Preparation and gas separation performance of supported carbon

Journal of Membrane Science 450 (2014) 469–477

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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Preparation and gas separation performance of supported carbon membranes with ordered mesoporous carbon interlayer Lin Li a, Chengwen Song a,b,n, Huawei Jiang a, Jieshan Qiu a, Tonghua Wang a,nn 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 School of Environment Science and Engineering, Dalian Maritime University, 1 Linghai Road, Dalian 116026, China

art ic l e i nf o

a b s t r a c t

Article history: Received 10 April 2013 Received in revised form 20 August 2013 Accepted 17 September 2013 Available online 25 September 2013

An ordered mesoporous precursor synthesized by the soft-templating approach has successfully been used to prepare a carbon interlayer between the thin separation layer and the support of carbon membranes. Morphology and pore structure characteristics of the ordered mesoporous carbon (OMC) interlayer were investigated by HRTEM, XRD, SEM and N2 adsorption techniques. Gas separation properties of resultant supported carbon membranes were evaluated by single gas permeation experiments. The results showed that the OMC interlayer can effectively reduce surface defects of the support with large pore sizes, improve the interfacial adhesion of the support to the thin separation layer, and further enhance the gas permeation properties of the supported carbon membranes by ordered and uniform mesoporous channels. The supported carbon membranes were synthesized by one-step coating on the support modified by the OMC interlayer and achieved O2, CO2 and H2 permeances of 74.5, 88.0 and 545.5 mol m 2 s 1 Pa 1 10 10, respectively. These are about 4 times higher than those without the interlayer and are very competitive with respect to other carbon membranes reported in literature. The results clearly indicate that this novel approach using the OMC as an interlayer to fabricate supported carbon membranes with macro–meso–microporous gradient structure have attractive potential for gas separation. & 2013 Elsevier B.V. All rights reserved.

Keywords: Carbon membranes Gas separation OMC Interlayer

1. Introduction Membrane-based gas separation has demonstrated outstanding advantages over conventional cryogenic distillation and pressure swing adsorption in terms of low energy consumption, low capital investments, simple and easy operation, and compact equipment [1]. Currently, the dominant membrane materials used in industrial gas separation applications are polymeric membranes. However, the encountered challenges of harsh environments during membrane operation and the limitation of the trade-off between permeance and selectivity of polymeric membranes have prompted the search for more robust materials with higher permeance and selectivity. Porous inorganic membranes, exhibiting both molecular sieving properties and better selectivity, thermal stability and chemical stability, such as zeolite membranes, silica membranes and carbon membranes, have gained

n Corresponding author at: State key Laboratory of Fine chemicals, Carbon Research Laboratory, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China. Fax: þ 86 411 84724342. nn Corresponding author. Fax: þ 86 411 84724342. E-mail addresses: [email protected] (C. Song), [email protected] (T. Wang).

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.09.032

considerable attention over the past several years. Silica membranes can selectively separate hydrogen from other gases but selectivity between similar-sized molecules, such as oxygen and nitrogen is not sufficient. Zeolite membranes can separate isomers, but it is difficult to obtain a large, crack-free zeolite membranes. Carbon membranes, as one of the most promising porous inorganic membranes, have demonstrated their outstanding role in gas separation processes [2–4]. It is not only due to their significant advantages that can separate gas molecules under harsh conditions (i.e., elevated temperature and pressure) but also due to their molecular sieving capability derived from their ultramicropore structure with the dimensions close to the size of permeating gas molecules, which help them to achieve a desirable and practicable selectivity [5–7]. Carbon membranes are typically prepared from inert or vacuum pyrolysis of various polymeric materials such as poly(furfuryl alcohol), phenolic resins, polyacrylonitrile, coal tar pitch, and polyimide. Among these, polyimide, synthesized by different types of dianhydrides and diamines, has been utilized extensively as the precursor to prepare carbon membranes. In general, carbon membranes can be grouped into two categories. Unsupported carbon membranes (flat, capillary or hollow fiber) [8–10] and supported carbon membranes (flat or tube) [11–13]. In the case of the former, carbon hollow fiber membranes are

