A method for preparing silicalite-1 and ZSM-5 molecular sieves

Abstract

The application discloses a preparation method of silicalite-1 molecular sieve and ZSM-5 molecular sieve. The molecular sieve has uniform crystal grain size and small crystal grain, which is beneficial to mass transfer and diffusion in the silicalite-1 molecular sieve and ZSM-5 molecular sieve crystals; the regular molecular sieve crystal grain is more beneficial to keeping the structure stable of the molecular sieve under the use condition and improving the service life of the molecular sieve. The method for synthesizing the silicalite-1 molecular sieve and the ZSM-5 molecular sieve by using guanidino acetic acid as an additive can prepare the silicalite-1 molecular sieve and the ZSM-5 molecular sieve crystals with the crystal grain size distribution of 140-160 nm and regular crystal shape.

Description

TECHNICAL FIELD

[0001] The present application relates to a preparation method for synthesizing small crystal grain silicalite-1 and ZSM-5 molecular sieves with uniform size and regular crystal form under fluorine-free conditions, and belongs to the technical field of molecular sieve synthesis and preparation. BACKGROUND

[0002] Full-silicon silicalite-1 molecular sieves and ZSM-5 molecular sieves are widely used as adsorbents, catalysts and catalyst carriers. Since small-sized molecular sieve crystals have shorter molecular sieve channels, which are beneficial to the mass transfer and diffusion of reactant molecules inside the molecular sieve crystals, the synthesis of small crystal grain molecular sieves has become a research hotspot in the field of molecular sieve synthesis. However, for the same kind of molecular sieve, when its crystal grain is too small, its thermal stability and hydrothermal stability will be poor. The thermal stability and hydrothermal stability of the molecular sieve are directly related to the crystal size of the molecular sieve, and also have a certain relationship with the perfection degree of the crystal structure of the molecular sieve. The more regular and less defective the molecular sieve crystal is, the more beneficial to the above-mentioned structural stability of the molecular sieve. When the molecular sieve is used as a catalyst, the surface defect sites (silicon hydroxyl nests) often become the points of initiating carbon deposition on the catalyst bed, and these points are more likely to exist on the surface of small crystal grains with irregular shape. Therefore, to make the molecular sieve catalyst have a longer service life, it is not only required that the corresponding molecular sieve crystal particles have a smaller size, but also more uniform particle size and regular crystal form.

[0003] So far, there is no suitable method reported to synthesize the molecular sieve with the above-mentioned crystal grain characteristics.

[0004] Making the crystallization process at a lower temperature, adding crystal seeds of a suitable form to the raw materials for synthesizing the molecular sieve, and treating the raw materials for synthesizing the molecular sieve at a lower temperature before the crystallization process to generate more crystal nuclei are the three major elements for preparing small crystal grain and uniform molecular sieve crystals known so far. However, a large number of research and practice results show that even if these three elements are applied at the same time, the above-mentioned synthesis target cannot be achieved without developing new innovative methods.

[0005] Xue et al. used S-1 suspension as crystal seeds and tetraalkylammonium bromide as template agent to synthesize ZSM-5 molecular sieves with a size of 100-400 nm (Solid State Sciences, 2012, 14(4):409).

[0006] Xue et al. achieved the controllable adjustment of crystal size in the range of 150-1000 nm by adjusting the amount of seed suspension. They found that the larger the amount of seed suspension added in the synthesis mixture, the smaller the crystal size of the obtained zeolite. However, they synthesized ZSM-5 zeolite at 175℃ by adding 10wt% of seed suspension in the synthesis mixture, although the crystal size range was 150-200 nm, the crystals were adhered to each other (Microporous and Mesoporous Materials, 2012, 156:29).

[0007] Zhang et al. synthesized ZSM-5 zeolite with size range of 500-700 nm by vapor-assisted crystallization method using 3-aminopropyltrimethoxysilane, [3-(2-aminoethyl) aminopropyl]-trimethoxysilane, aniline propyl trimethoxysilane as organic template. The crystal not only has large size, but also is micro-mesoporous hierarchical ZSM-5 zeolite aggregate (New J. Chem., 2014, 38(12):5808).

[0008] In order to synthesize ZSM-5 zeolite crystals with regular crystal shape and small crystal size, Qin et al. used nano S-1 zeolite as seed crystal and fluoride as reaction medium to synthesize ZSM-5 zeolite crystals with regular crystal shape, twin crystals, uniform particle size and size of 3 μm. However, when the synthesis conditions were optimized to reduce the crystal size to close to 150 nm, ZSM-5 zeolite nanocrystals adhered to each other were obtained (Advanced Functional Materials, 2014, 24(2):257).

