Article pubs.acs.org/IECR
Optimum Conditions for the Synthesis of CHA-Type Zeolite Membranes Applicable to the Purification of Natural Gas Saeed Shirazian and Seyed Nezameddin Ashrafizadeh* Research Laboratory for Advanced Separation Processes, Department of Chemical Engineering, Iran University of Science and Technology, Narmak 16846-13114, Tehran, Iran ABSTRACT: The synthesis and characterization of CHA-type zeolite membranes was carried out in this work. Chabazite seeds were synthesized by the hydrothermal treatment of zeolite Y. The obtained chabazite particles were evaluated in the adsorption of CO2 and CH4. CHA membranes were synthesized by a secondary growth method on α-alumina substrate. Four synthesis temperatures of 100, 120, 140, and 160 °C and a duration of 20 h were employed for the synthesis of membranes. The structures and morphologies of the synthesized CHA zeolite seeds and membranes were characterized using scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and energy-dispersive X-ray spectroscopy. Moreover, single-gas permeation analyses were performed to evaluate the potential of CHA membranes in capturing CO2 from CH4 as a model of natural gas. The results revealed that the gas permeation first decreased with increasing synthesis temperature from 100 to 140 °C and subsequently increased with further enhancement of synthesis temperature to 160 °C. Reverse trends were observed in the case of the CO2/CH4 ideal selectivity. Increasing the synthesis temperature caused the formation of an integrated zeolite layer at the surface of the substrate. The latter decreased the permeation through the membranes. The optimum conditions for the synthesis of CHA zeolite membranes were obtained as a synthesis temperature of 140 °C and a synthesis duration of 20 h. CHAtype zeolite membranes showed great capability for the removal of CO2 from CH4.
1. INTRODUCTION Currently, concerns about emissions of greenhouse gases are growing worldwide. Many efforts are made by researchers to mitigate the release of greenhouse gases into the atmosphere. Carbon dioxide is known as the most common greenhouse gas that is mostly released by industrial factories. The development of novel methods for the capture and removal of CO2 from gas streams has been a subject of great interest for the research community. Absorption of CO2 into amine aqueous solutions is currently used as the most common method for CO2 capture. Much work has been done regarding experimental and theoretical developments of amine absorption processes to improve the efficiency of this method.1,2 Membrane separation processes have shown great ability in gas separation. Membrane processes offer superior characteristics compared to amine absorption and other CO2 separation methods.3−6 Various membrane processes can be utilized for the removal of CO2 from gas mixtures including those using dense polymeric and ceramic membranes, zeolite membranes, and membrane contactors.1,2,7−10 The most common membrane tool for CO2 removal that has been extensively studied by researchers is membrane contactors (MCs). Membrane contactors are porous and provide a high surface area per contact volume between the gas and liquid phases. A large number of experimental and theoretical studies have been conducted on the development of this process.1,7−10 Another attractive and efficient membrane separation tool that can be used for the removal of CO2 from gas mixtures is zeolite membranes. Zeolite membranes are polycrystalline films of aluminosilicates deposited on porous supports such as alumina, titania, and zirconia. Zeolite membranes have shown great potential for separations based on molecular size and © 2014 American Chemical Society
shape because of the small pore size of zeolites, which is close to the kinetic diameters of small molecules. So far, various zeolite membranes such as zeolites Y, X, T, MFI, and A have been synthesized and tested for the separation of CO2 from gas streams.5,11−17 CHA is an important category of zeolites and has great potential for the capture and removal of CO2 from gas mixtures. Among the family of CHA zeolites, chabazite is a favorite for CO2 capture because of its superior properties toward CO2. The structure of chabazite consists of threedimensional pores including ellipsoidal-shaped cavities. In the chabazite structure, each cavity is entered by six apertures that are formed by 8-ring windows.18 The pores of chabazite zeolite have dimensions of 0.38 × 0.38 nm. Considering the kinetic diameters of CO2 (0.33 nm) and CH4 (0.38 nm), chabazite zeolite should be able to selectively separate CO2 from methane by a molecular sieving mechanism. Moreover, the affinity of CO2 toward chabazite zeolite has been reported to be higher than that of CH4. Therefore, chabazite zeolite membrane could be a great choice for the capture of CO2 from methane.3,4,6,19 A few studies have been conducted on the synthesis of chabazite zeolite membranes. This novel zeolite membrane was first introduced by Hasegawa and co-workers,20−23 who synthesized the membranes by a secondary growth method with the introduction of strontium ions for the promotion of zeolite formation. They prepared chabazite seeds by transforming the H form of zeolite Y in the presence of potassium Received: Revised: Accepted: Published: 12435