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preferable due to their low cost, high packing density and high separation performance from the standpoint of large-scale application. However, the brittleness of carbon hollow fiber membranes makes them difficult to handle, and limits their applications in membrane separation [14–17]. The supported carbon membranes are therefore regarded as the favored choice for commercial application of carbon membranes owing to their thin separation layer and high mechanical strength. In the preparation of supported carbon membranes with excellent separation performance, the pore structure and surface roughness of the support play important roles in the formation of a uniform and defect-free thin separation layer [18]. The pore diameter of support should be about an order of magnitude smaller than the thickness of separation layer [15] and the surface of the support should be polished to reduce surface flaws and improve interfacial adhesion before coating [18]. Besides the uniform pore size distribution in the support, it is also necessary to make the coating solution easier to cover the surface of the support completely and form a uniform thin separation layer with high gas separation performance. For those reasons, the small pore diameter (SPD) supports, having usually less than 0.2 μm with smooth surfaces, are preferably used. A defect-free thin separation layer could be formed on the support by one subsequent casting step in the fabrication of supported carbon membranes [17,19]. The large pore diameter (LPD) support with usually greater than 0.2 μm and a rougher surface does not generally produce high-quality supported carbon membranes because it is prone to initiate interfacial defects during the formation of separation layer. Thus, several coating– pyrolysis cycles are required to reduce the surface defects and form a defect-free separation layer [20,21] which greatly reduces the gas permeance of the obtained supported carbon membranes. As a result, the low gas permeance of supported carbon membranes obtained on LPD or SPD supports can hardly satisfy practical separation requirements [22]. Solving these problems is still a big challenge for the commercial application of supported carbon membranes. An effective solution may be to build a bridge between the thin separation layer and LPD support to improve their interfacial adhesion and enhance the structural stability and separation performance of supported carbon membranes. Ordered mesoporous carbon (OMC), a porous carbon material synthesized by nanocasting (hard-template) or self-assembly (soft-template) methods and carbonized at high temperature, can be a good candidate for this bridge. This is because of its regular mesoporous structure, narrow pore size distribution, high surface area, large pore volume and surface functional groups, among other properties [23–27]. An attempt to establish a framework of supported carbon membranes with macro–meso–microporous gradient structure was proposed in this study, in which the OMC was used as an interlayer (i.e., bridge) to link the thin separation layer and support by improving the interfacial adhesion between them. In this work, an ordered mesoporous precursor synthesized by the soft-template method was first coated on LPD coal-based carbon disk and carbonized to form the OMC interlayer. Then the poly(amic acid) (PAA), which exhibited an outstanding gas separation performance in preparation of carbon membranes due to its high thermal stability and high carbon residues, was coated on the top and carbonized to form a thin separation layer, and finally, the supported carbon membranes with unique macro–meso–microporous gradient structure were fabricated. The specific objective of the present study was to establish the interlayer as a bridge to link the thin separation layer and support and

discuss the influence of preparation parameters (e.g., R/F127, synthetic temperature, OMC precursor solution concentration) on structure and gas permeance of the OMC interlayer in detail. Also, gas permeance and selectivity of supported carbon membranes with the ordered mesoporous interlayer are compared to those without the interlayer. Results of this work indicate that introduction of the OMC interlayer between the separation layer and LPD support can greatly improve the gas permeance of the resultant supported carbon membranes by reducing the gas diffusion resistance through the membranes. With this novel approach, highly permeable and defect-free supported carbon membranes for gas separation can easily be derived.

