Preparation method of core-shell structure composite molecular sieve

Abstract

The application discloses a preparation method of a core-shell structure composite molecular sieve, and is characterized in that: ZSM-5 molecular sieve is pretreated by using tetraethyl ammonium hydroxide, then a silicon source, an aluminum source and water are added, and a mixture with a molar ratio of SiO2:A12O3:NaOH: tetraethyl ammonium hydroxide: water = 1:(0.04-0.08):(0.09-0.12):(0.1-0.13):(6-9) is obtained, the mixture is subjected to hydrothermal crystallization, and a core-shell structure composite molecular sieve with ZSM-5 molecular sieve as a core and beta molecular sieve as a shell is obtained through recovery. The preparation method has the advantages of low water-silicon ratio, high single-kettle yield and simple preparation process.

Description

Technical Field

[0001] This invention relates to a method for preparing molecular sieves, and more specifically to a method for preparing ZSM-5 and β composite molecular sieves. Background Technology

[0002] ZSM-5 molecular sieve is currently the most important mesoporous zeolite molecular sieve shape-selective catalyst in industry. It has a three-dimensional intersecting pore structure. The ten-membered ring channels parallel to the c-axis are linear, with a diameter of 0.51 × 0.56 nm; the channels parallel to the a-axis are Z-shaped, with a diameter of 0.54 × 0.56 nm. Due to its high silicon content, the silicon-to-aluminum ratio of ZSM-5 molecular sieve can range from 10 to pure silicon. This high silicon-to-aluminum ratio gives ZSM-5 molecular sieve good heat resistance and hydrothermal stability, as well as strong acid resistance and resistance to carbon deposition.

[0003] β-zeolite is a high-silica zeolite with a three-dimensional twelve-membered ring pore structure. Its structural features are a double six-membered ring unit cell structure with two four-membered rings and four five-membered rings. The main channel diameter is 0.56-0.75 nm. It has strong acidity and high framework stability.

[0004] Molecular sieve materials with both mesopores and macropores can be synthesized through different synthetic methods and template agents. These molecular sieves, possessing multiple structures and superimposed functions, can avoid the defects of single-pore structures. The hierarchical pore system can simultaneously provide pores of different sizes, which will greatly help in solving mass transfer problems and has broad application prospects in macromolecular catalysis, heavy oil cracking, catalyst supports, filtration and separation materials, battery materials, and thermal resistance materials.

[0005] The following literature reports the synthesis of ZSM-5@β core-shell composite molecular sieves.

[0006] Tong Weiyi et al. (“Synthesis and Characterization of Core-Shell Composite Molecular Sieves ZSM-5@Nano-β”, Journal of Chemical Research in Chinese Universities) reported that the surface of ZSM-5 core-phase molecular sieve was pretreated with polycationic electrolyte PDDA using ethylamine as a template agent to modulate the Zeta potential of the ZSM-5 surface; nano-β molecular sieves were then attached to the electrically modulated ZSM-5 surface and calcined to serve as seed crystals; then, the sieves were added to a crystallization mother liquor of nano-β shells with a molar ratio of tetraethylammonium hydroxide:SiO2:Al2O3:H2O = 9:25:0.35:800, and the ZSM-5@nano-β core-shell composite molecular sieve was obtained through secondary growth.

[0007] CN112110453A discloses a method for synthesizing ZSM-5@β core-shell composite molecular sieve. The method first synthesizes ZSM-5 zeolite particles using tetrapropylammonium hydroxide as a template agent; then, the ZSM-5 zeolite particles are dispersed in a mixture with a molar ratio of Al₂O₃:SiO₂:tetraethylammonium hydroxide:water = 1:(30-80):(8-30):(500-2000), and some water is evaporated by heating. The mixture is then spray-dried to obtain a solid powder. The solid powder undergoes a three-stage crystallization process, followed by filtration, washing, drying, and calcination to obtain the ZSM-5@β core-shell composite molecular sieve.

