Silicalite-1-coated ZSM-5 core-shell molecular sieve as well as preparation method and application thereof

Abstract

A Silicalite-1 (at) ZSM-5 core-shell molecular sieve takes a Silicalite-1 molecular sieve as a core layer and a ZSM-5 molecular sieve as a shell layer, the particle size is 90-140nm, and a preparation method of the Silicalite-1 (at) ZSM-5 core-shell molecular sieve comprises the following steps: (1) mixing a template agent, a first silicon source and water to obtain a core-layer molecular sieve synthesis system, and performing aging I and crystallization I to obtain an intermediate product; and (2) mixing the intermediate product obtained in the step (1), an aluminum source, a second silicon source, an alkali source, a zwitterion balancing agent and water to obtain a shell molecular sieve synthesis system, carrying out aging II and crystallization II, washing and drying the obtained solid, and roasting to obtain the Sillicalite-1-coated ZSM-5 core-shell molecular sieve. The molecular sieve is high in carbon capacity, simple in preparation method and low in cost, and can be used for a low-carbon alkane catalytic cracking reaction, a heavy aromatic hydrocarbon lightening reaction and a light hydrocarbon aromatization reaction.

Description

Technical Field

[0001] The present invention relates to a core-shell molecular sieve and a preparation method and application thereof, and specifically relates to a Silicalite-1@ZSM-5 core-shell molecular sieve and a preparation method and application thereof. Background Art

[0002] Both ZSM-5 molecular sieve and Silicalite-1 molecular sieve are molecular sieves with two-dimensional cross ten-membered ring channel structure and MFI type channel structure. The acidic function of ZSM-5 molecular sieve is closely related to the Al content in the framework. Its Si / Al ratio can be flexibly adjusted according to the requirements of the catalytic reaction. Due to its unique acid sites and channel structure, it is widely used in a series of reactions of light hydrocarbon conversion and is one of the important catalytic materials at present. However, in reactions such as aromatization of light hydrocarbons and catalytic cracking, the acid density of ZSM-5 molecular sieve is high, and it is easy to accumulate carbon and block the internal channels. Therefore, many studies have optimized the synthesis of ZSM-5 molecular sieve from the perspectives of grain size, silicon-aluminum ratio, pore structure, etc. However, the currently synthesized ZSM-5@Silicalite-1 core-shell molecular sieves have a relatively complicated synthesis process, a relatively fixed feed ratio, a low single-pot output, and a difficult recovery of the template agent. In addition, the raw material price is relatively expensive, which is not conducive to industrial production.

[0003] CN113753913A discloses a ZSM-5@Silicalite-1 core-shell molecular sieve and its preparation method and application, the method at least comprising: (1) crystallizing a raw material containing an aluminum source, a silicon source, a regulator, and a Silicalite-1 seed crystal I to obtain an intermediate product; (2) adding a template to the intermediate product, crystallizing II to obtain the ZSM-5@Silicalite-1 core-shell molecular sieve; in the step (1), the regulator is selected from at least one of an acid source and an alkali source. The method does not require the addition of ZSM-5 crystal nuclei, and can prepare ZSM-5@S-1 core-shell materials in a one-pot method using cheap silica sol as a silicon source. At the same time, the method can greatly reduce the H2O / SiO2 ratio to below 40, improve the single-pot yield, and obtain regular, uniform and monodispersed particles with a grain size of 0.2 to 5 μm.

[0004] CN112705248A discloses a core-shell MFI / MFI molecular sieve, comprising an MFI structure molecular sieve, a layered silicate material, a template R and an optional reaction product of a base and water. The molecular sieve has good molecular shape selectivity in the catalytic reaction and can be used in catalytic reactions with high requirements for reaction selectivity, such as the production of paraxylene and ethylbenzene, with high selectivity and conversion rate.

[0005] CN104556131A adopts microwave heating method and takes advantage of the characteristics of microwave rapid non-gradient heating. Through microwave vibration, SiO2 in the shell mother liquor is induced to the core phase molecular sieve, and the nucleation and growth are rapid to synthesize the core-shell molecular sieve. The ZSM-5 / Silicalite-1 core-shell molecular sieve can be obtained in a relatively short time by microwave heating technology.

