CHA ZEOLITE MEMBRANE AND METHOD OF PREPARING THE SAME
20230001360 · 2023-01-05
Assignee
Inventors
Cpc classification
B01D2323/08
PERFORMING OPERATIONS; TRANSPORTING
B01D53/228
PERFORMING OPERATIONS; TRANSPORTING
B01D2323/12
PERFORMING OPERATIONS; TRANSPORTING
B01D2323/24
PERFORMING OPERATIONS; TRANSPORTING
Y02C20/40
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
B01D69/10
PERFORMING OPERATIONS; TRANSPORTING
B01D67/0051
PERFORMING OPERATIONS; TRANSPORTING
International classification
Abstract
Disclosed are a CHA zeolite membrane and a method of preparing the same, and more particularly, a CHA zeolite membrane having high capacity to separate CO.sub.2/N.sub.2 and CO.sub.2/CH.sub.4 even under wet conditions using a membrane produced using a synthetic precursor having a controlled ratio of Si and Al, a method of preparing the same, and a method of capturing and removing carbon dioxide using the membrane.
Claims
1. A method of preparing a CHA zeolite membrane comprising: adding a support including a CHA particle seed layer deposited thereon into a synthetic solution for growing zeolite containing an organic structure-directing agent and a Si source, without containing an Al source; and performing hydrothermal synthesis.
2. The method of preparing a CHA zeolite membrane of claim 1, wherein the support is selected from the group consisting of alumina, polypropylene, polyethylene, polytetrafluoroethylene, polysulfone, polyimide, silica, glass gamma-alumina, mullite, zirconia, titania, yttria, ceria, vanadia, silicon, stainless steel and carbon.
3. The method of preparing a CHA zeolite membrane of claim 1, wherein the organic structure-directing agent comprises one or more selected from the group consisting of TMAdaOH (N,N,N-trimethyl adamantylammonium hydroxide), TMAdaBr (N,N,N-trimethyl adamantylammonium bromide), TMAdaF (N,N,N-trimethyl adamantylammonium fluoride), TMAdaCl (N,N,N-trimethyl adamantylammonium chloride), TMAdaI (N,N,N-trimethyl adamantylammonium iodide), TEAOH (tetraethylammonium hydroxide), TEABr (tetraethylammonium bromide), TEAF (tetraethylammonium fluoride), TEACl (tetraethylammonium chloride), TEAI (tetraethylammonium iodide), dipropylamine and cyclohexylamine.
4. The method of preparing a CHA zeolite membrane of claim 1, wherein the synthetic solution for growing zeolite has a molar ratio of TMAdaOH: SiO.sub.2: H.sub.2O: NaOH: Al(OH).sub.3 of 1 to 100: 100: 1000 to 20000: 1 to 100: 0 to 20.
5. The method of preparing a CHA zeolite membrane of claim 1, wherein the hydrothermal synthesis is performed at a temperature of 100 to 200° C. for 12 to 240 hours.
6. A CHA zeolite membrane prepared by the method of claim 1, wherein the CHA zeolite membrane has a porosity of a defect of 0.1 to 1% and a z-axis tortuosity (tortuosity in a thickness direction of the membrane) of the defect of 1 to 2.
7. The CHA zeolite membrane of claim 6, wherein the CHA zeolite membrane has a separation factor of CO.sub.2/N.sub.2 in the flue gas separation process containing water, of 10 to 30 and a separation factor of CO.sub.2/CH.sub.4 in the upgrading process of biogas containing water, of 10 to 80.
8. A method of separating CO.sub.2 from a mixture containing CO.sub.2 and a molecule selected from the group consisting of CH.sub.4, N.sub.2, O.sub.2, C.sub.2H.sub.4, C.sub.2H.sub.6, C.sub.3H.sub.6 and C.sub.3H.sub.8 using the CHA zeolite membrane of claim 6.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The patent or application file contains a least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
[0020] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
[0021]
[0022]
[0023]
[0024]
[0025]
[0026]
[0027]
[0028]
[0029]
[0030]
[0031]
[0032]
[0033]
[0034]
[0035]
[0036]
[0037]
[0038]
BEST MODE FOR CARRYING OUT THE INVENTION
[0039] Unless defined otherwise, all technical and scientific terms used herein have the same meanings as appreciated by those skilled in the field to which the present invention pertains. In general, the nomenclature used herein is well-known in the art and is ordinarily used.
[0040] In the production of a CHA zeolite membrane, it was found that, when a membrane is fabricated using a synthetic precursor having a controlled ratio of Si and Al, the zeolite membrane has optimized hydrophobicity and defect formation and thus high separation performance of CO.sub.2/N.sub.2 and CO.sub.2/CH.sub.4 even under wet conditions, unlike conventional zeolite membranes.
