Effect of Oxidation on Gas Permeation of Carbon Molecular Sieving

Apr 1, 1997 - The CMS membranes were exposed to air at 100 °C for 1 ... was decreased in the initial stage of exposure, while permselectivity was inc...
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Ind. Eng. Chem. Res. 1997, 36, 2134-2140

MATERIALS AND INTERFACES Effect of Oxidation on Gas Permeation of Carbon Molecular Sieving Membranes Based on BPDA-pp′ODA Polyimide Jun-ichiro Hayashi,† Masatake Yamamoto, Katsuki Kusakabe, and Shigeharu Morooka* Department of Chemical Science and Technology, Kyushu University, Fukuoka 812-81, Japan

A BPDA-pp′ODA polyimide film was formed on the outer surface of a porous alumina support tube and was then carbonized in an inert atmosphere at 600-900 °C. The resulting carbon molecular sieving (CMS) membranes were oxidized in O2-N2 mixtures at 300 °C or in CO2 at 800-900 °C, and the permeation properties were determined. The O2 oxidation increased both permeances and permselectivities. The CMS membranes were exposed to air at 100 °C for 1 month, and their resistance to oxidation was determined. The results show that permeance was decreased in the initial stage of exposure, while permselectivity was increased. Both properties were largely restored by heat-treatment in nitrogen at 600 °C for 1-4 h. This suggests that CMS membranes would be stable at 100 °C for months, when used in an atmosphere which contained only a small fraction of oxidants. Introduction Membrane separation represents an attractive energysaving process, since it includes no phase transformation steps. Some of the goals of membrane separation include removal of organic and water vapors, CO2 removal from natural gases, separation of air, purification of hydrogen, and separation of hydrocarbons having similar boiling points. A considerable amount of information has been accumulated especially for polymeric membranes (Robeson, 1991; Stern, 1994). Polyimide membranes based on 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) are capable of achieving both high selectivity and permeability primarily as a result of their bulky structure (Tanaka et al., 1992; Matsumoto et al., 1993; Stern, 1994) and are of great commercial use. Silica-based inorganic membranes have been also studied mainly for the separation of hydrogen from other gases (Yan et al., 1994; Morooka et al., 1995; Sea et al., 1996). Zeolite membranes have been studied for the separation of isomeric hydrocarbon, such as isobutane and n-butane, based on differences in molecular size (Kusakabe et al., 1996a and 1996b). These ceramic membranes might be used at elevated temperatures where polymeric membranes are not stable as in membrane reactors, but their industrial applications are rather limited at the present stage. In order to increase the stability of polymeric membranes, carbonization appears to be a useful technique (Koresh and Soffer, 1983; Verma and Walker, 1990; Hatori et al., 1992; Chen and Yang, 1994; Haraya et al., 1995; Hayashi et al., 1995; Jones and Koros, 1995a; Suda and Haraya, 1995). Koresh and Soffer (1983) first succeeded in producing a carbon molecular sieving * Author to whom correspondence should be addressed. Tel, +81-92-642-3551; Fax, +81-92-651-5606; e-mail, smorotcf@ mbox.nc.kyushu-u.ac.jp. † Present address: Center for Advanced Research of Energy Technology (CARET), Hokkaido University, Sapporo 060, Japan. S0888-5885(96)00767-1 CCC: $14.00

