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PI blends: Temperature effect on gas transport and separation performance
Journal of Membrane Science 597 (2020) 117703
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Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
CMS membranes from PBI/PI blends: Temperature effect on gas transport and separation performance Jos�e Manuel P�erez-Francisco a, b, Jos�e Luis Santiago-García a, María Isabel Loría-Bastarrachea a, Donald R. Paul c, Benny D. Freeman c, Manuel Aguilar-Vega a, * a
Unidad de Materiales, Centro de Investigaci� on Científica de Yucat� an, A.C., Calle 43, No. 130, C.P, 97205, M�erida, Yucat� an, Mexico Tecnol� ogico Nacional de M�exico/Instituto Tecnol� ogico Superior de Coatzacoalcos, Carretera Antigua Minatitl� an-Coatzacoalcos Km. 16.5, C.P., 96536, Coatzacoalcos, Veracruz, Mexico c McKetta Department of Chemical Engineering, University of Texas at Austin, 200 E St Stop C0400 Dean Keaton St, Austin, TX, 78712, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: CMS membrane Blend precursors Gas separation Entropic selectivity Graphenic stuctures
This study reports pure gas permeability and diffusion coefficients for carbon molecular sieve membranes (CMSM) derived from dense membranes based on blends of a rigid polyimide PI DPPD-IMM (PI) and poly benzimidazole (PBI), PI/PBI, at different concentrations. The permeability, diffusion and selectivity for PI/PBI blend CMS membranes was systematically tested as a function of concentration and temperature. CMS membrane derived from pure PI DPPD-IMM membrane (PI100-600) exhibited the highest permeability coefficients (PHe ¼ 960 Barrer, PCO2 ¼ 503 Barrer) and the highest separation factors (αO2/N2 ¼ 8.3 and αCO2/CH4 ¼ 56.5). Increasing PI concentration in the membranes blend precursors shows a positive effect on CMS membranes permeability, diffusion coefficients and selectivity. These results are attributed to an increase in micropores dimension that was reflected by the emergence of a d-spacing shift from 5.9 Å to 7.1 Å. It was also found that large concentrations of PBI (>50 wt%) in the precursor showed a negative effect on permeability and also in gas pair selectivity for carbon membranes which was attributed to carbazole-type strands densification of the structure. As PI con centration increases in PI/PBI CMS membranes, they present higher entropic selectivity and low energetic selectivity leading to the best permeability/selectivity relationship surpassing 2008 Robeson’s upper bound for O2/N2 and CO2/CH4 gas pairs.
1. Introduction Membrane technology is one of the processes in the gas separation industry that is growing fast and competes with cryogenic distillation and absorption processes [1,2]. Polymeric membranes are making in roads in gas separation technology because of their ease of fabrication, wide materials availability and some of them present a good per meability/selectivity relationship for some industrially interesting gas pairs [3–5]. For example, natural gas consumption has been growing in the last three decades and it is expected that for the year 2040, 30% of world’s primary energy will be obtained from natural gas [6]. Carbon dioxide is one of the most common natural gas impurities, with almost 20% of U.S. natural gas production with CO2 concentrations above 2 vol %, which is the allowable U.S. pipeline specification [7] to eliminate a potential cause of pipeline corrosion [8]. Therefore, CO2/CH4 separa tion is an important topic in the natural gas industry [9,10].
Nevertheless, gas separation is limited by an intrinsic trade-off between productivity and separation capacity because as the gas permeability increases, the separation factor decreases. This fact has been docu mented by Robeson and corroborated by others in the so called upper-bound relationship between permeability and gas separation factor [11]. On the other hand, carbon molecular sieve (CMS) membranes are promising materials for gas separation processes. They are formed by the pyrolysis of polymeric precursor membranes under inert atmosphere or controlled vacuum [12]. In recent years research has focused on these carbon membranes due to their excellent permeability and selectivity relationship that usually gives properties lying above the upper-bound limit of polymeric membranes [2,13]. The excellent relationship be tween permeability and selectivity is the result of chain packing im perfections during pyrolysis that lead to a nonhomogeneous carbon pore structure consisting of relatively wide open pores (7–20 Å) coexisting
* Corresponding author. E-mail address: [email protected] (M. Aguilar-Vega). https://doi.org/10.1016/j.memsci.2019.117703 Received 23 August 2019; Received in revised form 15 November 2019; Accepted 28 November 2019 Available online 30 November 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
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Journal of Membrane Science 597 (2020) 117703
with small selective pores (3–6 Å) [14,15]. The pore dimensions and their microstructural distribution depend on several parameters. Among them, the most important ones are precursor material selection, inert atmosphere, and pyrolysis conditions [16–18]. Several reports in the literature focus in the relationship between precursor materials and CMS membranes final properties [1,2,19]. Aromatic polyimides have been used as a preferred precursor for CMS membranes due to their high chemical and thermal stability, high carbon yield and outstanding in crease in gas separation properties [19–22]. Blending different polymers in order to obtained CMS membrane precursors that offer new features may show advantages for modulation of CMS membranes gas separation properties [23,24]. Several studies on CMS membranes derived from polymer blends have been reported in recent years, showing that CMSM derived from blends, in some cases show superior gas transport and separation properties than CMS membranes derived from their indi vidual components [23,25–28]. Hosseini et al. [23,25] reported CMSM derived from blends of polymers with similar gas permeability as polymeric membranes. In the first work, they elaborated CMS mem branes from Matrimid, Torlon, P84 and PBI. Their results showed that Matrimid blended with PBI was the best option to fabricate CMSM as the result of the gas pair selectivity increment (CO2/CH4 ¼ 203.9, H2/CO2 ¼ 33.44, N2/CH4 ¼ 7.99 and PN2 ¼ 2.78 Barrer). In the second work, they fabricated CMS membranes from Kapton, P84 HT, UIP-R and PBI under vacuum. They reported that CMSM obtained from PBI/Kapton blends at 10 7 Torr and 800 � C have good CO2/CH4 separation factors, offering enhanced selectivity in the range of up to 200. A number of researchers have studied the effect of operating tem perature on gas permeability and selectivity in CMS derived from polymeric membranes [29–32]. However, the temperature dependence of gas permeability coefficients and selectivity for CMS membranes derived from blend membranes is an issue where a more systematic investigation can contribute to understand the factors that lead to an increase in gas separation behavior [33,34]. Ning and Koros [35] reported gas transport properties and temper ature dependence of a CMS membrane derived from Matrimid, finding that gas permeability and diffusion coefficients follow an Arrhenius type relationship, while the selectivity decreases as temperature increases. In another work, Fu et al. [14] study the temperature dependence of CMS membranes derived from four novel 6FDA based polyimides. In both reports, the analysis of activation energies for permeation, diffusion and sorption gave information about permeability and selectivity changes with operating temperature. Additionally, they factored the diffusion selectivity into energetic and entropic selectivity in order to elucidate the reason why CMS membranes outperform the precursor polymeric membranes. Typically, CMS membranes possess high entropic selec tivity, which is related to rigid CMS pore structures and differences in penetrant shape [34,35]. Recently, Fu et al. [33] explained the effect of pyrolysis temperature on entropic selectivity of CMS membranes derived from a 6FDA based polyimide. Their results indicated that entropic selectivity increases as the pyrolysis temperature increases. They attributed the increase in gas selectivity based on size and shape discrimination ability showing that determining the changes in the entropic factor may be an important tool to understand carbon mem brane performance. Since the effect of temperature on the mechanism of improved gas permeability and selectivity in CMS membranes from polymer blends has not been thoroughly assessed to the best of our knowledge. In this study we report the gas transport and separation properties of CMS membranes from polymer blend precursors with a systematic increase in concentration of a closely packed low permeability polymer, Poly[2,2’(1,3-phenylene)-5,50 -bibenzimidazole], PBI, blended with an open structure, high permeability aromatic rigid polyimide Polyimide[3,8diphenylpyrene-1,2,6,7-tetracarboxylic dianhydride and 4,40 -methyl enebis(2-isopropyl-6-methylaniline)], PI. We also tested the effect of increasing temperature on pure gas permeability and diffusion co efficients, between 35 and 65 � C, on CMS membranes derived from PI/
PBI blends. The results are analyzed in terms of structural polymer dif ferences and blend concentration as well as structural changes upon carbonization. 2. Experimental 2.1. Materials Polyimide [3,8-diphenylpyrene-1,2,6,7-tetracarboxylic dianhydride and 4,40 -methylenebis(2-isopropyl-6-methylaniline)] (PI DPPD-IMM) was synthesized according to a previously reported method by San tiago-García et al. [36] Poly[2,2’-(1,3-phenylene)-5,50 -bibenzimida zole] (PBI) was supplied by PBI Performance Products Co. Insoluble material was eliminated from PBI by dissolving the raw polymer in NMP at 150 � C for 24 h. The viscous solution was filtered and poured in ethanol to obtain the purified polymer. 1-methyl-2-pyrrolidinone, NMP (99.5% Sigma-Aldrich) was used as received. The chemical structures of the polymers used for blends preparation are shown in Fig. 1. 2.2. Dense membrane preparation Polymeric dense membranes of PBI and blends with PI DPPD-IMM (PI) were prepared by solution casting according to our previous report [37]. 4 wt% polymer solutions with different PI/PBI compositions (100/0, 87.5/12.5, 75/25, 50/50, 25/75 and 0/100 wt%) were pre pared according to the following steps. First, PBI in the required quantity was dissolved in NMP at 150 � C using a magnetic stirrer for 24 h. Sub sequently, the polyimide was added and stirring was continued for another 24 h. The polymer solution was filtered through a 0.45 μm filter and then poured onto an aluminum plate surrounded by an aluminum ring. The plate was heated to 100 � C and an inverted glass funnel was placed over the solution to control solvent evaporation rate. The solu tion was allowed to evaporate for 24 h. The film was then removed and dried in a vacuum oven at 260 � C for 24 h. After that time, dense membranes were removed from the oven and they were cooled quickly to room temperature. Membrane thickness was measured using a Dig imatic (Toyo) indicator (IDC-112B-5) with an accuracy of 1 μm. The thickness recorded is an average value obtained from 10 different points of the membrane. The thickness of the films was in the range of 70 � 10 μm. 2.3. Elaboration of carbon molecular sieve membrane Samples of dense membranes, one inch square, were cut and placed inside a quartz tube. Dense membranes for PBI/PI blends were pyrolyzed to a final temperature of 600 � C. The pyrolysis process was performed in a three-zone tube furnace (Lindberg/Blue, model STF55346C-1) under an ultra-high purity (UHP) argon atmosphere. Fig. 2 describes the py rolysis system used. In order to reduce the oxygen concentration during the pyrolysis process, the quartz tube with the samples was maintained under a 300 cc/min flux of UHP argon for 2 h prior to the pyrolysis process. Fig. 3 shows the heating steps and rates used during the heating protocol [35]. During the pyrolysis an amorphous turbostratic structure is formed as the result of the thermal decomposition of the polymeric chains. Different authors report this turbostratic structures as disordered carbon hexagonal sheets with pores formed from packing imperfections. 600 � C was chosen as the final temperature for CMS PI/PBI blends since it has been reported that at 700 � C and 800 � C there is an important permeability reduction due to the collapse of the turbostratic structures [33,35,38]. CMS membranes derived from precursor blend membranes were named as PI(polyimide concentration)-600. For example, PI75-600 is the CMS membrane derived from the blend containing 75 wt% of PI DPPD-IMM polyimide and 25 wt% PBI.
