Preparation method and application of zsm-5 molecular sieve with lewis strong acid center

Abstract

This invention discloses a method for preparing ZSM-5 molecular sieves with Lewis strong acid centers and their applications. The method first involves surface treatment of the ZSM-5 molecular sieve to protect its original acid site structure; then, using an in-situ hydrothermal method combined with a slow-release precipitant, and by precisely controlling the hydrothermal reaction conditions and subsequent calcination process, the synergistic regulation of the state of zinc active species and the spatial distribution of acid sites in the catalytic material within the ZSM-5 molecular sieve is achieved, resulting in highly dispersed [ZnOH]. + The active species construct a novel Lewis strong acid center. Compared to traditional preparation methods such as impregnation, the molecular sieve prepared by the method of this invention has a stronger deep dehydrogenation capability, and can achieve synergistic optimization of dehydrogenation and aromatization functions, thereby effectively improving the alkane aromatization performance of the catalyst.

Description

Technical Field

[0001] The invention belongs to the field of petrochemical technology, specifically relating to a method for preparing and applying ZSM-5 molecular sieves with Lewis strong acid centers. Background Technology

[0002] Aromatic hydrocarbons, especially benzene, toluene, and xylene (the three benzenes), are indispensable key raw materials in the modern petrochemical industry and are widely used in synthetic fibers, engineering plastics, and high-end electronic chemicals. The main industrial methods for obtaining aromatic hydrocarbons still rely on naphtha catalytic reforming and steam cracking, which suffer from high energy consumption, high carbon emissions, and relatively fixed product distribution.

[0003] With the development of industrial structure, aromatization technology, which directly produces aromatics from alkanes, is becoming an important development direction for the petrochemical industry to optimize its raw material structure and improve economic efficiency due to its advantages such as low raw material cost and short process. This aromatization technology can convert low-value alkanes into high-value aromatics, and its key lies in the development of new catalysts with high activity, high selectivity, and excellent stability.

[0004] Currently, ZSM-5 molecular sieves with MFI topology are widely used due to their suitable pore structure, tunable acidity, and excellent hydrothermal stability. However, their dehydrogenation capacity is relatively weak, making it difficult to efficiently convert alkanes and intermediates into aromatics. To endow them with dehydrogenation function, metal components such as zinc, gallium, and tin are often introduced through methods such as impregnation, precipitation, and physical blending. Among these, zinc has become one of the promising elements for molecular sieve modification due to its excellent dehydrogenation activity and relatively low economic cost. Studies have shown that in alkane aromatization reactions, Lewis acid centers are key active sites for steps such as alkane activation, dehydrogenation, and cyclization. Furthermore, compared to weak or moderately strong acids, Lewis strong acid centers can more efficiently activate alkane CH bonds and stabilize key intermediates, thereby significantly improving aromatization performance. Therefore, constructing Lewis strong acid centers is key to improving the aromatization performance of ZSM-5 molecular sieves.

[0005] However, catalysts prepared by existing methods generally suffer from insufficient Lewis acid strength and uneven distribution of acid centers, which limits the further improvement of aromatization reaction efficiency. Although the acid properties can be modulated by post-treatment methods such as high-temperature steam treatment or acid treatment, these methods often fail to balance the high dispersion of active centers with the stability of catalyst structure. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing ZSM-5 molecular sieves with Lewis strong acid centers and their applications. This preparation method can construct new Lewis strong acid centers by introducing unique active sites, optimize the existence state and distribution of metal species in the catalyst, and enhance the synergistic effect of dehydrogenation and aromatization functions, thereby significantly improving the catalytic performance of the catalyst in alkane aromatization reactions.

[0007] A method for preparing ZSM-5 molecular sieves with Lewis strong acid centers mainly includes the following steps: (1) Add ZSM-5 molecular sieve and surface protectant to anhydrous ethanol, sonicate for 20 min, and stir continuously at room temperature for 6 h. After stirring, centrifuge and wash multiple times, and dry to obtain pretreated ZSM-5 solid powder. (2) The ZSM-5 pretreated in step (1) was mixed with the zinc precursor solution to obtain a suspension, and the suspension was ultrasonically treated for 20 min. (3) Add a slow-release precipitant to the suspension from step (2) and stir continuously for 20 minutes; (4) Transfer the suspension from step (3) to the reactor for hydrothermal reaction; (5) The suspension after hydrothermal reaction in step (4) is centrifuged and washed multiple times, dried to obtain solid catalyst powder, and then calcined by programmed temperature rise to obtain ZSM-5 molecular sieve with Lewis strong acid center.

