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Preparation and gas permeation properties on pure silica CHA-type zeolite membranes
Author’s Accepted Manuscript Preparation and gas permeation properties on pure silica CHA-type zeolite membranes Koji Kida, Yasushi Maeta, Katsunori Yogo
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To appear in: Journal of Membrane Science Received date: 30 June 2016 Revised date: 31 August 2016 Accepted date: 1 September 2016 Cite this article as: Koji Kida, Yasushi Maeta and Katsunori Yogo, Preparation and gas permeation properties on pure silica CHA-type zeolite membranes, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.09.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and gas permeation properties on pure silica CHA-type zeolite membranes Koji Kidaa, Yasushi Maetab, Katsunori Yogoa,b* a
Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai,
Kizugawa, Kyoto 619-0292, Japan b
Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma,
Nara 630-0192, Japan *
Corresponding author: Tel.: +81 774 75 2305; fax: +81 774 75-2318. [email protected]
Abstract Pure silica CHA-type zeolite (Si-CHA) membranes were synthesized by a hydrothermal secondary growth method on porous α-alumina supports. Using Si-CHA seed crystals as a crystalline nuclei allowed the formation of a dense Si-CHA layer. The Si-CHA membranes prepared in this study showed excellent gas permeance derived from their large pore volume and effective molecular sieve performance for gas separation. The Si-CHA membranes exhibited excellent H2 and CO2 permeance of 1.1 × 10−6 and 1.7 × 10−6 mol/m2sPa, respectively. The permeance ratio of H2/CH4 and CO2/CH4 were 34 and 54, respectively. The stability test in the presence of water vapor
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on the Si-CHA membranes was performed and the Si-CHA membranes demonstrated good water vapor resistance, more so than the topologically analogous aluminosilicate zeolite membrane (SSZ-13). The Si-CHA membranes were unlikely to present pore blockages by water adsorption because of their hydrophobic pore systems. Graphical abstract
Keywords: Zeolite membrane, Pure silica zeolite, CHA-type zeolite, Gas permeation, High CO2 permeance
1. INTRODUCTION Zeolite membranes have been studied in the separation of liquid mixtures or gas mixtures. Various structural types of zeolite membranes, such as MFI [1–3], LTA [4,5], FAU [6,7], CHA [8–11], DDR [12,13] and MOR [14-15], have been reported, and the preparation technique and separation properties have been investigated. Recently, highsilica zeolite membranes have attracted significant attention for gas separation. Highsilica zeolite membranes allow high thermal stability and chemical resistance. The gas
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permeance of high-silica zeolite membranes tend to be higher than that of aluminosilicate zeolite membranes because their large pore volumes are advantageous for gas diffusion. Additionally, high-silica zeolite membranes are unlikely to present pore blockages by water adsorption. Kosinov et al. reported that the low polarity of high-silica SSZ-13 membranes gives beneficial CO2 separation properties under hydrothermal conditions [8]. Endowing zeolite membranes with a resistance to water vapor is an extremely important factor to operate in gas separation. For the above reasons, research in recent zeolite membrane development for gas separation has shifted to developing zeolite membranes of higher Si/Al ratio [8,9]. Pure silica zeolites that have only Si–O–Si bonds in their framework are different from common aluminosilicate zeolites. Because there are no framework aluminum atoms and concomitantly no extra-framework counter cations, this maximizes the ability of guest species to diffuse in their pores. Silicalite-1 has received the most attention in zeolite membrane research [2-3]. Silicalite-1 membranes have MFI-type topology and three-dimensional intermediate pores (0.51 × 0.55 nm and 0.53 × 0.56 nm). However, their large pores are unsuitable for gas separation of small molecules, such as H2/CH4 and CO2/CH4. While Silicalite-1 membranes have excellent gas permeation properties, low separation selectivity can be obtained. Si-DDR membranes are also typical pure silica zeolite membranes, and have been reported to possess excellent gas separation performance [12-13]. Si-DDR has eight-membered ring pores (0.36 × 0.44 nm), which are appropriate in size for CO2/CH4 separation. Tomita et al. reported Si-DDR membranes to have a CO2/CH4 separation selectivity of more than 100 [13]. However, Si-DDR membranes often demonstrate low gas permeance. Because the DDR-type zeolite has a two-dimensional pore structure, this decreases the diffusivity of gas
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molecules. These pure silica zeolite membranes reported hitherto show some problems in selectivity or permeability for gas separation. Therefore, it is considered that new types of pure silica zeolite membranes are required to address the separation of small molecules. Pure silica CHA-type zeolite (Si-CHA) membranes can be expected to show high gas permeance and high separation selectivity. Si-CHA possess elliptical cages of large pore volume derived from its low framework density [16] and three-dimensional pore systems via eight-membered ring windows (0.38 × 0.38 nm). To the best of our knowledge, Si-CHA membranes possessing effective molecular sieve performance for gas separation have not been reported so far. The preparation method of Si-CHA is considered the reason for the poor gas separation performance. The first preparation method toward Si-CHA zeolite was first reported by Camblor et al.[16]; however, their synthesis method requires cumbersome procedures and precise composition adjustment of the precursor gel. In particular, Si-CHA was only formed from the solid-like precursor gel with a small quantity of water. Therefore, Si-CHA is one of the most difficult zeolites to form membrane structures. Our group has embarked on the development of Si-CHA zeolites, and to date, the simple synthetic procedure [17] and the CO2 adsorption properties [18] of Si-CHA crystals have been investigated. Here we report the preparation techniques and single gas permeation properties of SiCHA zeolite membranes. Si-CHA membranes were synthesized by a hydrothermal secondary growth method on porous α-alumina supports. We found that the use of SiCHA crystals as a nucleation agent allowed the formation of a dense Si-CHA layer. The various gas permeation properties of the Si-CHA membranes were studied. Additionally,
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the stability test in the presence of water vapor on Si-CHA membranes and the topologically analogous aluminosilicate zeolite membrane (SSZ-13) were performed.
