Method for synthesizing nano-zsm-5 molecular sieve with low silicon-aluminum ratio and application thereof

Abstract

A method for synthesizing low-silicon-to-aluminum ratio nano-ZSM-5 molecular sieves and their applications are disclosed, belonging to the technical field of catalyst preparation. The preparation method includes: uniformly mixing a portion of a silicon source, an aluminum source, a template agent, and water to obtain a gel; then, subjecting the gel to vigorous mixing via a microchannel reactor; finally, crystallizing the gel, and filtering, washing, drying, and calcining the crystallized product to obtain a Na-type molecular sieve, followed by ammonium exchange, filtration, washing, drying, and calcination to obtain an HZSM-5 molecular sieve. The molecular sieve prepared by this method has a relative crystallinity of over 95%, effectively solving the problem of uneven silicon-to-aluminum ratio distribution and the formation of impurities due to the low silicon-to-aluminum ratio. The molecular sieve prepared by this invention exhibits a nano-aggregate morphology with abundant intergranular mesopores, which facilitates rapid contact between the active sites of the catalyst and reactant molecules, resulting in good diffusion performance. The low-silicon-to-aluminum ratio hierarchical porous ZSM-5 molecular sieve prepared by this invention can be used in crude oil catalytic cracking reactions, significantly improving the reaction conversion rate.

Description

Technical Field

[0001] This invention belongs to the field of catalysis technology, specifically relating to a method and application for synthesizing low silica-to-alumina ratio hierarchical porous nano ZSM-5 molecular sieves. Background Technology

[0002] ZSM-5 zeolite / molecular sieve, as a key material in FCC catalysts / auxiliaries, plays a crucial role in increasing propylene production. Furthermore, ZSM-5 molecular sieves also contribute to octane rating in FCC applications, reflecting their excellent aromatization properties and significant potential for aromatic hydrocarbon production under fluidized bed reaction conditions. Comprehensive analysis of the structure and acid-catalyzed reaction characteristics of various novel zeolite materials reveals that for ZSM-5 molecular sieves, decreasing the Si / Al ratio increases microreactor activity; a low Si / Al ratio is most favorable for generating high cracking reaction activity, while nano-sizing significantly improves the selectivity of the aromatization reaction, which is beneficial for reducing coke yield and greatly extending catalyst lifetime. Experimental results also show that FCC auxiliary agents prepared using low Si / Al ratio or nano-sized ZSM-5 molecular sieves are beneficial for improving propylene yield in catalytic cracking reactions. Therefore, it can be concluded that developing nano-sized ZSM-5 molecular sieves with low Si / Al ratios (SiO2 / Al2O3 between 15-25) is key to the efficient production of low-carbon olefins (such as propylene) and aromatics through FCC technology.

[0003] However, the silica-to-alumina ratio (S / A) of synthesized nano-ZSM-5 molecular sieves in current literature is generally greater than 25, with only a few studies focusing on the synthesis of low S / A ZSM-5 zeolites. This is because, on the one hand, an S / A ratio less than 25 exceeds the phase region of ZSM-5 molecular sieves; on the other hand, the uneven distribution of S / A ratio in low S / A gels easily leads to the formation of impurity crystals. This technique will develop a method for forming a homogeneous alumina-rich gel by strong mixing of low-polymerization-degree silicates and aluminum salts through a microchannel reactor. This method can simultaneously and rapidly form nanocrystal nuclei under isothermal conditions, suppressing impurity crystal growth from both chemical composition and kinetic perspectives, and obtaining nano-ZSM-5 zeolites in a low S / A gel system. Summary of the Invention

[0004] To address the challenge of synthesizing nano-ZSM-5 molecular sieves at low silica-to-alumina ratios, this invention provides a novel method for synthesizing low-silica-to-alumina ratio nano-ZSM-5 molecular sieves. This method utilizes a microchannel reactor with each microchannel approximately 1.5 mm in diameter to achieve thorough and uniform mixing of the silica-to-alumina gel, effectively eliminating the effects of concentration and temperature gradients. Within a low silica-to-alumina ratio range, this method facilitates the synthesis of pure-phase, uniformly sized small-grained zeolites, successfully achieving a method for synthesizing low-silica-to-alumina ratio nano-ZSM-5 molecular sieves.

