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A porous ionic polymer bionic carrier in a mixed matrix membrane for facilitating selective CO2 permeability
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A porous ionic polymer bionic carrier in a mixed matrix membrane for facilitating selective CO2 permeability Chenhui Wang a, Fangyuan Guo a, He Li a, Jian Xu b, Jun Hu a, *, Honglai Liu a, Meihong Wang c, ** a
State Key Laboratory of Chemical Engineering and School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, China b Shanghai Institute of Measurement and Testing Technology, 1500 Zhangheng Road, Shanghai, 201203, China c Department of Chemical and Biological Engineering, The University of Sheffield, Western Bank, Sheffield, S10 2TN, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Mixed matrix membranes Porous ionic polymer PIM-1 CO2 separation Pyridine-based porous ionic polymer
Inspired by the most efficient CO2 transport across natural biological membranes, a novel bionic channel facilitating CO2 permeation was realized via adding a filtering porous ionic polymer (PIP) in a mixed matrix membrane (MMM). A series of polymer of intrinsic microporosity (PIM-1) based MMMs were fabricated by three pyridine-based PIPs (PIP-Py-X) with different Cl , Ac , and BF4 anions as fillers. PIP-Py-X PIPs show highly selective CO2 adsorption capacity and perfect interfacial compatibility with the PIM-1 matrix. More significantly, acting as carriers, the movable anions in the MMMs efficiently facilitated the transport of adsorbed CO2 across the membranes. As a result, the obtained PIM-Py-Ac-15 MMM exhibited an excellent permeability for CO2 of 6205 barrer and a CO2/N2 and CO2/CH4 selectivity of 62.5 and 56.1, respectively, far surpassing the 2008 Robeson upper bound and exceeding most of the reported advanced membranes. As a divertive of PIPs, this work opens a window for designing and fabricating promising MMMs for task-specific membrane separations.
1. Introduction Greenhouse effect caused by excessive carbon dioxide (CO2) emis sons has attracted great concern, increasing the importance of CO2 separation from flue gases [1]. Membrane separation has been shown to be one of the most efficient separation processes [2]. However, most membranes still suffer from the trade-off between permeability and selectivity, although great efforts have been devoted to improving sep aration performance [3–5]. In natural CO2 transport across biological membranes, two pathways, (1) the free diffusion of solution-diffusion mechanism and (2) the protein-based facilitated transport mechanism, endow the most efficient CO2 separation processes in the world [6]. It has been demonstrated that carrier proteins, such as aquaporin (AQP), can specifically and reversibly bind with CO2 molecules and transport them to the other side of the cell membrane via conformational variation [7,8]. Inspired by the accelerated CO2 transport by HCO3 -Cl- self-exchange permeability in a red cell’s membrane [9], we took a particular interest in membranes modified by ionic liquids (ILs) due to their excellent af finity towards CO2 as well as the exchangeable ions [10–13]. Coating ILs
on the support substrate is the simplest way to prepare supported ionic liquid membranes (SILMs) [14–16]. Among them, the best reported performance of CO2/N2 and CO2/CH4 selectivity were 151.4 and 72, and the highest CO2 permeability was 3352 barrer [17]. However, their insufficient stability, e. g. with a maximum operating pressure of no more than 0.25–0.3 MPa [18], makes large scale applications difficult [19–22]. Alternative, Ils composite polymer membranes (ILPMs) have been proposed by mixing Ils into the casting solutions [23], which showed an excellent performance, such as a CO2 permeability as high as 5800 barrer and a CO2/N2 selectivity as high as 30 [24]. Although the freely mobile IL carriers in membranes significantly enhanced the selectivity and permeability, the mechanical properties simultaneously decreased with increasing IL content, in general. Therefore, long-time operation remains difficult. Poly (ionic liquid)s membranes (PILMs), polymerized by IL-containing monomers, provided a method to avoid the mechanical problems even at high ILs content. However, these membranes can only enhance CO2/N2 selectivity at the sacrifice of permeability due to the low free volume caused by the linear polymer chains [25,26]. Therefore, great challenges remain to simultaneously improve the separation performance of permeability and selectivity, as
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Hu), [email protected] (M. Wang). https://doi.org/10.1016/j.memsci.2019.117677 Received 9 November 2019; Accepted 17 November 2019 Available online 21 November 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Chenhui Wang, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117677
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well as the mechanical stability. Mixed matrix membranes (MMMs), combining the advantages of matrix polymers and fillers, have been demonstrated as a promising strategy to meet the above challenges [27–29]. Using porous materials with ionic sites as the filler to produce MMMs could be a novel pathway when compared with other IL-based membranes. Porous ionic polymers (PIPs), a kind of cross-linked polymer with a polymer skeleton composed of ionic groups linked with rigid chemical bonds, aroused our attention [30–32]. Huang et al. reported a cationic microporous polymer network PyTTA-BFBIm-iCOF, after anion exchange, that exhibited a CO2 uptake of 17.6 wt% (4.0 mmol g 1) at 273 K and 1 bar [33]. The synthetic di versity endows PIPs with a high surface area, changeable counter-ions, and a high charge density. We anticipate the well-defined porosity in PIPs could provide CO2 transfer channels, while the charged polymer skeleton and moveable counter-ions could allow for novel CO2 transport through both “hopping” and “vehicle” mechanisms [32]. However, no promising works using PIPs as fillers to produce MMMs for facilitating CO2 separation have been reported, so far. Herein, we proposed a bionic facilitating CO2 separation strategy by using PIPs as fillers to produce MMMs. Because the polymer of intrinsic microporosity, PIM-1, shows extremely high gas permeability [34–36] and also good affinity towards triptycene-based porous organic poly mers, according to our previous work [37], we chose it as the matrix polymer. By using pyridine-based cationic PIPs with different Ac , BF4 , and Cl anions as fillers, the PIM-Py-X MMMs were produced to demonstrate their CO2 separation performance. The effects of the porosity and the type of anions were researched to understand the facilitating mechanism of CO2 separation. Due to the synergistic effects of each component in the MMMs, the CO2 separation performance of all three obtained PIM-1 based MMMs with PIPs as fillers were significantly enhanced, exceeding the 2008 Robeson upper bound for the CO2/N2 and CO2/CH4 gas pairs. As far as we know, PIM-Py-Ac-15 exhibits the best CO2 separation performance in most advanced membranes, with an excellent CO2 permeability of 6200 barrer, and a selectivity of CO2/N2 and CO2/CH4 as high as 62.7 and 56.1. Therefore, the bionic facilitating CO2 channels through PIM-1 based MMMs could be a promising strategy for CO2 separation.
The obtained brown powder was noted as PIP-Py-Cl.Then, TPFC-CH2Cl (0.30 g), pyridine (0.74 g), and acetonitrile (3.0 mL) were added to a flask and stirred at 90 � C for 24 h. The cooled mixture was filtered and briefly washed with methanol three times, then dried to afford PIP-Py-Cl. PIP-Py-Cl (0.05 g), NaAc (5 g), and deionized water (10 mL) were added to a 50 mL flask and stirred at room temperature for 24 h. The cooled mixture was isolated by filtration and subsequently washed thrice with deionized water. Next, the resultant PIP-Py-Ac was dried in 80 � C for 12 h. PIP-Py-BF4 was synthesized in a similar way, except the reactant NaAc was replaced by NaBF4 (5 g). 2.3. Fabrication of PIM-Py-X MMMs The PIP-Py-X PIPs were added to chloroform and ground in a plan etary ball mill at 500 rpm for 5 h. A mixture of the PIP-Py-X powder and PIM-1 polymer was then added to chloroform under an 1-h stirring, subsequently ultrasounded for another 1 h. The mass ratio of solvent to filler polymer was controlled at 19/1 (wt.%) and the mass ratio of filler adding to the MMMs ranged from 0 to 20 wt%. Thereafter, the solution was poured onto a home-made super flat glass Petri dish, and then left for 12 h, such that the chloroform evaporated at room temperature. After drying, the membranes were placed into a 100 � C vacuum oven to remove the remaining solvent. Finally, due to the excellent demolding properties of the membrane, it was easily lifted off the petri dishes. 2.4. Characterization
Triptycene and paraformaldehyde were purchased from SigmaAldrich. Formaldehyde and dimethoxymethane were purchased from Alfa Aesar and 1, 2-Dichloroethane was purchased from Aladdin. Phosphoric acid was purchased from Shanghai Macklin Biochemical Co., Ltd.
