Patent Application
20250065297
The present invention relates to methods of generating zeolites and methods of tiling zeolite nanosheet plates on polymer supports.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Zeolites are crystalline aluminosilicates containing ordered pores of molecular dimensions well-suited for molecular catalysis and separations in petrochemical, biochemical reactions, energy production processes. Zeolites with small and medium pore openings, such as the zeolite A (LTA) and MFI types, can achieve perfect selectivity to water and proton permeation over hydrated metal ions by molecule and ion sieving effects. Thus, zeolite membranes can be useful to high-efficiency proton-conduction in electrolyte solutions and desalination of concentrated brines for which traditional polymer membranes are inefficient. The MFI-type zeolites are particularly desirable for such applications because their pore openings (approximately 0.56 nm) are close to the maximum width (approximately 0.60 nm) allowing total rejection of metal ions and hence can achieve low resistances to water and proton (mainly H3O+) transport with high selectivity. The porous structure of zeolites makes them highly effective for catalysis due, at least in part, to the proportionally higher number of active sites for catalytic reactions arising from increased surface area.
In recent years, there have been successful syntheses of two-dimensional (2D) zeolite nanosheets (ZNs) with thicknesses of one or two unit-cells. These 2D ZNs maximize the accessibility of active surface sites and micropores for adsorbing molecules and catalyzing reactions. Meanwhile, the ultrathin ZNs minimize the diffusion length that overcomes molecular transport limitations in catalysis and separation processes. The 2D ZNs and their hierarchical assemblies thus offer significantly enhanced performances in heterogeneous catalysis and molecular separations. The 2D ZNs with large lateral-to-thickness aspect ratios can also laminate ultrathin membranes of desired orientations to reduce the transmembrane diffusion resistance and nonselective intercrystalline entrances leading to simultaneous improvements in permeability and selectivity.
Despite the tremendous potentials demonstrated on small-sized samples, the development of practical ZN adsorbents, catalysts, and membranes has been hampered by the lack of methods for efficient synthesis of redispersible activated ZNs with well-preserved micropore accessibility and surface properties. In the literature, medium pore size MWW and MFI type ZNs with thickness of one- or two-unit cells were obtained by exfoliation from layered zeolite precursors (LZPs) synthesized using diquaternary or triquaternary ammonium structure directing agents (SDAs). The exfoliated single crystalline ZNs are typically 100-400 nm in lateral lengths. These small-size ZNs tend to aggregate and deform, which can cause difficulties in reassembling useful macrostructures. For example, aggregation of ZNs often results in non-selective or less selective structures which allows for increased permeation through intercrystalline spaces rather than through the zeolite pores, thereby reducing the efficacy of the ZN membranes in separation applications. Recently, a nanoparticle-seeded secondary growth method was developed to directly produce isolated single crystalline MFI ZNs using diquaternary bis-1,5(tripropyl ammonium) pentamethylene diiodide (dC5) as the SDA. However, the as-synthesized ZN crystal contains an isotropic core grown from the spherical seed, which must be eliminated by tedious processes to achieve flat ZNs. Methods for obtaining flat ZNs by LZP exfoliation and seeded secondary growth methods are both prohibitively complex with extremely low yields. The difficult redispersion of activated ZNs also limits the fabrication of ZN-laminated membranes on economical polymer substrates. Therefore, a need still exists for an efficient method to synthesize ZN assemblies and for an efficient method to prepare highly dispersed open-pore ZN materials for tiling a polymer substrate with ZN plates to form molecular and ion separation membranes.
Accordingly, there is a need for a method for producing zeolite nanosheet assemblies that does not require removing the seed core to form a flat zeolite nanosheet. Additionally, there is a need for a method of producing zeolite nanosheet plates with large areas for use in application such as zeolite nanosheet plate-tiled polymer membranes.
Certain exemplary aspects of the invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be explicitly set forth below.
One aspect of the invention is directed to a method for synthesizing flower-like zeolite nanosheet (ZN) assemblies from pure-silica MFI (silicalite) ZN flake seeds, the method including (a) obtaining ZN flake seeds; and (b) growing single-crystal nanosheets from the ZN flake seeds to form ZN assemblies in a synthesis solution comprising a source of silica and a structure directing agent (SDA).
In one embodiment of the invention, the synthesis solution of step (b) comprises diquaternary bis-1,5(tripropyl ammonium) pentamethylene diiodide (dC5) as an SDA.
In one embodiment of the invention, the synthesis solution of step (b) comprises tetraethyl orthosilicalite (TEOS) as a source of silica. In a further embodiment, the synthesis solution of step (b) further comprises diquaternary bis-1,5(tripropyl ammonium) pentamethylene diiodide (dC5) as an SDA. In a yet further embodiment, the synthesis solution of step (b) further comprises a base.
In one embodiment of the invention, obtaining the ZN flake seeds in step (a) is preceded by producing the ZN flake seeds, wherein producing the ZN flake seeds includes (i) generating silicalite nanoparticles (NPs) in a silicalite synthesis solution comprising a source of silica and a structure directing agent (SDA) to produce NP seeds; (ii) growing single crystalline silicalite ZNs from the NP seeds using a ZN precursor solution comprising an SDA to produce seed-evolved ZNs; (iii) cleaning the seed-evolved ZNs; and (iv) fracturing the cleaned seed-evolved ZNs to produce ZN flake seeds.
In one such embodiment, the source of silica in the silicalite synthesis solution includes tetraethyl orthosilicate (TEOS).
In one such embodiment, the SDA of the silicalite nanoparticle seed synthesis solution comprises tetrapropyl ammonium hydroxide (TPAOH).
In one such embodiment of the invention, the SDA of the ZN precursor solution comprises diquaternary bis-1,5(tripropyl ammonium) pentamethylene diiodide (dC5).
In one such embodiment of the invention, the ZN precursor solution further comprises a source of silica. In a further embodiment thereof, the source of silica of the ZN precursor solution comprises tetraethyl orthosilicalite (TEOS).
In one such embodiment of the invention, the ZN precursor solution is hydrolyzed prior to step (ii).
In one such embodiment of the invention, cleaning the seed-evolved ZNs comprises subjecting the seed-evolved ZNs to at least one base treatment step.
In one such embodiment of the invention, cleaning the seed-evolved ZNs comprises subjecting the seed-evolved ZNs to at least one base-chloride treatment step.
In one such embodiment of the invention, fracturing the seed-evolved ZNs comprises ball milling. In a further embodiment, fracturing the seed-evolved ZNs comprises sonicated ball milling in water.
In one such embodiment of the invention, the synthesis solution of step (b) comprises diquaternary bis-1,5(tripropyl ammonium) pentamethylene diiodide (dC5) as an SDA.
In another embodiment where obtaining the ZN flake seeds in step (a) is preceded by producing the ZN flake seeds, wherein producing the ZN flake seeds comprises: (i) obtaining a ZN assembly; (ii) cleaning the ZN assembly; and (iii) fracturing the ZN assembly to produce ZN flake seeds.
In one such embodiment of the invention, obtaining a ZN assembly comprises obtaining a ZN assembly produced according to step (b)
In one such embodiment of the invention, cleaning the ZN assembly comprises subjecting the ZN assembly to at least one base treatment step.