[0009] Xiong et al. prepared ZSM-5 nanocrystal aggregates composed of 50-100 nm nanocrystals by aerosol-assisted method (RSC Adv., 2016, 6, 101365).

[0010] Xu et al. studied the effect of crystallization temperature on the synthesis of ZSM-5 zeolite. They found that as the crystallization temperature decreased from 170℃ to 100℃, the crystal size decreased from 140 nm to 103 nm. However, they found that the ZSM-5 zeolite synthesized at 100℃ had low crystallinity, poor crystal shape and many framework defects (Chemistry, 2018, 24(50):13136).

[0011] Zhang et al. synthesized ZSM-5 molecular sieve with L-lysine as additive, the size of which was 100-200 nm, but the crystal shape was not regular, and the crystal morphology was virus-like particles (Journal of the American Chemical Society, 2019, 141:3772).

[0012] Jia et al. synthesized ZSM-5 molecular sieve with 3-glycidoxypropyltrimethoxysilane as raw material, and the obtained ZSM-5 molecular sieve was a hierarchical ZSM-5 aggregate composed of small crystals (RSC Adv., 2020, 10, 29618).

[0013] Zhang et al. synthesized ZSM-5 crystals with a grain size range of 90-150 nm at 90℃ by reducing the crystallization temperature and adding seeds. Not only the grain size range was large, but the crystal morphology also showed irregular spherical shape (Chemical Engineering Journal 382 (2020) 122913).

[0014] Zhang et al. synthesized ZSM-5 crystal aggregates with a size of 50-110 nm and abundant intracrystalline mesopores using bis-1,6-(tripropylammonium) hexaethylene as a template (Crystals 2021, 11, 1247).

[0015] Zong et al. synthesized rod-like ZSM-5 crystals with a particle size distribution range of 300-500 nm by adding recycled mother liquor and S-1 seeds to the synthesis mixture (New Journal of Chemistry, 2021, 45(19):8582).

[0016] Zhao et al. reported a method for synthesizing ZSM-5 crystals by water / toluene phase transfer, and the obtained ZSM-5 crystals were ZSM-5 aggregates with a size of about 100-200 nm stacked by nanocrystals (Catalysts 2022, 12, 1216).

[0017] Although people have been trying to prepare ZSM-5 molecular sieve and Silicalite-1 molecular sieve with smaller size, uniform particle size and regular crystal shape for many years, as described above, no better method has been given to achieve this synthesis goal. The present invention proposes that guanidino acetic acid is used as an additive to synthesize silicalite-1 molecular sieve and ZSM-5 molecular sieve, and silicalite-1 molecular sieve and ZSM-5 molecular sieve crystals with a crystal size distribution of 140-160 nm and regular crystal shape can be prepared. SUMMARY

[0018] The preparation method of the silicalite-1 and ZSM-5 molecular sieves with uniform size, regular crystal form and small crystal grains synthesized by adding guanidoacetic acid as an additive, refers to adding guanidoacetic acid with a certain ratio into a raw material mixture for synthesizing silicalite-1 or ZSM-5 molecular sieves under a fluoride-free condition.

[0019] Further, in the above technical solution, the hydrothermal crystallization temperature is 160-200 ℃.

[0020] Further, in the above technical solution, the raw material mixture for synthesizing the silicalite-1 molecular sieve comprises a silicon source, and the silicon source is silicate, sodium silicate or silica sol.

[0021] Further, in the above technical solution, the raw material mixture for synthesizing the ZSM-5 molecular sieve comprises an aluminum source and a silicon source, the aluminum source is aluminum nitrate or sodium aluminate, and the silicon source is silicate, sodium silicate or silica sol.

[0022] Further, in the above technical solution, the atomic ratio of Al / Si in the raw material mixture for synthesizing the ZSM-5 molecular sieve is not higher than 0.08 and not equal to 0.

[0023] Further, in the above technical solution, the raw material mixture for synthesizing the molecular sieve comprises a silicon source, and the guanidoacetic acid is added in a molar ratio of 0.2-0.4 of guanidoacetic acid to silicon. The effective addition ratio of guanidoacetic acid is 0.2-0.4 mol of guanidoacetic acid per 1 mol of the silicon source, and the most effective and more economical ratio is 0.25-0.3 mol of guanidoacetic acid per 1 mol of the silicon source.

