A low-silica SAPO-34 molecular sieve, its preparation method and application

Abstract

This invention belongs to the field of molecular sieve preparation technology, specifically relating to a low-silica SAPO-34 molecular sieve, its preparation method, and its application. It includes the following steps: (1) gel preparation; (2) aging treatment; (3) variable-temperature crystallization: after aging treatment, a two-stage variable-temperature crystallization is performed. The first stage is high-temperature crystallization: the reactor temperature is raised to 170-210℃ and crystallized at a constant temperature for 4-10 hours; the second stage is low-temperature crystallization: after high-temperature crystallization, the temperature is naturally cooled to 100-150℃ and crystallized at a constant temperature for 38-44 hours; (4) post-treatment: after variable-temperature crystallization, the temperature is lowered to room temperature, the crystallized product is removed, centrifuged, washed with deionized water until neutral, dried, and calcined to obtain the low-silica SAPO-34 molecular sieve. The low-silica SAPO-34 molecular sieve of this invention has a good effect on improving catalytic lifetime and diene selectivity in the MTO reaction.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve preparation technology, specifically relating to a low-silica SAPO-34 molecular sieve, its preparation method, and its application. Background Technology

[0002] SAPO-34 molecular sieve is a phosphorus-silica-alumina molecular sieve belonging to the eight-membered ring chalcogenide CHA structure, with a cubic crystal system and an eight-membered ring pore size of 0.43 nm, which is similar to the molecular dynamic diameter of ethylene and propylene. SAPO-34 molecular sieve possesses suitable acid properties and pore structure, exhibiting excellent catalytic performance in methanol-to-olefins (MTO).

[0003] There are many methods for synthesizing SAPO-34 molecular sieves, such as hydrothermal synthesis, microwave synthesis, liquid-phase synthesis, and gas-phase transfer synthesis. Hydrothermal synthesis is the most commonly used. For example, Chinese patent CN101121529A discloses a rapid synthesis method for SAPO-34 molecular sieves, with a C2-C3 olefin selectivity of approximately 90% in the product. Chinese patent CN101633508A discloses a SAPO-34 molecular sieve and its synthesis method, which exhibits particularly high low-carbon olefin selectivity in methanol conversion (MTO) reactions.

[0004] Based on the nSi / n(Si+Al+P) molar ratio, SAPO-34 molecular sieves are classified into high-silica SAPO-34 molecular sieves and low-silica SAPO-34 molecular sieves. High-silica SAPO-34 molecular sieves generally refer to those with an nSi / n(Si+Al+P) molar ratio greater than 0.08. These molecular sieves are relatively easy to synthesize, but their MTO reaction stability is poor and their induction period is relatively long. Many studies have shown that low-silica SAPO-34 has a lower acid density and moderate acid strength, and its framework silicon mainly exists in the form of isolated Si(4Al), thus exhibiting superior MTO reaction performance and resistance to carbon deposition.

[0005] However, in the synthesis experiment, the synthesis of low-silicon SAPO-34 is more difficult than that of high-silicon SAPO-34. That is, simply reducing the amount of silicon source in the gel can reduce the silicon content of SAPO-34 within a certain range. Further reducing the amount of silicon source faces the problems of AFI structure impurity crystals and poor synthesis reproducibility. In addition, there are reports on the synthesis of low-silicon SAPO-34 molecular sieves, such as patent CN104556142A, a method for preparing low-silicon SAPO-34 molecular sieves, which includes the following steps: (1) mixing silicon source, phosphorus source and aluminum source evenly and drying to obtain a silicon-phosphorus-aluminum dry gel with a solid content of not less than 60%; (2) contacting and mixing the silicon-phosphorus-aluminum dry gel with a template agent to obtain a mixture; (3) placing the mixture in a closed reaction vessel under self-generated pressure to crystallize and recover the product. The crystallization process is carried out sequentially in three segments within a temperature range from low to high. The crystallization temperatures of adjacent segments are different, and the crystallization temperature of the later segment is at least 10°C higher than that of the earlier segment. The three crystallization temperatures are 20-90°C, 90-150°C, and 150-250°C, respectively. CN102336413A describes a method for synthesizing low-silica SAPO-34 molecular sieves, employing a two-step method. First, a high-silica SAPO-34 initial gel is prepared and isothermally crystallized at 150-210°C for 0.1-24 hours to obtain a gel system containing microparticles that form the primary and secondary structural units of the SAPO-34 molecular sieve. Then, an aluminum phosphate and organic amine gel is added to this system, and crystallization continues at 150-210°C for 1-48 hours, ultimately yielding low-silica SAPO-34 with a Si / Al molar ratio of 0.01-0.15. The above methods can obtain low-silica SAPO-34 molecular sieves, which improve diene selectivity and single-pass lifetime in MTO reactions. However, there are also many shortcomings: the molecular sieve preparation process is complicated: the two-step crystallization process in CN102336413A and the preparation of silicon-phosphorus-aluminum dry gel in CN104556142A; there are many uncontrollable factors: how to achieve secondary feeding in a high-pressure sealed reactor in CN102336413A, and the requirement that the solid content of silicon-phosphorus-aluminum dry gel in CN104556142A should not be less than 60%, etc.

