A catalyst for the cracking of C4 olefins to propylene and ethylene, its preparation method and application

Abstract

This invention discloses a catalyst for the cracking of C4 olefins to propylene and ethylene, its preparation method, and its application. The catalyst has a core-shell structure, with the core phase being a ZSM-5 molecular sieve with a composite pore structure, and the shell phase also being a ZSM-5 molecular sieve. In the ZSM-5 molecular sieve with the composite pore structure, the pore volume occupied by pores with a pore size <2 nm is denoted as 'a', the pore volume occupied by pores with a pore size of 2–50 nm is denoted as 'b', and the pore volume occupied by pores with a pore size >50 nm is denoted as 'c'; wherein b / a is 0.10–0.80, and c / a is 0.30–0.70. This catalyst is suitable for the cracking of C4 olefins to propylene and ethylene, and can significantly improve the conversion rate of C4 olefins and the selectivity of the products propylene and ethylene.

Description

Technical Field

[0001] This invention belongs to the field of catalytic cracking technology, specifically relating to a catalyst for the cracking of C4 olefins to propylene and ethylene, its preparation method, and its application. Background Technology

[0002] Propylene is an important basic organic chemical raw material, with most of it derived from steam cracking and catalytic cracking units. Driven by the rapid growth in demand for polypropylene and its derivatives, propylene demand has remained strong and grown at a rapid pace in recent years, thus it is considered a product with great market potential. To increase propylene production, various countries have developed other methods for producing low-carbon olefins, such as the butene disproportionation (OMT) process, which increases propylene production through the reaction of ethylene and butene; the propane dehydrogenation (PDH) process, which directly utilizes shale gas byproducts in chemical processes; and the methanol-to-propylene (MTP) technology, which uses coal-based methanol to produce propylene. Furthermore, utilizing the large quantities of C4 and C5 hydrocarbons generated by ethylene plants, FCC units, and MTO units to increase propylene production through catalytic cracking of olefins is also an important research and development direction for petrochemical companies worldwide.

[0003] Currently, the main industrialized olefin cracking processes for increasing propylene production include Asahi Kasei's Omega process and UOP's OCP process. Both processes use ZSM-5 molecular sieves as catalysts and mixed C4 / C5 monoolefins as byproducts from FCC, ethylene plants, or MTO as feedstocks. The OCP and Omega processes do not add any diluents during the reaction, and are characterized by high space velocity, small reactor size, and high propylene space-time yield. The catalyst regeneration cycle is 2-3 days, requiring frequent switching for regeneration. Asahi Kasei's Omega process was industrially applied in 2006. UOP combines methanol-to-olefins (MTO) with OCP, characterized by high propylene and ethylene yields and fewer byproducts. CN98813467.5 discloses a process using crystalline silicates with a silicon / aluminum atomic ratio of 180-1000 as catalysts at 500-600°C and a space velocity of 10-30 h⁻¹. -1 The following method describes a process for cracking C4 and / or other olefin-rich streams into light olefin-rich streams such as propylene, where the total amount of olefins remains essentially unchanged before and after the reaction. CN1284109A discloses a process for cracking C4 and higher olefins to produce propylene and ethylene, using a hydrothermally modified ZSM-5 molecular sieve with a silicon-aluminum molar ratio greater than 200 as the catalyst. In Example 3, the etherification conversion rate of C4 was 54%, while the propylene yield was only 29%.

[0004] Currently, olefin cracking technology still suffers from low olefin conversion rates and low yields of propylene and ethylene, especially in refineries where the total amount of C4 byproducts is relatively large. Therefore, it is necessary to develop C4 olefin cracking catalysts with better performance to promote greater progress in olefin cracking technology. Summary of the Invention

[0005] The technical problem this invention aims to solve is the low olefin conversion rate and low selectivity of propylene and ethylene products in existing C4 olefin cracking technologies, particularly for low-concentration C4 olefin feedstocks, which suffer from low feedstock utilization, numerous byproducts, poor economics, and product separation issues. This invention provides a catalyst for the cracking of C4 olefins to propylene and ethylene, its preparation method, and its applications. This catalyst is suitable for the cracking of C4 olefins to propylene and ethylene, especially for low-concentration C4 olefin cracking reactions in refineries, and can significantly improve the C4 olefin conversion rate and the selectivity of propylene and ethylene products.

[0006] The first aspect of this invention provides a catalyst for the cracking of C4 olefins to produce propylene and ethylene. The catalyst has a core-shell structure, with the core phase being a ZSM-5 molecular sieve having a composite pore structure, and the shell phase being a ZSM-5 molecular sieve. In the ZSM-5 molecular sieve with the core-shell composite pore structure, the pore volume occupied by pores with a pore size < 2 nm is denoted as 'a', the pore volume occupied by pores with a pore size of 2–50 nm is denoted as 'b', and the pore volume occupied by pores with a pore size > 50 nm is denoted as 'c'; wherein b / a is 0.10–0.80, preferably 0.30–0.80, and c / a is 0.30–0.70.

