A low-cost controllable preparation method of zsm-5 molecular sieve under process intensification and mother liquor recycling method

Abstract

This invention discloses a low-cost, controllable preparation method for ZSM-5 molecular sieves and a mother liquor recycling method under process intensification. First, a suspension containing a silicon source and an aluminum source are prepared, and then both are simultaneously added to a microfilm reactor for thorough mixing and reaction to obtain a precursor solution. The precursor solution is then subjected to hydrothermal crystallization, and the hydrothermal crystallization product is separated into a molecular sieve synthesis mother liquor and a solid molecular sieve. The solid molecular sieve is washed, dried, and calcined to obtain ZSM-5 molecular sieves. The molecular sieve synthesis mother liquor is mixed with supplemented silicon and aluminum source suspensions to obtain the precursor solution again, thus achieving mother liquor reuse. This invention can accelerate the formation of primary structural units of molecular sieves, shorten crystallization time, and reduce raw material costs and energy consumption in the molecular sieve preparation process. It also solves the problem of generating large amounts of high-salt, high-ammonia nitrogen wastewater during the conversion of sodium-form to hydrogen-form molecular sieves with catalytic properties, thereby reducing the post-treatment costs of molecular sieves.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve material preparation technology, specifically relating to a low-cost and controllable preparation method of ZSM-5 molecular sieve under process intensification and a method for recycling mother liquor. Background Technology

[0002] ZSM-5 molecular sieves are among the most systematically studied and widely used zeolite materials in industry. With the continuous expansion of their applications, both global demand and production of ZSM-5 molecular sieves are showing a significant upward trend. However, their synthesis process suffers from prominent problems such as high cost and pollution (especially wastewater). In industry, ZSM-5 is typically synthesized via a hydrothermal method in a sodium-containing system, followed by treatment with NH4+. + The exchange process transforms zeolite from its sodium form to its hydrogen form, which possesses catalytic properties. Therefore, the preparation process mainly generates two types of wastewater: crystallization wastewater (i.e., molecular sieve synthesis mother liquor) from the crystallization unit and modification wastewater from the modification unit. The latter includes high-salt wastewater, high-COD wastewater, and high-ammonia-nitrogen wastewater. The molecular sieve synthesis mother liquor is a strongly alkaline waste liquid containing a large amount of unreacted silicon and aluminum sources (the content can reach 10%-25% of the initial feed amount), as well as expensive organic template agents (accounting for more than 30% of the total raw material cost). It is worth noting that the template agent is both a major cost source and a major source of pollution in molecular sieve synthesis. Considering the efficient utilization of resources, economic benefits, and environmental protection, the recovery and utilization of the molecular sieve synthesis mother liquor is of great significance.

[0003] The reuse of molecular sieve synthesis mother liquor mainly includes three technical approaches: physical separation, chemical conversion, and biological treatment. Among them, direct reuse falls under the category of chemical conversion and has attracted much attention due to its simple operation and low cost. However, this method encounters several persistent technical challenges: (1) In the recycling process, including Na⁺, Fe 2+ and Ca 2+Impurities gradually accumulate, reducing the effectiveness of the structure-directing agent, promoting the formation of structural defects, and causing unwanted crystalline impurity phases; (2) The high viscosity and strong alkalinity of the mother liquor disrupt the uniformity of the reaction, leading to pH gradients and local supersaturation, which in turn triggers premature nucleation, gelation, or amorphous aggregation, ultimately reducing crystallinity and adversely affecting surface area, pore volume, and particle size distribution; (3) Residual aluminosilicate colloids or oligomers in the recovered mother liquor may adsorb or encapsulate primary zeolite nuclei, resulting in differences in crystallization kinetics, phase purity, and particle morphology between different batches; (4) The synthesis process of molecular sieves is highly sensitive to reaction conditions, especially in the mother liquor circulation system, where parameters such as silicon source, aluminum source, temperature, concentration, pH value, and template agent dosage significantly affect product performance. Traditional batch reactors suffer from problems such as insufficient control precision, low mixing efficiency, uneven temperature and concentration distribution, and large mass transfer resistance, which lead to the coupling of nucleation and crystal growth processes, making independent control difficult. These limitations often lead to problems such as uneven product composition, low crystallinity, poor pore order, and poor batch repeatability.

