively.
Tensile strengths and elongations at break were measured using a WDW-100 universal testing machine (Jinan MTS Test Technology Co. Ltd, China) at a speed of 25 mm min−1. All samples were prepared with widths of 10 mm and lengths of 60 mm [42]. Five pieces of each sample were tested, and an average value was obtained.
3. Results and discussion 3.1. Characterization of Zn/Ni-ZIF-8 Zn/Ni-ZIF-8 nanoparticles were synthesized according to a previous report [41], while ZIF-8 was prepared by a solvothermal method for control experiments. SEM revealed that ZIF-8 and Zn/Ni-ZIF-8 have nanocrystalline and porous packing structures with uniform crystal distributions (Fig. S2). The ZIF-8 crystals were determined to be 40–60 nm in size; however, Zn/Ni-ZIF-8 crystals were smaller (20–40 nm), which is probably due to their lower crystal-growth rate and the presence of Ni ions that are smaller than Zn ions. Moreover, the final amounts of Zn and Ni in Zn/Ni-ZIF-8 were 17.8% and 1.48%, respectively, as determined from elemental-analysis data (Table S1). As shown in Fig. S3, the powder X-ray diffraction patterns of Zn/Ni-ZIF-8 reveal a highly crystalline structure; these patterns are well matched with those simulated for ZIF-8 and are in agreement with the reported literature [41]. CO2-adsorption isotherms for the ZIF-8 and Zn/Ni-ZIF-8 nanoparticles at 298 K are displayed in Fig. 1; CO2 adsorption grew in a linear manner with increasing pressure. The CO2-adsorption isotherm and adsorption capacity of ZIF-8 were similar to those previously reported [44]. Zn/Ni-ZIF-8 exhibited a higher CO2-adsorption capacity than ZIF-8, highlighting its outstanding affinity for CO2, which is ascribable to dual metal ions (zinc and nickel) that exhibit different binding behaviour toward CO2 based on the π-complexation mechanism, resulting in asymmetric CO2 adsorption.
2.6. Gas-separation experiments Gas-permeation experiments were carried out using the constantpressure and variable-volume method. Fig. S1 displays a schematic diagram of the apparatus used for the gas-permeation experiments. The pure-gas permeation experiments were performed using CO2 and N2 with purities of 99.999%, and under pressure conditions that ranged from 1 to 5 bar at 25 °C. Mixed-gas permeation experiments were performed using a binary gas mixture (CO2/N2 = 20/80 vol%) at a pressure of 2 bar and a temperature of 25 °C. Before the first experiment, the tested gas was introduced for at least 3 h for degassing purposes. Between two subsequent experiments, the system was flushed with the latter gas for a time sufficient to guarantee the complete removal of the previous gas. H2, at flow rate of 30 mL min−1, was selected as the sweep gas, and the feed-gas flow rate was 60 mL min−1. The upstream and downstream flow rates were measured using soap-bubble flow meters. The downstream compositions were determined by gas chromatography (GC-2014C, Shimadzu) with a thermal conductivity detector. After the system reached steady state, each gas-permeation experiment was repeated more than five times. The permeability of each gas was calculated using Eq. (1) [43]:
Pi =
Qil ∆pi A
3.2. MMM characterization (1) Scheme 1 depicts the fabrication of a Zn/Ni-ZIF-8-PEBA MMM. As shown in Scheme 1, Zn(NO3)2·6H2O, Ni(NO3)2·6H2O, and MeIm are uniformly mixed, resulting in the immediate formation of a green dodecahedral. In this intermediate, a zinc atom (depicted in blue) is coordinated to four nitrogen atoms in MeIm to form a tetrahedron, and a nickel atom (green) is coordinated to six atoms (nitrogen atoms or solvent molecules) to produce an unstable sexadentate structure. The Ni-coordination mode is then transformed from metastable sexadentate to stable tetrahedral by the action of methanol to afford a purple dodecahedral structure [40]. Finally, these (as-prepared) particles are added to PEBA to produce a purple solution, and the Zn/Ni-ZIF-8-PEBA MMM is prepared by solution casting. The Zn/Ni-ZIF-8 is uniformly distributed in the PEBA matrix probably as a result of asymmetric repulsive forces that originate from the dual metal ions in the purple dodecahedral structure. Fig. S4 reveals that the BET surface area and micropore volume of Zn/Ni-ZIF-8 were 1839 m2 g−1 and 0.71 cm3 g−1,
where Pi is the permeability of gas ‘i’ (Barrer, 1 Barrer = 1 × 10−10 cm3 (STP) cm/(cm2 s cmHg)); Qi is the volumetric flow rate of gas ‘i’ (cm3 (STP)/s); l (cm) is the thickness of the membrane measured by a micrometre calliper, Δpi is the transmembrane-pressure difference (cmHg), and A is the effective membrane area (12.56 cm2). The ideal selectivity (αij) of the pure gas was calculated using Eq. (2) [43]:
α ij =
Pi Pj
The mixed-gas separation factor of gases ‘i and j’ lated using Eq. (3) [43]:
α ij* =
(2) (αij*)
was calcu-
yi / yj x i /x j
(3)
where x and y are the volumetric fractions of one component in the feed 40
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respectively, while those of ZIF-8 were 1228 m2 g−1 and 0.41 cm3 g−1, respectively, in agreement with the literature [45,46]. Zn/Ni-ZIF-8 exhibited the larger surface area, which may has more organic moiety and metal ions in external surface. Zn/Ni-ZIF-8 is possibly in tight contact with the macromolecular chains of PEBA through hydrogen bonding and van der Waals forces [47]. Moreover, chelation between the metal (zinc and nickel) nodes and the ester groups of PEBA, as well as hydrogen bonding between the PEBA chains and the imidazoles of Zn/Ni-ZIF-8, may also contribute to stronger interfacial interactions [48]. Fig. 2 shows the cross-sectional morphologies of the pure PEBA membrane and the Zn/Ni-ZIF-8-PEBA MMMs with various particle loadings. As shown in Fig. 2(a), the pure PEBA membrane is smooth, while the Zn/Ni-ZIF-8-PEBA MMMs are rough. This observation is attributed to the elimination of intermolecular hydrogen bonding and a decline in the crystallinity of the polyether and polyamide blocks following addition of Zn/Ni-ZIF-8 to the PEBA matrix [49,50]. As shown in Fig. 2(b-c), Zn/Ni-ZIF-8 particles are well dispersed in the MMMs at low loadings, and large conglomerates of particles as obvious MOFpolymer interface are not observable in SEM images. However, the slight agglomeration was observed when the Zn/Ni-ZIF-8 loading in PEBA exceed 15% (Fig. 2(d-e)), but there were no visible voids at the
Scheme 1. Depicting the fabrication of a Zn/Ni-ZIF-8-PEBA MMM. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).
