Functionalized carbon nanotubes mixed matrix membranes of polymers of intrinsic microporosity for gas separation
© Khan et al.; licensee Springer. 2012
Received: 12 July 2012
Accepted: 28 August 2012
Published: 6 September 2012
The present work reports on the gas transport behavior of mixed matrix membranes (MMM) which were prepared from multi-walled carbon nanotubes (MWCNTs) and dispersed within polymers of intrinsic microporosity (PIM-1) matrix. The MWCNTs were chemically functionalized with poly(ethylene glycol) (PEG) for a better dispersion in the polymer matrix. MMM-incorporating functionalized MWCNTs (f-MWCNTs) were fabricated by dip-coating method using microporous polyacrylonitrile membrane as a support and were characterized for gas separation performance. Gas permeation measurements show that MMM incorporated with pristine or functionalized MWCNTs exhibited improved gas separation performance compared to pure PIM-1. The f-MWCNTs MMM show better performance in terms of permeance and selectivity in comparison to pristine MWCNTs. The gas permeances of the derived MMM are increased to approximately 50% without sacrificing the selectivity at 2 wt.% of f-MWCNTs' loading. The PEG groups on the MWCNTs have strong interaction with CO2 which increases the solubility of polar gas and limit the solubility of nonpolar gas, which is advantageous for CO2/N2 selectivity. The addition of f-MWCNTs inside the polymer matrix also improved the long-term gas transport stability of MMM in comparison with PIM-1. The high permeance, selectivity, and long term stability of the fabricated MMM suggest that the reported approach can be utilized in practical gas separation technology.
KeywordsMixed matrix membrane Multi-walled carbon nanotubes Polymer of intrinsic microporosity
During the last two decades, significant improvements in the performance of polymeric materials for gas separation membranes have been made[1–6], and understanding of the relationships between the polymer structure and gas transport properties of polymeric membranes has been greatly advanced[2, 3]. Despite these advantages and progresses, polymeric membranes are restricted by the trade-off trend between gas permeability and selectivity, as shown by Robeson. Most of the researchers have paid special attention to the relationship between polymer structure and gas separation properties in order to improve membrane performance both in permeance and selectivity.
The permeability of a polymeric membrane is mainly controlled by the chain mobility, the packing density, and the free volume of the polymer structure. The introduction of rigid fillers having particle size close to the characteristic size of the macromolecules which form the selective polymer film can be the best technique to improve gas permeability by inhibiting molecular chain packing and increasing free volume. Therefore, mixed matrix membranes (MMM) defined as the synergistic combination of organic polymers with inorganic nanofillers (both permeable and impermeable) dispersed at the nanometer level have been studied as an alternative approach to solve the trade-off problem of polymeric membranes in gas separation[8, 9].
Proper material selection for both the polymer matrix and the inorganic phase is important in the development of MMM. It has been found that polymer properties as well as inorganic phase properties affect mixed matrix membranes morphology and thus influence the separation performance. Compared to the pure polymer membranes, many polymer-inorganic MMM show higher permeabilities without sacrificing or even improve gas selectivity. Pinnau and He reported an unexpected increase of gas permeability without loss of gas selectivity in a series of high-free-volume glassy polymers whereby inorganic nonporous nanoparticles, such as fumed silica or carbon black, were incorporated into the polymeric matrix. Some experimental studies used carbon nanotubes as inorganic nanofillers to fabricate MMM, and an improvement of the gas permeability compared to the neat polymer membranes was observed. For instance, Kim et al. reported on the addition of carbon nanotubes (CNTs) to poly(imide siloxane) membranes which resulted in increased O2, N2, and CH4 permeability. Cong et al. prepared the brominated poly(2, 6-diphenyl-1,4-phenylene oxide) composite membrane with single-walled CNTs or MWCNTs and found that the low concentration of CNTs addition increases the gas permeability without sacrificing the selectivity. Weng et al. fabricated the MWCNTs/PBNPI membrane. In their results both the permeabilities and the selectivities of H2, CO2, and CH4 improved significantly at high MWCNTs concentrations (>5 wt.%). Based on these investigations, one can conclude that the interaction between polymer matrix and nanotubes may disrupt the polymer chain packing thus enhancing gas diffusion due to introducing more free volume voids between the polymer chains and nanoscale defects on the polymer/nanofillers interface.
