A modified zsm-5 molecular sieve catalyst, its preparation method and application

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: preparing ZSM-5 molecular sieve without removing a template agent, then mixing the ZSM-5 molecular sieve with an organic solvent to obtain a liquid-solid mixture, adding an acid solution drop by drop to modify the liquid-solid mixture, and then separating, drying, calcining and ammonium ion exchanging to obtain the modified ZSM-5 molecular sieve catalyst. According to the method, the surface diffusion performance of the molecular sieve is improved by regulating the surface defects of the molecular sieve, and the catalytic efficiency, reaction life and ethylene and propylene selectivity of the molecular sieve are improved.

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] The processes used in the petrochemical and coal chemical industries to produce ethylene and propylene often generate large amounts of C4 / C5 olefins as byproducts. Olefins catalytic cracking (OCC) technology can convert these low-value-added byproducts into ethylene and propylene, promoting integrated refining and chemical production, connecting and supporting coal chemical processes, and improving carbon resource utilization and technological competitiveness. In recent years, olefin cracking using H-ZSM-5 molecular sieves as catalysts has become an important technological route for efficiently utilizing light hydrocarbon resources to produce high-value-added basic organic feedstocks. 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. Therefore, precisely controlling the molecular sieve diffusion pathway is a key challenge for further improving catalytic efficiency.

[0003] For a long time, intracrystalline diffusion behavior in molecular sieve catalysts has been considered the rate-determining step dominating the diffusion pathway. Our research group previously conducted a series of exploratory studies on how to regulate the intracrystalline diffusion pathway of ZSM-5 molecular sieves and improve the catalytic performance of OCC. The results showed that reducing grain size or creating hierarchical porous structures can effectively shorten the intracrystalline diffusion distance of molecular sieve catalysts, reduce intracrystalline diffusion resistance, and significantly improve their catalytic efficiency. Furthermore, due to the anisotropic diffusion of molecules within the channels of ZSM-5 molecular sieves, constructing molecular sieve morphologies with specific crystal faces and channel exposure can also improve diene yields. However, in reality, guest molecules also face diffusion resistance at the interface before entering the microporous channels. Researchers (Nature Mater., 2016, 15, 401-406) used microscopic imaging to study the diffusion of guest molecules in molecular sieves and found that as the adsorption and desorption processes proceeded, the concentration of probe molecules near the outer surface of the zeolite crystals showed a significant jump compared to the concentration at final equilibrium. Fan et al. (Langmuir, 2013, 29, 13943-13950) found that after nano-sizing MFI molecular sieves, the actual measured diffusion coefficient was lower than the theoretical value, and the calculated diffusion distance of the microporous configuration was longer than the molecular sieve grain size. These anomalies are mainly attributed to the increased surface diffusion resistance of guest molecules on the molecular sieve. Surface diffusion resistance is a crucial and significant factor affecting the diffusion path of molecular sieves, and it is widely present in various molecular sieve systems. However, there is currently a severe lack of strategies for controlling the surface diffusion resistance of ZSM-5 molecular sieves, and the impact of surface diffusion resistance on the catalytic performance of molecular sieves is even more unclear.

[0004] Therefore, it is crucial to find ways to improve the surface diffusion resistance of ZSM-5 molecular sieves. Summary of the Invention

[0005] To address the problems of high surface diffusion resistance of ZSM-5 molecular sieves in existing technologies, such as low diene selectivity and short catalyst lifetime in olefin catalytic cracking reactions to ethylene and propylene, 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, thereby improving its catalytic efficiency, reaction lifetime, and ethylene and propylene selectivity.