2
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Journal of Membrane Science 597 (2020) 117703
Fig. 1. Chemical structure of (a) PI DPPD-IMM and (b) PBI.
Fig. 2. Pyrolysis system schematic diagram.
2.4. CMS membrane characterization Wide angle X-ray diffraction (WAXD) measurements were performed on dense PBI/PI membranes and their CMS membranes using a Siemens 5000 X-ray diffractometer with CuKα radiation (wavelength 1.54 Å) operated at 40 kV and 15 mA between 4 and 60� (2θ). The average dspacing was determined based on the Bragg’s law according to the following equation: nλ ¼ 2d sinθ
(1)
where n is an integral number (1, 2, 3,…), λ denotes the X-ray wave length 1.54 A), d stands for the d-spacing value and θ indicates the diffraction angle. Raman spectra were acquired with a confocal Raman InVia micro scope, using a 50X objective at wavelength of 532 nm with a 1000 s exposure time. 2.5. Gas transport properties CMS membranes pure gas permeability coefficients were determined by a variable-pressure constant-volume method as described elsewhere [39]. Gas permeability coefficients were measured at 35, 45, 55 and 65 � C at 2 atm upstream pressure for He, O2, N2, CH4 and CO2. Each gas permeability coefficient was determined from the rate of pressure
Fig. 3. Heating protocol and step ramps used in this study.
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increase (dp/dt)ss at steady state using the following relationship: � � 273:15 VL dp � � P¼ 76 ATp0 dt ss
Journal of Membrane Science 597 (2020) 117703
correlated to distance between graphitic structures, which is the space available for gas molecules to move across the CMS membrane [16,40]. In the case of CMS membranes derived from PBI/PI polymer blends, the peak at 7.1 Å tends to increase in intensity and becomes more defined as PI concentration increase in the blend which indicates the existence of a larger space available for the gas molecules to flow through the mem brane. The presence of a second maxima in all the CMS membranes at 3.9 Å could be related to an evolution towards a more efficiently packed graphitic structure that occurs in both PI and it is the main structure in PBI. The latter structure may present a restriction to flow through the membrane for gas molecules with a larger kinetic diameter and it could be one of the facts that increase selectivity. The combination of two different maxima in the WADX of CMS membranes PI/PBI blends and the evolution towards higher d-space values with increasing amounts of PI may be related to the characteristic reported for the CMS membranes, high gas permeability for small kinetic diameter gases, such as He and O2 and high selectivity for gas pairs involving larger kinetic diameter gases such as CH4 or N2 [41,42].
(2)
where P is the gas permeability coefficient in Barrer (1 Barrer ¼ 10 10 cm3(STP)∙cm∙cm 2∙s 1∙cmHg 1), V is the downstream chamber vol ume(cm3), A refers to the effective membrane area (cm2), L is the membrane thickness (cm), T is the temperature (K). (dp/dt)ss is given in mmHg/s and the feed gas pressure in the upstream, p0, in mmHg, where downstream pressure is considered so low that the pressure difference across the CMS membrane is essentially the value of the upstream pressure p0. The apparent diffusion coefficient (D) was obtained by using the time-lag (θ) method by the relationship: D¼
L2 6θ
(3)
where θ is the time-lag. The apparent solubility coefficient (S) was calculated from the ratio between P and D coefficients: P S¼ D
3.1.1. Raman spectroscopy The Raman spectra of CMS membranes pyrolyzed to 600 � C are shown in Fig. 5. All carbon membranes show two predominant broad peaks at ~1580 cm 1 (G peak) and ~1316 cm 1 (D peak). These two peaks are related with defective structure of graphite (D peak) and to the regular graphite structure (G peak) [38,43]. These Raman results show a stable G peak width and intensity for all CMS membranes; in the case of carbon membranes with PI � 87.5 wt%, G peak shows a slight decre ment in the position (~1546-1576 cm 1). An increment in D peak in tensity as PI concentration increase is observed, which is an indicative that inclusion of PBI let to control the ordering of graphitic structures as was discussed in previous WAXD section and a more closed or open structures can be obtained just by controlling the PBI/PI concentration.
(4)
3. Results 3.1. Wide angle X-ray diffraction Fig. 4 shows wide angle X-ray diffraction (WAXD) patterns for CMS membranes and their polymeric blend precursors with a normalized intensity for a better comparison. All polymeric and carbon membranes show amorphous halos in their diffraction pattern. When CMS mem branes d-spacing maxima are compared with the ones of polymeric precursors, it can be observed that there are several changes in the amorphous halo. d-spacing maxima for dense polyimide and blend membranes are located between 4.4 and 5.4 Å; PI100 membrane and blends with PBI concentration lower than 50 wt% show d-spacing in the range of 5.1 to 5.4 Å [37]. After polymeric precursor pyrolysis, CMS membranes show amorphous halos for PI-600, PI87.5–600 and PI75 gradually splits into two maxima on each WAXD pattern. The first one is located between 5.9 and 7.1 Å, where the d-spacing at 7.1 Å is dominant for the CMS membrane PI100-600. Even though these d-spacing values cannot be used as indications of the interplanar distance, they can be
3.2. Gas transport properties Table 1 shows the gas permeability coefficients measured at 35 � C and 2 atm upstream pressure for dense polymeric PI DPPD-IMM (PI100) and PI/PBI blend membranes [37] that were used as precursors for CMS membranes preparation. Gas permeability coefficients for PI100 follow the order (PCO2 > PHe > PO2 > PCH4 > PN2), having high permeability coefficients and a moderate ideal selectivity. Polymeric blend PBI/PI membranes show a decrease in gas permeability coefficients as PBI
Fig. 4. WAXD normalized scattering patterns for CMS membranes pyrolyzed to 600 � C continuous lines) and their polymeric precursors (dotted lines).
Fig. 5. Raman spectra of PI, PBI and PI/PBI blends CMS membranes pyrolyzed to 600 � C. 4
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Journal of Membrane Science 597 (2020) 117703
Table 1 Permeability coefficients and gas separation performance at 35 � C and 2 atm of PI DPPD-IMM (PI) and PI/PBI blend membranes as reported [37]. Precursor
Permeability (Barrer) He
PI100 PI87.5 PI75 PI50 PI25 PBI PBIa a
N2
O2
167 175 119 54 17 2.4 –
75 68 31 6.6 0.8 0.071 0.18
21 18 7.4 1.1 – – 0.07
Ideal Selectivity CH4 32 25 7.8 0.9 – – 0.035
CO2 457 408 173 31 3.1 0.19 0.4
αO2/
αCO2/
N2
CH4
αCO2/ N2
3.5 3.8 4.2 5.9 – – 2.6
14.1 16.3 22.3 35.0 – – 11.4
21.3 23.0 23.8 28.2 – – 5.71
Giel, V. et al. Report these pure gas permeability values [44].
concentration in the blend increases. A typical trade-off behavior is observed since as PBI concentration increase in the blend, gas perme ability coefficients diminishes while ideal gas selectivity increases. Gas permeability coefficients for N2 and CH4 were not measured for PI25 and PBI since their permeabilities were below the limit of measurement of the permeation cell used. As a reference PBI pure gas permeability co efficients reported by Giel et al. [44] are given in Table 1. Pure gas permeability coefficient for CMS membranes derived from PI DPPD-IMM (PI) and their blends with PBI (PI/PBI) for He, N2, O2, CH4, and CO2 measured at 2 atm and 35 � C are shown in Table 2. Gas permeability coefficients for all CMS membranes are between two and three times those presented by their polymeric counterparts and follow the order PHe > PCO2 > PO2 > PN2 > PCH4, which correspond to the order of gas kinetic diameter [25,42]. As PI concentration increases in PI/PBI CMS membranes, a clear tendency of increasing gas permeability coef ficient, P, is seen, Fig. 6. The trend of increasing P presents a larger step increase around 75 wt% PI. This behavior is attributed to CMS structural evolution during pyrolysis, because as was reported by several authors [26,43,45,46], during pyrolysis, a continuous decomposition of polymer chains and partial elimination of elements other than carbon occur to produce a carbon rich material. Recently, new insights into CMS struc ture formation was reported by Adams et al. [43] and Hazazi et al. [38]. In the first work it was reported the formation of “pyridinic” and “pyr rolic” strand segments from polyimide precursors. They suggest that around 75–80% of the strands correspond to pyrrolic form, being the maxima formation of the pyrrolic strands at 600 � C. On the other hand, Shulamn and Lochte [47] reported that PBI thermal degradation in Argon atmosphere tends to form carbazole structures or carbon-nitrogen residue with a ladder polymer structure. Consequently, in the present work structural evolution of PI and PBI during pyrolysis results in the strands shown in Fig. 7. These rigid and aromatic proposed strands are consistent with mass loss during the pyrolysis process. According to Rungta et al. [45] and Adams et al. [43], formed strands tend to align into plates, which form the ultramicropore section. Plates consolidate into tighter structures and the spaces between parallel aromatic plates comprise the micropores. Thermal soak and cooling lead to a behavior where adjacent micropores can coalesce into a cellular structure, with shared ultramicropore walls between micropore cells. According with
Fig. 6. Pure gas permeability coefficients as PI concentration increases in PI/ PBI blend CMS membranes.
