Mfi / fau composite molecular sieve with synergistically regulated micro-mesopore-macropore structure and in-situ synthesis method thereof

Abstract

The present application relates to a kind of MFI / FAU composite molecular sieve with synergistic regulation micropore, mesopore and pore structure and its in-situ synthesis method.The molecular sieve is introduced into FAU molecular sieve seed after the pre-mixing of MFI molecular sieve, aluminum source, sodium hydroxide and water, and is crystallized under controlled hydrothermal conditions, finally obtains the composite material with multistage pore structure.The material has the advantage of controllable pore structure proportion (by pore volume, micropore 5-10%, mesopore 20-30%, macropore 60-80%), significantly improves the mass transfer efficiency and catalytic selectivity of macromolecular feed.The method does not need to use organic structure directing agent and mesopore template agent, has the advantages of green environmental protection, simple process and high yield.The catalytic cracking catalyst prepared by using the molecular sieve as active component and applied to catalytic cracking reaction of crude oil can realize the catalytic relay ability of complex parallel sequential reaction, the yield of low-carbon olefin is more than 30%, the conversion rate is more than 86%, showing the characteristics of high activity and high olefin yield.

Description

Technical Field

[0001] This invention belongs to the field of preparation technology of hierarchical porous molecular sieve materials, specifically relating to an MFI / FAU composite molecular sieve with synergistic regulation of micropore, mesopore, and macropore structures and its in-situ synthesis method. It is applicable to catalytic conversion processes involving macromolecular reactants, such as heavy oil and crude oil, and is particularly suitable for applications requiring fluidized catalytic cracking and catalytic pyrolysis. Background Technology

[0002] Molecular sieve materials are crystalline aluminosilicate or phosphoaluminate materials with regular microporous structures, playing a crucial role in processes such as catalytic cracking, hydrocracking, isomerization, and molecular sieve membrane separation. The core characteristic of molecular sieves lies in their highly ordered pore structure. Based on their pore size, molecular sieve materials can be classified into microporous molecular sieves (pore size less than 2 nm), mesoporous materials (pore size 2-50 nm), and macroporous materials (pore size greater than 50 nm). Molecular sieves with different pore sizes can effectively sieve molecules of varying sizes and provide a large number of active sites for catalytic reactions, thus exhibiting high selectivity and activity. However, traditional molecular sieves are mainly microporous, with pore sizes generally less than 2 nm. While this single microporous structure endows molecular sieves with excellent sieving capabilities, it also introduces limitations on mass diffusion. Especially in catalytic processes involving large molecular reactants, the diffusion resistance of micropores significantly affects the reaction rate and catalytic efficiency. Furthermore, although microporous molecular sieves have a higher surface area, most catalytic reactions occur at the pore surface. The timely detachment of product molecules is also limited by the microporous channels, easily leading to catalyst deactivation and reduced reaction efficiency. It is evident that molecular sieve materials with single-channel structures are gradually becoming insufficient to meet practical needs. How to improve the transport rate of reactants while maintaining a high specific surface area is an important direction for current molecular sieve research.

[0003] To address this issue, scientists have proposed introducing mesoporous and macroporous structures to construct hierarchical porous molecular sieves. MFI-type molecular sieves (such as ZSM-5) and FAU-type molecular sieves (such as Y-type molecular sieves) have attracted widespread attention due to their excellent catalytic performance and thermal stability. Patent CN108745410A discloses a method for preparing a phosphorus-containing hierarchical porous ZSM-5 / Y composite molecular sieve. This method involves mixing an alkali, an organic template agent, and deionized water, adding NaY molecular sieve, a silicon source, and a boron source, followed by crystallization, then ammonium ion exchange and phosphide impregnation to finally obtain a phosphorus-containing microporous-mesoporous ZSM-5 / Y composite molecular sieve. Patent CN115814848A discloses a ZSM-5 / Y composite molecular sieve, its preparation method, and its applications. This method involves crystallizing a ZSM-5 molecular sieve mother liquor, mixing it with a NaY molecular sieve mother liquor, and then performing a secondary crystallization to obtain a core-shell structured ZSM-5 / Y composite molecular sieve. Patents CN101767034A, CN115926833A, and CN117899810A all disclose ZSM-5 / Y molecular sieves. A paper (Petroleum Processing, 2015, 31(2):535-541.) successfully prepared a core-shell structured ZSM-5 / Y zeolite catalytic material using silicon and aluminum species formed by the depolymerization of industrial NaY zeolite under alkaline conditions. A paper (Journal of Materials Research, 2015, 30(16):2434-2446) reported the preparation of Y / ZSM-5 composite materials via a two-step hydrothermal crystallization method. A paper (Chemical Engineering Journal, 2021, 417:129172) reported the preparation of binder-free microporous-mesoporous Y / ZSM-5 zeolite composite materials via vapor phase transport (VPT) and subsequent dealumination treatment. The literature (Materials Letters, 2024, 360: 135994) reported the growth of NaY zeolite shells on ZSM-5 cores via hydrothermal crystallization, forming a nanocrystalline core-shell structure.