[0004] Patent CN108128789B discloses a method for utilizing the mother liquor in molecular sieve synthesis. The method involves separating the mother liquor by sedimentation of suspended solids to obtain a supernatant, and then adding lime slurry to the supernatant for desilication treatment. The desilication-treated supernatant is used as a clinker leaching adjustment solution in the sintering alumina production process, thus solving the problem of the inability to recycle the caustic alkali in the molecular sieve synthesis mother liquor. Patent CN107311200A achieves the recycling of Beta molecular sieve mother liquor by supplementing with fresh raw materials. However, the cumulative changes in the concentration of each component in the mother liquor with the number of cycles lead to impurity ions and organic decomposition products interfering with nucleation and crystal growth, resulting in fluctuations in component content. Patent CN115650250B discloses a method for the continuous synthesis of ZSM-5 molecular sieves. This method uses a gel protectant to precipitate the silicon and aluminum sources in the ZSM-5 molecular sieve mother liquor, preserving and protecting the remaining ZSM-5 microcrystals for use in the next batch of ZSM-5 molecular sieve synthesis. Furthermore, in an effort to reduce the manufacturing cost of molecular sieves from the raw material end, many researchers have used solid wastes such as fly ash and coal gangue as raw materials to prepare molecular sieves. While some progress has been made, most of these efforts face problems such as poor molecular sieve quality, difficulty in controlling impurity phases, and low batch stability. Therefore, from multiple dimensions such as efficient resource utilization, economic benefits, and environmental protection, developing advanced equipment that combines multiphase interface mixing enhancement and reaction process optimization, and constructing new green synthesis methods, is of great significance for achieving precise control of the molecular sieve preparation process, improving overall efficiency, and efficiently recovering and recycling mother liquor. This can not only significantly reduce the production cost of molecular sieves but also help mitigate their negative impact on the ecological environment throughout their entire life cycle. Summary of the Invention

[0005] This invention is proposed to address the problems of poor process control precision, low mixing efficiency, uneven mixing, and low crystallinity, poor pore structure, and poor batch repeatability in the traditional ZSM-5 molecular sieve synthesis method. Its purpose is to provide a low-cost, controllable preparation method for ZSM-5 molecular sieve and mother liquor recycling under process enhancement.

[0006] This invention is achieved through the following technical solution: A low-cost, controllable preparation method for ZSM-5 molecular sieves and a method for recycling the mother liquor under process intensification includes the following steps: S1. Preparation of precursor solution: At room temperature, a silicon source is ultrasonically dispersed in deionized water to form a silicon source suspension; The aluminum source is ultrasonically dispersed into the template agent solution to form an aluminum source suspension; The silicon-based suspension and the aluminum-based suspension were simultaneously added to the microfilm reactor and thoroughly mixed to obtain the precursor solution. S2, hydrothermal crystallization: The precursor solution obtained in step S1 is transferred to a hydrothermal crystallization kettle for hydrothermal crystallization. The hydrothermal crystallization product is separated into solid and liquid to obtain molecular sieve synthesis mother liquor and molecular sieve solid. Preparation of S3 and ZSM-5 molecular sieves: The molecular sieve solid obtained in step S2 was washed, dried and calcined to obtain ZSM-5 molecular sieve; S4. Reuse of molecular sieve synthesis mother liquor: Quantitative analysis was performed on the content of silicon, aluminum and template agent in the molecular sieve synthesis mother liquor obtained in step S2; Based on the quantitative analysis results and the proportions of each effective component in the precursor solution in step S1, the amount of silicon source, aluminum source, and template agent to be added is determined. Prepare supplementary silicon source suspension and supplementary aluminum source suspension according to the supplementary dosage; The silicon source suspension, the aluminum source suspension, and the molecular sieve synthesis mother liquor obtained in step S2 were simultaneously added to the micro liquid membrane reactor for mixing and reaction to obtain the reusable precursor colloid. The precursor solution is hydrothermally crystallized under the same conditions as in step S2 to achieve the recycling of the molecular sieve synthesis mother liquor.