[0041] Therefore, in one aspect, the present invention provides a method of preparing a CHA zeolite membrane including adding a support including a CHA particle seed layer deposited thereon to a synthetic solution for growing zeolite containing an organic structure-directing agent, and Si and Al sources, and performing hydrothermal synthesis, wherein a molar ratio of Si/Al in the synthetic solution for growing zeolite is 5 or more.
[0042] Therefore, in another aspect, the present invention provides a method of preparing a CHA zeolite membrane including adding a support including a CHA particle seed layer deposited thereon to a synthetic solution for growing zeolite containing an organic structure-directing agent, and a Si source, without containing an Al source, and performing hydrothermal synthesis.
[0043] In the present invention, the molar ratio of Si/Al may be 5 or more, preferably 5 to 1,000, more preferably 10 to 1000, 20 to 1,000, 10 to 500, and most preferably 20 to 200.
[0044] In the present invention, the properties of zeolite are changed depending on the Si/Al ratio of the synthetic solution. Thus, similarly, zeolites having a CHA structure are affected by the Si/Al ratio of the synthetic solution. CHA particles with a low Si/Al ratio are hydrophilic and have many defects, whereas CHA particles with a high Si/Al ratio are hydrophobic and have fewer defects. In the present invention, the hydrophobicity and defect formation of the membrane can be optimized by controlling the Si/Al ratio when synthesizing the CHA zeolite membrane based on this principle. This results in the production of a CHA zeolite membrane having optimal Si/Al content providing excellent CO.sub.2 separation performance under wet conditions.
[0045] Since the Si/Al ratio of the membrane affects not only the hydrophobicity of the membrane but also the formation of the defect structure, it is important to find a Si/Al ratio optimal for CO.sub.2 separation in a wet condition. In the present invention, CHA zeolite membranes with various Si/Al ratios are produced, and the CO.sub.2/N.sub.2 and CO.sub.2/CH.sub.4 separation performance of each membrane under wet conditions are identified. As a result, the conditions for the synthesis of CHA zeolite membranes that exhibit the best performance under wet conditions were found.
[0046] As the temperature increases, adsorption of water on the membrane decreases. For this reason, the high-temperature wet condition is relatively less affected by water than the low-temperature wet condition. Therefore, a CHA zeolite membrane having optimum temperature-dependent performance is present. This is determined by the Si/Al ratio, which is directly related to the hydrophobicity and defect formation of the membrane. For this reason, it is important to find the optimum Si/Al ratio depending on the temperature. In the present invention, a membrane having an optimized Si/Al ratio depending on the process temperature was produced, so that a CHA zeolite membrane specialized for the temperature of each process may be applied.
[0047] In the present invention, the support may include at least one selected from the group consisting of alumina, polypropylene, polyethylene, polytetrafluoroethylene, polysulfone, polyimide, silica, glass gamma-alumina, mullite, zirconia, titania, yttria, ceria, vanadia, silicon, stainless steel and carbon.
[0048] In the present invention, the organic structure-directing agent may include one or more selected from the group consisting of TMAdaOH (N,N,N-trimethyl adamantylammonium hydroxide), TMAdaBr (N,N,N-trimethyl adamantylammonium bromide), TMAdaF (N,N,N-trimethyl adamantylammonium fluoride), TMAdaCl (N,N,N-trimethyl adamantylammonium chloride), TMAdaI (N,N,N-trimethyl adamantylammonium iodide), TEAOH (tetraethylammonium hydroxide), TEABr (tetraethylammonium bromide), TEAF (tetraethylammonium fluoride), TEACl (tetraethylammonium chloride), TEAI (tetraethylammonium iodide), dipropylamine and cyclohexylamine.
[0049] In the present invention, the synthetic solution for growing zeolite growth may have a molar ratio of TMAdaOH: SiO.sub.2: H.sub.2O: NaOH: Al(OH).sub.3 of 1 to 100: 100: 1000 to 20000: 1 to 100: 0 to 20.
[0050] In the present invention, hydrothermal synthesis may be carried out at a temperature of 100 to 200 ° C. for 12 to 240 hours.
