A method for synthesizing a SAPO-34 zeolite molecular sieve having a square thin block morphology

Abstract

The application discloses a synthesis method of SAPO-34 zeolite molecular sieve with square thin block morphology, and is characterized by the fact that the prepared SAPO-34 zeolite molecular sieve has square thin block morphology. The SAPO-34 zeolite molecular sieve has high diene selectivity and long catalytic service life in a methanol-to-olefin reaction, and the thickness of the crystal block is concentratedly distributed at about 0.8 mu m. The synthesis method comprises the following steps: mixing a silicon source, an aluminum source, a phosphorus source, a template agent, water, a crystal seed and a surface modifier to form an initial gel; and then performing crystallization to prepare the SAPO-34 zeolite molecular sieve. The SAPO-34 zeolite molecular sieve in the application presents square thin block morphology, and is particularly suitable for being used as an active component of a catalyst in a methanol-to-olefin (MTO) reaction, can improve the yield of low-carbon olefins, and the catalyst has good stability and long service life.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve synthesis technology, specifically, it relates to a SAPO-34 zeolite molecular sieve with a cubic thin block morphology and its synthesis method. Background Technology

[0002] Low-carbon olefins (ethylene and propylene) are core raw materials for the organic chemical industry and hold a vital position in the modern petrochemical field. my country is a country rich in natural gas and coal but lacking in oil; therefore, developing new processes for producing olefins from coal or natural gas is of significant practical and strategic importance. Traditionally, olefins are mainly obtained through petroleum cracking of naphtha, with approximately 60-70% of propylene derived from ethylene co-production. Globally, ethylene demand is currently approaching saturation, while propylene demand continues to grow.

[0003] In 1984, Union Carbide Corporation (UCC) in the United States first synthesized aluminosilicate phosphate molecular sieve (SAPO molecular sieve) with a pore size of approximately 0.4 nm. Among the SAPO series, SAPO-34 zeolite molecular sieve is widely used in modern petroleum processing due to its excellent thermal and hydrothermal stability, moderate acidity, high specific surface area, and highly ordered microporous structure. Its performance in methanol-to-olefins (MTO) reactions is particularly outstanding, achieving a methanol conversion rate of up to 100%, a total selectivity of over 80% for ethylene and propylene, and minimal C5+ and aromatic byproducts. MTO technology has opened up new pathways for olefin production from non-petroleum resources such as coal, natural gas, and biomass, becoming the most likely alternative to traditional petroleum routes for low-carbon olefin production, and allowing for flexible adjustment of product ratios to meet market demands. Given my country's energy structure of "abundant coal, scarce oil, and limited gas," developing MTO technology is of great strategic significance for ensuring national energy security and promoting sustainable economic development.

[0004] Among numerous MTO catalysts, SAPO-34 zeolite molecular sieve has become the preferred catalyst due to its suitable acidity, excellent pore structure, and good catalytic stability. Its three-dimensional eight-membered ring pore structure is consistent with the topology of chalcogenide (CHA), with an effective pore diameter of 0.43-0.50 nm. This moderate acidity and small pore characteristic endow SAPO-34 zeolite molecular sieve with outstanding catalytic performance in the MTO reaction, achieving almost complete methanol conversion and high selectivity for ethylene and propylene, demonstrating broad application prospects.

[0005] Currently, SAPO-34 zeolite molecular sieves are mainly synthesized using a hydrothermal method. Although researchers have developed various novel preparation methods, their complexity or specialized equipment requirements hinder industrial application. Therefore, industrial production remains dominated by the hydrothermal method. This process involves mixing aluminum, phosphorus, and silicon sources with a template agent to form a gel, followed by crystallization in a reaction vessel. However, SAPO-34 zeolite molecular sieves synthesized via conventional hydrothermal methods suffer from uneven crystal size distribution, which affects their catalytic performance. Improving the hydrothermal synthesis process to enhance the activity and stability of SAPO-34 zeolite molecular sieves in MTO reactions remains a core focus in this field.

