A mesoporous pore size tunable SAPO-34 / pure silicon molecular sieve and a preparation method thereof

Abstract

The present application relates to the technical field of molecular sieve preparation, and particularly relates to a mesoporous pore size adjustable SAPO-34 / pure silicon molecular sieve and a preparation method thereof, which comprises the following steps: uniformly stirring an aluminum source, a silicon source, a phosphorus source, a structure directing agent and distilled water to obtain an initial gel; placing the initial gel in a reaction kettle and performing hydrothermal crystallization in a static oven; sequentially performing filtration, washing, drying and calcination on the obtained product to obtain a SAPO-34 molecular sieve; adding the SAPO-34 molecular sieve into a functional solution of a cationic surfactant, performing water bath heating, and sequentially performing filtration, washing, drying and calcination on the obtained product to obtain a mesoporous pore size adjustable SAPO-34 / pure silicon molecular sieve; wherein the functional solution of the cationic surfactant comprises a cationic surfactant, an organic alcohol, a silicon source, an organic base and distilled water. The preparation method of the present application significantly improves the service life of the SAPO-34 / pure silicon molecular sieve as a catalyst in MTO reaction and the selectivity of the SAPO-34 / pure silicon molecular sieve to low carbon olefins.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of molecular sieve preparation, and particularly relates to a mesoporous SAPO-34 / pure silicon molecular sieve with adjustable pore size and a preparation method thereof. BACKGROUND

[0002] Ethylene and propylene are basic organic raw materials for chemical industry, which are mainly obtained by thermal cracking of petroleum. In the context of the increasing shortage of petroleum resources, it is impossible to meet the growing market demand for low-carbon olefins by using petroleum as raw material. Therefore, it is urgent to seek a renewable and environmentally friendly method to produce light olefins. Methanol to olefins (MTO) as a non-petroleum route for producing high-value chemicals from carbon-rich raw materials such as coal, natural gas and biomass, provides a more efficient and cleaner method for preparing low-carbon olefins and has attracted much attention.

[0003] Silicon aluminum phosphate SAPO-34 molecular sieve has CHA topology structure, and its unique pore structure and acid characteristics have been proved to be the best commercial catalyst in MTO reaction. However, since the surface of SAPO-34 crystal contains a large number of invalid acid sites, carbon precursors are easily formed at the edges and corners of SAPO-34 crystal during the MTO reaction, which further hinders the diffusion of reactants into the crystal interior, the internal acid sites cannot be fully utilized, and the catalyst is quickly deactivated. In addition, the active sites on the surface of SAPO-34 have the disadvantage of poor selectivity in the reaction, which is not conducive to the generation of target products ethylene and propylene. Therefore, the high acid density on the outer surface of SAPO-34 crystal will have a negative impact on the catalytic reaction. At the same time, traditional zeolite catalysts will also encounter serious diffusion limitations, secondary reactions are intensified, active sites are covered and pore channels are blocked, which seriously affect the service life of the catalyst, which largely restricts the practical application of zeolite materials.

[0004] In view of the above problems, the existing low-acid-density mesoporous SAPO-34 is mainly prepared by reducing the silicon content in the gel precursor solution and adding a secondary template. However, this method is easy to form a large amount of AlPO4 and SAPO-5 heterocrystals. Therefore, it still faces great challenges to directly synthesize low-silicon mesoporous SAPO-34 molecular sieve.

[0005] Therefore, how to prepare low-silicon mesoporous SAPO-34 molecular sieve by a simple and repeatable method to improve the service life of the catalyst and the selectivity of low-carbon olefins has important practical application value and theoretical guiding significance in industry.

[0006] In view of this, the present application is proposed. SUMMARY

[0007] The application aims to provide a preparation method of SAPO-34 / pure silicon molecular sieve with adjustable mesopore size, which significantly improves the service life of the SAPO-34 / pure silicon molecular sieve as a catalyst in MTO reaction and the selectivity of the SAPO-34 / pure silicon molecular sieve to low-carbon olefins.

