Iron-containing ZSM-5 molecular sieve, preparation method thereof and n-tetradecane catalytic cracking method

Abstract

The invention relates to an iron-containing ZSM-5 molecular sieve, a preparation method thereof and an n-tetradecane catalytic cracking method, and the method comprises the following steps: S1, carrying out a first hydrothermal crystallization reaction on a first mixture containing a silicon source, an aluminum source, an alkali source, a seed crystal and water to obtain ZSM-5 molecular sieve mother liquor; and S2, mixing the ZSM-5 molecular sieve mother liquor, an iron source and water, and carrying out a second hydrothermal crystallization reaction on the obtained second mixture. According to the method disclosed by the invention, the iron-containing ZSM-5 molecular sieve with relatively excellent catalytic performance can be prepared.

Description

Technical Field

[0001] This application relates to an iron-containing ZSM-5 molecular sieve, its preparation method, and a method for catalytic cracking of n-tetradecane. Background Technology

[0002] Catalytic cracking is a process in which hydrocarbons are cracked in the presence of an acidic catalyst to produce low-carbon olefins such as ethylene, propylene, and butene, while simultaneously producing light aromatics. The reaction temperature is typically between 400-700℃. ZSM-5 molecular sieves, with their unique three-dimensional ten-membered ring pore structure, tunable acidity, and high thermal / hydrothermal stability, are the preferred active catalyst component for hydrocarbon catalytic cracking. However, with the diversification of catalytic cracking feedstocks and the increasing severity of reactions, the cracking performance of ZSM-5 molecular sieves is crucial for further improving the overall efficiency of catalytic cracking, especially in terms of improving the selectivity for low-carbon olefins under high conversion conditions, which has become a technical bottleneck. Therefore, developing ZSM-5 molecular sieves with high cracking activity and low-carbon olefin selectivity is of great significance for supporting the high-quality and sustainable development of the refining and chemical industry.

[0003] To further enhance the activity and selectivity of molecular sieves in catalytic cracking reactions, the introduction of dehydrogenation-active metal oxide centers onto ZSM-5 molecular sieves to construct bifunctional catalysts with both cracking and dehydrogenation centers has been extensively studied. Encapsulating metal oxides in molecular sieves is an important method for constructing bifunctional catalysts. It not only allows for high dispersion of metal oxides but also provides closer proximity between acid centers and metal oxide centers, resulting in superior synergistic effects. This facilitates the adsorption-diffusion of reactants and intermediates, enabling selective dehydrogenation of alkanes before cracking.

[0004] There has been considerable research on encapsulating metals into all-silica or high-silica MFI molecular sieves, but research on encapsulating metals into ZSM-5 molecular sieves is limited. This is because if metals are directly introduced into the synthesis system, they tend to form hydroxide precipitates in an alkaline environment. Ligand-assisted synthesis is one method for synthesizing encapsulated metal-silica-alumina MFI molecular sieves; however, this method generally uses organic reagents as ligands, such as alkanolamine complexes and hexamethylenediamine, which still presents problems such as the discharge of organic wastewater.

[0005] The synthesis of ZSM-5 molecular sieves (silicon-to-aluminum ratio 35-50) encapsulating metal oxides is generally carried out in an organic template system, regardless of the presence or absence of ligands. This is because the direct addition of metal salts can lead to the formation of metal hydroxide precipitates, which affects the alkaline synthesis system for amine-free ZSM-5 molecular sieve synthesis, resulting in difficult synthesis or low crystallinity. There are currently no reports on the synthesis of ZSM-5 molecular sieves (silicon-to-aluminum ratio 35-50) encapsulating metal oxides without templates or organic ligands. Summary of the Invention

[0006] The purpose of this disclosure is to provide an iron-containing ZSM-5 molecular sieve, its preparation method, and a method for catalytic cracking of n-tetradecane. This method can prepare an iron-containing ZSM-5 molecular sieve with superior catalytic cracking activity in an amine-free system.

[0007] To achieve the above objectives, the first aspect of this disclosure provides a method for preparing iron-containing ZSM-5 molecular sieves, the method comprising: S1. A first mixture containing silicon source, aluminum source, alkali source, seed crystal and water is subjected to a first hydrothermal crystallization reaction to obtain ZSM-5 molecular sieve mother liquor; S2. Mix the ZSM-5 molecular sieve mother liquor, iron source and water, and carry out a second hydrothermal crystallization reaction on the resulting second mixture.

