A method for preparing rod-shaped ZSM-5 molecular sieve

Abstract

The invention discloses a method for preparing a rod-shaped ZSM-5 molecular sieve, comprising the following steps: a step of crystallizing after uniformly mixing tetraethyl orthosilicate, tetrapropylammonium hydroxide, sodium hydroxide and water to obtain a seed; a step of adding aluminum nitrate nonahydrate to a mixed solution of tetraethyl orthosilicate and water adjusted to pH by hydrochloric acid, and obtaining a silicon-aluminum precursor solution after aging; a step of adding a seed to the silicon-aluminum precursor solution and adjusting the solution to neutrality with a sodium hydroxide solution, and obtaining a rod-shaped ZSM-5 molecular sieve after crystallization. Compared with the prior art, the rod-shaped ZSM-5 molecular sieve synthesized by the present invention has a regular morphology, uniform particle size, and does not need to add a special template or modifier, reduces the emission of alkaline wastewater in the separation process and nitrogen oxides in the roasting process, has a simple synthesis process, is low in cost, and has a high solid yield.

Description

Technical Field

[0001] The invention relates to a method for synthesizing a rod-shaped ZSM-5 molecular sieve, and belongs to the technical field of molecular sieve preparation. Background Art

[0002] ZSM-5 molecular sieve is a high-silicon zeolite molecular sieve synthesized by Mobil Corporation of the United States in 1972 (US3702886). Due to its unique pore structure, good thermal stability and strong acidity, it is widely used in adsorption, separation, catalysis and other fields. The morphology of ZSM-5 molecular sieve will affect the diffusion and reaction activity of reactant molecules in the molecular sieve crystal. Therefore, the synthesis of ZSM-5 molecular sieves with special morphology (such as rod-shaped ZSM-5 molecular sieves) has attracted widespread attention from domestic and foreign workers.

[0003] In the prior art, patent document CN109485066A adopts X(C2H5)(CH3)2N(CH2) n N(CH3)2 (C2H5) X (where n is 1-6, X is Br - or Cl - ) as a template to synthesize a rod-shaped ZSM-5 molecular sieve; Patent document CN110422858B adds ethylamine to assist in synthesizing a rod-shaped ZSM-5 molecular sieve. However, the above-mentioned prior art requires the use of special templates or modifiers, which will undoubtedly increase the production cost of the molecular sieve, and the nitrogen oxides generated by the decomposition of the template or modifier during the later calcination process will inevitably cause environmental pollution problems. Therefore, the development of a rod-shaped ZSM-5 molecular sieve synthesized in a neutral environment without special templates or additives is an important research direction for technicians. Summary of the invention

[0004] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for preparing a rod-shaped ZSM-5 molecular sieve. The synthesis system is a neutral environment, no organic template or special additive is used, the preparation cost of the rod-shaped ZSM-5 molecular sieve is reduced, and alkaline wastewater and NO are reduced. x Emission of exhaust gas.

[0005] In order to achieve the above objectives, the present invention adopts the following technical solution: a method for preparing a rod-shaped ZSM-5 molecular sieve, comprising the following steps:

[0006] (1) Seed synthesis

[0007] Tetraethyl orthosilicate, tetrapropylammonium hydroxide, sodium hydroxide and deionized water are fully stirred at a molar ratio of SiO2: TPAOH: NaOH: H2O=25: 3-8: 0-0.5: 500-2500, the solution is aged at 20-90° C. for 5-30 hours, then transferred into a high-pressure reactor, and crystallized at 50-120° C. for 1-7 days to obtain seed crystals with a particle size of 300-1000 nanometers;

[0008] (2) Preparation of silicon-aluminum precursor solution

[0009] Tetraethyl orthosilicate and deionized water were mixed and stirred at a molar ratio of 1:37.5, hydrochloric acid was slowly added dropwise to adjust the pH of the solution to 1.0, and after stirring at room temperature for 1 hour, aluminum nitrate nonahydrate was added according to a molar ratio of 30-500:1 of tetraethyl orthosilicate: aluminum nitrate nonahydrate, and aging was continued for 15 hours;

[0010] (3) Preparation of rod-shaped ZSM-5 molecular sieve

[0011] The seed crystals obtained in step (1) are weighed and slowly added into the silicon-aluminum precursor solution obtained in step (2) in a certain mass ratio, and the mixture is fully stirred at room temperature. A sodium hydroxide solution of not less than 8 mol / L is slowly added dropwise to make it neutral, and the mixture is fully stirred at room temperature. The resulting solution is then transferred to a high-pressure reactor and crystallized at 140-200° C. for 4-12 hours. After the crystallization is completed, the resulting product is centrifugally washed and dried, and then calcined at 400-600° C. for 3-8 hours to obtain a rod-shaped ZSM-5 molecular sieve.

