Synthesis method of nanocrystalline SSZ-39 molecular sieve

Abstract

The invention discloses a synthetic method of a nanocrystalline SSZ-39 molecular sieve, and belongs to the field of molecular sieve synthesis. The preparation method comprises the following steps: mixing and dissolving an alkali source and water, adding a template agent and a silicon source, continuously stirring until the template agent and the silicon source are completely dissolved, then adding an aluminum source and a seed crystal, and continuously stirring and aging to obtain alkaline gel; then transferring the alkaline gel into a reaction kettle containing a polytetrafluoroethylene lining for sealed crystallization, cooling after crystallization, separating mother liquor, centrifugally washing a solid product to be neutral, and drying and roasting to obtain the SSZ-39 molecular sieve. According to the present invention, the SSZ-39 molecular sieve with the particle size of less than 150 nm is obtained under the condition of the high initial gel silica-alumina ratio, the crystallization time can be significantly reduced, and the nanocrystalline SSZ-39 molecular sieve for selective catalytic reduction can be obtained by using the method, and has characteristics of high relative crystallinity and high specific surface area.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve synthesis, specifically relating to a method for synthesizing nanocrystalline SSZ-39 molecular sieve. Background Technology

[0002] Zeolite molecular sieves are a class of inorganic microporous materials with regular channel structures, widely used in gas adsorption and catalysis. When used as catalysts or adsorbents, the performance of molecular sieves is usually closely related to parameters such as crystal size and morphology. SSZ-39 molecular sieve, first discovered by Zones et al. (US-5958370), is an aluminosilicate molecular sieve with an AEI-type topology. Its internal framework consists of a three-dimensional structure of octagonal (8-R) pores, and it also has double six-membered rings (D6Rs) as secondary structural units. The alternating orientation of the double six-membered rings results in large pear-shaped channels with a pore size of 3.8 Å × 3.8 Å. Due to the unique channel structure of SSZ-39 molecular sieve, it exhibits excellent nitrogen oxide (NOx) resorption capacity. x Its selective catalytic reduction performance and methanol-to-olefins (MTO) performance are considered to be among the most promising small-pore zeolites in recent years.

[0003] The main drawback of SSZ-39 molecular sieves currently used for CO2 adsorption is that they have fewer exposed pores, which restricts the movement of CO2 within the crystal. Nanocrystals, on the other hand, can reduce diffusion restrictions and enhance mass transfer in this process. At the same time, the increased specific surface area increases the number of exposed active sites, thereby improving adsorption performance.

[0004] Public documents ( Chemistry Letters The document (Volume 45, Issue 8, 2016, 919-921) discloses a method for preparing SSZ-39 molecular sieves (~300 nm). Using USY molecular sieves as the sole aluminum source, SSZ-39 molecular sieves with smaller particle sizes can be prepared. However, USY molecular sieves are prone to generating pollutants during the preparation process and are expensive, making them unsuitable for industrial production.

[0005] A method for synthesizing nanoscale SSZ-39 molecular sieves has been disclosed in the published literature (Chem. Commun., 2016, 52, 6072), but the product has a low specific surface area, is not suitable for denitrification reactions, and the synthesis time is as long as nine days, which is not suitable for industrial production.

[0006] CN116199236A discloses a method for preparing SSZ-39 molecular sieves (200~300 nm) by pretreating all aluminum sources. This method involves pretreating Y-type molecular sieves at a temperature of 400~700 nm. oHydrothermal treatment at C20°C can stably prepare nanoscale SSZ-39 molecular sieves via crystallization, but this process consumes a lot of energy, produces products with low silicon-to-aluminum ratios, and is cumbersome, making it unsuitable for industrial production. Summary of the Invention

[0007] The purpose of this invention is to provide a low-cost method for synthesizing nanocrystalline SSZ-39 molecular sieves that requires no complex pretreatment steps and has a suitable synthesis time.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for synthesizing nanocrystalline SSZ-39 molecular sieve includes the following steps: 1) Mix the alkali source with deionized water and stir in a water bath at 25-80℃ until the alkali source is completely dissolved. Then add the template agent and silicon source and continue stirring until completely dissolved. Then add the aluminum source and seed crystals and continue stirring and aging at 25-80℃ to obtain an alkaline gel. 2) The alkaline gel was transferred to a reaction vessel with a polytetrafluoroethylene liner and sealed for crystallization. After crystallization, the mixture was cooled, the mother liquor was separated, the solid product was centrifuged and washed until neutral, and then dried and calcined to obtain SSZ-39 molecular sieve.

