A mesoporous ZSM-5 molecular sieve and its preparation method

Abstract

The present invention discloses a kind of mesoporous ZSM 5 molecular sieve and preparation method thereof, the preparation method comprises the following steps:Silicon source, aluminum source, alkali source, crystallization inducer, auxiliary agent, water are mixed to obtain crystallized gel, the crystallized gel is crystallized under hydrothermal conditions, washed, filtered, dried, roasted, and mesoporous ZSM 5 molecular sieve is obtained;Wherein, the silicon source is magadiite, the crystallization inducer is organic ammonium salt and seed crystal, and the auxiliary agent is selected from at least one of alcohol or amine. The molecular sieve obtained by the method of the present invention has the characteristics of high crystallinity, large specific surface area, and large micro-mesopore volume. The preparation method of the present invention adopts cheap silicon source, and in the case of adding a small amount of mesoporous template agent, mesoporous ZSM 5 molecular sieve can be synthesized, which is applied to light hydrocarbon catalytic cracking reaction with good low-carbon olefin selectivity.

Description

Technical Field

[0001] The invention belongs to the technical field of chemistry and chemical engineering, and relates to a mesoporous-rich ZSM-5 molecular sieve and a preparation method thereof. Background Art

[0002] As a porous material, ZSM-5 molecular sieve has long attracted significant attention in catalysis and adsorption due to its flexible and adjustable acidity, excellent thermal stability, and other properties. Pore structure plays a dominant role in the catalytic performance of molecular sieves. The micropores in the micro-mesoporous composite zeolite provide the primary acidic catalytic sites and shape-selective molecular transport, while the mesopores enhance the diffusion efficiency of molecules within and between crystals. The increased mesopores in the molecular sieve facilitate the rapid diffusion of light olefins from the pores, thereby improving the selectivity of light olefins.

[0003] "Study on the Preparation and Mechanism of Zeolite Molecular Sieve from Magadiyite Crystallization" ([D]. Dalian. Dalian University of Technology. 2020.) discloses the preparation of omega, Si-Nu-1, chabazite, and omega zeolite by hydrothermal crystallization using magadiyite as a silicon source in combination with TMABr, TMAOH, and choline. "Synthesis of Zeolite Molecular Sieve from Seed-Directed Magadiyite Crystallization" ([D]. Dalian. Dalian University of Technology. 2021.) discloses the preparation of omega, beta, and chabazite using magadiyite as a silicon source in combination with seed crystallization. Patent CN 113912077 A discloses a method for preparing mesoporous ZSM-5 molecular sieves by a seed crystal method. This method uses inexpensive water glass as a silicon source, aluminum sulfate as an aluminum source, and nano-metal oxides as pore-enlarging agents to synthesize mesoporous ZSM-5 molecular sieves via hydrothermal crystallization. Patent CN 113943008 A discloses a method for preparing ZSM-5 molecular sieves. The method uses silica sol as a silicon source and sodium hydroxide as an alkali source. After mixing and adjusting the silica sol to alkalinity, sodium aluminate, copper nitrate, and tetrapropylammonium bromide as templates are sequentially added. The resulting molecular sieve has high crystallinity, a large specific surface area, good hydrothermal stability, and strong resistance to carbon deposition. Patent CN 113880107A discloses a method for synthesizing and applying a dice-shaped ZSM-5 molecular sieve, primarily addressing the problem of the lack of macropores on the surface of ZSM-5 molecular sieve crystals obtained by direct synthesis in the prior art. The ZSM-5 molecular sieve of this invention exhibits a dice-shaped morphology, with multiple macropores on the outer surface of the grains, each with an opening diameter of 100-200 nm. Patent CN 111099620A improves the order of the mesopores within the mesoporous molecular sieve by pre-exposing the ZSM-5 molecular sieve to an ordered mesopore directing agent solution followed by alkaline treatment, thereby increasing the sample solid yield and crystallinity. Patent CN 113044852 A uses hemicellulose as a hard template to prepare a hierarchical ZSM-5 molecular sieve. Without introducing complex processes, the synthesized molecular sieve exhibits both ordered micropores and mesopores, forming a more diverse pore structure. Microporous and Mesoporous Materials (2015, 208:66-71) reports a method for preparing ZSM-5 molecular sieves by inducing the crystallization of magadiite using 1,6-hexanediamine as a template. The resulting product exhibits a blocky morphology. "Microporous and Mesoporous Materials" (018, 265: 63-69.) reported a preparation method using tetrapropylammonium salt as a template to induce the crystallization of magadiite to form ZSM-5 molecular sieve. The synthesized product has a blocky morphology."Microporous and Mesoporous Materials" (2022, 329: 111534.) reported the synthesis of a petal-shaped ZSM-5 molecular sieve with a rich mesoporous structure by co-induction with cetyltrimethylammonium bromide and TPABr. The molar ratio of cetyltrimethylammonium bromide to silicon source (calculated as silica) was 0.1. "Microporous and Mesoporous Materials" (2022, 345: 112252.) reported the synthesis of a flaky ZSM-5 molecular sieve by co-induction with cetyltrimethylammonium bromide and seed crystals. The effect of the amount of cetyltrimethylammonium bromide and seed crystals on the morphology of the molecular sieve product was investigated in detail. The molar ratio of cetyltrimethylammonium bromide to silicon source (calculated as silica) was not less than 0.03.

