A method for preparing hierarchical pore MFI molecular sieves based on metal species modulation

Abstract

The application relates to a method for preparing hierarchical-pore MFI molecular sieves based on metal species regulation, and belongs to the technical field of molecular sieve material preparation. The method mixes a silicon source, an aluminum source, an organic structure directing agent, a metal precursor and water to form a crystallization precursor gel, and controls the nucleation, primary crystal growth and secondary aggregation behavior of the MFI molecular sieves by regulating the metal type, metal addition amount and gel composition. After hydrothermal crystallization, separation, washing, drying and calcination, hierarchical-pore molecular sieves formed by MFI primary nanocrystal aggregation and simultaneously having MFI micropores and intercrystalline mesopores are obtained. The metal precursor contains at least one of Sn, Zn, Cu and Mo.

Description

Technical Field

[0001] This invention relates to the field of molecular sieve material preparation technology, specifically to a method for preparing hierarchical porous MFI molecular sieves by regulating metal species, and particularly to the preparation of hierarchical porous MFI molecular sieves with a structure in which micropores and intergranular mesopores coexist by introducing metal precursors into the hydrothermal synthesis system of MFI molecular sieves to regulate the nucleation, grain growth and secondary aggregation behavior of molecular sieves. Background Technology

[0002] ZSM-5 is an important silica-alumina molecular sieve with an MFI framework structure. Mobil Corporation of the United States disclosed ZSM-5 molecular sieve and its preparation method in US3702886. This molecular sieve possesses regular ten-membered ring intersecting channels, a high specific surface area, tunable acidity, and good thermal and hydrothermal stability, making it widely applicable in petroleum refining, fine chemicals, adsorption separation, and environmental remediation. Due to the uniform microporous channels of ZSM-5 molecular sieve, it can exert shape-selective effects on reactants, intermediates, and products, exhibiting excellent catalytic performance in reactions such as catalytic cracking, isomerization, and aromatization.

[0003] However, traditional ZSM-5 molecular sieves are primarily composed of micropores, with pore sizes typically less than 2 nm. The small pore size and long diffusion paths can restrict the transport of macromolecular reactants, intermediates, and products. For long-chain hydrocarbons, polymers, and other larger reactants, molecules struggle to fully penetrate the micropores and contact the internal active sites, resulting in some active sites remaining unutilized. Restricted diffusion of reaction intermediates and products within the micropores also prolongs their residence time, easily leading to over-reaction, carbon deposition, and pore blockage, thereby reducing catalytic efficiency, target product selectivity, and catalyst stability. Therefore, micropore diffusion limitation is a significant factor restricting the further application of ZSM-5 molecular sieves in macromolecular catalytic reactions.

[0004] To overcome the mass transfer limitations imposed by single micropores, introducing mesopores or macropores into ZSM-5 molecular sieves to construct a hierarchical pore structure where micropores and secondary channels coexist is an important approach to improving its diffusion performance. Hierarchical pore structures can shorten molecular diffusion paths, increase external specific surface area and accessibility of active sites, which is beneficial for alleviating mass transfer resistance in macromolecular reactions. In recent years, various methods have been developed for the preparation of hierarchical ZSM-5 molecular sieves, mainly including post-treatment methods such as desilication and dealumination, hard template methods, and soft template or surfactant-assisted synthesis methods.

[0005] Post-treatment methods typically involve first preparing the parent ZSM-5 molecular sieve, then selectively removing framework elements through acid or alkali treatment to form secondary channels. CN115140746A discloses a ZSM-5 molecular sieve with a tiered pore distribution, which, based on the parent molecular sieve, forms a tiered pore structure with micropores, mesopores, and macropores coexisting after alkali solution treatment. While such methods can improve the pore structure to some extent, the treatment process usually requires the use of acid or alkali solutions, increasing the number of process steps and washing burden, and potentially causing problems such as damage to the molecular sieve framework, decreased crystallinity, loss of acidic sites, and discharge of acidic or alkaline wastewater.

[0006] The hard-templating method involves introducing carbon materials, polymers, or other solid templates into the molecular sieve synthesis system, followed by crystallization and then removal of the template through calcination, oxidation, or other methods to form mesopores or macropores. The porous ZSM-5 molecular sieve synthesis method disclosed in CN105399110A involves adding hard templates such as carbon nanotubes to the synthesis system and removing the template through high-temperature treatment to obtain a porous structure. While the hard-templating method can regulate pore structure to some extent, it typically requires pre-treating the template, ensuring uniform dispersion of the template in the gel, and removing the template in subsequent steps. This makes the preparation process relatively complex, and the template removal process increases energy consumption and operating costs.