[0051] Typical CHA zeolites are promising for separating CO.sub.2 from large molecules such as N.sub.2 (associated with carbon capture after combustion) and CH.sub.4 (associated with natural gas/biogas upgrading). In particular, the pore size of CHA zeolites (0.37×0.42 nm.sup.2) can distinguish size differences between small molecules such as CO.sub.2 (0.33 nm) and larger N.sub.2 (0.364 nm) or CH.sub.4 (0.38 nm) molecules, and thus can separate CO.sub.2 through CHA membranes. In addition, the siliceous component of the CHA zeolite can reduce the adsorption capacity of smaller H.sub.2O molecules (0.265 nm), thus reducing the H.sub.2O permeation rate. This is highly preferable in ensuring excellent capacity of molecular sieves with selective CO.sub.2 permeance in the presence of water vapor. In fact, siliceous CHA films with a nominal Si/Al ratio of 100 (CHA_100) exhibited high CO.sub.2/N.sub.2 and CO.sub.2/CH.sub.4 separation performance, especially in the presence of water vapor (CO.sub.2/N.sub.2 separation factor (SF) of ˜13.4 and CO.sub.2/CH.sub.4 separation factor (SF) of ˜37 at 30° C.). These SFs were higher than the corresponding values (CO.sub.2/CH.sub.4 SFs of ˜5.2 and CO.sub.2/CH.sub.4 SFs of ˜31) under dry conditions. Such improvement could be ascribed to defect blocking by physisorbed water molecules. Preferably, under wet conditions, the siliceous, hydrophobic CHA membrane (CHA_100) could provide the high CO.sub.2 permeance of ˜1.1−1.4×10.sup.−7 mol.Math.m.sup.−2.Math.s.sup.−1.Math.Pa.sup.−1, which was as high as ˜14 to 19% of that through bare supports. Finally, the contribution of molecular transport through zeolitic and non-zeolitic parts was quantitatively analyzed by combining information extracted from image processing of fluorescence confocal optical microscopy images with a 1-dimensional permeation model. The result showed that ˜19% and ˜20% of the total CO.sub.2 permeance for CHA_100 were reduced due to transport inhibition by the physisorbed water molecules on the membrane surface and defects, respectively.
[0052] In the present invention, a secondary growth method was adopted for the fabrication of siliceous CHA membranes. First, a uniform CHA seed layer, which was mainly composed of plate-like CHA particles, was formed on a porous α-Al.sub.2O.sub.3 disc via a selective deposition method (Kim, E. et al., Angew. Chem., Int. Ed. 2013, 52, 5280-5284). The seed layer was further hydrothermally inter-grown to form a continuous, siliceous CHA membrane. For secondary growth, three different nominal Si/Al ratios of 20, 100, and infinity in the synthetic precursor were used. This approach was used to study the effect of the Si/Al ratio on the membrane properties systematically and, thus, the separation performances. Among the three, the most siliceous and thus most hydrophobic CHA membrane showed much improved separation performance for both CO.sub.2/N.sub.2 and CO.sub.2/CH.sub.4 mixtures under wet conditions compared to that under dry conditions. In addition, fluorescence confocal optical microscopy (FCOM) revealed that the CHA membranes showed distinctive features in the defect (crack) structure depending on the Si/Al ratios in the synthetic precursors. Image processing of the FCOM images provides correlation between the defect structure and the separation performance quantitatively. Furthermore, this relationship was used to elucidate the effect of the hydrophobicity and defect structure of the CHA membrane on the CO.sub.2 permselectivities in wet feeds. Finally, the CO.sub.2 permselectivity of the CHA membranes in the presence of H.sub.2O vapor in the feed was compared with those of CHA and other types of zeolite membranes in the literature, and CHA membranes have excellent performance under both dry and wet conditions were found.
[0053] In the present invention, it was found that the CHA zeolite membrane prepared by the method described above can provide and maintain a high CO.sub.2 separation capacity even in a feed condition containing water, regardless of the temperature or pressure of the flue gas.
[0054] Accordingly, in another aspect, the present invention provides a CHA zeolite membrane produced by the method, wherein the CHA zeolite membrane has a defect porosity of 0.1 to 1% and a z-axis tortuosity (tortuosity in a thickness direction of the defect) of 1 to 2.
[0055] In still another aspect, the present invention is directed to a method of separating CO.sub.2 from a mixture containing CO.sub.2 and a molecule selected from the group consisting of CH.sub.4, N.sub.2, O.sub.2, C.sub.2H.sub.4, C.sub.2H.sub.6, C.sub.3H.sub.6 and C.sub.3H.sub.8 using the CHA zeolite membrane.
[0056] In the present invention, the separation factor of CO.sub.2/N.sub.2 in the flue gas separation process containing water may be 10 to 30, and the separation factor of CO.sub.2/CH.sub.4 in the upgrading process of biogas containing water may be 10 to 80.
[0057] In the use of the CHA zeolite membrane according to the present invention, the separation of carbon dioxide includes all of separation, capturing and removal of carbon dioxide.
EXAMPLES
[0058] Hereinafter, the present invention will be described in more detail with reference to examples. However, it will be obvious to those skilled in the art that these examples are provided only for illustration of the present invention and should not be construed as limiting the scope of the present invention.