2. EXPERIMENTAL 2.1 Synthesis procedure of Si-CHA crystals and seeds Si-CHA crystals were synthesized using a modified method reported by Camblor et al. [16]. In a typical synthesis, tetraethyl orthosilicate (TEOS; Wako Pure Chemical Industries Ltd.) was dissolved in an aqueous solution of N,N,N-trimethyl-1adamantammonium hydroxide (TMAdaOH; SACHEM, Inc.), and this solution was stirred overnight. Thereafter the solution was heated, and mixed vigorously by hand until a dry powder formed. Hydrofluoric acid (HF; Wako Pure Chemical Industries Ltd.) in an equivalent molar ratio to TMAdaOH was added and the pH of the mixture gel was adjusted to neutral. Furthermore, the gel was heated and the water evaporated until the desired water content was reached. In this study, the final molar composition of the precursor gel was: 1.0 SiO2 : 0.8 TMAdaOH : 0.8 HF : 5.7 H2O. The precursor gel of this molar composition is forming paste-like precursor gel. The liquid-like precursor of higher water contents allows uniform coating to the alumina support. However, it was preferred to be below H2O / Si = 6 in order to prevent contamination of the STTtype zeolite [17]. This precursor gel was transferred to a Teflon-lined stainless steel autoclave and placed into an oven at 423 K for 72 h. The product was filtered, washed with deionized water, and calcined at 853 K for 12 h. The Si-CHA seeds for the preparation of the membranes were prepared by ball-milling of the Si-CHA crystals for several hours. In this research, the average size of the Si-CHA seeds was ~200 nm.
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2.2 Synthesis procedure of Si-CHA membranes Si-CHA membranes were obtained by hydrothermal synthesis via seed secondary growth. First, α-alumina porous support tubes (outside diameter: 10 mm, average pore size: 100 nm; NGK Insulators, Ltd.) were cut into 10-cm-long pieces and washed several times with boiling deionized water and dried overnight at 393 K. Second, the outside surface of the support tube was coated with Si-CHA seeds using a rubbing method. The seed/water paste was prepared by mixing Si-CHA seeds with deionized water at the mass ratio of 1:10, which was rubbed on the support surface with finger for few minutes. Third, the secondary growth gel was daubed on the seeded support tube, and aged at 423 K for 72 h in a Teflon-lined stainless steel autoclave. The secondary growth gel was prepared in the same manner and composition as the precursor gel for CHA crystals. The product was washed with deionized water, and dried at 373 K overnight. Both ends of the as-synthesized membranes were coated by glass paste (AP5346, Asahi Glass Co., Ltd.), leaving a 5-cm middle portion of the tube. These membranes were calcined at 853 K for 12 h, employing heating and cooling rates of 0.5 K/min to remove organic components and to seal the membrane ends. The calcined membranes were stored at 373 K prior to separation measurements. A second synthesis method was adopted whereby Si-CHA crystals (0.01 wt% based on the total SiO2) were added to the secondary growth gel during TEOS addition, which acts as a crystalline nuclei. The average size of Si-CHA crystals for the nucleation was ~2 μm. The membrane samples obtained in the usual procedure without using the Si-CHA crystals was denoted as M1, and the membrane samples prepared using a small amount of SiCHA crystals as a silicon source was denoted as M2.
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2.3 Synthesis of SSZ-13 seeds and membranes The SSZ-13 crystals were synthesized using a modified procedure reported by Robson et al. [19]. The molar composition of the precursor gel was: 1.0 SiO2 : 0.10 Na2O : 0.025 Al2O3: 0.40 TMAdaOH: 44 H2O. The mixture of sodium hydroxide (NaOH; Wako Pure Chemical), sodium aluminate (NaAlO2; Wako Pure Chemical), TMAdaOH and deionized water was stirred at room temperature. After 1 h, Fumed silica (0.007 μm; Sigma-Aldrich) was added to the solution, and this mixture was aged overnight with stirring maintained. This precursor gel was transferred to a Teflon-lined stainless steel autoclave and placed into an oven at 433 K for 96 h. After hydrothermal synthesis, the obtained products were filtered, washed with deionized water, and calcined at 853 K for 10 h. The SSZ-13 crystals were ball-milled for several hours, and used as SSZ-13 seeds. The average size of the SSZ-13 seed used as a prerequisite reagent for the preparation of the membranes was ~200 nm. SSZ-13 membranes were prepared in the same manner as the Si-CHA membranes. The secondary growth gel employed the same composition as that of the SSZ-13 crystals. Hydrothermal synthesis was performed in an oven at 443 K for 48 h. The resulting membranes were washed with deionized water, and soaked in deionized water overnight prior to drying overnight at 373 K. The membranes were calcined in air at 753 K for 6 h with heating and cooling rates of 0.5 K/min. SSZ-13 membranes are denoted as M3.
2.4 Characterization and separation measurements X-ray diffraction (XRD) patterns were recorded on a RINT2000 X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation at 40 kV and 20 mA. Diffraction data were
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collected in the range of 2θ = 3–45° with 0.02° steps. Field emission scanning electron microscopy (FESEM) images of surface morphology and thickness were obtained using an SU9000 electron microscope (Hitachi High-Tech, Japan) equipped with an energydispersive X-ray spectroscopy (EDS) detector. Nitrogen adsorption isotherms were measured at liquid nitrogen temperature (77 K) using a volumetric adsorption analyzer ASAP2420 (Micromeritics, Japan), with samples being degassed at 473 K under vacuum. Brunauer–Emmett–Teller (BET) model surface area (SBET), total pore volume (Vtotal), and micropore volume (Vmicro) were calculated from the nitrogen isotherms. The BET equation was used to calculate the surface area from adsorption data obtained at p/p0 = 0.05–0.2. The total pore volume was calculated from the amount of nitrogen adsorbed at p/p0 = 0.99 and the micropore volume was calculated by the t-plot method. Single gas permeation measurements were performed using the following probe gases: H2, CO2, N2, CH4, SF6, at 313 K. The apparatus for the gas permeation tests is shown in Figure 1. The membranes were activated at 473 K under flowing N2 (100 cc/min) over 12 h prior to the permeation measurements. In all permeation measurements, the pressure drop was kept constant at 0.1 MPa and the permeation side was kept constant at atmospheric pressure. The probe gases were passed through the inside of the membrane to the outside. Gas permeances were calculated as the permeate flow rate divided by the pressure drop and membrane area. The stability tests in the presence of water vapor were performed using an H2O/Ar mixture containing 10% H2O at 423 K. The pressure drop was maintained at 0.1 MPa, and the water vapor was supplied from the outside of the membrane for 5 h. After water vapor exposure, the membranes were dried using dry Ar at 423 K (100 cc/min) over 12
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h, and the single gas permeances were measured. The degree of deterioration of the membrane was confirmed by observing the change of gas permeance.
Figure 1. Schematic illustration of the apparatus for the single gas permeation tests.