[0007] According to the present invention, the mass ratio of the core phase to the shell phase in the catalyst is 1 to 5:1.

[0008] According to the present invention, in the ZSM-5 molecular sieve with a core-phase composite pore structure, the pore volume of pores with a diameter <2 nm accounts for 20% to 75% of the total pore volume, the pore volume of pores with a diameter of 2 to 50 nm accounts for 10% to 55% of the total pore volume, and the pore volume of pores with a diameter >50 nm accounts for 6% to 30% of the total pore volume, preferably 15% to 30%.

[0009] According to the present invention, the silicon-aluminum molar ratio (SiO2 / Al2O3) of the ZSM-5 molecular sieve with a nucleophase composite pore structure is 100 to 1000.

[0010] According to the present invention, the shell-phase ZSM-5 molecular sieve in the catalyst is a conventional microporous molecular sieve. The silicon-aluminum molar ratio (SiO2 / Al2O3) of the shell-phase ZSM-5 molecular sieve is 100–1000.

[0011] According to the present invention, in the catalyst, in the shell-phase ZSM-5 molecular sieve, pores with a diameter <2 nm occupy 85% to 100% of the total pore volume; pores with a diameter >50 nm occupy 0% to 15% of the total pore volume; and pores with a diameter of 2 to 50 nm occupy 0% to 10% of the total pore volume. Pores with a diameter >50 nm are stacked pores.

[0012] A second aspect of the present invention provides a method for preparing the above-mentioned catalyst, the method comprising the following steps:

[0013] a) The ZSM-5 molecular sieve raw powder was treated in an alkaline solution and then calcined to obtain the precursor.

[0014] b) Mix the template agent, aluminum source, silicon source, alkali source and the precursor described in step a), perform hydrothermal crystallization, a second calcination, then perform ammonium exchange, and a third calcination to obtain the catalyst.

[0015] According to the present invention, the alkali in step a) includes at least one of sodium hydroxide and sodium carbonate. The mass concentration of the alkali solution is 5% to 10%. The treatment temperature in the alkali solution is 60 to 100°C, preferably 60 to 90°C. The treatment time in the alkali solution is 2 to 4 hours. The alkali treatment can be carried out under stirring. After the alkali treatment, the solid is filtered and then dried. The drying temperature is 80 to 120°C, and the time is 8 to 10 hours.

[0016] According to the present invention, the conditions for the first calcination in step a) are: temperature 500-600°C, time 4-8 h. The precursor obtained after the first calcination is a hierarchical porous ZSM-5 molecular sieve.

[0017] According to the present invention, step a) ZSM-5 molecular sieve raw powder can be obtained using commercially available ZSM-5 molecular sieve or by self-production. The preparation method of ZSM-5 molecular sieve raw powder includes the steps of uniformly mixing template agent, aluminum source, silicon source and alkali source, hydrothermal crystallization, and calcination to obtain ZSM-5 molecular sieve raw powder.

[0018] According to the present invention, in the method for preparing the ZSM-5 molecular sieve raw powder, the template agent includes at least one selected from tetramethylammonium bromide, tetraethylammonium bromide, tetrapropylammonium bromide, and tetrapropylammonium hydroxide. The aluminum source includes at least one selected from aluminum nitrate, aluminum sulfate, and sodium aluminate. The silicon source includes at least one selected from water glass, silica sol, and tetraethyl orthosilicate. The alkali source includes at least one selected from sodium hydroxide and potassium hydroxide. The template agent is NH4. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4+ to water (calculated as H2O) is: + Al2O3:SiO2:OH - The ratio of H₂O is 0.1–0.5: 0.001–0.01: 1: 0.1–0.4: 5–10. The hydrothermal crystallization temperature is 80–200℃, and the crystallization time is 10–50 hours. An autoclave is preferred for hydrothermal crystallization. The pressure of the autoclave is not particularly limited and is self-generated pressure. After hydrothermal crystallization, the product is washed with water and dried. The drying temperature is 60–100℃, and the drying time is 10–20 hours. The calcination temperature is 400–500℃, and the time is 5–10 hours.

[0019] According to the present invention, in step b), the template agent comprises at least one selected from tetramethylammonium bromide, tetraethylammonium bromide, tetrapropylammonium bromide, and tetrapropylammonium hydroxide. The aluminum source comprises at least one selected from aluminum nitrate, aluminum sulfate, and sodium aluminate. The silicon source comprises at least one selected from water glass, silica sol, and tetraethyl orthosilicate. The alkali source comprises at least one selected from sodium hydroxide and potassium hydroxide. The template agent is NH4. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4+ to water (calculated as H2O) is: + Al2O3:SiO2:OH - : H2O=0.1~0.5: 0.001~0.01: 1: 0.1~0.4: 5~10.

