Preparation method and application of high-stability nanoscale ZSM-5 catalyst

Abstract

The application provides a preparation method of high-stability nanoscale ZSM-5 catalyst and application thereof, and comprises the following steps: (1) preparing full-silicon molecular sieve silicalite-1 by a pre-crystallization method with a template agent, a silicon source and water as raw materials; (2) adopting a seed method, taking the full-silicon molecular sieve silicalite-1 as seed crystals, mixing the seed crystals with reaction raw materials including a template agent, a silicon source, an aluminum source and water, and then treating again by a pre-crystallization method, and then preparing ZSM-5 catalyst by crystallization, ammonia exchange and metal loading in sequence. The preparation method obtains high-stability light alkane aromatization ZSM-5 catalyst with secondary small particles of about 50 nm stacked together. The catalyst can maintain activity for a long time in the process of propane anaerobic aromatization reaction, has low surface carbon, high aromatic hydrocarbon selectivity and high propane conversion rate; and the synthesis method reduces the use of template agents, is green and environment-friendly, and has good application prospect in the field of propane aromatization catalyst preparation.

Description

Technical Field

[0001] This invention belongs to the technical field of catalysts and reaction optimization, specifically, it relates to a method for preparing a highly stable nanoscale ZSM-5 catalyst and its application. Background Technology

[0002] Light aromatics (BTX) are a widely used chemical feedstock, used to reduce the octane number of gasoline and as an important intermediate in the production of surfactants, pesticides, pigments, and dyes. Industrially, BTX is primarily obtained through naphtha catalytic reforming. However, with the rapid development of the petrochemical industry and the shortage of fossil fuels, traditional BTX production methods can no longer meet the growing market demand. Against this backdrop, new process routes for producing BTX from light alkanes (C2-C6), such as propane aromatization, have attracted widespread attention. Furthermore, compared to naphtha, light alkane aromatization has broad application prospects due to its abundant feedstock and low process cost.

[0003] Zeolite molecular sieves such as H-ZSM-5, H-ZSM-8, H-ZSM-11, MCM-22, and L-type zeolites are favored for the aromatization of light alkanes due to their two-dimensional structure and pore size characteristics, and have been extensively studied. Among them, ZSM-5 catalysts, because their pores only allow molecules smaller than trimethylbenzene to pass through, exhibit superior selectivity for β-xylene (BTX) and are considered the most effective catalysts for propane aromatization. However, low-silica-alumina ratio HZSM-5 catalysts have stronger Brønsted acid content, which promotes side reactions such as hydrogen transfer and results in higher C1-C3 yields, leading to less than ideal BTX selectivity and poor catalyst stability. Conversely, high-silica-alumina ratio HZSM-5 catalysts have lower total acid content, resulting in lower reactivity. Furthermore, HZSM-5 catalysts have insufficient dehydrogenation and hydrogenation capabilities, leading to low reactivity in propane aromatization systems without metal loading.

[0004] Therefore, researchers are dedicated to improving the conversion rate and catalyst stability of the reaction by studying structural changes, acidity, and acidic site regulation of zeolite molecular sieves, hoping to find a light alkane aromatization catalyst that combines stability and selectivity. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing a highly stable nanoscale ZSM-5 catalyst and its application. This preparation method is green and environmentally friendly, requires low equipment, and the catalyst obtained has high feed conversion rate and high aromatic selectivity in light alkane aromatization systems, while also exhibiting good stability.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing a highly stable nanoscale ZSM-5 catalyst, the method comprising the following steps:

[0008] (1) All-silicone molecular sieve silicalite-1 was prepared using template agent, silicon source and water as raw materials;

[0009] (2) Using the seed crystal method, the all-silica molecular sieve silicalite-1 prepared in step (1) is used as the seed crystal. It is mixed with the reaction raw materials, including template agent, silicon source, aluminum source and water. After the pre-crystallization stage, it is successively crystallized, ammonia exchanged and metal loaded to prepare ZSM-5 catalyst.

[0010] This invention utilizes a method that uses all-silica molecular sieve silicalite-1 as a seed crystal. After pre-crystallization under water bath conditions, the mother liquor obtained is crystallized in a hydrothermal reactor to obtain a catalyst with a diameter of tens of nanometers. After ammonia exchange and metal loading, it exhibits a high conversion rate of light alkanes and maintains excellent reaction stability, thereby realizing the preparation of a highly stable nanoscale aromatized ZSM-5 catalyst.