In one such embodiment of the invention, cleaning the ZN assembly comprises subjecting the ZN assembly to at least one base-chloride treatment step.
In one such embodiment of the invention, wherein fracturing the ZN assembly comprises ball milling. In a further embodiment, fracturing the ZN assembly comprises sonicated ball milling in water
In one embodiment of the invention, growing single crystal nanosheets comprises growing multilayered ZN plates. In a further embodiment, the multilayered ZN plates comprise greater than or equal to 2 single-crystal ZN layers and less than or equal to 20 single crystal ZN layers.
Another aspect of the invention is directed to a method for producing a polymer support zeolite nanosheet plate-tiled (ZNPT) membrane comprising: (a) obtaining ZN plates; (b) dispersing the ZN plates in a polymer tiling solution comprising a solvent, an amount of dissolved polymer binders, and a binder solvent to form a ZN plate dispersion; (c) tiling the ZN plate dispersion onto a polymer substrate to form a ZN plate layer; (d) drying the ZN plate layer on the polymer substrate after step (c); and (e) curing the ZN plate layer coated on the polymer substrate after step (d) to form a polymer-support ZNPT membrane.
In one embodiment of the invention, step (a) for obtaining ZN plates is preceded by producing the ZN plates, wherein producing the ZN plates comprises: (i) obtaining a ZN assembly; (ii) cleaning the ZN assembly; and (iii) fracturing the ZN assembly in a liquid solvent to produce ZN plates. In one embodiment of the invention, the method further includes step (iv) for activating the ZN assembly prior to fracturing the ZN assembly. In one such embodiment, activating the ZN assembly comprises calcination in air at a temperature greater than or equal to 400° C.
In one such embodiment of the invention, obtaining a ZN assembly comprises synthesizing a ZN assembly according to the aspect of the invention detailed above.
In one such embodiment of the invention, cleaning the ZN assembly comprises subjecting the ZN assembly to at least one base treatment step.
In one such embodiment of the invention, cleaning the ZN assembly comprises subjecting the ZN assembly to at least one base-chloride treatment step.
In one such embodiment of the invention, fracturing the ZN assembly comprises ball milling. In a further embodiment of the invention, fracturing the ZN assembly comprises sonicated ball milling in an organic solvent. In a yet further embodiment of the invention, the organic solvent comprises ethanol.
In one embodiment of the invention, the amount of dissolved polymer binders of step (b) comprises polyvinylidene fluoride (PVDF).
In one embodiment of the invention, the solvent of step (b) comprises ethanol.
In one embodiment of the invention, the binder solvent of step (b) comprises dimethyl sulfoxide (DMSO). In a further embodiment, the solvent of step (b) comprises ethanol. In a yet further embodiment, the weight ratio of ethanol to DMSO is 2:1.
In one embodiment of the invention, the ZN plates comprise greater than or equal to 0.01 wt. % by weight of the polymer tiling solution of step (b).
In one embodiment of the invention, step (c) comprises: (i) placing the polymer substrate between the ZN plate dispersion and a downstream compartment; and (ii) applying a pressure driving force to the ZN plate dispersion to tile the ZN plates onto the polymer substrate using filtration coating. In one such embodiment, applying a pressure driving force comprises applying a downstream vacuum. In one such embodiment, applying a pressure driving force comprises applying an upper stream pressurization.
In one embodiment of the invention, step (d) comprises subjecting the ZN plate layer on the polymer substrate to a temperature greater than or equal to 80° C. for a period of time greater than or equal to 3 hours. In a further embodiment, step (d) further includes pulling a vacuum at a pressure less than or equal to 1.5 kPa during step (d).
In one embodiment of the invention, step (e) comprises subjecting the ZN plate layer on the polymer substrate to a temperature greater than or equal to 120° C. for a period of time greater than or equal to 3 hours. In a further embodiment, step (e) further includes pulling a vacuum at a pressure less than or equal to 24 kPa during step (e).
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. Similar reference numerals are used to indicate similar features throughout the various figures of the drawings.
One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
In one embodiment, the present invention is a self-seeded single-step synthesis method for efficient reproduction of flower-like ZN-assemblies comprising very large single-crystal nanosheets of about two unit-cells in thickness. Another embodiment of the invention is a method for forming ZN plates from a flower-like ZN assemblies. Yet another embodiment of the invention is a method for forming ZN plates from single crystal ZN seeds. Microscopic investigations were carried out to understand the ZN-seeded crystal growth mechanisms and unveil the microstructure and pore system of the ZN plates and ZN assemblies. The ZN materials were demonstrated with markedly enhanced molecular adsorption and transport properties using xylenes as probing molecules. Another embodiment of the invention is directed to a method for using ZNs and/or ZN plates to tile a polymer substrate.
With reference to
The pure-silica MFI (silicalite) ZNs used as ZN flake seeds for growing the first-generation ZN-assemblies may be synthesized by fracturing the seed-evolved ZNs produced according to the method reported by Jeon et al., Ultra-selective high-flux membranes from directly synthesized zeolite nanosheets, Nature. 543 (2017) 690-694. These seed-evolved ZNs were obtained by two separate steps of hydrothermal reactions. The first step was to generate silicalite nanoparticles (NP) seeds using a silicalite synthesis solution including an SDA, a source of silica, a solvent, and a base. The second step was to grow single crystalline silicalite zeolite nanosheets (ZNs) by mixing NP seeds and a ZN precursor solution including an SDA, a source of silica, a solvent, and a base. The detailed synthesis procedure is schematically depicted in
With regard to generating ZNP seeds, the SDA may include, for example, tetrapropylammonium hydroxide (TPAOH), tetrapropylammonium bromide (TPABr), other suitable SDAs, or a combination thereof. In one embodiment, the SDA is TPAOH. The source of silica may include, for example, silica dioxide (SiO2), tetraethyl orthosilicate (TEOS), aluminosilicates, some other suitable source of silica, or some combination thereof. In one embodiment, the source of silica is SiO2. The solvent may include, for example, water, some other polar solvent, or some combination thereof. The base may include, for example, a hydroxide, ammonia, some other suitable base, or some combination thereof. In one embodiment, the base is a hydroxide. The base may be included in an amount to achieve a target pH. In one embodiment, the target pH is 13. In one embodiment, the molar ratio of the reagents is 10 SDA:2.4 source of silica:0.87 hydroxide:114 water. In a further embodiment, the SDA is TPAOH and the source of silica is SiO2.
The synthesis of NP seeds may involve agitating the reagents for a period of time using means such as, for example, stirring, sonicating, other suitable means of mixing or agitating the reagents, or some combination thereof. The period of time for agitating the reagents may be greater than or equal to 1 hour, alternatively greater than or equal to 6 hours, or still alternatively greater than or equal to 12 hours. The stirring or agitation make take place at room temperature. Following agitation of the reagents for a defined period of time, the method of synthesizing NP seeds may include heating the reagents for a period of time. The period of time for heating the reagents may be greater than or equal to 8 hours, alternatively greater than or equal to 24 hours, still alternatively greater than or equal to 72 hours, or yet still alternatively greater than or equal to 144 hours. The period of time for heating the reagents may depend on the elevated temperature the reagents are heated to. For example, the reagents may be heated to a temperature greater than or equal to 50° C., alternatively a temperature greater than or equal to 100° C., or still alternatively a temperature greater than or equal to 140° C. Suitable methods for heating the reagents include, for example, an oil bath, a water bath, an oven, other suitable heating methods, or some combination thereof. In one embodiment, the reagents are heated without stirring.
