Rare earth-loaded walnut shell activated carbon catalyst and preparation method and application thereof in PE pyrolysis

Abstract

The invention relates to the technical field of catalysts, in particular to a rare-earth-loaded walnut shell activated carbon catalyst, a preparation method thereof and application of the rare-earth-loaded walnut shell activated carbon catalyst in PE pyrolysis. According to the catalyst, walnut shells are used as raw materials, high-specific-surface-area activated carbon is prepared through pretreatment and alkali activation, and then a hierarchical pore structure is constructed through phosphorus anchoring and carbon dioxide secondary activation; sequentially carrying out lanthanum rare earth loading and dopamine prime coating to enhance the interface stability, and introducing a silicon-oxygen-phosphorus bridge and a weak Bronsted acid site through surface phosphorus crosslinking; and finally, loading cerium rare earth and carrying out programmed reduction in a reducing atmosphere to form an active phase rich in oxygen vacancies. The obtained catalyst has high specific surface area and optimized pore distribution, and rare earth elements are synergistically dispersed by La / Ce. When the catalyst is applied to PE pyrolysis, the initial cracking temperature can be remarkably reduced, the C2-C4 light olefin selectivity can be improved, aromatic hydrocarbon byproducts and carbon deposit generation can be inhibited, the cyclic stability is good, low-temperature efficient catalysis is realized, and the economical efficiency and environmental protection property of the pyrolysis process are improved.

Description

Technical Field

[0001] This invention relates to the field of catalyst technology, and in particular to a rare earth-supported walnut shell activated carbon catalyst, its preparation method, and its application in PE pyrolysis. Background Technology

[0002] Polyethylene (PE), as the most produced general-purpose plastic, faces the critical challenge of efficient recycling and resource utilization of its waste for environmental protection and sustainable development. Pyrolysis technology can convert PE into fuels or chemicals, but non-catalytic pyrolysis processes typically require high temperatures (often above 450°C), resulting in high energy consumption, slow reaction rates, and a wide product distribution with low selectivity for light olefins (such as ethylene, propylene, and butene), while producing large amounts of alkanes and aromatics as byproducts, thus reducing product value. In non-catalytic pyrolysis, the random breakage of PE molecular chains easily triggers secondary reactions, such as condensation and aromatization, generating harmful substances like polycyclic aromatic hydrocarbons. This not only affects product purity but can also lead to equipment coking and catalyst deactivation.

[0003] To improve pyrolysis efficiency, researchers have attempted to introduce metal oxide catalysts (such as zeolites, alumina, or transition metal oxides) to lower the reaction temperature and regulate product distribution. However, conventional catalysts still have significant limitations in PE pyrolysis: First, the acidity sites are unevenly distributed, and strong acid centers are prone to excessive cracking or hydrogen transfer reactions, leading to decreased selectivity for light olefins and increased aromatic hydrocarbon byproducts; second, the catalyst has a simple pore structure, mainly micropores, which is not conducive to macromolecular diffusion and product desorption, causing coking and pore blockage, reducing catalyst lifetime; third, metal components are prone to sintering or loss at high temperatures, reducing active sites and resulting in poor cycle stability.

[0004] Furthermore, rare earth elements (such as lanthanum and cerium) are used to modify catalysts to improve olefin selectivity due to their unique electronic structure and redox properties. However, directly loading rare earth elements onto conventional supports (such as activated carbon) often results in low utilization of active sites due to unstable surface chemistry and poor rare earth dispersion. Simultaneously, the interaction between rare earth elements and the support is weak, making them prone to phase transitions or migrations at high pyrolysis temperatures, thus hindering the maintenance of long-term catalytic performance. Moreover, existing catalyst designs often focus on single-function optimization (such as simply adjusting acidity or pore size), lacking synergistic control over multi-level pore structures and surface chemical environments, thus failing to simultaneously achieve low-temperature activation, high olefin selectivity, and low byproduct generation.

[0005] Further analysis reveals that the free radical reaction pathways during PE pyrolysis are complex. If the acidic sites on the catalyst surface do not match the diffusion channels, secondary reactions of intermediates can be exacerbated, such as the repolymerization or aromatization of olefins, leading to increased coking and aromatic hydrocarbon byproducts. Existing catalysts struggle to precisely control the surface site distribution and pore structure at the molecular level, resulting in poor controllability of the reaction pathway and difficulty in balancing low-temperature activity, selectivity, and stability. Therefore, developing a novel catalyst with multi-level pores, stable surface anchoring sites, and synergistic catalytic functions is crucial for solving the current technical problems of PE pyrolysis. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a rare earth-supported walnut shell activated carbon catalyst, its preparation method, and its application in PE pyrolysis, so as to solve the problems of high initial cracking temperature, low selectivity of light olefins, high by-product aromatic hydrocarbons, and easy carbon deposition and deactivation of catalysts in the existing polyethylene pyrolysis process, which lead to low energy efficiency, low product value, and poor process economy.

