Method for synthesizing Ce-loaded ZSM-5 molecular sieve with assistance of graphene oxide

Abstract

The invention discloses a method for graphene oxide assisted synthesis of a Ce-loaded ZSM-5 molecular sieve, and relates to the technical field of molecular sieve preparation. The method comprises the following steps: firstly, introducing a cerium salt precursor into a graphene oxide system to realize loading and stabilization of a Ce element on the surface of graphene oxide; and then adding the compound into a ZSM-5 molecular sieve precursor, and inducing crystallization under a microwave hydrothermal condition to obtain the mesoporous ZSM-5 molecular sieve material with a regular structure. In the obtained material, the graphene oxide effectively improves the crystal morphology and mesoporous structure of the molecular sieve, and promotes the dispersion and fixation of Ce element in the skeleton and surface, thereby significantly improving the catalytic performance and stability of the material. The method disclosed by the invention has the advantages of simplicity and convenience in operation, short synthesis period and strong controllability; and the prepared Ce-loaded mesoporous ZSM-5 molecular sieve has a wide application prospect in the fields of hydrocarbon conversion and the like.

Description

Technical Field

[0001] This invention relates to the field of molecular sieve preparation technology, specifically to a method for synthesizing Ce-supported ZSM-5 molecular sieves with the assistance of graphene oxide. Background Technology

[0002] With the widespread use and disposal of plastic products, the environmental pressure from plastic waste is intensifying. Pyrolysis technology, as an important approach to the resource utilization of waste plastics, can break down high molecular weight hydrocarbons into low molecular weight hydrocarbons, among which olefins have high industrial application value. However, traditional pyrolysis processes suffer from problems such as uneven product distribution, severe coking, and poor selectivity, necessitating the development of efficient and stable catalysts to improve the pyrolysis reaction process.

[0003] ZSM-5 molecular sieves are widely used in plastic pyrolysis catalysis due to their unique pore structure and acidic sites. However, traditional microporous ZSM-5 exhibits drawbacks during catalysis, including limited diffusion, accelerated coking, and insufficient utilization of active sites. Furthermore, single ZSM-5 sieves struggle to achieve both high activity and high product selectivity during pyrolysis.

[0004] The specification CN115990509A discloses a method for preparing cerium-loaded ZSM-5 molecular sieves in Example 2. The method involves first impregnating, drying, and calcining graphene oxide, then adding cerium nitrate solution, and finally introducing the metal element through impregnation. This method is prone to causing the metal to agglomerate on the surface, and the preparation steps are cumbersome and time-consuming. Summary of the Invention

[0005] The purpose of this invention is to provide a method for the synthesis of Ce-supported ZSM-5 molecular sieves assisted by graphene oxide, solving the problems of cumbersome processes, long processing times, and unsatisfactory results in existing methods. This invention combines microwave hydrothermal treatment to achieve rapid synthesis of Ce-supported ZSM-5 molecular sieves and the formation of mesoporous structures.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for synthesizing Ce-supported ZSM-5 molecular sieves with graphene oxide assistance, characterized by comprising the following steps: S1. Graphene oxide is dispersed in deionized water to obtain a uniform dispersion; a cerium salt precursor is added to the dispersion to make Ce... 3+ Ce-GO composites were obtained by loading them onto the surface of graphene oxide. S2. Mix silicon source, aluminum source, template agent and deionized water, and stir for 1-2 hours to obtain precursor a; S3. Add the Ce-GO complex obtained in S1 to the precursor a in S2, stir thoroughly, and then place it in a microwave hydrothermal reactor. React at 160-190°C for 2-4 hours to induce the Ce-GO complex to form product b. S4. After calcining product b in a muffle furnace to remove the template agent and graphene oxide, ion exchange was performed to obtain Ce-supported ZSM-5 molecular sieve with a mesoporous structure.

[0007] A further technical solution is that, in step S1, the amount of graphene oxide added is 20–100 mg / 100 mL.

[0008] A further technical solution is that, in step S1, the cerium salt precursor is cerium nitrate or cerium acetate, and the mass ratio of the cerium salt precursor to graphene oxide is 0.5 to 10:1.

