J. Phys. Chem. C 2009, 113, 3767–3774
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Gas Permeation and Diffusion Characteristics of MFI-Type Zeolite Membranes at High Temperatures M. Kanezashi†,‡ and Y. S. Lin*,† Department of Chemical Engineering, Arizona State UniVersity, Tempe, Arizona 85287, and Department of Chemical Engineering, Hiroshima UniVersity, Higashi-Hiroshima 739-8527, Japan ReceiVed: May 23, 2008; ReVised Manuscript ReceiVed: December 11, 2008
MFI-type zeolite membranes were prepared by the template-free secondary growth method followed by onstream counter-diffusion or one-side chemical vapor deposition (CVD) modification to eliminate intercrystalline pores. Gas permeation and separation experiments were conducted on unmodified and modified membranes at 25-500 °C. For unmodified MFI-type zeolite membranes, single-gas permeation of H2, He, CO, and CO2 exhibits characteristics of Knudsen diffusion up to 500 °C, and adsorption of CO2 on MFI-type zeolite has a strong effect on ternary gas separation (H2, CO, and CO2) below 300 °C. Counter-diffusion CVD modification is effective in sealing the intercrystalline gaps resulting in defect-free MFI-type zeolite membranes. Permeation of nonadsorbing gases (He, H2, and CO) through counter-diffusion CVD-modified zeolite membranes also exhibits Knudsen diffusion characteristics with very small activation energies for diffusion (0.1-3 kJ mol-1), with gas permeance (diffusivity) decreasing with increasing molecular weight. For one-side CVD-modified MFI-type zeolite membranes, gas permeance (diffusivity) decreases and activation energy for diffusion increases with increasing molecular size because of the formation of an amorphous microporous silica layer. High-temperature gas permeation data on defect-free MFI-type zeolite membranes confirm the translational gas diffusion model for zeolites. 1. Introduction Zeolites are crystalline aluminosilicate materials having micropores (zeolitic pores) in their structures built by various connections of a TO4 (T ) Si or Al) tetrahedral. Small or intermediate pore zeolites such as A-type, DDR-type, or MFItype zeolites contain pores defined by 8- and 10-membered oxygen rings. Small gas permeation and diffusion data evaluated for these zeolites in a wide temperature range will provide an improved understanding of gas diffusion and permeation properties of these materials. This is also important for applications of these membranes in gas separation and reaction processes. For diffusion in zeolites, Xiao and Wei1 developed a theory that can predict the diffusivity of a single species in zeolite micropores from temperature, relative size of the gas molecule to the zeolite pore, and gas molecular loading in microporous material. For diffusion of small gases with weak adsorption affinity with zeolites or at high temperatures, molecules in zeolite pores retain their gas characteristics, though their movement is restricted and has to overcome the energy barrier imposed by the zeolite pore structure. Under these conditions, they predicted that for a given zeolite the activation energy depends strongly on the ratio of the kinetic diameter of the diffusing gas molecule to the zeolite pore diameter, λ ) dm/dp when λ > 0.5, and weakly on the ratio of the kinetic diameter to the Lennard-Jones length constant for molecules (dm/σm).1 This theory provides the basis for examining the effects of the size or molecular weight on gas permeation through zeolite membranes. A few research groups have reported small gas permeation characteristics for A-type zeolite membranes.2,3 Gas permeation * Corresponding author. E-mail:
[email protected]. Telephone: 1-480965-7769. Fax: 1-480-965-0037. † Arizona State University. ‡ Hiroshima University.
