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Factors Controlling Successful Formation of Mixed-Matrix Gas Separation Materials Rajiv Mahajan and William J. Koros* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1062
Prior work has suggested simple guidelines for matching transport characteristics of materials to form high-performance mixed-matrix materials for gas separation. Such materials comprise a dispersion of molecular sieving particles in a properly selected matrix polymer phase. Recent work has shown that these simple criteria are necessary but not sufficient to achieve the desired properties. The analysis presented here shows the need to optimize the transport properties of the interfacial region, i.e., the region between the bulk polymer and dispersed sieve phases. Guided by the need to optimize both the transport properties of the interfacial region and the matrix material selection criteria noted above, a new paradigm is recommended for matrix phase selection. The practicality of the paradigm is validated by the formation of mixed-matrix membranes with an appropriate polymer and sieve. These materials lead to the attractive predicted performances at low loading. For success at higher loading a zeolite “priming” protocol based on polymer-solvent sieve interactions is shown to be necessary. This modified protocol leads to success at intermediate and high dispersed-phase loading. Introduction and Background Overview. Nitrogen-enriched air production is an important application of gas separation membranes for small and intermediate capacities. To continue their growth in market share, membranes with improved transport properties are essential to economically achieve higher purities without loss in product recovery. Despite rapid advances in polymeric gas separation membrane performance in the 1980s, 9 years ago an “upper bound” tradeoff curve between O2 permeability and O2/N2 selectivity was constructed.1 This plot still defines the effective performance bounds for conventional soluble polymers that are easy to process into membranes. While greatly advantageous for economical membrane formation, the segmental flexibility of polymers limits their discriminating ability compared to more selective, but unprocessable rigid molecular-sieving media. Clearly, both types of materials have advantages and disadvantages. Mixed matrix materials comprising molecular sieve entities embedded in a polymer matrix offer the most viable approach around the current limitations on continued economical materials development for this application. These materials offer the potential to combine the easy processability of polymers with the superior gas separation properties of rigid molecularsieving materials. Mixed-Matrix Membranes. Current asymmetric composite hollow fibers consist of an inexpensive porous polymeric support coated with a thin, higher performance polymer. Similar in construction, mixed-matrix composite (MMC) membranes could replace the thin, higher performance polymeric layer with a tightly packed layer with (>30 vol %) molecular-sieving media, such as zeolite or carbon molecular sieves (CMS) supported within an appropriate polymeric matrix. This layer would also be asymmetric with a thin dense layer followed by an open substructure that then connects to * To whom correspondence should be addressed.
the porous polymeric support. In contrast to the many studies on conventional polymers in the past 25 years, only a few attempts to increase gas separation membrane performance with dense film mixed matrices of zeolite and rubbery or glassy polymers have been reported.2-7 Proper Material Selection. Material selection for both matrix and sieve phases is a key aspect in the development of mixed-matrix membranes.7 Polymermatrix selection determines minimum membrane performance, and the addition of properly selected molecular sieves can only improve membrane selectivity in the absence of defects. Intrinsically, the matrix polymer selected must provide a commercially acceptable performance, and this limits the choice of candidate materials greatly. For example, a mixed-matrix membrane using silicone rubber could exhibit a performance typical of intrinsic silicone rubber properties, an O2 permeability of 933 barrer and an O2/N2 selectivity of 2.1. Such properties lie substantially outside the commercially relevant region on the upper bound tradeoff curve for gas permeability and selectivity and are not attractive. On the other hand, selection of a highly impermeable matrix polymer would “starve” the sieves of the gas and also result in unattractive transport properties. In contrast, a glassy polymer such as Matrimid polyimide (PI), which has properties near the upper bound alluded to earlier and can be processed using current technology for formation of asymmetric membranes,8 is a good candidate matrix. Similarly, the choice of zeolite is also important. The molecular-sieving phase must accurately discriminate between the size and shape differences of spherocylindrical O2 and N2 molecules. Silicalite, commonly used in reported mixed-matrix membrane studies, is a hydrophobic zeolite possessing channels with dimensions between 5.2 and 5.8 Å. Likewise, zeolite 13X possesses an aperture of 10 Å.9 Clearly, these materials are not molecular sieves for O2 and N2 molecules with lengths of 3.75 and 4.07 Å, respectively. Zeolite 4A possesses
10.1021/ie990799r CCC: $19.00 © 2000 American Chemical Society Published on Web 06/29/2000
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2693
Figure 3. SEM showing the cross section of a PVAc-zeolite 4A mixed-matrix material. Figure 1. Predicted zeolite 4A-PI MMC membrane performance from Maxwell’s equation. The shaded boundary corresponds to the performance area typically thought to be commercially attractive based on current technology for forming asymmetric membranes with thin selective layers of 1000-2000 Å.
