Preparation method and application of ZSM-5 molecular sieve catalyst with low outer surface acidity

Abstract

The invention discloses a preparation method of a ZSM-5 molecular sieve catalyst with low outer surface acidity and application of the ZSM-5 molecular sieve catalyst in an isomerization upper-layer reaction of de-ethyl xylene, and belongs to the technical field of molecular sieve catalysts. The alkali-silica ratio in the synthesis process is reduced through precise regulation and control, and synthesis of the ZSM-5 molecular sieve with low outer surface acidity is realized; zinc nitrate is adopted as a precursor, the acidity of the ZSM-5 molecular sieve is modulated through a metal-acid center synergistic effect, and the side reaction activity such as transalkylation is reduced; the surface of the molecular sieve is coated with Silicalite-1 by using a seed crystal guiding liquid auxiliary method so as to further eliminate the outer surface activity of an upper agent. According to the method, side reactions in the deethylated xylene isomerization upper layer reaction are effectively inhibited, and a new path is opened up for industrial application of the efficient deethylated xylene isomerization upper layer agent.

Description

Technical Field

[0001] The invention belongs to the technical field of molecular sieve catalysts, in particular to the preparation of a low external surface acidic ZSM-5 molecular sieve catalyst and its application in the upper layer reaction of deethylated xylene isomerization. Background Art

[0002] With the rapid development of the petrochemical industry, deethylation-type xylene isomerization catalysts play a vital role in the separation and conversion of C8 aromatics. Conventional deethylation-type xylene isomerization reactions typically utilize a series of catalysts to improve reaction efficiency. ZSM-5 molecular sieves, with their relatively weak surface acidity and strong internal pore acidity, are typically used as the top layer catalyst. However, these catalysts exhibit a number of issues in the deethylation of ethylbenzene, including low ethylbenzene conversion and high activity in side reactions such as xylene isomerization and transalkylation (CN115025811A). Therefore, further reducing the surface acidity of the top layer catalyst and improving the distribution of acid sites within the pores to reduce side reaction activity have become current research priorities.

[0003] In recent years, the preparation of deethylated xylene isomerization top-layer agents often relies on synthesizing ZSM-5 molecular sieves at high alkali-silica ratios to drive the formation and growth of zeolite nuclei, improving their pore structure integrity and catalytic stability (CN112939014A, CN109336129A). However, a high alkali-silica ratio leads to excessively alkaline synthesis, which not only exacerbates the dissolution of silicon species in the zeolite framework and uneven aluminum distribution, severely affecting the acidity of the zeolite, but also induces the formation of impurities, reducing the crystallinity and structural integrity of the product. The presence of an organic microporous template can expand the range of alkali-silica ratios in the synthesis system, enabling the successful preparation of molecular sieves in low alkali-silica ratio synthesis systems with the addition of an organic template (CN115025811A). However, due to the lack of an organic microporous template in template-free synthesis systems, the alkali-silica ratio range is limited, making it often impossible to prepare ZSM-5 molecular sieves with excellent crystallinity at low alkali-silica ratios.

[0004] To enhance catalytic performance, researchers have attempted to precisely control the acidity of ZSM-5 zeolites by loading them with metal components (such as Pd and Ni) (CN103962165A, CN105597809A). The introduction of metal species selectively neutralizes strong acid sites, inhibiting carbon deposition and enhancing the activity of ethylbenzene deethylation by forming a synergistic metal-acid site network. However, during zeolite synthesis, zeolites synthesized with a high alkali-silicon ratio have low surface hydroxyl density and few defect sites, making it difficult for the metal precursor to be effectively anchored. This, coupled with uneven dispersion during loading, can lead to active site aggregation, exacerbating side reactions. Therefore, developing ultra-low alkali-silicon ratio synthesis technology and modifying the zeolite with appropriate metal components are key challenges in overcoming existing process limitations and achieving efficient deethylation-type xylene isomerization top layer catalysts. Furthermore, to minimize side reactions on the outer surface of the top layer catalyst, it is necessary to passivate the outer surface acidity. Traditional treatment methods, such as surface deposition, can create a shell that clogs the molecular sieve pores (CN102794193A). The high cost of preparing the core-shell Silicalite-1@ZSM-5 molecular sieve is also significant (CN118561292A).

