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High-Flux SAPO-34 Membrane for CO2/N2 Separation Shiguang Li* and Chinbay Q. Fan Office of Technology and InnoVation, Gas Technology Institute, 1700 S Mount Prospect Road, Des Plaines, Illinois 60018-1804
SAPO-34 membranes, synthesized by using multiple templates and reduced crystallization time, showed high CO2 permeability for separating CO2/N2 mixtures up to 230 °C. At a trans-membrane pressure drop of 138 kPa and an atmospheric pressure in the permeate side, one such membrane had a CO2 permeance of 1.2 × 10-6 mol/m2 · s · Pa () 3500 GPU) with a CO2/N2 separation selectivity of 32 for a 50/50 feed at 22 °C. At a feed pressure of 2.3 MPa (23 bar), the CO2 flux was as high as 75 kg/m2 h. The CO2/N2 separations were also investigated in part by using vacuum permeate pumping, where the membrane showed a CO2 permeance of 7.7 × 10-7 mol/m2 · s · Pa and a CO2 permeate concentration of 93% for an equimolar feed at 22 °C. At 105 °C, adding 8% water vapor to the feed had almost no effect on the CO2/N2 separation selectivity. The CO2 permeance decreased by 37%, but it was recovered by calcination. 1. Introduction The greatest concern to global warming is the emission of greenhouse gases, especially CO2 from various sources. According to the Environmental Protection Agency, the U.S. emitted 6.1 billion metric tons of CO2 to the atmosphere in 2007.1 Carbon dioxide is primarily emitted from fossil fuel combustion, natural gas sweetening, synthesis gas production, and certain chemical plants. Low-temperature distillation is the widely used commercial process for purification CO2 from high pressure gas streams containing >90% CO2. However, this method is not practical for atmospheric pressure flue gases containing about 15% CO2. Alkaline sorbents and scrubbing solutions are also employed to remove CO2 from various gas mixtures. Compared to these methods, membrane processes are less expensive, require less energy to operate, and do not need chemicals or regenerating absorbents to maintain. Also, membranes are compact and can be retrofitted onto the tail end of power-plant flue gas streams without complicated integration. Permeance and selectivity at the intended operating environment are the two basic criteria to determine whether a membrane can be effectively utilized for flue gas applications. Flue gases are at or slightly above atmospheric pressure. Creating adequate driving force accounts for most of the cost for membrane separation. The majority of previous studies have compressed feed gas (15-20 bar), while allowing the permeate to release at atmospheric pressure (designated as pressurized feed/atmospheric permeate mode). Under this mode, the feedgas and the post-separation compressors account for over 50% of the capital and operating costs.2 Another approach is to leave the feed gas close to atmospheric pressure and use a vacuum to draw the permeate (designated as atmospheric feed/vacuum permeate mode). Under this mode, the estimated cost of capturing CO2 is reduced by 65%.2 For CO2 separation from flue gas, it has been reported that a CO2/N2 selectivity of >70 and a CO2 permeance of >3.3 × 10-7 mol/(m2 · s · Pa) or 1000 GPU (GPU is an industrial unit equivalent to 10-6 cm3(STP)/(cm2 · s · cmHg)) are required for economic operation.3 Polymeric membranes have been successfully applied for the separation of CO2 from natural gas streams. However, poor stability at boiler exit temperature and * To whom correspondence should be addressed. E-mail: shiguang.li@ gastechnology.org. Phone: 847-544-3478. Fax: 847-768-0916.