these findings, carbazole type strands result in a more closed ultra microporous structure and more compact micropores than pyrrolic strands (See Fig. 8), which results in lower permeability when carbozole strands predominate in CMSM from PI/PBI blends. When pyrrolic strands are more common than carbazole strands (PI > 75 wt%) spaces between formed plates increase, resulting in an increment in gas permeability coefficients. The more open spacing between graphitic strata in CMS membranes from PI/PBI blends where PI > 75 wt% cor relates with the occurrence of a maxima at 7.1 Å observed by WADX. Ideal selectivity for gas pair (αCO2/CH4) in PI100-600 and PI87.5–600 CMS membranes reaches up to three times the selectivity of polymeric precursor membranes. It was also observed that CMS membranes from PI/PBI blends the ideal selectivity decreases with increasing PBI con centration. Thus, CMS membrane PI100-600 has the highest ideal gas pair selectivity (αO2/N2 ¼ 8.3, αCO2/N2 ¼ 31.0 and αCO2/CH4 ¼ 56.5) and also the highest CO2 and O2 permeability coefficients. This gas selec tivity drop with PBI concentration in PI/PBI blend CMS membranes can be attributed to a decrease between graphitic strata space, as is shown in Fig. 8, where the carbazole strands, derived from PBI, tend to form closed ultramicropores with similar dimension to gas molecules, which difficult the gas molecule discrimination process. Similar behavior was reported by Hosseini et al. [23,25] and Fu et al. [26] in CMS membranes derived from polymeric blend membranes, where the inclusion of a less permeable polymer in the pyrolyzed blend decreases the gas perme ability coefficients and selectivity in their CMS membranes. PI/PBI blends CMS membranes gas separation performance and their precursors for O2/N2 and CO2/CH4 pairs is shown in a Robeson plot, Fig. 9. After pyrolysis, carbon membranes show an enhancement in gas permeability coefficients and gas separation for all PI/PBI blends. PI100600, from pure polyimide, shows the best relationship between perme ability and selectivity coefficients for O2/N2 gas pair. O2 permeability coefficient has a moderate increment while O2/N2 ideal selectivity in crease around two times with respect to the polymeric precursors, the results of PI100-600 are above 2008 Robeson’s upper bound, as well as those from CMS PI87.5–600 and PI75-600. Analogous enhancement in separation performance was observed in CO2/CH4 gas pair for CMS membranes, where PI100-600 and PI87.5–600 membranes are above the 2008 Robeson’s upper bound, which actually is used as a reference since it was built with polymeric membranes permeability and selec tivity results. Therefore, in PI/PBI blends when PBI is below 25 wt% in the precursor materials an enhancement in both gas permeability co efficients and gas selectivity is observed. On the other hand, when both polymers are at the same concentration (PI50-600) or PBI is in larger
Table 2 Permeability coefficients at 35 � C and 2 atm for PI/PBI blend CMS membranes pyrolyzed at 600 � C. CMS membrane PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600 PBI-600
Permeability (Barrer) He 960 526 459 197 170 152
O2 134 98 39 24 15 7.5
N2 16.2 12.2 4.9 4.3 2.6 1.7
Ideal Selectivity CH4 8.9 7.3 3.1 2.0 1.5 1.0
CO2 503 359 109 84 47 24
αO2/
αCO2/
αCO2/
N2
CH4
N2
8.3 8.1 8.0 5.6 5.8 4.4
56.5 49.4 35.2 42 31.3 24
31.0 29.4 22.5 19.5 18.0 14.1
5
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Journal of Membrane Science 597 (2020) 117703
Fig. 7. Evolution of (a) PI and (b) PBI precursors into (c) pyrrolic and (d) carbazole strands during pyrolysis.
Fig. 8. CMS membranes formation from PI/PBI blend precursors. Adapted from Ref. [43].
concentration, pyrolysis to get a CMS membrane shows a minimum enhancement on gas selectivity even though gas permeability increases 2 to 3 times. Tables 3 and 4 show apparent diffusion and solubility coefficients for CMS membranes as well as their diffusion and solubility selectivity for O2/N2, CO2/CH4 and CO2/N2 gas pairs. The apparent diffusion coeffi cient (D) follows the order DO2 > DCO2 > DN2 > DCH4. PI100-600 membrane has the highest apparent diffusion coefficients. As pre sented in Table 4, blend composition in precursor membranes has a significant impact on diffusion coefficients while only a slight effect on solubility coefficients was observed. Diffusion coefficients decrease as PBI concentration increases in the blend. PI75-600 and PI50-600 CMS membranes have an important drop in diffusion coefficients. They drop more than 3 times below the one of PI100-600, apparently as a conse quence of an increase in structure packing due to a loss of the micro structure related to the observed d-spacing at 7.1 Å. Apparent solubility coefficients (S) for all carbon membranes have the order SCO2 > SCH4 > SO2 > SN2, following gas critical temperature order [42]. S in CMS membranes shows a small dependence with the composition showing a slight decrease as PBI concentration increases in PI/PBI blend.
Fig. 9. (a) O2/N2, (b) CO2/CH4 separation performance of CMS membranes from PI and PI/PBI and their polymeric precursors at 35 � C and 2 atm. PBI data from Giel, V. et al. [44]. P84 and PBI/P84(50/50) from Hosseini and Chung [25]. SBFDA-DMN Hazazi et al. [38].
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Table 3 Apparent diffusion coefficients for 600 membranes.
�
CMS
Diffusion coefficient (10-8 cm2/s)
O2
N2
CH4
CO2
αO2/N2
αCO2/CH4
αCO2/N2
PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600 PBI-600
9.0 7.1 2.9 1.9 1.36 0.73
1.40 1.07 0.46 0.38 0.25 0.18
0.23 0.20 0.08 0.07 0.05 0.03
8.6 6.3 2.3 1.9 1.2 0.61
6.4 6.6 6.3 5 5.4 4.0
38.7 31.5 28.8 27.1 24 20.3
6.1 5.9 5.0 5.0 4.8 3.4
Table 4 Apparent solubility coefficients for 600 membranes. CMSM
PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600 PBI-600
�
Diffusion Selectivity � � DA DB
3.4. PI/PBI CMS membranes apparent diffusion temperature dependence Fig. 11 shows the effect of temperature on apparent gas diffusion coefficients, Di, for CMS membranes derived from PI and PI/PBI blend precursors. Di increase as temperature increases, following an Arrhenius type behavior (equation (6)). � � EDi Di ¼ D0i exp (6) RT
C pyrolyzed CMS PI/PBI blend
Solubility coefficient 10 cm3(STP)/cm3cm Hg
2
O2
N2
CH4
CO2
αO2/N2
αCO2/CH4
αCO2/N2
14.9 14.0 13.5 12.8 10.9 10.3
11.7 11.4 10.6 11.3 10.4 9.4
38.8 33.6 38.8 32.9 30.0 31.7
58.5 50.5 47.6 44.1 39.5 39.0
1.3 1.2 1.3 1.1 1.0 1.1
1.5 1.5 1.2 1.3 1.3 1.2
5.0 4.4 4.5 3.9 3.8 4.1
CMS ideal gas selectivity α ¼ PPAB ¼
in CMS precursor blend. For each carbon membrane EPi follows the general trend CH4 > N2 > O2 > CO2 > He as is shown in Table 5. It is also seen that for CMS membranes EPi increases as the concentration of PBI increases in the CMS membrane precursors. Since EPi is the combination of diffusion activation energy and heat of solubility and since diffusion selectivity appear to be the principal factor for higher gas separation properties, a detailed analysis of diffusion selectivity. � � DA DB , will be performed in the next section.
C pyrolyzed CMS PI/PBI blend
Solubility Selectivity � � SA SB
Tables 7 and 8 present the values of activation energy for gas diffusion, EDi and the pre-exponential factor, D0i, for CMS membranes from PI/PBI blends. It can be observed that PBI concentration in CMS precursor affects EDi as a function of penetrant kinetic diameter since EDi follow the order CH4 > N2 > O2 > CO2 for PI100-600 and PI87.5–600. Additionally, it is observed that EDi increases as PBI concentration in creases in membrane precursors, showing an increment of their barrier properties and resulting in a higher EDi for gas diffusion through the CMS membranes where N2 and CH4 are the gases with the highest EDi increase as PBI concentration in the precursor goes up. It is clear from Fig. 11 that an important drop in Di occurs between PI87.5–600 and PI75-600 membranes, an indication of the shift lowering the gap of the graphitic structure as was discussed above.
� � � � DA SA can be viewed as the DB X SB
product of diffusion selectivity and solubility selectivity. In CMS mem � � branes diffusion selectivity, DDAB , is governed by the pore distribution and size and shape of the gas molecules. On the other hand, solubility � � selectivity, SSAB , depends on gas condensability and their interaction
3.5. PI/PBI CMS membranes diffusion selectivity: entropic and energetic selectivities
with the membrane material [22]. From the results in Tables 3 and 4, diffusion selectivity in these CMS membranes from PI/PBI blends is the main contributing factor to the overall permselectivity, which decreases as PBI increases in PI/PBI blends. Thus, diffusion selectivity appears as the principal factor governing gas separation properties in the CMS membranes from PI/PBI blends.
