Enhanced CO2 separation properties by incorporating poly(ethylene glycol)-containing polymeric submicrospheres into polyimide membrane

Journal of Membrane Science 473 (2015) 310–317

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Enhanced CO2 separation properties by incorporating poly(ethylene glycol)-containing polymeric submicrospheres into polyimide membrane Shaofei Wang a,b, Zhizhang Tian a,b, Jiangyan Feng b, Hong Wu a,b, Yifan Li a,b, Ye Liu a,b, Xueqin Li a,b, Qingping Xin a,b, Zhongyi Jiang a,b,n a

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 30 July 2014 Received in revised form 16 September 2014 Accepted 19 September 2014 Available online 28 September 2014

Poly(ethylene glycol)-containing polymeric submicrospheres (PEGSS) were synthesized via distillation precipitation polymerization and incorporated into polyimide (PI) matrix to prepare hybrid membranes. The PEGSS and hybrid membranes were characterized by Fourier transform infrared (FTIR) spectra, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Single gas permeabilities of N2, CH4 and CO2 were measured by using the time-lag method. The hybrid membranes showed enhanced CO2 permeability and CO2/N2, CO2/CH4 selectivities at low PEGSS loadings. The favorable affinity between PEGSS and CO2 greatly enhanced CO2 solubility and thus CO2 permeability. Whereas, N2 and CH4 permeabilities both decreased for the tortuous gas transport pathways by PEGSS incorporation. Particularly, PI–PEGSS(20) membrane, with 20 wt% PEGSS loading, showed 35% of increase in CO2 permeability and 104% of increase in CO2/N2 selectivity compared with those of pristine polyimide membrane. & 2014 Elsevier B.V. All rights reserved.

Keywords: Poly(ethylene glycol) Submicroshpere Polyimide Hybrid membrane CO2 separation

1. Introduction Energy-efficient and scalable carbon capture from large emission sources, e.g. flue gas, syngas and natural gas, stands as one of the greatest challenges [1]. Among various separation technologies, membrane gas separation has evolved as a green and affordable alternative [2] owing to its intrinsic advantages such as small footprint, low capital and operating costs [3]. Currently, polymers with good mechanical stability and scale-up simplicity constitute the most promising candidates for large-scale applications [4]. Polyimides are a family of glassy polymers synthesized via polymerization of various diamine and dianhydride monomers. In recent years, polyimides have attracted considerable attention due to their excellent CO2 separation performance, high chemical resistance, superior thermal stability and mechanical strength [5,6]. However, gas permeation properties of the existing polyimide materials need continuous exploitation to better meet the requirements for practical applications [7]. Great deal of strategies n Correspondence to: Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, 92# Weijin Road, District Nankai, Tianjin 300072, China. Tel./fax: þ86 22 23500086. E-mail address: [email protected] (Z. Jiang).

http://dx.doi.org/10.1016/j.memsci.2014.09.035 0376-7388/& 2014 Elsevier B.V. All rights reserved.

have been put forward to enhance the performance of polyimide membrane. Specific tailoring of the polyimide molecular structure to suppress polymer chain packing and increase fractional free volume has been demonstrated a commonly applied method [8,9]. Another facile and promising alternative is fabricating hybrid membranes. Hybrid membranes, comprising a polymer bulk phase and a dispersed filler phase, offer an approach to synergistically combine the favorable properties of two moieties and may open up additional opportunities for better exploration of the existing polyimides [10,11]. Therefore, a broad range of porous fillers with good size sieving ability (e.g. metal organic frameworks (MOFs) [12–14], zeolites [15], carbon molecular sieves [16]) have been attempted to endow the membranes with enhanced diffusivity and diffusivity selectivity. Meanwhile, incorporating fillers with polar groups that favorably interacted with CO2 may confer the membrane with enhanced CO2 solubility and thus permeability [5,17,18]. This approach may be more effective for glassy polymers with high intrinsic diffusivity selectivity. For instance, MCM-41 was functionalized with –SO3H and incorporated into Matrimid to fabricate hybrid membranes. The resultant membranes showed up to 31% increase in CO2 permeability and 14% increase in CO2/CH4 selectivity partially because of the polar –SO3H groups, which increased the affinity of fillers towards CO2 molecules [19]. Since

