Preparation method of catalyst for waste plastic cracking

Abstract

The invention discloses a preparation method of a catalyst for waste plastic cracking, and belongs to the technical field of environmental protection, and the preparation method comprises the following steps: dispersing silicon carbide nanowires and boron nitride nanosheets in a solvent, then adding alumina sol, stirring and drying by distillation to obtain a composite hard phase aggregate; carrying out ball-milling mixing on the composite hard-phase aggregate and ZSM-5 powder, then adding silica sol and water, carrying out spray drying after full mixing, and carrying out surface activation treatment on the obtained microspheres; dissolving cerous nitrate, zirconium nitrate, nickel nitrate and ammonium paramolybdate in water, adding an aluminum dihydrogen phosphate aqueous solution, boric acid and nano silicon dioxide sol, and uniformly stirring to obtain slurry; spraying and coating the slurry on the microspheres, carrying out stepped heat treatment, then treating in a humid atmosphere, and finally crystallizing. The obtained catalyst has high wear resistance and excellent catalytic activity.

Description

Technical Field

[0001] This invention relates to a method for preparing a catalyst for the pyrolysis of waste plastics, belonging to the field of environmental protection technology. Background Technology

[0002] With the increasingly serious problem of plastic pollution, converting waste plastics into high-value oil and gas fuels through catalytic cracking is a key technology for realizing their resource utilization. ZSM-5 molecular sieve, due to its regular micropores and tunable acidity, exhibits excellent catalytic activity and product selectivity in this process, making it an ideal core catalyst material.

[0003] However, industrial-scale applications, especially in continuous, high-efficiency fluidized bed or moving bed reactors, pose significant challenges to the overall performance of catalysts. To withstand intense particle collisions and friction, catalysts need to possess extremely high mechanical strength and wear resistance. Traditional methods involve using large amounts of inert binders (such as kaolin and silica sol) for shaping and reinforcement, but this inevitably covers some active sites and clogs molecular sieve channels, leading to decreased catalytic activity, increased internal diffusion resistance, and poorer product selectivity.

[0004] Meanwhile, to further enhance or modulate catalytic performance, it is often necessary to load active components such as metals onto ZSM-5. Conventional impregnation or ion exchange methods are difficult to achieve a uniform distribution of active components on complex shaped supports, and the interaction between the loaded components and the support is mostly physical adsorption or weak interaction. Under the harsh reaction environment of high temperature and high wear, these components are prone to loss, aggregation, or peeling off from the support surface, leading to rapid catalyst deactivation. Summary of the Invention

[0005] The first objective of this invention is to provide a method for preparing a catalyst for the pyrolysis of waste plastics, wherein the resulting catalyst has both high wear resistance and excellent catalytic activity.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a catalyst for the pyrolysis of waste plastics includes the following steps: S1. Disperse silicon carbide nanowires and boron nitride nanosheets in a solvent, then add aluminum sol, stir and evaporate to dryness to obtain a composite hard phase aggregate; S2. The composite hard phase aggregate is ball-milled and mixed with ZSM-5 powder, then silica sol and water are added. After thorough mixing, the mixture is spray-dried. The resulting microspheres are surface-activated to expose aluminum active defect sites. S3. Dissolve cerium nitrate, zirconium nitrate, nickel nitrate, and ammonium molybdate in water, then add aluminum dihydrogen phosphate aqueous solution, boric acid, and nano silica sol, and stir until uniform to obtain a slurry; S4. Spray the slurry onto the microspheres, first perform a stepped heat treatment under a nitrogen atmosphere, then treat it under a humid atmosphere, and finally perform crystallization treatment.

[0008] Preferably, in step S1, the mass ratio of silicon carbide nanowires, boron nitride nanosheets, and aluminum sol is (3-8):(5-10):1, and the evaporation temperature is 70-90℃.

[0009] Preferably, in step S2, the ratio of the composite hard phase aggregate, ZSM-5 powder, silica sol and water is (8-15)g:(40-80)g:(1-3)g:(80-200)mL.

[0010] Preferably, in step S2, the spray drying conditions are: inlet air temperature 180-220℃, outlet air temperature 90-100℃, and atomizer speed 15000-25000 rpm. The surface activation method is as follows: temperature 500-550℃, nitrogen as carrier gas, water vapor partial pressure 10-15kPa, gas flow rate 1-5L / min, 1-2h.