[0008] In step (1) of the above method, the surface protectant is either polyvinylpyrrolidone or polyethylene glycol; preferably, the surface protectant is polyvinylpyrrolidone.

[0009] In step (1) of the above method, the solid-liquid ratio of ZSM-5, surface protectant and anhydrous ethanol is 1g : (0.1-0.4)g : (60-100)mL; preferably, the solid-liquid ratio of ZSM-5, surface protectant and anhydrous ethanol is 1g : (0.2-0.3)g : (70-80)mL.

[0010] In step (2) of the above method, the solid-liquid ratio of ZSM-5 to zinc precursor solution is 1g : (60-100) mL; preferably, the solid-liquid ratio of ZSM-5 to zinc precursor solution is 1g : (70-80) mL.

[0011] In step (2) of the above method, the zinc ion concentration in the zinc precursor solution is 0.01-0.2 mol / L; preferably, the zinc ion concentration in the zinc precursor solution is 0.02-0.1 mol / L.

[0012] In step (2) of the above method, the zinc precursor is one of zinc nitrate, zinc chloride, zinc acetate or zinc sulfate; preferably, the zinc precursor is one of zinc nitrate or zinc chloride.

[0013] In step (3) of the above method, the slow-release precipitant is one of urea, hexamethylenetetramine or ethylenediamine; preferably, the slow-release precipitant is one of urea or ethylenediamine.

[0014] In step (3) of the above method, the molar ratio of the slow-release precipitant to zinc ions is (3-5):1; preferably, the molar ratio of the slow-release precipitant to zinc ions is (4-5):1.

[0015] In step (4) of the above method, the hydrothermal reaction temperature is 110-150℃ and the hydrothermal reaction time is 6-24h; preferably, the hydrothermal reaction temperature is 120℃ and the hydrothermal reaction time is 12-18h.

[0016] In step (5) of the above method, the specific process of programmed temperature calcination is as follows: the temperature is increased from room temperature to 600℃ at a rate of 2-5℃ / min in an air atmosphere, and then kept at 200℃, 400℃ and 600℃ for 2-3 hours respectively; preferably, the specific process of programmed temperature calcination is as follows: the temperature is increased from room temperature to 600℃ at a rate of 2℃ / min in an air atmosphere, and then kept at 200℃, 400℃ and 600℃ for 2 hours respectively.

[0017] This invention also provides an application of ZSM-5 molecular sieve with Lewis strong acid centers, mainly including the following steps: (a) The prepared catalyst powder is compressed into tablets, crushed and sieved to 20-40 mesh, and 0.4g of the above catalyst particles are loaded into a micro fixed bed reactor; (b) Introduce N2 as a carrier gas into the reactor, raise the reactor temperature to the target reaction temperature, and stabilize for 1 hour; (c) An alkane feedstock is introduced into the reactor to start the reaction, and the composition of the reaction products is analyzed online by gas chromatography.

[0018] In the above application step (b), the N2 flow rate is 10-50 mL / min and the aromatization reaction temperature is 500-600℃; preferably, the N2 flow rate is 20 mL / min and the aromatization reaction temperature is 550℃.

[0019] In the above application step (c), the feedstock is C4-C10 light alkanes, and the mass hourly space velocity (MHV) introduced into the reactor is 2-10 h⁻¹. -1 Preferably, the mass hourly space velocity (MHSV) introduced into the reactor is 3-5 h⁻¹. -1 .

[0020] Compared with the prior art, the present invention has the following advantages: This invention, through a unique preparation method, not only optimizes the existence state and spatial distribution of zinc species on ZSM-5 molecular sieves, but also yields more highly dispersed [ZnOH]. + The active species also construct new Lewis strong acid centers that are difficult to form using traditional methods, fundamentally enhancing the synergistic effect of dehydrogenation and aromatization functions, thus significantly improving aromatization performance and showing broad application prospects. Attached Figure Description

[0021] Figure 1 The NH3-TPD characterization results are for the catalysts prepared in Examples 1-5 and Comparative Examples 1-4.