Tailoring the free volume cavities by controlling the macromolecule's size and shape of the microporous polymer directly influences gas transport properties. In particular, a novel class of high free volume, glassy, ladder-type polymers, referred to as polymer of intrinsic microporosity (PIM), is a potential candidate for highly effective gas separation membranes comprising the capability for gas permeability and selectivity optimization by changing the polymer chain packing. McKeown et al.[17, 18] were the first to report this new class of rigid ladder-type polydioxanes containing highly contorted chains. Among these, PIM-1, containing the contorted spirobisindane unit, has attracted the most attention due to its relative ease of synthesizing high molecular weight polymers and the combining outstanding permeability with relatively moderate but technically attractive selectivity[19, 20], especially for O2/N2 and CO2/CH4 pairs, which shows the upper bound trade-off introduced by Robeson. However, a major drawback in the practical use of PIM-1 is the significant decay of its gas permeability with time.
In the present work, MMM were fabricated by loading of functionalized MWCNTs (f-MWCNTs) as an inorganic dispersed phase and PIM-1 as a polymer matrix. The MWCNTs were chosen because they had been proven to be promising nanofillers in tailoring polymeric material suited to be prescribed for application even at low incorporation. To the best of our knowledge, so far there is no literature available on using MWCNTs combined with PIM-1 as polymer matrix for gas separation. However, it is well documented that for sufficient enhancement of MMM performance, the dispersion of MWCNTs in the polymer matrix should be very fine, which means that the surface interaction between the filler and the polymer matrices should be strong. In response to that, the MWCNTs were functionalized with poly(ethylene glycol) (PEG) as a spacer via ‘grafting to’ method to facilitate their dispersion in the PIM-1 matrix. The as-prepared MMM were characterized for their morphology using scanning electron microscopy (SEM). Gas permeability, permselectivity, and long term membrane stability were studied by pure gas permeation measurements.
The monomer 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane (TTSBI, 97%) was obtained from ABCR (GmbH & Co. KG, Karlsruhe, Germany) and 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN, 99%) was kindly donated by Lanxess (Bitterfeld, Germany). TFTPN was sublimated twice under vacuum prior to use. Potassium carbonate (K2CO3, >99.5%) was dried overnight under vacuum at 120°C in order to ensure no moisture is trapped in it and then milled in a ball mill for 15 min. The MWCNTs were supplied by FutureCarbon GmbH (Bayreuth, Germany) (purity >98%, surface area of 250 m2/g, diameter varies from 12 to 15 nm, and number of walls was 8 to 12). Poly(ethylene glycol) of 200 g/mol and diethylbenzene (isomeric mixture) was purchased from Sigma-Aldrich (Sigma-Aldrich Logistik GmbH, Schnelldorf, Germany); dimethyl acetamide (DMAc, ≥99%), nitric acid (HNO3, 65% v/v), thionyl chloride (SOCl2, ≥99%), tetrahydrofurane (THF, ≥ 99,9%), methanol (≥99.9%), chloroform (CHCl3, 99.99%), and dioxane (≥99%) from Merck (Merck, KGaA, Darmstadt, Germany) were used as received.
The long-term stability test was carried out at a similar facility at constant pressure (2 bar) and 27°C. The gas flow rate was measured manually using a flow meter (Bioblock Scientific, Cedex, France). All data presented in this study were collected for at least three stamps of MMM of the same origin in order to obtain statistically valid results.
Functionalization of MWCNTs
(a) Oxidation of the MWCNTs
The MWCNTs were dispersed in HNO3 in a round bottom flask. After 30 min of sonication at room temperature, the solution was heated to 65°C and continuously stirred for 48 h. The dispersion was then filtered and washed with ample amounts of distilled water until the pH = 7 was achieved. Finally, the oxidized carbon nanotubes were dried under vacuum at 60°C for 72 h.
(b) Conversion of carboxylated MWCNTs to acyl chloride MWCNTs and esterification
The carboxyl groups of the oxidized MWCNTs were transformed into acid chloride groups by stirring the dispersion with SOCl2 for 24 h at 65°C. After reaction, the dispersion was generously washed with THF, filtered, and dried overnight at ambient temperature. Then acyl chloride-functionalized MWCNTs were added in PEG (200 g/mol) heated to 120°C and stirred for 48 h. Finally, functionalized carbon nanotubes were washed with THF and filtered through a Teflon® membrane (Sartorius Stedium Biotech GmbH, Göttingen, Germany) (0.2 μm pore size), followed by drying at ambient temperature for 48 h.