[0011] Preferably, in step S3, the aluminum dihydrogen phosphate aqueous solution contains 40-60% aluminum dihydrogen phosphate by mass. The mass ratio of cerium nitrate, zirconium nitrate, nickel nitrate, ammonium molybdate, aluminum dihydrogen phosphate aqueous solution, boric acid, and nano silica sol is (2-3):(1.5-2.5):(1-2):(0.5-1):(8-12):(4-7):(2-4); The resulting slurry has a solid content of 20-25 wt%.

[0012] Preferably, in step S4, the spray coating is carried out in a fluidized bed, and the fluidized bed is preheated to 50-60°C; The conditions for spray coating are: air inlet temperature 70-80℃, atomization pressure 0.2-0.5MPa, and slurry feed rate 3-10mL / min.

[0013] Preferably, in step S4, the step heat treatment method is as follows: first, heat to 200-220℃ at a heating rate of 5-10℃ / min and hold for 1-2 hours; then heat to 480-520℃ at a heating rate of 3-5℃ / min and hold for 3-5 hours.

[0014] Preferably, in step S4, the conditions for humid atmosphere treatment are: temperature 480-520℃, water vapor partial pressure 10-15kPa, gas flow rate 1-5L / min, 1-2h.

[0015] Preferably, in step S4, the crystallization treatment conditions are: heating to 600-650℃ at a heating rate of 3-5℃ / min, and holding at that temperature for 3-5 hours.

[0016] The second objective of this invention is to provide an application of a catalyst in the catalytic cracking of waste plastics. Specifically, the catalyst prepared by any of the above methods is used to catalytically crack waste plastics at 300-500℃ and 0.01-3MPa. The mass ratio of the catalyst to the waste plastics is (2-5):100, and the waste plastics are particulate matter with a particle size ≤10mm.

[0017] The beneficial effects of this invention are as follows: 1. This method overcomes the shortcomings of traditional impregnation or coating methods, such as weak bonding between the active component and the support and easy peeling, resulting in a significantly improved wear resistance of the catalyst. Components with significantly different coefficients of thermal expansion are pre-introduced into the microspheres. When the material is heated during heat treatment, the different expansion amplitudes of the components, but mutual constraint, generate internal micro-stress. At the heat treatment temperature, the slurry coating the surface of the microspheres melts, and the micro-stress field inside the microspheres provides a directional driving force for the molten active component, prompting it to penetrate and fill the pores and defects on the surface of the microspheres, achieving a close contact state.

[0018] Based on the tight interface formed by stress induction, by controlling the humid air atmosphere, the active precursor and the active sites (such as pretreated aluminum-rich defects) on the surface of the microspheres undergo hydrolysis-condensation reaction to generate stable Al-OP covalent bonds and achieve interfacial metal-oxygen-silicon / aluminum chemical bonding. This process transforms the physical tight contact into a strong chemical bond, which can effectively inhibit the migration and aggregation of active components at high temperatures, maintain high dispersion, and fundamentally solve the problem of easy peeling of active components, thus reducing process losses.

[0019] 2. During the spray coating and subsequent stress-induced transport processes, the slurry loading mainly occurs on the outer surface and shallow pores of the microspheres. This results in a radial gradient distribution of active components such as Ni, Mo, Ce, and Zr across the cross-section of the microspheres, characterized by "high concentration in the outer layer and low concentration in the inner layer." Meanwhile, the internal ZSM-5 support largely retains its pure silicon-aluminum framework structure. In the final crystallization stage, the amorphous interfaces within the catalyst crystallize and densify, transforming the amorphous interface phase into a well-crystallized, thermally stable phase, ensuring the durability of the interface under long-term high temperatures. Simultaneously, the crystallization process causes cracking or the formation of new mesopores / macropores. These channels connect with the original micropores of ZSM-5, ultimately forming a multi-level pore network penetrating the active layer and the molecular sieve, thus significantly optimizing the diffusion and mass transfer of reactants and products.

