Preparation method of monodisperse nano plate-shaped MFI type molecular sieve as well as product and application of monodisperse nano plate-shaped MFI type molecular sieve

Abstract

The invention provides a preparation method of a monodisperse nano plate-shaped MFI type molecular sieve, a product and application, and relates to the technical field of molecular sieve synthesis. The method comprises the following steps: pretreating a molecular sieve by fluoride to obtain a seed crystal suspension; sequentially adding an aluminum source, a template agent and a silicon source into water, uniformly stirring to obtain silicon-aluminum gel, then slowly adding the seed crystal suspension, and selectively adding and supplementing a fluorine source to obtain initial gel; and carrying out hydro-thermal treatment, washing, drying and roasting to obtain the monodisperse nano-plate-shaped MFI type molecular sieve. According to the method provided by the invention, the seed crystal selection of the nano molecular sieve synthesized by a seed crystal method can be expanded, the use of a template agent is reduced, multi-stage utilization of fluorine is realized, and the production cost is reduced; meanwhile, the pretreatment and crystallization time is short, the production period is short, the operation process is continuous, and industrial production of the molecular sieve is easy to realize. The obtained nano plate-shaped MFI type molecular sieve has high carbon deposition resistance, and can be used in the fields of crude oil / heavy oil catalytic cracking, carbon dioxide hydrogenation, propane dehydrogenation and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of molecular sieve synthesis, and relates to a synthesis method, product and application of a monodisperse nanoplate-shaped MFI type molecular sieve. Background Art

[0002] MFI molecular sieve has a unique two-dimensional pore structure. The basic structural unit is composed of eight five-membered rings and contains ten-membered rings. The skeleton is composed of two cross-pore systems. The straight cylindrical pore is elliptical, parallel to the b-axis direction, and the long axis is The short axis is The other is a vertical "Z"-shaped channel with a cross section close to a circle and a hole diameter of

[0003] In the process of molecular sieve synthesis, by controlling different synthesis conditions, MFI molecular sieves with different morphologies, pore structures and acid properties can be obtained, which in turn affects their catalytic performance. Synthesizing monodisperse nanoplate molecular sieves is an effective method to shorten the diffusion path length within the framework and improve the diffusion efficiency. Among them, the b-axis size of the monodisperse nanoplate MFI molecular sieve is at the nanometer level, the crystal thickness is thin, and the straight channel is short. At the same time, monodispersity increases the surface area of ​​the molecular sieve, which is beneficial to improve the diffusion rate of the gas, enhance the diffusion efficiency of the reactants and products, and reduce the carbon deposition rate.

[0004] Chinese patent CN117208924A discloses a method for preparing a nano-flaky MFI molecular sieve, which comprises the following steps: mixing tetraethyl orthosilicate, tetrapropylammonium hydroxide and water, aging for 5 to 9 hours to obtain an intermediate solution; mixing the intermediate solution, ammonium fluoride and water for 10 to 14 hours, and crystallizing for 1 to 4 days to obtain a nano-flaky MFI molecular sieve. This method introduces ammonium fluoride as a mineralizer into the MFI molecular sieve, and the morphology, b-axis thickness and number of twins of the nano-flaky MFI molecular sieve can be adjusted by changing the aging time and crystallization time.

[0005] Chinese patent CN116395712A discloses a structural defect-free MFI nano zeolite and a preparation method thereof, comprising the following steps: mixing a silicon source, an aluminum source, a template and water, subjecting the mixture to low-temperature microwave treatment for 60 to 150 minutes and high-temperature microwave treatment for 10 to 90 minutes, followed by dialysis purification to obtain a ZSM-5 zeolite crystallite aqueous solution; mixing a silicon source, a template and water in the same manner to obtain a Silicalite-1 zeolite crystallite aqueous solution; mixing the above two crystallite aqueous solutions with fluoride ions, adjusting the pH to 2, and then subjecting the mixture to hydrothermal treatment for 3 to 30 days to obtain nano-flaky MFI zeolite crystals.