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ions (K-CHA). They showed that the presence of strontium ions in the synthesis solution promotes the formation of zeolite film and that, without the application of strontium ions, poor zeolite membranes are formed. The membranes showed great separation performance in the pervaporation dehydration of alcohols. High separation factors and permeation fluxes were achieved that were relatively high compared to those obtained using common zeolite NaA membranes for the dehydration of alcohols. Li et al.24 synthesized chabazite zeolite membranes without the application of strontium ions. They claimed that this method is more economical than the method developed by the Hasegawa group for the synthesis of chabazite zeolite membranes. They also characterized the as-synthesized membranes for the dehydration of alcohols in a pervaporation process. However, poor separation performances were obtained in their work. The synthesis of chabazite zeolite membranes for gas separation requires the development of a sophisticated synthesis procedure. Defect-free membranes should be synthesized for gas separation purposes. To the best knowledge of the current authors, no studies have been published on the synthesis of chabazite zeolite membranes for gas separations. In this study, chabazite zeolite membranes were synthesized by a hydrothermal process on the surface of porous α-alumina supports, and their gas permeation properties were examined. Gas adsorption and single-gas permeation tests were then carried out for CO2 and methane to assess the ability of chabazite zeolite membranes to remove CO2 from natural gas.
Figure 1. Schematic representation of the gas permeation experimental setup.
conditions, the values of gas permeances are easily calculated by the equation m P= M w (pf − pp )At (1) where pp and pf are the permeate and feed pressures (Pa), respectively; A is the effective membrane area (m2); t is the permeation time (s); Mw is the molecular weight of the gas; and m is the mass of permeated gas. 2.3. Gas Adsorption Experiments. The synthesized chabazite powder was evaluated in the adsorption of CO2 and methane to obtain the affinity of chabazite toward the single gases. Gas adsorption data were collected with a staticvolumetric adsorption experimental system. The experiments were carried out at ambient temperature. The chabazite sample was then exposed to a vacuum at 100 °C for 12 h to remove the adsorbed impurities. 2.4. Synthesis of NaY and Chabazite Zeolite Powder. Chabazite was synthesized from zeolite NaY as the starting material. Zeolite NaY with a molar composition ratio of 4.62 Na2O/1 Al2O3/10 SiO2/180 H2O25 was prepared using the hydrothermal method. The synthesis procedure was the same as reported by Robson.25 The initial gel was prepared from two gels, namely, seed gel and feedstock gel. Si solution was prepared by mixing sodium silicate (Na2SiO3) with sodium hydroxide (NaOH) and DI water. The aluminate solution was prepared by mixing sodium aluminate (NaAlO2), sodium hydroxide, and DI water. The gel was then poured into a Teflon autoclave for crystallization at 100 °C for 8 h. The product was then filtered and dried at 110 °C for 3 h. Chabazite zeolite was prepared from the decomposition of zeolite Y which was subsequently ion-exchanged to synthesize a fully exchanged potassium chabazite (K-CHA). The procedure for the preparation of HY zeolite included the use of NH4Cl for the preparation of H-form of zeolite Y. Zeolite NaY and NH4Cl were mixed in DI water. The slurry was stirred at 90 °C for 1 h, filtered, and washed with deionized water. The ion-exchange operation was repeated three times to ensure the formation of fully exchanged zeolite. The resulting solid was dried in an oven at 100 °C for 5 h and then calcined in a furnace at 500 °C for 4 h to obtain the H-form of zeolite Y. The mechanism involves the removal of NH3 from NH4+, which leaves H+ as the cation in the zeolite structure. Chabazite zeolites were prepared as crystal seeds according to the reported solid-phase conversion route, as described in