Following heating the NP seed reagents for a period of time, the solution may be filtered and the filtrate subjected to additional heating for a period of time. The filtration may involve the use of a filter such as, for example, syringe filtration, column filtration, membrane filtration, vacuum filtration, some other suitable filtration technique, or some combination thereof. The period of time for heating the filtrate may be greater than or equal to 8 hours, alternatively greater than or equal to 24 hours, or still alternatively greater than or equal to 72 hours. The period of time for heating the filtrate may depend on the elevated temperature the filtrate is heated to. Heating the filtrate for a period of time may involve heating the filtrate to a temperature greater than or equal to 50° C., alternatively a temperature greater than or equal to 65° C., or still alternatively a temperature greater than or equal to 90° C. Suitable methods for heating the filtrate include, for example, an oil bath, a water bath, an oven, other suitable heating methods, or some combination thereof. In one embodiment, the filtrate is heated without stirring.
Following heating the filtrate for a period of time, the NP seed crystals may be washed using a solvent such as, for example, de-ionized (DI) water. This washing step may include a plurality of rinse steps and centrifugation steps to remove the supernatant liquid.
With regard to synthesizing single crystalline silicate ZNs, NP seeds are mixed with a ZN precursor solution. The NP seeds may include a NP synthesized using the method described above, another suitable ZNP, or some combination thereof. In one embodiment, the NP seeds are dispersed in solution, such as water, prior to mixing with the ZN precursor solution. With regard to the ZN precursor solution, the SDA may include, for example, dC5, diquaternary ammonium compounds, some other suitable SDA, or some combination thereof. In one embodiment, the SDA is dC5. The source of silica may include, for example, tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), tetrapropyl orthosilicate (TPrOS), tetrabutyl orthosilicate (TBOS), tetrapentyl orthosilicate (TPeOS), tetrahexyl orthosilicate (THOS), aluminosilicates, fumed silica, some other suitable source of silica, or some combination thereof. In one embodiment, the source of silica is TEOS. The solvent may include, for example, water, some other polar solvent, or some combination thereof. The base may include, for example, a hydroxide, ammonia, some other suitable base, or some combination thereof. In one embodiment, the base includes a hydroxide such as, for example, sodium hydroxide, potassium hydroxide, some other hydroxide containing salt, or some combination thereof. In one embodiment, the molar ratio of the reagents in the ZN precursor solution is 3.75 SDA:80 source of silica:20 hydroxide:9500 water. In a further embodiment, the SDA is dC5 and the source of silica is TEOS.
The ZN precursor solution may be hydrolyzed prior to synthesis of the ZNs for a period of time. In one embodiment, hydrolyzation includes air purging of approximately 50 m/min at room temperature. The period of time for hydrolysis may be greater than or equal to 1 hour, alternatively greater than or equal to 4 hours, or still alternatively greater than or equal to 16 hours. In embodiments where the source of silica is an orthosilicate (e.g., TMOS, TEOS, etc.), hydrolysis may reduce the concentration of alcohols in the ZN precursor solution. In one such embodiment, the hydrolysis is a gas-purged hydrolysis.
Following hydrolysis of the ZN precursor solution, the ZN precursor solution may be subjected to filtration and the filtrate mixed with a solution including a dispersion of NP seeds to form seed-evolved ZNs. The filtration may involve the use of a filter such as, for example, syringe filtration, column filtration, membrane filtration, some other suitable filtration technique, or some combination thereof. In one embodiment, the solution is syringe filtered through a GHP (polypropylene) filter with a pore size smaller than or equal to 0.45 μm. The molar ratio of silica between the NP seed dispersion and the ZN precursor solution may be greater than or equal to 50, alternatively greater than or equal to 200, or still alternatively greater than or equal to 50 and less than or equal to 1000. After mixing the NP seed dispersion solution and the ZN precursor solution, the mixture may be subjected to hydrothermal treatment for a period of time to form seed-evolved ZNs. In one such embodiment, hydrothermal treatment of the mixture includes heating the mixture to a temperature greater than or equal to 140° C. in an autoclave for a period of time. The period of time may be greater than or equal to 36 hours, alternatively greater than or equal to 36 hours and less than or equal to 96 hours, or still alternatively greater than or equal to 96 hours.
Depending on factors such as, for example, the period of time for hydrothermal treatment, more or less amorphous silica may be formed along with the seed-evolved ZNs. Generally speaking, lower hydrothermal treatment times results in a higher amount of amorphous silica formed. Additionally, the seed-evolved ZNs may include one or more organic residue adsorbed onto the zeolite. In embodiments where a non-negligible amount of amorphous silica is formed and/or organic residues remain on the seed-evolved ZNs, the hydrothermal treatment step may be followed by an optional base treatment. The optional base treatment may include, for example, mixing equal volumes of a dispersion containing the seed-evolved ZNs with a base solution having 0.1 M hydroxide ions (e.g., from a hydroxide salt such as potassium hydroxide, sodium hydroxide, etc.) to form a base treatment mixture and centrifuging the base treatment mixture to recover the ZN products. In one embodiment, the volume of the dispersion containing the seed-evolved ZNs and the volume of the base solution are each 1 mL. Depending on the amount of amorphous silica formed, the above base treatment steps may be repeated as needed until a white slurry including the seed-evolved ZNs can be obtained following centrifugation. In one embodiment, the above base treatment steps are repeated one time, alternatively greater than or equal to one time.
Following the above optional base treatment steps or as an alternative to the above optional base treatment steps, the white slurry containing the seed-evolved ZNs may be subjected to an optional base-chloride treatment step. In an optional base-chloride treatment step, the seed-evolved ZNs may be dispersed in a base-chloride cleaning solution having a volume approximately equal to the base treatment mixture. In one embodiment, the volume of the base-chloride cleaning solution may be 2 mL. This cleaning process may improve the usefulness of seed-evolved ZNs formed at least by removing the amorphous silica and organic residues adsorbed on ZN surfaces, which could shield the active nucleation sites, and by cleaving surface Si—O—Si that could create [≡Si—O(—)] sites to induce nucleation in different orientations in subsequent steps. In one embodiment, the base-chloride solution has 0.1 M hydroxide ions and 1 M chloride ions (e.g., potassium chloride, etc.), or alternatively 0.1 M hydroxide ions and 2 M chloride ions. In one embodiment, the base solution and the base chloride solution are made using hydroxide salts and chloride salts having the same cation. The dispersion of the white slurry containing the seed-evolved ZNs in the base-chloride cleaning solution may be stirred at room temperature for a period of time greater than or equal to 8 hours, alternatively greater than or equal to 24 hours, still alternatively greater than or equal to 72 hours, or yet alternatively greater than or equal to 168 hours. In some embodiments, such as those embodiments with longer treatment times with the base-chloride cleaning solution (e.g., greater than or equal to 72 hours), the base-chloride cleaning solution may be replaced with a fresh base-chloride cleaning solution one or more time. Following this dissolution step in the base-chloride cleaning solution, the seed-evolved ZNs may washed with DI water one or more time.
