Method for preparing nanosheet-shaped zsm-5 molecular sieve and application thereof

Abstract

The application provides a method for preparing nanosheet ZSM-5 molecular sieve and application, and the method comprises the following steps: mixing a silicon source, an optional alkali source and a first organic template to obtain a gel precursor; adding a dispersed phase aqueous solution of an aluminum source into the gel precursor to obtain a first gel solution; adding an aqueous solution or solid particles of a first inhibitor and a second inhibitor different from the first inhibitor into the first gel solution to obtain a second gel solution, and maintaining the pH value of the second gel solution at 8.5-12.5; adding a nanometer white liquid seed into the second gel solution, and performing ultrasonic treatment to obtain a third gel solution; placing the third gel solution into a hydrothermal reaction kettle, and performing three-stage temperature control crystallization to obtain nanosheet ZSM-5 molecular sieve; and wherein a second organic template different from the first organic template is added in step (1) or (2).

Description

Technical Field

[0001] This application relates to the field of molecular sieve material preparation technology, and more specifically, to a method for preparing nanosheet-like ZSM-5 molecular sieves and their applications. Background Technology

[0002] The main channels of ZSM-5 molecular sieves consist of two sets of intersecting ten-membered ring channels. Compared to zigzag channels (0.51 nm × 0.55 nm), the channel size along the b-axis is 0.53 nm × 0.56 nm, which is beneficial for the diffusion of reactant and product molecules. Reducing the thickness along the b-axis and c-axis to prepare sheet-like ZSM-5 molecular sieves can reduce the residence time of catalytic reaction products, shorten the diffusion path in the channels, and enhance resistance to coking. This can significantly improve catalyst stability, inhibit hydrogen transfer and side reactions such as oligomerization, cyclization, and aromatization, reduce coking rate, and extend catalyst lifetime. The morphology of sheet-like molecular sieves has a significant impact on catalyst performance. Due to their uniform and ordered micropores, large specific surface area, and high hydrothermal stability, they are widely used in catalysis and adsorption separation fields and have broad application prospects.

[0003] The hydrothermal method is commonly used for the laboratory synthesis and large-scale industrial production of molecular sieves. The vast majority of reports on the synthesis of ZSM-5 molecular sieves employ the hydrothermal method, using a variety of template agents, including amines, alcohols, alkanolamines, quaternary ammonium salts, and quaternary ammonium bases. The morphology of the synthesized ZSM-5 molecular sieves is affected by the raw materials (silicon source, aluminum source, alkali source, water dosage, and template agent), aging temperature and time, and crystallization conditions (crystallization temperature and time).

[0004] The charge distribution, size, and spatial structure of template molecules significantly influence the morphology of molecular sieves, contributing to their guiding properties. Furthermore, template molecules reduce the chemical potential for molecular sieve lattice formation by affecting the gelation and nucleation processes, thus promoting crystal formation both thermodynamically and kinetically. Therefore, the amount of template agent used is related to crystal nucleation and growth; lower amounts result in larger crystal grains, while higher amounts lead to smaller crystal grains and better physicochemical properties.

[0005] In the hydrothermal synthesis of zeolite molecular sieves, the silicate dissolution process involves almost no chemical bond changes and the reaction is minimally exothermic. Therefore, the crystallization rate and relative selectivity of the molecular sieve are largely kinetically controlled. The pH of the precursor solution is another key factor affecting molecular sieve synthesis, reflecting the OH- concentration and significantly influencing the crystallization rate and crystal morphology. Due to the different pKa points of silicates and organic SDAs, the solution pH directly affects their solubility and state. Adding ammonium salts to the synthetic gel indirectly lowers the gel's pH, and the degree of pH decrease depends on the type of ammonium salt added. The following patents all relate to the synthesis of ZSM-5 molecular sieves in systems using quaternary ammonium and quaternary ammonium salts as template agents:

[0006] European Patent EP 0173901 (1985) discloses a method for synthesizing ZSM-5. Its technical features include: pretreating a raw material mixture at 80°C for 6 hours, then crystallizing the pretreated material at 15% of a freshly prepared raw material mixture at 175°C for 8 hours to obtain ZSM-5 zeolite with a silicon-to-aluminum ratio of 60 and a grain size of approximately 100–300 nm. The amount of quaternary ammonium template agent (TPA, TEA) used (molar ratio R:SiO2) can be reduced to 0.05. The molar ratios of the feed materials are SiO2 / Al2O3 = 40–80, (Si+Al) / H2O = 0.06–0.09, R / SiO2 < 0.4, and H2O / OH... - =50~70. The pretreatment at 80℃ for 6 hours in this method is equivalent to preparing the directing agent by aging, and still requires the use of expensive quaternary ammonium template agents to achieve the purpose of synthesizing nano ZSM-5 zeolite.

[0007] US Patent 4606900 (1986) discloses a method for synthesizing ZSM-5. Its key feature is that, based on US Patent 3702886, an amorphous silica-alumina gel directing agent pre-prepared with tetrapropyl quaternary ammonium salt or quaternary ammonium base is added to a reaction system containing a large amount of template agent. This accelerates crystallization and increases crystallinity. By controlling the amount of the amorphous silica-alumina gel crystallization accelerator, this method can yield ZSM-5 zeolite with a particle size of 100 nm to 100 μm. The molar ratios specified in the patent are: SiO2 / Al2O3 < 5, Na2O / SiO2 = 2 × 10⁻¹⁰ to 2, TPA / SiO2 = 0.01 to 2.0, and H2O / OH... - =0.7~3000. This method relies on directing agents and expensive quaternary ammonium template agents to achieve the purpose of synthesizing nano ZSM-5 zeolite.

