Ind. Eng. Chem. Res. 1997, 36, 649-655
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MATERIALS AND INTERFACES Formation of a Y-Type Zeolite Membrane on a Porous r-Alumina Tube for Gas Separation Katsuki Kusakabe, Takahiro Kuroda, Atsushi Murata, and Shigeharu Morooka* Department of Chemical Science and Technology, Kyushu University, Fukuoka, 812-81 Japan
A porous R-alumina support tube, polished with a finely powdered X-type zeolite for use as seeds, was placed vertically in an autoclave containing an aqueous mixture of water glass and sodium aluminate. Hydrothermal synthesis was carried out at 90 °C for 24 h. A polycrystalline layer of Y-type zeolite was thus formed on the outer surface of the support tube. After washing and drying in air, permeances of single components and mixtures of CO2 and N2, as well as CH4, C2H6, and SF6, were determined. The CO2 permeance was higher than that of N2 at temperatures of 30-130 °C. When an equimolar mixture of CO2 and N2 was fed into the feed side, the CO2 permeance was nearly equal to that for the single-component system and the N2 permeance for the mixture was greatly decreased, especially at lower permeation temperatures. This was due to selective adsorption of CO2 in subnanometer micropores of the membrane. At 30 °C, the permeance of CO2 was higher than 10-7 mol‚m-2‚s-1‚Pa-1, and the permselectivity of CO2 to N2 was 20-100. Introduction Inorganic membranes which contain subnanometer pores are useful for the separation of gas mixtures under severe conditions where organic membranes are not functional. Gavalas et al. (1989) reported the first successful preparation of an amorphous silica membrane, which had a high hydrogen permselectivity, by chemical vapor deposition (CVD) in pores of a Vycor glass tube. Yan et al. (1994) and Morooka et al. (1995a) also prepared a silica membrane using tetraethyl orthosilicate as the silica source in macropores of an Ralumina tube. The membrane showed hydrogen permeance and hydrogen/nitrogen permselectivity higher than 10-8 mol‚m-2‚s-1‚Pa-1 and 1000 at 600 °C, respectively, with a low H2O permeation in the temperature range of 200-400 °C (Morooka et al., 1996). The solgel process constitutes another technique for producing silica-based membranes. Using this technique, Raman and Brinker (1995) prepared a membrane capable of separating carbon dioxide from methane. Amorphous silica membranes are fundamentally suited for hydrogen separation, which is achievable with a palladium membrane. Morooka et al. (1995b) prepared a palladium membrane by CVD using palladium acetate. A palladium membrane of 4-5 µm in thickness, formed in the macropores of an R-alumina support tube, was resistant to hydrogen embrittlement and mechanical damage. The permeance was as high as 10-6 mol‚m-2‚s-1‚Pa-1, and the selectivity of hydrogen to nitrogen was greater than 1000 at 500 °C. Recently, Peachey et al. (1996) prepared a palladium membrane on a tantalum sheet by electron beam evaporation. The permeance of the membrane was 10-6 mol‚m-2‚s-1‚Pa-1 when the hydrogen pressure difference was 0.1 MPa. This membrane was composed of metals and was * Author to whom all correspondence should be addressed. Telephone: 81-92-642-3551. Fax: 81-92-651-5606. E-mail:
[email protected]. S0888-5885(96)00519-2 CCC: $14.00
mechanically more flexible than a palladium membrane formed on a porous ceramic support. Zeolites, on the other hand, can be designed to separate hydrocarbon molecules. van Bekkum et al. (1994) prepared an MFI-type zeolite membrane on a porous stainless steel disk. It showed a high permselectivity of n-butane (n-C4H10) to i-butane (i-C4H10) at room temperature. Jia et al. (1993, 1994) reported on a zeolite membrane which showed a n-C4H10/i-C4H10 selectivity of approximately 50 to 20 °C. However, they reported no data at elevated temperatures. Yan et al. (1995) prepared an MFI membrane on an R-alumina porous disk. The n-C4H10/i-C4H10 permselectivity was 6.2 at 108 °C and 9.4 at 185 °C. Vroon et al. (1996) reported that an MFI-type membrane formed on an R-alumina support showed a n-C4H10/i-C4H10 permselectivity of 90 at 25 °C and 11 at 200 °C. To the contrary, a silicalite membrane produced by Bai et al. (1995) on an alumina tube showed a n-C4H10/i-C4H10 of 0.37 at 88 °C and 0.59 at 170 °C. Matsukata et al. (1994) also prepared zeolite