[0005] This invention involves uniformly mixing a silicon source, an aluminum source, a template agent, and water, followed by low-temperature aging treatment at 25-100℃ for 24-48 hours, and then hydrothermal crystallization at 100-190℃ for 8-120 hours to obtain nano-packed ZSM-5 zeolite with an average particle size of less than 500 nm. This is because low-temperature aging promotes nanocrystal nucleation, thus reducing the final grain size, while at high temperatures, primary nanoparticles combine through intragranular ripening to form larger nanoparticles, resulting in mesoporous intergranular deposits after calcination. To achieve the above objectives, this invention employs the following technical solution:

[0006] A method for synthesizing low silica-to-alumina ratio nano-ZSM-5 molecular sieves includes the following steps:

[0007] (1) Mix the silicon source, aluminum source, water and template agent evenly to obtain a mixed gel;

[0008] (2) The mixed gel obtained in step (1) is subjected to forced mixing using a microchannel reactor; the diameter of each microchannel in the microchannel reactor is 1.5±0.5 mm;

[0009] (3) Place the mixed gel obtained in step (2) in a hydrothermal reactor for crystallization treatment;

[0010] (4) The crystallized product obtained in step (3) is filtered, washed, dried and calcined to obtain Na-type ZSM-5 molecular sieve;

[0011] (5) The Na-type molecular sieve obtained in step (4) is subjected to ammonium exchange with an ammonium salt aqueous solution, and then filtered, washed, dried and calcined to obtain H-type ZSM-5 molecular sieve;

[0012] The molar ratio of the effective components SiO2, Al2O3, Na2O, template agent and H2O in the mixed gel is 1:(0.066-0.02):(0.005-0.03):(0.05-0.5):(30-180).

[0013] In the preferred step (1), the silicon source is selected from one or more of the following: sodium silicate nonahydrate, tetraethyl orthosilicate, water glass, solid silica gel, fumed silica, and silica sol, preferably solid silica gel.

[0014] The preferred aluminum source in step (1) is selected from one or more of the following: aluminum hydroxide, sodium aluminate, aluminum isopropoxide, boehmite, aluminum sulfate, and aluminum powder, preferably aluminum sulfate.

[0015] The template agent in step (1) is selected from one or more of methylamine, ethylenediamine, n-propylamine, n-butylamine, and tetrapropylammonium hydroxide, preferably n-butylamine.

[0016] The preferred strong mixing time in step (2) is 4 to 48 hours.

[0017] The preferred crystallization temperature in step (3) is: first, a low-temperature aging treatment at 25-100℃ for 24-48 hours, and then hydrothermal crystallization at 100-190℃ for 8-120 hours.

[0018] The preferred step (4) is a 0.4-0.8M ammonium chloride solution with an ammonium exchange temperature of 30-60℃ and 1-5 exchanges, preferably 0.4M, at 60℃, and 3 exchanges.

[0019] The preferred roasting temperature for steps (4) and (5) is 450-650℃ and the roasting time is 3-7h, further preferably 540-580℃ and 4-6h, and most preferably 540℃ and 6h.

[0020] The present invention also provides an application of the low silica-alumina ratio nano ZSM-5 molecular sieve in crude oil catalytic cracking.

[0021] The beneficial effects of this invention are as follows: A simple and easy-to-operate method is used, employing a microchannel reactor to strongly mix the crystallized gel. Each microchannel has a diameter of approximately 1.5 mm, ensuring thorough and uniform mixing of the silica-alumina gel and eliminating the effects of concentration and temperature gradients. Within a low silica-alumina ratio range, this method facilitates the synthesis of pure-phase, uniformly sized small-grained zeolites, successfully yielding a method for synthesizing low silica-alumina ratio nano-ZSM-5 molecular sieves with a relative crystallinity exceeding 95%. The ZSM-5 molecular sieve obtained using this method exhibits good conversion rates of 1,3,5-triisopropylbenzene and high hydrothermal stability, making it valuable for applications in crude oil catalytic cracking. Attached Figure Description

[0022] Figure 1 The images show X-ray diffraction (XRD) test patterns of the samples prepared in Comparative Examples 2, 1, 3, and 4 of this invention.

[0023] Figure 2 The nitrogen physisorption curves are shown for the samples prepared in Comparative Examples 1-2, Examples 1 and 3 of this invention.

[0024] Figure 3 The images shown are scanning electron microscope (SEM) images of the samples prepared in Comparative Examples 1 and 3 of this invention.