[0005] In summary, how to achieve efficient nucleation and directional growth of molecular sieves under low alkalinity conditions, and significantly reduce production costs by reducing the amount of alkali used; using the adaptive design of molecular sieve surface characteristics and metal loading process to solve the industry problem of balancing the dispersion and stability of active components; at the same time, how to effectively passivate the acidity of the catalyst outer surface has become a technical difficulty that urgently needs to be broken through in this field. Summary of the Invention

[0006] The present invention aims to provide a method for preparing a low external surface acidity ZSM-5 molecular sieve catalyst, which is suitable for the upper layer reaction of deethylation type xylene isomerization.

[0007] The present invention significantly reduces the alkali-silicon ratio (Na2O: SiO2) during the catalyst preparation process, significantly inhibits the formation of impurity crystals and maintains the uniformity of the framework aluminum, the acidity of the molecular sieve and the crystallinity of the molecular sieve, thereby achieving efficient nucleation and directional growth of the molecular sieve and significantly reducing production costs by reducing the amount of alkali used. Secondly, the loading of zinc nitrate optimizes the acidity of the molecular sieve and utilizes Zn 2+By combining strong acid sites to construct a metal (ZnO cluster)-acid (Brønsted acid) synergistic active center, the industry's difficult problem of balancing the dispersion and stability of the active components was solved. Silicalite-1 was then coated on the surface of the molecular sieve using a seed-guided liquid-assisted method to eliminate the activity of the catalyst's outer surface (i.e., passivate the acidity of the catalyst's outer surface). The catalyst prepared by the present invention exhibited excellent catalytic activity in the upper layer reaction of deethylated xylene isomerization, providing a new solution for the industrialization of high-efficiency catalysts.

[0008] The present invention provides a method for preparing a low external surface acidic ZSM-5 molecular sieve catalyst, comprising the following steps: (1) Sodium hydroxide, an aluminum source, and a silicon source are sequentially added to deionized water to prepare a gel, with the alkali-silicon ratio being controlled below 0.110; ZSM-5 molecular sieve seed crystals are then added, and the prepared gel is subjected to a hydrothermal crystallization reaction to prepare a Na-type ZSM-5 molecular sieve; (2) filtering and washing the Na-type ZSM-5 molecular sieve so that its pH value remains neutral at room temperature; (3) using NH4Cl solution to perform ion exchange on the synthesized Na-type ZSM-5 and calcining at high temperature to obtain HZSM-5 molecular sieve; (4) Immersing an equal volume of the HZSM-5 molecular sieve prepared above in a zinc nitrate solution, allowing it to stand and dry, and then calcining it at a high temperature; (5) A seed guiding liquid is prepared using silica sol, TPAOH, and deionized water; the HZSM-5 molecular sieve prepared in step (4) is added to the seed guiding liquid, dried, filtered, washed, and then calcined at high temperature to obtain a low external surface acidity Silicalite-1@ZSM-5 molecular sieve catalyst. The catalyst can be used as a deethylation type xylene isomerization top layer agent.

[0009] Furthermore, the molar ratio of the raw materials used in the preparation of ZSM-5 molecular sieve in step (1) is H2O: SiO2: Al2O3: Na2O = (10 ~ 12): 1: (0.033 ~ 0.045): (0.067 ~ 0.110).

[0010] Furthermore, in step (1), the silicon source is selected from one of silica sol, macroporous silica gel, and fumed silica, and the aluminum source is selected from one of sodium aluminate, pseudo-boehmite, and aluminum nitrate; preferably, the silicon source is silica sol, and the aluminum source is sodium aluminate.

[0011] Furthermore, the mass of the ZSM-5 molecular sieve seed crystals in step (1) is 2.0% to 5.0% of the total mass of the gel.

[0012] Furthermore, the temperature of the hydrothermal crystallization reaction of the molecular sieve in step (1) is 160°C to 190°C, and the synthesis time is 42 h to 72 h.

