A synthesis method and application of nano SAPO-34 molecular sieve

Abstract

The present application discloses a method for synthesizing a nano SAPO-34 molecular sieve, the steps of which are as follows: (1) water, an aluminum source, a silicon source, a phosphorus source and a template A are heated in a water bath at a temperature of 70-90°C to obtain an initial gel; the initial gel is cooled to room temperature, and a template B and a mesoporous directing agent are added to the initial gel to obtain a molecular sieve gel; (2) the molecular sieve gel is transferred to a reactor and subjected to a hydrothermal reaction. After the reaction is completed, the reactor is cooled to room temperature to obtain an initial product; (3) the initial product is washed, dried, ground and roasted to obtain a nano SAPO-34 molecular sieve. The application of the nano SAPO-34 molecular sieve prepared by the above-mentioned synthesis method in MTO is also disclosed. The present application can effectively improve the selectivity of light olefins during the MTO reaction, and reduce the synthesis time of the nano SAPO-34 molecular sieve, saving energy consumption.

Description

Technical Field

[0001] The invention relates to a method for preparing a molecular sieve, in particular to a method for synthesizing a nano SAPO-34 molecular sieve and application of the nano SAPO-34 molecular sieve in a MTO reaction. Background Art

[0002] As an important basic chemical raw material, ethylene and propylene are in increasing demand in the industry. They are mainly produced from petroleum. With the increasing shortage of petroleum resources and the environmental protection requirements of the chemical industry, a new energy-saving and efficient route is urgently needed to produce low-carbon olefins. Due to the wide source of methanol, the methanol-olefin process has gradually matured and gradually industrialized.

[0003] In the methanol to olefins reaction, the silicoaluminophosphate SAPO-34 molecular sieve has excellent selectivity for light olefins due to its special CHA cage structure, relatively mild acidity, hydrothermal stability and shape-selective catalysis through small pore entrances. However, due to the unique structure and long pores of the silicoaluminophosphate SAPO-34 molecular sieve, the byproducts and coke generated during the reaction are very likely to cover the active sites, thus greatly reducing the life of the catalyst.

[0004] Although the existing silicoaluminophosphate SAPO-34 molecular sieve has excellent selectivity for light olefins, its selectivity is still only 86%, resulting in relatively high side reactions. How to further improve the selectivity of light olefins in the MTO reaction still requires further research.

[0005] In addition, the existing SAPO-34 molecular sieve requires long-term high-temperature heating during the production process, which consumes a large amount of energy, limits the production efficiency of the SAPO-34 molecular sieve, and leads to a high production cost of the SAPO-34 molecular sieve. Summary of the invention

[0006] In order to improve the selectivity of light olefins during the MTO reaction, the present application first discloses a method for synthesizing a nano SAPO-34 molecular sieve, which comprises the following steps:

[0007] (1) After water, aluminum source, silicon source, phosphorus source and template A are stirred evenly, the stirring is maintained and the mixture is heated in a water bath at a temperature of 70-90° C. to obtain an initial gel; the temperature of the initial gel is lowered to room temperature, and then template B and mesopore directing agent are added to the initial gel, and the mixture is stirred evenly to obtain a molecular sieve gel;

[0008] The template agent A is tetraethylammonium hydroxide, the template agent B is triethylamine, and the mesoporous directing agent is hexadecyltrimethylammonium bromide;

[0009] (2) transferring the molecular sieve gel to a reactor and performing a hydrothermal reaction. After the reaction is completed, the reactor is cooled to room temperature to obtain an initial product;

[0010] (3) The initial product is washed, dried, ground and calcined to obtain nano SAPO-34 molecular sieve.

[0011] In the present application, when preparing the nano SAPO-34 molecular sieve, water, an aluminum source, a silicon source, a phosphorus source and a template A are first reacted at a certain temperature to obtain an initial gel. Due to the effect of the template A, a large number of tiny crystal nuclei based on tetraethylammonium hydroxide are formed in the initial gel, and then the template B and the mesoporous directing agent are added, so that a large number of crystal nuclei can exist in the molecular sieve gel. Since the present application adopts a method of adding the template step by step during the preparation of the molecular sieve gel, the formation of larger crystals is avoided, and in the subsequent hydrothermal reaction, these crystal nuclei can be used as the basis to form crystals with smaller particle size, thereby helping to reduce the length of the pores in the molecular sieve.

