N2 Separation Comprising a Poly

Article pubs.acs.org/IECR

Mixed-Matrix Membranes for CO2/N2 Separation Comprising a Poly(vinylamine) Matrix and Metal−Organic Frameworks Song Zhao, Xiaochang Cao, Zijian Ma, Zhi Wang,* Zhihua Qiao, Jixiao Wang, and Shichang Wang Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin Key Laboratory of Membrane Science and Desalination Technology, and State Key Laboratory of Chemical Engineering (Tianjin University), Synergetic Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, P. R. China S Supporting Information *

ABSTRACT: In this work, imidazolate framework ZIF-8 nanoparticles were synthesized and incorporated into a poly(vinylamine) (PVAm) coating solution. The chemical structure of a PVAm/ZIF-8 film was characterized by X-ray diffraction and attenuated total reflectance Fourier transform infrared spectroscopy techniques. Mixed-matrix membranes (MMMs) were developed by coating the PVAm/ZIF-8 mixed dispersion on a polysulfone (PSf) support membrane. Surface and cross-sectional scanning electron microscopy images of the MMMs were employed to observe the dispersion of nanoparticles in the polymer matrix. The CO2 permeance and CO2/N2 selectivity of the MMMs were evaluated by using a CO2/N2 mixture gas. Optimal preparation parameters for the MMMs were achieved by varying the incorporated ZIF-8 contents in the coating solution and the wet coating thickness. The effect of incorporated ZIF-8 on improving the gas permselectivity was revealed, and the mechanism of CO2-facilitated transport in the PVAm/ZIF-8/PSf membrane was analyzed. The CO2/N2 separation performance of PVAm/ZIF-8/PSf membranes with different wet coating thicknesses was also evaluated at 22−60 °C to analyze the changes of the gas permselectivity with increasing temperature.

1. INTRODUCTION Emission of greenhouse gases, particularly CO2, has usually been cited as one of the most important factors affecting global warming and climate change.1,2 Thus, the separation and capture of CO2 from flue gas mixture is a scientific challenge in chemical and environmental technology.2,3 Membrane separation is considered as an alternative technology to the current amino absorption for CO2 capture because of its low energy requirement and simplicity of operation. Polymeric membranes are currently the dominant materials for gas separation processes because of the fact that polymer membranes have low production cost and exhibit the desired mechanical flexibility to be processed into different modules.4 However, there is a limiting trade-off between the polymer membrane’s selectivity and permeability, as reported by Robeson in 1991 and 2008,5,6 which constraints the development of these polymeric membranes for gas separation application. Low permeance of the membrane requires a large membrane area and high capital cost, while low selectivity means high operating cost from the energy-intensive multistage process.7 To expand the industrial application of polymeric membranes, extensive research has been conducted to enhance the gas permeance and selectivity by synthesizing high-performance membrane materials and developing effective membrane fabrication technology. Over the last decades, mixed-matrix membranes (MMMs), composed of an inorganic dispersed phase inserted in a polymer matrix, have been identified to offer the significant potential to achieve higher gas permselectivity. Nowadays, polymer materials such as cellulose acetate, poly(ether sulfone), polycarbonate, poly(ether imide), and polyimide are commonly employed as the polymer matrix for fabricating gas separation membranes.8−13 Inorganic fillers including zeolite, carbon © XXXX American Chemical Society

molecular sieves, and silica nanoparticles have been used to fabricate the MMMs by a polymer blending technique.8−15 Although the MMMs have been reported to display high gas permeance or selectivity, there are still many technical challenges that need to be addressed for achieving membranes with better gas separation performance. Usually, because of the difference between the polymer and inorganic phase properties, fabricating ideal MMMs with no defect in the polymer−particle interface is difficult.16 The interface defects can be classified into interface voids, the rigidified polymer layer in the polymer−particle interface, and particle pore blockage. When polymer−particle interfacial adhesion is weak, the formed nonselective interfacial voids would reduce the apparent selectivity of the MMMs. When polymer−particle interfacial adhesion is strong, polymer rigidification occurs near the filler surface with a reduction in the free volume, leading to a decrease in gas permeance. When the porous particles are incorporated into the polymer matrix, total and partial pore blockage may form and cause a decrease in gas permeance. Thus, the separation properties of the MMMs are related to the particle type, surface chemistry of inorganic fillers, and chemical structure of the polymer. Selection for the polymer and inorganic fillers with suitable structure and functional groups is very important in the development of MMMs. In recent years, new materials known as metal−organic frameworks (MOFs) have attracted much research interest and shown promising application as storage media, adsorbents, and Received: December 8, 2014 Revised: April 19, 2015 Accepted: April 23, 2015

