N2 separation

Author’s Accepted Manuscript Porous Graphene Nanosheets Functionalized Thin Film Nanocomposite Membrane Prepared by Interfacial Polymerization for CO2/N2 Separation Hui Li, Xiaoxu Ding, Yatao Zhang, Jindun Liu www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(17)30965-1 http://dx.doi.org/10.1016/j.memsci.2017.08.046 MEMSCI15512

To appear in: Journal of Membrane Science Received date: 3 April 2017 Revised date: 2 August 2017 Accepted date: 18 August 2017 Cite this article as: Hui Li, Xiaoxu Ding, Yatao Zhang and Jindun Liu, Porous Graphene Nanosheets Functionalized Thin Film Nanocomposite Membrane Prepared by Interfacial Polymerization for CO2/N2 Separation, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.08.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Porous

Graphene

Nanocomposite

Nanosheets

Membrane

Functionalized Prepared

Thin

by

Film

Interfacial

Polymerization for CO2/N2 Separation Hui Li a, b, Xiaoxu Ding a, Yatao Zhang a, b*, Jindun Liu a, b a

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou

450001, P. R. China b

Zhengzhou Key Laboratory of Advanced Separation Technology, Zhengzhou 450001,

P. R. China

Corresponding author: Email address: [email protected]

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Abstract The inherent defects of porous graphene (PG) formed during reduction etching process could serve as nanopores, making PG emerge a potential application for the preparation of micrometre-sized separation membranes. Here, we introduced PG as inorganic nanofiller to fabricate thin film nanocomposite (TFN) membranes for CO2 capture via interfacial polymerization technique. The PG selective nanolayers not only possessed a good adhesion with polymers but also benefited from hydrogen bonding actions, simultaneously, thus ensuring the formation of high-efficiency molecular sieving passageway in the separation layer of membranes. Furthermore, the thin PG nanosheets were verified to have an significantly affect for permeability and selectivity of membranes (PG, 0.05 wt%, 1 bar), with exhibiting about 21% and 20.8% enhancement of the CO2 permeance and the CO2/N2 selectively compared to that of the membrane without PG separately. Simultaneously, the membrane also showed higher stability and the porous surface morphology of PG shortened greatly the gas transfer path. The approach offers a potential promising to exploit the ultra-thin film composite membrane for efficient gas separation. Keywords: porous graphene; interfacial polymerization; thin film nanocomposite (TFN) membranes; CO2 capture

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1. Introduction Along with the economic development, energy and environment have become two major problems which the world is facing nowadays. In particular, directly discharged of carbon dioxide will cause high pollution to the environment, while carbon dioxide has high agricultural and industrial value [1-4]. Thus, capture and separation of carbon dioxide have been considered as a strategic task for solving the greenhouse effect and using carbon resources efficiently [5,6]. Nowadays, cryogenic and adsorption methods to remove and purify carbon dioxide, are widely used [7]. However, both of which are inefficient and high cost [8].To seek a more effective method for the removal of carbon dioxide, gas membrane separation has attracted much attention as alternative means because it possesses tight structure, environment friendly, high efficiency and low-cost [9,10]. Currently, polymeric separation membrane, inorganic separation membrane and new block polymeric membrane are three popular traditional gas separation membranes, used extensively for separation of CO2. However, they still have some defects, hindering the application of industrialization. Inorganic separation membranes possess high penetration rate [11,12], and the performance of much of that have exceeded "Robeson upper bond". Yet high costs, crisp and structure defects have been a significant limit for the development of inorganic separation membranes. To date, the study for inorganic separation membranes is still confined to the preparation and characteristic in laboratory. Meanwhile, polymeric separation membranes also arouse more and more concern due to the low cost and easily fabricated for membrane materials [13,14]. Furthermore, polymeric membrane materials can effectively improve the membrane permeability and selectivity for CO2 after modification [15]. For