The seed-evolved ZNs, which may be formed according to the steps above, may then then be subjected to mechanical force sufficient to fracture and/or separate the ZN portion from the NP seed core. Applying a mechanical force may include methods such as, for example, milling, sonication, sonicated ball-milling, other suitable methods of applying mechanical force, or some combination thereof. In one embodiment, fracturing the seed-evolved ZNs includes sonicated ball milling in a solvent. In one such embodiment, the seed-evolved ZNs are dispersed in water for sonicated ball milling. In another such embodiment, the solution for sonicated ball milling is ethanol. Water may be used as a solvent in applications where the fractured ZN portions are later used as ZN flake seeds. Ethanol may be used as a solvent where the fractured ZN portions are ZN plates later used to tile a polymer substrate or a polymer membrane (discussed further below). The fractured flat ZN fragments (i.e., ZN flakes), which can be used as ZN seeds (i.e., ZN flake seeds) as discussed further below, were separated from the large core debris by one or more fractionation and rinse steps. The fractionation step may include at least one of centrifugation, filtration, sedimentation, some other suitable fractionation technique, or some combination thereof. Fractionation and rinsing may be followed by further ball-milling to generate smaller ZN flake seeds for greater seed population.
These ZN fragments may then be used as ZN flake seeds. In one embodiment, the ZN flake seeds may be subsequently dispersed in a synthesis solution including a source of silica, an SDA, a solvent, and a base. The SDA may include, for example, dC5, diquaternary ammonium compounds, some other suitable SDA, or some combination thereof. In one embodiment, the SDA is dC5. The source of silica may include, for example, tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), tetrapropyl orthosilicate (TPrOS), tetrabutyl orthosilicate (TBOS), tetrapentyl orthosilicate (TPeOS), tetrahexyl orthosilicate (THOS), fumed silica, aluminosilicates, some other suitable source of silica, or some combination thereof. In one embodiment, the source of silica is TEOS. The solvent may include, for example, water, some other polar solvent, or some combination thereof. The base may include, for example, a hydroxide, ammonia, some other suitable base, or some combination thereof. In one embodiment, the base includes a hydroxide such as, for example, sodium hydroxide, potassium hydroxide, some other hydroxide containing salt, or some combination thereof. In one embodiment, the synthesis solution had a molar ratio of components equal to 80 source of silica:3.75 SDA:20 hydroxide:12000 H2O to avoid undesirable nucleation and inhibit random growth. In a further embodiment, the source of silica is TEOS and the SDA is dC5. The mass ratio of silica between the ZN seeds and the source of silica in the precursor solution may be greater than or equal to 500, alternatively greater than or equal to 1000, or still alternatively greater than or equal to 2000.
The mixture of ZN flake-seeds and synthesis solution may then be subjected to hydrothermal treatment for a period of time to obtain a first-generation ZN-assembly. Generally speaking, the ZN seeds may function as a substrate from which secondary growth using the synthesis solution may occur. In one embodiment, the mixture of ZN seeds and synthesis solution are placed in an autoclave for autogenous reaction at 140° C. The period of time for hydrothermal treatment of the ZN seeds and the synthesis solution may be greater than or equal to 24 hours, alternatively greater than or equal to 96 hours, or still alternatively greater than or equal to 96 hours and less than or equal to 144 hours. After the hydrothermal treatment, the first-generation ZN-assemblies were separated by centrifuge and then thoroughly cleaned using a hydroxide cleaning solution. Following this cleaning, the first-generation ZN-assembly may be dried and characterized.
The reaction conditions used to synthesize the ZN assemblies may impact the structure of the ZN assemblies formed. In one embodiment, the ZN assembly has a predominantly planar structure and comprises multiple single-crystal ZN layers to form a ZN plate. The ZN plate may have a thickness expressed in ZN layers such as greater than or equal to 2 single-crystal ZN layers, alternatively greater than or equal to 10 single-crystal ZN layers, or still alternatively greater than or equal to 2 single-crystal ZN layers and less than or equal to 20 single-crystal ZN layers. In another embodiment, the first-generation ZN assembly formed had multiple intersecting planar structures that resembled a flower such as, for example, a rose-like structure or a spiral leaf-like structure. In a further embodiment, the flower-like structure may be comprised of one or more orthogonal ZN plate.
In some embodiments, an optional secondary cation addition step may be implemented during thermal treatment of the ZN-seeds and precursor solution to form a ZN-material incorporating isomorphous substitutes. In such embodiments, a source of one or more optional secondary element, such as a metal salt containing an optional secondary metal, may be dissolved or dispersed in water, the solution or dispersion including the one or more optional secondary metal may be mixed added to the mixture of ZN-seeds and precursor solution, and the resulting mixture may be subjected to hydrothermal treatment for a period of time. The solution or dispersion of the one or more optional secondary metal may be prepared or selected such that the atomic ratio of silicon to secondary metal(s) in the resulting ZN-material is greater than or equal to 1:1. The secondary metal may include, for example, aluminum, boron, tungsten, other suitable metals, or some combination thereof. In one embodiment, aluminum is included as a secondary framework metal ion from a sodium aluminate dissolved in a solution. The period of time for hydrothermal treatment of the mixture after introducing the optional secondary metal may be greater than or equal to 12 hours, or alternatively greater than or equal to 12 hours and less than or equal to 36 hours. It should be understood that further references to first-generation ZN assemblies may refer to embodiments incorporating one or more secondary metal as described above.
First-generation ZN assemblies, such as those formed using the method steps detailed above, may then be subjected to mechanical force sufficient to form ZN fragments which can serve as ZN flake seeds. Applying mechanical force sufficient to form ZN fragments may include methods such as, for example, milling, sonication, sonicated ball-milling, other suitable methods of applying mechanical force, or some combination thereof. In one embodiment, fracturing the ZN assemblies includes sonicated ball milling in a solvent. In one such embodiment, the ZN assemblies are dispersed in water for sonicated ball milling. In another such embodiment, the solution for sonicated ball milling is ethanol. Water may be used as a solvent in applications where the ZN fragments are later used as ZN flake seeds. Ethanol may be used as a solvent where the ZN fragments are ZN plates later used to tile a polymer substrate or a polymer membrane (discussed further below). These ZN fragments may be used as ZN flake seeds instead of or in addition to the NP seeds and/or the ZN flake seeds formed from fracturing the NP seed-evolved ZNs. The ZN seeds are mixed with a synthesis solution containing an SDA, a source of silica, a solvent, and a base. The SDA may include, for example, dC5, diquaternary ammonium compounds, some other suitable SDA, or some combination thereof. In one embodiment, the SDA is dC5. The source of silica may include, for example, tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), tetrapropyl orthosilicate (TPrOS), tetrabutyl orthosilicate (TBOS), tetrapentyl orthosilicate (TPeOS), tetrahexyl orthosilicate (THOS), aluminosilicates, fumed silica, some other suitable source of silica, or some combination thereof. In one embodiment, the source of silica is TEOS. The solvent may include, for example, water, some other polar solvent, or some combination thereof. The base may include, for example, a hydroxide, ammonia, some other suitable base, or some combination thereof. In one embodiment, the base includes a hydroxide such as, for example, sodium hydroxide, potassium hydroxide, some other hydroxide containing salt, or some combination thereof. In one embodiment, the synthesis solution had a molar ratio of components equal to 80 source of silica:3.75 SDA:20 hydroxide:12000 H2O to avoid undesirable nucleation and inhibit random growth. In a further embodiment, the source of silica is TEOS and the SDA is dC5. The mass ratio of silica between the ZN seeds and the source of silica in the precursor solution may be greater than or equal to 500, alternatively greater than or equal to 1000, or still alternatively greater than or equal to 2000. In one embodiment, the synthesis solution for the ZN seeds formed from first-generation ZN assemblies contains the same components in the same ratio as the synthesis solution for the ZN seeds formed from the NP seed-evolved ZNs.
