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PEBAX mixed matrix membranes for efficient CO2 capture
Separation and Purification Technology 166 (2016) 171–180
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Imidazole functionalized graphene oxide/PEBAX mixed matrix membranes for efficient CO2 capture Yan Dai a, Xuehua Ruan b, Zhijun Yan a, Kai Yang a, Miao Yu a, Hao Li a, Wei Zhao c, Gaohong He a,b,⇑ a
State Key Laboratory of Fine Chemicals, R&D Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian 116023, China School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China c School of Petrochemical Engineering, Shenyang University of Technology, Liaoyang 111003, China b
a r t i c l e
i n f o
Article history: Received 24 January 2016 Received in revised form 21 April 2016 Accepted 22 April 2016 Available online 23 April 2016 Keywords: CO2 Poly(ether-b-amide) MMMs Gas separation Functionalized graphene oxide
a b s t r a c t Mixed matrix membranes (MMMs) were composed of imidazole functionalized graphene oxide (ImGO), a CO2-philic nano-sheet inorganic material, and poly(ether-b-amide) (PEBAX) for CO2 capture. MMM doped with 0.8 wt.% ImGO exhibits the best CO2 separation performance, which shows the CO2/N2 selectivity up to 105.5 combined with CO2 permeability of 76.2 Barrer (1 Barrer = 1010 cm3(STP) cm cm2 s1 cmHg1), surpassing the Robeson Upper Bound of 2008. The selectivity of MMM for CO2/N2 increases by 46.0% compared to the Pristine PEBAX due to the interaction between CO2 and imidazole groups. With the increase of feed pressure, CO2 permeability increases significantly because of its higher solubility in polymer matrix and plasticization. It is effective to separation CO2 from N2 at lower temperature for MMMs because the apparent activation energy of the N2 permeation process in ImGO/PEBAX MMMs is much higher than that of CO2. Tg of MMMs are increased gradually because the polymer chain mobility is restricted by the presence of ImGO and a rigidified interface generates between polymer and filler. The mechanical properties have been significantly enhanced by the ImGO sheet as expected because of the presence of H-bonding. Having distinct improvement of CO2 separation performance, the ImGO/PEBAX MMMs indicates promising applications in CO2 capture processes. Ó 2016 Published by Elsevier B.V.
1. Introduction CO2 capture and storage is gaining tremendous interests for energy, economic and environmental perspectives [1,2]. Compared with conventional processes, membrane technology has presented excellent potential in CO2 capture due to many advantages, such as no phase change, low energy consumption, ease of scale up and small footprint [3,4]. Polymers are the most frequent membrane materials that successfully applied in industry. Therefore, there is a pressing demand to develop polymeric materials with high gas separation performance. However, polymeric membrane is always restricted by a trade-off between permeability and selectivity [5,6]. So modifications of polymeric membrane with a number of techniques including crosslinking [7], blending [8] and thermal annealing [9] are usually employed to solve the problem. Mixed matrix membranes (MMMs) comprised of a polymer as the continuous phase and an inorganic filler as the dispersed phase can offer one ⇑ Corresponding author at: State Key Laboratory of Fine Chemicals, R&D Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian 116023, China. E-mail address: [email protected] (G. He). http://dx.doi.org/10.1016/j.seppur.2016.04.038 1383-5866/Ó 2016 Published by Elsevier B.V.
of the most potentially effective solutions [10,11]. The incorporation of fillers with porous structure, such as carbon molecular sieves [12,13], zeolites [14] and metal organic frameworks (MOFs) [15] in polymeric membranes can combine the size selectivity of inorganic nanomaterials with the permeability, mechanical stability and ease of processibility of polymer [16]. Herein, MMMs have received tremendous attention over the last decade. Poly(ether-b-amide) (PEBAX), a commercial rubbery copolymer material with high gas permeability and selectivity, is a good candidate for membrane separation of CO2 from N2, H2 and CH4 [17– 20]. The chemical structure of PEBAX MH 1657 is shown in Fig. 1, in which PA is polyimide 6 block and PE is poly(ethylene oxide) block. PA segment in PEBAX supplies high mechanical strength to the membrane due to its crystallinity, while PE segment provides high CO2 permeability because of the high chain mobility and affinity with the polar molecules [16]. Some organic materials have been selected as modifier to improve the gas separation performance of PEBAX. Low molecular weight poly(ethylene glycol) (PEG) was used to improve the CO2 separation performance of PEBAX membrane whose CO2 permeability was increased two fold and the CO2/H2 selectivity was enhanced from 9 to almost 11 [21]. A high CO2 separation performance MMMs were prepared
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Group Co., Ltd. (China). Deionized water (DI water) was used in all the experiments. Gases (H2, N2, CH4 and CO2) of research grade were supplied by Dalian Institute of Chemistry and Physics (China). All chemicals were used as received. 2.2. Membrane preparation
Fig. 1. Chemical structure of PEBAX MH 1657 (the weight ratio of PA to PE (x:y) is about 40:60).
by incorporating room temperature ionic liquid (RTIL) into PEBAX showed a 300 Barrer of CO2 permeability and 40 CO2/N2 selectivity [22]. Recently, inorganic filling materials such as multiwall nanotube (MWNT), single wall nanotube (SWNT), 4A zeolite, carboxylic acid nanogels (CANs), and calcium lignosulfonate (CaLS) have received widespread attention due to their size selectivity [19,23–27]. Graphene oxide (GO) is a well-known two-dimensional material that presents excellent mechanical and thermal properties due to its unique one-atom-thick structure [28]. It is usually applied to the field of energy and environment, for example, supercapacitors and batteries [29–31]. Recent years, GO showed the significant potential in membrane for small-molecule separation [32]. It is found that the submicrometer-thick membranes prepared from GO can prevent completely from liquids, vapors and gases, including helium, but allow water to permeate [33]. In addition, ultrathin GO (<5 nm) membranes with high CO2 permeability and selectivity (CO2/N2, CO2/CH4 and CO2/H2) has been fabricated by spin-casting onto a porous polymeric substrate [34]. In the GO membranes, gas transfer channel can be controlled via different stacking methods, so the gas diffusion selectivity can be controlled [35]. GO/ PEBAX MMMs with 3.85 vol.% GO loading were prepared by solution casting method, but the membrane displayed lower CO2 permeability compared with pristine PEBAX membrane, while the mechanical properties of the membranes had been improved [36]. MMMs separation performance also can be enhanced by functional groups for CO2 such as ethylene oxide (EO) and amine carrier. Wu [37] reported the MMMs doped with GO nanosheets functionalized by PEG (contained EO groups) and polyethylenimine (PEI, contained amine carrier). The MMMs with potential application in CO2 capture showed a good performance, surpassing the upper bound limit of the Robeson’s work (2008) [6] under humidified state. In this study, we fabricated MMMs with PEBAX as matrix and imidazole functionalized graphene oxide (ImGO) as CO2-philic filler. The objective of this study was to develop a high CO2 permeability and selectivity of ImGO/PEBAX MMMs. The properties of the MMMs including morphology, thermal stability, crystallization, mechanical properties and content of elements were investigated. The effect of ImGO loading, feed pressure and temperature on CO2 permeability and selectivity was studied. 2. Experimental 2.1. Materials PEBAX MH1657 was purchased from Arkema (France). ImGO (>99 wt.%) was obtained from Nanjing XF Nanomaterials Co., Ltd. (China). The size of ImGO nanosheets was at the range of 0.5–1 lm. The loading of carboimidazole groups in ImGO was 7.2 wt.%. The chemical structure of ImGO was shown in Fig. 2. Ethanol (99.5 wt.%, anhydrous) was purchased from Sinopharm
3.0 wt.% PEBAX solution was prepared by dissolving polymer in the mixture of anhydrous ethanol/DI water (70/303.0 wt.% PEBAX solution wt.%) under reflux and stirring at 85 °C for 4 h [19,38–40]. The solution was filtered through a 5 lm polytetrafluoroethylene filter (MILLIPORE, USA). Certain amount of ImGO was added into the PEBAX solution and kept stirring overnight for good dispersion. Bubbles were removed by ultrasonication for 5 min with an ultrasonic bath (AS5150B, Tianjin Automatic Science Instrument Co., LTD, China). The PEBAX or ImGO/PEBAX solution was cast on a flat TeflonÒ plate with a glass ring attached. Membrane thickness (30– 50 lm) was controlled by the amount of solution added to the casting ring. Pristine PEBAX or ImGO/PEBAX membranes were obtained by drying the casting solution at 40 °C for 24 h. In order to remove residual solvent, the as-prepared membranes were further dried in a vacuum oven at 60 °C for 72 h. The ImGO loading and ImGO volume fraction (uf) were defined as Eqs. (1) and (2), respectively.
