Nanosingle-crystal SAPO-34 molecular sieve, preparation method and application thereof

Abstract

The application relates to the technical field of molecular sieve preparation, in particular to a nano single-crystal SAPO-34 molecular sieve and a preparation method and application thereof, which comprises the following steps: uniformly mixing an aluminum source, a silicon source, a phosphorus source, a structure directing agent and water to obtain an initial gel; drying the initial gel to obtain a dry gel; placing the dry gel in a reaction kettle to perform steam phase conversion; sequentially performing centrifugation, washing, drying and calcination on the steam phase conversion product to obtain the nano single-crystal SAPO-34 molecular sieve; wherein, during the steam phase conversion, a mixed solution of water and an organic alcohol is added in the reaction kettle, and the temperature of the steam phase conversion is 100-130 DEG C, and the time is 3-7 h. The preparation method has relatively mild reaction conditions, is simple in process, can realize industrial application, and effectively overcomes the problems that in the prior art, a secondary pore directing agent with high price needs to be added or the synthesized zeolite needs to be subjected to acid or alkali treatment, the operation steps are complicated, and the method cannot be massively applied in industry.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of molecular sieve preparation, in particular to a nano single crystal SAPO-34 molecular sieve and a preparation method and application thereof. BACKGROUND

[0002] Ethylene and propylene are important basic organic raw materials in modern petrochemical industry, and the technologies for increasing ethylene and propylene production and raw material alternatives have always been the focus of long-term attention and research of major oil companies and research institutions in the world.

[0003] The energy occurrence characteristics of China are relatively 'rich in coal, poor in oil and less in gas', and it is imminent to develop modern coal chemical industry and coal-to-olefin technology. The methanol-to-olefin (MTO) technology is the most promising new process route to replace oil-to-olefin at present, and the design and research of catalysts have always been the focus of researchers. Among numerous catalytic materials, SAPO-34 molecular sieve with CHA type topological structure is widely used in MTO reaction due to its large specific surface area, special pore structure, good thermal and hydrothermal stability and adjustable acidity.

[0004] However, the SAPO-34 molecular sieve with micron-sized crystal grain size and single pore (micropore) is easily obtained by using the traditional hydrothermal synthesis method, which seriously limits the mass transfer and diffusion of reactants and products, causing the catalyst to be rapidly deactivated due to rapid carbon deposition in a very short time, which greatly hinders industrial application.

[0005] In order to solve this key technical problem, introducing secondary pore structure into zeolite is an effective means to improve its diffusion efficiency and prolong the service life of the catalyst. At present, the most common method is to introduce secondary mesopore into zeolite by adding secondary template. In this case, the multi-level pores usually need expensive soft templates (such as silanized polymers, hydrophilic cationic polymers and surfactants) or hard templates (such as carbon nanotubes, carbon black and mesoporous carbon), and the complicated synthesis steps seriously restrict the practical application of zeolite materials.

[0006] In addition, secondary voids can also be made by extracting aluminum or silicon atoms in the zeolite framework through acid or alkali treatment, but the crystal structure of the zeolite is inevitably damaged, the acid characteristics of the zeolite are changed, and the removed silicon and aluminum fragments are easy to block the micropore channels of the zeolite, reducing the effective utilization rate of active sites.

[0007] In recent years, nano crystal self-assembly method has attracted much attention from scientists, and this method can effectively avoid the above problems. The intercrystalline mesopore is formed only by the self-assembly of nano crystals. However, this method has problems such as long crystallization time and uncontrollable crystallization size.

[0008] In view of this, the present application is proposed. SUMMARY

[0009] The present application aims to provide a kind of nanometer single crystal SAPO-34 molecular sieve and its preparation method and application nanometer single crystal SAPO-34 molecular sieve prepared by the present application contains abundant intercrystalline mesoporous structure, shorten the diffusion path of reactant and product, in MTO acid catalytic reaction It shows excellent catalytic performance, its catalytic life is greatly improved while also improving the selectivity of low carbon olefin.

