A small crystal saPO-34 molecular sieve and a preparation method thereof

Abstract

The application belongs to the technical field of material synthesis, and relates to small-grain SAPO-34 molecular sieve and a preparation method thereof, wherein the preparation method comprises the following steps: mechanically crushing large-grain SAPO-34 molecular sieve in low-cost organic template to obtain a seed template liquid; uniformly mixing an aluminum source with deionized water, adding a phosphorus source, adding a silicon source, and then mixing the seed template liquid, dynamic crystallization, separation, washing, drying, and removing the template. The method can be used for synthesizing SAPO-34 molecular sieve by using large-grain seeds, and the synthesized product has high crystallinity and high yield.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve synthesis technology, specifically relating to a method for preparing small-crystal SAPO-34 molecular sieve. Background Technology

[0002] In the early 1980s, Union Carbide Corporation of the United States successfully developed a novel molecular sieve system—the silica-alumina (SAPO-n) series of molecular sieves. These sieves consist of a three-dimensional framework structure composed of phosphorus-oxygen tetrahedra (PO4), aluminum-oxygen tetrahedra (AlO4), and silicon-oxygen tetrahedra (SiO4). After 40 years of development, the structural types of silica-alumina molecular sieves have reached more than 60, and they are widely used in C1 chemistry, environmental protection and emission reduction, and oil refining and chemical industries, providing material support for technologies such as comprehensive carbon resource utilization, clean environmental protection, and the transformation of oil refining into chemical engineering.

[0003] SAPO-34 molecular sieve is a silica-alumina phosphate molecular sieve with a chalcogenide (CHA) topology. Its framework structure consists of parallel double six-membered rings stacked in an ABC configuration. SAPO-34 exhibits eight-membered ring pores and a three-dimensional intersecting channel structure. The pore size is 0.38 nm × 0.38 nm, and an ellipsoidal CHA cage with dimensions of 1.0 nm × 0.67 nm × 0.67 nm is formed at the intersection of the eight-membered ring channels. Due to its suitable acid strength, unique pore structure, good stability, and hydrothermal stability, SAPO-34 molecular sieve has been proven to be an ideal catalyst for methanol-to-olefins (MTO) reactions. It also has wide applications in the selective reduction of nitrogen oxides, CO2 hydrogenation to olefins reactions, and the adsorption and separation of CO2 gas.

[0004] The commonly used synthesis method for SAPO-34 molecular sieve is hydrothermal synthesis, using water as a solvent. Commonly used organic template agents include tetraethylammonium hydroxide, triethylammonium, diethylammonium, morpholine, isopropylammonium, and mixtures thereof. The synthesized samples usually appear as cubic crystals.

[0005] The small pore size of SAPO molecular sieves provides a unique confinement effect for reactions, but these narrow channels also introduce significant mass transfer resistance, adversely affecting the catalyst's reactivity and lifespan. Studies have shown that synthesizing small-crystal SAPO-34 molecular sieves can not only shorten the residence time of reactant molecules within the pores and improve intracrystalline diffusion efficiency, but also enhance the connectivity between the pores and the outer surface, thereby increasing the utilization rate of active sites.

[0006] One method for synthesizing small-crystal SAPO-34 molecular sieves involves using seed crystals, such as finished molecular sieves or seed-directing liquids. However, using seed-directing liquids (or precursors) as seed crystals for SAPO-34 synthesis presents challenges. Since the directing liquids primarily contain primary or secondary structural units, not SAPO-34 molecular sieve nuclei, they are prone to generating other impurities during crystallization and require a long crystallization time. Furthermore, seed-directing liquids cannot be stored for extended periods, making them inconvenient to use. Using finished molecular sieves as seed crystals solves these problems.

[0007] CN 110342538 A discloses a method for synthesizing small-grained SAPO-34 molecular sieve with silicon silicate and its application. First, aluminum, phosphorus, silicon, organic amine, and deionized water are added to a container and mixed to obtain a homogeneous gel. This gel is then placed in a crystallization vessel and pre-crystallized at a higher temperature, followed by cooling to room temperature. Seed crystals are then added and mixed, and the mixture is heated further for crystallization. The crystallized product is centrifuged, washed, and dried to obtain the small-grained SAPO-34 molecular sieve with silicon silicate. This method requires adding SAPO-34 molecular sieve with a grain size less than 500 nm as seed crystals to the initial gel. However, the preparation of small-particle molecular sieves requires complex processes, leading to wastewater and waste gas emissions and increased operating costs.