In CMS membranes from PI/PBI blends, diffusion selectivity,
� � DA DB , is
being labeled as the main contributing factor to the overall permse lectivity. Consequently, higher diffusion selectivity leads to improved gas separation performance [33]. According to Singh and Koros [34] diffusion selectivity can be factored into an “energetic selectivity” and an “entropic selectivity” as described by equation (7). � � � � � � DA D0A EDA EDB SDA SDB EDA EDB ¼ exp ¼ exp ⋅exp (7) DB D0B RT R RT
3.3. PI/PBI CMS membranes gas permeability temperature dependence Activation energy and entropic selectivity extracted from. � � DA DB , as reported by Singh and Koros [34], can be a tool in order to
where: � � is the entropic selectivity and. exp SDA R SDB ¼ DD0A 0B � � EDA EDB exp is the energetic selectivity. RT
understand gas separation and transport properties in PI/PBI CMS membranes [33,35,48]. It involves the measurement of gas permeability and apparent diffusion coefficients as a function of temperature which are given below. CMS membranes gas permeability coefficients, P, for PI and PI/PBI blends were examined between 35 and 65 � C at 2 atm upstream pres sure. Fig. 10 shows gas permeability coefficients’ Arrhenius plots as a function of temperature for He, O2, N2, CH4 and CO2 of CMS membranes derived from PI and PI/PBI blends. It can be observed that P for all gases in CMS membranes increases with temperature following closely an Arrhenius type behavior as indicated by equation (5). � � EPi Pi ¼ P0i exp (5) RT
Energetic selectivity is referred to the difference in the diffusion activation energies of the two penetrants, while entropic selectivity re flects differences in the mobility of penetrant gases due to penetrant shape and CMS membranes structure capacity to restrict their rotational and internal vibrational degrees of freedom [34,35,48]. Table 9 summarizes energetic and entropic selectivity for PI/PBI blends CMS membranes for gas pairs O2/N2 and CO2/CH4 at a temper ature range between 35 and 65 � C, measured at 2 atm upstream pres sure. Values were calculated from Equation (7) and from EDi and D0i values reported in Tables 7 and 8 It can be observed that energetic and entropic selectivity have a clear dependence with PI concentration in the PI/PBI blends. CMS membranes energetic selectivity values drop as PI concentration increases in the membranes as the result of the increased distance between graphitic strata which in turn increases the population
The activation energy for permeation, EPi, and the corresponding pre-exponential factors, P0i, for He, O2, N2, CH4 and CO2 are shown in Tables 5 and 6. EPi depends on both penetrant size and PBI concentration 7
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Journal of Membrane Science 597 (2020) 117703
Fig. 10. Gas permeability coefficients temperature dependence for CMS membranes derived from PI and PI/PBI blend (a) He, (b) O2, (c) N2, (d) CH4 and (e) CO2. Table 5 Activation energy for Permeation for He, O2, N2, CH4 and CO2 in CMS mem branes from PI and PI/PBI blend precursors. CMS membranes PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600
Table 6 Permeation pre-exponential factors for He, O2, N2, CH4 and CO2 in CMS mem branes derived from PI and PI/PBI blend precursors.
Ep (kJ/mol) He
O2
N2
CH4
CO2
3.7 5.0 5.5 6.5 8.5
8.7 9.3 12.4 14.3 16.3
9.4 11.6 15.8 18.6 21.0
10.4 12.7 18.4 20.7 22.4
6.0 7.4 10.7 12.4 14.2
of larger micropores. A decrease in the difference in energy necessary for gas permeation and decrease of activation energy values for gas diffu sion through the CMS membrane results in a higher gas permeability and diffusion coefficients since there is a minimum resistance to gas flow. Thus, the compact CMS graphitic structures from PBI, that have a larger energetic selectivity value, show a drastic change with the presence of PI particularly above 75 wt% dropping rapidly towards low activation energy differences and they become small for gas pairs O2/N2 and CO2/
CMS membranes
P0 (barrer) He
O2
N2
CH4
CO2
PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600
4.1 � 103 3.7 � 103 3.9 � 103 2.5 � 103 4.8 � 103
4.0 � 103 3.7 � 103 4.9 � 103 6.3 � 103 8.6 � 103
6.4 � 102 1.1 � 103 2.3 � 103 6.0 � 103 9.5 � 103
5.1 � 102 1.0 � 103 4.2 � 103 7.4 � 103 9.5 � 103
5.2 � 103 6.5 � 103 7.2 � 103 1.1 � 104 1.2 � 104
CH4. According to that, lower energetic selectivity favors higher gas permeability and contributes to higher selectivity for the studied gas pairs. The last statement agrees with reports by different authors [14,33, 35,48]. Entropic selectivity shows an increase with increasing PI concen tration in CMS membranes from PI/PBI blends. This result agrees with an increase in diffusion selectivity and permeation selectivity for the studied gas pairs. As the value of entropic selectivity decreases towards 8
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Journal of Membrane Science 597 (2020) 117703
Fig. 11. Apparent diffusion coefficients temperature dependence for CMS membranes derived from PI and PI/PBI blend (a) O2, (b) N2, (c) CH4 and (d) CO2. Table 7 Activation energy for diffusion, EDi, for O2, N2, CH4 and CO2 in CMS membranes from PI/PBI blends. CMS membranes
Table 9 Energetic and entropic selectivity for CMS from PI/PBI membranes for gas pairs O2/N2 and CO2/CH4 in the temperature range of 35–65 � C.
EDi (kJ/mol)
PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600
CMSM
O2
N2
CH4
CO2
16.2 17.6 21.2 23.2 26.4
16.8 19.1 23.5 26.1 29.9
19.2 22.1 27.5 30.2 33.2
15.1 17.2 21.2 23.0 25.9
Energetic selectivity PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600 Entropic selectivity PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600
Table 8 Diffusion pre-exponential factors for O2, N2, CH4 and CO2 in CMS membranes from PI/PBI blends. CMS membranes PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600
D0 (cm2/seg) O2
N2
5.49 � 103 6.96 � 103 1.11 � 104 1.65 � 104 4.15 � 104
9.72 1.82 4.50 1.01 2.97
CH4 � 102 � 103 � 103 � 104 � 104
4.11 � 1.10 � 3.70 � 9.07 � 2.18 �
CO2/CH4
1.14–1.13 1.74–1.66 2.51–2.31 3.04–2.75 3.82–3.39
5.0–4.3 6.7–5.7 11.9–9.57 16.2–12.7 17.2–13.4
5.64 3.82 2.47 1.63 1.40
7.70 4.77 2.42 1.68 1.36
line. For CMS membranes from PI/PBI blends, the increase on this entropic selectivity factor is closely related with the presence of a differentiated graphitic structure, depicted in Fig. 8 as PBI is found in larger concentration and becomes the dominant phase in the blend. The difference in entropic selectivity between CMS membrane PI and those derived from PI/PBI blends seems to be due to the difference between these differentiated graphitic structures, indicating that the main contribution to the diffusion selectivity for CMS membranes comes from the entropic selectivity while low energetic selectivity contributes to a higher gas diffusion. These values agree with the results shown in the gas separation performance, where PI100-600, PI87.5–600 and PI75-600 CMS membranes had the best relationship between permeability and gas selectivity and they are located above the upper bound.
CO2 102 103 103 103 104
O2/N2
3.16 � 103 5.25 � 103 8.95 � 103 1.50 � 104 2.96 � 104
unity, the permeation selectivity decreases. In fact, in the case of the CO2/CH4 gas pair, entropic selectivity is close to unity in PI/PBI blends with larger content of PBI and selectivity is poor due to a similar mobility of the molecules. On the other hand, larger differences in molecules mobility are found for PI/PBI blends PI100-600 and PI87.5–600, which are the ones that lie above Robeson upper bound, while those for PI50-600 and PI25-600, which are closer to unity are found below the upper bound limit. The presence of PI increases entropic selectivity for O2/N2 and CO2/CH4 gas pairs. The result in dicates that differences in gas mobility restrictions have to be larger than 2.4 for the entropic selectivity to go above 2008 Robeson’s upper bound
4. Conclusion Carbon molecular sieve membranes, CMSM, were prepared by py rolysis at 600 � C from PI DPPD-IMM (PI) and PI/PBI blends. WAXD analysis revealed a change in the ultramicroporous structure of CMS 9
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Journal of Membrane Science 597 (2020) 117703
membranes as the concentration of PI in the precursor membrane in creases as the result of the formation of pyrrolic strands, which form more open ultramicropores in comparison with carbazole-type strands, derived from PBI. PBI concentration in the precursor showed a negative effect on permeability and also in gas pair selectivity for carbon mem branes which was attributed to carbazole-type strands densification. So that PI100-600 carbon membrane derived from pure polyimide had the highest gas permeability values (PCO2 ¼ 503 Barrer and PO2 ¼ 134 Barrer) and gas pair selectivity (αCO2/CH4 ¼ 56.5 and αO2/N2 ¼ 8.3). The effects of test temperature on gas permeability and diffusion coefficients were analyzed to provide insights into the factors governing gas sepa ration properties in CMS membranes from PI/PBI blends. An increase in permeation, as well as diffusivity, occurs with increasing testing tem perature. Carbon membranes derived from precursors with low PBI content exhibit entropic selectivity higher than 2.4, indicating that the discrimination ability of CMS membranes increases as PI concentration increases in blend precursors, producing an enhancement of gas permeability and separation factor in PI75-600, PI87.5–600 and PI100600 for O2/N2 and CO2/CH4 gas pairs, which surpass the upper bound. Clearly, the main contribution to the increase of diffusion selectivity and, therefore, the increment of permseletivity is the entropic selec tivity, which is the result of the presence of differentiated carbon structures that depend on blend composition. Further studies of the role of differences in carbon structures composition and carbon strands ar rangements in CMSM from polymer blend precursors have to be carried out to properly understand their effect in gas permeability and selectivity.
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Authors contribution statement Jos�e Manuel P� erez-Francisco: Responsible for preparation and par tial characterization and gas transport properties of CMSM membranes and discussion of the characterization results. Jos�e Luis Santiago-García: Responsible for polyimide synthesis characterization and polymeric blends preparation. María Isabel Loría-Bastarrachea: Responsible for characterization of polymeric membranes and CMSM blends and instrumental runs and characterization (WAXD, Raman and Thermal analysis). Donald R. Paul: Contribution on blend interactions and theoretical blend behavior expected as well as transport properties. Benny D. Freeman: Contribution on blend gas permeability and selectivity structure property relationships for the gas transport behavior. Manuel Aguilar-Vega: In charge of analysis of transport properties from temperature and overall project results. Declaration of competing interest The authors declare that they do not have any conflict of interest to declare. Acknowledgments J.M.P.F. gratefully acknowledges a scholarship from CONACyT (Mexico’s National Council for Science and Technology) under Grant 354777 and the financial support from Tecnol� ogico Nacional de M�exico (Grant 537.18-PD). M. Aguilar-Vega is grateful for a fellowship from Fulbright-Garcia-Robles (COMEXUS) for a sabbatical leave at UT Austin. The authors acknowledge Dr. Patricia Quintana for allowing access to The National Laboratory of Nano and Biomaterials (LANNBIO) from CINVESTAV-IPN, Merida Unit, grants FOMIX-YUCATAN 108160 and CONACYT 123913, in which X-ray diffraction analysis was carried out with the technical assistance of M. C. Daniel Aguilar-Trevineo.