[0006] The first aspect of this invention provides a method for preparing a modified ZSM-5 molecular sieve catalyst, comprising the following steps:

[0007] (1) Preparation of ZSM-5 molecular sieve without template agent removal;

[0008] (2) The ZSM-5 molecular sieve obtained in step (1) without removing the template agent is mixed with an organic solvent to obtain a liquid-solid mixture;

[0009] (3) Add acid solution dropwise to the liquid-solid mixture obtained in step (2) to modify it, and obtain a modified liquid-solid mixture;

[0010] (4) The modified liquid-solid mixture obtained in step (3) is separated, dried and calcined to obtain the catalyst precursor;

[0011] (5) The catalyst precursor obtained in step (4) is subjected to ammonium ion exchange to obtain the modified ZSM-5 molecular sieve catalyst.

[0012] Further, the ZSM-5 molecular sieve without template removal described in step (1) can be prepared using a conventional hydrothermal method. The template is selected from one or more of tetrapropylammonium bromide, tetrapropylammonium hydroxide, n-butylamine, ethylenediamine, ethylamine, tripropylamine, and n-butylamine; preferably, the template is selected from one or more of tetrapropylammonium bromide, tetrapropylammonium hydroxide, and ethylenediamine.

[0013] Furthermore, the silicon-aluminum atomic ratio of the ZSM-5 molecular sieve without template removal in step (1) is 200-600, preferably 300-400.

[0014] Furthermore, in step (2), the ratio of ZSM-5 molecular sieve without template agent removal to organic solvent is 1:1 to 1:5 g / mL.

[0015] Further, the organic solvent in step (2) is selected from one or more of acetone, butanone, methanol, ethanol, diethyl ether, petroleum ether, dichloromethane, n-hexane, and cyclohexane; preferably, the organic solvent is selected from one or more of acetone, diethyl ether, and ethanol.

[0016] Further, the acid used in the acid solution in step (3) is selected from one or more of hydrochloric acid, hydrofluoric acid, acetic acid, oxalic acid, nitric acid, and sulfuric acid; preferably, the acid used in the acid solution is selected from one or more of hydrochloric acid, hydrofluoric acid, and acetic acid. The mass concentration of the acid solution is 1% to 20%, preferably 5% to 15%.

[0017] Furthermore, in step (3), the mass ratio of the acid solution to the ZSM-5 molecular sieve in the liquid-solid mixture obtained in step (2) is 0.01:1 to 2:1, preferably 0.03:1 to 1:1.

[0018] Further, the conditions for the modification reaction in step (3) are as follows: the reaction temperature is 0 to 100°C and the reaction time is 5 to 60 min; preferably, the reaction temperature is 25 to 50°C and the reaction time is 10 to 30 min.

[0019] Further, the modified liquid-solid mixture described in step (4) can be separated into solids from the obtained mixture by any conventionally known solid-liquid separation method. After solid-liquid separation, washing and drying can be performed. 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. The drying can be carried out under normal pressure or under reduced pressure.

[0020] Further, in step (4), the roasting temperature is 400-800℃ and the roasting time is 3-20h; preferably, the roasting temperature is 500-650℃ and the roasting time is 5-10h.

[0021] Further, the specific operation of the ammonium ion exchange in step (5) is as follows: the catalyst precursor and the ammonium salt solution are ion exchanged at a solid-liquid mass ratio of 1:10 to 1:40 at 50 to 100°C for 1 to 10 hours, preferably at 70 to 90°C for 3 to 5 hours, and then the solid is separated. The exchange is then repeated 0 to 2 times in the above manner. The ammonium salt used for the exchange is selected from at least one of ammonium chloride, ammonium nitrate, ammonium carbonate, and ammonium sulfate; the concentration of ammonium ions in the ammonium salt solution is 0.5 to 2 mol / L.

[0022] A second aspect of the present invention provides a modified ZSM-5 molecular sieve catalyst, which is prepared by any of the preparation methods described in the first aspect.

[0023] Furthermore, the surface diffusion rate of the modified ZSM-5 molecular sieve catalyst is 1.40–4.00 s⁻¹. -1 The preferred time is 2.50–4.00 s. -1 .

[0024] Furthermore, the ZSM-5 molecular sieve is an H-ZSM-5 molecular sieve.