Thermal gravimetric analyses (TGA) of the PIPs and MMMs were analyzed by an STA 499 F3 Thermo-gravimetric Analyzer (Netzsch). Samples were heated with a constant heating rate of 10 � C min 1 to 800 � C under an N2 environment. N2 was used as the purge gas and its flow rate was controlled at 20 ml min 1. The chemical group of the PIPs were analyzed by Fourier transform infrared spectroscopy (FTIR) (NEXUS 470 (Thermo Nicolet)) with a scan range from 400 to 4000 cm 1. A field emission scanning electron microscope (FESEM), GeminiSEM 500 (Germany) was used to analyze the PIPs’ surface images. The surface of membranes were analyzed by Scanning Probe Microscope (*/Veeco/DI China). A Falion 60S (EDAX America) was used to analyze the element distribution of PIPs. The TEM images of the PIPs were analyzed with a High-Resolution Transmission Electron Microscope (HRTEM), JEM2100 (Japan). The surface area and pore size distribution of the mate rials was observed with a Micrometrics tristar II. N2 adsorption iso therms were performed at 77 K, ranging from 1.1 to 101 kPa. Before analysis, each sample was degassed for 3 h at 90 � C. The gas mole fractions on the permeate side were determined using a gas chromato graph equipped (Haixin GC950).
2.2. Synthesis of PIP-Py-X PIPs
2.5. Gas permeation measurements
The synthesis of the TPFC neutral porous organic polymer (POP) has been reported elsewhere [38]. Briefly, triptyene (1.28 g, 5 mmol) was dissolved in 10 mL dichloroethane in a 250 mL three-necked flask, and then 2.28 g of formaldehyde dimethyl acetal (FDA) (2 mmol) was added in the mixture. Anhydrous FeCl3 (4.88 g, 30 mmol) was then added. The three-necked flask was stirred for 5 h in a 45 � C oil bath and then stirred for 19 h at 80 � C. The mixture was cooled to room temperature, filtered, and then washed with methanol three times. The solid was placed in a Soxhlet apparatus and extracted with methanol for 24 h, and a brown solid of TPFC POP was obtained after drying. The TPFC powder was chloromethylated and ionized to produce the PIPs. (0.60 g) TPFC and (3.0 g) paraformaldehyde were successively added to a 350 mL flask, followed by (18 mL) acetic acid, (60 mL) concentrated hydrochloric acid, and (9 mL) phosphoric acid. The sealed flask was then held at 90 � C for three days. The cooled brown powder was filtered, followed by washing thrice with water and methanol and drying at 80 � C for 12 h.
Before gas permeation measurements, the membrane was cliped with an intermediate diameter of about 10 mm for testing. The thickness at different positions was measured with Vernier calipers and averaged. Gas permeability coefficients were measured at stable states and the average was calculated from at least three measurements. The definition of permeability (Pi, Barrer) was showed by equation (1), where Qi is the volume flow of gas “i” (cm3 s 1, STP); A is the area of the membrane (cm2); and Δpi is the difference of pressure between the permeation and feed sides of gas "i" (cmHg). L represents the thickness of the membrane (cm). According to our previous work, the single gas permeation test was carried out by a gas permeation evaluation apparatus [39]. A soap bubble flow meter was used to measure the CO2, N2 and CH4 fluxes. The gas permeability (P) and the ideal selectivity (S) of CO2/N2 and CO2/CH4 gas pairs were calculated by Eq. (1) and Eq. (2):
2. Experimental methods 2.1. Materials
P ¼ Qi =ðA � Δpi Þ � L ; 2
(1)
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Fig. 1. (a) Fourier transform infrared spectroscopy (FTIR) spectra and (b) CO2 adsorption-desorption isotherms at 273 K of PIP-Py-Cl, PIP-Py-Ac and PIP-Py-BF4.
2.6. Adsorption enthalpy calculation
(2)
Toth model was used to fit the CO2 adsorption isotherms under 273 K and 298 K. Their isotherms can be described by the Eq. (5).
The actual gas separation selectivity from the mixed gases of CO2/N2 and CO2/CH4 (1:1 v/v) was performed using a device similar to the pure gas permeation test described above. The total feed pressure was set as 2 bar. The gas molar fractions on the permeate side were analyzed using a gas chromatograph equipped with a thermal conductivity detector and He as a carrier gas. The determination include (1) He was used as a gas carrier, which made both peaks of CO2 and N2 can be detected. (2) For CO2/N2 gas pair, CH4 was used as a purge gas to fulfill the chamber of permeate side before the testing, since N2 is abundant in air and easily infiltrated into the device to dilute the CO2 concentration in the sample. For CO2/CH4 gas pair, N2 was used as a purge gas. The permeability and the separation factor of the mixed gas were calculated by Eq. (3), and Eq. (4): (3)
y =y α¼ A B xA =xB
(4)
=
P ¼ Qi =ðA � Δpi Þ � L � yi =xi
1
Q¼
Qm � ðB � PÞ t 1
=
� S ¼ Pi Pj :
ð1 þ B � PÞ t
(5)
where, Q is molar loading of CO2 (mol kg 1), Qm is saturation capacity of CO2 (mol kg 1), p is gas pressure (Pa), B and t are parameters in the Toth model. The adsorption enthalpy were calculate by the Clausius–Clapeyron equation (Eq. (6)). � � � � p1 ΔH 1 1 ¼ (6) ln R T1 T2 p2 where Ti represents the temperature for isotherm i, pi represents the pressure for isotherm i, R is 8.315 J (K mol) 1.
where y and x are the mole fractions of the permeate side and the feed side, respectively.
2.7. IAST selectivity calculation N2 adsorption isotherms were fitted by Single-site Langmuir model
Fig. 2. SEM and EDS mapping images of PIP-Py-Cl, PIP-Py-Ac and PIP-Py-BF4. 3
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(Eq. (7)). q ¼ qsat
bp 1 þ bp
(7)
where, q represents molar loading of gas components (mol kg 1), p is gas pressure (Pa),b represents a parameter in the single-site Langmuir isotherm (Pa 1) and qsat represents saturation capacity of gas compo nents (mol kg 1). CO2 adsorption isotherms were fitted by Dual-site Langmuir model (Eq. (8)), because both chemical and physical interaction should be taken into account. q ¼ q1;sat
b1 p b2 p þ q2;sat 1 þ b1 p 1 þ b2 p
(8)
where qsat,1, b1, qsat,2 and b2 are four parameters in the equation because two distinct adsorption sites are assumed to be existed. Ideal adsorbed solution theory (IAST) was used to calculate the adsorption selectivity (Eq. (9)) by pure-component isotherm fitting parameters. S¼
q1 =q2 p1 =p2
Fig. 3. Cross-section SEM images of (a) membrane of neat PIM-1, (b) MMM of PIM-Py-Cl-15, (c) MMM of PIM-Py-Ac-15 and (d) MMM of PIM-Py-BF4-15.].
(9)
rule (Fig. S3). Under the low adsorption capacity, the adsorption enthalpy of PIP-Py-Cl, PIP-Py-Ac and PIP-Py-BF4 was 42.3, 51.1 and 45.6 kJ mol 1, respectively (Fig. S3). With the highest adsorption enthalpy, Ac anions in PIP-Py-Ac exhibited the strongest interaction with CO2. Therefore, the anion in the PIPs played a significant role in the CO2 adsorption due to their affinity towards CO2 [42]. Accordingly, the IAST selectivity of the PIPs for CO2/N2 was also conformed to this order PIP-Py-Ac (25.7) >PIP-Py-BF4 (25.5) > PIP-Py-Cl (24.6).