[0007] Soft template methods or surfactant-assisted synthesis methods typically rely on organic structure-directing agents, quaternary ammonium salts, or long-chain surfactants to induce the formation of nanosheets, nanocrystal aggregates, or other hierarchical porous structures in molecular sieves. The hierarchical porous ZSM-5 molecular sieve synthesis method disclosed in CN103288100A introduces quaternary ammonium salt structure-directing agents, cationic surfactants, and seed gels into the synthesis system. After hydrothermal crystallization and calcination, a ZSM-5 molecular sieve with a layered structure and hierarchical pores is formed. While such methods are helpful in constructing hierarchical porous structures, they require large quantities of organic template agents or surfactants, resulting in high costs. The preparation or use of some template agents is also cumbersome, and subsequent calcination to remove organic components increases energy consumption and generates pollutant emissions.

[0008] In summary, while existing methods for preparing hierarchical ZSM-5 molecular sieves can alleviate micropore diffusion limitations to some extent, they still generally suffer from problems such as complex process steps, strong dependence on template agents or surfactants, large amounts of post-treatment acid and alkali wastewater, high energy consumption, and susceptibility of the framework and acidic sites to interference. How to construct hierarchical structures using more environmentally friendly, simple, and cost-effective methods while maintaining the ZSM-5 molecular sieve MFI framework structure and micropore characteristics, and reducing the use of complex template agents and the emission of acid and alkali wastewater and pollutants, is a key technical problem that urgently needs to be solved in this field. Summary of the Invention

[0009] To address the problems of complex process steps, strong dependence on template agents, large amounts of acid and alkali waste liquid in the preparation of existing hierarchical porous ZSM-5 molecular sieves, and susceptibility of the molecular sieve framework and acidic sites to influence, this invention provides a heteroatom-induced synthesis method for forming hierarchical porous ZSM-5 molecular sieves. This method uses tetrapropylammonium hydroxide as a structure-directing agent to introduce heteroatom sources such as Sn, Cu, Zn, or Mo into the ZSM-5 molecular sieve crystallization system. Through the regulation of nucleation, crystal growth, and nanocrystal assembly processes by heteroatoms, the resulting ZSM-5 molecular sieve forms a hierarchical porous structure with coexisting micropores and secondary channels, exhibiting nanoaggregate characteristics.

[0010] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0011] A synthetic method for forming heteroatom-induced hierarchical porous ZSM-5 molecular sieve includes the following steps:

[0012] A precursor gel was obtained by mixing a silicon source, an aluminum source, tetrapropylammonium hydroxide, a metal heteroatom source, and water. The precursor gel was then aged. The aged precursor gel was subjected to hydrothermal crystallization. The crystallized product was separated into solid and liquid phases, washed, dried at 100 °C for 12 h, and then calcined at 550 °C for 4 h in air to obtain a graded pore ZSM-5 molecular sieve.

[0013] The heteroatom M is selected from one or more of Sn, Cu, Zn, and Mo. During crystallization, the heteroatom regulates the nucleation, growth, and nanocrystal assembly behavior of the ZSM-5 molecular sieve, resulting in a hierarchical pore structure composed of micropores and secondary channels. This method does not require the addition of additional hard or soft templates for forming mesopores or macropores, nor does it require post-treatment with strong acids or bases to etch the parent molecular sieve.

[0014] The silicon source may be selected from one or more of tetraethyl orthosilicate, silica sol, sodium silicate, water glass, silica gel, fumed silica, silica gel, and colloidal silica. The aluminum source may be selected from one or more of sodium aluminate, aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum isopropoxide, aluminum sec-butoxide, aluminum hydroxide, boehmite, and aluminum sol.

[0015] The heteroatom source includes one or more of tin, copper, zinc, and molybdenum. The tin source is selected from one or more of tin chloride, stannous chloride, and tin sulfate; the copper source is selected from one or more of copper nitrate, copper chloride, and copper sulfate; the zinc source is selected from one or more of zinc nitrate, zinc chloride, and zinc sulfate; and the molybdenum source is selected from one or more of ammonium molybdate, sodium molybdate, and molybdenum chloride.

[0016] In a preferred embodiment, the molar ratio of each component in the precursor gel, based on SiO2, Al2O3, M, TPAOH, and H2O, is SiO2:Al2O3:M:TPAOH:H2O=1:(0.005-0.030):(0.003-0.030):(0.10-0.50):(10-60), wherein M is one or more of Sn, Cu, Zn, and Mo, and TPAOH is tetrapropylammonium hydroxide.