[0012] In the present application, because the alkalinity of organic template and the degree of dispersion in water have an important influence on the molecular sieve nucleation rate, template agent A is first added and then template agent B is added, because the dispersion of template agent A in water at room temperature is higher, so water-soluble is correspondingly larger, and the solubility of template agent A in every 100g water at room temperature is greater than 150g, and it is possible to be evenly dispersed in the system, and more crystal nuclei can be formed during heating. After the total amount of aluminum source, silicon source and phosphorus source is determined, the crystal nuclei in molecular sieve gel are more, and in subsequent hydrothermal reaction, the number of crystal grains formed is correspondingly more, and the grain size is correspondingly smaller. When the number of crystal nuclei is sufficient, template agent B is added again, because template agent B has stronger alkalinity and guiding effect, Al atoms and P atoms can be made to grow faster on the crystal nucleus surface, and the isomorphous substitution reaction after Si element enters molecular sieve framework is faster, so as to accelerate the crystallization process of molecular sieve. When template B is added first and template A is added later, or when template A and template B are added at the same time, because the solubility of template B is low, only 5.5 g can be dissolved in every 100 g of water at 30°C. In the process of preparing the initial gel, part of template B is not evenly dispersed and is in an aggregated state. During the process of heating to form crystal nuclei, only a small and dense number of crystal nuclei can be formed. When template B is continued to be added to complete the continued growth of the molecular sieve, the molecular sieve crystal nuclei are relatively aggregated and the number is small, resulting in a larger molecular sieve grain size and an uneven particle size distribution.

[0013] The addition of mesoporous directing agent hexadecyltrimethylammonium bromide can dissociate in water to form cationic C with hydrophobic groups. 19 H 42 N +, and can be adsorbed on the crystal surface of the molecular sieve nucleus, and can affect the pore structure of the molecular sieve through electrostatic and steric effects, increase the pore volume of the molecular sieve, and accelerate the transmission rate of the reaction products. SAPO-34 molecular sieve is a microporous molecular sieve. Without the addition of a mesoporous directing agent, SAPO-34 molecular sieve cannot produce a mesoporous or macroporous multi-level pore structure. The reaction product transmission rate of the microporous molecular sieve is slow, resulting in a faster carbon deposition rate and easy deactivation of the catalyst.

[0014] The nano SAPO-34 molecular sieve prepared by the present application has the following advantages: (1) The nano molecular sieve has short and long straight pores, has high stability and long life, has complete molecular sieve crystal form, uniform grain distribution, and high silicon aluminum content in the skeleton; (2) The molecular sieve is not easy to coke and deactivate during the catalytic process, has high selectivity, and the structure of the nano molecular sieve pores is conducive to the diffusion of the raw gas between the pores, thereby improving the catalyst stability and product selectivity. (3) It is more conducive to the loading amount of metal components and the realization of modification technology after the molecular sieve synthesis; (4) The nano molecular sieve has more acidic sites. The hydrothermal reaction time of the present application is shorter, which has greater cost advantages than the existing technology.

[0015] After being heated in a water bath at a constant temperature of 70-90°C, the temperature of the initial gel is lowered to room temperature to reduce the reaction heat of the system, so that the crystal nuclei can grow to a certain critical size and then stably exist in the supersaturated gel to prevent dissolution during the subsequent crystallization process.

[0016] Specifically, in step (1), the water is deionized water; the aluminum source is alumina, pseudo-boehmite or aluminum isopropoxide; the silicon source is fumed silica, silica sol or tetraethyl orthosilicate; and the phosphorus source is orthophosphoric acid.

[0017] The aluminum source can obtain a nano molecular sieve with uniform particle size distribution and regular morphology. The silicon source can facilitate the dispersion of silicon in water into the molecular sieve framework to form the acid center required for the catalytic reaction.

[0018] Furthermore, in step (1), the molar ratio of water, aluminum source, silicon source, phosphorus source, template A, template B and mesoporous directing agent is (50-70): (0.5-2): (0.4-1): (0.5-2): (1-2): (2-3): (0.1-1), wherein the aluminum source is calculated as aluminum element, the silicon source is calculated as silicon element, and the phosphorus source is calculated as phosphorus element.

[0019] When the molar ratio of water is relatively low, it will make it difficult for the silicon source, aluminum source and phosphorus source to be completely dissolved and cannot be evenly dispersed in the gel, resulting in the presence of granular silicon source and aluminum source in some areas, thereby causing a decrease in the number of crystal nuclei when preparing the initial gel and the enrichment of silicon, aluminum and phosphorus elements in local areas; if the molar ratio of water is too high, the crystallization energy consumption during the hydrothermal reaction will increase.

[0020] Since a large number of crystal nuclei are generated when the initial gel is formed, when the molar ratio of template A is relatively low, sufficient crystal nuclei cannot be formed in the initial gel, resulting in a larger average particle size of the molecular sieve during the subsequent crystallization reaction due to fewer crystal nuclei.

[0021] When the molar ratio of template B is relatively low, the proportion of silicon source, aluminum source and phosphorus source in the solution will increase accordingly, which will easily generate impure crystalline phases such as SAPO-5, affecting the MTO reaction performance. When the molar ratio of template B is relatively high, part of template B will not be completely dissolved. In the subsequent reaction, the undissolved template B will continue to dissolve and participate in the reaction, resulting in excessive particle size of some crystals, making the particle size distribution of the molecular sieve uneven, and also causing excessive waste of template.

[0022] When the molar ratio of the silicon source is relatively low, it is not conducive for silicon to enter the molecular sieve framework to form enough acid sites, affecting the reaction performance. When the molar ratio of the silicon source is relatively high, the silicon in the molecular sieve framework will be in an aggregated state, making the molecular sieve too acidic and reducing the selectivity of low-carbon olefins.

