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Crosslinkable mixed matrix membranes with surface modified molecular sieves for natural gas purification: II. Performance characterization under contaminated feed conditions
Journal of Membrane Science 377 (2011) 82–88
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Crosslinkable mixed matrix membranes with surface modified molecular sieves for natural gas purification: II. Performance characterization under contaminated feed conditions Jason K. Ward, William J. Koros ∗ Georgia Institute of Technology, School of Chemical & Biomolecular Engineering, Atlanta, GA 30332, USA
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Article history: Received 25 October 2010 Received in revised form 5 April 2011 Accepted 7 April 2011 Available online 14 April 2011 Keywords: Antiplasticization Physical aging Gas separation Mixed matrix Natural gas
a b s t r a c t Mixed matrix membranes (MMMs) composed of the crosslinkable polyimide PDMC and surface modified (SM) SSZ-13 have recently been shown to enhance carbon dioxide permeability and carbon dioxide/methane selectivity versus neat PDMC films by as much as 47% and 13%, respectively (Part I). The previous film characterization, however, was performed using ideal, clean mixed gas feeds. In this paper, PDMC/SSZ-13 MMMs are further characterized using more realistic mixed gases containing low concentrations (500 or 1000 ppm) of toluene as a model contaminant. Mixed matrix membranes are shown to outperform pure PDMC films in the presence of toluene with 43% greater carbon dioxide permeability and 12% greater carbon dioxide/selectivity at 35 ◦ C and 700 psia feed pressure. These results suggest that MMMs—in addition to exhibiting enhanced transport properties—may mitigate performance degradation due to antiplasticization effects. Moreover, the analyses presented here show that the reduction in separation performance by trace contaminant-accelerated physical aging can be suppressed greatly with MMMs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Widespread commercial acceptance of polymer membranes for gas separations has been hindered by a number of performancelimiting phenomena. Perhaps the most well-known limitation of polymeric membranes is the intrinsic permeability/selectivity tradeoff noted by Robeson [1,2]. This hurdle has prevented the development of the highly productive and selective materials sought for cost-effective gas separations. Another common phenomenon in polymer membranes that often leads to drastically reduced separation performance is termed “plasticization”. Frequently encountered in natural gas purification and other separations involving strongly interacting sorbents, the swollen matrix of a plasticized membrane leads to increased gas permeability with a simultaneous loss of selectivity [3–5]. Somewhat less studied phenomena that can affect polymer membrane performance include antiplasticization and physical aging. In an antiplasticized membrane, low concentrations of certain penetrants retard polymer chain segmental motion, leading to reduced permeabilities for all penetrants; the impact on selectivity can vary [6,7]. In some cases, increasing antiplasticizer concentration beyond certain levels can result in a conventional plasticization
∗ Corresponding author. Tel.: +1 4043852845. E-mail address: [email protected] (W.J. Koros). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.04.015
response. Several researchers have reported such behavior in their respective work involving a variety of polymers and antiplasticizers [8–10]. Physical aging involves the relaxation of polymer chains in glassy polymers to a more equilibrated state, resulting in the reduction of excess free volume. This matrix densification generally leads to diminished permeability and enhanced selectivity [11–18]. Physical aging can be considered to be a diffusive process involving loss of excess free volume in the polymer matrix. The rate of membrane aging is affected by temperature, membrane thickness, and even the presence of sorbed penetrants [14,16,17]. Since commercial membranes typically utilize thin selective layers—often at elevated temperatures—an understanding of the aging dynamics of a given polymer is critical when considering membrane performance over time. Research has shown that many limitations of pure polymer membranes can be overcome with proper design. Mixed matrix membranes (MMMs) have been studied for quite some time as a solution to the inherent permeability/selectivity tradeoff of pure polymer membranes. Some progress has been made in developing these nanocomposite materials for separation applications [19–23]. However, the majority of researchers’ efforts have been hindered by a variety of interfacial defects and nonidealities, including sieve-in-a-cage, leaky interface, and rigidified matrix defects [24,25]. Furthermore, crosslinkable polymer membranes have been shown to be quite effective against plasticizationinduced performance losses [26–30]. These concepts have been
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Fig. 1. Mixed gas CO2 permeability (a), CO2 /CH4 selectivity (b), and normalized CO2 permeability and CO2 /CH4 selectivity (c) and (d), respectively, for neat PDMC tested prior to exposure to 500 ppm (䊉) and 1000 ppm () toluene-contaminated feeds. Dashed lines represent data taken during actual exposure to toluene-containing feed at the 500 ppm () and 1000 ppm () levels. Data in (c) and (d) are normalized by neat PDMC exposed to 500 ppm (⊕) and 1000 ppm ( ) toluene at respective feed pressures. Permeabilities were calculated using fugacity at corresponding feed pressures.
combined to yield a membrane exhibiting simultaneous permeability and selectivity enhancements over the pure polymer, as well as plasticization resistance at carbon dioxide pressures up to 450 psia [26]. We recently extended this work with a surface modification technique that resulted in the formation of high-performance crosslinked MMMs suitable for carbon dioxide/methane separation [31]. These films are further characterized in this paper using gas mixtures containing low concentrations of toluene to simulate more realistic natural gas feeds. To our knowledge, no prior studies have specifically probed the effects of antiplasticization and physical aging on MMMs for the purpose of gas separations. These phenomena are the main topics of the current investigation. The improvements in transport properties and reductions in antiplasticization and trace contaminant-accelerated physical aging responses reported here and previously [31] suggest that the surface modification technique developed in this work may indeed lead to next-generation gas separation technology.
