A core-shell type SAPO-34@nano-SAPO-34 molecular sieve catalyst

Abstract

This invention provides a core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst, which is a core-shell SAPO-34@nano-SAPO-34 zeolite catalyst with microporous SAPO-34 as the core and low-silica nano-SAPO-34 as the shell; specifically, a layer of low-silica nano-SAPO-34 zeolite crystals is epitaxially grown on the surface of microporous SAPO-34. This invention effectively overcomes the disadvantage of the easy formation of impurity crystals in the current direct synthesis of low-silica SAPO-34, and significantly improves the transport and diffusion rates of reactants and products; in the MTO reaction, this invention can significantly improve the catalyst's lifespan and the selectivity for low-carbon olefins.

Description

Technical Field

[0001] This invention belongs to the field of inorganic materials, specifically relating to a core-shell type SAPO-34@nano-SAPO-34 molecular sieve catalyst. Background Technology

[0002] Low-carbon olefins (ethylene and propylene) are important organic raw materials in modern chemical industries, widely used in petrochemicals, fine chemicals, and other fields. In traditional production processes, ethylene and propylene are mainly obtained through the thermal cracking of naphtha at high temperatures. However, my country's oil reserves are very limited, requiring excessive reliance on imported oil. With increasingly scarce oil resources, producing low-carbon olefins from petroleum can no longer meet the growing market demand. In recent years, methanol-to-olefins (MTO) technology has proven to be the most effective alternative petroleum-based route for producing ethylene and propylene, effectively alleviating energy shortages. In recent decades, the design and development of highly efficient MTO catalysts has been a key focus of scientific research.

[0003] SAPO-34 molecular sieves, due to their small pore size (~0.38 nm) and CHA cage structure (0.94 nm in diameter), are often considered ideal solid acid catalysts in MTO reactions. Through optimization of reaction conditions, complete methanol conversion can be achieved, with ethylene and propylene yields exceeding 80% (Catal. Today, 2005, 106, 103-107). However, traditional microporous zeolite catalysts often face the challenge of excessively narrow micropores during the reaction process, severely limiting mass transfer and diffusion of reactant and product molecules, thus affecting catalyst lifespan. More importantly, the acid density and strength of zeolite significantly impact the selectivity for low-carbon olefins. High acid density and strength promote secondary reactions of olefins, easily forming polycyclic aromatic hydrocarbon derivatives such as anthracene, phenanthrene, and oxane; this is detrimental to improving the selectivity of the target product, low-carbon olefins. To address these problems, conventional modification methods involve constructing hierarchical pores within the zeolite while simultaneously reducing the Si / Al ratio to weaken the acid content. However, for SAPO-34 molecular sieves, when the Si / Al ratio in the gel precursor solution is too low, impurity phases (AlPO4 and SAPO-5) are easily formed (Chem. Phys. Lett., 2022, 794, 139513-139517; Micropor. Mespore. Mater., 20 13, 155-163), making it difficult to obtain pure-phase low-silica hierarchical porous SAPO-34 zeolite. Therefore, the direct preparation of low-silica hierarchical porous SAPO-34 zeolite via traditional synthetic routes still faces significant challenges. Currently, the most effective method is to introduce metal elements (such as K, Ce, Ni, Mg, Cr, etc.) to alter the acidity characteristics of SAPO-34 (J. Ind. Eng. Chem., 2011, 17, 755-761; J. Porous Mat., 2015, 22, 187-200), which can effectively reduce the acid density and acid strength of the zeolite. However, these metal-modified catalysts generally suffer from the following drawbacks: (1) Metal particles easily clog the micropores of zeolite, hindering the diffusion path of products and reactants. Furthermore, they reduce the pore volume and surface area of ​​the zeolite, while also decreasing the number of accessible active sites; (2) Metal-modified SAPO-34 catalysts, especially those containing toxic Cr metals, cause serious environmental pollution; (3) Sintering and agglomeration of metal particles during the reaction can lead to irreversible deactivation of the catalyst. These drawbacks have become the main obstacles to the industrial application of SAPO-34 catalysts. Therefore, how to prepare low-silica hierarchical porous SAPO-34 using a simple and easy-to-operate method remains challenging, but it has significant scientific and industrial practical value. Summary of the Invention

