Ind. Eng. Chem. Res. 1996, 35, 2999-3003
2999
Effects of Polyimide Pyrolysis Conditions on Carbon Molecular Sieve Membrane Properties Vincent C. Geiszler* and William J. Koros Separations Research Program, Center for Energy Studies, The University of Texas at Austin, Austin, Texas 78712
In previous research, carbon molecular sieve (CMS) membranes for gas separations have been produced using either a vacuum pyrolysis or an inert purge pyrolysis technique on a precursor which is often polymeric. This study compares both techniques using the same polyimide precursor material. Additional pyrolysis variables included the type of “inert” purge gas (argon, helium, and carbon dioxide), purge flow rate, and temperature. Vacuum pyrolysis produced more selective but less productive CMS membranes than the inert purge pyrolyzed membranes. “High” purge gas flow rates (i.e., 200 standard cubic centimeters per minute or cm3(STP)/min) produced a much higher permeability, but lower selectivity membrane compared to those produced in a “low” purge flow rate (20 cm3(STP)/min). By raising the pyrolysis temperature from 550 to 800 °C, the effective pore size was reduced, thereby making the CMS membranes more selective but less productive. Mixed gas tests using oxygen/nitrogen and hydrogen/nitrogen mixtures were used to evaluate membrane performance. Introduction To make gas separation membranes more economically attractive, research continues to seek materials that are both more selective and permeable. Molecular sieving materials have the potential to push the upper boundary of a permeability and selectivity tradeoff relationship (Koros, 1995). Such materials comprise carbon molecular sieves (CMS) and zeolites. This article focuses on the processing aspects of CMS membranes and how they affect final performance. Carbon membranes for various separations are typically produced by the pyrolysis of a polymeric precursor. Another synthetic method that has seldom been used is the compression sintering of granular carbon (Ash et al., 1976). Both inert purge and vacuum pyrolysis techniques have been used in separate studies but never compared. Soffer et al. produced CMS membranes by use of an argon atmosphere (1987). Hatori et al. produced macroporous carbons by pyrolysis of a Kaptontype polyimide film between graphite plates in flowing argon (1995). A nitrogen atmosphere was used by Chen and Yang to produce CMS membranes (1994) and by Rao and Sircar to produce nanoporous membranes (1993). To synthesize carbon hollow fibers, Linkov et al. stabilized and carbonized a polyacrylonitrile/poly(methyl methacrylate) copolymer in nitrogen (1994). Vacuum pyrolysis was used by Jones and Koros (1994) and Haraya and Suda (1995) for CMS hollow fiber membranes. To tailor the separation performance of carbon membranes, the pyrolysis temperature can be varied in accordance with the type of precursor material. It is desirable to keep the processing temperature low enough to prevent graphitization, especially for coke-forming precursor materials. For carbon membranes, processing temperatures are typically in the range 500-1000 °C, and CMS membrane synthesis temperatures fall within this range (see Table 1). This study examines how vacuum and inert purge pyrolysis of a precursor affect CMS membrane perfor* Author to whom correspondence should be addressed. Phone: (512) 471-1087. FAX: (512) 471-1720. E-mail:
[email protected].