[0023] Furthermore, in step (1), the water bath constant temperature heating time is 1-5h. If the stirring heating time is too short, the components in the gel cannot be evenly mixed to form a large number of crystal nuclei. After the template B and the mesoporous directing agent are added, the molecular sieve crystals grow excessively during the crystallization process, resulting in an excessively large particle size of the molecular sieve.

[0024] Furthermore, in step (2), the reaction temperature of the hydrothermal reaction is 150-250°C, and the reaction time is 6-18h. If the hydrothermal reaction temperature is too low, the crystallization rate will slow down and the crystallinity will decrease; if the hydrothermal reaction temperature is too high, it will affect the crystallization environment of the molecular sieve, accelerate the crystallization rate, generate a large amount of amorphous substances, and increase the side reactions in the MTO reaction. If the thermal reaction time is too short, crystals with complete pores cannot be formed; if the time is too long, energy will be wasted and the crystal size will become larger.

[0025] Furthermore, in the step (3), the initial product is washed by pure water filtration or high-speed centrifugation, the drying temperature is 100-110°C, and the drying time is 5-7h. The above-mentioned washing method can smoothly complete the washing of the product and reduce the loss of the initial product. Under the above-mentioned drying temperature and time, the water in the initial product can be discharged smoothly. When the drying temperature is too high, the steam generated by the water inside the crystal will spray out of the crystal at high speed, causing damage to the structure of the crystal. If the drying temperature is too low, the drying time will be extended and the efficiency will be reduced.

[0026] Specifically, in order to ensure that the structure of the molecular sieve remains intact and the organic components therein are completely removed, in step (3), the calcination temperature is 450-650° C. and the calcination time is 3-7 hours.

[0027] Secondly, the present application also discloses the use of a nano SAPO-34 molecular sieve prepared by any of the above-mentioned synthesis methods of a nano SAPO-34 molecular sieve in an MTO reaction, which comprises the following steps:

[0028] (1) loading the catalyst into the reactor of the MTO reaction device;

[0029] (2) introducing heated nitrogen into the reaction device, allowing the heated nitrogen to pass through the preheating furnace and the reaction furnace in sequence, preheating the preheating furnace and the reaction furnace, so that the temperature of the preheating furnace reaches 120-180° C., and the temperature of the reaction furnace reaches 500-600° C., and the preheating time is 0.8-2 hours;

[0030] (3) After the preheating of the reactor is completed, the temperature of the reactor is lowered to 380-480°C, and methanol is preheated in the preheating furnace and then introduced into the reactor for reaction, the reaction temperature is 400-500°C, and the reaction pressure is 0.05-0.2MPa. After the preheating of the reactor is completed and the temperature is lowered, the temperature of the reactor is lower than the subsequent reaction temperature.

[0031] Specifically, the mass space velocity of methanol during the reaction is 14-18h -1 .

[0032] In the above-mentioned MTO reaction, nano SAPO-34 molecular sieve is used as a catalyst for the reaction. Before the reaction, the preheating furnace and the reaction furnace are first preheated with nitrogen, and at the same time, the impurities adsorbed on the pores and surfaces of the SAPO-34 molecular sieve are blown away to avoid these impurities affecting the reaction and reducing the selectivity of light olefins during the reaction.

[0033] In the present application, during preheating, the reactor is first heated to 500-600°C for 0.8-2h, and then the temperature is lowered to a preset temperature of 400-500°C to accelerate the preheating of the nano SAPO-34 molecular sieve, so that the internal and external temperatures of the nano SAPO-34 molecular sieve can quickly tend to be consistent, thereby shortening the preheating time.

[0034] When the MTO reaction is carried out, methanol molecules are first dehydrated to produce dimethyl ether. The B acid active sites on the surface of the molecular sieve capture methanol molecules to generate surface methoxy, and the surface methoxy further combines with methanol to obtain dimethyl ether. Secondly, the first carbon-carbon bond will be formed during the reaction induction period, and the core is the generation of active toluene. Since olefins mainly come from the elimination of the alkyl chain of active toluene and the cracking reaction greater than C5, the reaction is an endothermic reaction, so higher temperatures are conducive to the positive reaction. Therefore, with the increase of temperature, the selectivity of olefins gradually increases. However, when the reaction temperature is too high, the carbon deposition rate will also be significantly accelerated, a large amount of carbon deposition will cover the active center, the acid center will be reduced, and the selectivity of olefins will gradually decrease. When the reaction temperature is too low, the methanol molecules cannot fully react with the catalyst, so that part of the methanol is not converted, and the selectivity will also be reduced.

