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Synthesis of Defect-Free FAU-Type Zeolite Membranes and Separation for Dry and Moist CO2/N2 Mixtures Xuehong Gu,† Junhang Dong,*,† and Tina M. Nenoff‡ Petroleum and Chemical Engineering Department, Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, and Chemical and Biological Technologies, Sandia National Laboratories, Albuquerque, New Mexico 87185-0734
In this paper, the effects of synthesis conditions, including the seeding approach, gel composition, and hydrothermal treatment process, on the quality of porous R-alumina-supported Y-type FAU membranes were studied. Defect-free, pure NaY-type zeolite membranes have been synthesized and investigated for separation of equimolar CO2/N2 mixtures under dry and moist conditions at atmospheric pressure. At room temperature, the CO2 selectivity was about 31.2 for the CO2/ N2 dry gas mixture with a CO2 permeance of 2.1 × 10-8 mol/m2‚Pa‚s. The addition of water vapor to the feed stream decreased the permeance for both CO2 and N2 in a temperature range of 23-200 °C. The existence of water vapor significantly enhanced the CO2 selectivity at 110200 °C but drastically lowered the CO2 selectivity below 80 °C. At 200 °C, with increasing water partial pressure, the CO2 selectivity increased and then decreased after reaching a maximum of 4.6 at a water partial pressure of 12.3 kPa. 1. Introduction Zeolites are crystalline aluminosilicate materials with well-defined subnanometer pores and unique surface properties appropriate for molecular separations. In the past 2 decades, polycrystalline membranes and films of various types of zeolite structures have been synthesized and employed for separations of gas and liquid mixtures with industrial importance. A few review papers regarding zeolite membranes have been published over the years.1-4 So far, most of the zeolites for constructing membranes possess 12-membered-ring (e.g., FAU and MOR), 10-membered-ring (e.g., MFI, MEL, and FER), and 8-membered-ring (e.g., LTA and SAPO-34) pore windows, which provide different pore sizes suitable for separating various molecular systems.5-13 Gas and liquid separation on zeolite membranes is primarily governed by the competitive adsorption and diffusion mechanisms. When the zeolite pore size falls between the molecular sizes of the feed components, a sizeexclusion mechanism can dominate the separation process.14,15 One of the main challenges in zeolite membrane development is to minimize the intercrystal pores formed inherently in polycrystalline zeolite films. The existence of intercrystal pores with sizes larger than the zeolitic pores is the major cause for decline in molecular separation efficiency.16 The FAU-type zeolites, including NaX (Si/Al ratio of 1:1.5) and NaY (Si/Al ratio >1.5), have a pore diameter (pd) of ∼0.74 nm suitable for separating large molecules, which cannot be handled effectively by MFI-type (pd ∼ 0.56 nm) and A-type (pd ∼ 0.42 nm) zeolites. Also, because of the low Si/Al ratio, the adsorbing properties of FAU-type zeolites can be readily altered by ion exchange to enhance the separation performance for specific molecular systems.17,18 However, the synthesis * To whom correspondence should be addressed. Tel.: (505) 835-5293. Fax: (505) 835-5210. E-mail:
[email protected]. † New Mexico Institute of Mining and Technology. ‡ Sandia National Laboratories.
of FAU-type zeolite membranes often encounters the problem of formation of impurity phases, commonly NaP, in the zeolite layer.19-21 The existence of NaP crystals in the FAU membrane layer can decrease the membrane permeability because of the small pore size of zeolite P (elliptical pores of 0.31 × 0.44 nm in the [100] direction and 0.26 × 0.49 nm in the [010] direction) and reduce the thermal stability of the membrane because of the cubic-to-tetragonal phase transformation of zeolite P at low temperatures, especially in dry conditions.22 To date, many efforts have been made on developing reproducible processes for the synthesis of defect-free, pure FAU-type zeolite membranes. Most of the FAU membrane synthesis methods involve seeding with NaY or NaX zeolite crystals on the substrate surface and subsequent densification of the seed layer by secondary growth through hydrothermal treatment in aluminosilicate gels or solutions. Kusakabe et al.10 employed a rubbing technique to seed the NaX zeolites on tubular R-alumina supports. After subsequent hydrothermal treatments, NaY zeolite membranes were obtained on the seeded surface. The synthesized NaY membranes were highly CO2-selective with CO2 separation factors of 20-100 at 30 °C for CO2/N2 mixtures. Kita et al.,20 however, reported that an impurity phase of zeolite P formed in the resultant membrane when a NaX seed layer was used, while pure FAU membranes were obtained from NaY seed layers. It was also observed that zeolite membranes grown from NaY seed layers of fine particles had fewer defects because the small size NaY seed particles could effectively enter the pores of the substrates.17 Nakolakis et al.23 reported the synthesis of high-quality NaX zeolite membranes by secondary growth from nanocrystalline ZSM-2 seed layers. Lassinantti et al.19 found that, in the synthesis of FAU films on alumina surfaces, the crystal structure of the zeolite films depended on the duration of the hydrothermal treatment. A prolonged hydrothermal treatment caused the formation of a NaP impurity phase in the zeolite films.