[0009] Furthermore, the alkali source is one or a mixture of two of sodium hydroxide and potassium hydroxide.

[0010] Furthermore, the template agent is one or a mixture of two of 1,1,3,5-tetramethylpiperidine hydroxide and 1,1,2,6-tetramethylpiperidine hydroxide.

[0011] Furthermore, the aluminum source is one or a mixture of several of sodium aluminate, sodium sulfate, and aluminum nitrate.

[0012] Furthermore, the silicon source is one or a mixture of several of the following: silica sol water glass, fumed silica, solid silica gel, and sodium silicate.

[0013] Furthermore, the molar ratio of each component in the alkaline gel is: SiO2:Al2O3:MOH:ROH:H2O=1:0.008~0.15:0.48~0.56:0.08-0.1:17~23, and the seed crystal content is 0.5~10wt% of the silicon source; wherein, MOH represents the alkaline source and ROH represents the template agent.

[0014] Furthermore, the aging time is 1 to 24 hours.

[0015] Furthermore, the crystallization temperature is 150~170℃, and the crystallization time is 24~120 hours. The crystallization method is either static crystallization or dynamic crystallization.

[0016] Furthermore, the roasting temperature is 550~650℃, and the roasting time is 4~6 hours.

[0017] Compared with existing SSZ-39 synthesis schemes, the advantages of this invention are as follows: 1. The molar ratio of each component in the alkaline gel of this invention is: SiO2:Al2O3:MOH:ROH:H2O=1:0.008~0.15:0.48~0.56:0.08-0.1:17~23. This ratio of raw materials is key to achieving the specific technical goal of "nanocrystallization." This invention may employ a relatively low alkalinity (template agent ratio). A lower OH⁻ concentration slows down the dissolution and deposition rate of silicates and aluminates, effectively inhibiting the Ostwald ripening process (i.e., the dissolution of small crystals and the growth of large crystals), thus obtaining uniform nanocrystals below 100 nm.

[0018] 2. This invention uses inexpensive and readily available sodium aluminate as the sole aluminum source, which greatly reduces the amount of chemical reagents used in the synthesis of SSZ-39 molecular sieve using molecular sieve as the sole aluminum source. Moreover, the synthesis time is moderate, which has the advantages of reducing synthesis costs and saving energy and reducing emissions.

[0019] 3. Compared with conventionally synthesized SSZ-39 molecular sieves, the SSZ-39 molecular sieves synthesized in this invention have smaller particle size and higher specific surface area. SSZ-39 molecular sieves with more mesoporous structures can be synthesized without the use of mesoporous template agents and post-treatment pore-forming conditions, which can reduce diffusion restriction and enhance the mass transfer process.

[0020] 4. This invention uses sodium aluminate as the sole aluminum source, which improves the crystallization rate of SSZ-39 molecular sieve. It can prepare nanocrystalline SSZ-39 molecular sieve with high crystallinity and high specific surface area at a higher silicon-to-aluminum ratio, in a shorter time, and with less template agent. Attached Figure Description

[0021] Figure 1 The image shows a scanning electron microscope (SEM) image of the SSZ-39 molecular sieve obtained in Example 1.

[0022] Figure 2 The X-ray diffraction pattern of the SSZ-39 molecular sieve obtained in Example 1 is shown.

[0023] Figure 3 The N2 adsorption-desorption curves of the SSZ-39 molecular sieve obtained in Example 1 are shown.

[0024] Figure 4 The image shows a scanning electron microscope (SEM) image of the SSZ-39 molecular sieve obtained in Example 2.

[0025] Figure 5 The image shows a scanning electron microscope (SEM) image of the SSZ-39 molecular sieve obtained in Example 3.

[0026] Figure 6 The image shows a scanning electron microscope (SEM) image of the SSZ-39 molecular sieve obtained in Example 4.

[0027] Figure 7 The image shows a scanning electron microscope (SEM) image of the SSZ-39 molecular sieve obtained in Example 5.

[0028] Figure 8 The image shows a scanning electron microscope (SEM) image of the SSZ-39 molecular sieve obtained in Example 6.