[0007] In the above technical solution, the silicon source is nano-silica prepared from silica gel solid waste in ZL202210253161.6.

[0008] In the above technical solution, the aluminum source is at least one of alumina, boehmite, boehmite, or aluminum hydroxide.

[0009] In the above technical solution, the template agent is at least one of tetrapropylammonium hydroxide, tetrapropylammonium bromide, tetrabutylammonium hydroxide, n-propylamine, n-butylamine, ethylamine, or isopropylamine.

[0010] In the above technical solution, the effective components in the precursor adhesive are calculated as SiO2, Al2O3, template agent, and water, and the proportions of each effective component are as follows: The molar ratio of SiO2 to Al2O3 is 20~1000:1; The molar ratio of template agent to SiO2 is 0.2~0.5:1; The molar ratio of water to SiO2 is 30~4000:1.

[0011] In the above technical solution, the microliquid membrane reactor mentioned in step S1 has the structure described in the patent "202220583676.8 - A multi-stage confined vortex mixing microreactor". The rotation speed of the microliquid membrane reactor is 2000 rpm to 6000 rpm, and the mixing time is 5 min to 30 min.

[0012] In the above technical solution, the hydrothermal crystallization conditions are hydrothermal crystallization at 150 ℃~200 ℃ for 6 h~48 h.

[0013] In the above technical solution, the equipment used for hydrothermal crystallization in step S2 is not particularly limited. Well-known hydrothermal crystallization equipment in the art can be used. Preferably, it is carried out in a hydrothermal reactor with a polytetrafluoroethylene liner or a high-pressure reactor equipped with a stirrer.

[0014] In the above technical solution, there is no special limitation on the solid-liquid separation method in step S2; centrifugation, filtration, or vacuum filtration can all be used.

[0015] In the above technical solution, step S3 involves washing until the pH is ≤7.

[0016] In the above technical solution, the drying time is not specifically limited, and drying to constant weight is sufficient. The preferred drying temperature is 60 °C to 80 °C.

[0017] In the above technical solution, the calcination conditions in step S3 are calcination at 450 ℃ to 650 ℃ for 4 h to 8 h.

[0018] In the above technical solution, the specific amount of deionized water and template agent solution mentioned in step S3 is limited by the amount and composition of the molecular sieve synthesis mother liquor, so as to ensure that the composition and ratio of the precursor solution obtained during the mother liquor recycling process are consistent with the composition and ratio during the initial molecular sieve synthesis.

[0019] A ZSM-5 molecular sieve prepared by the aforementioned method, wherein the specific surface area of ​​the ZSM-5 molecular sieve is 350 m². 2 / g~500m 2 / g, with a particle size of 0.3 μm~0.9 μm, and the molar ratio of SiO2 / Al2O3 in ZSM-5 molecular sieve is 30~1000:1.

