Gas Permeation Properties of High-Silica CHA-Type Zeolite Membrane

The polycrystalline CHA-type zeolite layer with Si/Al = 18 was formed on the porous α-Al2O3 tube in this study, and the gas permeation properties were determined using single-component H2, CO2, N2, CH4, n-C4H10, and SF6 at 303–473 K. The membrane showed permeation behavior, wherein the permeance reduced with the molecular size, attributed to the effect of molecular sieving. The separation performances were also determined using the equimolar mixtures of N2–SF6, CO2–N2, and CO2–CH4. As a result, the N2/SF6 and CO2/CH4 selectivities were as high as 710 and 240, respectively. However, the CO2/N2 selectivity was only 25. These results propose that the high-silica CHA-type zeolite membrane is suitable for the separation of CO2 from CH4 by the effect of molecular sieving.


Introduction
Zeolites are microporous aluminosilicate compounds, and they have been attracted much attention as the potential material for membranes. Zeolite membranes have been studied since the 1990s, and the MFI-type zeolite membranes were formed on the substrates by deposition and intergrowth of crystallites, which were nucleated in synthesis mixtures [1][2][3][4][5][6][7][8][9][10]. Geus et al. [1] prepared MFI-type zeolite membranes on several kinds of substrates and determined gas permeation properties through the membranes. Sano et al. [2] investigated MFI-type zeolite membranes by a hydrothermal process on porous stainlesssteel and alumina substrates and applied them to the separation of water/alcohol mixtures. The permeation and separation properties were also studied by several groups [11][12][13][14][15][16][17]. Moulijn and coworkers [11][12][13][14] estimated the permeation properties through the MFI-type zeolite membranes using a Maxwell-Stefan formulation containing the gas adsorption on zeolites and the diffusion inside the membrane. Morooka and coworkers [15,16] proposed an adsorption-diffusion model to discuss the effect of CO 2 adsorption on zeolite and diffusion in the membrane for CO 2 separation using Y-type zeolite membranes. Some kinds of zeolite membranes are available for the dehydration of many kinds of organic solvents in industries [18][19][20][21][22].
These mechanism studies have established that DDR-, CHA-, and AEI-type zeolite has favorable characteristics for the membrane material, such as small micropore diameter, large micropore volume, and the composition variability [23]. These features correspond to the permeation and separation performances and the acid and thermal stabilities of the membrane [24]. Tomita et al. developed the DDR-type zeolite membrane by the secondary growth of seeded crystals [20], and the several gas permeation properties were determined [25]. In particular, the CO 2 /CH 4 selectivity was as high as 2000 below 250 K. However, the selectivity decreased to 220 around 300 K. Noble and coworkers investigated CHA-type silica-aluminophosphate zeolite (SAPO-34) membranes [26][27][28][29][30], and the separation performances were determined for CO 2 -CH 4 , CO 2 -N 2 , Kr-Xe, and N 2 -CH 4 mixtures. The CO 2 /CH 4 selectivity was 171 at 295 K [28]. Some high-silica A high-silica CHA-type zeolite membrane was prepared on the outer surface of a porous α-Al 2 O 3 support tube by the combination of the secondary growth of seed particles and the structure conversion of FAU-type zeolite [36][37][38]. The seed particles were prepared by mixing sodium hydroxide (FUJIFILM Wako, Tokyo, Japan), sodium aluminate (FUJIFILM Wako, Tokyo, Japan), a N,N,N-trimethyl-1-adamantammonium hydroxide solution (SDA, 25%, Sachem Asia, Osaka, Japan), and ultra-stable Y-type zeolite particles (HSZ-390HUA, Tosoh, Tokyo, Japan). The molar composition of the solution was 40 SiO 2 :1 Al 2 O 3 :4 Na 2 O:8 SDA:800 H 2 O. The mixture was poured into a Teflon-lined stainless-steel autoclave, and a hydrothermal reaction was carried out at 433 K for 4 days. Solids were recovered by filtration, washed with distilled water, and dried overnight at 383 K to obtain seed particles. For the secondary growth, a synthesis solution was prepared by the same procedures as that for the seed particles, and the mixture was stirred at room temperature for 4 h. The molar composition of the mixture was 45 SiO 2 :1 Al 2 O 3 :4.5 Na 2 O:3.4 SDA:4500 H 2 O. The α-Al 2 O 3 tube was used as the support, and the properties were as follows: outer diameter = 2.0 mm; inside diameter = 1.5 mm; mean pore diameter = 0.3 µm; and porosity = 45%. The outer surface of the support tube was rubbed with the seed particles to implant seeds for nucleation, and the tube was added to the autoclave filled with 30 g of the synthesis solution. The autoclave was placed horizontally in an oven at 433 K for 20 h to form the polycrystalline high-silica CHA-type zeolite layer. After the autoclave was cooled to room temperature, the support tube was recovered, washed with distilled water, and dried at room temperature overnight. Finally, the tube was calcined in air at 773 K for 10 h to remove the SDA to obtain the high-silica CHA-type zeolite membrane.