[0017] In a preferred embodiment, the aging treatment is performed at a temperature of 20-60 °C for 2-24 h; the hydrothermal crystallization is performed at a temperature of 150-190 °C for 24-96 h.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] First, this invention introduces heteroatom sources such as Sn, Cu, Zn or Mo during the in-situ crystallization process of ZSM-5 molecular sieve. By regulating nucleation, crystal growth and nanocrystal assembly through heteroatoms, the molecular sieve forms a hierarchical pore structure during the crystallization process.

[0020] Secondly, the present invention can form a hierarchical pore structure without acid or alkali etching of the parent molecular sieve, and without the need to add additional hard or soft template agents. The process is more simplified and helps to reduce washing burden, energy consumption and emissions.

[0021] Third, the synthesis process of this invention is relatively simple, the process conditions are clear, and the raw materials are widely available, making it highly feasible.

[0022] Fourth, the hierarchical pore ZSM-5 molecular sieve obtained by this invention has a pore structure in which micropores and secondary channels coexist and exhibits nano-aggregate characteristics, which is beneficial to shorten the molecular diffusion path, improve the accessibility of active sites, and thus improve the mass transfer behavior of macromolecular reactants in the molecular sieve. Attached Figure Description

[0023] Figure 1 The XRD pattern of the graded pore size ZSM-5 molecular sieve obtained in Example 1;

[0024] Figure 2 The N2 adsorption-desorption isotherm of the graded porous ZSM-5 molecular sieve obtained in Example 1 is shown below.

[0025] Figure 3 This is a scanning electron microscope image of the graded pore size ZSM-5 molecular sieve obtained in Example 1;

[0026] Figure 4 The XRD pattern of the graded pore size ZSM-5 molecular sieve obtained in Example 2;

[0027] Figure 5The N2 adsorption-desorption isotherm of the graded porous ZSM-5 molecular sieve obtained in Example 2 is shown below.

[0028] Figure 6 This is a scanning electron microscope image of the graded pore size ZSM-5 molecular sieve obtained in Example 2;

[0029] Figure 7 The XRD pattern of the graded pore size ZSM-5 molecular sieve obtained in Example 3;

[0030] Figure 8 The N2 adsorption-desorption isotherm of the graded porous ZSM-5 molecular sieve obtained in Example 3 is shown below.

[0031] Figure 9 This is a scanning electron microscope image of the graded pore size ZSM-5 molecular sieve obtained in Example 3;

[0032] Figure 10 The XRD pattern of the graded pore size ZSM-5 molecular sieve obtained in Example 4;

[0033] Figure 11 The N2 adsorption-desorption isotherm of the graded porous ZSM-5 molecular sieve obtained in Example 4 is shown below.

[0034] Figure 12 This is a scanning electron microscope image of the graded pore size ZSM-5 molecular sieve obtained in Example 4;

[0035] Figure 13 The XRD pattern of the graded pore size ZSM-5 molecular sieve obtained in Example 5;

[0036] Figure 14 The N2 adsorption-desorption isotherm of the graded porous ZSM-5 molecular sieve obtained in Example 5 is shown below.

[0037] Figure 15 This is a scanning electron microscope image of the graded pore size ZSM-5 molecular sieve obtained in Example 5;

[0038] Figure 16 The XRD pattern of the graded pore size ZSM-5 molecular sieve obtained in Example 6;

[0039] Figure 17 The N2 adsorption-desorption isotherm of the graded porous ZSM-5 molecular sieve obtained in Example 6 is shown below.

[0040] Figure 18 This is a scanning electron microscope image of the graded pore size ZSM-5 molecular sieve obtained in Example 6;

[0041] Figure 19 The XRD pattern of the graded pore size ZSM-5 molecular sieve obtained in Example 7;

[0042] Figure 20The N2 adsorption-desorption isotherm of the graded porous ZSM-5 molecular sieve obtained in Example 7 is shown below.

[0043] Figure 21 This is a scanning electron microscope image of the graded pore size ZSM-5 molecular sieve obtained in Example 7;

[0044] Figure 22 The N2 adsorption-desorption isotherm of the graded porous ZSM-5 molecular sieve obtained in Example 8;

[0045] Figure 23 This is a scanning electron microscope image of the graded pore size ZSM-5 molecular sieve obtained in Example 8;

[0046] Figure 24 The XRD pattern of the ZSM-5 molecular sieve obtained in Comparative Example 1 is shown.

[0047] Figure 25 The N2 adsorption-desorption isotherm of the ZSM-5 molecular sieve obtained in Comparative Example 1 is shown below.