[0004] While existing methods have achieved the compositing of MFI and FAU structures to some extent, they still face numerous technical bottlenecks in constructing hierarchical porous structures. First, current processes generally rely on post-processing or physical mixing methods, making it difficult to achieve the synergistic in-situ construction of micropores, mesopores, and macropores in a single step. This results in uneven spatial distribution of pores and a lack of effective synergy between different pore levels, failing to meet the demands for efficient conversion of macromolecular reactants. Second, there is currently a lack of precise control mechanisms for the pore volume distribution of the three types of pores. Most technologies can only confirm the "existence" of the pore structure but cannot quantitatively control and optimize the ratio of micro-mesopores to macropores, limiting further improvements in the diffusion performance and catalytic stability of molecular sieves. Furthermore, existing technologies often rely on organic structure-directing agents or multiple hydrothermal treatment processes, resulting in complex synthesis procedures and high energy consumption. This not only increases manufacturing costs but also imposes an environmental burden, hindering green and large-scale applications. Therefore, there is an urgent need to develop a novel MFI / FAU composite molecular sieve material that requires no template agent, can achieve precise synergistic control of hierarchical pore structure under one-step hydrothermal conditions, and possesses both high catalytic activity and excellent mass transfer efficiency, in order to overcome the limitations of existing synthesis systems and meet the practical needs of complex molecular catalytic transformation. Summary of the Invention

[0005] The purpose of this invention is to provide a method for the in-situ synthesis of MFI / FAU composite molecular sieves with synergistic regulation of micropore, mesopore, and macropore structures, addressing the technical problems of low hierarchical pore construction efficiency, uncontrollable pore structure, insufficient diffusion performance, and poor adaptability to macromolecular reactants in existing molecular sieve materials. To achieve the above objective, this invention provides a method for the in-situ construction of MFI / FAU composite molecular sieves, characterized by the synergistic regulation and construction of micropore (0.5-0.8 nm), mesopore (5-50 nm), and macropore (60-140 nm) structures through the control of raw material ratios, hydrothermal treatment conditions, and the amount of FAU seed crystals introduced. This method enhances the diffusion efficiency to macromolecular guests while maintaining the excellent adsorption performance of the composite molecular sieve micropores. This method requires no organic structure-directing agents, uses little water, has a simple synthesis process, and is suitable for large-scale preparation.

[0006] The technical solution provided by this invention is as follows: an in-situ synthesis method for MFI / FAU composite molecular sieves with synergistic regulation of micropore-mesopore-macropore structure, comprising the following steps:

[0007] (1) Mix water, sodium hydroxide, MFI structured molecular sieve and aluminum source for 10-50 min to prepare mixture A;

[0008] (2) Add FAU structured molecular sieve seed crystals to the mixture A obtained in step (1) and mix for 10-30 min to obtain mixture B;

[0009] (3) The mixture B obtained in step (2) is subjected to hydrothermal treatment at 80-120°C for 9-24 hours;

[0010] (4) The mixture obtained in step (3) is washed and dried to obtain a multi-level pore MFI / FAU composite molecular sieve with micropore-mesopore-macropore.

[0011] In step (1) of this invention, the SiO2 / Al2O3 molar ratio of the MFI structure molecular sieve is not less than 18; the aluminum source is one or more of aluminum sulfate, sodium aluminate, aluminum hydroxide, boehmite, and aluminum isopropoxide.

[0012] In step (1) of this invention, the mixing method is conventional stirring, which is carried out at room temperature; conventional stirring is glass rod stirring or mechanical stirring.

[0013] In step (1) of this invention, the aluminum source is calculated as equivalent oxide aluminum oxide, and the mass ratio of water:sodium hydroxide:MFI structure molecular sieve:Al2O3 is (10-14):(1-3):(10-15):1; preferably 14:2:13:1.

[0014] In step (2) of this invention, the mass ratio of FAU structure molecular sieve seed crystals to MFI structure molecular sieves is (0.02 to 0.20): 1.

[0015] In step (2) of this invention, the mixing time is 10 to 30 minutes, and the mixing method is stirring or grinding, which is carried out at room temperature.

[0016] In step (3) of the present invention, the preferred temperature for hydrothermal treatment is 100°C and the time is 18h.

[0017] In step (4) of this invention, the solvent used for washing is deionized water, and the pH of the filtrate after washing is 6.5 to 7.5.

[0018] In step (4) of this invention, the drying temperature is 80-120℃ and the drying time is 8-24h.

[0019] In this invention, the MFI structure molecules are screened from ZSM-5 molecular sieve, Silicalite-1 molecular sieve, or TS-1 (titanium silicate molecular sieve), and the FAU structure molecules are screened from Y-type molecular sieve, X-type molecular sieve, or ultrastable Y-type molecular sieve.

