Mesoporous core-shell molecular sieve tandem β-zeolite catalysts, their preparation methods and applications

Abstract

This invention discloses a mesoporous core-shell molecular sieve tandem β-zeolite catalyst, its preparation method, and its application, belonging to the field of polyolefin hydrocracking catalyst technology. In this invention, Pt is confined within the micropores of a silicon-titanium molecular sieve core, while a mesoporous SiO2 shell is used to coat the silicon-titanium molecular sieve core. This tandem catalytic system, composed of a mesoporous core-shell molecular sieve catalyst and β-zeolite, exhibits excellent activity and selectivity in the hydrocracking reaction of polyethylene. During the hydrocracking reaction of polyethylene, the C-C bond cleavage reaction mainly occurs in the β-zeolite. Subsequently, through the adsorption of the mesoporous shell, intermediates are efficiently transferred into the micropores. Within the micropores, short-chain olefins can achieve directional hydrogenation due to their shape-selective effect. This invention optimizes the tiered mesoporous structure and establishes a method for preparing a core-shell structured mesoporous composite molecular sieve with controllable pore channels.

Description

Technical Field

[0001] This invention relates to the field of polyolefin hydrocracking catalyst technology, specifically to a mesoporous core-shell molecular sieve tandem β-zeolite catalyst and its preparation method and application. Background Technology

[0002] In today's society, plastics are widely used in various fields due to their excellent properties, greatly improving people's lives. However, the overuse of plastics has also led to increasingly serious environmental problems. The accumulation of plastic waste not only occupies a large amount of land resources but also causes profound damage to ecosystems. How to effectively treat these plastic wastes and achieve their recycling has become a global focus. Traditional methods of waste plastic treatment mainly include incineration and landfill, but these methods not only cause serious air pollution but also occupy a large amount of land resources. Chemical catalytic recycling, as an emerging strategy, is an effective way to solve the problem of waste plastic pollution. This method explores the intrinsic value of macromolecules in plastics, transforming them into higher-value chemicals, thereby achieving high-value recycling of waste plastics. Compared with traditional recycling methods, chemical catalysis is greener and more environmentally friendly, and can significantly improve the economic value of the products. Among them, tandem catalysis technology constructs an acid-metal site spatial isolation and functional regulation strategy, achieving independent and precise control of the cracking and hydrogenation processes by physically separating two types of active sites. This separation not only avoids the uncontrollable interaction between acidic sites and metal sites but also achieves independent and precise control of the cracking and hydrogenation processes. In this way, tandem catalysis technology can significantly improve the efficiency and selectivity of hydrocracking of polyolefin waste plastics, providing a new approach to overcome the limitations of traditional catalytic systems.

[0003] In recent years, significant progress has been made in the catalytic conversion of polyethylene, with tandem catalysis technology attracting widespread attention. For example, Liu et al. used Pt / WO3 / ZrO2 and HY zeolite as catalysts to hydrogenate plastics via tandem catalysis, obtaining liquid fuel products composed of gasoline, diesel, and kerosene with a yield as high as 85%. Even at 250℃, the conversion rate of low-density polyethylene (LDPE) was relatively low when using Pt / WO3 / ZrO2 alone as a catalyst. However, when using a mixture of Pt / WO3 / ZrO2 and HY zeolite as a catalyst, the conversion rate was significantly improved; almost all LDPE could be converted after 2 hours of reaction, demonstrating a strong synergistic effect between the two catalysts. However, while the aforementioned physically mixed catalysts can effectively isolate active sites, theoretically, they suffer from weakened thermodynamic coupling effects and prolonged intermediate migration paths, leading to increased side reactions and limiting the energy efficiency and product selectivity of the catalytic process.

[0004] Traditional polyethylene catalytic cracking is limited by the mass transfer-reaction contradiction caused by the single pore size. Although mesoporous materials are conducive to the diffusion of polyethylene macromolecules, they lack the shape-selective confinement effect for cracking intermediates (such as short-chain olefins). While microporous molecular sieves can regulate product distribution through shape-selective effect, they are difficult to accommodate the mass transfer space required for the initial cracking of polyethylene. Summary of the Invention

[0005] This invention provides a mesoporous / microporous core-shell molecular sieve tandem β-zeolite catalyst, its preparation method, and its application. It effectively solves the technical problems of existing polyethylene hydrocracking catalysts, such as mismatch between pore structure and reaction pathway, lack of shape-selective confinement effect for cracking intermediates, resulting in low catalytic reaction efficiency and poor product selectivity. This invention achieves spatiotemporal decoupling of the hydrocracking reaction through a mesoporous / microporous partitioned catalytic strategy. It not only achieves precise matching between catalyst pore structure and reaction pathway, but also synergistically leverages the high-efficiency mass transfer advantage of mesopores and the shape-selective catalytic characteristics of micropores, thereby spatially regulating the activity and selectivity of the products.