[0011] The present invention is further configured such that the template agent is preferably tetrapropylammonium hydroxide (TPAOH), the silicon source is preferably tetraethoxysilane (TEOS), and the aluminum source is preferably sodium aluminate; the template agent and silicon source in steps (1) and (2) may be the same or different.

[0012] The present invention is further configured such that, in step (1), the all-silica molecular sieve silicalite-1 is prepared by a pre-crystallization method, specifically including the following steps:

[0013] S1. Precrystallization stage: Mix the raw material template agent, silicon source and water, and age it in a water bath at 70-90℃ for 1-3 hours. After aging, add water to the initial mass, that is, the total mass of the reaction system returns to the mass before aging by adding water.

[0014] S2. Crystallization stage: The aged mixture is transferred to a crystallization kettle for crystallization reaction. The reaction temperature is 160-180℃ and the reaction time is 48-96h.

[0015] S3. Separation and washing: The crystallized product is separated by centrifugation and washed;

[0016] S4. Calcination stage: The separated and washed product is calcined at a heating rate of 1-5℃ / min, a calcination temperature of 500-600℃, and a holding time of 4-8h to obtain the all-silica molecular sieve silicalite-1.

[0017] Furthermore, in step (1), the molar ratio of raw material silicon source, template agent and water is 30SiO2:x TPAOH:900H2O, x = 3~12, preferably 3, 6, 9 or 12.

[0018] Preferably, in step (1), the aging temperature in the S1 pre-crystallization stage is 80°C and the aging time is 2h; the crystallization reaction temperature in the S2 crystallization stage is 170°C and the reaction time is 72h.

[0019] Preferably, in step (1) S3 separation and washing, the crystallized product is centrifuged at 5000-10000 r / min, more preferably 10000 r / min, and washed three times with ultrapure water.

[0020] Preferably, in step (1), during the S4 calcination stage, the product is heated at a rate of 2°C / min in a muffle furnace, the calcination temperature is 550°C, and the holding time is 6h.

[0021] The present invention is further configured such that, in step (2), the ZSM-5 catalyst is prepared by a seed crystal method, specifically including the following steps:

[0022] S1. Precrystallization stage: Mix template agent, silicon source, aluminum source and water, and add 2.5% to 10% by mass of all-silica molecular sieve silicalite-1. Age in a water bath at 70 to 90°C for 1 to 3 hours. After aging, add water to the initial mass. The mass fraction of silicalite-1 is the mass percentage of the total silicon source (calculated as SiO2) in the raw material.

[0023] S2. Crystallization stage: The aged mixture is transferred to a crystallization kettle for crystallization reaction. The reaction temperature is 160-180℃ and the reaction time is 36-96h.

[0024] S3. Separation and calcination: The crystallized product is separated by centrifugation, washed, and then calcined.

[0025] S4. Ammonia exchange: The product obtained in S3 is subjected to ammonia exchange in ammonium chloride solution, followed by calcination.

[0026] S5. Metal loading: The product obtained in S4 is impregnated with an equal amount of active metal in a metal compound, followed by calcination.

[0027] Furthermore, in step (2), the molar ratio of raw material silicon source, aluminum source, template agent and water is 60SiO2:xAl:12TPAOH:1800H2O, x = 1~5, preferably 1~3, more preferably 1.5~2.

[0028] Preferably, in step (2), the S1 pre-crystallization stage has an aging temperature of 80°C and an aging time of 2 hours.

[0029] Preferably, the crystallization reaction temperature of the S2 crystallization stage in step (2) is 170°C and the reaction time is 72h.

[0030] Preferably, in step (2) S3 separation and calcination, the crystallized product is centrifuged at 5000-10000 r / min, more preferably 10000 r / min, and washed three times with ultrapure water.

[0031] Preferably, in step (2) S4 ammonia exchange, the concentration of the ammonium chloride solution is 0.5–2 mol·L⁻¹. -1 More preferably 1 mol·L -1 The ammonia exchange time is 20-30 hours, more preferably 24 hours.

[0032] Furthermore, in step (2) S5 metal loading, the metal compound used for loading is selected from at least one of zinc nitrate, gallium nitrate, copper nitrate, and iron nitrate, with a loading amount of 0.5-6% by mass and an immersion time of 20-30 h, more preferably 24 h.