[0008] CN102874843A discloses a method for preparing nano-sized ZSM-5 molecular sieves using a seed crystal method. The invention is characterized by first uniformly mixing aluminum isopropoxide, tetraethyl orthosilicate, tetrapropylammonium hydroxide, and water, stirring and aging the mixture, then drying the aged sol-gel into a dry gel. The dry gel is then ground into powder and used as a seed crystal. Water, a silicon source, an aluminum source, and the seed crystal are then mixed and stirred until homogeneous, allowing it to statically crystallize in a reaction vessel to obtain nano-sized ZSM-5 molecular sieves.

[0009] CN110217804B discloses ZSM-5 molecular sieve and its preparation method, hydrogen-form ZSM-5 molecular sieve and its application, and a method for methanol conversion. The invention is characterized by: 1) a molar ratio of a first silicon source, a first aluminum source, a first alkali source, a first template agent, a seed crystal, urea, and water of 100:(0.2-2):(1-10):(1-10):(10-300):(1000-2000), followed by aging to obtain a gel; the gel is then subjected to low-temperature crystallization and high-temperature crystallization, and the solid product obtained by high-temperature crystallization is dried and calcined; the seed crystal is a spherical ZSM-5 molecular sieve containing a second template agent, obtained by crystallization at 100-135℃. The provided molecular sieve has low production cost and can improve the selectivity of propylene and butene and has a long service life when used in methanol conversion to olefins.

[0010] Literature reports that Xue et al. first prepared seed crystals by hydrothermal treatment at 80℃ for 72 h using TEOS and TPAOH as raw materials. Then, using silica sol as the silicon source and aluminum sulfate as the aluminum source, they added different amounts of seed crystals and crystallized at 175℃ for 24 h to obtain nanoscale ZSM-5 molecular sieves with different SiO2 / Al2O3 molar ratios. Studies have shown that the addition of seed crystals plays an important role in the formation of ZSM-5 nanocrystals and reduces the use of organic template agents, offering advantages in terms of economy and environmental protection. Chen et al. calcined aggregated nanoscale ZSM-5 and conventional ZSM-5 molecular sieves at high temperature to remove the template agent, treated them with alkali at 70℃ for a certain time, and used them as seed crystals. They added CTAB template agent and silica sol, crystallized at 120℃ for 24 h, and then crystallized at 170℃ for 12 h to obtain nanoscale ZSM-5 molecular sieves. The addition of CTAB can accelerate the crystallization of molecular sieves and is crucial for the formation of pure ZSM-5 molecular sieves. Firoozi et al. obtained nanoscale ZSM-5 molecular sieves by using aluminum nitrate as the aluminum source, TEOS as the silicon source, and TPAOH as the template agent, and then statically crystallizing them at low temperature for 60 hours in an oil bath at 100℃. Compared with the synthesized microcrystalline molecular sieve samples, the nanoscale ZSM-5 molecular sieves have a larger total specific surface area.

[0011] Chinese invention patent CN105883849 (application number 201610515849.1, 2016) discloses a method for synthesizing ZSM-5 molecular sieves with controllable morphology. Its technical features are: the use of composite template agents, one being tetrapropylammonium hydroxide and the other an organic amine; firstly, potassium hydroxide or sodium hydroxide and an aluminum source are dissolved in water and stirred evenly, then tetrapropylammonium hydroxide and the second template agent are added separately, stirred at room temperature, and then a silicon source is added to form a sol; then the sol is crystallized at 100–200℃ for 0.5–5 days, and after crystallization, the product is obtained by filtration, washing, and drying. Under certain synthesis ratios, by only changing the type and amount of template agent, ZSM-5 molecular sieves with different crystal sizes and shapes can be synthesized in a controlled manner. This method is simple to operate, has good reproducibility, and the synthesized product has a regular shape and uniform particle size distribution.

[0012] Chinese invention patent CN 107892308 (application number 201711319914.4, 2017) discloses a ZSM-5 molecular sieve and its preparation method. Its technical features are: providing a silicon source, an aluminum source, a quaternary ammonium template agent, a phosphorus template agent, ethanol, and water, and mixing them to obtain a mixed gel; crystallizing the mixed gel at 100–200℃ for 6–72 hours to obtain a reaction product, from which the desired ZSM-5 molecular sieve can be obtained. The molar ratio of SiO2 to Al2O3 in the mixed gel is (20-200) / 1, and the molar ratio of quaternary ammonium template agent, phosphorus template agent, ethanol, water and SiO2 is (0.001-0.5) / (0.001-0.5) / (1-50) / (0.2-50) / 1. This method for preparing ZSM-5 molecular sieve can prepare ZSM-5 molecular sieves with hierarchical porous structure containing both micropores and mesopores, high stability and good catalytic performance. Furthermore, the introduction of phosphorus further improves the catalytic performance of ZSM-5 molecular sieves.

[0013] Chinese patent application No. 201811579605.5 (2018) proposes a Fe-ZSM-5 zeolite molecular sieve that facilitates the diffusion of coking precursors and products in the methanol-to-propylene reaction, reduces the selectivity of the byproduct methane during the reaction, and exhibits a longer lifespan and higher propylene selectivity. Thin-film Fe-ZSM-5 molecular sieves were obtained by adding different amounts of nanocrystal seeds and adjusting the amount of mineralizer NH4F.

[0014] Chinese Patent Application No. 201710154093.7 discloses a molecular sieve, its preparation method, and its applications. The ZSM-5 molecular sieve is prepared using an amine-free system. The ZSM-5 molecular sieve has a silica-to-alumina ratio of 40-200. It is prepared using high silica-to-alumina ratio nanolayered ZSM-5 molecular sieves as seed crystals, under the condition that the molar ratio of the raw materials is equal to one. Compared to traditional template-free synthesis, the ZSM-5 molecular sieve synthesized in this invention has a high silica-to-alumina ratio greater than 40, and the silica-to-alumina ratio of the prepared sheet-like ZSM-5 molecular sieve can be adjusted within a wide range of 40-200.