[0006] The first objective of this invention is to provide a method for preparing a mesoporous core-shell molecular sieve tandem β-zeolite catalyst, comprising the following steps:

[0007] At 1℃~4℃, silicon and titanium sources are used as raw materials, and tetrapropylammonium hydroxide is used as a structure directing agent. The raw materials are dissolved in water to obtain a mixed solution. The mixed solution is subjected to a hydrolysis-condensation reaction at 60℃~80℃. The silicon and titanium sources are hydrolyzed to generate silanol and titanium hydroxyl groups, and then condensed to form a SiO2-TiO2 network. Isopropanol is added to obtain the silicon-titanium molecular sieve precursor.

[0008] (3-Trimercaptopropyl)-trimethoxysilane was hydrolyzed under alkaline conditions to obtain mercapto and silanol groups, and then Pt was added. 4+ The source, the thiol group will Pt 4+ The material is reduced to elemental Pt and anchored to the silane backbone via silanol groups to obtain a Pt-supported silane complex.

[0009] The Pt-supported silane complex was added to the silicon-titanium molecular sieve precursor, stirred and mixed at 60℃~80℃, and then crystallized at 160℃~180℃ to obtain a platinum-supported silicon-titanium molecular sieve seed solution.

[0010] Using hexadecyltrimethylammonium chloride and triethanolamine as templates, a pore-expanding agent solution of tetraethyl orthosilicate was introduced into the seed solution of the platinum-supported silicon-titanium molecular sieve. The interface self-assembly was carried out under stirring at 55℃~65℃ to form a mesoporous SiO2 shell layer. After calcination, a mesoporous core-shell molecular sieve was obtained. This sieve was then mixed with β-zeolite to obtain a mesoporous core-shell molecular sieve tandem with β-zeolite catalyst.

[0011] In a preferred embodiment, the mass ratio of the mesoporous core-shell molecular sieve to β-zeolite is 1:0.8~1.3.

[0012] In a preferred embodiment, the volume concentration of tetraethyl orthosilicate in the pore-expanding agent solution is 18% to 23%, and the volume ratio of the platinum-supported silicon-titanium molecular sieve seed solution to the tetraethoxysilane pore-expanding agent solution is 5:20 to 24.

[0013] In a preferred embodiment, the pore-expanding agent is 1-octadecene, decachloronaphthalene, or cyclohexane; the pore size of the mesoporous SiO2 shell is 2 nm to 20 nm.

[0014] As a preferred embodiment, the Pt 4+ The concentration of the source is 50mM~150mM, and the loading of Pt in the platinum-supported silicon-titanium molecular sieve seed crystal is 1.0wt%~2.0wt%.

[0015] In a preferred embodiment, the molar ratio of the silicon source, titanium source and tetrapropylammonium hydroxide is 1:0.02~0.05:0.2~0.4.

[0016] In a preferred embodiment, the calcination is performed at 500℃~550℃ for 4h~5h.

[0017] In a preferred embodiment, the silicon source is tetraethyl orthosilicate, the titanium source is tetrabutyl titanate, and the Pt... 4+ The source is H2PtCl6.

[0018] The second objective of this invention is to provide a mesoporous core-shell molecular sieve tandem β-zeolite catalyst, which is prepared using the above-described preparation method.

[0019] The third objective of this invention is to provide an application of the above-mentioned mesoporous core-shell molecular sieve tandem β-zeolite catalyst in the catalytic hydrocracking of polyethylene.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] This invention provides a method for preparing a mesoporous core-shell molecular sieve tandem β-zeolite catalyst. Pt is confined within the micropores of a silicon-titanium molecular sieve core, which is then coated with a mesoporous SiO2 shell. This mesoporous core-shell molecular sieve catalyst, combined with β-zeolite, forms a tandem catalytic system that exhibits excellent activity and selectivity in the hydrocracking reaction of polyethylene. During the polyethylene hydrocracking reaction, β-zeolite provides acidic sites, and the C-C bond cleavage reaction of polyethylene mainly occurs within the β-zeolite. Subsequently, intermediates are efficiently transferred into the micropores through adsorption by the mesoporous shell. Within the micropores, short-chain olefins can achieve directional hydrogenation due to their shape-selective effect. This invention optimizes the tiered mesoporous structure and establishes a method for preparing a core-shell structured mesoporous composite molecular sieve with controllable pore channels. The mesoporous core-shell molecular sieve prepared by this invention can enhance the diffusion of intermediate products during the catalytic process. Spatially, through a cross-scale mass transfer-reaction synergistic effect, the tandem catalytic system exhibits unique spatial adaptability.

[0022] This invention utilizes a micro / mesoporous ladder-pore core-shell structure catalyst with silica-coated zeolite (zeolite@SiO2). The mesoporous shell layer is responsible for adsorbing and transferring olefin intermediates, while simultaneously controlling the hydrogenation reaction to occur within the micropores. This micro / mesoporous partitioned catalysis strategy achieves spatiotemporal decoupling of the cracking-hydrogenation reaction. This strategy not only enables precise matching of the catalyst pore structure to the reaction pathway but also combines the efficient mass transfer advantages of mesopores with the shape-selective catalytic properties of micropores, allowing for spatial control of the product activity and selectivity. Attached Figure Description

[0023] Figure 1 This is a TEM image of the mesoporous core-shell molecular sieve of Example 1 of the present invention.

[0024] Figure 2 The images show the XRD patterns of the mesoporous core-shell molecular sieves of Examples 1 to 3 of this invention and the Pt@TS-1 prepared in Comparative Example 1.