The mixture of the ZN flake seeds formed from fracturing the first-generation ZN assembly fragments and the synthesis solution may be subjected to hydrothermal treatment for a period of time to form a second-generation ZN assembly. Generally speaking, the ZN seeds may function as a substrate from which secondary growth using the synthesis solution may occur. In one embodiment, the mixture of ZN seeds and synthesis solution are placed in an autoclave for autogenous reaction at 140° C. The period of time for hydrothermal treatment of the ZN flake seeds and the synthesis solution may be greater than or equal to 24 hours, alternatively greater than or equal to 96 hours, or still alternatively greater than or equal to 96 hours and less than or equal to 144 hours. After the hydrothermal treatment, the second-generation ZN-assemblies may be separated by centrifuge and then thoroughly cleaned using a hydroxide cleaning solution as discussed above. Following this cleaning, the second-generation ZN-assembly may be dried and characterized.
It should be understood that the method for obtaining a second-generation ZN assembly from a first-generation ZN assembly may be adapted to obtain a third-generation ZN assembly from a second-generation ZN assembly using similar steps, to obtain a fourth-generation ZN assembly from a third-generation ZN assembly using similar steps, and so on. In one embodiment, forming a second-generation or later ZN assembly (hereinafter a higher-generation ZN assembly) may use ZN fragments (i.e., ZN flake seeds) from only the preceding ZN assembly generation. In an alternate embodiment, forming a higher-generation ZN assembly may use ZN fragments from one or more previous generation ZN assembly (e.g., a third-generation ZN assembly may use ZN fragments from both first-generation and second-generation ZN assemblies). In yet another alternate embodiment, forming a higher-generation ZN assembly may use ZN fragments from the NP seed-evolved ZN and one or more previous generation ZN assembly. A major difference between forming the first-generation ZN assemblies and forming second-generation ZN assemblies and onwards is that the ZN seeds for second-generation ZN-assemblies and onwards may simply include the ZN fragments of one or more of the prior generation ZN assemblies. Put differently, formation of higher-generation ZN assemblies may allow for formation of ZN seeds without requiring further synthesis of NP seeds or NP seed-evolved ZNs.
Any of the ZN assemblies described above (i.e., first-generation ZN assemblies and higher-generation ZN assemblies) may be activated. In one embodiment, the ZN assembly may be activated by subjecting the ZN assembly to air at an elevated temperature for a period of time to calcinate the ZN assembly. Calcination may function to remove the SDA, such as dC5, from the zeolite pores to activate the zeolite without collapsing the structure of the ZN assembly. The elevated temperature may be greater than 400° C., alternatively greater than or equal to 500° C., or still alternatively greater than or equal to 600° C. The elevated temperature may be reached by gradually increasing the temperature of the environment. In one such embodiment, the temperature is increased at a rate of less than or equal to 5° C./min. The period of time for calcination may be greater than or equal to 3 hours, or alternatively greater than or equal to 6 hours. Following calcination, the temperature of the environment may be gradually reduced. In one such embodiment, the temperature following calcination is reduced at a rate of less than or equal to 5° C./min.
After calcination, the ZN assemblies may be subjected to dissociation or fracturing in liquid solvent to produce dispersed open-pore ZN plates for tiling a membrane on a polymer support. The dissociation or fracturing to form ZN plates may include, for example, sonication, milling, sonicated ball-milling, some other means of fracturing a zeolite, or some combination thereof. In one embodiment, fracturing the ZN assemblies includes sonicated ball milling in a solvent. In one such embodiment, the ZN assemblies are dispersed in water for sonicated ball milling. In another such embodiment, the solution for sonicated ball milling is ethanol. Ethanol may be used as a solvent where the ZN plates are later used to tile a polymer substrate or a polymer membrane. In one such embodiment, the calcined ZN assembly is fractured using sonicated ball milling in an ethanol bath. The ZN plates dissociated or fractured from the ZN assembly may be mixed with smaller ZN fragments. Larger ZN plates may be separated from ZN fragments using a fractionation technique including, for example, sedimentation, filtration, centrifugation, some other means of fractionating solids based on size, or some combination thereof.
With regard to
Preactivated ZN plates may be used for tiling on a polymer substrate to form a ZN plate-tiled (ZNPT) membrane. The polymer substrate may include a material such as, for example, polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polysulfone, polyacrylonitrile (PAN), Polytetrafluoroethylene (PTFE), Nafion®, other suitable polymer substrates, or some combination thereof. In one embodiment, the polymer substrate may be porous. Tiling a polymer substrate with the ZN plates may involve dispersing the ZN plates in a polymer tiling solution including a solvent, an amount of dissolved polymer binders, and a binder solvent. The solvent may be an organic solvent, such as ethanol, isopropanol, some other organic solvent, or some combination thereof. In one embodiment, the solvent is configured to be miscible with the binder solvent but not to dissolve the polymer binder. The polymer binder dissolved in the polymer tiling solution may function as adhesive for the polymer substrate and the ZN plates. The polymer binder may include, for example, polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polysulfone, polyacrylonitrile (PAN), Polytetrafluoroethylene (PTFE), Nafion®, other suitable nonionic and ionic polymers, or some combination thereof. In one such embodiment, the polymer binder dissolved in the polymer tiling solution comprises the same materials as the polymer substrate. In one embodiment, the binder solvent is dimethyl sulfoxide (DMSO). The polymer binder may first be dissolved in the binder solvent before adding the solvent. In one such embodiment, the binder solvent DMSO is used to dissolve a PVDF polymer binder prior to adding an amount of the solvent, which is an organic solvent, for controlling ZN plate content in suspension. The ratio of solvent to the binder solvent may be selected to be high enough to ensure that the polymer binder completely dissolves and low enough that the polymer substrate surface is not damaged during the membrane coating process (discussed further below). In one such embodiment, the weight ratio of solvent to the binder solvent is 2:1. In a further embodiment, the solvent is an organic solvent and the binder solvent is DMSO. In an even further embodiment, the organic solvent is ethanol and the weight ratio of ethanol to DMSO is 2:1. The weight percentage of the ZN plates dispersed in the tiling solution may be selected to high enough to minimize uncovered spots or pinholes in the ZN plate layer and low enough to minimize defects from misaligned ZN plates. In one such embodiment, the weight percentage of ZN plates in solution is greater than or equal to 0.01 wt. %, alternatively greater than or equal to 0.02 wt. %, or still alternatively greater than or equal to 0.01 wt. % and less than or equal to 0.06 wt. %. Alternatively, the weight percentage of ZN plates in solution may be less than or equal to 0.1 wt. %. Still alternatively, the weight percentage of ZN plates in solution may be less than or equal to 1 wt. %.