[0008] In a first aspect, the application provides a preparation method of SAPO-34 / pure silicon molecular sieve with adjustable mesopore size, which comprises the following steps:

[0009] S1, uniformly stirring an aluminum source, a silicon source, a phosphorus source, a structure-directing agent and distilled water to obtain an initial gel;

[0010] S2, placing the initial gel in a reaction kettle and performing hydrothermal crystallization in a static oven, and sequentially performing filtration, washing, drying and calcination on the obtained product to obtain SAPO-34 molecular sieve;

[0011] S3, adding the SAPO-34 molecular sieve into a functional solution of cationic surfactant, water-bath heating, and sequentially performing filtration, washing, drying and calcination on the obtained product to obtain SAPO-34 / pure silicon molecular sieve with adjustable mesopore size;

[0012] In the application, the functional solution of cationic surfactant comprises cationic surfactant, organic alcohol, silicon source, organic base and distilled water.

[0013] In step S3, the calcination is controlled at a temperature of 250-450 DEG C for 3-6 hours.

[0014] The application first prepares microporous SAPO-34 molecular sieve through a hydrothermal crystallization method, and then modifies and modifies the surface acidity of the SAPO-34 molecular sieve by growing a layer of mesoporous pure silicon molecular sieve on the surface of the SAPO-34 molecular sieve, which does not affect the active center inside the SAPO-34 crystal, at the same time, the surface acid is passivated, which significantly reduces the probability of side reactions, at the same time, the mesoporous channel of the pure silicon molecular sieve is connected with the microporous channel of the core zeolite, which can make the methanol molecules directly enter the zeolite channel through the ordered mesoporous channel for catalytic reaction; at the same time, by controlling the removal temperature of the shell mesoporous template agent, the SAPO-34 / pure silicon molecular sieve mesopore size is adjusted, and the pore size structure is optimized, the probability of side reactions is reduced, and the reaction performance as a catalyst is improved.

[0015] Research shows that calcination at 250-450 DEG C for 3-6 hours can precisely control the shell mesopore size in the range of 2.7-6.0.

[0016] Therefore, the SAPO-34 / pure silicon molecular sieve prepared by the application has excellent service life and high selectivity in MTO reaction.

[0017] As the preferred technical solution, in step S1, the molar ratio of Al2O3, SiO2, structure directing agent (SDA), P2O5 and H2O in the initial gel is (1.0-3.2):(0.3-2.5):(0.9-5.2):(0.6-6.13):(95.3-425.5), and preferably 1.54:0.45:1.47:3.50:232.5, 2.57:1.23:1.55:4.27:235.5 and 2.07:1.50:2.24:4.58:323.5.

[0018] As the preferred technical solution, in step S1, the present application does not strictly limit the specific composition of the aluminum source, silicon source, phosphorus source and structure directing agent, wherein the aluminum source includes any one of aluminum oxide, aluminum hydroxide, aluminum isopropoxide and aluminum sec-butoxide; the silicon source includes any one of fumed silica, tetraethyl orthosilicate and silica sol; the phosphorus source includes any one of phosphoric acid, ammonium monohydrogen phosphate and ammonium dihydrogen phosphate; and the structure directing agent includes any one of tetraethylammonium hydroxide, diethylamine, triethylamine and n-propylamine.

[0019] As the preferred technical solution, in step S2, compared with other synthesis methods, the hydrothermal crystallization method can effectively control the morphology and size of zeolite crystals by adjusting the synthesis parameters. The SAPO-34 molecular sieve prepared by using the hydrothermal crystallization method has the advantages of uniform crystal grain size and morphology. Specifically, during hydrothermal crystallization, the temperature is preferably 180-200℃, and the time is preferably 24-72h.

[0020] The product obtained by hydrothermal crystallization is sequentially subjected to filtration, washing, drying and calcination treatment, so as to obtain the SAPO-34 molecular sieve. The filtration and washing are mainly to wash the product to neutral. The drying is mainly to remove residual moisture, and therefore, the conditions for drying are not strictly limited, and the main purpose is to remove residual moisture. Specifically, during drying, the temperature can be controlled to be 90-110℃, and the time is 12-24h. During calcination, the temperature is preferably 550-650℃, and the time is preferably 6-12h.