[0004] The synthesis of multi-level pore ZSM-5 molecular sieve is difficult. The use of a large amount of mesoporous templates will lead to "phase separation" between the microporous phase and the mesoporous phase of the molecular sieve, and the crystallization process of the molecular sieve is difficult to control. The use of post-treatment methods to create mesopores will lead to uncontrollable mesopore formation and reduce the crystallinity of the product. The method of inducing the synthesis of ZSM-5 molecular sieves using the seed crystal method has been widely studied and applied, but the mesoporous specific surface area of ​​the product synthesized by this method is very small, generally not exceeding 100m 2 / g.

[0005] From the above, it can be seen that the preparation process of mesoporous-rich ZSM-5 molecular sieve is complicated, and expensive mesoporous templates are usually used, which will make the synthesis cost of the product too high and reduce the practical value of the product. Hexadecyltrimethylammonium salt is a commonly used template for preparing special morphology ZSM-5 molecular sieves, but the dosage is generally high. This type of ammonium salt is a typical surfactant. A high addition amount will produce a large number of bubbles coating the crystallized material during the dynamic crystallization process of the molecular sieve, which will have a serious impact on the reaction system and the crystallization process, limiting its scale-up preparation. In addition, the products obtained by converting magadiite into ZSM-5 molecular sieves are mostly blocky and do not have a typical mesoporous-rich structure. Therefore, it is very important to develop a green, simple and low-cost method for preparing mesoporous-rich molecular sieves. Summary of the Invention

[0006] Based on the above, the main purpose of the present invention is to provide a mesoporous ZSM-5 molecular sieve and a preparation method thereof. The preparation method uses magadiite as a silicon source and uses extremely low amounts of seed crystals and organic ammonium salts to synergistically assist crystallization to synthesize mesoporous ZSM-5 molecular sieves with different morphologies.

[0007] In order to achieve the above object, the present invention provides a method for preparing mesoporous ZSM-5 molecular sieve, which comprises the following steps:

[0008] A silicon source, an aluminum source, an alkali source, a crystallization inducer, an auxiliary agent, and water are mixed to obtain a crystallized gel, and the crystallized gel is crystallized under hydrothermal conditions, washed, filtered, dried, and calcined to obtain a mesoporous ZSM-5 molecular sieve; wherein the silicon source is magadiite, the crystallization inducer is an organic ammonium salt and a seed crystal, and the auxiliary agent is selected from at least one of an alcohol or an amine.

[0009] In the preparation method of the mesoporous-rich ZSM-5 molecular sieve of the present invention, the magadiite can be a natural mineral, a homemade or commercially available product, and can be a silicate or contain other metal elements.