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instance, PDMS/membranes were prepared successfully by the appropriate chemical modification of PDMS side chains, further increased the permeability of CO2 [16]. Wang et al., [17] would PDMS coat on PS support membranes, thus limited the reactants permeability to the support membrane interface, further improved dramatically the permeability of CO2. And according to the stronger affinity between polymer containing ether oxygen bond and CO2, Li et al., [18] fabricated an excellent composite membrane using rubbery polymer F127 containing ether oxygen bond as supporting membrane material and Pebax MH 1657 as separation layer, and the permeability of CO2 of that reached 1000 GPU. Despite of the separation performance of prepared membranes has a significant improvement by the above methods, it is still a big challenge of possessed a simultaneously high permeability and selectivity. Recently, to improve significantly gas separation performance, the incorporation of inorganic nanofillers as the dispersed phase into membranes to form mixed-matrix membranes (MMMs) gain widespread attention at latest years [19-22]. MMMs exhibit a potential promising to broken the Robeson’s upper bound of polymeric membranes by the synergic action of inorganic nanofillers due to their own intrinsic property including chemical, physical, mechanical [23,24]. However, most MMMs are homogeneous membrane with the thickness over 10 µm according to the previous reports because of the difficulty in avoiding the aggregation of inorganic nanofillers during membranes fabrication process [25-27]. Furthermore, the poor adhesion and compatibility between inorganic fillers and polymeric matrix can lead to non-selective voids in the active film interface, resulting in the gas separation performance degradation [28]. Therefore, in recent years, significant work has been applied toward the development of thin film nanocomposite (TFN) by interfacial polymerization (PI) to accelerate the

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efficient separation of industrial gases. TFN membranes is analogous to MMMs but the real benefit of its tailored skin and support layers discretely. In addition, PI has the ability to form ultrathin defect-free selective layers (0.1-1.0 µm) due to its selftermination property, and the flexibility of which makes it easier to large-scale in practical application. Several reviews on these TFN membranes have been recently published [29]. The majority of studies on TFN membranes have focused on their use in the field of water treatment, but TFN membranes also are of value as a way to improve the gas separation performance. For instance, a report by A. F. Ismail et al. reported a MWNTs/ polymer TFN membrane for gas separation, and greatly improving the carbon capture capacity from N2 and CH4 [30,31]. Wang et al. integrated the CO2-selective adsorptive silica nanoparticles with the polymer, and then fabricating TFN membranes by PI with improving the CO2/N2 separation performance [32]. Thus, the development of TFN membranes with high-flux and high-selectivity is a pressing need for costefficient CO2 capture. Graphene oxide (GO), which possesses atomic thickness [33], a variety of oxygen-containing functional groups, exceptional mechanical strength [34], is an excellent starting nanomaterial for developing size-selective, uniform and stable membranes [35-37]. However, pores that exclude larger molecules but are benefit to the diffusion of smaller molecules with low resistance, thus, which would benefit to gas permeation performance introduced into the material [37,38]. At the moment, though, it should be noted that it's still a very difficult work for prepared membranes to minimize the separating layer defect and make inorganic film thickness thinner. To solve these problems, we integrated the advantage of porous graphene (PG) prepared by a facile wet chemical process [23] and applied them to prepare thin film nanocomposite (TFN) membranes for CO2/N2 gases via interfacial polymerization. Here,

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the applied of hyperbranched amino-teminated polyoxypropylene (PEA) as water phase could improve effectively the film forming property and decreased the membrane defect. More important, the PEA was chosen due to the good affinity between ether linkage of PEA and CO2. In addition, the PG possessed numerous advantages including porous surface, oxygen-containing functional group and good dispersed in water phase, meanwhile PG could act as the inorganic fillers during the prepared process of membrane, further attained a uniform coating layer and improved greatly the membrane separation performance. In particular, interfacial polymerization was a remarkably simple route in the fabrication of TFN membranes and endowed them with continuous, ultrathin active films to improve the gas permeability. 2. Experimental 2.1 Chemicals and materials The graphite powder and amino-teminated polyoxypropylene (PEA, D230) were both purchased from Aladdin Industrial Corporation. Trimesoyl chloride (TMC, ≥99 %), potassium permanganate (KMnO4) and sodium hydroxide (NaOH) were all attained from J&K Scientific Ltd (China). Polysulphone ultrafiltration membrane (PS) with an average molecular weight of 6000 was purchased from Beijing Vontron Technology Development Co., Ltd and Sodium lauryl sulfate (SDS) were both applied from Tianjin Kemiou Chemical Reagent Co., Ltd. hydrochloric acid (HCl), hydrogen peroxide (H2O2, 30 wt%) and phosphoric acid (H3PO4, 85 wt%) were all supplied from Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd (China)., All the chemicals were used without further purification. 2.2 Preparation of graphite oxide (GO)