[0006] Chinese patent CN115403053B discloses a method for preparing monodisperse nano ZSM-5 with orthorhombic twins, comprising the following steps: mixing a nano seed emulsion obtained by crystallizing a silicon source, a template agent and a solvent with a silicon source, a template agent, an organic amine and a solvent, aging to obtain a mixture colloid, and then mixing with an aluminum source solution to obtain a silicon-aluminum mixed gel, aging to obtain a molecular sieve mother liquid, and crystallizing to obtain a monodisperse nano ZSM-5 molecular sieve with orthorhombic twins. This method can effectively adjust the thickness of the b-axis and orthorhombic twins and the length of the a-axis of the ZSM-5 molecular sieve by regulating the preparation conditions and addition amount of the nano seed emulsion and adjusting the crystallization conditions of the molecular sieve mother liquid.

[0007] As can be seen from the above, the current preparation method of monodisperse nanoplate-shaped MFI molecular sieves is relatively complicated, and the synthesis step usually requires multiple aging or aging of the raw materials. At the same time, to achieve monodisperse molecular sieves, it is usually necessary to add more expensive templates, and the production cost is high. In view of this, it is urgent to provide a synthesis method of monodisperse nanoplate-shaped MFI molecular sieves with a short production cycle, low production cost, and suitable for industrial production. Summary of the invention

[0008] The present invention aims to provide a method for synthesizing monodisperse nanoplate-shaped MFI molecular sieves by pretreating seed crystals. Fluoride is used to pretreat the agglomerate seed crystals, which preferentially acts on the twin interface to make the agglomerate seed crystals uniformly dispersed, change the properties of the seed crystals, and thus improve the uniformity and dispersibility of the product molecular sieve. Further changing the seed crystal pretreatment conditions, by adjusting the severity of the fluorine treatment, the grain size can be reduced and the number of crystal nuclei can be increased. Combined with the change of the molecular sieve crystallization conditions, the grain size of the molecular sieve product can be further controlled.

[0009] The object of the present invention is to provide a method for preparing a monodisperse nanoplate-like MFI molecular sieve, which specifically comprises the following steps:

[0010] (1) Pretreatment of seed crystals: preparing a fluoride aqueous solution and subjecting the molecular sieve used as seed crystals to different degrees of fluorine treatment to obtain a seed crystal suspension;

[0011] (2) Obtaining an initial gel: adding an aluminum source, a template agent, and a silicon source to water in sequence, stirring evenly to obtain a silica-alumina gel, slowly adding a seed suspension to the silica-alumina gel and optionally adding a supplementary fluorine source, stirring evenly at a certain temperature to obtain an initial gel;

[0012] (3) Synthesizing molecular sieves: The initial gel obtained in step (2) is subjected to hydrothermal treatment, washed, dried and then calcined to obtain a monodisperse nanoplate-shaped MFI molecular sieve; the liquid after washing is recovered and circulated to prepare a fluoride aqueous solution for treating seed crystals and synthesizing molecular sieves.

[0013] In the present invention, in step (1), the fluoride aqueous solution is an aqueous solution of one or a mixture of hydrogen fluoride, ammonium fluoride or sodium fluoride; the molecular sieve used as the seed is a nano ZSM-5 or Silicalite-1 molecular sieve, preferably a monodisperse nano molecular sieve. The mass ratio of the fluoride aqueous solution to the molecular sieve, that is, the liquid-to-solid ratio, is 5 to 50; the concentration of the fluoride aqueous solution is 0% to 45%. The temperature of the fluorine treatment is 15 to 95°C, and the dissolution time is 1 to 10 hours; the fluorine treatment process may or may not be assisted by external field enhancement, and the external field enhancement includes ultrasonic treatment. The pretreated seed molecular sieve particles are uniformly dispersed, and preferably the seed size is reduced and the number is increased on the basis of dispersion.

[0014] In the present invention, the feed ratio of the initial gel obtained in step (2) is that the silicon source is calculated as SiO2, and the aluminum source is calculated as Al2O3. The molar ratio of SiO2, template and Al2O3 in the initial gel is 1.0:0-2.00:0-0.80. The total amount of fluorine source used in step (1) and step (2) is the fluorine source in the initial gel, the total amount of water in step (1) and step (2) is the amount of water in the initial gel, and the molar ratio of SiO2, fluorine source and water in the initial gel is 1.0:0.20-1.00:20-50. In step (2), a supplementary fluorine source can be added selectively so that the fluorine source in the initial gel satisfies the above-mentioned proportional relationship. That is, if the concentration of the fluoride aqueous solution in step (1) is high (more fluorine source), step (2) does not need to add a supplementary fluorine source; if the concentration of the fluoride aqueous solution in step (1) is low, it is necessary to add a supplementary fluorine source in step (2). In step (2), the template is tetrapropylammonium bromide, the silicon source is white carbon black or tetraethyl orthosilicate, and the aluminum source is aluminum sulfate. In step (2), the silicon source, aluminum source, template, seed suspension and water are stirred at 25-40° C. for 1-2.5 hours at a stirring rate of 500-800 rpm.