[0020] The Ni-Mo component, abundant in the outer layer of the microspheres, intrinsically provides efficient C / C bond breaking (pyrolysis activity). The Ce-Zr component offers redox assistance and structural stability. Therefore, when waste plastic macromolecules come into contact with the catalyst, they are first adsorbed, activated, and initially pyrolyzed into smaller hydrocarbon fragments on the outer layer. The products of this initial pyrolysis diffuse into the internal ZSM-5 region. The strong acidity and selective micropores of ZSM-5 are intrinsic properties, dominating carbocation reactions (isomerization, aromatization, etc.) to generate high-value products. Detailed Implementation

[0021] Example 1: This example provides a method for preparing a catalyst for the pyrolysis of waste plastics, comprising the following steps:

[0022] S1. Disperse 5g of silicon carbide nanowires and 5g of boron nitride nanosheets in 50mL of ethanol by ultrasonication for 10min, then add 1g of aluminum sol (based on the dry weight of alumina), stir and evaporate to dryness at 80℃ to obtain a composite hard phase aggregate.

[0023] S2. 10g of composite hard phase aggregate and 55g of ZSM-5 powder (silicon-aluminum ratio = 100) are ball-milled and mixed for 1 hour. Then, a solution prepared by 100mL of deionized water and 2g of silica sol (based on the dry weight of silica) is added. After thorough mixing, the mixture is spray-dried at an inlet air temperature of 200℃, an outlet air temperature of 100℃, and an atomizer speed of 20000rpm to obtain microspheres.

[0024] The obtained microspheres were placed in a tube furnace and treated for 2 hours at 550°C, atmospheric pressure, and a nitrogen carrier gas atmosphere with a water vapor partial pressure of 10 kPa to selectively etch silicon atoms on the surface of the microspheres and expose aluminum-rich active defect sites.

[0025] S3. Dissolve 2.5g cerium nitrate, 2.0g zirconium nitrate, 1.5g nickel nitrate, and 0.8g ammonium molybdate in 50mL of deionized water. Add 10g of 50% aluminum dihydrogen phosphate aqueous solution and 5g boric acid. Stir and then add 3g of nano silica sol. Adjust the solid content to 25 wt% and stir evenly to obtain a slurry.

[0026] S4. Place the microspheres obtained in step S2 on a fluidized bed, preheat to 60°C, and then spray them using the slurry prepared in step S3. The spray inlet air temperature is 75°C, the atomization pressure is 0.3MPa, and the slurry feed rate is 5mL / min, so that a uniform and wet film is formed on the surface of the microspheres without sticking together.

[0027] Then, under a nitrogen atmosphere, the temperature was increased to 220℃ at 10℃ / min and held for 1 hour; then increased to 480℃ at 5℃ / min and held for 3 hours.

[0028] Maintain 480℃, switch the gas to humid air with a water vapor partial pressure of 15 kPa, with a total gas flow rate of 1 L / min, and process for 1.5 h.

[0029] Finally, switch the gas to dry air, heat it to 650℃ at a rate of 3℃ / min, and hold it at that temperature for 4 hours. Then let it cool naturally to below 200℃ before removing it.

[0030] Example 2: This example provides a method for preparing a catalyst for the pyrolysis of waste plastics, comprising the following steps:

[0031] S1. Disperse 5g of silicon carbide nanowires and 5g of boron nitride nanosheets in 50mL of ethanol by ultrasonication for 10min, then add 1g of aluminum sol, stir and evaporate to dryness at 80℃ to obtain a composite hard phase aggregate.

[0032] S2. 10g of composite hard phase aggregate and 55g of ZSM-5 powder (silicon-aluminum ratio = 100) are ball-milled and mixed for 1 hour. Then, a solution prepared by 100mL of deionized water and 2g of silica sol is added. After thorough mixing, the mixture is spray-dried at an inlet air temperature of 200℃, an outlet air temperature of 100℃, and an atomizer speed of 20000rpm to obtain microspheres.

[0033] The obtained microspheres were placed in a tube furnace and treated for 2 hours at 550°C, atmospheric pressure, and a nitrogen carrier gas atmosphere with a water vapor partial pressure of 10 kPa to selectively etch silicon atoms on the surface of the microspheres and expose aluminum-rich active defect sites.

[0034] S3. Dissolve 2.5g cerium nitrate, 2.0g zirconium nitrate, 1.5g nickel nitrate, and 0.8g ammonium molybdate in 50mL of deionized water. Add 10g of 50% aluminum dihydrogen phosphate aqueous solution and 5g boric acid. Stir and then add 3g of nano silica sol. Adjust the solid content to 25 wt% and stir evenly to obtain a slurry.

[0035] S4. Place the microspheres obtained in step S2 on a fluidized bed, preheat to 60°C, and then spray them using the slurry prepared in step S3. The spray inlet air temperature is 75°C, the atomization pressure is 0.3MPa, and the slurry feed rate is 5mL / min, so that a uniform and wet film is formed on the surface of the microspheres without sticking together.

