- Email: [email protected]
CH4 separation
Journal of Membrane Science 471 (2014) 338–346
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Alumina-supported AlPO-18 membranes for CO2/CH4 separation Ting Wu a,b, Bin Wang b, Zhanghui Lu b, Rongfei Zhou a,b,n, Xiangshu Chen b a State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China b College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 May 2014 Received in revised form 5 August 2014 Accepted 16 August 2014 Available online 23 August 2014
Selective AlPO-18 membranes were reproducibly synthesized on the outside surface of macroporous tubular α-alumina supports in a single hydrothermal synthesis step. The AlPO-18 crystals and membranes were characterized by SEM, XRD, adsorption and permeation measurements. Single-gas permeances of CO2, N2, CH4 and i-C4H10 decreased with increased kinetic diameter, but CO2 permeated faster than the smaller H2, because CO2 adsorbs more strongly. Eleven AlPO-18 membranes had an average CO2 permeance of (1.870.5) 10 7 mol/(m2 s Pa) and average CO2/CH4 separation selectivities of 101 716 at room temperature and a pressure drop of 0.1 MPa for the equimolar CO2/CH4 mixture. The membrane was also selective (selectivity ¼31) at 2 MPa feed pressure and had a CO2 permeance of 0.47 10 7 mol/(m2 s Pa) [CO2 flux of 15 kg/(m2 h)]. The CO2/CH4 selectivities decreased as temperature and pressure drop increased. The membranes stability was investigated for different calcination times, calcination temperature ramps, and following exposures to water vapor and liquid. & 2014 Elsevier B.V. All rights reserved.
Keywords: AlPO-18 Zeolite membrane CO2/CH4 separation Membrane stability Gas purification
1. Introduction Separation of carbon dioxide from methane is important because CO2 is a significant impurity in many natural gas wells and reduces the energy efficiency and is corrosive in the presence of moisture. Amine scrubbing is commonly used for CO2 separation. Compared to the method, membrane processes are less expensive, require less energy to operate and avoid the usage of chemicals or the need to regenerate absorbents. Polymeric membranes for CO2 separation of natural gas were installed in the 1980s [1], but high CO2 partial pressure from natural gas wells can plasticize polymeric membranes and decrease their separation performance [2]. Microporous inorganic membranes with pore size of less than 1 nm have been widely studied for gas separation due to their superior thermal and mechanical stability, good chemical resistance, and high pressure stability in comparison with polymeric membranes [3]. Microporous silica [4,5], carbon molecular sieve [6–8], metal organic frameworks (MOF) [9–11], and zeolite membranes [12–14] have been reported for gas (vapor) mixture separation. MOFs offer potential applications in gas separation because of the well-defined pore size, high surface area, and low framework density. Membranes such as MOF-5 [15], Bio-MOF-1 n Corresponding author at: State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China. E-mail address: [email protected] (R. Zhou).
http://dx.doi.org/10.1016/j.memsci.2014.08.035 0376-7388/& 2014 Elsevier B.V. All rights reserved.
[16] and zeolite imidazolate framework-8 (ZIF-8) [17], were CO2 selective. ZIF-8 membranes [17] displayed an unprecedented high CO2 permeance of 2.4 10 5 mol/(m2 s Pa), but the CO2/CH4 separation selectivities were only 4–7. Zeolite membranes are good candidates for gas separations due to their uniform-sized pores of molecular dimensions. Aluminosilicate zeolite membranes such as zeolite T [18,19] DDR [20,21], MFI [22–24] and FAU [25–27] were shown to separate CO2/CH4 and CO2/N2 mixtures. Zeolite T membranes [18] displayed a CO2/CH4 selectivity of 400, but with a relatively low permeance of 4.6 10 8 mol/(m2 s Pa) at a pressure drop of 0.1 MPa for an equimolar CO2/CH4 mixture. An higher permeance (7 10 8 mol/(m2 s Pa)) and selectivity ( 280) was obtained for a DDR membrane [20] for the same gas mixture. A MFI membrane [23] displayed a high permeance of 3 10 6 mol/(m2 s Pa) at a pressure drop of 2 MPa, but the CO2/CH4 selectivity was as low as 4. Microporous aluminophosphates (AlPOs) and Si-substituted aluminophosphates (SAPOs) are classes of crystalline materials built of equimolar AlO4 and PO4þ tetrahedral units or together with SiO4 units for SAPOs. SAPO-34 (CHA-type) membranes [28– 32] have been used for separations at high pressures, and CO2/CH4 selectivity was 70 and CO2 permeances were 1.2 10 6 mol/ (m2 s Pa) for a feed pressure of 4.6 MPa. The aluminophosphate LTA (AlPO-42) membranes [33] showed a H2/C3H8 selectivity of 146. The AEI-type AlPO-18 has a three-dimensional pore system possessing 8-membered intersecting channels with a diameter of
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
0.38 nm and a low framework density of 15.1 T/nm3. The pore properties of AEI framework indicate AlPO-18 membranes are good candidates for CO2 (kinetic diameter of 0.33 nm) separation over CH4 (kinetic diameter of 0.38 nm) by adsorption [34] or membrane [35] separation processes. Recently, AlPO-18 membranes were fabricated on the inside surface of small-pore (0.26 mm) stainless steel nanofilters by Carreon et al. [35]. Selective membranes were only obtained by twice hydrothermal synthesis steps; they had CO2 permeances of 6.6 10 8 mol/(m2 s Pa) and CO2/CH4 selectivities of 52–60 at 295 K and 138 kPa for an equimolar CO2/CH4 mixture. For practical application of a zeolite membrane, the fabrication cost, membrane stability, synthesis simplification and reproducibility should be taken into account. In the current study, AlPO-18 membranes were prepared on the outside surface of a symmetric macroporous alumina tube by a single hydrothermal synthesis step; this support is less expensive than those used for SAPO-34 and AlPO-18 membranes to date. Further, the stabilities of membranes exposed to water are important for separation of natural gas, which contains moisture. Uncalcined SAPO-34 membranes were stable in liquid water for at least 2 days [32], but calcined SAPO-34 membranes were sensitive to liquid water even for 15 min. In the present work, the stabilities of AlPO-18 membranes were investigated after exposure to water, multiple calcinations, long-term operation, and at higher pressures.
2. Experimental methods 2.1. Synthesis of AlPO-18 seeds All chemicals for seeds and membrane preparation were purchased from Sigma-Aldrich. Nanosized AlPO-18 seeds were synthesized according to the previously-described procedure [36–38]. In a typical synthesis, Al(i-C3H7O)3 (98%), TEAOH (35 wt% aqueous solution) and deionized water were mixed and stirred for 1 h to form a homogeneous solution. Then H3PO4 (85 wt% aqueous solution) was added and the resulting solution was stirred for 2 h at 313 K. The final gel had a molar composition of 1.0 Al2O3: 3.16 P2O5: 6.32 tetraethyl ammonium hydroxide (TEAOH): 186 H2O and was transferred to a Teflon-lined autoclave. The stationary hydrothermal synthesis was carried out in an oven at 423 K for 20 h. After the reaction mixture cooled below 343 K, the seeds were centrifuged at 6000 rpm for 20 min and then washed with DI water. The washing procedure was repeated three times and the resulting AlPO-18 seeds were dried overnight in an oven at 323 K. 2.2. Synthesis of AlPO-18 membrane AlPO-18 membranes were prepared on the outside surface of the macroporous α-alumina tubes. The α-alumina tubes (12 mm OD, 9 mm ID and 1.3 mm average pore size) from Nikkato were cut into 10-cm long pieces and polished using 800♯ sandpaper. The supports were washed several times with boiling DI water for 30 min and dried overnight at 373 K. After cleaning, the outside surface of α-alumina tubes was seeded by rubbing it with uncalcined AlPO-18 crystals. Membrane synthesis gel were prepared with Al(i-C3H7O)3, TEAOH and H3PO4 as aluminum, template and phosphate precursors and had a molar ratio of 1.0 Al2O3: 1.0 P2O5: 1.8 TEAOH: 120H2O. The seeded supports were placed vertically in an autoclave filled with 280 g synthesis gel. Hydrothermal treatment was carried out in a conventional oven at 488 K for a certain period. The as-synthesized membranes were washed by running tap water for 15 min, soaked in deionized water for 1 h, and dried overnight at 373 K. The membranes were calcined in a
339
temperature-programmed furnace in air at 723 K for 6 h with heating and cooling rates of 0.6 K/min. The calcined membranes were stored at 473 K prior to the separation tests. 2.3. Characterization and separation measurements AlPO-18 crystals and membranes were characterized using a field emission scanning electron microscopy (FE-SEM, Hitachi SU8020) at acceleration voltages of 5–10 KV. The crystal phases of the seeds and membranes were identified by XRD (Ultima IV) using Cu Kα radiation at 2θ from 51 to 451 and a step size of 0.051. Note that the membranes were not cut to make the XRD measurements. Carbon dioxide and CH4 adsorption isotherms were measured on AlPO-18 crystals in an Autosorb-1 (Quantachrome Corp., AS1-C-VP-RGA) system. Before adsorption measurements, the samples were calcined at 773 K for 6 h and degassed in vacuum at 493 K overnight. Single-gas permeation was measured as a function of pressure and temperature using a dead-end (retentate stream blocked) system without sweep gas for H2, CO2, N2, CH4, C2H6 and i-C4H10, similar to the measurements by Funke et al. [39]. The membranes were mounted in a stainless steel module, and sealed at each end with three silicone O-rings and two stainless steel rings. The feed gas flowed through the gap (6 mm) between membrane outside surface and module inside surface. The ideal selectivity is the ratio of the single-gas permeances. Separations were measured using the Wiche–Kallenbach method with helium as a sweep gas, or a pressure-drop was maintained across the membrane without sweep gas. The module temperature was controlled between 300 and 423 K and the feed pressure could be varied from 0.1 to 0.5 MPa. Mass flow controllers were used to mix pure gases in equimolar ratio, and the compositions of the feed, retentate and permeate streams were measured by a GC (J-science Lab. Co. Ltd., GC7100T) with a thermal conductivity detector. The flow rates for CO2 and CH4 and the He sweep gas were 150, 150 and 400 standard ml/min (SMLPM), respectively. Selectivity is the ratio of permeances. Some separations are carried out at higher temperatures and feed pressure (up to 2 MPa) without a sweep gas, as described previously [31,32]. The feed flow rate was up to 8000 SMLPM. High feed flow rates (3.2 m/s linear velocity at the membrane surface) were used to minimize concentration polarization [40]. Unless otherwise specified, the retentate pressure was kept to 0.13 MPa (absolute pressure) by a back pressure regulator and one by-pass line in the retentate side. A second GC with a thermal conductivity detector (Shimadzu GC-14C) was used to measure compositions of in the feed and retentate.