membranes. Recently, Funke et al. (1996) separated n-octane, i-octane, and n-hexane using an MFI-type zeolite membrane that was formed on a γ-alumina-coated R-alumina porous tube. In order to employ zeolite membranes in a practical manner, reproducibility in the membrane formation process is one of the most important factors. The effect of supporting substrates on permeation properties of zeolite membranes also represents critical information. Yan et al. (1995) reported that changes in membrane morphology occurred, for the same porous substrate, under different synthesis conditions. Kusakabe et al. (1996) using a hydrothermal reaction, produced an MFItype zeolite membrane on the outer surfaces of a porous R-alumina support tube. No direct relationship was found between film morphology and permselectivity. Membranes with a n-C4H10/i-C4H10 permselectivity of 5-10 and a n-C4H10 permeance of 10-8-10-7 mol‚m-2‚s-1‚Pa-1 were reproducible. Yan et al. (1996) © 1997 American Chemical Society
650 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997
plugged non-zeolite pores with coke and increased the n-C4H10/i-C4H10 permselectivity to 322-325 at a n-C4H10 permeance of 10-8 mol‚m-2‚s-1‚Pa-1. From the standpoint of a feasible industrial process, however, membranes which can separate carbon dioxide from nitrogen with high permeance and selectivity are not available. In this paper, we report the hydrothermal synthesis of a Y-type zeolite membrane on a porous support tube and its permeation properties especially with respect to mixtures of carbon dioxide and nitrogen. Experimental Section A porous R-alumina tube (2.8 mm o.d. and 1.9 mm i.d.) with an average pore size of 150-170 nm was used as the support of a zeolite membrane. Figure 1 shows the procedure for the synthesis. Water glass (Wako Pure Chemical, Na2O, 17-19%; SiO2, 35-38%; Fe, maximum 0.03%) was diluted in water, and then sodium aluminate (Wako Pure Chemical, Al/NaOH ) 0.81) followed by NaOH (Wako Pure Chemical, reagent grade) was added to the stirred solution. The final composition was Al2O3:SiO2:Na2O:H2O ) 1:10:14:798 in a molar basis, as used by Inoue et al. (1995) for synthesis of a Y-type zeolite, and this solution was stirred for 20 h at room temperature. Each support tube was cut to a length of 30 mm, and the outer surface of the tube was rubbed with NaX zeolite particles (200 mesh, F-9, Wako Pure Chemical) to implant crystal fragments as nucleation sites. Since X-type zeolite possesses the same crystal structure as Y-type zeolite, they are interchangeable as the nuclei of crystals. After the rubbing treatment, three tubes were fixed vertically with a Teflon holder in a 40 mL Teflon-coated autoclave. An aliquot of the solution (30 mL) was then placed in the autoclave, which was maintained at 90 °C for 24 h, unless otherwise noted. Both ends of the tube were open to circulation of the liquid. After synthesis, the tubes were washed thoroughly with distilled water until the pH of the rinse water reached neutrality. They were then dried in air at ambient temperature and were stored in a desiccator. The synthesis was repeated under the same conditions, and the reproducibility of membranes was evaluated. Membranes thus prepared were referred to as A, B, and C. The morphology of the membranes was observed by scanning electron microscopy (SEM; Hitachi S-900), and the crystal structure was determined by X-ray diffraction (XRD; Shimadzu XD-D1). Figure 2 shows a schematic diagram of the membrane holder, which was used for the permeation tests. Each end of the membrane was connected to a stainless steel tube with epoxy resin. The permeating portion of the membrane was 10-15 mm in length. Permeance to a single-component gas was measured at 30-130 °C using carbon dioxide (CO2), nitrogen (N2), methane (CH4), ethane (C2H6), and sulfur hexafluoride (SF6) and was calculated from the following equation:
permeance ) mole of gas transferred per unit time (1) (membrane area)(partial pressure difference) Helium was used as the sweep gas on the permeate side, and ambient pressure was maintained on both sides of the membrane. Permeance was possibly underestimated because of the countercurrent diffusion of helium. The partial pressure on the permeate side was maintained at less than 2 kPa by dilution with the sweep
Figure 1. Preparation procedure for Y-type zeolite membranes.