[0036] Then, under a nitrogen atmosphere, the temperature was increased to 220℃ at 10℃ / min and held for 1 hour; then increased to 500℃ at 5℃ / min and held for 2.5 hours.

[0037] Maintain at 500℃, switch the gas to humid air with a water vapor partial pressure of 15 kPa, with a total gas flow rate of 1 L / min, and process for 1.2 h.

[0038] Finally, switch the gas to dry air, heat it to 650℃ at a rate of 3℃ / min, and hold it at that temperature for 4 hours. Then let it cool naturally to below 200℃ before removing it.

[0039] Example 3: This example provides a method for preparing a catalyst for the pyrolysis of waste plastics, comprising the following steps:

[0040] S1. Disperse 5g of silicon carbide nanowires and 5g of boron nitride nanosheets in 50mL of ethanol by ultrasonication for 10min, then add 1g of aluminum sol, stir and evaporate to dryness at 80℃ to obtain a composite hard phase aggregate.

[0041] S2. 10g of composite hard phase aggregate and 55g of ZSM-5 powder (silicon-aluminum ratio = 100) are ball-milled and mixed for 1 hour. Then, a solution prepared by 100mL of deionized water and 2g of silica sol is added. After thorough mixing, the mixture is spray-dried at an inlet air temperature of 200℃, an outlet air temperature of 100℃, and an atomizer speed of 20000rpm to obtain microspheres.

[0042] The obtained microspheres were placed in a tube furnace and treated for 2 hours at 550°C, atmospheric pressure, and a nitrogen carrier gas atmosphere with a water vapor partial pressure of 10 kPa to selectively etch silicon atoms on the surface of the microspheres and expose aluminum-rich active defect sites.

[0043] S3. Dissolve 2.5g cerium nitrate, 2.0g zirconium nitrate, 1.5g nickel nitrate, and 0.8g ammonium molybdate in 50mL of deionized water. Add 10g of 50% aluminum dihydrogen phosphate aqueous solution and 5g boric acid. Stir and then add 3g of nano silica sol. Adjust the solid content to 25 wt% and stir evenly to obtain a slurry.

[0044] S4. Place the microspheres obtained in step S2 on a fluidized bed, preheat to 60°C, and then spray them using the slurry prepared in step S3. The spray inlet air temperature is 75°C, the atomization pressure is 0.3MPa, and the slurry feed rate is 5mL / min, so that a uniform and wet film is formed on the surface of the microspheres without sticking together.

[0045] Then, under a nitrogen atmosphere, the temperature was increased to 220℃ at 10℃ / min and held for 1 hour; then increased to 520℃ at 5℃ / min and held for 2 hours.

[0046] Maintain 520℃, switch the gas to humid air with a water vapor partial pressure of 15 kPa, with a total gas flow rate of 1 L / min, and process for 1 h.

[0047] Finally, switch the gas to dry air, heat it to 650℃ at a rate of 3℃ / min, and hold it at that temperature for 4 hours. Then let it cool naturally to below 200℃ before removing it.

[0048] Comparative Example 1: Basically the same as Example 1, except that the humid atmosphere treatment in step S4 was not performed.

[0049] Comparative Example 2: It is basically the same as Example 1, except that the slurry is directly sprayed onto the ZSM-5 powder.

[0050] The performance of the catalyst in the catalytic cracking of waste plastics was evaluated in a fixed-bed reactor (after steam aging treatment). The catalyst loading was 2g, the polyethylene waste plastic particles (3mm) loading was 100g, nitrogen was used as the carrier gas (1L / min), the reaction temperature was 350℃, the reaction pressure was 0.5MPa, and the reaction time was 3h. The obtained products were cooled, separated into gas and liquid components, and then quantitatively analyzed. The results are shown in Table 1.

[0051] Table 1

[0052] As shown in Table 1, due to the lack of heat treatment in a humid air atmosphere, the catalyst in Comparative Example 1 could not form stable Al-OP covalent bonds and interfacial metal-oxygen-silicon / aluminum chemical bonds. The migration and aggregation of active components at high temperatures hindered mass transfer, resulting in reduced accessibility of the initial pyrolysis products to the internal ZSM-5 region, leading to lighter products that were prone to coking. If the slurry was directly sprayed onto the ZSM-5 powder (Comparative Example 2), the "stress-induced transport" process could not be formed, and the active components would accumulate on the surface of the microspheres, easily leading to excessive pyrolysis and carbon deposition.