[0025] Figure 4 The images show the reaction test results of 1,3,5-triisopropylbenzene in the samples prepared by Comparative Examples 1-2, Examples 1 and 3 of this invention. Detailed Implementation

[0026] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments. The analysis method in the embodiments of the present application is as follows:

[0027] X-ray diffraction (XRD) analysis of the samples: The analytical instrument was a DX-2700B X-ray diffractometer manufactured by Dandong Haoyuan Instrument Co., Ltd. The measurement conditions were as follows: CuKα fluorescence radiation, tube voltage 40kV, tube current 30mA, scanning step size 0.02°, scanning range diffraction angle 2θ=5~40°, and scanning speed 6° / min.

[0028] Nitrogen physical adsorption characterization of samples: The analytical instrument was a four-station nitrogen physical adsorption analyzer JW-TB400 from Beijing Jingwei Gaobo Scientific Technology Co., Ltd. The test methods were as follows: 1) Pretreatment: 0.15g of the catalyst sample to be tested (powder samples required tableting) was placed in a quartz tube and vacuum-treated at 350℃ for 4h to remove moisture and impurities adsorbed by the ZSM-5 molecular sieve material; 2) Nitrogen adsorption / desorption experiments were conducted at -195.7℃; 3) The specific surface area of ​​micropores and mesopores of the sample was calculated using the Brunauer-Emmett-Teller (BET) equation, and the internal specific surface area, external surface area, and pore volume of the ZSM-5 molecular sieve sample were calculated using the t-plot method. The total pore volume was calculated as the N2 adsorption capacity at a relative pressure P / P0 = 0.99.

[0029] SEM characterization of the samples: The instrument used was a Hitachi SU8220 scanning electron microscope (SEM) manufactured by Hitachi, Japan. The analytical conditions were accelerating voltage 5 kV and 10 mA. The sample preparation method was as follows: the solid powder was dispersed in ethanol and ultrasonically treated for 5 min to obtain a suspension. The suspension was then dropped onto a conductive silicon wafer using a capillary tube, and the wafer was dried to allow the ethanol to evaporate before use. The sample was then sputter-coated with gold before testing.

[0030] The 1,3,5-triisopropylbenzene pyrolysis reaction of the catalyst was evaluated using the following method: The reaction was conducted in a laboratory-built pulsed microreactor, using a Shimadzu gas chromatograph equipped with an Al2O3 column (model SHIMADZU 2014C). The main experimental procedures included the following steps: 1) Sample pretreatment: After calcination to remove water, the molecular sieve catalyst sample was sieved to obtain 20-40 mesh catalyst particles. 0.15 g of these particles was loaded into a U-shaped microreactor with an inner diameter of 2 mm. 2) Sample activation: Nitrogen carrier gas was introduced, and the reactor temperature was set to 350℃, with the insulation zone temperature set to 200℃. The catalyst was activated at 350℃ for 1 h. 3) Reaction determination: The temperature rise program for the 1,3,5-triisopropylbenzene pyrolysis reaction column was selected. The injection volume of the 1,3,5-triisopropylbenzene feedstock was 0.4 μL. After the reaction, the residence time and area ratio of each sample were recorded. The area normalization method was used to quantitatively analyze the reactivity of the molecular sieve catalyst sample.

[0031] In this application, the dehydration pretreatment of zeolite molecular sieve adopts conventional drying or calcination methods. In the examples, the dehydration pretreatment of zeolite molecular sieve is carried out according to the following method: the catalyst is placed in a muffle furnace and heated to 540°C for calcination for 3 hours at room temperature for 2 hours.

[0032] Comparative Example 1

[0033] (1) Solution A: Add a mixture of 12g tetrapropylammonium hydroxide aqueous solution (TPAOH, 25wt%) and 12g deionized water dropwise to 8g tetraethyl orthosilicate, stirring until the tetraethyl orthosilicate is completely hydrolyzed; Solution B: Add 0.28g sodium aluminate to 8g deionized water and stir until homogeneous. Add solution B dropwise to solution A and stir until homogeneous to obtain a mixed gel;

[0034] (2) Place the mixed gel obtained in step (1) in a hydrothermal reactor, then place the reactor in an oven, first perform an aging treatment at 80°C for 24 hours, and then perform a hydrothermal crystallization treatment at 150°C for 48 hours.