[0008] CN 113955767 A discloses a method for preparing heterogeneous molecular sieves with crystal seeding liquid by mixing an aluminum source, phosphorus source, silicon source, template agent, and water to form a gel, stirring it evenly, and then adding an alkaline solution to the mixture. This application uses an inexpensive template agent to prepare nano-SAPO-34 molecular sieves with crystal sizes of 20-800 nm via hydrothermal synthesis. This method requires treating the heterogeneous molecular sieves with an alkaline solution, generating alkaline wastewater in the process.

[0009] Existing technologies for synthesizing small-crystal SAPO-34 molecular sieves using SAPO-34 molecular sieves as seed crystals either directly use small-particle-size SAPO-34 molecular sieves as seed crystals, treat commercial molecular sieves with alkaline solutions, or use expensive organic template agents, which increases production costs or environmental pressure. Summary of the Invention

[0010] The technical problem to be solved by this invention is the difficulty in using large-grained SAPO-34 molecular sieves as seed crystals to synthesize small-grained SAPO-34 molecular sieves in the prior art. This invention provides a simple and low-cost method for preparing small-grained SAPO-34 molecular sieves. The method adopts the traditional hydrothermal synthesis method and uses conventional large-grained SAPO-34 molecular sieves to synthesize plate-shaped small-grained SAPO-34 molecular sieves without using expensive organic template agent tetraethylammonium hydroxide.

[0011] In this invention, the grain thickness refers to the size of the grain in this dimension being smaller than the size of the other two dimensions perpendicular to it in three-dimensional space, and the grain size refers to the side length of the longest side of the grain surface perpendicular to the grain thickness direction.

[0012] In this invention, the grain size and thickness are determined by scanning electron microscopy (SEM). The measurement method is as follows: 50 grains are randomly selected, and the grain size and thickness are measured by measurement software. The minimum and maximum values ​​of the grain size and thickness are taken as their respective size ranges.

[0013] In this invention, the seed crystal size refers to the particle size measured using a laser particle size analyzer, with the testing reference standard being GB / T19077-2016 Particle Size Distribution by Laser Diffraction.

[0014] In this invention, the relative crystallinity is obtained by selecting the peak areas of the four strongest diffraction peaks in the XRD spectrum, using commercially available large-grain SAPO-34 molecular sieve as a reference sample, and calculating the ratio of the sum of the areas of the four strongest peaks of the synthesized sample to the sum of the corresponding peak areas of the commercial sample.

[0015] In this invention, the product yield is defined by calculating the initial material mass (the amount of silicon source, aluminum source, phosphorus source, and seed crystal added), the quantitative product mass (the mass after high-temperature calcination to remove the template agent), and dividing the product mass by the initial material mass.

[0016] This invention provides a method for preparing small-crystal SAPO-34 molecular sieves, comprising the following steps:

[0017] a) Disperse large-grain SAPO-34 molecular sieves in an organic template agent, and mechanically crush the large-grain SAPO-34 molecular sieves to obtain mixture A; the organic template agent is a low-cost organic template agent;

[0018] b) Mix the aluminum source with deionized water until homogeneous, for example, by stirring for 0.5 to 4 hours to obtain mixture B;

[0019] c) Add a phosphorus source to mixture B and stir, preferably for more than 1 hour, for example, 1 to 8 hours, to obtain mixture C;

[0020] d) Add a silicon source to the mixture C and stir for 0.5 to 6 hours to obtain mixture D;

[0021] e) Mixture D and mixture A together, and stir for 0.5 to 2 hours to obtain mixture E;

[0022] f) The mixture E is hydrothermally dynamically crystallized at 150-220°C for 12-120 hours. In one embodiment of the dynamic crystallization, the crystallization vessel is rotated at a speed of 5-50 r / min. After crystallization, the solid product is obtained by separation, such as centrifugation, washing, and drying, i.e., molecular sieve powder.

[0023] g) Remove the template agent from the molecular sieve powder, for example, by calcining it in air at 450-600°C for 2-8 hours to remove the template agent, to obtain SAPO-34 molecular sieve.