[0015] In the present invention, in step (3), the hydrothermal treatment temperature is 90-200° C. and the time is 4-120 h; the calcination process is heated to 500-600° C. at a heating rate of 2-10° C. / min and kept warm for 4-6 h.

[0016] The present invention also provides a monodisperse nanoplate-like MFI type molecular sieve prepared by the above method, wherein the monodisperse nanoplate-like MFI type molecular sieve comprises a monodisperse nanoplate-like ZSM-5 molecular sieve and / or a monodisperse nanoplate-like Silicalite-1 molecular sieve; the monodisperse nanoplate-like MFI type molecular sieve has no twins, a regular structure, and a uniform particle size, and its grain size has a c-axis length of 500nm to 5μm, an a-axis width of 50nm to 1.60μm, and a b-axis thickness of 50 to 600nm.

[0017] The present invention also provides application of the above method in industrial production, which can obtain at least 3 kg of monodisperse nanoplate-shaped ZSM-5 molecular sieve products at one time without generating impurity crystals.

[0018] The present invention also provides a heavy oil or crude oil catalytic cracking catalyst, wherein the active component is a monodisperse nanoplate-shaped MFI molecular sieve prepared by the above method; preferably, the active component: kaolin: binder: water = 0.4: 0.5: 0.1: 2.0 mass ratio is mixed, and the heavy oil or crude oil catalytic cracking catalyst is prepared through the steps of stirring at 80°C for 4 to 6 hours, drying at 100°C for 10 to 12 hours, roasting at 550°C for 4 to 6 hours, and hydrothermal aging at 800°C for 4 to 17 hours. The catalytic cracking catalyst has a low-carbon olefin selectivity of 35.18% and a coke yield of 2.69% under the reaction conditions of 590°C and a catalyst-to-oil ratio of 3; the low-carbon olefin refers to ethylene, propylene or butene.

[0019] The present invention proposes for the first time to pre-treat the molecular sieve used as a seed with different degrees of fluorine treatment. The pre-treatment can destroy the seed agglomerates to achieve the purpose of dispersing the seed. If the seed is not treated, a ZSM-5 molecular sieve with more twins will be synthesized. Moreover, changing the concentration of the fluoride aqueous solution in the pre-treatment conditions affects the apparent morphology and grain size of the molecular sieve product, wherein the higher the concentration of the fluoride aqueous solution in the pre-treatment, the better the dispersion of the molecular sieve product, the more uniform and smaller the grain size; changing the pre-treatment temperature affects the apparent morphology and grain size of the molecular sieve product, wherein the higher the pre-treatment temperature, the better the dispersion of the molecular sieve product, and the more uniform the grain size. The present invention unexpectedly discovered that the order of adding materials directly affects the uniformity of the gel, and thus affects the apparent morphology of the molecular sieve product. The order of first mixing the aluminum source, the template, the silicon source and water to evenly prepare the silica-alumina gel, and then adding the seed suspension to the silica-alumina gel can avoid the situation where the gel system is difficult to stir evenly. The present invention also found that the mixing temperature of the silica-alumina gel and the seed suspension affects the nucleation and growth process of the molecular sieve. The higher temperature makes the molecular sieve grow rapidly when the gel is not stirred evenly, causing the product to agglomerate and failing to obtain a monodisperse nanoplate-shaped molecular sieve. In particular, the method provided by the present invention can expand the selection of seeds for synthesizing nanomolecular sieves by the seed method, reduce the use of templates, realize multi-stage utilization of fluorine, and reduce production costs; at the same time, the production cycle is short, the operation process is continuous, and it is easier to realize the industrial production of molecular sieves.

