Modified zsm-5 molecular sieve catalyst, method for preparing the same, and use thereof

Abstract

The application discloses a modified ZSM-5 molecular sieve catalyst and a preparation method and application thereof. The preparation method of the modified ZSM-5 molecular sieve catalyst comprises the following steps: mixing ZSM-5 molecular sieve with a solvent to obtain a liquid-solid mixture; adding a modification precursor solution into the obtained liquid-solid mixture to perform a modification reaction, so as to obtain a modified liquid-solid mixture; performing drying and calcination on the modified liquid-solid mixture to obtain a catalyst precursor; and repeating the modification process for 2 to 10 times to obtain the modified ZSM-5 molecular sieve catalyst. According to the application, the surface diffusion performance of the molecular sieve is improved by regulating the properties of the surface hydroxyl and defects of the ZSM-5 molecular sieve, and the application of the molecular sieve in an olefin catalytic cracking reaction for preparing ethylene and propylene can improve the catalytic efficiency, the reaction life and the selectivity of the target product.

Description

Technical Field

[0001] This invention relates to the field of zeolite molecular sieve catalysts, specifically to a modified ZSM-5 molecular sieve catalyst, its preparation method, and its application. Background Technology

[0002] Ethylene and propylene, as building blocks of many important chemicals, are widely used in the production of polyethylene, polypropylene, and acrylonitrile, holding significant positions in industries such as packaging, textiles, electronics, and automobiles. Traditionally, the main technology for producing low-carbon olefins involves the catalytic cracking of petroleum products such as naphtha, petroleum gas, and condensate. In recent years, new coal chemical technologies for producing low-carbon olefins have seen unprecedented development, gradually becoming an important technological route for olefin production. However, these processes generate large amounts of carbon dioxide as a byproduct. 4+ Long-chain olefins. Therefore, olefin catalytic cracking (OCC) technology is used to process low-value-added byproduct C... 4+ The conversion of long-chain olefins into ethylene and propylene can not only promote integrated refining and chemical production, but also connect and support coal chemical industry and improve carbon resource utilization.

[0003] Olefin catalytic cracking is a typical acid-catalyzed reaction. In recent years, due to the suitable acidity and unique pore structure network of H-ZSM-5 molecular sieves, olefin cracking technology using H-ZSM-5 as a catalyst has developed rapidly. However, current OCC technology still has some drawbacks, such as rapid catalyst deactivation due to coking, numerous byproducts (approximately 300 types), and difficulty in improving diene (ethylene, propylene) yields. Further improving catalyst activity and diene selectivity is a major challenge for OCC technology. The reactant / product molecule diffusion pathways determined by the complex pore structure network of H-ZSM-5 molecular sieves have a significant impact on catalytic performance. On the one hand, the abundant microporous structure endows molecular sieve catalysts with excellent shape-selective catalytic performance, enabling them to obtain target products with high selectivity. On the other hand, the narrow channels of molecular sieves increase the diffusion resistance of reactant, intermediate, and product molecules, prolonging their residence time and increasing the probability of side reactions and coking, thereby reducing the selectivity and reaction stability of the target product. Therefore, how to precisely control the diffusion pathway of molecular sieves is the key to further improving catalytic efficiency.

[0004] Currently, there are three main methods to improve the diffusion resistance limitation of ZSM-5 molecular sieves: 1. Reducing the crystal size of ZSM-5 molecular sieves. This method can effectively shorten the intracrystalline diffusion distance of molecular sieve catalysts, reduce intracrystalline diffusion resistance, and significantly improve their catalytic efficiency. However, due to the separation difficulties in the production process of small-particle-size molecular sieves, the production cost is high. In addition, since the molecular sieve catalysts used in industry are all molded catalysts, small-particle-size molecular sieves require more binders during the molding process, further increasing the cost of industrial applications and hindering the industrial application of small-particle-size molecular sieves. 2. Creating ZSM-5 molecular sieve catalysts with hierarchical porous structures. This method can also reduce intracrystalline diffusion resistance and improve OCC catalytic performance. Currently, there are two main methods for preparing hierarchical porous ZSM-5 molecular sieves: the bottom-up method by adding mesoporous template agents during the preparation process and the top-down method by post-treatment of the molecular sieve. Among them, the bottom-up method requires the use of a large amount of expensive template agents, and the large-scale calcination process of template agents is prone to environmental pollution, making it difficult to apply to industrial production. The post-processing of top-down ZSM-5 molecular sieve preparation can easily damage the molecular sieve framework structure and has poor controllability. 3. Constructing molecular sieve morphologies with specific crystal planes and pore exposure. Due to the anisotropic diffusion of molecules within the pores of ZSM-5 molecular sieves, this method can also improve diene yield. However, the preparation of such ZSM-5 molecular sieves with specific morphologies requires the addition of expensive additives, which is not conducive to industrial-scale production from both cost and environmental perspectives.

