Large crystal zsm-11 molecular sieve and method for preparing the same

Abstract

The application relates to the technical field of chemical materials, and particularly provides a large-grain ZSM-11 molecular sieve and a preparation method thereof. The surface of the large-grain ZSM-11 molecular sieve is a sponge-like channel structure and the surface Brønsted acid sites are passivated, the grain size is 2-4 mu m, and the inner pore size is 30-350 nm. The preparation method of the large-grain ZSM-11 molecular sieve comprises the following steps: adding an alkali source into an aluminum source solution, then adding a template agent, and then adding a silicon source solution drop by drop to stir at room temperature-80 DEG C to obtain a mixed gel; then the mixed gel is placed in a hydrothermal crystallization kettle, and crystallization reaction is carried out at 100-180 DEG C for 6-48 h, and then drying and calcination are carried out to obtain the large-grain ZSM-11 molecular sieve. The method solves the technical problem in the prior art that the large-grain ZSM-11 molecular sieve prepared by the prior art has high surface Brønsted acid sites, so that carbon is easily accumulated in the initial contact process of reactants and catalysts in the application process, the molecular sieve channels are blocked, the reaction efficiency is low, and the service life of the catalyst is shortened.

Description

Technical Field

[0001] This invention relates to the field of chemical materials technology, and in particular to a ZSM-11 molecular sieve and its preparation method. Background Technology

[0002] ZSM-11 molecular sieve, a type of high-silica shape-selective molecular sieve, has wide applications in catalysis, pharmaceuticals, environmental protection, adsorption separation, chemical industry, and petrochemicals. It can be used as an ion exchange material, adsorbent, and catalyst. In catalysis, ZSM-11 has demonstrated excellent catalytic performance in reactions such as polyethylene to hydrocarbons, ethylene to propylene and butene, and methanol to low-carbon olefins. Furthermore, compared to the traditional ZSM-5, ZSM-11 exhibits superior catalytic performance in reactions such as the hydroisomerization of long-chain alkanes, the dehydration of glycerol to acrolein, catalytic cracking of heavy oil, the aromatization and isomerization of 1-hexene, the cracking of pentene to C2-C4 olefins, the alkylation of benzene, and the dehydrogenation of alkanes.

[0003] ZSM-11 molecular sieve is a three-dimensional topological molecular sieve with a regular pore structure. Its adjacent layers are mirror-symmetrically correlated, and it has straight ten-membered ring channels parallel to the a-axis and b-axis with a channel size of 5.4 × 5.3 Å. At the channel intersections, it forms a three-dimensional space different from ZSM-5. However, both have a large number of micropores with small pore sizes, which inhibit the diffusion of macromolecules and cause the catalyst to deactivate rapidly due to coking (Hui Liu, et al. Chinese Journal of Catalysis 2018;39: 167-180).

[0004] To overcome the diffusion defects of microporous molecular sieves, this can be achieved by reducing the crystal size of the molecular sieve or by preparing hierarchical porous molecular sieves. However, nanomolecular sieves, due to their significantly reduced crystal size, exhibit a decreased shape selectivity effect in the catalyst, impacting their application value. Molecular sieves with large crystallites and surface diffusion channels offer another effective way to reduce molecular diffusion resistance, combining the advantages of molecular sieves and mesoporous channels, thus exhibiting the strong acidity, hydrothermal stability, and superior diffusion performance of molecular sieves. Furthermore, ZSM-11 molecular sieves are constrained in reactions such as isomerization by their abundant Brønsted acid sites on the surface. Initial contact between reactants and catalysts can lead to carbon deposition, clogging the molecular sieve channels, resulting in low reaction efficiency and shortened catalyst lifetime. Therefore, developing ZSM-11 molecular sieves with surface Brønsted acid passivation and weakly acidic aluminum hydroxyl sites is a pressing and common technical challenge that urgently needs to be addressed. Summary of the Invention

[0005] The present invention provides a large-grain ZSM-11 molecular sieve, wherein the surface of the large-grain ZSM-11 molecular sieve has a sponge-like pore structure and the surface Brønsted acid sites are passivated; the grain size of the large-grain ZSM-11 is 2~4 μm and the pore size in the pores is 30~350 nm.

