Core-shell type MFI molecular sieve as well as preparation method and application thereof

Abstract

The invention discloses a core-shell type MFI molecular sieve as well as a preparation method and application thereof, and belongs to the technical field of molecular sieve catalysts. The core-shell type MFI molecular sieve comprises a nuclear phase and a shell layer coating the surface of the nuclear phase, the nuclear phase is metal-loaded Sne-Silicalite-1, the shell layer is HZSM-5, and the Sne-Silicalite-1 is a molecular sieve with a large number of defect anchoring sites formed after boron removal of a borosilicate MFI molecular sieve B-Silicalite-1. Functional boundaries of the core-shell type MFI molecular sieve are obviously distinguished and orderly joined, and the core-shell type MFI molecular sieve has gradient acid sites and can inhibit migration and agglomeration of internal metals, so that internal weak acid sites and external strong acid sites respectively catalyze different reactions on a reaction path, the reaction process is connected in series, and aromatization reaction is promoted.

Description

Technical Field

[0001] This application belongs to the field of molecular sieve catalyst technology, specifically relating to a core-shell type MFI molecular sieve and its preparation method, as well as the application of the core-shell type MFI molecular sieve as a catalyst. Background Technology

[0002] Aromatization is a process that converts low-molecular-weight non-aromatic hydrocarbons (such as C1-C5 alkanes or alkenes like methane, ethane, and propane) into high-value aromatic hydrocarbons using a catalyst at high temperatures. This technology can effectively enhance the economic value of light non-aromatic hydrocarbon resources and has important applications in the petrochemical and natural gas processing fields.

[0003] In aromatization technology, catalyst performance is crucial. Molecular sieves with MFI topologies (such as ZSM-5) show promising application prospects, as their excellent catalytic performance, high specific surface area, and abundant strong acid sites provide essential active centers for alkane activation and aromatization. However, their strong acidity also promotes unwanted side reactions leading to carbon deposition, which rapidly reduces catalyst activity and shortens its lifespan.

[0004] Currently, the synthesis of core-shell structures is one of the effective strategies for controlling the acidity of MFI molecular sieves and inhibiting carbon deposition. For example, using micron-sized pure silicon molecular sieve Silicalite-1 as the core phase and ZSM-5 molecular sieve nanolayers as the shell phase, a core-shell MFI molecular sieve with good catalytic performance can be obtained. However, since the interior is a pure silicon molecular sieve, it lacks anchoring sites, which means that after loading metal, the metal will migrate and agglomerate during the reaction, leading to sintering and reducing active sites. Summary of the Invention

[0005] In view of this, the primary objective of this application is to provide a core-shell type MFI molecular sieve, which uses Sne-Silicalite-1, a molecular sieve with numerous defect anchoring sites formed after deboronization of borosilicate MFI molecular sieve B-Silicalite-1, as the core. After loading metal, borosilicate MFI molecular sieve (HZSM-5) is coated on its surface, resulting in a core-shell type MFI molecular sieve with clearly defined and orderly connected functional domains and gradient acid sites. This inhibits the migration and aggregation of internal metals, allowing the internal weak acid sites and the external strong acid sites to catalyze different reactions in the reaction pathway, thus cascading the reaction.

[0006] To achieve the above objectives, this application adopts the following technical solution: One aspect of this application discloses a core-shell type MFI molecular sieve, comprising a core phase and a shell layer covering the surface of the core phase, wherein the core phase is metal-supported Sne-Silicalite-1 and the shell layer is HZSM-5, wherein Sne-Silicalite-1 is a molecular sieve that forms a large number of defect anchoring points after deboronization of borosilicate MFI molecular sieve B-Silicalite-1.

[0007] Another aspect of this application discloses a method for preparing the core-shell type MFI molecular sieve described in this application, the method being either method one or method two, wherein method one includes the following steps: Borosilicate MFI molecular sieve B-Silicalite-1 was prepared and deboronized to obtain molecular sieve Sne-Silicalite-1 with defect anchoring points. Metal was loaded onto the surface of Sne-Silicalite-1 to obtain M / Sne-Silicalite-1; subsequently, carbonization was performed to obtain MC / Sne-Silicalite-1. A core-shell type MFI molecular sieve, M / Sne-Silicalite-1@HZSM-5, was obtained by coating the surface of MC / Sne-Silicalite-1 with an HZSM-5 shell. The second method includes the following steps: Preparation of seed crystal B-Silicalite-1; The seed crystal B-Silicalite-1 was mixed with the shell crystallization mother liquor and then hydrothermally crystallized to obtain the Sne-Silicalite-1 / HZSM-5 catalyst. Metals were loaded onto the surface of the Sne-Silicalite-1 / HZSM-5 catalyst to obtain a core-shell type MFI molecular sieve M / Sne-Silicalite-1@HZSM-5.

[0008] Another aspect of this application discloses the use of core-shell MFI molecular sieves as described in this application, or core-shell MFI molecular sieves prepared by the method described herein, as catalysts in aromatization reactions.

[0009] The beneficial effects of this application are: In this application, borosilicate MFI molecular sieve B-Silicalite-1 is used as the core phase. The core phase undergoes deboronization (B) treatment to generate numerous defect sites for metal anchoring, resulting in a core-phase molecular sieve containing only weak acid sites. After metal loading, an HZSM-5 shell is formed on the core phase surface. By introducing Al, numerous strong acid sites appear on the catalyst after coating the shell, thus forming a core-shell MFI molecular sieve with clearly defined and orderly connected functional domains and a gradient of acid sites. Its outer layer consists of strong acid sites, which can catalyze further conversion to aromatics; while the inner layer consists of weak acid sites, which can anchor active metal species, forming strong metal-support interactions with the metal, inhibiting internal metal migration and aggregation, and catalyzing reactant activation. This allows the internal weak acid sites and the external strong acid sites to catalyze different reactions along the reaction pathway, distinguishing different reaction regions and linking different reactions in aromatization, thereby promoting the aromatization reaction. Attached Figure Description

[0010] Figure 1 The image shows the XRD pattern of the Mo / Sne-Silicalite-1@HZSM-5 sample from Example 1.

[0011] Figure 2 The images show SEM images of the Mo / Sne-Silicalite-1@HZSM-5 sample in Example 1 at different magnifications. Figure 2 The scale of (a) is 1 μm. Figure 2 The scale bar in (b) is 100 nm. Figure 2 The scale bar in (c) is 1 μm. Figure 2 The scale bar for (d) is 200 nm.