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DOI: 10.1021/ie504786x Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research catalysts.2,3,17 The MOFs are an emerging class of nanoporous materials comprising metal centers such as zinc and cupric ions connected by various organic linkers to create porous structures with tunable pore volume, surface area, and chemical property.18,19 Because of their large surface area, functional groups, and tunable structure, the MOFs are predicted to be highly attractive for gas separation and reported to have high CO2 adsorption capacity and selectivity over the nonpolar gases. Moreover, MOFs are appealing candidates as the fillers incorporated into the polymeric membrane to fabricate MMMs. First, the incorporation of MOFs into the polymeric matrix can, in principle, combine the size/shape selectivity of MOFs with the processability of polymers. Second, ligands with a broad variety of functionalities would allow the MOFs to have a certain affinity with chains of the polymer matrix, which makes it possible to avoid the interaction problems between two components. Third, because the pores in the MOFs could serve as high-speed transport channels for gases, MMMs could possibly overcome the trade-off limitation between the permeance and selectivity and show high separation performance. Zeolitic imidazolate frameworks (ZIFs) are a MOF subclass with tetrahedral networks that are analogous to zeolites but use transition metals linked by imidazolate ligands. In particular, the ZIFs have great promise for gas storage and separation because of their exceptional thermal, chemical, and water stabilities.20−23 Several studies have reported the experimental adsorption capacities of CO2 on ZIF-8 nanoparticles and validated the CO2 adsorption sites as above the three methyl rings and near the six imidazole rings.20,21 The ZIFs also gained attention as fillers for the MMMs because of their molecular sieving effect, facile synthesis, and good compatibility with polymers.22−27 The study of Song et al. incorporated the ZIF-8 nanoparticles into the polyimide matrix.24 The permeation tests showed enhanced CO2 permeability of the MMMs with negligible loss in CO2/N2 selectivity upon increasing the loading of ZIF-8 nanoparticles. The study of Nafisi and Hägg indicated that the incorporation of 30 wt % ZIF-8 into a 6FDAdurene diamine matrix enhanced the CO2 permeability by 50% because of the polymer chain interruption.26 However, the MMMs using MOFs as the filler in the published work mostly used rigid polymeric material as the polymer matrix, such as polysulfone (PSf), Matrimid polyimide, and Ultem poly(ether imide). In a certain pressure range, a flexible polymeric material was considered to be more suitable than a rigid polymer for preparing the MMMs because priming of the filler could be easily achieved by adsorbing a layer of polymer onto the surface, which could reduce the stress at the polymer−filler interface.28 Besides, when a flexible polymer is used as the matrix, the prepared MMMs can be fabricated into a composite membrane with a thin selective layer depositing on a substrate. The effect of dispersed fillers may show a more obvious effect when incorporated into a thinner film with a thickness of a few hundred nanometers. Thus, it is meaningful to investigate the incorporation of MOFs into a flexible polymer matrix and its effect on the separation performance of MMMs. As a facilitated transport membrane material, poly(vinylamine) (PVAm) shows the potential to achieve high permselectivity through a reversible reaction between reactive amino carriers and CO2 molecules.29,30 The corresponding membrane is often prepared as a composite membrane with a PVAm selective layer depositing on a substrate. However, the membrane with a pure PVAm selective layer often exhibits low

gas permeance because the intermolecular interaction reduces the effective permeating area and facilitated transport carrier.15 Several studies were conducted to improve the separation performance by incorporating polyaniline nanofillers15,19 or adding small molecules29−31 into the PVAm matrix. In this study, the MMMs were fabricated by incorporating ZIF-8 nanoparticles into the PVAm matrix, and MMMs with excellent CO2 permselectivity are expected to be obtained for the following reasons. First, the polymer PVAm has a high molecular weight and suitable flexibility with good membrane-forming ability. The flexible polymer chain segment of PVAm could reduce stress at the polymer−filler interface and deduce the interface defects in the MMMs. Second, the incorporation of ZIF-8 into the PVAm matrix may increase the free volume of the membrane and then enhance the gas permeance. Third, the pore aperture of ZIF-8 nanoparticles and functional groups of PVAm may facilitate CO2 transport and favor enhancement of the selectivity. Figure 1 shows the

Figure 1. Chemical structure of the polymer PVAm and the singlecrystal X-ray structure of ZIF-8.32

chemical structure of PVAm and the single-crystal X-ray structure of ZIF-8 (Zn(C3N2H3)2).32 ZIF-8 nanoparticles are made from the linking of zinc(II) cations and 2-methylimidazole anions, giving a theoretical pore aperture of 3.4 Å. During the investigation, the synthesized ZIF-8 nanoparticles were incorporated into the PVAm coating solution, and the MMMs were developed by coating the mixed dispersion of PVAm and ZIF-8 on a PSf support membrane. The chemical and morphological structures of the selective layer were characterized by X-ray diffraction, attenuated total reflectance Fourier transform infrared spectroscopy, and scanning electron microscopy techniques. The gas permselectivity for a CO2/N2 mixture was measured to investigate the separation performance of MMMs with different ZIF-8 loadings and wet coating thicknesses. The effect of the temperature on the gas permselectivity of MMMs was also investigated. In general, the objective of this work was to investigate the facilitated transport mechanism for separating a CO2/N2 mixture that exists in MMMs comprising ZIF-8 nanoparticles and a PVAm matrix.

2. EXPERIMENTAL SECTION 2.1. Materials. N-Vinylformamide (NVF), purchased from Aldrich Inc., was distilled under vacuum and stored at −15 °C. 2,2′-Azobis(2-methylpropionamidine) dihydrochloride (AIBA), from Aldrich Inc., was recrystallized from ethanol and stored at −15 °C. Zn(NO3)2·6H2O (99.0%), 2-methylimidazole (Hmim, 99.0%), methanol (99.8%), and dichloromethane (99.5%) were purchased from Aladdin Reagent Co., Ltd. (China), and used without further purification. The polysulfone (PSf) membrane (average cutoff molecular weight of 6000) was used as the B