behavior for a NaA zeolite (dp ∼ 0.41 nm) membrane is dominated by the molecular sieving mechanism, despite the presence of defects larger than the structural pores.2 However, no work on its gas permeation characteristics at high temperatures (>300 °C) has been reported due to the thermal stability of the A-type zeolite structure. Compared to A-type zeolite membranes, the highly siliceous DDR (deca-dodecasil 3R)-type zeolites can give reproducible gas permeation and diffusion data at high temperatures because of their highly thermally stable structure. A DDR-type zeolite structure is formed from a polyhedron with an oxygen 8-membered ring, and a pore diameter of this ring is 0.36 × 0.44 nm.4 In 2004, Tomita et al.5 successfully synthesized DDR-type zeolite membranes on porous alumina supports by the secondary growth method. Recently, Kanezashi et al.6 examined the gas permeation and diffusion behavior for DDR-type zeolite membranes in a wide temperature range (25-500 °C). Intracrystalline diffusivities of small gases (He, H2, CO2, and CO) in DDRtype zeolites were obtained from the permeation data under the conditions of negligible adsorption (at temperatures above 300 °C) to examine the effects of the size and molecular weight of permeating gases on the diffusion and permeation rate in DDRtype zeolite membranes. For high-quality DDR-type zeolite membranes, diffusivity (and gas permeance) is determined by the size of the permeating gases, and the activation energy for diffusion increases with increasing size of the permeating gases (He, 6.66 kJ mol-1 < H2, 9.62 kJ mol-1 < CO2, 12.8 kJ mol-1 < CO, 15.5 kJ mol-1).6 High-temperature diffusion data for the small gases in the DDR-type zeolites measured by the macroscopic membrane permeation method are consistent with the theory of translational gas diffusion in zeolites proposed by Xiao and Wei.1 Gas permeation and diffusion behavior for a MFI-type zeolite structure, which contains two channels, a straight channel along
10.1021/jp804586q CCC: $40.75 2009 American Chemical Society Published on Web 02/05/2009
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Figure 1. Temperature dependency of gas permeances for MFI-type zeolite membranes (closed symbols on solid line are H2 permeance, open symbols on broken line are He permeance).
the b-axis with circular openings of 0.54 × 0.56 nm and a sinusoidal channel along the a-axis with elliptical openings of 0.51 × 0.55 nm,7 has been studied extensively.8 Most work was focused on temperature and pressure dependency of gas permeance through zeolite membranes at temperatures below 300 °C, which could be well explained by the Stefan-Maxwell flux equation incorporating the surface diffusivity equation developed by Xiao and Wei1 and a proper equilibrium adsorption isotherm equation. However, experimental results showing the effects of the size or molecular weight of light gases on permeation (or diffusion) properties of MFI-type zeolite membranes are inconsistent. For example, Figure 1 shows the previously reported temperature dependency of He and H2 permeances for MFItype zeolite membranes.9-15 Some results summarized in Figure 1 show the permeances of He and H2 through the MFI-type zeolite membranes increase with increasing temperature, which is an activated diffusion mechanism.9-11,13 These results are not consistent with the theory of Xiao and Wei,1 i.e., the diffusion of small molecules including He (dm ) 0.26 nm) and H2 (dm ) 0.289 nm) in MFI-type zeolite channels is of the Knudsen type. In addition, despite the activation diffusion behavior for He and H2 molecules, the permeance of H2, with a larger kinetic diameter (dm ) 0.289 nm), is larger than that of He (dm ) 0.26 nm).13 The inconsistent results described above show a need for a more detailed study on permeation properties of small molecules through MFI zeolitic pore membranes, especially at high temperatures at which adsorption can be neglected. This paper reports on synthesis of high-quality MFI-type zeolite membranes and properties of gas permeation and diffusion of small gas molecules (He, H2, CO2, CO, and SF6 for comparison) for MFItype zeolite membranes in a wide temperature range up to 500 °C. The objective of this study is to clarify the effects of molecular weight and size of small gases and the size of zeolite pores on gas permeation and diffusion in zeolites and zeolite membranes. 2. Experimental Section 2.1. Preparation of MFI-Type Zeolite Membranes. Homemade porous R-alumina disks with thicknesses of 2 mm and diameters of 20 mm (average pore diameter, 0.2 µm; porosity, 45%) were used as supports. Before preparation of the MFItype zeolite membranes, the coating side of the R-alumina supports was polished with two types of SiC paper (500 and 800) and washed in an ultrasonic bath for 15 min. The supports