The morphology was observed using a scanning electron microscope (SEM, TM-1000, Hitachi High-Technologies, Tokyo, Japan), and the composition was analyzed by an energy-dispersive X-ray (EDX) analyzer attached with the SEM. The crystal structure of the membrane was identified by X-ray diffraction (XRD, Smart-Lab, Rigaku, Tokyo, Japan).

Gas Permeation Test
Both the ends of the support tube were connected to stainless-steel tubes with silicon resin (TSE3976-B, Momentive, Tokyo, Japan), and the outer surfaces of resin were wrapped with thermally shrinking tetrafluoroethylene and hexafluoropropylene copolymer (FEP) tubes (FEP-040, Junkosha, Osaka, Japan). The effective membrane area for permeation was 1.2 cm 2 . The membrane was fixed to a permeation cell, as shown in Figure 1, and the cell was placed in an electric furnace [39]. Single-component H 2 , CO 2 , N 2 , CH 4 , n-C 4 H 10 , and SF 6 , as well as binary mixtures of N 2 -SF 6 , CO 2 -N 2 , and CO 2 -CH 4 , were fed onto the outer surface of the membrane (feed side) at 100 mL min −1 , and either argon (for H 2 ) or helium (for the others) was introduced into the inside of the membrane (permeate side) at 10-50 mL min −1 as the sweep gas. The total pressures of the feed and permeate sides were kept at 300 and 101 kPa, respectively. In this study, the membrane was treated under N 2 flow at 473 K for 30 min to remove adsorbed water, and the test gas was fed onto the feed side. The pretreatment was carried out before each measurement. The gas composition was analyzed using a gas chromatograph with a thermal conductivity detector (Shimadzu GC-8A), and the gas flow rate was determined by a soap-film flowmeter. The permeance for component i, Q i , was calculated using the following equation: where N p is the molar flow rate of the outlet from the permeate side; S, the effective membrane area for permeation; y i , the mole fraction of component i in the outlet gas of the permeate side; P fi , the partial pressure of component i on the feed side; and P pi , the partial pressure of component i on the permeate side. The selectivity was defined as the ratio of the permeances in this study.

