Article pubs.acs.org/IECR
Zeolite-Filled Porous Mixed Matrix Membranes for Air Separation Jung-Tsai Chen,† Chien-Chung Shih,† Ywu-Jang Fu,‡ Shu-Hsien Huang,§ Chien-Chieh Hu,*,† Kueir-Rarn Lee,† and Juin-Yih Lai† †
R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan ‡ Department of Biotechnology, Vanung University, Chung-Li 32023, Taiwan § Department of Chemical and Materials Engineering, National Ilan University, I-Lan 26047, Taiwan ABSTRACT: This work introduces the preparation of zeolite-filled porous mixed matrix membranes (MMMs) and discusses their potential use for air separation. Porous polysulfone (PSF) matrix membranes were prepared using the nonsolvent-induced phase separation process. Scanning electron microscope and gas permeation experiments suggested the presence of closed pores in the porous matrix membranes. Furthermore, porous MMMs containing zeolite 4A particles as filler were prepared and characterized. The permeation properties of porous MMMs were highly dependent on the zeolite content. The introduction of zeolite particles distorted the closed pores in the PSF matrix and formed nonselective voids which resulted in very high oxygen flux but low O2/N2 selectivity. The annealing technique for modifying the nonselective pores present in the polymer−zeolite interface is also described in this study. The resulting annealed porous MMMs had the ability to separate O2 from N2 more effectively than traditional dense MMMs.
1. INTRODUCTION Oxygen-enriched combustion is considered to be an energyefficient process in power plants. Oxygen-enriched combustion needs a tremendous amount of oxygen enriched air. In air separation applications, membrane processes have become competitive in terms of both cost and performance, and therefore offer outstanding advantages over alternative oxygen supply technologies.1 Polymeric membranes, particularly those prepared from glassy polymers, have received considerable attention because they possess several advantages: high gas selectivity, good mechanical properties, and relatively economical processing capability.2−4 Although glassy polymers possess adequate selectivity, their permeability is still in need of improvement. Besides, due to the susceptibility of polymeric membranes to plasticization and thermal instability, their applications have been limited to separation processes where harsh conditions are not encountered.5−7 Zeolites and carbon-based molecular sieves have been considered as some of the materials for the preparation of inorganic membranes because of their molecular sieving property, thermal resistance, and chemical stability.8−12 However, these materials are expensive and difficult to process as membranes. Deficiencies in both polymeric and purely molecular sieving media suggest the need for a hybrid approach to gas separation membrane development and material processing. Organic−inorganic hybrid membranes have received much attention as potential membrane materials.13−16 Such hybrid membranes are typically composed of porous inorganic zeolite particles dispersed in a polymeric matrix. These so-called “mixed matrix membranes” can combine the excellent size-sieving capacity of low-cost zeolites and the processability of polymers into one membrane. Over the past few decades, much work has been done on mixed matrix membranes (MMMs). Pechar et al. dispersed © 2014 American Chemical Society
zeolite L in a matrix of 6FDA−6FpDA−DABA polyimide and documented that the resulting membranes displayed increased permeability without substantial improvements in selectivity.17 Sen et al. investigated the effect of zeolite loading on the performance of polycarbonate (PC)/zeolite 4A membranes.18 The permeabilities decreased with the zeolite loading, and they were lower compared with the pure PC membrane. As opposed to the permeabilities, the selectivities were higher than those of the pure PC membranes. Dual-layer polyethersulfone (PES)/ P84 hollow fiber membranes with a PES/zeolite β mixed matrix dense selective layer was successfully fabricated by Li et al.19 The newly developed dual-layer hollow fibers exhibited enhanced O2/N2 and CO2/CH4 selectivity of around 10− 20% compared with that of the dense PES films. Recent research has shown that the interfacial region, which is a transition phase between the polymer and zeolite phases, is of particular importance.20−22 The interaction between the polymer and the zeolite is of concern, as undesirable channels may be created between both phases if the polymer chains do not completely interact with the zeolite, which may deteriorate the gas separation performance of the membranes. Li et al. demonstrated that the adhesion between zeolite A and the polyethersulfone matrix could be improved by modifying the zeolite surface with a (3-aminopropyl)diethoxymethyl silane (APDEMS) coupling agent.23 Both the permeability and the selectivity of MMMs made from the modified zeolite were higher than those of MMMs made from the unmodified zeolite at 20 wt % zeolite loading because of a decrease in the degree of the partial pore blockage of the zeolite. They also pointed out Received: Revised: Accepted: Published: 2781