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mance for similar pyrolysis temperatures. Additionally, pyrolysis variables such as the processing temperature, purge gas flow rate, and residual oxygen concentration in the purge gas were examined. Experimental Section Making of CMS Membranes. The precursor was an integrally-skinned, asymmetric hollow fiber made of a polyimide. This polyimide was the product of three monomers: 2,4,6-trimethyl-1,3-phenylenediamine, 3,4: 3′,4′-biphenyltetracarboxylic acid dianhydride, and 5,5′[2,2,2,-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3isobenzofurandione whose structures are shown in Figure 1 (Jones and Koros, 1994). The precursor fibers were 157 ( 3 µm in inner diameter and 231 ( 3 µm in outer diameter. The outer surface exhibited a thin skin layer, whereas the inner surface did not. Before starting a pyrolysis trial, a batch of 6-8 fibers was secured to a stainless steel wire mesh by wrapping a thin wire around both. The secured sample was then placed in the middle of a quartz tube (full schematic of equipment shown in Figure 2) which resided in a Thermcraft tube furnace oven. The temperature of the furnace was measured using an Omega thermocouple and temperature controller. The quartz tube was sealed with a glass end with a stopcock and a Viton O-ring, held by a pinch-type clamp. The pyrolysis system was adapted for two different protocols: vacuum pyrolysis and inert purge pyrolysis. For the former protocol, a vacuum pump was used to reduce the pressure to under 0.1 mmHg, which was measured using a McLeod vacuum gauge. When using an inert purge protocol, gas was supplied to the quartz tube from cylinders at a controlled rate using an MKS Instruments, Inc., mass flow controller. After flowing through the quartz tube, the gas went to exhaust. Additionally, at the exhaust end, the oxygen level was monitored using an oxygen analyzer and the flow rate was measured using a soap film flowmeter. Although this system is similar to the one used in previous studies by Jones and Koros (1994, 1995), the permselective properties were somewhat different as a result of the subtle equipment changes. For the vacuum pyrolysis © 1996 American Chemical Society
3000 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 Table 1. Pyrolysis Temperature Ranges and Precursor Materials for CMS Membranes researcher(s)
precursor material
pyrolysis temperature range (°C)
reference
Bird and Trimm Chen and Yang Suda and Haraya Jones and Koros Soffer et al.
polyfurfuryl alcohol polyfurfuryl alcohol Kapton-type polyimide 6F-containing polyimide copolymer cellulose and derivatives, thermosetting polymers, and peach tar mesophase
700 500 500-1000 500-550 800-950
1983 1994 1995 (2 articles) 1994 1987
Figure 1. Monomers for polyimide used as the precursor for carbon molecular sieve membranes: (A) 2,4,6-trimethyl-1,3-phenylenediamine, (B) 3,4:3′,4′-biphenyltetracarboxylic acid dianhydride, and (C) 5,5′-[2,2,2,-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofurandione (Jones and Koros, 1994).
Figure 2. Schematic diagram of the pyrolysis furnace for both the vacuum and inert purge protocols.
technique, the permeate flux was higher while the selectivity was lower than previous results. For the purpose of comparing the two pyrolysis techniques, however, the equipment performed sufficiently well. Testing of CMS Membranes. Single fibers were potted and tested for their mixed gas selectivities and fluxes. The fiber potting technique and the mixed gas testing apparatus has been previously described (Jones and Koros, 1995). Results and Discussion Effect of Pyrolysis Technique. The effects on performance due to the vacuum pyrolysis process and the “inert” purge pyrolysis protocol were pronounced. This section will examine the use of purge gases at 200 cm3(STP)/min because the best inert purge results were achieved at that flow rate. Flowrate effects are examined in the next section. Additionally, the choice of “inert” purge gas was shown to affect the final membrane properties.
CMS membranes produced by vacuum pyrolysis at 550 °C showed higher O2/N2 and H2/N2 selectivities than those produced in inert gas atmospheres at the same temperature (see Table 2). As noted above, the resultant selectivities using nominally the same protocol are lower than those obtained previously (Jones and Koros, 1994, 1995) but still impressively high and reproducible at this new level. As is typical of more selective membrane materials, the permeate fluxes were lower than those of less selective CMS membranes. The fibers shown in Table 2 were pyrolyzed in 200 cm3(STP)/min flow rate of gas at approximately atmospheric pressure while the vacuum-pyrolyzed fibers were produced at an average pressure of only 0.02-0.03 Torr. By