[0022] Figure 2 The Py-IR characterization results are for the catalyst prepared in Example 3. Detailed Implementation

[0023] This invention provides a method for preparing ZSM-5 molecular sieves with Lewis strong acid centers. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0024] In the embodiments and comparative examples of the present invention, the programmed temperature calcination steps are as follows: the temperature is increased from room temperature to 600°C at a heating rate of 2°C / min in an air atmosphere, and then kept constant at 200°C, 400°C, and 600°C for 2 hours respectively.

[0025] Example 1 Preparation of ZSM-5-HT10 catalyst: 1 g of ZSM-5 molecular sieve and 0.2 g of polyvinylpyrrolidone were added to 75 mL of anhydrous ethanol, sonicated at 100 W for 20 min, and then stirred continuously at room temperature for 6 h. After stirring, the mixture was centrifuged and washed multiple times, and dried at 110 °C for 12 h to obtain pretreated ZSM-5 solid powder. 0.33 g of Zn(NO3)2·6H2O was dissolved in 60 mL of deionized water and stirred at room temperature for 20 min to obtain a homogeneous solution. 0.8 g of pretreated ZSM-5 was added to the above solution and ultrasonically treated at 100 W for 20 min; 0.33 g of urea was added to the above suspension and stirred for another 20 min. The resulting suspension was then transferred to a 100 mL reactor and subjected to hydrothermal reaction at 120 °C for 12 h; after the reactor cooled to room temperature, the suspension was removed and centrifuged and washed multiple times. After drying at 110 °C for 12 h, solid catalyst powder was obtained, which was then calcined by programmed temperature rise to obtain the ZSM-5-HT10 catalyst.

[0026] Example 2 Preparation of ZSM-5-HT20 catalyst: The catalyst was prepared according to the same method and steps as in Example 1, except that the amount of Zn(NO3)2·6H2O was 0.75g and the amount of urea was 0.74g.

[0027] Example 3 Preparation of ZSM-5-HT30 catalyst: The catalyst was prepared according to the same method and steps as in Example 1, except that the amount of Zn(NO3)2·6H2O was 1.28 g and the amount of urea was 1.27 g.

[0028] Example 4 Preparation of ZSM-5-HT40 catalyst: The catalyst was prepared according to the same method and steps as in Example 1, except that the amount of Zn(NO3)2·6H2O was 1.99g and the amount of urea was 1.98g.

[0029] Example 5 Preparation of ZSM-5-HT50 catalyst: The catalyst was prepared according to the same method and steps as in Example 1, except that the amount of Zn(NO3)2·6H2O was 2.98g and the amount of urea was 2.97g.

[0030] Example 6 Preparation of ZSM-5-HT10-CL catalyst: The catalyst was prepared according to the same method and steps as in Example 1, except that the zinc precursor was ZnCl2 and the amount used was 0.15g.

[0031] Example 7 Preparation of ZSM-5-HT10-EA catalyst: The catalyst was prepared according to the same method and steps as in Example 1, except that the slow-release precipitant was ethylenediamine, and the amount used was 1.34 g.

[0032] Example 8 Preparation of ZSM-5-HT10-T24 catalyst: The catalyst was prepared according to the same method and steps as in Example 1, except that the hydrothermal reaction time was 24 h.

[0033] Example 9 Preparation of ZSM-5-HT10-PG catalyst: The catalyst was prepared according to the same method and steps as in Example 1, except that the surface protectant was polyethylene glycol (PEG 200) in an amount of 0.1 g.

[0034] Comparative Example 1 Preparation of ZSM-5-H catalyst: The pretreated ZSM-5 powder described in Example 1 was directly subjected to programmed temperature calcination to obtain ZSM-5-H catalyst.

[0035] Comparative Example 2 Preparation of ZSM-5-IM30 catalyst: 1.28 g of Zn(NO3)2·6H2O was dissolved in 1.2 mL of deionized water to obtain a Zn precursor solution; the above solution was added dropwise to the pretreated ZSM-5 powder described in Example 1 by equal volume impregnation method, and the powder was dried at 110 °C for 12 h to obtain a solid catalyst powder, which was then calcined by programmed temperature rise to obtain the ZSM-5-IM30 catalyst.