where pi is partial pressure of gas i and qi represents the corresponding pure-component gas uptake amount at pi. For the simulated flue gas, the binary mixture contains 15% CO2 and 85% N2. 3. Results and discussion 3.1. Properties of pyridine-based PIPs
3.2. Properties of PIM-Py-Xs MMMs
Through a three-step synthesis procedure, including polymerization of triptycene and formaldehyde dimethyl acetal, followed by chlor omethylation and pyridine ionization, and finally anion exchange, the pyridine-based PIPs with different anions of PIP-Py-Cl, PIP-Py-Ac, and PIP-Py-BF4 were produced (Scheme. S1). The characteristic bands of the benzene ring at 1774 cm 1, CH2 group at around 2800–3000 cm 1 and 1467 cm 1, and the pyridine ring at 1384 cm 1 were observed in the FTIR comparative spectra. The structure information of TPFC POP and the pyridine-based PIPs were further revealed as proposed (Fig. 1a). More significantly, a new band at 1728 cm 1 in the FTIR spectrum of PIP-Py-Ac and a band at 1083 cm 1 in that of PIP-Py-BF4 confirmed the successful anion exchange. The SEM images showed that the powder of PIP-Py-Xs before ball milling were about 300–400 nm, consisting of 20–30 nm nanoparticle aggregates (Fig. S1 a-c). The even distribution of the N element in the EDS mapping images of PIP-Py-Xs demonstrated the successful incorporation of the pyridine group. Moreover, the highly dispersed O element in the EDS mapping image of PIP-Py-Ac and the F element in that of PIP-Py- BF4 provided clear evidence of the successful anion exchange of Ac and BF4 , respectively (Fig. 2). The microporous textures can be observed in their TEM images (Figs. S1d–f) and a more accurate description was revealed by their N2 adsorption isotherms (Fig. S2a). With a similar sharp increase at low pressure and a hysteresis at middle pressures, the N2 adsorption isotherms demonstrated the hi erarchical structure of the micropores and mesopores co-existing in the PIP-Py-Xs. After the anion exchange, the BET surface area decreased from 856 m2 g 1 of PIP-Py-Cl to 773 m2 g 1 of PIP-Py-Ac and 654 m2 g 1 of PIP-Py-BF4 (Table S1). Despite the decrease in the BET surface area, the CO2 adsorption capacity at 273 K showed a contrary increase (Fig. 1b), which was 3.31 and 2.89 mmol g 1 for PIP-Py-Ac and PIP-PyBF4, higher than 2.58 mmol g 1 for PIP-Py-Cl (Table S2). Consistent with the reported P [DADMA] PILs, the surface area was also in the order of Ac > BF4 > Cl [40]. Worthy of mentioning, the PIPs showed a similar rule for the effect of anions on the solubility of CO2 in ILs [41]. The adsorption enthalpy of CO2 on different anionic PIPs also proves this
As shown in Fig. S4, with different pyridine-based PIPs fillers addi tion into the PIM-1 matrix, numerous bumps and pits showed up, indi cating the formation of a much rougher surface compared with that of the neat PIM-1 membrane. These significant structural changes clearly prove the successful incorporation of the PIPs fillers. No obvious defects were observed (Fig. S5), and EDS mapping shows that the PIPs fillers were dispersed in the whole PIM-1 matrix with excellent compatibility (Fig. S6). Furthermore, the cross-section morphology of all the obtained MMMs showed a loose spongy mesh structure, which was quite different from the dense neat PIM-1 membrane (Fig. 3a). No significant particleaggregation was observed and all PIP fillers were evenly distributed over the wall of the meshes. Also, no visible void-defects between the PIM-1 matrix and fillers could be found, demonstrating that the general problem of interfacial void between fillers and the matrix polymer in the formation of MMMs was successfully overcome. We further revealed this good interfacial compatibility of the filler-polymer by quantum calcu lation based on Density Functional Theory (DFT). To simplify, the structure of the monomers of pyridine-based PIPs and PIM-1 were used to represent themselves. The calculation details can be found in the SI. After optimization, each triptyene group in the PIP-Py-Xs arranged itself closely to the PIM-1 through π-π interactions (Fig. 4). The distance and the dihedral angle between the two closed planes of PIP-Py-Cl and PIM-1 were 3.2 Å and 27.0� , the change was smaller (3.1 Å, 3.7� ) for PIP-Py-Ac and (3.4 Å, 14.2� ) PIP-Py-BF4, revealing they were almost parallel with each other. Accordingly, the binding energies (BEs) were calculated as high as 112.7, 126.0, and 149.4 kJ mol 1 for PIP-Py-Cl, PIP-Py-Ac, and PIP-Py-BF4 towards PIM-1, respectively. Therefore, the three PIPs fillers all had excellent compatibility towards the PIM-1 matrix, and no evidence of voids between the matrix membrane and the fillers occurred. Moreover, the intermolecular interactions between PIM-1 and PIP-Py-Xs, as well as the repulsive force among the charged skeleton of the PIPs, effectively prevented the adhesion among PIP-Py-Xs fillers, resulting in a high dispersion of the fillers in the PIM-1 matrix and the loose spongy mesh structure. More significantly, each anion of the PIP4
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Fig. 4. BEs of (a) PIP-Py-Cl, (b) PIP-Py-Ac and (c) PIP-Py-BF4 monomers towards the segment of PIM-1 calculated by DFT: C, grey for PIPs and green for PIM-1; H, white; O, red; N, blue; Cl, reseda; B, light red; F, light blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. (a) Permeability of CO2 and N2, (b) ideal selectivity of CO2/N2, (c) permeability of CO2 and CH4, (d) ideal selectivity of CO2/CH4 of membranes.
592 m2 g 1, and 577 m2 g 1, respectively, slightly higher than that of the neat PIM-1, 551 m2 g 1, due to the large surface area of the corre sponding PIPs fillers and the increased free volume of polymer matrix with the addition of filler particles. In addition, the TGA curves of the MMMs were also similar to PIM-1, with the main weight loss at about 500 � C (Fig. S8). Compared with the significant weight loss below 250 � C for PIP-Py-Cl and PIP-Py-Ac and 300 � C for PIP-Py-BF4, the lack of weight loss in the relevant temperature range in the TGA curves of the corresponding MMMs indicates the enhanced thermal stability of PIPs fillers due to the strong interaction between the PIM-1 matrix and the PIPs fillers.
Py-Xs fillers was relatively free from the skeleton, which provided a movable carrier for CO2 transportation through the membrane. Considering the specific structure of the MMMs, we anticipated the loose mesh structure would enhance the gas permeability, the PIP-Py-Xs fillers in the PIM-1 based MMMs would improve the CO2 selectivity, and, more importantly, free anions would provide a carrier to facilitate the CO2 transport. N2 adsorption-desorption isotherms of the obtained MMMs were similar to the neat PIM-1 membrane (Fig. S7), indicating that the characteristic hierarchical porosity of the PIM-1 matrix was retained after adding the PIPs fillers. The calculated BET surface areas of the PIMPy-Cl-15, PIM-Py-Ac-15, and PIM-Py-BF4-15 MMMs were 590 m2 g 1, 5
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Scheme. 1. Chemical structures of PIM-1 and PIPs fillers, and the schematic mechanism of a facilitating CO2 permeability in PIM-Py-X MMMs.
3.3. Gas separation performance
exhibited enhanced performance (Fig. 5c and d) and the maximum values were as high as 56.1, 40.4, and 39.9 for PIM-Py-Ac-15, PIM-Py-Cl15, and PIM-Py-BF4-15, respectively. The ideal CO2/CH4 selectivity of PIM-Py-BF4 was in accordance with the higher solubility of CH4 in BF4 ILs [41]. This further confirms the assumption that the properties of ILs can be preserved in PIPs, and in MMMs, accordingly. As shown in Fig. S10, the permeability of N2 and CH4 were almost as a constant with increasing the pressure, however the CO2 permeability declined in slow although the CO2 flux increased with the pressure. Generally, the Langmuir adsorption sites of the membrane surface would be occupied gradually with increasing pressure, which led to a relative decline of gas solubility and hence the permeability [47]. Meanwhile, the packing porosity of the polymeric membranes would also decrease with increasing pressure. Accordingly, both the ideal selectivity of CO2 to N2 and CO2 to CH4 showed a little decline. There fore, the low pressure is favorable for gas separation. In order to further evaluate the real gas separation selectivity of the obtained MMMs, the membrane separation performance of the mixed gases of CO2/N2 and CO2/CH4 (1:1 v/v) were demonstrated. As shown in Fig. S11 and listed in Table S4, the actual separation factor of the PIMPy-Ac-15 MMM were slightly lower than the ideal selectivity, because of the competitive permeation [48], but still reached as high as 41 and 37, respectively. Meanwhile, the CO2 permeability was as high as 5095 barrer for the CO2/N2 mixture, and 5300 barrer for the CO2/CH4 mixture, respectively, still surpassing the 2008 Robeson upper bounds. Therefore, the obtained MMMs with PIPs as fillers showed a good prospect in practical applications. As for the practical applications, the stability of the membrane is one of the most important factors. After running for 96 h, the CO2 separation performance of PIM-Py-Ac-15 MMM showed almost no changes (Fig. S12). Even after half a year, PIM-Py-Ac-15 MMM exhibited much more stable performance than the neat PIM-1 membranes. its CO2 permeability decreased from 6205 barrer to 4086 barrer within 180 days, showing a 34.2% loss. In contrast, the CO2 permeability of neat PIIM-1 decreased rapidly from the initial 2345 barrer to 1116 barrer after 180 days, showing a 52.4% loss. Therefore, the PIM-Py-Ac-15 membrane exhibited a much better stability. This result further indi cated that the arrangement of PIM-1 chain is disrupted by the π-π interaction between matrix PIM-1 phase and filler phase, the collapse of voids with aging is effectively inhibited, and the membrane durability is enhanced, accordingly (Table S5). To sum up, the gas separation performance of all three MMMs of PIM-Py-Cl-15, PIM-Py-Ac-15, and PIM-Py-BF4-15 surpassed the 2008 Robeson upper bounds of CO2/N2 and CO2/CH4 gas pairs. Especially, PIM-Py-Ac-15 showed the best gas separation performance with a CO2
The CO2, CH4, and N2 permeability were studied to reflect the po tential separation performance in the typical CO2 resources of biogas, natural gas, and flue gas. All MMMs showed higher CO2 permeability than the neat PIM-1 membrane (Fig. 5a, Table S3). The CO2 permeability increased with increasing PIPs loading, reaching as high as 5538, 6544, and 6230 barrer at an addition of 20 wt % forPIP-Py-Cl, PIP-Py-Ac, and PIP-Py-BF4, respectively. Nevertheless, the N2 permeability increased slightly at low PIPs loadings, but significantly increased when the PIPs loading exceeded 15 wt%. Thus, the ideal CO2/N2 selectivity of each MMMs increased with increasing PIPs loading and reached a maximum of 42.3, 62.5, and 46.9 for PIM-Py-Cl, PIM-Py-Ac, and PIM-Py-BF4 at 15 wt% loading, respectively (Fig. 5b). Among them, PIM-Py-Ac-15 exhibited the best CO2/N2 selectivity, 2.56 times higher than that of the neat PIM-1 membrane. Therefore, both CO2 permeability and CO2/ N2 selectivity were significantly enhanced in the MMMs by using PIPs as the filler. The effect of PIPs fillers on increasing the permeability of CO2 fol lows the order of PIM-Py-Ac-15 > PIM-Py-BF4-15 > PIM-Py-Cl-15 > PIM-1, which was consistent with the order of CO2 adsorption capacity of the PIP fillers (Fig. S9). Therefore, PIPs fillers play an important role in CO2 permeability. Since anions in PIPs are free from the skeleton and can move freely within a specific space, inspired by the transportation of CO2 molecules across the cell membrane by natural protein carriers, we proposed a novel bionic carrier mechanism with PIPs for facilitating CO2 permeability (Scheme. 1). It was revealed that CO2 molecules dissolved in imidazolium ILs accumulated around the anions [42], and, similarly, CO2 molecules dissolved in PIM-Py-Xs MMMs preferred binding with the free anions. Similar to the transfer process of HCO3 (CO2 dissolved in H2O) through the Band 3 anion transport protein in the erythrocyte membrane [9], anions in PIPs on one side of the membrane provided the binding sites for CO2 molecules and quickly transported them to another anion for exchanging. In this way, CO2 molecules quickly passed through the membrane via the facilitating channel (the red arrows in Scheme 1) rather than slowly diffusing from one side of the membrane to the other. Meanwhile, after releasing CO2, the anion migrated back to carry another CO2 molecule, forming a cycle anion transfer channel in the confined space (the green arrows in Scheme 1). Since the type of anion plays a significant role in the CO2 adsorption [42], the basic Ac anion transferred CO2 much faster due to the strong interaction between CO2 and the O in the acetic acid [43]. The dipolar interactions between fluorine in BF4 and CO2 can also improve the permeability of CO2, although it was weaker than the acid-base interaction [44–46]. Similarly, the CO2/CH4 ideal selectivity of the obtained MMMs also 6
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Fig. 6. Comparison of gas separation performance of PIM-1 based MMMs of this work with the 2008 Robeson upper bound.