Figure 2. SEM showing the cross section of a Matrimid-zeolite 4A mixed-matrix material.
an eight-sided aperture with an effective aperture size of 3.8 Å9 that falls between the lengths of the two molecules, so it is an attractive molecular sieve dispersed phase. Mixed-matrix membrane performance can be modeled using the following expression (Maxwell’s equation):7,10
Peff ) Pc
[
]
Pd + 2Pc - 2Φd(Pc - Pd) Pd + 2Pc + Φd(Pc - Pd)
(1)
Here Peff is the effective permeability of the mixedmatrix material, Φ is the volume fraction, and the subscripts d and c refer to the dispersed and continuous phases, respectively. To illustrate the potential of this approach, consider a zeolite 4A-Matrimid mixed-matrix membrane. The intrinsic polyimide properties are an O2 permeability of 1.3 barrer and an O2/N2 selectivity of 7.2 at 25 °C.11 Zeolite 4A crystals are estimated to have an O2 permeability of approximately 0.77 barrer and an O2/N2 selectivity of approximately 37 at 25 °C,12,13 which is markedly more selective than the polyimide. Expected zeolite 4A-PI defect-free membrane performance can be estimated using eq 1 by using the above properties for the continuous Matrimid and dispersed zeolite 4A phase. From membrane performance predictions shown
Figure 4. Predicted vs observed zeolite 4A-PVAc mixed-matrix membrane performance.
in Figure 1, increased addition of zeolite 4A simultaneously leads to large increases in the O2/N2 selectivity with small reductions in the predicted MMC membrane O2 permeability. Implementation Using Traditional Gas Separation Polymers. In addition to matching the sieve and polymer permeabilities, a more subtle barrier to success was discovered in early attempts to demonstrate the mixed-matrix concept. The model predictions assume a defect-free interface between the sieve and polymer. Previous researchers noted difficulties in achieving good sieve-to-polymer contact,2,5,6 which ultimately results in gas bypassing around the sieve. Our early experiments confirmed this phenomenon.13 We first investigated the zeolite 4A-Matrimid system and discovered that the permeability was roughly 3-fold higher than that for pure Matrimid instead of being slightly lower as predicted by eq 1. There was no improvement in selectivity. These results are consistent with the micrograph shown in Figure 2. The photomicrograph clearly shows that voids exist between the two materials, which allows the gas to simply bypass the sieve, resulting in higher permeability with no selectivity increase. These voids are probably caused by polymer chain delaminating from the sieve because of the rigid nature of the
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polymer. If the polymer is rigid, during membrane formation as the solvent leaves the polymer it could vitrify the system before all of the solvent has left the system. Shrinkage of the matrix due to solvent removal beyond this point can cause considerable stress in the matrix, because the rigid vitrified polymer is undergoing shrinkage. This stress is probably manifested as poor contact between the sieve and the polymer on the interface. Organosilane coupling agents were then used to ensure good adhesion between the sieve and the polymer. With this modification, photomicrographs showed an integral bond; however, the permeation results were still poor, possibly because of the rigid nature of the polymer. This indicated that a suitable flexible polymer with an affinity for the sieve surface may be a more viable alternative, at least for the initial proof of concept studies. Poly(vinyl acetate) Mixed-Matrix Membranes In this study mixed-matrix membranes were made with poly(vinyl acetate) (PVAc). Although PVAc is not an “upper bound” polymer, it still seems to be a good choice for the matrix material for several reasons. It is extremely flexible, has a low glass transition temperature (35 °C), is known to have an affinity for alumina (is, in fact, used as a binder in alumina processing),14 and has reasonable gas transport properties. We therefore expected good contact between PVAc and the aluminosilicate-type 4A zeolite as opposed to zeolite 4A-Matrimid membranes. Thus, the PVAc-4A system should be able to validate the mixed-matrix concept. Low Zeolite Loading. The sieves used were commercially available zeolite 4A crystals from Advanced Specialty Gas Equipment. The polymer used was also commercially available PVAc from Polysciences Inc. The molecular sieve media, zeolites (4A crystals), were dispersed in the solvent (initially dichloromethane). The mixture was rolled on a roll mill for around 15 min; it was then sonicated for 1 min (using an ultrasonic horn) and rolled for approximately 15 min more. At this time, the polymer (PVAc) was