Example
Preparation Example 1: Fabrication of CHA Membrane
[0059] Synthesis of Seed Particle and Seed Deposition
[0060] All-silica CHA (Si-CHA) zeolite particles were synthesized following a literature procedure (Diaz-Cabanas, M. J. et al., Synthesis and Structure of Pure SiO.sub.2 Chabazite: The SiO.sub.2 Polymorph with the Lowest Framework Density. Chem. Commun. 1998, 1881-1882). N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH, 25 wt % in water, Sachem Inc.) was poured into distilled (DI) water in a Teflon beaker. Ethanol (anhydrous 200 proof, ≥99.5%, Sigma-Aldrich) was subsequently added to the mixture, followed by the addition of tetraethyl orthosilicate (TEOS 98%, Sigma-Aldrich), which served as the silica source. The synthesis mixture was hydrolyzed overnight at room temperature and was further dried until the ratio of SiO.sub.2 to water reached ˜2.4. After that, the synthesis mixture became almost solid. Finally, an HF solution (48%, Sigma-Aldrich) was added dropwise to the solid-like synthesis mixture. The final molar composition was 10 SiO.sub.2: 5 TMAdaOH: 5 HF: 30 H.sub.2O. The resultant synthesis mixture was transferred to a Teflon liner in an autoclave for reaction. The autoclave containing the synthesis mixture was placed on a rotating rack in a preheated oven at 155° C., and after completing the reaction for 42 hours with rotation, the reaction was quenched with tap water. The resulting particles were recovered by repeating centrifugation and washing five times. In addition, the recovered particles were calcined at ˜600° C. for 12 hours at a ramp rate of 1° C.Math.min.sup.−1 under air flow (200 mL min.sup.−1).
[0061] The seed deposition procedure has been reported elsewhere (Kim, E. et al., Uniform Si-CHA Zeolite Layers Formed by a Selective Sonication-Assisted Deposition Method. Angew. Chem., Int. Ed. 2013, 52, 5280-5284; Kim, E. et al., On the Synthesis and Characterization of All-silica CHA Zeolite Particles. Microporous Mesoporous Mat. 2014, 184, 47-54). Briefly, —0.05 g of Si-CHA particles were added to ˜50 mL of anhydrous toluene in a specially designed glass reactor with flowing argon (˜20 mL.Math.min.sup.−1). After that, an α-Al.sub.2O.sub.3 disc sandwiched between two cover glasses was fixed on a comb-shaped Teflon holder and placed in the suspension inside the glass reactor. Detailed information about the placement of the disc on the Teflon holder can be found in (Choi, J. et al., MFI Zeolite Membranes from a- and Randomly Oriented Monolayers. Adsorption 2006, 12, 339-360). About 50 mL of anhydrous toluene was poured into the glass reactor under an argon environment. Subsequently, the glass reactor was sealed with Parafilm and ultra-sonicated (UC-10P, JEIO Tech, South Korea) for ˜20 min. The seeded α-Al.sub.2O.sub.3 disc was calcined at 450° C. for 4 hours at a ramp rate of 1° C.Math.min.sup.−1.
[0062]
[0063] CHA Film Growth
[0064] Three types of CHA zeolite membranes were synthesized by varying the Si/Al ratio in the synthetic precursor. First, a certain amount of TMAdaOH, Al(OH).sub.3 (for a finite Si/Al ratio), and CAB-O-SIL (CAB-O-Sil® M-5, Cabot Corporation) as a silica source were added to deionized water, and the mixture was further shaken using a shaking machine (Lab Companion, SI-300R, South Korea) overnight at room temperature. After that, a certain amount of NaOH was added to the well-mixed synthetic precursor, followed by shaking overnight at room temperature. The final molar compositions of the three synthetic precursors for secondary growth were 20 TMAdaOH: 100 SiO.sub.2: 4,400 H.sub.2O: 20 NaOH: x Al(OH).sub.3, where x=0, 1, and 5 correspond to the nominal Si/Al ratios of infinity, 100, and 20, respectively. The prepared synthetic precursor was poured into a Teflon liner where a seeded α-Al.sub.2O.sub.3 disc had been placed vertically with the help of a Teflon holder. The Teflon liner was placed in an autoclave and the autoclave was moved to an oven preheated to 160° C. The hydrothermal reaction for secondary growth was carried out at 160° C. for 6 days. After the reaction had finished, the autoclave was quenched with tap water. The as-synthesized membranes were recovered from the autoclave, washed with copious water, and dried at room temperature. The dried membranes were further calcined at 550° C. for 12 hours at a heating ramp rate of 0.5° C..Math.min.sup.−1 under air flow (200 mL.Math.min.sup.−1). For convenience, the calcined membrane samples are referred to as “CHA_x”, where x indicates the nominal Si/Al ratio in the synthetic precursor.