[0008] Optionally, in step S1, the molar ratio of the silicon source, the aluminum source, the alkali source, and the water in the first mixture is (20-300):1:(2-30):(300-2500), preferably (35-50):1:(2-6):(500-1000); wherein the silicon source is calculated as SiO2, the aluminum source is calculated as Al2O3, and the alkali source is calculated as alkali metal oxide; relative to the total weight of the silicon source calculated as SiO2, the amount of the seed crystal is 5-15% by weight, preferably 7-12% by weight.

[0009] Optionally, in step S1, the conditions for the first hydrothermal crystallization reaction include: a temperature of 20-190℃ and a time of 2-12 h; preferably, a temperature of 80-160℃ and a time of 2-4 h.

[0010] Optionally, step S2 includes: SS1. The iron source and water are mixed to obtain a third mixture; the molar ratio of the iron source and the water, calculated as Fe2O3, is 1:(100-1500), preferably 1:(100-800). SS2. The ZSM-5 molecular sieve mother liquor and the third mixture are mixed and then subjected to the second hydrothermal crystallization reaction; the molar ratio of the ZSM-5 molecular sieve mother liquor (calculated as Al2O3) to the third mixture (calculated as Fe2O3) is 1:(0.1-2.0), preferably 1:(0.2-1.0).

[0011] Optionally, in step S2, the conditions for the second hydrothermal crystallization reaction include: a temperature of 150-220°C and a time of 8-32 h; preferably, a temperature of 150-180°C and a time of 12-20 h.

[0012] Optionally, the method further includes: subjecting the solid product obtained from the second hydrothermal crystallization reaction to ammonium exchange, calcination, and aging treatment; The calcination conditions include: a temperature of 400-800℃, a time of 0.5-8h, and an atmosphere of air or water vapor; the aging treatment conditions include: a temperature of 750-850℃, and a time of 8-32h.

[0013] Optionally, 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.

[0014] The second aspect of this disclosure provides an iron-containing ZSM-5 molecular sieve prepared using the method provided in the first aspect of this disclosure.

[0015] Optionally, the iron-containing ZSM-5 molecular sieve contains iron active components distributed within the pores of the iron-containing ZSM-5 molecular sieve.

[0016] Optionally, the specific surface area of ​​the iron-containing ZSM-5 molecular sieve is 200-450 m². 2 / g, total pore volume is 0.130-0.240cm³. 3 ·g -1 The micropore volume is 0.100-0.200 cm³. 3 ·g -1 The mesopore volume is 0.010-0.100 cm³. 3 ·g -1 The relative crystallinity is 80-100%; In the iron-containing ZSM-5 molecular sieve, the molar ratio of SiO2 to Al2O3 is 20-300; Based on the total weight of the iron-containing ZSM-5 molecular sieve, the content of the iron active component, calculated as Fe2O3, is 1-3% by weight.

[0017] The third aspect of this disclosure provides a method for catalytic cracking of n-tetradecane using an iron-containing ZSM-5 molecular sieve provided in the second aspect of this disclosure, the method comprising: contacting n-tetradecane with the iron-containing ZSM-5 molecular sieve to carry out the catalytic cracking reaction.

[0018] Optionally, the conditions for the catalytic cracking reaction of n-tetradecane include: a temperature of 550-680℃ and a reaction mass hourly space velocity of 10-50 h⁻¹. -1 The reaction pressure is 0.8-1.2 MPa, and the agent-to-oil weight ratio is 1-5.

[0019] Through the above technical solution, this disclosure employs a two-step hydrothermal crystallization method to synthesize iron-containing ZSM-5 molecular sieves in an amine-free and organic ligand-free system, avoiding the discharge of amine nitrogen wastewater and achieving clean production of iron-containing ZSM-5 molecular sieves throughout the entire process. The prepared iron-containing ZSM-5 molecular sieves exhibit high conversion rates and selectivity for low-carbon olefins.

