Carbon Molecular Sieve Membranes Derived from Phenolic Resin

A thermosetting phenolic resin with a pendant sulfonic acid group was prepared by reacting a resol-type phenolic resin (PF) with a Novalak-type sulfon...
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Ind. Eng. Chem. Res. 2001, 40, 4801-4807

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Carbon Molecular Sieve Membranes Derived from Phenolic Resin with a Pendant Sulfonic Acid Group Weiliang Zhou, Makoto Yoshino, Hidetoshi Kita, and Ken-ichi Okamoto* Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan

A thermosetting phenolic resin with a pendant sulfonic acid group was prepared by reacting a resol-type phenolic resin (PF) with a Novalak-type sulfonated phenolic resin (SPF). Large amounts of gaseous molecules with similar and small size such as H2O and SO2 evolved in the range of 110 and 350 °C during the pyrolysis of this thermosetting phenolic resin (PF/SPF). Highly permeable carbon molecular sieve (CMS) membranes were obtained by pyrolysis of PF/ SPF(45/55) precursor membranes which were dip-coated on porous alumina tubes. For example, the membrane pyrolyzed at 500 °C for 1.5 h displayed H2, CO2, and O2 permeances of 1950, 800, and 240 [GPU (gas permeation units) ) 10-6 cm3(STP)‚s-1‚cm-2‚cmHg-1], respectively, and ideal H2/CH4, CO2/CH4, and O2/N2 separation factors of 65, 27, and 5.2 at 35 °C and 1 atm, respectively. Sulfonic acid groups linked to thermostable polymer chains might act as “bonded templates” and showed attractive potential in the preparation of CMS membranes. Introduction Microporous carbon membranes with a permselective layer are potentially attractive candidates in the field of gas separation, in terms of both selectivity and chemical and thermal stabilities. The permselective carbon layer can be either supported on porous substrate (alumina tube, sintered stainless steel flat plate, and macroporous carbon disk) or self-supported (flat sheet and hollow fiber). Mainly, two types of carbon membranes have been reported in the literature depending on the separation mechanism. One is a carbon molecular sieve (CMS) membrane.1 The other is a surface-selective diffusion carbon membrane.2 The gas separation performance of either CMS membranes or surface-selective diffusion membranes largely depends on the polymeric precursor, membrane formation method, pyrolysis variable, and posttreatment method. Great efforts have been made toward the preparation of high-performance carbon membranes. However, because of the complexity of the development of micropores during pyrolysis and carbon structure, the mechanism of micropore formation has not yet been well understood, and the empirical and tedious “trial and error” method has still been widely used to prepare carbon membranes. Among the factors determining the separation performance of carbon membranes, one of the most important factors is a polymeric precursor. The polymeric precursors of carbon membranes that have been reported to date mainly include polyimides, polypyrrolone, poly(furfuryl alcohol) (PFA), phenolic resin, poly(vinylidene chloride-co-vinyl chloride) (PVDCPVC), poly(vinylidene chloride-vinyl acrylate) terpolymer, cellulose derivate, etc. CMS membranes derived from polyimides were extensively studied by Koros et al.,3,4 Haraya et al.,5-7 Hayashi et al.,8,9 Kusuki et al.,10,11 Okamoto et al.,12,13 etc. Hollow fiber carbon membranes derived from a copolyimide (BPDA/6FDA* To whom correspondence should be addressed. E-mail: [email protected]. Fax: 81-836-85-9601. Tel.: 81-836-85-9660.