Synthesis and characterization of polymer of intrinsic microporosity (PIM-1)
PIM-1 was prepared by using a fast synthesis method originally developed by Du et al.. A slightly modified procedure for PIM-1 was carried out as follows: equimolar ratio of TTSBI and TFTPN was dissolved in DMAc to form an orange-red solution. The addition of K2CO3 (2.3 times with respect to -OH monomer concentration) caused the color change to yellow. The reaction mixture was immersed in an oil bath maintained at 150°C and kept under a continuous flow of argon. After 5 min the precipitate appeared and the reaction mixture became viscous, then diethylbenzene (DEB; same amount with respect to DMAc amount) was added into the reaction mixture, otherwise stirring could not be done easily. The stirring continued for a further 1 h, and the polymer was isolated by precipitation in methanol and filtered off. The polymer was boiled in water for several hours to remove salts and solvent residues, and then filtered off and dried overnight at 60°C. The dried polymer was dissolved in chloroform and reprecipitated again in methanol and then dried overnight at 60°C in vacuum to yield 90% PIM-1. The average molecular weight (Mw) and polydispersity (Mw/Mn) of the prepared PIM-1 is 2.21 × 105 (gmol−1) and 4.8, respectively, which are determined by size exclusion chromatography in chloroform against polystyrene standards.
Mixed matrix membrane preparation
Process parameters and membrane fabrication conditions
Casting solution composition
1 wt.% PIM-1; 99 wt.% CHCl3; 0.5, 1, 2, and 3 wt.% CNT with respect to polymer
Ambient temperature, 28°C
Air dried (ambient conditions for 24 h)
Results and discussion
Functionalization of MWCNTs
A similar procedure has been reported previously by other groups[25, 26]. The first step shows the oxidation of pristine MWCNTs with HNO3. The aims of the chosen acid treatment are disaggregation of nanotube bundles, dissolution of the catalysts, and the removal of by-products (e.g amorphous carbon). Further reaction between the oxidized carbon nanotubes and thionyl chloride leads to the formation of acyl chloride groups. The acyl chloride groups are highly reactive and can further react with poly(ethylene glycol) to form esters. The pristine and functionalized MWCNTs were characterized by TGA and FTIR .
Thermal gravimetric analysis
Although the reaction has been performed at high concentration of very low molecular weight PEG in the absence of the solvent, it is expected that during thorough washing processes, the entire unreacted PEG was washed out (washing was continued until constant mass loss in TGA was observed). Thus, the mass loss occurred only from the covalently attached PEG on the surface of the MWCNTs. Based on these results, the amount of PEG chains grafted on the MWCNTs was estimated (as above), and the concentration of PEG chains on the MWCNTs were calculated to be about 0.09 mmol/g.
Fourier transform infrared spectroscopy
Polymer (PIM-1) synthesis
The polymer was synthesized based on the modified rapid synthesis of PIM-1. DEB was introduced to this reaction system instead of toluene, which is different from previous polymerization protocols[17, 23], and seemed advantageous because of its higher boiling point (150°C). DEB was also added in the required minimal amounts to keep the mixture stirrable. The polymer was characterized by 1H nuclear magnetic resonance and IR spectroscopy.
Effect of pristine and f-MWCNTs on MMM morphology
Effect of f-MWCNTs content on MMM morphology
From these observations, the threshold limit for the addition of CNTs to the polymer matrix to prevent agglomeration is typically around 3 wt.%, and the optimum limit for the addition of f-MWCNTs is 2 wt.% which corresponds to the significant improvement in the separation properties of the MMM, as discussed in ‘Effects of f-MWCNTs content on MMM gas separation performance’ Section.
Gas permeation measurement
Effects of pristine and f-MWCNTs on MMM gas separation performance
Comparison of gas permeation results of PIM-1, MMM of f-MWCNTs and MWCNTs
Permeance (Nm3/(m2 h bar))
Pristine MWCNTs/PIM-1 MMM
Comparison of various gas pair selectivities of PIM-1 and MMM of f-MWCNTs and MWCNTs
Pristine MWCNTs/PIM-1 MMM
The CO2/CH4 selectivities have decreased in case of both f-MWCNTs and pristine MWCNTs-incorporated MMM. However, the observed CO2/CH4 selectivities are higher than that of MMM reported in literature using carboxyl-functionalized MWCNTs as inorganic filler in different glassy polymers (e.g., polyethersulfone and polyimide)[13, 33].
Gas permeation results of PIM-1, PIM-1 f-MWCNTs incorporated MMM
Permeance(Nm3/(m2 h bar))
0.5 wt.% f-MWCNTs/PIM-1
1 wt.% f-MWCNTs/PIM-1
2 wt.% f-MWCNTs/PIM-1
3 wt.% f-MWCNTs/PIM-1
Various gas pair selectivity of PIM-1 and PIM-1 MMM-incorporated f-MWCNTs
0.5 wt.% f-MWCNTs/PIM-1
1 wt.% f-MWCNTs/PIM-1
2 wt.% f-MWCNTs/PIM-1
3 wt.% f-MWCNTs/PIM-1
This could verify that the addition of pristine MWCNTs did not affect the selectivity of O2/N2. Interestingly, when PEG functionalization was applied onto MWCNTs, the selectivity of CO2/N2 of this MMM increased even at 1.0 wt.% loading of MWCNTs. This increase could be due to the higher compatibility of the polymer matrix with the surface of PEG-functionalized MWCNTs than that of pristine MWCNTs and preferential sorption of CO2 on the PEG chains of MWCNTs.
