Zsm-5 molecular sieve containing iron small crystal grains, and preparation method and application thereof

Abstract

The present disclosure relates to a ZSM-5 molecular sieve containing iron small crystal grains and a preparation method and application thereof, the method comprising: S1, mixing a silicon source, an aluminum source, an alkali source, seeds, a template agent and water to obtain a first mixture; S2, mixing an iron source, a peroxide, water and the first mixture and then performing a hydrothermal reaction; wherein the peroxide is selected from one or more of sodium persulfate, hydrogen peroxide and a Fenton reagent. The method of the present disclosure can prepare a ZSM-5 molecular sieve containing iron small crystal grains with a small crystal grain size, which is used in a light hydrocarbon catalytic cracking reaction and has a relatively optimal yield of low-carbon olefins.

Description

Technical Field

[0001] This application relates to a ZSM-5 molecular sieve containing iron-containing small crystals, its preparation method, and its application. Background Technology

[0002] Catalytic cracking refers to the process of converting hydrocarbons into low-carbon olefins such as ethylene, propylene, and butene under the action of a catalyst, while simultaneously producing light aromatics. The heavy oil catalytic cracking process for producing low-carbon olefins has long held a significant market share due to its advantages of strong feedstock adaptability and lower production costs. Since the mid-1980s, the Research Institute of Petroleum Processing (RIPP) of China Petroleum & Chemical Corporation (Sinopec) has been continuously researching this technology. In 1994, RIPP developed a catalytic cracking (DCC) process for producing low-carbon olefins from heavy oil (vacuum distillate, coking wax oil, and residue oil, etc.). After the successful industrialization of DCC technology, RIPP further developed a catalytic thermal cracking (CPP) process for directly producing ethylene and propylene from heavy oil, offering multiple production options including maximum ethylene production, maximum propylene production, and the simultaneous production of ethylene and propylene. Compared to steam cracking, the CPP process broadens the sources of ethylene feedstock, reduces ethylene feedstock costs, and, due to its lower reaction temperature compared to steam cracking, significantly reduces energy consumption. Light oil catalytic cracking mainly refers to the process of producing ethylene and propylene from light gasoline or naphtha in the presence of a catalyst. This process enables the conversion of petroleum resources into basic chemical feedstocks and will play an important role in future chemical refineries.

[0003] Introducing metal components into molecular sieves can be achieved through encapsulation. This method not only ensures uniform dispersion of the metal components within the molecular sieve but also leverages the shape-selective properties of the sieve for selective dehydrogenation, thereby reducing methane content and increasing selectivity for low-carbon olefins. Some studies have directly synthesized metal-encapsulated Pd-containing molecular sieves using ethylenediamine as a ligand, achieving the direct synthesis of high-silica molecular sieves with encapsulated metals. Characterization results show that the Pd particles after hydrogen reduction are approximately 1.5 nm in size and distributed at the intersection of the two sets of ten-membered ring channels in the MFI molecular sieve. However, while there is considerable research on encapsulating metals into all-silica or high-silica MFI molecular sieves, encapsulation into silica-alumina type MFI molecular sieves is relatively rare. This is because directly introducing metals into the synthesis system can lead to difficulties in synthesis or result in low activity, as metals readily form hydroxide precipitates in alkaline environments and readily interact with phosphorus. Summary of the Invention

[0004] The purpose of this disclosure is to provide a ZSM-5 molecular sieve containing small iron crystals, its preparation method, and its application. The method disclosed herein can prepare ZSM-5 molecular sieves with small iron crystal sizes, which can be used for catalytic cracking reactions of light hydrocarbons, resulting in a better yield of low-carbon olefins.

[0005] To achieve the above objectives, the first aspect of this disclosure provides a method for preparing ZSM-5 molecular sieves containing iron-containing small crystallites, the method comprising:

[0006] S1. Mix the silicon source, aluminum source, alkali source, seed crystal, template agent and water to obtain the first mixture;

[0007] S2. After mixing the iron source, peroxide, water and the first mixture, a hydrothermal reaction is carried out.

[0008] The peroxide is selected from one or more of sodium persulfate, hydrogen peroxide, and Fenton's reagent.

[0009] Optionally, step S2 includes:

[0010] SS1. The iron source, peroxide and water are mixed to obtain a second mixture; the molar ratio of the iron source, peroxide and water, calculated as Fe2O3, is 1:(0.2-2.5):(100-1500); preferably 1:(0.2-2):(500-800);

[0011] SS2. The first mixture and the second mixture are mixed and then subjected to the hydrothermal reaction; the molar ratio of the first mixture (calculated as Al2O3) to the second mixture (calculated as Fe2O3) is 1:(0.01-2.5), preferably 1:(0.4-1.8).