varying the type of pyrolysis atmosphere from vacuum to a flowing gas, the mechanism of the carbonization reaction seems to change. Dickens conducted degradation studies on polyethylene (1982a) and isotactic polypropylene (1982b). These studies showed that the activation energy of degradation decreases significantly as the pressure of the inert pyrolysis atmosphere increases, thereby indicating possible differences in reaction mechanisms. When pyrolyzed in a vacuum, the polyimide probably degraded via a unimolecular degradation mechanism. When an inert gas was used, the degradation process was “enhanced”, presumably due to increased gas phase heat and mass transfer. By accelerating the carbonization reaction, the inert gas molecules appeared to produce a more “open” porous matrix in the CMS membranes resulting in a higher permeability and less selective pore structure. At a pyrolysis temperature of 550 °C and a flow rate of 200 cm3(STP)/min, little difference was seen among the membranes pyrolyzed in the three purge gases: argon, helium, and carbon dioxide (see Table 3). In previous work on granular carbon molecular sieves by Air Products (Armor et al., 1992), the researchers concluded that “the use of helium with or without nitrogen improved the selectivity of the CMS when the (CMS posttreatment) modification step employed a volatile organic compound.” Although the measurements in Table 3 could indicate such a difference between helium and argon as purge gases, such a claim would not be statistically conclusive with this data. At 800 °C, however, carbon dioxide becomes more oxidative (Ergun and Menster, 1965). As a result, the resulting CMS membranes were more productive than the equipment could accurately measure. The apparent carbon morphology in the fibers processed at 800 °C had little flux resistance (O2 flux ≈ 6500 GPU) and no appreciable selective phenomena (O2/N2 selectivity ≈ 1.0). For convenience, one might envision this in terms of a more open pore structure; the range of pore sizes for molecular sieving phenomena, however, is difficult to resolve with current techniques. At a CO2 flow rate of 200 cm3(STP)/min, the average weight loss during pyrolysis increased from 33.1% to 43.0% when the pyrolysis temperature was increased, further supporting this hypothesis. Effect of Purge Gas Flow Rate. During the pyrolysis of a polymer, byproducts of different volatility
Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3001 Table 2. O2/N2 and H2/N2 Separation Performance for CMS Membranes Produced at 550 °C (and 200 cm3(STP)/min for Inert Gas Pyrolysis) pyrolysis atmosphere
sample size
O2 flux (GPU)
O2/N2 selectivity
sample size
H2 flux (GPU)
H2/N2 selectivity
vacuum argon helium carbon dioxide
8 samples from 4 batches 19 samples from 11 batches 9 samples from 5 batches 16 samples from 9 batches
25-50 71-284 73-140 75-306
7.4-9.0 2.8-6.1 4.7-6.1 2.6-6.1
5 samples from 4 batches 11 samples from 11 batches 8 samples from 5 batches 11 samples from 8 batches
372-473 451-713 428-676 400-654
64-110 6.8-31.2 15.2-35.7 10.4-30.0
Table 3. Permselective Properties of CMS Membranes Pyrolyzed at 550 °C in Argon, Helium, and Carbon Dioxide While Varying the Purge Gas Flow Rate pyrolysis atmosphere
flow rate (cm3(STP)/min)
sample size
O2 flux (GPU)
O2/N2 selectivity
argon argon helium helium carbon dioxide carbon dioxide
200 20 200 20 200 20
19 samples from 11 batches 4 samples from 3 batches 9 samples from 5 batches 2 samples from 2 batches 16 samples from 9 batches 8 samples from 5 batches
71-284 0.050-0.54 73-140 0.05-0.11 75-306 0.05-15
2.8-6.1 2.4-7.0 4.7-6.1 4.0-5.2 2.6-6.1 2.0-7.5
can be produced. Typical volatile byproducts include H2, H2O, CO, and CO2, as well as smaller amounts of HCN, CH4, and NH3 (depending on the polymer). For a polyimide, typical less volatile byproducts include benzene, toluene, phenol, and other heavier molecules that resemble portions of the polyimide chain. (Wright, 1981; Crossland et al., 1987; Johnston and Gaulin, 1969). When the nonvolatile byproducts are not removed quickly enough during pyrolysis, they can presumably degrade further and leave carbon deposits on the surface of the carbon, which can be considered to be a form of chemical vapor deposition (CVD). The flow rates of 20 and 200 cm3(STP)/min used in this study corresponded to superficial velocities of 1.02 and 10.2 cm/min. For all three purge gases used at a pyrolysis temperature of 550 °C, the permeate flux decreased by at least 2 orders of magnitude while the selectivity remained approximately the same (see Table 3). The differences caused by flow rate variation were more pronounced at the low pyrolysis temperature of 550 °C than at 800 °C. The temperature effects will be considered in the next section. The permeate flux