Effects of f-MWCNTs content on MMM gas separation performance
In order to systematically investigate the effect of f-MWCNTs incorporation on the MMM gas separation performance, MMM were fabricated with different loadings of f-MWCNTs. The permeance and respective permeability (calculated from the thickness of selective layer using membrane's cross section SEM image) of O2, N2, CO2, and CH4 at different f-MWCNTs loadings are shown in Table4. The order of gas permeance is CO2 > O2 > CH4 > N2. Gas permeance increased with f-MWCNTs loading from 0.5 to 2 wt.%. An increase in the free volume of the PIM-1 as a result of the disruption of the polymer chains packing due to the interaction between surface of PEG-functionalized MWCNTs and the PIM-1 chains might have contributed to the increment of the permeability. However, with 3 wt.% loading permeability of O2, N2, and CH4 decreased, except CO2, it was unchanged compared to 2 wt.% f-MWCNTs-incorporated MMM.
Permeation measurements used pure gas O2, N2, CO2, CH4 of 2 bar feed pressure at room temperature (27°C). Table5 shows the selectivity of O2/N2, CO2/N2, and CO2/CH4 for the PIM-1 and f-MWCNTs/PIM-1 MMM, measured at 2 bar. At 0.5 to 2 wt.% f-MWCNTs loading the selectivity of CO2/N2 and O2/N2 was found to be higher than for pure PIM-1. The high selectivity values of O2/N2 at 0.5 to 2 wt.% f-MWCNTs loading might have resulted due to the intimate interface interaction of f-MWCNTs-polymer chain. In contrary, the CO2/CH4 selectivity of the f-MWCNTs MMM is worse compared with the pure PIM-1 membrane. The functional groups on the MWCNTs have a significant role in gas selectivity. The PEG group, confirmed by FTIR in Figure4, has stronger interaction with a polar gas, such as CO2 than a nonpolar gas, e.g., N2. In that case the polar gas solubility can be enhanced and the gas permeability be increased which facilitates the improvement of the total CO2/N2 selectivity. However, there is no improvement in the CO2/CH4 selectivity as f-MWCNTs loading increased from 0.5 to 3 wt.% in the polymer matrix.
Effect of f-MWCNTs on long-term stability of MMM
Furthermore, Figure8b shows the relative permeance data for f-MWCNTs containing MMM, where the maximum total O2 permeance loss was found for the pure PIM-1 composite membrane studied. The trend of 0.5 wt.% of f-MWCNTs MMM is less significant, while for the 1 and 2 wt.% f-MWCNTs MMM, the loss of permeance is very limited. From these experimental results, it can be concluded that the addition of f-MWCNTs into the polymer matrix improved the long-term stability of PIM-1.
Mixed matrix membranes or MMM comprising MWCNTs as dispersed phase and PIM-1 as polymer matrix were successfully prepared. FTIR and TGA analysis have confirmed the successful PEG functionalization of MWCNTs. The SEM images of the prepared MMM revealed that the functionalized MWCNTs are well dispersed throughout the PIM-1 matrix compared to the one which is fabricated from pristine MWCNTs. It was shown that the improvement in homogeneous dispersion of MWCNTs in the MMM is due to covalent functionalization of MWCNTs with poly(ethylene glycol). With good interfacial adhesion and the absence of voids between f-MWCNTs and polymer matrix, the MMM show higher permeabilities which are coupled with increased CO2/N2 and O2/N2 selectivities. For MMM with 0.5 to 2 wt.% f-MWCNTs loading, the SEM of surface and cross-sectional images showed well dispersed f-MWCNTs throughout the polymer matrix, while at 3 wt.% of f-MWCNTs loading, the tubes agglomerated and formed domains or interface voids in the polymer matrix. Above a threshold of 2 wt.%, agglomeration and saturation of f-MWCNTs particle in the polymer matrix hinder the fast transport of gases. In addition, PEG functional groups on the surface of MWCNTs provide higher solubilities of CO2 in the MMM. The improvement in long-term stability was obtained as well.
The authors would like to thank Silke Dargel, Silvio Neumann, Petra Merten, Bahadir Gacal, Shahid Majeed, and Anne Schroeder for their technical support. This work was financially supported by the project ‘High Aspect Ratio Carbon-based Nanocomposites’ (HARCANA) within the European Community's 7th Framework Programme for Research and Technological Development under the grant agreement number NMP3-LA-2008-213277.
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