[0020] Compared with the prior art, the present invention adopts a method of in-situ seed treatment to assist synthesis, which has the characteristics of being easy to industrialize, such as simple operation, good repeatability, short process time, recycling of raw materials to reduce pollution, etc. By changing the severity of seed treatment and crystallization conditions, the size and morphology of the MFI molecular sieve can be adjusted, and finally a monodisperse, high-crystallinity, short b-axis, twin-free monodisperse nanoplate-shaped MFI molecular sieve is prepared. The monodisperse plate-shaped MFI molecular sieve prepared by the present invention has excellent heavy oil / crude oil catalytic cracking performance, showing high selectivity for light olefins and low coke yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the XRD spectrum of the ZSM-5 molecular sieve synthesized in Example 1.

[0022] Figure 2 This is the SEM image of the ZSM-5 molecular sieve synthesized in Example 1.

[0023] Figure 3 This is the XRD spectrum of the ZSM-5 molecular sieve synthesized in Example 2.

[0024] Figure 4 This is the SEM image of the ZSM-5 molecular sieve synthesized in Example 2.

[0025] Figure 5 This is the XRD spectrum of the ZSM-5 molecular sieve synthesized in Example 3.

[0026] Figure 6 This is the SEM image of the ZSM-5 molecular sieve synthesized in Example 3.

[0027] Figure 7 This is the SEM image of the ZSM-5 molecular sieve synthesized in Comparative Example 1.

[0028] Figure 8 This is the SEM image of the ZSM-5 molecular sieve synthesized in Comparative Example 2.

[0029] Fig. 9 This is the SEM image of the ZSM-5 molecular sieve synthesized in Comparative Example 3.

[0030] Fig.10 This is the SEM image of the ZSM-5 molecular sieve synthesized in Comparative Example 4.

[0031] Fig.11 This is the SEM image of the ZSM-5 molecular sieve synthesized in Comparative Example 5.

[0032] Fig.12 This is the SEM image of the seed crystal (Nanhua NKF-5-85H model molecular sieve).

[0033] Fig.13 for Fig.12 SEM image after fluorine treatment for 1 h. DETAILED DESCRIPTION

[0034] In order to more clearly understand the technical features, purposes and excellent effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be understood as limiting the scope of implementation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] [Example 1]

[0036] Dissolve ammonium fluoride in water to prepare an ammonium fluoride aqueous solution with a concentration of 40wt%; add 0.30g of commercial ZSM-5 molecular sieve seeds (the seeds use Nanhua NKF-5-85H molecular sieve) to 6.0g of ammonium fluoride solution, and stir magnetically for 3h at 50℃ to form a seed suspension; add 1.33g of aluminum sulfate, 1.06g of TPABr, and 6.01g of white carbon black to 58.75g of water in sequence, and stir at 300rpm for 30min at 25℃ to obtain a silica-alumina gel; pour the seed suspension into the silica-alumina gel, and continue to stir at 25℃ and 300rpm for 60min to obtain an initial gel. Transfer the initial gel to a hydrothermal crystallization kettle at 170℃ for crystallization for 24 hours; filter the filtrate with deionized water until the pH of the filtrate is 7, dry it in an oven at 100℃ for 10 hours, and calcine it in a muffle furnace at 550℃ for 6 hours to obtain a plate-like ZSM-5 molecular sieve crystal.

[0037] The plate-like ZSM-5 molecular sieve obtained in this example was characterized, and the XRD characterization graph and SEM characterization graph of the plate-like ZSM-5 molecular sieve in this example were obtained, which correspond to Figure 1 and Figure 2 .Depend on Figure 1 It can be seen that the samples all have characteristic diffraction peaks belonging to ZSM-5 (2θ=7.9°, 8.9°, 23.0°, 23.3°, 23.9°), indicating that ZSM-5 molecular sieves can be synthesized without the generation of impurity crystals. Figure 2 It can be seen that the average grain size (a-axis×b-axis×c-axis) of the plate-like ZSM-5 molecular sieve prepared in this example is 0.69 μm×0.26 μm×1.24 μm, with uniform particle size, regular structure and no twins.