introduced in the mixture, and the solution was rolled overnight. The solutions varied in solids (polymer plus sieve) content from 20 to 25 wt %. The solution was then cast as a film with a casting knife on a Teflon-coated surface (PVAc films are very hard to remove from a glass surface). The films were then dried overnight at room temperature under vacuum. Then, they were annealed at around 50 °C under vacuum for 1 day. Gas permeation measurements were made using equipment previously described.15 The membranes were loaded into a permeation cell. The entire system was then evacuated upstream and downstream for 12 h, after which gas was introduced to the upstream side. Upstream pressures varied from 40 to 90 psia. On the permeate side, the increasing pressure was measured with a Baratron pressure transducer (0-10 Torr) and plotted against time on a stripchart recorder. Steadystate stripchart data were used to determine the downstream pressure increase with time. All measurements were made at 35 °C. The results of the investigation are summarized in Table 1 . These results are important in supporting the concept for the mixed-matrix work. At 15% loading, definite improvement in oxygen-nitrogen selectivity is apparent, with a slight decline in perme-
Table 1. Mixed-Matrix Membrane Performance, Predicted vs Observed (Low Loading) at 35 °C membrane
RO2/N2
PO 2 (Barrer)
pure PVAc zeolite 4A (15 vol %) in PVAc (predicted) zeolite 4A (15 vol %) in PVAc (observed)
5.9 7.5 7.3-7.6
0.5 0.53 0.45
ability. Scanning electron micrographs (SEMs) indicate good contact between the sieve and the polymer (Figure 3). The experimental results also seem reasonably close to predicted results based on eq 1. When this work was extended to higher loading (∼25 vol %), most of the samples were defective. It was hypothesized that these failures were due to contact between ‘bare’ zeolite surfaces coupled with poor dispersion, both of which are more likely at higher loading and would result in the formation of nonselective nanometric-scale channels at all such sites. A possible solution to these problems could be priming the zeolites with a thin polymer coating before dispersal, because this should lead to better dispersion and no contact between bare zeolite surfaces. The next section details the extension of this work to higher loading of sieves. Extension to Higher Loading. Priming the zeolites seemed to be a viable option to extend this work to higher loading. Priming the zeolites involves adsorbing a layer of polymer onto the surface of the sieve. To achieve this, one needs to pay attention not only to polymer-sieve interactions but also to interactions of the polymer and sieve with the solvent. Thus, before the experimental details of the priming protocol are described, the next section outlines the importance and relevance of zeolite-solvent-polymer interactions to the priming protocol. Zeolite-Solvent-Polymer Interactions. The balance of three interactions, polymer-solvent, polymersieve, and solvent-sieve determines the extent of polymer adsorption from solution (which, in turn, determines the polymer sieve adhesion). A polymer molecule is more solvated in a good solvent and has a larger coil size than if it is in a poor solvent. Therefore, as the solvent power decreases, the dimension of the polymer molecule in solution decreases and the amount adsorbed increases, because the coil surface area occupied decreases.16 The solvent-sieve interaction is important because it is desirable for the solvent to desorb from the sieve surface when the polymer segments approach. Thus, an ideal system would be one where the sieve has a stronger affinity for the polymer than the solvent, while the polymer has a stronger affinity for the zeolite surface than the solvent. These relations were quantified with the help of Hildebrand solubility parameters for the polymer-solvent interaction,17 and using the liquid-solid interaction strength parameter o for alumina and silica for the solventsieve interaction,17 the higher the parameter, the stronger the interaction. Table 2 summarizes the Hildebrand solubility parameters for some common solvents and the polymer as well as the liquid-solid interaction strength parameter o for the same solvents interacting with alumina and silica. These relations were used to define the optimum solvent for our system, as well as subsequent steps of the priming protocol. Membrane Formation and Testing at High Sieve Loading. Based on the above-mentioned criteria, toluene seemed to be the most suitable solvent for the zeolite-PVAc system. A protocol was defined to prime
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2695 Table 2. Solubility and Interaction Parameters at 25 °C17 a. Hildebrand Solubility Parameters δ/MPa0.5 (Hildebrand solubility parameter) poly(vinyl acetate) toluene benzene methylene chloride