[0012] Figure 3 This is a TEM image of the Mo / Sne-Silicalite-1@HZSM-5 surface in Example 1. Figure 3 (a) shows the element distribution results of the mapping scan; Figure 3 (b) is a TEM image; Figure 3 (c) is a superimposed image of Al and Si signals from the mapping scan; Figure 3 (d) is the A1 signal diagram of the mapping scan; Figure 3 (e) is the Si signal diagram of the mapping scan.

[0013] Figure 4 This is a TEM image of Mo / Sne-Silicalite-1@HZSM-5 in Example 1. Figure 4 (a) shows the element distribution results of the mapping scan; Figure 4 (b) TEM image; Figure 4(c) shows the A1 signal diagram of the mapping scan; Figure 4 The middle (d) image shows the Mo signal from the mapping scan; Figure 4 (e) is the Si signal diagram of the mapping scan.

[0014] Figure 5 The image shows the BET curve of Mo / Sne-Silicalite-1@HZSM-5 in Example 1, where blue represents the adsorption curve and purple represents the desorption curve.

[0015] Figure 6 The image shows the pore size distribution of Mo / Sne-Silicalite-1@HZSM-5 in Example 1.

[0016] Figure 7 The NH3-TPD curves of Mo / Sne-Silicalite-1@HZSM-5 and Sne-Silicalite-1 in Example 1 are shown.

[0017] Figure 8 This is a TEM image of Mo / Sne-Silicalite-1@HZSM-5 in Example 2.

[0018] Figure 9 The catalyst conversion and product selectivity of Example 1 (Mo / Sne-Silicalite-1@HZSM-5), Comparative Example 1 (Mo / Si-25), and Comparative Example 2 (Mo / S-1) are compared. Figure 9 (a) Methane conversion rate Figure 9 (b) Benzene selectivity, Figure 9 (c) Toluene selectivity, Figure 9 (d)C8 selectivity, Figure 9 (e) Ethylene selectivity, Figure 9 (f) Ethane selectivity. Detailed Implementation

[0019] The embodiments of this application will be clearly and completely described below. The technical solutions in the embodiments described below are exemplary and only possible technical implementations of this application, not all possible implementations. Those skilled in the art can combine the embodiments of this application to obtain other embodiments without creative effort, and these embodiments are also within the protection scope of this application.

[0020] The first aspect of this application discloses a core-shell type MFI molecular sieve, which has a typical core-shell structure, namely an internal core region (i.e., the "core phase") and a surface region (i.e., the "shell") surrounding the core phase. The shell and the core phase differ in chemical composition and / or crystal properties, thus forming a clearly defined core-shell relationship in space. In this application, the core phase is metal-supported Sne-Silicalite-1, and the shell is HZSM-5, wherein Sne-Silicalite-1 is a borosilicate MFI molecular sieve B-Silicalite-1 that has formed a large number of defect anchoring points after deboronization.

[0021] This application uses borosilicate MFI molecular sieve (B-Silicalite-1) as its core, utilizing the orientation and instability of boron within it to form numerous defect anchoring sites (Sne-Silicalite-1) within the molecular sieve core through deboronization. After loading metal (M / Sne-Silicalite-1), a borosilicate MFI molecular sieve (HZSM-5) is coated onto its outer surface, resulting in a core-shell structured support with clearly defined and orderly connected functional domains and gradient acid sites. After loading metal onto the core-shell support, the internal defect anchoring sites form a strong metal-support interaction with the metal, inhibiting internal metal migration and aggregation. This allows the internal weak acid sites and the external strong acid sites to catalyze different reactions along the reaction pathway, thus cascading the reactions.

[0022] In some specific embodiments of this application, the overall morphology of the core-shell type MFI molecular sieve can generally be approximately spherical, ellipsoidal or other regular geometric shapes, but its core feature is the composite configuration of the core phase and the shell.

[0023] In some specific embodiments of this application, the size range of the core phase is 500 nm to 700 nm. It should be understood that "size" here, when the molecular sieve particles are generally spherical, mainly refers to their diameter. The size of the core phase can be any value or a range between 500 nm, 520 nm, 550 nm, 580 nm, 600 nm, 620 nm, 650 nm, 680 nm, and 700 nm, such as 520 nm-650 nm, 550 nm-680 nm, etc. The core phase can be understood as an MFI molecular sieve structure with a large number of defect sites after deboronization.

[0024] In some specific embodiments of this application, the thickness of the shell layer ranges from 100nm to 200nm, for example, it can be any value or a range between 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, and 200nm, such as 120nm to 180nm, 140nm to 190nm, etc. In this application, the shell layer should be understood as substantially uniformly covering the outer surface of the core phase, and its thickness refers to the average thickness in the direction perpendicular to the surface of the core phase.

[0025] In this application, the shell material is HZSM-5 molecular sieve. It should be understood that "HZSM-5" refers to a hydrogen-form zeolite molecular sieve with an MFI topology, in which the cations in the original synthetic state (usually sodium form) have been replaced with protons (H+) through ion exchange or other methods. + Thus, it possesses Brønsted acidity.

[0026] As described in this application, the core-shell type MFI molecular sieve is loaded with a metal component. The metal is selected from at least one of molybdenum (Mo), iron (Fe), gallium (Ga), and zinc (Zn). It should be understood that "at least one" includes, but is not limited to, the following: Mo, Fe, Ga, or Zn are loaded alone; or two, three, or all four of the above metals are loaded together in any proportion and combination, such as Mo-Fe combination, Ga-Zn combination, Fe-Ga-Zn combination, etc.

[0027] In some specific embodiments of this application, the total loading of the metal on the core-shell MFI molecular sieve, in mass percentage (wt%), is 3%-5%, for example, it can be any value or a range between 3.0wt%, 3.2wt%, 3.5wt%, 3.8wt%, 4.0wt%, 4.2wt%, 4.5wt%, 4.8wt%, and 5.0wt%, such as 3.5wt% to 4.5wt%, 3.8wt% to 4.8wt%, etc. The loading refers to the mass percentage of the metal element (regardless of its form as an element, oxide, or other compound) relative to the entire core-shell molecular sieve composite material after loading.