[0012] Optionally, in step S1, the molar ratio of the silicon source (calculated as SiO2), the aluminum source (calculated as Al2O3), the alkali source (calculated as alkali metal oxide), the template agent, and the water is (20-300):1:(1-10):(1-30):(400-2500), preferably (30-100):1:(2-8):(2-10):(500-1200);

[0013] Based on SiO2, the amount of the seed crystal is 3-12% by weight, preferably 8-10% by weight, relative to the total weight of the silicon source.

[0014] Optionally, in step S2, the conditions for the hydrothermal reaction include: a temperature of 150-220℃ and a time of 8-32h.

[0015] Optionally, the method further includes: subjecting the solid product obtained by hydrothermal treatment to ammonium exchange and calcination; the calcination conditions include: a temperature of 400-800℃, a time of 0.5-8h, and an atmosphere of air or water vapor.

[0016] Optionally, the silicon source includes one or more of silica gel, silicon dioxide, silica fume, and silicates;

[0017] The aluminum source includes one or more of sodium aluminate, SB powder, aluminum alkoxide, aluminum oxide, aluminum hydroxide, and aluminum sulfate;

[0018] The alkaline source includes sodium hydroxide and / or potassium hydroxide;

[0019] The seed crystals include one or more ZSM-5 molecular sieves with a silicon-to-aluminum ratio of 20-50, and the average particle size of the ZSM-5 molecular sieve is 0.5-2 μm.

[0020] The iron source is selected from one or more of ferric nitrate, ferric oxalate, ferric sulfate, and ferric hydroxide;

[0021] The template agent includes an organic nitrogen-containing template agent, preferably one or more of n-butylamine, tetrapropylammonium hydroxide and tetrapropylammonium bromide.

[0022] The second aspect of this disclosure provides a ZSM-5 molecular sieve containing iron-containing small crystallites prepared by the method provided in the first aspect of this disclosure.

[0023] Optionally, the ZSM-5 molecular sieve containing iron-containing small crystals has a structure in which the iron-active component is internally encapsulated within the ZSM-5 molecular sieve.

[0024] The iron-active component is distributed on the framework of the ZSM-5 molecular sieve, and the grain size of the iron-containing small-crystal ZSM-5 molecular sieve is 0.5-1μm.

[0025] Optionally, the specific surface area of ​​the ZSM-5 molecular sieve containing iron-containing small crystallites is 240-550 m². 2 / g, total pore volume is 0.150-0.28cm³. 3 ·g -1 The micropore volume is 0.140-0.20 cm³. 3 ·g -1 The mesopore volume is 0.010-0.120 cm³. 3 ·g -1 The relative crystallinity is 82-100%;

[0026] In the ZSM-5 molecular sieve containing iron-containing small crystallites, the molar ratio of SiO2 to Al2O3 is 20-300;

[0027] The content of the iron active component, calculated as Fe2O3, is 0.8-1.5% by weight relative to the total weight of the ZSM-5 molecular sieve containing iron-containing small crystallites.

[0028] This disclosure provides a third aspect of the application of the iron-containing small-crystal ZSM-5 molecular sieve provided in the second aspect of the present invention in the catalytic cracking reaction of light hydrocarbons.

[0029] Optionally, the conditions for the catalytic cracking reaction include: a temperature of 500-680℃ and a reaction mass hourly space velocity of 20-40 h⁻¹. -1 The reaction pressure is 0.8-1.2 MPa, and the agent-to-oil weight ratio is 1-2.

[0030] Through the above technical solution, the method disclosed herein synthesizes ZSM-5 molecular sieves with iron-containing small crystals by introducing peroxides into the template agent system. The prepared molecular sieve has a small crystal size and is used in the catalytic cracking reaction of light hydrocarbons, resulting in a better yield of low-carbon olefins.

[0031] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Attached Figure Description

[0032] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings:

[0033] Figure 1 This is the X-ray diffraction pattern of the iron-containing small-crystal ZSM-5 molecular sieve A prepared in Example 1 of this disclosure;

[0034] Figure 2 This is a scanning electron microscope image of the iron-containing small-crystal ZSM-5 molecular sieve A prepared in Example 1 of this disclosure;

[0035] Figure 3 This is a transmission electron microscope (TEM) image of the iron-containing small-crystal ZSM-5 molecular sieve A prepared in Example 1 of this disclosure;

[0036] Figure 4 This is a mapping image of the iron-containing small-crystal ZSM-5 molecular sieve A prepared in Example 1 of this disclosure;

[0037] Figure 5 This is the X-ray diffraction pattern of the iron-containing small-crystal ZSM-5 molecular sieve B prepared in Example 2 of this disclosure;

[0038] Figure 6 This is a scanning electron microscope image of the iron-containing small-crystal ZSM-5 molecular sieve B prepared in Example 2 of this disclosure. Detailed Implementation

[0039] The specific embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this disclosure.

