Large crystal grade hole molecular sieve and preparation method thereof

Abstract

The present application relates to the technical field of molecular sieve material, and particularly relates to a preparation method and application of large-grain hierarchical pore molecular sieve, wherein a template agent is mixed with deionized water and stirred until completely dissolved, then an alkali source is added, and stirring is continuously performed until the system is clarified, and the pH of the solution is adjusted; then the silicon source is added at a constant speed drop by drop, and stirring is performed until the solution is clarified; finally, the aluminum source is added, constant-temperature stirring is performed, pre-freezing is performed, vacuum freeze drying is performed, the temperature is increased to 100 DEG C, then steam solvent is added into the kettle, crystallization is performed, cooling to room temperature is performed, and then filtering, washing, drying and calcination are sequentially performed to obtain the large-grain hierarchical pore molecular sieve product. The present application adopts a "freeze drying-steam assisted crystallization" synergistic strategy for the initial gel, and the freeze-dried solid-phase precursor is directionally converted into a large-grain hierarchical pore structure under the action of a specific solvent through a solid-solid rearrangement path, so that the problems of low crystallization efficiency, poor controllability of grain and pore structure and non-green process in the prior art are solved.

Description

Technical Field

[0001] This invention relates to the field of molecular sieve materials technology, specifically to a method for preparing large-grain-size porous molecular sieves and their applications. Background Technology

[0002] Molecular sieves are a class of inorganic porous materials with regular pore structures, adjustable pore size distribution, and excellent thermal and chemical stability, playing a crucial role in petrochemical, fine chemical, and environmental engineering fields. Among them, large-grain molecular sieves, with their higher mechanical strength, superior structural stability, and controllable surface properties, have significant advantages over small-grain molecular sieves in scenarios requiring long-term stable operation, such as catalytic reactions and high-pressure adsorption separation. Furthermore, the introduction of hierarchical pore structures (possessing both micropores and mesopores / macropores) can effectively solve the problems of high mass transfer resistance and difficulty for large molecular reactants to enter the pores in traditional microporous molecular sieves, further expanding the application range of molecular sieves. Therefore, the preparation of large-grain hierarchical pore molecular sieves has become a research hotspot in the field of molecular sieve materials.

[0003] Currently, the preparation methods for large-grain molecular sieves are still mainly based on traditional hydrothermal synthesis, supplemented by improved processes such as solvothermal synthesis and microwave-assisted synthesis. The existing crystallization process for large-grain molecular sieves mainly adopts the traditional liquid-phase hydrothermal crystallization method. This process requires long-term crystallization under high temperature and high pressure conditions, which not only has high energy consumption and long production cycle, but also easily leads to excessive growth or agglomeration of grains during the crystallization process, making it difficult to accurately control the grain size. At the same time, the crystallization process of amorphous silicon-aluminum precursors in the liquid phase is mainly liquid-solid transformation, and the molecular sieve grows through layer-by-layer growth. The process has poor controllability and it is difficult to generate large-grain-level pore structures, resulting in a single pore structure and limited mass transfer performance of the product.

[0004] CN104418356A discloses a method for preparing large-grained ZSM-5 molecular sieves with an average grain size greater than 2 μm, regular morphology, and high crystallinity by controlling the order of adding aluminum source, silicon source, and two template agents. Although the composition of the initial gel can be controlled by changing the order of material feeding and adding alcohol solvents during molecular sieve synthesis, thereby synthesizing large-grained molecular sieves, existing technologies struggle to simultaneously control grain size and pore structure. Mesoporous structures form during the growth of molecular sieve grains, making it impossible to obtain large-grained porous molecular sieves.

