A Silicalite-1@ZSM-5 core-shell molecular sieve, its preparation method and application

Abstract

A Silicalite-1@ZSM-5 core-shell molecular sieve, with Silicalite-1 molecular sieve as the core layer and ZSM-5 molecular sieve as the shell layer, has a particle size of 90-140 nm. Its preparation method includes the following steps: (1) mixing a template agent, a first silicon source, and water to obtain a core-layer molecular sieve synthesis system, followed by aging I and crystallization I to obtain an intermediate product; (2) mixing the intermediate product obtained in step (1), an aluminum source, a second silicon source, an alkali source, an anion-cation balancer, and water to obtain a shell-layer molecular sieve synthesis system, followed by aging II and crystallization II. The resulting solid is then washed, dried, and calcined to obtain the Silicalite-1@ZSM-5 core-shell molecular sieve. This molecular sieve has strong carbon-holding capacity, a simple preparation method, and low cost. It can be used for catalytic cracking of low-carbon alkanes, lightening of heavy aromatics, and aromatization of light hydrocarbons.

Description

Technical Field

[0001] This invention relates to a core-shell molecular sieve, its preparation method, and its application; specifically, it relates to a Silicalite-1@ZSM-5 core-shell molecular sieve, its preparation method, and its application. Background Technology

[0002] Both ZSM-5 and Silicalite-1 molecular sieves possess a two-dimensional cross-linked ten-membered ring pore structure, while ZSM-5 exhibits an MFI-type pore structure. The acidity of ZSM-5 molecular sieves is highly dependent on the Al content in its framework, and its Si / Al ratio can be flexibly adjusted according to the catalytic reaction requirements. Due to its unique acidic sites and pore structure, it is widely used in a series of reactions involving the conversion of light hydrocarbons and is currently one of the important catalytic materials. However, in reactions such as light hydrocarbon aromatization and catalytic cracking, ZSM-5 molecular sieves exhibit high acid density, easily leading to carbon buildup and clogging of internal pores. Therefore, many studies have optimized the synthesis of ZSM-5 molecular sieves by focusing on grain size, Si / Al ratio, and pore structure. Currently, however, all synthesized ZSM-5@Silicalite-1 core-shell molecular sieves are produced. The synthesis process is relatively complex, the feed ratio is fixed, single-reactor yield is low, the template agent is difficult to recover, and the raw materials are expensive, making industrial production impractical.

[0003] CN113753913A discloses a ZSM-5@Silicalite-1 type core-shell molecular sieve, its preparation method, and its application. The method includes at least: (1) crystallizing a raw material containing an aluminum source, a silicon source, a modifier, and Silicalite-1 seed crystals to obtain an intermediate product; (2) adding a template agent to the intermediate product and crystallizing it to obtain the ZSM-5@Silicalite-1 type core-shell molecular sieve. In step (1), the modifier is selected from at least one of an acid source and an alkaline source. This method does not require the addition of ZSM-5 crystal nuclei and uses inexpensive silica sol as a silicon source. ZSM-5@S-1 core-shell materials can be prepared in a one-pot process. At the same time, this method can greatly reduce the H2O / SiO2 ratio to below 40, increase the yield per batch, and obtain regular, uniform, and monodisperse particles with a grain size of 0.2 to 5 μm.

[0004] CN112705248A discloses a core-shell type MFI / MFI molecular sieve, comprising an MFI structure molecular sieve, a layered silicate material, a template agent R, and optional reaction products of alkali and water. This molecular sieve exhibits good molecular shape selectivity in catalytic reactions and can be used in catalytic reactions requiring high reaction selectivity, such as in the production of p-xylene and ethylbenzene, with high selectivity and conversion rate.

[0005] CN104556131A employs microwave heating, taking advantage of the rapid, gradient-free heating characteristic of microwaves. Through microwave vibration, SiO2 in the shell mother liquor is induced onto the core phase molecular sieve, leading to rapid nucleation and growth, thus synthesizing a core-shell molecular sieve. The microwave heating technology can obtain ZSM-5 / Silicalite-1 type core-shell molecular sieves in a relatively short time.

[0006] The ZSM-5@Silicalite-1 core-shell molecular sieve synthesized by the above synthesis method uses aluminum-rich ZSM-5 molecular sieve as the core layer and grows Silicalite-1 on the outer layer. The abundant Brønsted acid sites inside easily cause catalyst coking and deactivation, resulting in weak carbon-holding capacity. In addition, the preparation process has problems such as strict selection of raw materials, non-recyclable template agent, high cost, and microwave heating and other methods not having prospects for industrial scale-up. Summary of the Invention

[0007] The purpose of this invention is to provide a Silicalite-1@ZSM-5 core-shell molecular sieve based on existing technology. This core-shell molecular sieve has a Silicalite-1 molecular sieve as the core layer and a ZSM-5 molecular sieve as the shell layer. It has strong carbon holding capacity, a simple preparation method, and low cost. It can be used for catalytic cracking of low-carbon alkanes, lightening of heavy aromatics, and aromatization of light hydrocarbons.