[0028] The second aspect of this application discloses a method for preparing the core-shell MFI molecular sieve described in the first aspect of this application, including two preparation routes. These two preparation routes differ slightly in their technical routes, but both aim to construct a core-shell MFI molecular sieve with a specific structure as described in the first aspect of this application.

[0029] Method 1 follows a sequential process of "modifying the core phase first, then constructing the shell." Specifically, it first prepares and processes the core phase material, then performs metal loading and specific carbonization treatment, and finally hydrothermally synthesizes the shell on the surface of the treated core phase material.

[0030] Method two employs a "simultaneous construction of the core and shell, followed by metal loading" approach. Specifically, it first prepares a core-phase seed crystal, then allows the seed crystal to grow directly in a shell precursor environment to form a core-shell structure, and finally loads metal onto the core-shell structure.

[0031] In some specific embodiments of this application, method one is used to prepare core-shell MFI molecular sieves, the core steps of which include: <Nuclear processing> Borosilicate MFI molecular sieve B-Silicalite-1 was prepared and subjected to deboronization treatment to obtain molecular sieve Sne-Silicalite-1 with a large number of defect anchoring points.

[0032] In some specific embodiments, the preparation of borosilicate MFI molecular sieve B-Silicalite-1 includes the following steps: (1) Mixing: Mix the silicon source, boron source, template agent, and water. The silicon source is selected from tetraethyl orthosilicate, sodium silicate, and silica sol; the boron source is boric acid; the template agent is selected from tetrapropylammonium hydroxide (TPAOH) and tetrapropylammonium bromide (TPABr), but is not limited thereto. The silicon source and boron source are mixed according to a silicon (Si) to boron (B) molar ratio (Si / B) of 1:(0.01~0.04). For example, it can be any ratio or a range between any two ratios, such as 1:(0.015~0.03), 1:(0.02~0.035), etc.

[0033] (2) Pretreatment: Stir the mixture thoroughly until volatile components such as alcohols (mainly generated from the hydrolysis of silicon source) volatilize, forming a solid powder.

[0034] (3) Aging: The solid powder is kept at a temperature of 80℃~120℃ for 18~30h.

[0035] (4) Crystallization: The aged material is left to stand (i.e., hydrothermal crystallization) at a temperature of 175℃~185℃ for 120 to 192 hours.

[0036] (5) Post-processing: The crystallized product is washed with water until neutral and dried to obtain seed crystal B-Silicalite-1.

[0037] (6) Calcination: The seed crystal B-Silicalite-1 is calcined at a temperature of 550℃-560℃ for 6-8 hours to form borosilicate MFI molecular sieve B-Silicalite-1.

[0038] In some specific embodiments, the deboronization treatment involves placing the molecular sieve in a protonated solution, utilizing the orientation and instability of boron to form numerous defect anchoring sites within the molecular sieve core. As a preferred example, the deboronization treatment is achieved by adding the molecular sieve to an ammonium chloride or ammonium nitrate solution, followed by stirring and heating, washing, drying, and calcination. In some specific examples, B-Silicalite-1 is ground and then added to an NH4Cl solution at a solid-liquid mass ratio of 1:20. The mixture is heated in a water bath at 80°C with magnetic stirring at 300-800 rpm for 16 hours, then removed. The removed solid is washed with deionized water until neutral, dried at 80-100°C for 24 hours, and finally calcined at 550-560°C for 3-6 hours. This process is repeated twice.

[0039] <Loading Metal> By loading metal (M) onto the surface of Sne-Silicalite-1, M / Sne-Silicalite-1 is obtained.

[0040] The loading of the metal can be carried out in a manner known in the art, such as impregnation or ion exchange, but is not limited thereto.

[0041] In some specific examples, the metal loading is achieved by an impregnation method, the specific steps of which include: immersing the molecular sieve in a metal precursor solution, heating and stirring at 40°C for 6-16 hours, raising the temperature to 80°C and stirring until dry, then drying and calcining the solid to achieve metal loading; wherein the metal precursor solution used is an aqueous solution of a soluble metal salt, and in some specific examples, the soluble metal salt is one of ammonium heptamolybdate, ferric nitrate, gallium nitrate, and zinc sulfate.

[0042] <Carbonization treatment> M / Sne-Silicalite-1 was carbonized to obtain MC / Sne-Silicalite-1. This carbonization process was carried out in a non-reactive gas atmosphere (such as nitrogen, rare gases, etc.), by heating the material to approximately 680-720°C, and then introducing a mixture of carbon source and nitrogen. The space velocity of the mixed gas was 3000-4000 ml·h. - ¹·g - ¹. The carbon source is one of C1-C5 alkanes or alkenes, specific examples including but not limited to methane, ethane, ethylene, propane, propylene, etc. The mixing volume of the carbon source and nitrogen can be optimized or adjusted according to the experimental purpose and research needs, for example, it can be 5-10:1.

[0043] <Shell Covering> The final product M / Sne-Silicalite-1@HZSM-5 was obtained by coating the surface of MC / Sne-Silicalite-1 with an HZSM-5 shell. The coating was achieved by hydrothermal synthesis of MC / Sne-Silicalite-1 (as a core-phase catalyst) in a ZSM-5 precursor solution.

[0044] The ZSM-5 precursor solution is prepared by mixing a silicon source, an aluminum source, a template agent, and water. The silicon source can be selected from tetraethyl orthosilicate, sodium silicate, or silica sol; the aluminum source can be selected from sodium aluminate or aluminum nitrate; the template agent can be selected from tetrapropylammonium hydroxide (TPAOH) or tetrapropylammonium bromide (TPABr), but is not limited to these. The silicon source and aluminum source are mixed according to a silicon (Si) to aluminum (Al) molar ratio (Si / Al) of 1:(0.02~0.06). For example, it can be any ratio or range between any two ratios from 1:0.02, 1:0.025, 1:0.03, 1:0.035, 1:0.04, 1:0.045, 1:0.05, 1:0.055, 1:0.06, such as 1:(0.03~0.05), 1:(0.025~0.045), etc. This Si / Al ratio range corresponds to the HZSM-5 shell with a specific acid strength.

