Patent Application
20240351876
The present disclosure relates to a porous membrane and a method of manufacturing the same, and more particularly, to a thermally stable porous membrane capable of securing thermal stability and long-term stability of gas separation performance at high temperatures, and a manufacturing method thereof.
Most hydrogen is produced as a mixture of carbon monoxide and carbon dioxide by reacting with water, which is called a water gas shift (WGS) reaction. This reaction is thermodynamically exothermic, but the reaction proceeds at a high temperature corresponding to 200-500° C. to secure a high reaction rate. Since it is an exothermic reaction, the equilibrium conversion rate to hydrogen at a high temperature is limited, and a separation membrane reactor capable of separating hydrogen generated through the reaction at the same time as the reaction has been proposed. This is because, in the conversion of the equilibrium state, only hydrogen is selectively removed through the wall of the membrane reactor, thereby converting the equilibrium state to a more generated state. Since the reaction proceeds at the high temperature as described above, it is essential to secure the technical ability for the high thermal stability of the membrane that acts as a wall in the membrane reactor.
Zeolitic Imidazolate Framework-8 (hereinafter referred to as ZIF-8) membranes have been formed with a pore size of 0.34 nm, which allows for the separation of hydrogen (0.289 nm) and carbon dioxide (0.33 nm) based on size differences. The separation capability of H2/CO2 by ZIF-8 membranes can be confirmed in various documents (R. Krishna and J. M. van Baten, J. Membr. Sci., 2010, 360, 323-333; Q. Liu et al., J. Am. Chem. Soc., 2013, 135, 17679-17682).
One of the synthesis methods for ZIF-8 membranes, the counter-diffusion method, involves controlling the ratio of the diffusion speed and reaction rate of Zn2+ ions and 2-methylimidazole molecules, which are components of ZIF-8. Zn2+ ions are located inside a porous support, while 2-methylimidazole molecules are outside, diffusing in opposite directions due to concentration differences. At the surface of the porous support, the two materials meet to form ZIF-8 particles, thereby synthesizing the membrane. The resulting ZIF-8 membrane exhibits high separation factors in the separation of propylene/propane (H. T. Kwon and H. K. Jeong, J. Am. Chem. Soc., 2013, 135, 10763-10768).
Although current technologies for ZIF membranes primarily focus on new methods of membrane synthesis, to apply them in actual Water-Gas Shift (WGS) reactions, it is necessary to ensure thermal and long-term stability of gas separation performance at high temperatures (200-500° C.).
In an effort to resolve these issues, the inventors have proposed “A Method for Manufacturing a Selective Porous Separation Membrane and a Gas Separation Method Using the Manufactured Porous Membrane” in Korean Registered Patent No. 10-1905862 (Patent Document 1). In Patent Document 1, a γ-Al2O3 layer with a different pore size (pore size: 5 nm) was coated on a porous α-Al2O3 support (pore size: 150 nm) to delay the diffusion speed of Zn2+ ions and selectively synthesize the ZIF-8 membrane inside the support.
However, the ZIF-8 membrane produced according to Patent Document 1 maintained stable gas separation performance at up to 300° C. for a maximum of 10 hours, which still does not meet the actual requirements.
An object of the present invention is to provide a thermally stable porous membrane capable of securing thermal stability and long-term stability of a gas separation performance at a high temperature, and a method for manufacturing the same.
The embodiment of the present invention provides a porous membrane comprising: a first Zeolitic Imidazolate Fragments (ZIFs) part formed on a surface of a porous support; and a second ZIFs part embedded in the porous support, wherein the second ZIFs part is formed in a state in which it penetrates from an interface between the first ZIFs part and the second ZIFs part to a predetermined depth.
Furthermore, the present invention provides a method for separating hydrogen from a mixed gas of H2 and CO2 or from synthesis gas (syngas) using the aforementioned porous membrane.
