A method for synthesizing pure silica CHA zeolite

Abstract

The present invention relates to a method for synthesizing pure-silica CHA zeolite, belonging to the technical field of zeolite molecular sieve preparation. In this method, ball-milled high-silica SSZ-13 zeolite is added to the initial gel as the initial seed to synthesize CHA zeolite, and then the synthesized ball-milled pure-silica CHA is used as the induced seed to iteratively synthesize pure-silica CHA zeolite with higher crystallinity. Compared with the prior art, the present invention has the characteristics that the use of HF is avoided both in the initial seed synthesis and the subsequent hydrothermal synthesis process, and has good industrial application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of zeolite molecular sieve preparation, and particularly relates to a preparation method of pure silica CHA molecular sieve. Background Art

[0002] In recent years, eight-membered ring molecular sieves (8MR) with a maximum pore diameter of 0.43 nm are ideal molecular sieves for separating CO2 (0.33 nm) and other larger gas molecules (such as N2 (0.364 nm) and CH4 (0.38 nm)), and have been favored by many researchers. For example, CHA, DDR, AEI, etc. Since CHA has a window diameter of 0.38 nm and has a preferential adsorption selectivity for CO2, it is very suitable for gas separation in a CO2-containing system. However, since H2O has a stronger adsorption affinity for CO2, the presence of H2O in the feed stream may weaken this preferential adsorption. Therefore, all-silica CHA (Si-CHA) molecular sieve with superhydrophobicity is a good candidate for separating CO2 in an H2O-containing system, and has broad application prospects in the fields of catalysis, adsorption and separation, etc.

[0003] During the synthesis of pure silica CHA molecular sieve, HF is usually added as a mineralizing agent and synthesized in a viscous solution with H2O / Si less than 7. Due to the great harm of hydrofluoric acid to the human body and the problems of poor uniformity in the synthesis of viscous solution, poor preparation repeatability and complex preparation process, etc., the synthesis of Si-CHA molecular sieve becomes difficult and is not conducive to industrial application. Patent CN104925825A discloses a preparation method of all-silica CHA molecular sieve, and this method uses fluoride ions as a mineralizing agent in the synthesis, which is not conducive to industrial production. Summary of the Invention

[0004] The purpose of the present invention is to hydrothermally synthesize CHA molecular sieve with high-silica SSZ-13 ground seeds as the initial seeds on the basis of the existing technology, and then use this molecular sieve as the induced seeds to iteratively synthesize pure silica CHA molecular sieve with higher crystallinity. Compared with the existing technology, the present invention avoids the use of HF both in the synthesis of the initial seeds and in the subsequent hydrothermal process, without using HF. The high-silica seeds used in the present invention are self-made in the laboratory, and the seeds are calcined and ground after the synthesis of the seeds.

[0005] The present invention provides a synthesis method of pure silica CHA molecular sieve, and the steps are as follows:

[0006] (1) Add the template N, N, N-trimethyl-1-adamantylammonium hydroxide (TMAdaOH), the base source and distilled water into a polytetrafluoroethylene bottle in proportion, and stir at room temperature until it becomes a uniform mixed solution;

[0007] (2) While stirring, continue to add a silicon source to the mixed solution in step (1). After stirring vigorously and evenly, age at room temperature;

[0008] (3) Continue to add ball-milled high-silica SSZ-13 seeds to the solution aged in step (2), and stir at room temperature for 2 - 10 minutes;

[0009] (4) Hydrothermally crystallize the mixed solution obtained in step (3) in a reaction kettle;

[0010] (5) Cool the product obtained in step (4) to room temperature, then centrifuge, wash, and dry to obtain CHA zeolite;

[0011] (6) Calcinate the obtained CHA zeolite to remove the template agent, and then perform ball milling to obtain ball-milled CHA zeolite;

[0012] (7) Continue to repeat steps (1), (2), (3), (4), and (5) to obtain pure-silica CHA zeolite, where the ball-milled high-silica SSZ-13 seeds added in step (3) are replaced with the ball-milled CHA zeolite prepared in step (6) or the ball-milled CHA zeolite in the subsequent iteration of step (6) as the inducing seeds.

[0013] Preferably, the silicon source is ammonium hexafluorosilicate or silica sol, tetraethyl orthosilicate or ammonium fluoride; the base source is ethylenediamine (EDA), triethylamine or sodium hydroxide, potassium hydroxide or ammonia water.

[0014] Preferably, when the base source is ethylenediamine, the initial gel molar ratio of the solution aged in step (2) is SiO2:TMAdaOH:EDA:H2O = (2 - 4):(1 - 2):(8 - 16):(60 - 120).

[0015] Preferably, the amount of seeds added in step (3) is 0.5% of the mass of SiO2.

[0016] Preferably, the aging time in step (2) is 10 - 16 hours.

[0017] Preferably, the crystallization temperature in step (4) is 150 - 170 °C, and the crystallization time is 24 - 36 h.

[0018] Preferably, the rotation speed of the ball mill in step (6) is 300 - 400 r / min, and the ball milling time is 3 - 4 h.