3. Results and discussion 3.1. Membrane preparation and morphology AlPO-18 membranes were prepared on the outside surface of macroporous alumina supports by a single hydrothermal synthesis. The XRD patterns of the uncalcined seeds, uncalcined membrane, and powder formed membrane synthesis are shown in Fig. 1. The XRD peaks for the seeds and powders done during membrane synthesis match closely with those of AlPO-18 crystals reported by Vilaseca et al. [36]. The high intensity of the XRD lines and the low background intensity indicate a high degree of crystallinity for the AlPO-18 crystals as shown in Figs. 1a and b. The peaks at two theta of 9.61, 12.71, 161, and 211 (Fig. 1c) were typical peaks of AlPO-18 crystals, indicating that the supported zeolite layers are pure AlPO-18. Compared with the powdery XRD
340
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
patterns, the XRD pattern for the membrane showed that the membrane was not strong preferred orientation. The SEM image in Fig. 2a indicates that the AlPO-18 seeds had the hexagonal-prism and plate-like morphology. The seeds were approximately 500 nm size and 50 nm thick. The similar morphologies of AlPO-18 crystals were also previously reported in the literatures [36,37] since the gel composition and synthesis conditions for the references and our seeds were very close. Carreon et al. [35] prepared AlPO-18 membrane using the same gel composition with our seed preparation and the supported AlPO18 layers also consisted of hexagonal-prism-shaped plates. The bottom crystals of membrane preparation had rectangular and plate-like morphology with 500 300 100 nm, as shown in Fig. 2b. AlPO-18 crystals with the similar plate-like morphology were also prepared by Wendelbo et al. [38].
After rubbing the seeds on the support, AlPO-18 seeds were randomly and sparsely covered the outside surface of alumina tubes as shown in Fig. 2c. The cross-sectional view in Fig. 2d shows that the seed layer as multilayers was 3 mm thick and some crystals were inside the pores of the support. SEM images of AlPO-18 membranes prepared with different synthesis time are shown in Fig. 3. After 3.5-h synthesis, the cuboid AlPO-18 crystals fully covered the support, but some voids were observed in SEM images in Fig. 3a and b. The crystals size and membrane intergrowth increased with synthesis time from 3.5 to 10 h, but the crack defects formed when synthesis time was further increased to 24 h from their surface images in Fig. 3a, c, e and g. The cuboid morphology for the surface crystals changed to the cubic morphology when synthesis time increased. The crosssectional SEM images (Fig. 3d, f and h) show membrane thickness of approximately 4, 10 and 8 mm for the membranes prepared for 5, 10 and 24 h, respectively. The thinner membrane for longer synthesis time may be due to the dissolution of crystal layer in the gel (pH 10) at 488 K. Similarly, SAPO-34 membrane [31] and NaA membrane [41,42] thickness decreased after prolonged synthesis. Note that AlPO-18 crystals grew into the pores in the crosssectional views (Fig. 3). The grown crystals probably derived from the deposited seeds in the macropores during the rubbing process because the support sizes (average pore size of 1.3 mm) are much larger than the AlPO-18 seeds (average particle size of 0.5 mm).
3.2. Adsorption isotherms
Fig. 1. XRD pattern of (a) AlPO-18 seeds, (b) accompanying powder with membrane synthesis and (c) AlPO-18 membrane.þindicates the peaks of alumina supports.
Adsorption isotherms for CO2 and CH4 on AlPO-18 seeds at 293 K are shown in Fig. 4. The adsorption amount for CO2 on AlPO18 powders is higher than that for CH4. The amount of CO2 and CH4 absorbed increased linearly with pressure up to 100 kPa at 293 K. The amount of CO2 adsorbed at 90 kPa was four times the amount of CH4 adsorbed. Both CO2 and CH4 adsorption amounts on AlPO-18 crystals decreased with increased temperature (Fig. 5). The CO2/CH4 adsorption selectivity increased slightly with temperature from 263 to 348 K. The highest CO2/CH4 adsorption selectivity on AlPO-18 was 4.6 at 263 K.
Fig. 2. SEM images of (a) AlPO-18 seeds, (b) accompanying powder with membrane synthesis, (c) surface view and (d) cross-sectional view of seeded support.
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
341
Fig. 3. SEM images of AlPO-18 membranes prepared at 488 K with synthesis time of (a), (b) 3.5 h, (c), (d) 5 h, (e), (f) 10 h and (g), (h) 24 h.
The isotherm was modeled by using the statistical isotherm of Ruthven which was described in the literature [43]: h im ðbif iÞm 1 ððmÞ=ðΩi þ 1ÞÞ Ωi qi; sat bif i þ∑m ¼ 2 ðm 1Þ! 1 ðð1Þ=ðΩi þ 1ÞÞ ð1Þ qi ¼ h i Ωi 1 þ bif i þ∑Ωi ðbif iÞm 1 ððmÞ=ðΩi þ 1ÞÞ m
from the Arrhenius plots of the adsorption equilibrium constants (Fig. 6). The heats of adsorption for CO2 and CH4 were approximately 22.9 and 19.0 kJ/mol, respectively.
where qi is the coverage, qsat is the saturation coverage and Ωi is the maximum capacity expressed in molecules per cage, and bi is the adsorption equilibrium constant: ΔS ΔHads ð2Þ b ¼ exp R RT
Single-gas permeances of H2, CO2, N2, CH4, C2H6 and i-C4H10, measured at 297 K and 0.2 MPa feed pressure for AlPO-18 membranes M1 and M2, are shown as a function of kinetic diameter in Fig. 7. Both membranes showed the same permeance order: CO2 4 H2 4N2 4 CH4 4C2H6 4 i-C4H10. The i-C4H10 permeances were close our detectable limit of 1.0 10 11 mol/(m2 s Pa). Even though the kinetic diameter of CO2 is larger than that of H2, the CO2 permeance through membrane M1 was 4.9 times higher than
m ¼ 2 ðmÞ!
1 ðð1Þ=ðΩi þ 1ÞÞ
The qsat value was obtained from the lower temperature (263 K) as shown in Table 1. The heat of adsorption was calculated
3.3. Single-gas permeation
342
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
H2 permeance. The ideal selectivities of the light gases at a feed pressure of 0.2 MPa through membranes M1 and M2 are shown in Table 2. These AlPO-18 membranes have high ideal selectivities for H2 over small hydrocarbons (H2/CH4 and H2/i-C4H10 ideal selectivities of 23 and 4 1000, respectively), indicating their potential for these separations. This is analogous to the single-gas permeances through other 8-ring zeolite membranes such as DDR [20], SSZ-13 [44], zeolite T [18] and SAPO-34 [45] membranes. The temperature dependence of the single-gas permeances for CO2, H2, N2 and CH4 from 293 to 423 K through membrane M3 is shown in Fig. 8. The single-gas permeance for CO2, CH4, and N2 decreased, but the H2 permeance had a minimum. The CO2 permeance decreased more than the CH4 permeance, so the ideal selectivity decreased from 106 to 31 as the temperature increased from 293 to 423 K. The H2/CH4 ideal selectivity increased with temperatures, and was 51 at 423 K.
3.4. CO2/CH4 separation Fig. 9 shows the CO2/CH4 selectivity and permeances through membrane M3 for an equimolar CO2/CH4 mixture as a function of temperature at 0.23 MPa feed pressure and 0.13 MPa retentate
Fig. 4. Adsorption isotherms for CO2 and CH4 on AlPO-18 powder at 293 K.
pressure, without sweep gas. The single-gas permeation is included in Fig. 9. The CO2 permeance in the mixture was almost the same as the single gas, but the CH4 permeance was lower in the mixture, thus the mixture selectivity was a little higher than the ideal selectivity. The CO2/CH4 selectivities decreased with increased temperature because that CO2 permeance decreased as temperature increased, but CH4 permeance was almost independent of temperature. The same trends were observed for membrane M10 using with sweep gas, as shown in Fig. 10. The CO2/CH4 selectivities and permeances through membrane M4 as a function of feed pressure without sweep gas are shown in
Table 1 The saturation coverage qsat and bi values as a function of temperature. qsat (mmol/g)
CO2 CH4
13.62 6.61
bi ( 10 5) (Pa 1) 263 K
273 K
293 K
323 K
348 K
6.71 0.818
5.13 0.674
2.42 0.42
1.03 0.163
0.536 0.0288
Fig. 6. Adsorption equilibrium constants as a function of temperature for CO2 and CH4 adsorption on AlPO-18.(* dashed lines were fixed linear lines with fixed qsat obtained at 263 K to calculate the heat of adsorption, here for CH4 we ignored the data of 348 K due to the absorption is lower).
Fig. 5. Adsorption isotherms on AlPO-18 powder at various temperatures for (a) CO2 and (b) CH4 as adsorbent.
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
Fig. 7. Single-gas permeances as a function of the molecular kinetic diameter for AlPO-18 membrane M1 and M2 at room temperature and pressure drop of 0.1 MPa.
343
Fig. 9. Selectivity and permeances through membrane M3 for an equimolar CO2/ CH4 mixture as a function of feed temperature at pressure drop of 0.1 MPa without sweep gas.
Table 2 The ideal selectivities for the AlPO-18 membranes M1and M2 at room temperature and pressure drop of 0.1 MPa. Ideal pairs
M1
M2
CO2/H2 CO2/CH4 CO2/N2 H2/N2 H2/CH4 H2/i-C4H10
5.0 115.2 14.3 2.9 23.0 4 1000
3.3 61.8 10.9 3.3 18.7 41000
Fig. 10. Selectivity and permeances through membrane M10 for an equimolar CO2/ CH4 mixture as a function of feed temperature at pressure drop of 0.1 MPa with sweep gas.
Fig. 8. Single-gas permeances as a function of temperature for membrane M3 at pressure drop of 0.1 MPa.