permeability of 6205 barrer, and an ideal CO2/N2 and CO2/CH4 selec tivity of 62.5 and 56.1, respectively. Even for the mixed gases, the actual separation performance of PIM-Py-Ac-15 was still beyond the upper bounds. The obtained MMMs surpassed the most of state-of-the-art membranes, no matter if they were an IL-based membrane or another type of MMM (Fig. 6, Tables S6 and S7). Although the selectivity of PIMPy-Ac-15 was not comparable to the supported ionic liquid membranes (SILMs), such as the reported highest CO2/N2 selectivity of [DMAPH] [EoAc]/PES of 151.4 [17], and the CO2/CH4 selectivity of [N2224]2 [maleate]/PES of 218.5 [49], SILMs usually suffered from much lower permeability of no exceeding of 3500 barrer. Considering the perme ability and stability are more important characteristics in large-scale membrane separation, the obtained MMMs possessing higher perme ability would be more promising candidates. Whereas for ILs composite polymer membranes (ILPMs), their high permeability, such as 6650 barrer for the [C2mim][NTf2]/PIM-1 [50] and 5800 barrer for the [Dems][TFSI]/PSF [24] were attractive, but their selectivity of CO2/N2 and CO2/CH4 were usually much lower. Compared with our previous work in amine modified POP as fillers, this work also better permeability and selectivity, demonstrating that facilitating channels formed by anion carriers of PIP were more effective than specific transport chains formed by neutral amine-POP [37]. Therefore, PIPs were demonstrated to be excellent fillers for fabricating promising MMMs with great sepa ration performance.
Declaration of competing interest There are no conflicts to declare. Acknowledgements This work is supported by the Natural Science Foundation of China (Nos. 91834301, 21676080, and 21878076). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117677. References [1] A.A. Olajire, CO2 capture and separation technologies for end-of-pipe applications a review, Energy 35 (2010) 2610–2628, https://doi.org/10.1016/j. energy.2010.02.030. [2] C. Castel, E. Favre, Membrane separations and energy efficiency, J. Membr. Sci. 548 (2018) 345–357, https://doi.org/10.1016/j.memsci.2017.11.035. [3] M.G. Buonomenna, J. Bae, Membrane processes and renewable energies, Renew. Sustain. Energy Rev. 43 (2015) 1343–1398, https://doi.org/10.1016/j. rser.2014.11.091. [4] X. Cheng, F. Pan, M. Wang, W. Li, Y. Song, G. Liu, H. Yang, B. Gao, H. Wu, Z. Jiang, Hybrid membranes for pervaporation separations, J. Membr. Sci. 541 (2017) 329–346, https://doi.org/10.1016/j.memsci.2017.07.009. [5] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) 390–400, https://doi.org/10.1016/j.memsci.2008.04.030. [6] S.J. Waisbren, J.P. Geibel, I.M. Modlin, W.F. Boron, Unusual permeability properties of gastric gland cells, Nature 368 (1994) 332–335, https://doi.org/ 10.1038/368332a0. [7] G.J. Cooper, R. Occhipinti, W.F. Boron, CrossTalk proposal: physiological CO2 exchange can depend on membrane channels, J. Physiol. 593 (2015) 5025–5028, https://doi.org/10.1113/JP270059. [8] H. Ji, H. Dong, Biological significance and topological basis of aquaporinpartnering protein-protein interactions, Plant Signal. Behav. 10 (2016), https:// doi.org/10.1080/15592324.2015.1011947. [9] A. Cetinkaya, S. Erdogan, Changes of HCO-3/Cl- exchanger activity during meiotic maturation in balb/c strain mouse oocytes and zygotes, J. Reprod. Dev. 54 (2008) 492–495, https://doi.org/10.1262/jrd.20029. [10] J. De Riva, J. Suarez-reyes, D. Moreno, I. Díaz, V. Ferro, International Journal of Greenhouse Gas Control Ionic liquids for post-combustion CO2 capture by physical absorption : thermodynamic , kinetic and process analysis, Int. J. Greenh. Gas Control 61 (2017) 61–70, https://doi.org/10.1016/j.ijggc.2017.03.019. [11] K.E. Gutowski, Industrial uses and applications of ionic liquids, Phys. Sci. Rev. (2018) 43–58, https://doi.org/10.1515/psr-2017-0191. [12] I. Barden, S. Einloft, Multivariate statistical evaluation of ionic liquids features for CO2 capture, Energy Procedia 114 (2017) 86–94, https://doi.org/10.1016/j. egypro.2017.03.1151. [13] A. Baghban, A.H. Mohammadi, M.S. Taleghani, Rigorous modeling of CO2 equilibrium absorption in ionic liquids, Int. J. Greenh. Gas Control 58 (2017) 19–41, https://doi.org/10.1016/j.ijggc.2016.12.009. [14] M.A. Pizzoccaro-Zilamy, M. Drobek, E. Petit, C. Tot�ee, G. Silly, G. Guerrero, M. G. Cowan, A. Ayral, A. Julbe, Initial steps toward the development of grafted ionic liquid membranes for the selective transport of CO2, Ind. Eng. Chem. Res. 57 (2018) 16027–16040, https://doi.org/10.1021/acs.iecr.8b02466. [15] F. Hassan Hassan Abdellatif, J. Babin, C. Arnal-Herault, L. David, A. Jonquieres, Grafting cellulose acetate with ionic liquids for biofuel purification membranes:
4. Conclusion In summary, a novel bionic CO2 permeability mechanism was real ized by using PIPs as the carrier in mixed matrix membranes. A series of PIM-1 based MMMs were prepared by incorporating fillers of pyridinebased PIPs with different anions. The PIP-Py-Xs fillers were uniformly dispersed in the PIM-1 based MMMs due to good affinity through π-π interactions without any defective voids. The resultant MMMs showed excellent performance both in CO2 permeability and selectivity and exceed the 2008 Robeson upper bound for CO2/N2 and CO2/CH4 gas pairs. Especially, the performance of the PIM-Py-Ac-15 MMM with a CO2 permeability of 6205 barrer, and a CO2/N2 and CO2/CH4 idea selectivity of 62.5 and 56.1, respectively, was the best in the reported state-of-theart membranes, due to the CO2 facilitating channel of the PIP-Py-Ac filler. More significantly, the actual separation factor of the PIM-PyAc-15 MMM for the mixed feed gases of CO2/N2 and CO2/CH4 (1:1 v/ v) was slightly lower than the ideal selectivity, but still reached as high as 41 and 37, respectively. Therefore, pyridine-based PIP fillers will be a prospective option for manufacturing MMMs with extraordinary specific gas separation performance. We anticipate the strategy of using PIPs as fillers in MMMs highlights a bionic way for facilitating selective permeability of molecules and ions.