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CO2 with double bond may react reversibly with transition metal ions to form a π-bonded complex, novel POSS–Matrimid–Zn2 þ hybrid membranes were prepared and an obvious increase of CO2 solubility was verified by CO2 sorption tests [20]. However, functional groups with strong polarity in membranes may increase polymer chain rigidity and decrease fractional free volume of membranes, which is detrimental to gas diffusivity [18]. Low molecular weight poly(ethylene glycol) (PEG) with abundant polar ether groups could produce dipole–quadrupole interactions with CO2 and has been recognized as an effective polymer to achieve high CO2 permeability and CO2/other gas selectivity [18,21,22]. Numerous efforts have been devoted to explore PEGcontaining blend membranes [23], copolymer membranes [24] and hybrid membranes [25,26] for efficient CO2 separation. Considering the moderate polarity of PEG and the favorable affinity between PEG and CO2, rationally designed PEG-containing fillers may act as solubility promoter without causing polymer chain rigidification in membranes. However, rare efforts have been devoted to employ PEG-containing filler in hybrid membranes, especially in glassy polymer matrix [27,28]. Therefore, in this study, PEG-containing polymeric submicrospheres (PEGSS) were synthesized as fillers to prepare hybrid membranes. Matrimid 5218 was utilized as the polymer matrix for its excellent mechanical stability and commercial availability. PEGSS with low molecular weight PEG are designed to tune the interaction between the filler and polymer matrix and may produce desirable polymer–filler interface morphology. Fourier transform infrared (FTIR) spectra, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements were conducted to acquire an in-depth insight of chemical structure and thermal properties of the hybrid membranes. N2, CH4 and CO2 diffusivities, solubilities and permeabilities of hybrid membranes with different filler loadings were measured to investigate the relevant transport property in these membranes. Moreover, effect of operating temperature on gas permeability was studied.

311

600 μL EGDMA and 0.036 g AIBN were added to a 100 ml flask with 80 ml acetonitrile. Then the mixtures were heated to boiling state and reacted for 60 min. Subsequently, the products were separated and purified by centrifugation at 8000 rpm for 10 min. Afterwards, the PEGSS were dried in a vacuum oven at 40 1C for 2 days before use. 2.3. Preparation of hybrid membranes Pristine PI and hybrid membranes were prepared by a solution casting method. To be specific, certain amount of as-synthesized PEGSS was first added into DMF solvent and sonicated for 1 h (10 KHz, below 30 1C) to get a homogeneous dispersion of PEGSS. Then PI powder was added to the suspension and stirred for 5 h. After sonication for one more hour, the suspension was cast onto a glass mold placed in an oven. The solvent evaporation process was conducted at 50 1C for 12 h. Afterwards, the membranes were treated for 12 h at 80 1C and another 48 h at 120 1C. The resultant membranes with 5574 mm thickness were denoted as PI–PEGSS (X), where X (5, 10, 20, 30) represents the mass percentage of PEGSS to PI. The densities of PEGSS and hybrid membranes were calculated based on the Archimedean principles after measuring the weight of the membranes in air and in ethanol by using a density meter. 2.4. Characterization of PEGSS and membranes 2.4.1. Transmission electron microscopy (TEM) The size as well as morphology of the synthesized PEGSS was observed by a JEOL, Tecnai G2 F20 transmission electron microscope. 2.4.2. Fourier transform infrared (FT-IR) spectroscopy FT-IR spectra for PEGSS, pristine PI and hybrid membranes were obtained by using a BRUKER Vertex 70 Fourier transform infrared spectrometer with scan range of 4000–400 cm  1 and resolution of 1.93 cm  1. The hybrid membranes were measured directly whereas the PEGSS were prepared in KBr pellets.

