Method for cracking alkanes to produce low carbon olefins by using a shell lewis acid-rich zsm-5@sn-mfi molecular sieve catalyst

Abstract

The application discloses a method for cracking alkanes to produce low-carbon olefins by using ZSM-5@Sn-MFI molecular sieve with a shell rich in Lewis acid, and the method comprises the following steps: synthesizing an aluminum-containing ZSM-5 core in advance by using a hydrothermal method, then introducing a tin source into a shell sol, and coating MFI molecular sieve with tin isomorphous substitution on the surface of the ZSM-5 core by epitaxial growth under certain crystallization conditions, and then performing washing, drying, calcination, ion exchange and calcination to obtain the ZSM-5@Sn-MFI core-shell molecular sieve. The core-shell molecular sieve prepared by the method is used for catalyzing alkanes to crack, the occurrence of a side reaction can be inhibited, the low-carbon olefin selectivity is good, the low-carbon olefin yield is increased from 15.8% of a ZSM-5 system to 33.3%, and the method is favorable for directional cracking of alkanes to prepare low-carbon olefins.

Description

Technical Field

[0001] This invention belongs to the field of catalysis and relates to a method for catalyzing the cracking of alkanes to produce more low-carbon olefins using ZSM-5@Sn-MFI molecular sieves with shells rich in Lewis acids. Background Technology

[0002] Ethylene, propylene, and other low-carbon olefins are indispensable basic chemical raw materials, playing a vital role in the petrochemical industry. They are widely used in the production of synthetic fibers, synthetic rubber, synthetic plastics, and other chemical products, and can also be used to synthesize high-value-added products such as pharmaceutical intermediates, fine chemicals, and high-end new materials. Ethylene and propylene mainly originate from petroleum steam cracking and catalytic cracking (FCC) processes. The main product of steam cracking is ethylene, with propylene as a byproduct. The product distribution of steam cracking is difficult to control, and the temperature of steam cracking reactors is typically above 800℃, requiring a large amount of thermal energy to provide the high temperatures required for the reaction, resulting in high energy consumption and carbon emissions. In the FCC process, diesel and kerosene are the main products, while ethylene, propylene, and other low-carbon olefins are only generated as byproducts.

[0003] Under the dual-carbon goals, the increasing prevalence of photovoltaic power generation and new energy vehicles has driven demand for high-end materials. my country's ethylene and propylene shortages are widening, while there is overcapacity in refined oil products such as gasoline and diesel. "Reducing oil consumption and increasing chemical production" has become an inevitable trend in the petrochemical industry. Developing green, low-energy-consumption, and high-efficiency technologies for producing low-carbon olefins has become a hot topic in the petrochemical field. Catalytic cracking for producing low-carbon olefins can significantly reduce reaction temperature due to the introduction of catalysts. Furthermore, by adjusting the catalyst composition and process parameters, product distribution can be flexibly adjusted, resulting in higher product yields. In addition, this process has a wide range of applicable raw materials, making it an important route for future low-carbon olefin production.

[0004] Catalyst design and development is one of the core aspects of this process. Currently, catalytic cracking catalysts mainly fall into two categories: metal oxide catalysts and molecular sieve catalysts. ZSM-5 molecular sieves are widely used in the petrochemical industry due to their excellent shape selectivity of microporous structure, good hydrothermal stability, and easily tunable acidity. However, ZSM-5 has a relatively strong acidity, which can easily exacerbate side reactions (olefin hydrogen transfer, polymerization, etc.) during catalytic cracking, reduce the yield of low-carbon olefins, and cause coking and carbon deposition in the pores, leading to catalyst deactivation. Controlling the acidity of ZSM-5 molecular sieves is an effective way to solve these problems. To this end, researchers explored different methods. CN113548674A discloses a modified ZSM-5 molecular sieve, its preparation method, and its application. Through alkaline earth metal modification and post-treatment with sulfuric acid and oxalic acid, the acid type can be controlled and adjusted, improving the selectivity of propylene in the cracking of n-tetradecane. CN117160528A discloses a preparation method and application of a Mn-Zn-P composite modified molecular sieve. This method disperses ZSM-5 molecular sieve in a solution containing manganese, zinc, phosphorus, etc., to form a mixed slurry, which is then dried. Calcination yielded a Mn-Zn-P composite modified molecular sieve, which can suppress the production of methane in the catalytic cracking reaction of naphtha and improve the yield of ethylene and propylene. CN115990513A discloses a catalyst for the catalytic cracking of olefins to produce propylene, its preparation method and application. The catalyst uses an equal volume impregnation method to support group VIII metals such as Fe, Co, and Pd on ZSM-5 molecular sieve, which increases the content of Lewis acid in the catalyst. The resulting metal-modified ZSM-5 has a high conversion rate and propylene selectivity in the catalytic cracking reaction of n-butene.