3. RESULTS AND DISCUSSION 3.1 Membrane preparation and single gas permeation It is known that defective membranes exhibit permeation properties based on the Knudsen diffusion mechanism. The permeance ratio of H2/SF6 is often used to affirm the presence of defects, with a H2/SF6 ratio of ~8.5 confirming Knudsen diffusion. Gas permeance of M1 and M2 membranes are shown in Figure 2. The permeation behavior based on Knudsen diffusion is marked by the line (―), which is calculated with reference to hydrogen permeance. The M1 membrane possessed permeation properties based on the Knudsen diffusion mechanism, indicating that this membrane type has defects, such as pinholes or cracks. Conversely, the M2 membrane showed good
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molecular sieve performance. The permeance ratio of H2/SF6 is over 200, indicating few defects were formed in the zeolite layer. The dependence of synthesis time on membrane performance was investigated. Gas permeance of Si-CHA membranes prepared as a function of hydrothermal synthesis time are shown in Figure 3. The SF6 permeance decreased with increasing synthesis time, indicating that the zeolite layer is densified by the secondary growth. The membrane prepared over a 24-h synthesis period showed permeation behavior based on the Knudsen diffusion mechanism with a H2/SF6 ratio of 9.2. Further increasing the synthesis time to 48 h significantly increases the H2/SF6 ratio to in excess of 100. The temperature dependence of the probe gas permeances through M2 membranes across the temperature range of 313–453 K is shown in Figure 4. The permeance of large molecules, such as CH4 and SF6 increased with increasing temperature, indicating that CH4 and SF6 molecules pass through M2 membranes based on the activated diffusion mechanism. The apparent activation energies of permeation were calculated by fitting the experimental gas permeance data to an Arrhenius equation. The temperature dependence of various gas permeances show good fits to the Arrhenius equation with activation energies of −1.4 kJ/mol for H2, −9.9 kJ/mol for CO2, −3.8 kJ/mol for N2, +2.8 kJ/mol for CH4 and +5.4 kJ/mol for SF6. Molecules larger than the eight-membered ring windows of Si-CHA (0.38 × 0.38 nm) showed positive activation energies.
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Figure 2. Gas permeation properties on Si-CHA membranes, (a) M1 and (b) M2. Permeation temperature is 313 K, pressure drop is 0.1 MPa.
Figure 3. Gas permeation properties on the Si-CHA membranes prepared as a function of synthesis time. Permeation temperature is 313 K, pressure drop is 0.1 MPa.
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Figure 4. Temperature dependence of gas permeance on M2 membranes (Arrhenius plot).
3.2 Morphologies and structures of Si-CHA membranes The membrane morphologies were investigated by FESEM. The top-view FESEM images for each membrane sample are shown in Figure 5. M1 membranes possessed a defective structure, the gaps between its crystals can be clearly observed. In contrast, M2 membranes formed after secondary growth are polycrystalline, composed of ~1 μm grains. Intergrowths between grains can be clearly observed, which implies tight grain boundaries. Defects, such as pinholes or cracks, were not found in the observed range. It is thought that heterogeneous growth occurs at the surface of the M2 membranes. The seeding method, whereby zeolite crystals are added in situ to the synthesis gel, is a typical method to reduce crystal size and crystallization time [20]. In this study, the presence of Si-CHA crystals in the secondary growth gel promoted the preferential growth of the Si-CHA seeded layer on the support surface. It is thought that the added Si-CHA crystals are broken down to their respective building units, such as cha and d6r
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units [16], during the secondary growth gel preparation and induced the rebuilding of the Si-CHA structure. In general, the seed material for nucleation is often added to basic precursor gels immediately prior to the hydrothermal synthesis. However, the secondary growth gel of the Si-CHA synthesis requires adjustment of the gel pH to neutral, which leads to more robust crystals for nucleation that retain structural integrity in neutral conditions. In this study, the seed crystals for nucleation were added before pH adjustment of the secondary growth gel to neutral. Incidentally, Zhang et al. have also reported the seeding effect on Si-CHA synthesis, and succeeded in the fast synthesis of sub-micron Si-CHA crystals (~500 nm) by the seeding method [21]. Although the pH of their precursor gel is also neutral, it is considered that the dissolution of the Si-CHA crystals for nucleation is promoted by a high synthesis temperature. The XRD patterns of M1 and M2 prepared as a function of synthesis time are shown in Figure 6 (a, b). The characteristic peaks of CHA-type zeolite and the α-alumina support are marked with open circles (○) and closed circles (●), respectively. M1 and M2 membranes display signs of crystallization at 12 h and 4 h, respectively. These patterns were in good agreement with CHA-type topology with no observed impurities, indicating that the membrane is in the pure form of CHA. The Si-CHA powders obtained from the membrane preparation of M1 and M2, which is the product recovered from the secondary growth gel, were also characterized. The XRD patterns of the M1 and M2 powders prepared as a function of synthesis time are shown in Figure 6 (c, d). The crystallization of M1 and M2 powders were confirmed to be slower than that of their respective M1 and M2 membranes. It is suggested that heterogeneous growth of the zeolite layer on the seeded support tube occurred, and the addition of seed Si-CHA crystals to the secondary growth gel increases the nuclei, generating dense crystal
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growth during the initial stage of the Si-CHA layer formation. M2 membrane growth was promoted, with nucleation and crystallization occurring over a shorter time frame and the resulting crystal growth resulting in membranes with few defects. Furthermore, the relative intensities of the typical peaks for each membrane were comparable with that of the typical peaks of their respective powders, indicating that the membranes have no preferred orientation.
Figure 5. Top-view field emission scanning electron microscopy (FESEM) images of (a) M1 and (b) M2 membranes. (Scale bar: 5 μm).
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Figure 6. X-ray diffraction patterns of Si-CHA membranes and powders prepared as a function of synthesis time, (a) M1 membranes, (b) M2 membranes, (c) M1 powders and (d) M2 powders. The characteristic peaks of CHA-type zeolite and the α-alumina support are marked with open circles (○) and closed circles (●), respectively.
3.3 Comparison with aluminosilicate CHA-type membranes SSZ-13 membranes were prepared as a reference to the Si-CHA membranes. As shown in Table 1, SSZ-13 membranes prepared in this work (M3) exhibited high H2 and CO2 permeance of 6.5 × 10−7 and 8.6 × 10−7 mol/m2sPa and the permeance ratio of H2/CH4 and CO2/CH4 gave values of 42 and 56, respectively. Conversely, Si-CHA membranes prepared in this work (M2) exhibited excellent H2 and CO2 permeance of 1.1 × 10−6 and 1.7 × 10−6 mol/m2sPa, which are higher than the other CHA-type membranes reported so far [8,11]. The permeance ratio of H2/CH4 and CO2/CH4 were 34 and 54, the molecular sieving effect can be observed clearly.