In an embodiment, the method of separating hydrogen using the porous membrane can be performed at temperatures ranging from 100 to 500° C.
Moreover, the present invention provides a method for manufacturing the porous membrane. This method includes the steps of pre-depositing and immobilizing a Zn precursor using a first concentration of a zinc salt solution on the surface and inside of the porous support; and reacting the Zn precursor-coated porous support with an imidazole or imidazole derivative solution, whereby a first ZIFs part is formed on the surface of the porous support and a second ZIFs part is embedded inside the porous support. The second ZIFs part is characterized by being formed to penetrate to a certain depth from the interface between the first ZIFs part and the second ZIFs part.
In this embodiment, the first ZIFs part has a thickness of 0.1 μm to 10 μm, preferably 1 μm to 5 μm, more preferably 2 μm to 4 μm, and most preferably approximately 3 μm. The second ZIFs part can penetrate to a depth of 10 μm to 500 μm from the interface between the first and second ZIFs parts, preferably 50 μm to 200 μm, more preferably 70 μm to 150 μm, and most preferably about 80 μm.
Controlling the thickness of the first and second ZIFs parts within these ranges ensures high thermal stability of the separation membrane, which acts as a wall in the membrane reactor. It has been found that even after polishing, which will be discussed later, the first ZIFs part maintains stable separation performance.
In this embodiment, the first ZIFs part and the second ZIFs part can be formed by first pre-depositing and immobilizing a Zn precursor using a first concentration of a zinc salt solution on the surface and inside of the porous support, followed by reacting the Zn precursor-coated porous support with an imidazole or imidazole derivative solution.
In this embodiment, the first concentration can range from 0.27 M to 0.58 M, preferably 0.38 M to 0.51 M, most preferably about 0.48 M.
The pre-deposition and immobilization of the Zn precursor can be achieved by immersing the porous support in the zinc salt solution for 1 to 12 hours, preferably 2 to 8 hours, most preferably 2 to 4 hours, and then drying the zinc precursor-coated porous support in a vacuum oven at temperatures ranging from 30 to 200° C., preferably 30 to 120° C., most preferably 40 to 60° C., and even more preferably about 50° C. for 1 to 24 hours, preferably 2 to 15 hours, most preferably 2 to 4 hours.
As the immersion time increases, the amount of Zn precursor adsorbed onto the porous support may also increase. Consequently, it is advisable to appropriately adjust the drying time and drying temperature in the vacuum oven to accommodate this increase.
The reaction with the imidazole or imidazole derivative solution can involve immersing the zinc precursor-coated porous support in the imidazole or imidazole derivative solution and then reacting it at temperatures between 5° and 200° C., preferably 110 to 130° C., most preferably about 120° C. for 1 to 72 hours, preferably 1 to 24 hours, most preferably 2 to 15 hours, and even more preferably about 2 to 4 hours to form the crystals of Zn and imidazole on the surface and inside of the porous support.
In one embodiment, the zinc salt may be selected from a group consisting of zinc nitrate, zinc acetate, zinc chloride, zinc sulfate, zinc bromide, and zinc iodide.
In another embodiment, the imidazole or imidazole derivative used may be selected from a group consisting of benzimidazole, 2-methylimidazole, 4-methylimidazole, 2-methylbenzimidazole, 2-nitroimidazole, 5-nitrobenzimidazole, and 5-chlorobenzimidazole.
In another embodiment, the solution of zinc salt or imidazole or imidazole derivative may be dissolved in one or more solvents selected from a group consisting of methanol, ethanol, propanol, isopropanol, tert-butanol, n-butanol, methoxyethanol, ethoxyethanol, dimethylacetamide, dimethylformamide, N-methyl-2-pyrrolidone (NMP), formic acid, nitromethane, acetic acid, and distilled water.
In another embodiment, the surface of the porous support may be polished a specific number of times. When the porous support surface is polished between 20 to 100 times, preferably 30 to 90 times, it can exhibit a H2/CO2 separation factor of 3.1 to 3.8 at 200° C.