Kanezashi and Lin were dip-coated in 1-2 wt % silicalite suspensions synthesized from a solution with a composition of 1 g of SiO2, 5 mL of 1 M TPAOH, and 0.07 g of NaOH that had been treated hydrothermally at 120 °C for 12 h.15,16 A silicalite seed layer was coated by dip coating with the silicalite suspension, followed by drying and calcination at 450 °C for 8 h and subsequently at 650 °C for 8 h (heating-cooling rate, 18 °C h-1). The silicalite seed layer was calcinated after each dip coating, with the cycle repeated three times to ensure adequate coverage of the seed layer.15 Silica sol for the secondary grown membrane was prepared by adding a given amount of fumed silica powder to a NaOH solution (1.5 wt %; 0.15 g of NaOH, 1 g of SiO2, and 10.5 g of H2O) at about 80 °C with vigorous stirring and then aged for 1.5 h.15,16 The alumina-silicalite supports were placed in the silicalite synthesis solution with the silicalite seed layer facing up in a Teflon-lined autoclave. The autoclave was placed in an oven at 180 °C for a period of time to allow silicalite crystals in the silicalite seed layer to grow into a continuous film. After cooling to room temperature, the membranes were removed from the autoclave and washed with distilled water. 2.2. Single-Gas Permeation-Separation Measurements for MFI-Type Zeolite Membranes. The morphologies and thicknesses of the supported MFI-type zeolite membranes were characterizedusingSEM(Philips,XL30).Gaspermeation-separation experiments were conducted on a gas permeation setup shown in Figure 2. The membrane was fixed in a stainless steel cell with the zeolite layer facing upstream and was sealed by graphite rings. The leakage flow, which comes from the graphite rings, is about 8 × 10-11 to 2 × 10-10 mol m-2 s-1 Pa-1, and at high temperatures (450-500 °C), the leakage flow slightly increases to about 10-20%.17 This leakage flow was negligible compared to the permeation flow through the zeolite membranes. The liquid bubblers shown in the schematic were absent during the permeation experiments but were used for liquid precursors during CVD modification described in section 2.3. In gas permeation-separation equipment, a single gas (He, H2, CO2, CO, SF6) or a gas mixture of H2, CO2, and CO of industrial grade was fed at a flow rate of 100 cm3 min-1 in the cross-flow mode over the zeolite film surface of the membrane disk at 300 kPa. The downstream surface of the zeolite membrane disk was swept by nitrogen at a flow rate of 12 cm3 min-1 under atmospheric pressure. The flow rates and compositions of the retentate and permeate were respectively measured or analyzed by bubble flow meters and gas chromatography with a TCD detector (5973 Agilent GC/MS system with a stainless steel column; HayeSep DB 100/120, Alletech). Permeation experiments were conducted at 25-500 °C (heating-cooling rate, 30 °C h-1). 2.3. CVD Modification for MFI-Type Zeolite Membranes. Some of the MFI-type zeolite membranes were modified by counter-diffusion CVD or one-side CVD in the setup shown in Figure 2, with addition of bubblers filled with tetraethoxysilane (TEOS, 98% Sigma-Aldrich) and water, respectively. In the case of counter-diffusion CVD modification, the carrier gas (nitrogen, 100 cm3 min-1) with TEOS vapor was fed to the zeolite film surface, while water vapor was fed to the alumina support side of the MFI -type zeolite membrane. The concentration of TEOS and/or water vapor was controlled by the bubbling temperature. In the case of one-side CVD modification, O2 was used as the carrier gas, and the flow rate was kept at 100 cm3 min-1 by a mass flow meter. The carrier gas with TEOS vapor was fed to the zeolite film surface. In this work, both bubblers were kept
MFI-Type Zeolite Membranes at High Temperatures
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Figure 2. Schematic of the experimental apparatus for gas permeation (without bubblers) and CVD modification (with bubblers included for precursors).
Figure 3. SEM images of a (a) cross section of the MFI-type zeolite membrane and (b) surface of the membrane after secondary growth.
at room temperature, and the CVD reaction temperature was controlled at 500 °C. The gas permeation experiments on the CVD-modified zeolite membranes were performed onstream during the modification. Single-gas permeation experiments were performed by replacing the feeds of TEOS and water containing carrier gases with the single permeating gas described in section 2.2. 3. Results and Discussion 3.1. Gas Permeation-Separation Properties for Unmodified MFI-Type Zeolite Membranes. SEM images of the cross section and surface of the MFI-type zeolite membranes after secondary growth are shown in Figure 3. As shown in these micrographs, crack-free continuous MFI-type zeolite film could be formed on porous alumina supports. The thickness of the secondary growth zeolite layer is about 7 µm. The grain sizes after secondary growth are about 200-400 nm and wellintergrown. XRD analysis confirmed the MFI structure of these zeolite membranes. Figure 4 shows single-gas permeation characteristics of unmodified MFI-type zeolite membranes at 25-500 °C. The broken line (no symbols) at the bottom of Figure 4 shows a reference curve to compare the slope of temperature dependency of the permeance with that of the Knudsen behavior. The permeance of H2, He, CO, and CO2 for an unmodified membrane increases with decreasing temperature, which is Knudsen-type temperature dependency, while that of SF6 slightly increases with increasing temperature. The permeance for these gases decreases with increasing molecular weight, not the molecule size. H2 and SF6 permselectivity at 200 °C is about 20, which is slightly larger than the Knudsen selectivity (8.5). Ternary-component gas (H2, CO, and CO2) separation for unmodified MFI-type zeolite membranes in dry conditions was
Figure 4. Temperature dependency of gas permeances for unmodified MFI-type zeolite membranes at 25-500 °C.