[0033] Preferably, in the calcination treatment of S3 to S5 in step (2), the heating rate in the muffle furnace is 1 to 5 °C / min, the calcination temperature is 500 to 600 °C, and the holding time is 4 to 8 h; more preferably, the heating rate is 2 °C / min, the calcination temperature is 550 °C, and the holding time is 6 h.

[0034] The second aspect of this invention provides a method for preparing the aforementioned highly stable nanoscale ZSM-5 catalyst. The ZSM-5 catalyst is composed of secondary small particles with a particle size of approximately 50 nm (30–80 nm), rich in intergranular mesopores, exhibiting well-preserved acidity, large specific surface area, good diffusion performance, and high stability. Its specific surface area reaches as high as 395 m². 2 ·g -1 .

[0035] A third aspect of this invention is the application of the highly stable nanoscale ZSM-5 catalyst in the aromatization of light alkanes, including propane aromatization. In the anaerobic aromatization of propane, the highly stable nanoscale ZSM-5 catalyst exhibits a propane conversion rate >80% over 5 hours, an aromatic selectivity >85%, a carbon deposition rate <5%, and an activity decrease <5%.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] This invention first synthesizes the all-silica molecular sieve Silicalite-1 with the aid of precrystallization, and then synthesizes nano-ZSM-5 molecular sieve again with the aid of Silicalite-1 seed crystals via the precrystallization method. The introduction of alcohol evaporation and water replenishment during the precrystallization process enables the synthesis of nano-ZSM-5 molecular sieve composed of secondary small particles of about 50 nm, compared with the micron-sized ZSM-5 molecular sieve synthesized by traditional methods.

[0038] Metal-loaded nano-ZSM-5 molecular sieves exhibited excellent reactivity in propane aromatization reactions, demonstrating high aromatic selectivity and propane conversion. The 5-hour propane conversion and total aromatic selectivity reached 88.4% and 86.8%, respectively, with minimal deactivation within 5 hours. This allows the sieve to maintain activity for an extended period during the oxygen-free aromatization of propane. Furthermore, its low surface carbon content results in significantly better catalytic performance than micron-sized ZSM-5 catalysts synthesized without seeding and pre-crystallization methods. The synthesis method also reduces the use of template agents, making it environmentally friendly and promising for application in the preparation of propane aromatization catalysts. Attached Figure Description

[0039] Figure 1 TEM image of the ZSM-5 molecular sieve catalyst prepared in Example 1;

[0040] Figure 2 The image shows a TEM image of the ZSM-5 molecular sieve catalyst prepared in Comparative Example 1.

[0041] Figure 3 TEM image of the ZSM-5 molecular sieve catalyst prepared in Comparative Example 2;

[0042] Figure 4 This is a TEM image of the ZSM-5 molecular sieve catalyst prepared in Comparative Example 3. Detailed Implementation

[0043] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. It should be understood that the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of the present invention.

[0044] Example 1

[0045] Silicalite-1, an all-silica molecular sieve, was synthesized using a pre-crystallization method with a feed ratio of 30SiO2:9TPAOH:900H2O. Specifically, 32.4g of ultrapure water and 9.15g of TPAOH (40wt%) were added to a 200ml beaker, and 12.5g of TEOS solution was slowly added dropwise while stirring. The mixture was stirred in an 80℃ water bath for 2 hours until the alcohol was almost completely evaporated, and water was added to bring the total mass back to the initial level. The mixture was then transferred to a polytetrafluoroethylene reactor and statically crystallized at 170℃ for 72 hours. The resulting mother liquor was centrifuged, washed three times, and dried at 110℃ for 12 hours. Finally, it was calcined in a muffle furnace at 550℃ for 6 hours to obtain Silicalite-1.

