Preparation method of ZSM-5@silicalite-1 core-shell structure molecular sieve with adjustable shell thickness

Abstract

The present application belongs to the technical field of inorganic material preparation, and particularly relates to a preparation method of ZSM-5@Silicalite-1 core-shell structure molecular sieve with adjustable shell thickness. The present application adopts one-pot method to synthesize ZSM-5@Silicalite-1 core-shell structure molecular sieve with adjustable shell thickness, and does not need any complex and tedious treatment process in the middle, so that the method is simple. Taking silicon and aluminum source as a starting point, the ZSM-5@Silicalite-1 core-shell structure molecular sieve is synthesized from bottom to top, which is more suitable for in-situ study on the crystallization mechanism of the core-shell structure molecular sieve. Moreover, the preparation method provided by the present application can directly realize the preparation of ZSM-5@Silicalite-1 core-shell structure molecular sieve with adjustable shell thickness by taking ZSM-5 as a core in the original system.

Description

Technical Field

[0001] This invention belongs to the field of inorganic material preparation technology, specifically relating to a method for preparing a ZSM-5@Silicalite-1 core-shell molecular sieve with adjustable shell thickness. Background Technology

[0002] Zeolite molecular sieves are crystalline aluminosilicate materials that have been widely used in catalysis, adsorption, and separation due to their regular, open molecular-scale micropores and excellent hydrothermal stability. Among them, ZSM-5 molecular sieves, with their unique three-dimensional channel system, exhibit good shape selectivity and a wide range of adjustable acidity, crystal morphology, and particle size, making them increasingly popular in catalytic cracking. However, acidic sites located on the outer surface or near the pore openings of the molecular sieve lack shape selectivity, often leading to side reactions and easy carbon deposition and deactivation, significantly reducing the selectivity and lifespan of the molecular sieve, thus limiting its application. Therefore, to enhance the selectivity and stability of ZSM-5 molecular sieves, it is necessary to control the morphology and passivate the acid on the outer surface.

[0003] Currently, surface modification of ZSM-5 mainly includes: (1) chemical vapor deposition (CVD) of the external functional groups on the zeolite surface with passivating agents. Although this method can mask the surface active sites to improve the shape selectivity of the molecular sieve, the passivating agent will inevitably reduce the activity of the molecular sieve because it will partially block the pores and narrow the pore openings. (2) epitaxial growth of a structurally compatible zeolite shell on the zeolite surface. For example, using ZSM-5 as the core substrate, an inert Si-MFI (Silicalite-1) molecular sieve shell can be epitaxially grown on its outer surface. While passivating the acid sites on the outer surface of ZSM-5, the disadvantage of pore blockage of the above deposition techniques can be effectively avoided. Although the theory and experiments for preparing Si-MFI@Al-MFI core-shell materials with interconnected channels and adjustable shell thickness are relatively mature, there are still problems such as complex synthesis steps, low raw material utilization and high requirements for industrial equipment. It is impossible to directly prepare Si-MFI@Al-MFI core-shell materials with adjustable shell thickness using ZSM-5 as the core in the original system, which greatly limits its application possibilities. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a method for preparing a ZSM-5@Silicalite-1 core-shell molecular sieve with adjustable shell thickness. The preparation method provided by this invention can directly achieve the preparation of ZSM-5@Silicalite-1 molecular sieves with adjustable shell thickness using ZSM-5 as the core within the original system.

[0005] To achieve the above objectives, the present invention provides the following technical solutions.

[0006] This invention provides a method for preparing a ZSM-5@Silicalite-1 core-shell molecular sieve with adjustable shell thickness, comprising the following steps:

[0007] (1) After mixing the aqueous solutions of aluminum source, silicon source and tetrapropylammonium hydroxide, the solution is subjected to a first aging and a first heat treatment in sequence to obtain a solution containing ZSM-5 molecular sieve; then, the solution containing ZSM-5 molecular sieve is mixed with ethanol and subjected to a second heat treatment to obtain a solution containing ZSM-5@Silicalite-1 core-shell molecular sieve.

[0008] (2) After mixing the silicon source and the aqueous solution of tetrapropylammonium hydroxide, the resulting mixture is subjected to a second aging process to obtain an aged solution;

[0009] (3) After mixing the aging liquid and the solution of the molecular sieve containing the ZSM-5@Silicalite-1 core-shell structure, the third heat treatment and calcination were carried out in sequence to obtain the molecular sieve with the ZSM-5@Silicalite-1 core-shell structure.

[0010] The shell thickness of the ZSM-5@Silicalite-1 core-shell molecular sieve is controlled by the first or the second control method.

[0011] The first control method is to adjust the mass ratio of the aging liquid to the solution containing the ZSM-5@Silicalite-1 core-shell structure while keeping the duration of the third heat treatment constant.

[0012] The second control method is to adjust the heating time of the third heat treatment while keeping the mass ratio of the aging liquid and the solution containing the ZSM-5@Silicalite-1 core-shell molecular sieve constant.

[0013] Preferably, the aluminum source includes one or more of aluminum isopropoxide, aluminum nitrate, and aluminum chloride;

[0014] The silicon source includes one or more of tetraethyl orthosilicate, fumed silica, tetraethyl orthosilicate, and sodium silicate.