[0010] In the method for preparing a mesoporous ZSM-5 molecular sieve of the present invention, the cation of the organic ammonium salt comprises three or four alkyl groups, at least one of which has a carbon number greater than or equal to eight, i.e., a tertiary ammonium salt or a quaternary ammonium salt. The anion of the organic ammonium salt is not specifically required and may be a halogen anion, a sulfate anion, a hydroxide ion, or the like.

[0011] Among them, preferably, the cation of the organic ammonium salt contains 4 alkyl groups, and the carbon number of at least one of the alkyl groups is greater than or equal to 12 and less than or equal to 16, such as at least one of dodecylethyldimethylammonium bromide, dodecylpropylenedimethylammonium bromide, dodecyltriethylammonium bromide, dodecyltrimethylammonium bromide, dodecyltrimethylammonium hydroxide, dodecyltrimethylammonium chloride, dodecyltrimethylammonium hydrogen sulfate, hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium hydroxide, hexadecyltrimethylammonium chloride and hexadecyltrimethylammonium hydrogen sulfate.

[0012] The preparation method of the mesoporous-rich ZSM-5 molecular sieve described in the present invention, wherein the seed crystal is a molecular sieve with an MFI crystal structure, which can be a homemade or commercially available product, and the silicon-aluminum ratio (i.e., SiO2 / Al2O3 ratio) is not specifically required, preferably 40 to pure silicon.

[0013] In the method for preparing the mesoporous-rich ZSM-5 molecular sieve of the present invention, the auxiliary agent is selected from at least one of a monohydric alcohol having a carbon number less than or equal to 4 and a cyclic aliphatic amine having a carbon number less than or equal to 8.

[0014] The preparation method of the mesoporous-rich ZSM-5 molecular sieve of the present invention, wherein the auxiliary agent is selected from at least one of methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol, piperidine, pyridine, piperazine, homopiperazine, hexamethyleneimine, heptamethyleneimine, cyclohexylamine, methylcyclohexylamine and ethylcyclohexylamine.

[0015] The preparation method of the mesoporous ZSM-5 molecular sieve of the present invention, wherein the silicon source is calculated as SiO2, the aluminum source is calculated as Al2O3, the alkali source is calculated as M2O, and the organic ammonium salt is calculated as the cationic R + The molar ratio of each component in the crystallized gel is: SiO2 / Al2O3=30-400, seed / SiO2=0.002-0.250, M2O / SiO2=0.05-0.50, R + / SiO2=0.003~0.03, H2O / SiO2=15~120, auxiliary agent / SiO2=0.001~0.03; wherein, M is an alkali metal.

[0016] The preparation method of the mesoporous-rich ZSM-5 molecular sieve of the present invention, wherein the seed crystal / SiO2=0.002-0.09.

[0017] The preparation method of the mesoporous-rich ZSM-5 molecular sieve of the present invention, wherein the crystallization temperature is 120-190° C., and the crystallization time is 12-120 hours.

[0018] Preferably, the crystallization temperature is 140-185° C., and the crystallization time is 24-72 hours.

[0019] In the method for preparing the mesoporous-rich ZSM-5 molecular sieve of the present invention, the aluminum source is selected from at least one of pseudo-boehmite, aluminum isopropoxide, sodium metaaluminate and aluminum sulfate.

[0020] In the method for preparing the mesoporous-rich ZSM-5 molecular sieve of the present invention, the alkali source is selected from at least one of sodium hydroxide and potassium hydroxide.