sensitivity to acid gases such as SO2 and SO3, limit their applications.4 They are only applicable to the coal power plants with wet SO2 scrubbers, which reduce the flue gas temperature below 57 °C (135 °F) and remove 90% of acid gases.5 Note that 75% of coal power plants (>600 plants) in the United States do not employ SO2 scrubbers,6 and the flue gas temperatures of these plants are typically higher than 100 °C. Consequently, currently membrane systems are not an option for most coalfired plants. An alternative approach is to develop membrane materials that are inherently stable at higher temperatures and harsh chemicals. Molecular sieve materials (such as zeolite) are one such class of materials for highly selective membranes that overcome problems associated with existing polymer materials, and that offer an opportunity to expand membrane technology. Zeolite membranes are multicrystalline materials synthesized as a dense layer on a porous support surface (R-Al2O3, γ-Al2O3, or stainless steel) and/or within the pores of the support. The porous supports can be thick but with large pores (0.1-5 µm). They provide mechanical strength without introducing additional mass transfer resistance. Because the zeolite membrane is an inorganic oxide and the underlying support is ceramic or metal, these membranes are far more robust than conventional polymeric membranes and they are usable in high-pressure environments. In addition, these membranes are stable to at least 400 °C, as well as in chemically corrosive conditions. In addition to their robustness, zeolite membranes are of interest because they can separate gas mixtures with high selectivity. Depending upon the type of zeolite, the mixture system, and the operating conditions, mixtures are separated in accordance with at least the following three principles or mechanisms: (1) molecular sieving, where larger molecules are unable to fit into the pores, and thus the smaller molecules preferentially permeate; (2) differences in diffusivity, where the smaller, less hindered type of molecule in a mixture diffuses faster than the larger ones; and (3) competitive adsorption, where one type of molecule is more strongly adsorbed on the zeolite and thus can dramatically inhibit permeation of another type of molecule. The kinetic diameters of CO2 and N2 are 0.33 and 0.364 nm, respectively. Thus, to obtain a high CO2 flux and a high CO2/N2 selectivity by way of differences in diffusivity responsible for the separation, zeolite membranes would have pores (diameters) of approximately 0.35-0.55 nm.
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SAPO-34 membranes, which have pore sizes of 0.38 nm, have shown to be effective for removal of CO2 from natural gas.7-16 They had CO2/CH4 separation selectivities higher than 170, with CO2 permeances as high as ∼2 × 10-6 mol/(m2 · s · Pa) at 22 °C and a feed pressure of 224 kPa.16 SAPO-34 is a silicoaluminophosphate having the composition SixAlyPzO2 where x ) 0.01-0.98, y ) 0.01-0.60, and z ) 0.01-0.52.16 The SAPO34 structure is formed by substituting silicon for phosphorus in the AlPO4, which has a neutral framework and exhibits no ion exchange capacity.17 Briend et al.18 found SAPO-34 was highly stable in humid atmospheres at temperatures over 100 °C. The objective of the current study was to determine the potential of SAPO-34 membranes for CO2 capture in flue gas treatment. To obtain high CO2 fluxes, SAPO-34 membranes were synthesized by using a seeded growth method with multiple templates and reduced crystallization time. CO2/N2 separations through SAPO-34 membranes using both pressurized feed/ atmospheric permeate and atmospheric feed/vacuum permeate modes were investigated. In addition to separations performed over a range of conditions including temperature up to 230 °C, separations were also carried out with 8% water vapor in the feed. 2. Experimental Methods 2.1. Membrane Synthesis and Characterization. SAPO34 membranes were prepared by secondary growth onto tubular porous R-Al2O3 (0.2-µm pores, US Filter) and stainless steel (0.27-µm pores, Mott Corporation) supports. About 1 cm on each end of the Al2O3 supports was glazed to prevent membrane bypass and to provide a sealing surface for o-rings. Similarly, nonporous, stainless steel tubes were welded onto each end of the stainless steel supports. Before synthesis, the supports were boiled in deionized water for 1 h and dried at 150 °C for 30 min. SAPO-34 Seeds Synthesis. In a typical synthesis, 6.8 g of Al(i-C3H7O)3 (>99%, Aldrich), 3.85 g of H3PO4 (85 wt % aqueous solution, Aldrich), and 20 g of deionized H2O were stirred for 2 h to form an homogeneous solution. Then, 1.13 g of Ludox AS-40 colloidal silica (40 wt % suspension in water, Sigma-Aldrich) was added, and the solution was stirred for 0.5 h. Next, 12.3 g of tetraethylammonium hydroxide (TEAOH, 20 wt % solution in water, Sigma-Aldrich) was added, and the solution was stirred for another 0.5 h. Finally, 1.37 g of dipropylamine (99%, Aldrich) and 1.34 g of cyclohexylamine (99%, Sigma-Aldrich) were added, and the solution stirred for 12 h at room temperature. The resulting solution was placed in an autoclave, and treated hydrothermally at 220 °C for 12 h, producing SAPO seeds. After they were cooled to room temperature, the seeds were centrifuged at 2200 rpm for 20 min and washed with water. This procedure was repeated 4 times. The resultant precipitate was dried overnight and calcined at 500 °C for 5 h. The calcination heating and cooling rates were 1.0 °C/min. SAPO-34 Membrane Synthesis. The synthesis gel molar ratio was 1.0 Al2O3/1.0 P2O5/0.45 SiO2/1.2 TEAOH/1.6 dipropylamine/100 H2O. In a typical synthesis, Al(i-C3H7O)3, H3PO4 and deionized H2O were stirred for 0.5 h to form an homogeneous solution; then Ludox AS-40 colloidal silica was added, and the resulting solution was stirred for another 0.5 h. Then, TEAOH and dipropylamine were added, and the solution stirred for 12 h at room temperature. The membranes were prepared by rubbing the inside surface of porous supports with dry, calcined SAPO-34 seeds. The rubbed porous supports, with their outside wrapped with Teflon tape, were then placed in an
autoclave and filled with synthesis gel. The hydrothermal treatment was carried out at 220 °C for 2-6 h, after which the membranes were washed with deionized water. The SAPO-34 membranes were calcined in air at 390 °C for 10 h to remove the templates from the SAPO framework. The calcination was carried out in a computer-controlled muffle furnace. The calcination heating and cooling rates were 0.6 °C/min. Characterization. One membrane was broken and analyzed by scanning electron microscopy (SEM, Hitachi S-3500 N). 2.2. Gas Permeation and Separation. Single-gas and mixture permeations were measured in a flow system. The membrane was mounted in a stainless steel module and sealed at each end with silicone o-rings. Mass flow controllers were used to mix pure CO2 and N2 gases. The pressure on each side of the membrane was independently controlled. The membrane module was placed in an oven so that separation could be carried out at elevated temperatures (up to 250 °C). Separation was also performed for a feed mixture containing 8 mol · % water vapor. In that case, the CO2/N2 mixture flowed through a water bubbler, which was immersed in a water bath. The humidity was controlled by adjusting the water batch temperature. Fluxes were measured using a bubble flow meter. The compositions of the feed and permeate streams were measured by a CARLE Series 400 gas chromatograph equipped with a thermal conductivity detector and HAYESEP-A column. The oven was kept at 60 °C. The permeance of the component i, Pi, is Pi )
Ji ∆pln ,i
(1)
where Ji is the flux through the membrane for component i. For the cross-flow configuration, because one component preferentially permeates through the membrane, the partial pressures in the feed and retentate are quite different. Therefore, a Log-mean pressure drop, ∆pln,i, was calculated by ∆pln ,i )
(pf,i - pr,i) ln[(pf,i - pp,i)/(pr,i - pp,i)]
(2)
where pf,i, pr,i, andpp,i are partial pressures for component i, in feed, retentate, and permeate sides, respectively. The permeability is the permeance multiplied by membrane thickness. The ideal selectivity is the ratio of the single-gas permeances, and sep the separation selectivity, Ri/j , is the ratio of the permeances for mixtures. 3. Results and Discussion 3.1. Characterization. The SAPO-34 seeds used to synthesize membranes were cubic and rectangular crystals with sizes ranging from 0.5 to 1.2 µm (Figure 1). SAPO-34 membranes were prepared with one synthesis step by using reduced crystallization time. The cross-sectional SEM micrograph of a SAPO-34 membrane prepared with a crystallization time of 6 h shows a continuous zeolite layer approximately 5-µm thick on R-Al2O3 support (Figure 2). The support is composed of two well-defined regions: a top ∼40 µm thick layer with 0.2 µm average pore size and a porous area with 0.8 µm average pore size. 3.2. Effect of Crystallization Time. Membrane M1, as shown in Table 1, prepared with crystallization time of 6 h on R-Al2O3 support had a CO2/N2 separation selectivity of 32 with CO2 permeances of 1.2 × 10-6 mol/(m2 · s · Pa) for a 50/50 feed mixture at 22 °C and under a feed pressure of 240 kPa and
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Figure 1. Scanning electron micrograph of SAPO-34 seeds.