[0020] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Attached Figure Description

[0021] 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: Figure 1 This is the X-ray diffraction pattern of the iron-containing ZSM-5 molecular sieve prepared in Example 1 of this disclosure; Figure 2 This is a scanning electron microscope image of the iron-containing ZSM-5 molecular sieve prepared in Example 1 of this disclosure; Figure 3 This is a transmission electron microscope (TEM) image of the iron-containing ZSM-5 molecular sieve prepared in Example 1 of this disclosure; Figure 4 This is a STEM-Mapping image of the iron-containing ZSM-5 molecular sieve prepared in Example 1 of this disclosure; Figure 5 This is the X-ray diffraction pattern of the iron-containing ZSM-5 molecular sieve prepared in Example 2 of this disclosure; Figure 6 Here is a scanning electron microscope image of the iron-containing ZSM-5 molecular sieve prepared in Example 2 of this disclosure; Figure 7 These are the X-ray diffraction patterns of the ZSM-5 molecular sieves prepared in Comparative Examples 1 and 2 of this disclosure; Figure 8 This is a scanning electron microscope image of the ZSM-5 molecular sieve prepared in Comparative Example 1 of this disclosure; Figure 9 The images show the H2-TPR diagrams of iron-containing ZSM-5 molecular sieve sample A prepared in Example 1 of this disclosure and control sample D1 prepared in Comparative Example 1.

[0022] Figure 10 This is a scanning electron microscope image of the iron-containing ZSM-5 molecular sieve prepared in Comparative Example 2 of this disclosure. Detailed Implementation

[0023] The following provides a detailed description of specific embodiments of this disclosure. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit this disclosure.

[0024] The first aspect of this disclosure provides a method for preparing iron-containing ZSM-5 molecular sieves, the method comprising: S1, subjecting a first mixture containing a silicon source, an aluminum source, an alkali source, a seed crystal and water to a first hydrothermal crystallization reaction to obtain a ZSM-5 molecular sieve mother liquor; S2, mixing the ZSM-5 molecular sieve mother liquor, an iron source and water, and subjecting the resulting second mixture to a second hydrothermal crystallization reaction.

[0025] The method disclosed herein employs a two-step hydrothermal crystallization process. First, a first mixture undergoes hydrothermal crystallization to form crystal nuclei before a second hydrothermal crystallization is performed by introducing a metal precursor iron source. This method does not affect the formation of crystal nuclei in the amine-free system, allowing the nuclei to further promote the growth of the molecular sieve. Furthermore, the introduction of the metal precursor effectively introduces iron into the framework of the ZSM-5 molecular sieve, avoiding the problem of low crystallinity that occurs when directly introducing an iron source for one-step hydrothermal crystallization. This method can produce iron-containing ZSM-5 molecular sieves with superior catalytic activity. Simultaneously, the method disclosed herein does not use template agents or ligands containing amine nitrogen, avoiding the discharge of amine nitrogen wastewater during the molecular sieve preparation process, thus achieving clean production of iron-containing ZSM-5 molecular sieves.

[0026] According to this disclosure, the amounts of each component in the first mixture can vary within a wide range. In one specific embodiment of this disclosure, in step S1, the molar ratio of the silicon source, the aluminum source, the alkali source, and the water in the first mixture is (20-300):1:(2-30):(300-2500), preferably (35-50):1:(2-6):(500-1000); wherein the silicon source is calculated as SiO2, the aluminum source as Al2O3, and the alkali source as an alkali metal oxide. The amount of the seed crystal is 5-15% by weight, preferably 7-12% by weight, relative to the total weight of the silicon source calculated as SiO2.

[0027] According to this disclosure, hydrothermal crystallization 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 S1, the conditions for the first hydrothermal crystallization reaction include: a temperature of 20-190°C and a time of 2-12 h; preferably, a temperature of 80-160°C and a time of 2-4 h. This disclosure does not specifically limit 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. By carrying out the first hydrothermal crystallization reaction under the above conditions, crystal nuclei can form at a more favorable synthesis temperature, thereby promoting the growth of molecular sieves.

[0028] In a preferred embodiment, step S2 includes: SS1, mixing the iron source and water to obtain a third mixture; the molar ratio of the iron source and water, calculated as Fe2O3, is 1:(100-1500), preferably 1:(100-800); SS2, mixing the ZSM-5 molecular sieve mother liquor with the third mixture and then performing the second hydrothermal crystallization reaction; the molar ratio of the ZSM-5 molecular sieve mother liquor, calculated as Al2O3, to the third mixture, calculated as Fe2O3, is 1:(0.1-2.0), preferably 1:(0.2-1.0). Within the above range of proportions, the presence of free iron ions or ferrous ions in the third mixture can be avoided, resulting in a more uniform dispersion of the iron active component in the prepared ZSM-5 molecular sieve without aggregated iron active components, thereby further improving the catalytic performance of the catalyst. In this disclosure, the ZSM-5 molecular sieve mother liquor, calculated as Al2O3, refers to Al2O3 provided by the aluminum source used to prepare the ZSM-5 molecular sieve mother liquor.