[0010] In the first aspect, the present application provides a kind of nanometer single crystal SAPO-34 molecular sieve preparation method, comprising the following steps:

[0011] S1, aluminum source, silicon source, phosphorus source, structure directing agent and water are uniformly mixed to obtain initial gel;

[0012] S2, the initial gel is dried to obtain dry gel after treatment;

[0013] S3, the dry gel is placed in a reaction kettle for vapor phase conversion;

[0014] S4, the vapor phase conversion product is sequentially centrifuged, washed, dried and calcined to obtain nanometer single crystal SAPO-34 molecular sieve;

[0015] Wherein, during the vapor phase conversion, the mixed solution of water and organic alcohol is added in the reaction kettle, and the temperature of vapor phase conversion is 100-130 DEG C, and the time is 3-7h.

[0016] The present application uses the mixed solution of water and organic alcohol as solvent in the process of vapor phase conversion to prepare SAPO-34 molecular sieve, and constructs SAPO-34 zeolite framework by sequentially controlling the formation of primary and secondary structural units.

[0017] Specifically, in the preparation process of dry gel, the water in the initial gel will promote SDA concentration after evaporation at high temperature, enhance the stability of secondary structural unit, and promote the generation of a large number of SAPO-34 zeolite nucleation centers;In the process of vapor phase conversion, first, the presence of organic alcohol can increase the pressure of reaction system, the required crystallization temperature (<130 DEG C) is significantly lower than that of traditional hydrothermal crystallization (>180 DEG C), and the crystallization time is shortened (3-7h), which reduces energy consumption;Second, organic alcohol can fill the microporous channel of zeolite and play the role of structure directing agent;More importantly, organic alcohol can form hydrogen bond with zeolite crystal nucleus, effectively inhibit the Ostwald growth of crystal, and the formed primary nanoparticles further assemble into SAPO-34 zeolite nanopolymer aggregate, successfully realize the non-classical construction of hierarchical pore zeolite.

[0018] Therefore, the reaction system of the present invention effectively regulates the nucleation rate and growth rate during the crystallization process. Due to the faster nucleation rate, more crystal nuclei are generated, resulting in the preparation of nano-single crystal SAPO-34 molecular sieves, which are aggregates with a cubic morphology formed by the orderly stacking of initial nanoparticles with a size of about 25 nm. Furthermore, the sample exhibits an obvious intercrystalline mesoporous structure.

[0019] As a preferred embodiment of this technical solution, in step S1, the aluminum source, silicon source, and phosphorus source are converted into Al2O3, SiO2, and P2O5, respectively, and the molar ratio of the aluminum source, silicon source, phosphorus source, structure directing agent, and water is (0.3-1.4):(0.1-0.5):(0.9-3.2):(0.1-9.5):(40-320).

[0020] Wherein, the aluminum source is any one of boehmite, aluminum isopropoxide, aluminum sec-butoxide, and aluminum hydroxide; the silicon source is any one of tetraethyl orthosilicate, silica sol, silica, and fumed silica; the phosphorus source is any one of phosphoric acid, ammonium dihydrogen phosphate, and ammonium monohydrogen phosphate; and the structure directing agent is any one of tetraethylammonium hydroxide, morpholine, diethylamine, and triethylamine.

[0021] In a preferred embodiment of this technical solution, in step S2, the initial gel is dried to obtain a dry gel. During the drying process, the temperature is controlled at 90-100℃ and the time is 20-30h to initially remove moisture from the gel, promote the concentration and enrichment of SDA, enhance the stability of secondary structural units, and promote the generation of a large number of SAPO-34 zeolite nucleation centers.

[0022] As a preferred embodiment of this technical solution, in step S3, the mass ratio of dry gel to mixed liquid in the reaction vessel is 1:0.15-1.30, so as to ensure the smooth progress of vapor phase conversion while further increasing the pressure of the reaction system and shortening the crystallization time.

[0023] In the mixture, the mass ratio of water to organic alcohol is 1:(0.3-2), and the organic alcohol used includes any one of methanol, ethanol, isopropanol and ethylene glycol.