[0045] In some preferred embodiments, the weight ratio of the nucleus catalyst (MC / Sne-Silicalite-1) to the ZSM-5 precursor solution is 1:(1.2~2.5), for example, it can be any ratio or a range between any two ratios of 1:1.2, 1:1.4, 1:1.6, 1:1.8, 1:2.0, 1:2.2, 1:2.5, for example, 1:1.4 to 1:2.2.

[0046] In some preferred embodiments, the specific hydrothermal synthesis process is as follows: first, maintain at 80-120°C for 18-30 hours (aging stage), and then stand at 175-185°C for 48-72 hours (crystallization).

[0047] Furthermore, it is understandable that after the shell coating, depending on the raw material, it may be necessary to further select whether to perform protonation treatment. For example, if Na is introduced into the aluminum source, subsequent protonation treatment is usually required. For specific protonation treatment, please refer to the boron removal treatment mentioned above.

[0048] In some other specific embodiments of this application, core-shell MFI molecular sieves are prepared using Method 2. Method 2 has the advantages of simplified synthesis process and stable product quality (particle size, crystallinity, acidity). Its core steps include: <Preparation of Seed Crystals> For specific details, please refer to the steps in Method 1 above to prepare and obtain seed crystal B-Silicalite-1.

[0049] Synchronous Synthesis of Core-Shell After mixing seed crystal B-Silicalite-1 with shell crystallization mother liquor (i.e., ZSM-5 precursor solution, the composition of which can refer to the preferred scheme described in Method 1), a Sne-Silicalite-1 / HZSM-5 catalyst with a core-shell structure is directly prepared through a similar hydrothermal crystallization process. This step allows the core phase (derived from seed crystal growth or transformation) and the shell phase (formed by mother liquid crystallization) to recombine in a one-step hydrothermal process, and simultaneously achieves deboronization to form defect anchoring points.

[0050] <Metal Load> Metal (M) is loaded onto the surface of the Sne-Silicalite-1 / HZSM-5 catalyst by impregnation or ion exchange to obtain the final product M / Sne-Silicalite-1@HZSM-5.

[0051] In some specific embodiments, the metal support is achieved using an ion exchange method. The specific steps are as follows: a soluble metal salt, Sne-Silicalite-1 / HZSM-5, and excess water are mixed, with a preferred solid-liquid ratio of 1:10. The mixture is placed in a flask, heated and stirred at 40°C for 6 hours, then heated and stirred at 80°C until dry, and finally dried at 100°C for 24 hours. Finally, it is calcined at 550°C for 8 hours to obtain the Mo / Sne-Silicalite-1@HZSM-5 catalyst.

[0052] The third aspect of this application discloses the application of core-shell MFI molecular sieves as catalysts in aromatization reactions, as described in the first aspect. It should be understood that "aromatization reaction" refers to the reaction in which low-molecular-weight non-aromatic alkanes (e.g., C1-C6 alkanes, alkenes, etc., including but not limited to methane, ethane, propane, butane, ethylene, propylene, etc.) are converted into aromatic hydrocarbons (e.g., benzene, toluene, xylene, etc.) under the action of a catalyst. As a preferred example, its application is particularly evident in alkane aromatization reactions (e.g., methane aromatization). The core-shell MFI molecular sieve of this application, due to its specific core-shell structure and the distribution of gradient acidic sites, can effectively anchor metals, inhibit internal metal migration and aggregation, significantly improve the selectivity and stability of the catalyst, and allow the internal weak acid sites and external strong acid sites to catalyze different reactions along the reaction pathway, distinguishing different reaction regions and cascading different reactions in aromatization, thereby promoting the aromatization reaction.

[0053] The present application will be further illustrated below with reference to specific embodiments. It should be noted that the specific embodiments below are for illustrative purposes only and do not limit the scope of the present application in any way.

[0054] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0055] In addition, unless otherwise specified, methods without detailed conditions or steps are conventional methods, and the reagents and materials used are commercially available.

[0056] Example 1: Synthesis of Mo / Sne-Silicalite-1@HZSM-5 1.1 Synthesis of nucleus-phase catalysts C8H was weighed in molar ratios. 20 O4Si:H3BO3:C 12 H 29 NO:H2O = 1.000:0.04:0.125:8.000, mixed in the order of boron source-water-template agent-silicon source, at which point the solution separates into layers; the solution is mechanically stirred at 400 r / min for 24 hours, the solution gradually becomes clear while ethanol evaporates, until a solid powder is formed; the solid powder is transferred to a hydrothermal reactor and aged at 100℃ for 24 hours, then kept still at 180℃ for 168 hours; after hydrothermal synthesis, the solid is washed with deionized water until neutral and dried at 100℃ for 24 hours; finally, it is calcined at 550℃ for 8 hours to obtain borosilicate MFI molecular sieve B-Silicalite-1.

[0057] B-Silicalite-1 was ground and added to a 1M NH4Cl solution at a solid-liquid mass ratio of 1:20. The mixture was heated in a water bath at 80°C and 400 rpm with magnetic stirring for 16 hours, then removed. The solid was washed with deionized water until neutral, dried at 100°C for 24 hours, and finally calcined at 550°C for 3 hours. This process was repeated twice to obtain Sne-Silicalite-1 molecular sieve with numerous defect anchoring sites after boron removal.

[0058] 1.2 Metal Loading and Carbide 1.2 g of ammonium heptamolybdate, 15.0 g of Sne-Silicalite-1, and excess water were mixed at a solid-liquid ratio of 1:10 and placed in a flask. The mixture was heated and stirred at 40 °C for 6 h, then heated and stirred at 80 °C until evaporated to dryness, and finally dried at 100 °C for 24 h. Finally, it was calcined at 550 °C for 8 h to obtain the metal-supported molecular sieve Mo / Sne-Silicalite-1.

[0059] Mo / Sne-Silicalite-1 was heated to 700°C under an inert gas atmosphere, and a methane:nitrogen (9:1) gas mixture was introduced at a space velocity of 3000 ml·h. -1 ·g -1 After carbonization for half an hour, the sample was removed to obtain MoC2 / Sne-Silicalite-1.

[0060] 1.3 Synthesis of Shell Catalysts C8H was weighed in molar ratios. 20 O4Si:NaAlO2:C 12 H 29 NO:H2O = 1.00:0.02:0.27:50.00. The aluminum source, water, template agent, and silicon source were added to a beaker in that order and the mixture was magnetically stirred at 400 rpm for 12 hours to obtain the ZSM-5 precursor solution.

