Supported composite zinc catalyst and application thereof in biodiesel

Abstract

The invention discloses a supported composite zinc catalyst and application thereof in biodiesel, and belongs to the technical field of catalyst preparation. The supported composite zinc catalyst provided by the invention comprises the following components: an AlPO4-SiO2 composite carrier, a zinc-cerium bimetallic active component supported on the carrier, and a hydrophobic modification layer coated outside the active component. The supported composite zinc catalyst has higher tolerance to moisture and free fatty acid in the waste grease, saponification side reaction is reduced, raw material pretreatment requirements are simplified, the catalyst is easy to recover and reuse, and production cost and environmental burden are remarkably reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of catalyst preparation, and particularly to a supported composite zinc catalyst and its application in biodiesel. Background Art

[0002] The global energy shortage and environmental pollution problems are becoming increasingly serious. As a renewable energy source, biodiesel has become an important alternative to petrochemical diesel due to its advantages such as good biodegradability, low sulfur emissions, and high flash point. Currently, biodiesel is mainly prepared through transesterification or esterification reactions. Among them, waste oils and fats (such as restaurant waste oil, kitchen waste oil, etc.) have become a research and industrial application hotspot due to their wide sources and low costs. However, waste oils and fats contain a relatively high amount of free fatty acids (FFA), which are prone to saponification reactions under conventional base-catalyzed conditions, resulting in catalyst deactivation, difficult product separation, and reduced reaction efficiency. Therefore, developing efficient and stable catalysts to improve the conversion rate of waste oils and fats is the key to promoting the industrial application of biodiesel.

[0003] Chinese Patent Application No. 202411320544.6 discloses a method for preparing biodiesel from waste oils and fats, specifically discloses a method for preparing a composite zinc catalyst using activated carbon as a carrier, and applies it to the conversion of waste oils and fats. The gas-containing activated carbon composite zinc catalyst is placed in the tube body of the activated carbon block. Through the up-and-down movement and "micro-jet" effect of the gas-containing activated carbon composite zinc catalyst, the high specific surface area and pore structure of the activated carbon block are further utilized to increase the adsorption and dispersion of carbon deposition, further improve the absorption and cleaning ability of carbon deposition, maintain the catalytic activity and stability of the catalyst, and thus further improve the production efficiency of biodiesel and the quality of biodiesel. However, during the catalytic reaction process, the gas-containing activated carbon gradually loses gas as the reaction time prolongs, and its structure is prone to collapse or the specific surface area decreases, affecting the long-term catalytic performance; and the micro-jet effect depends on specific hydrodynamic conditions. If the gas flow rate or pressure is not properly controlled, it may lead to uneven dispersion of the catalyst, reducing the contact efficiency between the catalyst and the reactants. Summary of the Invention

[0004] Based on the problems existing in the background art, the present invention provides a supported composite zinc catalyst and its application in biodiesel. The supported composite zinc catalyst in the present invention has higher tolerance to moisture and free fatty acids in waste oils and fats, reduces the occurrence of saponification side reactions, simplifies the raw material pretreatment requirements, and the catalyst is easy to recycle and reuse, significantly reducing the production cost and environmental burden.

[0005] The present invention is implemented through the following technical solutions:

[0006] The first aspect of the present invention discloses a supported composite zinc catalyst, which includes the following component parts: an AlPO4-SiO2 composite support, a zinc-cerium bimetallic active component supported on the support, and a hydrophobic modification layer coated on the outside of the active component.

[0007] Furthermore, the preparation method of the supported composite zinc catalyst includes the following steps:

[0008] S1. Mix aluminum oxide monohydrate with ammonium dihydrogen phosphate solution, add tetraethyl orthosilicate, stir at 50-60 °C for 2-3 hours, adjust the pH to 4-5, continue stirring for 6-10 hours, spray dry to form microspheres, and calcine at 350-400 °C for 3-4 h to obtain the AlPO4-SiO2 composite support;

[0009] S2. Mix cerium nitrate with zinc acetate solution, add the AlPO4-SiO2 composite support, impregnate under vacuum assistance for 6 h, filter, dry, heat up to 450-500 °C in a 5% H2 / N2 mixed atmosphere, calcine for 2-3 h, and cool to room temperature to obtain the metal-loaded catalyst;

[0010] S3. Immerse the metal-loaded catalyst in a silane coupling agent solution, stir and react at 60-70 °C for 20-40 min, filter, then add the metal-loaded catalyst to a poly(N-isopropylacrylamide) solution, continue stirring at room temperature for 0.5-1.5 h, add polytetrafluoroethylene micropowder, ultrasonically disperse for 15-30 min, filter to remove the solvent, and treat in an air atmosphere at 120-130 °C for 1-2 hours to obtain the product.