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CMS membranes from PBI/PI blends: Temperature effect on gas transport and separation performance Jos�e Manuel P�erez-Francisco a, b, Jos�e Luis Santiago-García a, María Isabel Loría-Bastarrachea a, Donald R. Paul c, Benny D. Freeman c, Manuel Aguilar-Vega a, * a
Unidad de Materiales, Centro de Investigaci� on Científica de Yucat� an, A.C., Calle 43, No. 130, C.P, 97205, M�erida, Yucat� an, Mexico Tecnol� ogico Nacional de M�exico/Instituto Tecnol� ogico Superior de Coatzacoalcos, Carretera Antigua Minatitl� an-Coatzacoalcos Km. 16.5, C.P., 96536, Coatzacoalcos, Veracruz, Mexico c McKetta Department of Chemical Engineering, University of Texas at Austin, 200 E St Stop C0400 Dean Keaton St, Austin, TX, 78712, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: CMS membrane Blend precursors Gas separation Entropic selectivity Graphenic stuctures
This study reports pure gas permeability and diffusion coefficients for carbon molecular sieve membranes (CMSM) derived from dense membranes based on blends of a rigid polyimide PI DPPD-IMM (PI) and poly benzimidazole (PBI), PI/PBI, at different concentrations. The permeability, diffusion and selectivity for PI/PBI blend CMS membranes was systematically tested as a function of concentration and temperature. CMS membrane derived from pure PI DPPD-IMM membrane (PI100-600) exhibited the highest permeability coefficients (PHe ¼ 960 Barrer, PCO2 ¼ 503 Barrer) and the highest separation factors (αO2/N2 ¼ 8.3 and αCO2/CH4 ¼ 56.5). Increasing PI concentration in the membranes blend precursors shows a positive effect on CMS membranes permeability, diffusion coefficients and selectivity. These results are attributed to an increase in micropores dimension that was reflected by the emergence of a d-spacing shift from 5.9 Å to 7.1 Å. It was also found that large concentrations of PBI (>50 wt%) in the precursor showed a negative effect on permeability and also in gas pair selectivity for carbon membranes which was attributed to carbazole-type strands densification of the structure. As PI con centration increases in PI/PBI CMS membranes, they present higher entropic selectivity and low energetic selectivity leading to the best permeability/selectivity relationship surpassing 2008 Robeson’s upper bound for O2/N2 and CO2/CH4 gas pairs.
1. Introduction Membrane technology is one of the processes in the gas separation industry that is growing fast and competes with cryogenic distillation and absorption processes [1,2]. Polymeric membranes are making in roads in gas separation technology because of their ease of fabrication, wide materials availability and some of them present a good per meability/selectivity relationship for some industrially interesting gas pairs [3–5]. For example, natural gas consumption has been growing in the last three decades and it is expected that for the year 2040, 30% of world’s primary energy will be obtained from natural gas [6]. Carbon dioxide is one of the most common natural gas impurities, with almost 20% of U.S. natural gas production with CO2 concentrations above 2 vol %, which is the allowable U.S. pipeline specification [7] to eliminate a potential cause of pipeline corrosion [8]. Therefore, CO2/CH4 separa tion is an important topic in the natural gas industry [9,10].
Nevertheless, gas separation is limited by an intrinsic trade-off between productivity and separation capacity because as the gas permeability increases, the separation factor decreases. This fact has been docu mented by Robeson and corroborated by others in the so called upper-bound relationship between permeability and gas separation factor [11]. On the other hand, carbon molecular sieve (CMS) membranes are promising materials for gas separation processes. They are formed by the pyrolysis of polymeric precursor membranes under inert atmosphere or controlled vacuum [12]. In recent years research has focused on these carbon membranes due to their excellent permeability and selectivity relationship that usually gives properties lying above the upper-bound limit of polymeric membranes [2,13]. The excellent relationship be tween permeability and selectivity is the result of chain packing im perfections during pyrolysis that lead to a nonhomogeneous carbon pore structure consisting of relatively wide open pores (7–20 Å) coexisting
* Corresponding author. E-mail address: [email protected] (M. Aguilar-Vega). https://doi.org/10.1016/j.memsci.2019.117703 Received 23 August 2019; Received in revised form 15 November 2019; Accepted 28 November 2019 Available online 30 November 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
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Journal of Membrane Science 597 (2020) 117703
with small selective pores (3–6 Å) [14,15]. The pore dimensions and their microstructural distribution depend on several parameters. Among them, the most important ones are precursor material selection, inert atmosphere, and pyrolysis conditions [16–18]. Several reports in the literature focus in the relationship between precursor materials and CMS membranes final properties [1,2,19]. Aromatic polyimides have been used as a preferred precursor for CMS membranes due to their high chemical and thermal stability, high carbon yield and outstanding in crease in gas separation properties [19–22]. Blending different polymers in order to obtained CMS membrane precursors that offer new features may show advantages for modulation of CMS membranes gas separation properties [23,24]. Several studies on CMS membranes derived from polymer blends have been reported in recent years, showing that CMSM derived from blends, in some cases show superior gas transport and separation properties than CMS membranes derived from their indi vidual components [23,25–28]. Hosseini et al. [23,25] reported CMSM derived from blends of polymers with similar gas permeability as polymeric membranes. In the first work, they elaborated CMS mem branes from Matrimid, Torlon, P84 and PBI. Their results showed that Matrimid blended with PBI was the best option to fabricate CMSM as the result of the gas pair selectivity increment (CO2/CH4 ¼ 203.9, H2/CO2 ¼ 33.44, N2/CH4 ¼ 7.99 and PN2 ¼ 2.78 Barrer). In the second work, they fabricated CMS membranes from Kapton, P84 HT, UIP-R and PBI under vacuum. They reported that CMSM obtained from PBI/Kapton blends at 10 7 Torr and 800 � C have good CO2/CH4 separation factors, offering enhanced selectivity in the range of up to 200. A number of researchers have studied the effect of operating tem perature on gas permeability and selectivity in CMS derived from polymeric membranes [29–32]. However, the temperature dependence of gas permeability coefficients and selectivity for CMS membranes derived from blend membranes is an issue where a more systematic investigation can contribute to understand the factors that lead to an increase in gas separation behavior [33,34]. Ning and Koros [35] reported gas transport properties and temper ature dependence of a CMS membrane derived from Matrimid, finding that gas permeability and diffusion coefficients follow an Arrhenius type relationship, while the selectivity decreases as temperature increases. In another work, Fu et al. [14] study the temperature dependence of CMS membranes derived from four novel 6FDA based polyimides. In both reports, the analysis of activation energies for permeation, diffusion and sorption gave information about permeability and selectivity changes with operating temperature. Additionally, they factored the diffusion selectivity into energetic and entropic selectivity in order to elucidate the reason why CMS membranes outperform the precursor polymeric membranes. Typically, CMS membranes possess high entropic selec tivity, which is related to rigid CMS pore structures and differences in penetrant shape [34,35]. Recently, Fu et al. [33] explained the effect of pyrolysis temperature on entropic selectivity of CMS membranes derived from a 6FDA based polyimide. Their results indicated that entropic selectivity increases as the pyrolysis temperature increases. They attributed the increase in gas selectivity based on size and shape discrimination ability showing that determining the changes in the entropic factor may be an important tool to understand carbon mem brane performance. Since the effect of temperature on the mechanism of improved gas permeability and selectivity in CMS membranes from polymer blends has not been thoroughly assessed to the best of our knowledge. In this study we report the gas transport and separation properties of CMS membranes from polymer blend precursors with a systematic increase in concentration of a closely packed low permeability polymer, Poly[2,2’(1,3-phenylene)-5,50 -bibenzimidazole], PBI, blended with an open structure, high permeability aromatic rigid polyimide Polyimide[3,8diphenylpyrene-1,2,6,7-tetracarboxylic dianhydride and 4,40 -methyl enebis(2-isopropyl-6-methylaniline)], PI. We also tested the effect of increasing temperature on pure gas permeability and diffusion co efficients, between 35 and 65 � C, on CMS membranes derived from PI/
PBI blends. The results are analyzed in terms of structural polymer dif ferences and blend concentration as well as structural changes upon carbonization. 2. Experimental 2.1. Materials Polyimide [3,8-diphenylpyrene-1,2,6,7-tetracarboxylic dianhydride and 4,40 -methylenebis(2-isopropyl-6-methylaniline)] (PI DPPD-IMM) was synthesized according to a previously reported method by San tiago-García et al. [36] Poly[2,2’-(1,3-phenylene)-5,50 -bibenzimida zole] (PBI) was supplied by PBI Performance Products Co. Insoluble material was eliminated from PBI by dissolving the raw polymer in NMP at 150 � C for 24 h. The viscous solution was filtered and poured in ethanol to obtain the purified polymer. 1-methyl-2-pyrrolidinone, NMP (99.5% Sigma-Aldrich) was used as received. The chemical structures of the polymers used for blends preparation are shown in Fig. 1. 2.2. Dense membrane preparation Polymeric dense membranes of PBI and blends with PI DPPD-IMM (PI) were prepared by solution casting according to our previous report [37]. 4 wt% polymer solutions with different PI/PBI compositions (100/0, 87.5/12.5, 75/25, 50/50, 25/75 and 0/100 wt%) were pre pared according to the following steps. First, PBI in the required quantity was dissolved in NMP at 150 � C using a magnetic stirrer for 24 h. Sub sequently, the polyimide was added and stirring was continued for another 24 h. The polymer solution was filtered through a 0.45 μm filter and then poured onto an aluminum plate surrounded by an aluminum ring. The plate was heated to 100 � C and an inverted glass funnel was placed over the solution to control solvent evaporation rate. The solu tion was allowed to evaporate for 24 h. The film was then removed and dried in a vacuum oven at 260 � C for 24 h. After that time, dense membranes were removed from the oven and they were cooled quickly to room temperature. Membrane thickness was measured using a Dig imatic (Toyo) indicator (IDC-112B-5) with an accuracy of 1 μm. The thickness recorded is an average value obtained from 10 different points of the membrane. The thickness of the films was in the range of 70 � 10 μm. 2.3. Elaboration of carbon molecular sieve membrane Samples of dense membranes, one inch square, were cut and placed inside a quartz tube. Dense membranes for PBI/PI blends were pyrolyzed to a final temperature of 600 � C. The pyrolysis process was performed in a three-zone tube furnace (Lindberg/Blue, model STF55346C-1) under an ultra-high purity (UHP) argon atmosphere. Fig. 2 describes the py rolysis system used. In order to reduce the oxygen concentration during the pyrolysis process, the quartz tube with the samples was maintained under a 300 cc/min flux of UHP argon for 2 h prior to the pyrolysis process. Fig. 3 shows the heating steps and rates used during the heating protocol [35]. During the pyrolysis an amorphous turbostratic structure is formed as the result of the thermal decomposition of the polymeric chains. Different authors report this turbostratic structures as disordered carbon hexagonal sheets with pores formed from packing imperfections. 600 � C was chosen as the final temperature for CMS PI/PBI blends since it has been reported that at 700 � C and 800 � C there is an important permeability reduction due to the collapse of the turbostratic structures [33,35,38]. CMS membranes derived from precursor blend membranes were named as PI(polyimide concentration)-600. For example, PI75-600 is the CMS membrane derived from the blend containing 75 wt% of PI DPPD-IMM polyimide and 25 wt% PBI.