[0038] [Example 2]

[0039] Dissolve ammonium fluoride in water to prepare an ammonium fluoride aqueous solution with a concentration of 40wt%; add 0.30g of the ZSM-5 molecular sieve synthesized in Example 1 as a seed to 6.0g of the ammonium fluoride solution, and stir magnetically at 50°C for 180min to form a seed suspension; add 1.33g of aluminum sulfate, 1.06g of TPABr, and 6.01g of white carbon black to 58.75g of water in sequence, and stir at 300rpm for 30min at 25°C to obtain a silica-alumina gel; pour the seed suspension into the silica-alumina gel, and continue stirring at 25°C and 300rpm for 60min to obtain an initial gel. Transfer the initial gel to a hydrothermal crystallization kettle at 170°C for crystallization for 24 hours; filter the filtrate with deionized water until the pH value of the filtrate is 7, dry it in an oven at 100°C for 10 hours, and calcine it in a muffle furnace at 550°C for 6 hours to obtain a plate-like ZSM-5 molecular sieve crystal.

[0040] The plate-like ZSM-5 molecular sieve obtained in this example was characterized, and the XRD characterization graph and SEM characterization graph of the plate-like ZSM-5 molecular sieve in this example were obtained, which correspond to Figure 3 , Figure 4 .Depend on Figure 3 It can be seen that the samples all have characteristic diffraction peaks belonging to ZSM-5 (2θ=7.9°, 8.9°, 23.0°, 23.3°, 23.9°), indicating that ZSM-5 molecular sieves can be synthesized without the generation of impurity crystals. Figure 4 It can be seen that the average grain size (a-axis×b-axis×c-axis) of the plate-like ZSM-5 molecular sieve prepared in this example is 1.41 μm×0.33 μm×2.66 μm, with uniform particle size, regular structure and no twins.

[0041] [Example 3]

[0042] A 500-fold magnification experiment was conducted based on Example 1. 1200 g of ammonium fluoride was dissolved in 1800 g of water, and after stirring evenly, 105 g of commercial ZSM-5 molecular sieve seeds (the seeds used were Nanhua NKF-5-85H molecular sieves) were poured in, and mechanically stirred at 50°C for 3 hours to form a seed suspension; 29.4 kg of water, 666 g of aluminum sulfate, 532 g of TPABr and 3 kg of white carbon were added to a 50 L kettle in sequence, and stirred at 300 rpm for 30 minutes at 25°C to obtain a silica-alumina gel; the pre-treated seed suspension was poured into the silica-alumina gel, and stirring was continued at 25°C and 300 rpm for 60 minutes to obtain a uniformly mixed initial gel. The internal initial gel temperature was raised by heat transfer oil to 170°C, and crystallization took 24 hours. After cooling and discharging, the material was filtered using a plate and frame filter press until the filtrate pH was 7. The material was dried in an oven at 100°C for 10 hours. The material was calcined in a muffle furnace at 550°C for 6 hours to obtain about 3 kg of monodisperse nanoplate-shaped ZSM-5 molecular sieve product.

[0043] The plate-like ZSM-5 molecular sieve obtained in this example was characterized, and the XRD characterization graph and SEM characterization graph of the plate-like ZSM-5 molecular sieve in this example were obtained, which correspond to Figure 5 , Figure 6 .Depend on Figure 5 It can be seen that the samples all have characteristic diffraction peaks belonging to ZSM-5 (2θ=7.9°, 8.9°, 23.0°, 23.3°, 23.9°), indicating that ZSM-5 molecular sieves can be synthesized without the generation of impurity crystals. Figure 6 It can be seen that the average grain size (a-axis×b-axis×c-axis) of the plate-like ZSM-5 molecular sieve prepared in this example is 0.86 μm×0.30 μm×1.64 μm, with uniform particle size, regular structure and no twins.

[0044] [Comparative Example 1]

[0045] Add 0.30g of commercial ZSM-5 molecular sieve seeds (the seeds are Nanhua NKF-5-85H molecular sieve) to 6.0g of 40wt% ammonium fluoride aqueous solution, and stir magnetically for 3h at 50℃ to form a seed suspension; add 1.33g of aluminum sulfate, 1.06g of TPABr and the seed suspension to 58.75g of water in turn and stir evenly; finally add 6.01g of white carbon black and stir at 300rpm for 30min at 25℃ to obtain the initial gel. Transfer the synthesized gel to a hydrothermal crystallization kettle and crystallize at 170℃ for 24 hours; filter the filtrate with deionized water until the pH value of the filtrate is 7, dry it in an oven at 100℃ for 10 hours, and calcine it in a muffle furnace at 550℃ for 6 hours to obtain agglomerated ZSM-5 molecular sieve crystals, see Figure 7 .