Figure 2. Details of membrane sealing.
gas. Permeance was determined for single-component gases and equimolar mixtures, and selectivity was defined by the ratio of permeances. Purity of the permeant gases was not further determined. Results and Discussion Formation of Membranes. Parts a-c of Figure 3 show the top surfaces of the membrane formed without implanting seeds. No continuous film was formed even after a 24-h synthesis. When seed particles were implanted, however, a continuous layer of zeolite was formed on the outer surface of the tube as shown in Figure 4a-c. There are two zones of zeolite in the fractured section. The top layer (I) is composed of zeolite polycrystals, and the inner layer (II) is the R-alumina support whose macropores are filled with deposits. The layer of the R-alumina support is the white part below layer II in Figure 4c. The crystal size and top layer thickness increased with increasing reaction time. Since the inside of the tube was not rubbed with the zeolite crystals, no continuous film was formed even after a 24-h synthesis. Figure 5 indicates the XRD patterns for the membranes as-produced by the hydrothermal synthesis. In order to demonstrate the membrane formation, the reaction period was varied in the range of 6-24 h. The XRD pattern of the membrane formed after 12 h was similar to the pattern of the purchased NaX-type zeolite particles. Peaks attributed to R-alumina became smaller for the 24-h synthesis. Crystals recovered from the bottom of the reactor also revealed the XRD pattern of NaX-type zeolite. Since the top layer consisted of zeolite polycrystals, the ability of the membrane seems to rely on a dense layer formed in the R-alumina macropores underneath the top polycrystalline layer. The diffraction patterns in Figure 5 contained the information from the crystal structure of both top and inner layers. The micropore volume of the zeolite crystals was determined by a sorption test using a constant-volume
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 651
Figure 3. Morphology of zeolite formed without seeding. Synthesis period: (a) 6 h; (b) 12 h; (c) 24 h.
sorption unit (Hayashi et al., 1995). Prior to sorption, the sample was degassed at 250 °C for 10 h. Equilibrium sorption data were correlated by the DubininAstakhov equation (Dubinin, 1960).
W ) W0 exp[1 - (A/E)n]
(2)
A ) RT ln(ps/p)
(3)
The micropore volume was then evaluated from sorption data for CO2, C2H6, n-C4H10, i-C4H10, and SF6 at 25 °C under 1-100 kPa. The liquid density of these gases was assumed to be 0.72, 0.32, 0.57, 0.55, and 1.32 Mg‚m-3,
Figure 4. Morphology of zeolite formed without seeding: (a) top surface; (b and c) fractured sections. Synthesis period: 24 h.
respectively (Lide and Kehiaian, 1994). In eq 2, W0 is the limiting volume of pores, the size of which is larger than the kinetic diameter of sorbed species. The best fit of the sorption data was obtained with n ) 3 for these present experiments. Figure 6 shows the relationship between micropore volume and kinetic diameter of sorbed species based on the Lennard-Jones 6-12 potential (Breck, 1974). The micropore volume of the Y-type membrane was approximately 0.45 mL‚g-1 for CO2 and 0.27 mL‚g-1 for SF6. The former value is in agreement with that reported by Breck (1974). Permeation Properties. Permeation was determined with single-component gases and equimolar
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Figure 5. XRD patterns of support, membranes, and zeolite crystals.
Figure 7. Effect of permeation temperature on the permeance of membrane A.