[0006] Existing research has shown that, compared to conventional cubic structures, the unique morphologies of SAPO-34 zeolite molecular sieves (such as hollow, nanosheets, and hierarchical pores) significantly enhance MTO catalytic performance primarily by shortening diffusion paths and altering carbon deposition mechanisms. Hollow structures form "carbon storage chambers," where large molecular carbon deposits preferentially accumulate, protecting the active sites within the pores; simultaneously, the thin walls drastically shorten the diffusion path. Nanosheets / nanoparticles, with their extremely short b-axis thickness or nanoscale dimensions, significantly reduce diffusion distances and carbon deposition rates. Furthermore, the introduction of mesopores into the microporous structure creates "highways," promoting rapid molecular diffusion, significantly improving mass transfer efficiency, and reducing secondary reactions. Compared to conventional micron-sized blocks (with long diffusion paths), the apparent diffusion coefficient of these unique morphologies (hollow / nano) can be increased by 1-2 orders of magnitude. Conventional catalysts are prone to carbon deposition clogging pores, leading to a "cage effect"; these unique morphologies transform the carbon deposits from large, pore-clogging graphitic carbon molecules into less toxic, lighter aromatic hydrocarbons, thereby delaying deactivation. Nanostructured hollow structures can be synthesized directly from industrial waste or through in-situ pore creation with metal assistance (such as Zn). The unique morphology primarily functions by optimizing mass transfer and carbon retention.

[0007] Previous studies have found that, in addition to specific surface area, acid center density, crystal phase composition, and grain size affecting the catalytic performance of SAPO-34 zeolite molecular sieves as MTO catalysts, grain morphology also significantly influences their MTO catalytic performance. Therefore, although some methods for preparing non-cubic SAPO-34 zeolite molecular sieves have been developed, enriching the preparation methods for SAPO-34 zeolite molecular sieves to some extent, most of these methods have numerous problems. Furthermore, the synthesis of SAPO-34 zeolite molecular sieves is hampered by the large variety, high cost, and large quantity of organic template agents used, leading to environmental pollution that limits its industrial application. Effectively addressing the issue of excessively long residence times of reactants and target products in traditional cubic SAPO-34 zeolite molecular sieves, resulting in catalyst deactivation, short lifespan in MTO reactions, and low diene selectivity in the obtained products, is a key research hotspot and important topic. Many researchers have studied low-cost, green SAPO-34 zeolite molecular sieves with unique morphologies. For example, Gong et al. [Gong J, Tong F, Zhang C, et al. Microporous and Mesoporous Materials, 2021, 331, 111664.] developed a biosynthetic route for preparing hierarchical, fluffy SAPO-34 zeolite molecular sieves. Commercially available bacterial cellulose aerogels were used as templates, and SiO2-loaded bacterial cellulose aerogels were used as silica sources. Crystallization was achieved using a simple hydrothermal treatment. Irina Shamanaeva et al. [Shamanaeva I, Strelova S, Solovyeva M, et al. Nanomaterials, 2022, 12, 4086.] prepared SAPO-34 zeolite molecular sieve nanocrystals with sizes ranging from 50 to 150 nm from two aluminum precursors (aluminum isopropoxide and aluminum lignite) using a steam-assisted crystallization (SAC) process at 200 °C for 5 hours. The proposed synthetic method is economical and efficient, and can be used to prepare highly crystalline SAPO-34 zeolite molecular sieve nanoparticles without the need for subsequent synthetic steps such as separating nanocrystals from the mother liquor.Fang Yiwen et al. [CN110092390, 2022] invented a method for preparing loose, porous spherical SAPO-34 zeolite molecular sieves. The prepared loose, porous spherical SAPO-34 molecular sieves have a large number of mesopores communicating with the outside and a small number of macropores in the bulk phase, exhibiting a hierarchical pore structure; the external specific surface area is significantly increased, and the number of active sites is significantly increased; it promotes the mass transfer between reactants and products, effectively reduces the carbon deposition rate in the catalytic reaction process, and prolongs the catalyst lifetime. Xu Jinghua et al. [CN116199239, 2025] invented a method for preparing SAPO-34 zeolite molecular sieves with low silica sheet morphology. The synthesized SAPO-34 product has both low acidity and extremely small nanosheet morphology. This molecular sieve exhibits extremely high selectivity for low-carbon olefins in the methanol-to-olefins reaction and excellent stability.