at 25% loading
o silica (liquid-solid interaction strength parameter for silica)
o alumina (liquid-solid interaction strength parameter for alumina)
0.26 0.29 0.34
0.33 0.36 0.44
the zeolite surface. The zeolites were dispersed in toluene for 15 min and then sonicated, followed by the addition of a small amount of polymer, again enough to provide a coating of up to 400 Å thick on each particle (calculated by estimating the surface area). This solution was stirred overnight, after which the bulk of the toluene and unadsorbed polymer was decanted. The remaining solution was injected in an excess of hexane (a nonsolvent) to force the PVAc onto the zeolite surface. This suspension was stirred for a few hours, at the end of which the hexane was decanted; the zeolites were dried and annealed under vacuum at 60 °C. These primed sieves were found to be “different” from the unprimed sieves by the fact that they were much easier to disperse in toluene. SEMs revealed no significant difference from unprimed sieves as expected, because the coating is smaller than the resolution of the SEM. These sieves were used to form mixed-matrix membranes initially at 15% loading in toluene and as expected resulted in properties very similar to the ones made earlier with 15% loading in dichloromethane. The techniques used for membrane formation and characterization were similar to the low loading samples. One significant observation was that sonication of primed zeolites during dispersal (as was shown by SEMs) resulted in poor contact in the final membranes, probably because of intense conditions experienced during sonication, which could result in delamination of the priming layer. Some variations were made in the priming protocol. The most important among these was annealing of zeolites in solution to lessen the contact between zeolites to prevent possible conglomeration. This did not lead to any noticeable difference. These primed sieves resulted in successful membranes at intermediate loading (∼25%). Another modification was needed to extend this work to still higher loading (∼40%), which was annealing at 100 °C instead of 50 °C as earlier. This was done to provide additional mobility to the matrix phase so that it could better access any inaccessible regions between the zeolites and relax defects near the sieve-matrix interface. The experimental results of the investigation along with theoretical predictions are summarized in Table 3 and plotted on the oxygen-nitrogen tradeoff curve in Figure 4. There is reasonable agreement between the experimental and predicted values; however, as the loading increases, the deviation becomes more significant. This could be due to inaccurate estimation of the zeolite permeability, because the zeolite permeability cannot be measured directly (a defect-free zeolite 4A
RO2/N2
membrane pure poly(vinyl acetate) mixed-matrix case at 15% loading
20.9 18.2 18.8 20.0
b. Liquid-Solid Interaction Strength Parameters at 25 °C17
toluene benzene methylene chloride
Table 3. Mixed-Matrix Membrane Performance Predicted vs Observed at 35 °C
at 40% loading
5.9 predicted observed predicted observed predicted observed
7.5 7.3-7.6 8.7 8.3-8.5 10.9 9.7-10.4
PO2 (Barrer) 0.5 0.53 0.45 0.55 0.4 0.55 0.28-0.35
membrane has not been made yet). Therefore, the zeolite permeability was estimated indirectly by taking the product of the diffusion coefficient and the solubility coefficient measured by gravimetric measurements based on gas sorption.7 These measurements are not significantly affected by defects. These measurements are also not very accurate because it is hard to obtain samples with a monodisperse crystal size, and size dispersion reduces the accuracy of diffusion coefficient estimations. Thus, there can be a difference between the actual and the estimated permeability. Nevertheless, we feel a more probable cause for these deviations is due to inhibition of polymer chain mobility near the sievepolymer interface, because of polymer adsorption onto the surface of the sieve. This effect has been seen by previous researchers working on related problems18 and could lead to reduced permeability around the interface. This effect, in turn, would lead to a reduced overall permeability. The model used (eq 1) assumes uniform polymer permeability throughout the matrix, and the hypothetical effects due to surface segment mobility inhibition would become more pronounced as the sieve loading increases. In any case, these results show clearly that the addition of suitable sieves in a suitable matrix can lead to improved transport properties. Conclusions and Future Work Recent