[0005] In existing technologies, hierarchical pore structures are mostly constructed through post-treatment modification (such as alkali treatment for desilication and acid treatment for dealumination) or the addition of organic porogens. Post-treatment modification methods easily damage the crystal structure integrity of molecular sieves, reducing their mechanical strength and stability; adding organic porogens increases preparation costs, and the subsequent removal of porogens requires high-temperature calcination, which is not only energy-intensive but may also generate pollutant emissions, contradicting the development trend of green chemistry. While optimizing gel ratios and adjusting crystallization temperature and time can improve crystal growth, such improvements are limited to fine-tuning of single process parameters and fail to fundamentally solve the problem of synergistic control of the three molecular sieve synthesis steps: initial gel composition, precursor drying, and crystallization growth. Some studies have attempted to use special drying or crystallization methods, but these have not been combined with the control of the feeding sequence, making it impossible to achieve efficient and precise preparation of large-grain hierarchical pore molecular sieves. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing large-grain-size porous molecular sieves with controllable grain size, excellent pore structure, and a green and efficient preparation process.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention proposes a method for preparing large-grain-size porous molecular sieves, comprising the following steps: S1. First, mix the template agent with deionized water and stir until completely dissolved to obtain an aqueous solution of the template agent. Then, add the alkali source and stir continuously until the system is clear. Adjust the pH of the solution to 9-12. Then, add the silicon source dropwise at a uniform rate and stir until the solution is clear. Finally, add the aluminum source and stir at a constant temperature to form a uniformly dispersed initial gel under the structural guidance of the template agent. S2, the initial gel prepared in step S1 is pre-frozen at -40~-80℃ for 6~12h to rapidly freeze the water in the gel to form ice crystals; then it is transferred to a vacuum freeze dryer, the pressure inside the freeze dryer is reduced to 1~10Pa, and vacuum freeze-drying is performed for 12~24h. The ice crystals are directly sublimated and removed to obtain an amorphous aluminosilicate precursor with uniform chemical composition, no agglomeration, and uniform particle size distribution. S3. The freeze-dried amorphous aluminosilicate precursor is loaded into a polytetrafluoroethylene crystallization vessel. After heating to 100°C, a vapor solvent is added to the vessel. The crystallization vessel is heated to the molecular sieve crystallization temperature for crystallization. After crystallization, the product is cooled to room temperature and then filtered, washed, dried and calcined to obtain a large-grain-grade porous molecular sieve product.

[0008] Furthermore, the type of the finished large-grain-size pore molecular sieve is ZSM-5, Y-type, Beta, Silicalite-1, ZSM-11, or ZSM-22 molecular sieve.

[0009] Furthermore, in step S1, the template agent is selected from one or more of tetraethylammonium hydroxide, tetrapropylammonium bromide, tetrabutylammonium hydroxide, and hexadecylammonium bromide.

[0010] Furthermore, in step S1, the alkali source is selected from one or two of sodium hydroxide and potassium hydroxide; The silicon source is selected from one or more of tetraethyl orthosilicate, silica sol, and water glass; The aluminum source is selected from one or more of aluminum isopropoxide, aluminum nitrate, and aluminum sulfate.

[0011] Furthermore, in step S1, the temperature of the constant-temperature stirring is 25~40℃, and the duration is 1 hour.

[0012] Furthermore, in step S3, the vapor solvent includes one or more of water, methanol, ethanol, ethylene glycol, acetone, acetonitrile, and acetic acid.

[0013] Furthermore, in step S3, the drying temperature is 100~120℃, and the drying time is 4~8h.

[0014] Furthermore, in step S3, the calcination temperature is 500~600℃ and the duration is 4~6h.

[0015] The present invention also includes a large-grain-size porous molecular sieve prepared by any of the preparation methods described herein.

[0016] Compared with existing technologies, this invention employs a synergistic strategy of "freeze-drying-steam-assisted crystallization" to prepare large-grain-level porous molecular sieves from the initial gel. In this method, the freeze-dried solid precursor is oriented to transform into a large-grain-level porous structure through a solid-solid rearrangement path under the action of a specific solvent. This solves the problems of poor precursor quality, low crystallization efficiency, poor controllability of grain and pore structure, and non-green process in existing technologies. It has important practical significance and economic value for promoting the industrial application of large-grain-level porous molecular sieves. Attached Figure Description

[0017] Figure 1 XRD patterns of molecular sieves prepared by different methods.

