A method for synthesizing nano Sn-MFI molecular sieve

Abstract

The present invention belongs to the field of molecular sieve material preparation and provides a method for synthesizing nano Sn-MFI molecular sieves. This method combines the liquid seed method and aerosol spray drying technology to synthesize nano-level Sn-MFI molecular sieves for the first time. The obtained molecular sieve has the advantages of small grain size, small amount of template agent, uniform dispersion of Sn species, and high catalytic activity. A silicon source, a tin source, deionized water and other substances are mixed and stirred for a certain time, a precursor powder is obtained by an aerosol spray device, and then a liquid seed dissolved in the template agent is added to synthesize a nano Sn-MFI molecular sieve. The method is simple to operate, has good repeatability, and is easy to industrialize.

Description

Technical Field

[0001] The present invention belongs to the field of molecular sieve catalyst preparation, specifically to the aerosol-assisted synthesis of nano Sn-MFI molecular sieves with the addition of liquid seed crystals. The nano Sn-MFI molecular sieves synthesized by this method exhibit excellent catalytic performance in the reaction of converting dihydroxyacetone to methyl lactate. Background Art

[0002] Zeolite molecular sieves, with their unique pore system, have been widely used in heterogeneous catalytic reactions. The introduction of heteroatoms into the molecular sieve framework can modulate their catalytic properties. Tin-silicon molecular sieves are the most widely studied class of heteroatom molecular sieves. Tin is a good Lewis acid, enhancing the electropositive carbonyl carbon of ketones, thereby facilitating oxidant attack and activation. Consequently, tin-containing molecular sieves have been widely used in transfer hydrogenation, cracking reactions, sugar isomerization, and epoxide ring-opening reactions.

[0003] In 2013, Zhang Kun et al. (CN 103641136 A) disclosed a method for synthesizing tin-containing layered ZSM-5 zeolite molecular sieves using hexadecyltrimethyl ammonium p-toluenesulfonate as a template and crystallizing at 150-200°C for 5-20 days.

[0004] In 2017, Li Landong et al. (CN 108097293 B) disclosed a method for synthesizing Sn-MFI structured zeolite molecular sieves in one step by a hydrothermal method using soluble tin salt, disodium ethylenediaminetetraacetic acid, sodium citrate, aluminum source and silicon source as raw materials.

[0005] In the above literature reports, the synthesis of tin-containing heteroatom molecular sieves all adopts conventional hydrothermal methods, which usually take a long time to crystallize and require the addition of a large amount of organic structure-directing agents, which will produce a lot of waste liquid that is harmful to the environment; or additional mineralizers, surfactants and other substances need to be added to accelerate the crystallization process.

[0006] In 2014, Dai Weili et al. (CN 104437607 A) disclosed a method for synthesizing a tin-containing molecular sieve using H-SSZ-13 molecular sieve powder as raw material, grinding it with tin source tin acetate or dimethyltin dichloride after steam treatment or steam-acid combined treatment, and calcining it at 350-650°C for 6-12h.

[0007] In 2022, Liu Xue et al. (CN 115448323 A) disclosed a synthesis method for silicon-tin molecular sieve, which comprises treating the silicon-germanium molecular sieve UTL with the template removed in an acid solution at 150-200°C to obtain a layered precursor, and then mixing it with a tin-containing aqueous solution and calcining it in air at 550-650°C for 6-10 hours.

[0008] In the above literature reports, the synthesis of tin-containing heteroatom molecular sieves all adopts the post-synthesis method, which often requires inorganic acid dealumination, has disadvantages such as environmental pollution and relatively dangerous operation. At the same time, the molecular sieve has poor hydrophobicity, which is not conducive to catalytic reactions.

[0009] In 2019, Xia Changjiu et al. (CN 112744832 A) disclosed a synthesis method of first aging a tin source, a liquid silicon source, an auxiliary agent and a solvent mixture, then mixing the aged sol with a solid silicon source and a pH regulator to obtain a solid precipitate, then calcining it to obtain tin silicon oxide, and finally mixing the tin silicon oxide, a template, a seed crystal, water and an inorganic ammonium source for crystallization to obtain a tin-containing molecular sieve.