[0008] The Silicalite-1@ZSM-5 core-shell molecular sieve provided by this invention has Silicalite-1 molecular sieve as the core layer and ZSM-5 molecular sieve as the shell layer, with a particle size of 90-140 nm.

[0009] This invention also provides a method for preparing Silicalite-1@ZSM-5 core-shell molecular sieves, comprising the following steps:

[0010] (1) The template agent, the first silicon source and water were mixed to obtain the core layer molecular sieve synthesis system, and the intermediate product was obtained by aging I and crystallization I.

[0011] (2) The intermediate product obtained in step (1), aluminum source, second silicon source, alkali source, anion and cation balancer and water are mixed to obtain a shell molecular sieve synthesis system. After aging II and crystallization II, the obtained solid is washed, dried and calcined to obtain the Sililcite-1@ZSM-5 core-shell molecular sieve.

[0012] This invention also provides an application of Silicalite-1@ZSM-5 core-shell molecular sieve in the catalytic cracking reaction of low-carbon alkane, the lightening reaction of heavy aromatics, and the aromatization reaction of light hydrocarbons.

[0013] The Silicalite-1@ZSM-5 core-shell molecular sieve provided by this invention is synthesized using a two-step crystallization method. First, an intermediate product is obtained from an all-silica molecular sieve synthesis system. Then, the intermediate product is added to a shell molecular sieve synthesis system. No template agent or seed crystals need to be added to the shell molecular sieve synthesis system. The Silicalite-1@ZSM-5 core-shell molecular sieve can be obtained by adding an anion-cation balancer and adjusting the feed ratio of the synthesis system. This invention uses inexpensive silica sol as the silicon source, has a simple preparation process, and low cost. Uniform small-particle-size core-shell molecular sieves can be obtained through a simple hydrothermal method. Attached Figure Description

[0014] Figure 1 X-ray diffraction pattern of the hydrogen-form Silicalite-1@ZSM-5 core-shell molecular sieve sample prepared in Example 1;

[0015] Figures 2-1 to 2-5 Scanning electron microscope (SEM) images of hydrogen-form Silicalite-1@ZSM-5 core-shell molecular sieve samples prepared in Example 1 and Comparative Examples 1-4, respectively.

[0016] Figures 3-1 to 3-7 Transmission electron microscopy images of the hydrogen-form Silicalite-1@ZSM-5 core-shell molecular sieve samples prepared in Examples 1 to 7, respectively.

[0017] Figure 4 Enlarged scanning electron microscope image of the hydrogen-form Silicalite-1@ZSM-5 core-shell molecular sieve sample prepared in Example 1;

[0018] Figure 5-1 The image shows the X-ray diffraction pattern of the intermediate product M obtained in Example 8. Figure 5-2 Here is a scanning electron microscope image of it. Detailed Implementation

[0019] The Silicalite-1@ZSM-5 core-shell molecular sieve provided by this invention uses Silicalite-1 molecular sieve as the core layer and ZSM-5 molecular sieve as the shell layer. Epitaxial growth of ZSM-5 molecular sieve on Silicalite-1 molecular sieve can effectively solve the problem of poor product desorption caused by blockage inside the molecular sieve pores. Using Silicalite-1 as the core layer can reduce the Brønsted acid content inside the core-shell molecular sieve, thereby inhibiting the strong cracking reaction of reactants inside the molecular sieve, and ensuring that the cracking reaction occurs on the ZSM-5 molecular sieve in the shell layer, thereby reducing the overall carbon deposition of the molecular sieve.

[0020] The Silicalite-1@ZSM-5 core-shell molecular sieve provided by this invention has small crystal size, certain microporous structure and suitable specific surface area and pore volume. It can be used for catalytic cracking of low-carbon alkane, lightening of heavy aromatics, and aromatization of light hydrocarbons. It has high reactivity and low selectivity for low-carbon hydrocarbons, and low carbon deposition.

[0021] The Silicalite-1@ZSM-5 core-shell molecular sieve provided by this invention has a Silicalite-1 molecular sieve as the core layer and a ZSM-5 molecular sieve as the shell layer, with a particle size of 90-140 nm, preferably 100-130 nm; the Silicalite-1 molecular sieve has a particle size of 40-80 nm, preferably 50-70 nm, and the ZSM-5 shell layer thickness is 30-70 nm, preferably 45-60 nm; the ZSM-5 molecular sieve has a silica-alumina ratio of 20-200, preferably 40-80; and the Silicalite-1@ZSM-5 core-shell molecular sieve has a total specific surface area of ​​300-470 m². 2 / g, preferably 420-450m 2 / g, with a microporous specific surface area of ​​260–390m² 2 / g, preferably 350-380m 2 / g, total pore volume is 0.20~0.60cm³ 3 / g, preferably 0.30~0.55cm 3 / g, micropore volume is 0.10~0.30cm³ 3 / g, preferably 0.16~0.28cm 3 / g.

