Graphene Oxide Nanosheets Based Novel Facilitated Transport

Article pubs.acs.org/IECR

Graphene Oxide Nanosheets Based Novel Facilitated Transport Membranes for Efficient CO2 Capture Guanying Dong,† Yatao Zhang,*,†,‡ Jingwei Hou,‡ Jiangnan Shen,§ and Vicki Chen‡ †

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, PR China UNESCO Centre for Membrane Science and Technology, University of New South Wales, Sydney, New South Wales 2052, Australia § Center for Membrane and Water Science, Ocean College, Zhejiang University of Technology, Hangzhou 310014, PR China ‡

ABSTRACT: A novel fixed carrier composite membrane was prepared via interfacial polymerization by using graphene oxide nanosheets (GO), hyperbranched polyethylenimine (HPEI), and trimesoyl chloride (TMC) coating on a polysulfone membrane. The interfacial polymerization was confirmed with SEM, TEM, ATR-FTIR, XPS, DSC, and water contact angle. Further gas separation tests with CO2/N2 (10:90 v:v) mixed gas confirmed that the addition of GO could significantly improve the CO2 permeance and CO2/N2 selectivity. The highest CO2 permeance in this work was 9.7 GPU, while the selectivity was over 80. A further gas separation test under different feed gas humidity confirmed the facilitated transport was the main mechanism of gas separation through the membrane, while the addition of GO into the membrane exhibited a synergistic effect with the gas carriers: the surface defects acted as molecule sieves, and the interlayer fixed flow channels ensured a high water content microenvironment to improve the reactivity between CO2 and amino based carriers. Besides, superior stability of the composite membrane was also testified.

1. INTRODUCTION With the rapid process of global industrialization, greater energy consumption will be anticipated during the 21st century. Among them, over 80% of the total consumption consists of the fossil fuels due to their wide availability and high energy density.1,2 Consistent with this trend, the greenhouse effect caused by excessive emission of CO2 into the atmosphere has become more serious. As a result, carbon capture, utilization, and storage (CCUS) is critical to mitigate the potential adverse effect of CO2 on the climate. At present, the CO2 capture researchers have focused on the following potential strategies: chemical/physical absorption, membrane separation, ionicliquids, electrochemical conversion, and enzymatic conversion. 3−14 Among them, membrane-based CO 2 capture technologies have advantages such as being more environmentally benign, lower energy costs, simplicity of design and operations, and good thermal and mechanical stability.4,15−17 Currently, the membranes used for CO2 capture and storage are mainly classified as porous inorganic, dense polymeric, or facilitated transport membranes based on their materials, structures, and separation mechanisms. The inorganic membranes usually exhibit high CO 2 permeability and selectivity and ideal resistance to harsh chemical conditions. However, the preparation of inorganic membranes is complicated and expensive. It involves strict control of processing conditions like temperature, pressure, and © 2016 American Chemical Society

mole fraction of the condensable species in the feed. As a result, its wider application is restricted.15 In terms of the organic membrane, on the one hand, the polymeric membrane performances are limited by the “upper bound” due to the inverse relationship between permeability and selectivity;18 on the other hand, CO2 plasticization and/or competitive sorption effects also result in low gas selectivity.19 As a result, much research has been devoted to the gas separation material and membrane preparation to overcome the “trade-off” constraints. So far, mixed matrix membranes (MMMs), incorporating nanoporous molecular sieves into polymers, have aroused much concern, which in principle combine the size/shape selectivity of nanoporous materials with the processability and mechanical stability of the polymers matrix,20,21 and the results have demonstrated superior separation performance over the traditional polymeric membranes. For example, Bae et al.22 synthesized MMMs containing Mg2(dobdc) nanocrystals for CO2/N2 separation. Such glassy PI polymer based membranes enhanced CO2 permeability (from 650 to 850 barrers) and CO2/N2 selectivity (from 14 to 23) comparing with pure polymer membranes. Similar Received: Revised: Accepted: Published: 5403

March 13, 2016 April 12, 2016 April 14, 2016 April 14, 2016 DOI: 10.1021/acs.iecr.6b01005 Ind. Eng. Chem. Res. 2016, 55, 5403−5414

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Industrial & Engineering Chemistry Research

Figure 1. Schematic illustration of (a) the reaction between HPEI and TMC and (b) the fabrication process for the HPEI/GO-TMC composite membrane.