In another embodiment, the membrane according to this invention can maintain its separation performance for 36 to 168 hours, preferably 72 to 120 hours, and most preferably for 72 hours, at temperatures ranging from 300 to 360° C.
As observed in the embodiments discussed, this invention involves depositing and immobilizing a Zn precursor on the surface and inside of a porous support, followed by the synthesis of a ZIF-8 membrane. This approach enables the formation of a separation membrane within the interior of the porous support, thus preventing direct exposure of the membrane to high-temperature gases. Consequently, compared to existing ZIF-8 membranes, this method enhances the thermal and long-term stability of gas separation performance at high temperatures.
The description about the presented exemplary embodiments is provided so as for those skilled in the art to use or carry out the present invention. Various modifications of the exemplary embodiments will be apparent to those skilled in the art. General principles defined herein may be applied to other exemplary embodiments without departing from the scope of the present disclosure. Accordingly, the present invention is not limited to the exemplary embodiments presented herein. The present invention shall be interpreted within the broadest meaning range consistent to the principles and new characteristics presented herein
In this specification, when one component is mentioned as being “on” another component, it implies that it can be directly formed on the other component or that a third component may intervene between them. Additionally, in the drawings, the thicknesses of membranes and regions are exaggerated for clear technical explanation.
Furthermore, terms such as “first,” “second,” and “third” are used to describe various components in different embodiments within this specification; however, these components should not be limited by these terms. These terms are merely used to distinguish one component from another. Therefore, a component referred to as “first” in one embodiment might be referred to as “second” in another. Each embodiment described and exemplified here also includes its complementary embodiments. Moreover, the term “and/or” used in this specification means that it includes at least one of the components listed before and after.
In the specification, the use of the singular does not exclude the plural unless explicitly stated otherwise by the context. Terms such as “comprise” or “include,” and any variations thereof, are intended to cover the specified features, numbers, steps, components, or combinations thereof, and should not be understood to exclude the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.
Additionally, detailed descriptions of well-known functions and configurations that are deemed to unnecessarily obscure the essence of the invention have been omitted in the following description of the invention.
In the present invention, the conventional CD method is further improved by pre-depositing a Zn precursor or source on the surface and inside of a porous α-Al2O3 support before a solvothermal reaction with a ligand solution and fixing it through drying. In particular, the resulting immobilization of the Zn precursor at the support surface and inside was effective to achieve high dispersion of the Zn precursor, thus ensuring growth of ZIF-8 particles deep inside the porous support. In particular, the pre-deposited Zn precursor plausibly induces the formation of a continuous separation layer on the support, thereby enabling the formation of a dense film inside the support. This membrane configuration was advantageous for resisting thermal decomposition and mechanical damage. In addition, in order to obtain an optimal separation membrane structure, important factors such as the loading amount and position of the Zn precursor that determine the separation membrane preparation were systematically investigated. In particular, the characterized membrane structure showed a high relationship with the H2/CO2 separation performance measured at various temperatures. We also evaluated the H2/CO2 separation ability of the structured ZIF-8 membrane at a WGS reaction temperature of 300° C. In addition, a long-term stability test was performed to evaluate the possibility of application of the ZIF-8 membrane in an integrated WGS membrane reactor, and the results were compared with those reported in the prior art. Finally, the effect of water vapor on membrane damage, introduced to simulate the WGS reaction conditions, was investigated.
ZIF-8 membranes have emerged as promising wall candidates for use in membrane reactors. This membrane can enhance the WGS reaction performance due to its excellent thermal stability and remarkable H2/CO2 sieving ability.