[0025] Figure 3 The images show the XRD patterns of the mesoporous core-shell molecular sieves of Examples 2, 2, and 3 of the present invention, and the Pt@TS-1 prepared in Comparative Example 1.

[0026] Figure 4 This is a comparison chart of the catalytic performance of the mesoporous core-shell molecular sieve tandem β-zeolite catalysts of Examples 1 to 3 of the present invention and the Pt@TS-1 tandem β-zeolite catalyst prepared in Comparative Example 1.

[0027] Figure 5 This is a comparison chart of the catalytic performance of the mesoporous core-shell molecular sieve tandem β-zeolite catalysts of Example 2, Comparative Example 2, and Comparative Example 3, and the Pt@TS-1 tandem β-zeolite catalyst prepared in Comparative Example 1. Detailed Implementation

[0028] To enable those skilled in the art to better understand and implement the technical solutions of this invention, the invention is further described below with reference to specific embodiments. However, the embodiments are not intended to limit the invention. Unless otherwise specified, the following test methods and detection methods are conventional methods; unless otherwise specified, the reagents and raw materials are commercially available.

[0029] To address the technical problems of low catalytic efficiency and poor product selectivity in existing polyethylene hydrocracking catalysts, such as the mismatch between pore structure and reaction pathway and the lack of shape-selective confinement effect for cracking intermediates, this invention provides a mesoporous / microporous core-shell molecular sieve tandem β-zeolite catalyst, its preparation method, and its applications.

[0030] The technical solution of the present invention will be described in detail below.

[0031] This invention provides a method for preparing a mesoporous core-shell molecular sieve tandem β-zeolite catalyst, comprising the following steps:

[0032] S1, using silicon and titanium sources as raw materials and tetrapropylammonium hydroxide as a structure directing agent, is dissolved in water at 1℃~4℃ to obtain a mixed solution. The mixed solution undergoes a hydrolysis-condensation reaction at 60℃~80℃, where silicon and titanium sources are hydrolyzed to generate silanol and titanium hydroxyl groups, which then condense to form a SiO2-TiO2 network. Isopropanol is added to obtain the silicon-titanium molecular sieve precursor.

[0033] S2, (3-trimercaptopropyl)-trimethoxysilane is hydrolyzed under alkaline conditions to obtain mercapto and silanol groups, and then Pt is added. 4+ The source, the thiol group will Pt 4+ The material is reduced to elemental Pt and anchored to the silane backbone via silanol groups to obtain a Pt-supported silane complex.

[0034] S3, the Pt-supported silane complex is added to the silicon-titanium molecular sieve precursor, stirred and mixed at 60℃~80℃, and then crystallized at 160℃~180℃ to obtain platinum-supported silicon-titanium molecular sieve seed solution.

[0035] S4, using hexadecyltrimethylammonium chloride and triethanolamine as templates, introduces a tetraethyl orthosilicate pore-expanding agent solution into the platinum-supported silicon-titanium molecular sieve seed solution, and performs interfacial self-assembly under stirring at 55℃~65℃ to form a mesoporous SiO2 shell layer. Calcination at 500℃~550℃ for 4h~5h yields a mesoporous core-shell molecular sieve, which is then mixed with β-zeolite to obtain a mesoporous core-shell molecular sieve tandem with β-zeolite catalyst.

[0036] In the above technical solution, Pt is confined within the micropores of the silicon-titanium molecular sieve core, while a mesoporous SiO2 shell is used to coat the silicon-titanium molecular sieve core. This mesoporous core-shell molecular sieve catalyst, combined with β-zeolite, forms a tandem catalytic system, exhibiting excellent activity and selectivity in the hydrocracking reaction of polyethylene. The mesoporous SiO2 shell is responsible for adsorbing and transferring olefin intermediates, while controlling the hydrogenation reaction to occur within the micropores. Through a micro / mesoporous partitioned catalytic strategy, spatiotemporal decoupling of the cracking-hydrogenation reaction is achieved. During the polyethylene hydrocracking reaction, β-zeolite provides acidic sites, and the C-C bond cleavage reaction of polyethylene mainly occurs in β-zeolite. Subsequently, with the adsorption of the mesoporous shell, intermediates are efficiently transferred to the micropores. Within the micropores, short-chain olefins can achieve directional hydrogenation due to their shape-selective effect. The mesoporous core-shell molecular sieve prepared in this invention can enhance the diffusion of intermediate products during the catalytic process. Spatially, through cross-scale mass transfer-reaction synergistic effects, the tandem catalytic system exhibits unique spatial adaptability. This invention not only enables precise matching of catalyst pore structure with reaction pathway, but also combines the high-efficiency mass transfer advantage of mesopores with the shape-selective catalytic properties of micropores to regulate the activity and selectivity of products from a spatial dimension.

[0037] To further improve the activity of the mesoporous core-shell molecular sieve-β-zeolite catalyst in the catalytic hydrocracking of polyethylene, the mass ratio of the mesoporous core-shell molecular sieve to β-zeolite is 1:0.8~1.3. The β-zeolite affects the catalytic rate. If the mass of β-zeolite is less than the specified 0.8, the catalytic efficiency is too slow; if the mass of β-zeolite is greater than the specified 1.3, the catalytic rate is too fast, and the generated olefin intermediates cannot undergo the next hydrogenation reaction in time, continuing to crack on the β-zeolite to produce small molecule gases.