[0010] In 2021, Liu Zhongwen et al. (CN 113479904 A) disclosed a method for synthesizing Sn-doped MFI zeolite molecular sieves by adding S-1 molecular sieve to a mixture of a desired silicon source, a soluble tin salt, a structure-directing agent, a base, and water, followed by crystallization of a polymer at 20-250°C for 0.1-48 hours. This method requires the addition of a base to enhance the alkalinity of the system and the addition of a polymer to accelerate the crystallization process.

[0011] In the above-mentioned literature reports, the synthesis of tin-containing heteroatom molecular sieves all uses the seed crystal method, which can relatively accelerate the crystallization process, inhibit the formation of impurities, and control the size of crystals. However, the synthesis steps are complex and may require the addition of additional additives.

[0012] In 2013, Zhang Xiongfu et al. (Chemical Engineering Journal, 2013, 218, 425-432) reported a method for rapidly preparing Sn-Beta zeolite molecular sieve from dry tin silicate gel using a dry gel method, using fumed silica as the silicon source, tin tetrachloride pentahydrate as the tin source, tetraethylammonium hydroxide as the template, and NH4F as the mineralizer.

[0013] The dry gel conversion method is also a commonly used method in the synthesis of molecular sieves, but its synthesis steps are cumbersome and often require a special reactor, which is not conducive to industrial production.

[0014] To date, no published literature has reported the rapid synthesis of nano-Sn-MFI molecular sieves using low template dosages. The present invention combines a liquid seeding method with an aerosol-assisted method to synthesize nano-Sn-MFI molecular sieves. A tin-silicon precursor is hydrolyzed under acidic conditions, and the solvent is evaporated using an aerosol spray technique. The resulting solid powder serves as the precursor for the molecular sieve. Liquid seed crystals dissolved in a template are then added to synthesize the Sn-MFI molecular sieve in a high-concentration system in a short period of time. This method is simple to operate, highly efficient, and reproducible, and has potential industrial applications. Summary of the Invention

[0015] To address the problems of large size, high template dosage, and long crystallization time associated with conventional hydrothermal synthesis of Sn-MFI molecular sieves, the present invention relates to an aerosol-assisted synthesis method for nano-Sn-MFI molecular sieves using liquid seed crystals. This method aims to reduce the size of the molecular sieve, lower the template dosage, and improve the dispersion of the tin species. The molecular sieve synthesized using this method exhibits excellent catalytic performance in the conversion of dihydroxyacetone to methyl lactate.

[0016] Research has found that while direct aerosol synthesis can produce Sn-MFI molecular sieves with relatively low template dosages, the resulting crystals are too large, hindering contact between reactants and active centers during the catalytic reaction, resulting in low catalytic activity. Furthermore, hydrothermal synthesis of Sn-MFI molecular sieves by direct seeding with liquid crystals fails to incorporate tin into the molecular sieve framework in a short period of time. Therefore, the aerosol-assisted synthesis of nano-Sn-MFI molecular sieves with liquid seeding provided by the present invention offers significant advantages.

[0017] The technical solutions of the present invention are as follows:

[0018] A method for synthesizing nano Sn-MFI molecular sieves, comprising the following steps:

[0019] A. Mix silica, a 25 wt% template TPAOH aqueous solution, and deionized water in a molar ratio of 1SiO2:0.2TPAOH:19H2O, stir evenly, and transfer to a stainless steel high-pressure hydrothermal reactor lined with polytetrafluoroethylene for crystallization at 100°C for 24 hours. After crystallization, cool to room temperature, wash the solid powder until it is neutral, and dry it in a constant temperature oven overnight to obtain S-1 molecular sieve containing TPAOH as a seed crystal.

[0020] B. Mix a silicon source, a tin source, hydrochloric acid, and deionized water, and stir at room temperature until a uniform solution is formed. The molar ratio of each substance is 1SiO2:0.0075-0.03SnO2:0.15HCl:35H2O. Use an aerosol generator at a temperature of 220°C to obtain a precursor powder.