[0053] Meanwhile, the wear index of the obtained catalyst was tested according to the standard method of NB / SH / T 0964, and the results are shown in Table 2.

[0054] Table 2

[0055] As can be seen from Table 2, without the humid atmosphere treatment in step S4 and by directly spraying the slurry onto the ZSM-5 powder, the resulting catalyst has a higher wear index due to the lack of chemical anchoring and the absence of a "stress-induced transport" matrix.

Claims

1. A method for preparing a catalyst for the pyrolysis of waste plastics, characterized in that, Includes the following steps: S1. Disperse silicon carbide nanowires and boron nitride nanosheets in a solvent, then add aluminum sol, stir and evaporate to dryness to obtain a composite hard phase aggregate; S2. The composite hard phase aggregate is ball-milled and mixed with ZSM-5 powder, then silica sol and water are added. After thorough mixing, the mixture is spray-dried to activate the surface of the resulting microspheres. S3. Dissolve cerium nitrate, zirconium nitrate, nickel nitrate, and ammonium molybdate in water, then add aluminum dihydrogen phosphate aqueous solution, boric acid, and nano silica sol, and stir until uniform to obtain a slurry; S4. Spray the slurry onto the microspheres, first perform a step-by-step heat treatment under a nitrogen atmosphere, then treat it under a humid atmosphere, and finally perform crystallization treatment.

2. The method for preparing the catalyst for waste plastic pyrolysis according to claim 1, characterized in that, In step S1, the mass ratio of silicon carbide nanowires, boron nitride nanosheets, and aluminum sol is (3-8):(5-10):1, and the evaporation temperature is 70-90℃.

3. The method for preparing the catalyst for waste plastic pyrolysis according to claim 1, characterized in that, In step S2, the ratio of the composite hard phase aggregate, ZSM-5 powder, silica sol and water is (8-15)g:(40-80)g:(1-3)g:(80-200)mL.

4. The method for preparing the catalyst for waste plastic pyrolysis according to claim 1, characterized in that, In step S2, the spray drying conditions are: inlet air temperature 180-220℃, outlet air temperature 90-100℃, and atomizer speed 15000-25000rpm. The surface activation method is as follows: temperature 500-550℃, nitrogen as carrier gas, water vapor partial pressure 10-15kPa, gas flow rate 1-5L / min, 1-2h.

5. The method for preparing the catalyst for waste plastic pyrolysis according to claim 1, characterized in that, In step S3, the aluminum dihydrogen phosphate aqueous solution contains 40-60% aluminum dihydrogen phosphate by mass. The mass ratio of cerium nitrate, zirconium nitrate, nickel nitrate, ammonium molybdate, aluminum dihydrogen phosphate aqueous solution, boric acid, and nano silica sol is (2-3):(1.5-2.5):(1-2):(0.5-1):(8-12):(4-7):(2-4); The resulting slurry has a solid content of 20-25 wt%.

6. The method for preparing the catalyst for waste plastic pyrolysis according to claim 1, characterized in that, In step S4, the spray coating is carried out in a fluidized bed, which is preheated to 50-60°C; The conditions for spray coating are: air inlet temperature 70-80℃, atomization pressure 0.2-0.5MPa, and slurry feed rate 3-10mL / min.

7. The method for preparing the catalyst for waste plastic pyrolysis according to claim 1, characterized in that, In step S4, the step heat treatment method is as follows: first, heat to 200-220℃ at a heating rate of 5-10℃ / min and hold for 1-2 hours; then heat to 480-520℃ at a heating rate of 3-5℃ / min and hold for 3-5 hours.

8. The method for preparing the catalyst for waste plastic pyrolysis according to claim 1, characterized in that, In step S4, the conditions for humid atmosphere treatment are: temperature 480-520℃, water vapor partial pressure 10-15kPa, gas flow rate 1-5L / min, 1-2h.

9. The method for preparing the catalyst for waste plastic pyrolysis according to claim 1, characterized in that, In step S4, the crystallization treatment conditions are: heating to 600-650℃ at a heating rate of 3-5℃ / min, and holding at that temperature for 3-5 hours.

10. The application of the catalyst prepared by the method according to any one of claims 1-9 in the catalytic cracking of waste plastics, characterized in that, Catalytic cracking of waste plastics at 300-500℃ and 0.01-3MPa.

Images