Block Copolymer Derived Membranes for Sustained Carbon Dioxide

Sep 13, 2010 - Kitty Nijmeijer and Matthias Wessling. Membrane Technology Group, Institute of Mechanics, Processes and Control Twente, University of ...
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Ind. Eng. Chem. Res. 2010, 49, 12051–12059

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Block Copolymer Derived Membranes for Sustained Carbon Dioxide-Methane Separations Sarah E. Querelle,† Liang Chen,‡ Marc A. Hillmyer,‡ and Edward L. Cussler*,† Department of Chemistry and Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, Minneapolis, Minnesota 55455

Kitty Nijmeijer and Matthias Wessling Membrane Technology Group, Institute of Mechanics, Processes and Control Twente, UniVersity of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

Using a reactive block polymer precursor, membranes with a bicontinuous nanostructure containing a poly((N,Ndimethylamino)ethyl methacrylate) phase within a cross-linked poly(cyclooctene) framework were formed. These membranes were evaluated for their CO2 selectivity over CH4. Low pressure experiments demonstrated that the CO2 and CH4 permeabilities remain the same with pure and mixed gas feeds. This shows that the membranes are able to maintain their selectivity for CO2. This selectivity was only slightly decreased at moderate temperature (35 °C) and higher pressures (up to 40 bar) using a 50/50 CO2/CH4 mixed gas feed. The bicontinuous block polymer membranes resist the plasticization effect by using a cross-linked phase to sustain the properties of the selective block, and not by changing chain mobility through new chemistry. These bicontinuous block polymer membranes may be valuable for CO2 removal from natural gas streams. 1. Introduction One major goal for synthetic membrane processes is costeffective purification of gas mixtures. The expense and energy use associated with conventional separation procedures such as cryogenic distillation, condensation, pressure swing adsorption, or amine absorption have spurred research into these more energy efficient processes.1 Membrane systems are attractive because they do not involve phase changes, do not require separation agents, and are typically compact (high area-tovolume ratio). Current examples include the separation of hydrogen in petrochemical plants, of nitrogen from air and of organic vapors from nitrogen.2 The success of membrane-based separations relies heavily on high gas fluxes and on sustainable membrane selectivity, which results from permeability differences of the gases under consideration. The permeability of a membrane for the gas of interest is, in turn, the product of the solubility and the diffusion coefficient of the gas in the membrane. Which factor is more important depends on the gas involved. In general, for condensable gases like water vapor, the membrane’s selectivity is dominated by differences in the solubility. For permanent gases like hydrogen, the selectivity is usually controlled by differences in the diffusion coefficient. This heuristic, promoted by Donald R. Paul and others, has been a reliable guide for membrane separations.3 The membrane’s potential to perform a separation is often inferred from the ratio of its permeabilities obtained for pure gases, called the “ideal selectivity”. The “real selectivity” is obtained from the exposure of the membrane toward a mixture of these gases. When exposed to gas mixtures, membranes based on glassy polymers frequently suffer from a drastic decrease in real selectivity, as compared to the ideal selectivities obtained from pure gas permeation experiments. The reasons for the * To whom correspondence should be addressed. Tel.: +16126251596. E-mail: [email protected]. † Department of Chemical Engineering and Materials Science. ‡ Department of Chemistry.

difference between the actual and ideal selectivity can vary. In some cases, it may be due to enhanced diffusion of gases, especially of the less permeable component, caused by an increase of the polymer chain segmental mobility associated with the sorption of the highly penetrating molecule (plasticization). In other cases, the loss of selectivity may occur because of competitive sorption between the gases. As a consequence, the presence of the less penetrating species induces a diminution in the permeability of the more permeable component in the membrane.4,5 In this work, we explore sustaining the properties of a permeable polymer by making it one phase in a bicontinuous structure. One method that tries to keep the selectivity when the membrane is exposed to a mixed gas is the chemical crosslinking but this process causes a decrease in permeability at the same time.6 A second method, by Paul and co-workers, uses pendant groups in the para position of polystyrene chains, such as methoxy groups to enhance permeability and selectivity with respect to the unsubstituted polymer.7 These efforts and others like them aim to adjust the free volume available for diffusion between adjacent polymer chains. They change the distance between the chains by less than 0.1 nm. In contrast, our goal is not to change the free volume of the permeable phase, but to sustain this phase so that it keeps the properties found with pure gases. We do so by trapping a continuous permeable phase tens of nanometers in size within a second continuous phase of a cross-linked polymer. We mean to ensure that the permeable phase is unaltered; we do not seek to tune its selectivity. Our strategy is applied to the separation of carbon dioxide from methane because the ideal selectivity of the current membranes is 80 or higher, but their real selectivity usually drops to 4 or lower.8 The carbon dioxide/methane separation is important as part of the treatment of natural gas. As supplies of natural gas dwindle, greater fractions of carbon dioxide are typically present. Currently, impure natural gas is pumped to a central processing plant, where the CO2 is separated by absorption into alkanol amine solutions. This process is effective

10.1021/ie100461k  2010 American Chemical Society Published on Web 09/13/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010 Table 1. Summary of the Composition and Characteristics of the Membranes Used in This Study