[0006] The large-grained ZSM-11 molecular sieve of the present invention is synthesized by hydrothermal synthesis, and its preparation method includes the following steps: Aluminum source, template agent, silicon source and deionized water are mixed and stirred at room temperature for 2-8 h to obtain a mixed gel. The mixed gel is placed in a hydrothermal crystallization kettle and reacted at 100-180 °C for 6-48 h. After drying and air calcination, the large-grained ZSM-11 molecular sieve is obtained.

[0007] The specific steps for obtaining the mixed gel include: Step 1): Adding an aluminum source to water at 20-40 °C to prepare solution A; Step 2): Adding an alkali source to solution A and mixing thoroughly, then adding a template agent sequentially to prepare solution B; the alkali source is preferably sodium hydroxide; Step 3): Dispersing a silicon source in a portion of deionized water by mechanical stirring to prepare a silicon source suspension; Step 4): Subsequently, under a strong shear flow field, adding solution B dropwise to the silicon source suspension to obtain a mixed liquid.

[0008] During the mixing and stirring process in steps 1) to 3), the stirring speed of the system is controlled at 400~1000 rpm, corresponding to a fluid shear rate of 300~1200 s. -1 In step 4), when adding solution B dropwise, the dropping rate of solution B should be strictly controlled to be 0.5~3.0 mL / min. After mixing, the mixture should be subjected to strong shear stirring at room temperature to 80 ℃ for 2~8 h.

[0009] In the mixed gel, the aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as Na2O. The molar ratio between SiO2 and Al2O3 is 1:(0.010~0.030), and the molar ratio between Na2O, water, template agent, and SiO2 is (0.02~0.12):(20~40):(0.1~0.3). The resulting ZSM-11 molecular sieve has a crystal size of 2~4 μm and a mesopore size of 30~350 nm.

[0010] Preferably, when the molar ratio of Na₂O, water, template agent, and SiO₂ is (0.03~0.05):(25~35):(0.15~0.25), i.e., the low-alkali control region, the resulting ZSM-11 molecular sieve has a grain size of 2~3 μm and a mesopore size of 50~120 nm. When the molar ratio of Na₂O, water, template agent, and SiO₂ is (0.06~0.09):(25~35):(0.15~0.25), i.e., the high-alkali control region, the resulting ZSM-11 molecular sieve has a grain size of 3.1~4 μm and a mesopore size of 120~300 nm.

[0011] As a core feature of this invention, the activated silicon source is modified by in-situ local surface carbonization. The modification method includes: mixing the silicon source with tetrapropylammonium hydroxide (TPAOH) accounting for 20% to 50% of the total mass of the template agent and drying it, so that the tetrapropylammonium hydroxide is uniformly coated on the surface of the silicon source particles; then heat-treating it at 450 to 650 °C for 1 to 4 h in a mixed atmosphere containing methane and nitrogen, so as to form a porous carbonization layer with a slow-release effect in situ on the surface of the silicon source particles; wherein the volume ratio of methane to nitrogen in the mixed atmosphere is 1:(2 to 6).

[0012] The beneficial effects of this invention are as follows:

[0013] (1) Molecular-level slow-release valve effect: Part of the TPAOH template agent is pyrolyzed and carbonized in situ under a reducing atmosphere, forming a porous carbon sacrificial layer on the surface of the silicon source. In the high-temperature alkaline hydrothermal crystallization system, the alkaline solution must slowly diffuse inward through the microporous channels of this carbonized layer, dissolving and continuously releasing the internal silicon framework at an extremely low rate, so that the concentration of active silicon is always maintained below the explosive nucleation critical point. This effectively suppresses disordered secondary nucleation, giving the crystals sufficient time to grow anisotropically around the remaining free template agent, thereby obtaining a uniform 2~4 μm large-grained molecular sieve.