[0024] In a preferred embodiment of this technical solution, step S4 specifically includes centrifuging, washing, and drying the product obtained from the vapor phase conversion to obtain a white solid powder; calcining the obtained white solid powder to obtain a nano-single-crystal SAPO-34 molecular sieve with abundant mesoporous structure, wherein the drying temperature is controlled at 90-120℃ for 24-36h; and the calcination temperature is controlled at 500-650℃ for 4-12h.

[0025] Secondly, the present invention also provides a nano-single crystal SAPO-34 molecular sieve prepared by the above preparation method, which should also fall within the protection scope of the present invention. Specifically, the nano-single crystal SAPO-34 molecular sieve has a crystal size of 10-50 nm and a mesopore diameter of 2-30 nm.

[0026] Thirdly, the application of the nano-single-crystal SAPO-34 molecular sieve prepared by the above method in the methanol-to-olefins reaction should also fall within the scope of protection of this invention.

[0027] The method for preparing nano-single-crystal SAPO-34 molecular sieve of the present invention has at least the following technical effects:

[0028] 1. This invention uses a vapor phase conversion method to prepare SAPO-34 molecular sieves. The SAPO-34 zeolite framework is constructed by sequentially controlling the formation of primary and secondary structural units. Specifically, during the preparation of the dry gel, the evaporation of water in the initial gel at high temperature promotes the concentration and enrichment of SDA, enhancing the stability of the secondary structural units and promoting the generation of numerous SAPO-34 zeolite nucleation centers. During the vapor phase conversion, firstly, the presence of organic alcohols increases the pressure of the reaction system, requiring a crystallization temperature (<130℃) significantly lower than the traditional hydrothermal crystallization temperature (>180℃), shortening the crystallization time (3-7h) and reducing energy consumption. Secondly, organic alcohols can fill the micropores of the zeolite, acting as structure-directing agents. More importantly, organic alcohols can form hydrogen bonds with the zeolite nuclei, effectively inhibiting Ostwald growth of the crystals. The resulting primary nanoparticles further assemble into SAPO-34 zeolite nanocrystalline aggregates, successfully achieving the non-classical construction of hierarchical porous zeolites.

[0029] 2. The preparation method of the present invention has relatively mild reaction conditions and simple process, which can be applied to industrial applications. It effectively overcomes the problem that the existing technology requires the addition of expensive secondary pore guiding agents or acid or alkali treatment of the synthesized zeolite, which is complicated and cannot be applied to a large scale in industry.

[0030] 3. The nano-single crystal SAPO-34 molecular sieve prepared by this invention contains abundant intercrystalline mesoporous structures, which shortens the diffusion path of reactants and products, and exhibits excellent catalytic performance in MTO acid catalysis. Its catalytic lifetime is greatly improved, and the selectivity of low carbon olefins is also improved. Attached Figure Description

[0031] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0032] Figure 1 This is a process flow diagram of the material prepared by the vapor phase inversion method according to the present invention;

[0033] Figure 2 These are the XRD patterns of Embodiment 1 and Comparative Example 1 of the present invention;

[0034] Figure 3 This is a SEM image of Embodiment 1 of the present invention;

[0035] Figure 4 This is a TEM image of Embodiment 1 of the present invention;

[0036] Figure 5 This is a SEM image of Comparative Example 1 of the present invention;

[0037] Figure 6 This is a SEM image of Comparative Example 2 of the present invention;

[0038] Figure 7 The N2 adsorption-desorption isotherm and corresponding pore size distribution diagram are shown in Example 1 of the present invention.

[0039] Figure 8 This is a comparison chart of catalyst lifetime and average selectivity for low-carbon olefins in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention. Detailed Implementation

[0040] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0041] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations according to this application. As used herein, the singular form includes the plural form unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this description, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0042] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. 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.

[0043] Example 1

[0044] S11. At room temperature, 6.12 g of aluminum isopropoxide was added to a mixture of 20.5 mL of tetraethylammonium hydroxide (TEAOH, 25 wt.%) and 8 mL of distilled water, and the mixture was magnetically stirred for 4 h. After complete dissolution, 1.4 mL of tetraethyl orthosilicate (TEOS) was added, and the mixture was stirred for 2 h. Subsequently, 2.0 mL of phosphoric acid (H3PO4, 85 wt.%) solution was slowly added, and the mixture was stirred for another 1 h to obtain the initial gel.