[0048] Figure 26 The image shows a scanning electron microscope (SEM) image of the ZSM-5 molecular sieve obtained in Comparative Example 1. Detailed Implementation

[0049] The present invention will be further described below with reference to specific embodiments. However, the following embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent substitutions or conventional adjustments made based on the technical concept of the present invention should be included within the scope of protection of the present invention.

[0050] This invention provides a synthetic method for forming hierarchical porous ZSM-5 molecular sieves induced by heteroatoms. The method uses silicon source, aluminum source, tetrapropylammonium hydroxide, heteroatom source, and water as raw materials, and involves mixing, aging, hydrothermal crystallization, solid-liquid separation, washing, drying, and calcination to obtain hierarchical porous ZSM-5 molecular sieves. The heteroatom M in the heteroatom source is selected from one or more of Sn, Cu, Zn, and Mo.

[0051] In a specific embodiment of the present invention, the silicon source may be selected from one or more of tetraethyl orthosilicate, silica sol, sodium silicate, water glass, silica gel, fumed silica, silica gel, and colloidal silica; the aluminum source may be selected from one or more of sodium aluminate, aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum isopropoxide, aluminum sec-butoxide, aluminum hydroxide, boehmite, and aluminum sol. The tetrapropylammonium hydroxide serves as a structure directing agent in the formation process of ZSM-5 molecular sieves.

[0052] In a specific embodiment of the present invention, the heteroatom source includes one or more of a tin source, a copper source, a zinc source, and a molybdenum source. The tin source may be selected from one or more of tin chloride, stannous chloride, and tin sulfate; the copper source may be selected from one or more of copper nitrate, copper chloride, and copper sulfate; the zinc source may be selected from one or more of zinc nitrate, zinc chloride, and zinc sulfate; and the molybdenum source may be selected from one or more of ammonium molybdate, sodium molybdate, and molybdenum chloride. The above salts may be anhydrous or hydrated.

[0053] In a specific embodiment of the present invention, the molar ratio of each component in the precursor gel, based on SiO2, Al2O3, M, TPAOH, and H2O, can be SiO2:Al2O3:M:TPAOH:H2O=1:(0.005-0.030):(0.003-0.030):(0.10-0.50):(10-60), where M is one or more of Sn, Cu, Zn, and Mo, and TPAOH is tetrapropylammonium hydroxide.

[0054] In a specific embodiment of the present invention, the aging treatment can be carried out at 20-60 °C for 2-24 h; the hydrothermal crystallization can be carried out at 150-190 °C for 24-96 h. After solid-liquid separation and washing, the crystallized product is dried at 100 °C for 12 h, and then calcined at 550 °C for 4 h in air atmosphere to obtain graded pore ZSM-5 molecular sieve.

[0055] This invention introduces heteroatom sources during the crystallization process of ZSM-5 molecular sieves, which can regulate the nucleation, crystal growth, and nanocrystal assembly processes of silicon and aluminum species, resulting in a hierarchical pore structure in which micropores and secondary channels coexist and exhibit nanoaggregate characteristics.

[0056] The technical solution of the present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0057] Example 1

[0058] Weigh 28.8 g of a 25% (w / w) tetrapropylammonium hydroxide aqueous solution and 23.1 g of tetraethyl orthosilicate. Mix the two solutions and stir overnight at room temperature until the system becomes clear, obtaining a silicon source solution. Add 69 g of deionized water, 0.42 g of Al(NO3)3·9H2O, and 0.39 g of SnCl4·5H2O to the silicon source solution and continue stirring for 30 h to obtain a precursor gel. The molar ratio of the components in the precursor gel is: SiO2:Al:Sn:TPAOH:H2O = 1:0.01:0.01:0.32:45.4. Transfer the obtained precursor gel to a stainless steel reactor with a polytetrafluoroethylene liner and hydrothermally crystallize at 150 °C for 2 days. After crystallization, the crystallized product was separated into solid and liquid phases and washed with deionized water until neutral. It was then dried at 100 °C for 12 h and calcined at 550 °C for 4 h in air to obtain Sn-induced hierarchical porous ZSM-5 molecular sieve.

[0059] Example 2

[0060] The difference between this embodiment and Example 1 is that the amount of SnCl4·5H2O added is 0.31 g, corresponding to a Si / Sn molar ratio of 125. The amounts of other raw materials, mixing methods, stirring times, hydrothermal crystallization conditions, drying conditions, and calcination conditions are all the same as in Example 1, resulting in Sn-induced hierarchical porous ZSM-5 molecular sieve.

