N2 separation performance

Journal of Membrane Science 589 (2019) 117246

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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Polyvinylamine/graphene oxide/PANI@CNTs mixed matrix composite membranes with enhanced CO2/N2 separation performance

T

Yonghong Wanga,b,*, Long Lia, Xinru Zhanga,b, Jinping Lia,b, Chengcen Liua, Nanwen Lic,**, Zongli Xied a

College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, China Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan, 030024, Shanxi, China c State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, Shanxi, China d CSIRO Manufacturing, Private Bag 10, Clayton South, Victoria, 3169, Australia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Mixed matrix membranes Graphene oxide Interlayer spacing Facilitated transport CO2 separation

Lamellar PANI@CNTs-GO materials were prepared by intercalating polyaniline-coated carbon nanotubes (PANI@CNTs) in graphene oxide (GO) layers to regulate the interlayer spacing. Mixed matrix membranes (MMMs) with tuned structures were achieved by coating a mixed dispersion of PANI@CNTs-GO and polyvinylamine (PVAm) on an asymmetric polysulfone (PSf) membrane for enhancing its CO2 separation performance. The Fourier-transform infrared spectroscopy results showed that PANI was successfully coated on the surface of the CNTs. The X-ray diffraction results confirmed the intercalation of GO sheets with PANI@CNTs. PANI@CNTs-GO was uniformly dispersed in PVAm, as confirmed by scanning electron microscopy. The attenuated total reflectance Fourier transform infrared spectroscopy results revealed that strong interfacial interactions were present between PVAm and PANI@CNTs-GO. The MMM loaded with 1 wt% PANI@CNTs-GO showed the best CO2 separation performance with a CO2 permeance of 170 GPU and a CO2/N2 selectivity of 122.4 at 1 bar under the pure gas condition. These values were much higher than those of the pristine PVAm membrane (CO2 permeance = 73 GPU and CO2/N2 selectivity = 45.7). This enhanced separation performance can be mainly attributed to the effect of the facilitated transport carriers from amine groups in the interlayer spacing and the molecular sieving effect of the interlayer spacing. The MMMs exhibited excellent long-term stability even under the mixed-gas conditions for over 300 h with a CO2 permeance of 264 GPU and a CO2/N2 selectivity of 149.8.

1. Introduction The rapid increase in energy demand has led to an increase in the global emission of CO2, which is the largest contributor to the global warming effect and extreme weather conditions across the globe [1,2]. Therefore, CO2 separation and capture are crucial for energy saving, environmental protection, and sustainable development. Compared to traditional CO2 separation methods such as absorption, adsorption, and cryogenic distillation, membrane separation has gained immense attention owing to its high energy efficiency, simple process equipment, ease of processing, environmental friendliness, small footprint, and cost-effectiveness [3,4]. The membranes used for CO2 capture and storage are mainly classified as inorganic porous membranes, dense polymeric membranes, and mixed matrix membranes (MMMs)

*

according to their material type and structure [5]. Polymeric membranes are the most widely used materials for gas separation processes owing to their low production cost and exceptional mechanical properties [6]. However, these membranes suffer from a trade-off between the permeability and selectivity [7,8]. MMMs, which consist of a polymer matrix and inorganic particles as the dispersed phase, show excellent permeability and selectivity. They show advantages of both polymers and inorganic particles, and have gained significant attention over the past few years [9,10]. In conventional MMMs, the solution-diffusion mechanism dominates the gas transport [11,12]. However, most of the MMMs containing porous fillers or nonporous fillers exhibit low permeability or gas selectivity, respectively because of the presence of interfacial voids, pore blocking, and the combination of pore blocking and agglomeration [13]. Hence,

Corresponding author. College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, China. Corresponding author. E-mail addresses: [email protected] (Y. Wang), [email protected] (N. Li).

**

https://doi.org/10.1016/j.memsci.2019.117246 Received 30 April 2019; Received in revised form 2 July 2019; Accepted 3 July 2019 Available online 04 July 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

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PANI is considered as a promising candidate to facilitate CO2 transport [13]. In this study, we first intercalated PANI-coated MWCNTs (PANI@CNTs) into GO interlayers to prepare PANI@CNTs-GO with an optimum interlayer spacing for CO2 transport. Then, PANI@CNTs-GO was added to a PVAm aqueous solution, followed by coating the mixture on an ultrafiltration polysulfone (PSf) substrate to obtain PVAm/ PANI@CNTs-GO MMMs for CO2/N2 separation. The X-ray diffraction (XRD) results revealed that CO2 molecules could freely contact the amine groups to facilitate the transport under a humidified state in the interlayer spacing of the MMMs. The effects of the filler content, feed pressure, wet-film thickness, and temperature on the gas separation performance of the MMMs were systematically investigated. The PVAm/PANI@CNTs-GO MMMs exhibited excellent separation performance because the reversible reaction of CO2 and amine groups could be achieved in a large region in the interlayer spacing, facilitating rapid CO2 transport. In addition, the interlayer spacing could also induce the molecular sieving effect for CO2/N2 separation.

the fabrication of MMMs with high gas separation performance is a challenge. The incorporation of carriers into MMMs can facilitate CO2 transport in them, thus improving their separation performance through reversible reactions between the carriers and CO2. On the other hand, the other gases inert to the carriers permeate only via the solution-diffusion mechanism [14-16]. Thus, these membranes exhibit high permselectivity for CO2 during the separation process. MMMs containing CO2 carriers, require water for CO2 transport. Water not only causes the swelling of the polymeric chain, but also acts as the reaction medium. In the case of wet membranes, CO2 reacts with the carriers (primary or secondary amine groups) and water to produce a (RNHCOO−) complex and HCO3− within the membrane according to the following reactions [16,17]: 2CO2 + 2RNH2 + H2O ⇌ RHNCOOH + RHN3+ + HCO3-