[0045] S12. The obtained initial gel was placed in an oven at 100°C and dried for 24 hours to obtain 13.6 g of dry gel.

[0046] S13. Place the obtained dry gel in a stainless steel reactor with a polytetrafluoroethylene liner (100 mL), add 2 g of distilled water and 4 g of ethanol mixture (Note: the dry gel is separated from the mixture), and then let it stand at 100 °C for 5 h to crystallize.

[0047] S14. Wash the crystallized sample until neutral, dry overnight to obtain a white solid powder; then calcine at 550℃ for 6 hours to obtain nano-single crystal SAPO-34 molecular sieve with abundant mesopores.

[0048] Example 2

[0049] S21. At room temperature, 2.10 g of boehmite was added to a mixture of 3.02 g of diethylamine and 8 mL of distilled water, and the mixture was magnetically stirred for 4 h. After complete dissolution, 1.25 g of silica sol (30 wt.%) was added, and the mixture was stirred for 2 h. Subsequently, 3.66 g of ammonium monohydrogen phosphate was slowly added, and the mixture was stirred for another 1 h to obtain the initial gel.

[0050] S22. The obtained initial gel was placed in an oven at 100°C and dried for 24 hours to obtain 8.52 g of dry gel.

[0051] S23. The obtained dry gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner (100 mL), and a mixture of 3 g distilled water and 1.2 g methanol was added (Note: the dry gel was separated from the mixture). The mixture was then allowed to stand at 120 °C for 5 h to crystallize.

[0052] S24. The crystallized sample was washed until neutral and dried overnight to obtain a white solid powder; then it was calcined at 550℃ for 6 hours to obtain a nano-single crystal SAPO-34 molecular sieve with abundant mesopores.

[0053] Example 3

[0054] S31. At room temperature, 7.35 g of aluminum sec-butoxide was added to a mixture of 3.52 g of diethylamine and 8 mL of distilled water, and the mixture was magnetically stirred for 4 h. After complete dissolution, 1.25 g of silica sol (30 wt.%) was added, and the mixture was stirred for 2 h. Subsequently, 4.10 g of ammonium dihydrogen phosphate was slowly added, and the mixture was stirred for another 1 h to obtain the initial gel.

[0055] S32. The obtained initial gel was placed in an oven at 100°C and dried for 24 hours to obtain 12.71 g of dry gel.

[0056] S33. Place the obtained dry gel in a stainless steel reactor with a polytetrafluoroethylene liner (100 mL), add 2 g of distilled water and 4.5 g of ethylene glycol mixture (Note: the dry gel is separated from the mixture), and then let it stand at 130 °C for 3 h to crystallize.

[0057] S34. The crystallized sample was washed until neutral and dried overnight to obtain a white solid powder; then it was calcined at 550℃ for 6 hours to obtain a nano-single crystal SAPO-34 molecular sieve with abundant mesopores.

[0058] Example 4

[0059] S41. At room temperature, 1.32 g of aluminum hydroxide was added to a mixture of 3.02 g of diethylamine and 8 mL of distilled water, and the mixture was magnetically stirred for 4 h. After complete dissolution, 0.40 g of gaseous SiO2 was added and stirred for 2 h. Subsequently, 3.66 g of monoammonium phosphate was slowly added and the mixture was stirred for another 1 h to obtain the initial gel.

[0060] S42. The obtained initial gel was placed in an oven at 100°C and dried for 24 hours to obtain 7.43 g of dry gel.

[0061] S43. Then, the obtained dry gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner (100 mL), and 2 g of distilled water and 4 g of propanol mixture were added (Note: the dry gel was separated from the mixture). The mixture was then kept static at 120 °C for 6 h to crystallize.

[0062] S44. Wash the crystallized sample until neutral, dry overnight to obtain a white solid powder; then calcine at 550℃ for 6 hours to obtain nano-single crystal SAPO-34 molecular sieve with abundant mesopores.