The present invention proposes a modified interdiffusion method for forming an unprecedented thermally stable ZIF-8 membrane. Modified interdiffusion methods include pre-deposition of a zinc precursor (zinc acetate dihydrate) and solvothermal reaction with a methanol solution of the subsequent ligand (2-methylimidazole). The pre-deposition and drying of the Zn precursor on the surface and inside of the porous support appears to play an important role in retarding the diffusion of the Zn source, leading to a primary contact between the Zn source and the ligand molecules in the porous support. As a result, it is likely that a continuous ZIF-8 layer has formed on the upper surface, more preferably at the interface with the particles of the porous support, and has reached a penetration depth (thickness) of approximately 80 μm. Therefore, this enables effective blocking and high dispersion of the support pores by the ZIF-8 particles.
In particular, the resulting ZIF-8 membrane exhibited a maximum H2/CO2 separation factor of about 8.7 at a WGS reaction temperature of about 300° C. Unlike conventional membrane structures, ZIF-8 particles were densely embedded inside the porous support to reduce the risk of mechanical damage. The unique membrane structure was also desirable to ensure excellent thermal stability for stable and sustainable H2/CO2 separation at 300° C. for up to 72 hours.
1. Zn Source Deposition on Porous α-Al2O3 Supports
A modified CD method involving a coating process using a Zn source and a solvothermal reaction was used to prepare the ZIF-8 membrane. To form a separation membrane, 4.2 g of zinc acetate dihydrate (Zn(CH3COO)2·2H2O, product number 383058, 98%, Sigma-Aldrich) was first dissolved in 40 mL of methanol (product number 179337, 99.8%, Sigma-Aldrich) to produce a high concentration Zn solution (0.48 M). For convenience, zinc acetate dihydrate is hereinafter referred to as ZA. In addition, a series of Zn solutions (0.09, 0.18 and 0.36 M) were prepared by proportionally reducing the amount of ZA. In a typical Zn source coating process, a directly prepared porous α-Al2O3 disk (diameter of about 21 mm, thickness of 2 mm) was immersed in the ZA solution for 3 hours with the polished surface facing upward. A detailed description of the polishing procedure will be omitted. The ZA-containing α-Al2O3 disk was then placed in a vacuum oven (OV-11, Jeio Tech Co.) to evaporate the methanol solvent. It was dried at 50° C. for 3 hours in Ltd. (Korea). The α-Al2O3 disks that were immersed and then dried in solutions with ZA concentrations of 0.48, 0.36, 0.18 and 0.09 M are designated Znα, Zn0.75α, Zn0.38a and Zn0.19%, respectively, where a represents the α-Al2O3 disk and the numbers between Zn and a represent the molar concentrations for the saturated molar concentration of 0.48 M.
To synthesize the ZIF-8 membrane, a ligand solution was used to induce crystallization and growth of ZIF-8 particles on a porous α-Al2O3 support. Briefly, 2.59 g of 2-methylimidazole (2-methylimidazole; product number M50850, 99%, Sigma-Aldrich) and 0.25 g of sodium formate (product number 247596, ≥99%, Sigma-Aldrich) were dissolved in 20 mL of methanol with vigorous stirring. The well-dissolved ligand solution was then transferred to a Teflon liner and the Znα support was placed horizontally in the 2-methylimidazole solution with the polishing surface up. This procedure used for Znα was equally applied to Zn0.75α, Zn0.38a and Zn0.19α samples. The Teflon liner was sealed in a stainless steel autoclave and the solvothermal reaction was carried out under static conditions at about 120° C. for 4 hours. After the reaction, the autoclave was quenched with tap water to stop the formation of the membrane. The recovered samples were rinsed with fresh methanol and dried under atmospheric conditions for about 2 hours. Finally, the dried sample was further heat treated overnight at about 160° C. in a convection oven (PL_HV_250, Pluskolab, Korea) to remove methanol residue. For convenience, the resulting samples are denoted as Z_Znα, Z_Zn0.75α, Z_Zn0.38α, and Z_Zn0.19a, where Z represents the ZIF-8 membrane synthesized from the corresponding Zn-containing samples (Znα, Zn0.75α, Zn0.38a and Zn0.19α)
That is, in the present invention, a process of depositing and immobilizing a Zn precursor on the surface and inside of the porous support is performed before synthesizing the ZIF-8 membrane. Next, a ZIF-8 membrane is synthesized through an interdiffusion method using a support on which the Zn precursor is deposited and fixed. When the porous support on which the Zn precursor is deposited and fixed is immersed in the 2-methylimidazole solution, Zn2+ ions are diffused outward from the support side, and at the same time, the diffusion of 2-methylimidazole proceeds from the bulk solution side to the support side. At this time, since Zn2+ ions and 2-methylimidazole have high reactivity with each other, crystals are formed almost simultaneously with the contact, thereby synthesizing a ZIF-8 membrane.