ImGO loading ðwt:%Þ ¼ /f ¼
mImGO 100% mImGO þ mPEBAX
mImGO =qImGO mImGO =qImGO þ mPEBAX =qPEBAX
ð1Þ ð2Þ
where mImGO and mPEBAX were the masses of ImGO and PEBAX, respectively; qImGO and qPEBAX were the densities of ImGO and PEBAX, respectively. The density of bulk ImGO (2.28 cm3 g1) was assumed. In this work, pristine PEBAX membrane and MMMs with different ImGO loading of 0.2, 0.5, 0.8 and 1.0 wt.% were prepared. According to the ImGO loading, membrane samples were named Pristine PEBAX, ImGO 0.2, ImGO 0.5, ImGO 0.8 and ImGO 1.0. 2.3. Membrane characterization 2.3.1. Scanning electron microscope (SEM) The surface and cross-section morphology of the membrane was observed by field-emission scanning electron microscope (FEI Nava NanoSEM 450). Membrane samples were coated with a thin gold layer for better conductivity by sputtering (20 mA, 30 s) prior to observation. 2.3.2. Thermogravimetric analysis (TGA) TGA (Q50, TA Instruments, USA) was used to test the thermal stability of the ImGO and ImGO/PEBAX mixed matrix membranes. Samples were heated from 25 to 800 °C at a heating rate of 10 °C min1 under N2 atmosphere (50 ml min1 of flow rate). Before the test, the samples were dried in a vacuum oven for 24 h at 60 °C for moisture removal. 2.3.3. Differential scanning calorimetry (DSC) Glass transition temperatures (Tg) of the membranes were analyzed by DSC (Q20, TA Instruments, USA) measurements which were carried out from 80 to 250 °C in N2 atmosphere (50 ml min1 of flow rate). 2.3.4. X-ray photoelectron spectroscopy (XPS) In order to analyze the element content of membranes, XPS patterns (Thermo ESCALAB 250, USA) were recorded using monochromatized Al Ka radiation (1486.6 eV).
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Fig. 2. Schematic structure of ImGO.
2.3.5. Mechanical properties The tensile strength and elongation at break of membranes were measured by the stretching tester (SANSCMT8102, Xinsansi, China). The size of all membrane samples are 50 5 mm. The tensile speed was 10 mm min1. The mechanical properties presented are average values of three samples.
upstream pressure and under vacuum, cmHg s1. ai=j is the selectivity of gas pair i/j.
2.4. Gas permeation experiments
3.1.1. Membrane morphology The surface morphology of Pristine PEBAX and ImGO 0.8 membranes are shown in Fig. 3. Compared with the Pristine PEBAX membrane (Fig. 3(a) and (b)), the dispersed ImGO can be found in PEBAX matrix clearly in Fig. 3(c) and (d). The surface of ImGO 0.8 membrane is smooth and there is no wrinkle caused by graphene oxide-carboimidazole as reported [37]. But, slightly aggregation of ImGO can be observed in the surface of mixed matrix membrane. The cross-section morphology of Pristine PEBAX and ImGO 0.8 are shown in Fig. 4. The ImGO nanosheets are distributed uniformly in polymer matrix. Compared with pristine PEBAX membrane, ImGO 0.8 presents a slightly wrinkled cross-section caused by the interface void between ImGO nanosheets and polymer matrix.
Gas permeability coefficients of N2, CH4, H2, and CO2 were measured using a constant volume, variable pressure method [41,42]. The above gases are listed in order of testing. The gas permeability coefficients were measured at feed pressure and temperature ranging from 0.2 to 0.8 MPa and 25–80 °C, respectively. The gas permeability coefficients and selectivities were calculated with the following equations:
P¼
ai=j
V dl p2 ART Pi ¼ Pj
dp1 dp1 dt ss dt leak
ð3Þ ð4Þ
where P is the gas permeability coefficient, Barrer (1 Barrer = 1010 cm3(STP) cm cm2 s1 cmHg1) Vd is the downstream volume, cm3, l is the membrane thickness, cm, p2 is the upstream absolute pressure, cmHg, A is the membrane area available for gas transport, cm2, R is the gas constant, 0.278 cmHg cm3 cm3(STP) K1, T is absolute temperature, K, dpdt1 and dpdt1 are the steady ss
leak
state rates of pressure rise in the downstream volume at fixed
3. Results and discussion 3.1. Membrane characterization
3.1.2. Thermal analysis TGA curves of ImGO, Pristine PEBAX, and MMMs are presented in Fig. 5. As shown in Fig. 5(a), the weight loss of ImGO under 100 °C is about 7 wt.%, which could be attributed to the evaporation of physically adsorbed water. The weight loss of about 27 and 7 wt.% at 100–200 and 200–260 °C can be ascribed to the
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Fig. 3. Surface SEM images of Pristine PEBAX ((a) 1300, (b) 20,000) and ImGO 0.8 membrane ((c) 1300, (d) 20,000).
Fig. 4. Cross-section SEM images of Pristine PEBAX ((a) 20,000, (b) 40,000) and ImGO 0.8 ((c) 20,000, (d) 40,000).