(CMS) membrane. Hayashi et al. (1995) coated the outer surface of a porous alumina support tube with a polymeric acid film, synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-oxydianiline (ODA). The film was imidized to a polyimide membrane (BPDA-pp′ODA), which was then carbonized at 500-900 °C. Carbonization above 600 °C significantly increased the permeance of He, CO2, CH4, N2, and C2H6. Hayashi et al. (1996a) were able to control the pore structure of the membrane by chemical vapor deposition of carbonaceous matter. Selectivities obtained with carbonized membranes are generally much higher than those of polymeric membranes (Koresh and Soffer, 1983; Jones and Koros, 1995a and 1995b; Hayashi et al., 1996a and 1996b). Usually, selectivity decreases with increasing permeability coefficient, and this relationship is called a tradeoff line. Hayashi et al. (1996a) found that the trade-off line of a carbonized membrane for O2/N2 systems was threefold higher in the direction of selectivity than that of the upper limit of polyimide membranes, based on the data reported in 1995 (Koros, 1995). Hayashi et al. (1996b) also showed that a carbonized membrane formed with a BPDA-pp′ODA polyimide membrane easily distinguished C2H4/C2H6 and C3H6/C3H8 systems. The C3H6/C3H8 selectivity was approximately 30 at a permeability coefficient of 70 Barrer (1 Barrer ) 3.4 × 10-16 mol‚m-1‚s-1‚Pa-1). Carbon surfaces are generally hydrophobic, but micropore walls of carbonized membrane are partially covered with oxygen-containing functional groups, thus giving the membrane a hydrophilic character. Jones and Koros (1995a) reported that micropores were gradually plugged with water at room temperature and that the permeance to nonpolar gases was decreased. This drawback was circumvented by coating the membrane with a hydrophobic film (Jones and Koros, 1995b). Geiszler and Koros (1996) showed that atmospheres and flow rates in the heat-treatment step strongly influenced H2/N2 and O2/N2 selectivities of CMS membranes. The © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2135 Table 1. Oxidation Conditions carbonization oxidation oxidation temp (°C) temp (°C) atmosphere 600-800 800 900 900 900 a

Figure 1. Membrane preparation procedure.

removal of residual oxygen at the part per million level caused a change in the pore size of the membranes. It is well-known that adsorption properties of CMS granules and fibers are determined by conditions of activation (Verma and Waker, Jr., 1990). CMS membranes are more sensitive to activation conditions than CMS granules and fibers, since scarce defects greatly inhibit the membrane performance. It is generally thought that CMS membranes can be used under conditions where polymeric membranes are not applicable. The stability of CMS membranes is, therefore, a major evaluation factor. To date, however, systematic studies on the stability and durability of CMS membranes are lacking. In the present study, a CMS membrane was formed by carbonizing a BPDA-pp′ODA polyimide membrane coated on a porous R-alumina tube at 600-900 °C. The membranes were treated under a variety of oxidative atmospheres, and changes in permeation rates and permselectivities were investigated. The durability of the membranes in air at 100 °C was examined for 1 month. Experimental Section Preparation and Characterization of CMS Membranes. Figure 1 shows the procedure for the formation of a membrane. Powdered BPDA and ODA were each suspended in N,N′-dimethylacetamide (DMAc) which had been distilled prior to use. The BPDA suspension was then added dropwise to the ODA under an inert atmosphere. The weight concentration of polyamic acid was approximately 9%. The chemical structures of BPDA and ODA, as well as of polyamic acid and polyimide, are shown elsewhere (Hayashi et al., 1995). A porous R-alumina tube manufactured by NOK Corp., Japan, was used as a support to provide mechanical strength for the membrane (Hayashi et al., 1995). The dimensions of the support tube were, outside diameter, 2.4 mm; inside diameter, 1.8 mm; void fraction, 0.48; and average pore size, 140 nm. The morphologies of the support tube and membranes were observed with a scanning electron microscope (SEM, Hitachi S-900). The BPDA-pp′ODA polyimide mem-

300 800 900 900 900

O2-N2a CO2 CO2 CO2 CO2

holding time

defects obsd by SEM

1-6 h 1-6 h 5-30 min 1h 3h

none none none partial peeling peeled off

Oxygen fraction ) 0.05-1.0.

