Separation Performance of Nanoporous Carbon Membranes

Feb 13, 2008 - Using a Ni/polyamideimide sol as a template, a nanoporous carbon membrane was prepared in a single coating by means of a novel techniqu...
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Separation Performance of Nanoporous Carbon Membranes Fabricated by Catalytic Decomposition of CH4 Using Ni/Polyamideimide Templates B. S. Liu,* N. Wang, F. He, and J. X. Chu Department of Chemistry, School of Science, Tianjin UniVersity, Tianjin 300072, People’s Republic of China

Using a Ni/polyamideimide sol as a template, a nanoporous carbon membrane was prepared in a single coating by means of a novel technique: the carbonization of a nickel-containing polyamideimide precursor combined with the catalytic decomposition of methane in a nitrogen atmosphere. The separation performance of the membrane was significantly better than that fabricated by a single coating and conventional pyrolysis, even as good as or better than those for three-coating carbon membranes. This procedure significantly reduced the cost of the membrane production and provided a promising technical way for the preparation of carbon membrane. The surface and cross-sectional morphologies, the composition, and the microstructure of the carbon membranes were characterized by means of SEM-EDX, AFM, and XRD techniques. The properties of the precursor and membranes were investigated by FT-IR absorption spectra and H2-TPD. 1. Introduction During the past two decades, inorganic membranes have attracted considerable attention due to their superior permselectivity and their stability at high temperatures and in corrosive environments.1,2 Nonpolymeric membranes, made with molecular sieving materials, such as silica,3 zeolites,4,5 carbon,6,7 and carbon-silica,8,9 have the potential to push the upper boundary of the permeability vs selectivity tradeoff relationship. Carbon is one of the most promising membrane materials for industrial applications because nanoporous carbon membranes (NPCMs) exhibit excellent mechanical strength and separation performance for small gas molecules. In addition, carbon molecular sieving membranes (CMSMs) have excellent structural stability10 because they do not suffer from the plasticization and compacting that usually occurs in polymeric membranes. In 1983, Koresh et al.11 prepared CMSMs with high permeability by the carbonization of cellulose. Since then, there have been many reports about preparation techniques and the diffusion mechanism for carbon membranes,12-20 including the selection and pretreatment of the precursor,21 the optimization of pyrolysis conditions,22,23 and the posttreatment of the membranes.24 Due to the shrinkage9,25 of the polymer materials during pyrolysis, the coating and pyrolysis procedures have to be repeated until an intact CMSM is obtained.26 This makes the production of carbon membranes very costly. The cost of a carbon membrane is reported to be between 1 and 3 orders of magnitude greater than that of a typical polymeric membrane.27 In addition, the deposition process must be carefully controlled in order to produce a membrane with a thickness below 20 ( 3 µm because catastrophic cracking occurs in the vicinity of this critical film thickness.6 Currently, the most popular precursor for manufacturing carbon membranes is polyimide although it gives the best separation for small gas molecules. It is very expensive and is sometimes commercially unavailable. In order to reduce the overall cost and pyrolysis times, several research groups have utilized other polymers,28 such as polyacrylonitrile (PAN), to prepare the CMSMs. However, the separation performance of these membranes is poor. In this work, NPCMs were prepared on an aluminum support using a combination of * To whom correspondence should be addressed. Tel.: 86-2227892471. Fax: 86-22-87892946. E-mail: [email protected].