[0038] To further obtain a molecular sieve with a stepped pore structure, the volume concentration of tetraethyl orthosilicate in the pore-expanding agent solution is 18%~23%, and the volume ratio of the platinum-supported silicon-titanium molecular sieve seed solution to the tetraethoxysilane pore-expanding agent solution is 5:20~24.

[0039] It should be noted that the pore-expanding agent used in this invention is 1-octadecene, decachloronaphthalene, or cyclohexane; the pore size of the mesoporous SiO2 shell is 2 nm to 20 nm. The silicon source is tetraethyl orthosilicate, the titanium source is tetrabutyl titanate, and the Pt... 4+ The source is H2PtCl6.

[0040] To further improve the catalytic activity of the catalyst, the Pt 4+ The concentration of the source is 50mM~150mM, and the loading of Pt in the platinum-supported silicon-titanium molecular sieve seed crystal is 1.0wt%~2.0wt%.

[0041] The technical effects of the present invention will be described in detail below through specific embodiments and comparative examples.

[0042] Example 1

[0043] A method for preparing a mesoporous core-shell molecular sieve tandem β-zeolite catalyst includes the following steps:

[0044] S1, 40.0 g tetraethyl orthosilicate, 2.6 g tetrabutyl titanate, 56.7 g tetrapropylammonium hydroxide and 8.7 g deionized water were mixed and stirred for 1 h in an ice-water bath at 2 °C to ensure complete dissolution. The mixture was then transferred to a water bath at 70 °C and stirred for another 3 h. 48.0 g isopropanol was then added, and stirring was continued for 1 h to obtain the silicon-titanium molecular sieve precursor.

[0045] S2, 0.2 g NaOH, 3.0 g deionized water and 0.2 g (3-trimercaptopropyl)-trimethoxysilane were stirred and hydrolyzed for 20 min, then 9.2 mL of 100 mM H2PtCl6 was added dropwise, and stirring was continued for 20 min to obtain Pt-supported silane complex;

[0046] S3, the Pt-supported silane complex was slowly added dropwise to the silicon-titanium molecular sieve precursor and stirred in a water bath at 70°C for 30 min; then transferred to a crystallization vessel and crystallized at 170°C for 24 h. After the reaction was completed, the obtained sample was washed five times by centrifugation with deionized water and then dispersed in 45 mL of deionized water to obtain the platinum-supported silicon-titanium molecular sieve seed solution, denoted as Pt@TS-1.

[0047] S4, weigh 24 mL of hexadecyltrimethylammonium chloride solution, 0.18 g of triethanolamine, and 20 mL of deionized water, and add them to a 100 mL round-bottom flask. Stir slowly in a 60 °C oil bath for 30 min. Add 5 mL of the platinum-supported silicon-titanium molecular sieve seed solution prepared above. Before adding the seed solution, ultrasonically disperse the seed solution for 1 h to prevent agglomeration. It is best to take the seed solution while stirring to ensure uniformity. The total seed addition time should be controlled to 10 min. After adding the seed solution, sonicate for 15 min and continue stirring for 60 min. Slowly introduce 22 mL of tetraethyl orthosilicate using a glass rod. - An octadecene solution was prepared, with the bottom of a glass rod flush with the liquid level in the flask. The entire process was controlled within 45 seconds, and the reaction was continued for 12 hours at 60°C and 150 rpm in an oil bath. After the reaction was completed, the upper oil phase was aspirated with a dropper, and the resulting aqueous product was washed twice with deionized water and twice with ethanol. The product was then calcined in a muffle furnace at 550°C for 4 hours to obtain a mesoporous core-shell molecular sieve, denoted as Pt@TS-1 / DMSN-S. 0.1 g of Pt@TS-1 / DMSN-S was mixed with 0.1 g of β-zeolite to obtain a mesoporous core-shell molecular sieve tandem with β-zeolite catalyst.

[0048] Example 2

[0049] A method for preparing a mesoporous core-shell molecular sieve tandem β-zeolite catalyst includes the following steps:

[0050] S1, 40.0 g tetraethyl orthosilicate, 2.6 g tetrabutyl titanate, 56.7 g tetrapropylammonium hydroxide and 8.7 g deionized water were mixed and stirred for 1 h in an ice-water bath at 2 °C to ensure complete dissolution. The mixture was then transferred to a water bath at 70 °C and stirred for another 3 h. 48.0 g isopropanol was then added, and stirring was continued for 1 h to obtain the silicon-titanium molecular sieve precursor.

[0051] S2, 0.2 g NaOH, 3.0 g deionized water and 0.2 g (3-trimercaptopropyl)-trimethoxysilane were stirred and hydrolyzed for 20 min, then 9.2 mL of 100 mM H2PtCl6 was added dropwise, and stirring was continued for 20 min to obtain Pt-supported silane complex;

[0052] S3, the Pt-supported silane complex was slowly added dropwise to the silicon-titanium molecular sieve precursor and stirred in a water bath at 70°C for 30 min; then transferred to a crystallization vessel and crystallized at 170°C for 24 h. After the reaction was completed, the obtained sample was washed five times by centrifugation with deionized water and then dispersed in 45 mL of deionized water to obtain the platinum-supported silicon-titanium molecular sieve seed solution, denoted as Pt@TS-1.