brane was prepared on the outer surface of the support tube by the dip-coating method. The coating-imidization cycle was repeated two or three times as reported previously (Hayashi et al., 1996b) and resulted in a pinhole-free BPDA-pp′ODA membrane. The membrane, typically 1-2-cm-long, was then placed coaxially in a horizontal R-alumina tube of 24-mm i.d. and was carbonized in a deoxygenated nitrogen stream at a flow rate of 100 mL(STP)‚min-1 by heating at a rate of 5 °C/ min to a maximum temperature of 600-900 °C and allowing it to cool down to ambient temperature with no isothermal period. When the membrane was further oxidized in O2-N2 mixtures or single component CO2, the temperature was maintained or decreased to a prescribed value without cooling to ambient temperature. The carrier was then changed to oxidizing agent as listed in Table 1. The stability of the CMS membrane was investigated by maintaining a membrane sample, which was prepared by carbonization at 700 °C, in air at 100 °C and by determining the permeation properties at 10, 20, and 30 days after the start of the exposure. To evaluate regeneration after long-term oxidation, the membrane was heat-treated in nitrogen at 600 °C for 1-4 h. Permeation through a membrane was evaluated at 35, 65, and 100 °C for single-component permeants: He (0.26 nm), CO2 (0.33 nm), O2 (0.346 nm), N2 (0.364 nm), CH4 (0.38 nm), C2H4 (0.39 nm), C2H6 (0.40 nm), C3H8 (0.43 nm), and SF6 (0.55 nm). The values in parentheses are kinetic diameters of permeants given by Breck (1974). Permeant gas was introduced without dilution into the outside of the tube, and argon was fed to the inside of the tube as the sweep gas. The total pressure on either side of the membrane was maintained at 101.3 kPa throughout the experiment. Concentration of permeant on the permeate side was maintained in the range 0.1-1 vol % by varying the sweep flow rate. Details of the experimental setup were reported elsewhere (Hayashi et al., 1995). The permeance of icomponent gas, P′(i), was obtained by dividing the flux by the difference in partial pressures on feed and permeate sides, pH(i)-pL(i). The partial pressure on the permeate side, pL(i), was calculated by logarithmically averaging the partial pressures of i-component on the inlet and outlet of the tube. The permselectivity of i-component relative to j-component was defined as P′(i)/ P′(j). The permeance was unaffected by sweep flow rate and total pressure under these experimental conditions. Permeances of carbonized membranes remained unchanged for bicomponent systems (Hayashi et al., 1996b). Microporous properties of the CMS were evaluated with a constant-volume sorption unit. Equilibrium sorption data were correlated by the Dubinin-Astakhov equation (Dubinin, 1960), and microporosity was determined from sorption isotherms of CO2, C2H6, n-C4H10, and i-C4H10 at 25 °C under pressures of 1-100 kPa. The saturated liquid densities of CO2, C2H6, n-C4H10, and i-C4H10 were assumed to be 0.72, 0.31, 0.57, and 0.55

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Figure 2. Effect of carbonization temperature on elemental distribution in membranes. The mass of the initial polyimide membrane is assumed as unity.

Figure 3. Effect of carbonization temperature on micropore volume distribution. 0, pores larger than CO2; b, pores larger than C2H6; 4, pores larger than n-C4H10; O, pores larger than i-C4H10.

Figure 4. Morphology of a CMS membrane prepared by carbonization at 700 °C and further oxidation in O2-N2 mixture (O2 fraction ) 0.1) at 300 °C for 3 h.

Mg‚m-3, respectively (Lide and Kehiaian, 1994). Details of the measurement have been reported previously (Hayashi et al., 1996a). Results and Discussion Elemental Analysis, Pore Volume, and Morphology. Figure 2 shows the elemental contents of the membranes which were heat-treated under an inert atmosphere. Hydrogen and oxygen were removed more intensively by heat-treatment at higher temperatures, but nitrogen which was contained in imide bonds was rather stable. Condensed polynuclear compounds containing pyrrol and pyridine rings appeared to be produced by the heat-treatment. Figure 3 shows changes in micropore volume with carbonization temperature. The volume of micropores, based on the mass of the initial polyimide, reached a maximum at 700 °C for all molecular probes used, and micropores larger than 0.5 nm disappeared as a result of heat-treatment at 900 °C. This suggests 700 °C and 800-900 °C represent optimum carbonization temperatures from the standpoint of permeation rate and permselectivity, respectively. Figure 4 shows the surface morphology of a CMS membrane produced at a carbonization temperature of 700 °C and further oxidized in an O2-N2 mixture (O2 fraction ) 0.1) at 300 °C for 3 h. The top surface was smooth without pinholes, and the thickness was 4-6 µm in most cases. A single carbonized layer was observed, even though the dip-coating/imidization process was repeated two or three times. This indicates that BPDA-pp′ODA was partially dissolved in DMAc even after imidization. The thickness and surface