2. MATERIALS AND METHODS 2.1. Materials. The materials used to form the aluminosilicate gel for the synthesis of NaY, HY, and chabazite zeolites were sodium aluminate (NaAlO2, BDH), aluminum nitrate [Al2(NO3)3·9H2O, Fluka], strontium nitrate [Sr(NO3)2, POCH S.A.], colloidal silica (Ludox 40%, DuPont), sodium silicate (Na2SiO3, Aldrich), potassium hydroxide (KOH, 85 wt %, Merck), sodium hydroxide (NaOH, 99 wt %, Merck), ammonium chloride (NH4Cl, Merck), and deionized (DI) water. 2.2. Gas Permeation Setup. Gas permeation tests are among the most important methods for the characterization of membrane performance. Moreover, the structures of assynthesized zeolite membranes can also be evaluated by gas permeation tests. In addition, the successful formation of a zeolite film over a substrate can be verified by permeation measurements. To carry out these experiments, a gas permeation setup was assembled as shown in Figure 1. Single gases of CO2 and CH4, each supplied by Technical Gas Services with a purity of 99.999%, were used as permeants in the experiments. A constant transmembrane pressure was applied throughout the experiments by means of a pressure regulator. The permeate flow rate was measured using a bubble flow meter (BFM). The transmembrane pressure was set at a constant value; i.e., the upstream pressure was set at 2 bar, and the downstream pressure was set at 1 bar. The permeation tests were carried out at a constant temperature of 298 K. Prior to commencing the permeation tests, a duration of 1 h was set to reach steady-state conditions, and the permeate was measured after 1 h. The constant-pressure method was used for the measurment of the permeation flow rate, in which variations in the permeate volume over time were calculated. The permeances of the synthesized membranes were measured using a BFM technique. Under steady-state 12436
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the literature.25 This synthetic approach involves direct transformation of Y zeolites into CHA zeolites in an alkaline medium. Herein, the H-form of zeolite Y (i.e., HY) was selected as a feedstock. 2.5. Membrane Preparation. Chabazite zeolite membranes were hydrothermally grown on the surface of porous disks of α-Al2O3 supports, which had a diameter of 21 mm and a porosity of 45%. The supports were seeded prior to the synthesis stage. The dip-coating method was utilized for the seeding of alumina supports. For this purpose, a suspension of 1 wt % chabazite zeolite was prepared, and the supports were horizontally immersed in the suspension for 30 s. The seeded supports were then removed and dried at 100 °C for 5 h. For the synthesis of zeolite membranes, a gel composition was prepared using aluminum nitrate, Ludox, strontium nitrate, potassium hydroxide, and distilled water. The composition of the gel for secondary growth was 12 SiO2/1 Al2O3/2 K2O/8 KNO3/1 SrO/780 H2O, on a molar basis.22 The details of the procedure for the preparation of synthesis gel have been reported elsewhere.20−23 The seeded supports were vertically placed in a Teflon autoclave into which the gel was poured. According to the publications of Hasegawa et al.,20−23 chabazite zeolite membranes can be prepared at synthesis temperatures between 120 and 180 °C and a synthesis duration of 20 h. Therefore, four CHA membranes were synthesized by hydrothermal treatment at various synthesis temperatures (T) and a fixed synthesis duration of 20 h. The membranes are denoted as CHA1, CHA2, CHA3, and CHA4 for synthesis temperatures of 100, 120, 140, and 160 °C, respectively. At the end of the crystallization reaction, the membranes were removed from the autoclave, washed with DI water, and air-dried at ambient temperature. Each membrane was prepared three times, and the results are presented as averages with a maximum standard deviation of 10%. 2.6. Methods of Characterization. The synthesized zeolite seeds including NaY and chabazite were characterized by X-ray diffraction (XRD) using an X-ray diffractometer (D5000, 35 kV, 20 mA, λ = 1.54056 Å, Siemens) with Cu radiation, while the step size (2θ) was set at 0.04°. The morphologies of the α-alumina supports and chabazite zeolite seeds and membranes (surface and cross-section) were analyzed by scanning electron microscopy (SEM), using a Vega 3 Tescan instrument equipped for energy-dispersive X-ray (EDX) spectroscopy. The final porosities of the used supports were measured by the imbibition method. Each porous disk was immersed in water, and the pore volume was calculated by subtracting the volume of adsorbed water from the total volume of the dry porous disk. Fourier transform infrared (FTIR) spectroscopy was performed for the structural analysis of the zeolite particles. The infrared spectra of the zeolites were recorded on a Perkin-Elmer FTIR spectrometer using the KBr technique in the vicinity of 400 and 4000 cm−1.