Figure 6. Relationship between micropore volume of zeolite and kinetic diameter of sorbed species.
mixed gases. After the membrane was air dried, it was fixed in the permeation test unit. The carrier (He) was introduced in the permeate side, the temperature was then raised to 130 °C for 1 h, and permeating gases were introduced to the feed side. Measurement was started after stabilizing the flow system for 6 h and was completed in about 10 h. The temperature was then decreased to 80 °C in 1 h, and the measurement was repeated. The temperature was further lowered to 30 °C, and the procedure was again repeated. As indicated in Figure 7, the permeance of CO2 was (1-3) × 10-7 mol‚m-2‚s-1‚Pa-1, which was equivalent to the permeances of H2 and Ar through an MFI-type membrane reported by Bai et al. (1995). Kusakabe et al. (1996) also reported that the permeance to single-component CO2 through an MFI-type membrane was similar to these values. The Y-type membrane in the present study was unique in that the N2 permeance was greatly retarded when the mixture of CO2 and N2 was fed at 30 °C. The permeance to N2 for a single-component system was (1-3) × 10-8 mol‚m-2‚s-1‚Pa-1, and that for a mixed-gas system was 9 × 10-9 mol‚m-2‚s-1‚Pa-1. The permeance to CH4 was also greatly decreased when the CO2/CH4 mixture was fed. Figure 8 shows the reproducibility of the membrane formation. The permeances of CO2 and CH4 were nearly half those shown in Figure 7, but the nitrogen permeance for the CO2/N2
Figure 8. Effect of permeation temperature on the permeance of membrane B. The vertical bar shows the error range.
system was decreased more than the case of Figure 7. The decrease in nitrogen permeance was significant also at 130 °C. It was questioned if permeances were affected by desorption of water and adsorption of impurities during the permeation test. Thus, CO2 and N2 permeances were determined as a function of time. An air-dried membrane was placed in the permeation test unit, the temperature was maintained at 30 °C for 30 min, and an equimolar mixture of CO2/N2 was introduced. Figure 9 shows the relationship between permeance and time. The CO2 and N2 permeances increased by the desorption of water in the initial stage of the measurement and then gradually decreased. However, the CO2/N2 selectivity did not greatly change with time, ranging from 50 to zero time to 75 after 15 h. Some impurities in the permeate gases might be adsorbed and partially occlude the zeolite pores. The data shown in Figures 7
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Figure 9. Effect of permeation period on the permeance of membrane C. Permeation temperature ) 30 °C. CO2/N2 system: 2, CO2; b, N2.
Figure 11. Effect of permeation temperature on selectivity: 4, 2, 3, and 1, membrane A; 0, 9, and ], membrane B.
Figure 10. Arrhenius plot of permeance for single-component systems.
and 8 were acquired in the range of 6-10 h after the start of the test. Figure 10 shows an Arrhenius plot of permeances to CO2, N2, CH4, C2H6, and SF6. The activation energies for permeances to CO2, C2H6, and SF6 were positive and were zero or slightly negative for N2 and CH4. Li and Hwang (1992) reported that the activation energy for CO2 permeance through macropores was negative. Thus, the permeation mechanism of the Y-type membrane is different from that of macroporous membranes. The MFI-type zeolite crystal possesses channels approximately 0.54 nm in diameter, which is comparable to the kinetic diameter of i-C4H10 (ca. 0.5 nm). This leads to retardation of i-C4H10 permeation through an MFI membrane (Kusakabe et al., 1996). The free aperture of the main channels in Y-type zeolite is 0.74 nm (Breck, 1974) and is much larger than the diameters of CO2 and N2 molecules. If the concentrations of CO2 and N2 in the micropores of the Y-type zeolite membrane are equal to those in the outside gas phase, these molecules permeate through the membrane at a low CO2/N2 selectivity. However, this was not the case. Carbon dioxide molecules adsorbed on the outside of the membrane migrate into micropores by surface diffusion. Nitrogen molecules, which are not adsorptive, penetrate into micropores by a translation-collision mechanism from the outside gas phase. The mouth of the micropores may be narrowed by adsorbed CO2 molecules,