[0015] CN106809859A discloses a method for synthesizing a sheet-like interlaced self-supporting structure nano-ZSM-5 molecular sieve. The method uses silica sol and sodium aluminate as silicon and aluminum sources, respectively. One or more organic compounds selected from guanine, morpholine, triethylenediamine, and guanazine are added during the synthesis. After mixing the raw materials, the mixture is gelled at 20-50℃ for 0.5-10 hours, and then transferred to a high-pressure reactor for crystallization at 120-180℃. The resulting ZSM-5 molecular sieve, after calcination, exhibits a sheet-like interlaced self-supporting structure.

[0016] CN 110872127A (2018) proposed a method for preparing nanosheet-like ZSM-5 molecular sieves. In the molecular sieve synthesis mother liquor, the molar ratio of alkali metal oxides, Al2O3, SiO2, template agent, and H2O is (0.01~0.15):(0.001~0.05):1:(0.01~0.2):(5~100). The aging temperature is 10℃~120℃, and the crystallization temperature is 120℃~200℃. The template agent is one or more of ethylamine, diethylamine, n-butylamine, ethylenediamine, and isopropylamine. The alkalinity is adjusted by one or more of sulfuric acid, hydrochloric acid, nitric acid, sodium hydroxide, potassium hydroxide, and their aqueous solutions to make the pH of the molecular sieve mother liquor 8.5-11.5. At the same time, one or more of ammonium fluoride, sodium fluoride, potassium fluoride, and urea are added as additives.

[0017] Chinese patent application number 201811433787.5 discloses a snowflake-shaped ZSM-5 molecular sieve. A template agent is dispersed in an acid-base regulator to obtain a template agent dispersion A. An aluminum source is then dissolved in dispersion A to obtain a uniformly mixed solution B. Finally, a silicon source is added to obtain a gel C. Gel C is evaporated and concentrated to obtain a gel with a mass of 50%–60% of the mass of gel C. The molar ratio is Al2O3:(8-200)SiO2:(20-60)M2O:(10-20)R:(200-1000)H2O, where M represents Na or K, and R represents the template agent. The structural formula of the template agent is: (p-C6H4)[CH2N(CH3)2C n H 2n-2N(CH3)3Br]2, where n takes the value of 6-12; the gel D is crystallized in the temperature range of 180℃~220℃ for 1~3 days, cooled to room temperature, and the product is filtered, washed, dried and calcined to obtain snowflake-shaped ZSM-5 molecular sieve.

[0018] CN 111056559 A (2019) proposed a method for preparing thin-film Zn / ZSM-5 molecular sieves for methanol aromatization. The template agent is tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, or n-butylamine, etc. The inducing agent is urea, ammonium fluoride, or a bis-headed quaternary ammonium salt type surfactant, etc. The complexing agent is anhydrous ethylenediamine or ammonia solution with a mass fraction of 28%, etc. Introducing one of the zinc sources—zinc nitrate, zinc sulfate, or zinc chloride—results in thin-film Zn / ZSM-5 molecular sieves. The catalyst is resistant to carbon deposition and exhibits stable performance. It is mainly used in methanol-to-aromatics processes, distinguishing it from mechanical mixing and equal-volume impregnation methods for introducing zinc.

[0019] Chinese Patent Application No. 201410575163.2 discloses a thin-film ZSM-5 molecular sieve and its synthesis method. The template agent, additives, mineralizer, silicon source, aluminum source, and acid-base regulator are added to a reaction vessel, stirred evenly, gel aged at room temperature for 0-120 hours, crystallized at 60-300℃ with stirring at 50-1000 rpm for 1-15 days, cooled to room temperature, washed, centrifuged, and dried to obtain thin-film ZSM-5 zeolite. The additives are at least one of the nitrogen-containing compounds diethanolamine, guanidine, imidazole, ammonium acetate, n-butylamine, n-hexylamine, methylamine, ethylamine, urea, triethylamine, and ethylenediamine. The mineralizer is a fluoride.

[0020] However, current synthesis of sheet-like ZSM-5 molecular sieves mainly relies on single template agents, especially quaternary ammonium template agents, which are expensive. Furthermore, the removal of template agents after crystallization and the use of inhibitors such as fluorides are highly toxic, causing serious environmental pollution and making industrial application difficult. The size and morphology of ZSM-5 molecular sieves are difficult to control during preparation, resulting in low crystallinity, poor stability, difficulty in separation, and poor dispersibility. Therefore, it is of great significance to find a way to prepare sheet-like ZSM-5 molecular sieves with controllable morphology.

[0021] The information disclosed above in the background section is only intended to enhance the understanding of the background art of the art described herein. Therefore, the background art may contain certain information that does not constitute prior art known to those skilled in the art in this country. Summary of the Invention

[0022] The main objective of this application is to provide a method for preparing nanosheet-like ZSM-5 molecular sieves. Compared with traditional methods for preparing ZSM-5 molecular sieves, this method requires the simultaneous action of two inhibitors and a template agent, precise control of pH value, and segmental crystallization to obtain pure-phase, impurity-free, and smooth ZSM-5 molecular sieves with altered a, b, and c axes. Furthermore, compared with samples synthesized using a single template agent, the product obtained by this method combines the advantages of nanoparticles and hierarchical pores, exhibiting a larger micropore specific surface area and micropore volume, higher hydrothermal stability, a wider silica-to-alumina ratio range, controllable morphology, and higher resistance to coking, thus broadening its application range in shape-selective catalysis.

