A core-shell structure multi-level pore ZSM-5@silicalite-1 molecular sieve, a preparation method and application thereof
By epitaxially growing a Silicalite-1 shell layer on a ZSM-5 molecular sieve to form a hierarchical porous molecular sieve with a core-shell structure, the problem of mass transfer and diffusion restricted by the microporous structure was solved, the catalytic activity and selectivity for low-carbon olefins were improved, and high-efficiency catalytic performance was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2024-02-22
- Publication Date
- 2026-03-17
AI Technical Summary
The microporous structure of existing ZSM-5 molecular sieves restricts the mass transfer and diffusion of macromolecules, resulting in a shortened catalytic lifetime and increased surface mass transfer resistance. Traditional methods may damage the molecular sieve framework or cause pore blockage.
A Silicalite-1 molecular sieve shell is epitaxially grown on ZSM-5 molecular sieve to form a multi-level porous molecular sieve with a core-shell structure. This retains the microporous structure and reduces surface mass transfer resistance, while improving diffusion performance through mesopores.
The catalyst achieved improved catalytic activity and selectivity for low-carbon olefins. It exhibited excellent catalytic performance and stability in naphtha catalytic cracking, with high selectivity for ethylene and propylene and an overall yield of up to 53%.
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Figure CN118062856B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of petrochemical molecular sieve catalyst technology, specifically relating to a core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve, its preparation method, and its application. The ZSM-5@Silicalite-1 molecular sieve of this invention can be used as a catalyst in the catalytic cracking of naphtha to produce ethylene and propylene and other low-carbon olefins, and has good catalytic activity, low-carbon olefin selectivity, and catalytic stability. Background Technology
[0002] Low-carbon olefins such as ethylene and propylene are important basic raw materials in the petrochemical industry, widely used in the production of synthetic resins, synthetic fibers, synthetic rubber, and various fine chemicals, and applicable to numerous fields including pharmaceuticals, environmental protection, coal chemicals, and petrochemicals. The traditional production process involves naphtha steam cracking, but this process has many drawbacks, such as high energy consumption, high CO2 emissions, and unpredictable product distribution. Utilizing molecular sieve catalytic cracking of naphtha to produce low-carbon olefins is a more environmentally friendly, feasible, and highly efficient production technology that meets market demands, offering advantages such as high reactivity, high product selectivity, and adjustable product distribution.
[0003] ZSM-5 molecular sieves are a common type of microporous crystalline material used in the catalytic cracking of naphtha, characterized by their narrow intrinsic micropores. The unfavorable mass transfer and diffusion of larger molecules limits the utilization efficiency of active sites within the micropores and shortens the catalytic lifetime of ZSM-5 molecular sieves. Therefore, it is essential to introduce mesopores into microporous ZSM-5 molecular sieves to form hierarchical pores, thereby alleviating the mass transfer and diffusion limitations of the sieve. The main synthetic methods for hierarchical porous molecular sieves include post-treatment methods and template agent methods. Post-treatment methods can be further subdivided into desilication and dealumination methods. Post-treatment methods can be destructive to the framework structure of the molecular sieve, affecting its acidity and stability. In contrast, the in-situ synthesis of hierarchical porous molecular sieves using template agents avoids these problems.
[0004] Compared to traditional microporous molecular sieves, hierarchical molecular sieves face a more serious problem: with the reduction in crystal size and the introduction of mesopores, the increased proportion of the outer surface (including mesopore walls) of the molecular sieve makes surface mass transfer resistance more prominent, and may even become the dominant factor affecting molecular sieve performance. Depending on the different causes of surface mass transfer resistance, surface etching and surface deposition methods can generally be used to reduce the surface mass transfer resistance of molecular sieves. However, the former is somewhat destructive to the framework structure of the molecular sieve, while the latter forms a core-shell structure, and the deposited shell may cause severe blockage of the molecular sieve channels. Therefore, developing a simple method to reduce the surface mass transfer resistance and effectively improve the catalytic performance of hierarchical ZSM-5 molecular sieves without affecting their framework structure and channel connectivity is of great significance. Summary of the Invention
[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve, its preparation method, and its applications. This core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve exhibits excellent catalytic diffusion performance for the catalytic cracking of naphtha to produce ethylene and other low-carbon olefins such as propylene. This molecular sieve is based on an ellipsoidal hierarchical porous ZSM-5 molecular sieve aggregate with intercrystalline mesoporous structures formed by the aggregation and stacking of nanocrystals. A Silicalite-1 molecular sieve shell is epitaxially grown on the outer layer of the nanocrystals. This shell retains the intrinsic microporous structure of the ZSM-5 molecular sieve while achieving surface passivation. Furthermore, the core and shell pore structures have good connectivity, resulting in a molecular sieve catalyst with good catalytic activity, low-carbon olefin selectivity, and catalytic stability.