2. Experimental The crosslinkable polyimide PDMC was used as the continuous matrix in MMMs containing 25% (w/w) as-received (AR) and surface modified (SM) SSZ-13 [26]. Preparation of MMM films generally involved: (1) treating molecular sieve particles with a surface modification procedure; (2) preparation of polymer/sieve casting mixture; and (3) casting the mixture to form a MMM film. Film performance was evaluated using clean (10% carbon dioxide/90% methane) and contaminated (500 or 1000 ppm toluene/10% carbon
dioxide/remainder methane) gas mixtures. Diverse aromatic contaminants can be present in feed gases depending upon the source, and toluene is an useful model compound often used to probe the effects of such contaminants [37]. Films exposed to toluenecontaminated feeds were heated at 75 ◦ C under vacuum for 72 h to remove adsorbed toluene. An isochoric (constant volume) method [32,33] was used to determine transport properties at varying feed pressures and 35 ◦ C. The details of the materials used here and the procedures used to prepare and analyze them are discussed in the preceding work [31]. 3. Results and discussion 3.1. Permeation characterization of neat PDMC films Neat PDMC dense films were first characterized with clean and toluene-contaminated feeds in order to establish a baseline for comparison to MMM results. Prior work showed that hollow fiber membrane transport was severely impaired at toluene levels above 500 ppm [34]. Since the aim of this study was to investigate MMM efficacy in the presence of contaminated feeds, the same concentrations were used here. Films were tested with feed pressures up to 700 psia except for the 1000 ppm feed, since feed gas cylinder pressure allowed testing only up to 500 psia. All tests were run at 35 ◦ C. 3.1.1. Antiplasticization Fig. 1 displays permeation data for films first tested with a clean feed and then exposed to feeds containing toluene. The films,
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Fig. 2. Mixed gas CO2 permeability (a), CO2 /CH4 selectivity (b), and normalized CO2 permeability and CO2 /CH4 selectivity (c and d), respectively, for neat PDMC tested with 500 ppm (䊉) and 1000 ppm () toluene-contaminated feeds. Dashed lines represent data taken after toluene exposure for the 500 ppm () and 1000 ppm () cases. Data in (c) and (d) are normalized by neat PDMC prior to exposure to toluene for films exposed to 500 ppm (⊕) and 1000 ppm ( ) toluene at corresponding feed pressures. Permeabilities were calculated using fugacity at corresponding feed pressures.
though prepared separately, exhibited virtually identical transport properties prior to toluene exposure as evidenced by the similar permeabilities and selectivities given in the figure. This agreement demonstrates the good sample-to-sample reproducibility we find in the absence of true trace contaminant effects. The downward trends in transport with regard to feed pressure, even in the absence of toluene exposure, are primarily due to dual-mode sorption effects [35,36]; however, the decline in permeability and slight selectivity enhancement in the presence of toluene cannot be explained by sorption effects alone. Normalized permeability and selectivity are included in the figure to better illustrate the impact of toluene on separation performance. Data for toluenecontaminated feeds are normalized by neat PDMC clean-feed data at corresponding feed pressures. Clearly the film exposed to 1000 ppm toluene was affected more significantly than the film exposed to 500 ppm, as evidenced by the greater reduction in permeability in the presence of 1000 ppm toluene. This behavior is most likely attributable to a combination of both dual mode sorption competition [3] and antiplasticization effects [34,37,38]. These observations are consistent with those reported elsewhere in which the blending of low concentrations of various low molecular weight additives to glassy polymer membranes resulted in substantial permeability losses and slight increases in selectivity [9]. 3.1.2. Trace contaminant-accelerated physical aging in neat PDMC films After testing with toluene-contaminated feeds, adsorbed toluene was removed according to the method described in Section
2. Toluene was considered fully desorbed when post-desorption outgassing rates during permeation testing were equivalent to those observed during initial testing with uncontaminated feeds (<1% of the steady-state carbon dioxide permeation rate). To ensure that the toluene desorption procedure did not affect separation performance, a PDMC film that had been tested but not exposed to toluene was treated with the desorption procedure and was subsequently tested again. The resulting permeabilities and selectivities for this control film were within ±3% and ±1%, respectively, of the fresh PDMC films. This suggests that the mild heating under vacuum to desorb toluene had little to no effect on PDMC transport properties. The films were then retested with clean gas feeds. Fig. 2 compares transport data for neat PDMC films pre- and post-toluene exposure. The films showed apparent aging at an accelerated rate (versus films exposed to only clean-feeds) as a result of testing with toluene as evidenced by the lower permeabilities and higher selectivities for the post-toluene tested films. The differing reductions in permeability and increases in selectivity for the films exposed to the different contaminant levels suggest the films experienced varying degrees of aging. It is particularly interesting that the 500 ppm exposed sample aged more extensively than did the 1000 ppm exposed sample. These differences cannot be attributed to variations in thermal annealing or other factors since the films were identically processed. While it may seem counter-intuitive for lower toluene feed concentrations (500 ppm vs. 1000 ppm) to result in more extreme physical aging upon removal of the contaminant, the differences between the two cases exceeded the ±3% uncertainty noted for the
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Fig. 3. Mixed gas CO2 permeability (a), CO2 /CH4 selectivity (b), and normalized CO2 permeability and CO2 /CH4 selectivity (c and d), respectively, for neat PDMC pre-toluene (䊉) and during 500 ppm toluene exposure (); AR-SSZ-13/PDMC MMMs pre-toluene () and during 500 ppm toluene exposure (); and SM-SSZ-13/PDMC MMMs pre-toluene () and during 500 ppm toluene exposure (). Data in (c) and (d) represent films during 500 ppm toluene exposure normalized by neat PDMC data prior to toluene exposure for neat PDMC (⊕); AR-SSZ-13/PDMC MMMs ( ); and SM-SSZ-13/PDMC MMMs ( ). Permeabilities were calculated using fugacity at corresponding feed pressures.