[0029] Figure 9 The image shows a scanning electron microscope (SEM) image of the SSZ-39 molecular sieve obtained in Comparative Example 1.

[0030] Figure 10 The image shows a scanning electron microscope (SEM) image of the SSZ-39 molecular sieve obtained in Comparative Example 2. Detailed Implementation

[0031] To provide a clearer understanding of the technical features, objectives, and beneficial effects of this invention, the technical solution of this invention will now be described in detail with reference to the accompanying drawings. However, this should not be construed as limiting the scope of implementation of this invention. All reagents used in this invention are commercially available reagents. The standard sample is the SSZ-39 molecular sieve synthesized in Comparative Example 1, and the relative crystallinity is defined as 100%.

[0032] The seed crystals used in the examples were SSZ-39 molecular sieves prepared in Comparative Example 1. Example 1

[0033] A method for synthesizing nanocrystalline SSZ-39 molecular sieve, the steps of which are as follows: 3.744 g of sodium hydroxide was added to 31.041 g of deionized water and stirred until dissolved. The mixture was stirred and homogenized at 25-35°C. Then, 11.448 g of a 25% (w / w) solution of 1,1,3,5-tetramethylpiperidine hydroxide (DMDMPOH) was added and stirred for ten minutes. Then, 36 g of a 30% (w / w) silica sol was added dropwise until the silica sol was completely hydrolyzed and homogenized. Finally, 0.3111 g of sodium aluminate and 0.54 g of seed crystals were added and stirred continuously for 2 hours. The final alkaline gel composition had a molar ratio of SiO2: Al2O3: NaOH: DMDMPOH: H2O: seed = 1: 0.009: 0.52: 0.1: 20: 0.05.

[0034] The alkaline gel was transferred to a reaction vessel lined with polytetrafluoroethylene and crystallized in a sealed container at 160°C for 72 hours. After crystallization, the product was subjected to solid-liquid separation. The solid product was washed with deionized water, centrifuged until neutral, and dried overnight in an oven at 100°C. The dried solid was then calcined in a muffle furnace at 600°C with a heating rate of 2°C / min to remove the template agent. After natural cooling, SSZ-39 molecular sieve was obtained.

[0035] XRD analysis showed that the product belongs to SSZ-39 molecular sieve ( Figure 2 The relative crystallinity is 95%, and the scanning electron microscope image is shown below. Figure 1 The results show that the particle size of the obtained crystallized samples is mainly distributed in the range of 60~90 nm, and the N2 adsorption-desorption curves ( Figure 3 The isotherm is of type I and IV, indicating that the sample contains both microporous and mesoporous structures, with a specific surface area of ​​760 m². 2 / g, micropore volume is 0.27 m 2 / g, mesopore volume is 0.08 m 2 / g. Example 2

[0036] A method for synthesizing nanocrystalline SSZ-39 molecular sieve, using the same feeding sequence and crystallization conditions as in Example 1, except that all silicon sources were replaced with solid silica gel. The final gel composition molar ratio was SiO2: Al2O3: NaOH: DMDMPOH: H2O: seed = 1: 0.009: 0.52: 0.1: 20: 0.05. After crystallization, the product was subjected to solid-liquid separation. The solid product was washed with deionized water, centrifuged until neutral, and dried overnight in an oven at 100°C. The dried solid was then calcined in a muffle furnace at 600°C at a heating rate of 2°C / min to remove the template agent. After natural cooling, SSZ-39 molecular sieve was obtained. XRD analysis showed that it belonged to SSZ-39 molecular sieve, with a relative crystallinity of 67.8%. Scanning electron microscopy image (…). Figure 4 The results show that the particle size is mainly distributed in the range of 80~90 nanometers. Example 3