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decomposition of the hydroxyl and imidazole functional groups in ImGO, respectively. The weight loss details for pristine PEBAX membrane and ImGO/PEBAX MMMs almost exhibit the same tendency, as shown in Fig. 5(b). The pyrolysis occurs between 300 and 450 °C, which can be assigned to the decomposition reaction of the main polymer chain [16]. Fig. 6 shows DSC curves (second cooling cycle) of pristine PEBAX membrane and ImGO/PEBAX MMMs. Fig. 6(a) presents two exothermic peaks at 13.4 and 204.6 °C for the membranes, which are assigned to the solidification temperature of the polyether block and polyamide block, respectively. The pristine PEBAX shows a Tg of 51.7 °C that agrees well with literatures [18,21,37]. The Tg of ImGO/PEBAX MMMs are higher than that of pristine PEBAX membrane, which increase gradually from 51.0 to 48.6 °C with the increase of ImGO loading from 0.2 to 1.0 wt.%, as shown in Fig. 6(b). The mobility of polymer chain is restricted by the presence of filler in PEBAX matrix. Meanwhile, there is a rigidified interface between polymer and filler [28,37]. Therefore, the presence of ImGO in PEBAX matrix results in the change of Tg as mentioned above. 3.1.3. XPS analysis XPS measurement was utilized to investigate the change of element (C, N and O) contents. The complete survey spectra of pristine PEBAX membrane and ImGO/PEBAX MMMs are displayed in Fig. 7 (a) and (b), in which the atom ratio of C, N and O is listed. Compared with pristine PEBAX membrane, the content of O element in ImGO 0.8 increases by 11.6% due to addition of ImGO. Meanwhile, the content of N element decreased by 36.1% compared to pristine PEBAX. There are plenty of hydroxyl and carbonyl in graphene oxide-carboimidazole. Herein, the presence of ImGO in polymer results in the change of element contents mentioned above. Fig 7(c) and (d) show the high resolution spectrum of C1s of pristine PEBAX membrane and MMMs, which were curve fitted by three peaks. The addition of ImGO increases the electron cloud density of C atoms [28]. Herein, the C1s component peaks shift from 286.2 eV (C@O), 283.7 eV (CAO) and 282.0 eV (CAN) to 284.0 eV, 282.6 eV and 281.2 eV respectively. 3.1.4. Mechanical properties The mechanical properties of the ImGO/PEBAX mixed matrix membranes were evaluated by stress-stain tests. It is expected that the mechanical property of the MMMs would be enhanced significantly by the imidazole functionalized graphene oxide sheets. The tensile strength and elongation at break of membranes as a
175
function of ImGO loading are shown in Fig. 8. The tensile strength of the MMMs is significantly increased compared to that of pristine PEBAX membrane because of the strong interfacial adhesion, which is caused by the H-bonding interactions formed between graphene and the polymer matrix [43]. For instance, with ImGO loading of 0.8 wt.%, the tensile strength increases from 8.47 to 13.53 MPa. Meanwhile, the elongation at break of membranes is enhanced because of the addition of ImGO. Elongation at break increases from 362.78% (pristine PEBAX) to 387.41% (ImGO 0.2) and then to 451.34% (ImGO 0.5). There is almost no significant change of elongation at break when ImGO loading is beyond 0.5 wt.%, which suggests that the cooperation of ImGO and PEBAX matrix is strong enough to enhance the mechanical properties of MMMs.
3.2. Gas separation performance 3.2.1. Effect of ImGO loading on CO2 separation performance As shown in Fig. 9(a), the MMMs have the highest permeability for CO2 and lower permeation of other gases, with a gas permeability sequence of CO2 > H2 > CH4 > N2. For CO2, the strongly sorbing gases, plasticization and solubility contribute to the higher permeability. The kinetic diameter of CO2 (0.33 nm) is lower than those of N2 (0.36 nm) and CH4 (0.38 nm), so CO2 is easily to diffuse in the ImGO/PEBAX MMMs than N2 and CH4. Furthermore, there are Lewis acid-Lewis base interactions between the negatively charged oxygen atoms of CO2 and N-containing organic heterocyclic molecules such as imidazole, pyridine and tetrazole [44,45]. So the interactions between CO2 and imidazole groups in ImGO play an important role in facilitating the CO2 molecules transport in ImGO/PEBAX MMMs. H2 permeability is higher due to the least kinetic diameter among the gases. CO2 permeabilities except N2 increase gradually with the increase of ImGO from 0.2 to 0.5 wt.%, but those decrease when ImGO loading exceeds 0.5 wt.%. For CO2, there are two opposite effects on gas permeability and selectivity caused by addition of ImGO nanosheets filler. On one hand, the interactions between the negatively charged oxygen atoms of CO2 and imidazole groups in ImGO facilitate CO2 transport in ImGO/PEBAX membrane. On the other hand, the kind of graphene material, such as graphene, graphene oxide and ImGO, are believed to impermeable. So the gas molecules can go through hardly [33]. Under the two opposite effects, a modest enhancement in CO2 permeability is observed at low ImGO loading. And CO2 permeability decreases when ImGO loading is over 0.5 wt.%. This phenomenon is similar to a recent
Fig. 5. TGA curves of ImGO (a) and membranes (b).
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Fig. 6. DSC curves of the Pristine PEBAX and ImGO/PEBAX MMMs: (a) whole temperature range and (b) low temperature zone.
Fig. 7. XPS spectra of Pristine PEBAX (a: complete survey spectrum, c: C1 s spectrum) and ImGO 0.8 (b: complete survey spectrum, d: C1 s spectrum).
paper by Althumayri et al. [46]. For the kind of impermeable nanosheets fillers, an enhancement in gas permeability is observed at very low graphene loading. And at high graphene loading, gas permeability decreases. However, there is no interactions between N2 and ImGO nanosheets filler, so N2 transport has been blocked in MMMs due to presence of ImGO in MMMs
The selectivities for CO2/N2, CO2/CH4 and CO2/H2 gradually increase with the increase in filler content, as displayed in Fig. 9 (b). For instance, CO2/N2 selectivity increase by 46.0% from 65.5 (Pristine PEBAX) to 95.6 (ImGO 1.0). ImGO/PEBAX MMMs doped with 0.8 wt.% ImGO demonstrate the best separation performance with a CO2 permeability of 64.0 Barrer combined with the CO2/N2,
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Fig. 8. Effect of ImGO loading on mechanic properties of ImGO/PEBAX MMMs.