[0021] The preparation method of the present invention can also be described in detail as follows:

[0022] 1) Mix magadiite with an aluminum source, water, an alkali source, and an organic ammonium salt, and vigorously stir at 20-80° C. for 0.5-48 hours;

[0023] 2) adding seed crystals and auxiliary agents to the mixed solution obtained in step 1), and vigorously stirring at 20-80° C. for 0.5-48 h to obtain a crystallized gel. The molar ratio of each component in the crystallized gel (silicon source as SiO2, aluminum source as Al2O3, alkali source as M2O, organic ammonium salt as cationic R + The ratio of the SiO2 content in the seed crystal is: SiO2 / Al2O3=30-400, seed crystal / SiO2=0.002-0.250, M2O / SiO2=0.05-0.50, R + / SiO2=0.003~0.03,H2O / SiO2=15~120,auxiliary agent / SiO2=0.001~0.03;

[0024] 3) performing a crystallization reaction under hydrothermal conditions, wherein the conditions for the crystallization reaction are: a temperature of 120 to 190° C. and a time of 12 to 120 h, and the reaction obtains a mesoporous ZSM-5 molecular sieve.

[0025] During the research process, the inventors discovered that using a natural layered silicate material such as magadiite as a silicon source, in the presence of extremely low seed addition and a small amount of organic ammonium salt, mesoporous ZSM-5 molecular sieves of different morphologies can be synthesized. This is due to the natural layered structure of magadiite, which can reduce the demand for long-chain ammonium salts in the process of forming layered molecular sieves. The ions in the organic ammonium salt play two functions in the reaction: one is to form an intercalation layer on the magadiite, increasing the reaction contact area of ​​the magadiite; the other is to play a synergistic role with the seed crystal to induce crystallization and obtain a molecular sieve product with a rich mesoporous morphology. To better achieve this goal, the introduction of an auxiliary agent can further assist the crystallization process and can prevent the aggregation and ripening process of the layered structure by forming hydrogen bonds with the surface hydroxyl groups.

[0026] The preparation method of the present invention uses magadiite as a silicon source and can directly synthesize a mesoporous ZSM-5 molecular sieve with high crystallinity, large specific surface area, large micro-mesopore volume and different morphologies in a single step. By using a cheap natural layered silicate material as a silicon source, an extremely low seed addition amount and a small amount of organic ammonium salt, pollutant emissions in the product post-processing process are reduced, effectively reducing product production costs and environmental protection costs, and having a strong practical application significance. The reduction in the amount of organic ammonium salt used and the introduction of an organic ammonium salt with a shorter carbon chain are both conducive to weakening its surfactant performance, reducing the generation of bubbles in the dynamic crystallization process of the molecular sieve, thereby reducing the impact on the molecular sieve crystallization process and being conducive to the amplified synthesis of the mesoporous ZSM-5 molecular sieve.

[0027] The mesoporous ZSM-5 molecular sieve obtained by the preparation method of the present invention has a very developed and complete meso-microporous channel system, which can improve the selectivity of light olefins when used in catalytic cracking reactions, and is expected to have very broad application prospects in the fields of catalytic cracking and catalytic cracking. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the XRD spectrum of magadiite;

[0029] Figure 2 is the SEM image of magadiite;

[0030] Figure 3 is the XRD spectrum of the molecular sieve prepared in Example 1;

[0031] Figure 4 is a SEM image of the molecular sieve prepared in Example 1;

[0032] Figure 5 is the XRD spectrum of the molecular sieve prepared in Example 2;

[0033] Figure 6 is a SEM image of the molecular sieve prepared in Example 2;

[0034] Figure 7 is the XRD spectrum of the molecular sieve prepared in Example 3;

[0035] Figure 8 is a SEM image of the molecular sieve prepared in Example 3;

[0036] Figure 9 is the XRD spectrum of the molecular sieve prepared in Example 4;

[0037] Figure 10 This is the SEM image of the molecular sieve prepared in Example 4. DETAILED DESCRIPTION

[0038] The following is a detailed description of the embodiments of the present invention: This embodiment is implemented based on the technical solution of the present invention, and provides detailed implementation methods and processes. However, the protection scope of the present invention is not limited to the following embodiments. The experimental methods in the following embodiments where specific conditions are not specified are generally based on conventional conditions.

[0039] The raw materials used in the embodiment are:

[0040] Aluminum source: pseudo-boehmite (purity 95%), aluminum isopropoxide (purity 99%), sodium metaaluminate (purity 99%), aluminum sulfate (purity 99%).