[0006] This invention first proposes a method for preparing a core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve, which includes the following steps:
[0007] 1) The microporous template agent, aluminum source, and silicon source are dissolved in water in sequence and heated and stirred under reflux at 80-120°C, and then aged to form a preliminary precursor solution;
[0008] 2) Add mesoporous template agent to the preliminary precursor solution prepared in step 1) and heat and stir under reflux at 80-120°C to form the final precursor solution;
[0009] 3) The final precursor solution prepared in step 2) is transferred to a reaction vessel for hydrothermal crystallization reaction. The precipitate obtained from the reaction is centrifuged, washed, dried and calcined to obtain nanocrystalline aggregated hierarchical porous ZSM-5 molecular sieve.
[0010] 4) Dissolve the microporous template agent, ethanol, and silicon source in water sequentially and stir continuously at room temperature until the solution does not separate into layers to form a Silicalite-1 molecular sieve precursor solution;
[0011] 5) The nanocrystal aggregated hierarchical porous ZSM-5 molecular sieve prepared in step 3) is added to the Silicalite-1 molecular sieve precursor solution prepared in step 4), ultrasonically dispersed and continuously stirred, and the resulting suspension is transferred to a reaction vessel for hydrothermal crystallization reaction. The resulting precipitate is centrifuged, washed, dried and calcined to obtain the core-shell structure hierarchical porous ZSM-5@Silicalite-1 molecular sieve.
[0012] Preferably, the microporous template agent in steps 1) and 4) is one or more of tetrapropylammonium hydroxide (TPAOH), tetrapropylammonium bromide (TPABr), tetrabutylammonium hydroxide (TBAOH), and tetrabutylammonium bromide (TBABr). Preferably, the mesoporous template agent in step 2) is 3-aminopropyltriethoxysilane (APTES) or other amino-containing organosilanes.
[0013] Preferably, the aluminum source in step 1) is one or more of aluminum nitrate, aluminum sulfate, sodium aluminate, and aluminum oxide. Preferably, the silicon source in steps 1) and 4) is one or more of tetraethyl orthosilicate, silica sol, and sodium silicate.
[0014] In a preferred embodiment of the present invention, the temperature of the hydrothermal crystallization reaction in step 3) is 150-200°C and the reaction time is 4-8 days.
[0015] In a preferred embodiment of the present invention, the temperature of the hydrothermal crystallization reaction in step 5) is 150-200°C and the reaction time is 1-3 days.
[0016] In a preferred embodiment of the present invention, the amount of silicon source is SiO2 and the amount of aluminum source is Al2O3. The molar ratio of each component in the final precursor solution of the hierarchical ZSM-5 molecular sieve satisfies SiO2:Al2O3:TPAOH:APTES:H2O=100:1:25:(2~10):(2000~4000).
[0017] Preferably, the amount of silicon source is SiO2, and the molar ratio of each component in the Silicalite-1 molecular sieve precursor solution satisfies SiO2:TPAOH:EtOH:H2O=(1~10):0.5:(5~10):(200~600).
[0018] This invention also provides a core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve prepared by the method described above. The hierarchical porous ZSM-5@Silicalite-1 molecular sieve has a Silicalite-1 shell epitaxially grown on the nanocrystal-aggregated hierarchical porous ZSM-5 molecular sieve crystals. The morphology of the hierarchical porous ZSM-5 molecular sieve is that the nanocrystals are aggregated and stacked to form ellipsoidal aggregates, and the Silicalite-1 molecular sieve covers the surface of the ZSM-5 molecular sieve nanocrystals. The size of the nanocrystals is 20-80 nm, the size of the ellipsoidal aggregates is 300-600 nm, and the thickness of the Silicalite-1 molecular sieve shell on the surface of the nanocrystals is 2-7 nm.