[0015] Preferably, the mass concentration of the aqueous solution of tetrapropylammonium hydroxide is 20-60%.

[0016] Preferably, in step (1), the molar ratio of silicon source to tetrapropylammonium hydroxide is 1:(0.2-0.45).

[0017] Preferably, the molar ratio of silicon source to ethanol in step (1) is 1:(4-10).

[0018] Preferably, the first heat treatment is a first microwave-assisted heat treatment; the first microwave-assisted heat treatment includes a low-temperature treatment and a medium-temperature treatment performed sequentially; the temperature of the low-temperature treatment is 70-95°C; the holding time is 60-180 min; the temperature of the medium-temperature treatment is 110-140°C; the holding time is 4-10 h.

[0019] Preferably, the second heat treatment is a second microwave-assisted heat treatment, and the third heat treatment is a third microwave-assisted heat treatment;

[0020] The temperatures of the second and third microwave-assisted heat treatments are independently 150–180°C; the holding times are 0.5–4 hours.

[0021] Preferably, the molar ratio of silicon to tetrapropylammonium hydroxide in the aging solution is 1:(0.2-0.45).

[0022] Preferably, after the third heat treatment, the system obtained from the third heat treatment is further subjected to solid-liquid separation, water washing, and drying in sequence.

[0023] Preferably, the calcination temperature is 500–600°C, and the holding time is 5–7 hours.

[0024] This invention provides a method for preparing a ZSM-5@Silicalite-1 core-shell molecular sieve with adjustable shell thickness, comprising the following steps: (1) mixing an aqueous solution of an aluminum source, a silicon source, and tetrapropylammonium hydroxide, and then performing a first aging and a first heat treatment sequentially to obtain a solution containing ZSM-5 molecular sieve; then, mixing the solution containing ZSM-5 molecular sieve with ethanol and performing a second heat treatment to obtain a solution containing a ZSM-5@Silicalite-1 core-shell molecular sieve; (2) mixing an aqueous solution of a silicon source and tetrapropylammonium hydroxide, and then performing a second aging of the resulting mixture to obtain an aged solution; (3) mixing the aged solution with a solution containing a ZSM-5@Silicalite-1 core-shell molecular sieve. After the solution of the molecular sieve with the ZSM-5@Silicalite-1 core-shell structure is mixed, it is subjected to a third heat treatment and calcination in sequence to obtain the ZSM-5@Silicalite-1 core-shell structure molecular sieve. The shell thickness of the ZSM-5@Silicalite-1 core-shell structure molecular sieve is controlled by a first control method or a second control method. The first control method is to control the mass ratio of the aging liquid and the solution containing the ZSM-5@Silicalite-1 core-shell structure molecular sieve while keeping the duration of the third heat treatment unchanged. The second control method is to adjust the heating time of the third heat treatment while keeping the mass ratio of the aging liquid and the solution containing the ZSM-5@Silicalite-1 core-shell structure molecular sieve unchanged.

[0025] Compared with existing ZSM-5@Silicalite-1 core-shell molecular sieves, this invention has the following advantages:

[0026] (1) A one-pot method for synthesizing ZSM-5@Silicalite-1 core-shell molecular sieves with adjustable shell thickness is simple, requiring no complex or cumbersome intermediate processing. Starting with silicon and aluminum sources, the bottom-up synthesis of ZSM-5@Silicalite-1 core-shell molecular sieves is more suitable for in-situ research on the crystallization mechanism of core-shell molecular sieves.

[0027] (2) The raw materials used are readily available, and inexpensive tetrapropylammonium hydroxide is used as a template agent. Only a few hours of heating are needed to obtain a core-shell structure molecular sieve with adjustable shell thickness.

[0028] (3) No unique additives are required, and molecular sieves with adjustable shell thickness can be obtained under relatively mild synthesis conditions with a yield of up to 100%.

[0029] (4) The shell thickness can be precisely controlled by the amount of silicon source added in step (3) (the silicon source can basically cover the ZSM-5 core) or the heating time. No special treatment is required for the original system, and the shell thickness can be easily and conveniently adjusted.

[0030] (5) The ZSM-5@Silicalite-1 core-shell molecular sieve obtained by the preparation method of the present invention is a layer of Silicalite-1 epitaxially grown on the surface of a ZSM-5 molecular sieve with a silicon-rich core and an aluminum-rich shell, that is, a ZSM-5@Silicalite-1 molecular sieve with a sandwich structure.

[0031] The results of the examples show that X-ray diffraction reveals that the ZSM-5@Silicalite-1 core-shell molecular sieve prepared in this invention exhibits typical characteristic diffraction peaks of MFI, demonstrating good crystallinity and the absence of amorphous and impurity phases. Transmission and scanning electron microscopy show that the silicon source added in step (3) has been epitaxially grown on the ZSM-5 surface, with lattice fringes on the outer surface exhibiting the same orientation as those inside the particles. Furthermore, characterization of the acid properties of the outer surface of the molecular sieve by non-aqueous titration of tert-butylamine, X-ray photoelectron spectroscopy, and 2,6-di-tert-butylpyridine infrared spectroscopy indicates that Silicalite-1 completely covers the outer surface of the ZSM-5 molecular sieve. Near-in-situ dynamic light scattering experiments reveal that the increase in particle size is consistent with the theoretical increase in particle size caused by the subsequent addition of a silicon source to completely cover the particle surface.