[0021] As the preferred technical solution, in step S3, the prepared SAPO-34 molecular sieve is added into a functional solution of the cationic surfactant, and under the condition of water bath heating, the silicon source is loaded on the surface of the SAPO-34 molecular sieve, and the obtained product is further subjected to filtering, washing, drying and calcination treatment, so that the SAPO-34 / pure silicon molecular sieve is obtained; wherein the functional solution of the cationic surfactant is mainly composed of the cationic surfactant, an organic alcohol, a silicon source, an organic base and distilled water, and the molar ratio of the cationic surfactant, the organic alcohol, the silicon source, the distilled water and the organic base is (1.0-1.5):(95.3-570.3):(1.32-10.3):(1012.4-3710.9):(18.2-157.1), and preferably 1.54:0.45:1.47:3.50:232.5, 2.57:1.23:1.55:4.27:235.5 and 2.07:1.50:2.24:4.58:323.5.

[0022] The cationic surfactant includes any one of cetyltrimethylammonium bromide (CTAB), polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer (P123) and polyoxyethylene polyoxypropylene ether triblock copolymer (F127); the organic alcohol includes any one of n-butanol, isopropyl alcohol and ethylene glycol; the organic base includes any one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, ammonia and n-butylamine; and the silicon source includes any one of tetraethyl orthosilicate, silica sol and tetra-n-butyl orthosilicate.

[0023] In addition, in order to achieve a better surface modification effect, in the preparation process, the mass ratio of the SAPO-34 molecular sieve to the silicon source is preferably 1:(1-3.2), so that a layer of mesoporous pure silicon molecular sieve is grown on the surface of the SAPO-34 molecular sieve, so that the SAPO-34 surface acid is passivated without affecting the acidity inside the SAPO-34 crystal.

[0024] Similarly, the drying conditions in step S3 are not strictly limited, and the main purpose is to remove residual moisture. For example, the temperature can be controlled to be 90-110°C during drying, and the time is 12-24h.

[0025] In a second aspect, the application also discloses the mesoporous SAPO-34 / pure silicon molecular sieve prepared by the preparation method, and the external shell thickness of the SAPO-34 / pure silicon molecular sieve is 10-60nm, and preferably 20-40nm, wherein the SAPO-34 molecular sieve in the SAPO-34 / pure silicon molecular sieve is a 1-2μm polycrystalline aggregate microsphere formed by stacking 50-100nm primary nanoparticles.

[0026] Meanwhile, the present application is not strictly limited to the type of pure silica molecular sieve in SAPO-34 / pure silica molecular sieve, including but not limited to SBA-15, SBA-16 and KIT-6.

[0027] The preparation method of the mesoporous pore size adjustable SAPO-34 / pure silica molecular sieve has at least the following beneficial effects:

[0028] 1. In the preparation method of the mesoporous pore size adjustable SAPO-34 / pure silica molecular sieve, the respective advantages of the core phase and the shell layer mesoporous material are effectively combined. On the one hand, by growing a layer of mesoporous pure silica molecular sieve on the surface of the SAPO-34 molecular sieve, the surface acidity of the SAPO-34 molecular sieve is modified and modified, the surface acid is passivated without affecting the active center inside the SAPO-34 crystal, which significantly reduces the probability of side reactions, and the mesoporous channel of the pure silica molecular sieve is connected with the microporous channel of the core phase zeolite, which can make the methanol molecules pass through the ordered mesoporous channel and directly enter the zeolite channel for catalytic reaction. On the other hand, the mesoporous structure is introduced into the shell layer, and by controlling the removal temperature of the shell layer mesoporous template agent, the mesoporous pore size of the SAPO-34 / pure silica molecular sieve is adjusted, thereby optimizing the pore size structure, improving the diffusion rate of the reactants and products, reducing the probability of side reactions, and further improving the reaction performance of the catalyst.