CO2/CH4 and CO2/H2 selectivities of 90.3, 25.1 and 14.1 respectively at 25 °C and 0.4 MPa. Therefore, the effects of feed pressure and temperature on the gas separation of ImGO 0.8 are discussed in the following part. 3.2.2. Effect of feed pressure on CO2 separation performance Fig. 10 shows the gas permeability and selectivity of the ImGO 0.8 as a function of feed pressure. The permeability of H2, N2 and CH4 are almost independent of the feed pressure, whereas that of CO2 raises obviously as the feed pressure increases, as shown in Fig. 10(a). CO2 with high critical temperature is easily condensed and thus, has higher solubility in rubbery polymer membrane as feed pressure increases. Therefore, the selectivities are 83.56– 105.47, 22.79–29.32 and 12.14–15.07 for CO2/N2, CO2/CH4 and CO2/H2, respectively in the feed pressure range of 0.2–0.8 MPa as shown in Fig. 10(b). 3.2.3. Effect of feed temperature on CO2 separation performance The gas permeability increases with feed temperature as shown in Fig. 11, because the kinetic energy of gas molecular and mobility of polymer chains increase with temperature [47,48]. In gas membrane separation process, the effect of feed temperature on permeability (P) can be described by the Arrhenius relationship [49]:
Ep P ¼ P0 exp RT
ð5Þ
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where P0 is the pre-exponential factor, R is the gas constant, 8.314 J mol1 K1, T is the absolute temperature, K, Ep is the apparent activation energy for the gas permeation process, kJ mol1. Based on the Arrhenius equation, the Ep for H2, N2, CH4 and CO2 is shown in Table 1. It is seen that the temperature greatly influences the gas permeability in an Arrhenius’ type plot shown in Fig. 11(a). N2 permeability changes the most evidently with temperature because the N2 has the highest Ep based on the Arrhenius’ equation. The selectivities for gas pairs tend to decrease significantly when temperature increases as presented in Fig. 11(b). CO2 solubility decays more significant than the H2, N2 and CH4 solubilities with increasing temperature in the rubbery polymer. Meanwhile, temperature plays more important role in the increase of the diffusivity of H2, N2 and CH4 than that of CO2. So the CO2/N2, CO2/H2 and CO2/CH4 selectivities decrease significantly. This suggests that it is effective to separate CO2 from other gases at lower temperature using ImGO/PEBAX MMMs. 3.3. Stability of ImGO/PEBAX MMMs To make the ImGO/PEBAX MMMs for industrial applications, membranes stability is important. Fig. 12 shows the CO2/N2 separation performance of the ImGO 0.8 membrane as a function of operating time at 0.4 MPa and 25 °C. The ImGO 0.8 membrane does not present any degradation tendency in gas permeability and selectivity during the entire period of long-term test (50 h), which implies that the ImGO/PEBAX MMMs are potential for CO2 capture in industrial applications. 3.4. Upper bound for CO2/N2 separation The permselectivities of the ImGO/PEBAXMMMs are compared with the Robeson’s upper bound (2008) [6] as presented in Fig. 13. The ImGO/PEBAX MMMs shows good improvements in CO2 permeability and CO2/N2 selectivity compared to the pristine PEBAX membrane. The behavior of the MMMs becomes much closer to the Robeson’s upper bound with the increase of ImGO loading. ImGO/PEBAX mixed matrix membrane with ImGO loading of 0.8 (ImGO 0.8) shows the best CO2/N2 separation performance, overcoming the Robeson’s upper bound (2008), when feed pressure exceeds 0.4 MPa. The reported dry PEBAX 1657 mixed matrix membranes separation performance for CO2 separation compared with our work is summarized in Table 2.
Fig. 9. Effect of ImGO loading on (a) gas permeability and (b) selectivity of ImGO/PEBAX MMMs at 0.4 MPa and 25 °C.
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Fig. 10. Effect of feed pressure on (a) gas permeability and (b) selectivity of ImGO 0.8 at 25 °C.
Fig. 11. Effect of operating temperature on (a) gas permeability and (b) selectivity of ImGO 0.8 at 0.4 MPa.
Table 1 Apparent activation energy (Ep) for permeation process calculated for ImGO 0.8 at 0.4 MPa. Penetrant
CO2
H2
CH4
N2
Ep (kJ/mol)
15.52
29.57
31.91
45.43
4. Conclusion A novel mixed matrix membrane was prepared by impregnating imidazole functionalized graphene oxide into PEBAX matrix to enhance for CO2 capture. The CO2 separation performance has been improved by the addition of ImGO. The selectivity of MMMs for CO2/N2 increases by 46.0% compared to pristine PBBAX membrane. Mixed matrix membrane doped with 0.8 wt.% ImGO exhibits the best performance with a CO2 permeability of 64.0 Barrer combined with CO2/N2 selectivity of 90.3 at 25 °C and 0.4 MPa. CO2 permeability significant increases because of its higher solubility in polymer matrix and plasticization with the feed pressure increase. Apparent activation energy of the N2 permeation process in ImGO/PEBAX MMMs is much higher than that of CO2. So it is effective to separation CO2 from N2 at lower temperature for
Fig. 12. Long-term CO2/N2 separation performance of ImGO 0.8 membrane at 0.4 MPa and 25 °C.
MMMs. The melting and decomposition temperature are almost the same when the addition of ImGO compared with pristine polymer. Tg of mixed matrix membranes are increased gradually
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Mr. Feng Bai at Dalian University of Technology for their helpful comments and discussions, as well as their effort of editing language cautiously. References
Fig. 13. Robeson’s plot from the CO2/N2 separation with the upper bound [6] and the data for the membranes prepared in this work.
Table 2 Comparison of CO2 separation performance between ImGO/PEBAX MMMs and other PEBAX MH1657 MMMs from literatures. P CO2 =P N2
Membrane
P CO2 / Barrer
PEBAX/CANs PEBAX/GO PEBAX/CaLS PEBAX/Functional GO PEBAX/PEG PEG-PEBAX PEBAX/MWNT PEBAX/GO PEBAX/SWNT PEBAX/POSS PEBAX/RTIL PEBAX/ATP
119 108 140 143
69 48.5 62 63
151 151 262.15 100 102 152 300 77 104 196.7 179 102.8 97
47 58.5 90.9 73 51 40 52 81 62 52.3 49.9 54
113.7
39.5
64.0 69.5 76.2
90.3 96.9 105.5
PEBAX-PEG-MWNT
PEBAX/4A zeolite
PEBAX/ImGO
Test condition
Ref.