[0022] The method for preparing Silicalite-1@ZSM-5 core-shell molecular sieve provided by this invention includes the following steps:

[0023] (1) The template agent, the first silicon source and water were mixed to obtain the core layer molecular sieve synthesis system, and the intermediate product was obtained by aging I and crystallization I.

[0024] (2) The intermediate product obtained in step (1), aluminum source, second silicon source, alkali source, anion and cation balancer and water are mixed to obtain a shell molecular sieve synthesis system. After aging II and crystallization II, the obtained solid is washed, dried and calcined to obtain the Sililcite-1@ZSM-5 core-shell molecular sieve.

[0025] Preferably, a hydrolysis promoter can be added to the core-layer molecular sieve synthesis system described in step (1). The hydrolysis promoter is an alcohol and / or a weak base, more preferably at least one of ethylene glycol, methanol, ethanol, and ammonia. The first silicon source is calculated as SiO2, the mass ratio of the first silicon source to the hydrolysis promoter is 1-5, the mass ratio of the first silicon source to the template agent is 0.1-5, and the mass ratio of the first silicon source to water is 1-10. Optionally, the upper limit of the mass ratio of the first silicon source to the hydrolysis promoter is independently selected from 5.0, 4.0, 3.0, and 2.0, and the lower limit is independently selected from 1.0, 4.0, 3.0, and 2.0. Optionally, the upper limit of the mass ratio of the first silicon source to the template agent is independently selected from 1.2, 1, 0.8, 0.6, and 0.5, and the lower limit is independently selected from 1, 0.8, 0.6, 0.5, and 0.1.

[0026] Preferably, step (1) includes a step of recovering the template agent by distillation or centrifugation after crystallization I. When recovering the template agent by distillation, the temperature is 60-120°C, preferably 80-100°C, and the distillation time is 0.5-6 hours, preferably 2-4 hours.

[0027] Preferably, in step (2), the cation-anion balancer is one of a strong acid-weak base salt, a strong base-weak acid salt, a strong acid-strong base salt, or a weak acid-weak base salt, and more preferably at least one of sodium acetate, sodium chloride, ammonium acetate, and ammonium chloride.

[0028] Preferably, the first silicon source and the second silicon source are each independently selected from at least one of tetraethyl silicate, silica sol, water glass, fumed silica, and silica gel powder; the aluminum source is at least one of aluminum sulfate, sodium aluminate, aluminum nitrate, and aluminum chloride; the alkali source is at least one of sodium hydroxide, potassium hydroxide, and ammonia water; and the template agent is at least one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, n-butylamine, tetrapropylammonium bromide, and 1,6-hexamethylenediamine.

[0029] Preferably, in step (2), the second silicon source is SiO2, the aluminum source is Al2O3, and the molar ratio of the second silicon source to the aluminum source is 20 to 200. Optionally, the upper limit of the molar ratio of the second silicon source to the aluminum source is independently selected from 200, 170, 100, 80, and 50, and the lower limit is independently selected from 40, 50, 80, 100, and 170.

[0030] Preferably, in step (2), the aluminum source is calculated as Al2O3, the alkali source is calculated as NaOH, and the mass ratio of the intermediate product: aluminum source: alkali source: anion and cation balancer: water is (50-300): 1: (1-10): (1-30): (50-200).

[0031] Preferably, the aging process I is carried out at a temperature of 10–50°C, preferably 25–35°C, for a time of 0.5–12 h, preferably 4–6 h; the crystallization process I is carried out at a temperature of 80–250°C, preferably 100–150°C, for a time of 18–72 h, preferably 24–48 h; the aging process II is carried out at a temperature of 10–50°C, preferably 25–35°C, for a time of 0.5–12 h, preferably 4–6 h; the crystallization process II is carried out at a temperature of 80–250°C, preferably 150–200°C, for a time of 18–72 h, preferably 24–48 h.

[0032] Optionally, the upper limit of the crystallization temperature I is independently selected from 150℃, 140℃, 130℃, and 120℃, and the lower limit is independently selected from 100℃, 120℃, 130℃, and 140℃. The upper limit of the crystallization time I is independently selected from 48h, 36h, and 24h, and the lower limit is independently selected from 12h, 24h, and 36h. Optionally, the upper limit of the crystallization temperature II is independently selected from 200℃, 180℃, 160℃, 140℃, and 120℃, and the lower limit is independently selected from 100℃, 120℃, 140℃, 160℃, and 180℃. The upper limit of the crystallization time II is independently selected from 36h, 24h, and 12h, and the lower limit is independently selected from 6h, 12h, and 24h.