[0063] Example 5

[0064] S51. At room temperature, 1.54 g of aluminum hydroxide was added to a mixture of 2.79 g of morpholine and 14 mL of distilled water, and the mixture was magnetically stirred for 4 h. After complete dissolution, 0.40 g of gaseous SiO2 was added and the mixture was stirred for 2 h. Subsequently, 3.66 g of monoammonium phosphate was slowly added and the mixture was stirred for another 1 h to obtain the initial gel.

[0065] S52. The obtained initial gel was placed in an oven and dried at 100°C for 24 hours to obtain 7.85g of dry gel.

[0066] S53. The obtained dry gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner (100 mL), and 3.5 g of distilled water and 1.7 g of ethylene glycol mixture were added (Note: the dry gel was separated from the mixture). The mixture was then kept static at 100 °C for 7 h to crystallize.

[0067] S54. Wash the crystallized sample until neutral, dry overnight to obtain a white solid powder; then calcine at 550℃ for 6 hours to obtain nano-single crystal SAPO-34 molecular sieve with abundant mesopores.

[0068] Example 6

[0069] S61. At room temperature, 1.54 g of aluminum hydroxide was added to a mixture of 3.12 g of morpholine and 14 mL of distilled water, and the mixture was magnetically stirred for 4 h. After complete dissolution, 0.40 g of gaseous SiO2 was added and stirred for 2 h. Subsequently, 3.66 g of monoammonium phosphate was slowly added and the mixture was stirred for another 1 h to obtain the initial gel.

[0070] S62. The obtained initial gel was placed in an oven and dried at 100°C for 24 hours to obtain 7.8 g of dry gel.

[0071] S63. The obtained dry gel was transferred to a stainless steel reactor with a polytetrafluoroethylene liner (100 mL), and a mixture of 3.0 g of distilled water and 2.3 g of ethanol was added (Note: the dry gel was separated from the mixture). The mixture was then allowed to stand at 110 °C for 4 h to crystallize.

[0072] S64. The crystallized sample was washed until neutral and dried overnight to obtain a white solid powder; then it was calcined at 600℃ for 12h to obtain a nano-single crystal SAPO-34 molecular sieve with abundant mesopores.

[0073] Comparative Example 1

[0074] Preparation of micron-sized SAPO-34 zeolite by hydrothermal synthesis

[0075] S1. At room temperature, 6.12 g of aluminum isopropoxide was added to a mixture of 20.5 mL of tetraethylammonium hydroxide (TEAOH, 25 wt.%) and 8 mL of distilled water, and the mixture was magnetically stirred for 4 h. After complete dissolution, 1.4 mL of tetraethyl orthosilicate (TEOS) was added, and the mixture was stirred for 2 h. Subsequently, 2.0 mL of phosphoric acid (H3PO4, 85 wt.%) solution was slowly added, and the mixture was stirred for another 1 h to obtain the initial gel.

[0076] S2. Place the initial gel in a stainless steel reactor with a polytetrafluoroethylene liner (100 mL) and crystallize at 180°C for 72 h.

[0077] S3. Wash the crystallized sample until neutral, dry overnight to obtain a white solid powder; then calcine at 550℃ for 6 hours to obtain micron-sized SAPO-34 molecular sieve.

[0078] Comparative Example 2

[0079] SAPO-34 zeolite was prepared using a vapor phase reforming method with water as the solvent.

[0080] S1. At room temperature, 4.0 g of aluminum isopropoxide was added to a mixture of 20.5 mL of tetraethylammonium hydroxide (TEAOH, 25 wt.%) and 8 mL of distilled water, and the mixture was magnetically stirred for 4 h. After complete dissolution, 1.4 mL of tetraethyl orthosilicate (TEOS) was added, and the mixture was stirred for 2 h. Subsequently, 2.0 mL of phosphoric acid (H3PO4, 85 wt.%) solution was slowly added, and the mixture was stirred for another 1 h to obtain the initial gel.