Fig. 11. Both CO2 and CH4 permeances decreased as pressure drop increased, and the decrease trend for CO2 permeance was larger than that for CH4 permeance. The CO2/CH4 selectivity exhibited a maximum of 90 at pressure drop of 0.2 MPa and dropped off to 71 at 0.5 MPa. The CO2 single-gas and mixture permeances were almost the same, whereas the CH4 mixture permeances were 75–85% of the single gas permeances at the range of feed pressure. Therefore the mixture selectivity was higher than the ideal selectivity.
Those CO2/CH4 separation results through the AlPO-18 membranes agree with the model proposed by Keizer et al. [46] This model described the light gas separation through MFI membranes, in which both molecular sizes relative to the zeolite pore and the relative adsorption strengths are considered to determine the faster permeating species in a binary mixture. Ideal CO2/CH4 selectivities of our AlPO-18 membrane are much greater than gas adsorption selectivities, indicating smaller CO2 molecule diffuse faster though the membrane pores. From these adsorption data (Figs. 5 and 6), the adsorption heat of CO2 over AlPO-18 powders (22.9 kJ/mol) were larger than that of CH4 (19.0 kJ/mol). This should result in the CO2–preferential coverage in a binary CO2/CH4 mixture, by which competitive adsorption inhibited CH4 adsorption and increased the CO2/CH4 selectivity. Thus, separation selectivities were higher than ideal selectivities, as shown in Fig. 11. As a conclusion, the CO2/CH4 mixture is separated through the small-pore (3.8 Å) AlPO-18 membranes by the combination of differences in diffusivity and competitive adsorption that enhance collaboratively the mixture selectivity. This is consistent with the separation phenomenon through the SAPO-34 membranes [44]. Membrane M1 was also tested up to 2 MPa feed pressure, and both permeance and selectivity decreased, as shown in Fig. 12. Although the CO2 permeance decreased with pressure drop, the CO2 permeate flux increased as feed pressure increased. At a pressure drop of 0.2 MPa, membrane M1 had a selectivity of 85 and a CO2 permeance of 1.04 10 7 mol/(m2 s Pa) [CO2 flux of
344
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
3.3 kg/(m2 h)]. The selectivity at 2 MPa was only 31, and the permaence was 0.47 10 7 mol/(m2 s Pa) [CO2 flux of 15 kg/ (m2 h)]. One of the reason for the decrease in CO2/CH4 selectivity at high pressures was that the small non-zeolite pores in crystal boundary increased CH4 permeance [from 1.2 to 1.5 10 9 mol/ (m2 s Pa)] as feed pressure increased from 0.2 MPa to 2 MPa. The decreases in CO2 permeance at 2 MPa could be attributed to the concentration polarization which normally became not be ignored
Table 4 CO2 permeance and selectivity for AlPO-18 membranes before and after the treatment by water vapor and liquid at room temperature with sweep gas. Membrane
a
M1 M9b a b
CO2 permeance (mol/m2 s Pa)
CO2/CH4 selectivity
Original
Original
1.1 1.9
Treatment repeat 1
2
3
1.2 0.57
1.6 0.33
1.6 0.30
95 100
Treatment repeat 1
2
3
108 126
86 50
82 55
Water vapor for 2 d at room temperature. Liquid water for 3 h at room temperature.
Fig. 11. Selectivity and permeances through membrane M4 for an equimolar CO2/ CH4 mixture as a function of pressure drop at room temperature with sweep gas.
Fig. 13. Selectivity and permeances through membrane M5 for an equimolar CO2/ CH4 mixture as a function of test time at room temperature and pressure drop of 0.1 MPa with sweep gas.
Fig. 12. Selectivity and permeances through membrane M1 for an equimolar CO2/CH4 mixture as a function of pressure drop at room temperature without sweep gas.
Table 3 Permeance and selectivity for the AlPO-18 membranes under optimized conditions with sweep gas. Membrane
M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 Average
Permeance 107 [mol/(m2 s Pa)] CO2
CH4
1.1 2.4 1.8 1.7 1.2 2.1 2.8 1.3 1.9 1.1 2.0 1.8 70.6
0.012 0.032 0.015 0.018 0.010 0.019 0.032 0.015 0.019 0.011 0.017 0.0182 70.007
CO2/CH4 Selectivity
95 75 115 93 125 111 87 85 100 100 120 1017 16
Fig. 14. Selectivity and permeances through membranes M6 (empty dots) and M7 (full dots) for an equimolar CO2/CH4 mixture as a function of calcination times and temperature change rates at room temperature and pressure drop of 0.1 MPa with sweep gas. (The first three calcinations had a standard temperature change of 0.5 K/ min and the 4th calcination temperature changes are 3 K/min and 10 K/min for membrane M6 and M7, respectively.).
when feed pressure increased for highly selective membranes [31,32]. For the inside SAPO-34 membrane, a Teflon insert was used to increase linear velocity of the feed gas along the membrane surface, and thus the concentration polarization was minimized at high pressures [31,32]. However, this method cannot be adopted for the outside membrane. The reproducibility of the AlPO-18 membranes was shown in Table 3 for CO2/CH4 separation at 293 K and 0.1 MPa of pressure drop for eleven membranes. These membranes were synthesized under the same conditions. The average CO2 permeance was
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
345
Table 5 Comparison of CO2/CH4 separation performance through zeolite membranes. Membrane type
Support
CO2 permeance (mol/m2 s Pa)
Selectivity
References
AlPO-18 AlPO-18 DDR T-type SAPO-34 SAPO-34
Symmertic alumina tubes, 1.3 mm pores Stainless steel tubes, 0.27 mm pores Symmertic alumina tubse, 0.6 mm pores Symmertic mullite tubes, 1.3 mm pores. Stainless steel tubes, 0.27 mm pores Asymetric alumina tubes, 0.1–0.2 mm pores
2.0 10 7 6.6 10 8 7 10 8 4.6 10 8 4.0 10 7 1.8 10 6
120 60 220 400 115 171
This work [35] [20] [18] [28] [29]
(1.870.6) 10 7 mol/(m2 s Pa), and the average CO2/CH4 separation selectivity was 101 716. Three membranes with high selectivities (M3, M5, M10) were also tested using the pressure-drop method. The selectivities were 12–15% lower than when the sweep-gas method was used, and the CO2 permeances were 15– 20% lower.
3.5. Membrane stability Separation performance of membranes before and after exposure to water vapor and liquid water at room temperature are shown in Table 4. After each treatment, the membrane was stored overnight in an oven at 523 K. Water vapor at room temperature had little effect on separation performance, as shown in Table 4 for membrane M1, which was exposed to room temperature water vapor for a total of 6 days. The permeance and selectivity both increased after 2 day exposure to water vapor, and after 6 day, the permeance increased by 45%, and the selectivity decreased by 14%. When membrane M9 was immersed in 500-ml water at room temperature for 3 h, selectivity increased but CO2 permeances decreased. And longer exposures decreased CO2 permeance but little influenced CH4 permeance, resulting in the decrease in selectivity. The SEM and XRD observations (not shown) indicated that the 3rd treated membrane by liquid water still had continuous zeolite layers, and the crystallinity of zeolite layers did not appear to change. The decrease in selectivity should be due to the small change in crystal layers such as pore blocking. The thermal stability of AlPO-18 membranes was investigated by changing calcination times, calcination temperature, and heating and cooling rate during calcination. The standard calcination procedure was carried out at 723 K for 6 h with heating and cooling rates of 0.6 K/min. Membranes M6 and M7 were calcined the first three times using the standard procedure and then calcined at heating and cooling rates of 3 and 10 K/min, respectively. After each cycle, room temperature separation was carried out using a sweep gas, and the results are shown in Fig. 13. The membrane permeances were stable or increased after the 2nd and 3rd calcinations, and selectivity increased for one membrane (M7) and decreased for the other (M6). Faster heating and cooling rates (the 4th calcination) decreased permeances and selectivities, particularly for membrane M7, as shown in Fig. 13. However, the nonzeolitic pores did not increase for our AlPO-18 membrane (M7) by the temperature change of 10 K/min from room temperature to 753 K since CH4 permeance changed little. A fresh membrane was heated at a ramp rate of 2 K/min for the first calcination, saving 60% calcination time. This membrane had a CO2/CH4 selectivity of 87, which is only slightly lower than the average value of 101 for the eleven membranes in Table 3. Template removal for most zeolite membranes [20,24,30] was carried out at heating rates of less than 1 K/min to avoid forming nonzeolitic pores. Poshusta et al.[45] reported that temperature changes of 30 K/min from 458 K to 300 K resulted in the decrease of CO2/CH4 selectivity from 26 to 19 in view of the SAPO-34 membrane. Although our AlPO-18 membranes were thermally stable at heating rates up to 10 K/min and at temperatures up to 753 K, lower heating rates are preferred to obtain higher permeance.
Membrane stability was also investigated by running CO2/CH4 separations with a sweep gas for 170 h, as shown in Fig. 14. The CO2/CH4 selectivity did not change and the CO2 permeance was relatively constant. 3.6. Comparison to the literature The comparison of CO2/CH4 separation performance through zeolite membranes are shown in Table 5. Our one-layer AlPO-18 membrane prepared using symmetric supports had both higher permeance and higher selectivity than the previously reported two-layer AlPO-18 membranes [35]. This membrane also showed a higher CO2 permeance but a lower CO2/CH4 selectivity than the reported zeolite DDR [20] and zeolite T [18] membranes. The asymmetric alumina supported SAPO-34 membranes [28,29] showed the highest permeance for CO2/CH4 separation together with good selectivity. The symmetric alumina support we used for AlPO-18 membrane preparation is much cheaper than those for AlPO-18 [35] and SAPO-34 [28,29] membrane preparation.
4. Conclusion (1) AlPO-18 membranes with high CO2/CH4 separation selectivities can be reproducibly prepared on symmetric alumina tubular supports in a single hydrothermal synthesis. (2) The best CO2/CH4 selectivity was 120 with a permeance of 2.0 10 7 mol/(m2 s Pa) at room temperature and a pressure drop of 0.1 MPa for an equimolar CO2/CH4 mixture. Membranes were also selective CO2/CH4 separation at high pressures up to 2 MPa feed pressure. (3) The separation performance of AlPO-18 membranes was independent of calcination times and temperature ramp rates (up to 3 K/min) and was not affected by exposure to water vapor at room temperature and test time for the investigated 170-h period. However, membranes lost permeance and selectivity to some extent after extended exposures to liquid water at room temperature.