carbonizing the polymers at high temperature and catalytic decomposition of methane with Ni/polyamideimide as a template. This novel technique was able to patch the defects formed during pyrolysis, and to prepare NPCMs via a single coating procedure. The separation performance and the diffusion mechanism of gases through these NPCMs were investigated. 2. Experimental Section 2.1. Preparation of Carbon Membrane. Tubular R-Al2O3 substrates (o.d. 1.3 cm, i.d. 0.9 cm, length 30 cm, pore diameter 0.8-1 µm, porosity 30%; from Zhibo Ceramic Co. Ltd, China), were modified by repeated dip-coating in a boehmite (γAlOOH) sol.29 The membrane tubes were dried at room temperature (RT) for 24 h and calcined at 500 °C for 5 h after each dip-coating process. A polymeric precursor containing 19 wt % polyamideimide and 0.5 wt % nickel was prepared by dissolving Ni(Ac)2‚4H2O in N-methylpyrrolidone (NMP) at 60 °C using a supersonic bath. Poly(trimellitic-anhydride chloride-co-4,4′-methylenedianiline) (428272-100g, Aldrich, USA) was then added with continuous stirring to form a transparent polymeric sol. A layer of the polymeric precursor was coated on the outer surface of the support by spinning the support against a sponge soaked with the precursor. The coated ceramic tube was allowed to gelatinate at RT for 24 h, and was then placed in a quartz tube. The temperature was increased from RT to 300 °C in a flow of nitrogen (45 mL/min), and was held at 300 °C for 1 h. The temperature was then raised at 1 °C/min to the desired carbonizing temperature (600-700 °C). After the nickelcontaining precursor was reduced at 650 °C in hydrogen (20 mL/min) for 1 h according to the report in the literature,30 the NPCMs were then formed on the support in an atmosphere of CH4/N2 (1:3 v/v). Finally, the NPCMs were treated in an atmosphere of nitrogen at 600 or 700 °C for 2 h. The preparation conditions for a variety of carbon membranes are listed in Table 1. 2.2. Characterization of NPCMs. NPCMs prepared by means of a conventional pyrolysis method and by the catalytic decomposition of CH4 were characterized by X-ray diffraction (XRD) on a BDX 3300 diffractmeter with Cu KR radiation at 30 kV and 20 mA. The peaks were identified using powder diffraction files from the 1998 ICCD PDF Database. Infrared

10.1021/ie071161f CCC: $40.75 © 2008 American Chemical Society Published on Web 02/13/2008

Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 1897 Table 1. Preparation Parameters for Different Nanoporous Carbon Membranesa membrane

M-times

M0 M01 M1 M2 M3

0 0 3 3 3

nickel (%) NA NA NA 0.5 0.5

temp (°C)

times

thickmess (µm)

600 600 600 600 700

1 2 1 1 1

13.9 8.5 4.7 6.0 4.1

a M-times, number of the ceramic tube modified repeatedly by the solgel technique; temp, temperature of membrane carbonization and CH4 decomposition; times, number of times that the carbonization was repeated.

Figure 1. Schematic diagram for the membrane separation module.

(IR) absorption spectra for the Ni/polyamideimide precursor were obtained with a Bio-Rad FTS 3000 spectrophotometer. The surface and cross-sectional morphologies of the NPCMs were observed by means of a scanning electron microscope (SEM, Philips XL-30 ESEM) operating at 20 kV and atomic force microscopy (AFM, a Digital Instruments Nanoscope-III A) using a tapping mode. The samples were treated according to the procedure reported previously.31 The C, O, Al, and Ni concentrations on the surface of the NPCMs were analyzed by energy-dispersive X-ray spectrometry (EDX). H2 temperatureprogrammed desorption (H2-TPD) of NPCMs was performed on a homemade system. A sample of ca. 30 mg was charged into a quartz reactor and pretreated at 150 °C in helium (25 mL/min) for 30 min. The sample was then exposed to H2 (18 mL/min) at RT for 1 h. After helium purging (to remove gasphase and physically adsorbed H2) to reach a stable baseline of H2, the sample was heated from RT to 850 °C at a heating rate of 10 °C/min and the amount of desorbed H2 was monitored by means of a thermal conductivity detector. A pure gas permeation measurement was performed in a twotube membrane apparatus with a quartz tube and a NPCM tube as the outer and inner tubes, respectively. A schematic diagram is shown in Figure 1. In order to avoid the effect of membrane water adsorption on the separation performance, the NPCMs were treated in a vacuum of 0.7 mmHg at 100 °C for 2 h prior to the gas permeation measurements. The pure sample gases, N2 (3.64 Å), CH4 (3.8 Å), CO2 (3.3 Å), He (2.6 Å), and H2 (2.89 Å) (the values in parentheses are the kinetic diameters of the molecules), were introduced into the outer tube and allowed to permeate through the membrane into the inner tube with a pressure difference of ca. 30 kPa between two tubes. The amount of permeated gas was measured with a soap-film flow meter. The permeance ratio of two gases is defined as the ideal separation factor. All the gas permeation experiments were repeated three times. No cracks or deterioration of NPCMs were found, indicating that the carbon membranes on the supported Al2O3 tube are mechanically strong and can withstand different pressures and temperatures.