reduction suggests that, at a low purge gas flow rate, the flow rate determined whether the pores were blocked by carbon deposited by the pyrolysis of the polyimide decomposition byproducts. Consistent with this point of view, previous CMS literature has noted that when the precursor (i.e., coconut shell) was pyrolyzed at a low temperature of 500 °C, “much of the porosity was blocked by pyrolytic decomposition products” (Braymer et al., 1994). The reduction in permeate flux suggests that pore blocking via carbon deposition was occurring either in the pores or on the fiber surface, but the exact location could not be determined from this study. Chihara and Suzuki also reported pore blocking when they pyrolyzed aromatic organics over a molecular-sieving activated carbon at a low temperature of 400 °C (1979). More recently, Hu and Vansant pyrolyzed an aliphatic compound with limited adsorption on an activated carbon and suggested that deposition occurred near the pore entrances (1995). Furthermore, Table 3 shows O2/N2 separation results but no H2/N2 results because the N2 concentrations were below the level detectable by the gas chromatograph. Although one could conclude that the reduction in flow rate led to an increase in the boundary layer thickness, the change must be understood in light of the pyrolysis equipment used. Blasius formulated (and Howarth later elaborated) the following model for determining the boundary layer thickness, δ, over a flat
Table 4. Variation in Average Boundary Thickness over a 30-cm Plate as a Function of Flow Rate and Gas Type at 550 °C flow rate (cm3(STP)/min)
pyrolysis atmosphere
20 20 20 200 200 200
argon carbon dioxide helium argon carbon dioxide helium
Reynolds average boundary number layer thickness (cm) 0.32 0.49 0.039 3.2 4.9 0.39
130 110 420 45 38 130
plate in a laminar flow regime (Geankoplis, 1983).
δ)
5.0x
xNRe
x
) 5.0
µx Fv∞
where x is the distance from the leading edge of the plate and NRe is the Reynolds number. Using this model, the boundary layer thickness for the 550 °C experiments was calculated, as seen in Table 4. The calculated boundary layer thickness was much larger than the 5.0 cm diameter of the pyrolysis tube. Therefore, a change in boundary layer thickness does not explain the phenomena of this system, although the calculations give some insight into what could have been happening. Apparently, laminar flow convection of heat and mass is sufficient to cause the major changes observed in membrane productivity and selectivity. Effect of Pyrolysis Temperature. The pyrolysis temperature was varied between two settings, a “high” temperature of 800 °C and a “low” temperature of 550 °C. The final pyrolysis temperature was reached using a “ramp-and-soak” method, whereby the temperature was raised with heating rates starting at 13.3 °C/min, slowing first to 3.6-3.9 °C/min and then to 0.25 °C/min before holding at the final temperature. Each final pyrolysis temperature was maintained for 2 h, and then the membranes were allowed to cool to room temperature in the furnace tube. By increasing the final pyrolysis temperature, the permeate flux decreased while the selectivity increased. The variation of pyrolysis temperature could have affected several parameters that determine the permselectivity of the carbon molecular sieve membranes: (1) the pyrolysis kinetics of the polyimide, (2) the pyrolysis kinetics of the polyimide degradation byproducts, and (3) the compactness of the turbostratic carbon structure. First, the effects of changes in the pyrolysis kinetics of the polyimide can be seen by comparing the vacuum pyrolysis results, thereby removing the influence of the hypothesized inert gas “activation” of the degradation reaction. As seen
3002 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 Table 5. A. Permselective Properties of CMS Membranes Produced by Vacuum Pyrolysis pyrolysis temperature (°C)
O2 flux (GPU)
O2/N2 selectivity
average percent weight loss
550 800
25-50 1.6-4.2
7.4-9.0 6.8-10.3
36.2 36.7
B. Average Weight Loss during Pyrolysis of CMS Membranes Produced in Different Purge Gases pyrolysis purge gas
pyrolysis temperature (°C)
purge flow rate (cm3(STP)/min)
average percent weight loss
helium helium helium carbon dioxide carbon dioxide carbon dioxide
550 800 550 550 800 550
200 200 20 200 200 20
36.1 40.6 33.9 33.1 43.0 29.9