[0013] Furthermore, the concentration of the NH4Cl solution in step (3) is 1 mol / L.

[0014] Furthermore, the high temperature calcination temperature in step (3) is 450°C to 550°C, and the calcination time is 4 h to 10 h.

[0015] Furthermore, the concentration of the zinc nitrate solution in step (4) is 1.0 wt% to 9.0 wt%; the standing time at room temperature is 12 h, the drying time is 12 h, the high-temperature roasting temperature is 450°C to 550°C, and the roasting time is 4 h to 10 h.

[0016] Furthermore, the molar ratio of the raw materials for preparing the seed guide liquid in step (5) is SiO2: TPAOH: H2O = 1: (0.2 ~ 0.5): (10 ~ 25).

[0017] Furthermore, in step (5), when preparing the seed guiding liquid, the stirring time at room temperature is 3 h, the hydrothermal crystallization temperature is 80°C ~ 130°C, and the synthesis time is 12 h ~ 48 h; after the kettle is taken out and quenched, no treatment is required and the suspension is directly stored for use as the seed guiding liquid.

[0018] Furthermore, the mass of the HZSM-5 molecular sieve added in step (5) is 10 wt% to 30 wt% of the prepared seed guide liquid.

[0019] Furthermore, in step (5), when preparing Silicalite-1@ZSM-5, the stirring time at room temperature is 1 h, the product is dried at room temperature, filtered and washed, and then calcined at a high temperature of 450°C to 550°C for 4 h to 10 h.

[0020] Furthermore, the average particle size of the prepared molecular sieve catalyst crystals is 1 nm to 1.5 nm, and the total specific surface area is 300 to 370 m 2 / g, and the micropore volume is 0.10 ~ 0.15 cm 3 / g.

[0021] The present invention provides the use of the low external surface acidity ZSM-5 molecular sieve catalyst in the upper layer reaction of deethylation type xylene isomerization.

[0022] The ZSM-5 molecular sieve catalyst prepared by the present invention has highly dispersed Zn species and extremely low external surface activity, and exhibits excellent catalytic activity in the upper layer reaction of deethylated xylene isomerization.

[0023] In the above application, the deethylated xylene isomerization upper layer reaction is carried out under hydrogen conditions, with a reaction pressure of 0.5 MPa to 1.5 MPa, a reaction temperature of 350°C to 500°C, and a mass space velocity of 5 h -1 ~ 50 hours -1 .

[0024] Beneficial effects of the present invention: (1) The present invention significantly inhibits the formation of impurity crystals and maintains the uniformity of the framework aluminum, the acidity of the molecular sieve and the crystallinity of the molecular sieve by reducing the alkali-silicon ratio to below 0.110; the hydroxyl density in the pores is increased, providing highly active anchoring sites for subsequent metal loading, which is of great significance in industrial production; (2) The present invention uses an equal volume impregnation method to load 0 ~ 7 wt% Zn species, using Zn 2+ The metal (ZnO cluster)-acid (Brønsted acid) synergistic active center is constructed by combining strong acid sites, which shows excellent catalytic activity in the upper layer reaction of deethylation-type xylene isomerization and has high industrial application value; (3) The present invention adopts a seed-guided liquid-assisted method to coat Silicalite-1 on the surface of the molecular sieve, which deactivates the activity of the catalyst outer surface while reducing pore blockage, and significantly improves the catalytic performance of the upper layer reaction of deethylated xylene isomerization. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The X-ray diffraction (XRD) patterns of the samples synthesized in Examples 1-2 and Comparative Example 1 are shown; Figure 2 The following are scanning electron microscope (SEM) images of the samples synthesized in Examples 1-2 and Comparative Example 1; Figure 3 The XRD patterns of the ZSM-5 molecular sieves synthesized in Examples 3 to 5 are shown below: Figure 4 The SEM images of the ZSM-5 molecular sieves synthesized in Examples 3 to 5 are shown; Figure 5 This is the XRD pattern of the ZSM-5 molecular sieve synthesized in Comparative Example 2; Figure 6 This is the SEM image of the ZSM-5 molecular sieve synthesized in Comparative Example 2. DETAILED DESCRIPTION

[0026] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention will be further described in detail with reference to the following embodiments and accompanying drawings. It should be noted that the present invention is not limited to the examples listed, but also includes any other known changes within the scope of the rights claimed by the present invention.