[0005] Therefore, it is crucial to find new methods to improve the diffusion resistance of molecular sieves. Summary of the Invention

[0006] To address the problem of high surface diffusion resistance of ZSM-5 molecular sieves in existing technologies, resulting in low diene yields in olefin catalytic cracking technology, this invention provides a modified ZSM-5 molecular sieve catalyst, its preparation method, and its application. This catalyst, when applied to olefin catalytic cracking technology, can effectively eliminate the surface diffusion resistance of ZSM-5 molecular sieves, improve molecular mass transfer efficiency, and inhibit secondary reactions such as polymerization and coking.

[0007] The first aspect of this invention provides a modified ZSM-5 molecular sieve catalyst, wherein the silicon-aluminum atomic ratio of the ZSM-5 molecular sieve is 100–500, preferably 300–500, and the surface diffusion rate of the modified ZSM-5 molecular sieve catalyst is 1.50–4.00 s⁻¹. -1 The preferred time is 2.40–4.00 s. -1 .

[0008] Furthermore, the ZSM-5 molecular sieve is an H-ZSM-5 molecular sieve.

[0009] A second aspect of this invention provides a method for preparing a modified ZSM-5 molecular sieve catalyst, comprising the following steps:

[0010] (1) Mix ZSM-5 molecular sieve with solvent to obtain a liquid-solid mixture;

[0011] (2) Add the modified precursor solution to the liquid-solid mixture obtained in step (1) to carry out the modification reaction and obtain the modified liquid-solid mixture;

[0012] (3) The modified liquid-solid mixture obtained in step (2) is dried and calcined to obtain the catalyst precursor;

[0013] (4) Repeat steps (1) to (3) to obtain the modified ZSM-5 molecular sieve catalyst.

[0014] Further, the ZSM-5 molecular sieve mentioned in step (1) is an H-ZSM-5 molecular sieve. The silicon-to-aluminum atomic ratio of the H-ZSM-5 molecular sieve is 100 to 500, preferably 300 to 500.

[0015] Further, the solvent in step (1) is selected from one or more of methanol, ethanol, petroleum ether, dichloromethane, n-hexane, cyclohexane, acetone, butanone, etc.; preferably, the solvent is selected from one or more of methanol, ethanol, and n-hexane.

[0016] Further, the ratio of the solvent to the ZSM-5 molecular sieve in step (1) is 10-50:1 mL / g, preferably 15-30:1 mL / g.

[0017] Further, the modified precursor source in step (2) is selected from one or more of tetraethyl orthosilicate, tetramethyl orthosilicate, trimethylchlorosilane, triethylchlorosilane, and tripropylchlorosilane; preferably, the modified precursor source is selected from tetraethyl orthosilicate. The concentration of the modified precursor source, calculated as SiO2, is 0.03–13 mg / mL.

[0018] Furthermore, in step (2), the ratio of the modified precursor source to the ZSM-5 molecular sieve in the liquid-solid mixture obtained in step (1) is 0.05 to 1:1 mL / g, preferably 0.05 to 0.5:1 mL / g.

[0019] Furthermore, the reaction described in step (2) needs to be carried out under reflux condensation conditions, with a reaction temperature of 30 to 100°C and a reaction time of 0.5 to 10 hours; preferably, the reaction temperature is 40 to 80°C and the reaction time is 2 to 4 hours.

[0020] Further, the modified liquid-solid mixture described in step (3) can be separated into solids from the obtained mixture by any conventionally known solid-liquid separation method, and the solids can be washed and dried after solid-liquid separation. The solid-liquid separation, washing, and drying can be carried out in any manner conventionally known in the art. Specifically, the solid-liquid separation can be performed, for example, by vacuum filtration. The washing can be performed, for example, by using deionized water. The drying temperature is 50–120°C, preferably 80–100°C, and the drying time is 4–24 hours, preferably 8–12 hours.

[0021] Furthermore, the roasting conditions in step (3) are: roasting in a muffle furnace at 500-650℃ for 4-10 hours.

[0022] Furthermore, the number of repetitions in step (4) is 2 to 10 times, preferably 3 to 6 times.

[0023] A third aspect of the present invention provides a modified ZSM-5 molecular sieve catalyst prepared by the method described in the second aspect, wherein the surface diffusion rate of the modified ZSM-5 molecular sieve catalyst is 1.50–4.00 s⁻¹. -1 The preferred time is 2.40–4.00 s. -1 .