[0020] In step (1) of this invention, water, sodium hydroxide, MFI structured molecular sieve, and aluminum source are mixed and stirred in one step, and the amount of each component and the stirring time are controlled. Sodium hydroxide provides an alkaline environment, which can effectively promote the depolymerization and condensation reaction of aluminum source, and at the same time etches silicon atoms in the MFI framework. The released silicon species further serve as the silicon source for the growth of FAU crystal phase. This step is a key step in realizing the in-situ construction of a hierarchical porous structure. When the ratio of NaOH and aluminum source is insufficient, the formation of mesopores and macropores is limited; if the amount is too large, it is easy to cause framework damage. The stirring time is controlled at about 30 minutes to achieve a relatively uniform dispersion of precursors. Insufficient or excessive stirring is not conducive to pore channel control. Then, FAU structured molecular sieve seed crystals are added to the mixture obtained in step (1). The introduction of FAU seed crystals can effectively provide nucleation sites, significantly shorten the formation cycle of FAU crystal phase, and thus accelerate the growth process of MFI / FAU composite structure. The amount of seed crystals should ideally be controlled between 2-20% of the MFI mass. Too little seed crystals will slow down the crystallization rate, while too much will easily form FAU-enriched phases, disrupting the synergistic pore structure. Hydrothermal treatment is then performed, and the temperature and time are crucial for pore structure development. If the treatment time is too short (<9 hours) or the temperature is too low (<80℃), mesopores and macropores will not develop completely, and the material will be predominantly micropore-based, limiting catalytic diffusion performance. Conversely, if the time is too long or the temperature is too high, it may lead to the collapse of the macropore structure, reducing the specific surface area. The preferred conditions are treatment at 100℃ for 18 hours, under which an MFI / FAU composite molecular sieve with a reasonable pore structure distribution and a coordinated ratio of the three types of pores can be obtained.

[0021] This invention also provides an MFI / FAU composite molecular sieve prepared by the above method, which has a multi-level pore structure of micropores-mesopores-macropores and a total specific surface area of ​​300 m². 2 ·g -1 ~400m 2 ·g -1 The specific surface area of ​​the micropores is between 200 and 300 m². 2 ·g -1 The micropore volume is between 0.08 and 0.14 cm³. 3 ·g -1 Mesoporous specific surface area is 50-150 m² 2 ·g -1 The mesopore volume is 0.20–0.50 cm³. 3 ·g -1 .

[0022] In terms of pore volume contribution from micropores, mesopores, and macropores, micropores account for 5%–10%, mesopores for 20%–30%, and macropores for 60%–80%. Macropores provide low-resistance channels for the rapid entry of macromolecular reactants and the desorption of products, effectively reducing diffusion limitations. Mesopores act as relay pathways connecting micropores and macropores, improving overall mass transfer efficiency. The microporous region maintains a high density of acidic sites, responsible for reaction activation. These three types of pores are spatially interconnected, synergistically achieving the functional integration of molecular sieves in "rapid entry / exit – selective catalysis – efficient conversion."

[0023] The MFI / FAU composite molecular sieve of this invention has a significantly increased yield of 92%–97%, while the yield of the traditional hydrothermal method is only 60%–80%. This yield is calculated based on the ratio of the mass of the synthesized molecular sieve to the mass of the solid raw materials (commercial MFI structured molecular sieve, aluminum source, NaOH, and FAU structured molecular sieve seed crystals). This invention effectively improves the synthesis efficiency of MFI / FAU composite molecular sieves.

[0024] This invention significantly reduces the amount of water added during the synthesis process, enabling the preparation of a multi-level porous MFI / FAU molecular sieve with micropores, mesopores, and macropores using only a minimal amount of water. The in-situ introduction of this multi-level porous structure addresses the accessibility and diffusion limitations of reactant molecules caused by the acidic sites in microporous molecular sieves. The reduced water content significantly improves the synthesis yield of the composite molecular sieve. Furthermore, the reduced water content also lessens the load on production equipment, thereby lowering the risks associated with the production process.

[0025] This invention also provides the application of the microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve catalyst obtained by the above method in the catalytic cracking of crude oil. In the application of Daqing crude oil catalytic cracking, the total low-carbon olefin yield reached 30.51%, of which the ethylene yield was 4.05%, the propylene yield was 16.39%, and the crude oil conversion rate was 86.07%.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] (1) Achieving synergistic regulation of three types of pore structures: This invention proposes a design concept that allows micropores, mesopores and macropores to co-occur in proportion within the pore volume. This breaks the limitation of traditional micro / mesoporous material structure and function separation, and realizes the spatial interconnection and functional gradient cooperation of the three types of pores, significantly improving the adaptability and diffusion efficiency of molecular sieves to macromolecular reactants.