10.1021/ie049263i CCC: $30.25 © 2005 American Chemical Society Published on Web 01/20/2005
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Generally, in zeolite membrane synthesis, the seed layer determines the crystal structure of the resultant zeolite membrane,24 while the secondary growth step controls the membrane density, thickness, and crystal phase purity. With the aim to enhance the FAU membrane quality and synthesis reproducibility, many attempts have been made to improve the seeding and secondary growth processes. For example, nanosized crystal seeds and electrophoretic deposition methods were used to improve the seed coverage and reduce the initial intercrystal voids in the seed layer;12,25 clear synthesis solutions and microwave heating technology were employed to achieve more uniform crystallization conditions during the hydrothermal treatment.24,26 Although a lot of progress has been made in the synthesis of FAU membranes, the reproducibility of the synthesis results and control of microdefects and zeolite phase purity still need to be further advanced. CO2 separation is one of the most studied applications for FAU-type zeolite membranes25,27 because of its industrial significance such as CO2 capture for carbon sequestration, natural gas purification, separation of product streams from water gas shift (WGS) reactions for hydrogen production, to name a few. The literature results of CO2 separation have been qualitatively consistent in showing that FAU-type zeolite membranes are CO2-selective over N2. However, large variations in CO2 and N2 permeances as well as separation factors were found among the reported values because of the significant differences in microstructure and materials chemistry of the FAU membranes.10,18,25-28 In this paper, the effects of the seeding method, synthesis gel chemistry, and conditions of hydrothermal treatment on the FAU membrane quality in terms of the crystal phase purity and CO2/N2 separation performance were systematically studied. A synthesis route has been developed for fabricating defect-free Y-type FAU membranes. The synthesized NaY membranes were investigated for the separation of dry and moist CO2/N2 gas mixtures in a temperature range of 23-200 °C. 2. Experimental Section 2.1. Membrane Preparation. Faujasite zeolite membranes were prepared by a seeding and secondary growth approach. Disc-shaped porous R-Al2O3 substrates (2-mm thickness, 29-mm diameter, with a mean pore size of about 0.10 µm and a porosity of 30-35%) were seeded with NaY zeolite crystals with a size of 1-1.5 µm. The NaY seed particles were synthesized from an aluminosilicate gel with a molar composition of 12.8:1:17:675 SiO2/Al2O3/Na2O/H2O. The temperature and time for hydrothermal synthesis of the seed particles were 90 °C and 12 h. The substrates were seeded either by rubbing with zeolite powders or by dip coating with zeolite suspensions. The zeolite seed suspension for dip coating was prepared by dispersing 0.1 g of the synthesized NaY powders in 25 mL of deionized water. The suspension was adjusted to pH 3-4 by adding a 1.0 M HNO3 solution. The suspension was stirred rigorously for 24 h before use. After dip coating, the coated disks were dried at 60 °C in air for 24 h. The aluminosilicate gel used for secondary growth had a composition of 10.7:1:18.8:850 SiO2/Al2O3/Na2O/H2O or otherwise indicated later in the paper. The synthesis gel was prepared by mixing water glass (27% SiO2 + 14% NaOH; Aldrich), sodium aluminate [50-56% Al +
40-45% Na (Fe < 0.05%); Riedel-deHaen], and sodium hydroxide (99.99%; Aldrich). The gel was rigorously stirred for 12 h and poured into an autoclave, where a substrate was mounted horizontally on a stand with its seeded side facing downward. Hydrothermal crystallization was carried out at 85-105 °C in the closed autoclave for 3-24 h. The hydrothermal treatment was repeated to minimize the intercrystal pores. The synthesized membranes were washed with deionized water and then dried at 60 °C in air for at least 24 h before any tests or characterizations. 2.2. Membrane Characterization. The crystal phase of the membranes was identified by X-ray diffraction (XRD; Rigaku, model D/MAX-II). The chemical compositions of the zeolite membranes and particles were determined by energy-dispersive spectrometry (EDS) and X-ray fluorescence (XRF; Phillips PW2400 wavelength-dispersive X-ray spectrometer with Rh and a window tube controlled by ×40 software), respectively. The morphology of the membranes was examined by scanning electron microscopy (SEM; JEOL JSM-5800LV). The permeance of pure nitrogen was measured by a transient permeation method as previously described.22 The separation of equimolar N2/CO2 gas mixtures with and without water vapor content was conducted by the Wicke-Kallenbach method. In the gas separation experiments, the feed flow rate of the N2/CO2 mixture was 36 cm3/min, and the helium flow rate on the sweep side was also 36 cm3/min. The water vapor was added to the feed stream by bubbling the N2/CO2 mixture through a water saturator. The partial pressure of the water vapor in the feed gas was varied by controlling the temperature of the water saturator. The gas composition was analyzed by gas chromatograph (HP 5890 II) equipped with a Porapak Q packed column. The gas permeance for component i (Pi) and the separation factor of CO2 over N2 (RCO2/N2) are defined as
