Development and characterization of micropores in carbon molecular sieve membrane for gas separation

Microporous and Mesoporous Materials 143 (2011) 78–86

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Development and characterization of micropores in carbon molecular sieve membrane for gas separation Ywu-Jang Fu a,⇑, Kuo-Sung Liao b, Chien-Chieh Hu b,c, Kueir-Rarn Lee b, Juin-Yih Lai b a

Department of Biotechnology, Vanung University, Chung-Li 32061, Taiwan R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan c Graduate School of Materials Applied Technology and Department of Chemical and Materials Engineering, Nanya Institute of Technology, Chung Li-32034, Taiwan b

a r t i c l e

i n f o

Article history: Received 27 January 2010 Received in revised form 19 January 2011 Accepted 7 February 2011 Available online 24 February 2011 Keywords: Carbon molecular sieve membrane Gas separation Micropore structure characterization

a b s t r a c t Defect-free carbon molecular sieve membranes were formed by pyrolysis of a polyimide precursor. The micropore structures of the carbon molecular sieve membranes were studied for different pyrolysis temperatures and atmospheres. From attenuated total reflectance Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, and density measurements on these materials, it appears that pyrolysis under vacuum and at high temperature increases the degree of carbonization and creates smaller micropores. Differential thermoanalysis and thermogravimetry coupled to mass spectroscopy were performed to understand the thermal stability and pyrolysis phenomena. The results indicated that, between 400 and 600 °C, large micropores were created by the release of large-molecule volatile fragments and then, between 460 and 1000 °C, ultramicropores formed due to liberation of small-molecule volatile products. Microporosity of the carbon molecular sieve membranes was studied using gas permeation, transmission electron microscope, positron annihilation lifetime spectroscopy, and analysis of argon adsorption isotherms via density functional theory. It was found that pyrolysis under vacuum and at high temperature produced large numbers of ultramicropores within a narrow pore-size distribution. The permeability and solubility data showed a strong dependence on microporosity. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction In recent decades there have been rapid developments in membrane gas-separation techniques because, compared to conventional gas-separation methods like cryogenic distillation and sorption, membrane separations offer flexible designs together with compactness and energy efficiency. As a result, membrane-based gas separations have become competitive in both performance and cost. In most commercial gas-separation applications membranes are polymers, and such polymeric materials should be highly permeable as well as highly selective. Typically, in choosing polymeric materials, tradeoffs must be made between permeability and selectivity [1]; consequently, to maintain the advantage in gas-separation applications, new generations of polymeric materials need to be developed. Attempts to improve membrane materials include chemical modifications of existing polymers, chemical/physical modifications of formed membranes, and syntheses of new polymers [2–6]. Although the inherent segmental flexibility of polymeric membranes makes them economical to prepare, that flexibility also limits both their discriminating ability for similarly sized penetrants and ⇑ Corresponding author. Tel.: +886 3 4515811; fax: +886 3 4510156. E-mail address: [email protected] (Y.-J. Fu). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.02.007

their performance stability under adverse conditions. Therefore, high-performance polymeric membranes suitable for commercial gas-separation applications are still lacking, even though attempts to improve membrane materials continue to be topics of research. The limitations of polymeric membranes have encouraged the development of alternative materials that can overcome the present challenges and competition in current gas-separation technologies. For instance, inorganic membranes show extremely attractive permeation performance and give selectivities that are about 10 times higher than conventional polymeric materials. Moreover, these materials tend to be much less affected by aggressive feeds [7]. It is clear that, to improve gas-separation performance, attention will focus mainly on the development of novel inorganic membranes. At the present time, there is a growing interest in developing materials that provide better selectivity, better thermal stability, and better chemical stability than polymeric membranes. In particular, attention has focused on zeolite and carbon materials that exhibit molecular sieve properties [8]. Among molecular sieving materials, carbon molecular sieve membranes (CMSMs) show attractive characteristics; for example, CMSMs have been successfully manufactured in different configurations with higher permeabilities than zeolite membranes at the same selectivity values.