[0006] Existing technologies for preparing low-silicon SAPO-34 either involve modifying different raw materials, including aluminum sources, silicon sources, and organic template agents, to explore the synthesis of low-silicon SAPO-34. However, the following problems still exist: some methods have stringent requirements for parameters such as raw material ratios and solid content; there are many uncontrollable factors during the synthesis process, resulting in poor product reproducibility; traditional isothermal crystallization is prone to problems such as the formation of impurity crystals or uneven silicon distribution; simply reducing the amount of silicon source cannot effectively reduce the silicon content while ensuring the purity of the product, thus limiting the full realization of the performance advantages of low-silicon SAPO-34. Summary of the Invention

[0007] The purpose of this invention is to provide a low-silica SAPO-34 molecular sieve, its preparation method and application, to obtain a low-silica SAPO-34 molecular sieve product, which has a good effect on improving catalytic lifetime and diene selectivity when applied to MTO reaction.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a low-silica SAPO-34 molecular sieve includes the following steps: (1) Gel preparation: The silicon source, phosphorus source, aluminum source and organic template agent are mixed evenly according to the molar ratio of Al2O3:(0.7-1.5)P2O5:(0.1-0.6)SiO2:(1.0-4.0) Organic template agent:(20-100)H2O to form a silicon-phosphorus-aluminum gel. (2) Aging treatment: The silica-alumina gel is transferred to a closed reaction vessel for aging treatment; (3) Temperature-variable crystallization: After the aging treatment, two-stage temperature-variable crystallization is carried out. The first stage is high-temperature crystallization: the reactor temperature is raised to 170-210℃ and crystallized at a constant temperature for 4-10 hours; the second stage is low-temperature crystallization: after the high-temperature crystallization is completed, it is naturally cooled to 100-150℃ and crystallized at a constant temperature for 38-44 hours. (4) Post-processing: After the temperature-variable crystallization is completed, the temperature is lowered to room temperature, the crystallized product is taken out, and after centrifugation, it is washed until neutral; dried and calcined to obtain low-silicon SAPO-34 molecular sieve.

[0009] This invention employs a variable-temperature crystallization process. The first stage involves high-temperature crystallization: the reactor temperature is programmed to 170-210℃ and maintained at this temperature for 4-10 hours, providing the necessary secondary structural units for the formation of the CHA topology. The second stage involves low-temperature crystallization: after high-temperature crystallization, the reactor is naturally cooled to 100-150℃ and maintained at this temperature for 38-44 hours, suppressing isomorphic substitution of silicon atoms in the later stages of crystallization and reducing the framework silicon content. This invention, using a variable-temperature crystallization process, effectively reduces the framework silicon content by more than 29% while maintaining the pure-phase structure of SAPO-34. The low-silicon characteristic reduces the acidic sites in the molecular sieve, suppresses hydrogen transfer side reactions, increases the diene selectivity in the MTO reaction to over 82%, and results in a high catalyst lifetime, which can even be extended to over 255 minutes, with a short induction period.

[0010] Furthermore, the silicon source is selected from at least one of silica sol and tetraethyl orthosilicate; the aluminum source is selected from at least one of boehmite and aluminum isopropoxide.