[0020] The beneficial effects of this invention are: This invention provides a low-cost, controllable preparation and mother liquor recycling method for ZSM-5 molecular sieves under process intensification. Utilizing the enhanced mixing and reaction capabilities of a microfilm reactor (MFR) within a micrometer-scale confined space, the method strengthens the contact and reaction between the solid silicon and aluminum sources and the template agent, accelerating the condensation of initial structural units ([SiO4], [AlO4]) and the formation of secondary structural units. This facilitates crystal nucleation and crystal growth, resulting in ZSM-5 molecular sieves with small, uniform particle size, regular morphology, and high crystallinity. The process intensification effect of the reactor enhances the mixing and reaction between the synthesis mother liquor and the added silicon, aluminum, and template agents during mother liquor recycling, avoiding uneven distribution of materials and pH in high-viscosity systems. This allows for direct utilization of the molecular sieve synthesis mother liquor while precisely controlling the chemical composition and microstructure of the ZSM-5 molecular sieve within a wide silicon-to-aluminum ratio range. The silicon source used in the preparation process is silica obtained from solid waste, and the aluminum source is a low-cost inorganic aluminum source. This method reduces the pretreatment time of the precursor solution to 15 minutes, which is more than 70% shorter than the traditional mechanical stirring (MS) method, and the crystallization time is reduced by more than 30%, thus lowering the raw material cost and energy consumption in the molecular sieve preparation process. Furthermore, the absence of sodium components in the synthesis system avoids the adverse effects of sodium metal enrichment on the recycling of the molecular sieve mother liquor. Simultaneously, it addresses the generation of large amounts of high-salt, high-ammonia-nitrogen wastewater during the conversion of the sodium form to the catalytically active hydrogen form of the molecular sieve, reducing the post-treatment cost of the molecular sieve. Attached Figure Description

[0021] Figure 1 The image shows the XRD pattern of the ZSM-5 molecular sieve prepared in Example 1 of this invention, where a is the initial sample and b is the recycled sample. Figure 2 SEM images of the ZSM-5 molecular sieve catalyst prepared in Example 1 of this invention, where a is the initial sample and b is the recycled sample; Figure 3 The nitrogen adsorption-desorption curve and pore size distribution of the ZSM-5 molecular sieve catalyst prepared in Example 1 of this invention are shown, where a is the initial sample and b is the recycled sample. Figure 4 The image shows the XRD pattern of the ZSM-5 molecular sieve catalyst prepared in Example 2 of this invention, where a is the initial sample and b is the recycled sample. Figure 5 SEM images of the ZSM-5 molecular sieve catalyst prepared in Example 2 of this invention, where a is the initial sample and b is the recycled sample; Figure 6 The image shows the XRD pattern of the ZSM-5 molecular sieve catalyst prepared in Example 3 of this invention, where a is the initial sample and b is the recycled sample. Figure 7 SEM images of the ZSM-5 molecular sieve catalyst prepared in Example 3 of this invention, where a is the initial sample and b is the recycled sample; Figure 8 The image shows the XRD pattern of the ZSM-5 molecular sieve catalyst prepared in Comparative Example 1 of this invention, where a is the initial sample and b is the recycled sample. Figure 9 SEM images of the ZSM-5 molecular sieve catalyst prepared in Comparative Example 1 of this invention are shown, where a is the initial sample and b is the recycled sample. Figure 10 These are TEM images of the initial ZSM-5 molecular sieve catalysts prepared in Example 1 and Comparative Example 1 of the present invention, where a is the sample of Example 1 and b is the sample of Comparative Example 1.

[0022] For those skilled in the art, other related figures can be obtained from the above figures without any creative effort. Detailed Implementation

[0023] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0024] In this invention, unless otherwise specified, all raw material components are commercially available products well known to those skilled in the art.