[0018] Figure 2 SEM images of the molecular sieves prepared in Example 1 and Comparative Example 1.

[0019] Figure 3 The nitrogen adsorption-desorption isotherms and pore size distribution diagrams are shown for the molecular sieves prepared in Example 1 and Comparative Example 1. Detailed Implementation

[0020] The technical solution of the present invention will be further described below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention. Example 1:

[0021] S1, 2.6g sodium hydroxide was dissolved in a mixed solution of 21.2g tetraethylammonium hydroxide (35%) and 16.2g deionized water. After stirring for 10 min, 30.0g silica sol (30%) was added dropwise at a uniform rate. After stirring at room temperature until clear, 3.75g aluminum nitrate was added and stirred for 1 h to obtain the initial gel. S2, the initial gel obtained in step S1 was pre-frozen at -80℃ for 6 hours, and then transferred to a vacuum freeze dryer. The freezing temperature was kept constant, and the gel was vacuum dried at 5Pa pressure for 18 hours to obtain an amorphous aluminosilicate precursor with uniform chemical composition, no agglomeration, and uniform particle size distribution. In step S3, the amorphous aluminosilicate precursor obtained in step S2 was transferred to a polytetrafluoroethylene crystallization vessel (the precursor was 5 cm away from the bottom of the vessel and did not directly contact the solvent). After heating to 100°C, 25 mL of acetonitrile was added to the vessel as a vapor solvent. Subsequently, the temperature of the crystallization vessel was raised to 140°C, and the product was obtained after crystallization at this temperature for 12 h. After cooling to room temperature, the product was obtained by filtration, washing, drying at 110°C for 6 h, and calcining at 550°C for 5 h. This molecular sieve was designated as Zeolite-1. Example 2:

[0022] S1, 1.5g sodium hydroxide was dissolved in a mixed solution of 3.3g tetrapropylammonium bromide and 48.3g deionized water. After stirring for 10 min, 18.4g tetraethyl orthosilicate was added dropwise at a uniform rate. After stirring until the solution was clear, 1.2g aluminum isopropoxide was added. The mixture was stirred at 40℃ for 1 h to obtain the initial gel. S2, the initial gel obtained in step S1 was pre-frozen at -60℃ for 6 hours to rapidly freeze the water in the gel to form ice crystals; then it was transferred to a vacuum freeze dryer, and the freezing temperature was kept constant and vacuum dried at 5Pa pressure for 18 hours to obtain an amorphous aluminosilicate precursor with no agglomeration and uniform particle size distribution. In step S3, the amorphous aluminosilicate precursor obtained in step S2 was transferred to a polytetrafluoroethylene crystallization vessel. After heating to 100°C, 20 mL of water was added to the vessel as a vapor solvent. Subsequently, the temperature of the crystallization vessel was raised to 180°C, and the product was obtained after crystallization at this temperature for 4 hours. After cooling to room temperature, the product was obtained by filtration, washing, drying at 100°C for 8 hours, and calcining at 500°C for 6 hours. This molecular sieve was designated as Zeolite-2. Example 3:

[0023] S1, dissolve 6.0g of potassium hydroxide in a mixed solution of 4.5g of tetrabutylammonium hydroxide and 35.0g of deionized water, stir for 15min until the pH value is 12; then add 48.8g of water glass dropwise at a uniform rate, stir until clear, add 3.42g of aluminum sulfate, and stir at 50℃ for 1h to obtain the initial gel.

[0024] S2, the initial gel obtained in step S1 is pre-frozen at -40℃ for 6 hours to rapidly freeze the water in the gel to form ice crystals; then it is transferred to a vacuum freeze dryer, the freezing temperature is kept constant, and it is vacuum dried at 10Pa pressure for 12 hours to obtain an amorphous aluminosilicate precursor without agglomeration.