[0027] In the examples, if no specific techniques and conditions are specified, the techniques described in the literature in this field or the product instructions were followed. Unless otherwise specified, the reagents and instruments used in the examples were commercially available or prepared by known methods. Example 1

[0028] Preparation of HZSM-5 molecular sieve 0.33 g of sodium hydroxide (96 wt%) was dissolved in 24.14 g of deionized water, followed by the addition of 1.58 g of sodium metaaluminate (41 wt% as alumina). The mixture was stirred for 5 minutes until clear, then 38 g of silica sol (30 wt%, SiO₂ mass fraction) was slowly added, followed by 0.3 g of seed crystals. The mixture was aged at room temperature for 45 minutes to obtain an initial gel. The initial gel was placed in a polytetrafluoroethylene crystallization reactor and crystallized at 170°C for 48 hours. After the reaction, the mixture was quenched, washed with deionized water until neutral, and dried at 100°C to obtain Na-type ZSM-5 molecular sieve. The synthesized Na-type ZSM-5 was used as a support for exchange in a 90°C NH₄Cl solution and washed with water multiple times until neutral. After drying at 100°C, the mixture was calcined in a muffle furnace at 550°C for 6 hours to obtain HZSM-5, designated Z-1. Example 2

[0029] Preparation of HZSM-5 molecular sieve 0.16 g of sodium hydroxide (96 wt%) was dissolved in 24.14 g of deionized water, followed by the addition of 1.58 g of sodium metaaluminate (41 wt% as alumina). The mixture was stirred for 5 minutes until clear, then 38 g of silica sol (30 wt%, SiO₂ mass fraction) was slowly added, followed by 0.3 g of seed crystals. The mixture was aged at room temperature for 45 minutes to obtain an initial gel. The initial gel was placed in a polytetrafluoroethylene crystallization reactor and crystallized at 170°C for 48 hours. After the reaction, the mixture was quenched, washed with deionized water until neutral, and dried at 100°C to obtain Na-type ZSM-5 molecular sieve. The synthesized Na-type ZSM-5 was used as a support for exchange in a 90°C NH₄Cl solution and washed with water multiple times until neutral. After drying at 100°C, the mixture was calcined in a muffle furnace at 550°C for 6 hours to produce HZSM-5, designated Z-2. Example 3

[0030] Zn-loaded seed-guided liquid coating of ZSM-5 molecular sieve 0.16 g of sodium hydroxide (96 wt%) was dissolved in 24.14 g of deionized water, followed by the addition of 1.58 g of sodium metaaluminate (41 wt% as alumina). The mixture was stirred for 5 minutes until clear, then 38 g of silica sol (30 wt%, SiO₂ mass fraction) was slowly added, followed by 0.3 g of seed crystals. The mixture was aged at room temperature for 45 minutes to obtain an initial gel. The initial gel was placed in a polytetrafluoroethylene crystallization reactor and crystallized at 170°C for 48 hours. After the reaction, the mixture was quenched, washed with deionized water until neutral, and dried at 100°C to obtain Na-type ZSM-5 molecular sieve. The synthesized Na-type ZSM-5 was used as a support for exchange in a 90°C NH₄Cl solution and washed with water multiple times until neutral. After drying at 100°C, the mixture was calcined at 550°C in a muffle furnace for 6 hours to produce HZSM-5. The ion-exchanged and calcined HZSM-5 molecular sieve was loaded with 1.0 wt% Zn (zinc nitrate as the zinc source) via an equal volume impregnation method. The mixture was allowed to stand for 12 hours, dried at 100°C, and calcined at 550°C for 6 hours. A solution was then prepared by mixing silica sol, TPAOH, and deionized water in a molar ratio of SiO₂:TPAOH:H₂O = 1:0.3:20. The solution was stirred for 3 hours and then loaded into a polytetrafluoroethylene crystallization reactor. Crystallization was carried out at 100°C for 24 hours. The reactor was then quenched and the resulting suspension was directly used as a seeding liquid. Subsequently, 25 wt% of the prepared HZSM-5 molecular sieve was added to the seeding liquid and stirred for 1 hour. The mixture was dried at room temperature, filtered, washed, and calcined at 550°C in a muffle furnace for 6 hours to obtain a deethylated xylene isomerization top layer agent, designated as Zn-1. Example 4