[0024] The preparation method for synthesizing the silicalite-1 and ZSM-5 molecular sieves with uniform size, regular crystal form and small crystal grains by using the method of the present application, the silicon source can be silicate (Example 1), sodium silicate (Example 2) or silica sol (Example 3), and when an aluminum source is needed, the aluminum source can be aluminum nitrate (Example 1) or sodium metaaluminate (Example 4). The obtained molecular sieve can be silicalite-1 (Al / Si=0) or ZSM-5 (Al / Si≠0) according to the atomic ratio of Al / Si in the raw material mixture of the molecular sieve.

[0025] The synthesis of the silicalite-1 or ZSM-5 molecular sieves with uniform size, regular crystal form and small crystal grains by using the method of the present application is not affected by the order of adding guanidoacetic acid into the raw material mixture of the molecular sieve, and the guanidoacetic acid can be added during the mixing of the raw materials of the molecular sieve (Examples 1-4) or before the crystallization of the mixture of the raw materials of the molecular sieve (Example 5).

[0026] The silicalite-1 and ZSM-5 molecular sieves of uniform size and perfect crystal form synthesized by the method of the present application are influenced to some extent by the low temperature pretreatment process before crystallization. When the low temperature pretreatment process (stirring at 80°C for 24h) is carried out before crystallization, the size of the molecular sieve crystals obtained is 140-160nm (Examples 1-6), and when the low temperature pretreatment process is not carried out before crystallization, the size of the molecular sieve crystals obtained is 200-300nm (Example 7).

[0027] The silicalite-1 and ZSM-5 molecular sieves of uniform size and perfect crystal form synthesized by the method of the present application are influenced to some extent by the low temperature pretreatment process before crystallization. When the low temperature pretreatment process (stirring at 80°C for 24h) is carried out before crystallization, the size of the molecular sieve crystals obtained is 140-160nm (Examples 1-6), and when the low temperature pretreatment process is not carried out before crystallization, the size of the molecular sieve crystals obtained is 200-300nm (Example 7).

[0028] Advantages

[0029] The significant advantage of the present application is that by using the method of the present application, i.e. by adding a certain amount of guanidino acetic acid to the synthesis raw materials of silicalite-1 and ZSM-5 molecular sieves, the uniformity of the molecular sieve crystals obtained and the regularity of the molecular sieve crystal structure can be significantly improved, the size of the molecular sieve crystals can be reduced, and the application performance of the corresponding molecular sieves can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 SEM electron micrographs of ZSM-5 molecular sieve crystals obtained from different examples and comparative examples, wherein,

[0031] A, B: obtained from Comparative Example 1; C, D: obtained from Example 1;

[0032] E, F: obtained from Example 7; G, H: obtained from Example 9;

[0033] Figure 2 SEM electron micrographs of ZSM-5 molecular sieve crystals or silicalite-1 molecular sieve crystals obtained from different comparative examples, wherein,

[0034] I: obtained from Comparative Example 3; J: obtained from Comparative Example 4;

[0035] K: obtained from Comparative Example 6; L: obtained from Comparative Example 7. DETAILED DESCRIPTION

[0036] The embodiments and significant benefits of the present application are illustrated by the following examples and comparative examples.

[0037] (1) Synthesis of ZSM-5 molecular sieve

[0038] Example 1

[0039] To a solution containing 0.1130 g of Al(NO3)3-9H2O and 1.16 g of water, 8.8 ml of TPAOH (25 wt%) solution was added at room temperature with vigorous stirring for 5 min, and then 5.0094 g of tetraethyl orthosilicate was slowly added dropwise to the above solution with stirring for 5 h. After adding 0.8434 g of guanidinoacetic acid, the mixture was stirred at 80°C for 24 h. Finally, the resulting mixture was charged into a 50 ml autoclave and crystallized at 170°C for 12 h at a rotation speed of 20 rpm. The solid obtained by crystallization was separated by centrifugation, dried, and calcined in air at 550°C for 4 h to obtain the target ZSM-5 molecular sieve.

[0040] Example 2

[0041] Example 1 was repeated, except that 5.095 g of Na2SiO3-5H2O was used instead of 5.0094 g of tetraethyl orthosilicate in Example 1, and the addition of 1.16 g of water was omitted.

[0042] Example 3

[0043] Example 1 was repeated, except that 4.800 g of silica sol (SiO2 content: 30 wt.%) was used instead of 5.0094 g of tetraethyl orthosilicate in Example 1, and the addition of 1.16 g of water was omitted.

