Catalyst for hydrogenation dearomatization of waste lubricating oil and preparation method thereof

Abstract

The invention discloses a catalyst for hydrogenation dearomatization of waste lubricating oil and a preparation method of the catalyst. The catalyst comprises a carrier and an active component loaded on the carrier, the carrier is a hierarchical pore channel with a (3-aminopropyl) triethoxysilane functional group grafted on the surface, and the carrier has a double pore channel structure with mesopores with the pore diameter of 8-12 nm and macropores with the pore diameter of 50-100 nm; the active components comprise Pt or Pd single atoms with the loading capacity of 0.1-0.5 wt% and Ni-Mo-P alloy nanoparticles, the molar ratio of P to (Ni and Mo) is (0.1-0.4): 1, the molar ratio of Ni to Mo is (2-4): 1, and the size of the Ni-Mo-P alloy nanoparticles is 2-5 nm. The preparation method comprises the steps of carrier functionalization pretreatment, monatomic loading through an atomic layer deposition technology, ternary complex precursor preparation, impregnation loading, step-by-step roasting, hydrogen reduction activation and the like. The catalyst has excellent hydrogenation dearomatization performance, and provides a new technical approach for efficient purification treatment of waste lubricating oil.

Description

Technical Field

[0001] The invention belongs to the technical field of catalysts, and in particular relates to a catalyst for hydrogenation and dearomatization of waste lubricating oil and a preparation method thereof. Background Art

[0002] With industrial development and the growing number of vehicles, the amount of waste lubricating oil generated has increased annually. Waste lubricating oil contains a large amount of aromatic compounds, which not only affect lubricant performance but also pollute the environment. Therefore, the development of efficient waste lubricating oil hydrodearomatization technology is of great significance for resource recycling and environmental protection.

[0003] Conventional hydrodearomatization catalysts typically utilize a Ni-Mo or Co-Mo bimetallic system supported on an alumina support. However, these traditional catalysts have numerous drawbacks: first, the metal active components are poorly dispersed on the support surface, easily forming large particles and resulting in low active site utilization; second, the support pore structure is simple, resulting in high mass transfer resistance and hindering the diffusion of reactants and products; third, the catalysts exhibit poor selectivity, making it prone to over-hydrogenation during aromatics removal, impacting product quality; and finally, the catalysts exhibit poor stability and are prone to deactivation during the reaction.

[0004] The catalyst preparation methods in the prior art mainly use the equal volume impregnation method or the co-precipitation method. These methods are difficult to accurately control the dispersion state and particle size of the active components. The carrier is usually commercial , its pore structure is relatively simple and cannot well meet the mass transfer requirements of macromolecular reactants. In addition, the metal components in the catalysts prepared by traditional methods are prone to agglomeration, forming large metal particles, which reduces the catalytic activity.

[0005] In recent years, single-atom catalysts have garnered widespread attention due to their unique electronic structure and maximized atomic utilization. Metal atoms in single-atom catalysts exist in a monodispersed state, providing unique catalytic active sites. Furthermore, alloy nanoparticles exhibit excellent catalytic performance due to their synergistic effects. However, effectively combining single atoms and alloy nanoparticles to construct a synergistic bifunctional catalytic system remains a technical challenge. Summary of the Invention

[0006] The purpose of the present invention is to provide a catalyst for waste lubricating oil hydrodearomatization and a preparation method thereof. The catalyst adopts a single atom-alloy nanoparticle dual-functional active component system, combined with a hierarchical pore carrier and surface functionalization technology, to achieve high activity, high selectivity and high stability of waste lubricating oil hydrodearomatization reaction, while having good anti-deactivation performance and long service life.

[0007] To achieve the above object, the technical solution adopted by the present invention is: The catalyst for hydrodearomatization of waste lubricating oil according to the present invention comprises a carrier and an active component supported on the carrier, wherein: The carrier is a hierarchical channel with 3-aminopropyltriethoxysilane functional groups grafted on the surface. , with a dual pore structure of mesopores with a pore size of 8-12nm and macropores with a pore size of 50-100nm; The active components include Pt or Pd single atoms and Ni-Mo-P alloy nanoparticles, wherein the loading amount of Pt or Pd single atoms is 0.1-0.5wt%, the loading amount of Ni-Mo-P alloy nanoparticles is 8-18wt%, the total molar ratio of P to Ni plus Mo is 0.1-0.4, the molar ratio of Ni to Mo is 2-4, and the size of the Ni-Mo-P alloy nanoparticles is 2-5nm.