November 12, 2013 January 23, 2014 February 3, 2014 February 3, 2014 dx.doi.org/10.1021/ie403833u | Ind. Eng. Chem. Res. 2014, 53, 2781−2789
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for 24 h and dried in a vacuum oven at 120 °C. In order to improve the interfacial adhesion between the polymer and the zeolite, annealing was applied to porous MMMs at different temperatures (180−250 °C) for a fixed time. The annealed porous MMMs were represented as Hxxxoo, where “H” means annealing and Arabic numerals xxx and oo represent the annealing temperature and zeolite loading, respectively. Characterization. A scanning electron microscope (SEM; Model S-3000, Hitachi) was used to observe the morphology of PSF membranes and MMMs. Light transmission experiments were performed to measure the onset time of the demixing of the cast film. The principle is that the light transmittance of the cast film would decrease with the appearance of optical inhomogeneity, which can be induced by solution demixing. The experimental setup and operating procedures were the same as those described by Reuvers.32 Thermogravimetric analysis (TGA; Perkin-Elmer TGA7) was carried out at temperatures ranging from 50 to 850 °C at a rate of 10 °C/ min in nitrogen atmosphere. The specific technique adopted to measure the free volume in polymers is the positron annihilation lifetime. For polymer applications, the orthopositronium (o-Ps) lifetime and its probability are related to the free-volume hole size and fraction, respectively. The positron annihilation lifetimes of polymeric membranes were determined by detecting the prompt γ-rays (1.28 MeV) from the nuclear decay that accompanied the emission of a positron from the 22Na radioisotope and the subsequent annihilation of the γ-rays (0.511 MeV). A fast−fast coincidence circuit of the positron annihilation lifetime (PAL) spectrometer with a lifetime resolution of 260 ps, as monitored by using a 60Co source, was used to record all the PAL spectra. Each spectrum was collected at a fixed total count of 1 × 106. All of the PAL spectra were analyzed by a finite-term lifetime analysis method using the PATFIT program. The shortest lifetime (τ1 ∼ 0.12 ns) is the lifetime of parapositronium, the intermediate lifetime (τ2∼0.4 ns) is due to free positron annihilation, and the long lifetime (τ3 ∼ 1−3 ns) is due to o-Ps annihilation. In the current PAL method, we employed the results of the o-Ps lifetime to obtain the mean free-volume hole radius by the following semiempirical equation:33
that using a large pore-size zeolite for MMMs would potentially offset the negative effects of partial pore blockage and polymer chain rigidification on permeability. Clarizia et al. compared APDEMS with other coupling agents.24 Diethanolamine, being a smaller coupling agent than APDEMS, was more effective to reduce the void between the zeolite and the polymer. The success of Grignard-treated zeolites in the polymer matrix highlighted that coupling of the polymer to the sieve surface was not a prerequisite for successful mixed matrix membrane formation. According to the above discussion, mixed matrix membranes are not without drawbacks, the most important being the limit to their separation capabilities. Because the zeolite particles are not a continuous separation layer, only a small improvement over polymeric membranes can be achieved. Funk and Lloyd introduced the concept of zeolite-filled microporous mixed matrix membranes and discussed their potential use for gas separations.25 Such membranes addressed the drawbacks of both zeolite films and dense mixed matrix membranes by providing a porous matrix for zeolite particles, allowing only the particles to participate in the separation. Permeation modeling of microporous MMMs shows potential for improvement over dense MMM performance. Phase separation is a viable method for making porous polymeric membranes.26−31 This work merged the mixed matrix concept with a nonsolvent-induced phase separation system and formed a “porous mixed matrix membrane” with small pores of narrow pore size distribution. The air separation performance of resultant membranes was investigated to see whether the performance of mixed matrix membranes could be improved much further with the formation of the porous structure.