[0025] A third aspect of the present invention provides the application of the above-mentioned modified ZSM-5 molecular sieve catalyst in the catalytic cracking reaction of olefins to produce ethylene and propylene.

[0026] Further, the reaction is as follows: the modified ZSM-5 molecular sieve catalyst is reacted with a raw material containing methanol and butene to obtain ethylene and propylene.

[0027] Furthermore, the reaction temperature is 400–700°C, and the reaction mass hourly space velocity is 5–50 h⁻¹. -1 The reaction pressure is 0.1–5 bar. Preferably, the reaction temperature is 450–600 °C and the reaction space velocity is 10–40 h⁻¹. -1 The reaction pressure is 0.5–2 bar.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1. The modified ZSM-5 molecular sieve catalyst provided by this invention has a pore structure, crystallinity, 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 the selectivity of target products, improve the yield of target products, extend the reaction lifetime, and reduce the degradation of low-value-added C4 catalysts. 4+ The efficient catalytic conversion of olefin feedstocks has become possible, facilitating industrial production.

[0030] 2. In the catalytic cracking reaction of olefins, there is a lack of methods to regulate the surface diffusion resistance of ZSM-5 molecular sieves. This invention employs an acid modification method to regulate the surface diffusion resistance of high silica-to-alumina ratio ZSM-5 molecular sieves without template removal. By controlling the amount of acid solution and the modification time, the resulting modified ZSM-5 molecular sieve significantly reduces its surface diffusion resistance and improves its catalytic efficiency while maintaining its internal crystal structure and diffusion properties. This provides a new method for regulating the surface diffusion resistance of ZSM-5 molecular sieves. Furthermore, the method of this invention is simple to operate, has low modification costs, and has significant potential for industrial-scale production. Attached Figure Description

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

[0032] Figure 2 The TEM characterization results of ZSM-5 molecular sieve before and after modification in Example 1 of this invention are shown below.

[0033] Figure 3 The results of pyridine infrared characterization of ZSM-5 molecular sieve before and after modification in Example 1 of this invention;

[0034] Figure 4 The diffusion performance characterization results of ZSM-5 molecular sieve before and after modification in Example 1 of this invention are shown below.

[0035] Figure 5 The diffusion performance characteristics of ZSM-5 molecular sieve before and after modification in Comparative Example 1 of this invention are shown.

[0036] Figure 6 The XRD characterization results of ZSM-5 molecular sieve before and after modification in Comparative Example 3 of this invention are shown below.

[0037] Figure 7 The results of pyridine infrared characterization of ZSM-5 molecular sieve before and after modification in Comparative Example 3 of this invention are shown. Detailed Implementation

[0038] The present invention will be further described below with reference to specific embodiments. However, the present invention is not limited to the following embodiments.

[0039] In this invention, XRD was used to determine the crystallinity of the molecular sieve before and after modification. 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 was thoroughly ground in a mortar, and scanning was performed at a speed of 1° / min, with a 2θ angle scanning range of 5°–50°.

[0040] In this invention, a high-resolution transmission electron microscope (TEM) was used to measure the morphology before and after modification. The TEM used was a Tecnai 20 STWIN, operating at 200 kV.

[0041] In this invention, pyridine infrared spectroscopy was used to determine the acidity of the molecular sieve before and after modification. The pyridine infrared spectroscopy was performed on a Nicolet 6700 infrared spectrometer using an MCT detector and a CaF2 window.

[0042] 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:

[0043] 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.

[0044] 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.

[0045]

[0046]

[0047]

[0048] Among them, A t / A (t→∞) It is the ratio of the characteristic peak area of ​​the adsorbed molecule at adsorption time t (in seconds) to that at adsorption equilibrium. 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.

[0049] The technical solution of the present invention will be further described below through specific embodiments.