[0008] in: The carrier's pore volume distribution is composed of 60-70% mesopores and 30-40% macropores, resulting in a specific surface area of ​​180-250 m² / g. The hierarchical pore design significantly improves mass transfer performance. The mesopores provide a large specific surface area and abundant active sites, while the macropores facilitate the rapid transport of macromolecular reactants. The synergistic effect of these two enhances catalytic efficiency.

[0009] Specifically, the total loading of Ni and Mo in the catalyst is 8-18wt%, of which the loading of Ni is 3-8wt% and the loading of Mo is 5-10wt%. By optimizing the metal loading, sufficient active sites are ensured while avoiding metal agglomeration caused by excessive loading.

[0010] Specifically, the Pt or Pd single atoms are anchored to the support surface via functional groups, resulting in Ni-Mo-P alloy nanoparticles with a size of 2-5 nm. The amino group in the 3-aminopropyltriethoxysilane functional group forms a coordination bond with the noble metal precursor, effectively anchoring the single atoms and preventing their aggregation. The introduction of P modulates the electronic structure of the Ni-Mo alloy, improving its hydrogenation activity and selectivity.

[0011] The present invention also provides a method for preparing the above catalyst, comprising the following steps: Carrier pretreatment, The support was functionalized with (3-aminopropyl)triethoxysilane at 80 °C for 4-6 h; Single atom loading, using atomic layer deposition technology to load Pt or Pd single atom precursors onto functionalized supports; phosphorus modified metal precursor preparation, and After complexation with citric acid, ammonium dihydrogen phosphate is added to prepare a P-Ni-Mo ternary complex precursor solution; impregnation loading, the precursor solution prepared in the step is brought into contact with the carrier for impregnation; stepwise calcination, the impregnated sample is dried, pre-calcined at 300°C for 2h, and mainly calcined at 500°C for 4h; reduction treatment, reduction at 400°C for 3h in a hydrogen atmosphere, to obtain a catalyst.

[0012] Specifically, the atomic layer deposition technology includes the steps of carrier pretreatment, precursor adsorption, first purge, reduction reaction, second purge and cycle repetition. The carrier is placed in a hot wall ALD reactor and heated to the reaction temperature under nitrogen protection; or The precursor vapor undergoes a chemical adsorption reaction with a pulse time of 0.5-2.0 seconds; unreacted precursors and by-products are removed by purging with an inert gas for 10-30 seconds; hydrogen is introduced for a reduction reaction to form metal atoms with a reduction pulse time of 1-5 seconds; the inert gas is purged again; and the above steps are repeated until the target loading amount is reached, usually 10-50 cycles.

[0013] Specifically, the atomic layer deposition process parameters are: reactor pressure of 1-10 Torr, carrier preheat temperature of 200-250°C, Pt precursor evaporation temperature of 80-100°C, and Pd precursor evaporation temperature of 120-150°C. The ALD reactor is a hot-wall reactor that provides uniform temperature distribution, ensuring uniform loading of single atoms.

[0014] Specifically, the step-by-step calcination process has a heating rate of 2-5°C / min and an air atmosphere. Slow heating is beneficial to the gradual decomposition of the precursor and the formation of metal oxides, avoiding structural damage caused by rapid heating.

[0015] The beneficial effects of the present invention are as follows: 1. The single-atom-alloy nanoparticle bifunctional system achieves a synergistic catalytic effect. Pt or Pd single atoms provide active sites for hydrogen dissociation, while Ni-Mo-P alloy nanoparticles provide active sites for hydrogenation. The synergistic effect of these two significantly enhances the activity and selectivity of hydrodearomatization.

[0016] 2. The hierarchical pore carrier design optimizes mass transfer performance. The mesopores provide a large specific surface area and abundant active sites, while the macropores promote rapid transport of reactants. The dual pore structure resolves the contradiction between activity and mass transfer, improving catalytic efficiency.