[0031] Zn-loaded seed-guided liquid coating of ZSM-5 molecular sieve 0.16 g of sodium hydroxide (96 wt%) was dissolved in 24.14 g of deionized water, followed by the addition of 1.58 g of sodium metaaluminate (41 wt% as alumina). The mixture was stirred for 5 minutes until clear, then 38 g of silica sol (30 wt%, SiO₂ mass fraction) was slowly added, followed by 0.3 g of seed crystals. The mixture was aged at room temperature for 45 minutes to obtain an initial gel. The initial gel was placed in a polytetrafluoroethylene crystallization reactor and crystallized at 170°C for 48 hours. After the reaction, the mixture was quenched, washed with deionized water until neutral, and dried at 100°C to obtain Na-type ZSM-5 molecular sieve. The synthesized Na-type ZSM-5 was used as a support for exchange in a 90°C NH₄Cl solution and washed with water multiple times until neutral. After drying at 100°C, the mixture was calcined at 550°C in a muffle furnace for 6 hours to produce HZSM-5. The ion-exchanged and calcined HZSM-5 molecular sieve was loaded with 3.0 wt% Zn (zinc nitrate as the zinc source) via an equal volume impregnation method. The mixture was allowed to stand for 12 hours, dried at 100°C, and calcined at 550°C for 6 hours. A solution was then prepared by mixing silica sol, TPAOH, and deionized water in a molar ratio of SiO2:TPAOH:H2O = 1:0.3:20. The mixture was stirred for 3 hours and then placed in a polytetrafluoroethylene crystallization reactor. Crystallization was carried out at 100°C for 24 hours. The reactor was then quenched and the resulting suspension was directly used as a seeding liquid. Subsequently, 25 wt% of the prepared molecular sieve was added to the seeding liquid and stirred for 1 hour. The mixture was dried at room temperature, filtered, washed, and calcined at 550°C in a muffle furnace for 6 hours to obtain a deethylated xylene isomerization top layer agent, designated as Zn-2. Example 5

[0032] Zn-loaded seed-guided liquid coating of ZSM-5 molecular sieve 0.16 g of sodium hydroxide (96 wt%) was dissolved in 24.14 g of deionized water, followed by the addition of 1.58 g of sodium metaaluminate (41 wt% as alumina). The mixture was stirred for 5 minutes until clear, then 38 g of silica sol (30 wt%, SiO₂ mass fraction) was slowly added, followed by 0.3 g of seed crystals. The mixture was aged at room temperature for 45 minutes to obtain an initial gel. The initial gel was placed in a polytetrafluoroethylene crystallization reactor and crystallized at 170°C for 48 hours. After the reaction, the mixture was quenched, washed with deionized water until neutral, and dried at 100°C to obtain Na-type ZSM-5 molecular sieve. The synthesized Na-type ZSM-5 was used as a support for exchange in a 90°C NH₄Cl solution and washed with water multiple times until neutral. After drying at 100°C, the mixture was calcined at 550°C in a muffle furnace for 6 hours to produce HZSM-5. The ion-exchanged and calcined HZSM-5 molecular sieve was loaded with 5.0 wt% Zn (zinc nitrate as the zinc source) via an equal volume impregnation method. The mixture was allowed to stand for 12 hours, dried at 100°C, and calcined at 550°C for 6 hours. A solution was then prepared by mixing silica sol, TPAOH, and deionized water in a molar ratio of SiO2:TPAOH:H2O = 1:0.3:20. The mixture was stirred for 3 hours and then placed in a polytetrafluoroethylene crystallization reactor. Crystallization was carried out at 100°C for 24 hours. The reactor was then quenched and the resulting suspension was directly used as a seeding liquid. Subsequently, 25 wt% of the prepared molecular sieve was added to the seeding liquid and stirred for 1 hour. The mixture was dried at room temperature, filtered, washed, and calcined at 550°C in a muffle furnace for 6 hours to obtain a deethylated xylene isomerization top layer agent, designated as Zn-3.