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Koresh and Soffer first formed carbon molecular sieve membranes by pyrolysis of organic precursors [9,10]. They investigated how high-temperature treatments influence development of the micropore structure. Under a vacuum or an inert atmosphere, increasing temperature results in gradual pore closure during pyrolysis. According to their results, by appropriately choosing a sequence of thermal treatments, membrane pore size can be continuously tuned. Since then, the influence of increasing pyrolysis temperature on the evolution of microporous carbon structure has been studied for CMSMs derived from different precursors [11–15]. Steel and Koros also found that a more compact carbon structure was achieved by increasing the pyrolysis temperature, by increasing the time of thermal soak, or by changing the precursor. The choice of polymeric precursor is important because pyrolysis of different precursors may yield different kinds of carbon membranes. Therefore, microporous carbon membranes have been synthesized from several different precursors, including polyimides [16], coal tar pitch [17], polyfurfuryl alcohol (PFA) [18], phenolic resin [19], phenol formaldehyde [20], cellulose [21], and polyetherimide [22]. Typically, the properties of microporous carbon molecular sieve membranes depend upon the structure and preparation of the precursors. Park et al. [23], investigating the relationship between chemical structure of precursors and gas-separation properties of their CMSMs, show that the microstructure of the precursors significantly affected the final gas-separation properties of the CMSMs. The increase in fractional free volume in the precursors by methyl substituents led to increases in the permeability coefficients of the precursors and their subsequent CMSMs. In addition to side-group substitution, bonded templates introduced into thermostable polymer chains were also observed and were attributed to attractive potential in manipulating CMSMs [24]. Shao et al. [25] reported that the casting solvent plays an important role in polyimide membrane morphology and separation performance. The crystalline structure in polyimide-N,N-dimethylformamide membranes and its effects on gas separation are quite different from those of amorphous polyimide–dichloromethane and polyimide-1-methyl-2-pyrrolidinone membranes. The CMSM structure can be significantly affected by the crystalline structures of precursors if the pyrolysis temperature is close to the degradation temperature of the polymeric precursor. CMSMs can be modified to improve their gas-separation properties or to solve problems inherent to their structures. Thus attention has focused on post-treatment of CMSM to optimize separation efficiency. Of post-treatment methods, post-oxidation in the oxygen-containing circumstance is believed to improve micropore characteristics (pore size, its distribution, and surface area) [26,27]. According to one study, CMSMs and nonporous polymeric membranes transport gases by similar solution–diffusion mechanisms [28]. The permeability of gases can be expressed as a product of the diffusion coefficient (D) and the sorption coefficient (S). Much work has been presented in which diffusion and sorption coefficients are shown to be closely related to the micropore structure in membranes. It is clear that the gas-separation mechanism is dominated by the micropore structure. In general, the gasseparation mechanisms of both zeolite membranes and CMSMs are molecular sieves; however, the gas permeability of a CMSM is always higher than that of a zeolite membrane. This might be because, even when the pore sizes of two membranes are similar, their structures may differ. Using transmission electron microscopy, Park et al. [23] observed disordered building units in CMSMs: the CMSMs were found to be amorphous carbon materials comprised of a carbon matrix plus micropores. Using X-ray diffraction, Kusakabe et al. demonstrated that CMSMs possess slit-shaped micropores [26]. The CMSMs derived from higher temperatures have lower

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interlayer spacings (dC) and higher crystalline thicknesses (LC) than those derived from lower temperatures. Similar to the work by Steel and Koros, the authors explored how the porosity of carbon materials and related effects influence gas-separation performance [29]. They showed that CMSMs are amorphous and microporous materials. From separation studies of gases, it appears that these materials have both ultramicropores (<7 Å) and large micropores. The ultramicropores are believed to be mainly responsible for molecular sieving, whereas the micropores provide negligible resistance to diffusion but provide high-capacity sorption sites for gas molecules. In addition, they proposed that the structures of CMSMs may be idealized as slit-like structures containing three characteristic dimensions: a jump length dimension (dk), a transverse dimension (dTV), and a critical dimension (dC). These three key dimensions are described by distributions that are characteristic properties of the CMSM, its preparation history, and its sources. Based on this hypothesis, we have attempted to estimate the values of these three characteristic dimensions, which are believed to be responsible for the gas-separation performance of CMSMs. Positron annihilation spectroscopy has been successfully used to estimate the pore-size distribution in polymeric membranes. It has also been used to detect defects in semiconductive materials and crystalline materials. When positrons are injected into a bulk material, they may form para-positronium, free positrons, and ortho-positronium. The annihilation lifetimes of these three particles are identified by s1, s2, and s3, respectively. In polymeric membranes, an empirical equation has been developed to correlate the longest lifetime (s3) with ultramicropore size [30]. However, owing to the high electron density of CMSMs, it is inappropriate to measure pore sizes in carbon materials using the relationship between ortho-positronium lifetime and pore size in polymeric materials [31]. Recently, researchers have tried to use the free-positron annihilation lifetime to estimate the micropore structure in CMSMs. Chakrabarti et al. found that s2 and I2 (the intensity of free positrons) decreased with heat treatment temperature, while the degree of graphitization increased [32]. Although there are considerable published data concerning the effects of pyrolysis conditions, precursor chemical structure, and post-treatment on gas-separation properties of CMSMs, there are few data on micropore structure. Because fundamental understanding of micropore structure development in CMSMs is still lacking, this work aims to systematically study the development of micropores in CMSMs by performing precursor pyrolysis under different conditions. We also used permeation and sorption testing to study the effects of micropore structure on gas separation.