control sample, so we believe it to be significant. This trend may be possibly related to behavior reported in other glassy polymer systems [39–42]. In that earlier work it was reported that while previously dilated polymers aged under vacuum showed expected decay in sorption capacity, the same polymers, when aged in the presence of low penetrant activities, maintained higher sorption capacities. It was suggested that low levels of sorbed penetrants can effectively serve as “molecular props”, hindering the permanent loss of free volume. Since the 1000 ppm toluene sample in Fig. 1 showed more antiplasticization than the 500 ppm sample, but less aging in the exposed samples, it appears that toluene may act both as an antiplasticizer and a “molecular prop” in some complex manner. The work by Lee et al. [37,38] for non-crosslinked Matrimid showed other complex effects that may also be related to those seen here. Specifically, it was noted that in the presence of high pressure carbon dioxide and methane mixtures, the combination of carbon dioxide and toluene could act as an antiplasticizer at lower toluene concentrations and as a plasticizer at higher concentrations. A transition from antiplasticization behavior at low contaminant levels to conventional plasticization behavior at higher levels was seen in the more flexible uncrosslinked Matrimid sample, with reductions in carbon dioxide/methane selectivities at the higher toluene feed concentrations. For the more rigid PDMC samples studied here, the permeability suppression and selectivity increases in Fig. 1 in the presence of both 500 and 1000 ppm toluene are consistent with antiplasticization alone, and an apparent reduced ability to age in the more dilated 1000 ppm exposed sample. The extreme reductions in carbon dioxide permeability and actual increase in carbon dioxide/methane selectivity in Fig. 2
after the removal of the antiplasticizing toluene agent is noteworthy and very different from the response after exposure of the uncrosslinked Matrimid exposed to only 544 ppm of toluene. In the case of Matrimid, after this exposure and retesting, the permeance of the fibers increased significantly and the selectivity was reduced by over 20% [37]. In that study, the samples were tested by simply changing to a toluene-free feed, rather than actually evacuating and heating to remove the sorbed toluene, so the two experiments are not identical. Nevertheless, in the current case, it is clear that physical aging and densification accompanies the toluene removal and mild thermal heating used to ensure complete removal of the contaminant. 3.2. Contaminant-induced antiplasticization in SSZ-13/PDMC mixed matrix membranes As discussed earlier in this paper, antiplasticization can have a deleterious impact on separation performance. It was proposed at the onset of this work that the presence of molecular sieves in MMMs may reduce or even eliminate the effective degree of antiplasticization in these films. This suggestion was based on the generally accepted notion that antiplasticization results from a loss in free volume and diminished penetrant mobility as sorbed molecule concentration increases within a polymer matrix up to a critical level, at which point swelling-induced plasticization becomes dominant. Simply put, since the sorbent molecules cannot access the pore network of a properly chosen molecular sieve, a smaller fraction of total membrane free volume should be accessible to a contaminant sorbent in a MMM than in a pure polymer
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Fig. 4. Mixed gas CO2 permeability (a), CO2 /CH4 selectivity (b), and normalized CO2 permeability and CO2 /CH4 selectivity (c and d), respectively, for neat PDMC pre- (䊉) and post-500 ppm toluene exposure (); AR-SSZ-13/PDMC MMMs pre- () and post-500 ppm toluene exposure (); and SM-SSZ-13/PDMC MMMs pre- () and post-500 ppm toluene exposure (). Data in (c) and (d) represent films after 500 ppm toluene exposure normalized by neat PDMC data prior to toluene exposure for neat PDMC (⊕); AR-SSZ-13/PDMC MMMs ( ); and SM-SSZ-13/PDMC MMMs ( ). Permeabilities were calculated using fugacity at corresponding feed pressures.