[0007] To achieve the above objectives, the present invention provides a method for preparing a rare earth-supported walnut shell activated carbon catalyst, comprising the following steps: S1 Walnut shell pretreatment: Wash, dry, crush and sieve the walnut shells to obtain pretreated walnut shell powder; S2 Preparation of walnut shell activated carbon: The pretreated walnut shell powder is impregnated with potassium hydroxide aqueous solution, dried, heated to 780-820℃ at 8-12℃ / min under nitrogen atmosphere and kept at 30-50min, cooled, acid-washed with hydrochloric acid until neutral, and dried to obtain walnut shell activated carbon. S3 Phosphorus Anchoring and Secondary Activation: The walnut shell activated carbon is impregnated with phosphoric acid solution and dried. Then, it is heated to 180-220℃ at 5℃ / min under nitrogen atmosphere and kept at 2-2.5h before cooling. Then, it is heated to 780-820℃ at 8-12℃ / min under carbon dioxide atmosphere and kept at 25-40min before cooling to obtain phosphorus anchored porous activated carbon. S4 First rare earth loading: The phosphorus-anchored porous activated carbon is impregnated in a lanthanum nitrate aqueous solution, dried, and then heated to 380-420℃ at 5℃ / min under a nitrogen atmosphere and held for 50-70min to obtain lanthanum-loaded phosphorus-anchored porous activated carbon. S5 Dopamine Primer: Dopamine hydrochloride was dissolved in a tris(hydroxymethyl)aminomethane buffer solution at pH 8.5, and the lanthanum-loaded phosphorus-anchored porous activated carbon was added. The mixture was stirred and reacted in the presence of air at 25°C for 5-7 hours. The mixture was then washed and dried at 60°C to obtain the dopamine primer porous activated carbon. S6 surface phosphorus crosslinking: 3-aminopropyltriethoxysilane was hydrolyzed in anhydrous ethanol / water system at 25°C for 50-70 min, and then reacted with the dopamine-coated porous activated carbon at 60°C for 1.5-2.5 h. After drying, it was transferred to an aqueous solution containing p-aminobenzenesulfonic acid and reacted at 25°C for 2 h. Then, 20 wt% phosphoric acid was added and the temperature was raised to 80°C and reacted for 50-70 min. After washing, it was dried at 60°C to obtain phosphorus crosslinked porous activated carbon. S7 Second Rare Earth Loading and Process Reduction: The phosphorus cross-linked porous activated carbon was impregnated in a cerium nitrate aqueous solution and dried. Then, under a mixed atmosphere of 5% hydrogen and 95% nitrogen, the temperature was first raised to 240-260℃ at 5℃ / min and held for 50-70 min, then raised to 460-500℃ and held for 50-70 min, and finally cooled to room temperature to obtain a rare earth-loaded walnut shell activated carbon catalyst.

[0008] Preferably, in step S2, the amounts of potassium hydroxide and deionized water are: 250-360g of potassium hydroxide and 500-720g of deionized water per 1000g of pretreated walnut shell powder, soaking for 10-14h, drying for 10-14h, and nitrogen flow rate of 180-220mL / min.

[0009] Preferably, in step S2, 80-120g of hydrochloric acid is added to every 2000g of deionized water during pickling, and the pickling is performed 2-3 times.

[0010] Preferably, in step S3, the amount of phosphoric acid used is 80-120g / 400g deionized water, which is soaked with 300g of walnut shell activated carbon at 25℃ for 3-5h; the flow rates of nitrogen and carbon dioxide are both 180-220mL / min.

[0011] Preferably, in step S4, the amount of lanthanum nitrate hexahydrate used is 24-36g / 300g of deionized water, which is impregnated with 300g of the phosphorus-anchored porous activated carbon for 1.5-2.5h and then dried for 10-14h.

[0012] Preferably, in step S5, the amount of dopamine hydrochloride used is 4-6g / 500g buffer solution, and after the reaction, it is washed three times with deionized water and dried at 60°C for 5-7h.

[0013] Preferably, in step S6, the amount of 3-aminopropyltriethoxysilane used is 25-35g, the mass of anhydrous ethanol and water is 300g and 30g respectively; the amount of p-aminobenzenesulfonic acid used is 8-12g / 200g water, and the amount of 20wt% phosphoric acid solution added is 80-120g.

[0014] Preferably, in step S7, the amount of cerium nitrate hexahydrate used is 24-36g / 300g deionized water, soaking for 1.5-2.5h, and drying for 10-14h; the mixed gas flow rate during programmed reduction is 180-220mL / min.

[0015] A rare earth-supported walnut shell activated carbon catalyst is obtained by the above-mentioned method for preparing rare earth-supported walnut shell activated carbon catalyst.

[0016] Preferably, the specific surface area of ​​the rare earth-supported walnut shell activated carbon catalyst is 1680-2030 m². 2 / g, micropore volume is 0.78-0.90cm³ 3 / g, mesopore volume is 0.52-0.70cm³ 3 / g.

[0017] Preferably, the molar ratio of lanthanum to cerium in the rare earth-supported walnut shell activated carbon catalyst is 0.92-0.96.

[0018] The application of a rare earth-supported walnut shell activated carbon catalyst in polyethylene pyrolysis is used to improve the selectivity of C2-C4 light olefins and reduce aromatic hydrocarbon by-products and coke deposits.

[0019] The beneficial effects of this invention are: This invention achieves highly efficient catalysis of polyethylene pyrolysis through the hierarchical pore structure design and surface functionalization modification of walnut shell activated carbon. Phosphorus anchoring treatment forms stable phosphorus- and oxygen-containing anchoring sites on the activated carbon surface, providing uniformly dispersed binding sites for rare earth elements, enhancing the stability of surface Lewis acid centers, thereby promoting the initial activation and directional breakage of PE molecular chains, effectively reducing the pyrolysis initiation temperature, and suppressing side reactions caused by random pyrolysis.