2
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Journal of Membrane Science 597 (2020) 117703
Fig. 1. Chemical structure of (a) PI DPPD-IMM and (b) PBI.
Fig. 2. Pyrolysis system schematic diagram.
2.4. CMS membrane characterization Wide angle X-ray diffraction (WAXD) measurements were performed on dense PBI/PI membranes and their CMS membranes using a Siemens 5000 X-ray diffractometer with CuKα radiation (wavelength 1.54 Å) operated at 40 kV and 15 mA between 4 and 60� (2θ). The average dspacing was determined based on the Bragg’s law according to the following equation: nλ ¼ 2d sinθ
(1)
where n is an integral number (1, 2, 3,…), λ denotes the X-ray wave length 1.54 A), d stands for the d-spacing value and θ indicates the diffraction angle. Raman spectra were acquired with a confocal Raman InVia micro scope, using a 50X objective at wavelength of 532 nm with a 1000 s exposure time. 2.5. Gas transport properties CMS membranes pure gas permeability coefficients were determined by a variable-pressure constant-volume method as described elsewhere [39]. Gas permeability coefficients were measured at 35, 45, 55 and 65 � C at 2 atm upstream pressure for He, O2, N2, CH4 and CO2. Each gas permeability coefficient was determined from the rate of pressure
Fig. 3. Heating protocol and step ramps used in this study.
3
J.M. P�erez-Francisco et al.
increase (dp/dt)ss at steady state using the following relationship: � � 273:15 VL dp � � P¼ 76 ATp0 dt ss
Journal of Membrane Science 597 (2020) 117703
correlated to distance between graphitic structures, which is the space available for gas molecules to move across the CMS membrane [16,40]. In the case of CMS membranes derived from PBI/PI polymer blends, the peak at 7.1 Å tends to increase in intensity and becomes more defined as PI concentration increase in the blend which indicates the existence of a larger space available for the gas molecules to flow through the mem brane. The presence of a second maxima in all the CMS membranes at 3.9 Å could be related to an evolution towards a more efficiently packed graphitic structure that occurs in both PI and it is the main structure in PBI. The latter structure may present a restriction to flow through the membrane for gas molecules with a larger kinetic diameter and it could be one of the facts that increase selectivity. The combination of two different maxima in the WADX of CMS membranes PI/PBI blends and the evolution towards higher d-space values with increasing amounts of PI may be related to the characteristic reported for the CMS membranes, high gas permeability for small kinetic diameter gases, such as He and O2 and high selectivity for gas pairs involving larger kinetic diameter gases such as CH4 or N2 [41,42].
(2)
where P is the gas permeability coefficient in Barrer (1 Barrer ¼ 10 10 cm3(STP)∙cm∙cm 2∙s 1∙cmHg 1), V is the downstream chamber vol ume(cm3), A refers to the effective membrane area (cm2), L is the membrane thickness (cm), T is the temperature (K). (dp/dt)ss is given in mmHg/s and the feed gas pressure in the upstream, p0, in mmHg, where downstream pressure is considered so low that the pressure difference across the CMS membrane is essentially the value of the upstream pressure p0. The apparent diffusion coefficient (D) was obtained by using the time-lag (θ) method by the relationship: D¼
L2 6θ
(3)
where θ is the time-lag. The apparent solubility coefficient (S) was calculated from the ratio between P and D coefficients: P S¼ D
3.1.1. Raman spectroscopy The Raman spectra of CMS membranes pyrolyzed to 600 � C are shown in Fig. 5. All carbon membranes show two predominant broad peaks at ~1580 cm 1 (G peak) and ~1316 cm 1 (D peak). These two peaks are related with defective structure of graphite (D peak) and to the regular graphite structure (G peak) [38,43]. These Raman results show a stable G peak width and intensity for all CMS membranes; in the case of carbon membranes with PI � 87.5 wt%, G peak shows a slight decre ment in the position (~1546-1576 cm 1). An increment in D peak in tensity as PI concentration increase is observed, which is an indicative that inclusion of PBI let to control the ordering of graphitic structures as was discussed in previous WAXD section and a more closed or open structures can be obtained just by controlling the PBI/PI concentration.
(4)
3. Results 3.1. Wide angle X-ray diffraction Fig. 4 shows wide angle X-ray diffraction (WAXD) patterns for CMS membranes and their polymeric blend precursors with a normalized intensity for a better comparison. All polymeric and carbon membranes show amorphous halos in their diffraction pattern. When CMS mem branes d-spacing maxima are compared with the ones of polymeric precursors, it can be observed that there are several changes in the amorphous halo. d-spacing maxima for dense polyimide and blend membranes are located between 4.4 and 5.4 Å; PI100 membrane and blends with PBI concentration lower than 50 wt% show d-spacing in the range of 5.1 to 5.4 Å [37]. After polymeric precursor pyrolysis, CMS membranes show amorphous halos for PI-600, PI87.5–600 and PI75 gradually splits into two maxima on each WAXD pattern. The first one is located between 5.9 and 7.1 Å, where the d-spacing at 7.1 Å is dominant for the CMS membrane PI100-600. Even though these d-spacing values cannot be used as indications of the interplanar distance, they can be
3.2. Gas transport properties Table 1 shows the gas permeability coefficients measured at 35 � C and 2 atm upstream pressure for dense polymeric PI DPPD-IMM (PI100) and PI/PBI blend membranes [37] that were used as precursors for CMS membranes preparation. Gas permeability coefficients for PI100 follow the order (PCO2 > PHe > PO2 > PCH4 > PN2), having high permeability coefficients and a moderate ideal selectivity. Polymeric blend PBI/PI membranes show a decrease in gas permeability coefficients as PBI
Fig. 4. WAXD normalized scattering patterns for CMS membranes pyrolyzed to 600 � C continuous lines) and their polymeric precursors (dotted lines).
Fig. 5. Raman spectra of PI, PBI and PI/PBI blends CMS membranes pyrolyzed to 600 � C. 4
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Table 1 Permeability coefficients and gas separation performance at 35 � C and 2 atm of PI DPPD-IMM (PI) and PI/PBI blend membranes as reported [37]. Precursor
Permeability (Barrer) He
PI100 PI87.5 PI75 PI50 PI25 PBI PBIa a
N2
O2
167 175 119 54 17 2.4 –
75 68 31 6.6 0.8 0.071 0.18
21 18 7.4 1.1 – – 0.07
Ideal Selectivity CH4 32 25 7.8 0.9 – – 0.035
CO2 457 408 173 31 3.1 0.19 0.4
αO2/
αCO2/
N2
CH4
αCO2/ N2
3.5 3.8 4.2 5.9 – – 2.6
14.1 16.3 22.3 35.0 – – 11.4
21.3 23.0 23.8 28.2 – – 5.71
Giel, V. et al. Report these pure gas permeability values [44].
concentration in the blend increases. A typical trade-off behavior is observed since as PBI concentration increase in the blend, gas perme ability coefficients diminishes while ideal gas selectivity increases. Gas permeability coefficients for N2 and CH4 were not measured for PI25 and PBI since their permeabilities were below the limit of measurement of the permeation cell used. As a reference PBI pure gas permeability co efficients reported by Giel et al. [44] are given in Table 1. Pure gas permeability coefficient for CMS membranes derived from PI DPPD-IMM (PI) and their blends with PBI (PI/PBI) for He, N2, O2, CH4, and CO2 measured at 2 atm and 35 � C are shown in Table 2. Gas permeability coefficients for all CMS membranes are between two and three times those presented by their polymeric counterparts and follow the order PHe > PCO2 > PO2 > PN2 > PCH4, which correspond to the order of gas kinetic diameter [25,42]. As PI concentration increases in PI/PBI CMS membranes, a clear tendency of increasing gas permeability coef ficient, P, is seen, Fig. 6. The trend of increasing P presents a larger step increase around 75 wt% PI. This behavior is attributed to CMS structural evolution during pyrolysis, because as was reported by several authors [26,43,45,46], during pyrolysis, a continuous decomposition of polymer chains and partial elimination of elements other than carbon occur to produce a carbon rich material. Recently, new insights into CMS struc ture formation was reported by Adams et al. [43] and Hazazi et al. [38]. In the first work it was reported the formation of “pyridinic” and “pyr rolic” strand segments from polyimide precursors. They suggest that around 75–80% of the strands correspond to pyrrolic form, being the maxima formation of the pyrrolic strands at 600 � C. On the other hand, Shulamn and Lochte [47] reported that PBI thermal degradation in Argon atmosphere tends to form carbazole structures or carbon-nitrogen residue with a ladder polymer structure. Consequently, in the present work structural evolution of PI and PBI during pyrolysis results in the strands shown in Fig. 7. These rigid and aromatic proposed strands are consistent with mass loss during the pyrolysis process. According to Rungta et al. [45] and Adams et al. [43], formed strands tend to align into plates, which form the ultramicropore section. Plates consolidate into tighter structures and the spaces between parallel aromatic plates comprise the micropores. Thermal soak and cooling lead to a behavior where adjacent micropores can coalesce into a cellular structure, with shared ultramicropore walls between micropore cells. According with
Fig. 6. Pure gas permeability coefficients as PI concentration increases in PI/ PBI blend CMS membranes.