[0023] According to one aspect of the present invention, a method for preparing nanosheet-like ZSM-5 molecular sieves is provided, comprising the following steps: (1) mixing a silicon source, an optional alkali source, a first organic template agent and a second organic template agent different from the first organic template agent to obtain a gel precursor; (2) adding an aqueous dispersion of an aluminum source to the gel precursor to obtain a first gel solution; (3) adding an aqueous solution or solid particles of a first inhibitor and a second inhibitor different from the first inhibitor to the first gel solution to obtain a second gel solution, and maintaining the pH value of the second gel solution at 8.5 to 12.5; (4) adding nano-white liquid seed crystals to the second gel solution and performing ultrasonic treatment to obtain a third gel solution; (5) placing the third gel solution in a hydrothermal reactor and performing three-stage temperature-controlled crystallization to obtain the nanosheet-like ZSM-5 molecular sieve; wherein, in step (1) or (2), a second organic template agent different from the first organic template agent is added.

[0024] Specifically, the above method may include the following steps: (1) uniformly mixing a silicon source and a first organic template agent, stirring at 20-100°C for 0.1-24 h, and crystallizing at 20-100°C for 50-240 h to obtain nano-white liquid seed crystals, wherein in the seed crystals, the molar ratio of SiO2 in the silicon source to the first organic template agent is 1:0.01-50; (2) uniformly mixing a silicon source, an optional alkali source, a first organic template agent, and a second organic template agent different from the first organic template agent, stirring at 20-100°C for 0.1-24 h to obtain a gel precursor; (3) uniformly mixing an aluminum source and water, stirring at 20-100°C for 0.1-12 h to obtain an aqueous dispersion of the aluminum source; (4) adding the aqueous solution of the aluminum source dropwise to the gel precursor, stirring at 20-100°C for 0.1-24 h to obtain a first gel solution; (5) adding a first inhibitor and a second organic template agent different from the first inhibitor... (6) Add the aqueous solution or solid particles of the second inhibitor of the agent to the first gel solution to obtain the second gel solution, and maintain the pH value of the second gel solution at 8.5-12.5; (7) Add the nano white liquid seed crystals to the second gel solution and sonicate at 20-80℃ for 0.1-24h to obtain the third gel solution, wherein the mass ratio of the seed crystals to SiO2 in the silicon source is 0-10; (8) Place the third gel solution in a hydrothermal reactor and perform three-stage temperature-controlled crystallization, wherein the first stage crystallization temperature is 0-60℃ and the crystallization time is 2-24h, the second stage crystallization temperature is 60-130℃ and the crystallization time is 5-18h, and the third stage crystallization temperature is 140-180℃ and the crystallization time is 8-24h; (9) After the crystallization in step (7) is completed, a molecular sieve slurry is obtained, and the slurry is subjected to solid-liquid separation, washing, drying and calcination to obtain the nanosheet ZSM-5 molecular sieve.

[0025] Preferably, in the above three-stage temperature-controlled crystallization, the first stage crystallization temperature is 0-60℃ and the crystallization time is 2-24h, the second stage crystallization temperature is 60-130℃ and the crystallization time is 5-18h, and the third stage crystallization temperature is 140-180℃ and the crystallization time is 8-24h.

[0026] Further, the molar ratio of each component in the third gel solution is SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source / H2O = 60~800:1:0~80:0~80:0~20:0~20:0~25:15~500.

[0027] Furthermore, the silicon source is selected from one or more of silica sol, tetraethyl orthosilicate, and coarse-porous silica gel.

[0028] Furthermore, the aluminum source is selected from one or more of boehmite, aluminum sulfate, aluminum nitrate, and sodium aluminate.

[0029] Further, the first inhibitor is selected from one of urea, methylene blue, vitamin C, glucose, polyacrylamide, sucrose, glycerol, polyethylene glycol, ammonium phosphate, ammonium bicarbonate, and polyurethane glycol; the second inhibitor is selected from one of NH4Br, NH4F, NH4I, (NH4)2SO4, NH4Cl, NH4NO3, CH3COONH4, triammonium phosphate, ammonium bicarbonate, and ammonium phosphate.

[0030] Furthermore, the alkali source is selected from one or more combinations of sodium hydroxide, potassium hydroxide, and ammonia water.

[0031] Further, the first organic template agent is selected from one of tetramethylammonium hydroxide, tetramethylammonium bromide, tetrapropylammonium hydroxide, tetrapropylammonium bromide, tetraethylammonium hydroxide, and tetraethylammonium bromide; the second organic template agent is selected from one of tetramethylammonium hydroxide, tetramethylammonium bromide, tetrapropylammonium hydroxide, tetrapropylammonium bromide, tetraethylammonium hydroxide, and tetraethylammonium bromide.

[0032] In the embodiments of the present invention, the above-mentioned method for preparing nanosheet-like ZSM-5 molecular sieves is simple, fast, easy to operate, and can be applied industrially. By employing the synergistic effect of dual inhibitor modification and template agent guidance, a cross-scale interconnected composite structure within the ZSM-5 molecular sieve crystal is constructed, broadening the silica-alumina ratio range of molecular sieve synthesis, adjusting the acidic active sites of the molecular sieve, increasing the specific surface area and micropore volume of the molecular sieve, improving the internal flow and diffusion performance of the molecular sieve, increasing the resistance to carbon deposition and stability, and exhibiting good mass transfer efficiency while maintaining shape selectivity, thus forming a method for preparing ZSM-5 molecular sieve catalysts with a directionally controllable hierarchical porous structure.