Figure 2. Surface (a, b) and cross-sectional (c, d) SEM images of NPCMs M1 (b, c) and M2 (a, d).

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Figure 3. EDX analysis of nanoporous carbon membrane M2.

Figure 5. XRD patterns of membrane (a) M1 and (b) M2.

Figure 4. FT-IR absorption spectra for the two types of precursors: (a) Ni/polyamideimide; (b) polyamideimide. Values in parentheses are for the nickel-containing precursors.

3. Results and Discussion 3.1. Thicknesses and Morphologies of NPCMs. The thicknesses of the NPCMs were estimated from the density (F ) 1.6 g/cm3)32 and the weight of the membrane (i.e., thickness ) weight/F‚area) and are listed in Table 1. The thickness of the carbon membrane obtained on a commercial (nonmodified) R-Al2O3 tube was large (13.9 µm), indicating that the polyamideimide precursor easily enters the large pores of the substrates and forms a thick carbon layer during pyrolysis. After the R-Al2O3 tube was modified with a sol-gel technique, a γ-Al2O3 film forms a pore diameter ca. 4 nm29 on the outer surface of R-Al2O3 tubes. The NPCM prepared on the surface of the γ-Al2O3 results in a thin top layer (4.7 µm) (Table 1, M1). The surface and cross-sectional SEM images (Figure 2) for membranes M1 and M2 reveal an intact, continuous, and homogeneous carbon structure with no cracking or other imperfections. The thickness of membrane M1 (Figure 2c) was ca. 5 µm, which is similar to the value estimated from the weight (4.7 µm). For the NPCM prepared by polyamideimide precursor carbonization and catalytic decomposition of CH4 (M2), the thickness of the membrane obtained from the SEM image was ca. 9.0 µm, which is larger than the value given in Table 1 (6.0 µm). Part of the carbon originates from chemical vapor deposition (CVD) of the carbon species (CHx) produced by the catalytic decomposition of CH4. An EDX analysis of the carbon membrane (Figure 3) revealed the existence of oxygen, nickel, and aluminum in addition to large amounts of carbon species. 3.2. FT-IR Spectra of Nickel-Containing and -Noncontaining Precursors. Figure 4 shows a typical FT-IR spectrum

Figure 6. H2-TPD spectra for membrane (a) M1 and (b) M3.