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2500, Japan) in reflection mode with 2θ scanned between 5° and 80° under an 8 kW power. The surface and cross-sectional morphologies of the PVAm/ PSf membrane and PVAm/ZIF-8/PSf MMMs were observed by SEM (Nova NanoSEM 430, FEI). The samples for crosssectional observation were prepared by peeling away the polyester nonwoven fabric, breaking in the liquid nitroge and sputtering with gold. The CO2/N2 permselectivity of the PVAm/PSf membrane and PVAm/ZIF-8/PSf MMMs were measured using a CO2/N2 (15/85 by volume) mixed gas by a set of test equipments (see Figure S1 in the Supporting Information, SI).35 The membrane sample with an effective area of 19.26 cm2 was mounted in a circular stainless steel cell. Prior to contacting the membrane, the feed gas and sweep gas (H2) were fed through a humidifier, and the gas saturated with water vapor was admitted to the permeation cell. The downstream pressure in the apparatus was maintained at atmospheric pressure. The feed gas (CO2/N2 mixed gas) flow rate and the sweep gas (H2) flow rate are 16 and 0.5 cm3 (STP)/s, respectively. The relatively high sweep flow rate can greatly weaken the back-diffusion effect of the permeate gas.35 The downstream pressure in the apparatus was maintained at atmospheric pressure. The fluxes of CO2 and N2 were calculated from the sweep gas flow rate and its composition, which was analyzed by a gas chromatograph equipped with a thermal conductivity detector (HP4890, Porapak N). The gas permeance is customarily expressed in units of GPU [1 GPU = 10−6 cm3 (STP)/cm2·s·cmHg = 3.35 × 10−10 mol/m2·s·Pa]. The selectivity (α) is calculated from the ratio of two gas permeances. Permeation experiments were carried out at room temperature (22 °C) with a feed pressure varying from 0.15 to 2.0 MPa, and steady-state permeation was assumed to have been reached when the sweep gas flow rate and its composition no longer changed with time. All error bars presented represent the standard errors of the performance of three membranes that were prepared under the same preparation conditions. In addition, it has been proven that the effect of back-diffusion of sweep gas on data analysis could be neglected.35

support membrane and supplied by Vontron Technology Co., Ltd. (China). 2.2. Synthesis of Poly(vinylamine) (PVAm). PVAm was synthesized according to the method reported in the literature.29 Typically, polymerization of PNVF was carried out in a flask containing NVF monomer, AIBA, and deionized water equipped with a stirrer at 50 ± 2 °C for 5 h under a nitrogen atmosphere. After polymerization, a certain concentration of hydrochloric acid was added to the flask, and acidic hydrolysis was carried out at 70 ± 2 °C for 2 h. Then, the solution was poured into a large quantity of ethanol to precipitate. After drying, the dried product was dissolved in a certain amount of deionized water and treated with excessive strongly basic anion-exchange resin. The molecular weight of the synthesized PVAm is 3.4 × 105 g/mol, which was measured with the viscosity method. The detailed synthesis route and chemical characterization of PVAm are elaborated on in our previous work.33 2.3. Synthesis and Characterization of ZIF-8. Here the ZIF-8 nanoparticles were synthesized using the rapid roomtemperature method developed by Cravillon et al.34 Typically, a solution of Zn(NO3)2·6H2O (2.933 g) in 200 mL of methanol is rapidly poured into a solution of Hmim (6.489 g) in 200 mL of methanol under stirring with a magnetic bar. After stirring for 1 h, the mixture is separated by centrifugation. The nanoparticles are finally obtained after washing with methanol several times and drying at 40 °C for at least 24 h. The synthesized ZIF-8 nanoparticles were characterized by powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy (FTS-6000, Bio-Rad of America). Morphological investigations were carried out using scanning electron microscopy (SEM; Nova NanoSEM 430, FEI). 2.4. Fabrication of the PVAm/PSf Membrane and PVAm/ZIF-8/PSf MMMs. The composite membranes for gas separation were fabricated on a PSf support membrane by a coating technique. The coating solution for fabricating a PVAm/PSf membrane was prepared by dissolving the PVAm powder into deionized water to obtain an aqueous solution with 2.0 wt % PVAm. The coating solutions for fabricating PVAm/ZIF-8/PSf MMMs were prepared by mixing PVAm, ZIF-8 nanoparticles, and deionized water, wherein the content of PVAm is kept as 2.0 wt % and the contents of the ZIF-8 nanoparticles in the total solid are varied as 9.1, 13.1, and 23.1 wt %, respectively. The coating solution was stirred for 12 h and allowed to stand for 24 h to release the bubbles. The membrane was prepared by coating the mixed dispersion on the PSf support membrane with a preset wet coating thickness and drying at 30 °C and 40% relative humidity in an artificial climate chamber (Climacell 222R) for approximately 24 h. The pure PVAm and PVAm/ZIF-8 films were prepared by spreading the coating solutions on a silicone rubber substrate, then drying for approximately 24 h at 30 °C and 40% relative humidity, and finally peeling off the substrate before the test. 2.5. Characterization of the PVAm/PSf Membrane and PVAm/ZIF-8/PSf MMMs. The chemical compositions of pure PVAm and PVAm/ZIF-8 films were characterized by an attenuated total reflectance Fourier transform infrared (ATRFTIR) spectrometer (FTS-6000, Bio-Rad of America). The spectra were collected at wavenumbers between 700 and 4000 cm−1 with a spectral resolution of 4 cm−1. The crystallinities of the pure PVAm and PVAm/ZIF-8 films were estimated by XRD spectroscopy using an X-ray diffractometer (D/MAX-

3. RESULTS AND DISCUSSION 3.1. Chemical Characterization of ZIF-8 Nanoparticles and PVAm/ZIF-8/PSf MMMs. Figure 2 shows the XRD patterns of the synthesized ZIF-8 nanoparticles, PVAm film,

Figure 2. XRD patterns of ZIF-8 nanoparticles, a PVAm film, and PVAm/ZIF-8 films with different loadings of ZIF-8 nanoparticles. C