Gas Permeation Test
Both the ends of the support tube were connected to stainless-steel tubes with silicon resin (TSE3976-B, Momentive, Tokyo, Japan), and the outer surfaces of resin were wrapped with thermally shrinking tetrafluoroethylene and hexafluoropropylene copolymer (FEP) tubes (FEP-040, Junkosha, Osaka, Japan). The effective membrane area for permeation was 1.2 cm 2 . The membrane was fixed to a permeation cell, as shown in Figure 1, and the cell was placed in an electric furnace [39]. Single-component H2, CO2, N2, CH4, n-C4H10, and SF6, as well as binary mixtures of N2-SF6, CO2-N2, and CO2-CH4, were fed onto the outer surface of the membrane (feed side) at 100 mL min −1 , and either argon (for H2) or helium (for the others) was introduced into the inside of the membrane (permeate side) at 10-50 mL min -1 as the sweep gas. The total pressures of the feed and permeate sides were kept at 300 and 101 kPa, respectively. In this study, the membrane was treated under N2 flow at 473 K for 30 min to remove adsorbed water, and the test gas was fed onto the feed side. The pretreatment was carried out before each measurement. The gas composition was analyzed using a gas chromatograph with a thermal conductivity detector (Shimadzu GC-8A), and the gas flow rate was determined by a soap-film flowmeter. The permeance for component i, Qi, was calculated using the following equation: where Np is the molar flow rate of the outlet from the permeate side; S, the effective membrane area for permeation; yi, the mole fraction of component i in the outlet gas of the permeate side; Pfi, the partial pressure of component i on the feed side; and Ppi, the partial pressure of component i on the permeate side. The selectivity was defined as the ratio of the permeances in this study.  Figure 2 shows the SEM images of the CHA-type zeolite membrane. The outer surface of the porous support tube was covered with a polycrystalline layer (thickness ≈ 3 μm, Si/Al = 18). The XRD pattern of the membrane contained both the peaks of the support tube and seed particles, as shown in Figure 3. These are identical to those reported previously [36][37][38]. This suggests that the polycrystalline CHA-type zeolite layer could be formed on the porous α-Al2O3 tube with high reproducibility.  Figure 2 shows the SEM images of the CHA-type zeolite membrane. The outer surface of the porous support tube was covered with a polycrystalline layer (thickness ≈ 3 µm, Si/Al = 18). The XRD pattern of the membrane contained both the peaks of the support tube and seed particles, as shown in Figure 3. These are identical to those reported previously [36][37][38]. This suggests that the polycrystalline CHA-type zeolite layer could be formed on the porous α-Al 2 O 3 tube with high reproducibility.     Figure 4 shows the influence of the kinetic diameter on the single-component gas permeance at 303 and 473 K. The permeance of CO2 was 5.1 × 10 -7 mol m -2 s -1 Pa -1 at 303 K. The permeance, except for H2, decreased, with increase in the diameter, and that reached 4.1 × 10 −11 mol m −2 s −1 Pa −1 for SF6. The diameter of the crystallographic channel aperture of the CHA-type zeolite is 0.38 nm [23], and the molecular diameters of H2, CO2, N2, CH4, n-C4H10, and SF6 are 0.289, 0.33, 0.364, 0.38, 0.43, and 0.55 nm, respectively [40]. Since H2, CO2, and N2 molecules are smaller than the channel diameter, those molecules can penetrate into and diffuse within the zeolite channels. Although the molecular diameter of SF6 is clearly larger than the channel sizes, SF6 was detected on the permeate side of the membrane. The marginal permeance of SF6 proposes that the membrane had intercrystalline boundaries. The unit cell of the high-silica CHA-type zeolite was shrunk by the air calcination, and the volume shrinkage degree was 0.6 vol % [37]. The small intercrystalline boundaries were produced by the unit cell shrinkage by the air calcination.  Figure 4 shows the influence of the kinetic diameter on the single-component gas permeance at 303 and 473 K. The permeance of CO 2 was 5.1 × 10 −7 mol m −2 s −1 Pa −1 at 303 K. The permeance, except for H 2 , decreased, with increase in the diameter, and that reached 4.1 × 10 −11 mol m −2 s −1 Pa −1 for SF 6 . The diameter of the crystallographic channel aperture of the CHA-type zeolite is 0.38 nm [23], and the molecular diameters of H 2 , CO 2 , N 2 , CH 4 , n-C 4 H 10 , and SF 6 are 0.289, 0.33, 0.364, 0.38, 0.43, and 0.55 nm, respectively [40]. Since H 2 , CO 2 , and N 2 molecules are smaller than the channel diameter, those molecules can penetrate into and diffuse within the zeolite channels. Although the molecular diameter of SF 6 is clearly larger than the channel sizes, SF 6 was detected on the permeate side of the membrane. The marginal permeance of SF 6 proposes that the membrane had intercrystalline boundaries. The unit cell of the high-silica CHA-type zeolite was shrunk by the air calcination, and the volume shrinkage degree was 0.6 vol % [37]. The small intercrystalline boundaries were produced by the unit cell shrinkage by the air calcination.   