these findings, carbazole type strands result in a more closed ultra microporous structure and more compact micropores than pyrrolic strands (See Fig. 8), which results in lower permeability when carbozole strands predominate in CMSM from PI/PBI blends. When pyrrolic strands are more common than carbazole strands (PI > 75 wt%) spaces between formed plates increase, resulting in an increment in gas permeability coefficients. The more open spacing between graphitic strata in CMS membranes from PI/PBI blends where PI > 75 wt% cor relates with the occurrence of a maxima at 7.1 Å observed by WADX. Ideal selectivity for gas pair (αCO2/CH4) in PI100-600 and PI87.5–600 CMS membranes reaches up to three times the selectivity of polymeric precursor membranes. It was also observed that CMS membranes from PI/PBI blends the ideal selectivity decreases with increasing PBI con centration. Thus, CMS membrane PI100-600 has the highest ideal gas pair selectivity (αO2/N2 ¼ 8.3, αCO2/N2 ¼ 31.0 and αCO2/CH4 ¼ 56.5) and also the highest CO2 and O2 permeability coefficients. This gas selec tivity drop with PBI concentration in PI/PBI blend CMS membranes can be attributed to a decrease between graphitic strata space, as is shown in Fig. 8, where the carbazole strands, derived from PBI, tend to form closed ultramicropores with similar dimension to gas molecules, which difficult the gas molecule discrimination process. Similar behavior was reported by Hosseini et al. [23,25] and Fu et al. [26] in CMS membranes derived from polymeric blend membranes, where the inclusion of a less permeable polymer in the pyrolyzed blend decreases the gas perme ability coefficients and selectivity in their CMS membranes. PI/PBI blends CMS membranes gas separation performance and their precursors for O2/N2 and CO2/CH4 pairs is shown in a Robeson plot, Fig. 9. After pyrolysis, carbon membranes show an enhancement in gas permeability coefficients and gas separation for all PI/PBI blends. PI100600, from pure polyimide, shows the best relationship between perme ability and selectivity coefficients for O2/N2 gas pair. O2 permeability coefficient has a moderate increment while O2/N2 ideal selectivity in crease around two times with respect to the polymeric precursors, the results of PI100-600 are above 2008 Robeson’s upper bound, as well as those from CMS PI87.5–600 and PI75-600. Analogous enhancement in separation performance was observed in CO2/CH4 gas pair for CMS membranes, where PI100-600 and PI87.5–600 membranes are above the 2008 Robeson’s upper bound, which actually is used as a reference since it was built with polymeric membranes permeability and selec tivity results. Therefore, in PI/PBI blends when PBI is below 25 wt% in the precursor materials an enhancement in both gas permeability co efficients and gas selectivity is observed. On the other hand, when both polymers are at the same concentration (PI50-600) or PBI is in larger
Table 2 Permeability coefficients at 35 � C and 2 atm for PI/PBI blend CMS membranes pyrolyzed at 600 � C. CMS membrane PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600 PBI-600
Permeability (Barrer) He 960 526 459 197 170 152
O2 134 98 39 24 15 7.5
N2 16.2 12.2 4.9 4.3 2.6 1.7
Ideal Selectivity CH4 8.9 7.3 3.1 2.0 1.5 1.0
CO2 503 359 109 84 47 24
αO2/
αCO2/
αCO2/
N2
CH4
N2
8.3 8.1 8.0 5.6 5.8 4.4
56.5 49.4 35.2 42 31.3 24
31.0 29.4 22.5 19.5 18.0 14.1
5
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Fig. 7. Evolution of (a) PI and (b) PBI precursors into (c) pyrrolic and (d) carbazole strands during pyrolysis.
Fig. 8. CMS membranes formation from PI/PBI blend precursors. Adapted from Ref. [43].
concentration, pyrolysis to get a CMS membrane shows a minimum enhancement on gas selectivity even though gas permeability increases 2 to 3 times. Tables 3 and 4 show apparent diffusion and solubility coefficients for CMS membranes as well as their diffusion and solubility selectivity for O2/N2, CO2/CH4 and CO2/N2 gas pairs. The apparent diffusion coeffi cient (D) follows the order DO2 > DCO2 > DN2 > DCH4. PI100-600 membrane has the highest apparent diffusion coefficients. As pre sented in Table 4, blend composition in precursor membranes has a significant impact on diffusion coefficients while only a slight effect on solubility coefficients was observed. Diffusion coefficients decrease as PBI concentration increases in the blend. PI75-600 and PI50-600 CMS membranes have an important drop in diffusion coefficients. They drop more than 3 times below the one of PI100-600, apparently as a conse quence of an increase in structure packing due to a loss of the micro structure related to the observed d-spacing at 7.1 Å. Apparent solubility coefficients (S) for all carbon membranes have the order SCO2 > SCH4 > SO2 > SN2, following gas critical temperature order [42]. S in CMS membranes shows a small dependence with the composition showing a slight decrease as PBI concentration increases in PI/PBI blend.
Fig. 9. (a) O2/N2, (b) CO2/CH4 separation performance of CMS membranes from PI and PI/PBI and their polymeric precursors at 35 � C and 2 atm. PBI data from Giel, V. et al. [44]. P84 and PBI/P84(50/50) from Hosseini and Chung [25]. SBFDA-DMN Hazazi et al. [38].
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Table 3 Apparent diffusion coefficients for 600 membranes.
�
CMS
Diffusion coefficient (10-8 cm2/s)
O2
N2
CH4
CO2
αO2/N2
αCO2/CH4
αCO2/N2
PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600 PBI-600
9.0 7.1 2.9 1.9 1.36 0.73
1.40 1.07 0.46 0.38 0.25 0.18
0.23 0.20 0.08 0.07 0.05 0.03
8.6 6.3 2.3 1.9 1.2 0.61
6.4 6.6 6.3 5 5.4 4.0
38.7 31.5 28.8 27.1 24 20.3
6.1 5.9 5.0 5.0 4.8 3.4
Table 4 Apparent solubility coefficients for 600 membranes. CMSM
PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600 PBI-600
�
Diffusion Selectivity � � DA DB
3.4. PI/PBI CMS membranes apparent diffusion temperature dependence Fig. 11 shows the effect of temperature on apparent gas diffusion coefficients, Di, for CMS membranes derived from PI and PI/PBI blend precursors. Di increase as temperature increases, following an Arrhenius type behavior (equation (6)). � � EDi Di ¼ D0i exp (6) RT
C pyrolyzed CMS PI/PBI blend
Solubility coefficient 10 cm3(STP)/cm3cm Hg
2
O2
N2
CH4
CO2
αO2/N2
αCO2/CH4
αCO2/N2
14.9 14.0 13.5 12.8 10.9 10.3
11.7 11.4 10.6 11.3 10.4 9.4
38.8 33.6 38.8 32.9 30.0 31.7
58.5 50.5 47.6 44.1 39.5 39.0
1.3 1.2 1.3 1.1 1.0 1.1
1.5 1.5 1.2 1.3 1.3 1.2
5.0 4.4 4.5 3.9 3.8 4.1
CMS ideal gas selectivity α ¼ PPAB ¼
in CMS precursor blend. For each carbon membrane EPi follows the general trend CH4 > N2 > O2 > CO2 > He as is shown in Table 5. It is also seen that for CMS membranes EPi increases as the concentration of PBI increases in the CMS membrane precursors. Since EPi is the combination of diffusion activation energy and heat of solubility and since diffusion selectivity appear to be the principal factor for higher gas separation properties, a detailed analysis of diffusion selectivity. � � DA DB , will be performed in the next section.
C pyrolyzed CMS PI/PBI blend
Solubility Selectivity � � SA SB
Tables 7 and 8 present the values of activation energy for gas diffusion, EDi and the pre-exponential factor, D0i, for CMS membranes from PI/PBI blends. It can be observed that PBI concentration in CMS precursor affects EDi as a function of penetrant kinetic diameter since EDi follow the order CH4 > N2 > O2 > CO2 for PI100-600 and PI87.5–600. Additionally, it is observed that EDi increases as PBI concentration in creases in membrane precursors, showing an increment of their barrier properties and resulting in a higher EDi for gas diffusion through the CMS membranes where N2 and CH4 are the gases with the highest EDi increase as PBI concentration in the precursor goes up. It is clear from Fig. 11 that an important drop in Di occurs between PI87.5–600 and PI75-600 membranes, an indication of the shift lowering the gap of the graphitic structure as was discussed above.
� � � � DA SA can be viewed as the DB X SB
product of diffusion selectivity and solubility selectivity. In CMS mem � � branes diffusion selectivity, DDAB , is governed by the pore distribution and size and shape of the gas molecules. On the other hand, solubility � � selectivity, SSAB , depends on gas condensability and their interaction
3.5. PI/PBI CMS membranes diffusion selectivity: entropic and energetic selectivities
with the membrane material [22]. From the results in Tables 3 and 4, diffusion selectivity in these CMS membranes from PI/PBI blends is the main contributing factor to the overall permselectivity, which decreases as PBI increases in PI/PBI blends. Thus, diffusion selectivity appears as the principal factor governing gas separation properties in the CMS membranes from PI/PBI blends.