[0008] In summary, previous work on the synthesis of SAPO-34 zeolite molecular sieves has included the preparation of SAPO-34 zeolite molecular sieves with special morphologies. Previous work has shown that SAPO-34 zeolite molecular sieves with special morphologies have superior MTO catalytic performance as MTO catalysts. Summary of the Invention

[0009] This invention addresses the technical problem that excessively long residence times of reactants and target products in traditional cubic SAPO-34 zeolite molecular sieves lead to catalyst deactivation, short catalyst lifetime in MTO reactions, and low diene selectivity in the resulting products. The invention provides a simple and widely applicable method for synthesizing SAPO-34 zeolite molecular sieves with a cubic, blocky morphology. The SAPO-34 zeolite molecular sieve prepared by this method exhibits a cubic, blocky morphology and demonstrates high diene selectivity and a long catalytic lifetime in MTO reactions.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: A method for synthesizing SAPO-34 zeolite molecular sieves with a cubic block morphology includes the following steps: (1) Mix the phosphorus source and water to obtain mixed solution A; (2) Add an aluminum source to the mixed solution A obtained in step (1) and mix to obtain mixed solution B; (3) Add template agent to the mixed solution B obtained in step (2) and mix to obtain mixed solution C; (4) Add silicon source, water and surface modifier to the mixed solution C obtained in step (3) and mix to obtain mixed solution D; (5) Add SAPO-34 zeolite molecular sieve raw powder (seed crystal) to the mixed solution D obtained in step (4) and mix to obtain the initial gel; (6) The initial gel obtained in step (5) is subjected to hydrothermal crystallization, and then the material after crystallization is cooled, centrifuged, dried or calcined to obtain SAPO-34 zeolite molecular sieve with a cubic thin block morphology.

[0011] As a preferred embodiment, in the above-mentioned method for synthesizing SAPO-34 zeolite molecular sieve with a cubic thin block morphology, the phosphorus source in step (1) is calculated as P2O5, and the molar ratio of phosphorus source to water is 0.05~20:0.1~100. Preferably, the molar ratio of phosphorus source to water is 0.85~9.97:0.9~80.

[0012] As a preferred embodiment, in the above-mentioned method for synthesizing SAPO-34 zeolite molecular sieve with a cubic thin block morphology, the aluminum source in step (2) is calculated as Al2O3, and the molar ratio of aluminum source to water is 0.01~10:0.1~100. Preferably, the molar ratio of aluminum source to water is 0.55~5.85:0.9~80.

[0013] Preferably, in the above-mentioned method for synthesizing SAPO-34 zeolite molecular sieve with a cubic block morphology, the template agent in step (3) is one or a mixture of MO (morpholine), TEAOH (tetraethylammonium hydroxide), TEA (triethylamine), and DEA (diethylamine).

[0014] Preferably, in the above-mentioned method for synthesizing SAPO-34 zeolite molecular sieve with a cubic block morphology, the molar ratio of template agent to water in step (3) is 0.01~10:0.1~100, and more preferably, the molar ratio of template agent to water is 0.01~5.00:0.9~80.

[0015] As a preferred embodiment, in the above-mentioned method for synthesizing SAPO-34 zeolite molecular sieve with a cubic block morphology, the silicon source in step (4) is SiO2, and the molar ratio of silicon source to water is 0~2.0:0.1~100; preferably, the molar ratio of silicon source to water is 0.1~1.5:0.9~80.

[0016] Preferably, in the above-mentioned method for synthesizing SAPO-34 zeolite molecular sieves with a tetragonal block morphology, the surface modifier is one or a mixture of 3-methylpyridine, 4-methylpyridine, and N,N-diethyl-1-propane, and the mass percentage of the surface modifier added is 0-5%.

[0017] Preferably, in the above-mentioned method for synthesizing SAPO-34 zeolite molecular sieves with a tetragonal thin block morphology, the silicon source is one or more of silica sol, TEOS (tetraethyl orthosilicate), silica fume, water glass, silica gel, and silica, preferably one or more of silica sol and TEOS; the aluminum source is one or more of boehmite, aluminum isopropoxide, alumina, aluminum hydroxide, and aluminum sulfate, preferably one or more of boehmite, aluminum isopropoxide, and aluminum sulfate; the phosphorus source is one or more of phosphoric acid, phosphorous acid, and diamine hydrogen phosphate, preferably one or more of phosphoric acid and phosphorous acid.

[0018] Preferably, in the above-mentioned method for synthesizing SAPO-34 zeolite molecular sieve with a cubic block morphology, the mass percentage of SAPO-34 zeolite molecular sieve raw powder added in step (5) is 1~5%.