A potential solution to this problem is to introduce graphene oxide (GO) into the facilitated transport membranes. The ideal properties of GO including intercalation capability, hydrophilic surface, and the molecule-sieving mechanism could potentially enhance the performance of the facilitated transport gas separation membrane.32,33 Synthesized from powdered flake graphite, the oxidized graphene nanosheets bear carboxyl, hydroxyl, and epoxide on their basal planes and edges, with potential usage for multifunctional nanocomposite materials with high chemical stability, strong hydrophilicity, and improved mechanical, thermal, and/or electronic properties.34,35 Recently, several successful attempts of GO based gas separation membranes have been reported, and it has been demonstrated that the laminar structures and the controlled structure defects on GO sheets could provide high efficient gas separation performance.36,37 So far, most previous works of GO based membrane focused on either the molecular-sieving of GO or microflow channels between laminar GO nanosheets inside the membrane. The opportunity to combine the architecture of GO structure with facilitated transport has not been explored. The hydrophilicity of GO could potentially accommodate the attachment of water molecules inside the membranes and thus provide an alternative route for higher efficient and stable gas separation performance. Thus, in this study, a novel fixed carrier thin film composite (TFC) membrane was prepared by interfacial polymerization with a water-soluble hyperbranched polyethylenimine (HPEI) and GO mixture and hexane-soluble trimesoyl chloride (TMC) on the polysulfone (PSf) support membrane. It is hypothesized that when the CO2/N2 penetrate through the composite membranes, the selective structural defects within GO nanosheets and the nanochannels between the GO sheets function as selective gas barriers, and meanwhile, the CO2 molecules react with the fixed carriers (amine groups) contained in membranes according to the facilitated transport mechanism. This study is focused on understanding the gas separation mechanism for the composite membrane and to

improvement was also observed with the ZIF-7 nanofillers containing MMMs.23 Several problems need to be solved, which include but not limited to the lack of the comprehensive understanding of the inorganic materials’ separation properties, the difficulty in preparation (e.g., aggregation control), and the operational stability due to the poor adhesion between the filler phases and the polymer supports.20,24 In another line of research, the facilitated transport membranes (FTMs) have attracted increasing attention. The mechanism is to incorporate carrier agents (usually amine based chemicals), which react reversibly with CO2 and water to produce −CO2−H2O complexes and HCO3− into the membrane matrix to facilitate the transport of CO2 through the membrane, while the transport fillers are inert to other gases to achieve the separation of CO2 from other feed gases (eq 1) CO2 + RNH 2 + H 2O−RHNCOOH + HCO3−

(1)

Such membranes exhibited ideal selectivity. The preparation is usually achieved through either blending or coating, and usually coating is a preferable approach due to its thinner separation layer that ensures smaller gas transport resistance.25−29 Deng et al. coated a polyvinyl amine/poly(vinyl alcohol) layer on a porous polysulfone supportive membrane. With the amino groups as the fixed carriers, the highest selectivity of CO2/N2 achieved was 174.30 In addition to the fixed carrier, the mobile carriers or the mixture of both fixed and mobile carriers have been integrated with the membrane process to achieve gas separation, and satisfactory lab scale gas separation can be achieved.25−27,31 As presented above, water molecules play an important role during the transport of CO2 through membranes. Actually, for the facilitated transport membranes, the affinity between water and membrane is usually quite low due to the physical adsorption of water onto the membrane surface, which will lead to evaporation and eventual loss of CO2 transport efficiency.26,32 Therefore, stably attached water molecules are preferred for longer gas separation membrane operational performance. 5404