[0029] 2. The preparation method significantly improves the service life of the SAPO-34 / pure silica molecular sieve as a catalyst in the MTO reaction and its selectivity for low-carbon olefins, and has the advantages of cheap and easily available raw materials, simple and easy-to-operate process flow, strong operability, etc. It has wide application prospects in solving the problem of energy shortage. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0031] Figure 1 The XRD spectrum of the mesoporous pore size adjustable SAPO-34 / pure silica molecular sieve in the present application;

[0032] Figure 2 The SEM image of the SAPO-34 molecular sieve in the present application;

[0033] Figure 3 The SEM image of the mesoporous pore size adjustable SAPO-34 / pure silica molecular sieve in the present application;

[0034] Figure 4 TEM image of the mesoporous SAPO-34 / pure-silica molecular sieve with adjustable pore size of the present application;

[0035] Figure 5 NH3-TPD graph of the mesoporous SAPO-34 / pure-silica molecular sieve with adjustable pore size of the present application and SAPO-34 molecular sieve. DETAILED DESCRIPTION

[0036] It should be noted that the following detailed description is illustrative only, and is intended to provide further description in addition to the description of the exemplary embodiments according to this application. Unless otherwise defined, 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 belongs.

[0037] It is to be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to this application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, steps, operations, devices, components and / or combinations thereof, but do not preclude the presence or addition of one or more other features, steps, operations, devices, components and / or combinations thereof.

[0038] The technical solutions of the present application will be described clearly and completely in connection with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0039] Example 1

[0040] S1, 5.04 g of aluminum isopropoxide was weighed and dissolved in a solution of 27.8 mL of distilled water and 20 mL of tetraethylammonium hydroxide, stirred uniformly to obtain a clear solution, then 2.6 mL of tetraethyl orthosilicate and 3.6 mL of 85 wt% phosphoric acid solution were sequentially added, stirred for 2 h to obtain an initial gel;

[0041] S2, the initial gel was moved into a stainless steel high-pressure reaction kettle, crystallized in a static oven at 180°C for 72 h, the obtained product was washed to neutral, dried in an oven at 90°C for 12 h, and calcined at 500°C for 12 h to obtain SAPO-34 molecular sieve;

[0042] S3, 4 g of SAPO-34 molecular sieve was added to 140 mL of distilled water, stirred for 0.5 h, then 2 g of P123, 35 mL of isopropyl alcohol, 2 mL of ammonia water and 1.4 mL of n-butyl silicate were added in turn, heated in a 45°C water bath for 4 h, the obtained product was filtered, washed, dried in a 90°C oven for 12 h, and calcined at 390°C for 5 h to obtain a mesoporous SAPO-34 / pure silica molecular sieve with adjustable pore size.

[0043] Example 2

[0044] S1, 1.72 g of aluminum hydroxide was weighed and added to a mixed solution of 18 mL of distilled water and 3.8 g of triethylamine, stirred to obtain a clear solution, then 3.0 mL of silica sol and 3.34 g of ammonium dihydrogen phosphate were added in turn, stirred for 2 h to obtain an initial gel;

[0045] S2, the initial gel was moved into a stainless steel high-pressure reaction kettle, crystallized in a 180°C static oven for 48 h, the obtained product was washed to neutral, dried in a 100°C oven for 12 h, and then calcined at 550°C for 7 h to obtain a SAPO-34 molecular sieve;

[0046] S3, 4 g of SAPO-34 molecular sieve was added to 140 mL of distilled water, stirred for 0.5 h, then 2.0 g of F127, 30 mL of n-butanol, 6.0 mL of tetrapropylammonium hydroxide and 1.4 mL of n-butyl silicate were added in turn, heated in a 60°C water bath for 4 h, the obtained product was filtered, washed, dried in a 90°C oven for 12 h, and then calcined at 380°C for 6 h to obtain a mesoporous SAPO-34 / pure silica molecular sieve with adjustable pore size.