[0035] In this application, the mass space velocity of methanol is 14-18h -1 As the space velocity increases, the selectivity of low-carbon olefins will also increase. However, when the space velocity is too high, the residence time of the reactants and intermediates in the catalyst will be greatly shortened, making it impossible for the reactants and intermediates to fully contact and react with the active sites of the molecular sieve, resulting in incomplete reaction, increased by-products, and reduced selectivity. When the space velocity is too low, the macromolecular components generated by the reaction will be blocked in the pores of the molecular sieve and carbonized. The carbon deposits generated by carbonization will cover the active sites of the molecular sieve, thereby ultimately affecting the conversion of methanol. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The methanol conversion rate curves of the molecular sieves prepared in the examples and comparative examples;

[0037] Figure 2 The selectivity curves of light olefins for the molecular sieves prepared in the examples and comparative examples are as follows;

[0038] Figure 3 This is a scanning electron microscope (SEM) image of 1# nano-SAPO-34 prepared in Example 1. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0040] When the nano SAPO-34 molecular sieves of the following examples and comparative examples were tested for MTO reaction performance, the following steps were followed: (1) the catalyst was loaded into the reactor of the MTO reaction device;

[0041] (2) introducing heated nitrogen into the reaction device, allowing the heated nitrogen to pass through the preheating furnace and the reaction furnace in sequence, preheating the preheating furnace and the reaction furnace, so that the temperature of the preheating furnace reaches 120-180° C., and the temperature of the reaction furnace reaches 500-600° C., and the preheating time is 1 hour;

[0042] (3) After the preheating of the reactor is completed, the temperature of the reactor is lowered to 380-480°C, and the methanol preheated in the preheating furnace is introduced into the reactor for reaction. The reaction temperature is 400-500°C and the reaction pressure is 0.05-0.2MPa.

[0043] Example 1

[0044] Synthesis of 1# Nano-SAPO-34 Molecular Sieve

[0045] (1) Weigh 60 mol of deionized water and 1 mol of tetraethylammonium hydroxide, place them in a beaker, stir them at a constant speed on a magnetic stirrer, add pseudo-boehmite and silica sol into the beaker, and then drop phosphoric acid. Boehmite, silica sol and phosphoric acid are converted into aluminum, silicon and phosphorus respectively, and the molar numbers of pseudo-boehmite, silica sol and phosphoric acid are 1 mol, 0.6 mol and 1 mol respectively. Keep stirring and heat at 80°C in a constant temperature water bath for 2 hours to obtain an initial gel; the temperature of the initial gel is lowered to room temperature, and then 2 mol of triethylamine and 0.5 mol of hexadecyltrimethylammonium bromide are added to the initial gel, and the molecular sieve gel is obtained after stirring for 2 hours.

[0046] (2) The obtained molecular sieve gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and crystallized at 200°C under autogenous pressure for 12 hours. After the reaction was completed, the reactor was cooled to room temperature to obtain a solid initial product. The initial product was washed with deionized water until neutral. After washing, the initial product was dried at 105°C for 6 hours and then ground. It was calcined in a muffle furnace at 550°C for 5 hours. A 1# nano SAPO-34 molecular sieve was obtained.

[0047] The catalyst was tested for MTO reaction performance. The test conditions were preheating furnace temperature of 150°C, preheating furnace temperature of 550°C for 1h, and then methanol was introduced after the temperature of the reactor was lowered to 430°C. The reaction temperature was 450°C, the reaction pressure was 0.2MPa, and the mass space velocity of methanol during the reaction was 16h -1 .

[0048] The nano SAPO-34 molecular sieve obtained in this example was scanned to obtain Figure 3 The SEM picture shown. Figure 3 As shown, the prepared nano SAPO-34 molecular sieve presents a regular cubic morphology with a side length of 200-500 nm, high crystallinity without defects, and uniform grain size, which is beneficial to improving the catalytic performance of the MTO reaction.

[0049] Example 2

[0050] Synthesis of 2# Nano-SAPO-34 Molecular Sieve

[0051] (1) Weigh 55 mol of deionized water and 1 mol of tetraethylammonium hydroxide, place them in a beaker, stir them at a constant speed on a magnetic stirrer, add pseudoboehmite and silica sol into the beaker, and drop phosphoric acid, which is converted into aluminum, silicon and phosphorus. The molar numbers of pseudoboehmite, silica sol and phosphoric acid are 1 mol, 0.6 mol and 1 mol, respectively. Keep stirring and heat at 75°C in a constant temperature water bath for 5 h to obtain an initial gel; reduce the temperature of the initial gel to room temperature, add 2 mol of triethylamine and 0.5 mol of hexadecyltrimethylammonium bromide to the initial gel, and stir for 2 h to obtain a molecular sieve gel.

[0052] (2) The obtained molecular sieve gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and crystallized at 160°C under autogenous pressure for 18 hours. After the reaction was completed, the reactor was cooled to room temperature to obtain a solid initial product. The initial product was washed with deionized water until neutral. After washing, the initial product was dried at 110°C for 5 hours and then ground. It was calcined in a muffle furnace at 480°C for 6 hours. A 2# nano SAPO-34 molecular sieve was obtained.

[0053] The catalyst was tested for MTO reaction performance. The test conditions were preheating furnace temperature of 170°C, preheating furnace temperature of 580°C for 0.9h, and then methanol was introduced after the temperature of the reactor was lowered to 450°C. The reaction temperature was 480°C, the reaction pressure was 0.2MPa, and the mass space velocity of methanol during the reaction was 14h. -1 .