[0014] (2) Construction of inner silicon and outer aluminum gradient and surface acid passivation: Due to the extremely slow release rate of the activated silicon source, a special self-assembly timing effect is triggered. The strongly alkaline solution B added contains a highly active aluminum source, which preferentially combines with the trace amount of active silicon released first at the nucleus growth interface, resulting in significant aluminum-rich characteristics on the periphery of the crystal nucleus and the outer surface of the grain. As crystallization progresses, the silicon continuously released in the middle and later stages gradually deposits and fills the interior of the crystal framework. The final molecular sieve has a Si-OH-Al framework with strong Brønsted acid sites embedded inside the large grains, while the aluminum species exposed on the outer surface mainly exist in the form of weakly acidic aluminum hydroxyl groups (Al-OH), achieving passivation of the Brønsted acid sites on the surface.

[0015] (3) In-situ hydrodynamic scouring of multi-level channels: introducing 300~1200 s under the alkalinity of a specific alkali source (Na2O). -1 The high shear flow field and strong hydrodynamic shear force break the dense network formed by the excessive polymerization of aluminosilicates, guiding the alkaline solution to perform efficient and non-uniform local desilication and dissolution on the growing framework. This results in the in-situ scouring of 30-350 nm highly interconnected sponge-like mesopores within the 2-4 μm ultra-large grains, greatly shortening the diffusion path. Attached Figure Description

[0016] Figure 1 This is the XRD pattern of the ZSM-11 molecular sieve prepared in this invention.

[0017] Figure 2 This is a SEM image of ZSM-11 obtained in this invention.

[0018] Figure 3 This is the OH infrared spectrum of ZSM-11 obtained by this invention. Detailed Implementation

[0019] To enable those skilled in the art to more clearly understand the purpose, technical solution, and advantages of the invention, the invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0020] When the raw materials used in the following examples or comparative examples involve activated silicon sources, the preparation method of the activated silicon source is as follows: Weigh 10.0 g of solid silica powder and mix it thoroughly with 6.0 g of tetrapropylammonium hydroxide (TPAOH) aqueous solution (converted to the mass of free pure TPAOH, accounting for approximately 35% of the total mass of the template agent used for crystallization). Stir and dry at 80 °C to uniformly coat the surface of the solid silica particles with TPAOH. Then, place the coated solid in a tube furnace and introduce a mixture of methane and nitrogen gas with a volume ratio of 1:4. Heat the mixture to 550 °C at a heating rate of 5 °C / min, and heat-treat at this temperature for 2 h. After cooling, collect the mixture to obtain a modified solid activated silicon source with an in-situ porous carbonized layer coated on the surface.

[0021] Example 1

[0022] Preparation of mixed gel: Dissolve 0.25 g of sodium aluminate in 20 g of deionized water at 25 °C to obtain solution A. Add 0.08 g of solid NaOH to solution A and mix thoroughly. Then add the remaining 11.1 g of free TPAOH template agent to obtain strongly alkaline solution B.

[0023] Phase-reversed strong shear mixing: The modified solid activated silicon source prepared in the above preparation example was placed in 15 g of deionized water and stirred in a mechanical stirrer (500 rpm, corresponding to a shear rate of approximately 450 s).-1 The mixture was dispersed under stirring to form a suspension. Then, a shear flow field was activated, and a droplet pump was used to add the strongly alkaline solution B dropwise into the activated silicon source suspension at an extremely slow rate of 1.0 mL / min. After the addition was complete, the mixture was stirred under strong shear at 500 rpm for 4 h at room temperature to obtain the initial mixed gel. The overall molar ratios of the components were: SiO2:Al2O3 = 1:0.015, with added Na2O:SiO2 = 0.03, H2O:SiO2 = 25, and TPAOH:SiO2 = 0.15.