membrane. In this case, permeability loss should be controlled in the MMM sample. This argument suggests that membranes con˚ should exhibit a less taining SSZ-13 (effective pore diameter 3.8 A) significant apparent antiplasticization response than pure polymer membranes when testing with toluene (kinetic diameter 5.92 A˚ [43]) even if the matrix of the polymer is affected by the contaminant since toluene should not be able to sorb into and clog the internal sieve passages that are accessible to carbon dioxide. To probe this possibility, neat PDMC films and 25% (w/w) SSZ13/PDMC MMMs containing either AR or SM sieves were prepared. These films were first tested with 10% carbon dioxide in methane and then tested with 500 ppm toluene and 10% carbon dioxide in methane. The results of these analyses are compared in Fig. 3. As mentioned previously, the general trend downward in permeability and selectivity is partially due to sorption competition. Again, antiplasticization contributes to this trend and is evidenced by the additional drop in permeability and slight increase in selectivity observed when testing with toluene-contaminated feed. The normalized permeability data in Fig. 3 compare properties before and during 500 ppm exposure in neat PDMC, AR-SSZ-13/PDMC MMMs, and SM-SSZ-13/PDMC MMMs relative to the neat PDMC before exposure to 500 ppm toluene. The MMMs with AR-SSZ-13 before 500 ppm toluene exposure show the largest relative increase in permeability; however, in the presence of 500 ppm toluene, the AR-SSZ-13/PDMC and SM-SSZ-13/PDMC show very similar permeability increases relative to the neat PDMC prior to exposure to the toluene contaminant. Moreover, mixed matrix samples in the presence of toluene show increases ranging from 1.5 to 1.3× higher than the neat PDMC prior to toluene exposure. Since the neat PDMC
sample in the presence of the 500 ppm toluene shows permeability of only 80% of the sample without contaminants, the benefit of MMMs in both AR and SM forms is quite significant. The absolute selectivities given in Fig. 3 suggest substantially better performance for SM-SSZ-13 MMMs. These films exhibited roughly 12% and 14% higher selectivities versus neat PDMC and ARSSZ-13 MMMs, respectively, in the presence of 500 ppm toluene at 700 psia. Moreover, SM-SSZ-13 MMMs exhibited an absolute carbon dioxide permeability of 74 barrers at the same conditions—a ca. 43% enhancement over neat PDMC at the most demanding 700 psia feed pressure condition with 500 ppm toluene. While this permeability is around 6% lower than that for AR-SSZ-13 MMMs, overall separation performance is better with a selectivity of 49 versus 43. The separation performance discussed here is consistent with previous observations for these materials [31]. 3.3. Contaminant-induced physical aging in SSZ-13/PDMC mixed matrix membranes Physical aging in polymer membranes leads to historydependent transport properties. To gain a better understanding of the potential impact of MMMs on aging—versus neat polymer membranes—the films analyzed in the previous section were again tested with clean mixed gas feeds after the sorbed toluene had been removed. This analysis allowed the immediate comparison of the rates of accelerated aging of the films tested with toluenecontaminated feeds. The results of this analysis are given in Fig. 4. It should first be noted that all films tested experienced some degree of accelerated physical aging relative to their pre-toluene
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of toluene—in the 500–1000 ppm range—in a carbon dioxide/methane mixture results in a more significant antiplasticization response and less physical aging in neat PDMC films. Subsequent characterization of MMMs with a feed mixture containing 500 ppm toluene suggests that MMMs composed of surface modified SSZ-13 are effectively less prone to the effects of antiplasticization than the neat PDMC sample. In the presence of 500 ppm toluene, the surface modified SSZ-13/PDMC sample showed the best combination of properties with high permeability and selectivity versus the neat PDMC sample and a similar permeability but higher selectivity versus the non-sized sample. Long-term permeation testing is warranted to clarify the true potential of MMMs with surface modified SSZ-13 to mitigate physical aging. Acknowledgements
Fig. 5. Mixed gas CO2 permeability (a) and CO2 /CH4 selectivity (b) from Fig. 4 normalized relative to respective values for each sample prior to exposure to 500 ppm toluene for neat PDMC ( ), AR-SSZ-13/PDMC MMMs ( ), and SM-SSZ-13/PDMC MMMs ( ) to show specific aging differences. Permeabilities were calculated using fugacity at corresponding feed pressures.
The authors would like to acknowledge funding for this work from ChevronTexaco Energy Technology Company, the National Science Foundation STC-CERSP (CHE-9876674), and King Abdullah University of Science and Technology (KAUST award no. KUS-I1011-21). References
exposure, as evidenced by the reductions in permeability and increases in selectivity for post-toluene versus pre-toluene testing. In all cases, however, the MMM samples both in AR and SM forms still show major permeability improvements over neat PDMC. The neat PDMC showed the greatest extent of aging, yielding a 30% decline in carbon dioxide permeability with a simultaneous 7% increase in selectivity over methane at 700 psia. Fig. 5 further considers the differences caused by the toluene exposure from a different perspective. In this case, the changes are represented relative to the respective performance in each case prior to exposure to the toluene contaminant. While the neat sample response is the same as in Fig. 4, the AR and SM samples are clearly different from this perspective. The AR-SSZ-13 and SM-SSZ-13 MMMs exhibit declines of 10% and 5% in permeability compared to the 30% loss for the neat PDMC at the same pressure. Percentage increases in selectivity for the AR and SM MMMs are similar but less than that observed for neat PDMC, which showed the most significant aging after removal of the toluene. Consistent with the situation during actual exposure to 500 ppm toluene, overall SM-SSZ-13 MMM post-toluene performance is superior to that of neat PDMC and AR-SSZ-13 MMMs. Indeed, the SM-SSZ-13 MMM combines attractive absolute permeabilities and selectivities in the presence of the antiplasticizing agent and relatively small percentage changes in permeabilities and selectivities after the aging process is complete following removal of the contaminant. As before, these differences cannot be attributed to variations in thermal annealing or other factors since the films were identically processed. The results presented here, therefore, suggest that MMMs may be able to reduce the extent of physical aging versus pure polymer membranes. Nevertheless, aging dynamics may not be entirely halted in these membranes. Instead, the rate of aging in these MMMs may be only retarded such that the effects of physical aging have been effectively prevented on the time scales investigated. Long-term testing of these materials—while outside the scope of the present work—would be greatly beneficial in probing the true nature of MMM resistance to membrane aging. 4. Conclusions A companion study of SSZ-13/PDMC MMMs was extended to probe the effects of toluene-contaminated feeds on separation performance. It was first shown that a higher concentration