[0011] Furthermore, in step S1, the mass-volume ratio of aluminum oxide monohydrate to ammonium dihydrogen phosphate solution is (100-160) g:(150-300) mL; the concentration of the ammonium dihydrogen phosphate solution is 0.5-0.6 M; the amount of tetraethyl orthosilicate is 6-10% of the mass of aluminum oxide monohydrate.

[0012] In step S1, aluminum oxide monohydrate and ammonium dihydrogen phosphate react under specific temperature and pH conditions, and tetraethyl orthosilicate is introduced to form a unique composite support structure. AlPO4 provides strong acidic sites and excellent thermal stability, and SiO2 increases the mechanical strength and specific surface area of the material. A significant synergistic effect is produced after the two are combined. The microstructure of the composite support presents a uniform microsphere morphology, the pore distribution is uniform, and the specific surface area is significantly increased, providing an ideal dispersion platform for the subsequent loading of the metal active component. The spray drying and high-temperature calcination processes further strengthen the structural stability of the support, effectively inhibiting the structural collapse of the support and the inactivation of active sites during the high-temperature reaction process.

[0013] Further, in step S2, the concentration of zinc acetate is 0.3 - 0.4 M, and the molar ratio of cerium nitrate to zinc acetate is 1:(4 - 8).

[0014] In step S2, the zinc-cerium bimetallic active component is uniformly loaded on the surface of the AlPO4-SiO2 composite support by vacuum-assisted impregnation. Zinc, as the main catalytic active center, has excellent Lewis acidity and can effectively activate methanol and oil molecules. Cerium, as a co-catalyst, can reduce the activation energy of the transesterification reaction, improve the reaction selectivity and conversion rate through electron transfer and synergy. Cerium also inhibits carbon deposition formation through oxygen vacancies and enhances the anti-poisoning ability of the catalyst.

[0015] Further, in step S3, the concentration of the silane coupling agent is 2 - 6 wt%;

[0016] The concentration of the poly(N-isopropylacrylamide) solution is 1 - 3 wt%;

[0017] The dosage of polytetrafluoroethylene micropowder accounts for 0.5 - 2% of the mass of the metal-loaded catalyst.

[0018] In step S3, first, the surface of the metal-loaded catalyst is modified with a silane coupling agent to improve the compatibility between the catalyst and the subsequent modifiers, and then a preliminary hydrophobic layer is constructed on the catalyst surface. Then, the thermosensitive poly(N-isopropylacrylamide) is added to endow the catalyst with thermosensitive regulation function. At the reaction temperature (65 - 75 °C), it shows a hydrophobic state, forming a selective channel to preferentially allow oil molecules to approach the active center while repelling water and polar impurities, effectively protecting the catalytic active sites. The thermosensitive regulation performance of the catalyst allows the raw material water content to be relaxed to 5%, reducing the pretreatment energy consumption. Additionally, polytetrafluoroethylene micropowder is used to further enhance the surface hydrophobicity and reduce the adverse contact between the catalyst and water and impurities.

[0019] The second aspect of the present invention discloses the application of the supported composite zinc catalyst in the catalytic preparation of biodiesel.

[0020] Further, the method for preparing biodiesel using the supported composite zinc catalyst includes the following steps: pretreat the waste oil to remove solid impurities; add anhydrous methanol and the supported composite zinc catalyst to the pretreated waste oil, and carry out transesterification reaction by heating and stirring. After the reaction is completed, cool the mixed system, filter to recover the supported composite zinc catalyst, let the filtrate stand and separate into layers to separate the biodiesel phase and the glycerol phase; then wash and carry out vacuum distillation on the biodiesel phase to obtain biodiesel.

[0021] Further, the waste oil includes one or more of restaurant waste oil, kitchen waste oil, animal fat, non-edible vegetable oil, and fried waste oil.

[0022] Further, the molar ratio of methanol to waste oil is (12 - 15):1;

[0023] The dosage of the supported composite zinc catalyst is 2 - 4% of the mass of the waste oil;

[0024] The reaction temperature is 50 - 70 °C, and the reaction time is 2 - 3 hours.