[0019] Preferably, the particle size of the ball-milled high-silica SSZ-13 seeds in step (3) is 200 - 300 nm.

[0020] Beneficial effects

[0021] First, the present invention uses highly siliceous SSZ-13 milled seeds as the initial seeds to hydrothermally synthesize CHA zeolite molecular sieves, and then uses these molecular sieves as the induced seeds to iteratively synthesize highly crystalline pure silica CHA zeolite molecular sieves. Compared with the prior art, the present invention avoids the use of HF both in the synthesis of the initial seeds and in the subsequent hydrothermal process.

[0022] Secondly, the highly siliceous milled seeds of the present invention can direct the synthesis towards the desired zeolite phase, reduce the synthesis time by bypassing the long induction period, and provide more crystal nuclei. The XRD results show no impurity peaks, the purity is 100%, and the crystallinity of the iteratively synthesized molecular sieve is greater than that of the initially synthesized Si-CHA molecular sieve. The ICP results show that the Si / Al of the iterative seeds is >2000. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 XRD pattern of the highly siliceous SSZ-13 seeds prepared by the present invention;

[0024] Figure 2 SEM image of the highly siliceous SSZ-13 seeds prepared by the present invention;

[0025] Figure 3 XRD pattern of the pure silica CHA molecular sieve prepared in Example 1 of the present invention;

[0026] Figure 4 SEM image of the pure silica CHA molecular sieve prepared in Example 1 of the present invention;

[0027] Figure 5 XRD pattern of the pure silica CHA molecular sieve prepared in Example 2 of the present invention;

[0028] Figure 6 SEM image of the pure silica CHA molecular sieve prepared in Example 2 of the present invention;

[0029] Figure 7 XRD pattern of the pure silica CHA molecular sieve prepared in Example 3 of the present invention;

[0030] Figure 8 SEM image of the pure silica CHA molecular sieve prepared in Example 3 of the present invention;

[0031] Figure 9 XRD pattern of the molecular sieve prepared in Example 4 of the present invention;

[0032] Figure 10 SEM image of the molecular sieve prepared in Example 4 of the present invention;

[0033] Figure 11 XRD pattern of the molecular sieve prepared in Comparative Example 1 of the present invention;

[0034] Figure 12 SEM image of the molecular sieve prepared in Comparative Example 1 of the present invention. Detailed implementation manners

[0035] Preparation of high-silica SSZ-13 seeds

[0036] (1) Add 0.0530 g of aluminum hydroxide, 0.529 g of sodium hydroxide, and 10.735 g of N,N,N-trimethyl-1-adamantylammonium hydroxide into a polytetrafluoroethylene bottle, heat and stir in a water bath until the aluminum hydroxide is completely dissolved, and then stop the water bath;

[0037] (2) After cooling to room temperature, add 36.268 g of water and continue to stir;

[0038] (3) Dropwise add 10.014 g of silica sol to the above-mentioned stirred solution, heat in a water bath until the solution becomes clear and transparent, stop the water bath, and age for 12 h. The initial molar ratio of the obtained sol is SiO2:TMAdaOH:NaOH:Al(OH)3:H2O = 105:20:23.62:1.05:4400;

[0039] (4) Load the aged solution from step (3) into a reaction kettle and crystallize at 160 °C for 96 h;

[0040] (5) After cooling the reaction kettle, centrifuge, wash, and dry the obtained solution;

[0041] (6) Calcinate the dried product at 550 °C for 12 h and ball mill for 3 h to obtain the ball-milled high-silica SSZ-13 seeds (the XRD and SEM images are shown in Figure 1 and Figure 2 ).

[0042] Example 1

[0043] Preparation of pure-silica CHA molecular sieve

[0044] (1) Add 21.134 g of N,N,N-trimethyl-1-adamantylammonium hydroxide, 11.149 g of water, and 12.121 g of ethylenediamine into a polytetrafluoroethylene bottle, and stir for 10 minutes to obtain a uniformly mixed solution;

[0045] (2) Add 9.089 g of ammonium hexafluorosilicate to the above-mentioned solution under stirring, stir vigorously, and age for 12 h. Calculate by converting the silicon source to SiO2. The initial composition of the obtained sol is SiO2:TMAdaOH:EDA:H2O = 1:0.5:4:30;

[0046] (3) Add 0.045 g of ball-milled high-silica SSZ-13 molecular sieve as seeds to the aged solution, and stir for 10 minutes. The addition amount of the seeds is 0.5% based on the amount of SiO2;

[0047] (4) Pour the above-mentioned uniformly mixed reaction solution into a reaction kettle and crystallize it at 160 °C for 48 h;

[0048] (5) After the reaction kettle is cooled to room temperature, centrifuge, wash, and dry the obtained solution;

[0049] (6) Calcinate the dried product in a muffle furnace at 550 °C for 12 h, and mill the calcined product for 3 hours to obtain milled CHA zeolite;

[0050] (7) Repeat the above steps (1)(2)(3)(4)(5)(6) once, and only replace the high-silica milled SSZ-13 seed crystal in step (3) with milled Si-CHA zeolite as the induced seed crystal, and hydrothermal synthesis can obtain pure-silica CHA zeolite with higher crystallinity (the XRD and SEM diagrams are shown in Figure 3 and Figure 4 ).