[0024] According to the present invention, in step a, the large-grain SAPO-34 molecular sieve is mixed with an organic template agent, and then the large-grain SAPO-34 molecular sieve is mechanically crushed in the organic template agent. The large-grain SAPO-34 molecular sieve can be mechanically crushed by ball milling, for example, by ball milling at a speed of 100-600 r / min for 1-8 h.

[0025] According to the present invention, steps a, b, c, d, and e can be performed at room temperature. The room temperature described in the present invention can be 10-40°C, for example, 20-30°C.

[0026] According to the present invention, the silicon source may be selected from one or more of silica sol, fumed silica or tetraethyl orthosilicate.

[0027] The aluminum source may be selected from one or more of boehmite, pseudoboehmite, alumina, aluminum hydroxide, or aluminum isopropoxide.

[0028] The phosphorus source can be one or more of orthophosphoric acid, phosphorous acid, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate.

[0029] The low-cost organic template agent may be selected from one or more of diethylamine, triethylamine, morpholine, or isopropylamine.

[0030] The large-grain SAPO-34 molecular sieve can be a commercially available or self-synthesized large-grain SAPO-34 molecular sieve. The grain size of the large-grain SAPO-34 molecular sieve is the same as that of conventional SAPO-34 molecular sieves, but larger than that of the small-grain SAPO-34 molecular sieves prepared by the method provided in this invention. The grain size of the large-grain SAPO-34 molecular sieve is greater than 2 micrometers, for example, 5 to 10 micrometers.

[0031] In one embodiment, the mechanical crushing of the large-grained SAPO-34 molecular sieve results in a particle size of 0.1–1.5 micrometers in mixture A, for example, a particle size of 0.18–1.28 micrometers. The particle size in mixture A is measured using laser particle size distribution.

[0032] The method for preparing small-crystal SAPO-34 molecular sieves provided by this invention has the following molar ratios of each component in the mixture E (also known as the initial gel mixture):

[0033] SiO2:Al2O3 = 0.2–0.8, for example, 0.4–0.8;

[0034] P2O5:Al2O3 = 0.6–1.2;

[0035] Template agent: Al2O3 = 0.05-4.0, for example 0.2-3 or 0.5-1.5;

[0036] H2O:Al2O3 = 20-100, for example 40-80; wherein SiO2, Al2O3 and P2O5 are the amounts of silicon, aluminum and phosphorus introduced from silicon, aluminum and phosphorus sources, converted into SiO2, Al2O3 and P2O5.

[0037] According to the present invention, based on the total weight of SiO2, Al2O3 and P2O5 in the raw materials (silicon source, aluminum source and phosphorus source), the proportion of seed crystals is 0.5 to 30% by weight, for example 5 to 15% by weight.

[0038] This invention provides a small-grained SAPO-34 molecular sieve, wherein the grain size and thickness are both less than 2 μm. In one embodiment, the grain thickness of the small-grained SAPO-34 molecular sieve is 0.05–0.9 μm, for example, 0.06–0.65 μm, and the grain size is 0.2–1.6 μm, for example, 0.2–1.5 μm. In another embodiment, the surface of the small-grained SAPO-34 molecular sieve grains perpendicular to the thickness direction is rectangular.

[0039] Compared with the prior art, the present invention has at least one of the following beneficial effects, preferably multiple or all of the following beneficial effects:

[0040] 1. The small-crystal SAPO-34 molecular sieve provided by this invention is synthesized using a traditional hydrothermal synthesis method, and does not require the addition of solvents other than water, crystal morphology modifiers or surfactants, which reduces the subsequent separation cost and makes it easier to realize the industrial production of molecular sieves.

[0041] 2. This invention uses an inexpensive organic template agent as the sole template agent, avoiding the use of the expensive organic template agent tetraethylammonium hydroxide, thus reducing production costs.

[0042] 3. In this invention, large-grain SAPO-34 molecular sieves are dispersed in an organic template agent for mechanical treatment, and can be directly added to the molecular sieve synthesis gel without washing or filtration, without introducing other impurities; the organic template agent is in full contact with the seed crystal, which is beneficial for the growth of silicon, phosphorus and aluminum atoms on the seed crystal surface during the subsequent crystallization process.

[0043] 4. This invention significantly improves the yield of SAPO-34 molecular sieve.

[0044] 5. This invention significantly improves the crystallinity of SAPO-34 molecular sieve and effectively inhibits the formation of SAPO-5 impurities.