[0046] Using Silicalite-1 molecular sieve as seed crystals at a dosage of 5 wt%, a pre-crystallization method was employed with seed crystal assistance. The feed ratio was 30SiO2:1Al:6TPAOH:900H2O. Specifically, 32.4 g of ultrapure water, 6.1 g of TPAOH (40 wt%), 0.248 g of NaAlO2 (Al2O3 content 41%), and 0.18 g of Silicalite-1 were added to a 200 ml beaker. While stirring, 11.875 g of TEOS solution was slowly added dropwise. The mixture was stirred in an 80°C water bath for 2 hours until the alcohol was almost completely evaporated, and water was added to the initial mass. The mixture was then transferred to a polytetrafluoroethylene reactor and statically crystallized at 170°C for 72 hours. The resulting mother liquor was centrifuged, washed three times, and dried at 110°C for 12 hours. After drying, the sample was calcined in a muffle furnace at 550℃ for 6 hours. The resulting sample was then subjected to ammonia exchange with 1M NH4Cl for 24 hours. After the exchange, the sample was calcined in a muffle furnace at 550℃ for 6 hours to obtain nano-sized HZSM-5.

[0047] 2wt% Zn / HZSM-5 was prepared by impregnation with a zinc nitrate solution (Zn(NO3)2) of a certain concentration. The impregnated catalyst was dried at 110℃ for 12h and calcined in a muffle furnace at 550℃ for 6h to obtain the highly stable nano-scale catalyst.

[0048] Example 2

[0049] Using the all-silica molecular sieve silicalite-1 from Example 1 as seed crystals at a dosage of 5 wt%, a pre-crystallization method was employed with seed crystal assistance. The feed ratio was 30SiO2:1Al:6TPAOH:900H2O. Specifically, 32.4 g of ultrapure water, 6.1 g of TPAOH (40 wt%), 0.248 g of NaAlO2 (Al2O3 content 41%), and 0.18 g of silicalite-1 were added to a 200 ml beaker. 11.875 g of TEOS solution was slowly added dropwise while stirring. The mixture was stirred in a 70°C water bath for 2 hours until the alcohol was almost completely evaporated, and water was added to the initial mass. The mixture was then transferred to a polytetrafluoroethylene reactor and statically crystallized at 170°C for 72 hours. The resulting mother liquor was centrifuged, washed three times, and dried at 110°C for 12 hours. After drying, the sample was calcined in a muffle furnace at 550℃ for 6 hours. The resulting sample was then subjected to ammonia exchange with 1M NH4Cl for 24 hours. After the exchange, the sample was calcined in a muffle furnace at 550℃ for 6 hours to obtain nano-sized HZSM-5. A 2wt% Zn / HZSM-5 catalyst was prepared by impregnation with a zinc nitrate solution (Zn(NO3)2) of a certain concentration. The impregnated catalyst was dried at 110℃ for 12 hours and then calcined in a muffle furnace at 550℃ for 6 hours to obtain the highly stable nano-sized catalyst.

[0050] Example 3

[0051] Using Silicalite-1, the all-silica molecular sieve from Example 1, as a seed crystal at a dosage of 5 wt%, a pre-crystallization method was employed with seed crystal assistance. The feed ratio was 30SiO2:1Al:6TPAOH:900H2O. Specifically, 32.4 g of ultrapure water, 6.1 g of TPAOH (40 wt%), 0.248 g of NaAlO2 (Al2O3 content 41%), and 0.18 g of Silicalite-1 were added to a 200 ml beaker. 11.875 g of TEOS solution was slowly added dropwise while stirring. The mixture was stirred in a 90°C water bath for 2 hours until the alcohol was almost completely evaporated, and water was added to the initial mass. The mixture was then transferred to a polytetrafluoroethylene reactor and statically crystallized at 170°C for 72 hours. The resulting mother liquor was centrifuged, washed three times, and dried at 110°C for 12 hours. The sample was calcined in a muffle furnace at 550℃ for 6 hours. The resulting sample was then subjected to ammonia exchange with 1M NH4Cl for 24 hours. After the exchange, it was calcined again in a muffle furnace at 550℃ for 6 hours to obtain nano-sized HZSM-5. A 2wt% Zn / HZSM-5 catalyst was prepared by impregnation with a zinc nitrate solution (Zn(NO3)2) of a certain concentration. The impregnated catalyst was dried at 110℃ for 12 hours and then calcined in a muffle furnace at 550℃ for 6 hours to obtain the highly stable nano-sized catalyst.