[0053] S4, weigh out 24 mL of hexadecyltrimethylammonium chloride solution and 0.18 g of... Triethanolamine and 20 mL of deionized water were added to a 100 mL round-bottom flask. The mixture was slowly stirred for 30 min in an oil bath at 60 °C. 5 mL of the previously prepared platinum-supported silicon-titanium molecular sieve seed solution was then added dropwise. Before adding the seed solution, it was ultrasonically dispersed for 1 h to prevent agglomeration. It was best to take the seed solution while stirring to ensure uniformity. The total dropwise time was controlled to 10 min. After the dropwise addition, the mixture was ultrasonicated for 15 min and stirred for another 60 min. 22 mL of a tetraethyl orthosilicate solution of decahydronaphthalene was slowly introduced using a glass rod, with the bottom of the rod flush with the liquid surface in the flask. The entire process was controlled to take 45 s. The reaction was continued for 12 h in an oil bath at 60 °C and 150 rpm. After the reaction, the upper oil phase was aspirated with a dropper. The resulting aqueous product was washed twice with deionized water and twice with ethanol, and then calcined in a muffle furnace at 550 °C for 4 hours. h, a mesoporous core-shell molecular sieve, denoted as Pt@TS-1 / DMSN-M, was obtained. 0.1g of Pt@TS-1 / DMSN-M was mixed with 0.1g of β-zeolite to obtain a mesoporous core-shell molecular sieve tandem with β-zeolite catalyst.

[0054] Example 3

[0055] A method for preparing a mesoporous core-shell molecular sieve tandem β-zeolite catalyst includes the following steps:

[0056] S1, 40.0 g tetraethyl orthosilicate, 2.6 g tetrabutyl titanate, 56.7 g tetrapropylammonium hydroxide and 8.7 g deionized water were mixed and stirred for 1 h in an ice-water bath at 2 °C to ensure complete dissolution. The mixture was then transferred to a water bath at 70 °C and stirred for another 3 h. 48.0 g isopropanol was then added, and stirring was continued for 1 h to obtain the silicon-titanium molecular sieve precursor.

[0057] S2, 0.2 g NaOH, 3.0 g deionized water and 0.2 g (3-trimercaptopropyl)-trimethoxysilane were stirred and hydrolyzed for 20 min, then 9.2 mL of 100 mM H2PtCl6 was added dropwise, and stirring was continued for 20 min to obtain Pt-supported silane complex;

[0058] S3, the Pt-supported silane complex was slowly added dropwise to the silicon-titanium molecular sieve precursor and stirred in a water bath at 70°C for 30 min; then transferred to a crystallization vessel and crystallized at 170°C for 24 h. After the reaction was completed, the resulting sample was washed five times by centrifugation with deionized water and then dispersed in 45 mL of deionized water to obtain the platinum-supported silicon-titanium molecular sieve seed solution, denoted as Pt@TS-1.

[0059] S4, weigh out 24 mL of hexadecyltrimethylammonium chloride solution and 0.18 g of... Triethanolamine and 20 mL of deionized water were added to a 100 mL round-bottom flask. The mixture was slowly stirred for 30 min in an oil bath at 60 °C. 5 mL of the previously prepared platinum-supported silicon-titanium molecular sieve seed solution was then added dropwise. Before adding the seed solution, it was ultrasonically dispersed for 1 h to prevent agglomeration. It was best to take the seed solution while stirring to ensure uniformity. The total dropwise time was controlled to 10 min. After the dropwise addition, the mixture was ultrasonicated for 15 min and stirred for another 60 min. 22 mL of a tetraethyl orthosilicate cyclohexane solution was slowly introduced using a glass rod, with the bottom of the rod level with the liquid surface in the flask. The entire process was controlled to take 45 s. The reaction was continued for 12 h in an oil bath at 60 °C and 150 rpm. After the reaction, the upper oil phase was aspirated with a dropper. The resulting aqueous product was washed twice with deionized water and twice with ethanol, and then calcined in a muffle furnace at 550 °C for 4 hours. h, a mesoporous core-shell molecular sieve, denoted as Pt@TS-1 / DMSN-L, was obtained. 0.1g of Pt@TS-1 / DMSN-L was mixed with 0.1g of β-zeolite to obtain a mesoporous core-shell molecular sieve tandem with β-zeolite catalyst.

[0060] To further illustrate the technical effects of the present invention, a comparative example is also provided, as follows:

[0061] Comparative Example 1

[0062] A method for preparing a platinum-supported silicon-titanium molecular sieve seed solution includes the following steps:

[0063] S1, 40.0 g tetraethyl orthosilicate, 2.6 g tetrabutyl titanate, 56.7 g tetrapropylammonium hydroxide and 8.7 g deionized water were mixed and stirred for 1 h in an ice-water bath at 2 °C to ensure complete dissolution. The mixture was then transferred to a water bath at 70 °C and stirred for another 3 h. 48.0 g isopropanol was then added, and stirring was continued for 1 h to obtain the silicon-titanium molecular sieve precursor.