Pi ) ni/Am(pi,f - pi,p)
(1)
RCO2/N2 ) (yCO2/yN2)p/(yCO2/yN2)f
(2)
where subscripts p and f denote the permeate and feed sides, respectively; ni is the number of moles of component i flowing through the membrane per unit time; pi is the partial pressure of component i; Am is the membrane area; and y is the mole fraction. 3. Results and Discussion 3.1. Effect of the Seeding Method. NaY zeolite particles were seeded on the alumina substrates by four different methods, including (1) rubbing rough substrate (unpolished) with dry NaY powders, (2) rubbing polished substrate (polished by 600-mesh SiC sand paper) with dry NaY powders, (3) rubbing polished substrate with a paste of NaY powders, and (4) dip coating on polished substrate with a NaY suspension. All of the seeded substrates were treated hydrothermally under identical conditions at 90 °C for 24 h. The obtained membranes are denoted as M1, M2, M3, and M4 corresponding to their substrates seeded by methods 1-4, respectively. Figure 1 shows the XRD patterns of zeolite membranes after a single-time hydrothermal treatment. M1 had a NaP phase without appreciable peaks of the FAU phase. M2 and M3 contained mixtures of NaP and FAU, with NaP the majority phase in M2 and FAU the majority
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Figure 2. SEM images of the substrate surface: (a) blank; (b) seeded.
Figure 1. XRD patterns of the zeolite membranes, the zeolite standards, and the R-alumina support: (a) M1; (b) M2; (c) M3; (d) M4; (e) NaP standard; (f) NaY (FAU) standard; (g) R-Al2O3 standard.
phase in M3, as indicated by the relative intensity of the XRD peaks. M4 was a virtually pure FAU membrane. The NaP zeolite is the primary competing phase in the hydrothermal synthesis of FAU-type zeolites.29 Previous studies on the influence of tribochemical treatment on the crystallization of the NaY zeolite revealed that excessive mechanical fracture of NaY seed crystals led to a dominant P phase after secondary growth by hydrothermal treatment.30 In the seeding method 1, dry NaY particles rubbed over the unpolished rough surface caused the most severe fracture of the seed crystals. Thus, a pure NaP film was formed after subsequent synthesis. The poor adhesion of dry seeds to the support that might result in a poor coverage of the seed layer could also contribute to the formation of the impurity NaP phase. The mechanical damage of the seed crystals was mitigated when rubbed on the polished surface, preserving some of the NaY seeds for growing the FAU structure. In the seeding method 3, the addition of water as a lubricant markedly reduced the fracture of NaY seed crystals compared to methods 1 and 2, which, in turn, drastically improved the purity of the synthesized NaY films. In the seeding method 4, the dip-coating process is nondestructive to the seed crystals. As expected, the seed layer formed by dip coating resulted in a virtually pure FAU-type zeolite membrane after secondary growth. The dip-coating method also has the advantages of good control in seed coverage and high reproducibility of the seeding effect. Therefore, in all of the experiments hereafter, the substrates were seeded with NaY crystals by dip coating under identical conditions. The SEM images of the polished R-alumina substrate surface before and after seeding with NaY particles are shown in Figure 2. 3.2. Effects of Precursor Composition and Hydrothermal Treatment Conditions. The crystallization rate and phase purity of the FAU-type zeolite during hydrothermal synthesis are influenced by many factors such as the Si/Al ratio and alkalinity of the aluminosilicate gel, precursor mixing and aging conditions, synthesis temperature and duration, etc.29-33 In this work, investigations were first carried out to identify the gel compositions and hydrothermal treatment temperature and duration for synthesizing pure FAU-type zeolite membranes. Then the synthesis condi-
Figure 3. XRD patterns of the zeolite membranes synthesized from different gels. Parts a-c are membranes obtained from gels i-iii, respectively.