Figure 1. Structure of the poly(norborenylethylstyrene-s-styrene)-b-poly((N,Ndimethylamino)ethyl methacrylate) P(N-s-S)-b-PDMAEMA block polymer and the cyclooctene (COE) used to obtain the cross-linked poly(norborenylethylstyrene-s-styrene)/poly(cyclooctene) P(N-s-S)/PCOE phase.

but energy intensive due to evaporation and loss of the solvent. The solvent’s corrosiveness and low oxidative stability cause additional problems. Because of moderate temperatures (0-60 °C) and moderate to high feed pressures (50 bar) in natural gas streams, membrane-based separations can separate the gas at the wellhead, reducing the pumping and transportation costs and possibly allowing the CO2 to be reinjected. Natural gas separation must also involve removal of water and organics (BTEX), accomplished by a second scrubbing with solvents like triethylene glycol.9,10 The membrane developed here does not deal with this separation. In this exploratory work, we focus on bicontinuous block polymer templated membranes where one cross-linked phase resists changes in the properties of the selectively CO2 permeable block when exposed to mixed gases at high pressures. We have already reported improvements in ammonia separation from nitrogen and hydrogen using a related approach.11 As in the ammonia experiments, the less permeable phase is a cross-linked poly(norborenylethylstyrene-s-styrene)/poly(cyclooctene) P(Ns-S)/PCOE, but this time the permeable phase is a poly((N,Ndimethylamino)ethyl methacrylate) (PDMAEMA). We rely on a reaction-induced spinodal decomposition, which can yield a variety of two phase morphologies depending on the relative rates of the chemical reaction and the phase separation.12 Our research will be successful if the ideal selectivity equals the real selectivity. In the experimental section below, we describe the preparation, characterization, and permeability measurements of this membrane. In the next results section, we report membrane performance data for low and high pressure mixed gas experiments and we compare the ideal and real selectivities experimentally determined. Finally, we discuss how a useful operating system based on this membrane can be developed. 2. Experimental Section The membranes used contain a less permeable cross-linked poly(norborenylethylstyrene-s-styrene)/poly(cyclooctene) P(Ns-S)/PCOE phase and a highly permeable poly((N,N-dimethylamino)ethyl methacrylate) (PDMAEMA) phase. This two-phase structure is obtain from the poly(norborenylethylstyrene-sstyrene)-b-poly((N,N-dimethylamino)ethyl methacrylate) P(Ns-S)-b-PDMAEMA block polymer and the cyclooctene (COE) shown in Figure 1. The procedure to synthesize the P(N-s-S)b-PDMAEMA block polymer, which serves as the structural template for membrane formation, is described elsewhere.13,14 1 H NMR spectra were acquired for the polymer on a Varian 300 VI spectrometer at room temperature using 1 wt % samples dissolved in deuterated chloroform (Cambridge). The P(N-sS)-Br size exclusion chromatography (SEC) experiment was performed at 35 °C in CHCl3 with a flow rate of 1 mL min-1

sample

P(N-s-S)-bPDMAEMA (mg)

COE (mg)

fPDMAEMA (wt %)

thickness (µm)

M143.7/185 M255.4/115 M355.2/25 M450.4/44

300 400 100 300

223 150 38 153.3

43.7 55.4 55.2 50.4

185 115 25 44

The first column identifies the membrane. The second and third give the amounts of raw materials used in the preparation. The PDMAEMA weight fraction in the membrane (fPDMAEMA) is given in the fourth column. The last column reports the membrane thickness.

using a Hewlett-Packard 1100 series liquid chromatograph equipped with three PL gel 5 µm mixed columns and a HewlettPackard 1047A refractive index detector. The SEC instrument was calibrated with polystyrene standards (Polymer Laboratories). P(N-s-S)-b-PDMAEMA SEC data were acquired on a SEC system containing 1 vol % N,N,N′,N′-tetramethylethylenediamine and consisting of a Wyatt Optilab RI detector, a Wyatt Dawn multiangle light scattering detector, and three Phenolgel columns of 105, 104, and 103 Å pore sizes. THF at 40 °C was used as the mobile phase with a flow rate of 1 mL min-1. The refractive index of this copolymer was estimated from the values of polystyrene and poly(dimethylaminoethyl acrylate) in THF at 0.130 mL g-1. Differential scanning calorimetry (DSC) experiments were conducted on a TA Instruments Q1000 with a heating rate of 10 or 20 °C min-1. Small-angle X-ray scattering (SAXS) experiments were conducted on a 230 cm custom built beamline at the University of Minnesota. Cu KR X-rays (λ ) 1.542 Å) were generated through a Rigaku RU-200BVH rotating anode fitted with a 0.2 × 2 mm2 microfocus cathode and Franks mirror optics. Polymer samples were placed in a rubber O-ring and sandwiched between two Kapton films (Dupont). The domain spacing (D) was calculated on the basis of the principal peak at q*: D ) 2π/q*. Transmission electron microscopy (TEM) micrographs were acquired on a JEOL 1210 with an accelerating voltage of 120 kV. Films were cryo-microtomed at -120 °C and then stained with a 4 wt % OsO4 aqueous solution for 15 min at room temperature. Membranes were made by adding 0.3 g P(N-s-S)-b-PDMAEMA in 1.6 mL of tetrahydrofuran (THF) and stirring until a homogeneous solution was obtained. Cyclooctene (COE) (0.23 g) was then added to the polymer solution and stirred for another 20 min. A solution of the second generation Grubbs metathesis catalyst (2.5 mg) in THF (0.4 mL) was quickly added to the mixture, stirred for 20 s, and the entire solution was cast onto an aluminum pan. Gelation occurred within 5 min. The mixture was evaporated and cured at room temperature overnight and then at 90 °C for 1 h. The transparent membrane was peeled off from the pan and dried under vacuum at room temperature. Membranes differing in their P(N-s-S)-b-PDMAEMA/COE ratio were investigated. The membranes’ formulations and characteristics are summarized in Table 1. The membrane thicknesses, measured with a micrometer, were reproducible to within 3 µm. 2.1. Low Pressure Gas Diffusion with Pure and Mixed Gases. Circular disks of the membrane cut from the larger cast sheets were mounted on a stainless steel diaphragm cell consisting of two 15 cm3 compartments separated by the membrane. The membrane package, comprising the block polymer film, a piece of filter paper, and a metallic grid, was mounted between two steel disks and sealed with a Buna N O-ring (Buna N, RT/Dygert International, Edina, MN). The inner diameter of the O-ring was 1 cm, giving a membrane area of 0.79 cm2. This assembly was placed between the two compartments of the cell.14 The pressure difference between