Figure 3. Revised Robeson plot for CO2/N2.19 Data point for SAPO-34 membrane is shown for comparison.
Figure 2. Cross-sectional SEM micrograph of a SAPO-34 membrane on R-Al2O3 support. Table 1. Comparison of CO2/N2 Separations at 22°C for a 50/50 Feed Mixture through SAPO-34 Membranes Prepared with Different Crystallization Times on r-Al2O3 Supports permeance
membrane
crystallization time (h)
mol/m2 · s · Pa
GPUa
separation selectivity
M1 M2 M3
6 4 2
1.2 × 10-6 1.5 × 10-6 IMb
3500 4500 IMb
32 21 1
a GPU is a unit used in industry: cm3(STP)/(cm2 · s · cmHg). b IM: Immeasurable.
1
GPU
)
10-6
atmospheric permeate. Decreasing the crystallization time to 4 h (membrane M2) increased the concentration of non-zeolite pores and, thus, decreased the CO2/N2 selectivity. However, its permeance was as high as 1.5 × 10-6 mol/(m2 · s · Pa) under the same test conditions. The higher CO2 permeance for membrane M2 was because it was thinner than M1. Further decreasing the crystallization time to 2 h (membrane M3) failed to produce a CO2 selective membrane. It appears that a continuous layer was not formed during such a short crystallization time. It should be noted that the permeances in GPU for M1 (3500) and M2 (4500) were much higher than that required for economic industrial operation (1000). The high CO2 permeances are the result of the presence of small zeolite crystals with narrow size distributions. Robeson19 recently revised the upper bound for CO2/N2 separation selectivities versus CO2 permeabilities (permeance × membrane thickness) of polymeric membranes at ∼22 °C (Figure 3). For comparison, data for SAPO-34 membrane M1 is also shown in Figure 3. The data point for the SAPO-34 membrane is significantly above this upper bound. The SEM thickness (5 µm) was used to calculate the permeability for Figure 3.
Figure 4. CO2 and N2 fluxes and CO2 permeate concentration at 22 °C of a CO2/N2 mixture (50/50) as a function of feed pressure through a SAPO34 membrane M2. The permeate pressure was kept at 102 kPa.
3.3. CO2/N2 Separations Using Pressurized Feed/ Atmospheric Permeate. Figure 4 shows CO2 and N2 fluxes for membrane M2 at 22 °C for an equimolar CO2/N2 mixture as a function of feed pressure. The permeate pressure was held at atmospheric pressure (102 kPa). Both CO2 and N2 fluxes increased with feed pressure because of the increases in adsorption coverage. Carbon dioxide permeates faster than N2 through membrane M2 because the smaller CO2 diffuses faster, and it has higher adsorption coverage than N2. Note that at a feed pressure of 23 bar, the CO2 flux was as high as 75 kg/m2 h. The CO2 permeate concentration increased from 85.8% to 89.5% as feed pressure increased from 2.4 to 4.5 bar, and remained almost constant at higher feed pressures. Figure 5 shows CO2 permeance and CO2/N2 separation selectivity through membrane M1 as a function of temperature. As the temperature increases, the CO2 adsorption coverage decreases and CO2 less effectively inhibits N2 adsorption. Thus, CO2/N2 separation selectivity decreased (Figure 5). However, our membrane still had a CO2/N2 separation selectivity of 6.2 at 200 °C. A steep decrease in selectivity was observed as temperature increased from 21 to 75 °C because the CO2 adsorption coverage decreased dramatically in this temperature range.7 At a typical supercritical bituminous power-plant flue gas temperature of 110 °C,2 membrane M1 had a CO2 permeance of 4.5 × 10-7 mol/(m2 · s · Pa) and a CO2/N2 separation selectivity of 10. To our knowledge, zeolite mem-
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Figure 5. CO2 and N2 permeances and CO2/N2 selectivity of a CO2/N2 mixture (50/50) through a SAPO-34 membrane M1 as a function of temperature. The feed and permeate pressures were 240 and 102 kPa, respectively.