[0041] Organic ammonium salts: cetyltrimethylammonium bromide (purity 99%), cetyltrimethylammonium hydroxide (purity 10%), cetyltrimethylammonium chloride (purity 97%), cetyltrimethylammonium hydrogen sulfate (purity 98%); dodecyltrimethylammonium bromide (purity 99%), dodecyltrimethylammonium chloride (purity 99%).

[0042] Auxiliary agents: methanol (purity 99%), ethanol (purity 97%), cyclohexylamine (purity 98%), hexamethyleneimine (purity 98%), homopiperazine (purity 98%).

[0043] Alkali source: sodium hydroxide (purity 99%), potassium hydroxide (purity 99%).

[0044] Seed solution: molecular sieve with MFI crystal structure can be prepared by the following method: tetraethyl orthosilicate, tetraethylammonium hydroxide and deionized water are fully mixed, and the molar ratio of the mixture (silicon source is calculated based on the effective component of SiO2 contained, template agent is calculated based on the organic cation R+ R + / SiO2=0.25, H2O / SiO2=30, and a crystallization reaction is carried out under hydrothermal conditions. The crystallization reaction conditions are: 90°C, time: 36h. Add 2 times the mass of deionized water to the above reaction product and mix evenly, i.e., dilute the effective seed crystal solubility in the reaction product to one-third of the original solubility, and set aside for later use. It should be noted that the raw materials, ratios, reaction conditions, dilution ratio, etc. used in the preparation of the seed crystal solution of the present invention are not limited to the above process, and purchased molecular sieve products can also be used to prepare the seed crystal solution.

[0045] Others: Deionized water.

[0046] Example 1

[0047] Dissolve 1.1g of sodium hydroxide, 0.76g of sodium metaaluminate, and 0.3g of hexadecyltrimethylammonium bromide in 100g of water, slowly add 10.0g of magadiite, and stir at 50°C for 2h. Then slowly add 4.0g of seed solution and 0.1g of methanol, and stir at 50°C for 3h to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 170°C for 48h. The product is washed, filtered, dried, and calcined to obtain a molecular sieve product.

[0048] The XRD spectrum of molecular sieve products is shown in Figure 3 The product has typical MFI structural characteristic peaks, as shown in the SEM image. Figure 4 The product is in the form of nanocrystalline accumulation. The molecular sieve is subjected to full pore specific surface area and porosity analysis, and its total specific surface area is 352m 2 / g, of which the micropore specific surface area is 252m 2 / g, and the mesopore volume is 0.17m 3 / g.

[0049] Example 2

[0050] Dissolve 1.1g of sodium hydroxide, 0.76g of sodium metaaluminate, and 0.5g of hexadecyltrimethylammonium bromide in 100g of water, slowly add 10.0g of magadiite, and stir at room temperature for 3 hours. Then slowly add 3.0g of seed solution and 0.15g of ethanol, and stir at room temperature for 5 hours to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 170°C for 48 hours. The product is washed, filtered, dried, and calcined to obtain a molecular sieve product.

[0051] The XRD spectrum of molecular sieve products is shown in Figure 5 The product has typical MFI structural characteristic peaks, as shown in the SEM image. Figure 6 The product is in the shape of nano needles, and its sheet thickness is about 20nm. The molecular sieve is subjected to full pore specific surface area and porosity analysis, and its total specific surface area is 372m 2 / g, of which the micropore specific surface area is 239m 2 / g, and the mesopore volume is 0.23m 3 / g.

[0052] Example 3

[0053] Dissolve 1.3g of sodium hydroxide, 0.76g of sodium metaaluminate, and 0.75g of hexadecyltrimethylammonium bromide in 100g of water, slowly add 10.0g of magadiite, and stir at 50°C for 2h. Then, slowly add 4.0g of seed solution and 0.1g of ethanol, and stir at 50°C for 3h to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 175°C for 36h. The product is washed, filtered, dried, and calcined to obtain a molecular sieve product.