2 bar/25 °C 0.7 MPa/35 °C 3 bar/25 °C 2 bar/30 °C
[26] [36] [27] [37]
1 bar/30 °C 0.3 bar/30 °C 1 MPa/25 °C 0.3 MPa/25 °C 33 psig/21 °C 1 bar/30 °C 1 bar/25 °C 4 bar/35 °C 10 bar/35 °C 2 bar/25 °C 2 bar/25 °C 2 bar/25 °C 5 kg cm2/ 25 °C 5 kg cm2/ 25 °C 0.4 MPa/25 °C 0.6 MPa/25 °C 0.8 MPa/25 °C
[20] [21] [23] [28] [25] [18] [22] [16] [24]
[19]
This work
because the polymer chain mobility has been restricted by the presence of ImGO and a rigidified interface has been generated between polymer and filler. The mechanical properties have been significantly enhanced by the imidazole functionalized graphene oxide sheet as expected because of the presence of H-bonding between graphene and polymer matrix. Having distinct improvement of CO2 separation performance, the ImGO/PEBAX MMMs indicates promising applications in CO2 capture processes. Acknowledgments We appreciate the financial support from the Program for Changjiang Scholars and the National Science Fund for Distinguished Young Scholars of China (21125628), Major National Scientific Instrument Development Project (21527812), and the National Science Fund of China (21236006). The authors are also thankful to Dr. Mo Li at Swiss Federal Institute of Technology Zurich and
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Imidazole functionalized graphene oxide/PEBAX mixed matrix membranes for efficient CO2 capture Yan Dai a, Xuehua Ruan b, Zhijun Yan a, Kai Yang a, Miao Yu a, Hao Li a, Wei Zhao c, Gaohong He a,b,⇑ a
State Key Laboratory of Fine Chemicals, R&D Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian 116023, China School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China c School of Petrochemical Engineering, Shenyang University of Technology, Liaoyang 111003, China b
a r t i c l e
i n f o
Article history: Received 24 January 2016 Received in revised form 21 April 2016 Accepted 22 April 2016 Available online 23 April 2016 Keywords: CO2 Poly(ether-b-amide) MMMs Gas separation Functionalized graphene oxide
a b s t r a c t Mixed matrix membranes (MMMs) were composed of imidazole functionalized graphene oxide (ImGO), a CO2-philic nano-sheet inorganic material, and poly(ether-b-amide) (PEBAX) for CO2 capture. MMM doped with 0.8 wt.% ImGO exhibits the best CO2 separation performance, which shows the CO2/N2 selectivity up to 105.5 combined with CO2 permeability of 76.2 Barrer (1 Barrer = 1010 cm3(STP) cm cm2 s1 cmHg1), surpassing the Robeson Upper Bound of 2008. The selectivity of MMM for CO2/N2 increases by 46.0% compared to the Pristine PEBAX due to the interaction between CO2 and imidazole groups. With the increase of feed pressure, CO2 permeability increases significantly because of its higher solubility in polymer matrix and plasticization. It is effective to separation CO2 from N2 at lower temperature for MMMs because the apparent activation energy of the N2 permeation process in ImGO/PEBAX MMMs is much higher than that of CO2. Tg of MMMs are increased gradually because the polymer chain mobility is restricted by the presence of ImGO and a rigidified interface generates between polymer and filler. The mechanical properties have been significantly enhanced by the ImGO sheet as expected because of the presence of H-bonding. Having distinct improvement of CO2 separation performance, the ImGO/PEBAX MMMs indicates promising applications in CO2 capture processes. Ó 2016 Published by Elsevier B.V.
1. Introduction CO2 capture and storage is gaining tremendous interests for energy, economic and environmental perspectives [1,2]. Compared with conventional processes, membrane technology has presented excellent potential in CO2 capture due to many advantages, such as no phase change, low energy consumption, ease of scale up and small footprint [3,4]. Polymers are the most frequent membrane materials that successfully applied in industry. Therefore, there is a pressing demand to develop polymeric materials with high gas separation performance. However, polymeric membrane is always restricted by a trade-off between permeability and selectivity [5,6]. So modifications of polymeric membrane with a number of techniques including crosslinking [7], blending [8] and thermal annealing [9] are usually employed to solve the problem. Mixed matrix membranes (MMMs) comprised of a polymer as the continuous phase and an inorganic filler as the dispersed phase can offer one ⇑ Corresponding author at: State Key Laboratory of Fine Chemicals, R&D Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian 116023, China. E-mail address: [email protected] (G. He). http://dx.doi.org/10.1016/j.seppur.2016.04.038 1383-5866/Ó 2016 Published by Elsevier B.V.
of the most potentially effective solutions [10,11]. The incorporation of fillers with porous structure, such as carbon molecular sieves [12,13], zeolites [14] and metal organic frameworks (MOFs) [15] in polymeric membranes can combine the size selectivity of inorganic nanomaterials with the permeability, mechanical stability and ease of processibility of polymer [16]. Herein, MMMs have received tremendous attention over the last decade. Poly(ether-b-amide) (PEBAX), a commercial rubbery copolymer material with high gas permeability and selectivity, is a good candidate for membrane separation of CO2 from N2, H2 and CH4 [17– 20]. The chemical structure of PEBAX MH 1657 is shown in Fig. 1, in which PA is polyimide 6 block and PE is poly(ethylene oxide) block. PA segment in PEBAX supplies high mechanical strength to the membrane due to its crystallinity, while PE segment provides high CO2 permeability because of the high chain mobility and affinity with the polar molecules [16]. Some organic materials have been selected as modifier to improve the gas separation performance of PEBAX. Low molecular weight poly(ethylene glycol) (PEG) was used to improve the CO2 separation performance of PEBAX membrane whose CO2 permeability was increased two fold and the CO2/H2 selectivity was enhanced from 9 to almost 11 [21]. A high CO2 separation performance MMMs were prepared
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Group Co., Ltd. (China). Deionized water (DI water) was used in all the experiments. Gases (H2, N2, CH4 and CO2) of research grade were supplied by Dalian Institute of Chemistry and Physics (China). All chemicals were used as received. 2.2. Membrane preparation
Fig. 1. Chemical structure of PEBAX MH 1657 (the weight ratio of PA to PE (x:y) is about 40:60).
by incorporating room temperature ionic liquid (RTIL) into PEBAX showed a 300 Barrer of CO2 permeability and 40 CO2/N2 selectivity [22]. Recently, inorganic filling materials such as multiwall nanotube (MWNT), single wall nanotube (SWNT), 4A zeolite, carboxylic acid nanogels (CANs), and calcium lignosulfonate (CaLS) have received widespread attention due to their size selectivity [19,23–27]. Graphene oxide (GO) is a well-known two-dimensional material that presents excellent mechanical and thermal properties due to its unique one-atom-thick structure [28]. It is usually applied to the field of energy and environment, for example, supercapacitors and batteries [29–31]. Recent years, GO showed the significant potential in membrane for small-molecule separation [32]. It is found that the submicrometer-thick membranes prepared from GO can prevent completely from liquids, vapors and gases, including helium, but allow water to permeate [33]. In addition, ultrathin GO (<5 nm) membranes with high CO2 permeability and selectivity (CO2/N2, CO2/CH4 and CO2/H2) has been fabricated by spin-casting onto a porous polymeric substrate [34]. In the GO membranes, gas transfer channel can be controlled via different stacking methods, so the gas diffusion selectivity can be controlled [35]. GO/ PEBAX MMMs with 3.85 vol.% GO loading were prepared by solution casting method, but the membrane displayed lower CO2 permeability compared with pristine PEBAX membrane, while the mechanical properties of the membranes had been improved [36]. MMMs separation performance also can be enhanced by functional groups for CO2 such as ethylene oxide (EO) and amine carrier. Wu [37] reported the MMMs doped with GO nanosheets functionalized by PEG (contained EO groups) and polyethylenimine (PEI, contained amine carrier). The MMMs with potential application in CO2 capture showed a good performance, surpassing the upper bound limit of the Robeson’s work (2008) [6] under humidified state. In this study, we fabricated MMMs with PEBAX as matrix and imidazole functionalized graphene oxide (ImGO) as CO2-philic filler. The objective of this study was to develop a high CO2 permeability and selectivity of ImGO/PEBAX MMMs. The properties of the MMMs including morphology, thermal stability, crystallization, mechanical properties and content of elements were investigated. The effect of ImGO loading, feed pressure and temperature on CO2 permeability and selectivity was studied. 2. Experimental 2.1. Materials PEBAX MH1657 was purchased from Arkema (France). ImGO (>99 wt.%) was obtained from Nanjing XF Nanomaterials Co., Ltd. (China). The size of ImGO nanosheets was at the range of 0.5–1 lm. The loading of carboimidazole groups in ImGO was 7.2 wt.%. The chemical structure of ImGO was shown in Fig. 2. Ethanol (99.5 wt.%, anhydrous) was purchased from Sinopharm
3.0 wt.% PEBAX solution was prepared by dissolving polymer in the mixture of anhydrous ethanol/DI water (70/303.0 wt.% PEBAX solution wt.%) under reflux and stirring at 85 °C for 4 h [19,38–40]. The solution was filtered through a 5 lm polytetrafluoroethylene filter (MILLIPORE, USA). Certain amount of ImGO was added into the PEBAX solution and kept stirring overnight for good dispersion. Bubbles were removed by ultrasonication for 5 min with an ultrasonic bath (AS5150B, Tianjin Automatic Science Instrument Co., LTD, China). The PEBAX or ImGO/PEBAX solution was cast on a flat TeflonÒ plate with a glass ring attached. Membrane thickness (30– 50 lm) was controlled by the amount of solution added to the casting ring. Pristine PEBAX or ImGO/PEBAX membranes were obtained by drying the casting solution at 40 °C for 24 h. In order to remove residual solvent, the as-prepared membranes were further dried in a vacuum oven at 60 °C for 72 h. The ImGO loading and ImGO volume fraction (uf) were defined as Eqs. (1) and (2), respectively.