2. Experimental 2.1. Preparation of polymeric precursor The repeating unit of polyimide used for precursor preparation is shown in Fig. 1. The molecular weight and polydispersity index

Fig. 1. Repeating unit of polyimide precursor.

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for polyimide were 3.7  104 g/mol and 1.67, respectively. Dense membranes were cast from a dichloromethane (Mallinckrodt) solution containing 18 wt.% polyimide (Alfa Aesar). The polymer solution was cast on a glass plate at a thickness of 600 lm, then the solvent was allowed to evaporate in ambient air for 24 h. After the solvent had evaporated, the membranes were further dried under vacuum at 250 °C for another 24 h. All membranes were between 70 and 100 lm thick. 2.2. Preparation of carbon molecular sieve membranes Two pyrolysis atmospheres were used in this work: one was a vacuum and the other was nitrogen flowing at 200 sccm. Pyrolysis was begun by heating precursor membranes in a furnace (Tender Scientific Co.) under vacuum (<1.33 kPa) for 1 h. Thereafter, the pyrolysis procedure was divided into three parts. First, the temperature was raised from ambient to 100 °C at a rate of 10 °C/min; the precursors were held at 100 °C for 10 min. Second, the temperature was increased from 100 to 300 °C at a rate of 5 °C/min and the precursors were held at 300 °C for 1 h. Third, the carbonization temperature was set to either 550 or 700 °C, and that temperature was maintained for 2 h. In this third stage the heating rate was 1 °C/min to avoid uncontrolled shrinkage of the membrane. After pyrolysis, the CMSMs were cooled naturally in the furnace to room temperature. To prevent aging effects from contact with oxygen, the CMSMs were stored under vacuum in a desiccator. In this work, a particular membrane is denoted by a letter-number code. The letter identifies the pyrolysis atmosphere (V indicates vacuum whereas N indicates a nitrogen atmosphere); the number identifies the carbonization temperature. For example, V550 denotes a carbon membrane that was fabricated under vacuum at 550 °C. 2.3. Characterization Attenuated total reflectance Fourier transform infrared (ATRFT-IR) spectra (Miracle-Dou, Perkin–Elmer Instrument) were used to determine the degree of carbonization of polyimide precursors. The weight loss during pyrolysis and the released gas during degradation were detected by differential thermoanalysis and thermogravimetry coupled to mass spectroscopy (DTA–TG–MS). The DTA– TG–MS measurements were performed simultaneously using an STA-409CD with Skimmer coupling from Netzsch; this instrument is equipped with a quadrupole mass spectrometer QMA 400 (max. 512 amu) from Balzers. The analysis was carried out at temperatures ranging from 50 to 900 °C using a heating rate of 5 °C/min in a helium atmosphere. A Panalytical wide-angle X-ray diffractometer (WAXD) (X’Pert Pro(MRD) PW3040/60) was used to measure the ordered dimensions and interchain spacings of polymer precursors and CMSMs at ambient temperature; Cu Ka radiation of wavelength 1.54 Å was used. The data were collected using a step size of 2h = 0.004° from 2° to 60°. Average d-spacings were determined based on Bragg’s law. Wide bands in the resulting spectra indicated that the structures were amorphous, and the maxima of these bands were taken as the average d-spacings. The density of the membrane was measured by buoyancy. A well-dried membrane sample was first weighed in air and then immersed in silicon oil at 25 °C; the difference in weight before and after immersion was determined. The specific volume of the membrane (V) was then calculated from the weight difference of the measurements divided by the density of silicon oil. Diameter shrinkage and weight loss were measured to determine variations in bulk structure resulting from pyrolysis. The diameter shrinkage (DS) and weight loss (WL) were calculated as follows:

Di  Df  100% Di Wi  W f WL ¼  100% Wi

DS ¼

ð1Þ ð2Þ

In (1) Di and Df are the diameters before and after pyrolysis, whereas in (2) Wi and Wf are the membrane weights before and after pyrolysis. Argon adsorption isotherms were obtained using a Micrometrics model ASAP2020 (Micrometrics, USA) adsorption analyzer. To get pore-size distributions for the CMSMs, the adsorption isotherms were analyzed by software utilizing concepts from density functional theory. The morphology of CMSMs was examined using high resolution transmission electron microscope (JEOL JEM-2010). The positron annihilation lifetimes of carbon molecular sieve membranes were determined by detecting prompt c-rays (1.28 MeV) from the nuclear decay that accompanies the emission of a positron from a 22Na radioisotope and the subsequent annihilation c-rays (0.511 MeV). A fast–fast coincidence circuit of positron annihilation lifetime (PAL) spectrometer with a lifetime resolution of 260 ps from a 60Co source was used to record all PAL spectra. Each spectrum was collected to a fixed total count of 2  106. All the PAL spectra were analyzed by a finite-term lifetime analysis method using the maximum entropy method. A gas permeation analyzer (Yanaco GTR10) was used to measure pure-gas permeability coefficients for the precursor and CMSMs to O2 and N2. The tests were carried out under isothermal conditions at 35 °C (±0.5 °C). The usual unit of permeability (P) is the barrer [1010 (cm3 (STP) cm)/(cm2 s cm Hg)]. Ideal selectivities were calculated as the ratio of permeability coefficients:

aA=B ¼

PA PB

ð3Þ

where PA and PB are the permeability coefficients of pure gases A and B. Permeation tests using CO2, CH4, C2H6, and C3H8 were also performed to probe the critical dimension dC. A microbalance (Cahn model D202 microbalance) was used to determine the solubility coefficient S. The permeability coefficient can be written as the product of the diffusion coefficient and the solubility coefficient:

P ¼DS

ð4Þ

so the diffusion coefficient was determined by rearranging (4):

D¼

P S

ð5Þ

For each membrane, permeation and sorption experiment were carried out on four samples for each gas. Therefore, the permeation parameters (P, S) were obtained with coefficient of variation ranging from 5% to 15%. 3. Results and discussion 3.1. Pyrolysis of polymeric precursor Producing a high performance CMSM is a difficult task because it involves many processing parameters that must be controlled and optimized. Micropores structure and the ability of CMSMs to separate gases are determined by pyrolysis temperature and pyrolysis atmosphere; therefore, these two parameters are most important and can be regarded as key factors in CMSM production. To identify temperatures for pyrolysis, we determined the weight loss that occurs when commercial polyimide is heated to 1000 °C; the data are shown in Fig. 2. The figure shows that from room temperature to 460 °C, the polyimide precursor lost 4% of its original weight; this loss is attributed to release of adsorbed water and residual solvent from the precursor. With increases in temperature

Y.-J. Fu et al. / Microporous and Mesoporous Materials 143 (2011) 78–86

Fig. 2. Thermogravimetric results from pyrolysis of a polyimide precursor membrane.

beyond 460 °C, Fig. 2 shows the weight of the sample decreased abruptly, with distinguishing weight-loss peaks occurring at 488 and 561 °C. An abrupt weight-loss means micropores begin to form. At 1000 °C the total weight loss was approximately 45%. Inspection of Fig. 2 shows that two main decompositions occurred during pyrolysis; therefore, we chose two temperatures, 550 and 700 °C, to explore the effects of pyrolysis temperature on the pore structure of CMSMs. Typically, the micropores structure of a CMSM is inseparably related to the degree of carbonization of the precursor, and the degree of precursor carbonization can be monitored by FT-IR. Fig. 3 shows ATR-FT-IR spectra from the polyimide precursor and from CMSMs prepared under different pyrolysis conditions. For the precursor, the absorption band at 1374 cm1 identifies a C–N stretch in imide groups whereas the bands at 1724 and 1780 cm–1 are C@O stretching in imide groups. The intensities of characteristic bands are weaker after pyrolysis of the precursor; further, band intensities are weaker in CMSMs obtained from high-temperature pyrolysis than in those CMSMs formed using low-temperature pyrolysis. This implies that the degree of carbonization in CMSMs increases with increasing pyrolysis temperature. Further, Fig. 3 shows that band intensity is lower in CMSMs prepared in vacuum than in those prepared in nitrogen. This suggests that pyrolysis under vacuum facilitates release of degradation compounds and

Fig. 3. ATR-FT-IR spectra for precursor membrane and CMSMs.