[0011] Furthermore, the aging conditions are as follows: first, dynamic aging at 10-15℃ and 200-250rpm for 10-14h, then raising the temperature to 30-35℃ and maintaining a speed of 200-250rpm for continued dynamic aging for 24-36h.

[0012] The aging process of this invention employs a two-stage aging method: low-temperature pre-nucleation and room-temperature growth. First, explosive nucleation of the gel system is induced at low temperature, generating a large number of uniform microcrystal nuclei. Then, stable growth of the nuclei is promoted under mild conditions. This aging process can produce a significant synergistic effect with the subsequent temperature-variable crystallization process. The high-density nuclei accumulated in the low-temperature stage provide an ideal growth base for temperature-variable crystallization, making the crystal growth more regular and orderly in the high-temperature crystallization stage, avoiding the generation of impurity crystals, and helping to obtain SAPO-34 molecular sieves with more uniform grain size and fewer skeletal defects. When applied to the MTO reaction, the diffusion path is uniform and the mass transfer rate is fast, significantly extending the catalytic lifetime and improving the diene selectivity.

[0013] Furthermore, the roasting conditions are as follows: First stage: raise from room temperature to 200-220℃ and hold at that temperature for 2.0-2.2 hours; Second stage: continue to raise to 400-420℃ and hold at that temperature for 3.0-3.4 hours; Third stage: continue to raise to 650-670℃ and hold at that temperature for 5.0-5.5 hours; Fourth stage: after roasting, cool down with the furnace to 100-150℃ and then remove the product, allowing it to cool naturally to room temperature.

[0014] Furthermore, the calcination conditions are as follows: First stage: heating from room temperature to 200-220℃ at a heating rate of 0.9-1.1℃ / min, and holding at that temperature for 2.0-2.2h; Second stage: heating to 400-420℃ at an extremely slow heating rate of 0.4-0.6℃ / min, and holding at that temperature for 3.0-3.4h; Third stage: heating to 650-670℃ at a heating rate of 2.0-2.2℃ / min, and holding at that temperature for 5.0-5.5h; Fourth stage: after calcination, cooling down in the furnace to 100-150℃ at a rate of 5-8℃ / min before removing the product and allowing it to cool naturally to room temperature.

[0015] The gradient roasting process of this invention works synergistically with the variable temperature crystallization process. Variable temperature crystallization has already constructed a complete CHA framework structure, while the three-stage heating in gradient roasting can match the distribution of framework defects formed during crystallization, preventing local collapse of the framework due to heat concentration and effectively protecting the microporous structure formed by variable temperature crystallization. In the MTO reaction, this synergistically optimized pore environment and acidic sites work together to improve the selectivity of ethylene and propylene, delay carbon deposition and deactivation, and achieve an overall improvement in catalytic performance.

[0016] Furthermore, the phosphorus source is selected from at least one of phosphoric acid and ammonium dihydrogen phosphate; the organic template agent is selected from at least one of triethylamine and tetraethylammonium hydroxide.

[0017] Further, the mixing sequence is as follows: first, add the aluminum source to deionized water and stir to form solution A; dilute the phosphorus source in deionized water and stir to form solution B; add solution B to solution A and stir to obtain sol C; add the silicon source to solution C and stir; finally, add the organic template agent and continue stirring until homogeneous to obtain silicon-phosphorus-aluminum gel.

[0018] The low-silica SAPO-34 molecular sieve prepared by the method described above.

[0019] The application of the low-silica SAPO-34 molecular sieve prepared by the aforementioned method in the methanol-to-low-carbon olefin reaction.

[0020] Furthermore, when the methanol conversion rate is >99%, the total selectivity of ethylene and propylene is not less than 82%, the catalyst life is not less than 225 min, and the silicon content is reduced by more than 29% compared with the traditional isothermal crystallization method.

[0021] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: 1. This invention, through an innovative temperature-varying crystallization process, overcomes the shortcomings of existing technologies and achieves efficient and stable synthesis of low-silica SAPO-34 molecular sieves without changing the original gel formulation.

[0022] 2. The present invention adopts a variable temperature crystallization process, which effectively reduces the silicon content of the framework by more than 29% while ensuring the pure phase structure of SAPO-34. The low silicon characteristic reduces the acid sites of the molecular sieve, suppresses the hydrogen transfer side reaction, increases the diene selectivity in the MTO reaction to more than 82%, extends the catalyst lifetime to more than 255 minutes, and has a short induction period.