Figure 4 shows the influence of the kinetic diameter on the single-component gas permeance at 303 and 473 K. The permeance of CO2 was 5.1 × 10 -7 mol m -2 s -1 Pa -1 at 303 K. The permeance, except for H2, decreased, with increase in the diameter, and that reached 4.1 × 10 −11 mol m −2 s −1 Pa −1 for SF6. The diameter of the crystallographic channel aperture of the CHA-type zeolite is 0.38 nm [23], and the molecular diameters of H2, CO2, N2, CH4, n-C4H10, and SF6 are 0.289, 0.33, 0.364, 0.38, 0.43, and 0.55 nm, respectively [40]. Since H2, CO2, and N2 molecules are smaller than the channel diameter, those molecules can penetrate into and diffuse within the zeolite channels. Although the molecular diameter of SF6 is clearly larger than the channel sizes, SF6 was detected on the permeate side of the membrane. The marginal permeance of SF6 proposes that the membrane had intercrystalline boundaries. The unit cell of the high-silica CHA-type zeolite was shrunk by the air calcination, and the volume shrinkage degree was 0.6 vol % [37]. The small intercrystalline boundaries were produced by the unit cell shrinkage by the air calcination. Moreover, the permeance of N 2 was 1.8 × 10 −8 mol m −2 s −1 Pa −1 at 473 K. After nine times heating and cooling treatment for determination of the permeation properties of single-component gases and binary mixtures, the permeance was 1.7 × 10 −8 mol m −2 s −1 Pa −1 . The identical permeances of N 2 indicates that the high-silica CHA-type zeolite membrane was stable for the thermal treatment. Figure 5 shows the effect of temperature on the permeances of the singlecomponent H 2 , CO 2 , N 2 , CH 4 , n-C 4 H 10 , and SF 6 at 303-473 K. The permeance of CO 2 was 5.1 × 10 −7 mol m −2 s −1 Pa −1 at 303 K and decreased with temperature. As a result, it was 1.3 × 10 −7 mol m −2 s −1 Pa −1 at 473 K. The permeances of H 2 and N 2 showed similar dependencies. Since these molecules are smaller than the channel diameter of the CHAtype zeolite, these molecules can adsorb on the zeolite channels. The adsorption amount decreased with temperature. Therefore, the permeances of H 2 , CO 2 , and N 2 were decreased with temperature by the reduction of the concentration difference between both the sides of the membrane. and 473 K.

Single-Component Gas Permeation
Moreover, the permeance of N2 was 1.8 × 10 −8 mol m −2 s −1 Pa −1 at 473 K. After nine times heating and cooling treatment for determination of the permeation properties of single-component gases and binary mixtures, the permeance was 1.7 × 10 −8 mol m −2 s −1 Pa −1 . The identical permeances of N2 indicates that the high-silica CHA-type zeolite membrane was stable for the thermal treatment. Figure 5 shows the effect of temperature on the permeances of the single-component H2, CO2, N2, CH4, n-C4H10, and SF6 at 303-473 K. The permeance of CO2 was 5.1 × 10 −7 mol m −2 s −1 Pa −1 at 303 K and decreased with temperature. As a result, it was 1.3 × 10 −7 mol m −2 s −1 Pa −1 at 473 K. The permeances of H2 and N2 showed similar dependencies. Since these molecules are smaller than the channel diameter of the CHA-type zeolite, these molecules can adsorb on the zeolite channels. The adsorption amount decreased with temperature. Therefore, the permeances of H2, CO2, and N2 were decreased with temperature by the reduction of the concentration difference between both the sides of the membrane. In contrast, for CH4, n-C4H10, and SF6, the diameters of which are identical or larger than the zeolite channels, the permeances increased with temperature. The effect of temperatures is described using the Arrhenius equation as follows: where Qi * and Ep are the pre-exponential factor and activation energy for permeation, respectively. The pre-exponential factors and activation energies of single-component gas permeation are listed in Table 1. The activation energies for n-C4H10 and SF6 were higher than those of the other gases. It is well known that the difference in the diffusivities is important for the permeation through membranes [11][12][13][14][15][16][17]. The higher activation energies of n-C4H10 and SF6 suggest that it is difficult for these molecules to permeate through the intercrystalline boundaries. Therefore, the high-silica CHA-type zeolite membrane had small and minimal intercrystalline boundaries.  