DOI: 10.1021/acs.iecr.6b01005 Ind. Eng. Chem. Res. 2016, 55, 5403−5414

Article


and a current of 25 mA was carried out to evaluate the pristine graphite and GO, employing a scanning rate of 0.04° min−1 in the range from 5 to 50°. 2.4.2. Transmission Electron Microscopy (TEM). A FEI model TECNAI G2 TEM (200 kV acceleration voltages) was applied to observe the morphology of the samples. Prior to the analysis, the samples were dispersed in solvent with sonication, and then the dispersed particles were transferred to a carbon film coated Cu microgrid (400 meshes) and dried. 2.4.3. Atomic Force Microscopy (AFM). An atom force microscope (DI Nanoscope IIIa, Veeco, USA) was employed to obtain morphological images of the GO nanosheets operating in tapping mode. Samples were prepared by dropping a few droplets of the GO nanosheets water solution on a freshly cleaved mica substrate and dried at room temperature. 2.5. Characterization of Membranes. 2.5.1. Scanning Electron Microscopy (SEM). The surface and the cross-section morphologies of the membranes were examined using a JSM6700F SEM scanning electron microscope (JEOL), operating at the voltage of 15 kV. The samples were dried at room temperature, and the cross-section samples were fractured after frozen in liquid nitrogen. Finally all SEM samples were sputtercoated with a thin layer of gold prior to scanning. 2.5.2. Transmission Electron Microscopy (TEM). The composite membranes were observed with a FEI model TECNAI G2 TEM operated at 200 kV. The samples were prepared on the carbon-coated TEM copper grids directly via the interfacial polymerization method. 2.5.3. Fourier Transforms Infrared Spectroscopy (FTIR). FTIR-ATR spectra were performed by using a Thermo Nicolet IR 560 spectroscope (Thermo Nicolet Corporation, USA) over a frequency range of 4000−600 cm−1. All spectra were obtained from 64 scans at a resolution of 2 cm−1. 2.5.4. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos AXIS Ultra DLD X-ray photoelectron spectrometer with a monochromatized Al Kα (15 kV, 10 mA) X-ray source and a hemispherical sector analyzer (HSA) to investigate the membrane surface composition. For calibration the C 1s peak was used (284.8 eV). 2.5.5. Differential Scanning Calorimeter (DSC) Measurement. The thermal properties and the glass transition temperature (Tg) of the composite membranes were analyzed with a DCS using a DSC60 detector at a heating rate of 5 °C/ min in the temperature range of −50−50 °C, and nitrogen was used as a purge gas with a flow rate of 20 mL/min. 2.5.6. X-ray Diffraction(XRD). X-ray diffraction (XRD) was carried out to investigate the crystal structure of membranes using an X-ray diffractometer (XRD, PAN Alytical X’Pert-Pro) in the range of 5−70° at a step size of 0.02°. 2.5.7. Water Contact Angle. The water contact angle (θ) was measured at 25 °C and 50% RH on a contact angle system (OCA20, Dataphysics Insruments, Germany) to further characterize the membrane hydrophility, 1 μL of deionized water was carefully dropped on the dry membrane surface, and the contact angle between the water and membrane was measured until no further change was observed. Each sample was tested at five random locations to minimize the experimental error, and then the average was reported. 2.5.8. Water Sorption Measurement. Water uptake from water vapor at 25 °C was measured with an electronic microbalance (AUY120, Shimadzu, Japan) in a constant temperature and humidity incubator at a varying relative

confirm the synergistic effect between the GO nanosheets and the fixed carriers inside the membrane.

2. EXPERIMENTAL SECTION 2.1. Materials. Natural graphite powders (ca. 45 μm) were obtained from Sinopharm Chemical Reagent and were used as purchased. The PSf ultrafiltration membrane (MWCO 50,000) was used as a support membrane, which was provided by Vontron Technology Co., Ltd., Beijing, China. Hyperbranched polyethylenimine (HPEI) (average Mw = 25,000) was purchased from Sigma-Aldrich. Trimesoyl chloride (TMC) and sodium dodecyl sulfate (SDS) were obtained from J&K. Sodium carbonate, hydrogen peroxide, absolute ethanol, and nhexane were all obtained from Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd., China. All the other chemicals (analytical grade) were purchased from Tianjin Kermel Chemical Reagent Technologies Co., Ltd., China and were used without further purification. The deionized (DI) water was used in this study. 2.2. Preparation of Graphene Oxide. GO was prepared from natural graphite powders based on an improved method.34 First, 3.0 g of graphite powders was oxidized in a mixture of 18 g of KMnO4, 360 mL of 98% H2SO4, and 40 mL of 85% H3PO4 (9:1) at the temperature of 50 °C for 22 h, and then the resulting pasty solution was slowly poured into the mixture of ice (1200 mL) and H2O2 (30 wt %, 20 mL). Then the yellow suspension solution was sonicated and centrifuged at 1000 rpm for 10 min; afterward, the GO suspension was centrifuged at 6000 rpm. Finally, the GO solids remaining at the bottom of the centrifuge tube went through three cycles of resuspension-centrifugation sequentially with DI water, 30 wt % HCl, and absolute ethanol in order to completely wash out chemical residuals. The wet GO was dewatered by vacuum drying under 50 °C. 2.3. Preparation of HPEI-GO/TMC Membrane by Interfacial Polymerization. In this study, the polysulfone membrane was used as the support. It was dipped into the 0.05 wt % sodium dodecyl sulfate solution at least for 24 h in order to remove the preservatives and to improve its wetting property, then washed by DI water, and dried. It was embedded in a circular groove and clamped between a stainless steel apparatus. The HPEI-GO/TMC composite membrane was prepared via an interfacial polymerization method, which has been presented above, and the overall membrane synthesis procedure is illustrated in Figure 1. Briefly, 3.0 g of HPEI, 10 mL of 0.5 wt % sodium dodecyl sulfate, 10 mL of 0.4 wt % sodium carbonate, and a certain volume (0, 5, 10, 15, 20 mL) of GO solution (1 mg mL−1) (the weight percentage of GO content relative to the weight of HPEI were 0, 0.16, 0.33, 0.50, 0.66) were dissolved into 10 mL of deionized water, which was designed as the water phase. The organic phase was prepared by dissolving 0.2 g of TMC into 30 mL of n-hexane. First, the water phase was poured onto the surface of support membrane, followed by removing the excess solution with tissue paper after 30 min. Then, the TMC solution was poured on the top of the support membrane and kept for 3 min, where the TFC layer was formed near the aqueous-oil interface. The possible interaction was shown in Figure 1(a). Finally, the target composite membrane was washed several times with water and n-hexane and dried under 70 °C for 12 h before further testing. 2.4. Characterization of GO. 2.4.1. X-ray Diffractometer Analysis (XRD). An X-ray diffractometer analysis (XRD, Philips X Pert-Pro) with copper Kα radiation under a voltage of 35 kV 5405