In the conventional general ZIF-8 synthesis, since the Zn precursor is not deposited and immobilized on the surface and inside of the support, Zn2+ ions are rapidly diffused to the outside through the pores of the support and meet 2-methylimidazole to form most of the membrane on the upper portion of the support. In contrast, in the present invention, the Zn precursor is deposited and immobilized on the surface and inside of the support, and then a ZIF-8 separation membrane is synthesized, so that a separation membrane is also formed inside the support.
Therefore, in the present disclosure, since the ZIF-8 membrane is positioned inside the porous support, the stability of the membrane against physical factors such as external impact may be increased. In addition, the existing ZIF-8 membrane directly contacts high-temperature H2 and CO2 gases to show low thermal stability and long-term stability. In contrast, in the case of the ZIF-8 membrane according to the present invention synthesized after depositing and immobilizing the Zn precursor on the surface and inside of the support, the membrane is formed inside the support and is not directly exposed to a high temperature gas, so that it may have higher thermal stability and long-term stability than the existing ZIF-8 membrane.
3. Measurement of H2/CO2 Separation Performance
The H2/CO2 separation performance of the ZIF-8 membrane in the custom-made permeation cell was evaluated by using the Wickke-Kalenbach method, and the total pressure of the feed and permeate sides was maintained at about 101 kPa. The separation performance of the membrane was investigated as a function of temperature using an oven (DX330, Yamato Scientific Co., Ltd., Japan). For reliability, a heat resistant Kalrez O-ring was used for the membrane seal during the separation measurements according to temperature up to 300° C., while a Viton O-ring was used for the measurement up to 200° C. In addition, the flow rate of equimolar H2/CO2 binary mixture supply was about 100 mL·min-1, whereas a helium flow rate of about 100 mL·min-1 was used at the permeate side to carry the permeate component for gas chromatography (GC); YL 6500, Young In Chromass, Korea) analysis. The molar composition of the permeate stream was analyzed using a pulsed discharge ionization detector (PDD). In addition, 5 mL·min-1 of CH4 was added as an internal standard prior to the GC measurement in order to stably estimate the molar flux of the permeate component.
The film shape and element distribution were analyzed using a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi Ltd, Japan) equipped with an energy dispersive X-ray spectrometer (energy-dispersive X-ray spectroscopy; EDX). X-ray diffraction (XRD) characterization was performed using a D/Max-2500V/PC X-ray diffractometer (Rigaku Co., Japan) with Cu-Kα radiation (λ=0.154 nm) generated at 40 kV and 100 mA in the range of 5°-40°. For reference, the crystallographic information files for ZIF-8 (deposit no. 602542) and ZnO are respectively Cambridge Crystallographic Data Center (CCDC, www.ccdc.cam.ac.obtained from uk) and Materials Studio 7.0 (Accelrys, USA). The corresponding simulated XRD patterns were obtained using Mercury software available at wmww.ccdc.cam.ac.uk. In addition, N2 adsorption isotherms were obtained at 77K using an ASAP 2020 instrument (Micromeritics Instruments Corporation, USA). Prior to measuring the N2 adsorption isotherms, α-Al2O3 disks, Znα and Z_Znα samples were degassed for 24 hours at 350, 80 and 200° C., respectively. The specific surface area was estimated by the Brunauer-Emmett-Teller (BET) method, and the micropores and the external surface area were calculated by the t-plot method. Finally, an automatic mercury porosimeter was used to analyze the pore size distribution and porosities of the α-Al2O3 disk and the ZIF-8 membrane (Autopore IV 9520, Micromeritics Instruments Corporation, USA).