[0033] Specifically, the method of this invention adds a combination of quaternary ammonium salt and quaternary ammonium base template agents to promote the formation of regular nanoscale crystals during the crystallization synthesis of molecular sieves. It also improves the crystallinity of the molecular sieve sample and the silicon source conversion rate. A three-step hydrothermal crystallization process is used, where low-temperature hydrothermal treatment promotes crystal nucleation and dispersion, while high-temperature hydrothermal treatment promotes crystal nucleus growth and regulates directional crystal growth. The purpose of introducing dual inhibitors is to achieve coupled modification and regulation: firstly, the coupled modification modulates the crystal growth direction and morphology, reducing the thickness along the b-axis and c-axis; secondly, it adjusts the pH of the colloidal solution to form a highly saturated and concentrated colloidal solution; and thirdly, it adjusts the Al atom distribution and reaction channel space of the molecular sieve, widening the silica-alumina ratio of the molecular sieve framework, reducing the number of acidic centers (strong acid content) in the molecular sieve, and adjusting the molecular sieve to have a suitable acid content to increase the pore size inside the molecular sieve crystal. This improves the diffusion performance of guest molecules within the molecular sieve, thereby significantly reducing the carbon deposition rate of the molecular sieve and improving the diffusion performance of the catalyst, resulting in higher methanol conversion activity and superior propylene selectivity. Compared with dual inhibitors, the introduction of a single inhibitor temporarily exhibits a "volcano-like" effect, but it cannot be uniformly dispersed on the crystal surface and is insufficient to modify and regulate the distribution of Al atoms in the molecular sieve and the space of the reaction channels. Attached Figure Description

[0034] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0035] Figure 1 The XRD pattern of ZSM-5 molecular sieve, a product according to one embodiment of the present invention;

[0036] Figure 2 This is a SEM image of the product ZSM-5 molecular sieve according to one embodiment of the present invention.

[0037] Figure 3 This is a SEM image of the product ZSM-5 molecular sieve according to another embodiment of the present invention.

[0038] Figure 4 The image shows a SEM image of the ZSM-5 molecular sieve product from Example 10.

[0039] Figure 5 The image shows a SEM image of the ZSM-5 molecular sieve product from Example 14. Detailed Implementation

[0040] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0041] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0042] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of this application described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0043] It should be understood that when an element (such as a layer, film, region, or substrate) is described as being "on" another element, the element may be directly on the other element, or there may be an intermediate element present. Furthermore, in the specification and claims, when an element is described as being "connected" to another element, the element may be "directly connected" to the other element, or "connected" to the other element via a third element.

[0044] As mentioned in the background section, existing technologies for preparing sheet-like ZSM-5 molecular sieves mainly employ a single template agent. To address this issue, this application provides a method for preparing nanosheet-like ZSM-5 molecular sieves in a typical embodiment. Compared to traditional methods for preparing ZSM-5 molecular sieves, this method requires the simultaneous action of two inhibitors and two quaternary ammonium template agents to prepare smooth crystal surface molecular sieves with varying dimensions along the three axes (a, b, and c) in a one-pot process. Segmented crystallization yields pure-phase ZSM-5 molecular sieves.

[0045] The present invention will be described in detail below through embodiments. Each embodiment only lists key technical indicators, but the present invention is not limited to these embodiments. In the following embodiments 1-14, embodiments 1-7 are inventive embodiments, and embodiments 8-14 are comparative embodiments.

[0046] The preparation process of the nano-white liquid seed crystals used in the following examples is as follows: 100g of tetraethyl orthosilicate (SiO2 content of 28.8%), 150g of tetrapropylammonium hydroxide, and 15g of deionized water are stirred in a closed reactor at 20-100°C for 0.1-24h to obtain a dispersion precursor solution. The dispersion precursor solution is transferred to a hydrothermal reactor and subjected to two-stage crystallization at 50°C / 36h and 80°C / 48h to obtain a seed crystal solution with a b-axis thickness of 20-80nm. In the seed crystals, the molar ratio of SiO2 in the silicon source to the first organic template agent is 1:0.01-50.

[0047] Example 1

[0048] 20g of silica sol (30% by mass), 0.4g of sodium hydroxide, and 20.336g of tetrapropylammonium hydroxide were dissolved sequentially in 2.0g of deionized water. After pretreatment at 25°C for 1 hour in a sealed reactor, a gel precursor was obtained. 0.1332g of aluminum sulfate and 5.0g of tetrapropylammonium bromide were mixed evenly and stirred at 25°C for 1 hour to obtain an aqueous dispersion of the aluminum source. The aqueous solution of the aluminum source was slowly added dropwise to the gel precursor, and stirring was continued at 20–100°C for 0.1–24 hours to obtain the first gel solution.

[0049] 1.44 g of urea and 0.316 g of ammonium chloride were added sequentially to the first gel solution, and the mixture was stirred at 20–100 °C for 0.1–24 h to obtain the second gel solution. 2.5 g of nano-white liquid seed crystals were added to the second gel solution, and the mixture was ultrasonically treated at 20–80 °C for 0.1–24 h to obtain the third gel solution. The molar ratio of each component was in the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source / H2O = 60–800:1:0–80:0–80:0–20:0–20:0–25:15–500.

[0050] The third gel solution was transferred to a hydrothermal reactor and subjected to three-stage dynamic crystallization at 60℃ / 10h, 120℃ / 6h, and 160℃ / 18h. After crystallization, solid-liquid separation, washing, drying, and calcination were performed to obtain ZSM-5 molecular sieve. Its silica-alumina ratio is 500. The XRD diffraction pattern of this sample is shown below. Figure 1 As shown. The SEM characterization of this sample is as follows. Figure 2 As shown in Table 1, the specific surface area of ​​the samples was measured by BET.