[0032] This invention provides a ZSM-5@Silicalite-1 core-shell structured molecular sieve with adjustable shell thickness, prepared by the above-mentioned technical solution. The diameter of the sieve is 130–180 nm, and the shell thickness can be arbitrarily adjusted within the range of 0–30 nm. Furthermore, the ZSM-5@Silicalite-1 molecular sieves with different shell thicknesses prepared by this invention exhibit good crystallinity, and Silicalite-1 is uniformly epitaxially grown on the surface of the ZSM-5 zeolite core. While passivating the acid sites on the outer surface of the ZSM-5 molecular sieve, the connectivity of the pores is ensured, effectively improving the shape selectivity, activity, and stability of the molecular sieve. Attached Figure Description

[0033] Figure 1 X-ray diffraction pattern of the ZSM-5@Silicalite-1 core-shell molecular sieve prepared in Example 1;

[0034] Figure 2 Transmission electron microscope image of the ZSM-5@Silicalite-1 core-shell molecular sieve prepared for Example 1;

[0035] Figure 3 Scanning electron microscope image of the ZSM-5@Silicalite-1 core-shell molecular sieve prepared for Example 1;

[0036] Figure 4 The image shows the non-aqueous titration of tert-butylamine with the ZSM-5@Silicalite-1 core-shell molecular sieve prepared in Example 1.

[0037] Figure 5 The 2,6-di-tert-butylpyridine infrared spectrum of the ZSM-5@Silicalite-1 core-shell molecular sieve prepared in Example 1;

[0038] Figure 6 X-ray diffraction patterns of the ZSM-5@Silicalite-1 core-shell molecular sieves prepared in Examples 2-6;

[0039] Figure 7 Images of non-aqueous titrations of tert-butylamine with ZSM-5@Silicalite-1 core-shell molecular sieves prepared in Examples 2-6;

[0040] Figure 8 X-ray diffraction pattern of ZSM-5 molecular sieve prepared for Comparative Example 1;

[0041] Figure 9 Scanning electron microscope image of the ZSM-5 molecular sieve prepared for Comparative Example 1;

[0042] Figure 10Image of non-aqueous titration of tert-butylamine with ZSM-5 molecular sieve prepared in Comparative Example 1;

[0043] Figure 11 The infrared spectrum of 2,6-di-tert-butylpyridine for ZSM-5 molecular sieve prepared in Comparative Example 1;

[0044] Figure 12 The X-ray diffraction pattern of the Silicalite-1 molecular sieve prepared in Comparative Example 2 is shown.

[0045] Figure 13 The infrared spectrum of 2,6-di-tert-butylpyridine for the Silicalite-1 molecular sieve prepared in Comparative Example 2;

[0046] Figure 14 This is a non-aqueous titration image of tert-butylamine from Silicalite-1 molecular sieve prepared in Comparative Example 2. Detailed Implementation

[0047] This invention provides a method for preparing a ZSM-5@Silicalite-1 core-shell molecular sieve with adjustable shell thickness, comprising the following steps:

[0048] (1) After mixing the aqueous solutions of aluminum source, silicon source and tetrapropylammonium hydroxide, the solution is subjected to a first aging and a first heat treatment in sequence to obtain a solution containing ZSM-5 molecular sieve; then, the solution containing ZSM-5 molecular sieve is mixed with ethanol and subjected to a second heat treatment to obtain a solution containing ZSM-5@Silicalite-1 core-shell molecular sieve.

[0049] (2) After mixing the silicon source and the aqueous solution of tetrapropylammonium hydroxide, the resulting mixture is subjected to a second aging process to obtain an aged solution;

[0050] (3) After mixing the aging liquid and the solution of the molecular sieve containing the ZSM-5@Silicalite-1 core-shell structure, the third heat treatment and calcination were carried out in sequence to obtain the molecular sieve with the ZSM-5@Silicalite-1 core-shell structure.

[0051] The shell thickness of the ZSM-5@Silicalite-1 core-shell molecular sieve is controlled by the first or the second control method.

[0052] The first control method is to adjust the mass ratio of the aging liquid and the solution containing the ZSM-5@Silicalite-1 core-shell molecular sieve while keeping the duration of the third heat treatment constant.

[0053] The second control method is to adjust the heating time of the third heat treatment while keeping the mass ratio of the aging liquid and the solution containing the ZSM-5@Silicalite-1 core-shell molecular sieve constant.

[0054] In this invention, an aqueous solution of an aluminum source, a silicon source, and tetrapropylammonium hydroxide is mixed and then subjected to a first aging and a first heat treatment to obtain a solution containing ZSM-5 molecular sieve. Then, the solution containing ZSM-5 molecular sieve is mixed with ethanol and subjected to a second heat treatment to obtain a solution containing a molecular sieve with a ZSM-5@Silicalite-1 core-shell structure.

[0055] In this invention, the aluminum source preferably includes one or more of aluminum isopropoxide, aluminum nitrate, and aluminum chloride, more preferably aluminum isopropoxide. In this invention, the silicon source preferably includes one or more of tetraethyl orthosilicate, fumed silica, tetraethyl orthosilicate, and sodium silicate, more preferably tetraethyl orthosilicate. In this invention, the mass concentration of tetrapropylammonium hydroxide is preferably 20-60%, more preferably 25-40%.