[0021] C. Add the seed crystals obtained in step A to the template, heat and stir at 80°C to dissolve, and obtain a clear template solution; add the template solution containing the seed crystals to the precursor powder and stir for 30 minutes to form a suspension; transfer to a stainless steel high-pressure hydrothermal reactor with a polytetrafluoroethylene liner and crystallize at 160°C for 6 to 48 hours;

[0022] D. After the crystallization is completed, the mixture is cooled to room temperature, the solid powder is washed to neutrality, and dried in a constant temperature oven overnight. After high temperature calcination, the nano Sn-MFI molecular sieve is obtained.

[0023] Furthermore, in step B, the silicon source used is any one of tetraethyl orthosilicate, silica sol, and fumed silica; and the tin source used is any one of tin tetrachloride, stannous chloride, and tin acetate.

[0024] Furthermore, in step C, the seed crystals must be dissolved in the template solution; the template used is any one of tetrapropylammonium hydroxide, tetraethylammonium hydroxide, and tetrapropylammonium bromide; the preferred amount of seed crystals added is 5 to 15 wt% of the precursor powder mass, and the preferred template / Si ratio is 0.05 to 0.15.

[0025] Furthermore, in step D, the drying temperature is 110° C., the calcination temperature is 550° C., and the calcination time is 6 h.

[0026] The beneficial effects of the present invention are as follows: the present invention uses an aerosol spray technology with added liquid crystal seeds to synthesize nano Sn-MFI molecular sieves. This method hydrolyzes under acidic conditions, which can avoid the rapid hydrolysis and precipitation of SnCl4 in an alkaline environment and the rapid agglomeration of silicon species and difficulty in stirring due to drastic changes in pH. Therefore, the synthesized molecular sieve has good Sn distribution; the catalyst synthesis cycle is short, and the amount of template added can be effectively reduced; the obtained molecular sieve crystals are nanometer-level, which can shorten the contact time between reactants and active sites in the catalytic process, and exhibit excellent catalytic performance in the reaction of converting dihydroxyacetone to methyl lactate. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the XRD pattern of the sample obtained in Example 1.

[0028] Figure 2 is a SEM image of the sample obtained in Example 1.

[0029] Figure 3 This is the UV-vis graph of the sample obtained in Example 1.

[0030] Figure 4 This is the SEM image of the sample obtained in Comparative Example 1.

[0031] Figure 5 This is the UV-vis graph of the sample obtained in Comparative Example 2.

[0032] Figure 6 This is the SEM image of the sample obtained in Comparative Example 3. DETAILED DESCRIPTION

[0033] The following describes a specific embodiment of the present invention in combination with the technical solution of the present invention and the accompanying drawings.

[0034] Comparative Example 1

[0035] According to the molar ratio of 1SiO2:0.0075SnO2:0.15HCl:35H2O, first 5.1g of tetraethyl orthosilicate, 0.065g of tin tetrachloride pentahydrate, 0.3g of hydrochloric acid (mass fraction of 36% to 38%), and 15.6g of deionized water were mixed and stirred until uniform. The precursor solution was passed through an aerosol generator to obtain a precursor powder; then, according to TPAOH / SiO2=0.10, tetrapropylammonium hydroxide solution (mass fraction of 25%) was added to the precursor powder and stirred at room temperature for 30 minutes to form a suspension; the resulting suspension was transferred to a stainless steel autoclave with a polytetrafluoroethylene lining, crystallized at 160°C in a constant temperature oven for 48h, cooled to room temperature, filtered and washed to neutrality, dried at 110°C in a constant temperature oven overnight, and then calcined at 550°C in a muffle furnace for 6h to obtain Sn-MFI molecular sieve. Figure 4 As shown, the crystal size of the Sn-MFI molecular sieve obtained by this method is above 2 microns.

[0036] In this comparative example, the nano-scale Sn-MFI molecular sieve cannot be obtained by directly adopting the aerosol-assisted method.