[0024] Hydrothermal crystallization and post-treatment: The mixed gel was transferred to a stainless steel hydrothermal reactor and crystallized at 160 °C for 24 h. After the reaction, the product was centrifuged, washed with deionized water until neutral, and dried at 110 °C overnight. Finally, the dried powder was placed in a muffle furnace and calcined at 550 °C for 6 h in air. During this process, the carbide sacrificial layer coating the silicon source surface and the microporous template agent inside the framework were completely burned off, yielding pure large-grained ZSM-11 molecular sieves. The large-grained ZSM-11 has a grain size of 2.5 μm, and its surface is rich in sponge-like channels with a pore size of 50 nm. The surface Al-OH peak area is significantly higher than the area of ​​the surface with Brønsted acid (Si-OH-Al sites).

[0025] The characterization results of the prepared large-grained ZSM-11 molecular sieve are as follows: Figures 1 to 3 .Depend on Figure 1 The XRD patterns of the ZSM-11 molecular sieves show strong diffraction peaks of the five-membered ring at 8-10 ° and 22-25 °, indicating that all the molecular sieves are ZSM-11 molecular sieves and there are no impurities. Figure 2 These are SEM images of the ZSM-11 molecular sieve prepared in this invention, where (a) and (b) are images at different magnifications. The images show a distinct mesoporous structure within each small crystallite. Figure 3 The image shows the OH spectrum of the ZSM-11 molecular sieve prepared in this invention. It can be seen that the Al-OH signal area on its surface is significantly higher than the Si-OH-Al area that produces Brønsted acid.

[0026] Examples 2 to 6

[0027] Except for adjusting the amount of sodium hydroxide added, changing the mechanical stirring speed, dropping speed, and corresponding gel ratio, the remaining steps are exactly the same as in Example 1. Specific process parameters, product structural parameters, and morphological characteristics are shown in Table 1.

[0028] Comparative Example 1

[0029] This comparative example is used to verify the microporous structure comparison of the products prepared without the addition of additional controlled alkali source. The molecular sieve preparation steps are as follows: 0.341 g of sodium aluminate was dissolved in 35 g of deionized water and mixed evenly to obtain solution A. Without adding additional NaOH solid (i.e., Na2O:SiO2=0.00), 17.27 g of TBAOH template agent and 17.34 g of unmodified conventional tetraethyl orthosilicate (TEOS) liquid were added directly to solution A. The mixture was stirred under conventional magnetic stirring (100 rpm, shear rate approximately 80 s⁻¹). -1 The solution is poured in quickly in one go and stirred at room temperature for 4 hours. Hydrothermal crystallization, separation, and air calcination at 550 °C are then performed using the same process as in Example 1.

[0030] Comparative Example 2

[0031] This comparative example is used to verify the technical effect under the same alkalinity when the activated solid silicon source and the inverted slow dropping process specified in this invention are not used. The chemical ratio of its initial mixed gel (NaOH is used as the alkalinity source, and Na2O:SiO2=0.04) is completely consistent with that of Example 2. The difference is that the mixing process is changed: the surface carbonized activated silicon source is not used, but 17.34 g of unmodified liquid tetraethyl orthosilicate (TEOS) is used directly. In step (3), a high shear flow field is not used, but conventional low-speed magnetic stirring (150 rpm, corresponding to a shear rate of about 120 s) is used. -1 Simultaneously, the TEOS liquid was rapidly poured into solution B, which was rich in alkali and template agent, within 10 seconds. The remaining hydrothermal crystallization and calcination conditions were exactly the same as in Example 2.