[0056] In this invention, the molar ratio of the silicon source to tetrapropylammonium hydroxide is preferably 1:(0.2-0.45), more preferably 1:(0.3-0.4). This invention uses tetrapropylammonium hydroxide as a template agent to obtain zeolites with MFI topology over a wide phase region, facilitating the adjustment of synthesis parameters and providing a wide range of raw material sources, which helps reduce production costs.

[0057] In this invention, the molar ratio of the silicon source to the aluminum source is preferably 1:(0.004-0.02), more preferably 1:(0.005-0.01).

[0058] In this invention, the molar ratio of silicon source to ethanol is preferably 1:(4-10), more preferably 1:6. This invention uses ethanol as a solvent modifier, which is widely available, lower in cost, and less harmful to the human body. More importantly, it eliminates the cumbersome process of removing tetraethyl orthosilicate from the original system to produce ethanol by adding other types of alcohol. Simply adding a certain amount of ethanol can achieve a 100% yield in the original ZSM-5 synthesis system, providing a crucial prerequisite and guarantee for obtaining ZSM-5@Silicalite-1 core-shell molecular sieves by adding a silicon source. Too much ethanol will slow down the crystallization rate; too little will prevent all the silica and alumina in the supernatant from being added to the zeolite surface. Therefore, limiting the molar ratio of ethanol to silicon in the original system to 1:(4-10) ensures both a certain crystallization rate and atom economy.

[0059] In this invention, the preferred temperature for the first aging is room temperature, and the preferred aging time is 4–48 hours. The first aging is preferably carried out under stirring conditions. The preferred stirring speed is 100–600 rpm, more preferably 200–400 rpm, and the preferred aging time is 0.5–48 hours, more preferably 1–20 hours; the first aging continues until the mixture is clear and transparent. The aging under the above conditions allows for complete hydrolysis of the silicon and aluminum sources, facilitating the subsequent acquisition of a core-shell structured ZSM-5@Silicalite-1 molecular sieve.

[0060] In this invention, the first heat treatment is preferably a first microwave-assisted heat treatment, which includes a low-temperature treatment and a medium-temperature treatment performed sequentially; the temperature of the low-temperature treatment is preferably 70-95°C, more preferably 80-90°C; the holding time is preferably 60-180 min, more preferably 80-120 min. In this invention, the temperature of the medium-temperature treatment is preferably 110-140°C, more preferably 120-130°C; the holding time is preferably 4-10 h, more preferably 6-8 h.

[0061] In this invention, the second heat treatment is preferably a second microwave-assisted heat treatment, the temperature of the second microwave-assisted heat treatment is preferably 150-180°C, more preferably 160°C; the holding time is preferably 0.5-4h, more preferably 1-3h.

[0062] In this invention, a silicon source and an aqueous solution of tetrapropylammonium hydroxide are mixed, and the resulting mixture is then aged a second time to obtain an aged solution.

[0063] In this invention, the molar ratio of silicon to tetrapropylammonium hydroxide in the aging liquid is preferably 1:(0.2-0.45), more preferably 1:(0.3-0.4).

[0064] In this invention, the temperature of the second aging is preferably room temperature, and the second aging is preferably carried out under stirring conditions.

[0065] In this invention, an aging liquid and a solution containing a molecular sieve with a ZSM-5@Silicalite-1 core-shell structure are mixed and then subjected to a third heat treatment and calcination in sequence to obtain a molecular sieve with a ZSM-5@Silicalite-1 core-shell structure.

[0066] The shell thickness of the ZSM-5@Silicalite-1 core-shell molecular sieve is controlled by the first or the second control method.

[0067] The first control method is to adjust the mass ratio of the aging liquid and the solution containing the ZSM-5@Silicalite-1 core-shell molecular sieve while keeping the duration of the third heat treatment constant.

[0068] The second control method is to adjust the heating time of the third heat treatment while keeping the mass ratio of the aging liquid and the solution containing the ZSM-5@Silicalite-1 core-shell molecular sieve constant.

[0069] In this invention, the aging solution preferably comprises 5% to 80% of the mass of a molecular sieve containing a ZSM-5@Silicalite-1 core-shell structure. This invention can control the thickness of the ZSM-5@Silicalite-1 shell layer by adjusting the mass fraction of the aging solution relative to the molecular sieve containing the ZSM-5@Silicalite-1 core-shell structure.

[0070] In this invention, the third heat treatment is preferably a third microwave-assisted heat treatment, the temperature of which is preferably 150-180°C, more preferably 160-170°C; and the holding time is preferably 0.5-4h, more preferably 1-2h.

[0071] In this invention, the first heat treatment, the second heat treatment, and the third heat treatment are preferably performed independently in a microwave reactor, preferably an Anton Paar microwave reactor.

[0072] The low-medium-high temperature synthesis method employed in this invention optimizes the final molecular sieve with a ZSM-5@Silicalite-1 core-shell structure. Controlling the low temperature and duration improves the nucleation rate, ensuring the final crystals are of nanoscale size; controlling the medium temperature and duration promotes ordered crystal growth, providing a good substrate for subsequent shell growth; controlling the high temperature and duration facilitates the effective attachment of the added silicon source, ensuring the final high-quality ZSM-5@Silicalite-1 core-shell molecular sieve.