Example 1: Characterization of Membrane
[0065] SEM images of CHA particles, layers and separators were obtained using a Hitachi S-4300. Prior to analysis, all samples were coated with Pt-sputtering (Hitachi S-4300). The crystallinity of the Si-CHA zeolites and orientation of the Si-CHA layers and separators were determined through XRD (Rigaku Model D/Max-2,500 V/PC diffractometer, Japan) in the θ/2θ array using Cu Kα radiation (40 kV, 100 mA, λ=0.154 nm). Powder XRD patterns of CHA zeolites were generated using the Mercury program (www.ccdc.cam.ac.uk). The CHA Zeolite Decision Information File (CIF) was downloaded from www.iza-online.org. The chemical content from the membrane surface to the portion of the α-Al.sub.2O.sub.3 support adjacent to the CHA film was analyzed by energy dispersive X-ray analysis (EDX, Hitachi S-4800). This was used to evaluate the hydrophobicity of the separator. In addition, the contact angle of the water droplets on the membrane sample at room temperature was measured using a light microscope (General Type Phoenix 300, Surface Electro Optics) and recorded over time. To investigate the inner structures of the CHA membranes, FCOM (Zeiss LSM-700) was used after the membranes had been impregnated with a fluorescent dye (fluorescein sodium salt, empirical formula: C.sub.20H.sub.10Na.sub.2O.sub.5, Sigma-Aldrich (Product No.: F6377)) having an approximate diameter of ˜1 nm. In the present invention, membrane staining was performed for 12, 24, and 96 hours. In addition, a simple screening test suitable for excluding highly defective membranes was used. This test involves placing several droplets of 1 mM fluorescent dye solution on the membrane surfaces and observing the dye distribution with time.
[0066] The Wicke-Kallenbach mode was adopted for measuring the permeation rates of CO.sub.2 and N.sub.2 (or CH.sub.4). The total pressures of both feed and permeate sides were maintained at 1 atm. In order to examine the effect of water on the membrane, the feed gas was humidified. Specifically, the partial pressures of CO.sub.2/N.sub.2/H.sub.2O (or CO.sub.2/CH.sub.4/H.sub.2O) were 48.5 kPa/48.5 kPa/3 kPa, respectively. A total flow rate of 100 mL.Math.min.sup.−1 of CO.sub.2 and N.sub.2 (or CH.sub.4) on a dry basis was fed to the membrane, and the CO.sub.2/N.sub.2 or CO.sub.2/CH.sub.4 mixture permeated through the membrane. The permeate was carried by He sweep gas (100 mL.Math.min.sup.−1) and was analyzed on-line by a gas chromatography (GC) system (YL 6100 GC for CO.sub.2/N.sub.2 and YL 6500 GC for CO.sub.2/CH.sub.4, Young Lin Instruments, South Korea) equipped with a packed column (6 ft×⅛″ Porapak T for CO.sub.2/N.sub.2 and 30 m×0.320 mm GS-GasPro for CO.sub.2/CH.sub.4) and a thermal conductivity detector (TCD) for CO.sub.2/N.sub.2 analysis and a pulsed discharge ionization detector (PDD) for CO.sub.2/CH.sub.4 analysis. For reliable analysis, ˜5 mL.Math.min.sup.−1 of CH.sub.4 for CO.sub.2/N.sub.2 and H.sub.2 for CO.sub.2/CH.sub.4 measurements were added to the swept permeate gas stream before reaching the GC system. In addition, permeation tests with respect to the different feed humidities and longer durations at 50° C. were conducted with CHA_100 and co. In order to evaluate the effect of the humidity on the separation performance of the membrane samples at 50° C., the permeation tests were conducted at relative humidities of 0%, 26%, 60%, and 100% at 50° C. for 12 hours each. The relative humidities of 0%, 26%, 60% and 100% at 50° C. correspond to water vapor partial pressures of approximately 0, 3, 7 and 12 kPa, respectively. The long-term stability tests were conducted at 50° C. at the saturated water vapor of ˜12 kPa. To expedite any deactivation, the membrane samples were heated to 200° C. between the long-term measurements at 50° C. Specifically, the permeation tests were performed at 50° C. for 3 days, then at 200° C. for 2 days, and then again at 50° C. for 3 days.
[0067] Formation of CHA Membranes
[0068]
TABLE-US-00001 TABLE 1 Sample Thickness (μm) CHA_20 1.6 ± 0.4 CHA_100 9.3 ± 0.9 CHA_∞ 2.3 ± 0.7
Example 2: CO.SUB.2 .Permselectivities of CHA Membranes
Example 2-1: CO.SUB.2./N.SUB.2 .Separation Performance Under Dry Conditions
[0069] Despite the continuity observed at SEM resolution, the CHA film synthesized with a nominal Si/Al ratio of 20 (CHA_20) showed very poor CO.sub.2/N.sub.2 separation performance under dry conditions. The maximum (max) CO.sub.2/N.sub.2 SF was estimated to be 2.2±0.2, which is relatively close to that (˜0.8) determined based on Knudsen diffusion (
[0070] In addition, the CO.sub.2 permeance through CHA_100 was higher than that through CHA_∞ (
[0071] Previously, Si-CHA zeolite seed particles have been synthesized in the presence of hydrofluoric acid (HF), where F.sup.− served as an anionic counterpart with respect to the cationic structure directing agent (SDA, here TMAda.sup.+). However, the direct adoption of the synthetic protocol for the secondary growth of the CHA seed layer would not work, apparently due to the dissolution of the pre-deposited CHA seed particles by HF. Recently, the use of TMAdaF as a SDA (which can be considered to have the combined role of TMAdaOH and HF) instead of the conventional TMAdaOH SDA has been shown to be effective for the fabrication of CHA membranes via secondary growth. Nevertheless, the use of TMAdaOH is highly desirable for manufacturing high-performance CHA membranes, because it does not require an additional, sophisticated procedure to obtain TMAdaF.