[0054] The XRD spectrum of molecular sieve products is shown in Figure 7 The product has typical MFI structural characteristic peaks, as shown in the SEM image. Figure 8 The product is in the form of nano-sheets with a thickness of about 5nm. The molecular sieve is subjected to full pore specific surface area and porosity analysis, and its total specific surface area is 397m 2 / g, of which the micropore specific surface area is 213m 2 / g, and the mesopore volume is 0.32m 3 / g.

[0055] Example 4

[0056] Dissolve 1.5g of sodium hydroxide, 0.76g of sodium metaaluminate, and 1.5g of hexadecyltrimethylammonium bromide in 130g of water, slowly add 10.0g of magadiite, and stir at 50°C for 2h. Then slowly add 5.0g of seed solution and 0.2g of ethanol, and stir at 50°C for 3h to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 170°C for 48h. The product is washed, filtered, dried, and calcined to obtain a molecular sieve product.

[0057] The XRD spectrum of molecular sieve products is shown in Figure 9 The product has typical MFI structural characteristic peaks, as shown in the SEM image. Figure 10 The product is in the shape of nanorods. The molecular sieve is subjected to full pore specific surface area and porosity analysis, and its total specific surface area is 429m 2 / g, of which the micropore specific surface area is 208m 2 / g, mesopore volume is 0.35m 3 / g.

[0058] Example 5

[0059] Dissolve 1.1g of sodium hydroxide, 0.76g of sodium metaaluminate, and 3.2g of hexadecyltrimethylammonium hydroxide in 100g of water, slowly add 10.0g of magadiite, and stir at 50°C for 2h. Then slowly add 4.0g of seed solution and 0.25g of cyclohexylamine, and stir at 50°C for 3h to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 170°C for 48h. The product is washed, filtered, dried, and calcined to obtain a molecular sieve product.

[0060] The XRD spectrum of the molecular sieve product shows that the product has typical MFI structural characteristic peaks and the product is in the form of nano-sheets. The molecular sieve is subjected to full pore specific surface area and porosity analysis, and its total specific surface area is 326m 2 / g, of which the micropore specific surface area is 232m 2 / g, and the mesopore volume is 0.19m 3 / g.

[0061] Example 6

[0062] Dissolve 1.1g of sodium hydroxide, 0.76g of sodium metaaluminate, and 1.5g of hexadecyltrimethylammonium hydrogen sulfate in 100g of water, slowly add 10.0g of magadiite, and stir at 50°C for 2h. Then, slowly add 4.0g of seed solution and 0.15g of cyclohexylamine, and stir at 50°C for 3h to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 170°C for 48h. The product is washed, filtered, dried, and calcined to obtain a molecular sieve product.

[0063] The XRD spectrum of the molecular sieve product shows that the product has typical MFI structural characteristic peaks and the product is in the shape of nano needles. The molecular sieve is subjected to full pore specific surface area and porosity analysis, and its total specific surface area is 325m 2 / g, of which the micropore specific surface area is 212m 2 / g, mesopore volume is 0.25m 3 / g.

[0064] Example 7

[0065] Dissolve 1.1g of sodium hydroxide, 0.76g of sodium metaaluminate, and 1.1g of hexadecyltrimethylammonium chloride in 100g of water, slowly add 10.0g of magadiite, and stir at 50°C for 2h. Then slowly add 4.0g of seed solution and 0.15g of hexamethyleneimine, and stir at 50°C for 3h to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 170°C for 48h. The product is washed, filtered, dried, and calcined to obtain a molecular sieve product.

[0066] The XRD spectrum of the molecular sieve product shows that the product has typical MFI structural characteristic peaks and the product is in the form of nanocrystalline accumulation. The molecular sieve is subjected to full pore specific surface area and porosity analysis, and its total specific surface area is 338m 2 / g, of which the micropore specific surface area is 222m 2 / g, mesopore volume is 0.20m 3 / g.