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GO was prepared according to the improved Hummers’ method [39]. Firstly, the mixture composed of 360 mL of concentrated sulfuric acid (H2SO4, 98 %) and 40 mL of phosphoric acid (H3PO4, 85 %) was placed in round-bottom flask. And then the roundbottom flask was transferred to oil bath maintained at 50 oC, operated magnetic stirring, for 22 h simultaneously. Before the oil bath pot reaching to 50 oC, 18 g KMnO4 and 3 g graphite powder were added to the mixture in turn. After 22 h, the reaction solution was emptied into the vessel containing 1200 mL ice water and 30 % for H2O2 solution (30 mL), followed by sonication treatment for 1 h. Later on, the mixture was separated by centrifugation at 1000 rpm for 10 min, attaining the supernatant and precipitates. And the supernatant was centrifuged again. Finally, attaining precipitates was washed with distilled water, 30 % for hydrochloric acid (HCL) and anhydrous ethanol sequentially at least three times. The GO sample was recovered, followed by dry with a vacuum desiccator. 2.3 Preparation of porous graphene (PG) The prepared graphite oxide (GO) was dispersed into deionized water (750 mL) to a final concentration of 0.6 mg/mL. And then 3 g NaOH was added to the solution, followed by magnetic stirring and reflux for 1 h. Subsequently, the finally attained mixture was treated by centrifugation at 13000 rpm for 10 min. After 10 min, the attained precipitate was dispersed into deionized water (750 mL) again, and then 7.5 mL hydrochloric acid (30 %, HCL) was added to the solution, which was carried out magnetic stirring and reflux for 1 h, immediately. Finally, the attained GO has been successively treated by acid and alkaline, and then washing three times with deionized water and acetone solution, respectively. The PG sample was recovered, followed by dry with a vacuum desiccator.

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2.4 Preparation of porous graphene (PG) thin film nanocomposite (PG-TFN) membranes The typical preparation method of PG-TFN membranes were as follows (Fig. 1): (1) A certain size of polysulfone (PS) membrane served as a substrate membrane, and then immersed into 0.5 wt% sodium lauryl sulfate (SDS) solution for 24 h. After 24 h, the membrane was taken out from SDS solution, followed by washing with deionized water to removal the excessive solution and then given an airing. Finally, the prepared membrane was fixed in membrane preparation device. (2) The prepared PG was dispersed into deionized water in a certain mass to weight percent, 0.03 wt%，0.05 wt%，0.07 wt%, of PG suspension, followed by ultrasonic using a ultrasonic Cellbreak device until the PG was homogeneous dispersion in deionized water, solution A. 0.1 g TMC was dissolved into a certain mass of n-Hexane solution to a final weight percent, 0.4 wt%, of TMC organic phase, solution B. Meanwhile, 1 wt% PEA aqueous phase was also confected successfully, solution C. Subsequently, the solution A was added into solution C, and then adding 0.4 wt% sodium carbonate solution to the mixture to removal generated hydrochloric acid for improving the molecular weight of generating polymer while 0.05 wt% Sodium lauryl sulfate (SDS) solution was also added to the mixture so as to improve the aqueous phase adsorption capacity of PS membrane, attaining solution D. Later on, the solution D was poured into PS membrane surface until the membrane was completely covered and allowed to keep for 10min. After the excessive solution D, the solution B was then poured onto the surface of amine-saturated PS membrane for at least 3 min. Then the excessive solution B was removed again. Finally, the treated PS membrane was cured in an oven at 70 oC for 13

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min and 30 oC for the least 12 h, respectively. The PG thin film nanocomposite membranes were recovered. In order to obtain a more comparable data, thin film nanocomposite (TFN) membrane was prepared without PG via the same as the preparation method and experiment condition using TMC solution as organic phase and PEA solution as aqueous phase.

Fig. 1. Scheme of preparation of PG-TFN membranes 2.5 Gas-Permeation measurement Binary CO2/N2 gas-permeation studies were carried out at ambient temperature via gas permeation apparatus. H2 was selected as the sweep gas to measure the permeability of CO2 and N2. In the typical experimental process, a certain pressure of feed gas (CO2/N2 mixed gas, 1:2 by volume) ranging from 1 bar to 5 bar was introduced into a water bottle to make water vapor saturation, and then passing through an empty bottle to remove the residual water. Simultaneously, the sweep gas was also humidified at room

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temperature. Subsequently, the permeate side gas was introduced into gas chromatography device by the sweep gas, and then the permeation rate and separation selectivity of mixed gas were measured, respectively. The total flow could be attained by a mass flow meter including mixed gas and sweep gas. The permeation rate of CO 2 and N2 was calculated using the following equations: (1) (2) (3)

Where sweep gas,

is the volume fraction of component m of mixed gas including feed and is the total flux included the flux of feed gas and sweep gas,

of component m,

is the flux

is the volume fraction of the feed side component m,

absolute pressure of feed gas, during the testing process, two sides of membrane,

is the

is the pressure of permeate side gas keeping constant is the pressure difference of component m located in the

is the permeation flux of component m,

is the effective

area of membrane, 19.62 cm2. And the following equation was used to calculate the separation factor of twocomponent gas (CO2/N2):

Where

is the separation factor of two-component gas (CO2/N2).