Figure 2. SEM image of the surface of the alumina supports used for the synthesis of CHA membrane.
presents an SEM image of the α-alumina support used in the synthesis of chabazite membranes. As shown, the supports had a pore size of about 1 μm. These supports were synthesized using the pressing method. 3.2. Characterization of Zeolite Seeds. The structures of as-synthesized zeolites NaY and CHA were verified by both FTIR and XRD analyses. The characterization of zeolites by FTIR spectroscopy can provide valuable information regarding the unit cells and bonds of zeolite materials. FTIR spectroscopy was applied to identify the structures of the zeolites and also monitor the reactions in the zeolite pores. The structural configurations of the zeolitic materials were determined from the vibrational frequencies of the zeolite lattices observed in the vicinity of 200 and 1500 cm−1.26 FTIR spectra of the synthesized NaY and chabazite zeolites are presented in Figure 3. As can be seen in Figure 3, the bands detected at wave numbers of 1135 and 725 cm−1 are assigned to the asymmetric and symmetric stretching modes, respectively, of internal tetrahedra. These peaks are seen in both spectra. It can also be seen that the bands at 1020 and 792 cm−1 are associated with the asymmetric and symmetric stretching modes, respectively, of external linkages in the zeolite structure.27 Moreover, the double ring-opening vibration at 566 cm−1 in the FTIR spectrum of zeolite NaY is a characteristic of faujasite zeolites.28 To further analyze the synthesized NaY and chabazite zeolites, X-ray diffraction was also carried out. XRD patterns for the NaY and chabazite powder samples are given in Figure 4. As can be observed, almost-pure zeolite NaY and chabazite crystals were obtained using hydrothermal and ion-exchange processes. Figure 5 also shows an SEM image of as-synthesized chabazite zeolite seeds. As reported by Robson,25 the crystal habit of chabazite zeolite, which belongs to the family of CHAtype zeolites, is either multifaceted or hexagonal platelets. The crystal size of zeolite chabazite was estimated from the SEM observations. As observed in Figure 5, the chabazite crystals were in the range of 1−2 μm.
3. RESULTS AND DISCUSSION 3.1. Characterization of Support. The support plays an important role in the synthesis and preparation of defect-free zeolite membranes. Therefore, the choice of an appropriate material and structure for the support is of vital importance in the successful synthesis of zeolite membranes. Alumina has been found to be a suitable material for the preparation of composite zeolite membranes. Viable alumina supports can be prepared using extrusion or pressing methods. Figure 2 12437
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Figure 3. FTIR spectra of the synthesized zeolite particles: (A) NaY zeolite, (B) chabazite zeolite.
3.3. Adsorption Analysis. To determine the affinities of CO2 and methane toward chabazite zeolite, adsorption data were obtained. The results of preliminary experiments that were carried out to determine the equilibrium time of adsorption are shown in Figure 6. As shown, the time required to reach equilibrium was about 2000 s for CO2 and about 8000 s for methane. Adsorption isotherms of CO2 and CH4 in chabazite zeolite are presented in Figure 7. It can be clearly seen that the amount of adsorbed CO2 was higher than that of adsorbed CH4 in chabazite. Increasing pressure enhanced the amount of adsorbed gas, in agreement with common adsorption isotherms. The adsorption behaviors of both gases in chabazite zeolite can be explained in terms of the kinetic diameters of the gases, as well as the pore size of chabazite. Because the CO2
kinetic diameter (3.3 Å) is smaller than the chabazite pore size (3.8 Å × 3.8 Å), CO2 molecules can easily pass through the cavities of chabazite and become adsorbed on the surface of the zeolite. On the other hand, the kinetic diameter of CH4 is 3.8 Å. As such, CH4 cannot easily pass through the cavities of chabazite zeolite, in comparison with CO2. It can be deduced that the affinity of CO2 toward the chabazite is thus higher than that of methane. Therefore, chabazite should be an excellent choice for the synthesis of membranes applicable to CO2 separation from natural gas. 3.4. Effect of Synthesis Temperature on the Performance of Membranes. The performance characteristics of the synthesized chabazite zeolite membranes were evaluated in terms of gas permeances and ideal selectivities. These parameters are typically used in the characterization of 12438
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Figure 4. XRD patterns of (a) NaY zeolite and (b) CHA zeolite.