Figure 5. Effect of oxidation on permeances of membrane carbonized at 600 °C. Permeation temperature ) 100 °C; b, asformed; O, oxidized in O2-N2 mixture (O2 fraction ) 0.2) for 1 h.

morphology of the membranes remained unchanged by oxidation under the other conditions, so long as the membranes were not peeled off. However, the permeation properties were dependent on carbonization temperature and oxidation conditions as described later. Permeances of Oxidized Membranes. Figures 5-8 show the effects of carbonization temperature and oxidization conditions on permeance. The closed circles indicate permeances for the membranes carbonized at 600 °C (Figure 5), 700 °C (Figures 6 and 7), and 800 °C (Figure 8) with no subsequent oxidation. As reported previously (Hayashi et al., 1995), the permeances decreased and the permselectivities increased with increasing carbonization temperature. Oxidation in O2-N2 mixtures (O2 fraction; 0.2 for Figures 5, 0.05-1.0 for Figure 6, 1.0 for Figures 7 and 8) increased permeances to all gases but often decreased

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Figure 6. Effect of oxidation on permeances of membrane carbonized at 700 °C. Permeation temperature ) 100 °C. b, asformed; 3, oxidized in O2-N2 mixture (O2 fraction ) 0.05) for 1 h; O, oxidized in O2-N2 mixture (O2 fraction ) 0.1) for 3 h; 0, oxidized in O2-N2 mixture (O2 fraction ) 0.2) for 1 h; 4, oxidized in O2 for 3 h.

Figure 7. Effect of oxidation on permeances of membrane carbonized at 700 °C. Permeation temperature ) 65 °C; b, as formed, O, oxidized in O2 for 3 h.

Figure 8. Effect of oxidation on permeances of membrane carbonized at 800 °C. Permeation temperature ) 65 °C. b, as formed; O, oxidized in O2 for 3 h; 0, oxidized in O2 for 6 h.

permselectivities, especially for molecules larger than 0.4 nm, as indicated in Figure 5. This suggests that one role of the oxidation is a broadening of the pore size distribution. Meanwhile, oxidation of pore walls could enhance adsorption of polar molecules and, as a result, permeation rates. This mechanism predicts that permeation rate of polar permeants would be increased. Wang et al. (1996) reported that the O2 permeance of a carbon membrane was increased by oxidation in O2N2 mixtures (O2 fraction ) 0.005-0.02) at 800-950 °C for 30-60 min. However, this adsorption effect was not definitely observed for the case of the present study. As shown in Figure 6, permeances were unselectively increased when the membrane was oxidized at higher O2 fractions.

Figure 9. Effect of oxidation on permeances of membrane carbonized at 800 °C. Permeation temperature ) 65 °C. b, as formed, O, oxidized in CO2 at 800 °C for 1 h; 0, oxidized in CO2 at 800 °C for 3 h; 4, oxidized in CO2 at 800 °C for 6 h.

Figure 10. Effect of oxidation on permeances of membrane carbonized at 900 °C. Permeation temperature ) 65 °C. b, as formed; O, oxidized in CO2 at 900 °C for 5 min; 0, oxidized in CO2 at 900 °C for 0.5 h; 4, oxidized in CO2 at 900 °C for 1.0 h.