[0037] Comparative Example 2

[0038] According to the molar ratio of 1SiO2:0.0075SnO2:0.15HCl:35H2O, first mix 5.1g of tetraethyl orthosilicate, 0.065g of tin tetrachloride pentahydrate, 0.3g of hydrochloric acid (mass fraction of 36% to 38%), and a certain amount of deionized water, stir until uniform, stir at 25°C until the solution is clear, take 2.1g of tetrapropylammonium hydroxide solution (mass fraction of 25%) and slowly drip it into the above solution, add a certain amount of deionized water, and continue stirring at 25°C until a uniform precursor solution is formed. Finally, add 5wt% of S-1 molecular sieve as a seed crystal, stir evenly and transfer to a polytetrafluoroethylene liner, load it into a stainless steel high-pressure reactor, and crystallize it at 160°C for 48h. After crystallization, filter and wash with deionized water to obtain a solid sample, put it into a 110°C oven to dry overnight, and then put it into a muffle furnace and roast it at 550°C for 6h to remove the template. Figure 5 As shown, the Sn-MFI molecular sieve obtained by this method has poor tin distribution, and a large amount of tin substances fail to enter the framework.

[0039] In this comparative example, direct addition of seed crystals to hydrothermal synthesis cannot yield an MFI molecular sieve with a better tin distribution.

[0040] Comparative Example 3

[0041] According to the molar ratio of 1SiO2:0.0075SnO2:0.15HCl:35H2O, first 5.1g of tetraethyl orthosilicate, 0.065g of tin tetrachloride pentahydrate, 0.3g of hydrochloric acid (mass fraction of 36% to 38%), and 15.6g of deionized water were mixed and stirred until uniform. The precursor solution was passed through an aerosol generator to obtain a precursor powder; according to the molar ratio of TPAOH / SiO2 of 0.10, the precursor powder was added to the precursor powder. Add tetrapropylammonium hydroxide (mass fraction of 25%), then add 5% mass fraction of S-1 seed crystals, stir at room temperature for 30 minutes to form a suspension; transfer the resulting suspension to a stainless steel autoclave with a polytetrafluoroethylene lining, crystallize at 160°C in a constant temperature oven for 48 hours, cool to room temperature, filter and wash until neutral, dry at 110°C in a constant temperature oven overnight, and then use a muffle furnace to calcine at 550°C for 6 hours to obtain Sn-MFI molecular sieve. Figure 6 As shown, the Sn-MFI molecular sieve obtained by this method has an uneven grain size.

[0042] In this comparative example, the aerosol-assisted synthesis of seed crystals that were not dissolved in the template agent could not obtain nano Sn-MFI molecular sieves with uniform size.

[0043] Example 1

[0044] According to the molar ratio of 1SiO2:0.0075SnO2:0.15HCl:35H2O, 5.1g of tetraethyl orthosilicate, 0.065g of tin tetrachloride pentahydrate, 0.3g of hydrochloric acid (mass fraction of 36% to 38%), and 15.6g of deionized water were first mixed and stirred until uniform. The precursor solution was passed through an aerosol generator to obtain a precursor powder; then tetrapropylammonium hydroxide (mass fraction of 25%) was added according to the molar ratio of TPAOH / SiO2 of 0.10. S-1 seed crystals with a mass fraction of 5% were stirred at 80°C for 1 hour to completely dissolve the seed crystals and obtain a clear solution; the above solution was added to the precursor powder and stirred at room temperature for 30 minutes to form a suspension; the resulting suspension was transferred to a stainless steel high-pressure reactor with a polytetrafluoroethylene liner, crystallized at 160°C in a constant temperature oven for 48 hours, cooled to room temperature, filtered and washed to neutrality, dried at 110°C in a constant temperature oven overnight, and then calcined at 550°C in a muffle furnace for 6 hours to obtain a nano Sn-MFI molecular sieve.

[0045] Example 2

[0046] The difference between this embodiment and embodiment 1 is that the amount of seed crystal added is 10 wt %. The rest is the same as embodiment 1.

[0047] Example 3

[0048] The difference between this embodiment and embodiment 1 is that the amount of seed crystal added is 15 wt %. The rest is the same as embodiment 1.

[0049] Example 4

[0050] The difference between this embodiment and embodiment 1 is that the molar ratio of the template added is TPAOH / SiO2=0.05, and the rest is the same as embodiment 1.

[0051] Example 5

[0052] The difference between this embodiment and embodiment 1 is that the molar ratio of the template added is TPAOH / SiO2=0.15, and the rest is the same as embodiment 1.