In contrast, for CH 4 , n-C 4 H 10 , and SF 6 , the diameters of which are identical or larger than the zeolite channels, the permeances increased with temperature. The effect of temperatures is described using the Arrhenius equation as follows: where Q i * and E p are the pre-exponential factor and activation energy for permeation, respectively. The pre-exponential factors and activation energies of single-component gas permeation are listed in Table 1. The activation energies for n-C 4 H 10 and SF 6 were higher than those of the other gases. It is well known that the difference in the diffusivities is important for the permeation through membranes [11][12][13][14][15][16][17]. The higher activation energies of n-C 4 H 10 and SF 6 suggest that it is difficult for these molecules to permeate through the intercrystalline boundaries. Therefore, the high-silica CHA-type zeolite membrane had small and minimal intercrystalline boundaries. As shown in Table 1, the order of the activation energies was CO 2 < N 2 < CH 4 < SF 6 . This suggests that the CO 2 /N 2 , CO 2 /CH 4 , and N 2 /SF 6 selectivities are higher at lower temperatures. The separation performances for these mixtures are examined in the next subsection. Figure 6 shows the permeances of N 2 and SF 6 for the equimolar mixture of them at 303-473 K. As same as for the single gas, the permeance of N 2 for the binary mixture decreased with temperature, while that of SF 6 showed the reverse dependency. Therefore, the N 2 /SF 6 selectivity was the highest (=710) at 303 K. The permeance of SF 6 for the mixture was the same as that for single gas, and the permeance of N 2 for the mixture was also identical at temperatures higher than 363 K. However, below 363 K, the permeances of N 2 for the mixture was lower than that for the single gas. As a result, the N 2 /SF 6 selectivity was 440 for the mixture. The lower N 2 permeances for the mixture was attributed to the weaker interaction of N 2 with the zeolite than SF 6 . The interaction potential of individual molecules can be described using the Lennard-Jones 12-6 equation. The depths of interaction potential are 0.59 kJ mol −1 and 1.85 kJ mol −1 for N 2 and SF 6 , respectively [41]. The deeper potential of SF 6 proposes that SF 6 molecules interact with zeolites more strongly than N 2 . In addition, SF 6 molecules permeated through the boundaries, as discussed above. Therefore, N 2 molecules, permeated through the boundaries, were inhibited by the SF 6 molecules, and the permeance of N 2 became lower compared to the single gas.

Binary Mixture Gas Permeation
n-C4H10 0.43 7.5 × 10 11.3 SF6 0.55 7.3 × 10 −9 13.2 As shown in Table 1, the order of the activation energies was CO2 < N2 < CH4 < SF6. This suggests that the CO2/N2, CO2/CH4, and N2/SF6 selectivities are higher at lower temperatures. The separation performances for these mixtures are examined in the next subsection. Figure 6 shows the permeances of N2 and SF6 for the equimolar mixture of them at 303-473 K. As same as for the single gas, the permeance of N2 for the binary mixture decreased with temperature, while that of SF6 showed the reverse dependency. Therefore, the N2/SF6 selectivity was the highest (=710) at 303 K. The permeance of SF6 for the mixture was the same as that for single gas, and the permeance of N2 for the mixture was also identical at temperatures higher than 363 K. However, below 363 K, the permeances of N2 for the mixture was lower than that for the single gas. As a result, the N2/SF6 selectivity was 440 for the mixture. The lower N2 permeances for the mixture was attributed to the weaker interaction of N2 with the zeolite than SF6. The interaction potential of individual molecules can be described using the Lennard-Jones 12-6 equation. The depths of interaction potential are 0.59 kJ mol −1 and 1.85 kJ mol −1 for N2 and SF6, respectively [41]. The deeper potential of SF6 proposes that SF6 molecules interact with zeolites more strongly than N2. In addition, SF6 molecules permeated through the boundaries, as discussed above. Therefore, N2 molecules, permeated through the boundaries, were inhibited by the SF6 molecules, and the permeance of N2 became lower compared to the single gas.  Figure 7 shows the permeation properties of CO2 and N2 for the equimolar mixture of them at 303-473 K. Although the permeance of N2 became the maximum at 343 K for the mixture, that of CO2 decreased with temperature. The CO2/N2 selectivity for the mixture was the highest at 303 K (=25) and decreased with temperature. As a result, the selectivity became only 9 at 473 K.