(1)

2CO2 + 2R2NH + H2O ⇌ R2NCOOH + R2HN2

(2)

+

+

HCO3-

Flue gas consists of approximately 20 vol% CO2 and 80 vol% N2, with saturated water vapor [17]. Solution-diffusion membranes show low selectivity and permeability towards flue gas. On the other hand, transport membranes with carriers show high selectivity and permeability towards flue gas under a humidified state and at low partial pressure driving forces [17]. Therefore, MMMs containing CO2 carriers show great potential for separating CO2 from water vapor-containing flue gas [18]. Polyvinylamine (PVAm) is an amine carrier-containing membrane used for CO2 separation because of its high gas permselectivity and easy film-forming properties [14,15]. However, the pristine PVAm membrane shows high crystallinity because of strong intermolecular interactions, which cause low effective permeating area and limited carrier transport [19,20]. Various efforts have been made to improve the separation performance of PVAm membranes by incorporating an inorganic filler into the PVAm matrix [21-23]. For example, Wang et al. [24] introduced CNTs into the PVAm matrix to prepare MMMs with a CO2 permeance of 104 GPU and a CO2/N2 selectivity of 15.7 at 5 bar. This improvement in the separation performance of these membranes as compared to the pristine PVAm membrane can be attributed to the presence of CNTs, which acted as gas transport channels. Zhang et al. [5] added hyperbranched polyethylenimine functionalized graphene oxide (HPEI-GO) into PVAm-CS to prepare HPEI-GO/CS-PVAm/PS membranes with a reasonable CO2 permeance of 31.3 GPU and a high CO2/N2 selectivity of 107. This high separation performance can be attributed to the good compatibility of HPEI-modified GO with the polymer matrix, but also the presence of amine carriers, which increased the permeance and selectivity of the membranes. On the basis of these results, it can be stated that the incorporation of PVAm can enhance the flue gas permselectivity of MMMs. However, since the carriers cannot readily contact CO2 and water molecules because of the hindrance caused by the crystal face of PVAm, the carriers move only in a confined space. Thus, this passive CO2 transport limits the separation performance of PVAm-based MMMs. According to the facilitated transport model, the carriers for CO2 transport should be dispersed homogeneously in the polymer matrix to achieve good gas separation performance. Furthermore, MMMs consist of a continuous space for CO2 transport, which enhances their permeance and selectivity [25]. Liao et al. [22] reported that traditional facilitated-transport carriers can only passively transfer CO2 through a small-range vibrating space, resulting in a poor separation performance. The presence of facilitatedtransport channels in MMMs can significantly enhance their active region for CO2 transport. In order to create a micro-environment where the active carriers can arbitrarily approach CO2 in MMMs, we fabricated high-performance MMMs by tuning the interlayer spacing of GO as continuous facilitatedtransport pathways. Polyaniline (PANI) has gained significant research attention owing to its easy synthesis, tunable properties, and good stability [26-28]. Owing to its amine group-bearing molecular chains,

2. Experimental 2.1. Materials PVAm (15 wt% aqueous solution, Mw=600, 000) was obtained from BASF. Hydrochloric acid (HCl, 37.2%), sulfuric acid (H2SO4, 98%), nitric acid (HNO3, 70%), aniline (99% purity), sodium nitrate (NaNO3), glutaraldehyde (GA, 50%), and ammonium persulfate (APS) were purchased from Tianjin Guangfu Fine Chemistry Institute (Tianjin, China). Hydrogen peroxide (H2O2, 30%) and potassium permanganate (KMnO4) were purchased from Sinopharm Group Chemical Reagent Co., Ltd. Graphite powder was received from Alpha Reagent Company. MWCNTs were supplied by Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences. Polysulfone (PSf) ultrafiltration membranes with an average molecular weight cut-off of 6,000, which were used as porous supports, were supplied by Vontron Technology Co. Ltd (China). N2 and CO2 with purities of 99.999% and the mixed gas (CO2/N2 = 20/80 vol%) were provided by the Shanxi Special Gases Company. All the chemicals were used as received without further purification. 2.2. Preparation of PANI@CNTs-GO MWCNTs were first treated with a H2SO4/HNO3 mixture according to a previously reported procedure under reflux conditions to obtain carboxylated MWCNTs (MWCNTs-COOH) [29]. Graphene oxide was synthesized from natural graphite powder using the improved Hummers' method [30]. Water-dispersible PANI@CNTs were prepared by an in-situ polymerization method developed by Jiménez et al. [31]. Specifically, MWCNTs-COOH (20 mg) were first added into 10 mL of HCl (1 mol L-1) followed by sonication at room temperature for 30 min to obtain a uniform dispersion. Aniline (200 mg) was then added to the MWCNTs-COOH suspension under ultrasonication for another 30 min. A mixture of APS (160 mg) and HCl (10 mL, 1 mol L-1) was then added to the above mixture dropwise under sonication at a temperature of about 16–20 °C for 4 h. A stable dispersion of PANI@CNTs with a concentration of 0.5 mg mL-1 was obtained by washing the mixture with deionized water and ethanol to adjust the pH at around 3. The obtained PANI@CNTs dispersion (20 mL, 0.5 mg mL-1) was added to a GO colloid (60 mL, 0.5 mg mL-1) and the resulting mixture was sonicated for another 4 h. The GO/PANI@CNTs mixture so obtained was then washed with water by centrifugation. After several washing-centrifugation cycles, the pH of the mixture was adjusted at around 5 to obtain a dispersion of PANI@CNTs-GO with a concentration of 1 mg mL-1 for use in membrane preparation. For further analysis, the obtained dispersion of PANI@CNTs-GO was filtered using a vacuum filter followed by vacuum drying at 60 °C for 24 h. 2