[0008] In the preparation technology of ZSM-5@β core-shell structured composite molecular sieve, ZSM-5 precursors are mostly synthesized using amine-containing template agents, and the high water-to-silica ratio results in low single-pot yield and complex preparation process. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the purpose of this invention is to provide a simple and effective method for preparing ZSM-5@β core-shell structured composite molecular sieves that features a low water-to-silicon ratio, high single-batch yield, and minimal complexity.

[0010] Therefore, the preparation method of the core-shell structured composite molecular sieve provided by the present invention is characterized by: pretreating ZSM-5 molecular sieve with tetraethylammonium hydroxide, then adding silicon source, aluminum source and water, and mixing evenly to obtain a mixture with a molar ratio of SiO2:Al2O3:NaOH:tetraethylammonium hydroxide:water = 1:(0.04-0.08):(0.09-0.12):(0.1-0.13):(6-9), and then hydrothermally crystallizing and recovering to obtain a core-shell structured composite molecular sieve with ZSM-5 molecular sieve as the core and β molecular sieve as the shell.

[0011] The ZSM-5 molecular sieve provided by this invention is preferably synthesized using an amine-free method, for example, using ZSM-5 molecular sieve industrially synthesized using the amine-free method at Sinopec Catalyst Qilu Branch as the seed crystal. Although ZSM-5 molecular sieve synthesized using an amine method and calcined to essentially remove the template agent can also be used as the seed crystal, the crystallinity will be somewhat reduced. The preferred silica-alumina ratio of the ZSM-5 molecular sieve is 26-140.

[0012] The preparation method provided by this invention involves modifying the surface of ZSM-5 molecular sieve with tetraethylammonium hydroxide, followed by the addition of an aluminum source. This enriches the tetraethylammonium hydroxide and aluminum source on the ZSM-5 surface, thereby promoting the growth of β-molecular sieves on the ZSM-5 surface. Preferably, the tetraethylammonium hydroxide is a 25-27% by mass solution.

[0013] The preparation method provided by this invention preferably uses low-alkalinity sodium aluminate and boehmite as the aluminum source. Boehmite, as part of the aluminum source, can compensate for the insufficient aluminum source provided by low-alkalinity sodium aluminate. The mass ratio of the components in the low-alkalinity sodium aluminate is Al2O3:NaOH:H2O = 1:(0.8-1.5):(4.8-9.5).

[0014] The preparation method provided by this invention preferably uses silica gel or silica fume as the silicon source. The preferred mass ratio of the ZSM-5 molecular sieve to the silicon source is (0.15-0.5):1, and the silicon source is calculated as SiO2. The preferred order of addition is aluminum source, water, and silicon source.

[0015] The preparation method provided by this invention employs a single-stage hydrothermal crystallization process. Conditions may include, for example, crystallization at 145°C for 72-96 hours in a rotary oven at a rotation speed of 3-10 r / min. The recovered product includes processes such as solid-liquid separation, washing, drying, and calcination. For instance, the solid-liquid separation can be performed using methods well-known in the art, such as centrifugation or filtration; the washing process uses distilled or deionized water; the drying conditions involve drying at 120°C for 3-24 hours in a forced-air drying oven; and the calcination conditions are as follows: a muffle furnace with a programmed temperature increase of 5.8°C / min to 550°C, calcined for 2-4 hours. The conditions for recovering the product are well-known and understood by those skilled in the art, and will not be elaborated upon here.

[0016] The preparation method provided by this invention has a low water-to-silicon ratio, high single-reactor yield, simple preparation process, high crystallinity, and good experimental reproducibility, and is expected to be applied in industry.