Figure 6. CO2 and N2 permeances of single gases and a CO2/N2 mixture (50/50) through a SAPO-34 membrane M1 as a function of temperature. The feed pressure was 102 kPa, and the permeate was under vacuum (5 kPa).
branes with CO2/N2 selectivity >5 at temperatures higher than 100 °C have not been reported. 3.4. CO2/N2 Separations Using Atmospheric Feed/ Vacuum Permeate. The CO2/N2 separation properties of the membrane was also evaluated with the feed gas at atmospheric pressure while drawing the permeate under a 5 kPa vacuum. As shown in Figure 6, over the temperature range of 22-230 °C, both CO2 and N2 permeances in single gases and an equimolar CO2/N2 mixture decreased as the temperature increased from 22 to 230 °C. The CO2 permeances were identical for single gas and mixture. Previous study for CO2/CH4 mixture suggested that the slower diffusing CH4 slows the fasterdiffusing CO2.9,12 This slowing-down effect was not observed for CO2 in CO2/N2 mixture probably because the differences in diffusivity between CO2 and N2 were smaller than those between CO2 and CH4. The N2 permeance was slightly higher for a single gas than a gas mixture, indicating that CO2 slightly inhibited N2 adsorption in the mixture. As a result, the separation selectivity was a bit higher than ideal selectivity (Figure 7). At 22 °C, the CO2 permeate concentration was 93%, and the CO2 flux was 5.2 kg/m2 h (CO2 permeance ) 7.7 × 10-7 mol/ m2 · s · Pa). This flux is higher than most pervaporation fluxes through zeolite membranes, even though the driving force is low. The CO2 concentration in the permeate decreased with temperature as shown in Figure 7, but it was still as high as 83% at 230 °C. At 110 °C, the CO2/N2 separation selectivity was 8 and CO2 permeance was 3 × 10-7 mol/(m2 · s · Pa).
Figure 7. Selectivities and CO2 permeate concentration of a CO2/N2 mixture (50/50) through a SAPO-34 membrane M1 as a function of temperature. The feed pressure was 102 kPa, and the permeate was under vacuum (5 kPa).
Figure 8. Effect of H2O vapor on CO2/N2 separation at 105 °C for a SAPO34 membrane on stainless steel support. The feed pressure was 102 kPa, and the permeate was under vacuum (5 kPa).