in Table 5A, there was a slightly greater average weight loss at the “high” temperature of 800 °C. The difference in weight loss was more pronounced in experiments using inert purge pyrolysis (see Table 5B). Second, the effect of temperature on the byproduct pyrolysis kinetics can be probed by observing the interaction of the flow rate and temperature variables. For a membrane produced at 550 °C in a 200 cm3(STP)/ min flow rate of helium, the permselective properties were an O2/N2 selectivity of 4.7-6.2 and an O2 flux of 73-140 GPU, as seen in Table 6. At 800 °C and the same helium flow rate, the O2/N2 selectivity improved to 4.6-9.0 while the O2 flux decreased to 1.4-6.9 GPU. When the helium flow rate was decreased at both temperatures, the O2 flux decreased. The decrease, however, was more dramatic at 550 °C, presumably because the pyrolysis byproducts degraded prior to complete removal and were plugging the membrane “pores”. At 800 °C, however, a thin layer of carbon could have been deposited inside the “pores” without sealing them. As noted earlier, other researchers have reported pore-plugging phenomena at low pyrolysis temperatures and carbon deposition at higher temperatures. Hu and Vansant modified the pore size of granular carbon molecular sieves by pyrolyzing 3-methylpentane (3-MP) in a post treatment step (1995). When the pyrolysis temperature of the 3-MP was reduced, the micropore diameter was reduced. In work by Chihara and Suzuki (1979), pores were blocked by the deposition of carbon by ethylbenzene and styrene pyrolysis on molecularsieving activated carbon at 400 °C. This produced a lower flux while maintaining the same selectivity. In several other studies (Braymer et al., 1994; Moore and Trimm, 1977), however, the average pore size of an activated carbon was successfully reduced without completely blocking the pores by coating with a pyrolyzed volatile organic at temperatures in the range from 500 K (227 °C) to 1240 K (967 °C). Both studies demonstrated pore size modification with minimal pore blocking using sorption selectivity measurements. In previous literature, however, when a CMS precursor (i.e., coconut char) was pyrolyzed at 500 °C, the re-
searchers noted that “much of the porosity was blocked by pyrolytic decomposition products.” (Braymer et al., 1994). The third way in which the processing temperature could have been affecting the separation properties of the CMS membranes was by the densification of the porous carbon matrix. A pair of fibers pyrolyzed in vacuum at 550 and 800 °C are compared in Table 7. The fibers pyrolyzed at the “high” temperature of 800 °C had an equal weight loss as those processed at 550°C (see Table 5A) but were smaller in outer diameter and had higher linear shrinkage. This produced a tighter (and more selective) porous carbon morphology that reduced the O2 flux. Similar densification behavior has been seen in the production of vitreous carbon by pyrolyzing phenolic resin (Pierson, 1993), polyfurfuryl alcohol, and polyphenylene oxide (Fitzer and Scha¨fer, 1970). Jones and Koros have also seen a similar effect on the properties of CMS membranes produced in a smaller temperature (500-550 °C) range in vacuum (1994). It is not clear which of the three phenomena was the dominant factor that caused higher permeability and lower selectivity as the pyrolysis temperature increased. Effect of Reducing Residual Oxygen. While the literature acknowledges that polymer pyrolysis is accelerated by oxygen levels like those present in air (Dine-Hart and Wright, 1971; Crossland et al., 1987), the low end concentration to which this is significant had not been tested. According to the supplier Wilson Oxygen, the argon that was used for the pyrolysis experiments contained 0.30 ppm oxygen and 0.64 ppm H2O. By adding a Baxter S/P Brand disposable oxygen trap immediately after the pressure regulator on the argon supply, the oxygen concentration was assumed to be effectively reduced to zero (a 99% reduction at a flow rate of 3 L/min was the manufacturer’s spec). The oxygen trap’s effect on the water concentration is unknown, although it is only a mild oxidizer at low temperatures (Dine-Hart and Wright, 1971; Ergun and Menster, 1965). (Note: The helium experiments in this study were performed without an oxygen trap.) CMS membranes produced while the oxygen trap was attached showed a reduced O2 flux and an increase in O2/N2 selectivity, as seen in Table 8. Similar results were seen during H2/N2 mixed gas tests, as seen in Table 9. Simplistically, by reducing the O2 concentration in the argon purge, the fibers were oxidized less and critical pore constrictions in the carbon matrix were smaller. The tighter morphology resulted in a lower flux and increased selectivity for both the O2/N2 and H2/ N2 mixed gas pairs. As shown by both the O2/N2 and H2/N2 results, the effect was more pronounced at 550 than at 800 °C. Conclusions Consideration of vacuum and inert purge pyrolysis techniques allows evaluation of the effects of key processing variables for CMS membranes for gas separations. Inert purge pyrolysis produced CMS membranes with higher permeate fluxes and lower selectivities than those produced using vacuum pyrolysis.