[0020] The dopamine primer layer constructs an organic-inorganic interface bridging layer on the activated carbon surface. Its catechol and amine functional groups form strong coordination interactions with rare earth elements and the support, improving the integrity and adhesion of the surface functional layer. This layer not only buffers thermal stress at high temperatures but also enhances the mechanical strength and regeneration reversibility of the catalyst, enabling the catalyst to maintain the structural stability of active sites and reduce performance degradation during multiple cycles.

[0021] Surface phosphorus crosslinking treatment forms a silicon-oxygen-phosphorus covalent bridge through silane hydrolysis and condensation reaction with phosphoric acid, and synergistically introduces weak Brønsted acid sites with aromatic sulfonic acids, forming acid-base synergistic microregions with rare earth Lewis acid centers. This finely tuned surface acidic environment can precisely control the free radical reaction pathway, promote the β-crack main reaction, and inhibit the hydrogen transfer and aromatization side reactions of olefins, thereby significantly improving the selectivity of light olefins and reducing the formation of aromatic hydrocarbons.

[0022] Following the second rare-earth loading, treatment under a reducing atmosphere generates an active phase rich in oxygen vacancies from the cerium species. These vacancies efficiently capture and migrate oxygen species, promoting free radical hydrogen extraction reactions and further reducing the cracking energy barrier. Synergistically with the previously introduced lanthanum species, a spatially ordered catalytic microenvironment is constructed, enhancing redox capabilities at low temperatures and ensuring efficient reaction under mild conditions.

[0023] The hierarchical pore structure (synergistic micropore and mesopore structure) precisely regulates the diffusion path of products through alkali activation and secondary carbon dioxide activation, reducing the residence time of intermediates within the pores and thus inhibiting secondary condensation and coking. This porous framework not only improves mass transfer efficiency but also enhances the catalyst's resistance to coking, enabling the reaction process to exhibit both high selectivity and long lifespan.

[0024] This invention constructs a catalytic system that integrates structure and function. The gradual construction of surface sites achieves a balanced match of acidity, redox properties, and diffusion performance, enabling high light olefin yields at lower temperatures in polyethylene pyrolysis, while also producing fewer byproducts and exhibiting good stability, thus improving the economic efficiency and environmental friendliness of the process. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0026] Example 1: Step S1: Walnut shell pretreatment Take 2000g of walnut shells and add 10kg of deionized water. Stir and wash at 25°C for 30min. After filtration, dry in a forced-air dryer at 105°C for 8h. Crush and pass through a 40-mesh sieve to obtain pretreated walnut shell powder. Step S2: Preparation of walnut shell activated carbon Dissolve 250g of potassium hydroxide in 500g of deionized water to form an activation solution. Add 1000g of pretreated walnut shell powder to the activation solution. Let it stand at 25°C for 10 hours to soak. After filtration, dry at 110°C for 10 hours. Place it in a quartz boat tube furnace and purge with nitrogen at 180mL / min. Heat to 780°C at 8°C / min and hold for 30 minutes. Cool to room temperature. Add the solid to 2000g of deionized water and 80g of hydrochloric acid. Stir and soak for 30 minutes. Filter. Repeat the acid washing twice, adding 80g of hydrochloric acid and 2000g of deionized water each time. Then wash in batches with deionized water until the pH of the washing solution is 7. Dry at 110°C for 10 hours to obtain walnut shell activated carbon. Step S3: Preparation of phosphorus-anchored porous activated carbon Take 80g of phosphoric acid and add 400g of deionized water to prepare a phosphoric acid solution. Add 300g of walnut shell activated carbon to the solution, soak at 25°C for 3h, filter, dry at 110°C for 5h, heat to 180°C at 5°C / min under nitrogen at 180mL / min and hold for 2h, cool to room temperature, then introduce carbon dioxide at 180mL / min, heat to 780°C at 8°C / min and hold for 25min, cool to room temperature to obtain phosphorus-anchored porous activated carbon. Step S4: First Rare Earth Loading 24g of lanthanum nitrate hexahydrate was dissolved in 300g of deionized water to form an impregnation solution. 300g of phosphorus-anchored porous activated carbon was dispersed in the impregnation solution and impregnated by shaking for 1.5h. The mixture was then filtered, dried at 110°C for 10h, and placed under a nitrogen atmosphere of 200mL / min. The temperature was increased to 380°C at 5°C / min and held for 50min. The mixture was then cooled to obtain lanthanum-loaded phosphorus-anchored porous activated carbon. Step S5: Dopamine primer Prepare a tris(hydroxymethyl)aminomethane buffer solution with a pH of 8.5. Dissolve 4g of dopamine hydrochloride in 500g of the above buffer solution, then add 300g of lanthanum-loaded phosphorus-anchored porous activated carbon. Stir and react for 5h in the presence of air at 25°C. Filter, wash 3 times with deionized water, and dry at 60°C for 5h to obtain dopamine-coated porous activated carbon. Step S6: Surface phosphorus crosslinking Mix 300g of anhydrous ethanol with 30g of deionized water, add 25g of 3-aminopropyltriethoxysilane, and hydrolyze at 25°C for 50min. Add 300g of dopamine-coated porous activated carbon to the solution, stir at 60°C for 1.5h, filter, dry at 60°C for 5h, and then add to a mixed solution of 200g of deionized water and 8g of p-aminobenzenesulfonic acid. Stir at 25°C for 2h, then add 80g of 20wt% phosphoric acid aqueous solution, heat to 80°C and continue to react for 50min. Filter, wash three times with deionized water, and dry at 60°C for 5h to obtain phosphorus cross-linked porous activated carbon. Step S7: Second Rare Earth Loading Dissolve 24g of cerium nitrate hexahydrate in 300g of deionized water. Add 300g of phosphorus-crosslinked porous activated carbon to the solution and impregnate for 1.5h. Filter the solution and dry at 110°C for 10h. Place the solution in a tube furnace and purge with a mixture of 5% hydrogen and 95% nitrogen at a flow rate of 180mL / min. Heat the solution to 240°C at 5°C / min and hold for 50min. Then heat the solution to 460°C and hold for 50min. Cool the solution to room temperature to obtain a rare earth-supported walnut shell activated carbon catalyst.