[0081] S2. Place the obtained initial gel in an oven and dry it at 100°C for 24 hours to obtain a dry gel;

[0082] S3. Place the obtained dry gel in a stainless steel reactor with a polytetrafluoroethylene liner (100mL), add 6g of distilled water (Note: the dry gel is separated from the water), and then let it stand at 130℃ for 7h to crystallize.

[0083] S4. Wash the crystallized sample until neutral, dry overnight to obtain a white solid powder; then calcine at 550℃ for 6 hours to obtain SAPO-34 molecular sieve.

[0084] Figure 1 This is a flow chart of the preparation process of the vapor phase inversion method of the present invention. The obtained initial dry gel is transferred to a reaction vessel for vapor phase inversion, which yields nano-single-crystal SAPO-34 molecular sieves with abundant mesopores.

[0085] Figure 2This is the X-ray diffraction (XRD) pattern of the nanocrystalline SAPO-34 molecular sieve of this invention. As can be seen from the figure, the prepared material is a typical SAPO-34 molecular sieve, with no other impurities formed. Compared with the traditional hydrothermal crystallization method, the characteristic diffraction peaks of the SAPO-34 crystals obtained by the vapor phase conversion method show a significant broadening, indicating that the SAPO-34 prepared by this invention has a relatively small grain size.

[0086] Figure 3 The image shows a SEM image of the SAPO-34 molecular sieve nanocrystals prepared in this invention. It can be observed that the SAPO-34 molecular sieve morphology consists of cubic aggregates formed by the orderly stacking of initial nanoparticles approximately 25 nm in size, with a size ranging from 100 to 250 nm. Figure 4 It can be observed that the nano-aggregates have single-crystal characteristics.

[0087] Figure 5 For Comparative Example 1, SAPO-34 molecular sieves with larger crystal sizes (~5 μm) are usually synthesized using the traditional hydrothermal synthesis method, with a crystal size 200 times larger than the former.

[0088] Figure 6 The image shows a SEM image of the sample prepared by vapor phase inversion using distilled water as a solvent in Comparative Example 2. As can be seen from the image, the morphology mainly consists of bulk SAPO-34 crystals and amorphous substances. This further illustrates that the addition of organic alcohols is a key factor in the synthesis of nanoporous single-crystal SAPO-34 molecular sieves with abundant mesopores.

[0089] Figure 7 The present invention provides the N2 adsorption-desorption isotherms and corresponding pore size distribution curves for the preparation of nanocrystalline SAPO-34. Figure 7 It can be seen that the sample exhibits a significant hysteresis loop at a relative pressure P / P0 > 0.4, indicating the presence of a distinct intergranular mesoporous structure. Combined with... Figure 7 The inset diagram shows that the mesopores are distributed between 3-40 nm. Calculations indicate that the sample's external specific surface area and pore volume are 128 m². 2 / g and 0.15cm 3 / g, further indicating that the nano-single crystal SAPO-34 molecular sieve prepared in this invention has abundant intercrystalline mesoporous structure. For the pore structure parameters of other embodiments and comparative examples, please refer to Table 1.

[0090] The present invention further uses the sample prepared above as a catalyst for the methanol-to-low-carbon olefin reaction and investigates the catalytic performance of the catalyst. The test results are detailed in Table 1.

[0091] The specific experimental method is as follows:

[0092] The reaction was carried out in a micro fixed-bed reactor under atmospheric pressure, with 0.40 g of catalyst loaded into the reactor. N2 was first introduced into the reactor at a flow rate of 50 mL / min, and the temperature was increased from room temperature to the reaction temperature (550 °C) at a rate of 5 °C / min for 1 h. The temperature was then reduced to 400 °C.

[0093] Then, methanol was introduced using a micro-metering pump, with the feed rate controlled at 0.5 mL / h and the space velocity maintained at 1 h. -1 The reaction products were analyzed using a gas chromatograph equipped with a flame ionization detector (FID).

[0094] Catalytic results Figure 8 The results show that the nanocrystalline SAPO-34 catalyst obtained by steam phase conversion in Example 1 of this application has a significantly longer catalytic lifetime (960 min) than the SAPO-34 catalyst obtained by hydrothermal synthesis in Comparative Example 1 (560 min) and by using only water as a solvent (Comparative Example 2) (240 min). Compared with Comparative Examples 1 and 2, the catalyst prepared in Example 1 showed an average increase of 2.1% and 7.9% in selectivity for low-carbon olefins (ethylene + propylene), respectively.