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Crosslinkable mixed matrix membranes with surface modified molecular sieves for natural gas purification: II. Performance characterization under contaminated feed conditions Jason K. Ward, William J. Koros ∗ Georgia Institute of Technology, School of Chemical & Biomolecular Engineering, Atlanta, GA 30332, USA
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Article history: Received 25 October 2010 Received in revised form 5 April 2011 Accepted 7 April 2011 Available online 14 April 2011 Keywords: Antiplasticization Physical aging Gas separation Mixed matrix Natural gas
a b s t r a c t Mixed matrix membranes (MMMs) composed of the crosslinkable polyimide PDMC and surface modified (SM) SSZ-13 have recently been shown to enhance carbon dioxide permeability and carbon dioxide/methane selectivity versus neat PDMC films by as much as 47% and 13%, respectively (Part I). The previous film characterization, however, was performed using ideal, clean mixed gas feeds. In this paper, PDMC/SSZ-13 MMMs are further characterized using more realistic mixed gases containing low concentrations (500 or 1000 ppm) of toluene as a model contaminant. Mixed matrix membranes are shown to outperform pure PDMC films in the presence of toluene with 43% greater carbon dioxide permeability and 12% greater carbon dioxide/selectivity at 35 ◦ C and 700 psia feed pressure. These results suggest that MMMs—in addition to exhibiting enhanced transport properties—may mitigate performance degradation due to antiplasticization effects. Moreover, the analyses presented here show that the reduction in separation performance by trace contaminant-accelerated physical aging can be suppressed greatly with MMMs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Widespread commercial acceptance of polymer membranes for gas separations has been hindered by a number of performancelimiting phenomena. Perhaps the most well-known limitation of polymeric membranes is the intrinsic permeability/selectivity tradeoff noted by Robeson [1,2]. This hurdle has prevented the development of the highly productive and selective materials sought for cost-effective gas separations. Another common phenomenon in polymer membranes that often leads to drastically reduced separation performance is termed “plasticization”. Frequently encountered in natural gas purification and other separations involving strongly interacting sorbents, the swollen matrix of a plasticized membrane leads to increased gas permeability with a simultaneous loss of selectivity [3–5]. Somewhat less studied phenomena that can affect polymer membrane performance include antiplasticization and physical aging. In an antiplasticized membrane, low concentrations of certain penetrants retard polymer chain segmental motion, leading to reduced permeabilities for all penetrants; the impact on selectivity can vary [6,7]. In some cases, increasing antiplasticizer concentration beyond certain levels can result in a conventional plasticization
∗ Corresponding author. Tel.: +1 4043852845. E-mail address: [email protected] (W.J. Koros). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.04.015
response. Several researchers have reported such behavior in their respective work involving a variety of polymers and antiplasticizers [8–10]. Physical aging involves the relaxation of polymer chains in glassy polymers to a more equilibrated state, resulting in the reduction of excess free volume. This matrix densification generally leads to diminished permeability and enhanced selectivity [11–18]. Physical aging can be considered to be a diffusive process involving loss of excess free volume in the polymer matrix. The rate of membrane aging is affected by temperature, membrane thickness, and even the presence of sorbed penetrants [14,16,17]. Since commercial membranes typically utilize thin selective layers—often at elevated temperatures—an understanding of the aging dynamics of a given polymer is critical when considering membrane performance over time. Research has shown that many limitations of pure polymer membranes can be overcome with proper design. Mixed matrix membranes (MMMs) have been studied for quite some time as a solution to the inherent permeability/selectivity tradeoff of pure polymer membranes. Some progress has been made in developing these nanocomposite materials for separation applications [19–23]. However, the majority of researchers’ efforts have been hindered by a variety of interfacial defects and nonidealities, including sieve-in-a-cage, leaky interface, and rigidified matrix defects [24,25]. Furthermore, crosslinkable polymer membranes have been shown to be quite effective against plasticizationinduced performance losses [26–30]. These concepts have been
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Fig. 1. Mixed gas CO2 permeability (a), CO2 /CH4 selectivity (b), and normalized CO2 permeability and CO2 /CH4 selectivity (c) and (d), respectively, for neat PDMC tested prior to exposure to 500 ppm (䊉) and 1000 ppm () toluene-contaminated feeds. Dashed lines represent data taken during actual exposure to toluene-containing feed at the 500 ppm () and 1000 ppm () levels. Data in (c) and (d) are normalized by neat PDMC exposed to 500 ppm (⊕) and 1000 ppm ( ) toluene at respective feed pressures. Permeabilities were calculated using fugacity at corresponding feed pressures.
combined to yield a membrane exhibiting simultaneous permeability and selectivity enhancements over the pure polymer, as well as plasticization resistance at carbon dioxide pressures up to 450 psia [26]. We recently extended this work with a surface modification technique that resulted in the formation of high-performance crosslinked MMMs suitable for carbon dioxide/methane separation [31]. These films are further characterized in this paper using gas mixtures containing low concentrations of toluene to simulate more realistic natural gas feeds. To our knowledge, no prior studies have specifically probed the effects of antiplasticization and physical aging on MMMs for the purpose of gas separations. These phenomena are the main topics of the current investigation. The improvements in transport properties and reductions in antiplasticization and trace contaminant-accelerated physical aging responses reported here and previously [31] suggest that the surface modification technique developed in this work may indeed lead to next-generation gas separation technology.