[0028] (2) The material prepared by the present invention provides abundant acidic active centers in the microporous region, mesopores serve as relay channels to improve mass transfer efficiency, and macropores provide low-resistance diffusion paths for macromolecular reactants, thereby realizing the synergistic function of multi-level pores.

[0029] (3) In-situ one-step construction of multi-level porous structures, with simple and green process: By introducing FAU seed crystals in-situ in the alkaline etching-aluminum-silicon co-source system and controlling the hydrothermal treatment conditions, multi-level porous structures can be generated simultaneously without organic template agents and additional mesoporous template agents, which has industrial advantages such as short process, low energy consumption and environmental friendliness. Attached Figure Description

[0030] Figure 1 The X-ray diffraction patterns are of the MFI / FAU composite molecular sieves obtained in Examples 1-3, Comparative Examples 1-2, and Comparative Example 4.

[0031] Figure 2 This is a transmission electron microscope image of the microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve obtained in Example 1.

[0032] Figure 3 This is a transmission electron microscope image of the microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve obtained in Example 2.

[0033] Figure 4 This is a transmission electron microscope image of the microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve obtained in Example 3.

[0034] Figure 5 This is a scanning electron microscope image of the microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve obtained in Example 3.

[0035] Figure 6 The image shows a transmission electron microscope image of the MFI / FAU composite molecular sieve obtained in Comparative Example 1.

[0036] Figure 7 The image shows a transmission electron microscope image of the MFI / FAU composite molecular sieve obtained in Comparative Example 2.

[0037] Figure 8 This is a transmission electron microscope image of the mechanically mixed molecular sieve obtained in Comparative Example 3.

[0038] Figure 9 This is a transmission electron microscope image of Comparative Example 4.

[0039] Figure 10 The nitrogen physical adsorption-desorption isotherm of the microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve obtained in Example 3 is shown below.

[0040] Figure 11 The image shows the pore size distribution of the microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve obtained in Example 3. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to specific embodiments. It should be noted that the scope of protection of the present invention is not limited to these specific embodiments.

[0042] Catalyst preparation: The sample obtained in the examples or comparative examples was mixed with 1 mol / L ammonium sulfate solution at a mass ratio of 1:10 and subjected to ion exchange at 80°C for 1 h. The mixture was then washed and dried. The above operation was repeated once. Finally, the mixture was calcined at 550°C for 4 h to obtain the hydrogen form sample after ammonium exchange. The hydrogen form sample was mixed with kaolin and silica sol at a mass ratio of 4:5:1, slurried, and shaped. After aging at 800°C and 100 wt% steam for 4 h, the catalytic cracking catalyst was obtained.

[0043] The catalytic cracking experiments in the examples and comparative examples were evaluated in a micro fixed-bed reactor. The reaction temperature was 590°C, the regeneration temperature was 650°C, the feedstock was 1.667 g over 70 s, and the catalyst loading was 5 g. The gaseous and liquid products were analyzed using an Agilent 8890 gas chromatograph. The results of the catalytic cracking of Daqing crude oil are shown in Table 2.

[0044] Example 1

[0045] This embodiment provides a microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve, the preparation method of which includes the following steps:

[0046] 5.6 g of deionized water, 0.7 g of sodium hydroxide, 5 g of ZSM-5 molecular sieve (MFI structure), and 0.625 g of sodium aluminate were mixed and stirred for 30 minutes. Then, 0.5 g of NaY molecular sieve (FAU structure) seed crystals were added and stirring continued for another 30 minutes. The mixture was then transferred to a 100 ml polytetrafluoroethylene-lined synthesis vessel, sealed, and placed in a 100°C oven for 24 hours. The solution was then washed with deionized water until the pH of the filtrate reached 7, and dried in a 100°C oven for 9 hours to obtain a multi-level pore structure MFI / FAU molecular sieve sample (named S1). The yield of this composite molecular sieve S1 was 97%.

[0047] The XRD pattern of the microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve sample (named S1) prepared in Example 1 is shown below. Figure 1 As shown, the transmission electron microscope (TEM) image is as follows: Figure 2 As shown in the figure. The hole structure data is shown in Table 1.

[0048] Table 1 shows that the total specific surface area of ​​the MFI / FAU composite molecular sieve S1 is 346 m². 2 ·g -1 The specific surface area of ​​the micropores is 268 m². 2 ·g -1 The micropore volume is 0.1 cm³.3 ·g -1 The mesoporous specific surface area is 100m². 2 ·g -1 The mesopore volume is 0.38 cm³. 3 ·g -1 The macropore volume is 1.22 cm³. 3 ·g -1 The proportions of micropores, mesopores, and macropores were 6%, 22%, and 72%, respectively.