2.6 Membrane characterization The cross-sectional morphologies as well as membrane surface were examined with scanning electron microscopy (SEM, JSM-6700F, JEOL, and Japan) operating at 10.0

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KV, respectively. The microstructure of the samples was studied by TEM (Tecnai G2 F30 S-Twin, Philips-FEI) operating at 200 kV. The chemical formation of PG was tested by a Thermo Nicolet IR200 Fourier transform infrared (FTIR) with a scan range of 400-4000 cm-1. The chemical composition and characterization of PSf composite membrane and PS substrate membrane surface were further respectively accomplished via X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo Fisher, USA) using Mg Kα as the radiation source. X-ray diffraction (XRD) analysis of PG and GO was carried out on a PAN Alytical X’Pert Pro (PANalytical, The Netherlands) in the scanning range of 2θ between 5° and 80o using copper Kα as the source of radiation, and a step size of 0.02o. The BET surface area and pore volume of GO and PG were evaluated by using nitrogen adsorption-desorption isotherms measured at 77 K on an ASAP 2420 Surface Area & Pore Size Analyzer. 3. Results and discussion 3.1 Characterization of membrane material (GO and PG) PG nanosheets were prepared with GO by acid-base treatment process. The morphologies of PG and GO were observed directly by Transmission Electron Microscope (TEM) characterization. Fig. 2 showed that the TEM images of GO and PG nanosheets. As shown in Fig. 2d, the pristine GO presented relatively smooth nanosheets compared with subnanometre-sized pores PG nanosheets, because the a small amount of NaOH was inadequate for the chemical reduction of the vast oxygen groups on the GO nanosheets. But strong deoxidization process could be completed via the reduction treatment, shown in Fig. 2 (a-c). Subnanometre-sized pores with irregular shape could be clearly saw on PG nanosheets surface, indicating that deoxidizing and decarburization process of GO surface could be occurred during the NaOH reduction.

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Furthermore, it could remove the generated carbonate of reduction process by acid treatment, further prevented lye leak back into the pores. In addition, subnanometresized pores on PG nanosheets surface might greatly increase the specific surface area of PG and the tortuous pathway of gas molecular diffusion [40-42]. Subsequently, the nitrogen adsorption-desorption isotherm was conducted to further investigated the nanoscale pore size and porosity of PG. It is clear that the PG has higher BET surface area and larger BJH pores than GO (Fig. 2d and Table 1). Furthermore, PG nanosheets with high surface area could contributed to the integration between polymer layer and PG, further minimized the separating layer defect.

Fig. 2. (a-b) TEM images of PG with various magnification (c) TEM images of GO (d) Nitrogen sorption isotherms of GO and PG and histogram of pore size distribution of PG.

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Table 1 Summary of specific surface area and porosity parameters of GO and PG SBET

Pore Volume

Average pore size

（m2 g-1）

(cm3 g-1)

(nm)

GO

43.0577

0.0342

4.9028

PG

377.8657

1.2482

11.0310

The chemical structure of graphite oxide (GO) and porous graphene (PG) were also monitored using a Fourier Transform Infrared Spectrometer. As shown in Fig. 3a, it could be distinctly seen that the vibration peaks of PG were in consistent with the main fingerprint functional groups, such as carboxyl group (C=O), hydroxyl radical (-OH). Meanwhile, GO was also observed by the same method. Subsequently, compared to the vibration peaks of GO and PG, the results revealed that GO and PG possessed respectively significant vibrational peaks at around 3440 cm-1and 1720 cm-1, which belonged to hydroxyl radical and carboxyl group in turn. In addition, the intense peak at around 3440 cm-1 was contributed to the stretching vibration of hydroxyl radical. On the other hand, the peak has a higher stretch at 1720 cm-1 for GO compared with that of PG, which manifested that the higher density of C=O existed on the as-prepared GO lamella. On the contrary, the PG surface holds a more hydroxyl radical. The X-ray diffraction patterns (XRD) of GO and PG were shown in Fig. 3b. The result of analyzing Fig. 3b demonstrated that PG attained a higher angle peak (11.1o) compared to that (10.4o) of GO, further revealed that the decrease of the d-spacing for PG nanosheets (0.80 nm). In addition, a sharp peak at 10.4o for GO and 11.1o for PG also implied that the ordered stacking of GO and PG sheets. And form that, further study found that the XRD patterns

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of PG was more rough and become more gently than that of GO, so for that reason we could think the surface morphology of PG had a significant difference.