tions, including the temperature, duration, and repetition times, were further tuned to improve the membrane quality as characterized by N2 gas permeation and CO2 separation. Alkalinity. To study the effect of alkalinity on FAU crystallization and membrane formation, the NaOH content of the aluminosilicate gel was varied with a fixed Si/Al molar ratio of 6.4: (i) 12.8:1:17:870 SiO2/ Al2O3/Na2O/H2O; (ii) 12.8:1:20:870 SiO2/Al2O3/Na2O/ H2O; (iii) 12.8:1:25:870 SiO2/Al2O3/Na2O/H2O. The hydrothermal treatments were conducted at 90 °C for 24 h. XRD examination of the membranes obtained from the three synthesis gels showed that the alkalinity of the precursors did not have a significant impact on the growth of FAU films in the tested range (see Figure 3). However, the membrane synthesized from gel (ii) appeared to have the smallest impurity phase of the NaP zeolite. Si/Al Ratio. The effect of the Si/Al molar ratio on FAU membrane formation was studied using aluminosilicate gels with Si/Al ratios of 9.6, 6.4, and 5.4. The NaOH and H2O contents were adjusted such that all of the gels had the same alkalinity and overall concentration of aluminosilicate. The hydrothermal synthesis was carried out at 90 °C for 24 h. The XRD examination (Figure 4) indicated that the high Si/Al ratio of 9.6 inhibited crystal growth of the seeded layer, which was also evidenced by the poor coverage of zeolite crystals on the substrate as observed by SEM (Figure 5a,b). However, a large amount of NaY particles were found on the bottom of the autoclave, suggesting that high Si/ Al ratios favored nucleation and crystallization in the bulk solution. Strong XRD peaks of the FAU phase were measured for the membranes obtained from the gels with Si/Al ratios of 5.4 and 6.4 (Figure 4b,c). The SEM images of these two membranes (Figure 5c,d) showed full coverage of zeolite crystals. However, large intercrystal voids were found in the zeolite layers.
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Figure 4. XRD patterns of zeolite membranes synthesized from gels with different Si/Al ratios. Synthesis gels: (a) 19.2:1:33.2: 1470 SiO2/Al2O3/Na2O/H2O (Si/Al ) 9.6); (b) 12.8:1:20:870 SiO2/ Al2O3/Na2O/H2O (Si/Al ) 6.4); (c) 10.7:1:18.7:850 SiO2/Al2O3/Na2O/ H2O (Si/Al ) 5.4). Table 1. Properties of Zeolite Membranes Synthesized at Different Temperatures synthesis temp (°C)
synthesis time (h)
N2 permeance (mol/m2‚Pa‚s)
zeolite layer
85 90 100 100 105
24 24 24 12 12
1.6 × 10-6 1.6 × 10-6 4.4 × 10-8 8.6 × 10-8 4.6 × 10-9
FAU FAU FAU + NaP FAU FAU + NaP
Synthesis Temperature. A synthesis gel with a composition of 10.7:1:18.7:850 SiO2/Al2O3/Na2O/H2O (Si/Al ) 5.4) was tested for membrane growth at 85, 90, 100, and 105 °C, respectively. At 85, 90, and 100 °C, the hydrothermal treatment was performed for a fixed time of 24 h. As shown in Table 1, pure FAU films were obtained at 85 and 90 °C, while a mixture of FAU and NaP phases was formed at 100 °C. The room-temperature N2 single-gas permeance values of the membranes
obtained at 85 and 90 °C were essentially the same as that of the seeded substrate (1.9 × 10-6 mol/m2‚Pa‚s), indicating that the zeolite crystals were not sufficiently intergrown (see Figure 5c,d). The density was significantly improved for the membrane synthesized at 100 °C (Figure 6a,b), as was also indicated by the drastically reduced N2 permeance (Table 1). The hydrothermal synthesis was further investigated at 100 and 105 °C for a shorter duration of 12 h. The results are also given in Table 1. By reduction of the hydrothermal treatment time, a pure FAU film was obtained at 100 °C with some sacrifice in membrane density, as suggested by the increased N2 permeance. The membrane obtained at 105 °C contained a NaP impurity phase but possessed a much higher density according to the very low N2 permeance. It was also found that, at 105 °C, NaP crystals always formed in the zeolite membrane even if the duration of the hydrothermal treatment was reduced to less than 3 h. The effects of the hydrothermal synthesis temperature and duration on the formation of NaP phase and the membrane integrity observed in this study agreed with the