[0019] Preferably, in the above-mentioned method for synthesizing SAPO-34 zeolite molecular sieve with a tetragonal block morphology, the hydrothermal crystallization temperature in step (6) is 110~290℃ and the time is 10~72h. More preferably, the hydrothermal crystallization temperature is 130~260℃ and the time is 16~54h.

[0020] The above-mentioned surface modifier addition mass percentage is defined as the ratio of the mass of the surface modifier to the sum of the masses of the phosphorus source (calculated as P2O5), aluminum source (calculated as Al2O3), and silicon source (calculated as SiO2) in the raw materials; the seed crystal addition mass percentage in step (5) is defined as the ratio of the seed crystal mass to the sum of the masses of the phosphorus source (calculated as P2O5), aluminum source (calculated as Al2O3), and silicon source (calculated as SiO2) in the raw materials.

[0021] Compared with the prior art, the present invention has the following beneficial effects: The SAPO-34 zeolite molecular sieve prepared in this invention has a cubic thin-film morphology. When used in methanol-to-olefins reactions, it can improve the yield of low-carbon olefins (ethylene and propylene), with the highest yield reaching 86.73%. Furthermore, the catalyst exhibits good stability and a long lifespan, with a maximum lifespan of 398 min. The optimal average thickness of the prepared SAPO-34 zeolite molecular sieve is 0.8 μm, and it possesses a cubic thin-film morphology. Attached Figure Description

[0022] Figure 1 SEM image of molecular sieve A prepared in [Comparative Example 1]; Figure 2 SEM image of molecular sieve C prepared in Example 2. Detailed Implementation

[0023] To provide a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the technical solutions of the present invention will be further illustrated below through embodiments. However, these embodiments should not be construed as limiting the scope of implementation of the present invention, and the scope of protection of the present invention is not limited by the embodiments.

[0024] In this invention, the SEM images were obtained using a JEOL JSM-IT200A scanning electron microscope from Japan.

[0025] In this invention, the yield of each product is expressed in molar amounts.

[0026] Comparative Example 1 Phosphoric acid (85 wt% as P2O5) and 95% deionized water were mixed and stirred at room temperature for 2 hours to dissolve and mix evenly. Then, boehmite (78 wt% as Al2O3) was added and stirred for 2 hours to form a gel. Triethylamine was then slowly and evenly added and stirred for 2 hours. Next, silica sol (30 wt% as SiO2) and the remaining 5% deionized water were slowly added and stirred for another 2 hours to obtain an initial gel mixture. The initial gel mixture had a molar ratio of SiO2:Al2O3:P2O5:TEA:H2O of 0.24:1:1.06:4.00:100. The initial gel mixture was placed in a polytetrafluoroethylene reactor liner, fixed on a homogeneous reactor, and heated to 190°C at 20 rpm for 180 minutes. Hydrothermal crystallization was carried out at 190°C for 36 hours to obtain the crystallized gel. After crystallization, the crystallized product is cooled, centrifuged, dried, and calcined at 550℃ for 6 hours to obtain a molecular sieve, denoted as molecular sieve A.

[0027] SEM image of molecular sieve A as shown Figure 1 As shown, although the surface of molecular sieve A is very smooth and has a regular cubic morphology, the average particle size of molecular sieve A is 10 μm and the particle size distribution is uneven.

[0028]

Example 1

[0029]

Example 2

[0030] SEM images of molecular sieve C are shown below. Figure 2 As shown, the surface of molecular sieve C is very smooth, with a cubic morphology, and the average thickness of molecular sieve C is 0.8 μm, with a uniform particle size distribution.

[0031]

Example 3

[0032]

Example 4

[0033]

Example 5

[0034]

Example 6

[0035]

Example 7

[0036] The molecular sieve prepared above was used as a catalyst for the MTO reaction. The process conditions used were as follows: a fixed-bed catalytic reactor was used; 300 mg of catalyst (40-60 mesh) was mixed with quartz sand at a mass ratio of 1:2 and then filled into a quartz reaction tube with an inner diameter of 8 mm; the reaction temperature was 460 °C; and the methanol feed weight hourly space velocity was 2 h⁻¹. -1 The pressure was 0.1 MPa, and the evaluation results are shown in Table 1. As can be seen from Table 1, the method of this invention can obtain SAPO-34 zeolite molecular sieves with a tetragonal thin-film morphology, high diene yield, and long lifetime.