[0035] (3) The crystallized product obtained in step (2) is filtered, washed, dried in an oven at 110°C, and then placed in a muffle furnace and calcined in an air stream at 540°C for 6 hours to obtain Na-type ZSM-5 molecular sieve.

[0036] (4) The Na-type molecular sieve obtained in step (3) is exchanged three times with 0.4M ammonium chloride solution at an exchange temperature of 50°C. Then it is filtered, washed, dried in an oven at 110°C, and then placed in a muffle furnace and calcined in an air stream at a programmed temperature of 540°C for 3 hours to obtain the H-type molecular sieve.

[0037] The sample obtained in this comparative example is designated as control sample 1#. XRD analysis showed that the sample contained mordenite impurities with a relative crystallinity of 67%.

[0038] Example 1

[0039] This embodiment provides a method for synthesizing and applying low silica-to-alumina ratio nano-ZSM-5 molecular sieves. The specific steps are as follows:

[0040] (1) Solution A: Add a mixture of 12g tetrapropylammonium hydroxide aqueous solution (TPAOH, 25wt%) and 12g deionized water dropwise to 8g tetraethyl orthosilicate, stirring until the tetraethyl orthosilicate is completely hydrolyzed; Solution B: Add 0.49g sodium aluminate to 8g deionized water and stir until homogeneous. Add solution B dropwise to solution A and stir until homogeneous to obtain a mixed gel;

[0041] (2) The mixed gel obtained in step (1) is repeatedly extruded and strongly mixed in a microchannel reactor for 1 hour to obtain a fine and uniform mixed gel. The mixture is then placed in a hydrothermal reactor and aged at 80°C for 24 hours. Then, it is subjected to hydrothermal crystallization at 150°C for 48 hours.

[0042] (3) The crystallized product obtained in step (2) is filtered, washed, dried in an oven at 110°C, and then placed in a muffle furnace and calcined in an air stream at 540°C for 6 hours to obtain Na-type ZSM-5 molecular sieve.

[0043] (4) The Na-type molecular sieve obtained in step (3) is exchanged three times with a 0.4M ammonium chloride solution at an exchange temperature of 50°C. After filtration, washing, and drying in an oven at 110°C, it is then calcined in a muffle furnace at a programmed temperature of 540°C in an air stream for 3 hours to obtain the H-type molecular sieve. The molar ratio of the effective components SiO2, Al2O3, Na2O, template agent, and H2O in the mixed gel is 1:0.05:0.033:0.3:30:0.05.

[0044] The sample obtained in this embodiment is designated as sample 1#. XRD analysis showed that the sample was a pure phase ZSM-5 zeolite with a relative crystallinity of 95%.

[0045] Example 2

[0046] The operation is the same as in Example 1, except that the gel ratio is changed: the molar ratio of the effective components SiO2, Al2O3, Na2O, template agent and H2O in the mixed gel is 1:0.04:0.033:0.3:30:0.05.

[0047] The sample obtained in this embodiment is designated as sample 2#. XRD analysis showed that the sample was a pure phase ZSM-5 zeolite with a relative crystallinity of 96%.

[0048] Example 3

[0049] The operation is the same as in Example 1, except that the molar ratio of the initial gel is changed: the molar ratio of the effective components SiO2, Al2O3, Na2O, template agent and H2O in the mixed gel is 1:0.04:0.033:0.3:30:0.07.

[0050] The sample obtained in this embodiment is designated as sample 3#. XRD analysis showed that the sample was a pure phase ZSM-5 zeolite with a relative crystallinity of 97%.

[0051] Example 4

[0052] The procedure is the same as in Example 1, except that the template agent is n-butylamine.

[0053] The sample obtained in this embodiment is designated as sample 4#. XRD analysis showed that the sample is a pure phase ZSM-5 zeolite with a relative crystallinity of 96%.

[0054] Example 5

[0055] The operation is the same as in Example 1, except that the silicon source is solid silicone.

[0056] The sample obtained in this embodiment is designated as sample 5#. XRD analysis showed that the sample was a pure phase ZSM-5 zeolite with a relative crystallinity of 97%.

[0057] Example 6

[0058] The samples obtained in the above embodiments were characterized by nitrogen physical adsorption. Taking comparative sample 1# and samples 1#-4# as examples, the results are shown in Table 1 and... Figure 2 As shown, taking comparative sample 1# as an example, after aging treatment during the synthesis process, the final synthesized sample exhibited a decrease in grain size and a significant increase in mesopore volume due to the aging treatment's benefits to nucleation. The hysteresis loop of the physical adsorption isotherm in the high specific pressure region also increased, demonstrating the presence of larger mesopores. Table 1 shows the relative crystallinity and nitrogen physical adsorption data of different samples.