2. Experimental 2.1. Materials Matrimid 5218 (PI), poly(ethylene glycol) methacrylate (PEGMA) and ethylene glycol dimethacrylate (EGDMA) with an average of 8.3 ethylene glycol units were purchased from Alfa Aesar China Co., Ltd. Methacrylic acid (MAA), 2, 20 -Azoisobutyronitrile (AIBN), N,N-dimethylformamide (DMF) and ethanol were purchased from Tianjin Guangfu Fine Chemical Engineering Institute. Acetonitrile was purchased from Tianjin Kewei Co., Ltd. Chemical structures of EGDMA, MAA and PEGMA are shown in Fig. 1.

2.4.3. Field emission scanning electron microscope (FESEM) Membrane cross-sectional morphology and dispersion of PEGSS in hybrid membranes were examined with a Nanosem 430 field emission scanning electron microscope operated at 10 kV. Before analyzing, membranes were cryogenically fractured in liquid nitrogen and then sputter-coated with a thin layer of gold. 2.4.4. X-ray diffraction (XRD) The crystal structure of the membranes were recorded on a Rigaku D/max 2500 v/pc X-ray diffractiometer (XRD) in the range of 5–501 at the scan rate of 3 1/min. The X-rays of 1.5406 Å wavelength were generated by a Cu Kα source.

2.2. Synthesis of PEGSS The PEGSS, a block copolymer named poly{[poly(ethylene glycol) methyl etheracrylate]-co-(acrylic acid)}, were synthesized by applying the precipitation polymerization method reported by Dai et al. [29]. In a typical synthesis, 600 μL PEGMA, 400 μL MAA,

2.4.5. Differential scanning calorimeter (DSC) measurements Glass transition temperatures (Tg) of the membranes were analyzed by a Netzsch DSC 200F3 calorimeter. The measurements were performed from 50 to 400 1C at a heating rate of 10 1C/min. Nitrogen was used as a purge gas with a flow rate of 20 ml/min. Tg was determined as the midpoint temperature of the transition in the DSC curve. 2.5. Gas permeation experiments

Fig. 1. Chemical structures of EGDMA, MAA and PEGMA.

A custom-built apparatus based on the well-established constant volume/variable pressure method was utilized [26] to

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measure the permeability, diffusivity and solubility of the gases in membranes. The apparatus for single gas testing comprises the same three membrane cells. The membrane cell is a circular stainless steel cell consisting of the top half and the bottom half, on which the highpressure side and low-pressure side are designed, respectively. The temperature of each membrane cell is controlled with a circulator bath. The circular membrane with effective area of 12.5 cm2 was mounted in a permeation cell prior to degassing the whole apparatus. Then permeant gas with known pressure was introduced on the high-pressure side, and the permeant pressure on the downstream side was monitored using a pressure transducer. In this study, N2, CH4 and CO2 were applied as the test gases. The experiments were conducted at 30 1C unless otherwise stated and the high-pressure side was maintained at 1 bar. Before analysis, the membranes were evacuated at least 8 h to remove previously dissolved species. For each membrane, the gases were tested in the order of N2, CH4, CO2 to eliminate the influence of condensable molecules. Gas permeation tests were conducted at least 3 times to get the average data. The ideal selectivity αA/B is defined as the PA/PB, where PA and PB represent the permeabilities of gases A and B, respectively. In this study, the uncertainties of gas permeability are within 78% and selectivity within 75%.