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the compartments, monitored as a function of time, was plotted according to

( )

∆p0 1 t ln )P 1 1 ∆p l A + Vu Vd

(

)

(1)

yCO2/yCH4 xCO2/xCH4

(3)

where xi and yi are the concentrations of component “i” in the upstream and downstream side, respectively. 3. Results

where A is the membrane area available for diffusion, Vu and Vd are respectively the upstream (highest pressure) and downstream (lowest pressure) compartment volumes, t is the time, ∆p0 is the pressure difference between the cell compartments at time zero, ∆p is that difference at time t, P is the permeability, and l is the membrane thickness. For pure gas experiments, both compartments were flushed for 1 h with the gas to be studied (CO2 or CH4). After flushing, 200 kPa (absolute) of this gas was put in both the upstream and downstream compartments. Then the downstream compartment was emptied to 100 kPa (absolute), the cell was sealed, and the data acquisition program was started. Mixed gas experiments were different. After the CO2 was used to flush the cell for 1 h, 200 kPa (absolute) of CO2 was put in both the upstream and downstream compartments. After CH4 was introduced to bring the total upstream pressure to 300 kPa (absolute), the cell was sealed and the data acquisition program was started. The membrane selectivity R was calculated from their permeability ratio according to R)

R)

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PCO2 PCH4

(2)

This relation was used for both pure and mixed gases. 2.2. High Pressure Gas Diffusion with Mixed Gases. The experiments in the low pressure cell were supplemented by a second set of experiments at higher pressures performed at 35 °C in a different stainless steel cell.15,16 The membrane package was composed of the block polymer film, a layer of filter paper, and a porous metallic plate supporting the membrane. This plate guarantees mechanical strength for the high pressure experiments. The effective membrane area was 11.47 cm2. The two compartments of the cell were sealed with Viton O-rings. In this case, gas permeation measurement started with evacuation of the whole cell. The mixed gas flowed through the upstream compartment at a constant high feed pressure and composition. The downstream compartment of volume 148 cm3 was initially under vacuum. The gas permeability through the membrane was calculated from the pressure increase over time in the downstream volume. The compositions of the feed and the permeate were also measured by means of a Varian gas chromatograph equipped with a HayeSep Q column. For experiments in this high pressure cell, new membranes (M450.4/44) were prepared. CO2/CH4 (50/50 vol %) (Praxair) was used as a feed. In every experiment, two similar membranes were measured simultaneously. Prior to permeation experiments, the cells were thoroughly degassed. Then, the permeances for N2 and O2 at 400 kPa (4 bar) were measured and used to check for defects in the membranes. After that, the membranes were degassed again for at least 1 h and they were exposed to the CO2/ CH4 mixture at 150 kPa (1.5 bar) for at least 6 h. Then, the mixed gas permeability of each component was evaluated. The experiments were completed by another 30 min degassing step and the measurement of the N2 permeance repeated (about 16 h). These steps were repeated up to the maximum CO2/CH4 gas pressure of 4 MPa (40 bar). The separation factor R was calculated according to