[0033] Preferably, the preparation method of the molecular sieve of the present invention further includes step (3), in which the Silicalite-1@ZSM-5 core-shell molecular sieve obtained in step (2) is subjected to ion exchange, washing, and calcination in a solution containing ammonium ions to obtain the hydrogen form Silicalite-1@ZSM-5 core-shell molecular sieve, which can then be applied to different catalytic reaction processes. Ion exchange and subsequent washing, drying, and calcination are conventional operations in the art and will not be described in detail here. The Silicalite-1@ZSM-5 core-shell molecular sieve prepared by the present invention can be used in the catalytic cracking reaction of low-carbon alkanes, the lightening reaction of heavy aromatics, and the aromatization reaction of light hydrocarbons, but is not limited to the above-mentioned reaction processes.

[0034] The present invention will now be described in detail with reference to the embodiments, but the present invention is not limited to these embodiments.

[0035] The raw materials and reagents used in the embodiments of this invention were all purchased commercially. The analytical methods used in the embodiments of this invention are as follows: X-ray diffraction pattern analysis was performed using the XRDWUX70 method. Nitrogen adsorption-desorption analysis was performed using the PABETALL method. SEM morphology analysis was performed using the SEMMORPHO method.

[0036] Example 1

[0037] Step (1): Under stirring conditions, tetraethyl silicate, tetrapropylammonium hydroxide, ethanol and deionized water were added to a reaction vessel to obtain a core layer molecular sieve synthesis system. The mass ratio of tetraethyl silicate (as SiO2) to tetrapropylammonium hydroxide was 1.1, the mass ratio of tetraethyl silicate (as SiO2) to ethanol was 4.6, and the mass ratio of tetraethyl silicate (as SiO2) to water was 1.8. After stirring evenly, the mixture was aged at 35°C for 6 hours and then transferred to a crystallization vessel for crystallization at 120°C for 48 hours. The crystallization reaction product was separated by centrifugation to obtain the recovered template agent and intermediate product.

[0038] Step (2): Add sodium aluminate, silica sol, sodium chloride, sodium hydroxide and deionized water to the intermediate product to obtain the shell molecular sieve synthesis system. The mass ratio of intermediate product: sodium aluminate (calculated as Al2O3): NaOH: sodium chloride: water is 108.3:1:4.17:4.1:118.3, and the molar ratio of silica sol (calculated as SiO2): sodium aluminate (calculated as Al2O3) is 40. After stirring evenly, age at 50℃ for 6h and then transfer to a crystallization kettle. Crystallize at 150℃ for 24h. Wash, dry and calcine the obtained solid to obtain Silicalite-1@ZSM-5 core-shell molecular sieve.

[0039] Step (3): Take 100g of the Silicalite-1@ZSM-5 core-shell molecular sieve obtained in step (2), perform ion exchange in 500ml of 1mol / L ammonium chloride solution, then wash, dry, and calcine to obtain the hydrogen form Silicalite-1@ZSM-5 core-shell molecular sieve. This is denoted as S1.

[0040] Figure 1 The powder X-ray diffraction pattern of S1 shows that it has a distinct MFI topology. Figure 2-1 The image is a scanning electron microscope image of S1, which has a regular morphology. Figure 3-1 The transmission electron microscope images show that the core-shell molecular sieve prepared in this invention has a distinct core-shell structure. Figure 4 The magnified transmission electron microscope image shows that it has a distinct core-shell structure, with a core particle size of 40-60 nm and a shell outer edge size of 20-40 nm.

[0041] Table 1 shows the characterization results of the S1 structure properties.

[0042] Example 2

[0043] Step (1): Under stirring conditions, tetraethyl silicate, n-butylamine, ethanol and deionized water were added to the reaction vessel to obtain the core layer molecular sieve synthesis system. The mass ratio of tetraethyl silicate (calculated as SiO2) to n-butylamine was 2.3, the mass ratio of tetraethyl silicate (calculated as SiO2) to ethanol was 3.5, and the mass ratio of tetraethyl silicate (calculated as SiO2) to water was 5.0. After stirring evenly, the mixture was aged at 35°C for 3 hours and then transferred to a crystallization vessel. The mixture was crystallized at 100°C for 48 hours. The crystallization reaction product was separated by distillation to obtain the recovered template agent and intermediate product. The distillation conditions were as follows: the intermediate product slurry was taken into a rotary evaporator and heated to 80°C at 2°C per minute at room temperature. The mixture was then rotary evaporated at 80°C for 4 hours. The rotation frequency of the rotary evaporator was 15 rpm.