reported temperature and time dependences of the FAU crystal growth rate and phase transformation behavior. In general, the rate of FAU crystallization increases with temperature and phase transformation from FAU to denser zeolite structures (i.e., NaP and ANA) increases with the hydrothermal treatment time.29,30 The chemical composition of the FAU membrane synthesized at 100 °C for 12 h was determined by EDS at three locations along the thickness of the membrane layer, including location A near the outer surface, B at the middle of the membrane thickness, and C near the zeolite/substrate interface. The Si/Al ratios at the three
Figure 5. SEM pictures of zeolite membranes obtained from gels with different Si/Al ratios: (a and b) membrane from gel with a Si/Al ratio of 9.6; (c and d) membrane from gel with a Si/Al ratio of 5.4.
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Figure 6. SEM pictures of the (a) surface and (b) cross section of the zeolite membrane synthesized at 100 °C for 24 h. Table 2. Zeolite Y (FAU) Membranes Synthesized under Various Durations of Hydrothermal Treatment and Repetition Times membrane no.
synthesis times (h)a
crystal structure
substrate M2 M3 M3,3 M3,6 M4,6 M6,6 M3,3,3 M3,3,6
none 1/(3) 1/(6) 2/(3, 3) 2/(3, 6) 2/(4, 6) 2/(6, 6) 3/(3, 3, 3) 3/(3, 3, 6)
Y Y Y Y Y Y Y Y+P
gas permeance in a binary system (50 °C, mol/m2‚Pa‚s) N2 CO2 1.7 × 10-7 2.4 × 10-7 2.6 × 10-7 5.4 × 10-8 1.6 × 10-8 4.9 × 10-9 2.0 × 10-9 1.3 × 10-8 7.9 × 10-9
1.3 × 10-7 1.9 × 10-7 2.1 × 10-7 1.3 × 10-7 1.2 × 10-7 7.8 × 10-8 3.9 × 10-8 9.2 × 10-8 7.7 × 10-8
separation factor of CO2/N2 (binary) at 50 °C 0.82 0.86 0.88 2.4 7.6 16 20 6.9 9.7
a The number above the slash indicates the number of repetitions; numbers in the brackets are the durations (hour) of each hydrothermal treatment. For example, “3/(3, 3, 6)” means that the membrane was obtained by three repetitions of hydrothermal treatment: 3 h for the first time, 3 h for the second time, and 6 h for the third time.
locations were within a close range from 1.7 to 1.8, which is in the Si/Al range of Y-type zeolites. The Si/Al ratio of the membrane layer was close to that of the NaY zeolite particles collected from the liquid phase measured by XRF. To further examine the quality of this membrane, separation of an equimolar CO2/N2 mixture was conducted. The membrane was selective toward CO2 because of its preferential adsorption in FAU-type zeolites, but the separation factor was low (RCO2/N2 ) 1.3), indicating a significant amount of nonseparative intercrystal pores in the membrane layer. To reduce the intercrystal pores, the membrane was hydrothermally treated for a second time at 100 °C for 12 h. After the second hydrothermal treatment, the N2 single-gas permeance was reduced drastically from 8.6 × 10-8 to 3.6 × 10-9 mol/m2‚Pa‚s. However, the growth of a NaP impurity phase appeared in the second hydrothermal treatment. The existence of NaP in the membrane layer might have also contributed to the reduction of N2 permeance. 3.3. Membrane Improvement. In hydrothermal synthesis of zeolite Y, the crystallization rate increases rapidly with increasing temperature from 70 to 110 °C.30 Above 90 °C, the crystallinity of zeolite Y reaches a maximum within 4 h. At 100 °C, the crystallization of the FAU phase is much faster than its competing phases of NaP and ANA in the early hours of synthesis. In the prolonged period of the hydrothermal synthesis, the small-pore NaP and ANA continue to grow and that consumes the FAU phase.29 In the following study, zeolite membranes were synthesized at 100 °C with shortened synthesis durations to preserve the pure FAU phase. The short-duration synthesis process was repeated to perfect the membrane density. The resultant