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with a thermal conductivity detector. The permeance and selectivity for the pure gas and binary gas mixtures were calculated from the outlet sweep gas flow rate and the composition of the permeate side. The permeance of each gas was calculated using Eq. (3):

2.3. PVAm/PANI@CNTs-GO MMMs fabrication The MMMs for gas separation were fabricated on a PSf support membrane using a coating technique. The coating solutions were prepared by mixing the PANI@CNTs-GO dispersion, PVAm (2.5 wt%), and deionized water. The weight percentage of PANI@CNTs-GO relative to PVAm was varied as 0.25, 0.5, 0.75, 1, and 1.25 wt%. In order to effectively utilize all the available NH2 groups in PVAm, the pH of the PVAm/PANI@CNTs-GO coating solution was maintained at 10 by adding 1 mol L-1 NaOH to it in order to achieve high CO2 separation. Prior to scratching the dense selective layer, the PSf support was first rinsed with a dilute sodium laurylsulfonate solution and then flushed with deionized water to remove any possible contaminants. This solution was uniformly coated on the ultrafiltration membrane with a coating applicator by adjusting its wet thickness to prepare the PVAm/ PANI@CNTs-GO MMMs. The membrane was then dried in an oven at 30 °C and 40 % relative humidity for at least 12 h. The resulting membranes were denoted as PVAm/PANI@CNTs-GO-X, where X represents the weight percentage of the filler relative to PVAm. For comparison, a pristine PVAm membrane, PVAm/CNTs-GO MMM, and PVAm/PANI@CNTs MMM were prepared using a 2.5 wt% PVAm aqueous solution using the same procedure as that used for preparing the PVAm/PANI@CNTs-GO MMMs.

Pi =

Qi Δpi A

(3)

where, Pi is the permeance of the gas “i” (GPU, 1 GPU = 10-6 cm3 (STP) cm-2 s-1 cmHg-1). Qi is the volumetric flow rate of the gas ‘i’ (cm3 (STP)/ s); Δpi is the transmembrane-pressure difference (cmHg), and A is the effective membrane area (12.56 cm2). The ideal selectivity (αij) of the pure gas was calculated using Eq. (4):

αij =

Pi Pj

(4)

The mixed-gas separation factor of gases ‘i and j’ (αij ) was calculated using Eq. (5): *

αij* =

yi / yj x i /xj

(5)

where x and y are the volumetric fractions of one component in the feed and permeate, respectively.

2.4. Characterization

3. Results and discussion

The Fourier transform infrared (FT-IR) spectra of the as-prepared materials were obtained using a Nicolet 6700 FT-IR spectrometer (Thermo Scientific, USA) over the wavelength range of 4000–650 cm1 at a resolution of 4 cm-1. The crystal structures of the as-prepared materials were characterized by XRD (Shimadzu, XRD-6000, Japan) at a scanning rate of 8° min-1 over the 2θ range of 5–60°. The attenuated total reflectance FT-IR (ATR-FTIR) spectroscopy analysis of the MMMs was carried out using a Nicolet 6700 spectrometer (Thermo Scientific, USA) over the 650–4000 cm-1 range at a resolution of 4 cm-1. Transmission electron microscopy (TEM) (JEOL JEM-2100F) was used to observe the microstructures of GO and PANI@CNTs-GO. Scanning electron microscopy (SEM) (Hitachi, SU8010, Japan) was used to observe the cross-sectional morphology of the membranes. The MMM samples for cross-sectional observation were prepared by peeling the selective layer from the polyester nonwoven fabric, breaking in the liquid nitrogen, and sputtering with gold.

3.1. Preparation and characterization of MMMs Scheme 1 shows the fabrication procedure of PANI@CNTs-GO. As shown in Scheme 1, first, MWCNTs-COOH was prepared. Then, aniline was oxidized, which acted as the cationic radical and oligomer and adsorbed on the surface of MWCNTs-COOH. PANI was prepared via the in-situ polymerization of aniline on the surface of MWCNTs-COOH to obtain PANI@CNTs according to a previously reported procedure [34]. The formation of PANI@CNTs can be attributed to the presence of the π-π* electron interaction between MWCNTs-COOH and aniline, as well as the hydrogen bond interaction between the carboxyl groups of MWCNTs-COOH and the amine groups of the aniline monomers [35]. Finally, PANI@CNTs was intercalated into the interlayer space of GO via ultrasonication to obtain a composite material (PANI@CNTs-GO) with interlayer spacing. In PANI@CNTs-GO, the carboxyl groups or hydroxyl groups on GO formed hydrogen bonds with the amine groups on PANI. In addition, the negatively charged GO could adsorb the positively charged PANI by electrostatic force so that PANI@CNTs with CO2 facilitated transport carrier could be intercalated into the GO interlayer to obtain a stable composite material. The IR results validated these strong interactions between PANI@CNTs and GO (Fig. S2). The bands at 1544 and 1291 cm-1 corresponding to the quinonoid units and C–N stretching vibration of PANI shifted to lower wavenumbers [36]. PANI@CNTs-GO with interlayer spacing is likely to exhibit the molecular sieving effect, which is beneficial for gas transport. Furthermore, a thin PANI layer was formed on the outer surface of the CNTs, which facilitated CO2 transport under humidified conditions. The FT-IR and XRD results confirmed the successful synthesis of PANI@CNTs-GO (Figs. S2 and S3). These results were consistent with those reported previously [34,37,38]. The XRD results revealed that the PANI@CNTsGO sample with the mass ratio of 1:3 showed a d-spacing of 0.825 nm, which was higher than that of pure GO (0.737), as shown in Fig. S3. This increase in the interlayer spacing (which is beneficial for CO2 transport) can be attributed to intercalation of PANI@CNTs into the GO interlayers. The increased interlayer spacing effectively enhanced the CO2 transport in the membrane. The TEM results (Fig. S5) revealed that pristine GO consisted of layered sheets, while PANI@CNTs were distributed uniformly as tubular structures on the surface of the GO sheets in the case of PANI@CNTs-GO. Furthermore, PANI@CNTs did not show aggregation. This confirms the intercalation of PANI@CNTs into the