[0017] 3. Surface functionalization technology enables precise anchoring of single atoms. The (3-aminopropyl)triethoxysilane functional group strongly anchors the noble metal atoms through chemical bonding, preventing their aggregation and inactivation during the reaction and significantly improving catalyst stability.

[0018] 4. Atomic layer deposition technology ensures uniform dispersion of single atoms. By precisely controlling the number of deposition cycles and process parameters, atomic-level dispersion of precious metals on the carrier surface is achieved, maximizing metal atom utilization.

[0019] 5. Phosphorus modification improves alloy performance. The introduction of phosphorus adjusts the electronic structure of the Ni-Mo alloy, enhances the metal-support interaction, improves the dispersion and stability of the alloy nanoparticles, and optimizes the hydrogenation selectivity.

[0020] 6. The step-by-step calcination process ensures the optimal catalyst structure. Through precise control of temperature and time, the orderly decomposition of the precursor and the uniform distribution of the active components are achieved, avoiding excessive agglomeration of the metal components. DETAILED DESCRIPTION

[0021] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to these embodiments. Those skilled in the art should recognize that the present invention encompasses all possible alternatives, improvements, and equivalents within the scope of the claims.

[0022] The hierarchical pore structure was determined by nitrogen adsorption-desorption (BJH model) and mercury intrusion porosimetry: the volume of mesopores (2-50 nm) accounted for 65.3±2.0%, the volume of macropores (50-100 nm) accounted for 34.7±2.0%, and the adsorption-desorption isotherm showed the superposition characteristics of H2-type hysteresis loop (mesopores) and H3-type hysteresis loop (macroporos), confirming the coexistence of dual pores.

[0023] The structure of the Ni-Mo-P alloy nanoparticles described below was verified by X-ray diffraction (XRD): a diffraction peak of the Ni-Mo-P alloy (111) crystal plane (PDF#00-054-0413) appeared at 2θ=44.5°±0.5°, with a half-peak width of 0.8° corresponding to a grain size of 3.2 nm. Or MoP (2θ = 43°) impurity phase peak, confirming that P exists in the alloy in the form of solid solution.

[0024] described The original support parameters were: surface area 200±20 m² / g, pore volume 0.8±0.1 cm³ / g, and average pore diameter 12 nm. After modification with 3-aminopropyltriethoxysilane, X-ray photoelectron spectroscopy revealed a nitrogen 1s binding energy of 399.8 eV, corresponding to an amino grafting density of 1.15±0.05 mmol / g, confirming successful functional group grafting.

[0025] (3-Aminopropyl)triethoxysilane: purity ≥98%.

[0026] The remaining raw materials were analytical grade reagents and were commercially available without special requirements.

[0027] Embodiment 1:

[0028] A method for preparing a catalyst for hydrodearomatization of waste lubricating oil comprises the following steps:

[0029] (1) Carrier pretreatment: 5g Disperse in 50 mL of anhydrous toluene, add 0.25 g of 3-aminopropyltriethoxysilane, and reflux at 80°C for 5 h. Filter and wash with toluene and ethanol until no silicon is detected in the filtrate, and dry at 120°C for 4 h.

[0030] (2) Single atom loading: Atomic layer deposition technology is used to load Pt single atoms: the functionalized carrier is placed in a hot wall ALD reactor and heated to 225 ° C under nitrogen protection. The precursor (evaporation temperature 90 ° C), pulse time 1.5 seconds, nitrogen purge 20 seconds, hydrogen reduction 3 seconds, nitrogen purge again 20 seconds, repeated 30 cycles, to obtain a Pt single atom loading of 0.3wt% of the carrier.

[0031] (3) Preparation of phosphorus-modified metal precursor: 、 Dissolve citric acid (3.84 g) in 100 mL of deionized water and stir at 80 °C until clear (a green complex is formed with a characteristic absorption peak at 620 nm on UV-Vis). Stirring was continued for 2 h to obtain a transparent precursor solution.

[0032] (4) Impregnation loading: The precursor solution obtained in step (3) was contacted with the Pt single atom support obtained in step (2) by an equal volume impregnation method for 4 h, and then dried at 80°C for 12 h.

[0033] (5) Step-by-step calcination: pre-calcination at 300°C at a heating rate of 3°C / min for 2 h, and then main calcination at 500°C at a heating rate of 3°C / min for 4 h in air atmosphere.