[0045] 6. This invention yields a flat, small-crystal SAPO-34 molecular sieve. The resulting flat, small-crystal SAPO-34 molecular sieve material has small crystal size, thin thickness, and high crystallinity. Compared to cubic molecular sieves with large crystal size, the flat, small-crystal molecular sieve can significantly improve the diffusion rate of reactants and products, reduce carbon deposition, and improve reaction performance; its higher crystallinity also results in higher hydrothermal stability during reactions. Attached Figure Description

[0046] Figure 1 The XRD patterns of SAPO-34 molecular sieves prepared in Example 1, Comparative Example 1, and Comparative Example 2 are shown.

[0047] Figure 2 The image shows a SEM image of the small-crystal SAPO-34 molecular sieve prepared in Example 1. It is evident that it possesses a cubic lamellar structure.

[0048] Figure 3 SEM image of SAPO-34 molecular sieve prepared for Comparative Example 1.

[0049] Figure 4 SEM image of SAPO-34 molecular sieve prepared for Comparative Example 2. Detailed Implementation

[0050] The following examples will further illustrate the present invention and are intended to help readers better understand the essence of the present invention and its beneficial effects, but should not be construed as limiting the scope of the present invention.

[0051] The commercially available large-grain SAPO-34 molecular sieve used in the examples and comparative examples was from the Nanjing Branch of Sinopec Catalyst Co., Ltd., with the following composition: SiO2 content 6.77 wt%, Al2O3 content 49.5 wt%, P2O5 content 43.5 wt%, Na2O content 0.23 wt%; relative crystallinity 100%, and grain size 5-10 micrometers.

[0052] Ball mill model: BM4 planetary ball mill.

[0053] In this embodiment, a MASTERSIZER 2000 laser particle size analyzer was used for analysis.

[0054] In the following examples, the room temperature is controlled at 25°C.

[0055] Example 1

[0056] 3.26 g of commercially available large-crystal SAPO-34 molecular sieve was dispersed in 7.45 g of diethylamine and ball-milled at 300 r / min for 2 h to obtain a slurry A of molecular sieve and template agent. 14.3 g of pseudoboehmite was mixed with 88.01 g of deionized water and stirred at room temperature for 30 min to obtain mixture B. 28.17 g of phosphoric acid solution was added to mixture B and stirred at room temperature for 2 h to obtain mixture C. 12.08 g of diethylamine was added to mixture C. g of silica sol was stirred for 2 hours to obtain mixture D; mixture D was mixed with slurry A and stirred for 30 minutes to obtain mixture E; finally, mixture E was crystallized at 200℃ for 48 hours with a crystallization vessel speed of 10 r / min. After crystallization, the mixture was centrifuged, washed, and dried to obtain a solid product, namely molecular sieve powder; then the molecular sieve powder was calcined in air at 550℃ for 5 hours to remove the template agent, resulting in small-crystal SAPO-34 molecular sieve, which was labeled as sample A.

[0057] Example 2

[0058] 3.20 g of commercially available large-crystal SAPO-34 molecular sieve was dispersed in 10.09 g of triethylamine and ball-milled at 300 r / min for 2 h to obtain a slurry A of molecular sieve and template agent. 14.03 g of boehmite was mixed with 86.25 g of deionized water and stirred at room temperature for 1 h to obtain mixture B. 27.64 g of phosphoric acid solution was added to mixture B and stirred at room temperature for 1 h to obtain mixture C. 11.23 g of... Silica sol was stirred for 2 hours to obtain mixture D. Mixture D was then mixed with slurry A and stirred for 30 minutes to obtain mixture E. Finally, mixture E was crystallized at 190℃ for 24 hours with a crystallization vessel rotation speed of 15 r / min. After crystallization, the mixture was centrifuged, washed, and dried to obtain a solid product, namely molecular sieve powder. Subsequently, the molecular sieve powder was calcined in air at 550℃ for 4 hours to remove the template agent, resulting in small-crystal SAPO-34 molecular sieve, which was labeled as sample B.