Figure 7 indicates that the CMS membrane carbonized at 700 °C was successfully improved by oxidization with O2 at 300 °C for 3 h. The permeance to CO2 was approximately 4 × 10-7 mol‚m-2‚s-1‚Pa-1, and the CO2/ CH4 selectivity was approximately 20 at 65 °C. These data are of the same order for CO2 permeance and selectivity as for TEOS-modified silica membranes prepared by Raman and Brinker (1995). The permeance of the oxidized membrane to SF6 at 65 °C was 4 × 10-11 mol‚m-2‚s-1‚Pa-1, which was four orders of magnitude smaller than that of CO2. This suggests that the pore volume was increased without widening the pore size distribution in this case. Figure 8 shows the effect of O2 oxidation on permeances of the membrane carbonized at 800 °C. The permeances were improved by oxidation but less intensely than for the membrane carbonized at 700 °C. This may be ascribed to the difference in rigidity of the carbonized membranes. Figures 9 and 10 show the effects of carbonization temperature and oxidization conditions on permeance. CO2 oxidation for 1 h at 800 °C and 5 min at 900 °C had no effect on permeance. However, excess oxidation abruptly expanded the pore size and decreased permselectivities for permeants larger than 0.4 nm. Thus, the control of micropore size was not achieved by CO2 oxidation at 800-900 °C. To compare permeation rates of CMS membranes to those of polyimide membranes, permeance values should be converted to permeability coefficient values, which are commonly used for polymeric membranes. Since the carbonized membranes prepared in the present study were homogeneous and 4-6 µm in thickness due to SEM observation, 4 × 10-7 mol‚m-2‚s-1‚Pa-1 for CO2 corre-

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Figure 11. Change in mass and elemental distribution in membranes during the stability test. The mass of initial CMS membrane, which was formed by carbonizing at 700 °C, is assumed as unity.

sponds to approximately 6 × 103 Barrer. The combination of a CO2 permeability coefficient of 6 × 103 Barrer and a CO2/CH4 selectivity of 20 for the O2 oxidized membrane shown in Figure 7 is beyond the trade-off curve between permeability coefficient and selectivity for polyimide membranes reported by Tanaka et al. (1992). The CO2/N2 selectivity of the same membrane was approximately 8, and the combination of the CO2 permeability coefficient and CO2/N2 selectivity is equivalent to the trade-off curve for polyimide membranes. Hayashi et al. (1996a) carbonized a BPDA-pp′ODA polyimide membrane and then further modified it by chemical vapor deposition using propylene as the carbon source. Their membrane was approximately 6 µm in thickness, and the CO2/N2 selectivity was 73.4 at a CO2 permeability coefficient of 400 Barrer at 35 °C. This combination is beyond the trade-off curve for CMS membranes, which exhibited much better performance than polyimide membranes (Hayashi et al., 1996b). The CMS membrane formed by O2 oxidation shown in Figure 7 is equivalent to polyimide membranes from the viewpoint of the trade-off curves, but its permeation rate exceeded that of polyimide membranes. Long-Term Stability of Membranes in Air at 100 °C. In order to evaluate the stability of CMS membranes, a membrane carbonized at 700 °C was exposed to air at 100 °C for 1 month. Since the use of carbon membranes is not recommended under an oxidative atmosphere, this test period may correspond to many months for industrial processes. Figure 11 shows the changes in elemental contents of the membrane during the stability test. The change in membrane mass was largely due to the loss of carbon. Oxygen or water in air reacted with the membrane, producing oxygencontaining functional groups such as carbonyl. Since the membrane mass decreased gradually, incorporation of oxygen and removal of carbon as CO2 occurred concurrently. When the membrane was heat-treated in nitrogen at 600 °C for 4 h, the membrane mass and its elemental distribution were further decreased. Figures 12 shows the effects of permeation temperature and period of exposure to air at 100 °C. The permeances increased with increasing permeation temperature, and no breakdown of the membrane during the 1-month test was observed. The activation energies for permeances, especially to larger molecules, were increased by the exposure, as shown in Figure 13. After

Figure 12. Effects of exposure period and permeation temperature on permeances of membrane carbonized at 700 °C. Permeation temperature: (a) 35 °C; (b) 65 °C; (c) 100 °C. Exposure period in air at 100 °C: b, initial; O, 9 days; 0, 20 days; 4, 30 days.