membranes were characterized by the separation of a dry CO2/N2 mixture. Table 2 presents the properties of membranes synthesized with varied durations of hydrothermal treatment and repetition times. The membranes obtained by single-time synthesis with synthesis durations of 3 and 6 h were N2-selective with separation factors close to that of the seeded substrate, indicating poor density of the membranes. Single-time synthesis with increased durations (e.g., 12 or 24 h) was unable to make satisfactory membrane improvement, as discussed earlier in the paper. The results in Table 2 show that the membrane quality was effectively improved by repeating the short-duration hydrothermal treatment. For the pure NaY membranes in Table 2, RCO2/N2 increased with a decrease in the single-gas permeance of N2. Membranes M4,6 and M6,6 appeared to possess the best quality in terms of the CO2/ N2 separation factor. The SEM pictures of the surface and cross section of membrane M4,6 are shown in Figure 7. The membrane thickness was about 3-4 µm. The synthesis processes using more than two shorttime hydrothermal treatments, however, appeared to be unnecessary or less effective for quality improvement. The N2 permeance and RCO2/N2 of membrane M3,3,6 were both significantly lower than those of M4,6 and M6,6. This is likely caused by the NaP impurity phase in M3,3,6, which lowered the gas permeance but had no influence on the CO2 selectivity. 3.4. CO2/N2 Separation. The membrane M4,6 was used to investigate CO2 separation from equimolar CO2/ N2 mixtures under dry and moist conditions. For separation of dry gas mixtures, the membrane was dried by purging with pure helium at 200 °C for 5 h before introducing the feed gas. All experiments were con-
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Figure 7. SEM pictures of membrane M4,6: (a) surface; (b) cross section.
Figure 8. Gas permeance and CO2 separation factor as functions of temperature for equimolar CO2/N2 mixtures under dry and moist conditions.
ducted at local atmospheric pressures (86 kPa) on both the feed and permeate sides. To study the effect of moisture on CO2 separation, experiments were first carried out for a CO2/N2 mixture with absolute humidity of 0.031, i.e., a water partial pressure of 2.64 kPa. The effect of water vapor on CO2 separation was investigated at 200 °C in a range of absolute humidity from 0 to 0.546 (or partial pressure of water vapor from 0 to 47 kPa). The separation results for the dry and humid CO2/N2 mixtures are presented in Figure 8. For the dry system, N2 permeance increased monotonically with increasing temperature from 23 to 200
°C, while the CO2 permeance increased to a maximum at around 50 °C and then decreased slightly. At room temperature, RCO2/N2 was about 31.2 with a CO2 permeance of 2.1 × 10-8 mol/m2‚Pa‚s. The separation factor declined constantly with increasing temperature and reached a low value of 1.9 at 200 °C. The temperature dependence of the CO2/N2 permeation behavior is similar to those observed in zeolite membrane separations for gas mixtures containing strongly and weakly adsorbing components.6,17 Because of the strong adsorption of CO2 in the zeolite cavities, permeation of the weakly adsorbing N2 is hindered to a great extent at relatively low temperatures, resulting in a high CO2/ N2 separation factor. As the temperature increases, the CO2 adsorption decreases, opening up more pore volume for transport of N2, which has higher mobility than CO2, and thus reducing the CO2 selectivity. Recent molecular dynamics simulation has shown that, at relatively low temperatures, surface diffusion is the dominant mechanism of CO2 transport in FAUtype zeolite membranes.34 The surface diffusion rate depends on both the gradient of surface occupancy of adsorbed molecules, which decreases with increasing temperature, and the surface diffusivity, which increases with temperature.6 These two factors adversely influenced the CO2 permeance as the temperature changed, resulting in the CO2 permeance-temperature profile shown in Figure 8, which initially increases with the temperature and then decreases with a further increase in the temperature. With the humidified CO2/N2 mixture, the permeance values of all three components, including H2O, CO2, and N2, increased with the temperature in the entire tested range, while RCO2/N2 experienced a maximum at about 140 °C. Because of the strong affinity of H2O to the extremely hydrophilic FAU zeolites, the decrease in H2O adsorption is limited at the low-temperature range (