[0044] Step (2): Add aluminum sulfate, silica, sodium chloride, sodium hydroxide and deionized water to the intermediate product to obtain the shell molecular sieve synthesis system. The mass ratio of intermediate product: aluminum sulfate (calculated as Al2O3): NaOH: sodium chloride: water is 216.7:1:8.31:8.37:328.9, and the molar ratio of silica (calculated as SiO2): aluminum sulfate (calculated as Al2O3) is 80. After stirring evenly, age at 35℃ for 3h and then transfer to a crystallization kettle. Crystallize at 150℃ for 36h. Wash, dry and calcine the obtained solid to obtain Silicalite-1@ZSM-5 core-shell molecular sieve.

[0045] Step (3): Take 100g of the Silicalite-1@ZSM-5 core-shell molecular sieve obtained in step (2), perform ion exchange in 500ml of 1mol / L ammonium chloride solution, then wash, dry, and calcine to obtain the hydrogen form Silicalite-1@ZSM-5 core-shell molecular sieve. This is denoted as S2.

[0046] Figure 3-2 The transmission electron microscope image of S2 shows that the core-shell molecular sieve prepared in this invention has a distinct core-shell structure.

[0047] Table 1 shows the characterization results of the S2 structure.

[0048] Example 3

[0049] Step (1): Under stirring conditions, tetraethyl silicate, tetrapropylammonium hydroxide, methanol and deionized water were added to a reaction vessel to obtain a core-layer molecular sieve synthesis system. The mass ratio of tetraethyl silicate (as SiO2) to tetrapropylammonium hydroxide was 3.0, the mass ratio of tetraethyl silicate (as SiO2) to methanol was 2.5, and the mass ratio of tetraethyl silicate (as SiO2) to water was 3.5. After stirring evenly, the mixture was aged at 35°C for 3 hours and then transferred to a crystallization vessel for crystallization at 100°C for 24 hours. The crystallization reaction product was separated by centrifugation to obtain the recovered template agent and intermediate product.

[0050] Step (2): Add aluminum sulfate, water glass, ammonium acetate, sodium hydroxide and deionized water to the intermediate product to obtain the shell molecular sieve synthesis system. The mass ratio of intermediate product: aluminum sulfate (calculated as Al2O3): NaOH: ammonium acetate: water is 216.45:1:8.2:14.36:322.21. The molar ratio of water glass (calculated as SiO2): aluminum sulfate (calculated as Al2O3) is 150. After stirring evenly, age at 35℃ for 3h and then transfer to a crystallization kettle. Crystallize at 150℃ for 48h. Wash, dry and calcine the obtained solid to obtain Silicalite-1@ZSM-5 core-shell molecular sieve.

[0051] Step (3): Take 100g of the Silicalite-1@ZSM-5 core-shell molecular sieve obtained in step (2), perform ion exchange in 500ml of 1mol / L ammonium chloride solution, then wash, dry, and calcine to obtain the hydrogen form Silicalite-1@ZSM-5 core-shell molecular sieve. This is denoted as S3.

[0052] Figure 3-3 The transmission electron microscope image of S3 shows that the core-shell molecular sieve prepared in this invention has a distinct core-shell structure.

[0053] Table 1 shows the characterization results of the S3 structure properties.

[0054] Example 4

[0055] Step (1): Under stirring conditions, tetraethyl silicate, tetrapropylammonium hydroxide, methanol and deionized water were added to a reaction vessel to obtain a core-layer molecular sieve synthesis system. The mass ratio of tetraethyl silicate (as SiO2) to tetrapropylammonium hydroxide was 1.5, the mass ratio of tetraethyl silicate (as SiO2) to methanol was 4.8, and the mass ratio of tetraethyl silicate (as SiO2) to water was 2.5. After stirring evenly, the mixture was aged at 50°C for 3 hours and then transferred to a crystallization vessel for crystallization at 150°C for 24 hours. The crystallization reaction product was separated by centrifugation to obtain the recovered template agent and intermediate product.

[0056] Step (2): Add aluminum trichloride, silica sol, sodium chloride, sodium hydroxide and deionized water to the intermediate product to obtain the shell molecular sieve synthesis system. The mass ratio of intermediate product: Al2O3:NaOH:sodium chloride:water is 215.8:1:8.73:9.86:322.04, and the molar ratio of silica sol (calculated as SiO2):Al2O3 is 40. After stirring evenly, age at 50℃ for 3h and then transfer to a crystallization kettle. Crystallize at 160℃ for 24h. Wash, dry and calcine the obtained solid to obtain Silicalite-1@ZSM-5 core-shell molecular sieve.