ImGO loading ðwt:%Þ ¼ /f ¼
mImGO 100% mImGO þ mPEBAX
mImGO =qImGO mImGO =qImGO þ mPEBAX =qPEBAX
ð1Þ ð2Þ
where mImGO and mPEBAX were the masses of ImGO and PEBAX, respectively; qImGO and qPEBAX were the densities of ImGO and PEBAX, respectively. The density of bulk ImGO (2.28 cm3 g1) was assumed. In this work, pristine PEBAX membrane and MMMs with different ImGO loading of 0.2, 0.5, 0.8 and 1.0 wt.% were prepared. According to the ImGO loading, membrane samples were named Pristine PEBAX, ImGO 0.2, ImGO 0.5, ImGO 0.8 and ImGO 1.0. 2.3. Membrane characterization 2.3.1. Scanning electron microscope (SEM) The surface and cross-section morphology of the membrane was observed by field-emission scanning electron microscope (FEI Nava NanoSEM 450). Membrane samples were coated with a thin gold layer for better conductivity by sputtering (20 mA, 30 s) prior to observation. 2.3.2. Thermogravimetric analysis (TGA) TGA (Q50, TA Instruments, USA) was used to test the thermal stability of the ImGO and ImGO/PEBAX mixed matrix membranes. Samples were heated from 25 to 800 °C at a heating rate of 10 °C min1 under N2 atmosphere (50 ml min1 of flow rate). Before the test, the samples were dried in a vacuum oven for 24 h at 60 °C for moisture removal. 2.3.3. Differential scanning calorimetry (DSC) Glass transition temperatures (Tg) of the membranes were analyzed by DSC (Q20, TA Instruments, USA) measurements which were carried out from 80 to 250 °C in N2 atmosphere (50 ml min1 of flow rate). 2.3.4. X-ray photoelectron spectroscopy (XPS) In order to analyze the element content of membranes, XPS patterns (Thermo ESCALAB 250, USA) were recorded using monochromatized Al Ka radiation (1486.6 eV).
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Fig. 2. Schematic structure of ImGO.
2.3.5. Mechanical properties The tensile strength and elongation at break of membranes were measured by the stretching tester (SANSCMT8102, Xinsansi, China). The size of all membrane samples are 50 5 mm. The tensile speed was 10 mm min1. The mechanical properties presented are average values of three samples.
upstream pressure and under vacuum, cmHg s1. ai=j is the selectivity of gas pair i/j.
2.4. Gas permeation experiments
3.1.1. Membrane morphology The surface morphology of Pristine PEBAX and ImGO 0.8 membranes are shown in Fig. 3. Compared with the Pristine PEBAX membrane (Fig. 3(a) and (b)), the dispersed ImGO can be found in PEBAX matrix clearly in Fig. 3(c) and (d). The surface of ImGO 0.8 membrane is smooth and there is no wrinkle caused by graphene oxide-carboimidazole as reported [37]. But, slightly aggregation of ImGO can be observed in the surface of mixed matrix membrane. The cross-section morphology of Pristine PEBAX and ImGO 0.8 are shown in Fig. 4. The ImGO nanosheets are distributed uniformly in polymer matrix. Compared with pristine PEBAX membrane, ImGO 0.8 presents a slightly wrinkled cross-section caused by the interface void between ImGO nanosheets and polymer matrix.
Gas permeability coefficients of N2, CH4, H2, and CO2 were measured using a constant volume, variable pressure method [41,42]. The above gases are listed in order of testing. The gas permeability coefficients were measured at feed pressure and temperature ranging from 0.2 to 0.8 MPa and 25–80 °C, respectively. The gas permeability coefficients and selectivities were calculated with the following equations:
P¼
ai=j
V dl p2 ART Pi ¼ Pj
dp1 dp1 dt ss dt leak
ð3Þ ð4Þ
where P is the gas permeability coefficient, Barrer (1 Barrer = 1010 cm3(STP) cm cm2 s1 cmHg1) Vd is the downstream volume, cm3, l is the membrane thickness, cm, p2 is the upstream absolute pressure, cmHg, A is the membrane area available for gas transport, cm2, R is the gas constant, 0.278 cmHg cm3 cm3(STP) K1, T is absolute temperature, K, dpdt1 and dpdt1 are the steady ss
leak
state rates of pressure rise in the downstream volume at fixed
3. Results and discussion 3.1. Membrane characterization
3.1.2. Thermal analysis TGA curves of ImGO, Pristine PEBAX, and MMMs are presented in Fig. 5. As shown in Fig. 5(a), the weight loss of ImGO under 100 °C is about 7 wt.%, which could be attributed to the evaporation of physically adsorbed water. The weight loss of about 27 and 7 wt.% at 100–200 and 200–260 °C can be ascribed to the
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Fig. 3. Surface SEM images of Pristine PEBAX ((a) 1300, (b) 20,000) and ImGO 0.8 membrane ((c) 1300, (d) 20,000).
Fig. 4. Cross-section SEM images of Pristine PEBAX ((a) 20,000, (b) 40,000) and ImGO 0.8 ((c) 20,000, (d) 40,000).