Fig. 2. Cross-sectional SEM images of a (a) pure PEBA membrane and Zn/Ni-ZIF-8-PEBA MMMs with Zn/Ni-ZIF-8 loading of (b) 5, (c) 10, (d) 15, and (e) 20%. Each inset displays the corresponding enlarged image. The yellow ellipses highlight agglomerated nanoparticles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). 41
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Fig. 4. Effect of filler content on the mechanical properties of the Zn/Ni-ZIF-8PEBA and ZIF-8-PEBA MMMs. Fig. 3. ATR-FTIR spectra of the pure PEBA membrane and the Zn-Ni-MeImPEBA, ZIF-8-PEBA, and Zn/Ni-ZIF-8-PEBA MMMs.
interfaces between the Zn/Ni-ZIF-8 agglomerates and the polymer. This demonstrates that Zn/Ni-ZIF-8 is compatible with the PEBA-polymer molecular chains, which is a result of the electron-rich properties of the Zn/Ni-ZIF-8 framework that facilitates strong electrostatic interactions between Zn/Ni-ZIF-8 and the PEBA-polymer chains. Consequently, these Zn/Ni-ZIF-8-PEBA MMMs are very dense and defect-free, and meet the requirements for gas-separation applications. The ATR-FTIR spectra of the membranes are displayed in Fig. 3, which reveal characteristic peaks corresponding to PEBA for all of the membranes. The peak at 3298 cm−1 is assigned to the amide -N-H stretching vibration, while the band at 1735 cm−1 is attributed to the -C=O stretching vibration of the carboxylic acid [51]. The absorption peak at 1637 cm−1 is ascribed to the out-of-plane H-N-C=O vibration of the amide, and the peak at 1106 cm−1 is attributed to the polyether C-O-C stretching vibration, while the band at 1366 cm−1 is associated with the amide C-N stretching mode [49]. The aforementioned peaks are consistent with the structure of PEBA2533, which is composed of 80% polyether and 20% polyamide [52]. The peaks at 1366 cm−1 and 1032 cm−1 observed in the spectra of the Zn-Ni-MeIm-PEBA MMMs, are assigned to the asymmetric and symmetric stretching vibrations of NO3–, respectively, and the peak at 825 cm−1 is attributed to the nitrate out-of-plane bending vibration [53]. The peak at 1597 cm−1 is ascribed to the skeletal vibration of MeIm. These spectra demonstrated that the Zn-Ni-MeIm-PEBA MMMs are mixtures of Zn(NO3)2·6H2O, Ni (NO3)2·6H2O, MeIm, and PEBA. In the case of the MMMs, the absorption peak at 3298 cm−1 was red shifted following the addition of ZIF-8 or Zn/Ni-ZIF-8 to PEBA, confirming that hydrogen bonding exists in these MMMs [50]. Fig. 5. Effect of filler loading on (a) the CO2 permeability and (b) CO2/N2 selectivity of the ZIF-8-PEBA and Zn/Ni-ZIF-8-PEBA MMMs.
3.3. Mechanical properties Except for gas-separation performance, MMMs are required to have good mechanical properties for industrial applications, because durability is very important to the overall membrane-operating process. Fig. 4 displays the effect of filler loading on tensile strength and elongation at break of the MMMs. As shown in Fig. 4, the tensile strength and elongation at break of the Zn/Ni-ZIF-8-PEBA increased slightly as the filler loading was increased to 10%, after which it sharply declined. However, the tensile strength and elongation at break of the ZIF-8PEBA MMM decreased steadily with increasing ZIF-8 loading. The maximum value of tensile strength and elongation at break for the Zn/ Ni-ZIF-8 MMMs are clearly higher than those of the ZIF-8-containing MMMs, which is ascribable to the homogeneous distribution of Zn/NiZIF-8 and its good adhesion to PEBA. The reductions in the tensile strength of the Zn/Ni-ZIF-8-PEBA MMM at filler loadings greater than 10% are due to aggregation.
3.4. Gas-separation performance 3.4.1. Effect of ZIF loading Fig. 5 shows the effect of filler loading on the CO2 permeability and CO2/N2 selectivity of the MMMs. Pure-gas permeation testing was performed at 2 bar. As shown in Fig. 5(a), the CO2 permeability of the Zn/Ni-ZIF-8-PEBA MMM increased dramatically with increasing Zn/NiZIF-8 loading, while the CO2 permeability of the ZIF-8-PEBA MMM increased slowly. The maximum CO2 permeability of the MMMs loaded with 20% Zn/Ni-ZIF-8 and ZIF-8 were 478 and 294 Barrer, respectively, which represent increases of 193% and 80% compared to the permeability of the pure PEBA membrane, respectively. This is ascribable to disrupted polymer-matrix-chain packing as a result of the introduction of Zn/Ni-ZIF-8 and ZIF-8, which resulted in an increase in chain 42
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resulting in an increase of the concentration of gas transported into the membrane matrix; consequently a high CO2 permeability was obtained. The CO2 permeability of the MMM was observed to increase with increasing Zn/Ni-ZIF-8 loading, from 255 to 521 Barrer as the Zn/Ni-ZIF8 loading was increased from 0% to 20%, which is ascribable to larger amounts of Zn/Ni-ZIF-8 particles leading to more free volume and surface voids, and, as a result, higher CO2 permeability. As showed in Fig. 6(b), CO2/N2 selectivity also increased with increasing feed pressure. The CO2/N2 selectivity of pure PEBA increased from 21.6 to 31.7 as the feed pressure was increased from 1 to 5 bar, whereas the CO2/N2 selectivity of the MMM loaded with 10% Zn/Ni-ZIF-8 increased from 39.6 to 51.8. This is possibly due to the semi-crystalline nature of the PEBA membrane, which is composed of both hard and soft segments that resist matrix compaction at higher pressures; consequently PEBA is not easily compacted with increasing pressure. Under these circumstances, CO2 permeability was observed to improve with increasing feed pressure [49]; meanwhile feed pressure had little influence on N2 permeability. Moreover, the gas-transport tortuosity increased following incorporation of Zn/Ni-ZIF-8 into the PEBA matrix [55], which resulted in an increase in CO2/N2 selectivity. Zn/Ni-ZIF-8 became increasingly agglomerated with increasing Zn/Ni-ZIF-8 loading (more than 10%), which led to larger free volumes and interfacial voids; consequently, reduced CO2/N2 selectivity was observed.
mobility and free volume [54]. The CO2 permeability of the Zn/Ni-ZIF8-PEBA MMMs were higher than those of the ZIF-8-PEBA MMMs because Zn/Ni-ZIF-8 possesses a larger BET surface area and micropore volume, as well as a higher affinity and adsorption capacity for CO2 due to interactions between the quadrupolar CO2 molecules and the positive charges of the unsaturated metal sites. As shown in Fig. 5(b), each MMM exhibited increased CO2/N2 selectivity with increasing Zn/Ni-ZIF-8 or ZIF-8 loading, to a maximum value of 42.8 or 33.8, respectively, at 10% loading. This is ascribable to good particle dispersion in the MMMs at low loadings and the associated molecular-sieving effect for CO2. In addition, as the ZIF-8 or Zn/ Ni-ZIF-8 loading in the polymer matrix exceeded 10%, the CO2/N2 selectivity began to decline as a result of agglomeration and the occurrences of surface voids. Clearly, the Zn/Ni-ZIF-8 MMMs exhibited CO2/N2 selectivity that were always higher than those of the ZIF-8containing MMMs, which is explained by the presence of active metal sites that can react with CO2 through π-complexation, resulting in a stronger affinity towards CO2 [39]. In addition, the zinc and nickel metals in the framework adsorb CO2 differently, which results in quick adsorption/desorption in the MMM and a higher CO2/N2 selectivity. 3.4.2. Effect of feed pressure Fig. 6 shows the effect of feed pressure on CO2 permeability and CO2/N2 selectivity for the pure PEBA membrane and the MMMs. As shown in Fig. 6(a), the CO2 permeability of pure PEBA membrane and the MMMs increase remarkably with increasing feed pressure. The CO2 permeability of the pure PEBA membrane increased from 118 to 255 Barrer as the feed pressure was increased from 1 to 5 bar, while the CO2 permeability of the MMM with 20% Zn/Ni-ZIF-8 loading increased from 438 to 574 Barrer. The reason for these observations involves the gas driving force, which is enhanced with increasing feed pressure,
3.4.3. Mixed-gas separation performance Separation efficiency is influenced by other gases in a practical setting; consequently, a binary gas mixture (CO2/N2 = 20/80 vol%) was examined. Fig. 7 displays the CO2 permeability and CO2/N2 selectivity of the pure membrane and the MMMs. Mixed-gas-separation
Fig. 7. (a) CO2 permeability and (b) CO2/N2 selectivity of as-prepared membranes as determined by pure-gas and mixed-gas testing at feed pressures of 2 bar.