[0025] In step S3, the amorphous aluminosilicate precursor obtained in step S2 was transferred to a polytetrafluoroethylene crystallization vessel. After heating to 100°C, 15 mL of ethanol was added to the vessel as a vapor solvent, and the product was crystallized at this temperature for 4 hours to obtain the product. After cooling to room temperature, the product was successively filtered, washed, dried at 120°C for 4 hours, and calcined at 600°C for 4 hours to obtain NaY molecular sieve, which was designated Zeolite-3. Example 4:

[0026] The preparation steps and conditions are the same as in Example 1, except that the acetonitrile in step S3 is replaced with ethylene glycol. This molecular sieve is designated as Zeolite-4.

[0027] Comparative Example 1 The initial gel preparation used the same molar ratio of raw materials as S1 in Example 1. A conventional hydrothermal crystallization method was employed. Aluminum nitrate was added to a certain amount of sodium hydroxide aqueous solution, mixed thoroughly, and then tetraethylammonium hydroxide was added. After stirring for 10 minutes, silica sol was added dropwise. Stirring was continued at room temperature for 4 hours. The initial gel was then transferred to a stainless steel reactor with a polytetrafluoroethylene liner and statically crystallized at 140°C in an oven for 72 hours (6 times the crystallization time of Example 1). After washing, centrifugation, drying, and calcination, Beta molecular sieves were obtained. The final product was named Reference-1.

[0028] Comparative Example 2 The initial gel preparation used the same raw material molar ratio as S1 in Example 2. Only the order of adding the silicon and aluminum sources was changed. The resulting initial gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and statically crystallized at 180°C for 24 hours in an oven (the crystallization time was 6 times that of Example 2). After washing, centrifugation, drying, and calcination, ZSM-5 molecular sieve was obtained. The final product was named Reference-2.

[0029] Comparative Example 3 The initial gel preparation used the same raw material molar ratio as S1 in Example 3. The order of feeding the silicon and aluminum sources was changed, and the resulting initial gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner. It was statically crystallized in an oven at 100°C for 12 hours (three times the crystallization time of Example 3). After washing, centrifugation, drying, and calcination, Y molecular sieve was obtained. The final product was named Reference-3.

[0030] Comparative Example 4 The initial gel was prepared using the same steps and conditions as S1 in Example 1. Subsequently, a conventional hydrothermal crystallization method was employed, transferring the initial gel to a stainless steel reactor lined with polytetrafluoroethylene (PTFE). Static crystallization was carried out in an oven at 140°C for 48 hours (four times the crystallization time of Example 1). After washing, centrifugation, drying, and calcination, Beta molecular sieves were obtained. The final product was named Reference-4.

[0031] Comparative Example 5 The preparation of the initial gel was carried out under the same steps and conditions as in Comparative Example 1. The preparation and crystallization of the amorphous aluminosilicate precursor were carried out under the same steps and conditions as in S2 and S3 of Example 1. The final product was named Reference-5.

[0032] Test Example 1 This test used X-ray diffraction to detect the molecular sieves of Examples 1-3 and Comparative Examples 1-3. The characteristic diffraction peaks of different samples are as follows: Figure 1 As shown, all exhibited characteristic diffraction peaks of specific molecular sieves. Compared with comparative examples 1-3, the molecular sieves of examples 1-3 showed a higher degree of crystallization, indicating that the "freeze-drying-steam-assisted crystallization" synergistic strategy can shorten the crystallization time, significantly improve the crystallinity of the molecular sieve, and prevent the formation of impurity crystals.

[0033] Test Example 2 In this test example, the morphology and particle size of Zeolite-1 and Reference-1 samples were examined using high-resolution scanning electron microscopy (SEM), resulting in SEM images of the Zeolite-1 and Reference-1 molecular sieves, as shown below. Figure 2 As shown, Reference-1 molecular sieve exhibits small crystallite aggregation and random stacking to form intercrystalline pores. In contrast, Zeolite-1 molecular sieve has a larger particle size, indicating that controlling the initial gel composition under the synergistic strategy of "freeze-drying-steam-assisted crystallization" can effectively prevent particle aggregation and achieve precise control of the large crystal structure.