[0052] Example 4

[0053] Using Silicalite-1, the all-silica molecular sieve from Example 1, as a seed crystal at a dosage of 5 wt%, a pre-crystallization method was employed with seed crystal assistance. The feed ratio was 60SiO2:1.5Al:12TPAOH:1800H2O. Specifically, 32.4 g of ultrapure water, 6.1 g of TPAOH (40 wt%), 0.186 g of NaAlO2 (Al2O3 content 41%), and 0.18 g of Silicalite-1 were added to a 200 ml beaker. 11.875 g of TEOS solution was slowly added dropwise while stirring. The mixture was stirred in an 80°C water bath for 1 hour until most of the alcohol evaporated, and water was added to the initial mass. The mixture was then transferred to a polytetrafluoroethylene reactor and statically crystallized at 170°C for 72 hours. The resulting mother liquor was centrifuged, washed three times, and dried at 110°C for 12 hours. After drying, the sample was calcined in a muffle furnace at 550℃ for 6 hours. The resulting sample was then subjected to ammonia exchange with 1M NH4Cl for 24 hours. After the exchange, the sample was calcined in a muffle furnace at 550℃ for 6 hours to obtain nano-sized HZSM-5. A 2wt% Zn / HZSM-5 catalyst was prepared by impregnation with a zinc nitrate solution (Zn(NO3)2) of a certain concentration. The impregnated catalyst was dried at 110℃ for 12 hours and then calcined in a muffle furnace at 550℃ for 6 hours to obtain the highly stable nano-sized catalyst.

[0054] Example 5

[0055] Using Silicalite-1, the all-silica molecular sieve from Example 1, as a seed crystal at a dosage of 5 wt%, a pre-crystallization method was employed with seed crystal assistance. The feed ratio was 30SiO2:1Al:6TPAOH:900H2O. Specifically, 32.4 g of ultrapure water, 6.1 g of TPAOH (40 wt%), 0.248 g of NaAlO2 (Al2O3 content 41%), and 0.18 g of Silicalite-1 were added to a 200 ml beaker. 11.875 g of TEOS solution was slowly added dropwise while stirring. The mixture was stirred in an 80°C water bath for 3 hours until the alcohol evaporated to dryness, and water was added to the initial mass. The mixture was then transferred to a polytetrafluoroethylene reactor and statically crystallized at 170°C for 72 hours. The resulting mother liquor was centrifuged, washed three times, and dried at 110°C for 12 hours. After drying, the sample was calcined in a muffle furnace at 550℃ for 6 hours. The resulting sample was then subjected to ammonia exchange with 1M NH4Cl for 24 hours. After the exchange, the sample was calcined in a muffle furnace at 550℃ for 6 hours to obtain nano-sized HZSM-5. A 2wt% Zn / HZSM-5 catalyst was prepared by impregnation with a zinc nitrate solution (Zn(NO3)2) of a certain concentration. The impregnated catalyst was dried at 110℃ for 12 hours and then calcined in a muffle furnace at 550℃ for 6 hours to obtain the highly stable nano-sized catalyst.

[0056] Comparative Example 1

[0057] Taking the commercial ZSM-5 catalyst (Si / Al ratio = 30) purchased from Nankai University as an example, 2wt% Zn / HZSM-5 was prepared by impregnation with a certain concentration of zinc nitrate solution (Zn(NO3)2). The impregnated catalyst was dried at 110℃ for 12h and calcined in a muffle furnace at 550℃ for 6h to obtain comparative sample 1.

[0058] Comparative Example 2

[0059] This comparative example only uses the seed crystal method, and does not employ the pre-crystallization method in the preparation of ZSM-5 molecular sieve catalysts.

[0060] Using the all-silica molecular sieve silicalite-1 from Example 1 as seed crystals, the addition amount was 5 wt%, and the feed ratio was 30SiO2:1Al:6TPAOH:900H2O. Specifically, 32.4 g of ultrapure water, 6.1 g of TPAOH (40 wt%), 0.248 g of NaAlO2 (Al2O3 content 41%), and 0.18 g of silicalite-1 were added to a 200 ml beaker, and 11.875 g of TEOS solution was slowly added dropwise while stirring. The mixture was stirred at room temperature for 6 h. Then, it was transferred to a polytetrafluoroethylene reactor and statically crystallized at 170 °C for 72 h. The resulting mother liquor was centrifuged, washed three times, and dried at 110 °C for 12 h. After drying, it was calcined in a muffle furnace at 550 °C for 6 h. The obtained sample was then subjected to ammonia exchange with 1 M NH4Cl for 24 h, and finally calcined in a muffle furnace at 550 °C for 6 h to obtain nano-sized HZSM-5. 2wt% Zn / HZSM-5 was prepared by impregnation with a certain concentration of zinc nitrate solution (Zn(NO3)2) in equal amounts. The impregnated catalyst was dried at 110℃ for 12h and calcined in a muffle furnace at 550℃ for 6h to obtain comparative sample 2.