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increases the degree of carbonization. The presence of the nitrogen adds a boundary resistance on membrane surface which hinder pyrolysis product removal. As previously discussed, higher pyrolysis temperature and vacuum atmosphere toward the development of the micropores. Pyrolysis is frequently used in the production of CMSMs because it results in products that have microporosities of molecular dimensions; such micropores are responsible for the molecular sieve performance of CMSMs. During pyrolysis of a precursor, byproducts of different volatilities are released, causing a large weight loss. The evolution of these byproducts was monitored by DTA–TG–MS and the results are shown in Figs. 4 and 5. Fig. 4 shows small-molecule volatile products appearing at 2 amu (hydrogen), 16 amu (methane), 18 amu (water), 28 amu (nitrogen or carbon monoxide), and 44 amu (carbon dioxide). Between 25 and 460 °C almost no volatile products appear other than water. This suggests that the small weight loss during the initial stages of pyrolysis is due to water loss. Fig. 4 shows that CH4 (16 amu), CO (28 amu), and CO2 (44 amu) are released from 550 to 900 °C; these molecules are formed by the degradation of imide and benzene rings [33]. The release of small-molecule volatile fragments results in ultramicropores; meanwhile, the development of ultramicropores continued from low to high pyrolysis temperature. The inset in Fig. 4 shows that release of hydrogen becomes

Fig. 4. DTA–TG–MS results for precursor membrane over the range 5–50 amu. The inset shows results in the range 0–5 amu.

Fig. 5. DTA–TG–MS results for precursor membrane over the range 50–250 amu.

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significant after 700 °C, where the precursor degraded distinctly to form an amorphous carbon material. Fig. 5 shows that all large-molecule volatile fragments are produced between 400 and 600 °C. This suggests that, during low-temperature pyrolysis, polyimide chains simply break into large-molecule fragments. Generally, the micropore structure of CMSMs is heterogeneous, as it is comprised of large micropores and ultramicropores. For polyimide-derived CMSMs, it can be concluded that the production of large-molecule volatile fragments at low pyrolysis temperatures favors the formation of large micropores. Besides, the large micropore just formed at low pyrolysis temperature. 3.2. Characterization of precursor and CMSMs Pyrolysis removes most of the heteroatoms that were originally present in the polymeric precursor; it also changes precursor geometry and leaves behind a stiff carbon matrix. Results from measurements of precursor and CMSM dimensions, weight, and bulk density are shown in Table 1. Essentially, weight loss and diameter shrinkage become more pronounced for polyimidederived CMSMs produced by pyrolysis performed in a vacuum at high temperatures. Furthermore, according to Table 1, pyrolysis in a vacuum at high temperature causes the density of CMSMs to increase. Perhaps pyrolysis under these conditions causes the polyimide chains to become more orderly and closely packed. In any case, these results imply that carbonization is higher for CMSMs obtained from pyrolysis performed under a vacuum at high temperatures. To characterize the d-spacing of the precursor and CMSMs pyrolyzed under different conditions, wide-angle X-ray diffraction data were collected; the results are shown in Fig. 6. Theoretically, one can expect the d-spacing to be the average spacing between the centers of chains in molecular materials. This value was 6.4 Å in the amorphous polyimide precursor. When the precursor was pyrolyzed to either 550 or 700 °C the spectra of these CMSMs showed a broad peak that appears at d-spacings of 4.5 and 3.9 Å, respectively. The temperature increase caused the d-spacing to decrease, but d-spacing did not seem to be affected by changing the pyrolysis atmosphere (vacuum or nitrogen). In addition, a small broad peak appeared at 44°, which is the same d-spacing as for planes in graphite. This peak at 44° was more pronounced for CMSMs at the higher pyrolysis temperature, suggesting that the CMSM structure became more graphite-like as the temperature was increased. This seems consistent with the results in Table 1. An idealized structure of a micropore in a CMSM can be estimated from dk, dTV, and dC. The characteristic dimension dk correlates with the effective diffusion jump length and can hypothetically be much larger than the transverse dimension (dTV). Values for dTV may indicate the degree to which gas molecules can diffuse through CMSMs. The critical dimension (dC), often referred to as an ultramicropore, is thought to allow molecular sieving of gas molecules [29]. According to same study, we can estimate the likely scale of dk in CMSMs by assuming the ratio of known nitrogen diffusion coefficients of carbon to type 4A zeolites