[0029] In one specific embodiment of this disclosure, in step S2, the conditions for the second hydrothermal crystallization reaction include: a temperature of 150-220°C and a time of 8-32 h; preferably, a temperature of 150-180°C and a time of 12-20 h. This disclosure, by carrying out the second hydrothermal crystallization reaction under the above conditions, introduces an iron source into the crystallization system based on the formation of crystal nuclei in the first step, without affecting the growth of crystal nuclei or the subsequent crystallization of the molecular sieve.

[0030] In one specific embodiment of this disclosure, step S1 further includes: cooling the product obtained from the first hydrothermal crystallization reaction to obtain the ZSM-5 molecular sieve mother liquor.

[0031] In one specific embodiment of this disclosure, the method further includes: subjecting the solid product obtained from the second hydrothermal crystallization reaction to ammonium exchange, calcination, and aging treatment; the calcination conditions include: a temperature of 400-800℃ and a time of 0.5-8h; preferably, a temperature of 500-600℃ and a time of 2-4h, in an air atmosphere or a water vapor atmosphere. Calcination can be carried out in equipment well known to those skilled in the art, such as a muffle furnace or a tube furnace. 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 specific embodiment, the solid obtained from ammonium exchange is dried before calcination; the drying conditions may include: a temperature of 100-150℃ and a time of 5-20h.

[0032] According to this disclosure, aging treatment is well known to those skilled in the art. In one specific embodiment, the aging treatment conditions include: a temperature of 750-850°C and a time of 8-32 hours. In this embodiment, the aging treatment causes the framework iron in the molecular sieve to detach from the framework and form reducing iron oxide, which is uniformly distributed in the channels. The iron oxide in the channels forms dehydrogenation centers and cracking centers, thereby increasing the production of low-carbon olefins.

[0033] The solid product obtained from the second hydrothermal reaction can be removed using methods well known to those skilled in the art. There are no specific limitations on the method of removing the solid, such as centrifugation, filtration, precipitation, etc.

[0034] 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.

[0035] 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.

[0036] The second aspect of this disclosure provides an iron-containing ZSM-5 molecular sieve prepared using the method provided in the first aspect of this disclosure.

[0037] In one specific embodiment of this disclosure, the iron-containing ZSM-5 molecular sieve contains iron active components distributed within its pores. The introduction of iron modulates the acidity of the ZSM-5 molecular sieve, enhancing its catalytic function in cracking and dehydrogenation. Introducing the iron active components into the pores of the molecular sieve further reduces the severity of light hydrocarbon catalytic cracking reactions, improving the conversion rate of saturated hydrocarbons and the selectivity of low-carbon olefins.

[0038] In one specific embodiment of this disclosure, the specific surface area of ​​the iron-containing ZSM-5 molecular sieve is 200-450 m². 2 / g, preferably 300-400m 2 / g, total pore volume is 0.130-0.240cm³. 3 / g, preferably 0.18-0.22cm 3 / g, micropore volume is 0.100-0.200cm³ 3 / g, preferably 0.120-0.180cm 3 / g, mesopore volume is 0.010-0.100cm³ 3 / g, preferably 0.030-0.080cm 3 The relative crystallinity is 80-100%, preferably 85-100%; in the iron-containing ZSM-5 molecular sieve, the molar ratio of SiO2 to Al2O3 is 20-300, preferably 27-50; based on the total weight of the iron-containing ZSM-5 molecular sieve, the content of the iron active component, calculated as Fe2O3, is 1-3% by weight, preferably 1-1.5% by weight. The molar ratio of SiO2 to Al2O3 can be obtained by X-ray fluorescence spectroscopy. The relative crystallinity is obtained using a Siemens D5005 X-ray diffractometer, 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, China Petroleum & Chemical Corporation is 100%). The specific surface area and pore volume parameters can be determined using a specific surface area analyzer according to 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 iron active components, calculated as Fe2O3, was determined by X-ray fluorescence spectroscopy.