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Cross-sectional FESEM images and EDS images of M2 and M3 membranes are shown in Figure 7. The interface between the membrane and the support can be clearly identified. The thickness of the crystal layer on M2 and M3 membranes is approximately 6 μm and 5 μm, respectively. While the two membranes possess similar membrane thickness, major differences in gas permeance is observed. The excellent permeance of the M2 membrane relate to the extremely large pore volume of the zeolite layer. According to the EDS analysis, the crystal layer of the M2 membrane is mainly composed of the Si component, indicating that this membrane has almost no counter cations in their extra-frameworks. The Al content of the M2 membrane surface was below the EDS limit of detection. Conversely, the crystal layer of the M3 membrane is composed of Si, Al and Na components and the calculated Si/Al ratio of the top surface layer is ~5.0, which is lower than the Si/Al ratio of 20 in the secondary growth gel. The low Si/Al ratio of the M3 membrane may relate to elution of the α-alumina support because the pH of the secondary growth gel is strongly basic. The Si-CHA and SSZ-13 powders obtained from the M2 and M3 preparation, which is the product recovered from the secondary growth gel prepared for 72h, were also characterized by N2 and water adsorption measurements, as summarized in Table 2. N2 adsorption isotherms (77 K) on the obtained powder samples are shown in Figure 8 (a). The M2 powder exhibited typical type-I adsorption isotherms with a high level of adsorbate capacity, indicating good microporosity. The surface area is estimated to be 820 m2/g (SBET) and the micropore volume estimated to be 0.30 cc/g. These values are in good agreement with our previous study [17]. The M3 powder also exhibited typical type-I adsorption isotherms. The surface area is estimated to be 600 m2/g and the micropore volume estimated to be 0.22 cc/g. These values are in agreement with a recent report by
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Kosinov [22]. The micropore volume of the M2 powder is about 25% higher than that of M3 powder. Conversely, H2O adsorption isotherms (313 K) on the obtained powder samples are shown in Figure 8 (b). Despite the pore volume of the M2 powder being larger than that of the M3 powder, the capacity for water uptake is greatly reduced. The M2 powder was confirmed to possess a highly hydrophobic pore system because the framework solely consists of Si-O-Si bonds. As the pore structure of the membrane samples is the same, the influence of pore topology on water uptake capacity is minor compared with the composition of the framework, which changes the amphiphilic properties of the membrane, thus with the M2 membrane being composed of silica components only, the framework is highly hydrophobic, thereby the M2 membrane is expected to hinder water uptake compared with the M3 membrane. However, the water uptake on the M2 powder was higher than that of our previous report (3 mol/g) [18], indicating that there are some lattice defects in the zeolite framework.
Table 1. Permeation properties of M2 and M3. Permeance a Vapor exposure
Selectivity b -6
2
(× 10 mol/m sPa) H2 CO2 H2/CH4 CO2/CH4 Before 1.1 1.7 34 54 M2 After 1.1 1.7 33 52 Before 0.65 0.86 42 56 M3 After 0.52 0.61 32 38 a The pressure drop was maintained at 0.1 MPa and the permeation side was kept constant at atmospheric pressure. b
Permeance ratio of single gas.
Table 2. Characteristics of M2 and M3 powders.
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M2 powder a b
SBET a /m2 g-1 820
M3 powder 600 BET surface area.
Vmicro b /cc g-1 0.30
Vtotal c /cc g-1 0.32
qH2O d /mol/kg 5.1
0.22
0.25
13.0
Micropore volume calculated by t-plot method.
c
Total pore volume calculated as the amount of N2 adsorbed at a relative pressure of 0.99. d
Amount of H2O loading at saturated vapor pressure at 313 K.
Figure 7. Cross-sectional FESEM images and energy dispersive x-ray mappings, (a) M2 and (b) M3 membranes. (Scale bar: 5 μm).
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Figure 8. Adsorption isotherms of (a) N2 (77 K), (b) H2O (313 K) on M2 and M3 powders.
3.4. Stability tests in the presence of water vapor on Si-CHA membranes. Focusing on the fact that Si-CHA has a small water uptake capacity, the stability in the presence of water vapor on the M2 membrane was investigated. Gas permeance of M2 and M3 membranes before and after vapor exposure testing are shown in Table 1. After water vapor exposure, the permeance of H2 and CO2 on the M3 membrane changed to 5.2 × 10−7 and 6.1 × 10−7 mol/m2sPa, which is a ~30–40% reduction. Generally, the aluminosilicate zeolite membranes are known to have decreased gas permeation performance by water adsorption in their micropores. In particular, the low Si/Al aluminosilicate membrane can easily adsorb water in air. Once the water molecule has adsorbed, it is difficult to detach from the zeolite pores without heating at elevated temperatures. Therefore, the aluminosilicate membrane was applied only to the separation of dry gases. In fact, the gas permeation performance of the M3 membrane decreased upon water vapor exposure, suggesting that water molecules were trapped in the micropores. Conversely, no change in the gas permeance was observed for the M2
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membrane after exposure to water vapor. Because the Si-CHA zeolite possesses hydrophobic three-dimensional pore systems with elliptical cages that are accessible via eight-membered ring windows, the Si-CHA membranes are unlikely to present any pore blockage by water adsorption. Therefore, Si-CHA membranes should allow for easy handling, such as bare storage in open atmospheres. Additionally, the Si-CHA membrane is expected to exhibit high separation performance even in the presence of water. However, further fundamental investigations related to the presence of water are necessary to understand the permeation and separation properties on Si-CHA membranes.
4. CONCLUSIONS In this study, dense pure silica CHA-type zeolite (Si-CHA) membranes were synthesized by a hydrothermal secondary growth method. Our research is the first to employ Si-CHA membranes possessing effective molecular sieve performance for gas separation. The presence of Si-CHA seed crystals at low doping levels in the secondary growth gel allowed the formation of a defect-free Si-CHA layer. The building units formed in the secondary growth gel preparation may act as the crystalline nuclei, and the secondary growth of Si-CHA crystals was accelerated on the seeded support surface. Si-CHA membranes showed excellent gas permeance properties and water vapor stability because the Si-CHA zeolite possesses hydrophobic three-dimensional pore systems with elliptical cages that are accessible via eight-membered ring windows. The advantage of pure silica zeolite membranes, including Si-CHA, is their ability to demonstrate high gas permeability. Furthermore, there is a possibility to handle largescale gases and reduce the apparatus size.
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ACKNOWLEDGMENTS This work was partially supported by the Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (funding agency: JST).
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J. Zhang, X. Liu, M. Li, C. Liu, D. Hu, G. Zeng, Y. Zhang,Y. Sun, Fast
synthesis of sub-micron all-silica CHA zeolite particles with seeding method, RSC Adv. 5 (2015) 27087–27090 [22]
N. Kosinov, C. Auffret, G.J. Borghuis, V.G.P. Sripathi, E.J.M. Hensen,
Influence of the Si/Al ratio on the separation properties of SSZ-13 zeolite membranes, J. Membr. Sci. 484 (2015) 140–145.