[0057] Step (3): Take 100g of the Silicalite-1@ZSM-5 core-shell molecular sieve obtained in step (2), perform ion exchange in 500ml of 1mol / L ammonium chloride solution, then wash, dry, and calcine to obtain the hydrogen form Silicalite-1@ZSM-5 core-shell molecular sieve. This is denoted as S4.

[0058] Figure 3-4 The transmission electron microscope image of S4 shows that the core-shell molecular sieve prepared in this invention has a distinct core-shell structure.

[0059] Table 1 shows the characterization results of the S4 structure.

[0060] Example 5

[0061] Step (1): Under stirring conditions, tetraethyl silicate, 1,6-hexanediamine, ethanol and deionized water were added to a reaction vessel to obtain a core-layer molecular sieve synthesis system. The mass ratio of tetraethyl silicate (as SiO2) to 1,6-hexanediamine was 0.8, the mass ratio of tetraethyl silicate (as SiO2) to ethanol was 4.2, and the mass ratio of tetraethyl silicate (as SiO2) to water was 6.5. After stirring evenly, the mixture was aged at 35°C for 3 hours and then transferred to a crystallization vessel for crystallization at 150°C for 24 hours. The crystallization reaction product was separated by centrifugation to obtain the recovered template agent and intermediate product.

[0062] Step (2): Add aluminum sulfate, water glass, ammonium acetate, sodium hydroxide and deionized water to the intermediate product to obtain the shell molecular sieve synthesis system. The mass ratio of intermediate product: aluminum sulfate (calculated as Al2O3): NaOH: ammonium acetate: water is 209.95:1:7.49:21.64:329.46, and the molar ratio of water glass (calculated as SiO2): aluminum sulfate (calculated as Al2O3) is 40. After stirring evenly, age at 35℃ for 3h and then transfer to a crystallization kettle. Crystallize at 180℃ for 24h. Wash, dry and calcine the obtained solid to obtain Silicalite-1@ZSM-5 core-shell molecular sieve.

[0063] Step (3): Take 100g of the Silicalite-1@ZSM-5 core-shell molecular sieve obtained in step (2), perform ion exchange in 500ml of 1mol / L ammonium chloride solution, then wash, dry, and calcine to obtain the hydrogen form Silicalite-1@ZSM-5 core-shell molecular sieve. This is denoted as S5.

[0064] Figure 3-5 The transmission electron microscope image of S5 shows that the core-shell molecular sieve prepared in this invention has a distinct core-shell structure.

[0065] Table 1 shows the characterization results of the S5 structure.

[0066] Example 6

[0067] Silicalite-1@ZSM-5 core-shell molecular sieves were prepared according to the method in Example 1, except that ethanol was not added in step (1) to obtain the hydrogen form Silicalite-1@ZSM-5 core-shell molecular sieve. This is denoted as S6.

[0068] Figure 3-6 The transmission electron microscope image of S6 shows that the core-shell molecular sieve prepared in this invention has a distinct core-shell structure.

[0069] Table 1 shows the characterization results of the S6 structure.

[0070] Example 7

[0071] Silicalite-1@ZSM-5 core-shell molecular sieves were prepared according to the method of Example 1, except that in step (2), the molar ratio of silica sol (calculated as SiO2): sodium aluminate (calculated as Al2O3) was 200, resulting in hydrogen-form Silicalite-1@ZSM-5 core-shell molecular sieves. This is denoted as S7.

[0072] Figure 3-7 The transmission electron microscope image of S7 shows that the core-shell molecular sieve prepared in this invention has a distinct core-shell structure.

[0073] Table 1 shows the characterization results of the S7 structure.

[0074] Example 8

[0075] The intermediate product was prepared according to step (1) of Example 1, except that the recovered template agent obtained in step (1) of Example 1 was used instead of tetrapropylammonium hydroxide. The intermediate product was denoted as M. The X-ray diffraction pattern and scanning electron microscope image of M are shown in [reference needed]. Figure 5-1 and 5-2 .

[0076] Depend on Figure 5-1 and 5-2 The characteristic peaks in the X-ray spectrum confirm that the intermediate product M possesses the MFI topology, and the scanning electron microscope images clearly show the spherical morphology with a particle size of approximately 60–100 nm. This indicates that the recovered template agent can be used to replace tetrapropylammonium hydroxide to synthesize all-silica molecular sieves.

[0077] Comparative Example 1

[0078] Silicalite-1 molecular sieves were prepared according to the method in Example 1, except that steps (2) and (3) were omitted. The intermediate product obtained in step (1) was washed, dried, and calcined to obtain Silicalite-1 type all-silica molecular sieve material. It is denoted as D1.

[0079] Figure 2-2The image shown is a scanning electron microscope image of D1, and Table 1 shows the characterization results of its structural properties.