[0039] The third aspect of this disclosure provides a method for catalytic cracking of n-tetradecane using an iron-containing ZSM-5 molecular sieve provided in the second aspect of this disclosure, the method comprising: contacting n-tetradecane with the iron-containing ZSM-5 molecular sieve to carry out the catalytic cracking reaction.

[0040] In one specific embodiment of this disclosure, the conditions for the catalytic cracking reaction include: a temperature of 550-680°C and a reaction mass hourly space velocity of 10-50 h⁻¹. -1 The reaction pressure is 0.8-1.2 MPa, and the agent-to-oil weight ratio is 1-5; preferably, the temperature is 550-600℃, and the reaction mass hourly space velocity is 35-50 h⁻¹. -1 The reaction pressure is 0.9-1 MPa, and the agent-to-oil weight ratio is 1-3.

[0041] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited thereto.

[0042] Unless otherwise specified, all reagents used in the examples and comparative examples were commercially available.

[0043] 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 40 kV, and excitation current 50 mA. 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. 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), RIPP 151-90, Science Press, 1990). 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 a liquid nitrogen temperature of -196℃, and measurements were taken of the samples under different relative pressures (Pa). p / p The adsorption and desorption of N2 under 0) conditions were determined. Desorption branching data were calculated using the BJH (Barrett-Joyner-Halenda) method to obtain the pore size distribution data of the sample. The transmission electron microscope was a JEM-2100 (200 kV) transmission electron microscope from Nippon Electron Ltd.; the test conditions were: accelerating voltage of 200 kV, and after sample processing, the crystal morphology and size of the molecular sieve samples were observed at magnifications of 5000-30000. STEM-Mapping test method and instrument: JEM-2100 (200 kV) transmission electron microscope, manufactured by Nippon Electron Ltd. The accelerating voltage of the electron microscope was 200 kV. The sample was dispersed in anhydrous ethanol and then dropped onto a sample grid with a diameter of 3 mm for observation and testing. H2-TPR testing method and instrumentation: Micromeritic AutochemⅡ 2920 temperature-programmed desorption apparatus. 0.2 g (20-40 mesh) of molecular sieve catalyst was weighed and loaded into a sample tube, placed in a thermal conductivity cell furnace, and heated to 550 °C at a rate of 20 °C / min using He as the carrier gas. The sample tube was then purged for 60 min to remove impurities adsorbed on the catalyst surface. The temperature was then lowered to 150 °C and held for 60 min. An H2-He mixture (10.02% H2 + 89.98% He) was then used for adsorption for 60 min, followed by purging with He for 120 min until the baseline stabilized. The temperature was then programmed to rise to 550 °C at a rate of 10 °C / min for desorption. The temperature was held for 30 min to complete desorption, and the gas composition change was detected using a TCD detector. 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.

[0044] Example 1 This embodiment uses the following steps to prepare iron-containing ZSM-5 molecular sieve A: (1) Add 0.99g sodium hydroxide (purity 98.3% by weight), 40.8g deionized water, 20g silica gel (solid content 90.63% by weight), and 6.78g low-alkali sodium aluminate (Na2O: 153.5 g / L, Al2O3: 102.5 g / L, density: 1.2475 g / cm³) in sequence while stirring. 3 1.97g of ZSM-5 seed crystals were thoroughly stirred to obtain the first mixture. After the first mixture was thoroughly mixed, it was transferred to a stainless steel kettle and subjected to the first hydrothermal crystallization reaction at 100℃ for 5h. After cooling to room temperature, the ZSM-5 molecular sieve mother liquor was obtained.