[0006] The ZSM-5@Silicalite-1 core-shell molecular sieve synthesized in the above synthesis method has an aluminum-rich ZSM-5 molecular sieve as the core layer and Silicalite-1 grown on the outer layer. The abundant B acid sites inside easily cause the catalyst to coke and deactivate, and the carbon holding capacity is weak. In addition, there are problems such as strict raw materials selected in the preparation process, non-recyclable template agents, high costs, and microwave heating methods that do not have prospects for industrial scale-up. Summary of the invention

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

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

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

[0010] (1) mixing a template, a first silicon source and water to obtain a core layer molecular sieve synthesis system, and performing aging I and crystallization I to obtain an intermediate product;

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

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

[0013] The Silicalite-1@ZSM-5 core-shell molecular sieve provided by the present invention is synthesized by a two-step crystallization method, wherein an intermediate product is first obtained by an all-silicon molecular sieve synthesis system, and then the intermediate product is added to a shell molecular sieve synthesis system, and no template agent and seed crystal are added to the shell molecular sieve synthesis system, and the Silicalite-1@ZSM-5 core-shell molecular sieve can be obtained by adding an anion-cation balance agent and adjusting the feed ratio of the synthesis system. The method of the present invention uses cheap silica sol as a silicon source, has a simple preparation process, and is low in cost, and a small-particle core-shell molecular sieve with uniform morphology can be obtained by a simple hydrothermal method. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0015] Figures 2-1 to 2-5 The scanning electron microscope photos of the hydrogen-type Silicalite-1@ZSM-5 core-shell molecular sieve samples prepared in Example 1 and Comparative Examples 1 to 4 are respectively;

[0016] Figures 3-1 to 3-7 Transmission electron microscope photos of hydrogen-type Silicalite-1@ZSM-5 core-shell molecular sieve samples prepared in Examples 1 to 7, respectively;

[0017] Figure 4 This is an enlarged scanning electron microscope photo of the hydrogen-type Silicalite-1@ZSM-5 core-shell molecular sieve sample prepared in Example 1;

[0018] Figure 5-1 is the X-ray diffraction spectrum of the intermediate product M obtained in Example 8, Figure 5-2 A scanning electron microscope photo of it. DETAILED DESCRIPTION

[0019] The Silicalite-1@ZSM-5 core-shell molecular sieve provided by the present invention has a Silicalite-1 molecular sieve as a core layer and a ZSM-5 molecular sieve as a shell layer. Epitaxial growth of a ZSM-5 molecular sieve on the Silicalite-1 molecular sieve can effectively solve the problem of blockage inside the molecular sieve pores causing difficulty in product desorption. Using Silicalite-1 as the core layer can reduce the B acid content inside the core-shell molecular sieve, thereby inhibiting the strong cracking reaction of the reactant inside the molecular sieve, and making the cracking reaction occur 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 the present invention has a small particle size, a certain microporous structure, and a suitable specific surface area and pore volume, and is used for catalytic cracking reactions of low-carbon alkanes, lightweight reactions of heavy aromatics, and aromatization reactions of light hydrocarbons. It has high reaction activity and selectivity for low-carbon hydrocarbons, and has low carbon deposition.

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

[0022] The preparation method of Silicalite-1@ZSM-5 core-shell molecular sieve provided by the present invention comprises the following steps:

[0023] (1) mixing a template, a first silicon source and water to obtain a core layer molecular sieve synthesis system, and performing aging I and crystallization I to obtain an intermediate product;

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

[0025] Preferably, a hydrolysis promoter may be added to the core layer molecular sieve synthesis system described in step (1), wherein the hydrolysis promoter is an alcohol and / or a weak base, more preferably at least one of ethylene glycol, methanol, ethanol, and ammonia water; the first silicon source is calculated as SiO2, the mass ratio of the first silicon source to the hydrolysis promoter is 1 to 5, the mass ratio of the first silicon source to the template is 0.1 to 5, and the mass ratio of the first silicon source to water is 1 to 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 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, in step (1), after crystallization I, a step of recovering the template by distillation or centrifugation is included. When the template is recovered by distillation, the temperature is 60-120°C, preferably 80-100°C, and the distillation time is 0.5-6h, preferably 2-4h.

[0027] Preferably, in step (2), the anion-cation balance agent 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, 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 independently selected from at least one of tetraethyl silicate, silica sol, water glass, white carbon black, 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 is at least one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, n-butylamine, tetrapropylammonium bromide, and 1,6-hexanediamine.

[0029] Preferably, in step (2), the second silicon source is calculated as SiO2, the aluminum source is calculated as 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, 50, and the lower limit is independently selected from 40, 50, 80, 100, 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 balance agent: water is (50-300): 1: (1-10): (1-30): (50-200).