[0064] S2, 0.2 g NaOH, 3.0 g deionized water and 0.2 g (3-trimercaptopropyl)-trimethoxysilane were stirred and hydrolyzed for 20 min, then 9.2 mL of 100 mM H2PtCl6 was added dropwise, and stirring was continued for 20 min to obtain Pt-supported silane complex;

[0065] S3, the Pt-supported silane complex was slowly added dropwise to the silicon-titanium molecular sieve precursor and stirred in a water bath at 70°C for 30 min; then transferred to a crystallization vessel and crystallized at 170°C for 24 h. After the reaction was completed, the resulting sample was washed five times by centrifugation with deionized water and then dispersed in 45 mL of deionized water to obtain the platinum-supported silicon-titanium molecular sieve seed solution, denoted as Pt@TS-1.

[0066] Comparative Example 2

[0067] A method for preparing a mesoporous core-shell molecular sieve tandem β-zeolite catalyst includes the following steps:

[0068] S1, 40.0 g tetraethyl orthosilicate, 2.6 g tetrabutyl titanate, 56.7 g tetrapropylammonium hydroxide and 8.7 g deionized water were mixed and stirred for 1 h in an ice-water bath at 2 °C to ensure complete dissolution. The mixture was then transferred to a water bath at 70 °C and stirred for another 3 h. 48.0 g isopropanol was then added, and stirring was continued for 1 h to obtain the silicon-titanium molecular sieve precursor.

[0069] S2, 0.2 g NaOH, 3.0 g deionized water and 0.2 g (3-trimercaptopropyl)-trimethoxysilane were stirred and hydrolyzed for 20 min, then 9.2 mL of 100 mM H2PtCl6 was added dropwise, and stirring was continued for 20 min to obtain Pt-supported silane complex;

[0070] S3, the Pt-supported silane complex was slowly added dropwise to the silicon-titanium molecular sieve precursor and stirred in a water bath at 70°C for 30 min; then transferred to a crystallization vessel and crystallized at 170°C for 24 h. After the reaction was completed, the obtained sample was washed five times by centrifugation with deionized water and then dispersed in 45 mL of deionized water to obtain the platinum-supported silicon-titanium molecular sieve seed solution, denoted as Pt@TS-1.

[0071] S4, weigh 24 mL of hexadecyltrimethylammonium chloride solution, 0.18 g of triethanolamine, and 20 mL of deionized water, and add them to a 100 mL round-bottom flask. Stir slowly for 30 min in an oil bath at 60 °C. Add 5 mL of the platinum-supported silicon-titanium molecular sieve seed solution prepared above. Before adding the seed solution, ultrasonically disperse the solution for 1 h to prevent agglomeration. It is best to take the seed solution while stirring to ensure uniformity. The total seed addition time should be controlled to 10 min. After addition, ultrasonicate for 15 min and continue stirring for 60 min. Slowly introduce 22 mL of tetraethyl orthosilicate decahydronaphthalene solution using a glass rod, ensuring the bottom of the glass rod is level with the liquid surface in the flask. The entire process should be controlled within 45 s. Continue the reaction for 12 h in an oil bath at 60 °C and 150 rpm. After the reaction, separate the upper oil phase from the aqueous phase. Continue to slowly introduce 22 mL of tetraethyl orthosilicate decahydronaphthalene solution into the aqueous phase using a glass rod. The decahydronaphthalene solution of mLTEOS was reacted for 12 h at 60 °C and 150 rpm in an oil bath. After the reaction was completed, the upper oil phase was aspirated with a dropper. The resulting aqueous product was washed twice with deionized water and twice with ethanol. It was then calcined in a muffle furnace at 550 °C for 4 h to obtain a mesoporous core-shell molecular sieve, denoted as Pt@TS-1 / DMSN-2. 0.1 g of Pt@TS-1 / DMSN-2 was mixed with 0.1 g of β-zeolite to obtain a mesoporous core-shell molecular sieve tandem with β-zeolite catalyst.

[0072] Comparative Example 3

[0073] A method for preparing a mesoporous core-shell molecular sieve tandem β-zeolite catalyst includes the following steps:

[0074] S1, 40.0 g tetraethyl orthosilicate, 2.6 g tetrabutyl titanate, 56.7 g tetrapropylammonium hydroxide and 8.7 g deionized water were mixed and stirred for 1 h in an ice-water bath at 2 °C to ensure complete dissolution. The mixture was then transferred to a water bath at 70 °C and stirred for another 3 h. 48.0 g isopropanol was then added, and stirring was continued for 1 h to obtain the silicon-titanium molecular sieve precursor.

[0075] S2, 0.2 g NaOH, 3.0 g deionized water and 0.2 g (3-trimercaptopropyl)-trimethoxysilane were stirred and hydrolyzed for 20 min, then 9.2 mL of 100 mM H2PtCl6 was added dropwise, and stirring was continued for 20 min to obtain Pt-supported silane complex;

[0076] S3, the Pt-supported silane complex was slowly added dropwise to the silicon-titanium molecular sieve precursor and stirred in a water bath at 70°C for 30 min; then transferred to a crystallization vessel and crystallized at 170°C for 24 h. After the reaction was completed, the obtained sample was washed five times by centrifugation with deionized water and then dispersed in 45 mL of deionized water to obtain the platinum-supported silicon-titanium molecular sieve seed solution, denoted as Pt@TS-1.