[0061] Example 3

[0062] The difference between this embodiment and Example 1 is that the amount of SnCl4·5H2O added is 0.52 g, corresponding to a Si / Sn molar ratio of 75. The amounts of other raw materials, mixing methods, stirring times, hydrothermal crystallization conditions, drying conditions, and calcination conditions are all the same as in Example 1, resulting in Sn-induced hierarchical porous ZSM-5 molecular sieve.

[0063] Example 4

[0064] The difference between this embodiment and Example 1 is that the amount of SnCl4·5H2O added is 0.78 g, corresponding to a Si / Sn molar ratio of 50. The amounts of other raw materials, mixing methods, stirring times, hydrothermal crystallization conditions, drying conditions, and calcination conditions are all the same as in Example 1, resulting in Sn-induced hierarchical porous ZSM-5 molecular sieve.

[0065] Example 5

[0066] Weigh 28.8 g of a 25% (w / w) tetrapropylammonium hydroxide aqueous solution and 23.1 g of tetraethyl orthosilicate. Mix the two solutions and stir overnight at room temperature until the system becomes clear, obtaining a silicon source solution. Add 69.0 g of deionized water, 0.42 g of Al(NO3)3·9H2O, and 0.27 g of Cu(NO3)2·3H2O to the silicon source solution and continue stirring for 30 h to obtain a precursor gel. The molar ratio of the components in the precursor gel is: SiO2:Al:Cu:TPAOH:H2O = 1:0.01:0.01:0.32:45.4. Transfer the obtained precursor gel to a stainless steel reactor with a polytetrafluoroethylene liner and hydrothermally crystallize at 150 °C for 4 days. After crystallization, the crystallized product was separated into solid and liquid phases and washed with deionized water until neutral. It was then dried at 100 °C for 12 h and calcined at 550 °C for 4 h in air to obtain Cu-induced hierarchical porous ZSM-5 molecular sieve.

[0067] Example 6

[0068] Weigh 22.6 g of a 25% (w / w) tetrapropylammonium hydroxide aqueous solution and 23.1 g of tetraethyl orthosilicate. Mix the two solutions and stir overnight at room temperature until the system becomes clear, obtaining a silicon source solution. Add 52.6 g of deionized water, 0.62 g of Al(NO3)3·9H2O, and 0.26 g of Zn(NO3)2·6H2O to the silicon source solution and continue stirring for 24 h to obtain a precursor gel. The molar ratio of the components in the precursor gel is: SiO2:Al:Zn:TPAOH:H2O = 1:0.015:0.008:0.25:35. Transfer the obtained precursor gel to a stainless steel reactor with a polytetrafluoroethylene liner and hydrothermally crystallize at 160 °C for 3 days. After crystallization, the crystallized product was separated into solid and liquid phases and washed with deionized water until neutral. It was then dried at 100 °C for 12 h and calcined at 550 °C for 4 h in air to obtain Zn-induced hierarchical porous ZSM-5 molecular sieve.

[0069] Example 7

[0070] Weigh 36.1 g of a 25% (w / w) tetrapropylammonium hydroxide aqueous solution and 23.1 g of tetraethyl orthosilicate. Mix the two solutions and stir overnight at room temperature until the system becomes clear, obtaining a silicon source solution. Add 82.4 g of deionized water, 0.33 g of Al(NO3)3·9H2O, and 0.58 g of SnCl4·5H2O to the silicon source solution and continue stirring for 36 h to obtain a precursor gel. The molar ratio of the components in the precursor gel is: SiO2:Al:Sn:TPAOH:H2O = 1:0.008:0.015:0.40:55. Transfer the obtained precursor gel to a stainless steel reactor with a polytetrafluoroethylene liner and hydrothermally crystallize at 170 °C for 3 days. After crystallization, the crystallized product was separated into solid and liquid phases and washed with deionized water until neutral. It was then dried at 100 °C for 12 h and calcined at 550 °C for 4 h in air to obtain Sn-induced hierarchical porous ZSM-5 molecular sieve.

[0071] Example 8

[0072] Weigh 28.8 g of a 25% (w / w) tetrapropylammonium hydroxide aqueous solution and 23.1 g of tetraethyl orthosilicate. Mix the two solutions and stir overnight at room temperature until the system becomes clear, obtaining a silicon source solution. Add 69 g of deionized water, 0.42 g of Al(NO3)3·9H2O, and 0.20 g of (NH4)6Mo7O to the above silicon source solution. 24 ·4H2O was added, and stirring was continued for 30 h to obtain the precursor gel. The molar ratio of the components in the precursor gel was: SiO2:Al:Mo:TPAOH:H2O=1:0.01:0.01:0.32:45.4. The obtained precursor gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and hydrothermally crystallized at 150 °C for 4 days. After crystallization, the crystallization product was subjected to solid-liquid separation and washed with deionized water until neutral; then dried at 100 °C for 12 h, and then calcined at 550 °C for 4 h in air atmosphere to obtain Mo-induced hierarchical porous ZSM-5 molecular sieve.