[0031] Preferably, the aging I has a temperature of 10-50°C, preferably 25-35°C, and a time of 0.5-12h, preferably 4-6h; the crystallization I has a temperature of 80-250°C, preferably 100-150°C, and a time of 18-72h, preferably 24-48h; the aging II has a temperature of 10-50°C, preferably 25-35°C, and a time of 0.5-12h, preferably 4-6h; the crystallization II has a temperature of 80-250°C, preferably 150-200°C, and a time of 18-72h, preferably 24-48h.

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

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

[0034] The present invention is described in detail below in conjunction with embodiments, but the present invention is not limited to these embodiments.

[0035] The raw materials and reagents in the examples of the present invention were purchased from commercial sources. The analysis methods in the examples of the present invention are as follows: XRDWUX70 analysis method was used for X-ray diffraction pattern analysis. PABETALL analysis method was used for nitrogen adsorption and desorption analysis. SEMMORPHO analysis method was used for SEM morphology analysis.

[0036] Example 1

[0037] Step (1): under stirring conditions, tetraethyl silicate, tetrapropylammonium hydroxide, ethanol and deionized water are added into a reaction kettle to obtain a core layer molecular sieve synthesis system, wherein the mass ratio of tetraethyl silicate (in terms of SiO2) to tetrapropylammonium hydroxide is 1.1, the mass ratio of tetraethyl silicate (in terms of SiO2) to ethanol is 4.6, and the mass ratio of tetraethyl silicate (in terms of SiO2) to water is 1.8; after stirring evenly, aging at 35°C for 6h, the mixture is transferred into a crystallization kettle and crystallized at 120°C for 48h; the crystallization reaction product is separated by centrifugation to obtain a recovered template and an intermediate product;

[0038] Step (2): adding sodium aluminate, silica sol, sodium chloride, sodium hydroxide and deionized water to the intermediate product to obtain a shell molecular sieve synthesis system, wherein the mass ratio of the 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, aging at 50°C for 6h, the mixture is transferred to a crystallization kettle and crystallized at 150°C for 24h. The obtained solid is washed, dried and calcined to obtain the Silicalite-1@ZSM-5 core-shell molecular sieve.

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

[0040] Figure 1 is the powder X-ray diffraction spectrum of S1, which has an obvious MFI topological structure; Figure 2-1 This is a scanning electron microscope photo of S1, which has a regular morphology; Figure 3-1 From the transmission electron microscope photo, it can be seen that the core-shell molecular sieve prepared by the present invention has an obvious core-shell structure; Figure 4 From the magnified transmission electron microscope photo, it can be seen that it has a distinct core-shell structure, with the core layer particle size of 40-60nm and the outer edge of the shell layer of 20-40nm.

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

[0042] Example 2

[0043] Step (1): under stirring conditions, tetraethyl silicate, n-butylamine, ethanol and deionized water are added to a reaction kettle to obtain a core layer molecular sieve synthesis system, wherein the mass ratio of tetraethyl silicate (in terms of SiO2) to n-butylamine is 2.3, the mass ratio of tetraethyl silicate (in terms of SiO2) to ethanol is 3.5, and the mass ratio of tetraethyl silicate (in terms of SiO2) to water is 5.0; after stirring evenly, the mixture is aged at 35°C for 3 hours and then transferred to a crystallization kettle for crystallization at 100°C for 48 hours; the crystallization reaction product is separated by distillation to obtain a recovered template and an intermediate product; the distillation conditions are: the intermediate product slurry is taken into a rotary evaporator, the temperature is increased to 80°C at 2°C per minute under room temperature, and rotary evaporated at 80°C for 4 hours, and the rotation frequency of the rotary evaporator is 15 rpm;

[0044] Step (2): adding aluminum sulfate, white carbon black, sodium chloride, sodium hydroxide and deionized water to the intermediate product to obtain a shell molecular sieve synthesis system, wherein the mass ratio of the 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 white carbon black (calculated as SiO2): aluminum sulfate (calculated as Al2O3) is 80. After stirring evenly, aging at 35°C for 3h, the mixture is transferred to a crystallization kettle and crystallized at 150°C for 36h. The obtained solid is washed, dried and calcined to obtain the Silicalite-1@ZSM-5 core-shell molecular sieve.

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

[0046] Figure 3-2 This is a transmission electron microscope photo of S2, from which it can be seen that the core-shell molecular sieve prepared in the present invention has an obvious core-shell structure.