[0019] The present invention also provides the application of the core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve, characterized in that the ZSM-5@Silicalite-1 molecular sieve is used as a catalyst in the catalytic cracking of naphtha to produce ethylene and other low-carbon olefins such as propylene.
[0020] This invention also provides the application of the core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve as a catalyst in the catalytic cracking of n-heptane to produce ethylene and propylene and other low-carbon olefins. The reaction gas for catalytic cracking includes n-heptane and water vapor, and the molar ratio of n-heptane to water vapor is 1:(0.5-4), and the mass hourly space velocity of n-heptane is 2-10 h⁻¹. -1 The pyrolysis temperature (catalyst bed temperature) is 500–650℃.
[0021] Compared with the prior art, the present invention has the following characteristics:
[0022] 1) Compared with the nanocrystalline ZSM-5 molecular sieve synthesized in the prior art, the present invention introduces intercrystalline mesopores in the molecular sieve to form hierarchical pores by using an additional mesoporous template agent, and the prepared hierarchical porous molecular sieve has a higher mesopore volume and a suitable micro-mesopore ratio.
[0023] The preparation method of the core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve of the present invention is relatively simple. A Silicalite-1 shell is epitaxially grown on the nanocrystalline aggregated hierarchical porous ZSM-5 molecular sieve crystals. The core-shell structure is continuous and unobstructed, realizing the surface passivation of the hierarchical porous ZSM-5 molecular sieve and thus reducing its surface mass transfer resistance. It has excellent catalytic reaction-diffusion performance for the catalytic cracking of n-heptane to produce ethylene and propylene and other low-carbon olefins (n-heptane is a common component with a high content in naphtha. Using the catalytic cracking reaction of n-heptane as a model reaction can well measure the naphtha catalytic cracking reaction performance of molecular sieves).
[0024] 2) The synthesized core-shell structure hierarchical porous ZSM-5@Silicalite-1 molecular sieve has a smooth pore structure. By adjusting the amount of silicon source, the shell thickness of the Silicalite-1 molecular sieve can be adjusted in the range of 2 to 7 nm.
[0025] 3) The synthesized core-shell hierarchical porous ZSM-5@Silicalite-1 molecular sieve retains the intrinsic microporous structure of ZSM-5 molecular sieve while achieving surface passivation. Furthermore, the core and shell pore structures exhibit good connectivity, resulting in a molecular sieve catalyst with excellent catalytic activity, selectivity for low-carbon olefins, and catalytic stability. It demonstrates excellent catalytic performance for the catalytic cracking of n-heptane to ethylene and propylene, achieving a conversion rate of over 92% for n-heptane, a selectivity of over 23% for ethylene, and a selectivity of over 34% for propylene, with a total yield of 53% for both ethylene and propylene. Attached Figure Description
[0026] Figure 1 The images show SEM images of the molecular sieves prepared in Examples 2 and 6-8 of this invention.
[0027] Figure 2 TEM images of the molecular sieves prepared in Examples 2 and 6-8 of this invention;
[0028] Figure 3 The images show STEM images and EDS line scans of the crystal edges of the molecular sieves prepared in Examples 2 and 6-8 of this invention. Detailed Implementation
[0029] The present invention will be further illustrated by the following embodiments. However, these embodiments do not constitute a limitation of the present invention.