[0073] Example 9

[0074] In this embodiment, silica sol was used as the silicon source and sodium aluminate as the aluminum source. 28.8 g of a 25% (w / w) tetrapropylammonium hydroxide aqueous solution and 22.2 g of a 30% (w / w) silica sol were weighed and mixed, then stirred at room temperature for 6 h to obtain a homogeneous silicon source system. 53.5 g of deionized water, 0.09 g of sodium aluminate, and 0.39 g of SnCl4·5H2O were added to the above system, and stirring was continued for 30 h to obtain a precursor gel. The molar ratio of the components in the precursor gel was: SiO2:Al:Sn:TPAOH:H2O = 1:0.01:0.01:0.32:45.4. The obtained precursor gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and hydrothermally crystallized at 150 °C for 2 days. After crystallization, the crystallized product was separated into solid and liquid phases and washed with deionized water until neutral. It was then dried at 100 °C for 12 h and calcined at 550 °C for 4 h in air to obtain Sn-induced hierarchical porous ZSM-5 molecular sieve.

[0075] Example 10

[0076] In this embodiment, silica was used as the silicon source and aluminum sulfate as the aluminum source. 28.8 g of a 25% tetrapropylammonium hydroxide aqueous solution, 6.7 g of silica, and 69.0 g of deionized water were weighed and mixed. The mixture was stirred at room temperature for 12 h to ensure thorough dispersion of the silica. 0.37 g of Al2(SO4)3·18H2O and 0.19 g of CuCl2·2H2O were added to the above system, and stirring was continued for 30 h to obtain the precursor gel. The molar ratio of the components in the precursor gel was: SiO2:Al:Cu:TPAOH:H2O = 1:0.01:0.01:0.32:45.4. The obtained precursor gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and hydrothermally crystallized at 150 °C for 4 days. After crystallization, the crystallized product was separated into solid and liquid phases and washed with deionized water until neutral. It was then dried at 100 °C for 12 h and calcined at 550 °C for 4 h in air to obtain Cu-induced hierarchical porous ZSM-5 molecular sieve.

[0077] Example 11

[0078] In this embodiment, sodium silicate was used as the silicon source and boehmite as the aluminum source. 28.8 g of a 25% (w / w) tetrapropylammonium hydroxide aqueous solution, 31.5 g of Na₂SiO₃·9H₂O, and 51.0 g of deionized water were weighed and mixed, then stirred at room temperature for 12 h to obtain the silicon source system. 0.10 g of boehmite and 0.18 g of ZnCl₂ were added to the above system, and stirring was continued for 30 h to obtain the precursor gel. The molar ratio of the components in the precursor gel was: SiO₂:Al:Zn:TPAOH:H₂O = 1:0.015:0.012:0.32:45.4. The obtained precursor gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and hydrothermally crystallized at 160 °C for 3 days. After crystallization, the crystallized product was separated into solid and liquid phases and washed with deionized water until neutral. It was then dried at 100 °C for 12 h and calcined at 550 °C for 4 h in air to obtain Zn-induced hierarchical porous ZSM-5 molecular sieve.

[0079] Example 12

[0080] In this embodiment, fumed silica is used as the silicon source and aluminum isopropoxide as the aluminum source. 22.6 g of a 25% (w / w) tetrapropylammonium hydroxide aqueous solution, 6.7 g of fumed silica, and 52.9 g of deionized water were weighed and mixed. The mixture was stirred at room temperature for 12 h to ensure thorough dispersion of the fumed silica. Then, 0.34 g of aluminum isopropoxide and 0.16 g of (NH₄)₆Mo₇O₇ were added to the above system. 24 ·4H2O was added, and stirring was continued for 36 h to obtain the precursor gel. The molar ratio of the components in the precursor gel was: SiO2:Al:Mo:TPAOH:H2O = 1:0.015:0.008:0.25:35. The obtained precursor gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and hydrothermally crystallized at 170 °C for 3 days. After crystallization, the crystallization product was subjected to solid-liquid separation and washed with deionized water until neutral; then dried at 100 °C for 12 h, and calcined at 550 °C for 4 h in air atmosphere to obtain Mo-induced hierarchical porous ZSM-5 molecular sieve.