[0050] Example 1

[0051] (1) Weigh 31.25g tetraethyl orthosilicate + 30g tetrapropylammonium hydroxide (25%) + 45g distilled water into a polytetrafluoroethylene-lined tube, mix and stir for 1 hour to obtain mixed solution A; weigh 10g distilled water + 0.1405g aluminum nitrate nonahydrate + 0.6g sodium hydroxide into a polytetrafluoroethylene-lined tube, mix and stir for 1 hour to obtain mixed solution B; then, add solution B to solution A, stir for 1 hour, transfer to a stainless steel crystallization kettle, and crystallize in an oven at 170℃ for 48 hours. After washing, centrifugation, and drying, obtain the untemplated ZSM-5 molecular sieve bulk (Na-ZSM-5-t, silicon-aluminum atomic ratio of 400). After calcination and ammonium exchange, the Na-ZSM-5-t molecular sieve yields the hydrogen-form ZSM-5 molecular sieve bulk (H-ZSM-5-t).

[0052] (2) Weigh 3 grams of the undetemplated ZSM-5 molecular sieve powder (Na-ZSM-5-t) obtained in step (1) and add it to 12 mL of acetone solution. Stir well to obtain a liquid-solid mixture.

[0053] (3) Slowly add 2g of hydrofluoric acid modified precursor solution (hydrofluoric acid mass concentration is 10%, and the ratio of hydrofluoric acid to ZSM-5 molecular sieve is 0.2:3g / g) to the liquid-solid mixture obtained in step (2), and stir at 25℃ for 5min to obtain the modified liquid-solid mixture.

[0054] (4) The modified liquid-solid mixture obtained in step (3) is washed, filtered, dried in an oven at 100°C for 8 hours and calcined in a muffle furnace at 550°C for 6 hours to obtain sodium-type modified ZSM-5 molecular sieve precursor.

[0055] (5) The sodium-type ZSM-5 molecular sieve precursor obtained in step (4) was ion-exchanged with a 1 mol / L ammonium chloride solution at a solid-liquid mass ratio of 1:10 at 80°C for 3 h. The solid was then separated, and the ion exchange was repeated twice in the above manner to obtain the hydrogen-type modified ZSM-5 molecular sieve catalyst (H-ZSM-5-F-1).

[0056] Figure 1 XRD characterization results for H-ZSM-5-t and H-ZSM-5-F-1 catalysts. Figure 1 The XRD pattern shows that, compared with the unmodified ZSM-5 molecular sieve (H-ZSM-5-t), the crystallinity and framework structure of the acid-modified ZSM-5 molecular sieve (H-ZSM-5-F-1) obtained in Example 1 did not change significantly, and it can still maintain the typical MFI topology. This indicates that the template agent that was not removed can effectively prevent the destruction of the molecular sieve framework structure by the acid modification process, so that the crystallinity and pore structure of the modified ZSM-5 molecular sieve catalyst obtained in Example 1 are consistent with those of H-ZSM-5-t.

[0057] Figure 2 The TEM characterization results of the H-ZSM-5-t and H-ZSM-5-F-1 catalysts are shown. The results indicate that the morphology and pore structure of the acid-modified ZSM-5 molecular sieve (H-ZSM-5-F-1) obtained in Example 1 show significant changes.

[0058] Figure 3 The pyridine infrared spectra of H-ZSM-5-t and H-ZSM-5-F-1 are shown. The results show that the acidity of the molecular sieve remains basically unchanged before and after modification, indicating that the H-ZSM-5-F-1 molecular sieve can maintain its acidic characteristics due to the protection of the template agent.

[0059] The diffusion properties of the molecular sieve catalyst before and after modification were studied using time-resolved in-situ infrared spectroscopy. In-situ infrared spectroscopy testing conditions: test temperature 100℃, nitrogen purging flow rate 800 mL / min, and partial pressure of butene molecules 1 mbar. Results are as follows: Figure 4 As shown, compared with H-ZSM-5-t, H-ZSM-5-F-1, while maintaining a consistent intracrystalline diffusion rate, showed a surface diffusion rate that increased from the initial 1.31 s⁻¹. -1 Improved to 2.60s -1 .