3.5. Effect of Water Vapor in the Feed. The water vapor concentration of flue gases from coal-fired power plants without wet SO2 scrubber is about 8 mol · %. Figure 8 shows that water vapor had almost no effect on the CO2/N2 separation selectivity at 105 °C for a 14% CO2/78% N2/8% H2O ternary mixture through a SAPO-34 membrane on stainless steel support. The CO2 permeance, however, decreased after water vapor had been introduced, and reached steady state in ∼23 h. The steady-state CO2 permeance was 63% of its original value. Poshusta et al.20 found humidity (0.6-0.9 mol %) had a strong effect on gas permeation through SAPO-34 membranes at room temperature. In contrast, SAPO-34 membrane showed better separation performance at 105 °C in the presence of water vapor. After water effect experiment, the membrane had been recalcined at 280 °C for 10 h and used again at 105 °C for separating a dry 15% CO2/85% N2 mixture. The CO2 permeance was restored, and the CO2/N2 separation selectivity was identical to that of the initial membrane. This is consistent with Briend et al.’s observation that SAPO-34 is stable in humid atmospheres at temperatures over 100 °C.18 3.6. Comparison to Zeolite Membrane Reported in the Literature. Large-pore, medium-pore, and small-pore zeolite membranes have been reported for CO2/N2 separation. Table 2 compares CO2 permeances and CO2/N2 selectivities with zeolite membranes reported in the literature. Kusakabe et al.,21 using a NaY zeolite membrane (FAU structure, 0.74-nm pore diameter), obtained a CO2/N2 selectivity of 20, and CO2 permeance of 1.6 × 10-7 mol/(m2 · s · Pa) at 30 °C for an equimolar CO2/ N2 mixture (Table 2). Guo et al.22 used a pressure drop across
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Table 2. Comparison of CO2/N2 Separations through Zeolite Membranes membrane/support FAU/alumina tube FAU/alumina disk silicalite-1/stainless steel net Na-ZSM-5/alumina tube NaA/carbon T-type/mullite tube DDR/alumina tube SAPO-34/alumina tube a
pore diameter (nm) 0.74 0.74 0.55 0.55 0.42 0.41 0.36 × 0.44 0.38
temp (°C) 30 50 20 35 22 35 29 22
permeance (mol/m2 · s · Pa) -7
0.4-3 × 10 3.9 × 10-8 7.0 × 10-7 1.0 × 10-7 3.4 × 10-7 4.6 × 10-8 6.0 × 10-8 1.2-1.5 × 10-6
selectivity
ref
20-100 20 68 40 6.0a 107 20b 21-32
Kusakabe et al.20 Gu et al.27 Guo et al.21 Shin et al.28 Zhou et al.23 Cui et al.24 van den Bergh et al.25 this study
Ideal selectivity based on single-gas permeations. b CO2/air separation.
a ZSM-5 (MFI structure, 0.55-nm pore diameter) membrane and obtained a CO2/N2 separation selectivity of 68 at room temperature (Table 2). Because both CO2 and N2 molecules are much smaller than the FAU and MFI pores, separation with these membranes was mainly based on competitive adsorption. Recently, Ohta et al.23 performed dynamic Monte Carlo simulations to predict CO2/N2 separations through various zeolite membranes with large, medium, and small pores. They found that membranes with small pores, large adsorption site, and high connectivity of pores had the highest CO2/N2 selectivity. The most studied small-pore zeolite membrane is NaA (0.42nm pore diameter). It has been commercialized for separating water from organics by pervaporation. However, gas separation selectivities are typically low for NaA membrane due to the presence of nonzeolite pores that result from intercrystalline boundaries or amorphous regions. For example, using a NaA/ carbon membrane, Zhou et al.24 observed that the CO2/N2 ideal selectivity was only 6 at room temperature and 100 kPa (Table 2). Unlike NaA membranes, some small-pore zeolite membranes have been successfully synthesized with low concentrations of nonzeolite pores and have shown good CO2/N2 selectivities. Cui et al.,25 using a T-type zeolite membrane (0.41-nm pore diameter), obtained a CO2/N2 composition selectivity of 107 and a CO2 permeance of 4.6 × 10-8 mol/(m2 · s · Pa) at 35 °C for a feed pressure of 100 kPa and vacuum on the permeate side (Table 2). Van den Bergh et al.26 obtained a CO2/air selectivity of ∼20 and a CO2 permeance of 6 × 10-8 mol/ (m2 · s · Pa) at 29 °C (Table 2) and a total feed pressure of 101 kPa for a DDR membranes (0.36 × 0.44-nm pore diameter). In contrast, our SAPO-34 membranes have high