[0047] Example 3

[0048] S1, 5.74 g of aluminum sec-butoxide was dissolved in a solution of 24 mL of distilled water and 20 mL of tetrapropylammonium hydroxide, stirred to obtain a clear solution, then 0.78 g of fumed SiO2 and 3.89 g of ammonium dihydrogen phosphate were added in turn, stirred for 2 h to obtain an initial gel;

[0049] S2, the initial gel was moved into a stainless steel high-pressure reaction kettle, crystallized in a 200°C static oven for 24 h, the obtained product was washed to neutral, dried in a 90°C oven for 12 h, and then calcined at 600°C for 4 h to obtain a SAPO-34 molecular sieve;

[0050] S3, take 3g SAPO-34 molecular sieve into 140mL distilled water, stir for 0.5h, then add 2.1g CTAB, 27mL n-propanol, 2mL n-butylamine and 1.4mL tetraethyl orthosilicate in turn, heat in 40℃ water bath for 4h, filter, wash and dry in 90℃ oven for 12h, then calcine at 350℃ for 5h, to obtain mesoporous SAPO-34 / pure silica molecular sieve with adjustable pore size.

[0051] Example 4

[0052] S1, weigh 1.05g alumina into a solution of 24mL distilled water and 4.5g diethylamine, stir until uniform to obtain a clear solution, then add 0.78g fumed SiO2 and 3.89g ammonium dihydrogen phosphate in turn, stir for 2h to obtain an initial gel;

[0053] S2, move the initial gel into a stainless steel high-pressure reaction kettle, crystallize in a 200℃ static oven for 24h, wash the obtained product to neutral, dry in a 100℃ oven for 12h, then calcine at 580℃ for 10h to obtain SAPO-34 molecular sieve;

[0054] S3, take 4.8g SAPO-34 molecular sieve into 135mL distilled water, stir for 0.5h, then add 2.1g P123, 18mL ethylene glycol, 2mL n-butylamine and 4.8g silica sol in turn, heat in a 50℃ water bath for 4h, filter, wash and dry in a 90℃ oven for 12h, then calcine at 405℃ for 5h to obtain mesoporous SAPO-34 / pure silica molecular sieve with adjustable pore size.

[0055] Example 5

[0056] S1, weigh 1.05g alumina into a solution of 30mL distilled water and 5.3g diethylamine, stir until uniform to obtain a clear solution, then add 0.78g fumed SiO2 and 4mL phosphoric acid solution in turn, stir for 2h to obtain an initial gel;

[0057] S2, move the initial gel into a stainless steel high-pressure reaction kettle, crystallize in a 200℃ static oven for 36h, wash the obtained product to neutral, dry in a 100℃ oven for 12h, then calcine at 650℃ for 3h to obtain SAPO-34 molecular sieve;

[0058] S3, 4 g of SAPO-34 molecular sieve was added to 150 mL of distilled water, stirred for 0.5 h, then 2.1 g of P123, 18 mL of ethylene glycol, 2 mL of n-butylamine and 4.8 g of silica sol were sequentially added, heated in a 50°C water bath for 4 h, and the obtained product was filtered, washed, dried in an oven at 110°C for 12 h, and then calcined at 420°C for 4 h to obtain a mesoporous SAPO-34 / pure silica molecular sieve with adjustable pore size.

[0059] Example 6

[0060] S1, 1.05 g of alumina was weighed and dissolved in a solution of 30 mL of distilled water and 5.3 g of diethylamine, stirred until uniform to obtain a clear solution, then 3.2 mL of tetraethyl orthosilicate and 4 mL of phosphoric acid solution were sequentially added, stirred for 2 h to obtain an initial gel;

[0061] S2, the initial gel was moved into a stainless steel high-pressure reaction kettle, crystallized in a 190°C static oven for 36 h, the obtained product was washed to neutral, dried in an oven at 100°C for 12 h, and then calcined at 580°C for 4 h to obtain a SAPO-34 molecular sieve;

[0062] S3, 4 g of SAPO-34 molecular sieve was added to 150 mL of distilled water, stirred for 0.5 h, then 2.1 g of P123, 18 mL of n-propanol, 2 mL of n-butylamine and 1.6 mL of n-butyl silicate were sequentially added, heated in a 50°C water bath for 4 h, and the obtained product was filtered, washed, dried in an oven at 110°C for 24 h, and then calcined at 500°C for 3 h to obtain a mesoporous SAPO-34 / pure silica molecular sieve with adjustable pore size.