2. EXPERIMENTAL DETAILS Materials. The polysulfone (PSF) used in this study, under the trade name of Udel-P-3500, was supplied by AMOCO Performance Products Inc. (Ridgefield, CT, USA). Zeolite 4A (Aldrich) was chosen as the filler. The pore size of zeolite 4A is 3.8 Å. The average size of zeolite 4A is 2.4 μm, and the size distribution is in the range 1.0−5.0 μm. The average size and size distribution of zeolite 4A were determined by dynamic light scattering (DLS; Zetasizer Nano ZS90, Malvern, U.K.) with a He−Ne laser (wavelength = 633 nm). N-Methyl-2pyrrolidinone (NMP) was purchased from the Aldrich Chemical Co. and used without further purification. Alcohols employed in the different aqueous mixtures, namely, methanol, ethanol, 1-propanol, and 1-butanol, were obtained from TEDIA Co., Inc. Preparation of Membranes. PSF was dissolved in NMP to form 20 and 25 wt % polymer solutions, and the solutions were then stored overnight for degassing purposes. The degassed solution was cast on a glass plate with a casting knife (300 μm), and then immersed in an alcohol bath for a period of time to get porous PSF membranes. Mixed matrix membranes were prepared from a polymer solution containing 25 wt % PSF and 75 wt % NMP, filled with 10 to 60 wt % (based on PSF) of total zeolite 4A, as follows. Zeolite particles were dispersed into NMP by sonication for 5 min to have a homogenized dispersion. Then a part of the total amount of polymer was added to the solution and stirred for 4 h to prevent the aggregation of zeolite particles; the remaining polymer was finally added and stirred for another 20 h. The casting process for MMMs was similar to that for pure PSF membranes. The resultant membrane was kept in a water bath
−1 ⎛ 2πR ⎞⎤ 1 1⎡ R τ3 = ⎢1 − + sin⎜ ⎟⎥ 2 ⎢⎣ R0 2π ⎝ R 0 ⎠⎥⎦
(1)
where τ3 (o-Ps lifetime) and R (hole radius) are expressed in nanoseconds and angstroms, respectively. R0 is equal to R + ΔR, where ΔR is a fitted empirical electron layer thickness (=1.66 Å). Permeation Study. A gas permeation analyzer (Yanaco GTR10) was used to measure the pure gas (O2 and N2) permeation for the membranes. The tests were carried out under isothermal conditions at 35 °C (±0.5 °C). Permeability and permeance are expressed as barrers (10−10 (cm3(STP) cm)/(cm2 s cmHg)) and gas permeation units (GPU; 10−6 (cm3(STP))/(cm2 s cmHg)), respectively. The ideal selectivity was calculated based on the ratio of the permeability coefficients: αA/B = PA /PB
(2)
where PA and PB are the respective permeability coefficients of pure gases A and B. 2782
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Figure 1. Cross-sectional SEM images of PSF membranes: coagulated (a) by immersion into a methanol bath, (b) by immersion into an ethanol bath, (c) by immersion into a propanol bath, and (d) by immersion into a butanol bath (d). (e) and (f) represent magnified skin regions of (c) and (d), respectively.