TrMPD), which was prepared by condensation of 3,3′,4,4′biphenylyltetracarboxylic dianhydride (BPDA) and 3,3′,4,4′-diphenylhexafluoroisopropylidenetetracarboxylic dianhydride (6FDA) with 2,4,6-trimethyl-1,3phenylenediamine (TrMPD), exhibited a high performance of O2/N2 separation, that is, an O2 permeance of 15-40 GPU (gas permeation units) and a separation factor of 11-14.3 Kusuki et al. developed a manufacturing method to continuously prepare asymmetric hollow fiber carbon membranes with a high performance of H2/ CH4 separation.10,11 Okamoto et al. reported an excellent performance of propylene/propane and 1,3-butadiene/ n-butane separation for carbonized hollow fiber membranes derived from BPDA-DDBT/BADA copolyimide, which was prepared by condensation of BPDA with 2,8(6)-dimethyl-3,7-diaminodiphenylthiophene 5,5-dioxide (DDBT) and 3,5-diaminobenzoic acid (BADA).12 They also reported on carbonized flat membranes from 6FDAbased polypyrrolone (6FDA-DABZ), which displayed an O2 permeance of 1.4 or 14 GPU and an O2/N2 separation factor of 9.1 or 11.13 Shiflett and Foley successfully prepared CMS membranes from PFA, which displayed an O2 permeance of 0.17 GPU and an O2/N2 separation factor of 30.14 Wang et al. developed CMS membranes from furfuryl alcohol by vapor deposition polymerization, which showed an O2 permeance of 2.52 GPU and an O2/N2 separation factor of 12.7.15 Centeno and Fuertes obtained a CMS membrane from PVDC-PVC having an O2 permeance of 1.4 GPU and an O2/N2 separation factor of 14 at 25 °C.16 They also reported a CMS membrane from Novalak-type phenolic resin (Novalak-PF), which exhibited an O2 permeance of around 3 GPU and an O2/N2 separation factor of 10 at 25 °C.17 More recently, they reported an adsorption-selective carbon membrane prepared first by pyrolysis of a thin commercial NovolakPF film followed by oxidation of the pyrolyzed membrane at 300-400 °C in air.18 Okamoto et al.13 and Wang et al.19 also obtained CMS membranes from resoltype phenolic resin (PF). Phenolic resin is one of the popular polymeric precursors for preparation of carbon membranes because of

10.1021/ie010402v CCC: $20.00 © 2001 American Chemical Society Published on Web 09/29/2001

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Figure 1. Formation of soluble PF and SPF prepolymers.

its heat resistance, thermosetting, and high carbon yielding properties. Sulfonic acid groups bonded to polymer chains are supposed to decompose at relatively low temperature and evolve small molecules or fragments such as SO2 and H2O during the pyrolysis. Thus, when combined with thermosetting phenolic resin, sulfonic acid groups will evolve small molecular gases or fragments and leave space in the thermoset matrix during the pyrolysis. Decomposition of sulfonic acid groups takes place before substantial carbonization takes place. This effect is equivalent to an increase of the free volume in the matrix. Membranes with molecular sieving properties may be prepared by this approach because of the quite large amount of evolved gases or fragments with similar small size during the pyrolysis. This study managed to explore a method of tailoring micropores in preparation of carbon membranes by introducing a decomposable sulfonic acid group into the phenolic resin and then to deepen the fundamental understanding of phenomena determining the micropore formation during the pyrolysis. Experimental Section Synthesis of Polymeric Precursor and Characterization. Phenol and formaldehyde at a given ratio were charged into a reactor, and a certain amount of an aqueous KOH solution was added to the reactor with stirring. The mixture was heated to 90-94 °C and reacted for a certain time. The reaction mixture was then gradually cooled to 60 °C and dehydrated under vacuum until the desired solid content was obtained. The reaction mixture was cooled, a formic acid solution was added to the mixture to control the pH at 5, and the resulting resol resin was stored for use. Sulfonated phenolic resin (SPF) was synthesized by slowly adding p-phenolsulfonic acid into a 37% formaldehyde solution, which was kept in an ice water bath. Then the PF and SPF were mixed at a desired ratio for precursor membrane preparation. The corresponding schematic of the chemical structures is shown in Figure 1. Thermogravimetry-mass spectrometry (TG-MS) was carried out on a Rigaku TG-8120-Shimadzu GCMS-QP 5050. Fourier transform infrared spectrometry (FTIR) was carried out on a Jasco FT/IR-610. Elemental analysis was measured by a Perkin-Elmer CHN coder.