Acknowledgments We gratefully acknowledge financial support by the National Natural Science Foundation of China (NSFC) Contracts 20906042 and 21366013, and the Jiangxi International Scientific Cooperation contract 20111BDH80023. And we acknowledge the CU group Dr. J. L. Falconer, Dr. H. H. Funke and Dr. R. D. Noble for their adsorption test and discussion improvement. References [1] R.W. Baker, Future directions of membrane gas separation technology, Ind. Eng. Chem. Res. 41 (2002) 1393–1411. [2] W.J. Koros, R. Mahajan, Pushing the limits on possibilities for large scale gas separation: which strategies? J. Membr. Sci. 175 (2000) 181–196.
346
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
[3] M. Anderson, H. Wang, Y.S. Lin, Inorganic membranes for carbon dioxide and nitrogen separation, Rev. Chem. Eng. 28 (2012). [4] Y. Gu, P. Hacarlioglu, S.T. Oyama, Hydrothermally stable silica–alumina composite membranes for hydrogen separation, J. Membr. Sci. 310 (2008) 28–37. [5] M. Kanezashi, T. Shioda, T. Gunji, T. Tsuru, Gas permeation properties of silica membranes with uniform pore sizes derived from polyhedral oligomeric silsesquioxane, AIChE J. 58 (2012) 1733–1743. [6] D.Q. Vu, W.J. Koros, S.J. Miller, High pressure CO2/CH4 separation using carbon molecular sieve hollow fiber membranes, Ind. Eng. Chem. Res. 41 (2002) 367–380. [7] B.J. Hinds, N. Chopra, T. Rantell, R. Andrews, V. Gavalas, L.G. Bachas, Aligned multiwalled carbon nanotube membranes, Science 303 (2004) 62–65. [8] Y.H. Sim, H. Wang, F.Y. Li, M.L. Chua, T.S. Chung, M. Toriida, S. Tamai, High performance carbon molecular sieve membranes derived from hyperbranched polyimide precursors for improved gas separation applications, Carbon 53 (2013) 101–111. [9] J.R. Li, R.J. Kuppler, H.C. Zhou, Selective gas adsorption and separation in metal-organic frameworks, Chem. Soc. Rev. 38 (2009) 1477–1504. [10] Y. Pan, Z. Lai, Sharp separation of C2/C3 hydrocarbon mixtures by zeolitic imidazolate framework-8 (ZIF-8) membranes synthesized in aqueous solutions, Chem. Commun. 47 (2011) 10275–10277. [11] A. Huang, N. Wang, C. Kong, J. Caro, Organosilica-functionalized zeolitic imidazolate framework ZIF-90 membrane with high gas-separation performance, Angew. Chem. 51 (2012) 10551–10555. [12] M. Tsapatsis, Materials science. Toward high-throughput zeolite membranes, Science 334 (2011) 767–768. [13] T.C. Pham, H.S. Kim, K.B. Yoon, Growth of uniformly oriented silica MFI and BEA zeolite films on substrates, Science 334 (2011) 1533–1538. [14] M. Yu, R.D. Noble, J.L. Falconer, Zeolite membranes: microstructure characterization and permeation mechanisms, Acc. Chem. Res. 44 (2011) 1196–1206. [15] S. Keskin, D.S. Sholl, Assessment of a metal organic framework membrane for gas separations using atomically detailed calculations: CO2, CH4, N2, H2 mixtures in MOF-5, Ind. Eng. Chem. Res. 48 (2009) 914–922. [16] J.A. Bohrman, M.A. Carreon, Synthesis and CO2/CH4 separation performance of Bio-MOF-1 membranes, Chem. Commun. 48 (2012) 5130–5132. [17] S.R. Venna, M.A. Carreon, Highly permeable zeolite imidazolate framework-8 membranes for CO2/CH4 separation, J. Am. Chem. Soc. 132 (2010) 76–78. [18] Y. Cui, H. Kita, K.-i. Okamoto, Preparation and gas separation performance of zeolite T membrane, J. Mater. Chem. 14 (2004) 924. [19] S.M. Mirfendereski, T. Mazaheri, M. Sadrzadeh, T. Mohammadi, CO2 and CH4 permeation through T-type zeolite membranes: effect of synthesis parameters and feed pressure, Sep. Purif. Technol. 61 (2008) 317–323. [20] T. Tomita, K. Nakayama, H. Sakai, Gas separation characteristics of DDR type zeolite membrane, Microporous Mesoporous Mater. 68 (2004) 71–75. [21] M. Kanezashi, J. O’Brien-Abraham, Y.S. Lin, K. Suzuki, Gas permeation through DDRtype zeolite membranes at high temperatures, AIChE J. 54 (2008) 1478–1486. [22] J.C. Poshusta, R.D. Noble, J.L. Falconer, Temperature and pressure effects on CO2 and CH4 permeation through MFI zeolite membranes, J. Membr. Sci. 160 (1999) 115–125. [23] L. Sandström, E. Sjöberg, J. Hedlund, Very high flux MFI membrane for CO2 separation, J. Membr. Sci. 380 (2011) 232–240. [24] S. Fan, J. Liu, F. Zhang, S. Zhou, F. Sun, Fabrication of zeolite MFI membranes supported by α-Al2O3 hollow ceramic fifibers for CO2 separation, J. Mater. Res. 28 (2013) 1870–1876. [25] Y. Hasegawa, T. Tanaka, K. Watanabe, B.-H. Jeong, K. Kusakabe, S. Morooka, Separation of CO2-CH4 and CO2-N2 systems using ion-exchanged Fau-type zeolite membranes with different Si/Al ratios, Korean J. Chem. Eng. 19 (2002) 309–313.
[26] J.C. White, P.K. Dutta, K. Shqau, H. Verweij, Synthesis of ultrathin zeolite Y membranes and their application for separation of carbon dioxide and nitrogen gases, Langmuir 26 (2010) 10287–10293. [27] K. Kusakabe, T. Kuroda, A. Murata, S. Morooka, Formation of a Y-type zeolite membrane on a porous α-alumina tube for gas separation, Ind. Eng. Chem. Res. 36 (1997) 649–655. [28] S. Li, J.L. Falconer, R.D. Noble, Improved SAPO-34 membranes for CO2/CH4 separations, Adv. Mater. 18 (2006) 2601–2603. [29] M.A. Carreon, S. Li, J.L. Falconer, R.D. Noble, Alumina-supported SAPO-34 membranes for CO2/CH4 separation, J. Am. Chem. Soc. 130 (2008) 5412–5413. [30] Y. Zhang, B. Tokay, H.H. Funke, J.L. Falconer, R.D. Noble, Template removal from SAPO-34 crystals and membranes, J. Membr. Sci. 363 (2010) 29–35. [31] E.W. Ping, R. Zhou, H.H. Funke, J.L. Falconer, R.D. Noble, Seeded-gel synthesis of SAPO-34 single channel and monolith membranes, for CO2/CH4 separations, J. Membr. Sci. 415-416 (2012) 770–775. [32] R. Zhou, E.W. Ping, H.H. Funke, J.L. Falconer, R.D. Noble, Improving SAPO-34 membrane synthesis, J. Membr. Sci. 444 (2013) 384–393. [33] A. Huang, F. Liang, F. Steinbach, T.M. Gesing, J. Caro, Neutral and cation-free LTA-type aluminophosphate (AlPO4) molecular sieve membrane with high hydrogen permselectivity, J. Am. Chem. Soc. 132 (2010) 2140–2141. [34] I. Deroche, L. Gaberova, G. Maurin, P. Llewellyn, M. Castro, P. Wright, Adsorption of carbon dioxide in SAPO STA-7 and AlPO-18: grand canonical monte carlo simulations and microcalorimetry measurements, Adsorption 14 (2008) 207–213. [35] M.L. Carreon, S. Li, M.A. Carreon, AlPO-18 membranes for CO2/CH4 separation, Chem. Commun. 48 (2012) 2310–2312. [36] M. Vilaseca, S. Mintova, V. Valtchev, T.H. Metzger, T. Bein, Synthesis of colloidal AlPO4-18 crystals and their use for supported film growth, J. Mater. Chem. 13 (2003) 1526. [37] H. van Heyden, S. Mintova, T. Bein, AlPO-18 nanocrystals synthesized under microwave irradiation, J. Mater. Chem. 16 (2006) 514. [38] R. Wendelbo, D. Akporiaye, A. Andersen, I.M. Dahl, H.B. Mostad, Synthesis, characterization and catalytic testing of SAPO-18, MgAPO-18, and ZnAPO-18 in the MTO reaction, Appl. Catal. A: Gen. 142 (1996) L197–L207. [39] H.H. Funke, B. Tokay, R. Zhou, E.W. Ping, Y. Zhang, J.L. Falconer, R.D. Noble, Spatially resolved gas permeation through SAPO-34 membranes, J. Membr. Sci. 409-410 (2012) 212–221. [40] A.M. Avila, H.H. Funke, Y. Zhang, J.L. Falconer, R.D. Noble, Concentration polarization in SAPO-34 membranes at high pressures, J. Membr. Sci. 335 (2009) 32–36. [41] X. Chen, W. Yang, J. Liu, L. Lin, Synthesis of zeolite NaA membranes with high permeance under microwave radiation on mesoporous-layer-modified macroporous substrates for gas separation, J. Membr. Sci. 255 (2005) 201–211. [42] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Sep. Purif. Technol. 25 (2001) 251–260. [43] R. Krishna, S. Li, J.M. van Baten, J.L. Falconer, R.D. Noble, Investigation of slowing-down and speeding-up effects in binary mixture permeation across SAPO-34 and MFI membranes, Sep. Purif. Technol. 60 (2008) 230–236. [44] H. Kalipcilar, T.C. Bowen, R.D. Noble, J.L. Falconer, Synthesis and separation performance of SSZ-13 zeolite membranes on tubular supports, Chem. Mater. 14 (2002) 3458–3464. [45] J.C. Poshusta, V.A. Tuan, E.A. Pape, R.D. Noble, J.L. Falconer, Separation of light gas mixtures using SAPO-34 membranes, AIChE J. 46 (2000) 779–789. [46] K. Keizer, A.J. Burggraaf, Z.A.E.P. Vroon, H. Verweij, Two component permeation through thin zeolite MFI membranes, J. Membr. Sci. 147 (1998) 159–172.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Alumina-supported AlPO-18 membranes for CO2/CH4 separation Ting Wu a,b, Bin Wang b, Zhanghui Lu b, Rongfei Zhou a,b,n, Xiangshu Chen b a State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China b College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 May 2014 Received in revised form 5 August 2014 Accepted 16 August 2014 Available online 23 August 2014
Selective AlPO-18 membranes were reproducibly synthesized on the outside surface of macroporous tubular α-alumina supports in a single hydrothermal synthesis step. The AlPO-18 crystals and membranes were characterized by SEM, XRD, adsorption and permeation measurements. Single-gas permeances of CO2, N2, CH4 and i-C4H10 decreased with increased kinetic diameter, but CO2 permeated faster than the smaller H2, because CO2 adsorbs more strongly. Eleven AlPO-18 membranes had an average CO2 permeance of (1.870.5) 10 7 mol/(m2 s Pa) and average CO2/CH4 separation selectivities of 101 716 at room temperature and a pressure drop of 0.1 MPa for the equimolar CO2/CH4 mixture. The membrane was also selective (selectivity ¼31) at 2 MPa feed pressure and had a CO2 permeance of 0.47 10 7 mol/(m2 s Pa) [CO2 flux of 15 kg/(m2 h)]. The CO2/CH4 selectivities decreased as temperature and pressure drop increased. The membranes stability was investigated for different calcination times, calcination temperature ramps, and following exposures to water vapor and liquid. & 2014 Elsevier B.V. All rights reserved.