[0017] The composite molecular sieve prepared by the method of this invention has the characteristics of an ordered composite structure with ZSM-5 as the core and β-zeolite as the shell. XRD characterization revealed obvious characteristic peaks of β-zeolite near 2θ = 22.5°, and obvious characteristic peaks of ZSM-5 molecular sieve near 2θ = 23° and 24°. SEM characterization showed that granular β-zeolite grew on and covered the surface of the bulk ZSM-5 molecular sieve, and STEM characterization revealed that bulk ZSM-5 molecular sieve was embedded beneath the surface of the granular β-zeolite. XRD, SEM, and STEM characterizations demonstrate that the method of this invention yields a composite molecular sieve with a β-zeolite shell and a ZSM-5 molecular sieve core structure.

[0018] The composite molecular sieve prepared by the method of this invention has the characteristics of an ordered composite with ZSM-5 as the core and β-molecular sieve as the shell. Its multiple structures and superimposed functions can avoid the defects of a single pore structure. The multi-level pore system can simultaneously provide pores of different sizes, which will be of great help in solving mass transfer problems and has broad application prospects in macromolecular catalysis, heavy oil cracking, catalyst supports, filtration and separation materials. Attached Figure Description

[0019] Figure 1 The image shows the XRD pattern of the composite molecular sieve prepared in Example 1.

[0020] Figure 2 The image shows the XRD pattern of the composite molecular sieve prepared in Example 2.

[0021] Figure 3 The image shows the XRD pattern of the composite molecular sieve prepared in Example 3.

[0022] Figure 4 The image shows the XRD pattern of the composite molecular sieve prepared in Example 4.

[0023] Figure 5 The XRD pattern of the molecular sieve prepared in Comparative Example 1 is shown.

[0024] Figure 6 The XRD pattern of the molecular sieve prepared in Comparative Example 2 is shown.

[0025] Figure 7 The XRD pattern of the molecular sieve prepared in Comparative Example 3 is shown.

[0026] Figure 8 The XRD pattern of the molecular sieve prepared in Comparative Example 4 is shown.

[0027] Figure 9 The image shows the SEM image of the composite molecular sieve prepared in Example 1.

[0028] Figure 10 The image shows the SEM image of the composite molecular sieve prepared in Example 2.

[0029] Figure 11 The image shows the SEM image of the composite molecular sieve prepared in Example 3.

[0030] Figure 12 The image shows the SEM image of the composite molecular sieve prepared in Example 4.

[0031] Figure 13 This is a STEM image of the composite molecular sieve prepared in Example 4. Detailed Implementation

[0032] The present invention will be further illustrated by the following examples, but these examples do not limit the scope of the invention.

[0033] In the examples and comparative examples:

[0034] Phase characterization was performed using X-ray diffraction (XRD) on a Philips X'Pert diffractometer with CuKα rays, λ = 0.15048 nm, a scanning range of 5° to 70°, and a scanning rate of 4 (°) / min.

[0035] The morphology of the samples was characterized by scanning electron microscopy (SEM) using a Hitachi S4800 field emission scanning electron microscope with an accelerating voltage of 5 kV. The energy dispersive spectroscopy spectra were collected and processed using Horiba350 software.

[0036] The morphology of the sample was characterized by scanning electron microscopy (STEM). The JEM-ARM200F atomic resolution transmission electron microscope (STEM-HAADF) of Japan Electron Ltd. was used with a resolution of 0.078 nm and an accelerating voltage of 200 kV. The sample suspension was dispersed on the grating.

[0037] Example 1

[0038] 3g of ZSM-5 with a silicon-to-aluminum ratio of 26 was mixed with 21.60g of tetraethylammonium hydroxide solution with a wt% of 25%, and stirred at room temperature for 1 hour. Then, 12.76g of low-biased solution and 0.42g of pseudoboehmite were added, and the mixture was stirred for 30 minutes. Then, 10.36g of water and 22.72g of silica gel were added, and the mixture was stirred until homogeneous to obtain a homogeneous mixture. The molar ratio of various components in the mixture was: SiO2:Al2O3:NaOH:tetraethylammonium hydroxide:water = 1:0.04:0.12:0.11:6.5, the mass ratio of low-biased solution Al2O3:NaOH:H2O = 1:1.5:9.5, and the mass ratio of ZSM-5 molecular sieve to silicon source was 0.15:1.