[0051] Example 2

[0052] The steps in Example 1 were repeated, except that the silicon source was replaced with coarse-porous silica gel, the amount of coarse-porous silica gel added was 5.88 g, the amount of sodium hydroxide added was changed to 0.05 g, the amount of tetrapropylammonium bromide added was changed to 5.711 g, the amount of tetrapropylammonium hydroxide added was changed to 40.672 g, the amount of aluminum sulfate added was changed to 0.1665 g, the first inhibitor and the second inhibitor were 2.40 g of urea and 0.36 g of ammonium bicarbonate, respectively, and the amount of seed crystals added was 2.5 g. The molar ratio of each component in the third gel solution was within the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source / H2O = 60-800:1:0-80:0-80:0-20:0-20:0-25:15-500. The resulting ZSM-5 molecular sieve had a silicon-to-aluminum ratio of 400. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve was obtained. The SEM characterization of this sample is as follows: Figure 3 As shown in Table 1, the specific surface area of ​​the samples was measured by BET.

[0053] Example 3

[0054] The steps in Example 1 were repeated, except that sodium hydroxide was not added, the second inhibitor was changed to ammonium nitrate (0.24 g), the first inhibitor urea was added (1.0 g), tetrapropylammonium bromide was added (5.33 g), tetrapropylammonium hydroxide was added (10.80 g), and aluminum sulfate was added (0.1233 g). The molar ratio of each component in the third gel solution was such that the ratio of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source = 60–800:1:0–80:0–80:0–20:0–20:5–10:15–500. The resulting ZSM-5 molecular sieve had a silica-to-alumina ratio of 555. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve was obtained. The SEM characterization of this sample is similar. Figure 3 The specific surface area of ​​the samples, as measured by BET, is shown in Table 1.

[0055] Example 4

[0056] The steps in Example 1 were repeated, except that the first inhibitor was changed to polyethylene glycol, with an addition amount of 1.44 g; the second inhibitor was changed to ammonium bicarbonate, with an addition amount of 0.36 g; the sodium hydroxide addition amount was changed to 4.0 g; the tetrapropylammonium bromide addition amount was changed to 5.33 g; the tetrapropylammonium hydroxide addition amount was changed to 14.8 g; and the aluminum sulfate addition amount was changed to 0.1281 g. This ensured that the molar ratio of each component in the third gel solution was within the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source / H2O = 60–800:1:0–80:0–80:0–20:0–20:0–25:15–500. The resulting ZSM-5 molecular sieve had a silica-to-alumina ratio of 520. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve was obtained. The SEM characterization of this sample is similar. Figure 3 The specific surface area of ​​the samples, as measured by BET, is shown in Table 1.

[0057] Example 5

[0058] The steps in Example 1 were repeated, except that the first inhibitor was glucose (2.4g), the second inhibitor was ammonium chloride (2.5g), the silicon source was tetraethyl orthosilicate (28.8% SiO2 content) (20.83g), the sodium hydroxide was 0.2g, the tetrapropylammonium bromide was 4.5g, the tetrapropylammonium hydroxide was 15.5g, and the aluminum sulfate was 0.2667g. This ensured that the molar ratio of each component in the third gel solution was within the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source / H2O = 60-800:1:0-80:0-80:0-20:0-20:0-25:15-500. The resulting ZSM-5 molecular sieve had a Si / Al ratio of 540. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve has been obtained. The specific surface area of ​​the sample, measured by BET, is shown in Table 1.

[0059] Example 6

[0060] The steps in Example 1 were repeated, except that the first inhibitor was changed to methylene blue, with an addition amount of 1.2 g; the second inhibitor was changed to ammonium nitrate, with an addition amount of 0.28 g; the addition amount of tetrapropylammonium bromide was changed to 5.33 g; the addition amount of sodium hydroxide was changed to 0.8 g; the addition amount of tetrapropylammonium hydroxide was changed to 10.168 g; and the addition amount of aluminum sulfate was changed to 0.111 g. This ensured that the molar ratio of each component in the third gel solution was within the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / alkali source MOH / H2O = 60–800:1:0–80:0–80:0–20:0–20:0–25:15–500. The resulting ZSM-5 molecular sieve had a silica-to-alumina ratio of 600. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve has been obtained. The specific surface area of ​​the sample, measured by BET, is shown in Table 1.

[0061] Example 7

[0062] The steps in Example 1 were repeated, except that the amount of deionized water added was changed to 10g, the amount of sodium hydroxide added was changed to 0.08g, the amount of tetrapropylammonium bromide added was changed to 3.523g, the amount of tetrapropylammonium hydroxide added was changed to 8.988g, and the amount of aluminum sulfate added was changed to 0.1667g, so that the molar ratio of each component in the third gel solution was within the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH / H2O in the alkali source = 60-800:1:0-80:0-80:0-20:0-20:0-25:15-500. The resulting ZSM-5 molecular sieve had a silica-to-alumina ratio of 400. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve has been obtained. The specific surface area of ​​the sample, measured by BET, is shown in Table 1.

[0063] Example 8

[0064] The steps in Example 1 were repeated, except that a single template agent was used instead of tetrapropylammonium hydroxide in the preparation of the first gel solution. The amount of tetrapropylammonium hydroxide added was changed to 10.988 g, the amount of aluminum sulfate added was changed to 0.1025 g, the amount of sodium hydroxide added was changed to 0.02 g, the amount of deionized water added was 10 g, the first inhibitor and the second inhibitor were 3.60 g of vitamin C and 0.54 g of polyethylene glycol, respectively, and the amount of seed crystals added was 3.5 g. This ensured that the molar ratio of each component in the third gel solution was within the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source / H2O = 60–800:1:0–80:0–80:0–20:0–20:0–25:15–500. The resulting ZSM-5 molecular sieve had a silica-to-alumina ratio of 650. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve has been obtained. The specific surface area of ​​the sample, measured by BET, is shown in Table 1.