[0073] In this invention, after the third heat treatment and before calcination, the system obtained from the third heat treatment is further subjected to solid-liquid separation, water washing, and drying in sequence.

[0074] In this invention, the solid-liquid separation method is preferably filtration, vacuum filtration or centrifugation; the centrifugation speed is preferably 5000-15000 rpm, more preferably 10000-12000 rpm, and the centrifugation time is preferably 1-30 min, more preferably 5-8 min.

[0075] In this invention, the water washing is preferably centrifugal washing, and the temperature of the centrifugal washing is preferably room temperature; the rotation speed of the centrifugal washing is preferably 5000-15000 rpm, more preferably 10000-12000 rpm; the number of water washings is preferably 2-7 times, more preferably 3-5 times; and the time of a single water washing is preferably 1-30 minutes, more preferably 5-10 minutes. During the centrifugal washing process, after adding water to the centrifuge tube containing the solid obtained from centrifugation, it is preferable to first perform ultrasonic treatment before centrifugal washing. The ultrasonic treatment is preferably performed in an ultrasonic machine, and the purpose of the ultrasonic treatment is to redisperse the solid material uniformly in the water to ensure that the water can fully extract impurities. In this invention, the drying method can be any drying method well known in the art, such as air drying, oven drying, or freeze drying. Specifically, the drying method used in this invention is preferably freeze drying, and the freeze drying time is preferably 6-24 hours, more preferably 8-12 hours. Freeze drying can avoid excessive aggregation during the drying process and facilitate subsequent characterization.

[0076] In this invention, the calcination temperature is preferably 500-600℃, more preferably 550℃; the holding time is preferably 5-7h, more preferably 6h.

[0077] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0078] Example 1

[0079] 1) At room temperature, 0.0227 g of aluminum isopropoxide was added to 4.63 g of tetraethyl orthosilicate and mixed evenly with stirring at 300 rpm. Then, 7.05 g of 25% tetrapropylammonium hydroxide aqueous solution was added and aged for 24 h. The mixture was then transferred to a microwave reactor and subjected to low-temperature microwave heat treatment at 90 °C for 90 min, followed by medium-temperature microwave heat treatment at 130 °C for 8 h to obtain a solution containing ZSM-5 zeolite.

[0080] 2) Add 6.13g of ethanol to the above solution containing ZSM-5 zeolite and stir at room temperature for 30min. Then transfer it to a microwave reactor and heat treat it at 160℃ for 1h to obtain a solution containing a molecular sieve with a ZSM-5@Silicalite-1 core-shell structure.

[0081] The solution containing the molecular sieve with the ZSM-5@Silicalite-1 core-shell structure was transferred to a centrifuge tube, which was then centrifuged at 10,000 rpm for 8 minutes at room temperature. The supernatant was discarded. Deionized water was added to the resulting solid material, and the mixture was dispersed evenly in an ultrasonic machine before centrifugation and washing. The supernatant was discarded. The centrifugation and washing were repeated 3 times, with each washing lasting 8 minutes. The mixture was then freeze-dried for 12 hours and calcined in a muffle furnace at 500℃ for 6 hours to obtain ZSM-5@Silicalite-1 core-shell molecular sieve crystals with a shell thickness of 13 nm.

[0082] In this embodiment, the yield of ZSM-5@Silicalite-1 is 100%.

[0083] The zeolite yield is calculated as follows: (mass of silica in ZSM-5@Silicalite-1 core-shell molecular sieve crystals + mass of alumina) / (mass of silica in tetraethyl orthosilicate + mass of alumina in aluminum isopropoxide) × 100%.

[0084] Figure 1 The X-ray diffraction pattern of the ZSM-5@Silicalite-1 core-shell molecular sieve prepared in Example 1. Figure 1 It can be seen that the crystal prepared in Example 1 has typical MFI diffraction peaks, good crystallinity and no amorphous or impurity phases.

[0085] Figure 2 Transmission electron microscopy (TEM) images of the ZSM-5@Silicalite-1 core-shell molecular sieve prepared in Example 1. The high-resolution images show that the lattice fringes around the molecular sieve are uniformly oriented, and the particle length is approximately 188 nm.

[0086] Figure 3 Scanning electron microscope image of the ZSM-5@Silicalite-1 core-shell molecular sieve prepared in Example 1. It can be seen that the outer surface of the molecular sieve prepared in Example 1 is smooth.

[0087] Figure 4 Images of non-aqueous potentiometric titrations of the ZSM-5@Silicalite-1 core-shell molecular sieve prepared in Example 1 with tert-butylamine. The amount of tert-butylamine used from the highest initial potential to the final potential equilibrium state represents the number of acidic sites accessible on the outer surface of the molecular sieve material. Figure 4 It can be seen that the molecular sieve prepared in Example 1 has extremely low acid content on its outer surface.

[0088] Figure 5The 2,6-di-tert-butylpyridine infrared spectrum of the ZSM-5@Silicalite-1 core-shell molecular sieve prepared in Example 1. Figure 5 We know that: 1616cm -1 The infrared adsorption band at this location can be attributed to the ring vibration of 2,6-di-tert-butylpyridine, which is commonly used to quantitatively evaluate the concentration of Brønsted acid sites on the outer surface of zeolites. The spectrum shows that the molecular sieve prepared in this example has virtually no Brønsted acid sites on its outer surface.