[0051] Example 2

[0052] The difference from Example 1 is that: in step (4), it is crystallized at 160 °C for 72 h.

[0053] Example 3

[0054] The difference from Example 1 is that: in step (4), it is crystallized at 160 °C for 96 h (the XRD and SEM diagrams are shown in Figure 5 and Figure 6 ).

[0055] Example 4

[0056] (1) Add 14.089 g of N,N,N-trimethyl-1-adamantylammonium hydroxide, 28.567 g of water, and 8.080 g of ethylenediamine to a polytetrafluoroethylene bottle, stir for 10 minutes to obtain a uniformly mixed solution;

[0057] (2) Add 6.059 g of ammonium hexafluorosilicate to the above solution under stirring, stir vigorously, and age for 12 h. Calculate based on converting the silicon source to SiO2. The initial composition of the obtained sol is SiO2:TMAdaOH:EDA:H2O = 1:0.5:4:47;

[0058] (3) Add 0.030 g of milled high-silica SSZ-13 zeolite as a seed crystal to the aged solution, stir for 10 minutes, and the addition amount of the seed crystal is 0.05% based on the amount of SiO2;

[0059] (4) Pour the above-mentioned uniformly mixed reaction solution into a reaction kettle and crystallize it at 160 °C for 48 h;

[0060] (5) After the reaction kettle is cooled to room temperature, the obtained solution is centrifuged, washed, and dried;

[0061] (6) The dried product is calcined in a muffle furnace at 550 °C for 12 h to obtain molecular sieve.

[0062] Comparative Example 1

[0063] The difference from Example 1 is that the seed added in step (3) is the un-milled SSZ-13 molecular sieve as the seed;

[0064] From Figure 11 the XRD, it can be seen that the molecular sieve prepared in Comparative Example 1 has no CHA peak pattern and is amorphous SiO2. From Figure 12 the SEM image, it can be seen that most of the synthesized molecular sieves are amorphous in morphology, with only a few square particles. The XRD ( Figure 3 ) of the Si-CHA prepared in Example 1 shows a better peak pattern, which conforms to the standard peak pattern of CHA and the crystallinity of the iteratively synthesized seed is higher. From the SEM image ( Figure 4 ), it can be seen that the synthesized molecular sieve has a standard square morphology and a higher purity. However, in the formulation of Example 4, H2O / Si is different. As H2O / Si increases, the molecular sieve synthesized by this formulation is not of the CHA configuration.

Claims

1. A method for synthesizing pure silica CHA zeolite, the steps are as follows: (1) Add the template N, N, N-trimethyl-1-adamantylammonium hydroxide (TMAdaOH), the alkali source and distilled water into a polytetrafluoroethylene bottle in proportion, and stir at room temperature until it becomes a homogeneous mixed solution; (2) While stirring, continue to add the silicon source to the mixed solution in step (1), stir vigorously until uniform, and age at room temperature; (3) Continue to add the ball-milled high-silica SSZ-13 seed crystals to the solution aged in step (2), and stir at room temperature for 2-10 minutes; (4) Hydrothermally crystallize the mixed solution obtained in step (3) in a reaction kettle; (5) Cool the product obtained in step (4) to room temperature, then centrifuge, wash and dry to obtain CHA zeolite; (6) Calcinate the obtained CHA zeolite to remove the template agent, and then perform ball milling to obtain ball-milled CHA zeolite; (7) Continue to repeat steps (1), (2), (3), (4), (5) to obtain pure silica CHA zeolite, where the ball-milled high-silica SSZ-13 seed crystals added in step (3) are replaced with the ball-milled CHA zeolite prepared in step (6) or the subsequent step (6) after iteration as the inducing seed crystals; the rotation speed of the ball mill in step (6) is 300-400 r / min, and the ball milling time is 3-4 h; the particle size of the ball-milled high-silica SSZ-13 seed crystals in step (3) is 200-300 nm.

2. The synthesis method of a pure silicon CHA molecular sieve according to claim 1, characterized in that, The silicon source is ammonium hexafluorosilicate or silica sol, tetraethyl orthosilicate or ammonium fluoride; the alkali source is ethylenediamine (EDA), triethylamine or sodium hydroxide, potassium hydroxide or ammonia water.

3. A method for synthesizing pure-silica CHA zeolite according to claim 1 or 2, characterized in that, When the alkali source is ethylenediamine, the initial gel molar ratio of the solution aged in step (2) is SiO2: TMAdaOH: EDA: H2O = (2-4): (1-2): (8-16): (60-120).

4. The synthesis method of a pure silicon CHA molecular sieve according to claim 1, wherein, The amount of seed crystals added in step (3) is 0.5% of the mass of SiO2.

5. A method for synthesizing pure silicon CHA zeolite according to claim 1, characterized in that, The aging time in step (2) is 10-16 hours.

6. A method for synthesizing pure-silica CHA zeolite according to claim 1, characterized in that, The crystallization temperature in step (4) is 150-170 °C, and the crystallization time is 24-36 h.

Images