[0005] In summary, existing methods for controlling the acidity of ZSM-5 molecular sieves mainly revolve around acid-base treatment or metal loading. However, acid-base treatment can damage the molecular sieve framework structure to some extent, reducing catalyst stability; furthermore, metal loading may clog the molecular sieve channels, and metal particles can agglomerate during high-temperature reactions, leading to reduced catalyst activity. This invention achieves the control of the acidity of ZSM-5 molecular sieves by constructing a Sn-MFI-coated core-shell molecular sieve, thereby suppressing side reactions and improving the yield of low-carbon olefins. Summary of the Invention

[0006] The purpose of this invention is to provide a method for catalyzing the cracking of alkanes with low-carbon olefins using a Lewis acid-rich ZSM-5@Sn-MFI molecular sieve. This method achieves the regulation of the acidity of the ZSM-5 molecular sieve, suppresses side reactions during the catalytic cracking of alkanes, improves the selectivity of low-carbon olefins, and thus achieves directional cracking of alkanes.

[0007] To achieve the above objectives, the technical solution adopted by this invention is as follows: A tin-isomorphically substituted MFI molecular sieve shell is coated onto the surface of a pre-synthesized ZSM-5 core (~200 nm) via epitaxial growth, thereby preparing a ZSM-5@Sn-MFI molecular sieve. The specific synthesis method includes the following steps:

[0008] (1) Synthesis of core ZSM-5: A certain amount of template agent and silicon source were dissolved in water and stirred at 80°C for 12-48h. Then, aluminum source was dissolved in a certain amount of sodium hydroxide solution and added to the above solution. After stirring evenly, the mixed sol was transferred to a hydrothermal reactor for crystallization. Crystallization was carried out at 140-200°C for 24-72h. After cooling, the mixture was separated by centrifugation, washed with water until neutral, dried, and calcined to obtain core ZSM-5 molecular sieve.

[0009] (2) Dissolve a certain amount of tin source in water, add silicon source and stir thoroughly, then add a certain amount of template agent and sodium hydroxide solution in sequence, stir at 80°C for 3 hours, add a certain amount of ethanol and core ZSM-5 molecular sieve to the above solution, sonicate and stir evenly.

[0010] (3) The mixed sol was then transferred to a hydrothermal reactor and hydrothermally crystallized at 100-200℃ for 12-72 hours.

[0011] (4) After cooling the product obtained in step (3), centrifuge it, wash it with deionized water until neutral, dry it, and calcine it to obtain sodium core-shell molecular sieve.

[0012] (5) The sodium-type core-shell molecular sieve was subjected to ion exchange with 1 mol / L NH4Cl solution, dried, and calcined to obtain a hydrogen-type core-shell molecular sieve with a shell rich in Lewis acid.

[0013] In step (1) of the above method, the molar ratio of each substance is SiO2:Al2O3:TPAOH:Na2O:H2O=1:(0.01-0.02):(0.2-0.5):(0.02-0.1):(20-200), more preferably, SiO2:Al2O3:TPAOH:Na2O:H2O=1:(0.01-0.02):(0.2-0.3):(0.05-0.1):(50-150).

[0014] In step (2) of the above method, the mass ratio of the core ZSM-5 molecular sieve to the SiO2 in the silicon source is 1:0.25-2.

[0015] In step (3) of the above method, the molar ratio of each substance is SiO2:SnO2:template agent:Na2O:ethanol:H2O=1:(0.002-0.05):(0.2-0.5):(0.01-0.05):(2-8):(20-200), more preferably, SiO2:SnO2:template agent:Na2O:ethanol:H2O=1:(0.0025-0.02):(0.2-0.3):(0.02-0.04):(2-4):(50-150).

[0016] In steps (1) and (2) of the above method, the template agent is tetrapropylammonium hydroxide; in steps (1) and (2) of the above method, the silicon source is tetraethyl orthosilicate; in step (1) of the above method, the aluminum source is aluminum nitrate nonahydrate; in step (2) of the above method, the tin source is tin tetrachloride pentahydrate.