[0020] According to the present invention, in step b), the hydrothermal crystallization temperature is 80–200°C, and the crystallization time is 10–50 h. The preferred equipment for hydrothermal crystallization is an autoclave. The pressure of the autoclave is not particularly limited and is self-generated pressure. After hydrothermal crystallization, the product is washed with water and dried. The drying temperature is 80–120°C, and the drying time is 8–12 hours. The second calcination temperature is 500–600°C, and the time is 4–8 h. The product of the second calcination is the core-shell type ZSM-5 molecular sieve raw powder.

[0021] According to the present invention, in the catalyst described in step b), the mass ratio of the core phase to the shell phase is 1 to 5:1.

[0022] According to the present invention, the ammonium exchange in step b) can be carried out using conventional methods. Preferably, the exchange is performed at 80–90°C with a 5–10% by weight aqueous solution of an ammonium salt. The ammonium salt is preferably ammonium nitrate.

[0023] According to the present invention, the temperature of the third calcination in step b) is 500-600°C and the time is 4-8 hours.

[0024] According to the present invention, the catalyst in step b) has a core-shell structure, wherein the core phase is a ZSM-5 molecular sieve with a composite pore structure, and the shell phase is a ZSM-5 molecular sieve. In the ZSM-5 molecular sieve with a core-shell composite pore structure, the pore volume occupied by pores with a pore size < 2 nm is a, the pore volume occupied by pores with a pore size of 2–50 nm is b, and the pore volume occupied by pores with a pore size > 50 nm is c; wherein b / a is 0.10–0.80, preferably 0.30–0.80, and c / a is 0.30–0.70.

[0025] According to the present invention, in the catalyst described in step b), the ZSM-5 molecular sieve with a core-phase composite pore structure has pores with a diameter <2 nm accounting for 20% to 75% of the total pore volume, pores with a diameter of 2 to 50 nm accounting for 10% to 55% of the total pore volume, and pores with a diameter >50 nm accounting for 6% to 30% of the total pore volume, preferably 15% to 30%.

[0026] A third aspect of the present invention provides the application of the above-described catalyst or the catalyst prepared by the above-described method in the reaction of C4 olefins cracking to produce propylene and ethylene.

[0027] According to the present invention, the application is a reaction for producing propylene and ethylene by cracking low-concentration C4 olefins. The low-concentration C4 olefin feedstock is derived from refinery C4 hydrocarbons. The mass concentration of the C4 hydrocarbons is 30%–50%. Further, the C4 hydrocarbons are a mixture of C4 hydrocarbons. The mixture of C4 hydrocarbons includes at least one C4 olefin and at least one C4 alkane. The mass concentration of the C4 hydrocarbons is 30%–50% for the C4 olefins and 50%–70% for the C4 alkanes.

[0028] According to the present invention, the reaction conditions are as follows: reaction temperature of 500–600°C, preferably 520–580°C; reaction pressure of 0–1.0 MPa, preferably 0.03–0.8 MPa; and C4 hydrocarbon weight hourly space velocity of 1–30 h⁻¹. -1 Preferably 2-20h -1 .

[0029] Compared with the prior art, the present invention has significant advantages and outstanding effects, mainly including:

[0030] (1) In this invention, the catalyst is a core-shell structure catalyst, with the core being a ZSM-5 molecular sieve having a composite pore structure and the shell being a conventional ZSM-5 molecular sieve. The inventors have discovered that when the pore volume ratio of mesopores to micropores in the ZSM-5 molecular sieve with the composite pore structure is 0.10–0.80, and the pore volume ratio of macropores to micropores is 0.30–0.70, the core-shell structure catalyst of this invention, when applied to the cracking of C4 olefins to propylene and ethylene, especially in the cracking of low-concentration C4 olefins to propylene, can significantly improve the conversion rate of C4 olefins and the selectivity of the products propylene and ethylene.

[0031] (2) In this invention, the catalyst is prepared using a two-step hydrothermal synthesis method to prepare a core-shell catalyst. First, ZSM-5 molecular sieve powder is modified by treating it with an alkaline solution to obtain a ZSM-5 molecular sieve with a composite pore structure. This composite pore structure molecular sieve powder is then uniformly dispersed in a raw material mixture and subjected to secondary hydrothermal crystallization to finally obtain a molecular sieve catalyst with a core-shell structure. The core-shell structure catalyst prepared by this invention can be applied to the cracking of C4 olefins to propylene and ethylene, especially in the cracking of low-concentration C4 olefins to propylene, and can significantly improve the conversion rate of C4 olefins and the selectivity of the products propylene and ethylene.