[0027] Example 2: Step S1: Walnut shell pretreatment Take 2000g of walnut shells and add 10kg of deionized water. Stir and wash at 25°C for 30min. After filtration, dry in a forced-air dryer at 105°C for 8h. Crush and pass through a 40-mesh sieve to obtain pretreated walnut shell powder. Step S2: Preparation of walnut shell activated carbon Dissolve 300g of potassium hydroxide in 600g of deionized water to form an activation solution. Add 1000g of pretreated walnut shell powder to the activation solution. Let it stand at 25°C for 12 hours to soak. After filtration, dry at 110°C for 12 hours. Place it in a quartz boat tube furnace and purge with nitrogen at 200mL / min. Heat to 800°C at 10°C / min and hold for 40 minutes. Cool to room temperature. Add the solid to 2000g of deionized water and 100g of hydrochloric acid. Stir and soak for 30 minutes. Filter. Repeat the acid washing twice, adding 100g of hydrochloric acid and 2000g of deionized water each time. Then wash in batches with deionized water until the pH of the washing solution is 7. Dry at 110°C for 12 hours to obtain walnut shell activated carbon. Step S3: Preparation of phosphorus-anchored porous activated carbon Take 100g of phosphoric acid and add 400g of deionized water to prepare a phosphoric acid solution. Add 300g of walnut shell activated carbon to the solution, soak at 25°C for 4h, filter, dry at 110°C for 6h, heat to 200°C at 5°C / min under nitrogen at 200mL / min and hold for 2h, cool to room temperature, then introduce carbon dioxide at 200mL / min, heat to 800°C at 10°C / min and hold for 30min, cool to room temperature to obtain phosphorus-anchored porous activated carbon. Step S4: First Rare Earth Loading 30g of lanthanum nitrate hexahydrate was dissolved in 300g of deionized water to form an impregnation solution. 300g of phosphorus-anchored porous activated carbon was dispersed in the impregnation solution and impregnated by shaking for 2h. The mixture was then filtered, dried at 110°C for 12h, and placed under a nitrogen atmosphere of 200mL / min. The temperature was increased to 400°C at 5°C / min and held for 60min. The mixture was then cooled to obtain lanthanum-loaded phosphorus-anchored porous activated carbon. Step S5: Dopamine primer Prepare a tris(hydroxymethyl)aminomethane buffer solution with pH 8.5. Dissolve 5g of dopamine hydrochloride in 500g of the above buffer solution, then add 300g of lanthanum-loaded phosphorus-anchored porous activated carbon. Stir and react for 6h in the presence of air at 25°C. Filter, wash 3 times with deionized water, and dry at 60°C for 6h to obtain dopamine-coated porous activated carbon. Step S6: Surface phosphorus crosslinking Mix 300g of anhydrous ethanol with 30g of deionized water, add 30g of 3-aminopropyltriethoxysilane, and hydrolyze at 25°C for 60min. Add 300g of dopamine-coated porous activated carbon to the solution, stir at 60°C for 2h, filter, dry at 60°C for 6h, and then add to a mixed solution of 200g of deionized water and 10g of p-aminobenzenesulfonic acid. Stir at 25°C for 2h, then add 100g of 20wt% phosphoric acid aqueous solution, heat to 80°C and continue to react for 60min. Filter, wash three times with deionized water, and dry at 60°C for 6h to obtain phosphorus cross-linked porous activated carbon. Step S7: Second Rare Earth Loading Dissolve 30g of cerium nitrate hexahydrate in 300g of deionized water. Add 300g of phosphorus-crosslinked porous activated carbon to the solution and impregnate for 2 hours. Filter the solution and dry at 110°C for 12 hours. Place the solution in a tube furnace and introduce a mixture of 5% hydrogen and 95% nitrogen at a flow rate of 200mL / min. Heat the solution to 250°C at 5°C / min and hold for 60 minutes. Then heat the solution to 480°C and hold for 60 minutes. Cool the solution to room temperature to obtain a rare earth-supported walnut shell activated carbon catalyst.