[0059] Example 3

[0060] 2.94 g of commercially available large-crystal SAPO-34 molecular sieve was dispersed in 6.26 g of isopropylamine and ball-milled at 300 r / min for 2 h to obtain a slurry A of molecular sieve and template agent. 14.87 g of pseudoboehmite was mixed with 101.82 g of deionized water and stirred at room temperature for 1 h to obtain mixture B. 21.97 g of phosphoric acid solution was added to mixture B and stirred at room temperature for 1 h to obtain mixture C. 5.08 g of gaseous phase was added to mixture C. Silica was stirred for 2 hours to obtain mixture D. Mixture D was then mixed with slurry A and stirred for 30 minutes to obtain mixture E. Finally, mixture E was crystallized at 190℃ for 24 hours with a crystallization vessel rotation speed of 15 r / min. After crystallization, the mixture was centrifuged, washed, and dried to obtain a solid product, namely molecular sieve powder. Subsequently, the molecular sieve powder was calcined in air at 550℃ for 4 hours to remove the template agent, resulting in small-crystal SAPO-34 molecular sieve, which was labeled as sample C.

[0061] Example 4

[0062] 3.09 g of commercially available large-crystal SAPO-34 molecular sieve was dispersed in 8.89 g of morpholine and ball-milled at 300 r / min for 2 h to obtain a slurry A of molecular sieve and template agent; 11.57 g of alumina was mixed with 93.72 g of deionized water and stirred at room temperature for 1 h to obtain mixture B; 28.24 g of phosphoric acid solution was added to mixture A and stirred at room temperature for 1 h to obtain mixture C; 7.57 g of... Silica sol was stirred for 2 hours to obtain mixture D. Mixture D was then mixed with slurry A and stirred for 1 hour to obtain mixture E. Finally, mixture E was crystallized at 210℃ for 24 hours at a crystallization reactor speed of 15 r / min. After crystallization, the mixture was centrifuged, washed, and dried to obtain a solid product, namely molecular sieve powder. Subsequently, the molecular sieve powder was calcined in air at 550℃ for 4 hours to remove the template agent, resulting in small-crystal SAPO-34 molecular sieve, which was labeled as sample D.

[0063] Example 5

[0064] 1.31 g of commercially available large-crystal SAPO-34 molecular sieve was dispersed in 5.98 g of diethylamine and ball-milled at 300 r / min for 2 h to obtain a slurry A of molecular sieve and template agent. 34.10 g of aluminum isopropoxide and 77.57 g of deionized water were mixed and stirred at room temperature for 1 h to obtain mixture B. 22.64 g of phosphoric acid solution was added to mixture B and stirred at room temperature for 1 h to obtain mixture C. 9.71 g of... Silica sol was stirred for 2 hours to obtain mixture D. Mixture D was then mixed with slurry A and stirred for 1 hour to obtain mixture E. Finally, mixture E was crystallized at 190℃ for 72 hours with a crystallization vessel rotation speed of 15 r / min. After crystallization, the mixture was centrifuged, washed, and dried to obtain a solid product, namely molecular sieve powder. Subsequently, the molecular sieve powder was calcined in air at 550℃ for 4 hours to remove the template agent, resulting in small-crystal SAPO-34 molecular sieve, which was labeled as sample E.

[0065] Example 6

[0066] 4.80 g of commercially available large-crystal SAPO-34 molecular sieve was dispersed in 10.09 g of triethylamine and ball-milled at 300 r / min for 2 h to obtain a slurry A of molecular sieve and template agent. 14.03 g of boehmite was mixed with 93.44 g of deionized water and stirred at room temperature for 1 h to obtain mixture B. 27.64 g of phosphoric acid solution was added to mixture B and stirred at room temperature for 1 h to obtain mixture C. 4.80 g of... Vaporized SiO2 was stirred for 2 hours to obtain mixture D. Mixture D was then mixed with slurry A and stirred for 1 hour to obtain mixture E. Finally, mixture E was crystallized at 190℃ for 72 hours at a crystallization vessel speed of 30 r / min. After crystallization, the mixture was centrifuged, washed, and dried to obtain a solid product, namely molecular sieve powder. Subsequently, the molecular sieve powder was calcined in air at 550℃ for 4 hours to remove the template agent, resulting in small-crystal SAPO-34 molecular sieve, which was labeled as sample F.

[0067] Comparative Example 1

[0068] 14.3 g of boehmite was mixed with 88.01 g of deionized water and stirred at room temperature for 30 min to obtain mixture A. 28.17 g of phosphoric acid solution was added to mixture A and stirred at room temperature for 2 h to obtain mixture B. 7.45 g of diethylamine was added to mixture B and stirred for 1 h to obtain mixture C. 12.08 g of silica sol was added to mixture C and stirred for 2 h to obtain mixture D. Finally, mixture D was crystallized at 200 °C for 48 h at a crystallization vessel speed of 10 r / min. After crystallization, the mixture was centrifuged, washed, and dried to obtain a solid product, namely molecular sieve powder. Subsequently, the molecular sieve powder was calcined in air at 550 °C for 5 h to remove the template agent, resulting in small-crystal SAPO-34 molecular sieve, which was labeled as sample G.