[0081] Comparative Example 1

[0082] This comparative example is a ZSM-5 molecular sieve sample without added heteroatoms.

[0083] Weigh 28.8 g of a 25% (w / w) tetrapropylammonium hydroxide aqueous solution and 23.1 g of tetraethyl orthosilicate. Mix the two solutions and stir overnight at room temperature until the system becomes clear, obtaining a silicon source solution. Add 69 g of deionized water and 0.42 g of Al(NO3)3·9H2O to the silicon source solution, without adding SnCl4·5H2O, and continue stirring for 30 h to obtain a precursor gel. The molar ratio of the components in the precursor gel is: SiO2:Al:TPAOH:H2O = 1:0.01:0.32:45.4. Transfer the obtained precursor gel to a stainless steel reactor with a polytetrafluoroethylene liner and hydrothermally crystallize at 150 °C for 2 days. After crystallization, the crystallized product was separated into solid and liquid phases and washed with deionized water until neutral. It was then dried at 100 °C for 12 h and calcined at 550 °C for 4 h in air to obtain ZSM-5 molecular sieve without Sn.

[0084] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

[0085] Characterization:

[0086] Figure 1 , Figure 4 , Figure 7 , Figure 10 , Figure 13 , Figure 16 , Figure 19 and Figure 24 The XRD patterns of each sample are shown. As can be seen from the figures, the samples obtained in Examples 1-7 and Comparative Example 1 all exhibit characteristic diffraction peaks of the ZSM-5 molecular sieve MFI framework structure, indicating that the obtained samples successfully formed the ZSM-5 molecular sieve structure, and no obvious impurity crystal peaks were observed. Combined with the XRD refinement results in Table 2, it can be seen that the cell parameters of the samples obtained in Examples 1-7 are within the reasonable range of ZSM-5 molecular sieves, with grain sizes of 20-104 nm. This indicates that the introduction of heteroatoms can affect the crystal growth process of ZSM-5 molecular sieves, causing the obtained samples to exhibit nanocrystalline characteristics.

[0087] Figure 2 , Figure 5 , Figure 8 , Figure 11 , Figure 14 , Figure 17 , Figure 20 , Figure 22 and Figure 25The N2 adsorption-desorption isotherms for each sample are shown. All samples exhibited a significant increase in adsorption capacity in the low relative pressure region, indicating that the obtained samples retained the microporous structure of the ZSM-5 molecular sieve. In the higher relative pressure region, the sample from the examples showed a more significant increase in adsorption capacity, indicating the presence of a certain amount of secondary channels in the samples. As shown in Table 1, the total pore volume of Examples 1-8 is 0.49-0.62 cm³. 3 / g, higher than 0.44 cm in Comparative Example 1. 3 / g; the external pore volume of Examples 1-8 is 0.33-0.46 cm³. 3 / g, higher than 0.27 cm in Comparative Example 1. 3 / g indicates that heteroatom-induced synthesis can improve the external pore volume and secondary pore contribution while maintaining the microporous structure. Specifically, the external specific surface area of ​​Examples 3 and 4 reached 142 m². 2 / g and 175 m 2 / g, with an external pore volume reaching 0.46 cm³. 3 / g and 0.44 cm 3 / g indicates that its hierarchical pore structure is more obvious.

[0088] Figure 3 , Figure 6 , Figure 9 , Figure 12 , Figure 15 , Figure 18 , Figure 21 , Figure 23 and Figure 26 The images show scanning electron microscope (SEM) images of each sample. As can be seen from the figures, the ZSM-5 molecular sieves obtained in the examples are mainly formed by the aggregation of smaller primary nanocrystals. The particle surfaces are relatively rough, and there are certain gaps between the crystals, exhibiting characteristics of nanoaggregates. Compared with Comparative Example 1, the crystallite stacking structure of the sample in the examples is more obvious. This morphological feature is corroborated by the increased external specific surface area and external pore volume in the N2 adsorption-desorption test, indicating that heteroatoms can regulate nucleation, crystal growth, and nanocrystal assembly during the crystallization process of ZSM-5 molecular sieves, thereby promoting the formation of hierarchical porous ZSM-5 molecular sieves with coexisting micropores and secondary channels.

[0089] Based on the above characterization results, it can be seen that by introducing heteroatom sources such as Sn, Cu, Zn, or Mo into the ZSM-5 molecular sieve synthesis system, this invention can induce the formation of a hierarchical porous structure with nano-aggregate characteristics in ZSM-5 molecular sieves without the addition of additional hard or soft template agents. The resulting molecular sieve retains both the MFI framework and microporous structure, while also possessing high external pore volume and secondary channels, which is beneficial for improving molecular diffusion and mass transfer within the molecular sieve particles.