[0080] Depend on Figure 5-2 and 2-2 The comparison shows that the crystal structure and morphology of the all-silica molecular sieve (M) prepared by the recovered template agent are consistent with those of the molecular sieve (D1) prepared by the novel template agent, indicating that the template agent obtained by distillation recovery can be reused.

[0081] Comparative Example 2

[0082] Sodium aluminate, silica sol, sodium chloride, sodium hydroxide, ZSM-5 seed crystals (silicon-to-aluminum ratio of 40), and deionized water were added to a reactor under stirring conditions to obtain a ZSM-5 molecular sieve synthesis system. The mass ratio of sodium aluminate (based on Al₂O₃):NaOH:sodium chloride:ZSM-5 seed crystals:water was 1:4.8:4.7:2.5:28, and the molar ratio of silica sol (based on SiO₂):sodium aluminate (based on Al₂O₃) was 40. After stirring evenly, the mixture was aged at 35°C for 8 hours and then transferred to a crystallization reactor. Crystallization was carried out at 150°C for 48 hours. The resulting solid was washed, dried, and calcined to obtain a ZSM-5 molecular sieve with a silicon-to-aluminum ratio of 40. After ion exchange and further washing and calcination, a hydrogen-form ZSM-5 molecular sieve was obtained, denoted as D2.

[0083] Figure 2-3 The image shown is a scanning electron microscope image of D2, and Table 1 shows the characterization results of its structural properties.

[0084] Comparative Example 3

[0085] ZSM-5@Silicalite-1 type core-shell molecular sieve, denoted as D3, was prepared according to the method of Example 1 in Chinese patent application CN105268472A. Table 1 shows the characterization results of its structural properties. Figure 2-4 The image shown is a scanning electron microscope image of D4, and Table 1 shows the characterization results of its structural properties.

[0086] Comparative Example 4

[0087] Silicalite-1@ZSM-5 core-shell molecular sieves were prepared according to the method in Example 1, except that sodium chloride was not added in step (2) to obtain the hydrogen form Silicalite-1@ZSM-5 core-shell molecular sieve. This is denoted as D4.

[0088] Figure 2-5 The image shown is a scanning electron microscope image of D4, and Table 1 shows the characterization results of its structural properties.

[0089] pass Figures 2-1 to 2-5It can be seen that the molecular sieve S1 prepared in Example 1 of the present invention has a regular hexagonal prism shape, the molecular sieve D1 prepared in Comparative Example 1 has a small spherical shape with a small particle size, the molecular sieve D2 prepared in Comparative Example 2 has a larger particle size, the molecular sieve D3 prepared in Comparative Example 3 has a large cubic molecular sieve, and the molecular sieve D4 prepared in Comparative Example 4 has an irregular shape and a large particle size due to the lack of the addition of anion and cation balancer during the preparation process.

[0090] Table 1

[0091]

[0092]

[0093] As can be seen from the data in Table 1, the molecular sieves prepared in Examples 1-7 of the present invention all have obvious microporous structures, and the microporous specific surface area and microporous volume are both high.

[0094] Test Example 1

[0095] The molecular sieve performance of Examples 1-7 and Comparative Examples 1-4 was tested.

[0096] Test conditions: Molecular sieves prepared in Examples 1-7 and Comparative Examples 1-4 were respectively packed into a small fixed-bed reactor. Model compounds of n-pentane and n-hexane in a volume ratio of 1:1 were introduced into the reactor to contact the molecular sieves and react. The reaction conditions were as follows: temperature 400℃, pressure 0.1 MPa, and mass hourly space velocity (HHSV) 0.5 h⁻¹. -1 The test lasted for 24 hours, and the results are shown in Table 2.

[0097] Table 2

[0098]

[0099] Test Example 2

[0100] The molecular sieves were tested under the conditions of Test Example 1, except that the reaction time was 120 h. The results are shown in Table 3.

[0101] Table 3

[0102]

[0103] As shown in Tables 2 and 3, the Silicalite-1@ZSM-5 core-shell molecular sieve prepared in this invention exhibits high reactivity and low selectivity for light hydrocarbon aromatization reactions, with high yields of propane and butane, good stability, and low carbon deposition.