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Figure 2. XRD patterns of (a) graphite and (b) graphene oxide.

Figure 3. TEM images and AFM image (with corresponding thickness) of GO nanosheets.

between the permeate chamber and the feed gas chamber, supported by a porous metal disk and sealed with rubber Orings. The feed gas, a mixture of CO2 and N2 (10:90 v:v), and the sweep gas, H2 (30.0 mL min−1), were supplied. Prior to entering the membrane rig, the feed gas was humidified by bubbling through a water bottle at 30 °C and then passed through empty bottles to remove the condensate water. The humidity of the feed gas was adjusted with the precise valves fixing on the upstream water bubbling bottles, while the relative

humidity (RH) from 40 to 90%. Prior to the measurements, the samples were degassed in the vacuum oven at 50 °C for 12 h. The sample weight was measured after each adjustment of the humidity, and the final result was recorded until the sample weight was constant. 2.6. Gas Permeation Experiment. The mixed gas permeation performance of the membranes was tested with the circular membrane gas permeation rig.38 A membrane (effective membrane area of 19.6 cm2) was sandwiched 5406


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Industrial & Engineering Chemistry Research humidity of the feed gas was measured online by a humidity analyzer. The component of the sweep gas was monitored with a gas chromatograph equipped with a thermal conductivity detector (Shimadzu, GC-2014). The permeation fluxes of CO2 and N2 were calculated from sweep gas flow rate and its composition. Permeance of a specie i was defined as the flux divided by the partial pressure differences between the upstream and downstream of the membrane and reported in the units of GPU (1 GPU = 10−6 cm3 (STP)/(cm2·s·cmHg)) and given the symbol Ri. The selectivity (α) was calculated from the permeability of CO2 (RCO2) and N2 (RN2), as expressed by α = RCO2/RN2. The permeation experiment was carried out with a feed pressure of 0.1 MPa, and the separation performance was recorded after the system had been stabilized.

3. RESULTS AND DISCUSSION 3.1. Characterization of GO. The XRD patterns of pristine graphite (PG) and GO are shown in Figure 2. In terms of GO, a strong and broad peak at 2θ = 10.1° was attributed to (001) reflections. The stacking order was also observed in GO with a layer-to-layer distance (d-spacing) of 0.875 nm, which was larger than that of PG (0.336 nm, 2θ = 26.5°) because of the intercalated water molecules between layers, leading to the interlayer spacing being expanded. Comparison of the XRD patterns of the two materials indicated that PG exhibited better crystallinity than GO, which could be attributed to the varying nanostructures.39−41 Besides, the small bump appearing between 20° and 25° confirmed the pristine graphite is not completely oxidized, and the low intensity broad peaks between 20° and 45° 2θ can be attributed to the sign of amorphous regions.42The morphology of as-formed GO nanosheets is shown in Figure 3. It was observed that the GO nanosheets tend to aggregate together to form multilayer agglomerates.42,43 In addition, there was the typical characteristic for GO, the sp3-bonded carbon atoms allocated in the graphene sheets which disrupt the double bond conjugation effectively, resulting in the waved sheets structures, resembling wrinkled silk.44 The AFM image showed an average thickness of 1.1 nm of the as-prepared GO nanosheets, that is, presenting the single-layer structure (Figure 3c). 3.2. Morphology and Microstructure of the Composite Membranes. The membrane characterization was carried out with the pure PSf membrane and the HPEI-TMC and the HPEI/GO-TMC (0.33 wt % GO) composite membranes. The surface and cross-sectional images of the composite membrane are shown in Figure 4. As shown in Figure 4(a1), the upper surface of the untreated membrane had a dense layer with some scratches, which could be caused during the membrane preparation process. In terms of its cross-section view (Figure 4(b1)), the whole membrane had an asymmetric structure, and a thick sponge-like membrane matrix was observed, which was coated with a thin dense layer. This was also observed in other studies.45,46 In comparison, after the treatment of HPEI-TMC, the membrane upper surface was smooth and defect-free (Figure 4 (a2)), and its thickness increased from 1.2 to 2.3 μm, indicating the HPEI-TMC layer was roughly 1.1 μm thick. During the cross-linking of HPEI and TMC, the aliphatic amine had a long molecular chain on HPEI; together with the low monomer concentration in aqueous and organic phase for HPEI and TMC, the cross-linking reaction had a lower reaction rate, allowing more time for the cross-linking layer to realign and to form a smooth surface.47 Figure 4(a3) and Figure 4(b3) reflected that when adding GO nanosheets into the water