[0095] Table 1 Test Results

[0096]

[0097] As shown in Table 1, the nano-single crystal SAPO-34 molecular sieves prepared in Examples 1-6 of this invention have excellent methanol catalytic activity, stability and low-carbon olefin selectivity in the methanol-to-olefins reaction.

[0098] Comparative Example 1 uses the traditional hydrothermal synthesis method to prepare SAPO-34 molecular sieve. Due to the presence of a large amount of water, the concentration of the reaction system is low. During the crystallization process, the nucleation rate and growth rate are slow, the crystallization time is long, and the resulting polycrystalline aggregates are large in size with small external surface area and pore volume. When used as a catalyst, its catalytic performance and catalytic lifespan are limited.

[0099] Comparative Example 2 uses the vapor phase conversion method, but only water is used as a solvent to prepare SAPO-34 molecular sieve. Although it can reduce the crystal size of SAPO-34 zeolite to a certain extent (~1μm), the silicon, aluminum and phosphorus sources in the dry gel cannot be completely converted into SAPO-34 zeolite. A large amount of amorphous substances are generated, which seriously affects the reaction performance of the catalyst.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing nanocrystalline SAPO-34 molecular sieves, characterized in that, Includes the following steps: S1. Mix aluminum source, silicon source, phosphorus source, structure directing agent and water evenly to obtain initial gel; S2. After drying the initial gel, a dry gel is obtained; S3. The dry gel is placed in a reaction vessel for vapor phase conversion; S4. The vapor phase conversion products were centrifuged, washed, dried and calcined in sequence to obtain nano-single crystal SAPO-34 molecular sieve; During the vapor phase conversion, a mixture of water and organic alcohol is added to the reactor, and the vapor phase conversion temperature is 100-130℃, and the time is 3-7h. In the mixture, the mass ratio of water to organic alcohol is 1:(0.3-2). The organic alcohol is any one of methanol, ethanol, isopropanol, and ethylene glycol; The nano-single crystal SAPO-34 molecular sieve is an aggregate with a cubic morphology formed by the orderly stacking of initial nanoparticles with a grain size of 10-50 nm, a size of 100-250 nm, and a mesopore diameter of 2-30 nm. In step S3, the mass ratio of the dry gel to the mixture in the reaction vessel is 1:(0.15-1.30).

2. The preparation method according to claim 1, characterized in that, In step S1, the aluminum source, silicon source, and phosphorus source are converted into Al2O3, SiO2, and P2O5 respectively for calculation purposes. The molar ratio of the aluminum source, silicon source, phosphorus source, structure directing agent, and water is (0.3-1.4):(0.1-0.5):(0.9-3.2):(0.1-9.5):(40-320).

3. The preparation method according to claim 1, characterized in that, The aluminum source is any one of boehmite, aluminum isopropoxide, aluminum sec-butoxide, and aluminum hydroxide. The silicon source is any one of tetraethyl orthosilicate, silica sol, silica fume, and fumed silica; The phosphorus source is any one of phosphoric acid, ammonium dihydrogen phosphate, and ammonium monohydrogen phosphate. The structure-directing agent is any one of tetraethylammonium hydroxide, morpholine, diethylamine, and triethylamine.

4. The preparation method according to claim 1, characterized in that, In step S2, the drying process is carried out at a temperature of 90-110℃ for 20-30 hours.

5. The preparation method according to claim 1, characterized in that, In step S4, during the drying process, the temperature is controlled at 90-120℃ and the time is 24-36 hours. During the roasting process, the temperature is controlled at 500-650℃ and the time is 4-12 hours.

6. The application of the nano-single-crystal SAPO-34 molecular sieve prepared by the preparation method according to any one of claims 1-5, characterized in that, Application of the nano-single crystal SAPO-34 molecular sieve in methanol-to-olefins reaction.

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