[0045] The molar ratio of silicon source (SiO2), aluminum source (Al2O3), alkali source (alkali metal oxide), and water is 55:1:4.68:589; 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. (2) Dissolve 1.13g of ferric nitrate nonahydrate in 10g of deionized water and stir thoroughly until clear and transparent to obtain a third mixture; wherein the molar ratio of iron source (calculated as Fe2O3) to deionized water is 1:400. (3) Add the third mixture to the ZSM-5 molecular sieve mother liquor, mix thoroughly and evenly, then transfer to a stainless steel kettle and carry out the second hydrothermal crystallization reaction at 170°C for 10 h; wherein, the molar ratio of ZSM-5 molecular sieve mother liquor (calculated as Al2O3) to the third mixture (calculated as Fe2O3) is 1:0.40. (4) The product obtained from the second hydrothermal crystallization treatment in step (3) was filtered, washed until pH=7-8, and then subjected to ammonium exchange. It was dried at 120℃ for 12h and then calcined at 550℃ for 2h in air to obtain iron-containing ZSM-5 molecular sieve, denoted as A. Its X-ray diffraction pattern is shown in 1. From the XRD pattern, it can be seen that the two-stage method can synthesize encapsulated iron ZSM-5 molecular sieve with a relative crystallinity of more than 80%. The scanning electron microscope image is shown in 1. Figure 2 See TEM and Mapping photos Figure 3 and Figure 4 As shown in the figure, the grain size of iron-containing ZSM-5 molecular sieve A is 1~2μm, and the iron species are evenly distributed.

[0046] Example 2 Iron-containing ZSM-5 molecular sieve B was prepared using the same method as in Example 1, except that the temperature of the first hydrothermal crystallization reaction in step (1) was 100°C and the time was 2 hours. The temperature of the second hydrothermal crystallization reaction in step (3) was 170°C and the time was 15 hours. The resulting iron-containing ZSM-5 molecular sieve is denoted as B.

[0047] Example 3 Iron-containing ZSM-5 molecular sieve C was prepared using the same method as in Example 1, except that in step (1), 0.46 g of sodium hydroxide (purity 98.3% by weight), 38.4 g of deionized water, 20 g of silica gel (solid content 90.63% by weight), and 9.88 g of low-alkali sodium aluminate (Na2O: 153.5 g / L, Al2O3: 102.5 g / L, density: 1.2475 g / cm³) were added sequentially under stirring. 3 ), 1.97g ZSM-5 seed crystals; the molar ratio of silicon source (SiO2), aluminum source (Al2O3), alkali source (alkali metal oxide), and water is 37.5:1:3.19:401; the amount of ZSM-5 seed crystals is 10g by weight relative to the total weight of silicon source (SiO2).

[0048] Its X-ray diffraction pattern is shown in Figure 5 Its scanning electron microscope image can be found here. Figure 6Therefore, by reducing the silicon-to-aluminum ratio in the feed, the crystallinity of the synthesized encapsulated iron molecular sieve is higher, while the grain size remains at 1~2μm.

[0049] Example 4 Iron-containing ZSM-5 molecular sieve D was prepared using the same method as in Example 1, except that the temperature of the first hydrothermal crystallization reaction in step (1) was 90°C and the time was 5 h, and the temperature of the second hydrothermal crystallization reaction in step (2) was 180°C and the time was 20 h. The prepared iron-containing ZSM-5 molecular sieve was denoted as D.

[0050] Example 5 Iron-containing ZSM-5 molecular sieve E was prepared using the same method as in Example 1, except that the amount of ZSM-5 seed crystals was 5% by weight relative to the total weight of the silicon source (calculated as SiO2); the silicon-aluminum ratio of the ZSM-5 seed crystals was 27, and the average particle size was 1-2 μm.

[0051] Example 6 Iron-containing ZSM-5 molecular sieve F was prepared using the same method as in Example 1, except that in step (2), 0.375 g of ferric nitrate nonahydrate was dissolved in 10 g of deionized water and stirred thoroughly until clear and transparent to obtain a third mixture; wherein, the molar ratio of iron source to water (calculated as Fe2O3) was 1:1403; and the molar ratio of ZSM-5 molecular sieve mother liquor (calculated as Al2O3) to the third mixture (calculated as Fe2O3) was 1:0.16.

[0052] Comparative Example 1 This comparative example illustrates the difference between the two-stage method for preparing iron-free ZSM-5 molecular sieves.