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A porous ionic polymer bionic carrier in a mixed matrix membrane for facilitating selective CO2 permeability Chenhui Wang a, Fangyuan Guo a, He Li a, Jian Xu b, Jun Hu a, *, Honglai Liu a, Meihong Wang c, ** a
State Key Laboratory of Chemical Engineering and School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, China b Shanghai Institute of Measurement and Testing Technology, 1500 Zhangheng Road, Shanghai, 201203, China c Department of Chemical and Biological Engineering, The University of Sheffield, Western Bank, Sheffield, S10 2TN, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Mixed matrix membranes Porous ionic polymer PIM-1 CO2 separation Pyridine-based porous ionic polymer
Inspired by the most efficient CO2 transport across natural biological membranes, a novel bionic channel facilitating CO2 permeation was realized via adding a filtering porous ionic polymer (PIP) in a mixed matrix membrane (MMM). A series of polymer of intrinsic microporosity (PIM-1) based MMMs were fabricated by three pyridine-based PIPs (PIP-Py-X) with different Cl , Ac , and BF4 anions as fillers. PIP-Py-X PIPs show highly selective CO2 adsorption capacity and perfect interfacial compatibility with the PIM-1 matrix. More significantly, acting as carriers, the movable anions in the MMMs efficiently facilitated the transport of adsorbed CO2 across the membranes. As a result, the obtained PIM-Py-Ac-15 MMM exhibited an excellent permeability for CO2 of 6205 barrer and a CO2/N2 and CO2/CH4 selectivity of 62.5 and 56.1, respectively, far surpassing the 2008 Robeson upper bound and exceeding most of the reported advanced membranes. As a divertive of PIPs, this work opens a window for designing and fabricating promising MMMs for task-specific membrane separations.
1. Introduction Greenhouse effect caused by excessive carbon dioxide (CO2) emis sons has attracted great concern, increasing the importance of CO2 separation from flue gases [1]. Membrane separation has been shown to be one of the most efficient separation processes [2]. However, most membranes still suffer from the trade-off between permeability and selectivity, although great efforts have been devoted to improving sep aration performance [3–5]. In natural CO2 transport across biological membranes, two pathways, (1) the free diffusion of solution-diffusion mechanism and (2) the protein-based facilitated transport mechanism, endow the most efficient CO2 separation processes in the world [6]. It has been demonstrated that carrier proteins, such as aquaporin (AQP), can specifically and reversibly bind with CO2 molecules and transport them to the other side of the cell membrane via conformational variation [7,8]. Inspired by the accelerated CO2 transport by HCO3 -Cl- self-exchange permeability in a red cell’s membrane [9], we took a particular interest in membranes modified by ionic liquids (ILs) due to their excellent af finity towards CO2 as well as the exchangeable ions [10–13]. Coating ILs
on the support substrate is the simplest way to prepare supported ionic liquid membranes (SILMs) [14–16]. Among them, the best reported performance of CO2/N2 and CO2/CH4 selectivity were 151.4 and 72, and the highest CO2 permeability was 3352 barrer [17]. However, their insufficient stability, e. g. with a maximum operating pressure of no more than 0.25–0.3 MPa [18], makes large scale applications difficult [19–22]. Alternative, Ils composite polymer membranes (ILPMs) have been proposed by mixing Ils into the casting solutions [23], which showed an excellent performance, such as a CO2 permeability as high as 5800 barrer and a CO2/N2 selectivity as high as 30 [24]. Although the freely mobile IL carriers in membranes significantly enhanced the selectivity and permeability, the mechanical properties simultaneously decreased with increasing IL content, in general. Therefore, long-time operation remains difficult. Poly (ionic liquid)s membranes (PILMs), polymerized by IL-containing monomers, provided a method to avoid the mechanical problems even at high ILs content. However, these membranes can only enhance CO2/N2 selectivity at the sacrifice of permeability due to the low free volume caused by the linear polymer chains [25,26]. Therefore, great challenges remain to simultaneously improve the separation performance of permeability and selectivity, as
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Hu), [email protected] (M. Wang). https://doi.org/10.1016/j.memsci.2019.117677 Received 9 November 2019; Accepted 17 November 2019 Available online 21 November 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Chenhui Wang, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117677
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well as the mechanical stability. Mixed matrix membranes (MMMs), combining the advantages of matrix polymers and fillers, have been demonstrated as a promising strategy to meet the above challenges [27–29]. Using porous materials with ionic sites as the filler to produce MMMs could be a novel pathway when compared with other IL-based membranes. Porous ionic polymers (PIPs), a kind of cross-linked polymer with a polymer skeleton composed of ionic groups linked with rigid chemical bonds, aroused our attention [30–32]. Huang et al. reported a cationic microporous polymer network PyTTA-BFBIm-iCOF, after anion exchange, that exhibited a CO2 uptake of 17.6 wt% (4.0 mmol g 1) at 273 K and 1 bar [33]. The synthetic di versity endows PIPs with a high surface area, changeable counter-ions, and a high charge density. We anticipate the well-defined porosity in PIPs could provide CO2 transfer channels, while the charged polymer skeleton and moveable counter-ions could allow for novel CO2 transport through both “hopping” and “vehicle” mechanisms [32]. However, no promising works using PIPs as fillers to produce MMMs for facilitating CO2 separation have been reported, so far. Herein, we proposed a bionic facilitating CO2 separation strategy by using PIPs as fillers to produce MMMs. Because the polymer of intrinsic microporosity, PIM-1, shows extremely high gas permeability [34–36] and also good affinity towards triptycene-based porous organic poly mers, according to our previous work [37], we chose it as the matrix polymer. By using pyridine-based cationic PIPs with different Ac , BF4 , and Cl anions as fillers, the PIM-Py-X MMMs were produced to demonstrate their CO2 separation performance. The effects of the porosity and the type of anions were researched to understand the facilitating mechanism of CO2 separation. Due to the synergistic effects of each component in the MMMs, the CO2 separation performance of all three obtained PIM-1 based MMMs with PIPs as fillers were significantly enhanced, exceeding the 2008 Robeson upper bound for the CO2/N2 and CO2/CH4 gas pairs. As far as we know, PIM-Py-Ac-15 exhibits the best CO2 separation performance in most advanced membranes, with an excellent CO2 permeability of 6200 barrer, and a selectivity of CO2/N2 and CO2/CH4 as high as 62.7 and 56.1. Therefore, the bionic facilitating CO2 channels through PIM-1 based MMMs could be a promising strategy for CO2 separation.
The obtained brown powder was noted as PIP-Py-Cl.Then, TPFC-CH2Cl (0.30 g), pyridine (0.74 g), and acetonitrile (3.0 mL) were added to a flask and stirred at 90 � C for 24 h. The cooled mixture was filtered and briefly washed with methanol three times, then dried to afford PIP-Py-Cl. PIP-Py-Cl (0.05 g), NaAc (5 g), and deionized water (10 mL) were added to a 50 mL flask and stirred at room temperature for 24 h. The cooled mixture was isolated by filtration and subsequently washed thrice with deionized water. Next, the resultant PIP-Py-Ac was dried in 80 � C for 12 h. PIP-Py-BF4 was synthesized in a similar way, except the reactant NaAc was replaced by NaBF4 (5 g). 2.3. Fabrication of PIM-Py-X MMMs The PIP-Py-X PIPs were added to chloroform and ground in a plan etary ball mill at 500 rpm for 5 h. A mixture of the PIP-Py-X powder and PIM-1 polymer was then added to chloroform under an 1-h stirring, subsequently ultrasounded for another 1 h. The mass ratio of solvent to filler polymer was controlled at 19/1 (wt.%) and the mass ratio of filler adding to the MMMs ranged from 0 to 20 wt%. Thereafter, the solution was poured onto a home-made super flat glass Petri dish, and then left for 12 h, such that the chloroform evaporated at room temperature. After drying, the membranes were placed into a 100 � C vacuum oven to remove the remaining solvent. Finally, due to the excellent demolding properties of the membrane, it was easily lifted off the petri dishes. 2.4. Characterization
Triptycene and paraformaldehyde were purchased from SigmaAldrich. Formaldehyde and dimethoxymethane were purchased from Alfa Aesar and 1, 2-Dichloroethane was purchased from Aladdin. Phosphoric acid was purchased from Shanghai Macklin Biochemical Co., Ltd.