Fig. 3. (a) FTIR spectra of PG and GO (b) XRD patterns of PG and GO. 3.2 Characterization of membranes The surface and cross-section morphology of TFN, PG-TFN were analyzed using the scanning electron microscopy in a sequence. As shown in Fig. 4a, it could be observed that the PS substrate membrane presented a relatively smooth surface. On the contrary, the surface morphology of TFN has a significantly change presenting a “branch-like” cross-linked valley in Fig. 4b. It could be attributed to the interfacial polymerization between PEA and TMC on the surface of PS substrate membrane. In addition, the thickness of PEA-TMC polymerization layer was measured, about 0.125 µm shown in Fig. 4b. Further observation for Fig. 4(c, d, e), the “branch-like” cross-linked morphology become indistinct for PG/PS mixed-matrix membrane compared with that of PG-TFN revealed that PG was dispersed homogeneously into polymer matrix by the interfacial polymerization reaction. Additional, the results images of cross-section (Fig. 4(c, d, e)) also indicated that the selective layer adhered to the support surface tightly and had no defect distinctly. It also observed through Fig. 4 that with the increase of PG

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loading ranging from 0.03 wt% to 0.07 wt%, the thickness of selective layers have a gradual increased from 0.125 µm to 0.30 µm.

Fig. 4. SEM images of the surface and the cross section of the tested membranes. (a) PS substrate membrane (b) TFN membranes with PEA concentration of 1 wt% (c, d, e) PGTFN membranes with PG concentration of 0.03 wt%, 0.05 wt% and 0.07 wt% respectively, PEA concentration: 1 wt%.

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To confirm the chemical structure of membranes under different conditions, the ATRFTIR was conducted. As shown in Fig. 5a, the result for the analysis of ATR-FTIR spectra demonstrated that there were three new, homologous and intense absorption bands at around 1647 cm-1, 1725 cm-1, 1111 cm-1 for TFN and PG-TFN, successively, compared with the single PS substrate membrane. Further analysis, the absorption band around 1647 cm-1, 1725 cm-1, 1111 cm-1 were assigned to H–N–C=O, C-O-C and OC=O, respectively. The above three functional groups indicated the presence of the hard segment of polyamide chains [43]. Thus, there was good reason for thinking that interfacial polymerization was carried out successfully on the surface of PS substrate membrane between PEA and TMC, and then formed polyamide selective layers containing ether oxygen group. On the other hand, it could be also clearly seen that the PG-TFN (PG, weight percent, 0.03 wt%, 0.05 wt%, 0.07 wt%) did not appear a new absorption band compared to TFN, which demonstrated the absence of chemical bond generating. Simultaneously, it should be noted that the peak at around 1647 cm-1 shift slightly to a lower frequency compared to that of TFN. We speculated that numerous hydrogen bonds formed between different groups on PG and the polyamide chain. For instance, H atoms almost became protons due to the strong electrostatic attraction of N atoms, and positive centers were formed in polyamide [44]. O atoms of PG provided several negative centers, resulting in hydrogen bonding between the H atoms of the polyamide and the O atoms of PG, which led to a redshift of FTIR characteristic peaks of polyamide [45].

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Fig. 5. (a) FTIR spectras of PS substrate membrane and PG-TFN membranes under different treatment conditions. (b) XPS survey spectras of PS substrate membrane, TFN and PG-TFN membranes (weight percent of PEA and PG, 1 wt% and 0.05 wt%). Apart from that, the quantitative XPS experiment was also performed to further investigate the compound structure, elemental composition and binding energy for PS, bare TFN and PG-TFN (weight percent of PEA and PG, 1 wt% and 0.05 wt%). The results survey spectra of membranes (Fig. 5b) revealed that a new obvious N 1s peaks was presented for PG-TFN and TFN, further indicated that the successful coating of polyamide selective layer on the top of PS substrate. In addition, the curve-resolved C 1s signal of PG-TFN (weight percent of PEA and PG, 1 wt% and 0.05 wt%) resulted in presenting three additional peaks located successively at a binding energy of 284.61 eV, 285.958 eV, 288.002 eV as shown in Fig. 6b. These generated peaks were orderly assigned to C-C, C-N and C=O groups on the surface of produced polymer. Subsequently, the O 1s peak was also resolved in Fig. 6b indicating that the presence of two additional peaks located at 531.091 eV, 532.341 eV in sequence. These produced peaks belonged to C=O and O-C=O. In sum, the observed results further revealed that the surface of PS substrate membrane generated polyamide selective layer containing

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ether oxygen group with the interfacial polymerization. Simultaneously, there were no new chemical bonds compared to Fig. 6a. The result agreed fairly well with ATR-FTIR analysis.