[0061] The MoC2 / Sne-Silicalite-1 obtained in section 1.2 was ground and added as the core phase to the aforementioned ZSM-5 precursor solution. The weight ratio of the core phase to the ZSM-5 precursor solution was 1:1.2. The mixture was sonicated for 15 min. Then, the mixture was aged in a hydrothermal reactor at 100°C for 24 hours and then kept still at 180°C for 72 hours. After the hydrothermal synthesis was completed, the solid was washed with deionized water until neutral and dried at 100°C for 24 hours. Finally, it was calcined at 550°C for 8 hours.

[0062] The calcined catalyst was removed, ground, and then added to a 1M NH4Cl solution at a solid-liquid mass ratio of 1:20. After heating in a water bath at 80℃ and 400r / min with magnetic stirring for 6 hours, the catalyst was removed, washed with deionized water until neutral, and dried at 100℃ for 24 hours. Finally, it was calcined at 550℃ for 3 hours to obtain the core-shell type MFI molecular sieve Mo / Sne-Silicalite-1@HZSM-5.

[0063] 1.4 Characterization and Performance Testing Figure 1 The XRD results show that the Mo / Sne-Silicalite-1@HZSM-5 in this embodiment has typical MFI molecular sieve crystal diffraction peaks.

[0064] Electron microscopy characterization revealed that the molecular sieve is spherical and submicron in size, with the nuclear molecular sieve Sne-Silicalite-1 having a size of approximately 500 nm to 700 nm. Figure 2 In sections a and b), the overall size of the molecular sieve Mo / Sne-Silicalite-1@HZSM-5 is approximately 700~900 nm. Figure 2 (c and d), based on which the shell thickness is estimated to be approximately 125 nm.

[0065] Figure 3 TEM mapping data showed that only Si and Al signals were detected on the surface of the core-shell molecular sieve, and no B signal was detected in the core molecular sieve, indicating that the core was completely encapsulated. Figure 4 The Mo signal in Mo / Sne-Silicalite-1@HZSM-5 is concentrated in the core, while the Al signal is distributed across the entire surface. In summary, the core-shell molecular sieve prepared by the method in this embodiment has an MFI-type structure with a complete shell, a uniform borosilicate core, and an aluminosilicate shell with a thickness of approximately 125 nm.

[0066] BET data show that the molecular sieve exhibits atypical hysteresis loops, indicating that the core-shell structure brings about a rich hierarchical pore structure. Figure 5 , Figure 6 (See Table 1). This hierarchical porous structure exhibits a shape-selective effect for aromatics while also promoting product diffusion.

[0067] Table 1 Pore size distribution of Mo / Sne-Silicalite-1@HZSM-5

[0068] Note: S in Table 1 BET The total specific surface area is calculated using BET theory, including the internal surface area of ​​all pores (micropores, mesopores, macropores) within the material and the external surface area of ​​the particles; S micro This refers to the internal surface area contributed solely by micropores (pore size < 2 nm); S ext S refers to the sum of the areas of the material's outer surface and the inner surfaces of its mesopores / macropores (i.e., the surface area of ​​all non-microporous portions). ext =S BET -S micro Vtotal refers to the total volume of all pores (micropores, mesopores, and macropores) per unit mass of material; Vmicro refers to the pore volume contributed solely by micropores (pore size <2nm). 'a' indicates calculation using the Brunauer-Emmett-Teller equation, and 'b' indicates calculation using the t-plot method.

[0069] according to Figure 7 and Figure 8It is known that the nuclear molecular sieve contains only weak acid sites, while the catalyst exhibits a large number of strong acid sites after the coating shell is applied. This indicates that the catalyst has a gradient of acid sites. The interior contains weak acid sites, which anchor active metal species to catalyze the activation of reactants, while the exterior contains strong acid sites, which can catalyze further conversion to aromatics. This catalyst, through its designed synthesis, differentiates different reaction regions, cascades different reactions in aromatization, and can promote the aromatization process.

[0070] Example 2: Synthesis of Mo / Sne-Silicalite-1@HZSM-5 2.1 Seed Crystal B-Silicalite-1 C8H was weighed in molar ratios. 20 O4Si:H3BO3:C 12 H 29 NO:H2O = 1.000:0.04:0.125:8.000, mixed in the order of boron source-water-template agent-silicon source, at which point the solution separates into layers; the solution is mechanically stirred at 400 r / min for 24 hours, the solution gradually becomes clear while ethanol evaporates, until a solid powder is formed, the solid powder is transferred to a hydrothermal reactor; the hydrothermal reactor is aged at 100℃ for 24 hours, and then kept still at 180℃ for 168 hours; after the hydrothermal synthesis is completed, the solid is washed with deionized water until neutral, and dried at 100℃ for 24 hours to obtain seed crystal B-Silicalite-1.

[0071] 2.2 Sne-Silicalite-1 / HZSM-5 After grinding B-Silicalite-1 seed crystals, they were added to the ZSM-5 precursor solution (same as in Example 1), with a seed crystal to precursor solution weight ratio of 1:2. After sonication for 15 minutes, the mixture was transferred to a hydrothermal reactor. The reactor was aged at 100°C for 24 hours, followed by static aging at 180°C for 72 hours. After hydrothermal synthesis, the extracted solid was washed with deionized water until neutral and dried at 100°C for 24 hours. Finally, it was calcined at 550°C for 8 hours.

[0072] The calcined catalyst was ground and added to a 1M NH4Cl solution at a solid-liquid mass ratio of 1:20. The mixture was heated in a water bath at 80°C and 400 rpm with magnetic stirring for 16 hours, then removed. The solid was washed with deionized water until neutral, dried at 100°C for 24 hours, and finally calcined at 550°C for 3 hours to obtain the molecular sieve Sne-Silicalite-1 / HZSM-5.

[0073] 2.3 Mo / Sne-Silicalite-1 catalyst 1.2 g of ammonium heptamolybdate, 15.0 g of Sne-Silicalite-1 / HZSM-5, and excess water were mixed at a solid-liquid ratio of 1:10 and placed in a flask. The mixture was heated and stirred at 40 °C for 6 h, then heated and stirred at 80 °C until evaporated to dryness, and finally dried at 100 °C for 24 h. Finally, the mixture was calcined at 550 °C for 8 h to obtain the core-shell type MFI molecular sieve Mo / Sne-Silicalite-1@HZSM-5.