[0054] Example 3

[0055] Synthesis of 3# Nano-SAPO-34 Molecular Sieve

[0056] (1) Weigh 52 mol of deionized water and 1 mol of tetraethylammonium hydroxide, place them in a beaker, stir them at a constant speed on a magnetic stirrer, add pseudoboehmite and silica sol into the beaker, and drop phosphoric acid, which is converted into aluminum, silicon and phosphorus. The molar numbers of pseudoboehmite, silica sol and phosphoric acid are 1 mol, 0.6 mol and 1 mol, respectively. Keep stirring and heat at 85° C. in a constant temperature water bath for 2 h to obtain an initial gel; reduce the temperature of the initial gel to room temperature, add 3 mol of triethylamine and 0.5 mol of hexadecyltrimethylammonium bromide to the initial gel, and stir for 2 h to obtain a molecular sieve gel.

[0057] (2) The obtained molecular sieve gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and crystallized at 250°C under autogenous pressure for 6 hours. After the reaction was completed, the reactor was cooled to room temperature to obtain a solid initial product. The initial product was washed with deionized water until neutral. After washing, the initial product was dried at 100°C for 6 hours and then ground. It was calcined at 600°C in a muffle furnace for 4.5 hours. 3# nano SAPO-34 molecular sieve was obtained.

[0058] The catalyst was tested for MTO reaction performance. The test conditions were preheating furnace temperature of 160°C, preheating furnace temperature of 600°C for 0.8h, and then methanol was introduced after the temperature of the reactor was lowered to 480°C. The reaction temperature was 500°C, the reaction pressure was 0.2MPa, and the mass space velocity of methanol during the reaction was 15h -1 .

[0059] Example 4

[0060] Synthesis of 4# Nano-SAPO-34 Molecular Sieve

[0061] (1) Weigh 58 mol of deionized water and 1 mol of tetraethylammonium hydroxide, place them in a beaker, stir them at a constant speed on a magnetic stirrer, add pseudo-boehmite and silica sol into the beaker, and drop phosphoric acid, which is converted into aluminum, silicon and phosphorus. The molar numbers of pseudo-boehmite, silica sol and phosphoric acid are 1 mol, 0.6 mol and 1 mol, respectively. Keep stirring and heat at 90° C. in a constant temperature water bath for 1.5 h to obtain an initial gel; reduce the temperature of the initial gel to room temperature, add 2 mol of triethylamine and 0.5 mol of hexadecyltrimethylammonium bromide to the initial gel, and stir for 2 h to obtain a molecular sieve gel.

[0062] (2) The obtained molecular sieve gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and crystallized under autogenous pressure at 220°C for 10 hours. After the reaction was completed, the reactor was cooled to room temperature to obtain a solid initial product. The initial product was washed with deionized water until neutral. After washing, the initial product was dried at 110°C for 5 hours and then ground. It was calcined at 630°C in a muffle furnace for 3.5 hours. A 4# nano SAPO-34 molecular sieve was obtained.

[0063] The catalyst was tested for MTO reaction performance. The test conditions were preheating furnace temperature of 120°C, preheating furnace temperature of 590°C for 1h, and then methanol was introduced after the temperature of the reactor was lowered to 380°C. The reaction temperature was 440°C, the reaction pressure was 0.2MPa, and the mass space velocity of methanol during the reaction was 17h -1 .

[0064] In the above embodiments, after adding the template B and the mesopore directing agent, the stirring time is 2 hours. In other embodiments, the stirring time is not specifically limited as long as the raw materials can be stirred evenly.

[0065] Example 5

[0066] This embodiment is basically the same as Embodiment 1, except that the molar ratio of water, aluminum source, silicon source, phosphorus source, template A, template B and mesopore directing agent is 60:1.5:0.8:1.5:1.5:2.5:0.8.

[0067] When preparing the initial gel, stir at 88° C. for 1.5 h. After adding triethylamine and hexadecyltrimethylammonium bromide to the initial gel, stir for 2.2 h.

[0068] The hydrothermal reaction temperature is 180°C, and the hydrothermal reaction time is 18 h.

[0069] The calcination temperature is 470°C and the calcination time is 6.5h.

[0070] During the MTO reaction, the reaction temperature is 410°C, the reaction pressure is 0.1 MPa, and the mass space velocity of methanol is 18 h -1 .

[0071] Example 6

[0072] This embodiment is basically the same as Embodiment 1, except that the molar ratio of water, aluminum source, silicon source, phosphorus source, template A, template B and mesopore directing agent is 65:0.8:0.4:0.8:1.8:2.8:0.7.

[0073] When preparing the initial gel, stir at 76°C for 2.2 h.

[0074] After adding triethylamine and hexadecyltrimethylammonium bromide to the initial gel, stirring was continued for 1.5 h.

[0075] The hydrothermal reaction temperature is 160°C, and the hydrothermal reaction time is 15 h.

[0076] The calcination temperature is 620°C and the calcination time is 5.1h.

[0077] During the MTO reaction, the reaction temperature is 420°C, the reaction pressure is 0.18 MPa, and the mass space velocity of methanol is 15.5 h -1 .