[0090] Table 1. Hole structure results of Examples 1-8 and Comparative Example 1

[0091] sample Specific surface area (m² / g) Microporous specific surface area (m² / g) External specific surface area (m2 / g) Total pore volume (cm3 / g) Micropore volume (cm3 / g) External pore volume (cm3 / g) Example 1 477 419 58 0.55 0.18 0.37 Example 2 488 406 82 0.55 0.18 0.37 Example 3 504 362 142 0.62 0.16 0.46 Example 4 517 341 175 0.58 0.14 0.44 Example 5 448 400 49 0.49 0.16 0.33 Example 6 475 408 67 0.51 0.17 0.34 Example 7 476 395 81 0.58 0.16 0.42 Example 8 440 367 73 0.52 0.15 0.37 Comparative Example 1 433 390 43 0.44 0.17 0.27

[0092] Table 2. XRD refinement results of Examples 1-7 and Comparative Example 1

[0093] sample a b c Grain size (nm) GOF Example 1 20.0230424 19.8543578 13.3591627 104 3.72 Example 2 20.0428368 19.8623696 13.3626310 54 3.09 Example 3 20.0729412 19.8819650 13.3749298 28 2.94 Example 4 20.0761372 19.8868692 13.3760873 20 2.92 Example 5 20.03097 19.85342 13.36158 65 1.54 Example 6 20.04620 20.01419 13.36236 68 1.36 Example 7 20.04524 19.87174 13.35808 39 1.21 Comparative Example 1 20.06024 19.87523 13.37109 79 1.52

Claims

1. A method for synthesizing heteroatom-induced hierarchical porous ZSM-5 molecular sieves, characterized in that, The steps include the following: (1) A precursor gel is obtained by mixing a silicon source, an aluminum source, a tetrapropylammonium hydroxide, a heteroatom source, and water; (2) The precursor gel is subjected to an aging treatment; (3) The aged precursor gel is subjected to hydrothermal crystallization; (4) After solid-liquid separation and washing, the crystallized product was dried at 100 °C for 12 h and then calcined at 550 °C for 4 h in air atmosphere to obtain graded pore ZSM-5 molecular sieve. Wherein, the heteroatom M in the heteroatom source is selected from one or more of Sn, Cu, Zn and Mo; the heteroatom source regulates nucleation, crystal growth and nanocrystal assembly during the crystallization process of ZSM-5 molecular sieve, so that the obtained ZSM-5 molecular sieve forms a hierarchical pore structure in which micropores and secondary channels coexist; the synthesis method does not add any additional hard or soft template agents for forming mesopores or macropores.

2. The synthesis method according to claim 1, characterized in that, The silicon source is selected from one or more of tetraethyl orthosilicate, silica sol, sodium silicate, water glass, silica fume, fumed silica, silica gel, and colloidal silica.

3. The synthesis method according to claim 1, characterized in that, The aluminum source is selected from one or more of sodium aluminate, aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum isopropoxide, aluminum sec-butoxide, aluminum hydroxide, boehmite, and aluminum sol.

4. The synthesis method according to claim 1, characterized in that, The heteroatom source includes one or more of tin, copper, zinc and molybdenum; the tin source is selected from one or more of tin chloride, stannous chloride and tin sulfate; the copper source is selected from one or more of copper nitrate, copper chloride and copper sulfate; the zinc source is selected from one or more of zinc nitrate, zinc chloride and zinc sulfate; and the molybdenum source is selected from one or more of ammonium molybdate, sodium molybdate and molybdenum chloride.

5. The synthesis method according to claim 1, characterized in that, The molar ratio of each component in the precursor gel, calculated as SiO2, Al2O3, metal M, TPAOH, and H2O, is SiO2:Al2O3:M:TPAOH:H2O=1:(0.005-0.030):(0.003-0.030):(0.10-0.50):(10-60), where M is one or more of Sn, Cu, Zn, and Mo, and TPAOH is tetrapropylammonium hydroxide.

6. The synthesis method according to claim 1, characterized in that, The aging treatment is performed at a temperature of 20-60 °C for 2-24 h; the hydrothermal crystallization is performed at a temperature of 150-190 °C for 24-96 h.

7. The synthesis method according to claim 1, characterized in that, The graded-pore ZSM-5 molecular sieve has an MFI framework structure and exhibits the characteristics of nano-aggregates formed by the aggregation of nanocrystals.

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