Keywords: AlPO-18 Zeolite membrane CO2/CH4 separation Membrane stability Gas purification
1. Introduction Separation of carbon dioxide from methane is important because CO2 is a significant impurity in many natural gas wells and reduces the energy efficiency and is corrosive in the presence of moisture. Amine scrubbing is commonly used for CO2 separation. Compared to the method, membrane processes are less expensive, require less energy to operate and avoid the usage of chemicals or the need to regenerate absorbents. Polymeric membranes for CO2 separation of natural gas were installed in the 1980s [1], but high CO2 partial pressure from natural gas wells can plasticize polymeric membranes and decrease their separation performance [2]. Microporous inorganic membranes with pore size of less than 1 nm have been widely studied for gas separation due to their superior thermal and mechanical stability, good chemical resistance, and high pressure stability in comparison with polymeric membranes [3]. Microporous silica [4,5], carbon molecular sieve [6–8], metal organic frameworks (MOF) [9–11], and zeolite membranes [12–14] have been reported for gas (vapor) mixture separation. MOFs offer potential applications in gas separation because of the well-defined pore size, high surface area, and low framework density. Membranes such as MOF-5 [15], Bio-MOF-1 n Corresponding author at: State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China. E-mail address: [email protected] (R. Zhou).
http://dx.doi.org/10.1016/j.memsci.2014.08.035 0376-7388/& 2014 Elsevier B.V. All rights reserved.
[16] and zeolite imidazolate framework-8 (ZIF-8) [17], were CO2 selective. ZIF-8 membranes [17] displayed an unprecedented high CO2 permeance of 2.4 10 5 mol/(m2 s Pa), but the CO2/CH4 separation selectivities were only 4–7. Zeolite membranes are good candidates for gas separations due to their uniform-sized pores of molecular dimensions. Aluminosilicate zeolite membranes such as zeolite T [18,19] DDR [20,21], MFI [22–24] and FAU [25–27] were shown to separate CO2/CH4 and CO2/N2 mixtures. Zeolite T membranes [18] displayed a CO2/CH4 selectivity of 400, but with a relatively low permeance of 4.6 10 8 mol/(m2 s Pa) at a pressure drop of 0.1 MPa for an equimolar CO2/CH4 mixture. An higher permeance (7 10 8 mol/(m2 s Pa)) and selectivity ( 280) was obtained for a DDR membrane [20] for the same gas mixture. A MFI membrane [23] displayed a high permeance of 3 10 6 mol/(m2 s Pa) at a pressure drop of 2 MPa, but the CO2/CH4 selectivity was as low as 4. Microporous aluminophosphates (AlPOs) and Si-substituted aluminophosphates (SAPOs) are classes of crystalline materials built of equimolar AlO4 and PO4þ tetrahedral units or together with SiO4 units for SAPOs. SAPO-34 (CHA-type) membranes [28– 32] have been used for separations at high pressures, and CO2/CH4 selectivity was 70 and CO2 permeances were 1.2 10 6 mol/ (m2 s Pa) for a feed pressure of 4.6 MPa. The aluminophosphate LTA (AlPO-42) membranes [33] showed a H2/C3H8 selectivity of 146. The AEI-type AlPO-18 has a three-dimensional pore system possessing 8-membered intersecting channels with a diameter of
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
0.38 nm and a low framework density of 15.1 T/nm3. The pore properties of AEI framework indicate AlPO-18 membranes are good candidates for CO2 (kinetic diameter of 0.33 nm) separation over CH4 (kinetic diameter of 0.38 nm) by adsorption [34] or membrane [35] separation processes. Recently, AlPO-18 membranes were fabricated on the inside surface of small-pore (0.26 mm) stainless steel nanofilters by Carreon et al. [35]. Selective membranes were only obtained by twice hydrothermal synthesis steps; they had CO2 permeances of 6.6 10 8 mol/(m2 s Pa) and CO2/CH4 selectivities of 52–60 at 295 K and 138 kPa for an equimolar CO2/CH4 mixture. For practical application of a zeolite membrane, the fabrication cost, membrane stability, synthesis simplification and reproducibility should be taken into account. In the current study, AlPO-18 membranes were prepared on the outside surface of a symmetric macroporous alumina tube by a single hydrothermal synthesis step; this support is less expensive than those used for SAPO-34 and AlPO-18 membranes to date. Further, the stabilities of membranes exposed to water are important for separation of natural gas, which contains moisture. Uncalcined SAPO-34 membranes were stable in liquid water for at least 2 days [32], but calcined SAPO-34 membranes were sensitive to liquid water even for 15 min. In the present work, the stabilities of AlPO-18 membranes were investigated after exposure to water, multiple calcinations, long-term operation, and at higher pressures.
2. Experimental methods 2.1. Synthesis of AlPO-18 seeds All chemicals for seeds and membrane preparation were purchased from Sigma-Aldrich. Nanosized AlPO-18 seeds were synthesized according to the previously-described procedure [36–38]. In a typical synthesis, Al(i-C3H7O)3 (98%), TEAOH (35 wt% aqueous solution) and deionized water were mixed and stirred for 1 h to form a homogeneous solution. Then H3PO4 (85 wt% aqueous solution) was added and the resulting solution was stirred for 2 h at 313 K. The final gel had a molar composition of 1.0 Al2O3: 3.16 P2O5: 6.32 tetraethyl ammonium hydroxide (TEAOH): 186 H2O and was transferred to a Teflon-lined autoclave. The stationary hydrothermal synthesis was carried out in an oven at 423 K for 20 h. After the reaction mixture cooled below 343 K, the seeds were centrifuged at 6000 rpm for 20 min and then washed with DI water. The washing procedure was repeated three times and the resulting AlPO-18 seeds were dried overnight in an oven at 323 K. 2.2. Synthesis of AlPO-18 membrane AlPO-18 membranes were prepared on the outside surface of the macroporous α-alumina tubes. The α-alumina tubes (12 mm OD, 9 mm ID and 1.3 mm average pore size) from Nikkato were cut into 10-cm long pieces and polished using 800♯ sandpaper. The supports were washed several times with boiling DI water for 30 min and dried overnight at 373 K. After cleaning, the outside surface of α-alumina tubes was seeded by rubbing it with uncalcined AlPO-18 crystals. Membrane synthesis gel were prepared with Al(i-C3H7O)3, TEAOH and H3PO4 as aluminum, template and phosphate precursors and had a molar ratio of 1.0 Al2O3: 1.0 P2O5: 1.8 TEAOH: 120H2O. The seeded supports were placed vertically in an autoclave filled with 280 g synthesis gel. Hydrothermal treatment was carried out in a conventional oven at 488 K for a certain period. The as-synthesized membranes were washed by running tap water for 15 min, soaked in deionized water for 1 h, and dried overnight at 373 K. The membranes were calcined in a
339
temperature-programmed furnace in air at 723 K for 6 h with heating and cooling rates of 0.6 K/min. The calcined membranes were stored at 473 K prior to the separation tests. 2.3. Characterization and separation measurements AlPO-18 crystals and membranes were characterized using a field emission scanning electron microscopy (FE-SEM, Hitachi SU8020) at acceleration voltages of 5–10 KV. The crystal phases of the seeds and membranes were identified by XRD (Ultima IV) using Cu Kα radiation at 2θ from 51 to 451 and a step size of 0.051. Note that the membranes were not cut to make the XRD measurements. Carbon dioxide and CH4 adsorption isotherms were measured on AlPO-18 crystals in an Autosorb-1 (Quantachrome Corp., AS1-C-VP-RGA) system. Before adsorption measurements, the samples were calcined at 773 K for 6 h and degassed in vacuum at 493 K overnight. Single-gas permeation was measured as a function of pressure and temperature using a dead-end (retentate stream blocked) system without sweep gas for H2, CO2, N2, CH4, C2H6 and i-C4H10, similar to the measurements by Funke et al. [39]. The membranes were mounted in a stainless steel module, and sealed at each end with three silicone O-rings and two stainless steel rings. The feed gas flowed through the gap (6 mm) between membrane outside surface and module inside surface. The ideal selectivity is the ratio of the single-gas permeances. Separations were measured using the Wiche–Kallenbach method with helium as a sweep gas, or a pressure-drop was maintained across the membrane without sweep gas. The module temperature was controlled between 300 and 423 K and the feed pressure could be varied from 0.1 to 0.5 MPa. Mass flow controllers were used to mix pure gases in equimolar ratio, and the compositions of the feed, retentate and permeate streams were measured by a GC (J-science Lab. Co. Ltd., GC7100T) with a thermal conductivity detector. The flow rates for CO2 and CH4 and the He sweep gas were 150, 150 and 400 standard ml/min (SMLPM), respectively. Selectivity is the ratio of permeances. Some separations are carried out at higher temperatures and feed pressure (up to 2 MPa) without a sweep gas, as described previously [31,32]. The feed flow rate was up to 8000 SMLPM. High feed flow rates (3.2 m/s linear velocity at the membrane surface) were used to minimize concentration polarization [40]. Unless otherwise specified, the retentate pressure was kept to 0.13 MPa (absolute pressure) by a back pressure regulator and one by-pass line in the retentate side. A second GC with a thermal conductivity detector (Shimadzu GC-14C) was used to measure compositions of in the feed and retentate.