[0039] The mixture was transferred to a reaction vessel and crystallized at 145℃ for 3 days. After filtration, washing, drying at 120℃ for 5 hours, and calcination at 550℃ for 3 hours, the template agent was removed to obtain the molecular sieve. XRD characterization ( Figure 1 A distinct characteristic peak of β-zeolite was found near 2θ = 22.5°, and distinct characteristic peaks of ZSM-5 zeolite were found near 2θ = 23° and 24°, as determined by SEM characterization. Figure 8 It was found that granular β-zeolites grew on the surface of the blocky ZSM-5 molecular sieve and were covered by it, proving that this molecular sieve has the structural characteristics of β-zeolite as shell and ZSM-5 molecular sieve as core.

[0040] Example 2

[0041] 10g of ZSM-5 with a silicon-to-aluminum ratio of 44 was mixed with 25.53g of tetraethylammonium hydroxide solution (wt%) of 25% and stirred at room temperature for 1 hour. Then, 12.07g of low-biological solution and 1.04g of boehmite were added and stirred for 30 minutes. Then, 20.67g of water and 22.72g of silica gel were added and stirred until homogeneous to obtain a mixture. The molar ratio of various components in the mixture was: SiO2:Al2O3:NaOH:tetraethylammonium hydroxide:water = 1:0.075:0.11:0.13:8.5, the mass ratio of low-biological solution Al2O3:NaOH:H2O = 1:0.8:4.8, and the mass ratio of ZSM-5 molecular sieve to silicon source was 0.5:1.

[0042] The mixture was transferred to a reaction vessel and crystallized at 145℃ for 4 days. After filtration, washing, drying at 120℃ for 3 hours, and calcination at 550℃ for 3 hours, the template agent was removed to obtain the molecular sieve. XRD characterization ( Figure 2 A distinct characteristic peak of β-zeolite was found near 2θ = 22.5°, and distinct characteristic peaks of ZSM-5 zeolite were found near 2θ = 23° and 24°, as determined by SEM characterization. Figure 10 It was found that granular β-zeolites grew on the surface of the blocky ZSM-5 molecular sieve and were covered by it, proving that this molecular sieve has the structural characteristics of β-zeolite as shell and ZSM-5 molecular sieve as core.

[0043] Example 3

[0044] 6g of ZSM-5 with a silicon-to-aluminum ratio of 137 was mixed with 25.53g of tetraethylammonium hydroxide solution (wt%) and stirred at room temperature for 1 hour. Then, 10.63g of low-biological solution and 0.68g of boehmite were added and stirred for 30 minutes. Finally, 12.12g of water and 22.72g of silica gel were added and stirred until homogeneous to obtain a mixture. The molar ratio of various components in the mixture was: SiO2:Al2O3:NaOH:tetraethylammonium hydroxide:water = 1:0.04:0.10:0.13:7. The mass ratio of low-biological solution was Al2O3:NaOH:H2O = 1:1.5:9.5. The mass ratio of ZSM-5 molecular sieve to silicon source was 0.3:1.

[0045] The mixture was transferred to a reaction vessel and crystallized at 145℃ for 4 days. After filtration, washing, drying at 120℃ for 3 hours, and calcination at 550℃ for 3 hours, the template agent was removed to obtain the molecular sieve. XRD characterization ( Figure 3 A distinct characteristic peak of β-zeolite was found near 2θ = 22.5°, and distinct characteristic peaks of ZSM-5 zeolite were found near 2θ = 23° and 24°, as determined by SEM characterization. Figure 11 It was found that granular β-zeolites grew on the surface of the blocky ZSM-5 molecular sieve and were covered by it, proving that this molecular sieve has the structural characteristics of β-zeolite as shell and ZSM-5 molecular sieve as core.