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

[0048] Example 3

[0049] Step (1): adding tetraethyl silicate, tetrapropylammonium hydroxide, methanol and deionized water into a reaction kettle under stirring conditions to obtain a core layer molecular sieve synthesis system, wherein the mass ratio of tetraethyl silicate (calculated as SiO2) to tetrapropylammonium hydroxide is 3.0, the mass ratio of tetraethyl silicate (calculated as SiO2) to methanol is 2.5, and the mass ratio of tetraethyl silicate (calculated as SiO2) to water is 3.5; after stirring evenly, aging at 35°C for 3h, transferring to a crystallization kettle, and crystallizing at 100°C for 24h; centrifuging and separating the crystallization reaction product to obtain a recovered template and an intermediate product;

[0050] Step (2): adding aluminum sulfate, water glass, ammonium acetate, sodium hydroxide and deionized water to the intermediate product to obtain a shell molecular sieve synthesis system, wherein the mass ratio of the intermediate product: aluminum sulfate (calculated as Al2O3): NaOH: ammonium acetate: water is 216.45:1:8.2:14.36:322.21, and the molar ratio of water glass (calculated as SiO2): aluminum sulfate (calculated as Al2O3) is 150. After stirring evenly, aging at 35°C for 3h, the mixture is transferred to a crystallization kettle and crystallized at 150°C for 48h. The obtained solid is washed, dried and calcined to obtain the Silicalite-1@ZSM-5 core-shell molecular sieve.

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

[0052] Figure 3-3 This is a transmission electron microscope photo of S3, from which it can be seen that the core-shell molecular sieve prepared in the present invention has an obvious core-shell structure.

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

[0054] Example 4

[0055] Step (1): under stirring conditions, tetraethyl silicate, tetrapropylammonium hydroxide, methanol and deionized water are added into a reaction kettle to obtain a core layer molecular sieve synthesis system, wherein the mass ratio of tetraethyl silicate (in terms of SiO2) to tetrapropylammonium hydroxide is 1.5, the mass ratio of tetraethyl silicate (in terms of SiO2) to methanol is 4.8, and the mass ratio of tetraethyl silicate (in terms of SiO2) to water is 2.5; after stirring evenly, aging at 50°C for 3h, the mixture is transferred into a crystallization kettle and crystallized at 150°C for 24h; the crystallization reaction product is separated by centrifugation to obtain a recovered template and an intermediate product;

[0056] Step (2): adding aluminum chloride, silica sol, sodium chloride, sodium hydroxide, and deionized water to the intermediate product to obtain a shell molecular sieve synthesis system, wherein the mass ratio of the intermediate product: Al2O3: NaOH: sodium chloride: water is 215.8:1:8.73:9.86:322.04, and the molar ratio of silica sol (in terms of SiO2): Al2O3 is 40. After stirring evenly, aging at 50°C for 3h, the mixture is transferred to a crystallization kettle and crystallized at 160°C for 24h. The obtained solid is washed, dried, and calcined to obtain Silicalite-1@ZSM-5 core-shell molecular sieve.

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

[0058] Figure 3-4 This is a transmission electron microscope photo of S4, from which it can be seen that the core-shell molecular sieve prepared in the present invention has an obvious core-shell structure.

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

[0060] Example 5

[0061] Step (1): adding tetraethyl silicate, 1,6-hexanediamine, ethanol and deionized water into a reaction kettle under stirring conditions to obtain a core layer molecular sieve synthesis system, wherein the mass ratio of tetraethyl silicate (in terms of SiO2) to 1,6-hexanediamine is 0.8, the mass ratio of tetraethyl silicate (in terms of SiO2) to ethanol is 4.2, and the mass ratio of tetraethyl silicate (in terms of SiO2) to water is 6.5; stirring evenly, aging at 35°C for 3h, and then transferring to a crystallization kettle for crystallization at 150°C for 24h; centrifuging and separating the crystallization reaction product to obtain a recovered template and an intermediate product;

[0062] Step (2): adding aluminum sulfate, water glass, ammonium acetate, sodium hydroxide and deionized water to the intermediate product to obtain a shell molecular sieve synthesis system, wherein the mass ratio of the 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, aging at 35°C for 3h, the mixture is transferred to a crystallization kettle and crystallized at 180°C for 24h. The obtained solid is washed, dried and calcined to obtain Silicalite-1@ZSM-5 core-shell molecular sieve.