3.1.2. FT-IR The chemical structure of PEGSS is determined by FT-IR and the spectrum is shown in Fig. 3(a). As described in previous study [29], the prepared submicrosphere is a block copolymer of poly{[poly (ethylene glycol) methyl etheracrylate]-co-(acrylic acid)}. The broad peak in the region of 3200–2750 cm  1 (O–H stretching vibrations) and the peak at about 1720 cm  1 (CQO stretching vibrations) indicate the presence of MAA. Two stretching vibrations between 1300 and 1050 cm  1 represent the ester group from PEGMA and EGDMA, respectively. The stretching vibration of vinyl ether between 1310 and 1020 cm  1 may be overlapped by ester group. Fig. 3(b) displays the FT-IR spectrum of the prepared membranes. As for pristine Matrimid, the imide group is characterized by the bands at around 1780 cm  1, 1713 cm  1 (assigned to the asymmetric stretch and symmetric stretch of CQO in the imide group, respectively) and 1371 cm  1 (assigned to stretching vibration of C–N in the imide group) [30]. Matrimid possesses both an imide carbonyl band and a benzophenone carbonyl band at 1670 cm  1. Also bending of C–CO–C groups is observed at 1296 cm  1 [31]. With the incorporation of PEGSS, new peaks at about 1100 cm  1 (attributed to vinyl ether group from PEGSS) start to emerge and the peak at 1097 cm  1 shifts to lower wave numbers (1085 cm  1 for PI–PEGSS(30)), which may indicate that hydrogen bonds are formed between –COOH in PEGSS and CQO in Matrimid [32].

3. Results and discussion 3.1. Characterization of PEGSS and membranes 3.1.1. TEM Fig. 2 displays the TEM image of the as-synthesized PEGSS. As can be observed, during the precipitation polymerization process, polymeric submicrospheres with smooth surface and relative uniform size in the range of 350–420 nm were successfully synthesized. The covalent crosslinking of EGDMA and interchain hydrogen bonding caused by carboxylic acid units both attribute to the resultant sphere morphology. The density of the PEGSS is measured as 1.338 g/cm3, which is slightly greater than that of pristine PI (1.227 g/cm3). A Brunauer–Emmett–Teller (BET) specific surface area of 7.04 m2/g determined by nitrogen adsorption/ desorption isotherm may also indicate a solid and non-porous structure of PEGSS.

Fig. 2. TEM images of the prepared PEGSS.

3.1.3. FESEM Cross-sectional morphology of the membranes observed by FESEM is displayed in Fig. 4. As shown in this figure, cross section of membranes is impacted by the incorporation of the PEGSS. A smooth and dense structure is observed in pristine Matrimid membrane (Fig. 4(a)), whereas hybrid membranes show a rougher cross section. At low PEGSS loadings, the fillers are dispersed in PI matrix without severe aggregation. As PEGSS content increases, e.g. PI–PEGSS(30) membrane (Fig. 4(e)), the abundant PEGSS (about 28 vol%) in the membranes tend to connect with each other and form clusters. The magnified graph of PI–PEGSS(10) in Fig. 4(f) suggests a good contact between polymeric submicrospheres and PI. No obvious micrometer-size voids are observed between PEGSS and PI. As is known, the “sieve-in-a-cage” morphology is frequently observed in glassy polymer–inorganic filler hybrid membranes due to the poor compatibility between the two phases [33]. In this study, the PEGSS are found to be dispersed in the glassy Matrimid without forming any gaps at the polymer– filler interfaces. This superiority may be attributed to the organic feature of the filler and the intermolecular hydrogen bonding between the carboxylic group of PEGSS and Matrimid [20], as illustrated in the FT-IR spectrum. 3.1.4. XRD Fig. 5 represents the XRD spectra of neat PI and hybrid membranes with different PEGSS loadings. For pristine PI membrane, two strong and broad peaks at 101 and 251 may indicate the semi-crystalline structure of the sample [34,35]. The incorporation of PEGSS decreases the intensity of these two peaks, leading to a higher degree of amorphous phase. This is because the interaction between PEGSS and PI may disrupt the original polymer chain packing and decrease its crystallinity. It can be also observed that no obvious peak shift takes place at 2θ ¼ 171 while the peak at 2θ ¼ 231 shifts to lower values in hybrid membranes. As the sharp XRD peak in the glassy polymer spectra is often used to estimate the d-spacing, these shifts may indicate an increase in intersegmental spacing by the PEGSS, which is anticipated to create more free volume.