The ZN plates may be tiled on the polymer substrate by filtration coating using a pressure driving force for filtration such as, for example, downstream vacuum or upper stream pressurization. The vacuum may use a downstream pressure of less than or equal to 0.1 bar for a period of time to pull the polymer tiling solution through the polymer substrate. In one embodiment, the period of time for vacuum filtration to tile the polymer substrate is approximately 3 minutes. The polymer substrate may be presoaked with the solvent to avoid strong capillary suction when loading the polymer tiling solution which may result in misaligned ZN plate deposition. In one such embodiment, the solvent is an organic solvent. The polymer tiling solution may be deposited on top of the polymer substrate, and vacuum may be pulled to filter the polymer tiling solution through the polymer substrate, thereby tiling the polymer substrate with the ZN plates. In one such embodiment, the polymer substrate may be mounted to a ceramic filter between a suspension reservoir and a downstream compartment, the polymer tiling solution may be deposited into the suspension reservoir on top of the polymer substrate, and vacuum may be pulled through a vacuum line to filter the polymer tiling solution from the suspension reservoir through the polymer substrate into the downstream compartment, thereby tiling the polymer substrate with the ZN plates (see
Following the vacuum filtration tiling process, the resulting ZNPT polymer substrate may be dried and/or cured. The temperatures for drying and curing depend on the type of polymer substrate, the type of binder, as well as the solvent(s) and/or binder solvent(s) used. In one embodiment, the ZNPT polymer substrate is dried at an elevated temperature under vacuum for a period of time. The drying process may involve a temperature greater than or equal to 80° C. for a period of time greater than or equal to 3 hours under a vacuum pressure of less than or equal to 1.5 kPa. In one embodiment, the ZNPT polymer substrate is cured at an elevated temperature under vacuum for a period of time. In a further embodiment, the ZNPT polymer substrate is cured after it is dried. The curing process may involve a temperature greater than or equal to 120° C. at a pressure less than or equal to 24 kPa for a period of time greater than or equal to 3 hours.
The morphological and microstructural characteristics of the ZN assemblies, ZN fragments, and other zeolite crystals of different growth stages were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) using a FEI Scios DualBeam microscope equipped with Ametek Octane Super EDAX.
The thicknesses of the ZN plates constituting the ZN-assemblies and the isolated single-crystalline ZN sheets were examined by a Veeco Dimension 3100 atomic force microscope (AFM) using height imaging/profiling under tapping mode.
The crystalline phase and orientation of the ZNs were identified by X-ray diffraction (XRD) using a PANalytical X'Pert Pro diffractometer with Cu Ku radiation (λ=1.5406 Å) and confirmed by transmission electron microscopy (TEM) and electron diffraction (ED) observations using a JEOL 2010F field emission electron microscope. The high-resolution TEM observations were also used to unveil the mechanisms of ZN-assembly formation and growth in the very early stages.
The XRD peak identification and crystal phase determination were based upon the standard spectrum of silicalite powders.
The porosity and surface area of the ZN-assemblies were examined by the N2 adsorption-desorption isotherms using an Miromeritics ASAP 2020 BET unit.
The dynamic adsorption behaviors of p-xylene and o-xylene vapors on the ZN-assemblies were investigated at 300 K by a Cahn 1000 Microbalance. The zeolite sample was degassed at about 573.15 K for overnight before each adsorption test. The gas phase xylene vapor pressure was around 0.67 kPa (or p/po approximately 0.5). The xylene adsorption data were used to analyze the accessibility of the micropores and mesopores in the ZN-assemblies. A sample of conventional silicalite crystals was also examined for xylene adsorption measurements for comparison.
First-generation ZNs and ZN assemblies were grown from seeds consisting of single crystalline silicalite ZN flakes (i.e., ZN fragments) of submicron lateral lengths and 4-nm thickness in its b-axis (
To investigate the secondary crystallization behavior of the ZN seeds, the solid products were retrieved after different reaction durations, which were microscopically analyzed. The reaction consisted of hydrothermal treatment of MFI ZN flake seeds dispersed in a synthesis solution and heated in an autoclave at 140° C. for a period of time. The synthesis solution included an SDA (dC5), a source of silica (TEOS), a solvent (water), and a base (potassium hydroxide) in a molar ratio of 3.75:80:20:12000. The period of time was varied to determine differences in synthesizing first-generation ZN assemblies at 12 hours, 24 hours, 48 hours, 72 hours, 96 hours, and 144 hours. The results of hydrothermal treatments for different periods of time were compared by analyzing samples using SEM images, TEM images, an AFM height survey.
After reacting for 12 hours, the SEM and TEM images revealed that the ZN flake seeds, which had irregular shapes and sizes around approximately 0.1 μm2 (
The AFM height survey revealed that the ZN seed seemingly formed a 10-20 nm tall central plateau in the premature rhombus sheet (
When the reaction duration was extended to 24 hours, TEM images showed that the ZN sheets further grew to rhombuses having an average size of approximately 7.5 μm2 (
When the reaction duration was extended to 48 hours, SEM imaging revealed that the ZN rhombus sheets further developed orthogonal ZN plates (
When the reaction duration was extended to 72 hours, more orthogonal ZN plates were formed and the orthogonal ZN plates were larger on average (
When the reaction duration was extended to 96 hours, the premature structures, e.g., the orthogonally-oriented ZN plates, and the amorphous nanoparticles seen in
Higher-generation ZN assemblies (i.e., second-generation and onward) were grown from ZN flake seeds generated by fracturing and/or dissociating the flower petals from the previous generation ZN assembly. The ZN plate petals could be readily dissociated or fractured by sonicated ball-milling in water assisted by >−4 mm zirconia milling-beads to harvest flat ZN plates having large areas (
After obtaining ZN flake seeds from the first-generation flower-like ZN assemblies, a single-step synthesis method was demonstrated for reproducing next generations of ZN flowers (
The results of sonicated ball milling and hydrothermal treatments to form second and third-generation ZN assemblies were analyzed using SEM images, TEM images, an AFM height surveys, ED patterns, and XRD spectra.
After subjecting the first-generation ZN assemblies (e.g., the flower-like structures discussed in Example 1) to sonicated ball milling, the resulting ZN plates were analyzed using SEM, TEM, and AFM height surveys. Upon inspection, the unfractionated flat ZN plates had typical areas larger than 1.5 μm×2.5 μm. The small ZN fragments used as ZN flake seeds were determined to have average side lengths less than or equal to 0.5 μm in side-lengths. The larger ZN plates could be further sonicated and/or ball-milled to increase the seed population if necessary.
The ZN plates separated from the flower-like assemblies were determined to be multilayered using TEM images and ED patterns of the plate edge (
The multilayered ZN plates were exfoliated to separate a single crystal sheet for ED analysis. ED patterns and lattice dimension of the exfoliated single crystal sheet confirms the [010] orientation. Exfoliation of the multilayered ZN plates did not happen appreciably during the sonicated treatments in the KOH solution, although a few single crystal sheets were observed in the suspension. However, limited exfoliation of the ZN-stacked plates occurred when the plates were treated by alternated freezing and thawing processes in organic solvents including iso-butanol, toluene, and xylenes. These exfoliated single nanosheets (
The second and third-generation ZN assemblies also formed flower-like structures such as the rose flower structure and the spiral leaf structure seen in the first-generation ZN assemblies. XRD spectra for the randomly packed ZN flowers and a glass-supported single layer ZN plates confirm the pure MFI zeolite phase and out-of-plane orientation in b-axis for the flat ZN plates.