[0067] Example 8

[0068] Dissolve 1.3g of sodium hydroxide, 0.25g of sodium metaaluminate, and 0.35g of dodecyltrimethylammonium bromide in 100g of water, slowly add 10.0g of magadiite, and stir at 50°C for 2h. Then slowly add 4.0g of seed solution and 0.15g of hexamethyleneimine, and stir at 50°C for 3h to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 170°C for 60h. The product is washed, filtered, dried, and calcined to obtain a molecular sieve product.

[0069] The molecular sieve was analyzed for its full pore specific surface area and porosity, and its total specific surface area was 373m 2 / g, of which the micropore specific surface area is 244m 2 / g, and the mesopore volume is 0.27m 3 / g.

[0070] Example 9

[0071] Dissolve 1.3g of sodium hydroxide, 0.25g of sodium metaaluminate, and 0.25g of dodecyltrimethylammonium chloride in 100g of water, slowly add 10.0g of magadiite, and stir at 50°C for 2h. Then slowly add 4.0g of seed solution and 0.17g of homopiperazine, and stir at 50°C for 3h to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 170°C for 72h. The product is washed, filtered, dried, and calcined to obtain a molecular sieve product.

[0072] The molecular sieve was analyzed for its full pore specific surface area and porosity, and its total specific surface area was 381m 2 / g, of which the micropore specific surface area is 232m 2 / g, and the mesopore volume is 0.29m 3 / g.

[0073] Comparative Example 1

[0074] Dissolve 1.1g of sodium hydroxide, 0.76g of sodium metaaluminate, and 1.0g of hexadecyltrimethylammonium bromide in 100g of water, slowly add 10.0g of magadiite, and stir at 50°C for 2h to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 170°C for 48h. Wash, filter, dry, and calcine the product to obtain a molecular sieve product.

[0075] The XRD spectrum of the molecular sieve product shows only a weak MFI structural characteristic peak and a strong MCM-41 mesoporous material characteristic peak.

[0076] Comparative Example 2

[0077] Dissolve 1.1g of sodium hydroxide and 0.76g of sodium metaaluminate in 100g of water, slowly add 10.0g of magadiite, and stir at 50°C for 2h. Then slowly add 4.0g of seed solution and stir at 50°C for 3h to obtain a crystallized gel. Transfer the crystallized gel to an autoclave and crystallize at 170°C for 48h. The product is washed, filtered, dried, and calcined to obtain a molecular sieve product.

[0078] The XRD spectrum of the molecular sieve product shows that the product has typical MFI structural characteristic peaks and the product has a regular hexagonal prism morphology with a total specific surface area of ​​278m 2 / g, of which the micropore specific surface area is 252m 2 / g, mesopore volume is 0.03m 3 / g.

[0079] The molecular sieve products obtained in Examples 1-4 and Comparative Examples 1-2 were subjected to calcination, ammonium exchange and other treatment processes to obtain hydrogen-type molecular sieve products, which were applied to the catalytic cracking reaction of n-heptane for performance evaluation. The reaction conditions were: 550°C, mass space velocity 2.0h -1 , nitrogen carrier gas flow rate 200 mL / h, the evaluation results are shown in Table 1.

[0080] Table 1

[0081] project Propylene selectivity (%) Ethylene selectivity (%) Example 1 27 13 Example 2 29 14 Example 3 23 24 Example 4 21 27 Comparative Example 1 19 11 Comparative Example 2 24 12

[0082] Comparison of the results in Example 1 and Comparative Example 1 shows that, when magadiite is used as the silicon source, hexadecyltrimethylammonium bromide alone is insufficient to produce a highly crystalline MFI molecular sieve product. Comparison of the results in Example 1 and Comparative Example 2 shows that, when magadiite is used as the silicon source, a highly crystalline MFI molecular sieve product can be produced using only seed crystals, but a mesopore-rich molecular sieve product cannot be obtained. The results in Table 1 show that the molecular sieve prepared by the preparation method of the present invention can improve the selectivity of ethylene and propylene in catalytic cracking processes.