potential for CO2 capture in flue gas treatment since their CO2 permeances were about 1-2 orders of magnitude higher than other reported small-pore zeolite membranes. Even at 110 °C, the permeances for our current membranes, either using the pressurized feed/ atmospheric permeate mode (4.5 × 10-7 mol/(m2 · s · Pa) as shown in Figure 5 or using the atmospheric feed/vacuum permeate mode (3 × 10-7 mol/(m2 · s · Pa) as shown in Figure 6, generally meet the requirement for economic industrial operation. Note that sensitivity studies indicate that high CO2 permeance is much more important than high selectivity in lowering the cost of CO2 capture.5 The CO2/N2 separation selectivity of our SAPO-34 membranes can be further improved by post-treatment to reduce the SAPO pores from the current 0.38 nm size to less than 0.364 nm (the kinetic diameter of N2) so as to enhance the differences in diffusivity or even have molecular sieving play a role in the separation process. 4. Conclusion SAPO-34 membranes have potential for CO2 capture in flue gas treatment because of the high CO2 permeance and reasonable high CO2/N2 selectivity; at a trans-membrane pressure drop of
138 kPa and an atmospheric pressure in the permeate side, a membrane had a CO2 permeances of 1.2 × 10-6 mol/m2 · s · Pa (3500 GPU) with CO2/N2 separation selectivity of 32 for a 50/ 50 feed at 22 °C. At a feed pressure of 23 bar, the CO2 flux was as high as 75 kg/m2 h. CO2/N2 separations were also effective when using vacuum permeate pumping and leaving the feed at atmospheric pressure. At 105 °C, addition of 8% water vapor to the feed had almost no effect on the CO2/N2 separation selectivity. The CO2 permeance decreased by 37% but could be recovered by calcination. For such a high-flux membrane, the CO2 permeances were generally meet the requirement for the economic industrial operation at 110 °C. The CO2/N2 separation selectivity, however, needs to be further improved by postsynthesis treatment. Acknowledgment We gratefully acknowledge support by the GTI IR&D (20904). We thank Dr. Jack Lewnard for his useful discussions. Literature Cited (1) http://www.epa.gov/climatechange/policy/intensitygoal.html. (2) Ho, M. T.; Allinson, G.; Wiley, D. E. Reducing the cost of CO2 capture from flue gases using membrane technology. Ind. Eng. Chem. Res. 2008, 47, 1562. (3) Huang, J.; Zou, J.; Ho, W. S. W. Carbon dioxide capture using a CO2-selective facilitated transport membrane. Ind. Eng. Chem. Res. 2008, 47, 1261. (4) Granite, E. J.; O’Brien, T. Review of novel methods for carbon dioxide separation from flue and fuel gases. Fuel Process. Technol. 2005, 86, 1423. (5) Merkel, T.; Lin, H.; Wei, X.; He, J.; Firat, B.; Amo, K.; Daniels, R.; Baker, R. A membrane process to capture CO2 from coal-fired power plant flue gas, NETL review meeting, March 24-26, 2009. (6) Form EIA-767 Database, Energy Information Administration website, 2005. (7) Li, S.; Falconer, J. L.; Noble, R. D. SAPO-34 Membranes for CO2/ CH4 separation. J. Membr. Sci. 2004, 241, 121–135. (8) Li, S.; Alvarado, G.; Noble, R. D.; Falconer, J. L. Effects of impurities on CO2/CH4 separations through SAPO-34 membranes. J. Membr. Sci. 2005, 251, 59. (9) Li, S.; Martinek, J.; Falconer, J. L.; Noble, R. D.; Gardner, T. Q. High-pressure CO2/CH4 separation using SAPO-34 membranes. Ind. Eng. Chem. Res. 2005, 44, 3220. (10) Li, S.; Falconer, J. L.; Noble, R. D. Improved SAPO-34 membranes for CO2/CH4 separations. AdV. Mater. 2006, 18, 2601. (11) Li, S.; Falconer, J. L.; Noble, R. D.; Krishna, R. Interpreting unary, binary, and ternary mixture permeation across a SAPO-34 membrane with loading-dependent Maxwell-Stefan diffusivities. J. Phys. Chem. C 2007, 111, 5075. (12) Li, S.; Falconer, J. L.; Noble, R. D. Krishna, Modeling permeation of CO2/CH4, CO2/N2, and N2/CH4 mixtures across SAPO-34 membrane with the Maxwell-Stefan equations. Ind. Eng. Chem. Res. 2007, 46, 3904. (13) Hong, M.; Li, S.; Falconer, J. L.; Funke, H. F.; Noble, R. D. Ionexchanged SAPO-34 zeolite membranes for light gas separations. Microporous Mesoporous Mater. 2007, 106, 140. (14) Li, S.; Falconer, J. L.; Noble, R. D. SAPO-34 membranes for CO2/ CH4 separations: effect of Si/Al ratio. Microporous Mesoporous Mater. 2008, 110, 310.