[0023] 3. The present invention adopts a combination of variable temperature crystallization process, aging treatment and gradient calcination, which has a synergistic effect. Molecular sieves have a good effect on improving catalytic lifetime and diene selectivity in MTO reaction.

[0024] 4. The method of the present invention has strong compatibility and is applicable to existing mature SAPO-34 molecular sieve synthesis equipment. No additional investment in modification costs is required, and it can be directly connected to industrial production systems.

[0025] 5. This invention suppresses the insertion of silicon atoms in the later stage of crystallization by using a high-temperature followed by low-temperature crystallization method, thus achieving the synthesis of low-silicon SAPO-34 molecular sieves. Compared with the isothermal crystallization route, the silicon content of the product can be reduced by 30% while maintaining the pure phase of SAPO-34. It also features long catalytic life and high diene selectivity. Attached Figure Description

[0026] Figure 1 The diagram shows the crystallization process of the present invention and conventional methods. a represents the conventional crystallization method, and b represents the crystallization method of the present invention.

[0027] Figure 2 The image shows an electron microscope image of the molecular sieve prepared in Example 1.

[0028] Figure 3 The image shows an electron microscope image of the molecular sieve prepared in Example 2.

[0029] Figure 4 Electron micrograph of the molecular sieve prepared in Comparative Example 1.

[0030] Figure 5 This is a schematic diagram showing the change in methanol conversion rate over reaction time for Comparative Example 1 and Examples 1-2.

[0031] Figure 6 This is a schematic diagram showing the change in diene selectivity with reaction time for Comparative Example 1 and Examples 1-2. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Example 1 This embodiment provides a low-silica SAPO-34 molecular sieve, the preparation method of which includes the following steps: (1) Gel preparation: Weigh 18g of boehmite and add it to 40g of deionized water. Stir rapidly for 1h to form solution A; Weigh 28.4g of 85wt% phosphoric acid and dilute it in 36g of deionized water. Stir for 1h to form solution B; Add solution B to solution A and stir for 1h to obtain sol C; Add 7.92g of 30% silica sol to solution C and stir for 1h. Then add 31.2g of triethanolamine (TEA) and stir evenly to obtain the initial crystallized sol. The molar ratio of each component is 0.3SiO2:1Al2O3:1P2O5:2.5TEA:40H2O.

[0034] (2) Aging treatment: The sol was placed in a 200mL sealed reaction vessel and dynamically aged for 12h at 10℃ and 200rpm. Then the temperature was raised to 30℃ and the dynamic aging was continued for 24h at 200rpm. (3) Variable temperature crystallization: The temperature is raised to 200℃ and kept constant for 4 hours, then naturally cooled to 120℃ and crystallized for another 44 hours.

[0035] (4) Post-processing: After the reactor is cooled to room temperature, the product is taken out, centrifuged and washed with water until neutral, dried at 110℃, and then calcined. First stage: the temperature is increased from room temperature to 200℃ at a rate of 1℃ / min and held for 2 hours; Second stage: the temperature is increased from 200℃ to 400℃ at a very slow rate of 0.5℃ / min and held for 3 hours; Third stage: the temperature is increased from 400℃ to 650℃ at a rate of 2℃ / min and held for 5 hours; Fourth stage: after calcination, the temperature is reduced to 100-150℃ with the furnace at a rate of 5℃ / min and then taken out and naturally cooled to room temperature to obtain low-silicon SAPO-34 molecular sieve raw powder S1.

[0036] Example 2 The difference between this embodiment and embodiment 1 is as follows: Step (3) Variable temperature crystallization parameters: constant temperature crystallization at 200℃ for 10h, natural cooling to 120℃ and continued crystallization for 38h to obtain low silicon SAPO-34 molecular sieve raw powder S2.