Fig. 6. Effect of feed pressure on (a) the CO2 permeability and (b) CO2/N2 selectivity of the pure PEBA membrane and the Zn/Ni-ZIF-8-PEBA MMMs. 43
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MMM was lower than that of the pure PEBA membrane. We assume that CO2-affinity sites were consumed by chelating reactions between metal ions and the polyether. As shown in Fig. 8(b), CO2/N2 selectivity declined from 45.2 for the Zn/Ni-ZIF-8-PEBA MMM to 19.9 for the Zn-NiMeIm-ZIF-8-PEBA MMM at 5 bar. It was noted that the Zn/Ni-ZIF-8PEBA MMM exhibited the highest CO2/N2 selectivity, which is ascribable to the Zn-Ni tetradentate structure of the Zn/Ni-ZIF-8 cage, which results in asymmetric adsorption/desorption and rapid permselectivity in the MMM. Scheme 2 displays the gas-transport pathway for a Zn/Ni-ZIF-8PEBA MMM. Gas molecules are transported through the membrane under differential pressure as the driving force. Like most MMMs [57], passing through the polymer matrix and the inorganic nanofiller phase mainly relies on the molecular solution-diffusion mechanism. For the Zn/Ni-ZIF-8 MMM, synergism between molecular sieving and asymmetric adsorption/desorption affects gas-separation performance. The structure of Zn/Ni-ZIF-8 is similar to that of ZIF-8; it has a pore size of 3.4 Å that lies between the kinetic diameters of CO2 (3.3 Å) and N2 (3.64 Å) that contributes to CO2/N2 selectivity. On the basis of the πcomplexation mechanism, zinc and nickel ions have different adsorption strengths toward CO2, which results in the asymmetric adsorption/ desorption of CO2. Four kinds of interactions exist between the metal ions and the CO2 molecules based on π-complexation [38,58]. As shown in Scheme 2 (I-II), CO2 can donate its lone pair of electrons or π electrons to the empty s or d orbitals of a metal ion. As shown in Scheme 2 (III), a metal ion can provide a lone electron to the CO2 π-bonding orbital, while Scheme 2 (IV) shows the CO2 π* antibonding orbital receiving electrons from a metal ion. This π-complexation reaction is reversible, which facilitates CO2 transport. Therefore, the Zn/Ni-ZIF-8PEBA MMMs exhibit high CO2 permeability and CO2/N2 selectivity.
performance was observed to follow the same trend as that for the pure gas. As shown in Fig. 7(a), the CO2 permeability of the mixed-gas was markedly lower than that of the pure gas; the maximum CO2 permeability of the MMM loaded with 20% Zn/Ni-ZIF-8 was 426 Barrer for the mixed gas and 478 Barrer for the pure gas. The decline in the CO2 permeability of the mixed gas is ascribable to N2 molecules in the feed component that interfere with the plasticizing effect of CO2 [54]. As shown in Fig. 7(b), the CO2/N2 selectivity of the mixed gas were lower than their corresponding ideal selectivity, with the maximum CO2/N2 selectivity of the MMM loaded with 10% Zn/Ni-ZIF-8 observed to be 42.8 for the pure gas and 41.9 for the mixed gas, which is due to competitive adsorption in the mixed gas [56]. Fig. S5 displays the stability of the 10% Zn/Ni-ZIF-8-PEBA MMM, which was tested with a simulated flue gas (CO2/N2 = 20/80 vol%). The MMM exhibited an average CO2 permeability of 282 Barrer and a CO2/N2 selectivity of 42.7, which fluctuated only slightly. MMM performance was found to be stable for 150 h, and no degradation in either CO2 permeability or CO2/N2 selectivity was observed, which confirms that there was no obvious change in membrane structure. Based on above discussion, Zn/Ni-ZIF-8-PEBA MMMs exhibit exceptional permeability and selectivity compared to ZIF-8-PEBA MMMs and the pure PEBA membrane. We assume that the combination pathways between Ni ions and MeIm directly affect gas-permeation performance. Fig. 8 shows the effect of the Ni-ion combination pathway on CO2 permeability and CO2/N2 selectivity. As shown in Fig. 8(a), the 5% Zn/Ni-ZIF-8-PEBA MMM exhibited better permeation performance than that of the 5% Zn-Ni-MeIm-PEBA MMM and the pure PEBA membrane, which is due to the π-complexation-facilitated transport between metal ions and CO2. However, the CO2 permeability of the Zn-Ni-MeIm-PEBA
3.5. Comparisons with Robeson's upper bound Fig. 9 compares the pure-gas separation performance of the asprepared MMMs in relation to the proverbial Robeson's upper bound. With the exception of the 5% Zn-Ni-MeIm-PEBA MMM, Fig. 9 reveals that all of the MMMs exhibit improvements in both CO2 permeability and CO2/N2 selectivity compared with the pure PEBA membrane. The anti-trade-off gas-separation phenomenon was observed for the Zn/NiZIF-8-PEBA MMMs, which exhibited improvements in CO2/N2 selectivity with elevated CO2 permeability. In particular, the performance of the 10% Zn/Ni-ZIF-8-PEBA MMM actually surpassed Robeson's upper bound, indicating its excellent prospects for CO2-capture and flue-gasseparation applications. 4. Conclusions Zn/Ni-ZIF-8 particles were synthesized as stable Zn-Ni framework structures that exhibited higher CO2 absorption capacities than ZIF-8 due to the dual metal sites present in these frameworks. Zn/Ni-ZIF-8PEBA MMMs were fabricated for CO2/N2 separation by the solution casting method; these membranes exhibited excellent mechanical properties as a result of good compatibility between the two phases. The optimum CO2 permeability and selectivity of the MMM loaded with 10% Zn/Ni-ZIF-8 was 408 Barrer and 51.8 at 5 bar, which represents an increase of 60% and 64% compared to those of the pure PEBA membrane, and surpasses Robeson's upper bound reported in 2008. The Zn/ Ni-ZIF-8-PEBA MMMs display outstanding permeation performance because Zn/Ni-ZIF-8 exhibits a stronger affinity for CO2, leading to upstream absorption in the MMM, fast transport in the membrane, and rapid downstream desorption, compared to those of N2. On the other hand, the CO2 permeability and CO2/N2 selectivity of the 5% Zn-NiMeIm-PEBA MMM was 236 Barrer and 19.9, respectively, which is lower than that of the Zn/Ni-ZIF-8 MMM due to the absence of the ZnNi framework structure. Moreover, the Zn/Ni-ZIF-8-PEBA MMM exhibited stable permselectivity in a simulated flue-gas test over a long
Fig. 8. Effect of Ni-ion combination pathway on (a) the CO2 permeability and (b) CO2/N2 selectivity for as-prepared membranes. 44
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Scheme 2. Depicting the gas-transport pathway for a Zn/Ni-ZIF-8 MMM.