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

[0064] Figure 3-5 This is a transmission electron microscope photo of S5, from which it can be seen that the core-shell molecular sieve prepared in the present invention has an obvious core-shell structure.

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

[0066] Example 6

[0067] Silicalite-1@ZSM-5 core-shell molecular sieve was prepared according to the method of Example 1, except that ethanol was not added in step (1), to obtain hydrogen-type Silicalite-1@ZSM-5 core-shell molecular sieve, which was recorded as S6.

[0068] Figure 3-6 This is a transmission electron microscope photo of S6, from which it can be seen that the core-shell molecular sieve prepared in the present invention has an obvious core-shell structure.

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

[0070] Example 7

[0071] Silicalite-1@ZSM-5 core-shell molecular sieve was prepared according to the method of Example 1, except that the molar ratio of silica sol (in terms of SiO2): sodium metaaluminate (in terms of Al2O3) in step (2) was 200, and the hydrogen-type Silicalite-1@ZSM-5 core-shell molecular sieve was obtained, which was denoted as S7.

[0072] Figure 3-7 This is a transmission electron microscope photo of S7, from which it can be seen that the core-shell molecular sieve prepared in the present invention has an obvious core-shell structure.

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

[0074] Example 8

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

[0076] Depend on Figure 5-1 and 5-2 It can be seen that the characteristic peaks of the X-ray spectrum prove that the intermediate product M has an MFI topological structure, and the scanning electron microscope photo can clearly observe the morphology of small spheres with a particle size of about 60 to 100 nm. This shows that the recycled template agent can replace tetrapropylammonium hydroxide to synthesize all-silicon molecular sieves.

[0077] Comparative Example 1

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

[0079] Figure 2-2Figure 1 is the scanning electron microscope photo of D1, and Table 1 is the characterization results of its structural properties.

[0080] Depend on Figure 5-2 and 2-2 By comparison, the crystal structure and morphology of the all-silicon molecular sieve (M) prepared by the recovered template are consistent with those of the molecular sieve (D1) prepared by the new template, indicating that the template recovered by distillation can be reused.

[0081] Comparative Example 2

[0082] Sodium aluminate, silica sol, sodium chloride, sodium hydroxide, ZSM-5 seed crystals (silicon-aluminum ratio of 40) and deionized water were added to a reactor under stirring conditions to obtain a ZSM-5 molecular sieve synthesis system, wherein the mass ratio of sodium aluminate (in terms of 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 (in terms of SiO2): sodium aluminate (in terms of Al2O3) was 40. After stirring evenly, the mixture was aged at 35°C for 8 hours and then transferred to a crystallization kettle for crystallization at 150°C for 48 hours. The obtained solid was washed, dried and calcined to obtain a ZSM-5 molecular sieve with a silicon-aluminum ratio of 40. After ion exchange, the hydrogen-type ZSM-5 molecular sieve was obtained by washing and calcining again. It is denoted as D2.

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

[0084] Comparative Example 3

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

[0086] Comparative Example 4

[0087] Silicalite-1@ZSM-5 core-shell molecular sieve was prepared according to the method of Example 1, except that no sodium chloride was added in step (2), to obtain hydrogen-type Silicalite-1@ZSM-5 core-shell molecular sieve, which was recorded as D4.

[0088] Figure 2-5 Figure 1 is a scanning electron microscope photo of D4, and Table 1 is 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 morphology of a uniform hexagonal prism, the molecular sieve D1 prepared in Comparative Example 1 is a small spherical shape with a smaller 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 is a cubic molecular sieve with a larger particle size, and the molecular sieve D4 prepared in Comparative Example 4 has an irregular morphology and a larger particle size due to the lack of addition of anion and cation balance agents during the preparation process.

[0090] Table 1

[0091]

[0092]

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

[0094] Test Example 1

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

[0096] Test conditions: A small fixed bed reactor was filled with the molecular sieves prepared in Examples 1-7 and Comparative Examples 1-4, and n-pentane and n-hexane model compounds with a volume ratio of 1:1 were introduced into the reactor as raw materials, contacted with the molecular sieves and reacted. The reaction conditions were as follows: temperature 400°C, pressure 0.1 MPa, mass space velocity 0.5 h -1 , time is 24h, test results are shown in Table 2.

[0097] Table 2

[0098]

[0099] Test Example 2

[0100] According to the conditions of Test Example 1, the molecular sieves were tested for performance, 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 by the present invention is used for light hydrocarbon aromatization reaction, has high reaction activity and low carbon hydrocarbon selectivity, high propane and butane yields, good stability, and low carbon deposition.