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Industrial & Engineering Chemistry Research and PVAm/ZIF-8 films. The main peaks of the ZIF-8 nanoparticles match well with the published XRD pattern.34,36 The diffraction peaks are observed at 2θ of 7.2°, 10.2°, 12.6°, 16.3°, and 17.9°, which correspond to the (011), (002), (112), (013), and (222) planes, respectively. A sharp diffraction peak is observed for the PVAm and PVAm/ZIF-8 films at 2θ of around 22.8°. As observed, there is no obvious change for the XRD pattern of the films with low ZIF-8 loadings. With 23.1 wt % nanoparticles, the diffraction peaks corresponding to ZIF-8 were observed in the XRD pattern of the PVAm/ZIF-8 film. The effects of incorporated nanoparticles on the film crystallinity were usually considered to be from two aspects. On the one hand, the incorporated ZIF-8 in the polymer matrix may increase the free volume and decrease the crystallinity of the film. On the other hand, with high ZIF-8 loading, the incorporated ZIF-8 may act as a nucleation site and thus enhance the crystallinity of the film.36 These two opposing effects determine whether the film crystallinity increase or decrease with increasing ZIF-8 loadings. Figure 3 shows the FTIR spectra of ZIF-8 nanoparticles, a PVAm film, and PVAm/ZIF-8 films. The spectrum of ZIF-8

in the SEM image, similar to the reported results.24,34 Any defect or agglomerates of ZIF-8 nanoparticles are not observable in the surface SEM images of the PVAm/PSf membrane and PVAm/ZIF-8/PSf MMMs (shown in Figure S3 in the SI), suggesting that the incorporated nanoparticles are wrapped by the polymer matrix. Cross-sectional SEM images of the PVAm/PSf and PVAm/ ZIF-8/PSf membranes were acquired to probe the dispersion of nanoparticles in the polymer matrix. As can be seen from Figure 4, PVAm/ZIF-8/PSf MMMs with different loadings of

Figure 4. Cross-sectional SEM images of PVAm/ZIF-8/PSf MMMs with different loadings of ZIF-8 nanoparticles. Figure 3. FTIR spectra of ZIF-8 nanoparticles, a PVAm film, and PVAm/ZIF-8 films with different loadings of ZIF-8 nanoparticles.

ZIF-8 nanoparticles display selective layers with similar apparent thicknesses of about 240 nm. With 9.1 and 13.1 wt % loadings, ZIF-8 nanoparticles exhibit a good dispersion in the PVAm matrix. However, with further increasing ZIF-8 loading in the polymer matrix to 23.1 wt %, agglomerate with a size of around 150 nm appears in the cross-sectional SEM image, which may interrupt the gas permeance through the MMMs. 3.3. Gas Separation Performance of PVAm/ZIF-8/PSf MMMs. 3.3.1. CO2/N2 Separation Performance of PVAm/ZIF8/PSf MMMs Incorporating Different Contents of ZIF-8 Nanoparticles. The gas separation performances of the PVAm/ PSf membrane and PVAm/ZIF-8/PSf MMMs were evaluated using a CO2/N2 mixed gas with changes in the feed pressure from 0.15 to 2.0 MPa. Figure 5 shows the gas separation performance of PVAm/ZIF-8/PSf MMMs incorporating different contents of ZIF-8 nanoparticles. With increasing feed pressure, the CO2 permeance of all of the membranes drops a lot, representing the characteristics of the facilitated transport membrane. As described in our previous studies,29,30 the primary amine groups in the PVAm matrix act as CO2 carriers and dominated the facilitated diffusion and reaction selectivity of the membranes. Especially, the rapid decrease of CO2 permeance within a pressure range of 0.15−1.1 MPa is mainly due to the tendency toward saturation of the carriers, while the CO2 permeance decrease gently at higher pressure, resulting from the CO2 transport dominated by solution diffusion when the carriers are almost saturated.29,38,39 Apart from the

nanoparticles exhibits the adsorption bands of the functional group of imidazole units. The peak at 3610 cm−1 is due to O− H of the adsorbed H2O.2 The absorption band at 2927 cm−1 is ascribed to the aromatic C−H stretch of the imidazole.24,37 The peak at 1585 cm−1 can be assigned to the CN stretch mode.37 The intense and convoluted bands in the spectral regions of 700−1350 and 1350−1500 cm−1 are ascribed to the plane bending and stretching of the imidazole ring, respectively.37 The ATR-FTIR spectrum of the PVAm film also matches the literature well.30 The bands around 3300, 1662, and 1585 cm−1 could be assigned to the N−H stretching vibration, the CO stretching vibration, and the N−H bending vibration, respectively.31 The ATR-FTIR spectra of PVAm/ZIF-8 films exhibit the features that are present for both ZIF-8 nanoparticles and a PVAm film. The intensities of the bands at 748, 821, and 1018 cm−1 grow with increasing ZIF-8 loading. The new peak at 1369 cm−1 appears in the spectra of PVAm/ZIF-8 films due to a combination of the bands. 3.2. Morphological Observation of PVAm/ZIF-8/PSf MMMs. The SEM image of ZIF-8 nanoparticles (shown in Figure S2 in the SI) show that the synthesized nanoparticles have approximately globular morphology with narrow size distribution. An average particle diameter of around 46 nm was estimated from statistical evaluation of the particles presented D