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Fig. 9. CO2/N2-separation performance of the as-prepared membrane compared to Robeson's upper bound.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21506140), the Joint Fund of Shanxi Provincial Coal Seam Gas (No. 2015012009), China Postdoctoral Science Foundation (No. 2016M601289), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No. 2015134), the Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province, and partially supported by the National Natural Science Foundation of China (Nos. 51603225 and U1510123) and the Hundred Talents Program of the Chinese Academy of Sciences.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2018.05.004. 45
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Mixed-matrix membranes based on Zn/Ni-ZIF-8-PEBA for high performance CO2 separation ⁎
T
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Xinru Zhanga,b, Tao Zhanga, Yonghong Wanga,b, , Jinping Lia,b, Chengcen Liua, Nanwen Lic, , Jiayou Liaoc a
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan 030024, Shanxi, China c State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Mixed-matrix membranes Zn/Ni-ZIF-8 CO2 separation Asymmetric adsorption/desorption Separation mechanism
Mixed-matrix membranes (MMMs) for CO2/N2 separation were prepared by incorporating nickel-substituted ZIF-8 (Zn/Ni-ZIF-8) into a polyether-block-amide (PEBA2533) polymer, in which the Zn and Ni ions in Zn/NiZIF-8 have different affinities for CO2 based on π-complexation; these MMMs exhibited exceptional molecular sieving properties compared to ZIF-8-PEBA MMMs. Scanning electron microscopy revealed that Zn/Ni-ZIF-8 nanoparticles were small and uniformly distributed within the PEBA, and attenuated total reflectance Fouriertransform infrared spectroscopy revealed good interfacial compatibility between the Zn/Ni-ZIF-8 and PEBA; consequently these MMMs exhibited excellent mechanical properties. The 10% Zn/Ni-ZIF-8-PEBA MMM showed a high CO2 permeability of 321 Barrer, and a CO2/N2 selectivity of 42.8 at 2 bar; these values were much higher than those of the 10% ZIF-8-PEBA MMM (266 Barrer and 33.8, respectively). We assume that different metal-ion affinities towards CO2 resulted in the asymmetric adsorption/desorption behaviour of these MMMs, which led to a significantly higher permselectivity for the Zn/Ni-ZIF-8-PEBA MMM. The MMM loaded with 10% Zn/Ni-ZIF-8 exhibited long-term CO2-separation-performance stability under mixed-gas conditions, with a CO2 permeability of 282 Barrer and a CO2/N2 selectivity of 42.7. Moreover, the permselectivity of the 10% Zn/Ni-ZIF-8-PEBA MMM under pure-gas condition surpassed Robeson's upper bound reported in 2008.
1. Introduction The development of industry and population growth have turned people's attentions to the issues of climate change and global warming [1]. Mitigating the atmospheric CO2-emission issue and employing effective methods for CO2 trapping are urgent priorities. CO2 is commonly removed from gas mixtures by chemical absorption [2] and cryogenic separation [3], as well as other methods [4–6]. Recently, membrane separation technology has offered an effective method for the removal of CO2 from flue gases [7–10], which is a consequence of its high efficiency, adaptability, operational simplicity, low energy consumption, low cost, and environmental friendliness [11,12]. Membranes are conventionally divided into inorganic membranes, polymer organic membranes, and organic-inorganic hybrid membranes according to their structure [13]. Inorganic membranes have the advantages of resisting harsh chemical cleaning, high temperature and wear resistance, high chemical stabilities and long lifetimes, and autoclavabilities [14]. These outstanding properties make inorganic ⁎
membranes good candidates for use in gas-separation applications. However, as a result of their costs and fragile structures, they are difficult to prepare on the large scale. Polymer materials such as polyimide [15], polyamide [16], poly(ethylene glycol) [17], poly(dimethyl siloxane) [18], poly(vinyl alcohol) [19], poly(ethylene oxide) [20], poly (vinylimidazolium) [21], and poly(ether block amide) [22] have been extensively used to prepare polymeric membranes for CO2 separation, and have received considerable attention because of their excellent mechanical performance, reproducibilities, and relatively economical processing capabilities. However, gas permeability and selectivity are restricted by the trade-off introduced by Robeson [23]. Inorganic particles have been embedded into polymeric matrices to produce mixed-matrix membranes (MMMs) [24] that can deliver both high permeability and selectivity due to the advantages of the two phases involved. Specifically, inorganic particles disrupt molecularchain packing, where a relaxed membrane structure is beneficial for gas transport, leading to enhanced permeability. Furthermore, the adapted inorganic-particle structure not only improved the affinity for the
Corresponding author at: College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China. Corresponding author. E-mail addresses: [email protected] (Y. Wang), [email protected] (N. Li).
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https://doi.org/10.1016/j.memsci.2018.05.004 Received 15 January 2018; Received in revised form 30 April 2018; Accepted 3 May 2018 Available online 04 May 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.
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2. Experimental
targeted gas, but also supplied diffusion channels to match the kinetic diameters of the gas molecules [25]. Exploiting defect-free MMMs is challenging because MMM fabrication is confronted by polymer-filler incompatibility issues. The formations of non-selective voids, a sieve-in cage morphology, polymer rigidification, and pore blockages are due to poor polymer-filler interactions that result in the deterioration of membrane performance [26]. Hence, the judicious selection of the polymer and inorganic filler is crucial for MMM development. Over the years, zeolites, silica, and metal-organic frameworks (MOFs), as inorganic MMM fillers, have been shown to improve membrane performance [27]. Zeolitic imidazolate frameworks (ZIFs), a subclass of MOF that is composed of inorganic metal clusters coordinated with organic imidazole ligands, have been extensively used as membranes in gas-separation applications. ZIF-8 [28–32], a member of this family with sodalite topology, is highly porous and thermally stable. Moreover, owing to the unique pore size (0.34 nm) and structure of this microporous material, molecular-sieving behaviour of ZIF-8 for CO2 has been extensively investigated in recent years. In addition, ZIF-8 is suitable for addition to gas-separating polymer-membrane materials due to its flexible framework [33–35]. PEBA block copolymers, which are synthesized from dicarboxylic-acid-terminated aliphatic polyamides and polyoxyalkylene glycols, exhibit excellent CO2-separation performance [36]. Hägg et al. [30] developed ZIF-8-PEBA MMMs for CO2/N2 separation; these MMMs were two-layer membranes composed of a polymer layer with a few dispersed inorganic particles, and an inorganic layer. The permeability was found to increase with increasing amounts of inorganic filler, while selectivity decreased slightly in single-gas experiments due to increments in the ZIF-8 selective-particle content. Mosleh et al. [37] synthesized a ZIF-8 suspension that was directly incorporated into a PEBA matrix in order to improve the gas-separation performance of the membrane. The CO2 and N2 permeabilities of the ZIF-8-PEBA MMM loaded with 30% ZIF-8 were increased by 111% and 99%, respectively. However, to the best of our knowledge, research on ZIF-8-PEBA MMMs is still in an exploratory stage. In recent years, many researchers have demonstrated that π-complexation between transition-metal ions and CO2 effectively enhances CO2 permeability and selectivity in the dry state [38,39]. Nickel-substituted ZIF-8 (Zn/Ni-ZIF-8) [40], in which nickel ions are in situ incorporated to the ZIF-8 skeleton without altering the ZIF-8 structure, combines molecular sieving with the π-complexation mechanism of transition-metal ions for CO2 separation. In Zn/Ni-ZIF-8, zinc and nickel ions exhibit different affinities towards CO2 due to their inherent characteristics that include the numbers of virtual orbitals, charges, and ionic radii, among others, resulting in the asymmetric adsorption of CO2-gas molecules. Moreover, Zn/Ni-ZIF-8 has an intersecting 3-D network, with a very high number of pores, a high surface area, and is compatible with polymers that are potential MMM fillers for CO2 separation [41]. Herein, we first employed Zn/Ni-ZIF-8 as a filler during the preparation of MMMs with PEBA block copolymers for high-performance gas-separation membranes. The effects of Zn/Ni-ZIF-8 loading on the mechanical properties and microstructure were investigated. The influences of filler loading and feed pressure on gas-permeation performance were also studied, and the asymmetric adsorption/desorption transport mechanism of these MMMs for CO2 is discussed on a preliminary basis by investigating the effects of different MMM Ni-ion combination pathways on gas-permeation performance of the as-prepared membranes. We found that Zn/Ni-ZIF-8 was uniformly dispersed in these MMMs, and that Zn/Ni-ZIF-8 exhibited good compatibility with the PEBA in these MMMs. As a consequence, a Zn/Ni-ZIF-8-PEBA MMM exhibited good CO2 permselectivity when compared to the ZIF-8-PEBA MMM, with long-term stability over 150 h.