[0040] The first aspect of this disclosure provides a method for preparing ZSM-5 molecular sieves containing iron-containing small crystallites, the method comprising:

[0041] S1. Mix the silicon source, aluminum source, alkali source, seed crystal, template agent and water to obtain the first mixture;

[0042] S2. After mixing the iron source, peroxide, water and the first mixture, a hydrothermal reaction is carried out.

[0043] The peroxide is selected from one or more of sodium persulfate, hydrogen peroxide, and Fenton's reagent.

[0044] This disclosure employs a template agent method and introduces a free radical initiator to prepare iron-containing small-crystal ZSM-5 molecular sieves. The prepared iron-containing small-crystal ZSM-5 molecular sieves have small crystal size and exhibit good catalytic effect when used in light hydrocarbon catalytic cracking reactions, which can further improve the conversion rate of saturated hydrocarbons and the selectivity of low-carbon olefins.

[0045] In a preferred embodiment of this disclosure, the peroxide is sodium persulfate or hydrogen peroxide, more preferably sodium persulfate. The aforementioned peroxide can simultaneously interact with both the metal and the molecular sieve precursor, promoting molecular sieve nucleation around the metal and encapsulating the metal within the molecular sieve.

[0046] In one specific embodiment of this disclosure, step S2 includes: SS1, mixing the iron source, peroxide, and water to obtain a second mixture; the molar ratio of the iron source, peroxide, and water, calculated as Fe2O3, is 1:(0.2-2.5):(100-1500); preferably 1:(0.2-2):(500-800); SS2, mixing the first mixture and the second mixture and then performing the hydrothermal reaction; the molar ratio of the first mixture, calculated as Al2O3, to the second mixture, calculated as Fe2O3, is 1:(0.01-2.5), preferably 1:(0.4-1.8). Within the above range of proportions, the second mixture can be free of free iron ions or ferrous ions, avoiding the presence of aggregated iron active components in the prepared molecular sieve, thereby further improving its catalytic performance.

[0047] In one specific embodiment of this disclosure, in step S1, the molar ratio of the silicon source (calculated as SiO2), the aluminum source (calculated as Al2O3), the alkali source (calculated as alkali metal oxide), the template agent, and the water is (20-300):1:(1-10):(1-30):(500-1500), preferably (30-100):1:(2-8):(2-10):(500-1200); the amount of the seed crystal (calculated as SiO2) relative to the total weight of the silicon source is 3-12% by weight, preferably 8-10% by weight.

[0048] According to this disclosure, hydrothermal treatment is well known to those skilled in the art and can be carried out in equipment familiar to those skilled in the art, such as a closed, heat-resistant, and pressure-resistant reactor. In one specific embodiment of this disclosure, in step S2, the conditions for the hydrothermal reaction include: a temperature of 150-220°C and a time of 8-32 hours; preferably, a temperature of 150-180°C and a time of 8-20 hours. This disclosure does not impose specific limitations on the pressure of the hydrothermal reaction; for example, it can be carried out under the autogenous pressure of the reaction system or under an applied pressure, preferably under the autogenous pressure of the reaction system.

[0049] In one specific embodiment of this disclosure, the method further includes: subjecting the solid product obtained from the hydrothermal treatment to ammonium exchange and calcination; the calcination conditions include: a temperature of 400-800°C, a time of 0.5-8 hours, and an atmosphere of air or water vapor; preferably, the temperature is 500-600°C, and the time is 2-4 hours. Calcination is well known to those skilled in the art and can be performed, for example, in a muffle furnace, a tube furnace, etc. In a preferred embodiment, the solid product is washed until neutral before ammonium exchange; the washing liquid can be any type of liquid that does not react with the solid product, such as deionized water. In another preferred embodiment, the solid obtained from ammonium exchange is dried before calcination; the drying conditions may include: a temperature of 100-150°C, and a time of 5-20 hours.

[0050] In one specific embodiment of this disclosure, the solid product obtained from hydrothermal treatment can be removed by methods such as centrifugation and precipitation.

[0051] According to this disclosure, the silicon source, aluminum source, and alkali source are well known to those skilled in the art. In one specific embodiment of this disclosure, the silicon source includes one or more of silica gel, silicon dioxide, silica fume, and silicates; the aluminum source includes one or more of sodium aluminate, SB powder, aluminum alkoxide, aluminum oxide, aluminum hydroxide, and aluminum sulfate; and the alkali source includes sodium hydroxide and / or potassium hydroxide.