[0032] (3) In this invention, the application of the catalyst for the cracking of C4 olefins to propylene and ethylene, by optimizing the reaction process conditions and using the core-shell structure catalyst of this invention with a composite pore structure as the core phase, can significantly improve the conversion rate of C4 olefins and the selectivity of the products propylene and ethylene, and is especially suitable for the cracking of low-concentration C4 olefins to propylene. The conversion rate of low-concentration C4 olefins in the raw material can reach more than 63%, and the selectivity of the products propylene and ethylene dienes is greater than 80%, achieving good technical results. Detailed Implementation

[0033] The present invention will be further illustrated by the following embodiments, but the scope of protection of the present invention is not limited by the embodiments.

[0034] In this invention, the pore distribution was determined using a TriStar 3000 physical adsorption instrument. After vacuum treatment at 300°C for 3 hours, the sample was placed in the instrument and liquid nitrogen was added for testing. The pore distribution of the sample was calculated using the Barret-Joyner-Halenda (BJH) model.

[0035] In the context of this specification, the following formula shall be used for calculation:

[0036] C4 olefin conversion (wt%) = (1 - mass of C4 olefin in product / mass of C4 olefin in feed) × 100%;

[0037] Propylene selectivity (wt%) = Mass of propylene generated in the product / (Mass of C4 olefins in the feedstock - Mass of C4 olefins in the product) × 100%;

[0038] Ethylene selectivity (wt%) = Mass of ethylene generated in the product / (Mass of C4 olefins in the feedstock - Mass of C4 olefins in the product) × 100%.

[0039] In each case, the C4 feedstock came from a blend of C4 feedstocks from the refinery, and the feedstock composition is shown in Table 2.

[0040] Example 1

[0041] Step a)

[0042] Preparation of ZSM-5 molecular sieve raw powder:

[0043] Tetramethylammonium bromide was used as the template agent, aluminum nitrate as the aluminum source, silica sol as the silicon source, and sodium hydroxide as the alkali source. NH4+ was used as the template agent. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The raw materials were mixed and stirred until they reached an emulsion state. The mixture was then transferred to a high-pressure autoclave and crystallized at 120°C for 35 hours. The synthesized product was washed with water, dried at 60°C for 20 hours, and calcined at 500°C for 5 hours to obtain ZSM-5 molecular sieve powder.

[0044] Precursor preparation:

[0045] The ZSM-5 molecular sieve raw powder was dispersed in a 5% sodium carbonate solution, stirred at 80°C for 3 hours, filtered, dried at 120°C for 8 hours, and calcined at 600°C for 4 hours to obtain the precursor hierarchical porous ZSM-5 molecular sieve.

[0046] Step b)

[0047] The materials were re-formulated with the same composition as the raw materials for the synthetic molecular sieve powder, but the total material content was reduced to 50%. Tetramethylammonium bromide was used as the template agent, aluminum nitrate as the aluminum source, silica sol as the silicon source, and sodium hydroxide as the alkali source. NH4+ was used as the template agent. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The ratio of H2O is 0.1:0.005:1:0.3:6. After mixing the above raw materials, add them to the precursor prepared in step a), stir until it becomes an emulsion, transfer it to a high-pressure reactor, crystallize it at 150°C for 10 hours, wash the synthesized product with water, dry it at 80°C for 12 hours, and calcine it at 600°C for 4 hours to obtain the desired core-shell ZSM-5 molecular sieve raw powder.

[0048] The core-shell ZSM-5 molecular sieve powder was exchanged with a 10% ammonium nitrate aqueous solution at 90°C. After washing and drying, it was calcined at 500°C for 8 hours, pressed into tablets, and sieved to obtain a catalyst for the cracking of C4 olefins to propylene and ethylene. The pore distribution of the core-phase molecular sieve is shown in Table 1. The pore distribution of the shell-phase molecular sieve is as follows: pores with a diameter <2 nm account for 87% of the total pore volume, pores with a diameter of 2–50 nm account for 3% of the total pore volume, and pores with a diameter >50 nm account for 10% of the total pore volume. The mass ratio of the core phase to the shell phase of the catalyst is 2:1.

[0049] The activity of the prepared catalyst in the cracking of C4 olefins to propylene and ethylene was evaluated using a fixed-bed catalytic reactor. The process conditions used were: reaction temperature 530℃, reaction pressure 0.02 MPa, and C4 hydrocarbon weight hourly space velocity 15 h⁻¹. -1 The evaluation results are shown in Table 3.

[0050] Example 2

[0051] Step a)

[0052] Preparation of ZSM-5 molecular sieve raw powder:

[0053] Using tetrapropylammonium hydroxide as a template agent, aluminum sulfate as the aluminum source, tetraethyl orthosilicate as the silicon source, and potassium hydroxide as the alkali source, the template agent is NH4+. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The raw materials were mixed and stirred until they reached an emulsion state. The mixture was then transferred to a high-pressure reactor and crystallized at 150°C for 25 hours. The synthesized product was washed with water, dried at 80°C for 15 hours, and calcined at 400°C for 10 hours to obtain ZSM-5 molecular sieve raw powder.