[0025] Example 1

[0026] A low-cost, controllable preparation method for ZSM-5 molecular sieves and a method for recycling the mother liquor under process intensification includes the following steps: S1. Weigh 6.00 g of nano-silica and ultrasonically disperse it in 60.00 g of deionized water to form a silicon source suspension; 0.18 g of nano-alumina was ultrasonically dispersed in 41.00 g of tetrapropylammonium hydroxide solution (tetrapropylammonium hydroxide mass concentration was 25 wt%) to form an aluminum source suspension; Subsequently, the silicon source suspension and aluminum source suspension were simultaneously added to the micro-liquid membrane reactor and mixed and reacted at 6000 rpm for 15 min to obtain the precursor solution. The effective components in the precursor solution were SiO2, Al2O3, template agent and water, and the ratio of each effective component was 1:0.017:0.5:50. S2. The precursor solution obtained in step S1 is transferred to a high-pressure autoclave with a polytetrafluoroethylene liner and hydrothermally crystallized at 150 °C for 24 h. After hydrothermal crystallization, the product is separated by centrifugation to obtain molecular sieve synthesis mother liquor and molecular sieve. The molecular sieve is further washed until pH≤7, dried in an oven, and then calcined at 550 °C for 6 h at a heating rate of 5 °C / min to obtain ZSM-5 molecular sieve. S3. Quantitatively analyze the content of silicon, aluminum, and template agent in the molecular sieve synthesis mother liquor obtained in step S2. The percentage content of silicon and aluminum (calculated as SiO2 and Al2O3) in the molecular sieve synthesis mother liquor is 0.9% and 0.006%, respectively, and the concentration of template agent is 28.6%. Based on the composition of the molecular sieve mother liquor, 0.176 g of nano-alumina is added to 27.0 g of tetrapropylammonium hydroxide solution (25 wt%) to form an aluminum source suspension, and 5.56 g of silica is ultrasonically dispersed in the remaining mother liquor with 4.75 g of deionized water added to form a silicon source suspension. The above-mentioned silicon source suspension and aluminum source suspension are simultaneously added to the microfilm reactor and mixed and reacted at 6000 rpm for 15 minutes. The precursor solution was obtained by minimizing the amount of time required to ensure that the effective components in the precursor solution during the mother liquor recycling process, calculated as SiO2, Al2O3, template agent, and water, were in the same proportions as those used in the initial preparation of ZSM-5 molecular sieve.

[0027] The XRD pattern of the first prepared ZSM-5 molecular sieve is as follows: Figure 1 As shown in Figure a, the characteristic diffraction peaks of the ZSM-5 molecular sieve with the MFI topology appear, indicating the successful preparation of the ZSM-5 molecular sieve; the morphology of the ZSM-5 molecular sieve is as follows. Figure 2 As shown in figure a, the particles have a regular morphology, a particle size of approximately 500 nm to 600 nm, and a specific surface area of ​​334 m². 2 / g, the molar ratio of SiO2 to Al2O3 in ZSM-5 molecular sieve is 60:1. For example... Figure 1 As shown in Figure b, the ZSM-5 molecular sieve prepared by recycling the mother liquor also exhibited the characteristic diffraction peaks of the pure-phase MFI structure of ZSM-5 molecular sieve, with a morphology as shown in Figure b. Figure 2 As shown in b, the morphology is basically the same as that of the first prepared ZSM-5 molecular sieve, with a particle size of approximately 300-600 nm and a specific surface area of ​​380 m². 2 / g, the molar ratio of SiO2 to Al2O3 in the ZSM-5 molecular sieve obtained by recycling the mother liquor is 56:1.

[0028] Example 2

[0029] A low-cost, controllable preparation method for ZSM-5 molecular sieves and a method for recycling the mother liquor under process intensification includes the following steps: S1. Weigh 12.00 g of nano-silica and ultrasonically disperse it in 130.00 g of deionized water to form a silicon source suspension. Weigh 0.136 g of nano-alumina and ultrasonically disperse it in 67.00 g of tetrapropylammonium hydroxide solution (25 wt%) to form an aluminum source suspension. Then, add the above silicon source and aluminum source suspensions to a micro-liquid membrane reactor simultaneously and mix and react at 6000 rpm for 15 min to obtain a precursor solution. The effective components in the precursor solution are SiO2, Al2O3, template agent, and water, and the ratio of each effective component is 1:0.0067:0.5:50. S2. The precursor solution obtained in step S1 is transferred to a high-pressure autoclave with a polytetrafluoroethylene liner and hydrothermally crystallized at 150 °C for 24 h. After hydrothermal crystallization, the product is separated by centrifugation to obtain molecular sieve synthesis mother liquor and molecular sieve. The molecular sieve is further washed until pH≤7, dried in an oven, and then calcined at 550 °C for 6 h at a heating rate of 5 °C / min to obtain ZSM-5 molecular sieve. S3. Quantitatively analyze the content of silicon, aluminum, and template agent in the molecular sieve synthesis mother liquor obtained in step S2. The percentage content of silicon and aluminum (calculated as SiO2 and Al2O3) in the molecular sieve synthesis mother liquor is 0.7% and 0.001%, respectively, and the concentration of template agent is 27.3%. Based on the composition of the molecular sieve mother liquor, 0.13 g of nano-alumina is added to 26 g of tetrapropylammonium hydroxide solution (25wt%) to form an aluminum source suspension, and 10.95 g of silica is ultrasonically dispersed in the remaining mother liquor with 2.5 g of deionized water added to form a silicon source suspension. The above-mentioned silicon source suspension and aluminum source suspension are simultaneously added to the micro-liquid membrane reactor and mixed and reacted at 6000 rpm for 15 min to obtain the precursor solution. Ensure that the effective components in the precursor solution during the mother liquor recycling process are in the same proportion as those in the initial ZSM-5 molecular sieve preparation, calculated as SiO2, Al2O3, template agent, and water.