Transmittance

Absorbance (a.u.)

S. Wang et al. / Journal of Membrane Science 473 (2015) 310–317

4000

3500

3000

2500

2000

1500

1000

Wave number (cm-1)

500

2200

313

PI PI-PEGSS(5) PI-PEGSS(10) PI-PEGSS(20) PI-PEGSS(30)

2000

1800

1600

1400

1200

1000

Wave number (cm-1)

Fig. 3. FT-IR spectrum of (a) PEGSS, and (b) the membranes.

Fig. 4. Cross-sectional FESEM images of (a) pristine PI, (b) PI–PEGSS(5), (c) PI–PEGSS(10), (d) PI–PEGSS(20), (e) PI–PEGSS(30), and (f) PI–PEGSS(10) at higher magnification.

Intensity (a.u.)

PI PI-PEGSS(5) PI-PEGSS(10) PI-PEGSS(20) PI-PEGSS(30)

10

20

2θ (deg) Fig. 5. XRD pattern of the membranes.

30

3.1.5. DSC The polymer chain mobility is often empirically correlated with glass transition temperatures. DSC may provide a fundamental insight into the polymer chain mobility around the polymer–filler interface [36]. Glass transition temperature of pristine PI and hybrid membranes detected by DSC is shown in Fig. 6. As reported in literatures [37–39], increased Tg was frequently found in polyimide hybrid membranes. This may be attributed to the strong interactions between fillers and polymer matrix or the polymer chain trapped in the pore of porous fillers [40]. Whereas in this study, Tg of hybrid membranes gradually decrease as the PEGSS loading increases. Particularly, PI–PEGSS (30) membrane with the highest PEGSS loading exhibits Tg at about 312.6 1C. The decline of Tg indicates that the incorporation of PEGSS does not cause rigidification of polymer chains. Instead, the PEGSS slightly enhance polymer chain mobility, which may be ascribed to the moderate interaction between PEGSS and PI.

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Heat Flow (mW/mg)

PI PI-PEGSS(5) PI-PEGSS(10) PI-PEGSS(20) PI-PEGSS(30)

o

325.5 C o

322.5 C o

320.0 C o

315.6 C o

312.6 C

100

150

200

250

300

350

400

Temperature (oC) Fig. 6. DSC pattern of the membranes.

Table 1 Gas permeability and ideal selectivity of pristine PI and hybrid membranes (tested at 1 bar, 30 1C). Membrane

P N2 a

P CH4 a

P CO2 a

αCO2 =N2

αCO2 =CH4

PIb PIc PId PI–PEGSS(5) PI–PEGSS(10) PI–PEGSS(20) PI–PEGSS(30)

0.22 0.16 0.210 0.188 0.145 0.134 0.124

0.21 0.15 0.180 0.178 0.170 0.163 0.157

7.29 5.39 6.31 6.73 7.50 8.21 5.85

33.14 33.47 30.05 35.84 51.64 61.24 49.78

37.71 35.93 35.06 37.89 44.04 50.29 37.39

Gas permeability (Barrer, 1 Barrer¼ 1  10  10 cm3 (STP) cm/(cm2 s cmHg)). Gas permeabilities and selectivities in Ref. [41]. c Gas permeabilities and selectivities in Ref. [42]. d Gas permeabilities and selectivities in this study. a