[0063] Comparative Example 1

[0064] The SAPO-34 molecular sieve was prepared by steps S1-S2 in Example 1.

[0065] Comparative Example 2

[0066] SAPO-34 molecular sieve

[0067] The SAPO-34 molecular sieve was prepared by steps S1-S2 in Example 1.

[0068] Pure silica molecular sieve

[0069] To 140 mL of distilled water, then 2 g of P123, 35 mL of isopropyl alcohol, 2 mL of ammonia water and 1.4 mL of n-butyl silicate were sequentially added, stirred for 0.5 h, heated in a 45°C water bath for 4 h, and the obtained product was filtered, washed, dried in an oven at 90°C for 12 h, and then calcined at 550°C for 4 h to obtain a pure silica molecular sieve;

[0070] Mechanical mixing of SAPO-34 + pure silica molecular sieve

[0071] SAPO-34 zeolite and pure silicon molecular sieve were mixed and stirred evenly at a solid / solid (mass ratio) ratio of 1:0.27 to obtain mechanically mixed SAPO-34 + pure silicon molecular sieve.

[0072] Comparative Example 3

[0073] 4 g of SAPO-34 molecular sieve was added to 140 mL of distilled water and stirred for 0.5 h. Then, 2 g of P123, 35 mL of isopropanol, 2 mL of hydrochloric acid and 1.4 mL of tetrabutyl orthosilicate were added in sequence. The mixture was heated in a water bath at 45 °C for 4 h. The resulting product was filtered, washed, dried in an oven at 90 °C for 12 h, and calcined at 390 °C for 5 h to obtain SAPO-34 / pure silica molecular sieve with adjustable mesoporous pore size.

[0074] The other steps and parameters are basically the same as in Example 1.

[0075] Comparative Example 4

[0076] Replace the P123 cationic surfactant with a single-headed quaternary ammonium salt cationic surfactant;

[0077] The other steps and parameters are basically the same as in Example 1.

[0078] Comparative Example 5

[0079] When preparing SAPO-34 molecular sieve, the calcination conditions are: calcination at 400℃ for 8 hours.

[0080] When preparing SAPO-34 / pure silicon molecular sieve with adjustable mesoporous pore size, the calcination conditions are: calcination at 500℃ for 4 hours;

[0081] The other steps and parameters are basically the same as in Example 1.

[0082] Comparative Example 6

[0083] The calcination conditions for preparing SAPO-34 molecular sieve were: calcination at 700℃ for 6 hours.

[0084] When preparing SAPO-34 / pure silicon molecular sieve with adjustable mesoporous pore size, the calcination conditions are: calcination at 200℃ for 6 hours;

[0085] The other steps and parameters are basically the same as in Example 1.

[0086] Figure 1 The image shows the XRD pattern of the SAPO-34 / pure silica molecular sieve prepared in Example 1, wherein... Figure 1The wide-angle XRD spectrum of A can be seen that the sample appears diffraction peaks at 2θ = 9.7°, 13.1°, 16.2°, 20.7°, 25.1° and 30.8°, corresponding to the characteristic diffraction peaks of SAPO-34, indicating that the deposition of a layer of pure silicon mesoporous material on the surface of SAPO-34 does not affect the crystal structure of SAPO-34.