The flower-like ZN assemblies were calcined to remove SDA and other organic residue before analyzing the samples using a BET to determine adsorption-desorption properties of various gases. The calcination process to activate the ZN assemblies involved heating the ZN assemblies to 600° C. in air for 8 hours. The N2 adsorption-desorption measurements were carried out for the ZN assemblies at 77 K before and after calcination.
A sample of conventional silicalite crystals was also examined by the BET for comparison. The conventional silicalite crystals were synthesized using an Al-free clear solution of TEOS as silica source and TPAOH as SDA. Both the ZN assembly and conventional crystal samples were thoroughly cleaned by the KOH solution before calcination activation. Before each adsorption measurements, the samples were dried at 393.15 K in a vacuum oven and further degassed at 523.15 K for 2.5 hours during measurements. These deep cleaning and evacuation processes were used to completely remove the amorphous silica and organic residues from the ZN surfaces and intra- and inter-ZN layer non-zeolitic pores.
The apparent transport diffusivity (DT, m2/s) of p-xylene and o-xylene for the flower-like ZN assemblies and conventional crystals of pure silicalite were estimated under the transient state of dynamic uptake process. The nonequilibrium adsorption amount was taken at 30 seconds, which is far less than the equilibrating times of the tested xylene vapors. The DT was estimated by the following uptake equation [3,4] with crystal sizes represented by the characteristic radius (=(3Vp)/Ap) for both multilayered ZN plates and the conventional crystals,
where Vp and Ap are the volume and total surface area of an individual zeolite particle (i.e., the ZN plate or conventional crystal); Q∞, Q30, and Q0 are adsorption amounts recorded at equilibrium, t=30 seconds, and t=0 seconds, respectively. The use of xylene loadings at 30 second time mark ensured that the transient condition was far before equilibrium. Because of the intricate structures and relatively broad crystal size distributions, the DT values were estimated using conservatively estimated upper and lower average size limits for each sample. For the ZN assemblies, the lower and upper limit average sizes of the constituting multilayered plates were (3 μm×3 μm×0.1 μm) and (5 μm×5 μm×0.06 μm), respectively; and for conventional crystals, these size limits were (2 μm×0.6 μm×0.2 μm) and (2.3 μm×0.8 μm×0.25 μm), respectively.
The flower-like second- and third-generation ZN assemblies were structurally stable during SDA removal by calcination at 600° C. and multiple cycles of adsorption-desorption tests. The flower-like open structures prevented the assembled ZN plates from collapsing that consequently preserved the inter-ZN spaces for facilitated molecular transport. SEM and TEM images of the flower-like ZN assemblies after calcination corroborate the structural integrity of the activated ZN assemblies after calcination (
The N2-adsorption-desorption isotherms at 77 K for the ZN assembly sample before and after thermal activation demonstrate BET surface areas for the ZN assemblies were 42 and 492 m2/g before and after SDA removal, respectively (
On the other hand, the microporosity with dp of 1.09-1.85 nm and mesoporosity with dp of greater than or equal to 4.7 nm were not observed in the conventional silicalite crystals (
To examine the accessibility of the zeolitic pores in the assemblies of multilayered ZN plates, dynamic adsorption was measured for p-xylene and o-xylene vapors at 300 K under a relative vapor pressure (p/po) of approximately 0.5 (where saturation pressure po=1.3 kPa) (
Table 1 below summarizes the equilibrium loading of xylene molecules in a unit cell (molc./u.c.) and the xylene apparent transport diffusivity (Dr) estimated by the uptake equation using the conventional characteristic radius (=(3Vp)/Ap) for particles of consideration. The MFI zeolites have theoretical xylene loading limit of 8 molc./u.c. around room temperature and p/po=0.5. The loading of o-xylene in the conventional crystals were consistent with literature reports, which were slightly smaller than the theoretical value due to some inaccessible sites for the bulkier o-xylene molecule. The loadings of both xylenes in the ZN-assemblies were larger than those in the conventional crystals. The p-xylene loading in the ZN assembly exceeded the limit of 8 molc./u.c. because of the full accessibility of internal sites and additional inter-sheet sites for adsorption. The improved micropore accessibility also enhanced the o-xylene loading in the ZN plates as compared to the conventional crystals.
Because of the complex structures and broad size distributions of the crystals, we estimated DT between ranges of average size limits for the ZN plates in assemblies and the conventional crystals based on SEM observations (see
However, the DT of o-xylene in the ZN-assembly was dramatically greater by about two orders of magnitude than that in the conventional crystals. Because of its larger kinetic size, o-xylene molecules could only enter the straight channels in b-axis and defuse extremely slowly through the large thickness of the conventional crystals (
The unique interconnected mesopore and micropore system and the consequent efficient molecular transport mechanisms of the ZN plates were further corroborated by the dynamic adsorption behaviors of p-/o-xylene equimolar vapor mixture. In the conventional silicalite crystals, the pseudo-DT derived from the mixture uptake curve was 10 times greater than that of the o-xylene but more than 20 times smaller than that of the p-xylene. Obviously, for the mixture, the intracrystalline diffusion of p-xylene is hindered by the slow-moving o-xylene in the long channels of the large crystals. In the ZN plates, the uptake rate and pseudo DT for the p-/o-xylene mixture were comparable to those of the p-xylene. This confirms that the diffusion of p-xylene in the multilayered ZN plates was much less affected by the coexisting o-xylene because of the effective transport and distribution of xylene molecules through the inter-sheet spaces that is blocked by the o-xylene molecules.
The crystals grown from the silicalite nanoparticle seeds (
The single crystalline ZN seeds were further downsized to approximately 0.2×0.2 μm2 (
The constituting single crystalline ZNs in the ZN plates are presumably bonded by condensation reaction (≡Si—OH+HO—Si≡→≡Si—O—Si≡+H2O) between adjacent ZN surfaces, which could occur during the hydrothermal crystallization and calcination processes. The multilayered ZN plates had no evidence of delamination after sonication for two weeks in KOH (pH of approximately 13) and HCl (pH of approximately 1.0) solutions, respectively. Exfoliation of ZNs from the calcined plates was not observed during intensive sonication after three cycles of freezing and thawing in toluene and water, respectively. However, a few isolated single crystal ZNs were found in sonicated suspensions in the 0.1M KOH+1.0 M KCl solution after such freezing and thawing treatments of the nonactivated ZN plates (
ZN plates were synthesized by sonicated ball milling of ZN assemblies produced by hydrothermal treatment of ZN flake seeds dispersed in a synthesis solution at 140° C. in an autoclave. The ZN assemblies were calcined at 600° C. for six hours to remove the SDA and activate the zeolite pores. For comparison purposes, ZN plates from assemblies activated via calcination were compared to nonactivated ZN plates using the same steps to produce each except for calcination. The ZN plates in the slurry (ZN mass@0.02 wt. %) were then dispersed in a polymer tiling solution including a polymer support (PVDF®0.06 wt. %), DMSO, and an organic solvent (ethanol), wherein the weight ratio of the ethanol to DMSO was 2:1. The PVDF substrate had a thickness of 125 μm, an effective pore size of approximately 0.45 μm, and a porosity (FPVDF) of 83% according to the manufacturer specifications. The pore size in the PVDF film shown by the SEM images (
Pinhole-free ZNPT thin membranes (
The ZNPT-PVDF membranes after curing were experimentally studied for proton-selective ion transport and ion conduction in aqueous solutions, which are critical to applications as ion exchange membranes (IEMs). The tests used vanadium redox flow battery (RFB) electrolyte solutions as examples that is also of practical interest to electric energy storages. The ion diffusion measurements were conducted in a permeator with solutions circulated over a membrane area of 2.5 cm2. The RFB single cell was used for electrochemical impedance spectroscopy (EIS), charge-discharge, and polarization curve measurements. The membrane working area was 1.2 cm×1.2 cm (=1.44 cm2) when mounted in the RFB cell. All the measurements were conducted at room temperature (approximately 25° C.) and ambient pressure.