Membrane Preparation and Gas Permeation Measurement. A one-step coating/pyrolysis (C/P) method was used to prepare pyrolyzed membranes. Porous R-alumina tubes (average pore size ) 0.14 µm, diameter ) 2.3 mm, porosity ) 40-48%) were used as the membrane supports. The supports were coated with 30 and 10 wt % methanol solutions of PF and PF/SPF(45/55), respectively, three times. PF/SPF(45/55) means that the weight ratio of PF to SPF is 45 to 55. The coated PF/SPF(45/55) precursor membranes were first air-dried at 70 °C for more than 5 h before the next layer was coated. The dried membranes were further dried in a vacuum at 110 °C for 10 h after all of the layers were coated. The coated PF precursor membranes were first air-dried at 70 °C for more than 10 h and further vacuum-dried at 110 °C for 10 h before the next layer was coated. Then they were pyrolyzed under a given temperature for 1.5 h at a heating rate of 5 K/min and a N2 flow of 100 mL/min. This procedure produced a single-layer carbon membrane, or the so-called one-step coating/pyrolysis method. Single-gas permeation experiments were carried out by a vacuum time-lag method at a feed pressure of 1 atm. Membranes were sealed with an epoxy resin at one end and a stainless steel tube at the other end and had an effective membrane area of 0.72 cm2. Feed gas was fed to the outer side of the membrane in a permeation cell. The core side of the membrane was maintained in a vacuum. The permeation flux is given in gas permeation units (GPU): GPU ) 10-6 cm3(STP)‚s-1‚cm-2‚ cmHg-1. Results and Discussion Precursor Synthesis. Synthesis of resol-type PF used in this study can be divided into three stages. As shown in Figure 1, the first stage of synthesis is the base-catalyzed hydroxymethylation. o- and p-(hydroxymethyl)phenol, (dihydroxymethyl)phenol, and a small amount of (trihydroxymethyl)phenol were formed in the first stage. Then, condensation occurred among (hydroxymethyl)phenols, and a low molecular weight prepolymer was formed. The polycondensation was further promoted under vacuum, and the liquid-soluble resoltype PF was formed at the third stage. Theoretically, 1.5 mol of formaldehyde is needed for the complete three-dimensional cross-linking of 1 mol of phenol. A higher proportion of formaldehyde is used in technical resins. On average, approximately 1.6 mol of formaldehyde is used.18 The poor mechanical strength of the precursor membrane is apt to cause cracks and defects during pyrolysis. Therefore, to reduce the brittleness of resol-type PF, the amount of phenol was raised in comparison with common resol-type PF. A phenol/ formaldehyde molar ratio of 1/1.3 was adopted in this study. KOH was used as the base catalyst at a KOH/ phenol molar ratio of 0.0225/1. Furthermore, to form a desired film of PF/SPF with acceptable strength and toughness, it is necessary to carefully control the amount of active hydroxymethyl group, i.e., to carefully control the cross-linking point that occurred between resol-type PF and SPF. The amount of active hydroxymethyl group is controlled by the following reaction conditions, i.e., heating rate, reaction temperature, reaction time, and the solid content of the final resoltype PF. In this study, the reaction mixture was heated to 90-94 °C within 65 min and maintained at that temperature for 120-150 min. The reaction mixture

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Figure 2. FTIR spectra of PF and PF/SPF(45/55) precursors. Table 1. Results of FTIR Analysis for PF and PF/SPF(45/55) PF peak O-H stretch CdC stretch Ar-OH stretch C-O-C asymmetric stretch C-O-C symmetric stretch C-H bending

PF/SPF wavenumber (cm-1) 3348 1610, 1481 1221 1148 1072 877-760

peak O-H stretch CdC stretch Ar-OH stretch SdO asymmetric stretch C-H bending O-H bending

wavenumber (cm-1) 3399 1594, 1475 1220 1031 884-776 591

was then gradually cooled to 60 °C and dehydrated under vacuum for about 4 h. The obtained resol-type PF had a solid content of 85-88%. The lower the p-phenolsulfonic acid/formaldehyde ratio, the higher the reactivity of SPF prepolymer and, consequently, the more brittle the resin cross-linked between resol-type PF and SPF. The resin synthesized by the phenolsulfonic acid/formaldehyde ratio of less than 1/0.5 was not suitable to prepare the desired precursor films. The preferred molar ratio of p-phenolsulfonic acid to formaldehyde was 1:0.60 to 1:0.66. The structures of cross-linked PF and PF/SPF were investigated by FTIR. Samples were prepared as described below. The methanol solutions of resol-type PF and PF/SPF(45/55) copolymer were cast in alumina and glass plates, respectively, and dried at 70 °C for 8 h and then at 110 °C under vacuum for 10 h. The FTIR spectra of PF and PF/SPF are shown in Figure 2. The corresponding results are listed in Table 1. The distinguished feature of the IR spectrum of PF/SPF is the presence of absorption at 1031 cm-1, which was assigned to symmetric SdO stretching caused by the introduction of -SO3H groups into the copolymer, and absorption at 591 cm-1, which was assigned to O-H bending caused by a hydrogen bond due to -SO3H groups. Another difference between PF and PF/SPF is that the bands of the 1148 cm-1 asymmetric C-O-C stretch and the 1072