[0060] comprehensive Figures 1 to 4 The test results show that the modified ZSM-5 molecular sieve catalyst (H-ZSM-5-F-1) obtained in this embodiment can not only maintain its framework structure and intracrystalline diffusion resistance, but also effectively improve its surface diffusion rate.

[0061] Example 2

[0062] Compared with Example 1, the difference is that in step (1), ethylenediamine is used as the template agent and the silicon-to-aluminum atomic ratio is 300.

[0063] The modified ZSM-5 molecular sieve catalyst obtained in Example 2 is designated as H-ZSM-5-F-2. The XRD pattern of H-ZSM-5-F-2 is similar to... Figure 1 Similarly, the crystallinity and framework structure did not change significantly, still exhibiting a typical MFI structure; the morphological characterization results were consistent with... Figure 2 Similar; pyridine infrared test results are similar to Figure 3 Similarly, the specific surface area of ​​the molecular sieve before and after modification was found to be almost unchanged.

[0064] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rate of butene molecules on H-ZSM-5-F-2 is 2.62 s⁻¹. -1 .

[0065] Example 3

[0066] Compared with Example 1, the difference is that in step (1), the silicon-aluminum feed ratio in the ZSM-5 molecular sieve preparation process is 300. In step (3), the acid solution is a hydrochloric acid solution (mass concentration of 10wt%).

[0067] The modified ZSM-5 molecular sieve catalyst obtained in Example 3 is designated as H-ZSM-5-F-3. The XRD pattern of H-ZSM-5-F-3 is shown in the figure. Figure 1 Similarly, the crystallinity and framework structure did not change significantly, still exhibiting a typical MFI structure; the morphological characterization results were consistent with... Figure 2 Similar; pyridine infrared test results are similar to Figure 3 Similarly, this indicates that the acidity of the modified molecular sieve obtained in Example 3 was not compromised.

[0068] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rate of C4 olefins on H-ZSM-5-F-3 is 2.55 s⁻¹. -1 .

[0069] Example 4

[0070] Compared with Example 1, the difference is that in step (3), the mass of the modified acid solution is 1g, the mass concentration of hydrofluoric acid is 10%, the ratio of hydrofluoric acid to ZSM-5 molecular sieve is 0.1:3g / g, and the modification temperature is 40℃; in step (5), the selected ammonium salt is ammonium nitrate solution.

[0071] The modified ZSM-5 molecular sieve catalyst obtained in Example 4 is designated as H-ZSM-5-F-4. The XRD pattern of H-ZSM-5-F-4 is similar to... Figure 1 Similarly, the crystallinity and framework structure did not change significantly, still exhibiting a typical MFI structure; the morphological characterization results were consistent with... Figure 2 Similar; pyridine infrared test results are similar to Figure 3 Similarly, this indicates that the acidity of the modified molecular sieve obtained in Example 4 was not compromised.

[0072] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rate of C4 olefins on H-ZSM-5-F-4 is 2.89 s⁻¹. -1 .

[0073] Example 5

[0074] Compared with Example 1, the difference is that in step (3), the mass of the modified acid solution is 2g, the mass concentration of hydrofluoric acid is 15%, the ratio of hydrofluoric acid to ZSM-5 molecular sieve is 0.5:1g / g, and the modification time is 10min.

[0075] The modified ZSM-5 molecular sieve catalyst obtained in Example 5 is denoted as H-ZSM-5-F-5. The XRD pattern of H-ZSM-5-F-5 is similar to... Figure 1 Similarly, the crystallinity and framework structure did not change significantly, still exhibiting a typical MFI structure; the morphological characterization results were consistent with... Figure 2 Similar; pyridine infrared test results are similar to Figure 3 Similarly, this indicates that the acidity of the modified molecular sieve obtained in Example 5 was not compromised.