[0049] Example 2

[0050] This embodiment provides a microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve, the preparation method of which includes the following steps:

[0051] 16.8 g of deionized water, 2.1 g of sodium hydroxide, 15 g of ZSM-5 molecular sieve (MFI structure), and 1.875 g of sodium aluminate were mixed and stirred for 30 minutes. Then, 1.5 g of NaY molecular sieve (FAU structure) seed crystals were added and stirring continued for another 30 minutes. The mixture was then transferred to a 100 ml polytetrafluoroethylene-lined synthesis vessel, sealed, and placed in a 100°C oven for 12 hours. The solution was then washed with deionized water until the pH of the filtrate reached 7, and dried in a 100°C oven for 12 hours to obtain a multi-level pore structure MFI / FAU molecular sieve sample (named S2). The yield of this composite molecular sieve S2 was 95%.

[0052] The XRD pattern of the microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve sample (named S2) prepared in Example 2 is shown below. Figure 1 As shown, the transmission electron microscope (TEM) image is as follows: Figure 3 As shown.

[0053] Table 1 shows that the total specific surface area of ​​the MFI / FAU composite molecular sieve S2 is 338 m². 2 ·g -1 The specific surface area of ​​the micropores is 210 m². 2 ·g -1 The micropore volume is 0.09 cm³. 3 ·g -1 The mesoporous specific surface area is 128 m². 2 ·g -1 The mesopore volume is 0.37 cm³. 3 ·g -1 The macropore volume is 0.72 cm³. 3 ·g -1 The proportions of micropores, mesopores, and macropores were 8%, 31%, and 61%, respectively.

[0054] Example 3

[0055] This embodiment provides a microporous-mesoporous-macroporous hierarchical MFI / FAU composite molecular sieve, the preparation method of which includes the following steps:

[0056] 22.4 g of deionized water, 2.8 g of sodium hydroxide, 20 g of ZSM-5 molecular sieve (MFI structure), and 2.5 g of sodium aluminate were mixed and stirred for 30 minutes. Then, 2.0 g of NaY molecular sieve (FAU structure) seed crystals were added and stirring continued for another 30 minutes. The mixture was then transferred to a 100 ml polytetrafluoroethylene-lined synthesis vessel, sealed, and placed in a 100°C oven for 18 hours. The solution was then washed with deionized water until the pH of the filtrate reached 7, and dried in a 100°C oven for 12 hours to obtain a multi-level pore structure of MFI / FAU (named S3). The yield of this composite molecular sieve was 95%.

[0057] The XRD pattern of the MFI / FAU composite molecular sieve sample (named S3) with a hierarchical pore structure of micropores, mesopores, and macropores is shown below. Figure 1 As shown, the transmission electron microscope (TEM) image is as follows: Figure 4 As shown, the scanning electron microscope (SEM) image is as follows: Figure 5 As shown, the physical adsorption-desorption isotherms are as follows: Figure 10 As shown, the aperture distribution diagram is as follows: Figure 11 As shown in Table 2, the evaluation results of catalytic cracking of Daqing crude oil are as follows.

[0058] Table 1 shows that the total specific surface area of ​​the MFI / FAU composite molecular sieve S3 is 340 m². 2 ·g -1 The specific surface area of ​​the micropores is 233 m². 2 ·g -1 The micropore volume is 0.09 cm³. 3 ·g -1 The mesoporous specific surface area is 137 m². 2 ·g -1 The mesopore volume is 0.43 cm³. 3 ·g -1 The macropore volume is 1.35 cm³. 3 ·g -1 The proportions of micropores, mesopores, and macropores were 5%, 23%, and 72%, respectively. Figure 10 It can be seen that the N2 adsorption-desorption isotherm of the MFI / FAU composite molecular sieve S3 belongs to the type IV isotherm, and a large hysteresis loop exists at relative pressures of 0.45–0.9, indicating that the composite molecular sieve is a typical micro-mesocomposite material. No adsorption saturation plateau appears at a relative pressure of 0.989, indicating the presence of a macroporous structure in the composite molecular sieve. Figure 11 It can be seen that the pore size distribution of the MFI / FAU composite molecular sieve is 0.5–0.8 nm, 5–50 nm, and 60–140 nm.

[0059] Comparative Example 1

[0060] 5.6 g of deionized water, 0.7 g of sodium hydroxide, 5 g of ZSM-5 molecular sieve (MFI structure), and 0.625 g of sodium aluminate were stirred for 30 minutes. Then, 0.5 g of NaY molecular sieve (FAU structure) seed crystals were added and stirring continued for another 30 minutes. The mixture was then transferred to a 100 ml polytetrafluoroethylene-lined synthesis reactor, sealed, and placed in a 100°C oven for 1 hour. The solution was then washed with deionized water until the pH of the filtrate reached 7, and dried in a 100°C oven for 9 hours to obtain a macroporous MFI / FAU composite molecular sieve sample (named D1). The XRD pattern is shown below. Figure 1 As shown, the transmission electron microscope (TEM) image is as follows: Figure 6 As shown in Table 2, the evaluation results of catalytic cracking of Daqing crude oil are as follows.