[0009] A further technical solution is that, in step S2, the silicon source is one of liquid silica gel and tetraethyl orthosilicate, the template agent is one of tetrapropylammonium hydroxide and tetrapropylammonium bromide, the molar ratio of silicon source to template agent is 3-4:1, and the amount of aluminum source is determined according to the required silicon-aluminum ratio of ZSM-5 molecular sieve.

[0010] A further technical solution is that, in step S3, after the Ce-GO complex is added to the precursor a in S2, it is ultrasonically stirred at room temperature for 4 to 6 hours.

[0011] A further technical solution is that, in step S3, the microwave hydrothermal power is 1000-2000W.

[0012] A further technical solution is that, in step S4, the calcination temperature is increased to 500-700 °C at a heating rate of 5-20 °C / min, and held for 5-7 hours.

[0013] A further technical solution is that, in step S4, the ion exchange uses a 1 mol / L NH4NO3 or NH4Cl solution as the ion exchanger, the mass ratio of the ion exchanger to product b is 20:1, the ion exchange temperature is 60-70 °C, and the ion exchange time is 5-7 hours.

[0014] A further technical solution is that, in step S4, the Ce-supported ZSM-5 molecular sieve is a typical MFI framework with a pore size in the range of 1–50 nm and a specific surface area of ​​400–600 m². 2 / g, with a particle size of 4–9 μm.

[0015] Reaction mechanism: The surface of graphene oxide contains a large number of oxygen-containing functional groups, which readily form negatively charged sheet structures in water, adsorbing Ce in the initial stage of the reaction.3+ The Ce-GO complex intermediate is formed by the ionization of Ce-GO. This intermediate not only improves the stability of cerium species in the precursor system but also provides a structural template for the subsequent formation of mesoporous structures. Introducing the Ce-GO complex into ZSM-5 molecular sieve precursor a, under microwave hydrothermal irradiation, the efficient penetration and rapid heating of the microwave field significantly enhance the condensation reaction rate between the silicon source and the template agent, enabling rapid nucleation and uniform growth of the MFI framework. Simultaneously, GO sheets are adsorbed onto the crystal surface, forming internal defects at their embedding sites, thereby inducing mesoporous structures and achieving efficient immobilization of Ce species. Furthermore, microwave heating leads to a more uniform temperature distribution within the system, resulting in a more uniform Ce content in the Ce-GO intermediate. 3+ It diffuses more easily to the periphery of the molecular sieve crystal nucleus, coordinating or weakly binding with the surface Si-OH groups, thus improving the uniformity of metal species loading. Ultimately, during the subsequent calcination process, graphene oxide is completely removed, and its original sites are transformed into regular mesopores, while Ce... 3+ Immobilization within the crystals prevents high-temperature migration and aggregation, resulting in Ce-supported ZSM-5 molecular sieves that possess not only a stable mesoporous structure but also highly dispersed metal active centers. This method, through the synergistic effect of GO structure induction and microwave field, achieves simultaneous regulation of mesoporous structure formation, uniform crystal growth, and metal loading control. This results in a final product with excellent specific surface area, hierarchical pore structure, and metal stability, significantly improving catalytic performance and lifespan.

[0016] The Ce-supported ZSM-5 molecular sieve described above is used as a catalyst in the thermal cracking process of polyolefins. During application, LDPE is first adsorbed onto the outer surface of the zeolite, and the L-acid sites on the catalyst dehydrogenate the polymer chain to generate olefin intermediates. Due to the introduction of cerium, Ce... 3+ / Ce 4+ The reversible redox properties of Ce provide electronic regulation during the dehydrogenation process of pyrolysis and cracking segments, promoting β-crack and dehydrogenation reactions of long-chain hydrocarbons, making it easier for macromolecules to break down into short-chain olefins. Simultaneously, the introduction of Ce alters the acidity distribution of ZSM-5, moderately reducing strong acid sites and increasing the ratio of weak acid sites to L-acid sites, thereby reducing excessive cracking and secondary aromatization reactions, inhibiting the formation of aromatics and coke deposits, and further promoting the formation and release of low-carbon olefins.