[0076] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rate of C4 olefins on H-ZSM-5-F-5 is 3.5 s⁻¹. -1 .

[0077] Comparative Example 1

[0078] (1) Weigh 31.25g tetraethyl orthosilicate + 30g tetrapropylammonium hydroxide (25%) + 45g distilled water into a polytetrafluoroethylene-lined tube, mix and stir for 1 hour to obtain mixed solution A; weigh 10g distilled water + 0.1405g aluminum nitrate nonahydrate + 0.6g sodium hydroxide into a polytetrafluoroethylene-lined tube, mix and stir for 1 hour to obtain mixed solution B. Then, add solution B to solution A, stir for 1 hour, transfer to a stainless steel crystallization kettle, and crystallize in an oven at 170℃ for 48 hours. After washing, centrifugation, and drying, obtain the untemplated ZSM-5 molecular sieve bulk (Na-ZSM-5-t, silicon-aluminum atomic ratio of 400). After calcination and ammonium exchange, the Na-ZSM-5-t molecular sieve yields the hydrogen-form ZSM-5 molecular sieve bulk (H-ZSM-5-t).

[0079] (2) Weigh 3 grams of the undetemplated ZSM-5 molecular sieve powder (Na-ZSM-5-t) obtained in step (1) and add it to 12 mL of acetone solution. Stir well to obtain a liquid-solid mixture.

[0080] (3) Slowly add 2g of acetone solution to the liquid-solid mixture obtained in step (2) and stir at 25°C for 5 minutes to obtain the modified liquid-solid mixture;

[0081] (4) The modified liquid-solid mixture obtained in step (3) is washed, filtered, dried in an oven at 100°C for 8 hours and calcined in a muffle furnace at 550°C for 6 hours to obtain sodium-type modified ZSM-5 molecular sieve precursor.

[0082] (5) The sodium-type ZSM-5 molecular sieve precursor obtained in step (4) was ion-exchanged with a 1 mol / L ammonium chloride solution at a solid-liquid mass ratio of 1:10 at 80°C for 3 h. The solid was then separated, and the ion exchange was repeated twice in the above manner to obtain the hydrogen-type modified ZSM-5 molecular sieve catalyst (H-ZSM-5-C-1).

[0083] Compared with the unmodified ZSM-5 molecular sieve (H-ZSM-5-t), the modified ZSM-5 molecular sieve (H-ZSM-5-C-1) obtained in Comparative Example 1 showed no significant changes in crystallinity and framework structure, and could still maintain the typical MFI topology. The acidity of the molecular sieve remained essentially unchanged before and after modification.

[0084] The diffusion properties of the molecular sieve catalyst before and after modification were studied using time-resolved in-situ infrared spectroscopy. In-situ infrared spectroscopy testing conditions: test temperature 100℃, nitrogen purging flow rate 800 mL / min, and partial pressure of butene molecules 1 mbar. Figure 5 As shown, compared with H-ZSM-5-t, the diffusion rate of H-ZSM-5-C-1 remained essentially unchanged at 1.32 s⁻¹. -1 .

[0085] Comparative Example 2

[0086] Compared with Example 1, the difference is that in step (3), the mass of the modified precursor hydrofluoric acid is 0.1 g. The resulting modified ZSM-5 molecular sieve catalyst is denoted as H-ZSM-5-C-2.

[0087] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rate of butene molecules on H-ZSM-5-C-2 and H-ZSM-5-t is basically the same, at 1.36 s⁻¹. -1 .

[0088] Comparative Example 3

[0089] Compared with Example 1, the difference is as follows: In step (1), the Si / Al ratio of ZSM-5 molecular sieve (H-ZSM-5-t-1) is 40. In step (2), the mass of molecular sieve is 1g; in step (3), the mass of modified acid solution hydrofluoric acid is 6g (the mass concentration of hydrofluoric acid is 40%, and the ratio of hydrofluoric acid to ZSM-5 molecular sieve is 2.4:1g / g), the modification temperature is 90℃, and the modification time is 30 minutes. The obtained modified ZSM-5 molecular sieve catalyst is denoted as H-ZSM-5-C-3.