cm-1 symmetric C-O-C stretch were observed in PF but not in PF/SPF, indicating that PF was mainly linked by ether linkages whereas PF/SPF was mainly linked by methylene linkages. Formation of methylene linkages in soluble PF was due to the strong alkaline condition, while neutral or weak acidic conditions resulted in ether linkages in cross-linked PF. On the other hand, the strong acidity of sulfonic acid groups of SPF resulted in methylene linkages in PF/SPF. PF and SPF in a coating solution are still low molecular weight oligomers. As methanol evaporates from the coated membrane surface, reaction between the -CH2OH of PF and the ortho hydrogen of SPF occurs preferentially over the reaction among -CH2OH groups of PF because of the high reactivity of ortho hydrogen of SPF, resulting in a homogeneous and highly cross-linked polymer. Differential scanning calorimetry (DSC) analysis measured under a N2 atmosphere from -120 to 200 °C showed no glass transition signal for cross-linked PF/SPF. The schematic structures of cross-linked PF and PF/SPF are shown in Figure 3. Thermal Decomposition and Gas-Evolving Behavior of Precursors. The understanding of the thermal decomposition and gas-evolving properties of precursors is instructive to determine the appropriate pyrolysis conditions. Figure 4 shows TG-MS results of PF and PF/SPF(45/55) copolymer during the pyrolysis under a He atmosphere at a heating rate of 5 K/min up to 820 °C. Samples were prepared by the same method as that in FTIR. As shown in TG-MS spectra in Figure 4b,c, samples were kept at 110 °C for 30 min to remove water adsorbed. Both samples showed a total weight loss of around 47%, as shown in Figure 4a. For PF, 10% weight loss occurred in the range of 170-300 °C, which was mainly attributed to H2O and CH3OH, followed by a weight loss of about 25% between 400 and 600 °C, which was mainly attributed to 2-methylphenol and 4-methylphenol (C7H8O). However, for PF/SPF(45/55), 27% weight loss occurring in the range of 110-450 °C

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Figure 3. Schematic structures of cross-linked (a) PF and (b) PF/ SPF precursors.

was attributed to H2O and SO2 evolving in the pyrolysis. Then CO2 and CO gradually evolving above 450 °C accounted for the remaining weight loss. In Figure 4c, the first H2O peak appearing in the range of 45-110 °C was attributed to the adsorbed water in the precursor because of hydrophilic -SO3H groups. Such a peak was not observed for PF without sulfonic acid groups. H2O was not observed after the sample was held at 110 °C for 10 min. However, H2O was immediately detected as the temperature increased after this holding period, and the evolving H2O peak reached a maximum at around 210 °C. This mainly resulted from the release of hydrate water due to the strong hydratability of sulfonic acid. The calculated amount of -SO3H in PF/SPF(45/55) was 24 wt %, indicating that 27% weight loss from 110 to 450 °C for PF/SPF(45/55) was mainly due to released hydrate water and decomposition of -SO3H. Mass spectra also showed that no vigorous decomposition occurred besides decomposition of the sulfonic acid group for PF/SPF(45/55) below 450 °C, whereas various pyrolysis fragments were detected and substantial decomposition occurred for PF below 450 °C. Even above 450 °C, PF/SPF(45/55) was still rather stable and only small amounts of H2O, CO2, and CO were gradually detected, whereas various fragments were continuously evolved for PF. This suggested that the PF/SPF(45/55) backbone was thermally more stable than the PF backbone. This result is consistent with the fact that PF was mainly linked by ether linkage while PF/SPF(45/55) was mainly linked by methylene linkage, as indicated in Figure 3, because methylene linkage is thermodynamically much more stable than ether linkage. In other words, the big fragments generated for PF indicated that cleavage of the backbone occurred. Thus, decomposition of PF/SPF(45/55) occurred mainly in the pendant sulfonic acid group below 450 °C, suggesting that pyrolysis below 450 °C hardly affected the skeleton of the matrix. Figure 5 shows the FTIR spectra of the PF/SPF(45/55) precursor and samples pyrolyzed at 500 and 800 °C. The pyrolyzed samples were prepared by heating the precursor films at a rate of 5 °C/min under a N2 flow of 100 mL/min and by holding it at 500 or 800 °C for 1.5 h. The peaks assigned to the broad O-H stretch (around 3399 cm-1), SdO asymmetric stretch (1031