2. Experimental 2.1. Materials Coal powder was provided by Ningxia Coal Mine. Triblock copolymer Pluronic F127 (PEO106–PPO70–PEO106) (MW ¼12,600) was purchased from Sigma–Aldrich Corp. Resorcinol (R) was supplied from Tianjin Damao Chemical Reagent Factory. Formaldehyde (37 wt%) (F) was bought from Shenyang Lianbang Chemical Reagent Factory. HCl (37 wt%) was obtained from Beijing Beihua Fine Chemical Co., Ltd. Ethanol was purchased from Tianjin Fuchen Chemical Reagent Factory. PAA was purchased from Tianjin Insulation Materials Factory. Dimethylacetamide (DMAc) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. All chemicals were used as received without any further purification. 2.2. Preparation of disk-shape support from cheap coal The coal was used to prepare the support, in which fine particles were blended with a binder and pressed at 20 MPa in a static press to form a disk-shape support of 40 mm in diameter and 2 mm in thickness. After drying, the disks were carbonized to 900 1C at the rate of 3 1C/min and held for 30 min in an Ar atmosphere and cooled down to room temperature naturally. Properties of resultant carbon disks are listed in Table 1. 2.3. Preparation of supported carbon membranes The polymeric precursor of the OMC was synthesized by an organic–organic self-assembly method using resorcinol (R) and formaldehyde (F) as the carbon precursor and F127 block polymers as the soft template, according to literature [28]. This mixture was used as the coating solution, which was diluted by dimethylacetamide (DMAc), to form an interlayer on the disk by the spin-coating method. After drying, it was carbonized to 800 1C at the rate of 1 1C/min in an Ar atmosphere and held for 2 h before cooling down to room temperature. Poly(amic acid) (PAA) precursors derived from pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) in DMAc were stirred at room temperature for 12 h under a nitrogen atmosphere to obtain a homogeneous and viscous PAA solution [29]. Then, the PAA solution was coated on the disk with the interlayer by spin-coating method to form the thin top layer. The supported carbon membranes were heated at room temperature at a heating rate of 2 1C/min to 400 1C and held at this temperature for 60 min and then continued to be heated at a rate of 2 1C/min to the temperature of 700 1C in the

Table 1 Properties of coal-based carbon disk supports. Sample

Diameter (mm)

Thickness (mm)

Porosity (%)

The largest pore size (μm)

N2 Permeance (mol m 2 s 1 Pa 1 10 10)

Support

40

2

33.9

0.71

22,000


flowing Ar at 200 ml/min. Subsequently, the membranes were cooled to ambient temperature. 2.4. Characterizations The pore structure characteristics of the support were tested by the bubble-pressure method with isopropanol as wetting liquid and nitrogen as permeation gas [30]. The porous structure of the OMC was characterized by the nitrogen sorption technique (Micromeritics ASAP 2020). X-ray diffraction (XRD) patterns of the OMC were recorded using a D/Max-2400 diffractometer (Cu Kα radiation, λ ¼1.54055 Å) in a range of diffraction angle 2θ from

471

01 to 51 to analyze the diffraction peaks of the OMC. The morphology of the carbon membranes and OMC was observed by scanning electron microscopy (KYKY-2800B), field emission scanning electron microscopy (NOVA NanoSEM 450) and highresolution transmission electron microscopy (Philips TECNAI G220). Except for SEM images in Fig. 9 obtained by characterizing the OMC layer, other characteristics of the OMC samples were obtained by analyzing OMC powder prepared under similar conditions. The gas permeation performance of the supported membranes was measured using single gases with high purity (H2, CO2, O2 and N2) at room temperature by the traditional variable

Intensity (a.u.)

Intensity (a.u.)

R/F127=370 R/F127=185 R/F127=123

20°C

30°C 40°C

R/F127=93 1

2

3

4

5

2 Theta (degree) Fig. 1. XRD patterns of the mesoporous carbons obtained from different molar ratios of R/F127.

0

1

2 3 2 Theta (degree)

4

5

Fig. 3. XRD patterns of mesoporous carbon materials obtained from different reaction temperatures at R/F127 ratio of 123.

Fig. 2. TEM images of mesoporous carbon materials obtained at different molar ratios of R/F127. (a) 185, (b) 123 and (c) 93.

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volume-constant pressure method [17]. The gas provided by compressed gas cylinder was introduced to the upper side of a stainless membrane cell and the pressure was regulated at 0.1 MPa by a pressure regulator. 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 relative standard deviation of experimental data obtained in this work was in the range of 2–8%.

3. Results and discussion 3.1. Effect of synthetic parameters on morphology and structure of polymeric precursor of ordered mesoporous carbon Two factors, molar ratios of resorcinol to F127 (R/F127) in the synthetic solution and synthetic reaction temperature, which affect the pore structure of the resultant mesoporous carbons, were investigated. Figs. 1 and 2 show the XRD patterns and TEM

Fig. 4. TEM images of mesoporous carbon materials were performed at (a) 20 1C, (b) 30 1C view along the [111] direction, (c) 30 1C view along the [100] direction, (d) 30 1C view along the [110] direction, and (e) 40 1C and R/F127 ratio of 123.


Volume adsorbed (cm3 STP /g)

500 400 300 200 100 0 0.0

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

1.0

5 10 Pore diameter (nm)

15

Pore volume (cm3g-1)

0.015

0.010

0.005

0.000

0

Permeance (mol·m-2·s-1·Pa-1×10-10)

Fig. 5. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distributions for mesoporous carbon synthesized at 30 1C.