As stated in the Introduction, membrane-based CO2/CH4 separations are often disappointing because the real selectivity, based on mixed gas feeds, is much lower than the “ideal selectivity” inferred from experiments with pure gases. Block polymer-based membranes are good candidates for superior selectivity because they can self-assemble into a variety of nanostructures depending on the volume fraction of the different blocks and on the processing used to prepare the membrane. In our case, we attempted to retain CO2 selectivity using a block polymer-templated membrane containing PDMAEMA and P(Ns-S)/PCOE nanoscopic and interpenetrating domains. The PDMAEMA phase has a good ideal selectivity for CO2 over CH4,17 while the more rigid P(N-s-S)/PCOE phase is expected to be much less permeable to both CO2 and CH4.7 3.1. Membrane Structure. We first consider measurements which elucidate the polymer and membrane structures. The 1H NMR spectra of the resulting P(N-s-S)-Br macroinitiator and P(N-s-S)-b-PDMAEMA block polymer indicate that these structures are well-defined. From the 1H NMR spectrum of P(Ns-S)-Br, signals can be attributed as follows (δ ppm): 6.2-7.3 (b, PhsH), 5.9-6.2 (b, -CHdCH-), 4.3-4.6 (b, -CHsBr), 2.8 (b, -CHsCHdCHsCH-), 2.5 (b, PhsCH2-), 0.6-2.0 (b, other CH and CH2). The peaks in the 1H NMR spectrum of P(N-s-S)-b-PDMAEMA are also identified as, (δ ppm): 6.2-7.3 (b, PhsH), 5.9-6.2 (b, -CHdCH-), 4.05 (b, OsCH2-), 2.55 (b, O-CH2sCH2), 2.27 (b, -N(CH3)2). Other signals were not specified. The molecular weights of the different blocks from 1H NMR and SEC characterizations are in good agreement. The P(N-sS)-Br number average molecular weight derived from 1H NMR end group analysis gives MnP(N-s-S)-Br ) 7 kg mol-1, while a CHCl3 SEC gives 6.5 kg mol-1 (PDI ) 1.45). The MnPDMAEMA determined from the 1H NMR spectrum of P(N-s-S)-b-PDMAEMA using the result for the P(N-s-S) block gives 22.4 kg mol-1. The MnP(N-s-S)-b-PDMAEMA characterized in a THF SEC gives 32.9 kg mol-1 (PDI ) 1.56), which using the result for the P(Ns-S)-Br suggests a PDMAEMA number molecular weight of 26.4 kg mol-1. In DSC analysis, the P(N-s-S)-Br homopolymer displays a glass transition temperature (TgP(N-s-S)-Br) at 88 °C, but the P(Ns-S)-b-PDMAEMA block polymer only exhibits a single (TgP(N-s-S)-b-PDMAEMA) at 29 °C, implying no phase segregation between the P(N-s-S) and PDMAEMA blocks. According to the literature,18 PDMAEMA (Mn > 100 kg mol-1) has a glass transition temperature (TgPDMAEMA) of 18 °C. If the P(N-s-S) and PDMAEMA blocks are completely miscible in the P(N-sS)-b-PDMAEMA block polymer, the Fox equation19 predicts a single (TgP(N-s-S)-b-PDMAEMA) at 32 °C based on the glass transition temperatures of the two blocks. This value is consistent with experiment, given that the (TgPDMAEMA) of a PDMAEMA with MnPDMAEMA ) 22.4 kg mol-1 could be lower than 18 °C. Additionally, the reported χ between PS and PDMAEMA is 0.055 at about 160 °C,20 so (χN) in P(N-s-S)-b-PDMAEMA at room temperature would be less than 11 at a volume fraction of PS around 30%. Hence, no phase segregation in this P(Ns-S)-b-PDMAEMA block polymer would be expected,21 and none is observed.

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Figure 2. DSC traces of M143.7/185 and M255.4/115 membranes (second heating run).

Figure 3. SAXS profiles of P(N-s-S)-b-PDMAEMA, M143.7/185, and M255.4/ at room temperature. The signals are shifted vertically.

115

DSC analyses on the resultant membranes are consistent (Figure 2). In M143.7/185, the Tg of PDMAEMA is embedded in the melting peak of PCOE. For the DSC analysis of M255.4/115 with less PCOE incorporated, the matrix P(N-s-S)/PCOE phase is nearly amorphous. The Tg endotherm observed around 5 °C is possibly associated with the Tg of PDMAEMA. The lower Tg for PDMAEMA versus the reported value of 18 °C can be attributed to the low molecular weight of PDMAEMA and to impurities like PCOE oligomers in the PDMAEMA phase. SAXS analysis of this copolymer agrees with the DSC. It gives no scattering peak between room temperature and 150 °C as in the scattering profile at room temperature shown in Figure 3. This suggests no phase separation between the P(Ns-S) and PDMAEMA blocks for the block polymer alone. SAXS experiments were also conducted to determine the morphology in the membranes. For example, membranes M143.7/185 and M255.4/115 give a single scattering peak (Figure 3), indicating microphase separation between the cross-linked P(N-s-S)/PCOE and PDMAEMA phases. The domain spacings inferred by the peak position (D ) 2π/q*) are 45 nm for M143.7/185 and 25 nm for M255.4/115. To probe this microstructure further, TEM experiments were conducted on cryo-microtomed slices cut from the membrane selectively stained with OsO4. The TEM images for sample M143.7/185 (Figure 4) show the two distinct regions typical for

Figure 4. TEM images of M143.7/185. These images show the bicontinuous microstructure of the block polymer membrane. The dark regions correspond to the P(N-s-S)/PCOE phase, and the bright areas are the PDMAEMA nanodomains.

these films. The dark regions correspond to the stained P(N-sS)/PCOE phase, while the bright areas are the PDMAEMA nanodomains. The structure generated is bicontinuous, with PDMAEMA nanodomains percolating through a P(N-s-S)/ PCOE cross-linked matrix.22,23 The PDMAEMA domain size, based on TEM images, approximately equal for all films, is estimated to be around 20 nm. 3.2. Permeability Experiments. The permeability data for all the membranes exhibit similar trends. As a representative example, data for the M355.2/25 block polymer membrane in single and mixed gas low pressure experiments are shown in Figure 5. Duplicate runs performed for all experiments (after an additional 20 min of flushing) show reproducibility better than 10%. For mixed gas experiments in the low pressure cell, the data correspond to the pressure change caused by an initial CH4 pressure difference at a constant CO2 pressure in the upstream and downstream compartments, i.e., the same initial CO2 partial pressure in both the upstream and downstream compartments. We assume that the pressure variation in both compartments is a consequence of CH4 diffusion only and that the gas flux is independent of the presence of the other gas. This is equivalent to assuming that the main terms in the matrix diffusion coefficient dominate and that the cross term diffusion