[0033] Comparative Example 1 Preparation of HZSM-5 molecular sieve under NaOH-free conditions 1.58 g of sodium metaaluminate (41 wt% as alumina) was dissolved in 24.14 g of deionized water and stirred for 5 minutes until clear. Then, 38 g of silica sol (30 wt% as SiO₂) was slowly added, followed by 0.3 g of seed crystals. The mixture was aged at room temperature for 45 minutes to obtain an initial gel. The initial gel was placed in a polytetrafluoroethylene crystallization reactor and crystallized at 170°C for 48 hours. After the reaction, it was quenched, washed with deionized water until neutral, and dried at 100°C to obtain the sample. The prepared sample was used as a support for exchange in a 90°C NH₄Cl solution and washed with water multiple times until neutral. After drying at 100°C, the sample was calcined at 550°C in a muffle furnace for 6 hours and designated Z-3.

[0034] Comparative Example 2 ZSM-5 molecular sieve coated with ethyl orthosilicate without Zn loading 0.16 g of sodium hydroxide (96 wt%) was dissolved in 24.14 g of deionized water, followed by the addition of 1.58 g of sodium metaaluminate (41 wt% as alumina). The mixture was stirred for 5 minutes until clear, then 38 g of silica sol (30 wt%, SiO₂ mass fraction) was slowly added, followed by 0.3 g of seed crystals. The mixture was aged at room temperature for 45 minutes to obtain an initial gel. The initial gel was placed in a polytetrafluoroethylene crystallization reactor and crystallized at 170°C for 48 hours. After the reaction, the mixture was quenched, washed with deionized water until neutral, and dried at 100°C to obtain Na-type ZSM-5 molecular sieve. The synthesized Na-type ZSM-5 was used as a support for exchange in a 90°C NH₄Cl solution and washed with water multiple times until neutral. After drying at 100°C, the mixture was calcined at 550°C in a muffle furnace for 6 hours to produce HZSM-5. Subsequently, a solution of ethyl orthosilicate and cyclohexane was prepared in a mass ratio of 1:5, and molecular sieves were added to the prepared solution and stirred evenly. After drying at 100 °C, the solution was placed in a muffle furnace and calcined at 450 °C for 6 h to obtain a deethylated xylene isomerization upper layer agent, which was recorded as Z-4.

[0035] Figure 1 X-ray diffraction (XRD) patterns of the ZSM-5 molecular sieves synthesized in Examples 1 and 2 and Comparative Example 1 show that the crystallinity of the prepared molecular sieves decreases with decreasing alkali-silicon ratio. Highly crystalline ZSM-5 molecular sieves were successfully prepared when NaOH addition levels were 0.33 g and 0.16 g, whereas no ZSM-5 molecular sieve was synthesized when NaOH addition was 0 g.

[0036] Figure 2 Scanning electron microscopy (SEM) images of the ZSM-5 molecular sieves synthesized in Examples 1 and 2 and Comparative Example 1 show that ZSM-5 molecular sieves can be prepared when NaOH addition amounts of 0.33 g and 0.16 g are both used, while the sample synthesized with 0 g of NaOH addition is amorphous. This conclusion is consistent with the XRD pattern analysis.

[0037] Figure 3 The XRD patterns of the ZSM-5 molecular sieves synthesized in Examples 3-5 are shown. All three curves exhibit strong crystal diffraction peaks, demonstrating that the ZSM-5 molecular sieves were successfully prepared, and that the shell structure formed by the seed-guide solution also exhibited a high-quality ZSM-5 molecular sieve crystalline form.

[0038] Figure 4 The SEM images of the ZSM-5 molecular sieves synthesized in Examples 3 to 5 are shown. As can be seen from the images, the shell structures formed by the seed guiding liquid are successfully coated on the surface of the ZSM-5 molecular sieve.