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(15) Carreon, M. A.; Li, S.; Falconer, J. L.; Noble, R. D. SAPO-34 seeds and membranes prepared using multiple structure directing agents. AdV. Mater. 2008, 20, 729. (16) Carreon, M. A.; Li, S.; Falconer, J. L.; Noble, R. D. Alumina supported SAPO-34 membranes for CO2/CH4 separations. J. Am. Chem. Soc. 2008, 130, 5412. (17) Szostak, R. Molecular SieVessPrinciples of Synthesis and Identification; Van Nostrand Reinhold: New York, 1989. (18) Briend, M.; Vomscheid, R.; Peltre, M. J.; Man, P. P.; Barthomeuf, D. Influence of the choice of the template on the short- and long-term stability of SAPO-34 zeolite. J. Phys. Chem. 1995, 99, 827. (19) Robeson, L. M. The upper bound revisited. J. Membr. Sci. 2008, 320, 390. (20) Kusakabe, K.; Kuroda, T.; Murata, A.; Morooka, S. Formation of a Y-type zeolite membrane on a porous R-alumina tube for gas separation. Ind. Eng. Chem. Res. 1997, 36, 649. (21) Poshusta, J. C.; Noble, R. D.; Falconer, J. L. Characterization of SAPO-34 membranes by water adsorption. J. Membr. Sci. 2001, 186, 25. (22) Guo, H.; Zhu, G.; Li, H.; Zou, X.; Yin, X.; Yang, W.; Qiu, S.; Xu, R. Hierarchical growth of large-scale ordered zeolite silicalite-1 membranes with high permeability and selectivity for recycling CO2. Angew. Chem., Int. Ed. 2006, 45, 7053.
(23) Ohtaa, Y.; Takabab, H.; Nakao, S. A combinatorial dynamic Monte Carlo approach to finding a suitable zeolite membrane structure for CO2/ N2 separation. Microporous Mesoporous Mater. 2007, 101, 319. (24) Zhou, Z.; Yang, J.; Zhang, Y.; Chang, L.; Sun, W.; Wang, J. NaA zeolite/carbon nanocomposite thin films with high permeance for CO2/N2 separation. Sep. Purif. Technol. 2007, 55, 392. (25) Cui, Y.; Kita, H.; Okamoto, K.-I. Preparation and gas separation performance of zeolite T membrane. J. Mater. Chem. 2004, 14, 924. (26) van den Bergh, J.; Zhu, W.; Gascon, J.; Moulijn, J. A.; Kapteijn, F. Separation and permeation characteristics of a DD3R zeolite membrane. J. Membr. Sci. 2008, 316, 35. (27) Gu, X.; Dong, J.; Nenoff, T. M. Synthesis of defect-free FAUtype zeolite membranes and separation for dry and moist CO2/N2 mixtures. Ind. Eng. Chem. Res. 2005, 44, 937. (28) Shin, D. W.; Hyun, S. H.; Cho, C. H.; Han, M. H. Synthesis and CO2/N2 gas permeation characteristics of ZSM-5 zeolite membranes. Microporous Mesoporous Mater. 2005, 85, 313.
ReceiVed for reView December 31, 2009 ReVised manuscript receiVed March 11, 2010 Accepted March 17, 2010 IE902082F