Thermal gravimetric analyses (TGA) of the PIPs and MMMs were analyzed by an STA 499 F3 Thermo-gravimetric Analyzer (Netzsch). Samples were heated with a constant heating rate of 10 � C min 1 to 800 � C under an N2 environment. N2 was used as the purge gas and its flow rate was controlled at 20 ml min 1. The chemical group of the PIPs were analyzed by Fourier transform infrared spectroscopy (FTIR) (NEXUS 470 (Thermo Nicolet)) with a scan range from 400 to 4000 cm 1. A field emission scanning electron microscope (FESEM), GeminiSEM 500 (Germany) was used to analyze the PIPs’ surface images. The surface of membranes were analyzed by Scanning Probe Microscope (*/Veeco/DI China). A Falion 60S (EDAX America) was used to analyze the element distribution of PIPs. The TEM images of the PIPs were analyzed with a High-Resolution Transmission Electron Microscope (HRTEM), JEM2100 (Japan). The surface area and pore size distribution of the mate rials was observed with a Micrometrics tristar II. N2 adsorption iso therms were performed at 77 K, ranging from 1.1 to 101 kPa. Before analysis, each sample was degassed for 3 h at 90 � C. The gas mole fractions on the permeate side were determined using a gas chromato graph equipped (Haixin GC950).
2.2. Synthesis of PIP-Py-X PIPs
2.5. Gas permeation measurements
The synthesis of the TPFC neutral porous organic polymer (POP) has been reported elsewhere [38]. Briefly, triptyene (1.28 g, 5 mmol) was dissolved in 10 mL dichloroethane in a 250 mL three-necked flask, and then 2.28 g of formaldehyde dimethyl acetal (FDA) (2 mmol) was added in the mixture. Anhydrous FeCl3 (4.88 g, 30 mmol) was then added. The three-necked flask was stirred for 5 h in a 45 � C oil bath and then stirred for 19 h at 80 � C. The mixture was cooled to room temperature, filtered, and then washed with methanol three times. The solid was placed in a Soxhlet apparatus and extracted with methanol for 24 h, and a brown solid of TPFC POP was obtained after drying. The TPFC powder was chloromethylated and ionized to produce the PIPs. (0.60 g) TPFC and (3.0 g) paraformaldehyde were successively added to a 350 mL flask, followed by (18 mL) acetic acid, (60 mL) concentrated hydrochloric acid, and (9 mL) phosphoric acid. The sealed flask was then held at 90 � C for three days. The cooled brown powder was filtered, followed by washing thrice with water and methanol and drying at 80 � C for 12 h.
Before gas permeation measurements, the membrane was cliped with an intermediate diameter of about 10 mm for testing. The thickness at different positions was measured with Vernier calipers and averaged. Gas permeability coefficients were measured at stable states and the average was calculated from at least three measurements. The definition of permeability (Pi, Barrer) was showed by equation (1), where Qi is the volume flow of gas “i” (cm3 s 1, STP); A is the area of the membrane (cm2); and Δpi is the difference of pressure between the permeation and feed sides of gas "i" (cmHg). L represents the thickness of the membrane (cm). According to our previous work, the single gas permeation test was carried out by a gas permeation evaluation apparatus [39]. A soap bubble flow meter was used to measure the CO2, N2 and CH4 fluxes. The gas permeability (P) and the ideal selectivity (S) of CO2/N2 and CO2/CH4 gas pairs were calculated by Eq. (1) and Eq. (2):
2. Experimental methods 2.1. Materials
P ¼ Qi =ðA � Δpi Þ � L ; 2
(1)
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Fig. 1. (a) Fourier transform infrared spectroscopy (FTIR) spectra and (b) CO2 adsorption-desorption isotherms at 273 K of PIP-Py-Cl, PIP-Py-Ac and PIP-Py-BF4.
2.6. Adsorption enthalpy calculation
(2)
Toth model was used to fit the CO2 adsorption isotherms under 273 K and 298 K. Their isotherms can be described by the Eq. (5).
The actual gas separation selectivity from the mixed gases of CO2/N2 and CO2/CH4 (1:1 v/v) was performed using a device similar to the pure gas permeation test described above. The total feed pressure was set as 2 bar. The gas molar fractions on the permeate side were analyzed using a gas chromatograph equipped with a thermal conductivity detector and He as a carrier gas. The determination include (1) He was used as a gas carrier, which made both peaks of CO2 and N2 can be detected. (2) For CO2/N2 gas pair, CH4 was used as a purge gas to fulfill the chamber of permeate side before the testing, since N2 is abundant in air and easily infiltrated into the device to dilute the CO2 concentration in the sample. For CO2/CH4 gas pair, N2 was used as a purge gas. The permeability and the separation factor of the mixed gas were calculated by Eq. (3), and Eq. (4): (3)
y =y α¼ A B xA =xB
(4)
=
P ¼ Qi =ðA � Δpi Þ � L � yi =xi
1
Q¼
Qm � ðB � PÞ t 1
=
� S ¼ Pi Pj :
ð1 þ B � PÞ t
(5)
where, Q is molar loading of CO2 (mol kg 1), Qm is saturation capacity of CO2 (mol kg 1), p is gas pressure (Pa), B and t are parameters in the Toth model. The adsorption enthalpy were calculate by the Clausius–Clapeyron equation (Eq. (6)). � � � � p1 ΔH 1 1 ¼ (6) ln R T1 T2 p2 where Ti represents the temperature for isotherm i, pi represents the pressure for isotherm i, R is 8.315 J (K mol) 1.
where y and x are the mole fractions of the permeate side and the feed side, respectively.
2.7. IAST selectivity calculation N2 adsorption isotherms were fitted by Single-site Langmuir model
Fig. 2. SEM and EDS mapping images of PIP-Py-Cl, PIP-Py-Ac and PIP-Py-BF4. 3
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(Eq. (7)). q ¼ qsat
bp 1 þ bp
(7)
where, q represents molar loading of gas components (mol kg 1), p is gas pressure (Pa),b represents a parameter in the single-site Langmuir isotherm (Pa 1) and qsat represents saturation capacity of gas compo nents (mol kg 1). CO2 adsorption isotherms were fitted by Dual-site Langmuir model (Eq. (8)), because both chemical and physical interaction should be taken into account. q ¼ q1;sat
b1 p b2 p þ q2;sat 1 þ b1 p 1 þ b2 p
(8)
where qsat,1, b1, qsat,2 and b2 are four parameters in the equation because two distinct adsorption sites are assumed to be existed. Ideal adsorbed solution theory (IAST) was used to calculate the adsorption selectivity (Eq. (9)) by pure-component isotherm fitting parameters. S¼
q1 =q2 p1 =p2
Fig. 3. Cross-section SEM images of (a) membrane of neat PIM-1, (b) MMM of PIM-Py-Cl-15, (c) MMM of PIM-Py-Ac-15 and (d) MMM of PIM-Py-BF4-15.].
(9)
rule (Fig. S3). Under the low adsorption capacity, the adsorption enthalpy of PIP-Py-Cl, PIP-Py-Ac and PIP-Py-BF4 was 42.3, 51.1 and 45.6 kJ mol 1, respectively (Fig. S3). With the highest adsorption enthalpy, Ac anions in PIP-Py-Ac exhibited the strongest interaction with CO2. Therefore, the anion in the PIPs played a significant role in the CO2 adsorption due to their affinity towards CO2 [42]. Accordingly, the IAST selectivity of the PIPs for CO2/N2 was also conformed to this order PIP-Py-Ac (25.7) >PIP-Py-BF4 (25.5) > PIP-Py-Cl (24.6).