[0037] Example 3 This embodiment provides a low-silica SAPO-34 molecular sieve, the preparation method of which includes the following steps: (1) Add 4g of 30% silica sol to a mixed template solution of 110.5g tetraethylammonium hydroxide aqueous solution (TEAOH, 20wt%) and 2.3g DEA (diethanolamine), stir rapidly for 1h, and label it as solution A. Dissolve boehmite in solution A and stir for 1h to obtain solution B. Weigh 28.4g of 85wt% phosphoric acid and slowly add it dropwise to solution B, stir for 1h to obtain a homogeneous initial crystallized sol. The addition ratio of each component is: 0.2SiO2:1Al2O3:0.95P2O5:1.57TEAOH:0.32DEA:40H2O.

[0038] (2) Aging treatment: The sol was placed in a 200mL sealed reaction vessel and dynamically aged for 12h at 10℃ and 200rpm. Then the temperature was raised to 30℃ and the dynamic aging was continued for 24h at 200rpm. (3) Variable temperature crystallization: The temperature is raised to 200℃ for 4 hours and then naturally cooled to 120℃ for 44 hours to continue crystallization.

[0039] (4) Post-processing: When the temperature of the high-pressure reactor drops to room temperature, take out the crystallized product, centrifuge and wash with deionized water until neutral, place the product in a constant temperature drying oven at 110℃ to dry, and carry out calcination treatment. First stage: heat from room temperature to 200℃ at a heating rate of 1℃ / min and hold for 2h; Second stage: heat from 200℃ to 400℃ at a very slow heating rate of 0.5℃ / min and hold for 3h; Third stage: heat from 400℃ to 650℃ at a heating rate of 2℃ / min and hold for 5h; Fourth stage: after calcination, cool down with the furnace to 100-150℃ at a rate of 5℃ / min and take out, and cool naturally to room temperature to obtain SAPO-34 molecular sieve raw powder S3.

[0040] Comparative Example 1 (Traditional isothermal crystallization) The difference between this comparative example and Example 1 is that: in step (3) the crystallization stage, a single constant temperature crystallization of 200℃ for 48h was used to obtain SAPO-34 molecular sieve raw powder D1.

[0041] Comparative Example 2 The difference between this comparative example and Example 3 is that: in step (3) the crystallization stage, a single constant temperature crystallization of 200℃ for 48h was used to obtain SAPO-34 molecular sieve raw powder D3.

[0042] Comparative Example 3 The difference between this comparative example and Example 1 is that the calcination conditions are as follows: the sample is placed directly into a muffle furnace at 650°C (without going through a low-temperature stage), calcined at a constant temperature for 5 hours, and then directly removed and rapidly cooled in air.

[0043] Comparative Example 4 The difference between this comparative example and Example 1 is that the calcination conditions are as follows: the temperature is directly increased to 650°C at a rate of 5°C / min, and calcined at a constant temperature for 5 hours. After calcination, the temperature is reduced to 100°C with the furnace at a rate of 5°C / min before being taken out and allowed to cool naturally to room temperature.

[0044] Comparative Example 5 The difference between this comparative example and Example 1 is that the calcination conditions are as follows: the temperature is increased directly from room temperature to 650°C at a heating rate of 1°C / min, and held at a constant temperature for 10 hours. After calcination, the temperature is reduced to 100°C with the furnace at a rate of 5°C / min before being removed and allowed to cool naturally to room temperature.

[0045] Comparative Example 6 The difference between this comparative example and Example 1 is as follows: the calcination conditions are as follows: the temperature is increased from room temperature to 200°C at a rate of 1°C / min and held for 2 hours; then the temperature is increased from 200°C to 400°C at a rate of 0.5°C / min and held for 3 hours; then the temperature is increased from 400°C to 650°C at a rate of 2°C / min and held for 2 hours; after calcination, the temperature is reduced to 100°C with the furnace at a rate of 5°C / min and then removed and allowed to cool naturally to room temperature.

[0046] Comparative Example 7 The difference between this comparative example and Example 1 is as follows: the calcination conditions are as follows: the temperature is increased from room temperature to 200°C at a rate of 5°C / min and held for 2 hours; then the temperature is increased from 200°C to 400°C at a rate of 5°C / min and held for 3 hours; then the temperature is increased from 400°C to 650°C at a rate of 5°C / min and held for 5 hours; after calcination, the temperature is reduced to 100°C with the furnace at a rate of 5°C / min and then removed and allowed to cool naturally to room temperature.