Highlights
Pure silica CHA-type zeolite (Si-CHA) membranes were synthesized on porous αalumina supports
Si-CHA membranes prepared in this study showed an effective molecular sieve performance for gas separation
Si-CHA membranes exhibited excellent gas permeance derived from their large pore volume.
Si-CHA membranes were unlikely to present pore blockages by water adsorption because of their hydrophobic pore systems.
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PII: DOI: Reference:
S0376-7388(16)30852-3 http://dx.doi.org/10.1016/j.memsci.2016.09.002 MEMSCI14722
To appear in: Journal of Membrane Science Received date: 30 June 2016 Revised date: 31 August 2016 Accepted date: 1 September 2016 Cite this article as: Koji Kida, Yasushi Maeta and Katsunori Yogo, Preparation and gas permeation properties on pure silica CHA-type zeolite membranes, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.09.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and gas permeation properties on pure silica CHA-type zeolite membranes Koji Kidaa, Yasushi Maetab, Katsunori Yogoa,b* a
Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai,
Kizugawa, Kyoto 619-0292, Japan b
Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma,
Nara 630-0192, Japan *
Corresponding author: Tel.: +81 774 75 2305; fax: +81 774 75-2318. [email protected]
Abstract Pure silica CHA-type zeolite (Si-CHA) membranes were synthesized by a hydrothermal secondary growth method on porous α-alumina supports. Using Si-CHA seed crystals as a crystalline nuclei allowed the formation of a dense Si-CHA layer. The Si-CHA membranes prepared in this study showed excellent gas permeance derived from their large pore volume and effective molecular sieve performance for gas separation. The Si-CHA membranes exhibited excellent H2 and CO2 permeance of 1.1 × 10−6 and 1.7 × 10−6 mol/m2sPa, respectively. The permeance ratio of H2/CH4 and CO2/CH4 were 34 and 54, respectively. The stability test in the presence of water vapor
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on the Si-CHA membranes was performed and the Si-CHA membranes demonstrated good water vapor resistance, more so than the topologically analogous aluminosilicate zeolite membrane (SSZ-13). The Si-CHA membranes were unlikely to present pore blockages by water adsorption because of their hydrophobic pore systems. Graphical abstract
Keywords: Zeolite membrane, Pure silica zeolite, CHA-type zeolite, Gas permeation, High CO2 permeance
1. INTRODUCTION Zeolite membranes have been studied in the separation of liquid mixtures or gas mixtures. Various structural types of zeolite membranes, such as MFI [1–3], LTA [4,5], FAU [6,7], CHA [8–11], DDR [12,13] and MOR [14-15], have been reported, and the preparation technique and separation properties have been investigated. Recently, highsilica zeolite membranes have attracted significant attention for gas separation. Highsilica zeolite membranes allow high thermal stability and chemical resistance. The gas
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permeance of high-silica zeolite membranes tend to be higher than that of aluminosilicate zeolite membranes because their large pore volumes are advantageous for gas diffusion. Additionally, high-silica zeolite membranes are unlikely to present pore blockages by water adsorption. Kosinov et al. reported that the low polarity of high-silica SSZ-13 membranes gives beneficial CO2 separation properties under hydrothermal conditions [8]. Endowing zeolite membranes with a resistance to water vapor is an extremely important factor to operate in gas separation. For the above reasons, research in recent zeolite membrane development for gas separation has shifted to developing zeolite membranes of higher Si/Al ratio [8,9]. Pure silica zeolites that have only Si–O–Si bonds in their framework are different from common aluminosilicate zeolites. Because there are no framework aluminum atoms and concomitantly no extra-framework counter cations, this maximizes the ability of guest species to diffuse in their pores. Silicalite-1 has received the most attention in zeolite membrane research [2-3]. Silicalite-1 membranes have MFI-type topology and three-dimensional intermediate pores (0.51 × 0.55 nm and 0.53 × 0.56 nm). However, their large pores are unsuitable for gas separation of small molecules, such as H2/CH4 and CO2/CH4. While Silicalite-1 membranes have excellent gas permeation properties, low separation selectivity can be obtained. Si-DDR membranes are also typical pure silica zeolite membranes, and have been reported to possess excellent gas separation performance [12-13]. Si-DDR has eight-membered ring pores (0.36 × 0.44 nm), which are appropriate in size for CO2/CH4 separation. Tomita et al. reported Si-DDR membranes to have a CO2/CH4 separation selectivity of more than 100 [13]. However, Si-DDR membranes often demonstrate low gas permeance. Because the DDR-type zeolite has a two-dimensional pore structure, this decreases the diffusivity of gas
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molecules. These pure silica zeolite membranes reported hitherto show some problems in selectivity or permeability for gas separation. Therefore, it is considered that new types of pure silica zeolite membranes are required to address the separation of small molecules. Pure silica CHA-type zeolite (Si-CHA) membranes can be expected to show high gas permeance and high separation selectivity. Si-CHA possess elliptical cages of large pore volume derived from its low framework density [16] and three-dimensional pore systems via eight-membered ring windows (0.38 × 0.38 nm). To the best of our knowledge, Si-CHA membranes possessing effective molecular sieve performance for gas separation have not been reported so far. The preparation method of Si-CHA is considered the reason for the poor gas separation performance. The first preparation method toward Si-CHA zeolite was first reported by Camblor et al.[16]; however, their synthesis method requires cumbersome procedures and precise composition adjustment of the precursor gel. In particular, Si-CHA was only formed from the solid-like precursor gel with a small quantity of water. Therefore, Si-CHA is one of the most difficult zeolites to form membrane structures. Our group has embarked on the development of Si-CHA zeolites, and to date, the simple synthetic procedure [17] and the CO2 adsorption properties [18] of Si-CHA crystals have been investigated. Here we report the preparation techniques and single gas permeation properties of SiCHA zeolite membranes. Si-CHA membranes were synthesized by a hydrothermal secondary growth method on porous α-alumina supports. We found that the use of SiCHA crystals as a nucleation agent allowed the formation of a dense Si-CHA layer. The various gas permeation properties of the Si-CHA membranes were studied. Additionally,
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the stability test in the presence of water vapor on Si-CHA membranes and the topologically analogous aluminosilicate zeolite membrane (SSZ-13) were performed.