For ion diffusion measurements, a solution of 4/7M VOSO4+4/7M H2SO4 was circulated in the feed side and a 1M MgSO4 solution was circulated in the permeate side for balancing ionic strength. The ion flux Ji (mol/cm2·h) was obtained from the time-dependency of permeate ion concentration (Cp,i) in the linear region (i.e., the slope dCp,i/dt) under constant feed concentration,
where the balancing solution volume in the permeate side was Vp=20 mL. The αH/V is defined as the ratio of proton flux (JH+) to vanadyl ion flux (Jv4+).
The area specific resistance (ASR) of the membrane was measured in 2M H2SO4 solution by electrical impedance spectroscopy (EIS, Reference-600, Gamry Inc., USA). After the ion diffusion and ASR measurements, the ZNPT-PVDF membrane was tested as an ion separator for the vanadium RFB. The RFB charge-discharge and polarization curves were determined and compared with the commercially available Nafion117® membrane, which is a widely used benchmark IEM for RFBs. The polarization curves were obtained within a short time to minimize change of the state of charge (SOC=CV2+(CV2++CV3+) during measurement, so that the total cell ASR (ASRc) can be estimated based on the linear section of the polarization curve.
The very large ZN plates were able to bridge over the surface pore openings and pinholes were effectively eliminated by the self-repairing effect of the vacuum-assisted filtration coating mechanism (
Based on the silicalite single crystal density (ρz=1.76 g/cm3) and the 7.69·10−5 g/cm2 ZN plate deposited on the surface, a thickness of 437 nm could be expected if the ZNPT layer were a single crystal. Because the volume of inter-ZN spaces (dis of approximately 1.4 nm) was less than or equal to 3% of the total micropore volume of the ZN plates, the density of individual ZN plates is almost same as a single crystal. Thus, it could be estimated that the ZNPT film was stacked by about 7 layers of ZN plates for an average ZN plate thickness of 60 nm. Consequently, the inter-ZNPT space widths (δip) in vertical direction were approximately 9 nm in the completely dry approximately 500-nm-thick ZNPT membrane. These nanometer-scale inter-ZN plate spaces were results of the microscopically uneven ZN plate surfaces (
The ZNPT membrane layer had a bimodal pore system including intra-ZN plate zeolitic pores (dp of approximately 0.56 nm) and inter-ZN plate mesopores (δip of approximately 9 nm). The inter-ZN micropores (dis of approximately 1.4 nm) allow ion transfer through the multilayered ZN plates but not are not expected to affect the selectivity because the zeolitic pores determine the accessibility to ions by ion sieving effects. Thus, in the ZNPT layer, protons transport through both intra-ZN plate zeolitic pores and inter-ZN plate spaces, but the metal ions could only diffuse through the inter-ZN plate spaces (
Results of ion diffusion tests demonstrated that, for the bare PVDF substrate, both Cp,H+ and Cp,V4+ increased rapidly after a relatively slow increase in the first approximately 0.3 hours (
The membrane tiled by nonactivated ZN plates (NA-ZNPT-PVDF) had thickness and surface morphology that were about the same as those of the ZNPT-PVDF membranes (
The Cp,H+ exhibited three distinct regions of time dependency for the ZNPT-PVDF membrane with increasing slopes (
The surfaces of ZN plates were chemically different before and after calcination. The nonactivated ZN plates contained dC5 at the zeolitic pore mouths of external surface and at inter-ZN spaces within the ZN plates while the activated ZN plates were free of organics. These could different ZN surface ionization/solvation and inter-ZN plate space evolution behaviors in the electrolytes between the activated and nonactivated ZN plates. The organic-free silica surfaces are expected to be more readily protonated to form higher surface ionicity than the organic-containing surfaces. Consequently, the proton and vanadyl ion diffusion rates through the inter-ZN plate and inter-ZN (i.e., intra-ZN plate) spaces could vary in the Non-Activated (NA)-ZNPT and ZNPT layers. The ZNPT-PVDF exhibited dramatically higher JH+ that was caused by diffusion through the zeolitic pores. Meanwhile, it showed a substantially reduced JV4+ as compared to the NA-ZNPT. The smaller JV4+ of ZNPT membrane could be attributed to the greater surface protonation in the ZNPT, which created larger resistances to VO2+ diffusion through the nanometer-scale inter-ZN plate spaces (δip of approximately 9 nm).
Unlike the ZNPT-PVDF, the Nafion117® membrane had no appreciable ramp of proton permeation rate (
Results of the EIS measurements (
The ZNPT-PVDF membrane was tested to function as an ion separator for the vanadium RFB equipped with carbon felt electrodes. The charge-discharge curves at constant current densities (i) and discharge polarization curves were measured using 10 mL catholyte solution containing 2M (VO2)2SO4 (V5+)+2M (H2SO4) and 10 mL anolyte solution with 2M VSO4 (V2+)+2M (H2SO4). The membrane ASR was reexamined by EIS measurements under the RFB operation conditions.
The substantial increase of ASR for Nafion117 in RFB operation conditions has been found to be primarily caused by the penetration of vanadium ions. The penetration and retention of vanadium ions contaminate the nanoscale water channels (dp of approximately 2.5 nm) that increases resistance to proton conduction in Nafion. Although vanadium ions can also enter the ZNPT layer through the very small amount of 9-nm-width inter-ZN plate spaces (εip of approximately 2.0%), proton conduction in the membrane is much less affected because the zeolitic channels as the main pathways for proton transport are inaccessible to metal ions. Improved VE and Pe are hence expected from the ZNPT-PVDF over the Nafion117 because the former offers smaller ASR in RFB operation conditions to reduce internal Ohmic losses (=i·ASR), especially at relatively high current densities.
While the present invention has been illustrated by the description of one or more embodiments thereof, and while the embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. The various features shown and described herein may be used alone or in any combination. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of Applicants' general inventive concept.
This application claims the benefit of the filing date of U.S. Provisional Application No. 63/297,442, filed on Jan. 7, 2022, and U.S. Provisional Application No. 63/427,428, filed on Nov. 22, 2022, the disclosures of which are incorporated herein by reference in their entireties.
This invention was made with government support under DE-SC0020011 awarded by the U.S. Department of Energy, and CBET-1935205 awarded by the National Science Foundation. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2023/010426 | 1/9/2023 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63297442 | Jan 2022 | US | |
| 63427428 | Nov 2022 | US |