[0078] Example 7

[0079] This embodiment is basically the same as Embodiment 1, except that the molar ratio of water, aluminum source, silicon source, phosphorus source, template A, template B and mesopore directing agent is 60:1.3:0.7:1.5:1.3:2.6:0.4.

[0080] When preparing the initial gel, stir at 70°C for 3 h.

[0081] After adding triethylamine and hexadecyltrimethylammonium bromide to the initial gel, stirring was continued for 3 h.

[0082] The hydrothermal reaction temperature is 245°C, and the hydrothermal reaction time is 8 h.

[0083] The calcination temperature is 580°C and the calcination time is 4.3h.

[0084] During the MTO reaction, the reaction temperature is 460°C, the reaction pressure is 0.15MPa, and the mass space velocity of methanol is 14.2h -1 .

[0085] Example 8

[0086] This embodiment is basically the same as Embodiment 1, except that the molar ratio of water, aluminum source, silicon source, phosphorus source, template A, template B and mesopore directing agent is 60:1.7:0.9:1.8:1.4:2.2:0.4.

[0087] When preparing the initial gel, stir at 82°C for 4.2 h.

[0088] After adding triethylamine and hexadecyltrimethylammonium bromide to the initial gel, stirring was continued for 1.2 h.

[0089] The hydrothermal reaction temperature is 170°C, and the hydrothermal reaction time is 12 h.

[0090] The calcination temperature is 500°C and the calcination time is 7h.

[0091] During the MTO reaction, the reaction temperature is 440°C, the reaction pressure is 0.2 MPa, and the mass space velocity of methanol is 17.5 h -1 .

[0092] Comparative Example 1

[0093] Synthesis of 5# Nano-SAPO-34 Molecular Sieve

[0094] (1) Weigh 60 mol of deionized water and 2 mol of triethylamine, place them in a beaker, stir them at a constant speed on a magnetic stirrer, add pseudo-boehmite and silica sol into the beaker, and drop phosphoric acid, which is converted into aluminum, silicon and phosphorus. The molar numbers of pseudo-boehmite, silica sol and phosphoric acid are 1 mol, 0.6 mol and 1 mol, respectively. Keep stirring and heat at 80° C. in a constant temperature water bath for 2 h to obtain an initial gel; reduce the temperature of the initial gel to room temperature, add 1 mol of tetraethylammonium hydroxide and 0.5 mol of hexadecyltrimethylammonium bromide to the initial gel, and stir for 2 h to obtain a molecular sieve gel.

[0095] (2) The obtained molecular sieve gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and crystallized at 200°C under autogenous pressure for 12 hours. After the reaction was completed, the reactor was cooled to room temperature to obtain a solid initial product. The initial product was washed with deionized water until neutral. After washing, the initial product was dried at 105°C for 6 hours and then ground. It was calcined in a muffle furnace at 550°C for 5 hours. A 5# nano SAPO-34 molecular sieve was obtained.

[0096] The catalyst was tested for MTO reaction performance. The test conditions were preheating furnace temperature of 150°C, preheating furnace temperature of 550°C for 1h, and then methanol was introduced after the temperature of the reactor was lowered to 430°C. The reaction temperature was 450°C, the reaction pressure was 0.2MPa, and the mass space velocity of methanol during the reaction was 16h -1 .

[0097] Since the solubility of triethylamine in water is much lower than that of tetraethylammonium hydroxide, when the order of adding template A and template B is adjusted, the number of crystal nuclei in the initial gel will be smaller, resulting in a faster growth rate of the molecular sieve and a final crystal size of 3-5 microns.

[0098] Comparative Example 2

[0099] Synthesis of 6# Nano-SAPO-34 Molecular Sieve

[0100] (1) Weigh 60 mol of deionized water and 1 mol of tetraethylammonium hydroxide, place them in a beaker, stir them at a constant speed on a magnetic stirrer, add pseudo-boehmite and silica sol into the beaker, and drop phosphoric acid, which is converted into aluminum, silicon and phosphorus, and the molar numbers of pseudo-boehmite, silica sol and phosphoric acid are 1 mol, 0.6 mol and 1 mol respectively, then add 2 mol of triethylamine and 0.5 mol of hexadecyltrimethylammonium bromide, and stir for 4 hours to obtain a molecular sieve gel. In this comparative example, the molecular sieve gel is directly prepared at room temperature without heating and preparing the initial gel.

[0101] (2) The obtained molecular sieve gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and crystallized at 200°C under autogenous pressure for 12 hours. After the reaction was completed, the reactor was cooled to room temperature to obtain a solid initial product. The initial product was washed with deionized water until neutral. After washing, the initial product was dried at 105°C for 6 hours and then ground. It was calcined at 550°C in a muffle furnace for 5 hours. A 6# nano SAPO-34 molecular sieve was obtained.

[0102] The catalyst was tested for MTO reaction performance. The test conditions were preheating furnace temperature of 150°C, preheating furnace temperature of 550°C for 1h, and then methanol was introduced after the temperature of the reactor was lowered to 430°C. The reaction temperature was 450°C, the reaction pressure was 0.2MPa, and the mass space velocity of methanol during the reaction was 16h -1 .