Table 1 Characteristics of precursor membrane and CMSMs. Membrane Polyimide V500 N500 V700 N700

Weight loss (%) – 40.5 39.3 44.3 42.5

Diameter shrinkage (%) – 28.6 27.9 33.8 31.7

Fig. 6. WAXD patterns for precursor membrane and CMSMs.

(2.32 (1010) cm2 s–1 at 35 °C) [34] is proportional to the ratio of the square of their dk values: 2

½DN2 carbon ðdk Þcarbon  ½DN2 zeolite 4A ðd2k Þzeolite 4A

ð6Þ

where (dk)zeolite 4A  11.4 Å. Results for the diffusion coefficients and dk of the four CMSMs appear in Table 2. Table 2 shows that the dk of CMSMs decreased in the order N550 > V550 > V700 > N700. Undoubtedly, higher pyrolysis temperatures produce CMSMs with lower dk values, especially for CMSMs prepared in vacuum. As in previous results of Figs. 4 and 5, the release of large-molecule volatile fragments at low pyrolysis temperature caused bigger dk values, but the release of smallmolecule volatile fragments at high pyrolysis temperature resulted in smaller dk. In this work, the dk value was much bigger than in a previous study [29]. According to other reports [35,36], oxygen undergoes irreversible chemisorption on carbon membranes, so these differences in dk may be due primarily to differences in the effects of aging in the presence of oxygen and nitrogen. For a fixed membrane thickness, a higher dk means a lower dC value in the direction of diffusion; therefore, resistance to diffusion decreases when dk increases. Consequently, one can assume that high permeability coefficients occur in CMSMs that have large values for dk. To estimate the dTV dimensions of CMSMs, argon equilibrium sorption measurement was performed. Argon adsorption

Table 2 Nitrogen permeability, solubility, diffusivity, and characteristic dimension for CMSMs. Bulk density (g/cm3) 1.24 1.32 1.26 1.54 1.52

Membrane

V550 N550 V700 N700

P Barrer

S

D  108

cm3 ðSTPÞ cm3 atm

cm2 s

79.4 89.9 20.7 24.1

4.3 3.6 7.6 8.3

14.0 19.0 2.1 2.2

(dk)CMSM Å 280.0 326.2 108.8 111.0

Y.-J. Fu et al. / Microporous and Mesoporous Materials 143 (2011) 78–86

Fig. 7. Argon adsorption isotherm for CMSMs.

isotherms presented in Fig. 7. It is clear to conclude that CMSMs derived from various conditions show type I adsorption isotherm in accordance with IUPAC classification. The adsorption isotherm implied that CMSMs only consist of carbon matrix and micropore. Adsorption isotherms were analyzed by software utilizing

Fig. 8. Pore size distributions derived from 87 K Ar isotherms of CMSMs prepared under different conditions: (a) vacuum at 550 or 700 °C; and (b) nitrogen atmosphere at 550 or 700 °C.

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concepts from density functional theory to extract the microporesize distributions of CMSMs; the results appear in Fig. 8. This figure basically shows that the dTV dimension has a multimodal distribution, and the dTV dimensions fall between 4 and 14 Å. The number of pores with characteristic dimensions in the range 4–14 Å increases when the final pyrolysis temperature increased from 550 to 700 °C. The release of small-molecule volatile products at the higher temperature (700 °C) promotes formation of more pores with dTV in the range 4–7 Å. Further, CMSMs formed in a nitrogen atmosphere show a dTV distribution that is broader and has a higher pore volume than those formed in vacuum. Note that gasmolecule diffusion in CMSMs derived from a nitrogen atmosphere will be much easier. Although membrane analyses, such as presented above, provide insight into the morphology of micropores in CMSM, those analyses are not adequate to determine the critical dimension dC, which is responsible for molecular sieving. TEM is a well-known technique to acquire microstructure morphology of material. The TEM images of V550 and V700 CMSMs represent in Fig. 9a and b, respectively. Unexpectedly, distinct morphology cannot be identified in these TEM images even though CMSMs derived from nitrogen atmosphere. Gas molecules of different dimensions can be

Fig. 9. TEM images of vacuum-derived CMSMs prepared under different pyrolysis temperature: (a) 550 °C; and (b) 700 °C.