3. Results and discussion 3.1. Membrane preparation and morphology AlPO-18 membranes were prepared on the outside surface of macroporous alumina supports by a single hydrothermal synthesis. The XRD patterns of the uncalcined seeds, uncalcined membrane, and powder formed membrane synthesis are shown in Fig. 1. The XRD peaks for the seeds and powders done during membrane synthesis match closely with those of AlPO-18 crystals reported by Vilaseca et al. [36]. The high intensity of the XRD lines and the low background intensity indicate a high degree of crystallinity for the AlPO-18 crystals as shown in Figs. 1a and b. The peaks at two theta of 9.61, 12.71, 161, and 211 (Fig. 1c) were typical peaks of AlPO-18 crystals, indicating that the supported zeolite layers are pure AlPO-18. Compared with the powdery XRD
340
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
patterns, the XRD pattern for the membrane showed that the membrane was not strong preferred orientation. The SEM image in Fig. 2a indicates that the AlPO-18 seeds had the hexagonal-prism and plate-like morphology. The seeds were approximately 500 nm size and 50 nm thick. The similar morphologies of AlPO-18 crystals were also previously reported in the literatures [36,37] since the gel composition and synthesis conditions for the references and our seeds were very close. Carreon et al. [35] prepared AlPO-18 membrane using the same gel composition with our seed preparation and the supported AlPO18 layers also consisted of hexagonal-prism-shaped plates. The bottom crystals of membrane preparation had rectangular and plate-like morphology with 500 300 100 nm, as shown in Fig. 2b. AlPO-18 crystals with the similar plate-like morphology were also prepared by Wendelbo et al. [38].
After rubbing the seeds on the support, AlPO-18 seeds were randomly and sparsely covered the outside surface of alumina tubes as shown in Fig. 2c. The cross-sectional view in Fig. 2d shows that the seed layer as multilayers was 3 mm thick and some crystals were inside the pores of the support. SEM images of AlPO-18 membranes prepared with different synthesis time are shown in Fig. 3. After 3.5-h synthesis, the cuboid AlPO-18 crystals fully covered the support, but some voids were observed in SEM images in Fig. 3a and b. The crystals size and membrane intergrowth increased with synthesis time from 3.5 to 10 h, but the crack defects formed when synthesis time was further increased to 24 h from their surface images in Fig. 3a, c, e and g. The cuboid morphology for the surface crystals changed to the cubic morphology when synthesis time increased. The crosssectional SEM images (Fig. 3d, f and h) show membrane thickness of approximately 4, 10 and 8 mm for the membranes prepared for 5, 10 and 24 h, respectively. The thinner membrane for longer synthesis time may be due to the dissolution of crystal layer in the gel (pH 10) at 488 K. Similarly, SAPO-34 membrane [31] and NaA membrane [41,42] thickness decreased after prolonged synthesis. Note that AlPO-18 crystals grew into the pores in the crosssectional views (Fig. 3). The grown crystals probably derived from the deposited seeds in the macropores during the rubbing process because the support sizes (average pore size of 1.3 mm) are much larger than the AlPO-18 seeds (average particle size of 0.5 mm).
3.2. Adsorption isotherms
Fig. 1. XRD pattern of (a) AlPO-18 seeds, (b) accompanying powder with membrane synthesis and (c) AlPO-18 membrane.þindicates the peaks of alumina supports.
Adsorption isotherms for CO2 and CH4 on AlPO-18 seeds at 293 K are shown in Fig. 4. The adsorption amount for CO2 on AlPO18 powders is higher than that for CH4. The amount of CO2 and CH4 absorbed increased linearly with pressure up to 100 kPa at 293 K. The amount of CO2 adsorbed at 90 kPa was four times the amount of CH4 adsorbed. Both CO2 and CH4 adsorption amounts on AlPO-18 crystals decreased with increased temperature (Fig. 5). The CO2/CH4 adsorption selectivity increased slightly with temperature from 263 to 348 K. The highest CO2/CH4 adsorption selectivity on AlPO-18 was 4.6 at 263 K.
Fig. 2. SEM images of (a) AlPO-18 seeds, (b) accompanying powder with membrane synthesis, (c) surface view and (d) cross-sectional view of seeded support.
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
341
Fig. 3. SEM images of AlPO-18 membranes prepared at 488 K with synthesis time of (a), (b) 3.5 h, (c), (d) 5 h, (e), (f) 10 h and (g), (h) 24 h.
The isotherm was modeled by using the statistical isotherm of Ruthven which was described in the literature [43]: h im ðbif iÞm 1 ððmÞ=ðΩi þ 1ÞÞ Ωi qi; sat bif i þ∑m ¼ 2 ðm 1Þ! 1 ðð1Þ=ðΩi þ 1ÞÞ ð1Þ qi ¼ h i Ωi 1 þ bif i þ∑Ωi ðbif iÞm 1 ððmÞ=ðΩi þ 1ÞÞ m
from the Arrhenius plots of the adsorption equilibrium constants (Fig. 6). The heats of adsorption for CO2 and CH4 were approximately 22.9 and 19.0 kJ/mol, respectively.
where qi is the coverage, qsat is the saturation coverage and Ωi is the maximum capacity expressed in molecules per cage, and bi is the adsorption equilibrium constant: ΔS ΔHads ð2Þ b ¼ exp R RT
Single-gas permeances of H2, CO2, N2, CH4, C2H6 and i-C4H10, measured at 297 K and 0.2 MPa feed pressure for AlPO-18 membranes M1 and M2, are shown as a function of kinetic diameter in Fig. 7. Both membranes showed the same permeance order: CO2 4 H2 4N2 4 CH4 4C2H6 4 i-C4H10. The i-C4H10 permeances were close our detectable limit of 1.0 10 11 mol/(m2 s Pa). Even though the kinetic diameter of CO2 is larger than that of H2, the CO2 permeance through membrane M1 was 4.9 times higher than
m ¼ 2 ðmÞ!
1 ðð1Þ=ðΩi þ 1ÞÞ
The qsat value was obtained from the lower temperature (263 K) as shown in Table 1. The heat of adsorption was calculated
3.3. Single-gas permeation
342
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
H2 permeance. The ideal selectivities of the light gases at a feed pressure of 0.2 MPa through membranes M1 and M2 are shown in Table 2. These AlPO-18 membranes have high ideal selectivities for H2 over small hydrocarbons (H2/CH4 and H2/i-C4H10 ideal selectivities of 23 and 4 1000, respectively), indicating their potential for these separations. This is analogous to the single-gas permeances through other 8-ring zeolite membranes such as DDR [20], SSZ-13 [44], zeolite T [18] and SAPO-34 [45] membranes. The temperature dependence of the single-gas permeances for CO2, H2, N2 and CH4 from 293 to 423 K through membrane M3 is shown in Fig. 8. The single-gas permeance for CO2, CH4, and N2 decreased, but the H2 permeance had a minimum. The CO2 permeance decreased more than the CH4 permeance, so the ideal selectivity decreased from 106 to 31 as the temperature increased from 293 to 423 K. The H2/CH4 ideal selectivity increased with temperatures, and was 51 at 423 K.
3.4. CO2/CH4 separation Fig. 9 shows the CO2/CH4 selectivity and permeances through membrane M3 for an equimolar CO2/CH4 mixture as a function of temperature at 0.23 MPa feed pressure and 0.13 MPa retentate
Fig. 4. Adsorption isotherms for CO2 and CH4 on AlPO-18 powder at 293 K.
pressure, without sweep gas. The single-gas permeation is included in Fig. 9. The CO2 permeance in the mixture was almost the same as the single gas, but the CH4 permeance was lower in the mixture, thus the mixture selectivity was a little higher than the ideal selectivity. The CO2/CH4 selectivities decreased with increased temperature because that CO2 permeance decreased as temperature increased, but CH4 permeance was almost independent of temperature. The same trends were observed for membrane M10 using with sweep gas, as shown in Fig. 10. The CO2/CH4 selectivities and permeances through membrane M4 as a function of feed pressure without sweep gas are shown in
Table 1 The saturation coverage qsat and bi values as a function of temperature. qsat (mmol/g)
CO2 CH4
13.62 6.61
bi ( 10 5) (Pa 1) 263 K
273 K
293 K
323 K
348 K
6.71 0.818
5.13 0.674
2.42 0.42
1.03 0.163
0.536 0.0288
Fig. 6. Adsorption equilibrium constants as a function of temperature for CO2 and CH4 adsorption on AlPO-18.(* dashed lines were fixed linear lines with fixed qsat obtained at 263 K to calculate the heat of adsorption, here for CH4 we ignored the data of 348 K due to the absorption is lower).
Fig. 5. Adsorption isotherms on AlPO-18 powder at various temperatures for (a) CO2 and (b) CH4 as adsorbent.
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
Fig. 7. Single-gas permeances as a function of the molecular kinetic diameter for AlPO-18 membrane M1 and M2 at room temperature and pressure drop of 0.1 MPa.
343
Fig. 9. Selectivity and permeances through membrane M3 for an equimolar CO2/ CH4 mixture as a function of feed temperature at pressure drop of 0.1 MPa without sweep gas.
Table 2 The ideal selectivities for the AlPO-18 membranes M1and M2 at room temperature and pressure drop of 0.1 MPa. Ideal pairs
M1
M2
CO2/H2 CO2/CH4 CO2/N2 H2/N2 H2/CH4 H2/i-C4H10
5.0 115.2 14.3 2.9 23.0 4 1000
3.3 61.8 10.9 3.3 18.7 41000
Fig. 10. Selectivity and permeances through membrane M10 for an equimolar CO2/ CH4 mixture as a function of feed temperature at pressure drop of 0.1 MPa with sweep gas.
Fig. 8. Single-gas permeances as a function of temperature for membrane M3 at pressure drop of 0.1 MPa.