[0004] In view of the above, the purpose of this invention is to provide a core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst, which uses SAPO-34 as a physical support and epitaxially grows a layer of low-silicon pure-phase SAPO-34 nanocrystals on its surface, thereby improving the catalyst lifetime while also achieving high selectivity for low-carbon olefins.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is to provide a core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst, wherein the core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst is a core-shell SAPO-34@nano-SAPO-34 zeolite catalyst material with microporous SAPO-34 as the core and low-silica nano SAPO-34 as the shell.

[0006] The core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst is specifically formed by epitaxially growing a layer of low-silica nano-SAPO-34 zeolite crystals on the surface of microporous SAPO-34, with a shell thickness of 150-250 nm.

[0007] The microporous SAPO-34 has a grain size of 5–10 μm; the shell low-silicon SAPO-34 nanocrystals have a grain size of 30–50 nm.

[0008] The preparation method of the core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst specifically includes the following steps:

[0009] (1) The core phase SAPO-34 was prepared by hydrothermal synthesis. Aluminum source, silicon source, phosphorus source and structure directing agent (SDA) were added to distilled water in sequence, and the molar ratio of the components was: Al2O3:SiO2:P2O5:SDA:H2O = 1:0.2~1.0:0.7~1.3:4.1~7.9:100.2~300.5. After stirring thoroughly, the resulting mixed solution was transferred to a stainless steel reactor with a polytetrafluoroethylene liner for hydrothermal crystallization reaction to obtain the core phase SAPO-34 molecular sieve.

[0010] (2) At room temperature, the core-phase SAPO-34 molecular sieve obtained in step (1) was directly added to the gel precursor solution for synthesizing nano-SAPO-34 zeolite, with the following molar ratio: Al2O3:SiO2:P2O5:SDA:H2O = 1:0.06~0.13:1.5~5.2:2~10:90~543. After stirring for several hours, the resulting mixed solution was transferred to a stainless steel reactor with a polytetrafluoroethylene liner for hydrothermal crystallization. Then, the crystallized product was washed until neutral, dried, and left overnight. After calcination, SAPO-34@nano-SAPO-34 molecular sieve catalytic material with a core-shell structure was obtained.

[0011] In step (1), the aluminum source used to synthesize the nucleus-phase SAPO-34 molecular sieve is one of aluminum isopropoxide, aluminum sec-butoxide, alumina, or aluminum hydroxide; the silicon source is one of tetraethyl orthosilicate, fumed silica, or silica sol; the phosphorus source is one of ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, or phosphoric acid; the microporous template agent is one of tetraethylammonium hydroxide, diethylamine, or triethylamine; the hydrothermal crystallization temperature is 180–200 °C, and the crystallization time is 24–96 h.

[0012] In step (2), the solid-phase mass ratio of the nucleus phase SAPO-34 to the nano-SAPO-34 gel precursor solution of Al2O3 is 1 to 30:1.

[0013] In step (2), the aluminum source used to synthesize SAPO-34@nano-SAPO-34 is one of boehmite, alumina, or aluminum isopropoxide; the silicon source is one of tetraethyl orthosilicate, silica sol, or fumed silica; the phosphorus source is one of phosphoric acid or ammonium hydrogen phosphate; and the microporous template agent is one of tetraethylammonium hydroxide, triethylamine, diethylamine, or morpholine.

[0014] In step (2), the crystallization temperature of the synthesized SAPO-34@nano-SAPO-34 molecular sieve is 160-200℃, and the crystallization time is 12-72h.

[0015] In step (2), the calcination temperature for synthesizing SAPO-34@nano-SAPO-34 is 500-600℃, and the calcination time is 4-12h.

[0016] The core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst provided by this invention can be applied to the MTO reaction.