where pi is partial pressure of gas i and qi represents the corresponding pure-component gas uptake amount at pi. For the simulated flue gas, the binary mixture contains 15% CO2 and 85% N2. 3. Results and discussion 3.1. Properties of pyridine-based PIPs
3.2. Properties of PIM-Py-Xs MMMs
Through a three-step synthesis procedure, including polymerization of triptycene and formaldehyde dimethyl acetal, followed by chlor omethylation and pyridine ionization, and finally anion exchange, the pyridine-based PIPs with different anions of PIP-Py-Cl, PIP-Py-Ac, and PIP-Py-BF4 were produced (Scheme. S1). The characteristic bands of the benzene ring at 1774 cm 1, CH2 group at around 2800–3000 cm 1 and 1467 cm 1, and the pyridine ring at 1384 cm 1 were observed in the FTIR comparative spectra. The structure information of TPFC POP and the pyridine-based PIPs were further revealed as proposed (Fig. 1a). More significantly, a new band at 1728 cm 1 in the FTIR spectrum of PIP-Py-Ac and a band at 1083 cm 1 in that of PIP-Py-BF4 confirmed the successful anion exchange. The SEM images showed that the powder of PIP-Py-Xs before ball milling were about 300–400 nm, consisting of 20–30 nm nanoparticle aggregates (Fig. S1 a-c). The even distribution of the N element in the EDS mapping images of PIP-Py-Xs demonstrated the successful incorporation of the pyridine group. Moreover, the highly dispersed O element in the EDS mapping image of PIP-Py-Ac and the F element in that of PIP-Py- BF4 provided clear evidence of the successful anion exchange of Ac and BF4 , respectively (Fig. 2). The microporous textures can be observed in their TEM images (Figs. S1d–f) and a more accurate description was revealed by their N2 adsorption isotherms (Fig. S2a). With a similar sharp increase at low pressure and a hysteresis at middle pressures, the N2 adsorption isotherms demonstrated the hi erarchical structure of the micropores and mesopores co-existing in the PIP-Py-Xs. After the anion exchange, the BET surface area decreased from 856 m2 g 1 of PIP-Py-Cl to 773 m2 g 1 of PIP-Py-Ac and 654 m2 g 1 of PIP-Py-BF4 (Table S1). Despite the decrease in the BET surface area, the CO2 adsorption capacity at 273 K showed a contrary increase (Fig. 1b), which was 3.31 and 2.89 mmol g 1 for PIP-Py-Ac and PIP-PyBF4, higher than 2.58 mmol g 1 for PIP-Py-Cl (Table S2). Consistent with the reported P [DADMA] PILs, the surface area was also in the order of Ac > BF4 > Cl [40]. Worthy of mentioning, the PIPs showed a similar rule for the effect of anions on the solubility of CO2 in ILs [41]. The adsorption enthalpy of CO2 on different anionic PIPs also proves this
As shown in Fig. S4, with different pyridine-based PIPs fillers addi tion into the PIM-1 matrix, numerous bumps and pits showed up, indi cating the formation of a much rougher surface compared with that of the neat PIM-1 membrane. These significant structural changes clearly prove the successful incorporation of the PIPs fillers. No obvious defects were observed (Fig. S5), and EDS mapping shows that the PIPs fillers were dispersed in the whole PIM-1 matrix with excellent compatibility (Fig. S6). Furthermore, the cross-section morphology of all the obtained MMMs showed a loose spongy mesh structure, which was quite different from the dense neat PIM-1 membrane (Fig. 3a). No significant particleaggregation was observed and all PIP fillers were evenly distributed over the wall of the meshes. Also, no visible void-defects between the PIM-1 matrix and fillers could be found, demonstrating that the general problem of interfacial void between fillers and the matrix polymer in the formation of MMMs was successfully overcome. We further revealed this good interfacial compatibility of the filler-polymer by quantum calcu lation based on Density Functional Theory (DFT). To simplify, the structure of the monomers of pyridine-based PIPs and PIM-1 were used to represent themselves. The calculation details can be found in the SI. After optimization, each triptyene group in the PIP-Py-Xs arranged itself closely to the PIM-1 through π-π interactions (Fig. 4). The distance and the dihedral angle between the two closed planes of PIP-Py-Cl and PIM-1 were 3.2 Å and 27.0� , the change was smaller (3.1 Å, 3.7� ) for PIP-Py-Ac and (3.4 Å, 14.2� ) PIP-Py-BF4, revealing they were almost parallel with each other. Accordingly, the binding energies (BEs) were calculated as high as 112.7, 126.0, and 149.4 kJ mol 1 for PIP-Py-Cl, PIP-Py-Ac, and PIP-Py-BF4 towards PIM-1, respectively. Therefore, the three PIPs fillers all had excellent compatibility towards the PIM-1 matrix, and no evidence of voids between the matrix membrane and the fillers occurred. Moreover, the intermolecular interactions between PIM-1 and PIP-Py-Xs, as well as the repulsive force among the charged skeleton of the PIPs, effectively prevented the adhesion among PIP-Py-Xs fillers, resulting in a high dispersion of the fillers in the PIM-1 matrix and the loose spongy mesh structure. More significantly, each anion of the PIP4
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Fig. 4. BEs of (a) PIP-Py-Cl, (b) PIP-Py-Ac and (c) PIP-Py-BF4 monomers towards the segment of PIM-1 calculated by DFT: C, grey for PIPs and green for PIM-1; H, white; O, red; N, blue; Cl, reseda; B, light red; F, light blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. (a) Permeability of CO2 and N2, (b) ideal selectivity of CO2/N2, (c) permeability of CO2 and CH4, (d) ideal selectivity of CO2/CH4 of membranes.
592 m2 g 1, and 577 m2 g 1, respectively, slightly higher than that of the neat PIM-1, 551 m2 g 1, due to the large surface area of the corre sponding PIPs fillers and the increased free volume of polymer matrix with the addition of filler particles. In addition, the TGA curves of the MMMs were also similar to PIM-1, with the main weight loss at about 500 � C (Fig. S8). Compared with the significant weight loss below 250 � C for PIP-Py-Cl and PIP-Py-Ac and 300 � C for PIP-Py-BF4, the lack of weight loss in the relevant temperature range in the TGA curves of the corresponding MMMs indicates the enhanced thermal stability of PIPs fillers due to the strong interaction between the PIM-1 matrix and the PIPs fillers.
Py-Xs fillers was relatively free from the skeleton, which provided a movable carrier for CO2 transportation through the membrane. Considering the specific structure of the MMMs, we anticipated the loose mesh structure would enhance the gas permeability, the PIP-Py-Xs fillers in the PIM-1 based MMMs would improve the CO2 selectivity, and, more importantly, free anions would provide a carrier to facilitate the CO2 transport. N2 adsorption-desorption isotherms of the obtained MMMs were similar to the neat PIM-1 membrane (Fig. S7), indicating that the characteristic hierarchical porosity of the PIM-1 matrix was retained after adding the PIPs fillers. The calculated BET surface areas of the PIMPy-Cl-15, PIM-Py-Ac-15, and PIM-Py-BF4-15 MMMs were 590 m2 g 1, 5
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Scheme. 1. Chemical structures of PIM-1 and PIPs fillers, and the schematic mechanism of a facilitating CO2 permeability in PIM-Py-X MMMs.
3.3. Gas separation performance
exhibited enhanced performance (Fig. 5c and d) and the maximum values were as high as 56.1, 40.4, and 39.9 for PIM-Py-Ac-15, PIM-Py-Cl15, and PIM-Py-BF4-15, respectively. The ideal CO2/CH4 selectivity of PIM-Py-BF4 was in accordance with the higher solubility of CH4 in BF4 ILs [41]. This further confirms the assumption that the properties of ILs can be preserved in PIPs, and in MMMs, accordingly. As shown in Fig. S10, the permeability of N2 and CH4 were almost as a constant with increasing the pressure, however the CO2 permeability declined in slow although the CO2 flux increased with the pressure. Generally, the Langmuir adsorption sites of the membrane surface would be occupied gradually with increasing pressure, which led to a relative decline of gas solubility and hence the permeability [47]. Meanwhile, the packing porosity of the polymeric membranes would also decrease with increasing pressure. Accordingly, both the ideal selectivity of CO2 to N2 and CO2 to CH4 showed a little decline. There fore, the low pressure is favorable for gas separation. In order to further evaluate the real gas separation selectivity of the obtained MMMs, the membrane separation performance of the mixed gases of CO2/N2 and CO2/CH4 (1:1 v/v) were demonstrated. As shown in Fig. S11 and listed in Table S4, the actual separation factor of the PIMPy-Ac-15 MMM were slightly lower than the ideal selectivity, because of the competitive permeation [48], but still reached as high as 41 and 37, respectively. Meanwhile, the CO2 permeability was as high as 5095 barrer for the CO2/N2 mixture, and 5300 barrer for the CO2/CH4 mixture, respectively, still surpassing the 2008 Robeson upper bounds. Therefore, the obtained MMMs with PIPs as fillers showed a good prospect in practical applications. As for the practical applications, the stability of the membrane is one of the most important factors. After running for 96 h, the CO2 separation performance of PIM-Py-Ac-15 MMM showed almost no changes (Fig. S12). Even after half a year, PIM-Py-Ac-15 MMM exhibited much more stable performance than the neat PIM-1 membranes. its CO2 permeability decreased from 6205 barrer to 4086 barrer within 180 days, showing a 34.2% loss. In contrast, the CO2 permeability of neat PIIM-1 decreased rapidly from the initial 2345 barrer to 1116 barrer after 180 days, showing a 52.4% loss. Therefore, the PIM-Py-Ac-15 membrane exhibited a much better stability. This result further indi cated that the arrangement of PIM-1 chain is disrupted by the π-π interaction between matrix PIM-1 phase and filler phase, the collapse of voids with aging is effectively inhibited, and the membrane durability is enhanced, accordingly (Table S5). To sum up, the gas separation performance of all three MMMs of PIM-Py-Cl-15, PIM-Py-Ac-15, and PIM-Py-BF4-15 surpassed the 2008 Robeson upper bounds of CO2/N2 and CO2/CH4 gas pairs. Especially, PIM-Py-Ac-15 showed the best gas separation performance with a CO2
The CO2, CH4, and N2 permeability were studied to reflect the po tential separation performance in the typical CO2 resources of biogas, natural gas, and flue gas. All MMMs showed higher CO2 permeability than the neat PIM-1 membrane (Fig. 5a, Table S3). The CO2 permeability increased with increasing PIPs loading, reaching as high as 5538, 6544, and 6230 barrer at an addition of 20 wt % forPIP-Py-Cl, PIP-Py-Ac, and PIP-Py-BF4, respectively. Nevertheless, the N2 permeability increased slightly at low PIPs loadings, but significantly increased when the PIPs loading exceeded 15 wt%. Thus, the ideal CO2/N2 selectivity of each MMMs increased with increasing PIPs loading and reached a maximum of 42.3, 62.5, and 46.9 for PIM-Py-Cl, PIM-Py-Ac, and PIM-Py-BF4 at 15 wt% loading, respectively (Fig. 5b). Among them, PIM-Py-Ac-15 exhibited the best CO2/N2 selectivity, 2.56 times higher than that of the neat PIM-1 membrane. Therefore, both CO2 permeability and CO2/ N2 selectivity were significantly enhanced in the MMMs by using PIPs as the filler. The effect of PIPs fillers on increasing the permeability of CO2 fol lows the order of PIM-Py-Ac-15 > PIM-Py-BF4-15 > PIM-Py-Cl-15 > PIM-1, which was consistent with the order of CO2 adsorption capacity of the PIP fillers (Fig. S9). Therefore, PIPs fillers play an important role in CO2 permeability. Since anions in PIPs are free from the skeleton and can move freely within a specific space, inspired by the transportation of CO2 molecules across the cell membrane by natural protein carriers, we proposed a novel bionic carrier mechanism with PIPs for facilitating CO2 permeability (Scheme. 1). It was revealed that CO2 molecules dissolved in imidazolium ILs accumulated around the anions [42], and, similarly, CO2 molecules dissolved in PIM-Py-Xs MMMs preferred binding with the free anions. Similar to the transfer process of HCO3 (CO2 dissolved in H2O) through the Band 3 anion transport protein in the erythrocyte membrane [9], anions in PIPs on one side of the membrane provided the binding sites for CO2 molecules and quickly transported them to another anion for exchanging. In this way, CO2 molecules quickly passed through the membrane via the facilitating channel (the red arrows in Scheme 1) rather than slowly diffusing from one side of the membrane to the other. Meanwhile, after releasing CO2, the anion migrated back to carry another CO2 molecule, forming a cycle anion transfer channel in the confined space (the green arrows in Scheme 1). Since the type of anion plays a significant role in the CO2 adsorption [42], the basic Ac anion transferred CO2 much faster due to the strong interaction between CO2 and the O in the acetic acid [43]. The dipolar interactions between fluorine in BF4 and CO2 can also improve the permeability of CO2, although it was weaker than the acid-base interaction [44–46]. Similarly, the CO2/CH4 ideal selectivity of the obtained MMMs also 6
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Fig. 6. Comparison of gas separation performance of PIM-1 based MMMs of this work with the 2008 Robeson upper bound.