5. Structural Properties of ZIF-8 Membrane Synthesized by Plugging Grains Inside α-Al2O3 Disk
The planar SEM image of
The XRD analysis shown in
First, elemental line scanning provided significant Zn signals in the top continuous membrane, but relatively weak signals deep inside the support (
A series of ZIF-8 membranes were then prepared by reducing the concentration of the ZA solution from a saturation value of 0.48 M to 0.36, 0.18 or 0.09 M. This was done to gain insight into the effect of ZA concentration on the structural properties of the final membrane. All ZA-containing α-Al2O3 supports showed similar XRD patterns consistent with standard anhydrous zinc acetate, demonstrating successful formation of Zn source coated composite structures (
In addition, the relationship between the Zn precursor weight and the resulting ZIF-8 film weight was explored to explain the role of Zn precursor loading on film formation and properties. As previously reported, the theoretical weight of the Zn source and the ZIF-8 particles inside the porous support can be calculated assuming that the pores of the α-Al2O3 disk (estimated by the porosity in Table 2) in the used methanol solution were occupied by all ZA, and the occupied ZA was completely converted to ZIF-8 (D. J. Babu, G. W. He, J. Hao, M. T. Vandat, P. A. Schouwink, M. Mensi, K. V. Agrawal, Adv. Mater. 31 (2019) 1900855). First, it was confirmed that the theoretical ZA weights calculated for Znα and Znxa (x=0.75, 0.38, 0.19) monotonously increased as the ZA concentration increased (
In addition, the correlation between the membrane structure and the separation performance was investigated. To this end, equimolar H2/CO2 separation performance of Z_Znα and Z_Znxα (x=0.19, 0.38, 0.75) was considered at an industrially relevant temperature of 200° C. As shown in
Then, the micropore structures of the α-Al2O3 disk, Znα, and Z_Znα samples were analyzed to obtain more detailed microstructural properties. The N2 physisorption isotherm at 77 K (
Table 1 shows the measured weights of ZA and generated ZIF-8 loaded for Z_Znα and Z_Znxα (x=0.75, 0.38 and 0.19) as a function of ZA concentration.
Table 2 shows the Textural properties of the α-Al2O3 disk, Znα and Z_Znα samples, along with the weight of ZIF-8 particles synthesized in the methanol medium and Z_α as reference.
Table 3 shows the estimated and measured weight of Z_Znα, and the expected weight of Z a was calculated based on the micropore surface area.
In Table 2, the surface area of the Znα on which the Zn precursor is deposited and immobilized is reduced compared to the existing α-Al2O3 disk. Thereafter, Z_Znα synthesized by solvothermal reaction of Znα has an increased surface area since ZIF-8 membranes having micropores are formed on the outside and inside of the disk support. As a result of the previous study, Z_α showed that since the ZIF-8 membrane was mainly present only outside the disk support, the surface area of Z_Znα in which the ZIF-8 membrane was present was much larger even outside and inside the disk support.