[0017] This invention employs an epitaxial growth method. First, synthesized microporous SAPO-34 is added as a core phase to a precursor solution containing low-silicon nano-SAPO-34 using a conventional hydrothermal crystallization method. During the hydrothermal crystallization process, silicon, aluminum, and phosphorus sources in the solution grow around the surface of the core phase SAPO-34 under the action of a structure-directing agent, ultimately resulting in SAPO-34 nanocrystals completely encapsulating the surface of the core phase SAPO-34, thus forming a SAPO-34@nano-SAPO-34 molecular sieve catalytic material with a core-shell structure.

[0018] The beneficial effects of this invention are as follows: the prepared core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst uses SAPO-34 as a physical support, inducing secondary growth of low-silicon nano-SAPO-34 crystals on its surface, thus constructing a homocrystalline core-shell structure. This method effectively overcomes the drawback of impurity crystals easily forming during the direct synthesis of low-silicon SAPO-34. Furthermore, the loosely packed shell nanocrystals form a rich intercrystalline porous structure, which can significantly improve the transport and diffusion rates of reactants and products. In the MTO reaction, this invention can significantly improve the catalyst's lifespan and the selectivity for low-carbon olefins. The preparation method of this invention has the advantages of readily available raw materials, simple process, relatively mild conditions, and high reproducibility, enabling industrial application and showing broad application prospects in solving energy shortage problems. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the preparation process of the material prepared in Example 1 of the present invention;

[0020] Figure 2 The XRD patterns of nano-SAPO-34 (a), SAPO-34 (b), and SAPO-34@nano-SAPO-34 (c) prepared in Example 1 of this invention are shown.

[0021] Figure 3 SEM images of SAPO-34 (a), SAPO-34@nano-SAPO-34 (b and c), and SAPO-34@nano-SAPO-34 (d) crushed by 20 MPa extrusion, prepared in Example 1 of the present invention;

[0022] Figure 4 This is a SEM-EDS image of SAPO-34@nano-SAPO-34 prepared in Example 1 of this invention;

[0023] Figure 5 The graph shows the MTO catalytic reaction performance of SAPO-34 prepared in Example 1 of this invention.

[0024] Figure 6The graph shows the MTO catalytic reaction performance of SAPO-34@nano-SAPO-34 prepared in Example 1 of this invention. Implementation

[0025] The specific embodiments of the present invention will be further described below with reference to examples. These examples are only used to more clearly illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Various changes, modifications, substitutions, and variations made by those skilled in the art to these embodiments without departing from the principles and spirit of the present invention should be included within the scope of protection of the present invention.

[0026] Example 1: Preparation of SAPO-34@nano-SAPO-34

[0027] (1) Weigh 6.00g of aluminum isopropoxide and dissolve it in 8mL of diethylamine and 30mL of distilled water. Under vigorous stirring, add 2mL of tetraethyl orthosilicate and 2.0mL of 85wt% phosphoric acid solution in sequence. Then, transfer the well mixed solution to a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat it at 170℃ for 20h. The crystallized product is washed with distilled water until neutral and dried at 90℃ for 12h to obtain SAPO-34 molecular sieve.

[0028] (2) Dissolve 4.08 g of aluminum isopropoxide in 18 mL of 25 wt% tetraethylammonium hydroxide and 4 mL of distilled water solution; after complete dissolution, add 0.2 mL of tetraethyl orthosilicate and 4 mL of 85 wt% phosphoric acid solution in sequence; then, put 4 g of SAPO-34 molecular sieve obtained in step (1) into it; after stirring thoroughly, put the mixed solution into a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat it at 180°C for 48 h; wash the obtained product with distilled water until neutral to obtain SAPO-34@nano-SAPO-34 zeolite catalyst material.

[0029] like Figure 1 The diagram shows the preparation process flow of SAPO-34@nano-SAPO-34 of the present invention. First, a core-phase SAPO-34 molecular sieve is prepared, and then a layer of low-silicon SAPO-34 zeolite nanocrystals is epitaxially grown on its surface to prepare a SAPO-34@nano-SAPO-34 zeolite catalytic material with a core-shell structure.