[0044] Example 4

[0045] Example 1 was repeated, except that 0.0247 g of NaAlO2 was used instead of 0.1130 g of Al(NO3)3-9H2O in Example 1, and the amount of water added was changed from 1.16 g to 1.21 g.

[0046] Example 5

[0047] Example 1 was repeated, except that the addition of guanidinoacetic acid was performed after stirring at 80°C for 23 h. After adding guanidinoacetic acid, the stirring was continued for 1 h, and then the autoclave was charged for crystallization.

[0048] Example 6

[0049] Example 1 was repeated, but the 8.8 ml TPAOH (25 wt%) solution therein was replaced by a mixture obtained by adding 0.14 g of calcined nanosilicalite-1 molecular sieve as seed to 8.8 ml of TPAOH (25 wt%) solution and stirring at 80°C for 10 h.

[0050] Example 7

[0051] Example 1 was repeated, but the 8.8 ml TPAOH (25 wt%) solution therein was replaced by a mixture obtained by adding 0.14 g of calcined nanosilicalite-1 molecular sieve as seed to 8.8 ml of TPAOH (25 wt%) solution and stirring at 80°C for 10 h.

[0052] Example 8

[0053] Example 1 was repeated, but the crystallization temperature was changed to 190°C and the crystallization time was changed to 8 h.

[0054] Example 9

[0055] Example 1 was repeated, but the 8.8 ml TPAOH (25 wt%) solution therein was replaced by a mixture obtained by adding 0.14 g of calcined nanosilicalite-1 molecular sieve as seed to 8.8 ml of TPAOH (25 wt%) solution and stirring at 80°C for 10 h.

[0056] Comparative Example 1

[0057] Example 1 was repeated, but the guanidinoacetic acid was not added.

[0058] Comparative Example 2

[0059] Example 8 was repeated, but the pretreatment process of stirring at 80°C for 24 h was retained, but the guanidinoacetic acid was not added.

[0060] Comparative Example 3

[0061] Example 1 was repeated, but 0.1808 g of Al(NO3)3-9H2O was added and tetramethylguanidine was used instead of guanidinoacetic acid in the same molar amount.

[0062] Comparative Example 4

[0063] Example 10 was repeated except that no guanidinoacetic acid was added.

[0064] (2) Synthesis of Silicalite-1 molecular sieve

[0065] Example 10

[0066] Example 10 was repeated except that no guanidinoacetic acid was added.

[0067] Comparative Example 5

[0068] Example 10 was repeated except that no guanidinoacetic acid was added.

[0069] Comparative Example 6

[0070] Example 10 was repeated except that no guanidinoacetic acid was added, but instead one third of the molar amount of guanidinoacetic acid used in Example 10 was replaced by dodecylguanidine hydrochloride.

[0071] Comparative Example 7

[0072] Example 10 was repeated except that no guanidinoacetic acid was added, but instead one half of the molar amount of guanidinoacetic acid used in Example 10 was replaced by polyhexamethylene biguanide hydrochloride.

[0073] Table 1 compares the crystallite size and crystallinity of the molecular sieves obtained in each of the examples and comparative examples

[0074]

Claims

1. A method for preparing a silicalite-1 molecular sieve, characterized by: The guanidino acetic acid is added to a raw material mixture for synthesizing a molecular sieve by a hydrothermal method; the raw material mixture for synthesizing the molecular sieve comprises a silicon source and TPAOH; and the amount of the guanidino acetic acid added is such that the molar ratio of guanidino acetic acid to silicon is 0.2-0.

4.

2. A method for preparing a ZSM-5 molecular sieve, characterized by: The guanidino acetic acid is added to a raw material mixture for synthesizing a molecular sieve by a hydrothermal method; the raw material mixture for synthesizing the molecular sieve comprises a silicon source; and the amount of the guanidino acetic acid added is such that the molar ratio of guanidino acetic acid to silicon is 0.2-0.

4.

3. The method of claim 1 or 2, wherein the method comprises: The hydrothermal crystallization temperature is 160-200 DEG C.

4. The method of claim 1, wherein: The raw material mixture for synthesizing the silicalite-1 molecular sieve comprises a silicon source; and the silicon source is a silicate, sodium silicate or silica sol.

5. The method of claim 2, wherein: The raw material mixture for synthesizing the ZSM-5 molecular sieve comprises an aluminum source and a silicon source; the aluminum source is aluminum nitrate or sodium aluminate; and the silicon source is a silicate, sodium silicate or silica sol.

6. The method of claim 5, wherein: The atomic ratio of Al / Si in the raw material mixture for synthesizing the molecular sieve is not higher than 0.08.

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