2.5. Gas permeation experiments The gas separation performances of the membranes were evaluated using the constant pressure/variable volume method [30,32], and the schematic of the gas permeation apparatus (fabricated in-house) is shown in Fig. S1 [33]. The membrane was mounted in a circular stainless steel cell (effective membrane area is 12.56 cm2). Before contacting the membrane, the feed gas was introduced into a humidifier (40 °C) for saturation with water vapor and was then passed through a water knockout (25 °C) to remove the residual water. The sweep gas was also simultaneously introduced into the same apparatus. H2 was used as the sweep gas for the CO2/N2 mixed gas (CO2/N2 = 20/80 vol %) and pure CO2 and N2 (99.999%) gas tests. The flow rates of the feed and sweep gases were 60 and 30 mL min-1, respectively. The sweep gas was used to remove any components that permeated the membrane at the permeate side. The upstream pressure was varied from 1 to 9 bar, while the downstream pressure in the apparatus was maintained at the atmospheric pressure. Permeation experiments were carried out at 25 °C, and steady-state permeation was assumed to have been reached when the outlet sweep gas flow rate and its composition no longer changed with time. The flow rate of the outlet sweep gas was determined using a soap film flow meter, and the composition was analysed by a gas chromatograph (Shimadzu, GC-2014C, Japan) equipped 3

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Scheme 1. Schematic of the preparation of PANI@CNTs-GO.

along the interlayer channels in the humidified state. The bands at around 3238 and 1663 cm-1 in Fig. 1 can be assigned to the stretching vibration of primary amine groups and C]O stretching vibration in PVAm because of the incomplete hydrolysis of amide groups [7], respectively. These bands shifted to lower wavenumbers in the case of the PVAm/PANI@CNTs-GO MMMs, indicating the existence of hydrogen bonding and electrostatic interactions between PVAm and PANI@CNTs-GO. Thus, excellent compatibility was observed between PANI@CNTs-GO and PVAm. As can be observed from Fig. 2, the MMMs did not show any significant aggregation. The thickness of the selective layer was maintained at around 725 ± 32 nm for the pristine PVAm, PVAm/[email protected], and PVAm/PANI@CNTs-GO-1 membranes. The PVAm/[email protected] MMM showed a slightly higher selective layer thickness of 930 ± 50 nm. This increase in the thickness can be ascribed to the high viscosity of the coating solution loaded with a relatively high PANI@CNTs-GO content. Moreover, the cross-section of the PVAm membrane was smooth and crack- and defect-free. This confirms the dense structure of the PVAm membrane (Fig. 2(a)). However, the cross-section of the MMMs became rough after loading PANI@CNTs-GO. As shown in Fig. 2(b)–(c), the PVAm matrix of the MMMs with a PANI@CNTs-GO loading of up to 1 wt% was wrapped tightly with a layered structure of PANI@CNTs. This can be attributed to the formation of intermolecular hydrogen bonds between GO and PVAm. However, with a further increase in the PANI@CNTsGO content of the matrix up to 1.25 wt%, the membrane showed only slight aggregation, as shown in Fig. 2(d).

interlayers of GO. Moreover, the interlayer spacing of the samples could be controlled by adjusting their PANI@CNTs:GO mass ratio, which created sufficient space for CO2 transport. As shown in Fig. S4, the dspacing of the GO nanosheets increased from 0.737 nm to more than 0.794 nm after the intercalation of PANI@CNTs into them. Moreover, the d-spacing values of the samples increased with an increase in their PANI@CNTs contents. The highest d-spacing of 0.85 nm was achieved at the PANI@CNTs:GO mass ratio of 1:1. However, when the PANI@CNTs content increased to 75 wt%, the peak corresponding to the d-spacing of GO disappeared because of the exfoliation of GO by PANI@CNTs. These results indicate that rod-shaped one-dimensional materials can control the interlayer spacing of nanosheet materials. This is consistent with the results reported previously [39]. The PVAm/PANI@CNTs-GO MMMs were fabricated by solution casting onto the PSf membrane substrate. As shown in Scheme 2, an atrovirens-coloured aqueous dispersion of PANI@CNTs-GO was added to a PVAm solution to obtain a mazarine-coloured mixed-dispersion (Fig. S7), indicating that the de-doping process occurred in the PVAm alkaline solution. This demonstrates that a homogenous dispersion of the PANI@CNTs-GO can be obtained in PVAm aqueous solutions, which has also been confirmed in previous studies [13]. The MMMs were prepared by coating the mixture of PANI@CNTs-GO and PVAm on porous PSf membranes with a controlled wet film thickness. The layered PANI@CNTs-GO with interlayer spacing was uniformly distributed in the PVAm matrix and showed good interfacial adhesion to it. This is attributed to both the hydrogen bonding and electrostatic interactions between the carbonyl or carboxyl groups on the GO sheets and the amine groups on PVAm, as confirmed by the ATR-FT-IR results (Fig. 1). This resulted in the formation of defect-free organic-inorganic hybrid membranes. Jin et al. demonstrated that the selective layer of MMMs consist of parallel-stacked and random-stacked GO laminates [40]. It was found that GO could be assembled into laminar structures with fast and selective transport channels, which exhibited excellent CO2/N2 separation performance owing to their molecular-sieving interlayer spacing and straight diffusion pathways. In the PANI@CNTsGO MMMs, the interlayer spacing of PANI@CNTs-GO served as CO2 transport pathways as a result of the size sieving effect for CO2/N2, and the PANI layer between the GO interlayers facilitated CO2 transport