[0053] Example 6

[0054] The difference between this embodiment and embodiment 1 is that the crystallization time is 6 hours, and the rest is the same as embodiment 1.

[0055] Example 7

[0056] The difference between this embodiment and embodiment 1 is that the crystallization time is 12 hours, and the rest is the same as embodiment 1.

[0057] Example 8

[0058] The difference between this embodiment and embodiment 1 is that the crystallization time is 18 hours, and the rest is the same as embodiment 1.

[0059] Example 9

[0060] The difference between this embodiment and embodiment 1 is that the crystallization time is 24 hours, and the rest is the same as embodiment 1.

[0061] Example 10

[0062] The difference between this embodiment and embodiment 1 is that the crystallization time is 36 hours, and the rest is the same as embodiment 1.

[0063] Example 11

[0064] The difference between this embodiment and embodiment 9 is that SnO2 / SiO2 in the precursor solution is 0.015, and the rest is the same as embodiment 9.

[0065] Example 12

[0066] The difference between this embodiment and embodiment 9 is that SnO2 / SiO2 in the precursor solution is 0.03, and the rest is the same as embodiment 9.

[0067] Example 13

[0068] The sample prepared in Comparative Example 1 was used to catalyze the conversion of dihydroxyacetone into methyl lactate. The reaction conditions were 10 mmol of dihydroxyacetone, 5 ml of ethanol, 0.1 g of catalyst, and reaction at 90°C for 3 h. The yield of methyl lactate was 38.5%. Under the same reaction conditions, when the sample prepared in Comparative Example 2 was used as a catalyst, the yield of methyl lactate was 22.5%. Under the same reaction conditions, when the sample prepared in Comparative Example 3 was used as a catalyst, the yield of methyl lactate was 37.9%. Under the same reaction conditions, when the sample prepared in Example 1 was used as a catalyst, the yield of methyl lactate was increased to 62.3%.

[0069]

Claims

1. A method for synthesizing nano Sn-MFI molecular sieves, characterized in that: Here are the steps: A. Mix silica, a 25 wt% template TPAOH aqueous solution, and deionized water in a molar ratio of 1SiO2:0.2TPAOH:19H2O, stir evenly, and transfer to a stainless steel high-pressure hydrothermal reactor lined with polytetrafluoroethylene for crystallization at 100°C for 24 hours. After crystallization, cool to room temperature, wash the solid powder until it is neutral, and dry it in a constant temperature oven overnight to obtain S-1 molecular sieve containing TPAOH as a seed crystal. B. Mix a silicon source, a tin source, hydrochloric acid, and deionized water, and stir at room temperature until a uniform solution is formed. The molar ratio of each substance is 1SiO2:0.0075-0.03SnO2:0.15HCl:35H2O. Use an aerosol generator at a temperature of 220°C to obtain a precursor powder. C. Add the seed crystals obtained in step A to the template, heat and stir at 80°C to dissolve, and obtain a clear template solution; add the template solution containing the seed crystals to the precursor powder and stir for 30 minutes to form a suspension; transfer to a stainless steel high-pressure hydrothermal reactor with a polytetrafluoroethylene liner and crystallize at 160°C for 6 to 48 hours; D. After the crystallization is completed, the mixture is cooled to room temperature, the solid powder is washed to neutrality, and dried in a constant temperature oven overnight. After high temperature calcination, the nano Sn-MFI molecular sieve is obtained.

2. The synthesis method according to claim 1, wherein In step B, the silicon source used is any one of tetraethyl orthosilicate, silica sol, and fumed silica; the tin source used is any one of tin tetrachloride, stannous chloride, and tin acetate.

3. The synthesis method according to claim 1, wherein In step C, the template used is any one of tetrapropylammonium hydroxide, tetraethylammonium hydroxide, and tetrapropylammonium bromide.

4. The synthesis method according to claim 1, characterized in that In step C, the amount of seed crystal added is 5-15 wt% of the precursor powder mass, and the template / Si is 0.05-0.

15.

5. The synthesis method according to claim 1, characterized in that In step D, the drying temperature is 110° C., the calcination temperature is 550° C., and the calcination time is 6 h.

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