[0025] Further, the recovered supported composite zinc catalyst can be used in the next round of biodiesel preparation reaction.

[0026] The catalyst of the present invention is aimed at low-quality waste oils such as catering waste oil with high acid value. The high molar ratio of methanol and an appropriate amount of catalyst effectively promote the transesterification reaction equilibrium. A conversion rate of over 95% can be obtained in 2.5 hours at a temperature of 65 - 75 °C. The self-regulating characteristics of the catalyst enable it to adapt to complex waste oil systems, significantly reducing the raw material pretreatment requirements faced by traditional alkaline catalysts. The catalyst after the reaction is easy to recover and reuse. It has achieved a high yield of 89.5% even after 10 cycles of use, greatly improving the production economy.

[0027] The beneficial effects of the present invention:

[0028] The supported composite zinc catalyst in the present invention adopts a three-level structure (support - active component - hydrophobic modification layer). The AlPO4 - SiO2 composite support utilizes the interaction between aluminum hydroxide monohydrate and ammonium dihydrogen phosphate under specific temperature and pH conditions, and forms a microsphere support with excellent pore structure and high specific surface area by adding tetraethyl orthosilicate, providing a good dispersion platform for the subsequent metal active components. In the zinc - cerium bimetallic active component, zinc acts as the main active center to promote the transesterification reaction, while cerium enhances the catalytic activity and stability through its redox characteristics. The two act synergistically to improve the catalytic efficiency. After the preliminary hydrophobic treatment with a silane coupling agent, poly(N-isopropylacrylamide) forms a temperature-responsive interface on the catalyst surface, and combines with the highly hydrophobic characteristics of polytetrafluoroethylene micropowder to achieve intelligent regulation of the catalytic environment. N-isopropylacrylamide has typical phase transition characteristics. It is in a hydrophobic state at the reaction temperature (65 - 75 °C), preferentially adsorbing non-polar substances and repelling water and sugar impurities, thereby protecting the active center from impurity interference. When the temperature drops to room temperature, N-isopropylacrylamide turns into a hydrophilic state, facilitating the separation and recovery of the catalyst from the product. Polytetrafluoroethylene micropowder has extremely high chemical stability and can withstand acids, alkalis, and most organic solvents. It is not easily degraded even under the harsh conditions of biodiesel synthesis, protecting the catalyst structure from chemical erosion.

[0029] Compared with traditional alkaline catalysts, the supported new composite zinc catalyst of the present invention has higher tolerance to moisture and free fatty acids in waste oils and fats, reduces the occurrence of saponification side reactions, simplifies the requirements for raw material pretreatment, and the catalyst is easy to recycle and reuse, significantly reducing production costs and environmental burdens. Detailed implementation manners

[0030] The technical solutions of the present invention will be further described in detail below in conjunction with specific embodiments, but the protection scope of the present invention is not limited to the following embodiments.

[0031] Example 1

[0032] A preparation method of a supported composite zinc catalyst includes the following steps:

[0033] S1. Mix 300 g of aluminum oxide monohydrate with 500 mL of ammonium dihydrogen phosphate solution (0.5 M), add 24 g of tetraethyl orthosilicate, stir at 60 °C for 2 hours, adjust the pH to 4.5, continue stirring for 8 hours, spray dry to form microspheres, and calcine at 400 °C for 3.5 h to obtain an AlPO4-SiO2 composite support;

[0034] S2. Mix cerium nitrate with zinc acetate (0.4 M) solution, the molar ratio of cerium nitrate to zinc acetate in the mixed solution is 1:6, add the AlPO4-SiO2 composite support, impregnate under vacuum assistance for 6 h, filter, dry, heat up to 500 °C in a 5% H2 / N2 mixed atmosphere, calcine for 2 h, and cool to room temperature to obtain a metal-supported catalyst;

[0035] S3. Immerse the metal-supported catalyst in a 4% ethanol solution of silane coupling agent, stir and react at 70 °C for 20 min, filter, then add the metal-supported catalyst to a 2% ethanol / aqueous solution of poly(N-isopropylacrylamide), continue stirring at room temperature for 1 h, add polytetrafluoroethylene micropowder (the dosage accounts for 1% of the mass of the metal-supported catalyst), ultrasonically disperse for 20 min, filter to remove the solvent, and treat in an air atmosphere at 120 °C for 2 hours to obtain the product.