Claims

1. A Silicalite-1@ZSM-5 core-shell molecular sieve, characterized in that: The molecular sieve has Silicalite-1 molecular sieve as the core layer and ZSM-5 molecular sieve as the shell layer, and the particle size is 90-140nm.

2. The core-shell molecular sieve according to claim 1, characterized in that The particle size of the Silicalite-1 molecular sieve is 40 to 80 nm, preferably 50 to 70 nm, and the thickness of the ZSM-5 molecular sieve is 30 to 70 nm, preferably 45 to 60 nm.

3. The core-shell molecular sieve according to claim 1, characterized in that: The silicon-aluminum ratio of the ZSM-5 molecular sieve is 20-200.

4. The core-shell molecular sieve according to claim 1, characterized in that: The total specific surface area of ​​the Silicalite-1@ZSM-5 core-shell molecular sieve is 300 to 470 m 2 / g, preferably 420-450m 2 / g, micropore specific surface area is 260~390m 2 / g, preferably 350 to 380 m 2 / g, total pore volume is 0.20~0.60cm 3 / g, micropore volume is 0.10~0.30cm 3 / g.

5. A method for preparing the Silicalite-1@ZSM-5 core-shell molecular sieve according to claim 1, characterized in that: The following steps are involved: (1) mixing a template, a first silicon source and water to obtain a core layer molecular sieve synthesis system, and performing aging I and crystallization I to obtain an intermediate product; (2) The intermediate product obtained in step (1), an aluminum source, a second silicon source, an alkali source, an anion-cation balance agent and water are mixed to obtain a shell molecular sieve synthesis system, and after aging II and crystallization II, the obtained solid is washed, dried and calcined to obtain the Sililcalite-1@ZSM-5 core-shell molecular sieve.

6. The method according to claim 5, 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.

7. The method according to claim 6, characterized in that The hydrolysis accelerator is at least one of ethylene glycol, ethanol, methanol and ammonia water.

8. The method according to claim 5, characterized in that The anion-cation balance agent 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.

9. The method according to claim 5, characterized in that The anion-cation balance agent is at least one of sodium acetate, sodium chloride, ammonium acetate and ammonium chloride.

10. The method according to claim 5, characterized in that The first silicon source and the second silicon source are independently selected from at least one of tetraethyl silicate, silica sol, water glass, white carbon black, 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; The template agent is at least one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, n-butylamine, tetrapropylammonium bromide and 1,6-hexanediamine.

11. The method according to claim 5, characterized in that The step (1) includes a step of recovering the template by distillation or centrifugation after crystallization I.

12. The method according to claim 5, characterized in that The first silicon source is calculated as SiO2, the mass ratio of the first silicon source to the template is 0.1-5, and the mass ratio of the first silicon source to water is 1-10.

13. The method according to claim 6, characterized in that The first silicon source is calculated as SiO2, and the mass ratio of the first silicon source to the hydrolysis accelerator is 1 to 5.

14. The method according to claim 5, characterized in that The second silicon source is calculated as SiO2, the aluminum source is calculated as Al2O3, and the molar ratio of the second silicon source to the aluminum source is 20-200.

15. The method according to claim 5, 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 balance agent: water is (50-300): 1: (1-10): (1-30): (50-200).

16. The method according to claim 5, characterized in that The aging I is performed at a temperature of 10 to 50°C, preferably 25 to 35°C, for a time of 0.5 to 12 hours, preferably 4 to 6 hours; the crystallization I is performed at a temperature of 80 to 250°C, preferably 100 to 150°C, for a time of 18 to 72 hours, preferably 24 to 48 hours; The aging II has a temperature of 10 to 50°C, preferably 25 to 35°C, and a time of 0.5 to 12 hours, preferably 4 to 6 hours; the crystallization II has a temperature of 80 to 250°C, preferably 150 to 200°C, and a time of 18 to 72 hours, preferably 24 to 48 hours.

17. The method according to claim 11, characterized in that The temperature of the distillation recovery template agent is 60-120° C., preferably 80-100° C., and the distillation time is 0.5-6 hours, preferably 2-4 hours.

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

19. Use of the Silicalite-1@ZSM-5 core-shell molecular sieve described in any one of claims 1-4 and 18 in catalytic cracking reactions of light alkanes, lightweighting reactions of heavy aromatics, and aromatization reactions of light hydrocarbons.

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