[0024] The fourth aspect of this invention provides the application of the above-mentioned modified ZSM-5 molecular sieve catalyst in the catalytic cracking of olefins to produce ethylene and propylene.

[0025] Furthermore, the olefin catalytic cracking reaction is as follows: the modified ZSM-5 molecular sieve catalyst described above is reacted with a feedstock containing butene to obtain ethylene and propylene.

[0026] Furthermore, the reaction temperature is 300–700°C, and the reaction space velocity is 5–50 h⁻¹. -1 The reaction pressure is 0.1–3 bar. Preferably, the reaction temperature is 500–600 °C and the reaction space velocity is 20–35 h⁻¹. -1 The reaction pressure is 0.5 to 1 bar.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] 1. The modified ZSM-5 molecular sieve catalyst provided by this invention has a pore structure, crystallinity, surface acidity, and apparent morphology comparable to the unmodified ZSM-5 molecular sieve. Furthermore, while maintaining a substantially unchanged intracrystalline diffusion rate, the surface diffusion rate of the molecular sieve is significantly improved. When applied to olefin catalytic cracking reactions, it can effectively improve catalytic efficiency, increase target product selectivity, improve target product yield, and extend reaction lifetime, thus reducing the degradation of low-value-added C4 catalysts. 4+The efficient catalytic conversion of olefin feedstocks has become possible, facilitating industrial production.

[0029] 2. In the catalytic cracking reaction of olefins, traditional methods for regulating the diffusion performance of ZSM-5 molecular sieves mainly focus on modifying intracrystalline diffusion properties. However, this invention employs liquid-phase deposition to modify the surface properties of high-silicon-to-aluminum ratio ZSM-5 molecular sieves. By controlling the amount of the modified precursor and the number of modification cycles, the resulting modified ZSM-5 molecular sieve significantly reduces surface diffusion resistance and improves catalytic efficiency while maintaining the basic intracrystalline structure and diffusion performance of the sieve. This provides a new approach for regulating the diffusion-reaction performance of ZSM-5 molecular sieves. Furthermore, the method of this invention is simple to operate, more economical, and environmentally friendly, possessing significant potential for industrial-scale production. Attached Figure Description

[0030] Figure 1 The XRD characterization results of ZSM-5 molecular sieve before and after modification in Example 1 of this invention are shown below.

[0031] Figure 2 The results of argon physical adsorption characterization of ZSM-5 molecular sieve before and after modification in Example 1 of this invention are shown.

[0032] Figure 3 These are TEM images of ZSM-5 molecular sieves before and after modification in Example 1 of this invention;

[0033] Figure 4 The images show SEM images of ZSM-5 molecular sieves before and after modification in Example 1 of this invention.

[0034] Figure 5 The NH3-TPD characterization results of ZSM-5 molecular sieve before and after modification in Example 1 of this invention;

[0035] Figure 6 The results of pyridine infrared characterization of ZSM-5 molecular sieve before and after modification in Example 1 of this invention;

[0036] Figure 7 The diffusion performance characterization results of ZSM-5 molecular sieve before and after modification in Example 1 of this invention are shown below.

[0037] Figure 8 The NH3-TPD characterization results of ZSM-5 molecular sieves before and after modification in Comparative Example 1 of this invention are shown below.

[0038] Figure 9 The diene yields of ZSM-5 molecular sieves before and after modification in the catalytic cracking of olefins to produce ethylene and propylene in Example 1 of this invention;

[0039] Figure 10The diene selectivity of ZSM-5 molecular sieve before and after modification in the catalytic cracking of olefins to prepare ethylene and propylene in Example 1 of this invention. Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments. However, the present invention is not limited to the following embodiments.

[0041] Unless otherwise specified, the raw materials and ZSM-5 molecular sieve bodies used in the embodiments of the present invention were all purchased commercially.

[0042] In this invention, XRD and argon physical adsorption were used to determine the crystallinity and pore structure properties of the molecular sieve before and after modification, respectively. XRD characterization was performed on a Bruker D8 Advance diffractometer. The experimental parameters were: tube voltage 40 kV, tube current 40 mA, Cu(Kα) (λ = 5.1540589 nm). Before testing, the sample needed to be thoroughly ground in a mortar at a temperature of 1° min. -1 The scanning speed was measured, with a 2θ angle scanning range of 5°–50°. Argon physical adsorption was tested using a Micromeritics TriStar3000 surface area analyzer. Before testing, the sample was treated under vacuum at 350°C for 3 hours.