conducted to examine the effect of adsorption of CO2 molecules by zeolitic pores or surfaces on gas permeation characteristics. Figure 5 shows the temperature dependency of gas permeances for unmodified MFI-type zeolite membranes at 25-500 °C. Permeances (H2, CO, and CO2) obtained by ternary-component gas separation are quite similar to those for single-gas permeation above 300 °C. However, a clear difference between these permeances was observed below 300 °C. Permeances of H2 and CO for ternary-component gas separation decrease drastically with decreasing temperature, lowering the H2 permselectivity (H2 and CO). On the other hand, the permeance of CO2 for ternary-component gas separation is similar to that for singlegas permeation. It is expected that CO2 preferential adsorption by MFI-type zeolitic pores should occur because of a much stronger affinity between CO2 molecules and the MFI-type
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Figure 5. Temperature dependency of gas permeances for unmodified MFI-type zeolite membranes (closed symbols on the solid line are gas permeances for single permeation, open symbols on the broken line are gas permeances for ternary-component gas separation). Feed gas composition is H2/CO/CO2, 1:1:1; Pup, 300 kPa; and Pdown, 100 kPa.
zeolite pores. The H2 and CO molecules do not have such a strong affinity. Because CO2 adsorption on MFI-type zeolitic pores decreases with increasing temperature,18 it is expected that H2 separation performance for ternary-component gas separation should follow that of single-gas permeation at high temperatures; there is no effect on CO2 adsorption in other gas permeation performances above 300 °C. This also indicates that small gas molecules can freely diffuse even in the presence of other molecules in a zeolite channel. Because this unmodified zeolite membrane includes some intercrystalline gaps, which can strongly affect the diffusion property, further study to clarify the diffusion property through zeolite channels in the presence of other gases will be investigated by ternary-component gas separation (diffusion at different temperatures, pressures, and feed gas compositions) on the CVD-modified membrane. At low temperatures, the zeolitic pores are filled by adsorbed CO2 molecules that block H2 and CO molecules from permeating as freely through these pores in the membrane. In the steady state, single-gas permeance through a zeolite membrane can be obtained by eq 18
F)
φ L(Pf - Pp)
∫qq Dc( dd lnln Pq )dq p
f
(1)
where qp and qf are the concentrations of the permeating gas in the zeolite membrane at the feed and permeate side, respectively, φ is the ratio of the membrane porosity to the tortuosity factor, and Dc is the gas diffusivity in zeolite. The above equation correlates the permeance to the diffusivity, sorption equilibrium properties, membrane thickness, and upstream and downstream pressures. For gas-zeolite systems with a linear adsorption isotherm (q ) KP, where K is the adsorption equilibrium constant), eq 1 is deduced to
φ F ) (DcK) L
(2)
For diffusion of small gases with weak adsorption affinity with zeolites, Xaio and Wei1 proposed the following gas translational diffusion model to predict diffusivity in zeolite pores
Dc )
R 8RT z πM
1⁄2
( )
( )
exp
-Ed RT
(3)
where Ed is the activation energy for diffusion of the gas in the micropores, which is determined by the relative size of the
Figure 6. Calculated R/z and activation energies of gas diffusion for unmodified MFI-type zeolite membranes as a function of the ratio of the kinetic diameter of the diffusion gas molecule to the zeolite pore diameter λ ) dm/dp.