[0074] Example 3: Synthesis of Fe / Sne-Silicalite-1@HZSM-5 3.1 Synthesis of nucleus-phase catalysts C8H was weighed in molar ratios. 20 O4Si:H3BO3:C 12 H 29 NO:H2O = 1.000:0.03:0.125:8.000, mixed in the order of boron source-water-template agent-silicon source, at which point the solution separates into layers; the solution is mechanically stirred at 400 r / min for 24 hours, the solution gradually becomes clear while ethanol evaporates, until a solid powder is formed; the solid powder is transferred to a hydrothermal reactor and aged at 100℃ for 24 hours, then kept still at 180℃ for 168 hours; after hydrothermal synthesis, the solid is washed with deionized water until neutral and dried at 100℃ for 24 hours; finally, it is calcined at 550℃ for 8 hours to obtain borosilicate MFI molecular sieve B-Silicalite-1.

[0075] B-Silicalite-1 was ground and added to a 1M NH4Cl solution at a solid-liquid mass ratio of 1:20. The mixture was heated in a water bath at 80°C and 400 rpm with magnetic stirring for 16 hours, then removed. The solid was washed with deionized water until neutral and dried at 100°C for 24 hours, finally calcined at 550°C for 3 hours. This process was repeated twice to obtain Sne-Silicalite-1, a nucleated molecular sieve with numerous defect anchoring sites after boron removal.

[0076] 3.2 Metal Loading and Carbide 0.4 g of ferric nitrate, 15 g of Sne-Silicalite-1, and excess water were mixed at a solid-liquid ratio of 1:10 and placed in a flask. The mixture was heated and stirred at 40 °C for 6 h, then heated and stirred at 80 °C until evaporated to dryness, and finally dried at 100 °C for 24 h. Finally, the mixture was calcined at 550 °C for 8 h to obtain the metal-supported molecular sieve Fe / Sne-Silicalite-1.

[0077] Fe / Sne-Silicalite-1 was heated to 700℃ under an inert gas atmosphere, and a mixture of methane and nitrogen (9:1, v / v) was introduced at a space velocity of 3000 ml·h. -1 ·g -1 After carbonization for half an hour, the sample was removed to obtain FeC / Sne-Silicalite-1.

[0078] 3.3 Synthesis of Shell Catalysts C8H was weighed in molar ratios. 20 O4Si:NaAlO2:C 12 H 29 NO:H2O = 1.00:0.04:0.27:50.00, added to a beaker in the order of aluminum source-water-template agent-silicon source and magnetically stirred at 400 r / min for 12 hours to obtain ZSM-5 precursor solution.

[0079] The nucleus-phase catalyst FeC / Sne-Silicalite-1 was ground and added to the ZSM-5 precursor solution at a weight ratio of 1:1.5. The mixture was sonicated for 15 min. Subsequently, the mixture was aged in a hydrothermal reactor at 100℃ for 24 hours, followed by static aging at 180℃ for 72 hours. After hydrothermal synthesis, the extracted solid was washed with deionized water until neutral and dried at 100℃ for 24 hours. Finally, it was calcined at 550℃ for 8 hours.

[0080] The calcined catalyst was ground and added to a 1M NH4Cl solution at a solid-liquid mass ratio of 1:20. After heating in a water bath at 80℃ and 400r / min with magnetic stirring for 6 hours, the catalyst was removed, washed with deionized water until neutral, and dried at 100℃ for 24 hours. Finally, it was calcined at 550℃ for 3 hours to obtain the core-shell type MFI molecular sieve Fe / Sne-Silicalite-1@HZSM-5.

[0081] Example 4: Ga / Sne-Silicalite-1@HZSM-5 4.1 Synthesis of nucleus-phase catalysts C8H was weighed in molar ratios. 20 O4Si:H3BO3:C 12 H 29NO:H₂O = 1.000:0.04:0.125:8.000, mixed in the order of boron source-water-template agent-silicon source, at which point the solution separates into layers. The solution is mechanically stirred at 400 r / min for 24 hours, gradually becoming clear while ethanol evaporates, until a solid powder is formed. The solid powder is transferred to a hydrothermal reactor and aged at 100℃ for 24 hours, followed by static aging at 180℃ for 168 hours. After hydrothermal synthesis, the extracted solid is washed with deionized water until neutral and dried at 100℃ for 24 hours. Finally, it is calcined at 550℃ for 8 hours to obtain borosilicate MFI molecular sieve B-Silicalite-1.

[0082] B-Silicalite-1 was ground and added to a 1M NH4Cl solution at a solid-liquid mass ratio of 1:20. The mixture was heated in a water bath at 80°C with magnetic stirring at 500 rpm for 16 hours, then removed. The solid was washed with deionized water until neutral and dried at 80°C for 24 hours, finally calcined at 550°C for 3 hours. This process was repeated twice to obtain Sne-Silicalite-1, a nucleated molecular sieve with numerous defect anchoring sites after deboronization.

[0083] 4.2 Metal Loading and Carbide 0.3 g gallium nitrate, 15.0 g Sne-Silicalite-1, and excess water were mixed at a solid-liquid ratio of 1:10 and placed in a flask. The mixture was heated and stirred at 40 °C for 6 h, then heated and stirred at 80 °C until evaporated to dryness, and finally dried at 100 °C for 24 h. Finally, it was calcined at 550 °C for 8 h to obtain the metal-supported molecular sieve Ga / Sne-Silicalite-1.

[0084] Ga / Sne-Silicalite-1 was heated to 700°C under an inert gas atmosphere, and a mixture of methane and nitrogen (9:1, v / v) was introduced at a space velocity of 3000 ml·h. -1 ·g -1 After carbonization for half an hour, the sample was removed to obtain GaC / Sne-Silicalite-1.

[0085] 4.3 Synthesis of Shell Catalysts C8H was weighed in molar ratios. 20 O4Si:AlNO3:C 12 H 29 NO:H2O = 1.00:0.04:0.27:50.00, added to a beaker in the order of aluminum source-water-template agent-silicon source and magnetically stirred at 400 r / min for 12 hours to obtain ZSM-5 precursor solution.