[0017] Compared with the prior art, the beneficial effects of the present invention are: The microwave hydrothermal synthesis method enables rapid and uniform heating of the system, significantly shortens the crystallization time, promotes uniform crystal nucleus formation, and improves the crystallinity and morphology controllability of the product. Compared with traditional heating methods, the crystals formed under microwave action are more uniform in size and have more controllable defects.

[0018] The surface of graphene oxide is rich in oxygen-containing functional groups, which readily form negatively charged sheet structures in aqueous systems, enabling efficient adsorption of Ce. 3+ This process forms a stable Ce-GO complex intermediate. This intermediate not only significantly improves the stability of Ce species in the precursor system, but also lays the foundation for the uniform dispersion of subsequent metals and the formation of mesoporous structures.

[0019] Introducing the Ce-GO intermediate into ZSM-5 precursor a allows the GO sheets to adsorb and embed into the surface of the forming crystal, creating structural defects within the crystal. After calcination to remove the GO, the GO is transformed into a mesoporous structure, giving the molecular sieve both microporous and mesoporous characteristics, significantly improving its diffusion performance.

[0020] Under microwave irradiation, the internal temperature distribution of the reaction system becomes more uniform, significantly improving the migration ability of Ce species in the Ce-GO intermediate. This makes it easier for Ce species to diffuse around the ZSM-5 crystal nucleus and coordinate or weakly bind with the surface Si-OH groups. This process improves the uniformity of metal loading, avoids local enrichment, and ultimately forms CeO. x The metal is highly dispersed. Since the metal is introduced during the microwave synthesis stage, the repetitive steps of impregnation-drying-calcination in existing technologies are avoided, saving time and making the process more controllable.

[0021] During the subsequent calcination process, graphene oxide was completely removed, and its original sites were transformed into regular mesopores, while Ce... 3+ After being coordinated and fixed on the framework surface, the metal sites are difficult to migrate or aggregate even under high temperatures. The Ce / ZSM-5 constructed in this way has a stable mesoporous structure, highly dispersed metal sites, and excellent thermal stability, making the catalyst outstanding in both catalytic performance and service life.

[0022] The prepared Ce-supported mesoporous ZSM-5 molecular sieve exhibits excellent catalytic performance in the pyrolysis of waste plastics, which can promote the efficient cracking of long-chain polymers into light olefins, improve product selectivity and gas conversion efficiency, and realize the high-value resource utilization of waste plastics. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention.

[0024] Appendix Figure 1 This is a flowchart of the cerium-supported mesoporous ZSM-5 molecular sieve catalyst synthesized with the assistance of graphene oxide in this invention.

[0025] Appendix Figure 2The results of Ce loading of the cerium-supported mesoporous ZSM-5 molecular sieve catalyst synthesized with the assistance of graphene oxide in the embodiments of the present invention are obtained by inductively coupled plasma mass spectrometry (ICP-MS).

[0026] Appendix Figure 3 The X-ray powder diffraction spectrum of the cerium-supported mesoporous ZSM-5 molecular sieve synthesized with the assistance of ZSM-5 molecular sieve and graphene oxide in the embodiments of the present invention is shown.

[0027] Appendix Figure 4 This is a scanning electron microscope image of the cerium-supported mesoporous ZSM-5 molecular sieve synthesized with the assistance of ZSM-5 molecular sieve and graphene oxide in an embodiment of the present invention.