[0030] Example 1:
[0031] Preparation process of microporous ZSM-5 molecular sieve:
[0032] 0.2499 g of aluminum nitrate was dissolved in 15 mL of deionized water to prepare an aluminum nitrate solution. 8.33 mL of tetrapropylammonium hydroxide was added to a three-necked flask, followed by the slowly added aluminum nitrate solution. The mixture was heated and refluxed at 90 °C for 10 min with stirring. Then, 7.43 mL of tetraethyl orthosilicate was added, and the mixture was heated and refluxed at 90 °C for 12 h with stirring. The mixture was then allowed to stand at room temperature for 12 h. The resulting gel-like precursor was then transferred to a 100 mL stainless steel reactor and hydrothermally crystallized at 170 °C for 5 days. Finally, the product was centrifuged, washed, dried at 80 °C for 24 h, and calcined in a tubular furnace at 600 °C in air for 6 h. After cooling, the product was ground and sieved to obtain a 40–60 mesh microporous ZSM-5 molecular sieve, designated HZSM-5, which was stored in a sealed container at room temperature.
[0033] The process of catalytic cracking of n-heptane to produce ethylene and propylene:
[0034] Weigh 0.05g of the above molecular sieve and mix it evenly with 0.2g of 40-60 mesh quartz sand. Then, fill the isothermal section in the middle of a quartz reaction tube with an inner diameter of 8mm. A mixed reaction gas consisting of n-heptane and water vapor is introduced through a bubbler. The volume ratio of n-heptane to water vapor is 1:1, and the mass hourly space velocity of n-heptane is 8h⁻¹. -1 The catalytic cracking reaction was carried out in a fixed-bed reactor at 600℃. After 2 hours and 5 hours of reaction, the reaction products were analyzed by online gas chromatography, and the results are as follows:
[0035]
[0036] Example 2:
[0037] Preparation process of hierarchical porous ZSM-5 molecular sieve:
[0038] 0.2499 g of aluminum nitrate was dissolved in 15 mL of deionized water to prepare an aluminum nitrate solution. 8.33 mL of tetrapropylammonium hydroxide was added to a three-necked flask, followed by the slowly added aluminum nitrate solution. The mixture was heated and refluxed at 90 °C for 10 min with stirring. Then, 7.43 mL of tetraethyl orthosilicate was added, and the mixture was heated and refluxed at 90 °C for 12 h with stirring. The mixture was then allowed to stand at room temperature for 12 h. Next, 0.40 mL of 3-aminopropyltriethoxysilane was added to the three-necked flask, and the mixture was heated and refluxed at 90 °C for 8 h with stirring. The resulting gel-like precursor was transferred to a 100 mL stainless steel reactor and hydrothermally crystallized at 170 °C for 5 days. Finally, the product was centrifuged, washed, dried at 80 °C for 24 h, and calcined in a tubular furnace at 600 °C in air for 6 h. After cooling, the product was ground and sieved to obtain a 40–60 mesh multi-level porous ZSM-5 molecular sieve, designated HZ-S-0, which was then stored in a sealed container at room temperature.
[0039] The SEM and TEM images of the HZ-S-0 molecular sieve prepared in Example 2 are as follows: Figures 1-2 As shown in the figure, the hierarchical porous ZSM-5 molecular sieve morphology is indeed an ellipsoidal aggregate formed by the mutual aggregation and stacking of nanocrystals, with intercrystalline mesopores formed between the nanocrystals.
[0040] The process of catalytic cracking of n-heptane to produce ethylene and propylene:
[0041] Weigh 0.05g of the above molecular sieve and mix it evenly with 0.2g of 40-60 mesh quartz sand. Then, fill the isothermal section in the middle of a quartz reaction tube with an inner diameter of 8mm. A mixed reaction gas consisting of n-heptane and water vapor is introduced through a bubbler. The volume ratio of n-heptane to water vapor is 1:1, and the mass hourly space velocity of n-heptane is 8h⁻¹. -1 The catalytic cracking reaction was carried out in a fixed-bed reactor at 600℃. After 2 hours and 5 hours of reaction, the reaction products were analyzed by online gas chromatography, and the results are as follows:
[0042]
[0043] Examples 3-5:
[0044] Preparation process of Silicalite-1 molecular sieve precursor solution:
[0045] Solution A was prepared by uniformly mixing 1.7 mL of tetrapropylammonium hydroxide and 14.6 mL of deionized water; solution B was prepared by uniformly mixing a certain amount of tetraethyl orthosilicate and 1.6 mL of ethanol; solution B was then added dropwise to solution A using a dropper, and the mixture was stirred continuously for 3–6 h until no layering occurred, thus obtaining the Silicalite-1 molecular sieve precursor solution.