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aperture of ZIF-8 would allow the transport of gas molecules with smaller kinetic diameter (such as CO2, 3.3 Å) and block the large molecules (such as N2, 3.64 Å).24 Thus, the PVAm/ ZIF-8/PSf membranes could selectively promote the transport of CO2, resulting in an increase in the CO2 permeance and CO2/N2 selectivity. Second, the incorporation of ZIF-8 nanoparticles disrupts the inherent organization of the polymer chains and increases the distance between polymer chains, which contributes to the overall gas diffusion and enhanced gas permeance. Similar results have also been reported for other MMMs, such as ZIF-8/polyimide and ZIF-8/poly(1,4-phenylene ether−ether sulfone) membranes.23−25 Finally, the incorporated ZIF-8 nanoparticles may enhance the compaction resistance of the MMMs by restricting the chain segment motion, which may also favor an increase of the CO 2 permeance at high feed pressure. N2 transports through the membrane only following the solution-diffusion mechanism because of no chemical reaction between N2 and the membrane. Thus, the changes in the N2 permeance with increasing feed pressure and the incorporation of ZIF-8 nanoparticles are mainly influenced by membrane compression and the interfacial gaps between ZIF-8 nanoparticles and the PVAm matrix. At low feed pressure, the N2 permeances of the MMMs are similar and higher than that of the PVAm/PSf membrane, which may be attributed to the disruption of polymer chains by the incorporated nanoparticles. With increasing feed pressure, the MMMs with 9.1 and 13.1 wt % ZIF-8 nanoparticles show a decrease in the N2 permeance, similar to that of the PVAm/PSf membrane, which may be due to compaction of the polymer matrix. The interfacial gap between ZIF-8 nanoparticles and the PVAm matrix has little influence on the N2 permeance because of the uniform dispersion of ZIF-8 nanoparticles in the MMMs with low ZIF-8 nanoparticle contents. However, as the ZIF-8 loading reaches 23.1 wt %, the N2 permeance increases a lot at or above 1.6 MPa, which may be ascribed to several aspects, including CO2-induced plasticization, membrane compression, and the interfacial gap between ZIF-8 nanoparticles and the PVAm matrix. At high feed pressure, dissolution of a large quantity of CO 2 within the polymer matrix would enhance the intersegmental mobility and increase the polymer free volume, which was called CO2-induced plasticization in the literature.29 As the ZIF-8 loading reaches 23.1 wt %, the polymer content in the selective layer is low and may be easily influenced by CO2induced plasticization, leading to an increase of the N2 permeance. The aggregated nanoparticles in the PVAm/ZIF8/PSf (23.1%) membrane may be another reason for the high N2 permeance because the nonselective voids between aggregated nanoparticles would make N2 permeate through the membrane more readily. These aspects may lead to a sharp increase of the N2 permeance at 1.6 MPa. With the feed pressure increasing from 1.6 to 2.0 MPa, the N2 permeance of the PVAm/ZIF-8/PSf (23.1%) membrane changes little, which may result from further membrane compression. The high N2 permeance causes the low CO2/N2 selectivity of the PVAm/ ZIF-8/PSf (23.1%) membrane at high feed pressure. Besides, the estimated gas permeabilities of the membranes were also calculated by the gas permeance and apparent thickness of the selective layer, and the results are shown in the SI. As presented in Figure S4 in the SI, because of the similar apparent selective layer thicknesses, the gas permeabilities of the MMMs with different ZIF-8 loadings show a similar tendency with that of gas permeances. Compared with the

Figure 5. Gas separation performance of PVAm/ZIF-8/PSf MMMs incorporating different contents of ZIF-8 nanoparticles: (a) CO2 and N2 permeances; (b) CO2/N2 selectivity. Feed gas: CO2/N2 mixture (15/85 by volume). Wet coating thickness: 50 μm.

facilitated transport effect, the decrease in the CO2 permeance may also arise from membrane compression with increasing differential pressure. Exposure of the membrane to high pressure would lead to compaction of the polymer structure, which would decrease the amount of free volume and subsequently reduce the mobility of the penetrating molecules.40 The incorporation of ZIF-8 nanoparticles obviously increases the gas permeance and CO2/N2 selectivity of the membranes. As can be seen from Figure 5a, the CO2 permeance increases steadily with increasing ZIF-8 loadings in the MMMs. The N2 permeance of the MMMs changes little when the ZIF-8 nanoparticle contents are below 13.1 wt %. However, with 23.1 wt % loading of ZIF-8 nanoparticles, the N2 permeance of the MMMs exhibits a sharp increase at high feed pressure. With a change in the N2 permeance, the CO2/N2 selectivity of the MMMs (shown in Figure 5b) increases significantly by incorporating 13.1 wt % ZIF-8 nanoparticles into the PVAm matrix. However, with 23.1 wt % loading of ZIF-8 nanoparticles, the CO2/N2 selectivity of the MMMs decreases to a lower value than that of the PVAm/PSf membrane at a feed pressure of over 1.0 MPa. Thus, taking into account the gas permselectivity, the MMMs with 13.1 wt % ZIF-8 nanoparticles present the optimal separation performance. Compared with the PVAm/PSf membrane, the CO2 permeance and CO2/N2 selectivity of the PVAm/ZIF-8/PSf (13.1%) membrane increase about 325% and 65% at 0.15 MPa, 130% and 47% at 0.6 MPa, and 79% and 140% at 2.0 MPa, respectively. The increases in the CO2 permeance and CO2/N2 selectivity with the incorporation of ZIF-8 nanoparticles could be explained by the following aspects. First, the 3.4 Å pore E


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50, and 100 μm wet coating thicknesses display selective layers with apparent thicknesses of 141, 246, and 387 nm, respectively. Figure 7 presents the CO2 permeance, N2 permeance, and CO2/N2 selectivity of PVAm/ZIF-8/PSf (13.1%) MMMs with