[0046] [Comparative Example 2]

[0047] Add 0.30g of commercial ZSM-5 molecular sieve seeds (the seeds are Nanhua NKF-5-85H molecular sieve) to 6.0g of 40wt% ammonium fluoride aqueous solution, and stir magnetically at 50°C for 3h to form a seed suspension; add 1.33g of aluminum sulfate, 1.06g of TPABr, and 6.01g of white carbon black to 58.75g of water in sequence, and stir at 300rpm for 30min at 80°C to obtain a silica-alumina gel; pour the seed suspension into the silica-alumina gel and continue stirring at 80°C and 300rpm for 60min to obtain an initial gel. Transfer the initial gel to a hydrothermal crystallization kettle at 170°C for crystallization for 24 hours; filter the filtrate with deionized water until the pH value of the filtrate is 7, dry it in an oven at 100°C for 10 hours, and calcine it in a muffle furnace at 550°C for 6 hours to obtain agglomerated ZSM-5 molecular sieve crystals, see Figure 8 .

[0048] [Comparative Example 3]

[0049] To 58.75g of water, 1.33g of aluminum sulfate, 1.06g of TPABr and 6.01g of white carbon black were added in sequence, and the mixture was stirred at 300rpm for 30min at 25°C to obtain a silica-alumina gel. To the silica-alumina gel, 6.0g of a 40wt% aqueous solution of ammonium fluoride and 0.30g of commercial ZSM-5 molecular sieve seeds without any pretreatment (the seeds used were Nanhua NKF-5-85H model molecular sieves) were added to obtain an initial gel. The initial gel was transferred to a hydrothermal crystallization kettle and crystallized at 170°C for 24 hours; deionized water was filtered until the filtrate had a pH of 7, dried in an oven at 100°C for 10 hours, and calcined in a muffle furnace at 550°C for 6 hours to obtain ZSM-5 molecular sieve crystals with more twins, as shown. Fig. 9 .

[0050] [Comparative Example 4]

[0051] Ammonium fluoride is dissolved in water to prepare an ammonium fluoride aqueous solution with a concentration of 20wt%; 0.30g of commercial ZSM-5 molecular sieve seeds (the seeds are Nanhua NKF-5-85H molecular sieve) is added to 6.0g of ammonium fluoride solution, and magnetic stirring is carried out at 50°C for 180min to form a seed suspension; 1.33g of aluminum sulfate, 1.06g of TPABr, and 6.01g of white carbon are added to 58.75g of water in sequence, and stirred at 300rpm for 30min at 25°C to obtain a silica-alumina gel; the seed suspension is poured into the silica-alumina gel and 1.2g of ammonium fluoride solid is added, and stirring is continued at 25°C and 300rpm for 60min to obtain an initial gel. The initial gel was transferred to a hydrothermal crystallization reactor and crystallized at 170°C for 24 hours; deionized water was filtered until the filtrate pH = 7, dried in an oven at 100°C for 10 hours, and calcined in a muffle furnace at 550°C for 6 hours to obtain a ZSM-5 molecular sieve sample. Fig.10 .pass Fig.10 It can be observed that the morphology of the molecular sieve sample is relatively messy, and it has plate-like, twinned and agglomerated ZSM-5 molecular sieve crystals; and the average grain size (a-axis × b-axis × c-axis) of the molecular sieve prepared by this comparative example is 1.03μm × 0.30μm × 1.68μm, and the dimensions of each axis are larger than the plate-like molecular sieve sample obtained in Example 1.