[0087] From the wide-angle XRD spectrum of B, it can be seen that the sample appears a strong diffraction peak at 2θ = 1.0, 1.7 and 2.0, and two relatively weak diffraction peaks, which can be respectively considered as the diffraction of (100), (110) and (200) crystal faces belonging to the p6mm space group, proving that the sample has an ordered SBA-15 mesoporous channel structure. Figure 1

[0088] Figures 2-3 The scanning electron microscope (SEM) images of Comparative Example 1 and Example 1 materials, respectively, can be seen from the wide-angle XRD spectrum of B, Figure 2 It can be seen that the SAPO-34 zeolite is a polycrystalline aggregate of spherical morphology formed by the accumulation of primary nanocubes (~ 100 nm). It can be observed from the local magnified area that the surface of the primary nanocubes is smooth and angular. When a layer of pure silicon zeolite is deposited, the surface particles of SAPO-34 adhere to each other, and the boundary is blurred Figure 3 It can be further observed by transmission electron microscopy (TEM) Figure 4 ), and the shell thickness is about 23 nm, thus confirming that the SAPO-34 / pure silicon zeolite with a composite type can be successfully prepared by the secondary growth synthesis strategy.

[0089] The SAPO-34 / pure silicon zeolite prepared in the application was subjected to NH3-TPD analysis, and the results are shown in Figure 5 Compared with the parent SAPO-34, the strong acid amount (~ 460 ℃) of the SAPO-34 / pure silicon zeolite catalytic material is obviously reduced, indicating that the acid sites on the surface of SAPO-34 are passivated, thereby achieving the effect of modifying and modifying the surface acidity.

[0090] The nitrogen adsorption-desorption of the composite zeolite obtained in Examples 1-6 and Comparative Examples 1-6 was further characterized, and the results are shown in Table 1.

[0091] Table 1 Nitrogen adsorption-desorption characterization test results

[0092]

[0093] As can be seen from Table 1, with the increase of the mesoporous template removal temperature, the mesoporous pore size gradually increases. Therefore, by controlling the mesoporous template removal temperature, the shell mesoporous pore size can be precisely controlled in the range of 2.7-6.0.​

[0094] The prepared molecular sieve is further used as a catalyst for a methanol to light olefin (MTO) reaction, and the catalytic performance of the catalyst is investigated, and the test results are shown in Table 2.

[0095] The specific experimental method is as follows:

[0096] The MTO reaction performance evaluation is carried out in a fixed bed micro-reactor device under normal pressure.

[0097] The catalyst powder is crushed by extrusion to prepare 20-40 mesh particles, which are loaded into a reaction tube, and activated at 550 DEG C for 2h in a N2 gas flow with a flow rate of 50 mL / min before the reaction, and then reduced to the reaction temperature; the loading amount of the catalyst is 0.4g, and the reaction temperature is 400 DEG C; then a trace metering pump is used to introduce the methanol raw material, so that the mass space velocity is maintained at 1 h -1 ; the product after the reaction is analyzed on a gas chromatograph equipped with a hydrogen flame ionization detector (FID).

[0098] Table 2 Catalytic effect data

[0099]

[0100] As shown in Table 2, the mesoporous pore size adjustable composite SAPO-34 / pure silicon molecular sieve prepared in Examples 1-6 has excellent methanol catalytic activity, stability and light olefin selectivity.

[0101] The SAPO-34 zeolite prepared in Comparative Example 1 has a large number of invalid acid sites on the surface, which promotes the generation of carbon precursors and is attached to the outer surface of the molecular sieve, thereby further hindering the diffusion of the reactants into the crystal interior, the internal acid sites cannot be fully utilized, and the catalytic activity is lost.

[0102] The zeolite catalytic material prepared in Comparative Example 2 by mechanical mixing method can improve the selectivity of light olefins to a certain extent, but significantly shortens the service life of the catalyst, indicating that the catalytic material prepared by the mechanical mixing method cannot completely coat the SAPO-34, resulting in a large number of exposed acid sites on the surface, thereby affecting the reaction performance of the catalyst.

[0103] In Comparative Example 3, the use of an acidic cationic surfactant functional solution changes the pH value of the synthesis system, and the silicon element in the SAPO-34 framework is easily removed in the slightly acidic solution, which destroys the crystallinity of the zeolite, and further affects the catalytic performance.