[0017] In the above method steps (1), (4) and (5), the drying conditions are a drying temperature of 80-150℃ and a drying time of 6-24h, more preferably a drying temperature of 100-120℃ and a drying time of 8-12h.

[0018] In steps (4) and (5) of the above method, the calcination conditions are: calcination temperature of 450-800℃, calcination time of 3-12h, and heating rate of 2-10℃·min. -1 More preferably, the calcination temperature is 550-650℃, the calcination time is 3-6h, and the heating rate is 2-5℃·min. -1 .

[0019] In step (5) of the above method, the ion exchange conditions are: the mass ratio of core-shell molecular sieve to NH4Cl solution is m(core-shell molecular sieve):m(NH4Cl)=1:10, the ion exchange temperature is 80℃, the ion exchange time is 4h, and the number of ion exchanges is 2.

[0020] This invention also provides the application of ZSM-5@Sn-MFI molecular sieve in the catalytic cracking of hydrocarbons to produce low-carbon olefins. The application process mainly includes the following steps:

[0021] (1) Press the prepared ZSM-5@Sn-MFI molecular sieve into tablets, sieve to 20-40 mesh, and take 1.5g of the above catalyst and load it into a fixed bed reactor;

[0022] (2) In a N2 atmosphere, heat the reactor to the required reaction temperature and stabilize it for 1 hour;

[0023] (3) Introduce raw materials to start the reaction, collect the gas, and analyze the composition of the product using gas chromatography.

[0024] In the above-mentioned application of core-shell molecular sieves, the catalytic alkane cracking reaction to produce low-carbon olefins uses C5-C10 alkanes as raw materials and N2 as the carrier gas, wherein the mass hourly space velocity (HSV) of the alkane is 0.5-8 h⁻¹. -1 The N2 flow rate is 5-60 mL / min. -1 The cracking reaction temperature is 450-650℃, and more preferably, the mass hourly space velocity of the alkane is 0.5-4h. -1 The N2 flow rate is 10-40 mL / min. -1 The pyrolysis reaction temperature is 500-600℃.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The catalytic material prepared by the ZSM-5@Sn-MFI molecular sieve synthesis method provided by the present invention introduces a large number of Lewis acid sites in the shell layer, forming a surface acid shield, reducing the surface acidity of the molecular sieve, while retaining the core cleavage active sites, thus realizing the regulation of the acidity of ZSM-5 molecular sieve.

[0027] (2) In the application of catalytic alkane cracking to produce low-carbon olefins, the generation of by-products was suppressed and the selectivity of low-carbon olefins was improved, so that the core-shell molecular sieve achieved a low-carbon olefin yield of 33.3% while maintaining a high conversion rate of 82.7%. Attached Figure Description

[0028] Figure 1 Transmission electron micrographs of the molecular sieves obtained in Comparative Example 1(a) and Example 1(b).

[0029] Figure 2 The images show the pyridine infrared spectra of the molecular sieves obtained in Comparative Example 1, Example 1, and Example 2. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0031] Comparative Example 1

[0032] Preparation of hydrogen-form ZSM-5

[0033] 9.53 g of tetrapropylammonium hydroxide and 15.61 g of tetraethyl orthosilicate were dissolved in 32.34 g of water and stirred at 80 °C for 24 h. Then, 0.29 g of Al(NO3)3·9H2O was dissolved in 3 g of sodium hydroxide solution (10 wt%) and added to the above solution. The mixture was stirred at 80 °C for another 1 h. The resulting sol was then transferred to a hydrothermal reactor and hydrothermally crystallized at 180 °C for 24 h. After cooling, the mixture was centrifuged, washed repeatedly with water until neutral, dried, and calcined to obtain sodium-type ZSM-5 molecular sieve. The obtained molecular sieve was mixed with 1 mol / L NH4Cl solution and stirred at 80 °C for 4 h for ion exchange. This process was repeated twice to form ammonium-type ZSM-5 molecular sieve. The ammonium-type ZSM-5 molecular sieve was then dried and calcined to obtain hydrogen-type ZSM-5 molecular sieve.

[0034] Its TEM image and Py-IR spectrum are as follows: Figure 1 , Figure 2 As shown.