[0046] Example 4

[0047] 10g of ZSM-5 with a silicon-to-aluminum ratio of 137 was mixed with 25.53g of tetraethylammonium hydroxide solution (wt%) and stirred at room temperature for 1 hour. Then, 11.70g of low-biased solution and 0.55g of boehmite were added and stirred for 30 minutes. Finally, 20.31g of water and 22.72g of silica gel were added and stirred until homogeneous to obtain a mixture. The molar ratio of various components in the mixture was: SiO2:Al2O3:NaOH:tetraethylammonium hydroxide:water = 1:0.04:0.11:0.13:8.5. The mass ratio of low-biased solution was Al2O3:NaOH:H2O = 1:1.5:9.5. The mass ratio of ZSM-5 molecular sieve to silicon source was 0.5:1.

[0048] The mixture was transferred to a reaction vessel and crystallized at 145℃ for 4 days. After filtration, washing, drying at 120℃ for 3 hours, and calcination at 550℃ for 3 hours, the template agent was removed to obtain the molecular sieve. XRD characterization ( Figure 4 A distinct characteristic peak of β-zeolite was found near 2θ = 22.5°, and distinct characteristic peaks of ZSM-5 zeolite were found near 2θ = 23° and 24°. These were characterized by SEM. Figure 12 It was discovered that granular β-zeolites grew and covered the surface of the bulk ZSM-5 molecular sieve, as characterized by STEM. Figure 13 It was discovered that granular β-zeolite was embedded with blocky ZSM-5 zeolite. This proves that this zeolite has a structural feature of β-zeolite as the shell and ZSM-5 zeolite as the core.

[0049] Comparative Example 1

[0050] 2g of ZSM-5 with a silicon-to-aluminum ratio of 26 was mixed with 21.60g of tetraethylammonium hydroxide solution (wt%) of 25% and stirred at room temperature for 1 hour. Then, 12.76g of low-biased solution and 0.42g of boehmite were added, and the mixture was stirred for 30 minutes. Next, 10.36g of water and 22.72g of silica gel were added, and the mixture was stirred until homogeneous. The mixture was then placed in a reaction vessel and crystallized at 145℃ for 3 days. After filtration, washing, drying at 120℃ for 5 hours, and calcination at 550℃ for 3 hours, the mixture was characterized by XRD. Figure 5 No characteristic peaks of ZSM-5 molecular sieve were observed.

[0051] In this comparative example, the ratio of ZSM-5 molecular sieve to silicon source is 0.1:1, which is less than 0.15:1 in Example 1. This causes the entire ZSM-5 molecular sieve seed crystal to dissolve during the synthesis process, resulting in the failure of the synthesis of the target product.

[0052] Comparative Example 2

[0053] 15g of ZSM-5 with a silicon-to-aluminum ratio of 137 was mixed with 25.53g of a 25% wt% tetraethylammonium hydroxide solution and stirred at room temperature for 1 hour. Then, 10.63g of a low-polarity solution and 0.68g of pseudoboehmite were added, and the mixture was stirred for 30 minutes. Next, 12.12g of water and 22.72g of silica gel were added, and the mixture was stirred until homogeneous. The mixture was then placed in a reaction vessel and crystallized at 145℃ for 4 days. After filtration, washing, drying at 120℃ for 3 hours, and calcination at 550℃ for 3 hours, the mixture was characterized by XRD. Figure 6 No characteristic peaks of β molecular sieve were observed.

[0054] In this comparative example, the ratio of ZSM-5 molecular sieve to silicon source is 0.75:1, which is greater than the 0.3:1 in Example 3. This leads to the failure of the synthesis of the shell β molecular sieve, and thus the failure of the synthesis of the target product.

[0055] Comparative Example 3

[0056] 20g of ZSM-5 with a silicon-to-aluminum ratio of 137 was mixed with 25.53g of a 25% wt% tetraethylammonium hydroxide solution and stirred at room temperature for 1 hour. Then, 11.70g of a low-polarity solution and 0.55g of pseudoboehmite were added, and the mixture was stirred for 30 minutes. Next, 20.31g of water and 22.72g of silica gel were added, and the mixture was stirred until homogeneous. The mixture was then placed in a reaction vessel and crystallized at 145℃ for 4 days. After filtration, washing, drying at 120℃ for 3 hours, and calcination at 550℃ for 3 hours, the mixture was characterized by XRD. Figure 7 No characteristic peaks of β molecular sieve were observed.