for the two kinds of precursors. The absorption band at 16301700 cm-1 is attributed to the -CdO stretching vibrations in the amide groups.33 For the nickel-containing precursor, the position of the absorption band shifted 12 cm-1 lower (from 1682 to 1670 cm-1) and the intensity of the band is also lower, indicating that the carbonyl interacts with the Ni2+ ion through a coordination bond. The strong absorption band at 3400-3500 cm-1 and the weaker band at ca. 1540 cm-1 can respectively be assigned to N-H stretching and bending vibrations in the secondary amide.34 The -NH stretching peak shifted from 3468 to 3440 cm-1 due to interaction of the Ni2+ ion with the -NH group. The -CH stretching vibration bands (2975-2840 cm-1)33,23 renained almost constant after the introduction of Ni2+ ions. This suggests that the Ni2+ ions mainly interact with -Cd O and -NH groups. 3.3. XRD Analysis of Nanoporous Carbon Membrane. The XRD patterns for membranes M1 and M2 are shown in Figure 5. For M1, in addition to the diffraction peaks of the R-Al2O3 support,35 there is a small broad amorphous carbon peak at 2θ ) 26.4°, indicating that the formation of the matrix carbon layer originated from the pyrolysis of the polymeric precursor (Figure 5a). For M2, in addition to the diffraction peak at 2θ ) 26.4° with a d spacing of 0.338 nm, there are three peaks at 2θ ) 44.62, 47.36, and 51.86°, which can be attributed to either elemental Ni0 36 or amorphous carbon37 (Figure 5b). This means that the CHx species formed during the decomposition of the CH4 on the active nickel sites combines with the aromatic rings formed during the polyamideimide pyrolysis to form local “conjugated” aromatic graphitic planes, similar to CH4 dehy-

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Figure 7. Permeance of gases through membranes vs kinetic diameter of molecules. Values in parentheses are the square root data of the gas molecular weight.

Figure 8. Permeance of gases through membrane M1 (solid) and M3 (hollow) as a function of the pressure difference between outer and inner tubes.

droaromatization over 3% Mo/ZSM-5 catalyst.38 The appearance of the peak at d spacing of 0.192 nm (2θ ) 47.36°) is representative of the carbon-carbon spacing in graphitic planes.39 Kishore et al.40 characterized the structure of carbon membranes made from phenol-formaldehyde resin using Raman spectroscopy and XRD. They found two kinds of carbon: disordered or amorphous carbon and graphitic carbon. For Saran-based carbon, with increasing carbonization temperature the pore mouths shrunk.41 The [002] peak at 2θ ) 25.2° is identical to the spacing between graphite sheets with an interlayer spacing of 0.353 nm, which is similar to our observations in this work. 3.4 H2-TPD Spectra for Membrane M1 and M3. In order to probe hydrogen adsorption properties and activated action of elemental nickel during gas permeance, H2-TPD spectra for both membranes M1 and M3 were observed, as shown in Figure 6. For M1, there are two peaks of hydrogen desorption at 73.0 and 752 °C, attributed to the adsorption of molecular and atomic

hydrogen in the microporous carbon, respectively. For M3, in addition to aforementioned desorption peaks, two remarkable desorption peaks observed in the 200-600 °C range can be correlated to hydrogen adsorption on active Ni0 sites. 3.5. Relationship of the Permeance of NPCM with Kinetic Diameter of Gas. The relationship between the gas permeance through the membranes and the kinetic diameter is shown in Figure 7. For the NPCM prepared by direct pyrolysis of the polyamideimide precursor (Figure 7a), the gas permeance decreased in proportion to the square root of the molecular weight of the gas. This indicates that the carbon membrane generated via a single coating pyrolysis procedure at 600 °C has a large pore size distribution. The diffusion of gas through the membrane was mainly controlled by Knudsen diffusion as evidence of the agreement between the ideal separation factor and theoretical values of Knudsen diffusion (Table 2). The permeance of CO2 through membrane M1 was the lowest due to its relatively high square root data of molecular weight. The separation performance of this membrane is similar to the observations of Fuertes et al.25 (Table 2). In order to improve the performance of the membrane, multiple coating had been employed for other research groups.25,42 For the carbon membrane obtained from the catalytic decomposition of CH4 over Ni/polyamideimide (Figure 7b), the permeance of N2 (3.64 Å) was lower than that of CO2 (3.3 Å) and the ideal separation factors for H2/N2 and H2/CH4 (4.5 and 3.4 in Table 2) are somewhat higher than the theoretical Knudsen values (R ) 3.7 and 2.8). This increase may partially be attributed to a molecular sieving contribution to the separation mechanism of membrane M2. It has been reported that the pore diameter of carbon membranes can be controlled by adjusting the carbonization temperature and that CH4 more favorably decomposes on active nickel sites under high-temperature conditions.22 Membrane M3 was prepared under those conditions, and the permeance of all gases through membrane M3 (Figure 7c) is much lower than that through M1 and M2. The ideal separation factors for H2/N2 and H2/CH4 are 24.0 and 47.7, respectively. These results are as good as or better than the separation efficiency obtained for a three-coating carbon membrane (Table 2).42-44 The results also indicate that the diffusion of gas through membrane M3 is mainly controlled by the mechanism of molecular sieving. In other words, the permeance of different gases through the membrane, to a large extent, depends on their kinetic diameters, which will be discussed further in section 3.6. However, as an extraordinary example, the permeability of H2 and He through NPCMs did not exactly follow the magnitude of the kinetic gas diameter, as shown in Figure 7. According to reports in the literature, the solubility (S, a thermodynamic factor) of He was much less than that of H2, in agreement with the magnitude of the Lennard-Jones potential /κ for He (/κ ) 10.2 K) and for H2 (/κ ) 60 K)45 although the diffusivity (D, a kinetic factor)