PVAm/PSf membrane, the PVAm/ZIF-8/PSf MMMs show higher CO2 and N2 permeabilities. With increasing ZIF-8 loading, the CO2 permeability increases gradually because of accelerated diffusion caused by both interrupted polymer chain packing and the appropriate pore aperture of ZIF-8. The N2 permeability changes little at a low ZIF-8 loading but increases a lot when the ZIF-8 loading reaches 23.1%, most likely because of the interfacial gaps formed between aggregated nanoparticles. With increasing feed gas pressure, the CO 2 permeability of the membranes decrease gradually as a result of carrier saturation while the N2 permeability changes little because of its transport dominated by the solution-diffusion mechanism. The high CO2 permeability at low feed gas pressure is most likely due to the presence of an abundant free carrier. With further increasing feed gas pressure, the carrier becomes saturated and the CO2 permeability becomes low and decreases gently because its transport is mainly dominated by the solution-diffusion mechanism.41 However, the above calculated results are based on the apparent thickness of the selective layer. Actually, the coating solution usually penetrates into the pores of the substrate during fabrication of the composite membrane. Thus, the thickness of the selective layer measured by SEM images may be the apparent thickness but not the effective thickness in consideration of the penetration of the coating solution into the pores of the substrate. In the study of Gurr et al., a cross-linked poly(dimethylsiloxane) layer was coated on the substrates and the apparent layer thickness was measured as 180 nm by SEM analysis.42 However, on the basis of the known permeability of cross-linked poly(dimethylsiloxane) and the measured gas permeance, the effective thickness of the layer is estimated to be on the order of 1.3 μm. Thus, if considering penetration of the coating solution, the effective thickness of the selective layer would be larger than the apparent thickness measured by SEM analysis, and thus the actual gas permeability might be much larger than the calculated gas permeability shown in Figure S4 in the SI. Because it is difficult to determine penetration of the coating solution into the pore of the substrate, the gas separation performances of the prepared membranes are mainly evaluated by gas permeance, which is independent of the thickness of the selective layer. 3.3.2. CO2/N2 Separation Performance of PVAm/ZIF-8/PSf MMMs with Different Wet Coating Thicknesses. A defect-free thin selective layer is crucial for enhancing the permselectivity of the composite membrane. Generally, the membrane with a thinner selective layer would display a higher gas permeance. Thus, the effect of the wet coating thickness on the permselectivity of the PVAm/ZIF-8/PSf membrane was evaluated. Figure 6 shows the cross-sectional SEM images of PVAm/ZIF-8/PSf (13.1%) MMMs with different wet coating thicknesses. It can be seen that the MMMs prepared with 30,

Figure 7. Gas separation performance of PVAm/ZIF-8/PSf (13.1%) membranes with different wet coating thicknesses: (a) CO2 and N2 permeances; (b) CO2/N2 selectivity. Feed gas: CO2/N2 mixture (15/ 85 by volume).

different wet coating thicknesses. The MMMs with a thinner selective layer usually have a higher gas permeance because of reduction of the transport resistance. Besides, with increasing feed pressure, the MMMs with thinner selective layers display a more rapid decrease in the CO2 permeance. When the feed pressure is above 0.6 MPa, the MMMs with the thinnest selective layers show an obvious increase in the N2 permeance and a decrease in the CO2/N2 selectivity. This may be because the polymer segments/chains in a thinner selective layer might become more flexible and results in a higher degree of plasticization.43 Besides, when ZIF-8 nanoparticles are incorporated into a thinner polymer selective layer, the probability of forming nonselective voids may increase, leading to an increase in the N2 permeance. Thus, the MMMs with 30 μm wet coating thicknesses show a rapid decrease in the CO2/ N2 selectivity at high feed pressure. Compared with the MMMs with 50 μm wet coating thicknesses, the MMMs with 100 μm wet coating thicknesses displayed much lower CO2 permeance at the low feed pressure. With increasing feed pressure, the CO2 permeance of the membrane with a thicker selective layer drops more slowly. In the previous work, Yuan et al. stated that the thinner the selective layer is, the larger the influence of the CO2 partial pressure on the facilitated diffusion term will be, which makes an important contribution to the CO2 permeance of the membrane.29 Thus, with increasing feed pressure, the CO2

Figure 6. Cross-sectional SEM images of PVAm/ZIF-8/PSf (13.1%) MMMs with different wet coating thicknesses. F


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Figure 8. Effect of the temperature on the gas separation performance of PVAm/ZIF-8/PSf (13.1%) membranes with different wet coating thicknesses: (a) CO2 permeance; (b) N2 permeance; (c) CO2/N2 selectivity. Feed gas: CO2/N2 (15/85 by volume). Feed gas pressure: 0.3 MPa.

permeances mentioned above, the membrane with 50 μm wet coating thickness showed a higher CO2/N2 selectivity than that with 100 μm wet coating thickness. 3.3.3. CO2/N2 Separation Performance of PVAm/ZIF-8/PSf MMMs under Different Temperatures. The gas separation performances of PVAm/ZIF-8/PSf membranes with different wet coating thicknesses were tested under different temperatures using a CO2/N2 mixed gas (15/85 by volume). In the practical application, the feed gas pressure is usually set as 0.3 MPa for CO2/N2 separation in comprehensive consideration of the driving force and energy consumption.45,46 Thus, the experiment was carried out at 0.3 MPa to investigate the effect of the temperature on the membrane separation performance. To control the testing temperature, the gas pipelines and membrane cell were held at a constant temperature (ranging from 22 to 60 °C) inside a thermostatic housing. Figure 8 shows the effect of the temperature on the gas separation performance of PVAm/ZIF-8/PSf (13.1%) membranes with different wet coating thicknesses. It can be seen that the gas permselectivities of membranes with different wet coating thicknesses show different tendencies with increasing temperature. With increasing temperature from 22 to 60 °C, the membrane with a thinner selective layer shows an apparent decrease in the CO2 permselectivity, while the membrane with a thicker selective layer shows a relatively stable separation performance. Specifically, when the temperature increases from 22 to 60 °C, the membrane with 30 μm wet coating thickness shows a sharp decrease in both the CO2 permeance and CO2/ N2 selectivity. The membrane with 50 μm wet coating thickness shows little change in the permselectivity with increasing temperature from 22 to 40 °C but shows a decrease in the CO2 permeance and CO2/N2 selectivity with increasing temperature from 40 to 60 °C. When the temperature increases from 22 to