Example 2.2: CO.SUB.2./N.SUB.2 .Separation Performance Under Wet Conditions
[0072] Considering the third main component, H.sub.2O vapor, in the CO.sub.2-containing streams, the CO.sub.2 permselectivities of the three CHA membranes under wet conditions are investigated. Specifically, CHA_20, which showed poor CO.sub.2/N.sub.2 separation performance (maximum SF of ˜2.2 at 30° C.) under dry conditions, also exhibited a low CO.sub.2/N.sub.2 separation performance (maximum SF of ˜1.6 at 100° C.) under wet conditions (
[0073] Despite previous results concerning water-assisted CO.sub.2 permselectivity, the monotonic decrease in the CO.sub.2/N.sub.2 SF of the SSZ-13 membrane with increased temperature makes it undesirable for practical use. In particular, considering the plausible temperature fluctuations of flue gas streams around the representative temperature (˜50° C.), establishment of maximum separation performance in the temperature range of ˜50-75° C. is highly desirable. In this respect, CHA_∞ prepared in the present invention, which had a CO.sub.2/N.sub.2 SF greater than 10 in the presence of H.sub.2O vapor in a temperature range of ˜50-75° C., meets the above-mentioned requirement for practical use.
[0074] The improved CO.sub.2/N.sub.2 separation performance under wet conditions can be ascribed to the hydrophobic surface of the CHA membranes. Because of the smaller size of the H.sub.2O molecule (0.265 nm), preventing the entry of H.sub.2O into the CHA zeolite pores is not a sound approach for minimizing the H.sub.2O permeation rate. Instead, the control of the hydrophobicity of the surface of the CHA membranes is effective for reducing the adsorption of H.sub.2O and, thus, its permeation rate. The EDX analysis shown in
[0075] To clarify the hydrophobicities of the CHA membranes, the contact angles of water droplets on the membrane surfaces are measured (
[0076] It was noted that the dry CO.sub.2/N.sub.2 SF of CHA_∞ was higher that of CHA_100 (
[0077] The good separation performance of CHA membranes under wet conditions (
[0078] The long-term stability test of CHA_100 in the simulated CO.sub.2/N.sub.2 feed at 50° C. with a relative humidity of 100% (
Example 3: CO.SUB.2./CH.SUB.4 .Separation Performance of CHA Membrane
[0079] In addition to the CO.sub.2/N.sub.2 separation performance, the CO.sub.2/CH.sub.4 separation performance of CHA_100 and _∞ was also measured (
[0080] At this point, the monotonically decreasing trend of CO.sub.2/N.sub.2 and CO.sub.2/CH.sub.4 SFs with an increase in temperature is not desirable for reliable use. Maximum performance should be achieved at ˜50° C., a representative flue gas and biogas temperature, and should be insensitive to small changes in temperature. The low Si/Al ratio in the synthetic precursor could contribute to the formation of more defects in the zeolite membranes (as observed in CHA_20 in the present invention), possibly because of the relatively high polarity and repulsion of the growing zeolite grains. Concerning CO.sub.2/N.sub.2 and CO.sub.2/CH.sub.4 separation through SSZ-13 membranes under wet conditions, the permeances of CO.sub.2, N.sub.2, and CH.sub.4 all decreased at temperatures below 100° C., but the corresponding SFs were increased because of the greater inhibition of the slowly permeating, larger N.sub.2 and CH.sub.4 molecules by H.sub.2O. The Si/Al ratio in the synthetic precursor significantly affects the hydrophobicity of the final membrane surface and constituents, whereas defect formation is a sensitive function of thermal processing. As both factors influence the final performance, the identification of defects and the elucidation of their effects on the performance are also important for understanding and achieving high CO.sub.2 permselectivities in realistic feeds (here, water-vapor-containing CO.sub.2 streams).
Example 4: Investigation of Defects in CHA Membranes through FCOM
[0081] In addition to the measurement of CO.sub.2 permselectivities of CHA_20, _100, and _∞, the defect structures were imaged via FCOM to understand the relationship between the structures and separation performance FCOM analysis is an attractive technique for studying the structure of defects embedded in a membrane without damaging the membrane structure.