[0065] Example 9

[0066] The steps in Example 1 were repeated, except that a single template agent was used instead of tetrapropylammonium hydroxide in the preparation of the first gel solution. The amount of tetrapropylammonium hydroxide added was changed to 20.546 g, the amount of aluminum sulfate added was changed to 0.118 g, the amount of sodium hydroxide added was changed to 1.0 g, the amount of deionized water added was 8 g, the first inhibitor and the second inhibitor were 1.60 g of methylene blue and 0.54 g of ammonium phosphate, respectively, and the amount of seed crystals added was 1.5 g. This ensured that the molar ratio of each component in the third gel solution was within the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source / H2O = 60–800:1:0–80:0–80:0–20:0–20:0–25:15–500. The resulting ZSM-5 molecular sieve had a silica-to-alumina ratio of 570. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve has been obtained. The specific surface area of ​​the sample, measured by BET, is shown in Table 1.

[0067] Example 10

[0068] The steps in Example 1 were repeated, except that only one inhibitor was used: ammonium chloride (10.0 g), silica sol (30% by mass) (40 g), sodium hydroxide (0.2 g), tetrapropylammonium bromide (4.5 g), tetrapropylammonium hydroxide (35.5 g), and aluminum sulfate (0.296 g). This ensured that the molar ratio of each component in the third gel solution was within the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source / H2O = 60–800:1:0–80:0–80:0–20:0–20:0–25:15–500. The resulting ZSM-5 molecular sieve had a silica-to-alumina ratio of 450. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve was obtained. The SEM characterization of this sample is as follows: Figure 4 As shown in Table 1, the specific surface area of ​​the samples was measured by BET.

[0069] Example 11

[0070] The steps in Example 1 were repeated, except for a two-stage crystallization process. 5.0 g of nano-white liquid crystals were added to the second gel solution, and the mixture was ultrasonically treated at 20–80 °C for 0.1–24 h to obtain a third gel solution. This third gel solution was transferred to a hydrothermal reactor, and without further low-temperature pretreatment, two-stage dynamic crystallization was performed at programmed temperatures of 120 °C / 6 h and 160 °C / 18 h. After crystallization, solid-liquid separation, washing, drying, and calcination were performed to obtain ZSM-5 molecular sieve. Its silica-alumina ratio is 500. The XRD diffraction pattern of this sample is shown below. Figure 1 As shown, ZSM-5 molecular sieve was obtained. The specific surface area of ​​the sample, measured by BET, is shown in Table 1. The molar ratio of each component is in the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source / H2O = 60~800:1:0~80:0~80:0~20:0~20:0~25:15~500.

[0071] Example 12

[0072] The steps in Example 1 were repeated, except that the proportions of each component were changed. The amount of silica sol added was 20.01 g, tetrapropylammonium bromide added was 0.2663 g, tetrapropylammonium hydroxide added was 81.344 g, aluminum sulfate added was 0.0666 g, the first and second inhibitors were 12.03 g urea and 21.39 g ammonium chloride, respectively, the amount of ammonium hydroxide added was 8.0 g, the amount of water was 180 g, and the amount of seed crystals added was 15.0 g. The molar ratio of each component in the third gel solution was 1000:1:100:100:25:25:50:10, corresponding to SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template / second organic template / first inhibitor / second inhibitor / MOH / H2O in the alkali source. The resulting ZSM-5 molecular sieve had a silica-alumina ratio of 1000. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve has been obtained. The specific surface area of ​​the sample, measured by BET, is shown in Table 1.

[0073] Example 13

[0074] The steps in Example 1 were repeated, except that segmented crystallization was not used. 5.0 g of nano-white liquid crystals were added to the second gel solution, and the mixture was ultrasonically treated at 20–80 °C for 0.1–24 h to obtain a third gel solution. This third gel solution was transferred to a hydrothermal reactor, and the temperature was programmed to 160 °C for crystallization for 18 h. After crystallization, solid-liquid separation, washing, drying, and calcination were performed to obtain ZSM-5 molecular sieve. Its silica-alumina ratio is 500. The XRD diffraction pattern of this sample is shown below. Figure 1 As shown, ZSM-5 molecular sieve was obtained. The specific surface area of ​​the sample, measured by BET, is shown in Table 1. The molar ratio of each component is in the range of SiO2 in silicon source / Al2O3 in aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in alkaline source / H2O = 60~800:1:0~80:0~80:0~20:0~20:0~25:15~500.

[0075] Example 14

[0076] The steps in Example 1 were repeated, except that no inhibitors were added. The silicon source was silica sol (30% by mass), with an addition amount of 40 g. The amount of sodium hydroxide added was changed to 0.1 g, the amount of tetrapropylammonium bromide added was changed to 9.5 g, the amount of tetrapropylammonium hydroxide added was changed to 25.5 g, and the amount of aluminum sulfate added was changed to 0.296 g. The molar ratio of each component in the third gel solution was within the range of SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkali source / H2O = 60–800:1:0–80:0–80:0–20:0–20:0–25:15–500. The resulting ZSM-5 molecular sieve had a silica-to-alumina ratio of 450. The XRD diffraction pattern of the final sample was similar to... Figure 1 This indicates that ZSM-5 molecular sieve was obtained. The SEM characterization of this sample is as follows: Figure 5 As shown in Table 1, the specific surface area of ​​the samples was measured by BET.

[0077] Table 1. BET specific surface area, pore volume, and percentage of ZSM-5 molecular sieve

[0078]

[0079]

[0080] The ZSM-5 molecular sieves prepared in Examples 1-14 were subjected to acidity determination using a BELCAT-II atmospheric pressure chemisorption analyzer to measure the acidity of the prepared ZSM-5 molecular sieves. The amount of silicon source generated in the prepared ZSM-5 molecular sieves was measured, and the silicon source conversion rate of the ZSM-5 molecular sieves was obtained by dividing the amount of silicon source generated by the amount of silicon source added during the preparation of the ZSM-5 molecular sieves. The above measurement results are recorded in Table 2.