[0069] Comparative Example 2

[0070] 14.3 g of boehmite was mixed with 88.01 g of deionized water and stirred at room temperature for 30 min to obtain mixture A. 28.17 g of phosphoric acid solution was added to mixture A and stirred at room temperature for 2 h to obtain mixture B. 7.45 g of diethylamine was added to mixture B and stirred for 1 h to obtain mixture C. 12.08 g of silica sol was added to mixture C and stirred for 2 h to obtain mixture D. 3.26 g of commercial large-grain SAPO-34 molecular sieve was added to mixture D and stirred for 1 h to obtain mixture E. Finally, mixture E was crystallized at 200 °C for 48 h at a crystallization vessel speed of 10 r / min. After crystallization, the mixture was centrifuged, washed, and dried to obtain a solid product, namely the molecular sieve powder. Subsequently, the molecular sieve powder was calcined in air at 550 °C for 5 h to remove the template agent, yielding small-grain SAPO-34 molecular sieve, which was labeled as sample H.

[0071] Comparative Example 3

[0072] 3.26 g of commercially available large-crystal SAPO-34 molecular sieve was dispersed in 7.45 g of diethylamine and ball-milled at 300 r / min for 2 h to obtain a slurry A of molecular sieve and template agent. 12.08 g of silica sol was mixed with 88.01 g of deionized water and stirred at room temperature for 30 min to obtain mixture B. 14.3 g of pseudoboehmite was added to mixture B and stirred at room temperature for 1 h to obtain mixture C. 28.17 g of phosphoric acid was added to mixture C. The solution was stirred at room temperature for 2 hours to obtain mixture D; mixture D was mixed with slurry A and stirred for 30 minutes to obtain mixture E; finally, mixture E was crystallized at 200℃ for 48 hours with a crystallization vessel speed of 10 r / min. After crystallization, the mixture was centrifuged, washed, and dried to obtain a solid product, namely molecular sieve powder; subsequently, the molecular sieve powder was calcined in air at 550℃ for 5 hours to remove the template agent, resulting in small-crystal SAPO-34 molecular sieve, which was labeled as sample I.

[0073] Comparative Example 4

[0074] 3.26 g of commercial large-grain seed crystals were dispersed in 40 g of deionized water and ball-milled at 300 r / min for 2 h to obtain a molecular sieve-water mixture slurry A; 14.3 g of boehmite was mixed with 48.01 g of deionized water and stirred at room temperature for 30 min to obtain mixture B; 28.17 g of phosphoric acid solution was added to mixture B and stirred at room temperature for 2 h to obtain mixture C; 7.45 g of diethylamine was added to mixture C and stirred for 1 h to obtain mixture D; [The text abruptly ends here, likely due to an incomplete sentence or missing information.] 12.08 g of silica sol was added to liquid D and stirred for 2 h to obtain mixed liquid D; mixed slurry A was mixed with mixed liquid D and stirred for 1 h to obtain mixed liquid E; finally, mixed liquid E was crystallized at 200℃ for 48 h with a crystallization kettle speed of 10 r / min. After crystallization, the solid product was obtained by centrifugation, washing and drying, i.e., molecular sieve raw powder; then the molecular sieve raw powder was calcined in air at 550℃ for 5 h to remove the template agent, and small-crystal SAPO-34 molecular sieve was obtained and labeled as sample J.

[0075] The proportions of each embodiment and comparative example are shown in Table 1, and the properties of the resulting products are shown in Table 2.