[0077] S4, weigh 24 mL of hexadecyltrimethylammonium chloride solution, 0.18 g of triethanolamine, and 20 mL of deionized water, and add them to a 100 mL round-bottom flask. Stir slowly for 30 min in an oil bath at 60 °C. Add 5 mL of the platinum-supported silicon-titanium molecular sieve seed solution prepared above. Before adding the seed solution, ultrasonically disperse the solution for 1 h to prevent agglomeration. It is best to take the seed solution while stirring to ensure uniformity. The total seed addition time should be controlled to 10 min. After addition, ultrasonicate for 15 min and continue stirring for 60 min. Slowly introduce 22 mL of tetraethyl orthosilicate decahydronaphthalene solution using a glass rod, ensuring the bottom of the glass rod is level with the liquid surface in the flask. The entire process should be controlled within 45 s. Continue the reaction for 12 h in an oil bath at 60 °C and 150 rpm. After the reaction, separate the upper oil phase from the aqueous phase. Continue to slowly introduce 22 mL of tetraethyl orthosilicate decahydronaphthalene solution into the aqueous phase using a glass rod. A solution of decahydronaphthalene from mLTEOS was prepared, and the reaction was continued for 12 h at 60 °C and 150 rpm in an oil bath. After the reaction was completed, the oil phase and the aqueous phase were separated. 22 mL of the decahydronaphthalene solution from TEOS was slowly introduced into the aqueous phase using a glass rod, and the reaction was continued for 12 h at 60 °C and 150 rpm in an oil bath. The upper oil phase was aspirated with a dropper, and the resulting aqueous product was washed twice with deionized water and twice with ethanol. The product was then calcined in a muffle furnace at 550 °C for 4 h to obtain a mesoporous core-shell molecular sieve, denoted as Pt@TS-1 / DMSN-3. 0.1 g of Pt@TS-1 / DMSN-3 was mixed with 0.1 g of β-zeolite to obtain a mesoporous core-shell molecular sieve tandem with β-zeolite catalyst.

[0078] The mesoporous core-shell molecular sieve tandem β-zeolite catalysts prepared in Examples 1-3, the Pt@TS-1 tandem β-zeolite catalyst in Comparative Example 1, and the mesoporous core-shell molecular sieve tandem β-zeolite catalysts prepared in Comparative Examples 2-3 were used in the catalytic hydrocracking of polyethylene. Specifically, a 25 mL stainless steel high-pressure reactor equipped with a magnetic stirrer was used. 0.2 g of the mesoporous core-shell molecular sieve tandem β-zeolite catalyst and 1 g of low-density polyethylene (LDPE) were added to the reactor for physical mixing. The hydrogenation reactor was cleaned three times with H2 (99.9%), then the H2 was pressurized to 1 MPa and rapidly heated to 240 °C and held at that temperature for 1 h. After the reaction was completed, the reactor temperature was allowed to drop to room temperature, and then the reactor was placed in a -20 °C cryogenic constant-temperature reaction bath for condensation. The gas in the reactor was collected using a gas collection bag. The liquid phase product in the reactor was extracted using a mixture of dichloromethane and n-hexadecane (external standard). The remaining solid portion was dried and weighed to calculate the conversion rate. Gaseous products were analyzed using gas chromatography-flame ionization detector (GC-FID) with an SH-Alumina BOND / Na2SO4 column. Liquid products were analyzed using gas chromatography-flame ionization detector (GC-FID) with an MTX-1 column.

[0079] The performance of the mesoporous core-shell molecular sieves and mesoporous core-shell molecular sieve tandem β-zeolite catalysts provided in Examples 1 to 3, the Pt@TS-1 tandem β-zeolite catalyst in Comparative Example 1, and the mesoporous core-shell molecular sieve tandem β-zeolite catalysts prepared in Comparative Examples 2 to 3 were tested, and the results are as follows.

[0080] Figure 1 This is a TEM image of the mesoporous core-shell molecular sieve of Example 1 of the present invention. Figure 1 It can be clearly observed that the TS-1 crystal nuclei are all uniformly wrapped in a silicon dioxide shell.

[0081] Figure 2 The images show the XRD patterns of the mesoporous core-shell molecular sieves of Examples 1 to 3 of this invention and the Pt@TS-1 prepared in Comparative Example 1. Figure 3 The images show the XRD patterns of the mesoporous core-shell molecular sieves of Examples 2, 2, and 3 of this invention, and the Pt@TS-1 prepared in Comparative Example 1. Figure 2 and Figure 3XRD analysis was used to study the crystal structures of different mesoporous core-shell molecular sieve samples. The results showed that the prepared mesoporous core-shell molecular sieves exhibited characteristic diffraction peaks belonging to TS-1 microporous molecular sieves in the ranges of 2θ = 7.8°~8.7° and 22°~25°, corresponding to the (011), (020), (051), (303), (313), and (532) crystal planes, respectively. Notably, the diffraction peak intensity of TS-1 / MSN decreased after being encapsulated in a silica shell, indicating a decrease in the overall crystallinity of the sample. Furthermore, no Pt diffraction peaks were observed in any of the mesoporous core-shell molecular sieve samples, indicating that the Pt particles were highly uniformly dispersed.

