CNTs) hybrid membranes

CNTs) hybrid membranes

Accepted Manuscript Preparation and enhanced gas separation performance of Carbon /Carbon nanotubes (C/CNTs) hybrid membranes Lin Li, Chengwen Song, D...

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Accepted Manuscript Preparation and enhanced gas separation performance of Carbon /Carbon nanotubes (C/CNTs) hybrid membranes Lin Li, Chengwen Song, Dawei Jiang, Tonghua Wang PII: DOI: Reference:

S1383-5866(17)31437-5 http://dx.doi.org/10.1016/j.seppur.2017.07.019 SEPPUR 13876

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

5 May 2017 9 July 2017 9 July 2017

Please cite this article as: L. Li, C. Song, D. Jiang, T. Wang, Preparation and enhanced gas separation performance of Carbon /Carbon nanotubes (C/CNTs) hybrid membranes, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/j.seppur.2017.07.019

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Preparation and enhanced gas separation performance of Carbon /Carbon nanotubes (C/CNTs) hybrid membranes Lin Lia, Chengwen Songa,b.*, Dawei Jianga, Tonghua Wanga,** a

State key Laboratory of Fine chemicals, Carbon Research Laboratory, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian, 116024, China

b

College of Environmental Science and Engineering, Dalian Maritime University, 1 Linghai Road, Dalian 116026, China

Abstract: Carbon/carbon nanotubes (C/CNTs) hybrid membranes are successfully fabricated by pyrolyzing poly(amic acid) (PAA) precursors incorporated with CNTs. The morphology and gas separation performance of the hybrid carbon membranes are analyzed by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and single gas permeation test (H2, CO2, O2, N2, and CH4). The effects of CNTs properties (CNTs type, concentration, length and diameter) of on gas separation performance of the hybrid carbon membranes are systemically investigated. The results show the combination of carbon molecular sieve membranes (CMSMs) with CNTs exerts a significantly favourable effect on the enhancement of gas separation performance. Carbon membrane incorporated by multi-walled carbon nanotubes (MWCNTs) shows higher permeabilities but lower selectivities than that embedded by single-walled carbon nanotubes (SWCNTs). After acid treatment, the improvement on gas permeabilities by MWCNTs tunnels is not significant when facing the compaction of membrane

* Corresponding author: Tel/Fax: +86-411-84724342. E-mail: [email protected] (C.Song), ** Corresponding author: Tel/Fax: +86-411-38993968. E-mail: [email protected] (T. Wang)

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structure. With the increase of MWCNTs concentrations, the hybrid carbon membrane exhibits significant enhancement in gas permeabilities. In addition, MWCNTs with long length and large diameter usually produce high gas permeabilities. Key words: PMDA-ODA; Carbon membranes; Gas separation; Carbon nanotubes 1. Introduction Membrane-based gas separation technologies, offering higher energy efficiency, lower operational cost and smaller footprint, have demonstrated great potentials as promising processes for various industrial applications, particularly in hydrogen recovery, carbon capture, air separation, natural gas purification and olefin/paraffin separation during the past few decades [1]. Generally, the successful applications of gas separation membranes in these industrial processes extensively depend upon the properties of membrane materials [2]. At present, the dominant membrane materials used in gas separation fields are polymeric membranes because of their advantages of low cost, good mechanical properties, and easy processability. Despite many advantages, the encountered challenges of harsh environments during membrane operation processes, and the trade-off limitation between permeability and selectivity of polymeric membranes have prompted the search for more robust materials with higher separation property [3]. Behind those driving forces, carbon molecular sieve membranes (CMSMs), possessing random nanoporous carbon networks and co-existing molecular sieving characteristics typically obtained by pyrolyzing precursor polymers under vacuum or an inert atmosphere at high temperatures, have aroused considerable attention for their higher thermal and chemical stability, excellent separation property [4].