[0089] Example 2

[0090] 1) At room temperature, 0.0227 g of aluminum isopropoxide was added to 4.63 g of tetraethyl orthosilicate and mixed evenly with stirring at 300 rpm. Then, 7.05 g of 25% tetrapropylammonium hydroxide aqueous solution was added and aged for 24 h. The mixture was then transferred to a microwave reactor and subjected to low-temperature microwave heat treatment at 90 °C for 90 min, followed by medium-temperature microwave heat treatment at 130 °C for 8 h to obtain a solution containing ZSM-5 zeolite.

[0091] 2) Add 6.13g of ethanol to the above solution containing ZSM-5 zeolite and stir at room temperature for 30min. Then transfer it to a microwave reactor and heat treat it at 160℃ for 1h to obtain a solution containing a molecular sieve with a ZSM-5@Silicalite-1 core-shell structure.

[0092] 3) Add 0.353 g of 25 wt% tetrapropylammonium hydroxide aqueous solution to 0.232 g of tetraethyl orthosilicate, age at room temperature for 4 h, then add to the molecular sieve solution synthesized in step 2) and stir at room temperature for 2 h (i.e., add 5% of the original system mass of the aging solution). Then, transfer to a microwave reactor and perform high-temperature microwave heat treatment at 160 °C for 1 h to obtain a solution of ZSM-5@Silicalite-1 core-shell structured molecular sieve with a thicker shell. The milky white solution was transferred to a centrifuge tube, which was then placed in a centrifuge and centrifuged at 10,000 rpm for 8 minutes at room temperature. The supernatant was discarded, and deionized water was added to the resulting solid material. The mixture was then dispersed evenly in an ultrasonic machine and centrifuged and washed again. The supernatant was discarded, and the mixture was centrifuged and washed three times, with each centrifugation and washing lasting 8 minutes. The mixture was then placed in a freeze dryer and dried for 12 hours. Finally, it was calcined in a muffle furnace at 500°C for 6 hours to obtain ZSM-5@Silicalite-1 molecular sieve crystals with a thicker shell (shell thickness of 14.5 nm).

[0093] In this embodiment, the yield of ZSM-5@Silicalite-1 is 100%.

[0094] The zeolite yield is calculated as follows: (mass of silica in ZSM-5@Silicalite-1 core-shell molecular sieve crystals + mass of alumina) / (mass of silica in tetraethyl orthosilicate from two feedings + mass of alumina in aluminum isopropoxide) × 100%.

[0095] Example 3

[0096] The only difference from Example 2 is that in step 3), "adding 0.353g of 25wt% tetrapropylammonium hydroxide aqueous solution to 0.232g tetraethyl orthosilicate, aging at room temperature for 4h, and then adding it to the molecular sieve solution synthesized in step 2) and stirring at room temperature for 2h" is replaced with "adding 0.705g of 25wt% tetrapropylammonium hydroxide aqueous solution to 0.463g tetraethyl orthosilicate, aging at room temperature for 4h, and then adding it to the molecular sieve solution synthesized in step 2) and stirring at room temperature for 2h", that is, adding 10% of the original system mass of aging solution.

[0097] The shell thickness of the obtained product is 16 nm.

[0098] In this embodiment, the yield of ZSM-5@Silicalite-1 is 100%.

[0099] Example 4

[0100] The only difference from Example 2 is that in step 3), "adding 0.353g of 25wt% tetrapropylammonium hydroxide aqueous solution to 0.232g tetraethyl orthosilicate, aging at room temperature for 4h, and then adding it to the molecular sieve solution synthesized in step 2) and stirring at room temperature for 2h" is replaced with "adding 1.410g of 25wt% tetrapropylammonium hydroxide aqueous solution to 0.926g tetraethyl orthosilicate, aging at room temperature for 4h, and then adding it to the molecular sieve solution synthesized in step 2) and stirring at room temperature for 2h", that is, adding 20% ​​of the original system mass of aging solution.

[0101] The shell thickness of the obtained product is 19 nm.

[0102] In this embodiment, the yield of ZSM-5@Silicalite-1 is 100%.

[0103] Example 5

[0104] The only difference from Example 2 is that in step 3), "adding 0.353g of 25wt% tetrapropylammonium hydroxide aqueous solution to 0.232g tetraethyl orthosilicate, aging at room temperature for 4h, and then adding it to the molecular sieve solution synthesized in step 2) and stirring at room temperature for 2h" is replaced with "adding 2.820g of 25wt% tetrapropylammonium hydroxide aqueous solution to 1.852g tetraethyl orthosilicate, aging at room temperature for 4h, and then adding it to the molecular sieve solution synthesized in step 2) and stirring at room temperature for 2h", that is, adding 40% of the original system mass of aging solution.

[0105] The shell thickness of the obtained product is 25 nm.

[0106] In this embodiment, the yield of ZSM-5@Silicalite-1 is 100%.

[0107] Example 6

[0108] The only difference from Example 2 is that in step 3), "adding 0.353g of 25wt% tetrapropylammonium hydroxide aqueous solution to 0.232g tetraethyl orthosilicate, aging at room temperature for 4h, and then adding it to the molecular sieve solution synthesized in step 2) and stirring at room temperature for 2h" is replaced with "adding 5.640g of 25wt% tetrapropylammonium hydroxide aqueous solution to 3.704g tetraethyl orthosilicate, aging at room temperature for 4h, and then adding it to the molecular sieve solution synthesized in step 2) and stirring at room temperature for 2h", that is, adding 80% of the original system mass of aging solution.