[0083] Of course, the present invention may have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art may make various corresponding changes and modifications based on the present invention, but these corresponding changes and modifications should all fall within the scope of protection of the present invention.

Claims

1. A method for preparing mesoporous ZSM-5 molecular sieve, characterized in that: The following steps are involved: A silicon source, an aluminum source, an alkali source, a crystallization inducer, an auxiliary agent, and water are mixed to obtain a crystallized gel, the crystallized gel is crystallized under hydrothermal conditions, washed, filtered, dried, and calcined to obtain a mesoporous ZSM-5 molecular sieve; wherein the silicon source is magadiite, the crystallization inducer is an organic ammonium salt and a seed crystal, the seed crystal is a molecular sieve having an MFI crystal structure, and the auxiliary agent is selected from at least one of an alcohol or an amine; Silicon source is calculated as SiO2, aluminum source is calculated as Al2O3, alkali source is calculated as M2O, organic ammonium salt is calculated as cation R + The molar ratio of each component in the crystallized gel is: SiO2 / Al2O3=30-400, seed / SiO2=0.002-0.250, M2O / SiO2=0.05-0.50, R + / SiO2=0.003~0.03, H2O / SiO2=15~120, auxiliary agent / SiO2=0.001~0.03; wherein, M is an alkali metal.

2. The method for preparing mesoporous ZSM-5 molecular sieve according to claim 1, characterized in that: The cation of the organic ammonium salt contains 3 or 4 alkyl groups, and the carbon number of at least one of the alkyl groups is greater than or equal to 8.

3. The method for preparing mesoporous ZSM-5 molecular sieve according to claim 2, characterized in that: The cation of the organic ammonium salt contains four alkyl groups, and at least one of the alkyl groups has a carbon number greater than or equal to 12 and less than or equal to 16.

4. The method for preparing mesoporous ZSM-5 molecular sieve according to claim 3, characterized in that: The organic ammonium salt is selected from at least one of dodecylethyldimethylammonium bromide, dodecylpropylenedimethylammonium bromide, dodecyltriethylammonium bromide, dodecyltrimethylammonium bromide, dodecyltrimethylammonium hydroxide, dodecyltrimethylammonium chloride, dodecyltrimethylammonium hydrogen sulfate, hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium hydroxide, hexadecyltrimethylammonium chloride and hexadecyltrimethylammonium hydrogen sulfate.

5. The method for preparing mesoporous ZSM-5 molecular sieve according to claim 1, characterized in that: The auxiliary agent is selected from at least one of a monohydric alcohol having a carbon number less than or equal to 4 and a cyclic aliphatic amine having a carbon number less than or equal to 8.

6. The method for preparing mesoporous ZSM-5 molecular sieve according to claim 1, characterized in that: The auxiliary agent is selected from at least one of methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol, piperidine, pyridine, piperazine, homopiperazine, hexamethyleneimine, heptamethyleneimine, cyclohexylamine, methylcyclohexylamine and ethylcyclohexylamine.

7. The method for preparing mesoporous ZSM-5 molecular sieve according to claim 1, characterized in that: Seed crystal / SiO2=0.002-0.

09.

8. The method for preparing mesoporous ZSM-5 molecular sieve according to claim 1, characterized in that: The crystallization temperature is 120-190° C., and the crystallization time is 12-120 hours.

9. The method for preparing mesoporous ZSM-5 molecular sieve according to claim 1, characterized in that: The crystallization temperature is 140-185° C., and the crystallization time is 24-72 hours.

10. The method for preparing mesoporous ZSM-5 molecular sieve according to claim 1, characterized in that: The aluminum source is selected from at least one of pseudo-boehmite, aluminum isopropoxide, sodium metaaluminate and aluminum sulfate; and the alkali source is selected from at least one of sodium hydroxide and potassium hydroxide.

11. A mesoporous ZSM-5 molecular sieve obtained according to the preparation method according to any one of claims 1 to 10.

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