[0039] Figure 5This is the XRD pattern of the ZSM-5 molecular sieve synthesized in Comparative Example 2; it can be seen from the figure that the synthesized sample has a strong crystal diffraction peak, indicating that the ZSM-5 molecular sieve can be successfully synthesized.

[0040] Figure 6 This is a SEM image of the ZSM-5 molecular sieve synthesized in Comparative Example 2; it can be seen from the figure that ethyl orthosilicate is successfully coated on the surface of the ZSM-5 molecular sieve.

[0041] Application Example 1 In order to investigate the catalytic performance of the molecular sieve prepared by the present invention, the ZSM-5 molecular sieve catalyst synthesized in all the examples and comparative examples was selected for the upper layer reaction of deethylated xylene isomerization to compare their catalytic performance. The specific steps are as follows: (1) Catalytic performance evaluation: 1 g of catalyst was ground and sieved (20-40 mesh) and loaded into the middle section of the reaction tube. Quartz sand was filled into the upper and lower sections. The catalytic reaction was carried out at a temperature of 450 °C and a H2 flow rate of 15 ml / min. The raw materials (including 93% by mass of m-xylene and 7% by mass of ethylbenzene) were introduced into the fixed-bed reactor through a micropump. The reaction pressure was 0.8 MPa and the mass space velocity was 25 h / min. -1 .

[0042] (2) The products were analyzed using a GC-950 chromatograph equipped with a flame ionization detector (FID) and a Zhongke Antai PEG-20M (50 m × 0.32 mm × 0.5 μm) column. Finally, the catalytic performance results were calculated based on the chromatographic data.

[0043] Table 1 Catalytic performance of all catalysts in Application Example 1

[0044] Table 1 shows the catalytic performance of all catalysts in Application Example 1. The catalytic results show that after 48 h of reaction under the conditions described in Application Example 1, the deethylated xylene isomerization top layer agent prepared in Example 4 (sample Zn-2 coated with seed guide liquid and loaded with 3.0 wt% Zn) exhibited higher catalytic activity than other catalysts: the ethylbenzene conversion rate was approximately 97.1%, the xylene yield was approximately 99.7%, and the isomerization activity was approximately 1.5%.

[0045] In summary, ZSM-5 molecular sieve was not synthesized in Comparative Example 1 and had no catalytic performance. The lack of Zn loading in Comparative Example 2 resulted in low acidity inside the catalyst and low ethylbenzene conversion; and the ethyl orthosilicate coating effect in Comparative Example 2 was not as good as the seed guiding liquid of the present invention, so the outer surface acidity of the catalyst of Comparative Example 2 was strong, resulting in increased side reactions. From the effects of the embodiments of the present invention, it can be seen that the seed guiding liquid coating can effectively reduce the acidity of the outer surface of the catalyst, reduce the isomerization activity of the side reaction meta-xylene, and improve the xylene yield; when the Zn loading is 3.0%, the catalyst has strong acidity inside, and Zn can be evenly distributed inside the catalyst without gathering to form clusters, which greatly enhances the conversion of ethylbenzene inside the catalyst.

[0046] The above content is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific embodiments of the present invention should not be considered to be limited to these. For those skilled in the art of the art to which the present invention belongs, they can make a number of simple deductions, substitutions, modifications or changes without departing from the principles and spirit of the present invention. Such changes should be deemed to fall within the scope of patent protection determined by the claims submitted by the present invention.