[0032] The specific surface area and pore structure parameters of each embodiment and comparative example were obtained by low-temperature nitrogen adsorption measurement, and the results are shown in the table below:

[0033] Table 1

[0034] Example 1 0.03 500 rpm / 1.0 mL / min 92 2.1 55 335 65 0.13 0.12 Rich aluminum hydroxyl / B acid passivation Example 2 0.04 600 rpm / 1.5 mL / min 90 2.5 85 320 86 0.12 0.15 Rich aluminum hydroxyl / B acid passivation Example 3 0.05 800 rpm / 2.0 mL / min 91 2.9 115 315 98 0.12 0.18 Rich aluminum hydroxyl / B acid passivation Example 4 0.06 900 rpm / 2.2 mL / min 88 3.2 135 305 112 0.11 0.21 Rich aluminum hydroxyl / B acid passivation Example 5 0.07 1000 rpm / 2.5 mL / min 86 3.5 180 290 135 0.1 0.24 Rich aluminum hydroxyl / B acid passivation Example 6 0.09 1200 rpm / 3.0 mL / min 85 3.9 280 275 160 0.09 0.29 Rich aluminum hydroxyl / B acid passivation Comparative Example 1 0 100 rpm / pour in all at once 94 1.8 Mesoporous 355 12 0.14 0.02 Predominantly strong β-acids / unpassivated Comparative Example 2 0.04 150 rpm / 15.0 mL / min 78 0.8 ~1.2 The apertures are disordered 245 45 0.08 0.08 Predominantly strong β-acids / unpassivated

[0035] As can be seen from the data comparison in Table 1, this invention achieves significant technological progress compared to existing technologies through its unique process of synergistic control of additional alkalinity and strong shear flow field. Comparing Comparative Example 1 with Examples 1 to 6, it can be seen that without the addition of solid NaOH (alkaline source) and under conventional low-shear stirring (100 rpm), the product exhibits only a traditional pure microporous structure with a mesopore volume of only 0.02 mL / g. However, as the additional Na2O:SiO2 molar ratio increases from 0.03 to 0.09, coupled with a high-speed shear flow field of 500~1200 rpm, the mesopore volume of the product systematically increases from 0.12 mL / g to 0.29 mL / g, and the mesopore area also increases from 65 m² / g. 2 / g increased significantly to 160m 2 / g. This indicates that the unique strong shear flow field of this invention can break the dense network of aluminosilicates, guiding the strong alkaline solution to locally and efficiently dissolve and desilicate the growing crystal framework. This results in the in-situ scouring of highly interconnected, sponge-like hierarchical channels within the large grains, completely solving the technical problem of hindered mass transfer and diffusion of macromolecules within the large grains of traditional ZSM-11 molecular sieves. More importantly, this invention achieves a dual technological breakthrough in controlled grain growth and acid passivation of the outer surface through a combination of slow release of a solid activated silicon source and phase-reversed slow droplet addition via sequential self-assembly.

[0036] A rigorous univariate comparison was conducted between Comparative Example 2 and Example 2 at the same alkalinity. It can be seen that Comparative Example 2, due to the use of a traditional liquid silicon source and its rapid, one-time pouring, resulted in severe secondary nucleation, a sharp drop in crystallinity to 78%, and grain fragmentation and shrinkage to 0.8–1.2 μm, with densely packed strong Broønsted acid sites exposed on the outer surface. In contrast, Example 2 introduced a solid activated silicon source with locally carbonized surface modification, acting as a molecular-level slow-release valve. The B solution was slowly added in reverse phase at a rate of 1.5 mL / min, allowing the crystals to grow to 2.5 μm around the template agent. Because the silicon species were released very slowly, the highly active aluminum source preferentially accumulated at the crystal nucleus interface in the early stages of crystallization. With the continued release of silicon in the middle and later stages, the strong Broønsted acid sites were deeply embedded within the grains, resulting in the outer surface of the grains primarily exposing weakly acidic aluminum hydroxyl groups (achieving complete passivation of the Broønsted acid). This unique physicochemical characteristic enables the large-grain molecular sieve of the present invention to significantly reduce mass transfer resistance in subsequent catalytic reactions by utilizing its internal multi-level pores, and to fundamentally eliminate carbon deposition and coking caused by strong acid sites on the outer surface during the initial condensation phase of cracking, thereby achieving a leapfrog improvement in catalyst life.