Fig. 11. Both CO2 and CH4 permeances decreased as pressure drop increased, and the decrease trend for CO2 permeance was larger than that for CH4 permeance. The CO2/CH4 selectivity exhibited a maximum of 90 at pressure drop of 0.2 MPa and dropped off to 71 at 0.5 MPa. The CO2 single-gas and mixture permeances were almost the same, whereas the CH4 mixture permeances were 75–85% of the single gas permeances at the range of feed pressure. Therefore the mixture selectivity was higher than the ideal selectivity.
Those CO2/CH4 separation results through the AlPO-18 membranes agree with the model proposed by Keizer et al. [46] This model described the light gas separation through MFI membranes, in which both molecular sizes relative to the zeolite pore and the relative adsorption strengths are considered to determine the faster permeating species in a binary mixture. Ideal CO2/CH4 selectivities of our AlPO-18 membrane are much greater than gas adsorption selectivities, indicating smaller CO2 molecule diffuse faster though the membrane pores. From these adsorption data (Figs. 5 and 6), the adsorption heat of CO2 over AlPO-18 powders (22.9 kJ/mol) were larger than that of CH4 (19.0 kJ/mol). This should result in the CO2–preferential coverage in a binary CO2/CH4 mixture, by which competitive adsorption inhibited CH4 adsorption and increased the CO2/CH4 selectivity. Thus, separation selectivities were higher than ideal selectivities, as shown in Fig. 11. As a conclusion, the CO2/CH4 mixture is separated through the small-pore (3.8 Å) AlPO-18 membranes by the combination of differences in diffusivity and competitive adsorption that enhance collaboratively the mixture selectivity. This is consistent with the separation phenomenon through the SAPO-34 membranes [44]. Membrane M1 was also tested up to 2 MPa feed pressure, and both permeance and selectivity decreased, as shown in Fig. 12. Although the CO2 permeance decreased with pressure drop, the CO2 permeate flux increased as feed pressure increased. At a pressure drop of 0.2 MPa, membrane M1 had a selectivity of 85 and a CO2 permeance of 1.04 10 7 mol/(m2 s Pa) [CO2 flux of
344
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
3.3 kg/(m2 h)]. The selectivity at 2 MPa was only 31, and the permaence was 0.47 10 7 mol/(m2 s Pa) [CO2 flux of 15 kg/ (m2 h)]. One of the reason for the decrease in CO2/CH4 selectivity at high pressures was that the small non-zeolite pores in crystal boundary increased CH4 permeance [from 1.2 to 1.5 10 9 mol/ (m2 s Pa)] as feed pressure increased from 0.2 MPa to 2 MPa. The decreases in CO2 permeance at 2 MPa could be attributed to the concentration polarization which normally became not be ignored
Table 4 CO2 permeance and selectivity for AlPO-18 membranes before and after the treatment by water vapor and liquid at room temperature with sweep gas. Membrane
a
M1 M9b a b
CO2 permeance (mol/m2 s Pa)
CO2/CH4 selectivity
Original
Original
1.1 1.9
Treatment repeat 1
2
3
1.2 0.57
1.6 0.33
1.6 0.30
95 100
Treatment repeat 1
2
3
108 126
86 50
82 55
Water vapor for 2 d at room temperature. Liquid water for 3 h at room temperature.
Fig. 11. Selectivity and permeances through membrane M4 for an equimolar CO2/ CH4 mixture as a function of pressure drop at room temperature with sweep gas.
Fig. 13. Selectivity and permeances through membrane M5 for an equimolar CO2/ CH4 mixture as a function of test time at room temperature and pressure drop of 0.1 MPa with sweep gas.
Fig. 12. Selectivity and permeances through membrane M1 for an equimolar CO2/CH4 mixture as a function of pressure drop at room temperature without sweep gas.
Table 3 Permeance and selectivity for the AlPO-18 membranes under optimized conditions with sweep gas. Membrane
M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 Average
Permeance 107 [mol/(m2 s Pa)] CO2
CH4
1.1 2.4 1.8 1.7 1.2 2.1 2.8 1.3 1.9 1.1 2.0 1.8 70.6
0.012 0.032 0.015 0.018 0.010 0.019 0.032 0.015 0.019 0.011 0.017 0.0182 70.007
CO2/CH4 Selectivity
95 75 115 93 125 111 87 85 100 100 120 1017 16
Fig. 14. Selectivity and permeances through membranes M6 (empty dots) and M7 (full dots) for an equimolar CO2/CH4 mixture as a function of calcination times and temperature change rates at room temperature and pressure drop of 0.1 MPa with sweep gas. (The first three calcinations had a standard temperature change of 0.5 K/ min and the 4th calcination temperature changes are 3 K/min and 10 K/min for membrane M6 and M7, respectively.).
when feed pressure increased for highly selective membranes [31,32]. For the inside SAPO-34 membrane, a Teflon insert was used to increase linear velocity of the feed gas along the membrane surface, and thus the concentration polarization was minimized at high pressures [31,32]. However, this method cannot be adopted for the outside membrane. The reproducibility of the AlPO-18 membranes was shown in Table 3 for CO2/CH4 separation at 293 K and 0.1 MPa of pressure drop for eleven membranes. These membranes were synthesized under the same conditions. The average CO2 permeance was
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
345
Table 5 Comparison of CO2/CH4 separation performance through zeolite membranes. Membrane type
Support
CO2 permeance (mol/m2 s Pa)
Selectivity
References
AlPO-18 AlPO-18 DDR T-type SAPO-34 SAPO-34
Symmertic alumina tubes, 1.3 mm pores Stainless steel tubes, 0.27 mm pores Symmertic alumina tubse, 0.6 mm pores Symmertic mullite tubes, 1.3 mm pores. Stainless steel tubes, 0.27 mm pores Asymetric alumina tubes, 0.1–0.2 mm pores
2.0 10 7 6.6 10 8 7 10 8 4.6 10 8 4.0 10 7 1.8 10 6
120 60 220 400 115 171
This work [35] [20] [18] [28] [29]
(1.870.6) 10 7 mol/(m2 s Pa), and the average CO2/CH4 separation selectivity was 101 716. Three membranes with high selectivities (M3, M5, M10) were also tested using the pressure-drop method. The selectivities were 12–15% lower than when the sweep-gas method was used, and the CO2 permeances were 15– 20% lower.
3.5. Membrane stability Separation performance of membranes before and after exposure to water vapor and liquid water at room temperature are shown in Table 4. After each treatment, the membrane was stored overnight in an oven at 523 K. Water vapor at room temperature had little effect on separation performance, as shown in Table 4 for membrane M1, which was exposed to room temperature water vapor for a total of 6 days. The permeance and selectivity both increased after 2 day exposure to water vapor, and after 6 day, the permeance increased by 45%, and the selectivity decreased by 14%. When membrane M9 was immersed in 500-ml water at room temperature for 3 h, selectivity increased but CO2 permeances decreased. And longer exposures decreased CO2 permeance but little influenced CH4 permeance, resulting in the decrease in selectivity. The SEM and XRD observations (not shown) indicated that the 3rd treated membrane by liquid water still had continuous zeolite layers, and the crystallinity of zeolite layers did not appear to change. The decrease in selectivity should be due to the small change in crystal layers such as pore blocking. The thermal stability of AlPO-18 membranes was investigated by changing calcination times, calcination temperature, and heating and cooling rate during calcination. The standard calcination procedure was carried out at 723 K for 6 h with heating and cooling rates of 0.6 K/min. Membranes M6 and M7 were calcined the first three times using the standard procedure and then calcined at heating and cooling rates of 3 and 10 K/min, respectively. After each cycle, room temperature separation was carried out using a sweep gas, and the results are shown in Fig. 13. The membrane permeances were stable or increased after the 2nd and 3rd calcinations, and selectivity increased for one membrane (M7) and decreased for the other (M6). Faster heating and cooling rates (the 4th calcination) decreased permeances and selectivities, particularly for membrane M7, as shown in Fig. 13. However, the nonzeolitic pores did not increase for our AlPO-18 membrane (M7) by the temperature change of 10 K/min from room temperature to 753 K since CH4 permeance changed little. A fresh membrane was heated at a ramp rate of 2 K/min for the first calcination, saving 60% calcination time. This membrane had a CO2/CH4 selectivity of 87, which is only slightly lower than the average value of 101 for the eleven membranes in Table 3. Template removal for most zeolite membranes [20,24,30] was carried out at heating rates of less than 1 K/min to avoid forming nonzeolitic pores. Poshusta et al.[45] reported that temperature changes of 30 K/min from 458 K to 300 K resulted in the decrease of CO2/CH4 selectivity from 26 to 19 in view of the SAPO-34 membrane. Although our AlPO-18 membranes were thermally stable at heating rates up to 10 K/min and at temperatures up to 753 K, lower heating rates are preferred to obtain higher permeance.
Membrane stability was also investigated by running CO2/CH4 separations with a sweep gas for 170 h, as shown in Fig. 14. The CO2/CH4 selectivity did not change and the CO2 permeance was relatively constant. 3.6. Comparison to the literature The comparison of CO2/CH4 separation performance through zeolite membranes are shown in Table 5. Our one-layer AlPO-18 membrane prepared using symmetric supports had both higher permeance and higher selectivity than the previously reported two-layer AlPO-18 membranes [35]. This membrane also showed a higher CO2 permeance but a lower CO2/CH4 selectivity than the reported zeolite DDR [20] and zeolite T [18] membranes. The asymmetric alumina supported SAPO-34 membranes [28,29] showed the highest permeance for CO2/CH4 separation together with good selectivity. The symmetric alumina support we used for AlPO-18 membrane preparation is much cheaper than those for AlPO-18 [35] and SAPO-34 [28,29] membrane preparation.
4. Conclusion (1) AlPO-18 membranes with high CO2/CH4 separation selectivities can be reproducibly prepared on symmetric alumina tubular supports in a single hydrothermal synthesis. (2) The best CO2/CH4 selectivity was 120 with a permeance of 2.0 10 7 mol/(m2 s Pa) at room temperature and a pressure drop of 0.1 MPa for an equimolar CO2/CH4 mixture. Membranes were also selective CO2/CH4 separation at high pressures up to 2 MPa feed pressure. (3) The separation performance of AlPO-18 membranes was independent of calcination times and temperature ramp rates (up to 3 K/min) and was not affected by exposure to water vapor at room temperature and test time for the investigated 170-h period. However, membranes lost permeance and selectivity to some extent after extended exposures to liquid water at room temperature.