Figure 4. (a) TG curves of PF and PF/SPF(45/55) precursors. (b) MS spectra of gases evolved during pyrolysis of the PF precusor. (c) MS spectra of gases evolved during pyrolysis of the PF/SPF(45/55) precursor.

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Figure 5. FTIR spectra of the PF/SPF(45/55) precursor and PF/SPF(45/55) pyrolyzed at 500 and 800 °C. Table 2. Elemental Analysis Results of the PF Precursor, PF/SPF(45/55) Precursor, and Pyrolyzed PF/SPF(45/55) sample PF PF/SPF(45/55) PF/SPF-I PF/SPF-II PF/SPF-III

preparation condition precursor precursor 500 °C, 1.5 h 500 °C, 1.5 h, two timesb 500 °C, 1.5 h, three times

Ca 70.78 56.42 81.99 84.20

H

S

O

K

5.86 22.30 1.06 3.97 9.70 29.22 0.69 3.64 3.03 10.65 0.69 3.35 2.72 9.04 0.69

84.24 3.37 2.56

9.14 0.69

a All elements were given by weight percent. b Two and three times mean that the same pyrolysis procedure was repeated two and three times.

cm-1), and O-H bending (591 cm-1) in the PF/SPF(45/55) precursor disappeared after the pyrolysis at 500 °C, indicating decomposition of -SO3H. The peaks assigned to the Ar-OH stretch (1206 cm-1) and CdC stretch (1593 and 1434 cm-1) were clearly detected, although they became broader than those of the PF/ SPF(45/55) precursor, suggesting that the matrix still had polymeric characteristics. These peaks completely disappeared after pyrolysis at 800 °C, suggesting that carbonization substantially proceeded above 500 °C. Table 2 lists the elemental analysis results of the precursor and pyrolyzed samples. The pyrolyzed samples were prepared by heating the precursor films at 500 °C for 1.5 h at a heating rate of 5 K/min and a N2 flow of 100 mL/min and were then ground to powder. The K element was assumed to remain in the samples by the amount used in the precursor synthesis. The O content was calculated by subtracting the sum of the C, H, S, and K contents from the total weight. The C content of PF/SPF(45/55) is 15% less than that of PF because -SO3H groups were introduced into the copolymer. By pyrolysis, the S and O contents of PF/SPF-I decreased significantly and the C content increased drastically. By the repeated pyrolysis (PF/SPF-II), the C content increased by 2% and the O content decreased by 1.5% whereas the H and S contents decreased slightly. There