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Generally, gas separation property of CMSMs usually depends on the nature of their nanoporous structures. As previous reported, CMSMs possess the unique combination of micropore (6–20 Å) and ultramicropore (<6 Å) networks. The micropores facilitate gas sorption, while the ultramicropores are responsible for molecular sieving [5]. Due to the unique bi-modal nanoporous structure, CMSMs demonstrate outstanding separation ability exceeding the upper-bound limit of polymeric membranes. Up to now, though great progress has been made in the field of CMSMs, the gas permeability for pure CMSMs available still now cannot satisfy the requirements of commercial applications [6]. Moreover, from economical consideration, the permeability of CMSMs should be maximized without sacrificing gas selectivity or even improve selectivity so as to provide attractive commercial values [7]. Among various reported methods in previous papers, incorporating functional materials into membrane precursors to fabricate hybrid CMSMs have been regarded as an effective strategy to further improve the gas permeability of pure CMSMs [8]. Many materials such as thermally labile polymer [9-11], metals [12,13], inorganic particles [14], porous materials [15-19] and heteroatom [20] have been widely adopted as the functional materials to fabricate the hybrid CMSMs. The resultant hybrid CMSMs combine the molecular sieve ability of pure CMSMs with the advantages of those functional materials, such as a reduced gas diffusion resistance or strong interaction with permeation molecules, and demonstrate improved separation performance compared to pure CMSMs. Carbon nanotubes (CNTs), as potential functional materials, have attracted much attention since the discovery of rapid transport of gases in CNTs due to low transport resistance in their inherent channels. The outstanding characteristics of CNTs motivate many researchers to explore highly permeable, selective hybrid CMSMs by embedding CNTs into membrane precursors [21].

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Tseng et al. obtained multi-walled carbon nanotubes (MWCNTs)/carbon nanocomposite thin films by incorporating (MWCNTs) into polyimide (PI) precursor solution. The carbon membrane exhibited higher CO2 flux of 866.6 Barrer, which was 2-4 times of magnitude higher than that of pure carbon membrane [22]. Rao et al. prepared polyetherimide (PEI)/multi-walled carbon nanotubes (MWCNTs) composite carbon membrane, and the CO2 permeability was about 27 times of PEI carbon membrane, reaching 1463 Barrer [23]. Although several pioneering works on hybrid CMSMs incorporated by CNTs have demonstrated their great potentials in gas separation, they did not investigate the effect of the properties of CNTs on enhanced gas separation performance systematically. Moreover, to our knowledge, almost no related studies are reported in recent years. Based on this investigation, the main purpose in the present work is to fabricate high performance C/CNTs hybrid membranes by introducing CNTs in poly(amic acid) (PAA) precursors, and systemically investigate the effect of CNTs properties (type, concentration, length and diameter) on gas separation performance of the resulting hybrid carbon membranes. These findings are expected to help us better understand and gain an insight into the role of CNTs in enhancing gas separation performance of carbon membranes. 2. Experimental 2.1 Preparation of C/CNTs hybrid membranes The preparation procedure of C/CNTs hybrid membrane was shown in Figure 1. SWCNTs (SC1) and three types of MWCNTs (MC1, MC2, and MC3 ) manufactured by chemical vapor deposition method were purchased from Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China) with detailed specifications as follows: SC1 (diameter: < 2 nm, length: 5-15 μm); MC1 (diameter: 40-60 nm, length: 1-2 μm); MC2 (diameter: 40-60 nm, length: 5-15 μm); MC3 (diameter: 10-20 nm, length: 5-15 μm). CNTs used in this work were treated by mixed acid according to the following

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procedure: About 4 g CNTs were added into 100 mL HNO3/H2SO4 (1:3, v/v) mixed solution. Then, the solution was kept at 60 oC for 1 h with vigorous stirring, followed by thorough washing with distilled water until the filtrate became neutral. After drying at 60 oC for 10 h, the acid-treated CNTs were obtained for further experiment. The CNTs (0.19 g, 0.40 g and 0.64 g) were dispersed in 18.9 mL of N,N-Dimethylacetamide (DMAc) under stirring and ultrasonic treatment for 2 h, respectively. The obtained suspension was mixed with 15 g of poly(amic acid) (PAA) solution (24 wt%) derived from pyromellitic dianhydride (PMDA) and 4,4’-oxydianiline (ODA) with continuous stirring and ultrasonic treatment for 4 h at room temperature to prepare casting solution with 5 wt.%, 10 wt.%, and 15 wt.% of CNTs content in PAA/CNTs matrix, respectively. Free-standing PAA/CNTs composite membranes were fabricated by a casting method in a dust-free environment. About 15 g of the above solution was casted on a glass plate and dried at 40 oC for 12 h at the relative humidity RH < 40% to obtain the PAA/CNTs composite membranes. The as-made composite membranes were cut into coin-like sheets with a diameter of ca. 3.5 cm before the subsequent heat treatment. Then, the PAA/CNTs composite membranes were fixed between graphite blocks and pyrolyzed in a tubular furnace from room temperature to 400 oC at a heating rate of 2 oC/min and held at this temperature for 60 min in flowing Ar of 200 mL/min, and then continued to be heated to 700 oC for 2 h at a rate of 2 oC/min in the flowing Ar of 200 mL/min. Subsequently, the membranes were naturally cooled to ambient temperature. The thickness of the obtained C/CNTs composite membranes were around 55 μm. 2.2 Characterizations The morphology of the carbon membranes was observed by scanning electron microscopy (NOVA NanoSEM 450) and high-resolution transmission electron microscopy (Philips TECNAI GZ20). Gas permeabilities of carbon membranes were measured using single gases with high purity (H2, CO2, O2, N2, and CH4) and binary gases (CO2/N2 =50/50 vol%) at room temperature by the traditional variable volume-constant pressure method. The gas provided by compressed gas cylinder