[0012] The rod-shaped ZSM-5 molecular sieve obtained by the method of the present invention has a good crystal structure and a regular morphology, and the length and thickness can be flexibly regulated by adjusting the particle size of the seed crystal in the synthesis method of the present invention. The method of the present invention promotes the synthesis of the rod-shaped ZSM-5 molecular sieve by adding seed crystals. The synthesis method does not use a special template or modifier, which reduces production costs while avoiding the emission of nitrogen oxides produced by the decomposition of the template or modifier during the roasting process, and is environmentally friendly. In addition, unlike the prior art, the present invention synthesizes the ZSM-5 molecular sieve under a neutral environment, reducing the generation of a large amount of alkaline wastewater during centrifugal washing. At the same time, the neutral environment is more conducive to the utilization of silicon-aluminum raw materials, and the solid yield of the synthetic molecular sieve is high, which is very suitable for industrial large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 The X-ray diffraction pattern of the rod-shaped ZSM-5 molecular sieve prepared in Example 1;

[0014] Figure 2 This is a scanning electron microscope photograph of the rod-shaped ZSM-5 molecular sieve prepared in Example 1;

[0015] Figure 3 This is a transmission electron microscope photograph of the rod-shaped ZSM-5 molecular sieve prepared in Example 1. DETAILED DESCRIPTION

[0016] In the method of the present invention, a Japanese Rigaku D / max2500 X-ray diffractometer is used to characterize the crystal structure of the prepared rod-shaped ZSM-5 molecular sieve. The instrument uses Cu target Kα rays (λ=0.154056 nm) as the radiation light source, the tube voltage is 40 kV, and the tube current parameter is 30 mA. Its scanning speed is 20°·min -1 , step length 0.01°, scanning range 5-50°; JEM-2100F field emission transmission electron microscope and JSM-7001F field emission scanning electron microscope were used to test the morphology of the synthesized rod-shaped ZSM-5 molecular sieve. Among them, the sample was fully ground before being characterized by transmission electron microscope, and then an appropriate amount of the sample was dispersed in anhydrous ethanol medium and ultrasonicated for 40 min. After being fully dispersed, the obtained suspension was dropped on the copper mesh carbon support film, and after natural drying, the photo was collected under the condition of electron acceleration voltage of 200 kV; before being characterized by scanning electron microscope, the sample was placed on the test table with conductive glue, and gold was sprayed before testing.

[0017] In order to facilitate the understanding of the present invention, the embodiments of the present invention are further described below by preferred specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. The details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0018] Example 1.

[0019] (1) Seed synthesis

[0020] TPA, tetrapropylammonium hydroxide, sodium hydroxide and deionized water were stirred at a molar ratio of SiO2: TPAOH: NaOH: H2O = 25: 8: 0.1: 550, and the resulting solution was aged at 25°C for 30 hours. Then, the solution was transferred to a high-pressure reactor and crystallized at 100°C for 1 day to obtain seed crystals with a particle size of 340 nanometers.

[0021] (2) Preparation of silicon-aluminum precursor solution

[0022] Tetraethyl orthosilicate and deionized water were fully stirred at a molar ratio of 1:37.5, hydrochloric acid was slowly added dropwise to adjust the pH of the solution to 1.0, and after stirring at room temperature for 1 hour, aluminum nitrate nonahydrate was added at a molar ratio of 30:1 of tetraethyl orthosilicate: aluminum nitrate nonahydrate and aging was continued for 15 hours.

[0023] (3) Preparation of rod-shaped ZSM-5 molecular sieve

[0024] The seed crystals obtained in step (1) were weighed and slowly added to the silicon-aluminum precursor solution obtained in step (2), with the mass ratio of the two being 1:100. After being fully stirred at room temperature, 12 mol / L sodium hydroxide solution was slowly added dropwise to make it neutral, and fully stirred at room temperature, and then the resulting solution was transferred to a high-pressure reactor and crystallized at 170°C for 7 hours. After the crystallization was completed, the resulting product was centrifugally washed and dried, and then calcined at 550°C for 6 hours to obtain a rod-shaped ZSM-5 molecular sieve.