[0043] In this invention, NH3-TPD and pyridine infrared spectroscopy were used to determine the acidity of the molecular sieve before and after modification. The NH3-TPD experiment was used to test the acidity properties of the molecular sieve, and was performed on a Micromeritics Auto Chem II 2920 fully automated chemisorption analyzer. Detection was conducted using a Pfeiffer Vacuum ThermoStar GSD 301 T2 mass spectrometer (MS), with a mass-to-charge ratio (m / e) of 16 selected as the mass spectral signal for NH3. The pyridine infrared spectroscopy was performed on a Nicolet 6700 infrared spectrometer using an MCT detector and a CaF2 window.

[0044] In this invention, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were used to measure the morphology before and after modification. The SEM characterization experiments were conducted on a Hitachi S-4800 SEM at a voltage of 15 kV. The HRTEM was a Tecnai 20 STWIN with an operating voltage of 200 kV.

[0045] In this invention, time-resolved in-situ infrared spectroscopy is used in conjunction with the following principles and methods to measure the diffusion performance of different adsorbed molecules on molecular sieve catalysts:

[0046] The testing principle is as follows: When organic molecules are irradiated with infrared light, the chemical bonds or functional groups within the molecules undergo vibrational absorption. Different chemical bonds or functional groups absorb at different frequencies and will occupy different positions on the infrared spectrum, thus providing information about the types of chemical bonds or functional groups present in the molecule. By monitoring the absorption peak area of ​​the molecule on the infrared spectrum, the concentration change of the molecule on the adsorbent can be detected, thereby enabling the measurement of the diffusion performance of the molecule on the molecular sieve catalyst.

[0047] The test method is as follows: Molecular sieve catalyst was pressed into 10mm sheets and placed in an in-situ cell. N2 was introduced, and the mixture was pretreated at 500℃ for 2 hours. The temperature was then lowered to the measurement temperature, and background spectra were collected. Diluted adsorbed molecules were then introduced into the in-situ cell with N2, and the adsorption amount of the molecular sieve was tracked over time until adsorption saturation. Subsequently, pure N2 was introduced, and the desorption amount of the catalyst was tracked over time until complete desorption. The experimental results were fitted according to Fick's first and second laws, using Equations I, II, and III.

[0048]

[0049]

[0050]

[0051] Among them, A t / A (t→∞) A is the ratio of the characteristic peak area of ​​the adsorbed molecule at adsorption time t (in seconds) to that at adsorption equilibrium; t / A ∞ | √t→0 D is the ratio of the transient to the final characteristic peak area of ​​the adsorbed molecule during the initial adsorption stage; eff / L 2 The apparent diffusion rate of the molecular sieve catalyst is the overall diffusion flux per unit time and unit distance, expressed in seconds (s). -1 D intra / L 2 The intracrystalline diffusion rate is the diffusion flux within the micropores of a molecular sieve per unit time and distance, expressed in seconds (s). -1 α / L is the surface diffusion rate, which is the surface diffusion flux per unit time and unit distance, and its unit is s. -1 ;D eff D intra Both α and ρ are diffusion coefficients, which represent the amount of substance passing through a unit area per unit time when the concentration gradient is one unit, with units of m. 2 / s; L is the diffusion distance, in meters.

[0052] The technical solution of the present invention will be further described below through preferred specific embodiments.

[0053] In the embodiments and comparative examples of this invention, the ZSM-5 molecular sieve is first converted to the H-form using ammonium chloride solution before use. The operation method is a conventional ammonium exchange method, and the specific steps are as follows: 2g of molecular sieve catalyst is dispersed in 80mL of ammonium chloride solution with a concentration of 1mol / L, stirred at 90℃ for 2h, filtered and washed, the filter cake is placed in a 90℃ oven to dry overnight, and then calcined at 550℃ for 6h to obtain the hydrogen form ZSM-5 molecular sieve.

[0054] Example 1

[0055] (1) Weigh 2 grams of hydrogen-type ZSM-5 molecular sieve bulk (ZSM-5-p, silicon-aluminum atomic ratio of 300) and add it to 30 mL of n-hexane solution. Stir well to obtain a liquid-solid mixture.

[0056] (2) Transfer the liquid-solid mixture obtained in step (1) to a three-necked round-bottom flask, and then slowly add 0.6 mL of tetraethyl orthosilicate modified precursor solution (the concentration of tetraethyl orthosilicate as SiO2 is 0.004 g / mL, and the ratio of tetraethyl orthosilicate to ZSM-5 molecular sieve is 0.3:1 mL / g) to the round-bottom flask, and reflux and condense the reaction at 60°C for 2 h to obtain the modified liquid-solid mixture;

[0057] (3) The modified liquid-solid mixture obtained in step (2) was filtered and washed, then dried in an oven at 100°C for 8 hours and calcined in a muffle furnace at 550°C for 6 hours to obtain the catalyst precursor.