Figure 8. As mentioned earlier, four synthesis temperatures were employed for membrane preparation. As can be seen, the membranes synthesized at 100 and 120 °C (CHA1 and CHA2) exhibited the highest gas permeances for both CO2 and methane. However, the CO2 permeances were higher than the CH4 permeances for both synthesis temperatures. Enhancement of the synthesis temperature to 140 °C (CHA3) reduced the gas permeances of both gases dramatically. A further increase in the synthesis temperature to 160 °C increased the gas permeances (CHA4) slightly. The lowest gas permeance was thus observed for the membrane synthesized at 140 °C. The effects of synthesis temperature on the ideal selectivity, which is defined as the ratio of the individual permeances for single gases, are also shown in Figure 8. A reverse trend, compared with the permeances, can be observed for the variation of ideal selectivities with synthesis temperature. At first, the ideal selectivity increased slightly with an increase in synthesis temperature from 100 to 120 °C. The ideal selectivity then increased considerably when the synthesis temperature reached 140 °C. The ideal selectivity decreased thereafter for the synthesis temperature of 160 °C.
Figure 5. SEM image of the synthesized chabazite zeolite powder.
membrane performance. The effects of synthesis temperature on the permeances of both CO2 and CH4 gases are depicted in 12439
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Figure 6. Adsorption kinetics for CO2 and CH4 adsorption in chabazite powder.
Figure 7. Adsorption isotherms for CO2 and CH4 in chabazite zeolite powder.
temperature increases. However, enhancement of the synthesis temperature is not always favorable for membrane synthesis because temperature should be well adjusted and controlled for the formation of a densly packed zeolite film on the surface of the substrate. Enhancement in the rate of crystal growth results in the promotion of zeolite layer integration, which is responsible for the separation of CO2 and CH4 gases. That also decreases the density of the zeolite film on the substrate, because of an increase in the voids in the zeolite layer.16,29 SEM images of the membranes synthesized at 100 and 120 °C are shown in Figure 10. It can be clearly seen that neither dense nor integrated zeolite layers formed on the surface of the substrate. Moreover, some amorphous phases were detected on the surface of the membranes. This observation can be attributed to the low synthesis temperature, which was not appropriate for the integration of the zeolite layer (active layer).
Further analyses are required for the justification of these behaviors of the membrane permeances and ideal selectivities. Analysis of the surface morphology can assist in describing membrane performances in gas separation. An SEM image of a seeded support prior to the secondary growth is presented in Figure 9. As can be observed, the surface of the support was properly covered with the chabazite seeds. The coverage of the substrate surface with zeolite seeds has been confirmed to be an important parameter for the synthesis of high-quality zeolite membranes. In fact, implanted seeds promote the formation of a zeolite layer on the surface of the support. SEM images of the surface of chabazite zeolite membranes at various synthesis temperatures are presented in Figures 10 and 11. Synthesis temperature has a significant effect on the performance and morphology of zeolite membranes. The latter controls the rate of crystal growth in the secondary growth method. The rate of crystal growth increases as the synthesis 12440
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Figure 8. Effects of synthesis temperature on CO2 and CH4 permeances and selectivity (synthesis duration = 20 h).