Figure 4. SEM characterization of membrane surface and cross-section for (a1) and (b1): the pure PSf support membrane, (a2) and (b2): the HPEI-TMC membrane, and (a3) and (b3): the HPEI/GO-TMC membrane.

phase, the membrane morphology changed as compared to the HPEI-TMC composite membrane. The wrinkled structure could be attributed to the dispersed GO sheets and several GO sheets agglomerating on the membrane surface. The addition of GO also increased the skin layer thickness compared with the HPEI-TMC membrane (from 1.1 to 1.7 μm). This was probably derived from the accumulation of GO nanosheets; besides, the GO nanosheets residing in the polymer could undermine the polymer chain packing to a certain degree because of its unique inorganic characteristic, thus decreasing the compactness of the selective layer.48 TEM images of the HPEI-TMC and HPEI/GO-TMC (0.33 wt % GO) composite membranes were introduced for further morphological characterization and shown in Figure 5. Some particles were found on the HPEI-TMC membrane surface at high magnification, which may represent the cross-linking structure of some macromolecules. The laminated structure of GO nanosheets was clearly observed in Figure 5(d), in which the interlayer space could provide more molecular-sieving channels.49 The ATR-FTIR spectra of PSf support (a), HPEI-TMC (b), and HPEI-GO/TMC (0.33 wt % GO) membranes (c) are shown in Figure 6. Comparing with the substrate membrane, the FTIR spectrum of the composite membranes (b and c) exhibits evident strong new peaks. The bands near 1470 and 3370 cm−1 were assigned to the stretching of C−N and N−H arising from HPEI. The peak at 1550 cm−1 was associated with N−H (amide II) deformation vibration, and the peak at 1630 cm−1 corresponded to the CO stretching (amide I) for −NHCO−, indicating the generation of polyamide during the polymerization process. All these observations confirmed the occurrence of the interfacial polymerization between HPEI and 5407


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Figure 7. XPS spectra of (a) the pure PSf support membrane; (b) the HPEI-TMC membrane; and (c) the HPEI/GO-TMC membrane.

Table 1. Content of Carbon, Oxygen, Nitrogen, Chlorine and Sulfur in (a) the Pure PSf Support Membrane, (b) the HPEI-TMC Composite Membrane, and (c) the HPEI/GOTMC (0.33 wt % GO) Composite Membrane

Figure 5. TEM images of (a,b) HPEI-TMC and (c,d) HPEI/GOTMC composite membranes at different magnifications.

atomic%

a

b

c

C1s O1s N1s Cl2p S2p

15.78 81.59

68.27 8.19 21.96 1.10 0.48

70.21 11.19 15.84 2.09 0.68

2.63

the percent of N−H decreased in the HPEI-GO/TMC composite membrane. The peaks of sodium, sulfur, and chlorine could be assigned to the acid acceptor, Na2CO3, PSf, and TMC, respectively. DSC was usually used to evaluate the chain mobility around the polymer−filler interface correlating with the glass transition temperature (Tg). Figure 8 shows the DSC curves of HPEITMC and HPEI/GO-TMC composite membranes with various GO content. For the HPEI-TMC membrane, an endothermic peak was observed at 11.7 °C, whereas the Tg of HPEI/GOTMC composite membranes were lower than that of the HPEI-

Figure 6. FTIR spectra of (a) the pure PSf support membrane; (b) the HPEI-TMC membrane; and (c) the HPEI/GO-TMC membrane.