[0103] When preparing molecular sieve gel, due to the lack of an intermediate water bath heating process, the molecular sieve gel cannot form crystal nuclei during the initial stirring, and the subsequent high-temperature crystallization process of the molecular sieve will cause the growth rate of the molecular sieve to be greater than the nucleation rate, and the final crystal size of the molecular sieve is 2-4 microns.

[0104] Comparative Example 3

[0105] Synthesis of 7# Nano-SAPO-34 Molecular Sieve

[0106] (1) Weigh 60 mol of deionized water and 1 mol of tetraethylammonium hydroxide, place them in a beaker, stir them at a constant speed on a magnetic stirrer, add pseudo-boehmite and silica sol into the beaker, and drop phosphoric acid, which is converted into aluminum, silicon and phosphorus. The molar numbers of pseudo-boehmite, silica sol and phosphoric acid are 1 mol, 0.6 mol and 1 mol, respectively. Keep stirring and heat at 80° C. in a constant temperature water bath for 2 h to obtain an initial gel; reduce the temperature of the initial gel to room temperature, add 1 mol of triethylamine and 0.5 mol of hexadecyltrimethylammonium bromide to the initial gel, and stir for 2 h to obtain a molecular sieve gel.

[0107] (2) The obtained molecular sieve gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and crystallized at 200°C under autogenous pressure for 18 hours. After the reaction was completed, the reactor was cooled to room temperature to obtain a solid initial product. The initial product was washed with deionized water until neutral. After washing, the initial product was dried at 105°C for 6 hours and then ground. It was calcined in a muffle furnace at 550°C for 5 hours. 7# nano SAPO-34 molecular sieve was obtained.

[0108] The catalyst was tested for MTO reaction performance. The test conditions were preheating furnace temperature of 150°C, preheating the reaction furnace temperature at 550°C for 1h, lowering the reaction furnace temperature to 450°C to start the reaction, the reaction pressure was 0.2MPa, and the mass space velocity of methanol during the reaction was 16h-1 .

[0109] As the molar ratio of triethylamine as template B is reduced, SAPO-5 impurity crystals are generated during the crystallization process, which is not conducive to the forward progress of the subsequent catalytic reaction, and the service life and selectivity of the catalyst are reduced.

[0110] Comparative Example 4

[0111] Synthesis of 8# Nano-SAPO-34 Molecular Sieve

[0112] 1) Weigh 60 mol of deionized water and 2 mol of triethylamine, place them in a beaker, stir them at a uniform speed on a magnetic stirrer, add pseudo-boehmite and silica sol into the beaker, and drop phosphoric acid, which is converted into aluminum, silicon and phosphorus. The moles of pseudo-boehmite, silica sol and phosphoric acid are 1 mol, 0.6 mol and 1 mol respectively, keep stirring, heat at 80°C in a constant temperature water bath for 2 hours to obtain an initial gel; reduce the temperature of the initial gel to room temperature, add 1 mol of tetraethylammonium hydroxide to the initial gel, and stir for 2 hours to obtain a molecular sieve gel. In this embodiment, hexadecyltrimethylammonium bromide as a mesopore directing agent is not added.

[0113] (2) The obtained molecular sieve gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and crystallized at 200°C under autogenous pressure for 12 hours. After the reaction was completed, the reactor was cooled to room temperature to obtain a solid initial product. The initial product was washed with deionized water until neutral. After washing, the initial product was dried at 105°C for 6 hours and then ground. It was calcined in a muffle furnace at 550°C for 5 hours. A 5# nano SAPO-34 molecular sieve was obtained.

[0114] The catalyst was tested for MTO reaction performance. The test conditions were preheating furnace temperature of 150°C, preheating furnace temperature of 550°C for 1h, and then methanol was introduced after the temperature of the reactor was lowered to 430°C. The reaction temperature was 450°C, the reaction pressure was 0.2MPa, and the mass space velocity of methanol during the reaction was 16h -1 .

[0115] Comparative Example 5

[0116] Synthesis of 9# Nano-SAPO-34 Molecular Sieve

[0117] The preparation method of this comparative example is exactly the same as that of Example 1.

[0118] The catalyst was tested for MTO reaction performance. The test conditions were preheating furnace temperature of 150°C, preheating furnace temperature of 550°C for 1h, and then methanol was introduced after the temperature of the reactor was lowered to 430°C. The reaction temperature was 450°C, the reaction pressure was 0.2MPa, and the mass space velocity of methanol during the reaction was 8h -1 .

[0119] The mass space velocity of MTO reaction was increased from 14h -1 Adjusted to 8h -1 The selectivity of ethylene and propylene decreased by 2.94% and 2.71% respectively, and the selectivity of alkanes increased. This is because when the space velocity is small, the reaction products will be over-cracked in the reactor, resulting in a decrease in selectivity.

[0120] The molecular sieves prepared in the above embodiments and comparative examples were tested.