[0037] Table 1

Claims

1. A method for synthesizing SAPO-34 zeolite molecular sieves with a cubic thin-film morphology, characterized in that... Includes the following steps: (1) Mix the phosphorus source and water to obtain mixed solution A; (2) Add an aluminum source to the mixed solution A obtained in step (1) and mix to obtain mixed solution B; (3) Add template agent to the mixed solution B obtained in step (2) and mix to obtain mixed solution C; (4) Add silicon source, water and surface modifier to the mixed solution C obtained in step (3) and mix to obtain mixed solution D; (5) Add SAPO-34 zeolite molecular sieve powder to the mixed solution D obtained in step (4) and mix to obtain the initial gel; (6) The initial gel obtained in step (5) is subjected to hydrothermal crystallization, and then the material after crystallization is cooled, centrifuged, dried or calcined to obtain SAPO-34 zeolite molecular sieve with a cubic thin block morphology.

2. The method for synthesizing SAPO-34 zeolite molecular sieves with a tetragonal thin-film morphology as described in claim 1, characterized in that... The phosphorus source mentioned in step (1) is calculated as P2O5, and the molar ratio of phosphorus source to water is 0.05~20:0.1~100. Preferably, the molar ratio of phosphorus source to water is 0.85~9.97:0.9~80.

3. The method for synthesizing SAPO-34 zeolite molecular sieves with a tetragonal thin-film morphology as described in claim 1, characterized in that... The aluminum source mentioned in step (2) is Al2O3, and the molar ratio of aluminum source to water is 0.01~10:0.1~100. Preferably, the molar ratio of aluminum source to water is 0.55~5.85:0.9~80.

4. The method for synthesizing SAPO-34 zeolite molecular sieves with a tetragonal thin-film morphology as described in claim 1, characterized in that... The template agent mentioned in step (3) is one or a mixture of morpholine, tetraethylammonium hydroxide, triethylamine, and diethylamine.

5. The method for synthesizing SAPO-34 zeolite molecular sieves with a tetragonal thin block morphology as described in claim 1, characterized in that... The molar ratio of the template agent to water in step (3) is 0.01~10:0.1~100. Preferably, the molar ratio of the template agent to water is 0.01~5.00:0.9~80.

6. The method for synthesizing SAPO-34 zeolite molecular sieves with a tetragonal thin-film morphology as described in claim 1, characterized in that... The silicon source mentioned in step (4) is SiO2, and the molar ratio of silicon source to water is 0~2.0:0.1~100; preferably, the molar ratio of silicon source to water is 0.1~1.5:0.9~80.

7. The method for synthesizing SAPO-34 zeolite molecular sieves with a tetragonal thin-film morphology as described in claim 1, characterized in that... The surface modifier is one or a mixture of 3-methylpyridine, 4-methylpyridine, and N,N-diethyl-1-propane, and the mass percentage of the surface modifier added is 1-5%.

8. The method for synthesizing SAPO-34 zeolite molecular sieves with a tetragonal thin block morphology as described in claim 1, characterized in that... The silicon source is one or more of silica sol, tetraethyl orthosilicate, silica fume, water glass, silica gel, and silica, preferably one or more of silica sol and TEOS; the aluminum source is one or more of boehmite, aluminum isopropoxide, alumina, aluminum hydroxide, and aluminum sulfate, preferably one or more of boehmite, aluminum isopropoxide, and aluminum sulfate; the phosphorus source is one or more of phosphoric acid, phosphorous acid, and diamine hydrogen phosphate, preferably one or more of phosphoric acid and phosphorous acid.

9. The method for synthesizing SAPO-34 zeolite molecular sieves with a tetragonal thin block morphology as described in claim 1, characterized in that... The mass percentage of SAPO-34 zeolite molecular sieve powder added in step (5) is 1~5%.

10. The method for synthesizing SAPO-34 zeolite molecular sieve with a tetragonal thin block morphology as described in claim 1, characterized in that... The hydrothermal crystallization temperature in step (6) is 110~290℃ and the time is 10~72h. Preferably, the hydrothermal crystallization temperature is 130~260℃ and the time is 16~54h.

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