[0030] like Figure 2The image shows the X-ray diffraction (XRD) pattern of the SAPO-34@nano-SAPO-34 prepared in this invention. As can be seen from the image, the prepared SAPO-34@nano-SAPO-34 catalytic material only exhibits the characteristic diffraction peaks of SAPO-34 molecular sieves, without any other impurity phases. However, without the addition of the core phase SAPO-34, AlPO4-18 impurities appear in the obtained nano-SAPO-34 sample, indicating that the presence of the core phase SAPO-34 is a key factor in the preparation of pure-phase low-silicon SAPO-34 nanocrystals.

[0031] like Figure 3 The image shown is a scanning electron microscope (SEM) image of SAPO-34@nano-SAPO-34 obtained in Example 1. Figure 3 (a) It can be seen that the prepared core phase SAPO-34 exhibits a uniform cubic morphology with a size of approximately 5 μm, and its surface is very smooth and dense. From Figure 3 Figures (b) and (c) show that the surface of the prepared SAPO-34 cubic material is composed of a loose and porous shell structure formed by the accumulation of nanocrystals (~50 nm). Figure (d) further shows that the sample has a distinct core-shell structure after being crushed by extrusion at 20 MPa, with a shell thickness of approximately 210 nm.

[0032] Figure 4 The image shows the scanning electron microscope X-ray energy dispersive spectroscopy (SEM-EDS) image of SAPO-34@nano-SAPO-34 obtained in Example 1. The analysis results show that the Si / Al ratio of the core phase SAPO-34 is 0.33, while the shell has a lower Si / Al ratio of 0.18, indicating that low-silicon nano SAPO-34 molecular sieves can be synthesized by preparing core-shell zeolite materials.

[0033] The samples prepared above were used as zeolite catalysts for the MTO reaction, and the reaction performance of the catalysts was investigated.

[0034] The reaction was carried out in a micro fixed-bed reactor at atmospheric pressure, with 0.40 g of catalyst (20-40 mesh) loaded into the reaction tube. Under a N2 atmosphere (50 mL / min), the temperature was increased from room temperature to 400 °C at a rate of 5 °C / min for 1 h. Then, methanol was introduced using a micro-metering pump, with a mass hourly space velocity (WHSV) maintained at 1 h⁻¹. -1 The reaction products were analyzed using a gas chromatograph equipped with a flame ionization detector (FID).

[0035] Catalytic results as follows Figure 5 , 6As shown in the figure, SAPO-34@nano-SAPO-34 exhibits superior catalytic performance compared to the core-phase SAPO-34 catalyst. Its catalytic lifetime (methanol conversion rate > 99.0%) is twice that of the core-phase SAPO-34, and the average selectivity for low-carbon olefins is increased by 2.32%, indicating that this catalytic material has high methanol reactivity, stability, and low-carbon olefin selectivity.

[0036] Example 2: Preparation of SAPO-34@nano-SAPO-34

[0037] (1) Weigh 1.41 g of alumina and add it to a mixed solution of 8 mL of triethylamine and 30 mL of distilled water. Under vigorous stirring, add 2 mL of tetraethyl orthosilicate and 2.0 mL of 85 wt% phosphoric acid solution in sequence. Then, transfer the well mixed solution to a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat it at 180 °C for 48 h. The crystallized product is washed with distilled water until neutral and dried at 90 °C for 12 h to obtain SAPO-34 molecular sieve.

[0038] (2) Dissolve 6.12 g of aluminum isopropoxide in 18 mL of 25 wt% tetraethylammonium hydroxide and 4 mL of distilled aqueous solution. After complete dissolution, add 0.2 mL of tetraethyl orthosilicate and 4 mL of 85 wt% phosphoric acid solution. Then, add 6.0 g of the SAPO-34 molecular sieve obtained in step (1). After thorough mixing, place the mixed solution in a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat at 170°C for 24 h. Wash the obtained product with distilled water until neutral to obtain SAPO-34@nano-SAPO-34 zeolite catalyst.