[0052] According to this disclosure, the iron source can be a water-soluble ferric salt and / or ferrous salt. In one specific embodiment of this disclosure, the iron source is selected from one or more of ferric nitrate, ferric sulfate, and ferric hydroxide.

[0053] In one specific embodiment of this disclosure, the seed crystals include one or more ZSM-5 molecular sieves with a silicon-to-aluminum ratio of 20-50, such as, but not limited to, ZPR molecular sieves and ZSP molecular sieves; the average particle size of the ZSM-5 molecular sieve is 0.5-2 μm, preferably 1-2 μm. In this disclosure, the average particle size of the ZSM-5 molecular sieve refers to the average particle size estimated by performing SEM analysis on the molecular sieve and measuring the particle size of 50 randomly selected particles from its SEM image, using a HITACHI S-4800 SEM instrument.

[0054] According to this disclosure, the template agent is an organic nitrogen-containing template agent, preferably including one or more of aminoalkane, nitrogen-containing organic hydroxide and organic ammonium bromide; more preferably including one or more of n-butylamine, tetrapropylammonium hydroxide and tetrapropylammonium bromide.

[0055] The second aspect of this disclosure provides a ZSM-5 molecular sieve containing iron-containing small crystallites prepared by the method provided in the first aspect of this disclosure.

[0056] In one specific embodiment of this disclosure, the ZSM-5 molecular sieve containing iron microcrystals has a structure in which iron active components are internally encapsulated within the ZSM-5 molecular sieve; the iron active components are distributed on the framework of the ZSM-5 molecular sieve, and the grain size of the ZSM-5 molecular sieve containing iron microcrystals is 0.3-0.8 μm.

[0057] The grain size of the iron-containing small-grained ZSM-5 molecular sieve disclosed herein is further preferably 0.5-0.8 μm.

[0058] In this disclosure, "the iron-active component is distributed on the framework of the ZSM-5 molecular sieve" means that at least a portion of the Si and Al on the ZSM-5 molecular sieve framework is replaced by Fe. The introduction of iron can modulate the acidity of the ZSM-5 molecular sieve, improve its catalytic function in cracking and dehydrogenation, and introduce the iron-active component into the interior of the molecular sieve in an encapsulated manner, further reducing the severity of the catalytic cracking reaction of light hydrocarbons and improving the conversion rate of saturated hydrocarbons and the selectivity of low-carbon olefins. In this disclosure, "the outer surface of the iron-containing small-crystal ZSM-5 molecular sieve particles does not contain iron-active components" means that the iron-active component is not distributed on the outer surface of the molecular sieve, and the outer surface of the iron-active component is not connected to the outer surface of the ZSM-5 molecular sieve. Compared to the iron-active component existing on the outer surface of the molecular sieve, the iron-active component of the molecular sieve in this disclosure is more uniformly distributed inside the molecular sieve, which is more conducive to the selective dehydrogenation of alkanes into olefins in the molecular sieve channels, rather than dehydrogenation on the outer surface of the molecular sieve.

[0059] In this disclosure, the iron-active component may include skeletal iron and / or Fe2O3.

[0060] In one specific embodiment of this disclosure, the specific surface area of ​​the ZSM-5 molecular sieve containing iron-containing small crystallites is 240-550 m². 2 / g, preferably 320-370m 2 / g, total pore volume is 0.150-0.28cm³. 3 ·g -1 The preferred size is 0.16-0.24cm. 3 ·g -1 The micropore volume is 0.140-0.200 cm³. 3 ·g -1 The preferred size is 0.15-0.18cm. 3 ·g -1 The mesopore volume is 0.010-0.120 cm³. 3 ·g -1 The preferred size is 0.01-0.015cm. 3 ·g -1 The relative crystallinity is 82-100%, preferably 85-100%; in the ZSM-5 molecular sieve containing iron small crystals, the molar ratio of SiO2 to Al2O3 is 20-300; relative to the total weight of the ZSM-5 molecular sieve containing iron small crystals, the content of the iron active component, calculated as Fe2O3, is 0.3-3% by weight, preferably 0.8-1.5% by weight.

[0061] The molar ratio of SiO2 to Al2O3 was determined using X-ray fluorescence spectroscopy. The relative crystallinity was determined using a Siemens D5005 X-ray diffractometer, with the ZSM-5 molecular sieve standard from the Research Institute of Petroleum Processing (i.e., the relative crystallinity of the ZSM-5 molecular sieve standard from the Research Institute of Petroleum Processing, China Petroleum & Chemical Corporation, is 100%. Specific surface area and pore volume parameters were determined using a specific surface area analyzer based on the N2 adsorption principle and the BET calculation method (see Petrochemical Analytical Methods (RIPP Test Methods), RIPP 151-90, Science Press, 1990). The content of the iron active component, calculated as Fe2O3, was determined using X-ray fluorescence spectroscopy.