Fig. 6. (a) XPS survey spectras of O1s, C1s, N 1s of TFN membrane. (b) XPS survey spectras of O1s, C1s of PG-TFN membrane. (Weight percent of PEA, 1 wt%). Table 2 XPS data of TFN membrane and PG-TFN membranes element

functional TFN membrane (eV) PG-TFN membrane (eV) group C1s C-N 286.335 285.958 C=O 287.926 288.002 O1s C=O 531.08 531.091 O-C=O 532.29 532.341 And more notably, XPS characterization for PS substrate membrane, TFN and PG-TFN membranes could also indicate the presence of hydrogen bonding between GO and

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polyamide chains. It could be clearly seen from the table 2 that the binding energy for C 1s had a slightly decreased shifting from 286.335 eV (C-N), 287.926 eV (C=O) to 285.958 eV, 288.002 eV between PG-TFN and TFN, respectively. In stark contrast, that of O 1s had an increased shifting from 531.08 eV (C=O), 532.29 eV (O-C=O) to 531.091 eV, 532.341 eV. These results showed a decreased electron cloud density of C atoms while an increased electron cloud density of O atoms, suggesting that the O atoms of the polyamide could form hydrogen bonds with the H atoms of PG [46].

Fig. 7. XRD spectra of TFN membrane and PG-TFN membranes with different PG content. XRD analysis was also employed to monitor the effect of PG on the microstructure of PG-TFN membranes as shown in Fig. 7. The XRD pattern for the pristine TFN membrane showed a characteristic peak at 17.7o and 26.1o, which indicated the semicrystalline structure of the sample. And the peak around 22.9° resulted from the crystalline region of polyamide segments. As PG concentrations increased, the polyamide peak was shifted from 22.9° for the pristine TFN membrane to a smaller angle of 22.0°, implying that the increased d spacing. It reflects that the introduced of PG increases the intermolecular periodic chain-to-chain distance of polyamide, resulting

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in a looser chain packing [47,48]. The polymer chains distortion is anticipated to create more free volume [49]. 3.3 Gas Separation Experiments 3.3.1 The effect of feed pressure for gas separation performance To provide a much more indicative view for gas permeation performance about TFN and PG-TFN, the detail data was shown in Fig. 8(a, b) under different pressures ranging from 1 bar to 5 bar. It could be clearly found that the CO2 permeation has a significant decreased with the gradual increase for pressures, but that of N2 has no a sharp decreased. The reason was as follows: (1) the relatively relationship between feed gas pressure and CO2 permeation rate could be illuminated by the dual-mode sorption model [50,51]. Namely, a part of solute molecules for CO2 dissolving into polymer obeyed the Henry law. However, another part of that was adsorbed into the micropore of polymer following the Langmuir regular. And it was consistent with the permeation rate curve of Langmuir law (2) the membrane-transport process of N2 conformed to the dissolution-diffusion model, so it was often thought that the solute molecule permeation rate was no significant change with the partial pressure of testing component increasing. But it should be noted that the N2 permeation presented a lightly rising trend in 5 bar for TFN and PG-TFN (PG, 0.05 wt%). The experimental results of N2 permeation in 5 bar for TFN and PG-TFN were out of the rule. This might be a reason that a high N2 concentration in the PG-TFN membrane leaded to excessive polymer swelling, resulting in an accelerated increase in segmental mobility of the polymer chains leading to higher diffusion coefficients for N2 in this case, further resulted in the increase of permeation rate at 5 bar [52].

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Meanwhile, the analysis results for Fig. 8c revealed that all membranes showed a gradual decrease in CO2/N2 selectivity with pressure, which agree with the results previously reported in the literature [23]. The reason is that when higher pressure is applied, the hydrostatic pressure of the membrane is also increased, causing compaction of the polymer matrix which decreases the membrane free volume and penetrant diffusivity, further leading to the reduction in CO2 permeability [53]. However, the permeability of light gases, such as H2 and N2 are independent of pressure, further leading to the little change of N2 permeation, thus which lead to the significantly decreased of selectivities for CO2/N2 [54].