permeability of 6205 barrer, and an ideal CO2/N2 and CO2/CH4 selec tivity of 62.5 and 56.1, respectively. Even for the mixed gases, the actual separation performance of PIM-Py-Ac-15 was still beyond the upper bounds. The obtained MMMs surpassed the most of state-of-the-art membranes, no matter if they were an IL-based membrane or another type of MMM (Fig. 6, Tables S6 and S7). Although the selectivity of PIMPy-Ac-15 was not comparable to the supported ionic liquid membranes (SILMs), such as the reported highest CO2/N2 selectivity of [DMAPH] [EoAc]/PES of 151.4 [17], and the CO2/CH4 selectivity of [N2224]2 [maleate]/PES of 218.5 [49], SILMs usually suffered from much lower permeability of no exceeding of 3500 barrer. Considering the perme ability and stability are more important characteristics in large-scale membrane separation, the obtained MMMs possessing higher perme ability would be more promising candidates. Whereas for ILs composite polymer membranes (ILPMs), their high permeability, such as 6650 barrer for the [C2mim][NTf2]/PIM-1 [50] and 5800 barrer for the [Dems][TFSI]/PSF [24] were attractive, but their selectivity of CO2/N2 and CO2/CH4 were usually much lower. Compared with our previous work in amine modified POP as fillers, this work also better permeability and selectivity, demonstrating that facilitating channels formed by anion carriers of PIP were more effective than specific transport chains formed by neutral amine-POP [37]. Therefore, PIPs were demonstrated to be excellent fillers for fabricating promising MMMs with great sepa ration performance.
Declaration of competing interest There are no conflicts to declare. Acknowledgements This work is supported by the Natural Science Foundation of China (Nos. 91834301, 21676080, and 21878076). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117677. References [1] A.A. Olajire, CO2 capture and separation technologies for end-of-pipe applications a review, Energy 35 (2010) 2610–2628, https://doi.org/10.1016/j. energy.2010.02.030. [2] C. Castel, E. Favre, Membrane separations and energy efficiency, J. Membr. Sci. 548 (2018) 345–357, https://doi.org/10.1016/j.memsci.2017.11.035. [3] M.G. Buonomenna, J. Bae, Membrane processes and renewable energies, Renew. Sustain. Energy Rev. 43 (2015) 1343–1398, https://doi.org/10.1016/j. rser.2014.11.091. [4] X. Cheng, F. Pan, M. Wang, W. Li, Y. Song, G. Liu, H. Yang, B. Gao, H. Wu, Z. Jiang, Hybrid membranes for pervaporation separations, J. Membr. Sci. 541 (2017) 329–346, https://doi.org/10.1016/j.memsci.2017.07.009. [5] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) 390–400, https://doi.org/10.1016/j.memsci.2008.04.030. [6] S.J. Waisbren, J.P. Geibel, I.M. Modlin, W.F. Boron, Unusual permeability properties of gastric gland cells, Nature 368 (1994) 332–335, https://doi.org/ 10.1038/368332a0. [7] G.J. Cooper, R. Occhipinti, W.F. Boron, CrossTalk proposal: physiological CO2 exchange can depend on membrane channels, J. Physiol. 593 (2015) 5025–5028, https://doi.org/10.1113/JP270059. [8] H. Ji, H. Dong, Biological significance and topological basis of aquaporinpartnering protein-protein interactions, Plant Signal. Behav. 10 (2016), https:// doi.org/10.1080/15592324.2015.1011947. [9] A. Cetinkaya, S. Erdogan, Changes of HCO-3/Cl- exchanger activity during meiotic maturation in balb/c strain mouse oocytes and zygotes, J. Reprod. Dev. 54 (2008) 492–495, https://doi.org/10.1262/jrd.20029. [10] J. De Riva, J. Suarez-reyes, D. Moreno, I. Díaz, V. Ferro, International Journal of Greenhouse Gas Control Ionic liquids for post-combustion CO2 capture by physical absorption : thermodynamic , kinetic and process analysis, Int. J. Greenh. Gas Control 61 (2017) 61–70, https://doi.org/10.1016/j.ijggc.2017.03.019. [11] K.E. Gutowski, Industrial uses and applications of ionic liquids, Phys. Sci. Rev. (2018) 43–58, https://doi.org/10.1515/psr-2017-0191. [12] I. Barden, S. Einloft, Multivariate statistical evaluation of ionic liquids features for CO2 capture, Energy Procedia 114 (2017) 86–94, https://doi.org/10.1016/j. egypro.2017.03.1151. [13] A. Baghban, A.H. Mohammadi, M.S. Taleghani, Rigorous modeling of CO2 equilibrium absorption in ionic liquids, Int. J. Greenh. Gas Control 58 (2017) 19–41, https://doi.org/10.1016/j.ijggc.2016.12.009. [14] M.A. Pizzoccaro-Zilamy, M. Drobek, E. Petit, C. Tot�ee, G. Silly, G. Guerrero, M. G. Cowan, A. Ayral, A. Julbe, Initial steps toward the development of grafted ionic liquid membranes for the selective transport of CO2, Ind. Eng. Chem. Res. 57 (2018) 16027–16040, https://doi.org/10.1021/acs.iecr.8b02466. [15] F. Hassan Hassan Abdellatif, J. Babin, C. Arnal-Herault, L. David, A. Jonquieres, Grafting cellulose acetate with ionic liquids for biofuel purification membranes:
4. Conclusion In summary, a novel bionic CO2 permeability mechanism was real ized by using PIPs as the carrier in mixed matrix membranes. A series of PIM-1 based MMMs were prepared by incorporating fillers of pyridinebased PIPs with different anions. The PIP-Py-Xs fillers were uniformly dispersed in the PIM-1 based MMMs due to good affinity through π-π interactions without any defective voids. The resultant MMMs showed excellent performance both in CO2 permeability and selectivity and exceed the 2008 Robeson upper bound for CO2/N2 and CO2/CH4 gas pairs. Especially, the performance of the PIM-Py-Ac-15 MMM with a CO2 permeability of 6205 barrer, and a CO2/N2 and CO2/CH4 idea selectivity of 62.5 and 56.1, respectively, was the best in the reported state-of-theart membranes, due to the CO2 facilitating channel of the PIP-Py-Ac filler. More significantly, the actual separation factor of the PIM-PyAc-15 MMM for the mixed feed gases of CO2/N2 and CO2/CH4 (1:1 v/ v) was slightly lower than the ideal selectivity, but still reached as high as 41 and 37, respectively. Therefore, pyridine-based PIP fillers will be a prospective option for manufacturing MMMs with extraordinary specific gas separation performance. We anticipate the strategy of using PIPs as fillers in MMMs highlights a bionic way for facilitating selective permeability of molecules and ions.
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