[0054] Precursor preparation:

[0055] The ZSM-5 molecular sieve raw powder was dispersed in an 8% sodium hydroxide solution, stirred at 60°C for 4 hours, filtered, dried at 80°C for 10 hours, and calcined at 500°C for 8 hours to obtain the precursor hierarchical porous ZSM-5 molecular sieve.

[0056] Step b)

[0057] The materials were re-formulated in the same proportions as the raw materials used in the above-mentioned synthesis of molecular sieve powder, with the same total amount of materials. Tetrapropylammonium hydroxide was used as the template agent, aluminum sulfate as the aluminum source, tetraethyl orthosilicate as the silicon source, and potassium hydroxide as the alkali source. NH4+ was used as the template agent. +The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The ratio of H2O to H2O is 0.5:0.001:1:0.4:10. After mixing the above raw materials, add them to the precursor prepared in step a), stir until it becomes an emulsion, transfer it to a high-pressure reactor, crystallize it at 150°C for 25 hours, wash the synthesized product with water, dry it at 120°C for 8 hours, and calcine it at 550°C for 6 hours to obtain the desired core-shell ZSM-5 molecular sieve raw powder.

[0058] The core-shell ZSM-5 molecular sieve powder was exchanged with a 5% ammonium nitrate aqueous solution at 90°C. After washing and drying, it was calcined at 600°C for 4 hours, pressed into tablets, and sieved to obtain a catalyst for the cracking of C4 olefins to propylene and ethylene. The pore distribution of the core-phase molecular sieve is shown in Table 1. The pore distribution of the shell-phase molecular sieve is as follows: pores with a diameter <2 nm account for 92% of the total pore volume, pores with a diameter of 2–50 nm account for 3% of the total pore volume, and pores with a diameter >50 nm account for 5% of the total pore volume. The mass ratio of the core phase to the shell phase of the catalyst is 1:1.

[0059] The activity of the prepared catalyst in the cracking of C4 olefins to propylene and ethylene was evaluated using a fixed-bed catalytic reactor. The process conditions used were: reaction temperature 530℃, reaction pressure 0.02 MPa, and C4 hydrocarbon weight hourly space velocity 15 h⁻¹. -1 The evaluation results are shown in Table 3.

[0060] Example 3

[0061] Step a)

[0062] Preparation of ZSM-5 molecular sieve raw powder:

[0063] Using tetraethylammonium bromide as a template agent, sodium aluminate as the aluminum source, water glass as the silicon source, and sodium hydroxide as the alkali source, the template agent is NH4. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The raw materials were mixed and stirred until they reached an emulsion state. The mixture was then transferred to a high-pressure autoclave and crystallized at 180°C for 30 hours. The synthesized product was washed with water, dried at 100°C for 10 hours, and calcined at 400°C for 10 hours to obtain ZSM-5 molecular sieve raw powder.

[0064] Precursor preparation:

[0065] The ZSM-5 molecular sieve raw powder was dispersed in a 10% sodium carbonate solution, stirred at 90°C for 2 hours, filtered, dried at 100°C for 8 hours, and calcined at 500°C for 8 hours to obtain the precursor hierarchical porous ZSM-5 molecular sieve.

[0066] Step b)

[0067] The materials were re-formulated with the same composition as the above-mentioned synthetic molecular sieve raw material, but the total material content was reduced to 20%. Tetraethylammonium bromide was used as the template agent, sodium aluminate as the aluminum source, water glass as the silicon source, and sodium hydroxide as the alkali source. NH4+ was used as the template agent. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The above raw materials are mixed with H2O = 0.3:0.01:1:0.1:10. The mixture is then added to the precursor prepared in step a), stirred until it becomes an emulsion, and then transferred to a high-pressure reactor. The mixture is crystallized at 180°C for 30 hours. The synthesized product is washed with water, dried at 120°C for 8 hours, and calcined at 550°C for 6 hours to obtain the desired core-shell ZSM-5 molecular sieve powder.

[0068] The core-shell ZSM-5 molecular sieve powder was exchanged with a 10% ammonium nitrate aqueous solution at 90°C. After washing and drying, it was calcined at 500°C for 8 hours, pressed into tablets, and sieved to obtain a catalyst for the cracking of C4 olefins to propylene and ethylene. The pore distribution of the core-phase molecular sieve is shown in Table 1. The pore distribution of the shell-phase molecular sieve is as follows: pores with a diameter <2 nm account for 85% of the total pore volume, pores with a diameter of 2–50 nm account for 5% of the total pore volume, and pores with a diameter >50 nm account for 10% of the total pore volume. The mass ratio of the core phase to the shell phase of the catalyst is 5:1.