[0053] (1) While stirring, add 0.99g sodium hydroxide (purity 98.3% by weight), 50.8g deionized water, 20g silica gel (solid content 90.63% by weight), and 6.78g low-alkali sodium aluminate (Na2O: 153.5 g / L, Al2O3: 102.5 g / L, density: 1.2475 g / cm³) in sequence. 3 1.97g of ZSM-5 seed crystals were thoroughly stirred to obtain the first mixture. After thorough mixing, the mixture was transferred to a stainless steel autoclave and hydrothermally treated at 100℃ for 5 hours. The temperature was then increased to 170℃ and hydrothermally treated for 10 hours. The molar ratio of silicon source (SiO2), aluminum source (Al2O3), alkali source (alkali metal oxide), and water is 55:1:4.68:589; 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. (2) The product obtained from the hydrothermal treatment in step (1) was filtered, washed until pH=7-8, and then subjected to ammonium exchange. It was dried at 120℃ for 12h and then calcined at 550℃ for 2h in air to obtain the parent ZSM-5 molecular sieve, denoted as D1. Its X-ray diffraction pattern is shown in [reference needed]. Figure 7 Scanning electron microscope images can be found Figure 8 .

[0054] The H2-TPR spectra of iron-free ZSM-5 molecular sieve D1 and iron-containing ZSM-5 molecular sieve A are shown below. Figure 9 As shown in the figure, the iron-free ZSM-5 molecular sieve D1 maintains a high degree of crystallinity, but since ZSM-5 molecular sieve D1 has no reducing species, its H2-TPR spectrum shows no reduction peak. Although ZSM-5 molecular sieve A contains iron species, its H2-TPR spectrum also shows no obvious reduction peak, indicating that the iron species is non-reducible framework iron existing on the framework.

[0055] Comparative Example 2 This comparative example illustrates the effect of the order of adding the iron source on the synthesis of iron-containing ZSM-5 molecular sieves.

[0056] A synthesis slurry was prepared according to a molar ratio of silicon source (SiO2), aluminum source (Al2O3), alkali source (alkali metal oxide), water, and Fe2O3 of 55:1:4.68:589:0.40, yielding a first mixture. The first mixture was first subjected to hydrothermal crystallization at 100℃ for 5 hours, followed by a second hydrothermal crystallization at 170℃ for 10 hours. The resulting product was filtered, washed until pH=7-8, subjected to ammonium exchange, dried at 120℃ for 12 hours, and then calcined at 550℃ for 2 hours in air to obtain an iron-containing ZSM-5 molecular sieve, denoted as D2. Its SEM image is shown below. Figure 10 As shown in the figure, the crystallinity of the synthesized molecular sieve is low. This is because the iron source, when added in the first step, affects the alkalinity of the molecular sieve synthesis system, which prevents a large amount of silica gel from depolymerizing, thus affecting the crystallinity of the synthesized molecular sieve.

[0057] Comparative Example 3 This comparative example illustrates the difference between hydrothermal crystallization treatment and direct iron addition in the synthesis of iron-containing ZSM-5 molecular sieves.

[0058] A synthesis slurry was prepared according to a molar ratio of silicon source (SiO2), aluminum source (Al2O3), alkali source (alkali metal oxide), water, and Fe2O3 of 55:1:4.68:589:0.40, yielding a first mixture. The first mixture was then subjected to hydrothermal crystallization at 170°C for 10 hours. After filtration, washing to pH 7-8, ammonium exchange, and drying at 120°C for 12 hours, it was then calcined at 550°C for 2 hours in air to obtain an iron-containing ZSM-5 molecular sieve, denoted as D3.

[0059] Test case The 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 is a self-made hydrothermal aging device of the Petroleum Research Institute. The state of the aging at 800℃ for 17 hours is used to simulate the balancing agent in industrial operation.

[0060] The ZSM-5 molecular sieves prepared in the examples and comparative examples were used as catalysts in 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 20 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 2.74 hr. -1 The molecular sieve tablets were sieved into 20-40 mesh particles. The feed amount of n-tetradecane was 1.56 g, the molecular sieve loading was 2.0 g, and the agent-to-oil ratio was 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.

[0061] The micro-reaction conversion rate X of the raw material and the yield S of the product are calculated using the following formulas. i :

[0062] Table 1

[0063] As can be seen from the above, the method disclosed herein can prepare iron-containing ZSM-5 molecular sieves in an amine-free system. These molecular sieves exhibit good selectivity for low-carbon olefins when used in the catalytic cracking reaction of n-tetradecane.

[0064] 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.

[0065] 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.

[0066] 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 an iron-containing ZSM-5 molecular sieve, the method comprising: S1, subjecting a first mixture containing a silicon source, an aluminum source, an alkali source, seeds and water to a first hydrothermal crystallization reaction to obtain a ZSM-5 molecular sieve mother liquor; S2, mixing the ZSM-5 molecular sieve mother liquor, an iron source and water, and subjecting the obtained second mixture to a second hydrothermal crystallization reaction.