[0028] Appendix Figure 5 The figures show the changing trends of LDPE pyrolysis product distribution and gaseous product composition under different catalyst systems in the embodiments of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0030] Example 1 A method for synthesizing Ce-supported ZSM-5 molecular sieves with the assistance of graphene oxide includes the following steps: 60 mg of graphene oxide was weighed and dispersed in 100 mL of deionized water. After sonication at room temperature for 0.5 hours, a uniform graphene oxide dispersion was obtained. 100 mg of cerium nitrate hexahydrate was weighed and added to the graphene oxide dispersion. After stirring at room temperature for 4 hours, a uniform dispersion was obtained. The Ce-GO complex was obtained by centrifugation. Weigh 0.3 g NaOH, add 8.2 mL tetrapropylamine hydroxide and 20 mL deionized water, and stir to form a homogeneous solution; add 0.14 g sodium aluminate and 23 mL liquid silica gel to the solution, and stir at room temperature for 2 hours to obtain precursor a; The Ce-GO complex was added to precursor a and sonicated at room temperature for 4 hours to obtain mixture b. Mixture b was transferred to a polytetrafluoroethylene digestion vessel and kept at 170 °C for 3 hours under microwave irradiation with a power of 1200 W. The product was a hydrothermal product. The hydrothermal product was filtered, washed, and dried overnight. The hydrothermal product was placed in a muffle furnace and heated to 550 °C at a rate of 10 °C / min, then held at that temperature for 5 hours to remove the template agent and graphene oxide. 1 mol / L NH4NO3 was used as the ion exchanger, with a mass ratio of ion exchanger to hydrothermal product of 20:1. The ion exchange temperature was 70 °C, and the ion exchange was performed for 5 hours. The product was a 0.2 wt% Ce-supported mesoporous molecular sieve, named 0.2 wt% Ce / ZSM-5.

[0031] Example 2 Unlike Example 1, the amount of cerium nitrate hexahydrate added in step 1 above is 150 mg, and the resulting product is a 0.3 wt% Ce-loaded mesoporous molecular sieve, named 0.3 wt% Ce / ZSM-5.

[0032] Example 3 Unlike Example 1, the amount of cerium nitrate hexahydrate added in step 1 above is 50 mg, and the resulting product is a 0.1 wt% Ce-loaded mesoporous molecular sieve, named 0.1 wt% Ce / ZSM-5.

[0033] Comparative Example 1 Compared with Example 1, this comparative example differs from Example 1 in that, in step 4 above, the hydrothermal product is placed in a dynamic stainless steel high-pressure reactor (200 r·min). -1 In the process, after maintaining the temperature at 100 °C for 24 h, the temperature was increased to 170 °C for crystallization for 72 h. The resulting product was a mesoporous molecular sieve supported on 0.2 wt% Ce, named CH-0.2 wt% Ce / ZSM-5 (CH = Conventional Hydrothermal).

[0034] like Figure 3 The XRD patterns shown indicate that Ce / ZSM-5, CH-0.2wt%Ce / ZSM-5, and ZSM-5 exhibit the same characteristic diffraction peaks, confirming that the prepared molecular sieves possess a typical MFI topology. Due to the low and dispersed loading of Ce species, no Ce species-related diffraction peaks were observed. Figure 4 As can be seen, compared with CH-0.2wt%Ce / ZSM-5, the ZSM-5 and Ce / ZSM-5 molecular sieve samples are uniformly prismatic with a particle size of 7-8 μm. After adding the Ce-GO complex, uniformly dispersed scaly protrusions appeared on the surface of the prepared molecular sieve.

[0035] Example 4 A reaction process for the pyrolysis of low-density polyethylene to recover light olefins using cerium-supported mesoporous ZSM-5 molecular sieves synthesized via a graphene oxide-assisted microwave hydrothermal method includes the following steps: Weigh 0.4 g of 0.2wt% Ce / ZSM-5 catalyst, mix it evenly with 2 g of LDPE, and place it in a microwave tube furnace; The protective gas was set to argon, the reaction temperature was set to 400 ℃, and the temperature was maintained for 5 minutes.

[0036] A microwave tube furnace is connected to a gas chromatograph for qualitative and quantitative analysis of product distribution.

[0037] Example 5 Unlike Example 4, the catalyst in step 1 above is 0.3wt% Ce / ZSM-5.

[0038] Example 6 Unlike Example 4, the catalyst in step 1 above is 0.1 wt% Ce / ZSM-5.

[0039] Comparative Example 2 This comparative example is compared with Example 4, except that the catalyst in step 1 above is commercially available ZSM-5.

[0040] Comparative Example 3 This comparative example is compared with Example 4, except that the catalyst in step 1 above is CH-0.2wt%Ce / ZSM-5.

[0041] Comparative Example 4 This comparative example is compared with Example 4, except that no catalyst is added in step 1 above.