[0058] (4) Repeat the above steps 4 times to obtain the modified ZSM-5 molecular sieve catalyst, denoted as Zm-1.

[0059] ZSM-5-p and Zm-1 were characterized by XRD and argon physical adsorption, and the results are as follows: Figure 1 and Figure 2 As shown. By Figure 1 The XRD patterns show that, compared with the unmodified ZSM-5 molecular sieve (ZSM-5-p), the modified ZSM-5 molecular sieve (Zm-1) obtained in this example does not show significant changes in crystallinity and framework structure, and still possesses a typical MFI structure. Figure 2 The argon physical adsorption curves show that the adsorption isotherms of the ZSM-5 molecular sieve before and after modification are almost identical, indicating that the pore structure of the molecular sieve before and after modification is the same. Simultaneously, the specific surface area of ​​the molecular sieve before and after modification is almost unchanged. Therefore, it can be seen that the crystallinity and pore structure of the modified ZSM-5 molecular sieve prepared in Example 1 are not affected.

[0060] The morphology of ZSM-5-p and Zm-1 was characterized by TEM and SEM, and the results are as follows: Figure 3 andFigure 4 As shown, the morphology of ZSM-5 molecular sieve remained unchanged before and after modification.

[0061] The acidity of ZSM-5-p and Zm-1 was characterized by NH3-TPD and pyridine infrared spectroscopy, and the results are as follows: Figure 5 and Figure 6 As shown, the acidity of ZSM-5 molecular sieve before and after modification was not damaged.

[0062] The diffusion properties of C4 olefins on ZSM-5-p and Zm-1 were tested using time-resolved in-situ infrared spectroscopy. The in-situ infrared spectroscopy conditions were: test temperature 100℃, nitrogen purging flow rate 800 mL / min, and C4 olefin partial pressure 1 mbar. The results are as follows: Figure 7 As shown, compared with ZSM-5-p, Zm-1, while maintaining a consistent intracrystalline diffusion rate, exhibits a surface diffusion rate that increases from 1.12 s⁻¹. -1 (ZSM-5-p) increased to 2.40s -1 .

[0063] comprehensive Figures 1 to 7 The test results show that the modified ZSM-5 molecular sieve catalyst (Zm-1) obtained in this embodiment can effectively improve the surface diffusion performance without destroying the molecular sieve framework structure and intracrystalline diffusion resistance.

[0064] Example 2

[0065] Compared with Example 1, the difference is that in step (1), the silicon-aluminum ratio of ZSM-5 molecular sieve is 400, the solvent is selected from ethanol, and in step (2), the modification temperature is 40℃.

[0066] The modified ZSM-5 molecular sieve catalyst obtained in Example 2 is denoted as Zm-2. The XRD pattern of Zm-2 is... Figure 1 Similarly, the crystallinity and framework structure did not change significantly, still exhibiting a typical MFI structure; the argon physical adsorption results were consistent with... Figure 2 Similarly, the morphological characterization results are the same as Figure 3 , 4 Similarly; the results of NH3-TPD and pyridine infrared tests are the same as those of... Figure 5 , 6 Similarly, the specific surface area of ​​the molecular sieve before and after modification was found to be almost unchanged.

[0067] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rate of C4 olefins on modified Zm-2 is 2.42 s⁻¹. -1 .

[0068] Example 3

[0069] Compared with Example 1, the difference is that in step (1), the silicon-aluminum ratio of ZSM-5 molecular sieve is 500 and the solvent is selected from ethanol. In step (2), the modified precursor is triethylchlorosilane (concentration of 0.0036 g / mL based on SiO2).

[0070] The modified ZSM-5 molecular sieve catalyst obtained in Example 3 is denoted as Zm-3. The XRD pattern of Zm-3 is... Figure 1 Similarly, the crystallinity and framework structure did not change significantly, still exhibiting a typical MFI structure; the argon physical adsorption results were consistent with... Figure 2 Similarly, the morphological characterization results are the same as Figure 3 , 4 Similarly; the results of NH3-TPD and pyridine infrared tests are the same as those of... Figure 5 , 6 Similarly, the specific surface area of ​​the molecular sieve before and after modification was found to be almost unchanged.

[0071] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rate of C4 olefins on modified Zm-3 is 2.51 s⁻¹. -1 .

[0072] Example 4

[0073] Compared with Example 1, the differences are as follows: in step (1), the solvent is methanol and the volume of methanol is 60 mL; in step (2), the volume of the modified precursor tetraethyl orthosilicate is 0.4 mL; and in step (4), the modification is repeated 6 times.