[0035] Comparative Example 2

[0036] Sn-MFI molecular sieves were prepared using the same method and steps as Comparative Example 1, except that Al(NO3)3·9H2O in Comparative Example 1 was replaced with an equimolar amount of SnCl4·5H2O.

[0037] Comparative Example 3

[0038] This comparative example uses a commercial H-type ZSM-5 molecular sieve, purchased from Tianjin Nanhua Catalyst Co., Ltd., model number NKF-5D-38JZ.

[0039] Example 1

[0040] ZSM-5@Sn-MFI core-shell molecular sieve

[0041] (1) Synthesis of core ZSM-5 molecular sieve: 9.53 g tetrapropylammonium hydroxide and 15.61 g tetraethyl orthosilicate were dissolved in 32.34 g water and stirred at 80 °C for 24 h. Then, 0.29 g Al(NO3)3·9H2O was dissolved in 3.00 g sodium hydroxide solution (10 wt%) and added to the above solution. The mixture was stirred at 80 °C for 1 h. The mixed sol was then transferred to a hydrothermal reactor for crystallization at 180 °C for 24 h. After cooling, the mixture was centrifuged, washed with water multiple times until neutral, dried at 100 °C for 24 h, and calcined at 650 °C for 3 h to obtain core ZSM-5 molecular sieve.

[0042] (2) Dissolve 0.351g of SnCl4·5H2O in 80.19g of water, add 10.40g of tetraethyl orthosilicate and stir thoroughly, then add 6.35g of tetrapropylammonium hydroxide and 6.00g of sodium hydroxide solution (1.2wt%) in sequence, stir at 80℃ for 3h, add 9.20g of ethanol and 3.00g of core ZSM-5 molecular sieve to the above solution, sonicate for 1h, and then continue stirring at 80℃ for 1h;

[0043] (3) The mixed sol was then transferred to a hydrothermal reactor and hydrothermally crystallized at 180°C for 24 hours.

[0044] (4) After cooling the product obtained in step (3), centrifuge it, wash it with deionized water until neutral, dry it at 100°C for 24 hours, and calcine it at 650°C for 3 hours to obtain sodium molecular sieve.

[0045] (5) The molecular sieve obtained in step (4) is mixed with 1 mol / L NH4Cl solution and stirred at 80°C for 4 h for ion exchange. After repeating twice, an ammonium core-shell molecular sieve is formed. Then, it is dried at 100°C for 24 h and calcined at 650°C for 3 h to obtain a hydrogen core-shell molecular sieve.

[0046] Its TEM image and Py-IR spectrum are as follows: Figure 1 , Figure 2 As shown.

[0047] Example 2

[0048] ZSM-5@Sn-MFI core-shell molecular sieves were prepared according to the same method and steps as in Example 1, except that the amount of SnCl4·5H2O used in step (2) was 0.176g.

[0049] Its py-IR spectrum is as follows Figure 2 As shown.

[0050] Example 3

[0051] ZSM-5@Sn-MFI core-shell molecular sieves were prepared according to the same method and steps as in Example 1, except that the amount of SnCl4·5H2O used in step (2) was 0.088g.

[0052] Example 4

[0053] ZSM-5@Sn-MFI core-shell molecular sieves were prepared according to the same method and steps as in Example 1, except that the amount of SnCl4·5H2O used in step (2) was 0.044g.

[0054] Example 5

[0055] ZSM-5@Sn-MFI core-shell molecular sieves were prepared according to the same method and steps as in Example 1, except that the crystallization temperature in step (3) was 100°C.

[0056] Example 6

[0057] ZSM-5@Sn-MFI core-shell molecular sieves were prepared according to the same method and steps as in Example 1, except that the crystallization temperature in step (3) was 120°C.

[0058] Example 7

[0059] ZSM-5@Sn-MFI core-shell molecular sieves were prepared according to the same method and steps as in Example 1, except that the crystallization time in step (3) was 12 h.

[0060] Example 8

[0061] ZSM-5@Sn-MFI core-shell molecular sieves were prepared according to the same method and steps as in Example 1, except that the crystallization time in step (3) was 48 h.

[0062] Application examples

[0063] The molecular sieves obtained in Comparative Examples 1-3 and Examples 1-8 were used for catalytic alkane cracking. The evaluation device used was a micro fixed-bed reactor, and the specific process is as follows:

[0064] (1) Press the obtained molecular sieve powder into tablets, sieve to 20-40 mesh, and take 1.50g of the above catalyst and load it into a fixed bed reactor;

[0065] (2) Control the N2 carrier gas flow rate to 20.0 mL·min -1 In a N2 atmosphere, the reactor is heated to the required reaction temperature and stabilized for 1 hour.