[0042] like Figure 5 As shown in Figure (a): Selectivity and conversion, the left side of the figure uses a bar chart to represent the selectivity distribution (%) of three types of pyrolysis products: coke, liquid products, and gaseous products under different catalyst systems. The right side uses a line graph to represent the overall conversion rate of LDPE pyrolysis (gaseous product selectivity % + liquid product selectivity %). In the presence of Ce-supported ZSM-5 molecular sieve catalyst, the selectivity of gaseous products is significantly improved, reaching the highest level in the 0.3 wt% Ce / ZSM-5 molecular sieve sample (gase selectivity 85.8%, product conversion rate 99.8%).

[0043] Figure (b): Composition of gaseous products and selectivity for low-carbon olefins. The bar chart on the left shows the distribution of each component in the gaseous products under different catalytic systems, including: C6+, methane, H2, alkanes (C2-C5), and olefins (C2-C4). The line chart on the right shows the selectivity for olefins (C2-C4). The selectivity for olefins (C2-C4) increases with increasing Ce loading, reaching a maximum of 75.7% under the 0.3wt% Ce / ZSM-5 molecular sieve sample. Meanwhile, at the same Ce loading, the 0.2wt% Ce / ZSM-5 molecular sieve shows higher selectivity for low-carbon olefins (C2-C4) than the CH-0.2wt% Ce / ZSM-5 sample.

[0044] Although the invention has been described herein with reference to several illustrative embodiments, it should be understood that many other modifications and implementations can be devised by those skilled in the art, which will fall within the scope of this disclosure. More specifically, various variations and modifications can be made to the solutions within the scope of this disclosure, the drawings, and the claims. Besides variations and modifications, other uses will be apparent to those skilled in the art.

Claims

1. A method for synthesizing Ce-supported ZSM-5 molecular sieves with graphene oxide assistance, characterized in that, Includes the following steps: S1. Graphene oxide is dispersed in deionized water to obtain a uniform dispersion; a cerium salt precursor is added to the dispersion, and the mixture is stirred or sonicated for 3–4 hours to allow the cerium salt precursor to react with the cerium. 3+ Ce-GO composites were obtained by loading them onto the surface of graphene oxide. S2. Mix silicon source, aluminum source, template agent and deionized water, and stir for 1-2 hours to obtain precursor a; S3. Add the Ce-GO complex obtained in S1 to the precursor a in S2, stir thoroughly, and then place it in a microwave hydrothermal reactor. React at 160-190°C for 2-4 hours to induce the Ce-GO complex to form product b. S4. After calcining product b in a muffle furnace to remove the template agent and graphene oxide, ion exchange was performed to obtain Ce-supported ZSM-5 molecular sieve with a mesoporous structure.

2. The method according to claim 1, characterized in that: In step S1, the amount of graphene oxide added is 20–100 mg / L.

3. The method according to claim 1, characterized in that: In step S1, the cerium salt precursor is cerium nitrate or cerium acetate, and the mass ratio of the cerium salt precursor to graphene oxide is 0.5 to 10:

1.

4. The method according to claim 1, characterized in that: In step S2, the silicon source is one of liquid silica gel and tetraethyl orthosilicate, and the template agent is one of tetrapropylammonium hydroxide and tetrapropylammonium bromide. The molar ratio of the silicon source to the template agent is 3 to 4:

1.

5. The method according to claim 1, characterized in that: In step S3, after the Ce-GO complex is added to precursor a in S2, it is ultrasonically stirred at room temperature for 4 to 6 hours.

6. The method according to claim 1, characterized in that: In step S3, the microwave hydrothermal power is 1000-2000W.

7. The method according to claim 1, characterized in that: In step S4, the calcination temperature is increased to 500-700 °C at a heating rate of 5-20 °C / min, and held for 5-7 hours.

8. The method according to claim 1, characterized in that: In step S4, the ion exchange uses a 1 mol / L NH4NO3 or NH4Cl solution as the ion exchanger, the mass ratio of the ion exchanger to product b is 20:1, the ion exchange temperature is 60-70℃, and the ion exchange time is 5-7 hours.

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