[0025] like Figure 1 As shown, the ZSM-5 molecular sieve synthesized by the method of the present invention has five characteristic diffraction peaks representing the ZSM-5 structure at 2θ=7-10 and 22-25°, indicating that the present invention achieves the synthesis of ZSM-5 molecular sieve in a neutral environment, which is significantly different from the prior art where ZSM-5 molecular sieves are mostly synthesized in a strong alkaline environment. Figure 2 It can be clearly seen that the ZSM-5 molecular sieve synthesized by the method of the present invention is in the shape of regular long rods. Figure 3 It can be further known that the rod-shaped ZSM-5 molecular sieve synthesized in this embodiment has a length of 4 micrometers and a thickness of 400 nanometers.

[0026] Example 2.

[0027] (1) Seed synthesis

[0028] Tetraethyl orthosilicate, tetrapropylammonium hydroxide, sodium hydroxide and deionized water were fully stirred at a molar ratio of SiO2: TPAOH: NaOH: H2O = 25: 7: 0.2: 1000, and the resulting solution was aged at 40°C for 12 hours. Then, the solution was transferred to a high-pressure reactor and crystallized at 90°C for 3 days to obtain seed crystals with a particle size of 500 nanometers.

[0029] (2) Preparation of silicon-aluminum precursor solution

[0030] Tetraethyl orthosilicate and deionized water were fully stirred at a molar ratio of 1:37.5, hydrochloric acid was slowly added dropwise to adjust the pH of the solution to 1.0, and after stirring at room temperature for 1 hour, aluminum nitrate nonahydrate was added at a molar ratio of 130:1 of tetraethyl orthosilicate: aluminum nitrate nonahydrate and aging was continued for 15 hours.

[0031] (3) Preparation of rod-shaped ZSM-5 molecular sieve

[0032] Weigh the seed crystals obtained in step (1) and slowly add them to the silicon-aluminum precursor solution obtained in step (2), with the mass ratio of the two being 3.5:100. After fully stirring at room temperature, slowly drop 8.5 mol / L sodium hydroxide solution to make it neutral, and fully stir at room temperature, then transfer the resulting solution to a high-pressure reactor and crystallize at 180°C for 6 hours. After the crystallization is completed, the resulting product is centrifugally washed and dried, and then calcined at 500°C for 8 hours to obtain a rod-shaped ZSM-5 molecular sieve with a length of 4.5 microns and a thickness of 500 nanometers.

[0033] Example 3.

[0034] (1) Seed synthesis

[0035] TPA, tetrapropylammonium hydroxide, sodium hydroxide and deionized water were fully stirred at a molar ratio of SiO2: TPAOH: NaOH: H2O = 25: 6.5: 0.25: 1300, and the resulting solution was aged at 55°C for 10 hours. Then, the solution was transferred to a high-pressure reactor and crystallized at 80°C for 7 days to obtain seed crystals with a particle size of 690 nanometers.

[0036] (2) Preparation of silicon-aluminum precursor solution

[0037] Tetraethyl orthosilicate and deionized water were fully stirred at a molar ratio of 1:37.5, hydrochloric acid was slowly added dropwise to adjust the pH of the solution to 1.0, and after stirring at room temperature for 1 hour, aluminum nitrate nonahydrate was added at a molar ratio of 210:1 of tetraethyl orthosilicate: aluminum nitrate nonahydrate and aging was continued for 15 hours.

[0038] (3) Preparation of rod-shaped ZSM-5 molecular sieve

[0039] Weigh the seed crystals obtained in step (1) and slowly add them to the silicon-aluminum precursor solution obtained in step (2), with the mass ratio of the two being 5:100. After fully stirring at room temperature, slowly drop a high-concentration sodium hydroxide solution to make it neutral, and fully stir at room temperature. Then transfer the resulting solution to a high-pressure reactor and crystallize it at 200°C for 4 hours. After the crystallization is completed, the resulting product is centrifugally washed and dried, and then calcined at 450°C for 8 hours to obtain a rod-shaped ZSM-5 molecular sieve with a length of 6.5 microns and a thickness of 550 nanometers.

[0040] Example 4.

[0041] (1) Seed synthesis

[0042] TEOS, tetrapropylammonium hydroxide, sodium hydroxide and deionized water were stirred at a molar ratio of SiO2: TPAOH: NaOH: H2O = 25: 4.4: 0.35: 1800, and the resulting solution was aged at 90°C for 5 hours. The solution was then transferred to a high-pressure reactor and crystallized at 120°C for 1 day to obtain seed crystals with a particle size of 850 nanometers.

[0043] (2) Preparation of silicon-aluminum precursor solution

[0044] Tetraethyl orthosilicate and deionized water were fully stirred at a molar ratio of 1:37.5, hydrochloric acid was slowly added dropwise to adjust the pH of the solution to 1.0, and after stirring at room temperature for 1 hour, aluminum nitrate nonahydrate was added at a molar ratio of 320:1 of tetraethyl orthosilicate: aluminum nitrate nonahydrate and aging was continued for 15 hours.