[0082] Considering that the dyeing process is kinetically relevant, the dyeing duration was systematically varied from 12 hours through 24 hours to 96 hours (
[0083] Although the FCOM images provide detailed information about the defect structure, the dyeing process required for FCOM characterizations also can reveal the degree of undesired defects. The inset in
[0084] Combining the FCOM characterization (defect structure inside the CHA membranes) with the water contact angle results (hydrophilicity of the surface on the CHA membranes), a simple and rapid but reliable evaluation tool that can determine the success of a membrane fabrication was developed. Specifically, a dye solution, which was originally allowed to diffuse into the defect structure in the CHA membranes for FCOM characterization, was dropped onto the membrane surface and its diffusion into the membrane was tracked with time (
Example 5: Elucidation of CO.SUB.2 .Separation Performance Based on Quantitative Membrane Properties
[0085] To date, the qualitative analysis of defects in zeolite membranes based on the FCOM technique has mainly focused on identifying the defect density and distribution and correlating these factors with the final separation performance However, quantitative information about the size and number of defects and association with the final membrane performance would be useful. Thus, a quantitative analysis of defects in CHA_20, _100 and _∞ was conducted based on an approach reported in a previous study by the present inventors. The resulting processed defects are schematically illustrated in
[0086]
TABLE-US-00002 TABLE 2 Tortuosity x- y- z- Area Porosity Sample direction direction direction fraction.sup.a (%).sup.b Ref. SSZ-13 2.17 2.23 1.10 6.85 × 10.sup.−2 0.27 1 Membrane CHA_20 3.02 3.10 1.35 5.36 × 10.sup.−2 0.24 Present invention CHA_100 2.55 2.56 1.58 8.17 × 10.sup.−2 0.30 Present invention CHA_∞ 3.81 3.69 1.24 4.86 × 10.sup.−2 0.13 Present invention
[0087] As already inferred from the CO.sub.2 separation performance in
[0088] Compared to that under dry conditions, the membrane separation performance under wet conditions is a strongly couple function of the zeolite structure, surface hydrophobicity/hydrophilicity, and size/number of defects. Thus, summarized all possible membrane properties relevant to the permeation behavior of CHA_100 and _∞ and their CO.sub.2/N.sub.2 permeation results at 30° C. are summarized in Table 3, in which those of another SSZ-13 membrane (for convenience, referred to as SSZ-13 in this section) are also given for comparison. First, the effects of defects (defect (crack) porosity and defect size) on the CO.sub.2/N.sub.2 SFs of CHA_∞ and SSZ-13 were considered. The porosity difference between the two membranes (0.08% for CHA_∞ and 0.15% for SSZ-13 in Table 2) had a significant effect on their respective dry maximum CO.sub.2/N.sub.2 SFs of 7.3 and 4.6. This observation is consistent with the relationship between the porosity and dry maximum CO.sub.2/N.sub.2 SF for CHA_20 and _∞. More specifically, given the similar area fraction of CHA_20 and _∞ (Table 3), the crack size was critical in determining the final separation performance. Thus, the crack size of CHA_100, which could not be determined from its FCOM images, will be less than ˜3.1 nm to achieve the dry maximum CO.sub.2/N.sub.2 SF of 5.2, because the corresponding crack density was higher than that of CHA_∞ and SSZ-13.
TABLE-US-00003 TABLE 3 Dry Wet CO.sub.2 CO.sub.2 100- Wet/ Water permeance × Dry permeance × Wet Wet/Dry Wet/Dry Dry Thick- contact Defect 10.sup.7 CO.sub.2/ 10.sup.7 CO.sub.2/ CO.sub.2 CO.sub.2 CO.sub.2/ Membrane ness angle size Defect (mol .Math. m.sup.−2 .Math. N.sub.2 (mol .Math. m.sup.−2 .Math. N.sub.2 permeance permeance N.sub.2 sample (μm) (°) (nm) density s.sup.−1 .Math. Pa.sup.−1) SF s.sup.−1 .Math. Pa.sup.−1) SF (%) (%) SF Ref. SSZ-13 ~3.5 78 1.8 Middle 1.4 4.6 0.37 10 26 74 2.2 Hong S. et al.1) CHA_100 ~9.3 80 3.1 Relatively 2.3 5.2 1.4 13.4 61 39 2.6 Preparation High Example 1 CHA_∞ ~2.3 25 1.3 Middle 1.5 7.3 0.37 9.7 25 75 1.3 Preparation Example 1 1) Hong, S. et al., Healing of Microdefects in SSZ-13 Membranes via Filling with Dye Molecules and Its Effect on Dry and Wet CO.sub.2 Separations. Chem. Mater. 2018, 30, 3346-3358.
[0089] For a fair comparison, the permeation behavior caused by the change from dry to wet conditions was focused. Specifically, the CO.sub.2 permeance (wet CO.sub.2 permeance) and CO.sub.2/N.sub.2 SF 30° C. (wet CO.sub.2/N.sub.2 SF) under wet conditions were compared with those (dry CO.sub.2 permeance and dry CO.sub.2/N.sub.2 SF) under dry conditions (Table 3); the respective ratios are referred to as wet/dry CO.sub.2 permeance and wet/dry CO.sub.2/N.sub.2 SF, respectively.