[0037] A method for synthesizing nanocrystalline SSZ-39 molecular sieve, the preparation steps are the same as in Example 1, only some parameters are changed, as follows: 1.92 g of sodium hydroxide was dissolved in 17.23 g of deionized water and stirred until homogeneous at 25-35°C. Then, 6.36 g of a 25% (w / w) solution of 1,1,3,5-tetramethylpiperidine hydroxide (DMDMPOH) was added and stirred for ten minutes. Next, 20 g of a 30% (w / w) silica sol was added dropwise until the silica sol was completely hydrolyzed and homogeneous. Finally, 0.2113 g of sodium aluminate and 0.3 g of seed crystals were added, and the mixture was stirred and aged for 2 hours. The final gel composition molar ratio of SiO2:Al2O3:NaOH:DMDMPOH:H2O:seed = 1:0.011:0.48:0.1:20: 0.05. After crystallization, the product was subjected to solid-liquid separation. The solid product was washed with deionized water, centrifuged until neutral, and dried overnight in an oven at 100℃. The dried solid was then calcined in a muffle furnace at 600℃ with a heating rate of 2℃ / min to remove the template agent. After natural cooling, SSZ-39 molecular sieve was obtained. XRD analysis showed that it belonged to SSZ-39 molecular sieve with a relative crystallinity of 73.4%. Scanning electron microscopy image (…). Figure 5 The results show that the particle size is mainly distributed in the range of 90~150 nanometers. Example 4

[0038] A method for synthesizing nanocrystalline SSZ-39 molecular sieve, the preparation steps are the same as in Example 1, only some parameters are changed, as follows: 3.6 g of sodium hydroxide was dissolved in 26.2512 g of deionized water and stirred until homogeneous at 25-35°C. Then, 9.1584 g of a 25% (w / w) solution of 1,1,3,5-tetramethylpiperidine hydroxide (DMDMPOH) was added and stirred for ten minutes. Next, 36 g of a 30% (w / w) silica sol was added dropwise until the silica sol was completely hydrolyzed and homogeneous. Finally, 0.3111 g of sodium aluminate and 0.54 g of seed crystals were added, and the mixture was stirred and aged for 2 hours. The final gel composition molar ratio was SiO2: Al2O3: NaOH: DMDMPOH: H2O: seed = 1: 0.009: 0.52: 0.08: 20: 0.05. After crystallization, the product was subjected to solid-liquid separation. The solid product was washed with deionized water, centrifuged until neutral, and dried overnight in an oven at 100℃. The dried solid was then calcined in a muffle furnace at 600℃ with a heating rate of 2℃ / min to remove the template agent. After natural cooling, SSZ-39 molecular sieve was obtained. XRD analysis showed that it belonged to SSZ-39 molecular sieve with a relative crystallinity of 90.1%. Scanning electron microscopy image (…). Figure 6 The results show that the particle size is mainly distributed in the range of 50~90 nanometers. Example 5

[0039] A method for synthesizing nanocrystalline SSZ-39 molecular sieve, the preparation steps are the same as in Example 1, only the crystallization time is changed to 36 hours, and it is determined that it belongs to SSZ-39 molecular sieve ( Figure 9 The relative crystallinity is 71.3%, and the scanning electron microscope image is shown below. Figure 7 The results show that the particle size is mainly distributed in the range of 60~100 nanometers. Example 6

[0040] A method for synthesizing nanocrystalline SSZ-39 molecular sieves, the preparation steps are the same as in Example 1, except that the aging temperature is changed to 60℃. It was determined that it belongs to SSZ-39 molecular sieves, with a relative crystallinity of 97.0%. Scanning electron microscopy image (…). Figure 8 The results show that the particle size is mainly distributed in the range of 80~130 nanometers.

[0041] Comparative Example 1 This comparative example uses the crystal transformation method to prepare SSZ-39 molecular sieves. The specific preparation steps are as follows: 1.6 g of sodium hydroxide was added to 10.812 g of a 25% (w / w) solution of 1,1,3,5-tetramethylpiperidine hydroxide and stirred until dissolved. The mixture was thoroughly mixed at 25–35°C. Then, 0.7762 g of deionized water was added, and the mixture was stirred for ten minutes. Next, 18.164 g of a 30% (w / w) silica sol was added dropwise until the silica sol was completely hydrolyzed and thoroughly mixed. Finally, 0.7242 g of USY molecular sieve with a silica-to-alumina ratio of 5.4 was added, and the mixture was stirred and aged for 2 hours. The final gel composition molar ratio was SiO2: Al2O3: NaOH: DMDMPOH: H2O = 1: 0.017: 0.4: 0.17: 12. After crystallization, the product was subjected to solid-liquid separation. The solid product was washed with deionized water, centrifuged until neutral, and dried overnight in an oven at 100℃. The dried solid was then calcined in a muffle furnace at 600℃ with a heating rate of 2℃ / min to remove the template agent. After natural cooling, SSZ-39 molecular sieve was obtained. XRD analysis showed that it belonged to SSZ-39 molecular sieve with a relative crystallinity of 100%. Scanning electron microscopy image (…). Figure 9 The results show that the particle size is mainly distributed in the range of 1 to 6 micrometers.