Claims

1. A method for preparing a low external surface acidic ZSM-5 molecular sieve catalyst, characterized in that The following steps are involved: (1) Sodium hydroxide, an aluminum source, and a silicon source are sequentially added to deionized water to prepare a gel, and the alkali-silicon ratio is controlled to be below 0.110; Then, ZSM-5 molecular sieve seed crystals were added, and the prepared gel was subjected to a hydrothermal crystallization reaction to prepare Na-type ZSM-5 molecular sieve; (2) filtering and washing the Na-type ZSM-5 molecular sieve so that its pH value remains neutral at room temperature; (3) using NH4Cl solution to perform ion exchange on the synthesized Na-type ZSM-5 and calcining at high temperature to obtain HZSM-5 molecular sieve; (4) Immersing an equal volume of the HZSM-5 molecular sieve prepared above in a zinc nitrate solution, allowing it to stand and dry, and then calcining it at a high temperature; (5) using silica sol, TPAOH and deionized water to prepare a seed guiding liquid; adding the HZSM-5 molecular sieve prepared in step (4) to the seed guiding liquid, drying, filtering and washing, and then calcining at high temperature to obtain a low external surface acid Silicalite-1@ZSM-5 molecular sieve catalyst.

2. The method for preparing a low external surface acidic ZSM-5 molecular sieve catalyst according to claim 1, wherein: The molar ratio of the raw materials used in the preparation of ZSM-5 molecular sieve in step (1) is H2O: SiO2: Al2O3: Na2O = (10 ~ 12): 1: (0.033 ~ 0.045): (0.067 ~ 0.110); the silicon source is selected from one of silica sol, coarse-pore silica gel and fumed silica, and the aluminum source is selected from one of sodium metaaluminate, pseudo-boehmite and aluminum nitrate.

3. The method for preparing a low external surface acidic ZSM-5 molecular sieve catalyst according to claim 1, wherein: The mass of the ZSM-5 molecular sieve seed crystals in step (1) is 2.0% to 5.0% of the total mass of the gel; the temperature of the hydrothermal crystallization reaction of the molecular sieve is 160°C to 190°C, and the synthesis time is 42 h to 72 h.

4. The method for preparing a low external surface acidic ZSM-5 molecular sieve catalyst according to claim 1, wherein: The concentration of the NH4Cl solution in step (3) is 1 mol / L; the high-temperature calcination temperature is 450°C to 550°C, and the calcination time is 4 h to 10 h.

5. The method for preparing a low external surface acidic ZSM-5 molecular sieve catalyst according to claim 1, wherein: The concentration of the zinc nitrate solution in step (4) is 1.0 wt% to 9.0 wt%; the standing time at room temperature is 12 h, the drying time is 12 h, the high-temperature roasting temperature is 450°C to 550°C, and the roasting time is 4 h to 10 h.

6. The method for preparing a low external surface acidic ZSM-5 molecular sieve catalyst according to claim 1, characterized in that: The molar ratio of the raw materials for preparing the seed crystal guide liquid in step (5) is SiO2: TPAOH: H2O = 1: (0.2 ~ 0.5): (10 ~ 25); When preparing the seed crystal guide liquid, the stirring time at room temperature is 3 h, the hydrothermal crystallization temperature is 80 ℃ ~ 130 ℃, and the synthesis time is 12 h ~ 48 h; after the kettle is taken out and quenched, no treatment is required and the suspension is directly stored for use as the seed crystal guide liquid.

7. The method for preparing a low external surface acidic ZSM-5 molecular sieve catalyst according to claim 1, wherein: The mass of HZSM-5 molecular sieve added in step (5) is 10 wt% to 30 wt% of the prepared seed guide solution; when preparing Silicalite-1@ZSM-5, the stirring time at room temperature is 1 h, and it is dried at room temperature, filtered and washed, and then calcined at a high temperature of 450°C to 550°C for 4 h to 10 h.

8. A low external surface acidic ZSM-5 molecular sieve catalyst prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The average particle size of the prepared molecular sieve catalyst crystals is 1 nm ~ 1.5 nm, and the total specific surface area is 300 ~ 370 m 2 / g, and the micropore volume is 0.10 ~ 0.15 cm 3 / g.

9. Use of the low external surface acidic ZSM-5 molecular sieve catalyst according to claim 8 in the upper layer reaction of deethylated xylene isomerization.

10. The use according to claim 9, characterized in that: The deethylated xylene isomerization upper layer reaction is carried out under hydrogen conditions, with a reaction pressure of 0.5 MPa to 1.5 MPa, a reaction temperature of 350°C to 500°C, and a mass space velocity of 5 h -1 ~ 50 hours -1 .

Images