3.2. Gas permeation performance of MMMs 3.2.1. Effect of PANI@CNTs-GO loading Fig. 3 shows the pure gas separation performance of the MMMs with different PANI@CNTs-GO loadings in the humidified state (100% relative humidity). Compared to the pristine PVAm membrane, the MMMs exhibited not only higher CO2 permeance but also higher CO2/ N2 selectivity. As shown in Fig. 3, both the CO2 permeance and CO2/N2 selectivity of the MMMs increased steadily with an increase in the PANI@CNTs-GO loading from 0 to 1 wt%. However, a decrease in the CO2/N2 selectivity was observed when the PANI@CNTs-GO loading 4

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Scheme 2. Schematic of the fabrication of the PVAm/PANI@CNTs-GO MMMs.

function as the facilitated transport carriers. As shown in Fig. S6, the CO2 permeance of the PVAm/PANI@CNTs-GO MMMs increased with an increase in their PANI@CNTs content (d-spacing increment). However, the CO2/N2 selectivity of the PVAm/PANI@CNTs-GO MMMs first increased with an increase in the PANI@CNTs content up to 20 wt% and then decreased rapidly. This indicates that excess PANI@CNTs reduced the CO2/N2 selectivity of the MMMs. The PVAm/PANI@CNTsGO MMM with the PANI@CNTs content of 25 wt% (mass ratio = 1:3) showed optimum CO2 permeance and CO2/N2 selectivity of 171 GPU and 122.4, respectively. At this mass ratio, PANI@CNTs-GO showed a d-spacing of 0.825. This demonstrates that the d-spacing of 0.825 nm was suitable for CO2 transport and prevented the diffusion of N2 because of the molecular size or shape sieving effect, and facilitated transport effect of amine groups. Furthermore, the presence of lamellar PANI@CNTs-GO increased the length of the tortuous gas diffusion path in the PVAm matrix, thus improving the CO2 diffusivity selectivity as compared N2 diffusivity (because of the large molecular size of N2). These results are consistent with those reported by Jiang et al. for functionalized graphene oxide-based MMMs [42].

Fig. 1. ATR-FT-IR spectra of the pristine PVAm membrane and PVAm/ PANI@CNTs-GO MMMs.

was increased beyond 1 wt% in spite of the increased CO2 permeance. At higher loadings, the aggregation of PANI@CNTs-GO created interfacial voids between PANI@CNTs-GO and PVAm, which consequently reduced the CO2/N2 selectivity of the membranes [41]. The MMM loaded with 1 wt% PANI@CNTs-GO showed the best gas separation performance with a CO2 permeance of 171 GPU and a CO2/N2 selectivity of 122.4. These values were 2.3 and 2.7 times higher than those of the pristine PVAm membrane, respectively. The increase in the CO2 permeance can be attributed to the fact that the incorporation of PANI@CNTs-GO disrupted the packing of the PVAm chains and increased the distance between them, which resulted in enhanced gas diffusion through the membrane. Moreover, the increased interlayer spacing caused by the incorporation of PANI@CNTs-GO provided a large number of CO2 transport pathways, thus improving the CO2 permeance of the membrane. With an increase in the PANI@CNTs-GO loading up to 1 wt%, the increase in the CO2/N2 selectivity of the MMMs was mainly caused by the molecular sieving effect of the interlayer spacing. In these pathways, the amine groups of PANI@CNTs

3.2.2. Effect of operating pressure The effect of the feed pressure on the separation performance of the PVAm/PANI@CNTs-GO MMMs was investigated using a pure gas (CO2 and N2) in the humidified state. The feed pressure was varied from 1 to 9 bar. As shown in Fig. 4(a), the CO2 permeance of the membranes first decreased rapidly with an increase in the feed gas pressure up to 5 bar and then changed slowly with the further increase in the feed gas pressure. This is because the feed gas pressure effects the availability of the facilitated transport carriers (the amine groups) in membranes, and these carriers tend to saturate with an increase the feed gas pressure because of the reversible reaction of CO2 with secondary amine groups [1,15]. The rapid decrease in the CO2 permeance with an increase in pressure from 1 to 5 bar was mainly due to the decrease in the number of facilitated transport carriers. With a further increase in the pressure, the CO2 transport was mainly dominated by the solution-diffusion mechanism and the carriers became almost saturated. As a result, at 5

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Fig. 2. Cross-sectional SEM images of (a) the pristine PVAm membrane and the PVAm/PANI@CNTs-GO MMMs with the loadings of (b) 0.5 wt%, (c) 1 wt% and (d) 1.25 wt%. Wet film thickness: 50 μm.

Fig. 3. Effect of the filler loading on the CO2 permeance and CO2/N2 selectivity of the PVAm/PANI@CNTs-GO MMMs. Wet film thickness: 50 μm; feed gas: pure CO2 and N2; 25 °C; pressure: 1 bar.

high pressures, an increase in pressure showed a very little effect on the CO2 permeance of the membranes. The decrease in the CO2 permeance of the MMMs at high feed pressures can be attributed to their dense polymeric chain packing, which resulted in a decrease in their free volume, thus reducing their gas penetration [43]. On the other hand, the N2 permeance of the membranes changes slightly with an increase in the feed pressure. Similar behaviour were observed in previous studies for PVAm/ZIF-8/PSf composite membranes (the N2 permeance showed less dependence on the feed pressure) [6]. Therefore, both the CO2 permeance and CO2/N2 selectivity of the PVAm/PANI@CNTs-GO MMMs decreased significantly with an increase in the feed pressure.

Fig. 4. Effect of the operating pressure on the (a) CO2 permeance and (b) CO2/ N2 selectivity of the pristine PVAm membrane and PVAm/PANI@CNTs-GO MMMs. Wet film thickness: 50 μm; feed gas: pure CO2 and N2; 25 °C.