For better understanding, a schematic view of each immersion and surface polishing process is shown in
To dissolve and remove the buried ZA precipitate, the immersion time of Znα in methanol was further extended to 10 to 30 min or 60 min. The resulting 130-Znα and I60-Znα samples showed attenuated XRD peak intensities, indicating a significant reduction in the amount of ZA (
7. It Relates to the H2/CO2 Separation Performance and Thermal/Mechanical Stability of a ZIF-8 Membrane
Next, the temperature-dependent H2/CO2 separation performance of the ZIF-8 membrane for equimolar H2/CO2 mixtures was investigated over a wide temperature range (30-200° C.,
Instead of forming an existing well-intergrown separation layer or membrane on top of the porous support, embedding, more precisely plugging, the ZIF-8 particles inside the support was strategically effective to minimize the aforementioned thermally induced ZIF-8 structural damage while maintaining the integrity of the original ZIF-8. In addition, the mechanical stability of Z_Znα was examined by mechanically polishing or polishing the continuous separation layer formed on the upper surface. This allowed us to determine the role of surface interlocking and dense ZIF-8 particles in the separation of H2/CO2 at high temperature. For convenience, the polished Z_Znα membrane is referred to as Z_Znα-Gx, where x represents the number of mechanical polishing steps. In particular, the mechanical polishing of the ZIF-8 membrane did not impair the H2/CO2 separation performance, and the membrane maintained a H2/CO2 separation ability that steadily improved as the temperature increased. More surprisingly, both intact Z_Znα and Z_Znα-G100 membranes have similar H2/CO2 SF (e.g., 7.5 for Z_Znα-G100 versus 7.7 for Z_Znα at 200° C.) and H2 permeability (e.g., 1.6×10−7 mol··s−1 m−2·Pa−1 at 200° C. versus 1.3×10−7 mol·s−1 m−2·Pa−1 for Z_Znα-G100). Thus, the well-preserved gas separation capacity of Z_Znα-G100 provided further evidence for the critical and positive contribution of the buried or plugged interlocking membrane particles of H2/CO2 inside the α-Al2O3 disk to the robustness of the high temperature Z_Zn α separation capacity.
In a previous study (D. J. Babu, G. W. He, J. Hao, M. T. Vandat, P. A. Schouwink, M. Mensi, K. V. Agrawal, Adv. Mater. 31 (2019) 1900855), the γ-Al2O3 layer with a smaller pore size (about 5 nm) served as a diffusion barrier to limit the growth of ZIF-8 particles inside the α-Al2O3 disk in the conventional CD scheme. Compared to the composite membrane structure revealed by the SEM and EDX results in
Although the buried separation layer provided mechanical stability, the ZIF-8 crystalline membrane (this ZIF-8 membrane is referred to as Z_γα to match the sample labeling in the present invention) still underwent thermally induced separation after about 10 consecutive hours of separation at 300° C.
In view of this structural discrepancy, the long-term thermal stability of the Z_Znα membrane in the present invention was further evaluated in relation to the H2/CO2 separation performance.
In particular, long-term stability is a prerequisite for implementing an integrated WGS membrane reactor.
Initially, ZIF-8 crystals have been reported to have abnormal thermal stability under inert gas flow. However, the ZIF-8 framework is susceptible to accelerated carbonization or local pyrolysis in an oxidizing or reducing atmosphere. As a result, the high sensitivity to temperature and the environment reduced the application of ZIF-8 membranes under industrially relevant conditions. However, unlike the conventional ZIF-8 framework collapsed due to heat treatment, the structure of ZIF-8 was maintained for the Z_Znα film after treatment at 300° C. for 72 hours, as shown in
In a typical WGS reaction, the vapor is high in volume and coexists with the H2/CO2 product stream. The ZIF-8 framework has been reported to be vulnerable to hydrothermal conditions because water molecules that can serve as proton donors separate Zn—N bonds. Therefore, the wet H2/CO2 separation performance of the Z_Znα membrane was further evaluated as a function of temperature. For reliability, another Z_Znα membrane sample (not used in
ZIF-8 is known to be hydrophobic, but it showed moderate water vapor adsorption capacity. However, at an operating temperature of 100° C. or higher, since the effect of water vapor adsorption was alleviated, the diffusion of H2 and CO2 through the ZIF-8 membrane was easy (