[0062] The third aspect of this disclosure provides the application of the iron-containing small-crystal ZSM-5 molecular sieve provided in the second aspect of this disclosure in the catalytic cracking reaction of n-tetradecane and the catalytic cracking reaction of light hydrocarbons.

[0063] In one specific embodiment of this disclosure, the conditions for the catalytic cracking reaction include: a temperature of 500-680℃, a reaction pressure of 0.8-1.2MPa, and a catalyst-to-oil weight ratio of 1-1.8; preferably, the temperature is 540-600℃, the reaction pressure is 1-1.2MPa, and the catalyst-to-oil weight ratio is 1.28-1.92.

[0064] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited thereto.

[0065] The silicon-aluminum molar ratio is the molar ratio of SiO2 to Al2O3, which is detected by X-ray fluorescence spectroscopy. The instrument is a Rigaku Electric Co., Ltd. 3013 X-ray fluorescence spectrometer. The test conditions are: tungsten target, excitation voltage 40kV, and excitation current 50mA.

[0066] Grain size: The grain size was estimated by performing SEM analysis on the molecular sieve and measuring the particle size of 50 randomly selected particles from the SEM image and calculating the average value. The SEM instrument model was HITACHI S-4800.

[0067] Relative crystallinity: Based on the ZSM-5 molecular sieve standard sample from the Research Institute of Petroleum Processing (i.e., the relative crystallinity of the ZSM-5 molecular sieve standard sample from the Research Institute of Petroleum Processing of China Petroleum & Chemical Corporation is 100%), the crystallinity was measured using a Siemens D5005 X-ray diffractometer.

[0068] Specific surface area: Determined using a specific surface area analyzer based on the N2 adsorption principle and the BET calculation method (see Petrochemical Analysis Methods (RIPP Test Methods), RIPP151-90, Science Press, 1990).

[0069] Method for testing pore volume: The sample is evacuated to a vacuum of 1.33 × 10⁻⁶ at 350°C. -2 The pressure was maintained at a constant temperature and pressure for 15 hours. Tests were conducted at liquid nitrogen temperature -196℃ to measure the adsorption and desorption of N2 by the sample under different relative pressures (p / p0). Desorption branching data were calculated using the BJH (Barrett-Joyner-Halenda) method to obtain the pore size distribution data of the sample.

[0070] The scanning electron microscope (SEM) used was a HITACHI S-4800. The testing conditions were: accelerating voltage of 20.0 kV, sample preparation, and observation of the crystal morphology and size of the molecular sieve samples at magnifications of 5000-30000.

[0071] The transmission electron microscope was a JEM-2100 (200kV) transmission electron microscope manufactured by Nippon Electron Ltd.; the test conditions were: accelerating voltage of 200kV, sample preparation, and observation of the crystal morphology and size of the molecular sieve samples at magnifications of 5000-30000.

[0072] STEM-Mapping test method and instrument: JEM-2100 (200kV) transmission electron microscope, manufactured by Nippon Electron Ltd. The accelerating voltage of the electron microscope was 200kV. The sample was dispersed in anhydrous ethanol and then dropped onto a sample grid with a diameter of 3mm for observation and testing.

[0073] Test method for the content of iron active components, calculated as Fe2O3: Semi-quantitative analysis of iron element is performed by measuring the intensity of characteristic spectral lines of each element by XRF.

[0074] Example 1

[0075] This embodiment uses the following steps to prepare iron-containing small-crystal ZSM-5 molecular sieve A:

[0076] (1) Add 0.99g sodium hydroxide, 40.8g deionized water, 20g silica gel, 6.77g low-alkali sodium aluminate (Na2O: 156.3g / L, Al2O3: 103.8g / L), 1.97g ZSM-5 seed crystals and 1.98g n-butylamine in sequence under stirring, and stir thoroughly to obtain the first mixture;

[0077] The molar ratio of silicon source (SiO2), aluminum source (Al2O3), alkali source (alkali metal oxide), n-butylamine, and water is 55:1:4.675:4.95:589; the amount of ZSM-5 seed crystals is 10% by weight relative to the total weight of silicon source (SiO2); the silicon-aluminum ratio of ZSM-5 seed crystals is 27, and the average particle size is 1-2 μm.