formation of another zeolitic phase that coexists with the chabazite zeolitic phase. According to the results reported by Hasegawa et al.,22 at high temperature (in the vicinity of 150 and 160 °C), MER-type zeolite is formed along with chabazite zeolite. The formation of MER zeolite would reduce the separation performance of the membrane because of larger pore size of MER-type zeolite compared to chabazite. Therefore, the synthesis temperature of 140 °C was selected as the optimum and safe temperature for the preparation of chabazite zeolite membranes in this work. An SEM image of the cross section of the membrane synthesized at 140 °C is presented in Figure 12. As shown, a dense zeolite layer with a thickness of about 5 μm formed on the surface of the porous substrate. Moreover, Figure 12 indicates that no significant intrusion of zeolite layer into the pores of the support occurred. Elemental analysis of the zeolite layer was carried out by energy-dispersive X-ray (EDX) spectroscopy. The results of EDX analysis are shown in Figure 13. The basic elements of chabazite zeolite including Al, Si, and K were detected by EDX analysis. Moreover, the Si/Al ratio of the zeolite layer was estimated to be about 2, which is the intrinsic ratio of chabazite zeolite. It should be pointed out that the peaks detected for Au were due to the gold coating applied before EDX analysis. Comparisons among the results obtained in this work with those provided in previous works on SAPO-34 zeolite membranes for the separation of CO2/CH4 were carried out to assess the gas permeation results. Because SAPO-34 zeolite has a structure similar to that of chabazite and both are CHAtype zeolites, such comparisons should be helpful. The results are provided in Table 1 in terms of permeance and ideal selectivity. It should be pointed out that the results for membrane CHA3 synthesized at 140 °C are compared with those of other works. As can be seen, ideal selectivities between 16 and 25 have been reported for CO2/CH4 by SAPO-34 zeolite membranes. It is also revealed that the results of this
Figure 9. SEM image of a support seeded with chabazite seeds.
These membranes (CHA1 and CHA2) exhibited poor performance in the separation of CO2 and CH4 (see Figure 8). SEM images of the membranes synthesized at 140 and 160 °C are shown in Figure 11. As can be seen, integrated zeolite films were deposited on the surface of the support for these membranes. Membrane CHA3 also showed the best separation performance for CO2 removal (see Figure 8). An SEM image of the membrane synthesized at 160 °C is also presented in Figure 11A. Again, a dense zeolite layer can be observed for this synthesized CHA membrane. This membrane (CHA4) exhibited a poorer separation performance compared with CHA3 membrane. This can be explained by the effect of high temperature on zeolite crystallization. Increasing the synthesis temperature would result in the formation of cracks during membrane synthesis, which, in turn, would reduce the separation performance of the zeolite membrane. Another reason for such a behavior of the CHA4 membrane is the 12441
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Figure 11. SEM images of membranes synthesized at (A) 160 and (B) 140 °C. Figure 10. SEM images of chabazite membranes synthesized at (A) 100 and (B) 120 °C.
work are in the range of the SAPO-34 results for CO2/CH4 separation. However, further modifications in membrane synthesis would result in a better gas separation performance for chabazite zeolite membranes compared to SAPO-34 membranes, because the synthesis of SAPO-34 membrane is more complicated. It is also useful to point out that chabazite membrane is also appropriate for removing water vapor from gas streams because the nature of CHA zeolite is hydrophilic and it can easily discriminate the permeation of water vapor from methane. Moreover, the kinetic diameter of water (0.26 nm) is less than the pore size of CHA zeolite, and water can easily pass through the pores of the zeolite. However, it is also reported that CHAtype zeolite membranes are not stable when exposed to water for a long time.4
Figure 12. SEM image of a cross section of chabazite membrane synthesized at 140 °C.
4. CONCLUSIONS The effect of synthesis temperatures (100, 120, 140, and 160 °C) at a synthesis duration of 20 h on the CO2/CH4 ideal selectivity and gas permeances of synthesized chabazite zeolite
membranes were studied in this work. The membranes were synthesized by a secondary growth method. The chabazite seeds for the membrane synthesis were obtained through ion 12442
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Figure 13. EDX analysis of a membrane synthesized at 140 °C.