[0036] Comparative Example 3 Preparation of ZSM-5-PM30 catalyst: 0.33 g of Zn(NO3)2·6H2O was dissolved in 60 mL of deionized water and stirred at room temperature for 20 min to obtain a homogeneous solution. 0.33 g of urea was added and stirring was continued for 20 min. The resulting suspension was transferred to a 100 mL reactor and subjected to hydrothermal reaction at 120 °C for 12 h. After the reactor cooled to room temperature, the suspension was removed and centrifuged and washed multiple times. After drying at 110 °C for 12 h, a solid catalyst powder was obtained. The powder was then calcined by programmed temperature rise to obtain ZnO. The ZnO and ZSM-5-H were thoroughly mixed at a mass ratio of 3:7 using a physical mixing method and ground for 20 min to obtain the ZSM-5-PM30 catalyst.

[0037] Comparative Example 4 Preparation of ZSM-5-DM30 catalyst: 1.28 g of Zn(NO3)2·6H2O was dissolved in 60 mL of deionized water and stirred at room temperature for 20 min to obtain a homogeneous solution; 0.8 g of the pretreated ZSM-5 powder described in Example 1 was added to the above solution and ultrasonically treated at 100 W for 20 min; ammonia was added dropwise to the above suspension by direct precipitation method under continuous stirring until the pH of the suspension was 9, and after standing for 30 min, it was centrifuged and washed multiple times, dried at 110 °C for 12 h to obtain solid catalyst powder, and then calcined by programmed temperature rise to obtain ZSM-5-DM30 catalyst.

[0038] Catalyst characterization The catalysts prepared in Examples 1-5 and Comparative Examples 1-4 were characterized by NH3-TPD, and the results are as follows: Figure 1As shown. The catalysts obtained by the preparation methods in the embodiments of the present invention (Examples 1-5) all possess weak acid (100-200℃), medium-strong acid (300-400℃), and strong acid (500-600℃) sites. The amount of strong acid first increases and then decreases with increasing Zn content, reaching its maximum at a Zn content of 24 wt% (Example 3). In contrast, pure molecular sieves (Comparative Example 1) and catalysts prepared by impregnation, physical mixing, and direct precipitation methods (Comparative Examples 2-4) all possess only weak and medium-strong acid sites, but no strong acid sites. These results indicate that the preparation method of the present invention can form new strong acid centers in ZSM-5.

[0039] The catalyst prepared in Example 3 was characterized by Py-IR, and the results are as follows: Figure 2 As shown. This catalyst exhibits Brønsted acid (-1545 cm⁻¹) at desorption temperatures of 150, 250, and 350 °C. -1 ) and Lewis acid (-1455cm) -1 The characteristic absorption peaks of the catalyst indicate that the weak acid and medium-strong acid centers are composed of both Brønsted acid and Lewis acid. Furthermore, when the desorption temperature is increased to 550°C, the characteristic absorption peaks of Lewis acid can still be observed, further proving that new Lewis strong acid centers can be formed by the preparation method described in this invention.

[0040] Application examples The catalysts prepared in Examples 1-9 and Comparative Examples 1-4 were used for alkane aromatization reactions. The specific performance evaluation process is as follows: (1) The prepared catalyst powder is compressed into tablets, crushed and sieved to 20-40 mesh, and 0.4g of the above catalyst particles are packed into a micro fixed bed reactor. (2) Use N2 as carrier gas and introduce it into the reactor at a flow rate of 20 mL / min. Set the reactor temperature to 550℃ and stabilize for 1 h. (3) With a mass hourly space velocity of 3h -1 The reaction was initiated by introducing alkane feedstock at a rate of [missing information - likely a specific rate or percentage]. The composition of the reaction products was analyzed online by gas chromatography, with samples taken every 1 hour, and the reaction was continued for 5 hours. The feedstock conversion rates and the yields of trienes (ethylene, propylene, and butene) and tribenzenes (benzene, toluene, and xylene) are shown in Tables 1 and 2.