[0039] Example 3: Preparation of SAPO-34@nano-SAPO-34

[0040] (1) Weigh 1.20 g of alumina and add it to 8 mL of diethylamine and 30 mL of distilled aqueous solution. Under vigorous stirring, add 4 mL of tetraethyl orthosilicate and 2.0 mL of 85 wt% phosphoric acid solution. Then, transfer the well-mixed solution to a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat it at 170 °C for 20 h. The crystallized product is washed with distilled water until neutral and dried at 90 °C for 12 h to obtain SAPO-34 molecular sieve.

[0041] (2) Dissolve 4.08 g of aluminum isopropoxide in 18 mL of 25 wt% tetraethylammonium hydroxide and 4 mL of distilled aqueous solution. After complete dissolution, add 0.2 mL of tetraethyl orthosilicate and 4 mL of 85 wt% phosphoric acid solution. Then, add 4.0 g of the SAPO-34 molecular sieve obtained in step (1). After thorough mixing, place the mixed solution in a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat at 180°C for 48 h. Wash the obtained product with distilled water until neutral to obtain SAPO-34@nano-SAPO-34 zeolite catalyst.

[0042] Example 4: Preparation of SAPO-34@nano-SAPO-34

[0043] (1) Weigh 0.98 g of aluminum hydroxide and add it to 8 mL of diethylamine and 30 mL of distilled aqueous solution. Under vigorous stirring, add 0.48 g of fumed silica and 2.0 mL of 85 wt% ammonium hydrogen phosphate solution. Then, transfer the well-mixed solution to a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat it at 200 °C for 24 h. Wash the crystallized product with distilled water until neutral and dry it at 90 °C for 12 h to obtain SAPO-34 molecular sieve.

[0044] (2) Dissolve 1.10 g of pseudoboehmite in 22 mL of 25 wt% tetraethylammonium hydroxide and 4 mL of distilled aqueous solution. After complete dissolution, add 0.2 mL of tetraethyl orthosilicate and 4 mL of 85 wt% phosphoric acid solution. Then, add 4.5 g of the SAPO-34 molecular sieve obtained in step (1). After thorough mixing, place the mixed solution into a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat at 180°C for 48 h. Wash the obtained product with distilled water until neutral to obtain SAPO-34@nano-SAPO-34 zeolite catalyst.

[0045] Example 5: Preparation of SAPO-34@nano-SAPO-34

[0046] (1) Weigh 7.56 g of aluminum sec-butoxide and dissolve it in 8 mL of diethylamine and 30 mL of distilled water. Under vigorous stirring, add 1.20 g of silica sol and 2.2 mL of 85 wt% ammonium dihydrogen phosphate solution. Then, transfer the well-mixed solution to a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat it at 200 °C for 24 h. Wash the crystallized product with distilled water until neutral and dry it at 90 °C for 12 h to obtain SAPO-34 molecular sieve;

[0047] (2) Add 2.2g of alumina to 6.0g of morpholine and 4mL of distilled aqueous solution. After stirring evenly, add 0.2mL of tetraethyl orthosilicate and 4mL of 85wt% phosphoric acid solution. Then, add 4g of the SAPO-34 molecular sieve obtained in step (1). After stirring evenly, place the mixed solution into a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat at 180℃ for 48h. Wash the obtained product with distilled water until neutral to obtain SAPO-34@nano-SAPO-34 zeolite catalyst.