[0030] The XRD pattern of the first prepared ZSM-5 molecular sieve is shown below. Figure 4 As shown in Figure a, the characteristic diffraction peaks of the ZSM-5 molecular sieve with the MFI topology appear, indicating the successful preparation of the ZSM-5 molecular sieve; the morphology of the ZSM-5 molecular sieve is as follows. Figure 5As shown in a, the particles have regular morphology, a particle size of approximately 200–500 nm, and a specific surface area of ​​390 m². 2 / g, the molar ratio of SiO2 to Al2O3 in ZSM-5 molecular sieve is 120:1. For example... Figure 4 As shown in b, the ZSM-5 molecular sieve prepared by recycling the mother liquor also exhibited the characteristic diffraction peaks of the pure-phase MFI structure of ZSM-5 molecular sieve, with a morphology as shown in Figure b. Figure 5 As shown in b, the morphology is basically the same as that of the first prepared ZSM-5 molecular sieve, with a particle size of approximately 200 nm to 400 nm and a specific surface area of ​​387 m². 2 / g.

[0031] Example 3

[0032] A low-cost, controllable preparation method for ZSM-5 molecular sieves and a method for recycling the mother liquor under process intensification includes the following steps: S1. Weigh 6.00 g of nano-silica and ultrasonically disperse it in 60.00 g of deionized water to form a silicon source suspension. Weigh 0.18 g of nano-alumina and ultrasonically disperse it in 41.00 g of tetrapropylammonium hydroxide solution (25 wt%) to form an aluminum source suspension. Then, add the above silicon source and aluminum source suspensions to a micro-liquid membrane reactor and mix and react at 3000 rpm for 15 min to obtain a precursor solution. The effective components in the precursor solution are SiO2, Al2O3, template agent, and water, and the ratio of each effective component is 1:0.017:0.5:50. S2. The precursor solution obtained in step S1 is transferred to a high-pressure autoclave with a polytetrafluoroethylene liner and hydrothermally crystallized at 150 °C for 24 h. After hydrothermal crystallization, the product is separated by centrifugation to obtain molecular sieve synthesis mother liquor and molecular sieve. The molecular sieve is further washed until pH≤7, dried in an oven, and then calcined at 550 °C for 6 h at a heating rate of 5 °C / min to obtain ZSM-5 molecular sieve. S3. Quantitatively analyze the content of silicon, aluminum, and template agent in the molecular sieve synthesis mother liquor obtained in step S2. The percentage content of silicon and aluminum (calculated as SiO2 and Al2O3) in the molecular sieve synthesis mother liquor is 1.0% and 0.002%, respectively, and the concentration of template agent is 33.2%. Based on the composition of the molecular sieve mother liquor, 0.18 g of nano-alumina is added to 24.4 g of tetrapropylammonium hydroxide solution (25 wt%) to form an aluminum source suspension, and 5.95 g of silica is ultrasonically dispersed in 8.3 g of deionized water to form a silicon source suspension. The above-mentioned silicon source suspension, aluminum source suspension, and molecular sieve synthesis mother liquor are simultaneously added to a micro-liquid membrane reactor and mixed and reacted at 3000 rpm for 15 min to obtain a precursor solution. Ensure that the effective components in the precursor solution during the mother liquor recycling process are in the same proportion as those in the initial ZSM-5 molecular sieve preparation, calculated as SiO2, Al2O3, template agent, and water.