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decomposition of the hydroxyl and imidazole functional groups in ImGO, respectively. The weight loss details for pristine PEBAX membrane and ImGO/PEBAX MMMs almost exhibit the same tendency, as shown in Fig. 5(b). The pyrolysis occurs between 300 and 450 °C, which can be assigned to the decomposition reaction of the main polymer chain [16]. Fig. 6 shows DSC curves (second cooling cycle) of pristine PEBAX membrane and ImGO/PEBAX MMMs. Fig. 6(a) presents two exothermic peaks at 13.4 and 204.6 °C for the membranes, which are assigned to the solidification temperature of the polyether block and polyamide block, respectively. The pristine PEBAX shows a Tg of 51.7 °C that agrees well with literatures [18,21,37]. The Tg of ImGO/PEBAX MMMs are higher than that of pristine PEBAX membrane, which increase gradually from 51.0 to 48.6 °C with the increase of ImGO loading from 0.2 to 1.0 wt.%, as shown in Fig. 6(b). The mobility of polymer chain is restricted by the presence of filler in PEBAX matrix. Meanwhile, there is a rigidified interface between polymer and filler [28,37]. Therefore, the presence of ImGO in PEBAX matrix results in the change of Tg as mentioned above. 3.1.3. XPS analysis XPS measurement was utilized to investigate the change of element (C, N and O) contents. The complete survey spectra of pristine PEBAX membrane and ImGO/PEBAX MMMs are displayed in Fig. 7 (a) and (b), in which the atom ratio of C, N and O is listed. Compared with pristine PEBAX membrane, the content of O element in ImGO 0.8 increases by 11.6% due to addition of ImGO. Meanwhile, the content of N element decreased by 36.1% compared to pristine PEBAX. There are plenty of hydroxyl and carbonyl in graphene oxide-carboimidazole. Herein, the presence of ImGO in polymer results in the change of element contents mentioned above. Fig 7(c) and (d) show the high resolution spectrum of C1s of pristine PEBAX membrane and MMMs, which were curve fitted by three peaks. The addition of ImGO increases the electron cloud density of C atoms [28]. Herein, the C1s component peaks shift from 286.2 eV (C@O), 283.7 eV (CAO) and 282.0 eV (CAN) to 284.0 eV, 282.6 eV and 281.2 eV respectively. 3.1.4. Mechanical properties The mechanical properties of the ImGO/PEBAX mixed matrix membranes were evaluated by stress-stain tests. It is expected that the mechanical property of the MMMs would be enhanced significantly by the imidazole functionalized graphene oxide sheets. The tensile strength and elongation at break of membranes as a
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function of ImGO loading are shown in Fig. 8. The tensile strength of the MMMs is significantly increased compared to that of pristine PEBAX membrane because of the strong interfacial adhesion, which is caused by the H-bonding interactions formed between graphene and the polymer matrix [43]. For instance, with ImGO loading of 0.8 wt.%, the tensile strength increases from 8.47 to 13.53 MPa. Meanwhile, the elongation at break of membranes is enhanced because of the addition of ImGO. Elongation at break increases from 362.78% (pristine PEBAX) to 387.41% (ImGO 0.2) and then to 451.34% (ImGO 0.5). There is almost no significant change of elongation at break when ImGO loading is beyond 0.5 wt.%, which suggests that the cooperation of ImGO and PEBAX matrix is strong enough to enhance the mechanical properties of MMMs.
3.2. Gas separation performance 3.2.1. Effect of ImGO loading on CO2 separation performance As shown in Fig. 9(a), the MMMs have the highest permeability for CO2 and lower permeation of other gases, with a gas permeability sequence of CO2 > H2 > CH4 > N2. For CO2, the strongly sorbing gases, plasticization and solubility contribute to the higher permeability. The kinetic diameter of CO2 (0.33 nm) is lower than those of N2 (0.36 nm) and CH4 (0.38 nm), so CO2 is easily to diffuse in the ImGO/PEBAX MMMs than N2 and CH4. Furthermore, there are Lewis acid-Lewis base interactions between the negatively charged oxygen atoms of CO2 and N-containing organic heterocyclic molecules such as imidazole, pyridine and tetrazole [44,45]. So the interactions between CO2 and imidazole groups in ImGO play an important role in facilitating the CO2 molecules transport in ImGO/PEBAX MMMs. H2 permeability is higher due to the least kinetic diameter among the gases. CO2 permeabilities except N2 increase gradually with the increase of ImGO from 0.2 to 0.5 wt.%, but those decrease when ImGO loading exceeds 0.5 wt.%. For CO2, there are two opposite effects on gas permeability and selectivity caused by addition of ImGO nanosheets filler. On one hand, the interactions between the negatively charged oxygen atoms of CO2 and imidazole groups in ImGO facilitate CO2 transport in ImGO/PEBAX membrane. On the other hand, the kind of graphene material, such as graphene, graphene oxide and ImGO, are believed to impermeable. So the gas molecules can go through hardly [33]. Under the two opposite effects, a modest enhancement in CO2 permeability is observed at low ImGO loading. And CO2 permeability decreases when ImGO loading is over 0.5 wt.%. This phenomenon is similar to a recent
Fig. 5. TGA curves of ImGO (a) and membranes (b).
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Fig. 6. DSC curves of the Pristine PEBAX and ImGO/PEBAX MMMs: (a) whole temperature range and (b) low temperature zone.
Fig. 7. XPS spectra of Pristine PEBAX (a: complete survey spectrum, c: C1 s spectrum) and ImGO 0.8 (b: complete survey spectrum, d: C1 s spectrum).
paper by Althumayri et al. [46]. For the kind of impermeable nanosheets fillers, an enhancement in gas permeability is observed at very low graphene loading. And at high graphene loading, gas permeability decreases. However, there is no interactions between N2 and ImGO nanosheets filler, so N2 transport has been blocked in MMMs due to presence of ImGO in MMMs
The selectivities for CO2/N2, CO2/CH4 and CO2/H2 gradually increase with the increase in filler content, as displayed in Fig. 9 (b). For instance, CO2/N2 selectivity increase by 46.0% from 65.5 (Pristine PEBAX) to 95.6 (ImGO 1.0). ImGO/PEBAX MMMs doped with 0.8 wt.% ImGO demonstrate the best separation performance with a CO2 permeability of 64.0 Barrer combined with the CO2/N2,
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Fig. 8. Effect of ImGO loading on mechanic properties of ImGO/PEBAX MMMs.