Claims

1. A large-grained ZSM-11 molecular sieve, characterized in that, The surface of the large-grained ZSM-11 molecular sieve has a sponge-like pore structure and is passivated by Brønsted acid sites; the grain size of the large-grained ZSM-11 is 2~4 μm, and the pore size in the pores is 30~350 nm.

2. A method for preparing large-grained ZSM-11 molecular sieves, characterized in that, Includes the following steps: 1) Add aluminum source to water at 20-40 ℃ and stir to obtain solution A; 2) Add the alkali source to solution A and mix thoroughly, then add the template agent and stir to obtain solution B; 3) Place the activated silicon source in water and stir to obtain solution C; 4) Add solution B dropwise to solution C and continue stirring at room temperature to 80 ℃ for 2-8 h to obtain a mixed gel; 5) The mixed gel was placed in a hydrothermal crystallization vessel and crystallized at 100-180 °C for 6-48 h, and then dried and calcined to obtain large-grained ZSM-11 molecular sieves with sponge-like pores on the surface and passivated Brønsted acid sites on the surface.

3. The method for preparing large-grain ZSM-11 molecular sieve as described in claim 2, characterized in that: The aluminum source is calculated as Al2O3, the activated silicon source as SiO2, and the alkali source as Na2O. The molar ratio of SiO2 to Al2O3 in the mixed gel is 1:(0.010~0.030), and the molar ratio of Na2O, water, template agent and SiO2 is (0.02~0.12):(20~40):(0.1~0.3).

4. The method for preparing large-grain ZSM-11 molecular sieve as described in claim 2 or 3, characterized in that: The alkaline source is at least one of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium formate, and sodium acetate.

5. The method for preparing large-grain ZSM-11 molecular sieve as described in claim 2 or 3, characterized in that: The aluminum source is selected from at least one of sodium aluminate, aluminum sulfate, aluminum nitrate, aluminum chloride, aluminum isopropoxide, boehmite, and aluminum hydroxide.

6. The method for preparing large-grain ZSM-11 molecular sieve as described in claim 2 or 3, characterized in that: The activation involves in-situ localized surface carbonization modification of the silicon source; the silicon source is selected from at least one of tetraethyl orthosilicate, methyl orthosilicate, silica sol, water glass, silica, and fumed silica.

7. The method for preparing large-grain ZSM-11 molecular sieve as described in claim 6, characterized in that: The preparation steps of the in-situ surface local carbonization modified activated silicon source include: mixing the silicon source with tetrapropylammonium hydroxide accounting for 20%~50% of the total mass of the template agent and drying it so that the tetrapropylammonium hydroxide is uniformly coated on the surface of the silicon source particles; then heat-treating it at 450~650 °C for 1~4 h in a mixed atmosphere containing a safe ratio of methane and nitrogen to form a porous carbonization layer with a slow-release effect in situ on the surface of the silicon source particles; wherein, the volume ratio of methane to nitrogen in the mixed atmosphere is 1:(2~6).

8. The method for preparing large-grain ZSM-11 molecular sieve as described in claim 2 or 3, characterized in that: The template agent is selected from one or a mixture of several of tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, tetrapropylammonium bromide, tetrabutylammonium bromide, and hexadecyltrimethylammonium bromide.

9. The method for synthesizing large-grain ZSM-11 molecular sieve as described in claim 2 or 3, characterized in that: During the stirring process in steps 1) to 3), the stirring speed is controlled at 400~1000 rpm, and the fluid shear rate is 300~1200 s. -1 In step 4), when adding solution B, the dropping rate should be controlled to be 0.5~3.0 mL / min.

Images