[0082] Figure 4 This invention describes the hydrocracking reaction of low-density polyethylene (LDPE) by mixing mesoporous core-shell molecular sieves with different pore sizes and shell thicknesses with β-zeolite under reaction conditions of 240℃ and 1MPa H2 pressure. After 1 h of reaction, the activity and product distribution characteristics of each catalyst were systematically analyzed. The experimental results show that the performance of mesoporous core-shell molecular sieves with different pore sizes is significantly better than that of the Pt@TS-1 catalyst in Comparative Example 1. Among them, Pt@TS-1 / DMSN-M exhibits the best catalytic performance, with an LDPE conversion rate as high as 94.7% and a liquid product yield of 65.9%.

[0083] Figure 5 The different shell thicknesses of the catalysts in this invention have a significant impact on their catalytic performance. When the catalyst has only a single-layer shell structure (Examples 1-3), its catalytic activity is significantly better than that of the traditional Pt@TS-1 catalyst (Comparative Example 1), indicating that a suitable shell structure can effectively improve catalytic performance. However, when the shell thickness is increased to two layers, i.e., the catalyst prepared in Comparative Example 2, its performance tends to be similar to that of the Pt@TS-1 catalyst, indicating that the increase in shell thickness may inhibit the accessibility of active sites to some extent. Notably, when the shell thickness is further increased to three layers, i.e., the catalyst prepared in Comparative Example 3, its activity decreases significantly. This may be because the excessively thick shell hinders the mass transfer process between the reactants and active sites, leading to a decrease in catalytic efficiency. This result indicates that the shell thickness of the catalyst needs to be optimized, and an excessively thick shell is not conducive to improving catalytic performance.

[0084] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for preparing a mesoporous core-shell molecular sieve tandem β-zeolite catalyst, characterized in that, Includes the following steps: At 1℃~4℃, silicon and titanium sources are used as raw materials, and tetrapropylammonium hydroxide is used as a structure directing agent. The materials are dissolved in water and undergo hydrolysis and condensation reaction at 60℃~80℃. The silicon and titanium sources are hydrolyzed to generate silanol and titanium hydroxyl groups, which are then condensed to form a SiO2-TiO2 network. Isopropanol is then added to obtain the silicon-titanium molecular sieve precursor. (3-Trimercaptopropyl)-trimethoxysilane was hydrolyzed under alkaline conditions to obtain mercapto and silanol groups, and then Pt was added. 4+ The source, the thiol group will Pt 4+ The Pt element was reduced to Pt and anchored to the silane framework via silanol groups to obtain a Pt-supported silane complex. The Pt-supported silane complex was added to the silicon-titanium molecular sieve precursor and mixed thoroughly, then crystallized at 160℃~180℃ to obtain a platinum-supported silicon-titanium molecular sieve seed solution; the Pt 4+ The source concentration is 50 mM to 150 mM, and the Pt loading in the platinum-supported silicon-titanium molecular sieve seed crystals is 1.0 wt% to 2.0 wt%. Using hexadecyltrimethylammonium chloride and triethanolamine as templates, a pore-expanding agent solution of tetraethyl orthosilicate was introduced into the seed solution of the platinum-supported silicon-titanium molecular sieve. Interfacial self-assembly was carried out at 55℃~65℃ to form a mesoporous SiO2 shell layer. Calcination yielded a mesoporous core-shell molecular sieve, which was then mixed with β-zeolite to obtain a mesoporous core-shell molecular sieve tandem with β-zeolite catalyst. The mesoporous SiO2 shell layer was a single-layer shell structure. The mass ratio of the mesoporous core-shell molecular sieve to β-zeolite was 1:0.8~1.

3. In the tetraethyl orthosilicate pore-expanding agent solution, the volume concentration of tetraethyl orthosilicate is 18%~23%, and the volume ratio of the platinum-supported silicon-titanium molecular sieve seed solution to the tetraethoxysilane pore-expanding agent solution is 5:20~24; the pore-expanding agent is 1-octadecene, decachloronaphthalene, or cyclohexane; the pore size of the mesoporous SiO2 shell is 2nm~20nm.

2. The preparation method according to claim 1, characterized in that, The molar ratio of the silicon source, titanium source and tetrapropylammonium hydroxide is 1:0.02~0.05:0.2~0.

4.

3. The preparation method according to claim 1, characterized in that, The calcination is carried out at 500℃~550℃ for 4h~5h.

4. The preparation method according to claim 1, characterized in that, The silicon source is tetraethyl orthosilicate, the titanium source is tetrabutyl titanate, and the Pt... 4+ The source is H2PtCl6.

5. A mesoporous core-shell molecular sieve tandem β-zeolite catalyst, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 4.

6. The application of the mesoporous core-shell molecular sieve tandem β-zeolite catalyst according to claim 5 in the catalytic hydrocracking of polyethylene.

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