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was introduced to the upper side (feed side) of a carbon membrane that was tightly sealed in stainless steel membrane cell by an epoxy glue and O-shape gasket at a regulated pressure of 0.1MPa. In order to prevent damage to brittle carbon membrane during test process, the carbon membrane is put on the perforated plate, which is designed in the stainless steel membrane cell. The permeation side of the membrane cell was kept at ambient pressure. The gas flux on the permeation side was measured by a gas microflowmeter. The gas flux on the permeation side was measured by a gas microflowmeter. The effective area of the membranes is around 6 cm2. To ensure precision of testing, measurements were conducted more than 3 times with more than three different samples prepared under the same condition and the reported final results are the averaged ones. The permeability coefficient of every gas species was calculated from Eq. (1) [24]. The ideal separation factor (gas selectivity) for a gas pair is obtained from the ratio of the permeabilities of two single gases, as shown in Eq. (2) [25].

P

F A  p / l

(1)

where P is the permeability in the standard unit of Barrer (1 Barrer = 10-10 cm3(STP) cm/cm2 s cm Hg = 7.5×10-18 m2 s-1 Pa-1) for membrane materials; F is the gas flux permeating through the membrane; A and l are the effective membrane area and membrane thickness, respectively; ΔP is the pressure difference between the feed side and the permeation side.

S A/ B 

PA PB

(2)

3. Results and discussion 3.1 Morphology of C/CNTs hybrid membrane Figure 2a and b show the SEM images of the surface and cross-section of PAA/CNTs composite membrane containing 10 wt% acid-treated MWCNTs (MC2) synthesized in this work.

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As observed, the PAA/CNTs composite membrane is smooth and homogeneous on appearance, and MWCNTs are disordered dispersed in the polymeric matrix. After pyrolysis, the surface of C/CNTs hybrid membrane is not flat due to membrane shrinkage during heat treatment. On the cross-section images, we also can see that CNTs are tightly embedded into the carbon matrix without any obvious defects or cracks along the boundaries of interfacial gaps (Figure 2c and d). Figure 3a shows the HRTEM image of the above C/CNTs hybrid membrane. It can be seen that MWCNTs are dispersed very well in carbon matrix in the form of single tubes. The average outer diameter of MWCNTs is about 40 nm, and show one-dimensional beam structure. The top part of CNTs shows the ‘opening’ status. The HRTEM image of the MWCNTs shows the waving structure of graphitic sheets at a short range [26], and the carbon matrix from the pyrolysis of the polyimide belongs to amorphous carbon materials. There are obvious interfacial gaps between MWCNTs and carbon matrix, which create more spaces for the diffusion of gas molecules. In addition, the one-dimensional tubular structures of MWCNTs also provide more effective gas transport channels [27]. Figure 3b presents the selected area diffraction (SAED) pattern of MWCNTs, it is clear that the MWCNTs has a relatively clear ring structure, which implies the MWCNTs possess a low degree of graphite, in good agreement with the results from HRTEM analysis [28]. 3.2 Effect of CNTs type on gas separation performance Generally, CNTs can be synthesized as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The two kinds of CNTs usually possess different chemical and physical properties, which will influence gas separation performance of the resultant C/CNTs hybrid membranes ultimately. Here, C/CNTs hybrid membranes by embedding SWCNTs (SC1) or MWCNTs (MC2) into carbon matrix are prepared. For comparison, pure carbon