[0061] Comparative Example 2

[0062] 5.6 g of deionized water, 0.7 g of sodium hydroxide, 5 g of ZSM-5 molecular sieve (MFI structure), and 0.625 g of sodium aluminate were stirred for 30 minutes. Then, 0.5 g of NaY molecular sieve (FAU structure) seed crystals were added and stirring continued for another 30 minutes. The mixture was then transferred to a 100 ml polytetrafluoroethylene-lined synthesis reactor, sealed, and placed in a 100°C oven for 3 hours. The solution was then washed with deionized water until the pH of the filtrate reached 7, and dried in a 100°C oven for 9 hours to obtain a macroporous MFI / FAU composite molecular sieve sample (named D2). The XRD pattern is shown below. Figure 1 As shown, the transmission electron microscope (TEM) image is as follows: Figure 7 As shown in Table 2, the evaluation results of catalytic cracking of Daqing crude oil are as follows.

[0063] Comparative Example 3

[0064] The ZSM-5 molecular sieve (MFI structure) and the NaY molecular sieve (FAU structure) seed crystals used in Example 1 were mechanically mixed at a mass ratio of 83%:17% to obtain the mechanically mixed molecular sieve sample (named D3). The XRD pattern is shown below. Figure 1 As shown, the transmission electron microscope (TEM) image is as follows: Figure 8 As shown in Table 2, the evaluation results of catalytic cracking of Daqing crude oil are as follows.

[0065] Comparative Example 4

[0066] 22.4 g of deionized water, 2.8 g of sodium hydroxide, and 20 g of ZSM-5 molecular sieve (MFI structure) were stirred for 30 minutes. Then, 2.5 g of sodium aluminate and 2.0 g of NaY molecular sieve (FAU structure) seed crystals were added and stirring continued for another 30 minutes. The mixture was then transferred to a 100 ml polytetrafluoroethylene-lined synthesis reactor, sealed, and placed in a 100°C oven for 18 hours. The solution was then washed with deionized water until the pH of the filtrate reached 7, and dried in a 100°C oven for 12 hours to obtain the MFI / FAU composite molecular sieve sample (named D4). The XRD pattern is shown below. Figure 1 As shown, the transmission electron microscope (TEM) image is as follows: Figure 9 As shown in Table 2, the evaluation results of the catalytic cracking of Daqing crude oil are presented. The only difference between the synthesis steps of Comparative Example D4 and Example S3 is that the aluminum source (sodium aluminate) in Comparative Example D4 is added in a later step (2), that is, the aluminum source is added simultaneously during the FAU seed introduction stage. In Example S3, the aluminum source is mixed with water, NaOH, and ZSM-5 molecular sieve (MFI structure) in step (1).

[0067] Depend on Figure 1 It can be seen that the diffraction peaks observed at 2θ = 7.9°, 8.9°, 23.0°, 23.3°, and 23.9° correspond to orthorhombic MFI structure molecular sieves (PDF number: 00-044-0003), while the diffraction peaks observed at 2θ = 15.64°, 18.66°, 20.36°, 23.63°, 27.04°, 30.73°, 31.79°, and 34.05° correspond to cubic FAU structure molecular sieves (PDF number: 00-043-0168). The simultaneous appearance of characteristic diffraction peaks from both MFI and FAU structure molecular sieves proves that the S1, S2, and S3 materials prepared in Examples 1-3 are all MFI / FAU composite materials, indicating the successful preparation of MFI / FAU composite molecular sieves. Figure 2-4 It can be seen that the MFI / FAU composite molecular sieves S1, S2, and S3 prepared in Examples 1-3 clearly exhibit a microporous-mesoporous-macroporous structure.

[0068] Table 1. Hole structure data of Examples 1-3 and Comparative Examples 1-4

[0069]

[0070]

[0071] illustrate:

[0072] S BET Total specific surface area calculated using the BET method;

[0073] S micro: Specific surface area of ​​micropores calculated using the t-plot method;

[0074] S meso : Mesoporous specific surface area calculated using the BJH method; V micro Micropore volume calculated using the t-plot method;

[0075] V meso Mesoporous pore volume calculated using the BJH method;

[0076] V macro Macropore volume calculated using mercury porosimetry;

[0077] Micropore ratio: V micro / (V micro +V meso +V macro );

[0078] Mesoporous percentage: V meso / (V micro +V meso +V macro );

[0079] Percentage of large holes: V macro / (V micro +V meso +V macro );