[0033] The XRD pattern of the first prepared ZSM-5 molecular sieve is shown below. Figure 6 As shown in Figure a, the characteristic diffraction peaks of the ZSM-5 molecular sieve with the MFI topology appear, indicating the successful preparation of the ZSM-5 molecular sieve; the morphology of the ZSM-5 molecular sieve is as follows. Figure 7 As shown in a, the particles have regular morphology, a particle size of approximately 500–900 nm, and a specific surface area of ​​320 m². 2 / g, the molar ratio of SiO2 to Al2O3 in ZSM-5 molecular sieve is 43:1. For example... Figure 6 As shown in b, the ZSM-5 molecular sieve prepared by recycling the mother liquor also exhibited the characteristic diffraction peaks of the pure-phase MFI structure of ZSM-5 molecular sieve, with a morphology as shown in Figure b. Figure 7 As shown in b, the morphology is basically the same as that of the first prepared ZSM-5 molecular sieve, with a particle size of approximately 400 nm to 700 nm and a specific surface area of ​​335 m². 2 / g.

[0034] Comparative Example 1 The difference from Example 1 is that in step S1, the equipment used for mixing and reacting the silicon source suspension and the aluminum source suspension is replaced by a conventional mechanical stirring device instead of a micro-liquid membrane reactor. The stirring speed of the device is 600 rpm. Other operations are the same as in Example 1.

[0035] The XRD pattern of the ZSM-5 molecular sieve obtained in Comparative Example 1 is shown below. Figure 8 Figure a shows the characteristic diffraction peaks of ZSM-5 molecular sieve, and the morphology of the obtained molecular sieve is as follows. Figure 9 Figure a shows spherical or elliptical particles with irregular boundaries and broken crystals, with a particle size of approximately 900 nm and a surface area of ​​341 m². 2 / g, total pore volume is 0.22 cm³3 / g; The ZSM-5 circulating molecular sieve prepared in this comparative example exhibits diffraction peaks of impurity phases, the specific spectrum of which is shown in [reference needed]. Figure 8 b shows that Figure 9 b indicates that the morphology of the circulating molecular sieve prepared in this comparative example is an irregular morphology with incompletely crystallized silica-alumina gel, and the particle size is smaller than that of comparative example 1, approximately 300 nm ~ 400 nm, with a total specific surface area of ​​247 m². 2 / g.

[0036] Overall, compared with the ZSM-5 molecular sieve synthesized by micro-liquid membrane mixing enhancement, the ZSM-5 molecular sieve synthesized in the comparative ratio has lower crystallinity and poorer morphology. After two cycles, a large amount of aluminosilicates appears, and the cycle stability and controllability are poor.

[0037] This invention utilizes a microfilm reactor as a process intensification device to achieve controllable preparation of ZSM-5 molecular sieves with small particle size, regular morphology, and high crystallinity in a sodium-free system, as well as direct recycling of the molecular sieve mother liquor. It accelerates the formation of primary structural units of the molecular sieve, shortens crystallization time, and reduces raw material costs and energy consumption in the molecular sieve preparation process. Furthermore, this invention solves the problem at its source of generating large amounts of high-salt, high-ammonia-nitrogen wastewater during the conversion of the sodium form to the catalytically active hydrogen form of the molecular sieve, thus reducing the post-treatment costs of the molecular sieve.