Binary Mixture Gas Permeation
Compared to the single gas, the temperature dependency of CO2 for the mixture was similar, although the permeance for the mixture was slightly higher at all temperatures. The permeance of N2 was almost the same that for the single gas at higher temperatures than 373 K, while it was lower below 363 K. These are typical permeation properties when molecules are transferred and separated by the preferential adsorption of CO2. Similar properties were also observed for the CO2-N2 separation using FAU-type zeolite membranes [15,16]. Morooka and coworkers explained the permeation properties by the preferential CO2 adsorption and the overtaking N2 by CO2 [15].  Figure 7 shows the permeation properties of CO 2 and N 2 for the equimolar mixture of them at 303-473 K. Although the permeance of N 2 became the maximum at 343 K for the mixture, that of CO 2 decreased with temperature. The CO 2 /N 2 selectivity for the mixture was the highest at 303 K (=25) and decreased with temperature. As a result, the selectivity became only 9 at 473 K.  Figure 8 shows the effect of temperatures on the permeation properties of CO2 and CH4 for the equimolar mixture of them. The permeances of CO2 and CH4 for the mixture were 5.3 × 10 −7 and 2.2 × 10 −9 mol m −2 s −1 Pa −1 at 303 K, respectively. The CO2/CH4 selectivity was 240 for the mixture. The permeance of CO2 decreased with temperature, although that of CH4 showed the reverse dependency. As a result, the CO2/CH4 selectivity reduced to 43 at 473 K. Comparing to the single gases, the permeances of CO2 and CH4 for the Compared to the single gas, the temperature dependency of CO 2 for the mixture was similar, although the permeance for the mixture was slightly higher at all temperatures. The permeance of N 2 was almost the same that for the single gas at higher temperatures than 373 K, while it was lower below 363 K. These are typical permeation properties when molecules are transferred and separated by the preferential adsorption of CO 2 . Similar properties were also observed for the CO 2 -N 2 separation using FAU-type zeolite membranes [15,16]. Morooka and coworkers explained the permeation properties by the preferential CO 2 adsorption and the overtaking N 2 by CO 2 [15]. Figure 8 shows the effect of temperatures on the permeation properties of CO 2 and CH 4 for the equimolar mixture of them. The permeances of CO 2 and CH 4 for the mixture were 5.3 × 10 −7 and 2.2 × 10 −9 mol m −2 s −1 Pa −1 at 303 K, respectively. The CO 2 /CH 4 selectivity was 240 for the mixture. The permeance of CO 2 decreased with temperature, although that of CH 4 showed the reverse dependency. As a result, the CO 2 /CH 4 selectivity reduced to 43 at 473 K. Comparing to the single gases, the permeances of CO 2 and CH 4 for the mixture were almost identical. The permeation properties cannot be explained by only the preferential CO 2 adsorption, as discussed in Figure 7. Since the molecular size of CH 4 is similar to the channel diameter of the CHA-type zeolite, the diffusion of CH 4 within the zeolite channel was slower than N 2 , as show in Figure 4. It is known that the i-C 4 H 10 molecules hinder the diffusion of n-C 4 H 10 for the separation of butane isomers using MFI-type zeolite membranes [39]. The CH 4 molecules within the zeolite channels may hinder the diffusion of CO 2 molecules.  