Figure 13. Effects of exposure to air at 100 °C and post-heattreatment in nitrogen at 600 °C for 4 h on activation energies for permeances of membrane carbonized at 700 °C.

the post-heat-treatment in nitrogen, the activation energies for permeances to He, CO2, and N2 were restored to the initial values for the membrane. How-

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Conclusions

Figure 14. Effects of exposure to air at 100 °C and post-heattreatment in nitrogen at 600 °C for 4 h on permeances of membrane carbonized at 700 °C. Permeation temperature ) 65 °C.

A BPDA-pp′ODA polyimide membrane was formed on a porous R-alumina support tube and carbonized at 600-900 °C in an inert atmosphere. The resulting membranes were oxidized in mixtures of O2-N2 at 300 °C or in CO2 at 800-900 °C. The O2 oxidation at low temperature was more controllable than the CO2 oxidation at high temperature. The CO2 permeance of the membrane oxidized under optimum conditions was increased to approximately 4 × 10-7 mol‚m-2‚s-1‚Pa-1, and the CO2/CH4 selectivity was approximately 20 at 65 °C. To evaluate the stability of carbon membranes, a membrane carbonized at 700 °C was exposed to air at 100 °C for 1 month. This condition was selected to shorten the period of stability test which had to be carried out under a scarce oxygen concentration. The permeances decreased in the initial stage of the oxidation but were largely restored by a post-heat-treatment at 600 °C for 1-4 h. Chemical analyses and pore size distribution of oxidized membranes will be discussed in subsequent reports. Acknowledgment This work was supported by the Ministry of Education, Science, Sports and Culture, Japan; The New Energy and Industrial Technology Development Organization (NEDO); and The Research Institute of Innovative Technology for the Earth (RITE). Support from the Kyocera Corp. and NOK Corp. is also acknowledged.

Figure 15. Effects of exposure to air at 100 °C and post-heattreatment in nitrogen at 600 °C for 4 h on permselectivities of membrane carbonized at 700 °C. Permeation temperature ) 65 °C.

ever, those for CH4 and C2H6 were higher than the initial values, even after the post-heat-treatment. Figure 14 shows changes in permeances at 65 °C during the long-term oxidation and post-heat-treatment. Permeances to larger molecules were considerably decreased in the initial stage of the oxidation but were largely recovered by the post-heat-treatment. The permselectivities for CO2/N2 and CO2/CH4 systems, as typical cases, at a permeation temperature of 65 °C are shown in Figure 15. The CO2/N2 selectivity was restored, and the CO2/CH4 and N2/C2H4 (not shown) selectivities were somewhat increased after the heattreatment. As indicated in Figure 11, approximately 20% of the initial mass of the CMS membrane was lost as a result of oxidation and post-heat-treatment, while the thickness of the membrane remained unchanged. If the micropore volume of the membrane is increased, the permeances would be increased during the longterm oxidation. However, this prediction is not supported by the experimental results. Thus, plausible effects of long-term oxidation are as follows: (1) Oxygen in air reacts with the membrane and forms oxygen-containing functional groups, which are continuously decomposed to CO2. (2) The presence of surface oxides reduces the aperture of micropores, decreases permeances, and increases permselectivities, especially for larger permeants, such as C2H6. (3) The formation and removal of surface oxides gradually restructure the carbon layers and somewhat narrow the micropores permanently.

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Received for review December 3, 1996 Revised manuscript received February 4, 1997 Accepted February 5, 1997X IE960767T

X Abstract published in Advance ACS Abstracts, April 1, 1997.