[0081] Table 2. Acidity and Conversion Rate of ZSM-5 Molecular Sieves

[0082]

[0083]

[0084] The ZSM-5 molecular sieves prepared in Examples 1-14 were applied to the methanol-to-propylene reaction. The reaction conditions were: temperature 460-480℃, pressure at atmospheric pressure, ZSM-5 molecular sieve loading of 2g, and methanol mass hourly space velocity of 2h⁻¹. -1 The weight ratio of raw water to raw methanol was 1:0.7, and the reaction results are shown in Table 3.

[0085] Table 3

[0086]

[0087] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0088] The results in Tables 1 and 2, comparing Examples 1-7 and 8-14, show that the ZSM-5 molecular sieve prepared by the method of the present invention has improved pore surface area, pore size, acid content and silicon source conversion rate.

[0089] Table 3 shows that, comparing Examples 1-7 with Examples 8-14, the ZSM-5 molecular sieve prepared using the method of this invention exhibits higher methanol conversion and higher propylene selectivity in the methanol-to-propylene reaction. Furthermore, the ZSM-5 molecular sieve prepared in Examples 1-7 shows a methanol conversion exceeding 99.65% and a propylene selectivity exceeding 50.30% in the methanol-to-propylene reaction, demonstrating better catalytic performance. Additionally, comparing the SEM images of the ZSM-5 molecular sieves prepared in Examples 1-4 with those in Examples 10 and 14... Figure 2-5 As can be seen, the ZSM-5 molecular sieve prepared by the method of the present invention has a small particle size, a uniform and regular morphology, and good dispersibility and stability.

[0090] In summary, the ZSM-5 molecular sieve prepared by the method of the present invention, especially the ZSM-5 molecular sieve prepared by the method of the present invention using dual inhibitors and dual templates, three-stage crystallization at specific temperatures and times, and specific reagent ratios, exhibits improved pore surface area, pore size, silicon source conversion rate, and catalytic performance.

[0091] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing nanosheet-like ZSM-5 molecular sieves, comprising the following steps: (1) Mix the silicon source, optional alkali source, and first organic template agent to obtain a gel precursor; (2) Add the aqueous dispersion of the aluminum source to the gel precursor to obtain the first gel solution; (3) Add an aqueous solution or solid particles of the first inhibitor and a second inhibitor different from the first inhibitor to the first gel solution to obtain a second gel solution, and maintain the pH value of the second gel solution at 8.5~12.5; (4) Add nano-white liquid seed crystals to the second gel solution and perform ultrasonic treatment to obtain a third gel solution; (5) The third gel solution is placed in a hydrothermal reactor and subjected to three-stage temperature-controlled crystallization to obtain the nanosheet ZSM-5 molecular sieve; In step (1) or (2), a second organic template agent different from the first organic template agent is added; The molar ratio of each component in the third gel solution is SiO2 in the silicon source / Al2O3 in the aluminum source / first organic template agent / second organic template agent / first inhibitor / second inhibitor / MOH in the alkaline source / H2O = 60~800:1:0~80:0~80:0~20:0~20:0~25:15~500, wherein the amounts of the first organic template agent, the second organic template agent, the first inhibitor, and the second inhibitor are not 0; Step (1) is performed at 20~100℃ for 0.1~24h; Step (2) is performed at 20~100℃ for 0.1~24h; The ultrasonic treatment in Step (4) is performed at 20~80℃ for 0.1~24h; In the three-stage temperature-controlled crystallization in Step (5), the first stage crystallization temperature is 60-80℃ and the time is 2~24h, the second stage crystallization temperature is 120℃ and the time is 6~48h, and the third stage crystallization temperature is 160~220℃ and the time is 6~48h. The first inhibitor is selected from one of urea, methylene blue, vitamin C, glucose, polyacrylamide, sucrose, glycerol, polyethylene glycol, ammonium phosphate, and ammonium bicarbonate; the second inhibitor is selected from one of NH4Br, NH4F, NH4I, (NH4)2SO4, NH4Cl, NH4NO3, CH3COONH4, ammonium bicarbonate, and ammonium phosphate.

2. The method according to claim 1, characterized in that, The nano-white liquid seed crystal is obtained by uniformly mixing the silicon source and the first organic template agent in step (6), stirring at 20~100℃ for 0.1~24h, and crystallizing at 20~100℃ for 50~240h. In the seed crystal, the molar ratio of SiO2 in the silicon source to the first organic template agent is 1:0.01~50.

3. The method according to claim 1, characterized in that, The dispersed phase aqueous solution of the aluminum source is obtained by uniformly mixing the aluminum source and water in step (7) and stirring at 20~100℃ for 0.1~12h.

4. The method according to any one of claims 1-3, characterized in that, The silicon source is selected from one or more of silica sol, tetraethyl orthosilicate, and coarse-porous silica gel.

5. The method according to any one of claims 1-3, characterized in that, The aluminum source is selected from one or more of boehmite, aluminum sulfate, aluminum nitrate, and sodium aluminate.

6. The method according to any one of claims 1-3, characterized in that, The alkali source is selected from one or more combinations of sodium hydroxide, potassium hydroxide, and ammonia water.

7. The method according to any one of claims 1-3, characterized in that, The first organic template agent is selected from one of tetramethylammonium hydroxide, tetramethylammonium bromide, tetrapropylammonium hydroxide, tetrapropylammonium bromide, tetraethylammonium hydroxide, and tetraethylammonium bromide; the second organic template agent is selected from one of tetramethylammonium hydroxide, tetramethylammonium bromide, tetrapropylammonium hydroxide, tetrapropylammonium bromide, tetraethylammonium hydroxide, and tetraethylammonium bromide.

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