[0046] In Example 2, the volume of tetraethyl orthosilicate used was 1.5 mL; in Example 3, the volume of tetraethyl orthosilicate used was 3.0 mL; and in Example 4, the volume of tetraethyl orthosilicate used was 4.5 mL.
[0047] Example 6:
[0048] Preparation process of core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve:
[0049] Weigh 1.5g of the hierarchical ZSM-5 molecular sieve powder described in Example 2 and add it to the Silicalite-1 molecular sieve precursor solution described in Example 3. Disperse the mixture ultrasonically for 0.5h and stir continuously for 2h. Transfer the solution to a 100mL stainless steel reactor and hydrothermally crystallize at 180℃ for 1d. The resulting product is centrifuged, washed, dried at 80℃ for 24h, and calcined in a tubular furnace at 600℃ in air atmosphere for 6h. After cooling, grind and sieve to obtain the 40-60 mesh core-shell structure hierarchical ZSM-5@Silicalite-1 molecular sieve, designated HZ-S-2, and stored in a sealed container at room temperature.
[0050] The SEM and TEM images of the HZ-S-2 molecular sieve prepared in Example 6 are as follows: Figures 1-2 As shown in the figure, compared with the sample prepared in Example 2, the nanocrystal particles of the hierarchical porous molecular sieve are larger and the volume of intercrystalline mesopores is smaller, indicating that the Silicalite-1 molecular sieve shell is successfully coated on the surface of the ZSM-5 molecular sieve nanocrystals.
[0051] The process of catalytic cracking of n-heptane to produce ethylene and propylene:
[0052] Weigh 0.05g of the above molecular sieve and mix it evenly with 0.2g of 40-60 mesh quartz sand. Then, fill the isothermal section in the middle of a quartz reaction tube with an inner diameter of 8mm. A mixed reaction gas consisting of n-heptane and water vapor is introduced through a bubbler. The volume ratio of n-heptane to water vapor is 1:1, and the mass hourly space velocity of n-heptane is 8h⁻¹. -1 The catalytic cracking reaction was carried out in a fixed-bed reactor at 600℃. After 2 hours and 5 hours of reaction, the reaction products were analyzed by online gas chromatography, and the results are as follows:
[0053]
[0054] Example 7:
[0055] Preparation process of core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve:
[0056] Weigh 1.5g of the hierarchical ZSM-5 molecular sieve powder described in Example 2 and add it to the Silicalite-1 molecular sieve precursor solution described in Example 4. Disperse the mixture ultrasonically for 0.5h and stir continuously for 2h. Transfer the solution to a 100mL stainless steel reactor and hydrothermally crystallize at 180℃ for 1d. The resulting product is centrifuged, washed, dried at 80℃ for 24h, and calcined in a tubular furnace at 600℃ in air atmosphere for 6h. After cooling, grind and sieve to obtain the 40-60 mesh core-shell structure hierarchical ZSM-5@Silicalite-1 molecular sieve, designated HZ-S-4, and stored in a sealed container at room temperature.
[0057] The SEM and TEM images of the HZ-S-4 molecular sieve prepared in Example 7 are as follows: Figures 1-2 As shown in the figure, compared with the sample prepared in Example 6, the nanocrystal particles of the hierarchical porous molecular sieve continued to increase in size, the outer edges of the crystals gradually became smoother, and the volume of the intercrystalline mesopores inside the aggregates continued to decrease, indicating that the thickness of the Silicalite-1 shell epitaxially grown from ZSM-5 molecular sieve nanocrystals has increased.