[0080] As can be seen from the pore structure data in Table 1, the MFI / FAU composite molecular sieves prepared in Examples 1-3 have a hierarchical pore structure, mainly containing micropores, mesopores, and macropores. In terms of pore volume, micropores account for 5%–10%, mesopores for 20%–30%, and macropores for 60%–80%. The MFI / FAU composite molecular sieves have a large mesopore volume, with a mesopore specific surface area of ​​over 50 m² / m³. 2 ·g -1 ~150m 2 ·g -1 Between 0.20 cm³, the mesopore volume is... 3 ·g -1 ~0.50cm 3 ·g -1 Between. Among them, the mesoporous pore volume of S3 reaches 0.43 cm. 3 / g, macropore volume 1.35cm³ 3 / g. This gradient pore structure originates from the synergistic effect of aluminum source premixing in step (1) and hydrothermal treatment for 18 hours in step (3). The multi-level pore structure of the MFI / FAU composite molecular sieve, consisting of micropores, mesopores, and macropores, not only has excellent adsorption and shape-selective catalytic performance, but also enhances the diffusion efficiency of macromolecular guests and reduces the occurrence of side reactions during catalysis.

[0081] Comparative Examples 1-2 (D1, D2) failed to form macroporous structures (0% macropore ratio) due to insufficient hydrothermal treatment time (only 1 hour and 3 hours, respectively). D1 exhibited an extremely high mesopore ratio (71%) but lacked macropores; D2 also had a high mesopore ratio (77%) and a higher micropore ratio (23%) than the examples, but still lacked macroporous structures. This severely limited its diffusion efficiency for macromolecular reactants. Comparative Example 3 (D3) was a mechanically mixed sample, and its pore structure characteristics were significantly different: it had the highest total specific surface area (587 m²). 2 ·g -1 However, it is mainly contributed by micropores (S) micro =554m 2 ·g -1 V micro =0.22cm 3 ·g -1 Mesopores account for an extremely low percentage (only 8%), and the pore volume is small (0.11 cm). 3 ·g -1 Although the macropore content (75%) was relatively high (due to the accumulation of pores in the mechanical mixture), it lacked in-situ connectivity and synergistic effects with micropores / mesopores. Comparative Example 4 (D4) showed severe pore structure degradation due to a change in the order of aluminum source addition (delayed to step 2): the mesopore volume sharply decreased to 0.20 cm³. 3 ·g -1 (0.43cm compared to S3) 3 ·g -1 (Decreased by approximately 53.5%), with the macropore volume collapsing to 0.31 cm. 3 ·g -1 (Compared to S3, 1.35cm) 3 ·g -1 The proportion of mesopores decreased by approximately 77%, and the proportion of macropores dropped to 50%. The dramatic reduction in the volume of both mesopores and macropores indicates that the aluminum source premixing step is crucial for constructing a well-developed and proportionally balanced hierarchical porous structure.

[0082] Table 2 Evaluation results of catalytic cracking of Daqing crude oil for the examples and comparative samples.

[0083]

[0084] Table 2 shows the catalytic cracking evaluation results, indicating that sample S3 prepared in Example 3 exhibits significant performance advantages in the catalytic cracking of Daqing crude oil. The total low-carbon olefin (ethylene + propylene + total butene) yield of S3 reached 30.51%, significantly higher than that of comparative examples D1 (24.93%), D2 (24.69%), mechanically mixed sample D3 (24.15%), and comparative example D4 with delayed aluminum source addition (27.89%). The crude oil conversion rate of S3 (86.07%) was also significantly higher than that of comparative examples D1 (82.48%), D2 (83.34%), mechanically mixed sample D3 (82.99%), and comparative example D4 with delayed aluminum source addition (83.27%). This result fully demonstrates that the MFI / FAU composite molecular sieve catalyst S3, with its microporous-mesoporous-macroporous hierarchical pore structure, can effectively improve the yield of low-carbon olefins while maintaining high catalytic activity.

[0085] Comparative Examples D1 and D2 suffered from insufficient hydrothermal treatment time (only 1-3 hours), resulting in incomplete development of mesoporous and macroporous structures and limited diffusion performance. Mechanically mixed sample D3 exhibited low catalytic efficiency due to the inability to achieve spatial synergy of multi-level pores through physical mixing. While Comparative Example D4 used the same raw material ratio, the delayed addition of the aluminum source in step (2) disrupted the synchronous process of alkaline etching of the MFI molecular sieve and aluminum source depolymerization in step (1), leading to a sharp reduction in mesoporous pore volume of 53.5% (0.20 vs 0.43 cm⁻¹). 3 / g), macropore volume collapse 77% (0.31 vs 1.35cm) 3 / g), the proportion of macropores decreased from 72% to 50%. This imbalance in pore structure directly resulted in D4 having a crude oil conversion rate and olefin yield that, although better than D1-D3, were still significantly lower than S3, confirming the necessity of aluminum source premixing in step (1) for constructing gradient channels. The superior performance of S3 stems from its unique micropore (5%)-mesopore (23%)-macropore (72%) synergistic system: macropores provide rapid diffusion channels for crude oil macromolecules, mesopores serve as intermediate product transport hubs, and micropores ensure the efficient utilization of cracking active sites. This three-level channel relay mechanism ultimately achieved a conversion rate of 86.07% and a low-carbon olefin yield of 30.51%, breaking through the performance bottleneck of existing technologies.