[0028] Example 3: Step S1: Walnut shell pretreatment Take 2000g of walnut shells and add 10kg of deionized water. Stir and wash at 25°C for 30min. After filtration, dry in a forced-air dryer at 105°C for 8h. Crush and pass through a 40-mesh sieve to obtain pretreated walnut shell powder. Step S2: Preparation of walnut shell activated carbon Dissolve 360g of potassium hydroxide in 720g of deionized water to form an activation solution. Add 1000g of pretreated walnut shell powder to the activation solution. Let it stand at 25°C for 14h for soaking. After filtration, dry at 110°C for 14h. Place it in a quartz boat tube furnace, purge with nitrogen at 220mL / min, heat to 820°C at 12°C / min and hold for 50min. Cool to room temperature, add the solid to 2000g of deionized water, add 120g of hydrochloric acid, stir and soak for 30min, filter, repeat acid washing 3 times, each time adding 120g of hydrochloric acid and 2000g of deionized water. Then wash in batches with deionized water until the pH of the washing solution is 7. Dry at 110°C for 14h to obtain walnut shell activated carbon. Step S3: Preparation of phosphorus-anchored porous activated carbon 120g of phosphoric acid was added to 400g of deionized water to prepare a phosphoric acid solution. 300g of walnut shell activated carbon was added to the solution and soaked at 25°C for 5h. The solution was then filtered and dried at 110°C for 7h. The temperature was increased to 220°C at 5°C / min under nitrogen at 220mL / min and held for 2.5h. The solution was then cooled to room temperature and carbon dioxide was introduced at 220mL / min. The temperature was increased to 820°C at 12°C / min and held for 40min. The solution was then cooled to room temperature to obtain phosphorus-anchored porous activated carbon. Step S4: First Rare Earth Loading 36g of lanthanum nitrate hexahydrate was dissolved in 300g of deionized water to form an impregnation solution. 300g of phosphorus-anchored porous activated carbon was dispersed in the impregnation solution and impregnated by shaking for 2.5h. The mixture was then filtered, dried at 110°C for 14h, and placed under a nitrogen atmosphere of 200mL / min. The temperature was increased to 420°C at 5°C / min and held for 70min. The mixture was then cooled to obtain lanthanum-loaded phosphorus-anchored porous activated carbon. Step S5: Dopamine primer Prepare a tris(hydroxymethyl)aminomethane buffer solution with pH 8.5. Dissolve 6g of dopamine hydrochloride in 500g of the above buffer solution, then add 300g of lanthanum-loaded phosphorus-anchored porous activated carbon. Stir and react for 7h in the presence of air at 25°C. Filter, wash 3 times with deionized water, and dry at 60°C for 7h to obtain dopamine-coated porous activated carbon. Step S6: Surface phosphorus crosslinking Mix 300g of anhydrous ethanol with 30g of deionized water, add 35g of 3-aminopropyltriethoxysilane, and hydrolyze at 25°C for 70min. Add 300g of dopamine-coated porous activated carbon to the solution, stir at 60°C for 2.5h, filter, dry at 60°C for 7h, and then add to a mixed solution of 200g of deionized water and 12g of p-aminobenzenesulfonic acid. Stir at 25°C for 2h, then add 120g of 20wt% phosphoric acid aqueous solution, heat to 80°C and continue to react for 70min. Filter, wash three times with deionized water, and dry at 60°C for 7h to obtain phosphorus cross-linked porous activated carbon. Step S7: Second Rare Earth Loading Dissolve 36g of cerium nitrate hexahydrate in 300g of deionized water. Add 300g of phosphorus-crosslinked porous activated carbon to the solution and impregnate for 2.5h. Filter the solution and dry at 110°C for 14h. Place the solution in a tube furnace and purge with a mixture of 5% hydrogen and 95% nitrogen at a flow rate of 220mL / min. Heat the solution to 260°C at 5°C / min and hold for 70min. Then heat the solution to 500°C and hold for 70min. Cool the solution to room temperature to obtain a rare earth-supported walnut shell activated carbon catalyst.

[0029] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that: in step S3, the impregnation with phosphoric acid added to deionized water was omitted, and the sample was directly impregnated with deionized water at 25°C for 4 hours, followed by drying at 110°C for 6 hours, heating to 200°C at 5°C / min under nitrogen at 200 mL / min and holding for 2 hours, then heating to 800°C at 10°C / min under carbon dioxide at 200 mL / min and holding for 30 minutes, and then proceeding to the subsequent steps after cooling, with the remaining conditions being the same as in Example 2.

[0030] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that the dopamine primer treatment in step S5 is omitted, the reaction of tris(hydroxymethyl)aminomethane buffer and dopamine hydrochloride is omitted, the sample obtained in step S4 directly enters step S6, and the other conditions are the same as in Example 2.

[0031] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the surface phosphorus crosslinking treatment of 3-aminopropyltriethoxysilane hydrolysis and condensation, p-aminobenzenesulfonic acid fixation and subsequent addition of phosphoric acid in step S6 is not performed. The sample obtained in step S5 directly enters step S7, and the remaining conditions are the same as in Example 2.

[0032] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that the second rare earth loading in step S7 and the programmed reduction in a mixed atmosphere of 5% hydrogen and 95% nitrogen by volume are not performed. The sample obtained in step S6 is the comparative example sample, and the other conditions are the same as those in Example 2.