12000 10000

O2 N2

8000 6000 4000 2000 0

60 65 70 75 OMC solution concentration (wt %)

Fig. 6. Effect of solution concentration on gas permeation performance of the LPD support with OMC interlayer at the spin-coating speed of 3000 rpm.


images of the mesoporous carbons obtained from different molar ratios of R/F127, which were adjusted from 370 to 93. There is no diffraction peak on the XRD pattern at the high molar ratios of R/ F127 (Z185) in the synthetic solution, indicating that high molar ratios of R/F127 is unsuitable to form ordered pore structure in the obtained mesoporous carbons. The TEM image in Fig. 2a proves that the pore structure of mesoporous carbons fabricated by high molar ratios of R/F127 have a worm-like pore structure. When the molar ratio of R/F127 reaches 123, a strong reflection peak can be clearly observed (at 2θ¼ 0.51–11) on the XRD pattern, which suggests that a high-ordered pore structure is formed in the prepared mesoporous carbons [28]. The TEM image (Fig. 2b) also gives the obvious evidence that there is an ordered pore structure. With further decrease of the molar ratio of R/F127 to 93, the order degree of mesoporous carbon decreases or disappears. This is attributed to the excess presence of the F127 block copolymer resulting in a phase transition to the worm-like pore structure [28,31]. The TEM image in Fig. 2c also shows the new worm-like pore structure is formed again in the resultant mesoporous carbon at a low molar ratio of R/F127. These results suggest that controlling a proper molar ratio of R/F127 is required for the formation of a highly ordered pore structure in the resultant mesoporous carbon. Figs. 3 and 4 show XRD patterns and TEM images of the mesoporous carbon materials prepared at 20 1C, 30 1C and 40 1C and R/F127 ratio of 123. When reaction temperature is controlled at 20 1C, no obvious diffraction peak was observed from the XRD pattern. The reason may be attributed to the fact that most of the block polymers exist in the solutions as unimers at low temperature, which are hard to interact with resorcinol–formaldehyde (RF) [32]. Moreover, the sol–gel polycondensation of resorcinol (R) and formaldehyde (F) is slow and produces low cross-linked degree polymers at 20 1C, which cannot form a stable skeleton structure after carbonization. TEM images also show the disordered structure of the mesoporous carbon, as shown in Fig. 4a. With the increase of reaction temperature to 30 1C, a strong peak at 2θ¼0.71 appeared, which revealed that an excellent ordered mesoporous structure was formed in the carbon matrix after the block polymers were aggregated to produce micelles [32]. The representative TEM images and corresponding Fourier diffractograms (Fig. 4b–d) show that mesoporous carbon materials synthesized at 30 1C contain a high degree of periodicity over large domains viewed from the [100], [110], and [111] directions. This demonstrates that the mesostructure has highly ordered body-centered cubic Im3m structure [28,33]. As the synthesis temperature reaches 40 1C, the diffraction peak is no longer observed from XRD pattern and a worm-like structure was formed (Fig. 4e). The reason may be that higher reaction temperature improves the hydrophobic character of the block polymers, causing a structural change in the mesoporous carbon leading to the formation of the disordered structure [34]. Apparently, the pore structure property of mesoporous carbon can be optimized by adjusting to an appropriate reaction temperature. N2 adsorption isotherms and pore size distribution of mesoporous carbon materials synthesized at 30 1C are presented in Fig. 5. It is clearly shown that the isotherms show type-IV curves with a sharp capillary condensation step at P/P0 ¼0.40– 0.85, illustrating this material has typical mesoporous channels. The BET surface areas and pore volumes are calculated to be 596.56 m2 g 1 and 0.47 cm3 g 1, respectively. The pore size distribution obtained from the adsorption branch of the N2 isotherm using the Barret–Joyner–Halenda (BJH) method was very narrow, centering around 6.5 nm, which is consistent with the TEM images (Fig. 4c and d). It indicates that the resultant carbon material has a well-ordered structure with uniform mesopores.

473

12000 10000

O2 N2

8000 6000 4000 2000 0

3000

4000

5000

6000

Spin speed (rpm) Fig. 7. Effect of spin speed on permeation performance of interlayer at OMC solution concentration of 70 wt%.