b

3.2. Gas separation performances 3.2.1. Gas permeation performances Gas permeation properties of pristine PI and hybrid membranes were investigated by using N2, CH4 and CO2 gases at 1 bar and 30 1C. The permeabilities of the above gases in pure PI membrane are presented in Table 1, which are similar to those reported in other literatures [41,42]. Compared with rubbery polymers with low packing efficiency, glassy PI membrane has higher diffusivity selectivity (i.e. size sieving ability). Considering the kinetic diameter, the molecular dimensions increase in the order of CO2 (0.330 nm) oN2 (0.364 nm) oCH4 (0.380 nm). This is also the order of the decreased permeability in PI membrane. With the incorporation of PEGSS, N2 and CH4 permeabilities in hybrid membranes both exhibit a decreasing trend. Whereas, CO2 permeability first increases from PI to PI–PEGSS(20) then suffers a sharp decrease in PI–PEGSS(30) membrane, of which the CO2 permeability is even lower than that of pristine PI membrane. The ideal selectivities for CO2/N2 and CO2/CH4 both follow the same trend as CO2 permeability. Compared with pristine PI membrane, PI–PEGSS (20) membrane with the highest separation performance exhibits CO2/N2 selectivity increment of 104% and CO2/CH4 increment of 35%. Gas diffusivity and solubility coefficients are calculated to investigate the transport properties and presented in Table 2. As for the diffusivity coefficients, N2, CH4 and CO2 all display a decreasing trend. It may seem counterintuitive, since the XRD results shows decreased crystallinity in all hybrid membranes, which may indicate enhanced gas diffusivity coefficients [43]. However, the decline of diffusivity can be explained as follows:

in PI–PEGSS hybrid membranes, gas diffusion takes place through three phases: the polymer matrix, the filler and the polymer–filler interface. As stated above, the decreased crystallinity caused by PEGSS incorporation may increase the gas diffusivity in the polymer matrix. Moreover, considering the good compatibility between the polymer matrix and filler, no evident interfacial voids or “sieve-in-a-cage” morphology are observed. Meanwhile, the decreased Tg values of hybrid membranes may be an indication of looser chain packing around the filler and may result in increased gas diffusivity at the interface. Integrating the above factors, the decrease of diffusivity coefficients should be attributed to the low gas permeability in the non-porous polymeric filler. The low permeable filler in the hybrid membranes will result in tortuous gas transport pathways and decrease the gas diffusivity in membranes. Particularly, at high PEGSS loadings, the PEGSS tend to connect with each other (as observed by FESEM images); thus the decrease of gas diffusivity may be more prominent. Compared with pristine PI membrane, gas solubilities all increase in hybrid membranes: N2 and CH4 solubilities only show moderate increase, whereas CO2 solubility in PI–PEGSS(20) is 1.97 times that of pristine PI membrane. This should be explained by the following two reasons. First, the disrupted PI chain packing caused by PEGSS incorporation will create more polar sites and increase the gas solubilities in PI. Second, the incorporated PEGSS with polar EO units may serve as absorbents for gas molecules. Considering the critical temperature of three gases, CO2 with good condensability will be easily absorbed on the PEGSS. Moreover, owing to the dipole–quadrupole interactions between EO and CO2 [18], CO2 solubility increments are more prominent. It should be mentioned that, to better exploit adsorbents as fillers for hybrid membranes, the micro-environment of adsorbents, especially the polymer chain mobility in the vicinity of fillers should be readily manipulated. In many studies [37,39], the rigidified polymer chains around the filler will lead to decreased gas diffusivity and retard the gas molecules towards fillers, which may be detrimental to fully exploit the function of fillers. Whereas in this study, the moderate interactions between polymer matrix and fillers may lead to looser PI chain packing around PEGSS, which helps the PEGSS to take effects. However, the CO2 solubility of PI–PEGSS(30) is slightly lesser than that of PI–PEGSS(20). This may be explained by the aggregation of PEGSS, which will lead to decreased surface area and decreased contact sites for CO2 molecules. Based on the variation in gas diffusivity and solubility, it can be deduced that the decrease in diffusivity lead to the decreased N2 and CH4 permeabilities. While the significant increase in CO2 solubility offsets the decrease in gas diffusivity, thus CO2 permeability increases. Therefore, in this study, significant improvement of CO2/N2 and CO2/CH4 selectivity is acquired for hybrid membranes. The increase of CO2/N2 selectivity is more remarkable than that of CO2/CH4. This may be because the critical temperature difference between CO2 (304.1 K) and N2 (126.2 K) is larger than that of CO2 and CH4 (190.6K) [26].