[0076] Table 1. Composition ratios of each embodiment and comparative example mixture E (n represents moles, m represents mass)

[0077]

[0078] Table 2 Yields and sample properties of products from each example

[0079] Example 1 A 89.2 112 0.21～1.54 0.08～0.56 Example 2 B 86.9 109 0.33～1.48 0.09～0.63 Example 3 C 87.5 115 0.26～1.32 0.08～0.59 Example 4 D 90.3 109 0.21～1.28 0.12～0.65 Example 5 E 85.6 103 0.38～1.57 0.15～0.87 Example 6 F 93.2 121 0.20～1.04 0.06～0.34 Comparative Example 1 G 73.1 78 8.82～12.34 7.53～11.67 Comparative Example 2 H 78.4 86 0.84～5.48 0.78～4.32 Comparative Example 3 I 82.3 92 0.83～3.35 0.21～2.38 Comparative Example 4 J 84.1 95 0.62～3.07 0.14～1.96

Claims

1. A method for preparing small-crystal SAPO-34 molecular sieves, comprising the following steps: a) Mix large-grain SAPO-34 molecular sieve with an organic template agent, and then mechanically crush the mixture to obtain mixture A. The organic template agent is selected from one or more of diethylamine, triethylamine, morpholine, or isopropylamine. b) The aluminum source and deionized water are mixed evenly to obtain mixture B; c) Add a phosphorus source to mixture B and stir to obtain mixture C; d) Add a silicon source to mixture C and stir to obtain mixture D; e) Mixture D and mixture A are mixed and stirred to obtain mixture E, wherein mixture E has the following molar ratio: SiO2:Al2O3=0.2~0.8, P2O5:Al2O3=0.6~1.2, template agent:Al2O3=0.05~4.0, H2O:Al2O3=20~100; wherein SiO2, Al2O3 and P2O5 are the amounts of silicon, aluminum and phosphorus introduced from silicon source, aluminum source and phosphorus source, converted into SiO2, Al2O3 and P2O5; the seed crystal is 0.5~30% by weight of the total weight of SiO2, Al2O3 and P2O5 in silicon source, aluminum source and phosphorus source raw materials; f) The mixture E is hydrothermally dynamically crystallized at 150~220℃ for 12~120 h. After crystallization, the mixture is separated, washed and dried to obtain molecular sieve powder. g) Remove the template agent from the molecular sieve powder to obtain SAPO-34 molecular sieve.

2. The method for preparing small-crystal SAPO-34 molecular sieve according to claim 1, characterized in that, The particle size in mixture A is 0.18 to 1.28 micrometers, and the particle size is measured using laser particle size distribution.

3. The method for preparing small-crystal SAPO-34 molecular sieve according to claim 1 or 2, characterized in that, The mechanical crushing process involves ball milling a mixture of large-grain SAPO-34 molecular sieve and organic template agent in a ball mill at a speed of 100-600 r / min for 1-8 h.

4. The method for preparing small-crystal SAPO-34 molecular sieve according to claim 1, characterized in that, The stirring described in step c is carried out at room temperature for 1 to 8 hours. The stirring described in step d is carried out at room temperature for 0.5 to 6 hours. The stirring described in step d shall be carried out for 0.5 to 2 hours. The template removal agent described in step g is calcined in air at 450~600℃ for 2~8 h.

5. The method for preparing small-crystal SAPO-34 molecular sieve according to claim 1, characterized in that, The silicon source is one or more of silica sol, fumed silica, or tetraethyl orthosilicate; the aluminum source is one or more of boehmite, pseudoboehmite, alumina, aluminum hydroxide, or aluminum isopropoxide; and the phosphorus source is one or more of orthophosphoric acid, phosphorous acid, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate.

6. The method for preparing small-crystal SAPO-34 molecular sieve according to claim 1, characterized in that, The large-grain SAPO-34 molecular sieve has a grain size greater than 2 micrometers.

7. The method for preparing small-crystal SAPO-34 molecular sieve according to claim 1, characterized in that, The mixture E has the following molar ratios: SiO2:Al2O3 = 0.4~0.8, P2O5:Al2O3 = 0.6~1.2, template agent:Al2O3 = 0.2~3, H2O:Al2O3 = 40~80; wherein SiO2, Al2O3 and P2O5 are the amounts of silicon, aluminum and phosphorus introduced from silicon source, aluminum source and phosphorus source, converted into SiO2, Al2O3 and P2O5.

8. The method for preparing small-crystal SAPO-34 molecular sieve according to claim 1, characterized in that, The seed crystal is 5-15% by weight of the total weight of SiO2, Al2O3 and P2O5 in the silicon source, aluminum source and phosphorus source raw materials.

9. The small-crystal SAPO-34 molecular sieve obtained according to claim 1.

10. The small-grained SAPO-34 molecular sieve according to claim 9, characterized in that, The SAPO-34 molecular sieve has a lamellar structure.

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