membrane materials development has failed to exceed the polymeric O2/N2 upper bound tradeoff curve in the past 9 years. A MMC membrane-processing scheme based on molecular-sieving materials is attractive if a proper molecular sieve and polymer matrix is selected for the given separation. Using Maxwell’s equation, estimates of MMC membrane performance indicate that significant improvements should be achievable with elimination of nonselective defects. Matching of transport properties, promotion of molecular adsorption of the polymer onto the sieve surface, and polymer flexibility during membrane formation seem to be keys to the success of the mixed-matrix materials. The PVAc-zeolite 4A system (which matches these criteria) showed considerable improvement over the neat polymer and validated the mixed-matrix concept, although there were some deviations from the predicted behavior at high loading of sieves. For the mixed-matrix approach to achieve its full potential, the polymer matrix should be closer to the upper bound (because it provides the base case on which we improve). The insight provided by the PVAc-zeolite 4A system could be used to guide future work to work with more traditional gas separation polymers (closer to the upper bound). As mentioned above, there are two key requirements that would be necessary for success in addition to matching of sieve and polymer transport properties: molecular adsorption of the polymer onto
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the sieve surface, and polymer flexibility during membrane formation. The principles (based on polymersolvent-sieve interactions) used to define the priming protocol for the PVAc-zeolite 4A system are general enough to define a protocol for the chosen polymerzeolite pair. The additional requirement of polymer flexibility during formation would be harder to achieve because polymers close to the upper bound are inherently rigid, but this can still be achieved by the use of suitable plasticizing mechanisms. Acknowledgment The authors gratefully acknowledge the support of Medal Inc. Literature Cited (1) Robeson, L. M. Correlation of Separation Factor Versus Permeability for Polymeric Membranes. J. Membr. Sci. 1991, 62, 165. (2) Su¨er, M. G.; Bac¸ , N.; Yilmaz, L. Gas Permeation Properties of Polymer-Zeolite Mixed Matrix Membranes. J. Membr. Sci. 1994, 91, 77. (3) Kulprathipanja, S.; Neuzil, R. W.; Li, N. N. Separation of Fluids by Means of Mixed Matrix Membranes. U.S. Patent 4,740,219, 1988. (4) Jia, M.; Peinemann, K.-V.; Behling, R.-D. Molecular Sieving Effect of the Zeolite Filled Silicone Rubber Membranes in Gas Permeation. J. Membr. Sci. 1991, 57, 289. (5) Duval, J.-M.; Folkers, B.; Mulder, M. H. V.; Desgrandchamps, G.; Smolders, C. A. Adsorbent Filled Membranes for Gas Separations. J. Membr. Sci. 1993, 80, 189. (6) Gu¨r, T. M. Permselectivity of Zeolite Filled Polysulfone Gas Separation Membranes. J. Membr. Sci. 1994, 93, 283. (7) Zimmerman, C. M.; Singh, A.; Koros, W. J. Tailoring Mixed Matrix Composite Membranes for Gas Separations. J. Membr. Sci. 1997, 137, 145.
(8) Ekiner, O. M.; Hayes, R. A. Phenylindane-Containing Gas Separation Membranes. U.S. Patent 5,015,270, 1991. (9) Yang, R. T. Gas separation by adsorption processes; Butterworth Publications: Stoneham, MA, 1987. (10) Robeson, L. M.; Noshay, A.; Matzner, M.; Merriam, C. N. Physical Property Characteristics of Polysulfone/Polydimethylsiloxane Block Copolymers. Angew. Makromol. Chem. 1973, 29, 47. (11) Mahajan, R.; Zimmerman, C. M.; Koros, W. J. Fundamental and practical aspects of mixed matrix gas separation membranes. Polymer membranes for gas and vapor separation Chemistry and Material Science; ACS Symposium Series 733; American Chemical Society: Washington, DC, 1999. (12) Ka¨rger, J.; Ruthven, D. M. Diffusion in zeolites and other microporous solids; Wiley-Interscience Publications: New York, 1992. (13) Ruthven, D. M.; Derrah, R. I. Diffusion of Monatomic and Diatomic Gases in 4A and 5A Zeolites. J. Chem. Soc., Faraday Trans. 1975, 71, 2031. (14) Reed, J. S. Principles of ceramics processing; Wiley: New York, 1995. (15) O’Brien, K. C.; Koros, W. J.; Barbari, T. A. A New Technique for the Measurement of Multi-Component Gas Transport through Polymeric Films. J. Membr. Sci. 1986, 29, 229. (16) Sato, T.; Ruch, T. Stabilization of Colloidal Dispersions by Polymer Adsorption; Marcel Dekker: New York, 1980. (17) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983. (18) Moaddeb, M. Formation of Composite Membranes Based on Silica Occlusion of Surface Pores. Ph.D. Thesis, The University of Texas at Austin, Austin, TX, 1997.
Received for review November 8, 1999 Revised manuscript received March 14, 2000 Accepted March 15, 2000 IE990799R