[0038] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. The applicant declares that the above descriptions are merely specific implementations of this invention, but the scope of protection of this invention is not limited thereto. Those skilled in the art should understand that any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this invention fall within the scope of protection and disclosure of this invention.

Claims

1. A low-cost, controllable preparation method for ZSM-5 molecular sieves under process intensification and a method for recycling the mother liquor, characterized in that: Includes the following steps: S1. The silicon source is ultrasonically dispersed in deionized water to form a silicon source suspension; the aluminum source is ultrasonically dispersed in the template agent solution to form an aluminum source suspension; the silicon source suspension and the aluminum source suspension are simultaneously added to the micro-liquid membrane reactor and mixed and reacted to obtain the precursor gel. S2. The precursor solution obtained in step S1 is subjected to hydrothermal crystallization. The hydrothermal crystallization product is separated into solid and liquid to obtain molecular sieve synthesis mother liquor and molecular sieve solid. S3. The molecular sieve solid obtained in step S2 is washed, dried and calcined to obtain ZSM-5 molecular sieve; S4. Simultaneously add the silicon source suspension, the aluminum source suspension, and the molecular sieve synthesis mother liquor obtained in step S2 into the micro-liquid membrane reactor for mixing and reaction to obtain the reusable precursor solution; repeat steps S2 and S3 with the reusable precursor solution.

2. The method for low-cost, controllable preparation of ZSM-5 molecular sieve and mother liquor recycling under process enhancement as described in claim 1, characterized in that: The silicon source is nano-silica prepared from silica gel solid waste in ZL202210253161.6; the aluminum source is at least one of alumina, boehmite, boehmite or aluminum hydroxide; the template agent is at least one of tetrapropylammonium hydroxide, tetrapropylammonium bromide, tetrabutylammonium hydroxide, n-propylamine, n-butylamine, ethylamine or isopropylamine.

3. The method for low-cost, controllable preparation of ZSM-5 molecular sieve and mother liquor recycling under process intensification as described in claim 1, characterized in that: The effective components in the precursor solution, calculated as SiO2, Al2O3, template agent, and water, have the following proportional relationship: The molar ratio of SiO2 to Al2O3 is 20~1000:1; The molar ratio of template agent to SiO2 is 0.2~0.5:1; The molar ratio of water to SiO2 is 30~4000:

1.

4. The method for low-cost, controllable preparation of ZSM-5 molecular sieve and mother liquor recycling under process intensification as described in claim 1, characterized in that: In steps S2 and S4, the rotation speed of the microfilm reactor is 2000 rpm to 6000 rpm, and the mixing time is 5 min to 30 min.

5. The method for low-cost, controllable preparation of ZSM-5 molecular sieve and mother liquor recycling under process intensification as described in claim 1, characterized in that: The hydrothermal crystallization conditions are 150 ℃ to 200 ℃ for 6 h to 48 h.

6. The method for low-cost, controllable preparation of ZSM-5 molecular sieve and mother liquor recycling under process intensification as described in claim 1, characterized in that: The calcination conditions are 450 ℃ to 650 ℃ for 4 h to 8 h.

7. The method for low-cost, controllable preparation of ZSM-5 molecular sieve and mother liquor recycling under process intensification as described in claim 1, characterized in that: The method for determining the amount of silicon source, aluminum source and template agent added in the silicon source suspension and aluminum source suspension is as follows: First, the content of silicon, aluminum and template agent in the molecular sieve synthesis mother liquor obtained in step S2 is quantitatively analyzed; then, based on the quantitative analysis results, the amount of silicon source, aluminum source and template agent added is determined according to the proportion of each effective component in the precursor solution obtained in step S1.

8. A ZSM-5 molecular sieve catalyst prepared by the method according to any one of claims 1-7, characterized in that: The specific surface area of the ZSM-5 molecular sieve is 350 m 2 / g~500 m 2 / g, the particle size is 0.3 μm~0.9 μm, and the molar ratio of SiO2 / Al2O3 in the ZSM-5 molecular sieve is 30~1000:1.

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