diffusing gas molecule to the zeolite pore size and shape of diffusing molecule, M is the molecular weight of the permeating gas, R is about 1 nm for diffusion in a MFI-type zeolite, and z is the diffusion coordination number (4 for a MFI-type zeolite). As shown in eq 3, the diffusivity in zeolites is determined and can be predicted by the molecular weight, M, and the activation energy for diffusion, Ed. For diffusion of small molecules with a weak adsorption affinity with zeolite pores or at high temperatures, K ) 1/RT, inserting eq 3 into eq 2 gives
F)
φR 8 L z πMRT
(
1⁄2
)
( )
exp
-Ed RT
(4)
The activation energy for diffusion (Ed) and values of R/z for He, H2, CO, and CO2 were obtained by regressing eq 4 with the experimental permeation data at different temperatures above 300 °C (to ensure validity of eq 2 and K ) 1/RT). Wirawan and Creaser19 investigated the CO2 adsorption-desorption behavior on silicalite-1 and cation-exchanged ZSM-5 zeolites in a wide temperature range. From the data on the adsorption isotherm at 200 °C, the amount of CO2 adsorption on silicalite-1 is low (less than 0.05 mmol/g at PCO2 ) 1 atm), and the results for the temperature-programmed desorption (TPD) and FTIR show negligible CO2 adsorption above 300 °C. Similar data for SF6 were not calculated because its permeation data were only measured below 200 °C. In this temperature range, there is an adsorption effect between the SF6 molecules and MFI-type zeolite structure.18,20-23 Values of R/z and Ed for the four compounds as a function of the relative size of the gas molecule to the zeolite pore size, λ ) dm/dp, are plotted in Figure 6. To calculate R/z, a MFI-type zeolite porosity of 0.2523 was used as the membrane porosity. The membrane thickness was estimated by SEM (about 7 µm), and a tortuosity factor of 1 was used in the calculation. It was found that R/z is essentially independent of λ. The results agree with the theoretical model of Xiao and Wei1 with respect to the dependency of these two parameters on λ. The value of R/z measured in this work for the MFI-type zeolite membranes is about 10-11 m, which is smaller than that for the MFI-type zeolites (2.5 × 10-10 m).1 If a tortuosity factor of 5 is used, values of R/z obtained would be the same as the values reported by Xiao and Wei.1 This indicates that gas diffusion through polycrystalline zeolite film follows a much more tortuous path as compared to diffusion into zeolite crystals. Diffusivity is calculated from a gas permeation flux through a polycrystalline
MFI-Type Zeolite Membranes at High Temperatures
Figure 7. Temperature dependency of gas permeances for counterdiffusion CVD-modified MFI-type zeolite membranes at 50-500 °C (same membrane used in Figure 4).
zeolite membrane, containing many randomly oriented MFI zeolite crystals close packed with minimum intercrystalline gaps. Therefore, gas passing through such a polycrystalline membrane may follow a tortuous path because of the random orientation of the zeolite crystals. It should be noted that similar to other macroscopic methods the determination of the absolute value of diffusivity depends on accurate measurements of the macroscopic length (and tortuosity) of the zeolite subjects (crystals, pellets, and membranes). The aforementioned discussion shows that there is a certain degree of uncertainty in the absolute value of R/z for the MFItype zeolite as it depends on membrane thickness, porosity, tortuosity, and permeation area. It is important to show that the value of R/z is constant and does not vary with the relative size of the gas molecule to the zeolite pore size. There is only a minor difference in the activation energies for diffusion of He (0.22 kJ mol-1), H2 (0.23 kJ mol-1), CO2 (0.43 kJ mol-1), and CO (1.11 kJ mol-1). Because the permselectivity of H2 and SF6 for unmodified membranes is not large, the presence of intercrystalline gaps in unmodified zeolite membranes is expected. Their presence can strongly affect the gas permeation behavior through zeolitic pores. Because of the presence of intercrystalline pores in membranes, the unmodified membranes have an average pore diameter (dp) larger than that of the MFI-type zeolitic pores. This gives smaller values for λ ) dm/dp for all of the molecules. In order to show true gas permeation (diffusion) characteristics for the MFI-type zeolites, MFI-type zeolite membranes should be modified to eliminate possible intercrystalline pores. 3.2. CVD Modification and Gas Permeation Properties of Modified Membranes. It is known that many good quality, defect-free, polycrystalline zeolite membranes include not only zeolitic pores but also intercrystalline gaps. These intercrystalline gaps are microporous with a pore size larger than that of the zeolitic pores. To obtain reliable data for gas permeation through the zeolitic pores, these intercrystalline gaps should be sealed. In this work, two types of CVD modification techniques were applied to seal the intercrystalline gaps in the zeolite membrane: counter-diffusion CVD modification24 and one-side CVD modification. The use of a silica precursor of TEOS with a molecular size larger than that of the zeolitic pores ensures that the product of CVD conducted in the proper configuration seals the intercrystalline gaps rather than being deposited within the zeolite pores.