2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 98%) were received from Alfa Aesar, and 2-methylimidazole (MeIm, 99%) was obtained from Aladdin. PEBA polymer (grade 2533) was provided by Arkema Inc., France. Methanol (MeOH, 98%) was purchased from the Sinopharm Chemical Reagent Co., Ltd., and N,N-dimethylacetamide (DMAc) was purchased from the Tianjin Damao Chemical Reagent Plant. N2 and CO2, with purities of 99.999%, and the mixed gas (CO2/N2 = 20/80 vol%) were provided by the Shanxi Special Gases Company. All chemicals were used as received without further treatment. 2.2. Synthesis of Zn/Ni-ZIF-8 Zn/Ni-ZIF-8 was synthesized by a solvothermal method [40]. The synthesis solution with molar ratio of Zn2+:Ni2+:MeIm = 1:1:16 was prepared as follows. Specifically, Zn(NO3)2·6H2O (1.188 g), Ni (NO3)2·6H2O (1.172 g), and MeIm (5.296 g) were dissolved separately in 100 mL methanol (designated as solution A, solution B and solution C respectively). The solution A and B were added into solution C rapidly and continuously stirred at 25 °C for 24 h. A violet powder was acquired by centrifugation at 10,000 rpm for 10 min; this powder was washed with methanol three times to remove guest molecules from the pores of the material, after which it was dried under vacuum at 80 °C. For comparison, ZIF-8 was prepared by dissolving Zn(NO3)2·6H2O (1.188 g) and MeIm (2.648 g) in methanol; the molar ratio of two reactants in this solution was 1:8. ZIF-8 was synthesized from this solution using a similar process to that used for the preparation of Zn/NiZIF-8 [41]. The yields of Zn/Ni-ZIF-8 and ZIF-8 particles, based on the limiting zinc reactant, were about 57% and 48%, respectively. 2.3. MMM fabrication To prepare the Zn/Ni-ZIF-8-PEBA MMMs, PEBA2533 was added to N,N-dimethylacetamide (DMAc) such that a 6% PEBA solution was obtained. At the same time, specific amounts of Zn/Ni-ZIF-8 were dispersed in DMAc with sonication for 2 h, to produce Zn/Ni-ZIF-8 suspensions. Each suspension was separately added to the aforementioned PEBA solution, and stirred for 8 h at 67 °C. The mixture was then poured onto a clean glass plate, and the cast membrane was placed in a vacuum oven at 30 °C for 24 h, and then dried at 50 °C for about another 24 h. The temperature was then gradually increased to 70 °C and maintained at this temperature for 24 h to remove residual solvent. Thereafter, each membrane was peeled from the glass plate and dried at 60 °C to constant weight prior to characterization or gas-permeation performance testing. For comparison, pure PEBA membrane and ZIF-8-PEBA MMMs were fabricated using similar procedures. In addition, a 5% Zn-Ni-MeIm-PEBA MMM was prepared in order to investigate the effect of the Ni-ion-combination pathway on gas-separation performance, as follows. Zn(NO3)2·6H2O (12 mg), Ni (NO3)2·6H2O (11.6 mg), and MeIm (6.5 mg) were added to DMAc (3 g) and sonicated for 2 h to produce a homogeneous dispersion. This dispersion was then added to a 6% PEBA solution in DMAc and stirred for 8 h at 67 °C. The Zn-Ni-MeIm-PEBA MMM was then fabricated in a similar manner to that described for the fabrication of the Zn/Ni-ZIF-8PEBA MMMs. The dry thicknesses of all membranes used for gas separation was 30 ± 3 µm. X% filler-PEBA refers to the percentage (X) of the filler in the PEBA matrix. 2.4. Characterization The crystal structure of Zn/Ni-ZIF-8 was characterized by X-ray diffraction (XRD, Shimadzu, XRD-6000, Japan) at a scanning rate of 39
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4° min−1 and a 5–65° angular range. Attenuated total internal reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy of the MMMs was conducted using a Nicolet 6700 spectrometer (Thermo Scientific, USA) over the 650–4000 cm−1 range at a resolution of 4 cm−1. N2 adsorption-desorption isotherms were acquired at 77 K in order to determine surface areas and pore volume by the BrunauerEmmett-Teller (BET) method using a JW-BK122W surface area and micropore size analyser (JWGB Sci & Tech Co., Ltd., Japan). Before analysis, each sample was outgassed under vacuum at 180 °C for 8 h. The morphological structures of the Zn/Ni-ZIF-8 and Zn/Ni-ZIF-8-PEBA MMMs were examined by scanning electron microscopy (SEM) (Hitachi, SU8010, Japan) at 3 kV, together with energy-dispersive X-ray spectroscopy (EDXS) for the analysis of elemental composition and content. The CO2-adsorption isotherms of ZIF-8 and Zn/Ni-ZIF-8 were acquired in the 0–100 kPa range at 298 K on a NOVA 1200e adsorption analyser (Quantachrome, USA). Samples were degassed at 300 °C and 10−2 Pa for 10 h prior to any experiment.
Fig. 1. CO2-adsorption isotherms for ZIF-8 and Zn/Ni-ZIF-8 nanoparticles at 298 K.
2.5. Mechanical properties
and permeate, respectively.
Tensile strengths and elongations at break were measured using a WDW-100 universal testing machine (Jinan MTS Test Technology Co. Ltd, China) at a speed of 25 mm min−1. All samples were prepared with widths of 10 mm and lengths of 60 mm [42]. Five pieces of each sample were tested, and an average value was obtained.