was no substantial variation in the C, H, S, and O contents by the third-time pyrolysis (PF/SPF-III). Potential of the Pendant Sulfonic Acid Groups in Forming CMS Membranes. Thermal decomposition of PF/SPF(45/55) implied that the preparation of pyrolyzed membranes using PF/SPF(45/55) as the precursor material has two advantages compared to those derived from PF. One is the evolution of gases with small and similar size such as H2O, SO2, CO2, and CO. This might be the basis to manipulate CMS membranes. The other is the smaller collapse of the matrix skeleton, which might be helpful to prepare highly permeable CMS membranes. Precursor and pyrolyzed membranes from PF and PF/ SPF(45/55) were prepared to check the potential of the pendant sulfonic acid groups in forming CMS membranes. PF-based and PF/SPF(45/55)-based membranes were prepared by coating a 30 wt % methanol solution of PF and a 10 wt % methanol solution of PF/SPF(45/ 55), respectively, three times. These membranes were subjected to the one-time pyrolysis (one-step C/P method) under a specified temperature (given in parentheses in Table 3) for 1.5 h. The gas permeation results are shown in Table 3. The PF precursor membrane showed very low gas permeances even to He and H2. For the PF membrane pyrolyzed at 500 °C [hereafter abbreviated as PF(500)], the O2 permeance was only 0.13 GPU and the ideal O2/N2 separation factor was 1.8. PF(600) showed no appreciable improvement in gas permeation compared to the PF precursor membrane. The reason might be that cleavage of the backbone led to densification of the matrix, and as a result, evolved pyrolysis fragments did not effectively contribute to micropore formation. The PF/SPF(45/55) precursor membrane displayed much larger gas permeances than the PF precursor, probably because the linear segment of SPF resulted in less-dense packing of the polymer matrix. However, both the O2 permeance and the ideal O2/N2 separation factor were still low.

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Table 3. Comparison of Gas Permeation Properties for PF and PF/SPF(45/55) Precursors and Pyrolyzed Membranes at 35 °C and 1 atm (Permeance Units: GPU)a membrane

RHe

RH2

RCO2

RO2

PF precursor PF(500) PF(600) PF/SPF precursor PF/SPF(250) PF/SPF(350) PF/SPF(450) PF/SPF(500) PF/SPF(550) PF/SPF(800)

0.076 16 0.53 4.8 16.6 132 420 910 660 91

0.056 10.5 0.091 3.2 18.3 250 860 1950 1150 98

× 0.29 × 0.12 2.1 94 330 800 460 15.8

× 0.13 × 0.059 0.54 20 74 240 120 4.3

a

RH2/ RCO2/ RO2/ RCH4 RCH4 RN2 × 157 × 74 260 112 113 65 71 138

× 4 × 3 30 43 44 27 28 22

× 1.8 × 1.8 6.3 5.9 6.3 5.2 5.4 5.1

× refers to permeances of less than 0.001 GPU.

Figure 7. SEM photograph of a cross-sectional view of PF/SPF(500).

Figure 6. TG curves of PF/SPF(45/55) pyrolyzed at 350 and 500 °C for 1.5 h.

Interestingly, compared to its precursor membrane, PF/SPF(250) displayed high gas permeances and ideal separation factors. The pyrolysis at 350 °C enhanced the gas permeance drastically, whereas the ideal separation factor increased a little or hardly changed depending on the gas pairs. This is probably due to formation of a microporous structure as a result of decomposition of sulfonic acid groups. A further increase in the pyrolysis temperature up to 500 °C enhanced gas permeance much more, accompanied a slight decrease in the separation factor. Judging from the gas-evolving behavior of PF/SPF shown in Figure 4c, decomposition of sulfonic acid groups almost completed up to 350 °C. The increase in the pyrolysis temperature from 350 to 500 °C caused an increase in weight loss by about 10 wt %, accompanied by the evolution of H2O, CO2, and CO gases, resulting in further development of an interpenetrating micropore structure. This may be the reason for the significant increases in gas permeance observed for membranes pyrolyzed around 500 °C. Figure 6 shows TG results for PF/SPF(45/55), which were carried out by the same heating programs as those of PF/SPF(350) and PF/SPF(500). Both cases showed the same weight loss of 24 wt % in the range of 110-350 °C and around 3 wt % during their holding periods. An increase by 10% weight loss between PF/SPF(350) and PF/SPF(500) seemed to play an important role in the development of the micropore structure, resulting in high gas permeance for PF/SPF(500). The scanning