[0033] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that the secondary activation process of introducing carbon dioxide at 200 mL / min and heating to 800°C at 10°C / min and holding for 30 min is omitted in step S3. Only the treatment of heating to 200°C at 5°C / min and holding for 2 h with nitrogen at 200 mL / min is retained. The other conditions are the same as in Example 2.

[0034] Comparative Example 6: The difference between Comparative Example 6 and Example 2 is that the order of introducing rare earth salts in steps S4 and S7 is reversed. Specifically, in step S4, cerium nitrate hexahydrate is used for the first rare earth loading and the temperature is raised to 400°C at 5°C / min and held for 60 min in nitrogen. In step S7, lanthanum nitrate hexahydrate is used for the second rare earth loading and the temperature is raised to 250°C at 5°C / min and held for 60 min under the conditions of 5% hydrogen and 95% nitrogen by volume, and then raised to 480°C and held for 60 min. The other conditions are the same as in Example 2.

[0035] Comparative Example 7: The difference between Comparative Example 7 and Example 2 is that p-aminobenzenesulfonic acid was not added to 200g of deionized water in step S6, while the other operations, such as hydrolyzing 3-aminopropyltriethoxysilane for 60min, stirring at 60°C for 2h, and then adding 20wt% phosphoric acid and reacting at 80°C for 60min, remained unchanged. The other conditions were the same as in Example 2.

[0036] Performance testing: Catalytic application: 0.9 g of catalyst and 17.1 g of polyethylene particles (average particle size 0.8 mm) were mixed in a stainless steel drum for 10 min and packed into a quartz fixed bed with an inner diameter of 30 mm and a packing height of 50 mm. Nitrogen gas was introduced at a rate of 100 mL / min. The temperature was increased to 450 °C at a rate of 10 °C / min and held for 45 min. The gaseous products were collected online and sent to a gas chromatograph (flame hydrogen detector) for quantitative analysis of the selectivity of C2-C4 olefins and the distribution of alkanes and by-product aromatics. After the reaction, the catalyst was regenerated by calcining with carbon dioxide at 750 °C for 30 min. The reaction was repeated 5 times to evaluate stability.

[0037] Specific surface area and pore structure characterization: The samples were degassed under vacuum at 200°C for 10 h and measured under liquid nitrogen at 77 K. BET was taken in the linear region of P / P0 = 0.05-0.30. Micropores were modeled using t-plot and DFT, and mesopores were modeled using the BJH desorption branch. The specific surface area SSA, micropore volume Vmic, and mesopore volume Vmeso were reported. The results are shown in Table 1.

[0038] Thermogravimetric analysis and initial pyrolysis temperature: After mixing polyethylene and catalyst according to the charging ratio, a total of 10 mg was taken for TG-DTG. The temperature was increased from 25°C to 800°C at a nitrogen atmosphere of 100 mL / min and a heating rate of 10°C / min. T5% was defined as the initial pyrolysis temperature. T5%, Tmax, and char residue were reported. The results are shown in Table 1.

[0039] Quantification of C1-C5 hydrocarbons in fixed-bed pyrolysis products: Chromatographic quantification was performed according to GB / T 13610-2020, using FID detection with helium carrier gas, programmed temperature rise: 35°C for 5 min, then increased to 180°C at 10°C / min and held for 5 min. Calibration was performed using a mixed standard gas containing methane, ethane, ethylene, propane, propylene, n-butane, isobutane, 1-butene, trans-2-butene, cis-2-butene, and 1,3-butadiene. The mole fraction was calculated using the normalization method, and the light olefin selectivity S(C2-C4) was defined as n(C2H4+C3H6+∑C4H8 isomers) / ∑n(C1-C5)×100%. The results are shown in Table 1.

[0040] Quantification of aromatic hydrocarbon byproducts: Benzene, toluene, ethylbenzene, xylene, etc. were quantified by FID according to the chromatographic conditions of HJ 583-2010. Combined with the chromatographic procedure for quantifying C1-C5 hydrocarbons in fixed-bed pyrolysis products, the mole fraction was calculated using the external standard method, and the Σ aromatic hydrocarbon content was reported. The results are shown in Table 1.

[0041] Coking amount and regeneration stability: After the reaction, 2 mg of catalyst was sampled and subjected to air atmosphere TG, heated from 25°C to 800°C at a rate of 10°C / min. The mass fraction of coking corresponding to the oxidation weight loss was calculated according to GB / T 33047.1-2016. After treatment under regeneration conditions, the fixed-bed reaction was repeated 5 times. The S (C2-C4), benzene series compounds and coking amount were recorded each time. The selectivity retention rate and coking change of the 5 cycles were reported. The results are shown in Table 1.

[0042] Rare earth loading determination: Inductively coupled plasma mass spectrometry was used to report the mass fractions of La and Ce and the La / Ce molar ratio.