[0058] The process of catalytic cracking of n-heptane to produce ethylene and propylene:
[0059] Weigh 0.05g of the above molecular sieve and mix it evenly with 0.2g of 40-60 mesh quartz sand. Then, fill the isothermal section in the middle of a quartz reaction tube with an inner diameter of 8mm. A mixed reaction gas consisting of n-heptane and water vapor is introduced through a bubbler. The volume ratio of n-heptane to water vapor is 1:1, and the mass hourly space velocity of n-heptane is 8h⁻¹. -1 The catalytic cracking reaction was carried out in a fixed-bed reactor at 600℃. After 2 hours and 5 hours of reaction, the reaction products were analyzed by online gas chromatography, and the results are as follows:
[0060]
[0061] Example 8:
[0062] Preparation process of core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve:
[0063] Weigh 1.5g of the hierarchical ZSM-5 molecular sieve powder described in Example 2 and add it to the Silicalite-1 molecular sieve precursor solution described in Example 5. Disperse the mixture ultrasonically for 0.5h and stir continuously for 2h. Transfer the solution to a 100mL stainless steel reactor and hydrothermally crystallize at 180℃ for 1d. The resulting product is centrifuged, washed, dried at 80℃ for 24h, and calcined in a tubular furnace at 600℃ in air atmosphere for 6h. After cooling, grind and sieve to obtain the 40-60 mesh core-shell structure hierarchical ZSM-5@Silicalite-1 molecular sieve, designated HZ-S-6, and stored in a sealed container at room temperature.
[0064] The SEM and TEM images of the HZ-S-6 molecular sieve prepared in Example 8 are as follows: Figures 1-2As shown in the figure, compared with the sample prepared in Example 7, the nanocrystal particles of the hierarchical porous molecular sieve are larger, and the volume of the intercrystalline mesopores inside the aggregates becomes very small. Combined with Examples 6 to 8, it can be fully demonstrated that the micro-mesopore ratio and the thickness of the Silicalite-1 shell are adjustable in the preparation of the core-shell structure hierarchical porous ZSM-5@Silicalite-1 molecular sieve.
[0065] The process of catalytic cracking of n-heptane to produce ethylene and propylene:
[0066] Weigh 0.05g of the above molecular sieve and mix it evenly with 0.2g of 40-60 mesh quartz sand. Then, fill the isothermal section in the middle of a quartz reaction tube with an inner diameter of 8mm. A mixed reaction gas consisting of n-heptane and water vapor is introduced through a bubbler. The volume ratio of n-heptane to water vapor is 1:1, and the mass hourly space velocity of n-heptane is 8h⁻¹. -1 The catalytic cracking reaction was carried out in a fixed-bed reactor at 600℃. After 2 hours and 5 hours of reaction, the reaction products were analyzed by online gas chromatography, and the results are as follows:
[0067]
[0068]
[0069] STEM images and EDS line scans of the crystal edges of the molecular sieves prepared in Examples 2 and 6-8 are shown below. Figure 3 As shown, EDS line scan analysis of the molecular sieve crystal edges reveals that the Si and Al element signals of the hierarchical ZSM-5 molecular sieve prepared in Example 2 disappear at the same position at the crystal edge. In contrast, the Si element signal of the core-shell hierarchical ZSM-5@Silicalite-1 molecular sieves prepared in Examples 6-8 extends further outward than the Al element signal at the crystal edge. This distance corresponds to the thickness of the Silicalite-1 molecular sieve epitaxially surrounding the ZSM-5 molecular sieve nanocrystals. The figure shows that the shell thickness of the Silicalite-1 molecular sieve is adjustable within the range of 2-10 nm. Compared to Examples 6 and 7, Example 8, due to its thicker Silicalite-1 molecular sieve shell, has already affected its catalytic performance. Therefore, controlling the Silicalite-1 molecular sieve shell layer on the nanocrystal surface to 2-7 nm is more beneficial for the catalytic cracking of n-heptane to produce ethylene and propylene, and other low-carbon olefins.
[0070] The above content provides a further detailed description of the present invention in conjunction with specific preferred embodiments. It should not be construed that the specific embodiments of the present invention are limited to this. For those skilled in the art, several simple deductions and substitutions can be made without departing from the concept of the present invention, and all such deductions and substitutions should be considered as falling within the scope of patent protection determined by the submitted claims.