[0086] The GaC / Sne-Silicalite-1 from section 4.2 was ground and added to the ZSM-5 precursor solution at a weight ratio of 1:2.5. The mixture was sonicated for 15 min. Then, the mixture was aged in a hydrothermal reactor at 100 °C for 24 hours and then kept still at 180 °C for 72 hours. After the hydrothermal synthesis was completed, the solid was washed with deionized water until neutral and dried at 100 °C for 24 hours. Finally, it was calcined at 550 °C for 8 hours to obtain Ga / Sne-Silicalite-1@HZSM-5.

[0087] Example 5: Zn / Sne-Silicalite-1@HZSM-5 5.1 Synthesis of nucleus-phase catalysts C8H was weighed in molar ratios. 20 O4Si:H3BO3:C 12 H 29 NO:H2O = 1.000:0.04:0.125:8.000, mixed in the order of boron source-water-template agent-silicon source, at which point the solution separates into layers; the solution is mechanically stirred at 400 r / min for 24 hours, the solution gradually becomes clear while ethanol evaporates, and then a solid powder is formed, which is transferred to a hydrothermal reactor; the hydrothermal reactor is kept stationary at 180℃ for 168 hours; after the hydrothermal synthesis is completed, the solid is washed with deionized water until neutral and dried at 80℃ for 24 hours; finally, it is calcined at 550℃ for 8 hours to obtain borosilicate MFI molecular sieve B-Silicalite-1.

[0088] The catalyst was ground and added to a 1M NH4Cl solution at a solid-liquid mass ratio of 1:20. After heating in a water bath at 80°C and 500 rpm for 16 hours with magnetic stirring, the solid was removed, washed with deionized water until neutral, dried at 80°C for 24 hours, and finally calcined at 550°C for 3 hours. This process was repeated twice to obtain the nucleated molecular sieve Sne-Silicalite-1, which formed a large number of defect anchoring sites after deboronization.

[0089] 5.2 Metal Loading and Carbide 0.3 g zinc sulfate, 15.0 g Sne-Silicalite-1 and excess water were mixed at a solid-liquid ratio of 1:10 and placed in a flask. The mixture was heated and stirred at 40 °C for 6 h, then heated and stirred at 80 °C to evaporate to dryness, and dried at 100 °C for 24 h. Finally, it was calcined at 550 °C for 8 h to obtain the metal-supported molecular sieve Zn / Sne-Silicalite-1.

[0090] Zn / Sne-Silicalite-1 was heated to 700°C under an inert gas atmosphere, and a mixture of methane and nitrogen (9:1, v / v) was introduced at a space velocity of 3000 ml·h. -1 ·g -1 After carbonization for half an hour, the catalyst was removed to obtain ZnC / Sne-Silicalite-1 catalyst.

[0091] 5.3 Synthesis of Shell Catalysts C8H was weighed in molar ratios. 20 O4Si:AlNO3:C 12 H 29 NO:H2O = 1.00:0.04:0.27:50.00, added to a beaker in the order of aluminum source-water-template agent-silicon source and magnetically stirred at 400 r / min for 12 hours to obtain ZSM-5 precursor solution.

[0092] ZnC / Sne-Silicalite-1, used as the core phase in section 5.2, was ground and added to the ZSM-5 precursor solution at a weight ratio of 1:2.5. The mixture was sonicated for 15 min. Subsequently, it was aged in a hydrothermal reactor at 100℃ for 24 hours, followed by static aging at 180℃ for 72 hours. After hydrothermal synthesis, the extracted solid was washed with deionized water until neutral and dried at 100℃ for 24 hours. Finally, it was calcined at 550℃ for 8 hours to obtain the core-shell MFI molecular sieve Zn / Sne-Silicalite-1@HZSM-5.

[0093] Comparative Example 1: Mo / Si-25 The catalyst synthesized in this comparative example follows the same procedure as in Example 1, except that no carbonization was performed after metal loading, and no shell layer was synthesized. All other process steps and parameters are the same as in Example 1.

[0094] The catalyst synthesis steps in this comparative example are as follows: (1) Synthesis of catalysts C8H was weighed in molar ratios. 20 O4Si:H3BO3:C 12 H 29NO:H2O = 1.000:0.04:0.125:8.000, mixed in the order of boron source-water-template agent-silicon source, at which point the solution separates into layers; the solution is mechanically stirred at 400 r / min for 24 hours, the solution gradually becomes clear while ethanol evaporates, until a solid powder is formed, the solid powder is transferred to a hydrothermal reactor; the hydrothermal reactor is aged at 100℃ for 24 hours, and then kept still at 180℃ for 168 hours; after the hydrothermal synthesis is completed, the solid is washed with deionized water until neutral, and dried at 100℃ for 24 hours; finally, it is calcined at 550℃ for 8 hours to obtain borosilicate MFI molecular sieve B-Silicalite-1.

[0095] B-Silicalite-1 was ground and added to a 1M NH4Cl solution at a solid-liquid mass ratio of 1:20. The mixture was heated in a water bath at 80°C and 400 rpm with magnetic stirring for 16 hours, then removed. The solid was washed with deionized water until neutral, dried at 100°C for 24 hours, and finally calcined at 550°C for 3 hours. This process was repeated twice to obtain the catalyst Si-25.

[0096] (2) Metal load 1.2 g of ammonium heptamolybdate, 15.0 g of Si-25, and excess water were mixed at a solid-liquid ratio of 1:10 and placed in a flask. The mixture was heated and stirred at 40 °C for 6 h, then heated and stirred at 80 °C until evaporated to dryness, and finally dried at 100 °C for 24 h. Finally, the mixture was calcined at 550 °C for 8 h to obtain the catalyst Mo / Si-25.

[0097] Comparative Example 2: Mo / S-1 Commercially available pure silica molecular sieves were used as the carrier. 1.2 g of ammonium heptamolybdate, 15.0 g of molecular sieve, and excess water were mixed at a solid-liquid ratio of 1:10, and the mixture was placed in a flask and heated and stirred at 40 °C for 6 h. Then, it was heated and stirred at 80 °C until evaporated to dryness, and then dried at 100 °C for 24 h. Finally, it was calcined at 550 °C for 8 h to obtain the catalyst Mo / S-1.