[0061] Comparative Example 3

[0062] This comparative example only used the pre-crystallization method and did not add all-silica molecular sieve silicalite-1 as a seed crystal.

[0063] Samples were prepared using a feed ratio of 30SiO2:1Al:6TPAOH:900H2O. Specifically, 32.4g of ultrapure water, 6.1g of TPAOH (40wt%), and 0.248g of NaAlO2 (Al2O3 content 41%) were added to a 200ml beaker. 12.5g of TEOS solution was slowly added dropwise while stirring. The mixture was stirred in an 80℃ water bath for 2 hours until the alcohol was almost completely evaporated, and water was added to the initial mass. The mixture was then transferred to a polytetrafluoroethylene reactor and statically crystallized at 170℃ for 72 hours. The resulting mother liquor was centrifuged, washed three times, and dried at 110℃ for 12 hours. After drying, the sample was calcined in a muffle furnace at 550℃ for 6 hours. The resulting sample underwent ammonia exchange with 1M NH4Cl for 24 hours, followed by calcination in a muffle furnace at 550℃ for 6 hours to obtain nano-sized HZSM-5. 2wt% Zn / HZSM-5 was prepared by impregnation with a certain concentration of zinc nitrate solution (Zn(NO3)2) in equal amounts. The impregnated catalyst was dried at 110℃ for 12h and calcined in a muffle furnace at 550℃ for 6h to obtain comparative sample 3.

[0064] Example 6 Catalyst Evaluation

[0065] The microstructure of the catalysts in Examples 1-5 and Comparative Examples 1-3 was analyzed, and the results are as follows: Figure 1 and Figure 2-4 The images shown are TEM images of Example 1 and Comparative Examples 1-3, respectively. The catalyst microstructures obtained in Examples 2-5 are similar to those in Example 1. The catalysts prepared using the seed crystal method and pre-crystallization method of this invention exhibit significantly altered particle size; the nano-ZSM-5 molecular sieves are composed of secondary small particles with diameters of tens of nanometers. In contrast, the catalyst particles obtained without using the seed crystal method and pre-crystallization method of this invention all have particle sizes exceeding several hundred nanometers.

[0066] The above catalyst was evaluated in the aromatization of propane in a fixed-bed reactor at 550°C and atmospheric pressure. 0.3 g of 20-40 mesh catalyst was charged into a quartz tube reactor, and the reaction was carried out at 20 mL / min. -1 The reactor temperature was raised from room temperature to 550°C under N2 flow protection. Propane was used at a propane:nitrogen ratio of 3:7 at a flow rate of 3000 mL·g. -1 ·h -1 The sample was injected at space velocity. The reaction products were analyzed by a combination of gas chromatography and liquid chromatography, and the results are shown in Table 1.

[0067] Table 1. Reaction results of the catalysts prepared in Examples 1-5 and Comparative Examples 1-3 after 5 hours.

[0068]

[0069]

[0070] As shown in Table 1, compared with the comparative example, the catalyst prepared in the embodiments of the present invention, while maintaining high aromatic selectivity, significantly improved propane conversion and exhibited very low activity decline after 5 hours. Propane conversion at 5 hours was >80%, aromatic selectivity >85%, carbon deposition rate <5%, and activity decline <5%. Furthermore, the catalyst prepared under the conditions of Example 1 showed the best performance, with a propane conversion rate as high as 88.4% after 5 hours. The present invention prepared a ZSM-5 molecular sieve composed of secondary small particles with a particle size of tens of nanometers. Compared with the larger particle size of ZSM-5 molecular sieves in the prior art, the smaller particle size results in higher diffusion performance and reduced carbon deposition, thereby further improving the stability of the catalyst.

[0071] This application provides a detailed description, the purpose of which is to enable those skilled in the art to understand and implement the content of this application, but it should not be used to limit the scope of protection of this application. All equivalent changes or modifications made in accordance with the spirit and essence of this application should be included within the scope of protection of this application.