[0121] Conversion rate and selectivity of light olefins of nano SAPO-34 molecular sieve: The catalyst was tested for methanol to olefins (MTO) reaction performance in a fixed bed reactor customized by Beijing Kemenuo Company. The product was tested by gas chromatograph (GC 2014, Shimadzu), and the conversion rate and selectivity were calculated based on the test results.

[0122] Each test data is made Figure 1 and Figure 2 .

[0123] Figure 2 From left to right in the figure are Examples 1, 2, 3, 4, and Comparative Examples 1, 2, 3, 4, 5.

[0124] Compared with Comparative Examples 1, 2, 3, and 4, each embodiment can significantly shorten the crystallization reaction time of the molecular sieve by heating at a certain temperature and forming a large number of crystal nuclei in advance when preparing the molecular sieve gel, and can significantly reduce energy consumption while ensuring a small crystal size.

[0125] Figure 1 It is shown that when the methanol conversion rate is 95%, due to the reduction in particle size, the molecular sieve can have more active sites, so that the reactant methanol can be completely converted, and at the same time, the uniform radial distribution of the skeleton components can further improve the selectivity of light olefins, avoid the secondary reaction of the reaction intermediates in the pores, and reduce the probability of coking and carbon deposition of the nano molecular sieve. The catalytic life of the molecular sieves of Examples 1, 2, 3, and 4 is significantly longer than that of Comparative Examples 1, 2, 3, and 4.

[0126] The selectivity of various products generated by the MTO reaction of Example 1 and Comparative Example 5 are listed in the following table.

[0127]

[0128] It can be seen from the above table that at high space velocity, the selectivity of light olefins can be increased by 6.18%.

[0129] like Figure 2As shown, the selectivity of light olefins of Examples 1, 2, 3, 4 and Comparative Examples 1, 2, 3, 4 are 91.98%, 91.13%, 90.31%, 90.15%, 85.10%, 86.06%, 86.83% and 85.80% respectively. Compared with the comparative examples, the selectivity of the examples is significantly improved. The selectivity of light olefins of Examples 5-8 is 90.96%, 91.41%, 92.1% and 90.21% respectively.

Claims

1. A method for synthesizing nano SAPO-34 molecular sieve, characterized in that: The following steps are involved: (1) After water, aluminum source, silicon source, phosphorus source and template A are stirred evenly, the stirring is maintained and the mixture is heated in a water bath at a temperature of 70-90° C. to obtain an initial gel; the temperature of the initial gel is lowered to room temperature, and then template B and mesoporous directing agent are added to the initial gel and stirred evenly to obtain a molecular sieve gel; The molar ratio of water, aluminum source, silicon source, phosphorus source, template A, template B and mesopore directing agent is (50-70): (0.5-2): (0.4-1): (0.5-2): (1-2): (2-3): (0.1-1), wherein the aluminum source is calculated as aluminum element, the silicon source is calculated as silicon element, and the phosphorus source is calculated as phosphorus element; The template agent A is tetraethylammonium hydroxide, the template agent B is triethylamine, and the mesoporous directing agent is hexadecyltrimethylammonium bromide; (2) transferring the molecular sieve gel to a reactor for hydrothermal reaction. After the reaction is completed, cooling the reactor to room temperature to obtain an initial product; (3) The initial product is washed, dried, ground and calcined to obtain nano SAPO-34 molecular sieve.

2. The synthesis method according to claim 1, characterized in that In step (1), the water is deionized water; the aluminum source is alumina, pseudo-boehmite or aluminum isopropoxide; the silicon source is fumed silica, silica sol or tetraethyl orthosilicate; and the phosphorus source is orthophosphoric acid.

3. The synthesis method according to claim 1, characterized in that In step (1), the constant temperature heating time in the water bath is 1-5 h.

4. The synthesis method according to claim 1, characterized in that In step (2), the reaction temperature of the hydrothermal reaction is 150-250° C., and the reaction time is 6-18 h.

5. The synthesis method according to claim 1, characterized in that In step (3), the initial product is washed by pure water filtration or high-speed centrifugation, the drying temperature is 100-110°C, and the drying time is 5-7h.

6. The synthesis method according to claim 1, characterized in that In step (3), the calcination temperature is 450-650°C and the calcination time is 3-7h.

7. Application of the nano SAPO-34 molecular sieve prepared by the synthesis method according to any one of claims 1 to 6 in MTO reaction, characterized in that: The following steps are involved: (1) Loading the catalyst into the reactor of the MTO reaction unit; (2) introducing heated nitrogen into the reaction device, allowing the heated nitrogen to pass through the preheating furnace and the reaction furnace in sequence, preheating the preheating furnace and the reaction furnace, so that the temperature of the preheating furnace reaches 120-180°C, and the temperature of the reaction furnace reaches 500-600°C, and the preheating time is 0.8-2 h; (3) After the preheating of the reactor is completed, the temperature of the reactor is lowered to 380-480°C, and the methanol preheated in the preheating furnace is introduced into the reactor for reaction. The reaction temperature is 400-500°C and the reaction pressure is 0.05-0.2MPa.

8. The use according to claim 7, characterized in that: The mass space velocity of methanol during the reaction is 14-18h -1 .

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