[0036] Comparative example 1

[0037] A preparation method of a biochar-supported composite zinc catalyst includes the following steps:

[0038] S1. Wash and dry the activated carbon to obtain pretreated activated carbon;

[0039] S2. Mix cerium nitrate with zinc acetate (0.4 M) solution. The molar ratio of cerium nitrate to zinc acetate in the mixed solution is 1:6. Add the pretreated activated carbon, and perform vacuum-assisted impregnation for 6 h. Then filter, dry, heat up to 500 °C in a 5% H2 / N2 mixed atmosphere, calcine for 2 h, and cool to room temperature to obtain the biochar-supported composite zinc catalyst.

[0040] Comparative Example 2

[0041] A preparation method of a catalyst (without hydrophobic modification layer based on Example 1) includes the following steps:

[0042] S1. Mix 300 g of aluminum oxide monohydrate with 500 mL of ammonium dihydrogen phosphate solution (0.5 M), add 24 g of tetraethyl orthosilicate, stir at 60 °C for 2 hours, adjust the pH to 4.5, continue stirring for 8 hours, spray-dry to form microspheres, and calcine at 400 °C for 3.5 h to obtain the AlPO4-SiO2 composite support;

[0043] S2. Mix cerium nitrate with zinc acetate (0.4 M) solution. The molar ratio of cerium nitrate to zinc acetate in the mixed solution is 1:6. Add the AlPO4-SiO2 composite support, perform vacuum-assisted impregnation for 6 h, filter, dry, heat up to 500 °C in a 5% H2 / N2 mixed atmosphere, calcine for 2 h, and cool to room temperature to obtain the supported composite zinc catalyst.

[0044] Comparative Example 3

[0045] A preparation method of a catalyst (not including the treatment with poly-N-isopropylacrylamide and polytetrafluoroethylene micropowder based on Example 1) includes the following steps:

[0046] S1. Mix 300 g of aluminum oxide monohydrate with 500 mL of ammonium dihydrogen phosphate solution (0.5 M), add 24 g of tetraethyl orthosilicate, stir at 60 °C for 2 hours, adjust the pH to 4.5, continue stirring for 8 hours, spray-dry to form microspheres, and calcine at 400 °C for 3.5 h to obtain the AlPO4-SiO2 composite support;

[0047] S2. Mix cerium nitrate with zinc acetate (0.4 M) solution. The molar ratio of cerium nitrate to zinc acetate in the mixed solution is 1:6. Add the AlPO4-SiO2 composite support, perform vacuum-assisted impregnation for 6 h, filter, dry, heat up to 500 °C in a 5% H2 / N2 mixed atmosphere, calcine for 2 h, and cool to room temperature to obtain the metal-supported catalyst;

[0048] S3. Immerse the metal-supported catalyst in a 4% ethanol solution of silane coupling agent, stir and react at 70 °C for 20 min, filter, and treat in an air atmosphere at 120 °C for 2 hours to obtain the product.

[0049] Comparative Example 4

[0050] A preparation method of a catalyst (using single zinc loading based on Example 1 and not containing cerium nitrate) includes the following steps:

[0051] S1. Mix 300 g of aluminum oxide monohydrate with 500 mL of ammonium dihydrogen phosphate solution (0.5 M), add 24 g of tetraethyl orthosilicate, stir at 60 °C for 2 hours, adjust the pH to 4.5, continue stirring for 8 hours, spray dry to form microspheres, and calcine at 400 °C for 3.5 h to obtain an AlPO4-SiO2 composite support;

[0052] S2. Add the AlPO4-SiO2 composite support to a zinc acetate (0.4 M) solution, impregnate under vacuum assistance for 6 h, filter, dry, heat up to 500 °C in a nitrogen atmosphere, calcine for 2 h, and cool to room temperature to obtain a metal-loaded catalyst;

[0053] S3. Immerse the metal-loaded catalyst in a 4% ethanol solution of silane coupling agent, stir and react at 70 °C for 20 min, filter, then add the metal-loaded catalyst to a 2% ethanol / aqueous solution of poly(N-isopropylacrylamide), continue stirring at room temperature for 1 h, add polytetrafluoroethylene micropowder (the dosage accounts for 1% of the mass of the metal-loaded catalyst), ultrasonically disperse for 20 min, filter to remove the solvent, and treat in an air atmosphere at 120 °C for 2 hours to obtain the product.