Claims
1. A method for preparing a core-shell hierarchical pore ZSM-5@Silicalite-1 zeolite, characterized in that, The multi-level pore ZSM-5@Silicalite-1 molecular sieve is prepared by epitaxial growth of a Silicalite-1 shell on a nano-crystal aggregate multi-level pore ZSM-5 molecular sieve crystal, the multi-level pore ZSM-5 molecular sieve has a nano-crystal aggregate morphology, and the Silicalite-1 molecular sieve covers the surface layer of the nano-crystal of the ZSM-5 molecular sieve; the size of the nano-crystal is 20-80 nm, the size of the ellipsoidal aggregate is 300-600 nm, and the thickness of the Silicalite-1 molecular sieve shell of the surface layer of the nano-crystal is 2-7 nm. The preparation method comprises the following steps: 1) sequentially dissolving a micropore template agent, an aluminum source and a silicon source in water and heating and stirring under reflux at 80-120 ℃, and aging to form a preliminary precursor solution; 2) adding a mesopore template agent to the preliminary precursor solution prepared in step 1) and heating and stirring under reflux at 80-120 ℃ to form a final precursor solution; the micropore template agent is tetrapropylammonium hydroxide (TPAOH), and the mesopore template agent is 3-aminopropyl triethoxysilane (APTES); the amount of the silicon source is calculated based on SiO2, the amount of the aluminum source is calculated based on Al2O3, and the molar ratio of the components in the final precursor solution of the multi-level pore ZSM-5 molecular sieve satisfies SiO2:Al2O3:TPAOH:APTES:H2O = 100:1:25:(2-10):(2000-4000); 3) transferring the final precursor solution prepared in step 2) to a reaction kettle to perform a hydrothermal crystallization reaction, centrifuging, washing, drying and calcining the obtained precipitate to obtain a nano-crystal aggregate multi-level pore ZSM-5 molecular sieve; 4) sequentially dissolving a micropore template agent, ethanol and a silicon source in water and continuously stirring at room temperature until the solution is not layered to form a Silicalite-1 molecular sieve precursor solution; the micropore template agent is tetrapropylammonium hydroxide (TPAOH); the amount of the silicon source is calculated based on SiO2, and the molar ratio of the components in the Silicalite-1 molecular sieve precursor solution satisfies SiO2:TPAOH:EtOH:H2O = (1-10):0.5:(5-10):(200-600); 5) putting the nano-crystal aggregate multi-level pore ZSM-5 molecular sieve prepared in step 3) into the Silicalite-1 molecular sieve precursor solution prepared in step 4), ultrasonic dispersing and continuously stirring, and transferring the obtained suspension to a reaction kettle to perform a hydrothermal crystallization reaction, and centrifuging, washing, drying and calcining the obtained precipitate to obtain a core-shell structure multi-level pore ZSM-5@Silicalite-1 molecular sieve.
2. The production method according to claim 1, characterized by, In step 1), the aluminum source is one or more of aluminum nitrate, aluminum sulfate, sodium metaaluminate and aluminum oxide, and in steps 1) and 4), the silicon source is one or more of tetraethyl orthosilicate, silica sol and sodium silicate.
3. The production method according to claim 1, characterized by, In step 3), the temperature of the hydrothermal crystallization reaction is 150-200 ℃, and the reaction time is 4-8 days.
4. The production method according to claim 1, characterized by, The temperature of the hydrothermal crystallization reaction in the step 5) is 150-200 ℃, and the reaction time is 1-3 days. 5.A core-shell structured hierarchical porous ZSM-5@Silicalite-1 molecular sieve prepared by the method of any one of claims 1-4.
6. The use of the core-shell structured hierarchical-porous ZSM-5@Silicalite-1 zeolite of claim 5, characterized in that, The ZSM-5@Silicalite-1 molecular sieve is applied as a catalyst in catalytic cracking of naphtha to produce low-carbon olefins including ethylene and propylene.
7. Use of the core-shell structured hierarchical-pore ZSM-5@Silicalite-1 zeolite of claim 5 as a catalyst in catalytic cracking of n-heptane to produce light olefins including ethylene and propylene, the reaction gas for catalytic cracking comprising n-heptane and water vapor, and the molar ratio of n-heptane to water vapor being 1: (0.5-4), the mass space velocity of n-heptane being 2-10 h-1, and the temperature of the catalyst bed being 500-650 ℃. -1