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3.2. Effect of preparation conditions on gas permeance of ordered mesoporous interlayer The ordered mesoporous interlayer was prepared by coating the polymeric precursor of the OMC on the LPD support. Effects of preparation conditions and pore structure characteristics of the obtained mesoporous carbons on gas permeation properties of mesoporous interlayer were investigated and discussed. Fig. 6 shows O2 and N2 gas permeance of the LPD support with OMC interlayer prepared at the spin-coating speed of 3000 rpm within the concentration range of 60–75 wt%. High gas permeance of the interlayer was not achieved at low solution concentration (60 and 65 wt%) because the coating solution could easily infiltrate the pore of the support and prevent the formation of a regular interlayer. By increasing the concentration to 70 wt%, O2 and N2 permeance are improved to 9236 and 9324 mol m 2 s 1 Pa 1 10 10, indicating that the relatively high viscosity of coating solution is beneficial to form a regular coating layer. However, the higher concentration (higher than 75 wt%) might cause some defects (e.g., cracks) to form on the interlayer after pyrolysis due to the excessive thickness. Therefore, the suitable concentration of the coating solution is required to form a defect-free thin mesoporous interlayer on the LPD support with high gas permeance. In addition, the LPD support modified by the OMC interlayer shows lower gas selectivity to O2 and

N2. This is because the gas diffusive mechanism in the modified support obeys the Knudsen diffusion mechanism. Fig. 7 shows the effect of spin-coating speed on O2 and N2 gas permeance of the LPD support with OMC interlayer at OMC solution concentration of 70 wt%. The O2 and N2 permeance of the OMC interlayer are almost the highest at the spin-coating speed of 3000 rpm, and are reduced to 6084, and 6374 mol m 2 s 1 Pa 1 10 10, respectively, with the increase of the spin-coating speed to 4000 rpm. And then the O2 and N2 permeance begin to rise again as the spin-coating speed rises to 6000 rpm. As is known, a low spincoating speed could form a thick interlayer on the support which will crack after pyrolysis at the high temperature [35]. At the high spincoating speed, the formed interlayer might be too thin to cover the surface of support completely due to the high centrifugal force. Hence, it is necessary to control a proper spin-coating speed for preparing a uniform and defect-free thin interlayer on the LPD support with high gas permeance. Table 2 shows gas permeation properties of the OMC interlayer with two typical pore structure (worm-like and ordered mesoporous structure) synthesized in this study. The interlayer with the ordered mesoporous structure demonstrated nearly 200% higher O2 and N2 permeance than that with the worm-like structure. It indicates that the type of pore structure should greatly influence the gas permeance of derived mesoporous interlayer. The ordered mesoporous interlayer

Table 2 Comparison of gas permeance of OMC interlayer with MCM-type mesoporous membranes reported in literature. Sample

O2 Permeance (mol m 2 s 1 Pa 1 10 10)

N2 Permeance (mol m 2 s 1 Pa 1 10 10)

Reference

Worm-like interlayer Order mesoporous interlayer MCM-48 MCM-48 MCM-41(“LUS”)

3411 7362 – – –

3638 8357 2000 860–2540 193,000

In this work In this work [36] [37] [38]

Fig. 8. SEM micrographs of support (a) surface (b) cross and support modified by ordered mesoporous carbon interlayer (c) surface (d) cross.


Fig. 8 shows the typical SEM images of surfaces and the cross section of the support and that modified by OMC interlayer. The surface morphology of the porous support was rather rough (Fig. 8a and b), which makes it hard to use for preparing defectfree supported carbon membranes. After being modified by OMC, the surface of the support is completely covered with an OMC interlayer (Fig. 8c and d) and becomes more smooth and defectfree. In addition, the order and uniform mesoporous channels in the interlayer enhanced the gas permeation properties of the supported carbon membrane. The typical SEM images of the composite carbon membranes are shown in Fig. 9. Three parts can be distinguished in the cross section: micropore separation layer, mesoporous interlayer and macroporous support. The separation layer with a thickness of ca. 2 mm is very smooth and almost defect-free, and is more compact than the interlayer and support. The interlayer, which is ca. 4 mm in thickness, connects tightly with the separation layer and support. This gives evidences that a defect-free composition carbon membrane with macro–meso–microporous gradient structure can be prepared easily in one coating step on support modified by OMC interlayer. Fig. 10 shows the gas separation performance of supported carbon membranes with ordered mesoporous interlayer and worm-like interlayer. The permeance of H2, CO2, O2 and N2 of supported carbon membranes obtained with the ordered mesoporous interlayer were 545.5, 88.0, 74.5 and 7.15 mol m 2 s 1 Pa 1 10 10, respectively, which were about 1.2–1.8 times higher than those with the wormlike interlayer. In addition, supported carbon membranes obtained with the ordered mesoporous interlayer still demonstrate their advantages in H2/N2, CO2/N2 and O2/N2 selectivity (76.3, 12.3 and 10.4, respectively), which are about 1.1–1.5 times higher than those with the worm-like interlayer. All this indicates that the support modified by the ordered mesoporous interlayer is favorable to prepare