[0109] The shell thickness of the obtained product is 35 nm.

[0110] In this embodiment, the yield of ZSM-5@Silicalite-1 is 100%.

[0111] Figure 6 X-ray diffraction patterns of ZSM-5@Silicalite-1 molecular sieves with different shell thicknesses prepared in Examples 2-6. Figure 6 It can be seen that the crystals prepared in Examples 2 to 6 of this embodiment have typical characteristic diffraction peaks of MFI, good crystallinity and no amorphous or impurity phases.

[0112] Figure 7 Images show non-aqueous potentiometric titrations of ZSM-5@Silicalite-1 molecular sieves with different shell thicknesses prepared in Examples 2-6 using tert-butylamine. The amount of tert-butylamine used from the highest initial potential to the final potential equilibrium state represents the number of acidic sites accessible on the outer surface of the molecular sieve material. It can be seen that the molecular sieves prepared in Examples 2-6 all have extremely low acid content on their outer surface.

[0113] Comparative Example 1

[0114] 1) At room temperature, 0.0227 g of aluminum isopropoxide was added to 4.63 g of tetraethyl orthosilicate and mixed evenly with stirring at 300 rpm. Then, 7.05 g of 25% tetrapropylammonium hydroxide aqueous solution was added and aged for 24 h. The mixture was then transferred to a microwave reactor and subjected to low-temperature microwave heat treatment at 90 °C for 90 min, followed by medium-temperature microwave heat treatment at 130 °C for 8 h to obtain a solution containing ZSM-5 zeolite.

[0115] The solution containing ZSM-5 zeolite was transferred to a centrifuge tube, which was then placed in a centrifuge and centrifuged at 10,000 rpm for 8 minutes at room temperature. The supernatant was discarded, and deionized water was added to the resulting solid material. The mixture was then dispersed evenly in an ultrasonic machine and centrifuged again. The supernatant was discarded, and the mixture was washed with water three times, with each washing lasting 8 minutes. The mixture was then placed in a freeze dryer and dried for 12 hours to obtain ZSM-5 molecular sieve crystals.

[0116] In this embodiment, the yield of ZSM-5 was 66%.

[0117] The zeolite yield is calculated as follows: (mass of silicon dioxide in ZSM-5 crystals + mass of alumina) / (mass of silicon dioxide in tetraethyl orthosilicate + mass of alumina in aluminum isopropoxide) × 100%.

[0118] Figure 8 The X-ray diffraction pattern of nuclear ZSM-5 prepared for Comparative Example 1. Figure 8 It can be seen that the crystal prepared in Comparative Example 1 has typical MFI diffraction peaks, good crystallinity, and no amorphous or impurity phases.

[0119] Figure 9 Scanning electron microscope image of nuclear ZSM-5 prepared for Comparative Example 1. From Figure 9 It can be seen that the outer surface of the molecular sieve core prepared in Comparative Example 1 is rough, which contrasts sharply with the smooth outer surface of the ZSM-5@Silicalite-1 molecular sieve.

[0120] Figure 10 Images of the non-aqueous potentiometric titration of nuclear ZSM-5 prepared in Comparative Example 1 with tert-butylamine. The amount of tert-butylamine used from the highest initial potential to the final potential equilibrium represents the number of acidic sites accessible on the outer surface of this molecular sieve material. Figure 10 It can be seen that the nuclear ZSM-5 prepared in Comparative Example 1 has a certain number of acid sites on its outer surface.

[0121] Figure 11 The infrared spectrum of 2,6-di-tert-butylpyridine of nuclear ZSM-5 prepared in Comparative Example 1, 1616 cm⁻¹ -1The infrared adsorption band at this location can be attributed to the ring vibration of 2,6-di-tert-butylpyridine, and is commonly used to quantitatively evaluate the concentration of Brønsted acid sites on the outer surface of zeolites. Figure 11 It can be seen that the outer surface of the ZSM-5 core prepared in Comparative Example 1 has obvious Brønsted acid sites, which is in stark contrast to the molecular sieves prepared in Examples 1-6, which have virtually no Brønsted acid sites on their outer surface.

[0122] Comparative Example 2

[0123] At room temperature, 7.05 g of 25 wt% tetrapropylammonium hydroxide aqueous solution was added to 4.63 g of tetraethyl orthosilicate and mixed evenly at a stirring speed of 300 rpm for 24 h. Then, the mixture was transferred to a microwave reactor and subjected to low-temperature microwave heat treatment at 90 °C for 90 min, followed by medium-temperature microwave heat treatment at 130 °C for 8 h to obtain a solution containing Silicalite-1 zeolite.

[0124] The solution containing Silicalite-1 zeolite was then transferred to centrifuge tubes, which were placed in a centrifuge and centrifuged at 10,000 rpm for 8 minutes at room temperature. The supernatant was discarded, and deionized water was added to the resulting solid material. The mixture was then dispersed evenly in an ultrasonic machine and centrifuged again. The supernatant was discarded, and the mixture was washed with water three times, with each washing lasting 8 minutes. The mixture was then placed in a freeze dryer and dried for 12 hours to obtain Silicalite-1 molecular sieve crystals.