Acknowledgments We gratefully acknowledge financial support by the National Natural Science Foundation of China (NSFC) Contracts 20906042 and 21366013, and the Jiangxi International Scientific Cooperation contract 20111BDH80023. And we acknowledge the CU group Dr. J. L. Falconer, Dr. H. H. Funke and Dr. R. D. Noble for their adsorption test and discussion improvement. References [1] R.W. Baker, Future directions of membrane gas separation technology, Ind. Eng. Chem. Res. 41 (2002) 1393–1411. [2] W.J. Koros, R. Mahajan, Pushing the limits on possibilities for large scale gas separation: which strategies? J. Membr. Sci. 175 (2000) 181–196.
346
T. Wu et al. / Journal of Membrane Science 471 (2014) 338–346
[3] M. Anderson, H. Wang, Y.S. Lin, Inorganic membranes for carbon dioxide and nitrogen separation, Rev. Chem. Eng. 28 (2012). [4] Y. Gu, P. Hacarlioglu, S.T. Oyama, Hydrothermally stable silica–alumina composite membranes for hydrogen separation, J. Membr. Sci. 310 (2008) 28–37. [5] M. Kanezashi, T. Shioda, T. Gunji, T. Tsuru, Gas permeation properties of silica membranes with uniform pore sizes derived from polyhedral oligomeric silsesquioxane, AIChE J. 58 (2012) 1733–1743. [6] D.Q. Vu, W.J. Koros, S.J. Miller, High pressure CO2/CH4 separation using carbon molecular sieve hollow fiber membranes, Ind. Eng. Chem. Res. 41 (2002) 367–380. [7] B.J. Hinds, N. Chopra, T. Rantell, R. Andrews, V. Gavalas, L.G. Bachas, Aligned multiwalled carbon nanotube membranes, Science 303 (2004) 62–65. [8] Y.H. Sim, H. Wang, F.Y. Li, M.L. Chua, T.S. Chung, M. Toriida, S. Tamai, High performance carbon molecular sieve membranes derived from hyperbranched polyimide precursors for improved gas separation applications, Carbon 53 (2013) 101–111. [9] J.R. Li, R.J. Kuppler, H.C. Zhou, Selective gas adsorption and separation in metal-organic frameworks, Chem. Soc. Rev. 38 (2009) 1477–1504. [10] Y. Pan, Z. Lai, Sharp separation of C2/C3 hydrocarbon mixtures by zeolitic imidazolate framework-8 (ZIF-8) membranes synthesized in aqueous solutions, Chem. Commun. 47 (2011) 10275–10277. [11] A. Huang, N. Wang, C. Kong, J. Caro, Organosilica-functionalized zeolitic imidazolate framework ZIF-90 membrane with high gas-separation performance, Angew. Chem. 51 (2012) 10551–10555. [12] M. Tsapatsis, Materials science. Toward high-throughput zeolite membranes, Science 334 (2011) 767–768. [13] T.C. Pham, H.S. Kim, K.B. Yoon, Growth of uniformly oriented silica MFI and BEA zeolite films on substrates, Science 334 (2011) 1533–1538. [14] M. Yu, R.D. Noble, J.L. Falconer, Zeolite membranes: microstructure characterization and permeation mechanisms, Acc. Chem. Res. 44 (2011) 1196–1206. [15] S. Keskin, D.S. Sholl, Assessment of a metal organic framework membrane for gas separations using atomically detailed calculations: CO2, CH4, N2, H2 mixtures in MOF-5, Ind. Eng. Chem. Res. 48 (2009) 914–922. [16] J.A. Bohrman, M.A. Carreon, Synthesis and CO2/CH4 separation performance of Bio-MOF-1 membranes, Chem. Commun. 48 (2012) 5130–5132. [17] S.R. Venna, M.A. Carreon, Highly permeable zeolite imidazolate framework-8 membranes for CO2/CH4 separation, J. Am. Chem. Soc. 132 (2010) 76–78. [18] Y. Cui, H. Kita, K.-i. Okamoto, Preparation and gas separation performance of zeolite T membrane, J. Mater. Chem. 14 (2004) 924. [19] S.M. Mirfendereski, T. Mazaheri, M. Sadrzadeh, T. Mohammadi, CO2 and CH4 permeation through T-type zeolite membranes: effect of synthesis parameters and feed pressure, Sep. Purif. Technol. 61 (2008) 317–323. [20] T. Tomita, K. Nakayama, H. Sakai, Gas separation characteristics of DDR type zeolite membrane, Microporous Mesoporous Mater. 68 (2004) 71–75. [21] M. Kanezashi, J. O’Brien-Abraham, Y.S. Lin, K. Suzuki, Gas permeation through DDRtype zeolite membranes at high temperatures, AIChE J. 54 (2008) 1478–1486. [22] J.C. Poshusta, R.D. Noble, J.L. Falconer, Temperature and pressure effects on CO2 and CH4 permeation through MFI zeolite membranes, J. Membr. Sci. 160 (1999) 115–125. [23] L. Sandström, E. Sjöberg, J. Hedlund, Very high flux MFI membrane for CO2 separation, J. Membr. Sci. 380 (2011) 232–240. [24] S. Fan, J. Liu, F. Zhang, S. Zhou, F. Sun, Fabrication of zeolite MFI membranes supported by α-Al2O3 hollow ceramic fifibers for CO2 separation, J. Mater. Res. 28 (2013) 1870–1876. [25] Y. Hasegawa, T. Tanaka, K. Watanabe, B.-H. Jeong, K. Kusakabe, S. Morooka, Separation of CO2-CH4 and CO2-N2 systems using ion-exchanged Fau-type zeolite membranes with different Si/Al ratios, Korean J. Chem. Eng. 19 (2002) 309–313.
[26] J.C. White, P.K. Dutta, K. Shqau, H. Verweij, Synthesis of ultrathin zeolite Y membranes and their application for separation of carbon dioxide and nitrogen gases, Langmuir 26 (2010) 10287–10293. [27] K. Kusakabe, T. Kuroda, A. Murata, S. Morooka, Formation of a Y-type zeolite membrane on a porous α-alumina tube for gas separation, Ind. Eng. Chem. Res. 36 (1997) 649–655. [28] S. Li, J.L. Falconer, R.D. Noble, Improved SAPO-34 membranes for CO2/CH4 separations, Adv. Mater. 18 (2006) 2601–2603. [29] M.A. Carreon, S. Li, J.L. Falconer, R.D. Noble, Alumina-supported SAPO-34 membranes for CO2/CH4 separation, J. Am. Chem. Soc. 130 (2008) 5412–5413. [30] Y. Zhang, B. Tokay, H.H. Funke, J.L. Falconer, R.D. Noble, Template removal from SAPO-34 crystals and membranes, J. Membr. Sci. 363 (2010) 29–35. [31] E.W. Ping, R. Zhou, H.H. Funke, J.L. Falconer, R.D. Noble, Seeded-gel synthesis of SAPO-34 single channel and monolith membranes, for CO2/CH4 separations, J. Membr. Sci. 415-416 (2012) 770–775. [32] R. Zhou, E.W. Ping, H.H. Funke, J.L. Falconer, R.D. Noble, Improving SAPO-34 membrane synthesis, J. Membr. Sci. 444 (2013) 384–393. [33] A. Huang, F. Liang, F. Steinbach, T.M. Gesing, J. Caro, Neutral and cation-free LTA-type aluminophosphate (AlPO4) molecular sieve membrane with high hydrogen permselectivity, J. Am. Chem. Soc. 132 (2010) 2140–2141. [34] I. Deroche, L. Gaberova, G. Maurin, P. Llewellyn, M. Castro, P. Wright, Adsorption of carbon dioxide in SAPO STA-7 and AlPO-18: grand canonical monte carlo simulations and microcalorimetry measurements, Adsorption 14 (2008) 207–213. [35] M.L. Carreon, S. Li, M.A. Carreon, AlPO-18 membranes for CO2/CH4 separation, Chem. Commun. 48 (2012) 2310–2312. [36] M. Vilaseca, S. Mintova, V. Valtchev, T.H. Metzger, T. Bein, Synthesis of colloidal AlPO4-18 crystals and their use for supported film growth, J. Mater. Chem. 13 (2003) 1526. [37] H. van Heyden, S. Mintova, T. Bein, AlPO-18 nanocrystals synthesized under microwave irradiation, J. Mater. Chem. 16 (2006) 514. [38] R. Wendelbo, D. Akporiaye, A. Andersen, I.M. Dahl, H.B. Mostad, Synthesis, characterization and catalytic testing of SAPO-18, MgAPO-18, and ZnAPO-18 in the MTO reaction, Appl. Catal. A: Gen. 142 (1996) L197–L207. [39] H.H. Funke, B. Tokay, R. Zhou, E.W. Ping, Y. Zhang, J.L. Falconer, R.D. Noble, Spatially resolved gas permeation through SAPO-34 membranes, J. Membr. Sci. 409-410 (2012) 212–221. [40] A.M. Avila, H.H. Funke, Y. Zhang, J.L. Falconer, R.D. Noble, Concentration polarization in SAPO-34 membranes at high pressures, J. Membr. Sci. 335 (2009) 32–36. [41] X. Chen, W. Yang, J. Liu, L. Lin, Synthesis of zeolite NaA membranes with high permeance under microwave radiation on mesoporous-layer-modified macroporous substrates for gas separation, J. Membr. Sci. 255 (2005) 201–211. [42] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Sep. Purif. Technol. 25 (2001) 251–260. [43] R. Krishna, S. Li, J.M. van Baten, J.L. Falconer, R.D. Noble, Investigation of slowing-down and speeding-up effects in binary mixture permeation across SAPO-34 and MFI membranes, Sep. Purif. Technol. 60 (2008) 230–236. [44] H. Kalipcilar, T.C. Bowen, R.D. Noble, J.L. Falconer, Synthesis and separation performance of SSZ-13 zeolite membranes on tubular supports, Chem. Mater. 14 (2002) 3458–3464. [45] J.C. Poshusta, V.A. Tuan, E.A. Pape, R.D. Noble, J.L. Falconer, Separation of light gas mixtures using SAPO-34 membranes, AIChE J. 46 (2000) 779–789. [46] K. Keizer, A.J. Burggraaf, Z.A.E.P. Vroon, H. Verweij, Two component permeation through thin zeolite MFI membranes, J. Membr. Sci. 147 (1998) 159–172.