[0041] Table 1. Hexane aromatization performance of catalysts prepared in Examples 1-9 and Comparative Examples 1-4 catalyst Conversion rate (%) Triene yield (%) Triphenyl yield (%) Example 1 99.9 2.1 70.1 Example 2 99.9 2.5 72.1 Example 3 99.9 3.1 80.1 Example 4 99.9 3.4 69.9 Example 5 99.9 4.4 57.9 Example 6 99.8 2.5 68.4 Example 7 99.9 2.3 69.5 Example 8 99.9 2.1 66.5 Example 9 99.8 2.3 67.9 Comparative Example 1 98.2 34.9 11.7 Comparative Example 2 99.9 3.1 66.4 Comparative Example 3 99.9 1.7 67.4 Comparative Example 4 99.9 1.5 62.2 Table 2. Octane aromatization properties of the catalysts prepared in Examples 1-9 and Comparative Examples 1-4 catalyst Conversion rate (%) Triene yield (%) Triphenyl yield (%) Example 1 99.9 1.8 72.3 Example 2 99.9 1.5 75.4 Example 3 99.9 1.2 83.1 Example 4 99.9 2.5 71.7 Example 5 99.5 3.8 60.8 Example 6 99.9 2.3 70.2 Example 7 99.9 1.9 61.2 Example 8 99.9 1.8 67.6 Example 9 99.8 2.1 69.5 Comparative Example 1 97.1 25.5 10.4 Comparative Example 2 99.9 2.9 68.8 Comparative Example 3 99.9 2.3 69.7 Comparative Example 4 99.9 3.1 64.5

Claims

1. A method for preparing ZSM-5 molecular sieves with Lewis strong acid centers, characterized in that, The preparation method includes the following steps: (1) Add ZSM-5 molecular sieve and surface protectant to anhydrous ethanol, sonicate for 20 min, and stir continuously at room temperature for 6 h. After stirring, centrifuge and wash multiple times, and dry to obtain pretreated ZSM-5 solid powder. (2) The ZSM-5 pretreated in step (1) was mixed with the zinc precursor solution to obtain a suspension, and the suspension was ultrasonically treated for 20 min. (3) Add a slow-release precipitant to the suspension from step (2) and stir continuously for 20 minutes; (4) Transfer the suspension from step (3) to the reactor for hydrothermal reaction; (5) The suspension after hydrothermal reaction in step (4) is centrifuged and washed multiple times, dried to obtain solid catalyst powder, and then calcined by programmed temperature rise to obtain ZSM-5 molecular sieve with Lewis strong acid center.

2. The preparation method according to claim 1, characterized in that, The solid-liquid ratio of ZSM-5, surface protectant and anhydrous ethanol in step (1) is 1g : (0.1-0.4)g : (60-100)mL.

3. The preparation method according to claim 2, characterized in that, The surface protectant mentioned in step (1) is either polyvinylpyrrolidone or polyethylene glycol.

4. The preparation method according to claim 1, characterized in that, In step (2), the solid-liquid ratio of ZSM-5 to the zinc precursor solution is 1 g : (60-100) mL; the zinc ion concentration in the zinc precursor solution is 0.01-0.2 mol / L.

5. The preparation method according to claim 4, characterized in that, The zinc precursor mentioned in step (2) is one of zinc nitrate, zinc chloride, zinc acetate or zinc sulfate.

6. The preparation method according to claim 1, characterized in that, The molar ratio of the slow-release precipitant to zinc ions in step (3) is (3-5):

1.

7. The preparation method according to claim 6, characterized in that, The slow-release precipitant mentioned in step (3) is one of urea, hexamethylenetetramine, or ethylenediamine.

8. The preparation method according to claim 1, characterized in that, The hydrothermal reaction temperature in step (4) is 110-150℃, and the hydrothermal reaction time is 6-24h.

9. The preparation method according to claim 1, characterized in that, The specific process of the programmed temperature rise calcination in step (5) is as follows: the temperature is raised from room temperature to 600℃ at a rate of 2-5℃ / min in an air atmosphere, and then kept constant at 200℃, 400℃ and 600℃ for 2-3 hours respectively.

10. An application of ZSM-5 molecular sieve with Lewis strong acid centers, characterized in that, The ZSM-5 molecular sieve is prepared by the preparation method according to any one of claims 1-9, and its application is as follows: using the ZSM-5 molecular sieve with Lewis strong acid centers for the aromatization reaction of C4-C10 light alkanes, at a reaction temperature of 500-600℃ and a mass hourly space velocity of 2-10 h⁻¹. -1 .

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