[0090] The XRD pattern of H-ZSM-5-C-3 is as follows: Figure 6 As shown, although it still possesses a typical MFI structure, its crystallinity is clearly reduced and its framework structure is obviously disrupted; pyridine infrared results indicate that the acidity of the modified ZSM-5 molecular sieve is significantly reduced (see...). Figure 7 The problem may be due to excessively high acid concentration and long reaction time, leading to excessive etching of the silicon atoms on the surface of the molecular sieve by the acid, causing severe damage to the molecular sieve structure. In existing technologies, excessively high acid concentration, excessively high modification temperature, and excessively long modification time result in significant damage to the microporous structure of the molecular sieve, severe loss of active centers, and a significant weakening of the catalytic performance of the modified ZSM-5 molecular sieve.

[0091] Time-resolved in-situ infrared spectroscopy results show that the surface diffusion rate of butene molecules on H-ZSM-5-C-3 is significantly lower than that on H-ZSM-5-t-1, decreasing by 0.5 s⁻¹. -1 .

[0092] Example 6

[0093] The modified ZSM-5 molecular sieve catalysts obtained in Examples 1-5 and Comparative Examples 1-3 were applied to the catalytic cracking of butene to produce propylene and ethylene, respectively. The yields of propylene and ethylene are shown in Table 1, the selectivity of ethylene and propylene 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 (H-ZSM-5-t) (after 100 h of reaction) is shown in Table 1.

[0094] Catalytic cracking reaction conditions for olefins: reaction temperature 550℃, catalyst dosage 0.3g, C4 ＝ Feed flow rate: 15 mL / min, WHSV = 30 h -1 .

[0095] For the reaction results of Examples 1, 2, 3, 4, and 5 (H-ZSM-5-F-1, H-ZSM-5-F-2, H-ZSM-5-F-3, H-ZSM-5-F-4, and H-ZSM-5-F-5), compared with the ZSM-5 molecular sieve bulk (H-ZSM-5-t), the modified reaction stability was significantly improved, and the selectivity of the target products (ethylene and propylene) in the reaction process was also significantly improved. That is, the modified ZSM-5 molecular sieve catalyst exhibits superior catalytic performance.

[0096] For the reaction results of Comparative Examples 1 and 2 (H-ZSM-5-C-1 and H-ZSM-5-C-2), 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.

[0097] For the reaction results of Comparative Example 3 (H-ZSM-5-C-3), compared with the high silica-alumina ratio ZSM-5 molecular sieve modified under normal temperature and low acid solution concentration conditions, the selected low silica-alumina ratio molecular sieve showed significantly lower olefin cracking reaction yield, selectivity and stability before and after modification under high temperature and high acid solution concentration conditions.

[0098] Table 1

[0099]

[0100] Table 2

[0101]

[0102]

[0103] 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 method for preparing a modified ZSM-5 molecular sieve catalyst, comprising the following steps: (1) Preparation of ZSM-5 molecular sieve without removal of template agent; (2) The ZSM-5 molecular sieve obtained in step (1) without removing the template agent is mixed with an organic solvent to obtain a liquid-solid mixture; (3) Add acid solution dropwise to the liquid-solid mixture obtained in step (2) to modify it, and obtain a modified liquid-solid mixture; (4) The modified liquid-solid mixture obtained in step (3) is separated, dried and calcined to obtain the catalyst precursor; (5) The catalyst precursor obtained in step (4) is subjected to ammonium ion exchange to obtain the modified ZSM-5 molecular sieve catalyst; The conditions for the modification reaction in step (3) are as follows: the reaction temperature is 25~50 ℃ and the reaction time is 10~30 min; The mass concentration of the acid solution mentioned in step (3) is 1%~20%; In step (3), the mass ratio of the acid solution to the ZSM-5 molecular sieve in the liquid-solid mixture obtained in step (2) is 0.01:1~2:1; The acid used in the acid solution in step (3) is selected from one or more of hydrochloric acid, hydrofluoric acid, acetic acid, oxalic acid, nitric acid, and sulfuric acid.