[0074] The modified ZSM-5 molecular sieve catalyst obtained in Example 4 is designated Zm-4. The XRD pattern of Zm-4 is... Figure 1 Similarly, the crystallinity and framework structure did not change significantly, still exhibiting a typical MFI structure; the argon physical adsorption results were consistent with... Figure 2 Similar; morphological characterization results are the same as Figure 3 , 4 Similarly; the results of NH3-TPD and pyridine infrared tests are the same as those of... Figure 5 , 6 Similarly, the specific surface area of ​​the molecular sieve before and after modification was found to be almost unchanged.

[0075] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rate of C4 olefins on modified Zm-4 is 3.62 s⁻¹. -1 .

[0076] Comparative Example 1

[0077] (1) Weigh 2 grams of ZSM-5 molecular sieve and add it to 30 mL of n-hexane solution. Stir well to obtain a liquid-solid mixture.

[0078] (2) Transfer the liquid-solid mixture obtained in step (1) to a three-necked round-bottom flask, then slowly add 1.2 mL of n-hexane-modified precursor to the round-bottom flask, and reflux and condense the reaction at 60°C for 2 h to obtain the modified liquid-solid mixture.

[0079] (3) After the modified liquid-solid mixture from step (2) is filtered and washed, it is dried in an oven at 100°C for 8 hours and calcined in a muffle furnace at 550°C for 6 hours to obtain ZSM-5 molecular sieve with modified surface structure.

[0080] (4) Repeat the above steps 4 times to obtain the modified ZSM-5 molecular sieve catalyst, denoted as Zc-1.

[0081] The ZSM-5 molecular sieves before and after modification in Comparative Example 1 were characterized by XRD and argon physical adsorption. The results are consistent with those obtained by... Figure 1 and Figure 2 Similarly, compared to the unmodified ZSM-5 molecular sieve, the modified ZSM-5 molecular sieve in this comparative example showed no significant changes in crystallinity and framework structure, still exhibiting a typical MFI structure. The adsorption isotherms of the ZSM-5 molecular sieves before and after modification were almost identical, indicating that the pore structures of the molecular sieves were the same. Simultaneously, the specific surface area of ​​the molecular sieves before and after modification was found to be almost unchanged. This demonstrates that the crystallinity and pore structure of the modified ZSM-5 molecular sieve in this example were not affected. The morphology of the ZSM-5 molecular sieves before and after modification was characterized using TEM and SEM techniques, and the results are consistent with... Figure 3 and Figure 4 Similarly, the morphology of the molecular sieve remained unchanged before and after modification. NH3-TPD and pyridine infrared results showed that the acidity of the modified ZSM-5 molecular sieve in this comparative example was not compromised.

[0082] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rates of C4 olefins on Zc-1 and ZSM-5-p are basically the same, at 1.12 s⁻¹. -1 .

[0083] Comparative Example 2

[0084] Compared with Example 1, the difference is that in step (2), the volume of the modified precursor tetraethyl orthosilicate is 0.04 mL. The resulting modified ZSM-5 molecular sieve catalyst is designated as Zc-2.

[0085] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rates of C4 olefins on Zc-2 and ZSM-5-p are basically the same, at 1.15 s⁻¹. -1 .

[0086] Comparative Example 3

[0087] Compared with Example 1, the difference is that in step (1), the Si / Al ratio of ZSM-5 molecular sieve (ZSM-5-p-2) is 50, and in step (2), the volume of the modified precursor tetraethyl orthosilicate is 4 mL. The resulting modified ZSM-5 molecular sieve catalyst is denoted as Zc-3.

[0088] XRD pattern of Zc-3 and Figure 1 Similarly, the crystallinity and framework structure did not change significantly, still exhibiting a typical MFI structure; the argon physical adsorption results were consistent with... Figure 2 Similarly, the morphological characterization results are the same as Figure 3 , 4 Similarly, the specific surface area of ​​the molecular sieve before and after modification was almost unchanged. However, NH3-TPD test results showed that the acidity of the modified ZSM-5 molecular sieve decreased by 30% (see...). Figure 8 This may be because when the silicon-to-aluminum ratio is low enough, the deposited SiO2 readily reacts with the acidic sites on the molecular sieve surface, thus reducing the overall acidity. Meanwhile, elemental analysis results show that the SiO2 deposition amount of Zc-3 molecular sieve is 5.1% compared to ZSM-5-p-2.

[0089] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rate of C4 olefins on ZSM-5-p-2 is 0.91 s⁻¹. -1 In comparison, the surface diffusion rate of Zc-3 decreased significantly to 0.61 s⁻¹. -1 In the existing technology, excessive SiO2 deposition, while modifying the acidity of the outer surface, also blocks the surface channels of ZSM-5 molecular sieve, thereby reducing its surface diffusion rate.