3.2.2. Temperature dependence of gas transport properties To further investigate the gas transport properties of the membranes, operating temperature on gas permeation is also studied. Gas permeation tests were conducted at 1 bar and different temperatures (30, 40, 50, 60 1C). The results are presented in Table 3. As can be observed, N2, CH4 and CO2 permeabilities all increase with the increase in temperature. Because the mobility of gas molecules will increase at elevated temperature, it will enhance the driving force for diffusion. In addition, the increase of temperature will lead to more flexible polymer chains, thus creates more free volume cavities for gas transport. However, the flexible polymer chains will lower the size sieving ability of

S. Wang et al. / Journal of Membrane Science 473 (2015) 310–317

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Table 2 Gas diffusivity and solubility coefficients of pristine PI and hybrid membranes (tested at 1 bar, 30 1C). Membrane

DN2 a

S N2 b

DCH4 a

SCH4 b

DCO2 a

SCO2 b

SCO2 =N2

SCO2 =CH4

DCO2 =N2

DCO2 =CH4

PI PI–PEGSS(5) PI–PEGSS(10) PI–PEGSS(20) PI–PEGSS(30)

2.66 2.25 1.75 1.52 1.25

0.08 0.08 0.08 0.09 0.10

1.02 0.94 0.86 0.81 0.71

0.18 0.19 0.20 0.20 0.22

3.78 3.65 3.43 3.23 2.96

1.67 1.85 2.18 3.30 3.15

21.14 22.11 26.39 28.82 25.09

9.46 9.72 11.03 12.58 8.99

1.42 1.62 1.96 2.12 2.36

3.71 3.90 3.99 4.01 4.16

a b

Diffusivity coefficient [cm2/s]  108. Solubility coefficient [cm3(STP)/cm3 cmHg]  102.

Table 3 Gas permeability and ideal selectivity of pristine PI and hybrid membranes at different temperatures (tested at 1 bar). Membrane

Temperature (1C)

P N2 a

P CH4 a

P CO2 a

αCO2 =N2

αCO2 =CH4

PI

30.00 40.00 50.00 60.00 30.00 40.00 50.00 60.00

0.21 0.28 0.37 0.49 0.13 0.22 0.32 0.47

0.18 0.22 0.28 0.34 0.16 0.21 0.27 0.33

6.31 7.42 8.68 10.26 8.21 10.42 12.46 14.27

30.11 26.50 23.46 20.98 61.27 47.36 38.94 30.36

35.06 33.73 31.01 30.18 50.37 49.62 46.15 43.24

PI–PEGSS(20)

a

Gas permeability (Barrer, 1 Barrer¼ 1  10  10 cm3 (STP) cm/(cm2 s cmHg)). 1

4x10

N2 CH4

1

Permeability (Barrer)

2x10

given by the following expression: P ¼ P 0 expð Ep =RTÞ where P is the permeability of the gas, P0 refers to the preexponential factor, R is the gas constant and T is the absolute temperature. The activation energies determined from the plots of permeability vs. reciprocal temperature show that the permeability coefficients obey the Arrhenius equation. The Ep values are calculated and presented in Table 4. As described in previous study [44], the Ep values of the gases are typically governed by two main factors: molecular size and interaction with the polymer. The hybrid membrane shows higher Ep values than those of pristine PI membrane, which indicates that the hybrid membranes are more sensitive to temperature. This increase in activation energy permeation accompanied by the enhanced chain mobility at elevated temperatures may also prove the tortuous transport pathway in hybrid membranes [45].

CO2

-1

4x10

-1

2x10

3.00

3.05

3.10

3.15

3.20

3.25

3.30

-1

1000/T (K ) Fig. 7. Effect of temperature on gas transport properties of PI membrane (solid symbols) and PI–PEGSS(20) membrane (open symbols) at 1 bar.