[0057] In this comparative example, the ratio of ZSM-5 molecular sieve to silicon source is 1:1, which is greater than the 0.5:1 ratio in Example 4. This leads to the failure of the synthesis of the shell β molecular sieve, and thus the failure of the synthesis of the target product.

[0058] Comparative Example 4

[0059] 10g of ZSM-5 with a silicon-to-aluminum ratio of 44 and 0.23g of NaCl were added to 100g of 0.5% PDDA solution and stirred for 30min. After filtration and drying at 120℃, the solution was added to 200g of 5% β-zeolite nanocrystal suspension for pre-adhesion for 30min, filtered, and dried at 120℃ to obtain the seed crystals. The seed crystals were then calcined at 540℃ for 5h. 20.20g of silica, 0.38g of sodium aluminate, 66.78g of 26.46% TEAOH solution (wt%), and 1.89g of H2O were added and stirred for 2h. The mixture was then placed in a reactor for hydrothermal crystallization for 3d, filtered, washed, dried at 120℃ for 3h, and calcined at 550℃ for 3h.

[0060] In this comparative example, the synthesis steps reported by Tong Weiyi et al. (“Synthesis and Characterization of Core-Shell Composite Molecular Sieves ZSM-5@Nano-β”, Journal of Chemical Research in Chinese Universities) were adopted. The formulation was identical to the reported one, except that the water-to-silicon ratio was 8.5 (same as in Example 2). XRD characterization ( Figure 8 The absence of characteristic peaks for β-zeolite indicates that the synthesis of shell β-zeolite has failed. In other words, reducing the water-to-silicon ratio to 8.5 will not allow the synthesis of ZSM-5@β core-shell zeolite.

Claims

1. A method for preparing a core-shell structured composite molecular sieve, characterized in that: The ZSM-5 molecular sieve is pretreated with tetraethylammonium hydroxide, then a silicon source, an aluminum source and water are added, and the mixture is uniformly mixed to obtain a mixture with a molar ratio of SiO2:Al2O3:NaOH: tetraethylammonium hydroxide: water = 1: (0.04-0.08): (0.09-0.12): (0.1-0.13): (6-9), which is hydrothermally crystallized and recovered to obtain a core-shell structure composite molecular sieve with ZSM-5 molecular sieve as the core and beta molecular sieve as the shell; the core-shell structure composite molecular sieve is characterized by SEM, and the surface of the blocky ZSM-5 molecular sieve is covered with granular beta molecular sieve; the core-shell structure composite molecular sieve is characterized by STEM, and the surface of the granular beta molecular sieve is embedded with the blocky ZSM-5 molecular sieve.

2. The production method according to claim 1, characterized by, The ZSM-5 molecular sieve is synthesized by an amine-free method.

3. The production method according to claim 1, characterized by, The ZSM-5 molecular sieve has a silicon-aluminum ratio of 26-140.

4. The production method according to claim 1, characterized by, The aluminum source is low-alkalinity sodium metaaluminate and pseudo-boehmite.

5. The production method according to claim 4, characterized by, In the low-alkalinity sodium metaaluminate, the mass ratio of the components is Al2O3:NaOH:H2O = 1: (0.8-1.5): (4.8-9.5).

6. The production method according to claim 1, characterized by, The silicon source is silica gel or white carbon black.

7. The production method according to claim 1, characterized by, The mass ratio of the ZSM-5 molecular sieve to the silicon source is (0.15-0.5): 1, and the silicon source is calculated as SiO2.

8. The production method according to claim 1, characterized by, The tetraethylammonium hydroxide is a 25-27% by mass solution.

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