[0090] Comparative Example 4

[0091] Compared with Example 1, the difference is that in step (4), the repetition is repeated once. The resulting modified ZSM-5 molecular sieve catalyst is denoted as Zc-4.

[0092] ZSM-5-p and Zc-4 molecular sieves were characterized by XRD and argon physical adsorption. The results are consistent with... Figure 1 and Figure 2Similarly, compared with the unmodified ZSM-5 molecular sieve, the modified ZSM-5 molecular sieve in this comparative example showed no significant changes in crystallinity and framework structure, still exhibiting a typical MFI structure. The adsorption isotherms of the ZSM-5 molecular sieves before and after modification were almost identical, indicating that the pore structure of the molecular sieves before and after modification was the same. Simultaneously, the specific surface area of ​​the molecular sieves before and after modification was found to be almost unchanged. Therefore, it can be seen that the crystallinity and pore structure of the modified ZSM-5 molecular sieve in this example were not affected. The morphology of the ZSM-5 molecular sieves before and after modification was characterized using TEM and SEM techniques, and the results are consistent with... Figure 3 and Figure 4 Similarly, the morphology of the molecular sieve remained unchanged before and after modification. NH3-TPD and pyridine infrared results showed that the acidity of the modified ZSM-5 molecular sieve in this comparative example was not compromised.

[0093] Time-resolved in-situ infrared spectroscopy results show that, compared with ZSM-5-p, the diffusion rate of C4 olefins on the surface of Zc-4 is basically the same, at 1.16 s⁻¹. -1 .

[0094] Example 5

[0095] The modified ZSM-5 molecular sieve catalysts obtained in Examples 1-4 and Comparative Examples 1-4 were applied to the catalytic cracking of butene to produce propylene and ethylene, respectively. Product C 2-3 The yields of olefins are shown in Table 1, and product C... 2-3 The selectivity of olefins is shown in Table 2, and the performance improvement rate of the catalysts obtained in each example compared with the corresponding unmodified ZSM-5 molecular sieve (after 100 h of reaction) is shown in Table 1.

[0096] Catalytic cracking reaction conditions for olefins: reaction temperature 550℃, catalyst dosage 0.3g, C4 ＝ Feed flow rate: 15 mL / min, WHSV = 30 h -1 .

[0097] The reaction results of the catalyst (Zm-1) in Example 1 are as follows: Figure 9 and Figure 10 As shown in the figure, compared with the ZSM-5 molecular sieve bulk (ZSM-5-p), the reaction stability of Zm-1 is significantly improved, and the selectivity of the target products (ethylene and propylene) in the reaction process is also significantly improved. That is, the surface structure modified ZSM-5 molecular sieve catalyst exhibits superior catalytic performance.

[0098] For the reaction results of Examples 2, 3, and 4 (Zm-2, Zm-3, and Zm-4), compared with the ZSM-5 molecular sieve bulk (ZSM-5-p), the reaction stability of the modified catalyst was significantly improved, and the selectivity of the target products (ethylene and propylene) in the reaction process was also significantly improved. That is, the surface-structure modified ZSM-5 molecular sieve catalyst exhibits superior catalytic performance.

[0099] For the reaction results of Comparative Examples 1, 2, and 4 (Zc-1, Zc-2, and Zc-4), compared with the ZSM-5 molecular sieve bulk, the activity, target product selectivity, and product stability of the molecular sieve catalysts obtained in the comparative examples are comparable to those of the unmodified molecular sieve catalysts.

[0100] For the reaction results of Comparative Example 3 (Zc-3), compared with the high silica-alumina ratio ZSM-5 molecular sieve, the selected low silica-alumina ratio molecular sieve showed significantly lower olefin cracking reaction yield, selectivity and stability before and after modification.

[0101] Table 1

[0102]

[0103] Table 2

[0104]

[0105] The embodiments described above are merely detailed descriptions of the technical solutions of the present invention, but the present invention is not limited to the above embodiments, that is, the present invention does not depend on the steps described in the above embodiments to be implemented. In summary, any improvements made to the present invention by those skilled in the art, including the substitution of the raw materials and additives described in the present invention, the selection of specific implementation methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A modified ZSM-5 molecular sieve catalyst, characterized in that, The ZSM-5 molecular sieve has a silicon-to-aluminum atomic ratio of 100-500, and the modified ZSM-5 molecular sieve catalyst has a surface diffusion rate of 2.40-4.00 s⁻¹. -1 The preparation method of the modified ZSM-5 molecular sieve catalyst includes the following steps: (1) Mix ZSM-5 molecular sieve with solvent to obtain a liquid-solid mixture; (2) Add the modified precursor solution to the liquid-solid mixture obtained in step (1) to carry out the modification reaction, and obtain the modified liquid-solid mixture; (3) The modified liquid-solid mixture obtained in step (2) is dried and calcined to obtain the catalyst precursor; (4) Repeat steps (1) to (3) to obtain the modified ZSM-5 molecular sieve catalyst; In step (2), the modified precursor source is selected from one or more of tetraethyl orthosilicate, tetramethyl orthosilicate, trimethylchlorosilane, triethylchlorosilane, and tripropylchlorosilane.