[0069] The activity of the prepared catalyst in the cracking of C4 olefins to propylene and ethylene was evaluated using a fixed-bed catalytic reactor. The process conditions used were: reaction temperature 530℃, reaction pressure 0.02 MPa, and C4 hydrocarbon weight hourly space velocity 15 h⁻¹. -1 The evaluation results are shown in Table 3.

[0070] Example 4

[0071] Step a)

[0072] Preparation of ZSM-5 molecular sieve raw powder:

[0073] Tetramethylammonium bromide was used as the template agent, aluminum nitrate as the aluminum source, silica sol as the silicon source, and sodium hydroxide as the alkali source. NH4+ was used as the template agent. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The raw materials were mixed and stirred until they reached an emulsion state. The mixture was then transferred to a high-pressure autoclave and crystallized at 120°C for 35 hours. The synthesized product was washed with water, dried at 60°C for 20 hours, and calcined at 500°C for 5 hours to obtain ZSM-5 molecular sieve powder.

[0074] Precursor preparation:

[0075] The ZSM-5 molecular sieve raw powder was dispersed in a 5% sodium carbonate solution, stirred at 80°C for 3 hours, filtered, dried at 120°C for 8 hours, and calcined at 600°C for 4 hours to obtain the precursor hierarchical porous ZSM-5 molecular sieve.

[0076] Step b)

[0077] The materials were re-formulated with the same composition as the raw materials for the synthetic molecular sieve powder, but with the total material content reduced to 25%. Tetramethylammonium bromide was used as the template agent, aluminum nitrate as the aluminum source, silica sol as the silicon source, and sodium hydroxide as the alkali source. NH4+ was used as the template agent. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The above raw materials are mixed with H2O = 0.5:0.002:1:0.4:5. The mixture is then added to the precursor prepared in step a), stirred until it becomes an emulsion, and then transferred to a high-pressure reactor. The mixture is crystallized at 180°C for 30 hours. The synthesized product is washed with water, dried at 120°C for 8 hours, and calcined at 550°C for 6 hours to obtain the desired core-shell ZSM-5 molecular sieve powder.

[0078] The core-shell ZSM-5 molecular sieve powder was exchanged with a 10% ammonium nitrate aqueous solution at 90°C. After washing and drying, it was calcined at 500°C for 8 hours, pressed into tablets, and sieved to obtain a catalyst for the cracking of C4 olefins to propylene and ethylene. The pore distribution of the core-phase molecular sieve is shown in Table 1. The pore distribution of the shell-phase molecular sieve is as follows: pores with a diameter <2 nm account for 90% of the total pore volume, pores with a diameter of 2–50 nm account for 2% of the total pore volume, and pores with a diameter >50 nm account for 8% of the total pore volume. The mass ratio of the core phase to the shell phase of the catalyst is 4:1.

[0079] The activity of the prepared catalyst in the cracking of C4 olefins to propylene and ethylene was evaluated using a fixed-bed catalytic reactor. The process conditions used were: reaction temperature 530℃, reaction pressure 0.02 MPa, and C4 hydrocarbon weight hourly space velocity 15 h⁻¹. -1 The evaluation results are shown in Table 3.

[0080] Comparative Example 1

[0081] Tetramethylammonium bromide was used as the template agent, aluminum nitrate as the aluminum source, silica sol as the silicon source, and sodium hydroxide as the alkali source. NH4+ was used as the template agent. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The raw materials were mixed and stirred until they reached an emulsion state. The mixture was then transferred to a high-pressure reactor and crystallized at 150°C for 10 hours. The synthesized product was washed with water, dried at 80°C for 12 hours, and calcined at 600°C for 4 hours to obtain ZSM-5 molecular sieve.

[0082] The ZSM-5 molecular sieve was exchanged with a 10% ammonium nitrate aqueous solution at 90°C. After washing and drying, it was calcined at 500°C for 8 hours, pressed into tablets, and sieved to obtain the desired catalyst for the cracking of C4 olefins to propylene and ethylene. In the catalyst, pores with a diameter <2 nm accounted for 87% of the total pore volume, pores with a diameter of 2–50 nm accounted for 3% of the total pore volume, and pores with a diameter >50 nm accounted for 10% of the total pore volume.

[0083] The activity of the prepared catalyst in the cracking of C4 olefins to propylene and ethylene was evaluated using a fixed-bed catalytic reactor. The process conditions used were: operating temperature of 530℃, operating pressure of 0.02 MPa, and weight hourly space velocity of 15 h⁻¹. -1 The evaluation results are shown in Table 3.