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membrane is also fabricated by the same procedure. As shown in Figure 4, for pure carbon membrane, the gas permeabilities are found to be in the following order: H2 >CO2 >O2 >N2 >CH4, which is inversely in proportion to the kinetic diameter of selected gases, evidencing that the gas transport through pure carbon membrane follows the molecular sieving mechanism [29]. When CNTs (SWCNTs or MWCNTs) are incorporated into the carbon membrane, significant enhancement in gas permeabilities is observed. Between them, the carbon membrane embedded by MWCNTs demonstrates significantly higher gas permeabilities than that incorporated by SWCNTs, and does not show much sacrifice in gas selectivities. Therefore, we can conclude that MWCNTs are more effective than SWCNTs in enhancing gas permeabilities. In addition, it is noteworthy that CO2 permeabilities increase remarkably from 321 Barrer to 2626 Barrer for SWCNTs and 6661 Barrer for MWCNTs, respectively, which both exceed H2 permeabilities (2563 Barrer for SWCNTs and 5824 Barrer for MWCNTs), and thus the order of gas permeabilities is changed as CO2>H2>O2> N2>CH4. As we know, carbon membranes usually have a complex pore structure that combines both ultramicropores and micropores. The ultramicropores are mainly responsible for molecular sieving while the micropores provide high capacity adsorption sites for gas molecules [30]. By incorporating CNTs, interfacial gaps between CNTs and carbon matrix are produced (bigger interfacial gaps resulting from MWCNTs than SWCNTs), which may create more spaces for the transport of gas molecules compared with pure carbon membrane. Moreover, CNTs usually show strong affinity towards CO2 due to their unique 1D nanostructure with conjugated π bonds, which helps to capture CO2 molecules, and thus improve CO2 permeabilities of hybrid carbon membranes [31]. 3.3 Effect of acid treatment on gas separation performance

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The effects of acid treatment on gas separation performance of C/CNTs hybrid membranes are investigated using the membranes incorporated without and with acid-treated MWCNTs (MC2). As shown in Figure 5, the improvement on gas permeabilities of carbon membrane is not obvious after acid treatment, and only small molecular gases, such as H2, CO2 show the slightly increase in gas permeabilities, while other gases, such as O2, N2 and CH4, demonstrate a slightly decline trend on the permeabilities. However, the hybrid carbon membrane incorporated with acid-treated MWCNTs achieved higher selectivities compared to that without acid treatment. The reason for this phenomenon may be due to the fact that acid treatment can open up part of the MWCNTs (Figure 3a), and these open-ended MWCNTs may serve as gas diffusion tunnels, which allow gas molecules pass through rapidly, and thus increase the gas permeabilities of hybrid carbon membrane. However, acid treatment can also introduce polar carboxylic acid groups and hydroxyl groups, which can be identified by the peak at 3432 cm-1 and 1709 cm-1 from the FTIR spectrum (Figure 6). The hydrophilic surface makes MWCNTs compatible with the network of crosslinked PI and enhanced their affinity [27]. Thus, the interfacial gaps between MWCNTs and carbon matrix are reduced, which hinders the transport larger molecular gases, and results in the descent of their gas permeabilities and remains higher selectivities. Therefore, the enhanced gas permeabilities by MWCNTs tunnels can be weakened when facing the compaction of membrane structure. 3.4 Effect of MWCNTs concentrations on gas separation performance Hybrid carbon membranes obtained by embedding MWCNTs into carbon matrix are expected to exhibit enhanced gas permeabilities. In order to investigate systematically the effect of MWCNTs loading on the gas separation performance of the C/CNTs hybrid membranes, a series of hybrid carbon membranes are fabricated at the MWCNTs (MC2) concentrations of 5, 10 and 15 wt%,