permeance of a thinner membrane declines at a higher rate. The N2 permeances of the MMMs with 50 and 100 μm wet coating thicknesses decrease slowly with increasing feed pressure and do not show an apparent plasticization phenomenon. According to variation of the CO2 and N2 permeances mentioned above, the membrane with a 100 μm wet coating thickness shows a lower CO2/N2 selectivity than that with a 50 μm wet coating thickness. The low CO2/N2 selectivity of the membrane with a 100 μm wet coating thickness may be due to its low CO2 permeance compared with membranes with thinner selective layers. With a decrease in the wet coating thickness from 100 to 50 μm, the CO2 permeance increases a lot while the N2 permeance increases very little. The results indicate that the selective layer thickness exerts more influence on the transport of CO2 than that of N2, which could be explained from the following factors. On the one hand, a decrease in the selective layer thickness reduces the transport resistance, leading to increases in both the CO2 and N2 permeances. On the other hand, a decrease in the selective layer thickness accelerates the solubility and diffusion of the CO2 molecules in the PVAm matrix. As reported, there was a significant decrease of the glass transition temperature with a reduction in the thickness for ultrathin films, suggesting that the polymer chain in the microscale entity of thin films might be much more flexible than that in thick films.43,44 The increase in the mobility of the polymer chain could weaken the interaction between carriers in PVAm. Thus, the solubility and diffusion of CO2 molecules in the PVAm matrix may increase by the increased interaction between CO2 and the carriers, while the N2 permeance may change little under this condition partly because of its transport dominated by the solution-diffusion mechanism and partly because of the competition adsorption between CO2 and N2. Because of variation of the CO2 and N2 G


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Table 1. Separation Performance Comparison of the Membrane Prepared in This Work with Other MMMs Incorporating ZIFs and Other PVAm-Based Composite Membranes feed gas

RCO2(GPU)

α(CO2/N2)

PCO2 (P)

ref

PVAm

ZIF-8

filler

CO2/N2(15 vol % CO2)

ZIF-8 ZIF-8 ZIF-8 piperazine methylcarbamate polyaniline nanorods poly(vinyl alcohol)

CO2/N2 CO2/N2 CO2/N2 CO2/N2 CO2/N2 CO2/N2 CO2/N2

83 72 23 17 44 63 164 50 94

0.165 (1.1) 0.3 (2.0) 0.35 (1.0) 0.026 (0.26) 0.14 (0.69) 0.32 (1.6) 0.40 (2.0) 0.225 (1.5) 0.15 (1.5)

this work

Matrimid 6FDA-durene Ultem PVAm PVAm PVAm PVAm

297 173 20 43 18 211 197 160 48

polymer

(35 (10 (20 (20 (20 (15 (10

vol vol vol vol vol vol vol

% % % % % % %

CO2) CO2) CO2) CO2) CO2) CO2) CO2)

10 26 27 30 33 52 53

PCO2 = CO2 partial pressure of the feed gas (MPa); P = feed gas pressure.

60 °C, the CO2 permeance of the membrane with 100 μm wet coating thickness increases first and then decreases, while the N2 permeance increases a lot, resulting in a decrease in the CO2/N2 selectivity with increasing temperature from 40 to 60 °C. The effect of the temperature on the gas separation performance of the membranes could be explained from the following aspects. First, the polymer structure may be densified under high temperature, which results in a reduction of the chain mobility and simultaneously causes a decrease in the gas permeance.47,48 Second, the increase in the temperature would enhance the gas diffusion rate and thus accelerate gas transport in the membrane. Third, because of the existing fixed carrier in the PVAm matrix, the temperature may influence the gas permselectivity by changing the reaction of amine groups with CO2 molecules. With increasing temperature, the chemical equilibrium constant of the reaction between amine groups and CO2 molecules would decrease, resulting in a decrease of the CO2 permeance and CO2/N2 selectivity.49,50 Besides, an increase of the reaction rate between CO2 and carriers with increasing temperature may also have an effect on the CO2 permselectivity. Thus, the effect of the temperature on the membrane separation performance would be affected by the above-mentioned aspects, and the degree of their influence may vary for membranes with different wet coating thicknesses. Generally, it was reported that the polymer chain in the microscale entity of thin films might be much more flexible than that of thick films.43,44 Thus, with increasing temperature, densification of the polymer structure may have a great impact on the membrane with a thinner selective layer, which would lead to a decrease in both the CO2 and N2 permeance. Moreover, as the temperature increases, a decrease in the chemical equilibrium constant of the reaction between CO2 and carriers would result in a decrease of both the CO2 permeance and CO2/N2 selectivity.49,50 The combined effects of the above factors may result in a decrease of the gas permeance and CO2/ N2 selectivity of the membrane with 30 μm wet coating thickness with increasing temperature from 22 to 60 °C. For membranes with thicker selective layers (50 and 100 μm wet coating thicknesses), with increasing temperature from 22 to 40 °C, an increase in the gas diffusion rate may also exert a greater influence on the gas permeance because of the original larger gas transport resistance than that with a thinner selective layer. Combined with the effects of the gas diffusion rate, polymer densification, and chemical reaction, the CO 2 permselectivity of the membrane with 50 μm wet coating thickness changes little with increasing temperature from 22 to 40 °C, while the membrane with 100 μm wet coating thickness