[0043] Table 1 Performance Test Results

[0044] Data Analysis: As can be seen from the data in Examples 1-3 in Table 1, with the gradual optimization of the hierarchical pore structure and the ordered construction of surface sites, the cracking initiation temperature of polyethylene gradually decreases, the main peak temperature advances accordingly, the selectivity of light olefins remains at a stable high level, and the by-products of aromatic hydrocarbons are suppressed, with minimal coking and cycle decay. This may be due to the following reasons: the compatible distribution of phosphate anchoring sites and lanthanum at the micropore openings provides a suitable Lewis acid; the polydopamine primer acts as a bonding layer, improving the integrity of the organic-inorganic interface; the silicon-oxygen-phosphorus bridge formed by 3-aminopropyltriethoxysilane and phosphoric acid enhances the stability of the surface layer and finely adjusts the acidity; and the post-introduced and reduced cerium generates oxygen-containing vacancies on the mesopore wall, promoting free radical hydrogen extraction and β-cleavage. The hierarchical pores shorten the diffusion path and weaken secondary condensation, maintaining the main reaction channel, thus achieving a comprehensive performance of low-temperature cracking initiation, high olefin content, and low aromatization.

[0045] As can be seen from the data in Example 2 and Comparative Example 1 in Table 1, after removing the phosphoric acid anchoring, the fixation and dispersion of rare earth elements on the carbon surface deteriorated, the cracking initiation temperature increased, the selectivity of light olefins decreased, and the amount of aromatic hydrocarbons and coke increased. The main reason is that the lack of phosphorus-containing and oxygen-containing anchor sites leads to instability of the Lewis acid centers at the lanthanum-carbon interface, weakening the synergistic effect between the subsequent organic layer and rare earth elements, and making olefins prone to retraction and alkyl transfer. Therefore, phosphorus anchoring is a prerequisite for the subsequent construction of multilayer sites.

[0046] As can be seen from the data in Table 1 for Example 2 and Comparative Example 2, the lack of a polydopamine primer weakens both the low-temperature cracking and selectivity advantages, while reducing cycle retention. The main reason for this is the disappearance of the catechol / amine dual coordination sites provided by polydopamine, which weakens the adhesion of the subsequent silane condensation layer and aromatic sulfonic acid, making interface defects more susceptible to exposure at high temperatures and inducing secondary reactions. Therefore, a polydopamine primer is crucial for ensuring the durability and reversible regeneration of the surface functional layer.

[0047] As can be seen from the data in Example 2 and Comparative Example 3 in Table 1, when only a polydopamine primer is applied without the silane-phosphoric acid-aromatic sulfonic acid composite layer, the selectivity of light olefins decreases and the selectivity of aromatic hydrocarbons increases. The main reason is the lack of Si-OP bridges and the coupling microregions between the ortho-position weak Brønsted acid and Lewis acid, making it easier for cleavage radicals to undergo condensation and aromatization on the surface. Therefore, the synergy between silane and aromatic sulfonic acid-phosphoric acid is key to suppressing secondary aromatization.

[0048] As can be seen from the data in Example 2 and Comparative Example 4 in Table 1, omitting the second rare earth loading and programmed reduction resulted in unfavorable changes in crack initiation temperature, aromatic hydrocarbons, and coke deposition. The main reason for this is that without subsequent cerium introduction and reduction, stable oxygen-containing vacancies are difficult to form on the surface, limiting oxygen migration and dehydrogenation processes, making it difficult to maintain the high-olefin pathway at lower temperatures. This indicates that post-cerium reduction to generate defect sites is a key step in achieving low-temperature, high-selectivity operation.

[0049] As can be seen from the data in Table 1 for Example 2 and Comparative Example 5, the removal of secondary carbon dioxide activation significantly reduced mesopore size, resulting in limited diffusion and the lowest selectivity for light olefins and the highest coking. This is mainly because the pore size distribution shifted towards micropores, making product desorption difficult and increasing secondary condensation and aromatization. This indicates that hierarchical pore engineering in both in-situ alkali activation and secondary carbon dioxide activation is equally important for kinetics and selectivity.

[0050] As can be seen from the data in Example 2 and Comparative Example 6 in Table 1, changing the order of lanthanum / cerium introduction resulted in a certain decrease in low-temperature crack initiation and selectivity, but not to the point of failure. The main reason is that introducing cerium first and then lanthanum disrupts the spatial sequence of lanthanum-phosphorus anchor sites and cerium reduction to defect sites, reducing the adjacency of oxygen-containing vacancies with weakly acidic microregions and weakening the synergistic effect.

[0051] As can be seen from the data in Table 1 for Example 2 and Comparative Example 7, the selectivity and aging resistance of light olefins deteriorated without the addition of p-aminobenzenesulfonic acid. The main reason for this is the absence of the ortho-position weak Brønsted acid, leading to excessive migration of cleavage radicals and increased hydrogen transfer, resulting in intensified aromatization and coking. Therefore, the juxtaposition of aromatic sulfonic acid and the silicon-phosphorus bridge is the core of low aromaticity.