[0078] (2) Dissolve 1.13g of ferric nitrate nonahydrate in 10g of deionized water, add 0.14g of sodium persulfate, and stir thoroughly until clear and transparent to obtain a second mixture; wherein, the molar ratio of iron source, sodium persulfate and water (calculated as Fe2O3) is 1:0.2:200, and the molar ratio of aluminum source (calculated as Al2O3) to iron source (calculated as Fe2O3) is 1:0.40;

[0079] (3) Add the second mixture to the first mixture, mix thoroughly and evenly, then transfer to a stainless steel autoclave and perform hydrothermal treatment at 170°C for 12 hours;

[0080] (4) The product obtained from the hydrothermal treatment in step (3) was filtered, washed until pH = 7-8, and then subjected to ammonium exchange. After drying at 120℃ for 12h, it was calcined at 550℃ for 2h in air to obtain small-crystal ZSM-5 molecular sieve encapsulated with iron, denoted as A. Its structural parameters are listed in Table 1, and its X-ray diffraction pattern is shown in Table 1. Figure 1 Scanning electron microscope images can be found Figure 2 See STEM and Mapping photos Figure 3 and Figure 4 .

[0081] Example 2

[0082] This embodiment uses the same method as in Example 1 to prepare iron-containing small-crystal ZSM-5 molecular sieve B. The only difference is that in step (2), 1.13g of ferric nitrate nonahydrate is dissolved in 10g of deionized water, and 0.28g of sodium persulfate is added. The mixture is stirred thoroughly until it becomes clear and transparent, resulting in a second mixture. The molar ratio of iron source (calculated as Fe2O3), sodium persulfate, and water is 1:0.4:200, and the molar ratio of aluminum source (calculated as Al2O3) to iron source (calculated as Fe2O3) is 1:0.40. The resulting encapsulated iron-containing small-crystal ZSM-5 molecular sieve is denoted as B. Its structural parameters are listed in Table 1, and its X-ray diffraction pattern is shown in Table 1. Figure 5 Scanning electron microscope images can be found Figure 6 .

[0083] Example 3

[0084] This embodiment uses the same method as in Example 1 to prepare iron-containing small-crystal ZSM-5 molecular sieve C. The only difference is that in step (1), 1.53g sodium hydroxide, 105g deionized water, 20g silica gel, 3.38g low-alkali sodium aluminate (Na2O: 156.3g / L, Al2O3: 103.8g / L), 1.97g ZSM-5 seed crystals, and 1.43g n-butylamine are added sequentially under stirring and stirred thoroughly to obtain the first mixture.

[0085] The molar ratio of silicon source (SiO2), aluminum source (Al2O3), alkali source (alkali metal oxide), n-butylamine, and water is 110:1:9.35:11:2420; the amount of ZSM-5 seed crystals is 10% by weight relative to the total weight of silicon source (SiO2); the silicon-aluminum ratio of ZSM-5 seed crystals is 27, and the average particle size is 1-2 μm.

[0086] Example 4

[0087] This embodiment uses the same method as in Example 1 to prepare iron-containing small-crystal ZSM-5 molecular sieve D. The only difference is that (1) 1.23g sodium hydroxide, 41.9g deionized water, 20g silica gel, 5.33g low-alkali sodium aluminate (Na2O: 156.3g / L, Al2O3: 103.8g / L), 1.97g ZSM-5 seed crystals, and 1.98g n-butylamine are added sequentially under stirring and stirred thoroughly to obtain the first mixture;

[0088] The molar ratio of silicon source (SiO2), aluminum source (Al2O3), alkali source (alkali metal oxide), n-butylamine, and water is 70:1:4.675:9.9:589; the amount of ZSM-5 seed crystals is 10% by weight relative to the total weight of silicon source (SiO2); the silicon-aluminum ratio of ZSM-5 seed crystals is 27, and the average particle size is 1-2 μm.

[0089] (2) Dissolve 1.08g of ferric nitrate nonahydrate in 10g of deionized water, add 0.56g of sodium persulfate, and stir thoroughly until clear and transparent to obtain a second mixture; wherein the molar ratio of iron source, sodium persulfate and water (calculated as Fe2O3) is 1:0.8:200, and the molar ratio of aluminum source (calculated as Al2O3) to iron source (calculated as Fe2O3) is 1:0.40.

[0090] Example 5

[0091] This embodiment uses the same method as in Example 1 to prepare iron-containing small crystals. The only difference is that in step (2), 1.13g of ferric nitrate nonahydrate is dissolved in 22.5g of deionized water, and 1.61g of sodium persulfate is added. The mixture is stirred thoroughly until it is clear and transparent to obtain a second mixture. The molar ratio of iron source (calculated as Fe2O3), sodium persulfate and water is 1:2.3:450, and the molar ratio of aluminum source (calculated as Al2O3) to iron source (calculated as Fe2O3) is 1:0.4.

[0092] Comparative Example 1

[0093] Iron-containing small-crystal ZSM-5 molecular sieve D1 was prepared using the same method as in Example 2, except that the free radical initiator sodium persulfate was not added.