3. Results and discussion 3.1. Characterization of Zn/Ni-ZIF-8 Zn/Ni-ZIF-8 nanoparticles were synthesized according to a previous report [41], while ZIF-8 was prepared by a solvothermal method for control experiments. SEM revealed that ZIF-8 and Zn/Ni-ZIF-8 have nanocrystalline and porous packing structures with uniform crystal distributions (Fig. S2). The ZIF-8 crystals were determined to be 40–60 nm in size; however, Zn/Ni-ZIF-8 crystals were smaller (20–40 nm), which is probably due to their lower crystal-growth rate and the presence of Ni ions that are smaller than Zn ions. Moreover, the final amounts of Zn and Ni in Zn/Ni-ZIF-8 were 17.8% and 1.48%, respectively, as determined from elemental-analysis data (Table S1). As shown in Fig. S3, the powder X-ray diffraction patterns of Zn/Ni-ZIF-8 reveal a highly crystalline structure; these patterns are well matched with those simulated for ZIF-8 and are in agreement with the reported literature [41]. CO2-adsorption isotherms for the ZIF-8 and Zn/Ni-ZIF-8 nanoparticles at 298 K are displayed in Fig. 1; CO2 adsorption grew in a linear manner with increasing pressure. The CO2-adsorption isotherm and adsorption capacity of ZIF-8 were similar to those previously reported [44]. Zn/Ni-ZIF-8 exhibited a higher CO2-adsorption capacity than ZIF-8, highlighting its outstanding affinity for CO2, which is ascribable to dual metal ions (zinc and nickel) that exhibit different binding behaviour toward CO2 based on the π-complexation mechanism, resulting in asymmetric CO2 adsorption.
2.6. Gas-separation experiments Gas-permeation experiments were carried out using the constantpressure and variable-volume method. Fig. S1 displays a schematic diagram of the apparatus used for the gas-permeation experiments. The pure-gas permeation experiments were performed using CO2 and N2 with purities of 99.999%, and under pressure conditions that ranged from 1 to 5 bar at 25 °C. Mixed-gas permeation experiments were performed using a binary gas mixture (CO2/N2 = 20/80 vol%) at a pressure of 2 bar and a temperature of 25 °C. Before the first experiment, the tested gas was introduced for at least 3 h for degassing purposes. Between two subsequent experiments, the system was flushed with the latter gas for a time sufficient to guarantee the complete removal of the previous gas. H2, at flow rate of 30 mL min−1, was selected as the sweep gas, and the feed-gas flow rate was 60 mL min−1. The upstream and downstream flow rates were measured using soap-bubble flow meters. The downstream compositions were determined by gas chromatography (GC-2014C, Shimadzu) with a thermal conductivity detector. After the system reached steady state, each gas-permeation experiment was repeated more than five times. The permeability of each gas was calculated using Eq. (1) [43]:
Pi =
Qil ∆pi A
3.2. MMM characterization (1) Scheme 1 depicts the fabrication of a Zn/Ni-ZIF-8-PEBA MMM. As shown in Scheme 1, Zn(NO3)2·6H2O, Ni(NO3)2·6H2O, and MeIm are uniformly mixed, resulting in the immediate formation of a green dodecahedral. In this intermediate, a zinc atom (depicted in blue) is coordinated to four nitrogen atoms in MeIm to form a tetrahedron, and a nickel atom (green) is coordinated to six atoms (nitrogen atoms or solvent molecules) to produce an unstable sexadentate structure. The Ni-coordination mode is then transformed from metastable sexadentate to stable tetrahedral by the action of methanol to afford a purple dodecahedral structure [40]. Finally, these (as-prepared) particles are added to PEBA to produce a purple solution, and the Zn/Ni-ZIF-8-PEBA MMM is prepared by solution casting. The Zn/Ni-ZIF-8 is uniformly distributed in the PEBA matrix probably as a result of asymmetric repulsive forces that originate from the dual metal ions in the purple dodecahedral structure. Fig. S4 reveals that the BET surface area and micropore volume of Zn/Ni-ZIF-8 were 1839 m2 g−1 and 0.71 cm3 g−1,
where Pi is the permeability of gas ‘i’ (Barrer, 1 Barrer = 1 × 10−10 cm3 (STP) cm/(cm2 s cmHg)); Qi is the volumetric flow rate of gas ‘i’ (cm3 (STP)/s); l (cm) is the thickness of the membrane measured by a micrometre calliper, Δpi is the transmembrane-pressure difference (cmHg), and A is the effective membrane area (12.56 cm2). The ideal selectivity (αij) of the pure gas was calculated using Eq. (2) [43]:
α ij =
Pi Pj
The mixed-gas separation factor of gases ‘i and j’ lated using Eq. (3) [43]:
α ij* =
(2) (αij*)
was calcu-
yi / yj x i /x j
(3)
where x and y are the volumetric fractions of one component in the feed 40
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respectively, while those of ZIF-8 were 1228 m2 g−1 and 0.41 cm3 g−1, respectively, in agreement with the literature [45,46]. Zn/Ni-ZIF-8 exhibited the larger surface area, which may has more organic moiety and metal ions in external surface. Zn/Ni-ZIF-8 is possibly in tight contact with the macromolecular chains of PEBA through hydrogen bonding and van der Waals forces [47]. Moreover, chelation between the metal (zinc and nickel) nodes and the ester groups of PEBA, as well as hydrogen bonding between the PEBA chains and the imidazoles of Zn/Ni-ZIF-8, may also contribute to stronger interfacial interactions [48]. Fig. 2 shows the cross-sectional morphologies of the pure PEBA membrane and the Zn/Ni-ZIF-8-PEBA MMMs with various particle loadings. As shown in Fig. 2(a), the pure PEBA membrane is smooth, while the Zn/Ni-ZIF-8-PEBA MMMs are rough. This observation is attributed to the elimination of intermolecular hydrogen bonding and a decline in the crystallinity of the polyether and polyamide blocks following addition of Zn/Ni-ZIF-8 to the PEBA matrix [49,50]. As shown in Fig. 2(b-c), Zn/Ni-ZIF-8 particles are well dispersed in the MMMs at low loadings, and large conglomerates of particles as obvious MOFpolymer interface are not observable in SEM images. However, the slight agglomeration was observed when the Zn/Ni-ZIF-8 loading in PEBA exceed 15% (Fig. 2(d-e)), but there were no visible voids at the
Scheme 1. Depicting the fabrication of a Zn/Ni-ZIF-8-PEBA MMM. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).
Fig. 2. Cross-sectional SEM images of a (a) pure PEBA membrane and Zn/Ni-ZIF-8-PEBA MMMs with Zn/Ni-ZIF-8 loading of (b) 5, (c) 10, (d) 15, and (e) 20%. Each inset displays the corresponding enlarged image. The yellow ellipses highlight agglomerated nanoparticles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). 41
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Fig. 4. Effect of filler content on the mechanical properties of the Zn/Ni-ZIF-8PEBA and ZIF-8-PEBA MMMs. Fig. 3. ATR-FTIR spectra of the pure PEBA membrane and the Zn-Ni-MeImPEBA, ZIF-8-PEBA, and Zn/Ni-ZIF-8-PEBA MMMs.