[0052] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing a rare earth supported walnut shell activated carbon catalyst, characterized in that, The method comprises the following steps: S1 walnut shell pretreatment: washing, drying, crushing and sieving the walnut shell to obtain pretreated walnut shell powder; S2 walnut shell activated carbon preparation: impregnating the pretreated walnut shell powder with potassium hydroxide aqueous solution, drying, and then heating to 780-820℃ at a rate of 8-12℃ / min under nitrogen atmosphere for 30-50min, and then performing hydrochloric acid washing until neutral, and drying to obtain walnut shell activated carbon; S3 phosphorus anchoring and secondary activation: impregnating the walnut shell activated carbon with phosphoric acid solution, drying, and then heating to 180-220℃ at a rate of 5℃ / min under nitrogen atmosphere for 2-2.5h, cooling, and then heating to 780-820℃ at a rate of 8-12℃ / min under carbon dioxide atmosphere for 25-40min, and then cooling to obtain phosphorus-anchored porous activated carbon; S4 first rare earth loading: impregnating the phosphorus-anchored porous activated carbon in lanthanum nitrate aqueous solution, drying, and then heating to 380-420℃ at a rate of 5℃ / min under nitrogen atmosphere for 50-70min to obtain lanthanum-loaded phosphorus-anchored porous activated carbon; S5 dopamine primer coating: dissolving dopamine hydrochloride in a tris-hydroxymethyl aminomethane buffer solution with a pH of 8.5, adding the lanthanum-loaded phosphorus-anchored porous activated carbon, stirring and reacting at 25℃ in the presence of air for 5-7h, washing, and drying at 60℃ to obtain dopamine primer-coated porous activated carbon; S6 surface phosphorus crosslinking: hydrolyzing 3-aminopropyl triethoxysilane in anhydrous ethanol / water system at 25℃ for 50-70min, and then reacting with the dopamine primer-coated porous activated carbon at 60℃ for 1.5-2.5h, drying, and then transferring into an aqueous solution containing p-aminobenzenesulfonic acid, reacting at 25℃ for 2h, adding 20wt% phosphoric acid, and heating to 80℃ for 50-70min, washing, and drying at 60℃ to obtain phosphorus-crosslinked porous activated carbon; S7 second rare earth loading and programmed reduction: impregnating the phosphorus-crosslinked porous activated carbon in cerium nitrate aqueous solution and drying, and then heating to 240-260℃ at a rate of 5℃ / min under a mixed gas atmosphere of 5% hydrogen and 95% nitrogen for 50-70min, and then heating to 460-500℃ for 50-70min, and then cooling to room temperature to obtain a walnut shell activated carbon catalyst loaded with rare earth.

2. The method of claim 1, wherein the rare earth supported walnut shell activated carbon catalyst is prepared by the steps of: (a) mixing walnut shell activated carbon powder with a rare earth solution; (b) drying the mixture; (c) calcining the dried mixture; and (d) washing the calcined mixture with distilled water. In step S3, the amount of phosphoric acid is 80-120g per 400g of deionized water, and the walnut shell activated carbon is impregnated with the phosphoric acid at 25℃ for 3-5h.

3. The method of claim 1, wherein the rare earth supported walnut shell activated carbon catalyst is prepared by the steps of: (a) mixing walnut shell activated carbon powder with a rare earth solution; (b) drying the mixture; (c) calcining the dried mixture; and (d) washing the calcined mixture with distilled water. In step S4, the amount of lanthanum nitrate hexahydrate is 24-36g per 300g of deionized water, and the phosphorus-anchored porous activated carbon is impregnated with the lanthanum nitrate hexahydrate for 1.5-2.5h, and then dried for 10-14h.

4. The method of claim 1, wherein the rare earth supported walnut shell activated carbon catalyst is prepared by the steps of: (a) mixing walnut shell activated carbon powder with a rare earth solution; (b) drying the mixture; (c) calcining the dried mixture; and (d) washing the calcined mixture with distilled water. In step S5, the amount of dopamine hydrochloride is 4-6g per 500g of buffer solution.

5. The method of claim 1, wherein the rare earth supported walnut shell activated carbon catalyst is prepared by the steps of: a) mixing walnut shell activated carbon powder with a rare earth solution; b) drying the mixture; c) calcining the dried mixture; and d) reducing the calcined mixture. In step S6, the amount of 3-aminopropyl triethoxysilane is 25-35g, the mass of anhydrous ethanol and water is 300g and 30g, respectively, the amount of p-aminobenzenesulfonic acid is 8-12g per 200g of water, and the amount of 20wt% phosphoric acid solution added is 80-120g.

6. The method of claim 1, wherein the rare earth supported walnut shell activated carbon catalyst is prepared by the steps of: a) mixing walnut shell activated carbon powder with a rare earth solution; b) drying the mixture; c) calcining the dried mixture; and d) reducing the calcined mixture. The amount of cerium nitrate hexahydrate in the step S7 is 24-36 g / 300 g deionized water, and the impregnation time is 1.5-2.5 h, and the drying time is 10-14 h.

7. A rare earth supported walnut shell activated carbon catalyst, characterized in that, The preparation method of the rare earth-loaded walnut shell activated carbon catalyst is obtained by any one of claims 1-6.

8. The rare earth supported walnut shell activated carbon catalyst of claim 7, wherein, The specific surface area of the said rare earth loaded walnut shell activated carbon catalyst is 1680-2030 m 2 / g, the micropore volume is 0.78-0.90 cm 3 / g, the mesopore volume is 0.52-0.70 cm 3 / g.

9. The rare earth supported walnut shell activated carbon catalyst of claim 7, wherein, The molar ratio of lanthanum to cerium in the rare earth-loaded walnut shell activated carbon catalyst is 0.92-0.

96.

10. Use of the rare earth supported walnut shell activated carbon catalyst according to claim 7, characterized in that, For improving the selectivity of C2-C4 light olefins and reducing the by-product of aromatic hydrocarbon and carbon deposition.

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