2. The preparation method according to claim 1, characterized in that, The template agent mentioned in step (1) is selected from one or more of tetrapropylammonium bromide, tetrapropylammonium hydroxide, n-butylamine, ethylenediamine, ethylamine and tripropylamine.

3. The preparation method according to claim 2, characterized in that, The template agent is selected from one or more of tetrapropylammonium bromide, tetrapropylammonium hydroxide, and ethylenediamine.

4. The preparation method according to claim 1, characterized in that, The silicon-aluminum atomic ratio of the ZSM-5 molecular sieve without template removal in step (1) is 200~600.

5. The preparation method according to claim 4, characterized in that, The silicon-aluminum atomic ratio of the ZSM-5 molecular sieve without template removal in step (1) is 300~400.

6. The preparation method according to claim 1, characterized in that, In step (2), the ratio of ZSM-5 molecular sieve without template removal to organic solvent is 1:1 to 1:5 g / mL; and / or, The organic solvent is selected from one or more of acetone, butanone, methanol, ethanol, diethyl ether, petroleum ether, dichloromethane, n-hexane, and cyclohexane.

7. The preparation method according to claim 6, wherein the organic solvent is selected from one or more of acetone, diethyl ether, and ethanol.

8. The preparation method according to claim 1, characterized in that, The acid used in the acid solution in step (3) is selected from one or more of hydrochloric acid, hydrofluoric acid, and acetic acid; and / or, The mass concentration of the acid solution is 5% to 15%.

9. The preparation method according to claim 1, characterized in that, In step (3), the mass ratio of the acid solution to the liquid-solid mixture obtained in step (2) is 0.03:1 to 1:

1.

10. The preparation method according to claim 1, characterized in that, In step (4), the roasting temperature is 400~800 ℃ and the roasting time is 3~20 h.

11. The preparation method according to claim 10, characterized in that, In step (4), the roasting temperature is 500~650 ℃ and the roasting time is 5~10 h.

12. The preparation method according to claim 1, characterized in that, In step (5), the ammonium ion exchange temperature is 50~100℃; the ammonium ion exchange time is 1~10 h; and / or, The ammonium salt solution used for the ammonium ion exchange has an ammonium ion concentration of 0.5~2 mol / L.

13. The preparation method according to claim 12, characterized in that, In step (5), the ammonium ion exchange temperature is 70~90℃ and the ammonium ion exchange time is 3~5 h.

14. The modified ZSM-5 molecular sieve catalyst prepared by any of the preparation methods described in claims 1-13.

15. The catalyst according to claim 14, characterized in that, The surface diffusion rate of the modified ZSM-5 molecular sieve catalyst is 1.40~4.00 s⁻¹. -1 .

16. The catalyst according to claim 15, characterized in that, The surface diffusion rate of the modified ZSM-5 molecular sieve catalyst is 2.50~4.00 s. -1 .

17. The application of the modified ZSM-5 molecular sieve catalyst according to any one of claims 14-16 in the catalytic cracking reaction of olefins to produce ethylene and propylene.

18. The application according to claim 17, characterized in that, The reaction temperature is 400~700 ℃, and the reaction mass hourly space velocity is 5~50 h⁻¹. -1 The reaction pressure is 0.1~5 bar.

19. The application according to claim 18, characterized in that, The reaction temperature is 450~600 ℃, and the reaction mass hourly space velocity is 10~40 h⁻¹. -1 The reaction pressure is 0.5~2 bar.

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