Figure 8 shows the effect of temperatures on the permeation properties of CO2 and CH4 for the equimolar mixture of them. The permeances of CO2 and CH4 for the mixture were 5.3 × 10 −7 and 2.2 × 10 −9 mol m −2 s −1 Pa −1 at 303 K, respectively. The CO2/CH4 selectivity was 240 for the mixture. The permeance of CO2 decreased with temperature, although that of CH4 showed the reverse dependency. As a result, the CO2/CH4 selectivity reduced to 43 at 473 K. Comparing to the single gases, the permeances of CO2 and CH4 for the mixture were almost identical. The permeation properties cannot be explained by only the preferential CO2 adsorption, as discussed in Figure 7. Since the molecular size of CH4 is similar to the channel diameter of the CHA-type zeolite, the diffusion of CH4 within the zeolite channel was slower than N2, as show in Figure 4. It is known that the i-C4H10 molecules hinder the diffusion of n-C4H10 for the separation of butane isomers using MFI-type zeolite membranes [39]. The CH4 molecules within the zeolite channels may hinder the diffusion of CO2 molecules. The influence of the CO2 concentration in the feed mixture was determined to check the interaction between CO2 and CH4 molecules during the permeation. Figure 9 shows the influences of the CO2 concentration in the feed mixtures on the permeation properties of CO2 and CH4 for binary mixtures of them at 303 K. If CH4 inhibits the permeation of CO2, the permeance of CO2 would be reduced at the dilute CO2 concentration. However, the permeances of CO2 and CH4 were independent of the CO2 concentration in the feed mixture, and the selectivity was almost constant at 200-260. The constant permeances of CO2 and CH4 propose that the CO2 and CH4 molecules did not interact each other during the membrane permeation. Therefore, it is considered that CO2 and CH4 molecules diffuse in the different passes, such as zeolite micropores and inter-crystalline boundaries. The influence of the CO 2 concentration in the feed mixture was determined to check the interaction between CO 2 and CH 4 molecules during the permeation. Figure 9 shows the influences of the CO 2 concentration in the feed mixtures on the permeation properties of CO 2 and CH 4 for binary mixtures of them at 303 K. If CH 4 inhibits the permeation of CO 2 , the permeance of CO 2 would be reduced at the dilute CO 2 concentration. However, the permeances of CO 2 and CH 4 were independent of the CO 2 concentration in the feed mixture, and the selectivity was almost constant at 200-260. The constant permeances of CO 2 and CH 4 propose that the CO 2 and CH 4 molecules did not interact each other during the membrane permeation. Therefore, it is considered that CO 2 and CH 4 molecules diffuse in the different passes, such as zeolite micropores and inter-crystalline boundaries. Figure 10 compares the CO 2 separation performance to previous reports [25,27,28,32,34,42,43]. The CO 2 permeance and selectivity of our membrane were relatively high for CO 2 -CH 4 mixtures, and the performances could be plotted on the trade-off line of SAPO-34, CHA-, and AEI-type zeolite membranes [27,28,[32][33][34]. It is considered that the similar CO 2 separation performances of the membranes are attributed to the similar crystal structures [44]. Although the DDR-type zeolite membrane showed extremely high CO 2 selectivity below 300 K, the selectivity at temperature higher than 300 K was almost the same as those of CHA-and AEI-type zeolite membranes [25]. The CO 2 selectivities of the FAU-type zeolite membranes were lower compared to those membranes [42]. On the contrary, the FAU-type zeolite membranes showed higher CO 2 permeance and selectivity  [42,43]. The selectivity of our membrane was an order of magnitude lower than those of the FAU-type zeolite membranes and comparable to those of the DDR-type zeolite membrane around 300 K. The Si/Al ratio of the DDR-and CHA-type zeolite membranes were more than 15, and the amount of the counter-cation was much less. In contrast, the FAU-type zeolite membrane contained many cations because of low Si/Al ratio (Si/Al < 2). These propose that the effect of molecular sieving such as the DDR-, CHA-, and AEI-type zeolite membranes is effective for the CO 2 separation from CH 4 , while the selective adsorption of CO 2 is necessary for CO 2 -N 2 mixtures.