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used to probe CMSMs, much like ‘molecular rulers’, to estimate critical pore dimensions in ultramicroporous materials. The permeability coefficients of various gases in CMSMs are shown in Table 3. The critical dimensions were less than 4 Å for V550 and N550 and less than 3.8 Å for V700 and N700. Based on the order of ethane permeability, CMSM cut-off diameters decreased as follows: V550 > N550 > N700 > V700. This indicates that pyrolysis temperature significantly affects dC of the resultant CMSMs. Further, the decrease in dC suggests that increasing pyrolysis temperature is an effective way to produce CMSMs with high selectivity. Positron annihilation lifetime spectroscopy, which is based on measurements of lifetimes of positrons in annihilation c-ray spectra, is useful for detecting the evolution of micropores in solids. Usually, the ortho-positronium annihilation lifetime (s3) is used to estimate the pore-size distribution. The ortho-positronium annihilation lifetime (s3) for precursor membrane is 1.7 ns. However, for carbon materials, most ortho-positronium annihilations occur on the surface, which is inadequate to resolve a third distinct component (s3) in the spectra. In such situations, one would consider estimating characteristics of micropores from the free-positron annihilation lifetime (s2); however, a quantitative empirical equation that relates micropore size to s2 is not yet available. Therefore, in this work, we used the free-positron annihilation lifetime distribution to estimate micropore characteristics. Free-positron annihilation lifetime (s2) distributions for CMSMs are shown in Fig. 10. Values for s2 are proportional to pore size and can provide information about all different micropores. Note in Fig. 10 that, when the pyrolysis temperature is increased, the s2 distribution curve shifts to smaller lifetimes, becomes narrower, and increases in intensity. This is consistent with the results in Figs. 4, 5 and 8. CMSM derived from high pyrolysis temperature, no pyrolysis at low temperature, just released small-molecule volatile fragments which formed smaller and narrower micropores. Comparing Fig. 4 with Fig. 5, the release amount of small-molecule

Table 3 Molecular probe results for CMSMs. Gas

Kinetic diameter (Å)

CO2 N2 CH4 C2H6 C3H8

3.30 3.64 3.80 4.00 4.30

Permeability (barrer) V550

N550

V700

N700

350.22 79.40 28.60 36.70 0.00

428.16 89.80 23.87 21.56 0.00

93.78 20.74 0.03 0.00 0.00

128.29 24.10 3.57 0.01 0.00

Fig. 10. Free-positron lifetime distributions derived from PAL spectra for CMSMs.