[0054] Test Example

[0055] The catalysts prepared in Example 1 and Comparative Examples 1-4 were used for the preparation of biodiesel. The specific steps were as follows: Collect the waste cooking oil generated by a certain catering enterprise, remove solid impurities through simple sedimentation and filtration, and measure the acid value to be 10.54 mg KOH / g, the water content to be 0.86%, the density to be 0.92 g / cm 3 , and the viscosity to be 40.5 mm2 / s (40 °C); Add anhydrous methanol (the molar ratio of methanol to waste oil is 13:1) and a 3% supported composite zinc catalyst based on the mass of the waste oil to the pretreated oil, stir and react at 60 °C for 2 hours. After the reaction is completed, cool the mixed system, filter to recover the supported composite zinc catalyst, let the filtrate stand and separate into layers to separate the biodiesel phase and the glycerol phase; then wash and subject the biodiesel phase to vacuum distillation to obtain biodiesel.

[0056] The catalysts prepared in Example 1 and Comparative Examples 1-4 were used for the preparation of biodiesel continuously for 10 times, and the biodiesel yields (%) were measured. The results are shown in Table 1.

[0057] Table 1 Recycling performance of different catalysts for preparing biodiesel from waste cooking oil

[0058]

[0059]

[0060] Meanwhile, the performance of the biodiesel products prepared by the catalysts in Example 1 and Comparative Examples 1-4 in the first and tenth cycles was tested. The specific test results are shown in Table 2.

[0061] Table 2 Performance of biodiesel products prepared by different catalysts in the first and tenth cycles

[0062]

[0063] It can be seen from the data in Table 1 that the supported composite zinc catalyst prepared in Example 1 exhibits excellent stability during the recycling process. After 10 cycles of use, the biodiesel yield of this catalyst still remains at 89.5%, only a 6.1% decrease compared to the initial yield. In contrast, the yields of the catalysts in Comparative Examples 1-4 decreased by 54.6%, 49.3%, 41.2%, and 57.0% respectively. This indicates that the supported composite zinc catalyst prepared by the present invention has significant advantages in cyclic stability. It can be seen from the data in Table 2 that the biodiesel prepared by the supported composite zinc catalyst in Example 1 after 10 cycles still meets the requirements of GB 25199-2017. In contrast, the biodiesel prepared by other catalysts fails to meet the requirements in multiple indicators and cannot meet the quality requirements for commercial applications. The data in Table 1 and Table 2 fully demonstrate the unique advantages of the supported composite zinc catalyst prepared by the present invention in treating waste oils and fats, which are not only manifested in terms of yield and cyclic stability, but also in ensuring product quality.

[0064] The catalyst used in Comparative Example 1 is a biochar-supported catalyst without a hydrophobic protective layer. The initial yield is 92.4%, but it drops sharply to 37.8% after 10 cycles, with an average decrease of about 5.5 percentage points per cycle. The yield decay curve shows an accelerating downward trend, and the decay is more severe during the 6th - 10th cycles. This indicates that although the biochar carrier has a certain specific surface area and pore structure, in the absence of surface modification, the active metal is prone to loss, and the carrier itself has a strong adsorption capacity for impurities in high-acid-value kitchen waste oils, resulting in the rapid occupation of catalytic active sites.

[0065] The catalyst used in Comparative Example 2 has no surface modification. The initial yield is 94.2%, and it drops to 44.9% after 10 cycles. Although this catalyst uses the same composite support as in Example 1, due to the lack of a surface hydrophobic modification layer, its cyclic stability is significantly inferior to that in Example 1.

[0066] The catalyst used in Comparative Example 3 was only modified with a silane coupling agent, with an initial yield of 94.8%, which decreased to 53.6% after 10 cycles. Compared with the catalyst of Comparative Example 2, this catalyst showed better cycle stability, but was still significantly lower than that of Example 1. This indicates that the hydrophobicity provided by the silane coupling agent can protect the active sites to a certain extent, but its protection effect is not as good as that of the hydrophobic modification layer composed of PNIPAM and polytetrafluoroethylene micropowder.

[0067] The catalyst used in Comparative Example 4 was loaded with a single zinc, with an initial yield of 91.5%, which decreased to 34.5% after 10 cycles, showing the largest decrease in yield. This indicates that cerium, as a promoter, has an important stabilizing effect on the zinc active component, and the presence of cerium enhances the anti-poisoning ability of the zinc active sites and inhibits the agglomeration of the active metal through a synergistic effect.