[0052] [Comparative Example 5]

[0053] Ammonium fluoride is dissolved in water to prepare an ammonium fluoride aqueous solution with a concentration of 40wt%; 0.30g of commercial ZSM-5 molecular sieve seeds (the seeds are Nanhua NKF-5-85H molecular sieve) is added to 6.0g of ammonium fluoride solution, and magnetic stirring is carried out for 180min at room temperature of 25°C to form a seed suspension; 1.33g of aluminum sulfate, 1.06g of TPABr, and 6.01g of white carbon are added to 58.75g of water in sequence, and stirred at 300rpm for 30min at room temperature of 25°C to obtain a silica-alumina gel; the seed suspension is poured into the silica-alumina gel, and stirring is continued for 60min at 25°C and 300rpm to obtain an initial gel. The initial gel was transferred to a hydrothermal crystallization reactor and crystallized at 170°C for 24 hours; deionized water was filtered until the filtrate pH = 7, dried in an oven at 100°C for 10 hours, and calcined in a muffle furnace at 550°C for 6 hours to obtain a plate-like ZSM-5 molecular sieve sample with twins and agglomerates. Fig.11 .

[0054]

Application Example 1

[0055] The monodisperse nanoplate-shaped ZSM-5 molecular sieve prepared in Example 1 was used as an active component, and mixed according to the mass ratio of active component: kaolin: binder: water = 0.4: 0.5: 0.1: 2.0, and the heavy oil catalytic cracking catalyst was prepared after stirring at 80°C for 4h, drying at 100°C for 10h, calcining at 550°C for 6h, and hydrothermal aging at 800°C for 4h. After testing, the catalytic cracking catalyst had a selectivity of 35.18% for light olefins (ethylene, propylene, butene) and a coke yield of 2.69% under the reaction conditions of 590°C and a catalyst-oil ratio (catalyst to crude oil mass ratio) of 3. The olefin selectivity = olefin yield / heavy oil selectivity; wherein, olefin yield = olefin mass / heavy oil mass; heavy oil selectivity = (1-unreacted heavy oil mass) / heavy oil mass; in summary, olefin selectivity = olefin mass / (1-unreacted heavy oil mass).

[0056]

Application Example 2

[0057] The monodisperse nanoplate-shaped ZSM-5 molecular sieve product prepared by enlarging the scale in Example 3 was used as an active component, and mixed according to the mass ratio of active component: kaolin: binder: water = 0.4: 0.5: 0.1: 2.0, and the crude oil catalytic cracking catalyst was prepared through the steps of stirring at 80°C for 4h, drying at 100°C for 10h, calcining at 550°C for 6h, and hydrothermal aging at 800°C for 4h. According to the test, the catalytic cracking catalyst has a selectivity of 34.30% for light olefins (ethylene, propylene, butene) and a coke yield of 3.09% under the reaction conditions of 590°C and a catalyst-to-oil ratio (catalyst to crude oil mass ratio) of 3.

[0058] The present invention particularly notes that monodisperse nanocrystal seeds are the key to synthesizing monodisperse nanoplate-shaped MFI molecular sieves, but they have strong aggregation characteristics, are expensive and difficult to use on a large scale. Fluorine treatment is used to make nanoaggregate seeds ( Fig.12 ) preferentially dissolves and disperses from the grain boundaries ( Fig.13 ) to obtain monodisperse nanocrystalline seeds; adding the fluorine-containing seed suspension to the silica-alumina gel plays the role of a mineralizer and controls the oriented growth of the molecular sieve, which is beneficial to the growth of the plate-like nano-molecular sieve. Comparing the embodiment with the comparative example, it can be concluded that the morphological difference of the molecular sieve is attributed to the morphology after the seed treatment, and is greatly affected by the uniformity of the silica-alumina gel in the synthesis system.

[0059] Comparative Examples 1 to 2 show that controlling the order of adding materials can improve the uniformity of the synthetic gel, wherein adding the fluorine-containing seed suspension at the end and controlling the stirring temperature can effectively improve the uniformity of the synthetic gel. Comparative Examples 3 to 5 show that controlling the time, concentration, temperature and other conditions of fluorine treatment can obtain monodisperse nanocrystalline seeds with uniform size, wherein the longer the seed pretreatment time, the higher the concentration of the fluorine-containing aqueous solution, and the higher the temperature (within a certain range), the better the dispersion of the obtained molecular sieve product and the smaller the average grain size.