electron microscopy (SEM) image of the cross section for PF/SPF(500) displayed a membrane thickness of 3-4 µm, as shown in Figure 7. Accurate thickness was difficult to assess because some of the precursor penetrated into the pores of the support. There might be some variation in thickness for other PF/SPF membranes compared to PF/SPF(500). However, large differences in thickness should not occur because precursor membranes were prepared under the same preparation conditions. The viscosity of the 30 wt % PF solution was similar to that of the 10 wt % PF/SPF(45/55) solution because the molecular weight of PF was lower than that of PF/SPF(45/55). This might be the reason why the PF(500) membrane showed a similar membrane thickness to that of PF/SPF(500). On the other hand, PF/SPF(800) exhibited quite low gas permeances compared with PF/SPF(500) and PF/ SPF(550). This may be due to the significant decrease in the pore size and/or reduction in the fraction of the micropore (or porosity), as is often observed for carbon membranes prepared by high-temperature pyrolysis.7,10,12,13,17 It is noted that the very excellent gas separation performance of the carbonized PF/SPF(45/ 55) membranes compared with carbonized PF membranes originated from the unique pyrolysis behavior of PF/SPF(45/55), namely, the evolution of gases with small and similar size and relatively the high thermal stability of the polymer skeleton. The permeance of CMS membranes is largely controlled by the porosity, which may depend on the amount of gas evolved during the pyrolysis and the structural stability and rigidity of the polymer matrix. The pore size and its distribution, which are mainly responsible for the permselectivity of CMS membranes, may depend on the size of the gas evolved during the pyrolysis. Comparison of O2 permeance and O2/N2 separation factor for various CMS membranes is shown in Figure 8. Membrane performances were compared by gas permeances because some asymmetry even in a dense membrane could lead to an overestimation of permeability coefficients. Furthermore, it is difficult to say if one membrane is homogeneous without the data of poresize distribution. On the other hand, in the viewpoint of practical use, comparison by gas permeances will give more direct and useful information than that by gas permeabilities. Membranes from PF/SPF(45/55) displayed higher O2 permeances by a factor of more than 20 than CMS membranes from other phenolic resins

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Literature Cited

Figure 8. Comparison of O2 permeance and O2/N2 separation factor for CMS membranes derived from various polymeric precursors: circle in box, PFA14 (23 °C); 3, Resol-PF19 (23 °C); &, PFA21 (25 °C); 2, Novalak-PF17 (25 °C); ^, 6FDA-based polypyrrolone13 (dense and flat, 50 µm, 35 °C); 1, BPDA/6FDA-TrMPD3 (20 °C); 4, 6FDA-mPD13 (dense and flat, 50 µm, 35 °C); b, PVDC-PVC16 (25 °C); 0, Kapton6,7 (dense and flat, 22 µm, 35 °C); cross in box, BPDA-DDBT/BADA12 (35 °C); ., BPDA-ODA8 (65 °C); O, PF/ SPF (this study; 35 °C); ±, Resol-PF13 (35 °C). Abbreviations: mPD, m-phenylenediamine; Kapton, pyromellitic dianhydride (PMDA)4,4′-oxydianiline (ODA); BPDA, 3,3′,4,4′-biphenyltetracarboxylic dianhydride.

(Resol-PF13,19 and Novalak-PF17). This high permeance was attributed to the thermal decomposition behavior of PF/SPF(45/55). The separation factors of the present membranes were not as high as those of the other CMS membranes. This may be due to the presence of some defects acting as pinholes, because the present membranes were prepared by the one coating/pyrolysis cycle method. Repeating the coating/pyrolysis cycle may be effective to prepare defectless membranes. The preliminary study showed that the O2/N2 separation factor was significantly enhanced by keeping the O2 permeance reasonably high by repeating the C/P cycle. For example, an O2 permeance of 30 GPU and an O2/N2 separation factor of 12 were achieved at present. Further study is in progress. Conclusion 1. Cross-linked PF was mainly linked by ether linkages, whereas PF/SPF(45/55) was mainly linked by methylene linkages. 2. Preparation of pyrolyzed membranes using PF/ SPF(45/55) as the precursor material has two advantages: the evolution of gases with small and similar size such as H2O, SO2, CO2, and CO and the high thermal stability of the matrix skeleton. 3. Pyrolyzed membranes from PF/SPF(45/55) showed higher O2 permeances than those of membranes from other precursor materials found in the literature. When being introduced into thermostable polymer chains, sulfonic acid groups might act as “bonded templates” during the pyrolysis and have attractive potential in manipulating CMS membranes. Acknowledgment This work was supported partly by a Grant-in-Aid for Development Scientific Research (No. 10450296) from the Ministry of Education, Science, and Culture of Japan.

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Received for review May 3, 2001 Revised manuscript received July 26, 2001 Accepted July 30, 2001 IE010402V