[0034] Test Example 3 This test case used an ASAP 2020 fully automated physical adsorption analyzer to detect the pore structure of Zeolite-1 and Reference-1. Nitrogen adsorption-desorption isotherms and pore distribution diagrams are shown below. Figure 3 As shown, the mesopore volume of the Zeolite-1 molecular sieve with a pore size of 2-4 nm is significantly higher than that of the Reference-1 molecular sieve, indicating that compared with the Reference-1 molecular sieve, Zeolite-1 has more abundant and more concentrated intracrystalline mesopores, forming a typical micropore-mesopore hierarchical pore structure.

[0035] As can be seen from the above embodiments and comparative examples, the preparation method of the present invention can accurately prepare large-grain-grade porous molecular sieves with different grain sizes and mesopore diameters. Compared with the traditional hydrothermal crystallization method, it has advantages such as high crystallization efficiency, high crystallinity, concentrated pore size distribution, and green process. It can achieve directional control of product structure, and thus can be widely used in the fields of adsorption separation and catalytic conversion.

[0036] While specific embodiments of the present invention have been described above, those skilled in the art should understand that the specific embodiments described are merely illustrative and not intended to limit the scope of the invention. Modifications and variations made by those skilled in the art in accordance with the spirit of the invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a large-grain-size porous molecular sieve, characterized in that: Includes the following steps: S1. First, mix the template agent with deionized water and stir until completely dissolved to obtain an aqueous solution of the template agent. Then, add the alkali source and stir continuously until the system is clear. Adjust the pH of the solution to 9-12. Then, add the silicon source dropwise at a uniform rate and stir until the solution is clear. Finally, add the aluminum source and stir at a constant temperature to form a uniformly dispersed initial gel under the structural guidance of the template agent. S2, the initial gel prepared in step S1 is pre-frozen at -40~-80℃ for 6~12h to rapidly freeze the water in the gel to form ice crystals; then it is transferred to a vacuum freeze dryer, the pressure inside the freeze dryer is reduced to 1~10Pa, and vacuum freeze-drying is performed for 12~24h. The ice crystals are directly sublimated and removed to obtain an amorphous aluminosilicate precursor with uniform chemical composition, no agglomeration, and uniform particle size distribution. S3. The freeze-dried amorphous aluminosilicate precursor is loaded into a polytetrafluoroethylene crystallization vessel. After heating to 100°C, a vapor solvent is added to the vessel. The crystallization vessel is heated to the molecular sieve crystallization temperature for crystallization. After crystallization, the product is cooled to room temperature and then filtered, washed, dried and calcined to obtain a large-grain-grade porous molecular sieve product.

2. The method for preparing a large-grain-size porous molecular sieve according to claim 1, characterized in that: In step S1, the template agent is selected from one or more of tetraethylammonium hydroxide, tetrapropylammonium bromide, tetrabutylammonium hydroxide, and hexadecylammonium bromide.

3. The method for preparing a large-grain-size porous molecular sieve according to claim 1, characterized in that: In step S1, the alkali source is selected from one or two of sodium hydroxide and potassium hydroxide; The silicon source is selected from one or more of tetraethyl orthosilicate, silica sol, and water glass; The aluminum source is selected from one or more of aluminum isopropoxide, aluminum nitrate, and aluminum sulfate.

4. The method for preparing a large-grain-size porous molecular sieve according to claim 1, characterized in that: In step S1, the temperature of the constant-temperature stirring is 25~40℃, and the duration is 1 hour.

5. The method for preparing a large-grain-size porous molecular sieve according to claim 1, characterized in that: In step S3, the vapor solvent includes one or more of water, methanol, ethanol, ethylene glycol, acetone, acetonitrile, and acetic acid.

6. The method for preparing a large-grain-size porous molecular sieve according to claim 1, characterized in that: In step S3, the drying temperature is 100~120℃ and the drying time is 4~8h.

7. The method for preparing a large-grain-size porous molecular sieve according to claim 1, characterized in that: In step S3, the calcination temperature is 500~600℃ and the duration is 4~6h.

8. A large-grain-size porous molecular sieve prepared by any of the preparation methods described in claims 1-7.

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