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respectively. As shown in Figure 7a. When 5% of MWCNTs are embedded into the carbon membrane, the hybrid carbon membrane exhibits significant enhancement in gas permeabilities. Further increasing MWCNTs concentrations, the gas permeabilities still demonstrate a gradually ascending trend without saturation phenomenon. Especially for CO2 permeability, it increases with MWCNTs loading and reaches 9332 Barrer at 15 wt% MWCNTs loading. We also notice that the MWCNTs-enhanced gas permeabilities accompany with the reduced H2/N2, O2/N2 and CO2/CH4 selectivities compared to the pure carbon membrane (Figure 7b). The reason can be explained by the fact that interface gaps weaken the molecular sieve ability of hybrid carbon membrane, making the selectivities decline [32]. On the contrary, CO2/N2 selectivity slight increases with respect to increasing MWCNTs concentrations, which is ascribed to the hybrid carbon membrane containing MWCNTs can interact with CO2, and allow CO2 molecules to transport faster, thus increase the CO2/N2 selectivity [33]. It is noteworthy that the fast transportation of CO 2 molecules does not produce high CO2/CH4 selectivity. The phenomenon can be explained by that CH4 molecules are more readily adsorbed on carbon materials than N2 molecules [34]. Increasing MWCNTs concentration from 0 to 15%, the CH4 permeability increases from 0.3 to 12 Barrer (40 times), while the N2 permeability only increases from 14 to 335 Barrer (24 times). Therefore, CO2/CH4 selectivity does not demonstrate the same changing trend as CO2/N2 selectivity. 3.5 Effect of MWCNTs lengths on gas separation performance Figure 8a shows the effect of MWCNTs lengths on the gas permeabilities of hybrid carbon membrane. As observed, the gas permeabilities of carbon membrane incorporated by MC2 are superior to those of carbon membrane embedded by MC1. We think the possible explanation of this phenomena is due to the fact that the bigger interfacial gaps are formed using longer MWCNTs,

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which provide more high capacity adsorption sites for gases transport, so that the gas permeabilities are improved [35]. Moreover, the carbon membrane embedded by MC2 also demonstrates high gas selectivities (Figure 8b), which is attributed to high gas permeanbilities of H2 and CO2, arising from their small molecular diameters and high affinity towards CO2 of MWCNTs [36]. 3.6 Effect of the MWCNTs diameters on gas separation performance Figure 9a show the effect of the diameters of MWCNTs on gas permeabilities. We can see that the gas permeabilities of carbon membrane doped by MC2 are higher than those of carbon membrane doped by MC3. This may be due to large diameter of MWCNTs have greater internal channels, which are usually much larger than the kinetic diameters of the tested gases, allowing gas molecules pass through rapidly. Moreover, large diameter of MWCNTs can create bigger interface gaps between MWCNTs and carbon matrix, which will form more shortcuts for gas molecule diffusion, and subsequently increase gas permeabilities. Meanwhile, we also notice that H2 /N2, O2 /N2, CO2/CH4 and CO2/N2 selectivities of carbon membrane doped by MC2 are slightly lower than that of carbon membrane doped by MC3 (Figure 9b). This can be explained by the fact that larger inside channels of MWCNTs and interface gaps are more advantageous to the pass of gas molecules, which weaken the molecular sieve ability of hybrid carbon membrane [37]. 3.7 Evaluation of gas separation performance of the C/CNTs hybrid membrane In order to evaluate the gas separation performance of the C/CNTs hybrid membrane obtained in this work, a well-known Robeson’s upper bound (2008) is adopted for CO2/N2 gas pairs (Figure 10). As observed, the CO2/N2 permeation data of the C/CNTs hybrid membrane achieves significant enhancement compared with the pure carbon membrane, and surpasses Robeson’s upper bound (2008).

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Additionally, we compare gas permeability and selectivity of the C/CNTs hybrid membranes with carbon membranes embedded by various functional materials reported in literatures. As summarized in Table 1, pure CMSMS usually shows high selectivities, but low permeabilities [38,39]. Incorporating by MWCNTs, the hybrid carbon membranes demonstrate great advantage in gas permeabilities [22,23], which are all higher than those of carbon membranes embedded by other functional materials, such as Poly(benzimidazole) [10], Ag [13], FeO [12], SBA-15 [6], ZSM5 [18], Zeolite KY [19] and Boehmite [40]. As for those three carbon membranes doped by MWCNTs, the C/CNTs hybrid membrane obtained in this study exhibits highest gas permeabilities. Despite being lower selectivity than carbon membranes reported in Refs. [23], the significant improvement on gas permeabilities (4.6, 1.7 and 8.4 times for CO2, O2 and N2, respectively) still demonstrates a good competitiveness. Binary gases (CO2-N2 gas mixture) permeation experiments are also carried out to further assess the separation performance of the C/CNTs hybrid membranes. As shown in Figure 11, with the increase of MWCNTs concentration, the separation performance of the C/CNTs hybrid membranes indicates the similar changing trend with the single gas permeation experiments. Although the gas permeability and selectivity are slightly lower than the corresponding values obtained at the single gas permeation experiments due to the competitive diffusion of the two gases through the membrane pores, the C/CNTs hybrid membrane still exhibits a great attraction for its commercial application in gas separation. 4. Conclusions In this study, we have successfully fabricated C/CNTs hybrid membranes by pyrolyzing poly(amic acid) (PAA) precursors incorporated with CNTs, and investigated the effects of the