2. EXPERIMENTAL 2.1 Synthesis procedure of Si-CHA crystals and seeds Si-CHA crystals were synthesized using a modified method reported by Camblor et al. [16]. In a typical synthesis, tetraethyl orthosilicate (TEOS; Wako Pure Chemical Industries Ltd.) was dissolved in an aqueous solution of N,N,N-trimethyl-1adamantammonium hydroxide (TMAdaOH; SACHEM, Inc.), and this solution was stirred overnight. Thereafter the solution was heated, and mixed vigorously by hand until a dry powder formed. Hydrofluoric acid (HF; Wako Pure Chemical Industries Ltd.) in an equivalent molar ratio to TMAdaOH was added and the pH of the mixture gel was adjusted to neutral. Furthermore, the gel was heated and the water evaporated until the desired water content was reached. In this study, the final molar composition of the precursor gel was: 1.0 SiO2 : 0.8 TMAdaOH : 0.8 HF : 5.7 H2O. The precursor gel of this molar composition is forming paste-like precursor gel. The liquid-like precursor of higher water contents allows uniform coating to the alumina support. However, it was preferred to be below H2O / Si = 6 in order to prevent contamination of the STTtype zeolite [17]. This precursor gel was transferred to a Teflon-lined stainless steel autoclave and placed into an oven at 423 K for 72 h. The product was filtered, washed with deionized water, and calcined at 853 K for 12 h. The Si-CHA seeds for the preparation of the membranes were prepared by ball-milling of the Si-CHA crystals for several hours. In this research, the average size of the Si-CHA seeds was ~200 nm.
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2.2 Synthesis procedure of Si-CHA membranes Si-CHA membranes were obtained by hydrothermal synthesis via seed secondary growth. First, α-alumina porous support tubes (outside diameter: 10 mm, average pore size: 100 nm; NGK Insulators, Ltd.) were cut into 10-cm-long pieces and washed several times with boiling deionized water and dried overnight at 393 K. Second, the outside surface of the support tube was coated with Si-CHA seeds using a rubbing method. The seed/water paste was prepared by mixing Si-CHA seeds with deionized water at the mass ratio of 1:10, which was rubbed on the support surface with finger for few minutes. Third, the secondary growth gel was daubed on the seeded support tube, and aged at 423 K for 72 h in a Teflon-lined stainless steel autoclave. The secondary growth gel was prepared in the same manner and composition as the precursor gel for CHA crystals. The product was washed with deionized water, and dried at 373 K overnight. Both ends of the as-synthesized membranes were coated by glass paste (AP5346, Asahi Glass Co., Ltd.), leaving a 5-cm middle portion of the tube. These membranes were calcined at 853 K for 12 h, employing heating and cooling rates of 0.5 K/min to remove organic components and to seal the membrane ends. The calcined membranes were stored at 373 K prior to separation measurements. A second synthesis method was adopted whereby Si-CHA crystals (0.01 wt% based on the total SiO2) were added to the secondary growth gel during TEOS addition, which acts as a crystalline nuclei. The average size of Si-CHA crystals for the nucleation was ~2 μm. The membrane samples obtained in the usual procedure without using the Si-CHA crystals was denoted as M1, and the membrane samples prepared using a small amount of SiCHA crystals as a silicon source was denoted as M2.
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2.3 Synthesis of SSZ-13 seeds and membranes The SSZ-13 crystals were synthesized using a modified procedure reported by Robson et al. [19]. The molar composition of the precursor gel was: 1.0 SiO2 : 0.10 Na2O : 0.025 Al2O3: 0.40 TMAdaOH: 44 H2O. The mixture of sodium hydroxide (NaOH; Wako Pure Chemical), sodium aluminate (NaAlO2; Wako Pure Chemical), TMAdaOH and deionized water was stirred at room temperature. After 1 h, Fumed silica (0.007 μm; Sigma-Aldrich) was added to the solution, and this mixture was aged overnight with stirring maintained. This precursor gel was transferred to a Teflon-lined stainless steel autoclave and placed into an oven at 433 K for 96 h. After hydrothermal synthesis, the obtained products were filtered, washed with deionized water, and calcined at 853 K for 10 h. The SSZ-13 crystals were ball-milled for several hours, and used as SSZ-13 seeds. The average size of the SSZ-13 seed used as a prerequisite reagent for the preparation of the membranes was ~200 nm. SSZ-13 membranes were prepared in the same manner as the Si-CHA membranes. The secondary growth gel employed the same composition as that of the SSZ-13 crystals. Hydrothermal synthesis was performed in an oven at 443 K for 48 h. The resulting membranes were washed with deionized water, and soaked in deionized water overnight prior to drying overnight at 373 K. The membranes were calcined in air at 753 K for 6 h with heating and cooling rates of 0.5 K/min. SSZ-13 membranes are denoted as M3.
2.4 Characterization and separation measurements X-ray diffraction (XRD) patterns were recorded on a RINT2000 X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation at 40 kV and 20 mA. Diffraction data were
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collected in the range of 2θ = 3–45° with 0.02° steps. Field emission scanning electron microscopy (FESEM) images of surface morphology and thickness were obtained using an SU9000 electron microscope (Hitachi High-Tech, Japan) equipped with an energydispersive X-ray spectroscopy (EDS) detector. Nitrogen adsorption isotherms were measured at liquid nitrogen temperature (77 K) using a volumetric adsorption analyzer ASAP2420 (Micromeritics, Japan), with samples being degassed at 473 K under vacuum. Brunauer–Emmett–Teller (BET) model surface area (SBET), total pore volume (Vtotal), and micropore volume (Vmicro) were calculated from the nitrogen isotherms. The BET equation was used to calculate the surface area from adsorption data obtained at p/p0 = 0.05–0.2. The total pore volume was calculated from the amount of nitrogen adsorbed at p/p0 = 0.99 and the micropore volume was calculated by the t-plot method. Single gas permeation measurements were performed using the following probe gases: H2, CO2, N2, CH4, SF6, at 313 K. The apparatus for the gas permeation tests is shown in Figure 1. The membranes were activated at 473 K under flowing N2 (100 cc/min) over 12 h prior to the permeation measurements. In all permeation measurements, the pressure drop was kept constant at 0.1 MPa and the permeation side was kept constant at atmospheric pressure. The probe gases were passed through the inside of the membrane to the outside. Gas permeances were calculated as the permeate flow rate divided by the pressure drop and membrane area. The stability tests in the presence of water vapor were performed using an H2O/Ar mixture containing 10% H2O at 423 K. The pressure drop was maintained at 0.1 MPa, and the water vapor was supplied from the outside of the membrane for 5 h. After water vapor exposure, the membranes were dried using dry Ar at 423 K (100 cc/min) over 12
8
h, and the single gas permeances were measured. The degree of deterioration of the membrane was confirmed by observing the change of gas permeance.
Figure 1. Schematic illustration of the apparatus for the single gas permeation tests.