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Figure 5. Diffusion data for single and mixed gas experiments across the M355.2/25 block polymer membrane: CO2 (b); CH4 (9); CH4 in CO2 (∆).

coefficients equal zero.24 The linearity of the data in Figure 5, indicates that the flux of species across the membrane quickly reaches the steady state in comparison to changes in the donating and receiving compartments. For thicker films or membranes with lower permeabilities, this assumption may not be valid early in the experiment and a linear trend may be observed only after some time lag.25 In our case, there is no evidence of such a lag. Gas permeance, permeability and selectivity can be extracted from plots like that shown in Figure 5. These data from the low pressure cell are reported for all membranes in Table 2. They show CO2 permeabilities ranging from 46.9 to 56.3 barrer and selectivities from 4.6 to 12.0. These values do not agree closely with data previously reported giving PCO2 ) 22 barrer and RCO2/CH4 ) 22.17 These differences are probably due to crosslinking and water.26 Gas diffusion in a polymer membrane depends on free volume. When the material is dry, this volume is small. In a hydrated membrane, the swelling induced by water may enlarge the free volume and give more mobility to the polymer chains. At the same time, CO2 dissolves easily in water. Both factors can alter selectivity. For example, permselectivity for CO2 over N2 of plasma-grafting PDMAEMA membranes onto microporous polyethylene substrate was measured in both dry and water swollen conditions. In the dry membrane, the selectivity of CO2 over N2 was 30, while for the water swollen membrane it reached 130.27 Table 2 shows that the real selectivity obtained from our mixed gas experiments can equal the ideal selectivity found as the ratio of pure gas permeabilities. More specifically, the CH4 diffusion data in Figure 5 are the same with and without the presence of CO2 in the cell. This effect depends dramatically on membrane composition. As Table 1 shows, membrane M143.7/185 has a PDMAEMA weight fraction of 43.7 wt %, while the fPDMAEMA of M255.4/115 and M355.2/25 is about 55 wt %. As shown in Table 2, M255.4/115 and M355.2/25 membranes have lower CH4 permeabilities than M143.7/185. However, the CO2 permeability is nearly the same for all these membranes. Going from M143.7/185 to M255.4/115 membranes changes two parameters. First, the amount of selective PDMAEMA phase is increased, though its domain size remains the same (about 20 nm). This increases the surface per unit area of the CO2 permeable phase and improves the permeability. Second, the amount of scaffold is reduced, decreasing the impermeable P(Ns-S)/PCOE domain size. Thus, as the size of the less permeable domain diminishes and the size of the more selective block is kept constant, the membrane should become more selective and it does. The compositions of the membranes M255.4/115 and M355.2/25 are close to each other (Table 1) and their permeabilities and

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selectivities are almost the same. However, the membrane M355.2/25 exhibited a permeance about 4 times greater than the sample M255.4/115. The M355.2/25 membrane is thinner by about the same factor. Thus, these membranes’ fluxes can be increased by reducing their thicknesses. The surface per unit area occupied by each component in these two membranes should not vary strongly and their gas separation performances remain in the same range. However, according to Table 2, going from M255.4/115 to M355.2/25 the CO2 and CH4 permeabilities respectively decrease by 20 and 15%. A slight compositional modification of the membrane could have an effect on its gas separation response. Indeed, the PDMAEMA weight fraction diminution, which is the more permeable block, comes with a decrease in CO2 and CH4 permeabilities. In the same time, because of its poorer separation properties, a slight increase in the scaffold content can have a negative impact on the membrane selectivity. This last comment explains the more pronounced diminution of the CO2 permeability relative to CH4 observed in this case. The permeability and selectivity of the sample M450.4/44 are shown in Figure 6 for pressures ranging from 1.5 to 40 bar in mixed gas permeation experiments at 35 °C. This membrane exhibits a CO2 permeability of 54.7 barrer and a selectivity of 11.3 at 1.5 bar and 35 °C for a 50.1/49.9 CO2/CH4 feed. These results are in good agreement with the low pressure experiments reported in Table 2. The results in Figure 6 also show that the CO2 permeability increases by 15% when the total feed pressure is increased from 1.5 to 40 bar, an increase of 2500%. The CH4 permeability increases almost 100%, so the ideal mixed gas selectivity decreases by a factor of 2 over the same pressure range. These results are explored in more detail in the discussion that follows. 4. Discussion The results above show that the bicontinuous block polymer membranes retain more of the ideal CO2/CH4 selectivity in mixed gas studies in contrast to conventional membranes. This was also observed for the NH3/H2 and NH3/N2 separations as reported earlier.11 The block polymer chemistry used to achieve these gains makes continuous permeable phases that are 20 nm in size held in place by a continuous impermeable phase. These phases are much larger than the free volumes, less than a nanometer in size, which are manipulated to change selectivity. Here, our goal is not improving selectivity, but using block polymer structure to sustain selectivity already achieved. Our partial success underscores the need to understand how this bicontinuous structure is formed. A qualitative picture is as follows. Initially, the block polymer chains and the monomer are evenly distributed in the solvent and the metathesis reaction of P(N-s-S)-b-PDMAEMA and COE generate small cross-linked regions, where the PDMAEMA chains are segregated from the reacted P(N-s-S)/PCOE domains. When cross-linked regions overlap, a morphology with low interfacial energy, like a bicontinuous phase, is favored between the PDMAEMA and the cross-linked P(N-s-S)/PCOE domains. After gelation, the morphology does not evolve further, but the polymerization of the free COE remaining monomers keeps going in the P(N-sS)/PCOE domains until a high cross-linking density is achieved. These films have a cross-linked matrix that prevents the PDMAEMA phase from strongly swelling, thereby allowing the membrane to remain CO2 selective in mixed gas experiments. However, the CO2 permeability is lower than expected for uncross-linked PDMAEMA, likely due to the P(N-s-S)/PCOE content and cross-linked nature.