[0042] Comparative Example 2 This comparative example uses the crystal transformation method to prepare SSZ-39 molecular sieves. The specific preparation steps are as follows: 0.72 g of sodium hydroxide was added to 10.812 g of a 25% (w / w) solution of 1,1,3,5-tetramethylpiperidine hydroxide. After stirring at 25-35°C for ten minutes, 16.107 g of a 40% (w / w) silica sol was added dropwise until the silica sol was completely hydrolyzed and the mixture was homogeneous. The mixture was then placed in an 80°C water bath to evaporate excess water. After natural cooling, 1.3413 g of USY molecular sieve with a silicon-to-aluminum ratio of 10.45 was added and stirred until homogeneous. The final gel composition molar ratio of SiO2:Al2O3:NaOH:DMDMPOH:H2O = 1:0.017:0.18:0.17: 3.3 After crystallization, the product was subjected to solid-liquid separation. The solid product was washed with deionized water, centrifuged until neutral, and dried overnight in an oven at 100℃. The dried solid was then calcined in a muffle furnace at 600℃ with a heating rate of 2℃ / min to remove the template agent. After natural cooling, SSZ-39 molecular sieve was obtained. XRD analysis showed that it belonged to SSZ-39 molecular sieve with a relative crystallinity of 85.1%. Scanning electron microscopy image (…). Figure 10 The results show that the particle size is mainly distributed in the range of 180~250 nanometers.

[0043] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for synthesizing nanocrystalline SSZ-39 molecular sieve, characterized in that, Includes the following steps: 1) Mix the alkali source with water and stir in a water bath at 25-80℃ until the alkali source is completely dissolved. Then add the template agent and silicon source and continue stirring until completely dissolved. Then add the aluminum source and seed crystals and continue stirring and aging at 25-80℃ to obtain an alkaline gel. 2) The alkaline gel was transferred to a reaction vessel with a polytetrafluoroethylene liner and sealed for crystallization. After crystallization, the mixture was cooled, the mother liquor was separated, the solid product was centrifuged and washed until neutral, and then dried and calcined to obtain SSZ-39 molecular sieve.

2. The method for synthesizing nanocrystalline SSZ-39 molecular sieve according to claim 1, characterized in that, The alkaline source is one or a mixture of two of sodium hydroxide and potassium hydroxide.

3. The method for synthesizing nanocrystalline SSZ-39 molecular sieve according to claim 1, characterized in that, The template agent is one or a mixture of two of 1,1,3,5-tetramethylpiperidine hydroxide and 1,1,2,6-tetramethylpiperidine hydroxide.

4. The method for synthesizing nanocrystalline SSZ-39 molecular sieve according to claim 1, characterized in that, The aluminum source is one or a mixture of sodium aluminate, sodium sulfate, and aluminum nitrate.

5. The method for synthesizing a nanocrystalline SSZ-39 molecular sieve according to claim 1, characterized in that, The silicon source is one or a mixture of several of the following: silica sol water glass, fumed silica, solid silica gel, and sodium silicate.

6. The method for synthesizing nanocrystalline SSZ-39 molecular sieve according to claim 1, characterized in that, The molar ratio of each component in the alkaline gel is: SiO2:Al2O3:MOH:ROH:H2O=1:0.008~0.15:0.48~0.56:0.08-0.1:17~23, and the seed crystal content is 0.5~10wt% of the silicon source; wherein, MOH represents the alkaline source and ROH represents the template agent.

7. The method for synthesizing nanocrystalline SSZ-39 molecular sieve according to claim 1, characterized in that, The aging time is 1 to 24 hours.

8. The method for synthesizing nanocrystalline SSZ-39 molecular sieve according to claim 1, characterized in that, The crystallization temperature is 150~170℃, and the crystallization time is 24~120 hours.

9. The method for synthesizing nanocrystalline SSZ-39 molecular sieve according to claim 1, characterized in that, The roasting temperature is 550~650℃, and the roasting time is 4~6 hours.

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