2. The method of claim 1, wherein, In step S1, in the first mixture, the molar ratio of the silicon source, the aluminum source, the alkali source and the water is (20-300) : 1 : (2-30) : (300-2500), preferably (35-50) : 1 : (2-6) : (500-1000) ; wherein the silicon source is calculated as SiO 2, the aluminum source is calculated as Al 2 O 3, and the alkali source is calculated as alkali metal oxide; The amount of the seeds is 5-15 wt%,preferably 7-12 wt%, relative to the total weight of the silicon source calculated as SiO 2.

3. The method of claim 1, wherein, In step S1, the first hydrothermal crystallization reaction is carried out at a temperature of 20-190 ℃ for 2-12 h;preferably, at a temperature of 80-160 ℃ for 2-4 h.

4. The method of claim 1, wherein, Step S2 comprises: SS1, mixing the iron source and water to obtain a third mixture;the molar ratio of the iron source calculated as Fe 2 O 3 and the amount of water is 1 : (100-1500), preferably 1 : (100-800) ; SS2, mixing the ZSM-5 molecular sieve mother liquor and the third mixture and then subjecting the mixture to the second hydrothermal crystallization reaction;the molar ratio of the ZSM-5 molecular sieve mother liquor calculated as Al 2 O 3 and the amount of the third mixture calculated as Fe 2 O 3 is 1 : (0.1-2.0), preferably 1 : (0.2-1.0).

5. The method of claim 1, wherein, In step S2, the second hydrothermal crystallization reaction is carried out at a temperature of 150-220 ℃ for 8-32 h;preferably, at a temperature of 150-180 ℃ for 12-20 h.

6. The method of claim 1, wherein, The method further comprises subjecting the solid product obtained in the second hydrothermal crystallization reaction to ammonium exchange, calcination and aging treatment; The calcination is carried out at a temperature of 400-800 ℃ for 0.5-8 h in an air atmosphere or a water vapor atmosphere;the aging treatment is carried out at a temperature of 750-850 ℃ for 8-32 h.

7. The method of claim 1, wherein, The silicon source includes one or more of silica gel, silicon dioxide, white carbon black and silicate; The aluminum source includes one or more of sodium metaaluminate, SB powder, aluminum alcoholate, aluminum oxide, aluminum hydroxide and aluminum sulfate; The alkali source includes sodium hydroxide and / or potassium hydroxide; The seeds include one or more of ZSM-5 molecular sieves with a silica-alumina ratio of 20-50, and the average particle size of the ZSM-5 molecular sieves is 0.5-2 μm; The iron source is selected from one or more of ferric nitrate, ferric oxalate, ferric sulfate and ferric hydroxide. 8.An iron-containing ZSM-5 molecular sieve prepared by the method of any one of claims 1-7.

9. The iron-containing ZSM-5 molecular sieve of claim 8, wherein, The iron active component in the iron-containing ZSM-5 molecular sieve is distributed in the pores of the iron-containing ZSM-5 molecular sieve.

10. The iron-containing ZSM-5 molecular sieve of claim 8, wherein, The iron-containing ZSM-5 molecular sieve has a specific surface area of 200-450 m 2 / g, a total pore volume of 0.130-0.240 cm 3 ·g -1 , a micropore volume of 0.100-0.200 cm 3 ·g -1 , a mesopore volume of 0.010-0.100 cm 3 ·g -1 , and a relative crystallinity of 80-100%. In the iron-containing ZSM-5 molecular sieve, the molar ratio of SiO2 to Al2O3 is 20-300. The content of the iron active component, calculated as Fe2O3, is 1-3% by weight, based on the total weight of the iron-containing ZSM-5 molecular sieve.

11. A process for catalytically cracking n-tetradecane using the iron-containing ZSM-5 molecular sieve of any of claims 8-10, the process comprising: The iron-containing ZSM-5 molecular sieve is contacted with n-tetradecane to perform a catalytic cracking reaction.

12. The method of claim 11, wherein, The conditions of the catalytic cracking reaction of n-tetradecane include: temperature of 550-680℃, reaction mass space velocity of 10-50h -1 , reaction pressure of 0.8-1.2MPa, and weight ratio of catalyst to oil of 1-5.

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