[0084] Comparative Example 2

[0085] Tetramethylammonium bromide was used as the template agent, aluminum nitrate as the aluminum source, silica sol as the silicon source, and sodium hydroxide as the alkali source. NH4+ was used as the template agent. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH -The raw materials were mixed and stirred until they reached an emulsion state. The mixture was then transferred to a high-pressure autoclave and crystallized at 120°C for 35 hours. The synthesized product was washed with water, dried at 60°C for 20 hours, and calcined at 500°C for 5 hours to obtain ZSM-5 molecular sieve powder.

[0086] The molecular sieve powder was dispersed in a 5% sodium carbonate solution, stirred at 80°C for 3 hours, filtered, dried at 120°C for 8 hours, and calcined at 600°C for 4 hours to obtain a multi-level porous ZSM-5 molecular sieve.

[0087] The hierarchical ZSM-5 molecular sieve was exchanged with a 10% ammonium nitrate aqueous solution at 90°C. After washing and drying, it was calcined at 500°C for 8 hours, pressed into tablets, and sieved to obtain the desired catalyst for the cracking of C4 olefins to propylene and ethylene. In the catalyst, pores with a diameter <2 nm accounted for 47% of the total pore volume, pores with a diameter of 2–50 nm accounted for 28% of the total pore volume, and pores with a diameter >50 nm accounted for 25% of the total pore volume.

[0088] The activity of the prepared catalyst in the cracking of C4 olefins to propylene and ethylene was evaluated using a fixed-bed catalytic reactor. The process conditions used were: operating temperature of 530℃, operating pressure of 0.02 MPa, and weight hourly space velocity of 15 h⁻¹. -1 The evaluation results are shown in Table 3.

[0089] Comparative Example 3

[0090] Preparation of ZSM-5 molecular sieve raw powder:

[0091] Tetramethylammonium bromide was used as the template agent, aluminum nitrate as the aluminum source, silica sol as the silicon source, and sodium hydroxide as the alkali source. NH4+ was used as the template agent. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The raw materials were mixed and stirred until they reached an emulsion state. The mixture was then transferred to a high-pressure reactor and crystallized at 150°C for 10 hours. The synthesized product was washed with water, dried at 80°C for 12 hours, and calcined at 600°C for 4 hours to obtain ZSM-5 molecular sieve raw powder.

[0092] The materials were re-formulated with the same composition as the raw materials for the synthetic molecular sieve powder, but the total material content was reduced to 50%. Tetramethylammonium bromide was used as the template agent, aluminum nitrate as the aluminum source, silica sol as the silicon source, and sodium hydroxide as the alkali source. NH4+ was used as the template agent. + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH.- The molar ratio of NH4 to water is: + Al2O3:SiO2:OH - The ratio of H2O to 0.1:0.005:1:0.3:6 was added, along with 2 wt% methylcellulose as a pore-forming agent. After the above raw materials were mixed evenly, the synthesized ZSM-5 molecular sieve powder was evenly dispersed in the material. After thorough stirring, the mixture was transferred to a high-pressure reactor and crystallized at 150°C for 10 hours. The synthesized product was then washed with water, dried at 80°C for 12 hours, and calcined at 600°C for 4 hours to obtain the desired core-shell ZSM-5 molecular sieve powder.

[0093] The core-shell ZSM-5 molecular sieve powder was exchanged at 90°C with a 10% ammonium nitrate aqueous solution, washed, dried, and then calcined at 500°C for 8 hours. After pressing and sieving, the desired catalyst for the cracking of C4 olefins to propylene and ethylene was obtained.

[0094] The catalyst is a core-shell molecular sieve, with a conventional molecular sieve as the core and a composite pore structure molecular sieve as the shell. The pore distribution of the shell-phase composite pore structure molecular sieve is as follows: pores with a diameter <2 nm account for 35% of the total pore volume, pores with a diameter of 2–50 nm account for 43% of the total pore volume, and pores with a diameter >50 nm account for 22% of the total pore volume. The pore distribution of the shell-phase molecular sieve is as follows: pores with a diameter <2 nm account for 87% of the total pore volume, pores with a diameter of 2–50 nm account for 3% of the total pore volume, and pores with a diameter >50 nm account for 10% of the total pore volume.

[0095] The activity of the prepared catalyst in the cracking of C4 olefins to propylene and ethylene was evaluated using a fixed-bed catalytic reactor. The process conditions used were: operating temperature of 530℃, operating pressure of 0.02 MPa, and weight hourly space velocity of 15 h⁻¹. -1 The evaluation results are shown in Table 3.