Claims

1. A method for preparing a highly stable nanoscale ZSM-5 catalyst, characterized in that, Includes the following steps: (1) All-silica molecular sieve silicalite-1 was prepared using template agent, silicon source and water as raw materials; Specifically, the pre-crystallization method is used for preparation. In the pre-crystallization stage, the raw material template agent, silicon source and water are mixed and aged in a water bath at 70~90℃ for 1~3 hours to evaporate alcohol. After aging, water is added to the initial mass. (2) Using the seed crystal method, the all-silica molecular sieve silicalite-1 is used as a seed crystal and mixed with the reaction raw materials, including the template agent, silicon source, aluminum source and water. After a pre-crystallization stage, the mixture is then subjected to crystallization, ammonia exchange and metal loading to prepare the ZSM-5 catalyst. In the pre-crystallization stage, the template agent, silicon source, aluminum source and water are mixed, and 2.5% to 10% of the all-silica molecular sieve silicalite-1 is added. The mixture is aged in a water bath at 70 to 90°C for 1 to 3 hours to evaporate the alcohol. After aging, water is added to the initial mass. The template agent is tetrapropylammonium hydroxide.

2. The preparation method according to claim 1, characterized in that, The silicon source is tetraethoxysilane, and the aluminum source is sodium aluminate.

3. The preparation method according to claim 2, characterized in that, Step (1) involves the pre-crystallization method to prepare the all-silica molecular sieve silicalite-1, which includes the following steps: S1. Pre-crystallization stage: Mix the raw material template agent, silicon source and water, age in a water bath at 70~90℃ for 1~3 hours to evaporate the alcohol, and add water to the initial mass after aging. S2. Crystallization stage: The aged mixture is transferred to a crystallization reactor for crystallization reaction at a temperature of 160~180℃ for a reaction time of 48~96h. S3. Separation and washing: Centrifuge and wash the crystallized product; S4. Calcination stage: The separated and washed products are calcined to obtain the all-silica molecular sieve silicalite-1.

4. The preparation method according to claim 3, characterized in that, In step (1), the molar ratio of raw material silicon source, template agent and water is 30 SiO2:x TPAOH:900 H2O, x=3~12.

5. The preparation method according to claim 2, characterized in that, Step (2) includes the following steps: S1. Precrystallization stage: Mix template agent, silicon source, aluminum source and water, add 2.5%~10% by mass of all-silica molecular sieve silicalite-1, age in a water bath at 70~90℃ for 1~3 hours to evaporate alcohol, and add water to the initial mass after aging. S2. Crystallization stage: The aged mixture is transferred to a crystallization reactor for crystallization reaction at a temperature of 160~180℃ for 36~96h. S3. Separation and calcination: The crystallized product is separated by centrifugation, washed, and then calcined. S4. Ammonia exchange: The product obtained in S3 is subjected to ammonia exchange in ammonium chloride solution, followed by calcination. S5. Metal loading: The product obtained in S4 is impregnated with an equal amount of active metal in a metal compound, followed by calcination.

6. The preparation method according to claim 5, characterized in that, In step (2), the molar ratio of raw material silicon source, aluminum source, template agent and water is 60:SiO2:x Al:12 TPAOH:1800 H2O, x=1~5.

7. The preparation method according to claim 5, characterized in that, In step (2), the S1 pre-crystallization stage has an aging temperature of 80℃ and an aging time of 2h; the S2 crystallization stage has a crystallization reaction temperature of 170℃ and a reaction time of 72h.

8. The preparation method according to claim 5, characterized in that, In step (2), during the S4 ammonia exchange, the concentration of the ammonium chloride solution is 0.5~2 mol·L⁻¹. -1 The ammonia exchange time is 20-30 hours. In S5 metal loading, the metal compound used for loading is selected from at least one of zinc nitrate, gallium nitrate, copper nitrate, and iron nitrate, with a loading amount of 0.5-6 wt% and an impregnation time of 20-30 h; In the calcination treatment of S3~S5, the heating rate in the muffle furnace is 1~5℃ / min, the calcination temperature is 500~600℃, and the holding time is 4~8h.

9. A highly stable nanoscale ZSM-5 catalyst, characterized in that, The catalyst is prepared using any one of the preparation methods described in claims 1-8.

10. The application of the highly stable nanoscale ZSM-5 catalyst according to claim 9, characterized in that, Used for aromatization reactions of light alkanes, including propane aromatization reactions.

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