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properties of CNTs (type, concentration, length, diameter) of on gas separation performance of the hybrid carbon membranes. Carbon membrane incorporated by CNTs shows enhanced gas permeabilities, and MWCNTs are more effective than SWCNTs. Although acid treatment can open the blocked ends of MWCNTs, it is not significant on the improvement of gas permeabilities with the help of MWCNTs inner channels when facing the compaction of membrane structure. High loading concentration of MWCNTs favors the improvement of gas permeabilities of hybrid carbon membrane. Larger dimensions (length and diameter) of MWCNTs usually create bigger interfacial gaps, and thus enhance gas permeabilities. Acknowledgement This work was supported by the National Natural Science Foundation of China (21476034, 21376037, 21676044, 21276035) and the State Key Laboratory of Separation Membranes and Membrane Processes (Tianjin Polytechnic University) (No. M2-201509). Reference [1] C. Song, T. Wang, X. Wang, J. Qiu, Y. Cao, Preparation and gas separation properties of poly(furfuryl alcohol)-based C/CMS composite membranes, Sep. Purif. Technol. 58 (2008) 412-418. [2] P.S. Goh, A.F. Ismail , S.M. Sanip, B.C. Ng, M. Aziz, Recent advances of inorganic fillers in mixed matrix membrane for gas separation, Sep. Purif. Technol. 81 (2011) 243–264. [3] L. Li, C. Song, H. Jiang, J. Qiu, T. Wang. Preparation and gas separation performance of supported carbon membranes with ordered mesoporous carbon interlayer, J. Membr. Sci. 450 (2014) 469-477

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carbon

molecular

sieve

membranes—Permeation and adsorption studies, Chem. Eng. Res. Des. 92 (2014) 2668–2680.

18

Table 1 Comparison of separation performance of C/CNTs hybrid membranes with that of other carbon membranes reported in earlier studies Precursor

Permeability (Barrer)

Incorporated

Selectivity

Permeation temperature(oC)

Reference

materials

H2

CO2

O2

N2

CH4

H2/N2

CO2/N2

O2/N2

CO2/CH4

Polyimide

-

1487

314.8

97.7

9.1

-

163.1

34.6

10.7

-

Polybenzimidazole

-

-

759.9

-

-

7.24

-

-

-

105

1337.0

305.5

113.20

15.71

5.84

85.10

19.45

7.21

52.31

35

[10]

Ag

637

95.5

-

2.85

1.43

224

-

-

67

35

[13]

Cellulose

FeO

280

110

30

8.3

4

33.7

13.3

3.6

27.5

30

[12]

Polyetherimide

SBA-15

667.5

222.5

57.8

8.9

8.9

11.5

25

7.5

7.5

30

[6]

Polyetherimide

ZSM5

264.2

30.7

13.0

3.1

-

85.8

10.0

4.2

-

50

[18]

Matrimid

Zeolite KY

-

266

-

-

2.15

-

-

-

124

-

[19]

Phenolic resin

Boehmite

2047

1148

153

32.7

-

65.6

35.1

4.7

-

20

[40]

Polyimide

MWCNTs

-

866.6

694.0

213.2

-

-

4.1

3.3

-

26

[22]

polyetherimide

MWCNTs

-

1463

723.6

30

-

-

48.8

24.2

-

22

[23]

Polyimide

MWCNTs

5824

6661

1259

253

161

23.02

26.33

4.98

41.37

25

This work

Polyimide poly(aryl ether ketone)

Poly(benzimi dazole)

19

Room temperature Room temperature

[38] [39]