Table 2. Separation Performance for Nanoporous Carbon Membranes Obtained under Different Conditionsa H2/CH4 Knudsen value polyimide (M1)b BPDA-pPDAb Ni/polyimide (M2)b Ni/polyimide (M3)b PFA or PhRc,d

H2/N2

H2/CO2

He/N2

4.69

1.32

1.65

1.25

1.9 3.842 3.0 9.2 6.242

1.5 1.4525 1.0 4.8 NA

1.3 1.8425 1.3 2.4 NA

2.83

3.74

2.6 NA 3.4 47.7 1.0-4.044

Ideal Separation Factors 3.8 4.8 NA NA 4.5 3.4 24.0 10.0 22.642 1.943

CH4/CO2

N2/CO2

a All data were measured at room temperature (22 °C) in this work except for those denoted with references; the errors for our measured separation factors were (0.2. b Single coating membrane. c Three coating membrane. d PFA, poly(furfuryl) alcohol; PhR, phenolic resin.

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the permeance of gas through membrane M1 is significantly higher than that through membrane M3 in the range of 10-90 kPa. This is due to the fact that M1 has large nanopores that allow large gas molecules to pass through the membrane. A scheme of the microstructure for the formation of the carbon membranes is shown in Figure 9. For M1, the pyrolysis of the polymeric precursor forms large voids or pores (Figure 9a) due to weak cross-linking or nonconjugation action between the polymeric chains. The AFM image of membrane M1 (Figure 10a) also verifies that the carbon membrane has large voids with a striped structure. For M2, the CHx species generated by the catalytic decomposition of CH4 move to the defect sites that formed during pyrolysis when small gas molecules, such as N2 and CO2, escape. They then cross-link with aromatic rings of the polymeric chains to form a layer of “conjugated” carbon matrix structure, as shown in Figure 9b. Furthermore, with the enhancement of carbonization temperature, the rate of CH4 decomposition increases on active Ni0 sites and the generated CHx species can more effectively combine with carbon atoms on aromatic rings to form an intact carbon membrane via a single coating procedure. An AFM image of M3 (Figure 10c) also confirmed the integrality and uniformity of membrane formed at 700 °C. Using this procedure eliminates the shrinkage of the carbon membrane during pyrolysis at high temperature (Figure 10b,c), and the separation performance of the NPCM is improved (Figure 7). Thereby, the permeance of gases through membrane M3 was lower than that through M1 and M2 (Figures 7 and 8) and the separation factor of H2/CH4 significantly increased. Arrhenius plots of the permeance of gases, such as H2, CO2, N2, and CH4, versus temperature are shown in Figure 11. The apparent activation energy, Ep, for different gases can be calculated from the following formula:46