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Table 2. Permeances, Permeabilities, and Selectivities for Pure (CO2 or CH4) and Mixed (CH4 in CO2) Gas Experiments Conducted at Low Pressures and 25 °C sample

gas

permeance (m s-1)

permeability (m2 s-1)

permeability (barrer)

ideal selectivity

selectivity in mixed gas system

M143.7/185

CO2 CH4 CH4 in CO2 CO2 CH4 CH4 in CO2 CO2 CH4 CH4 in CO2

2.17 × 10-7 4.63 × 10-8 4.70 × 10-8 3.79 × 10-7 3.23 × 10-8 3.18 × 10-8 1.45 × 10-6 1.27 × 10-7 1.24 × 10-7

4.02 × 10-11 8.56 × 10-12 8.70 × 10-12 4.36 × 10-11 3.72 × 10-12 3.66 × 10-12 3.63 × 10-11 3.18 × 10-12 3.10 × 10-12

51.9 11 11.2 56.3 4.8 4.7 46.9 4.1 4

4.7

4.6

M255.4/115 M355.2/25

At the initial stage of the PIPS process, some COE monomers and the catalyst are possibly located within both the P(N-s-S)/ THF and PDMAEMA/THF domains. As in reactive graft copolymers,28 the highly cross-linked structure obtained from the polymerization between P(N-s-S) blocks and the COE monomers through the in situ metathesis reactions should yield to bicontinuous morphology. Despite the weak segregation between P(N-s-S) (PCOE) and PDMAEMA, the phase separation between the P(N-s-S)/PCOE and PDMAEMA domains occurs to minimize interfacial energy when the cross-linking is high enough. At the same time, χ(PDMAEMA/(P(N-s-S)/PCOE)) is presumably large enough so any PCOE oligomers formed in the PDMAEMA/THF phase can segregate from PDMAEMA domains before gelation. We expect that this picture is qualitatively correct but incomplete. We expect that future work will sharpen the details. As one example,13 a study with a P(N-s-S)-b-PLA block copolymer and a DCPD monomer showed that the catalyst concentration did not affect the membrane morphology but did influence the resulting mechanical strength. In this earlier work, a low catalyst loading gives membranes with reduced mechanical strength even though all norbornene groups should be crosslinked. Here, a low catalyst loading results in insufficient crosslinking because of catalyst degradation. As second example, the real selectivity observed for surface cross-linked PDMAEMA on microporous polysulfone membrane is 7.8 for a 0.5 mol fraction of CO2 in the feed at 308 kPa.29 While this performance is not as good as that observed here, the chemistry of XDC may be cheaper and easier to operate at large scale. For economic reasons, transport rates in membrane separation processes have to be high. Composites membranes composed of a thin cationic PDMAEMA layer and a microporous support for gas separations were prepared by coating the polymer onto polysulfone substrate.30 Many works reported on the preparation of PDMAEMA composite membranes for different kinds of applications.31-33 The main objective of this study was to

Figure 6. Permeability and selectivity as a function of the total feed pressure at 35 °C using gas mixtures: CO2 (b); CH4 (9); selectivity (×). The gas composition was 50.1/49.9 CO2/CH4 for measurements at 1.5, 10, and 20 bar and 50.3/49.7 CO2/CH4 for measurements at 30 and 40 bar.

11.7

12

11.4

11.7

identify new materials with abilities to perform enhanced permselection without focusing on the practical formulation of a high surface to volume membrane. From a manufacturing point of view, membranes consisting of a thin layer supported by a porous substructure would be better for practical applications. Materials developed here present the potential to be transformed into a realistic membrane with a reasonable selective layer thickness. In addition, the results in this paper raise two important and very different points. The first comes from a major and implicit assumption in our experiments that the transport of one gas is independent of the concentration gradients of the other gases. The second point is the practical value of the results and the future research implied. Each of these points is discussed below. In the solution diffusion model used to describe gas permeation, it is assumed that the gases dissolve in the membrane and then diffuse through it. The separation between several gases is due to differences between their solubilities in the membrane and between their rates of diffusion. The diffusion of each gas is described by Fick’s law: Ji ) -Di

dci ) -Di∇Ci dx

(4)

where Ji is the flux of component “i”, ci is the concentration of component “i”, and Di is the diffusion coefficient. Here, the implicit assumption is that membrane transport may be described by a binary form of Fick’s law. The diffusion coefficient Di in eq 4 will be a function of the polymer structure and the kinetic diameter of the gas. However, the results presented above for the low pressure gas permeation cell make a major implicit assumption that may compromise our conclusions. This assumption is that ternary diffusion effects are negligible. The assumption is important in the following way. Because we are measuring the diffusion of CH4 across a fixed membrane in the presence of a second gas, diffusion is described by the flux equations: -J1 ) D11∇C1 + D12∇C2