Figure captions Figure 1. Schematic of the preparation procedure for C/CNTs hybrid membrane. Figure 2. (a) Surface (b) cross-section SEM images of PAA/CNTs composite membranes, (c) surface (d) cross-section SEM images of C/CNTs hybrid membrane. Figure 3. (a) HRTEM image of C/CNTs hybrid membrane, and (b) selected area diffraction (SAED) pattern of MWCNTs. Figure 4. The effect of CNTs types on (a) gas permeabilities and (b) selectivities of C/CNTs hybrid membranes. Figure 5. The effect of acid treatment on (a) gas permeabilities and (b) selectivities of C/CNTs hybrid membranes. Figure 6. FTIR spectra of untreated MWCNTs and acid-treated MWCNTs. Figure 7. Effect of CNTs concentrations on (a) gas permeabilities and (b) selectivities of C/CNTs hybrid membranes. Figure 8. Effect of the CNTs lengths of on (a) gas permeabilities and (b) selectivities of C/CNTs hybrid membranes. Figure 9. Effect of MWCNTs diameters on (a) gas permeabilities and (b) selectivities of C/CNTs hybrid membranes. Figure 10. Separation performance of carbon membrane versus Robeson’s upper bound (2008). Figure 11. Mixed gas separation performance of C/CNTs hybrid membranes for a CO2:N2 (50:50 vol%) feed gas mixture.

20

Figure 1 Lin Li, et.al. to Separation and Purification Technology

21

Figure 2 Lin Li, et.al. to Separation and Purification Technology

22

Figure 3 Lin Li, et.al. to Separation and Purification Technology

23

Figure 4 Lin Li, et.al. to Separation and Purification Technology

10000 Pure CMSM SC1 MC2

Permeability (Barrer)

(a) 1000

100

10

1

H2

O2

CO2

N2

CH4

80 70

(b)

Selectivity

60 50

Pure CMSM SC1 MC2

40 30 20 10 0 H2/N2

O2/N2

CO2/N2

24

CO2/CH4

Figure 5 Lin Li, et.al. to Separation and Purification Technology

8000 (a)

MC2 (without acid treatment)

Permeability (Barrer)

MC2

6000

4000

2000

0

H2

CO2

O2

N2

CH4

50 (b)

Selectivity

40 MC2 (without acid treatment) MC2

30 20 10 0 H2/N2

O2/N2

CO2/N2

25

CO2/CH4

Figure 6 Lin Li, et.al. to Separation and Purification Technology

Transmittance

MWCNTs

Acid-treated MWCNTs

4000 3500 3000 2500 2000 1500 1000 -1

Wavenumber (cm )

26

500

Figure 7 Lin Li, et.al. to Separation and Purification Technology

10000 H2 CO2 O2

1000

N2 CH4

100

10

1

0

5%

10%

70 (b)

15%

H2/N2 CO2/N2

60 Selectivity

Permeability (Barrer)

(a)

O2/N2 CO2/CH4

50 40 30 20 10 0 0

10%

5%

27

15%

Figure 8 Lin Li, et.al. to Separation and Purification Technology

MC1 MC2

6000 5000 4000 3000 2000 1000 0

H2

CO2

O2

CH4

N2

50 (b)

40 Selectivity

Permeability (Barrer)

7000 (a)

MC1 MC2

30 20 10 0 H2/N2

O2/N2

CO2/N2

28

CO2/CH4

MC3 MC2

7000 (a) 6000 5000 4000 3000 2000 1000 0

H2

CO2

O2

N2

CH4

50 (b)

40 Selectivity

Permeability (Barrer)

Figure 9 Lin Li, et.al. to Separation and Purification Technology

MC3 MC2

30 20 10 0 H2/N2

O2/N2

CO2/N2

29

CO2/CH4

Figure 10 Lin Li, et.al. to Separation and Purification Technology

1000 200

obe

son

's u p

CO2/N2

100

8R

10

1

per

bou

nd

MC2

Pure carbon membrane

1

10

100

1000

10000

CO2 permeability (Barrer)

30

100000

Figure 11 Lin Li, et.al. to Separation and Purification Technology

35 30

1000

25 20

100 15 10

CO2

10

N2 CO2/N2

5

1

0 5%

10%

31

15%

Selectivity

Permeability (Barrer)

10000

Highlight  Hybrid membranes are fabricated by pyrolyzing poly(amic acid) embedded with CNTs.  Hybrid membranes exhibit significantly improvement on gas separation performance.  MWCNTs are more effective than SWCNTs in enhancing gas permeabilities.  Gas permeabilities increase with the increase of MWCNTs concentrations.  Larger dimensions of MWCNTs demonstrate enhanced gas permeabilities.

32