CO2/CH4 and CO2/H2 selectivities of 90.3, 25.1 and 14.1 respectively at 25 °C and 0.4 MPa. Therefore, the effects of feed pressure and temperature on the gas separation of ImGO 0.8 are discussed in the following part. 3.2.2. Effect of feed pressure on CO2 separation performance Fig. 10 shows the gas permeability and selectivity of the ImGO 0.8 as a function of feed pressure. The permeability of H2, N2 and CH4 are almost independent of the feed pressure, whereas that of CO2 raises obviously as the feed pressure increases, as shown in Fig. 10(a). CO2 with high critical temperature is easily condensed and thus, has higher solubility in rubbery polymer membrane as feed pressure increases. Therefore, the selectivities are 83.56– 105.47, 22.79–29.32 and 12.14–15.07 for CO2/N2, CO2/CH4 and CO2/H2, respectively in the feed pressure range of 0.2–0.8 MPa as shown in Fig. 10(b). 3.2.3. Effect of feed temperature on CO2 separation performance The gas permeability increases with feed temperature as shown in Fig. 11, because the kinetic energy of gas molecular and mobility of polymer chains increase with temperature [47,48]. In gas membrane separation process, the effect of feed temperature on permeability (P) can be described by the Arrhenius relationship [49]:
Ep P ¼ P0 exp RT
ð5Þ
177
where P0 is the pre-exponential factor, R is the gas constant, 8.314 J mol1 K1, T is the absolute temperature, K, Ep is the apparent activation energy for the gas permeation process, kJ mol1. Based on the Arrhenius equation, the Ep for H2, N2, CH4 and CO2 is shown in Table 1. It is seen that the temperature greatly influences the gas permeability in an Arrhenius’ type plot shown in Fig. 11(a). N2 permeability changes the most evidently with temperature because the N2 has the highest Ep based on the Arrhenius’ equation. The selectivities for gas pairs tend to decrease significantly when temperature increases as presented in Fig. 11(b). CO2 solubility decays more significant than the H2, N2 and CH4 solubilities with increasing temperature in the rubbery polymer. Meanwhile, temperature plays more important role in the increase of the diffusivity of H2, N2 and CH4 than that of CO2. So the CO2/N2, CO2/H2 and CO2/CH4 selectivities decrease significantly. This suggests that it is effective to separate CO2 from other gases at lower temperature using ImGO/PEBAX MMMs. 3.3. Stability of ImGO/PEBAX MMMs To make the ImGO/PEBAX MMMs for industrial applications, membranes stability is important. Fig. 12 shows the CO2/N2 separation performance of the ImGO 0.8 membrane as a function of operating time at 0.4 MPa and 25 °C. The ImGO 0.8 membrane does not present any degradation tendency in gas permeability and selectivity during the entire period of long-term test (50 h), which implies that the ImGO/PEBAX MMMs are potential for CO2 capture in industrial applications. 3.4. Upper bound for CO2/N2 separation The permselectivities of the ImGO/PEBAXMMMs are compared with the Robeson’s upper bound (2008) [6] as presented in Fig. 13. The ImGO/PEBAX MMMs shows good improvements in CO2 permeability and CO2/N2 selectivity compared to the pristine PEBAX membrane. The behavior of the MMMs becomes much closer to the Robeson’s upper bound with the increase of ImGO loading. ImGO/PEBAX mixed matrix membrane with ImGO loading of 0.8 (ImGO 0.8) shows the best CO2/N2 separation performance, overcoming the Robeson’s upper bound (2008), when feed pressure exceeds 0.4 MPa. The reported dry PEBAX 1657 mixed matrix membranes separation performance for CO2 separation compared with our work is summarized in Table 2.
Fig. 9. Effect of ImGO loading on (a) gas permeability and (b) selectivity of ImGO/PEBAX MMMs at 0.4 MPa and 25 °C.
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Fig. 10. Effect of feed pressure on (a) gas permeability and (b) selectivity of ImGO 0.8 at 25 °C.
Fig. 11. Effect of operating temperature on (a) gas permeability and (b) selectivity of ImGO 0.8 at 0.4 MPa.
Table 1 Apparent activation energy (Ep) for permeation process calculated for ImGO 0.8 at 0.4 MPa. Penetrant
CO2
H2
CH4
N2
Ep (kJ/mol)
15.52
29.57
31.91
45.43
4. Conclusion A novel mixed matrix membrane was prepared by impregnating imidazole functionalized graphene oxide into PEBAX matrix to enhance for CO2 capture. The CO2 separation performance has been improved by the addition of ImGO. The selectivity of MMMs for CO2/N2 increases by 46.0% compared to pristine PBBAX membrane. Mixed matrix membrane doped with 0.8 wt.% ImGO exhibits the best performance with a CO2 permeability of 64.0 Barrer combined with CO2/N2 selectivity of 90.3 at 25 °C and 0.4 MPa. CO2 permeability significant increases because of its higher solubility in polymer matrix and plasticization with the feed pressure increase. Apparent activation energy of the N2 permeation process in ImGO/PEBAX MMMs is much higher than that of CO2. So it is effective to separation CO2 from N2 at lower temperature for
Fig. 12. Long-term CO2/N2 separation performance of ImGO 0.8 membrane at 0.4 MPa and 25 °C.
MMMs. The melting and decomposition temperature are almost the same when the addition of ImGO compared with pristine polymer. Tg of mixed matrix membranes are increased gradually
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Mr. Feng Bai at Dalian University of Technology for their helpful comments and discussions, as well as their effort of editing language cautiously. References
Fig. 13. Robeson’s plot from the CO2/N2 separation with the upper bound [6] and the data for the membranes prepared in this work.
Table 2 Comparison of CO2 separation performance between ImGO/PEBAX MMMs and other PEBAX MH1657 MMMs from literatures. P CO2 =P N2
Membrane
P CO2 / Barrer
PEBAX/CANs PEBAX/GO PEBAX/CaLS PEBAX/Functional GO PEBAX/PEG PEG-PEBAX PEBAX/MWNT PEBAX/GO PEBAX/SWNT PEBAX/POSS PEBAX/RTIL PEBAX/ATP
119 108 140 143
69 48.5 62 63
151 151 262.15 100 102 152 300 77 104 196.7 179 102.8 97
47 58.5 90.9 73 51 40 52 81 62 52.3 49.9 54
113.7
39.5
64.0 69.5 76.2
90.3 96.9 105.5
PEBAX-PEG-MWNT
PEBAX/4A zeolite
PEBAX/ImGO
Test condition
Ref.
2 bar/25 °C 0.7 MPa/35 °C 3 bar/25 °C 2 bar/30 °C
[26] [36] [27] [37]
1 bar/30 °C 0.3 bar/30 °C 1 MPa/25 °C 0.3 MPa/25 °C 33 psig/21 °C 1 bar/30 °C 1 bar/25 °C 4 bar/35 °C 10 bar/35 °C 2 bar/25 °C 2 bar/25 °C 2 bar/25 °C 5 kg cm2/ 25 °C 5 kg cm2/ 25 °C 0.4 MPa/25 °C 0.6 MPa/25 °C 0.8 MPa/25 °C
[20] [21] [23] [28] [25] [18] [22] [16] [24]
[19]
This work
because the polymer chain mobility has been restricted by the presence of ImGO and a rigidified interface has been generated between polymer and filler. The mechanical properties have been significantly enhanced by the imidazole functionalized graphene oxide sheet as expected because of the presence of H-bonding between graphene and polymer matrix. Having distinct improvement of CO2 separation performance, the ImGO/PEBAX MMMs indicates promising applications in CO2 capture processes. Acknowledgments We appreciate the financial support from the Program for Changjiang Scholars and the National Science Fund for Distinguished Young Scholars of China (21125628), Major National Scientific Instrument Development Project (21527812), and the National Science Fund of China (21236006). The authors are also thankful to Dr. Mo Li at Swiss Federal Institute of Technology Zurich and
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