Fig. 8. Effect of pressure and the amount of introduced PG for gas permeation and selectivity of PG-TFN membranes. (a) CO2 permeation (b) N2 permeation (c) CO2/N2

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selectivity and (d) Robeson upper bound between the CO2 permeability and the CO2/N2 selectivity of TFN membranes prepared in this work compared with various other membranes. Membrane preparation: 1 wt% PEA, 0.4 wt% TMC 3.3.2 The effect of the amount of introduced PG for gas separation performance Further analysis for Fig. 8(a, b), the results showed that the CO2 and N2 permeation performance for PG-TFN presented an upward tendency with the increasing amount of introduced PG ranging from 0.03 wt% to 0.05 wt% and a slight decreased when the amount of introduced PG shifted 0.05 wt% into 0.07 wt%. In particular, PG-TFN exhibited ∼21 % enhancement of the CO2 permeation performance under given conditions (PG, 0.05 wt%, and 1 bar) compared with that of TFN, but that of N2 has no obvious change. The major reasons were as follows: The CO2 permeability increased with the increase in PG concentration initially, it could be attributed to that introduced PG disrupted the orderly stacked of polymer chain, further increased the free volume of membrane, and with the gradually increase of PG concentration, the oxygen-containing groups content (ether oxygen group, carboxyl) of PG surface was also gradually increased, which have an high affinity for CO2, shown in Fig. 8 [18,55]. Thus, this process synergy effects between PG and oxygen-containing groups increased the CO2 permeability. But the further increase of the PG loading could exacerbate the stack of PG layers (Fig 4), thus leaded to more tortuous gas transport and blocked gas transport quickly, thus leaded to the decrease of the CO2 and N2 permeability when the amount of introduced PG shifted 0.05wt% into 0.07wt%. This phenomenon has also been reported [23,45,56]. Meanwhile, it is also worth noting that the decrease of the CO2 and N2 permeability could be also attributed to the negative influence of the membrane thickness, as the increase of membrane thickness is usually result of the increase of gas

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transfer resistance, and the facilitated effect of PG is significantly decreased with the increasing membrane thickness [17]. Furthermore, it would be specially mentioned that the pore in PG surface also played a key role for improving the separation performance. The PG sheets and pore rim could be functional during interfacial polymerization process, and the analysis results for Fig 6 also revealed that there were hydrogen bond between O atom of ether oxygen groups and H atom of PG, thus indicating that chemical functionalization of the PG pore rim could significantly improve the selectivity of CO2 over N2 [57]. And PG was negatively charged with a large number of carboxyl (Fig 2), and carbon dioxide had stronger polarity and quadrupole compared with nitrogen, thus enhanced the electrostatic interaction with carbon dioxide, and further accelerated CO2 permeating through the pore [58]. In addition, the subnanometre-sized pores on the surface of PG nanosheets might increase the specific surface area, the tortuous pathway for small gas molecule diffusion, and hinder the big gas molecule diffusion in PG selective thin films [40,41]. Furthermore, PG nanosheets with high surface area could also contributed to the integration between polymer layer and PG, further minimized the separating layer defect [42].

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Fig. 9. Mechanism of gas molecules through PG-TFN membranes Meanwhile, the result image (Fig. 8c) showed the separation factor of CO2/N2 for PGTFN (PG, 0.05 wt%) has an apparently improved and in particular, presented about 20.8 % enhancement compared with that of TFN at 1 bar. Beside the above mentioned reasons, it is also a vitally important factor that single-layered PG nanosheets would be assembled into a multi-layer laminates driven by the PG-polyamide hydrogen-bonding interactions (Fig. 6), and the analysis of Fig. 10 showed that the interlayer spacing of PG laminates given by TEM is approximately 0.34 nm, which is in the range of the molecular kinetic diameters of industrial gases, N2 (0.36 nm), CO2 (0.33 nm). Thus, the membrane with PG laminates enhanced the molecular-sieving properties. In addition, it could be also clearly saw from fig. 11a that the membranes showed the highest selectivity for CO2/N2 when introducing 0.05 wt% PG. But increasing the numbers of PG laminates could improve the CO2/N2 selectivity initially, while the further increase of the PG number reduced the CO2/N2 selectivity. This tendency accords with the permeability of CO2 and N2. But it’s important to note that the crystallinity and rigidification of the polymer chains with the increasing the numbers of PG laminates

24

could have also important impact [59]. Base on above, we speculated that the gas separation performance has an intimate connection with the amount of introduced PG.

Fig. 10. (a-b) TEM images of cross-section of PG-TFN membrane (PG, 0.05 wt%), (the yellow solid lines are eye-guiding lines indicating the PG laminates). In addition, the separation performance of membranes prepared in this work was also compared with the previous reported. The result was showed in the Robeson’s upper bound relationship plot (Fig. 8d). Besides, compared to the TFN, PG-TFN introduced the different amount of PG were closer to the upper bound line. Thus, it provided a potential opportunity to prepared high-efficiency gas separation membrane through regulating the amount of two-dimensional inorganic nano-fillers.