Claims

1. An in-situ synthesis method for MFI / FAU composite molecular sieves with synergistic regulation of microporous-mesoporous-macroporous structures, characterized in that, The method includes the following steps: (1) Mix water, sodium hydroxide, MFI structured molecular sieve and aluminum source for 10-50 min to prepare mixture A; the SiO2 / Al2O3 molar ratio of the MFI structured molecular sieve is not less than 18; the aluminum source is calculated as equivalent oxide aluminum oxide, and the mass ratio of water:sodium hydroxide:MFI structured molecular sieve:Al2O3 is (10-14):(1-3):(10-15):1; (2) Add FAU structured molecular sieve seed crystals to mixture A obtained in step (1) and mix for 10 to 30 min to obtain mixture B; by mass ratio, FAU structured molecular sieve seed crystals: MFI structured molecular sieve = (0.02 to 0.20): 1; (3) The mixture B obtained in step (2) is subjected to hydrothermal treatment at 80-120 °C for 9-24 h; (4) The mixture obtained in step (3) is washed and dried to obtain an MFI / FAU composite molecular sieve with micropores, mesopores and macropores. The micropores account for 5% to 10%, the mesopores account for 20% to 30%, and the macropores account for 60% to 80% based on pore volume.

2. The in-situ synthesis method of MFI / FAU composite molecular sieve with synergistic regulation of microporous-mesoporous-macroporous structure according to claim 1, characterized in that, In step (1), the aluminum source is one or more of aluminum sulfate, sodium aluminate, aluminum hydroxide, boehmite, and aluminum isopropoxide; the aluminum source is calculated as equivalent oxide aluminum oxide, and the mass ratio of water: sodium hydroxide: MFI structure molecular sieve: Al2O3 is 14:2:13:

1.

3. The in-situ synthesis method of the MFI / FAU composite molecular sieve with synergistic regulation of microporous-mesoporous-macroporous structure according to claim 1, characterized in that, In step (1), the mixing time is 30 min.

4. The in-situ synthesis method of MFI / FAU composite molecular sieve with synergistic regulation of microporous-mesoporous-macroporous structure according to claim 1, characterized in that, In step (3), the hydrothermal treatment temperature is 100 ℃ and the time is 18 h.

5. The in-situ synthesis method of the MFI / FAU composite molecular sieve with synergistic regulation of microporous-mesoporous-macroporous structure according to claim 1, characterized in that, In step (4), the solvent used for washing is deionized water, and the pH of the filtrate after washing is 6.5 to 7.5; the drying temperature is 80 to 120 ℃, and the drying time is 8 to 24 h.

6. The in-situ synthesis method of MFI / FAU composite molecular sieve with synergistic regulation of microporous-mesoporous-macroporous structure according to claim 1, characterized in that, The yield of the MFI / FAU composite molecular sieve was 92%–97%.

7. The in-situ synthesis method of MFI / FAU composite molecular sieve with synergistic regulation of microporous-mesoporous-macroporous structure according to claim 1, characterized in that, The MFI structure molecules are screened from ZSM-5 molecular sieve, Silicalite-1 molecular sieve or TS-1 (titanium silicate molecular sieve), and the FAU structure molecules are screened from Y-type molecular sieve, X-type molecular sieve or ultra-stable Y-type molecular sieve.

8. The MFI / FAU composite molecular sieve with a synergistically regulated microporous-mesoporous-macroporous structure prepared by the method according to any one of claims 1 to 7, characterized in that, The MFI / FAU composite molecular sieve has a multi-level pore structure containing micropores, mesopores, and macropores; based on pore volume, micropores account for 5% to 10%, mesopores account for 20% to 30%, and macropores account for 60% to 80%.

9. The MFI / FAU composite molecular sieve according to claim 8, characterized in that, The MFI / FAU composite molecular sieve has a pore size distribution range of 0.5–0.8 nm, 5–50 nm, and 60–140 nm, and a total specific surface area of ​​300–400 m². 2 ·g -1 The specific surface area of ​​the micropores is 200–300 m². 2 ·g -1 The micropore volume is 0.08–0.14 cm³. 3 ·g -1 The mesoporous specific surface area is 50–150 m². 2 ·g -1 The mesopore volume is 0.20–0.50 cm³. 3 ·g -1 .

10. The application of the MFI / FAU composite molecular sieve according to claim 8 or 9 in the catalytic cracking of crude oil, characterized in that, The total yield of low-carbon olefins is ≥30%, of which the yield of ethylene is ≥4%, the yield of propylene is ≥16%, the yield of butene is ≥10%, and the crude oil conversion rate is ≥86%.

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