2. The modified ZSM-5 molecular sieve catalyst according to claim 1, characterized in that, The silicon-aluminum atomic ratio of the ZSM-5 molecular sieve is 300~500.

3. The modified ZSM-5 molecular sieve catalyst according to claim 1, characterized in that, The ZSM-5 molecular sieve is H-ZSM-5 molecular sieve.

4. A method for preparing the modified ZSM-5 molecular sieve catalyst according to any one of claims 1-3, comprising the following steps: (1) Mix ZSM-5 molecular sieve with solvent to obtain a liquid-solid mixture; (2) Add the modified precursor solution to the liquid-solid mixture obtained in step (1) to carry out the modification reaction, and obtain the modified liquid-solid mixture; (3) The modified liquid-solid mixture obtained in step (2) is dried and calcined to obtain the catalyst precursor; (4) Repeat steps (1) to (3) to obtain the modified ZSM-5 molecular sieve catalyst; In step (2), the modified precursor source is selected from one or more of tetraethyl orthosilicate, tetramethyl orthosilicate, trimethylchlorosilane, triethylchlorosilane, and tripropylchlorosilane.

5. The preparation method according to claim 4, characterized in that, The solvent mentioned in step (1) is selected from one or more of methanol, ethanol, petroleum ether, dichloromethane, n-hexane, cyclohexane, acetone, and butanone.

6. The preparation method according to claim 4, characterized in that, The solvent mentioned in step (1) is selected from one or more of methanol, ethanol, and n-hexane.

7. The preparation method according to claim 4, characterized in that, The ratio of the solvent to the ZSM-5 molecular sieve in step (1) is 10~50:1 mL / g.

8. The preparation method according to claim 4, characterized in that, The ratio of the solvent to the ZSM-5 molecular sieve in step (1) is 15~30:1 mL / g.

9. The preparation method according to claim 4, characterized in that, The concentration of the modified precursor solution in step (2), calculated as SiO2, is 0.03~13 mg / mL.

10. The preparation method according to claim 4, characterized in that, In step (2), the modified precursor source is selected from tetraethyl orthosilicate.

11. The preparation method according to claim 4, characterized in that, In step (2), the ratio of ZSM-5 molecular sieve in the modified precursor solution and the liquid-solid mixture obtained in step (1) is 0.05~1:1 mL / g.

12. The preparation method according to claim 4, characterized in that, In step (2), the ratio of ZSM-5 molecular sieve in the modified precursor solution and the liquid-solid mixture obtained in step (1) is 0.05~0.5: 1 mL / g.

13. The preparation method according to claim 4, characterized in that, The reaction described in step (2) needs to be carried out under reflux condensation conditions, with a reaction temperature of 30~100 ℃ and a reaction time of 0.5~10 h.

14. The preparation method according to claim 4, characterized in that, The reaction described in step (2) needs to be carried out under reflux condensation conditions, with a reaction temperature of 40~80 ℃ and a reaction time of 2~4 h.

15. The preparation method according to claim 4, characterized in that, In step (3), the roasting temperature is 500~650 ℃ and the roasting time is 4~10 h.

16. The preparation method according to claim 4, characterized in that, The number of repetitions in step (4) is 2 to 10.

17. The preparation method according to claim 4, characterized in that, The number of repetitions in step (4) is 3 to 6.

18. The application of the modified ZSM-5 molecular sieve catalyst according to any one of claims 1-3 or the modified ZSM-5 molecular sieve catalyst prepared by any one of claims 4-17 in the catalytic cracking of olefins to produce ethylene and propylene.

19. The application according to claim 18, characterized in that, The reaction conditions are as follows: reaction temperature 300~700℃, reaction space velocity 5~50 h⁻¹ -1 The reaction pressure is 0.1~3 bar.

20. The application according to claim 18, characterized in that, The reaction conditions are as follows: reaction temperature 500~600℃, reaction space velocity 20~35 h⁻¹ -1 The reaction pressure is 0.5~1 bar.

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