TMC. Besides, the peaks located at 2840, 2930 cm−1 were designated as the asymmetric and symmetric vibrations of CH2.50 In terms of the HPEI-GO/TMC membrane, the characteristic peak of GO appearing at 1240 cm−1 represented C−O stretching and vibration in C−O−C/C−O−H groups, which confirmed the presence of GO in HPEI-GO/TMC composite membranes. The chemical changes that occurred as a result of in situ interfacial polymerization on PSf support were further confirmed by XPS analysis. The XPS results are shown in Figure 7. Compared with the PSf support membrane, the nitrogen-containing functionality peaks are clearly observed in Figure 7 (b) and (c), which could be assigned to the active skin layer of the composite membranes containing both amino groups and amide bonds after interfacial polymerization of HPEI and HPEI-GO (0.33 wt % GO) with TMC. In addition, the content of oxygen and carbon in the composite membranes increases from 8.19%, 68.27% to 11.19%, 70.21%, while the content of nitrogen decreases from 21.96% to 15.84% before and after introducing GO into the water phase (Table 1), which suggested that the percent of the CO bond increased while

Figure 8. DSC curves of HPEI/GO-TMC composite membranes with different GO content (a, b, c, d, e: 0, 0.16 wt %, 0.33 wt %, 0.50 wt %, 0.66 wt %). 5408


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Industrial & Engineering Chemistry Research TMC composite membrane, indicating that the incorporation of GO nanosheets did not generate the rigidified interface; in contrast, the GO nanosheets slightly enhanced the polymer chain mobility, which may be caused by the moderate interaction between GO nanosheets and polymer chains,51 but the Tg increased gradually with the increase of GO content for HPEI/GO-TMC composite membranes, from 3.8 to 8.2 °C. This phenomenon can be attributed to the large amount of hydrogen bond between the GO sheets and polymer chains which restricted the “moderate interaction” slightly. The crystal structures of composite membranes are provided in Figure 9. For the HPEI-TMC membrane, the peaks at 17°

Figure 10. Static water contact angle of HPEI/GO-TMC composite membranes with various GO content.

Figure 9. XRD spectra of HPEI-TMC and HPEI/GO-TMC composite membranes with different GO content.

and 26° indicate the semicrystalline structure of the sample,52 and the peak around 23° results from the crystalline region of polyamide segments. Compared with other membranes, no new peaks or obvious peak shift took place. In addition, the characteristic peak of GO nanosheets disappeared in the XRD curves, showing the homogeneous dispersion of GO in aqueous phase. To investigate the surface hydrophility of the prepared membranes, surface contact angles of membranes were measured. As shown in Figure 10, the as-prepared membranes showed superior hydrophility with the highest contact angle of 55°. The contact angle decreased from 30° to 27° when adding no more than 0.33 wt % GO, indicating that the hydrophility of composite membranes was enhanced slightly; however, the contact angle increased with adding more GO, and it may be attributed to the agglomeration of GO on membrane surface. As previously mentioned, water molecules are indispensable for the facilitated transport mechanism, and the affinity between membrane and water is closely related with longtime stability. Thus, to further explore the difference between HPEITMC and HPEI/GO-TMC composite membranes on retaining water, a dynamic vapor sorption experiment was conducted (Figure 11). The net change in mass for HPEI-TMC and HPEI/GO-TMC composite membranes at 90% RH were 20% and 31%, showing that the addition of GO improved the water sorption behavior. This phenomenon may arise from the hydrogen bonding between water and abundant oxygenic functional groups.53 Besides, the water sorption and desorption curves for the HPEI-TMC membrane were nearly overlapped,

Figure 11. Water sorption isotherms for different composite membranes as a function of relative humidity (RH) at 25 °C.

while some hysteresis appeared for the HPEI/GO-TMC membrane, in agreement with other GO membranes.54 We speculated that the water sorbed by the HPEI-TMC membrane was mostly covering the outside, and the hysteresis could also be explained by the strong interaction between water and GO sheets and by the confinement of narrow GO sheets, leading to more hindrance for the desorption of trapped water molecules. 3.3. Separation Performance of the Composite Membranes. The membrane gas separation performance was tested with CO2/N2 mixed gas (10:90 v:v), and the results of gas permeance and selectivity as a function of different GO content are shown in Figure 12. Overall, the as-prepared membranes showed relatively low CO2 and N2 permeance, which could be attributed to the thick skin layer, as the increase of membrane thickness usually gives rise to the increase of gas transport resistance, and the facilitated effect is sensitively decreased with increasing membrane thickness.55 With the increase of GO content, the N2 permeance declined steadily, while the highest CO2 permeance and CO2/N2 selectivity were achieved with 0.33 wt % GO membrane. Compared with the HPEI-TMC membrane, the addition of GO into the coating layer significantly improved the CO2 permeance and CO2/N2 5409


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Figure 12. (a) CO2 and N2 permeances and (b) CO2/N2 selectivity of the composite membrane as a function of GO content, sweep gas flow rate 0.5 mL/s, at 30 °C and 0.1 MPa.