3.3. Morphology and gas separation performance of supported carbon membranes with macro–meso–microporous gradient structure

a supported carbon membrane with a uniform and defect-free thin top layer and high gas separation performance. Additionally, Fig. 10 also presents the relationship between gas permeance of the carbon membrane and the kinetic diameter of permeation gases. The order of gas permeance for the tested gases, H2 4CO2 4O2 4N2, is inversely related to the order of the kinetic diameter of the tested gases, proving that the gas diffusion through carbon membranes follows the molecular sieving mechanism. In order to evaluate gas separation performance of supported carbon membranes obtained using the novel method, gas permeance and selectivity were compared to those without interlayer prepared in this work, as well as with other carbon membranes reported in literature. As shown in Table 3, three cycle coating/ carbonization procedures are needed for preparing defect-free carbon membranes using a support without an interlayer and the thickness of a separation layer is ca. 5 mm. Its gas permeance of O2, CO2 and H2 is 14.4, 25.8 and 133 mol m 2 s 1 Pa 1 10 10, respectively, and the CO2 permeance is about 1.79 times higher

1000

Worm-like interlayer Order mesoporous interlayer

H2

100 CO2

O2

10 N2

1 2.8

3.0

3.2

3.4

3.6

Kinetic diameter (Å) 100 Worm-like interlayer Order mesoporous interlayer

80 Selectivity

with the hexagonal structure and regular arrays of uniform channels provided readily accessible diffusion paths for gas molecules and exhibited the high gas permeance [28], while the worm-like interlayer with the random interconnective nano-channels gave rise to high gas diffusion resistance and lead to the low gas permeance. Compared to other mesoporous membranes, N2 permeance of the interlayer with the order mesoporous structure in this work were still 3–4 times higher than those of MCM-48 membranes [36,37], although it was lower than that of the MCM-41 (“LUS”) membrane [38]. This clearly indicates that an interlayer with the order mesoporous structure has a great advantage as interlayer in preparing supported carbon membranes.

475

60 40 20 0

H2/N2

CO2/N2

O2/N2

Fig. 10. Gas permeability (a) and selectivity (b) of supported carbon membranes with the order mesoporous interlayer and worm-like interlayer.

Fig. 9. SEM micrographs of supported carbon membrane (a) surface (b) cross.

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Table 3 Comparison of gas separation performance of supported carbon membranes with those reported in literature. Precursor

Coating time

Thickness of separation layer (μm)

Selectivity of (A/B)c

Selectivity

Permeance of Ad (mol m 2 s 1 Pa 1 10 10)

Reference

PMDA–ODA polyimidea

1

2

O2/N2 CO2/N2 H2/N2

10.4 12.3 76.3

74.5 88.0 545.5

In this work

PMDA–ODA polyimideb

3

5

O2/N2 CO2/N2 H2/N2

10.2 18.3 94

14.4 25.8 133

In this work

BPDA–ODA polyimide

3

6

O2/N2

9.7

30

[41]

Matrimid polyimide

1

11

CO2/N2 H2/N2

1.86 4.46

41 98.2

[42]

Polyimidee

–

–

O2/N2 CO2/N2

12 58

83 351

[14]

6FDA/BPDA–DAM Polyimidee

–

–

O2/N2

7.1

137

[16]

Matrimids 5218e

–

–

O2/N2 CO2/N2 H2/N2

5.5 21.83 146.5

2.2 8.8 58.8

[40]

Phenolic resin

1

35

O2/N2 H2/N2

1.8–2.8 24.1–39.5

2.52–3.48 40.1–51.2

[19]