[0059]

[0060] Example 8

[0061] Using 1,3,5-triisopropylbenzene as the reactant, the conversion rate of the catalyst prepared above in the catalytic cracking reaction of 1,3,5-triisopropylbenzene was characterized by catalytic cracking reaction.

[0062] The conversion rates of each molecular sieve catalyst are calculated using the following formulas, where A 正 Percentage of chromatographic area for residual 1,3,5-triisopropylbenzene: Conversion rate of 1,3,5-triisopropylbenzene: C = 100% - A 正

[0063] The reaction conditions were: reaction temperature 300-500℃. Product analysis was performed using gas chromatography with a flame ionization detector (FID). Reaction data are as follows: Figure 4 As shown in Table 2, the test results show that the conversion rate of almost all catalysts increases with increasing temperature. Compared with control sample 1#, sample 3#, which exhibits a small crystallite packing morphology, has a 1,3,5-triisopropylbenzene conversion rate of 90%, indicating that the low silica-to-alumina ratio hierarchical porous ZSM-5 molecular sieve catalyst prepared in this invention has high conversion activity in macromolecular catalytic cracking reactions.

[0064] Table 2. Conversion rates of 1,3,5-triisopropylbenzene in different samples.

[0065]

[0066] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for synthesizing ZSM-5 molecular sieves with a low silica-to-alumina ratio, characterized in that, Includes the following steps: (1) Mix the silicon source, aluminum source, water and template agent evenly to obtain a mixed gel; (2) The mixed gel obtained in step (1) is subjected to forced mixing using a microchannel reactor; the diameter of each microchannel in the microchannel reactor is 1.5±0.5mm; (3) Place the mixed gel obtained in step (2) in a hydrothermal reactor for crystallization treatment; (4) The crystallized product obtained in step (3) is filtered, washed, dried and calcined to obtain Na-type ZSM-5 molecular sieve; (5) The Na-type molecular sieve obtained in step (4) is subjected to ammonium exchange with an ammonium salt aqueous solution, and then filtered, washed, dried and calcined to obtain H-type ZSM-5 molecular sieve; The molar ratio of the effective components SiO2, Al2O3, Na2O, template agent and H2O in the mixed gel is 1: (0.066-0.04): (0.005-0.03): (0.05-0.5): (30-180).

2. The method for synthesizing a low silica-to-alumina ratio hierarchical porous ZSM-5 molecular sieve according to claim 1, characterized in that: The silicon source is one or more of sodium silicate nonahydrate, tetraethyl orthosilicate, water glass, solid silica gel, silica fume, and silica sol; the aluminum source is one or more of aluminum hydroxide, sodium aluminate, aluminum isopropoxide, boehmite, aluminum sulfate, and aluminum powder.

3. The method for synthesizing low silica-to-alumina ratio ZSM-5 molecular sieve according to claim 1, characterized in that: The template agent is one or more of methylamine, ethylenediamine, n-propylamine, n-butylamine, and tetrapropylammonium hydroxide.

4. The method for synthesizing a low silica-to-alumina ratio hierarchical porous ZSM-5 molecular sieve according to claim 1, characterized in that: The crystallization temperature is first subjected to low-temperature aging treatment at 25-100℃ for 24-48 hours, and then hydrothermal crystallization at 100-190℃ for 8-120 hours.

5. The method for synthesizing a low silica-to-alumina ratio hierarchical porous ZSM-5 molecular sieve according to claim 1, characterized in that: The ammonium salt aqueous solution is a 0.4-0.8M ammonium chloride solution, the ammonium exchange temperature is 30-60℃, and the number of exchange cycles is 1-5.

6. The method for synthesizing a low silica-to-alumina ratio hierarchical porous ZSM-5 molecular sieve according to claim 1, characterized in that: The roasting temperature is 540-580℃, and the roasting time is 4-6 hours.

7. The application of the H-type ZSM-5 molecular sieve prepared by the method according to any one of claims 1-5, characterized in that: Application of the H-type ZSM-5 molecular sieve in the catalytic cracking reaction of 1,3,5-triisopropylbenzene.

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