[0068] Finally, it should be noted that the above embodiments only represent several implementation modes of the present invention and are not intended to limit the present invention. For those of ordinary skill in the art, any modifications, equivalent replacements, improvements, etc. made without departing from the concept of the present invention should be included within the protection scope of the present invention. Therefore, the protection scope of this invention patent shall be subject to the appended claims.

Claims

1. A supported composite zinc catalyst, characterized in that The invention comprises the following components: an AlPO4-SiO2 composite carrier, a zinc-cerium bimetallic active component loaded on the carrier, and a hydrophobic modification layer coated on the outside of the active component.

2. The supported composite zinc catalyst according to claim 1, characterized in that The preparation method of the supported composite zinc catalyst comprises the following steps: S1. Alumina monohydrate was mixed with ammonium dihydrogen phosphate solution, ethyl orthosilicate was added, stirred at 50-60 ° C for 2-3 hours, the pH was adjusted to 4-5, stirring was continued for 6-10 hours, spray dried to form microspheres, calcined at 350-400 ° C for 3-4h to obtain an AlPO4-SiO2 composite carrier; S2. The cerium nitrate and zinc acetate solution were mixed, the AlPO4-SiO2 composite carrier was added, vacuum-assisted impregnation was performed for 6 hours, filtered, dried, heated to 450-500°C in a 5% H2 / N2 mixed atmosphere, calcined for 2-3 hours, and cooled to room temperature to obtain a metal-supported catalyst; S3. Immerse the metal-supported catalyst in the silane coupling agent solution, stir the reaction at 60-70°C for 20-40 minutes, filter, then add the metal-supported catalyst to the poly (N-isopropylacrylamide) solution, continue stirring at room temperature for 0.5-1.5 hours, add polytetrafluoroethylene powder, ultrasonically disperse for 15-30 minutes, filter to remove the solvent, and treat in an air atmosphere at 120-130°C for 1-2 hours to obtain.

3. The supported composite zinc catalyst according to claim 2, characterized in that In step S1, the mass volume ratio of aluminum oxide monohydrate to ammonium dihydrogen phosphate solution is (100-160) g: (150-300) mL; the concentration of the ammonium dihydrogen phosphate solution is 0.5-0.6 M; and the amount of ethyl orthosilicate used is 6-10% of the mass of aluminum oxide monohydrate.

4. The supported composite zinc catalyst according to claim 2, characterized in that In step S2, the concentration of zinc acetate is 0.3-0.4M, and the molar ratio of cerium nitrate to zinc acetate is 1:(4-8).

5. The supported composite zinc catalyst according to claim 2, characterized in that In step S3, the concentration of the silane coupling agent is 2-6 wt %; The concentration of the poly (N-isopropylacrylamide) solution is 1-3 wt %; The amount of polytetrafluoroethylene powder used accounts for 0.5-2% of the mass of the metal-loaded catalyst.

6. Use of a supported composite zinc catalyst as claimed in any one of claims 1 to 5, characterized in that: The supported composite zinc catalyst was used to catalyze the preparation of biodiesel.

7. The use of the supported composite zinc catalyst according to claim 6, characterized in that: The method for preparing biodiesel with a supported composite zinc catalyst comprises the following steps: pre-treating waste oil to remove solid impurities; adding anhydrous methanol and a supported composite zinc catalyst to the pre-treated waste oil, heating and stirring to cause an ester exchange reaction, cooling the mixed system after the reaction is completed, filtering and recovering the supported composite zinc catalyst, allowing the filtrate to stand and separate into layers, and separating a biodiesel phase and a glycerol phase; and washing and reducing-pressure distillation of the biodiesel phase to obtain biodiesel.

8. The use of the supported composite zinc catalyst according to claim 7, characterized in that: Waste oils and fats include one or more of waste catering oil, waste kitchen oil, animal fat, non-edible grade vegetable oil, and waste frying oil.

9. The use of the supported composite zinc catalyst according to claim 7, characterized in that: The molar ratio of methanol to waste oil is (12-15):1; The dosage of the supported composite zinc catalyst is 2-4% of the mass of the waste oil; The reaction temperature is 50-70°C and the reaction time is 2-3 hours.

10. The use of the supported composite zinc catalyst according to claim 7, characterized in that: The recovered supported composite zinc catalyst can be used for the next round of biodiesel preparation reaction.

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