[0034] (5) Reduction treatment: In a hydrogen atmosphere (flow rate 50 mL / min), the temperature was raised to 400 °C at 5 °C / min for 3 h, and the catalyst was obtained after cooling.

[0035] Example 2:

[0036] The preparation method of a catalyst for hydrodearomatization of waste lubricating oil differs from that of Example 1 only in that: (1) Carrier pretreatment: 5g Disperse in 50 mL of anhydrous toluene, add 0.15 g of 3-aminopropyltriethoxysilane, and reflux at 80°C for 4 h. Filter and wash with toluene and ethanol in sequence until no silicon is detected in the filtrate, and dry at 120°C for 4 h.

[0037] (2) Single atom loading: Atomic layer deposition technology is used to load Pt single atoms: the functionalized carrier is placed in a hot wall ALD reactor and heated to 200 ° C under nitrogen protection. The precursor (evaporation temperature 135 ° C), pulse time 1.0 s, nitrogen purge 15 s, hydrogen reduction 2 s, 25 cycles, to obtain a Pd single atom loading of 0.2 wt% of the carrier.

[0038] (3) Preparation of phosphorus-modified metal precursor: 、 Dissolve citric acid (3.84 g) in 100 mL of deionized water and stir at 80 °C until clear (a green complex is formed with a characteristic absorption peak at 620 nm on UV-Vis). Stirring was continued for 2 h to obtain a transparent precursor solution.

[0039] Example 3: The preparation method of a catalyst for hydrodearomatization of waste lubricating oil differs from that of Example 1 only in that: (1) Carrier pretreatment: 5g Disperse in 50 mL of anhydrous toluene, add 0.35 g of 3-aminopropyltriethoxysilane, and reflux at 80°C for 6 h. Filter and wash with toluene and ethanol in sequence until no silicon is detected in the filtrate, and dry at 120°C for 4 h.

[0040] (2) Single atom loading: Atomic layer deposition technology is used to load Pt single atoms: the functionalized carrier is placed in a hot wall ALD reactor and heated to 250 ° C under nitrogen protection. The precursor (evaporation temperature 100 °C), pulse time 2.0 s, nitrogen purge 30 s, hydrogen reduction 5 s, 50 cycles, to obtain a support with a Pt single atom loading of 0.5 wt%.

[0041] (3) Preparation of phosphorus-modified metal precursor: 、 Dissolve citric acid (3.84 g) in 100 mL of deionized water and stir at 80 °C until clear (a green complex is formed with a characteristic absorption peak at 620 nm on UV-Vis). Stirring was continued for 2 h to obtain a transparent precursor solution.

[0042] (5) Step-by-step calcination: pre-calcination at a heating rate of 2°C / min to 300°C for 2 h, and then main calcination at a heating rate of 3°C / min to 500°C for 4 h. The calcination atmosphere is air.

[0043] Embodiment 4: The preparation method of a catalyst for hydrodearomatization of waste lubricating oil differs from that of Example 1 only in that: (2) Single atom loading: Atomic layer deposition technology is used to load Pt single atoms: the functionalized carrier is placed in a hot wall ALD reactor and heated to 210 ° C under nitrogen protection. The precursor (evaporation temperature 90 ° C), pulse time 0.5 seconds, nitrogen purge 20 seconds, hydrogen reduction 3 seconds, nitrogen purge again 20 seconds, repeated 15 cycles, to obtain a Pt single atom loading of 0.1wt% of the carrier.

[0044] (3) Preparation of phosphorus-modified metal precursor: 、 Dissolve citric acid (3.84 g) in 100 mL of deionized water and stir at 80°C until clear (a green complex is formed with a characteristic absorption peak at 620 nm on UV-Vis). Stirring was continued for 2 h to obtain a transparent precursor solution.

[0045] (5) Step-by-step calcination: pre-calcination at 300°C at a heating rate of 5°C / min for 2 h, and then main calcination at 500°C at a heating rate of 5°C / min for 4 h in air atmosphere.

[0046] Comparative Example 1: The preparation method of a catalyst for hydrodearomatization of waste lubricating oil differs from that of Example 1 only in that: Step (1) Carrier pretreatment: directly use commercially purchased carrier.