(5)

-J2 ) D21∇C1 + D22∇C2

(6)

where “1” and “2” refer to CH4 and CO2, respectively, and Ji is the flux of gas “i” relative to the fixed membrane, i.e., species “3”, which does not appear directly in these equations. In our experiments, the initial pressure of CO2 is constant, and hence ∇C2 is initially zero. However, if the cross term diffusion coefficient D21 is nonzero, then the gradient of CH4 concentration ∇C1 may result in a flux J2 of CO2, and the pressure difference we use to calculate the CH4 permeability would, in fact, be a function of the permeability of both species, i.e., of (D11H1 + D21H2).

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Table 3. List of Some Potential Polymers and the Effect of Their Structure on Permeability and Selectivity in CO2/CH4 Pure Gas Separationa polymer family poly(sulfone) poly(imide) poly(carbonate) poly(phenylene oxide) polymer of intrinsic microporosity poly(pyrrolone) poly(acetylene) silicon poly(phosphazene) poly(ether) a

sample abbreviation 36

PSF TM HF PSF37 DM PSF36 6FDA-ODA38 PMDA-DAF38 6FDA-2.6-DATr39 PC40 TM HF PC41 TBr HF PC40 PPO42 1.06 Br PPO42 0.91 Br PPO42 PIM-143 PIM-743 6FDA-TADPO44 PMP45 PTMSP46 PDMS47 PPZ48 PPZ-SAPO48 semicrystalline PEO49

RCO2/CH4

PCO2 (barrer) 5.6 72 2.1 23 0.1 42.5 6.8 111 32 50 108 68 2300 1100 27.6 10700 18000 3800 71 17 8.1

• + • + • ++ + + ++ + ++ ++ • ++ ++ ++ + • •

22 24 30 61 72 46 19 24 36 17 17 20 18.4 17.7 51.1 3.7 4.3 3.1 15.3 57.7 51

+ + + ++ ++ + • + + • • + • • ++ • ++ ++

Meaning of symbols used in this table: less than encouraging, -; moderate potential, •; encouraging, +; high potential, ++.

However, both theory and experiments are inconsistent with this. Ternary diffusion effects are known in gases with dramatically different molecular weights and when the gaseous solutions are concentrated.24 Here, while the gaseous solutions are sometimes concentrated, the solutions within the membranes are always dilute. In this case, the cross term diffusion coefficients D12 and D21 are expected to be small. Thus we are confident on the first point: the analysis summarized by eq 1 is reliable. We can now focus on the second question raised by the results obtained in this work. Since the end of the 1980s, membranes for CO2/CH4 separation have been typically based on cellulose acetate (CA), produced by Natco (Cynara), UOP (Separex), and Kvaerner (Grace Membrane Systems). Because of plasticization,8 CA membranes exhibit a mixed gas selectivity that is significantly below the ideal selectivity of 28 at 25 °C.34 More selective polyimides (PI) membranes introduced by Air Products, Air Liquide, and Ube sought to displace these systems (PCO2 of 13 barrer and ideal selectivity of 32.5 at 60 °C).34 However, CA membranes remain the industrial leader mainly because they have an average lifetime of five years, longer than PI.35 Although several hundred membranes with improved permeability and ideal selectivity have been reported in the past few years, maintaining real selectivity during operation remains a problem. Consequently, research efforts must focus on membrane materials that exhibit not only high selectivity but also high reliability and low cost. The next generation membranes must show real selectivities for CO2 above 20, withstand elevated pressures, and resist cracking. This research is only one step toward this goal. The experiments summarized by Figure 6 show that the retained selectivity with increasing pressure is modest, ranging from 7 to 12. However, this selectivity changes less than that observed for other membranes.30 In other words, the use of these block polymer templated membranes with bicontinuous network phases does seem to be an attractive route for sustaining membrane selectivity. The challenge is to define a polymer that could serve as the CO2 selective phase in the block polymer. Table 3 lists potential polymers, giving the CO2 permeability and the ideal selectivity. Membranes are roughly classified into four categories: less than encouraging, -; moderate potential, b; encouraging, +; high