[0098] The catalysts from Example 1, Comparative Examples 1 and 2 were used in a fixed-bed reactor to study the catalytic reaction under atmospheric pressure. The specific steps are as follows: The core of the reactor consists of a tubular furnace and a variable-diameter quartz tube (upper half with outer diameter equal to inner diameter, lower half with outer diameter equal to inner diameter). The quartz tube contains a perforated plate filled with catalyst. During the experiment, the reactant gas is precisely delivered into the reactor via a mass flow controller. A type K thermocouple is used for temperature monitoring, and the external quartz tube prevents direct contact with the reactant gas, thus avoiding unnecessary catalyst reactions. When using a fixed-bed reactor, after filling with 0.5 g of catalyst, it is heated to 500 °C at a rate of 10 °C / min under an argon atmosphere. This temperature is maintained for 30 minutes to remove any potential impurities and pre-treat the catalyst. Subsequently, the temperature is further increased to the reaction temperature (700 °C) at a rate of 10 °C / min. Once 700 °C is reached, the inlet gas is switched to a mixture of methane and nitrogen (CH4:N2 ratio of 9:1), with the total flow rate maintained at 15 sccm. Nitrogen is used as an internal standard gas to accurately measure the methane conversion rate. After 8 minutes of reaction, the composition of the gaseous products was monitored and analyzed online using TCD and FID, and the internal and external standard methods were used to evaluate the conversion and selectivity of the catalyst.

[0099] The results are as follows Figure 9 As shown, there is an induction period at the beginning of the reaction, resulting in a low benzene yield at the first test point. The reaction activity then increases. Overall, Example 1 demonstrates significantly enhanced stability compared to the comparative example, with a selectivity for benzene compounds increased by over 10%. This is attributed to the introduction of appropriate acid sites into the core-shell MFI molecular sieve prepared in this application, which alters the reaction pathway, promotes aromatic hydrocarbon formation, and reduces coke production.

[0100] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A core-shell type MFI molecular sieve, comprising a core phase and a shell layer covering the surface of the core phase, characterized in that, The core phase is metal-supported Sne-Silicalite-1, and the shell is HZSM-5. Sne-Silicalite-1 is a borosilicate MFI molecular sieve B-Silicalite-1 that has been deboronized to form a large number of defect anchoring points.

2. The core-shell type MFI molecular sieve as described in claim 1, characterized in that, The core phase has a size of 500-700 nm, and the shell has a thickness of 100-200 nm. And / or, the metal is at least one of molybdenum, iron, gallium, and zinc; And / or, the metal loading is 3wt% to 5wt% based on the total mass of the core-shell MFI molecular sieve.

3. A method for preparing the core-shell type MFI molecular sieve as described in claim 1 or 2, characterized in that, The method is divided into Method 1 or Method 2, wherein Method 1 includes the following steps: Borosilicate MFI molecular sieve B-Silicalite-1 was prepared and deboronized to obtain molecular sieve Sne-Silicalite-1 with defect anchoring points. Metal was loaded onto the surface of Sne-Silicalite-1 to obtain M / Sne-Silicalite-1; subsequently, carbonization was performed to obtain MC / Sne-Silicalite-1. A core-shell type MFI molecular sieve, M / Sne-Silicalite-1@HZSM-5, was obtained by coating the surface of MC / Sne-Silicalite-1 with an HZSM-5 shell. The second method includes the following steps: Preparation of seed crystal B-Silicalite-1; The seed crystal B-Silicalite-1 was mixed with the shell crystallization mother liquor and then hydrothermally crystallized to obtain the Sne-Silicalite-1 / HZSM-5 catalyst. Metals were loaded onto the surface of the Sne-Silicalite-1 / HZSM-5 catalyst to obtain a core-shell type MFI molecular sieve M / Sne-Silicalite-1@HZSM-5.

4. The method as described in claim 3, characterized in that, The preparation of the seed crystal B-Silicalite-1 includes the following steps: The silicon source, boron source, template agent and water are mixed and stirred thoroughly until the ethanol evaporates to form a solid powder. The solid powder is aged at 80-120℃ for 18-30h, then left to stand at 175-185℃ for 120-192h. After being washed with water until neutral, it is dried to obtain seed crystal B-Silicalite-1. The borosilicate MFI molecular sieve B-Silicalite-1 is formed by calcining the seed crystal B-Silicalite-1 at 550-560℃ for 6-8 hours.

5. The method as described in claim 4, characterized in that, The silicon source is one of tetraethyl orthosilicate, sodium silicate, and silica sol; the boron source is boric acid; the template agent is one of tetrapropylammonium hydroxide and tetrapropylammonium bromide; wherein the silicon source and the boron source are mixed in a Si:B molar ratio of 1:(0.01~0.04).

6. The method as described in claim 3, characterized in that, The deboron removal process is achieved by adding molecular sieves to an ammonium chloride or ammonium nitrate solution, followed by stirring, heating, washing, drying, and calcination.

7. The method as described in claim 3, characterized in that, The carbonization process in Method 1 involves heating the M / Sne-Silicalite-1 catalyst to 680-720°C under a non-reactive atmosphere, and then introducing a mixed gas of carbon source and nitrogen at a space velocity of 3000-4000 ml·h. -1 ·g -1 It is achieved through a reaction, wherein the carbon source is one of the C1-C5 alkanes or alkenes.

8. The method as described in claim 3, characterized in that, In Method 1, the HZSM-5 shell is synthesized by adding MC / Sne-Silicalite-1 to the ZSM-5 precursor solution via hydrothermal synthesis.

9. The method as described in claim 8, characterized in that, The ZSM-5 precursor solution is prepared by mixing a silicon source, an aluminum source, a template agent, and water. The silicon source is one of tetraethyl orthosilicate, sodium silicate, or silica sol; the aluminum source is one of sodium aluminate or sodium nitrate; and the template agent is one of tetrapropylammonium hydroxide or tetrapropylammonium bromide. The silicon source and aluminum source are mixed in a molar ratio of Si to Al of 1:(0.02~0.06). And / or, the weight ratio of the MC / Sne-Silicalite-1 and ZSM-5 precursor solutions is 1:(1.2~2.5). And / or, the hydrothermal synthesis process is as follows: first, maintain at 80-120℃ for 18-30h, and then stand at 175-185℃ for 48-72h.

10. The application of the core-shell MFI molecular sieve as described in claim 1 or 2, or the core-shell MFI molecular sieve prepared by the method described in claims 3-9, as a catalyst in aromatization reactions.

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