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Fig. 5. Effect of the wet film thickness on (a) the CO2 permeance and (b) CO2/ N2 selectivity of the MMM loaded with 1 wt% PANI@CNTs-GO. Feed gas: pure CO2 and N2; 25 °C.

Fig. 6. Effect of the operating temperature on (a) the CO2 permeance and (b) CO2/N2 selectivity of the pristine PVAm membrane and MMMs loaded with 1 wt% PANI@CNTs-GO. Wet film thickness: 50 μm; feed gas: pure CO2 and N2; pressure: 1 bar.

3.2.3. Effect of wet film thickness It is well-known that the performance of a membrane is directly related to its thickness. Generally, membranes with a thin selective layer show high gas permeance [1,6]. However, membranes with very thin selective layers show low CO2/N2 selectivity. Fig. 5 shows the effect of the wet film thickness on the CO2 permeance and CO2/N2 selectivity of the PVAm/PANI@CNTs-GO MMMs. As expected, the CO2 permeance of the PVAm/PANI@CNTs-GO MMMs decreased sharply with an increase in the wet film thickness, as shown in Fig. 5(a). The CO2 permeance of the membrane with the wet film thickness of 30 μm was 180 GPU at 1 bar, which is about 1.8 times higher than that of the membrane with the film thickness of 150 μm (96 GPU). The increase in the CO2 permeance of the membranes with thin selective layers can be attributed to their short gas transport pathways and low mass transfer resistance. Moreover, a decrease in the thin selective layer improved the solubility and diffusion of CO2 molecules in the PVAm matrix. As shown in Fig. 5(b), the CO2/N2 selectivity of the membranes first increased with an increase in the wet film thickness up to 50 μm and then decreased dramatically with a further increase in the wet film thickness. The PVAm/PANI@CNTs-GO MMM with a wet film thickness of 50 μm showed the maximum CO2/N2 selectivity of 122.4 at 1 bar. The MMMs with very thin selective layers showed low CO2/N2 selectivity because of their low carrier group concentrations [6].

PANI@CNTs-GO MMMs reduced from 171 to 81 GPU, whereas the CO2/N2 selectivity decreased from 122.4 to 43.7. This can be attributed to the fact that the crystalline structure of PVAm densifies at higher temperatures, which in turn inhibits the polymer segment mobility of the membrane [45,46]. In addition, with an increase in the operating temperature, water vaporized from the membranes, restricting the facilitated transport of CO2 [22]. In addition, a decrease in the water retention hindered the mobility of amine carriers and the reaction rates of CO2 with the carriers, resulting in a decrease in the permselectivity of the membranes [47]. Gao et al. [48] also reported that both the CO2 permeance and CO2/N2 selectivity of PVAm-based MMMs decrease with an increase in the operating temperature mainly because of the evaporation of the water present in them. At higher temperatures, the permselectivities of the PVAm/PANI@CNTs-GO MMMs were significantly higher than that of the pristine PVAm membrane. This indicates that the amine carriers in the interlayer spacing did not hydrolyse easily with the vaporisation of water from the MMMs because of the strong interaction between PANI@CNTs-GO and the PVAm matrix [49].

3.3. Mixed gas separation performance Generally, the optimal feed pressure for CO2 capture from flue gas ranges from 1 to 3 bar [50]. In this study, the practical separation performances of the membranes were investigated using a gas mixture of CO2 and N2 (20/80 vol%) at 1 bar. Fig. 7 shows the CO2 separation performances of the membranes under the pure gas and mixed gas condition in the humidified state. Both the mixed-gas- and pure gasseparation performances showed similar trends. As shown in Fig. 7(a), the CO2 permeance of the mixed gas was significantly higher than that of the pure gas; the maximum CO2 permeance of the MMM loaded with 1.25 wt% PANI@CNTs-GO was 323 GPU under the mixed gas condition

3.2.4. Effect of operation temperature The separation of CO2 from flue gas in power plants (in which the temperature of flue gas is about 40-50 °C) is usually affected by the operating temperature [44]. In order to evaluate the applicability of the membranes for practical applications, we investigated the effect of the operating temperature on their separation performance, and the results are shown in Fig. 6. As expected, with an increase in the operating temperature from 25 to 80 °C, both the CO2 permeance and CO2/N2 selectivity of the membranes decreased. When the operating temperature was increased from 25 to 80 °C, the CO2 permeance of the PVAm/ 7

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Fig. 8. Gas separation performance of the MMMs with different fillers at the loading of 1 wt%. Wet film thickness: 50 μm; feed gas: pure CO2 and N2; 25 °C; pressure: 1 bar.

effect of the interlayer spacing for small CO2 molecules (kinetic diameter: 3.3 Å) and facilitated the transport of amine groups (Fig. S6). The possible gas-transport pathways for the PVAm/PANI@CNTs-GO MMMs are shown in Scheme 3. PANI@CNTs-GO was introduced into PVAm to construct interlayer spacing for CO2 transport in the membranes, in which PANI@CNTs was intercalated in the interlayer spacing of GO. It was distributed in PVAm in a parallel or random orientation. In the case of CO2 transport in the MMMs, apart from the CO2 facilitated transport of the PVAm matrix, which improved their CO2/N2 selectivity, the interlayer spacing also contributed significantly to the improvement in their permselectivity because of the shape sieving potential for CO2/N2 separation. Besides, PANI@CNTs, which was embedded in the GO interlayer spacing, also facilitated the CO2 transport in the MMMs owing to the reversible reaction between CO2 and amine groups. As shown in Scheme 3(a), CO2 molecules could also transport through the tortuous spacing because of the molecular sieving effect. The large increase in the permselectivity of the MMMs can be attributed to the high porosity of PANI@CNTs-GO in their inner tube. The relatively large N2 (kinetic diameter: 3.6 Å) molecules experienced more diffusion hindrance compared to the small CO2 molecules (kinetic diameter: 3.3 Å). As shown in Scheme 3(b), CO2 molecules could diffuse rapidly through PANI@CNTs-GO with the interlayer spacing, since this straight interlayer spacing functioned as a high-speed road-like pathway for CO2 molecules. These molecules readily reacted with amine groups reversibly, thus improving the CO2/N2 separation performance of the membranes. Therefore, the PVAm/PANI@CNTs-GO MMMs exhibited high CO2 permeance and CO2/N2 selectivity as compared to the other membranes.