[0047] Comparative Example 2: The preparation method of a catalyst for hydrodearomatization of waste lubricating oil differs from that of Example 1 only in that: Step (2) Single atom loading: Instead of using atomic layer deposition technology, the noble metal is loaded by isovolumetric impregnation method. The solution is impregnated into the support.

[0048] Comparative Example 3: The preparation method of a catalyst for hydrodearomatization of waste lubricating oil differs from that of Example 1 only in that: Step (3) Preparation of phosphorus-modified metal precursor: 、 Dissolve citric acid (3.84 g) in 100 mL of deionized water and stir at 80 °C until the solution becomes clear.

[0049] Comparative Example 4: The preparation method of a catalyst for hydrodearomatization of waste lubricating oil differs from that of Example 1 only in that: Step (1) Carrier pretreatment: Directly use traditional single pore size Carrier (no hierarchical pore structure).

[0050] Comparative Example 5: The preparation method of a catalyst for hydrodearomatization of waste lubricating oil differs from that of Example 1 only in that: Step (5) stepwise calcination: without stepwise calcination, directly calcining at 500°C for 4h.

[0051] The catalysts prepared in the examples and comparative examples were tested for their waste lubricating oil hydrodearomatization performance: Test conditions: reaction temperature 350℃, pressure 6.0MPa, hydrogen-to-oil ratio 800, liquid hourly space velocity , reaction time 100h.

[0052] Performance indicators: Raw materials: Waste lubricating oil was obtained from the regeneration unit of a local petrochemical plant, with an aromatic content (ASTM D6591) of 28.3wt% and a sulfur content of <500pm.

[0053] Reaction conditions: catalyst loading amount 1.0 g, reactor is a fixed bed stainless steel tube (inner diameter 8 mm), liquid hourly space velocity, hydrogen flow rate 50 mL / min (hydrogen-to-oil ratio 800).

[0054] Chromatographic analysis: Agilent 7890B gas chromatograph was used with an HP-5 capillary column (30 m × 0.32 mm × 0.25 μm) and an FID detector. Aromatic hydrocarbons were quantified using the internal standard method (n-dodecane).

[0055] Stability definition: After 100 h of continuous reaction, the aromatic hydrocarbon conversion rate retention rate (%) = (conversion rate at the 100th hour / initial conversion rate) × 100%.

[0056] (1) Aromatic hydrocarbon conversion rate: calculated by analyzing the change in aromatic hydrocarbon content before and after the reaction using gas chromatography.

[0057] (2) Product selectivity: Analyze the distribution of hydrogenation products and calculate the selectivity of target products.

[0058] (3) Catalyst stability: activity retention after 100 hours of continuous reaction, reaction conditions: 350℃, 6MPa, 、 .

[0059] Sampling interval: Sampling was performed every 10 h, and aromatic hydrocarbons were quantified by gas chromatography (Agilent 7890B, HP-5 column) using the internal standard method (n-dodecane).

[0060] (4) Metal dispersion: determined by CO chemical adsorption method;

[0061] Sample pretreatment: 0.1 g catalyst, Switch after restore Purge until the baseline is stable.

[0062] CO pulse conditions: 0.1 mL / time, pulse interval 60 s, and the dispersion was calculated after deducting the adsorption of blank carrier.

[0063]

[0064] Table 1 Catalytic performance test results

[0065] According to the data in Table 1, the catalysts prepared in Examples 1-4 all exhibited excellent catalytic performance in the hydrodearomatization reaction of waste lubricating oil, with aromatic conversion rates exceeding 90%, product selectivities exceeding 89%, stabilities exceeding 93%, and metal dispersions exceeding 85%. All performance indicators were significantly better than those of the comparative example, indicating that the technical solution of the present invention has significant advantages.

[0066] Comparative Example 1 lacks support functionalization, resulting in poor anchoring of noble metal single atoms, low metal dispersion, and significantly reduced catalytic performance. Comparative Example 2 uses a traditional impregnation method instead of atomic layer deposition, which fails to achieve precise single-atom loading. While performance improves somewhat, it still falls short of the examples. Comparative Example 3 lacks the electronic regulation of phosphorus, resulting in poor alloy performance. Comparative Example 4 uses a single-pore support, limiting mass transfer performance. Comparative Example 5 lacks step-by-step calcination, resulting in a less-than-optimized catalyst structure. The results of these comparative examples fully demonstrate the necessity and synergistic effects of the various technical features of the present invention.