3.2.3. Comparison with other Matrimid-based hybrid membranes To acquire an overview of CO2, N2 and CH4 permeation properties of Matrimid-based hybrid membranes, a comparison is implemented and the data are displayed in Table 5. Though only moderate CO2 permeability is acquired in this study, the CO2/N2 selectivity is higher than those in most previous studies. This should be attributed to the increased EO units in the hybrid membranes, which increase the membrane solubility selectivity. It can be also seen that the N2/CH4 selectivity is less than 1, which means PEGSS show preferential affinity towards CH4 [41]. Hopefully, this study could provide a facile approach to enhance the CO2/N2 selectivity in glassy polyimide membranes.

4. Conclusion Table 4 Activation energy for permeation (kJ mol  1). Membrane

N2

CH4

CO2

PI PI–PEGSS(20)

23.62 34.79

18.04 19.89

13.55 15.47

membranes and lead to a decrease in gas selectivity, which can also be observed in this table. Comparing these two gas pairs, a notable decrease (  50%) in gas selectivity is observed for CO2/N2. This is because CO2 molecule has high solubility in the polymer materials for its high condensability, as temperature increases, CO2 solubility will decrease and result in lower solubility selectivity. Fig. 7 presents the effect of varying feed temperatures on N2, CH4 and CO2 permeabilities. The temperature dependence of gas permeabilities can be further described by using the Arrhenius equation which relates the gas permeability to the operating temperature via the activation energy of permeation (Ep), and is

In the present study, novel PEG-containing polymeric submicrospheres (PEGSS) were utilized as versatile fillers to prepare hybrid membranes for efficient CO2 capture. The hybrid membranes show enhanced CO2 permeability due to the favorable affinity of PEGSS towards CO2 molecules. The decreased crystallinity and increased polymer chain mobility may also contribute to the easy access of CO2 molecules towards PEGSS. While N2 and CH4 permeabilities decrease because the non-porous fillers prolong gas transport pathways in membranes. Compared to pristine PI membrane, PI–PEGSS(20) membrane exhibits 35% of increment in CO2 permeability and 104% of increment in CO2/N2 selectivity, which shows promising prospects of incorporating PEG-containing fillers into polyimide membranes. It should be mentioned that due to the nonporous feature of the PEG-containing submicrospheres in this study, the enhancement of gas permeability is still not prominent enough. Further studies should be conducted to develop porous PEG-containing fillers for preparing more permeable CO2 separation membranes.

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Table 5 Comparison of CO2, N2 and CH4 permeation properties in different Matrimid-based hybrid membranes. Filler

CO2 permeability (Barrer)

CO2/N2 selectivity

CO2/CH4 selectivity

N2/CH4 selectivity

Temperature (1C)

Pressure (bar)

Reference

Mesoporous silica spheres SO3H-MCM-41 ZIF-8 POSS POSS–Zn Cu–BPY–HFS MIL-53 TiO2 C60 Carbon aerogel PEGSS

15.3 11.2 16.6 5.3 4.0 10.4 51.0 10.5 3.79 13.34 8.21

40.3 37.4 19.0 27.9 30.8 33.4 28.3 11.4 23.7 39.2 61.2

53.6 40.5 35.8 37.2 57.6 27.4 46.4 11.4 34.8 47.8 50.3

1.33 1.08 1.88 1.33 1.87 0.82 1.64 1.19 1.44 1.21 0.82

35 25 22 35

1.75 10 4 10

[40] [19] [13] [20]

35 35 35 35 35 30

3.9 2 2 10 2.6 1

[41] [46] [31] [39] [37] This study

Acknowledgments The authors gratefully acknowledge the financial support from the National High Technology Research and Development Program of China (2012AA03A611), the National Science Fund for Distinguished Young Scholars (No. 21125627), and Program of Introducing Talents of Discipline to Universities (B06006).

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