Membranes 2021, 11, x FOR PEER REVIEW 8 of 10 Figure 9. Influence of the feed gas composition on the gas permeation properties for the binary mixtures of CO2 and CH4 303 K. Figure 10 compares the CO2 separation performance to previous reports [25,27,28,32,34,42,43]. The CO2 permeance and selectivity of our membrane were relatively high for CO2-CH4 mixtures, and the performances could be plotted on the trade-off line of SAPO-34, CHA-, and AEI-type zeolite membranes [27,28,[32][33][34]. It is considered that the similar CO2 separation performances of the membranes are attributed to the similar crystal structures [44]. Although the DDR-type zeolite membrane showed extremely high CO2 selectivity below 300 K, the selectivity at temperature higher than 300 K was almost the same as those of CHA-and AEI-type zeolite membranes [25]. The CO2 selectivities of the FAU-type zeolite membranes were lower compared to those membranes [42]. On the contrary, the FAU-type zeolite membranes showed higher CO2 permeance and selectivity for CO2-N2 mixtures [42,43]. The selectivity of our membrane was an order of magnitude lower than those of the FAU-type zeolite membranes and comparable to those of the DDR-type zeolite membrane around 300 K. The Si/Al ratio of the DDR-and CHA-type zeolite membranes were more than 15, and the amount of the counter-cation was much less. In contrast, the FAU-type zeolite membrane contained many cations because of low Si/Al ratio (Si/Al < 2). These propose that the effect of molecular sieving such as the DDR-, CHA-, and AEI-type zeolite membranes is effective for the CO2 separation from CH4, while the selective adsorption of CO2 is necessary for CO2-N2 mixtures. Figure 9. Influence of the feed gas composition on the gas permeation properties for the binary mixtures of CO 2 and CH 4 303 K. Figure 9. Influence of the feed gas composition on the gas permeation properties for the binary mixtures of CO2 and CH4 303 K. Figure 10 compares the CO2 separation performance to previous reports [25,27,28,32,34,42,43]. The CO2 permeance and selectivity of our membrane were relatively high for CO2-CH4 mixtures, and the performances could be plotted on the trade-off line of SAPO-34, CHA-, and AEI-type zeolite membranes [27,28,[32][33][34]. It is considered that the similar CO2 separation performances of the membranes are attributed to the similar crystal structures [44]. Although the DDR-type zeolite membrane showed extremely high CO2 selectivity below 300 K, the selectivity at temperature higher than 300 K was almost the same as those of CHA-and AEI-type zeolite membranes [25]. The CO2 selectivities of the FAU-type zeolite membranes were lower compared to those membranes [42]. On the contrary, the FAU-type zeolite membranes showed higher CO2 permeance and selectivity for CO2-N2 mixtures [42,43]. The selectivity of our membrane was an order of magnitude lower than those of the FAU-type zeolite membranes and comparable to those of the DDR-type zeolite membrane around 300 K. The Si/Al ratio of the DDR-and CHA-type zeolite membranes were more than 15, and the amount of the counter-cation was much less. In contrast, the FAU-type zeolite membrane contained many cations because of low Si/Al ratio (Si/Al < 2). These propose that the effect of molecular sieving such as the DDR-, CHA-, and AEI-type zeolite membranes is effective for the CO2 separation from CH4, while the selective adsorption of CO2 is necessary for CO2-N2 mixtures. Figure 10. Comparison of the CO2 separation performance to previous reports [25,27,28,32,34,42,43]. Figure 10. Comparison of the CO 2 separation performance to previous reports [25,27,28,32,34,42,43].

Conclusions
The polycrystalline CHA-type zeolite layer with Si/Al = 18 was formed on the porous α-Al 2 O 3 tube in this study, and the gas permeation properties were determined using single-component H 2 , CO 2 , N 2 , CH 4 , n-C 4 H 10 , and SF 6 at 303-473 K. The permeance was 5.1 × 10 −7 mol m −2 s −1 Pa −1 for CO 2 at 303 K and the permeance reduced with increasing the molecular size. Moreover, the permeances of H 2 , CO 2 , and N 2 decreased with temperature, while those of CH 4 , n-C 4 H 10 , and SF 6 showed reverse trends. The gas separation tests were also carried out using binary mixtures of N 2 -SF 6 , CO 2 -N 2 , and CO 2 -CH 4 . The membrane showed the high separation performance for the mixtures, and the N 2 /SF 6 , CO 2 /N 2 , and CO 2 /CH 4 selectivities were 710, 25, and 240, respectively.