interfaces between the Zn/Ni-ZIF-8 agglomerates and the polymer. This demonstrates that Zn/Ni-ZIF-8 is compatible with the PEBA-polymer molecular chains, which is a result of the electron-rich properties of the Zn/Ni-ZIF-8 framework that facilitates strong electrostatic interactions between Zn/Ni-ZIF-8 and the PEBA-polymer chains. Consequently, these Zn/Ni-ZIF-8-PEBA MMMs are very dense and defect-free, and meet the requirements for gas-separation applications. The ATR-FTIR spectra of the membranes are displayed in Fig. 3, which reveal characteristic peaks corresponding to PEBA for all of the membranes. The peak at 3298 cm−1 is assigned to the amide -N-H stretching vibration, while the band at 1735 cm−1 is attributed to the -C=O stretching vibration of the carboxylic acid [51]. The absorption peak at 1637 cm−1 is ascribed to the out-of-plane H-N-C=O vibration of the amide, and the peak at 1106 cm−1 is attributed to the polyether C-O-C stretching vibration, while the band at 1366 cm−1 is associated with the amide C-N stretching mode [49]. The aforementioned peaks are consistent with the structure of PEBA2533, which is composed of 80% polyether and 20% polyamide [52]. The peaks at 1366 cm−1 and 1032 cm−1 observed in the spectra of the Zn-Ni-MeIm-PEBA MMMs, are assigned to the asymmetric and symmetric stretching vibrations of NO3–, respectively, and the peak at 825 cm−1 is attributed to the nitrate out-of-plane bending vibration [53]. The peak at 1597 cm−1 is ascribed to the skeletal vibration of MeIm. These spectra demonstrated that the Zn-Ni-MeIm-PEBA MMMs are mixtures of Zn(NO3)2·6H2O, Ni (NO3)2·6H2O, MeIm, and PEBA. In the case of the MMMs, the absorption peak at 3298 cm−1 was red shifted following the addition of ZIF-8 or Zn/Ni-ZIF-8 to PEBA, confirming that hydrogen bonding exists in these MMMs [50]. Fig. 5. Effect of filler loading on (a) the CO2 permeability and (b) CO2/N2 selectivity of the ZIF-8-PEBA and Zn/Ni-ZIF-8-PEBA MMMs.
3.3. Mechanical properties Except for gas-separation performance, MMMs are required to have good mechanical properties for industrial applications, because durability is very important to the overall membrane-operating process. Fig. 4 displays the effect of filler loading on tensile strength and elongation at break of the MMMs. As shown in Fig. 4, the tensile strength and elongation at break of the Zn/Ni-ZIF-8-PEBA increased slightly as the filler loading was increased to 10%, after which it sharply declined. However, the tensile strength and elongation at break of the ZIF-8PEBA MMM decreased steadily with increasing ZIF-8 loading. The maximum value of tensile strength and elongation at break for the Zn/ Ni-ZIF-8 MMMs are clearly higher than those of the ZIF-8-containing MMMs, which is ascribable to the homogeneous distribution of Zn/NiZIF-8 and its good adhesion to PEBA. The reductions in the tensile strength of the Zn/Ni-ZIF-8-PEBA MMM at filler loadings greater than 10% are due to aggregation.
3.4. Gas-separation performance 3.4.1. Effect of ZIF loading Fig. 5 shows the effect of filler loading on the CO2 permeability and CO2/N2 selectivity of the MMMs. Pure-gas permeation testing was performed at 2 bar. As shown in Fig. 5(a), the CO2 permeability of the Zn/Ni-ZIF-8-PEBA MMM increased dramatically with increasing Zn/NiZIF-8 loading, while the CO2 permeability of the ZIF-8-PEBA MMM increased slowly. The maximum CO2 permeability of the MMMs loaded with 20% Zn/Ni-ZIF-8 and ZIF-8 were 478 and 294 Barrer, respectively, which represent increases of 193% and 80% compared to the permeability of the pure PEBA membrane, respectively. This is ascribable to disrupted polymer-matrix-chain packing as a result of the introduction of Zn/Ni-ZIF-8 and ZIF-8, which resulted in an increase in chain 42
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resulting in an increase of the concentration of gas transported into the membrane matrix; consequently a high CO2 permeability was obtained. The CO2 permeability of the MMM was observed to increase with increasing Zn/Ni-ZIF-8 loading, from 255 to 521 Barrer as the Zn/Ni-ZIF8 loading was increased from 0% to 20%, which is ascribable to larger amounts of Zn/Ni-ZIF-8 particles leading to more free volume and surface voids, and, as a result, higher CO2 permeability. As showed in Fig. 6(b), CO2/N2 selectivity also increased with increasing feed pressure. The CO2/N2 selectivity of pure PEBA increased from 21.6 to 31.7 as the feed pressure was increased from 1 to 5 bar, whereas the CO2/N2 selectivity of the MMM loaded with 10% Zn/Ni-ZIF-8 increased from 39.6 to 51.8. This is possibly due to the semi-crystalline nature of the PEBA membrane, which is composed of both hard and soft segments that resist matrix compaction at higher pressures; consequently PEBA is not easily compacted with increasing pressure. Under these circumstances, CO2 permeability was observed to improve with increasing feed pressure [49]; meanwhile feed pressure had little influence on N2 permeability. Moreover, the gas-transport tortuosity increased following incorporation of Zn/Ni-ZIF-8 into the PEBA matrix [55], which resulted in an increase in CO2/N2 selectivity. Zn/Ni-ZIF-8 became increasingly agglomerated with increasing Zn/Ni-ZIF-8 loading (more than 10%), which led to larger free volumes and interfacial voids; consequently, reduced CO2/N2 selectivity was observed.
mobility and free volume [54]. The CO2 permeability of the Zn/Ni-ZIF8-PEBA MMMs were higher than those of the ZIF-8-PEBA MMMs because Zn/Ni-ZIF-8 possesses a larger BET surface area and micropore volume, as well as a higher affinity and adsorption capacity for CO2 due to interactions between the quadrupolar CO2 molecules and the positive charges of the unsaturated metal sites. As shown in Fig. 5(b), each MMM exhibited increased CO2/N2 selectivity with increasing Zn/Ni-ZIF-8 or ZIF-8 loading, to a maximum value of 42.8 or 33.8, respectively, at 10% loading. This is ascribable to good particle dispersion in the MMMs at low loadings and the associated molecular-sieving effect for CO2. In addition, as the ZIF-8 or Zn/ Ni-ZIF-8 loading in the polymer matrix exceeded 10%, the CO2/N2 selectivity began to decline as a result of agglomeration and the occurrences of surface voids. Clearly, the Zn/Ni-ZIF-8 MMMs exhibited CO2/N2 selectivity that were always higher than those of the ZIF-8containing MMMs, which is explained by the presence of active metal sites that can react with CO2 through π-complexation, resulting in a stronger affinity towards CO2 [39]. In addition, the zinc and nickel metals in the framework adsorb CO2 differently, which results in quick adsorption/desorption in the MMM and a higher CO2/N2 selectivity. 3.4.2. Effect of feed pressure Fig. 6 shows the effect of feed pressure on CO2 permeability and CO2/N2 selectivity for the pure PEBA membrane and the MMMs. As shown in Fig. 6(a), the CO2 permeability of pure PEBA membrane and the MMMs increase remarkably with increasing feed pressure. The CO2 permeability of the pure PEBA membrane increased from 118 to 255 Barrer as the feed pressure was increased from 1 to 5 bar, while the CO2 permeability of the MMM with 20% Zn/Ni-ZIF-8 loading increased from 438 to 574 Barrer. The reason for these observations involves the gas driving force, which is enhanced with increasing feed pressure,
3.4.3. Mixed-gas separation performance Separation efficiency is influenced by other gases in a practical setting; consequently, a binary gas mixture (CO2/N2 = 20/80 vol%) was examined. Fig. 7 displays the CO2 permeability and CO2/N2 selectivity of the pure membrane and the MMMs. Mixed-gas-separation
Fig. 7. (a) CO2 permeability and (b) CO2/N2 selectivity of as-prepared membranes as determined by pure-gas and mixed-gas testing at feed pressures of 2 bar.