Claims

1. A method for preparing Silicalite-1@ZSM-5 core-shell molecular sieve, characterized in that, Includes the following steps: (1) The template agent, the first silicon source and water are mixed to obtain the core layer molecular sieve synthesis system, and the intermediate product is obtained by aging I and crystallization I; (2) The intermediate product obtained in step (1), aluminum source, second silicon source, alkali source, anion and cation balancer and water are mixed to obtain a shell molecular sieve synthesis system. After aging II and crystallization II, the obtained solid is washed, dried and calcined to obtain the Silicalite-1@ZSM-5 core-shell molecular sieve. The cation-anion balancer is one of the following: a strong acid-weak base salt, a strong base-weak acid salt, a strong acid-strong base salt, or a weak acid-weak base salt. The first silicon source is SiO2, the mass ratio of the first silicon source to the template agent is 0.1~5, and the mass ratio of the first silicon source to water is 1~10; The second silicon source is SiO2, the aluminum source is Al2O3, and the molar ratio of the second silicon source to the aluminum source is 20~200. The Silicalite-1@ZSM-5 core-shell molecular sieve has Silicalite-1 molecular sieve as the core layer and ZSM-5 molecular sieve as the shell layer, with a particle size of 90~140nm.

2. The method according to claim 1, characterized in that, Add a hydrolysis promoter to the core layer molecular sieve synthesis system described in step (1), wherein the hydrolysis promoter is an alcohol and / or a weak base.

3. The method according to claim 2, characterized in that, The hydrolysis accelerator is at least one of ethylene glycol, ethanol, methanol, and ammonia.

4. The method according to claim 1, characterized in that, The anion and cation balancer is at least one of sodium acetate, sodium chloride, ammonium acetate, and ammonium chloride.

5. The method according to claim 1, characterized in that, The first silicon source and the second silicon source are each independently selected from at least one of tetraethyl silicate, silica sol, water glass, fumed silica, and silica gel powder; The aluminum source is at least one of aluminum sulfate, sodium aluminate, aluminum nitrate, and aluminum chloride; The alkaline source is at least one of sodium hydroxide, potassium hydroxide, and ammonia water; The template agent is at least one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, n-butylamine, tetrapropylammonium bromide, and 1,6-hexanediamine.

6. The method according to claim 1, characterized in that, Step (1) includes the step of recovering the template agent by distillation or centrifugation after crystallization.

7. The method according to claim 2, characterized in that, The first silicon source is SiO2, and the mass ratio of the first silicon source to the hydrolysis promoter is 1 to 5.

8. The method according to claim 1, characterized in that, The aluminum source is calculated as Al2O3, the alkali source is calculated as NaOH, and the mass ratio of the intermediate product: aluminum source: alkali source: anion and cation balancer: water is (50~300):1:(1~10):(1~30):(50~200).

9. The method according to claim 1, characterized in that, The aging process I is carried out at a temperature of 10~50℃ for 0.5~12h; the crystallization process I is carried out at a temperature of 80~250℃ for 18~72h. The aging process II is carried out at a temperature of 10~50℃ for 0.5~12h; the crystallization process II is carried out at a temperature of 80~250℃ for 18~72h.

10. The method according to claim 1, characterized in that, The aging process I is carried out at a temperature of 25~35℃ for 4~6 hours; the crystallization process I is carried out at a temperature of 100~150℃ for 24~48 hours. The aging process II is carried out at a temperature of 25~35℃ for 4~6 hours; the crystallization process II is carried out at a temperature of 150~200℃ for 24~48 hours.

11. The method according to claim 6, characterized in that, The distillation temperature for recovering the template agent is 60~120℃, and the distillation time is 0.5~6h.

12. The method according to claim 6, characterized in that, The distillation temperature for recovering the template agent is 80~100℃, and the distillation time is 2~4h.

13. The method according to claim 1, characterized in that, The Silicalite-1 molecular sieve has a particle size of 40~80nm, and the ZSM-5 molecular sieve has a thickness of 30~70nm.

14. The method according to claim 1, characterized in that, The Silicalite-1 molecular sieve has a particle size of 50~70nm, and the ZSM-5 molecular sieve has a thickness of 45~60nm.

15. The method according to claim 1, characterized in that, The molar ratio of silica to alumina in the ZSM-5 molecular sieve is 20 to 200.

16. The method according to claim 1, characterized in that, The total specific surface area of ​​the Silicalite-1@ZSM-5 core-shell molecular sieve is 300~470m². 2 / g, with a microporous specific surface area of ​​260~390m² 2 / g, total pore volume is 0.20~0.60cm³ 3 / g, micropore volume is 0.10~0.30cm³ 3 / g.

17. The method according to claim 1, characterized in that, The total specific surface area of ​​the Silicalite-1@ZSM-5 core-shell molecular sieve is 420~450m². 2 / g, with a microporous specific surface area of ​​350~380m² 2 / g, total pore volume is 0.20~0.60cm³ 3 / g, micropore volume is 0.10~0.30cm³ 3 / g.

18. Silicalite-1@ZSM-5 core-shell molecular sieve prepared by any one of claims 1-17.

19. The application of the Silicalite-1@ZSM-5 core-shell molecular sieve of claim 18 in the catalytic cracking reaction of low-carbon alkane, the lightening reaction of heavy aromatics, or the aromatization reaction of light hydrocarbons.

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