In
Although higher operating temperatures are preferred for H2/CO2 separation, most ZIF-8 membranes in the literature were tested in relatively moderate temperature ranges below 150° C. The low thermal stability of the previously mentioned conventional polycrystalline ZIF-8 membrane hinders practical use under industrially relevant conditions. In fact, a polymer membrane having high processability and low manufacturing cost is a competitive candidate for H2/CO2 separation in a low-temperature region. Compared to other ZIF-8 membranes, the Z_Znα in the present invention exhibited a significant H2/CO2 SF at a high temperature of 300° C. More importantly, the Z_Znα membrane exhibited and maintained significant H2/CO2 SF for up to 72 hours continuous measurement with an H2/CO2 SF of about 8.0 at a WGS reaction temperature of 300° C. The Z_Znα in the present invention showed the highest hydrothermal stability against H2/CO2 SF among the ZIF films reported so far (
Finally, in
Preferably, the first ZIFs portion (1st ZIFs) may have 0.1 μm to 10 μm, 1 μm to 5 μm, preferably 2 μm to 4 μm, and more preferably 3 μm. At this time, even if the surface of the first ZIFs part (1st ZIFs) formed on the upper portion of the porous support (α-Al2O3 disk) is polished, sufficient separation performance can be maintained thanks to the second ZIFs part (2nd ZIFs) inside the porous support.
In addition, the second ZIFs portion 2 nd ZIFs may be formed to penetrate from the interface between the first ZIFs portion 1 st ZIFs and the second ZIFs portion 2 nd ZIFs to a depth of 10 μm to 500 μm, 50 μm to 200 μm, preferably 70 μm to 150 μm, and more preferably 80 μm.
As described above, since the second ZIFs unit (2nd ZIFs) are formed to penetrate into the porous support to a predetermined depth, high thermal stability of the membrane may be secured.
Zinc acetate, which is used in the present invention as a metal source for ZIF-8 formation, was intentionally pre-deposited and immobilized on a porous α-Al2O3 disk to mitigate diffusion into the bulk solution during the CD method. In particular, the pre-deposited Zn source was important for growing ZIF-8 particles that block the pores of the α-Al2O3 disk. This manufacturing concept results in embedded membrane grains to constitute a continuous membrane construction. The resulting ZIF-8 membrane exhibited a maximum H2/CO2 separation performance of about 8.7 at a WGS reaction temperature of 300° C. Interestingly, H2/CO2 separation performance was almost proportional to the amount of buried ZIF-8 particles. These results show that the pre-deposition of the Zn source controls the membrane structure to control the separation performance. The buried ZIF-8 layer also improved the mechanical and thermal resistance of the membrane. More importantly, the ZIF-8 membrane particles embedded deep inside the α-Al2O3 disk formed a membrane structure about 80 μm thick, and surprisingly resisted the harsh high temperature H2/CO2 stream. Specifically, the well-constructed ZIF-8 membrane achieved stable H2/CO2 separation performance at a WGS reaction temperature of 300° C. for up to 72 hours, while the ZIF-8 membrane prepared using the conventional CD method failed. Nevertheless, the inflow of water vapor significantly damaged the ZIF-8 membrane particles after 3 hours at 300° C., indicating the need for additional strategies for water-resistant ZIF-8 particle formation. The present invention demonstrates an easy strategy for enhancing membrane thermal durability and emphasizes the potential of ZIF-8 membranes as membranes or membrane reactor walls for hot streams.
The description of the presented exemplary embodiments is provided so as for those skilled in the art to use or carry out the present disclosure. Various modifications of the exemplary embodiments may be apparent to those skilled in the art, and general principles defined herein may be applied to other exemplary embodiments without departing from the scope of the present disclosure. Accordingly, the present disclosure is not limited to the exemplary embodiments suggested herein, and shall be interpreted within the broadest meaning range consistent to the principles and new characteristics presented herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2023-0043081 | Mar 2023 | KR | national |