volatile fragments is higher than large-molecule volatile fragments, and then high pyrolysis temperature created more micropores (higher intensity). An alternative explanation is that, during high-temperature pyrolysis, some types of micropores may fragment into smaller ones. Small pores with narrow pore-size distributions suggest that CMSMs derived from high-temperature pyrolysis should have high selectivity but low permeability. Fig. 10 also shows that the effects of pyrolysis atmosphere on micropore morphology are not as significant as the effects of pyrolysis temperature. The results in Fig. 10 are partially consistent with the results from other analyses presented earlier in this section, but they can perfectly predict the gas separation behavior of CMSMs. It is clearly that positron annihilation lifetime spectroscopy is a powerful skill to analysis the micropore structure of CMSMs. 3.3. Effects of micropores on gas permeation properties of CMSMs The permeability of a penetrant in a CMSM or in a polymer may be obtained from the product of the diffusion coefficient, D, and the sorption coefficient, S. Therefore, we can use diffusion and sorption coefficients to estimate the gas permeation behavior of CMSMs. The Langmuir isotherm is commonly used to define sorption isotherms for gases in porous molecular sieving materials. The adsorption isotherms for N2 and O2 on CMSMs as a function of pressures up to 28 atm are presented graphically in Fig. 11. These are all of type I. Nitrogen (Fig. 11b) is weakly adsorbed compared to oxygen (Fig. 11a), possibly because oxygen has a smaller kinetic diameter and more affinity for CMSMs than nitrogen. But in both cases the adsorption capabilities increase substantially when the polyimide precursor is converted into a microporous amorphous carbon material. Further, the adsorption capacities of CMSMs increase strongly with pyrolysis temperature; however, the effects of pyrolysis atmosphere on adsorption capacity were negligible. Apparently, pyrolysis alters the number of micropores and the micropore-size distribution, and the formation of micropores by pyrolysis plays an important role in altering the adsorption capacity of CMSMs. Specifically, CMSMs derived from high-temperature pyrolysis exhibit high adsorption capacities. Transport properties for O2/N2 were studied as functions of pyrolysis temperature and atmosphere; results are shown in Fig. 12. The precursor membrane and the CMSMs exhibit quite different separation properties, which can be attributed to their structural differences. CMSMs derived from high-temperature (V700 and N700) pyrolysis have large numbers of small micropores, as shown in Figs. 8 and 10, leading to low permeability and high selectivity. In contrast, low-temperature pyrolyzed CMSMs exhibit high permeability and low selectivity. It may be the case that, in addition to increasing micropore size and size distribution, lowtemperature pyrolysis induces formation of large-molecule volatile fragments. Referring to Fig. 10, CMSMs pyrolyzed in a nitrogen atmosphere have more and larger micropores. This suggests that CMSMs pyrolyzed in nitrogen should exhibit high permeability and low selectivity when compared to pyrolysis under vacuum. Steel et al. prepared CMSMs with the same polymer pyrolyzed under similar conditions as we use [11,29]. The oxygen permeability is about 320 and 20 berrer and O2/N2 selectivity is about 8 and 13 for CMSMs pyrolyzed under 550 and 800 °C, respectively. The oxygen permeability of CMSMs in this work, V550 is about 340 berrer and V700 is about 130 berrer, are higher then Steel’s. However, the O2/N2 selectivity, V550 is 4.3 and V700 is 6.5, is much lower. The differences in gas separation performance between Steel et al. and this work arise from the variations in precursor membrane characteristic and pyrolysis conditions. According to the solution–diffusion mechanism, the ideal selectivity can be expressed, as in (4), as a ratio of diffusion and sorption

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Y.-J. Fu et al. / Microporous and Mesoporous Materials 143 (2011) 78–86 Table 4 Selectivity parameters for precursor membrane and CMSMs. Membrane

P O2 /P N2

SO2 /SN2

DO2 /DN2

Polyimide V550 N550 V700 N700

6.0 4.3 4.2 6.5 6.3

1.38 1.08 1.10 1.09 1.11

4.36 3.98 3.79 5.97 5.64

4. Conclusions In this work the microporosity of carbon molecular sieve membranes derived from pyrolysis of a polyimide precursor was systematically investigated. Development and characterization of micropores in CMSMs were characterized using ATR-FT-IR, WAXD, DTA–TG–MS, density balance, adsorption analyzer, and positron annihilation lifetime spectroscopy. We found that the conditions under which pyrolysis are performed can considerably alter the microporosity of the resulting carbon materials. By controlling these conditions, it is possible to tailor CMSMs to obtain the desired permeability and selectivity. According to our micropore structure analyses, CMSMs pyrolyzed under vacuum at high temperature possess a large number of small micropores with narrow pore-size distributions. The permeabilities of the CMSMs decreased in the order N550 > V550 > N700 > V700; this ordering of permeation performance is consistent with the micropore structure. The ideal selectivity of CMSMs is dominated by diffusive selectivity. Acknowledgements The authors wish to sincerely thank the Ministry of Economic Affairs and the National Science Council of Taiwan, ROC, for financially supporting this project. Fig. 11. Sorption isotherms for precursor membrane and CMSMs derived from pyrolysis under different sorption atmospheres: (a) oxygen; (b) nitrogen.

coefficients for gases A and B in CMSMs. The ideal selectivity data are presented in Table 4. The table shows that solubility selectivities (SO2/SN2) are close to one, suggesting that selectivities of CMSMs are controlled by diffusive selectivities. It is obvious that the micropore structure, tuned by pyrolysis conditions, dominate the gas selectivities of CMSMs.

Fig. 12. Permeation results for precursor membrane and CMSMs.

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