selectivity. This observation confirmed the synergistic effect between GO and the facilitated transport mechanism. The presence of GO inside the membrane may have the following effects: first, the hydrophilicity of GO sheets could retain more water molecules inside the membrane (as shown in Figure 11). The facilitated transport membrane requires water molecules to conduct the reaction between CO2 and carriers. Meanwhile, the dissolved CO2 could form aqueous HCO3− ions in an aqueous environment which could more efficiently react with HPEI to facilitate the transport.32 Second, the structural defects on the GO sheets provided an extra size exclusion effect due to the different dynamic diameters (0.33 nm for CO2 and 0.36 nm for N2). Lastly, the GO nanosheets in membranes exhibited improved CO2 separation property at high RH levels, because the strong affinity betweeen intercalated water and CO2 molecules enhanced CO2 solubility but reduced CO2 diffusivity greatly.32,54 All these aspects led to higher CO2 permeance and CO2/N2 selectivity. However, when the GO content exceeded 0.33 wt %, the CO2 permeance and selectivity decreased with increasing GO content, and the reduction of CO2 permeance was much more than that of N2 permeance. This observation could be attributed to, on the one hand, highly stacked GO nanosheets, where the tortuosity determined by GO thickness and deformation of the thin-layered structures would increase the resistance of gas diffusion through the membrane, and the microflow channels could be blocked by multilayers of GO sheets, which significantly increased the gas transport resistance through the membrane. On the other hand, the hydrophility of composite membrane declined with increasing GO content (Figure 10), which compromised the CO2 uptake immediately. To fully illustrate the transport mechanism of the mixed gas through the HPEI/GO-TMC composite membrane, the schematic in Figure 13 shows the synergistic effect of the GO and the facilitated transport membrane. As mentioned above, the presence of GO not only improved the water sorption behavior but also enhanced CO2 diffusion into the interlayer because of the strong interaction between CO2 molecules and intercalated water. Thus, the preferential adsorption for CO2 of GO and the facilitated transport for CO2 are beneficial to the production of high CO2 permeance and CO2/N2 selectivity. Table 2 shows the CO2 separation performance of the facilitated transport membrane of the current study and results reported by other researchers. It contains both CO2/N2 separation and CO2/CH4 separation results. However, due to dramatically different operation conditions, it is difficult to make a direct comparison, but the data provides a general comparison across a range of facilitated transport membranes.

Figure 13. Schematic showing the transport mechanism of mixed gas through the HPEI/GO-TMC composite membrane.

Generally, the permeance and selectivity in this work is comparable or higher than other facilitated transport membranes including PVAm, DAMA, PEI, chitosan, and VSA-SA membranes.56−61 Zhang et al.62 reported a very high permeance membrane with up to 169 GPU. However, the high permeance was only obtained under very low feed gas pressure. When the feed pressure increased to 0.013 MPa (10 cmHg), the permeance reduced to less than 10% of its original level, indicating limited CO2 carrier capacity inside the membranes. In terms of mobile carrier membranes, they usually exhibited much higher selectivity.63−65 However, the evaporation of liquid carrier was an issue for long-term operational stability for these membranes. 3.4. Effect of Relative Humidity on the Separation Performance of the Composite Membranes. Figure 14 illustrates the effect of relative humidity on CO2 permeance and CO2/N2 selectivity of HPEI-TMC and HPEI/GO-TMC composite membranes at 30 °C and 0.1 MPa feed pressure. Within the test conditions, both the CO2 permeance and CO2/ N2 selectivity increased monotonically with higher relative humidity before plateauing, while the N2 permeance showed a slight decrease. Higher water content in the feed gas raised the water retention inside the membrane, leading to an increase of the mobility of the CO2 carriers and the reaction rates of CO2 with the carriers. Thus, the CO2 transport through the membrane was enhanced. On the other hand, with the increase of water content in the feed gas, the competitive adsorption between water and N2 resulted in a reduced diffusion rate through the membrane and led to lower N2 permeance, and the increasing relative humidity may be enough to block membrane permeable spaces, because of membrane swelling, thereby 5410


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Industrial & Engineering Chemistry Research Table 2. Comparison of Membrane Separation Performance of Facilitated Transport Membranes membrane HPEI/TMC coated on PSf membrane HPEI/TMC/GO coated on PSf membrane PVAm/PVA coated on PSf membrane PAAm/PVA coated on PSf membrane DAMA plasma grafted on PE substrate PEI/PVA blended membrane swollen and dry chitosan membrane VSA-SA coated on PSf membrane polyvinylpyrrolidone membrane ethylenediamine in PAA/PVA membrane amino acid ionic liquid in hydrophilic PTFE membrane arginine salt solution in chitosan membrane (150 degrees)

permeance (GPU)

selectivity

feed pressure (MPa)

feed gas (CO2 volume percent)