3. RESULTS AND DISCUSSION Formation and Air Separation Application of Porous PSF Membrane. Liquid−liquid nonsolvent-induced phase separation is a viable method for making porous polymeric membranes. In this work, we used the nonsolvent-induced phase separation (NIPS) method to get a suitable porous structure of zeolite-filled porous mixed matrix membranes. Figure 1 shows the morphology of the PSF membranes obtained by immersing the PSF/NMP cast film in different alcohol baths. SEM pictures of the membrane cross section
show that the membrane (Figure 1a), which was prepared by immersion into a methanol bath, has a graded pore structure from skin to sublayer. The skin region of the membrane consists of teardrop voids and dense layer. The membrane also has macrovoids in the sublayer. On the contrary, two membranes (Figure 1c,d), which were prepared by immersion into propanol and butanol baths, respectively, have closed pore structures in the whole cross section. The formation of the membrane structure can be related to the demixing characteristics. Among the precipitation rates of the film immersed into 2783
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effect, through the porous PSF membrane also increases by 9 times compared with that through the dense membrane. This is strong evidence that the formation of macropores reduced the gas transport resistance dramatically. The presence of macropores would have resulted in a decrease in the ideal separation factor. Also, the separation factor will decrease further when zeolite particles are added into the polymer to form MMMs. A more selective porous membrane is needed if we hope that the separation factor of porous MMMs does not decrease too much. It is well-known that the separation factor of membranes increases with increasing polymer concentration of the casting solutions. Therefore, when the polymer concentration increases from 20 to 25 wt % for preparing MMMs to get an applicable porous membrane, its separation factor is similar to that of the dense membrane, as shown in Table 1. Formation and Air Separation Application of Porous MMMs. SEM images of the interior of porous MMMs were taken. Cross-sectional images reveal that zeolites are welldistributed throughout the membrane, as shown in Figure 3. Few areas of agglomeration existed when the zeolite loading increased to 50 wt %. As with the previous discussion, even closed pore structures formed in porous MMMs. For a low zeolite loading, the zeolite particle is located in the center of several pores (Figure 3a), and it has effectively broken the wall of closed pores. Zeolite particle aggregation reduces the integrity of the closed pore structure and creates some channels (Figure 3d) when the zeolite loading increases. The particle real content of MMM is shown in Figure 4. For MMM with 30 wt % zeolite 4A loading, the particle real content calculated from TGA data was 28.3 wt %. The experimental zeolite 4A content was close to the practical addition. Figure 5 shows the results of oxygen permeability, oxygen permeance, and O2/N2 selectivity of porous MMMs prepared from the zeolite 4A−PSF with 0− 50 wt % loading. The oxygen permeabilities increase with the zeolite loading, but the selectivities decrease. The trend of permeance and permeability are the same, but the degree of variation is small for permeance. The pore size of zeolite 4A (3.8 Å) was bigger than the kinetic diameter of both oxygen (3.46 Å) and nitrogen (3.64 Å), so both gases can pass through the pore of zeolite base on size discrimination. However, zeolite 4A still can separate the oxygen and nitrogen due to the size and molecular shape difference between those two molecules. Some undesirable channels were formed between the zeolite and the polymer interphase, and gas may pass though these channels. We cannot at present clearly define the effect of undesirable channels and zeolite on the gas permeation gate and which one was the dominator. Therefore, the increase in the permeability with the loading could be explained by taking into account a combination of two factors. The first factor means that gas transport through the zeolite particle can occur, and the zeolite particle has a higher permeability than the polymer phase. The second factor is a change which may be induced by the zeolite particle in the organic/inorganic interphase. The loss of selectivity could be due to the generation of small nonselective voids existing between the zeolite particles and the organic/inorganic interphase. The permeability increases and the selectivity decreases significantly when the zeolite loading is more than 30 wt %. This indicates that higher zeolite loading (>30 wt %) will create more nonselective voids; SEM images (Figure 3) prove this result. Effect of Annealing on Porous MMMs. According to the data in Figure 5, it should be noted that the permeability increases from 7.6 (0 wt %) to 278.0 (50 wt %) barrers, but the
the four coagulant baths used in the experiment, the precipitation rate of the film immersed into a methanol bath just after casting has the fastest rate because of the rapid exchange between methanol and NMP. When the number of carbon atoms of alcohol decreased, the solubility parameter difference between NMP and alcohol decreased and the molecular size of alcohol would become small. This resulted in a faster demixing rate and created a graded pore structure in the membrane. Also, a closed pore structure was obtained when the faster demixing rate had slowed down. To get more insight into the demixing rate change, a light transmission experiment was performed. Figure 2 shows that the change in the type of
Figure 2. Light transmission curves obtained after immersion of the cast film into methanol and butanol.