Table 6. Permselective Properties of CMS Membranes Produced Using a Helium Inert Purge Pyrolysis pyrolysis temperature (°C)
helium flow rate (cm3(STP)/min)
sample size
O2 flux (GPU)
O2/N2 selectivity
550 550 800 800
200 20 200 20
9 samples from 5 batches 2 samples from 2 batches 10 samples from 4 batches 5 samples from 3 batches
73-140 0.05-0.11 1.4-6.9 1.4-3.2
4.7-6.1 4.0-5.2 4.6-9.0 7.0-11.4
Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3003 Table 7. Permselective and Dimensional Properties of CMS Membranes Produced Using a Vacuum Pyrolysis Protocol (2 Samples Each Tested from Batches 3/9/95 89XAB-17V and 3/15/95 89XAB-23V) pyrolysis temperature (°C) 550 800
O2 flux (GPU)
O2/N2 selectivity
inner, outer diameters (µm)
average percent linear shrinkage (%)
18.0-27.3 8.70-9.50 100, 155 ( 2.5 1.61-2.29 7.75-12.5 100, 150 ( 2.5
15.1 20.3
Table 8. Influence of Oxygen Trap Usage during Pyrolysis on the Permselective Properties of CMS Membranes Used for an O2/N2 Mixed Gas Separation. These Fibers Were Produced Using a 200 cm3(STP)/min Argon Purge during Pyrolysis pyrolysis O2 temperature trap used? (°C) 550 550 800 800
no yes no yes
sample size 19 samples from 11 batches 11 samples from 7 batches 11 samples from 4 batches 5 samples from 2 batches
O2 flux O2/N2 (GPU) selectivity 71-284 66-204 3.0-7.6 1.7-2.9
2.8-6.1 3.6-6.5 7.8-9.8 8.7-11.2
Table 9. Influence of Oxygen Trap Usage during Pyrolysis on the Permselective Properties of CMS Membranes Used for an H2/N2 Mixed Gas Separation. These Fibers Were Produced Using a 200 cm3(STP)/min Argon Purge during Pyrolysis pyrolysis O2 temperature trap (°C) used? 550 550 800 800
no yes no yes
sample size 11 samples from 11 batches 7 samples from 7 batches 5 samples from 3 batches 4 samples from 2 batches
H2 flux H2/N2 (GPU) selectivity 451-713 372-626 47-61 39-54
6.8-31.2 13-26 82-188 156-247
Helium, argon, and carbon dioxide performed as effectively similar purge gases at 550 °C, but at 800 °C, a carbon dioxide purge produced a highly porous, nonselective membrane by oxidizing the carbon. Reductions in the purge gas flow rate from 200 to 20 cm3(STP)/min caused a decrease in the permeate flux of the CMS membranes, presumably by the deposition of carbon either on the membrane surface or in the pores. An increase in the pyrolysis temperature from 550 to 800 °C caused a significant decrease in the permeate flux and a significant increase in the selectivity for CMS membranes produced by both vacuum and inert purge pyrolysis techniques. The removal of residual oxygen at the ppm level reduced the apparent pore size of CMS membranes, resulting in higher selectivities and lower permeate fluxes. Acknowledgment The authors would like to gratefully acknowledge the financial support of Texaco, Inc. in the form of fellowship support for V.C.G. Also, the support of the work by the Separations Research Program is appreciated. Nomenclature GPU ) gas permeation unit × 10-6[1 cm3(STP)/ cm2‚s‚cmHg]
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Received for review December 15, 1995 Revised manuscript received March 5, 1996 Accepted March 6, 1996X IE950746J
X Abstract published in Advance ACS Abstracts, August 15, 1996.