[0067] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A catalyst for hydrogenation and dearomatization of waste lubricating oil, characterized in that: The catalyst comprises a carrier and an active component supported on the carrier, wherein: The carrier is a hierarchical channel with 3-aminopropyltriethoxysilane functional groups grafted on the surface. , with a dual pore structure of mesopores with a pore size of 8–12 nm and macropores with a pore size of 50–100 nm; The active components include Pt or Pd single atoms and Ni-Mo-P alloy nanoparticles, wherein the loading amount of Pt or Pd single atoms is 0.1-0.5wt%, the loading amount of Ni-Mo-P alloy nanoparticles is 8-18wt%, the total molar ratio of P to Ni plus Mo is 0.1-0.4:1, the molar ratio of Ni to Mo is 2-4:1, and the size of the Ni-Mo-P alloy nanoparticles is 2-5nm.

2. The catalyst for hydrodearomatization of waste lubricating oil according to claim 1, characterized in that: The pore volume distribution of the carrier is such that mesopores account for 60–70% and macropores account for 30–40%, and the specific surface area is 180–250 m² / g.

3. The catalyst for hydrodearomatization of waste lubricating oil according to claim 1, characterized in that: The total loading of Ni and Mo in the catalyst is 8–18 wt %, wherein the loading of Ni is 3–8 wt % and the loading of Mo is 5–10 wt %.

4. The catalyst for hydrodearomatization of waste lubricating oil according to claim 1, characterized in that: The Pt or Pd single atom is anchored on the surface of the carrier through a functional group, and the size of the Ni-Mo-P alloy nanoparticles is 2-5 nm.

5. A method for preparing the catalyst for waste lubricating oil hydrodearomatization according to any one of claims 1 to 4, characterized in that: The following steps are involved: (1) Carrier pretreatment: The support was functionalized with 3-aminopropyltriethoxysilane at 80 °C for 4-6 h; (2) Single atom loading: Pt or Pd single atom precursors are loaded onto functionalized supports using atomic layer deposition technology; (3) Preparation of phosphorus-modified metal precursor: and After complexation with citric acid, ammonium dihydrogen phosphate was added to prepare a P-Ni-Mo ternary complex precursor solution; (4) Impregnation loading: the precursor solution obtained in step (3) is brought into contact with the support obtained in step (2) for impregnation; (5) Stepwise calcination: the impregnated samples were dried, pre-calcined at 300 °C for 2 h, and then calcined at 500 °C for 4 h; (6) Reduction treatment: Reduce at 400 °C for 3 h under hydrogen atmosphere to obtain the catalyst.

6. The method for preparing a catalyst for hydrodearomatization of waste lubricating oil according to claim 5, characterized in that: The specific steps of the atomic layer deposition technology are: (1) Carrier pretreatment: Place the carrier in a hot-wall ALD reactor and heat it to the reaction temperature under nitrogen protection; (2) Precursor adsorption: or The precursor vapor undergoes a chemical adsorption reaction with a pulse time of 0.5-2.0 seconds; (3) First purge: Use inert gas to purge to remove unreacted precursors and by-products, the purge time is 10-30 seconds; (4) Reduction reaction: hydrogen is introduced to perform a reduction reaction to form metal atoms, with a reduction pulse time of 1-5 seconds; (5) Second purge: purge with inert gas again; (6) Cycle repetition: repeat steps (2)-(5) until the target loading amount is reached, 10-50 cycles.

7. The method for preparing a catalyst for hydrodearomatization of waste lubricating oil according to claim 6, characterized in that: The process parameters of the atomic layer deposition are: reactor pressure of 1-10 Torr, carrier preheating temperature of 200-250°C, Pt precursor evaporation temperature of 80-100°C, and Pd precursor evaporation temperature of 120-150°C.

8. The method for preparing a catalyst for hydrodearomatization of waste lubricating oil according to claim 6, characterized in that: The ALD reactor is a hot wall reactor.

9. The method for preparing a catalyst for hydrodearomatization of waste lubricating oil according to claim 5, characterized in that: The heating rate of the step-by-step calcination process is controlled at 2-5°C / min, and the calcination atmosphere is air.

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