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Figure 8. Temperature dependency of CO2 permeances for MFI-type zeolite membranes at 25-500 °C (closed symbols on solid line are before counter-diffusion CVD modification, open symbols on broken line are after counter-diffusion CVD modification).
Figure 7 shows single-gas permeation characteristics of counter-diffusion CVD-modified MFI-type zeolite membranes at 50-500 °C. The permeance of the counter-diffusion CVDmodified membrane of SF6 at 200 °C for 6 h is about 10% of that for unmodified membrane, while those of He, H2, and CO at 500 °C decrease about 30-50%. The permselectivity of H2 and SF6 at 200 °C increases from 20 to 270, indicating that some intercrystalline gaps were completely plugged. CVD modification does not change the slope of the permeance versus temperature curves shown in Figure 7 for He and H2 but lowers the slope for CO. CVD modification increases substantially the temperature dependence of the permeance for SF6. For CO2 permeation, the temperature dependency of CO2 permeance for counter-diffusion CVD-modified membranes has a minimum at 300 °C as shown in Figure 8. The slope between 50 and 300 °C is much larger than that of Knudsen diffusion, while above 300 °C the permeance of CO2 increases with increasing temperature in a manner similar to that of activated permeation. This trend was also reported in the literature.17,25,26 Because CO2 molecules are thought to be adsorbed in the MFI channels,18 with increasing temperature the mass-transport mechanism should be shifted from the surface diffusion regime to the activated gaseous diffusion regime. Some intercrystalline gaps, through which CO2 can permeate by Knudsen diffusion, could be narrowed or eliminated by amorphous silica after CVD modification, resulting in enhancement of CO2 adsorption effects. Therefore, the gas translational diffusion model proposed by Xiao and Wei1 cannot be applied for CO2 data for the modified membranes because of enhancement of CO2 adsorption, which invalidates eq 2 and K ) 1/RT. For the CVDmodified MFI-type zeolite membrane, the permeation data for H2, He, and CO are obtained and analyzed next. The calculated values of R/z and activation energies for counter-diffusion CVD-modified MFI-type zeolite membranes as a function of λ ) dm/dp are shown in Figure 9. The values of R/z are clearly independent of λ before and after counterdiffusion CVD modification. Compared with the activation energy data for the unmodified MFI-type zeolite membranes (Figure 6), CVD modification results in an increase in activation energy for diffusion of CO molecules only. The activation energy for diffusion of He and H2 in the high-quality MFI-type zeolite membranes obtained by counter-diffusion CVD modification is approximately zero, indicating that diffusion is dominated by the Knudsen mechanism.
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Figure 9. Calculated R/z and activation energies of gas diffusion for counter-diffusion CVD-modified MFI-type zeolite membranes as a function of the ratio of the kinetic diameter of the diffusion gas molecule to the zeolite pore diameter λ ) dm/dp.
Figure 11. Calculated R/z and activation energies of gas diffusion for one-side CVD-modified MFI-type zeolite membranes as a function of the ratio of the kinetic diameter of the diffusion gas molecule to the zeolite pore diameter λ ) dm/dp.
Figure 10. Temperature dependency of gas permeances for one-side CVD-modified MFI-type zeolite membranes at 50-500 °C.
Figure 12. Kinetic diameter dependency of measured diffusivity for MFI-type zeolite membranes at 500 °C before and after CVD modification (open symbols on broken line are calculated diffusivity at room temperature by MD simulation33).
Figure 10 shows single-gas permeation characteristics of oneside CVD-modified MFI-type zeolite membranes at 50-500 °C. One-side CVD modification was conducted on a MFI-type zeolite membrane that was prepared under the same conditions (same batch) as the membrane shown in Figure 4. Membrane quality (absolute value of He and H2 permeance and H2 and SF6 permselectivity) is similar to that shown in Figure 4. Oneside CVD modification for 6 h results in a 4-, 5-, and 200-fold decrease of the permeances for He, H2, and CO, respectively. The H2 and CO permselectivity at 500 °C is about 200, which is much higher than the Knudsen ratio (3.74), indicating a molecular-sieving effect between H2 and CO molecules. The permeance of SF6 is below the detection limit of the permeation equipment (