2. Experimental The crosslinkable polyimide PDMC was used as the continuous matrix in MMMs containing 25% (w/w) as-received (AR) and surface modified (SM) SSZ-13 [26]. Preparation of MMM films generally involved: (1) treating molecular sieve particles with a surface modification procedure; (2) preparation of polymer/sieve casting mixture; and (3) casting the mixture to form a MMM film. Film performance was evaluated using clean (10% carbon dioxide/90% methane) and contaminated (500 or 1000 ppm toluene/10% carbon
dioxide/remainder methane) gas mixtures. Diverse aromatic contaminants can be present in feed gases depending upon the source, and toluene is an useful model compound often used to probe the effects of such contaminants [37]. Films exposed to toluenecontaminated feeds were heated at 75 ◦ C under vacuum for 72 h to remove adsorbed toluene. An isochoric (constant volume) method [32,33] was used to determine transport properties at varying feed pressures and 35 ◦ C. The details of the materials used here and the procedures used to prepare and analyze them are discussed in the preceding work [31]. 3. Results and discussion 3.1. Permeation characterization of neat PDMC films Neat PDMC dense films were first characterized with clean and toluene-contaminated feeds in order to establish a baseline for comparison to MMM results. Prior work showed that hollow fiber membrane transport was severely impaired at toluene levels above 500 ppm [34]. Since the aim of this study was to investigate MMM efficacy in the presence of contaminated feeds, the same concentrations were used here. Films were tested with feed pressures up to 700 psia except for the 1000 ppm feed, since feed gas cylinder pressure allowed testing only up to 500 psia. All tests were run at 35 ◦ C. 3.1.1. Antiplasticization Fig. 1 displays permeation data for films first tested with a clean feed and then exposed to feeds containing toluene. The films,
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Fig. 2. Mixed gas CO2 permeability (a), CO2 /CH4 selectivity (b), and normalized CO2 permeability and CO2 /CH4 selectivity (c and d), respectively, for neat PDMC tested with 500 ppm (䊉) and 1000 ppm () toluene-contaminated feeds. Dashed lines represent data taken after toluene exposure for the 500 ppm () and 1000 ppm () cases. Data in (c) and (d) are normalized by neat PDMC prior to exposure to toluene for films exposed to 500 ppm (⊕) and 1000 ppm ( ) toluene at corresponding feed pressures. Permeabilities were calculated using fugacity at corresponding feed pressures.
though prepared separately, exhibited virtually identical transport properties prior to toluene exposure as evidenced by the similar permeabilities and selectivities given in the figure. This agreement demonstrates the good sample-to-sample reproducibility we find in the absence of true trace contaminant effects. The downward trends in transport with regard to feed pressure, even in the absence of toluene exposure, are primarily due to dual-mode sorption effects [35,36]; however, the decline in permeability and slight selectivity enhancement in the presence of toluene cannot be explained by sorption effects alone. Normalized permeability and selectivity are included in the figure to better illustrate the impact of toluene on separation performance. Data for toluenecontaminated feeds are normalized by neat PDMC clean-feed data at corresponding feed pressures. Clearly the film exposed to 1000 ppm toluene was affected more significantly than the film exposed to 500 ppm, as evidenced by the greater reduction in permeability in the presence of 1000 ppm toluene. This behavior is most likely attributable to a combination of both dual mode sorption competition [3] and antiplasticization effects [34,37,38]. These observations are consistent with those reported elsewhere in which the blending of low concentrations of various low molecular weight additives to glassy polymer membranes resulted in substantial permeability losses and slight increases in selectivity [9]. 3.1.2. Trace contaminant-accelerated physical aging in neat PDMC films After testing with toluene-contaminated feeds, adsorbed toluene was removed according to the method described in Section
2. Toluene was considered fully desorbed when post-desorption outgassing rates during permeation testing were equivalent to those observed during initial testing with uncontaminated feeds (<1% of the steady-state carbon dioxide permeation rate). To ensure that the toluene desorption procedure did not affect separation performance, a PDMC film that had been tested but not exposed to toluene was treated with the desorption procedure and was subsequently tested again. The resulting permeabilities and selectivities for this control film were within ±3% and ±1%, respectively, of the fresh PDMC films. This suggests that the mild heating under vacuum to desorb toluene had little to no effect on PDMC transport properties. The films were then retested with clean gas feeds. Fig. 2 compares transport data for neat PDMC films pre- and post-toluene exposure. The films showed apparent aging at an accelerated rate (versus films exposed to only clean-feeds) as a result of testing with toluene as evidenced by the lower permeabilities and higher selectivities for the post-toluene tested films. The differing reductions in permeability and increases in selectivity for the films exposed to the different contaminant levels suggest the films experienced varying degrees of aging. It is particularly interesting that the 500 ppm exposed sample aged more extensively than did the 1000 ppm exposed sample. These differences cannot be attributed to variations in thermal annealing or other factors since the films were identically processed. While it may seem counter-intuitive for lower toluene feed concentrations (500 ppm vs. 1000 ppm) to result in more extreme physical aging upon removal of the contaminant, the differences between the two cases exceeded the ±3% uncertainty noted for the
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Fig. 3. Mixed gas CO2 permeability (a), CO2 /CH4 selectivity (b), and normalized CO2 permeability and CO2 /CH4 selectivity (c and d), respectively, for neat PDMC pre-toluene (䊉) and during 500 ppm toluene exposure (); AR-SSZ-13/PDMC MMMs pre-toluene () and during 500 ppm toluene exposure (); and SM-SSZ-13/PDMC MMMs pre-toluene () and during 500 ppm toluene exposure (). Data in (c) and (d) represent films during 500 ppm toluene exposure normalized by neat PDMC data prior to toluene exposure for neat PDMC (⊕); AR-SSZ-13/PDMC MMMs ( ); and SM-SSZ-13/PDMC MMMs ( ). Permeabilities were calculated using fugacity at corresponding feed pressures.