3. RESULTS AND DISCUSSION 3.1 Membrane preparation and single gas permeation It is known that defective membranes exhibit permeation properties based on the Knudsen diffusion mechanism. The permeance ratio of H2/SF6 is often used to affirm the presence of defects, with a H2/SF6 ratio of ~8.5 confirming Knudsen diffusion. Gas permeance of M1 and M2 membranes are shown in Figure 2. The permeation behavior based on Knudsen diffusion is marked by the line (―), which is calculated with reference to hydrogen permeance. The M1 membrane possessed permeation properties based on the Knudsen diffusion mechanism, indicating that this membrane type has defects, such as pinholes or cracks. Conversely, the M2 membrane showed good
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molecular sieve performance. The permeance ratio of H2/SF6 is over 200, indicating few defects were formed in the zeolite layer. The dependence of synthesis time on membrane performance was investigated. Gas permeance of Si-CHA membranes prepared as a function of hydrothermal synthesis time are shown in Figure 3. The SF6 permeance decreased with increasing synthesis time, indicating that the zeolite layer is densified by the secondary growth. The membrane prepared over a 24-h synthesis period showed permeation behavior based on the Knudsen diffusion mechanism with a H2/SF6 ratio of 9.2. Further increasing the synthesis time to 48 h significantly increases the H2/SF6 ratio to in excess of 100. The temperature dependence of the probe gas permeances through M2 membranes across the temperature range of 313–453 K is shown in Figure 4. The permeance of large molecules, such as CH4 and SF6 increased with increasing temperature, indicating that CH4 and SF6 molecules pass through M2 membranes based on the activated diffusion mechanism. The apparent activation energies of permeation were calculated by fitting the experimental gas permeance data to an Arrhenius equation. The temperature dependence of various gas permeances show good fits to the Arrhenius equation with activation energies of −1.4 kJ/mol for H2, −9.9 kJ/mol for CO2, −3.8 kJ/mol for N2, +2.8 kJ/mol for CH4 and +5.4 kJ/mol for SF6. Molecules larger than the eight-membered ring windows of Si-CHA (0.38 × 0.38 nm) showed positive activation energies.
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Figure 2. Gas permeation properties on Si-CHA membranes, (a) M1 and (b) M2. Permeation temperature is 313 K, pressure drop is 0.1 MPa.
Figure 3. Gas permeation properties on the Si-CHA membranes prepared as a function of synthesis time. Permeation temperature is 313 K, pressure drop is 0.1 MPa.
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Figure 4. Temperature dependence of gas permeance on M2 membranes (Arrhenius plot).
3.2 Morphologies and structures of Si-CHA membranes The membrane morphologies were investigated by FESEM. The top-view FESEM images for each membrane sample are shown in Figure 5. M1 membranes possessed a defective structure, the gaps between its crystals can be clearly observed. In contrast, M2 membranes formed after secondary growth are polycrystalline, composed of ~1 μm grains. Intergrowths between grains can be clearly observed, which implies tight grain boundaries. Defects, such as pinholes or cracks, were not found in the observed range. It is thought that heterogeneous growth occurs at the surface of the M2 membranes. The seeding method, whereby zeolite crystals are added in situ to the synthesis gel, is a typical method to reduce crystal size and crystallization time [20]. In this study, the presence of Si-CHA crystals in the secondary growth gel promoted the preferential growth of the Si-CHA seeded layer on the support surface. It is thought that the added Si-CHA crystals are broken down to their respective building units, such as cha and d6r
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units [16], during the secondary growth gel preparation and induced the rebuilding of the Si-CHA structure. In general, the seed material for nucleation is often added to basic precursor gels immediately prior to the hydrothermal synthesis. However, the secondary growth gel of the Si-CHA synthesis requires adjustment of the gel pH to neutral, which leads to more robust crystals for nucleation that retain structural integrity in neutral conditions. In this study, the seed crystals for nucleation were added before pH adjustment of the secondary growth gel to neutral. Incidentally, Zhang et al. have also reported the seeding effect on Si-CHA synthesis, and succeeded in the fast synthesis of sub-micron Si-CHA crystals (~500 nm) by the seeding method [21]. Although the pH of their precursor gel is also neutral, it is considered that the dissolution of the Si-CHA crystals for nucleation is promoted by a high synthesis temperature. The XRD patterns of M1 and M2 prepared as a function of synthesis time are shown in Figure 6 (a, b). The characteristic peaks of CHA-type zeolite and the α-alumina support are marked with open circles (○) and closed circles (●), respectively. M1 and M2 membranes display signs of crystallization at 12 h and 4 h, respectively. These patterns were in good agreement with CHA-type topology with no observed impurities, indicating that the membrane is in the pure form of CHA. The Si-CHA powders obtained from the membrane preparation of M1 and M2, which is the product recovered from the secondary growth gel, were also characterized. The XRD patterns of the M1 and M2 powders prepared as a function of synthesis time are shown in Figure 6 (c, d). The crystallization of M1 and M2 powders were confirmed to be slower than that of their respective M1 and M2 membranes. It is suggested that heterogeneous growth of the zeolite layer on the seeded support tube occurred, and the addition of seed Si-CHA crystals to the secondary growth gel increases the nuclei, generating dense crystal
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growth during the initial stage of the Si-CHA layer formation. M2 membrane growth was promoted, with nucleation and crystallization occurring over a shorter time frame and the resulting crystal growth resulting in membranes with few defects. Furthermore, the relative intensities of the typical peaks for each membrane were comparable with that of the typical peaks of their respective powders, indicating that the membranes have no preferred orientation.
Figure 5. Top-view field emission scanning electron microscopy (FESEM) images of (a) M1 and (b) M2 membranes. (Scale bar: 5 μm).
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Figure 6. X-ray diffraction patterns of Si-CHA membranes and powders prepared as a function of synthesis time, (a) M1 membranes, (b) M2 membranes, (c) M1 powders and (d) M2 powders. The characteristic peaks of CHA-type zeolite and the α-alumina support are marked with open circles (○) and closed circles (●), respectively.
3.3 Comparison with aluminosilicate CHA-type membranes SSZ-13 membranes were prepared as a reference to the Si-CHA membranes. As shown in Table 1, SSZ-13 membranes prepared in this work (M3) exhibited high H2 and CO2 permeance of 6.5 × 10−7 and 8.6 × 10−7 mol/m2sPa and the permeance ratio of H2/CH4 and CO2/CH4 gave values of 42 and 56, respectively. Conversely, Si-CHA membranes prepared in this work (M2) exhibited excellent H2 and CO2 permeance of 1.1 × 10−6 and 1.7 × 10−6 mol/m2sPa, which are higher than the other CHA-type membranes reported so far [8,11]. The permeance ratio of H2/CH4 and CO2/CH4 were 34 and 54, the molecular sieving effect can be observed clearly.