Fig. 7. (a) CO2 permeance and (b) CO2/N2 selectivity of the pristine PVAm membrane and PVAm/PANI@CNTs-GO MMMs under the mixed and pure gas conditions. Wet film thickness: 50 μm; feed gas: pure CO2 and N2, CO2/N2 mixed gas (20/80 vol%); 25 °C; pressure: 1 bar.

and 192 GPU under the pure gas condition. As shown in Fig. 7(b), the CO2/N2 selectivities of the membranes under the mixed gas condition were higher than their corresponding ideal selectivities. The maximum CO2/N2 selectivity of the MMM loaded with 1 wt% PANI@CNTs-GO was 155.1 under the mixed gas condition and 122.4 under the pure gas condition. This difference in the permselectivities under the mixed and pure gas conditions can be mainly ascribed to the competitive penetration effect. The presence of CO2 and N2 in the MMMs reduced their CO2 pressure, thus their CO2 permeance was predominantly caused by facilitated transport at low pressures [51,52]. On the basis of the gas separation performance of the MMMs, it can be stated that their PANI-containing interlayer spacing acted as a fast and selective transport pathway and reduced the mass transfer resistance of CO2 molecules through them. In order to further confirm this, we investigated the effect of different fillers on the gas separation performance of the MMMs to elucidate the effect of the interlayer spacing in them. As shown in Fig. 8, the CO2 permeance increased from 73 GPU for the PVAm membrane to 102 and 140 GPU for PVAm/CNTsGO and the PVAm/PANI@CNTs MMM, respectively. On the other hand, the CO2/N2 selectivity increased from 45.7 for the PVAm membrane to 62.3 and 74 for PVAm/CNTs-GO and the PVAm/PANI@CNTs MMM, respectively. The improvement in the gas separation performance of the MMMs (as compared to the pristine PVAm membrane) can be attributed to the fact that the smooth walls of MWCNTs in the MMMs as nanochannels contributed to their high CO2 permeance, whereas the high aspect ratio of the GO sheets increased their CO2/N2 selectivity [53]. Furthermore, PANI molecular chains contain secondary amine groups, which can react with CO2 molecules to facilitate CO2 transport in MMMs. As can be observed from Fig. 8, both the permeance and selectivity of the PVAm/PANI@CNTs-GO MMMs were much higher than those of PVAm/CNTs-GO and the PVAm/PANI@CNTs MMM. This is ascribed to the synergistic effect, which combined the molecular sieving

3.4. Durability of PVAm/PANI@CNTs-GO MMMs The long-term separation performance of the PVAm/PANI@CNTsGO MMM with the optimized filler loading of 1 wt% was also investigated using the CO2/N2 mixed gas (20/80 vol%) under 1 bar feed gas pressure at 25 °C. The membrane performance was continuously tested for 300 h, and the results are shown in Fig. 9. The PVAm/PANI@CNTs-GO MMM with the loading of 1 wt% showed a high and stable separation performance with an average CO2 permeance of 264 GPU and CO2/N2 selectivity of 149.8. The performance of the membrane did not deteriorate over the entire testing period of 300 h. This indicates that the membrane showed a very good long-term stability. On the other hand, the pristine PVAm composite membrane exhibited significant performance degradation after 120 h under similar testing conditions [13]. This further confirms the strong hydrogen bond and electrostatic interactions between PANI@CNTs-GO and the PVAm matrix. Thus, the facilitated transport carriers in the interlayer spacing did not hydrolyse with water vaporisation from the MMM and the pH 8

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Scheme 3. Schematic of the possible gas-transport pathways for PVAm/PANI@CNTs-GO MMMs. (a) high-speed transport interlayer spacing; (b) tortuous transport interlayer spacing.

Fig. 9. Separation performance stability of the PVAm/PANI@CNTs-GO MMM loaded with 1 wt% PANI@CNTs-GO. Wet film thickness: 50 μm; feed gas: CO2/ N2 mixed gas (20/80 vol%); 25 °C; pressure: 1 bar.

Fig. 10. CO2/N2 separation performances of the PVAm/PANI@CNTs-GO MMMs and previously reported PVAm MMMs. 25 °C; pressure: 1 bar.

decrease during the long term operation. However, the amine groups of the pristine PVAm membrane hydrolysed with a decrease in its pH because of the gradual evaporation of water from it [49].

PVAm/CNTs [24], PVAm/HPEI-GO/CS [5], PVAm/EDA [14], and PVAm/PPO [54] facilitated transport membranes reported previously. This can be attributed to the fact the construction of amine carriercontaining interlayer spacings in MMMs can improve their CO2 transport as compared to the N2 transport under the humidified state.