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Fig. 11. (a) Effect of PG concentrations for the membrane permselectivity. (b) Stability of PG-TFN membranes (Membrane preparation: 1 wt% PEA, 0.05 wt% PG) up to 48 h. These membranes were measured at 1 bar and 30 °C. 3.4 Stability of PG-TFN membrane The gas separation membranes possessed outstanding stability was crucial for realistic application. The stability of long time running was evaluated for 48 h under the condition of 1 bar and 30 oC. The PG-TFN with the 0.05 wt% PG was tested. The result image (Fig. 11b) indicated that the separation performance of PG-TFN had no significant volatility, thus also revealed that the membrane was stable. The superior stability mainly attributed to the introduced of PG, further enhancing the mechanical properties of membrane and making the defect minimization. Equally important, the membrane prepared method also plays a key role for the stability. 4. Conclusion In this work, the PG-TFN was fabricated by interfacial polymerization between PEA containing porous graphene (PG) and TMC. And CO2 permeation could reach 70 GPU, presenting 21% enhancement while the CO2/N2 selectively could reach 130, showed a 20.8% increase, comparing with that of TFN at 1 bar (PG, 0.05 wt%). The results revealed that the introduced PG has a distinct effect for the permselectivity of membranes. Thus, inspired by this study, two-dimensional material (MoS2, Mxenes) has similar layered structure and should possess also equivalent effects or better. In addition, the interfacial polymerization method for fabricating gas separation membrane is still at the laboratory research stage and many inadequacies are exists, such as a small variety of functional monomers, performance deviation and etc. Thus, this reminded us of the

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urgent to seek new function materials and explore innovative ways to improve the separation performance of membrane. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21376225 and 21476215), Program for Science & Technology Innovation Talents in Universities of Henan Province (16HASTIT004), and Excellent Youth Development Foundation of Zhengzhou University (No. 1421324066). References: [1] L. Zhang, N. Xu, X. Li, S. Wang, K. Huang, W.H. Harris, W.K.S. Chiu, High CO 2 permeation flux enabled by highly interconnected three-dimensional ionic channels in selective CO2 separation membranes, Energy & Environmental Science, 5 (2012) 8310-8317. [2] J. Hou, G. Dong, B. Xiao, C. Malassigne, V. Chen, Preparation of titania based biocatalytic nanoparticles and membranes for CO2 conversion, Journal of Materials Chemistry A, 3 (2015) 33323342. [3] N. Kielland, C.J. Whiteoak, A.W. Kleij, Stereoselective Synthesis with Carbon Dioxide, Advanced Synthesis & Catalysis, 355 (2013) 2115-2138. [4] D. Yu, S.P. Teong, Y. Zhang, Transition metal complex catalyzed carboxylation reactions with CO2, Coordination Chemistry Reviews, 293 (2015) 279-291. [5] S. Zhao, Z. Wang, Z. Qiao, X. Wei, C. Zhang, J. Wang, S. Wang, Gas separation membrane with CO 2 facilitated transport highway constructed from amino carrier containing nanorods and macromolecules, Journal of Materials Chemistry A, 1 (2012) 246-249. [6] Y. Bae, R.Q. Snurr, Development and Evaluation of Porous Materials for Carbon Dioxide Separationand Capture, Angewandte Chemie International Edition, 50 (2011) 11586-11596. [7] A. Hart, N. Gnanendran, Cryogenic CO2 capture in natural gas, Energy Procedia, 1 (2009) 697-706. [8] X. Gao, X. Zou, H. Ma, S. Meng, G. Zhu, Highly Selective and Permeable Porous Organic FrameworkMembrane for CO2 Capture, Advanced Materials, 26 (2014) 3644-3648. [9] P. Luis, T. Van Gerven, B.V. Der Bruggen, Recent developments in membrane-based technologies for CO2 capture, Progress in Energy and Combustion Science, 38 (2012) 419-448. [10] Y. Zhang, X. Dai, G. Xu, L. Zhang, H. Zhang, J. Liu, H. Chen, Modeling of CO2 mass transport across a hollow fiber membrane reactor filled with immobilized enzyme, Aiche Journal, 58 (2012) 2069-2077. [11] M. Peratitus, Porous Inorganic Membranes for CO 2 Capture: Present and Prospects, Chemical Reviews, 2 (2014) 1413–1492. [12] N.C. Nwogu, E. Gobina, M.N. Kajama, Improved Carbon Dioxide Capture Using Nanostructured

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Graphical abstract

Highlights 1. Porous graphene as inorganic nanofiller was used to fabricate thin film nanocomposite membranes. 2. The permeability and selectivity of membranes have been improved after adding porous graphene nanosheets. 3. This work could provide a potential approach for the thin film nanocomposite membrane for gas separation.

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