Phenolic resin

1

3

O2/N2

10.8

6.8

[43]

Poly(furfuryl alcohol)

3

21.3

O2/N2 H2/N2

30.4 331

0.557 6.05

[44]

Poly(furfuryl alcohol)

1

10

O2/N2

12.5

1.97

[30]

CO2/N2 H2/N2

31.4 347

4.92 54.55

a

PMDA–ODA polyimide was coated on support with interlayer. PMDA–ODA polyimide was coated on support without interlayer. Selectivity of A gas to B gas in the following separation system. d Permeance of A gas. e Hollow fiber carbon membranes. b c

than O2 permeance. In the preparation of the carbon membranes using the support modified by OMC interlayer, defect-free carbon membranes can be fabricated by only one coating step and the thickness of separation layer is reduced to ca. 2 mm. Its gas permeance of O2, CO2 and H2 is enhanced to 74.5, 88.0 and 545.5 mol m 2 s 1 Pa 1 10 10 respectively, which is almost 4 times higher than those without interlayer. These results indicate that the OMC has great advantage in acting as an interlayer to prepare high-quality supported carbon membranes. In the case of CO2 permeance, it is only 1.18 times higher than O2 permeance. This phenomenon may be attributed to high transport resistance caused by CO2 adsorption in the OMC interlayer [39], which affects the CO2 permeance more significantly than for O2. Compared to carbon hollow fiber membranes in earlier studies, carbon membranes derived in this work demonstrate certain advantage than those in Refs. [16 and 40] , but slightly lower than those in Ref. [14]. The permeance and selectivity of tested gases in this work are higher than those of supported carbon membranes derived from BPDA–ODA polyimide [41], Matrimid polyimide [42], and phenolic resin [19,43]. Despite being lower selectivity than supported carbon membranes reported in Refs. [30 and 44], the significant increase (10–37 times and 90–133 times, respectively) of gas permeance of carbon membranes derived in this work exhibits a great attraction for its commercial application in gas separation.

bridge) between the thin separation layer and LPD support to improve their interfacial adhesion and the structural stability in the preparation of supported carbon membranes. By this way, defect-free supported carbon membranes with macro–meso– microporous gradient structure were fabricated on a LPD support and showed an excellent gas permeance for O2, CO2 and H2 gases and selectivity for H2/N2, CO2/N2 and O2/N2 gas pairs compared to the carbon membranes without interlayer prepared in our work and other carbon membranes reported in literature. In the fabrication of the OMC interlayer, the precursors with the highly ordered mesoporous structure was synthesized and optimized by adjusting to an appropriate reaction temperature and molar ratio of R/F127. Controlling a suitable concentration of coating solution and spincoating speed is required to form a defect-free and uniform mesoporous interlayer on the LPD support with high gas permeance. The novel approach, which adapts OMC as an interlayer to fabricate the composite carbon membranes with unique macro– meso–microporous gradient structure on the LPD support, will provide the possibility of preparing supported carbon membranes with high gas permeance from a support with large pore size and surface flaws and speed up the commercial application of carbon membranes for gas separation.

Acknowledgments 4. Conclusions An ordered mesoporous precursor synthesized by the softtemplate approach can be used as the mesoporous interlayer (i.e.,

This work was supported by the National Natural Science Foundation of China (No. 20776024, 20836006, 20976021, 211760 63, 21276035), the National High-tech Research and Development Project of China (2009AA03Z215, 2012AA03A611), The Scientific