[0094] (1) Add 0.99g sodium hydroxide, 40.8g deionized water, 20g silica gel, 6.77g low-alkali sodium aluminate (Na2O: 156.3g / L, Al2O3: 103.8g / L), 1.97g ZSM-5 seed crystals and 1.98g n-butylamine in sequence under stirring, and stir thoroughly to obtain the first mixture;

[0095] The molar ratio of silicon source (SiO2), aluminum source (Al2O3), alkali source (alkali metal oxide), n-butylamine, and water is 55:1:4.675:4.95:589; the amount of ZSM-5 seed crystals is 10% by weight relative to the total weight of silicon source (SiO2); the silicon-aluminum ratio of ZSM-5 seed crystals is 27, and the average particle size is 1-2 μm.

[0096] (2) Dissolve 1.13g of ferric nitrate nonahydrate in 10g of deionized water and stir thoroughly until clear and transparent to obtain a second mixture; after high-temperature crystallization, iron-containing ZSM-5 molecular sieve is obtained, denoted as D1, and its structural parameters are listed in Table 1.

[0097] Test case

[0098] The iron-containing ZSM-5 molecular sieves prepared in the examples and comparative examples were subjected to aging treatment. The specific aging treatment method is as follows: the aging furnace was a self-made hydrothermal aging device from the Institute of Petroleum Research, and the state after aging at 800℃ for 17 hours was used to simulate the balancing agent during industrial operation.

[0099] The iron-containing ZSM-5 molecular sieves prepared in the examples and comparative examples were used as catalysts in the catalytic cracking reaction of light hydrocarbons to carry out the catalytic cracking reaction of n-tetradecane. The specific method is as follows: the reaction was carried out in a fixed-bed reactor, the feedstock was n-tetradecane, the carrier gas was nitrogen at a flow rate of 30 mL / min, the reaction temperature was 550 °C, the regeneration temperature was 600 °C, the reaction pressure was 0.1 MPa, and the weight hourly space velocity was 20 hr. -1 The molecular sieve tablets were sieved into 20-40 mesh particles, with a loading of 2.0 g and an oil-to-dosage ratio of 1.28. Samples were taken for analysis after 900 s of reaction, and material balance calculations were performed. The product distribution is shown in Table 1.

[0100] The micro-reaction conversion rate X of the raw material and the yield S of the product are calculated using the following formulas. i :

[0101]

[0102]

[0103]

[0104]

[0105] Table 1

[0106]

[0107] As shown in Table 1, the iron-containing small-crystal ZSM-5 molecular sieve prepared by the template agent method and the introduction of a free radical initiator has a relatively high crystallinity of approximately 80-105% and a small crystal size of 0.5-0.8 μm. Scanning electron micrographs of the iron-containing ZSM-5 molecular sieves prepared in Examples 1-3 show that the crystal size of the iron-containing ZSM-5 molecular sieve prepared by the method of this disclosure is 0.5-1 μm, which belongs to the category of small-crystal molecular sieves. STEM-Mapping images of the iron-containing ZSM-5 molecular sieves prepared in Examples 1-3 show that the iron active component in the iron-containing ZSM-5 molecular sieve prepared by the method of this disclosure is uniformly distributed.

[0108] Furthermore, according to the data in Tables 1 and 2, when the iron-containing small-crystal ZSM-5 molecular sieve prepared using the method of this disclosure is used in the catalytic cracking reaction of light hydrocarbons, a high micro-reaction conversion rate and a better yield of low-carbon olefins can be obtained, and the yield of propylene can be improved. A comparison between Examples 1-2 and Example 3 shows that when the molar ratio of silicon source (based on SiO2), aluminum source (based on Al2O3), alkali source (based on alkali metal oxides), template agent, and water is (30-100):1:(2- When the ratio of 8):(2-10):(500-1200) is 1:(0.2-2):(500-800), a higher micro-reaction conversion rate and propylene yield can be obtained. According to the comparison between Example 1 and Example 5, when the molar ratio of the iron source, peroxide and water (calculated as Fe2O3) is 1:(0.2-2):(500-800), a higher micro-reaction conversion rate and propylene yield can be obtained. According to the comparison between Example 1 and 2, when the silica-alumina ratio is low, the molecular sieve is more acidic and a higher micro-reaction conversion rate can be obtained.