3.5. Comparison of gas separation performances As discussed above, building interlayer spacing with facilitated transport carriers is one an effective approach for improving the CO2/ N2 separation and long-term operating stability of membranes. The CO2 fast transport pathways of the membranes could be tuned by adjusting their filler loadings. The performance of the PVAm/PANI@CNTs-GO MMM at the loading of 1 wt% was compared with the other previously reported PVAm MMMs supported on porous membranes. All the separation performance measurements of the membranes were carried out under similar CO2 partial pressure and mixed gas conditions. As shown in Fig. 10, the PVAm/PANI@CNTs-GO MMM with the loading of 1 wt% showed higher CO2 permeance and CO2/N2 selectivity than the pristine PVAm membrane. It showed a CO2 permeance of 275 GPU and a selectivity of 155.1, which were remarkably higher than those of the

4. Conclusions MMMs with interlayer spacing were prepared by coating a thin layer of a mixture of PANI@CNTs-GO and PVAm onto a porous support for CO2 separation. The compatibility between the PVAm matrix and PANI@CNTs-GO was enhanced by electrostatic interactions and hydrogen bonding, which resulted in the formation of a defect-free thin layer on the PSf membrane and the uniform dispersion of PANI@CNTsGO in the MMMs. The interlayer spacing in the MMMs facilitated CO2 transport but hindered N2 transport because of the molecular sieving effect as well as the reversible reaction between CO2 and amine groups. 9

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Thus, the MMMs showed excellent gas separation performance. The MMM loaded with 1 wt% PANI@CNTs-GO exhibited a permeance of 170 GPU with a selectivity of 122.4 at 1 bar and outperformed the pristine PVAm membrane and previously reported PVAm-based membranes. This can be attributed to the synergistic effect of both the molecular sieving and reversible reaction in the interlayer spacing. Moreover, the MMM loaded with 1 wt% PANI@CNTs-GO exhibited long-term stability with an average CO2 permeance of 264 GPU and a high CO2/N2 selectivity of 149.8 in a gas mixture during more than 300 h of testing.

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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21506140), the Joint Fund of Shanxi Provincial Coal Seam Gas (No. 2015012009), China Postdoctoral Science Foundation (No. 2016M601289), the Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province, and the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No. 2015134) and was partially supported by the National Natural Science Foundation of China (No. U1510123) and the Hundred Talents Program of the Chinese Academy of Sciences. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.117246. References [1] J.Y. Liao, Z. Wang, M. Wang, C.Y. Gao, S. Zhao, J.X. Wang, S.C. Wang, Adjusting carrier microenvironment in CO2 separation fixed carrier membrane, J. Membr. Sci. 511 (2016) 9–19. [2] S.F. Wang, Y.Z. Wu, N. Zhang, G.W. He, Q.P. Xin, X.X. Wu, H. Wu, X.Z. Cao, M.D. Guiver, Z.Y. Jiang, A highly permeable graphene oxide membrane with fast and selective transport nanochannels for efficient carbon capture, Energy Environ. Sci. 9 (2016) 3107–3112. [3] L. Olivieri, S. Meneguzzo, S. Ligi, A. Saccani, L. Giorgini, A. Orsini, A. Pettinau, M.G. De Angelis, Reducing ageing of thin PTMSP films by incorporating graphene and graphene oxide: effect of thickness, gas type and temperature, J. Membr. Sci. 555 (2018) 258–267. [4] S. Zhao, Z. Wang, Z.H. Qiao, X. Wei, C.X. Zhang, J.X. Wang, S.C. Wang, Gas separation membrane with CO2-facilitated transport highway constructed from amino carrier containing nanorods and macromolecules, J. Mater. Chem. 1 (2013) 246–249. [5] Y.J. Shen, H.X. Wang, J.D. Liu, Y.T. Zhang, Enhanced performance of a novel polyvinylamine/chitosan/graphene oxide mixed matrix membrane for CO2 capture, ACS Sustain. Chem. Eng. 3 (2015) 1819–1829. [6] S. Zhao, X.C. Cao, Z.J. Ma, Z. Wang, Z.H. Qiao, J.X. Wang, S.C. Wang, Mixed-matrix Membranes for CO2/N2 separation comprising a poly(vinylamine) matrix and metal-organic frameworks, Ind. Eng. Chem. Res. 54 (2015) 5139–5148. [7] P.Y. Li, Z. Wang, Y.N. Liu, S. Zhao, J.X. Wang, S.C. Wang, A synergistic strategy via the combination of multiple functional groups into membranes towards superior CO2 separation performances, J. Membr. Sci. 476 (2015) 243–255. [8] L.L. Dong, M.Q. Chen, J. Li, D.J. Shi, W.F. Dong, X.J. Li, Y.X. Bai, Metal-organic framework-graphene oxide composites: a facile method to highly improve the CO2 separation performance of mixed matrix membranes, J. Membr. Sci. 520 (2016) 801–811. [9] T.S. Chung, L.Y. Jiang, Y. Li, S. Kulprathipanja, Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation, Prog. Polym. Sci. 32 (2007) 483–507. [10] R.D. Noble, Perspectives on mixed matrix membranes, J. Membr. Sci. 378 (2011) 393–397. [11] S. Janakiram, M. Ahmadi, Z.D. Dai, L. Ansaloni, L.Y. Deng, Performance of nanocomposite membranes containing 0D to 2D nanofillers for CO2 separation: a review, Membranes 8 (2018) 24. [12] M. Vinoba, M. Bhagiyalakshmi, Y. Alqaheem, A.A. Alomair, A. Pérez, M.S. Rana, Recent progress of fillers in mixed matrix membranes for CO2 separation: a review, Separ. Purif. Technol. 188 (2017) 431–450. [13] J. Zhao, Z. Wang, J.X. Wang, S.C. Wang, High-performance membranes comprising polyaniline nanoparticles incorporated into polyvinylamine matrix for CO2/N2 separation, J. Membr. Sci. 403–404 (2012) 203–215. [14] S.J. Yuan, Z. Wang, Z.H. Qiao, M.M. Wang, J.X. Wang, S.C. Wang, Improvement of CO2/N2 separation characteristics of polyvinylamine by modifying with ethylenediamine, J. Membr. Sci. 378 (2011) 425–437.

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