Research Project of Education Department of Liaoning Province (L2013203), and the Fundamental Research Funds for the Central Universities (3132013085). Special thanks go to Dr. and Prof. Christopher T. Williams, the seasky scholar of Dalian University of Technology, and Dr. Yanan Zhao for their discussions and help in English polishing. References [1] S.M. Saufi, A.F. Ismail, Fabrication of carbon membranes for gas separation—a review, Carbon 42 (2004) 241. [2] A.F Ismail, L.I.B. David, A review on the latest development of carbon membranes for gas separation, J. Membr. Sci. 193 (2001) 1. [3] J. Su, A.C. Lua, Effects of carbonisation atmosphere on the structural characteristics and transport properties of carbon membranes prepared from Kaptons polyimide, J. Membr. Sci. 305 (2007) 263. [4] C.J. Anderson, S.J. Pas, G. Arora, S.E. Kentish, A.J. Hill, S.I. Sandler, G.W. Stevens, Effect of pyrolysis temperature and operating temperature on the performance of nanoporous carbon membranes, J. Membr. Sci. 322 (2008) 19. [5] Q.L. Liu, T.H. Wang, C.H. Liang, B. Zhang, S.L. Liu, Y.M. Cao, J.S. Qiu, Zeolite married to carbon—a new family of membrane materials with excellent gas separation performance, Chem. Mater. 18 (2006) 6283. [6] L. Li, T.H. Wang, Q.L. Liu, Y.M. Cao, J.S. Qiu., A high CO2 permselective mesoporous silica/carbon composite membrane for CO2 separation, Carbon 50 (2012) 5186. [7] B. Zhang, T.H. Wang, Y.H. Wu, Q.L. Liu, S.L. Liu, S.H. Zhang, J.S. Qiu, Preparation and gas permeation of composite carbon membranes from poly(phthalazinone ether sulfone ketone), Sep. Purif. Technol. 60 (2008) 259. [8] T.H. Wang, B. Zhang, J.S. Qiu, Y.H. Wu, S.H. Zhang, Y.M. Cao, Effects of sulfone/ ketone in poly(phthalazinone ether sulfone ketone) on the gas permeation of their derived carbon membranes, J. Membr. Sci. 330 (2009) 319. [9] J. Petersen, M. Matsuda, K. Haraya, Capillary carbon molecular sieve membranes derived from Kapton for high temperature gas separation, J. Membr. Sci. 131 (1997) 85. [10] M. Yoshinoa, S. Nakamuraa, H. Kitaa, K. Okamotoa, N. Taniharab, Y. Kusukib, Olefin/paraffin separation performance of carbonized membranes derived from an asymmetric hollow fiber membrane of 6FDA/BPDA–DDBT copolyimide, J. Membr. Sci. 215 (2003) 169. [11] M.B. Shiflett, H.C Foley, Ultrasonic deposition of high selectvity nanoporous carbon membranes, Science 285 (1999) 1902. [12] C.W. Song, T.H. Wang, H.W. Jiang, X.Y. Wang, Y.M. Cao, J.S. Qiu, Gas separation performance of C/CMS membranes derived from poly(furfuryl alcohol) (PFA) with different chemical structure, J. Membr. Sci. 361 (2010) 22. [13] C.W. Song, T.H. Wang, X.Y. Wang, J.S. Qiu, Y.M. Cao, Preparation and gas separation properties of poly(furfuryl alcohol)-based C/CMS composite membranes, Sep. Purif. Technol. 58 (2008) 412. [14] C.W. Jones, W.J. Koros, Carbon molecular sieve gas separation membranes—I. Preparation and characterization based on polyimide precursors, Carbon 32 (1994) 1419. [15] D.Q. Vu, W.J. Koros, S.J. Miller, High pressure CO2/CH4 separation using carbon molecular sieve hollow fiber membranes, Ind. Eng. Chem. Res. 41 (2002) 367. [16] R. Singh, W.J. Koros, Carbon molecular sieve membrane performance tuning by dual temperature secondary oxygen doping (DTSOD), J. Membr. Sci. 427 (2013) 472. [17] X. He, M.B. Hagg, Hollow fiber carbon membranes: investigations for CO2 capture, J. Membr. Sci. 378 (2011) 1. [18] W. Wei, S. Xia, G. Liu, X. Gu, W. Jin, N. Xu, Interfacial adhesion between polymer separation layer and ceramic support for composite membrane, AIChE J. 56 (2010) 1584–1592. [19] W. Wei, G. Qin, H. Hu, L. You, G. Chen, Preparation of supported carbon molecular sieve membrane from novolac phenol–formaldehyde resin, J. Membr. Sci. 303 (2007) 80. [20] H.J. Lee, M. Yoshimune, H. Suda, K. Haraya, Gas permeation properties of poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) derived carbon membranes prepared on a tubular ceramic support, J. Membr. Sci. 279 (2006) 372. [21] C.J. Andersona, S.J. Pas, G. Arorad, S.E. Kentisha, A.J. Hill, S.I. Sandler, G. W. Stevensa, Effect of pyrolysis temperature and operating temperature on the performance of nanoporous carbon membranes, J. Membr. Sci. 322 (2008) 19.

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