In CMS membranes from PI/PBI blends, diffusion selectivity,
� � DA DB , is
being labeled as the main contributing factor to the overall permse lectivity. Consequently, higher diffusion selectivity leads to improved gas separation performance [33]. According to Singh and Koros [34] diffusion selectivity can be factored into an “energetic selectivity” and an “entropic selectivity” as described by equation (7). � � � � � � DA D0A EDA EDB SDA SDB EDA EDB ¼ exp ¼ exp ⋅exp (7) DB D0B RT R RT
3.3. PI/PBI CMS membranes gas permeability temperature dependence Activation energy and entropic selectivity extracted from. � � DA DB , as reported by Singh and Koros [34], can be a tool in order to
where: � � is the entropic selectivity and. exp SDA R SDB ¼ DD0A 0B � � EDA EDB exp is the energetic selectivity. RT
understand gas separation and transport properties in PI/PBI CMS membranes [33,35,48]. It involves the measurement of gas permeability and apparent diffusion coefficients as a function of temperature which are given below. CMS membranes gas permeability coefficients, P, for PI and PI/PBI blends were examined between 35 and 65 � C at 2 atm upstream pres sure. Fig. 10 shows gas permeability coefficients’ Arrhenius plots as a function of temperature for He, O2, N2, CH4 and CO2 of CMS membranes derived from PI and PI/PBI blends. It can be observed that P for all gases in CMS membranes increases with temperature following closely an Arrhenius type behavior as indicated by equation (5). � � EPi Pi ¼ P0i exp (5) RT
Energetic selectivity is referred to the difference in the diffusion activation energies of the two penetrants, while entropic selectivity re flects differences in the mobility of penetrant gases due to penetrant shape and CMS membranes structure capacity to restrict their rotational and internal vibrational degrees of freedom [34,35,48]. Table 9 summarizes energetic and entropic selectivity for PI/PBI blends CMS membranes for gas pairs O2/N2 and CO2/CH4 at a temper ature range between 35 and 65 � C, measured at 2 atm upstream pres sure. Values were calculated from Equation (7) and from EDi and D0i values reported in Tables 7 and 8 It can be observed that energetic and entropic selectivity have a clear dependence with PI concentration in the PI/PBI blends. CMS membranes energetic selectivity values drop as PI concentration increases in the membranes as the result of the increased distance between graphitic strata which in turn increases the population
The activation energy for permeation, EPi, and the corresponding pre-exponential factors, P0i, for He, O2, N2, CH4 and CO2 are shown in Tables 5 and 6. EPi depends on both penetrant size and PBI concentration 7
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Fig. 10. Gas permeability coefficients temperature dependence for CMS membranes derived from PI and PI/PBI blend (a) He, (b) O2, (c) N2, (d) CH4 and (e) CO2. Table 5 Activation energy for Permeation for He, O2, N2, CH4 and CO2 in CMS mem branes from PI and PI/PBI blend precursors. CMS membranes PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600
Table 6 Permeation pre-exponential factors for He, O2, N2, CH4 and CO2 in CMS mem branes derived from PI and PI/PBI blend precursors.
Ep (kJ/mol) He
O2
N2
CH4
CO2
3.7 5.0 5.5 6.5 8.5
8.7 9.3 12.4 14.3 16.3
9.4 11.6 15.8 18.6 21.0
10.4 12.7 18.4 20.7 22.4
6.0 7.4 10.7 12.4 14.2
of larger micropores. A decrease in the difference in energy necessary for gas permeation and decrease of activation energy values for gas diffu sion through the CMS membrane results in a higher gas permeability and diffusion coefficients since there is a minimum resistance to gas flow. Thus, the compact CMS graphitic structures from PBI, that have a larger energetic selectivity value, show a drastic change with the presence of PI particularly above 75 wt% dropping rapidly towards low activation energy differences and they become small for gas pairs O2/N2 and CO2/
CMS membranes
P0 (barrer) He
O2
N2
CH4
CO2
PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600
4.1 � 103 3.7 � 103 3.9 � 103 2.5 � 103 4.8 � 103
4.0 � 103 3.7 � 103 4.9 � 103 6.3 � 103 8.6 � 103
6.4 � 102 1.1 � 103 2.3 � 103 6.0 � 103 9.5 � 103
5.1 � 102 1.0 � 103 4.2 � 103 7.4 � 103 9.5 � 103
5.2 � 103 6.5 � 103 7.2 � 103 1.1 � 104 1.2 � 104
CH4. According to that, lower energetic selectivity favors higher gas permeability and contributes to higher selectivity for the studied gas pairs. The last statement agrees with reports by different authors [14,33, 35,48]. Entropic selectivity shows an increase with increasing PI concen tration in CMS membranes from PI/PBI blends. This result agrees with an increase in diffusion selectivity and permeation selectivity for the studied gas pairs. As the value of entropic selectivity decreases towards 8
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Journal of Membrane Science 597 (2020) 117703
Fig. 11. Apparent diffusion coefficients temperature dependence for CMS membranes derived from PI and PI/PBI blend (a) O2, (b) N2, (c) CH4 and (d) CO2. Table 7 Activation energy for diffusion, EDi, for O2, N2, CH4 and CO2 in CMS membranes from PI/PBI blends. CMS membranes
Table 9 Energetic and entropic selectivity for CMS from PI/PBI membranes for gas pairs O2/N2 and CO2/CH4 in the temperature range of 35–65 � C.
EDi (kJ/mol)
PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600
CMSM
O2
N2
CH4
CO2
16.2 17.6 21.2 23.2 26.4
16.8 19.1 23.5 26.1 29.9
19.2 22.1 27.5 30.2 33.2
15.1 17.2 21.2 23.0 25.9
Energetic selectivity PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600 Entropic selectivity PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600
Table 8 Diffusion pre-exponential factors for O2, N2, CH4 and CO2 in CMS membranes from PI/PBI blends. CMS membranes PI100–600 PI87.5–600 PI75–600 PI50–600 PI25–600
D0 (cm2/seg) O2
N2
5.49 � 103 6.96 � 103 1.11 � 104 1.65 � 104 4.15 � 104
9.72 1.82 4.50 1.01 2.97
CH4 � 102 � 103 � 103 � 104 � 104
4.11 � 1.10 � 3.70 � 9.07 � 2.18 �
CO2/CH4
1.14–1.13 1.74–1.66 2.51–2.31 3.04–2.75 3.82–3.39
5.0–4.3 6.7–5.7 11.9–9.57 16.2–12.7 17.2–13.4
5.64 3.82 2.47 1.63 1.40
7.70 4.77 2.42 1.68 1.36
line. For CMS membranes from PI/PBI blends, the increase on this entropic selectivity factor is closely related with the presence of a differentiated graphitic structure, depicted in Fig. 8 as PBI is found in larger concentration and becomes the dominant phase in the blend. The difference in entropic selectivity between CMS membrane PI and those derived from PI/PBI blends seems to be due to the difference between these differentiated graphitic structures, indicating that the main contribution to the diffusion selectivity for CMS membranes comes from the entropic selectivity while low energetic selectivity contributes to a higher gas diffusion. These values agree with the results shown in the gas separation performance, where PI100-600, PI87.5–600 and PI75-600 CMS membranes had the best relationship between permeability and gas selectivity and they are located above the upper bound.
CO2 102 103 103 103 104
O2/N2
3.16 � 103 5.25 � 103 8.95 � 103 1.50 � 104 2.96 � 104
unity, the permeation selectivity decreases. In fact, in the case of the CO2/CH4 gas pair, entropic selectivity is close to unity in PI/PBI blends with larger content of PBI and selectivity is poor due to a similar mobility of the molecules. On the other hand, larger differences in molecules mobility are found for PI/PBI blends PI100-600 and PI87.5–600, which are the ones that lie above Robeson upper bound, while those for PI50-600 and PI25-600, which are closer to unity are found below the upper bound limit. The presence of PI increases entropic selectivity for O2/N2 and CO2/CH4 gas pairs. The result in dicates that differences in gas mobility restrictions have to be larger than 2.4 for the entropic selectivity to go above 2008 Robeson’s upper bound
4. Conclusion Carbon molecular sieve membranes, CMSM, were prepared by py rolysis at 600 � C from PI DPPD-IMM (PI) and PI/PBI blends. WAXD analysis revealed a change in the ultramicroporous structure of CMS 9
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Journal of Membrane Science 597 (2020) 117703
membranes as the concentration of PI in the precursor membrane in creases as the result of the formation of pyrrolic strands, which form more open ultramicropores in comparison with carbazole-type strands, derived from PBI. PBI concentration in the precursor showed a negative effect on permeability and also in gas pair selectivity for carbon mem branes which was attributed to carbazole-type strands densification. So that PI100-600 carbon membrane derived from pure polyimide had the highest gas permeability values (PCO2 ¼ 503 Barrer and PO2 ¼ 134 Barrer) and gas pair selectivity (αCO2/CH4 ¼ 56.5 and αO2/N2 ¼ 8.3). The effects of test temperature on gas permeability and diffusion coefficients were analyzed to provide insights into the factors governing gas sepa ration properties in CMS membranes from PI/PBI blends. An increase in permeation, as well as diffusivity, occurs with increasing testing tem perature. Carbon membranes derived from precursors with low PBI content exhibit entropic selectivity higher than 2.4, indicating that the discrimination ability of CMS membranes increases as PI concentration increases in blend precursors, producing an enhancement of gas permeability and separation factor in PI75-600, PI87.5–600 and PI100600 for O2/N2 and CO2/CH4 gas pairs, which surpass the upper bound. Clearly, the main contribution to the increase of diffusion selectivity and, therefore, the increment of permseletivity is the entropic selec tivity, which is the result of the presence of differentiated carbon structures that depend on blend composition. Further studies of the role of differences in carbon structures composition and carbon strands ar rangements in CMSM from polymer blend precursors have to be carried out to properly understand their effect in gas permeability and selectivity.
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Authors contribution statement Jos�e Manuel P� erez-Francisco: Responsible for preparation and par tial characterization and gas transport properties of CMSM membranes and discussion of the characterization results. Jos�e Luis Santiago-García: Responsible for polyimide synthesis characterization and polymeric blends preparation. María Isabel Loría-Bastarrachea: Responsible for characterization of polymeric membranes and CMSM blends and instrumental runs and characterization (WAXD, Raman and Thermal analysis). Donald R. Paul: Contribution on blend interactions and theoretical blend behavior expected as well as transport properties. Benny D. Freeman: Contribution on blend gas permeability and selectivity structure property relationships for the gas transport behavior. Manuel Aguilar-Vega: In charge of analysis of transport properties from temperature and overall project results. Declaration of competing interest The authors declare that they do not have any conflict of interest to declare. Acknowledgments J.M.P.F. gratefully acknowledges a scholarship from CONACyT (Mexico’s National Council for Science and Technology) under Grant 354777 and the financial support from Tecnol� ogico Nacional de M�exico (Grant 537.18-PD). M. Aguilar-Vega is grateful for a fellowship from Fulbright-Garcia-Robles (COMEXUS) for a sabbatical leave at UT Austin. The authors acknowledge Dr. Patricia Quintana for allowing access to The National Laboratory of Nano and Biomaterials (LANNBIO) from CINVESTAV-IPN, Merida Unit, grants FOMIX-YUCATAN 108160 and CONACYT 123913, in which X-ray diffraction analysis was carried out with the technical assistance of M. C. Daniel Aguilar-Trevineo.
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