Fig. 6. Effect of feed pressure on (a) the CO2 permeability and (b) CO2/N2 selectivity of the pure PEBA membrane and the Zn/Ni-ZIF-8-PEBA MMMs. 43
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MMM was lower than that of the pure PEBA membrane. We assume that CO2-affinity sites were consumed by chelating reactions between metal ions and the polyether. As shown in Fig. 8(b), CO2/N2 selectivity declined from 45.2 for the Zn/Ni-ZIF-8-PEBA MMM to 19.9 for the Zn-NiMeIm-ZIF-8-PEBA MMM at 5 bar. It was noted that the Zn/Ni-ZIF-8PEBA MMM exhibited the highest CO2/N2 selectivity, which is ascribable to the Zn-Ni tetradentate structure of the Zn/Ni-ZIF-8 cage, which results in asymmetric adsorption/desorption and rapid permselectivity in the MMM. Scheme 2 displays the gas-transport pathway for a Zn/Ni-ZIF-8PEBA MMM. Gas molecules are transported through the membrane under differential pressure as the driving force. Like most MMMs [57], passing through the polymer matrix and the inorganic nanofiller phase mainly relies on the molecular solution-diffusion mechanism. For the Zn/Ni-ZIF-8 MMM, synergism between molecular sieving and asymmetric adsorption/desorption affects gas-separation performance. The structure of Zn/Ni-ZIF-8 is similar to that of ZIF-8; it has a pore size of 3.4 Å that lies between the kinetic diameters of CO2 (3.3 Å) and N2 (3.64 Å) that contributes to CO2/N2 selectivity. On the basis of the πcomplexation mechanism, zinc and nickel ions have different adsorption strengths toward CO2, which results in the asymmetric adsorption/ desorption of CO2. Four kinds of interactions exist between the metal ions and the CO2 molecules based on π-complexation [38,58]. As shown in Scheme 2 (I-II), CO2 can donate its lone pair of electrons or π electrons to the empty s or d orbitals of a metal ion. As shown in Scheme 2 (III), a metal ion can provide a lone electron to the CO2 π-bonding orbital, while Scheme 2 (IV) shows the CO2 π* antibonding orbital receiving electrons from a metal ion. This π-complexation reaction is reversible, which facilitates CO2 transport. Therefore, the Zn/Ni-ZIF-8PEBA MMMs exhibit high CO2 permeability and CO2/N2 selectivity.
performance was observed to follow the same trend as that for the pure gas. As shown in Fig. 7(a), the CO2 permeability of the mixed-gas was markedly lower than that of the pure gas; the maximum CO2 permeability of the MMM loaded with 20% Zn/Ni-ZIF-8 was 426 Barrer for the mixed gas and 478 Barrer for the pure gas. The decline in the CO2 permeability of the mixed gas is ascribable to N2 molecules in the feed component that interfere with the plasticizing effect of CO2 [54]. As shown in Fig. 7(b), the CO2/N2 selectivity of the mixed gas were lower than their corresponding ideal selectivity, with the maximum CO2/N2 selectivity of the MMM loaded with 10% Zn/Ni-ZIF-8 observed to be 42.8 for the pure gas and 41.9 for the mixed gas, which is due to competitive adsorption in the mixed gas [56]. Fig. S5 displays the stability of the 10% Zn/Ni-ZIF-8-PEBA MMM, which was tested with a simulated flue gas (CO2/N2 = 20/80 vol%). The MMM exhibited an average CO2 permeability of 282 Barrer and a CO2/N2 selectivity of 42.7, which fluctuated only slightly. MMM performance was found to be stable for 150 h, and no degradation in either CO2 permeability or CO2/N2 selectivity was observed, which confirms that there was no obvious change in membrane structure. Based on above discussion, Zn/Ni-ZIF-8-PEBA MMMs exhibit exceptional permeability and selectivity compared to ZIF-8-PEBA MMMs and the pure PEBA membrane. We assume that the combination pathways between Ni ions and MeIm directly affect gas-permeation performance. Fig. 8 shows the effect of the Ni-ion combination pathway on CO2 permeability and CO2/N2 selectivity. As shown in Fig. 8(a), the 5% Zn/Ni-ZIF-8-PEBA MMM exhibited better permeation performance than that of the 5% Zn-Ni-MeIm-PEBA MMM and the pure PEBA membrane, which is due to the π-complexation-facilitated transport between metal ions and CO2. However, the CO2 permeability of the Zn-Ni-MeIm-PEBA
3.5. Comparisons with Robeson's upper bound Fig. 9 compares the pure-gas separation performance of the asprepared MMMs in relation to the proverbial Robeson's upper bound. With the exception of the 5% Zn-Ni-MeIm-PEBA MMM, Fig. 9 reveals that all of the MMMs exhibit improvements in both CO2 permeability and CO2/N2 selectivity compared with the pure PEBA membrane. The anti-trade-off gas-separation phenomenon was observed for the Zn/NiZIF-8-PEBA MMMs, which exhibited improvements in CO2/N2 selectivity with elevated CO2 permeability. In particular, the performance of the 10% Zn/Ni-ZIF-8-PEBA MMM actually surpassed Robeson's upper bound, indicating its excellent prospects for CO2-capture and flue-gasseparation applications. 4. Conclusions Zn/Ni-ZIF-8 particles were synthesized as stable Zn-Ni framework structures that exhibited higher CO2 absorption capacities than ZIF-8 due to the dual metal sites present in these frameworks. Zn/Ni-ZIF-8PEBA MMMs were fabricated for CO2/N2 separation by the solution casting method; these membranes exhibited excellent mechanical properties as a result of good compatibility between the two phases. The optimum CO2 permeability and selectivity of the MMM loaded with 10% Zn/Ni-ZIF-8 was 408 Barrer and 51.8 at 5 bar, which represents an increase of 60% and 64% compared to those of the pure PEBA membrane, and surpasses Robeson's upper bound reported in 2008. The Zn/ Ni-ZIF-8-PEBA MMMs display outstanding permeation performance because Zn/Ni-ZIF-8 exhibits a stronger affinity for CO2, leading to upstream absorption in the MMM, fast transport in the membrane, and rapid downstream desorption, compared to those of N2. On the other hand, the CO2 permeability and CO2/N2 selectivity of the 5% Zn-NiMeIm-PEBA MMM was 236 Barrer and 19.9, respectively, which is lower than that of the Zn/Ni-ZIF-8 MMM due to the absence of the ZnNi framework structure. Moreover, the Zn/Ni-ZIF-8-PEBA MMM exhibited stable permselectivity in a simulated flue-gas test over a long
Fig. 8. Effect of Ni-ion combination pathway on (a) the CO2 permeability and (b) CO2/N2 selectivity for as-prepared membranes. 44
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Scheme 2. Depicting the gas-transport pathway for a Zn/Ni-ZIF-8 MMM.
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Fig. 9. CO2/N2-separation performance of the as-prepared membrane compared to Robeson's upper bound.
time.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21506140), the Joint Fund of Shanxi Provincial Coal Seam Gas (No. 2015012009), China Postdoctoral Science Foundation (No. 2016M601289), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No. 2015134), the Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province, and partially supported by the National Natural Science Foundation of China (Nos. 51603225 and U1510123) and the Hundred Talents Program of the Chinese Academy of Sciences.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2018.05.004. 45
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