J(i) ) A(i) exp(-Ep/RT) Figure 9. Scheme of the microstructure during the formation of nanoporous carbon membranes.

of He was larger than that of H2 (He ) 0.26 nm, H2 ) 0.289 nm). As a result, the permeance of H2 was higher than that of He. 3.6. Effect of Pressure and Temperature on the Gaseous Permeance. The permeance of the different gases through membranes M1 and M3 as a function of the pressure difference between the tubes is shown in Figure 8. Except for hydrogen,

where J is the gas permeance (mol/m2‚s‚Pa), A is the frequency factor in the Arrhenius equation, R is the ideal gas constant, and T is the temperature (K). The permeance of both N2 (3.64 Å) and CH4 (3.8 Å) increased with temperature, indicating that gas transport through the micropores in the carbon membrane took place according to the mechanism of activation or molecule sieving. In this case, the apparent activation energy (Figure 11) increased with the increase of the molecular kinetic diameter. The apparent activation energy of N2 and CH4 through membrane M3 was 7.23 and 10.31 kJ/mol, respectively, similar to the results reported in the literature.47 The permeance of

Figure 10. AFM images of nanoporous carbon membranes: (a) M1; (b) M2; (c) M3.

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Figure 11. Permeance of gases through membrane M3 as a function of temperature.

CO2 within range of 19-120 °C has a maximum value around 50 °C, which is in agreement with the literature.26 The reason for this phenomenon is a combination of gas adsorption and diffusion. At low temperature, surface diffusion is the dominant mechanism, whereas at high temperature the transport occurs mainly by diffusion in the gas phase. At intermediate temperatures, both mechanisms contributed to the overall transport through the membrane.48 In addition, the diffusion of hydrogen through membrane M3 should be related to the microstructure of NPCMs. The XRD analysis of sample revealed that the distance between atoms in neighboring planes was ca. 0.338 nm (Figure 5), significantly larger than the kinetic diameter of H2 (0.289 nm). Therefore, the apparent activation energy of H2 through membrane M3 was nearly zero, and Kim et al.49 has verified the close relationship of gas permeance to the average d-spacing value of carbon membranes. In the meantime, H2TPD spectra illustrated that the effect of active Ni0 sites on the permeance of hydrogen can be ignored in the 20-120 °C range. 4. Conclusion By addition of Ni(Ac)2‚4H2O to a polyamideimide precursor, an intact and crack-free NPCM was prepared by a combination of carbonizing the polyamideimide precursor and the catalytic decomposition of methane. The AFM images and XRD results of sample revealed that this single coating technique was able to control the pore sizes in the NPCMs and patch the defects caused by the escape of small gas molecules during pyrolysis at high temperature. The formation of a high-quality NPCM was related to the interaction of CHx radical with aromatic rings of the polymeric chains to form a layer of “conjugated” carbon matrix structure. From an economic standpoint, this is a promising method to reduce the fabrication cost of NPCMs. Acknowledgment We gratefully acknowledge the financial support of the Scientific Research Foundation (SRF) for Returned Overseas Chinese Scholars (ROCS), State Education Ministry (SEM), People’s Republic of China (2002-247). We also acknowledge the analytical center of Tianjin University for the XRD, FT-IR, and SEM-EDX and Prof. A. S. C. Cheung of Hong Kong University for AFM characterization of the samples. Literature Cited (1) Hsieh, H. P. Inorganic membrane for separation and reaction. Membrane Science and Technology; Elsevier: Amsterdam, 1996; Chapter 3. (2) Saracco, G.; Neomagus, H. W. J. P.; Versteeg, G. F. Hightemperature membrane reactors: potential and problem. Chem. Eng. Sci. 1999, 54, 1997.

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ReceiVed for reView August 26, 2007 ReVised manuscript receiVed December 10, 2007 Accepted December 17, 2007 IE071161F