[0045] (3) Preparation of rod-shaped ZSM-5 molecular sieve

[0046] Weigh the seed crystals obtained in step (1) and slowly add them to the silicon-aluminum precursor solution obtained in step (2), with the mass ratio of the two being 7:100. After fully stirring at room temperature, slowly drop 12 mol / L sodium hydroxide solution to make it neutral, and fully stir at room temperature, then transfer the resulting solution to a high-pressure reactor and crystallize at 140°C for 12 hours. After the crystallization is completed, the resulting product is centrifugally washed and dried, and then calcined at 600°C for 5 hours to obtain a rod-shaped ZSM-5 molecular sieve with a length of 7.2 microns and a thickness of 620 nanometers.

[0047] Example 5.

[0048] (1) Seed synthesis

[0049] TEOS, tetrapropylammonium hydroxide, sodium hydroxide and deionized water were fully stirred at a molar ratio of SiO2: TPAOH: NaOH: H2O = 25: 3.1: 0.48: 2500, and the resulting solution was aged at 25°C for 24 hours. Then, the solution was transferred to a high-pressure reactor and crystallized at 100°C for 1 day to obtain seed crystals with a particle size of 1000 nanometers.

[0050] (2) Preparation of silicon-aluminum precursor solution

[0051] TEOS and deionized water were fully stirred at a molar ratio of 1:37.5, hydrochloric acid was slowly added dropwise to adjust the pH of the solution to 6.1, and after stirring at room temperature for 1 hour, aluminum nitrate nonahydrate was added at a molar ratio of EOS:aluminum nitrate nonahydrate of 480:1, and aging was continued for 15 hours.

[0052] (3) Preparation of rod-shaped ZSM-5 molecular sieve

[0053] Weigh the seed crystals obtained in step (1) and slowly add them to the silicon-aluminum precursor solution obtained in step (2), with the mass ratio of the two being 10:100. After fully stirring at room temperature, slowly drop 11 mol / L sodium hydroxide solution to make it neutral, and fully stir at room temperature, then transfer the resulting solution to a high-pressure reactor and crystallize at 160°C for 9 hours. After the crystallization is completed, the resulting product is centrifugally washed and dried, and then calcined at 550°C for 6 hours to obtain a rod-shaped ZSM-5 molecular sieve with a length of 9.3 microns and a thickness of 700 nanometers.

Claims

1. A method for preparing a rod-shaped ZSM-5 molecular sieve, characterized in that Without using a template agent, a rod-shaped ZSM-5 molecular sieve is prepared by adding a seed crystal in a neutral hydrothermal synthesis system, comprising the following steps: (1) Seed Synthesis TEOS, tetrapropylammonium hydroxide, sodium hydroxide and deionized water are mixed in a certain proportion and stirred to form a solution. The solution is aged at 20-90° C. for 5-30 hours and then transferred to a high-pressure reactor for crystallization at 50-120° C. for 1-7 days to obtain seed crystals; wherein the molar ratio of the various materials is SiO2:TPAOH:NaOH:H2O=25:3-8:0-0.5:500-2500; (2) Preparation of silicon-aluminum precursor solution The tetraethyl orthosilicate and deionized water are mixed and stirred at a molar ratio of 1:37.5, and then the pH value of the solution is adjusted to 1.0 with hydrochloric acid. After stirring at room temperature for 1 hour, aluminum nitrate nonahydrate is added and the aging is continued for 15 hours to obtain a silicon-aluminum precursor solution; the molar ratio of the tetraethyl orthosilicate and aluminum nitrate nonahydrate is 30 to 500:1; (3) Preparation of rod-shaped ZSM-5 molecular sieve A certain amount of seed crystals prepared in step (1) is weighed and slowly added to the silicon-aluminum precursor solution prepared in step (2), and the mixture is fully stirred at room temperature. A sodium hydroxide solution with a concentration of not less than 8 mol / L is slowly added dropwise to adjust the mixed solution to neutrality, and the mixture is stirred at room temperature until it is completely mixed. The obtained solution is then transferred to a high-pressure reactor and crystallized at 140 to 200° C. for 4 to 12 hours. After the crystallization is completed, the obtained product is centrifugally washed and dried, and then calcined at 400 to 600° C. for 3 to 8 hours to obtain a rod-shaped ZSM-5 molecular sieve; the mass ratio of the seed crystals to the silicon-aluminum precursor solution is 3 to 10:100.

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