6.4

51.4

0.1

CO2/N2 (10%)

9.7 22 24 1−10 0.99−3.96 3.1 6.12 169 17−40

81.3 174−194 80 130 130−230 69 524.5 48.1 400−1500

CO2/N2 (10%) CO2/N2 (10%) CO2/N2 (20%) CO2/N2 (2.7−58%) CO2/N2 (5.8−34%) CO2/H2/N2 (10%) CO2/CH4 (50%) CO2/CH4 (50%) CO2/N2

30 57 58 59 60 61 62 63

28

80

0.1 0.2−1.5 0.1 0.0047 (CO2 partial pressure) 0.0066 0.15 0.106 0.0016 0.007−0.04 (CO2 partial pressure) 0.1

CO2/N2

64

19.75

516

0.15

CO2/H2/N2 (10%)

65

ref current work

Figure 14. Effect of relative humidity on CO2 separation performance, (a) and (b): HPEI-TMC and (c) and (d): HPEI/GO-TMC (0.33 wt % GO) composite membranes, sweep gas flow rate 0.5 mL/s.

Figure 15. Stability test of the HPEI/GO-TMC (0.33 wt % GO) composite membrane under (a) humidified feed stream and (b) dry feed stream.

limiting N2 permeation.66 The hydration of GO with higher feed gas humidity also had an effect on the permeation performance because of the structural change of the GO sheets

itself.56 Due to the insertion of water, the GO interlayer distance expanded with higher humidity. As a result, CO2 molecules could diffuse into the GO interlayer and combine 5411


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Industrial & Engineering Chemistry Research Notes

with the carriers (amine groups) more easily. However, the expended interlayer also increased the gas diffusion distance due to an increase of the flow channel tortuosity, which could reduce the gas permeance, especially for N2. The variation of the GO interlayer was accompanied by substantial topography alterations at high humidity, exhibiting a granular morphology with protrusions and valleys, which could lead to negative effects on CO2 and N2 permeance. 3.5. Stability of the HPEI/GO-TMC Composite Membrane. To examine the separation performance stability of the HPEI/GO-TMC composite membrane under humidified and dry stream, the membrane with the best CO2 separation performance, containing 0.33 wt % GO, was tested continuously using CO2/N2 mixed gases (10/90 by volume) for 100 h at 30 °C and 0.1 MPa. As shown in Figure 15a, the separation performance of the HPEI/GO-TMC composite membrane shows roughly constant CO2 permance and CO2/N2 selectivity with the feed gas humidified; besides, the CO2 permance and CO2/N2 selectivity, as a function of time, exhibit a similar trend. The reasons for long-time stability are probably related to the hydrophilic surface of GO flakes and the intercalated phenomenon, which enable CO2 to be adsorbed strongly; besides, when operated under specific conditions, the GO structure, such as interlayer spacing, intercalated water, remains unchanged. It is with these factors that no deterioration in membrane performance was occurring, while the membrane exhibited an unsatisfied performance under dry feed stream (Figure 15b). Both the CO2 permeance and CO2/N2 selectivity declined with time as a result of the absence of water (i.e., the carriers) and great substructure resistance of GO. In contrast with the membrane performance at dry state, membranes tested under a humidified condition showed more superior dimensional stability. Therefore, high water−vaportolerant property of GO is crucial for high CO2 permselectivity and longtime stability.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially sponsored by the National Natural Science Foundation of China (No. 21376225), Program for Science & Technology Innovation Talents in Universities of Henan Province (16HASTIT004), and Excellent Youth Development Foundation of Zhengzhou University (No. 1421324066).

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5. CONCLUSIONS In summary, a novel facilitated transport membrane was successfully synthesized through interfacial polymerization and the introduction of GO nanosheets. The GO nanosheets enhanced the separation performance because of their unique physical, chemical, and structural properties. Compared with the HPEI-TMC composite membrane, the addition of GO significantly improved the CO2 permeance and CO2/N2 selectivity. This work also examined the effect of feed gas humidity on gas permeance, confirming the facilitated transport was the main gas separation mechanism within the gas separation membrane, i.e. higher water content improved the reactivity of the carrier and CO2, leading to the enhanced CO2 permeance. This work confirmed the synergistic effect between GO nanosheets and facilitated transport membranes. The GO sheets not only possessed a molecular sieving mechanism but also provided fixed flow channels with more attached water molecules. All these aspects could benefit the separation of CO2, and this technology could be used for other gas separation membranes as well.

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REFERENCES

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Graphene Oxide Nanosheets Based Novel Facilitated Transport

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