[0125] In this embodiment, the yield of Silicalite-1 was 62%.

[0126] The zeolite yield is calculated as follows: (mass of silicon dioxide in Silicalite-1 crystals) / (mass of silicon dioxide in the added tetraethyl orthosilicate) × 100%.

[0127] Figure 12 The X-ray diffraction pattern of Si-MFI prepared in Comparative Example 2 is shown. Figure 12 It can be seen that the crystal prepared in Comparative Example 2 has typical characteristic diffraction peaks of MFI, good crystallinity, and no amorphous or impurity phases.

[0128] Figure 13 The infrared spectrum of 2,6-di-tert-butylpyridine for Si-MFI prepared in Comparative Example 2 is shown at 1616 cm⁻¹. -1 The infrared adsorption band at this location can be attributed to the ring vibration of 2,6-di-tert-butylpyridine, which is commonly used to quantitatively evaluate the concentration of Brønsted acid sites on the outer surface of zeolites. The spectrum shows that the Si-MFI prepared in Comparative Example 2 has no Brønsted acid sites on its outer surface, consistent with the results of the ZSM-5@Silicalite-1 molecular sieves prepared in Examples 1-6.

[0129] Figure 14The image shows a non-aqueous potentiometric titration of Si-MFI prepared in Comparative Example 2 with tert-butylamine. The amount of tert-butylamine used from the highest initial potential to the final potential equilibrium state represents the number of acidic sites accessible on the outer surface of the molecular sieve material. It can be seen that the Si-MFI prepared in Comparative Example 2 has extremely low acid content on the outer surface, consistent with the results of ZSM-5@Silicalite-1 molecular sieves prepared in Examples 1-6.

[0130] As can be seen from the above examples and comparative examples, the present invention can easily and efficiently prepare ZSM-5@Silicalite-1 core-shell molecular sieves with adjustable shell thickness, expand the current synthesis methods of ZSM-5@Silicalite-1 molecular sieves, and provide a simpler and easier method.

[0131] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing a ZSM-5@Silicalite-1 core-shell molecular sieve with adjustable shell thickness, comprising the following steps: (1) After mixing the aqueous solutions of aluminum source, silicon source and tetrapropylammonium hydroxide, the solution is subjected to a first aging and a first heat treatment in sequence to obtain a solution containing ZSM-5 molecular sieve; then, the solution containing ZSM-5 molecular sieve is mixed with ethanol and subjected to a second heat treatment to obtain a solution containing ZSM-5@Silicalite-1 core-shell molecular sieve. (2) After mixing the silicon source and the aqueous solution of tetrapropylammonium hydroxide, the resulting mixture is subjected to a second aging process to obtain an aged solution; (3) After mixing the aging liquid and the solution of the molecular sieve containing the ZSM-5@Silicalite-1 core-shell structure, the third heat treatment and calcination were carried out in sequence to obtain the molecular sieve with the ZSM-5@Silicalite-1 core-shell structure. The shell thickness of the ZSM-5@Silicalite-1 core-shell molecular sieve is controlled by the first or the second control method. The first control method is to adjust the mass ratio of the aging liquid to the solution containing the ZSM-5@Silicalite-1 core-shell structure while keeping the duration of the third heat treatment constant. The second control method is to adjust the heating time of the third heat treatment while keeping the mass ratio of the aging liquid and the solution containing the ZSM-5@Silicalite-1 core-shell molecular sieve constant. The molar ratio of silicon source to ethanol in step (1) is 1:(4-10).

2. The preparation method according to claim 1, characterized in that, The aluminum source includes one or more of aluminum isopropoxide, aluminum nitrate, and aluminum chloride; The silicon source includes one or more of tetraethyl orthosilicate, fumed silica, and sodium silicate.

3. The preparation method according to claim 1, characterized in that, The aqueous solution of the tetrapropylammonium hydroxide has a mass concentration of 20-60%.

4. The preparation method according to claim 1 or 2, characterized in that, In step (1), the molar ratio of silicon source to tetrapropylammonium hydroxide is 1:(0.2-0.45).

5. The preparation method according to claim 1, characterized in that, The first heat treatment is a first microwave-assisted heat treatment; the first microwave-assisted heat treatment includes a low-temperature treatment and a medium-temperature treatment performed sequentially; the temperature of the low-temperature treatment is 70-95°C; the holding time is 60-180 min; the temperature of the medium-temperature treatment is 110-140°C; the holding time is 4-10 h.

6. The preparation method according to claim 1, characterized in that, The second heat treatment is a second microwave-assisted heat treatment, and the third heat treatment is a third microwave-assisted heat treatment; The temperatures of the second and third microwave-assisted heat treatments are independently 150–180°C; the holding times are 0.5–4 hours.

7. The preparation method according to claim 1, characterized in that, The molar ratio of silicon to tetrapropylammonium hydroxide in the aging solution is 1:(0.2-0.45).

8. The preparation method according to claim 1, characterized in that, After the third heat treatment and before calcination, the system obtained from the third heat treatment is further subjected to solid-liquid separation, water washing, and drying in sequence.

9. The preparation method according to claim 1, characterized in that, The calcination temperature is 500–600℃, and the holding time is 5–7 hours.

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