[0066] (3) The reaction begins by introducing the raw materials, wherein the mass hourly space velocity of the alkane is 2 h⁻¹. -1 The gaseous and liquid products were collected through a drainage gas collection tank and a condenser, respectively. The products were collected every 1 hour, and the content of each component in the gaseous and liquid products was quantitatively analyzed by gas chromatography.

[0067] The conversion rates, yields, and selectivity of the major products are shown in Table 1. (In Table 1, "BTX" represents benzene, toluene, and xylene; "trienes" includes ethylene, propylene, and butene.)

[0068] Table 1 shows the pyrolysis performance of the molecular sieves prepared in the comparative examples and embodiments.

[0069]

[0070] Reaction temperature: 550℃, alkane mass hourly space velocity: 2h-1 。

Claims

1. A method for catalyzing the cracking of alkane to produce low-carbon olefins using ZSM-5@Sn-MFI molecular sieves with shells rich in Lewis acids, characterized in that, The synthesis method includes the following steps: (1) Synthesis of core ZSM-5: A certain amount of template agent and silicon source were dissolved in water and stirred at 80°C for 12-48 h. Then, aluminum source was dissolved in a certain amount of sodium hydroxide solution and added to the above solution. After stirring evenly, the mixed sol was transferred to a hydrothermal reactor for crystallization. Crystallization was carried out at 140-200°C for 24-72 h. After cooling, the mixture was separated by centrifugation, washed with water until neutral, dried, and calcined to obtain core ZSM-5. (2) Dissolve a certain amount of tin source in water, add silicon source and stir thoroughly. The mass ratio of core ZSM-5 molecular sieve to SiO2 in silicon source is 1:0.25-2. Then add a certain amount of template agent and sodium hydroxide solution in sequence, stir at 80°C for 3 h, add a certain amount of ethanol and core ZSM-5 molecular sieve powder to the above solution, sonicate and stir evenly. (3) The mixed sol was then transferred to a hydrothermal reactor. The molar ratio of each substance in the mixed sol was SiO2:SnO2:template agent:Na2O:ethanol:H2O=1:(0.002-0.05):(0.2-0.5):(0.01-0.05):(2-8):(20-200). The mixture was hydrothermally crystallized at 100-200°C for 12-72 h. (4) After cooling the product obtained in step (3), centrifuge it, wash it with deionized water until neutral, dry it, and calcine it to obtain sodium core-shell molecular sieve. (5) The sodium-type core-shell molecular sieve was subjected to ion exchange with 1 mol / L NH4Cl solution, dried, and calcined to obtain a hydrogen-type core-shell molecular sieve with a shell rich in Lewis acid.

2. The method according to claim 1, characterized in that, The molar ratio of each substance in step (1) is SiO2:Al2O3:TPAOH:Na2O:H2O=1:(0.01-0.02):(0.2-0.5):(0.02-0.1):(20-200).

3. The method according to claim 1, characterized in that, The template agent mentioned in steps (1) and (2) is tetrapropylammonium hydroxide, the silicon source mentioned in steps (1) and (2) is tetraethyl orthosilicate, the aluminum source mentioned in step (1) is aluminum nitrate nonahydrate, and the tin source mentioned in step (2) is tin tetrachloride pentahydrate.

4. The method according to claim 1, characterized in that, The drying conditions described in steps (1), (4) and (5) are a drying temperature of 80-150°C and a drying time of 6-24 h.

5. The method according to claim 1, characterized in that, The calcination conditions described in steps (4) and (5) are: calcination temperature of 450-800°C, calcination time of 3-12 h, and heating rate of 2-10°C·min. -1 .

6. The method according to claim 1, characterized in that, In step (5), the ion exchange conditions are as follows: the mass ratio of core-shell molecular sieve to NH4Cl solution is m(core-shell molecular sieve):m(NH4Cl)=1:10, the exchange temperature is 80°C, the exchange time is 4 h, and the number of exchanges is 2.

7. The application of the ZSM-5@Sn-MFI molecular sieve prepared by the method described in claim 1 in the catalytic cracking of alkane to produce low-carbon olefins.

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