[0047] Comparative Example 8 The difference between this comparative example and Example 1 is: aging treatment: the sol was placed in a 200mL sealed reaction vessel and dynamically aged for 36h at 30℃ and 200rpm.

[0048] Performance Evaluation 1. The atomic composition of Examples 1-3 and Comparative Examples 1-2 was determined using XRF, as shown in Table 1.

[0049] Table 1. Elemental composition determined by XRF characterization.

[0050] XRF characterization results show that the method of the present invention can effectively reduce the silicon content of the framework by 29%. SEM images show that the crystal sizes of the examples and comparative examples are similar.

[0051] A product with nSi / n(Si+Al+P) < 8% can be called low-silicon SAPO-34. This technical solution can further reduce the silicon content based on the low-silicon SAPO-34 formula without increasing the cost.

[0052] 2. The MTO catalytic performance of the products from the examples and comparative examples was evaluated using a fixed-bed reactor. Evaluation conditions: catalyst loading 1g, reactant was 80% methanol aqueous solution, and mass hourly space velocity (WHSV) 3h. - ¹ Nitrogen was used as the carrier gas (flow rate 150 mL / min), the reaction temperature was 450 °C, and the products were analyzed online by gas chromatography. The results are shown in Table 2 below. The lifetime was calculated with a sampling interval of 5 minutes.

[0053] Table 2. Methanol conversion reaction lifetime and product selectivity of the examples and comparative examples.

[0054] Through Table 1-2 and Figure 1-6 It can be seen that the silicon content in Examples 1 and 2 is significantly lower than that in Comparative Example 1, and the diene selectivity and catalytic lifetime are both greatly improved, demonstrating the significant advantages of the method of the present invention. (See Table 2 and...) Figure 5-6 Under the premise of comparable diene selectivity, the catalytic lifetime of the examples is significantly extended.

[0055] In Comparative Example 1, a single isothermal crystallization at 200°C for 48 hours was used. Due to the lack of temperature regulation, the crystal growth was too fast, resulting in uneven grain size and increased defects, which significantly reduced the catalytic lifetime and diene selectivity compared to Example 1.

[0056] In Comparative Example 2, isothermal crystallization was performed in the template agent system of Example 3. Although the diene selectivity was comparable to that of Example 3, the lifetime was significantly shorter than that of Example 3, indicating that variable temperature crystallization plays a key role in extending the catalyst lifetime.

[0057] In Comparative Example 3, the sample was placed directly into a muffle furnace at 650℃ and rapidly cooled. The severe thermal shock caused a large number of cracks or even local collapse in the molecular sieve framework, resulting in a significant decrease in specific surface area. Consequently, the lifetime and diene selectivity decreased significantly.

[0058] In Comparative Example 4, the template agent was removed too violently by rapidly heating to 650°C at a rate of 5°C / min. The concentrated heat release caused skeleton defects and disordered distribution of acid centers, resulting in a significant reduction in lifetime and selectivity compared to Example 1.

[0059] In Comparative Example 5, a one-step slow heating to 650°C was used, which avoided rapid cooling, but lacked a 400°C constant temperature buffer stage. The decomposition process of the template agent was still not smooth enough, the integrity of the skeleton was slightly inferior, and the lifespan and selectivity were lower than those of Example 1.

[0060] In Comparative Example 6, gradient calcination was performed, but the final holding time at 650°C was only 2 hours. The template agent was not completely removed, and residual carbon species in the pores covered the acidic sites, which affected the initial activity and reduced the lifetime and selectivity compared to Example 1.

[0061] In Comparative Example 7, the rapid heating at 5℃ / min through each isothermal platform resulted in an excessively fast heating rate, leading to concentrated decomposition of the template agent, damage to the crystal microstructure, and a decrease in lifetime and selectivity.

[0062] In Comparative Example 8, the crystal nuclei were aged at 30°C for 36 hours. Due to the lack of a low-temperature pre-nucleation stage, the number of crystal nuclei generated was small and uneven, resulting in larger and unevenly distributed crystal sizes, longer diffusion paths, and lower lifetimes and selectivity than in Example 1.