[0090] For CHA_∞, it may be reasonably speculated that water vapor was adsorbed in the defects and further adsorbed on the membrane surface of the less hydrophobic CHA_∞. Assuming that all of the defects were occupied and blocked by water vapor, for CHA_∞, the reduction in CO.sub.2 permeation (i.e., wet/dry CO.sub.2 permeance) was caused by defect blocking (˜5%;
[0091] Subsequently, the values of the wet/dry CO.sub.2/N.sub.2 SF for SSZ-13, CHA_100, and CHA_∞ are 2.2, 2.6, and 1.3, respectively. It appears that the water-blocking defects in SSZ-13 and CHA_100 contributed to increasing their molecular sieving abilities, as indicated by the higher wet/dry CO.sub.2/N.sub.2 SFs of 2.2 and 2.6, respectively. However, CHA_∞, which had the lowest degree of defects as indicated by the highest dry maximum CO.sub.2/N.sub.2 SF of 7.3, was unlikely to gain a marked benefit from the blocking of defects with water molecules, because the defect effects were much less significant than those of the other two membranes. Instead, a significant loss in CO.sub.2 permeance without any improvement of CO.sub.2/N.sub.2 SFs occurred for less hydrophobic CHA_∞ by surface inhibition (up to ˜56%) because of the physisorbed water molecules.
[0092] The absolute values of the CO.sub.2 permeances obtained through the zeolitic and non-zeolitic parts were investigated (
Example 6: Comparison in Separation Performance between CHA 100 and ∞
[0093]
[0094] According to the present invention, a uniform CHA seed layer was prepared via the physical attachment of plate-like Si-CHA seed particles to an α-Al.sub.2O.sub.3 disc. The limited accessibility for seed particles to reach the surface of the α-Al.sub.2O.sub.3 disc was key to obtaining a uniform seed layer. Despite being the minor portion, the plate-like particles dominated the seed layer. The subsequent secondary growth of the seed layer with a synthetic precursor that had nominal Si/Al ratios of 100 and infinity allowed for the formation of continuous CHA membranes (denoted CHA_100 and _∞, respectively).
[0095] The synthesized CHA membranes showed modest CO.sub.2 SFs (5.2 for CHA_100 and 7.3 for CHA_∞ at 30° C.) when using dry CO.sub.2/N.sub.2 binary mixtures. However, they showed improved CO.sub.2/N.sub.2 SFs as high as 13.4 for CHA_100 and 9.7 for CHA_∞ at 30° C., when using a wet feed. Such performance improvement was ascribed to the defect blocking by physisorbed water molecules and pronounced for more defective CHA_100. The trend observed for CO.sub.2/N.sub.2 separation was similar to that for CO.sub.2/CH.sub.4 separation, but the dry and wet CO.sub.2/CH.sub.4 SFs were comparable to each other. Considering the CO.sub.2 molar flux (relevant to CO.sub.2 recovery) and CO.sub.2/N.sub.2 SF (relevant to CO.sub.2 purity) in a coupled way, CHA_100 showed a marked activity for CO.sub.2 capture in the temperature range of 30-75° C. (representative of flue gas temperatures). In addition, the degree of CO.sub.2 permeance reduction (75%) due to the presence of water vapor in the feed was much larger for less hydrophobic CHA_∞ (water contact angle: 25°) than that (39%) observed for CHA_100 (water contact angle: 80°), supporting that hydrophobic membrane constituents have desirable roles on membrane performance. In particular, the CO.sub.2 molar fluxes through zeolitic parts and non-zeolitic parts could be effectively deciphered by complementing the information extracted from the image processing of FCOM images with a one dimensional permeation model. This indicates that increasing hydrophobicity was desirable for securing high CO.sub.2 permselectivities under a realistic water vapor-containing feed condition, where defects with proper size/density could be well blocked by physically adsorbed water molecules.
INDUSTRIAL APPLICABILITY
[0096] According to the present invention, by producing a CHA zeolite membrane having a high Si/Al ratio, a CHA zeolite membrane having high CO.sub.2/N.sub.2 and CO.sub.2/CH.sub.4 separation performance even under wet conditions can be obtained.
[0097] In addition, the CHA zeolite membrane has an effect of exhibiting high separation performance even under feed conditions containing water, regardless of the temperature and pressure of the flue gas.
[0098] Although specific configurations of the present invention have been described in detail, those skilled in the art will appreciate that this description is provided to set forth preferred embodiments for illustrative purposes and should not be construed as limiting the scope of the present invention. Therefore, the substantial scope of the present invention is defined by the accompanying claims and equivalents thereto.