Claims

1. A method for preparing monodisperse nanoplate-like MFI molecular sieves, characterized in that: The specific steps include: (1) Pretreatment of seed crystals: preparing a fluoride aqueous solution and subjecting the molecular sieve used as seed crystals to different degrees of fluorine treatment to obtain a seed crystal suspension; (2) obtaining an initial gel: adding an aluminum source, a template agent, and a silicon source to water in sequence, stirring uniformly to obtain a silica-alumina gel, slowly adding a seed suspension to the silica-alumina gel and optionally adding a supplementary fluorine source, stirring uniformly at a certain temperature to obtain an initial gel; (3) Synthesizing molecular sieves: The initial gel obtained in step (2) is subjected to hydrothermal treatment, washed, dried and then calcined to obtain a monodisperse nanoplate-shaped MFI molecular sieve; the liquid after washing is recovered and circulated to prepare a fluoride aqueous solution for treating seed crystals and synthesizing molecular sieves.

2. The method according to claim 1, characterized in that In step (1), the fluoride aqueous solution is an aqueous solution of one or a mixture of hydrogen fluoride, ammonium fluoride or sodium fluoride; the molecular sieve used as the seed is a nano ZSM-5 or Silicalite-1 molecular sieve; the mass ratio of the fluoride aqueous solution to the molecular sieve, that is, the liquid-to-solid ratio is 5 to 50; the concentration of the fluoride aqueous solution is 0% to 45%; the temperature of the fluorine treatment is 15 to 95° C., and the dissolution time is 1 to 10 hours; the fluorine treatment process may or may not be assisted by external field enhancement, and the external field enhancement includes ultrasonic treatment.

3. The method according to claim 1, characterized in that In step (2), the initial gel is obtained by charging a material ratio of SiO2 as the silicon source and Al2O3 as the aluminum source, and the molar ratio of SiO2, template and Al2O3 in the initial gel is 1.0:0-2.00:0-0.80; The total amount of fluorine source used in step (1) and step (2) is the fluorine source in the initial gel, the total amount of water in step (1) and step (2) is the water amount in the initial gel, and the molar ratio of SiO2, fluorine source and water in the initial gel is 1.0:0.20~1.00:20~50.

4. The method according to claim 1, characterized in that: In step (2), the template is tetrapropylammonium bromide, the silicon source is white carbon black or tetraethyl orthosilicate, and the aluminum source is aluminum sulfate; in step (2), the silicon source, aluminum source, template, seed suspension and water are stirred at 25-40° C. for 1-2.5 h, and the stirring rate is 500-800 rpm.

5. The method according to claim 1, characterized in that: In step (3), the hydrothermal treatment temperature is 90-200° C. and the time is 4-120 h; the calcination process is heated to 500-600° C. at a heating rate of 2-10° C. / min and kept at that temperature for 4-6 h.

6. The monodisperse nanoplate-shaped MFI molecular sieve prepared by the method according to any one of claims 1 to 5, characterized in that: The molecular sieve comprises a monodisperse nanoplate-shaped ZSM-5 molecular sieve and / or a monodisperse nanoplate-shaped Silicalite-1 molecular sieve; the molecular sieve has no twins, a regular structure, and a uniform particle size, and its grain size has a c-axis length of 500nm to 5μm, an a-axis width of 50nm to 1.60μm, and a b-axis thickness of 50 to 600nm.

7. Application of the method according to any one of claims 1 to 5 in industrial production, characterized in that: At least 3 kg of monodisperse nanoplate-shaped ZSM-5 molecular sieve products can be obtained at one time without generating impurity crystals.

8. A heavy oil or crude oil catalytic cracking catalyst, characterized in that: The active component of the catalyst includes the monodisperse nanoplate-shaped MFI type molecular sieve prepared by any one of the methods of claims 1-5.

9. The catalyst according to claim 8, characterized in that The active components are mixed in a mass ratio of kaolin: binder: water = 0.4:0.5:0.1:2.0, and the catalyst is stirred at 80°C for 4-6 hours, dried at 100°C for 10-12 hours, calcined at 550°C for 4-6 hours, and hydrothermally aged at 800°C for 4-17 hours to prepare a heavy oil or crude oil catalytic cracking catalyst.

10. The catalyst according to claim 8, characterized in that Under the conditions of reaction temperature of 590° C. and catalyst-to-oil ratio of 3, the selectivity of the catalyst for light olefins reaches 35.18% and the coke yield is 2.69%; the light olefins refer to ethylene, propylene or butene.

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