[0063] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing low-silica SAPO-34 molecular sieve, characterized in that, Includes the following steps: (1) Gel preparation: The silicon source, phosphorus source, aluminum source and organic template agent are mixed evenly according to the molar ratio of Al2O3:(0.7-1.5)P2O5:(0.1-0.6)SiO2:(1.0-4.0) Organic template agent:(20-100)H2O to form a silicon-phosphorus-aluminum gel. (2) Aging treatment: The silica-alumina gel is transferred to a closed reaction vessel for aging treatment; (3) Temperature-variable crystallization: After the aging treatment, two-stage temperature-variable crystallization is carried out. The first stage is high-temperature crystallization: the reactor temperature is raised to 170-210℃ and crystallized at a constant temperature for 4-10 hours; the second stage is low-temperature crystallization: after the high-temperature crystallization is completed, it is naturally cooled to 100-150℃ and crystallized at a constant temperature for 38-44 hours. (4) Post-processing: After the temperature-variable crystallization is completed, the temperature is lowered to room temperature, the crystallized product is taken out, and after centrifugation, it is washed until neutral; dried and calcined to obtain low-silicon SAPO-34 molecular sieve.

2. The method for preparing low-silica SAPO-34 molecular sieve according to claim 1, characterized in that, The silicon source is selected from at least one of silica sol and tetraethyl orthosilicate; the aluminum source is selected from at least one of boehmite and aluminum isopropoxide.

3. The method for preparing low-silica SAPO-34 molecular sieve according to claim 1, characterized in that, The aging conditions are as follows: first, dynamically age at 10-15℃ and 200-250rpm for 10-14h, then raise the temperature to 30-35℃ and maintain the speed at 200-250rpm for another 24-36h of dynamic aging.

4. The method for preparing low-silica SAPO-34 molecular sieve according to claim 1, characterized in that, The roasting conditions are as follows: First stage: raise the temperature from room temperature to 200-220℃ and hold for 2.0-2.2 hours; Second stage: continue to raise the temperature to 400-420℃ and hold for 3.0-3.4 hours; Third stage: continue to raise the temperature to 650-670℃ and hold for 5.0-5.5 hours; Fourth stage: after roasting, cool the furnace to 100-150℃ and remove the product, then allow it to cool naturally to room temperature.

5. The method for preparing low-silica SAPO-34 molecular sieve according to claim 4, characterized in that, The calcination conditions are as follows: First stage: raise the temperature from room temperature to 200-220℃ at a heating rate of 0.9-1.1℃ / min and hold for 2.0-2.2 hours; Second stage: continue to raise the temperature to 400-420℃ at a very slow heating rate of 0.4-0.6℃ / min and hold for 3.0-3.4 hours; Third stage: continue to raise the temperature to 650-670℃ at a heating rate of 2.0-2.2℃ / min and hold for 5.0-5.5 hours; Fourth stage: after calcination, cool the furnace at a rate of 5-8℃ / min to 100-150℃ and remove the furnace, allowing it to cool naturally to room temperature.

6. The method for preparing low-silica SAPO-34 molecular sieve according to claim 1, characterized in that, The phosphorus source is selected from at least one of phosphoric acid and ammonium dihydrogen phosphate; the organic template agent is selected from at least one of triethylamine and tetraethylammonium hydroxide.

7. The method for preparing low-silica SAPO-34 molecular sieve according to claim 1, characterized in that, The mixing sequence is as follows: first, add the aluminum source to deionized water and stir to form solution A; dilute the phosphorus source in deionized water and stir to form solution B; add solution B to solution A and stir to obtain sol C; add the silicon source to solution C and stir; finally, add the organic template agent and continue stirring until homogeneous to obtain silicon-phosphorus-aluminum gel.

8. A low-silica SAPO-34 molecular sieve prepared by the preparation method according to claims 1-7.

9. The application of the low-silica SAPO-34 molecular sieve prepared by the method described in claims 1-7 in the methanol-to-low-carbon olefin reaction.

10. The application of the low-silica SAPO-34 molecular sieve according to claim 9 in the methanol-to-olefins reaction, characterized in that, When the methanol conversion rate is greater than 99%, the total selectivity of ethylene and propylene is not less than 82%, the catalyst life is not less than 225 min, and the silicon content is reduced by more than 29% compared with the traditional isothermal crystallization method.

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