Table 1. Comparison between the Current Results and Those Reported in the Literature for CO2/CH4 Separation Using CHAType Zeolite Membranes membrane SAPO-34 SAPO-34 SAPO-34 chabazite
support
temperature (K)
α-Al2O3 α-Al2O3 α-Al2O3 α-Al2O3
300 298 297 298
CO2 permeance (mol m−2 s−1 Pa−1) 2.4 1.5 2.5 4.0
× × × ×
CH4 permeance (mol m−2 s−1 Pa−1)
−8
−9
1.3 × 10 9 × 10−9 1 × 10−8 1.5 × 10−8
10 10−7 10−7 10−7
AUTHOR INFORMATION
Corresponding Author
*Tel.: +98 21 77240496. Fax: +98 21 77240495. E-mail: ashrafi@iust.ac.ir. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research council at Iran University of Science and Technology (IUST) is gratefully acknowledged for its financial support during the course of this research.
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ref
19 16 25 27
30 31 4 this work
(2) Merkel, T. C.; Wei, X.; He, Z.; White, L. S.; Wijmans, J. G.; Baker, R. W. Selective Exhaust Gas Recycle with Membranes for CO2 Capture from Natural Gas Combined Cycle Power Plants. Ind. Eng. Chem. Res. 2012, 52 (3), 1150−1159. (3) Funke, H. H.; Chen, M. Z.; Prakash, A. N.; Falconer, J. L.; Noble, R. D. Separating molecules by size in SAPO-34 membranes. J. Membr. Sci. 2014, 456, 185−191. (4) Li, S.; Falconer, J. L.; Noble, R. D. SAPO-34 membranes for CO2/CH4 separation. J. Membr. Sci. 2004, 241 (1), 121−135. (5) Yin, X.; Chu, N.; Yang, J.; Wang, J.; Li, Z. Thin zeolite T/carbon composite membranes supported on the porous alumina tubes for CO2 separation. Int. J. Greenhouse Gas Control 2013, 15, 55−64. (6) Zhou, R.; Ping, E. W.; Funke, H. H.; Falconer, J. L.; Noble, R. D. Improving SAPO-34 membrane synthesis. J. Membr. Sci. 2013, 444, 384−393. (7) Al-Marzouqi, M.; El-Naas, M.; Marzouk, S.; Abdullatif, N. Modeling of chemical absorption of CO2 in membrane contactors. Sep. Purif. Technol. 2008, 62 (3), 499−506. (8) Al-Marzouqi, M. H.; El-Naas, M. H.; Marzouk, S. A. M.; AlZarooni, M. A.; Abdullatif, N.; Faiz, R. Modeling of CO2 absorption in membrane contactors. Sep. Purif. Technol. 2008, 59 (3), 286−293. (9) Shirazian, S.; Ashrafizadeh, S. N. Mass Transfer Simulation of Carbon Dioxide Absorption in a Hollow-Fiber Membrane Contactor. Sep. Sci. Technol. 2010, 45 (4), 515−524. (10) Shirazian, S.; Moghadassi, A.; Moradi, S. Numerical simulation of mass transfer in gas−liquid hollow fiber membrane contactors for laminar flow conditions. Simul. Modell. Pract. Theory 2009, 17 (4), 708−718. (11) Hasegawa, Y.; Kusakabe, K.; Morooka, S. Effect of temperature on the gas permeation properties of NaY-type zeolite formed on the inner surface of a porous support tube. Chem. Eng. Sci. 2001, 56 (14), 4273−4281. (12) Hasegawa, Y.; Watanabe, K.; Kusakabe, K.; Morooka, S. The separation of CO2 using Y-type zeolite membranes ion-exchanged with alkali metal cations. Sep. Purif. Technol. 2001, 22−23, 319−325.
exchange and recrystalliztion of zeolite Y. The membranes and seeds were characterized by SEM, FTIR, XRD, and EDX analyses to ensure the formation of zeolite membranes. The performances of the membranes were determined in the permeation of CO2 and CH4 gases through the membranes. The results revealed that the maximum ideal selectivity was obtained at the synthesis temperature of 140 °C. This is because the zeolite layer was densly deposited on the whole surface of the support. Increasing the synthesis temperature to 160 °C resulted in a reduction of the performance of the membranes. The optimum conditions for the synthesis of highquality chabazite zeolite membranes was identified as a synthesis temperature of 140 °C and a synthesis duration of 20 h.
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CO2/CH4 ideal selectivity
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dx.doi.org/10.1021/ie501810e | Ind. Eng. Chem. Res. 2014, 53, 12435−12444