which block N2 molecules from entering the pores. This selection mechanism is plausible for micropores with a width of up to six molecules (Furukawa et al., 1996). When CO2 molecules are strongly adsorbed on the pore wall, on the other hand, the CO2 permeation rate will be low even if CO2 is concentrated in the pore. Thus, balances between pore size and molecule size and between adsorptivity and mobility as well as differences in polarity of competitive species are important to attain both high permeance and selectivity. As shown in Figure 10, the permeance to SF6 was higher than the permeances to CH4 and N2 at 130 °C but was lower at 30 °C. This suggests that the mobility of SF6 molecules was rate-controlling at 30 °C. Figure 11 shows selectivities of CO2/N2 and CO2/CH4 for single-component and equimolar mixture systems. At 30 °C, CO2/N2 selectivity for the CO2/N2 mixture was 20-100 for membranes A and B (50-75 for membrane C), and CO2 permeance was 10-7-10-6 mol‚m-2‚s-1‚Pa-1. These values for selectivity and permeance are unusually high compared to those for the MFI-type membrane (Kusakabe et al., 1996). The CO2/N2 selectivity decreased with increasing permeation temperature but was 8-15 at 130 °C. As indicated in Figure 12, permeance was not directly related to the kinetic diameter of permeants given by Breck (1974). The relationship between permeance and molar mass also was not appreciable. Funke et al. (1996) indicated that relative permeance through an MFI-type zeolite membrane was not predictable on the basis of the size and shape of permeates alone. Vroon et al. (1996) also found that the flux of nonadsorptive molecules was suppressed by the presence of adsorptive molecules. These conclusions are in agreement with the results of this study, although these workers evaluated the effect of adsorption using hydrocarbons which were adsorbed on a MFItype zeolite. Permeance through a membrane with a thickness δ is given as D/δRT, where D is the effective diffusivity of permeant in zeolite. A considerable amount of effective diffusivity data have been reported in the literature, and these are often scattered over several orders of magnitude (Breck, 1974; Ka¨rger and Ruthven,
654 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 R ) gas constant, J‚mol-1‚K-1 T ) temperature, K W ) volume of sorbed molecules as liquid, m3 W0 ) limiting volume of pores, m3 Greek Letters R(i/j) ) selectivity of i-component to j-component δ ) membrane thickness, m
Literature Cited
Figure 12. Relationship between permeance and kinetic diameter of permeates. Permeation temperature: 0 and 9, 30 °C; 4 and 2, 80 °C; O and b, 130 °C. Closed symbols and open symbols mean membranes A and B, respectively.
1992). Kapteijn et al. (1995) reported very large values in an MFI-type zeolite membrane, while Masuda et al. (1996) reported very small values for C6-C8 hydrocarbons in Y-type zeolite crystals. Distribution in diffusivities is wider for larger hydrocarbon molecules, but diffusivities of smaller molecules are within an acceptable range. Kapteijn et al. (1995) reported diffusivities for CH4 in silicalite to be 10-10-10-8 m2‚s-1, irrespective of the method of determination. From Figures 7 and 8, CO2 permeance is on the order of 10-7 mol‚m-2‚s-1‚Pa-1. If the membrane thickness is taken as 20 µm from Figure 5b, the diffusivity of CO2 in the membrane is calculated as 5 × 10-9 m2‚s-1. This value is in the range of CH4 diffusivity in silicalite (Kapteijn et al., 1995). Conclusions A Y-type zeolite membrane was formed on a porous R-alumina support tube. The synthesis was performed several times, and the membranes produced separated CO2 from N2 at a permeance on the order of 10-7 mol‚m-2‚s-1‚Pa-1 and a selectivity of 20-100 at 30 °C. Membrane C showed a CO2 permeance of (3-9) × 10-7 mol‚m-2‚s-1‚Pa-1 and a CO2/N2 selectivity of 50-75 at 30 °C. This rapid and selective permeation was due to the pore-size-controlled adsorption. It was presumed that carbon dioxide molecules adsorbed in narrow pores obstructed the penetration of nonadsorptive nitrogen molecules from the outside. Acknowledgment This work was financially supported by the Ministry of Education, Science, Sports and Culture, New Energy and Industrial Technology Development Organization, Research Institute of Innovative Technology for the Earth, and Japan Fine Ceramics Center. We are especially appreciative of support by the Kyocera Corp. The NOK Corp. kindly provided the porous supports. Nomenclature A ) absorption potential, J‚mol-1 D ) effective diffusivity, m-2‚s-1 E ) characteristic energy for adsorption, J‚mol-1 n ) exponent defined in eq 2 p ) vapor pressure, Pa ps ) saturated vapor pressure, Pa
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Received for review August 19, 1996 Revised manuscript received December 4, 1996 Accepted December 10, 1996X IE960519X
X Abstract published in Advance ACS Abstracts, January 15, 1997.