control sample, so we believe it to be significant. This trend may be possibly related to behavior reported in other glassy polymer systems [39–42]. In that earlier work it was reported that while previously dilated polymers aged under vacuum showed expected decay in sorption capacity, the same polymers, when aged in the presence of low penetrant activities, maintained higher sorption capacities. It was suggested that low levels of sorbed penetrants can effectively serve as “molecular props”, hindering the permanent loss of free volume. Since the 1000 ppm toluene sample in Fig. 1 showed more antiplasticization than the 500 ppm sample, but less aging in the exposed samples, it appears that toluene may act both as an antiplasticizer and a “molecular prop” in some complex manner. The work by Lee et al. [37,38] for non-crosslinked Matrimid showed other complex effects that may also be related to those seen here. Specifically, it was noted that in the presence of high pressure carbon dioxide and methane mixtures, the combination of carbon dioxide and toluene could act as an antiplasticizer at lower toluene concentrations and as a plasticizer at higher concentrations. A transition from antiplasticization behavior at low contaminant levels to conventional plasticization behavior at higher levels was seen in the more flexible uncrosslinked Matrimid sample, with reductions in carbon dioxide/methane selectivities at the higher toluene feed concentrations. For the more rigid PDMC samples studied here, the permeability suppression and selectivity increases in Fig. 1 in the presence of both 500 and 1000 ppm toluene are consistent with antiplasticization alone, and an apparent reduced ability to age in the more dilated 1000 ppm exposed sample. The extreme reductions in carbon dioxide permeability and actual increase in carbon dioxide/methane selectivity in Fig. 2
after the removal of the antiplasticizing toluene agent is noteworthy and very different from the response after exposure of the uncrosslinked Matrimid exposed to only 544 ppm of toluene. In the case of Matrimid, after this exposure and retesting, the permeance of the fibers increased significantly and the selectivity was reduced by over 20% [37]. In that study, the samples were tested by simply changing to a toluene-free feed, rather than actually evacuating and heating to remove the sorbed toluene, so the two experiments are not identical. Nevertheless, in the current case, it is clear that physical aging and densification accompanies the toluene removal and mild thermal heating used to ensure complete removal of the contaminant. 3.2. Contaminant-induced antiplasticization in SSZ-13/PDMC mixed matrix membranes As discussed earlier in this paper, antiplasticization can have a deleterious impact on separation performance. It was proposed at the onset of this work that the presence of molecular sieves in MMMs may reduce or even eliminate the effective degree of antiplasticization in these films. This suggestion was based on the generally accepted notion that antiplasticization results from a loss in free volume and diminished penetrant mobility as sorbed molecule concentration increases within a polymer matrix up to a critical level, at which point swelling-induced plasticization becomes dominant. Simply put, since the sorbent molecules cannot access the pore network of a properly chosen molecular sieve, a smaller fraction of total membrane free volume should be accessible to a contaminant sorbent in a MMM than in a pure polymer
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Fig. 4. Mixed gas CO2 permeability (a), CO2 /CH4 selectivity (b), and normalized CO2 permeability and CO2 /CH4 selectivity (c and d), respectively, for neat PDMC pre- (䊉) and post-500 ppm toluene exposure (); AR-SSZ-13/PDMC MMMs pre- () and post-500 ppm toluene exposure (); and SM-SSZ-13/PDMC MMMs pre- () and post-500 ppm toluene exposure (). Data in (c) and (d) represent films after 500 ppm toluene exposure normalized by neat PDMC data prior to toluene exposure for neat PDMC (⊕); AR-SSZ-13/PDMC MMMs ( ); and SM-SSZ-13/PDMC MMMs ( ). Permeabilities were calculated using fugacity at corresponding feed pressures.