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Cross-sectional FESEM images and EDS images of M2 and M3 membranes are shown in Figure 7. The interface between the membrane and the support can be clearly identified. The thickness of the crystal layer on M2 and M3 membranes is approximately 6 μm and 5 μm, respectively. While the two membranes possess similar membrane thickness, major differences in gas permeance is observed. The excellent permeance of the M2 membrane relate to the extremely large pore volume of the zeolite layer. According to the EDS analysis, the crystal layer of the M2 membrane is mainly composed of the Si component, indicating that this membrane has almost no counter cations in their extra-frameworks. The Al content of the M2 membrane surface was below the EDS limit of detection. Conversely, the crystal layer of the M3 membrane is composed of Si, Al and Na components and the calculated Si/Al ratio of the top surface layer is ~5.0, which is lower than the Si/Al ratio of 20 in the secondary growth gel. The low Si/Al ratio of the M3 membrane may relate to elution of the α-alumina support because the pH of the secondary growth gel is strongly basic. The Si-CHA and SSZ-13 powders obtained from the M2 and M3 preparation, which is the product recovered from the secondary growth gel prepared for 72h, were also characterized by N2 and water adsorption measurements, as summarized in Table 2. N2 adsorption isotherms (77 K) on the obtained powder samples are shown in Figure 8 (a). The M2 powder exhibited typical type-I adsorption isotherms with a high level of adsorbate capacity, indicating good microporosity. The surface area is estimated to be 820 m2/g (SBET) and the micropore volume estimated to be 0.30 cc/g. These values are in good agreement with our previous study [17]. The M3 powder also exhibited typical type-I adsorption isotherms. The surface area is estimated to be 600 m2/g and the micropore volume estimated to be 0.22 cc/g. These values are in agreement with a recent report by
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Kosinov [22]. The micropore volume of the M2 powder is about 25% higher than that of M3 powder. Conversely, H2O adsorption isotherms (313 K) on the obtained powder samples are shown in Figure 8 (b). Despite the pore volume of the M2 powder being larger than that of the M3 powder, the capacity for water uptake is greatly reduced. The M2 powder was confirmed to possess a highly hydrophobic pore system because the framework solely consists of Si-O-Si bonds. As the pore structure of the membrane samples is the same, the influence of pore topology on water uptake capacity is minor compared with the composition of the framework, which changes the amphiphilic properties of the membrane, thus with the M2 membrane being composed of silica components only, the framework is highly hydrophobic, thereby the M2 membrane is expected to hinder water uptake compared with the M3 membrane. However, the water uptake on the M2 powder was higher than that of our previous report (3 mol/g) [18], indicating that there are some lattice defects in the zeolite framework.
Table 1. Permeation properties of M2 and M3. Permeance a Vapor exposure
Selectivity b -6
2
(× 10 mol/m sPa) H2 CO2 H2/CH4 CO2/CH4 Before 1.1 1.7 34 54 M2 After 1.1 1.7 33 52 Before 0.65 0.86 42 56 M3 After 0.52 0.61 32 38 a The pressure drop was maintained at 0.1 MPa and the permeation side was kept constant at atmospheric pressure. b
Permeance ratio of single gas.
Table 2. Characteristics of M2 and M3 powders.
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M2 powder a b
SBET a /m2 g-1 820
M3 powder 600 BET surface area.
Vmicro b /cc g-1 0.30
Vtotal c /cc g-1 0.32
qH2O d /mol/kg 5.1
0.22
0.25
13.0
Micropore volume calculated by t-plot method.
c
Total pore volume calculated as the amount of N2 adsorbed at a relative pressure of 0.99. d
Amount of H2O loading at saturated vapor pressure at 313 K.
Figure 7. Cross-sectional FESEM images and energy dispersive x-ray mappings, (a) M2 and (b) M3 membranes. (Scale bar: 5 μm).
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Figure 8. Adsorption isotherms of (a) N2 (77 K), (b) H2O (313 K) on M2 and M3 powders.
3.4. Stability tests in the presence of water vapor on Si-CHA membranes. Focusing on the fact that Si-CHA has a small water uptake capacity, the stability in the presence of water vapor on the M2 membrane was investigated. Gas permeance of M2 and M3 membranes before and after vapor exposure testing are shown in Table 1. After water vapor exposure, the permeance of H2 and CO2 on the M3 membrane changed to 5.2 × 10−7 and 6.1 × 10−7 mol/m2sPa, which is a ~30–40% reduction. Generally, the aluminosilicate zeolite membranes are known to have decreased gas permeation performance by water adsorption in their micropores. In particular, the low Si/Al aluminosilicate membrane can easily adsorb water in air. Once the water molecule has adsorbed, it is difficult to detach from the zeolite pores without heating at elevated temperatures. Therefore, the aluminosilicate membrane was applied only to the separation of dry gases. In fact, the gas permeation performance of the M3 membrane decreased upon water vapor exposure, suggesting that water molecules were trapped in the micropores. Conversely, no change in the gas permeance was observed for the M2
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membrane after exposure to water vapor. Because the Si-CHA zeolite possesses hydrophobic three-dimensional pore systems with elliptical cages that are accessible via eight-membered ring windows, the Si-CHA membranes are unlikely to present any pore blockage by water adsorption. Therefore, Si-CHA membranes should allow for easy handling, such as bare storage in open atmospheres. Additionally, the Si-CHA membrane is expected to exhibit high separation performance even in the presence of water. However, further fundamental investigations related to the presence of water are necessary to understand the permeation and separation properties on Si-CHA membranes.
4. CONCLUSIONS In this study, dense pure silica CHA-type zeolite (Si-CHA) membranes were synthesized by a hydrothermal secondary growth method. Our research is the first to employ Si-CHA membranes possessing effective molecular sieve performance for gas separation. The presence of Si-CHA seed crystals at low doping levels in the secondary growth gel allowed the formation of a defect-free Si-CHA layer. The building units formed in the secondary growth gel preparation may act as the crystalline nuclei, and the secondary growth of Si-CHA crystals was accelerated on the seeded support surface. Si-CHA membranes showed excellent gas permeance properties and water vapor stability because the Si-CHA zeolite possesses hydrophobic three-dimensional pore systems with elliptical cages that are accessible via eight-membered ring windows. The advantage of pure silica zeolite membranes, including Si-CHA, is their ability to demonstrate high gas permeability. Furthermore, there is a possibility to handle largescale gases and reduce the apparatus size.
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ACKNOWLEDGMENTS This work was partially supported by the Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (funding agency: JST).
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Highlights
Pure silica CHA-type zeolite (Si-CHA) membranes were synthesized on porous αalumina supports
Si-CHA membranes prepared in this study showed an effective molecular sieve performance for gas separation
Si-CHA membranes exhibited excellent gas permeance derived from their large pore volume.
Si-CHA membranes were unlikely to present pore blockages by water adsorption because of their hydrophobic pore systems.
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