[0109] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0110] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0111] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A method for preparing ZSM-5 molecular sieves containing iron-containing small crystallites, the method comprising: S1. A silicon source, an aluminum source, an alkali source, a seed crystal, a template agent, and water are mixed to obtain a first mixture; wherein the molar ratio of the silicon source (based on SiO2), the aluminum source (based on Al2O3), the alkali source (based on alkali metal oxide), the template agent, and the water is (20-300):1:(1-10):(1-30):(400-2500); and the amount of the seed crystal relative to the total weight of the silicon source is 3-12% by weight (based on SiO2). S2. Mix the iron source, peroxide, and water to obtain a second mixture; the molar ratio of the iron source, peroxide, and water, calculated as Fe2O3, is 1:(0.2-2.5):(100-1500). The first mixture and the second mixture are mixed and then subjected to a hydrothermal reaction; the molar ratio of the first mixture (calculated as Al2O3) to the second mixture (calculated as Fe2O3) is 1:(0.01-2.5). The peroxide is selected from one or more of sodium persulfate, hydrogen peroxide, and Fenton's reagent; the ZSM-5 molecular sieve containing iron crystals has a structure in which iron active components are encapsulated internally by the ZSM-5 molecular sieve; the iron active components are distributed on the framework of the ZSM-5 molecular sieve.

2. The method according to claim 1, wherein, Step S2 includes: SS1. The iron source, peroxide, and water are mixed to obtain a second mixture; the molar ratio of the iron source, peroxide, and water, calculated as Fe2O3, is 1:(0.2-2):(500-800). SS2. The first mixture and the second mixture are mixed and then subjected to the hydrothermal reaction; the molar ratio of the first mixture (calculated as Al2O3) to the second mixture (calculated as Fe2O3) is 1:(0.4-1.8).

3. The method according to claim 1, wherein, In step S1, the molar ratio of the silicon source (calculated as SiO2), the aluminum source (calculated as Al2O3), the alkali source (calculated as alkali metal oxide), the template agent, and the water is (30-100):1:(2-8):(2-10):(500-1200). Based on SiO2, the amount of seed crystals used is 8-10% by weight relative to the total weight of the silicon source.

4. The method according to claim 1, wherein, In step S2, the conditions for the hydrothermal reaction include: a temperature of 150-220℃ and a time of 8-32h.

5. The method according to claim 1, wherein, The method further includes: subjecting the solid product obtained by hydrothermal treatment to ammonium exchange and calcination; the calcination conditions include: a temperature of 400-800℃, a time of 0.5-8h, and an atmosphere of air or water vapor.

6. The method according to claim 1, wherein, The silicon source includes one or more of silica gel, silicon dioxide, silica fume, and silicates; The aluminum source includes one or more of sodium aluminate, SB powder, aluminum alkoxide, aluminum oxide, aluminum hydroxide, and aluminum sulfate; The alkaline source includes sodium hydroxide and / or potassium hydroxide; The seed crystals include one or more ZSM-5 molecular sieves with a silicon-to-aluminum ratio of 20-50, and the average particle size of the ZSM-5 molecular sieve is 0.5-2 μm. The iron source is selected from one or more of ferric nitrate, ferric oxalate, ferric sulfate, and ferric hydroxide; The template agent includes an organic nitrogen-containing template agent.

7. The method according to claim 6, wherein, The template agent includes one or more of n-butylamine, tetrapropylammonium hydroxide, and tetrapropylammonium bromide.

8. The ZSM-5 molecular sieve containing iron-containing small crystals prepared by the method according to any one of claims 1-7.

9. The iron-containing small-grained ZSM-5 molecular sieve according to claim 8, wherein, The ZSM-5 molecular sieve containing iron-containing small crystals has a structure in which the iron-active components are internally encapsulated within the ZSM-5 molecular sieve. The iron-active component is distributed on the framework of the ZSM-5 molecular sieve, and the grain size of the iron-containing small-crystal ZSM-5 molecular sieve is 0.5-0.8 μm.

10. The iron-containing ZSM-5 molecular sieve according to claim 8, wherein, The specific surface area of ​​the ZSM-5 molecular sieve containing iron-containing small crystallites is 240-550 m². 2 / g, total pore volume is 0.150-0.28cm³. 3 ·g -1 The micropore volume is 0.140-0.20 cm³. 3 ·g -1 The mesopore volume is 0.010-0.120 cm³. 3 ·g -1 The relative crystallinity is 82-100%; In the ZSM-5 molecular sieve containing iron-containing small crystallites, the molar ratio of SiO2 to Al2O3 is 20-300; The content of the iron active component, calculated as Fe2O3, is 0.8-1.5% by weight relative to the total weight of the ZSM-5 molecular sieve containing iron-containing small crystals.

11. The application of the iron-containing small-crystal ZSM-5 molecular sieve according to any one of claims 8-10 in the catalytic cracking reaction of n-tetradecane.

12. The application according to claim 11, wherein, The conditions for the catalytic cracking reaction include: a temperature of 500-680℃ and a reaction mass hourly space velocity of 20-40 h⁻¹. -1 The reaction pressure is 0.8-1.2 MPa, and the agent-to-oil weight ratio is 1-2.

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