[0104] In the comparative example 4, the P123 cationic surfactant is replaced by a single head quaternary ammonium salt cationic surfactant, which can prepare the SAPO-34 / pure silica molecular sieve material, but the pore structure of the surface shell is disordered, the pore size is difficult to control, and the mesopore of the shell layer and the micropore of the core phase SAPO-34 have poor connectivity, which seriously hinders the transmission and diffusion of the reactant molecules.

[0105] In summary, the acid of the SAPO-34 surface is modified and modified by depositing the pure silica molecular sieve on the surface of the SAPO-34, without affecting the active center in the SAPO-34 crystal, the surface acid is passivated, which significantly reduces the probability of side reactions; in addition, the mesoporous structure is introduced in the shell layer, and by controlling the removal temperature of the shell layer mesoporous template, the SAPO-34 / pure silica molecular sieve mesopore size is adjusted, and the pore size structure is optimized, and the diffusion rate of the reactants and products is improved. Therefore, the preparation method of the present application significantly improves the service life of the SAPO-34 / pure silica molecular sieve as a catalyst in the MTO reaction and its selectivity to low-carbon olefins.

[0106] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or replace some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a mesoporous pore size tunable SAPO-34 / pure silica molecular sieve, characterized in that, The method comprises the following steps: S1, stirring the aluminum source, silicon source, phosphorus source, structure directing agent and distilled water uniformly to obtain an initial gel; S2, placing the initial gel in a reaction kettle and performing hydrothermal crystallization in a static oven, and then performing filtration, washing, drying and calcination on the obtained product in sequence to obtain SAPO-34 molecular sieve; S3, adding the SAPO-34 molecular sieve into a functional solution of surfactant, heating in a water bath, and then performing filtration, washing, drying and calcination on the obtained product in sequence to obtain mesoporous SAPO-34 / pure silicon molecular sieve with adjustable pore size; The functional solution of surfactant comprises any one of cetyltrimethylammonium bromide, polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer and polyoxyethylene polyoxypropylene ether triblock copolymer. In step S3, the calcination is controlled at a temperature of 250-450 DEG C for 3-6 hours.

2. The production method according to claim 1, characterized by, In step S1, the molar ratio of Al2O3, SiO2, structure directing agent, P2O5 and H2O in the initial gel is (1.0-3.2):(0.3-2.5):(0.9-5.2):(0.6-6.13):(95.3-425.5).

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

4. The method of claim 1, wherein, In step S2, the hydrothermal crystallization is controlled at a temperature of 180-200 DEG C for 24-72 hours.

5. The preparation method according to claim 1, characterized in that, In step S2, the drying is controlled at a temperature of 90-110 DEG C for 12-24 hours. The calcination is controlled at a temperature of 550-650 DEG C for 6-12 hours.

6. The method of claim 1, wherein, In step S3, the molar ratio of surfactant, organic alcohol, silicon source, distilled water and organic base is (1.0-1.5):(95.3-570.3):(1.32-10.3):(1012.4-3710.9):(18.2-157.1).

7. The preparation method according to claim 1, characterized in that, In step S3, the mass ratio of SAPO-34 molecular sieve to silicon source is 1:(1-3.2).

8. The method of claim 1, wherein, The organic alcohol comprises any one of n-butanol, isopropyl alcohol and ethylene glycol. The organic base comprises any one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, ammonia water and n-butylamine. The silicon source comprises any one of tetraethyl orthosilicate, silica sol and n-butyl orthosilicate.

9. A mesoporous pore size tunable SAPO-34 / pure-silica molecular sieve, characterized in that, The SAPO-34 / pure silicon molecular sieve prepared by the method of any one of claims 1-8 has an external shell thickness of 10-60 nm. In the SAPO-34 / pure silicon molecular sieve, the SAPO-34 molecular sieve is a 1-2 μm polycrystal aggregate microsphere formed by stacking 50-100 nm primary nanoparticles.

10. The adjustable mesopore size SAPO-34 / pure silica molecular sieve of claim 9, wherein, The pure silicon molecular sieve in the SAPO-34 / pure silicon molecular sieve includes any one of SBA-15, SBA-16 and KIT-6.

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