potential, ++. This highlights ways where improvements may be achieved in CO2 removal with the judicious choice of membrane processes. Looking closer at Table 3, we see glassy polymeric materials that include poly(sulfone)s, poly(imide)s, poly(carbonate)s, and poly(phenylene oxide)s.50-52 Gas transport can be modified by the introduction on these polymers of polar substituents or bulky and rigid side groups. Attractive membranes can also be prepared using ultramicroporous structures.43,53-55 However, exposure of these glassy membranes to CO2 leads to performance deterioration. Unparalleled gas transport properties have been achieved with disubstituted acetylene-based polymers,45,46 but these polymers usually have poor chemical resistance causing the loss of their performance over time. Perhaps putting these materials within self-assembled block polymer structures would sustain these properties. In addition to glassy polymeric membranes, rubbery materials that are less affected by swelling and plasticization are an alternative to prepare membranes. Silicon membranes have received considerable interest due to their high intrinsic permeabilities,47 but these materials did not show high selectivities. Poly(phosphazene)s48 and poly(ether)s49 can show ideal CO2/ CH4 selectivities up to 60, considerably higher than commercially available CA and PI membranes. Groups such as ether oxygens,49,56 nitriles,57 acetates,58 and carbonates59 can be employed. Carbonyl groups improve CO2 sorption and solubility selectivity but also give glassy polymer at 35 °C.60 Ether oxygens in ethylene oxide (EO) units appear to provide a good compromise between CO2 separation and permeation properties.61,62 These materials also offer the possibility for significant improvement within a block polymer structure. Considering the advantages and drawbacks of each materials previously mentioned, we urge future research on PEOcontaining block polymers for CO2/CH4 separations. Combining new block polymer structures with this breath of experiences may produce a major breakthrough showing how a high ideal selectivity can be sustained under operating conditions. 5. Conclusion Block polymer templated membranes that are more permeable to CO2 than CH4 for both pure and mixed gases were prepared. These membranes were obtained with the use of a reactive block

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polymer containing a chemically cross-linkable block. Metathesis reactions were employed to cross-link the copolymer with a tough PCOE polymer. The polymerization induced phase separation during the ring-opening metathesis polymerization of the COE in the presence of the P(N-s-S)-b-PDMAEMA block polymer gives continuous PDMAEMA nanodomains in a crosslinked P(N-s-S)/PCOE matrix. The size of the more permeable domains (PDMAEMA), around 20 nm, does not change with the formulation chosen. On the other hand, the size of the less permeable (P(N-s-S)/PCOE) domains does depend on formulation. Thereby, selectivity loss is avoided when the membrane is faced to mixed gases at low pressure. When the pressure is increased to 40 bar, the selectivity drops but by less than is typical. Extending the work to block polymers with more selective blocks seems an attractive alternative. More precisely, the use of a copolymer including a block with high ideal selectivity but strongly prone to plasticization and a second block with high robustness appears a strong approach. Acknowledgment This work was supported by the MRSEC Program of the National Science Fundation under Award Numbers DMR0212302 and DMR-081985. We thank Mark Amendt for preparation of M450.4/44 membrane. S.E.Q. also thanks Jeroen Ploegmakers, Katja Simons, and Sander Reijerkerk from the group Membrane Science & Technology of the University of Twente (NL) for support during the high pressure gas permeation experiments. Supporting Information Available: Diffusion data for single and mixed gas experiments across M143.7/185 and M255.4/115 block polymer membranes. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Baker, R. W.; Lokhandwala, K. Natural Gas Processing with Membranes: An Overview. Ind. Eng. Chem. Res. 2008, 47, 2109. (2) Noble, R. D.; Stern, S. A. Membrane separations technology: principles and applications; Elsiever Science: . Amsterdam, 1995. (3) Paul, D. R. Evolution of gas separation membranes. PMSE Prepr. 2009, 100, 103. (4) Visser, T.; Masetto, N.; Wessling, M. Materials dependence of mixed gas plasticization behavior in asymmetric membranes. J. Membr. Sci. 2007, 306, 16. (5) Visser, T.; Wessling, M. When Do Sorption-Induced Relaxations in Glassy Polymers Set In. Macromolecules 2007, 40, 4992. (6) Shao, L.; et al. Transport properties of cross-linked polyimide membranes induced by different generations of diaminobutane (DAB) dendrimers. J. Membr. Sci. 2004, 238, 153. (7) Puleo, A. C.; Muruganandam, N.; Paul, D. R. Gas sorption and transport in substituted polystyrenes. J. Polym. Sci., Part B: Polym. Phys. 1989, 27, 2385. (8) Donohue, M. D.; Minhas, B. S.; Lee, S. Y. Permeation behavior of carbon dioxide-methane mixtures in cellulose acetate membranes. J. Membr. Sci. 1989, 42, 197. (9) Landreau, B.; Drying process for gases making use of glycol including the separation of gaseous effluents. Eur. Pat. Appl. EP 770667 A1, 1997. (10) Roberts, D. L. Elimination of organic emissions in dehydration of natural gas. US 5399188 A, 1995. (11) Phillip, W. A.; et al. Seeking an ammonia selective membrane based on nanostructured sulfonated block copolymers. J. Membr. Sci. 2009, 337, 39. (12) Inoue, T. Reaction-induced phase decomposition in polymer blends. Prog. Polym. Sci. 1995, 20, 119. (13) Chen, L.; et al. Robust Nanoporous Membranes Templated by a Doubly Reactive Block Copolymer. J. Am. Chem. Soc. 2007, 129, 13786. (14) Phillip, W. A.; et al. Gas and water liquid transport through nanoporous block copolymer membranes. J. Membr. Sci. 2006, 286, 144.

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ReceiVed for reView March 1, 2010 ReVised manuscript receiVed June 30, 2010 Accepted July 26, 2010 IE100461K