[0048] Example 6: Preparation of SAPO-34@nano-SAPO-34

[0049] (1) Weigh 5.56 g of aluminum isopropoxide and dissolve it in 8 mL of diethylamine and 30 mL of distilled water. Under vigorous stirring, add 1.20 g of silica sol and 2.2 mL of 85 wt% ammonium dihydrogen phosphate solution. Then, transfer the well-mixed solution to a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat it at 180 °C for 36 h. Wash the crystallized product with distilled water until neutral and dry it at 90 °C for 12 h to obtain SAPO-34 molecular sieve;

[0050] (2) Add 2.2g of alumina to 7.5g of triethylamine and 4mL of distilled aqueous solution. After stirring evenly, add 0.2mL of tetraethyl orthosilicate and 4mL of 85wt% phosphoric acid solution. Then, add 5g of the SAPO-34 molecular sieve obtained in step (1). After stirring evenly, place the mixed solution into a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heat at 190℃ for 36h. Wash the obtained product with distilled water until neutral to obtain SAPO-34@nano-SAPO-34 zeolite catalyst.

[0051] The core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst prepared in this invention uses SAPO-34 as the core phase, which serves only as a physical support. Low-silica nano-SAPO-34 crystals are induced to grow secondaryly on its surface, constructing a homomorphic core-shell structure. This invention can significantly improve the catalyst's lifespan and selectivity for low-carbon olefins in the MTO reaction. The preparation method of this invention has advantages such as readily available raw materials, simple process, relatively mild conditions, and high reproducibility, enabling industrial application and showing broad application prospects in solving energy shortage problems.

Claims

1. A core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst, characterized in that: The core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst is a core-shell SAPO-34@nano-SAPO-34 zeolite catalytic material with microporous SAPO-34 as the core and low-silicon nano-SAPO-34 as the shell; Specifically, a layer of low-silicon nano-SAPO-34 zeolite crystal is epitaxially grown on the surface of microporous SAPO-34, and the shell layer has a thickness of 150-250 nm; The microporous SAPO-34 has a grain size of 5-10 μm, and the shell layer of low-silicon SAPO-34 nanocrystal has a grain size of 30-50 nm. The core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst has the core phase SAPO-34 serving as a physical carrier, inducing the secondary growth of low-silicon nano-SAPO-34 crystals on the surface of the core phase SAPO-34, and forming a homomorphous core-shell structure; and the shell layer of nanocrystal is loosely packed to form a rich intercrystalline pore structure.

2. The core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst according to claim 1, characterized in that: The preparation method of the core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst is as follows: (1) The core phase SAPO-34 is prepared by a hydrothermal synthesis method; specifically, an aluminum source, a silicon source, a phosphorus source and a structure directing agent are sequentially added into distilled water in a molar composition ratio of Al2O3:SiO2:P2O5:SDA:H2O = 1:0.2-1.0:0.7-1.3:4.1-7.9:100.2-300.5; the obtained solution is uniformly stirred, and then is moved into a stainless steel reaction kettle with a polytetrafluoroethylene lining to perform a hydrothermal crystallization reaction, so as to obtain the core phase SAPO-34 molecular sieve; the hydrothermal crystallization temperature is 180-200 ℃, and the crystallization time is 24-96 h; (2) The core phase SAPO-34 molecular sieve obtained in step (1) is added into a gel precursor solution for synthesizing nano-SAPO-34 zeolite at room temperature, in a molar composition ratio of Al2O3:SiO2:P2O5:SDA:H2O = 1:0.06-0.13:1.5-5.2:2-10:90-543; after stirring for several hours, the obtained solution is moved into a stainless steel reaction kettle with a polytetrafluoroethylene lining to perform a hydrothermal crystallization reaction; then, the crystallized product is washed to neutral, dried overnight, and calcined to obtain the core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst; the hydrothermal crystallization temperature is 160-200 ℃, and the crystallization time is 12-72 h; In step (2), the mass ratio of the core phase SAPO-34 molecular sieve to the aluminum oxide in the nano-SAPO-34 zeolite gel precursor is 1-30:

1.

3. The core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst according to claim 2, characterized in that: In step (2), the shell layer of the core-shell SAPO-34@nano-SAPO-34 catalyst has a thickness of 210 nm.

4. The core-shell SAPO-34@nano-SAPO-34 molecular sieve catalyst according to claim 2, characterized in that: In step (2), the calcination temperature is 500-600 ℃.

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