alcohol can shift the PSF/NMP/alcohol system from delayed demixing (butanol) to instantaneous demixing (methanol), consistent with the observation that methanol as a coagulant can induce a graded pore structure. Even though the two membranes derived from propanol and butanol have closed pore structures in the whole cross section, these structures can also be distinguished from each other, as shown in Figure 1. The membrane in Figure 1d has even closed pores with 3−5 μm pore size, which is similar to the size of zeolite particles. Therefore, we chose butanol as a coagulant to prepare PSF/zeolite porous MMMs. The permeation properties of the dense and porous PSF membrane are listed in Table 1. The oxygen permeability, the value in parentheses, through the porous PSF membrane, which was prepared from casting a solution with a PSF concentration of 20 wt %, increases by 29 times compared with that through the dense membrane. The permeability considered the effect of membrane thickness. The permeance, ignoring the thickness Table 1. O2/N2 Separation Properties of Pure PSF Membranes membrane
PO2
αO2/N2
dense PSF porous PSF, 20 wt % porous PSF, 25 wt %
0.07a (1.4)b 0.65 (40.9) 0.13 (7.6)
5.6 3.6 5.1
P is permeance in GPU (1 GPU = 1 × 10−6 cm3(STP)/cm2 s cmHg). P is permeability in barrers (1 barrer = 1 × 10−10 cm3(STP) cm/cm2 s cmHg). a b
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Figure 3. Cross-sectional (left) and magnified cross-sectional (right) SEM images of porous MMMs with (a) 20, (b) 30, (c) 40, and (d) 50 wt % zeolite loading.
Figure 6 illustrates the effect of varying annealing temperatures on the oxygen permeability, oxygen permeance, and O2/N2 selectivity for porous MMMs with 50 wt % zeolite 4A in PSF. The increase in temperature leads to lower permeability or permeance, while this effect is more pronounced at higher
selectivity decreases from 5.1 to 1.4. The substantial change in the permeation performance means that we can obtain porous MMMs with appropriate air separation performance if we can tune the microstructure of the membrane. Annealing is a wellknown method for tuning the microstructure of membranes. 2785
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Figure 4. Thermogravimetric analysis of polysulfone membrane and MMM.
Figure 6. Separation properties vs annealing temperature for 50 wt % zeolite 4A−PSF porous MMMs.
annealing temperature to improve the air separation property of porous MMMs. Positron annihilation lifetime measurements were made to confirm the microstructural changes in porous MMMs during annealing. PAL results for the pristine and the porous MMM annealed at 200 °C are listed in Table 2. There are two kinds of pores that affect the gas transport: one is the free volume in the wall of the pore, and the other is the pore existing in the zeolite/polymer interphase. Clearly, both the free volume (τ3) and the pore size (τ4) of the annealed porous MMM are smaller than those of the pristine membrane, suggesting the possibility of permeability decreasing and selectivity increasing due to annealing. The comparison of the I3 and I4 of the pristine and annealed MMM is also interesting. The amounts of free volume (I3) and pores (I4) decrease with annealing. This indicates that a part of the free volume and pore vanishes during the annealing process, and it improves the selectivity of the MMM but reduces the permeation rate. Figure 8 shows the results of permeability, permeance, and selectivity of the porous MMMs prepared from zeolite 4A−PSF with 0−60 wt % loading and which is annealed at 200 °C. Oxygen and nitrogen permeabilities or permeances increase with the zeolite loading, and the O2/N2 selectivity remains constant at zeolite loading of