[0096] Table 1. Pore distribution of nucleophase molecular sieve catalysts in Examples 1-4

[0097]

[0098] Table 2

[0099]

[0100] Table 3

[0101]

[0102] The specific embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combining the various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A catalyst for the cracking of C4 olefins to produce propylene and ethylene, characterized in that, The catalyst has a core-shell structure, with the core phase being a ZSM-5 molecular sieve having a composite pore structure and the shell phase being a ZSM-5 molecular sieve. In the ZSM-5 molecular sieve with a core-phase composite pore structure, the pore volume occupied by pores with a pore size < 2 nm is a, the pore volume occupied by pores with a pore size of 2~50 nm is b, and the pore volume occupied by pores with a pore size > 50 nm is c; where b / a is 0.10~0.80 and c / a is 0.30~0.

70. In ZSM-5 molecular sieves with a core-phase composite pore structure, pores with a diameter <2 nm account for 20%~75% of the total pore volume, pores with a diameter of 2~50 nm account for 10%~55% of the total pore volume, and pores with a diameter >50 nm account for 6%~30% of the total pore volume. In the shell-phase ZSM-5 molecular sieve, pores with a diameter of <2 nm account for 85% to 100% of the total pore volume; pores with a diameter of >50 nm account for 0 to 15% of the total pore volume; and pores with a diameter of 2 to 50 nm account for 0 to 10% of the total pore volume.

2. The catalyst according to claim 1, characterized in that, The mass ratio of the core phase to the shell phase is 1 to 5:

1.

3. The catalyst according to claim 1, characterized in that, In ZSM-5 molecular sieves with a core-phase composite pore structure, pores with a diameter >50 nm account for 15% to 30% of the total pore volume.

4. A method for preparing the catalyst according to any one of claims 1 to 3, comprising the following steps: a) The ZSM-5 molecular sieve raw powder was treated in an alkaline solution and then calcined to obtain the precursor. b) Mix the template agent, aluminum source, silicon source, alkali source and the precursor described in step a), perform hydrothermal crystallization, a second calcination, then perform ammonium exchange, and a third calcination to obtain the catalyst.

5. The preparation method according to claim 4, characterized in that, The alkali mentioned in step a) includes at least one of sodium hydroxide and sodium carbonate; And / or, in step a), the temperature of the alkaline solution treatment is 60~100℃; the treatment time in the alkaline solution is 2~4h; And / or, the conditions for the first calcination in step a) are: temperature 500~600℃, time 4~8h.

6. The preparation method according to claim 5, characterized in that, Step a) The temperature for treatment in the alkaline solution is 60~90℃.

7. The preparation method according to claim 4 or 5, characterized in that, In step b), The template agent includes at least one of tetramethylammonium bromide, tetraethylammonium bromide, tetrapropylammonium bromide, and tetrapropylammonium hydroxide; And / or, the aluminum source includes at least one of aluminum nitrate, aluminum sulfate, and sodium aluminate; And / or, the silicon source includes at least one of water glass, silica sol, and tetraethyl orthosilicate; And / or, the alkali source includes at least one of sodium hydroxide and potassium hydroxide; And / or, template agent with NH4 + The aluminum source is calculated as Al2O3, the silicon source as SiO2, and the alkali source as OH. - The molar ratio of NH4+ to water (calculated as H2O) is: + Al2O3:SiO2:OH - : H2O=0.1~0.5: 0.001~0.01: 1: 0.1~0.4: 5~10.

8. The preparation method according to claim 4, characterized in that, In step b), the hydrothermal crystallization temperature is 80~200℃, and the crystallization time is 10~50h; And / or, in step b), the temperature of the second calcination is 500~600℃ and the time is 4~8h; And / or, in step b), the temperature of the third calcination is 500~600℃ and the time is 4~8h.

9. The use of a catalyst according to any one of claims 1 to 3 or a catalyst prepared by any one of claims 4 to 8 in the reaction of C4 olefins cracking to produce propylene and ethylene.

10. The application according to claim 9, characterized in that, The application is a reaction to produce propylene and ethylene by cracking low-concentration C4 olefins; the low-concentration C4 olefin feedstock comes from C4 hydrocarbons in a refinery; the mass concentration of C4 olefins in the C4 hydrocarbons is 30%~50%.

11. The application according to claim 10, characterized in that, The C4 hydrocarbon is a C4 mixture; the C4 mixture includes at least one C4 olefin and at least one C4 alkane.

12. The application according to claim 11, characterized in that, In the C4 hydrocarbons, the mass concentration of C4 olefins is 30%~50%, and the mass concentration of C4 alkanes is 50%~70%.

13. The application according to claim 9, characterized in that, The reaction conditions are as follows: reaction temperature of 500-600℃; and / or reaction pressure of 0-1.0 MPa; and / or C4 hydrocarbon weight hourly space velocity of 1-30 h⁻¹. -1 .

14. The application according to claim 13, characterized in that, The reaction conditions are as follows: reaction temperature of 520~580℃; and / or reaction pressure of 0.03~0.8MPa; and / or C4 hydrocarbon weight hourly space velocity of 2~20h⁻¹. -1 .

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