membrane. In this case, permeability loss should be controlled in the MMM sample. This argument suggests that membranes con˚ should exhibit a less taining SSZ-13 (effective pore diameter 3.8 A) significant apparent antiplasticization response than pure polymer membranes when testing with toluene (kinetic diameter 5.92 A˚ [43]) even if the matrix of the polymer is affected by the contaminant since toluene should not be able to sorb into and clog the internal sieve passages that are accessible to carbon dioxide. To probe this possibility, neat PDMC films and 25% (w/w) SSZ13/PDMC MMMs containing either AR or SM sieves were prepared. These films were first tested with 10% carbon dioxide in methane and then tested with 500 ppm toluene and 10% carbon dioxide in methane. The results of these analyses are compared in Fig. 3. As mentioned previously, the general trend downward in permeability and selectivity is partially due to sorption competition. Again, antiplasticization contributes to this trend and is evidenced by the additional drop in permeability and slight increase in selectivity observed when testing with toluene-contaminated feed. The normalized permeability data in Fig. 3 compare properties before and during 500 ppm exposure in neat PDMC, AR-SSZ-13/PDMC MMMs, and SM-SSZ-13/PDMC MMMs relative to the neat PDMC before exposure to 500 ppm toluene. The MMMs with AR-SSZ-13 before 500 ppm toluene exposure show the largest relative increase in permeability; however, in the presence of 500 ppm toluene, the AR-SSZ-13/PDMC and SM-SSZ-13/PDMC show very similar permeability increases relative to the neat PDMC prior to exposure to the toluene contaminant. Moreover, mixed matrix samples in the presence of toluene show increases ranging from 1.5 to 1.3× higher than the neat PDMC prior to toluene exposure. Since the neat PDMC
sample in the presence of the 500 ppm toluene shows permeability of only 80% of the sample without contaminants, the benefit of MMMs in both AR and SM forms is quite significant. The absolute selectivities given in Fig. 3 suggest substantially better performance for SM-SSZ-13 MMMs. These films exhibited roughly 12% and 14% higher selectivities versus neat PDMC and ARSSZ-13 MMMs, respectively, in the presence of 500 ppm toluene at 700 psia. Moreover, SM-SSZ-13 MMMs exhibited an absolute carbon dioxide permeability of 74 barrers at the same conditions—a ca. 43% enhancement over neat PDMC at the most demanding 700 psia feed pressure condition with 500 ppm toluene. While this permeability is around 6% lower than that for AR-SSZ-13 MMMs, overall separation performance is better with a selectivity of 49 versus 43. The separation performance discussed here is consistent with previous observations for these materials [31]. 3.3. Contaminant-induced physical aging in SSZ-13/PDMC mixed matrix membranes Physical aging in polymer membranes leads to historydependent transport properties. To gain a better understanding of the potential impact of MMMs on aging—versus neat polymer membranes—the films analyzed in the previous section were again tested with clean mixed gas feeds after the sorbed toluene had been removed. This analysis allowed the immediate comparison of the rates of accelerated aging of the films tested with toluenecontaminated feeds. The results of this analysis are given in Fig. 4. It should first be noted that all films tested experienced some degree of accelerated physical aging relative to their pre-toluene
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of toluene—in the 500–1000 ppm range—in a carbon dioxide/methane mixture results in a more significant antiplasticization response and less physical aging in neat PDMC films. Subsequent characterization of MMMs with a feed mixture containing 500 ppm toluene suggests that MMMs composed of surface modified SSZ-13 are effectively less prone to the effects of antiplasticization than the neat PDMC sample. In the presence of 500 ppm toluene, the surface modified SSZ-13/PDMC sample showed the best combination of properties with high permeability and selectivity versus the neat PDMC sample and a similar permeability but higher selectivity versus the non-sized sample. Long-term permeation testing is warranted to clarify the true potential of MMMs with surface modified SSZ-13 to mitigate physical aging. Acknowledgements
Fig. 5. Mixed gas CO2 permeability (a) and CO2 /CH4 selectivity (b) from Fig. 4 normalized relative to respective values for each sample prior to exposure to 500 ppm toluene for neat PDMC ( ), AR-SSZ-13/PDMC MMMs ( ), and SM-SSZ-13/PDMC MMMs ( ) to show specific aging differences. Permeabilities were calculated using fugacity at corresponding feed pressures.
The authors would like to acknowledge funding for this work from ChevronTexaco Energy Technology Company, the National Science Foundation STC-CERSP (CHE-9876674), and King Abdullah University of Science and Technology (KAUST award no. KUS-I1011-21). References
exposure, as evidenced by the reductions in permeability and increases in selectivity for post-toluene versus pre-toluene testing. In all cases, however, the MMM samples both in AR and SM forms still show major permeability improvements over neat PDMC. The neat PDMC showed the greatest extent of aging, yielding a 30% decline in carbon dioxide permeability with a simultaneous 7% increase in selectivity over methane at 700 psia. Fig. 5 further considers the differences caused by the toluene exposure from a different perspective. In this case, the changes are represented relative to the respective performance in each case prior to exposure to the toluene contaminant. While the neat sample response is the same as in Fig. 4, the AR and SM samples are clearly different from this perspective. The AR-SSZ-13 and SM-SSZ-13 MMMs exhibit declines of 10% and 5% in permeability compared to the 30% loss for the neat PDMC at the same pressure. Percentage increases in selectivity for the AR and SM MMMs are similar but less than that observed for neat PDMC, which showed the most significant aging after removal of the toluene. Consistent with the situation during actual exposure to 500 ppm toluene, overall SM-SSZ-13 MMM post-toluene performance is superior to that of neat PDMC and AR-SSZ-13 MMMs. Indeed, the SM-SSZ-13 MMM combines attractive absolute permeabilities and selectivities in the presence of the antiplasticizing agent and relatively small percentage changes in permeabilities and selectivities after the aging process is complete following removal of the contaminant. As before, these differences cannot be attributed to variations in thermal annealing or other factors since the films were identically processed. The results presented here, therefore, suggest that MMMs may be able to reduce the extent of physical aging versus pure polymer membranes. Nevertheless, aging dynamics may not be entirely halted in these membranes. Instead, the rate of aging in these MMMs may be only retarded such that the effects of physical aging have been effectively prevented on the time scales investigated. Long-term testing of these materials—while outside the scope of the present work—would be greatly beneficial in probing the true nature of MMM resistance to membrane aging. 4. Conclusions A companion study of SSZ-13/PDMC MMMs was extended to probe the effects of toluene-contaminated feeds on separation performance. It was first shown that a higher concentration
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