shows a slight increase in the CO2 permeance and basically unchanged CO2/N2 selectivity. With a further increase in the temperature from 40 to 60 °C, polymer densification may be more evident and a decrease in the chemical equilibrium constant of the reaction between CO2 and carriers would also become apparent, which results in a decrease of the CO2 permeance. The N2 permeance increases mainly because of a further increase in the gas diffusion rate with increasing temperature. The decrease in the CO2/N2 selectivity with increasing temperature from 40 to 60 °C is attributed to changes in the CO2 and N2 permeances. Overall, with increasing temperature from 22 to 60 °C, the membranes with 50 and 100 μm wet coating thicknesses display relatively stable separation performances compared to that with 30 μm wet coating thickness. 3.4. Comparative Study with Literature Data of the MMMs with ZIFs and PVAm-Based Composite Membranes. The CO2 permselectivities of the MMMs obtained in this work are compared with those of the membranes incorporating ZIFs and other PVAm-based composite membranes reported in the literature. Considering that the calculated gas permeability would be influenced by the accuracy of the effective thickness of the selective layer, the membrane performances for the comparison are given as CO2 permeances, which are independent of the film thickness. Table 1 lists the mixed gas permeation results of the membranes and references. It can be seen that the gas separation performance of the MMMs using ZIFs as fillers and a rigid polymer as the matrix is mostly evaluated below 1.0 MPa. Under similar gas pressure, the PVAm/ZIF-8/PSf membrane shows higher CO2 permeance and CO2/N2 selectivity than other MMMs using ZIFs as fillers. Besides, compared with asymmetric MMMs, composite MMMs possess more potential for industrial membrane fabrication because of the facile preparation process and the small required quantity of membrane materials.51 Compared with PVAm-based composite membranes, the gas permselectivity of the PVAm/ZIF-8/PSf membrane was higher than those of several modified PVAm/PSf membranes but lower than those modified by piperazine and methylcarbamate, which displayed high permselectivity possibly because of the introduced amine or ester groups. Besides, the isoreticular metal−organic framework (IRMOF1) was also synthesized and incorporated into the PVAm matrix to fabricate the MMMs. The PVAm/IRMOF-1 film and PVAm/IRMOF-1/PSf membrane were characterized by XRD and SEM analysis, which can be seen in Figures S5 and S6 in the SI. The results of gas separation measurement indicated that the PVAm/IRMOF-1/PSf membrane displayed much H



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lower CO2 permeance than the PVAm/ZIF-8/PSf membrane (see Figures S7 and S8 in the SI), which may be ascribed to the morphological structure and water stability of IRMOF-1 nanoparticles.

ASSOCIATED CONTENT

S Supporting Information *

Gas permeation measurement, morphological observation, SEM images, CO2 and N2 gas permeability, fabrication and evaluation of PVAm/IRMOF-1/PSf membranes, and XRD patterns. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ie504786x.

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REFERENCES

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4. CONCLUSIONS In this work, ZIF-8 nanoparticles were synthesized and incorporated into the PVAm matrix to fabricate the MMMs for gas separation. From the experimental observation, the ZIF8 nanoparticles were completely wrapped and uniformly dispersed into the polymer matrix when the incorporated contents were below 13.1 wt %. Compared with the PVAm/PSf membrane, the CO2 permeance and CO2/N2 selectivity of PVAm/ZIF-8/PSf membranes were significantly enhanced, which could be ascribed to several aspects including the selective transport of a CO2 molecule by the pore aperture, the disruption of inherent organization of the polymer chains, and the interfacial gaps of polymer−nanoparticles. Taking account the gas permselectivity, the MMMs with 13.1 wt % ZIF-8 nanoparticles present optimal gas separation performance. Compared with the PVAm/PSf membrane, the CO2 permeance and CO2/N2 selectivity of the PVAm/ZIF-8/PSf (13.1%) membrane increase about 325% and 65% at 0.15 MPa and 79% and 140% at 2.0 MPa, respectively. The structure and gas separation performance of PVAm/ZIF-8/PSf (13.1%) membranes prepared with different wet coating thicknesses were also characterized. The PVAm/ZIF-8/PSf MMMs prepared with 30, 50, and 100 μm wet coating thicknesses display selective layers with apparent thicknesses of 141, 246, and 387 nm, respectively. The MMMs with 50 μm wet coating thicknesses show medium CO2 permeance and the highest selectivity. The effect of the temperature on the gas separation performance of PVAm/ZIF-8/PSf (13.1%) membranes with different wet coating thicknesses was also conducted. With increasing temperature from 22 to 60 °C, the membranes with 50 and 100 μm wet coating thicknesses display relatively stable separation performances compared to that with 30 μm wet coating thickness.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 02227404533. Fax: +86 02227404496. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by grants from the National Natural Science Foundation of China (Grant 21306130), the Major State Basic Research Development Program of China (973 Program, Grant 2009CB623405), the Science & Technology Pillar Program of Tianjin (Grant 10ZCKFSH01700), and the Program of Introducing Talents of Discipline to Universities (Grant B06006). I


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J


N2 Separation Comprising a Poly

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